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Zoology 


Frontispiece A colorful tidepool community on 
a rocky shore ofF the coast of California. The 
beautiful pink and orange sea anemones are 
so-called because of their resemblance to flow- 
ers. Perched on a rock are two edible mussels, 
Mytilus edulis. In the lower left-hand corner 
is a hermit crab with its soft tail tucked into an 
empty shell for protection. As the crab increases 
in size, it moves into a larger shell, and wher- 
ever it goes the shell travels with it. (Photo 
courtesy of M. Woodbridge Williams.) 


SEVENTH EDITION 


College 
Zoology 


ROBERT W. HEGNER 

Ph.D., Sc.D. 

LATE PROFESSOR OF PROTOZOOLOGY IN 

THE SCHOOL OF HYGIENE AND PUBLIC HEALTH 

OF JOHNS HOPKINS UNIVERSITY 


KARL A. STILES 

M.S., Ph.D. 

PROFESSOR AND HEAD, DEPARTMENT OF 
ZOOLOGY, MICHIGAN STATE UNIVERSITY 


THE MACMILLAN COMPANY 
NEW YORK 


Seventh edition 
© Copyright, The Macmillon Company, 1959 


All rights reserved. No part of this book may be re- 
produced or utilized in any form or by any means, 
electronic or mechanical, including photocopying, 
recording or by any information storage and 
retrieval system, without permission in writing from 

the Publisher. 


Eighth Printing, 1966 


Previous editions © copyright 1912, 1926, 1931, 
1936, 1942 and 1951 by The Macmillon Company 


Renewal copyrights 1940 by Robert W. Hegner and 
1954 and 1959 by Jane Z. Hegner 


Library of Congress catalog card number: 59-5186 


The Macmillon Company, New York 
Coilier-Macmillan Canada, Ltd., Toronto, Ontario 


Printed in the United States of America 


Preface 


HE excellent reception and wide use ac- 
corded the sixth edition of College Zoology 
were very encouraging. The marked im- 
provements in the seventh edition should 
increase its usefulness as a textbook for be- 
ginning students in college zoology. The 
entire book has been reillustrated and re- 
vised; some parts have been rewritten, others 
added, and still others reorganized to make 
it a comprehensive, stimulating, and up-to- 
date work of zoological science. 

A serious effort has been made to achieve 
a good balance between structure, function, 
and principles. The early chapters deal 
broadly with such subjects as classification, 
protoplasm, cellular structure and function, 
and the fundamental aspects of metabolism. 
Thus, these chapters provide an introduc- 
tion to principles that apply throughout the 
Animal Kingdom. However, the basic plan 
of the book has not been altered materially, 
for it is believed that most teachers of gen- 
eral zoology prefer the approach in which 
animals are considered from the simple to 
the complex, including man. There are 
many advantages to this plan of instruc- 
tion: (1) it aids in teaching the scientific 
method, which involves the deduction of 
general principles from many facts; (2) stu- 
dents find it psychologically more satisf}'- 
ing to proceed from the simple to the com- 
plex, and they better retain the idea of the 
division of labor in living things when it is 
presented to them in this order; (3) a back- 
ground in the study of the invertebrates 
helps one to understand the vertebrates; 
(4) although students may have a superfi- 
cial acquaintance with the frog, they ac- 
tually know little about its biology, so there 
is serious doubt concerning the validity of 
the argument that the frog should be stud- 
ied first because of the student's familiarity 
with it; ( 5 ) the great complexity of the frog 
makes its study difficult; therefore, for psy- 
chological reasons as well as for logical ones, 
it should not be studied first; and (6) the 
simple-to-complex approach best introduces 
the student to the principle of organic 
evolution. 


VI 


PREFACE 


Despite all the reasons given for the se- 
quence of material in this book, some ex- 
cellent teachers of zoology prefer the 
method in which the frog and most of the 
basic principles are studied before consider- 
ing representative types in phylogenetic or- 
der. Doubtless different paths may be used 
to reach the same goal. If this approach is 
preferred, the order of chapters should be: 
1, 23, 2, 8, 31, 32, 33, 34, 35, 36, 37; then 
these should be followed by the phylo- 
genetic studies starting with Chapter 3. 
This text is designed to be so flexible that 
the arrangement of the chapters can be al- 
tered in any way to suit the teaching phi- 
losophy of the instructor. 

Most teachers of zoology agree that the 
students who take the introductor)^ course 
in college zoolog\' may be divided into three 
groups: (1) those who will major in this 
field; (2) those who wish to do further work 
to prepare themselves for teaching in high 
schools, or for the medical sciences; and 
(3) those for whom this is a terminal course 
as a part of their general education. A con- 
scious attempt has been made to develop a 
textbook that will satisfactorily serve all 
three groups. 

The introductory course in zoology should 
give the student a knowledge of animals 
that will add greatly to his interest in life; 
it should present the various subjects in 
such a way that he can apply the principles 
of zoologv' to man so as to obtain a better 
understanding of man's place in nature; and 
it should furnish a good idea of the many 
more or less direct relations between man 
and the other animals. In College Zoology 
a definite effort has been made to meet these 
requirements. Reference is made in various 
chapters to human anatomy and physiology, 
especially in Chapters 31 to 34. At the end 
of most chapters, the direct relations of the 
animals under consideration to man are 
presented. 

The discussion of the animal phyla has 
purposely been made more comprehensive 
than is customary to enable each instructor 


to make a choice of representatives of the 
groups; he can select those that best imple- 
ment his own educational philosophy. Ad- 
mittedly, it would be difficult for the aver- 
age student to master the material for all 
forms treated. 

All chapters have been revised to clarify 
the presentation and improve the read- 
abilitv. A few of the more conspicuous new 
features are as follows: hundreds of superior 
drawings by one artist possess a style de- 
signed for clarity, to attract the student's at- 
tention, stimulate his imagination, and im- 
press his memory. The labels have been 
printed, and the margins are in a straight 
line. Also many new photographs have been 
added, including electron micrographs and 
a color photograph of a tidepool com- 
munity. Wherever possible, the drawings 
were based on actual dissection, and the 
photographs are those of living animals. 
Decorative headpieces for the 38 chapters 
suggest the themes of the respective chap- 
ters and also contribute something of in- 
structional value. This edition with over 
1400 illustrations, grouped in 467 figures, 
tries to tell the story of zoolog}' by means 
of the graphic arts. Many legends for the 
illustrations were rewritten and are more 
descriptive than in the previous edi- 
tions. 

The chapters on the invertebrate ph\'la 
do not merely form a survey of these groups, 
but they illustrate the progression of levels 
of organization through evolution. This edi- 
tion contains more material of human inter- 
est and emphasizes the socially significant 
application of zoologv. 

A photograph and line drawing with full 
discussion of the newly discovered deep-sea 
mollusk, Neopilina, is included. This is con- 
sidered to be a more incredible discovery 
than Latimeria, the living coelacanth. The 
explanation of osmosis is in keeping with 
present-day thinking. The newest concepts 
on animal beha\ior have been included. 
Recent advances in organic evolution have 
been incorporated. Consideration is also 


PREFACE 


VH 


given to some of the problems of human 
flight into outer space. 

There is much more emphasis on the 
ecology of communities and populations, 
natural histor\', parasitology, and the scien- 
tific method. The physiologic content has 
been increased, and there is much more 
emphasis on biological principles. New 
material has been added on experimental 
embryology. There is somewhat more em- 
phasis on economic zoology. New sections 
have been added on the enzymes, vitamins, 
hormones, gene action and genetic effect of 
fallout, and many other subjects. A constant 
effort has been made to achieve better in- 
tegration of subject matter throughout the 
text. 

The problem of revising classification is 
always one of the most difficult encountered, 
for the specialists themselves are not in 
agreement. In the matter of classification of 
animals, an author cannot be all things to 
all people. Even a beginning zoology stu- 
dent should learn that there is no such thing 
as a definitive classification of animals. 
However, the classification of each major 
group in this text was checked by an au- 
thority. 

All phylogenetic trees (dendograms) have 
been redrawn to bring them into harmony 
with the newest concepts of animal phy- 
logeny. The number of species in the various 
groups is based mostly on information ob- 
tained by correspondence with authorities. 
If the numbers seem large, it is because 
taxonomists are continually making studies 
that result in an increase in the numbers 
of new species described. 

The references at the ends of the chapters 
give the student a ready entrance to the 
literature; these have been greatly increased 
in number. 

The glossar}' has been made much more 
comprehensive than those usually found in 
introductory texts, because vocabulary stud- 
ies provide evidence that words are of great 
importance in the learning process. We 
keep an object in mind by means of a word 


or symbol; in fact, languages have developed 
from such simple beginnings. A single word 
recalls an experience, as well as a complex 
of ideas associated with it. Therefore, a 
glossary enables the student to learn the 
present meaning of the scientific term as 
well as its origin. By the inclusion of syl- 
labification and accent marks, the student 
is helped in the pronunciation of these 
terms as well. 

A very complete index is also provided so 
that the reader can easily find the informa- 
tion he wishes. 

In an effort to achieve the highest degree 
of authenticity in a subject as broad as gen- 
eral zoology, a specialist in a given field can 
best exercise the critical judgment necessar}' 
for the evaluation of facts in a particulai 
field. Such help was sought and received in 
great measure, as the acknowledgments be- 
low testify. 


ACKNOWLEDGMENTS 

The excellent spirit of cooperation shown 
by the writer's colleagues was a heart-warm- 
ing experience. The friendly and generous 
help of many eminent specialists proves 
that they are interested in improving the 
teaching of general zoology. Their contribu- 
tion guarantees a higher degree of authen- 
ticity than would othenvise have been pos- 
sible. In a very real sense this book has been 
a team effort. 

Above all I am appreciative of the many 
hours of conscientious effort spent by my 
wife, Nettie R. Stiles, in the exacting work 
of editing, proofreading, and indexing. Mrs. 
Olivia Jensen Ingersoll has not only con- 
tributed her outstanding talent as an artist 
in the preparation of all the drawings, but 
as a zoologist she has shown a consistent 
interest in her work which has made for 
clarity in the illustrations. 

Helpful suggestions and critical com- 
ments were made by the following persons 


Vlll 


PREFACE 


whose names are synonymous with scholar- 
ship: Hans Ris, Franz Schrader (Chapter 2), 
C.E. Packard (4 and 6), L.S. West (7), 
M.W. de Laubenfels (9), Libbie H. 
Hyman, J.F. Mueller (10), Libbie H. Hy- 
man (11 and 12), G.R. LaRue (13), R.W. 
Pennak (14), Olga Hartman, A.W. Bell 
(15), T.W. Porter, A.L. Goodrich, Thomas 
Park (16), H.L. King, R.L. Fisher, T.W. 
Porter, J.B. Gerberich (18), B.J. Kaston 
(19), W.J. Clench, R.D. Turner, E.P. 
Cheatum (20), Libbie H. Hyman (21), 
T.H. Bullock (22), J.C. Braddock (23), 
V.C. Applegate, R.C. Ball, C.W. Greaser 
(24), L.P. Schultz, L.M. Ashley, R.A. Fen- 
nell (25), L.P. Schultz, P.L Tack (26), 
CM. Bogert, M.M. Hensley (27 and 28), 
L.M. Ashley (28), G.J. Wallace, A. Wet- 
more, M.D. Pirnie (29), H.E. Anthony, 
R.H. Manville, R.H. Baker (30), C.F. Cairy 
(31), C.F. Cairy, E. Hackel (32), C.F. 
Cairy (33), R.L. Watterson, J.R. Shaver 
(34)', H.O. Goodman, E. Hackel (35), J.R. 
Shaver, J.E. Smith (36), J.C. Braddock, 
A.N. Bragg, R.H. Baker (37), and A.N. 
Bragg (Glossary). 

In addition, the following teachers gave 
assistance in the preparation of the book: 
J.C. Braddock and W.J. Clench. 


I am deeply indebted to the many instruc- 
tors who filled out questionnaires and to the 
graduate assistants who made valuable sug- 
gestions based on their classroom experi- 
ence with this textbook. 

Edwin Ingersoll and other members of 
the Department of Zoology of Miami Uni- 
versity, Oxford, Ohio, gave cooperation and 
help to the artist, Olivia Jensen Ingersoll. 

Other persons who assisted in various 
ways were Bernadette McCarthy Henderson 
and Norman and Patricia Harris. 

The radiograph of a rattlesnake on page 
418 is reproduced by courtesy of the Air 
Forces Institute of Patholog}'. 

Finally, much credit should go to the 
many critical students who refuse to accept 
everything they read on the printed page 
as gospel. 

Because the author has been the final 
judge of all that is presented in this book, 
he alone is responsible for errors or misin- 
terpretations of fact. Suggestions for im- 
provement are not only welcome but greatly 
appreciated. 


Karl A. Stiles 


East Lansmg, Michigan 


Contents 


1. Introduction 1 

2. Protoplasm and Cellular 
Organization 14 

3. Phylum Protozoa. One- 
Ceiled Animals 30 

4. Phylum Protozoa. Flagel- 
lates 42 

5. Phylum Protozoa. One- 
Celled Parasites 50 

6. Phylum Protozoa. Ciliates 54 

7. Relations of Protozoa to 

Man 67 

8. Introduction to the Meta- 

zoa 78 

9. Phylum Porifera. Simple 
Multicellular Animals 92 

10. Phylum Coelenterata 
(Cnidaria). Simple Tissue 
Animals 102 

11. Phylum Ctenophora. 

Comb Jellies 130 

12. Phylum Platyhelminthes. 
Simple Organ-System 
Animals 133 

13. Phylum Nemathelmin- 

thes, Phylum Nemato- 
morpha, and Phylum 
Acanthocephala. Round- 
worms 1 49 

14. Miscellaneous Minor Phyla 163 


X 

15. 


Phylum Annelida. Seg- 
mented Worms 


16. Phylum Arthropoda. 
Crayfish, Crabs, Barna- 
cles, Water Fleas, Sow 
Bugs, and Others 

17. Phylum Arthropoda. Pe- 
ripatus. Centipedes, and 
Millipedes 

18. Phylum Arthropoda. In- 
sects 

19. Phylum Arthropoda. 
Spiders and Their Allies 

20. Phylum Mollusca. Snails, 
Squids, Octopuses, and 
Others 

21. Phylum Echinodermata. 
Starfishes, Sea Urchins, 
Sea Cucumbers, Sea 
Lilies, and Others 

22. Phylum Chordata. Am- 
phioxus, Tunicates, Verte- 
brates, and Others 

23. A Representative Verte- 
brate. Frog 

24. Class Agnatha (Jawless 
Vertebrates). Lampreys 
and Hagfishes 

25. Class Chondrichthyes. 
Cartilaginous Fishes 


CONTENTS 

26. Class Osteichthyes. Bony 

170 Fishes 378 

27. Class Amphibia. Frogs, 
Toads, Salamanders, and 
Others 396 


197 


222 


315 


326 


360 


368 


28. Class Reptilia. Turtles, 
Lizards, Snakes, Croco- 
diles, and Others 405 

29. Class Aves. Birds 432 


228 30. Class Mammalia. Mam- 
mals 463 

267 31. Skeletal Systems and 

Movement 498 

32. Metabolism and Trans- 

281 port in Animals 509 

33. Coordination and Be- 
havior 543 

303 ^^* Reproduction and Devel- 
opment 558 


35. Heredity 


38. History of Zoology 
Glossary 


572 


36. The Origin and History of 
Animal Life 601 

37. Ecology and Zoogeogra- 
phy 628 


Index 


652 
661 
691 


College 
Zoology 


CHAPTER 1 


Introduction 


/jooLOGY is the science that deals with ani- 
mals. It is an old, old science, almost as old 
as man himself. According to current esti- 
mates, man has been living on this planet 
for about a million years, and the science of 
zoology began with his curiosity about life. 

Murals on the walls of rock shelters pic- 
ture the life of people who lived in the 
Sahara Desert between 8000 and 3000 b.c. 
Like many prehistoric people, these early 
artists showed an interest in animal life by 
portraying the various birds and mammals 
that were so closely associated with their 
sundval. 

Animals play a vital role in the survival of 
man today: they feed, clothe, and provide 
him with a means of transportation. His- 
tory, poetry, music, and literature are en- 
riched with references to our animal life. 
Holmes philosophized about ''The Cham- 
bered Nautilus," Saint-Saens composed "A 
Grand Zoological Fantasy," and Frost wrote 
a poem entitled "The Need of Being Versed 
in Country Things." 

Our science had its beginning in the 
earliest times because man had a curiosity 
about animal life and made an effort to place 
living things in groups based on their simi- 
larities. Through the centuries we have con- 
tinued to study the many forms of animals, 
until today more than a million have been 
described and named. 

And probably most zoologists would agree 
with the statement made by St. Augustine 
more than 1400 years ago, when he said: 
"Man wonders over the restless sea, the 
flowing water, the sight of the sky, and 
forgets that of all wonders, man himself is 
the most wonderful." Man is truly a re- 
markable machine, highly complex, and still 
not too well understood as a biologic organ- 
ism. 

Regardless of your role in the world's af- 
fairs, your life is not only enriched by a 
knowledge of living things, but this informa- 
tion will help you in understanding some of 
the most challenging problems of our times, 

1 


COLLEGE ZOOLOGY 


such as population growth, disease, the ef- 
fects of radiation on hfe, and man's survival 
in outer space. 

To provide a background for the study of 
animal life, a brief consideration is given to 
each of the following topics: 

1. The name and distinguishing characteristics 
of each large group of animals. 

2. The features common to all animals, with 
emphasis on the unity of animal life as 
shown by the universal presence of the liv- 
ing substance, protoplasm. 

3. Conditions under which animals live, habi- 
tats. 

4. The value and method of classifying ani- 
mals, classification. 

5. The scope of zoology. 

6. The scientific method and how it aids in 
formulating scientific principles. 

7. The influence of zoology on intellectual 
progress, and its practical value. 


VARIETY AND UNITY 
OF ANIMAL LIFE 

Variety of animal life 

Everyone is familiar with many of our 
common animals and knows something 
about where and how they live; but few 
people realize how many different kinds of 
animals there are and how greatly they 
vary in size, shape, structure, and habits. 
It is easy to observe the larger types such 
as cats, birds, frogs, and even some of the 
smaller ones such as earthworms and flies, 
but a considerable part of the animal king- 
dom consists of forms so minute that they 
can be seen only with the aid of the micro- 
scope. Then there are forms that live in the 
soil, in the ocean, or in other places where 
we do not ordinarily see them. 

No one knows exactly how many different 
kinds of animals there are now in existence, 
but we do know that more than one million 
have been described by zoologists. Fortu- 
nately for us, although they differ from each 


other sufficiently to be recognized as dis- 
tinct kinds (species), they possess char- 
acteristics in common and can be arranged 
in groups. The principal groups are called 
phyla (singular, phylum). Zoologists are not 
in agreement with respect to the number of 
phyla into which the animal kingdom should 
be divided, but usually 11 are studied in 
some detail in a beginning zoology course. 
Representatives of some of the phyla are 
shown in Fig. 430. Besides these, there are a 
few groups of animals of more or less uncer- 
tain relationships such as the Rotifera and 
Br}Ozoa. 

For each phylum, in the brief outline pre- 
sented here, the approximate number of 
known living species is given. Figure 1 shows 
that the Arthropoda comprise about three- 
fourths of all the species of animals. We 
shall find later (Chap. 16, Fig. 130) that 
about 97 per cent of the Arthropoda are in- 
sects. Among the other phyla, the Mollusca 
(snails, clams, etc.), Chordata (fish, birds, 
mammals, etc.), and Protozoa (one-celled 
animals) are the most numerous. The num- 
bers given are estimates by specialists, but no 


Figure 1. There are approximately 1,116,300 
known living species in the entire animal kingdom. 
Of these, 875,000 or approximately 78 per cent are 
arthropods, leaving 241,300 species to account for 
the other animals. 


INTRODUCTION 


one knows exactly how many species have 
been described in any phylum. 

Synopsis of ihe phyla 

Our survey of the animal kingdom will 
treat only the 11 most important phyla out 
of the 20 or more which compose it. These 
11 phyla include about 98 per cent of all 
species of animals. The estimates of num- 
bers of living species are from authorities, 
but new forms are being named all the time, 
so all figures must be regarded as tentative. 

1. Phylum Protozoa 

These animals (30,000 species) are mostly 
microscopic in size, and each consists of a 
single cell or of simple colonies of cells. 
They live in fresh water, in the sea, in the 
soil, and in other moist places, and as para- 
sites on or within the bodies of other ani- 
mals. Some of them, such as the malarial 
organisms and the dysentery amoeba, are 
important in our study because they pro- 
duce disease in man. 

2. Phylum Porifera 

The sponges or pore bearers (5000 spe- 
cies) live only in water— most of them in 
salt water. The body wall is perforated with 
many pores and is usually supported by a 
skeleton of spicules of calcium carbonate, 
silica, or spongin. The commercial bath 
sponge consists of spongin. 

3. Phylum Coelenterata 

Most of the coelenterates (10,000 species) 
also live in salt water. They are the hydroids, 
polyps, jellyfishes, sea anemones, and corals. 
A common fresh-water type is the hydra. 
Coelenterates are radially symmetrical, pos- 
sess single gastrovascular cavities, and are 
provided with peculiar stinging capsules 
called nematocysts. 

4. Phylum Ctenophora 

The ctenophores (100 species) are mostly 
free-swimming marine animals that resem- 


ble the coelenterate jellyfishes and are com- 
monly called sea walnuts or comb jellies. 
They are biradially symmetrical. 

5. Phylum. Platyhelminthes 

These are wormlike, unsegmented, bi- 
laterally symmetrical animals (10,000 spe- 
cies) known as flatworms. Certain tape- 
worms and flukes are serious parasites of 
man and lower animals. Other flatworms 
live on land, in the sea, and a few live in 
fresh water, including planaria, the type usu- 
ally studied in general zoology. 

6. Phylum. Nemathelminthes — Nematodes 
The threadworms or roundworms (12,- 

000 species) are likewise unsegmented and 
bilaterally symmetrical. They possess both a 
mouth and an anus. Many of them are free- 
living, that is, they live in salt water, fresh 
water, or in the soil; but others are parasites 
in plants and animals, such as the hook- 
worm, roundworm, and trichinella of man. 

7. Phylum. Annelida 

The body of an annelid consists of a row 
of little rings or segments; hence the mem- 
bers of this phylum (13,500 species) are 
known as segmented worms. The earthworm 
and leech are common representatives. Salt 
water, fresh water, and the soil serve as habi- 
tats. 

8. Phylum Arthropoda 

The joint-footed animals belong to this 
phylum (875,000 species); they are about 
three times as numerous in species as all 
other animals. The principal groups of 
arthropods are the crustaceans, including 
the lobsters, crayfishes, crabs, and barnacles; 
the centipedes and millipedes with their 
many pairs of legs; the insects, such as but- 
terflies, bees, beetles, bugs; and the arach- 
noids, represented by spiders, scorpions, 
mites, and ticks. 

9. Phylum Mollusca 

Snails, slugs, clams, and oysters are com- 
mon mollusks; others are known as squids. 


COLLEGE ZOOLOGY 


nautili, cuttlefish, and octopi; the phylum 
includes at least 90,000 species. An organ 
characteristic of most of them is a muscular 
foot that usually serves as an organ of loco- 
motion. An enclosing envelope, the mantle, 
is also present. The soft body of many mol- 
lusks, such as the oyster and snail, is pro- 
tected by a shell of calcium carbonate which 
is secreted by the mantle. 

10. Phylum Echinodermata 

A characteristic of most members of this 
group (5000 species) is a spiny skin. It in- 
cludes the starfishes, brittle stars, sea urchins, 
sea cucumbers, and sea lilies. All are marine 
in habit and radially symmetrical; a skeleton 
of calcium carbonate is often present. Loco- 
motion is usually accomplished by means of 
tube feet. 

11. Phylum Chordata 

Except for a few primitive species, the 
chordates (65,700 species) are vertebrates; 
that is, their axial support is made up of 
small bones or vertebrae and is known as the 
vertebral column, or backbone. Vertebrates 
are the most highly developed of all ani- 
mals. They may be divided into 7 classes as 
follows: ( 1 ) the cyclostomes or lamprey eels 
and hagfishes, (2) the cartilaginous fishes, or 
sharks and rays, (3) the common bony 
fishes, (4) the amphibians or frogs, toads, 
and salamanders, (5) the reptiles or alli- 
gators, lizards, snakes, and turtles, (6) the 
birds, and (7) the mammals or four-footed 
animals. The birds and mammals differ from 
the others in that they are warm-blooded; 
that is, their body temperature is constant 
and about 100° F, regardless of the tempera- 
ture of the surrounding medium; whereas 
reptiles, amphibians, fish, and other animals 
are called "cold-blooded" because their body 
temperature varies with that of their en- 
vironment. Actually, cold-blooded is a poor 
name to apply to these animals, for in sum- 
mer the blood of a grasshopper may be 
warmer than that of a man. These so-called 
cold-blooded forms are really animals with- 


out a temperature-controlling mechanism. 
The headpiece at the beginning of this 
chapter helps us to realize how varied ani- 
mal life is, but only a study which we 
are going to make of each of the 11 phyla 
just described can furnish a true idea of the 
remarkable diversities exhibited by the hun- 
dreds of thousands of different kinds of 
animals. 


Unity of animal life 

There is a tremendous variety of animal 
life among the more than one million dif- 
ferent species, yet all these exhibit some fea- 
tures in common. Some common charac- 
teristics will be mentioned here, but they 
cannot be appreciated fully until they are 
studied later in more detail. Many of these 
characteristics are similar to those of plants 
and to those of nonliving matter, but when 
taken together they furnish a means of dis- 
tinguishing animals from all other things. 

Composition 

The essential substance of which all plants 
and animals are composed is known as pro- 
toplasm. Nonliving things do not contain 
protoplasm. 

Structure 

The protoplasm in plants and animals is 
divided into units called cells; nonliving 
things are not divided into cells. 

Form 

Animals are so constant in form that they 
can usually be distinguished from one an- 
other by this characteristic alone. Plants are 
less constant in form, but more so than most 
nonliving things. 

Movement 

Animals can move their parts, and most of 
them are capable of locomotion. Plants, with 
few exceptions, are incapable of locomotion, 
and the same is true of nonliving things. 


INTRODUCTION 


Irritability 

Animals are irritable and respond quickly 
to changes in their surroundings, such as 
changes in temperature and in light. Plants 
respond less quickly, and nonliving things 
do not respond at all. 

Metabolism 

Animals and plants are "machines" that 
run themselves. This is due to the processes 
of metabolism whereby protoplasm is broken 
down to furnish energy and is built up again 
out of food. Animals require other animals 
and plants for food, whereas plants are able 
to manufacture their own food from non- 
living materials. The ability to transform 
environmental material into its own specifi- 
cally organized and active substance is one 
thing that distinguishes living from nonliv- 
ing matter. 

Growth 

Animals and plants grow as a result of the 
building up of protoplasm within the cells. 
Nonliving things may increase in size, but 
the new material is added to the outside. 

Reproduction 

Animals reproduce others of their kind. 
In general, nonliving bodies cannot repro- 
duce their kind. 

The unity of animal life is thus clearly 
c\ident in composition, structure, form, 
movement, irritability, metabolism, growth, 
and reproduction. 

ANIMAL HABITATS 

Most areas on the surface of the earth are 
inhabited by animals. We are familiar with 
many species that live on land; with fresh- 
water inhabitants, such as fish and frogs; and 
with salt-water types, such as seals, whales, 
and sharks. Parasites that live on or within 
the bodies of other animals are less well 
known. The four major habitats of animals 


that are briefly described here are salt water, 
fresh water, land, and other organisms. A 
more detailed account of animal habitats is 
presented in a later chapter on Ecology and 
Zoogeography. 

Salt-water animals 

About 72 per cent of the earth's surface is 
covered by the sea; this salt water serves as 
a home for vast numbers of different kinds 
of aquatic animals. As a rule salt-water ani- 
mals cannot live in fresh water or on land. 
Furthermore, they do not roam over the 
ocean at will, but are restricted to definite 
habitats. For example, a large number of 
animals are found only on the beaches; some 
live on sand beaches and others on mud 
beaches; some are attached to rocks and 
others live among seaweeds. The open ocean 
is thickly populated with animals; many are 
able to swim about, but others float near 
the surface and are carried from place to 
place by waves and currents. As a rule each 
species seeks a certain depth and does not 
move up or down beyond a more or less 
narrow range. 

Plants and animals that live in the sea 
usually sink to the bottom when they die. 
On this account the sea bottom is a favor- 
able habitat for scavengers, and a distinct 
group of animals lives in this debris. Each 
of these sea habitats— the beaches, open 
ocean, and sea bottom— may be subdivided 
into several minor habitats, which indicates 
how restricted animals really are in the 
character of their environment. The study 
of the relation of living things to their en- 
vironment is called ecology. The marine 
animals alone may be divided into about 
50 groups, according to the nature of the 
environments in which they live. 

Fresh-water animals 

Fresh-water animals live in lakes, ponds, 
pools, rivers, streams, swamps, and bogs. 
Some prefer flowing water, and others prefer 


COLLEGE ZOOLOGY 


standing water. They may swim about freely, 
float on the surface, or crawl on the bottom, 
and among the water plants. Each species 
occurs in a definite type of minor habitat. 
Such factors as the swiftness of the stream, 
the character of the vegetation, the depth of 
the water, and the nature of the bottom de- 
termine what species of animals are present. 

Terrestrial animals 

We are more familiar with animals that 
live on land than with aquatic species. Those 
on land are called terrestrial animals. Many 
live on the surface; others burrow beneath 
the surface, thus becoming subterrestrial; 
many make their homes in trees (arboreal 
species) and in other plants; and a few, 
known as aerial animals, spend a large part 
of their time in the air. The surface-dwelling 
animals may prefer either wet or dry ground, 
humus, sand, or rocks. Subterrestrial animals 
are profoundly influenced by the character 
of the soil in which they live. Plant-dwelling 
animals may live in evergreen (coniferous) 
or in deciduous trees, on the bark or in the 
wood, in dead wood or in living wood, on 
the fruit or among the leaves. Aerial animals 
may fly or simply glide through the air, or 
may be carried about passively by some 
balloonlike contrivance. 

Parasitic animals 

A parasite is an organism that lives the 
whole or part of its life on or within another 
organism of a different species, from which 
it obtains its food. Parasites occur among 
both plants and animals. Many animals that 
live in water are parasitized by other animals 
that creep over them or are attached to their 
surfaces. Most parasites of terrestrial animals 
live within the bodies of their victims; they 
are inhabitants of the digestive tract, the 
blood, and the muscle. Almost every large 
group in the animal kingdom contains para- 
sites, but the parasites are mostly protozoans, 
flatworms, roundworms, annelids, insects, 
mites, and ticks. 


Adaptations of animals 
to their environment 

A study of the relation of animals to their 
environment reveals many ways in which 
they are adapted to the particular habitat 
in which they flourish. These adaptations in- 
volve all organs and all physiologic processes 
that make up the activities of the animal. 
Different animals are adapted to similar con- 
ditions in different ways. Thus aquatic in- 
sects and fish are able to move and breathe 
under water, but the methods by which 
these activities are accomplished are very 
different. A review of the structure and be- 
havior of any animal will show how wonder- 
fully it is adapted to life in its particular ei> 
vironment. Each species of animal, however, 
is not adapted to a certain habitat to the ex- 
clusion of other species— many species of 
animals and plants may live in one habitat. 
Animals, when associated together, form 
what are known as animal communities. An 
attempt has been made by students of ecol- 
ogy to classify these communities. It is a 
comparatively simple matter to determine 
what species of animals occupy a certain 
habitat, but it is more difficult to work out 
the actual physiologic relations between the 
animal and the various factors in its en- 
vironment — only a beginning has been made 
in this direction. 

Maintenance of the individual 

We have already noted that each species 
of animal is limited to some particular type 
of habitat. The problems involved in merely 
existing in these habitats are many and 
varied. In the first place, each animal must 
protect itself from competitors, enemies, and 
harmful physical agents. It must find proper 
food and then capture and ingest it. Phys- 
iologic processes within the body must 
bring about digestion, transportation, and 
assimilation of this nutritive material. Other 
processes within the body must lead to lib- 
eration of energy for the animal's various ac- 
tivities. Oxygen must be taken in and carbon 


INTRODUCTION 


dioxide expelled. Secretions for digestive and 
other purposes must be elaborated, and poi- 
sonous excretions discarded. Only the fittest 
among each species sur\ave in the desperate 
struggle for existence. 

Maintenance of the race 

The ability of an animal to maintain itself 
in its habitat is not enough; it would soon 
die out if others of its kind were not repro- 
duced. As a matter of fact, the powers of 
reproduction of animals are enormous; any 
species would soon overrun the world if all 
offspring were to grow to maturity and repro- 
duce their kind. The struggle for existence, 
due largely to limits in space and food sup- 
ply, is responsible for the destruction of 
most of the young that are brought into the 
world each year. The number of each species 
of animal is thus kept more or less constant 
from year to year. Occasionally a species be- 
comes extinct, such as the passenger pigeon 
(Fig. 334), or unusually abundant, as the 
lemming, but ordinarily a state approaching 
equilibrium exists in nature with respect to 
the number and character of the animals 
present in any locality. 

SCIENTIFIC CLASSIFICATION 
OF ANIMALS 

When a large number of dissimilar objects 
are collected, it is natural to place them in 
groups according to the presence or ab- 
sence of certain characteristics. This is called 
classification. The science of classification is 
known as taxonomy. Animals may be classi- 
fied in several ways. 

Artificial classification 

This groups animals according to some 
superficial resemblance in structure, color, 
habitat, etc. For example, certain animals 
are called aquatic because they live in the 
water; others are called terrestrial, because 
they live on land; some are called carnivo- 


rous because they eat flesh; others are called 
herbivorous because they live on vegetable 
food; and still others are called omnivorous 
because they devour both animal and vege- 
table matter. This is called artificial classifi- 
cation, and it is often convenient to use. 

Natural classification 

For all scientific work, natural classifica- 
tion is employed. This is based on similarity 
in structure, physiology, embr}'ology, and 
other factors. Natural classification is based 
on the principle of evolution and is an effort 
to show true genetic relationships of ani- 
mals. A number of large divisions of the 
animal kingdom known as phyla are recog- 
nized by zoologists. Each phylum is made up 
of one or more classes, each class of one or 
more orders, each order of families, each 
family of genera, and each genus of species. 

A phylum is a wide group of animals hav- 
ing some characteristics in common. A class 
is a somewhat narrower group, composed of 
individuals which have not only the struc- 
tures peculiar to the phylum, but additional 
common structural characteristics. An order 
is a still smaller group in which the individ- 
uals have the same phylum and class char- 
acteristics, and, in addition, some common 
characteristics peculiar to the order. Like- 
wise, the family, genus, and species repre- 
sent smaller and smaller groups of individ- 
uals which possess the characteristics of the 
larger groups, but, in addition, each has its 
own identifying characteristics. 

The timber wolf, for example, belongs to 
the species lupus of the genus Canis. This 
genus and others, such as the genus Vulpes, 
which contains the red fox, constitute the 
family Canidae. The Canidae are included 
with the bears (family Ursidae), the seals 
(family Phocidae), and a number of other 
groups of flesh-eating animals in the order 
Carnivora. Nineteen related orders, of which 
the Carnivora form one, are placed in the 
class Mammalia. Mammals possess hair and 
mammary glands; these characteristics dis- 
tinguish them from the six other classes 


8 


COLLEGE ZOOLOGY 


that make up the subphylum Vertebra ta or 
animals possessing vertebral columns. The 
subphylum Vertebrata, together with three 
other subphyla usually called primitive chor- 
dates, are grouped together in the phylum 
Chordata, which contains animals possess- 
ing at some time in their existence an in- 
ternal rodlike support known as the note- 
chord (Fig. 207, p. 324). 

Classification of a species 

The scientific name of any animal con- 
sists of the terms used to designate the genus 
and species; the first letter of the genus 
name is a capital, but the first letter of the 
species name is always a small letter. The 
genus and species names are commonly 
followed by the name of the zoologist who 
wrote the first valid description of that par- 
ticular species. The scientific name of the 
timber wolf is therefore written Canis lupus 
Linnaeus. 

The complete classification of the timber 
wolf may be shown in outline in the follow- 
ing manner: 

Animal Kingdom (consists of all known ani- 
mals) 
Phylum Chordata (animals possessing noto- 
chords ) 
Subphylum Vertebrata (chordates with 
vertebral columns) 
Class Mammalia (vertebrates with 
mammary glands) 
Order Carnivora (mammals that eat 
flesh) 
Family Canidae (carnivores that 
walk on their toes) 
Genus Canis (Canidae with 
round pupils in their eyes) 
Species lupus [lupus means 
wolf) Fig. 368 

The classification of man, which is the 
same as that of the wolf up to the order, is 
as follows: 

Phylum Chordata 

Subphylum Vertebrata 
Class Mammalia 


Order Primates (possess four limbs, each 

with five digits which usually end 

in nails, not claws) 

Family Hominidae (As compared 

with apes, the brain is larger; the 

face more vertical; lower jaw less 

protruding; and the teeth more 

evenly sized. The hair is long on 

the head, but scant on the rest 

of the body. The legs are longer 

than the arms; the thumbs are 

well developed; and the big toe 

is not opposed to the other 

digits.) 

Genus Homo (man) 

Species sapiens (means reason- 
ing) . Thus it will be seen that 
the scientific name of man is 
Homo sapiens Linnaeus. 

Latin or Latinized names are used for 
genera and species. The genus name is a 
noun, and the species name is usually an 
adjective. Intermediate terms such as sub- 
order, subfamily, subgenus, and subspecies 
are also in use. The typical grizzly bear, for 
example, is named Ursus horribilis, but large 
specimens with long ears occur in central 
California that belong to the subspecies 
Ursus horribilis californicus. 

What is a species? 

The exact meaning of the term species is 
rather difficult to explain. A species consists 
of a group of animals that mate with one 
another and that resemble one another 
more than they do individuals in other 
groups of animals. All members of a species 
possess certain characteristics in common, 
but differ from one another in various re- 
spects. For example, all wolves of the species 
Canis lupus (timber wolves) are large, 
with a body about 55 inches long, a tail 
about 10 inches long, and a weight of 
about 100 pounds. Their color is gray, vary- 
ing to blackish on the back and tawny on 
the belly. Timber wolves vary among them- 
selves: in the density of their color (some 
are lighter than others), in the length of 
the body and tail, in weight, and in other 


INIRODUCTION 


characteristics; but they breed with one an- 
other and are more Hke each other than 
they are like other wolves. The prairie wolf 
or coyote {Canis latrans), in contrast, is 
smaller and more slender, with a body about 
49 inches long, a tail about 16 inches long, 
and a weight of only about 25 pounds. Its 
color is tawny, clouded with black, and its 
tail is tipped with black. Timber wolves and 
prairie wolves, as their common names indi- 
cate, live in different types of habitats. 

The following is a good definition of a 
species: A species may be defined as con- 
sisting of groups of interbreeding natural 
populations, which may differ markedly 
among themselves, yet resemble each other 
more closely than the members of any other 
groups, and which are reproductively iso- 
lated from other such groups. 

Origin of modem classification 

Many attempts to classify animals were 
made before the present system was per- 
fected. The Greek scientist Aristotle (384- 
322 B.C., p. 652) attempted to classifv ani- 
mals according to their similarities in 
structure and succeeded so well that practi- 
cally no improvements were made until the 
time of Linnaeus (1707-1778, p. 654). This 
Swedish scientist, instead of giving animals 
common names which might be used for 
different species in different localities, estab- 
lished a universal system of classification; 
this is the binomial nomenclature still in 
use, and gave for each species a concise de- 
scription in Latin. He succeeded in listing 
4378 different species of animals and plants. 
His greatest work entitled Systema Naturae 
was published in 1735. It passed through 12 
editions, and the tenth (1758) has been 
agreed upon as the basis for zoological no- 
menclature. The work of Linnaeus stimu- 
lated other naturalists to discover and name 
new species of animals. At first this was the 
only end in view, but at the present time 
taxonomists are interested mainly in the 
evolution of animals in general, and espe- 
cially in tlie groups which they are studying. 


Rules of nomenclature 

In 1901 the International Congress of 
Zoology organized an International Commis- 
sion on Zoological Nomenclature, which has 
served since that time. The Commission has 
prepared a set of International Rules of 
Zoological Nomenclature; these rules apply 
to family, subfamily, generic, subgeneric, 
specific, and subspccific names. They cover 
the formation, derivation, and correct spell- 
ing of zoological names, the author's name, 
the law of priority and its application, and 
the rejection of names. According to these 
rules, zoological and botanical names are 
independent; and the same genus and spe- 
cies name may be applied to both an animal 
and a plant, although this is not recom- 
mended. Scientific names must be Latin 
or Latinized. Family names are formed by 
adding idae to the stem of the name of the 
type genus. Generic names should consist 
of a single word, written with a capital 
initial letter, and italicized. The names of 
species are adjectives, agreeing grammati- 
cally with the generic name, or substantives 
in the nominative, in apposition with the 
generic name, or substantives in the geni- 
tive; they should be italicized. The author 
of a scientific name is the first person to 
publish the name with a definition or de- 
scription of the organism. If a new genus is 
proposed, it is necessary to publish a de- 
scription of it, to designate a type species of 
the genus to describe it, and to tell the col- 
lection in which it has been placed. The list 
of International Rules of Zoological Nomen- 
clature was published in a text titled Pro- 
cedure in Taxonomy, 1956, by Schenk and 
McMasters. 


Derivation of terms 

Every subject has its own vocabulary 
which must be learned by the student. New 
terms have more meaning and are easier to 
remember if their derivation is known. For 
this reason, the derivations of many of the 
common scientific terms used in zoology are 


10 

given in this book; some are in the text 
proper, but more appear in the Glossary. 
Most of our scientific terms came from 
Greek (Gr.) and Latin (L.) words. 

SCOPE OF ZOOLOGY 

Fields of the zoological sciences 

Zoology (Gr. zoioir, animal; logos, dis- 
course) is the science of animals, whereas 


COLLEGE ZOOLOGY 

botany is the science of plants. The com- 
bined study of animals and plants forms the 
science known as biology. The facts about 
animals alone and the methods of studying 
them have become so numerous that one 
man in his lifetime can master and become 
an authority on only one, or at most, a few 
phases of the subject. It has, therefore, been 
found necessary and convenient to divide 
zoology into a number of sciences. Some of 
the principal subdivisions of zoology are in- 
dicated in Fig. 2). 


CYTOLOGY 

Structure and 
functions with- 
in cells 


ANATOMY 
Structure of the 
animal body 


PATHOLOGY 

Nature of diseases, 
causes and symptoms 


PHYSIOLOGY 

Functions of organisms 

ECOLOGY 

Relations of organisms 
to their environment 


PALEONTOLOGY 
Fossil organisms 


SOCIOLOGY 

Animal societies 
including man 


HISTOLOGY 

Microscopic structure 
of tissues and organs 


TAXONOMY 
Classification of 
organisms 


GENETICS 
Heredity 

EMBRYOLOGY 
Developmental stages 
of organisms 


ZOOGEOGRAPHY 
Geographical distribu- 
tion of animals 


Figure 2. Some of the main subdivisions of zoology with concise definitions. 


Many other zoological fields are recog- 
nized other than those in Fig. 2. These are 
often devoted to a study of a group of ani- 
mals of special interest or importance. For 
example, parasitology is the study of para- 
sitic organisms; protozoology, of Protozoa; 
entomology, of insects; malacology, of mol- 
lusks; ichthyology, of fish; herpetology, of 
reptiles and amphibians; ornithology, of 
birds; mammalogy, of mammals; medical 
zoology of animals that affect the health of 
man, etc. 


Certain zoological sciences are involved in 
the study of each of the animal types de- 
scribed in this book. These include particu- 
larly those dealing with gross structure 
(anatomy), microscopic structure (histol- 
ogy), cellular structure (cytology), develop- 
ment of the individual (embryology), func- 
tion (physiology), behavior (psychology), 
classification (taxonomy), and origin (phy- 
logeny). Certain zoological sciences of a 
more general nature are considered in sepa- 
rate chapters; these are nutrition, coordina- 


E^TRODUCTION 


11 


tion and behavior, the relations of animals 
to their environment (ecology), the geo- 
graphic distribution of animals (zoogeogra- 
phy), heredity (genetics), reproduction and 
development, the origin and history of ani- 
mal life (organic evolution), and the history 
of zoology. 

Science and its methods 

One of the objectives of a course in zool- 
ogy is to gain an understanding of the 
scientific method. The method of science 
involves primarily (1) being aware that a 
problem exists, (2) formulating a supposi- 
tion (hypothesis) on the basis of a rela- 
tively small amount of information, (3) 
testing the correctness of the hypothesis by 
securing more facts by direct observation or 
experimentation, (4) arranging the facts 
observed in some orderly manner to deter- 
mine relationships, and (5) drawing valid 
conclusions. It is by this logical procedure 
that most of our zoological principles have 
been developed. 

The scientific method involves skillful 
handling of the material being studied, care- 
ful observations, controlled experiments if 
possible, close attention to detail, clear 
thinking in drawing conclusions, and the 
modification of conclusions when further 
facts make this necessary. This is the method 
of discovery. 

Attitudes are also very important in solv- 
ing problems by the scientific method. They 
include (1) intellectual honesty, that is, 
freeing oneself of prejudice and admitting 
an error when facts indicate that there is 
one, (2) openmindedness about a subject, 

(3) cautiousness in reaching conclusions, 

(4) a willingness to repeat experiments (the 
facts obtained by one experimenter must be 
verified by others as well as himself, so that 
conclusions are confirmed), and (5) vigi- 
lance for the occurrence of possible flaws in 
hypotheses, theories, evidences, and conclu- 
sions. 

Anyone can make discoveries in zoology 


with very little training, and few human ex- 
periences can furnish such a thrill as that 
of making an original discovery. 

Principles of zoology 

Zoological principles are scientific theories, 
facts, and laws of wide application. It is 
possible to make a list of zoological princi- 
ples and to discuss them with the aid of 
photographs or laboratory material, but the 
best method of learning them is to study 
animals and deduce principles after a suf- 
ficient amount of original data has been 
accumulated. This book has been prepared 
with this aim in view. After each chapter has 
been studied and the appropriate laboratory 
studies have been completed, a careful re- 
view should be made of the knowledge thus 
obtained, and a list of zoological principles 
prepared. For example. Chapter 3 is devoted 
to the class Sarcodina of the phylum Pro- 
tozoa, and the amoeba is employed as a 
typical species. After studying this species 
and possibly other Sarcodina in the labora- 
tory and reading the account in this book, 
one of the principles which will be evident 
is that every member of the class Sarcodina 
consists of a single cell. From this principle 
we may derive the subordinate principle 
that among the Sarcodina a single cell car- 
ries on all of the physiologic processes nec- 
essary for maintaining the individual and 
the race. When all classes of the Protozoa 
have been studied in the chapters that fol- 
low, principles that are applicable to the 
entire phylum may be deduced. Later in 
the course, principles that apply to several 
phyla and finally to the entire animal king- 
dom may be formulated. 

Zoology and human progress 

The study of animals has been of great 
intellectual and practical value to man. It 
has enabled him to recognize the unity of 
all living things and to determine his place 
in nature. Zoological knowledge has made it 


12 

possible for man to adjust himself more 
successfully to his environment. It has freed 
him from many superstitions (Fig. 3) and 
fears by explaining, one by one, the mys- 
teries that had held him in bondage for 
many centuries. Studies of living things have 
revealed the ever changing nature of the 
vvodd of life and have furnished a simple 
explanation, namely, organic evolution, that 
has revolutionized modern thought. A stu- 


Figure 3. West Africa medicine man and as- 
sistants. One of his superstitious treatments is to 
take the fin of a fish, the tail of a rat, the head 
of a snake, and the foot of a fowl; tie them to- 
gether in a bundle; place the bundle beneath the 
nose of a patient and ask him to inhale deeply; his 
headache is supposed to disappear. 

dent of zoology (1) learns about himself 
through the study of animals; (2) learns the 
scientific method, which will effectively as- 
sist him throughout his entire life no mat- 
ter in what field his labors fall; and (3) 
gains an esthetic appreciation of nature that 
can be acquired in no other way. 

Value of zoology 

The practical value of zoology can hardly 
be overestimated. Zoology and botany form 


COLLEGE ZOOLOGY 

the basis of medicine, dentistry, veterinary 
medicme, medical technology, nursing, op- 
tometry, medical dietetics, museum work, 
zoological teaching, zoological research, agri- 
culture, and conservation. Biological studies 
are responsible for our pure water, pure food, 
balanced diet, and protection against animal 
parasites and disease agents. Recently ac- 
quired knowledge of heredity has revolu- 
tionized plant and animal breeding and has 
had some effect on that of human beings. 
What were once considered to be inexhausti- 
ble resources in this country have for some 
years been in need of conservation. Only 
with the aid of a broad knowledge of biol- 
ogy can our conservation program be carried 
out successfully. 

A state approaching equilibrium exists on 
the earth with respect to the association of 
plants and animals. In this world of living 
organisms, a terrific struggle for space and 
food is continually going on, and the situa- 
tion that results is extremely complex. Part 
of this struggle involves human beings. Man 
is associated with other animals in many 
ways; some are of value to him, others arc 
of no particular use, and a few are decidedly 
harmful. 

Use of animals for scientific research 

Lower animals are largely used for scien- 
tific research, and much that is learned in 
this way can be translated more or less di- 
rectly into human terms. Thus a large part 
of what we know about heredity has been 
learned from the study of fruit flies, and 
most of the work on vitamins has been done 
with rats. Experiments on animals have 
given us much of our knowledge of physio- 
logic processes and have enabled us to de- 
velop effective methods of surgery. Drugs 
are first tested on animals before being used 
for human treatment, and many new drugs 
have been discovered as a result of animal 
experimentation. Millions of diabetics are 
alive today because of the experimental 
work which sacrificed the lives of only about 
30 dogs. The lower animals also benefit 


INTRODUCTION 


13 


from the research on them. Without ani- 
mal experimentation there might be no 
protection against rabies, smallpox, diph- 
theria, typhoid and undulant fevers, and 
many other diseases which plague the ani- 
mal world. The value of lower animals in 
scientific work generally cannot be overem- 
phasized. 

Food and animal products 

Animals are very useful to man because 
of their value as food. Almost every phylum 
or class of the larger animals contains at 
least a few species that reach our tables. 
These include especially the shellfish, lob- 
sters, crabs, shrimps, fish, turtles, frogs, 
birds, and mammals. We depend largely, of 
course, on domesticated birds and mammals 
for our supply of meat. Animal products are 
hardly less important; among these are 
sponges, corals, pearls and pearl buttons, 
honey, beeswax, silk, tortoise shell, feathers, 
fur, and leather. 

Harnifid animals 

Destructive animals fall principally into 
two types, predaceous animals and parasites. 
We need not fear direct attacks of predatory 
animals, but many useful wild and domestic 
animals are killed by them. Parasites not 
only destroy or make unhealthy large num- 
bers of useful wild and domestic animals, 
but also attack man, and every year bring 
sickness or death to millions of human be- 
ings. These parasites are mostly protozoans, 
flatworms, roundworms, and insects. The 
insects, mites, and ticks not only attack man 


directly, but many also carry disease germs 
from nonhuman animals to man, from man 
to man, or from animal to animal. A few 
animals, including certain insects, spiders, 
scorpions, fishes, and snakes, are poisonous 
to man. More details regarding the relations 
of the various types of animals to man are 
presented in the chapters which follow, 

SELECTED COLLATERAL 
READINGS 

The books listed here and in other chapters 
comprise a few selected works and are intended 
only as suggestions to the beginning student. 
Many of the texts cited have extensi\e bibliog- 
raphies which give a ready entrance into the 
zoological literature. The following works in- 
clude taxonomic reviews of the animal king- 
dom: 

Caiman, W.T. The Classification of Animals: 
An Introduction to Zoological Taxonomy. 
Methuen, London, 1949. 

Hyman, L.H. The Invertebrates: Protozoa 
Through Ctenophora. McGraw-Hill, New 
York. 1940. 

Manville, R.H. "The Principles of Taxonomy." 
Turtox News, 30: No. 1 and No. 2, 19 52^ 

Mayr, E., Linsley, E.G., and Usinger, R.L. 
Methods and Principles of Systematic Zool- 
ogy. McGraw-Hill, New York, 1953. 

Schenk, E.T., and McMasters, J.H. Procedure 
in Taxonomy. Stanford Univ. Press, Stan- 
ford, 1956. 

Simpson, G.G. The Principles of Classification 
and a Classification of Mammals. Bull. Am. 
Mus. Nat. Hist., Vol. 85, New York, 1945. 


PROTOPLASM: THE PHYSICAL 
BASIS OF LIFE 


CHAPTER 2 


o«;i 


Protoplasm and 

Cellular 

Organization 


What is life? 

This is a question the biologists have been 
trying to answer for centuries. As a matter 
of fact, biology may be defined as the sci- 
ence of life. A fly buzzing about on a win- 
dow pane is certainly alive, but after it is 
swatted successfully, it is just as certainly 
dead; life has departed from it. The most 
obvious change that has taken place in the 
fly is the loss of its ability to move and to 
take in food. It has lost the power to respond 
in any way to stimuli; for example, we can 
poke it with a pencil without observing any 
reaction. Evidently the visible activities of 
the fly have ceased. As we shall see later, the 
cessation of visible activities is due to the 
cessation of activities within the substance 
of the body. This living substance is known 
as protoplasm. As long as protoplasm is able 
to carry on its activities, it is alive; when 
these activities cease, it is no longer alive. 
Therefore, life may be studied in terms of 
the activities of protoplasm. 

Most of our present knowledge of biology 
is attributable to a century of work on the 
chemistry and structure of protoplasm. In 
fact, if we want to know what makes the 
heart beat, a cell divide, or any other normal 
function of the body, we seek explanations 
in terms of the protoplasm that is in all liv- 
ing cells. Since disease and aging result from 
changes in the normal activities of proto- 
plasm, understanding of normal protoplasm 
is one of the best approaches to understand- 
ing disease, for diseases are, in the final anal- 
ysis, problems of protoplasm. 

Physical organization 
of protoplasm 

The structure of protoplasm cannot be 
seen with the naked eye, hence we can learn 
about it only with the aid of a microscope. 
The amoeba, to be studied later, affords an 
excellent opportunity to make actual ob- 


14 


PROTOPLASM AND CELLULAR ORGANIZATION 


15 


servations on naked living protoplasm. A few 
bodies can be seen in living protoplasm, but 
most of the structures are practically color- 
less. This makes it necessary to treat it with 
dyes which stain certain parts. Many differ- 
ent dyes have been employed and numerous 
methods have been devised for the study of 
protoplasm. While most of these result in 
the death of the protoplasm, the structure is 
probably not changed very much. 

When examined with a microscope, pro- 


toplasm usually looks like a grayish jelly in 
which may be embedded granules and glob- 
ules of various sizes and shapes. It differs 
under various conditions; usually it is about 
the consistency of glycerin, somewhat vis- 
cous but capable of flowing. Protoplasm may 
exist as a sol that streams easily, or as a 
more solid gel; under certain conditions it 
may change from a sol to a gel, or a gel to 
a sol, and back again; this is the unique prop- 
erty of a colloid. 


sc»«m»b.Ci^ 


Figure 4. Colloidal states. Ultramicroscopic structure of a sol and a gel (diagrammatic). Left, 
a sol state. The colloidal particles are represented as circles of different diameters and the water 
particles (molecules) as dots. Such a solution has the physical properties of a liquid. Right, a gel 
state. The colloidal particles adhere together to form a continuous network. Such a substance 
has the physical properties of a semisolid substance (jellylike), which tends to be elastic. The 
arrows show that the sol and gel states are reversible under appropriate conditions. 


Many minute granules can be seen in 
protoplasm with the aid of a microscope. 
When the protoplasm is in a liquid or sol 
state, the granules may be observed moving 
about. This is known as Brownian move- 
ment, having been discovered by an English 
botanist, Robert Brown, in 1827. This type 
of movement is due to invisible particles 
striking against larger granules. It also oc- 
curs in water and other liquids and is not 
necessarily a sign of life. 

Certain knowledge of the fine structure 
of protoplasm has been contributed by the 
physical chemists. They tell us that proto- 
plasm is a colloid and all life is associated 
with the colloid state. Many of the prop- 
erties of protoplasm depend on the fact that 


it is a colloid, a mixture in which compara- 
tively large but still invisible particles are 
suspended in a liquid medium, that is, they 
do not settle out. The particles are estimated 
to range in size from O.OOOI to O.OOOOOI mm. 
in diameter. Colloid suspensions often have 
a sticky, gluelike consistency; this accounts 
for the name, which comes from a Greek 
word that means glue. Changes in proto- 
plasm from a sol to the gel condition and 
back again may be explained on the basis of 
the distribution of the colloid particles. If 
the particles are more or less evenly distrib- 
uted in a liquid medium, as in Fig. 4, the 
mixture flows easily and is in the sol state, 
but if the particles are arranged so as to 
form a meshwork, with the liquid medium 


16 

enclosed by the meshes, the mixture does 
not flow but is in a solid or semisolid gel 
condition. Colloidal suspensions and the sol 
and gel conditions are not confined to proto- 
plasm; for example, jello forms a colloidal 
suspension in water— when warm it is in a 
fluid sol condition, but when cool it changes 
to a solid or semisolid gel condition. As in 
protoplasm, either condition may be 
changed back into the other. Some other 
colloidal substances are mayonnaise, cream, 
butter, glue, and soap. 

Consideration of all the known details of 
the fine structure of protoplasm goes beyond 


Figure 5. Electron microscope. It uses a beam of 
electrons and magnetic fields, which take the place 
of light and lenses. Magnifications of over 100,000 
times (diameters) may be obtained. This microscope 
is useful in studying the smallest living things such 
as viruses and the submicroscopic structures of cells. 
(Photo courtesy of George Jennings, Michigan De- 
partment of Health.) 


COLLEGE ZOOLOGY 

the scope of this book. However, the elec- 
tron microscope reveals that it is far, far 
more complex than the studies made with 
the light microscope led us to suspect. Life 
now appears to result from the complex in- 
terrelations of the microscopic and ultra- 
microscopic components of protoplasm. 

Chemical composition 
of protoplasm 

When protoplasm is studied chemically, 
it is round to be built up of the same ele- 
ments that occur in nonliving materials. 
The 20 elements listed below appear to be 
essential to protoplasm: 


ESSENTIAL 


PER CENT IN 


ELEMENTS 


SYMBOLS 


PROTOPL.\SM 


Oxygen 


(O) 


63.00 


Carbon 


(C) 


20.00 


Hydrogen 


(H) 


10.00 


Nitrogen 


(N) 


2.50 


Calcium 


(Ca) 


2.50 


Phosphorus 


(P) 


1.14 


Potassium 


(K) 


0.11 


Sulfur 


(S) 


0.14 


Chlorine 


(CI) 


0.10 


Fluorine 


(F) 


0.10 


Sodium 


(Na) 


0.10 


Magnesium 


(Mg) 


0.07 


Iron 


(Fe) 


0.01 


Copper 


(Cu) 


Trace 


Cobalt 


(Co) 


Trace 


Zinc 


(Zn) 


Trace 


Silicon 


(Si) 


Trace 


Manganese 


(Mn) 


Trace 


Iodine 


(I) 


Trace 


Nickel 


(Ni) 


Trace 


These elements, with the exception of 
oxygen, are generally combined to form 
compounds. Compounds can be divided 
into inorganic and organic. Organic com- 
pounds occur in nature only in living plants 
and animals, or in their products and re- 
mains. Inorganic compounds are principally 
water and salts, and organic compounds are 
principally proteins, fats, and carbohydrates. 
The percentages of these different com- 
pounds in protoplasm are on the average as 
follows: 


PROTOPLASM AND CELLULAR ORGANIZATION 


17 


COMPOUNDS 


PER CENT 


Water 


80.0 


Proteins 


12.0 


Nucleic acid 


2.0 


Fats 


3.0 


Carbohydrates 


1.0 


Steroids 


0.5 


Inorganic salts 


1.0 


Other substances 


0.5 


Compounds are made up of one or more 
molecules of the same kind; for example, 
water, sugar, and carbon dioxide are com- 
pounds. Molecules are so small that one 
computation shows that there are about 
1,000,000 molecules in a single bacterium. A 
molecule is the smallest particle of a sub- 
stance that possesses the chemical nature of 
that substance. For example, a molecule of 
water can be subdivided, but it ceases to be 
water when it is broken down into the 2 ele- 
ments, hydrogen and oxygen, of which it is 
composed. Elements, such as hydrogen and 
oxygen, are known as atoms. More than 100 
different elements or kinds of atoms are 
known. Many of these atoms can combine 
in various ways to form molecules; for ex- 
ample, 2 atoms of hydrogen, combined with 
1 atom of oxygen, produce 1 molecule of 
water; 1 atom of carbon and 2 atoms of 
oxygen combine to form 1 molecule of car- 
bon dioxide. Evidently there are vastly 
greater numbers of different kinds of mole- 
cules than of different kinds of atoms. Like- 
wise, molecules of different kinds may be 
mixed together in various combinations so 
as to produce many more different kinds of 
substances than there are different kinds of 
molecules. Atoms and molecules are ordi- 
narily indicated by means of symbols which 
provide a sort of chemical shorthand. Thus 
hydrogen is indicated by the letter H, and 
oxygen by the letter O. The molecular for- 
mula of water is written as H^O, since each 
molecule of water is made up of 2 atoms of 
hydrogen and 1 of oxygen. Carbon is indi- 
cated by the letter C, and the molecular 
form of carbon dioxide is COo. The com- 
binations of atoms, or molecules, are usu- 


ally written in the form of chemical equa- 
tions, such as the following: 

Hydrogen Oxygen Water 

2U2 -f 02 -> ZHsO 

Carbon 
Sugar Oxygen Dioxide Water 

CeHi^Oe + 6O2 -> 6CO. + 6H0O 

This reaction is reversible, as indicated by 
the following equation: 

6CO2 4- 6H«0 -^ C«Hi206 + 6O2 

Reversible reactions are indicated bv two 
arrows as follows: 

CaHiaOe + 6O2 ^ 6CO2 + 6H,0 

Water is the most common compound 
in protoplasm, making up from about 60 to 
96 per cent of it. Water is ingested in greater 
amounts than all other substances com- 
bined, and it is the chief excretion. It is the 
vehicle of the principal foods and excretion 
products, for most of these are dissolved as 
they enter or leave the body. Actually, there 
is hardly a physiologic process in which water 
is not of fundamental importance. 

Inorganic salts are essential for life proc- 
esses. They are present in solution in the 
protoplasm and in body fluids. In body 
fluids they are very similar in concentration 
to the salts in sea water. Although small in 
amount, they are important since certain 
salts in certain proportions are necessary for 
normal life activities. For example, if the 
calcium content of the blood is lowered suf- 
ficiently, convulsions and death ensue; and 
if sodium, calcium, and potassium are not 
properly balanced, the muscles of the heart 
do not function normally. The presence of 
certain salts is quite obvious to us, since 
calcium phosphate and calcium carbonate 
make up about 65 per cent of bone. 

The three principal classes of organic 
compounds in protoplasm are known as 
carbohydrates, fats, and proteins. Carbo- 
hydrates and fats are composed entirely of 
carbon, hydrogen, and oxygen; protein has 


18 

in addition nitrogen, sulphur, and phos- 
phorus. Common carbohydrates are starch 
and sugar. The word carbohydrate is de- 
rived from the Latin term carbo, meaning 
coal, and the Greek term hydor, meaning 
water. Coal is a form of carbon. Carbo- 
hydrates are compounds of carbon, hydro- 
gen, and oxygen in which the ratio of hy- 
drogen and oxygen atoms is the same as 
that in water, that is, 2 of hydrogen to 1 of 
oxygen (H2O). Carbohydrates are stored in 
the body in a form called glycogen, espe- 
cially in the liver and the muscle cells. They 
are particularly valuable as fuel for the body, 
but are also used in the structure of proto- 
plasm. One of the simple carbohydrates, a 
sugar called glucose, seems to be of particu- 
lar importance, probably as a fuel. If there 
is too little glucose present, nerves and mus- 
cles become more irritable, and death may 
follow convulsions, just as when the calcium 
content of the blood becomes too low. If the 
sugar content of the blood is too high, a 
disease known as diabetes results; this condi- 
tion can be corrected by injection of the 
hormone insulin. 

Fats differ from carbohydrates in the 
structure of their molecules. Less oxygen is 
present in proportion to the carbon and 
hydrogen. This is evident when the formulas 
of a carbohydrate and a fat are contrasted. 


Carbohydrate 


Fat 


Fats, like carbohydrates, serve principally as 
fuel, and much fat is stored in the body 
where it can be used when needed. When 
deposited just beneath the skin, it insulates 
the body, since it is a poor conductor of 
heat. 

Proteins are the primary constituents of 
protoplasm. Their molecules are much larger 
than those of fats and carbohydrates; a 
common protein (hemoglobin) in our red 
blood corpuscles, for example, has the ap- 
proximate formula C3032H48160872N78oSsFe4, 

which means that each molecule is built up 
of 6 different kinds of atoms, totaling about 
10,000. Since protoplasm is composed largely 


COLLEGE ZOOLOGY 

of proteins, we need plenty of protein in our 
food; and since different parts of the body, 
such as the liver and muscles, contain differ- 
ent kinds of proteins, we require food con- 
taining various types of proteins. Common 
animal proteins are present in meat, fish, 
milk, and eggs, and common plant proteins 
in peas, beans, and peanuts. 

Proteins play the leading role in the 
chemical composition of protoplasm; fats 
and carbohydrates serve principally as fuels. 
Fats and carbohydrates cannot be converted 
into proteins in the body, but proteins can 
be converted into carbohydrates, carbo- 
hydrates into fats, and fats into carbohy- 
drates. 

Metabolism and growth 

The term metabolism is used to include 
all chemical changes that take place in the 
protoplasm. Growth in any living thing in- 
volves a complex series of changes. The 
chemical compounds which make up the 
bodies of animals are extremely unstable; 
they are constantly breaking down into sim- 
pler substances or becoming more complex 
by the addition of new materials. There is 
no time during the life of any individual, 
even after growth ceases, when elaborate 
chemical reactions are not taking place. 
Metabolism is the term used to include 
this great complex of incessant chemical 
changes. Those processes which use energy 
to build up compounds are said to be ana- 
bolic; those by which substances are broken 
down, thereby releasing energy, are termed 
catabolic. 

Animals are primarily catabolic organisms. 
They cannot make organic compounds from 
simple inorganic substances; in this respect 
they differ from plants, which manufacture 
sugar (glucose) from carbon dioxide and 
water, in the presence of light energy and 
chlorophyl. The green plants obtain carbon 
dioxide (CO2) from the air, water (H2O) 
from the soil, and energy from light. Chloro- 
phyl, an additional substance, which is re- 
sponsible for the green color of plants is also 


PROTOPLASM AND CELLULAR ORGANIZATION 


19 


necessary. We do not know how chlorophyl 
is able to convert the hght energy into chem- 
ical energy, nor how this chemical energy is 
used to synthesize glucose from carbon 
dioxide and water. 

Because this synthesis is dependent on 
light, it is called photosynthesis. The photo- 
synthetic equation is written as follows: 


CO2 4- H.O + Light 


-f Chlorophyl 


Glucose + Oxygen 


We know that the above equation is no 
more than a statement of input and output. 
Chemical studies involving the use of "la- 
beled" carbon dioxide reveal that there are 
probably dozens of intermediate chemical 
processes. 

Since animals must have organic food, 
plant products are necessary either directly 
or in the form of protoplasm built up by 
other animals out of plant food. Before 
animal growth is possible, food must be con- 
verted into living substance. 

Digestion is the process by which food 
materials are broken down into simpler sub- 
stances so they can be absorbed. This is a 
nutritive process, and, while not a part of 
metabolism as defined above, it is necessary 
if metabolism is to continue. Material can- 
not be absorbed unless it is in a liquid condi- 
tion. Water may be absorbed without 
change. Many mineral salts are easily ab- 
sorbed, the process depending on their con- 
centration. Carbohydrates must be broken 
down into simple sugars, such as glucose, 
before their absorption is possible. This is 
accomplished with the help of complex 
substances produced by the protoplasm, 
which are known as enzymes.* Fats must be 
broken down by enzymes into glycerin and 
fatty acids before they can be absorbed. Pro- 
teins are likewise acted upon by enzymes, 
eventually becoming amino acids, which 

* The importance of enzymes in life processes 
cannot be overemphasized. The modern biochemist 
is inclined to believe that living things are chiefly a 
matter of enzymatic reactions. It has been estimated 
that there are 3000 to 5000 different enzymes in a 
cell. 


are absorbable. In very small animals, di- 
gested food does not need to be transported 
very far in order to become distributed 
throughout the body, but in larger animals 
some sort of circulatory system is necessar}' 
for this purpose. 

Assimilation, an important part of ana- 
bolism, is the process of converting absorbed 
material into protoplasm. During this proc- 
ess comparatively simple materials are built 
up into more complex compounds with the 
aid of enzymes produced by the protoplasm; 
that is, the protoplasm manufactures en- 
zymes which convert digested and absorbed 
materials into more protoplasm. The result 
is replacement of the protoplasm that is 
broken down; and after this has been re- 
placed, growth takes place. 

Energy is defined as the ability to do 
work, to produce a change in matter; it may 
take the form of motion, heat, light, or elec- 
tricity. Energy is derived ultimately from 
sunlight and is stored in the molecules of 
food as chemical energy. Chemical reactions 
inside the body occur, changing the chem- 
ical energy to heat, motion, or some other 
kind of energy. Under experimentally con- 
trolled conditions, the amount of energy en- 
tering and leaving any given system may be 
determined and compared. It is always 
found that energy is neither created nor de- 
stroyed, but only changed from one form to 
another. This generalization is known as the 
Law of the Conservation of Energy. This 
law applies to living as well as nonliving sys- 
tems. 

Energy is contained in the organic mole- 
cules in protoplasm and in stored substances 
in the body and is liberated when these 
molecules are broken down by oxidation. A 
simple example of the oxidative process is 
as follows: 

Carbon 
Sugar Oxygen dioxide Water 
CflHiaOe + 6O2 -> 6CO2 -f 6H2O + Energy 

According to this equation, oxygen splits the 
sugar molecule into carbon dioxide and 
water, thereby liberating energy. Oxidation 


20 


COLLEGE ZOOLOGY 


is a breaking down of protoplasm and there- 
fore a catabolic process. 

This gaseous metabolism of the proto- 
plasm, including absorption of oxygen, and 
elimination of carbon dioxide, is known as 
cellular respiration. In small aquatic ani- 
mals, oxygen is obtained from the surround- 
ing water, and carbon dioxide is given off 
into the same water. In many larger animals, 
a respiratory system is necessary to take in 
oxygen and to expel carbon dioxide. The 
transportation of both these gases is one of 
the functions of the circulatory system. 

Carbon dioxide is a waste product of me- 
tabolism, an excretion. Other waste prod- 
ucts due to catabolic processes are water, 
inorganic salts, and nitrogenous salts such as 
urea. These may be cast out directly into 
the surrounding water by small aquatic ani- 
mals, or they may be carried by a circulatory 
system to an excretory system, the function 
of which is to extract waste products and 
expel them from the body. 

Some of the energy liberated by oxidation 
may be used in the production of substances 
known as secretions that are of use to the 
animal. Certain types of protoplasm may be 
specialized for this purpose and concen- 
trated in glands. Glands secrete sweat, diges- 
tive juices, milk, poison, the shells of eggs, 
and many other substances with which we 
are familiar. They also secrete, into the 
blood, substances that have a remarkable in- 
fluence on our growth and behavior; these 
are called hormones and will be considered 
later. Biological processes involve not only 
continual energy transformations but var}'- 
ing energy levels. 

Irritability or excitability 

One of the fundamental properties of 
protoplasm is its irritability. This property 
is responsible for the reactions of an animal 
to changes in surrounding conditions. The 
change that brings about the reaction is 
known as a stimulus, and the reaction as a 
response. Most stimuli are external changes 


in the environment, but certain stimuli 
such as hunger seem to arise from within. 
Some of the common types of stimuli arc 
mechanical (for example, contact), chem- 
ical, thermal (changes in temperature), and 
photic (for example, changes in intensity 
or color of light). The stimulus may be and 
often is extremely small as compared with 
the magnitude of the response. The response 
may depend on the nature of the protoplasm 
stimulated; for example, it may appear as a 
movement if muscle is excited, or as a secre- 
tion if gland cells receive the stimulus. The 
transmission of the excitation from one part 
of the protoplasm to another is called con- 
duction. Conduction is a general attribute 
of protoplasm, but the protoplasm of nerves 
is speciahzed for this purpose. 

CELLULAR ORGANIZATION 

Division of the 
protoplasm into cells 

In one phylum of animals, the Protozoa, 
the protoplasm is continuous, but in all 
other animals the body is divided into units 
called cells, which contain the protoplasm. 
We owe the term cell to an Englishman 
named Robert Hooke, who, in 1665, de- 
scribed as "little boxes or cells" those spaces 
surrounded by walls which he observed in 
cork and pith with his new microscope. 
Since the essential substance in cells is the 
protoplasm and not the wall, the term was 
an unfortunate choice. The protoplasm of 
cells is of two principal kinds: (1) cyto- 
plasm and (2) nucleus. A cell may be de- 
fined as a small mass of protoplasm consist- 
ing of cytoplasm and a nucleus, which are 
enclosed by membranes. 

Size, shape, and 
number of cells 

Cells vary in size; some are extremely 
small, for example, blood parasites are as 
small as ^5,000 of an inch, whereas others. 


PROTOPLASM AND CELLULAR ORGANIZATION 


21 


like the egg of a bird, are very large. The 
large size of some egg cells is due chiefly to 
the accumulation of an enormous quantity 
of reserve food material, and not to the 
protoplasm they contain. Cells differ in 
shape (Fig. 43, p. 85); they may be col- 
umnar, flat, spherical, stellate, or long and 
thin. There are trillions of cells in a complex 
animal; there are about 9.2 billion in the 
gray matter of the human brain alone. On 
the other hand, certain animals (proto- 


zoans) may consist of a single cell. The size 
of an animal usually depends not upon the 
size of the cells but upon the number. 

Structure of cells 


The nucleus and certain other bodies can 
sometimes be seen in living cells when 
viewed under the higher powers of a com- 
pound microscope, but special preparation 
is necessary to make visible most of the 


Vacuole 


fCenfrosphere 
Centrosome < 


Nucleoplasm 

Chromatin 
Nucleus — 
Nucleolus 

Nuclear membrane 
Golgi apparatus 

Mifochondria 

Fat droplet 
Cytoplasm 

Cell membrane 


Figure 6. Diagram of a generalized animal cell, that is, one showing the structures found in 
various animal cells; all these parts are not necessarily present in all cells. The shape of mito- 
chondria may change, depending on the phase of cellular activity. 


structure. This is accomplished by treating 
living cells with dyes or by killing and then 
staining them. The electron microscope has 
greatly increased our knowledge concerning 
the smallest structures of the cell (Fig. 7). 
A diagram of the structure of a stained 
animal cell containing most of the bodies 


that may be observed in cells of various types 
is presented in Fig. 6. The animal cell is 
surrounded by a thin cell membrane. A 
rigid cell wall outside of the limiting cell 
membrane is characteristic of plant cells, but 
is rare in animals. The most conspicuous 
body in the cell is the nucleus. This is 


22 


COLLEGE ZOOLOGY 


Figure 7. Electron micrograph of a cross section of a sea urchin egg, magnification 15,600 
times. Note the nucleus (N) with the dark nucleolus (Nc) to the left; it has been demonstrated 
that the dark wavy nuclear membrane (M) has "holes" in it, covered with a thin membrane. 
The yolk granule (Y) barely visible under a light microscope is seen plainly, and the protoplasmic 
reticulum (R) is well shown. (Electron micrograph courtesy of B.A. Afzelius, reprinted by 
permission of Experimental Cell Research, 8:155, 1955, and Academic Press Inc., New York.) 


bounded by a nuclear membrane. Within 
the nucleus is a colorless fluid, the nuclear 
sap (nucleoplasm), in which there is a sub- 
stance that has a strong afhnity for certain 
dyes; this is known as chromatin. Some nu- 
clei contain an intensely staining, spherical 
body, the nucleolus. 

Various types of bodies may occur in the 
cytoplasm. Often near the nucleus is lo- 


cated a specialized portion of the proto- 
plasm, the centrosphere, in the center of 
which are one or two deeply staining bodies, 
the centrioles. The term centrosome in- 
cludes the centrosphere together with the 
centrioles. Spherical vesicles of liquid of var- 
ious sizes, called vacuoles, may or may not 
be present. Spherical or rod-shaped mito- 
chondria contain enzymes which are in- 


PROTOPLASM AND CELLULAR ORGANIZATION 


23 


volved in cellular respiration; other enzymes 
in the mitochondria function in chemical 
reactions which produce and store energy in 
the cell. By special staining methods the 
Golgi apparatus is sometimes made visible; 
its function is not definitely known. Cyto- 
plasmic inclusions of various sorts that are 
not considered parts of the living proto- 
plasm may also be present; these are pig- 
ment granules, starch granules, fat globules, 
and other nutritive, secretory, or excretory 
material. 

Experiments indicate that neither the cy- 
toplasm nor the nucleus can exist long with- 
out the other. For example, if the single 
cell of the protozoan is deprived of its nu- 
cleus, the remaining cytoplasm may con- 
tinue to move for a few hours and may 
ingest food, but all its activities soon cease, 
and death ensues. Both nucleus and cyto- 
plasm are necessary for normal cellular ac- 
tivities, due probably to the exchange of 
substances between them. 

Passage of materials 
through cell membrane 

The living animal cells throughout the 
body are inhabitants of tissue fluid. Tissue 
fluid is probably of much the same composi- 
tion as the sea water in which animal life is 
thought to have originated. All materials en- 
tering a cell must pass through the fluid 
surrounding each individual cell before they 
reach the cell membrane. 

The best-known physical process which 
enables water and other substances to en- 
ter the cell is diffusion. Diffusion is defined 
as the movement of molecules from a re- 
gion of high concentration to one of lower 
concentration, brought about by the in- 
herent heat energy of the molecules. The 
rate of diffusion depends mainly on the size 
of the molecule and the temperature. Diffu- 
sion is fundamental to many biologic phe- 
nomena, and examples of it in everyday life 
are familiar to all of us. For example, if a 
tablespoonful of household ammonia is 


spilled on the floor, the odor will soon be 
noticed in all parts of the room. The mole- 
cules of ammonia have become evenly dis- 
tributed throughout the entire room. 

This same principle holds true if the sub- 
stance is a solid, such as a small lump of 
sugar dropped in a jar of water. The sugar 
dissolves and the individual sugar molecules 
(solute) diffuse from their original position 
in the jar of water (solvent) and spread 
evenly throughout the liquid (Fig. 8A). 
The individual sugar molecules move in a 
straight line until they bump into another 
molecule; then they rebound and move in 
another direction. 

Diffusion of a solute can be modified oi 
prevented by the presence of a membrane. 
A membrane is permeable if it permits water 
and all solutes to pass through, impermeable 
if it will permit no substances to pass, and 
semipermeable ( differentially permeable ) if 
it will allow some but not all substances to 
diffuse through. This makes it clear that 
permeability is a property of the membrane, 
not the diffusing substance. The dense sur- 
face film on the outside of an animal cell, 
the cell membrane, is semipermeable. One 
of the principle functions of the cell mem- 
brane is that of regulating the passage of 
materials into and out of the cell. Certain 
liquids and dissolved substances can pass 
through the cell membrane and others can- 
not. Mineral nutrients dissolved in water 
pass through the cell membrane by diffu- 
sion. Water passes through the cell mem- 
brane by osmosis, a special form of diffusion. 

Osmosis may be defined as diffusion of 
a solvent through a semipermeable mem- 
brane. In biological processes the solvent is 
almost universally water. Osmosis is a kind 
of one-directional diffusion as explained in 
Fig. 8B. It plays an important role in the 
life processes of cells, both plant and ani- 
mal, because of the indispensable functions 
of water in a cell. Why does osmosis occur 
in the living animal cell? It is because the 
cell contains solutes such as sugars, salts, 
and others, which reduce the concentration 


24 


of water molecules to a point lower than 
that of the tissue fluid in which the cell is 
immersed. Hence, in accordance with the 
principle of osmosis, the water moves from 
the region of higher concentration (tissue 
fluid) to the region of lower concentration 
(cell protoplasm). Cell membranes are im- 
permeable to many substances that we eat, 
such as starch, because they must be di- 
gested, that is, made soluble, before they can 
be absorbed into the cells. For definitions 


COLLEGE ZOOLOGY 

of the terms isotonic, hypertonic, and hypo- 
tonic see the Glossary. 

Contrary to a common misconception, ex- 
change of foods, wastes, and respiratory gases 
between cells and fluids in animal bodies is 
not by osmosis. The chief factor in the 
transport of these substances is ordinary 
diffusion. When water molecules move 
either inward or outward by osmosis they do 
not carry other molecules along with 
them. 


Semipermeable membrane 
(permeable to water 
molecules, impermeable 
to sugar molecules) 

Membrane permeable 
to all substances 

Difference in levels of 
iquids when chambers 
separated by semiperme- 
able membrane measures 
osmotic pressure 

Water molecule:.. 
Sugar molecule: tv 


chamber a, sugar solution placed in chamber b 


Figure 8. Diagram to illustrate ordinary diffusion and osmosis. A, ordinary diffusion. The 
battery jar is divided into two chambers, a and b, by a permeable membrane which offers prac- 
tically no hindrance to the diffusion of both water and sugar molecules (particles). In ordinary 
diffusion, any kind of molecule tends to diffuse (move) from where it is more abundant, per 
volume of space, to where the molecule is less abundant. Diffusion through a permeable mem- 
brane continues until every component reaches equal concentration; therefore in diagram A, 
the water and sugar molecules are of equal concentration on both sides of the permeable mem- 
brane. B, osmosis. The battery jar is divided into two chambers, a and h, by a semipermeable 
membrane, that is, one that is permeable to water but hinders the passage of sugar molecules. 
Under these conditions, water molecules will diffuse through the membrane more rapidly into 
chamber b than into chamber a. In accordance with the law of diffusion, the water molecules 
move in greater numbers from the place of higher water molecule concentration (higher water 
diffusion pressure) to the region of lower water molecule concentration (lower water diffusion 
pressure). Diffusion through a semipermeable membrane is known as osmosis. Osmosis in living 
things has almost always to do with the movement of water through a semipermeable membrane. 


Cell division 

Reproduction is a fundamental property 
of protoplasm, and cell division is a type of 
reproduction. For many years after cells were 
discovered, division of the nucleus, which 
precedes cell division, was supposed to take 


place by a process which we call aniitosis tc 
distinguish it from mitosis. 

Amitosis means a sort of mass division ol 
the nucleus. This type of nuclear division h 
rare and of little importance. As a rule, the 
protoplasm in a cell grows until the cell 
reaches a certain size; then the cell divides 


PROTOPLASM AND CELLULAR ORGANIZATION 


25 


This is called mitosis. The two daughter 
cells proceed to grow, and they in turn di- 
vide, and so on, generation after generation. 
Many cells, however, notably those formed 
during the development of eggs, grow very 
little or not at all during the period between 
successive divisions. Why cells divide when 
they do is not known, but the relative quan- 
tities of nucleoplasm and cytoplasm are usu- 
ally maintained in each kind of cell. It has 
been suggested that when the cytoplasm 
reaches a volume too great for the nucleus, 
division begins. 

Interphase {"resting") cell 

A cell that is not undergoing division has 
been called a "resting" cell. However, it is 
anything but a resting cell in the true sense 
of the word. It is carrying on all the life 
processes of any living cell, and a more ap- 
propriate name for it is an interphase cell. 
This period in the life of a cell is one in 
which no visible structural changes are tak- 
ing place in the nucleus. This stage is not 
considered one of the phases of mitosis, al- 
though there is no sharp line of demarca- 
tion between the late telophase and inter- 
phase as shown in Fig. 9. The description of 
the generalized animal cell (Fig. 6) is that 
of a typical interphase cell. 

Alitosis 

Cell division involves a series of processes 
of considerable complexity and of great 
significance. The nucleus divides first and 
then the cytoplasm. Constant reference to 
Fig. 9 will make clear the following brief ac- 
count of mitosis in a typical cell. Four stages 
are recognized. 

1. Prophase: the mitotic figure arises and each 
chromosome appears to be split longitudi- 
nally (Fig. 9); actually, each chromosome 
has duplicated itself. 

2. Metaphase: the duplicated chromosomes be- 
come located in the equatorial plane of the 
mitotic figure (Fig. 9). 

3. Anaphase: the halves of the duplicated 
chromosomes separate and move as two 


groups to opposite ends of the mitotic 
spindle (Fig. 9). 
4. Telophase: two daughter nuclei are formed 
and the cell body divides (Fig. 9). 

These 4 stages will now be described in more 
detail. 

Prophase 

The chromatin in the interphase nucleus 
(Fig. 9) may appear to be in the form of 
isolated granules or a network of granules. 
However, there is good evidence to indicate 
that the chromatin is actually in the form of 
fine threads which are much coiled. The 
modern view is that the so-called granules 
are actually a mass of very fine coils. What 
may appear to be chromatin granules of the 
interphase nucleus of some cells now can be 
seen as distinct threadlike structures (Fig. 
9). These threads (chromonemata) are 
really double (Fig. 10). The chromonemata 
go through a process of spiralization, which 
is accompanied by a shortening and thicken- 
ing of the chromosome. These chromosomes 
are characteristic in size, shape, and number, 
depending on the species of animal to which 
the dividing cell belongs. While this is hap- 
pening, a halo of radiating fibers appears 
around the centrosphere, thus forming an 
aster. The two centrioles then separate and 
migrate to opposite ends of the cell, each 
with an aster about it (Fig. 9). Between the 
asters and the nuclear membrane, a number 
of fibers become visible in fixed material 
(Fig. 9). The nuclear membrane breaks 
down and disappears; and the fibers, extend- 
ing from the asters across the nuclear space, 
form a spindle. 

Metaphase 

During this phase of mitosis the duplicated 
("split") chromosomes become located in 
the equatorial plane of the spindle (Fig. 
9). The two daughter chromosomes pro- 
duced from one are identical with each 
other and with the chromosome from which 
they developed. 


Astral ray 
Centriole- 


Nuclear membrane 
Chromosome 
Nucleoplasm 
Cell membrane 


Aster 


Daughter cells in 
INTERPHASE STAGE 


LATER PROPHASE 


Spindle fiber 

Astral ray 
Chromosome 


LATE TELOPHASE 


LATER PROPHASE 


k 


Spindle fiber 


EARLY TELOPHASE 


\ 


METAPHASE 


y 


ANAPHASE 


Figure 9. Animal mitosis. Typical stages in the mitotic division of one somatic cell into two; 
diagrammatic. Spiralization and centromere are shown in Fig. 10. 


26 


PROTOPLASM AND CELLULAR ORGANIZATION 


27 


Anaphase 

The daughter halves of the dupHcated 
chromosomes now move to opposite ends 
of the spindle (Fig. 9). Spindle fibers are 
attached to the chromosomes at definite 
points. The movement of the daughter 
chromosomes is due to the contraction of 
these spindle fibers. 

Telophase 

The daughter nuclei are now recon- 
structed (Fig. 9). The chromosomes return 
to the state in which they existed before 
mitosis began, a nuclear membrane appears, 
and the astral rays disappear. The cell body 
divides into two by a constriction which 
arises as a furrow at right angles to the spin- 
dle. This furrow becomes deeper, until fi- 
nally the cytoplasm is divided into two. 

The time required for nuclear and cyto- 
plasmic division varies with the type of cell 
and the temperature. At a temperature of 
39° C, the mesenchyme cells of a chick 
that were being grown in tissue culture di- 
vided as follows: prophase, 5 to 50 minutes, 
usually over 30 minutes; metaphase, 1 to 15 
minutes, usually 2 to 10 minutes; anaphase, 
1 to 5 minutes, usually 2 to 3 minutes; telo- 
phase to cytoplasmic division, 2 to 13 min- 
utes, usually 3 to 6 minutes; telophase re- 
construction of daughter nuclei, 30 to 120 
minutes; total 70 to 180 minutes. Cyto- 
kinesis (cytoplasmic division) is usually 
quite rapid. Moving pictures of dividing 
cells prove that nuclear mitosis occupies 
most of the time, whereas division of the 
cytoplasm is accomplished very quickly. 

Many variations occur in the structure and 
mitotic division of nuclei and in the division 
of the cytoplasm. For example, in many of 
the Protozoa and in certain cells of some 
other animals, the mitotic apparatus is built 
up within the nuclear membrane. Some pro- 
tozoans and animals above the protozoans 
in the scale of life produce a type of cell 
that is capable of developing under certain 
conditions into an organism like the parent; 


cells of this type are called gametes or germ 
cells in contrast to the rest of the cells of 
the body, which are known as somatic cells. 
The description of mitosis presented here 
applies to the division of body (somatic) 
cells. Mitosis, during the development of 
gametes, may differ in several very impor- 
tant features from that of somatic cells. 
These differences will be described later. 

Chromosomes 

Every species of animal has a definite 
number of chromosomes that appear when 
the cells of its body undergo mitosis. Thus 
there are 4 in the nematode worm, Paras- 
caris equorum; 8 in the fruit fly, Drosophila 
melanogaster; and as many as 168 in the 
brine shrimp, Artemia. An even number of 
chromosomes is characteristic of most ani- 
mals, but some forms have an odd number. 
Chromosomes vary considerably in both size 
and shape. Typically they are rodlike, but 
some appear to be spherical. They may be 
less than i-looo mm. or more than Y^q mm. 
in length. The chromosomes that appear 
during mitosis in the cells of an animal may 
differ in size and shape; when such differ- 
ences are visible they are not only charac- 
teristic of all cells of that animal, but also 
of the species. These differences are mostly 
in length, the thickness usually being con- 
stant. 

A chromosome is not a homogeneous 
mass of dark-staining material as it appears 
to be in many preparations, but it has a com- 
plex structure. In the interphase (Fig. 10), 
in some cases, it can be observed that the 
chromosome consists of at least two thin 
chromatin threads, the chromonemata (sin- 
gular, chromonema * ) ; the chromonema is 
the basic unit of the chromosome. The two 
chromonemata are often so closely applied 
to each other along their entire lengths that 

* The thread or strand visible in the light micro- 
scope is called a chromonema, but the electron 
microscope reveals that each chromonema is subdi- 
vided into thin fibers. 


28 


COLLEGE ZOOLOGY 


^ Telophase 


m 


Y Anaph 

\ 


Centromere 


ase 


Metaphase 


Chromatids 


Prophase 


Figure 10. Structure of a chromosome during mitosis. Note spiralization (coiling) of chro- 
monema throughout the cycle. The interphase chromosome consists of at least two chromo- 
nemata. In the early prophase chromosome, note the two distinct chromonemata and how they 
shorten by coiling; in the last stage of the prophase, observe that the chromonema of each half 
chromosome (chromatid) has been duplicated. Metaphase shows that each half chromosome 
is composed of two chromonemata. Anaphase shows that daughter halves of the duplicated 
chromosome separate and move to opposite ends of the mitotic spindle. In the telophase the 
chromosome forms from two daughter chromonemata. The chromosome is linearly differentiated 
into a variety of genes, qualitatively different from one another insofar as they affect the develop- 
ment of traits. The centromere has been indicated by a clear circle; this is the point of spindle- 
fiber attachment. (After General Cytology by De Robertis, Nowinski, and Saez. Second edition. 
Copyright 1954 by Saunders Company.) 


they appear and behave as a single structure. 
As the prophase progresses, the chromosome 
thickens and shortens, probably due to the 
chromonemata becoming more tightly 
coiled, like a spring (Fig. 10). 

The primary significance of mitosis is the 
separation of the longitudinally duplicated 
chromosomes into two identical groups, 
constituting two daughter nuclei. The gen- 


eral result is that every cell in the body con- 
tains the same number of chromosomes of 
the same size, shape, and quality. 

Chromosomes have a persistent individ- 
uality. Those that appear during the pro- 
phases of mitosis are the same as those that 
took part in the reconstruction of the nu- 
cleus in the telophase of the preceding di- 
vision. In some cases the chromosomes are 


PROTOPLASM AND CELLULAR ORGANIZATION 


29 


distinct throughout the interphase stage of 
the nucleus. Observations indicate that 
chromosomes do not move at random dur- 
ing the interphase stage, but form a sort of 
mosaic with respect to one another in a 
definite order. That chromosomes retain 
their individuahty and genetic continuity 
from generation to generation is indicated 
by breeding experiments. 

Discovery of cells 
and protoplasm 

As noted previously, we owe the term cell 
to Hooke, who in 1665 described the struc- 
ture of cork and pith. Many other early 
investigators who used the compound micro- 
scope, which was then being developed, re- 
ported the presence of cells in all sorts of 
plants and animals. In 1674, a Dutch micro- 
scopist, Leeuwenhoek, discovered unicellular 
animals, the Protozoa. For many years the 
cell wall was considered the important part 
of the cell, but later the protoplasm within 
the wall was recognized as the essential cel- 
lular substance. A nucleus had been seen in 
cells, but was not recognized as a regular 
constituent until 1833, when an English 
botanist, Robert Brown, made this general- 
ization and called it by that name. Two 
years later, in 1835, Dujardin, a French pro- 
tozoologist, described the semi-fluid sub- 
stance in unicellular animals and coined the 
term sarcode. Not until 1840 were the cell 
contents called protoplasm by Purkinje; and 
in 1846 the term was also used by the Ger- 
man von Mohl for the "slime" that is pres- 
ent in plant cells. In the meantime, the 
German botanist Schleiden in 1838 and the 
zoologist Schwann in 1839 concluded that 
all plants and animals are made up of simi- 


lar cellular units. Another German zoologist, 
Max Schultze, in 1861, furnished the final 
proof that protoplasm is the essential living 
substance. 

Cell theory 

The modern cell theory may be expressed 
thus: organisms are made up of cells and 
cell products, or are free single cells. Ex- 
amples of the products of cells are the inter- 
cellular substances of plants and animals. 
The cell is not only the unit of structure but 
also of function. A human being begins life 
as a cell (the fertilized egg) which by multi- 
plication and differentiation develops into a 
complex, multicellular organism. The cell 
principle has exerted an important influence 
on the development of all biology. 


SELECTED COLLATERAL 
READINGS 

DeRobcrtis, E.D.P., Nowinski, W.W., and 
Saez, F.A. General Cytology. Saunders, 
Philadelphia, 1954. 

Gerard, R.W. Unresting Cells. Harper, New 
York, 1949. 

Heilbrunn, L.V. An Outline of General Phy- 
siology. Saunders, Philadelphia, 1952. 

Hughes, A. The Mitotic Cycle. Academic Press, 
New York, 1952. 

Schrader, F. Mitosis. Columbia Univ. Press, 
New York, 1953. 

Sharp, L.W. Fundamentals of Cytology. Mc- 
Graw-Hill, New York, 1943. 

Symposium. Fine Structure of Cells. Intersci- 
ence Publishers, New York, 1955. 

Wyckoff, W.G. The World of the Electron 
Microscope. Yale Univ. Press, New Haven, 
1958. 


w. 


CHAPTER 3 


OJI 


Phylum Protozoa. 

OneCelled 

Animals 


30 


E have briefly discussed protoplasm, 
the substance of which all living organisms 
are composed. It is tremendously complex 
both in its chemical and physical nature and 
is found throughout the animal kingdom. 

If we think of the development of ani- 
mal life in terms of increasing levels of 
complexity (biologic levels of organization), 
then the typical protozoan represents the 
first level because it is usually only a spe- 
cialized bit of protoplasm surrounded by a 
membrane. A higher level consists of the 
simple multicellular animals, the sponges, 
because they are a little more complex in 
structure. Degrees of increasing complexity 
in structure and function are found among 
the many-celled animals, from differentia- 
tion of tissues (tissue level) to the forma- 
tion of organs (organ level). Finally, there 
is the highest development of the organ sys- 
tem (organ-system level), which is found in 
the most complex animals, including man. 
We could begin the study of animal life 
with one of the many-celled animals such as 
the earthworm, grasshopper, frog, or cat; 
but we shall start with the simplest animals, 
the protozoans, and then study the animal 
kingdom in approximately the order we 
think it appeared on the earth. This plan 
gives you the best opportunity to note the 
gradual increase in complexity of structure 
with varying levels of biologic organization, 
from protozoan to mammal. 

Insofar as structure is concerned, a single- 
celled protozoan is comparable, in some re- 
spects, to the individual cells of the body of 
a many-celled animal, but the physiology of 
the protozoan is comparable to the whole 
body of the multicellular animal. The single- 
celled protozoan can reproduce, show irrita- 
bility, metabolize, and perform the necessary 
biological functions of life characteristic of 
many-celled organisms. One of the intrigu- 
ing things about protozoans is the fact that 
a single cell can carry on all the basic life 
processes. 

One of the simplest protozoans is Amoeba 
proteus. Its structure, physiologic processes, 


PHYLUM PROTOZOA. ONE-CELLED ANIMALS 


31 


behavior, and habitat, will be studied in de- 
tail later in this chapter, 

CHARACTERISTICS OF 
THE PROTOZOA 

One who examines a bit of pond scum 
under the microscope for the first time feels 
as though he were discovering a new world. 
The protozoans that become visible as a 
result of magnification do not come within 
our everyday experience because they are 
microscopic in size. If enormous numbers of 
them are crowded together, they may im- 
part their color to the water in which they 
live, as the green species Euglena some- 
times does to a fresh-water pond. However, 
few species are large enough to be seen with 
the naked eye when only one specimen is 
present. 

Active protozoans are unable to live where 
it is dry, but they are abundant almost every- 
where in water or in moist places. Fresh- 
water ponds, lakes, and streams abound in 
them; billions live in the sea; the soil often 
teemxS with them to a depth of several inches 
where it is moist; and large numbers live on 
the outside or within the bodies of other 
animals. 

Most protozoans are unicellular, that is, 
they consist of a single cell; but a few con- 
sist of groups of cells. If they are composed 
of a group of cells, the cells are not differen- 
tiated into tissues. The Protozoa are the 
most primitive of the large groups of ani- 
mals and stand in contrast to most of the 
others, which are many-celled tissue animals. 
It seems quite remarkable that such minute 
organisms are capable of maintaining them- 
selves in a world inhabited by so many larger 
and more complex animals. 

In spite of the small size and vast num- 
bers of species of Protozoa, it is not difficult 
to arrange them in classes, orders, families, 
genera, and species. The Protozoa are di- 
vided into 4 classes on the basis of the struc- 
ture they possess for locomotion. One exam- 


ple from each class is described in the fol- 
lowing chapters. The 4 classes of Protozoa 
are as follows: 

Class 1. Sarcodina. Type: Amoeba proteus. 

Protozoa that move by means of 

false feet called pseudopodia. 
Class 2. Mastigophora. Type: Euglena viridis. 

Protozoa that move by means of 

whiplike processes called flagclla. 
Class 3. Sporozoa. Type: Monocystis lum- 

brici. 

Protozoa without motile organelles, 

but with a spore stage in their life 

cycle. 
Class 4. Ciliata. Type: Paramecium cauda- 

tum. 

Protozoa that move by means of cilia. 


AMOEBA PROTEUS 

Habitat and preparation 
for study 

From "amoeba to man" is a common ex- 
pression often seen in the popular press, sug- 
gesting that all living animals are found be- 
tween these extremes, with the amoeba 
representing the lowest form of life and 
man the highest. Whether or not this ex- 
pression is true is open to question, and 
after you have made a comprehensive study 
of animal life you will understand why this 
is said. Amoebas live in many different habi- 
tats, such as fresh water, the sea, the soil, 
and as parasites within other animals, in- 
cluding man. A common large fresh-water 
species, and one that is usually selected to 
introduce the phylum Protozoa, is Amoeba 
proteus (Gr. amoibe, change; Proteus, a sea 
god in classical mythology who had the 
power of changing his shape). The amoeba 
(Fig. 11) lives in fresh-water ponds and 
streams. It can often be found on the under- 
side of dead lily pads and other vegetation 
in shallow water. 

If material containing amoebas is studied 
under a microscope, some of the activities 


32 


COLLEGE ZOOLOGY 


Pseudopodium 


Cell membrane 

Endoplasm 

Ectoplasm 


Figure 11. Structure of Amoeba. Arrows indicate direction of movement. 


and a little of the structure of the animals 
can be observed. By changing the conditions 
with respect to temperature, light, etc., one 
can study their behavior. To obtain a satis- 
factory idea of the structure of the organ- 
isms, it is necessary to kill them and treat 
them vi'ith certain dyes which stain some of 
the parts, thus making them visible or more 
distinct than they appear in a living animal. 

Structure (morphology) 

Amoeba proteus (Fig. 11) is only about 
i/ioo inch (0.25 mm.) in length. It appears 
under the microscope as an irregular, gray- 
ish particle of animated jelly that is continu- 
ally changing its shape by thrusting out and 
withdrawing little fingerlike processes. Two 
types of cytoplasm are recognizable in the 
amoeba, the central part of the body appears 
to consist of granular protoplasm called 
endoplasm; surrounding the endoplasm is 
a thin layer of clear protoplasm called ecto- 
plasm. Although the ectoplasm is sur- 
rounded by only a very thin external elastic 
cell membrane, yet it has been observed 
that amoebas crawl over each other and 
never fuse. Within the endoplasm several 


bodies may be seen that are larger than the 
ordinary granules. One of these, the nucleus, 
is not easy to see in the living animal, but 
when stained it appears to be disk-shaped 
and filled with chromatin granules. The 
nucleus is thought to play an important part 
in such fundamental activities of the cell as 
growth, manufacture and use of foodstuffs, 
and formation of new cells. If an amoeba is 
cut into two pieces, the part containing the 
nucleus may continue to live and reproduce, 
but the one without the nucleus cannot re- 
produce itself and soon dies. 

A clear, bubblelike body can often be 
seen lying near the nucleus; this is known as 
the contractile vacuole (Fig. 11), because 
at more or less regular intervals it is carried 
to the surface, where it contracts and forces 
its fluid contents out of the body. Other 
vacuoles may often be seen in the endo- 
plasm; these may be temporary and may 
contain food bodies in process of digestion, 
or they may be more or less permanent. 

When an amoeba is examined with higher 
magnification, streaming movements may be 
observed in the endoplasm, indicating that 
this part of the protoplasm is in a liquid 
(sol) condition. 


PHYLUM PROTOZOA. ONE-CELLED ANIMALS 


33 


Physiology 

An amoeba exhibits all activities necessary 
to maintain itself, and which are characteris- 
tic of higher animals. It moves about; cap- 
tures, ingests, and digests food; egests undi- 
gested matter; absorbs and assimilates the 
products of digestion; secretes and excretes 
various substances; respires; grows; repro- 
duces itself; and responds to changes in its 
environment. These facts indicate that the 
amoeba is physiologically a very complex 
organism. 

Amoeboid movement 

Amoebas move from place to place, cap- 
ture other organisms, and ingest solid parti- 
cles of food by means of fingerlike protru- 
sions of the body known as pseudopodia 
(singular pseudopodium). These pseudo- 
podia may arise at any point on the surface 
of the animal. The formation of the pseudo- 
podium looks simple, but it has not yet been 
explained with certainty in spite of detailed 
investigations by some of our best zoologists. 
When a pseudopodium is formed, a blunt 
projection appears, which consists of ecto- 
plasm. Granular endoplasm can be seen 
flowing into this. The entire amoeba moves 
forward in the direction of the pseudopo- 
dium. Several pseudopodia may form at the 
same time; usually one becomes large and 
effective, and the others become smaller and 
disappear. Actually, the amoeba moves along 
by thrusting out pseudopodia and then flow- 
ing into them. It has been observed to move 
at the rate of one inch per hour, but the 
rate of movement varies with the tempera- 
ture, increasing up to a temperature of about 
30° C, but ceasing at 33° C. 

Many cells in multicellular (metazoan) 
animals, including man, exhibit typical 
amoeboid movements. For example, the 
white blood corpuscles in our own blood, 
which are known as leukocytes, move from 
place to place by means of pseudopodia and 
are even able to work their way through the 


walls of blood vessels. Leukocytes also en- 
gulf and destroy disease germs by means of 
their pseudopodia, a process known as 
phagocytosis. 

The two principal theories that have been 
proposed to explain the formation of pseu- 
dopodia are based on ( I ) changes in surface 
tension, and (2) changes in the viscosity of 
the cytoplasm. The subject is too complex 
to be considered here in detail; further in- 
formation can be obtained in advanced 
books on zoology and in scientific journals. 
There is still much to be learned about 
amoeboid movement, but when the true ex- 
planation is found it may give the key not 
only to the formation of pseudopodia but 
to the movement of flagella, cilia, and even 
muscular contraction. 

Food 

The amoeba feeds principally on minute 
animals and plants. Not every object en- 
countered is ingested; a distinct selection of 
food particles is evident (Fig. 12). It seems 
rather surprising that the amoeba is able to 
capture such rapidly swimming creatures as 
the flagellate Chilomonas (Fig. 12A) and 
ciliates such as the paramecium; the former, 
however, is a favorite type of food. A Para- 
mecium is sometimes held and actually cut 
in two by the pseudopodia of the amoeba for 
the purpose of ingestion. 

Ingestion 

Food may be engulfed at any point on the 
surface of the body (see headpiece) of the 
amoeba, but it is usually taken in at what 
may be called the temporary anterior end, 
that is, the part of the body extended to- 
ward the direction of the animal's locomo- 
tion. 

A food cup is usually formed in the follow- 
ing way (Fig. 12D): pseudopodia enclose 
the food particle from the sides; then thin 
sheets of cytoplasm cover the top and the 
bottom, thus entirely surrounding it. Often 
when the prey is active, a large food cup is 


34 


COLLEGE ZOOLOGY 


Figure 12. Amoeba. Ingestion of food. A, successive positions of a pseudopodium of an 
amoeba capturing a flagellate, Chilomonas. B, ingesting a cyst of a flagellate. C, ingesting a plant 
filament. D, a food cup for ingesting a flagellate superimposed on a food cup containing a 
ciliate. (A after Kepner and Taliaferro; B after Jennings; C after Rhumbler; D after Becker.) 


formed and the victim is enclosed without 
being touched; in this manner a dozen or 
more flagellates may be ingested in one 
food cup. A small amount of water is taken 
in with the food, so that a vacuole is formed 
with walls which were formerly part of 
the cell membrane on the outside of the 
body, and contents consisting of a parti- 
cle of nutritive material suspended in water. 
The whole process of food taking occupies 


one or more minutes, depending on the 
character of the food and the temperature. 
It increases in rapidity up to 25° C, and 
decreases to zero at about 33° C. The 
amoeba is not always successful in accom- 
plishing what it undertakes, but when it does 
not capture its prey at once, it seems to show 
a persistence usually attributed only to 
higher organisms (Fig. 13). 

Feeding occurs only when the amoebas 


/•-Carbon M 

v^Food ^ 

Amoeba encounters 
food and carbon 
particle 


2J^ minutes 
later a food 
cup is formed 


7^ minutes 
later 

(The carbon is not ingested) 


8 minutes 
later 


Figure 13. Amoeba proteus exhibiting food selection. The arrows indicate the direction of the 
movement of the protoplasm in the pseudopodia. (After Schaeffer.) 


PHYLUM PROTOZOA. ONE-CELLED ANIMALS 


35 


are attached to some solid object. At certain 
times any animal or plant that is not too 
large may be ingested, but when several 
species are present, selection is evident, since 
the small flagellate Chilomonas (Fig. 21) is 
engulfed more readily than the larger ciliate 
Colpidium; and the flagellate Monas is rarely 
taken if Chilomonas or Colpidium is avail- 
able. As many as 50 to 100 chilomonads may 
be ingested in a single day. When amoebas 
are fed exclusively on chilomonads, they 
grov/ and multiply for a few days but soon 
die; whereas when fed exclusively on Colpi- 
dium they grow large, become sluggish, and 
multiply slowly, but do not die. The amoeba 
may live for 20 days or more without food, 
but it decreases in volume until it is only 
about 5 per cent of its original size. 

Digestion 

The food vacuole (food chamber) (Fig. 
11) serves as a sort of temporary stomach. 
Digestive fluids (enzymes) are secreted into 
it by the surrounding cytoplasm. The con- 
tents are at first acid and then become 


alkaline. In man, as we shall see later, food 
materials encounter an acid medium in the 
stomach and an alkaline medium in the in- 
testine. Chilomonads remain alive in the 
food vacuoles from 3 to 18 minutes and are 
digested in from 12 to 24 hours. Proteins, 
fats, sugars, and starches are broken down. 
The digested material diffuses out of the 
vacuoles into the cytoplasm, with the vac- 
uole decreasing in size until only indigesti- 
ble matter remains. This is eventually 
eliminated. 

Egestion 

Indigestible and sometimes partially di- 
gested particles are egested at any point on 
the surface of the amoeba, there being no 
special opening to the exterior for this waste 
matter. Usually such particles are heavier 
than the cytoplasm of the amoeba; and as 
the animal moves forward, they lag behind, 
finally passing out at the end away from the 
direction of movement; that is, the amoeba 
flows away, leaving the indigestible solids 
behind (Fig. 14). 


New cell membrane 


Figure 14. Amoeba verrucosa. Part of a specimen showing three stages in the egestion of an 
indigestible particle; development of a new cell membrane prevents loss of endoplasm. (After 
Howland.) 


Assimilation 

The digested material absorbed into the 
cytoplasm is built up into protoplasm, that 
is, it is assimilated, and growth results. 

Dissimilation ( catabolism ) 

The energy for the work done by the 
amoeba comes from the breaking down of 
complex molecules of protoplasm by oxida- 


tion or physiologic burning. The products 
of this slow combustion are the energy of 
movement, heat, and residual matter. Ordi- 
narily the residual matter consists of solids, 
fluids consisting mainly of water, some min- 
eral substances, urea, and carbon dioxide. 
Thus it will be seen that the products of 
respiration are included in this residual 
matter. 


36 


COLLEGE ZOOLOGY 


Secretion 

Very little is known about secretion in 
the amoeba. Undoubtedly digestive fluids are 
secreted into the food vacuoles. Other sub- 
stances of use in the life processes of the 
animal may also be secreted. 

Excretion 

The amoeba probably gets rid of most of 
its excretory matter, including urea and 
carbon dioxide, through the general surface 
of the body. The contractile vacuole may 
serve in part for excretion, but its primar}' 
function is to regulate the v^^ater content of 
the cell body. Water enters the body with 
the food; it is a by-product of oxidation; and 
it also passes into the cell through the gen- 
eral surface. The contractile vacuole is 
formed by the fusion of minute droplets of 
liquid. Its "wall" is not usually permanent; 
it is a condensation membrane that disap- 
pears at each contraction. It forms in various 
parts of the body, often near the nucleus, 
and is carried toward the posterior end. The 
discharge of the contractile vacuole to the 
outside seems to take place through the up- 
per surface and for that reason cannot ordi- 
narily be seen. 

Respiration 

The amoeba requires oxygen for metabo- 
lism and must get rid of carbon dioxide. This 
interchange corresponds to the internal 
respiration of cells in higher animals. That 
oxygen is necessary for the life of the amoeba 
can be proved by replacing it with hydrogen; 
movements cease after 24 hours; if air is 
then introduced, movement begins again; if 
not, death ensues. Oxygen dissolved in water 
is taken in, and carbon dioxide passes out 
through the surface of the amoeba. The 
contractile vacuole may take part in carry- 
ing carbon dioxide to the outside. 

Reproduction 

Ordinarily the amoeba builds up proto- 
plasm more rapidly than it breaks it down; 


and when full size is reached, it reproduces 
by the simple process of dividing into two 
amoebas. This method of reproduction is 
called binary fission (Fig. 15). The nucleus 
divides by mitosis (Fig. 15); the prophase 
lasts 10 minutes, the metaphase probably 
less than 5 minutes, the anaphase about 10 
minutes, and the telophase about 8 minutes. 
The nuclear membrane disappears during 
the metaphase. The body of the amoeba, at 
the time of division, becomes spherical and 
covered with small pseudopodia; it elongates 
and separates into two during the telophase 
stage in mitosis. The time required for the 
entire process depends on the temperature; 
at 24° C. it takes about 33 minutes, and 
under laboratory conditions, the amoeba di- 
vides every few days. 

Development in the amoeba is simply a 
matter of growth; the rate of growth is 
rapid just after division, and then gradually 
decreases until the size for division is once 
again reached, which takes on the average 
about three days. Potentially, the amoeba is 
"immortal," for if it reproduces by fission, 
there is no death from old age. If death oc- 
curs, it results only from an accident. 

Behavior 

The activities of the amoeba involving 
changes in shape, formation of pseudopodia, 
locomotion, capture of food, etc., constitute 
its behavior. These activities are due largely 
to changes in the animal's environment and 
possibly in part to internal changes such as 
"hunger." The environmental change is 
called a stimulus, and the animal's reaction, 
a response. The amoeba responds to a num- 
ber of types of stimuli, including those due 
to changes in contact, light, temperature, 
chemicals, and electricity. Movement toward 
a stimulus is called a positive reaction and 
away from a stimulus, a negative reaction. 

Contact 

The amoeba when touched with a small 
rod will cease locomotion for a time and 


Early anaphase, 
daughter chromosomes 
have separated; faint 
spindle fibers present 


Late prophase 


Metaphase: the nuclear membrane 
is disappearing, chromosomes lining 
up at equatorial plate 

Figure 15. The amoeba, reproducing by binary fission and showing both external appearance 
and the division of the nucleus by mitosis. Begin study with the interphase stage and follow 
the arrows. In the center of the diagram, the time in minutes for each stage is shown. Highly 
magnified. (After Chalkley and Daniel.) 


37 


38 


COLLEGE ZOOLOGY 


then move away, thus exhibiting a negative 
reaction to contact or mechanical shock 
(Fig. 16). If, however, a floating specimen 
touches a solid object, it will react positively 
and move toward the object. 


::'.:i^^^SaU 


Chemicals 

Choice of food by the amoeba is probably 
largely the result of reactions to chemicals; 
a positive reaction results in ingestion and a 
negative reaction in movement away from 


Amoeba touches drop 
of salt solufioi) 


Amoeba touches a rod 


}^-y(^y 


and quickly moves away 


forms pseudopodia around ifr 


and turns to 
ovoid the obstacle 


Light 


Negative reaction of amoeba to strong 
light. The direction of the beam of light 
was changed at intervals. The amoeba in 
each case changed its direction so as to 
avoid the light. Time: 9:40 to 9:55 A.M. 


Figure 16. Amoeba. Reactions to various stimuli. Arrows show direction of movement. 


the food particle. The amoeba reacts nega- 
tively to various chemicals such as table salt 
(sodium chloride), acetic acid, cane sugar, 
and methyl green (Fig. 16). 

Light 

The amoeba will orient itself in respect to 
the direction of the rays of a strong light 
and move away from it (Fig. 16), but it 
may react positively to a very weak light. 

Temperature 

As noted above, the rate of locomotion of 
the amoeba depends on the temperature of 


the medium. An increase in temperature re- 
sults in movements away from the stimulus, 
that is, in a negative response. If the tem- 
perature is decreased sufficiently, movements 
cease. 

Conclusions 

These examples of behavior of the amoeba 
show that it is irritable and that stimuli are 
conducted to all parts of its cell body. Its 
reactions to stimuli are of undoubted value 
to the individual and to the preservation of 
the species since the negative reactions, pro- 
duced in most cases by injurious agents such 


PHYLUM PROTOZOA. ONE-CELLED ANIMALS 


39 


as strong chemicals, heat, and mechanical 
impacts, carry the animal out of danger. 

The data thus far obtained indicate that 
factors are present in the behavior of the 
amoeba "comparable to the habits, reflexes, 
and automatic activities of higher organ- 
isms" (Jennings). 


OTHER SARCODINA 

The amoeba was first reported by Roesel 
in 1755, although what species he saw is in 
doubt. Our type. Amoeba proteus, has been 
described as "a shapeless mass of proto- 
plasm," but this is incorrect. Although it is 
continually changing its shape, it has definite 
characteristics such as a disk-shaped nucleus, 
blunt pseudopodia, and often longitudinal 
ridges on the surface. Many amoebas from 
fresh water, salt water, soil, and as parasites 
in other animals, have been described; some 
have been placed in the genus Amoeba, and 
the rest have been assigned to other 
genera. 

Pelomyxa palustris is a large species that 
may reach a diameter of 2 mm.; it contains 
many nuclei and moves along without defi- 
nite pseudopodia. Another large species, 
Pelomyxa carolinensis, which is sometimes 
referred to as Chaos chaos or giant amoeba, 
may be obtained from biological supply 
houses. This species is from 50 to 500 times 
the volume of Amoeba proteus; it may reach 
a length of from 2 to 5 mm. (^100 to 2%oo 
inch), and can be seen with the naked eye. 
Pelomyxa carolinensis usually contains from 
300 to 400 nuclei, and from 3 to about 12 
contractile vacuoles. Instead of dividing into 
2 daughter amoebas, it generally divides into 
3. Parasitic amoebas are described in a later 
chapter. 

Several types of common fresh-water Sar- 
codina are protected by shells. Arcella (Fig. 
17) secretes its shell, but Difflugia (Fig. 17) 
builds a shell of minute grains of sand. In 
both types, pseudopodia are thrust out 
through a circular opening in the shell; they 


serve, as in Amoeba, for purposes of loco- 
motion and obtaining food. Another inter- 
esting fresh-water species is sometimes called 
the sun animal, Actinophrys (Fig. 17) be- 
cause of its stiff radiating pseudopodia. This 
is abundant among aquatic plants. The ray- 
like pseudopodia are stiff because each con- 
tains an axial filament to keep it rigid. 

Most of the 8000 or more Sarcodina live 
in the sea. The Foraminifera, of which Glo- 
bigerina is an example, construct a perfo- 
rated shell, usually of calcium carbonate, 
through which slender pseudopodia project 
(Fig. 17). Radiolaria also possess slender 
pseudopodia; many build elaborate skeletons 
of silica (Fig. 40). 


CLASSIFICATION OF 
THE SARCODINA 

(For reference purposes only) 

Class Sarcodina includes mostly marine 
Protozoa which are free-living. They move and 
capture food by means of pseudopodia. A shell 
or skeleton may be present. Nutrition is holo- 
zoic (subsisting on other organisms) and re- 
production is principally by binary fission. 
About 8000 species have been described. The 
two subclasses and four orders are described 
as follows: 

Subclass 1. Rhizopoda (Gr. rhiza, root; 
pous, foot). Typically creeping 
forms with lobose pseudopodia, 
but no central filament. 

Order 1. Amoebina. Amoebalike. Short, 
lobose pseudopodia. Some spe- 
cies (Gymnamoebac: Gr. gym- 
nos, naked) are naked, whereas 
other species (Thccamoebae: 
Gr. theke, case) are covered by 
a simple shell with one opening. 
Exs. Amoeba proteus and 
Arcella vulgaris ( Fig. 17). 

Order 2. Foraminifera (L. forc/mcn, open- 
ing; fero, bear). With simple 
or chambered perforated shell 
and from one to many branched 


Top view 


Pseudopodia 


Side view 
Arcelia, with a secreted shell 


Actlnophrys, with needle- 
shaped pseudopodia 


Pseudopodia 


G/obigerina, with several com- 
partments of different sizes 


OdI 


Difflugia, with a shell 
of sand grains 


Figure 17. Various representatives of Sarcodina. Highly magnified. 


40 


PHYLUM PROTOZOA. ONE-CELLED ANIMALS 


41 


pseudopodia. Mostly marine. 
Ex. Globigerina bulloides (Fig. 
17). 
Subclass 2. Actinopoda (Gr. aktis, ray; 
pous, foot). Typically floating 
forms with radiating, un- 
branched pseudopodia, each 
with central filament. 

Order 1. Heliozoa (Gr. helios, sun; 
zoion, animal). Pseudopodia 
are thin, radially arranged, and 
usually supported by axial 
threads; spherical; chiefly in 
fresh water. Ex. Actinophrys sol 
(Fig. 17). 

Order 2. Radiolaria (L. radiolus, a little 
ray). Marine. Often spherical; 
pseudopodia raylike; protoplasm 
divided into inner and outer 
parts by a perforated capsule; 
usually a skeleton of silica or 
strontium sulfate. Ex. Acan- 
thometron elasticum (Fig. 40). 


Subclass 3. Mycetozoa (L. mykes, fungus; 
zoion, animal). Slime animals. 
Adult phase consists of a sheet 
of multinucleate protoplasm up 
to several inches in width. They 
are found on decaying organic 
matter, such as rotting leaves 
and wood. The mycetozoa pro- 
duce resistant spores that sur- 
vive dry conditions. 

SELECTED COLLATERAL 
READINGS 

Calkins, G.N. The Biology of the Protozoa, 
Lea & Febiger, Philadelphia, 1933. 

Cushman, J. A. Foraminifera, Their Classifica- 
tion and Economic Use. Har\'ard Univ. 
Press, Cambridge, 1948. 

Hagelstein, R. The Mycetozoa of North Amer- 
ica. Published by author, Mineola, N.Y., 
1944. 


CHAPTER 4 


f ^# 


OJl 


w 


Phylum Protozoa. 
Flagellates 


EiE flagellates are protozoa that differ 
from the amoeba in that they usually pos- 
sess a definite shape and a front end from 
which arise one or more whiplike locomotor 
organelles called flagella (singular flagel- 
lum). The flagella are also used to capture 
food and to serve as sense receptors for ex- 
ploring the surroundings. The flagellates are 
abundant in puddles, ponds, and swamps. 
Unfortunately, most of them are so small 
that they are difficult to study. The euglenas, 
however, are comparatively large and exhibit 
most of the characteristics peculiar to the 
class. 

One of the flagellates, the Phytomona- 
dida, includes a number of colonial species 
that can be arranged in a series, from a sim- 
ple aggregation of cells as in Spondylomorum 
(Fig. 401, p. 562), to a very complex colony 
such as Volvox (Fig. 22). These flagellates 
and certain others are of particular interest 
since they combine the characteristics of 
both plants and animals and are frequently 
claimed by botanists. Many different species 
live in fresh and salt water. With diatoms, 
they constitute an important part of the 
food supply for very small aquatic animals. 
Many flagellates are parasitic in man, lower 
animals, and plants. Some parasitic forms 
are mentioned here, but a fuller discussion 
will be found in a subsequent chapter. 


EUGLENA VIRIDIS 


Habitat 

One of the common species of the genus 
Euglena, usually Euglena viridis, ordinarily 
serves as a type of the class Mastigophora. 
Euglenas are common in fresh-water ponds, 
to which they give a greenish tinge if present 
in sufficient numbers. They are usually found 
in collections of pond weeds and thrive in 
the laboratory in a jar on the window sill 
where there is plenty of indirect sunlight. 
Over 150 species have been described in the 


42 


PHYLUM PROTOZOA. FLAGELLATES 


43 


genus Euglena; these differ from one another 
in size, shape, behavior, and structural de- 
tails. 

Morphology 

Euglena viridis (Fig. 18) is 0.1 mm. or 
less in length, blunt at the anterior end, and 


pointed at the posterior end. Figure 18 pre- 
sents the structural features of this species. 
The peripheral layer of cytoplasm is a thin 
elastic membrane, the pellicle. This pellicle 
has parallel, spiral thickenings that give it 
a striated appearance. It is rigid enough to 
maintain the shape of the body, but suf- 
ficiently flexible to allow euglenoid move- 


Cell mou^h 
Cell gullet 

Flageltum 

Reservoir 
Pellicle 


OJI 


Nucleus 
Endosome 

•■Chloroplast 

Paramylum body 


Figure 18. Euglena viridis. Diagram of a stained specimen showing structure. 


ments. Near the anterior end is a funnel- 
shaped depression, the cell mouth 
(cytostome), that leads into the cell gullet 
( cytopharynx ) . The euglena does not eat 
solid food as these terms might imply. The 
cytopharynx is enlarged at the base to form 
a vesicle called the reservoir, adjacent to 
which is located a contractile vacuole, which 
discharges its contents into the reservoir and 
out through the cytopharynx. 

Near the anterior end of the body is an 
orange-red eye spot which is part of a light- 
sensitive organelle and probably serves in 
orienting the euglena to light. A flagellum, 
which arises from two axial filaments within 
the body, extends out of the cytostome. The 
electron microscope shows that the flagellum 
consists of a core of two axial filaments sur- 
rounded by a sheath of protoplasm. Near 
the center of the euglena is an oval or spheri- 
cal nucleus containing a central body, the 
endosome. The function of the endosome is 
controversial. Suspended in the cytoplasm 
are also a number of green bodies, the 


chromatophores, which are known as chloro- 
plasts. This green color is due to the pres- 
ence of chlorophyl. In Euglena viridis the 
chloroplasts are slender and radiate from a 
central point. In each chloroplast of some 
species of Euglena there is a pyrenoid, which 
is probably a center for the formation of a 
starchlike substance called paramylum. Para- 
mylum bodies may also be free in the cyto- 
plasm in the form of disks, rods, and links. 
Paramylum is produced by photosynthesis 
and represents reserve food material. 

Physiology 

Nutrition 

Euglenas obtain their nutriment largely 
by photosynthesis, a process that takes place 
within the chloroplasts; however, it has not 
been proved that any species of Euglena can 
grow in light without a trace of organic 
material such as a peptone. In the dark the 
euglenas can live on organic compounds that 
are dissolved in water; under these condi- 


44 


COLLEGE ZOOLOGY 


tions the chloroplasts and pyrenoids degen- 
erate and disappear. Although the euglenas 
do not capture and eat other organisms, 
there are colorless animal-like flagellates 
which do eat protozoans, algae, and 
diatoms. 

Locomotion 

In swimming, the flagellum beats back 
and forth, moving the animal forward. A 
spiral path is followed, resulting in a straight 
course through the trackless water, provided 
no stimulus interferes. Although euglenas 
possess a definite shape, they are charac- 
terized by wormlike movements involving 
waves of contraction to which the term 
euglenoid movement has been applied 
(Fig. 20). 

Reactions to light 

Euglenas are easily stimulated by changes 
in the direction of the light. Most species 
swim toward an ordinary light such as that 
from a window; and if a culture is examined, 


most of the animals will be found on the 
side toward the brightest light. This is of 
distinct advantage to the animal since light 
is necessary for the process of photosyn- 
thesis, just as is true in plants. However, 
euglenas will swim away from the direct rays 
of the sun; direct sunlight will kill them if 
they are exposed to it for a long time. If a 
drop of water containing euglenas is placed 
in the direct sunlight with one half shaded, 
the euglenas will avoid the shady part as 
well as the direct sunlight, both of which 
are unfavorable to them. They will remain 
in a small band between the two, in the 
light best suited for them, their optimum 
(Fig. 19). By shading various portions of 
the body of a euglena, it has been found 
that the region of the eye spot is especially 
sensitive. It should be noted that when a 
euglena is swimming through the water, it 
is this anterior end which first encounters 
regions of different light intensity; the ani- 
mals give the avoiding reaction when they 
enter less favorable areas. 


Shaded side of vessel 


Euglenas gather in 
intermediate region 


Direction of light 


Figure 19. Euglena. Reaction to light. The euglenas gather in the intermediate region across 
the middle, where the light intensity is most favorable for them. (After Jennings.) 


Reproduction 

Reproduction in Euglena takes place by 
binary longitudinal fission (Fig. 20). The 
nucleus divides in two by mitosis; then the 
anterior organelles such as the reservoir are 
duplicated; and the animal divides longitudi- 
nally, that is, in an antero-posterior direc- 
tion, splitting the cell into two equal parts. 
The old flagellum may be retained by one 
half, while a new flagellum is developed by 
the other. Often longitudinal division takes 


place while the animals are in the encysted 
condition (Fig. 20). These animals are said 
to be encysted when they have become al- 
most spherical and are surrounded by a 
gelatinous wall which they have secreted. In 
this condition, periods of drought are suc- 
cessfully passed, the animals becoming active 
again when water is encountered. Usually 
in laboratory cultures, cysts are present on 
the sides of the dish. Before encystment, 
the flagellum is thrown off, but a new one is 
produced when activity is again resumed. 


PHYLUM PROTOZOA. FLAGELLATES 


45 


One cyst usually contains two euglenas, al- 
though further multiplication by longitud- 


inal division may produce 4, 16, or 32 young 
euglenas in a single cyst. 


Stages In longitudinol fission 


Euglenoid movement 


Division v/ithin a cyst 

Figure 20. Reproduction and euglenoid movement in Euglena vhidis. 


OTHER MASTIGOPHORA 

The relations of flagellates to man are dis- 
cussed in Chapter 7, where accounts will be 
found of species that live in drinking water, 
in the soil, and in the blood streams and in- 
testines of human beings. A few other types 
of particular interest are as follows. 

Chilomonas (Fig. 21) is a species that is 
very common in nature and in laboratory 
cultures; it constitutes a large part of the 
food of Amoeba proteus. It is about 35 
microns long and has two flagella at the an- 
terior end. It does not possess chromato- 
phores, but absorbs nutriment through the 
surface of the body. 

Among the flagellates that live in the sea 
are species of the genus Noctiluca (Fig. 21), 
which sometimes occur in such enormous 


numbers that, due to their orange color, the 
water looks like tomato soup. One quart of 
sea water may contain more than three mil- 
lion individuals. Even more striking is the 
appearance of the sea when one travels over 
it at night. Noctiluca is luminescent and 
glows with a bluish or greenish light when 
agitated. One can read the time on his 
watch when it is held a foot away from a 
glassful of these flagellates. Incidentally, this 
light is not accompanied by production of 
heat, and hence it is generated without the 
loss of heat energy, which is something man 
has not been able to do in making artificial 
light. Many other animals and certain plants 
possess a similar power of producing light 
without wasting energy, for example, the 
fireflies and their larvae, the glow^'orms. 
Gymnodinium (Fig. 21) is a dinoflagellate 


Chilomonas 


OJI 


Codosiga 


Gymnodinium 


Masfigamoeba 


Phacus 


Noctiluca 


46 


Figure 21. Representatives of various orders of flagellates. All highly magnified. {Noctiluca 
after Jahn and Jahn.) 


PHYLUM PROTOZOA. FLAGELLATES 


47 


of which one species {brevis) may occur in 
such great numbers as to cause a periodic 
red tide in coastal waters. The red tide may 
appear anywhere in the world, in both 
tropical and temperate waters. The mis- 
nomer red tide is the popular name for the 
brownish-amber discoloration of sea water 
caused by this microscopic flagellate. Under 
certain conditions it reproduces at a fantas- 
tic rate; sixty million have been counted in 
a single quart. 

The organism produces a toxic substance 
that is fatal to fish. The tiny pest also re- 
leases an airborne "poison gas" which ir- 
ritates the human respiratory tract and may 
cause coughing, sneezing, and even shortness 
of breath. 

During the red tide off the coast of Florida 
in 1952 and again in 1954 enormous num- 
bers of fish died, and the shore was littered 
for miles with stinking fish. An extensive red 
tide results in the loss of tremendous 
amounts of sea food. 

Another dinoflagellate, Gonyaulax, also 
causes waters to appear a rusty red at times 
because of its great numbers. Gonyaulax 
catenella is known to have been the cause 
of disastrous poisoning in man. Several kinds 
of shellfish along the Pacific Coast feed on 
them, thus making the shellfish poisonous 
for human consumption. In 1941, there 
were 346 cases of poisoning with 24 deaths. 
Since 1941 state laws forbid the gathering 
of shellfish during the season of the red 
waters. Experiments have shown the toxin 
to be about ten times as potent as strych- 
nine, which is used for poisoning mice. 

The genus Mastigamoeba (Fig. 21) in- 
cludes species that live in fresh water or in 
the soil. They not only possess a flagellum, 
but also form pseudopodia with which they 
ingest food particles. 

Many flagellates are very complex in 
structure, especially certain species that live 
in the intestine of termites (white ants) 
such as those shown in Fig. 447, p. 639. The 
relations between these flagellates and the 
termites in which they live are described in 
Chapter 37. 


RELATIONS BETWEEN 
SARCODINA AND MASTIGOPHORA 

Many zoologists believe that flagellates 
evolved from the green algae among plants 
and that the Sarcodina arose from a flagel- 
late or flagellatelike organism. Many green 
flagellates such as Volvox (Fig. 22) can 
hardly be separated from green algae. A close 
relation between amoebas and flagellates is 
indicated by the fact that in certain species 
both amoeboid and flagellate stages occur in 
the same life cycle. Also certain types of 
flagellates such as Mastigamoeba possess 
both flagella and pseudopodia. Probably not 
all Sarcodina arose from the flagellates; some 
doubtless have evolved from other Sarco- 
dina. 


CLASSIFICATION OF 
THE MASTIGOPHORA 

{For reference purposes only) 

Class Mastigophora. These bear one or more 
flagella in the adult stage. They may be amoe- 
boid in shape but are generally covered with 
a pellicle. Many of them are parasitic. Binary 
fission is usually longitudinal division. No sex- 
ual reproduction is known in many of the 
genera. Two subclasses may be recognized ac- 
cording to their principal method of nutrition. 
The members of the subclass Phytomastigina 
are mostly holophytic, although some are sa- 
prozoic and may be in part holozoic. Those of 
the subclass Zoomastigina are primarily holo- 
zoic. 

Subclass 1. Phytomastigina (Or. phyton, 
plant; mastix, whip). Plantlike; 
chromatophores usually present; 
often a red eye spot. 

Order 1. Chrysomonadina. Small; 1 or 2 
flagella; some colonial. Ex. 
Uroglenopsis americana (Fig. 
39). 

Order 2. Cryptomonadina. One or 2 
flagella and usually 1 or 2 chro- 
matophores. Ex. Chilomonas 
Paramecium (Fig. 21). 


Asexual reproductive cell 

Cell bodies of 
somatic cells 

Daughter 
colony 

Protoplasmic 
strand 

Flogellum 
Gelatinous matrix 


Side view of 
somatic cells 


ASEXUAL REPRODUCTION Stages 
in the development of daughter 
colonies 


Sperm bundle (surface view) 

Sperm bundle (side view) 
Unfertilized egg 


SEXUAL REPRODUCTION 

Stages in the development 

of sperm bundles 

Fertilization of egg 


Free sperm bundle 
Zygote 


Zygote 


SEXUAL REPRODUCTION Stages in development, 
fertilization and encystment of eggs 

Figure 22. Volvox, the largest of the colonial flagellates, is a sphere of from 500 to 40,000 
cells. The whole organism barely attains the size of a pinhead. When the flagella vibrate, the 
organism revolves through the water. Its reproduction is described on page 562. 


48 


PHYLUM PROTOZOA. FLAGELLATES 


49 


Order 3. Dinoflagellina. Two flagella, 
usually 1 forward and the other 
in a groove around the body; 
mostly marine. Ex. Noctiluca 
scintillans (Fig. 21). 

Order 4. Phytomonadina. Cellulose body 
wall; no cytostome; many colo- 
nial. Ex. Volvox globatoT (Fig. 
22). 

Order 5. Euglenoidina. Usually 1 or 2 
flagella, a cytostome and cyto- 
pharynx; often chromatophores 
and eye spot. Exs. Euglena and 
Phacus (Fig. 21). 
Subclass 2. Zoomastigina (Gr. zoion, ani- 
mal; mastix, whip) . Animal-like; 
no chromatophores; no sexual re- 
production known. 

Order 1. Rhizomastigina or Pantosto- 
matida. Colorless; amoeboid; 1 
flagellum. Ex. Mastigamoeba 
aspera (Fig. 21). 

Order 2. Protomonadina. Colorless; often 
amoeboid; 1 to 3 flagella. Ex. 
Codosiga (Fig. 21). 

Order 3. Polymastigina. Mostly intestinal 


inhabitants; 3 to 8 flagella; some 
bilaterally symmetrical. Ex. Gi- 
ardia lamblia (Fig. 37, p. 72). 
Order 4. Hypermastigina. Intestinal in- 
habitants of termites (p. 638) 
and cockroaches; many flagella; 
often very complex. Ex. Spiro- 
trichonympha flagellata (Fig. 
447). 


SELECTED COLLATERAL 
READINGS 

Allen, W.E. "Red Water in La Jolla Bay 
(California) in 1945." Trans., Am. Micro- 
scopical Soc, 55:149-153, 1946. 

Gojdics, Mary. The Genus Euglena. Univ. of 
Wisconsin Press, Madison, 1953. 

Hall, R.P. Protozoology. Prentice-Hall, Engle- 
wood Cliffs, N.J., 1953. 

Jahn, T.L. "The Euglenoid Flagellates." Quart. 
Rev. BfoZ., 21:246-274, 1946. 

Pennak, R.W. Fresh-water Invertebrates of the 
United States. Ronald Press, New York, 
1953. 


CHAPTER 5 


OJI 


Phylum Protozoa. 

OneCelled 

Parasites 


HE class Sporozoa lives on or within other 
animals from which it derives nutrition, and 
therefore these animals are classified as para- 
sites. They are not as well known as other 
types of protozoans, but they may be found 
in animals ranging in complexity from sim- 
ple invertebrates to man. The life cycles of 
many sporozoans involve different species 
of hosts. A host is any plant or animal on 
or within which a parasite lives and from 
which it obtains its nourishment. The var- 
ious stages of development of the sporozoans 
are very interesting, and the methods by 
which they are transmitted from one host 
to another are quite remarkable. They may 
also cause the death of their host, including 
man, and are therefore very important to 
our welfare and economy. 

The life cycle is usually complicated, in- 
volving the production of resistant stages 
which in some cases are called spores. The 
spore may be a spindle-shaped case contain- 
ing sporozoites (Fig. 23). Spores serve as an 
infective stage in the life cycle. They often 
pass out of one host in the feces and enter 
another host in contaminated food or drink; 
or they may be sucked out of one host by a 
bloodsucking animal, such as an insect, and 
inoculated into another animal by this in- 
termediate host. 


MONOCYSTIS LUMBRICI-A 

SPOROZOAN PARASITE 

OF EARTHWORMS 


50 


Monocystis lumbrici illustrates many of 
the characteristics of the Sporozoa (Fig. 23). 
It is a parasite almost invariably found in 
the seminal vesicles of the common earth- 
worm. The stages that are usually present 
are (1) the trophozoite, (2) cysts contain- 
ing two individuals, or gametes and spores 
in various phases of development, and (3) 
isolated spores. 

Monocystis is easily obtained for study. 


PHYLUM PROTOZOA. ONE-CELLED PARASITES 


51 


A living or preserved earthworm should be 
pinned down and a slit made in the body 
wall from about the tenth to the fifteenth 
segment; the whitish bodies that extrude 
are the seminal vesicles. Parts of these should 
be pinched off with forceps and teased out 
well with dissection needles on a slide, in 
a drop of 0.7 per cent table salt (NaCl) 
solution. A cover glass should be placed on 


Young trophozoite 


the preparation, which should then be ex- 
amined under the microscope. 

The life cycle of Monocystis is briefly out- 
lined as follows (Fig. 23). The spores are 
taken into the earthworm's digestive tract 
where the sporozoites are set free. Each 
sporozoite penetrates a bundle of develop- 
ing sperm cells in the testis of the earth- 
worm and is then termed a trophozoite. 


Sporozoite enters developing 
sperm cells in testes 


Trophozoite grows and 
migrates to seminal vesicle 


Cyst wa 


Spores containing sporozoites 
escape and are eaten by 
another worm 


Trophozoites associate 
in pairs and form a 
cyst wo I 


Spores 


Gametes 
Zygotes 


Zygotes secrete a spore 
wall and divide to form 
8 sporozoites in each spore 


Gametes are produced 


Gametes fuse (fertilization) 
to form zygotes 


Figure 23. Life cycle of Monocystis, a sporozoan that lives in the common earthworm. All 
highly magnified but not drawn to scale. 


Here it lives at the expense of the cells 
among which it lies. The sperms of the 
earthworm, which are deprived of nourish- 
ment by the parasite, slowly shrivel up, be- 


coming tiny filaments on the surface of the 
trophozoite, making it resemble a ciliated 
organism. The trophozoite grows and then 
migrates to a seminal vesicle. Here two 


52 


COLLEGE ZOOLOGY 


trophozoites come together and are sur- 
rounded by a cyst wall. Each then divides, 
producing a number of small cells called 
gametes. The gametes unite in pairs to form 
zygotes. It is probable that the gametes 
produced by one of the trophozoites do not 
fuse with each other but with gametes pro- 
duced by the other trophozoite enclosed in 
the same cyst. Each zygote becomes lemon- 
shaped and secretes a thin hard wall about 
itself. It is now known as a spore. The 
nucleus of the spore divides successively into 


2,4, and finally 8 daughter nuclei, each of 
which, together with a portion of the cyto- 
plasm, becomes a sporozoite. 

OTHER SPOROZOA 

Four subclasses and five orders of Sporo- 
zoa are usually recognized by zoologists. 
Some of the species of great importance to 
man are described in Chapter 7. Two types 
that are easily obtained for study are greg- 
arines and coccidians. 


OJI 


ORDER HAEMOSPORIDIA 
Sporozoite of Lankesferella 
in frog erythrocyte 


ORDER COCCIDIA 
Oocyst of Isospora 
hominis from human 
intestine 


ORDER 
MICROSPORIDIA 
Spores of Thelohania 
coniejeani from cray- 
fish muscle 


ORDER GREGARINIDA 
Two trophozoites (attached 
end to end) of Gregarina 
blatiarum from cockroach 
intestine 


ORDER HAEMOSPORIDIA 
Gametocyte of Hoemoprofeus 
in bird erythrocyte 


Figure 24. Types of sporozoa. 


Gregarines may be obtained from the in- 
testines of grasshoppers, cockroaches, and 
meal worms. Spores are swallowed by these 
insects from which sporozoites escape. These 
penetrate the cells of the intestinal wall, and 
trophozoites develop from them. The 
trophozoites, after undergoing a period of 
growth, break out into the intestine, where 
they unite end to end (Fig. 24) . The rest of 
the life cycle is similar to that of Monocystis. 

Coccidia are most easily obtained for 
study from the rabbit. Oocysts may be found 
in the feces of a large proportion of these 


animals. They consist of a single cell when 
passed, but if the material is placed in a 5 
per cent aqueous solution of potassium di- 
chromate to inhibit the growth of bacteria, 
four spores, each containing two sporozoites, 
will develop within each cyst in about three 
days. 


ORIGIN OF THE SPOROZOA 

Parasitic protozoans, no doubt, evolved 
from free-living species or from other para- 


PHYLUM PROTOZOA. ONE-CELLED PARASITES 


55 


sitic species that had free-Hving ancestors. 
The origin of the Sporozoa is obscure. The 
different groups included in the class may 
have arisen from different classes. Those 
with amoeboid sporozoites may have evolved 
from amoeboid ancestors, and those with 
flagellated sporozoites from flagellate an- 
cestors. Coccidia probably originated from 
gregarines and the blood-inhabiting Haemo- 
sporidia from the Coccidia. 


CLASSIFICATION OF 
THE SPOROZOA 

(For reference purposes only) 

Class Sporozoa. These are among the most 
widely distributed of all animal parasites; mem- 
bers of almost every large group in the animal 
kingdom are parasitized by one or more species. 
They are greatly modified, due to their para- 
sitic existence. These modifications have re- 
sulted in the absence of locomotor organelles, 
mouth, anal pore, and contractile vacuoles. 
Food is absorbed directly from the host, and 
respiration and excretion take place by diffu- 
sion through the cell membrane. Many organs 
of the host may be parasitized, especially the 
digestive tract, kidneys, blood, muscles, and 
connective tissues. The 4 subclasses and 5 
orders are as follows: 


Subclass 1. Telosporidia. Spores produced 
at end of life of trophozoite; no 
polar capsule nor polar filament. 

Order 1. Gregarinida. Common parasites 
of insects; at first intracellular, 
but later often free in cavities. 
Ex. Monocystis lumbrici (Fig. 
23). 

Order 2. Coccidia. Parasites of verte- 
brates and invertebrates; one 
species in man. Ex. Isospora 
hominis (Fig. 24). 

Order 3. Haemosporidia. Parasites in 
blood cells of vertebrates, and 
in bodies of invertebrates. Ex. 
Malaria organisms of man (Fig. 
33, p. 69). 
Subclass 2. Cnidosporidia (Gr. knide, net- 
tle). Spores with one to four 
polar capsules, with a coiled 
polar filament. 

Order 1. Myxosporidia. Principally para- 
sites of fish. Ex. Myxidium lie- 
berkuhni. 

Order 2. Microsporidia. Spores extremely 
small; usually with one polar 
capsule; insects most frequently 
infected. Ex. Nosema apis, and 
Thelohania contejeani. 
Subclass 3. Acnidosporidia. Simple spores 
without polar capsules; some 
parasitic in vertebrates. 
Subclass 4. Haplosporidia. In lower verte- 
brates and invertebrates. Ex. 
Haplosporidium nemertis. 


CHAPTER 6 


N/ \/ ^/ V ex. c--. ^-- V 

^^•^••••t, \.**.V>**«M <i^*/|^***« »,,«4Sm^ •..-H-*^* ■•••^Slk-*-** *i^^fr"**« •mX.P**^ 


HE ciliates are relatively large and the 
most complex in structure of the Protozoa, 
The slipperlike animal Paramecium cauda- 
tum is ordinarily used as a type since it is 
easy to obtain for study and reaches the 
comparatively great length of about 0.3 mm. 
Many other species of ciliates may be found 
in cultures made from pond weeds and de- 
caying plant and animal infusions. 

The ciliates (class Cilia ta) are distin- 
guished from other protozoans in possessing 
cilia, which are sometimes modified into 
cirri, during a part or all of their life cycle. 
In most of them the nuclear material is 
separated into a large macronucleus and one 
or more smaller micronuclei. Most ciliates 
are free-living in fresh or salt water. Some, 
however, are important parasites of man and 
other animals. 


Phylum Protozoa. 

Ciliates 


PARAMECIUM CAUDATUM- 
A FRESH-WATER CILIATE 

Paramecia are common animals usually 
found in pond water which contains con- 
siderable decaying vegetation. This little ani- 
mal was among the first living things seen 
with the newly invented microscope in the 
seventeenth century. The paramecium then 
became of biological interest and has con- 
tinued to be investigated down through the 
years. Recently, it and related ciliates have 
been used for studies of nutrition, respira- 
tion, cancer, heredity, sex, behavior, and 
ecology. 

The protozoans generally play a consider- 
able role in aquatic food chains. Many spe- 
cies use dissolved nutrients in the water and 
serve as food for small many-celled animals, 
which in turn are eaten by larger animals. 


54 


Morphology 

The 10 well-known species of paramecia 
differ from one another in size, shape, and 
structure. The following description applies 


PHYLUM PROTOZOA. CILIATES 


55 


principally to Paramecium caudatum (Fig. 
25) and Param.ecium. aurelia. Paramecium, 
caudatum ranges from about 0.15 mm, to 0.3 
mm. in length, and Paramecium aurelia 
from about 0.12 mm. to 0.2 mm. Figure 25 
indicates the shape of a specimen of P. 
caudatum. The anterior end is blunt and 
the posterior end more pointed. The greatest 
width is behind the center of the body. 


A depression extends from the anterior 
end, obliquely backward, ending just pos- 
terior to the middle of the animal; this is 
the oral groove. The cell mouth (cyto- 
stome) is situated near the end of the oral 
groove. It opens into a short tube, the cell 
gullet (cytopharynx), which passes obliquely 
downward and posteriorly into the endo- 
plasm. The side containing the oral groove 


Anterior end 

Cilium 
Trichocysts 
Food vacuole 3 


Contractile vacuole 
Pellicle 


vJral groove - 
Micronucleus- 
Macronucleus 

Cell mouth — 
Cell gullet 


Ectoplasm 
Endoplasm 


Food vacuole 2 


Bacteria being ingested 


Food vacuole forming. 
Cell anus^ 


i^JiSlS- 


^■wm^. 


t— ' 


wm, 


Radiating canal of C. vacuole 
Food vacuole 1 


OJt 


Figure 25. Left, spiral path of a free-swimming paramecium. Note that these animals rotate 
on their long axes at the same time that they are moving forward. Right, drawing designed to 
show the structure of Paramecium caudatum. Numbered food vacuoles show progress of digestion 
and absorption. 


56 


COLLEGE ZOOLOGY 


may be designated oral, and the opposite 
side, aboral. The motile organelles are fine 
hairlike cilia regularly arranged over the 
surface. As in the amoeba, two types of 
cytoplasm are visible: an outer compara- 
tively thin clear layer, the ectoplasm; and 
an inner granular mass, the endoplasm. Be- 
sides these, a distinct elastic membrane, the 
pellicle, is present on the outer surface of 
the ectoplasm. One large contractile vacu- 
ole is usually situated near each end of the 
body, close to the aboral surface; and a 


variable number of food vacuoles may be 
seen. The nuclei are two in number, a large 
macronucleus concerned with vegetative 
functions, and a smaller micronucleus that 
is important in reproduction; these are sus- 
pended in the endoplasm near the mouth 
opening. A temporary opening called the 
cell anus can be observed only when undi- 
gested particles are discharged. It is situated 
posterior to the oral groove, and it always 
reforms at the same point on the surface of 
the body. 


Cilium 


Pellicle 


Trichocyst 
opening 


Basal body 
of cilium 


TrichocysJ 


fnferciliary 
fibril 


Figure 26. PuTameciurn. Diagram showing structure of the pellicle: the hexagonal areas are due 
to ridges; the cilia extend out from the center of the hexagonal areas, each being attached to 
a basal body; the basal bodies are located on longitudinal fibers; these fibers appear to constitute 
the mechanism by which the activity of the cilia is coordinated. The carrot-shaped trichocysts 
are attached by delicate threads to the ridges. Highly magnified. (After Lund.) 


The endoplasm occupies the central part 
of the body. Most of the larger granules con- 
tained within it are shown by microchemical 
reactions to be reserve food particles; they 
flow from place to place, indicating that the 
endoplasm is of a fluid nature. The ecto- 
plasm does not contain any of the large gran- 


ules characteristic of the endoplasm, since 
its density prevents their entrance. In this 
respect the two kinds of cytoplasm resemble 
the ectoplasm and endoplasm of the amoeba. 
If a drop or two of 35 per cent alcohol ia 
added to a drop of water containing para- 
mecia, the pellicle will be raised in some 


PHYLUM PROTOZOA. CILIATES 


57 


specimens in the form of a blister. Under 
the higher powers of the microscope the 
pelhcle is then seen to be made up of a great 
number of hexagonal areas produced by 
ridges on the surface (Fig. 26). 

The distribution of the motile organelles, 
the cilia, corresponds to the arrangement of 
the striations on the pellicle, since one 
cilium projects from the center of each 
hexagonal area (Fig. 26). These hairlike 
structures occur on all parts of the body, 
those at the posterior end being slightlv 
longer than elsewhere. Cilia are outgrowths 
of the cell protoplasm and arise from basal 
bodies. The arrangement of the cilia within 
the cytophar}'nx is rather complicated; they 
guide the food particles that are swept 
within their reach. 

Physiology 

Physiologic processes similar to those de- 
scribed for the amoeba occur in the Parame- 
cium. Paramecia defend themselves from 
enemies, capture and ingest food, digest it, 
build up protoplasm, react to stimuli, carry 
on processes of respiration and excretion, 
and reproduce. 

Offense and defense 

The Paramecium feeds principally on bac- 
teria and on other protozoans which as a 
rule are smaller than itself. No special weap- 
ons of offense appear to be necessary for 
the capture of food. Paramecia, however, are 
attacked and used for food by other proto- 
zoans and by larger animals, so they have 
a real need for weapons of defense; the 
trichocysts appear to answer this purpose. 
These carrot-shaped structures are embedded 
in the ectoplasm just beneath the surface 
as shown in Figs. 25 and 26. These bodies 
are oriented perpendicularly to the surface. 
A small amount of iodine or acetic acid, 
when added to a drop of water containing 
paramecia, causes the discharge of the tri- 
chocysts to the exterior. After the explosion, 
the animal is surrounded by a halo of long 


threads. Evidence that the trichocysts are 
probably weapons of defense is furnished 
when a paramecium encounters another spe- 
cies of ciliate, Didinium; see headpiece on 
first page of this chapter, which shows a 
Paramecium attached to and being eaten by 
a didinium. If the seizing organ of this pro- 
tozoan becomes fastened in the paramecium, 
a great number of trichocysts are discharged 
near the place of the injury. If the parame- 
cium is a large one, it frequently succeeds in 
making its escape. However, it is probable 
that the usual function of the trichocvsts is 
to hold the paramecium to the substratum 
while it is feeding on bacteria. A ciliate is at 
a disadvantage during feeding if it does not 
have anchorage because the ciliaiy currents 
used to gather food cause it to whirl 
around. 

Nutrition 

Paramecia do not possess chlorophyl and 
hence are unable to manufacture food by 
photosynthesis as euglenas do. One species, 
Paramecium bursaria, which contains min- 
ute unicellular green plants in its endoplasm, 
will reproduce in a solution of salts alone if 
it is kept in the light. This is a case of mu- 
tualism in which a mutually beneficial rela- 
tionship exists between two different or- 
ganisms. Paramecia do not ingest ever}' small 
object that reaches the cytostome; they take 
in certain particles and not others. For ex 
ample, if different species of bacteria are 
present, they may feed on one species and 
not on another. 

Bacteria, yeasts, small protozoans, and 
algae are captured with the aid of the cilia 
in the oral groove. By the direction of their 
beating, they produce a sort of current that 
drives a steady stream of water toward the 
cytostome. Food particles that are swept into 
the cvtostome are carried down into the 
cytophar}'nx; they are then driven onward 
by the cilia in the cytopharynx and are fi- 
nally gathered together near the end into 
a food vacuole (Fig. 25). When this vacuole 
has reached a certain size, it is rdc'^scd into 


58 


COLLEGE ZOOLOGY 


Anterior 


Yellow-green 


Red (food vacuole forming) 


Blue-green (acid) 


-Yellow (alkaline) 


■Orange 


■Red-orange 


-Waste material 
Red-orange 


Posterior 

Figure 27. Paramecium caudatum. Diagram illustrating cyclosis and the process of digestion. 
To indicate the path of the food vacuole within the paramecium, some yeast cells stained with 
Congo red (a red dye) should be placed near the animal. The yeast cells are taken into the 
body, where a food vacuole is formed. The arrows in the figure of the paramecium indicate the 
path of the food vacuole within the body (cyclosis). The change in color of the Congo red 
to blue-green indicates that the vacuole became acid soon after it was formed. As the vacuole 
with the yeast cells circulates through the body, it changes back to a red-orange color, indicating 
that the vacuole and its contents are becoming less acid. Chemical tests have proved that the 
vacuole with its contents actually becomes definitely alkaline even though the red-orange color 
would not prove this, for Congo red is an indicator of acidity only, and cannot be used to test 
for alkalinity. We can assume from experiments that in the digestion of food the vacuole is 
first acid and then alkaline. 


the surrounding cytoplasm; and the forma- 
tion of another vacuole is begun. 

A food vacuole is a droplet of water with 
food particles suspended within it. As soon 
as one is separated from the cytopharynx, it 
is swept away by the rotary streaming move- 
ment of the endoplasm, known as cyclosis 
(Fig. 27). This carries the food vacuole 
around a definite course, which begins just 
behind the cytopharynx, passes posteriorly, 
then forward and aborally, and finally pos- 
teriorly to near the oral groove. In the course 
of this journey, digestion takes place. During 
the cyclosis the food is digested by enzymes 


from the endoplasm, and the food vacuoles 
become smaller. The digested food is either 
stored, used for a vital activity, or is built 
up into protoplasm. 

Regulation of water content 

Two contractile vacuoles are present in 
Paramecium caudatum, occupying definite 
positions, one near each end of the body. 
They lie in the inner layer of the ecto- 
plasm and communicate with a large por- 
tion of the body by means of a system of 
radiating canals, 6 to 11 in number (Fig. 
25). These canals fill with liquid, then dis- 


PHYLUM PROTOZOA. CILIATES 


59 


charge their contents to form the vacuole, 
which in turn ejects the hquid to the ex- 
terior. After each discharge a new contrac- 
tile vacuole is formed. 

The rate of contraction varies with the 
activity of the animal, temperature, and the 
concentration of salts in the surrounding 
water. Since the body of the paramecium is 
surrounded by a semipermeable membrane, 
and the concentration of water molecules 
on the outside of the membrane is greater 
than that inside as a consequence of dis- 
solved substances in the protoplasm, water 
is continually entering the cell. The con- 
tractile vacuoles function primarily to re- 
move the excess water and in so doing re- 
move excretory waste products of metab- 
olism from the body. 

Excretion 

Excretion is a type of activity in which 
the waste products of metabolism are elimi- 
nated from protoplasm. Most of the waste 
products of paramecia appear to diffuse to 
the exterior through the pellicle, but nitrog- 
enous substances have been detected in the 
contractile vacuoles, which may therefore be 
excretory in function. 

Respiration 

This process in the paramecium corre- 
sponds to the internal (cell) respiration of 
man. Oxygen, dissolved in the water, diffuses 
through the surface of the body into the 
protoplasm, just as oxygen is taken from the 
blood by our red blood corpuscles. Carbon 
dioxide diffuses out of the body through the 
surface, although the contractile vacuoles 
probably discharge some of the carbon 
dioxide as well as nitrogenous wastes. 

Locomotion 

The cilia are fine protoplasmic processes 
that cover the body of the paramecium; the 
effective strokes of all the cilia force the 
animal forward or backward. Since they beat 
obliquely, the animal rotates on its long 
axis as it swims forward (Fig. 25). The 


swerving of the body away from the oral side 
is due to the fact that the cilia in the oral 
groove beat more strongly than others, but 
the rotation of the animal on its long axis 
enables it to follow a more or less straight 
course in forming large spirals. 

The remarkable coordination of the cilia 
is probably made possible by a mechanism of 
tiny fibrils just beneath the pellicle which 
connect one cilium with another (Fig. 26). 
The fibrils concentrate at one point near the 
cell gullet to form a "neuromotor" center. 
If this center is experimentally destroyed, 
the cilia fail to beat in a coordinated man- 
ner; this results in the loss of coordination 
in movements. 

Behavior 

As in the amoeba and the euglena, 
changes in the environment serve as stimuli 
to which paramecia respond in various 
ways. 

Avoiding reaction 

One of the most common responses of 
the Paramecium is known as the avoidmg 
reaction (negative response) (Fig. 28). 
When a free-swimming paramecium en- 
counters a harmful chemical such as strong 
salt, it may reverse its cilia and swim back- 
ward for a short distance; then its rotation 
decreases in rapidity, and it swerves toward 
the aboral side more strongly than under 
normal conditions. Its posterior end then 
becomes a sort of pivot upon which the 
animal swings in a circle. During this revolu- 
tion, samples of the surrounding medium are 
brought into the oral groove. Wlien a sam- 
ple no longer contains the stimulus to which 
it reacts negatively, the cilia resume their 
normal beating and the animal moves for- 
ward again. If this movement once more 
brings it into the region of the harmful 
chemical, the avoiding reaction is repeated; 
this goes on as long as the animal receives 
the stimulus to which there is a negative 
reaction. 


60 


COLLEGE ZOOLOGY 


24°C-28°C 
Selects the optimum temperature 

Figure 28. Behavior of the paraniecium to various conditions in its environment. 


Reactions to stimuli 

The Paramecium not only reacts to chem- 
icals, but to contact, to changes in tem- 
perature, to light, to electric current, and 
to other stimuli. If the anterior end of a 
Paramecium, which is more sensitive than 
the other parts of the body, is touched with 
a glass rod, the avoiding reaction is given. 
Frequently, a paramecium when swimming 
slowly comes to rest with its cilia in contact 


with an object; this positive response often 
brings it into an environment rich in food. 
Paramecia do not respond in any way to 
ordinary visible light, but give the avoiding 
reaction when ultraviolet rays are thrown 
upon them. 

The optimum temperature for the Para- 
mecium lies, under ordinary conditions, be- 
tween 24° C. and 28° C. A number of ani- 
mals placed on a slide which is heated at 


PHYLUM PROTOZOA. CILIATES 


61 


one end will swim about in all directions, 
giving the avoiding reaction where stimu- 
lated, until they become oriented and move 
toward the cooler end. This is the method of 
trial and error; that is, the animal tries all 
directions until one is discovered which al- 
lows it to escape from the region of injuri- 
ous stimulation. 

Chemicals of various sorts and in various 
concentrations have a striking effect on the 
Paramecium. For example, sodium chloride 
(salt) solution when allowed to flow under 
a cover glass (Fig. 28) repels any specimens 
that encounter it. Weak acetic acid attracts 
the Paramecium, that is, it reacts positively, 
and mercuric chloride kills it. 

Frequently a paramecium may be stimu- 
lated in more than one way at the same 
time. For example, a specimen which is in 
contact with a solid is acted upon by tem- 
perature, chemicals, heat, and other stimuli. 
The physiologic condition of a paramecium 
determines the character of its response. 
This physiologic state is a dynamic condi- 
tion, changing continually with the processes 
of metabolism going on within the living 
substance of the animal. 

Reproduction 

The Paramecium usually multiplies by 
simple binary fission. This process is inter- 
rupted occasionally by a temporary union 
(conjugation) of two individuals and a sub- 
sequent mutual nuclear fertilization. 

Binary fission 

In binary fission the animal divides trans- 
versely (Fig. 29). It is an asexual process in 
which one fully grown individual divides 
into two daughters without leaving a pa- 
rental corpse. First the micronucleus under- 
goes mitosis and its substance is equally 
divided between the two daughter micronu- 
clei; these separate and finally come to lie 
one near each end of the body. The macro- 
nucleus elongates and then divides trans- 
versely by amitosis. The cytopharjnx pro- 


duces a bud which develops into another 
cytopharnj-x. A new contractile vacuole 
arises near the anterior end of the body, 
another just back of the middle line. While 
these events are taking place, a constriction 
appears near the middle of the longitudinal 
diameter of the body; this cleavage furrow 
becomes deeper and deeper until only a 
slender thread of protoplasm holds the two 
halves of the body together. This connec- 
tion is finally severed and the two daughter 
paramecia are freed from each other. The 
entire process occupies about two hours. 
The time, however, varies considerably, de- 
pending upon the temperature of the water, 
the quality and quantity of food, and other 
factors. The daughter paramecia increase 
rapidly in size, and at the end of 24 hours 
divide again if the temperature remains at 
15° to 17° C; if the temperature is raised 
to 17° to 20° C, two divisions may take 
place in one day. A paramecium under op- 
timum conditions may reproduce at the rate 
of 600 or more generations per year. If all 
the descendants of one individual were to 
live and reproduce at a normal rate, they 
would soon equal the earth in volume. Un- 
der the usual natural conditions, they do 
not increase at any such fantastic rate be- 
cause of lack of food, internal physiology, 
low temperatures, drought, or falling prey 
to other animals. 

Conjugation 

Ordinarily the paramecium multiplies by 
binar}' fission for long periods of time, but at 
intervals this may be interrupted by the sex- 
ual process of conjugation (Fig. 30). When 
two paramecia conjugate, their oral surfaces 
are opposed, and a protoplasmic bridge is 
constructed between them. During conjuga- 
tion the pairs continue to swim about. As 
soon as this union is effected, the micro- 
nuclei pass through a series of stages in 
which the chromosome number is reduced 
to one-half; this has been likened to the 
maturation processes of metazoan germ cells, 
see page 80. These are illustrated and de- 


62 


COLLEGE ZOOLOGY 


Micronucleus in mitosis, 
macronucleus elongates 


\ 


Micronucleus begins mitosis 


y 


Micronucleus divides, macronucleus 
pulls in two, new cell mouth and 
two new contractile vacuoles appear 


Two daughter paramecia 

Figure 29. Division of Paramecium caudatum (binary fission). The first stage in the process 
of division is shown at the top of diagram. 


scribed in Fig. 30. During conjugation there 
is an interchange of micronuclei. The migra- 
tory micronucleus (Fig. 30) is smaller than 
the stationary micronucleus and may be 
considered comparable to the nucleus of a 
male germ cell. Its fusion with the stationary 
micronucleus resembles the fusion of male 
and female nuclei in the eggs of higher ani- 
mals at the time of fertilization. Conjuga- 
tion is similar to fertilization in that there 
is a mixture of nuclear materials from two 
individuals, and some authors consider the 
fusion of micronuclei of conjugating individ- 
uals as true fertilization. However, after con- 
jugation the animals continue to reproduce 
by asexual division in contrast to higher 
animals in which there is only sexual repro- 
duction. 

If the paramecia are kept in a constant 
medium, for example, hay infusion, they 
undergo a period of physiologic depression 


about every three months as shown by the 
decrease in their rate of division. Semian- 
nual periods also occur, and recovery from 
these does not take place if the animals arc 
kept under constant conditions, or conjuga- 
tion is prevented; the protoplasm degener- 
ates and becomes vacuolated, and the ani- 
mals lose their energies and finally die. This 
suggests that conjugation is essential for 
continued asexual reproduction. 

Experiments have been performed on one 
species which seem to show that in a varied 
environment neither conjugation nor death 
from old age necessarily occurs. Thus in one 
experiment, a culture of Paramecium was 
carried through a period of over 25 years 
without the intervention of conjugation, by 
changing the character of the medium daily. 
During this time there were over 25,000 
generations, and there was no evidence of a 
decline in the vitality of the organisms as 


PHYLUM PROTOZOA. CILIATES 


63 


Micronucleus and animal 
divide twice to produce 
four small paramecia 


Two paramecia unite 
by their oral grooves 


Macronuclei degenerate, 
micronuclei divide 


Four micronuclei become macronuclei, 
three degenerate, one remains 


Three micronuclei degenerate 


OJI 


The fusion micronucleus 
divides three times 


K 


The animals separate 
Only one member 
of the pair is shown 


Remaining micronucleus divides 
unequally and smaller micro- 
nuclei are exchanged 


The two micronuclei in each fuse 


Figure 30. Conjugation in Paramecium caudatum. The first stage in conjugation is shown at 
the top and at the middle of the diagram. Not all stages are shown. In the interest of clarity, 
the macronuclei are omitted from the third stage; actually they do not disappear completely 
until after the conjugating animals have separated. 


indicated by the rate of division. The cycle 
may thus be prolonged by employing a 
varied culture medium. However, it is now 
known that this paramecium does undergo 
a process of self-fertilization called autogamy 
at regular intervals, but some other ciliates, 
over a period of years, do not undergo either 
conjugation or autogamy. Therefore, nuclear 
reorganization may be essential in some 
ciliates but not in others. 


Mating types in conjugation 

At one time it was thought that there 
were no differences between the two para- 
mecia which join in conjugation, but now 
it is known that there are mating types 
within each species. For example, in Parame- 
cium aurelia at least two mating t\pes 
("sexes") occur, designated I and II. Mem- 
bers of mating type I do not conjugate with 
each other but will conjugate with members 


64 


COLLEGE ZOOLOGY 


of mating type II; and members of mating 
type II do not conjugate with each other but 
vill conjugate with members of mating 
type I, To determine the character of speci- 
mens of an unknown mating type, it is nec- 
essary to add them to a culture containing 
mating type I. If they conjugate with mem- 
bers of mating type I, they belong to mating 
type II; if they do not, they belong to mat- 
ing type I. 


OTHER CILIATA 

Ciliata are widespread in nature. Many 
species live in fresh water; other species oc- 
cur in the sea, in the soil, and upon or within 
the bodies of other animals. 

Most of the ciliates are not parasites and 
are therefore said to be free-living. Those 
shown in Fig. 31 are representative of some 
of the more interesting free-living forms. 

Tetrahymena, a small ciliate (Fig. 31) 
grows in a culture medium free from all 
other microscopic organisms and has about 
the same nutritional requirements as man 
himself. Studies show it must have a diet of 
vitamins, amino acids, salts, and sugar. This 
suggests that even a single-celled animal has 
physiologic processes nearly as complex as 
those in man. This tiny organism is playing 
an increasingly important role in physiologic 
and genetic research. 

Stentor is trumpet-shaped, bluish in color, 
has a beaded macronucleus, and cilia spirally 
arranged around the "mouth"; it may be 
free-swimming or attached, and at the an- 
terior end is a complicated disk of cilia. 
Stylonychia has a flattened body and groups 
of cilia fused to form cirri, which are used 
as "legs" in creeping. Vorticella resembles 
an inverted bell attached to a stalk. 

Parasitic ciliates 

One well-known parasitic species of ciliate 
lives in man (Fig. 38). Many domesticated 
and wild animals, both terrestrial and 
aquatic, are parasitized by ciliates, most of 


which live in the digestive tract. Species be- 
longing to the genus Opalina are common 
in the rectum of certain frogs and toads; 
Opalina (Fig. 31) contains many nuclei of 
one type. One ciliate lives in the intestine 
of the earthworm. About 40 species of 
ciliates live in the first and second chambers 
of the stomach of cattle, and some are very 
complex in structure. These may be only 
mess mates (commensals) without either 
benefit or harm to cattle. Two common spe- 
cies of ciliates creep about on the bodies of 
certain aquatic animals and frequently oc- 
cur on the fresh-water hydra; these are 
Kerona (Fig. 32) and Trichodina. Many of 
the Suctoria are parasitic; one {Podophrya) 
is of particular interest because it parasitizes 
other ciliates. 


CLASSIFICATION OF 
THE CILIATA 

{For reference purposes only) 

Ciliata possess cilia at some stage in their 
life cycle. In most of them the nuclear mate- 
rial is separated into a large macronucleus and 
a smaller micronucleus. Most of them are free- 
living in fresh water or the sea, but many are 
parasites of other animals. They may be sepa- 
rated into two classes and four orders as fol- 
lows : 

Class 1. Ciliata. Cilia present throughout life; 
"tentacles" absent. 

Order 1. Holotricha. Cilia typically of 
equal length all over the body; 
no adoral cilia. Ex. Paramecium 
caudatum (Fig. 25) and Chilo- 
donella (Fig. 31). 

Order 2. Spirotricha. Cilia covering en- 
tire body, an adoral zone of 
either large cilia or membranel- 
les, which are wound to the left 
along the oral groove. Exs. 
Stentor coeruleus (Fig. 31) and 
Balantidium. 

Order 3. Hypotricha. Dorsoventrally flat- 
tened; with cilia, cirri, and 
membranelles. Ex. Stylonychia 
mytilus (Fig. 31). 


Nuclei 


Macronuclei 


Tetrahymena 


Chilodonelfa 


Opalina 


Oui \ 

Vortkella Stylonychia Stentor 

Figure 31. Some representative ciliated protozoans. Highly magnified. 


65 


66 


COLLEGE ZOOLOGY 


OJl 


Kerona from Hydra 


Nycfotherus from 
rectum of frog 


Podophrya with 
tentacles 


Figure 32. Some ciliates of special interest. Kerona, a commensal which lives on the hydra. 
Nyctothems, a parasite in the rectum of the frog. Vodophrya, ciliated only in young stages, but 
the adult, as shown, has "tentacles." In feeding, the tubular tentacles are attached to its prey, 
then the suctorian sucks the cytoplasm of its victim into its own body. 


Order 4. Peritricha. Body cilia generally 
absent; adoral spiral zone wound 
around the peristome. Body typ- 
ically vase- or bell-shaped; 
mostly stalked, often colonial. 
Ex. Vorticella (Fig. 31). 
Class 2. Suctoria. Cilia when young; "tenta- 
cles" in adult stage. Adults attached 
by stalk. Ex. Podophrya (Fig. 32). 


SELECTED COLLATERAL 
READINGS 

Buchsbaum, Ralph. Animals Without Back- 
bones. Univ. Chicago Press, Chicago, 1948. 


Carter, G.S. A General Zoology of the Inverte- 
brates. Sidgwiek and Jackson, London, 1951. 

Grant, M.P. Microbiology and Human Prog- 
ress. Rinehart, New York, 1953. 

Hutner, S.H., and Lwoff, A. Biochemistry and 
Physiology of Protozoa, 2 vols. Academic 
Press, New York, 1955. 

Hyman, L.H. The Invertebrates: Protozoa 
Through Ctenophora. McGraw-Hill New 
York, 1940. 

Jahn, T.L., and Jahn, F.F. How to Know the 
Protozoa. W.C. Brown, Dubuque, Iowa, 
1949. 

Kudo, R.R. Protozoology. Thomas, Springfield, 
111., 1946. 

Wichterman, Ralph. The Biology of Parame- 
cium. Blakiston, New York, 1953. 


I 


OJI 


CHAPTER 7 


Relations of 

Protozoa 

to Man 


-T is obvious to anyone that the larger ani- 
mals, such as cattle, pigs, horses, and dogs, 
are important to the human economy and 
health, but the relation of the minute Pro- 
tozoa to the well-being of man is not so 
evident. Nevertheless, protozoans play an 
important role. They exist in large numbers, 
but of greatest interest to us are those that 
live as parasites in the bodies of man and 
other animals; those that render water unfit 
to drink, fertilize the soil; and those that 
have built up large parts of the earth's crust 
with their skeletons. 


PROTOZOA PARASITIC 
IN MAN 

Approximately 15 different species of 
Protozoa have been found living as parasites 
within the human body. While the majority 
of these have relatively little effect upon 
their hosts, certain species cause some of the 
worst diseases in man. This is especially true 
in tropical areas where millions of people 
die each year as a result of protozoan infec- 
tions. As a matter of convenience, the proto- 
zoan parasites of man may be divided into 
two general groups: (1) those which inhabit 
blood and tissue and (2) those found in the 
digestive tract and related cavities and pas- 
sages. 


Blood and tissue Protozoa 

Of the blood and tissue parasites, the 
human malarial organism is by far the most 
important. Large numbers of people die 
from malaria each year, and the enfeebled 
condition of those chronically ill from the 
disease represents a tremendous economic 
loss where malaria control is neglected or 
haphazardly carried out. 

Although malaria is probably the most 
devastating disease, with regard to preva- 
lence, mortality, sickness, and economic loss, 
it appears to be coming under control. If 
malaria were finally licked, it would be due 

67 


68 


COLLEGE ZOOLOGY 


to the combined efforts of zoologists, physi- 
cians, and engineers. 

There are three important kinds of hu- 
man malaria. Benign tertian malaria, caused 
by Plasmodium vivax, is characterized by an 
attack of fever every 48 hours. Quartan ma- 
laria, caused by Plasmodium malariae, usu- 
ally produces fever every 72 hours; while 
pernicious malaria, caused by Plasmodium 
falciparum, manifests an irregular tempera- 
ture curve; sometimes the fever is practically 
continuous. Plasmodium falciparum is the 
greatest killer of man, sometimes striking in 
an unusual fashion, with symptoms resem- 
bling typhoid or apoplexy. 

The life cycles of all three species are es- 
sentially similar and may be summarized as 
follows : 

Malaria is transmitted through the bite 
of a female mosquito; the male mosquito 
cannot suck blood because he lacks piercing 
mouth parts. Animal malarias are carried by 
various species, but only mosquitoes of the 
genus Anopheles can transmit the parasite 
to man. 

When the insect's mouth parts (Fig. 146, 
p. 247) pierce the skin, a certain amount 
of saliva containing anticoagulants leaves 
the salivary glands and passes into the 
wound (Fig. 33). If the mosquito is har- 
boring malarial parasites in its glands, this 
gives an opportunity for some of them to 
pass into the human blood stream. The in- 
fective stage is called a sporozoite. The 
sporozoites do not invade the blood corpus- 
cles directly, but penetrate certain tissue 
cells, where they grow, multiply, and go 
through at least two cycles. The persistence 
of the tissue-cell forms provides the reservoir 
from which relapses occur. Sooner or later 
some of the parasites are released into the 
blood stream, where they proceed to enter 
the red blood cells. Each parasitized ery- 
throcyte contains as a rule a single Plasmo- 
dium. The latter assumes at first a ring 
shape, then an irregular form that soon fills 
the cell. In both stages the organism is 
termed a trophozoite. The time required for 
its development will depend upon the spe- 


cies concerned. The mature trophozoite 
eventually divides into a number of daugh- 
ter cells called merozoites, which are then 
released into the blood stream by rupture 
of the red blood cell. Each merozoite in- 
vades a new erythrocyte, where it becomes a 
trophozoite. Each time the cycle is repeated, 
a larger number of erythrocytes is involved; 
when these burst, chills and fever occur. 

Eventually, a few merozoites, instead of 
becoming trophozoites, develop into game- 
tocytes. These are potential gametes, but as 
long as they remain in the human host they 
undergo no further development. They are 
of no significance to the human host as they 
float harmlessly in the circulating blood; 
their survival depends upon their being 
sucked up by a mosquito of the appropriate 
type. Gametocytes which pass into a mos- 
quito's stomach become active at once. 
There are two kinds: the female gametocyte, 
which develops into a single spherical egg 
or female gamete; and the male, which, on 
the other hand, undergoes exflagellation, a 
process by which a number of slender male 
gametes (sperms) grow out radially from 
the surface of the cell and finally float free. 
Union of a male gamete with an egg pro- 
duces a zygote, which, because it has the 
ability to move about, is called an ookinete. 
The ookinete (fertilized egg) migrates 
through the stomach epithelium and takes 
up a position in the stomach wall. There its 
nucleus divides and redivides, resulting in 
the formation of a great number of sporo- 
zoites, which, for a time, are contained 
within a swelling called an oocyst. The 
stomach of an infected mosquito often bears 
a considerable number of these oocysts 
clearly visible on its exterior surface. 

The oocysts finally rupture, and the sporo- 
zoites are released into the insect's body 
cavity. Sporozoites are motile, and many 


Figure 33. Facing page, life cycle of Plasmodium 
vivax, one of the protozoans causing malaria in 
man. Diagram of a mosquito's body and a human 
blood vessel showing asexual stages in the blood 
vessel and sexual stages in the mosquito. 


Ookinete 
Fertilizafion of egg 


Stomach 


Young 
oocyst 
Developing 
oocyst 


Mature oocyst 
with sporozoites 


Sexual cycle in mosquito 
(sporogony) about two weeks 


Merozoites 


Release of 
merozoites 


'■■.•■■■ .'r 


^^ 


' 'I.'.' ' 


our asexua 


cycle in man 
(schizogony) 


stages 

Parasite enters 
Red blood cell 


Nuclear division Later stage Ring stage 

in schizont I __ Trophozoites ' 


69 


70 


COLLEGE ZOOLOGY 


succeed in making their way to the mos- 
quito's sahvary glands. The next time the 
mosquito takes a blood meal, sporozoites 
are injected into the human body. 

Adequate control of human malaria in- 
volves at least three approaches: 

1. Treatment of infected humans by use of 
appropriate antimalarial drugs. 

2. Protection of uninfected individuals by use 
of screens, nets, gloves, and the application 
of solutions to the skin which are capable 
of repelling the mosquito. 

3. Elimination of the mosquito, either by use 
of insecticides such as DDT, or by drainage 
and other engineering operations calculated 
to destroy its breeding places. 


Because of control measures, malaria is 
no longer an endemic disease in the United 
States. In 1951 the National Malaria Society 
went out of existence because its goal had 
been attained. This was certainly a remark- 
able achievement in public health history. 

Other blood and 
tissue parasites 

Blood- and tissue-inhabiting forms are 
found among the flagellates. Two important 
types are ( 1 ) the trypanosomes and ( 2 ) the 
leishmanias. 

The genus Trypanosoma is widespread in 
nature and may be found in the blood of 


Figure 34. Trypanosoma gambiense, the parasitic flagellate of African sleeping sickness in man. 
Form on left, stage in man; and form on right, stage in the tsetse fly. Highly magnified. 


many common mammals, birds, reptiles, 
fishes, and amphibians. Man is host to three 
well-known species: Try^panosoma gam- 
biense (Fig. 34); Trypanosoma rhodesiense, 
which causes two forms of African sleeping 
sickness; and Trypanosoma cruzi, the causa- 
tive agent of Chagas' disease, which occurs 
in Central and South America. 

African sleeping sickness is transmitted by 
the bite of both sexes of the tsetse fly. When 
an infected insect takes its blood meal, the 
slender, flagellated trypanosomes pass by 
way of the fly's proboscis into the human 
host. Early stages of the disease are charac- 
terized by fever and swelling of the lymph 
glands in the neck. Involvement of the nerv- 
ous system results progressively in drowsi- 
ness, coma, emaciation (through inability 


to take food), and finally death. Treatment 
with naphuride, tr}'parsamide, or pentami- 
dine is fairly successful, especially if the pa- 
tient is reached early in the course of the 
disease. Recent experiments with antrycide 
give promise of dramatic results in the treat- 
ment of tr}'panosomiasis in Africa. The 
prevalence of sleeping sickness has greatly 
hindered the development of natural re- 
sources in many of Africa's richest tropical 
areas. In rare instances the infection has 
been known to be transmitted by sexual 
contact. 

South American trypanosomiasis or 
Chagas' disease behaves quite differently. 
Only the acute stages are serious, and gen- 
eral involvement of the nervous system does 
not take place. The trypanosomes are trans- 


RELATIONS OF PROTOZOA TO MAN 


71 


Figure 35. Leishmania donovani, the parasitic flagellate of kala-azar in Asia. Form on left, 
stage in man; forms in middle and to right, stages in the sandfly (Phlebotomus). Highly magnified. 


mitted by bloodsucking kissing bugs. Infec- 
tion takes place by way of the bugs' feces, 
which the patient probably rubs into the 
puncture wound after the insect has ceased 
feeding. Besides the flagellated form which 
may be found in the blood stream, a spheri- 
cal, nonflagellated stage is frequently en- 
countered in the tissues. The muscular tissue 
of the heart is particularly susceptible, and 
death from cardiac failure is common. Kiss- 
ing bugs feed on other animals besides man, 
the armadillo being a particularly important 
reservoir of infection. 

Leishmania infections are also of more 
than one type. Kala-azar or dumdum fever, 
a disease common in India, northern Africa, 
and parts of South America, is caused by 
Leishmania donovani (Fig. 35). It is trans- 
mitted by the bites of small sandflies. Per- 
sons suffering from kala-azar usually show 
enlargement of both spleen and liver, gen- 


eral emaciation, and a peculiar darkening of 
the skin. Various antimony compounds are 
used in treatment. Oriental sore is caused by 
Leishmania tropica. It occurs in northern 
Africa, southern Asia, and southern Europe. 
Espundia, caused by Leishmania brasilienis, 
is limited to Central and South America. All 
types of leishmaniasis are carried by sandflies. 

Parasites of the digestive tract 

The intestinal- and cavity-inhabiting 
forms include representatives of all 4 classes 
of Protozoa. Entamoeba (formerly Enda- 
moeha) gingivalis lives in the human mouth. 
It is sometimes found associated with pyor- 
rhea, though it is not considered the primary 
cause of that condition. Kissing is the com- 
monest mode of transmission. Over 50 per 
cent of the population is infected. In the 
intestine, and especially in the colon, a num- 


Cyst wall 


Food vacuole 
containing 
red blood ce 

Nuclei 


Chromatoid body 


Pseudopodium 


Cyst 


Trophozoite 


Figure 36. Entamoeba (formerly Endamoeba) histolytica, a human intestinal amoeba that 
causes amoebic dysentery and amoebic liver abscess. Highly magnified. 


72 


COLLEGE ZOOLOGY 


ber of related species may occur. Entamoeba 
histolytica (Fig. 36), the causative agent of 
amoebic dysentery, is the only serious dis- 
ease-producing parasite of this group. About 
10 per cent of the general population is in- 
fected, but fortunately most of them are 
merely carriers; that is, the entamoebas are 
present but do no damage, and hence no 
symptoms appear. Occasionally, however, en- 
tamoebas invade the intestinal wall and form 
abscesses, which later rupture and become 
persistent ulcers, resulting in diarrhea and 
dysentery. Occasionally they are carried to 
other parts of the body, such as the brain or 
liver, where abscess formation may cause 
death. Infected persons pass the cysts in 
their feces. These cysts gain access to new 
hosts by contaminated food or water, soiled 


hands, or the activities of flies. Though more 
prevalent in the tropics, amoebiasis is fairly 
common in the temperate zones, and out- 
breaks have even been reported in arctic 
regions. 

Several drugs have been found effective 
in treating amoebic dysentery, the choice of 
which depends on the location of the infec- 
tion and the condition of the patient. 

There are other intestinal amoebas of less 
importance such as Entamoeba coli, which 
is a harmless form often found associated 
with histolytica. Dientamoeba fragilis is 
characterized by the presence of two nuclei 
in the active state; it does not form cvsts. 
Mild diarrhea appears to have been traced 
to an infection with Dientamoeba. 

There are four flagellates which live in 


Ciardia lamblia Trichomonas bominis 

Figure 37. Intestinal flagellates of man. Highly magnified. 


RELATIONS OF PROTOZOA TO MAN 


73 


the digestive tract: Trichomonas tenax lives 
in the tartar of the human mouth; and 
Chilomastix mesnili. Trichomonas hominis 
(Fig. 37) and Giardia lamblia (Fig. 37) in- 
habit the intestine. Chilomastix is a pear- 
shaped organism with four conspicuous fla- 
gella at its broad anterior end. Its cysts are 
lemon-shaped. Trichomonas hominis is of 
similar contour, but bears from three to five 
anterior flagella besides a longitudinal un- 
dulating membrane. There is also a central 
axostyle which protrudes caudally as a dis- 
tinct tail. Trichomonas never forms cysts. 

Giardia lamblia (Fig. 37) is an odd-look- 
ing protozoan with two anterior nuclei and 
a number of backwardly directed flagella. 
The cell has one flat side by which it may 
adhere to an epithelial cell. This species 


sometimes causes mechanical interference 
with absorption, particularly fats; and the 
presence of large amounts of unabsorbed 
fats causes diarrhea. The cyst of Giardia is 
elongate, with four nuclei grouped at one 
end. Atabrine is an effective drug in the 
treatment of giardiasis. 

Trichomonas vaginalis is an inhabitant 
primarily of the vagina and may cause in- 
flammation of this organ. It closely resem- 
bles Trichomonas hominis (Fig. 37). 

The largest protozoan found in the hu- 
man intestinal tract is Balantidium coli 
(Fig. 38). This large motile organism pene- 
trates the membrane lining of the colon, 
where it frequently causes ulcers. It is a 
definite disease-producing organism and may 
produce symptoms resembling acute amoe- 


Oo 


Cell mouth 
Contractile vacuole 

Food vacuoles 
Micronucleus 

Macronucleus 


Contractile vacuole 
Anal pore 


Cyst 


Trophozoite 

Figure 38. Balantidium coli, a ciliate parasitic in man. Highly magnified. 


bic dysentery. Infected persons pass large 
spherical cysts, easily identified by micro- 
scopic examination. 

The Sporozoa are represented among the 
intestinal forms by Isospora hominis, a 
rather rare form, sometimes accused of caus- 
ing a diarrheic condition. 


PROTOZOA PARASITIC IN 
OTHER ANIMALS 

Both game and domestic animals harbor 
a variety of protozoan parasites, which affect 
directly or indirectly the economic well-be- 
ing of man. Trichomonas foetus is an im- 


74 


COLLEGE ZOOLOGY 


portant cause of abortion in cattle; while 
Trichomonas gallinae produces a fatal in- 
fection in pigeons and other birds. Tricho- 
monas gallinarum inhabits the lower intes- 
tine of chickens and turkeys, and, in chronic 
cases, invades the liver. Giardia infections 
may cause severe diarrhea in both rabbits 
and dogs. Blackhead in turkeys is caused by 
an amoeboid flagellate, Histomonas melea- 
gridis. 

Balantidium coli is common in pigs and 
in chimpanzees, both of which probably 
serve as reservoirs for human infection. 

Trypanosome diseases of animals are com- 
mon in tropical areas. Horses, camels, cattle, 
pigs, dogs, and monkeys are susceptible to 
nagana, an African disease, caused by Try- 
panosoma brucei and transmitted by tsetse 
flies. Dourine or horse "syphilis" is also 
caused by a trypanosome, as is mal de Ca- 
deras, a South American disease of horses. 
Trypanosoma evansi is responsible for surra, 
a serious disease of horses, dogs, elephants, 
camels, and other species. Trypanosoma 
lewisi is a parasite almost universally found 
in rats. Bats, mice, sheep, goats, and other 
animals also harbor trypanosome infections. 

Malaria is by no means confined to man. 
Several species of Plasmodium occur in 
birds; also monkeys, bats, squirrels, buffalo, 
and antelope become infected. At least thir- 
teen species occur in reptiles, and two have 
been reported in amphibians. 

Of particular interest is the malarialike 
disease, red-water fever of cattle, also called 
Texas fever; this is caused by a sporozoan, 
Babesia higemina, which parasitizes the red 
blood cells. This species is transmitted by 
a tick, Boophilus (Fig. 167, p. 275). The 
infection passes from the mother tick into 
her eggs and is thus present in the larvae 
which emerge from them. Each new genera- 
tion is therefore able to transmit the disease 
to new bovine hosts. This is the first case in 
which it was shown that an arthropod trans- 
mitted a protozoan disease agent. The dis- 
covery of this fact by Smith and Kilbourne 
in 1893 was an important early contribution 
to medical entomology. 


Sporozoa are abundantly present in ani- 
mal hosts. Coccidial infections of the intes- 
tine are particularly destructive to rabbits 
and birds; and Nosema bombycis, a sporo- 
zoan parasite of the silkworm caterpillar, 
once threatened the silk industry of the 
entire world. Infested caterpillars that did 
not succumb gave rise to moths which laid 
infected eggs, and thus the infection con- 
tinued. Pasteur studied the problem (1865- 
1870) and discovered that diseased eggs may 
be detected by microscopic examination. 
Such eggs can be destroyed before hatching, 
and a healthy generation of caterpillars 
thereby assured. 

Nosema apis is also a destructive parasite 
of honey bees. Many harmful insects are un- 
doubtedly held somewhat in check by proto- 
zoan parasites. 

PROTOZOA IN 
WATER SUPPLIES 

Water for drinking may not only be con- 
taminated by Protozoa of fecal origin, but 
it may also be unpalatable because of the 
multiplication of various free-living proto- 
zoans under natural conditions. This is espe- 
cially likely to occur when the water is con- 
fined in a quiet open reservoir before being 
released for use. Uroglenopsis (Fig. 39) 
forms spherical colonies, the individuals of 
which are embedded in the periphery of a 
gelatinous matrix. Dinobryon (Fig. 39) is a 
branching flagellate, which occurs more 
commonly in alkaline regions. A colony of 
Synura (Fig. 39) consists of from two to 
fifty individuals, arranged in radial fashion. 

Among several protozoans known to make 
water unfit for drinking, Uroglenopsis is pos- 
sibly the worst since it imparts a fishy odor 
like cod-liver oil. Similar odors result from 
the presence of Eudorina, Pandorina, Vol- 
vox, and Glenodinium. Both Synura and 
Pelomyxa produce an odor like ripe cucum- 
bers. Bursaria gives off an odor like a salt 
marsh, while a culture of Peridinium smells 
like clam shells. The fishy odor of Dino- 


RELATIONS OF PROTOZOA TO MAN 


75 


Uroglenops'is Dinobryon Synura 

Figure 39. Colonial flagellates that may render water unfit to drink. Highly magnified. 


bry'on is more like that of rockweed. Cera- 
tium produces a vile stench. ChlamydomO' 
nas and Mallomonas are less objectionable 
as their odors have an aromatic quality. 
Waters which harbor Cryptomonas may 
even smell like "candied violets." 

All these odors are due to aromatic oils 
which are produced by the organisms during 
growth and liberated when they die and 
undergo decomposition. Treatment of the 
water supply with copper sulfate is stand- 
ard procedure for the control of these bad- 
smelling protozoans. 

Some species of aquatic Protozoa, on the 
other hand, are really beneficial to man. 
Kudo (1947) states that since protozoans 
feed extensively on bacteria, they prevent 
the bacteria from reaching the saturation 
point, that is, from becoming so numerous 
that they cease to multiply and thus no 
longer perform the important function of 
destroying waste materials which continu- 
ally pollute the waters. Protozoa therefore 
help indirectly in the purification of water. 

In many aquatic situations. Protozoa serve 
as food for small insects, crustaceans, and 
the like; these organisms, in turn, are fed on 
extensively by many species of fish. Of 
course, other protozoans live as parasites of 


fish. The Mj'xosporidia, especially, cause the 
death of large numbers of commercially 
important species. The relations of the Pro- 
tozoa to aquatic biology are therefore many 
and varied. 


SOIL PROTOZOA 

Soil fertility is affected not only by bac- 
terial action, but also by the protozoan 
fauna which may be present. It has long 
been held that the Protozoa probably reduce 
the numbers of nitrogen-fixing bacteria and 
thus limit the production of nitrates so es- 
sential for soil fertility. Between 200 and 
300 species have been identified from soils, 
the small flagellates being the most com- 
mon. Amoeboid types and ciliatcs follow in 
the order named. Most soil-inhabiting Pro- 
tozoa are found near the surface, the great- 
est concentration being at a depth of about 
Ys inch, and few are found at depths of 12 
to 18 inches. Very few are ever found in 
subsoil. Certain well-adapted species show 
a surprising geographical distribution. 
Amoeba proteus, for example, has been 
found in soils from almost every part of 
the world. Trinema is represented in soils 


76 


COLLEGE ZOOLOGY 


from Spitzbergen, Greenland, England, 
Japan, Australia, St. Helena, Barbados, 
Mauritius, Africa, and the Argentine. Most 
species of Testacea favor soils with a high 
moisture content. A large amount of organic 
matter, combined with small soil particles, 
favors a rich protozoan fauna, as well as a 
high bacterial content. There appears to be 
a definite relation between the number of 
Protozoa and the number of bacteria present 
since the latter serve as food for the Proto- 
zoa. This relationship may have an impor- 
tant bearing on soil fertility. 


PROTOZOA AND GEOLOGY 

Though most protozoans leave no sub- 
stantial remains after the death of the cell, 
at least two large groups of Sarcodina de- 
velop skeletal structures capable of being 
preserved as fossils. These are the Radiolaria 
(Fig. 40) and the Foraminifera, sometimes 
shortened to forams. Because species of this 
type have existed since very early times, a 
great number of distinct rock strata of the 
earth bear evidence of protozoan life. The 


Skeleton of radiolarion 


Imng radTolarlan 


Figure 40. Radiolarians are marine animals, usually with a skeleton of silicon which sinks to 
the ocean floor when they die, forming a layer called radiolarian ooze. Left, helmet-shaped skeleton 
of Podocyrtis. Right, Acanthometron. Highly magnified. 


forams, especially, are of great importance 
to oil geologists in analyzing the results of 
drilling operations. 

Under present conditions, as in the past, 
the skeletons of ocean-dwelling forms are 
continually sinking to the bottom to form 
ever growing layers of ooze. The greater por- 
tion of the bottom of the Atlantic Ocean, an 


area of perhaps 20 million square miles, is 
covered with an ooze formed of skeletons of 
the genus Globigerina. Eventual compac- 
tion of such ooze results in the production 
of limestone in the form of chalk. An impor- 
tant chalk deposit in Alabama and Missis- 
sippi, undoubtedly created in this way, is 
approximately 1000 feet thick in certain 


RELATIONS OF PROTOZOA TO MAN 


77 


places. The chalk cliffs of Dover, which 
have played an important part in the de- 
fense of England, are deposits composed 
mostly of the shells of forams. The stone in 
the Egyptian pyramids is made up of the 
skeletons of very large forms of the genus 
Carnerina (formerly Nummulites) . 

In various shore and deep-sea regions, 
ooze of radiolarian origin is equally abun- 
dant. Approximately 2,290,000 square miles 
in the Pacific and Indian oceans are covered 
by this material. Radiolarian ooze may be- 
come sedimentary rock and may be buried 
under other types of rock. Radiolarians, like 
the forams, are of great importance to geolo- 
gists. 


SELECTED COLLATERAL 
READINGS 

Chandler, A.C. Introduction to Parasitology. 
Wiley, New York, 1955. 

Gerberich, J.B. "An Annotated Bibliography 
of Papers Relating to the Control of Mos- 
quitoes by the Use of Fish." Am. Midland 
Naturalist, 36:87-131, 1946. 

Mackie, T.T., Hunter, G.W., and Worth, C.B. 
Manual of Tropical Medicine. Saunders, 
Philadelphia, 1954. 

Russell, P.P., West, L.S., and Manwcll, R.D. 
Practical Malariology. Saunders, Philadel- 
phia, 1946. 

Warshavv, L.J. Malaria: The BiograpJiy of a 
Killer. Rinehart, New York, 1949, 


LEVELS OF ORGANIZATION 


CHAPTER 8 


Introduction to 
the Metazoa 


Animals show different levels of organiza- 
tion, and organization increases in complex- 
ity from one level to another as they are 
studied from the structurally simple to the 
most complex. Our plan of study is to con- 
sider animals in approximately the order in 
which they have evolved. The patterns of 
organization do not suddenly appear fully 
formed, but are usually foreshadowed some- 
where before becoming definitive. For ex- 
ample, a hint of tissue, but not true tissue, 
in the sponge, emerges as a definite tissue- 
level in the jellyfish. 

Our studies of the Protozoa have given us 
insights into the cell-level of organization, 
and we will consider in increasing complex- 
ity the following additional levels of organ- 
ization which exist in all the other animal 
phyla collectively called Metazoa. The 
tissue-level of organization includes a group 
of specialized cells similar in structure and 
associated together to perform some definite 
function; at the organ-level, there is co- 
operative specialization of groups of tissues 
to form an organ which performs one or 
more special functions; at the organ-system- 
level, there is close cooperation among sev- 
eral organs to perform some general func- 
tion such as digestion. 

This brief evolutionary history of levels of 
organization with gradual increasing com- 
plexity from one-celled protozoans to the 
most complex metazoans covers a time span 
of several hundred million years. 


THE METAZOA AND 
PROTOZOA COMPARED 


78 


The somatic cells of metazoans are not all 
alike as in the colonial protozoans, but differ 
from one another both in structure and in 
function. The body cells are not independ- 
ent as in most of the protozoans, but are 
dependent upon one another. This is the 


INTRODUCTION TO THE METAZOA 


79 


result of a division of labor among the 
somatic cells. 

However, there is no sharp line between 
Metazoa and Protozoa. The colonial proto- 
zoans are composed of many cells, which, as 
in Volvox (Fig. 22), are differentiated into 
germ and body (somatic) cells. However, 
the somatic cells do not show any division 
of labor (specialization); this distinguishes 
such complex Protozoa from the Metazoa. 
There is a considerable number of animals 
intermediate between Protozoa and Meta- 
zoa, but we do not find in any of the pro- 
tozoans the high degree of specialization 
which results in the various types of somatic 
tissues, such as nerves and muscles. 

THE METAZOA 

The differences between the Protozoa and 
the more complex Metazoa are so great that 
we will consider some of the more impor- 
tant metazoan characteristics before study- 
ing the various metazoan phyla. 

In our metazoan study, the following sub- 
jects are of fundamental importance: (1) 
the origin of germ cells, (2) the methods of 
reproduction, (3) differentiation of somatic 
cells and formation of tissues, (4) the asso- 
ciation of different tissues to form organs 
and of different organs to form systems, and 
(5) variations in the forms of animals cor- 
related with differences in symmetry, meta- 
merism, and the character of the append- 
ages. 

Differentiation of germ 
cells and somatic cells 

The body of a true metazoan is always 
composed of germ cells (gametes) and so- 
matic cells. The germ cells serve for repro- 
ductive purposes only; the somatic cells 
form a distinct body which carries on all 
the functions characteristic of animals, ex- 
cept sexual reproduction. The mature germ 
cells are either female or male. Female germ 


cells are known as eggs (ova), and male 
germ cells as sperms (spermatozoa). When 
the germ cells become mature, they may 
separate from the body, giving rise to a new 
generation, whereas the somatic cells die. 

Methods of reproduction 

Reproduction is one of the fundamental 
properties of protoplasm. Two types may be 
recognized: asexual and sexual. In the fol- 
lowing paragraphs some of the common 
methods of reproduction are listed, but varia- 
tions of each type will be encountered in 
the study of the metazoans. A general ac- 
count is presented in Chapter 34. 

Asexual reproduction 

This is reproduction without sexual 
(germ) cells, that is, without eggs or sperms. 
The principal methods of asexual reproduc- 
tion are fission and budding. The amoeba, 
Paramecium, and many other protozoans re- 
produce by binary fission. In budding, as in 
the hydra (Fig. 50), there is usually an out- 
growth from the parent, a bud, which sepa- 
rates while still small and grows into an 
adult. 

Sexual reproduction 

This type of reproduction is by means of 
sexual (germ) cells. Usually a female cell, 
the egg, fuses with a male cell, the sperm. 

When eggs develop normally without be- 
ing fertilized by spermatozoa, the process is 
known as parthenogenesis. For example, the 
eggs of plant lice (aphids. Fig. 157, p. 258) 
and water fleas {Daphnia, Fig. 110) may de- 
velop normally without fusing with sperma- 
tozoa. 

Dioecious animals are either male or fe- 
male; each possesses only one type of repro- 
ductive organ that gives rise to either eggs or 
sperms. Most species of higher animals are 
of this type. 

Monoecious ( hermaphroditic ) animals 
are provided with both male and female re- 
productive organs, and produce both eggs 


80 


COLLEGE ZOOLOGY 


and sperms; actually they are double-sexed. 
In some species the eggs of an individual are 
fertilized by sperms from the same individ- 
ual; this is called self-fertilization. In other 
species the eggs of one individual are fertil- 
ized by the spermatozoa from another indi- 
vidual; this is called cross-fertilization. The 
earthworm is a common hermaphroditic 
animal. 

Oviparous animals are those that lay eggs 
which hatch outside the body of the mother; 
for example, birds. 

Viviparous animals usually give birth to 
young that develop from eggs within the 
body of the mother and are nourished 
from her blood stream; for example, mam- 
mals. 

Ovoviviparous animals produce eggs that 
hatch within the mother's body, but are not 
nourished by the mother's blood stream 
through a placenta; for example, certain 
sharks and reptiles. 

Origin of mature eggs 
and spermatozoa 

Oogenesis 

The primordial germ cells that give rise 
to eggs or ova multiply by mitosis (see pp. 
24-27). As a result, many oogonia are pro- 
duced. The number of oogonia that arise 
from each primordial germ cell may be defi- 
nite as in some insects, or indefinite as in 
most of the metazoans. As illustrated in 
Fig. 404, p. 576, the oogonia grow in size 
and are then called primary oocytes. In- 
stead of dividing into two cells of equal size, 
each oocyte undergoes an unequal division; 
the larger daughter cell is known as a sec- 
ondary oocyte, and the smaller daughter 
cell, as the first polar body. The first polar 
body may disintegrate, or divide into two 
cells which eventually disintegrate. The sec- 
ondary oocyte divides unequally, producing 
a large cell, which we recognize as a mature 
egg, and a small cell, called the second polar 
body, which disintegrates. During the divi- 


sions of the primary and secondary oocytes, 
the number of chromosomes is reduced to 
one-half that in the oogonia. The signifi- 
cance of this will be discussed in the chapter 
on heredity. The end result of the divisions 
of the oocytes is a mature egg, and the 
process is referred to as maturation (mei- 
osis). In many species eggs are laid before 
the two maturation divisions occur; eggs 
are considered immature until after the 
polar bodies are formed. 

Spermatogenesis 

The stages (Fig. 404) in the origin of the 
male sex cells, the spermatozoa, are very 
similar to those of the eggs (Fig. 404). An 
indefinite number of spermatogonia are pro- 
duced by the primordial male germ cells. 
These increase in size and then are called 
primary spermatocytes. By the division of 
each primar)' spermatocyte, two similar sec- 
ondary spermatocytes are produced, and not 
one functional and one nonfunctional cell 
as when the primary oocyte divides. Like- 
wise, when the secondary spermatocytes di- 
vide, each produces two functional cells; 
these are called spermatids. The spermatids 
change without further division into sperms 
(spermatozoa). During the maturation of 
the male sex cells, the number of chromo- 
somes is reduced to one-half the number in 
the spermatogonia. 

Much scientific study has contributed to 
our knowledge of the sperm and its function. 
It is a highly specialized cell that can neither 
grow nor divide; it consists essentially of a 
condensed nucleus with means for locomo- 
tion and penetration of the egg. It plays no 
part in the physiology of the animal that pro- 
duces it; its only function is the production 
of a new individual. 

Fertilization 

The final result of the oocyte divisions is 
one mature egg, and of the spermatocyte di- 
visions, four sperms (Fig. 404). The mature 
ovum now becomes the center of the inter- 


INTRODUCTION TO THE METAZOA 


81 


esting process of fertilization. The sperm 
sometimes enters the egg before the polar 
bodies are formed, and sometimes after- 
ward. The sperm brings into the egg a 
nucleus, a centrosome, and a very small 
amount of cytoplasm. A mitotic figure soon 
develops and moves toward the center of the 
egg. The egg nucleus also moves in this di- 
rection, and finally both the male and fe- 
male nuclei are brought together in the 
midst of the spindle produced about the 
sperm nucleus. Their union forms the 
zygote nucleus. This completes the process 
usually known as fertilization. In this proc- 
ess the most important result appears to be 
the union of two nuclei, one of maternal 
origin, the other of paternal origin. 

Chromosome reduction 

It is now possible to point out the result 
of the reduction in the number of chromo- 
somes which takes place during maturation. 
It has already been stated that every species 
of animal has a definite number of chromo- 
somes in its somatic cells, two of each kind. 
This number remains constant, generation 
after generation. Now, if the mature egg con- 
tained this somatic number of chromosomes 
and the sperm brought into it a like number, 
the animal which developed from the fer- 
tilized egg would possess in its somatic cells 
twice as many chromosomes as its parents. 
However, the number is kept constant by 
reduction during the maturation divisions 
(Fig. 404), so that both egg and sperm con- 
tain only one-half the number of the somatic 
cells and primordial germ cells. The union 
of egg and sperm again establishes the nor- 
mal number of chromosomes possessed by 
the parents. 

Embryo, embryogeny, and 
embryology 

An embryo is a young animal that passes 
its developmental stages within the egg or 
within the mother's uterus. Embryogeny is 


the study of the development of particular 
organisms. Embryology is that branch of 
biology which deals with the development of 
an embr}'0. 

Cleavage 

The division of the fertilized egg (zygote) 
into a number of cells (blastomeres) is 
known as cleavage. The chromatin material 
in the zygote nucleus becomes organized 
into chromosomes. Each chromosome dupli- 
cates itself; these daughter chromosomes are 
so arranged on the first cleavage spindle that 
each daughter nucleus receives either the 
original or the duplicate of each chromo- 
some. After nuclear division, comes the di- 
vision of the zygote into two blastomeres. 
This means that each blastomere will re- 
ceive one of each chromosome of parental 
origin, and one of each chromosome of ma- 
ternal origin. Further divisions insure a like 
distribution to every cell of the body. The 
blastomeres do not separate, as do the 
daughter cells produced by the binary divi- 
sion of the Paramecium, but remain at- 
tached to one another. The resemblance of 
the group of blastomeres to a mulberr\' sug- 
gests the term morula which is sometimes 
used in describing the egg during the early 
cleavage stages. 

Several types of cleavage patterns are rec- 
ognizable. If the eggs contain relatively little 
yolk, the entire zygote divides into 2, 4, 8, 
etc., blastomeres. If these daughter cells are 
approximately equal in size, the process is 
known as equal holoblastic (total) cleavage. 
Such a cleavage pattern is characteristic of 
the eggs of starfish and amphioxus (Fig. 41 ) . 
If the daughter cells are of unequal size, the 
process is known as unequal holoblastic 
(total) cleavage; this cleavage pattern is il- 
lustrated by the frog egg except during the 
first two or three cleavages (Figs. 41 and 
237). If the eggs contain a considerable 
amount of yolk, the entire egg is not divided 
into cells; only restricted portions of the 
cytoplasm undergo cleavage. If cell divi- 
sion is restricted to a small cap or disk on 


82 


COLLEGE ZOOLOGY 


one side of the egg (as in birds), the process 
is spoken of as meroblastic or discoidal 
cleavage (Fig. 41). If cleavage is restricted 


to a layer of cytoplasm around the entire 
egg as in insects, it is spoken of as superfi- 
cial cleavage (Fig. 41). 


Egg 


First 


(fertilized) 


cleavage 


o 


"55 (U 


y^TTvN. 


^f^^ik. .^••*^. 


O _ D) 


/•'■ * '\ 


aV^\ 


^|5 


|; • ) 


(3© 


CT (1) 


X^-D 


Blastula 
(surface) 


Blastula 
(section) 


Egg with slight yolk (starfish) 


Gastrula 
(section) 


o 


</> 


: - 


(1) 


o 


o 


n) 


J3 


3 
C7 


o 

> 


(U 


n 


O 


c 


(U 


X 


_) 


<j 


.2 o 

to U 


Egg with moderate yolk (frog) 


Egg with much yolk (reptiles, birds, most fishes,- some invertebrates as the squid) 


Yolk 


Yolk 


Centra! yolk mass (most arthropods) 

Figure 41. Top three rows: types of cleavage; blastulae, and gastrulae of vertebrates. A, archen- 
teron (gastrocoel). B, blastocoel (segmentation cavity). Bottom row: cleavage; blastulae and 
gastrulae of arthropods. 


Blastula 

As cleavage advances, a cavity becomes 
noticeable in the center of the egg (Fig. 41), 
enlarging as development proceeds until the 
whole resembles a hollow rubber ball, the 
rubber being represented by a single layer of 
cells. At this stage the embryo is called a 
blastula, the cavity the blastocoel (segmen- 
tation cavity), and the cellular layer the 


blastoderm. The blastula resembles some- 
w^hat a single colony of Volvox (Fig. 22). 
The blastula wall in some cases is more than 
one cell thick, and the segmentation cavity 
may be lacking. 

Gastrula 

The cells on one side of the blastula begin 
to invaginate (infold) (Fig. 41) into the 


INTRODUCTION TO THE METAZOA 


83 


interior. The blastocoel is gradually obliter- 
ated during invagination, while a new cavity, 
the gastrocoel ( archenteron ) is established 
and is bounded by the invaginated cells. The 
gastrocoel lined by endoderm represents the 
future gut cavity. The developmental stage 
is now called a gastrula, and the process by 
which it developed from the blastula is 
termed gastrulation. In the embr)-ogeny of 
many species of animals, invagination does 
not occur, but certain cells of the blastula di- 
vide and fill up the segmentation cavity as 
in the hydra (Fig. 56, p. 116). 

Germ layers 

The gastrula of the simple metazoan 
(sponges, coelenterates) consists of two 

Sperm 


germ layers, an outer ectoderm and an in- 
ner endoderm. These phyla are said to be 
diploblastic. However, in the most complex 
animals, a third layer, the mesoderm, arises 
as a result of gastrulation. Thus all the 
higher metazoans arc triploblastic. The 
origin of the mesoderm varies in different 
groups, originating either as a result of multi- 
plication of a few special blastomeres which 
may be recognized in the early cleavage 
stages, or from pouches arising from the 
walls of the archenteron. All the tissues and 
organs of the body are differentiated from 
these germ (embryonic) layers. The struc- 
tures that develop from the germ layers are 
indicated in Fig. 42. 

Egg 


Zygote 
(fertilized egg) 


Centra 

nervous 

system 


Epidermis 


Sense 
receptors 


ENDODERM 


MESODERM 


Notochord 
(eventuc 
surround 
by verteb 


Dermis 
(in part) 

Muscles 


Most of 

reproductive 

system 


Circulatory 
system 

Skeleton 


Peritoneum an 
mesenteries 


Connective tiss 
of digestive a 
other organs of 
endodermal origin 


Figure 42. 


Excretory 
system 

Simplified diagram of the embr>onic differentiation in a vertebrate. 


Lining of 

respiratory 

system 

Lining of 

urinary 

bladder 

Most of 

digestive 

epithelium 

Lining of 
ddle ear 


Thyroid 

and 

thymus 


84 


COLLEGE ZOOLOGY 


Coelom 

The coelom (Fig. 92, p. 174) is a body 
cavity that is present in most triploblastic 
animals; it is, by definition, a cavity or a 
series of cavities completely bounded by 
mesoderm. The importance of the coelom, 
both morphologically and physiologically, 
will be discussed later. 

Organogeny 

Organogeny is concerned largely with how 
tissues, as structural units, are arranged to 
make organ systems during embrj'onic de- 
velopment. Yet it also deals with the forma- 
tion of the specialized tissues which make 
up organs. The characteristic tissue making 
up an organ system, for example, the nerv- 
ous system, is derived from ectoderm, but 
other tissue types from other germ layers 
are involved in the development of the 
nervous system as a whole. 

Larvae and their 
metamorphosis 

Many of the animals with which we are 
familiar, such as mammals and birds, are 
very much like their parents when they are 
born or hatch from the egg; but among 
lower vertebrates, such as the frogs and 
toads, and in most of the invertebrates, the 
animal that is born or hatches from the egg 
is very different from its parents and is 
known as a larva. Common larvae are the 
tadpoles of frogs, the grubs of beetles, the 
maggots of flies, and the caterpillars of but- 
terflies. Many larvae do not develop grad- 
ually into adults, but change rather abruptly 
from the larval to the adult stage, a process 
known as metamorphosis. Numerous exam- 
ples of larvae and their metamorphosis will 
be encountered in our studies of the Meta- 
zoa. 

DIFFERENTIATION OF 
SOMATIC CELLS: TISSUES 

Several types of somatic (body) cells can 
be distinguished in metazoans by differences 


in shape, structure, and function; cells of 
the same type are grouped together as a tis- 
sue. A tissue is a group of similar cells so 
specialized that they perform a common 
function. The study of tissues is called his- 
tology. Some of the simple metazoans pos- 
sess only two kinds of somatic tissues; others 
are made up of a great number. The many 
different kinds of somatic tissues may be 
classified according to their structure and 
functions into 5 groups. 

Epithelial tissue 

Epithelial tissue (Fig. 43) consists of 
cells which cover the surfaces of the body, 
both without and within, such as the skin 
and the lining of the digestive tube. It may 
be protective, absorptive, secretive, or ex- 
cretive in function. Epithelial tissue may be 
flat (squamous), cuboidal, or columnar, 
and may form a single layer or several layers 
(stratified). It may be ciliated or noncili- 
ated. Nutritive material may pass through 
an epithelial tissue into the body, while ex- 
cretory products may pass through it on 
their way out; it may contain the end organs 
of the sensory apparatus, and may protect 
delicate tissues from a harmful environment. 
Examples: epidermis and gastrodermis of 
the hydra (p. 107), lining of coelom in the 
frog and other animals (Fig. 213, p. 331), 
and lining of intestine (Fig. 43). 

Connective tissues 

These tissues (Fig. 43) may be encoun- 
tered in almost any part of the body; they 
are the supporting or uniting structures of 
the body. Their chief functions are (1) to 
bind together various parts of the body and 
(2) to form rigid structures capable of re- 
sisting shocks and pressures of various kinds. 
These tissues consist largely of intercellular 
substances such as fibers, cartilage, and bone 
produced by the cells, either within or out- 
side the cell. The fibrous connective tissues 
occur throughout the entire body, connect- 
ing the cells to one another and binding the 


INTRODUCTION TO THE METAZOA 


85 


tissues into organs such as muscles and 
nerve trunks. The tendons which unite mus- 
cles to bones consist of connective tissue; 
cartilage and bone are supporting connec- 


tive tissues. Cartilage is either clear (hya- 
line) or contains fibers (fibrous). Bone is 
a hard intercellular substance containing 
much calcium and phosphorus. 


NERVOUS 


Simple 
squamous 
(surface view) 

VASCULAR 


Cuboidal 


Nonciliated Ciliated 
columnar columnar 
Vertical section 


Stratified 
squamous 


Erythrocytes 
CONNECTIVE 


Leucocytes ■ 


Tendon (fibrous) 
MUSCULAR 


Bone 


Smooth 


Smooth muscle 

Columnar 
epithelium 

Nervous 


Connective 


Vascular 


Squamous 
epithelium 


Skeletal 


J 


WX9^ 


i<M^^^A 


Cartilage (hyaline) 


Cardiac 


One organ (as the 
intestine) consists 
of many tissues 


Figure 43. Types of tissue. 


86 


COLLEGE ZOOLOGY 


Muscle or contractile tissue 

Muscle tissue (Fig. 43) is composed of 
cells specialized for contraction. Muscle cells 
possess fibrils which are able to contract 
with great force. The fibrils are usually of 
two kinds: (1) striated and (2) smooth, 
without striations. The latter are found in 
smooth muscles of the simpler inactive 
animals, and in those internal organs of 
higher organisms not under voluntary con- 
trol, such as the walls of the blood vessels 
and the urinary bladder; they are therefore 
also known as involuntary muscles. Striated 
muscles are of two types : ( 1 ) skeletal mus- 
cles, which are, for the most part, under 
control of the brain and are hence called 
voluntary muscles; and (2) cardiac muscle, 
which occurs in the heart and is involun- 
tary. 

Nervous tissue 

Nervous tissue is composed of cells spe- 
cialized for the reception of stimuli and the 
transmission of impulses (Fig. 43). All pro- 
toplasm is irritable, as in the amoeba, but 
in the metazoans certain cells are specialized 
for the sole purpose of performing nervous 
functions. This is the most highly special- 
ized tissue in animals. 


Vascular tissue 

Vascular tissue (Fig. 43) is a fluid tissue 
which is composed of white blood cells 
(leukocytes), red blood corpuscles (erythro- 
cytes), blood platelets, liquid plasma, and 
lymph. Blood plasma transports various 
substances to the cells of the body; erythro- 
cytes carry oxygen to the tissues; and the 
leukocytes may move about somewhat like 
amoebas and engulf bacteria and other parti- 
cles that get into the blood plasma. The 
tissue fluid is an accessory to the blood 
proper; it arises from the blood by diffusion 
through the walls of the capillaries into the 


tissue spaces, and it is the fluid medium in 
which the individual cells live. Lymph con- 
sists of tissue fluid and leukocytes which 
have entered the lymphatic vessels; these 
vessels return the lymph to the blood 
stream. 

ORGANS AND SYSTEMS 
OF ORGANS 

An organ is an aggregate of tissues ar- 
ranged in a characteristic structural plan, 
which performs one or more special func- 
tions. For example, the human intestine 
(Fig. 43) is a digestive organ; it consists of 
a variety of tissues, including epithelial, con- 
tractile (muscles), nervous (nerves), vas- 
cular (blood), and fibrous connective. How- 
ever, the lining of epithelium is of primary 
importance, for the digestive glands are an 
outgrowth of it. 

Protozoa carry on physiologic processes 
without the presence of definite organs, but 
in most of the metazoans many organs are 
usually necessary for the performance of a 
single function; for example, the proper di- 
gestion and absorption of food in man re- 
quire a large number of organs collectively 
known as the digestive system. Similarly, 
other sets of organs are associated for carry- 
ing on other functions. The principal sys- 
tems of organs in man and in other higher 
animals, and their chief functions, are as 
follows. 

Digestive system (digestion 
and absorption) 

In the mouth, the teeth, assisted by the 
tongue, masticate food. Salivary glands and 
glandular cells of the mouth furnish saliva. 
The food passes through the pharynx and 
esophagus into the stomach where mucin 
and gastric juices are added to it; here it 
undergoes mechanical and chemical changes. 
Digestion continues in the small intestine. 


INTRODUCTION TO THE METAZOA 


87 


which secretes secretins and receives pan- 
creatic fluid from the pancreas and bile 
from the Hver. Absorption occurs in the 
small intestine, and both digestion and ab- 
sorption continue in the large intestine. Un- 
digested material and other wastes are elimi- 
nated by the large intestine. 

Circulatory system 
(transportation of food, 
oxygen, and waste products) 

Blood has many functions. It carries 
oxygen from the lungs to the tissues, food 
material to the tissues, hormones and other 
internal secretions to various parts of the 
body, and metabolic wastes to the excretor}' 
organs; it maintains a normal temperature 
in warm-blooded animals, and aids in main- 
taining an internal fluid pressure. The heart 
receives blood from the veins and forces it 
through the arteries. Arteries carry blood to 
the tissues, and veins carry it away from the 
tissues. Lymphatic ducts and lymphatic 
capillaries carry lymph. Tissue fluid (fluid 
surrounding cells) transports nourishment 
from the blood to the tissues, and metabolic 
wastes from the tissues to the blood. 


Respiratory system (taking 
in oxygen and eliminating 
carbon dioxide) 

Air enters the respiratory system by way 
of the mouth or nostrils, passing through 
the larynx, which contains the vocal cords; 
then on through the trachea, and the bron- 
chial tubes, into the lungs, where external 
respiration takes place; here the blood gains 
about 8 per cent of oxygen and loses about 
7 per cent of its carbon dioxide. Internal 
respiration involves passage of oxygen from 
the blood to the tissue fluid and thence into 
the tissues, and the passage of carbon diox- 
ide from the tissues to the tissue fluid and 
thence to the blood. 


Excretory system 
(elimination of waste 
products of metabolism) 

The kidneys extract urine from the blood; 
urine consists largely of water and urea. 
Urine passes through the ureters into the 
bladder, which acts as a storage reservoir, 
and from the bladder to the outside through 
the urethra. The lungs, digestive tract, and 
skin also serve as excretory organs. 

Muscular system (motion 
and locomotion) 

Muscles receive stimuli and respond to 
them and are capable of contraction and 
recovery. Striated skeletal muscles operate 
the bones and produce motion. Smooth 
visceral muscles bring about movements of 
the viscera. Cardiac muscles are responsible 
for the beating of the heart. 

Skeletal system (support, 
protection, attachment) 

All vertebrates and many other animals 
have firm frameworks or skeletons that give 
support and protection to the bodies and 
may provide places for attachment of mus 
cles. 

Nervous system (sensation, 
conduction, and correlation) 

The nervous system enables an animal to 
become aware of its environment, to see, 
hear, smell, taste, and feel. It correlates the 
different parts of the body, exerts control 
over the internal organs, and is responsible 
for human thought and conduct. Tlie cen- 
tral nervous system consists of the brain and 
spinal cord. The peripheral nervous system 
comprises the organs of special sense and the 
nerves connecting the central nervous sys- 
tem with various parts of the body; these 


88 


COLLEGE ZOOLOGY 


are the cranial, spinal, and autonomic 
nerves. The autonomic nerves innervate the 
heart, glands, and smooth muscular tissue. 
They are influenced by the emotions, but 
are not under the control of the will. 

Reproductive system 
(reproduction) 

The essential organs of reproduction are 
the ovaries, in which eggs develop, and the 
testes, in w^hich the sperms are produced. 
Accessory organs include those that supply 
yolk and other secretions, ducts that carry 
the germ cells or young to the outside, and 
copulatory organs necessary to insure fer- 
tilization. 


Putty 


Ball 


-Amoeba 
Asymmetry 


Volvox 

Spherical 
(Universal) 


FORMS OF ANIMALS 

Differences in the forms of animals are 
due principally to differences in symmetry, 
metamerism, and the character of the ap- 
pendages. 

Symmetry 

Symmetry refers to the arrangement of 
parts in relation to planes and straight lines. 
Animals are either symmetrical or asym- 
metrical (Fig. 44). The symmetrical ani- 
mals may be divided into three types: (1) 
spherically (universally) symmetrical, (2) 
radially symmetrical, and (3) bilaterally 
symmetrical. 


Canister 


Boat 


Hydra 
Radial 


Man 
Bilateral 


FiGtniE 44. Types of symmetry in animals; illustrated by common objects. 


INTRODUCTION TO THE METAZOA 


89 


In asymmetrical animals, the body can- 
not be divided by planes into similar parts; 
in other words, the body has no definite 
form or arrangement of parts. Many proto- 
zoans and most sponges are asymmetri- 
cal. 

A radially symmetrical animal possesses a 
number of similar parts called antimeres, 
which radiate out from a central axis like 
the spokes of a wheel. It is possible to draw 
a number of planes through a central axis 
dividing the body of these animals into equal 
parts (Fig. 44). The hydra (Fig. 44) is an 
example; its tentacles are similar and radiate 
out from the mouth. Some simple sponges 
(Fng. 46), the majority of the coelenterates 
(Fig. 61), and most of the adult echino- 
derms are radially symmetrical. Radial sym- 
metry is best suited to sessile animals, since 
the similarity of the antimeres enables them 
to obtain food or repel enemies from all 
sides. 

The body of a bilaterally symmetrical 
animal is so constructed that the chief or- 
gans are generally arranged in pairs on either 
side of an axis, passing from the head (an- 
terior end) to the tail (posterior end). 
There is only one plane through which the 
body can be divided into two similar parts. 
An upper or dorsal surface and a lower or 
ventral surface are recognizable, as well as 
right and left sides. In most of the bilateral 
animals, the anterior end is differentiated 
into a head, which contains a concentration 
of nervous tissue and which is supplied with 
numerous sense organs. This modification 
is termed cephalization. Bilateral symmetry 
is characteristic of the most successful ani- 
mals living at the present time, including all 
vertebrates and most invertebrates. 

Some animals are spherical, as, for exam- 
ple, certain Protozoa. Such an animal shows 
approximate spherical symmetry. It is 
symmetrical around the axis of a sphere 
like a ball. It can be divided into two similar 
parts by a cut in any direction through the 
center. 

Spherical synmietr}' is disadvantageous. 


since such an animal can show only an in- 
definite kind of locomotion. Most spherical 
animals are free-floating as the radiolarians; 
or they progress by a rolling movement as 
the volvox. 

It is doubtful if perfect s\mmctry is to be 
found ann\hcrc in the animal kingdom. 
Animals said to show spherical symmetry 
usually only approach a spherical form. 
There are traces of bilateral symmetry in 
the various radially symmetrical animals. Al- 
though the human form is considered a 
good example of bilateral symmetry, every- 
one knows that the right and left sides of the 
human body are not identical. Nevertheless, 
the zoological concept of symmetn,' is of 
great importance in the study of animals; 
this will become evident as the different 
groups are studied. 

Metamerism 

Metameric animals have bodies composed 
of more or less similar parts, or they have 
organs arranged in a linear series along the 
main axis. Each part is called a metamere, 
somite, or segment. In many animals meta- 
merism is not shown by the external struc- 
tures, but is exhibited by the internal or- 
gans; this is true of the vertebrates, which 
have the vertebrae of the backbone, the 
ribs, and nerves metamerically arranged. The 
earthworm (Fig. 91) is a good illustration 
of both external and internal metamerism; 
the body consists of a great number of simi- 
lar segments; and the ganglia of the nerve 
cord, the chambers of the bodv cavitv, the 
blood vessels, and the excretory organs are 
segmcntally arranged. 

The earthworm may serve also as an ex- 
ample of an animal with homonomous seg- 
mentation, since the mctameres are similar. 
The crayfish (Fig. Ill), on the other hand, 
is a heteronomous animal, since division of 
labor has resulted in dissimilarity of the 
mctameres of different regions of the body. 
The vertebrates, including man, are all 
heteronomous. 


90 


COLLEGE ZOOLOGY 


Appendages 

The external appendages of animals are 
outgrowths of the body, which are used for 
locomotion, obtaining food, protection, res- 
piration, and many other purposes. They are 
greatly modified for their various functions, 
and these modifications furnish excellent 
material for the study of homologous and 
analogous organs. 


Homologous organs 

These are organs that are usually funda- 
mentally similar in structure (Fig. 433) and 
always the same in embryologic develop- 
ment, having their origin in a common an- 
cestral type. For example, the forelimbs of 
the frog, the wings of birds, and the arms of 
man serve to distinguish their bearers from 
one another; nevertheless, these structures 
are homologous, since they are morpholog- 
ically equivalent. Homologous organs may 
have similar functions, for example, the legs 
of a man and the hindlegs of a horse; or 
they may have different functions, for exam- 
ple, the arms of a man and the wings of a 
bird. 


Analogous organs 

Similar functions may make nonhomol- 
ogous organs resemble each other. Such or- 
gans are said to be analogous. For example, 
the wings of butterflies and birds are analog- 
ous because they are both used for flight, but 
they are not homologous because they have 
neither the same fundamental structure nor 
the same embryonic origin. 


THE ORGANISM AS 
A WHOLE 

We have been discussing cells, tissues, 
organs, and systems of organs as though they 
are independent. Nothing, however, is more 
certain than that these parts act together as 


a unit— the organism. The cells are not in- 
dependent. In many cases actual protoplas- 
mic bridges connect cells as in Volvox (Fig. 
22) and certain epithelial tissues. Even 
closer union of cytoplasm and nuclei is ef- 
fected in multinucleate cells, such as those 
of skeletal muscle; this type of cell is 
called a syncytium. Cytoplasmic connections 
are not necessary, however, since substances 
may pass from cell to cell by diffusion 
through their membranes, and thus one cell 
may have a profound influence on neighbor- 
ing cells. Other relations between cells are 
brought about by various organs and sys- 
tems, such as the nerves of the nervous 
system and the blood of the circulatory sys- 
tem. 

Zoologists spend a large part of their time 
studying the structures and functions of the 
parts of animals; this is necessary for a 
proper understanding of the whole. The 
whole, however, differs from the sum of its 
parts; the parts cooperate to maintain the 
whole in its struggle to maintain itself and 
the race. Reproduction, embryonic and 
larval development, reactions to changes in 
external conditions, the appearance of in- 
herited characteristics, and organic evolu- 
tion itself are all manifestations of the or- 
ganism as a whole. 

THE ORIGIN OF 
THE METAZOA 

We know very little about the relation- 
ships of the major groups of animals, but 
it is interesting to speculate about their 
origin. The exact origin of the metazoans 
is unknown, but zoologists hold the opin- 
ion that they must have evolved from single- 
celled organisms; since many of the cells of 
the lower metazoans possess flagella, it seems 
probable that the flagellates were their an- 
cestors. Certain colonial protozoans now 
living resemble what the metazoan ancestors 
may have been like. Proterospongia (Fig. 
428, p. 605) possess two very conspicuous 


INTRODUCTION TO THE METAZOA 


91 


spongelike characteristics: (1) flagellated 
collar cells and (2) amoeboid wandering 
cells. It is not difficult to imagine Protero- 
spongid developing into a sponge. Sponges, 
however, do not seem to occupy a place in 
the main line of evolution. Another type of 
metazoan ancestor could have been an 
organism similar to a spherical protozoan 
colony such as Volvox (Fig. 22). The blas- 
tula stage (Fig. 41) is represented in the 
development of many metazoans. But how 
could the metazoans have evolved from a 
hollow ball of cells? Invagination may have 
occurred, resulting in a gastrula with two 
layers of cells and a cavity, the gastrocoel 
(Fig. 41). However, the arguments in favor 
of this hypothesis are not impressive. Many 
coelenterates (p. 106) resemble a modified 
gastrula such as in the adult hydra, others 
resemble the hollow ball filled with cells as 
in the embryonic stage of hydra (Fig. 56, p. 
116). The metazoans may have developed 
from a type of larva, for example, the 
planula (p. 119), which is characteristic of 


many of the lower metazoans. The origin of 
the Metazoa from a two-layered primitive 
planula is a hypothesis which many biolo- 
gists favor (Fig. 430). The subject will be 
discussed in some detail after more has been 
learned about the various groups of Meta- 
zoa. 


SELECTED COLLATERAL 
READINGS 

Baitsell, G.A. Human Biology. McGraw-Hill, 

New York, 1950. 
Huettner, A.F. Comparative Embryology of 

the Vertebrates. Macmillan, New York, 

1949. 
Kimber, D.C., Gray, C.E., Stackpolc, C.E., 

and Leavell, L.C. Textbook of Anatomy and 

Physiology. Macmillan, New York, 1956. 
Maximow, A.A., and Bloom, W. A Textbook 

of Histology. Saunders, Philadelphia, 1957. 
Stiles, Karl A. Handbook of Histology. Blakis- 

ton Division, McGraw-Hill, New York, 

1956. 


CHAPTER 9 


OM 


Phylum Porifera. 
Simple Multicellular 

Animals 


HE phylum Porifera or pore bearers are 
commonly known as sponges. For centuries 
they were thought to be plants, but eventu- 
ally their animal nature was discovered. This 
is not as strange as it seems, for some of the 
fresh-water forms are green, due to the fact 
that they contain many one-celled plants 
(algae); therefore they appear distinctly 
plantlike. Sponges are considered to be an 
ancient group of animals that belong near 
the bottom of the animal series but not in 
the direct line of evolution of the more com- 
plex animals. Even though they are not in 
the direct line of evolution, their study is 
of great interest for they show a multicellu- 
lar organization that is intermediate between 
true protozoans and typical metazoans. 

Sponges are usually attached and station- 
ary animals in the adult stage, distribution 
being brought about largely by the actively 
swimming flagellated larvae, or by currents 
of water which carr}' the young from place 
to place before they become attached. The 
thousands of different species vary greatly 
in shape, size, structure, and geographic 
distribution. Most of the poriferans, includ- 
ing the bath sponges, live in the sea, but a 
few belonging to a single family, Spongilli- 
dae, are fresh-water inhabitants. 

Sponges are more complex than Protozoa, 
and in them, division of labor among soma- 
tic cells has resulted in myocytes and other 
cellular specialization, but there is no group- 
ing and coordination of specialized cells to 
form definite tissues. Therefore, sponges 
have not advanced beyond the cell-level of 
organization, although they are multicellular 
animals. Of particular interest in sponges 
are: (1) the lack of definite germ layers so 
characteristic of most metazoans, (2) the 
complicated systems of canals and flagellated 
chambers, and (3) the formation of spongin 
5nd various types of spicules. 


92 


PHYLUM PORIFERA. SIMPLE MULTICELLULAR ANIMALS 


93 


Elephant's ear sponge 


Venus's flower-basket 


Glass rope 
sponge 


Finger sponge 


Fresh-water sponge 


Figure 45. Some types of fresh-water and marine sponges showing various shapes. The figures 
are not drawn to scale. 


LEUCOSOLENIA-A 
SIMPLE SPONGE 

Leucosolenia is a simple sponge (Fig. 46), 
whitish or yellowish in color. It is attached to 
rocks at the seashore, just below low-tide 
mark, and consists of a number of horizon- 
tal tubes from which branches extend up 
into the water. These branches have an 
opening, the osculum, at the distal end, 
and buds and branches project from their 
sides. The cavity within each branch is 
known as the central cavity (spongocoel). 
A large number of three-pronged (triradi- 
ate) spicules are embedded in the soft tis- 
sues of the body wall; these serve to 
strengthen the body and hold it upright. 

The body wall (Fig. 47) is usually said 
to consist of two layers of cells: an outer 


layer composed of dermal amoebocytes,* 
and an inner layer consisting of flagellated 
collar cells (choanocytes), with mesoglea 
(often jelly-like material) between, in which 
are many amoeboid wandering cells. Ac- 
cording to de Laubenfels, the term ''layer 
of cells" must be used with reservations, for 
no true epithelial layers of cells exist in the 
sponge, such as are present in other meta- 
zoan animals. The outer and inner lavers 
appear to be firm in sponges that are hard- 
ened in fixing solutions, but in life there is 
a mesoglea, sometimes nearly as liquid as 
water, and again almost cartilaginous in 
density, with cells crawling through it or 
clustered on its external and extensive inter- 

* These cells are so termed by de Laubenfels be- 
cause he reports that the cells on the external sur- 
face of the sponge are actually amoebocytes which 
do not occupy any one permanent position. 


94 


COLLEGE ZOOLOGY 


Osculum 


Figure 46. Above, a simple ascon type of sponge 
(Leucosolenia) . A small colony. Right, a young 
sponge of the ascon type. Highly magnified. The 
arrows indicate the course of water into and out of 
the sponge. 

nal surfaces. It is always much eroded with 
chambers and canals of various sizes con- 
nected to the exterior by pores and oscula. 
The mesoglea is often so fluid that a strong 
oscular current throws up transparent chim- 
neys from it which are maintained erect by 
the forces of the current alone. 

The central cavity is lined by a single 
layer of collar cells (Fig. 47); these choano- 
cytes are in loose contact with one another 
and resemble the cells of the flagellated 
protozoans (Fig. 21). The flagella of these 
collar cells beat constantly, creating a cur- 
rent of water. 

If a little coloring matter is placed in the 
water, it will be drawn into the animal 
through minute incurrent pores in the body 
wall and pass out through the osculum, 
which is therefore an exhalent opening and 


Spicule 


Pore 


not a mouth. Sponges are the only animals 
in which the large opening is limited to an 
outward current of water. 

SCYPHA-A SYCON SPONGE 

Morphology 

Scypha appears to be the correct generic 
name of the sponge that occurs along our 
eastern coast which was formerly called 
Grantia* It is a comparatively simple marine 
type of sponge, permanently attached by one 
end to rocks and other solid objects. It varies 
in length from Vi inch to almost an inch and 

* Grantia has been recorded three times from the 
Gulf of St. Lawrence and is abundant in Europe, 
but as yet has not been reported for the United 
States. 


PHYLUM PORIFERA. SIMPLE MULTICELLULAR ANIMALS 


95 


Dermd amoebocyte 
Incurrent pore 

Food particle 
Amoebocyte 
Porocyte 
Mesoglea 

Choanocyte 
Central cavity 


Figure 47. Diagrammatic cross section, designed to show the cellular structure of the body 
wall of a simple sponge {Leucosolenia) . Highly magnified. 


resembles in sbape a slender vase that bulges 
slightly near the center. The osculum is sur- 
rounded by a circlet of straight spicules, 
and smaller spicules protrude from other 
parts of its body. The body wall is perforated 
by numerous incurrent pores. 

Scypha has one large central cavity 
(spongocoel) (Fig. 48), which leads from 
the base of the sponge up to the osculum 
at the distal end. Around the central cavity, 
the thick body wall is built up of elongated, 
sack-shaped, flagellated chambers. Each of 
these is perpendicular to the central cavity, 
like the bristles on a bottle brush. The large 
exhalent opening of each chamber (apo- 
pyle) empties into the central cavity. These 
chambers do not fit closely together; there 
are narrow spaces between them. Water is 
drawn into these spaces and then into the 
chambers through many inhalent openings 
(prosopyles) which abundantly pierce the 
walls of the chambers. The flow of water 
through the sponge is produced by the un- 
correlated but constant beating of the 
flagella of the collar cells (choanocytcs), 
which more or less completely line the in- 
side of each chamber. 


In the wall of the flagellated chambers 
there occur (1) inhalent openings (pros- 
opyles), (2) jellylike material called me- 
soglea (mesenchyme), (3) spicules, and (4) 
numerous amoeboid cells. The latter are of 
three types: (1) pore cells surround pores 
and mav close them in a muscularlike man- 
ner; (2) scleroblasts manufacture spicules, 
which are mineral skeletal structures abun- 
dantly present; (3) archeocytes are embry- 
onic amoebocytes with blunt pseudopodia, 
which can produce other types of cells, par- 
ticularly reproductive cells. 

The soft body wall is supported and pro- 
tected by a skeleton consisting of a great 
number of spicules composed of carbonate 
of lime (Fig. 49). Four varieties of spicules 
are always present: (1) long straight mon- 
axon rods guarding the osculum, (2) short 
straight monaxon rods surrounding the in- 
current pores, (3) triradiatc spicules always 
found embedded in the body wall, and (4) 
T-shaped spicules lining the central cavity. 
Spicules are formed within scleroblasts ( Fig. 
49). A slender organic axial thread is first 
built up within the cell; around this is de- 
posited calcareous matter; the whole spicule 


96 


COLLEGE ZOOLOGY 


Central cavity 
Surface pore 


Flagellated 
chamber 


RHAGON 


Figure 48. Types of canal systems of sponges; diagrammatic sections. The sycon type is 
derived, theoretically, by the outpushing of the wall of the ascon type of sponge into saclike 
chambers. Note how much each chamber of the sycon type of sponge resembles the single 
chamber of a simple sponge. The rhagon type, like the bath sponge, is more complex with an 
elaborate system of canals and flagellated chambers. The arrows indicate the course of water 
through the various types of sponges. (Ascon and sycon types after Minchin; rhagon type after 
Parker and Haswell.) 


is then enslieathed by an envelope of organic 
matter like that composing the axial thread. 

Physiology 

Scypha lives on minute organisms and 
small particles of organic matter drawn into 
it by the back-and-forth beating of the 
flagella on the choanocytes, but little or no 
digestion occurs within choanocvtes. The 
food particles are engulfed by amoebocytes 
where they are digested. Digestion, as in a 
protozoan, is intracellular. Distribution of 
the nutriment is accomplished by diffusion 
of digested food from cell to cell, aided by 
the amoeboid wandering cells. 

Excretory matter is discharged through 
the general body surface, assisted probably 


by the amoeboid wandering cells, and pos- 
sibly by the collar cells. Respiration, like- 
wise, takes place, in the absence of special 
organs, by means of the cells of the body 
wall. 

Sponges are usually considered to be very 
quiet and sluggish, but actually they are 
among the most active and energetic of all 
animals, working night and day to create 
the currents of water that bring food and 
oxygen into the body and carry away waste 
matter; they are veritable living dynamos. 
The amount of water that passes through 
the body of a sponge is tremendous; for ex- 
ample, an average-sized sponge draws about 
45 gallons of water through its canal system 
in a single day. 

True nerve tissue in sponges has not been 


PHYLUM PORIFERA. SIMPLE MULTICELLULAR ANIMALS 


97 


demonstrated and their behavior is what 
one would expect in the absence of nerves.* 
However, they are able to respond to certain 
stimuli; the response, as in the Protozoa, is 
one-celled. The pores and oscula are sur- 
rounded by contractile cells (myocytes) 
which are able to close these openings. A 
finger placed in an osculum may be squeezed 
with the force of a grip of the hand by a 
man. Apparently the myocytes respond to 
direct stimulation, since no nervous tissue 
is present. The entire body may contract and 
then expand. Reactions to stimuli are very 
slow since they depend upon the funda- 
mental properties of protoplasm, that is, 
conductivity and contractility. Since proto- 
plasm can only contract and not extend it- 
self, most movement must be due to con- 
traction of the protoplasm; and when cells 
elongate, it is due to the transverse contrac- 
tion of protoplasm that decreases the width 
of the cell and causes it to become longer. 
But usually a return to normal by contractile 
cells is due to simple relaxation and a con- 
sequent return to normal shape. 

Reproduction 

Reproduction in Scypha takes place by 
both sexual and asexual methods. In the 
latter case, a bud arises near the point of 
attachment, finally breaks free, and takes up 
a separate existence. 

The sexual reproductive cells in sponges 

* While it is true that Tuzet and deCeccatty 
have reported the presence of a primitive nervous 
system in some sponges, this has not been con- 
firmed at this writing by others, although several in- 
vestigators in America are working on the problem. 
The evidence of these two investigators for inter- 
preting the cells in question as nerve cells comes 
entirely from cytologic work. To the author's knowl- 
edge there is no unequivocal staining method for 
distinguishing nerve cells from other types of cells. 
As their critics have pointed out, the diffuse net- 
work of neuronlike cells demonstrated can be inter- 
preted as connective tissue cells. Tuzet and deCec- 
catty are now well aware that it will be necessary to 
have parallel evidence from physiologic studies in 
order to prove the presence of nerve cells in sponges. 
Until such proof is available, the writer will assume 
that the question of nervous tissue in sponges is still 
an open one. 


lie in the jellylike layer (the mesoglea) of 
the body wall. Both eggs and sperms occur 
in a single individual; i.e., Scypha is monoe- 
cious (hermaphroditic). Tlie fertilized egg 
segments by 3 vertical divisions into a pyra- 
midal plate of 8 cells. A horizontal division 
now cuts off a small cell from the top of 
each of the 8, the result being a layer of 8 
large cells crowned by a layer of 8 small cells. 
The cells now become arranged about a cen- 
tral cavity, producing a blastulaiikc sphere. 
The small cells multiply rapidly and de- 
velop flagella, while the large cells become 
granular. The small cells become partially 
grown over by the others, forming a struc- 
ture called the amphiblastula (Fig. 49); this 
escapes from the parent as a flagellated 
larva. After the larva swims about for several 
days it becomes attached to a solid object 
and begins growth as a young sponge. 

A peculiarity in the embryogeny of cer- 
tain sponges is this: the flagellated cells of 
the larva do not become the outer (dermal) 
layer, as do the flagellated cells of certain 
higher animals, but they produce the layer 
of choanocytes; and the nonflagellated cells 
do not become the inner (gastral) epithe- 
lium, as do the similarly situated cells in the 
coelenterates, but they produce the dermal 
layer as well as the middle region. No sponge 
has anything like an ectoderm or an endo- 
derm as do the other metazoans. 


OTHER PORIFERA 

Form, size, and color 

Sponges may be simple, thin-walled, 
tubular structures like Leucosolenia (Fig. 
46), or massive and more or less irregular 
in shape. Many sponges are indefinite masses 
of tissue encrusting the stones, shells, sticks, 
or plants to which they are attached; others 
are more regular in shape and attached to 
the sea bottom by means of masses of 
spicules. The form exhibited by the mem- 
bers of certain species may vary somewhat, 
depending on whether they develop in shal- 


98 


COLLEGE ZOOLOGY 


low or deep water; for example, Microciona 
in shallow water forms a thin encrustation 
on rocks, while in deep water the colonies 
become massive and reach a height of as 
much as 6 inches. Some are branched like 
trees, others are shaped like gloves, cups, or 
domes. The majority are irregular and with- 
out symmetry, although some are radially 
symmetrical. Sponges vary in size from spe- 
cies no larger than a pinhead to species that 
are as big as barrels 8 feet in diameter. 
Sponges are highly variable in color; some 
are white or gray, and others are yellow, 
orange, red, green, blue, purple, and velvety 
black. 

Canal systems 

If it had not been for the development 
of elaborate canal systems, sponges would 
have remained in the simple condition of 
Leucosolenia and would never have been 


able to become massive in size. The canal 
system furnishes a highway for food through 
the body and for transportation of excretory 
matter out of the body. Three types are usu- 
ally recognized (Fig. 48): (1) the simplest 
or ascon type, as in Leucosolenia, (2) the 
sycon type, as in Scypha, and (3) the 
rhagon (leucon) type in which there are a 
number of small chambers lined with 
choanocytes. 

Skeletons 

The skeletons of sponges consist of car- 
bonate of lime or silica (a mineral sub- 
Stance akin to glass) in the form of spicules, 
or of spongin in the form of fibers more or 
less closely united (Fig. 49). Spongin is a 
substance chemically related to human hair. 
It is secreted by flask-shaped cells (spongo- 
blasts). Spicules are deposited in cells 
(scleroblasts, Fig. 49), and more than one 


Calcareous spicules 


Siliceous spicules 


Spongin with 
spicules 


Spongin without 
spicules 


Side view 


Thick wait 
with spicules 

Germinal cells 


Development of 
spicules 


Gemmules 

(asexual 

reproduction) 


Free swimming larva 

(amphiblastula of 

sexual reproduction) 


Figure 49. Parts of sponges. Spicules from various genera. Gemmules of a freshwater sponge. 
(Amphiblastula after Parker and Haswell.) 


PHYLUM PORIFERA. SIMPLE MULTICELLULAR ANIMALS 


99 


cell may take part in the formation of a 
single spicule. The work required to build 
a sponge skeleton is almost unbelievable. 
The silica present in solution in sea water 
is about P/2 parts in 100,000; hence, to ex- 
tract an ounce of skeleton, at least a ton of 
sea water must be drawn through the pores 
of the sponge and forced out again through 
the oscula. 

Amoeboid wandering cells 

The amoebocytes (Fig. 47) in the meso- 
glea between the cell layers in the body 
wall of sponges give rise to reproductive 
cells and to several types of somatic cells 
such as pigment cells, food-storage cells, 
scleroblasts, and spongoblasts. 


gemmules are the chief means of identifica- 
tion. A gemmule (Fig. 49) consists of a 
number of cells from the middle layer of 
the body wall, which are gathered into a 
ball, and surrounded by a chitinous shell 
reinforced by spicules. They are formed dur- 
ing the summer and autumn. In the spring 
the gemmules develop into new sponges 
and are hence of value in carrying the sponge 
through a period of adverse conditions such 
as the winter season. 

More than 20 species of fresh-water 
sponges occur in this country. Spongilla 
lacustris is the most abundant; it prefers 
running water. 

ORIGIN AND RELATIONS 
OF THE SPONGES 


Regeneration 

In many sponges, if an individual is cut 
into pieces, each piece will grow into a nor- 
mal animal, a process known as regenera- 
tion. Cuttings of bath sponges in Florida 
may increase from IVi cubic inches to llVi 
cubic inches in two months. The wool 
sponges of the Caribbean Sea may grow to 
be IVi feet in diameter. The remarkable 
regenerative power of sponges is demon- 
strated when certain species are broken up 
and strained through fine bolting cloth so 
as to dissociate the cells; the cells will fuse 
on the bottom of a dish to form sponge- 
lets, which in the course of several days 
acquire canals, flagellated chambers, and a 
skeleton; and later, they will also develop 
reproductive bodies. 


Fresh-water sponges 

These all belong to the family Spongilli- 
dae. They are usually found in clear water, 
encrusting stones, sticks, and plants, and 
are often yellow, brown, or green in color. 
These sponges reproduce by formation of 
gemmules, and the characteristics of these 


Sponges are many-celled animals in which 
the somatic cells are somewhat differen- 
tiated for the performance of special func- 
tions; that is, division of labor among the 
somatic cells has developed. Although there 
is relatively little specialization of the 
somatic cells, the sponges represent a con- 
siderable advance over the condition exist- 
ing even among such complex protozoans 
as Volvox (Fig. 22). 

Despite the fact that sponges are many- 
celled animals and contain hints of tissues, 
there are no organs as in most of the higher 
animals, and no digestive cavity is present. 
It is thought that the sponges have devel- 
oped from some protozoan group, probably 
the Choanoftagellata. They resemble the 
colonial protozoans in many ways, such as 
in the digestion of particles of food within 
cells, and in the formation of skeletal spic- 
ules by single cells. They suggest especially 
certain flagellates that are colonial and pos- 
sess collar cells, like Proterospongia (Fig. 
428, p. 605). 

Although the sponges are well enough 
adapted to their environment to have lived 
their primitive way of life for millions of 
years, they do not appear to be in the direct 


100 


COLLEGE ZOOLOGY 


line of development, of more complex ani- 
mals (Fig. 430). 

RELATIONS OF PORIFERA TO 
MAN AND OTHER ANIMALS 

Sponges are mostly beneficial to man. 
They supply him with the sponges of com- 
merce, which are the spongin skeletons of 
certain species living chiefly near the shore 
of the Mediterranean Sea, the coast of Aus- 
tralia, the Bahama Islands, Cuba, and 
Florida. Only the eastern Mediterranean is 
superior as a sponge-producing region to 
that around Florida. 

Sponge culture, that is, growing sponges 
from cuttings, is little practiced today, due 
to the difficulties in preventing theft of the 
crop. Fishing for sponges is carried on by 
divers either with or without a diving suit; 
the latter are known as skin divers. Com- 
mercial diving for sponges is dangerous be- 
cause most of the shallow waters have been 
"fished out" and the deeper regions must be 
worked; these often cause the divers to suf- 
fer from a pressure disease called the bends. 
Sharks are another danger to the diver. 
There is a good market for the natural 
sponge, although it must compete with the 
cellulose and rubber substitutes. 

Bath sponges in their living state resem- 
ble internally a piece of raw beef liver in 
both consistency and color. Externally they 
are black or blackish in color. 

Boring sponges occur in shallow water 
near shores all over the world. They form ir- 
regular masses and are a bright sulfur yel- 
low in color. Their name has reference to 
their habit of attaching themselves to the 
shells of oysters, clams, etc., and boring 
them so full of holes that the animals within 
are killed and in time the shells are en- 
tirely broken up. 

Some sponges are poisonous; certain forms 
are as dangerous as poison ivy, when touched 
by man, and produce similar results. Other 
sponges when alive give off a strong unpleas- 


ant odor, and many contain sharp spiny 
spicules. Probably for these reasons as well 
as for purposes of concealment, certain spe- 
cies of crabs place sponges on their backs 
or on their legs. Other animals find the 
body of the sponge an excellent place in 
which to retreat for protection. 

Of ornamental interest is a sponge known 
as Venus's-flower-basket, which builds up a 
beautiful skeleton of "spun glass" in the 
form of a cylinder about a foot long. Sponges 
of this type live in the sea, where they are 
fastened in the mud of the sea bottom by a 
mass of long threads at one end. 

Some sponges are of economic importance 
in that the siliceous spicules form large flint 
deposits. 


CLASSIFICATION OF 
THE PORIFERA 

{For reference purposes only) 

Sponges are all sessile animals in the adult 
stage, and asymmetrical or radially symmetrical 
in form. Their many cells are loosely arranged 
into two, more or less definite, layers, between 
which are amoeboid wandering cells. Neither 
organs nor mouth is present, the cells acting 
mostly independently. The soft tissues of 
sponges are usually held in place by skeletons 
of spicules or spongin. The bodies of sponges 
contain pores, canals, chambers, and a central 
cavity, through which currents of water flow. 
Collar cells, the choanocytes, line some of the 
body cavities. The 5000 or more living species 
of sponges are marine, except for about 150 
species which comprise the fresh-water family 
Spongillidae (Fig. 45). Three classes and 12 
orders are recognized as follows: 

Class 1. Calcispongiae. Shallow-water species, 
comparatively simple in structure. 
Calcareous spicules make up the prin- 
cipal skeleton. Ascon, sycon, and 
simple rhagon types are present. 
Order 1. Asconosa. Sponges of ascon 
type, or ascon t}'pe at first, 
changing directly into rhagon. 
Ex. Leucosolenia (Fig. 46). 


PHYLUM PORIFERA. SIMPLE MULTICELLULAR ANIMALS 


101 


Order 2. Syconosa. Sponges of sycon 
type, or sycon type at first, 
changing into rhagon. Ex. 
Scypha. 
Class 2. Hyalospongiae. Mostly deep-sea spe- 
cies. Siliceous spicules make up the 
principal skeleton. The architecture is 
very much openwork, with structural 
parts often at right angles to each 
other. The flagellate chambers are 
consistently of a rhagon type, which 
is only slightly modified from the 
sycon type. 
Order 1. Hexasterophora. Many spicules 
are starlike in shape. Ex. 
Euplectella, Venus's-flower-bas- 
ket (Fig. 45). 
Order 2. Amphidiscophora. No astral 
spicules, instead there are am- 
phidisks. Ex. Hyalonema, glass 
rope sponge (Fig. 45). 
Class 3. Demospongiae. Dominant t}'pe at 
present; often massive and brightly 
colored. The skeleton may comprise 
siliceous spicules, spongin fibers, both 
of these, or neither. The architecture 
is always compact, with flagellate 
chambers consistently of a highly de- 
veloped rhagon type. 
Order 1. Carnosa. Skeleton principally or 
entirely organic colloidal jelly. 
Small spicules sometimes pres- 
ent. Ex. Chondrosia. 
Order 2. Choristida. Skeleton principally 
of spicules with 4 rays radiating 
from a central point. Ex. Geo- 
dia. 
Order 3. Epipolasida. Somewhat spherical 
sponges, with monaxon spic- 
ules, which radiate from a cen- 
tral region within the sponge. 
Ex. Tethya. 
Order 4. Hadromerina. Pin-shaped spic- 


ules. Some kinds excavate gal- 
leries into calcareous material, 
such as ovstcr shells. Ex. Cliona. 

Order 5. Halichondrina. Double-pointed 
spicules, plumose or confused 
arrangement. Ex. Halichondria, 
the common "crumb of bread" 
sponge. 

Order 6. Poecilosclerina. Many kinds of 
spicules present. Often also 
some spongin. Ex. Microciona. 

Order 7. Haplosclerina. As in Halichon- 
drina, but with reticulate, topi- 
cally fibrous skeletons. Ex. Hali- 
clona, the "finger" sponge (Fig. 
45). 

Order 8. Keratosa. No spicules; skeleton 
of well-developed spongin fibers. 
Ex. Spongia, the bath sponge 
(Fig. 45). 


SELECTED COLLATERAL 
READINGS 

de Laubenfels, M.W. A Guide to the Sponges 
of Eastern North America. Univ. of Miami 
Press, Florida, 1953. 

Hyman, L.H. The Invertebrates Through 
Ctenophora. McGraw-Hill, New York, 1940. 

MacGinitie, G.E., and MacGinitie, N. Nat- 
ural History of Marine Animals. McGraw- 
Hill, New York, 19-19. 

Pennak, R.W. Fresh-Water Invertebrates of 
the United States. Ronald Press, New York, 
1953. 

Reese, A.M. Outlines of Economic Zoology. 
Blakiston, Philadelphia, 1942. 

Wilson, H.V., and Penny, J.T. "The Regenera- 
tion of Sponges (Microciona) from Disso- 
ciated Cells." /. Exp. Zoo/., 56:73-147, 
1930. 


CHAPTER TO 


'■'^*^i,: 


"%« 


Mi 

0" 


'S^p^ 


Phylum Coelenterata 

(Cnidaria). Simple 

Tissue Animals 


102 


HE phylum Coelenterata contains a large 
number of interesting animals, but most of 
the 10,000 or more species live in salt water 
and are seen alive by only a small proportion 
of those interested in animal life. However, 
they exhibit the characteristics of the lower 
Metazoa to good advantage; and one type, 
the hydra, common in fresh water, affords 
excellent material for laboratory study. Cer- 
tain types that live in the sea and that serve 
well as examples of the larger division of the 
phylum are also described briefly and illus- 
trated; these include the colonial hydroid 
Obelia, the hydrozoan jellyfish Gonionemus, 
the scyphozoan jellyfish Aurellia* the sea 
anemone Metridium, and the coral Astran- 
gia. The sponges do not exhibit well-devel- 
oped tissues, but the coelenterates have 
reached a definite tissue-level of organiza- 
tion. Another advance in their level of or- 
ganization over the sponges is that the cells 
are more highly specialized and integrated; 
they are more receptive to stimuli and are 
capable of a great variety of responses. 

The coelenterates may be used to illus- 
trate many important biological phenomena 
such as budding, certain types of behavior, 
regeneration, grafting, colony formation, 
metagenesis, and polymorphism. These are 
all exhibited by the Hydrozoa, hence it is 
suggested that this class be studied more 
thoroughly than the Scyphozoa and Antho- 
zoa. 

Coelenterates are radially symmetrical 
animals. The principal axis extends from 
the mouth to the base; similar parts are 
arranged around this axis in a circle. The 
body wall consists of two layers of cells, 
between which is a noncellular substance, 
the mesoglea (Fig. 58). Within the body is 
a single gastrovascular cavity (Fig. 51). The 
coelenterates are provided with stinging 
capsules called nematocysts (Fig. 54). 

The phylum contains three classes: (1) 
the Hydrozoa, including the fresh-water 
polyps, the small jellyfishes, the hydroid 

* Usually incorrectly spelled Amelia. The original 
and hence correct spelling is Aurellia. 


PHYLUM COELENTERATA (cNIDARIA). SIMPLE TISSUE ANIMALS 


105 


zoophytes, and a few stony hydroids; (2) 
the Scyphozoa, mostly large jellyfishes; and 
(3) the Anthozoa, which include the sea 
anemones and most of the stony and horny 
corals. 

HYDRA-A FRESH- 
WATER HYDROZOAN 

Hydras are simple coelenterates, abun- 
dant in fresh-water ponds and streams. Nine 
known species occur in the United States. 
They are easily seen with the naked eye, 
are usually 2 to 20 mm. in length, and re- 
semble a short thread frazzled at the unat- 
tached distal end. The great variation in 
length exhibited by hydras at different times 
is due to the fact that both body and tenta- 
cles are capable of remarkable expansion 
and contraction because of specialized con- 
tractile fibers. The coelenterates we know as 
hydras were named after the mythological 
nine-headed dragon slain by Hercules. 

Hydras are of particular interest in that 
their adult organization corresponds roughly 
to the gastrula of higher animals. Thus they 
may be regarded as the living counterparts 
of some remote ancestor of the higher meta- 
zoans. They are of further interest in that 
they exhibit a complex organization in a 
so-called simple animal, but there is little 
division of labor. The work performed by 
organs in higher animals is thrown upon the 
tissues and individual cells in the hydra. As 
with most "simple" animals, anyone who 
studies these cells and tissues carefully 
comes to realize that the supposed simplicity 
of the hydra is largely fallacious. 


Gross morphology 

The body of the hydra (Fig. 50) resem- 
bles an elastic tube which may be extended 
to a length of 2 cm. At the distal end is a 
circlet of tentacles, usually 6 or 7, and as 
many as 10 in some species. Some hydras 
have extremely extensible tentacles, which 


may stretch out to several times the body 
length; they have a stalk portion well set off 
from the rest of the body. 

The tentacles are capable of remarkable 
extension, and may stretch out from small 
blunt projections to very thin threads 7 cm. 
or more in length (Fig. 50). They move 
independently, capturing food and bringing 
it to the mouth. Their number varies con- 
siderably, increasing with the size and age 
of the animal. 

The part of the body which is usually at- 
tached to some object is known as the foot 
or basal disk and is referred to as the aboral 
(opposite the mouth) end. The foot secretes 
a sticky substance, and not only anchors 
the animal when at rest, but also ser\'es as a 
locomotor organ. The foot may also secrete 
a gas bubble enclosed by a film of mucus. 
This bubble raises the animal to the surface, 
where it spreads out like a raft, the h}dra 
hanging from the underside. In the com- 
mon brown species Hydra oligactis,*" the 
aboral region is a stalk, and the distal region 
constitutes a sort of stomach; these two re- 
gions together are known as the body col- 
umn. A conical elevation, the hypostome, 
occupies the oral (mouth) end of the body. 
The hypostome is surrounded by tentacles 
already mentioned and has an opening at 
the top, the mouth. WTien the mouth is 
contracted, as during rest or digestion, it is 
a minute circular pore, but when swallowing 
objects, it and the surrounding hypostome 
can dilate to a rclativclv enormous diameter. 

Frequently specimens of the hydra are 
found which possess buds in various stages 
of development (Fig. 50). This is a form of 
asexual reproduction, characterized by the 
fact that many parent cells go to make up 
the new individual, which is in contrast to 
sexual reproduction, in which the new indi- 
vidual arises from a single cell, the fertilized 
egg. Sexual reproduction also occurs in the 
hydra. Reproductive organs or gonads (Fig. 
50) may be observed on specimens of the 

* Genus Pelmatohydra discarded, not sufficiently 
distinct. 


Expanded 


Contracted 


Figure 50. Hydra. Asexual reproduction on the left. Sexual reproduction on the right. Note 
both sexual and asexual reproduction in top middle individual. Arrow points to sperms being 
discharged from the testes. The egg is fertilized while still attached to the parent. 


104 


PHYLUM COELENTERATA ( CNIDARIA ) . SIMPLE TISSUE ANIMALS 


105 


hydra in the summer or autumn. The stimu- 
lus for the formation of gonads in some 
species appears to be a sudden change in 
temperature, either rising or falhng. Both an 
ovary and testes (Fig. 50) are produced on 
a single individual in some species; the for- 
mer is knoblike, occupying a position about 
one-third the length of the animal above 
the basal disk. The testes, usually several to 
many in number, are conical or rounded 
elevations located near the tentacles of the 
animal. The stalk never has gonads on it. 

Histology 

The body wall 

The hydra consists of two cellular layers: 
an outer thin layer, the epidermis; and an 
inner layer, the gastrodermis, about twice as 
thick as the outer (Fig. 52). Formerly the 
terms ectoderm and endoderm were applied 
to these layers and are still retained in some 
textbooks, but these terms are strictly ap- 
plicable to the germ layers of an embryo, 
and therefore cannot properly be used to 
designate the differentiated epithelial tissues 
of an adult animal. Both layers are com- 
posed primarily of epitheliomuscular cells. 
A thin space containing a jellylike material, 
the mesoglea, separates the epidermis from 
the gastrodermis. Although in many coelen- 
terates the mesoglea constitutes a large part 
of the bulk of the body, in the hydra it is 
thin, especially toward the oral end of the 
body and in the tentacles, while in the cen- 
ter of the basal disk it is lacking altogether. 
It serves as a basement membrane for the 
epithelial cells and a place for attachment of 
their muscle processes. Hence in the hydra 
it serves as a supporting layer. Both body and 
tentacles are hollow, the single central space 
being known as the gastrovascular cavity. 

The following outline shows the cellular 
elements of the several layers: 

Epidermis 

1. Epitheliomuscular cells. 

2. Sensory ccl-s. 


3. Other nerve cells. 

4. Interstitial cells. 

5. Cnidoblasts. 

6. Germ cells (eggs and sperms). 

Supporting Layer (Mesoglea) 

This layer is nonccllular, but is traversed by 
migrating cells and crossed by intercellular 
bridges and nerve cell processes (fibers). 

Gastrodermis 

1. Epitheliomuscular or nutritive cells (var- 
iously differentiated in different regions). 

2. Gland cells (several types). 

3. Sparse interstitial cells, which migrate from 
epidermis as needed, and transform into 
gland cells between stalks of nutritive cells. 

4. Sensory cells. 

5. Other nerve cells. 

6. Cnidoblasts (temporary migrants through 
this layer on the way to their final location) . 

The primary component of both epider- 
mis and gastrodermis is the epitheliomus- 
cular cell, which extends the full height of 
the epithelial layer and supports the other 
elements. Its proportions vary greatly with 
the expansion or contraction of the animal. 

The epitheliomuscular cells of the epider- 
mis have their polygonal outer surfaces ce- 
mented together in wavy borders to form 
a continuous membrane over the animal, in- 
terrupted only where stinging or sense cells 
come to the surface. 

The epitheliomuscular cells of the gastro- 
dermis line the entire wall of the gastro- 
vascular cavity. The character of these cells 
is subject to wide variation in different re- 
gions, but since they are all concerned with 
either digestion or absorption of food mate- 
rial, they are nutritive in function, and 
therefore are called nutritive cells. The 
stomach cells form pseudopodia, flagella, 
and food vacuoles at their free ends; their 
bases are drawn out into extensions, and 
often contain contractile fibers. 

On the center of the hypostome, the 
gastrodermal cells are either filled with small 
secretion granules, or they have a fine spongy 
texture, the two conditions alternating in 


106 


COLLEGE ZOOLOGY 


Nemafocysts 
on tentacle 


Hypostome 

Epidermis 
Interstitial cells 
Gastrovascular cavity 

Testis 
Gastrodermis 

Longitudinal and 
circular muscles 


rmis- 


Nerve network 
Egg in ovary 


Cross section 


Bud 
Mesoglea 


Basal disk 


Aboral pore 
Closed Open 


Figure 51. Hydra. Parts cut away and sections to show structure. (Redrawn from a drawing 
by Justus F. Mueller; prepared exclusively for this text.) 


adjacent cells. These are the mucous gland 
cells. Their contractile fibers form the 
sphincter around the mouth, and they assist 
in swallowing food and in preparing it for 
digestion. 


Whereas the nutritive cells rest on the 
mesoglea, the gland cells are tear-shaped and 
wedged in between the free ends of the 
nutritive cells. These gland cells occur 
abundantly in the stomach and in the hy- 


PHYLUM COELENTERATA (cNIDARIA). SIMPLE TISSUE ANIMALS 


107 


Interstitial cells- 
Nematocyst- 

Sensory cell- 


Supporting 
cell nucleus- 
Interstitial cells- 
Nerve eel 

Longitudinal 
muscle fiber- 

Mesoglea- 


Nufritive cell 
Gland cell 


Sensory cell 
-Gland cell 


Young gland cell 


— Food 
vacuole 


Food being 
engulfed 


Pseudopodium 


Figure 52. Longitudinal section of the body wall of the hydra, highly magnified to show the 
structure of the epidermis, mesoglea, and gastrodermis. Note that the mesoglea is free from 
nerve cell bodies although nerve cell fibers pass through it. (Redrawn from a drawing by Justus 
F, Mueller; made expressly for this text.) 


postome; sparsely in the stalk and basal 
disk. 

The interstitial cells are small rounded 
cells with clear cytoplasm and a relatively 
large nucleus containing one or two nucleoli. 
Their cytoplasm lacks specialized structure; 
hence they are undifferentiated. Mitotic 
figures are frequent. 

It is thought by some that the interstitial 
cells represent a sort of embryonic tissue 
carried over into the adult, that they can 
differentiate into any of the specialized cells 
of the hydra, and that hence they are the 
chief agents in reconstructing tissues in 
growth, budding, regeneration, etc. The in- 
terstitial cells also form the primordial germ 
cells of the gonads, and replace worn-out 
gland cells in the gastrodermis; but whether 
they have any other significance is a de- 
bated point. It has been shown conclusively 
that regeneration, as well as bud formation, 
takes place by a rearrangement of the differ- 
entiated epitheliomuscular cells of both lay- 
ers, and that the interstitial cells, at least 


locally, play only a subordinate part in the 
process. Whether cnidoblasts, germ cells, 
and gland cells arise from a common stem 
cell or from several types of interstitial cells 
is not known. 

The muscular system 

The muscular system of the hydra con- 
sists primarily of two layers of contractile 
fibers applied to opposite surfaces of the 
supporting mesoglea. The outer muscle layer 
is longitudinal and is formed by the con- 
tractile fibers of the epidermal cells, while 
the inner muscle layer is circular and is de- 
rived from the contractile fibers of the 
gastrodermal cells. 

The circular muscle layer contracts slowly, 
performing movements of a peristaltic char- 
acter, while the external longitudinal layer is 
capable of rapid response. Thus the two 
muscle layers of hydra already foreshadow in 
a dim way the visceral and skeletal muscles 
of higher animals. 


108 


COLLEGE ZOOLOGY 


The nervous system 

In the coelenterate we find for the first 
time true nerve cells such as are found in 
all the higher animals. The nervous system 
of the hydra consists of three general types 
of cells which are called: (1) conducting 
and motor nerve cells, (2) sensory cells, 
and (3) sensory nerve cells. These are dis- 
tributed throughout the body in such a way 
as to form a network in the epidermis; this 
can be demonstrated by a special stain in 
the living animal. Although often called a 
"nerve net," the elements of the system 
have not been demonstrated to be continu- 
ous. Nervous elements are present in the 


gastrodermis, but in such small numbers 
that if a gastrodermal nerve network exists, 
it is certainly much more diffuse than that 
of the epidermis. It is presumed that the 
two systems are interconnected by fibers 
passing through the mesoglea, and certain 
workers claim they have observed such 
fibers. Circumstantial evidence for their ex- 
istence is afforded by the fact that in certain 
movements of the animal, inner and outer 
layers of muscles work in coordination. 

Most of the conducting and motor nerve 
cells (Fig. 53) of the hydra have several 
processes; they conduct impulses in any di- 
rection, and thus differ from the neurons of 


Flagella 

Food vacuole 
Flagella 


Vacuole 


Nuclei 


Gastrodermal 
gland cell 


Circular 
muscle fiber 


Interstitial cells 


Figure 53. Principal cell types of the hydra. (Redrawn from a drawing by Justus F. Mueller; 
made expressly for this text.) 


PHYLUM COELENTERATA (cNIDARIA). SIMPLE TISSUE ANIMALS 


109 


higher animals in that they are not polar- 
ized. 

The nerve cells of the epidermis lie just 
external to the longitudinal muscle layer, 
between the bases of the supporting cells, 
and their processes interlace to form the so- 
called nerve net which extends over the en- 
tire body from the tip of the tentacles to 
the basal disk. Formerly it was thought that 
these processes were continuous from one 
cell to the next, but careful studies have 
shown that although the endings of fibers 
lie very close to each other, they do not join. 
Occasionally, however, nerve endings ap- 
pear to be fused with other nerve cells, mus- 
cle processes, or bodies of epitheliomuscular 
cells. 

Response to stimuli. The nervous system 
of higher animals is synaptic, that is, the 
nerve cells are usually separate, and the im- 
pulse must go from the endings of one nerve 
cell to those of another. In a synaptic sys- 
tem the direction of the nerve impulse is 
controlled and is normally conducted in one 
direction. In the hydra, there is probably 
little directional control of nervous impulses, 
and the resulting reactions are similar to 
those that would take place if there were 
only a protoplasmically continuous network 
of nervous tissue. 

The greatest concentration of nerve ele- 
ments occurs around the hypostome, where 
the fibers pass in a circular direction to form 
a loosely organized nerve ring. Another 
somewhat similar concentration of nerve 
fibers appears in the foot. 

The sensory cells (Fig. 53) consist of 
slender, threadlike, specialized nerve cells, 
lying in both epidermis and gastrodermis, 
between the epitheliomuscular cells. They 
frequently bear a hairlike process or some 
other specialized structure at their tips. 
Basally they usually divide into two or more 
fibers which may connect either with the 
nerve plexus or with muscle fibers. The sen- 
sory cells of the gastrodermis are said to be 
more abundant toward the foot region. The 
epidermal sensory cells are found mainly on 


the hypostome and inner part of the tenta- 
cles, and on the basal disk; these parts are 
the most sensitive to external stimuli. 

The nematocysts 

These nematocysts (stinging capsules) 
(Fig. 54) are present on all parts of the 
epidermis of the hydra except on the basal 
disk; they are most numerous on the tenta- 
cles. Each is formed inside an interstitial 
cell, which is then known as a cnidoblast. 
On the general body surface, cnidoblasts are 
mostly wedged in between the outer edges 
of the supporting cells, but on the tentacles 
and hypostome, cnidoblasts lie within the 
bodies of the epitheliomuscular cells, which 
are then known as host cells. On the tenta- 
cles the host cells are large and each con- 
tains a battery of stinging capsules, consist- 
ing of one or two large nematocysts (pene- 
trants) surrounded by a number of smaller 
types. 

Four kinds of nematocysts occur in the 
hydra as follows: (1) The largest is the 
penetrant, which, before it is discharged, is 
pear-shaped and occupies almost the entire 
cnidoblast in which it lies. Within it is a 
coiled tube, at the base of which are three 
large and a number of small spines. Three 
rows of minute spines spiral along the out- 
side of the thread when discharged. (2) 
Volvents are small pear-shaped nematocysts, 
containing a thread, which, when dis- 
charged, coils tightly around the hairs or 
bristles of its prey. (3) The oval glutinant 
is large and has a long thread that bears 
minute spines. (4) Tlie small glutinant is 
a straight unarmed thread. The first two 
types are of special help in capturing prey; 
the others secrete a sticky substance pos- 
sibly used in locomotion as well as in food 
getting. 

Projecting from the cnidoblast, near the 
outer end of the nematocyst, is a hairlike 
process, the cnidocil. Nematocysts may be 
exploded by adding a little acetic acid or 
methyl green to the water. The lid forming 
the apex of the cyst is thrown off, and the 


110 


COLLEGE ZOOLOGY 


DIschorged thread 
Dissolved chitin 
Chitin 


Cnidoblast 
Fibril 


Undischarged 
nematocyst 


Nucleus 


Nematocyst penetrating the sclerotized 
covering of an insect 


Nematocyst discharged 
Figure 54. Penetrant nematocyst of the hydra. Note cnidoblast on right. 


tube which is coiled within is then everted. 
First the base of the tube with the large 
spines appears, and then the rest of the tube 
rapidly turns inside out. Penetrants are able 
to penetrate the tissues of other animals 
only when they are discharged with great 
speed, and before eversion is completed. 
Even the extremely firm sclerotized covering 
of insects may be punctured by these struc- 
tures. 

For a long time, touching the cnidocil 
was considered the cause of the explosion of 
the nematocysts, and for this reason the 
cnidocil is known as the trigger. One can 
easily prove, however, that mechanical 
shocks have no influence upon the nemato- 


cysts. The discharge of the capsule is prob- 
ably triggered by a suitable stimulus, pre- 
sumbly chemical, applied to the cnidocil. 
However, the mechanism of setting off the 
explosion is still not understood, but once 
the process has started, the energy for ever- 
sion of the thread is provided by the progres- 
sive swelling of the shaft wall itself. It has 
been clearly shown that as the thread everts, 
it not only increases in diameter, but also 
markedly elongates. In the discharging ne- 
matocyst, the inverted filament comes into 
immediate contact with external water only 
at the advancing tip of the shaft. In isolated 
and dried nematocysts, discharge will not 
proceed unless the advancing tip is supplied 


PHYLUM COELENTERATA (cNIDARIA). SIMPLE TISSUE ANIMALS 


111 


with water, and the rate of aversion can be 
controlled by the rate at which moisture is 
supplied. Hence swelling at this point must 
be responsible for the progressive eversion 
of the thread. 

Since nematocysts are discharged by di- 
rect stimuli, and not as a result of nervous 
control, they are independent effectors. 

An animal "shot" by nematocysts is im- 
mediately paralyzed and sometimes killed 
by a poison that has been called hypnotoxin, 
which is injected into it through the 
tube. 

Cnidoblasts are developed from intersti- 
tial cells (Fig. 52), which appear in nests or 
clusters in the epidermis of the stomach re- 
gion. Before the nematocyst is completely 
developed, the cnidoblast in which it is 
formed migrates to the part of the body 
where it is needed. Here the cell matures, 
developing the cnidocil. The function of the 
cnidoblast is limited to the formation of the 
nematocyst, and possibly the cnidocil. Since 
the tube of the nematocyst cannot be re- 
turned to the capsule, nor can another nem- 
atocyst be developed by the cnidoblast, 
the cnidoblast perishes with the loss of its 
nematocyst, and a new cnidoblast must be 
formed from an interstitial cell to replace 
the one that has been used. 

DifFerentiation of the 
body regions 

The hypostome 

The muscle layers of the hypostome con- 
sist of the epidermal muscle fibers which 
radiate from the mouth and the gastroder- 
mal fibers which surround it. The hypostome 
is rich in nervous elements and is the most 
sensitive region of the body. Cnidoblasts 
and a few interstitial cells also occur here. 
The gastrodermal layer of the hypostome is 
thrown into large deep folds when the 
mouth is contracted, so that a cross section 
of the oral cone shows a star-shaped 
"throat," which has been mistaken for the 


mouth by some authors. The folds contain 
the mucous gland cells. The secretion of 
these cells is poured out over the food in 
swallowing and is a necessary forerunner of 
gastric digestion. Food introduced directly 
into the stomach through a pipette, or by 
an incision, without first coming in contact 
with these cells, is not digested. Hence di- 
gestion in the hydra is dependent upon an 
enzyme system which follows an orderly 
sequence of events. 

The tentacles 

There is a poorly developed "sphincter" 
formed by the gastrodermal fibers at the base 
of the tentacles. The tentacles can be rapidly 
elongated by pumping fluid from the gastro- 
dermal cavity into them, and the "sphinc- 
ter" probably influences the entrance or 
escape of this fluid. 

Stomach-reproductive and stalk region 

The epidermis of this region is about 
twice as thick as that of the hypostome, and 
harbors the great bulk of the interstitial cells. 
This is the region of nematocyst formation, 
and of testes, ovaries, and buds. The epithe- 
liomuscular cells form the supporting cells 
of testis and ovary. The gastrodcrmis of this 
region is the chief digestive organ, effecting 
both extracellular and intracellular digestion. 
Correlated with this digestive activity is the 
presence of many enzymatic gland cells; 
their secretion reduces the food to a broth 
of fine particles. The particles are then fished 
out by the nutritive cells with their flagella 
and taken into food vacuoles, where diges- 
tion is completed. Thus digestion in the 
hydra reflects certain features of the process 
as it occurs in both sponges and protozoans 
on the one hand, and in the higher meta- 
zoans on the other. Although the cavity of 
the stomach and stalk is continuous, the 
hydra confines large food objects within the 
stomach by muscular action and does not 
permit them to enter the stalk. 

Since the stalk is primarily a region of ex- 
tension and motility, its structure is adapted 


112 


COLLEGE ZOOLOGY 


to maximum elasticity. The mesoglea here 
reaches its greatest thickness; and both mus- 
cle layers are well developed, particularly 
the longitudinal. 

The basal disk 

The epidermis of the foot consists of 
columnar epitheliomuscular cells. Their 
outer portions are filled with refractive 
globules, which store the sticky mucus these 
cells elaborate. Their bases are provided with 
muscle fibers which radiate from the center 
of the disk. 

The mesoglea is lackmg over a small area 
at the center of the disk, and here epidermis 
and gastrodermis come in direct contact. 
This is the region of the aboral pore, which 
is opened when the hydra suddenly releases 
its hold on the substratum, but is completely 
closed during attachment. The probable 
function of the pore is to enable the hydra 
to "blast" itself loose from the sticky mucus 
secreted by the foot. This is accomplished by 
a "peeling" action of the cells at the pe- 
riphery of the foot, plus a simultaneous 
strong expulsion of water from the gas- 
trovascular cavity through the aboral 
pore. 


Physiology 

Food 

The food of the hydra consists principally 
of small animals that live in the water, such 
as Cyclops, annelids, and insect larvae. Large 
specimens may ingest aquatic animals as big 
as young fish and tadpoles. Bits of meat may 
be ingested when offered to them in a lab- 
oratory aquarium. The hydra normally rests 
with its basal disk attached to some object, 
and its body and tentacles extended into 
the water. In this position it occupies a 
considerable amount of hunting territory. 
Any small animal swimming within touch 
of a tentacle is at once shot full of pene- 
trants, affixed by glutinants, or grappled by 
volvents. 


Ingestion 

The tentacle which has captured the prey 
bends toward the mouth with its load of 
food. The other tentacles not only assist in 
this, but may use their nematocysts in quiet- 
ing the victim. The mouth often begins to 
open before the food has reached it. The 
edges of the mouth gradually enclose the 
organism and force it into the gastrovascular 
cavity. The body wall contracts behind the 
food and forces it down. Frequently organ- 
isms many times the size of the hydra are 
successfully ingested. 

Reactions to food 

It is common to find hydras that will not 
react to food when it is presented to them. 
This is due to the fact that these animals 
will eat only when a certain interval of time 
has elapsed after their last meal. The phy- 
siologic condition of the hydra, therefore, 
determines its response to the food stimulus. 
The collision of an aquatic organism with 
the tentacle of the hydra is not sufficient to 
cause the food-taking reaction, since it has 
been found that not only a mechanical 
stimulus, but also a chemical stimulus must 
be present. A very hungry hydra will go 
through the food-taking movements when 
it is excited by a chemical stimulus alone, 
such as beef juice. 

Digestion 

Immediately after the ingestion of food, 
the gland cells in the gastrodermis show 
signs of great activity; their nuclei enlarge 
and become granular. This is accompanied 
by the formation of enzymes which are dis- 
charged into the gastrovascular cavity and 
begin at once the digestion of the food. The 
action of the digestive juices is made more 
effective by the churning of the food as the 
animal expands and contracts. The flagella 
extending out into the central cavity also 
aid in the breakdown of the food by creat- 
ing currents. This method of digestion dif- 
fers from that of the amoeba, paramecium, 


PHYLUM COELENTERATA (cNIDARIA). SIMPLE TISSUE ANIMALS 


113 


and sponge in that it is carried on outside 
the cell, that is, extracellular. However, in- 
tracellular digestion also takes place in the 
hydra; the pseudopodia thrust out by the 
gastrodermal cells (Fig. 52) seize and en- 
gulf particles of food; these particles are 
then further digested in the cells. The di- 
sested food is absorbed and stored by the 
g^strodermis. 

One species of hydra, Chlorohydra viridis- 
sima, is green in color because of the pres- 
ence of a unicellular alga, Chlorella vulgaris, 
in the gastrodermal cells. As in Paramecium 
bursaria, the plant uses some of the waste 
products of metabolism of the hydra, and 
the hydra uses some of the oxygen resulting 
from the process of photosynthesis in the 
plant. This condition is one of mutualism. 

Egestion 

All indigestible material is egested from 
the mouth. This is accomplished by a very 

Hanging fronn the 
surface of water 


sudden squirt which throws the debris some 
distance. 

Respiration and excretion 

Oxygen diffuses into the cells from the 
water in which the hydra lives, and carbon 
dioxide diffuses out of the cells. The met- 
abolic waste products are excreted through 
the general body surfaces. 

Behavior 

Spontaneous movements 

All the movements of the hydra are the 
result of the contraction of the contractile 
fibers and are produced by two kinds of 
stimuli, internal, or spontaneous, and ex- 
ternal. Spontaneous movements may be ob- 
served when the animal is attached and un- 
disturbed. At intervals of several minutes, 
the body, or tentacles, or both contract sud- 
denly and rapidly, and then slowly expand 


Locomotion by a series 
of somersaults 


Figure 55. Sketches showing the hydra feeding and its methods of locomotion. Somersaulting 
is the most rapid method of locomotion. 


114 


COLLEGE ZOOLOGY 


in a new direction; hungry specimens are 
more active than well-fed individuals. 

Locomotion 

When going from one place to another, 
the hydra uses several methods. One is a 
gliding movement, with the basal disk slowly 
sliding over the object to which the animal 
is attached. A second method is a "measur- 
ing worm" type of movement (Fig. 55), in 
which the hydra bends over and attaches its 
tentacles to a region, slides its basal disk up 
close to them, and then releases the tenta- 
cles, assuming an upright position. Another 
method is by turning somersaults. The ani- 
mal releases its basal disk and moves it com- 
pletely over and attaches it to a new region. 
Such end-over-end movements are repeated 
again and again. At other times the hydra 
travels by a method seldom observed. It 
moves from place to place in an upside down 
position, using its tentacles as legs. To rise 
to the surface of water, it may form a gas 
bubble on its basal disk, which helps to 
carry it upward. 

Contact 

Hydra reacts to various kinds of special 
stimuli. Mechanical shocks, such as jarring 
the watch glass containing the specimen, or 
agitating the surface of the water, cause a 
rapid contraction of a part of or the whole 
animal. This is followed by a gradual ex- 
pansion until the original condition is re- 
gained. 

Mechanical stimuli may be localized or 
nonlocalized. The one just mentioned is of 
the latter type. Local stimulation may be 
accomplished by touching the body or ten- 
tacles with the end of a fine glass rod. It 
has been noted that the stimulation of one 
tentacle may cause the contraction of all the 
tentacles, or even the contraction of both 
tentacles and body. This shows that there 
must be some sort of transmission of stimuli 
from one tentacle to another and to the 
body. The structure of the nervous system 
would make this possible. 


Light 

There is no well-defined response to light, 
although the final result is quite decisive. If 
a dish containing hydras is placed so that 
the illumination is not equal on all sides, 
the animals will collect in the brightest 
region. However, if the light is too strong, 
they will congregate in a place where the 
light is less intense. The hydra therefore 
has an optimum with regard to light. The 
movement into or out of a certain area is 
accomplished by a method of trial and error. 
When put in a dark place, the hydra be- 
comes restless and moves about in no defi- 
nite direction; but if white light is encoun- 
tered, its locomotion becomes less rapid and 
finally ceases altogether. The value of such 
a reaction is considerable, since the small 
animals that serve as food for it are attracted 
to well-lighted areas. 

Other stimuli 

The reactions of the hydra to changes in 
temperature are also indefinite, although in 
many cases they enable the animal to escape 
from a heated region. An attached hydra, 
when subjected to a weak, constant electric 
current, bends toward the anode, its body 
finally becoming oriented with the basal 
disk toward the cathode and the anterior 
end toward the anode. The hydra does not 
react to currents of water. 

The physiologic condition of an animal 
determines to a large extent the kind of reac- 
tions produced not only spontaneously, but 
also by external stimuli. It determines 
whether the hydra creeps upward to the 
surface and toward the light, or sinks to the 
bottom; how it reacts to chemicals and to 
solid objects; whether it remains quiet in a 
certain position, or reverses this position and 
undertakes a laborious tour of exploration. 

Reproduction 

Reproduction takes place in the hydra 
both asexually and sexually; in the former 
case by budding, in the latter by production 


PHYLUM COELENTERATA (cNIDARIA). SIMPLE TISSUE ANIMALS 


115 


of fertilized eggs. Asexual and sexual repro- 
duction may both occur at the same time in 
an individual. 

Budding (Fig. 50) 

Asexual (sexless) reproduction by a proc- 
ess of budding is a common occurrence in 
the hydra. Several buds are often found on 
a single animal. Superficially the bud first 
appears as a slight bulge in the body wall. 
This pushes out rapidly as a projection which 
soon develops a circlet of blunt tentacles 
about its outer end. The cavities of both 
stalk and tentacles are at all times directly 
connected with that of the parent. When 
full grown, the bud becomes detached and 
leads a separate existence; this requires about 
two days when conditions are favorable. 
Budding may occur at almost any season. 

Sexual reproduction 

Both ova and spermatozoa appear to de- 
velop from interstitial cells. Some species of 
the hydra form both sperms and eggs in one 
individual, but in others only one sex oc- 
curs. There may be as many as 20 or 30 
testes; each is a conical outgrowth. The sex- 
ual state can be induced in some species by 
lowering the temperature; this accounts for 
the appearance in Hydra oligactus of sex 
organs in the autumn and during early win- 
ter. 

Spermatogenesis. The male germ cells of 
the hydra are formed in little conical or 
rounded elevations called testes, which pro- 
ject from the surface of the body (Fig. 51). 
An indefinite number of interstitial cells 
collect locally into a mass, causing the epi- 
dermis of the animal to bulge. Each of these 
interstitial cells is a primordial germ cell; 
it gives rise by mitosis to a variable number 
of spermatogonia; these divide to form pri- 
mary spermatocytes, which give rise by divi- 
sion to secondary spermatocytes; these 
divide, producing spermatids which trans- 
form into spermatozoa. The mature sper- 
matozoa swim about in the distal end of the 
testis and finally escape to the exterior 


through one or more small fissures in the 
protective covering. In most hydras definite 
nipples are formed on the testes, through 
which the sperms escape (Fig. 50). The 
mature spermatozoa swim about in the 
water searching for an egg. 

Oogenesis. The egg is an interstitial cell 
which becomes large and spherical and pos- 
sesses a large nucleus (Fig. 51). Several in- 
terstitial cells begin to enlarge to form 
ovocytes but one finally incorporates the 
others. As the ovum grows it becomes scal- 
lop-shaped, due to confinement between the 
columns of the supporting cells. When fi- 
nally it attains full growth, it becomes 
spherical; but it is still surrounded by epi- 
dermal cells, which stretch enormously to 
cover the egg and still remain rooted to the 
mesoglea. (Illustrations showing a layer of 
epithelial cells covering the egg, but separate 
from the mesoglea, are incorrect; although 
such false interpretations in sections are easy 
to make.) Maturation now takes place. Two 
polar bodies are formed, the first being 
larger than the second. During matura- 
tion the number of chromosomes is re- 
duced from the somatic number 12 to 6; 
this occurs at the end of the growth period. 
Now an opening appears in the epidermis 
and the egg is forced out, becoming free on 
all sides except where it is attached to the 
parent. 

Fertilization. Fertilization usually occurs 
about as soon as the egg is extruded. Several 
sperms may penetrate the egg membrane, 
but only one enters the egg itself. The sperm 
brings a nucleus containing 6 chromosom.es 
into the egg. The male and female nuclei 
unite, forming the fusion nucleus. 

Embryology. The cleavage, which now be- 
gins, is total and regular. A well-defined 
cleavage cavity is present at the end of the 
third cleavage, the eight-cell stage. V.^hen 
the blastula is completed, it resembles a 
hollow sphere with a single layer of epithelial 
cells composing its wall (Fig. 56). These 
cells may be called the primitive ectoderm. 
By mitotic division they form endoderm cells 


116 


COLLEGE ZOOLOGY 


Mesoglea 

Epidermis 
of parent 


Mature egg 


Young hydra emerging 


Figure 56. Stages in the development of the hydra. 


whicli drop into the cleavage cavity, com- 
]5lctely filling it. The early gastrula, there- 
fore, is a solid sphere of cells differentiated 
into a single outer layer, the ectoderm, and 
an irregular central mass, the endoderm 
(Fig. 56). The ectoderm secretes two en- 
velopes around the gastrula; the outer is a 
thick chitinous shell which may be covered 
with sharp projections; the inner is a thin 
gelatinous membrane. Different species of 
hydras can be identified by the structure of 
their shells. 

Hatching. The embryo in this condition 
separates from the parent and falls to the 
bottom, where it remains unchanged for 
several weeks. Then interstitial cells make 
their appearance. A subsequent resting pe- 
riod is followed by the breaking away of 
the outer chitinous envelope and the elonga- 
tion of the escaped embryo (larva). Meso- 
glea is now secreted between the ectoderm 
and endoderm cells, and these layers dif- 
ferentiate to form the adult epithelial tis- 


sues: the epidermis and gastrodermis. A 
circlet of tentacles arises at one end and a 
mouth appears in their midst. The young 
hydra thus formed grows into the adult 
condition. 


Regeneration 

An account of the phenomena of regen- 
eration is appropriate at this place, since the 
power of animals to restore lost parts was 
first discovered by Trembley in 1740 in the 
hydra. This investigator found that if hydras 
were cut into 2, 3, or 4 pieces, each part 
would grow into an entire animal. Other 
experimental results obtained by Trembley 
are that if a hydra is split longitudinally into 
2 or 4 parts, each part becomes a perfect 
polyp; that when the head end is split in 
two, and the parts are separated slightly, a 
"two-headed" animal results; and that a 
specimen when turned inside out is able to 
readjust itself to these new conditions forced 


PHYLUM COELENTERATA (CNmARIA). SIMPLE TISSUE ANIMALS 


117 


upon it. If a hydra remains turned inside 
out, the cells of the epidermis and gastro- 
dermis migrate past each other through the 
mesoglea until they regain their original 
positions. 

Regeneration may be defined as replace- 
ment of lost parts. It takes place not only 
in the hydra, but in many other coelen- 
terates, and in some of the representatives 
of almost every phylum of the animal king- 
dom. The hydra, however, is one of the 
types that have been most widely used for 
experimentation. Pieces of the hydra that 
measure only six thousandths of an inch in 
diameter are capable of becoming entire 
animals. The tissues in some cases restore 
the lost parts by multiplication of their cells; 
in other cases, they are worked over directly 
into a new but smaller individual. 

Grafting 

Parts of one hydra may easily be grafted 
upon another; in this way many bizarre ef- 
fects have been produced. Parts of two 
hydras of different species have also been 
united successfully. 

Space v/ill not permit a detailed account 
of the many interesting questions involved 
in the phenomena of regeneration and graft- 
ing, but enough has been given to indicate 
the nature of the process. The ability to 
regenerate lost parts is obviously of benefit 
to the animal. Such an animal, in many 
cases, will succeed in the struggle for exist- 
ence under adverse conditions. Regeneration 
takes place continually in all animals; for ex- 
ample, new cells are produced in the epi- 
dermis of man to take the place of those 
that are no longer able to perform their 
proper functions. Both internal and external 
factors have an influence upon the rate of 
regeneration and upon the character of the 
new part. Temperature, food, light, gravity, 
and contact are some of the external fac- 
tors. In man, various tissues are capable of 
regeneration; for example, the skin, muscles, 
nerves, blood vessels, and bones. Lost parts. 


however, are not restored in man because 
the growing tissues do not coordinate prop- 
erly. A decrease in regenerative power seems 
to be correlated with the increase in com- 
plexity of animal types. The inability of the 
more complex forms to replace lost parts 
appears to be the price of specialization. 

OTHER COELENTERATA 

As in the case of the hydra, coelenterates 
in general are diploblastic and possess nema- 
tocysts. Contractile fibers are present in a 
more or less concentrated condition. Nerve 
cell processes ( fibers ) and sensory cells are 
characteristic structures; they may be few 
in number and scattered, as in the hydra, 
or numerous and concentrated. Tlie two 
principal types of coelenterates are the polyp 
and the jellyfish or medusa. These are 
fundamentally similar in structure, but 
are variously modified. Both polyps and 
medusae are radially symmetrical. Although 
the medusae may, upon superficial examina- 
tion, appear to be very different from the 
polyps, they are constructed on the same 
general plan. Both have similar parts, the 
most noticeable difference being the enor 
mous quantity of mesoglea present in the 
medusa. The water content of a medusa is 
very high; that of Aurellia is about 96 per 
cent. 

Digestion in coelenterates is both extra- 
and intracellular; enzymes are discharged 
into the gastrovascular cavity for maceration 
of food organisms. Tlie particles are trans- 
ported to various parts of the body by cur- 
rents in the gastrovascular cavity and are 
then taken up by the gastrodermal cells and 
passed over to the epidermal cells. Both 
respiration and excretion are performed by 
the general surface of the epidermis and 
gastrodermis. There is no endoskeleton, but 
the stony masses built up by the coral polyps 
support the soft tissues to a certain extent. 
The nervous tissue and sensory organs pro- 
vide for perception of various kinds of 


118 


COLLEGE ZOOLOGY 


stimuli and for conduction of impulses from 
one part of the body to another. Coelen- 
terates are generally sensitive to light inten- 
sities, changes in temperature, mechanical 
stimuli, chemical stimuli, and gravity. Re- 
production is both asexual, by budding and 
fission, and sexual, by means of eggs and 
spermatozoa. 

Obelia— a colonial hydroid 

Obelia (Fig. 57) lives in water up to 240 
feet in depth, along our eastern coast from 
Long Island Sound to Labrador, on the 
Pacific Coast, and other parts of the world. 
If you can imagine a hydra budding with- 
out the buds detaching from the parent 
body, and then imagine these new individ- 
uals specializing for certain functions as 
feeding and reproduction, it will be easy to 
understand the development of Obelia. It is 
attached to the substratum by a rootlike 
mass ( hydrorhiza ) , from which arise up- 
right branches (hydrocauli). Hydralike feed- 
ing members (hydranths) and reproductive 
members (gonangia) are given off from 
the hydrocaulus, as shown in Fig. 57. The 
soft parts are protected by a cellophanelike, 
chitinous covering (perisarc), which is 
ringed at intervals and expands around the 
hydranths to form the hydrothecae and 
around the gonangia to form the gonothe- 
cae. The soft parts ( coenosarc ) are attached 
to the perisarc by small strands. 

The hydranths resemble the hydra some- 
what in structure and function, but these are 
specialized for feeding only. The tentacles, 
however, are solid and about 30 in number. 
The central axis ( blastostyle ) of the gonan- 
gium gives rise to buds that develop into 
medusae; these escape through the opening 
in the end of the gonotheca. The free- 
swimming medusae produce either eggs or 
spermatozoa. The fertilized egg (zygote) 
develops into a ciliated, free-swimming larva 
(planula) which soon becomes attached to 
a stone and grows into a polyp type of col- 
ony that reproduces asexually by budding. 


Metagenesis 

The alternation of a generation that re- 
produces only asexually by division or bud- 
ding, with a generation which reproduces 
only sexually by means of eggs and sperms, 
as in Obelia, is known as metagenesis. The 
polyp and medusa stages are not equally 
prominent in all Hydrozoa; for example, the 
medusa in some species is degenerate or 
inconspicuous, as in Obelia, whereas in other 
species the polyp generation is only slightly 
developed, as in Gonionemus. 

Gonionemus— a hydrozoon 
medusa 

Gonionemus is a jellyfish (Fig. 58), com- 
mon along the eastern coast of the United 
States. It measures about Vz inch in diam- 
eter, and bears around the margin from 16 
to 80 or more hollow tentacles which bend 
at a sharp angle near the tip. The gonads 
are brown. The convex or aboral surface is 
the exumbrella; the concave or oral surface, 
the subumbrella; this is partly closed by a 
perforated membrane, the velum. Water is 
taken into the subumbrellar cavity and is 
then forced out through the central opening 
in the velum by the contraction of the body; 
this propels the animal in the opposite di- 
rection, a sort of jet propulsion. Hanging 
down into the subumbrellar cavity is the 
manubrium with the mouth at one end sur- 
rounded by 4 frilled oral lobes. The mouth 
leads to the gastrovascular cavity in the mid- 
dle of the bell, where 4 radial canals ex- 
tend to a ring canal which lies near the mar- 
gin of the umbrella. 

The cellular layers are similar to those in 
the hydra, but the mesoglea is extremely 
thick and gives the animal a jellylike con- 
sistency. Suspended beneath the radial 
canals are the sinuously folded reproductive 
organs or gonads. One individual produces 
either eggs or spermatozoa; therefore Go- 
nionemus is dioecious. A ciliated planula de- 
velops from the fertilized egg; it is at first 


Gonad 


Hydranth 


Medusa 

from male 

colony 


BLASTULA 7 


OOI 


j/ PLANULA 


MATURE FEMALE 
COLONY 


YOUNG COLONY 


Figure 57. Life history and structure of Obelia, a colonial marine hydroid. The colony consists 
of polyps of two types, the feeding hydranths and reproductive gonangia. Both hydranths and 
gonangia are developed by asexual budding from stems attached to a branching rootlike tangle 
called the hydrorhiza. Sperms and eggs are produced by medusae which bud from gonangia of 
different colonies, that is, the colonies are dioecious. The embryo develops into a ciliated free- 
swimming planula larva; this attaches to the substratum and forms a new colony. The three 
kinds of individuals — the feeding polyps (hydranths), the asexual reproductive polyps, and the 
sexual reproductive medusae — illustrate polymorphism. 


11^ 


120 


COLLEGE ZOOLOGY 


Exumbrella 
Mesoglea - 


Gastrovascular cavity 
Subumbrella 

Ring canal 
Nerve ring 
Tentacle 

Velum 


Adhes 


Epidermis 

Gastrodermis 


-Manubrium 
Radial canal 

— Gonad 


Mouth 


Oral lobe 


Statocyst 


Tentacular bulb 


OJI 


Figure 58. Diagram of a hydrozoan medusa with part cut away so as to show the internal 
structure. Natural size, one inch in diameter. 


free-swimming, but soon becomes fixed to 
some object and grows into a polyp. 

Physalia— a polymorphic 
colonial hydrozoan 

Physalia is a colony of hydrozoan polyps. 
A colony containing two kinds of individ- 
uals is said to be dimorphic; one containing 
more than two kinds, polymorphic. Some of 
the most remarkable cases of polymorphism 
occur among the Hydrozoa. Physalia, or the 
Portuguese man-of-war (Fig. 59), for exam- 
ple, consists of a gas-filled float (pneumato- 
phore) with a sail-like crest, from which a 


number of polyps hang down into the water. 
Some of these polyps are nutritive (gastro- 
zooid), others are feelers ( dactylozooid ) , 
and others are reproductive zooids (gono- 
zooid ) . 

The surface of the float shimmers with 
beautiful iridescent colors: blues, pinks, 
violets, and purples, and the crest may glow 
with vivid carmine. The different types of 
zooids in Physalia arise from a single planula 
larva and bud off from a section of the 
coenosarc just beneath the float. This 
strange animal occurs in the Gulf Stream 
from Florida northward; specimens are often 
cast up on the shore. It has no effective 


PHYLUM COELENTERATA (cNIDARIA). SIMPLE TISSUE ANIMALS 


121 


Figure 59. Portuguese man-of-war. The Portu- 
guese man-of-war floats on the surface of tropical 
seas. Hanging down from the float are long tentacles 
loaded with nematocysts. Many fish are captured 
by these streamerlike tentacles. The colony shown 
has just caught a fish; arrow points to fish. How 
ever, one species of fish of the genus Nomeus swims 
about among the tentacles of the Portuguese man- 
of-war with impunity. It appears to be immune to 
the poison of the stinging cells, possibly because it 
eats the tentacles of its host. These fish dart out to 
grasp a small food animal and hasten back amid 
the safety of the tentacles to devour it. The tenta- 
cles protect the fish, and particles of food not eaten 
by the fish are engulfed by the Portuguese man-of- 
war. (Courtesy of N.Y. Zoological Society.) 

locomotor organs but is carried from place 
to place by currents in the water or by 
winds blowing against the pneumatophore. 
The stinging dactylozooids may be over 60 
feet in length, with nematocysts powerful 
enough to inflict serious and even fatal in- 
jury on man. The dactylozooids are able to 
catch large fish; and by means of contrac- 
tion, they draw them up to the gastrozooids; 
these enclose the prey in a digestive sac by 
spreading their lips over it. 


Aurellia— a scyphozoan 
medusa 

Most of the larger jcllyfishcs belong to the 
class Scyphozoa. They can be distinguished 
easilv from the hvdrozoan medusae by the 
presence of notches, usually 8, in the margin 
of the umbrella, and by the absence of a 
distinct velum. The scyphozoan jellyfishcs 
usually range from an inch to 3 or 4 feet in 
diameter. The giant jellyfish, Cyanea, which 
lives in the cold water of the north Atlantic, 
has been found with a disk up to IVi feet 
in diameter, tentacles 120 feet long, and 
weighing up to one ton. The Scyphozoa are 
usually found floating near the surface of the 
sea, though some of them are attached to 
rocks and seaweeds. There is an alternation 
of generations in their life history, but the 
asexual stage is subordinate. 

Aurellia (Fig. 60) is white or bluish, with 
pink gonads. It differs from Gonionemus 
and other hvdrozoan medusae in the ab- 
sence of a velum, the characteristics of the 
canal system, the position of the gonads, 
and the arrangement and morphology- of the 
sense organs. 

The oral arms hang down from the square 
mouth, which opens into a short gullet; 
this leads to a rectangular central enteron. 
Gastric pouches extend laterally from 4 
sides of the enteron. Within each gastric 
pouch is a gonad and a row of small gastric 
filaments bearing nematocysts. Numerous 
radial canals, some of which branch several 
times, lead from the enteron to a ring canal 
at the margin. The 8 sense organs of Aurel- 
lia lie between the marginal lappets and 
are known as tentaculocysts; these are con- 
sidered to be organs of equilibrium. In 
addition, each tentaculocyst bears a pigment 
spot which is sensitive to light. 

The food of Aurellia consists of small 
particles which are carried along the radial 
canals by currents produced by the beating 
of cilia with which some of the gastrodermal 
cells are provided and are ingested by gastro- 
dermal cells. The physiologic processes in 


122 


COLLEGE ZOOLOGY 


Gastral filament £^^^^00 

Subgenital pit 
Gastrodermis 
Mesoglea 


Epidermis Gonad 


Gonad 

Radial cana 
Lappet 

Tentacle 


SEXUAL 
REPRODUCTION 


Strobila 


medusa (section) 


t/t 

E 


D 

O 

c 
o 

</> 
0) 
O) 

o 


y Blastula 


Gastrula 
(section) 


Scyphlstoma 


Attached stage 


Planula swims free 
of oral arm 


ASEXUAL 
REPRODUCTION 

Figure 60. Life cycle of the jellyfish Aurellia. Longitudinal sections through gastrula stage. 
Vertical section through adult. 


Aurellia are, in general, similar to those of 
the hydra. 

The gonads are frill-like organs lying in 
the floor of the gastric pouches. The egg de- 
velops into a free-swimming planula which 
becomes attached to some object, and devel- 
ops into an elongated and deeply constricted 
polyp, known as the scyphistoma stage 
(Fig. 60). The scyphistoma becomes di- 
vided into disks, resembling a pile of 
saucers; at this stage it is known as a strobila. 
Each disk develops tentacles; and, separat- 
ing itself from those below, it swims away 
as a small medusa called an ephyra. The 


ephyra gradually develops into an adult 
jellyfish. 

Metridium— a sea anemone 

A common representative of the class An- 
thozoa is the sea anemone (see colored 
frontispiece at beginning of text). Metri- 
dium dianthus (Fig. 61), is an anemone 
which fastens itself to the piles of wharves 
and to solid objects in tide pools along the 
north Atlantic coast. It is a cylindrical ani- 
mal with a crown of hollow tentacles, ar- 
ranged in a number of circlets about the 


PHYLUM COELENTERATA (CNmARIA). SIMPLE TISSUE ANIMALS 


123 


slitlike mouth. The color is variable, but 
usually brownish or yellowish. The skin is 
soft but tough. At either side of the gullet 
( stomodaeum ) is a ciliated groove called 
the siphonoglyph. The internal body cavity 
consists of 6 radial chambers; between these 
chambers are 6 pairs of thin double parti- 
tions called primary septa or mesenteries. 
Water passes from one chamber to another 


through pores (ostia) in these septa, and 
all are open below the gullet. Smaller septa 
project out from the body wall into the 
chambers, but do not reach the gullet; these 
are secondary septa. Tertiary septa lie be- 
tween the primaries and secondaries. There 
is a considerable variation in the number, 
position, and size of the septa. 

The free edges of the septa below the 


Tentacles 


Siphonoglyph 


Oral disk 


Cross section 
through gullet 

Figure 61. Structure of the sea anemone, Metridium, a representative of the class Anthozoa. 
Left, cross section through the gullet shows the arrangement of the septa. Right, a part of the 
body has been cut away to show the internal structure. 


gullet in the enteron are expanded int^o 
thickened structures called digestive fila- 
ments. These bear the gland cells that se- 
crete digestive enzymes. Near the base these 
filaments bear long delicate threads called 
acontia. The acontia are armed with gland 
cells and nematocysts. Near the edge of the 
septa are the gonads. The animals are 


dioecious. Asexual reproduction occurs by 
budding, by fragmentation at the edge 
of the basal disk, and by longitudinal 
fission. 

Sea anemones are among the most beauti- 
ful and conspicuous inhabitants of tide pooh 
along the seacoast. When fully expanded 
they form a sea garden filled with flowerlike 


124 


COLLEGE ZOOLOGY 


crowns of various colors, resemDiing not so 
much the anemones after which they were 
named, but more closely chrysanthemums or 
dahlias. When greatly disturbed, these sensi- 
tive "flowers" may be drawn into a shape- 
less mass, and the long white acontia threads 
bearing stinging capsules are extended 
through minute pores in the body wall to 
drive away enemies. 

In their natural habitat, sea anemones are 
far from flowerlike. They serve as death traps 
for any small animal that comes within 
reach of their tentacles. They may be beauti- 
ful in color but they wield their batteries of 
stinging capsules with deadly effect. The 
paralyzed prey is carried through the greedy 
mouth, down the gullet, and into the en- 
teron, which is hardly more than a digestive 
sac. The food is digested by enzymes se- 
creted by the cells of the digestive filaments 


and absorbed by the gastrodermis. Undi- 
gested wastes are ejected through the 
mouth. Saville-Kent's anemone, which lives 
on the Great Barrier Reef of Australia, is 
two feet across and is inhabited by small 
red and white fish; these swim in and out 
through the mouth without being injured 
in any way by the stinging capsules. 

Astrangia— a coral polyp 

Astrangia danae is a white coral polyp 
that inhabits the waters of our north Atlan- 
tic coast. Another species, Astrangia insig- 
nifica, occurs along the Pacific Coast; the 
polyps of this species are orange and the 
coral is red. A number of individuals live 
together in colonies attached to rocks near 
the shore. Each polyp looks like a small sea 
anemone. Each polyp secretes a calcareous 


Figure 62. Colony of Astrangia which hves in the waters of the north Atlantic Coast. These 
corals secrete protective limestone cups into which the delicate polyps can retract. (Courtesy of 
George G. Lower.) 


skeleton within which the animal rests. The 
corals on display in all museums are simply 
skeletons of coral polyps. Although Astran- 
a cuplike skeleton less than Vi 


gia builds 


inch in height, it produces large masses ot 
coral in the course of centuries. The physio- 
logic processes of corals are much like those 
of other coelenterates. 


PHYLUM COELENTERATA (cNIDARIA). SIMPLE TISSUE ANIMALS 


125 


1IIIIIIIII Ectoderm 
Iv/fS^^-x-:! Endoderm 
Mesoglea 
■Tentacle 

Mouth 


Hydra (Hydroid polyp) 


Jellyfish (Medusa) 


Figure 63. Basic plans of the three chief forms of coelenterates. The mouths of the hydra 
and sea anemone are held upward, but the jellyfish swims with mouth down. For purposes of 
comparison, however, the jellyfish is drawn with its mouth up. 


ORIGIN AND RELATIONS 
OF THE COELENTERATA 

The coelenterata probably arose from a 
two-layered animal (Fig. 430). We can, of 
course, only speculate regarding their origin 
and differentiation. A hypothesis based on 
our present knowledge is that coelenterates 
developed from a free-swimming ciliated 
form, something like the planula larvae of 
certain hydroids (Fig. 57). This became 
modified into a gastrula form with a body 
wall consisting of an outer ectoderm, protec- 
tive and sensor)' in function, and an inner 
endoderm, digestive and absorptive in func- 
tion. Between these layers a jellylike connec- 
tive tissue, the mesoglea, appeared. The 
gastrula ancestor possessed a central cavity, 
the gastrocoel, a mouth, and a sense organ 
opposite the mouth. The muscle cells and 
nervous system were in a primitive stage of 
differentiation. Tentacles grew out from 
such an ancestral form, resembling some- 
what a medusa. The larvae of these medusa- 
like ancestors may have become attached 
and then modified into hydroid polyps. Ac- 
cording to the above hypothesis, the hydra is 
not a primitive type, but a coelenterate, well 


developed histologically, that has lost its 
medusa stage. 

RELATIONS OF THE 
COELENTERATA TO MAN 

Coelenterates are of considerable eco- 
nomic importance, though probably little 
used as food by man. However, some scypho- 
zoan coelenterates are eaten in the Orient, 
and two species of Anthozoa arc eaten in 
Italy under the name of Ogliolc. Precious 
corals (Fig. 66), usually bright red or pink, 
are made into necklaces and other tvpcs of 
jewelry. 

Coral polyps build various types of reefs, 
atolls, and islands. These are confined to 
waters at least 60° F., principally in tropical 
seas. The best-known coral islands are the 
Maldive Islands of the Indian Ocean, Wake 
Island, Marshall Islands, the Fiji Islands of 
the Pacific Ocean, and those located in the 
Bahama Islands region. Bermuda is a coral 
island and the houses are built of coral 
blocks mined from certain areas. 

The Mariana Islands are coral islands of 
historic interest, for it was from an airfield 
on one of them (Tinian Island) that the 


126 


COLLEGE ZOOLOGY 


atomic bombers took off for the bombing of 
Hiroshima and Nagasaki in Japan. Many of 
the finest landing strips on the Pacific is- 


lands have been paved with coral. Coral has 
also been used for making roads and side- 
walks. 


High tide 


yLagooDy 
:\::Coral reef;w/Vw:v':; 


Low tic 


e 


^ 


A/J^^y 


/^^^ 


>Vol 


came IS 


land^ 


Lagoor 
::v.Coral reef'- 


Cross section of a fringing reef 


Cross section of a barrier reef 


High tide 
Low tide 


OOI 


■■^:^" " Lagoon ^"•:^^^^:;i:^" . /- fj}^^ 


/<C^:V."-l-i'-:V^^;=^Coral reef v^^::^^•::^v:;V:^v^^v^^ 


Cross section of an atoll reef 

Figure 64. Coral reefs are grouped in three general classes: fringing, barrier, and atoll. The 
fringing reef lies close against the shore; the barrier reef lies off shore and is separated from the 
land by a lagoon; the atoll reef is a barrier reef which encloses a lagoon; it may be a continuous 
reef, but it is usually divided by water channels, extending through it from the ocean to the 
lagoon, as shown in this illustration. Vegetation grows on accumulated debris. One coral reef 
is known to be 690 feet in depth. (Modified from drawings by P.G. McCurdy.) 


There are three types of coral reef forma- 
tions: (1) fringing reef, (2) barrier reef, 
and (3) atoll reef (Fig. 64). 

A fringing reef is a ridge of coral built up 
from the sea bottom, located so near to 
land that no navigable channel exists be- 
tween it and the shore. Frequently, breaks 
occur in the reef and irregular channels and 
pools are created which are often inhabited 
by many different kinds of animals, some of 
them brilliantly colored. 

A banier reef is separated from the shore 


by a wide deep channel which may afford 
passage for relatively large boats. These 
coral reefs may constitute a great danger to 
shipping, however. The Great Barrier Reef 
of Australia, which is the largest, is an enor- 
mous coral structure, 1350 miles in length, 
and, in places, 25 to 90 miles from the main- 
land of Australia. The channel is from 60 
to 150 feet deep. A barrier reef may en- 
tirely surround an island. 

An atoll is one or more islands, consisting 
of a belt of coral reef surrounding a central 


PHYLUM COELENTERATA (CNmARIA). SIMPLE TISSUE ANIMALS 


127 


Figure 65. A fringing reef. Two Island, Pacific Ocean. The light streak around the island is 
coral, submerged in very shallow water. The gray area outside of that is also coral but in 
deeper water, and the black area outside of that is very deep sea water. This is one of the 
many islands which were used by the armed forces during World War II. (Courtesy of Major 
Dennis G. Cooper.) 


lagoon. There are many of these in the 
mid-Pacific with lagoons of a few hundred 
yards to miles in diameter. The atoll of 
Bikini, of atomic and hydrogen bomb test 
fame, has a lagoon area of 280 square miles 
and a land area of only 2.87 square miles. 
Wake Island and Tarawa are atolls which 
figured prominently in World War II. 

The horseshoe atoll of West Texas is well 
known as a prolific source of oil production. 
It is now buried under thousands of feet of 
rocks and existed as an atoll in the shallow 
seas which covered west Texas many mil- 
lions of years ago. The horseshoe atoll is the 
largest limestone petroleum reservoir in 
North America. It is from 70 to 90 miles 
across and as much as 3000 feet thick. More 
than 5000 wells have been drilled into the 
reef mass and over 300,000,000 barrels of 
oil have been produced to date. 

Barrier reefs and atolls have been built 
by the epidermal cells of countless numbers 
of small polyps, each one secreting its cup- 
shaped skeleton. The polyps die and new 
generations secrete new calcareous cups 
upon the old ones; only the surface of the 
coral mass is alive. 


CLASSIFICATION OF 
THE COELENTERATA 

(For reference purposes only) 

This phylum includes polyps, jellyfishes, sea 
anemones, and corals. All have a body wall 
consisting of two layers of cells, between which 
is a jell}like substance, the mesoglea, which 
may or may not contain cells. Within the body 
is a single gastrovascular cavity. The epidermis 
is derived from ectoderm, and the gastrodermis 
from endoderm. They are called acoelomates 
because they do not possess a second body 
cavity, the coelom. All coelenteratcs are pro- 
\ided with nematocysts. 

About 10,000 species of coelcntcrates have 
been described. They may be grouped into 
three classes and two subclasses as follows: 

Class I. Hydrozoa. Hydroid polyps and medu- 
sae with a velum; mesoglea noncel- 
lular; solitary or colonial; mostly 
marine. Exs. Hydra, Obelia, Gonione- 
miis, and Physcilia (p. 121). 

Class 2. Scyphozoa. True medusae. No dis- 
tinct velum; usually 8 notches in the 
margin of the umbrella; mesoglea 
cellular; polyp stage absent or re- 
duced. Ex. Aurellia. 


128 


COLLEGE ZOOLOGY 


ORGAN PIPE CORAL Tuhipora 


SEA FAN Gorgonia 


PRECIOUS CORAL 
Corall'iiim 


BRAIN CORAL 
Menandrina slnuosa 


Figure 66. Some interesting types of coral. Note that some of the precious coral polyps are 
expanded as they are when feeding. 


Class 3. Anthozoa. Corals, sea anemones, etc. 
Solitary or colonial; polyps only, no 
medusae; distal end, an oral disk; 
gullet well developed; gastrovascular 
cavity divided by mesenteries; meso- 
glea cellular. 
Subclass 1. Alcyonaria. Corals, sea pens, etc. 
Colonial; skeleton present; 8 
pinnate tentacles; 8 mesen- 
teries; 1 siphonoglyph, ventral. 
Exs. Corallium, precious coral 
(Fig. 66), Pcnnatula, a sea 
feather (Fig. 66). 
^•ubclass 2. Zoantharia. Sea anemones, true 


corals, etc. Solitary or colonial; 
with or without skeleton; not 8 
tentacles. Exs. Metridium (Fig. 
61), Astrangia. 


SELECTED COLLATERAL 
READINGS 


Allee, W.C. The Social Life of Animals. Nor- 
ton, New York, 1958. 

Berrill, N.J. "The Indestructible Hydra," Scien- 
tific American, 197:118-125, 1957. 


PHYLUM COELENTERATA (cNIDARIA). SIMPLE TISSUE ANIMALS 


129 


Carter, G.S. A General Zoology of the Inverte- 
brates. Macmillan, New York, 1953. 

Coker, R.E. This Great and Wide Sea. Univ. 
North Carolina Press, Chapel Hill, N.C., 
1947. 

Crowder, W. Between the Tides. Dodd, Mead, 
New York, 1931. 

Gardiner, J.S. Coral Reefs and Atolls. Macmil- 
lan, London, 1931. 

Hickson, S.J. "Coelenterata." Cambridge Nat- 
ural History. Vol. 1, Macmillan, London, 
1906, pp. 243-411. 

Johnson, M.E., and Snook, H.J. Seashore Ani- 
mals of the Pacific Coast. Macmillan, New 
York, 1935. 


Mueller, J.F. "Some Observations on the 
Structure of Hydra, with Particular Refer- 
ence to the Muscular System," Trans. Am. 
Microscopic Soc. 69:133-147, 1950. 

Rickctts, E.F., and Calvin, Jack. Between 
Pacific Tides. Stanford Univ. Press, Stanford, 
1952. 

Robson, E.A. "Nematocysts of Corynactis: The 
Activity of the Filament During Discharge," 
Quart. J. of Microscop. Scierice 94:229- 
235, 1953. 

Roughley, T.C. Wonders of the Great Barrier 
Reef. Scribner's, New York, 1947. 

Yonge, CM. A Year on the Great Barrier Reef. 
Putnam, New York, 1930. 


CHAPTER 11 


OJI 


Phylum Ctenophora. 
Comb Jellies 


HE phylum Ctenophora (comb bearers) 
includes a small group of about 100 species 
of exclusively marine animals that resemble 
coelenterate jellyfishes. They are widely dis- 
tributed, being especially abundant in warm 
seas. Ctenophores are beautifully iridescent 
in sunlight, and they glow like electric light 
bulbs at night, due to their luminescence. 

Ctenophores are commonly called sea 
gooseberries or sea walnuts (Fig. 67) be- 
cause of their shape; or comb jellies because 
of the comblike locomotor organs arranged 
in 8 rows which extend as meridians from 
pole to pole. They are biradially symmetri- 
cal, since the parts, though in general radi- 
ally disposed, lie half on one side and half 
on the other side of a median longitudinal 
plane (Fig. 67). The mouth is situated at 
one end (oral), and a sense organ (stato- 
cyst) at the opposite or aboral end. 

Most ctenophores possess two solid, con- 
tractile tentacles which emerge from blind 
pouches opposite each other; these are 
covered with glue cells (colloblasts), which 
produce a secretion of use in capturing the 
small animals they eat. Their food consists 
of fish eggs, molluscan larvae, and small 
pelagic invertebrates. The Bureau of Fish- 
eries reports that large numbers of oyster 
larvae are killed by ctenophores. 

Ctenophores are hermaphroditic. The ova 
and spermatozoa are formed on the walls of 
the digestive canals just beneath the cili- 
ated bands. The eggs and sperms pass to 
the outside by way of the mouth. The fer- 
tilized eggs usually develop directly into the 
adult. 

As in coelenterate jellyfishes, the cellular 
layers of ctenophores constitute a very small 
part of the body, most of it being composed 
of the transparent jellylike mesoglea. A 
thin ciliated epidermis, derived from the 
ectoderm, covers the exterior and lines the 
pharynx (stomodaeum); and a gastrodermis 
derived from the endoderm, also ciliated, 
lines the stomach and the gastrovascular 
canals associated with it. 

Scattered cells and muscle fibers lie in the 


130 


PHYLUM CTENOPHORA. COMB JELLIES 


131 


ABORAL VIEW 


Oral end 
SIDE VIEW 


Sense 


Mouth 


Figure 67. The structure of a typical ctenophore. Side view of Hormiphora. (After Chun.) 
Aboral view of Pleurobrachia. (From Lankester's Treatise.) 


mesoglea; this "layer" of cells in the cteno- 
phores resembles that in certain coelen- 
terates but represents a higher grade of de- 
velopment. All the systems are of tissue 
grade in construction except that there are 
indications of reproductive ducts in some 
forms. 

Ctenophores differ from the coelenterates 
in having ciliated bands, aboral sense organs, 
mesenchymal muscles, more definite organ- 
ization of the digestive system with anal 
pores, pronounced biradial symmetry', and 
no nematocysts. They probably evolved 
from primitive coelenterate ancestors, but 
can no longer be combined with that 
phylum. 

Pleurobrachia pileus is white or rose- 


colored, ovoidal, about 2 cm. long, and 
possesses tentacles about 15 cm. long; it oc- 
curs from Long Island to Greenland, on the 
Pacific Coast, and in Europe. Mnemiopsis 
leidyi is a transparent, luminescent species, 
about 10 cm. long, that lives along our east- 
ern sea coast. It is often parasitized by a 
minute sea anemone 1.5 mm. long. Some 
bizarre forms occur among the ctenophores, 
for example, Cestus veneris (Venus's- 
girdle), headpiece, page 130, may be two 
inches wide and over three feet long, trans- 
parent, but showing green, blue, and violet 
colors; it swims by muscular movements of 
the ribbonlike body as well as by the beating 
of the elongated swimming plates. It lives 
in tropical seas but is sometimes carried 


132 


COLLEGE ZOOLOGY 


north along our Atlantic Coast by the Gulf 
Stream. 


SELECTED COLLATERAL 
READINGS 

Harvey, E.N. Bioluminescence. Academic 
Press, New York, 1952. 


Hyman, L.H. The Invertebrates: Protozoa 

Through Ctenophora. McGraw-Hill, New 

York, 1940. 
Mayer, A.G. The Medusae of the World. 

Carnegie Inst., Washington, 1910. 
Mayer, A.G. Ctenophores of the Atlantic Coast 

of North America, Pub. 162. Carnegie Inst., 

Washington, 1912. 


CHAPTER 12 


4l\o*t 


Phylum 

Platyhelminthes. 

Simple Organ-System 

Animals 


HE Platyhelminthes are much flattened 
dorsoventrally and hence are known as flat- 
worms. Among them are both free-living 
and parasitic species; the former live prin- 
cipally in fresh or salt water; the latter are 
mostly endoparasitic. The parasitic flat- 
worms are known as flukes or trematodes, 
and tapeworms or cestodes. They arc widely 
distributed among human beings and other 
vertebrates; they are often pathogenic, and 
sometimes bring about the death of the 
host. Free-living flatworms of North Amer- 
ica live in springs, ponds, and streams, or in 
bodies of salt water. 

Flatworms exhibit many advances over 
the coelenterates and ctenophores. They are 
definitely bilaterally symmetrical, a charac- 
teristic common to most of the animals 
above them in the scale of life. This type of 
symmetry is correlated with various modifi- 
cations both in structure and physiolog). 
The flatworms possess a third embryonic 
tissue; hence their structures are derived 
from ectoderm, endoderm, and mesoderm 
(Fig. 92). The mesoderm gives rise to all 
tissue between the epidermis and intestine, 
except the nervous tissue. 

Like most of the animal characters, the 
mesoderm has been foreshadowed in the 
more primitive animal groups. Its early be- 
ginnings are probably represented by some 
of the mesenchyme cells of the coelen- 
terates. However, mesenchyme is not gen- 
erally considered true mesoderm until, as 
in the flatworms and other more complex 
animals, it is more massive and gives rise to 
definite structures such as muscles. The 
Platyhelminthes is the lowest phylum of 
animals built on an organ-system level of 
complexity. It is also the first phylum in 
which there is a distinct head with sense 
organs and central nervous system. The 
commonest free-living species of the flat- 
worms are called planarians and for this rea- 
son a planarian has been chosen for special 
description. 

Two entire classes of flatworms are para- 
sites, some of which, such as the tapeworms, 

133 


134 


COLLEGE ZOOLOGY 


are well known, at least by name. Parasitic 
species are of great interest and very impor- 
tant economically. Their mode of life has 
brought about various specializations, such 
as enormously increased powers of reproduc- 
tion, and extremely complicated life cycles, 
involving in certain cases three or four dif- 
ferent species of hosts and intermediate 
hosts. The relations of some species to hu- 
man health and to the rearing of domesti- 
cated animals constitute a large part of 
what is known as economic zoology and 
medical zoology. 

Three classes of Platyhelminthes may be 
recognized: these are (1) Turbellaria, (2) 
Trematoda, and (3) Cestoda. Most of the 
Turbellaria are free-living and inhabit either 
fresh or salt water; a few live in moist soil, 
and a few are parasitic. The trematodes and 
cestodes are all parasitic. 

PLANARIAN-A FRESH- 
WATER FLATWORM 

The commonest fresh-water planarian in 
the United States is Dugesia tigrina (Fig. 
68). It lives on water plants in ponds, and 
along the shores of ponds, lakes, and rivers, 
and in small streams under stones. Its up- 
per surface is brown or mottled and irreg- 
ularly spotted with white, and its under sur- 
face is white or grayish. The body is 
bilaterally symmetrical, broad and blunt at 
the anterior end, and pointed at the pos- 
terior end, and may reach a length of from 
15 to 18 mm. 

The anterior end of the animal is quite 
distinctly the head. At each side of the 
head is a sharp projecting auricle. It con- 
tains a variety of sense cells. A pair of eyes 
(Fig. 68) is present on the dorsal surface 
near the anterior end. The mouth is not on 
the head, but near the middle of the ventral 
surface. It opens into a cavity which con- 
tains a muscular tube, the pharynx (Fig. 
68), attached only at its proximal end. The 
pharynx consists of a complex of muscle 


layers and many gland cells. By means of 
the muscles, the phar}-nx can be thrust out 
of the mouth some distance when feeding. 
On the ventral side, posterior to the pharynx 
is a smaller opening, the genital pore; this 
is present only in sexually mature individ- 
uals. The ventral surface of the body is 
covered with cilia, which play some part in 
locomotion; however, the chief method of 
locomotion is by almost imperceptible mus- 
cular contractions. 

Planarians, like other flatworms, possess a 
mesoderm. The tissues of mesodermal 
origin, lying between the body wall and the 
intestine, consist of a fibrous mesh, in which 
are embedded fixed cells whose processes 
anastomose, and free cells that can move 
about in amoeboid manner. This mesoder- 
mal network of connective tissue is called 
parenchyma (mesenchyme). The well-de- 
veloped muscular, nervous, digestive, ex- 
cretor\% and reproductive systems are con- 
structed in such a way as to function 
without the coordination of a circulatory 
system, respiratory system, coelom, and 
anus, which are present in many more com- 
plex animals. The digestive system consists 
of a mouth, a pharnyx, and an intestine of 
three main trunks with a large number of 
small lateral extensions (Fig. 68). 

The food of planaria consists of animals, 
living or dead. The pharjnx is protruded 
into the food; and by a sucking action, 
microscopic particles are detached and 
drawn into the digestive cavity. Digestion 
occurs only within cells lining the simple 
intestine. There is only one opening to the 
digestive cavity; as in coelenterates, the undi- 
gested matter is ejected through the mouth. 

Figure 68. Facing page, a planarian. Diagram 
on upper left shows the digestive and nervous sys- 
tems. Diagram on upper right shows part of the 
reproductive system; and at anterior left side, part of 
the excretory system. Actually these systems exhibit 
bilateral symmetry, but portions of each are shown 
to conserve space. Lower, schematic diagram of 
pharynx. (After L.H. Hyman, The American Biology 
Teacher, 1956.) 


Testis - 


Oviduct 


Brain 

Excretory network 

Flame ce 

Ovary 


Digestive tract 


Transverse nerve 


Longitudina 
nerve cord 
Sperm duct 


Yolk gland 


Pharynx 
Mouth 

Opening of 
pharynx 

Copulatory sac 


Genital chamber 
Genital pore 


Lateral branch of 
digestive tract 


Digestive and nervous 
systems 


Excretory and 
reproductive systems 


w.}}m})}}}}}}))}m777m. 


Pharynx at rest in pharyngeal 
chamber (side viev/) 


Pharynx protruded 
through mouth 


135 


136 


COLLEGE ZOOLOGY 


Nucleus 
Cytoplasm 


Cell lumen 
Cilia 


Flame cell 


Excretory duct 


Excretory 
tubule 


Figure 69. Excretory system of a fresh-water planarian. On the right is shown a single flame 
cell attached to a portion of an excretory duct. Arrows in flame cell indicate direction of flow 
of materials. {Left after L.H. Hyman, The American Biology Teacher, 1956.) 


Circulation of the digested food is accom- 
plished within the branches of the digestive 
system and in the fluid-filled spaces in the 
parenchyma. 

The excretory system consists of a com- 
plex network of small tubes on each side, 
from which flame cells branch (Figs, 68 and 
69). The flame cell (Fig. 69) is large and 
hollow, with a group of flickering cilia ex- 
tending into the central cavity, which create 
a current and force the collected fluid 
through the tubules which open on the sur- 
face by several minute pores. The muscular 
system consists principally of three sets of 
muscles, a circular layer just beneath the 
epidermis, then a longitudinal layer imme- 
diately below the circular muscle cells, and 
dorsoventral muscles lying in the paren- 
chyma (Fig. 70). 

There is a well-developed nervous system 
(Fig. 68), consisting of an inverted V- 
shaped mass of tissue, the brain, and two 
ventral longitudinal nerve cords connected 
by transverse nerves. From the brain, nerves 
pass to various parts of the anterior end of 


the body. The highly pigmented eyes are 
sensitive to light but do not form an image. 
Reproduction is by fission or by the sexual 
method. An animal may divide transversely; 
each part then becomes reorganized into a 
complete planarian. Each individual pos- 
sesses both male and female sexual organs 
(Fig. 68), that is, it is hermaphroditic, but 
self-fertilization is not known to occur, and 
cross-fertilization is certainly the rule. The 
development is direct, without a larval stage. 
Some fresh-water planarians show remark- 
able powers of regeneration (Fig. 72). If 
such an individual is cut in two, the anterior 
end will regenerate a new tail, while the 
posterior part will develop a new head. A 
section from the middle of the body will 
regenerate both a head at the anterior end 
and a new tail at the posterior end. No dif- 
ficulty is experienced in grafting pieces from 
one animal to another. 

Axial gradients 

Planarians are animals that illustrate ad- 
mirably the theory of axial gradients. The 


PHYLUM PLATYHELMINTIIES. SIMPLE ORGAN-SYSTEM ANIMALS 


137 


Pharyngeal chamber 
Pharynx 

Lunnen 


— Epidermis 


Parenchyma 


Intestinal epithelium 


Longitudinal muscle fibers 

Circular muscle fibers 

Epidermis 

Cilia 

Histological detail of a 
portion of cross section 

Figure 70. Cross section through the pharjngeal region of a planarian. (This cross section 
was drawn from a histological preparation provided through the courtesy of L.H. Hyman.) 


primary axis or axis of polarity is an imag- 
inary line extending from the anterior to 
the posterior end of the body. In the plana- 
rian the head has a relatively high rate of 
metabolism and dominates the rest of the 
body. Experiments have shown that a gra- 
dient of metabolic activity proceeds from 
the anterior to the posterior end. For exam- 
ple, if planarians are cut into 4 pieces, the 


anterior piece will be found to use up more 
oxygen and give off more carbon dioxide 
than any of the others; the second piece 
comes next in its rate of metabolism; the 
third piece next; and the tail piece gives the 
lowest rate of all. Thus an axial gradient is 
demonstrated in the metabolism of the 
animal from the anterior to the posterior 
end; its significance is controversial. 


138 


COLLEGE ZOOLOGY 


Epidermis 


Nucleus 


Light-sensitive 
cell 

Pigment cell 


Pigment granules 


Nerve to brain 


Figure 71. Section through eye of the planarian. The pigmented cells form a cup which 
insulates the light-sensitive cells within it against all light except that which enters through the 
open side of the cup, as shown by the arrow. Since the direction of the light determines which 
light-sensitive cells arc stimulated, the planarian can determine the direction from which the 
light comes. No visual image is possible with such an organ. 


Figure 72. Planaria. Diagrams illustrate stages in the process of regeneration. A, B, specimen 
cut into two parts; the head (A) regenerates another tail (dotted) (Ai) and finally regains 
its normal shape (A2); B regenerates another head (B^) and lengthens into a normal specimen 
(B2); C, a spht head, regenerates two heads (Ci). 


OTHER PLATYHELMINTHES 

Flatworms differ greatly among them- 
selves due largely to the fact that the Turbel- 
laria are for the most part free-living, 
whereas the Trematoda and Cestoda are all 
parasitic in habit. Turbellarians probably 
exhibit the typical organization of the phy- 


lum, the trematodes and cestodes being 
modified considerably for a parasitic exist- 
ence. The epidermis is ciliated in the turbel- 
larians, but in the trematodes and cestodes 
there is no epidermis; they are covered with 
a thick cuticle. Sense organs are probably 
present in all. 

In the Turbellaria and Trematoda there 


PHYLUM PLATYHELMINTHES. SIMPLE ORGAN-SYSTEM ANIMALS 


139 


IS a saclike intestine with a single opening, 
which serves both as mouth and anus. In 
the simplest forms, the intestine is un- 
branched; but in others, branches occur that 
may penetrate to all parts of the body, thus 
rendering a circulatory system unnecessary. 
However, in certain trematode families, 
channels filled with fluid occupy a consider- 
able part of the body. The fluid surging back 
and forth as a result of muscular contrac- 
tions may in effect serve as a transport sys- 
tem. The Cestoda have lost the intestine 
and absorb nutriment through the general 
surface of the body. An excretory system oc- 
curs in almost all of the flatworms; and in 
some, it is very complicated. The most char- 
acteristic feature of this system is the flame 
cell. The nervous system consists of a net- 
work with a concentration of nervous tissue 
at the anterior end, the brain, and several 
longitudinal nerve cords. Flatworms are 
characterized by a complex reproductive 
system. 

Fasciola hepatica— a 
parasitic trematode 

Fasciola is known as the sheep liver fluke. 
It lives as an adult in the bile ducts of the 
livers of sheep, cows, pigs, and many other 
herbivores. Olsen has estimated that on the 
Gulf coast alone there is a yearly loss of 44 
tons of condemned livers and 58 tons of 
other meat, not to mention the mortality of 
livestock, especially among calves, reduction 
in milk production, and lessened breeding. 
Human infections of the liver fluke are 
relatively rare, but this is probably due to 
infrequent exposure to the parasite rather 
than to its failure to develop in man. Water 
cress is one of the commonest means of 
human infection. In Cuba human infections 
are reported common, and in some years 
reach epidemic proportions. 

The mouth of Fasciola lies in the middle 
of a muscular disk, the oral sucker (Fig. 73). 
The ventral sucker serves as an organ of at- 
tachment. Between the mouth and the ven- 


tral sucker is the genital pore through which 
the eggs pass to the exterior. The excretory 
pore (Fig. 73) lies at the extreme posterior 
end of the body. 

The digestive system consists of a mouth, 
phar) nx, short esophagus, and intestine with 
two main branches (Fig. 73). The excretory 
system is similar to that of planarians, but 
only one main tube and one exterior open- 
ing are present. The nervous system con- 
sists of a small ganglion at the anterior end 
of the body which gives off a few longitudi- 
nal nerves. Sense organs are almost lacking. 
Complex muscle layers lie just beneath the 
cuticle. 

The body of the liver fluke is covered with 
a thick, heavy, elastic cuticle. The paren- 
chyma is a loose tissue lying between the 
body wall and the digestive tract; within it 
are embedded the various internal organs 
described above, as well as the reproductive 
system. 

Except in the schistosomes and one other 
group, both male and female reproductive 
organs are present in every adult fluke (Fig. 
75 ) ; they are extremely well developed, and, 
as in planarians, quite complex. One liver 
fluke may produce as many as 500,000 eggs; 
and, since the bile ducts in the liver of a 
single sheep may contain more than 200 
adult flukes, there may be 100 million eggs 
formed in one parasitized animal. The eggs 
segment in the uterus of the fluke, then 
pass through the bile ducts of the sheep into 
its intestine, and finally are carried out of 
the sheep's body with the feces. Those eggs 
(Fig. 73) that encounter water produce 
ciliated larvae (miracidia) that swim about 
until they encounter a certain fresh-water 
snail, into which they burrow. Here, in 
about two weeks, they change into saclike 
sporocysts containing germ balls or em- 
br}Os. Each germ ball within the sporocyst 
develops into a second kind of larva (redia). 
These usually give rise to one or more gen- 
erations of daughter rediae, after which they 
produce a third kind of larva with a long tail, 
known as a cercaria. The cercariae leave the 


I — Mouth 
i-Oral sucker 
Pharynx 


U^«^"* Mehlis' gland |-Yolk duct 

■Sperm duct 


■Yolk gland 
■Testis 


Ventral sucker 
Ovary 


Excretory 
pore 

Intestine 
"Lourer's canal 

•Ootype ADULT TREMATODE FROM BILE DUCTS 
OF SHEEP, MAN, AND OTHER ANIMALS 


Eggs develop, 
miracidia hatch 
and burrow 
into snails 


ENCYSTED STAGE 
(METACERCARIA) 
Dissected free 
from cyst 


Cercariae emerge 
and encyst ^•^^•* 
on plants 


OJI 

CERCARIA 


MIRACIDIUM 


Germ ball 


REDIA FROM 
SNAIL 


REDIA FROM 
SNAIL 


SPOROCYST 
FROM SNAIL 


140 


Figure 73. Life cycle and structure of the liver fluke of sheep, Fasciola hepatica. 


PHYLUM PLATYHELMINTHES. SIMPLE ORGAN-SYSTEM ANIMALS 


141 


body of the snail, swim about in the water 
for a short time, and then encyst on a leaf 
or blade of grass. The encysted cercaria is 
called a metacercaria. If the leaf or grass is 
eaten by a sheep, the metacercariae escape 
from their cyst wall and make their way 
from the sheep's digestive tract to the bile 
ducts, where they develop into mature flukes 
in about 6 weeks. 

The great number of eggs produced by a 
single fluke is necessary because many eggs 
do not reach water; the majority of the 
larvae do not find the particular kind of 
snail necessary for their further develop- 
ment; and the metacercariae to which the 
successful lar\'ae give rise have little chance 
of being devoured by a sheep. The genera- 
tions within the snail, of course, increase 
greatly the number of larvae which may de- 
velop from a single egg. This complicated 
life history should also be looked upon as 
enabling the fluke to gain access to new 
hosts. The liver fluke is not so prevalent in 
the sheep of this countr}' as in those of 
Europe. 

Clonorchis sinensis is an important hu- 
man parasite, especially in certain parts of 
Japan and China. An illustration (Fig. 74) 
is included here, since in some ways this ani- 
mal is easier to study than Fasciola hepatica, 
and specimens may be obtained from biolog- 
ical supply houses. This species lives in the 
bile ducts of man, cats, dogs, and other 
mammals. The eggs are passed in the feces; 
the early larvae live in snails; the cercariae 
enter various species of fresh-water fish, 
where they become metacercariae which are 
infective to man; man and other animals are 
infected by eating uncooked, parasitized fish. 

Pork tapeworm (Taenia solium) 

The pork tapeworm lives as an adult in 
the digestive tract of man. A nearly related 
species, the beef tapeworm, is also a parasite 
of man. The pork tapeworm is long and 
consists of a knoblike "head," the scolex 
(Fig. 76), and a great number of similar 


parts, the proglottids, arranged in a linear 
series. The animal clings to the inner wall 
of the intestine by means of hooks and 
suckers located on the scolex. No hooks, 
however, are present on the scolex of the 
beef tapeworm. Behind the scolex of the 
pork tapeworm is a short neck followed by a 
string of proglottids which gradually in- 
crease in size from the anterior to the 
posterior end. The worm may reach a length 
of 10 feet and contain 800 or 900 proglot- 
tids. Since the proglottids are budded off 
from the neck, those at the posterior end 
are the oldest. 

No digestive tract is present, the digested 
food in the intestine of the host being ab- 
sorbed through the body wall. The ner\'ous 
system (Fig. 76) is similar to that of the 
planarians and the liver fluke, but not so 
well developed. Longitudinal excretory 
canals (Fig. 76), which have branches end- 
ing in flame cells, open at the posterior end 
of the worm and carrv metabolic waste out 
of the body. 

A mature proglottid is almost completely 
filled with reproductive organs (Fig. 76). 
The eggs develop into 6-hooked embryos 
(Fig. 76) while still within the proglottid. 
If they are then eaten by a pig, they escape 
from their envelopes and bore their way 
through the wall of the intestine into the 
blood or lymph vessels to be carried even- 
tually to the voluntar)' muscles, brain, or 
eyes, where they form cysts. A scolex is de- 
veloped from the cyst wall (Fig. 76). The 
larva is known as a bladder worm or cysti- 
cercus at this stage (Fig. 76). If insuffi- 
ciently cooked pork containing cysticerci is 
eaten by man, the bladder is thrown off, and 
the scolex, which develops, becomes fas- 
tened to the wall of the human intestine, 
and a series of proglottids is developed. Man 
can also serve as the intermediate host if ova 
are ingested or enter the stomach as a re- 
sult of reverse peristalsis. Since the cysti- 
cercus may be located in the brain or eyes, 
infection with this parasite may be a seriou§ 
matter. 


Oral sucker 
Pharynx 


Genird pore Uterus 
Yolk gland 


Ovory Ootype 


Testis Excretory blade 
Sperm duct 


Mouth 
htestlne 


Excretory pore 


Ventral sucker yolk duct Peminal receptacle 

Seminal vesicle tourer's con^l 

ADULT TREMATODE FROM BILE DUaS "* 


Adult trematode sheds 

eggs which ore carried 

out of the body in feces 


Man becomes infected by eating poorly cooked 
or raw fish with cysts C^^ 


Cercarioe emerge from 
Snail and encyst in 
muscles of certain 
freshwater fishes 


Eggs containing 
miracidia ore ingested 
by snail host 


K 


Encysted stage 
(mefocercoria) 
dissected free 
of cyst 
Ventral sucker- 


c^ 


Sporocyst stage 
from snail 

Young cercaria 

— Germ ball 


®JI Cercana Redio stage from snail 

Figure 74. Life cycle and structure of the human liver fluke, Clonorchis sinensis. 


142 


PHYLUM PLATYHELMINTHES. SIMPLE ORGAN-SYSTEM ANIMALS 


143 


Mai 


Testis 

Ventral 
sucker 

Oral 
sucker 


Adult schistosomes in blood vessels 
of bladder and urinary tract 


Eggs are passed in urine 
and hatch into miracidia 
which enter snail 


Cercariae emerge from 
snail and penetrate the 
exposed skin of man 


\ 


Cercaria 


Free-swimming 
miracidium 


Sporocyst stage in snail 

Figure 75. Life cycle of a human blood fluke (Schistosoma haematobium). The large male 
is carrying the small female. This is an ancient human parasite, especially common in many 
parts of Africa and lower Egypt. 


Why is the tapeworm not digested in the 
human intestinal juices (enzymes)? It is be- 
Heved that the tapeworm is protected from 
the action of digestive enzymes by means of 
an anti-enzyme mechanism. 

ORIGIN AND RELATIONS OF 
THE PLATYHELMINTHES 

The flatworms, especially the Turbellaria, 
resemble the coelenterates in certain re- 
pects which indicate coelenteratelike ances- 
tors. There is usually a single opening for 
ingestion of food and egestion of waste ma- 


terial; a nerve net reminiscent of the coelen- 
terates; and the parenchymal connective tis- 
sue is similar to the cellular mesoglea in 
higher coelenterates and ctenophorcs. The 
chief differences between the classes of flat- 
worms appear to be due to their free-living 
or parasitic character. 

RELATIONS OF THE 
PLATYHELMINTHES TO MAN 

The free-living flatworms are of very little 
importance to man, but many of the trema- 
todes and cestodes are dangerous parasites, 


Rostellum 
Hook 


Proglottids 
budding off 


Embryo from ripe 
proglottid in feces of man 


14^ 


Gravid proglottid 
Figure 76. Life cycle and structure of the pork tapeworm. Taenia. 


PHYLUM PIAl YHELMINTHES. SIMPLE ORGAN-SYSTEM ANIMALS 


145 


SOME COMMON TREMATODES OF MAN 


SCIENTIFIC 
NAME 


DEVELOP- 
MENTAL STAGES 


GEOGRAPHICAL 
DISTRIBUTION 


DEFINITIVE 
HOSTS 


INTESTINAL 
FLUKES 


Fasciolopsis 
buski 


In snails and on 
water-grown 
vegetables 


China, Indo China, 
Formosa, Suma- 
tra, India 


Man, pigs, in 
China, Formosa, 
Siam 


Heterophyes 
heterophyes 


In snails and fresh- 
water fish 


Egypt, China, 
Japan 


Man, cat, dog, in 
Egypt 


LIVER FLUKES 


Clonorchis 
sinensis 
(Fig. 74) 


In snails and fresh- 
water fish 


China, Japan, 
Korea, French 
Indo-China, 
Philippines 


Man, cat, dog, and 
fish-eating mam- 
mals 


Opisthorchis 
felineus 


In snails and fresh- 
water fish 


Europe, Siberia 


Man, cat, dog 


LUNG FLUKES 


Paragonimus 
westermani 


In snails and fresh- 
water crabs 


Japan, China, 
Philippines, 
America 


Man, tiger, dog, cat, 
mustelids, pig 


Schistosoma 
haemato- 
bium 

(Fig. 75) 


In snails 


Africa, Near East, 
Portugal, Aus- 
tralia 


Man, monkey 


BLOOD FLUKES 


Schistosoma 
mansoni 


In snails 


Africa, West In- 
dies, N. and S. 
America 


Man 


Schistosoma 
japonicum 


In snails 


Japan, China, 
Philippines 


Man, cat, dog, pig, 
etc. 


the trematodes especially in Africa, Asia 
Minor, Arabia, West Indies, Brazil, and 
Venezuela; the cestodes throughout the 
world. The scientific names, location of de- 
velopmental stages, geographic distribution 
and hosts of some of the species that live 
in man are presented in the accompanying 
tables. Hydatid cysts represent a larval stage 
of the tapeworm, Echinococcus granulosus. 
They occur in the liver and other organs of 
man, cattle, horses, sheep, etc., and may at- 
tain the size of a child's head. One abdom- 
inal cyst has been reported which contained 
42 liters of fluid. Within each cyst the 
germinal layer may give rise to thousands of 
brood capsules or daughter cysts in which 
scolices develop. Operative measures only 
are effective in treatment. 

Certain schistosome cercariae incapable of 


infecting man cause a severe swimmer's itch 
(dermatitis) when they penetrate the skin 
of bathers who have become sensitized by 
repeated exposures. This condition is com- 
mon in the north central states, in southern 
Canada, as well as in some other parts of 
the United States, Europe, and India. Swim- 
mer's itch can be avoided by swimming in 
deep water. The first symptom of swimmer's 
itch is a prickly sensation followed by the 
development of ver\' itchy pimples, which 
sometimes become pustular. The itch or- 
ganism can be controlled in small bodies of 
water by the use of copper sulfate to kill the 
snail in which the cercariae spend part of 
their life cycle. One midwest state used over 
25 tons of copper sulfate in a recent summer 
to destroy infected snails to free the beaches 
from swimmer's itch. 


Cysf wall 


Eggs develop into hydatid cysts in 
liver and other organs 


intestine 


Scolex with hooks 
and suckers 


Carnivorous mammals (dog) eat 
tissues containing hydatid cysts. 
Adult tapeworms develop in 


/ 


Eggs passed in feces 
are accidentally eaten 
by various herbivorous 
mammals (sheep, swine) 
and man 


Mature proglottid 


Adult in intestine of 
carnivorous mammals 


Raw or partially cooked infected 
fish eaten by man and other 
mammals 


Copepods eaten by fish 


Eggs passed 
feces 


Eggs hatch into ciliated 
larvae in water 

Co'pepod eats 
•.; larvae 


Larva from fish muscU 

146 


Larva from 
copepod 


Adult in intestine of man and 
other mammals 


PHYLUM PLATYHELMINTHES. SIMPLE ORGAN-SYSTEM ANIMALS 


147 


SOME COMMON CESTODES OF M.\N 


SCIENTIFIC NAME 


DEVELOPMENTAL 
STAGES 


GEOGRAPHIC 

DISTRIBUTION 


HOSTS 


Dibothriocephalus 
latus 
(Fig. 77) 


In copcpods and fresh- 
water fish 


Widespread 


Man, dog, cat, bear, 
fox (intestine) 


Echinococcus 
granulosus 
(Fig. 77) 


In hver, brain, lungs, 
of man, pig, sheep 


Widespread 


Dog and other car- 
nivores (intestine) 


Taenia saginata 


Muscles of cattle 


World-wide 


Man 


Taenia solium 
(Fig. 76) 


In muscles of pig, and 
man, accidentally 


World-wide 


Man 


CLASSIFICATION OF THE 
PLATYHELMINTHES 

{For reference purposes only) 

The Platyhelminthes are animals character- 
ized as being unsegmented, triploblastic, and 
bilaterally symmetrical. The body is flattened 
dorsoventrally. No anus (usually) or coelom is 
present. They have no skeletal, circulatory, or 
respiratory systems; but they have an excretory 
system with many flame cells. They have a 
head, sense organs, and a central nervous sys- 
tem which consists of a brain and two longitu- 
dinal nerve cords. Most flatworms are herma- 
phroditic. 

The principle classes, subclasses, and orders 
are as follows: 

Class 1. Turbellaria. Mostly free-living; epi- 
dermis at least partly covered with 
cilia, with rodlike rhabdites and many 
mucous glands. No suckers. 
Order 1. Acoela. Marine; no intestine. 

Ex. Polychoerus caudatus. 
Order 2. Rhabdocoela. Intestine simple, 
unbranched. Ex. Stenostomum 
leucops. 
Order 3. AUoeocoela. Usually cylindroid; 
intestine straight or with short 


Figure 77. Facing page, upper part of illustra- 
tion, life cycle of the dog tapeworm, Echinococcus 
granulosus. Lower part of illustration, life cycle of 
the broad tapeworm of man, Dibothriocephalus 
latus (formerly Diphyllobothrium latum). 


branches; mostly marine. Ex. 
Prorhynchus. 
Order 4. Tricladida. Intestine of three 
main trunks, each with many 
lateral branches. Ex. Dugesia 
tigrina (Fig. 68). 
Order 5. Polycladida. Marine; central di- 
gestive ca\ity with many irreg- 
ular branches. Ex. Stylochus 
ellipticus. 
Class 2. Trematoda. Parasitic; intestine pres- 
ent; no cilia on adult; cuticle present; 
suckers on ventral surface. 
Order 1. Monogenea. External or semiex- 
ternal parasites; direct de- 
velopment with no asexual 
multiplication. Ex. Benedenia 
(Epibdella) melleni. 
Order 2. Digenea. Internal parasites; an 
asexual generation in life cycle. 
Two or more hosts required, 
with alternation of hosts. Exs. 
Fasciola hepatica (Fig. 73); 
CloTiorchis sinensis (Fig. 74). 
Class 3. Cestoda. Endoparasites; no intestine; 
no cilia on adult; cuticle present; usu- 
ally proglottids. Ex. Taenia solium 
(Fig. 76). 


SELECTED COLLATERAL 
READINGS 

Chandler, A.C. Introduction to Parasitology. 
Wiley, New York, 1955. 


148 


COLLEGE ZOOLOGY 


Craig, C.F., and Faust, E.G. Clinical Parasitol- 
ogy. Lea & Febiger, Philadelphia, 1951. 

Faust, E.G. Animal Agents and Vectors of 
Human Disease. Lea & Febiger, Philadelphia, 
1955. 

Hyman, L.H. The Invertebrates: Platyhelmin- 
thes and Rhynchocoela. McGraw-Hill, New 
York, 1951. 


Mackie, T.T., Hunter, G.W., III, and Worth, 
G.B. Manual of Tropical Medicine. Saun- 
ders, Philadelphia, 1945. 

Wardle, R.A., and McLeod, J. A. The Zoology 
of Tapeworms. Univ. of Minnesota Press, 
Minneapolis, 1952. 


K 


CHAPTER 13 


OJI 


Phylum 
Nemathelminthes, 

Phylum 
Nematomorpha, and 

Phylum 

Acanthocephala. 

Roundworms 


Iematodes are one of man's worst ene- 
mies. Their activities are less spectacular 
than those of insects, but they are nearly as 
detrimental. They are universally present in 
the sea, in the fresh water, and in the soil. 
They occur also as parasites of plants and 
animals, including man. It is estimated that 
roundworms cause millions of dollars of 
crop damage every year. On the other hand, 
some of the many free-living species are 
known to be beneficial. Together, the para- 
sitic and free-living roundworms form a 
subdivision of the animal kingdom which is 
of the utmost importance to man because 
it affects his well-being and economy. 

Formerly, three different groups of round- 
worms were placed together in a single phy- 
lum. However, this arrangement was un- 
satisfactory to most zoologists, who preferred 
to classify them separately as the phylum 
Nemathelminthes, phylum Nematomorpha, 
and the phylum Acanthocephala. These 
three groups will be considered in the same 
chapter, even though divided into three 
separate phyla, for they do have some char- 
acteristics in common. 


PHYLUM NEMATHELMINTHES 

The Nemathelminthes are called unseg- 
mented roundworms to distinguish them 
from the flatvvorms and segmented annelids. 
They are, typically, long slender animals, 
usually with a smooth glistening surface, and 
tapering at one or both ends. In size, they 
range from ^4 2 5 oi an inch to 4 feet in 
length. 

Many of them are so important economi- 
cally and undergo such amazingly complex 
life histories that a knowledge of some of 
the parasitic species of animals is of great 
interest to everyone. For this reason, a brief 
description of Ascaris is presented, as well 
as an account of a number of the more im- 
portant parasitic species. These include the 
hookworm, trichina worm, pinworm, whip- 
worm, filaria, eye worm, and guinea worm. 

149 


Ovary 


Oviduct 


OJI 


PHYLA NEMATHELMINTHES, NEMATOMORPHA, AND ACANTHOCEPHALA 


151 


Ascaris lumbricoides— a 
roundworm parasitic in man 

Anatomy 

This is the common roundworm parasitic 
in the intestine of man. The sexes are sepa- 
rate. The female is the larger and measures 
up to 16 inches in length and to V4 inch 
in diameter. The body has a dorsal and 
ventral, narrow white line, running its en- 
tire length, and a broader lateral line on 
either side. The tough cuticle is smooth and 
marked with fine striations. The mouth 
opening is in the anterior end and is sur- 
rounded by one dorsal and two lateroven- 
tral lips. In the male, near the posterior end, 
is the cloacal opening, from which extend 
two chitinous rods, the penial spicules, of 
use during copulation. Many ventral, prea- 
nal, and postanal papillae are also present 
in the male. The male is considerably 
smaller and more slender than the female; 
one of its best distinguishing characteristics 
is the sharply curved posterior end. In the 
female, the vulva or genital pore is located 
ventrally at about one-third the length of 
the body from the anterior end. 

The body contains a straight digestive 
tract and other organs (Fig. 78). Between 
the intestine and the body wall is a body 
cavity, which is called a pseudocoel. It is 
not a true coelom because there is no true 
mesodermal epithelium covering the intes- 
tine. The digestive tract is very simple. A 
small mouth cavity opens into the mus- 
cular esophagus, or pharynx, which is from 
10 to 15 mm. long. The esophagus draws 
fluids from the intestinal contents of the 
host into the long nonmuscular intestine, 
and the nutriment is absorbed through the 
walls. The posterior portion of the intestine 
is known as the rectum in the female, which 
discharges through the anus. But in the male 
the intestine and reproductive system open 


Figure 78. Facing page, female Ascaris. Side view 
of a specimen on left, and a dissection on the right 
to show the internal organs. 


into a common passage way, the cloaca, and 
the opening to the outside probably should 
be called a cloacal opening; however, it is 
usually termed an anus. 

The excretory system consists of two 
longitudinal tubes, one in each lateral line 
(Fig. 79); these open to the outside by a 
single excretory pore situated near the 
anterior end in the midventral body 
wall. 

A ring of nervous tissue surrounds the 
esophagus and gives off two large nerve 
cords, one dorsal, the other ventral, and a 
number of other smaller strands and connec- 
tions. 

Reproduction 

The male reproductive organs are a single, 
coiled, threadlike testis, from which a vas 
deferens leads to a wider tube, the seminal 
vesicle; this is followed by the short, mus- 
cular, ejaculatory duct which opens into 
the cloaca. In the female (Fig. 78) lies a 
Y-shaped reproductive system. Each branch 
of the Y consists of a coiled threadlike 
ovary, which is continuous with the 
oviduct and uterus. The uteri of the two 
branches unite into a short muscular tube, 
the vagina, which opens to the outside 
through the vulva. Fertilization takes place 
in the oviduct. The egg is then surrounded 
by a thick, rough-surfaced shell, and passes 
out through the vulva. The genital tubules 
of a female worm may contain as many as 
27 million eggs in various stages of develop- 
ment at one time, and each mature female 
lays about 200,000 eggs per day. 

The eggs of the ascaris are laid inside of 
the intestine of the host and pass out in the 
feces. They are very resistant; if deposited 
on the soil they may remain alive for many 
months. Embryos are formed, under favor- 
able conditions, in about 14 days. Infection 
with the ascaris results from ingesting eggs 
containing embr^'os. The eggs are usually 
carried to the mouth with either food or 
water, or by accidental transfer of soil con- 
taining them. They do not regularly hatch in 
the stomach but pass on to the small intes- 


152 


COLLEGE ZOOLOGY 


Dorsal nerve 


ricie 


Body wall-| Epidermal syncytiu 
Muscle cell 


Ovary 


Growth zone 


(^ Germinal zone 

Rod border of intestine 
Intestinal epithelium 
Lumen of intestine 
Excretory tube 
Lateral line 

Oviduct 
Uterus 

Egg 

Pseudocoel 
Muscle cell process 

Longitudinal muscle fibrils 

Ventral nerve 


Protoplasmic process 

Cytoplasm 

Nucleus 


Contractile muscle fibrils 


Figure 79. Cross section of a female AscaHs in the upper figure. Details of a muscle cell in 
the lower figure. 


tine, where they begin to hatch within a 
few hours. 

The newly hatched larvae burrow into the 
wall of the small intestine and enter the 
veins or lymphatic vessels. If the larvae 
pass into the lymphatics, they are eventually 
carried into the blood and to the right side 
of the heart. They may also be carried in 
the blood to the liver and then to the right 


side of the heart. From here they pass on 
to the lungs, where they pass into the air 
passages, after which they move through the 
trachea, throat, esophagus, and stomach, to 
the small intestine again. This journey 
through the host requires about 10 days. 
They become mature worms in the intestine 
in about IVi months. 
Ascaris worms found in man and pigs are 


PHYLA NEMATHELMINTHES, NEMATOMORPHA, AND ACANTHOCEPHALA 


153 


structurally indistinguishable, but they dif- 
fer physiologically in that the human ascaris 
eggs do not usually produce mature worms 
in pigs and vice versa. 

How does the intestinal 
parasite resist digestion? 

This is a very interesting and perplexing 
question. That they resist digestion when 
alive is common knowledge. However the 
mechanism is by no means clear although 
much research has been done on it. There 
are two schools of thought: (1) that there 
is a passive anti-enzyme action, due to the 
chemical constitution of the parasite, which 
makes it resistant to enzyme action, and (2) 
that the parasite secretes chemical sub- 
stances by means of which the host's diges- 
tive enzymes are neutralized or in- 
hibited. 

In the case of nematodes there is some 
experimental evidence for the production of 
enzyme-inhibiting substances. More specifi- 
cally, there is recent proof of an interfer- 
ence with the digestion of dietary protein 
by intestinal juices because of the anti- 
enzymatic action of the ascaris. This strongly 
suggests that there is an anti-enzyme se- 
creted by the ascaris, which counteracts the 
effect of the host's digestive enzymes. Of 
course, the thick cuticle covering this worm 
provides some protection from the host's 
digestive juices. 

The results of investigations on the resist- 
ance of tapeworms to digestion have been 
inconclusive. 


Free-living roundworms 

The vinegar eel 

The vinegar eel Turbatrix (Anguillula) 
aceti is a free-living nematode, easily ob- 
tained at any time of the year from the bot- 
tom of a cider vinegar barrel. It is visible to 
the naked eye when held before a bright 
light and exhibits characteristic nematode 


movements. Heating the vinegar for a min- 
ute kills and straightens vinegar eels, and 
clearing them in phenol, after fixing, reveals 
the internal organs. 

The female worm is about 2 mm. and 
the male about 1.4 mm. in length. Most of 
the anatomical features of a female are 
shown in Fig. 80. The eggs are not only 
fertilized within the body of the female but 
also develop there. The thin egg membrane 
ruptures in the uterus and the young are 
born in an active condition, that is, the 
vinegar eel is ovoviviparous. One female 
may produce as many as 45 larvae. Males 
and females are equal in number; they may 
live for 10 months or more. 

Other free-living roiindworins 

Free-living nematodes are mostly small, 
a large specimen being only one cm. in 
length. Many of them possess an oral spear 
with which they puncture the roots of 
plants, badly injuring economically valu- 
able species, as well as others. Several hun- 
dred millions of dollars of damage result 
every year from these attacks. They live in 
almost every conceivable type of environ- 
ment, such as wet sand or mud, aquatic 
vegetation, standing or running water, the 
soil, the sea, tap water, fruit juices, and 
moist places almost everywhere. They have 
been found in such varied habitats as Pike's 
Peak, Alpine snows, the Antarctic Ocean, 
and in hot springs. "If all the matter in the 
universe, except the nematodes, were swept 
away, our world would still be dimly recog- 
nizable, and if, as disembodied spirits, we 
could then investigate it, we should find its 
mountains, hills, vales, rivers, lakes, and 
oceans represented by a film of nematodes. 
The location of towns would be decipher- 
able, since for every massing of human be- 
ings there would be a corresponding massing 
of certain nematodes. Trees would still stand 
in ghostly rows representing our streets and 
highways. The location of the various plants 
and animals would still be decipherable, 
and, had we sufficient knowledge, in many 


154 


COLLEGE ZOOLOGY 


Esophagus Body cavity (pseudocoel) 


Seminal receptacle 
Recfum 


Moufh Esophageal Infesfine Egg Uferus Developing Vulva 
bulb larva 

Figure 80. Internal structure of the free-living vinegar eel. Parts of digestive and reproductive 
systems are shown. Natural size 2 mm. in length. 


cases even their species could be determined 
by an examination of their erstwhile nema- 
tode parasites" (Cobb). 

Parasitic plant nematodes 

Nematodes live in or on many different 
kinds of plants, causing enormous economic 
loss. More than 1000 different species of 
nematodes are known to attack plants. The 
damage these tiny parasites do on American 
farms amounts to $500,000,000 a year. The 
common garden roundworm Meloidogyne 
lives in roots of over 1700 species of plants. 
The worms lay their eggs either directly in 
the roots or in the nearby soil. Certain 
nematodes stimulate plant tissue to form 
knotlike galls on the roots. Others enter 
leaves and move about eating the contents 
of the cells. Some worms stay on the surface 
of the plant, bury their heads into the tis- 
sue, and suck out the juice. These parasites 
sap the plant's vigor, open the way for bac- 
teria and fungi, and injure growing points. 
The only means of control of these plant 
parasites is by crop rotation, soil sterilization, 
and development of resistant varieties of 
plants. A lima bean has recently been de- 
veloped by scientists which is nematode re- 
sistant. 

Other parasitic roundworms 

Among the representative roundworms 
that may be found in other vertebrate ani- 
mals are the cecum worm of chickens, the 
dog ascarid, the horse roundworm, and 
horsehair worms. 


The dog ascarid 

Toxocara canis is especially prevalent in 
puppies which became infected through the 
placenta. Dogs become infected by swallow- 
ing the eggs. The larvae migrate through the 
body as do those of Ascaris in man. Dogs 
acquire an immunity as a result of the infec- 
tion; and after three or four months, the 
worms are cast out and susceptibility to fur- 
ther infection is greatly reduced. 

The cecum worm of chickens 

Heterakis gallinae (Fig. 81) lays eggs, 
which are passed in the feces of infected 
birds and are swallowed by other birds; the 
young that hatch from these eggs in the 
small intestine move on into the ceca. They 
do not seriously injure the fowls but are 
of great economic importance since they 
carry with them a protozoan parasite, His- 
tomonas meleagridis, which is the causative 
agent of the disease of turkeys known as 
blackhead. 

The horse roundworm 

Horse strong}'le (Strongylus vulgaris) is 
world-wide in distribution but especially 
prevalent in warm countries. It lives in the 
cecum or colon, attached by the mouth to 
the mucosa, from which it sucks blood. Loss 
of blood results in anemia. The eggs of 
Strongylus are deposited in the feces, where 
they give rise to infective larvae. These, 
when ingested by a horse, migrate to the 
posterior mesenteric artery, where an aneu- 
rysm may be produced; they then move on 
to the cecum where they become encysted 
in the submucosa; and finally they break out 


PHYLA NEMATHELMINTHES, NEMATOMORPHA, AND ACANTHOCEPHALA 


155 


Giant kidney worm, 
Dioctophyma renale, 
from fish-eating 
mammals 


Free-living horsehair worm, 
Paragordius varius 


Figure 81. Roundworms, parasitic and free-living. 


into the lumen, attach themselves to the 
mucosa, and develop into adults. 

PARASTIC ROUNDWORMS 

OF MAN 


Ascaris lumbricoides 

Human beings are parasitized by at least 
45 species of roundworms, some of which 
are widespread and cause great suffering and 
thousands of deaths annually. Ascaris lum- 
bricoides is an important human parasite. 
One survey in the tropics showed that in 
over 200 natives studied, only one did not 
suffer from ascariasis. \Vhen large numbers 
of the ascaris larvae pass through the lungs, 
inflammation is set up and generalized pneu- 
monia may result. The adults may be pres- 
ent in the intestine in such large numbers 
as to produce fatal intestinal obstruction. 
One thousand to five thousand worms have 
been recorded in a patient, but even a hun- 


dred worms can cause a blockage that is 
fatal. One of the most frequent complaints 
of a patient suffering from ascariasis is 
abdominal pain or discomfort. Nervous 
svmptoms such as headache or convulsions 
may appear as a result of the secretion of 
toxic substances by the worms. Fortunately, 
several drugs are available which remove 
the worms. The amount of infection with 
ascarids is a measure of the sanitation pres- 
ent in the region. In some areas and eco- 
nomic strata in the United States there is 
still considerable worm infection. Ascariasis 
occurs more frequently in children than in 
adults because of the carelessness of chil- 
dren with regard to sanitary matters. Infec- 
tion can be prevented by enforcing sanitary 
practices. 

Hookworms 

The hookworms, Ancylostoma duodenale, 
and Necator americanus (Fig. 82) are also 
widespread and injurious; about 95 per cent 


156 


COLLEGE ZOOLOGY 


of the hookworms in the United States are 
of the latter species. The larvae develop in 
moist earth and usually find their way into 
the bodies of human beings by boring 
through the skin of the foot. They enter a 
lymph or blood vessel and pass to the 
heart; from the heart they reach the lungs, 
where they make their way through the 
air passages into the windpipe (trachea), 
and thence into the intestine. The adults 
attach themselves to the walls of the intes- 
tine and by suction they feed upon the blood 
and tissue juices. In the case of the dog 
hookworm and probably also of the human 
hookworm, blood is continuously being 
sucked into the body of the worm and ex- 
pelled from the anus in the form of drop- 
lets, consisting mainly of red corpuscles. 
Calculations indicate that a single worm 
may withdraw blood from the host at the 
rate of 0.8 ml. in 24 hours. When the in- 
testinal wall is punctured, a small amount of 
poison is poured into the wound by the 
worm. This poison prevents the blood from 
coagulating and therefore results in a con- 
siderable loss of blood, even after the worm 
has left the wound. The victims of the 
hookworm are anemic and subject to other 
diseases because of malnutrition. Hookworm 
disease is not as serious in this country as 
it once was, although there is still some in- 
fection in the southeastern coastal states. 
However, hookworm disease is very preva- 
lent in large areas of the tropics where soil 
and climate favor these parasites. In fact, 
hookworm is considered the most impor- 
tant parasitic intestinal worm of man. 
Hookworm disease can be cured by several 
drugs, but tetrachloroethylene or hexylresor- 
cinal are commonly used by physicians. The 
most important preventive measure is the 
disposing of human feces in rural districts, 
mines, brickyards, etc., in such a manner as 
to avoid pollution of the soil, thus giving 
the eggs of the parasites contained in the 
feces of infected human beings no oppor- 
tunity to hatch and develop to the infective 
larval stage. 


Trichina worms 

Trichinella spiralis causes the disease of 
human beings, pigs, and rats which is called 
trichinosis. Estimates in 1953 placed the 
number of persons in the United States in- 
fested with trichinae at several million, un- 
diagnosed cases at several hundred thou- 
sand, and animal deaths at several thousand. 
The parasites enter the human body when 
inadequately cooked meat from an infected 
pig or bear is eaten (Fig. 83). The larvae 
soon become mature in the human intestine, 
and each mature worm deposits from 1500 
to 2500 living larvae. These larvae are either 
placed directly into the lymph or blood ves- 
sels by the female worms, or they burrow 
through the intestinal wall; they eventually 
encyst in muscular tissue in various parts 
of the body. As many as 15,000 encysted 
parasites have been counted in a single gram 
of muscle. 

Pigs acquire the disease chiefly by eating 
restaurant meat scraps and slaughterhouse 
garbage; however, infected rats may be a 
source of infection also. The incidence of 
trichinosis among hogs fed with raw garbage 
is almost 20 times higher than the inci- 
dence among other hogs. In this country, 
it has not been found practical for the gov- 
ernment to inspect pork for the trichina 
worm. The only protection is to avoid eat- 
ing pink pork. Pink color in freshly cooked 
pork is evidence of inadequate cooking. 

It has been stated that there is a death 
rate of about 5 per cent of those who ac- 
tually show symptoms for trichinosis; specific 
treatment is lacking. ACTH hormone and 
cortisone provide relief from symptoms and 
may prevent a fatal outcome. Trichinosis 
seems to be a greater problem in the United 
States than in most countries. However, it 
is on the decrease because of a relatively 
recent law (1953), in most states, which re- 
quires the cooking of garbage before feeding 
it to hogs. Recent experiments have shown 
that a dose of 25,000 rep (rep is a unit of 
radiation) of gamma radiation is sufficient 


Hookworm, 
Necator amerkanus, 
from intestine 


Hookworm 
embryo 


Whipworm, 
Trichuris trkhiura, 
from cecum 


Guinea worm, Dracunculus 
medinensis, being wound 
on a stick 


Figure 82. Some roundworms that live in man. 


157 


158 


COLLEGE ZOOLOGY 


Adult female becomes 
embedded in small 
intestine of host and 
produces larvae 


Infected pork 
scraps 


Larva enters lymph 
or blood vessels 


MuscU 


Encysted larva 


Poorly cooked or raw infected meat is 
eaten by a new host (man, pig, rat); 
larvae are freed and become adults 


Figure 83. Life cycle of Trichinella. 


to destroy the Trichinella larvae in pork and 
make it safe for human consumption even 
though the meat is not thoroughly cooked. 
Further studies are needed to determine the 
practicality of radiation in the control of 
trichinosis. 

Pinworms 

The pinworm of man, Enterobius ver- 
micularis (Fig. 82), measures from 9 to 12 
mm. in length, is world-wide in distribution, 
and lives in the adult stage in the upper 
part of the large intestine. Pinworm infec- 
tion is nearly a universal experience of man- 
kind in infancy, since nearly all children 
become infected. Most cases show no symp- 
toms and many children get over the disease 
without treatment. In some cases, however, 
the infection persists, even into adulthood. 
Sometimes over 5000 worms are present in 
a single person. Sample surveys of white 
children in the United States and Canada 
revealed that 30 to 60 per cent were in- 
fected. Colored races are less susceptible. 
Species of pinworms, closely related to the 


one that lives in man, occur in monkeys 
and apes. 

Whipworms 

Trichuris trichiura (Fig. 82) lives primarily 
in the cecum and appendix of man. Its 
body is draw^n out anteriorly into a long 
slender, whiplike process. There is no inter- 
mediate host. The eggs escape in the feces 
and ripen outside of the body. Ripe eggs 
when swallowed hatch in the intestine, and 
the larvae become located in the cecum. 
Heavy infections may cause abdominal dis- 
comfort, anemia, and bloody stools. It has 
been estimated that there are 355.1 million 
people in the world infected with whip- 
worms. Sanitary disposal of human excre- 
ment breaks the life cycle and prevents the 
spread of this parasite. 

Filaria worms 

Wuchereria bancrofti is a species of filaria 
that is widely spread in tropical countries. 
The larvae of this species are about i-^oo 


PHYLA NEMATHELMINTHES, NEMATOMORPHA, AND ACANTHOCEPHALA 


159 


inch long (Fig. 82). Mosquitoes, which are 
active at night, suck up these larvae with 
the blood of the infected person. The larvae 
of the worms develop in the body of a mos- 
quito, make their way into the mouth parts, 
and enter the blood of the mosquito's next 
victim. From the blood they migrate to the 
lymphatics, where they become adults and 
obstruct the lymph passages, often causing 
serious disturbances. This may result in a 
condition called elephantiasis (Fig. 84). 
The limbs, scrotum, or other regions of the 
body swell up to an enormous size. Infec- 
tion with this parasite is common in man, 
especially in the South Pacific islands, the 
West Indies, South America, and west and 
central Africa. A recent survey on St. Croix, 
one of the Virgin Islands, showed 16 per 
cent of the people suffering with filariasis. 
Thousands of World War II service men 


Figure 84. Elephantiasis due to Wuchereria ban- 
crofti. This woman lives on a South Pacific Island. 
The infection started in the left arm when she was 
33 years of age, and at 38 both arms and legs were 
affected. This photograph was taken at the age of 
43. The filaria worms block che lymph vessels, which 
results in the diversion of lymph into the tissues 
and the enormous growth of connective tissue. 
(Courtesy of W.A. Robinson.) 


contracted filariasis in the islands of the 
South Pacific. These men made good re- 
covery and none showed the severe symp- 
toms exhibited in old chronic cases among 
native inhabitants. Another interesting spe- 
cies of filaria is the eye worm, Loa loa, of 
West Africa (Fig. 82). The adult migrates 
around the body through the subdermal con- 
nective tissue and sometimes across the eye- 
ball. No severe pathological lesions are pro- 
duced. 

Guinea worm 

Dracunculus medinensis, the guinea 
worm, is a common human parasite in 
tropical Africa, Arabia, India, South Amer- 
ica, and the West Indies. It has been known 
for centuries and is probably the "fiery 
serpent" mentioned by Moses (Numbers 
21). The adult female, which may reach a 
length of over three feet, is usually located 
in the subcutaneous tissue of the arms, legs, 
and shoulders. The young larvae are dis- 
charged from the worm and escape through 
an opening in the human skin when that 
part of the body is submerged in water. The 
larvae may be eaten by the water flea 
[Cyclops), and man becomes infected by 
swallowing the water fleas in drinking water. 
The method of extracting the worm, prac- 
ticed by natives for hundreds of years, is to 
roll it up gradually on a stick, a few turns 
each day, until the entire worm has been 
drawn from the body (Fig. 82). Serious 
poisoning of the host occurs if the worm is 
broken. 

Beneficial nematodes 

Research has discovered a beneficial 
roundworm that attacks insects, and there 
is good evidence that this nematode can be 
used in pest control because it carries a type 
of bacteria that quickly kills many insects. 
The nematode acts as a microsyringe to in- 
troduce the bacterium into the infected in- 
sect's body cavity. This bacterium has proved 
deadly not only to codling moths but to at 


160 


COLLEGE ZOOLOGY 


least 35 other kinds of insects, including 
the corn earworm, boll weevil, pink boll- 
worm, vegetable weevil, cabbage worm, and 
white fringed beetle. 

Origin and relations of 
tFie Nemathelminthes 

The Nemathelminthes seem to occupy a 
rather isolated position in the animal series. 
In many respects they resemble the platy- 
helminthes; they are unsegmented and pos- 
sess excretory canals but no flame cells. 
There is an absence of cilia. In the nema- 
todes, we encounter, for the first time, ani- 
mals with two openings in the digestive 
tract, a mouth and an anus. The sexes are 
usually separate, whereas in the Platyhel- 
minthes hermaphroditism is the rule. The 
parasitic roundworms evolved from free- 
living roundworms. 


crickets to the water is problematical. How- 
ever, one investigator discovered that when 
crickets found near the edge of a lake were 
placed in the water, a horsehair worm in- 
variably escaped from the cricket's body in 
less than a minute, but crickets collected 
100 to 300 feet from the lake did not yield 
worms. Paragordius varius is a horsehair 
worm common in North America (Fig. 
81). 

The Nematomorpha are like the Nema- 
thelminthes because of body form, presence 
of cuticle, simple musculature, and absence 
of segmentation. However, in Nematomor- 
pha there are important differences, such 
as a body cavity which is nearly filled with 
parenchyma, a degenerate digestive tract in 
adults, a single nerve cord, and a cloaca. 
No circulatory, respiratory, or excretory or- 
gans are present in the Nematomorpha. The 
physiology of this group is not well under- 
stood. 


PHYLUM NEMATOMORPHA 
OR HORSEHAIR WORMS 

The horsehair worms 

The name "horsehair" comes from a 
popular superstition, not yet dead, that this 
roundworm develops from horsehairs that 
fall into water. Actually their life cycle is 
as follows. The adults live in fresh water 
where the eggs are laid. The larv^ae that 
hatch from the eggs penetrate the body of 
some aquatic insect larva. These worms 
migrate to the body cavity and continue to 
develop until they escape as young adults 
or juveniles. They have been found in dif- 
ferent stages of development in the body 
cavities of beetles, crickets, and grass- 
hoppers, all of which are terrestrial, suggest- 
ing that these land forms may eat aquatic 
insects containing the minute roundworm 
larvae. Infested crickets appear to migrate to 
the edge of water, and, if caught by a wave, 
Paragordius emerges from their bodies 
within 20 to 50 seconds. What brings the 


PHYLUM ACANTHOCEPHALA 
(SPINY-HEADED WORMS) 

The spiny-headed worms 

These peculiar parasitic worms (Fig. 85) 
belong to the phylum Acanthocephala, a 
name that means spiny-headed and refers 
to a retractile proboscis armed with rows of 
recurved hooks. They live in the intestine 
of vertebrates and are attached to the wall 
by a protrusible proboscis covered with re- 
curved hooks; they vary in length from less 
than an inch to more than a foot. The body 
of most species is elongate, flattened, and 
capable of extension. No digestive tract is 
present at any stage in their life cycle, food 
being absorbed directly from the host's in- 
testine. The sexes are separate, and the re- 
productive systems are complex. Species 
have been reported in the United States 
from fish, turtles, birds, rats, mice, pigs, 
squirrels, dogs, and man. 

The Acanthocephala differ from the Nem- 


PHYLA NEMATHELMINTHES, NEMATOMORPHA, AND ACANTHOCEPHALA 


161 


Proboscis with hooks 


Lemnisci 


Testes 


Body cavity 


Figure 85. Spiny-headed worm, Acanthocephala: internal structure. These intestinal parasites 
are found in all classes of vertebrates. Occasionally the worms cause perforation of the gut of 
the host, resulting in its death. 


athelminthes in the absence of a digestive 
tract, presence of a proboscis, circular mus- 
cles, ciliated excretory organs, and certain 
peculiarities in the reproductive organs. 


CLASSIFICATION OF THE 

NEMATHELMINTHES, 

NEMATOMORPHA, AND 

ACANTHOCEPHALA 

{For reference purposes only) 

Phylum Nemathelminthes (Aschelminthes). 

These roundworms are unsegmented, three- 
germ-layered (triploblastic), bilaterally sym- 
metrical animals. The body is cylindrical and 
elongate with tough resistant cuticle. Digestive 
tract is a straight tube with mouth and anus 
at opposite ends of the body. Body wall with 
longitudinal muscles only; space within body 
is a pseudocoel. There are no respiratory nor 
circulatory organs; excretory organs are simple, 
consisting of one, two, or none. The nerve ring 
around the esophagus connects with other 
nerves or cords. No cilia are present and the 
sexes are usually separate. One class and 5 
orders are listed here as follows: 

Class 1. Nematoda. With intestine but no 
proboscis. Body cavity not lined with 


epithelium; gonads continuous with 
their ducts; no cloaca in female; 
lateral lines present. 

Order 1. Ascaroidea. Free-living or para- 
sitic; with three prominent lips. 
Ex. Ascaris lumbricoides (Fig. 
78). 

Order 2. Strongyloidea. Parasitic; male 
with copulatory bursa supported 
by muscular rays; esophagus 
club-shaped and without pos- 
terior bulb. Ex. Necator Ameri- 
canus (Fig. 82). 

Order 5. Filarioidea. Parasitic; filiform 
worms without lips; esophagus 
without bulb, the anterior por- 
tion being muscular and the 
posterior glandular. Ex. Wuc/ic- 
reria bancrofti (Fig. 82). 

Order 4. Dioctophymoidea. Parasitic; 
moderate to very long nema- 
todes; mouth without lips, sur- 
rounded by 6, 12, or 18 papillae; 
esophagus elongated without 
bulb. Ex. Dioctophyma renale 
(Fig. 81). 

Order 5. Trichuroidca or Trichinelloidea. 
Parasitic. Body filiform anteri- 
orly; mouth without lips; eso- 
phageal portion of body more 
or less distinct; esophagus a 
cuticular tube embedded in a 


162 


COLLEGE ZOOLOGY 


single row of cells. Ex. Trichi- 
nelld spiralis (Fig. 83). 

Phylum Nematomorpha or Gordiacea, Very 
long slender cylindrical worms; gonoducts in 
both sexes enter the intestine; lateral nerve 
cords absent. Parasitic as juveniles, adult free- 
living. Ex. Paragordius (Fig. 81). 

Phylum Acanthocephala. Parasitic; without 
intestine but with spiny proboscis. Ex. Lep- 
torhynchoides thecatus (Fig. 85). 


SELECTED COLLATERAL 
READINGS 

Baer, J.G. Ecology of Animal Parasites. Univ. 
of Illinois Press, Urbana, 111., 1952. 


Chandler, A.C. Introduction to Parasitology. 
Wiley, New York, 1955. 

Chitwood, B.C. and others. An Introduction 
to Nematology. Monumental Pub. Co., 
Baltimore, 1950. 

Goodey, T. Plant Parasitic Nematodes. Dut- 
ton,'New York, 1933. 

Hull, T.G., and others. Diseases Transmitted 
from Animals to Man. Thomas, Springheld, 
111., 1946. 

Hyman, L.H. The Invertebrates: Acanthoce- 
phala, Aschelminthes and Entoprocta. Mc- 
Graw-Hill, New York, 1951. 

Pearse, A.S. Introduction to Parasitology. 
Thomas, Springfield, 111., 1942. 

Shipley, A.E. "Nemathelminthes." Cambridge 
Natural History, Vol. 2. Macmillan, London, 
1896. 


A 


CHAPTER 14 


i 1 


Oil 


Miscellaneous 
Minor Phyla 


NUMBER of groups of animals are con- 
sidered together here because their relation- 
ships to other animals and to each other are 
rather uncertain. Tliis chapter is something 
of an invertebrate catchall; it contains sev- 
eral groups that in prehistoric times were 
large, but at present are represented by 
relatively few types. However, an introduc- 
tion to general zoology would be incomplete 
without at least a look at these interesting 
though highly specialized groups. 

PHYLUM NEMERTINA 

(RHYNCHOCOELA)- 

RIBBON WORMS 

The members of the phylum are called 
ribbon worms because they are long and 
flattened dorsoventrally (Fig. 86). They 
range in length from less than an inch to 
over 90 feet. Most live in the sea, but a few 
inhabit fresh water and land. 

The most important anatomic features of 
the nemertines are the presence of (1) a 
long retractile proboscis, which lies in a 
proboscis sheath just above the digestive 
tract and may be everted and used as a 
tactile, and a defensive organ; (2) a blood 
vascular system, usually consisting of a me- 
dian dorsal and two lateral trunks; and (3) 
a complete digestive tract with both mouth 
and anal openings. The circulator}' system 
is encountered here for the first time. 

The nemertines resemble the free-living 
flatworm or Turbellaria in being bilaterally 
symmetrical, in having flame cells, unseg- 
mented and contractile bodies, and in lack- 
ing a true coelom and respirator}' system. 
However, they differ from the flatworms in 
that they have a complete digestive system 
with mouth and anus, a functional cir- 
culatory system, a less complex reproduc- 
tive system, and they are usually dioecious. 
Nemertines feed on other animals, both 
dead and alive. They live, as a rule, coiled 
up in burrows in mud, sand, or under stones, 
but some of them frequent patches of sea- 

163 


164 


COLLEGE ZOOLOGY 


Normal worm 


Id 


Figure 86. Lineus: regeneration in a ribbon worm. A, section cut from body. B, its appearance 
after 12 days. C, the same after 30 days, cut in 3 pieces. Successive cuttings and regenerations 
indicated by arrows. This worm occurs around Long Island Sound. (After Coe.) 


weed. Locomotion is effected by the cilia 
which cover the surface of the body, by 
contractions of the body muscles, or by the 
attachment of the proboscis and a subse- 
quent drawing forward of the body. The 
adults have great powers of regeneration 
and some reproduce in warm weather by 
fragmentation of the body (Fig. 86). If, for 
example, Lineus socialis, which is only 100 
mm. in length, is cut into as many as 100 
pieces, each piece will regenerate a minute 
worm within four or five weeks. These mi- 
nute worms may again be cut into pieces 
that regenerate, and these in turn may be 
cut up, and so on, until miniature worms 
result which are less than ^-200.000 the vol- 
ume of the original worm. 

PHYLUM ROTIFERA 
(ROTATORIA) 

The Rotifera or Rotatoria are commonly 
known as wheel animals (Fig. 87) . The com- 
mon name refers to the beating of the cilia 
on the head, which suggests the rotation of 
wheels. These cilia aid in locomotion and 
draw food into the mouth. The rotifers are, 
generally speaking, the smallest of the Meta- 


zoa. Because the rotifers possess fantastic 
forms and brilliant colors they usually at- 
tract the attention of amateur microsco- 
pists. Most are inhabitants of fresh water, 
but some are marine, and a few parasitic. 
The tail-like foot is often forked and ad- 
heres to objects by means of a secretion 
from the cement glands. The body is usu- 
ally cylindrical and is covered by a shell-like 
cuticle. 

Protozoa, other minute organisms, and 
debris, used as food, are swept by the cilia 
through the mouth into the pharynx, the 
lower end of which forms the very charac- 
teristic grinding organ called the mastax. 
Here chitinlike jaws, which are constantly 
at work, break up the food. The movements 
of these jaws easily distinguish a living 
rotifer from other animals. A short esoph- 
agus leads into the stomach. The food 
is digested in the glandular stomach, or in 
the stomach and intestine, depending on 
the species. Undigested particles pass 
through the intestine into the cloaca and 
out of the cloacal opening ("anus"). The 
excretory system consists of two long tubes 
with flame cells at intervals along their 
sides. These tubes open into a bladder 
which contracts at intervals, forcing the 


MISCELLANEOUS MINOR PHYLA 


165 


Dorsa! view 


Lateral view 


Figure 87. Phylum Rotifera. General structure of a female rotifer. Left, dorsal view. Right, 
lateral view of the same animal. 


contents into the cloaca and then out the 
cloacal opening. This bladder, besides being 
excretory in function, doubtless serves to 
maintain a proper water balance in the ani- 
mal, just as the contractile vacuole does in 
the protozoans. 

The sexes of rotifers are separate. The 
males are known in only a few species; and 
where found, they are usually smaller than 
the females, and often degenerate. Two 
kinds of eggs are laid: the summer eggs, 
which develop parthenogenetically, are 
thin-shelled and commonly of 2 sizes; the 
>arger produce females and the smaller pro- 


duce males. The winter eggs, which are fer- 
tilized, develop into females and have thick 
shells which protect the contents during un- 
favorable weather. 

One peculiarity of rotifers is their power 
to resist desiccation. Certain species, if 
dried slowly, secrete gelatinous envelopes 
which prevent further drying; in this condi- 
tion they live through seasons of drought 
and may be subjected to extremes of tem- 
perature without dying. 

The resemblances between certain rotifers 
and the trochophore larvae of certain mol- 
lusks, annelids, and other animals to be 


166 


COLLEGE ZOOLOGY 


described later is quite striking. This has 
led to the theory that the rotifers are ani- 
mals somewhat closely related to the ances- 
tors of the mollusks, annelids, and certain 
other groups. However, some of the most 
competent investigators believe that these 
resemblances of certain trochophore larvae 
are purely coincidental, the result of adapta- 
tive radiation and of no evolutionary signifi- 
cance. It appears more likely that the rotifers 
have originated from a primitive turbel- 
larian. 

Some common rotifers are Epiphanes 
senta (formerly called Hydatina senta), a 
species used widely for experimental pur- 
poses; Asplanchna, which often occurs in 
enormous numbers in the plankton of the 
Great Lakes; Floscularia, which lives in a 
transparent tube and has a beautiful corona 
with 5 knobbed lobes; Melicerta which 
builds for itself a tube of spherical pellets; 
and Philodina (Fig. 87), characterized by a 
slender rose-colored body. 

The rotifers eat microscopic organisms 
which they convert into their own tissues. 
In turn the rotifers may serve as food for 
larger species and eventually, through fish, 
serve as food for man. Thus the rotifers may 
serve an important part in a fresh-water food 
chain. 


PHYLUM BRYOZOA 
(POLYZOA) 

The Bryozoa, a name that means "moss 
animals," are so-called because they appear 
plantlike. They are mostly colonial, and 
resemble hydroids in form, but they are 
more advanced in internal structure (Fig. 
88). The majority of them live in the sea, 
but a few inhabit fresh water. Bugula is a 
common marine genus, and Plumatella is 
the most common fresh-water genus. A col- 
ony of Plumatella is made up of cylindrical, 
more or less branched, tubes. These tubes 
protect the soft parts of the body. The an- 
terior end of the body of Plumatella consists 


of a rounded ridge called a lophophore; this 
bears a horseshoe-shaped double row of 
tentacles. These tentacles are from 40 to 60 
in number, hollow, and ciliated. When 
these tentacles are spread out in the water, 
the cilia cause currents that sweep micro- 
scopic food organisms into the mouth. The 
mouth, esophagus, stomach, cecum, intes- 
tine and anus of Plumatella are shown in 
Fig. 88. Between the digestive tract and the 
body wall is a true coelom which is lined 
with a peritoneum. There are no respiratory, 
circulatory, or excretory organs. Bryozoans 
are hermaphroditic. The larvae of some of 
them resemble a trochophore (Fig. 107). 
This suggests an ancient origin from some 
annelid stock. 

Certain fresh-water Bryozoa produce disk- 
like buds (Fig. 88) which secrete a hard 
chitinous shell and are known as statoblasts. 
These survive when the animal dies in the 
fall or during a drought, giving rise to a new 
colony in the spring or when the wet season 
returns. 

Of special interest is the fouling of pipes 
by certain fresh-water bryozoans. They form 
thick crusts inside pipes, and dead colonies 
sometimes break loose, become fragmented, 
and clog small pipes and meters. 

In prehistoric times there were many more 
species than are living today. Since their 
first appearance in the Cambrian period (an 
early geologic period ) , bryozoans have made 
substantial contributions to layers of cal- 
careous rock in every geologic period. 

PHYLUM BRACHIOPODA 
(LAMP SHELLS) 

The Brachiopoda are marine animals liv- 
ing within a calcareous bivalve shell (Fig. 
89). They are usually attached to some ob- 
ject by a muscular stalk called the peduncle. 
Because of their shell, they were long re- 
garded as mollusks. The valves of the shell, 
however, are dorsal and ventral instead of 
lateral as in the bivalve mollusks. The name 


MISCELLANEOUS MINOR PHYLA 


167 


Statoblast of 
Plumaiella 


Lophopfiore 
Mouth 

Anus 


Infestlne 


Stomach 


Cecum 


Statoblast 


Plumafella 


Figure 88. Phylum Bryozoa. Plumatella, a com- 
mon fresh-water bryozoan. Below, drawing showing 
the structure of a single individual (a zooid). En- 
larged. Above, statoblast of Plumatella. 

"lamp shell" refers to the resemblance of 
the shells to the oil lamps of the Romans. 
Within the shell is a conspicuous structure, 
the lophophore, which consists of t^vo coiled 
ridges, called arms; these bear ciliated ten- 
tacles. Food is drawn into the mouth by 
the lophophore. A true coelom is present, 
within which lie the stomach, digestive 
gland, short blind intestine, and the 
"heart." 

The group Brachiopoda is extremely old, 
dating since Cambrian time; and, although 
found in all seas today, brachiopods were 
formerly more numerous in species and of 


much greater variety in form than at present. 
Some of them, for example Lingula, are ap- 
parently the same today as they were in the 
Ordovician period, estimated at over 400 
million years ago. Lingula (see headpiece, p. 
163) is thought to be the oldest animal 
genus known; it is called a "living fossil" 
because it has not changed, on the outside 
at least, during long geologic periods. 


PHYLUM CHAETOGNATHA 
(ARROW WORMS) 

The Chaetognatha are marine animals 
which swim about near the surface of the 
sea (Fig. 89). The best-known genus is 
Sagitta, the arrow worm. Tlie bilaterally 
symmetrical body consists of three regions, 
head, trunk, and tail. Lateral and caudal 
fins are present. There is a distinct coelom, 
a digestive tract with mouth, intestine, and 
anus, a well-developed nervous system, two 
eyes, and other sensory organs. There are 
no circulatory, respirator}-, or excretory or- 
gans. The mouth has a lobe on either side 
provided with bristles which are used in 
capturing the minute animals and plants 
that serve as food. The members of the 
group are hermaphroditic. Many species are 
very widely distributed. 

PHYLUM GASTROTRICHA 

To this group belong certain microscopic 
animals that live in both fresh and salt 
water and are often abundant among algae 
and debris upon which they feed. They 
range in length from 0.06 to 1.5 mm. The 
gastrotrichs resemble some ciliate protozoans 
(Fig. 89). The body is indistinctly divided 
into head, neck, trunk, and toes. The 
mouth, which is at the anterior end, is sur- 
rounded by oral bristles; locomotion is ac- 
complished by longitudinal bands of cilia 
on the ventral surface. On the dorsal surface 
there are many slender spines. The intestine 


168 


COLLEGE ZOOLOGY 


Phylum 
Brochiopoda 


Phylum 

Gastrotricha 

Chaefonoius 


Phylum 
Echinodera 
Echinoderes dujardinl 


Bristtes 

Brain 

Mouth 


Ventral 
ganglion 


fin- 


V 

Phylum 

Phoronidea 

PAoronis 


Oviduct 
Ovary 

Intestine 


Female genital 

pore 

Anus 


Testis 

Male genita 
pore 


Phylum 

Mesozoa 

R/jopofuro 


Phylum 
Chaetognatha 
Sagitfa hexaplera 


Figure 89. Phylum Brachiopoda. Ventral view of the shell (one-half natural size). Phylum 
Mesozoa; Rhopalura, a parasite of the brittle star. Phylum Gastrotricha; Chaetonotus, a free- 
living species. Phylum Phoronidea; Phoronis, removed from its tube. Phylum Echinodera; Echino- 
deres dujardini, a marine species. Phylum Chaetognatha; Sagitta hexaptera, an arrow worm 
(6 mm. in length). 


is a straight tube leading to an anus near 
the posterior end of the body. The excretory 
organs are a pair of coiled tubes with a 
flame cell at the inner end of each. The 
eggs are very large. There is no larval stage. 
About 200 species of Gastrotricha are 
known. Chaetonotus (Fig. 89) is a typical 
gastrotrich. 


mals of sedentary habit, that live in tubes. 
The larva, called an actinotrocha, is free- 
swimming and resembles a trochophore. The 
adults are unsegmented, coelomate, and 
hermaphroditic. They possess a horseshoe- 
shaped lophophore, U-shaped digestive tract, 
two ciliated nephridia, and a vascular sys- 
tem which contains red blood corpuscles. 


PHYLUM PHORONIDEA 

Most of the species in this group, about 15 
in all, belong to the genus Phoronis (Fig. 
89). They are small, wormlike, marine ani- 


PHYLUM KINORHYNCHA 
(ECHINODERA) 

The Echinodera are very small marine 
worms that range from 0.18 to 1 mm. in 


MISCELLANEOUS MINOR PHYLA 


169 


length. They Hve in the mud or sand on the 
bottom of either deep or shallow water. The 
body consists of a series of 13 or 14 rings, 
two of which form the head, which is en- 
circled by spines and has a short retractile 
proboscis. There are two excretory' organs, 
each consisting of a flame cell connected to 
a flagellated ciliated duct opening dorsally 
on ring 9. Echinoderes dujardini (Fig. 89) 
is reddish in color; it lives in mud and less 
often among algae in the north Atlantic 
Ocean. 


PHYLUM MESOZOA 

These are small slender animals, with the 
simplest structures of any metazoan. They 
are parasites; and their simplicity may be 
partly the result of modifications due to a 
parasitic existence. They live in the internal 
spaces and tissues of squids, flatworms, star- 
fishes, annelids, and other invertebrates. The 
body consists of an outer layer of cells en- 
closing one or more reproductive cells. The 
life cycle is complicated by an alternation 
of sexual and asexual generations. Dicyema 
lives in the nephridia of the octopus, and 
Rhopalura (Fig. 89) parasitizes the gonads 
of the brittle star. 

The Mesozoa resemble some colonial 
protozoans in that they have external cilia, 
digestion which occurs in external cells, 
and special reproductive cells as in Volvox. 
The Mesozoa are unlike the typical meta- 


zoans in that their two-cell layers are not 
comparable with the ectoderm and en- 
doderm of typical metazoan animals, and 
they have no internal digestive tract. The 
name Mesozoa implies that these organisms 
are intermediate between the Protozoa and 
Metazoa, which, indeed, they may actually 
be. Either they are intermediate between 
unicellular and multicellular animals or else 
degenerate forms. Some of the best authori- 
ties believe that their characters are chiefly 
primitive and not the result of parasitic 
degeneration. 


SELECTED COLLATERAL 
READINGS 

Bassler, R.S. "The Bryozoa, or Moss Animals." 
Smithsonian Inst. Ann. Rept., 1920. 

Hyman, L.H. The Invertebrates: Acanthoce- 
phala, Aschelminthes, and Entoprocta. Mc- 
Graw-Hill, New York, 1951. 

Johnson, M.E., and Snook, H.J. Seashore Ani- 
mals of the Pacific Coast. Macmillan, New 
York, 1927. 

MacGinitie, G.E., and MacGinitie, N. Natural 
History of Marine Animals. McGraw-Hill, 
New York, 1949. 

Michael, E.L. "Classification and Vertical Dis- 
tribution of the Chaetognatha of the San 
Diego Region." Univ. Calif. Pub. 7.ool., 
1911. 

Ward, H.B., and Whipple^ G.C. Freshwater 
Biology. Wiley, New York, 1918. (New 
edition in press.) 


A^ 


CHAPTER 15 


pps"- 


Phylum Annelida. 
Segmented Worms 


lnnelids (Fig. 90) are usually called seg- 
mented worms in order to distinguish them 
from flatworms and roundworms, which are 
not segmented. The body consists of a 
linear series of similar parts, which are 
known as segments, somites, or metameres. 
These are usually visible externally as rings; 
the rings of an earthworm's body and the 
vertebrae of man's backbone are evidences 
of segmentation. 

Most annelids are marine, but many live 
in fresh water, in the soil, or in other moist 
places. Earthworms, sandworms, and leeches 
are common examples. Most everyone is 
familiar with the common earthworm for it 
is distributed all over the earth except in 
regions where the soil is nearly pure sand 
and in mountain regions where the soil is 
scanty and poor. For the most part earth- 
worms are nocturnal. During the day they 
are usually hidden in their burrows, but at 
night they come out to feed. Apart from its 
being not difficult to obtain, the earthworm 
provides an opportunity for studying annelid 
characteristics under advantageous condi- 
tions. 

Metamerism, both external and internal, 
is very conspicuous; the coelom is large and 
obvious; several systems of organs, such as 
the circulatory and nervous systems, are well 
developed; and the details of behavior, re- 
generation, and embryonic development are 
well known. Several other annelids are very 
briefly described; included in these are the 
sandworm, Neanthes virens,"^ which is rep- 
resentative of the class Polychaeta, and the 
leech, Hirudo medicinalis, of the class Hiru- 
dinea. 


170 


* There seems to be a rather common misconcep- 
tion that Neanthes is the generic name for a group 
of annehds formerly called Nereis. Both genera 
Neanthes and Nereis are old, well established, with 
many species attributed to each. Zoologists have 
sometimes identified specimens as Nereis when more 
critical determination would have shown that 
Neanthes was correct. Figure 99 was drawn from 
an actual specimen which came from a container 
labeled Nereis, but it was positively identified by 
Olga Hartman as Neanthes. 


PHYLUM ANNELIDA. SEGMENTED WORMS 


171 


OUGOCHAETA 
(Earthworm) 


POLYCHAETA 
(Lugworm) 


HIRUDINEA 
(Leech) 

Figure 90. Representatives of 4 classes of segmented worms. They are varied in form and 
widely distributed. The figures are not drawn to scale. 


LUMBRICUS TERRESTRIS- 
AN EARTHWORM 

The common earthworm, Lumbricus ter- 
restris, serves well to illustrate the principal 
characteristics of the annelids. Figure 91 
shows many of the structural features of a 
segmented worm. 

Earthworms are soft and naked, and hence 
must live in moist earth; for this reason also 
they venture out of their burrows chiefly on 
damp nights. They are never "rained down" 
but are "rained up" out of their burrows 
when these are flooded. The burrows usually 
extend about two feet underground. Earth- 


worms can force their way through soft 
earth, but must eat their way through 
harder soil. The earth which has been eaten 
passes through the digestive tract and is 
deposited on the surface as castings. 

External anatomy 

The body of Lumbricus is cylindrical and 
varies in length from about 6 inches to 1 
foot. The ventral surface is slightly flat- 
tened, and the dorsal surface is darker col- 
ored than the ventral surface. The segments 
(somites), of which there are over 100, are 
easily determined externally because of the 


Prostomium 


Mouth cavity 


Cerebral gangffon "brain' 
Circumpharyngeal connective 

Pharynx 
Pharyngeal muscle 


Septum 
"Heart" 

Nephridium 

_ Nephridiopore 

Seminal receptacle 
Esophagus 

Testis 
Sperm funne 

Seminal vesicle 

Cut edge of seminal vesicle 

Vas efferent 


Intestine 


Typhlosole 


Vas deferens 
Ovary 
Egg funne 

Oviduct 
Egg sac 
Crop 

Dorsal blood vessel 


Gizzard 


Ventral nerve cord 


Figure 91. Anterior part of an earthworm with dorsal body wall removed to show the 
internal organs. 


172 


PHYLUM ANNELIDA. SEGMENTED WORMS 


173 


grooves extending around the body. At the 
anterior end a fleshy lobe, the prostomium 
(Fig. 94), projects over the mouth; this is 
not here considered a true segment, although 
some authors regard the prostomium as the 
first true segment. It is customar)' to number 
the segments, beginning at the anterior end, 
since both external and internal structures 
bear a constant relation to them. Segments 
31 or 32 to 37 are swollen in mature worms, 
forming a saddle-shaped enlargement, the 
clitellum, of use during reproduction. Every 
segment, except the first and last, bears 4 
pairs of chitinous bristles, the setae (Fig. 
92); these may be moved by retractor and 
protractor muscles and are renewed if lost. 
The setae on segment 36, in mature worms, 
are modified for reproductive purposes. 

The body is covered by a thin transparent 
cuticle secreted by the cells lying just be- 
neath it. The cuticle protects the body from 
physical and chemical injury; it contains nu- 
merous pores to allow the secretions from 
unicellular glands to pass through; and it is 
marked with fine striations, causing the 
surface to appear iridescent. 

A number of external openings of various 
sizes allow the entrance of food into the 
body, and the exit of feces, excretory prod- 
ucts, reproductive cells, etc. ( 1 ) The mouth 
is a crescentic opening situated in the ven- 
tral half of the first segment (Fig. 94); it 
is overhung by the prostomium. (2) The 
oval anal opening lies in the last segment. 
(3) The openings of the sperm ducts or 
vasa deferentia are situated on each side of 
segment 15 (Fig. 91); they have swollen 
lips, and a slight ridge extends posterioriy 
from them to the clitellum. (4) The open- 
ings of the oviducts are small round pores, 
one on either side of segment 14; eggs pass 
out of the body through them. (5) The 
openings of the seminal receptacles appear 
as 2 pairs of minute pores concealed within 
the grooves which separate segments 9 and 
10, and 10 and 11. (6) A pair of nephridio- 
pores (Fig. 92), the external apertures of 
the excretory organs, open on every seg- 


ment except the first 3 and the last. They 
are usually situated immediately anterior to 
the outer setae of the inner pair. (7) The 
body cavity or coelom communicates with 
the exterior by means of dorsal pores. One 
of these is located in the middorsal line at 
the anterior edge of each segment from 8 
or 9 to the posterior end of the body. 

Internal anatomy 

If a specimen is cut open from the an- 
terior to the posterior end by an incision 
passing through the body wall, a general 
view of the internal structures (Fig. 91 ) may 
be obtained. The body is essentially a double 
tube (Fig. 91); the body wall constituting 
the outer, and the straight digestive tract, 
the inner; between the two is a cavity, the 
coelom. The external segmentation corre- 
sponds to an internal division of the coe- 
lomic cavity into compartments by means 
of partitions called septa, which lie beneath 
the grooves (Fig. 91). The digestive tract 
passes through the center of the body and is 
suspended in the coelom by the partitions. 
Septa are absent between segments 1 and 
2 and incomplete between segments 3 and 
4, and 17 and 18. The walls of the coelom 
are lined with an epithelium termed the 
peritoneum (Fig. 92), which is derived 
from the mesoderm. 

The coelomic cavity is filled with a 
colorless fluid which flows from one com- 
partment to another when the body of 
the worm contracts, thus producing a sort 
of circulation. This is possible since a 
large opening is present in the median ven- 
tral part of each septum. In segments 9 to 
16 are the reproductive organs; running 
along the upper surface of the digestive 
tract is the dorsal blood vessel; and just 
beneath it lie the ventral blood vessels and 
nerve cord. The body wall contains 2 layers 
of muscles. The outer layer lies beneath the 
epidermis and consists of circular muscle 
tissue. The muscle fibers are long and spin- 
dle-shapedj when they contract, the diam- 


Coetenterate 

GastrovascuIoT 
cavity 


Flatwomj 
Lumen 


Roundworm 


Annetid 


PseucJocoet 


CoeTom' 


I Mesoderm ^ Endoderm 


The relationship of body cavities to germ cell layers 


Intestinal epithelium" 

Typhlosole - 
lumen- 

Peritoneum - 


.^.^ssSSSi 


Dorsal blood vessel 

Chloragogue cells 
Coelom 


Muscle 


Epidermis 

Nephridium (section) 
Circular muscle 
Longitudinal muscle 
Ventral nerve cord 
Subneural blood vesse 


NepHridiopore 
Tubule of nephridium 

Nephrostome 


Lateral neural blood vessel 
Ventral blood vessel 


Figure 92. Top, cross sections of a coelenterate, flatworm, roundworm, and annelid, designed 
to show the relationship of body cavities to the germ layers. Bottom, cross section of an 
earthworm, which illustrates the advances in complexity of structure, correlated with the appear- 
ance of a coelom and the development of systems of organs. Left side of drawing shows sec- 
tioned parts of nephridium as they actually appear, and right side shows an earthworm nephridium 
as it appears in a dissection. Rarely does a cross section show all four pairs of setae. 


174 


PHYLUM ANNELroA. SEGMENTED WORMS 


175 


eter of the body becomes smaller and the 
worm longer. Under the circular layer is a 
thick longitudinal layer with muscle fibers 
lying parallel to the length of the worm; 
when these contract, the diameter of the 
body becomes greater and the worm shorter. 

Digestive system 

The digestive tract (Fig. 91) consists of 
(1) a mouth (buccal) cavity in segments 
1 to 3; (2) a thick muscular pharynx lying 
in segments 4 and 5; (3) a narrow, straight 
tube, the esophagus, which extends through 
segments 6 to 14; (4) a thin-walled enlarge- 
ment, the crop ( proventriculus ) , in seg- 
ments 15 and 16; (5) a thick muscular- 
walled gizzard in segments 17 and 18; and 
(6) a thin-walled intestine extending from 
segment 19 to the anal opening. The intes- 
tine is not a simple cylindrical tube, its 
dorsal wall is infolded, forming an internal 
longitudinal ridge, the typhlosole ( Fig. 92 ) ; 
this increases the digestive surface. Sur- 
rounding the digestive tract and dorsal blood 
vessel is a layer of chloragogue cells (Fig. 
92). The functions of these cells are not 
known with certainty, but in lumbricids and 
some other groups they nourish the develop- 
ing eggs. Since chloragogue cells can synthe- 
size urea, they are thought to be also ex- 
cretory. Three pairs of calciferous glands 
lie at the sides of the esophagus in segments 
10 to 12; actually, the first pair are storage 
pouches, but the second and third are true 
glands. Their primary function is excretion 
of calcium; neutralization of acid foods is 
probably an incidental function. 

The food of the earthworm consists prin- 
cipally of pieces of leaves and other vegeta- 
tion, particles of animal matter, and soil; 
this material is gathered at night, at which 
time the worms are active. They crawl out 
on the surface of the ground and hold fast 
to the tops of their burrows with their tails, 
exploring the neighborhood. Food parti- 
cles are drawn into the mouth cavity by 
suction produced when the pharyngeal 
cavity is enlarged by the contraction of the 


muscles which extend from the pharynx 
to the body wall. 

In the pharynx, the food receives a secre- 
tion from the phar}'ngeal glands; it then 
passes through the esophagus to the crop, 
where it is stored temporarily. The gizzard 
is a grinding organ; in it the food is broken 
up into minute fragments by being squeezed 
and rolled about. Solid particles, such as 
grains of sand, which are frequently swal- 
lowed, probably aid in this grinding proc- 
ess. The food then passes on to the intes- 
tine, where most of the digestion and 
absorption takes place. 

Digestion in the earthworm is very simi- 
lar to that of higher animals. Enzymes aid 
in the breakdown of food; these include 
amylase which acts upon carbohydrates, 
cellulase which acts upon cellulose, pepsin 
and trypsin which act upon proteins, and 
lipase which acts upon fats. The digested 
food is absorbed through the wall of the 
intestine, assisted by the amoeboid activity 
of some of the epithelial cells. Upon reach- 
ing the blood, the absorbed food is carried 
to various parts of the body. Absorbed food 
also makes its way into the coelomic cavity 
and is carried directly to those tissues bathed 
by the coelomic fluid. In one-celled animals, 
and in such metazoans as the hydra, planaria, 
and ascaris, no circulatory system is neces- 
sary, since the food is either digested within 
the cells or comes into direct contact with 
them; but in large complex animals, a spe- 
cial system of organs must be provided to 
bring about the proper distribution of di- 
gested food. 

Circulatory system 

The blood of the earthworm is contained 
in a complicated system of tubes which 
ramify to all parts of the body (Figs. 91 
and 93). A number of these tubes are large 
and centrally located; these give off branches 
which likewise branch, finally ending in ex- 
ceedingly thin tubules, the capillaries. The 
blood consists of a plasma in which are 
suspended a great number of colorless 


176 


COLLEGE ZOOLOGY 


amoeboid cells (corpuscles), which corre- 
spond to white corpuscles in man. Its red 
color is due to a respiratory pigment termed 
hemoglobin which is dissolved in the 
plasma. In vertebrates the hemoglobin is 
located in the blood corpuscles. 

There are 5 longitudinal blood vessels. 
These main vessels and their connectives are 
shown in Fig. 93 and are as follows: (1 "> the 
dorsal vessel, (2) the ventral (subintes- 


Anterior 


tinal) vessel, (3) the subneural vessel, (4) 
two lateral neural vessels, (5) five pairs of 
aortic arches (hearts) in segments 7 to 11, 
(6) -two lateral esophageal vessels, (7) 
segmental vessels from the ventral vessel to 
the nephridia, body wall, and intestine, (8) 
parietal vessels connecting the dorsal and 
subneural vessels in the intestinal region, 
(9) branches to the dorsal vessel from the 
intestine, and (10) a typhlosolar vessel 


Dorsal vessel- 


Valve- 


Esophagus 

Aortic aich ("heart") 

•Ventral vessel 
Nerve cord 

Lateral neural vessel 
Subneural vessel 


Valve between dorsal 
and t/phlosolor vessels 


Typhlosolar v. 


Typhlosole 
Intestine 

Afferent 
intestinal v. 
Segmental v. 

Nerve cord- 


-Dorsal vessel 

-Dorsal intestinal v. 
(efferent intestinal) 


Body wall capillaries 

Afferent>^ 

Nephridial vessels 
Efferent-^ 


Parietal vessel 

Ventral vessel 

(subintestinal) 
Lateral neural vessel 

Subneural vessel 


Figure 93. Earthworm circulatory system. A, one pair of "hearts" and other vessels. B, a sec- 
tion to show the structure of a valve. C, a third-dimensional view of two cuts through the 
earthworm to show the general scheme of the circulation. (A and B modified from Bell; C, after 
Bell, and a drawing by the Department of Zoology, Kansas State College.) 


PHYLUM ANNELIDA. SEGMENTED WORMS 


ii: 


from the dorsal vessel supplies the dorsal 
half of the intestine. 

The dorsal vessel (Fig. 93) serves the 
function of a true heart in that it is a pump 
with valves; and the aortic arches, the so- 
called hearts, act as a pressure-regulating 
mechanism, receiving blood in spurts from 
the dorsal vessel, and then contracting to 
force the blood under a steady pressure into 
the ventral vessel. Blood is forced forward 
by wavelike contractions of the dorsal ves- 
sel, beginning at the posterior end and 
traveling quickly anteriorly. These contrac- 
tions are said to be peristaltic; they have 
been likened to the action of the fingers in 
the operation of milking a cow. Valves 
(Fig. 93) in the walls of the dorsal vessel 
prevent the return of blood from the an- 
terior end. In segments 7 to 11, the blood 
passes from the dorsal vessel into the hearts, 
which force it forward and backward in the 
ventral trunk. Valves in the heart prevent 
the backward flow. From the ventral vessel 
the blood passes to the body wall, the in- 
testine, and the nephridia. The flow in the 
subneural vessel is toward the posterior end, 
then dorsally through the parietal vessels 
into the dorsal vessel. The anterior region 


receives blood from the dorsal and ventral 
vessels. The blood which is carried to the 
body wall and the skin receives oxygen 
through the cuticle and is then returned to 
the dorsal vessel by way of the subneural 
vessel and the parietal connectives. 

The exchange of materials between the 
blood and the tissue cells takes place in 
minute tissue spaces. Blood plasma and a 
few corpuscles, which constitute the tissue 
fluid, pass from the capillaries into these 
tissue spaces, where the cells are bathed 
and the interchange occurs. The tissue fluid 
collects waste products of cellular metab- 
olism and makes its way back again into the 
blood stream. 

Respiration 

The earthworm possesses no organized 
respiratory system, but it obtains oxygen and 
gets rid of carbon dioxide through the moist 
skin. Respiration can be carried on in air 
and also in water as experiments have 
shown. Many capillaries lie just beneath the 
cuticle, making transfer of gases essentially 
as it is done in a gill or lung. The oxygen 
passes into the blood and combines with the 
hemoglobin. The hemoglobin of the earth- 


Cerebral ganglia "brain" Pharynx Lateral nerve 


Tactile nerve 
Prostomium 


OJI 

Mouth 


Mouth 
cavity 


Circumpharyngeal 
connective 


Subpharyngeal 
ganglion 


Nerve 
cord 


Figure 94. Earthworm. Side view of anterior end showing the cerebral gangha and large; 
nerves. (After Hess.) 


178 


COLLEGE ZOOLOGY 


worm is inefEcient as an oxygen-transporting 
substance compared to the hemoglobin of 
man. 

Excretory system 

Most of the excretory matter is carried 
out of the body by a number of coiled tubes 
termed nephridia (Figs, 91 and 92), a pair 
of which are present in every segment ex- 
cept the first three and the last. A nephri- 
dium occupies part of two successive seg- 
ments; in one is a ciliated funnel, the 
nephrostome, which is connected by a thin 
ciliated tube with the major portion of the 
structure in the segment posterior to it. 
Three loops make up the coiled portion of 
the nephridium. The cilia on the nephro- 
stome and in the nephridium create a cur- 
rent which draws in waste material from 
the coelomic fluid; other waste is received 
directly from blood vessels surrounding the 
nephridium. These excretory products (am- 
monia, urea, creatine) are eventually car- 
ried out through the nephridiopore. Chlora- 
gogue cells may store excretory matter 
temporarily before releasing it into the 
coelomic fluid. The nephridia serve the 


same function in the earthworm that 
the kidneys do in man. 

Nervous system 

The nervous system is concentrated (Figs. 
91, 94, and 95). There is a bilobed mass 
of nervous tissue, the "brain" or cerebral 
ganglia, on the dorsal surface of the pharynx 
in segment 3. This is connected by 2 cir- 
cumpharyngeal connectives with a pair of 
subpharyngeal ganglia which lie beneath 
the pharynx. From the latter, the ventral 
nerve cord extends posteriorly near the 
ventral body wall. The ventral nerve cord 
enlarges into a ganglion in each segment 
and gives off 3 pairs of nerves in every seg- 
ment posterior to segment 4. Each ganglion 
really consists of 2 ganglia fused together. 
Near the dorsal surface of the ventral nerve 
cord are 3 longitudinal giant fibers. The 
brain and nerve cord constitute the central 
nervous system; the nerves which pass 
from and to them represent the peripheral 
nervous system. 

The nerves of the peripheral nervous sys- 
tem are either motor or sensory. Motor 
nerve fibers (Fig. 95) are extensions from 


Sensory cell (receptor) 
Sensory fiber 


-Dorsal giant fiber 


Association neuron 
Lateral nerve 


Epidermis 

Longitudinal muscle (effector) 

Figure 95. Diagram of sensory and motor neurons of the ventral nerve cord of an earthworm, 
showing their connections with the skin and the muscles to form a reflex arc. 


Ventral giant cells 
Motor neuron cell body 


PHYLUM ANNELroA. SEGMENTED WORMS 


179 


cells in the ganglia of the central nervous 
system. They pass out to the muscles or 
other organs; and, since impulses sent along 
them give rise to movements, the cells of 
which they are a part are said to be motor 
nerve cells. The sensory fibers originate 
from nerve cells in the epidermis and carry 
impulses into the ventral nerve cord. The 
peripheral nervous system is composed of 
elements which have definite connections 
in the nerve cord. 

The functions of nervous tissue are recep- 
tion, conduction, and stimulation. These are 
usually performed by nerve cells called 
neurons. The neuron theory assumes that 
there is no nerve fiber independent of a 
nerve cell, that the nerve cell body with 
all of its processes is a unit, called the 
neuron. There is no protoplasmic con- 


tinuity of one neuron with another; the 
relation between the neurons is probably 
contact of the terminals of one neuron with 
those of another. 

The reflex is considered the functional 
unit of the nervous system. The apparatus 
required for a simple reflex in the body of 
an earthworm is represented in Fig. 95. A 
sensory neuron, lying at the surface of the 
body, sends a fiber into the ventral nerve 
cord, where it branches out; these branches 
meet but are not continuous with branches 
from an association neuron lying in the 
ventral nerve cord. The association neuron 
is in contact with a motor neuron that sends 
fibers into a reacting organ, which in this 
case is a muscle. These fibers extending to 
the reacting organ are called motor fibers; 
those leading to the ventral nerve cord are 


Mucous secretion 
Cuticle 

Epithelial 
cell 


Gland cell pore 
Supporting cell 


•^ens' 

photoreceptor 
(light sensitive cell) 

Nucleus" 

Nerve fiber 


Sensory cell of 
sense organ 

Gland cell 

Basal membrane 
Nerve fiber 

Figure 96. Diagram of the epidermis of the earthworm showing sense organs. 


termed sensory fibers. The first neuron or 
receptor receives the stimulus and produces 
the nerve impulse which is carried on to the 
association neuron; the association neuron 
in turn transmits the impulse to a motor 
neuron which has processes extending to 
an effector such as a muscle or other organ. 

Any action which takes place through 
such a reflex arc is termed a reflex act. 

Within the ventral nerve cord are associa- 
tion neurons whose fibers serve to connect 
structures within one ganglion or two suc- 
ceeding ganglia. These neurons are doubt- 
less responsible for the muscular waves which 
pass from the anterior to the posterior end 


of the worm during locomotion. The three 
giant fibers, which lie in the dorsal part of 
the ventral nerve cord throughout almost 
its entire length, are connected by means of 
fibrils with nerve cells in the ganglia, and 
probably distribute the impulses that cause 
a worm to contract its entire body when 
stimulated. The earthworm's behavior is 
largely a matter of reflex acts. 

Behavior due to simple reflexes are as 
mechanical as the reflection of light from a 
mirror, they often save animals from in- 
jury and even from death. Fortunately, we 
ourselves do not have to think before we 
pull our hand away from a hot stove, or 


180 


COLLEGE ZOOLOGY 


before we close our eyes when we see an 
object about to strike us in the face. 

Sense organs. The sensitiveness of the 
earthworm to hght and other stimuli is due 
to the presence of a great number of epi- 
dermal sense organs. The two main types 
of epidermal receptors are the light-sensitive 
cells (photoreceptor cells) and the sense 
organs (Fig. 96), composed of a group of 
sensory cells surrounded by supporting cells. 
These sense organs are connected with the 
central nervous system by means of nerve 
fibers and communicate with the outside 
world through sense hairs which penetrate 
the cuticle. In addition to these sensory or- 
gans there are also free endings of nerve 
fibers between the cells of the epithelium. 
More of these sense organs occur at the 
anterior and posterior ends than in any 
other region of the body. 

Reproductive system 

The earthworm is not known to reproduce 
asexually although it has great powers of 
regeneration of lost parts. Mating takes 
place at night and requires two or three 
liours. Both male and female sexual organs 
occur in a single earthworm (Fig. 91). The 
female system consists of: (1) a pair of 
ovaries in segment 13; (2) a pair of ovi- 
ducts, which open by a ciliated funnel in 
segment 13 and pass to the exterior in seg- 
ment 14; (3) an egg sac, which is a small 
diverticulum of the septum associated wath 
the funnel; and (4) two pairs of seminal 
receptacles, in segments 9 and 10. The male 
organs are: (1) two pairs of minute glove- 
shaped testes in segments 10 and 11, and 
back of each, (2) a ciliated sperm funnel 
which is connected to (3) a tiny duct, the 
vas eflEerens. The two ducts on each side 
connect to (4) a vas deferens, that leads 
to the outside. The testes and funnels are 
contained in (5) the seminal vesicles, con- 
spicuous saclike structures which surround 
the testes and in which the sperms mature. 
Self-fertilization does not take place, but 
sperms are transferred from one worm to 


another during a process called copulation. 
Two worms come together, as shown in 
Fig. 98; then spermatozoa from the seminal 
vesicles of each worm are expelled. They 
pass along the seminal grooves into the 
seminal receptacles of the other worm. The 
worms then separate. When the time for 
egg laying approaches, the glandular clitel- 
lum secretes a bandlike mucous tube which 
is forced forward by movements of the 
worm. Eggs are discharged through the 
oviducts, and sperms through the openings 
of the seminal receptacles into the space 
between this tube and the body wall. The 
tube is then forced forward over the an- 
terior end; its ends become closed, and a 
cocoon, about the size of an apple seed, is 
thus fonned containing fertilized eggs which 
develop within the cocoon into minute 
worms. The reciprocal fertilization insures 
cross-fertilization in the earthworm. 

The eggs of the earthworm are holoblas- 
tic, but cleavage is unequal. A hollow blas- 
tula is formed, and a gastrula is produced 
by invagination. The mesoderm develops 
from tw^o of the blastula cells called meso- 
blasts. These cells divide, forming two meso- 
blastic bands which later become the epithe- 
lial lining of the coelom. There is no 
swimming stage such as occurs in the 
marine annelids. The embryo escapes from 
the cocoon as a small worm in about two 
to three weeks. 

Regeneration and grafting 

Earthworms have considerable powers of 
regeneration. No more than 5 new segments 
will regenerate at the anterior end, and no 
"head" will regenerate if 15 or more seg- 
ments have been cut off. A posterior piece 
may regenerate a "head" of 5 segments 
(Fig. 98B); or, in certain cases, a tail (Fig. 
98C). Such a double-tailed worm slowly 
starves to death. An anterior piece regener- 
ates a tail. Three pieces from several worms 
may be united to make a long worm (Fig. 
98D); two pieces may fuse, forming a worm 


PHYLUM ANNELIDA. SEGMENTED WORMS 


181 


' '^.w... 


Figure 97. Earthworm cocoons deposited in cornstalk compost. Arrows point to cocoons. 
(Photo courtesy of R.C. Ball.) 


with two tails; and an anterior piece may be 
united with a posterior piece to make a 
short worm. In such regeneration experi- 
ments, the parts are held together by threads 
until they become united. Regeneration 
probably does not contribute to the survival 
of the earthworm as much as it does to 
planarians and starfishes. 

Behavior 

The external stimuli that have been most 
frequently employed in studying the be- 
havior of earthworms are those dealing with 
contact, chemicals, and light. 

Reactions to mechanical stimuli 

Mechanical stimulation, if continuous and 
not too strong, calls forth a positive reaction; 
the worms live where their bodies come in 
contact with solid objects; they respond to 
the stimulus of mechanical contact such 
as the walls of their burrows. Reactions to 
sounds are not due to the presence of a 
sense of hearing, but to the contact stimuli 


produced by vibrations. Darwin showed that 
musical tones produced no response; but if 
a flower pot containing earthworms was 
placed upon a piano and a note was struck, 
the worms immediately drew back into their 
burrows. This result was due to vibrations. 

Reactions to chemicals 

In certain cases, reactions to chemicals re- 
sult in bringing the animal into regions of 
favorable food conditions or turning it 
away from unpleasant substances. Moisture, 
which is necessary for respiration and con- 
sequently for the life of the earthworm, 
causes a positive reaction, when it comes in 
contact with the body; no positive reactions 
are produced by chemical stimulation from 
a distance. Negative reactions, on the other 
hand, such as moving to one side or back 
into the burrow, are produced even when 
certain unpleasant chemical agents are still 
some distance from the body. These reac- 
tions are quite similar to those caused by 
contact stimuli. Darwin explained the prefer- 
ence of the earthworm for certain kinds of 


182 


COLLEGE ZOOLOGY 


d 


c 


B 

Figure 98. Earthworm. Diagrams illustrating copulation and regeneration. A, a pair in copula- 
tion. Because this usually occurs at night, it is not often observed. B, a new anterior end 
(dotted) that has regenerated in place of an anterior end removed. C, a new posterior end 
(dotted) that has regenerated in place of an anterior end removed. D, a long worm produced 
by grafting together parts of three worms. (A, courtesy of General Biological Supply House, Inc., 
Chicago; B, C, D after Morgan.) 


food by supposing that the discrimination 
between edible and inedible substances was 
possible when they were in contact with the 
body. This would resemble the sense of taste 
as present in the higher animals. 

ReacfioTts to light 

No definite visual organs such as eyes 
have been discovered in earthworms; never- 
theless, these animals are very sensitive to 
light, as is proved by the fact that a sudden 
illumination at night will often cause them 
to quickly snap back into their burrows. 
This sensitiveness to light is due to the 


photoreceptor cells (Fig. 96) that are con- 
centrated especially in the anterior and pos- 
terior ends, and are found in every segment 
of the body. Each of these light-sensitive 
cells contains a transparent "lens" that 
focuses light on the neurofibrils which 
ramify through the cell. By means of these 
photoreceptor cells, very slight differences 
in the intensity of the light are distinguished. 
If a choice of two illuminated regions is 
given, the one more faintly lighted is se- 
lected in the majority of cases. A positive 
reaction to faint light has been demon- 
strated for the manure worm, Eisenia foe- 


PHYLUM ANNELroA. SEGMENTED WORMS 


183 


tida; this positive reaction to faint light may 
account for the emergence of the worms 
from their burrows at night. It is an inter- 
esting fact that although the worms react 
negatively to sunlight, they respond posi- 
tively to red light and may be collected at 
night with the use of such a light. 

Physiologic state 

From the foregoing account, it might be 
inferred that only external stimuli are fac- 
tors in the behavior of the earthworm. This, 
however, is not the case, since the physio- 
logic condition, which depends largely upon 
previous stimulation, determines the char- 
acter of the response. Different physiologic 
states may be recognized, ranging from a 
state of rest in which slight stimuli are not 
effective, to a state of great excitement 
caused by long-continued and intense stimu- 
lation, in which condition, slight stimuli 
cause violent responses. By physiologic states 
we mean the varying internal physiologic 
conditions of the organism as distinguished 
from permanent anatomic conditions. Such 
different internal physiologic conditions can 
be inferred from the behavior of the animal. 

Learning in earthworms 

Whether or not learning occurs in proto- 
zoans, or in such simple metazoans as 
sponges and hydras, is uncertain. But at 
the stage in evolution represented by the 
earthworm, experiments indicate that this 
animal is capable of what psychologists call 
"latent memory," or the storing of impres- 
sions until a later time when they may be 
useful. 

In one experiment, worms could escape 
from a lighted chamber by entering the bot- 
tom of a branched passageway constructed 
of glass tubing in the form of a "T." If 
the worms turned to the right at the top of 
the "T," they entered a dark moist cham- 
ber filled with damp earth and moss, a 
favorable environment for an earthworm. 
If they turned left, they encountered an 
electric shock. In the eariy trials, they turned 


to the left as often as to the right. At the 
end of 20 days, they turned to the left only 
5 times out of 20, and at the end of 40 days 
they were turning left only once out of 20 
trials. 

OTHER ANNELIDA 

Annelids differ from the other groups of 
"worms" in the following respects: (1) the 
body is divided into a linear series of similar 
segments, often visible externally because of 
grooves that encircle the body, and inter- 
nally because of partitions called septa; (2) 
the body cavity between the digestive tract 
and body wall is a true coelom; (3) the 
mouth opens in the first segment and is 
overhung by the prostomium; (4) the nerv- 
ous system consists of a preoral ganglion, the 
"brain," often bilobed, and a pair of ventral 
nerve cords, typically with a pair of ganglia 
in each segment; (5) usually, a nonchitinous 
cuticle on the surface of the body; chitinized 
bristles or setae are present. 

The sandworm 

Neanthes virens is a common polychaete 
that lives in burrows in the sand or mud of 
the seashore at tide level. By day it rests 
in its burrow, but at night it extends its 
body in search of food or may leave the bur- 
row entirely. 

The body is flattened dorsoventrally and 
may reach a length of 18 inches or more, 
with 100 to 200 or more segments. The 
head is well developed. Above the mouth is 
the prostomium (Fig. 99) which bears a 
pair of terminal tentacles, 2 pairs of simple 
eyes, and, on either side, a thick palp. The 
first segment is the peristomium; from each 
side of this arise 4 tentacles. Small animals 
are captured by a pair of strong chitinous 
jaws which are everted with part of the 
pharynx when Neanthes is feeding. Behind 
the head are a variable number of segments 
each bearing a fleshy outgrowth on either 
side, the parapodium (Fig. 99). 


Jaw — 

Horny 
paragnaths 


DORSAL VIEW OF INTERNAL 
ANATOMY OF ANTERIOR END 


■- Peristom 
tentacles 

Parapodium 

Nephridium 
Dorsal cirrus 

Esophagus 
Esophageal 
gland 
Notopodium 


DORSAL VIEW OF HEAD WITH 
PHARYNX EVERTED 


Dorsal blood 


Coelom- 
Epidermis- 

184 


Aciculum 

Intestine 

Neuropodium 

Ventral cirrus 
Ventral 
blood V. 

Ventral nerve 
cord 

Dorsal blood vessel 

Longitudinal muscle 
ntestine 

Ventral blood vessel 
Ventral nerve cord 

Circular muscle 


PARAPODIUM (Posterior view] 


Cuticle 


PHYLUM ANNELIDA. SEGMENTED WORMS 


185 


The body wall consists of an outer cuticle, 
which is secreted by the cells of the epi- 
dermis just beneath it, and several mus- 
cular layers under the epidermis. The body 
cavity between the body wall and the intes- 
tine is a coelom lined with peritoneal 
epithelium. The digestive system (Fig. 99) 
consists of the mouth, pharynx, esophagus, 
with an esophageal gland on either side 
opening into it, and a straight stomach-in- 
testine extending to the anus. 

The circulatory system comprises a dorsal 
vessel and a ventral vessel, with branches to 
capillaries in the body wall and intestine. 
Almost ever}' segment, except the peristo- 
mium and the anal segment, contains a pair 
of nephridia. In the head is a cerebral 
ganglion, the "brain." This is joined by a 
pair of circumesophageal connectives to a 
pair of subesophageal ganglia and is fol- 
lowed by a ventral nerve cord with a pair 
of ganglia in each segment. The sexes are 
separate. Ova or sperms arise from the wall 
of the coelom. A trochophore larva develops 
from the fertilized egg. 

Polychaetes 

The principal characteristics of the classes 
Oligochaeta and Polychaeta are exhibited by 
the earthworm (Fig. 92) and the sandworm 
(Fig. 99) respectively. However, many varia- 
tions from these types occur. 

The polychaetes consist largely of free- 
living marine annelids in which typical an- 
nelidan characters occur. The body tends to 
be long and wormlike, and somewhat de- 
pressed to a cylindrical shape in cross sec- 
tion. It consists of a prostomial or head re- 
gion, and a trunk. Segmentation is well 
marked both internallv and externallv. The 
outer cuticle is usually soft and moist and is 
dependent on a wet environment for the 
prevention of desiccation. The digestive sys- 
tem consists of a straight tube with an an- 

FiGURE 99. Facing page, Neanthes, the sand- 
worm. Left, anterior end of the body with dorsal 
wall removed. Right, some details of structure. 


teroventral mouth and a posterodorsal anus. 
The circulatory system has a dorsal vessel 
where the blood moves forward, and a 
ventral one where it moves backward, to- 
gether with transverse vessels. The nervous 
system has a dorsal "brain" in or near the 
prostomium, and paired ventral ganglia in 
a laddcrlike arrangement. A giant nerve fiber 
system is usually present, consisting of longi- 
tudinal strands that extend parallel to the 
ventral nerve cord and function for rapid 
response reactions. Nephridia are segmental 
and are present in most body segments. 
Most polychaetes are dioecious, with the 
two sexes resembling each other; gonads 
may occur in many segments, and ova may 
be produced in enormous numbers. The 
lateral appendages or parapodia are formed 
by outpocketings of the lateral body walls; 
they are usually conspicuous and variously 
provided with fleshy structures such as cirri, 
scales, and gills. The setae occur in bundles; 
they are formed from secretions of special- 
ized cells, and they function in locomotion, 
tube building, food gathering, and other im- 
portant ways. The fertilized egg develops 
into a trochophore. 

In detail, however, there is remarkable 
diversity among the polychaetes so that the 
characters named above can be regarded 
only as generalizations. Such common 
names for families as the following illustrate 
the variations in shape and structure that 
mav occur: sea mouse, scale worms, fire- 
worms, glass worms, proboscis worms, bam- 
boo worms, gold crowns, gooseberry worms, 
lugworms, feather dusters, and shield worms. 
The variable structure of polychaetes makes 
possible adaptations to many ocean habitats. 

Most polychaetes are free-living, but many 
are partly or wholly parasitic; most are ma- 
rine but many others live in water varying 
in saltiness from briny to fresh; a few are ter- 
restrial. Metamerism may be homonymous 
(with successive rings alike), but usually 
there is considerable departure from this 
structure. In Chaetopterus (Fig. 100), parts 
of the parapodia are modified to function as 
suction disks, as a food-ball organ, as water- 


186 


COLLEGE ZOOLOGY 


Suckers .•• 


Adult in tube 


Tentacle 
Mouth 


Seta 


Nofopodium 


Paropodia 


Food ball 
feeding organ 


Dorsal view of 
anterior end 


Figure 100. A marine polychaete (Chaetopterus) feeding in its tube. The arrows indicate the 
direction of water currents. {Left after Enders; right after Lankester.) 


pumping fans, etc. In the feather duster 
worms the peristomium or first segment is 
enormously developed to form the feathery 
tentacular crown, or food-gathering organ, 
or to form also the operculum that serves 
to close the end of the tube when the animal 
is withdrawn. The tubes of the polychaetes 
are nearly as variable and characteristic of 
the different species as are the body parts; 
the basic structures formed by the worms 
may be spun threads (modified setal secre- 
tions as in some of the scale worms), trans- 
parent horny tubes, tough leathery tubes, 
calcareous tubes, or clear glasslike tubes 
(some serpulids). Extraneous materials such 
as sand particles, shells, and sticks, are fre- 
quently used and sometimes selected with 
precision in regard to size, color, and weight, 
so that some intelligence has been credited 
to certain tube-dwelling worms. 

Polychaeta differ from Oligochaeta in be- 
ing largely marine instead of fresh-water or 


terrestrial; parapodia are typically well de- 
veloped, and the setae are numerous instead 
of few; the prostomium or some of the first 
few segments are often highly differentiated 
to form a cephalic region of considerable 
proportions; sexes are usually separate, with 
gonads present in a large and variable num- 
ber of segments. Fertilization of ova is 
typically external; development is by spiral 
cleavage and through a pelagic trochophore. 
In certain species, for example Autolytus, 
the body, which is only 15 mm. long, may 
produce buds at the posterior ends, thus 
forming a linear row of offspring (Fig. 101), 
each of which acquires a head before sepa- 
rating from the parent. There are thousands 
of species of polychaetes. They are known in 
all seas and at all recorded depths, but they 
are most abundant in the upper 180 feet. 

The Pacific palolo worm, Eunice (Fig. 
102), first became known from the Samoan 
Islands, where it attracted the attention of 


PHYLUM ANNELIDA. SEGMENTED WORMS 


187 


Aufolyius, a Polychaete reproducing by budding at the posterior end. 


Aeolosoma, a fresh-water oligochaete reproducing by transverse division. 

Figure 101. Asexual reproduction in annelids. Top, a marine polychaete budding at the posterior 
end. Bottom, a fresh-water oligochaete showing a budding segment. (After Mensch.) 


the missionaries because it was eaten by the 
natives; also because it appeared periodically 
in certain localities in enormous numbers for 
only a few hours. It makes its appearance 
(swarms) almost invariably in the months 
of October and November, and usually at 
the time of the third quarter of the moon. 
Other important factors, such as the velocity 
of the wind and the stage of sexual matur- 
ity, may account for a departure from this 
time to produce lesser swarms at other moon 
phases during these two months. The pos- 


terior half of the worm breaks off from the 
parent worm and swims to the surface. The 
enclosed eggs and sperms are shed into the 
sea in the early morning, and in some local- 
ities in such enormous numbers that the 
surface of the sea has been likened to a 
thick noodle soup. The eggs develop into 
young larvae rapidly, and in three days sink 
to the bottom. Other palolo worms occur in 
different parts of the world, particularly in 
warm seas. The Atlantic palolo swarms in 
June and July. 


&:iiM^^^r0: tAa\e v/ith sexual 
y^^;^v;;j;^v region detached 


Mature 
fema 


Sexual segments sv/im 
to surface; eggs and 
sperms are discharged 


Parent worm 
regenerating 
sexual region 

Sexual reproduction of the palolo worm, 
Eunice, a polychaete 

Figure 102. The Pacific palolo worm, Eunice viridis, has its burrows in coral reefs; it pro- 
duces many posterior segments filled with eggs or sperms which are periodically cast off. 


188 


COLLEGE ZOOLOGY 


A tube-dwelling species is Chaetopterus 
(Fig. 100). When full grown it may reach 
15 to 30 cm. (12 inches); the body is highly 
luminescent and consists of three distinct 
regions. The U-shaped, opaque, parchment- 
like tube may be 50 cm. long; it lies com- 
pletely hurried in mud or sand, except for 
the two distal orifices. The worm maintains 
its position in the spacious tube by means 
of the long anterior notopodia (Fig. 100) 
and three ventral suckers that are formed by 
the median fusion of three pairs of neuro- 
podia. A powerful current of water may be 
set up by three muscular fan segments, 
formed by the median fusion of three pairs 
of notopodia. Other remarkable modifica- 
tions include the food-ball organ (Fig. 100) 
that is formed by the fusion of a pair of 
notopodia and serves to carry mucous food 
balls to the mouth. This polychaete is world- 
wide in its distribution; it usually occurs 
where there are broad sand flats and little 
current. It is found along the Atlantic Coast 
from North Carolina to Cape Cod. 


Archiannelida 

The Archiannelida are aberrant marine 
polychaetes, characterized largely for the per- 
sistence of larval features such as ciliar)' 
rings and lack of setae, or reduction of organ 
systems. Whether these traits are primitive 
or degenerate is not known. Polygordius ap- 
pendiculatus (Fig. 103) lives in the sandy 
shores of the Atlantic and Mediterranean 
coasts. It is about one inch long and indis- 
tinctly segmented externally. The prosto- 
mium bears a pair of cephalic tentacles, and 
the posterior end bears two anal tentacles. 
A pair of ciliated pits, one on either side of 
the prostomium, probably serve as sense or- 
gans. The development of Polygordius in- 
cludes a trochophore stage. The adult 
develops from the trochophore by the 
growth and elongation of the anal end. This 
elongation becomes segmented; and, by con- 
tinued growth the larva transforms into the 
adult. 

The archiannelids number only about 45 


Cephalic 
tentacles 


Head 


Faint exfernol segmentation 


Young trochophore 
larva 


Elongation of 
trochophore larva 


Transformation stages of larva 

Figure 103. Stages in development of Polygordius appendiculatus. one of the Archiannelida. 
(After Fraipont.) 


PHYLUM ANNELIDA. SEGMENTED WORMS 


189 


species in 10 genera; they have originated in 
various ways, and are thus a heterogeneous 
assemblage and not to be regarded as a 
unit. 

Another aberrant group of polychaetes in- 
cludes the family Myzostomidae. All are 
parasites of echinoderms, notably sea lilies 
(crinoids); in size they range from 0.5 to 9 
mm. The body is oval and depressed with 
few segments. Individuals are protandric, 
that is, the smaller younger ones function as 
males, and later, with increase in size and 
age, become females; cross-fertilization is 
thus insured. The egg gives rise to a swim- 
ming trochophore. 


Oligochaetes 

The members of the class Oligochaeta are 
mostly terrestrial, but some inhabit fresh 
water; no parapodia, and few setae are pres- 
ent, and the head has no distinct appen- 
dages. They are hermaphroditic, but no 
trochophore larva develops from the egg. 
The earthworm is the best-known species. 
Among the interesting species of oligo- 
chaetes are those of Aeolosoma (Fig. 101), 
which are only 1 mm. long and spotted with 
red oil globules in the integument. They live 
among algae, consist of from 7 to 10 seg- 
ments, and reproduce asexually by trans- 


FiGURE 104. A giant earthworm is shown being pulled from its burrow in the wet river slopes 
of Gippsland, Australia. Although the giant earthworm, in popular accounts, is said to be 12 
feet long, scientific reports give 7 feet as the length. (Courtesy of Australian News and In- 
formation Bureau.) 


190 


COLLEGE ZOOLOGY 


verse fission. Nais is light brown in color, 
2 to 4 mm. long, and consists of from 15 to 
37 segments. It lives among algae and may 
reproduce by budding. Tubifex tubifex is 
reddish in color and about 4 cm. long. It 
lives in a tube from which the posterior end 
projects and waves back and forth. Often 
large numbers occur in patches on muddy 
bottoms. 

Among the smallest of oligochaetes are 
species of Chaetogaster that may be only 
0.44 mm. long. The largest ones are known 
from South Australia, where Megascolides 
austrdis may attain a length of 7 feet. The 
number of segments in oligochaetes varies 
from 7 in Aeolosoma to over 600 in Rhino- 
drilus. 

Leeches 

The class Hirudinea contains annelids 
that are usually flattened dorsoventrally, but 
differ externally from the flatworms in being 
distinctly segmented. They differ from other 
annelids in the lack of setae (except in one 
genus), and in the presence of copulatory 
organs and genital openings on the ventral 
side. Leeches (Fig. 105) are abundant in 
fresh water but also occur in salt water and 
on land. Many of them are brilliantly col- 
ored and bear elaborate color patterns. We 
commonly think of leeches as bloodsuckers; 
large numbers, however, are predaceous, 
that is, they do not act as bloodsucking 
parasites, but devour other small animals 
such as earthworms and mollusks. They are 
themselves preyed upon by birds such as the 
bittern, reptiles, flatworms, and other ani- 
mals. 

External annulation is not indicative of 
the true number of segments; there may be 
several external annulations for every seg- 
ment as shown by internal organs (Fig. 
106). 

The principal characteristics are exhibited 
by Hirudo medicinalis, which is about 4 
inches long but is capable of great contrac- 
tion and elongation. The suckers are used 


as organs of attachment. Figure 106 illus- 
trates the principal structures of a leech. The 
digestive tract is fitted for digestion of the 
blood of vertebrates, which forms the prin- 
cipal food of some leeches. The mouth lies 
in the anterior sucker and is provided with 
three jaws armed with chitinous teeth for 
biting. Blood is sucked up by the dilatation 
of the muscular pharynx. The short esopha- 
gus leads from the pharynx into the crop, 
which has 1 1 pairs of lateral branches. Here 
the blood is stored until digested in the 
small globular stomach. Because of its enor- 
mous crop, a leech is able to ingest three 
times its own weight in blood; and, since it 
may take as long as 9 months to digest this 
amount, meals are few and far between. 

Respiration is carried on mainly through 
the surface of the body. Waste products 
are extracted from the blood and coelomic 
fluid by 17 pairs of nephridia. Leeches are 
hermaphroditic, but the eggs of one animal 
are fertilized by sperms from another leech. 
Copulation and formation of a cocoon are 
similar to those processes in the earthworm. 
Other leeches carry their eggs on the ventral 
side, and some deposit them on stones. 

Metamerism 

The biological principle of body segmen- 
tation is called metamerism. This is ex- 
hibited in the true annelids and is here en- 
countered for the first time. This type of 
structure is of considerable interest since 
the most successful groups in the animal 
kingdom, the Arthropoda and Vertebrata, 
have their parts metamerically arranged. 
How this condition has been brought about 
is still doubtful, but many theories have 
been proposed to account for it. According 
to one view, the body of a metameric ani- 
mal has evolved from that of a non-seg- 
mented animal by transverse fission. The 
individuals thus produced remained united 
end to end and gradually became integrated 
both structurally and physiologically so that 
their individualities were united into one 


PHYLUM ANNELIDA. SEGMENTED WORMS 


191 


Medicinal leech attached to ar 


m 


Partly extended medicinal leech (ventral view) 


Piacobdella attached 
to neck of turtle 


Piacobdella with 
eggs (ventral view) 


Figure 105. Leeches are commonly called bloodsuckers. A full-grown medicinal leech is four 
inches in length. Piacobdella, common on turtles, is about one inch long. 


complex individuality. Some zoologists main- 
tain that the segmental arrangement of or- 
gans, such as nephridia, blood vessels, and 
reproductive organs, has arisen by division 
of a single ancestral organ, and not by for- 
mation of new organs as the fission theory 
demands. 

The coeiom 

The coeiom (Fig. 92) is a body cavity 
lined with tissue of mesodermal origin; from 
it the excretory organs open; and from its 
embryonic walls, the reproductive cells or- 
iginate. The importance of the coeiom 
should be clearly understood since it has 
played a prominent role in the progressive 
development of complexity of structure. The 


appearance of this cavity between the diges- 
tive tract and body wall brought about great 
physiologic changes; it is correlated with the 
origin of nephridia for transporting waste 
products out of the body, and of reproduc- 
tive ducts for the exit of eggs and sperms. 
The coeiom also affected the distribution of 
digested food within the body, since it con- 
tains a fluid which takes up material ab- 
sorbed by the digestive tract and carries it 
to the tissues. Excretor)' matter finds its way 
into the coelomic fluid and thence out of the 
body through the nephridia. 

So important is the coeiom considered by 
most zoologists that the Metazoa are fre- 
quently separated into two groups: (1) the 
Acoelomata without a coeiom, and (2) the 
Coelomata with a coeiom. The Porifera, 


Jaws around mouth (ventral) 
Eyes 

Muscular pharynx 


■Anterior sucker 
Cerebral ganglia ("brain") 


1st diverticulum of crop 


Prostate 

Penis 

Mole opening 

Ovary 

Vagina 
Female opening 

Vas deferens 

Ventral nerve cord 
Testis 


Nephridtum 


Anus 


Clitellum 


Crop 


Nerve cord ganglion 


lOth diverticulum 

of CTOp 


Posterior sucker 


Figure 106. Dorsal view to show the segmentation and internal anatomy of the leech. Part 
of the crop is cut away on the left side to show the ventral nerve cord and reproductive organs. 
The numbers on the right indicate the internal segmentation or somites as shown by the 
nerve ganglia. 


192 


PHYLUM ANNELIDA. SEGMENTED WORMS 


193 


Coelenterata, Ctenophora, and Platyhelmin- 
thes are undoubtedly Acoelomata. Likewise 
the Annelida, Echinodermata, Arthropoda, 
Mollusca, and Chordata are certainly Coelo- 
mata. The Nemathelminthes and related 
phyla belong to the Pseudocoelomata. 

Trochophore 

The term trochophore has been applied 
to the larval stages of a number of marine 
animals. The figures of the trochophores of 
the polychaete Eupomatus (Fig. 107) and 
of Polygordius (Fig. 103) are sufficient to 
indicate the characteristics of this larva. 


Many other marine annelids pass through 
a trochophore stage during their life his- 
tory; those that do not are supposed to 
have lost this step during the course of 
evolution. 

Since a trochophore also appears in the 
development of animals belonging to other 
phyla, for example, Mollusca and Br}^ozoa, 
and resembles very closely certain Rotifera, 
the conclusion has been reached by some 
embr)'ologists that these groups of animals 
are all descended from a common hypo- 
thetical ancestor called a trochozoan. Strong 
arguments have been advanced both for and 
against this theory. 


Apical organ 


Esophagus 


Mesenchyme 


Stomach 
Ciliated finQ 


Larval nephn'dium 
Otocyst 


Blastocoel 
Ciliated ring 

Anos 
Anal vesicle 


Figure 107. Trochophore larva of a polychaete, Eupomatus, side view. 


ORIGIN AND RELATIONS 
OF THE ANNELIDA 

The annelids comprise the polychaetes, 
archiannelids, oligochaetes, and leeches. 
Formerly the archiannelids were regarded as 
ancestral annelids. It was hypothesized that 
both polychaetes and oligochaetes evolved 
from them. Since the discovery that the 
archiannelids show some larval features that 
resemble larval polychaetes, it is not known 
whether they are primitive or degenerative. 
Hartman prefers to regard the Archiannclida 
as an Appendix of Polychaeta. 


The polychaetes are by far the oldest, 
largest, and most diversified of the annelids. 
The origin of aquatic and terrestrial oligo- 
chaetes from an ancestral, generalized 
polychaete is likely. The leeches, in turn, 
have many features in common with oligo- 
chaetes; their peculiar modifications are the 
result of parasitism. 

RELATIONS OF THE 
ANNELIDA TO MAN 

Of the influence of segmented worms on 
human welfare, that of the earthworm and 


194 


COLLEGE ZOOLOGY 


leeches is the most obvious. Earthworms are 
widely used as bait for fishing; various 
methods have been used to drive them out 
of their burrows so that they can be collected 
in large numbers. These include use of an 
electric current, jarring the soil by beating 
a stick driven into it, and pouring a solution 
of chemicals such as mercuric chloride 
(poison) on the ground. Raising earthworms 
as bait for fishing has become quite profit- 
able in some resort districts. 


Figure 108. Diagram showing the burrow and 
castings of an earthworm. 

Charles Darwin demonstrated, by careful 
observations extending over a period of 40 
years, the great economic importance of 
earthworms. One acre of ground may con- 
tain over 50,000 earthworms. The feces of 
these worms are the little heaps of black 
earth called castings (Fig. 108) which strew 
the ground; they are especially noticeable 
early in the morning. Darwin estimated that 
more than 18 tons of earthly castings may 
be carried to the surface in a single year on 
one acre of ground; and in 20 years, a layer 
three inches thick would be transferred from 
the subsoil to the surface. By this means 
objects are covered up in the course of a few 
years. The continuous honeycombing of the 
soil by earthworms makes the land more 
porous and insures better penetration of air 
and moisture. The mixing of soil and organic 
matter in the digestive tract of the earth- 
worm should contribute something to in- 
creasing humus; however, the claim that the 
addition of earthworms to an unproductive 


soil will greatly increase its fertility is false. 
Earthworms may also be harmful. They 
disfigure lawns and golf courses with their 
castings and may serve as intermediate hosts 
of parasitic worms. For example, they are 
intermediate hosts in the life cycle of a 
cestode of chickens, Amoebotaenia, and in 
that of a pig lungworm of the nematode 
genus Metastrongylus; and they are passive 
carriers of the nematode worm Sy7igamus 
trachea, which causes gapes in fowls. 

As a transporter of soil, the lugworm, a 
species of Arenicola (Fig. 90), a polychaete, 
is even more effective than the earthworm. 
The amount of sand brought to the surface 
on 19 measured areas was 82,423 castings to 
an acre; the average amount of sand brought 
up to the surface each year on these areas 
was about 1911 tons to the acre, which, if 
spread evenly, would form a layer about 13 
inches deep. Other observations made at 
different places showed about 34 to 38 cast- 
ings to the square yard; the amount brought 
up was estimated to be about 3700 tons to 
the acre in a year, or equivalent to a layer 
about 24 inches thick. Species of lugworms 
aire widely used as bait in all places where 
they are found. A bed where fishermen con- 
stantly dig may contain about three million 
worms; removal of a few thousand a day 
produces no noticeable effect. In certain 
bays of New England, it is estimated that 
12^2 million worms (species of Neanthes 
and Glycera) are picked up by diggers in 
one year. At one time, a digger may collect 
about 350 worms. 

Oyster pests include polychaete worms of 
the genus Polydora; they cause mud blisters 
in the nacreous layers of the shells and ren- 
der the oysters unsalable; or the oyster may 
be weakened, if not actually killed. Oyster 
growers call it "worm disease." In some re- 
gions where oyster culture once flourished, it 
had to be discontinued, or different methods 
had to be introduced, such as rearing the 
spat (young oysters) on elevated or only 
partially submerged surfaces. Not only 


PHYLUM ANNELIDA. SEGMENTED WORMS 


195 


oysters, but other bivalved mollusks may 
be attacked; also years of low infesta- 
tion may be followed by years of heavy 
mortality. 

Sedentary polychaetes are among the more 
conspicuous agents that cause fouling on the 
bottoms of ships, dikes, and various harbor 
installations. They not only cause destruc- 
tion of the building materials, but add to the 
submerged weight so that the speed of a 
vessel is materially lessened. Periodic dv)'- 
docking of vessels in harbor cities is required 
to clear the hulls of fouling organisms. 

As reef-building agents, some sand- and 
lime-concreting, tube-building polychaetes 
are important in some parts of the world, 
changing shore contours, building up land 
masses, and transporting vast amounts of 
inert materials. As a result of selective ac- 
tion in the construction of tubes or mat- 
rices, some reefs or bars are likely to be 
pure sand or lime particles of homogeneous 
size. 

Use of certain polychaetes as food, such 
as the palolo, is of interest since there are 
certain traditional rites attending such 
feasts. The annual occurrence of swarms is 
predictable within narrow limits in certain 
regions of the south Pacific. Since the por- 
tions taken consist of almost pure yolk-laden 
eggs, a highly nutritive food is available. In 
oriental countries a large echiurid worm is 
collected, dried, and used as food. 

The widespread occurrence of fireworms 
(species of the polychaete family Amphino- 
midae) found along tropical shores is of 
interest to man largely because of the in- 
juries that may be inflicted. The worms are 
sometimes large, as much as a foot long, 
with striking color patterns and brilliant 
displays, creeping conspicuously over rocky 
surfaces. The unwary collector who picks 
them up is startled by severe burning from 
the contact. The injuries are produced by 
the harpoonlike bristles that penetrate hu- 
man skin. 

The use of the leech (Fig. 105) in medi- 


cine was based on the theory that many 
illnesses were due to "bad blood," either 
locally or generally; bloodletting, as the 
practice is called, was thus considered a cure 
for many ailments. Today in modern med- 
ical practice, transfusions of blood into the 
body are a common procedure, instead of 
bloodletting, to get rid of "bad blood." How- 
ever, so common was leeching in olden times 
that doctors were often called leeches. Not 
only Hirudo medicinalis but other species 
were used in various parts of the world. 
Wordsworth's interesting poem, "The Leech 
Gatherer" was based on the medicinal use 
of the leech. Bloodletting by leeches is now 
extremely rare in this country. In addition 
to such therapeutic use, the leech has been 
used as a drug, supposedly, to cure loss or 
graying of hair and other symptoms of old 
age. 

Leeches may be very annoying, especially 
in tropical regions where they live among 
dense vegetation and may attach themselves 
in large numbers to human beings and other 
animals. It has been said that such leeches 
caused much discomfort to the soldiers of 
Napoleon when they invaded North Africa. 
The salivary glands of leeches produce a 
substance termed hirudin, which prevents 
clotting of blood while the leech is feeding. 
For this reason a wound made by a leech 
may bleed for some time after the leech has 
detached itself. Hirudin is used in modern 
medicine as an anticoagulant. 


CLASSIFICATION OF THE 
PHYLUM ANNELIDA 

{For reference purposes only) 

Phylum Annelida. Annelids are bilaterally 
symmetrical, segmented worms; the body cavity 
is a true coelom; the nervous system consists 
of a dorsal brain and a pair of ventral nerve 
cords with, topically, a pair of ganglia in each 
segment; the digestive tract is a straight tube 
with a mouth that is anterior and ventral, and 


196 


COLLEGE ZOOLOGY 


an anus that is posterior and dorsal; the mus- 
cular system consists of an outer circular and 
an inner longitudinal series. The sperms and 
eggs are derived from mesoderm. Cleavage of 
the egg is spiral unless obscured by excessive 
yolk. Four classes are recognized as follows: 

Class 1. Polychaeta. Marine; parapodia well 
developed and provided with setae 
that are variously modified; prosto- 
mium and first few segments some- 
times highly cephalized; sexes usually 
separate; larva typically a trocho- 
phore. Ex. Neanthes virens (Fig. 
99). 
Class 2. Archiannelida. A small heterogeneous 
group, most nearly related to Poly- 
chaeta; therefore some zoologists pre- 
fer to make it an appendix to the 
class Polychaeta rather than give 
it a separate class status. It is charac- 
terized largely for loss of morphologic 
characters such as distinct parapodia 
or setae, and retention of larval ones 
such as ciliary rows. Mainly marine, 
littoral, and sometimes living in 
brackish to fresh water. Usually dioe- 
cious or sometimes hermaphroditic 
larva, a trochophore, or development 
direct. Ex. Dinophilus (Fig. 90). 
Class 3. Oligochaeta. Terrestrial or fresh- 
water; without parapodia, and setae 
few; head not well developed; herma- 
phroditic; no trochophore larva. Ex. 
Lumbriciis terrestris (Fig. 90). 
Class 4. Hirudinea. Parasitic or predaceous; 
mostly fresh-water or terrestrial; with- 
out parapodia or setae; body with 33 
segments plus prostomium; posterior 
and often an anterior sucker; herma- 
phroditic; coelom reduced by en- 
croachment of connective tissue. Ex. 
Ilirudo medicinalis (Fig. 105). 


SELECTED COLLATERAL 
READINGS 

Bahl, K.N. Pheretima, An Indian Earthworm. 
Lucknow Publishing House, Lucknow, India, 

1947. 

Ball, R.C., and Curry, L.L. "Culture and 
Agricultural Worth of Earthworms." Bull. 
222, Michigan State Univ., East Lansing, 
Mich., 1956. 

Beddard, F.E. "Oligochaetes (Earthworms, 
etc.) and Hirudinea (Leeches)." Cambridge 
Natural History. Macmillan, London, 1896. 

Bell, A.W. "The Earthworm Circulatory Sys- 
tem." Turtox News, 25:89-94, 1947. 

Borradaile, L.A. and Potts, F.A. The Inverte- 
brata. Cambridge Univ. Press, New York, 
1958. 

Buchsbaum, Ralph. Animals Without Back- 
bones. Univ. Chicago Press, Chicago, 1948. 

Darwin, C. The Formation of Vegetable 
Moidd Through the Action of Worms, with 
Observations on Their Habits. Murray, Lon- 
don, 1881. 

Grove, A.}. "On the Reproductive Processes of 
the Earthworm, Lumbricus terrestris.'' 
Quart. J. Microscop. Sci., 69:245-290, 1925. 

Harvev, E.N. Bioluminescence. Academic Fress, 
New York, 1952. 

Miner, R.W. Field Book of Seashore Life. 
Putnam, New York, 1950. 

Moore, J. P. "The Control of Blood-sucking 
Leeches, with an Account of the Leeches of 
Palisades Interstate Park." Roosevelt Wild 
Life Bull, Syracuse Univ., 2:1-55, 1923. 

Robertson, J.D. "The Function of the Calcif- 
erous Glands of Earthworms." /. of Exper. 
Biol. (British), 13:279-297, 1936. 

Stephenson, J. The Oligochaeta. Clarendon 
Press, Oxford, 1930. 

Wilson, E.B. "The Embr\'ology of the Earth- 
worm." /. of Morph., 3:387-462, 1889. 


CHAPTER 16 


OJI 


Sfr* ^ir* 


Phylum Arthropoda. 

Crayfish, Crabs, 

Barnacles, Water 

Fleas, Sow Bugs, 

and Others 


HE arthropods are joint-footed animals. 
To this phylum belong the lobsters, crabs, 
water fleas, barnacles, millipedes, centi- 
pedes, scorpions, spiders, mites, and insects 
(Fig. 109). An arthropod is bilaterally 
symmetrical, and consists of a longitudinal 
series of segments; on all or some is a pair 
of appendages. An animal of this phylum is 
covered with a hardened exoskeleton, con- 
taining chitin which is flexible at intervals 
to provide movable joints. It possesses a 
nervous system of the annelid type and has 
a coelom which is small or absent in the 
adult; the body cavity is a hemocoel filled 
with blood. 

The arthropods comprise about 78 per 
cent of all known species of animals (Fig. 
1). They are the dominant animals on the 
earth, if numbers of different species are 
accepted as criteria of dominance. The va- 
riety of the multitudes of arthropods seems 
infinite, but the fundamental plan of struc- 
ture is the same. The common cravfish ex- 
hibits to excellent advantage the character- 
istics of the class Crustacea as well as of 
arthropods in general. The segmented ap- 
pendages of the crayfish are particularly 
interesting since they seem to have devel- 
oped from a common type but have become 
greatly modified for the performance of 
various functions. Many arthropods, includ- 
ing the crayfish, possess compound eyes— a 
type of visual organ very different from 
those of other invertebrates and vertebrates. 
Other biological phenomena exhibited by 
the crayfish and worthy of special mention 
are the power of regeneration, autotomy, 
habit formation, and superficial cleavage of 
the fertilized egg. Many other Crustacea are 
of great biological interest and of economic 
importance. 


CAMBARUS-A CRAYFISH 

The crayfish (crawfish) is found in fresh- 
water lakes, streams, ponds, and swamps 
over most of the world. The genus Cam- 
barus is common in the central and eastern 

197 


198 


COLLEGE ZOOLOGY 


Figure 109. Representatives of the major classes of arthropods, showing body divisions and 
appendages. The lines suggest possible relationships. The figures are not drawn to scale. 


states, and Astacus in the western United 
States. The lobster Homarus americanus, 
differs in structure from the crayfish only in 
minor details. In Europe the most common 
crayfish is Astacus fluviatilis. 

External anatomy 

Exoskeleton 

The outside of the body is covered by a 
hard cuticle containing chitin* and impreg- 

* The best-known component of the cuticle is 
chitin, a nitrogenous polysaccharide; it is a very re- 


nated with lime salts. This exoskeleton (Fig. 
112) is thinner and flexible at the joints, 
allowing movement. 


sistant substance that is insoluble in water, alcohol, 
dilute acids, alkalies, and the digestive juices of 
many animals. Formerly it was thought that the 
chitin was responsible for the hardness of the 
cuticle; now, however, it is definitely known that 
the hardness of the cuticle is due to nonchitinous 
substances. The softer parts of the cuticle usually 
contain more chitin than the harder parts. The 
hard parts of the cuticle are said to be "sclerotized," 
not chitinized. 

Chitin also occurs in some sponges, hydroids, 
bryozoans, brachiopods, annelids, and mollusks. 


PHYLUM ARTHROPODA 


199 


Ostracoda 
(Ostracod) 


Anostraca 
(Fairy shrimp) 


Figure 110. Representative crustaceans. The lines suggest possible relationships. The figures 
are not drawn to scale. 


Regions of the body 

The body consists of two distinct regions, 
an anterior rigid portion, the cephalothorax, 
and a posterior series of segments, the abdo- 
men. The entire body is segmented, but the 
joints, except one, have been obhterated on 
the dorsal surface of the cephalothorax. 

Exoskeletal structures of a segment 

A typical segment consists of a convex 
dorsal plate, the tergum, a ventral transverse 


bar, the sternum, and plates projecting down 
at the sides, the pleura (Fig. 113). 

Cephalothorax 

The cephalothorax consists of segments 
1-12,* which are enclosed dorsally and later- 

* Many textbooks give 1 3 segments; but accord- 
ing to Snodgrass and other authorities, the anten- 
nules of arthropods are developmentally and phylo- 
genctically different from the appendages posterior 
to them. The antennules arise from a structure 
which appears to be a homologue of the prostomium 


Abdomen- 


L ^V \\^^\^. Uropod 


out 


Figure 111. Crayfish, showing the external characteristic structures of most of the class 
Crustacea. 


Tactile sefa 


Waxy layer 
Rigid layer 


Flexible layer 


Epidermis- 


Flexible membrane of jo 


Basement membrane 


Figure 112. Diagram of the body wall of an arthropod, showing some of its modifications. 
The rigid layer is replaced by a flexible membrane in places where movement occurs. 


200 


PHYLUM ARTHROPODA 


201 


' Protopodite 


Swimmeret - 


Exopodite 


Tergum 


Pleuron 


^ Endopodite 


Figure 113. Diagram of a cross section of the third abdominal segment of the crayfish. 


ally by a cuticular shield, the carapace. An 
indentation, termed the cervical groove, 
runs across the middorsal region of the cara- 
pace and obliquely forward on either side. 
The anterior pointed extension of the cara- 
pace is known as the rostrum. Beneath this 
on either side is an eye at the end of a mov- 
able stalk. The mouth is situated on the ven- 
tral surface near the posterior end of the 
head region. It is partly obscured by the 
neighboring appendages. The carapace of 
the thorax is separated into three parts by 
branchiocardiac grooves: a median dorsal 
longitudinal strip, the areola, and two large 
convex flaps, one on either side, the bran- 
chiostegites, which protect the gills beneath 
them. 

Abdomen 

In the abdomen there are 6 segments 
and a terminal body extension, the telson, 
bearing on its ventral surface the longitud- 
inal anal opening. Whether or not the tcl- 


of the annelids and is a region, in a phylogenetic 
sense, that has not come under the influence of 
metamerism; therefore the first pair of serially me- 
tameric appendages of the crayfish is the antennae. 
The antennules are actually prostomial sense organs, 
like the eyes, and hence not homologous with the 
other true appendages. It will be noted that the 
numbers used in the discussion of the appendages 
are 1 less in value than those of texts which list 19 
pairs of homologous appendages. 


son is a true segment is still in dispute; we 
shall adopt the view that it is not. The first 
abdominal segment (13) is smaller than the 
others and lacks the pleura. Segments 14-18 
are sheathed as described above. 

Appendages 

Every segment of the body bears a pair 
of jointed appendages. These are all varia- 
tions of a common type consisting of a 
basal region, the protopodite, which bears 
2 branches, an inner endopodite, and an 
outer exopodite. Beginning at the anterior 
end, the appendages are arranged as follows 
(Fig. 115). In front of the mouth are the 
antennae; the mouth possesses a pair of 
mandibles, behind which are the first and 
second maxillae; the thoracic region bears 
the first, second, and third maxillipeds, the 
chelipeds (pincers), and 4 other pairs of 
walking legs; beneath the abdomen are 5 
pairs of swimmerets, some of which are 
much modified. The sixth abdominal seg- 
ment bears greatly flattened appendages 
termed uropods. Tlie accompanying table 
(pp. 206) gives brief descriptions of the dif- 
ferent appendages, and shows the modifica- 
tions concerned with differences in function. 
The functions of some of the appendages 
are still in doubt. 

Three kinds of appendages can be dis- 
tinguished in the adult crayfish: (1) foliace- 


Circumesophageal connective 


Antennule 
Antenna 


stomach 

Ophthalmic 

artery 

Antennary 
artery 

Testis 
Ostium 


Pericardial 
sinus 


First abdominal appendage 
(for sperm transfer) 


Flexor muscle 
Extensor muscle 


Abdominal ganglion 


Uropod 
Anus 


Telson 


Ventral nerve cord 

Dorsal abdominal artery 
,^ Ventral abdominal artery 


202 


Figure 114. Internal structure of a male crayfish, 


PHYLUM ARTHROPODA 


203 


ous, the second maxilla, (2) biramous, the 
swimmerets, and (3) uniramous, the walk- 
ing legs. All these appendages have probably 
been derived from a single type, the modifi- 
cations being correlated with the functions 
performed by them. The biramous type may 
represent the condition from which the 
other types developed as shown in Fig. 115. 
The uniramous walking legs, for example, 
pass through a biramous stage during their 
embr)'ologic development. Again, the bira- 
mous embryonic maxillipeds are converted 
into the foliaceous type by expansion of 


their basal segments. Other types of appen- 
dages undergo similar changes. 

Structures that have a similar fundamental 
structure, regardless of function, due to des- 
cent from a common ancestor, are said to be 
homologous. The highly specialized cheli- 
peds, walking legs, jaws, and other structures 
of the crayfish have evidently developed 
from a fundamental type and have become 
different in function. When homologous 
structures are repeated in a series the condi- 
tion is known as serial homology. This is a 
most striking example of serial homology 


Generalized Biramous Appendage 
1. Antenna (touching, tasting) 


18. Uropod (swimming) 


[ Protopodife 


M^i EndoDodite 


13. First abdominal 
oppendage of male 
(copulating) 

First abdominal 

appendage of 

female 

(rudimentary) 


11. Fourth walking leg (walking) 

8. First wolking leg (pinching) 
Exopodite 


Figure 115. Homology and evolution of crayfish appendages. All are believed to have been 
derived from a generalized two-branched (biramous) appendage consisting of protopodite, en- 
dopodite, and exopodite. This basic plan of structure has been modified (specialized) for the 
various uses noted. The appendages demonstrate in a striking way the changes that occur in 
the evolution of structures. 


204 


COLLEGE ZOOLOGY 


and is one of the reasons why crayfish ap- 
pendages are usually studied in some detail. 

Internal anatomy 

The body of the crayfish (Fig. 114) con- 
tains all of the important systems of organs 
characteristic of the higher animals. The 
coelom is not large but is restricted to the 
cavities enclosing the gonads and the ex- 
cretory green glands. Certain organs are 
metamerically arranged, such as the nervous 
system; others, like the excretory organs, 
are concentrated into a small space. The 
systems of organs and their functions will be 
presented in the following order: ( 1 ) diges- 
tive, (2) circulatory, (3) respirator)', (4) 
excretory, (5) nervous, (6) sense, (7) mus- 
cular, and (8) reproductive. 

Digestive system 

The digestive tract of Cambarus consists 
of the following parts: 

1. The mouth opens on the ventral sur- 
face between the jaws. 

2. The esophagus is a short tube leading 
from the mouth to the stomach. 

3. The stomach is a large cavity divided 
by a constriction into an anterior cardiac 
chamber and a smaller posterior pyloric 
chamber. In the cardiac chamber are three 
hard teeth (chitinous ossicles) of use in 
grinding the food and collectively known 
as the gastric mill. The teeth are able to 
move one upon another; and, being con- 
nected with powerful muscles, are effective 
in grinding up the food. On either side of 
the pyloric chamber a duct enters from the 
digestive glands and above is the opening 
of the small cecum. 

About 10 to 30 days before molting, two 
calcareous bodies, known as gastroliths, are 
present in the lateral walls of the cardiac 
chamber of the stomach. During the molt 
these are shed into the stomach where they 
may be dissolved. When this occurs they 
are probably used in the calcification of the 
new exoskeleton. However, a high percent- 


age of the gastroliths are lost in the shedding 
process, and in these cases there is no pos- 
sibility of the re-use of the lime which they 
contain. 

4. A short midgut. 

5. The intestine is a small tube that 
passes through the abdomen and opens to 
the outside through the anus on the ventral 
surface of the telson. 

6. The digestive glands ("liver") are sit- 
uated in the thorax and abdomen, one on 
each side. Each consists of 3 lobes com- 
posed of a great number of small tubules. 
The glandular epithelium lining these tub- 
ules produces a pancreaticlike enzyme which 
may pass into the hepatic ducts and thence 
into the midgut. 

Nutrition 

Food. The food of the crayfish is made up 
principally of living animals such as snails, 
tadpoles, insect lar\'ae, small fish, and frogs, 
but decaying organic matter is also eaten. 
Crayfishes also prey upon others of their 
kind. They feed at night, being more active 
at dusk and daybreak than at any other 
time. Their method of feeding may be ob- 
served in the laboratory if a little fresh meat 
is offered to them. The maxillipeds and 
maxillae hold the food while it is being 
torn and crushed into small pieces by the 
mandibles. It then passes through the 
esophagus into the stomach. The coarser 
parts are ejected through the mouth. 

Digestion. In the cardiac chamber of the 
stomach, the food is ground up by the 
teeth of the gastric mill. When fine enough, 
it passes through the strainer which lies be- 
tween the cardiac and pyloric portions of 
the stomach. This strainer consists of two 
lateral folds and a median ventral one which 
bear hairlike processes and allow passage of 
only liquids or very fine particles. In the 
midgut, the food is mixed with the secre- 
tion from the digestive glands brought in 
by way of the hepatic ducts. From the mid- 
gut some of the dissolved or partially di- 
gested food passes into the digestive glands. 


PHYLUM ARTHROPODA 


205 


DESCRIPTIVE TABLE OF THE HOMOLOGOUS APPENDAGES OF THE CRAYFISH * 


SEGMENT 
NUMBER AND 

NAME 
OF APPENDAGE 


PROTOPODITE 


EXOPODITE 


ENDOPODITE 


FUNCTION 


1 

Antenna 


2 segments; excretory 
pore in basal seg- 
ment 


Broad, thin, dagger- 
like lateral projec- 
tion 


Three basal segments, 
and long many- 
jointed "feeler" 


Touch; taste 


2 
Mandible 


1 segment; a heavy 
jaw 


Not present 


Small; 3 segments of 
palp 


Biting food 


3 
1st Maxilla 


2 thin lamellae ex- 
tending inward 


Not present 


A small outer lamella 


Food handling 


4 
2d Maxilla 


2 bilobed lamellae; a 
broad plate, the 
epipodite 


Dorsal half of plate, 
the scaphognathite 


1 segment; small, 
pointed 


Creates current of 
water in gill cham- 
ber; food handling 


5 

1st Maxilliped 


2 thin segments ex- 
tending inward; 
epipodite extend- 
ing outward 


A long basal segment 
bearing a many- 
jointed filament 


Small; 2 segments 


Taste; touch; holds 
food 


6 
2d Maxilliped 


2 segments; a basal 
coxopodite bearing 
a gill, and a basip- 
odite bearing the 
exopodite and en- 
dopodite 


Similar to 5 


5 segments; the basal 
one long and fused 
with the basipodite 


Similar to 5 


7 
sd Maxilliped 


Similar to 6 


Similar to 5 


Similar to 6; but 
larger 


Similar to 5 


8 
1st Walking Leg 
(Chela, 
Cheliped or 
Pincer) 


2 segments; coxop- 
odite, and basip- 
odite 


Not present 


5 segments, the term- 
inal two forming a 
powerful pincer 


Pincer for offense and 
defense; aids in 
walking; touch 


9 

2d Walking Leg 

(Pereiopod) 


Similar to 8 


Not present 


As in 8, but not so 
heavy 


Walking; grasping 


10 
3d Walking Leg 


Similar to 8; coxop- 
odite of female 
contains genital 
pore 


Not present 


Similar to 9 


Similar to 9 


11 

4th Walking Leg 


Similar to 8 


Not present 


Similar to 9, but no 
pincer at end 


Walking 


12 
5 th Walking Leg 


Similar to 8; coxop- 
odite of male beais 
genital pore 


Not present 


Similar to 11 


Walking; cleaning ab- 
domen and eggs 


13 
1st Abdominal 
(1st Pleopod or 
Swimmeret) 


Reduced in female; 
in male, protop- 
odite and endop- 
odite, fused to- 
gether, forming an 
organ for transfer- 
ing sperm 


206 


COLLEGE ZOOLOGY 


DESCRIPTIVE TABLE OF THE HOMOLOGOUS 

(Continued) 


APPENDAGES OF THE CRAYFISH * 


SEGMENT 
NUMBER AND 

NAME 
OF APPENDAGE 


PROTOPODITE 


EXOPODITE 


ENDOPODITE 


FUNCTION 


14 
2d Abdominal 
(2d Pleopod or 
Swimmeret) 


In female 2 segments 


In female many- 
jointed filament 


In female many- 
jointed filament 


In female as in 15; in 
male modified for 
transferring sperm 
to female 


15 
3d Abdominal 
(3d Pleopod or 
Swimmeret) 


2 segments 


Many-jointed fila- 
ment 


Many-jointed fila- 
ment 


Creates current of 
water; in female 
used for attach- 
ment of eggs and 
young 


16 
4th Abdominal 
(4th Pleopod 
or Swimmeret) 


2 segments 


As in 15 


As in 15 


As in 15 


17 
5th Abdominal 
(5 th Pleopod 
or Swimmeret) 


As in 16 


As in 15 


As in 15 


As in 15 


18 
6th Abdominal 
(Uropod) 


1 short, broad seg- 
ment 


Flat oval plate di- 
vided by transverse 
groove into two 
parts 


Flat oval plate 


Swimming 


* The antennules are not included in this table because they are considered by Snodgrass and other au- 
thorities as prostomial sense organs, as are the eyes. 


which not only form the digestive enzymes 
but also absorb some of the products of 
digestion. Undigested particles pass on into 
the posterior end of the intestine, where 
they are gathered together into feces and 
pass through the anus. 

Circulatory system 

Blood. The blood plasma, into which the 
absorbed food passes, is an almost colorless 
liquid, but contains hemocyanin, a bluish 
respiratory pigment that contains copper in- 
stead of iron. There are suspended in the 
plasma a number of amoeboid cells, the 
blood corpuscles. The principal functions 
of the blood are transportation: it transports 
food materials from one part of the body 
to another, oxygen from the gills to the 
various tissues, carbon dioxide to the gills, 
and waste products to the excretory organs. 


If a crayfish is wounded, the blood thickens, 
forming a clot; it is said to coagulate. This 
clogs the opening and prevents loss of 
blood. 

Blood vessels. The principal blood vessels 
(Figs. 114 and 116) are a heart, 7 main 
arteries, and a number of spaces called 
sinuses. 

Heart. The heart is a muscular-walled, 
saddle-shaped sac lying in the pericardial 
sinus in the median dorsal part of the thorax. 
It may be considered a dilatation of a dorsal 
vessel, resembling that of the earthworm. 
Blood enters the heart through three pairs 
of valves called ostia, one dorsal, one lateral, 
and one ventral. 

Arteries. Five arteries arise from the an- 
terior end of the heart. 

1. The ophthalmic artery is a median 
dorsal tube passing fonvard over the stomach 


PHYLUM ARTHROPODA 


207 


and supplying the cardiac portion, the eso- 
phagus, and the head. 

2, 3. The two antennary arteries arise one 
on each side of the ophthalmic artery. They 
pass forward, outward, and downward, and 
then branch, sending a gastric artery to the 
cardiac part of the stomach, and arteries to 
the antennae, excretory organs, muscles, and 
to other cephalic tissues. 

4, 5. The two hepatic arteries leave the 
heart below the antennary arteries. They 
lead directly to the digestive glands. 

6. The dorsal abdominal artery is a me- 
dian tube leading backward from the ventral 
part of the heart and supplying the dorsal 
region of the abdomen. It branches near its 
point of origin, giving rise to the sternal 
artery; this leads directly downward, and, 
passing between the nerve cords connecting 
the fourth and fifth pairs of thoracic ganglia, 
it divides into two arteries (Fig. 114). One 
of these, the ventral thoracic artery, runs 
for^vard beneath the ner\'e chain and sends 
branches to the ventral thoracic region and 
to appendages 2 to 12; the other, the ventral 
abdominal artery, runs backward beneath 
the nerve chain and sends branches to the 
ventral abdominal region. 

Sinuses. The blood passes from the small- 
est arteries into spaces lying in the midst 
of the tissues, called sinuses. The pericardial 
sinus has already been mentioned. The 
thorax contains a large ventral blood space, 
the sternal sinus, and a number of branchio- 
cardiac sinuses that lead from the bases of 
the gills, up the inner sides of the thoracic 
wall, to the pericardial sinus. A perivisceral 
sinus surrounds the digestive tract in the 
cephalothorax. 

Blood flow. The heart by means of rhyth- 
mic contractions forces the blood through 
the arteries to all parts of the body. Valves 
are present in every artery where it leaves 
the heart; they prevent the blood from 
flowing back. The finest branches of these 
arteries, open into spaces between the tis- 
sues, and the blood eventually reaches the 
sternal sinus. From here it passes into 


afferent channels of the gills and into the 
gill filaments, where the carbon dioxide is 
given off and oxygen is taken in from the 
water in the branchial chambers. It then 
returns by way of the efferent gill channels, 
passes into the branchiocardiac sinuses, 
thence to the pericardial sinus, and finally 
through the ostia into the heart. The valves 
of the ostia allow the blood to enter the 
heart, but prevent it from flowing back into 
the pericardial sinus. 

The crayfish thus has an open (lacunar) 
blood system in which the blood is distrib 
utcd to blood spaces (sinuses) before being 
returned to the heart. There are no veins as 
in vertebrates. 

Respiratory system 

Breathing in the crayfish is by means of 
plumy gills. Between the branchiostegites 
and the body wall are the branchial cham- 
bers containing the gills (Fig. 116). At the 
anterior end of the branchial chamber there 
is a channel in which the gill bailer ( scaph- 
ognathite) of the second maxilla moves 
back and forth, forcing the water out 
through the anterior opening. Water flows 
in through the posterior opening of the 
branchial chamber and ventrally. 

There are two rows of gills, named accord- 
ing to their points of attachment. The outer- 
most, the podobranchs, are fastened to the 
coxopodites of certain appendages; and the 
inner double row, the arthrobranchs, arise 
from the membranes at the bases of these 
appendages. In Astacus there is a third row, 
the plcurobranchs, attached to the walls of 
the thorax. The podobranchs consist of a 
basal plate covered with delicate setae and 
a central axis bearing a thin, longitudinally 
folded, corrugated plate on its distal end, and 
a featherlike group of branchial filaments. 
Each arthrobranch has a central stem, on 
each side of which extends a number of fila- 
ments, causing the entire structure to re- 
semble a plume. Attached to the base of 
the first maxilliped is a broad thin plate, the 
epipodite (Fig. 115), which has lost its 


208 


COLLEGE ZOOLOGY 


Carapace 


Pericardial sinus 


Heart 


Branchiostegite 


Muscle 


Int