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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.) 




Ph.D., Sc.D. 





M.S., Ph.D. 



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 


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 



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 

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- 

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 



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 

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- 

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. 


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 



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 

Karl A. Stiles 

East Lansmg, Michigan 


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 



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 

18. Phylum Arthropoda. In- 

19. Phylum Arthropoda. 
Spiders and Their Allies 

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

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 


26. Class Osteichthyes. Bony 

170 Fishes 378 

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







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 


36. The Origin and History of 
Animal Life 601 

37. Ecology and Zoogeogra- 
phy 628 






/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 

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- 

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, 



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- 

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 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 

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. 


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- 

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. 


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 

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. 


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


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


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. 


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. 



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. 


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. 


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. 


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. 


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 

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 


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 


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. 


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 



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 

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

Animal Kingdom (consists of all known ani- 
Phylum Chordata (animals possessing noto- 
chords ) 
Subphylum Vertebrata (chordates with 
vertebral columns) 
Class Mammalia (vertebrates with 
mammary glands) 
Order Carnivora (mammals that eat 
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 


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 


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 

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 


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. 


Fields of the zoological sciences 

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


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). 


Structure and 
functions with- 
in cells 

Structure of the 
animal body 


Nature of diseases, 
causes and symptoms 


Functions of organisms 


Relations of organisms 
to their environment 

Fossil organisms 


Animal societies 
including man 


Microscopic structure 
of tissues and organs 

Classification of 


Developmental stages 
of organisms 

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- 



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- 

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 


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 


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 

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 



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- 

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, 


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- 

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 and 



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- 




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. 


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 


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.) 


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: 


































































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 









Nucleic acid 








Inorganic salts 


Other substances 


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 

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 


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. 



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 

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 


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- 

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 

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 



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 

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 

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- 

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: 

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 



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. 


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. 



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 


Centrosome < 


Nucleus — 

Nuclear membrane 
Golgi apparatus 


Fat droplet 

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 



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- 



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 

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 


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 


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 

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 



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. 


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 


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. 


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 

Nuclear membrane 
Cell membrane 


Daughter cells in 


Spindle fiber 

Astral ray 




Spindle fiber 






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. 





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. 


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. 


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- 

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. 



^ Telophase 


Y Anaph 







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 



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 

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. 


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, 




Phylum Protozoa. 




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 

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



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


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 

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 

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- 


Protozoa without motile organelles, 

but with a spore stage in their life 

Class 4. Ciliata. Type: Paramecium cauda- 


Protozoa that move by means of cilia. 


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 




Cell membrane 



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. 




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 

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 

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. 


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. 


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- 

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 



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 

2J^ minutes 
later a food 
cup is formed 

7^ minutes 

(The carbon is not ingested) 

8 minutes 

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



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. 


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 


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 


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 




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. 


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. 


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. 


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. 


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. 


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.) 




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. 



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 


and quickly moves away 

forms pseudopodia around ifr 

and turns to 
ovoid the obstacle 


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). 


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. 


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 


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 



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). 


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 

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 

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). 


(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 


Side view 
Arcelia, with a secreted shell 

Actlnophrys, with needle- 
shaped pseudopodia 


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


Difflugia, with a shell 
of sand grains 

Figure 17. Various representatives of Sarcodina. Highly magnified. 




pseudopodia. Mostly marine. 
Ex. Globigerina bulloides (Fig. 
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. 


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., 


f ^# 



Phylum Protozoa. 

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 

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. 



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 




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


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 






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. 



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- 



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 


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 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. 



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. 


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 









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



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. 


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- 


{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- 

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. 

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 



Gelatinous matrix 

Side view of 
somatic cells 

in the development of daughter 

Sperm bundle (surface view) 

Sperm bundle (side view) 
Unfertilized egg 


Stages in the development 

of sperm bundles 

Fertilization of egg 

Free sperm bundle 


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. 




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. 

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. 


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, 



Phylum Protozoa. 



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 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. 



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 



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 



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. 


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. 


Sporozoite of Lankesferella 
in frog erythrocyte 

Oocyst of Isospora 
hominis from human 

Spores of Thelohania 
coniejeani from cray- 
fish muscle 

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

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 


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



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. 


(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. 

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- 

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. 


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. 



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 

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. 



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



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 

Food vacuole 3 

Contractile vacuole 

vJral groove - 

Cell mouth — 
Cell gullet 


Food vacuole 2 

Bacteria being ingested 

Food vacuole forming. 
Cell anus^ 



t— ' 


Radiating canal of C. vacuole 
Food vacuole 1 


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. 



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. 




Basal body 
of cilium 



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 



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. 


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 


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 





Red (food vacuole forming) 

Blue-green (acid) 

-Yellow (alkaline) 



-Waste material 


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- 



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 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. 


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. 


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. 


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

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 



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 



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. 


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. 


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- 



Micronucleus in mitosis, 
macronucleus elongates 


Micronucleus begins mitosis 


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- 

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 



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 


The fusion micronucleus 
divides three times 


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 



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. 


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. 


{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 

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






Oui \ 

Vortkella Stylonychia Stentor 

Figure 31. Some representative ciliated protozoans. Highly magnified. 





Kerona from Hydra 

Nycfotherus from 
rectum of frog 

Podophrya with 

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). 


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, 

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

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




Relations of 


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. 


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- 

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 




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. 

Fertilizafion of egg 



Mature oocyst 
with sporozoites 

Sexual cycle in mosquito 
(sporogony) about two weeks 


Release of 

'■■.•■■■ .'r 


' 'I.'.' ' 

our asexua 

cycle in man 


Parasite enters 
Red blood cell 

Nuclear division Later stage Ring stage 

in schizont I __ Trophozoites ' 




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 

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 

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- 



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 
red blood ce 


Chromatoid body 




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



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 

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. 



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- 


Cell mouth 
Contractile vacuole 

Food vacuoles 


Contractile vacuole 
Anal pore 



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. 


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- 



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- 

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. 


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- 



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 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 



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. 


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 



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- 


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, 



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 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 



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 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- 

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 

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- 

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 



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 

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- 

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 


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. 


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. 


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- 



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 

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. 


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 



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). 






"55 (U 


^f^^ik. .^••*^. 

O _ D) 

/•'■ * '\ 



|; • ) 


CT (1) 




Egg with slight yolk (starfish) 




: - 

















.2 o 

to U 

Egg with moderate yolk (frog) 

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



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. 


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. 


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



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 


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. 


(fertilized egg) 








by verteb 

(in part) 


Most of 





Peritoneum an 

Connective tiss 
of digestive a 
other organs of 
endodermal origin 

Figure 42. 


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

Lining of 



Lining of 



Most of 



Lining of 
ddle ear 







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 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 

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- 


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 



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. 


(surface view) 



Nonciliated Ciliated 
columnar columnar 
Vertical section 



Leucocytes ■ 

Tendon (fibrous) 



Smooth muscle 










Cartilage (hyaline) 


One organ (as the 
intestine) consists 
of many tissues 

Figure 43. Types of tissue. 



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- 

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 


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 

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. 



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 

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 



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 

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- 







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


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 





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



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- 

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 

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 

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. 


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 




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 

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. 


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- 

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. 


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 



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- 


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

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

the Vertebrates. Macmillan, New York, 

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, 




Phylum Porifera. 
Simple Multicellular 


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. 




Elephant's ear sponge 

Venus's flower-basket 

Glass rope 

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 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. 




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 



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



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 



Dermd amoebocyte 
Incurrent pore 

Food particle 

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 



Central cavity 
Surface pore 



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. 


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 

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 



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 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. 


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- 



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 

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 


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 

Spongin without 

Side view 

Thick wait 
with spicules 

Germinal cells 

Development of 




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.) 



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. 



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 



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


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 


{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). 



Order 2. Syconosa. Sponges of sycon 
type, or sycon type at first, 
changing into rhagon. Ex. 
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- 
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" 

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. 

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


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, 

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, 







Phylum Coelenterata 

(Cnidaria). Simple 

Tissue Animals 


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- 

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. 



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 


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 



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. 




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. 


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: 


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). 


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 



on tentacle 


Interstitial cells 
Gastrovascular cavity 


Longitudinal and 
circular muscles 


Nerve network 
Egg in ovary 

Cross section 


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 

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- 



Interstitial cells- 

Sensory cell- 

cell nucleus- 
Interstitial cells- 
Nerve eel 

muscle fiber- 


Nufritive cell 
Gland cell 

Sensory cell 
-Gland cell 

Young gland cell 

— Food 

Food being 


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 

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. 



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 


Food vacuole 



gland cell 

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.) 



higher animals in that they are not polar- 

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 

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 

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 

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 



DIschorged thread 
Dissolved chitin 




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- 

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 



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 

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 



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 



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 


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. 


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, 



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 

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. 


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 

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. 


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. 



in a new direction; hungry specimens are 
more active than well-fed individuals. 


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. 


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- 

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. 


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 takes place in the hydra 
both asexually and sexually; in the former 
case by budding, in the latter by production 



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- 

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 

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 
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 




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 


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 



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 

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. 


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. 


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 

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 



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 

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. 


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 

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 




from male 







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. 




Mesoglea - 

Gastrovascular cavity 

Ring canal 
Nerve ring 





Radial canal 

— Gonad 


Oral lobe 


Tentacular bulb 


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 



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 

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 



Gastral filament £^^^^00 

Subgenital pit 

Epidermis Gonad 


Radial cana 




medusa (section) 








y Blastula 



Attached stage 

Planula swims free 
of oral arm 


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 

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 



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 



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 

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 



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. 



1IIIIIIIII Ectoderm 
Iv/fS^^-x-:! Endoderm 


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. 


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. 


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 

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 



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- 

High tide 

:\::Coral reef;w/Vw:v':; 

Low tic 






came IS 


::v.Coral reef'- 

Cross section of a fringing reef 

Cross section of a barrier reef 

High tide 
Low tide 


■■^:^" " 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 



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. 


(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. 




SEA FAN Gorgonia 


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. 


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. 



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., 

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, 

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. 



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 

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 





Oral end 



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 

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 

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 



north along our Atlantic Coast by the Gulf 


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. 





Simple Organ-System 


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 

Two entire classes of flatworms are para- 
sites, some of which, such as the tapeworms, 




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. 


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 - 



Excretory network 

Flame ce 


Digestive tract 

Transverse nerve 

nerve cord 
Sperm duct 

Yolk gland 


Opening of 

Copulatory sac 

Genital chamber 
Genital pore 

Lateral branch of 
digestive tract 

Digestive and nervous 

Excretory and 
reproductive systems 


Pharynx at rest in pharyngeal 
chamber (side viev/) 

Pharynx protruded 
through mouth 





Cell lumen 

Flame cell 

Excretory duct 


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 

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 



Pharyngeal chamber 


— Epidermis 


Intestinal epithelium 

Longitudinal muscle fibers 

Circular muscle fibers 



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. 






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). 


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 



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 

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 

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 

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 

U^«^"* Mehlis' gland |-Yolk duct 

■Sperm duct 

■Yolk gland 

Ventral sucker 


"Lourer's canal 


Eggs develop, 
miracidia hatch 
and burrow 
into snails 

Dissected free 
from cyst 

Cercariae emerge 
and encyst ^•^^•* 
on plants 




Germ ball 





Figure 73. Life cycle and structure of the liver fluke of sheep, Fasciola hepatica. 



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 

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§ 

Oral sucker 

Genird pore Uterus 
Yolk gland 

Ovory Ootype 

Testis Excretory blade 
Sperm duct 


Excretory pore 

Ventral sucker yolk duct Peminal receptacle 

Seminal vesicle tourer's con^l 


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 


Encysted stage 
dissected free 
of cyst 
Ventral sucker- 


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. 








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 




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. 


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. 


The free-living flatworms are of very little 
importance to man, but many of the trema- 
todes and cestodes are dangerous parasites, 


budding off 

Embryo from ripe 
proglottid in feces of man 


Gravid proglottid 
Figure 76. Life cycle and structure of the pork tapeworm. Taenia. 










In snails and on 

China, Indo China, 
Formosa, Suma- 
tra, India 

Man, pigs, in 
China, Formosa, 


In snails and fresh- 
water fish 

Egypt, China, 

Man, cat, dog, in 


(Fig. 74) 

In snails and fresh- 
water fish 

China, Japan, 
Korea, French 

Man, cat, dog, and 
fish-eating mam- 


In snails and fresh- 
water fish 

Europe, Siberia 

Man, cat, dog 



In snails and fresh- 
water crabs 

Japan, China, 

Man, tiger, dog, cat, 
mustelids, pig 


(Fig. 75) 

In snails 

Africa, Near East, 
Portugal, Aus- 

Man, monkey 



In snails 

Africa, West In- 
dies, N. and S. 



In snails 

Japan, China, 

Man, cat, dog, pig, 

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 


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 

Copepods eaten by fish 

Eggs passed 

Eggs hatch into ciliated 
larvae in water 

Co'pepod eats 
•.; larvae 

Larva from fish muscU 


Larva from 

Adult in intestine of man and 
other mammals 









(Fig. 77) 

In copcpods and fresh- 
water fish 


Man, dog, cat, bear, 
fox (intestine) 

(Fig. 77) 

In hver, brain, lungs, 
of man, pig, sheep 


Dog and other car- 
nivores (intestine) 

Taenia saginata 

Muscles of cattle 



Taenia solium 
(Fig. 76) 

In muscles of pig, and 
man, accidentally 




{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- 

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 
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. 
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 
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). 


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



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, 

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. 





Nematomorpha, and 




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. 


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 

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. 







Ascaris lumbricoides— a 
roundworm parasitic in man 


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 

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- 


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- 



Dorsal nerve 


Body wall-| Epidermal syncytiu 
Muscle cell 


Growth zone 

(^ Germinal zone 

Rod border of intestine 
Intestinal epithelium 
Lumen of intestine 
Excretory tube 
Lateral line 



Muscle cell process 

Longitudinal muscle fibrils 

Ventral nerve 

Protoplasmic process 



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 



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- 

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 

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 



Esophagus Body cavity (pseudocoel) 

Seminal receptacle 

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- 

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 

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 



Giant kidney worm, 
Dioctophyma renale, 
from fish-eating 

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. 



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 


The hookworms, Ancylostoma duodenale, 
and Necator americanus (Fig. 82) are also 
widespread and injurious; about 95 per cent 



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 

Necator amerkanus, 
from intestine 


Trichuris trkhiura, 
from cecum 

Guinea worm, Dracunculus 
medinensis, being wound 
on a stick 

Figure 82. Some roundworms that live in man. 




Adult female becomes 
embedded in small 
intestine of host and 
produces larvae 

Infected pork 

Larva enters lymph 
or blood vessels 


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 


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. 


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 



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- 

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 

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 



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. 

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- 


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 


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- 



Proboscis with hooks 



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. 





{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. 

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 



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). 


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, 



i 1 


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. 




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- 




Normal worm 


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. 


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 

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 



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 



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- 

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 


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 

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. 


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 



Statoblast of 








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 

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. 


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. 


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 







Echinoderes dujardinl 












Female genital 




Male genita 




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 

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. 


Most of the species in this group, about 15 
in all, belong to the genus Phoronis (Fig. 
89). They are small, wormlike, marine ani- 


The Echinodera are very small marine 
worms that range from 0.18 to 1 mm. in 



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 


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 


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., 

Ward, H.B., and Whipple^ G.C. Freshwater 
Biology. Wiley, New York, 1918. (New 
edition in press.) 




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- 

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- 


* 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. 






Figure 90. Representatives of 4 classes of segmented worms. They are varied in form and 
widely distributed. The figures are not drawn to scale. 


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 


Mouth cavity 

Cerebral gangffon "brain' 
Circumpharyngeal connective 

Pharyngeal muscle 



_ Nephridiopore 

Seminal receptacle 

Sperm funne 

Seminal vesicle 

Cut edge of seminal vesicle 

Vas efferent 



Vas deferens 
Egg funne 

Egg sac 

Dorsal blood vessel 


Ventral nerve cord 

Figure 91. Anterior part of an earthworm with dorsal body wall removed to show the 
internal organs. 




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- 








I Mesoderm ^ Endoderm 

The relationship of body cavities to germ cell layers 

Intestinal epithelium" 

Typhlosole - 

Peritoneum - 


Dorsal blood vessel 

Chloragogue cells 



Nephridium (section) 
Circular muscle 
Longitudinal muscle 
Ventral nerve cord 
Subneural blood vesse 

Tubule of nephridium 


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. 




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 



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- 


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- 



Aortic aich ("heart") 

•Ventral vessel 
Nerve cord 

Lateral neural vessel 
Subneural vessel 

Valve between dorsal 
and t/phlosolor vessels 

Typhlosolar v. 


intestinal v. 
Segmental v. 

Nerve cord- 

-Dorsal vessel 

-Dorsal intestinal v. 
(efferent intestinal) 

Body wall capillaries 


Nephridial vessels 

Parietal vessel 

Ventral vessel 

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.) 



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. 


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 







Figure 94. Earthworm. Side view of anterior end showing the cerebral gangha and large; 
nerves. (After Hess.) 



worm is inefEcient as an oxygen-transporting 
substance compared to the hemoglobin of 

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 


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 



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 


Gland cell pore 
Supporting cell 


(light sensitive cell) 


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 



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 



' '^.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. 


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 






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- 



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 

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 


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 — 



■- Peristom 


Dorsal cirrus 



Dorsal blood 






Ventral cirrus 
blood V. 

Ventral nerve 

Dorsal blood vessel 

Longitudinal muscle 

Ventral blood vessel 
Ventral nerve cord 

Circular muscle 

PARAPODIUM (Posterior view] 




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. 


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- 



Suckers .•• 

Adult in tube 





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 



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 


Sexual segments sv/im 
to surface; eggs and 
sperms are discharged 

Parent worm 
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. 



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. 


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 

The archiannelids number only about 45 



Faint exfernol segmentation 

Young trochophore 

Elongation of 
trochophore larva 

Transformation stages of larva 

Figure 103. Stages in development of Polygordius appendiculatus. one of the Archiannelida. 
(After Fraipont.) 



species in 10 genera; they have originated in 
various ways, and are thus a heterogeneous 
assemblage and not to be regarded as a 

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. 


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.) 



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 

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- 


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- 

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. 

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. 


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 



Medicinal leech attached to ar 


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 

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) 

Muscular pharynx 

■Anterior sucker 
Cerebral ganglia ("brain") 

1st diverticulum of crop 



Mole opening 


Female opening 

Vas deferens 

Ventral nerve cord 





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. 




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. 


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 

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 



Ciliated finQ 

Larval nephn'dium 

Ciliated ring 

Anal vesicle 

Figure 107. Trochophore larva of a polychaete, Eupomatus, side view. 


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. 


Of the influence of segmented worms on 
human welfare, that of the earthworm and 



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 



oysters, but other bivalved mollusks may 
be attacked; also years of low infesta- 
tion may be followed by years of heavy 

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 

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 

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. 


{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 



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. 
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). 


Bahl, K.N. Pheretima, An Indian Earthworm. 
Lucknow Publishing House, Lucknow, India, 


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, 

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. 



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 


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 




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 


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. 




(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). 


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 


L ^V \\^^\^. Uropod 


Figure 111. Crayfish, showing the external characteristic structures of most of the class 

Tactile sefa 

Waxy layer 
Rigid layer 

Flexible layer 


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. 




' Protopodite 

Swimmeret - 




^ 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 


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. 


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 








First abdominal appendage 
(for sperm transfer) 

Flexor muscle 
Extensor muscle 

Abdominal ganglion 



Ventral nerve cord 

Dorsal abdominal artery 
,^ Ventral abdominal artery 


Figure 114. Internal structure of a male crayfish, 



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 

First abdominal 

appendage of 



11. Fourth walking leg (walking) 

8. First wolking leg (pinching) 

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. 



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 

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. 


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. 












2 segments; excretory 
pore in basal seg- 

Broad, thin, dagger- 
like lateral projec- 

Three basal segments, 
and long many- 
jointed "feeler" 

Touch; taste 


1 segment; a heavy 

Not present 

Small; 3 segments of 

Biting food 

1st Maxilla 

2 thin lamellae ex- 
tending inward 

Not present 

A small outer lamella 

Food handling 

2d Maxilla 

2 bilobed lamellae; a 
broad plate, the 

Dorsal half of plate, 
the scaphognathite 

1 segment; small, 

Creates current of 
water in gill cham- 
ber; food handling 


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 

2d Maxilliped 

2 segments; a basal 
coxopodite bearing 
a gill, and a basip- 
odite bearing the 
exopodite and en- 

Similar to 5 

5 segments; the basal 
one long and fused 
with the basipodite 

Similar to 5 

sd Maxilliped 

Similar to 6 

Similar to 5 

Similar to 6; but 

Similar to 5 

1st Walking Leg 
Cheliped or 

2 segments; coxop- 
odite, and basip- 

Not present 

5 segments, the term- 
inal two forming a 
powerful pincer 

Pincer for offense and 
defense; aids in 
walking; touch 


2d Walking Leg 


Similar to 8 

Not present 

As in 8, but not so 

Walking; grasping 

3d Walking Leg 

Similar to 8; coxop- 
odite of female 
contains genital 

Not present 

Similar to 9 

Similar to 9 


4th Walking Leg 

Similar to 8 

Not present 

Similar to 9, but no 
pincer at end 


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 

1st Abdominal 
(1st Pleopod or 

Reduced in female; 
in male, protop- 
odite and endop- 
odite, fused to- 
gether, forming an 
organ for transfer- 
ing sperm 












2d Abdominal 
(2d Pleopod or 

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 

3d Abdominal 
(3d Pleopod or 

2 segments 

Many-jointed fila- 

Many-jointed fila- 

Creates current of 
water; in female 
used for attach- 
ment of eggs and 

4th Abdominal 
(4th Pleopod 
or Swimmeret) 

2 segments 

As in 15 

As in 15 

As in 15 

5th Abdominal 
(5 th Pleopod 
or Swimmeret) 

As in 16 

As in 15 

As in 15 

As in 15 

6th Abdominal 

1 short, broad seg- 

Flat oval plate di- 
vided by transverse 
groove into two 

Flat oval plate 


* 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 vessels. The principal blood vessels 
(Figs. 114 and 116) are a heart, 7 main 
arteries, and a number of spaces called 

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 



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 

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 




Pericardial sinus 





gland — 



nerve cord 











Sterna! sinus 

Artery to leg 

Figure 116. Cross section of a crayfish showing arrangement of the gills and some of the 
internal organs. 

branchial filaments. Crayfishes do not drink 
when in water; the water diffuses through 
the gills into the body. 

Excretory system 

The excretor}- organs are a pair of rather 
large bodies, the green glands (Fig. 114), 
situated in the ventral part of the head an- 
terior to the esophagus. Each green gland 
consists of a glandular portion which is 
green in color, a thin-walled dilatation, the 
bladder, and a duct opening to the exterior 
through an excretory pore on the basal seg- 
ment of the antenna. 

Nervous system 

The general structure of the nervous sys- 
tem (Fig. 114) of the crayfish is in many 
respects similar to that of the earthworm but 
further developed in the head and thorax. 

The central nervous system includes a dorsal 
ganglionic mass, the brain ( supraesophageal 
ganglia), in the head, and two circume- 
sophageal connectives which pass to the 
subesophageal ganglion. This is the most 
anterior ganglion of the ventral nerve cord. 
Following the subesophageal are 6 ganglia 
in the thorax and 6 ganglia in the abdomen, 
all joined to each other by the ventral nerve 

The brain is a compact mass, larger than 
that of the earthworm, which supplies the 
eyes, antcnnules, and antennae with nerves. 

The ganglia and connectives of the ventral 
nerve cord are more intimately fused than 
in the earthworm; it is difficult to make out 
the double nature of the connectives except 
between thoracic ganglia 4 and 5, where the 
sternal artery passes through. 

The visceral nervous system consists of 



an anterior visceral nerve which arises from 
the ventral surface of the brain; it is joined 
by a nerve from each circumesophageal con- 
nective, and, passing back, it branches upon 
the dorsal wall of the pyloric part of the 
stomach, sending a lateral nerve on each 
side to unite with an inferolateral nerve 
from the stomatogastric ganglion. 

Sense organs 

Eyes. The compound eyes of the crayfish 
are situated at the end of movable stalks 
which extend out, one from under each side 
of the rostrum (Fig. 114). The external 

convex surface of the eye is covered by a 
modified portion of the transparent cuticle 
called the cornea. This cornea is divided by 
a large number of fine lines into 4-sided 
areas termed facets. Each facet is but the ex- 
ternal part of a long slender visual rod 
known as an ommatidium. 

Sections (Fig. 117, A) show the com- 
pound eye to be made up of similar om- 
matidia lying side by side, but separated 
from one another by a layer of dark pigment 
cells. The average number of ommatidia in a 
single eye is 2500. 

Two ommatidia are shown in Fig. 118. 

Cornea y , 
Facet A/ 


Lighf rays 



Nerve fibers 
Optic ganglia 

Connective tissue 
Optic ganglion 
Optic nerve 


Figure 117. Crayfish. A, entire eye in longitudinal section to show its general structure. 
B, diagram of part of a compound eye showing in strong light how the light rays are absorbed 
by the pigment surrounding the ommatidia; only those that pass through the center such as 
A-A', B-^', etc., reach the nerve fibers. This results in a separate image from each ommatidium. 
(A after Borradaile and Potts; B after Lubbock.) 

Beginning at the outer surface, each om- 
matidium consists of the following parts: 
(1) a cornea (lens); (2) two corneagen 
cells which secrete the cornea; (3) a crys- 
talline cone formed by four cone cells; (4) 
two retinular cells surrounding the crystal- 
line cone; (5) several retinular cells which 
form a central rhabdom where they meet; 

and (6) a number of black basal pigment 
cells around the base of the retinular cells. 
Fibers from the optic nerve enter at the 
base of the ommatidium and communicate 
with the inner ends of the retinular cells. 

Yision. The eyes of the crayfish are sup- 
posed to produce a mosaic or apposition 
image; this is illustrated in Fig. 117, B, where 



the ommatidia are represented by A-E, and 
the fibers from the optic nerve by A'-E'. 
The rays of Hght from any point, A, B, or C, 
will all encounter the dark pigment cells sur- 
rounding the ommatidia and be absorbed, 
except the ray which passes directly through 
the center of the cornea as D or E; this ray 
will penetrate to the retinulae and thence to 
the fibers of the optic nerve. Thus the 
retinula (plural, retinulae) of one om- 
matidium receives a single resultant impres- 
sion from the light which reaches it. But the 
adjacent ommatidia, being directed to a dif- 
ferent though adjoining region of the outer 
world, may transmit a different impression, 
and the stimuli from all of the ommatidia 

which make up a compound eye will corres- 
pond in greater or less degree to the whole 
of the visible outer world which subtends 
their several optic axes. This means that 
each visual unit responds to a fragment of 
the total field and that these fragmentary 
images are fitted together into a single gen- 
eral picture. However, the image formed by 
this type of eye is never very good; it func- 
tions best at short distances— the arthropod 
is always near-sighted. 

When the pigment surrounds the om- 
matidia (Fig. 118, A), vision is as described 
above; but it has been found that in weak 
light the pigment migrates partly toward the 
outer and partly toward the basal end of the 


Cornea (lens) 
Corneagen ce 

Distal retina 
pigment cells 

•Crystalline cone 



Basal retinal 
pigment cells 


Nerve fibers 


Figure 118. Longitudinal sections of two ommatidia of the crayfish. A, one ommatidium in 
the light; pigment extended so that it completely surrounds the ommatidium, isolating it from 
its neighbors. B, ommatidium in the dark; pigment in the basal pigment cells is withdrawn. 
(After Bernhards. from Parker.) 



ommatidia (Fig. 118, B). When this occurs 
the ommatidia no longer act separately, but 
a continuous image is thrown on the retinu- 
lar layer. This is called a superposition im- 
age, which is much less distinct than the 
apposition image, but it is more sensitive to 
weak intensities of light. 

Statocysts. The statocysts of Cambarus 
are chitin-lined sacs situated one in the 
basal segment of each antennule. In the base 
of the statocyst is a ridge with many fine 
sensory hairs which are innervated by a sin- 
gle nerve fiber. Among these hairs are a 
number of large grains of sand, the stato- 
liths, which are placed there by the crayfish. 
Beneath the sensory cushion are glands 
which secrete a substance for the attachment 
of the statoliths to the hairs. 

The statocyst for many years was con- 
sidered an auditory organ, but later investi- 
gations have proved that it is an organ of 
equilibrium. The contact of the statoliths 
with the statocyst hairs determines the orien- 
tation of the body while swimming, since 
any change in the position of the animal 
causes a change in the position of the stato- 
hths under the influence of gravity. When 
the crayfish changes its exoskeleton in the 
process of molting, the statocyst is also shed. 
Individuals that have just molted, or have 
had their statocysts removed, lose much of 
their powers of orientation. Perhaps the 
most convincing proof of the function of 
equilibrium is that furnished by experi- 
ments. Shrimps, which had just molted and 
were therefore without statoliths, were 
placed in filtered water. When supplied with 
iron filings, the animals filled their stato- 
cysts with them. A strong electromagnet was 
then held near the statocyst, and the shrimp 
took up a position corresponding to the 
resultant of the two pulls, that of gravity 
and that of the magnet. 

Endocrine glands 

The sinus gland located in the base of the 
eye stalk produces two hormones, and pos- 
sibly more. These hormones appear to con- 

trol the spread of pigment granules in the 
chromatophores in the compound eyes and 
in the body epidermis. They govern to a 
greater or lesser extent, metabolic rate, 
growth and viability. They also regulate the 
frequency of molting, and are necessary for 
normal deposition of calcium salts in the 
exoskeleton. These hormones are distributed 
by the blood stream, as in the vertebrates. 
It has been observed in experimental ani- 
mals that removal of the sinus glands 
shortened the life of crustaceans. 

Muscular system 

In the crayfish the complex muscles are 
all attached to the inner surface of the skele- 
ton, instead of constituting a part of the 
body wall as in the coelenterates and anne- 
lids, or being external to the skeleton as in 
man. The largest muscles in the body of the 
crayfish are situated in the abdomen (Fig. 
114) and are used to bend that part of the 
animal forward upon the ventral surface of 
the thorax, thus producing backward loco- 
motion in swimming. Other muscles extend 
the abdomen in preparation for another 
stroke. Muscles of considerable size are 
situated in the thorax and within the tubu- 
lar appendages, especially the chclipcds. A 
comparison of the skeleton and muscles of 
the crayfish with those of man is interesting. 

Reproductive system 

Sexes are normally separate in the crayfish, 
there being only a few cases on record where 
both male and female reproductive organs 
were found in a single specimen. 

Male reproductive organs. The male or- 
gans consist of two white testes partially 
fused into three lobes and on each side a 
long coiled sperm duct, the vas deferens, 
which opens through the base of the fifth 
walking leg. The testes lie just beneath the 
heart (Fig. 114) . They constitute a soft body 
possessing two anterior lobes and a median 
posterior extension. 

Spermatogenesis. The primiti\'e germ 
cells within the testis pass through two 





Breaking plane 

Figure 119. Left, different views of crayfish sperms, greatly magnified. Right, posterior view 
of basal region of second walking leg of the crayfish, showing the line where the break occurs 
in autotomy, and the special muscle concerned. Arrow indicates the direction of pull of the 
muscle involved in the reflex. 

maturation divisions and then develop into 
sperms. These are flattened spheroidal bodies 
when enclosed within the testis or vas 
deferens; but, if examined in water or some 
other liquid, they are seen to uncoil, finally 
becoming star-shaped (Fig. 119). 

The sperms remain in the testes and vasa 
deferentia until copulation takes place. As 
many as two million sperms are contained 
in the vasa deferentia of a single specimen. 

Female reproductive organs. The two 
ovaries resemble the testes in form and are 
similarly located in the body (Fig. 116). A 
short oviduct leads from near the center of 
the side of each ovary to the external open- 
ing in the base of the third walking leg. 

Oogenesis. The primitive germ cells in the 
walls of the ovary grow in size and become 
surrounded by a layer of small cells, the 
follicle, which eventually breaks down, al- 
lowing the eggs to escape into the central 
cavity of the ovary. At the time of laying, 
the ova pass out through the oviduct. 

Fertilization and development. The 
sperms are transferred from the male to the 
seminal receptacle of the female during 
copulation, which usually takes place be- 
tween early spring and autumn. The seminal 
receptacle is a cavity in a fold of cuticle be- 
tween the fourth and fifth pairs of walking 
legs. The female lays her eggs several weeks 
to several months after copulation; the ova 

are fertilized by the sperms when laid. The 
eggs are fastened to the swimmerets with a 
sticky substance and are aerated by being 
moved back and forth through the water. 

The cleavage of the egg is superficial, and 
the embryo appears first as a thickening on 
one side. The eggs hatch in 2 to 20 weeks, 
and the larvae cling to the egg stalk. In 
about 2 to 7 days they shed their euticular 
covering, a process known as molting. Cast- 
ing off the covering of the body is not pe- 
culiar to the young, but occurs in adult cray- 
fishes as well as in young and adults of 
many other arthropods. In the larval cray- 
fish the cuticle of the first stage becomes 
loosened and drops off. In the meantime, 
the epidermal cells have secreted a new 
covering. Molting is necessary before growth 
can proceed since the exoskeleton is hard 
and nonelastic. In adults it also is a means 
of getting rid of an old worn-out coat and 
acquiring a new one. Over a period of some 
time before the molt, a quantity of calcium 
from the old exoskeleton is absorbed and 
distributed by the blood, especially to the 
stomach, where it is deposited in calcareous 
bodies, the gastroliths. The formation of 
gastroliths is under the control of an endo- 
crine gland. The young stay with the mother, 
attached to her swimmerets with their 
chelae (see headpiece at beginning of this 
chapter) until they can shift for themselves. 



They molt at least 6 times during the first 
summer. Before the new exoskeleton hardens 
an increase in body bulk occurs, probably 
due to absorption of unusual quantities of 
water before molting. Several days after a 
molt, the animal remains in hiding, thus 
avoiding enemies while it is in a relatively 
defenseless condition. It takes several weeks 
for the new shell to become as completely 
hardened as the one cast off. 


The crayfish and many other crustaceans 
have the power of regenerating lost parts, 
but to a much more limited extent than 
such animals as the hydra and earthworm. 
Experiments have been performed upon al- 
most every one of the appendages as well 
as the eye. The second and third maxilli- 
peds, the walking legs, the swimmerets, and 
the eye have all been injured or cut off at 
various times, and the lost parts were sub- 
sequently renewed. Many species of crayfish 
of various ages have been used for these ex- 
periments. The growth of regenerated tissue 
is more frequent and rapid in young speci- 
mens than in adults. 

The new structure is not always like that 
of the one removed. For example, when the 
annulus containing the seminal receptacle 
of an adult is extirpated, another is regen- 
erated; although this is as large as that of 
the adult, it is comparable in complexity 
only to that of an early larval stage. A more 
remarkable phenomenon is the regeneration 
of an apparently functional antennalike or- 
gan in place of a degenerate eye which was 
removed from the blind crayfish Cambarus 
pellucidus testii. In this case a nonfunc- 
tional organ was replaced by a functional 
one of a different character. The regenera- 
tion of a new part which differs from the 
part removed is termed heteromorphosis. 

Self-amputation ( autotomy ) 

Perhaps the most interesting anatomic 
structure connected with the regenerative 

process in Cambarus is the definite break- 
ing point near the bases of the walking legs 
(Fig. 119). If a cheliped is grasped or in- 
jured, it is broken off by the crayfish at the 
breaking plane. The other walking legs, if 
injured, may be thrown off at the free joint 
between the second and third segments. A 
new leg as large as the one lost develops 
from the end of the remaining stump. This 
breaking off of the legs at a definite point 
is known as autotomy, a phenomenon that 
occurs also in a number of other animals. 
The breaking plane in decapod crustaceans 
is near the base of the legs. The leg is flexed 
by a special (autotomizer) muscle; con- 
tinued pull of this muscle separates the leg 
at the breaking point. The muscles are not 
damaged, and a membrane develops across 
the inside of the leg on the proximal side 
of the breaking place. There is a small hole 
in the membrane through which nerves and 
blood vessels pass, but this hole is quickly 
stopped by a blood clot. About S days later 
regeneration begins by an outward growth 
of the cells which lined the exoskeleton. 

Autotomy is an adaptation which pre- 
vents undue loss of blood when a leg is 
sacrificed to escape an enemy. 

As in the earthworm, the rate of regenera- 
tion depends upon the amount of tissue 
removed. If one cheliped is amputated, a 
new one regenerates less rapidly than if both 
chelipeds and some of the other walking 
legs are removed. 


When at rest, the crayfish usually faces 
outward from its place of concealment and 
extends its antennae. In this position it 
may learn the nature of any approaching 
object without being detected. Activity at 
this time is reduced to the movements of a 
few of the appendages and the gills; the gill 
bailers of the second maxillae move back 
and forth bailing water out of the forward 
end of the gill chambers; the swimmerets are 
in constant motion creating a current of 



water; the maxillipeds are likewise kept mov- 
ing; and the antennules and antennae are 
in continual motion exploring the surround- 

Crayfishes are more active between dusk 
and dawn than during the daytime. At this 
time they venture out of their hiding places 
in search of food. 


Locomotion is accomplished by either 
walking or swimming. Crayfishes are able to 
walk in any direction, forward usually, but 
also sidewise, obliquely, or backward. In 
walking, the fourth pair of legs is most 
effective and bears nearly the entire weight 
of the animal; the fifth pair serves as props 
and to push the body forward; the second 
and third pairs are less efficient for walking 
since they are modified to serve as grasping 
organs and as toilet implements. Swimming 
is not resorted to under ordinary conditions, 
but only when the animal is frightened or 
shocked. In such a case the crayfish extends 
the abdomen, spreads out the uropods and 
telson, and, by sudden contractions of the 
flexor abdominal muscles, bends the abdo- 
men and darts backward. The swimming 
reaction apparently is not voluntary, but is 
almost entirely reflex. 


The crayfish, either at rest or in motion, 
is in a state of unstable equilibrium and 
must maintain its body in the normal posi- 
tion by its own efforts. The force of gravity 
tends to turn the body over. From a large 
number of experiments, it has been proved 
that the statocysts are the organs of equi- 
librium. The structure of these organs is 
described on page 211. The contact of the 
statoliths with the sensor)' hairs furnishes 
the stimulus which causes the animal to 
maintain an upright position. 

When placed on its back, the crayfish has 
some difficulty in righting itself. Two 
methods of regaining its normal position are 
employed. The usual method is that of rais- 

ing itself on one side and allowing the body 
to tip over by the force of gravity. The sec- 
ond method is that of contracting the flexor 
abdominal muscles, which causes a quick 
backward flop, bringing the body right side 
up. In general, the animals right themselves 
by the easiest method when placed on their 
backs; and this is found to depend usually 
upon the relative weight of the two sides 
of the body. When placed upon a surface 
which is not level, they take advantage, after 
a few experiences, of the inclination by turn- 
ing toward the lower side. 

Senses and their location 

The crayfish has more highly developed 
sense organs than the annelids. The sense of 
touch in crayfishes is perhaps the most 
valuable, since it aids them in finding food, 
avoiding obstacles, and in many other ways. 
Touch organs are located in specialized 
hairlike bristles or setae (Fig. 112) on vari- 
ous parts of the body. These are especially 
abundant on the mouth parts, chelae, cheli- 
peds, and edge of telson. Vision in crayfishes 
is undoubtedly of real value to the animal 
in detecting moving objects. No reactions 
to sound have ever been observed in cray- 
fishes, and apparently they do not hear. The 
reactions formerly attributed to hearing are 
probably due to touch reflexes. In aquatic 
animals it is so difficult to distinguish be- 
tween reactions of taste and smell that 
these senses are both included in the term 
chemical sense. The end organs of this 
sense are found in hairs located on the an- 
tennules, tip of antennae, mouth parts, and 
other places. 

Reactions to stimuli 

Contact. Positive reactions to contact are 
exhibited to a marked degree by crayfishes; 
the animals seek to place their bodies in 
contact with a solid object if possible. The 
normal position of the crayfish when at rest 
under a stone is such as to bring its side or 
dorsal surface in contact with the walls of 
its hiding place. This, no doubt, is of dis- 



tinct advantage since it places the animal in 
a position of safety. 

Light. Light of various intensities in the 
majority of cases causes the crayfish to re- 
treat. Individuals prefer colored lights to 
white. Negative reactions to light play an 
important role in the animal's life, since 
they influence it to seek a dark place where 
it is concealed from its enemies. 

Chemicals. The reactions of the crayfish 
to food are due in part to a chemical sense. 
Positive reactions result from stimulation by 
food substances. For example, if meat juice 
is placed in the water near an animal, the 
antennae move slightly and the mouth parts 
perform vigorous chewing movements. The 
meat juice causes general restlessness and 
movements toward the source of the stimu- 
lation, but the animals seem to depend 
chiefly on touch for the accurate localization 
of food. Acids, salts, sugar, and other chem- 
icals produce a sort of negative reaction, 
indicated by the animal scratching the cara- 
pace, rubbing the chelae, or pulling at the 
part stimulated. 

Habit forming 

It has been shown by certain simple ex- 
periments that crayfishes are able to form 
habits and to modify them. They learn by 
experience and modify their behavior slowly 
or quickly, depending upon their familiarity 
with the situation. The chief factors in the 
formation of such habits are the chemical 
sense (probably both smell and taste), 
touch, sight, and the muscular sensations re- 
sulting from the direction of turning. Experi- 
ments show that the animals are able to 
learn a path even when the possibility of 
following a scent is excluded. 

Cave crayfishes 

There are at least 12 different species of 
cave crayfishes in the United States; some 
are restricted to the waters of a single cave, 
such as Mammoth Cave, Kentucky. 

Cave species are interesting because of 

their striking modifications. All are blind, 
the eyes are atrophied and the eye stalks are 
more or less undeveloped. Pigmentation is 
absent, and the body is light-colored. They 
are mostly small species; the chelae are not 
well developed. The antennae are long and 
highly specialized as tactile organs. 


Almost every pond, lake, or stream con- 
tains crustaceans of many species, and salt 
water is likewise inhabited by a large variety 
of forms. A few live on land. Often crusta- 
ceans are very abundant. Only a few of the 
more common or more interesting species 
can be mentioned here. 

Fairy shrimps, Eubranchipus, reach a 
length of about one inch, live in fresh-water 
pools, and are common. They are semitrans- 
parent, pinkish, and swim on their backs 
(Fig. 110). Eggs laid in the summer be- 
come buried in the mud and are able to 
withstand dr}ang and the winter cold. They 
hatch the following spring. One species of 
fairy shrimp lives in water more salty than 
that of the sea. 

Water fleas, Daphnia, are oval, laterally 
compressed crustaceans with a prominent 
beak on the under side of the head, and a 
sharp caudal spine (Fig. 110). They are 
about 2 mm. long and are very common in 
fresh water. The soft body is enclosed in 
a bivalve shell through which can be seen 
a regularly beating heart, in front of which 
is a single eye, and behind which is a brood 
chamber full of eggs. 

The ostracods are common in fresh and 
salt water and widely distributed. The bi- 
valve shell (Fig. 110) makes it appear like a 
microscopic clam if the appendages are not 
seen. They swim with the first pair of legs. 
Only females are known in certain genera; 
these lay eggs which develop without fertili- 
zation, that is, they are parthenogenetic. 

The modern cyclops is not a giant with a 
single eye in his forehead like the Cyclops of 



Greek mythology, but is a very successful 
one-eyed little crustacean that lives in fresh 
water or in the sea (Fig. 110). Cyclops 
viridis is common in small fresh-water ponds, 
measures from 1.5 to 5 mm. in length, and 
is usually greenish in color. The eye is red. 
Often on each side of the tail is a sac full 
of eggs. Certain relatives of the cyclops live 
in the sea; although minute in size, they are 
often so numerous that they color the water 
pink and furnish the principal food of certain 
whales. Cyclops serve as the intermediate 
host for the broad tapeworm of man and the 
guinea worm. 

Goose barnacles (Fig. 110), so called be- 
cause barnacle geese were once supposed to 
have hatched from them, often attaching 
themselves to the bottoms of ships. They are 

fringe-legged crustaceans. They were con- 
sidered by early zoologists to be mollusks be- 
cause they live within a calcareous shell that 
they secrete. This barnacle possesses a long 
stalk by which it is attached to seaweed or 
other floating objects. It is found in both the 
Atlantic and Pacific oceans. 

The rock barnacles, Balanus, possess a 
thick shell but no stalk (Fig. 120). They 
attach themselves to rocks and other sta- 
tionary objects, to the shells in which hermit 
crabs live, and to other animals; the body of 
the whale often becomes intensely irritated 
by them. When the tide goes out, they close 
the 6 plates of their shells for protection. 
While under water they thrust out their 
delicate fringed legs and kick minute or- 
ganisms into their mouths. 

Rootlike processes 


-•■• Oj; 

Sacculina on crab 

Balanus (rock barnacle) 
Figure 120. Highly magnified crustaceans. Left, rock barnacles are common marine crustaceans 
which live permanently attached. One is shown with appendages extended and fcedmg. The 
other is withdrawn into its shell for protection from enemies. Right, Sacculina, a curious marmc 
crustacean, which in the adult stage is parasitic on a crab. The crab is represented by dotted 
lines, and the parasite body from which the roots penetrate the tissues of the host by solid Imes. 

The root-headed barnacles are parasitic. 
The best-known species, Sacculina (Fig. 
120), attaches itself when young to a ma- 
rine crab, between the thorax and abdomen. 

on the ventral surface. It then loses most 
of its organs, sends rootlike processes into 
the body of the crab, and becomes a mere 
sac. The host's body becomes so completely 



parasitized that the whole physiology is 
seriously affected. 

Many interesting crustaceans belong to 
the subclass Malacostraca. A few will be 
described here, beginning with the sow bugs. 
A terrestrial species Oniscus (Fig. 110), 
about 16 mm. long, is slate-colored, spotted 
with white, and is common under stones, 
bark of logs, etc. It breathes with gills and 
must therefore live in a moist place. The 
body is oval and flat, which enables it to 

creep into crevices; and the "legs" are ap- 
proximately equal in size. 

Beach fleas live on beaches where they 
bury themselves in the sand. They can leap 
with agility but do not bite. The common, 
long-horned, beach flea (Fig. 121), is about 
one inch long, has a laterally flattened body 
like that of a flea, and has legs adapted for 
leaping. It feeds on decaying animal and 
vegetable matter and is a valuable scaven- 
ger on many sand beaches. 

Fiddler crab 

Edible or blue crab 

Figure 121. Representative malacostracans in their respective habitats. 

The fresh-water scud, Gammarus (Fig. 
110), is also an amphipod, but lives in fresh- 
water ponds and streams and swims instead 
of leaps. It is whitish and about 15 mm. 

Our most important edible crustacean is 
the American lobster, Homarus, a near rela- 
tive of the crayfish. The American lobster 
has probably been more intensively studied 

than any other marine animal with the pos- 
sible exception of the American oyster 
(Ostrea). These lobsters live in the sea 
along the Atlantic Coast. They are not red 
when alive, but usually dark green with 
darker spots and yellowish underneath. They 
may grow to a length of over 2 feet and 
to a weight of over 30 pounds, but most of 
those caught in lobster pots are less than 



10 inches long and weigh less than 2 pounds. 
The west coast lobster (Panulirus) is com- 
monly called a spiny lobster because its 
skeleton is provided with many needle-sharp 

Shrimps and prawns are also decapods. The 
edible shrimp is slender, with whiplike 
antennae, and is about 5 cm. long. It is 
very agile, swimming backward with quick 
jerks of the fin at the posterior end of the 
body. It eats both animal and vegetable 
matter. Shrimps are constantly being preyed 
upon by fish and other marine animals as 
well as by man. Prawns are smaller than 
shrimps. A common species, which is dis- 
played in Fig. 121, lives among rock weed 
and eel grass on muddy bottoms along the 
Atlantic Coast from Massachusetts to 

Hermit crabs (see colored frontispiece at 
beginning of text) are famous because they 
live in the empty shells of marine snails. As 
they grow larger, they must move from one 
shell into a larger one. Frequently the shell 
is covered with coelenterates. The abdomen 
of the crab within the shell is soft and 
twisted to fit the coils, and one pair of 
abdominal appendages develops into hooks 
which anchor the body in the shell; the 
other abdominal appendages become de- 
generate. The right-hand pincer, which is 
used to capture and crush its prey, is con- 
structed so as to close the opening in the 
shell. The other smaller pincer fills any 
crevice in the opening left by the other claw. 
A common one is Fagurus, which lives in 
rock pools and shallow water along the 
beach from Maine to South Carolina. 

Many species of crabs are edible, but one 
species, Callinectes sapidus (Fig. 121), is 
usually given this doubtful honor. The short 
and broad body is about 7 cm. long and 17 
cm. wide. The shell is dark green and the 
feet are blue. Edible crabs are common on 
muddy bottoms in shallow water from Cape 
Cod to Louisiana. In California the most 
important edible crab is Cancer magister. 
Crabs that are considered edible in one 

locality may not be eaten in other regions. 

Spider crabs are noted for their long 
spidery legs. The common species, Libinia 
emarginata, is about 7 cm. long and lives 
on mud flats and oyster beds along the 
Atlantic Coast from Maine to Florida, and 
in California. The shell is pointed in front 
and covered with a dense growth of 
chitinous "hairs" which give it a furry ap- 
pearance. The legs of a Japanese spider crab 
reach an enormous length; individuals of 
this species may measure 12 feet across when 
spread out. 

Fiddler crabs (Fig. 121) are accustomed 
to wave their large claw back and forth as 
though playing a violin, hence the common 
name. They form colonies which live in 
burrows that are dug in the mud or sand in 
salt marshes. In certain species, the males 
are very pugnacious and fight each other 
with great vigor. 


The Arthropoda probably evolved from 
an annelidlike ancestor. 

The Onychophora (Fig. 124) seem to 
resemble most closely this ancestral condi- 
tion. They possess a thin cuticle, a continu- 
ous and muscular body wall, no joints, one 
pair of jaws, and appendages on the first 
segment. In the other groups of arthropods, 
development of a rigid exoskeleton with 
joints brought about a change in the dis- 
tribution of the muscles from the continu- 
ous type forming a muscular body wall, as 
in the annelids, to the discontinuous type in 
which the muscles are separately and di- 
versely developed for the movement of spec- 
ial segments. 

The crustaceans, insects, centipedes, and 
millipedes appear to have developed along 
one line, since they have so much in com- 
mon; and the arachnids along another line, 
since none of their appendages have de- 
veloped into antennae, and none possess 



mandibles. The classes of crustaceans have 
become different from one another, prin- 
cipally as a result of specialization of ap- 
pendages, shortening of the body, and de- 
velopment of a carapace. Internal changes 
are associated with these external modifi- 
cations. Orders of insects are arranged in 
the phylogenetic tree (p. 229) to suggest 
possible relationships. The centipedes are 
more closely related to the insects than are 
the millipedes. 


The name trilobite (Fig. 122) means 
"three-lobed" and refers to the fact that the 
dorsal surface is divided by longitudinal fur- 
rows into three lobes. They were covered by 
a hard shell. On the head was a pair of an- 
tennae, 4 pairs of two-branched appendages, 
and often a pair of compound eyes. 

These primitive fossil arthropods date 
from early geologic time. Although trilobites 

Figure 122. Fossil trilobites; these extinct arthropods lived in warm primeval seas about 
500,000 million years ago. They dominated hfe on earth for a span many, many times as long 
as man's whole existence. (Courtesy of American Museum of Natural History.) 

probably did not give rise directly to any 
other group of arthropods, they appear to 
be most closely related to crustaceans. 


Crustacea are of considerable value as 
food for man, either directly or indirectly. 
The smaller species may be present in 
enormous numbers in both fresh water 

and salt water and constitute an important 
part of the food chain of many fish and 
other aquatic animals that eventually come 
to our table. Commercially, the shrimp is 
the most important of the crustaceans as 
human food; crabs, lobsters, and crayfishes 
follow in this order. Blue or edible crabs are 
eaten extensively in certain regions; they are 
called hard-shelled crabs except just after 
molting, when they become soft-shelled. The 
crayfish, especially the soft-shelled individ- 
ual, is very popular among fishermen as 



bait. The soft-shelled crayfishes may be 
kept soft for a week or more on ice. Re- 
frigeration slows metabolism so that the 
shell develops slowly. 

The crayfish is used for food, especially 
in Europe and on the Pacific Coast. The 
spiny lobster of the Pacific Coast may reach 
a weight of 10 pounds. Shrimps and prawns 

Figure 123. A Cape Cod lobsterman removing a prize lobster from a wooden trap. These traps 
are baited, weighted with a brick, tied to the frame, as shown, and set in the ocean to harvest 
the lobster crop. (Courtesy of Mike Roberts Color Productions.) 

are marine species whose large abdominal 
muscles are sold in the fresh condition or 
canned. The shrimp industry in the United 
States is especially important in Louisiana 
and California. Certain crustaceans may 
become pests when present in large num- 
bers. Thus in some parts of our southern 
states, crayfishes damage cotton and other 
crops by devouring the plants; they occa- 
sionally burrow into levees and weaken 
them. Sow bugs, which also feed on vegeta- 

tion, may become pests in greenhouses and 
fields when sufficiently numerous. 

Although some crustaceans are parasites 
of aquatic animals, none is a parasite of 
man or other land animals. Copepods serve 
as intermediate hosts of several parasitic 
worms in man; for example, certain species 
of Cyclops for the guinea worm and other 
species of Cyclops for tapeworms. Crayfish 
and crabs act as second intermediate hosts 
for the lung flukes. 




{For reference purposes only) 

Class Crustacea are arthropods, most of 
which hve in water and breathe by means of 
gills. Symmetry is bilateral; they are triploblas- 
tic, and the body consists of a longitudinal 
series of segments. The body is divided into 
head, thorax, and abdomen, or the head and 
thorax may be fused, forming a cephalothorax. 
The head consists of different numbers of 
fused segments in different groups; it bears 
two pairs of antennae (feelers), one pair of 
mandibles (jaws), and two pairs of maxillae. 
The appendages are jointed. Only five sub- 
classes and one order are listed here. 

Subclass 1. Branchiopoda. This is the most 
primitive group of crustaceans. 
Free-swimming; thoracic ap- 
pendages leaflike and respira- 
tory; usually a carapace. Ex. 
Eubranchipus vernalis (Fig. 

Subclass 2. Ostracoda. Free-swimming, cara- 
pace bivalved; appendages not 
leaflike. Ex. Eucypris virens 
(Fig. 110). 

Subclass 3. Copepoda. Free-swimming, par- 
asitic, or commensal; no com- 
pound eyes; typically with 6 
pairs of thoracic legs. Ex. 
Cyclops (Fig. 110). 

Subclass 4. Cirripedia. Barnacles. Adults 
sessile and attached, or para- 
sitic; no compound eyes in 
adults; carapace enclosing body, 
usually with limb plates; mostly 
hermaphroditic. Ex. Lepas 
(Fig. 110). 

Subclass 5, Malacostraca (Fig. 121). Mostly 
large, but some small, such as 

the sow bugs. Fig. 110; usually 4 
segments in head, 8 in thorax, 
and 6 in abdomen; gastric mill 
in stomach. Only one order 
listed here. 

Order Decapoda. Lobsters, crayfish, shrimps, 
crabs, etc. Carapace large, covering thorax; 
eyes on stalks; 5 pairs of walking legs. Ex. 
Orconectes, crayfish (Fig. Ill); Hoynarus, 

Class Trilobita. The trilobites (Fig. 122) 
were marine animals, probably allied to the 
crustaceans; they are all extinct. The best- 
known species, Triarthrus becki, occurs in the 
Utica shale (Lower Silurian) of New York 


Borradaile, L.A., and Yapp, W.B. Manual of 
Elementary Zoology. Oxford Univ. Press, 
New York, 1958. 

Huxley, T.H. The Crayfish, an Introduction 
to the Study of Zoology. Kegan, Paul, 
Trench, Trubner, London, 1880. 

Pennak, R.W. Freshwater Invertebrates of the 
United States. Ronald Press, New York, 

Pratt, H.S. A Manual of Common Inverte- 
brate Animals. Blakiston, Philadelphia, 1935. 

Snodgrass, R.E. "Evolution of the Annelida, 
Onychophora, and Arthropoda." Smith- 
sonian Misc. Collections, 97:1-159, 1938. 

Snodgrass, R.E. A Textbook of Arthropod 
Anatomy. Comstock Publishing Associates, 
Ithaca, N.Y., 1952. 

Ward, H.B., and Whipple, G.C. Fresh-water 
Biology. Wiley, New York, 1918. 

Wilson, R.C. "A Review of the Southern Cali- 
fornia Spring Lobster Fishery." California 
Fish and Game, 34:71-80, 1948. 



Phylum /Vrthropoda. 



and Millipedes 


Tor convenience, three classes of arthro- 
pods are considered together in this chapter. 
The Onychophora (the name means claw- 
bearing) are rare; and, although the Chilo- 
poda (centipedes) and Diplopoda (milli- 
pedes) are abundant in certain localities, 
other types of arthropods considered in 
Chapter 18 reveal the characteristics of this 
phylum to better advantage. 

The onychophorans resemble more 
closely than any other animals what is be- 
lieved to have been the ancestral condition 
of the arthropods. They possess a thin cuti- 
cle, a continuous muscular body wall, no 
joints, one pair of jaws, a tracheal respira- 
tory system, and a series of nephridial open- 


Peripatus (Fig. 124), a representative 
species, is about two or three inches in 
length, with a cylindrical body, but with- 
out a distinct head. These animals are 
especially interesting because they obviously 
exhibit both arthropod and annelid charac- 
teristics, as well as peculiarities of their own. 
Unfortunately, they are gradually disap- 
pearing and hence becoming more difficult 
to observe. However, the group furnishes 
an excellent example of discontinuous dis- 
tribution. Species have been reported from 
Central America, Mexico, the West Indies, 
and the southern hemisphere. Even in the 
area where a species occurs, specimens are 

Figure 124. Facing page, Peripatus, an onychoph- 
oran. Top, drawing to show external structure. 
Bottom, photo of living animal. It is a walking 
wormlike animal, which is neither an annelid nor a 
typical arthropod. Because it has both annelid and 
arthropod characteristics, it is the only living ani- 
mal that comes near to being a common rela- 
tive to annelids and arthropods. Therefore it is 
considered by some as a connecting link between 
the two phyla. (Photo reproduced by permission 
from The Biotic World and Man, by L.J. and M.J. 
Milne, p. 498. Copyright, 1952, by Prentice-Hall, 
Inc., Englewood Cliffs, N.J.) 




present in only a few of the many available 
habitats. This seems to indicate that this 
group once had a continuous distribution 
but that it has disappeared throughout most 
of its range and is on the road to extinction. 
Peripatus lives in crevices of rock, under 
bark and stones, and in other dark moist 
places and is active only at night. When 
irritated, it throws a jet of slime, sometimes 
to a distance of almost a foot, from a pair of 
glands which open on the oral papillae. 

This slime sticks to everything but the body 
of the animal itself; it is used principally to 
capture flies, wood lice, termites, and other 
small animals; and, in addition, is a weapon 
of defense. A pair of modified appendages 
serve as jaws and tear the food to pieces. 

Most of the 70 known species are vivipa- 
rous, and a single large female may produce 
30 or 40 young in a year. These young resem- 
ble the adult when born, differing mainly 
in size and color. 

f^^o;^ oS^.o oo ^ooq .0° S(?.o:p ° 0. 

.6^£p .0. 9 oP° qO o?°ei^?n ° 

Teeth Antenna 
Oral papilla 

Figure 125. Peripatus. Left, ventral view, the legs are stubby and unlike those of typical 
arthropods, but the claws are arthropodlike. In each jaw are embedded two backward-pointing, 
clawlike teeth. Right, lateral view, the "head" bears two extensible antennae, near the base of 
which is a pair of simple eyes. The numerous papillae which cover the whole body give it a 
velvety texture. 

The internal anatomy is a combination 
of annelidlike and arthropodlike structures. 
The chief systems of organs are arranged as 
in annelids; the nephridia are paired and 
segmental in distribution, and the reproduc- 
tive organs are supplied with cilia. Cilia do 
not occur in arthropods. The arthropod 
characteristics include jaws derived from ap- 
pendages, a body cavity that is a hemocoel, 

tracheae, and the almost complete absence 
of a coelom around the digestive tract. 
Peripatus differs from both annelids and 
other arthropods in the possession of a single 
pair of jaws, in scant metamerism, in the 
arrangement of the tracheal openings, in 
the texture of the skin, and in the separate 
nerve cords with no well-developed ganglia. 
Hence, the onychophorans do not fit the 



phylum Arthropoda very well, and are, 
therefore, sometimes placed in a separate 


The Chilopoda are called centipedes (Fig. 
126). The body is flattened dorsoventrally 
and, in different species, consists of from 
15 to 173 segments, each of which bears one 

pair of legs, except the first which has legs 
modified as poison claws, and the last two, 
which usually lack appendages. Their prey 
consists of insects, worms, mollusks, and 
other small animals which are killed with 
their poison claws and then chewed with 
their mandibles. The antennae are long, con- 
sisting of at least 12 segments. The internal 
anatomy of a common centipede is shown in 
Fig. 127. 

Figure 126. Centipedes have many legs. In the East Indies there is a giant centipede nearly 
a foot long. The photo shows a smaller form, common in the United States. (Courtesy of N.Y. 
Zoological Society.) 

Centipedes are swift-moving creatures. 
Many of them live under the bark of logs, 
or under stones. Some of the poisonous 
centipedes of tropical countries belong to 
the genus Scolopendra. They may reach a 
foot in length, and their bite is painful and 
even dangerous to man. The common house 
centipede (Scutigera) has 15 pairs of 
very long legs, and lives in damp places such 
as basements. It is not only harmless to man, 
but really beneficial for it feeds on insects. 


The Diplopoda are the millipedes. The 
body is subcylindrical and consists of from 
about 25 to more than 100 segments, accord- 
ing to the species (Fig. 128). All segments 

bear two pairs of legs except the thorax, on 
which the number is reduced to one pair. 

The mouth parts are a pair of mandibles 
and a pair of maxillae. One pair of short 
antennae and clumps of simple eyes are 
usually present. There are olfactory hairs on 
the antennae and a series of scent glands that 
secrete an objectionable fluid which is used 
in defense. In fact, there is a species in 
Micronesia that ejects for several inches 
such a highly irritating fluid that it will 
cause temporary blindness. The breathing 
tubes (tracheae) are usually unbranched; 
they develop in tufts from pouches which 
open just in front of the legs. 

Millipedes move very slowly in spite of 
their numerous legs. Some are able to roll 
themselves into a spiral or ball. They live in 
dark, moist places, and feed, principally, on 

^ tubule 


Poison claw- 

i — Salivary gland 
^Digestive tract 

nerve cord 


?* Digestive tract 

Sperm duel 


I opening 


Figure 127. Drawings of centipede showing the internal organs and external features of 



Genital opening 

Abdominal segment 
showing two pairs 
of legs 


Figure 128. A millipede, the name means "thousand-legged" but there are never that many 
legs. Millipedes are shy animals which hide in dark places to avoid the light. They can easily 
be distinguished from centipedes by the subcylindrical body and the two pairs of legs on most 

decaying vegetable matter, but sometimes 
on living plants, and may thus be destructive 
to gardens. The sexes are separate, and the 
eggs are laid in a nest made of damp earth. 
The young have 6 segments and only three 
pairs of legs when they hatch, and resemble 

wingless insects. Other segments are added 
just in front of the anal segment at succes- 
sive molts during growth. Common exam- 
ples of this class are Spirobolus and Julus; 
the latter occurs all over the United States, 
especially in meadows and gardens. 



Phylum Artliropnda. 


NSECTS are more numerous in species than 
all other animals combined. Over 850,000 
species have been described; and, no doubt, 
hundreds of thousands remain to be dis- 
covered (Fig. 130). They live in almost 
every conceivable type of environment: on 
land and in water, from the Arctic to the 
tropics; and their structure, habits, and life 
cycles are correspondingly modified. Never- 
theless, it is possible to separate this vast 
assemblage into orders, families, etc., al- 
though there is no unanimity of opinion 
with respect to the number that should be 
recognized and the names that should be 
applied (Fig. 129). 

The insects are such a large and varied 
group that many specialties have grown up 
in the general field of entomology, for ex- 
ample, medical and economic entomology. 
Because insects differ so widely in habit, 
physiology, and morphology and exhibit so 
many interesting adaptations, only those 
areas of entomology are included that deal 
briefly with life cycles, adaptive modifica- 
tions, types of coloration, and social life. 

The class Insecta are air-breathing arthro- 
pods with bodies divided into head, thorax, 
and abdomen. The head bears one pair of 
antennae, and the thorax three pairs of legs, 
and usually one or two pairs of wings in the 
adult stage. 

In many ways the grasshopper is a very 
favorable species as a type for detailed study. 
It is abundant and easily secured; it is com- 
paratively large and hence excellent material 
for dissection; it is one of the least special- 
ized of all insects, and, therefore, exhibits 
better than most other forms the essential 
features of insects, both externally and in- 
ternally. The grasshopper also has several 
conspicuous adaptations, such as leathery 


Figure 129. Facing page, representatives of some 
orders of insects. The lines suggest possible relation- 
ships. The figures are not drawn to scale. (Drawn 
specifically for this text from a diagram prepared by 
R.L. Fischer, Curator of Insects, Michigan State 
University. ) 







Figure 130. There are about 875,000 known living species of arthropods in the world. Of 
these approximately 850,000 species are insects, and 25,000 species are other arthropods. Insects 
comprise about 97 per cent of the species of known arthropods. 

forewings, enlarged hindlimbs, auditor}' or- 
gans, and structures for making sounds. It 
is of considerable economic importance. 


External anatomy 

Like the crayfish, the grasshopper is cov- 
ered by an exoskeleton (Fig. 131) which 
protects the delicate systems of organs 
within. This exoskeleton is the cuticle, 
which contains chitin and is divided into 
a linear row of segments. As in the crayfish, 
the cuticle is soft in certain regions, thus al- 
lowing movements of such structures as the 
abdomen, wings, legs, and antennae. These 
softer regions are known as sutures. The 
body wall consists of the cuticle beneath 
which is a layer of cells, the epidermis, 
which secretes it, and under this is a base- 
ment membrane. Each segment is made up 
of separate plates (pieces), which are known 
as sclerites; usually some of the sclerites of a 
typical segment cannot be distinguished 
because the sutures are indistinct or absent. 

In the grasshopper the body is divided into 
three groups of segments that constitute the 
head, thorax, and abdomen. 


The head is composed of fused segments 
(Fig. 132). These are not visible in the 
adult, but may be observed in the embryo, 
and are indicated by the paired appendages 
of the adult. The dorsal region of the head is 
known as the vertex; the front portion is 
called the frons; and the sides are the 
cheeks or genae. The rectangular sclerite be- 
low the frons is the clypeus. On either side 
of the head is a compound eye, and on top 
of the head and near the inner edge of each 
compound eye is a simple eye (ocellus). 

Mouth parts 

The food of the grasshopper consists of 
vegetation which it bites off and grinds up 
by means of its chewing mouth parts (Fig. 
132). There is a labrum or upper lip at- 
tached to the ventral edge of the clypeus. 
Beneath this is the membranous tonguelike 
organ, the hypopharynx. On either side is a 
hard jaw or mandible, with a toothed sur- 



-*■ Heacl->-<r- 



> ■ < 


Compound eye 


Mesothorax Metafhorax Hindwing 




Figure 131. External features of the grasshopper, a good example of a generalized insect. 
This is a female. 

face fitted for grinding. Behind the mandi- 
bles are a pair of maxillae, consisting of sev- 
eral parts and with sensory palps at the sides. 
The labium or lower lip has slender palps 
at the sides. The labium of insects appears 
to have evolved from the lateral union of 
two appendages resembling the biramous 
limbs of a crustacean. The maxillae obvi- 
ously have also arisen from this type of ap- 
pendage. The labrum and labium serve to 
hold food between the mandibles and maxil- 
lae, which move laterally and grind it. The 
maxillary and labial palpi are supplied with 
sense organs that probably serve to distin- 
guish different kinds of food. 


The simple eye (Fig. 133) consists of a 
group of visual cells, the retina, pigment, 
and a transparent lens, a modification of the 

Compound eye 

The compound eye (Fig. 132) is covered 
by a transparent part of the cuticle, the 
cornea, which is divided into a large num- 
ber of hexagonal pieces, the facets. Each 
facet is the outer end of a unit known as an 
ommatidium. Such a structure gives mosaic 
vision as described in the crayfish (p. 209). 
Some insects, possibly the grasshopper, are 
able to distinguish colors. 


These are threadlike in form and consist 
of many segments. Tactile hairs and olfac- 
tory pits are present on them; and this con- 
dition, combined with the ability of the 
insect to move them about, makes them 
efficient sense organs. 


This portion of the body is separated from 
the head and abdomen by flexible joints and 


Compound eye 




La brum — 
Maxillary palp 
Labia! palp 




Labial palp 



Maxillary palp 



Figure 132. Grasshopper head and mouth parts. 





Vitreous body 




Figure 133. Ocellus or simple eye of the honey bee in longitudinal section. (Redrawn from 
Entomology, by J.W. Folsom. Copyright 1906 by The Blakiston Company.) 

consists of three segments: an anterior 
prothoTax, a middle mesothorax, and a pos- 
terior metathorax. Each segment bears a pair 
of legs; and the mesothorax and metathorax 
each bear a pair of wings. On either side of 
the mesothorax and metathorax is a spira- 
cle, an opening into the respiratory system. 
A typical segment consists of the dorsal 
tergum composed of 4 fused sclerites in a 
row; a lateral pleuron made up of 3 sclerites 
on each side, and a single ventral sclerite, 
the sternum. 

Prothorax. The saddlelike pronotum of 
the prothorax is large and extends down on 
either side; its 4 sclerites are indicated by 
tranverse grooves. The sternum bears a 

Mesothorax. In this segment the tergum 
is small, but the sclerites of the pleuron are 
distinct. The sternum is large. 

Metathorax. This resembles the meso- 


Each leg (Fig. 131) consists of a longitu- 
dinal series of segments as follows: the coxa 
articulates with the body; then comes the 
small trochanter fused with the femur, 
the tibia, and the tarsus. The femora of the 
metathoracic legs are enlarged to contain 
the muscles used in jumping. The tarsus at 
the end of each leg consists of three visible 
segments; the one adjoining the tibia has 
three pads on the ventral surface, and the 
terminal segment bears a pair of claws be- 

tween which is a fleshy pad or lobe, the 
pulvillus. The claws and pulvilli are used 
in clinging to any kind of surface. 


The wings of insects (Figs. 134 and 135) 
arise from the region between the tergum 
and pleuron as a double layer of epidermis, 
which secretes the upper and lower cuticu- 
lar surfaces. Between these are tracheae, 
around which spaces occur, and the ciiticle 
thickens; they (Fig. 134C) later become 
the longitudinal wing veins. The veins are 
of value in strengthening the wings. They 
differ in number and arrangement in differ- 
ent species of insects but are so constant in 
individuals of certain species that they are 
very useful for purposes of classification. 
The mesothoracic wings of the grasshopper 
are leathery and not folded; they serve as 
covers for the metathoracic wings which lie 
beneath them. The latter are thin and 
folded like a fan. 


The slender abdomen consists dorsally of 
11 segments; those at the posterior extremity 
being modified for copulation or egg laying. 
Along the lower sides of the abdomen, there 
are 8 pairs of small openings (spiracles) 
through which the animal breathes. In the 
grasshopper, the sternum of segment one is 
fused with the metathorax; on either side of 
this segment there is an oval tympanic mem- 
brane covering an auditory sac. Segments 2 



Sc, Scj 


Costa „C 

Subcosta _Sc 

Radius ^__„.R 

Media M 

Cubitus. Cu 

Ist anal vein .1A 

2nd anal vein 2A 

3rd anal vein 3A 


Blood space 

Figure 134. A, generalized insect wing showing the chief veins. B, cross section of wing. 
C, enlarged cross section of wing showing a vein which consists of the outer surface of the 
wing, blood space, and trachea. (A from The Wings of Insects, by J.H. Comstock. Copyright 
1916 by The Comstock Publishing Company.) 

Dorsal longitudinal muscle 

-Tergosternal muscle 


Figure 135. Movement of wings in flight. A, the wings are elevated on the pleural wing 
processes by the depression of the tergum due to the contraction of the tergosternal muscles. 
The hind margins of the wings are deflected. B, the wings are lowered by the elevation of the 
tergum due to the contraction of the dorsal, longitudinal muscles. Hind margins of wings are 
elevated. (After Snodgrass.) 

to 8 are unmodified. In the male, the 
sternum of segment 9 is elongated ventrally, 
giving an upward twist to the abdomen. The 
end of the female abdomen is more taper- 
ing than that of the male and forms the 
ovipositor, an egg-laying apparatus, 

internal anatomy 
and physiology 

The systems of organs within the body 
of the insect (Fig. 136) lie in the body 
cavity, which is filled with blood and is a 



hemocoel, not a coelom. All of the systems 
characteristic of higher animals are repre- 

Muscular system 

The muscles are of the striated type, very 
soft and delicate, but strong. They are seg- 
mentally arranged in the abdomen but not 
in the head and thorax. The most con- 
spicuous muscles are those that move the 
mandibles, the wings, the metathoracie legs, 
and the ovipositor. 

Digestive system 

The principal parts of the digestive tract 
(Fig. 136) are the foregut, midgut, and 
hindgut. The foregut consists of (1) the 
mouth, on each side of which opens a 
salivary gland that produces an enzyme- 
containing secretion; (2) a tubular eso- 
phagus, which enlarges into (3) a crop in 
the mesothoracic and metathoracie seg- 
ments. This leads into (4) the proven- 
triculus, which is a grinding organ (gizzard). 
Next is the midgut, which is the ventriculus 
(stomach), reaching posteriorly into the ab- 
domen; and into it, 6 double cone-shaped 
pouches, the gastric ceca, pour the digestive 
enzymes they secrete. The products of diges- 
tion are absorbed through the wall of the 
stomach and pass into the blood around it. 
From this point, the food is carried through 
the circulatory system to the cells, where it 
is utilized. Then the hindgut is made up of 
(1) the ileum, into the anterior end of 
which the delicate Malpighian tubules open 
and (2) the colon, which expands into the 
(3) rectum and opens through the (4) 
anus. Since both the foregut and the hind- 
gut are lined with cuticle, little absorption 
takes place in them. 

Circulatory system 

This (Fig. 136) is not a closed system of 
blood vessels as in vertebrates and some in- 
vertebrates, but consists of a single tube lo- 
cated in the abdomen just under the body 
wall in the middorsal line, and of spaces 

(sinuses). The heart is divided into a row 
of chambers; into the base of each opens a 
pair of ostia. These ostia are closed by valves 
when the heart contracts. The pericardial 
cavity, in which the heart lies, is formed by 
a transverse diaphragm beneath it. Blood 
enters the heart and is forced anteriorly 
through the aorta into the hemocoel, where 
it bathes all the organs. The blood system is 
an open one, as is that of other arthropods, 
for there are no capillaries or veins. The 
blood serves chiefly to carry food and wastes; 
there is a separate respiratory system. The 
blood consists of a clear plasma in which are 
suspended white blood cells that act as 
phagocytes to remove foreign organisms and 
other substances. 

Respiratory system 

The respiratory system (Fig. 137) con- 
sists of a network of tubes, the tracheae, 
that communicate with every part of the 
body. The tracheae consist of a single layer 
of cells lined with a layer of cuticle, which 
is thickened to form spiral rings that pre- 
vent the tracheae from collapsing. A tracheal 
branch extends from each spiracle to a 
longitudinal trunk on each side of the body. 
The finest tracheae, the tracheoles, are con- 
nected directly with the tissue to which they 
supply oxygen and from which they carry 
away carbon dioxide. The smallest trache- 
oles contain fluid in which oxygen dis- 
solves before actually reaching the cells; 
this fluid serves in internal respiration like 
the blood in other animals. In the grass- 
hopper and certain other insects, some of 
the tracheae become expanded into thin- 
walled air sacs which are easily compressed 
and thus aid in the movement of air. Con- 
traction and expansion of the body expels 
air from and draws it into the tracheal 

The utilization of oxygen in the metabolic 
processes of the cell, with the production 
of carbon dioxide, is accomplished through 
the action of respiratory enzymes called the 
cytochrome system. The cytochrome sys- 






Circumesophageal connective 


Subesophageal ganglion 

Salivary duct 

Salivary gland 

Third thoracic ganglion 
Ventral nerve cord 

First abdominal ganglion 
Gastric cecum 


Malpighian tubules 




Seminal receptacle 

Genital opening 


Optic lobe 










— Rectum 


Figure 136. Internal organs of a grasshopper as seen with the left side of the body wall 
removed; tracheae not included. 




Air sac Thoracic spiracle Dorsal tracheal trunk Eighth abdominal spiracle 

■Lateral tracheal trunk 
^Ventral tracheal trunk 

Thoracic spiracle First abdominal spiracle Air sac 

Cell membrane Tracheole on muscle fiber 


Figure 137. A, diagram of the tracheae in the body of a grasshopper. The tracheal system 
consists of air-filled tubes which branch into others. The arrows indicate that the grasshopper 
inhales through spiracles located in the anterior part of the body and exhales through those 
limited to its abdomen. B, a large tracheal trunk and some of its branches. (A redrawn from 
College Entomology, by E.O. Essig. Copyright 1942 by The Macmillan Company.) 

tern is localized in the mitochondria of the 

Excretory system 

The organs of excretion (Fig. 136) are 
the Malpighian tubules that are coiled 
about in the hemocoel and open into the 
anterior end of the hindgut. These tubules 
remove metabolic wastes; for example, uric 
acid is taken from the blood that fills the 
hemocoel, condensed in the tubules to crys- 
tals and discharged into the hindgut for 
evacuation through the anus. The conserva- 
tion of water in this process results from its 
reabsorption through the tubules. The re- 
moval of wastes in the dry state is charac- 
teristic of small land animals that have only 
a limited water supply. 

Nervous system 

The nervous system (Fig. 138) includes a 
brain (supraesophageal ganglion), dorsally 

located in the head, consisting of three pairs 
of ganglia fused together. These ganglia 
supply the eyes, antennae, and other head 
organs. The brain joins, by two connectives 
around the esophagus, to the subesophageal 
ganglion. This ganglion consists of the three 
anterior pairs of ganglia of the ventral ner\'e 
chain fused together and supplies the mouth 
parts. The ventral nerve chain continues 
with a pair of large ganglia in each thoracic 
segment. The ganglia in the metathoracic 
segment are particularly large and represent 
the ganglia of this segment and of the first 
abdominal segment fused. Five pairs of 
ganglia are present in the abdomen. The pair 
in the second abdominal segment comprises 
the pairs from the second and third abdom- 
inal segments fused together, and the pair 
in the seventh segment represents the 
ganglia of the seventh to the eleventh seg- 
ments combined. Connected with the brain 
are ganglia of the so-called sympathetic 



( autonomic ) nervous system which controls 
the "involuntary" movements of the diges- 
tive tract, heart, aorta, and reproductive 

Supraesophageal ganglion 

SeTise organs. Grasshoppers possess or- 
gans of sight, hearing, touch, taste, and 
smell. The compound eye and ocellus (Fig. 
132) have already been noted. Vision by 

Optic lobe 
Subesophageal ganglion 

Nerve cord 
—Abdominal ganglion 

Figure 138. The grasshopper nervous system in dorsal view. (Redrawn from Principles of 
Insect Morphology, by R.E. Snodgrass. Copyright 1935 by McGraw-Hill Book Co., Inc.) 

means of the compound eyes has been de- 
scribed in the crayfish. The ocelli are thought 
to be primarily organs of light perception, 
although it is possible that they may form 
crude images at close range. The pair of 
auditory organs are located on the sides of 
the tergite of the first abdominal segment. 
Each consists of a tympanic membrane 
(tympanum) stretched with an almost cir- 
cular sclerotized ring; sound vibrations in 
the air set the tympanic membrane in mo- 
tion, and this in turn affects a slender point 
beneath the membrane which is connected 
to sensory nerve fibers. Some of the insects 
hear sounds beyond the range of the human 
ear. Sound is produced by grasshoppers by 
rubbing the tibia of the hindleg with its 
rough surface against a wing vein which 
causes it to vibrate. The antennae are sup- 
plied with the principal organs of smell. 
Organs of taste are located on the mouth 
parts. The hairlike organs of touch are 
present on various parts of the body but 
particularly on the antennae. 

Reproductive system 

Female grasshoppers can easily be dis- 
tinguished from males because of the pres- 

ence of the ovipositor (Fig. 136). In the 
female there are two ovaries. Each consists 
of several tapering egg tubules called 
ovarioles, which, however, do not possess a 
lumen (Fig. 139). The ovarioles contain 
oogonia and oocytes arranged in a linear 
series, nurse cells, and other tissue cells. The 
oocytes grow as they proceed posteriorly 
down the ovariole, hence the ovariole be- 
comes gradually larger toward the posterior 
end. The ovarioles of each ovary are at- 
tached posteriorly to an oviduct into which 
the eggs are discharged. The two oviducts 
unite to form a short vagina which leads to 
the genital opening between the plates of 
the ovipositor. A tubular seminal receptacle 
( spermatheca ) , which connects with the 
dorsal wall of the vagina, receives the 
spermatozoa during copulation and releases 
them when the eggs are fertilized. 

In the male are two testes in which 
spermatozoa develop (Fig. 139). These are 
discharged into a vas deferens. The two 
vasa deferentia unite to form an ejaculatory 
duct which runs through the penis, at the 
end of which is the sperm-escape opening. 
Accessory glands are present at the anterior 
end of the ejaculatory duct. 




Vas deferens 


Accessory giand 
Seminal vesicle 

Oviduct with 
egg pouch 

Seminal receptacle 

Spermathecal gland 
Accessory gland 

Ejaculatory duct^ 



Figure 139. Diagrams of the reproductive systems of insects in general, that is, typical repro- 
ductive organs found in various insects; but all the organs shown are not present in all species. 

Embryonic development 
and growth 

The eggs are fertilized at the time they 
are deposited by the entrance of sperma- 
tozoa through an opening called the micro- 
pyle in one end of the eggshell. One sperm 
nucleus unites with the nucleus of the ma- 
ture egg; a blastoderm is formed around the 
periphery of the egg from which an embryo 
develops (Fig. 41). The young grasshopper 
that hatches from the egg is called a nymph 
(Fig. 140). It resembles its parent but has a 
large head compared with the rest of the 
body and lacks wings. As it grows its body 
becomes too large for the inflexible exoskelc- 
ton, and the latter is shed periodically. Wings 
are gradually developed from wing buds, and 
the adult condition is finally assumed. This 
type of development is called gradual meta- 


The most conspicuous differences in the 
life cycles of various types of insects are as- 
sociated with the kind of metamorphosis 
involved. There is no metamorphosis in cer- 
tain species, a gradual metamorphosis in 
some as the grasshopper, incomplete meta- 
morphosis in others (dragonfly or mayfly), 
and a complete metamorphosis in the most 
specialized groups. As variations in insect 
life cycles are almost infinite, we select a 
few typical common species for descriptive 

A primitive wingless insect 
without metamorphosis 

Campodea staphylinus (order Thysanura) 
is a delicate, whitish species (Fig. 140) 
about Vs inch long, that lives under stones 



and leaves, in rotten wood, humus, and 
other damp places. The young that hatch 
from the eggs look like miniature adults. 
As they grow they molt a number of times, 
finally reaching sexual maturity and adult 

A grasshopper with 
gradual metamorphosis 

This is a type of development in which 
the young are strikingly like the adult in 
general form of body and in manner of life. 
However, there is a gradual growth of the 
body and wings, but these changes take 
place gradually and are not very great be- 
tween any successive stages (Fig. 140). 

A dragonfly and mayfly 

with incomplete metamorphosis 

This is a type of metamorphosis in which 
accessory organs or gills occur in the aquatic 
naiads, and the adults are aerial. In this type 
of metamorphosis the changes that take 
place in the form of the body are greater 
than in gradual metamorphosis, but much 
less marked than in complete metamor- 
phosis. The metamorphosis of a dragonfly 
(order Odonata) is incomplete. The naiad 
stage of the dragonflv does not have ex- 
ternal gills, but has rectal gills and breathes 
by alternately drawing in and expelling 

The metamorphosis of the mayfly is in- 
complete. Mating takes place during flight, 
after which the female lays several masses 
on a stone in the water; each mass contains 
from 80 to 300 eggs. The eggs hatch in 
about a month and the young that emerge 
are called naiads. The naiads live under the 
water, where they breathe by means of 
tracheal gills and feed on minute plants 
(diatoms and algae). Growth is accom- 
panied by 27 molts and requires from 6 to 9 
months. When ready to venture into the air, 
the naiad swims to the surface; a split ap- 
pears along the back; and a gauzy-winged 

adult flies out to a nearby object where it 
rests from 18 to 24 hours. Then it molts 
again and is ready to fly into the air and 
find a mate. The adult life of both males 
and females is but a few hours or days. 
Therefore, the scientific name of the order, 
which means "living but a day," was well 
chosen. The mayflies fly to lights in im- 
mense numbers, in towns along rivers or 
lakes. One river town has an authentic rec- 
ord of a pile of dead mayflies 8 feet deep, 
which formed in one night around an elec- 
tric light pole. 

A butterfly with 
complete metamorphosis 

Pieris rapae (order Lepidoptera), the 
white cabbage butterfly, is one of our com- 
monest species. The larvae or caterpillars 
(Fig. 143) feed on the leaves of certain 
plants, and therefore the eggs must be laid 
on them. These plants are cabbage, turnip, 
mustard, horse-radish, radish, etc. The larvae 
will die if they hatch out on the wrong type 
of plant. The butterfly probably distin- 
guishes one plant from another by means of 
an olfactory sense. The eggs are laid, one by 
one, few in number, on a single plant. The 
eggs are bullet-shaped and covered with 
ridges and depressions. They are fastened to 
the leaf by the flat end. The larva eats its 
way out of the distal end of the egg shell and 
then proceeds to devour the rest of the shell. 
It begins at once to chew holes in the leaves 
of the host plant with its jaws, and when it 
can grow no larger within its cuticular cover- 
ing, a split appears in the back near the 
anterior end, and the larva crawls out and 
expands because of the elasticity of the new 
exoskeleton. The caterpillar's legs are of 
two kinds: 3 jointed pairs on the thorax, and 
5 unjointed, temporary prolegs on the abdo- 

FiGURE 140. Facing page, three types of life cycles 
found in insects: without metamorphosis, gradual 
metamorphosis, and incomplete metamorphosis. See 
Fig. 141 for complete metamorphosis, a fourth type 
of life cycle. 


Without Metamorphosis 




Gradual Metamorphosis 










from water 

ncomplete Metamorphosis 






Adult ^••i *^ 

(June beetle) 



Complete Metamorphosis 







Figure 141. An insect with complete metamorphosis, June beetle. 

men. It possesses 6 small simple eyes on the 
head but no compound eyes. The green cab- 
bage caterpillars resemble very closely the 
leaves on which they feed and are therefore 
difficult to detect by birds and other enemies 
(probably protective coloration). 

When full grown (about 1V4 inches in 
length), the caterpillars attach themselves 
with a silken thread from their silk glands to 
the underside of a leaf of the host plant or 
beneath some other object. Butterfly cater- 
pillars do not spin cocoons as many moth 
caterpillars do. After a time the body be- 
comes shorter and thicker; the skin splits 
down the back and is pushed off at the 
posterior end; and a greenish-colored pupa 
is revealed (Fig. 143). The pupa does not 
feed; but, within it, violent activity is going 
on. The digestive system changes from one 
fitted for solid food to one that can utilize 
liquid food; the muscular system of the 
crawling larva becomes modified for pur- 
poses of flight; the nervous system is made 
over; wings grow out from pads of larval 
tissue; and the reproductive organs grow to 

maturity. For these purposes, fat stored up 
by the larva is largely utilized. When this is 
all accomplished, requiring about 10 days, 
the pupal skin splits, the adult butterfly 
emerges, spreads its wings, and after they 
have become dry, flies away. The adult 
butterfly possesses a long proboscis coiled 
beneath the head that can be extended so 
as to probe the corollas of flowers for nectar, 
which is sucked into the food reservoir. 

Many variations occur in the 4 types of 
metamorphosis just described. For example, 
among the termites one caste contains 
nymphs that are sexually mature; the naiads 
of mayflies, dragonflies, damsel flies, and 
stone flies are aquatic; the nymphs of grass- 
hoppers, crickets, cockroaches, and chinch 
bugs are terrestrial; the nymphs of the 
"seventeen-year locust" live in the ground 
for 13 to 17 years before becoming adult; 
aphids may be ovoviviparous or oviparous, 
and their eggs may be fertilized or may de- 
velop without fertilization; the larvae of 
many beetles are called grubs; many moth 
caterpillars spin cocoons in which to pupate; 



Figure 142. Millions of mayflies swarmed up from the river in St. Paul, Minnesota, to 
"plaster" lighted objects in the area. An attendant at a gas filling station covered his head for 
protection while servicing cars. The next morning, the short-lived flies were piled up seven 
inches high around the station. Their dead bodies were removed from the premises with a 
snow pusher. (Courtesy of Wide World Photos.) 

the cockroach secretes an egg case to pro- 
tect her eggs; the larvae of flies are known as 
maggots; and certain species, especially cer- 
tain hymenopeterans, stimulate the forma- 
tion of plant galls in which the larvae live. 


The wings, legs, mouth parts, antennae, 
digestive tract, and respiratory system are 
among the structures most conspicuously 




End view 

Side vi 


Larva (caterpillar) 
Figure 143. The life cycle of the imported cabbage butterfly. 

modified so as to adapt insects to their en- 


The mesothorax and metathorax, each, 
bear a pair of wings in most insects. Certain 
simple species (Thysanura, Fig. 129) do not 
possess wings; others (lice and fleas, Fig. 
160) are adapted to parasitic life and the 
wings are degenerate. The flies (Diptera) 
each have a pair of clubbed threads called 
balancers (halteres) in place of the meta- 
thoracic wings. Wings enable their owners 
to fly rapidly from place to place, to escape 
from enemies, and to find a bountiful food 
supply. The success of insects in the struggle 
for existence is in part attributable to the 
presence of wings. Modifications in wing 
venation come about by reduction or by 
addition. In the beetles (Coleoptera, Fig. 
131) the forewings are sheathlike and are 
called elytra. The forewings of Orthoptera 
(grasshoppers, etc.. Fig. 131) are leathery 
and are known as tegmina. The number of 
wing beats differs according to the species. 
Thus yellow swallow-tailed butterflies aver- 

age about 6 beats per second, dragonflies 
about 30, house flies about 160, honey bees 
about 400 and the wings of the humming- 
birds make about 750 beats per second in 
forward flight. In contrast, man can perform 
a fast-finger piano trill at about 10 beats per 


Legs are used for various purposes and are 
highly modified for special functions. Run- 
ning insects, such as the ground beetle, 
possess long, slender legs (Fig. 154); the 
mantis has its forelegs fitted for grasping 
(Fig. 154); the hindlegs of the grasshopper 
are used in leaping (Fig. 131); the forelegs 
of the mole cricket are modified for digging; 
and the legs of the water bug are fitted for 
swimming. Many other types of modifica- 
tions could be mentioned. 

The legs of the honey bee (Fig. 144) are 
perhaps as remarkably adapted for a variety 
of purposes as those of any living insect. 
Honey bees are easily obtained and studied 
in the laboratory, and hence are selected 
here for further description. The prothoracic 



legs possess two useful structures, the pollen 
brush and the antenna cleaner. The femur 
and the tibia are clothed with branched 
hairs for gathering pollen. The surface of 
the first tarsal joint is covered with bristles, 
constituting a cylindrical pollen brush which 
is used to brush up and collect pollen within 
reach of these legs. On the anterior edge of 
the tibia is a flattened movable spine, the 
velum, which fits over a curved indentation 
in the proximal tarsal segment. This entire 
structure is called the antenna cleaner, and 
the row of teeth which lines the indentation 
is known as the antenna comb. The last 

tarsal joint of every leg bears a pair of 
notched claws which enable the bee to ob- 
tain a foothold on rough surfaces. Between 
the claws is a fleshy glandular lobe, the 
pulvillus; its sticky secretion makes it pos- 
sible for the bee to cling to smooth objects. 
Tactile hairs are also present. 

The middle or mcsothoracic legs are pro- 
vided with a pollen brush, but instead of an 
antenna cleaner, a spur is present at the 
distal end of the tibia. This spur is used to 
dislodge wax from the wax pockets on the 
ventral side of the abdomen and to remove 
pollen from the pollen basket. 

Leg segments: 

C =Coxa 
Tr =Trochanter 
F =Femur 
Ti =Tibia 
Ta = Tarsus 

Fore- and hindwings 
hooked together 

Antenna deaner (comb 
and velum) in use 

Antenna comb ' 

Pollen brush 

Prothoracic legs 

Mesothoracic legs 

Metathoracic legs 

Figure 144. Bee adaptations. Legs of the honey bee worker showing many of the structural 
modifications adapting them for gathering pollen, manipulating wax, cleaning the antennae, 
and other functions. 



The hind- or metathoracic legs possess 
three very remarkable structures, the pollen 
basket, the pollen packer, and the pollen 
combs. The pollen basket consists of a con- 
cavity in the outer surface of the tibia with 
rows of curved bristles along the edges. By 
storing pollen in this basketlike structure, it 
is possible for the bee to spend more time in 
the field, and to carry a larger load on each 
trip. On the inner edge of the distal end of 
the tibia is a row of stout bristles, the 
pecten. Opposing the distal end of the tibia 
is the proximal end of the metatarsus bear- 
ing a plate or lip, the auricle. The auricle 
glides over the outer surface of the pecten 
and presses against the oblique outer sur- 
face of the end of the tibia when the joint 
between these structures is flexed backward. 
The auricle and the pecten, working to- 
gether, constitute the pollen packer, since 
their manipulations force the sticky pollen 
masses into the pollen basket. In loading the 
pollen baskets, the pollen brushes of the 

mesothoracic pair of legs collect pollen from 
the brushes of the front legs and from other 
parts of the body, and are themselves 
cleaned by being drawn between the pollen 
combs of the hindlegs. Each pollen comb is 
then scraped over the pecten of the opposite 
leg, the sticky pollen being deposited on the 
outer surface of the pecten or falling on the 
upper surface of the auricle. The leg is then 
flexed backward at this joint, the auricle 
squeezing the pollen outward and upward, 
and thus packing it into the pollen 

Mouth parts 

The mouth parts of insects are in most 
cases fitted either for chewing (mandibulate) 
or sucking (suctorial). The grasshopper 
possesses typical mandibulate mouth parts 
(Fig. 145). The mandibles of insects that 
live on vegetation are adapted for crushing; 
those of carnivorous species are usually 

(Orthoptera, Coleoptera) 

W^^ Labrum |a 

(Homoptera, Hemipfera) 







Figure 145. Some modifications of the fundamental mouth parts in insects. Note the high 
degree of speciahzation which adapts the animals to different methods of feeding. 



sharp and pointed, being fitted for piercing 
and sucking. Suctorial mouth parts are 
adapted for piercing the tissues of plants or 
animals and sucking juices. The mouth parts 
of the honey bee are suctorial, but highly 
modified. In the female mosquito (Fig. 
146), the labrum and hypopharynx com- 

bined form a sucking tube; the mandibles 
and maxillae are piercing organs; the hypo- 
pharynx carries saliva; and the labium consti- 
tutes a sheath in which the other mouth 
parts lie when not in use. The proboscis of 
the butterflies and moths is a sucking tube 
formed by the maxillae (Fig. 145). 

Salivary duct 


Figure 146. Mouth parts of a female mosquito showing the modifications adapting them for 
piercing and sucking. The mouth parts which are shown in soHd black are those used in stinging. 

The mouth parts of insects are of con- 
siderable importance from an economic 
standpoint, since insects that eat solid food 
can be destroyed by spraying the food with 
poisonous mixtures, whereas those that suck 
juices must be smothered with gases or 

killed by substances acting as direct contact 
poisons. The newer insecticides known as 
general purpose compounds are very effec- 
tive for they act either as a stomach or a con- 
tact poison, depending on the manner in 
which the insect encounters them. 




The antennae of insects are usually tactile, 
auditory, or olfactory in function. Interest- 
ing experiments by von Frisch have led him 
to the conclusion that bees can select certain 

odors; for example, they can select an odor 
derived from orange peel from among 43 
others. He has also demonstrated that bees 
can find feeding places through a sense of 
smell. Experiments based on the removal of 
antennae indicate that the olfactory sense 

Figure 147. Antenna of Coleoptera showing variations within a single order of insects. A, 
Elateridae, click beetle. B, Lampyridae, firefly. C, Gyrinidae, whirligig beetle. D, Silphidac, 
Necrophorm, burying beetle. E, Curcuhonidae, weevil. F, Scarabaeidae, lamellicorn beetle. All 
highly magnified. 

organs are located on this structure. Anten- 
nae differ in form and structure (Fig. 147), 
and often the antennae of the male differ 
from those of the female. 

Digestive tract 

Of the internal organs of insects, the 
digestive tract and respiratory system are of 

particular interest. The digestive tract is 
modified according to the character of the 
food; that of the grasshopper is typical of 
vegetable-eating insects. Suctorial insects, 
like the butterflies and moths, are provided 
with a muscular pharynx which acts as a 
pumping organ, and a crop for the storage 
of juices. 



Labial pa 


Maxillae fused to form proboscis Malpighian tubule 

Figure 148. Digestive tract of a sucking insect. Diagram of the internal organs from the 
left side. 



Respiratory system 

The respiratory system of insects (Fig. 
137) is in general like that of the grass- 
hopper, but modifications occur in many 
species, especially in the larvae of those that 
live in water. Aquatic naiads, in many cases, 
do not have spiracles but get oxygen by 
means of threadlike or leaflike outgrowths 
at the sides or posterior end of the body, 
termed tracheal gills. 

Special adaptations 

One has only to study the structure, 
physiology, and behavior of an insect to dis- 
cover adaptative modifications. A few in- 
teresting examples are: (1) the walking stick 
(Fig. 149) not only resembles a dead twig 
but has the habit of feigning death; (2) the 
male cricket possesses a highly differentiated 
sound-producing apparatus consisting of a 
file (Fig. 149) on the base of one wing and 
a scraper on the other; when the wings are 
held up over the body, the file is rubbed over 
the scraper, producing the pleasant call of 
the cricket. The rate at which these calls are 
made is proportional to the temperature. 
The significant part of the sounds produced 
by some classes of insects is a frequency so 
high as to be inaudible to the human ear; 
(3) dragonfly naiads breathe by means of 
rectal gills which line the enlarged posterior 
end of the digestive tract and remove oxygen 
from the water that is drawn in and expelled 
from this cavity; the labium of the naiad 
(Fig. 149) is much elongated and can be 
extended rapidly from its folded resting po- 
sition beneath the head so as to impale its 
prey on the hooks at the end; (4) mosquito 
larvae obtain air through a tube that is 
thrust through the surface of the water; 
(5) water striders (Fig. 149) have long, 
slender legs which do not break through 
the surface film as they skim about over the 
water; (6) fireflies are provided with an or- 
gan capable of emitting light; the females 

and larvae, known as glowworms, are also 
luminescent; (7) click beetles leap (click) 
by means of the action of a prosternal proc- 
ess in a metasternal groove; (8) dung 
beetles, including the sacred scarab of the 
Egyptians (Fig. 149), roll up balls of dung 
in which an egg is laid and on which the 
larva feeds; (9) the larvae of most caddis- 
flies (Fig. 149) build portable protective 
cases of sand grains or vegetable matter 
fastened together with silk; (10) certain 
hornets build nests of wood pulp (Fig. 149) 
—they were the first papermakers; and (11) 
gall wasps stimulate plants to develop ab- 
normal growths called galls, presumably 
caused by growth-stimulating substances 
secreted by the insect. 


It is now well established that hormones 
play an important part in the regulation of 
the activities of insects, for example, hor- 
mones control both metamorphosis and 
molting. Experiments have shown that the 
retention of juvenile characters and develop- 
ment of adult structures are controlled by 
hormones secreted by the brain, prothoracic 
gland, and corpora allata. The corpus al- 
latum is a gland which lies behind the brain. 
If this gland is removed from the nymph 
of the bug Rhodnius, at a certain time, 
molting is prevented. 


Everyone knows that many insects are 
brilliantly colored, especially the butterflies, 
moths, and beetles. Coloration of some in- 
sects differs with the season, and one brood 
may have one color pattern and a later 
brood a very different one. Such insects are 
said to be seasonally dimorphic (two types), 
trimorphic (three types), or polymorphic 
(more than three types). In certain species 
the males and females are differently colored; 



Walking stick 

Labium in place 

Dung beetle (Scarab) 
pushing a ball of dung 


Labium of 

dragonfly naiad 

(Odonata) capturing 


Water strider 

Oak apple gall 
of gall wasp 

Caddisfly larva 
with abdomen in 
pebble covered case 

Paper nest of 

hornet (Hymenoptera) 

A musician, the 
snowy tree cricket 

Figure 149. Special structural and physiologic adaptations of insects. 

that is, they are sexually dimorphic. This 
often occurs, for example, in butterflies of 
the genus Papilio. Often insects are pro- 
tected from their enemies by their colora- 
tion. Examples of what are known as pro- 
tective coloration, protective mimicry, etc., 
are common among insects. 


Various types of association occur among 
animals. Many protozoans live in colonies. 
The same is true of coelenterates, where the 
members of a colony may differ conspicu- 
ously from one another. In many cases ani- 



mals band together for mutual defense, or 
congregate in one place for breeding pur- 
poses, or are attracted to a limited area 
because of the presence of food. Birds are 
often gregarious during the breeding season 
and migration. Certain mammals unite in 
herds partly for protection; for example, 
male bison, when attacked, form a circle 
around the cows and calves. The more com- 
plex societies involve division of labor; the 
principal types of activity are ( 1 ) reproduc- 
tion, (2) obtaining food, and (3) defending 
the colony. Many of the most interesting 
examples of social life occur among the 
wasps, bees, ants, and termites. 

Wasps and bees 

Wasps and bees may be solitary or social. 
Solitary wasps and bees dig a hole in the 

ground or in wood, or construct a nest of 
mud. Wasps provision their nests with cater- 
pillars or other arthropods that they have 
paralyzed; and bees provide pollen ("bee 
bread") to furnish proteins for the growth 
of the larvae. After laying an egg in the nest, 
they close the entrance and give their off- 
spring no more parental care. 


Bumble bees (Fig. 129) and honey bees 
(Fig. 150) are types of social bees. The 
fertilized queen bumble bee lives through 
the winter. In the spring she lays a few eggs 
in a cavity in the ground, from which 
workers develop. The workers are infertile 
females. They carry on all of the activities 
of the colony except laying eggs. At the end 
of the summer, males (drones) and fertile 

Larva Pupa 


Larva fed 



Portion of 

Larva fed 
"bee bread" 

Larva fed 
"bee bread" 

Drone (from 

unfertilized egg) 

Figure 150. Life cycle of the honey bee showing growth stages and three adult castes, consist- 
ing of worker, drone, and queen. Cells of honeycomb in nature are arranged parallel to floor 
of hive. 



females (queens) hatch from some of the 
eggs. These mate and the sperm receptacles 
of the queens are filled with sperms. The 
workers and drones then die, and the race is 
maintained during the winter by the queens 
alone. Honey bees exhibit an even more 
complex social organization. 

"Language" of the honey bee 

Our knowledge of how bees communicate 
is a fascinating discovery, and one that has 
astonished everyone. Observers of honey 
bees have always realized that they had 
some system of communicating with each 
other. However, it remained for the brilliant 
Austrian zoologist, von Frisch, to show 
clearly that their main method of broadcast- 
ing a source of nectar or pollen is not verbal, 
but depends on rhythmic movements and 

A bee informs other bees in her hive of a 
rich source of nectar found near it by 
means of a round dance (Fig. 151). This 
dance gives no indication of direction, but 
the bees know the food is to be found close 

to the hive. The specific odor of the plant 
visited, which is on the body of the inform- 
ing bee, tells her companions the kind of 
flower for which to search. 

If food is farther away than about 165 to 
330 feet, the round dance is replaced by the 
tail-wagging dance (Fig. 151). This dance 
not only informs hive mates of a good source 
of food and its characteristic odor, but also 
the distance and direction in which it will be 
found. As the distance between feeding 
place and hive increases, the number of 
straight runs within the tail-wagging dance 
decreases. The direction of the straight run 
on the honeycomb indicates the direction of 
the nectar source in relation to the sun. 

The dances are closely watched by other 
bees in the hive, who then go out to find 
the source of food. 

Remarkable as the dance "language" of 
the bees is, other studies in progress on bee 
behavior may prove even more interesting. 
Many other social insects besides the honey 
bees undoubtedly have a means of com- 
munication, but its exact nature awaits dis- 
covery through further studies. 







Round dance 

Tail-wagging dance 

Figure 151. The behavior of the bee during the round and tail- wagging dance. When the 
round dance is performed, the bee turns in a circle, once to the left, then once to the right, 
repeating the dance in one place for about one-half minute. During the tail-wagging dance, the 
bee runs a short distance straight ahead wagging the abdomen, then makes a complete 360- 
degree turn to the left, runs ahead once more and turns right; this is repeated over and over. 
(After von Frisch.) 



Other senses of honey bees 

Are bees sensitive to colors? Apparently 
they can distinguish four colors: blue-green, 
yellow-green, blue-violet, and ultraviolet. 
Ultraviolet is invisible to man. This empha- 
sizes the well-established fact that color vis- 
ion in the honey bee is different from that in 
man. Too often biologists have made the 
false assumption that a red flower would 
appear red to all other animals. 

Something is known about taste in bees. 
They can distinguish salt, sour, sweet, and 
bitter. Honey bees are able to determine dif- 
ferent degrees of sweetness. 

It is also known that bees can distinguish 
solid objects, that is, a solid triangle from 
three parallel lines. 


Many ants live a complicated social life. 
The colony, unlike bees, contains several 
fertile females, the queens, and at certain 
periods fertile males, the drones. Infertile 
females may be of several types: (1) soldiers 
to guard the colony, (2) workers to gather 
food, (3) workers to care for the eggs and 
young, etc. These different types of indi- 
viduals are morphologically different, the 
species being very polymorphic. 


The most complex social life of all insects 
is that of the termites (Fig. 152). The 
colony contains three principal types or 


Nymph may develop 

into one of 3 or 

more castes 

Worker "*" *^ ^'^'^ Soldier 

Figure 152, Castes and life cycle of the termite. Note that it feeds on dead wood (p. 638); 

every year termites do great damage to buildings and books in their search for cellulose to eat. 



castes: (1) sexuals (kings and queens), (2) 
workers, and (3) soldiers. The first type, the 
reproductive individuals, may possess func- 
tional wings, small nonfunctional wings, or 
no wings. The winged kings (males) and 
queens (females) leave the colony, mate, 
lose their wings at a particular breaking 
point, and start a new colony. 

The second caste consists of the male and 
female workers, and the third caste of male 
and female soldiers. They have no wings and 
no functional sex organs. The workers are 
more numerous than any other caste. They 
care for the eggs and young, feed and tend 
the queen, obtain food, cultivate fungus in 
special chambers in certain species, excavate 
tunnels and galleries, construct mounds, and 
perform other duties. 

The soldiers are the most highly special- 
ized. Two castes may be present: one has a 
large body, strong head, and huge mandi- 

bles for driving away intruders; the other 
carries on chemical warfare by means of a 
pore in the head through which a repellent 
fluid may be ejected. That the soldiers are 
not very successful is indicated by the fact 
that over 100 species of other insects, ara- 
chnids, centipedes, and millipedes live regu- 
larly as guests in the nests of termites. 


Beneficial insects 

Insects of importance to human welfare 
have been mentioned in the preceding 
pages. Some of them are beneficial, but more 
are injurious. Among the beneficial insects 
are those that produce honey, wax, silk, lac, 
and cochineal; those that cross-fertilize (pol- 





Adult female 


Figure 153. A beneficial insect, the silkworm. 

lenize) flowers; and those that destroy in- 
jurous insects either by devouring or para- 
sitizing them. Injurious insects include farm 
and household pests, and species that trans- 
mit disease agents. 
The honey bee (Fig. 150) produces about 

250,000,000 pounds of honey in the United 
States every year. We also use about 10 
million pounds of beeswax annually. Silk- 
worms (Fig. 153) spin about 1000 feet of 
thread to make each cocoon, and about 
25,000 cocoons are necessary to manufacture 



one pound of silk. The number of silkworms 
that are working for man is indicated by the 
fact that about 50 million pounds of silk are 
used in the world every year. Lac insects 

belong to the family Coccidae; they secrete 
a wax known as shellac. The dye known as 
cochineal is made from the dried bodies of 
a scale insect that lives on cactus; it is no 


Antlion lying in 
wait at bottom 
of its pit 

* -./ 

Ground beetle 

Tiger beetle 

ladybird beetle 




Figure 154. Beneficial insects. The figures are not drawn to scale. (After U.S. Department of 
Agriculture Yearbook, 1952.) 

longer of much value, aniline dyes having 
largely taken its place. 

Bees are among the most valuable of the 
insects that pollenize flowers. For example. 

apples, pears, blackberries, raspberries, and 
clover depend upon them. Certain other in- 
sects also perform this important function. 
This means that many of our important food 



plants could not exist without insects. An 
interesting example is furnished by the 
Smyrna fig which could not be grown in 
California until the minute fig insect (Fig. 
448) was introduced to pollenize the 

Predaceous insects 

Predaceous insects are of benefit because 
they devour vast numbers of other insects, 
most of which are injurious. Ground beetles, 
tiger beetles, antlions, and lady beetles, all 

Hymenopteran ovipositing in on aphid (Homoptero) 

Cocoons (pupae) of a braconid 
fly (Hymenoptera) on the larva 
(hornv/orm) of the tomato 
sphinx moth (Lepidoptera) 


ovipositing in egg 
of codling moth 

Pyrgota fly (Diptera) 
ovipositing in abdomen 
of June beetle (Coleoptera) 

Figure 155. Some insects that show a parasite-host relationship. It is roughly estimated that 
there are about 11,000 species of parasitic insects in North America. Insects are probably their 
own worst enemies. (After U.S. Department of Agriculture Yearbook, 1952.) 

shown in Fig. 154, are common types. The 
Australian ladybird beetle (Fig. 154) was 
introduced into California because it was 
known to devour the fluted scale and to 
protect successfully the orange and lemon 
trees from this species. Other predaceous 

species have also been introduced with favor- 
able results. 

Insects that live as parasites (Fig. 155) on 
other insects are also beneficial to man. 
They usually lay their eggs in the larvae, and 
the young that hatch from these eggs 



slowly devour and finally kill their host be- 
fore it becomes an adult. Some parasitic in- 
sects furnish examples of what is known as 
polyembryony; each of their eggs produces 
not one but many larvae, as many as 395 
having been reported from a single egg. 
Sometimes parasitic insects parasitize other 
parasitic insects and these in turn may be 
parasitized, and so on. Thus we have pri- 
mary, secondary, tertiary, and quaternary 
parasites, a condition known as hyper- 
parasitism. There is certainly truth in the 
following humorous lines: 

Big fleas have little fleas 
Upon their backs to bite 'em 
And little fleas have lesser fleas, 
And so ad infinitum. 

Many insects are of the scavenger type, 
and vast quantities of dead animal and 
vegetable materials are eaten by them, thus 
preventing decay and obnoxious odors. Blow 
flies are especially effective, since they lay 
enormous numbers of eggs, and the larvae 
that hatch from them are extremely vora- 
cious. As Linnaeus remarked, a fly can devour 
the carcass of a horse more quickly than a 







Figure 156. Life cycle of the housefly (Muscd domestica). This fly, like insects in general, has 
a tremendous reproducti\-e potential. It has been said that if this potential were unchecked, the 
descendants of a pair of flies in 10 years would weigh more than the earth. 

lion can. Water-scavenger beetles, burying 
beetles, and dung beetles (Fig. 149), in- 
cluding the sacred scarab of the Egyptians 
are all scavengers. 

A study of the effects of certain maggots 
as an aid in the healing of wounds has led 
from maggot therapy to chemical therapy in 
which synthetic allantoin is used directly in 

Insects harmful to plants 

Insect pests, according to estimates by the 
United States Department of Agriculture, 
do about $4,000,000,000 damage annually to 
farm crops, forests, stored foodstuffs, and 
domesticated animals. Some are native pests, 
such as the potato beetle (Fig. 157), Rocky 
Mountain locust, and army worm; others 

Chinch bug 

Codling moth 

Figure 157. Insects harmful to plants. 

Rose aphid 




have been introduced from other countries, 
such as the San Jose scale, European corn 
borer, cotton boll weevil, Japanese beetle, 
and Mediterranean fruit fly. Among the 
sucking insects that may be classified as farm 
pests are the aphids, scale insects, stink 
bugs, Hessian fly maggots, and leafhoppers. 
The chinch bug is especially notorious be- 
cause of its injuries to corn and small grain. 
Chewing insects of importance include wire- 
worms, white grubs, the European corn 
borer, flat-headed borers, bark borers, alfalfa 
weevils, corn-ear worms, and cotton boll 
weevils. Stored grain is destroyed in large 
quantities by beetles of various kinds, es- 

pecially by weevils and by caterpillars of 

Insects injurious to 
domestic animals 

Domestic animals are often seriously in- 
jured by insects. Biting lice, such as the 
chicken louse (Fig. 158) may feed on 
feathers and cause loss of flesh by their con- 
stant irritation. Sucking lice are even more 
injurious. Among the flics, the horn fly is a 
bloodsucking species and a serious pest of 
cattle. The larvae of the horse botflies (Fig. 
158) may cause series disturbances to the 

Larva hatches, migrates through body 
to skin of back 

Adult fly 
lays eggs 
on hairs 
of cow 

Cattle grub 

Pupates in 

Larva and air 
hole in skin 

Sheep "tick" 

Larva hatches, 
migrates to 

Horse botfly 

Larva "bot" 
in stomach 


Pupates in ground 

Adult botfly emerges, lays eggs 
on hairs of horse 

Chicken louse (Mollophago) 

Figure 158. Insects injurious to domestic animals. Each year livestock pests cost this country 
about $500,000,000. Figures of insects are not drawn to scale. 



Stomach. The cattle grubs of the ox warble 
fly (Fig. 158) cut holes in the skin of cattle 
and damage the hide; the animals also lose 

Household insect pests 

Insects that are unwelcome guests in the 
house (Fig. 159) are sometimes only annoy- 
ing, but they may become destructive. Food 
may be spoiled by cockroaches, ants, fruit 
flies, and weevils; clothing, carpets, furs, and 
feathers may be injured by clothes moths 
and carpet beetles. Among the piercing in- 
sects that are annoying are stable flies, bed- 
bugs, and mosquitoes. 

Chemical control of household insect 
pests involves the use of chemicals that kill 
or repel them. 

Insects that transmit 
human diseases 

No doubt insects that carry diseases (Fig. 
160) are one of the greatest enemies of man 
and affect human welfare most profoundly. 
Some of the more important species are the 
house flies that spread the bacteria of ty- 
phoid and of summer diarrhea; mosquitoes 
that transmit malaria, yellow fever, dengue, 
and filariasis; fleas that convey the bacteria 
of bubonic plague from rat or ground 
squirrel to man; body lice (cooties) that are 
responsible for transmission of t\phus fever; 
and tsetse flies that are the vectors of Afri- 
can sleeping sickness in man, and nagana 
and other diseases in domestic animals. 

Our federal and state governments and 
educational institutions all recognize the 
necessity of controlling injurious insects, and 
hence economic entomology has become 
one of the most important activities in the 
scientific field, both pure and applied. De- 
partments of health devote a considerable 
part of their funds and efforts to the subject 
of insect control, especially the control of 
house flies and mosquitoes. 


{For reference purposes only) 

Insects are divided into orders principally 
on the basis of the following characteristics: 
( 1 ) with or without metamorphosis; if meta- 
morphosis occurs, whether gradual, incomplete, 
or complete; ( 2 ) type of mouth parts, and ( 3 ) 
number and type of wings. Minor features are 
also of service. Some of the entomologists di- 
vide a few of the orders listed below still 
further, but to simplify we are giving only 25 
as follows: 

1. Protura. 

2. Thysanura. Bristletails, etc. 

3. Collembola. Springtails 

4. Orthoptera. Grasshoppers, etc. 

5. Isoptera. Termites 

6. Neuroptera. Aphislions, etc. 

7. Ephemeroptera. Mayflies. 

8. Odonata. Dragonflies 

9. Plecoptera. Stoneflies 

10. Corrodentia. Booklice, etc. 

1 1 . Mallophaga. Bird and other biting lice 

12. Embioptera. Embiids 

13. Thysanoptera. Thrips 

14. Anoplura. Sucking lice 

1 5. Hemiptera. Bugs 

16. Homoptera. Plant lice, etc. 

17. Dermaptera. Earwigs 

18. Coleoptera. Beetles 

19. Strepsiptera. Stylopids 

20. Mecoptera. Scorpionflies and others 

21. Trichoptera. Caddisflies 

22. Lepidoptera. Moths and butterflies 
25. Diptera. Flies 

24. Siphonaptera. Fleas 

25. Hymenoptera. Bees, wasps, ants, etc. 

Order 1. Protura (Or. proto, first; uro, 
tail). Primitive; wingless; no metamorphosis; 
mouth parts insectlike; without antennae or 
true eyes; abdomen of 12 segments; live in 
damp places; about 30 species. Ex. Acerentulus 

Order 2. Thysanura (Or. thysanos, tassel). 
Firebrat (Fig. 129). Primitive, wingless; no 
metamorphosis; chewing mouth parts; 11 ab- 

Silverfish ^ 

Book louse, "death watch*,' 



American cockroac 


Clothes moth 


Little black ant 

Yellow mealworm beetle Cheese skipper 
{Coleoptera) (DipJera) 

Carpet beetle 

Figure 159. Household insect pests. Figures are not drawn to scale. 





Kissing bug (Hemiptera), 
vector of Chaga's disease 

Oriental rat flea 
(Siphonaptera), vector 
of bubonic plague 

Deer fly (Diptera), 
vector of tularemia 


Body louse, "cootie" (Anoplura); 
vector of typhus, relapsing and 
trench fevers 

Tsetse fly (Diptera), 
vector oif African 
sleeping sickness 

Sand fly (Diptera), 
vector of kala-azor 
and oriental sore 

Figure 160. Some insects that transmit human diseases. The figures are not drawn to scale. 

dominal segments; usually two or three long, 
filiform, segmented, caudal appendages; less 
than 20 species known from the United States. 
Ex. Lepisma saccharina (Fig. 159), silver- 

Order 3. Collembola (Gr. kolla, glue; ballo, 
put). Springtails (Fig. 129). Primitive wing- 
less insects; chewing or sucking mouth parts; 
4 segments in antennae; no metamorphosis; 
usually no tracheae, compound eyes, Mal- 
pighian tubules, nor tarsi; 6 abdominal seg- 
ments; springing organ present in most species 

on ventral side of fourth abdominal segment; 
sticky tubelike projection on ventral surface of 
first abdominal segment. Ex. Achorutes nivi- 
cola, snow flea. 

Order 4. Orthoptera (Gr. orthos, straight; 
pteron, wing). Grasshoppers, etc. Metamor- 
phosis gradual; chewing mouth parts; typically 
two pairs of wings; forewings often thickened 
and parchmentlike, called tegmina (singular, 
tegmen); hindwings folded like fan beneath 
forewings; in some, wings vestigial or absent; 
6 common families as follows: 



Cursoria. Walking or running 
Phasmatidae. Walkingsticks (Fig. 149) 
Mantidae. Mantes (Fig. 154) 
Blattidae. Cockroaches (Fig. 159) 
Saltatoria. Leaping 

Tettigoniidae. Long-homed grasshoppers 

Gr\'llidae. Crickets 

Locustidae. Short-horned grasshoppers 

Order 5. Isoptera (Or. isos, equal; pteron, 
wing). Termites or "white ants." Metamor- 
phosis gradual; chewing mouth parts; two pairs 
of long, narrow wings laid flat on back, or 
wingless; abdomen joined broadly to thorax; 
social insects living in colonies (p. 253). Ex. 
Reticulitermes flavipes (Fig. 152). 

Order 6. Neuroptera (Gr. neuron, nerve). 
Dobson or hellgrammite, aphislions, antlions, 
and others. Metamorphosis complete; chewing 
mouth parts; 4 similar membranous wings, 
usually with many veins and cross veins; no 
abdominal cerci; larvae carnivorous, some with 
suctorial mouth parts; tracheal gills usually 
present on aquatic larvae. Ex. Lacewing (Fig. 

Order 7. Ephemeroptera (Ephemerida) 
(Gr. ephemeros, living but a day). Mayflies 
(Fig. 129). Metamorphosis incomplete; mouth 
parts of adult, vestigial; two pairs of mem- 
branous, triangular wings; forewings larger 
than hindwings; caudal filament and cerci very 
long. Ex. Ephemera simulans, which is an im- 
portant fish food. 

Order 8. Odonata (Gr. odous, tooth). 
Dragonflies and damselflies. Metamorphosis 
incomplete; chewing mouth parts; two pairs 
of membranous wings; hindwings as large as 
or larger than forewings; large compound eyes; 
small antennae; no cerci; naiads aquatic; both 
naiads and adults, predaceous. Ex. Anax Junius. 

Order 9. Plecoptera (Gr. pleko, fold). 
Stoneflies (Fig. 129). Metamorphosis incom- 
plete; chewing mouth parts, often undeveloped 
in adults; two pairs of wings, the hindwings 
usually larger and folded beneath forewings; 
tarsus with three segments; naiad aquatic, often 
with tufts of tracheal gills. Ex. Allocapnia 

Order 10. Corrodentia (Psocoptera) (L. 
corrodenSy gnawing). Psocids and booklice 
(Fig. 159). Metamorphosis gradual; chewing 
mouth parts; wingless or with two pairs of 

membranous wings that have few, prominent 
veins; forewings larger than hindwings; wings, 
when at rest, held over body like sides of a 
roof. Ex. Liposcelis divinatorius. 

Order 11. Mallophaga (Gr. mallos, wool). 
Biting lice (Fig. 158). Gradual metamorphosis; 
chewing mouth parts; wings absent; eyes de- 
generate. Ex. Menopon gallinae, common 
chicken louse. 

Order 12. Embioptera (Gr. embios, lively). 
Embiids (Fig. 129). Metamorphosis gradual; 
chewing mouth parts; wingless or with two 
pairs of delicate, membranous wings, contain- 
ing few veins, and folded on the back when at 
rest; cerci of two segments. Ex. Embia texana. 

Order 13. Thysanoptera (Gr. thysanos, 
fringe). Thrips (Fig. 129). Metamorphosis 
gradual; rasping mouth parts; wingless or with 
two pairs of similar, long, narrow, membranous 
wings with few or no veins, and fringed with 
long hairlike structures; prothorax large and 
free; tarsi with two or three segments terminat- 
ing in a bladderlike, protrusible vesicle. Ex. 
Thrips tabaci, onion thrips. 

Order 14. Anoplura (Gr. anoplos, unarmed; 
oura, tail). Sucking lice. Gradual metamorpho- 
sis; piercing and sucking mouth parts; wing- 
less; ectoparasites on mammals; eyes poorly 
developed or absent; tarsus with one segment 
bearing a single, large, curved claw adapted for 
clinging to hair of host. Ex. Pediculus humanus 
corporis (Fig. 160), body louse. 

Order 15. Hemiptera (Gr. hemi, half). 
True bugs. Metamorphosis gradual; piercing 
and sucking mouth parts; wingless or with two 
pairs of wings; forewings thickened at base; 
labium forms a jointed beak in which the 
slender, piercing maxillae and mandibles move; 
a few families as follows: 


Corixidae. Water boatmen 
Notonectidae. Back swimmers 
Belostomatidae. Water bugs 
Gerridae. Water striders (Fig. 149) 


Miridae. Leaf bugs 

Cimicidae. Bedbugs (Fig. 159) 

Reduviidae. Kissing bugs (Fig. 160) 

Tingidae. Lace bugs 

Lygacidae. Chinch bugs (Fig. 157) 



Coreidae. Squash bugs 
Pentatomidae. Stink bugs (Fig. 129) 

Order 16. Homoptera (Gr. homos, same). 
Cicadas, leafhoppers, aphids, scales (Fig. 157). 
Metamorphosis gradual; mouth parts for pierc- 
ing and sucking; usually two pairs of wings of 
uniform thickness held over back, like sides 
of roof; a few families as follows: 

Cicadidae. Cicadas 

Cercopidae. Spittle insects (Fig. 129) 
Membracidae. Treehoppers 
Cicadellidae. Leafhoppers 
Aphididae. Plant lice (Fig. 157) 
Phylloxeridae. Phylloxcrids 
Aleyrodidae. Whiteflies 
Coccidae. Scale insects 

Order 17, Dermaptera (Gr. derma, skin). 
Earwigs (Fig. 129). Metamorphosis gradual; 
chewing mouth parts; wingless or with one or 
two pairs of wings; forewings small, leathery, 
and meeting in a straight line along back; 
hindwings large, membranous, and folded 
lengthwise and crosswise under forewings; 
forcepslike cerci at posterior end of abdomen. 
Ex. Forficula auricularia. 

Order 18. Coleoptera (Gr. koleos, sheath). 
Beetles (Figs. 149, 154 and 157). Metamor- 
phosis complete; chewing mouth parts; wingless 
or usually with two pairs of wings; forewings 
hard and shcathlike (elytra) and hindwings 
membranous and folded under elytra; prothorax 
large and movable; some of the families are 
as follows: 

Cicindelidae. Tiger beetles (Fig. 154) 
Carabidae. Ground beetles (Fig. 154) 
Hydrophilidae. Water scavengers 
Silphidae. Carrion beetles 
Staphylinidae. Rove beetles 
Dermestidae. Dermestids 
Tenebrionidae. Darkling beetles, such as yel- 
low mealworm beetles (Fig. 159) 
Coccinellidae. Ladybird beetles (Fig. 154) 
Scarabaeidae. Lamellicorn beetles (Fig. 149 
Dytiscidae. Diving beetles 
Gyrinidae. Whirligig beetles 
Lampyridae. Fireflies 
Meloidae. Blister beetles 
Elateridae. Click beetles 
Buprestidae. Wood borers 

Lucanidae. Stag beetles 
Cerambycidae. Long-homed beetles 
Chrysomelidae. Leaf beetles (Fig. 157) 
Curculionidae. Snout beetles, such as cotton 

boll weevil (Fig. 157) 
Scolytidae. Engraver beetles 

Order 19. Strepsiptera (Gr. strepsis, a turn- 
ing). St\'lopids (Fig. 129). Hypermetamorpho- 
sis; mouth parts vestigial or absent; endopara- 
sitic in other insects; male with raspbcrrylike 
eyes, tiny club-shaped forewings and large fan- 
shaped membranous hindwings; female, larva- 
like, eyeless, wingless, and legless; nutrition by 
absorption; life cycle complex. As the descrip- 
tion indicates, the Strepsiptera are in many re- 
spects the most unique of all the insects. Ex. 
Xenos wheeleri, a parasite of the wasp Polistes. 

Order 20. Mecoptera (Gr. mekos, length). 
Scorpionflies (Fig. 129). Metamorphosis com- 
plete; chewing mouth parts; antennae long and 
slender; head prolonged into beak; wingless or 
with two pairs of long, narrow, membranous 
wings; some males with clasping organ at 
caudal end resembling sting of scorpion. Ex. 
Panorpa rufescens. 

Order 21. Trichoptera (Gr. thrix, hair). 
Caddisflies (Figs. 129 and 149). Metamorpho- 
sis complete; vestigial mouth parts in adult; 
two pairs of membranous wings clothed with 
long, silky hairs; many aquatic larvae build 
portable cases. Ex. Phryganea interrupta. 

Order 22. Lepidoptera (Gr. lepis, scale). 
Butterflies, skippers, and moths (Figs. 143, 
1 57 and 1 59 ) . Metamorphosis complete; suck- 
ing mouth parts; wingless or with two pairs of 
membranous wings covered with overlapping 
scales; sucking apparatus coiled underneath 
head; lar\ae have chewing mouth parts and 
are called caterpillars; sometimes spin cocoon; 
pupa often called a chrysalis. A few families are 
as follows: 

Families of Moths 

Tmeidae. Clothes moth (Fig. 159) 
Tortricidae. Leaf rollers 
Sphingidae. Hawk moths 
Geometridae. Measuring worms 
Lymantriidae. Tussock moths 
Phalaenidae (Noctuidae). Owlet moths 
Arctiidae. Tiger moths 
Citheroniidae. Regal moths 



Saturniidae. Giant silkworm moths 
Bombycidae. Silkworm moths (Fig. 153) 

Families of Skippers, and Butterflies 

Hesperiidae. Skippers 

Papilionidae. Swallowtails 

Pieridae. Whites and yellows (Fig. 143) 

Nymphalidae. 4-footed butterflies 

Lycaenidae. Gossamer-wings 

Order 23. Diptera (Gr. dis, two). Flies 
(Fig. 156). Metamorphosis complete; piercing 
and sucking mouth parts forming proboscis; 
wingless or with one pair of membranous fore- 
wings; hindwings represented by knobbed 
threads called halteres; larvae known as mag- 
gots (Fig. 156); larval skin sometimes serves 
as a cocoon and called a puparium; some of the 
families are as follows: 

Tipulidae. Crane flies 

Tendipedidae (Chironomidae). Midges 

Psychodidae. Sand flies (Fig. 160) 

Culicidae. Mosquitoes 

Cecidomyidae. Gall gnats 

Syrphidae. Flower flies 

Trypetidae. Fruit flies 

Drosophilidae. Pomace flies 

Oestridae. Bot flies (Fig. 158) 

Calliphoridae. Blow flies 

Simuliidae. Black flies 

Tabanidae. Deer flies (Fig. 160) 

Bombyliidae. Bee flies 

Asilidae. Robber flies 

Sarcophagidae. Flesh flies 

Tachinidae. Tachinid flies 

Muscidae. Houseflies (Fig. 156) 

Hippoboscidae. Louse flies 

Braulidae. Bee lice 

Order 24. Siphouaptera (Gr. siphon, sucker; 
a, without; pteron, wing). Fleas. (Fig. 160). 
Metamorphosis complete; piercing and sucking 
mouth parts; wingless; body laterally com- 
pressed; head small; no compound eyes; legs 
adapted for leaping; ectoparasites of mammals, 
a few birds. Ex. Ctenocephalides felis, cat 

Order 25. Hymenoptera (Gr. hymen, mem- 
brane). Ants, bees, wasps, etc. Metamorphosis 
complete; chewing or sucking mouth parts; 
wingless or with two pairs of membranous 

wings, forewings larger, venation reduced, 
wings on each side held together by hook-j 
(hamuli); females usually with sting, piercer, 
or saw; some parasitic on other insects; som*': 
of the families are as follows: 

Tenthredinidae. Sawflies 
Braconidae. Braconids 
Ichneumonidae. Ichneumons 
Cynipidae. Gall wasps 
Ghalcididae. Chalcids 
Formicidae. Ants (Fig. 159) 
Vespidae. Wasps 
Sphecidae. Digger wasps 
Andrcnidae. Mining bees 
Megachilidae. Leaf cutters 
Bombidae. Bumble bees 
Apidae. Honey bees (Fig. 150) 


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Destructive and Useful Insects. McGraw- 
Hill, New York, 1951. 

Roeder, K.D. Insect Physiology. Wiley, New 
York, 1953. 

Ross, H.H. A Textbook of Entomology. Wiley, 
New York, 1956. 



Scheer, B.T. (ed.). Recent Advances in In- 
vertebrate Physiology. Univ. of Oregon Pub- 
lications, Eugene, Ore., 1957. 

Snodgrass, R.E. Principles of Insect Morphol- 
ogy. McGraw-Hill, New York, 1935. 

Steinhaus, E.A., and Smith, R.F. Ann. Rev. of 
Entomol. Annual Reviews, Inc., Stanford, 

von Frisch, Karl. The Dancing Bees. Harcourt, 
Brace, New York, 1955. 

. Bees, Their Vision, Chemical Senses, 

and Language. Cornell Univ. Press, Ithaca, 

N.Y., 1950. 
West, L.S. The House Fly. Comstock, Ithaca, 

N.Y., 1951. 
Wheeler, W.M. The Social Insects, Their 

Origin, and Evolution. Harcourt, Brace, New 

York, 1928. 
Wigglesworth, V.B. The Principles of Insect 

Physiology. Methuen, London, 1950. 



Phylum Arthropoda. 
Spiders and 
Their Allies 

HE class Arachnoidea received its name 
from the Greek word arachne, which means 
spider. The name is appropriate since spi- 
ders are the most abundant members of the 
class. However, it is a heterogeneous group 
consisting of spiders, king crabs, scorpions, 
harvestmen, mites, ticks, and several minor 
groups. These animals differ markedly from 
one another, but agree in several important 
respects: (1) they have no antennae; (2) 
there are no true mandibles; (3) the first 
pair of appendages are nippers, termed 
chelicerae; and (4) usually the body can be 
divided into an anterior part, the cephalo- 
thorax, and a posterior part, the abdomen. 
Some have very interesting adaptations such 
as the spinning apparatus of the spiders. 

The Arachnoidea, for convenience, are 
here classified into one class and four sub- 

The spiders show interesting arthropod 
modifications, especially for breathing, for 
obtaining and digesting juices, and for spin- 
ning webs. A general knowledge of the other 
groups is worth while. Since king crabs rep- 
resent an ancient group of animals, they are 
sometimes called "living fossils"; scorpions 
are poisonous to man and are often referred 
to in literature; harvestmen, or daddy long- 
legs, are frequently encountered in fields; 
and many mites and ticks are either di- 
rectly injurious to man and domesticated 
animals, or they carry disease germs. 

Ordinarly, arachnoids are not noticeable; 
and we would hardly know they existed were 
it not for the conspicuous webs spun by 


External anatomy 

The body of the spider (Fig. 162) con- 
sists of a cephalothorax and an unsegmented 

There are 6 pairs of appendages attached 
to the cephalothorax. Antennae are absent; 
sensory functions are in part performed by 




(King crab) 

(Garden spider) 




the pedipalps and walking legs. The first pair 
of appendages are called chelicerae (Figs. 
162 and 163). In many species, they are 
composed of two parts: a basal segment and 

a terminal claw or fang. Poison glands are 
situated in the chelicerae of the tarantulas, 
but in most spiders they are in the cephalo- 
thorax. The poison (venom) they secrete 



Abdomen - 


Poison gland 

Sucking stomach 









Silk glands 

Seminal receptacle 

Figure 162. Internal structure of a spider as seen with the left side of the body removed. 
(Modified from Lueckart.) 

passes through ducts that open on the fangs 
of the chelicerae; the venom is used to kill 
their prey and as a means of defense; it is 
strong enough to kill small animals and, in 
some species, to injure and kill larger ani- 
mals. Although spiders in general have for 
centuries been considered very poisonous, 
the black widow is the only one in this coun- 
try capable of causing death in man. The 
second pair of leglike appendages are the 
pedipalpi; their basal parts, called "maxil- 
lae," are used as jaws to press or chew the 
food. The pedipalpi of the mature male are 
also used to transfer sperms to the female. 
There are 4 pairs of walking legs (Fig. 
164) . Each leg consists of 7 joints: ( 1 ) coxa, 
(2) trochanter, (3) femur, (4) patella, (5) 
tibia, (6) metatarsus, (7) tarsus. It is 

Figure 161. Facing page, some representatives of the 
class Arachnoidea. The figures are not drawn to scale. 

terminated by two- or three-toothed claws 
(Fig. 163), and often a pad of hairs, the 
claw tuft, which enables the spider to run on 
ceilings and walls. 

The sternum lies between the legs, and a 
labium is situated between the "maxillae." 
The eyes, usually 8, are on the front of the 
head (Fig. 163). The mouth is a minute 
opening between the bases of the pedipalpi; 
it serves for ingestion of liquids only. 

The abdomen is connected with the 
cephaiothorax by a slender waist ( peduncle ). 
Near the anterior end of the abdomen, on 
the ventral surface, is the genital opening, 
which in some female spiders is covered by 
a flat plate called the epigynum. On either 
side of the epigynum is the slitlike open- 
ing of the respiratory organs or book lungs. 
Some spiders also possess tracheae which 
open to the outside through spiracles near 
the posterior end of the ventral surface (Fig. 



Anterior view of head 


Poison gland 




Tip of leg 
with claws 

Anterior view showing 
poison glands 

Accessory spinning 



Cocoon of garden 

Ventral view of posterror end 
of abdomen with spinnerets 

Figure 163. Structural details of spiders and a cocoon. 

162). Just back of the tracheal opening are 
three pairs of tubercles or spinnerets (Figs. 
162 and 163) used for spinning threads. The 
anus lies posterior to the spinnerets. 

Internal anatomy 
and physiology 

The spider feeds mainly on insects; and 
because it can ingest only liquid food, the 
solid parts are liquefied by the action of 
powerful digestive enzymes contained in a 
fluid which is regurgitated into or over its 
prey. Actually, spiders ingest food in two 

ways: (1) those with weak jaws puncture 
the body of the insect with their fangs, and 
then alternate between injecting digestive 
fluid through this hole, and sucking back the 
liquefied tissues, until only an exoskeleton 
remains; (2) those with strong jaws crush 
the insect into small pieces between their 
jaws as the digestive enzymes are regurgi- 
tated over them. Only a small mass of indi- 
gestible material, such as the sclerotized 
parts, remains to be discarded. In feeding, 
suction is produced mainly by the enlarge- 
ment of the sucking stomach. 
The digestive system is made up of a 



mouth and pharynx, followed by the hori- 
zontal esophagus that leads into the sucking 
stomach, which in turn opens into the true 
stomach which gives off 5 pairs of ceca or 
blind tubes in the cephalothorax. The in- 
testine passes almost straight through the 
abdomen; it is enlarged at a point where 
ducts bring into it a digestive fluid from 
the "hver," and again near the posterior 
end, where it forms a sac, the stercoral 
pocket, connected with the rectum, which 
terminates in the anus. Tubes called 
Malpighian tubes enter the intestine near 
the posterior end. The digestive tract in the 
abdomen is surrounded by a large digestive 
gland or "Hver." This gland secretes a fluid 
resembling pancreatic juice, which pours 
into the intestine through ducts. 

The circulatory system consists of a heart, 
arteries, veins, and a number of spaces or 
sinuses. The heart is situated in the abdo- 
men and is dorsal to the digestive tract. It 
is a muscular contractile tube lying in a 
sheath, the pericardium. The heart opens 
into the pericardium, usually, by three pairs 
of openings (ostia). It gives off posteriorly a 
caudal artery, anteriorly an anterior aorta 
which branches and supplies the tissues in 
the cephalothorax, and three pairs of ab- 
dominal arteries. The blood, which is color- 
less and contains mostly amoeboid corpus- 
cles, passes from the arteries into sinuses 
among the tissues, and is carried to the 
book lungs, where it is oxygenated; it then 
passes to the pericardium by way of the 
"pulmonary veins"; and finally enters the 
heart through the ostia. 

Respiration is carried on by tracheae and 
book lungs; the latter are peculiar to arach- 
nids. There are usually two book lungs, 
which, according to Kaston, have often 
been incorrectly described in textbooks. He 
states that each book lung consists of a 
blood-filled chamber with an air vestibule 
posterior to it. This air vestibule communi- 
cates to the outside through a slit in the 
body wall. From the front wall of the vesti- 
bule, there have been invaginated many 
parallel, narrow, air pockets, which have 

pushed forward into the blood-filled cham- 
ber; these may be considered tracheae which 
have a flattened fan shape instead of the 
tubular shape usually associated with 
tracheae. Air entering through the slit in the 
body wall circulates through the air pockets 
where oxygen is taken up by the blood. 
Tubular tracheae are also usually present but 
do not ramify to all parts of the body as in 
the insects. 

The excretory organs are paired Mal- 
pighian tubules, which open into the intes- 
tine, and one or two pairs of coxal glands 
in the floor of the cephalothorax. The coxal 
glands are sometimes degenerate, and their 
openings are difficult to find; they are 
homologous with the green glands of the 
crayfish (Fig. 114). 

The nervous system consists of a bilobed 
ganglion brain above the esophagus, a large 
subesophageal ganglionic mass, and the 
nerves which run to various organs. Sensory 
hairs, over the body and appendages, rep- 
resent the principal sense organs. There are 
usually 8 simple eyes (Fig. 163), which dif- 
fer in size and arrangement in different 
species. In only a few families are distinct 
images formed; in the others the eyes func- 
tion primarily for perception of degrees of 
light, and moving objects such as prey. 

Very fine, erect, hairlike processes set in 
sockets have been considered organs of hear- 
ing, but the evidence is still insufficient. The 
sense of smell is well developed, and an 
organ of taste is located in the pharynx. 

The sexes are separate, and the testes or 
ovaries form a network of tubes in the abdo- 
men. The sperms are ejected upon a special 
"sperm web," then picked up by the pedi- 
palps and transferred to the seminal recep- 
tacles of the female in mating. There is 
courtship activity before mating which varies 
with the species. Sometimes the female kills 
and eats the male after mating. The eggs 
are not fertilized until laid; the sperms move 
from the seminal receptacle to fertilize the 
eggs as they pass through the "uterus ex- 
ternus" on their way out of the body of the 
female. The eggs are laid in a silk cocoon, 



Jumping spicier 

House spider 

Figure 164. Some common spiders. 

Crab spider 

which is attached to the web or to a plant, or 
carried about by the female. The young leave 
the cocoon after hatching, as soon as they 

Figure 165. The web of an orb-weaving spider. 
(Photo by Spencer. Courtesy of Nature Magazine.) 

can run about. Several molts occur before 

The spinning organs of spiders are three 
pairs of appendages called spinnerets (Figs. 
162 and 163); they are pierced by hundreds 
of microscopic tubes through which a fluid, 
secreted by a number of abdominal silk 
glands, passes to the outside and hardens in 
the air, forming a thread. These threads are 
used to build snares, spin webs, and to 
form cocoons. Some spiders, while hunting, 
play out a dragline. Spiderlings disperse 
themselves by spinning a long thread on 
which they are carried away by the wind. 

Many spiders possess an accessory spin- 
ning organ, the cribellum (Fig. 163) in ad- 
dition to the spinnerets. A special kind of 
silk is emitted from this organ and is combed 
out by a row of bristles on the fourth 
metatarsus. The silk threads of spiders are 
stronger than steel threads of the same size. 
They are no longer used as cross hairs in 
the eye pieces of optical instruments; mod- 
ern technology has produced better mater- 
ials for the purpose. 

An orb web, such as is shown in Fig. 165, 
is spun in the following manner. A thread 
is stretched across the space selected for the 



web; then from a point on this thread, other 
threads are drawn out and attached in 
radiating hnes. These threads all become dry 
and smooth. On this foundation, a spiral 
of sticky thread is spun. The spider stands 
in the center of the web or retires to a nest 
at one side and waits for an insect to be- 
come entangled in the sticky thread; it then 
rushes out and spins threads about its prey 
until its struggles cease. 

Many spiders do not spin webs, but 
wander about capturing insects, or lie in 
wait for them in some place of concealment. 

Some spiders of 
special interest 

Crab spiders (Fig. 164), although not all 
are shaped like crabs, have the habit of walk- 
ing sideways. Some are white or yellow in 
color, and are said to favor flowers of simi- 
lar color. Jumping spiders (Fig. 164) have 
stout front legs for capturing prey; they are 
famous for their peculiar antics during mat- 
ing. Tarantulas (Fig. 169) are the giants of 
the spider world, reaching a body length of 
IVi inches and a leg spread of 9 or 10 inches. 
Spiders of this size are able to capture small 
birds. Tarantulas, under laboratory condi- 
tions, are known to have lived over 20 years. 
The trap-door spider digs a tunnel in the 
ground about 6 inches deep and closes it 
with a hinged door, not for the purpose of 
trapping other animals, but to protect itself 
from intruders. Wolf spiders do not wait 
for prey to come to them, but go hunting for 
their victims. They care for their young by 
transporting them on their backs. 

More than 30,000 species of spiders are 
known, of which about 3000 species live in 
the United States. Only the black widow 
and the tarantulas (Fig. 169) are harmful to 
man in this country; and contrary to popular 
belief, except for the tropical ones, the 
tarantulas are not particularly dangerous. 
The brightly colored garden spider is one of 
the more beautiful species. House spiders 
are considered a nuisance but are not dan- 


King or horseshoe crabs 

Some of the strangest animals on earth 
are these peculiar "living fossils" (Fig. 166), 
whose close relatives died millions of years 
ago. The common name horseshoe crab re- 
fers to the heavy horseshoe-shaped carapace. 
Actually, it is not a crab. The horseshoe 
crab occurs along the Atlantic Coast from 
Maine to Yucatan. It lives in shallow water 
along the shores. Here it shoves its way 
through sand and mud, where it hunts for 
worms, bivalves, and other small animals on 
which it feeds. 

Respiration is carried on with the aid of 
book gills. The respiratory pigment of the 
blood is hemocyanin which contains cop- 
per, although this metal is recoverable only 
in about a hundredth part in a million parts 
of sea water. Oxygen combines with the 
copper pigment to give the blood a blue 
color when it is exposed to the air. 


Scorpions (Fig. 166) are rapacious arach- 
nids measuring from Vi to 8 inches in 
length. They live in tropical and subtropical 
regions, hiding during the daytime, but 
running about actively at night. They cap- 
ture insects, spiders, and other small ani- 
mals. Larger animals are paralyzed by the 
sting on the end of the tail. This sting does 
not serve as a weapon of defense unless the 
scorpion is hard pressed; and it is not used 
to sting itself to death, as is often stated, 
since its poison has no effect upon its own 
body. The vital statistics of the Arizona 
State Department of Health show that dur- 
ing a twenty-year period, there were 64 
deaths from scorpion sting. The state of 
Durango, Mexico, reports more than 1700 
deaths from scorpions over a period of 41 
years. These records indicate that some 
scorpions are killers and should be treated 
with respect. 

The mating activities of scorpions are very 









Book gills 



(Horseshoe crab) 

ventral view 

dorsal view 

Figure 166. Horseshoe crab, Limulus, (formerly Xiphosura); ventral view. Natural size about 
15 inches long. It is called a living fossil because it has undergone little change during long geologic 
periods. Scorpion, dorsal view. The poison gland is contained in the telson. 

curious and include a sort of dance. Scor- 
pions are viviparous. The young ride about 
upon the back of the female for about a 
week and then shift for themselves. They 
reach maturity in about 5 years. 

Scorpions are one of the oldest forms of 
life still found on this earth. Fossilized 
specimens from many parts of the world 
show that they have remained essentially 
unchanged for hundreds of millions of years. 
Fossil scorpions resemble present-day forms 
(Fig. 166) in possessing a pair of chelicerae, 
a pair of pedipalps, four pairs of legs, and a 
poison gland. Thus it can be seen why mod- 
ern scorpions may well be described as "liv- 
ing fossils." 

Mites and ticks 

Mites and ticks (Fig. 167) are found al- 
most everywhere. Some live in fresh water, 
others in salt water; some live on the ground, 
others on vegetation; some live on the out- 
side of the bodies of other animals, and 
others burrow into them. Animals and 
plants, either living or dead, serve as food 
for them; the parasitic species live largely 
on blood. The common names of certain 
families (p. 279) give some idea of the dif- 
ferent types of ticks and mites. Some of 
those important to man are described be- 



Chigger mite 

Spotted fever tick 

Chicken mite 

Vector of relapsing fever 

Figure 167. Some parasitic ticks and mites. 

Human itch mite 

Texas cattle tick 


gorged with blood 


Most of the Arachnoidea are not only 
harmless, but they feed largely on injurious 
insects and are to a certain degree beneficial. 
A few species damage food plants; others 
attack man and domestic animals directly; 
and a few transmit disease germs. King 
crabs are trapped in great numbers and 
ground up for fertilizer. 

The red spider, Tetranychus, is a green- 
house pest that is destructive to many 
species of plants; it may become a pest also 
in cotton fields. The clover mite injures 
clover and fruit trees, and the pear-leaf 
blister mite damages pears and apples. 

Scorpions are noted for their poisonous 
sting; but they are mostly confined to the 
tropics and some are not very poisonous. 
The black widow spider, Latrodectus (Fig. 
168), appears to be the only spider in the 
United States whose bite needs to be feared. 
The adult can be recognized by its glossy 
black color, globose abdomen, and the red- 
dish spot on the under surface of the ab- 
domen usually shaped like an hour glass, 
but sometimes like a triangle or two tri- 

Although not dangerous, the chigger mites 
or red bugs (Fig. 167) produce a very dis- 
tressing itch which may continue for several 
days; an immunity may be acquired to their 
invasions of the skin. One of the itch mites, 



Ventral view of 
female showing 

Figure 168. Black widow spiders in their snare. The female may kill and eat the male after 

mating, but this is not always true. 

Sarcoptes (Fig. 167), causes the so-called 7- 
year itch in man, as well as mange in dogs, 
cats, and many other kinds of animals. 
Domestic animals are infested by a num- 

ber of species of mites and ticks. The 
chicken mite (Fig. 167) is a serious pest of 
poultry. Scab mites attack horses, dogs, etc. 
The sheep scab mite may seriously injure 

>. Tli*^'!- ■i'M' V' ii ii" iii i i m»'r i Vit^ 'i ii' * i « f>' a ^' i'^' • — -''--^ 

Figure 169. Tarantula, the largest of the spiders. The tarantula does not spin a web, but 
stalks its prey like a lion. It is found in our southwestern and western states. Although feared 
by many people, experiments on species in this country have shown their "bites" to be no 
more harmful than a bee sting. However, it is true that some South American tarantulas are 
more injurious. (Courtesy of N.Y. Zoological Society.) 



sheep; it causes intense irritation, loss of 
wool, and decreased vitality. Unless these 
animals are properly treated, they may die. 
Several important vectors of disease oc- 
cur among the ticks. Rocky Mountain spot- 
ted fever of man is due to Dermacentor, 
which transmits germs (rickettsial organ- 
isms) from rodents and larger mammals to 
man; the female ticks pass the disease germs 
to their offspring through the egg. One 
symptom of this disease is the appearance of 
spots on the wrists, ankles, trunk, and face. 
An eastern variety of spotted fever occurs 
in country districts from New York to Flor- 
ida; cases have also been found in the Mid- 
dle West. A vaccine has been developed 
that confers immunity for about one year. 
The Rocky Mountain wood tick is also re- 
sponsible for tick paralysis, which appears to 
be due to a poisonous salivary secretion 
(toxin) injected by the tick, and which may 
prove fatal to children. Another disease 
known to be transmitted by ticks as well as 
by certain insects (Fig. 160) is rabbit fever, 
or tularemia. Relapsing fever, characterized 
by alternating periods of fever and normal 
temperatures, occurs in various parts of the 
world, including Texas, Kansas, Montana, 
Utah, and California. Apparently a different 
species of tick of the genus Ornithodoros 
transmits the disease germs (spirochetes) in 
each locality. Texas fever in cattle is trans- 
mitted from diseased animals to healthy 
ones by the cattle tick (Boophilus) (Fig. 
167) which passes the infective agent (spor- 
ozoan) to her offspring through the egg. 
Among the continents of the world, Africa 
b especially burdened with more than its 
share of different kinds of ticks. 


(For reference purposes only) 

Class Arachnoidea. No antennae; no true 
mandibles; body usually of 2 divisions, cephalo- 
thorax and abdomen; 6 pairs of appendages on 
cephalothorax; first pair of appendages are 

nippers, termed chelicerae; 1 pair of pedipalpi 
variously modified; and 4 pairs of legs. 

Order 1. Xiphosura (Or. xiphos, sword; 
oura, tail). King or horseshoe 
crabs (Fig. 166). Crablike; 
cephalothorax horseshoe-shaped; 
tail or telson long and spikelike. 
Ex. Limulus polyphemus (for- 
merly Xiphosura). 
Subclass 1. Arachnida (Gr. arachne, spider) . 
Spiders, scorpions, mites, etc. 
No antennae; no true mandi- 
bles; first pair of appendages 
are chelicerae; cephalothorax 
and abdomen usually evident. 

Order 1. Scorpionida (L. scorpio, scor- 
pion). Scorpions (Fig. 166). 
Elongated; long abdomen of 1 3 
segments; sting at end of 
tail. Ex. Centruroides * gertsch, 

Order 2. Pedipalpi (L. pes, foot; palpo, 
touch gently). Whip scorpions, 
etc. (Fig. 161). Pedipalps thick 
and strong; first pair walking 
legs with many-jointed tactile 
flagellum. Ex. Mastigoproctus 
giganteus, the garoon. 

Order 3. Araneae. Spiders (Fig. 164). 
Cephalothorax distinct; abdo- 
men usually unsegmcnted; 
cephalothorax and abdomen 
joined by a narrow waist; cheli- 
cerae small, a poison duct in 
the terminal fang; book lungs, 
some with tracheae; spinnerets 
on abdomen; chiefly terrestrial. 
Some of the common families 
are as follows: 

1. Theraphosidae. Tarantulas. Large, hair)', 
hunting spiders, with 2 pairs of lungs, con- 
fined to the warmer regions of the globe, 
and hiding in holes in the ground or in 
crevices on tree trunks, etc. Sometimes 
brought to northern cities on bunches of 
bananas. Ex. Dugesiella hentzi of our 
south and southwest. 

2. Ctenizidae. Trap-door spiders. Moderately 
large spiders with 2 pairs of lungs and 
with stout strong legs. They live in under- 

* Formerly called Centrums. 



ground tunnels lined with silk and fitted 
with trap doors. Ex. Pachylomerides au- 
douini of our south, and Bothriocyrtum 
californicum of the west coast. 

3. Dictynidae. Hackled-band weavers. These 
spiders possess an accessory spinning organ, 
the cribellum (Fig. 163). The special silk 
emitted from this organ is combed out 
by the calamistrum of the fourth meta- 
tarsus. The irregular webs are constructed 
among plants or under debris. Ex. Dictyna 
muraria of wide distribution. 

4. Pholcidae. Long-legged spiders. Most of 
these are pale, small-bodied, weak-jawed, 
long-legged spiders, which shun the light. 
Their irregular webs are built in base- 
ments and similar situations. Ex. Pholcus 
phalangioides of wide distribution. 

5. Theridiidae. Comb-footed spiders. A very 
large family of spiders, most of whose 
members are small in size. The fourth 
tarsus is provided with a row of serrated 
bristles forming a comb, used for flinging 
a very viscid silk which swathes the prey 
captured in their irregular webs. Ex. Theri- 
dion tepidariorum (Fig. 164), the cosmo- 
politan and common house spider which 
builds cobwebs in the corners of rooms. 
Latrodectus mactans (Fig. 168), the black 
widow, which is found in most of North 
and South America, but commonest in 
the warmer regions. 

6. Linyphiidae. Sheet-web weavers. Small 
spiders which build webs usually provided 
with a more or less horizontal or curved 
sheet from the under surface of which they 
suspend themselves upside down. Many 
build among plants, but large numbers are 
found among the dead leaves and other 
litter close to the forest ground. They are 
more abundant in temperate and arctic 
regions than in subtropical and warmer 
zones. Ex. Linyphia marginata, the filmy 
dome spider. 

7. Araneidae. Orb weavers. A very large family 
of spiders, the members of which, if they 
build a web at all, construct a cartwheel- 
like structure known to all who have 
walked through the woods in late summer. 
While the foundation and radii lines are 
not sticky, the spiral thread is quite viscid 
and serves to snare the prey. The spiders 

have strong jaws and are usually beautifully 
marked. Ex. Argiope aurantia (Fig. 161), 
the black and orange garden spider, which 
builds its web in grass and among bushes 
exposed to sunlight, and in the hub of 
which it stands head down. 

8. Agelenidae. Funnel-web weavers. These 
spiders build a more or less horizontal 
sheet web, over which they run in an up- 
right position, and at one side of which 
they hide in a cone-shaped retreat. The 
threads are not sticky, and the spider does 
not swathe the prey, but depends upon 
lightning speed to run out and seize in its 
jaws any insect that happens to touch the 
threads. Ex. Agelenopsis naevia, which 
builds its web most commonly in grass, 
but also among shrubbery and stone 

9. Pisauridae. Nursery-web weavers. Most of 
these are large spiders which build no 
snares, but hunt their prey along water 
courses, in grassy areas, and forests. The 
large spherical egg sac is carried about by 
the mother under her sternum. Shortly be- 
fore the emergence of the progeny, most 
species fasten the sac among some leaves 
and construct a nursery web around it, 
standing guard until the spiderlings 
emerge. Ex. Pisaurina mira. 

10. Lycosidae. Wolf spiders. These are me- 
dium to very large spiders, with keen eye- 
sight, strong legs, and powerful jaws. 
They run about actively, both day and 
night, hunting their prey. Some hide in 
natural cavities in the ground, others con- 
struct vertical burrows somewhat like those 
of the trap-door spiders. The males of 
many species go through elaborate court- 
ship dances before the females. Ex. Lycosa 
carolinensis, our largest species. 

11. Gnaphosidae. Ground spiders. The ma- 
jority of these are medium-sized and run 
very rapidly over the ground, usually hid- 
ing under stones and logs. They build no 
snares, but may construct retreats in 
which they hide and in which the egg 
sac is fastened. 

12. Thomisidae. Crab spiders. These spiders 
walk sidewise as readily as forward or back- 
ward, and many have short stocky bodies. 
While most are dark-colored and live on 



bark, or among dead leaves on the ground, 
a few are conspicuously light-colored and 
frequent flowers. To some extent they can 
change from yellow to white, or vice versa, 
according to the color of the flower on 
which they wait to seize insects with their 
powerful front legs. Ex. Misumena vatia 
(Fig. 164). 

13. Salticidae. Jumping spiders. These sun- 
loving spiders are more abundant in the 
tropics than in temperate regions. They 
have the largest eyes, the keenest vision, 
and are among the most beautifully 
adorned of all spiders. They build no snares, 
but stalk and pounce upon their prey, 
hunting only in daylight. There is much 
sexual dimorphism, and males perform 
elaborate courtship dances before the 
much duller-hued females. Ex. Salticus 
scenicus (Fig. 164), common on fences 
and the outside of buildings. 

Order 4. Palpigrada ( L. palpo, touch gen- 
tly; gradior, walk), Palpigrada 
(Fig. 161). Minute; abdomen 
segmented; long caudal filament 
with bristles; 1 family. Ex. 
Koenenia wheeleri, Texas. 

Order 5. Pseudoscorpionida {Gx.pseudes, 
false). Pseudoscorpions (Fig. 
161). Small; flattened pedipalps 
scorpionlike. Ex. Chelifer can- 
croides, house scorpion. 

Order 6. Solpugida (L. solifuga, sun-flee- 
ing). Solpugids (Fig. 161). 
Head and thorax distinct; large 
chelate chelicerae; respiration 
by tracheae. Ex. Eremobates 
pallipes, southern states west of 

Order 7. Phalangida (Gr. phalangion, 
long-legged spider). Harvest- 
men or daddy longlegs (Fig. 
161). Body short and ovoid; 
cephalothorax unsegmented; ab- 
domen segmented; pedipalps 
long and leglike; legs usually 
very long and slender. Ex. 
Liobuniim vittatum. 

Order 8. Acarina (Gr. akares, mite). 
Ticks and mites (Fig. 167). 
Small; body short and thick; 

cephalothorax and abdomen 
fused; abdomen unsegmented; 
larva with 3 pairs of legs, adult 
with 4 pairs; free-living or para- 
sitic; world-wide. Some of the 
principal families are as fol- 

1. Argasidae. Soft ticks. Ex, Ornithodoros mou- 
bata: vector of relapsing fever in man. 

2. Ixodidae. Hard ticks. Ex, Boophilus annula- 
tus: vector of Texas fever in cattle. 

3. Eriophyidae. Gall mites. Ex. Phyllocoptes 
pyri: Pear-leaf blister mite. 

4. Dcmodecidae. Skin mites. Ex. Demodex fol- 
liculoruTn: human face mite. 

5. Dermanyssidae. Chicken mites and others. 
Ex. Dermanyssus gallinae: chicken mite. 

6. Sarcoptidae. Itch mites. Ex. Sarcoptes 
scabiei: human 7-year itch mite. 

7. Trombidiidae. Harvest mites and chiggers. 
Ex. Eutrombicula alfreddugesi: North Amer- 
ican chigger mite or red bug. 

8. Tetrarhynchidae. Red spiders. Ex. Tetrarhy- 
nchus telarius: red spider. 

9. Hydrachnidae. Fresh-water mites. Ex. Hy- 
drachna geographical parasitic on aquatic 

Subclass 2, Pycnogonida (Gr. pyknos, 
thick; gony, joint). Sea spiders 
(Fig, 161 ) , Marine; body small; 
legs very long; abdomen rudi- 
mentary, Ex. Pycnogonum lit- 
torale: sea spider. 

Subclass 3. Tardigrada (L. tardus, slow; 
gradior, walk). Water bears 
(Fig. 161). Microscopic; usu- 
ally aquatic; body cylindrical, 
unsegmented; 4 pairs of clawed 
legs; respiratory, excretory, and 
circulatory organs absent. Ex. 
Macrobiotus hufelandi: fresh- 
water water bear. 

Subclass 4. Pentastomida (Gr, pente, five; 
stoma, mouth). Wormlike 
arachnids (Fig. 161). Parasitic; 
body long and ringed but not 
segmented; respiratory, excre- 
tory, and circulatory organs ab- 
sent. Ex. Linguatula serrata: a 
parasite of mammals. 



Baker, E.W., and Wharton, G.W. Introduc- 
tion to Acawlogy. Macmillan, New York, 

Comstock, J.H., and Gertsch, W.J. The Spider 

Book. Doubleday, New York, 1940. 
Emans, E.V. About Spiders. Button, New 

York, 1940. 


Fabre, J.H. The Life of the Spider. Dodd 

Mead, New York, 1919. 
Gertsch, W.J. American Spiders. Van Nos- 

trand' New York, 1949. 
Kaston, B.J., and Kaston, E. How to Know 

the Spiders. W. C. Brown, Dubuque, Iowa, 

Savory, T.H. The Biology of Spiders. Sidgwick 
and Jackson, London, 1928. 


■^.rt •■••- r^'.v.'^^^. ^'oi^ 


Phylum Mollusca. 

Snails, Squids, 


and Others 

HE soft-bodied animals, which comprise 
the phylum Mollusca, include the snails, 
slugs, clams, mussels, oysters, octopuses, and 
squids (Fig. 170). Most of them are bilater- 
ally symmetrical; and, with the exception of 
one class, are unsegmented. Many possess 
shells of calcium carbonate. At first sight, 
mussels, clams, snails, and squids do not 
appear to have much in common, but closer 
examination reveals several structures which 
are possessed by all. One of these is an organ 
called the foot, which in the snail is usually 
used for creeping over surfaces; in the clam, 
generally, for plowing through the mud; and 
in the squid for seizing prey. In each there 
is a space called the mantle cavity between 
the main body and an enclosing envelope, 
the mantle. The anus opens into the mantle 

Mollusks are among the most abundant of 
all animals and may be found on land, in 
fresh water, and in the sea. Many of the 
90,000 species of this phylum are of eco- 
nomic importance. They serve as food for 
man— oysters, clams, and scallops; they pro- 
vide material for making pearl buttons; and 
they produce pearls. Some species are in- 
jurious. Although the various types of mol- 
lusks differ widely in appearance and struc- 
ture, they can be reduced to a single plan 
(Fig. 171). The varied activities of the dif- 
ferent types indicate how well they are 
adapted to their environments both mor- 
phologically and physiologically. The larvae 
of all mollusks, except the cephalopods, are 
particularly interesting because they pass 
through trochophorc and veliger stages, 
which suggests that they are related to the 
annelids and certain other phyla. 

The mollusks are divided into 6 classes, 
according to symmetr}' and the character of 
the foot, shell, mantle, gills, muscles, radula, 
and nervous system. The mollusks follow 
the arthropods in this discussion, not be- 
cause they are more complex or because they 
belong in this position, but for convenience, 
lliey could just as well come before the 


(chambered nautilus) 

Hypothetical ancestor 









Mouth 1 





Figure 171. Modifications in the molluscan body plan as illustrated by representatives of the 
5 classes. Note how the shell (heavy lines), the foot (stippled), and the digestive tract vary in 
position in the different groups. (After Animals Without Backbones, by Ralph Buchsbaum. 
Second edition. Copyright 1948 by University of Chicago Press.) 


Clams usually lie partly buried in the 
muddy or sandy bottoms of lakes or streams. 
They burrow and move from place to place 
by means of the foot (Fig. 172), which can 
be extended from the anterior end of the 
shell. Water, loaded with oxygen and food 
material, is drawn in through a slitlike 
opening at the posterior end, called the 
ventral or incurrent siphon; and excretory 
substances and feces, along with deoxy- 
genated water, are carried out through a 
smaller dorsal or excurrent siphon. 


The shell consists of two parts called 
valves. Concentric ridges called lines of 
growth appear on the outside of each valve; 
these represent the intervals of rest between 
successive periods of growth, the annual 
lines being more conspicuous. The umbo is 

Figure 170. Facing page, representative mollusks. 
The lines suggest possible relationships. The fig- 
ures are not drawn to scale. (Based on a diagram 
by William J. Clench and Ruth D. Turner, Mu- 
seum of Comparative Zoology, Harvard University. 
Made expressly for this book.) 

the first part of the shell to develop and is 
produced in the late veliger stage; it is 
usually corroded by the carbonic and humic 
acids in water. 

The outer epithelium of the mantle se- 
cretes the shell, which consists of three layers 
(Fig. 173): (1) an outer, thin, horny layer, 
the periostracum which serves to protect the 
underlying layers from the acids in the 
water and gives the exterior of the shell 
most of its color; (2) a middle portion of 
crystals of lime (calcium carbonate) called 
the prismatic layer; and (3) an inner 
nacreous layer (mother-of-pearl), which is 
made up of many horizontal layers of cal- 
cium carbonate, and produces an iridescent 

Anatomy and physiology 

The valves of the shell are held together 
by two large transverse muscles called an- 
terior and posterior adductors, and a dorsal, 
ligamentous, elastic tissue hinge (Fig. 172). 
The two folds of the dorsal wall of the clam 
which line the valves are called the mantle. 
The space between the mantle flaps, con- 
taining the two pairs of gill plates, the foot, 
and the visceral mass is called the mantle 

•Anterior adductor 
Anterior retractor 

- Posterior retractor 
Posterior adductor 

Incurrent siphon- 
Excurrent siphon 

Lines of 


Mantle line 


Figure 172. Freshwater clam. A, inside view of a valve showing point of attachment of 
mantle and points of attachment of muscles. B, external view of a livnig animal with foot 
protruding from the shell and arrows showing direction of water flow through the siphons. 

Periostracum Prismatic layer Nacreous layer 

(mother of pearl) 


Mantle < 

Ciliated outer 
c epithelium 

Foreign body (sand or 
a parasite) between 
shell and mantle 

Pearl formed by 
secretion of nacre 
around foreign body 

Figure 173. Enlarged cross section of the shell and mantle of a fresh-water clam. Note the 
pearl being formed between the shell and mantle by the nacre-secreting cells. 





Food is brought into the mantle cavity 
(Fig. 174) of the clam by the water circu- 
lating inward through the ventral incurrcnt 
siphon. Water containing suspended minute 
plants, animals, and debris passes over the 
gills, and the small suspended food particles 
adhere to the mucus which covers the gills. 
The smaller particles now caught in the 
mucus are carried by the beating cilia of the 
gills to the ventral edge, where this mass of 
material is transferred to the labial palps. 
These flaps of tissue which surround the 
mouth serve as a sorting mechanism that 
selects the materials to be utilized. These 
food materials are then carried into a deep 
groove between the labial palps and thence 

directly into the mouth. The food passes 
from the mouth through the short esopha- 
gus into the bulbous stomach, which is con- 
nected by ducts with a large digestive gland 
("liver"). This gland surrounds the stomach 
and is the chief source of digestive enzymes. 
The intestine is given off from the ventral 
side of the stomach, descending into the 
visceral mass, where it makes a loop. Then 
it ascends parallel to its first portion and 
turns sharply backward and out of the 
visceral mass, through the pericardium and 
through the heart itself, where it becomes 
the rectum. It finally ends at the anus which 
opens near the excurrent siphon, and the 
feces are carried away in the outgoing cur- 
rent of water. 

Renopericardial pore 

Pericardial cavity 
Anterior aorta 
Digestive gland 

Anterior adductor 

Kidney pore 



Posterior aorta 

Posterior retractor muscle 

Posterior adductor 





Cerebropleural ganglion 
Pedal ganglion 
Cerebrovisceral connective 

Visceral ganglion 
Intestine Incurrent siphon 


Figure 174. The internal anatomy of a freshwater dam in the right valve as viewed from 
the left side. 

The details of digestion in fresh-water 
clams are not very well known. However, as 
is true in most animals that eat fine particles 
of food, digestion appears to be partly 

Amoeboid cells are present throughout 
the digestive tract. From microscopic ex- 
aminations, it is believed that such cells pass 
through the wall of the tract, engulf food 
particles, and digest it; then leave the in- 



testine and return to the tissue spaces. The 
large digestive gland produces a digestive 
secretion containing the enzyme amylase; 
this secretion passes through ducts that 
empty into the stomach. 

In one family (Unionidae) of the fresh- 
water clams, there is a carbohydrate-digest- 
ing enzyme set free in the stomach by dis- 
solution of a gelatinous rod (crystalline 
style) which lies in a pouch off the intes- 
tine and projects into the stomach. 


The circulatory system consists of a dorsal 
heart, blood vessels, and spaces called 

sinuses. The heart lies in the pericardium 

(Fig. 175). The ventricle drives the blood 
forward through the anterior aorta and back- 
ward through the posterior aorta. Part of 
the blood passes into the mantle, where it is 
oxygenated, and then returns directly to the 
heart. The rest of the blood circulates 
through numerous spaces in the body and is 
finally collected by a vein, which lies just 
beneath the pericardium. From here the 
blood passes into the kidneys, then into the 
gills, and finally through the auricles into 
the ventricle. Nutriment and oxygen are 
carried by the blood to all parts of the body, 
and carbon dioxide is disposed of in the 

Hinge ligament- 

Pericardial cavity- 




^-^'-°'^ °' cn^^^Kyw 



mB I^M I \i foi fi 







Water tube of gill 

Mantle cavity 



Figure 175. Cross-section of a fresh-water clam through the region of the heart. 

mantle and gills; other waste products of 
metabolism are transported to the kidneys. 

Respiration and excretion 

Although the entire body surface is in 
contact with water and doubtless functions 

in respiration, the greater part of the oxy- 
gen-carbon dioxide exchange occurs in the 
gills and mantle. A pair of gills hang down 
into the mantle cavity on either side of the 
foot (Fig. 175). Each gill consists of two 
plates or lamellae made up of a large number 



of vertical gill filaments (Fig. 176), strength- 
ened by chitinous rods and connected to 
one another by horizontal bars. Cross or 
interlamellar partitions between the two 
lamellae divide the gill into many vertical 
water tubes. Dorsally, the water tubes of 
each gill join a common suprabranchial 
chamber. The gills function in respiration by 
water circulating through their interiors. It 

enters through the incurrent siphon and 
flows over the gills. On the surfaces of the 
gills there are many microscopic openings, 
water pores, through which the water is 
driven by ciliary action into the water tubes. 
The water then passes dorsally in the water 
tubes to the suprabranchial chambers and 
along them to the excurrent siphon. Ex- 
change of respiratory gases takes place 

jprabranchial chamber 
■Excurrent siphon 

•Incurrent siphon 

Gill filament 

Interlamellar partition 

tjkT' Afferent vein 

— Efferent vein 

: — Water pore 

I ir - -r ^ 

Water pores 
Chitinous rod 

Water tube 

Figure 176. Respiratory system of a fresh-water clam. Left, a whole gill. Right, horizontal 
section through a gill showing arrangement of gill filaments, blood vessels, and water tubes. 
Arrow shows direction of water currents. 

through the walls of blood spaces located 
in the partitions of the water tubes. 

The two kidneys (nephridia) lie just be- 
neath the pericardial cavity. Each nephrid- 
ium is folded upon itself and differentiated 
into glandular (dark spongy mass) and 
bladderlike portions. The structure and re- 
lationships are such that one end of the 
nephridium opens into the pericardial cavity 
and the other end (external) opens into a 
suprabranchial chamber. Liquid wastes 

within the pericardial cavity may enter the 
tubule; or metabolic wastes carried by 
the circulating blood may be removed by 
the cells in the glandular portion of the 

Nervous system and sense organs 

Three pairs of ganglia are present (Fig. 
174): cerebropleural ganglia, pedal ganglia, 
and visceral ganglia. The sensor}' structures 
include light receptors in the siphon mar- 



gins; a small vesicle (statocyst) containing 
a calcareous concretion (statolith) lies a 
short way behind the pedal ganglia; it is an 
organ of equilibrium. A thick patch of yel- 
low epithelial cells (osphradium) covers 
each visceral ganglion. The osphradia are 
thought by most writers to be useful in de- 
tecting foreign materials in water. The 
edges of the mantle are provided with 

sensory cells, probably sensitive to contact 
and light. 


Clams are usually either male or female; 
a few are hermaphroditic. The reproductive 
organs are situated in the visceral mass (Fig. 
174). Some clams have interesting life 

(end view) 


Adductor muscle 

Glochidium ^ 

(side view) n ^rom exhalent siphon 
» ^ »' \J o . ^^^ 

••*.*♦ O o.o 

Glochidia discharged ■♦• yN 

n gills 

Glochidia become encysted 
in gills and fins of fish 

Adult female clam 

Fertilized eggs develop 

into glochidia in 

..,.^ modified outer gill 

(brood pouch) 

Young clam 

Figure 177. Life cycle of a fresh-water clam. The larval clam or glochidium passes out of 
the excurrent siphon of a female, attaches itself to the gills of a fish; finally, the glochidium 
drops from the fish host to take up a free-living existence as a young clam. 

histories in which the young stage is para- 
sitic on fish (Fig. 177). The eggs develop 
into a peculiar larva known as a glochidium, 
a modified veliger (Fig. 177). In Anodonta 
the eggs are usually fertilized in August, 
and the glochidia which develop from them 
remain in the gills of the mother all winter. 
In the following spring they are discharged; 
and, if they chance to come in contact with 
the external parts of a fish, this contact 
stimulus causes them to seize hold of it bv 
closing the valves of their shells. The skin 
of the fish grows around them, forming 
"worms" or "blackheads." After a parasitic 
life within the tissues of the fish, from three 
to many weeks, the young clam is liberated 
and takes up a free existence. As a result of 
this parasitic habit, clams are widely dis- 
persed by the migrations of the fish. 


General characteristics 
of moliusks 

The bodies of moliusks are soft and gen- 
erally covered by a moist integument. They 
are therefore fitted for life in water or in 
moist places. The mantle is a fold of the 
body wall which secretes the shell. If there 
are two lobes, a bivalve shell is produced as 
in the mussel. If only one lobe is present, 
a univalve shell is formed as in snails. A 
modified coelom is usually recognizable in 
the adult as the pericardial cavity and the 
cavities of the reproductive organs. 

Moliusks eat both vegetable and animal 
food. Jaws are present in most gastropods 
and all cephalopods. A rasping organ, the 



Radula Vy.- 

Figure 178. Radula of a gastropod. Left, diagram of longitudinal section through anterior end 
of a snail. Right, dorsal view of a few teeth from radula of a snail; the one on extreme left is 
a central tooth. The radula is pressed against food and moved rapidly back and forth, rasping 
off small particles. 

radula (Fig. 178), exists usually in the 
mouth cavity or pharynx of all mollusks, 
except the bivalves; it consists of rows of 
chitinous teeth which tear up the food by 
being drawn across it. Respiration takes 
place primarily in the gills and in the man- 
tle. Most fresh-water and land snails (pul- 
monale gastropods) take air into the vas- 
cularized mantle cavity, which thus serves 
the purpose of a lung; or, they breathe 

The sexes are usually separate, though 
certain groups are hermaphroditic. The 
number of eggs that develop in some mol- 
lusks is very great; for example, nearly 500 
million in the oyster in a single season. In 
all such cases, the eggs when laid are sub- 
jected to the dangers of the ocean currents 
and numerous enemies. They also pass 
through a metamorphosis after hatching. 
Other mollusks lay very few eggs, for exam- 
ple, Lymnaea, 20 to 100 and Mesodon, 40 
to 100. 

The development of the eggs of most 
mollusks includes a trochophore larval stage 
(Fig. 179), which later develops into a 
veliger larva, so called because of the pres- 
ence of a band of cilia, the velum, in front 
of the mouth. The velum is an organ of 
locomotion and is somewhat responsible 
for the dispersion of the species. However, 
the major factor in the dispersal of most 
marine bivalves is oceanic currents. Con- 

siderable importance is attached to the 
presence of a trochophore in the develop- 
mental history of certain mollusks, and 
many embryologists are inclined to consider 
this stage an indication of the ancestral 
condition. According to this view, the mol- 
lusks and annelids, which pass through a 
trochophore stage in their ontogeny, have 
been derived from a common ancestor. 

A newly found, primitive, 
deep-sea mollusk 

The Danish "Galatheae Expedition" 
dredging off the v^^est coast of Costa Rica 
(9°23'N., 89°32'W.) found among the 
many animals collected some extraordinar}^ 
unidentified deep-sea mollusks. The 10 liv- 
ing specimens were dredged up from a depth 
of about 11,778 feet, or a little over two 
miles. This new limpet-like mollusk was re- 
ported by H. Lemche in Februar}', 1957. 
Many zoologists regard the finding of Neopi- 
lina galatheae as a far more important dis- 
covery than that of the living coelacanth, 
found a few years ago off the coast of 

Neopilina is a living representative of the 
class Monoplacophora. The class name was 
originally proposed to cover a group of ex- 
tinct primitive paleozoic mollusks. Now it 
includes a living mollusk, which is radically 
different from all other mollusks in that it 




Trochophore larva 

Veliger larva 

Figure 179. Two stages in the development of a mollusk; both are free-swimming larvae. 
Both annelids and mollusks have a trochophore larval stage, and many zoologists interpret this 
to mean that both groups of animals have developed from a common ancestor (Fig. 107). 

is internally segmented and not quite bi- 
laterally symmetrical. The segmentation 
violates one of the general criteria by which 
mollusks are most readily known, that of an 
unsegmented body plan. Neopilina (no 
common name) is the generic name of this 
new mollusk. 

The new mollusk has a single shell, and 
several pairs of internal organs, such as 
auricles, excretory organs, and nephridia— 
all evidences of internal segmentation. 
There may still be other internal organs 
which are segmented, but further study of 
the internal anatomy will be necessary to 
determine this. There are 5 pairs of strong 
double muscles on the inner surface of the 
shell, and the animal has 5 pairs of gills 
(Fig. 180). It possesses a well-developed 

Neopilina apparently belongs to a very 
ancient stock. Its early fossil ancestors are 
already known, and further study should 
throw additional light on our knowledge of 
molluscan evolution. 


The coat-of-mail shells (Fig. 170) are pro- 
tected by a shell of 8 transverse plates, which 
are arched above and overlapping like shin- 
gles on a roof. These plates are not evidence 
of metamerism. When detached from the 
rocks to which they cling, chitons roll up 
like an armadillo, with the soft parts prac- 
tically covered by the hard shell. They live 
on rocky seashores mostly in water less than 
25 fathoms in depth and cling tightly to 
the rocks by means of the suction of the 


Gastropods (Figs. 181 and 182) live in 
fresh or salt water and on land. A few are 
parasitic on other animals. Land snails must 
protect themselves from drying at certain 
seasons; they retire into their shell as far as 
possible and secrete a parchmentlike wall 
(epiphragm) across the opening, which pre- 








Figure 180. Top, photograph of a segmented mollusk, Neopilina galatheae, a newly found 
primitive mollusk which represents a class that probably existed about 450,000,000 years ago; 
it is truly a "living fossil." The largest specimen collected measured about 37 mm. in length, 
33 mm. in width, and 14 mm. in height. (Photo courtesy of Henning Lemche, Zoological 
Museum, University of Copenhagen, who published the first report on Neopilina.) Bottom, 
drawing from a sketch by Henning Lemche of the most primitive mollusk. 

vents evaporation. Locomotion in snails is 
very interesting. A slime gland at the for- 
ward end of the foot deposits a film of 
mucus on which the snail moves by means 
of wavelike contractions of the foot muscles. 
It thus lays its own pavement ahead of 

itself, which is always the same, whether 
the path is rough or smooth, uphill or down- 
hill. Progress is therefore always about the 
same; it may be two inches per minute, 10 
feet per hour, and 240 feet per day, provided 
the animal keeps going continuously. 



Seminal receptacle 

Hermaphroditic duct 

Albumen gland 




Genital pore 

Cerebral ganglia 


Salivary gland 

Figure 181, Internal structure of a snail; dorsal view. 

Fresh-water snails are numerous in creeks 
and pools. Snail shells may coil in two direc- 
tions—clockwise or counterclockwise. These 
are distinguished by the terms dextral and 
sinistral. The type of coiling can be deter- 
mined by holding the shell with the aperture 
toward the observer: if the opening is on 
the right as in Busycon canaliculatum (Fig. 
182), then it is dextral, but if on the left, 
then it is sinistral. The difference is the 
same as that between a "right-handed" and 
a "left-handed" screw. Some fresh-water 
snails possess gills with which they breathe 
under water, others are pulmonates and 
have a lung cavity so that they must come 
to the surface from time to time for air 
when the water is warm, otherwise cutane- 
ous respiration may be adequate. Many 
gastropods— land, fresh-water, and marine — 
serve as intermediate hosts for blood and 
liver flukes. 

Land slugs (pulmonates) are closely re- 
lated to the land snails, but are completely 
naked. To keep from drying up, slugs must 
live in a moist place such as is afforded 
under boards and stones and in holes in the 
ground. At night they feed on vegetation. 
Some sea slugs (nudibranchs) live among 
seaweeds, which they may resemble so 
closely that it is practically impossible to see 
them, while others are quite conspicuous. 

Whelks and periwinkles are among the 
commonest of the smaller marine snails. 
The largest marine snail in America is the 
queen conch which lives in the Atlantic, 
being especially common along the shores 
of the Florida Keys and the West Indies. 
The shell may be a foot long and weigh 5 
pounds. Also called a conch is the channel 
shell which hermit crabs find very satisfac- 
tory as a place in which to live. The spiral 
characteristic of the snail shell is absent 



• Busycon 

Sea water 

(Nudibranch) ' 

Empty shell of' ' ' . .' 
Urosalpinx '" ' ■:.■ 
' • (Oyster drill) .•■.•..•.. •.;.•■::•.':;:'. V} 

(Sea slug, sea hare) 

•" • ■ •".'• Strombus- 
(Giant conch) 


Figure 182. Representative gastropods. 

from that of the limpet, and what is left is 
a rather high-arched disk. Limpets chng 
tenaciously to rocks. The sea butterflies or 
wing-footed mollusks (pteropods) spend 
their lives near the surface of the open sea. 
They live in vast schools, sometimes cover- 
ing the sea for many miles. Whales feed 
on them to such an extent that they are 
known as "whale food." 

Squids, octopuses, and 
other cephaiopods 

The cephaiopods are the most highly de- 
veloped class of mollusks. 

A common squid along the eastern coast 
of North America from Maine to South 
Carolina is known as Loligo pealii (Fig. 
183). The foot of this species consists of 

10 arms bearing suckers, and a funnel 
(siphon). The arms are used for capturing 
prey. The funnel is the principal steering and 
locomotor organ; if it is directed forward, 
the jets of water forced through it propel 
the animal backward; if directed backward, 
the animal is propelled forward. Thus the 
jet propulsion principle was old in nature 
before it was applied to the powering of 
airplanes. The mantle in the posterior re- 
gion is extended into triangular fins which 
may propel the squid slowly fonvard or back- 
ward by undulator}^ movements. 

The pen ( shell ) of this squid is a feather- 
shaped plate concealed beneath the skin of 
the back (anterior surface). The true head 
is the short region between the arms and 
the mantle collar; it contains two large eyes. 
The digestive system includes a muscular 

Ventral surface 




Grasping arm 

— Funnel 

-Branchial artery 
Bronchial heart 


Dorsal surface 
Figure 183. Internal structure of a squid as seen with body wall and arms removed from 
right side. 




pharynx (buccal mass), esophagus, salivary 
glands, stomach, cecum, intestine, rectum, 
liver, and a small "pancreas." There are two 
powerful horny jaws in the phar)'nx, and a 
radula is also present. Above the rectum is 
the ink sac, with a duct which opens near 
the anus; this is a protective adaptation. 
When the squid is attacked, it emits a cloud 
of inky fluid through the funnel to provide 
what had been thought to be a "smoke 
screen" for its escape. However, recent stud- 
ies show that the squid does not eject a large 
cloud cover of ink as has been supposed; 
it discharges just enough ink to color a 
volume of water its own size. An enemy in 
pursuit often mistakes the ink for the squid; 
meanwhile the squid escapes. 

The blood of the squid is contained in a 
double, closed, vascular system. Two gills 

and two nephridia are present. The nervous 
system consists of a number of ganglia, 
mostly in the head. The sensory organs are 
two very highly developed eyes, two stato- 
cysts, and probably two olfactory organs. 
The eyes are large, and, superficially, some- 
what similar to those of vertebrates (Figs. 
184 and 397). 

In squids the sexes are separate. 

Squids are especially famous for their color 
changes. Pigment cells filled with blue, 
purple, red, and yellow color are present in 
the skin; and when these become larger or 
smaller, the color changes rapidly as though 
the animal were blushing. These chameleon- 
like changes in color harmonize with the 
color of the background, resulting in partial 
concealment of the animal. 

Near the coast of Newfoundland, giant 


Ciliary muscle 

Optic ganglion 

Figure 184. Diagram of a section through the eye of a squid. This eye is constructed on the 
same principle as a camera, which consists of a dark chamber to which hght is admitted only 
through an opening (pupil) in the diaphragm (iris). Behind this opening is a lens which 
focuses the hght on the retina. It can form real images, as does the eye of the vertebrate, but 
differs from the vertebrate eye in that light is received directly on the visual part of the retina 
without having to traverse nerves and cell bodies before it strikes the light receptors as in the 
vertebrate eye. 

squids are occasionally encountered. These 
may be 50 feet or more in total length, with 
arms as large as a man's legs, and suckers as 
big as teacups. Probably certain sea serpent 
stories are founded on the sudden and unex- 
pected appearance of these monsters. The 
giant squid is the largest living invertebrate 
animal known. 

The cuttlefish has a short oval body bor- 
dered by fins that are usually united behind 
and possesses arms, two of which are much 
longer than the others. Its internal calcare- 
ous shell is the cuttlebone used as a bill 
sharpener for caged birds. Its ink has pro- 
vided the sepia pigment used by artists for 
hundreds of years. 






Gas chamber 

' — Siphuncle 
Chamber partition 

Figure 185. Chambered (pearly) nautilus with shell cut away to the midline to show the 
animal and the many gas filled chambers in which it lived at successive stages in its growth. 
The siphuncle is a limy tube that encloses a cord of living tissue which extends from the last 
chamber to the first one formed. It lives in the south Pacific Ocean. Natural size up to 10 
inches in diameter. 

The chambered nautilus (Fig. 185) was 
immortalized by Oliver Wendell Holmes in 
his great poem, "The Chambered Nautilus," 
in which he called it "the ship of pearl." It 
has a shell that is coiled like a watch spring 
and is divided by cross walls into a series of 
compartments. A new and larger chamber is 
built when the old compartment is out- 
grown, and a new wall is secreted behind it. 
The head bears 60 to 90 tentacles without 

Octopuses (devilfishes) (Fig. 186) live in 
dark crevices and in coral reefs. Most of 
them are not large enough to harm a hu- 
man being, but the giant octopus of the 
Pacific reaches a diameter of about 30 feet 

and can be dangerous. These are feared by 
pearl divers. 

Bivalve mollusks 

Bivalves (Fig. 172) may live in fresh 
water or in the sea. More than 500 species 
of fresh-water clams live in the United 
States and many species of oysters have 
also been described. Adult oysters are unable 
to move about, being attached by the left 
valve to some solid object. Oysters feed in 
much the way fresh-water clams do. A single 
oyster may deposit approximately a half 
billion eggs in one season; these develop in 





Figure 186. 1 he common octopus is a ccphalopod without a shell. It pulls itself over the 
rocks with its arms or it can move by expelling water from its funnel. The sucker-bearing arms 
are used to seize the animals on which it feeds. (Courtesy of D.P. Wilson, Plymouth, England.) 

the oyster's gills into little ciliated spheres, 
called spat. 

Pearls are sometimes found in our edible 
oysters, but these are not nacreous, and 
therefore of little value. The most valuable 
pearls come from pearl oysters, which are 
not closely related to the edible oyster. The 
pearl is the result of an injury to the mollusk 
caused by either an organism or a foreign 
particle which embeds itself in the fleshy 
part of the bivalve and causes an irritation. 
The irritation stimulates the animal to de- 
posit layer upon layer of nacre around the 
intruding body to form the pearl; the amount 
of deposition is in direct proportion to the 
degree of irritation. The Japanese produce 
artificial pearls by inserting a foreign body 
into the mantle of the bivalves which are 
kept in cages until pearls are produced. The 
mollusk requires from 3 to 4 years to form 
a pearl of considerable size and 7 years to 

form a large one. Because the inner layer 
of the shell is composed of the same nacre- 
ous substance as the pearl, it is called 


The Monoplacophora appear to be the 
most primitive class of mollusks and have 
changed the least from the ancestral condi- 
tion. The gastropods have changed to a 
short creeping type, with a spiral visceral 
hump revolved through an angle of 180°. 
The pelecypods separated from the rest of 
the phylum at an early date; they became 
flattened laterally and developed a large 
bilobed mantle that secretes a shell of two 
valves, a large mantle cavity containing gills, 
a burrowing foot in place of the creeping 




^i:::-//0^' Lifhophaga, ' ''• 
■X>\-^v;/^:Jv;^ Rock -bo ring mussel 


Figure 187. Some marine bivalves showing adaptations. Mytilus. the edible mussel, attached 
by byssal threads to a wooden pier. Mya, the mud clam, a burrowing form with a long siphon. 
Contrary to a general misconception, the "head" end of a clam is the end opposite the siphons. 
The walrus is said to feed almost entirely on this clam. Yoldia is capable of leaping through 
the water; note the united retractible siphons. Lithophaga, the rock-boring mussel, is said to 
secrete an acid to dissolve the rock into which it burrows. 

type, and no true head. The cephalopods 
have become free-swimming animals with 
the foot modified into prehensile tentacles 
and with the brain and sense organs highly 
developed. The dominant view of the rela- 
tion of the mollusks to other phyla is based 
on the presence of the trochophore lar\'ae 
among both mollusks and annelids, which 
indicates that these may have been derived 
from the same ancestral type. Some of the 
fossil relatives of Neopilina may well have 
been the connecting link between the mol- 
lusks and the segmented annelid worms and 


Most of the mollusks may be considered 
beneficial to man; some are not. Slugs are 

sometimes injurious in greenhouses and 
gardens; shipworms {Teredo, Fig. 188) bur- 
row into the bottom of wooden vessels, 
wharfs, and piles, weakening and destroying 
them. Some of the larger octopuses (Fig. 
186) have the reputation of killing human 
beings, but are probably not as black as 
they are painted. Octopuses are very good 
food; many people prefer them to oysters. 
They are clean animals; but, because of 
prejudices, Americans do not use them for 
food as much as do other peoples, with the 
exception of one species found along the 
southern coast of California, which is much 
sought after for food. 

The value of mollusks as scavengers is 
little appreciated. The fresh-water clams, 
for example, are continually ingesting or- 
ganic particles and thus purifying the water 
in which they live. Mollusks, however, are 



Figure 188. A, shipworms (Teredo) exposed in their burrows in a piece of wood that has 
been spht open. Teredo is a bivalve, but the two shells which are used for boring enclose only 
a very small part of the anterior end of the body. The shipworm feeds on wood particles and 
minute organisms. B, damage done to a wharf pile section, driven March, 1944, and removed 
August, 1945. Every year these mollusks do millions of dollars worth of damage to wooden 
wharf pilings and to ships. (B courtesy of William F. Clapp Laboratories.) 

most favorably known as food, especially the 
bivalves. Aborigines of our own shores used 
oysters in great quantities long before the 
white settlers came to America. Proof of 

Figure 189. A pearl in a fresh- water clam; arrow 
points to pearl. A fresh- water pearl has sold for as 
much as $10,000. Pearls are protective secretions, 
made of the same substance (nacre) that lines the 
bivalve shell. (Courtesy of F.L. Clark.) 

the popularity of the oyster with the Indian 
is found in the many piles of shells, that 
have been found around onetime camping 
grounds. Some of these piles contain tens of 
thousands of bushels of shells, which in 

recent years have been mined for use in 
road building and for manufacture of lime. 
Oysters exceed in value any other kind of 
marine animals used as food by man. About 
30 million bushels are gathered annually in 
the United States. Oyster culture is being 
carried on with success. The soft-shell or 
long-neck clam, Mya arenaria, and the hard- 
shell clam, Mercenaria mercenaria, are 
both widely used marine bivalves. The 
edible clam is used for the famous Cape 
Cod chowder. The edible mussel Mytilus 
edulis (see colored frontispiece at beginning 
of text) is eaten extensively in Europe but 
not very much in this country. Only the 
large adductor muscles of scallops (Pecten, 
Fig. 170) are eaten. In certain parts of 
Europe snails are considered a delicacy; and 
one type of gastropod, the abalone, is a 
common article of food on our western 
coast. Squids (Fig. 183), cuttlefishes, and 
octopuses are esteemed by palates in the 
south of Europe and in the Orient. In the 
United States, it is only because of prejudice 
that people make less use of these clean 
animals for food than do people of other 
countries. But squids are used by the ton 
for fish bait in America. 



The giant clam (Tridacna) of the tropics 
may reach a length of three feet and weigh 
500 pounds. The large shells are used as 
cradles for babies by some natives of the 
East Indies. 

Among the products of value derived from 
mollusks are pearls and peari buttons. Pearis 
are found especially in pead oysters {Pinc- 
tada) in Ceylon, India, Japan, and north 
Australia. They also occur in the common 
oyster and in clams, but these are never of 
high value. The shells of fresh-water mussels 
are used for the manufacture of pearl but- 
tons and many tons are collected annually 
in the United States, mainly from the Mis- 
sissippi and Ohio rivers and their tributaries. 
Overfishing of mussels for buttons has seri- 
ously depleted them. The U.S. Fish and 
Wildlife Service has conducted many experi- 
ments in an effort to improve mussel fishing. 


[For reference purposes only) 

Mollusks are unsegmented invertebrates (ex- 
cept the class Monoplacophora), and without 
jointed appendages. They usually possess a 
shell which is secreted by a mantle. A mus- 
eular foot of some sort is generally present. 
The classification of the more than 90,000 
species is based on the characteristics of the 
foot, mantle, shell, radula, and respiratory 
organs. Six classes, three subclasses, and eight 
orders are described here as follows: 

Class 1. Monoplacophora (bearing one flat 
shell). This name was originally pro- 
posed to cover a group of extinct 
paleozoic mollusks, but now contains 
a remarkable living form Neopilina 
"(Fig. 180). A small disklike foot, a 
single shell, but the body is divided 
into segments with pairs of muscle 
scars on the inner surface of the shell, 

* This classification is according to W.J. Clench 
and R.D. Turner, Museum of Comparative Zoology, 
Harvard University. 

and paired auricles, nephridia, and 
breathing organs. This is the only 
class of segmented mollusks. 
Order 1. Tryblidiacea. Shell is spoon- or 
cap-shaped. The animal has a 
head region and 5 well-devel- 
oped metamercs which ha\e 
paired auricles, nephridia, and 
comblike gills (ctenida). The 
extraordinary deep-sea form, 
Neopilina galatheae, first re- 
ported in 1957, is the only 
known, living, segmented mol- 
Class 2. Amphineura (Or. amphi, both; 
neuron, nerve). Chitons (Fig. 170). 
Marine; elongate body; head re- 
duced; shell of 8 plates or none; no 
tentacles; and bilateral symmetry 
Order 1. Polyplacophora. Chitons. Ellip- 
tical body; large flat foot; shell 
a middorsal row of 8 plates, 
surrounded by a fleshy girdle; 
6 to 80 pairs of gills in groove 
around foot. Sexes separate. Exs. 
Tonicella and Chiton. 

Order 2. Aplacophora. Wormlike forms; 
integument thick; lacks any evi- 
dence of shell, except tiny limy 
spicules in the mantle; and 
rudimentary foot. Ex. Neo- 
Class 3. Scaphopoda (Gr. skaphe, boat; pons, 
foot). Tooth shells or tusk shells 
(Fig. 170). Marine; body enclosed 
in tubular shell, open at both ends, 
no gills, delicate tentacles, and dioe- 
cious. Ex. Dentalium entale; from 
Cape Cod northward. 
Class 4. Pelecypoda (Gr. pelekys, hatchet; 
pous, foot ) . ( Lamellibranchiata ) . 
The pclecypods are familiar forms to 
most people because of their eco- 
nomic importance. Bivalve mollusks, 
clams, mussels, oysters, scallops, nut 
clams, cockles, shipworms, etc. (Figs. 
170, 174, 187, 188). Marine or in 
fresh water; mantle secretes shell, usu- 
ally two valves; head, eyes, tentacles; 
radula absent; foot usually wedge- 



shaped and adapted for ploughing; 
usually two gills on either side of 
the mantle cavity. Entirely aquatic. 
Order 1. Taxodonta. The mantle mar- 
gins are united ventrally and 
posteriorly with openings for 
siphon and foot; gills usually 
absent, and hinges reduced or 
absent. Shells with many simi- 
lar teeth on hinge margin; usu- 
ally two equal adductor muscles. 
Entirely marine. Ex. Yoldia 
limatula in deep water from 
Connecticut northward. 
Order 2. Anisomyaria. The mantle mar- 
gins are usually separate, ven- 
trally and posteriorly; and the 
siphons are lacking, or only 
slightly developed. Teeth var- 
ious; anterior adductor muscle 
small or none; posterior adduc- 
tor, a large powerful muscle. 
Ostrea, edible oysters; Mytilus, 
sea mussel; Pecten, frcquentlv 
called scallop; Pinctada (for- 
merly Margaritana) are marine 
forms, and sometimes called 
pearl oysters. 
Order 3. Eulamellibranchia. The mantle 
more or less connected ventrally 
and behind; siphons generally 
well developed. Hinge teeth 
usually small in number and 
unlike in form; adductor mus- 
cles equal, or anterior muscle 
smaller. Marine, brackish, and 
fresh water. Exs. Mercenaria 
(formerly Venus), quahog or 
hard-shell clam; Ensis, razor 
clams; Mya aenaria, sand clam; 
Teredo, shipworm; Pisidium, 
free-living in fresh water; and 
Unio, Anodonta, Lampsilis, 
fresh-water clams, shells used 
for buttons. 
Class 5. Gastropoda (Gr. gastro, belly; pons, 
foot). Snails, slugs, whelks, etc. 
(Figs. 170, 181, and 182). Foot flat 
for creeping, head distinct, with eyes 
and tentacles; shell if present of one 
piece (one valve); shell usually spiral 
but uncoiled, reduced, or absent in 

some; radula present in all but a few 
parasitic species; trochophore and 
usually vcliger larvae. 
Subclass 1. Prosobranchia. Mostly marine 
snails, but fresh-water and land 
forms are represented (Fig. 
170). Respiration usually by 
gills which are situated in the 
mantle cavity anterior to the 
heart. If gills arc absent then 
respiration may be by means of 
mantle, or pulmonary chamber. 
Exs. Strombus gigas, giant 
conch; Helicina orbiculata, a 
terrestrial southern species 
which is frequently arboreal; 
Fissurella, keyhole limpets, aba- 
lones, oyster drills; and^Crepid- 
ula, slipper or boat shells. 
Subclass 2. Opisthobranchia. Strictly ma- 
rine (Figs. 170 and 182). Small 
shell or none; gills, if present, 
are gituated posterior to the 
heart. Hermaphroditic. Exs. 
Clione, pteropods or sea butter- 
flies, the foot may be expanded 
into two fins used in swimming; 
the sea hare and nudibranchs 
(snails without shells). 
Subclass 3. Pulmonata (Fig. 170) (Gr. 
pulmones, lungs). Mostly fresh- 
water and land snails. No gills; 
mantle cavity serves as a pul- 
monary sac (lung); shell usu- 
ally present, sometimes rudi- 
mentary or absent; one or two 
pairs of tentacles; mostly ter- 
restrial, many in fresh water, a 
few marine; mostly vegetarian, 
a few carnivorous. Exs. Lym- 
naea stagnalis, fresh-water spe- 
cies; Helix, European garden 
snails, introduced into America; 
Anon, slugs with no shell; 
Limax, with rudimentary shell 
in mantle; and Testacella hali- 
otidea, a slug that lives in 
greenhouses and preys on earth- 
Class 6. Cephalopoda (Gr. kephale, head; 
pons, foot). Squids, cuttlefish, oc- 
topuses, and nautili (Fig. 170). All 



marine; head large; eyes large and 
often complex (Fig. 184); radula 
present; the foot is modified into 8 
or 10 prehensile arms in the octopuses 
and squids, but many tentacles in 
the nautili; muscular funnel (si- 
phon); shell external, internal, or 
none; and dioecious. 
Order 1. Tetrabranchia. Calcareous shell, 
closely coiled, tentacles numer- 
ous and without suckers; eyes 
without lens; no chromato- 
phores; no ink sac; two pairs 
of gills and two pairs of ne- 
phridia. Ex. Nautilus pompilius, 
the pearly or chambered nau- 
tilus (Fig. 185). 
Order 2. Dibranchia. Shell absent or re- 
duced, and internal, calcareous, 
or horny, not coiled; body cylin- 
drical or globose; one pair of 
gills, one pair of ncphridia; 
tentacles 8 to 10, with suckers 
or hooks; eyes with lens; chro- 
matophores and ink sac present. 
Ex. Loligo pealii, squid (Fig. 


Abbott, R.T., American Sea Shells. Van Nos- 
strand. New York, 1954. 

Clench, W.}., Turner, R.D., et al. Johnsonia 
Monographs of the Marine Mollusks of the 
Western Atlantic, 1941 to date. Department 
of Mollusks, M.C.Z., Harvard Univ., Cam- 

Coker, R.T., and Clark, A.F. "Natural History 
and Propagation of Fresh Water Mussels." 
Bull. U.S. Bur. Fisheries, 37:77-181,. 

Grave, B.H. "Natural History of the Ship- 
worm, Teredo navalis, at Woods Hole, 
Mass." Biol. Bull, 55:260-282, 1928. 

MacGinitie, G.E., and MacGinitie, N. Natural 
History of Marine Animals. McGraw-Hill, 
New York, 1949. 

Pilsbry, Henry A. "Land Mollusca of North 
America (North of Mexico)." Acad. Nat. 
Sci. Phila. Monograph 3, Vol. 1, Parts 1 and 
and 2, 1939 and 1940; Vol. 2, Parts 1 and 2, 
1946 and 1948. 

Pratt, H.S. A Manual of the Common Inverte- 
brate Animals. Blakiston, Philadelphia, 

Prosser, C.L., et al. Comparative Animal 
Physiology. Saunders, Philadelphia, 1950. 

Robson, G.C. A Monograph of the Recent 
Cephalopoda. British Museum, London, 

Shrock, R.R., and Twenhofel, W.H. Principles 
of Invertebrate Paleontology. McGraw-Hill, 
New York, 1953. 

Stiles, Karl A., and Stiles, Nettie R. "The 
Pearl, A Biological Gem." Bios, 14:3-16, 






Sea Urchins, 

Sea Cucumbers, 

Sea Lihes, and Others 

HE echinoderms, which are spiny-skinned, 
consist of the starfishes, brittle stars, sea 
urchins, sea cucumbers, and sea lihes (Fig. 
190). They are all marine animals and con- 
stitute a considerable proportion of the ani- 
mal life of the seashore. The starfish is the 
best-known type, but the sea urchin and sea 
cucumber are also quite well known to the 
seashore visitor. 

The echinoderms are of particular inter- 
est because: (1) they change from a bilater- 
ally symmetrical larva to a radially sym- 
metrical adult; (2) they have remarkable 
powers of autotomy (self-mutilation) and 
regeneration of lost parts; (3) the eggs are 
especially suitable for extensive experimenta- 
tion on artificial parthenogenesis; (4) they 
have a water-vascular system which includes 
organs known as tube feet, and (5) they 
have internal skeletons of calcareous plates. 
There are 5 classes of living echinoderms. 



The starfish does not resemble a fish, and 
a more appropriate name would be "sea- 
star" because of its habitat and shape; but 
the name "starfish" is the one by which this 
animal is best known. On the upper or 
aboral surface are many spines of various 
sizes, pedicellariae at the base of the spines, 
a madreporite which is the entrance to the 
water-vascular system, and the anal open- 
ing. On the oral surface are a mouth cen- 
trally situated in the membranous peristome 
and 5 grooves (ambulacral), 1 in each ray, 
from which 2 or 4 rows of tube feet extend. 

The skeleton is made up of calcareous 
plates or ossicles (Fig. 193), bound together 
by muscle and connective tissue fibers. The 
spines are short and blunt and covered by 
epidermis. Around their bases are many 
whitish modified spines called pedicellariae. 
These are little jaws; they look like tiny 
scissor-blades mounted on a stalk, which, 




Figure 190. Representatives of the living classes of echinoderms. The figures are not drawn 
to scale. 

when irritated, may be opened and closed 
by several sets of muscles. Their function is 
to protect the dermal branchiae, to prevent 
debris and small organisms from collecting 
on the surface, and to capture food. The 
rays may be flc}4,ed slowly by a few muscle 
fibers in the body wall. The tube feet are 
also supplied with muscle fibers. 

Water-vascular system 

The water-vascular system (Fig. 191) is a 
division of the coelom, peculiar to echino- 

derms. Beginning with the madreporite, 
the following structures are encountered: 
the stone canal running downward enters 
the ring canal, which encircles the mouth; 
from this canal 5 radial canals, 1 in each 
ray, pass outward just above the ambulacral 
grooves. The radial canals give off side 
branches from which arise the tube feet and 
ampullae (Fig. 193). There are 9 small 
spherical swellings on the inner wall of the 
ring canal which open into its lumen; these 
are called Tiedemann's bodies. No function 
is known for these bodies. 




Stone canal 
Ring canal 

Tiedemann's body 

Transverse cana 

Tube foot 
Radial cana 

Figure 191. Starfish. Diagram of a part of the water vascular system; one of the radial canals 
is cut off at the base and the other four near the base. 

The water-vascular system provides an 
hydraulic pressure mechanism for locomo- 
tion. The starfish walks by means of its tube 
feet. Extension of the foot is brought about 
by contraction of the bulblike ampulla, 
forcing fluid into the cavity of the foot. 
Contraction of the foot muscles causes the 
fluid to run back into the ampulla. 

The action of the tube foot is usually 
that of stepping forward. The foot, from a 
contracted position, points forward; it then 
elongates as the muscles of the ampulla con- 
tract, thus forcing fluid into the tube foot; 
and the bottom of the foot, the sucker, be- 
comes pressed against the substrate. The 
action of certain muscles in the foot pro- 
vides a forward thrust of the animal's body; 
thus the total effect of the forward thrusts 
of the many tube feet produces the move- 
ment of the starfish. After the forward 
thrust, the sucker is detached from the sub- 
strate and the foot contracts and points 
forward again to start the process over. All 
the tube feet act in a coordinated way by 
extending in the same direction, but not at 
the same time. The starfish advances slowly, 
only about six inches in a minute. 

The tube feet are not only used for loco- 

motion, but for clinging to rocks and for 
capturing and handling food. 

Digestive system 

The digestive tract (Fig. 193) is short and 
greatly modified. The mouth opens into a 
very short esophagus which leads into a 
thin-walled sac, the stomach. The stomach 
consists of two parts— a larger oral portion 
and a small aboral or pyloric portion. From 
the pyloric portion, a tube passes into each 
ray, then divides into two branches, each 
of which possesses a large number of lateral 
pouches; these branches are called pyloric 
or hepatic ceca. They are green in color. 
Above the stomach is the slender intestine, 
which opens to the outside through the 
anus. Two branched pouches, brown in color, 
arise from the intestine and are known as 
intestinal ceca. 

The food of the starfish consists of fish, 
oysters, mussels, barnacles, clams, snails, 
worms, crustaceans, etc. Digestion is chiefly 
extracellular. The stomach and pyloric ceca 
secrete several digestive enzymes. Undi- 
gested matter is ejected through the mouth. 
The intestinal ceca secrete a brownish mate- 



rial of unknown function, possibly excretory. 
The fluid in the coelom is kept in motion 
by cilia and carries the absorbed food to all 
parts of the body. 


Excretion is accomplished by the amoebo- 
cytes in the coelomic fluid, which pass to 
the outside of the body through the walls 
of the dermal branchiae. 


Respiration is carried on by means of the 
dermal branchiae (papulae), which look 
like a soft furry substance, on the aboral 
surface of the rays. This appearance is 
caused by the outpouchings of the thin lin- 
ing of the body cavity through minute open- 
ings in the skeleton. These dermal branchiae 
are covered with cilia on both the inside 
and outside. The external cilia keep a cur- 

FiGURE 192. A starfish opening a clam. The starfish attaches its tube feet to the two shells 
of the clam and, by a continuous pull, virtually at right angles to the surface of each shell, 
eventually opens it. Then the stomach of the starfish is everted through the mouth and brought 
in contact with the soft parts of the clam (Fig. 190), and the bivalve is actually digested in its 
own shell. Note that this starfish has lost one ray. (Courtesy of George G. Lower.) 

rent of oxygenated water passing over the 
branchiae on the outside, and the internal 
cilia cause the body fluid to flow out into 
the branchiae. While the body fluid is in 
the branchiae, an exchange of oxygen and 
carbon dioxide takes place exactly as it does 
in our own lungs when the blood flows past 
the tiny air sacs in them. 

Nervous system and sense organs 

Besides many nerve cells which lie among 
the epidermal cells, there are ridges of nerv- 

ous tissue, the radial nerve cords (Fig. 
193), running along the ambulacral grooves, 
and uniting with an oral nerve ( circumoral ) 
ring encircling the mouth. In each ray there 
is (1) a radial nerve cord, (2) a pair of 
nerves that are aboral to the radial nerve 
cord, and (3) a nerve cord in the aboral 
peritoneum. The tube feet are the principal 
sense organs. They receive fibers from the 
radial nerves. At the end of each ray is a 
small, soft, tactile tentacle and a light-sensi- 
tive eye spot. The dermal branchiae are 
probably sensory also. 

Pyloric cecum 


Perihemal cana 
Radial nerve 
Ambulacral groove 

Oral surface of ray 
shov/ing tube feet 


Dermal bronchia 

w^a Ambulacral ossicle 
nMiPrrrr . — Radiol canal 

ood vessel 

Figure 193. General structure of the starfish. The disk and aboral surface are removed from 
two rays. Below, one ray is shown in cross section, without gonads. 





The sexes of starfishes are separate. The 
reproductive organs are branched structures, 
two in the base of each arm (Fig. 193). 
The female has been known to release as 
many as IVz million eggs in two hours, and 
200 million eggs may be liberated in a 

season. A male produces many times that 
number of sperms. The eggs of many star- 
fishes are fertilized in the water and de- 
velop into a type of larva called a bipin- 
naria (Fig. 194) that has bilateral sym- 
metry before it attains the radial symmetry 
of an adult. 


/ Young starfish 


of egg 





Figure 194. The life cycle of the common starfish. Note that in the later larval stages the 
starfish has bilateral symmetry before it attains radial symmetry. 


The starfish has remarkable powers of 
regeneration. A single arm with part of the 
disk will regenerate an entire body. In all 

species tested, arms cut off at any level are 
regenerated, although at a slow rate. If an 
arm is injured, it is usually cast off near the 
base at the fourth or fifth ambulacral 
ossicle. This is autotomy. 





The principal characteristics of echino- 
derms have been described in our account 
of the starfish. Three orders and about 20 
famihes of starfishes are recognized. About 
1200 species are known. Usually 5 or multi- 
ples of 5 rays are present. These rays may be 
long or short, sometimes so short that the 
body resembles a 5-sided pad. Starfishes are 
common marine animals all over the world 
and may be found in both shallow and deep 

Brittle stars and basket stars 

Brittle stars (Fig. 190) and basket stars 
have slender or branched flexible rays; the 
tube feet have largely lost their locomotor 
function and serve as sensory and respiratory 
organs. Food consists of minute organisms 
and decaying organic matter lying on the sea 
bottom. Locomotion is comparatively rapid. 
The rays bend like a whip and enable ani- 
mals belonging to certain species to "run," 
cling, and probably swim. The term "brittle 

star" is derived from the fact that these ani- 
mals break off their arms when they become 

Sea urchins 

A common type of sea urchin (Fig. 190) 
is Arbacia punctulata, a purple-colored spe- 
cies that lives in both shallow and deep 
water from Cape Cod to southern Mexico. 
It is somewhat globular in shape. The test 
(shell) (see head piece, p. 303) is made up 
of calcareous plates which bear movable 
spines about 25 mm. long. There is a system 
of plates in the ray, 5 pairs of columns of 
ambulacra] plates, so called because they 
are penetrated by tube feet and 5 pairs of 
columns of interambulacral plates (see 
headpiece). These correspond to the same 
regions on the starfish, assuming that the 
rays are folded back on its aboral surface. 
Most of the skeletal plates bear spines which 
are attached by muscles and move freely on 
little knoblike elevations called tubercles. 
The food consists of plant and animal mat- 
ter which falls to the sea bottom and is 
ingested by means of a complicated struc- 
ture known as Aristotle's lantern (Fig. 195). 

Genital pore- 



Ring can^ ^ 

Coelom — -, - - — 
Tube feet 





— Genital pore 

Stone canal 



Figure 195. Sea urchin, Arbacia, showing both external and internal structure. 



The water-vascular system consists of a 
madreporite, a stone canal, a ring canal, and 
5 radial canals which extend meridionally, 
connecting with the tube feet (Fig. 195). 
Transverse canals leading to the tube feet 
and ampullae are given off by the radial 
canals. Respiration takes place in most 
echinoids through 10 branched pouches 
situated on the area surrounding the mouth. 
Echinocardium is a heart-shaped urchin, 
and Echinarachnius resembles a silver dollar. 
Sea urchins are variously colored; they may 
be white, purple, green, yellowish green, 
gray, black, etc. Some species possess very 
long poisonous spines, such as Diadema of 
Florida and the West Indies. 

Sea cucumbers 

The sea cucumber differs from other 
echinoderms in that it has an elongated 
body, and lies on its side. One type of sea 
cucumber (Fig. 190) sometimes used for 
study is Cucumaria frondosa. This species 
is abundant on the coast of Maine. Sea 
cucumbers live sluggishly on the sea bottom 
or burrow in the surface mud or sand with 
only the ends exposed. Instead of a test or 
skeleton of spine-bearing plates, Cucumaria 
has a muscular body wall containing very 
small calcareous plates. The digestive tract 
(Fig. 196) consists of a mouth, a short 
esophagus, a small muscular stomach, and a 

Radiat canal 

longitudinal muscle' 

Genital duct 


Retractor muscle 
Ring canal 
Polion vesicle 

Longitudinal muscle 

Figure 1%. Sea cucumber, Cucumaria, showing the tube feet and internal structure. 



long looped intestine, the posterior end of 
which is a muscular enlargement called the 
cloaca ending at the posterior anus. The 
food of most sea cucumbers consists of 
organic particles extracted from the sand or 
mud which is taken into the digestive tract. 
From 10 to 30 of the tube feet surrounding 
the mouth are modified as tentacles for 
procuring food. Respiration is carried on 
through the cloaca. Connected to the cloaca 
are two long-branched tubes, respiratory 
trees. The muscular cloaca pumps water in 
and out of the tree, which serves both as a 
respiratory and excretory organ. The cloaca 
and respiratory trees also function as ex- 
cretory organs. 

Some sea cucumbers are long, slender, and 
wormlike. Their colors are varied— brown, 
yellowish, reddish, whitish, black, pink, 
purplish, etc. One Puget Sound species, 
Psolus chitinoides, has an orange-colored 
body and crimson-colored neck and tenta- 

Sea lilies or feather stars 

These (Fig. 190) are called crinoids. In 
some species they are attached to the sea 
bottom by a long jointed stalk. Their rays, 
5 or 10 in number, are often branched near 
the base and bear smaller branches called 
pinnules along their sides, giving them a 
feathery appearance. About 630 living species 
of crinoids are known. They live in both 
shallow and deep water. Fossil remains of 
crinoids are very abundant in some lime- 
stone formations. 


Both starfishes and sea cucumbers prac- 
tice autotomy. Starfishes break off injured 
arms at a particular point; and the sea 
cucumbers, Thyone, when irritated, may, by 
violent contractions, cast out through the 
cloacal opening most of the viscera; only the 
ends are left inside to regenerate. In both 

cases, the lost parts are soon regenerated. 
One investigator found that of 150 evis- 
cerated sea cucumbers {Thyone), all but 5 
lived and replaced the parts successfuhy. 
Starfishes with regenerating arms as shown 
in Fig. 197 are often encountered in nature. 

Artificial parthenogenesis 

The eggs of echinoderms have been used 
extensively for the study of artificial par- 
thenogenesis, that is, the activation of de- 
velopment in eggs by artificial means instead 
of by the penetration of spermatozoa. Loeb 
succeeded in obtaining actively swimming 
embryos from sea-urchin eggs by changing 
the chemical constitution of the sea water. 
Later it was found that the eggs of various 
species of animals, particularly those of 
starfishes, sea urchins, and frogs, could be 
induced to develop when subjected to a 
number of agents including heat, acids, and 
potassium and sodium chloride. Electrify- 
ing, shaking, and pricking some eggs with 
a needle also stimulate development. The 
efficacy of the agent differs for different 
types of eggs, each type responding to one 
agent more readily than to others. Artificially 
stimulated eggs may give rise to embryos 
and larvae; and in some cases, such as the 
starfishes, sea urchins, and frogs, they may 
produce adult animals. Even fatherless rab- 
bits have been obtained by stimulating un- 
fertilized rabbits' eggs to develop, and then 
replacing them in female rabbits. Thus eggs 
that normally require union with spermato- 
zoa to initiate development may give rise to 
mature animals just as do parthcnogenetic 


Echinoderms and coclenterates, because 
of their radial symmetn,', were at one time 
placed together in a group called Radiata. 



Figure 197. Four-rayed starfish is regenerating one new ray which appears as a small bud 
growing from the disk. (Courtesy of N.Y. Zoological Society.) 

The anatomy of the adult and the structure 
of the larvae, however, show that these phyla 
really occupy widely separated positions in 
the animal kingdom. The adult echinoderms 
cannot be compared with any other group 
of animals, and we must look to the larvae 
for signs of relationship. The bilateral larva 
is either a modification for a free-swimming 
life or an indication of the condition of its 
ancestors. The latter view is accepted by 
most zoologists. The ancestors of echino- 
derms were doubtless bilateral, wormlike 
animals which became radial and took up 
sessile habits secondarily, and present-day 
free-living echinoderms are probably derived 
from a fixed ancestor whose symmetry they 
still retain. 


The value of echinoderms in their rela- 
tions to other lower animals is problematic. 
Apparently, however, they are of consider- 
able importance to man. In the Orient, sea 

cucumbers are dried in the sun and sold as 
beche-de-mer or trepang for use especially 
in soup. The gonads of the sea urchin and 
the eggs of the starfish are also eaten in 
certain tropical regions. 

The dried skeletons of echinoderms have 
been crushed and used as a fertilizer be- 
cause of the high calcium and nitrogenous 

Starfish are very destructive in oyster beds 
since they succeed in pulling open the 
shells and eating large numbers of these 
bivalves (Fig. 192). A starfish has been ob- 
served to eat 10 oysters or clams in a day. 
Two control measures are in general use 
at the present time: (1) in some regions a 
moplike tangle of threads (Fig. 198) is 
dragged across the oyster beds and the star- 
fish that grab onto these with their pedicel- 
lariae are removed from the water and 
destroyed; and (2) the most efficacious 
method of killing starfish is to spread quick- 
lime over the oyster beds in a strength that 
is harmless to the oysters but is death to 
the starfish. 



Figure 198. Starfish collected by dragging a mop 
of threads behind a boat. (Courtesy of General 
Biological Supply House, Inc.) 


(For reference purposes only) 

Echinodenns are radially symmetrical as 
adults but bilaterally symmetrical as larvae; no 
segmentation; the body wall usually contains 
calcareous plates that form an endoskeleton; 
nervous system with oral nerve ring and radial 
nerves; sexes usually separate; a water-vascular 
system, including tube feet, is usually present. 
The 6000 known species are placed in 5 
classes as follows: 

Class 1. Crinoidea (Gr. krinon, lily). Sea 
lilies (Fig. 190). Adults usually with 
5 branched rays and with pinnules; 
cuplike calyx; tube feet suckerlcss; 
aboral pole sometimes with cirri, but 
only about 60 out of 630 existing 
species possess a stalk for temporary 
or permanent attachment; few mod- 
ern species but many fossils. Ex. An- 
tedon tenella, sea lily. 

Class 2. Asteroidea (Or. aster, star). Star- 
fishes (Fig. 190). Adults typically 
with 5 rays; rays usually not sharply 
marked off from disk; ambulacral 
grooves open with tube feet; madre- 
porite aboral; respiration by dermal 
branchiae. Ex. Asterias forbesi, star- 

Class 3. Ophiuroidea (Or. op/iis, snake). Brit- 
tle stars (Fig. 190). Typically 5 rays; 
rays sharply marked off from distinct 
disk; flexible rays; no ambulacral 
grooves; no pedicellariae; madreporite 
oral. Ex. Ophioderrna brevispinum, 
brittle star. 

Class 4. Echinoidea (Gr. echinos, spiny). Sea 
urchins (Fig. 190). Body hemispheri- 
cal, egg-shaped, or disk-shaped; no 
free rays; skeleton of calcareous plates 
forming a test, bearing movable 
spines; usually three-jawed pedicel- 
lariae; tube feet with suckers. Ex. 
Echinarachnius parma, sand dollar. 

Class 5. Holothurioidea (Gr. holothourion, 
sea cucumber). Sea cucumber (Fig. 
190). Adult body long, ovoid, and 
soft, with muscular wall; retractile 
tentacles around mouth; body wall 
usually contains calcareous plates; no 
rays; no spines nor pedicellariae; tube 
feet usually present; cloaca usually 
with respiratory tree. Ex. Thyone 
briareus, common sea cucumber. 


Clark, A.H. "Sea Lilies and Feather Stars." 
Sinithsonian Misc. Collections, 72, No. 7, 

Coe, W.R., "Echinoderms of Connecticut." 
Connecticut State Geological and Natural 
History Survey, Bull. 19, 1912. 

Harvey, E.B. The American Arbacia and Other 
Sea Urchins. Univ. Press, Princeton, 1956. 

Hyman, L.H. The Invertebrates: Echinoder- 
tnata. McGraw-Hill, New York, 1955. 

Jennings, H.S. "Behavior of the Starfish Aste- 
rias forreri DcLoriol." Univ. Calif. Pub. 
Zoo/., 4:53-185. 1907. 


Johnson, M.E., and Snook, H.J. Seashore 
Animals of the Pacific Coast. Macmillan, 
New York, 1927. 

MacGinitie, G.E., and MacGinitie, Nettie. 
Natural History of Marine Animals. Mc- 
Graw-Hill, New York, 1949. 


Mead, A.D. "Natural History of the Starfish." 

Bull. U.S. Fish Commission, 19, 203-224, 

Mortensen, Theodore. Handbook of the 

Echinoderms of the British Isles. Oxford 

Univ. Press, Oxford, 1927. 


Phylum Chordata. 




and Others 

HE phylum Chordata includes the verte- 
brate animals such as the mammals, birds, 
reptiles, amphibians, fishes, elasmobranchs, 
cyclostomes, and a number of marine forms 
that are less well known. All of these animals 
are characterized by: 

1. A skeletal axis, the notochord at some 
stage in the life cycle (Fig. 207). 

2. Paired gill slits connecting the pharynx with 
the exterior at some stage in the life cycle. 
All chordates up to and including the 
fishes carry on respiration by means of gills 
throughout life. In the higher vertebrates, 
gill slits or traces of them are usually pres- 
sent only in embryonic or larval stages. In 
mammals the gill slits never open. 

3. A central nerve cord which contains a 
cavity or system of cavities; it is dorsal to 
the digestive tract. 

These chordate characters all appear at some 
stage in development, and they may persist, 
change, or disappear in the adult. 

Figure 199 shows some of the funda- 
mental differences in the body plan of an 
achordate and a chordate. 


In many respects the chordates differ 
widely from one another, and it is cus- 
tomary to separate them into 4 subphyla: 

1. Hemichordata.* Two classes of wormlike 

a. Enteropneusta. Acorn (tongue) worm, 
wormlike with many gill slits. 

b. Pterobranchia. Very small chordates 
with one pair of gills or none. 

* Some zoologists question the presence of a true 
notochord in the hemichordates, and they would re- 
move them from the Chordata and place them in an 
independent invertebrate phylum, but embryology is 
the crucial thing and can be interpreted differently 
depending on what are regarded as the really defin- 
ing features of a notochord. Because the experts do 
not agree, classification of the hemichordates mus>^ 
be considered controversial. 




Figure 199. Diagram showing some of the fundamental differences in the body plan of {top) 
an achordate or invertebrate (annelid) and [bottom] a chordate (snake) in the location of the 
digestive system, heart, and nervous system. The arrows indicate the direction of the flow of blood. 

2. Urochordata (Tunicata). Tunicates (sea 
squirts) and a number of other marine 

3. Ccphalochordata. Two families of fishlike 
animals called lancelets. 

4. Vertebrata. Animals with backbones. 

The subphylum Vertebrata includes most 
of the chorda tes (Fig. 200), but the other 
three subphyla, often called protochordates, 
are of considerable interest since they are 
more primitive and hence give us some idea 
of the character of the animals from which 
the vertebrates probably developed. The 
primitive chordate most frequently studied 
in general zoology laboratories is the am- 
phioxus of the subphylum Ccphalochordata. 
The amphioxus exhibits chordate charac- 
teristics (notochord, gill slits, and a dorsal 
tubular nerve cord) so clearly that it illus- 
trates well the basic chordate characteristics. 
In the next chapter a detailed description 
of the frog is presented as an introduction 
to the subphylum Vertebrata. 


The Ccphalochordata comprise about 30 
species of marine animals of which Branchi- 
ostoma lanceolatus, commonly known as the 
amphioxus, lives in the waters of tropical 
and temperate seacoasts (Fig. 201). The 
amphioxus is of special interest, since it 

exhibits the characteristics of the chordates 
in a simple condition. Furthermore, it may 
be similar to some ancient ancestor of the 

The amphioxus is about two inches long. 
The semitransparent body is pointed at both 
ends and laterallv compressed. It is found 
near the shore, where it burrows in the clean 
sand with its head or tail, and conceals all 
but the anterior end (see headpiece, p. 315). 
It sometimes leaves its burrow at night and 
swims about by means of rapid lateral move- 
ments of the body. When it ceases to move, 
it falls on its side. 

External anatomy 

The amphioxus (Fig. 201) is shaped like 
a fish but has no lateral fins and no distinct 
head. Along the middorsal line is a low 
dorsal fin which extends the entire length 
of the body and widens at the posterior end 
into a caudal fin. The caudal fin extends 
forward on the ventral surface to form the 
short ventral fin. Both dorsal and ventral 
fins are strengthened by rods of connective 
tissue called fin rays. In front of the ven- 
tral fin, the lower surface of the body is 
flattened, and on each side is an expansion 
of the integument called the metapleural 
fold (Fig. 202). 

The body wall is divided into V-shaped 
muscle segments, the myotomes; these are 
separated from one another by septa of 



OSTEiCHTHYES (sunfish) 

bony fishes 

• .• •....•.■.•.ei)" 

MAMMALIA (squirrel) 

AMPHIBIA (frog) 


(primitive-jawed fishes) 

(jawless vertebrates) 

Hypothetical Chordate Ancestor 

Figure 200. A simplified family tree of the vertebrates. The ancestor to the most primitive 
jawless vertebrates (class Agnatha), which are fossil ostracoderms, is unknown. The primitive 
jawed placoderm is also a fossil form. (The ostracoderm and placoderm after A.S. Romer.) 

connective tissue. The myotomes on one 
side of the body alternate with those on the 
other side. The muscle fibers contained in 
them are longitudinal; and since they are 
attached to the connective tissue partitions, 
they are able to produce the lateral move- 
ments of the body used in swimming. 

The mouth proper is an opening in a 
membrane posterior in the oral hood. The 
anus is situated on the left side of the bodv 
near the base of the caudal fin. The 
atriopore is just anterior to the ventral fin; 
it is a ventral opening through which water 
used in respiration passes to the outside. 



■Nerve cord 
Fin ray 

Dorsal fin 

Velar tentacle 


Oral tentacle 

Gill slit of pharynx 
^Gill bar of pharynx 

Caudal fin 

' — Ventral fin 

Figure 201. Amphioxus, an animal that illustrates the three fundamental chordate charac- 
teristics. An adult with part of body wall removed from the left side to show the general struc- 
ture. Natural size about two inches long. 

Internal anatomy 
and physiology 


The amphioxus has a well-developed 
notochord (Figs. 201 and 202), which is 
the main support of the body. This is a 
rod of connective tissue lying near the 
dorsal surface and extending almost the 
entire length of the body. The notochord 
is composed of vacuolated cells which are 
made turgid by their fluid contents, and are 
therefore rigid. Other skeletal structures are 
the connective tissue rods which form the 
fin rays, and similar structures that support 
the oral tentacles (cirri) of the oral hood 
and gill bars. 

Digestive system 

The food of the amphioxus consists of 
minute organisms which are carried into the 
mouth with the current of water produced 
by cilia on the gills. The mouth is an open- 
ing in a membrane, the velum, and may be 
closed by circular muscle fibers which sur- 
round it. Twelve sensory-oral or velar 
tentacles protect the mouth, and when 
folded across it, act as a strainer, thus pre- 
venting entrance of coarse solid objects. 
The funnel-shaped vestibule is the cavity 

of the oral hood. The 22 ciliated tentacles 
which project from the edge of the oral 
hood are provided with sensory cells. The 
inner wall of the oral hood bears a number 
of ciliated lobes and is known as the wheel 
organ because, during life, its cilia appear 
to produce a rotatory movement. Water is 
drawn into the mouth chiefly by the action 
of the gill cilia. 

The mouth opens into a large, laterally 
compressed pharynx. A ciliated middorsal 
groove in the pharynx is called the hyper- 
branchial groove. A ventral groove, the 
hypobranchial groove (endostyle), is also 
present. The endostyle consists of a median 
ciliated region with a glandular portion on 
both sides. The glands secrete strings of 
mucus in which food particles become en- 
tangled as the mucus passes up the gills. 
The cilia drive this mucus forward and up- 
ward, by way of the gills and two peri- 
pharyngeal grooves, into the hyperbranchial 
groove. From here it is carried by the hyper- 
branchial cilia into the intestine. Other 
food particles are caught by mucus produced 
in the hyperbranchial groove and then car- 
ried posteriorly by the action of cilia to the 
intestine. A ventral finger-shaped outpocket- 
ing of the intestine is known as the "liver," 
but it might better be called the midgut 
cecum, since it is an outgrowth of the mid- 



gut. Although it secretes digestive enzymes, 
it is not known to function as the vertebrate 
liver. The intestine leads directly to the 

Respiratory system 

The pharynx (Fig. 202) is attached 
dorsally and hangs down into a cavity called 
the atrium. The atrium is not the coelom. 

Fin ray 

Dorsal root nerve 
Nerve cord 
Central canal 
Ventral root nerve 
Sclerocoel (coelom) 

Dorsal aorta 
Hyperbranchial groove 


Gill bar 

Gill slit 



Egg in ovary 


Transverse muscle 


Hypobranchial groove 
Subendostylar coelom 
Ventral aorta 
Metapleural fold 
Metapleural cavity 

Figure 202. A cross section of the amphioxus in the pharyngeal region, showing various in- 
ternal structures, including some of the coelomic cavities. 

but it is lined with an ectodermal epithe- 
lium and is really external to the body, as 
has been proved by the study of its de- 
velopment. Water, which is carried into 
the pharynx by way of the mouth, passes 
through the gill slits into the atrium and 

out of the atriopore. The gill slits are sepa- 
rated by gill bars; these are ciliated and 
supported by rods of connective tissue. 
Respiration takes place as the water, driven 
by the cilia, flows through the gill slits; the 
phar)'ngeal mechanism and gill slits are 



probably of equal importance in the func- 
tion of food catching. 

Circulatory system 

The circulatory system is similar to that 
in a higher chordate such as the fish, but 
lacks a heart. Besides the definite blood 
vessels, there are tissue spaces into which 
the colorless blood escapes. The subintes- 
tinal vein collects blood loaded with nutri- 
ment from the intestine and carries it for- 
ward into the hepatic portal vein, and 
thence to the liver. The hepatic vein leads 
from the liver to the ventral aorta. Blood is 
forced by the rhythmic contractions of the 
ventral aorta into the afferent branchial 
arteries, which are situated in the gill bars, 
and then through the efferent branchial 
arteries into the paired dorsal aortae. It 
passes back into the median dorsal aorta, 
and finally by way of intestinal capillaries 
into the subintestinal vein. The blood is 
oxygenated during its passage through the 
gill slits. The direction of the blood flow, 
backward in the dorsal and forward in the 
ventral vessel, is the same as that of the 
vertebrates, but the reverse of that in in- 
vertebrates such as annelids. 


The reduced coelom is represented in the 
adult by cavities around the digestive tract. 
The position of the coelomic cavities is 
shown in the pharyngeal region in Fig. 202. 

Excretory system 

The excretory organs, although often de- 
scribed as ciliated, are actually flagellated 
nephridia ( protonephridia ) situated near 
the dorsal region of the phar\'nx. Each 
nephridium bears several clusters of soleno- 
cytes, which are flagellated cells extending 
out of a tube in which the flagella play. 
About 100 pairs of protonephridia connect 
the dorsal coelom with the atrial cavity. 
These protonephridia are not homologous 
to the tubules of the vertebrate kidney. 

Nervous system 

The amphioxus possesses a central nerve 
cord (Fig. 202) lying entirely above the di- 
gestive tract in contrast to the ventral nerve 
cords of annelids and arthropods. It rests on 
the notochord and is almost as long. A mi- 
nute central canal traverses its entire length 
and widens at the anterior end to form a 
brain vesicle, which is the only trace of a 
brain present. An olfactory pit opens into 
this vesicle in young specimens. At the an- 
terior end of the nerve cord is a black pig- 
mented spot called an "eye spot," although 
it is not sensitive to light. The "eyes" con- 
sist of numerous single light-sensory cells, 
each with a pigment cup in the ventral wall 
of the nerve cord. Two pairs of sensory 
nerves arise from the cerebral vesicle and 
supply the anterior region of the body. The 
rest of the nerve cord gives off nerves on 
opposite sides, but alternating with one an- 
other. These nerves are of two kinds: (1) 
dorsal nerves with a sensory function which 
pass to the skin and ( 2 ) ventral nerves with 
a motor function which enter the myotomes. 
The sense organs include the olfactory pit, 
sensory cells in the epidermis on the oral 
and velar tentacles, "eyes," and possibly the 
"eye spot." 


In the amphioxus the sexes are separate. 
The paired gonads (Fig. 201) project into 
the atrium. Eggs and sperms are discharged 
into the atrial cavity and reach the exterior 
through the atriopore. Fertilization takes 
place externally in the water. The cleavage 
is holoblastic as in the starfish. 


Subphylum Hemichordata 

The hemichordates, traditionally, have 
been considered the lowest chordates, but 
some recent authorities do not regard them 
as chordates at all. They say the so-called 



"notochord" is not a true notochord, but is 
what they call a stomocord. However, as ex- 
plained in the footnote (p. 315), this is still 
an open question. 

The acorn or tongue worms are the com- 
mon names by which the hemichordates are 

known. They are soft-bodied animals and 
most of them live in shallow water along the 
seashore. Some species have persistent and 
unpleasant odors. The external features of 
one are shown in Fig. 203. 
Three regions may be distinguished: a 


Gill slits 


Figure 203. An acorn worm, Saccoglossus (Dolichoglossus) kowalevskii, a species that lives on 
sand flats from Massachusetts Bay to Beaufort, N.C. Natural size about 7 inches long. 

proboscis, a collar, and a trunk, which make 
up most of the body. Paired lateral gill slits 
are present in the anterior part of the trunk. 
Figure 204 shows diagrammatically the prin- 
cipal internal structures of another species. 
The mud or sand in which the animals live 
is taken into the mouth and forced slowly 
through the digestive tract, where nutriment 
is extracted from the organic matter con- 
tained in it— a process similar to digestion 
in the earthworm. Respiratory, circulator)', 
and nervous systems are present. The sexes 
are separate. In some species each egg de- 
velops into a free-swimming larva called a 
tornaria. The resemblance of the tornaria 
to the larvae of echinoderms is quite striking 
and has led to one hypothesis of the origin 
of the vertebrates (Chap. 36). 

Subphylum Urochordata 

The tunicates all live in the sea. They are 
either free-swimming or attached; they are 
widely distributed and occur at all levels 
from near the surface to a depth of over 

three miles. They range in size from about 
YioQ inch to over a foot in diameter. Some 
are brilliantly colored. The adult in some 
species (Fig. 205) is saclike and has received 
the common name "sea squirt" because 
when irritated it may eject water through 
two openings in the unattached end. The 
term Tunicata was formerly applied to mem- 
bers of the group on account of a cuticu- 
lar outer covering known as a tunic or 

The chordate characteristics of tunicates 
were not recognized until the development 
of the egg and metamorphosis of the larva 
were fully investigated. It was then dis- 
covered that the typical larva (Fig. 206), 
which is about V4 inch long and resembles a 
frog tadpole, possesses ( 1 ) a distinct noto- 
chord, (2) a dorsal neural tube in the tail 
enlarging in the trunk and ending in a 
vesicle, which is considered the forerunner 
of the brain of the vertebrates, and (3) a 
pharyngeal sac which opens to the exterior 
by innumerable ciliated gill slits. The tail 
propels the larva fonvard by lateral strokes. 



Glomerulus — i 

■*- -< Collar ■ 

Dorsal pore 

-Trunk *- 

Dorsal nerve cord 

I — Dorsal blood vessel 
Respiratory pharynx 

1 Gonad 

Proboscis coelom 



Gill slit-" 

'-Nutritive pha 
Ventral blood vessel 
Ventral nerve cord 

Figure 204. Acorn worm [Glossobalanus] . A, longitudinal section through the middle line. 
Some zoologists call the short anterior structure, which has long been called a notochord, a 
"stomochord," but this new name does not rule out the possibility of its being homologous with 
the notochord of the vertebrates. B, cross section through the trunk region. (A modified from 
Bullock; B after Bullock.) 

IncorrenT siphon 


Pharyngeal slit 

Endostyle with 


Excurrent siphon 

Nerve ganglion 

Atrial cavity 

Duct of gonad 

T^ Esophagus 

I Ovary 

— Testis 

Digestive gland 

Figure 205. Internal structure of a tunicate {Molgula). Tunic, mantle, and pharynx removed 
from left side. Arrows indicate flow of water currents through the animal. 




After a short existence as a free-swimming 
organism, the larva becomes attached to 
some object by three projections on the an- 
terior end, which secrete a sticky fluid. It 
then undergoes a retrogressive metamor- 
phosis, during which the tail with the noto- 
chord disappears, and the nervous system is 
reduced to a ganglion. 

The typical adult tunicate (Fig. 205) is 
attached by a base or stalk and surrounded 
by a thick, tough, elastic membrane, the 
tunic. This is composed of a celluloselike 
substance, a material rarely found in ani- 
mals but common in plants. The tunic is 
lined by a membranous mantle which con- 
tains muscle fibers and blood vessels. At the 

distal end are two external openings: one 
is the "mouth" or incurrent siphon (bran- 
chial opening), into which a current of 
water passes; the other is the excurrent 
siphon (atrial opening) through which the 
water escapes to the outside. This current 
of water brings food into the digestive tract, 
furnishes oxygen for respiration, and carries 
away gametes and excretory substances. 
Within the test and mantle is the atrial cav- 
ity which contains the internal organs. At the 
base of the incurrent siphon is a velum, 
forming a sensory sieve, through which in- 
coming water and food must pass. Below the 
velum is a pharyngeal sac; on the ventral 
side of this is the endostyle, a pharyngeal 


Dorsal nerve cord 



Gill slit 

Figure 206. The free-swimming larva of a tunicate, showing all three chordate characteristics: 
notochord, dorsal nerve cord, and gill slits. 

groove, lined with mucous and ciliated cells. 
Microscopic plants and animals are en- 
tangled in mucus secreted by the endostyle 
and move downward into the esophagus. 
This leads to the stomach which connects 
with the intestine. The digestive tube is 
bent upon itself and opens into the atrial 
cavity through the anus. A digestive gland 
connects by a duct to the stomach. A single 
nerve ganglion lies between the two siphons, 
with nerves to various structures. Located 
near this is a neural gland which may have 
an endocrine function. 

The circulatory system consists of a tubu- 
lar heart, to each end of which is connected 

a large vessel, and each of the vessels gives 
off branches to various structures. One un- 
usual feature of the circulation in these 
forms is that the direction of blood flow is 
reversed at short intervals. 

Tunicates are hermaphroditic, but they 
are usually self-sterile so that sexual repro- 
duction requires two animals. Some species 
also reproduce asexually by budding. 

Subphylum Vertebrata 

The Vertebrata are chordates having a 
segmental backbone or vertebral column. 
They also possess an axial notochord at some 



time in their lives. Although this notochord 
persists in some of the lower vertebrates, it 
becomes modified by an investment of car- 
tilage (Fig. 207) which becomes segmented 
and constitutes the vertebral column. In the 

higher vertebrates the vertebral column is 
made up of a series of bodies called verte- 
brae, and the notochord disappears before 
the adult stage is reached. Seven classes of 
vertebrates are recognized. 

Nerve cord 

Notochord sheaths 

Dorsal aorta 

— Cartilage 

Figure 207. Notochord of a young dogfish. Cross section shows nerve cord and sheaths of 

Plan of structure 

The vertebrates resemble the other 
chordates in their metamerism and bilateral 
symmetry, in the possession of a coelom, a 
notochord, and gill slits at some stage in 
their existence, and a dorsal nerve tube. 
They differ from other chordates and resem- 
ble one another in the possession of cartilag- 
inous or bony vertebrae, usually two pairs 
of appendages, an internal and jointed 
skeleton, a ventrally situated heart with at 
least two chambers, and red corpuscles in 
the blood. 

The body of a vertebrate may be divided 
into a head, usually a neck, and a trunk. 
In many species there is a posterior exten- 
sion, the tail. Two pairs of lateral append- 
ages are generally present, the thoracic (pec- 
toral fins, forelimbs, wings, or arms) and 
the pelvic (pelvic fins, hindlimbs). The 
limbs support the body, serve in locomotion, 
and usually have other special functions. 

The plan of structure of a vertebrate can 
be presented most clearly with diagrams 
showing longitudinal and cross sections 

through the body (Figs. 208 and 213). As 
in the amphioxus, the nerve cord is dorsal 
but extends in front of the end of the noto- 
chord and enlarges into a brain. The noto- 
chord becomes invested by the vertebrae. 
The coelom is large. The digestive canal 
forms a more or less convoluted tube which 
lies in the body cavity. The liver, pancreas, 
and spleen are situated near the digestive 
canal. In the anterior trunk region are the 
lungs and heart. The kidneys and gonads 
lie above the digestive canal. 

Classes of Vertebrata 

The principal classes of vertebrates having 
living representatives are: 

Class 1. Agnatha (L. a, without; Gr. gnathos, 
jaws). Lampreys and hags (Fig. 
240). Cold-blooded (poikilother- 
mous), fishlike vertebrates without 
scales, jaws, or lateral fins. 

Class 2. Chondrichthyes (Elasmobranchii) 
(Gr. chondros, cartilage; ichthys, 
fish). Sharks, rays, skates, and chi- 



Mouth cavity 
Brain— I 

Gill slit -| 



Genital duct 

Remnant of notochord 

Nerve cord 


Pericardial covity 

Cloacal opening 
Urinary bladder 


Figure 208. 

Stomach—' Bile duct-" 
A longitudinal vertical section of a generalized vertebrate, showing the plan of 

maeras. Cold-blooded vertebrates 
with jaws, a cartilaginous skeleton, 5 
to 7 pairs of gills in separate clefts, a 
persistent notochord, placoid scales, 
and paired fins. 

Class 3. Osteichthyes (Gr. osteon, bone; 
ichthys, fish). Fishes (Fig. 200). 
Cold-blooded vertebrates with jaws, 
bony skeleton, 4 pairs of gills in com- 
mon cavity under opercula, skin usu- 
ally with cycloid or ctenoid scales, 
usually with paired fins. 

Class 4. Amphibia (Gr. amphi, both; bios, 
life). Frogs, toads, and salamanders 
(Fig. 200). Cold-blooded vertebrates, 
skin moist, no external scales, mostly 
with pentadactyl (5-fingered) limbs; 
young usually aquatic and breathe by 
gills; adults usually lose the gills and 
breathe by means of lungs. 

Class 5. Reptilia (L. repere, to crawl). Tur- 
tles, sphenodon, lizards, snakes, and 
crocodiles (Fig. 200). Cold-blooded 
vertebrates breathing by means of 
lungs and usually having a dry scaly 
skin; respiration always by lungs. 

Class 6. Aves (L. avis, bird). Birds (Fig. 
200). Warm-blooded vertebrates 
with the forelimbs modified into 
wings and the body covered with 

Class 7. Mammalia (L. mamma, breast). 
Hairy quadrupeds, whales, squirrels, 
bats, monkeys, and man (Fig. 200). 
Warm-blooded vertebrates with a 
hairy covering at some stage in their 
existence; the young nourished after 
birth by the secretion of the mam- 
mary glands of the mother. 


Berrill, N.J. The Origin of Vertebrates. Claren- 
don Press, Oxford, 1955. 

Bullock, T.H. "The Anatomical Organization 
of the Nervous System of Enteropneusta." 
Quar. J. Microscop. Sci., 86:55-111, 1945. 

Carson, R.L. The Sea Around Us. Oxford 
Univ. Press, New York, 1951. 

Morgan, T.H. "The Development of Balano- 
glossus." /. Morphol. and Physiol, 9:1-86, 

Ritter, W.E., and Davis, B.M. "Studies on the 
Ecology, Morphology and Speciology of the 
Young of Some Enteropneusta of Western 
North America." Univ. Calif. Pub. Zool., 
1:171-210, 1904. 

Willey, A. Amphioxus and the Ancestry of the 
Vertebrates. Columbia Univ. Press, New 
York, 1894. 



«%P^** Ojl 

A Representative 




HE frog is used for laboratory study more 
often than any other animal as a vertebrate 
type. A knowledge of its structures and 
physiologic processes helps in understanding 
the vertebrates in general and gives a back- 
ground for the study of more complex forms, 
including man. The following account of 
the structure, physiology, and development 
of the frog applies to any common species 
such as the leopard frog, Rana pipiens, or 
the bullfrog, Rana catesbeiana. The prin- 
cipal subjects considered are the external 
features, histology and physiology of the 
skin, structure of the systems of organs, 
processes of digestion, respiration, internal 
transport by blood and lymph, disposal of 
wastes, mechanical support and movement 
by bones and muscles, control of the body 
exercised by nerves and sense organs, ecol- 
ogy, behavior, reproduction, and embryonic 

Many structural and physiologic charac- 
teristics of the frog are similar to those of 
man, hence it is suggested that comparisons 
be made from time to time. Chapters 31 to 
34, which contain general considerations of 
nutrition, skeletal structures and movement, 
coordination and behavior, and reproduction 
and development, might profitably be in- 
cluded with the study of the frog. 

The leopard frog lives in or near fresh- 
water lakes, ponds, and streams, and is dis- 
tributed over the North American continent 
except on the Pacific slope. The frog leaps 
on land and swims in water. The hind- 
limbs are large and powerful; when the frog 
is on land they are folded up, but to propel 
the body through the air they are suddenly 
extended. Likewise in swimming, the hind- 
limbs are alternately folded up and ex- 
tended; and during their backward stroke, 
the toes are spread apart, increasing the 
webbed surface so as to offer more resistance 
to the water. Frequently frogs float on the 
surface with just the tip of the nose exposed 
and with the hindlegs hanging down. When 
disturbed in this position, the frog dives 
under water, the hindlimbs are flexed, a 



movement which withdraws the body, and 
the forehmbs direct the animal downward; 
then the hindHmbs are extended again, com- 
pleting the dive. 

Frogs croak mostly during the breeding 
season, but also at other times of the year, 
especially in the evening or when the at- 
mosphere becomes damp. Croaking may 
take place either in air or under water. In 

the latter case, the air is forced from the 
lungs, past the vocal cords, into the mouth 
cavity, and back again. 

The principal enemies of frogs are snakes, 
turtles, cranes, herons, other amphibians, 
and man. The excellence of frog legs for the 
table has resulted in their widespread de- 
struction, and this has been augmented by 
the capture of great numbers for use in 

Figure 209. Feet of a male frog, Rana pipiens. A, forefoot showing enlarged first digit (thumb), 
which is most developed during the breeding season. B, hindfoot showing web. 

scientific investigations. Tadpoles fall prey 
to aquatic insects, fish, and waterfowl, and 
relatively few of them reach maturity. 

External anatomy 

The body of the frog may be divided into 
the head and trunk (Fig. 217); there is no 
neck region. The eyes usually protrude from 
the head, but are drawn into their orbits 
when the frog closes its eyelids. Behind each 
eye there is a flat eardrum ( tympanic mem- 
brane). A pair of nostrils (external nates) 
is situated on the dorsal surface near the end 
of the snout. In the dorsal midline, just in 
front of the eyes in some specimens, is a 
light area called the brow spot, which, in 
the embryo, was connected with the brain. 
The mouth of the frog extends from one side 
of the head to the other. The cloacal open- 
ing, or what is sometimes called the anus, is 
situated at the posterior end of the body. 

The forelimbs are short and serve to hold 
up the anterior part of the body. The hands 
possess 4 digits and the rudiment of a fifth. 

the thumb. In the male, the inner digit is 
thicker than the corresponding digit of the 
female, especially during the breeding sea- 
son (Fig. 209). The hindlimbs are folded 
together when the frog is at rest. The 5 toes 
are connected by a web, making the foot 
an efficient swimming organ. 

Body covering 

The skin is smooth and loosely attached 
to the body. Along either side of the body, 
behind the eyes, is a ridge formed by a 
thickening of the skin; this is called the 
dorsolateral fold (dermal plica). The skin 
is colored by scattered pigment granules in 
the epidermis and pigment cells known as 
chromatophores in the dermis. The chroma- 
tophores are of several different kinds, the 
most important being those which contain 
black or yellow pigment. There are also in- 
terference cells that contain whitish cr}'stals. 
There is no green pigment in the frog's skin. 
Frogs are usually protectively colored by 
resembling their surroundings. The color of 



Stratum corneum 

Stratum germinativum — i^ 
Pigment cells 

Mucous gland 

Stratum spongiosum 
Poison gland 

Stratum compactum 


Subcutaneous tissue- 


.- .. . .'...^is!?' ....' ■••;.•.•.•.....":.... ..'^ ■■■•••.. v!v' 


Blood cell 
■Blood vessel 

Figure 210. Section of frog skin showing microscopic structure (histology). Vertical section. 
(After Laboratory Explorations in General Zoology, by Karl A. Stiles. Third edition. Copyright 
1955 by The Macmillan Company.) 

the dorsal and lateral surfaces is darker than 
the ventral surface, which is whitish. 
Changes in color may occur as described on 
page 398. 

Histologically, the skin consists of two 
layers as in other vertebrates (Fig. 210), an 
outer epidermis and an inner dermis. There 
are several layers of cells in the epidermis 
(stratified epithelium, Fig. 43). That on the 
outside, the stratum corneum, is horny and 
consists of broad, thin cells, the squamous 
epithelium. Beneath this lies a transitional 
zone of polygonal cells, which in turn rest 
upon the basal columnar cells of the stratum 
germinativum. During molting the stratum 
corneum is shed. New cells are being con- 
tinually formed by those of the columnar 
layer, and the outward pressure of these 
cells brings about the flattening of the 
surface layers. 

The dermis consists of two layers, a loose 
outer layer, the stratum spongiosum, which 
contains the dermal glands and pigment 
cells (chromatophores), and a dense inner 
layer of connective tissue, the stratum com- 
pactum, which contains white and yellow 
fibers, a few smooth muscle cells, blood ves- 
sels, and nerves. Beneath the dermis is a 
subcutaneous layer of loose connective tis- 
sue, which is divided into two layers by 

lymph spaces and serves to attach the skin 
to the body wall by septa. 

The skin is richly provided with glands. 
These are of two principal types: mucous 
glands, which are more numerous and small, 
and poison glands, which are larger and less 
common. These glands open to the outside 
bv means of ducts. The mucus-covered sur- 
face of the frog makes him slippery, which 
often helps him to escape the grasp of 
enemies. Each gland consists of an epithelial 
layer of secreting cells, outside of which are 
muscle fibers and connective tissue. The 
mucous glands may be present in great num- 
bers, as many as 60 to each square millimeter 
of surface. Mucus is formed in the secreting 
epithelium, discharged into the lumen of 
the gland, and forced through the duct to 
the surface of the skin by the muscle cells. 
The poison glands secrete a whitish fluid 
with a burning taste, which serves as a 
means of protection against enemies. The 
expulsion of this secretion may be stimulated 
by rough handling or chloroforming a frog. 

The skin in man and 
other vertebrates 

The skin in vertebrates, in general, is 
chiefly protective and sensory, but it may 



also carry on respiration, secretion, and ex- 
cretion. Secretion and excretion take place 
by means of glands, which may be simple as 
the mucous glands of amphibians and fishes, 

or complex as the sweat, oil, and mammary 
glands. The skin often produces outgrowths 
such as hair, feathers, nails, hoofs, claws, 
scales, teeth, and bony plates. 



Sweat gland 
(in section) 

Hair bulb 
Hair papilla 



Duct of 
sweat gland 

Duct of 



ebaceous gland 

Hair muscle 
Hair root 

Tubule of 
sweat gland 

Blood vessel 
Fat cells 

Figure 211. Skin of man showing cell layers, fibers, and other parts. Vertical section. 

In man the skin (Fig. 211) protects the 
deeper tissues from drying, from injury, and 
from the invasion of bacteria and other 
organisms. It contains the end organs of 
many sensory nerves. The balance between 
heat production and heat dissipation is ef- 
fected largely by the skin, since skin offers 
a large surface for radiation and evaporation 
of sweat, and it contains a large amount of 
blood. About 87.5 per cent of the body heat 
passes out through the skin as compared 
with 10.7 per cent through the lungs, and 
1.8 per cent in the urine and feces. Sebace- 
ous glands occur everywhere except on the 
palms of the hands and soles of the feet. 
They are compound alveolar glands with 
ducts that usually open into a hair follicle. 
Their secretion, sebum, is a fatty, oily sub- 
stance that keeps the skin and hair flexible 

and covers the skin with a layer that pre- 
vents too rapid evaporation of water. Sweat 
glands are distributed over the surface of the 
skin, with the exception of the margins of 
the lips, the skin under the nails, and the 
glans penis. They are tubular, with the in- 
ner portion coiled into a ball. The average 
amount of sweat secreted in 24 hours is 
about 16 to 20 fluid ounces. Sweat contains 
some waste substances, but it is particularly 
important because of the heat that is 
necessary to evaporate it. 

General internal anatomy 

If the body wall of the frog is split open 
in the ventral midline, from the posterior 
end of the body to the angle of the jaw, the 
organs in the body cavity or coelom will be 



exposed (Fig. 212). The heart hes within 
the sadike pericardium; it is partially sur- 
rounded by the three lobes of the reddish- 
brown liver. The two lungs lie, one on either 
side, near the anterior end of the abdominal 
cavity. Coiled about within the body cavity 
are the stomach and intestine. The kidneys 
are flat reddish bodies attached to the dorsal 
body wall; they lie outside the coelom, just 
behind a thin membrane, the peritoneum. 
The two testes of the male are small ovoid 

organs suspended by membranes and lying 
at the sides of the digestive tract. The 
ovaries and oviducts of the female occupy 
a large part of the body cavity during the 
breeding season. The coelom is lined with a 
membrane of mesodermal origin, the 
peritoneum. The reproductive organs and 
digestive tract are suspended by double 
layers of peritoneum called mesenteries 
(Fig. 213). 
In man, a diaphragm separates the tho- 


Opening of 



Right atrium 
Left atrium 
Sinus venosus 


Gall bladder 
teric artery 

portal vein 

Coelom - 
Small intestine 

abdommal vein 

Large intestine 

^>^^ External nares 

^^^^ Internal nares 

Mouth cavity 


Opening of 

vocal sac 


Systemic arch 

Nerve cord 

vena cava 


moral vein' 

Dorsal aorta 

Fat body 







Urinary ducf 

Renal portal vein 
Iliac artery 



Figure 212. Internal structure of a frog. 



racic and abdominal cavities. The thoracic 
cavity contains the esophagus, trachea, 
lungs, heart, and blood vessels. The abdom- 
inal cavity contains the stomach, spleen, 
pancreas, liver, gall bladder, kidneys, and 
large and small intestines. The lower part 
of the abdominal cavity is called the pelvic 
cavity; this contains the bladder, rectum, 
and some of the reproductive organs. 

Digestive system 

The principal functions of the digestive 
system are to receive, digest, and absorb 
food and eliminate some wastes. 

The food of the frog consists principally 
of living worms and insects. These are usu- 
ally captured by the extensile tongue, which 
can be thrown forward as shown in the 

Subcutaneous lymph space 

Central cana 
Dorsal aorta 
Posterior vena cava 




Visceral peritoneum 


Subcutaneous lymph space 

Spinal cord 




Visceral peritoneum 

Lumen of Infestino 

Parietal peritoneum 

Abdominal vein 

Figure 213. Diagram of a cross section of the body of a frog through the kidneys and gonads 
to show relation of the peritoneum (broken hues) to other organs. 

illustration at the head of this chapter 
(headpiece). The object adheres to the 
tongue, which is covered with a sticky secre- 
tion, and is then drawn into the mouth. No 
attention is paid to objects that are not mov- 
ing. Large insects are pushed into the mouth 

with the forefeet. In case the object swal- 
lowed proves undesirable, it can be ejected 
through the mouth. 

The mouth cavity is large. The tongue 
lies on the floor of the cavity with its an- 
terior end attached to the jaw and its forked 

bile duct 
Cystic duct 


bile duct 


Pancreatic duct 

Common bile duct 

Figure 214. Liver, gall bladder, pancreas, stomach, and part of the intestine of a frog. 



posterior end lying free. When a lymph 
space beneath the tongue is filled, the 
tongue is thrown forward for capturing in- 
sects. The teeth are conical in shape; they 
are borne by the upper jaw and two bones 
in the roof of the mouth called vomers. 
Teeth are used only for holding food and 
not for chewing it. New teeth replace those 
that may be lost. 

The esophagus opens into the mouth 
cavity by a horizontal slit. Its inner surface 
bears longitudinal folds, which give it the 
remarkable powers of distension necessary 
for swallowing large animals for food. His- 
tologically it resembles the stomach. The 
stomach is crescent-shaped and lies mostly 
on the left side of the body. The anterior 
or cardiac end is larger than the esophagus. 
It decreases in size toward the posterior or 
pyloric end, where it joins the small intes- 
tine. The stomach is held in place by a dor- 
sal fold of the peritoneum, and a ventral 
fold of the peritoneum. 

The walls of the stomach are thick, con- 
sisting of 4 layers: (1) the outer thin peri- 
toneum, the serosa or serous membrane; (2) 

a tough muscular layer; (3) a spongy layer, 
the submucosa; and (4) an inner folded 
mucous layer, the mucosa. The mucosa or 
mucous membrane is an inner lining, with 
many glands, both one-celled and many- 
celled. These glands are tubular in shape and 
sometimes branched. They are formed by 
the invagination of the epithelium of the 
stomach lining. Those near the cardiac end 
of the stomach are longer and differ histo- 
logically from those near the pyloric end. 

The two largest digestive glands are the 
pancreas and liver (Fig. 214). The pancreas 
lies between the duodenum and the stom- 
ach. It is a much branched tubular gland, 
which secretes an alkaline digestive fluid and 
empties it into the common bile duct. The 
liver is a large, three-lobed, reddish gland, 
which secretes an alkaline digestive fluid 
called bile. This fluid is stored in the gall 
bladder until food enters the intestine, then 
it passes into the duodenum through the 
common bile duct. 

The anterior portion of the small intestine 
is known as the duodenum; this leads to the 
much coiled ileum, which widens abruptly 

Peritoneum (serosa) 
Lamina propria 


Blood vessel 
Submucosa - 

Circular muscle 

Longitudinal muscle- 



;.va:»::||:-.:.- • :. fl 

r-.V?-.-: •..•.•.•::• ■•1:1 

■■ . .-ii-- I: ■■ -.: ■•: • ■ --^ ■•/. 

iJu-..:V :-. ■•■■ ■•■■■■\Sl 


ithelial cell 
Goblet cell 

Blood cells 

Lumen of intestine 

Muscle layer (muscularis) 

Figure 215. Diagram of a small portion of a cross section of the frog intestine showing its 
histology. (After Laboratory Explorations in General Zoology, by Karl A. Stiles. Third edition. 
Copyright 1955 by The Macmillan Company.) 



into the large intestine. The digestive canal, 
the urinary bladder, and the reproductive 
ducts open into an enlarged cavity called the 
cloaca. Waste products and reproductive 
cells pass from the cloaca to the outside 
through the cloacal opening. In the frog 
the cloacal opening is often, but incorrectly, 
called the anus. An anus is the posterior 
opening of only the digestive system. The 
cloacal opening is the common posterior 
aperture through which the products of the 
intestine, kidneys, and reproductive organs 
pass to the outside of the body. The intes- 
tine is held in place by a dorsal fold of the 
peritoneum. The layers of cells that make up 
the intestmal wall (Fig. 215) consist first of 
a ver}' thin outer coat of peritoneum. Be- 
neath this is a layer of longitudinal muscle 
fibers, then a thicker layer of circular muscle 
fibers; next comes a connective tissue layer, 
the submucosa, containing numerous blood 
vessels, separated more or less from the in- 
nermost layer, the mucosa, by a thin layer of 
fibrous connective tissue, the lamina (tu- 
nica ) propria. The mucosal epithelium con- 
sists of two types of cylindrical cells forming 
a single layer: (1) absorptive cells, which 

are narrow, and (2) goblet cells, which 
produce a slippery mucus. The mucosa is 
thrown into many folds but no true villi nor 
definite glands and crypts are present as in 
higher vertebrates. 

Digestion in vertebrates is fully treated in 
Chapter 32. 

Respiratory system 
and respiration 

The primary functions of the respiratory 
system are to provide oxygen to the tissues 
and to get rid of excess carbon dioxide. 

Two kinds of respiration may be recog- 
nized: (1) external respiration, whereby 
oxygen in the air enters the body and is 
transported to the cells, and carbon dioxide 
is carried away from the cells to the outside 
of the body. There are two successive phases 
to this process: (a) breathing, which brings 
oxygen and blood together in the lungs, and 
(b) transportation of oxygen and carbon 
dioxide between the lungs and cells. (2) In- 
ternal respiration, during which the blood 
supplies oxygen to, and takes carbon dioxide 
from the cells of the body. Oxygen in the 

External nares 

Nasal cavity 




Mouth cavity Glottis Lung 

Figure 216. Respiratory movements of the frog. In diagram at left, the external nares are 
open and air enters the mouth cavity. In diagram at right, the external nares are closed, the floor 
of the mouth cavity is raised, and air is forced into the lungs. Labels have been omitted from 
the very short larynx and bronchus. Arrows show pathway of air to lungs. 



lungs unites readily with the hemoglobin in 
the red corpuscles. The hemoglobin com- 
bines with the oxygen to form a compound 
which is then transported by the blood from 
the respiratory organs to the capillaries, 
where it breaks up, the oxygen being ab- 
sorbed by the tissues. Carbon dioxide from 
the tissues is carried to the lungs and dis- 
charged to the outside. 

External respiration is carried on by gills 
in most aquatic vertebrates and by lungs in 
terrestrial vertebrates. Respiration in the 
frog, as just described, is carried on largely 
by the lungs, but takes place also, to a con- 
siderable extent, through the skin. As shown 
in Fig. 216, air passes through the nostrils or 
external nares into the nasal cavity and then 
through the internal or posterior nares into 
the mouth cavity. The external nares are 
then closed, the floor of the mouth is raised, 
and the air is forced through the glottis into 
a short tube, the lar\'nx, then into a very 
short tube, the bronchus, and thence into 
the lungs. Air is expelled from the lungs 
into the mouth cavity by the contraction of 
the muscles of the body wall. In addition to 
the skin and lungs, some gaseous exchange 
takes place through the mucous membrane 
lining the mouth. 

The air in the mouth cavity is changed 


Figure 217. The leopard frog {Rana pipiens) is 
one of the most common amphibians in North 
America. Note the expanded vocal sacs between ear 
and shoulder; these are found only in the male. 
(Courtesy of American Museum of Natural History.) 

by throat movements. The glottis remains 
closed, while the floor of the mouth is al- 
ternately raised and lowered. Air is thus 
drawn in and expelled through the nares. 

The lungs (Fig. 218) are ovoid sacs with 
thin elastic walls. The inner surface of the 
lungs is divided by a network of partitions 
into many minute chambers called alveoli. 
Blood capillaries are numerous in the walls 

Larynx (voice box) /!! 
Glottis ^ 

Pulmonary artery 
Pulmonary vein 

Cartilage in 
wall of glottis 

Ventral view of larynx 

(voice box) opened to 

show vocal cords 

Cross section of lung 
showing inner partitions 

Figure 218. Respiratory organs of the bullfrog. 



of these alveoli, where oxygen diffuses into 
the blood and carbon dioxide is released into 
the lung. 

The larynx (voice box) is strengthened by 
cartilages. Across it are stretched two elastic 
bands, the vocal cords (Fig. 218). The 
croaking of the frog is produced by the vibra- 
tions of the free edges of the vocal cords, 
due to expulsion of air from the lungs. The 
laryngeal muscles regulate the tension of the 
cords, and hence the pitch of the sound. 
Many male frogs have a pair of vocal sacs 
which open into the mouth cavity. (Fig. 
217); they serve as resonators to increase the 
volume of sound. 

The subject of respiration in vertebrates is 
treated more fully in Chapter 32. 

Circulatory system and 
internal transport 

The chief function of the circulatory sys- 
tem is to distribute body fluids to all the 
cells, maintaining the tissue fluid which 
bathes them in about the same state at all 

The circulatory system of the frog consists 
of a heart, arteries, veins, and lymph spaces. 
The liquid portion of the blood, which is 
known as plasma, contains three kinds of 
corpuscles: red corpuscles (erythrocytes), 
white corpuscles (leucocytes), and spindle 
cells (thrombocytes). The blood plasma 
carries food and waste matter in solution. 
It coagulates under certain conditions, form- 
ing a clot of fibrin and corpuscles, and a 
liquid called serum. The power of coagula- 
tion is an adaptation of great importance. 

since the clot soon closes a wound and thus 
prevents bleeding to death. 

The red corpuscles (erythrocytes) are el- 
liptical, flattened cells (Fig. 219) containing 
a respiratory pigment called hemoglobin. 
Hemoglobin combines with oxygen in the 
capillaries of the respiratory organs and gives 
it out to the tissues of the body. The white 
corpuscles (leucocytes) are of several types 
(Fig. 219); they vary in size, and most are 
capable of independent amoeboid move- 
ment. Certain kinds (phagocytes) are of 
great value to the animal since they engulf 
small bodies such as bacteria, frequently pre- 
venting the multiplication of pathogenic or- 
ganisms and helping to overcome infectious 
diseases. White corpuscles also aid in the re- 
moval of broken-down tissue. The throm- 
bocytes are usually spindle-shaped. They are 
unstable; and when brought in contact with 
foreign substances, they break down, releas- 
ing an enzyme, thrombin, which changes 
fibrinogen into the insoluble fibrin so neces- 
sary for blood clotting. Blood corpuscles 
arise principally in the marrow of the bones. 
They also increase in numbers by division 
while in the blood vessels. Some white cor- 
puscles are probably formed in the spleen, a 
gland in which worn-out red corpuscles are 

The heart (Fig. 220) is the central pump- 
ing station of the circulatory system. It is 
composed of a conical, muscular ventricle; 
two thin-walled atria,* one on the right, the 

* Auricles, according to the old tenninology. In 
human anatomy, only the ear-shaped lobe of the 
atrium is called the auricle. The plural of atrium is 

Erythrocyte Lymphocyte Eosinophil Basophil Neutrophil Thrombocyte 

Figure 219. Types of blood cells of the frog. 



Carotid arch- 

•Carotid gtand 
-Aortic arch 

Right anterior vena cava- 
Left anterior vena cava 
'ulmonary veins 

5^— Pulmocutaneous 
arch •>{ 

Entrance of 



Entrance of 
M ^ ^ ■ W sinus venosus 
r . . y /aV:/ V'.:] 1 Spiral valve ,:■ • 
•■ '.\.' A [.' ■ ^.j I Left atrium — Br 
^•/^^•''r' '-v / '^'g^^ atrium—^ 

Sinus venosus 


Semilunar valve 
Posterior vena cava 

Right atrium 

Ventral view dissected 

Dorsal viev/ 

Figure 220. Heart of the frog. The arrows indicate the direction in which the blood flows. 
In ventral view, unlabeled opening is to pulmocutaneous arch. 

other on the left; a thick-walled tube, the 
conus arteriosus, which arises from the base 
of the ventricle; and a thin-walled triangu- 
lar sac, the sinus venosus, on the dorsal side. 

The arteries (Fig. 221) carry blood away 
from the heart. The conus arteriosus divides 
near the anterior border of the atria into two 
vessels as shown in Fig. 220. Each branch is 
called a truncus arteriosus, and each gives 
rise to the following three arteries: 1. The 
common carotid divides into the external 
carotid (lingual), which supplies the tongue 
and neighboring parts, and the internal 
carotid, which gives off the palatine artery 
to the roof of the mouth, the cerebral 
carotid to the brain, and the ophthalmic 
artery to the eye. Where the common 
carotid branches is a swelling called the 
carotid gland; this body serves to equalize 
the blood flow, especially in the internal 
carotid artery. 

2. The pulmocutaneous artery branches, 
forming the pulmonary artery, which passes 

to the lungs and the cutaneous artery. The 
latter gives off the auricularis, which is dis- 
tributed to the lower jaw and neighboring 
parts, the dorsalis, which supplies the skin 
of the back, and the lateralis, which supplies 
the skin of the sides. Most of these branches 
carry blood to the respiratory organs— lungs, 
skin, and mouth. 

3. The aortic (systemic) arches, after 
passing outward and around the digestive 
tract, unite to form the dorsal aorta. Be- 
fore the union of the two aortic arches, 
several branches are given off, two of which 
are: (a) the occipitovertebral artery, which 
gives off branches dorsally that supply the 
backbone and posterior part of the skull; 
(b) the subclavian artery, located just pos- 
terior to the occipitovertebral, arising at 
about the level of the shoulder and extend- 
ing into the forelimbs as the brachial artery. 
The dorsal aorta gives off the coeliacomeseu- 
teric artery; this divides, forming the coeliac, 
which supplies the stomach, pancreas, and 



liver; and the anterior mesenteric, which is 
distributed to the intestine, spleen, and 
cloaca. Posterior to the origin of the 
coeliacomesenteric, the dorsal aorta gives off 
several renal arteries which supply the kid- 
neys. A small posterior mesenteric artery 
arises near the posterior end of the dorsal 
aorta and passes to the large intestine; and 
in the female, to the uterus. The dorsal aorta 
finally divides into two common iliac ar- 
teries, which are distributed to the ventral 
body wall, the rectum, bladder, the anterior 
part of the thigh (femoral artery), and 
other parts of the hindlimbs (sciatic artery). 

The veins (Fig. 221) return blood to the 
heart. The blood from the lungs is collected 
in the pulmonary veins and poured into the 
left atrium. Venous blood is carried to the 
sinus venosus by three large trunks: the two 
anterior venae cavae and the posterior vena 
cava. The anterior vanae cavae receive blood 
from the ( 1 ) external jugulars, which collect 
blood from the tongue, thyroid, and neigh- 
boring parts; (2) the innominates, which 
collect blood from the head by means of 
the internal jugulars and from the shoulder 
by means of the subscapulars; and (3) the 
subclavians, which collect blood from the 
forelimbs by means of the brachial, and 
from the side of the body and head by 
means of the musculocutaneous veins. The 
posterior vena cava receives blood from the 
kidneys by means of 4 to 6 pairs of renal 
veins; from the reproductive organs by 
means of spermatic or ovarian veins; and 
from the liver by means of two hepatic 

The veins which carry blood to the kid- 
neys constitute the renal portal system. The 
renal portal vein receives the blood from the 
hindlimbs by means of the sciatic and 
femoral veins, and from the body wall by 
means of the dorsolumbar vein. While it is 
true that the blood systems of the various 
vertebrates are built on the same general 
plan, there is no renal portal system in 

The liver receives blood from the hepatic 

portal system. The femoral veins from the 
hindlimbs divide, and their ventral branches 
unite to form the ventral abdominal vein. 
The ventral abdominal vein collects blood 
from the bladder, ventral body wall, and 
heart. The hepatic portal vein carries blood 
into the liver from the stomach, intestine, 
spleen, and pancreas. This passage of the 
venous blood from the intestinal tract 
through the liver before entering the main 
circulation makes it possible for the liver 
to add or remove substances from the blood 
as physiologic needs may require. 

Circulation in the frog takes place in the 
following manner: the sinus venosis con- 
tracts, forcing the nonoxygenated venous 
blood into the right atrium (Fig. 220). 
Oxygenated blood from the lungs passes into 
the left atrium. Then both atria contract 
and force their contents into the ventricle. 
Formerly, it was thought that when the 
ventricle contracted, the spiral valve de- 
flected nonoxygenated blood from the right 
side into the pulmocutaneous arch and oxy- 
genated blood from the left side into the 
carotid and aortic arches. But experiments 
have proved that the two blood streams mix. 
Therefore, it must be assumed that mixed 
blood is pumped to all parts of the frog's 
body. The blood is prevented from flowing 
back into the heart by means of valves (Figs. 
220 and 384). Respiration through the skin 
of the frog, both in water and on land, is 
thought to compensate, at least in part, for 
failure of all nonoxygenated blood to be 
pumped to the lungs. 

The blood that is thus forced through 
the arteries makes its way into tubular blood 
vessels that become smaller and smaller until 
the extremely narrow capillaries are reached 
(Fig. 383, p. 529). Here food and oxvgen 
are delivered to the tissues, and waste prod- 
ucts are taken up from the tissues. The 
renal portal system carries blood to the 
kidneys, where urea and similar impurities 
are taken out. The hepatic portal system 
carries blood to the liver, where bile and 
glycogen are formed. The blood brought to 



Internal jugular 

External jugular 



Ant. vena cava 
Conus arteriosus 

Sinus venosus 


Posterior vena cava 
Hepatic porta 


External carotid (lingual) 


Internal carotid 

Carotid gland 








Anterior mesenteric 

Dorsal aorta 
Posterior mesenteric 
Renal portal 

Ventral abdomina 






^' ' ' ^^ ' ' • Splenic 






Figure 221. Circulatory system of the bullfrog in ventral view, showing the larger arteries and 
veins in relation to the internal organs. The arrows indicate the direction in which the blood flows, 



the lungs and skin is oxygenated and then 
carried back to the heart. The passage of 
blood through the capillaries can easily be 
observed in the web of the frog's foot or in 
the tail of the tadpole. 

The lymphatic system of the frog includes 
many lymph vessels of various sizes that 
form networks co-extensive with blood ves- 
sels but are difficult to see. The frogs and 
toads, unlike other vertebrates, have several 
lymph spaces between the skin and the 
body. Four lymph hearts, two near the third 
vertebra and two near the end of the verte- 
bral column, force the lymph by pulsations 
into the internal jugular and a branch of the 
renal portal veins. The watery lymph which 
is colorless contains leucocytes and various 
constituents of blood plasma. 

Relations of the hearts of 
vertebrates to respiration 

In the fish (Fig. 222) the heart consists 
of a single muscular ventricle and a single 
thin-walled atrium. Blood enters the atrium 
from the body and, when the atrium con- 
tracts, passes into the ventricle; it is pre- 
vented by valves from returning to the 
atrium. The ventricle forces it into arteries 
leading to the gills; here it is oxygenated 
and carried directly to the body tissues be- 
fore again returning to the atrium. Valves 
prevent it from flowing back into the ven- 

In Amphibia (Fig. 220) there are two 
atria. Nonoxygenated venous blood from 
the body enters the right atrium and oxy- 
genated blood from the lungs flows into the 
left atrium. Both atria contract and force 
their contents into the single ventricle. The 
ventricle forces the blood through the conus 
and truncus arteriosus, into the carotid, 
aortic (systemic), and pulmocutaneous 

In most reptiles there are two atria, and 
the ventricle is partly divided into two 
chambers (Fig. 222). Nonoxygenated ven- 
ous blood from the body entering the right 
atrium is thus kept more or less separated 

from the oxygenated blood that flows into 
the left ventricle from the lungs. When the 
ventricle contracts, the nonoxygenated blood 
is forced through the pulmonary arteries to 
the lungs, and through the left aorta into 
the dorsal aorta; and the oxygenated blood 
is forced through the right aortic arch which 
merges into the dorsal aorta. Thus the 
dorsal aorta contains a mixture of both non- 
oxygenated and oxygenated blood. 

In birds and mammals (Fig. 222), the 
ventricle is completely separated into two 
chambers, forming a four-chambered heart; 
and thus the nonoxygenated and oxygenated 
blood are kept entirely separate, hence pro- 
viding an efficient pulmonary circulation. 

Excretory system and 
disposal of wastes 

A certain amount of substance resulting 
from the breaking down of living matter is 
excreted by the skin, lungs, liver, and in- 
testinal walls; but most of it is taken from 
the blood in the kidney. From the kidney, 
it passes through the urinary* (mesoneph- 
ric) duct, and then into the cloaca. It 
may be voided at once through the cloacal 
opening or stored in the bladder temporarily 
(Figs. 223 and 224). The kidneys lie dorsal 
to the peritoneum in the subvertebral lymph 
space. They are composed of connective 
tissue containing a large number of urinifer- 
ous tubules, each of which begins in a renal 
corpuscle. The renal corpuscle consists of a 
coiled mass of thin-walled blood vessels, the 
glomerulus, and an enclosing, thin, double- 
walled cup called Bowman's capsule. It acts 
as a selective filter which removes organic 
wastes (especially urea), excess inorganic 
salts, and water from the body. The liquid 
waste collected in the kidney is called urine. 
It is carried by the uriniferous tubules to a 
collecting tubule and thence into the uri- 
nary duct. Ciliated funnels called nephro- 
stomes occur in the ventral portion; these 

* Sometimes incorrectly called a ureter. The true 
ureter is found only in the reptiles, birds, and mam- 



O Nonoxygenated o ^^^ To 

# Oxygenated 



Sinus venosus 




To body 

To body 
To lungs 

From body 






Figure 222. Diagrams showing the comparative structure and evolution of the heart among 
different types of vertebrates; valves omitted. The arrows indicate the direction in which the 
blood flows. 

drain wastes from the coelom. The nephro- 
stomes in tadpoles are connected with the 
uriniferous tubules, but they open into 
branches of the renal vein in the adult. The 
nephrostome mechanism for removal of 
wastes probably represents a stage in the evo- 
lution of the kidney. Urogenital arteries and 
the renal portal vein (Fig. 221) bring blood 
into the kidney. Blood leaves the kidney by 
way of the renal veins. 

Reproductive system 

The function of the reproductive system 
is the maintenance of the species from one 
generation to the next. In the frog the sexes 
are separate. The male can be distinguished 
from the female by the greater thickness of 
the first digit of his forelimbs (Fig. 209). 
The sperms of the male arise in the testes 
(Fig. 224), pass into the kidneys through 



the vasa efferentia, then to the urinary duct 

by a route that differs in different species. 
The urinary duct is dilated at the posterior 
end in some species to form the seminal 

vesicle. The sperms then pass from the uri- 
nary duct or seminal vesicle, as the case may 
be, to the outside through the cloacal open- 



Fat body 

with egg 


Part of oviduct 

Adrenal gland 
Renal vein 



vena cava 

Urinary duct 



oacol opening 


Figure 223. Urogenital system of the female frog in ventral view; left ovary removed. 

The eggs arise in the ovaries of the female 
(Fig. 223); and during the breeding season, 
they break through the ovarian walls into the 
body cavity. There they are moved anteriorly 
by the beating of cilia which cover the 
peritoneum. The cilia at the entrance to the 
oviduct, the ostium, create currents which 
draw the eggs into the convoluted oviduct; 
they are carried down the oviduct into the 
thin-walled distensible uterus by action of 
the cilia in the oviduct. The glandular wall 
of the oviduct secretes the gelatinous coats 
of the eggs. The fertilization and develop- 
ment of the eggs will be described later. 

Just in front of each reproductive organ 
is a yellowish, glove-shaped fat body which 
probably constitutes reserve supplies of food 
that serve the animal during its period of 

Reproductive organs of man 

The reproductive organs of human beings 
(Fig. 225) resemble those of the frog rather 
closely. In the male, the two testes produce 
sperms and internal secretions. Within the 
testes are seminiferous tubules which unite 
to form the epididymis, a much coiled tub- 
ule about 20 feet long. The epididymis leads 
into the sperm duct (vas deferens); this 
duct joins the duct of a seminal vesicle to 
form an ejaculatory duct. The ejaculator}' 
ducts open into the urethra. Near this point 
is situated a gland about the size of a chest- 
nut, the prostate gland. On either side of 
the gland is a body about the size of a pea, 
the bulbourethral ( Cowper's ) gland. Sperms 
are formed in the testes and pass down the 
vasa deferentia. Secretions are supplied by 



Faf body 

Posterior vena cava 
Vasa efferentia 


Urinary duct 



Adrenal gland 
— Renal vein 

— Bladder 

Seminal vesicle 



Figure 224. Urogenital system of the male frog in ventral view. The seminal vesicle is poorly 
developed in the leopard frog. 

the seminal vesicles, Cowper's glands, and 
the prostate gland; these are ultimately 
added to the sperms and constitute the 
seminal fluid (semen). The semen flows 

Urinary Sperm ducf 

through the ejaculatory ducts into the 
urethra and thence out of the body through 
the penis. 

In the female, there are two ovaries 



Urinary 1 \ 
bladder— 1— A 




yy / 11 1^ 





Labium majus 

Figure 225. Diagram of a median section of the human male and female reproductive organs 
showing their relation to the urinary bladder and urethra. 



which produce eggs and internal secretions, 
two oviducts or Fallopian tubes about 4 
inches long, a uterus, a vagina, and the ex- 
ternal genitalia. The ovaries are almond- 
shaped and weigh from 2 to 3V2 grams. They 
contain at birth about 70,000 developing egg 
cells, most of which later disappear. The 
ova discharged from the ovary into the 
coelomic cavity then enter the oviduct and 
are carried to the uterus; here, if an egg is 
fertilized, it remains during embryonic de- 

A general account of reproduction and de- 
velopment is contained in Chapter 34. 

Skeletal system 

The principal functions of the skeleton 
are mechanical support, motion, and protec- 
tion. The internal supporting framework of 





Suprascapula y. 

Radio-ulna ^ 


the frog's body is an endoskeleton consisting 
largely of cartilage and bone. Cartilage 
(gristle) is a tissue in which the cells are 
embedded in a matrix that they secrete; it 
is firm, tough, and elastic (Fig. 43). Bone 
tissue is a connective tissue impregnated 
with mineral salts. About 58 per cent of 
bone is calcium phosphate, 7 per cent cal- 
cium carbonate, and 33 per cent organic 
matter such as cells and gelatinous sub- 
stances. The endoskeleton supports the soft 
parts of the body, furnishes points of at- 
tachment for the muscles, and protects 
some of the delicate organs such as the 
brain, spinal cord, and eyes. 

A study of the skeleton reveals an inter- 
esting story of the relation of structure to 
function. The ridges, lines, depressions, and 
protuberances on a bone all have some func- 
tional significance and are, therefore, mean- 



















Figure 226. Skeleton of the frog. 



ingful to the student of gross anatomy. 

There are two main subdivisions of the 
skeleton: (1) the axial and (2) the appen- 
dicular skeleton. 

The axial skeleton comprises the skull, 
vertebral column, and sternum.* The ap- 
pendicular skeleton consists of the pectoral 

and pelvic girdles, and the bones of the 
limbs which they support. 

The cartilage and bones of the skull may 
be grouped into two main divisions: (1) the 
brain case, and auditory and olfactory cap- 
sules, which constitute the cranium; and 
(2) the visceral skeleton (Fig. 228). 

Maxilla — 



Vomerine teeth 
Palatine - 







J Columella i,v^ 

^Quadratojugal -^ ■ 


Quadrate cartilage 
Occipital condyle 
Foramen magnum 

Figure 227. Skull of the bullfrog in dorsal and ventral views. 



A large part of the cranium consists of 
cartilage. The bones are either ossifications 
of the cartilage, the cartilage bones, or have 
developed into connective tissue without 
passing through a cartilage stage, the mem- 
brane bones. The spinal cord passes through 
a large opening, the foramen magnum, in 
the posterior end of the cranium. On either 
side of this opening is a convexity of the 
exoccipital bones called the occipital con- 
dyle, Vk'hich articulates in life in a concavity 
of the first vertebra and enables the frog 
to move its head. 

The cranial bones of the dorsal side are 
the prootics, which enclose the inner ears; 
the frontoparietals, which form most of the 

* The sternum may be regarded as a part of the 
appendicular skeleton because it supposedly orig- 
inated in the early history of the land vertebrates 
from the pectoral girdle. However, it may be con- 
sidered a part of the axial skeleton because of its 
medial (central) location. 

roof of the cranium; the sphenethmoid, 
which forms the posterior wall of the nasal 
cavity; and the nasals, which lie above the 
nasal capsules. The ventral surface of the 
cranium discloses the central, dagger-shaped 
parasphenoid and the vomers, which bear 
the vomerine teeth (Fig. 227). 

Visceral skeleton 

The jaws, hyoid, and cartilages of the 
larynx, which constitute the visceral skele- 
ton, are preformed in cartilage and then 
strengthened by ossifications. The upper 
jaw (maxilla) consists of a pair of premaxil- 
lae, a pair of maxillae, and a pair of quad- 
ratojugals. The maxillae and premaxillac 
bear teeth. The lower jaw (mandible) con- 
sists of a pair of angulosplenials and a pair 
of dentary bones which overlap the angulo- 
splenials and extend forward to meet a pair 
of mentomecklian bones. The visceral arches 
are represented in the adult by the hyoid 
and its processes (Fig. 228 ) . 














Hyoid (ventral view) 
Figure 228. Lateral view of skull and hyoid of bullfrog. 

Vertebral column 

The vertebral column (Fig. 226) or back- 
bone consists of 9 vertebrae and a bladelike 
posterior extension, the urostyle. Each verte- 
bra consists of a basal centrum, which is 
concave in front and convex behind, and a 
neural arch through which the spinal cord 

passes. The neural arch possesses a short 
neural spine, a transverse process on each 
side (except on the first vertebra), and a pair 
of articular processes called zygapophyses at 
each end. The vertebrae are held together 
by ligaments and move on one another by 
means of the centra and zygapophyses. The 



y^ ^ Omosternum 
-' — Scapula 

Glenoid fossa 








Piectorot girdle and sternum (ventral viev/j 

Figure 229. Girdles of the frog. 

Pelvic girdle (side view) 

vertebral column thus serves as a firm axial 
support which also allows bending of the 
body. A frog has no ribs. 

* Although in this text the sternum is regarded 
as a part of the axial skeleton, it is described with 
the pectoral girdle because of the functional relation- 
ship of the two structures. 

Appendicular skeleton 

The pectoral girdle and sternum * (Fig. 
229) support the forclimbs, serve as attach- 
ments for the muscles that move the fore- 
limbs, and protect the organs lying within 
the anterior portion of the trunk. They are 
composed partly of bone and partly of 



cartilage. The suprascapulae lie above the 
vertebral column, and the rest of the girdle 
passes downward on either side and unites 
with the sternum in the ventral middle line. 
The principal parts of the pectoral girdle 
are: the suprascapulae, scapulae, clavicles, 
coracoids, and epicoracoids; of the sternum: 
the episternum, omosternum, mesosternum, 
and xiphisternum. The end of the long bone 
of the forelimb ( humerus ) lies in a concav- 
ity in the scapula and coracoid called the 
glenoid fossa. 

The pelvic girdle supports the hindlimbs. 
It consists of two sets of three parts each: 
the ilium, the ischium, and the pubis. The 
pubis is cartilaginous. The anterior end of 
each ilium is attached to one of the trans- 
verse processes of the ninth vertebra. Where 
the parts of each half of the pelvic girdle 
unite, there is a concavity called the 
acetabulum, in which the head of the long 
leg bone (femur) lies. 

The forelimb consists of a humerus which 
articulates with the glenoid fossa of the 

pectoral girdle at its proximal end and with 
the radioulna at its distal end. The bone of 
the forearm (radioulna) consists of the 
radius and ulna fused. The wrist contains 6 
bones arranged in two rows, each consisting 
of three small bones (carpals). The palm of 
the hand is supported by 4 proximal meta- 
carpal bones, followed by digits 2 and 3 
which consist of 2 phalanges each, and by 
digits 4 and 5 which consist of 3 phalanges 
each, and the rudimentary thumb is repre- 
sented by the first metacarpal. 

The hindlimb consists of (1) a femur 
(thighbone), (2) a tibiofibula (the tibia 
and fibula fused) or lower leg bone, (3) 
four tarsal bones, the astragalus, the calcan- 
eum, and 2 smaller bones, (4) the 5 
metatarsals of the sole of the foot, (5) the 
phalanges of the digits, and (6) the prehal- 
lux ( calcar ) of the accessory digit. The two 
pairs of limbs differ in size but have similar 
component bones. This is easily seen in the 
accompanying table. 





Humerus (upper arm) 


Femur (thighbone) 


Radioulna (forearm) 

2 fused 

Tibiofibula (lower leg bone or 


2 fused 

Carpals (wrist) 


Tarsals (ankle) 


Metacarpals (palm of hand) 


Metatarsals (sole of foot) 


Phalanges (fingers) 


Phalanges (toes) 

14 + prehallux 

The skeletons of animals in general and 
of man in particular are described in Chap- 
ter 31. 

Muscular system, motion, 
and locomotion 

The main function of the muscular sys- 
tem is to cause movement by contraction. 
Muscles (Figs. 230 and 231) are of three 
principal types— smooth, cardiac, and stri- 
ated, which is of a skeletal type; these dif- 
fer in their microscopic structure and func- 
tion. Smooth muscle is involuntary muscle. 
Cardiac muscle is "involuntary" striated 
muscle that occurs in the vertebrate heart 

only. Striated muscle of the skeletal type- 
that is, the muscles of the external muscular 
system — is voluntary muscle. 

Muscle fibers may be 4 or 5 cm. long and, 
when bound together by connective tissue, 
form the larger bundles known as muscle. 
Skeletal muscles are usually attached by one 
or both ends to bones either directly or by 
means of a tendon, which is an inelastic 
band of connective tissue (Fig. 43). The 
two ends of a muscle are designated by dif- 
ferent terms: the origin is the end attached 
to a relatively immovable part; the insertion 
is the movable end. Usually the origin is near 
the spinal axis of the body, while the inser- 
tion is peripheral. A muscle which bends one 

Maxilla (bone) 

Tympanic ring 

Latissimus dorsi 
Triceps brachii 
(medial head) 


Depressor mandibularis 

Dorsalis scapulae 

Longissimus dorsi 

External oblique 
Cutaneous abdominis 

Rectus anticus femoris 

Vastus externus 
Fascia lata 

Semimembranosus /////' * 

Ilium (bone) 


Rectus anticus femoris 
Vastus externus 
Vastus internus^ 
Adductor longus 
Adductor magnus 
Gracilis major 

Tibialis anticus 



Head muscles 
Figure 230. The muscles of the bullfrog, dorsal view. 


Maxilla (bone) 
Dentary (bone) 


Depressor mandibulcris 

Triceps brachii 
(lateral head) 

Triceps (medial head) 
External oblique 


Vastus internus ////// /xy 

Adductor mag n us /////////yC 
Adductor maanus II i /''///^■^ ^ ' 

Gracilis minor 

Tibio-fibulo (bone 

Gracilis major 
Gracilis minor 

Tendon of Achilles 
Astragalus (bone) 

Figure 231. The muscles of the bullfrog, ventral view. 




part upon another, as the leg upon the 
thigh, is a flexor; one that straightens out 
a part, as the extending of the foot, is an 
extensor; one that draws a part toward the 
midhne of the body is an adductor; one that 
moves a part away from the midhne of the 
body is an abductor; one that lowers a part 
is a depressor; one that raises a part is a 
levator; and one that rotates one part on 

another is a rotator. The movements of an 
organ depend on the origin and insertion of 
the muscles and the nature of the articula- 
tions of its bones with each other and with 
other parts of the body (Fig. 232). 

The muscles of the hindlimb of the frog 
are usually selected for study to illustrate the 
methods of action of muscles in general. 
The accompanying table gives the name, 


of biceps muscle 

Extension of arm 

by contraction of 

triceps muscle 

Figure 232. Movements produced by muscles. Contraction is the only action produced by a 
muscle. Note the two opposing muscles in the arm and bones to which they are attached. 

origin, insertion, and action of some of the 
muscles of the hindlimb. Figure 230 shows 
that the triceps femoris arises from three 
distinct heads; namely, vastus internus, rec- 
tus anticus femoris, and vastus externus. 

The following are a few of the muscles of 
the other parts of the body: the rectus ab- 
dominis extends longitudinally along the 
ventral side of the trunk; the external 

oblique covers most of the sides of the 
trunk; the transverse lies beneath the ex- 
ternal oblique and serves to contract the 
body cavity; the pectoralis major moves the 
forelimbs; and the mylohyoid raises the 
floor of the mouth cavity during respiration. 
The muscles of animals in general and 
of man in particular are described in Chap- 
ter 31. 









Ilium, just in front of 

Just below head of tibia 

Flexes leg; and adducts 

Adductor magnus 

Ischium and pubis 

Distal end of femur 

Adducts thigh and leg 

Adductor longus 

Ventral part of ilium 

Joins adductor magnus to 
attach on femur 

Adducts thigh and leg 

Triceps femoris 

From three heads, one 
acetabulum, and two 
heads from ilium 

Upper end of tibiofibula 

Abducts thigh and extends 

Gracilis major 

Posterior margin of 

Proximal end of tibio- 

Adducts thigh; flexes leg 

Gracilis minor 

Tendon behind ischium 

Joins tendon of gracilis 
major and tibiofibula 

Adducts thigh; flexes leg 


Dorsal half of ischium 

Proximal end of tibiofibula 

Adducts thigh; flexes leg 

Biceps (iliofibularis) 

Dorsal side of ilium 


Adducts thigh; flexes leg 


Distal end of femur; 
tendon of triceps 

Tendon of Achilles 

Flexes leg, extends foot 

Tibialis posticus 

Posterior side of tibio- 

Proximal end of astragalus 

When foot is flexed, acts 
as an extensor 

Tibialis anticus longus 

Distal end of femur 

Proximal end of astragalus 
and calcaneus (ankle 

Extends leg; flexes foot 


Distal end of femur 

Distal end of tibiofibula; 
head of calcaneus 

Extends leg 

Extensor cruris 

Distal end of femur 

Anterior surface of tibio- 

Extends leg 

Nervous system 

The principal function of the nervous 
system is coordination of parts of the body 
that are widely separated. 

The nervous svstem of vertebrates is more 
complex than that of any other animals. 
It is composed of two main subdivisions: 
(1) a central nervous system consisting of 
the brain and spinal cord and (2) a 
peripheral ner\'ous system includes the cere- 
bral and spinal nerves together with the 
automatic system. The brain is protected by 
the cranium of the skull. The brain consists 
of 6 main parts when viewed dorsally (Fig. 
234): (1) two olfactory lobes with nerves 

to the nostrils; (2) two cerebral hemis- 
pheres; (3) diencephalon (between-brain) 
which has a dorsal pineal body (epiphysis); 
(4) two optic lobes; (5) cerebellum, a nar- 
row transverse portion of the brain; and (6) 
medulla (medulla oblongata) which joins 
the spinal cord. 

On the ventral side of the diencephalon 
the optic nerves cross each other to form the 
optic chiasma. Just posterior to the optic 
chiasma is the infundibulum, a large median 
projection. The pituitary gland (hypophy- 
sis) is the flattened ventral end of the in- 

The cavities within the brain are shown 
in Fig. 234. The cavities of both the brain 

Spinal nerves 

Vagus X 

Auditory VIII 
Facial VII 

Trigeminal V 1 

Oculomotor III 
Nasal sac 

Optic II 

Trochlear IV 
Abducens VI — ' 
Gasserian ganglion 

Tympanum — 
Glossopharyngeal IX-* 
Brachial enlargement — ' 
Autonomic ganglion- 
Brachial nerve 

Lumbar enlargement 
' — Autonomic nerve trunk 

Figure 233. Nervous system of the bullfrog, dorsal view. 

Olfactory nerve I 

Olfactory lobe 


Optic II — 
Optic chiasma 
Pineal body 

Optic lobe 

Trochlear IV 
■Trigeminal V 
Facial Vll 
Auditory VllI 

Vagus X 
Glossopharyngeal IX 
Spinal nenre I 
Nerve (spinal) cord 

y^ oJi 

Olfactory lobe 


First ventricle 
Second ventricle 

Foramen of 

Third ventrici 

. y Infundibulum 
'^ — Pituitary body 

Cerebral aqued 

(aqueduct of 


Abducens VI 

Fourth ventricle 

Centra! canal 

Figure 234. 

dorsal view. 

Brain of the frog. A, dorsal view. B, ventral view. C, ventricles (cavities) in 




and spinal cord are filled with a fluid ap- 
propriately called the cerebrospinal fluid. 
Although this fluid looks somewhat like 
lymph, the two are not identical in com- 

The peripheral nervous system (Fig. 233) 
includes 11 pairs of cranial nerves in the 
frog and a number of pairs of spinal nerves. 
Older textbooks give only 10 pairs of cranial 
nerves, but the discovery of the terminal 
nerve (nervus terminalis) makes it neces- 
sary to revise the former figure. The terminal 
nerve is found in many vertebrates, repre- 
senting every class except the Agnatha and 

Gray matter 
White matter 
Dorsal root 
Ventral root 

birds. Little is known about it, but it appears 
to be sensory in function. The origin, dis- 
tribution, and function of the cranial nerves 
are indicated in the accompanying table. 

The spinal cord is a thick tube connected 
directly with the brain; it passes through 
the neural arches of the vertebral column. 
The spinal nerves arise from the spinal cord 
in pairs, one on either side in each body 
segment, and pass out between the verte- 
brae. Each nerve has two roots (Fig. 235), 
a dorsal root (sensory), and a ventral loot 
(motor). The dorsal root possesses a gan- 
glion containing nerve cells. Its fibers carry 


Central cana 

Sensory nerve fiber ending 
in skin (receptor) 

Motor nerve ending 
in muscle (effector) 

Figure 235. Paths of sensory and motor nerve fibers. Arrows indicate direction of nerve im- 
pulses. A reflex arc from sensory nerve ending by way of the spinal cord to muscles is shown at 
lower right; also connections to and from the brain. 

impulses toward the spinal cord from var- 
ious parts of the body and are therefore 
sensory. The fibers of the ventral root carr\' 
impulses from the spinal cord to the tissues 
and are therefore motor. The structure of 
the nerve cells (neurons, Fig. 236) is similar 
to that of the earthworm. The direction of 
the nervous impulses is indicated by arrows 
in Fig. 235. 

On each side of the spinal cord is a chain 
of ganglia which is connected at various 
places with the central nervous system. This 

is known as the autonomic nervous system, 

once called the sympathetic nervous system, 
but now that term is reser\'ed for one sub- 
division of the autonomic system. These 
ganglia send nerves chiefly to the digestive 
tract, circulatory system, and glandular or 

Visceral and other activities of which we 
are not ordinarily conscious are regulated 
and controlled by the autonomic system; 
these include movements of the stomach 
and intestine, glandular activities, and the 






;r i V\'f'M i ' i ' ii'i| i. i.ii.ii ^^ 'i'i » i .' i 'a7l i< : i i' i M'ir '| i'ii m.X) 

Cell body 

Dotted lines indicate great length 
Myelin Node of Ranvier 

■'-««|OUJL '".III 1^1,11 I' PlI'MilL"". ,".1 III 

Terminal branches of axon" 

Figure 236. A motor neuron from the spinal cord of man. 

beating of the heart. The autonomic nerv- 
ous system is composed of two main subdi- 
visions, termed ( 1 ) parasympathetic ( crani- 
osacral) and (2) sympathetic (thoracolum- 
bar), but the details of these divisions are 
beyond the scope of this book. 

It will be sufficient in this place to point 
out certain selected points concerning the 
nervous system of the frog, since general ac- 
counts of nervous tissue, nervous activity, 
and the nervous system of vertebrates are 
presented elsewhere. 









Forebrain * 

Lining of nose 

Probably sensory 



Olfactory lobe 

Lining of nasal cavities 





Retina of eye 




Ventral side of midbrain 

Four muscles of eye 




Dorsal side of midbrain 

Superior oblique muscle 
of eye 




Side of medulla 

Skin of face, mouth, and 
tongue, and muscles of 

Sensory and motor, 
mostly sensory 



Ventral side of medulla 

External rectus muscle 
of eye 




Side of medulla 

Chiefly to muscles of face 

Motor and sensory, 
mostly motor 



Side of medulla 

Inner ear 




Side of medulla 

Muscles and membranes 
of pharynx, and tongue 

Sensory and motor 



Side of medulla 

Larynx, lungs, heart, 
esophagus, stomach, 
and intestines 

Sensory and motor 


Spinal accessory 
(not present in 

Side of medulla 

Chiefly muscles of 

Sensorv' and motor 


Hypoglossal (not 
present in frog) 

Ventral side of medulla 

Muscles of tongue and 


According to Hcrrick. it originates from both telencephalon and diencephalon in most species. 




The brain (Fig. 234) has two large olfac- 
tory lobes which are fused together, two 
large cerebral hemispheres, diencephalon, 
two large optic lobes, a very small cerebel- 
lum, and a medulla (medulla oblongata), 
which is produced by the broadening of the 
spinal cord. 

The functions of the different parts of the 
frog's brain have been partially determined 
by experiments in which the parts were re- 
moved and the effects upon the animals 
observed. There is evidence that the cerebral 
hemispheres are involved in associate mem- 
ory, but the frog has a very low "l.Q." The 
cerebrum is the seat of intelligence and vol- 
untary control in higher animals. When the 
diencephalon is removed with the cerebral 
hemispheres, the frog loses the power of 
spontaneous movement. When the optic 
lobes are removed, the spinal cord becomes 
more irritable; this shows that these lobes 
have an inhibiting influence on the reflex 
activity of the spinal cord. In man the 
cerebellum is a center of coordination, but 
experiments on the frog cerebellum have 
produced conflicting results. Many activities 
are still possible when everything but the 
medulla is removed. The animal breathes 
normally, snaps at and swallows food, leaps 
and swims regularly, and is able to right 
itself when thrown on its back. Extirpation 
of the posterior region of the medulla re- 
sults in early death of the frog. The brain 
as a whole controls the actions effected by 
the nerve centers of the spinal cord. "The 
higher centers of the brain are comparable 
to the captain of a steamer, who issues or- 
ders to the man running the engine, when 
to start and when to stop, and who has his 
hand on the wheel so as to guide the course 
of the vessel" (Holmes). 

Spinal cord 

The spinal cord (Fig. 233) extends back- 
ward from the medulla and ends in the 
urostyle. It is surrounded by two mem- 
branes, an outer dura mater and an inner 
pia mater. The cord is composed of a cen- 

tral mass of gray matter (Fig. 235) consist- 
ing mainly of nerve cells, and an outer mass 
of white matter made up chiefly of nerve 
fibers. A median fissure occurs both in the 
dorsal and in the ventral side of the cord, 
and a central canal lies in the gray matter 
and communicates anteriorly with the cavi- 
ties of the brain. 

Spinal nerves 

The relation of the spinal nerves to the 
spinal cord and the paths taken by nervous 
impulses are indicated in Fig. 235. There are 
10 pairs of spinal nerves in the frog (Fig. 
233). Each arises by a dorsal and a ventral 
root, which arise from the gray matter of 
the cord. The two roots unite to form a 
trunk, which passes out between the arches 
of adjacent vertebrae. The two largest 
nerves are ( 1 ) the brachials, each of which 
is composed of the second pair of spinal 
nerves and branches from the first and third 
pairs— these are distributed to the forelimbs 
and shoulders; and (2) the sciatics, which 
arise from plexuses composed of the seventh, 
eighth, and ninth spinal nerves, and are dis- 
tributed to the hindlimbs. 

Autonomic system 

This system (Fig. 233) consists of two 
principal trunks, which begin at the cranium 
and extend posteriorly, one on each side of 
the vertebral column. Each trunk is pro- 
vided with 10 ganglionic enlargements at 
the points where branches from the spinal 
ner\'es unite with it. The nerves of the auto- 
nomic system are distributed to internal 
organs and regulate many functions that are 
not under the control of conscious or volun- 
tary action, such as heart beat, secretions, 
movements of the digestive tract, and res- 
piratory, urogenital, reproductive systems, 
and others. 

Sense organs 

The principal sense organs are the eyes, 
ears, and olfactory organs. There are many 
smaller structures on the surface of the 



tongue, and on the floor and roof of the 
mouth, which probably function as organs 
of taste. In the skin are also many sensory 
nerve endings which receive contact, chem- 
ical, temperature, and light stimuli. 

Olfactory organs 

The olfactory nerves (Fig. 234) extend 
from the olfactory lobe of the brain to the 
nasal cavities, where they are distributed to 
the epithelial lining. The importance of the 
sense of smell in the life of the frog is not 


The inner ear of the frog lies within the 
auditory capsule and is protected by the 
prootic (Fig. 227) and exoccipital bones. It 
is supplied by branches of the auditory 
nerve. There is no external ear in the frog. 
The middle ear is a cavity which communi- 
cates with the mouth cavity through the 
Eustachian tube and is closed externally by 
the eardrum (tympanic membrane). 

A rod, the columella, extends across the 
cavity of the middle ear from the eardrum to 
the inner ear. The vibrations of the eardrum 
produced by sound waves are transmitted to 
the inner ear through the columella. The 
sensory end organs of the auditory nerve are 
stimulated by the vibrations, and the im- 
pulses carried to the brain give rise to the 
sensation of sound. The inner ears serve also 
as organs of equilibrium. Frogs, from which 
they are removed, cannot maintain an up- 
right position. 


The eyes of the frog resemble those of 
man in general structure and function, but 
differ in certain details. The eyeballs lie in 
cavities (orbits) in the sides of the head. 
They may be rotated by 6 muscles and also 
pulled into the orbit. The upper eyelid does 
not move independently. The lower eyelid 
consists of the lower eyelid proper, fused 
with the transparent third eyelid or nictitat- 
ing membrane. The lens is large and almost 
spherical. It cannot be changed in form or 

in position, and is therefore fitted for view- 
ing objects distinctly at a certain definite 
distance. Movements are noted much 
oftener than form. The amount of light 
that enters the eye can be regulated by the 
contraction of the pupil. The retina of the 
eye is stimulated by the rays of light which 
pass through the pupil; and the impulses, 
which are carried through the optic nerve 
to the brain, give rise to sensations of sight. 

Endocrine glands 

The frog body, like that of other verte- 
brates, is influenced tremendously by hor- 
mones which are produced by the endocrine 
(ductless) glands. These internal secretions 
pass directly into the blood. Some of the 
endocrines influence other glands, the rate 
of growth, even the whole organism, with 
its behavior and physiologic and structural 
characteristics. The endocrine glands of 
vertebrates are discussed in Chapter 33. 


The activities of the frog are such as to 
enable it to exist within the confines of its 
habitat. The ordinary movements are those 
employed in leaping, diving, crawling, bur- 
rowing, and maintaining an upright posi- 
tion. These and most of its other activities 
may be resolved into a series of inborn 
reflexes; they are commonly said to be 
"instinctive." Inborn reflexes result in the 
faculty to act in such a way as to produce 
certain ends, without foresight of the ends, 
and without previous education in the per- 

Some of the movements of the frog are 
due to internal causes, but many are re- 
sponses to external stimuli. Frogs are sensi- 
tive to light, and recent experiments have 
shown that skin responses to light come by 
the way of the eyes and pituitary gland. 
The reaction to light causes the animal to 
orient its body so that it faces the source 
and is in line with the direction of the rays. 
Nevertheless, frogs tend to congregate in 



shady places. Frogs also seem to be stimu- 
lated by contact as shown by their tendency 
to crawl under stones and into crevices. 
Other factors probably have some influence 
upon this reaction. Temperature and other 
stimuli modify the responses both to light 
and to contact. 

Investigators who have studied the be- 
havior of frogs have come to the conclusion 
that they are very stupid animals, but it is 
possible to teach them certain things, and 
habits once formed are not easily changed. 
For example, Yerkes found that a frog could 
learn to follow a path in a labyrinth after 
about 100 trials. If we consider the power 
to learn by individual experience as evi- 
dence of intelligence, we must attribute a 
primitive sort of intelligence to the frog. 

Life cycle of the frog 

Frogs lay their eggs in water in the early 
spring. The male clasps the female firmly 
with his forelegs just behind her forelegs. 
This is one of the strongest seasonal inborn 
reflexes (instincts) of the male frog; he will 
even clasp one's finger when caught during 
this time of the year. After the male has 
been carried about by the female for several 
days, the eggs pass from the uterus out of 
the cloaca. As the eggs are extruded by the 
female, they are fertilized by the sperms 
which the male discharges over them. The 
male then releases his grip on the female 
and leaves her. Each female lays several 
hundred or more eggs, but some of these 
fail to develop, and others are eaten by ene- 

The jelly which surrounds and protects 
the eggs soon swells up through absorption 
of water. The frog egg is well adapted to 
hatch in the cold pond water of early spring. 
The upper black surface of the egg absorbs 
the sun's heat like a black coat. The trans- 
parent jelly covering holds heat like the glass 
of a greenhouse. 

Cleavage takes place as indicated in Fig. 
237. Some of the cells, called macromeres, 
are large because of a bountiful supply of 

yolk; others, the micromeres, are smaller. A 
blastula is formed by the development of 
a cavity, the blastocoel, near the center of 
the egg. Gastrulation is modified in the frog's 
egg because of the amount of yolk present. 
The dark side of the egg gradually grows 
over the lighter portion untfl only a circu- 
lar area called the yolk plug is visible. The 
gastrula contains two germ layers, an outer 
ectoderm and an inner endoderm. A third 
layer, the mesoderm, soon appears between 
the other two and splits into two, an inner 
splanchnic layer, which forms the support- 
ing tissue and musculature of the digestive 
canal, and an outer parietal layer, which 
forms the connective tissue, muscle, and 
peritoneum of the body wall. The cavity be- 
tween these two mesodermal layers is the 

Soon after gastrulation, the neural groove 
appears, on either side of which is a neural 
fold. The neural folds grow together at the 
top, forming a tube which later develops 
into the brain and spinal cord of the em- 
bryo. The neural groove lies along the me- 
dian dorsal line, and the embr\'0 now length- 
ens in this direction. The region where the 
yolk plug was situated lies at the posterior 
end. On either side near the anterior end 
two gill arches appear; and in front of each 
of these a depression arises which unites 
with its fellow and moves to the ventral 
surface, becoming the ventral sucker. An 
invagination soon appears just above the 
ventral sucker; this is an oval pit which de- 
velops into the mouth. 

The invagination, which becomes the 
cloacal opening (anus) appears beneath the 
tail at the posterior end. On either side 
above the mouth, a thickening of the ecto- 
derm represents the beginning of the eye, 
and just above the gills appear the invagina- 
tions which form the vesicles of the inner 
ears. The markings of the muscle segments 
show through the skin along the sides of 
the body and tail. 

The embryo moves about within the egg 
membranes partly by means of cilia, but 
these soon disappear after hatching (Fig. 





Animal pole 

2-cell stage 


Older embryo in section 

Gill arches 

"Anal" pit 

Older embryo with neural groove closed 

I ^f^^iT'^^C^''^?:: Neural fold 

-Neural groove 

Primitive streak 

- .. Endoderm 

■ ■ :^. Blastocoe 
— ' ' - Gastrocoel y^ 

Vegetal pole 

4-cell stage 

8-cell stage 

in section 


Early gastrula 
Figure 237. Early development of the frog. (After Huettner.) 

Early gastrula 
in section 




Older tadpole (side view) 

Older tadpole (side view) 


Newly hatched 
tadpole (side view) 

Older tadpole (ventral 
view dissected to show 
intestine and gills) 


Frog after 

Stage in metamorphosis 
Figure 238. Life cycle of the frog from egg to adult. 

Resorption of gills 
Development of lungS 

Advanced tadpole juSt 
before metamorphosis 

238). The tadpole breaks out of the mem- 
branes, hves for a few days on the yolk in 
the digestive tract, and then feeds on algae 
and other vegetable matter. The external 
gills grow out into long branching tufts. A 
skin fold or operculum grows over the ex- 
ternal gills, which then degenerate, and are 
replaced by internal gills; water enters the 
mouth, passes through the gill slits, and 

out of an opening on the left side of the 
body, the spiracle. 

The hindlimbs appear first; later the fore- 
limbs break out. The tail decreases in size 
because it is gradually reabsorbed as the end 
of the larval period approaches. The gills 
are reabsorbed, and the lungs develop to 
take their place. Finally the form resembling 
that of the adult frog is acquired (Fig. 238). 




(See Chapter 27 for additiojial collateral 

Carroll, P.L., and Horner, W.F. An Atlas of 
the Frog. Mosby, St. Louis, 1940. 

Gaupp, E.A. Ecker und R. Wiedersheim's 
Anatomie des Frosches. Vieweg, Brunswick, 
Germany, 1896-1904. 

Holmes, S.J. The Biology of the Frog. Macmil- 

lan, New York, 1927. 
Marshall, A.M. The Frog: An Introduction to 

Anatomy, Histology and Embryology. Mac- 

millan, London, 1928. 
Pope, C.H. Amphibians and Reptiles of the 

Chicago Area. Chicago Natural History 

Museum, Chicago, 1947. 
Stuart, Richard R. The Anatomy of the Bull 

Frog. Denoyer-Geppert, Chicago, 1940. 


Class Agnatlia 

(lawless Vertebrates). 

Lampreys and 


HE animals in the order Cyclostomata 
(Fig. 240) represent a primitive level of 
vertebrate development. The name means 
round mouth. These eel-shaped vertebrates, 
without jaws or paired fins, and with only 
one olfactory pit, are commonly known as 
lampreys, hagfishes, and slime eels. 

The suborder M^'xinoidea, or hagfishes, 
are all marine; and the suborder Petromy- 
zontia, or lampreys, occur in both salt water 
and fresh water. The cyclostomes usually 
feed on the blood and tissue fluid of fishes 
which they attack with their rasping mouth 
(Fig. 239). The cyclostomes are not only 
interesting chordates, but some are of great 
economic importance. 


Petromyzon marinuSy the sea lamprey 
(Fig. 240), a rather unpleasant animal, is a 
modified survivor of some of the first verte- 
brates that lived on the earth. It inhabits 
the waters along the Atlantic Coast of North 
America, Great Lakes, the coasts of Europe, 
and the west coast of Africa. It swims about 
near the bottom by undulations of its body, 
or, when in a strong current, progresses by 
darting suddenly forward and attaching it- 
self to a rock by means of its suctorial 
mouth. In the spring adult lampreys ascend 
the rivers to spawn. 

External anatomy 

The marine lamprey reaches a length of 
about three feet. Land-locked populations 
such as those in the Great Lakes attain a 
maximum size of only two feet. The body 
of the lamprey is nearly cylindrical, except 
at the posterior end where it is laterally 
compressed. There is no exoskeleton. The 
skin is soft and slimy, made so by secretions 
from epidermal glands. It is a mottled 
greenish-brown in color. A row of segmental 
sense pits, the lateral line, is located on 
each side of the bodv and on the head. The 




Figure 239. A lake trout showing a typical lamprey scar; arrow points to scar. This fish has 
been the backbone of the Great Lakes fishing industry. (Courtesy of U.S. Fish and Wildlife 
Service. ) 

mouth (Fig. 241) lies at the bottom of a 
suctorial disk, the oral (buccal) funnel, and 
is held open by a ring of cartilage. Around 
the oral funnel are a number of papillae 

and horny "teeth." At the apex of the oral 
funnel is the mouth, through which the 
pistonlike tongue protrudes; it bears horny 
teeth. On each side of the head is an eye, 

Figure 240. Female sea lamprey; a jawless vertebrate, 20% inches long, showing the charac- 
teristically mottled back of a sexually mature adult. Note the laterally placed eye, behind 
which are the gill slits. The sea lamprey is an eel-like parasite that preys on fish. (Courtesy of 
Institute for Fisheries Research, Michigan Department of Conservation.) 

and posterior to the eye, 7 gill slits. Be- 
tween the eyes on the dorsal surface is a 
single opening, the nasal opening (Fig. 
242). The anus opens on the ventral sur- 
face near the posterior end; just behind it is 
the urogenital opening in the end of a small 
papilla. There are two dorsal fins and one 
caudal fin (Fig. 240). 

Skeletal system 

The notochord of Petromyzon persists as 
a well-developed structure in the adult (Fig. 
242). In the trunk region it is supplemented 

by small cartilaginous neural arches. Cartila- 
ginous rays hold the fins upright. The or- 
gans in the head are supported by a car- 
tilaginous cranium and a cartilaginous 
branchial basket. 

Muscular system 

The muscles in the walls of the trunk and 
tail are segmental, in a ^ -shaped arrange- 
ment, similar to the fishes. The tongue is 
moved by large retractor and smaller pro- 
tractor muscles. The buccal funnel is oper- 
ated by a number of radiating muscles. 



Digestive system 

The adult Petromyzon lives chiefly on the 
blood of fishes. The expansion of the oral 
funnel (Fig. 241) causes the mouth to act 
like a sucker and enables the animal to cling 
to stones or to fasten itself to fishes such as 
shad, sturgeon, cod, and mackerel in the 
ocean, and lake trout, whitefish, yellow pike- 
perch, and carp in the Great Lakes. With its 
rasplike tongue, it files a hole through the 
scales and flesh of its victim and sucks out 
the blood. 

Figure 241. The head of a sea lamprey showing 
the oral funnel which serves as a suction cup by 
which it attaches itself to its prey. It is by means of 
the sharply pointed horny teeth inside the oral fun- 
nel and the rasplike tongue that it can penetrate 
through the scales and flesh of its victim. Arrow 
points to pistonlike tongue. (Courtesy of Institute 
for Fisheries Research, Michigan Department of 

The mouth cavity opens at its posterior 
end into two tubes (Fig. 242), an upper 
one, the esophagus, and the ventral one, the 
pharynx. A fold, the velum, at the anterior 

end of the pharynx prevents the passage of 
food into the respiratory system. 

There is no distinct stomach. The 
posterior end of the esophagus is separated 
from the straight intestine only by a valve. 
A fold in the intestine called the typhlosole 
forms a sort of spiral valve. The digestive 
tract ends at the small anus. A liver is 
present, but there is usually no bile duct in 
the adult; it is not definitely known whether 
or not there is a pancreas. 

Circulatory system 

Petromyzon possesses a heart, a number 
of veins and arteries, and many lymphatic 
sinuses. The heart (Fig. 242) lies in the 
pericardial cavity, and consists of a ventricle 
which forces the blood into the arteries and 
an atrium which receives the blood from 
the veins. A renal portal system is absent. 

Respiratory system 

Respiration is carried on by means of 7 
pairs of gill pouches, which open to the out- 
side by the gill slits and internally to the 
pharynx. Each gill pouch contains numer- 
ous gill filaments that contain many capil- 
laries in which the blood is oxygenated by 
the water in the pouch. In the adult lam- 
prey, water is taken into the gill pouches 
through the external gill slits and is dis- 
charged through the same openings (Fig. 
242). This method, which is unlike that in 
the true fishes, is necessary because the 
lamprey, when attached to its food by its 
oral funnel, cannot take water through the 
mouth. However, in a larval lamprey, the 
water used in respiration passes in through 
the mouth and out the gill slits as in fishes. 

Nervous system 

The brain (Fig. 242) of the adult lamprey 
is very primitive. The forebrain consists of 
a large pair of olfactory lobes; behind these 
are the small cerebral hemispheres attached 




Oral funnel 

Mouth cavity — 
Protractor musci 

of tongue 


Tongue cartilag 


Inferior jugular 


Cross section through 
gill pouches (arrows 
indicate v/oter current) 

Retractor muscle 

of tongue 

External gill slit 

Gill filaments 

Internal gill slit 

Nerve cord 
■'^Jugular vein 

Dorsal aorta 

Ventral aorta 



Sinus venosus 

Hepatic vein 

Dorsal aorta 



Olfactory sac 

Pituitary sac 


Dorsal aorta 
Nerve cord 


Jugular vein 

cardinal vein 




cardinal vein 

Nerve cord 



Figure 242. Petromyzon, an adult lamprey. Shown in cross section and with left side of body 
removed to demonstrate the structure. 

to the diencephalon; ventral to the latter is 
a broad infundibulum, and above it a pineal 
structure. On the midbrain is a pair of large 
optic lobes. In the hindbrain, the rudimen- 
tary cerebellum is a small, dorsal, transverse 
band, but the ventral medulla oblongata 
(medulla) is fairly well developed. There 
are only 10 pairs of cranial nerves as the 
terminal nerve is absent. The nerve cord is 

flat and lies on the floor of the neural canal. 
There are no sympathetic ganglia, and the 
autonomic system consists only of an in- 
testinal plexus linked with the brain. 

Sense organs 

Organs of taste, smell, equilibrium, and 
sight are present in the lamprey. The end 



organs of taste are situated between the gill 
pouches on the pharyngeal wall. The organ 
of smell is an olfactory sac (Fig. 242) 
which lies in the nasal capsule; this com- 
municates with the outside by a nasal open- 
ing on the dorsal surface between the eyes. 
The olfactory sac gives off ventrally a tube 
of unknown function, called the pituitary 


The "ears" (balancing organs) of Petro- 
myzon, which lie in the auditory capsule, 
have only two semicircular canals instead of 
the usual three. The hagfish has only one. 
The eyes of the adult lamprey, though primi- 
tive, are excellent visual organs. Besides the 
paired eyes there is a well-developed median 
pineal eye just behind the nasal opening. 

Endocrine glands 

Where the pituitary sac comes in contact 
with the infundibulum of the brain, it gives 
off numerous small follicles which become 
separated, forming the pituitary gland. The 
endostyle of the larva, which has been stud- 
ied by means of radio-iodine, is the fore- 
runner of the thyroid gland of the adult 

Urogenital system 

The excretory and reproductive systems 
are so closely united in the lamprey that it 
is customary to treat them together as the 
urogenital system. The kidneys lie along 
the dorsal wall of the body cavity, and each 
pours its secretions by means of the urinary 
duct into the urogenital sinus, and thence 
to the outside through the urogenital open- 
ing. The sexes are separate in the adult; 
however, the immature gonad is hermaphro- 
ditic, but later becomes male or female for 
an individual. The single gonad fills most 
of the abdominal cavity at the time of sex- 
ual maturity. There is no genital duct; eggs 
or sperms break out into the coelom, make 
their way through two genital pores into 
the urogenital sinus, and then pass out 

through the urogenital opening into the 
water, where fertilization occurs. 


The sea lampreys become sexually mature 
in May or early June; then both sexes mi- 
grate into streams, sometimes "hitchhiking" 
on a passing boat. They seek a gravelly bot- 
tom under moderately fast-flowing water; 
and, by means of the oral hood, move 
stones on the bottom of the stream to form 
a shallow rounded depression called a nest. 
The female then fastens to a stone in the 
nest and the male attaches to the female by 
the use of their oral funnels. Partly en- 
twined, they move back and forth as sperms 
and eggs are discharged, fertilization taking 
place in the water. Each female sea lamprey 
produces 24,000 to 107,000 eggs, depending 
on her size. The average female lays 62,500 
eggs and dies after spawning. The eggs hatch 
out into larvae, known as "ammocoetes," 
in 20 to 21 days. The blind harmless larvae 
make their way out of the nest and drift 
downstream until quiet water is reached. 
Here they dive and burrow into the bottom 
if it is mud or silt. The larval period spent 
in burrows is, recent studies indicate, at 
least 7 to 8 years, and possibly longer in 
duration. Inspiration of water for respirator)' 
purposes appears to be largely responsible 
for drawing of food organisms in the mouth 
of the larvae. Thereafter, food particles are 
carried forward to the esophagus by action 
of the cilia in certain areas of the pharynx. 
An endostyle on the floor of the pharynx 
secretes mucus which entangles the food, as 
in the amphioxus. 

The ammocoetes lies buried in mud and 
sand and probably keeps its skin free from 
bacteria, fungi, and other parasitic growths 
by means of an integumentary secretion. In 
the winter of the seventh or eighth year, the 
larval lamprey undergoes a metamorphosis, 
after which it migrates to the sea, or, if it is 
in the Great Lakes region, to one of the 
lakes, where growth to sexual maturity takes 



Figure 243. American brook lampreys spawning. (Courtesy of Institute for Fisheries Research, 
Michigan Department of Conservation.) 

place. It is at this stage that the parasitic 
species begins to feed on fish. 


The nonparasitic brook lampreys of North 
America, Entosphenus lamottenii, breed in 
the spring. They move stones by means of 
their oral hoods until a space is cleared on 
the bottom where a number of them may 
congregate. A male clings to the head of a 
female for a moment, winds his tail about 
her body, and discharges spermatozoa over 
the eggs when they are extruded. There is 
a long larval stage in development. After 
spawning the adults probably take no food 
and soon die; they are therefore not injuri- 
ous to fishes. 


Hagfishes live in the mud of the sea bot- 
tom, down to a depth of nearly 350 fathoms. 
They are very destructive to fishes, especially 

those caught on lines or in nets; they bore 
their way into the body and eat out the 
soft parts. Cod and flounders are the fish 
usually attacked. 

In the hagfishes the same individual pro- 
duces first sperms and then later eggs. 
Growth to the adult is direct, that is, with- 
out a larval stage. 




The chordate characters of the cyclo- 
stomes are obvious. Some, such as a continu- 
ous notochord and many gill slits, are simi- 
lar to the primitive characters of the 
amphioxus. Others are of a more advanced 
nature, such as a distinct head, cranium, 
better-developed brain, and cartilaginous 
neural arches. Cyclostomcs are less advanced 
than fishes as indicated by the absence of 
hinged jaws, paired limbs, true teeth, and 
complete vertebrae. 




The flesh of the lamprey, in many parts 
of Europe, has been a popular food for cen- 
turies. Work has been done in the United 
States to make lampreys palatable as food, 
or to find other commercial use for them, 
but so far these efforts have met with little 
or no success. Larval lampreys sometimes 
serve as bait for both commercial and sport 
fishermen. Some adult lampreys are serious 
enemies of valuable food and game fishes. 
The Atlantic lamprey {Petromyzon marinus) 
is found in all the Great Lakes and now 
threatens their important commercial fish 
resources. It passed the Niagara Falls bar- 
rier by way of the Welland Canal. Lake 
Huron and Lake Michigan's trout fishing 
has been practically destroyed by the lam- 
prey, and Lake Superior's potentiality is 
30 per cent below that of former years. 
In 1946, commercial fishermen took a catch 
of lake trout from Lake Michigan of about 
5,500,000 pounds but the catch in 1953 
was only 482 pounds. The lampreys not 
only kill many fish by feeding on their blood, 
but they inflict injuries on many leaving 
scars (Fig. 239) and impairing their com- 
mercial value. Some large fish show several 

States adjoining the Great Lakes and the 
United States and Canadian governments 
are trying to work out methods to control 
the sea lamprey. At present the electric 
barriers (weirs) are the most dependable 
devices in use for sea lamprey control. How- 
ever, recent experiments with a chemical 
compound have been reported uncondition- 
ally successful. Electric weirs, chemical 
treatment, and other control measures 
should eventually make the sea lamprey 
problem less serious. But it is doubtful that 
this pest will ever be completely removed 
from the Great Lakes. The St. Lawrence 
Seaway probably means that the lamprey 
will always be a troublemaker in the Great 

Lakes, for it doubtless will be brought in 
from the oceans in increased numbers, at- 
tached to the bottoms of big ships. 

The migration of the sea lamprey into 
the Great Lakes is acting as a great disturber 
of the balance in nature, as is demonstrated 
by the important influence on the large com- 
mercial fisheries of these extremely produc- 
tive waters. 


(For reference purposes only) 

Class Agnatha includes the orders which 
constitute the ostracoderms of the Silurian 
and Devonian geological periods. The ostraco- 
derms are known only from their fossil remains. 

Order 1. Cyclostomata (the living cyclo- 
stomes). Hagfishes and lampreys. 
Eel-shaped; skin smooth, without 
scales; no lateral fins, functional jaws, 
or genital ducts; mouth suctorial, 
with horny teeth; 1 olfactory pit; 7 
or more pairs of gill clefts. 

The cyclostomes are the most 
primitive of all living vertebrates. 
Two suborders are recognized as fol- 
lows : 
Suborder 1. Petromyzontia (Gr. petra, 
rock; myzon, sucker). Lampreys. 
One family; nasal opening in 
front of eyes; mouth suctorial; 
7 pairs of gill slits. Exs. Petromy- 
zon marinus, sea lamprey (Fig. 
240), Atlantic Coast from 
Chesapeake Bay northward, 
and, in Europe, landlocked in 
the Finger Lakes; Entosphenus 
tridentatus, Pacific Coast, south- 
ern California to Alaska; Ich- 
thyomyzon, brook lampreys, 
central North America, includ- 
ing the Great Lakes; Enfos- 
phenus lamottenii, nonparasitic 
lampreys, eastern and midwest- 
em states. 
Suborder 2. Myxinoidia (Gr. myxa, 
slime) . Slime eels and hagfishes. 



One family; three genera; nasal 
opening terminal; four tentacles 
on either side of mouth; oral 
funnel absent. Exs. Myxijie 
limosa, slime eel, Atlantic Coast; 
Polistotrema, southern Cali- 
fornia to Alaska; and Bdellos- 
toma (Eptatretus) , Chile. 


Applegate, V.C. Natural History of the Sea 
Lamprey, Petromyzon marinus in Michigan. 
Special Scientific Report, Fisheries No. 55, 

U.S. Fish and Wildlife Service, 1950. 

-, and Moffett, J.W. "The Sea Lamprey.' 

Scientific Ainerican, 192:36-41, 1955. 
Gage, S.H. The Lake and Brook Lampreys o. 

New York. Wilder Quarterly Century Book 

Comstock, Ithaca, N.Y., 1893. 
Hubbs, C.L. "The Life-Cycle and Growth oi 

Lamprevs." Papers Mich. Acad. Sci., 4:587- 

603, 1924. 
Lennon, R.E. Feeding Mechanism of the Sea 

Lamprey and Its Effects on Host Fishes 

U.S. Fish and Wildlife Service. Fish. Bull.^ 

56:245-293, 1954. 
Reynolds, T.E. "Hydrostatics of the Suctorial 

Mouth of the Lamprey." Univ. Calif. Pub. 

Zoo/., 37:15-34, 1931. 


Class Chondi ichthyes. 



HE Chondrichthyes (cartilage fishes), also 
called elasmobranchs, are the sharks, rays, 
and skates. They are the most generalized 
of the living vertebrates that have com- 
plete vertebrae, movable jaws, and paired 


The common dogfish shark, known since 
Aristotle's time as being ovoviparous, is 
abundant in the waters off the coasts of 
New England and northern Europe. It is 
of special biologic interest because many of 
the basic vertebrate features are present in 
this shark in simple form, and this helps 
one to understand the more complex sys- 
tems of higher vertebrates. This is an an- 
cient group of fishes and is represented by 
many fossil remains. 

External anatomy 

The body is spindle-shaped and about 
IVi feet long. There are two dorsal fins, 
one behind the other, each with a spine at 
the anterior end, two pectoral fins, and two 
pelvic fins. The pelvic fins in the male 
possess cartilaginous appendages known as 
claspers. The tail (caudal fin) is heterocer- 
cal (Fig. 260). The mouth is a transverse 
slit on the ventral surface of the head. On 
either side, above the mouth is an eye, and 
each nostril on the ventral side of the head 
opens into a blind pouch, the olfactory sac. 
Anterior to each pectoral fin are 6 gill slits, 
the first of which is situated back of the eye 
and modified as a spiracle. Between the 
pelvic fins is the cloacal opening, some- 
times called an anus. 

The gray-colored skin is covered with 
placoid scales (Fig. 245). For a dorsal view 

Figure 244. Facing page, the internal organs of 
the spiny dogfish. The veins are in sohd black, the 
arteries in outline. 


^ Artery 



Afferent bronchia 
Ventral aorta 

Conus arteriosus 
Heart ' 

Common cardina 
(duct of Cuvier) 

Pectoral fin 


Cloacal opening 
Accessory urinary duct 

Caudal artery 

Caudal vein 


Pelvic fin 


Gill slit 






'Anterior cardinal 



Centrum of 


Dorsal aorta 

Vas deferens 

and urinary 






Rectal gland 

Renal portal 




Nerve (spinal) cord 




see the headpiece at the beginning of this 
chapter. Over the jaws the placoid scales 
are modified as teeth with their points di- 
rected backward and are used for holding 
and tearing prey. A placoid scale consists of 
a bony basal plate, with a spine in the center 
composed of dentine, and is covered with a 
hard enamel-like dentine. The method of 
embryonic development of the scale and its 

dentinal nature indicate that placoid scales 
are homologous with vertebrate teeth. Al- 
though the best evidence denies the pres- 
ence of enamel on the surface of the placoid 
scale, the enamel-forming organ is present 
as in developing teeth. The homology of the 
teeth and scales is due to the fact that the 
mouth lining is inturned skin and hence 
possesses skin structures. 


Basal plate 

Figure 245. Detail of a placoid scale (dermal denticle) as seen in section. Because of the 
pointed scales, the skin is rasplike in texture. Thus, if a shark brushes against a man in the 
water, it could inflict a severe wound in his skin. (After Kerr.) 

Skeletal system 

The skeleton is composed entirely of 
cartilage (gristle). The cartilaginous skele- 
ton in the elasmobranchs is in all probability 
a degenerate and not a primitive characteris- 
tic as was formerly believed. There are two 
main subdivisions of the skeleton: (1) the 
axial and (2) the appendicular. The axial 
skeleton consists of the vertebral column and 
the skull. The vertebrae are hour-glass- 
shaped (amphicoelous), and the notochord 
persists in the lens-shaped spaces between 
them. The skull is much more highly de- 
veloped than that of the cyclostomes. It is 
composed of ( 1 ) the cranium or brain case; 
(2) two large anterior nasal capsules and 
two posterior auditory capsules; and (3) the 
visceral skeleton, made up of the jaws, the 
hyoid arch, and 5 branchial arches support- 
ing the gill region. The appendicular skele- 
ton consists of the cartilages of the fins and 
those of the pectoral and pelvic girdles 
which support them. 

Digestive system 

The digestive tract is longer than the 
body. Following the mouth (Fig. 244) is a 
large pharynx into which open the spiracles 
and gill slits. The phar\'nx leads into the short 
wide esophagus which opens into the 
U-shaped stomach. The posterior end of the 
stomach ends at a circular sphincter muscle, 
the pyloric valve. The intestine follows and 
terminates in the cloaca and cloacal open- 
ing. A slender, fingerlike rectal gland, which 
apparently secretes mucus, attaches dorsally 
at the junction near the point where the 
small and large intestines join. Within the 
intestine is a spiral fold of mucous mem- 
brane called the spiral valve (Fig. 246), 
which prevents a too rapid passage of food 
and thus allows increased absorption. The 
liver is large and consists of two long lobes; 
its secretion, the bile, is stored up in a gall 
bladder and empties through the bile duct 
into the intestine. A pancreas and spleen 
are also present. 



Figure 246. Intestine of a dogfish shark cut open to show the structure of the spiral valve. 
The arrows inside valve show the direction of movement of food. 

Circulatory system 

As in the cyclostomes and most of the 
true fishes, the heart contains venous blood 
only (Fig. 244). It is pumped through the 
ventral aorta and thence into the afferent 
branchial arteries, becoming oxygenated in 
the capillaries of the gills. It then passes into 
the efferent branchial arteries, which carry 
it to the dorsal aorta. The dorsal aorta sup- 
plies the various parts of the body. Veins 
carry the blood back to the heart, opening 
into the sinus venosus. Other veins, called 
the hepatic portal system, transport the 

blood from the digestive canal, pancreas, 
and spleen to the liver from which hepatic 
sinuses return it to the sinus venosus. A 
third system, the renal portal system, con- 
veys the blood from the posterior part of the 
body to the kidneys. Blood leaves the kid- 
neys by way of several renal veins, emptying 
into the posterior cardinal sinuses which 
return it to the sinus venosus. 

Respiratory system 

Respiration is carried on by means of gills 
(Fig. 247). These are folds of mucous mem- 

Papilla of esophagus 

Gitl slit 

Gill slif 

•Visceral arch 

Gill raker 

Visceral arch 

Pectoral girdle 

Gill ray 
■Gill filament 

Gill slit 

Figure 247. Respiratory structures in the dogfish shark. The left side of the phar>nx is cut 
lengthwise and laid open to show the gills and other structures. 



brane well supplied with capillaries and 
borne by the hyoid arch and first 4 branchial 
arches. They are supported by these arches 
and by gill rays. Water entering the mouth 
passes between the branchial arches and out 
through the gill slits and spiracles, thus 
bathing the gills and supplying oxygen to 
the branchial blood vessels. 

Nervous system 

The brain (Figs. 248 and 249) is more 
highly developed than that of the cyclo- 
stomes. It possesses two remarkably large 
olfactory lobes, a cerebrum of two hemi- 
spheres, a pair of optic lobes, and a cere- 
bellum which projects backward over the 

medulla oblongata. There are 11 pairs of 
cranial nerves if the terminal is included. 
The nerve (spinal) cord is a dorsoventrally 
flattened tube with a narrow central canal; 
it is protected by the vertebral column. 
Spinal nerves arise from its sides in pairs. 

Sense organs 

The two olfactory sacs are characteristi- 
cally large in elasmobranchs. The ears are 
membranous sacs, each with three semicir- 
cular canals; they lie within the auditory 
capsules. The eyes are well developed. Along 
each side of the head and body is a longi- 
tudinal groove called the lateral line; it 
contains a canal with numerous openings 

Olfactory sac 

Olfactory bulb 
Olfactory lobe 

Optic lobe 

Fourth ventricle 


Nerve (spinal) 

Olfactory tract i 

Pineal body 


oblique muscle 
Optic II 

Trochlear IV 

Rectus muscle 

-^ — Superior 

Oculomotor III 


rectus muscle 

Abducens VI 

Trigeminal V 

+ facial VII 


-Auditory VIII 

Vagus X 
Spinal nerve 1 


Gill cleft 


Figure 248. Dorsal view of the brain and cranial nerves of the dogfish shark. Tl:e Roman 
numerals are used to identify the cranial nerves. 



Pinea! body- 
Lateral ventricfe 

Optic !obe 


—Fourth ventricfe 

Optic chiosma 
Third ventricle 

Medulla oblongata 

Aqueduct of Sylvius 


Figure 249. Longitudinal section between the first and second ventricles of the brain of the 

dogfish shark showing its structure. 

to the surface. Inside the canal are sensory 
hair cells connected to a branch of the 
tenth cranial nerve. On the surface of the 
head are also sensory canals, which open 
into pores containing pit organs with sensory 

each oviduct is called a shell gland, and a 
posterior part is enlarged in the dogfish to 
form a "uterus" in which the young de- 
velop (Fig. 250). The oviducts have separate 
openings into the cloaca. 

Urogenital system 

The dogfish shark possesses two ribbon- 
like kidneys (Fig. 250), one on either side 
of the dorsal aorta. Their secretion is car- 
ried by small ducts into a larger one, the 
urinary ( mesonephric ) duct which empties 
into a urogenital sinus; it then passes out of 
the body through the cloacal opening. A 
series of yellowish bodies called adrenals 
are located on the medial border of the 

The sexes are separate. The sperms of the 
male arise in two testes and are carried by 
the vasa eflFerentia to the much convoluted 
vasa deferentia which empty into the uro- 
genital sinus. During copulation the sperms 
are transferred to the oviducts of the female 
with the aid of the claspers. 

The eggs of the female arise in the paired 
ovaries, which are attached to the dorsal 
wall of the abdominal cavity. They break 
out into this cavity and enter the funnel- 
like opening, the ostium, common to both 
oviducts. An expanded anterior portion of 


The cartilaginous fishes now living are all 
that remain of a type that once dominated 
the ancient seas. Most of them occur in the 
warm waters of the tropics. The sharks are 
the largest of all fishes; they are to fishes 
what elephants are to land animals. The 
whale shark {Rhineodon typicus) is from 
40 to 50 feet long. The sharks are the larg- 
est living vertebrates with the exception of 
the whales. Among the interesting species 
is the great white shark, Carcharodon car- 
charias, which reaches a length of 36Vi feet 
and has earned the name of man-eater by 
occasionally devouring a human being. One 
of the most peculiar sharks is the hammer- 
head (Fig. 251); its head is shaped like the 
head of a mallet, with an eye on each side. 
The sawfish (Fig. 251) is abundant in the 
Gulf of Mexico and reaches a length of from 
10 to 20 feet. The saw is about 5 feet long; 
it is used for the capture of its prey; it 
swings the "saw" back and forth in a school 
of fishes to injure some of them sufficiently 



Oviduct ■ 


Rudimentary ostium and oviduct 

Ostium i 


Vasa efferentia 



Shell gland- 

Urinary duct and 
vas deferens 


mesonephric duct 

minal vesicle 

nary duct 

Sperm sac 


Urogenital papilla 
Oviduct opening 
Urinary papilla 



Figure 250. The urogenital systems of the dogfish shark. 

for capture; it is also used as a weapon of 

The sting ray, Dasyatis (Fig. 251), lives 
half buried in the sand along the seacoast. 
Its whiplike tail bears a barbed spine, which 
is provided with a poison gland; it makes a 
painful wound when driven into the hand 
or even through the side of a shoe into the 

The electric eel, Electrophorus electricus, 
is an eel-shaped fish of the Amazon and 
Orinoca basins. It is an extreme example of 
specialization for the greater part of the 

dorsal half of the body behind the head is 
occupied by the electric organ. This huge 
mass of electric tissue is made up of about 
70 columns of electroplates, each containing 
no fewer than 6000 cells in a series. In water 
an electric eel four feet long can produce 
up to 600 volts in potential. The maximum 
power output is about 1000 watts. The dis- 
charge of the electric organ may occur a 
second or more apart and continue for more 
than an hour without fatigue to the animal. 
It is suJSicient to disable a fairly large animal; 
thus it may serve as an effective weapon for 



Sting ray 

Hammerhead shark 

Figure 251. Some cartilaginous fishes showing extreme variation in form. The sting ray has 
pectoral fins so oversized that when the ray is swimming, the fins look like the wings of a bird 
in flight. 

securing food or for offense and defense. A 
horse wading through a shallow river where 
the electric eel lives has received enough 
shock to cause it to throw its rider. 




The cartilaginous fishes exhibit a number 
of structural advances over the cyclostomes; 
they possess paired fins, a lower jaw, gill 
arches, and placoid scales. Among the peculi- 
arities which separate the cartilaginous fishes 
from the bony fishes (Osteichthyes) are the 
absence of lungs, bones, air bladder, true 
scales, and the presence of skeletal charac- 
teristics which are not found in bony fishes. 


Sharks feed chiefly on crustaceans, squids, 
fish, other aquatic animals, and not on hu- 
man beings as one might infer from some 
newspaper accounts. However, there are reli- 
able reports of shark attacks on swimmers in 
all warm seas, but only an occasional attack 
in temperate waters. Most of these attacks 
have occurred near beaches or reefs, and a 
few in Lake Nicaragua by fresh-water sharks 
living in this lake. In recent years, an ever 
increasing number of sports enthusiasts, 
equipped with swimfins, snorkels, and aqua- 
lungs, have moved out to new underwater 
frontiers. Some of those who have been too 
venturesome have been killed by sharks. 



Sharks injure the nets of fishermen and 
destroy large numbers of lobsters, crabs, and 
food fishes. In certain parts of the world, 
especially the Orient, the smaller sharks and 
some skates are used for food— fresh, salted, 
and dried. In America, much prejudice exists 
against use of shark meat for food. However, 
in California they are being sold fresh as 
"grayfish." Also, dogfish are now being 
canned in the United States under a trade 
name and may become an important addi- 
tion to our list of fish foods. The fins of 
sharks and rays are considered a delicacy in 
certain Oriental countries. Sharkskin leather 
is of some commercial importance in the 
manufacture of shoes and handbags. Shark- 
skin, tanned with the scales on, is called 
shagren, and has been used as an abrasive. 
It is also used for binding books and as a 
covering for jewel boxes. The extraction of 
shark-liver oil is also being carried on. The 
cub shark, for example, possesses a liver that 
constitutes about 16 per cent of the total 
weight of the animal and yields a consider- 
able quantity of oil. This oil has been used 
principally in the tanning industry for 
leather. However, during World War II, 
the liver of the dogfish shark was America's 
chief source of vitamin A, and many shark 
livers are still used for this purpose. The 
pituitar}^ gland provides an extract for med- 
ical use. The sting ray is often a nuisance 
to bathers, and an occasional death results 
from the injur}' inflicted by its spines. 


(For reference purposes only) 

Class 1. Chondrichthyes ( Elasmobranchii ) . 

The chief characteristics of this class 

* This classification is according to Leonard P. 
Schultz, Curator of Fishes, United States National 
Museum, Smithsonian Institution. The endings for 
superorders, orders, and suborders for the cartila- 

are the presence of a cartilaginous 
skeleton; persistent notochord; pla- 
coid scales; spiral valve in intestine; 
two-chambered heart; claspers in 
male; no gill cover, pyloric ceca, or 
air bladder; mouth, a transverse 
opening on ventral side of head; tail 
heterocercal. Approximately 600 spe- 
cies according to Schultz. 
Superorder 1. Selachiica (Or. selachos, 
shark). Sharks, slender and 
cylindrical, with gill slits on 
the side. 
Order 1. Heterodontia. Ex. Bullhead 

Order 2. Hexanchida. Exs. Frilled and 

cow sharks. 
Order 3. Lamnida. Exs. Carpet, whale, 
mackerel, thresher, basking, cat, 
gray, and hammerhead sharks. 
Order 4. Squalida. Mostly under 8 feet 
long; carnivorous; voracious; but 
seldom attack man. Exs. Squalus 
acanthias, dogfish shark; bram- 
ble, saw, and angle sharks. 
Superorder 2. Hypotrematica. 

Order 1. Rajida. Rays and skates (Fig. 
251). Flattened dorsoventrally, 
with gill slits beneath. These are 
highly specialized sharks, 
adapted for life on the bottom 
of the seas. The rays are ovovi- 
viparous; they have a long 
whiplike tail, usually without a 
trace of a caudal fin, and near 
the midlength of the tail is a 
long sharp-pointed barbed spine, 
connected with poison glands. 
The skates are oviparous; they 
have a short thick "tail" with- 
out the poison spine, and the 
caudal fin is represented by a 
low dermal fold. Ex. Dasyatis 
sabina, sting ray. 
Class 2. Holocephali (Gr. holos, whole; 
kephale, head). Elephant fishes and 

ginous and bony fishes are those accepted by a 
unanimous vote of the ichthyologists at the meetings 
of the American Society of Ichthyologists and Herpe- 
tologists in Salt Lake City, June, 1950. 



chimaeras. The latter (Fig. 251) are 
grotesque-looking creatures named 
after the fire-breathing monster of 
Greek mythology. One olfactory sac; 
gills covered by operculum; no spira- 
cles or cloaca; adults nearly scale- 
less. Ex. Chimaera (Fig. 251). 


(See also Chapter 26) 

Daniel, J.F. The Elasmobranch Fishes. Univ. 
of Cal. Press, Berkeley, 1934. 



fc • ^ ^ 

r ^ ^ ^ ^ 


Class Osteichthyes. 
Bony Fishes 


HE Osteichthyes (bony fishes) are the 
true fishes. They range from the ordinary 
fish, such as the yellow perch, to the unusual 
lungfish. They are aquatic animals; one 
adaptation to their habitat is the gills which 
serve as respiratory organs. They are found 
in all the waters of the world, from the sur- 
faces to great ocean depths. Usually, they 
have an exoskeleton of scales or bony plates 
which furnishes protective covering for their 
bodies. They swim by means of fins. Their 
bodies are usually streamlined, but some are 
grotesque in shape; and others possess amaz- 
ing luminescent organs. 


External anatomy 

The yellow perch, Perca flavescens (Fig. 
253), inhabits the fresh-water streams and 
lakes of the northeastern United States and 
ranges west to the Mississippi Valley. Its 
body is about a foot long and is divisible 
into head, trunk, and tail. There are two 
dorsal fins, a caudal fin, a single median 
anal fin just posterior to the anus, two 
lateral pelvic fins, and two lateral pectoral 
fins. On each side of the body is a lateral 
line. The head bears a mouth with well- 
developed jaws armed with teeth, a pair of 
lateral eyes, a pair of external nares in front 
of each eye, and gill covers (opercula), be- 
neath which are the gills. The skin is pro- 
vided with a number of scales, which are 
arranged like the shingles on the roof of a 
house, and protect the fish from mechanical 
injury. Mucous glands are abundant in the 
skin, and they produce the "slime" (mucus) 
which makes the fish slippery. 

Figure 252. Facing page, representatives of the 
bony fishes. The lines suggest possible relationships. 
The figures are not drawn to scale. (Based on a 
diagram by Leonard P. Schultz, Curator of Fishes, 
United States National Museum, Smithsonian Insti- 
tution, and made expressly for this book.) 


(Bony fithet) 

(Lobefinned fish) 




Locomotor organs 

The body of the perch and of most other 
fishes is streamHned and offers Httle resist- 
ance to the water through which the animal 
swims. By means of an air bladder, it is 
kept at the same weight as that of the water 
it displaces. The fish is thus able to remain 
stationary without much muscular exertion. 
The principal locomotor organ is the tail. 
By alternating contractions of the muscular 
bands on the sides of the trunk and tail, the 
tail with its caudal fin is lashed from 
one side to the other, thus enabling the fish 

to swim. Similar movements are employed 
in sculling a boat, when one oar at the stern 
is moved from side to side. 

The fins are integumentary expansions 
supported by bony or cartilaginous rays. The 
paired lateral fins (pectoral and pelvic) are 
used as oars in swimming when the fish is 
moving slowly. They also aid the caudal fin 
in steering. Movement up or down results 
from holding the lateral fins in certain posi- 
tions—obliquely backward with the anterior 
edge higher for the ascent, and obliquely 
forward for the descent. 

Fishes must maintain their equilibrium 

Lateral line 
Exfernal nares 


Spinous dorsal fin 

Soft dorsal fin 


Branchiostega! membrane 
Pectoral fin 

Pelvic fin 

Anal fin 


Caudal fin 

Figure 253. External features of a typical bony fish, the yellow perch. 

in some way, since the back is the heaviest 
part of the body and tends to turn them 
over. The dorsal, anal, and caudal fins in- 
crease the vertical surface of the body, and, 
like the keel of a boat, assist the animal in 
maintaining an upright position. The paired 
lateral fins are also organs of equilibrium, 
acting as balancers. However, experimenta- 
tion shows that if one or two of the paired 
fins are removed, the fish soon learns to com- 
pensate for their loss. 

Skeletal system 

The exoskeleton of the perch includes 
scales and fin rays. The scales develop in 
pouches in the dermis. They are arranged 
in oblique rows and overlap like the shingles 
on the roof of a house, thus forming an ef- 
ficient protective covering. The posterior 
edge of each scale which extends out from 

under the preceding scale is toothed and 
therefore rough to the touch. Scales of this 
kind are called ctenoid scales (Fig. 261). 
The fin rays support the fins. Those of the 
spinous dorsal fin (Fig. 254) and of the 
anterior edge of the anal and pelvic fins are 
unjointed and unbranched spines. The 
caudal, pectoral, pelvic, soft dorsal, and 
anal fins are supplied with jointed, and, usu- 
ally branched, soft fin rays. 

The bones of the endoskeleton are shown 
in Fig. 254. They include the skull, vertebral 
column, ribs, pectoral girdle, and the inter- 
spinous bones which aid in supporting the 
unpaired fins. The body of the fish is, to a 
considerable extent, supported by the sur- 
rounding water; consequently, the bones do 
not need to be so strong as those of land 
animals, like birds and mammals, which 
support the entire weight of the body. 

The vertebrae are simple and compara- 



Spinous dorsal fin Soft dorsal fin Caudal fin 


Quadrat© girdle 


Pectoral fin 
False rib 

Anal fin 
Interspinous bones 

Figure 254. The skeleton of a fish (yellow perch). Note that the skeleton centers about the 
vertebral column and skull. 

tively uniform in structure. Ribs are at- 
tached by ligaments to the abdominal verte- 
brae and serve as a protecting framework 
for the body cavity and its contents. There 
is no sternum. The skull consists of a large 
number of parts— some bone, others car- 
tilage. The visceral skeleton is composed of 
7 paired arches more or less modified. The 
first or mandibular arch forms the jaws. 
The second or hyoid arch is modified as a 
support for the gill covers. Arches 3 to 7 
support the gills and are known as gill 
arches. The first 4 bear spinelike ossifica- 
tions, the gill rakers, which act as a sieve to 
intercept solid particles and keep them away 
from the gills. 

The appendicular skeleton is represented 
in the perch by pectoral and pelvic girdles 
and fins associated with them; and median 
fins (Fig. 254). The pelvic girdle is not very 
typical in form, being degenerate or pos- 
sibly primitive. 

Muscular system 

The principal muscles are those used in 
locomotion, respiration, and in obtaining 
food. The movements of the body employed 
in swimming are produced by 4 longitudinal 
bands of muscles, one heavy band on each 
side along the back, and a thinner band on 
each side of both trunk and tail. These are 
arranged in zigzag myotomes. Weaker mus- 
cles move the gill arches, operculum, hyoid, 
and jaws. 

Digestive system 

The aquatic insects, mollusks, and small 
fishes that constitute a large part of the 
food of the perch are captured by the jaws 
and held by the many conical teeth. Teeth 
are borne on the mandibles and premaxillae, 
and on the roof of the mouth. They are not 
used for chewing food, but only for holding 
it. A rudimentary tongue projects from the 



Mouth cavity 

Ventral aorta 


Sinus venosus 

Pericardial cavity 

Gall bladder 

Pelvic fin 

Olfactory nerve 


Optic lobe 


— Skull 

Nerve (spinal) cord 


Air bladder 

Spinous dorsal fin 

Dorsal aorta 

Air bladder 



— Rib 

Urogenital opening 


Neural spine 

Urinary bladder 
Urinary duct 

Soft dorsal fin 

Anal fin 

Caudal vein — 
Caudal artery — 

'—Neural spine 
Nerve (spinal) cord 

Figure 255. General structure of a yellow perch. 

floor of the mouth cavity; it is not capable 
of independent movement, but functions as 
a tactile organ. The mouth cavity is fol- 
lowed by the pharynx, on either side of 
which are 4 gill slits. Food passes directly 
to the stomach through a short esophagus. 

The stomach, intestine, liver, gall bladder, 
pyloric ceca, and anus are shown in Fig. 
255. The pancreas is located in the first loop 
of the intestine; however, it is so diffuse 
that it is not usually seen in gross dis- 



Circulatory system 

T/ie blood of the perch contains oval 
nucleated red corpuscles and amoeboid 
white corpuscles. The heart lies in a portion 
of the coelom, the pericardial cavity, be- 
neath the pharynx. Circulation in the perch, 
which is similar to that in the dogfish shark, 
is shown through the heart and gills in Fig. 
256. Circulation is much slower in fishes 
than it is in the higher vertebrates. 

Respiratory system 

The perch breathes with 4 pairs of gills 
supported by the first 4 gill arches. Each gill 
bears a double row of gill filaments, which 
are abundantly supplied with capillaries. 
The afferent branchial artery (Fig. 256) 
brings the blood from the heart to the gill 
filaments; here an exchange of gases takes 
place. The carbon dioxide, with which the 
blood is loaded, passes out the gill, and a 

Afferent branchial artery-. Hepatic vein 

Gill capillaries 




-Hepatic portal vein 
-Intestinal capillaries 

Head capillaries 
Ventral aorta 
Anterior cardinal vein 

Sinus venosus 
■Bulbus arteriosus 

lena\ portal vein 
Kidney capillaries 
Posterior cardinal vein 

Figure 256. Diagram of the main blood vessels of a fish (yellow perch) as seen in lateral view. 
The unstippled parts represent oxygenated blood and the stippled, nonoxygenated blood. 

supply of oxygen is taken in from the con- 
tinuous stream of water, which enters the 
pharynx through the mouth and bathes the 
gills on its way out through the gill slits. 

The oxygenated blood is collected into 
the efferent branchial artery and is carried 
about the body. The gills are protected from 
external injury by the gill covering or 
operculum (Fig. 253), and from solid parti- 
cles which enter the mouth by the gill 
rakers. Because oxygen is taken up by the 
capillaries of the gill filaments, a constant 
supply of fresh water is necessary for the 
life of the fish. If it is deprived of water 
entirely, respiration is prevented, and the 
fish dies of suffocation. 

The air bladder is a comparatively large, 
thin-walled sac lying in the dorsal part of 
the body cavity. It is filled with gas and is a 

hydrostatic organ or "float"; in certain 
fishes, but not in perch, it may also aid in 
respiration. The gas contained in it is a 
mixture of oxygen, nitrogen, and carbon 
dioxide, and is derived from the blood ves- 
sels in its walls. The air bladder decreases 
the specific gravity, making the body of the 
fish equal in weight to the amount of water 
it displaces. The fish, therefore, is able to 
maintain a stationary position without mus- 
cular effort. The amount of gas within the 
air bladder depends upon the pressure of the 
surrounding water; and, in some way, it is 
regulated by the fish according to depth. If 
a fish is suddenly brought to the surface 
from a great depth, the air bladder which 
was under considerable pressure is suddenly 
relieved, and therefore expands, often forc- 
ing the stomach out of the mouth. In some 



Figure 257. Winterkill of fish due to suffocation. This is a scene in spring on the shore of a 
northern lake. The first complete freezing over of a lake seals the water away from further 
contact with the air, an important source of oxygen. Then when the snow on top of the ice 
becomes deep enough to cut off the light necessary for photosynthesis in plants, an additional 
source of oxygen is eliminated. The fish, robbed of their life-sustaining oxygen, die, as is 
shown in this photograph. (Photo by Ouradnik. Courtesy of Institute for Fisheries Research, 
Michigan Department of Conservation.) 

fishes the air bladder may serve in sound 
production. It is lunghke in the lungfishes. 

Excretory system 

The kidneys lie just beneath the backbone 
in the abdominal cavitv. They extract urea 
and other waste products from the blood. 
Two thin tubes, the urinary ( mesonephric ) 
ducts or "ureters," carry the excretory matter 
into a urinary bladder (Fig. 255), where it 
is stored for a time and then expelled 
through the urogenital opening, which is 
located just posterior to the anus. 

Nervous system 

The brain of the perch (Figs. 255 and 
392) is more highly developed than that of 
the cyclostome or shark. The 4 chief divi- 

sions are well marked: the cerebrum, optic 
lobes, cerebellum, and the medulla. The 
brain gives off cranial nerves to the sense 
organs and other parts of the anterior por- 
tion of the body. The nerve (spinal) cord 
lies above the centra of the vertebral column 
and passes through the neural arches of the 
vertebrae. Spinal nerves arise from the sides 
of the spinal cord. 

Sense organs 

The principal sense organs are: (1) cu- 
taneous sense organs, (2) olfactory sacs, 
(3) ears, and (4) eyes. The integument, es- 
pecially that of the lips, serves as an organ of 
touch. Dermal barbels on some fishes such 
as the catfishes also function as sensory or- 
gans for locating food. The lateral line con- 
tains sensory cells which serve to detect 



vibrations in water and pressure stimuli. 
There are also numerous other cutaneous 
sense organs. 

The two olfactory sacs lie in the anterior 
part of the skull and communicate to the 
outside by a pair of openings in front of each 
eye. They are not connected with the mouth 
cavity and take no part in respiration. The 
inner surface is thrown up into folds which 
contain many sense cells. Ability to detect 
odors lies in the olfactory sacs. 

The ear consists of the membranous 
labyrinth only. As in the cyclostome and 
shark, the sound waves are transmitted by 
the bones of the skull to the fluid within the 
labyrinth. Three semicircular canals (Fig. 
396) are present, and the sacculus contains 
concretions of calcium carbonate called ear 
stones or otoliths. Experiments indicate 
that goldfishes can hear. The ear serves 
both as an organ of hearing and an organ 
of equilibrium. 

The eye of the perch differs in several 
respects from that of the terrestrial verte- 
brates. The eyelids are absent in bony fishes 
since the water keeps the eyeball moist and 
free from foreign objects. The cornea is flat- 
tened and of about the same refractive 
power as the water. The lens is almost spher- 
ical. The pupil is usually larger than that of 
other vertebrates and allows the entrance of 
more light rays; this is necessary, since semi- 
darkness prevails at moderate depths. When 
at rest the eye focuses clearly at about 15 
inches, but it can detect the movement of 
objects much farther away. To focus on dis- 
tant objects the lens is puhed backward. 
Many fishes are nearsighted. However, the 
sharks that pursue rapidly moving prey have 
lenses that are set for distant vision. Recent 
evidence makes it appear that fishes can 
distinguish colors; therefore, it is possible 
that gaudy colors on hook lures are an aid 
to success in fishing. 

Reproductive system 

The sexes are separate. The single ovary 
is probably the result of a fusion of two 

ovaries in the embryo. The ovary or testes 
lie in the body cavity. The germ cells pass 
through the reproductive ducts and out of 
the urogenital opening. Perch migrate in the 
spring from the deep waters of lakes and 
ponds, where they have spent the winter, to 
the shallow waters near shore. The female 
lays many thousands of eggs in a long rib- 
bonlike mass. The male fertilizes the eggs by 
depositing sperm (milt) over them. Very 
few eggs develop because of the numerous 
animals such as other fishes and aquatic 
birds which feed on them. 


The cmbryogeny of the goldfish is substi- 
tuted here for the perch because it is better 
known, yet the development of the perch is 
similar. The young goldfish hatches from the 
egg in about 3 to 14 days, depending upon 
the temperature of the water. The egg passes 
through the stages shown in Fig. 258. A large 
part of the egg consists of yolk. A proto- 
plasmic accumulation which forms a slight 
projection at one end is called the germinal 
disk ( blastodisk ) . Cleavage of the germinal 
disk takes place, and the blastoderm pro- 
duced gradually grows around the yolk. The 
embryo appears as a thickening of the edge 
of the blastoderm; this grows in size at the 
expense of the yolk. After a tjme the head 
and tail become free from the yolk, and the 
young fish breaks out of the egg membranes. 
The young fish lives at first upon the yolk 
in the yolk sac, but it is soon able to obtain 
food from the water. 


External features 

Form of the body 

The bodies of the majority of fishes are 
spindle-shaped and laterally compressed as in 
the perch— a form that offers slight resist- 
ance to progress through the water. Varia- 




Embryo of 32 somites 

Early blastoderm 


overgrowing yolk- 

Embryo of 25 somites 


Embryo of 18 somites 

Embryo with 
optic vesicles 

Early embryo 

Figure 258. Early embryology of a bony fish, the goldfish. (After Helen I. Battle, Ohio J. Sci. 
40:85, 1940.) 

tions in form are correlated with the habits 
of the fish. For example, flounders (Fig. 
262) have flat bodies and are adapted for 
life on the sea bottom; and eels (Fig. 252) 
have long cylindrical bodies which enable 
them to enter holes and crevices. 

Fins and tail 

Fins, according to some zoologists, arise 
in the embryo as median and lateral folds of 
the integument (Fig. 259), which are at first 
continuous. Later, parts of the folds disap- 
pear and the isolated dorsal, caudal, anal. 



Dorsal fin f o d 

Ventral fold 

Lateral folds 

Dorsal finsn 


Tail fin 

Pectoral fin 

Pelvic fin Anus Anal fin 

Figure 259. Diagrams illustrating the Enfold theory of the origin of fins. A, the continuous 
folds of the paired and unpaired fins in the embryo. B, parts of the continuous folds disappear 
to form the permanent fins. (After Wiedersheim. ) 

pelvic, and pectoral fins persist. The pelvic 
fins vary considerably in position. In the 
perch (Fig. 253) they are situated beneath 
the pectoral fins; in the fresh-water catfish, 
they are just in front of the anus and are 
called abdominal; and in certain other 
species they are in the throat region. 

The shape of the caudal fin and the 
terminal portion of the tail differs in the 
main groups of fishes and is therefore of 
importance in classification (Fig. 260). 

The three main types of caudal or tail fins 
found in fishes are: heterocercal, diphycercal, 
and homocercal. The heterocercal tail is 
found in modern sharks; it is two-lobed, 
with the vertebral column extending into the 
larger dorsal lobe. The stroke of the asym- 

metrical heterocercal tail forces the anterior 
part of the body downward. This type is 
therefore of advantage to and characteristic 
of those fishes that have ventrally situated 
mouths and feed on the bottom. 

In the diphycercal type the vertebral col- 
umn extends straight back to the tip of the 
body, with the tail fin developed symmetric- 
ally above and below it; the living lungfishes 
have tail fins of this type. The homocercal 
fin is externally symmetrical, but the internal 
structure shows that the backbone extends 
into the dorsal lobe. The stroke of the homo- 
cercal tail forces the fish straight forward. 
It is characteristic of those fishes with a 
terminal mouth and is the type possessed by 
most bony fishes. 

Heterocercal Diphycercal Homocercal 

Figure 260. Types of tails in fishes. 



The diphycercal tail has long been con- 
sidered the primitive type from which the 
others were derived, but this is not true. 
In most cases diphycercal tails are shown by 
fossil history to have been derived from 
heterocercal ones, and the homocercal type 
is also of heterocercal origin. 


The scales of fishes form a protecting 
exoskeleton. They are of three principal 
types: (1) ganoid, (2) cycloid, and (3) 
ctenoid (Fig. 261). Ganoid scales are usu- 
ally rhomboid- or diamond-shaped. They 
have layers of ganoin deposited on a layer 

of bone. Ganoid scales occur in gars, pikes, 
sturgeons, and their allies; these are often 
called ganoid fishes. Cycloid and ctenoid 
scales are arranged in overlapping rows as 
described for the perch. Cycloid scales are 
nearly circular with concentric rings about 
a central point; they are characteristic of the 
more primitive teleosts. Ctenoid scales are 
similar to cycloid scales, but the part which 
extends out from under the neighboring 
scales bears small spines; these are generally 
found in the higher teleosts. In many fishes 
the scales develop into large spines or fuse to 
form bony plates which are protective. Some 
fishes such as the catfishes are scaleless. 

Ganoid (gar pike) 

Ctenoid (perch) 

Cycloid (northern pike) 

Figure 261. The different types of scales on bony fishes. Ctenoid scale shows winter growth 
rings (numbered), which are used to determine age in years. The winter rings show slower 
growth periods which result from low food supply. (Photomicrographs of cycloid and ctenoid 
scales courtesy of Institute for Fisheries Research, Michigan Department of Conservation.) 


The general impression is that fishes are 
not brightly colored, but many, especially 
those in tropical waters, are exceedingly bril- 
liant. The colors are due to pigments within 
special dermal cells called chromatophores 
or to reflection and iridescence resulting 
from the physical structure of the scales 
which contain cr\'stals of guanine. The pig- 
ments are red, orange, yellow, or black, but 
other colors may be produced by a combina- 
tion of chromatophores; for example, yellow 
and black when blended give brown. Usu- 

ally the colors are arranged in a definite pat- 
tern consisting of transverse or longitudinal 
stripes and spots of various sizes. Coral-reef 
fishes have long been famous for their bril- 
liant colors, and many fresh-water fishes of 
the temperate zone exhibit bright hues dis- 
tributed so as to form striking and intricate 
patterns (e.g., the rainbow darter). 

The dispersion or concentration of the 
pigment in the chromatophores of certain 
fishes results in changes in coloration. These 
changes are due to incident light reflected 
from surrounding surfaces and act through 
the eye and nervous system on the differ- 



Figure 262. Protective resemblance in the flounder. The flounder changes in pattern and 
color to blend with its surroundings by dispersion or concentration of pigment bodies. Even 
when placed on such an unnatural background as that on the right, the resemblance is striking. 
(Courtesy of A.M. Winchester.) 

ently colored chromatophores. The changes 
are therefore dependent upon the color of 
the fish's environment and are often pro- 
tective because they help to conceal the ani- 
mal. The change in color is slow in some 
fishes, but usually it only takes a few min- 
utes, as in the case of the flounders (Fig. 
262), which respond to pattern of back- 
ground as well as simple illumination. In this 
respect flatfishes probably exceed even the 
famed chameleon. Male fishes are often 
more brightly colored than the females, es- 
pecially during spawning activities. 

Sound production in fishes 

A surprisingly large number of fishes can 
produce sounds audible to man; these noises 
are used either to bring the sexes together 
or to warn or startle enemies. Fishes make 
various sounds in various ways. 

Certain sculpins vibrate their gill covers 
against the sides of their heads to produce a 
humming note. The hogfish grunts by 
gnashing its pharjngeal teeth. The sea robin 
produces a grunt by means of special mus- 
cles in the air bladder. In open water, the 
croaker probably makes more noise than any 
other kind of fish, it can be heard 30 feet 
down. The sounds are produced by the ac- 
tion of special muscles on the air bladder. 
Croakers are edible fish found along our 
southern Atlantic Coast. 

Naval men during World War II had 
more than an academic interest in undersea 
sounds; at first, some of the noises made by 

fishes and other sea animals threw subma- 
rine detection by radar into confusion. 

Fishes of ancient lineage 

Among the ancient fishes were those 
known as the lobefins, because of the thick, 
lobe-shaped, paired fins. The lobefins con- 
tain skeletal structures that correspond with 
similar bony structures in the appendages of 
true land forms. Descendants of the ancient 
stocks from which these fishes were derived 
migrated onto land at a later time to give 
rise to land vertebrates. Lobefinned fishes 
were long considered extinct until one was 
caught off the coast of Africa in 1938. This 
was a coelacanth {Latimeria) , and about a 
dozen more have been collected since then 
(Fig. 263). The chances look good for more 
coelacanths to be hooked in the future, so 
that eventuallv we will have a more detailed 
knowledge of them. As a matter of fact, their 
internal structure is being carefully studied 
by Professor Millot of Paris, and we should 
soon know more about the anatomy of these 
most interesting creatures. 

The coelacanth is the only surviving mem- 
ber of the ancient crossopterygian fishes 
which gave rise to the amphibians. Here we 
have a valuable link with the past that gives 
us an understanding of an important group 
of vertebrates, previously known only from 

Many of the ancient fishes were covered 
with ganoid scales. The sturgeons (Fig. 252) 
and garpikes are protected in this way. Only 



Figure 263. An ancient fish [Latimeria), first caught off the coast of Africa in 1938; sup- 
posedly extinct for milhons of years. It is a "Hving fossil/' estimated to have lived in the 
Devonian period. Length about 5 feet. Note the thick lobe-shaped fins. (Courtesy of Sport and 
General Press Agency, Ltd., London.) 

two species of paddlefishes are now known: 
Polyodon lives in the Mississippi Valley, and 
Psephurus in China. The fresh-water dogfish, 
Amia calva, is the only existing species of 
the family Amiidae. 

Fishes that live in caves 

Six species of cave fishes hve in the subter- 
ranean streams of the cave region of In- 
diana, Kentucky, and Missouri. They are 
small, but of special interest because the 
eyes of some are rudimentary and covered 
with a thick skin. Cavefish [Amblyopsis) 
(Fig. 252) is common in the River Styx of 
Mammoth Cave. Did the sightless cave 
dwellers lose their eyes after they took up 
cave life, or were they blind fishes before 
they began their life underground? The an- 
swer is, we do not know. 

Flying fish 

Sixty-five or more species of flying fish live 
in warm seas (Fig. 252). Some are able to 
leave the water and, rising in the air a few 
yards, "fly" a distance of from a few rods to 
more than Vs mile. Contrary to a popular 
belief, the pectoral fins do not force the fish 

forward, but simply sustain the body in the 
air. It is the tail fin only that is used for 
fonvard propulsion when the fish skims the 
water surface during the "take-off." 


The true eels (Fig. 252) should not be 
confused with the so-called lamprey eels 
which are cyclostomes. The single species of 
North American fresh-water eel, Anguilld 
rostrata occurs in the streams of the Atlantic 
Coast. It is long and slender, and its scales 
are inconspicuous. The dorsal, caudal, and 
anal fins are continuous. Eels enter the sea 
in the autumn to spawn, after which they 
die. The eggs are laid in deep water off the 
Bermuda shore. The young develop in the 
sea and then migrate up the rivers. 


The famous nest-building stickleback 
(Fig. 266), has 5 large spines on its back. 
The nest is built of sticks fastened together 
with threads secreted by a gland in the male. 
The female lays eggs in the nest; the male 
then enters the nest and fertilizes them, after 
which he guards them from intruders. 




Seahorses (Fig. 252) are small and do not 
look much like fish, the head resembling 
that of a horse. They swim by means of the 
dorsal and pectoral fins, holding themselves 
in a vertical position; they move at a snail's 
pace. Often they cling to objects with their 
prehensile tail. The eggs are carried in a 
brood pouch of the male until they hatch. 
The sea dragon of Australia (Fig. 264) is 

Figure 264. Australian sea dragon, Phyllopteryx, 
the most bizarre of all seahorses. The postures and 
antics of these fishes are as distinctive and grotesque 
as their appearance. The leaflike extentions from 
the body tend to conceal the fish among seaweed. 
Natural size up to 12 inches long. (Courtesy of 
the American Museum of Natural History.) 

provided with leaflike appendages of skin 
which have a remarkable resemblance to 
seaweed among which they live. These fishes 
represent a more specialized stage in evolu- 
tion than the streamlined fishes, 

Porcupine fishes 

These inhabitants of tropical seas are 
covered with movable spines, hence their 
name. They live on the bottom among sea- 
weeds and corals, and, when disturbed, in- 
flate their bodies by swallowing water or air 
(Fig. 266), in which condition they are not 
easily injured by their enemies. 


Living on the bottom of the Atlantic, 
Indian, and Pacific oceans are about a 
dozen genera of extremely large-mouthed 
fishes known as anglers (Fig. 265). Lophius, 
the fishing frog or goosefish, occurs along 
the Atlantic Coast of North America. Its 
long dorsal ray is inserted on the snout and 
serves as a fishing rod with "bait." The latter 
consists of dermal tentacles. Their wormlike 
appearance attracts other fishes, which are 
engulfed into the big mouth cavity as the 

r-^ " " " " " " ^ ■ ■ ■ ■ 





,1. i 


Figure 265. An anglerfish from the deep sea; a 
fish that fishes. Note large mouth with sharp teeth 
and luminous dermal tentacles projecting from the 
upper and lower jaws. The one on top of the snout 
is called a rod with a luminous "bait" at the tip. 
(Courtesy of the American Museum of Natural 


jaws are quickly opened. This fish reaches a 
length of over three feet and has a mouth 
more than a foot wide. 

Deep-sea fishes 

Many families of fishes contain deep-sea 
species (Fig. 265) which are often curiously 
modified. Some have very large eyes, which 
enable them to catch as many rays of light 
as possible; these eyes probably serve in con- 
nection with luminescent organs. Others 
have small or rudimentary eyes and are 
blind; they depend upon organs of touch 
instead of eyes. Many have large mouths 
with long sharp teeth and enormous stom- 
achs. The luminescent organs are variously 
distributed over the body. One type consists 
of a cup of secretory cells covered by a cellu- 
lar lens. The secretion is luminous, and in 
certain cases acts as a lure; in others, it prob- 
ably enables the fish to see in the dark 
abyss of the ocean. 


The ability of the lungfishes to breathe 
air is suggestive of an intermediate stage be- 
tween fishes and amphibians. Furthermore, 
the Australian lungfish is able to "walk" 
along the bottom of the rivers in which it 
lives by using its paired fins as legs. Yet in 
spite of such specializations in the lungfishes 
which might lead one to conclude that they 
were a connecting link between water and 
land animals, the over-all evidence points 
clearly to the fact that these vertebrates 
have never been in the direct line of evolu- 
tion leading from fishes to the first land- 
living vertebrates. The lungfishes are now 
regarded as an ancient group that has 
changed little through the recent geologic 

The lungfish has an opening between the 
nasal sac and the mouth cavity, a persistent 
unconstricted notochord, and an air bladder 
which opens into the pharynx and functions 
as a lung. The Australian lungfish Neocera- 


todus (Fig. 252) lies on the bottom of 
stagnant pools and feeds on small animals; 
occasionally it comes to the surface in order 
to change the air in its single lung. Because 
of this lung it can exist in water unfit for 
fishes that breathe entirely with gills. 

The African lungfishes, Protopterus, live 
in the marshes of central Africa. During the 
dry summer season they burrow about 18 
inches into the mud, where a cocoon of 
slime is secreted; here they remain inactive, 
breathing with lungs and living on fat 
stored in the kidneys and gonads until the 
rainy season comes again. The South Amer- 
ican lungfish Lepidosiren also hibernates 
in the mud during the dry season. 

Fossil fishes 

A large number of species of fish are 
known only from their fossil remains. The 
earliest fish remains consist of spines and 
scales from the lower Silurian or Ordovician 
strata of the earth's crust, which were laid 
down probably over 300 million years ago. 
The Devonian age is called the "Age of 
Fishes" because of the predominance of 
fishes over the other animals that lived at 
that time. A considerable portion of the 
Osteichthyes are fossils: 4 of the 7 families 
of the Neoceratodida (Dipnoi); 6 of the 7 
families of the Crossopter^'giida; 41 of the 
43 families of Chondrosteica; 9 of the 11 
families of Holosteica; and but 28 of the 402 
families of Teleosteica. The study of fossil 
fishes is very important because of the light 
these prehistoric forms shed upon the af- 
finities of modern species. 


Although a few fishes are injurious be- 
cause they destroy valuable food fishes and 
other useful aquatic animals, many are of 
use to man, serving either as food or as a 
means of recreation. Among the fresh-water 

Top minnow (mosquito destroyer) 

Brook stickleback 


Deep-sea angler fish 

Remora (sucker for 
attaching to other animals) 

Lantern fish 

FiGURt 266. Some bony fishes to show the great variety of forms in the modern fish group. 
The codfish, herring, and tuna are of great economic importance. 




game fishes are the yellow perch, various 
species of trout, pilces, muskellunge, and bass 
(Fig. 252). Marine game fish include the 
tarpon (Fig. 266), sea bass, and tuna. 

Marine food fish are of great value. Her- 
ring (Fig. 266), in vast quantities, are 
smoked, salted, pickled, and packed as 
sardines. Mackerel are caught in enormous 
numbers. The flounder family (Fig. 262) 
contains halibuts, soles, plaice, and turbots. 
Codfish (Fig. 266) is especially valuable, 
together with other members of its family 
such as pollacks, haddocks, and hakes. The 
average annual catch of codfish is over two 
billion pounds. Cod-liver oil is also the prin- 
cipal source of vitamin A and vitamin D. 
Rivaling the codfish in value are the salmon 
of the Pacific Coast, which are caught and 
canned in large quantities. In fresh water, 
live whitefish, lake trout, catfish and perch- 
all important food fishes. The eggs of stur- 
geons and certain other fishes are made into 
caviar, especially in Russia. The use of fish 
meal has become important in the fertilizer 
and pet-food industries. 

Fishing is becoming increasingly popular 
as a recreation for millions of people. In one 
midwest state alone over a million fishing 
licenses are sold each year. The money spent 
by fishermen in pursuing their sport runs 
into manv millions of dollars. Enormous 
sums are spent by the federal and state gov- 
ernments in rearing fish and in stream and 
lake improvement work. 

Many food fish have decreased markedly 
in numbers due to overfishing, pollution of 
streams and lakes by sewage and chemical 
wastes, and other causes. For this reason the 
federal and state governments have placed 
certain restrictions on fishing and have also 
undertaken to propagate certain species arti- 
ficially. These include fresh-water species 
such as whitefish, lake trout, pike perch, 
and bass, and marine species such as codfish, 
haddock, salmon, flounders, and sardines of 
the Pacific Coast. 

Among the fish of benefit to man should 
be mentioned the top minnows {Gambusia, 

Fig. 266) which feed voraciously on mos- 
quito larvae. These are placed in bodies of 
fresh water to prevent the breeding of mos- 
quitoes that transmit malaria and yellow 

The scales of the garpike are used for 
jewelry and novelties. 

In recent years, considerable use has been 
made of fishes for experimental animals, 
especially in the fields of genetics, embryol- 
ogy, animal behavior, and pharmacology. 

There has been a tremendous growth of 
interest in tropical fishes; many of them are 
popular for the home aquarium. Pet shops 
now stock manv kinds of fishes for both 
scientists and hobbvists. 


(For reference purposes only) 

About 40,000 living species of fishes are 
known from the entire world according to 
Schultz; of these, about 168 families and about 
3300 species occur in North America. The 
families shown in Fig. 252 are some of the 
better known representatives of the orders 
listed below: 

Class 1. Osteichthyes (bony fishes). 
Subclass 1. Choanichthyes. 

Order 1. Neoceratodida. Ex. lungfishes 

(Fig. 252). 
Order 2. Crossopterygiida. Ex. lobe- 
finned fishes. 
Suborder 1. Coelacanthiina. Ex. 
Latimeria (Fig. 263). 
Subclass 2. Actinopterygii. Ex. rayed fins. 
Superorder 1. Chondrosteica (cartilage 
and bone). 
Order 1. Polypterida. Ex. lobefinned 

fishes (Fig. 252). 
Order 2. Acipenserida. Ex. sturgeons 
(Fig. 252). 
Superorder 2. Holosteica (bone and car- 

* TTiis classification is according to Leonard P. 
Schultz, Curator of Fishes, United States National 
Museum, Smithsonian Institution, 



** Order 



Order 1. Lepisosteida. Ex. gars. 
Order 2, Amiida. Ex. bowfin (Fig. 
Superorder 3. Teleosteica (perfected 

**Order 1. Isospondylida. Exs. herring, 
salmon, deep-sea fishes, tar- 
pon, shad, whitefish, and 
lake herring. 
Order 2. Bathyclupeida. Ex. deep-sea 

3. Mormyrida. 

4. Ateleopida. 

5. Giganturida. 

6. Lyomerida. 

7. Ostariophysida. Exs. cat- 
fishes, minnows, characins 
(Fig. 252), suckers, carp, 

8. Anguillida. Ex. eels (Fig. 

9. Heteromida. Ex. spiny eels. 
Order 10. Synbranchiida. Ex. mud 


Order 11. Cyprinodontida. Exs. 

toothed carps, top minnows. 

Order 12. Salmopercida. Ex. trout 

Order 13. Berycomorphida. Ex. Bery- 
coid fishes. 

Order 14. Zeomorphida. Ex. John 
**Order 15. Anacanthida. Exs. codfishes, 
haddock, pollack, burbot. 

Order 16. Thoracosteida. Ex. stickle- 

Order 17. Solenichthyida. Exs. trum- 
pet and pipefishes, sea 

Order 18. Allotriognathida. Exs. moon- 
fishes, ribbonfishes. 
**Order 19. Percomorphida. Exs. perches, 
basses, gobies, blennies (Fig. 
252), snappers, parrot fishes, 
and butterfly fishes. 

Order 20. Scleropareida. Exs. scorpion 

and rockfishes. 
Order 21. Cephalacanthida. Ex. gurn- 
ards (Fig. 252). 
**Order 22. Pleuronectida. Exs. flat- 

fishes, soles, flounders, hali- 

Order 23. Icosteida. Ex. ragfishes. 

Order 24. Chaudhurida. 

Order 25. Discocephalida. Exs. Shark 
suckers, rcmoras. 

Order 26. Plectognathida. Exs. puffers, 
filefishes, trunkfishes (Fig. 

Order 27. Gobiesocida. Ex. clingfishes. 

Order 28. Batrachoidida. Ex. toad- 

Order 29. Pediculatida. Exs. anglers, 
gooscfish (Fig. 252). 


Axclrod, H.R., and Schultz, L.P. Handbook of 

Tropical Aquarium Fishes. McGraw-Hill, 

New York, 1955. 
Berg, L.S. Classification of Fishes Both Recent 

and Fossil. Edwards, Ann Arbor, Mich., 

Brown, M.E. The Physiology of Fishes. Vols. 

1 and 2. Academic Press, New York, 1957. 
Curtis, B. The Life Story of the Fish: His 

Morals and Manners. Harcourt, Brace, New 

York, 1949. 
Hubbs, C.L., and Lagler, K.F. Fishes of the 

Great Lakes Region. Cranbrook Institute of 

Science, Bloomfield Hills, Mich., 1947. 
Jordan, D.S. A Guide to the Study of Fishes. 

Holt, New York, 1905. 
Lagler, K.F. Freshwater Fishery Biology, W.C. 

Brown, Dubuque, Iowa, 1956. 
LaGorce, J.O. The Book of Fishes. National 

Geographic Society, Washington, 1939. 
Norman, J.R. A History of Fishes. Bern, Lon- 
don, 1931. 
Schultz, L.P., with Stern, Edith. The Ways of 

Fishes. Van Nostrand, New York, 1948. 
Treassler, D.K. Marine Products of Commerce. 

Chemical Catalogue Co., New York, 1923. 
Walton, Izaak. The Compleat Angler. Luck 

Company, London, 1653. 

** Orders of special economic importance. 



Class Amphibia. 

Frogs, Toads, 


and Others 

HE common amphibians are the frogs, 
toads, and salamanders (Fig. 267). They 
spend part or all of their existence in the 
water or in damp places. Most lay their 
eggs in the water; and the larvae, which 
breathe with gills, are known as tadpoles or 
pollywogs. Some amphibians are often con- 
fused with reptiles, especially the lizards, 
because of their similarity of form, but al- 
most all reptiles possess scales and are not 
slimy, whereas amphibians usually have a 
smooth slimy skin without scales except in 
a few rare species. 


There are 10 orders of extinct Amphibia, 
and three orders of living forms, which are 
as follows: 

1. The Apoda (Gymnophiona), which are 
commonly called caecilians, are wormlike 
amphibians inhabiting tropical and sub- 
tropical regions. 

2. The Caudata are amphibians with tails; 
they include the mud puppies, sirens, and 

3. The Salientia are frogs and toads, which are 
tailless in the adult stage. 

The United States is a paradise for the 
student of Amphibia because it contains 
large numbers of species and individuals. 
Since all amphibians require moisture, they 
should be looked for in or near bodies of 
fresh water and in moist places such as 
under logs and stones in damp woods. 
Among the most interesting features of am- 
phibians are their ability to change color, 
their powers of regeneration, their varied and 
often curious breeding habits, their methods 
of spending the winter, and their poisonous 
secretions. As is the case with birds, more 
amphibians are heard than seen; hence it is 
advisable to become acquainted with the call 
notes of the various species. Many of these 
can be easily learned from the field record- 
ings made and narrated by Charles M. 
Bogert in 1958. 



(Red-backed salamander) 



(Extinct Eryops) 

Figure 267. Some orders and families of amphibians. Probable relationships of the more im- 
portant living families are indicated by lines; the lines are broken where greatest doubt exists. 
(Based on diagram by Charles M. Bogert, Curator of Amphibians and Reptiles, American 
Museum of Natural History; made expressly for this text.) 




Color and color changes 

The colors in the skin of amphibians are 
due to scattered pigment granules in the 
epidermis and to pigment cells in the dermis. 
The latter are usually brown, black, yellow, 
or red and are contained in cells called 
chromatophores. The power of changing 

color is possessed by most amphibians, and 
especially by frogs. The common leopard 
frogs are supplied with black pigment cells 
called melanophores and with interference 
cells that contain whitish crystals, golden 
pigment cells, and sometimes red pigment 
The black melanophores are branching 


Melanophore with pigment 
dispersed throughout cell 

Pigment beginning 
to concentrate 

Pigment concentrated 
in body of cell 

Figure 268. Stages in the changes of pigment-bearing cells (melanophores) in the skin of 
the frog. The color variation from the fully expanded to the completely contracted melanophore 
is black to gray. 

cells as shown in Fig. 268. When the pig- 
ment is dispersed, it covers a larger area and 
consequently gives the skin a darker color. 
When the pigment is concentrated the skin 
becomes lighter. These changes in the color 
of the skin are shown in Fig. 269. The yel- 
low pigment is contained in spherical golden 
cells. There is no green pigment; the green 
color results from a combined effect of light 
reflected from the granules of the interfer- 
ence cells and the yellow pigment through 
which the light passes. Most of the color 
changes are due to changes in the concentra- 
tion of the black and yellow pigments. 

Color changes are brought about primar- 
ily by a hormone called intermedin,* which 
is secreted by the intermediate lobe of the 
pituitary gland. Intermedin causes a dis- 

* Intermedin causes dispersion of pigment in the 
melanophores of fishes, amphibians, and reptiles, but 
appears to have no effect on the pigmentation of 
warm-blooded birds and mammals. 

persion of pigment granules. Light is the 
chief stimulus; it acts through the eye. Ex- 
perimentally blinded frogs show a reduced 
capacity to change color. In a bright light 
the skin of the frog becomes light in color, 
whereas in the dark it changes to a darker 

Changes in color are due to both external 
and internal conditions; temperature is an 
important external factor. When the tem- 
perature is raised, the pigment becomes 
more concentrated, and the skin changes to 
a lighter color. When the temperature is 
lowered, the pigment becomes expanded, 
and a darker color results. It is evident that 
changes in the skin color of the frog are in 
part due to a hormone (intermedin) and in 
part to the nervous system. Usually the 
color changes are such as to cause the frog 
to resemble more closely its surroundings; 
thus it becomes less conspicuous and is pro- 
tectively colored. 



Figure 269. Leopard frogs. Dark color phase on 
the left, and hght color phase on the right. When 
the animal is in darkness, the hormone intermedin 
is secreted into the blood stream; the pigment is 
then dispersed, causing the dark color phase. But 
when the retina of the eye is stimulated by light, 
nerve impulses pass to the intermediate lobe of the 
pituitary gland, causing the production of intermedin 
to be suppressed, which results in the light color 
phase. (Photo courtesy of Douglas Eastwood.) 


The power of regenerating lost parts is 
remarkably well developed in many Am- 
phibia. For example, the foot of a two-year- 
old axolotl was cut off, and in 12 weeks a 
complete foot was regenerated in its place. 
The newt (Fig. 270 A) has been observed to 
regenerate both limbs and tail. The tailless 
amphibians are apparently unable to regen- 
erate lost parts to any considerable extent, 
except in the early stages. As a general rule, 
the younger tadpoles regenerate limbs or a 
tail more readily than older specimens. 
There is a distinct advantage in this posses- 
sion of the power of regeneration, since am- 
phibians often escape from their enemies 
with mutilated limbs or tails; but they are 
not permanently inconvenienced by the loss, 
since new parts rapidly grow out. 

Breeding habits 

Most Amphibia are oviparous; and their 
eggs, as in the leopard frog, are fertilized 
by the male after extrusion. In some of the 
tailed forms, however, the eggs are fertilized 
before they are laid. A few species bring 
forth their young alive; for example, the al- 
pine salamander, Saiamaijdra. 

Several curious breeding habits are ex- 
hibited by certain species. The male obstet- 
rical toad (Fig. 267), carries the egg strings 
with him, wound about his hindlimbs; and 
when the tadpoles are ready to emerge, he 
takes to the water and allows them to escape. 

The eggs of the Surinam toad (Fig. 267) 
are placed on the back of the female during 
copulation, are held there by a sticky secre- 
tion, and are gradually enveloped by the 
skin. Within these epidermal pouches, the 
eggs develop and the tadpole stage is passed; 
then the young toads escape as air-breathing 
aquatic animals. 

The American bell toad (Fig. 267) found 
in California is unique in that fertilization is 
internal. The male grasps the female around 
the pelvis, and by use of an external taillike 
copulatory organ the sperms are deposited 
in the cloaca of the female. 

The brooding or marsupial tree frogs of 
Venezuela, Gastrotheca, have a pouch with 
an opening in the posterior part of the trunk 
in which the eggs are placed and the young 
are reared. The female of another species 
of tree frog carries her eggs in a depression 
on her back until they are almost ready for 


Many amphibians bury themselves in the 
mud at the bottom of ponds in the autumn 
and remain there in a dormant condition 
until the following spring. During this 
period of hibernation, the vital processes 
are reduced; no air is taken into the lungs, 
since all necessary respiration occurs through 
the skin; no food is eaten, but the physio- 



logic activities are carried on by the use 
of nutriment stored in the body; and the 
temperature of the animal is only slightly 
above that of the surrounding medium. The 
body temperature of all cold-blooded verte- 
brates— cyclostomes, elasmobranchs, fishes, 
amphibians, and reptiles— varies with the 
surrounding medium. Frogs cannot be en- 
tirely frozen, as is often reported, since death 
ensues if the heart is frozen. In v^^arm coun- 
tries many amphibians seek a moist place 
of concealment; they pass the hotter part of 
the year in a quiet, torpid condition; they 

Poisonous amphibians 

The poison glands of the leopard frog 
have already been mentioned. Certain sala- 
manders and newts are also provided with 
poison glands. As a means of defense the 
poison is very effective, since an animal that 
has once felt the effects of an encounter 
with a poisonous amphibian will not soon 
repeat the experiment. Some of the most 
poisonous species, for example Salamandra 
salamandra, are said to be warningly colored. 
Dogs and cats that catch and bite Bufo 
marinus often die from the toxic effects of 
their secretions. 

Fossil amphibians 

From fossils, it has been determined that 
amphibians first appeared during the Age 
of Fishes in the Devonian period (p. 617). 
From that time on they increased so rapidly 
in numbers that the Late Paleozoic or Car- 
boniferous period is spoken of as the Age of 
Amphibians. The Paleozoic amphibians are 
known as Stegocephali, a term that refers to 
the covered or mailed head, roofed over by 
dermal bones. Stegocephali were salamander- 
like animals that probably lived in fresh 
water or on land. Some of them are called 
labyrinthodonts because the dentine of their 
teeth is much folded. Primitive reptiles 
(cotylosaurs) and perhaps mammals stem- 

med directly from stegocephalians, and the 
stegocephalians themselves came from cros- 
sopterygian ancestors. 


Legless amphibians 

The family Caeciliidae includes over 50 
species of wormlike or snakelike legless Am- 
phibia (Fig. 266). They inhabit the tropical 
regions of the Americas, Africa, and Asia. 
They burrow in moist ground with their 
strong heads and possess eyes that are small 
and concealed. A sensory tentacle, which can 
be protruded from between the eyes and the 
nose, aids the animal in crawling about. 

Giant salamanders 

The family Cr\ptobranchidae contains 
two genera of giant salamanders. The Amer- 
ican hellbender, Cryptobranchus alleganien- 
sis (Fig. 267), occurs only in the streams of 
the eastern United States; it reaches a length 
of from 18 to 27 inches. The giant salaman- 
der of Japan is the largest living amphibian, 
reaching a length of over 5 feet. 


Tlie tiger salamander, Ambystoma ti- 
grinum, occurs from New York to California 
and south to central Mexico and reaches a 
length of from 6 to 10 inches. In some parts 
of its geographic range, it fails to metamor- 
phose and reproduces while it is in a larval 
state. Such a larval form is called an 
axolotl; it was long considered a separate 
species because the external gills persisted 
into the adult. However, if an axolotl is fed 
beef thyroid, even one or two meals, it de- 
velops into a land animal; it loses its gills and 
becomes an air-breathing salamander. This 
is not now thought to be a case of retarded 
evolution, but a secondary specialization for 
arid regions. Nonmetamorphosing forms 




Figure 270. A, eastern newt, Diemictylus vindescens [Wi inches long). B, the "congo eel," 
Amphiuma means (32 inches long) is a semilarval type of amphibian. (Courtesy of N.Y. 
Zoological Society.) 

arose in all probability from stocks that did 
undergo metamorphosis. Feeding the hor- 
mone (thyroxin) reverses this specialization. 


The newts belong to the salamander fam- 
ily. The crimson-spotted newt, Diemicty- 
lus viridescens, lives in the water as a larva; 
but when it is about one inch long it loses 
its gills and usually lives on land for about 
one or two years. During its terrestrial life 
it is a bright coral red in color and is known 
as the red eft. It then returns to the water 
and changes to the adult coloration of yel- 
lowish green with black spots on the under 
surface and a row of black-bordered crimson 
spots on both sides. The cold slimy skin of 

the salamanders gave rise to the belief in 
medieval times that the salamanders could 
live in fire and not be injured by it. The 
skin of the fire salamander of Europe se- 
cretes a particularly poisonous substance. 
This species is black with bright yellow spots 
and therefore very conspicuous; its colors are 
supposed to warn other animals that it is 


The family Bufonidae includes over 100 
species of toads, most of which belong to the 
genus Bufo. About 15 species of this genus 
have been reported from the United States. 
The common toad of the northeastern 
United States, Bu)0 terrestris (formerly 



Figure 271. Necturus, commonly called mud 
puppy. An aquatic species about 1^/2 feet long. 
(New York Zoological Society Photo.) 

americanus) (Fig. 272), possesses a rough, 
warty skin, but it does not cause appearance 
of warts upon the hands of those who han- 
dle it, as is often supposed. Toads secrete a 
milky poisonous fluid by means of glands in 
the skin, which protects them from many 
animals that would otherwise be important 
enemies. During the day they remain con- 
cealed in some dark damp place, but at night 
they hop about, feeding upon worms, snails. 

Figure 272. The common American toad. Note 
warty skin, bright markings, and jewel-like eye. Nat- 
ural size 3V2 inches long. (Courtesy of the American 
Museum of Natural History.) 

and especially insects, which they capture 
with their sticky tongues as frogs do. 

Tree frogs 

Tree frogs are usually jirboreal amphib- 
ians with adhesive disks en their toes and 
fingers that usually enable them to climb 
trees. They are provided with large vocal 
sacs and have a correspondingly loud voice; 
see headpiece at beginning of this chapter 
for illustration of vocal sacs distended with 
air. Of the more than 180 species, 21 occur 
in the United States and about 130 in Cen- 
tral and South America. The common tree 
frog, Hyla versicolor (Fig. 267), is about two 
inches long. Other common tree frogs are 
the spring peeper, Hyla crucifer, and the 
cricket frog, Acris gryllus. 

True frogs 

The family Ranidae contains the true 
frogs. These occur in all parts of the globe 
except Australia, New Zealand, and southern 
South America. Only one genus, Rana, and 
about 16 species live in the United States. 
Of these, the leopard frog (Fig. 217) is the 
most common. The bullfrog Rana cates- 
beiana (Fig. 267) is the largest of the family 
in this country, often reaching a body length 
of 6 or 8 inches. Bullfrogs usually remain in 
or near water. They possess a deep bass voice 
like that of a bull. The tadpoles do not be- 
come frogs the first year, as do those of the 
leopard frog, but transform during the sec- 
ond or even the third year. Other true frogs 
include the green frog Rana clamitans, the 
eastern wood frog Rana sylvatica, and the 
pickerel frog Rana palustris. 


Amphibians are virtually all beneficial to 
man. Many are so rare as to be of little value, 
but the frogs and toads are of considerable 
importance. Frogs have been and are now 



H ^mmp' <*■ ^ 

ss^^^'V.. >■-. . " jfe'aSWWI^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^H 

Figure 273. Hy/d andersonii, a tree frog. (Courtesy of N.Y. Zoological Society.) 

used extensively for laboratory dissections, 
physiologic experiments, human pregnancy 
tests, pharmacology, and for fish bait. Mud 
puppies {Necturus) also serve as teaching 
material; one firm in Chicago sells 2500 or 
more per year for this purpose. The skins 
of frogs are used for glue and book bindings. 
Frog legs are eagerly sought as an article 
of food; more than three million pounds are 
eaten every year in the United States. Mud 
puppies are also edible. In Japan, the giant 
salamander is much esteemed as an article 
of food. 

Hellbenders are considered poisonous by 
many people, but they are not dangerous to 
man. Many superstitious beliefs are held 
about amphibians, such as: salamanders are 
not injured by fire, a croaking frog predicts 
rain, and the toad has a jewel in its head. 
In China, the skin of the toad is used as a 
medicine; its use may have some therapeutic 
value, since certain glands contain a digitalis- 
like secretion which increases the blood pres- 
sure when injected into human beings. 

Frogs and toads are widely recognized as 
enemies of injurious insects. The toads are 
of special value, since they live in gardens 
where insects are most injurious. In France 
the gardeners even buy toads to aid them in 
keeping obnoxious insects under control. 
Bufo marinus has been introduced in the 

tropics, especially where sugar cane is grown, 
to control insects. 

Frog farming has been promoted for pleas- 
ure and profit, but it has generally proved 
a great disappointment to those who en- 
gaged in the enterprise. Many so-called 
"farms" are only favorable marshes where 
natural reproduction is encouraged. Artificial 
rearing of frogs is not practicable because it 
is difficult to find a satisfactory supply of 
food; unless the frogs of different sizes are 
separated, the large ones eat the smaller indi- 
viduals; losses from predators and disease 
may be high where there is a great concen- 
tration of frogs; and the selling price has 
been inadequate to make the business profit- 
able. Most of the money in frog farming has 
been made by unscrupulous promoters who 
sold breeding stock and books on frog rais- 
ing at exorbitant prices. 


{For reference purposes only) 

Class Amphibia. About 2500 different spe 
cies of living Amphibia are known, a number 
very much smaller than that of the other prin- 
cipal classes of vertebrates. Approximately 60 



belong to the order Apoda (Gymnophiona), 
about 200 to the Caudata, and approximately 
1740 to the Salientia. 

Order 1. Apoda (Gr. a, not; podos, foot) or 
Gymnophiona (Gr. gymnos, 
naked). Caecilians (Fig. 267). 
Wormlike; no limbs or limb gir- 
dles; sometimes with small scales 
embedded in skin; tail short or 
Family 1. Cacciliidae. Ex. Ichthyophis 
glutinosa, blindworm. 
Order 2. Caudata (L. cauda, tail). Tailed 
Amphibia. With a tail; without 
scales; usually two pairs of 
Family 1, Cryptobranchidae. Hellben- 
ders. Ex. Cryptobranchus al- 
leganiensis, American hell- 
bender (Fig. 267). 
Family 2. Ambystomidae. Ex. Amby- 
stoma tigrinum, tiger salaman- 
der (Fig. 267). 
Family 3. Salamandridae. Salamanders 
and newts. Ex. Diemictylus 
viridescens, eastern newt (Fig. 
Family 4. Amphiumidae. Ex. Amphiuma 

means, Congo eel (Fig. 267). 
Family 5. Plethodontidae. Lungless sal- 
amanders. Ex. Plethodon cin- 
ereus, red-backed salamander 
(Fig. 267). 
Family 6. Proteidae. Ex. Necturus macu- 

losus, mud puppy (Fig. 271 ) . 
Family 7. Sirenidae. Ex. Siren lacertina, 
greater siren (Fig. 267). Eel- 
shaped amphibians having 
small forclimbs, but lacking 
hindlimbs and pelvis and hav- 

ing permanent external gills as 

well as lungs. 

Order 3, Salientia (L. salio, leap). Tailless 

Amphibia. Without a tail; without 

scales; two pairs of limbs; without 

external gills or gill openings in 


Family 1. Pelobatidae. Ex. Scaphiopus 

holbrookii, spadefoot toad. 
Family 2. Bufonidae. Ex. Bufo terrestris, 

toad (Fig. 272). 
Family 3. Hylidae. Ex. Hyla versicolor, 

tree frog (Fig. 267). 
Family 4. Ranidae. Ex. Rana pipiens, 
leopard frog (Fig. 217). 


(See also Chapter 23) 

Barbour, T. Reptiles and Amphibians, Their 

Habits and Adaptations. Houghton Mifflin, 

Boston, 1934. 
Bishop, S.C. Handbook of Salamanders. The 

Salamanders of the United States, of Canada. 

and Lower California. Comstock, Ithaca, 

N.Y., 1943. 
Noble, G. Kingslev. Biology of the Amphibia. 

McGraw-Hill, New York, 1951. Reprinted, 

Dover Publications, 1955. 
Oliver, J.A. The Natural History of North 

American Amphibians and Reptiles. Van 

Nostrand, New York, 1955. 
Stebbins, R.C. Amphibians and Reptiles of 

Western North America. McGraw-Hill, New 

York, 1954. 
Wright, A.H., and Wright, A.A. Handbook of 

Frogs and Toads: Of the United States and 

Canada. Comstock, Ithaca, N.Y., 1949. 



Class Reptilia. 

Turtles, Lizards, 

Snakes, Crocodiles, 

and Others 

HE reptiles constitute one of the most in- 
teresting, and, in general, one of the least- 
known classes of the vertebrates. They are 
cold-blooded; usually covered with scales, 
and, frequently, with bony plates; and they 
always breathe by lungs. The popular no- 
tion that reptiles are slimy is erroneous. 
Contrary also to general belief, very few- 
reptiles, at least those in the United States, 
are dangerous to man; the majority are harm- 
less, and many are even beneficial. The rep- 
tiles living today are but a fraction of the 
vast hordes that inhabited the earth's sur- 
face in prehistoric times. In fact, of the 
approximately 16 orders of reptiles now 
recognized by herpetologists, only 4 possess 
living representatives, and one of these in- 
cludes a nearly exterminated species con- 
fined to New Zealand. The 4 orders of living 
reptiles are as follows: 

Order 1. Chelonia ( Testudinata ) . Turtles 
and tortoises. 

Order 2. Rhynchocephalia. Sphenodon, a liz- 
ardlike reptile confined to New Zealand. 

Order 3. Squamata. Lizards, and snakes. 

Order 4. Crocodilia. Crocodiles, alligators, 
gavials, and caimans. 

The reptiles are better adapted for living 
on land than amphibians. Some of the ad- 
vances shown by reptiles over amphibians 
are: ( 1 ) a dry scaly skin which is an adapta- 
tion for a complete existence on land, (2) 
limbs better suited for rapid locomotion, (3) 
partial or complete separation of the ven- 
tricle resulting in further separation of the 
oxygenated and nonoxygenated blood in the 
heart, (4) well-ossified skeleton, (5) some 
form of copulatory organ which is necessary 
for internal fertilization, (6) eggs with shells 
suited for development on land and protec- 
tive embryonic membranes to prevent dry- 
ing, another adaption for life on land. 

Reptiles are most abundant in the warmer 
regions of the world; very few live in the 
colder parts of the temperate zone, and none 
in the Arctic or Antarctic regions. Neverthe- 
less, the United States is well supplied with 




both species and individuals. About 243 
species of all the reptiles that have been 
described are known to occur in this coun- 
try. Reptiles occupy an important place in 
the vertebrate series because their anatomy 
is intermediate between that of a typical 
amphibian on the one hand and that of a 
typical bird on the other. Comparison of the 
structure and physiology of reptiles with 
those of amphibians and birds is well worth 
while. The poisonous nature of certain rep- 
tiles, the enormous size of some of the pre- 
historic species, and the relations of reptiles 
to man are among the most interesting gen- 
eral features of the class. 


The turtle has been selected as a repre- 
sentative reptile. The body is so constructed 
that it is adapted to live either in the water 
or on land. Although it is slow-moving on 
land, it can swim quite rapidly. 

External features 

The turtle is distinguished from all other 
animals by the shell which is broad and 
flattened and protects the vital organs (Fig. 
275). Even the head, limbs, and tail can be 
more or less completely withdrawn into the 
shell. The neck is long and very flexible; the 
head is flattened dorsoventrally. The mouth 
is large, but instead of teeth, horny plates 
form the margin of the jaws; they are used 
to crush their food. The external nares (nos- 
trils) are placed together, near the anterior 
end of the snout. The eyes, situated one on 
each side of the head, are each guarded by 
three eyelids: (1) a short, thick, opaque 
upper lid; (2) a longer, thin lower lid; and 
(3) a transparent nictitating membrane, 
which moves over the eyeball from the an- 
terior corner of the eye. Just behind the 
angle of the jaw on either side is a thin 
tympanic membrane. The limbs usually 
possess 5 digits each; most of the digits are 
armed with large horny claws that are useful 

in crawling, climbing, or digging. The skin 
is thin and smooth on the head, but thick, 
tough, scaly, and much wrinkled over the 
exposed parts of the body. 


Since the life of the turtle is influenced so 
strongly by the skeleton, this system will be 
briefly described first. 

The shell (Fig. 275) consists of a convex 
dorsal armor, the carapace, and a flattened 
ventral armor, the plastron; these are 
strongly bound together on each side by 
bony bridges varying in width with the 
species. Both carapace and plastron are usu- 
ally covered by a number of symmetrically 
arranged horny plates, called scutes 
(shields); the scutes do not correspond 
either in number or arrangement to the 
bony plates beneath them. The number and 
shape of the scutes vary according to the 
species but are usually constant in individ- 
uals of the same species. The horny scutes 
of the hawksbill turtle furnish the tortoise 
shell of commerce. Beneath the scutes are a 
number of bony plates formed by the dermis 
and closely united by sutures. 

The vertebrae and ribs are usually con- 
solidated with the bony carapace; no ster- 
num is found in these forms. Soft-shelled 
turtles (Fig. 280) have a leathery shell 
which is not divided into scutes, and it con- 
tains little bony substance. 

Digestive system 

Turtles feed on both plants and animals; 
some are entirely vegetarian. The animals 
preyed upon are waterfowl, small mammals, 
and many kinds of invertebrates. The flex- 
ible neck enables the turtle to rest on the 
bottom and reach out in all directions for 
food. The jaws of large snapping turtles are 
powerful enough to amputate a finger, or 
even a hand. 

The digestive organs are simple. The 
broad soft tongue is attached to the floor of 
the mouth cavity; it is not protrusible. The 


Fossil Labyrinthodontia 

(Snake-necked turtle) 

Figure 274. Some orders and families of reptiles. The lines indicate probable relationships. 
(Based on a diagram by Charles M. Bogert, Curator of Amphibians and Reptiles, American 
Museum of Natural History; made expressly for this book.) 





Edge of dorsal 

Opening for 
foreleg r- 

Opening for \ 
hindleg '■ 


Horny scutes 

Bony plates 
(scutes removed) 

Horny scutes 

Bony plates 
(scutes removed) 

Plastron (ventral viev/) 

Corapace (dorsal view) 

Figure 275. Turtle shell showing the external horny scutes and the bony plates beneath. The 
living epidermis, which covers the bony plates, produces the horny scutes. 

two posterior nares are situated in the an- 
terior part of the roof of the mouth. At the 
base of the tongue is a longitudinal slit, the 
glottis, and a short distance back of the an- 
gle of the jaw are the openings of the 
Eustachian tubes. The pharynx is thin- 
walled and very distensible; it leads into the 
more slender and thick-walled esophagus. 
The stomach opens by a pyloric valve into 
the small intestine; this is separated from 
the large intestine bv the ileocecal valve. 
The terminal portion of the digestive canal 
is the rectum; it opens into the cloaca. 
There is no intestinal cecum. 

The liver discharges bile into the intes- 
tine through the bile duct. Several pancre- 
atic ducts lead from the pancreas to the 

Circulatory system 

The reptilian heart (Fig. 222), except in 
the case of the Crocodilia, consists of two 

atria and a single ventricle which is divided 
into two by an incomplete septum. In the 
crocodilians the longitudinal septum in the 
ventricle is complete, forming a 4-cham- 
bered heart. The venous blood from the 
body (Fig. 276) is carried by the posterior 
vena cava and the two anterior venae cavae 
into the sinus venosus and thence into the 
right atrium. From here it passes into the 
right side of the ventricle, and when the lat- 
ter contracts, it is forced out through the 
pulmonary artery which sends a branch to 
each lung and through the left aorta which 
conveys blood to the viscera, and into the 
dorsal aorta. 

The blood which is oxygenated in the 
lungs is returned by the pulmonary veins to 
the left atrium and thence into the left side 
of the ventricle. This blood is pumped out 
through the right aortic arch, which merges 
into the dorsal aorta. Because the septum 
dividing the ventricle into two is incomplete, 
the blood that enters the right aortic arch is 







Ventral cervical 


Right aortic arch 


Anterior vena cava 
Sinus venosus 
Posterior vena cava 
Right atrium 


Gall bladder 

Posterior vena cava 


Small intestine 


Anterior mesenteric 

Large intestine 





External jugular 

nternal jugular 


Left aortic arch 





Left atrium 


Hepatic portal 





Dorsal aorta 


Renal portal 

Accessory bladder 


Cloacal opening 

Figure 276. The internal structure of a turtle. 

a mixture of oxygenated blood from the left 
atrium and venous blood from the right 

Certain species of turtles have a well-de- 
veloped renal portal system; the hepatic 
portal system shows an advance in develop- 
ment over the condition as described in the 

Respiratory system 

Turtles breathe by means of lungs. Air 
enters the mouth cavity by way of the nasal 
passages. The glottis opens into the larynx, 
through which the air passes into the trachea 
or windpipe. The trachea divides, sending 

one bronchus to each lung. The lungs are 
more complicated than those of the am- 
phibians. The bronchi branch out a num- 
ber of times, and the lung cavity is broken 
up into many spaces so that the respiratory 
surface is greatly increased. 

The presence of a hard rigid shell, in tur- 
tles, makes general expansion and contrac- 
tion of the body impossible. Turtle respira- 
tion, therefore, presents some unusual prob- 
lems. It was formerly thought that it had a 
breathing mechanism similar to amphibians, 
but this is not the case; the turtle has its 
own unique method of breathing. Inspira- 
tion is accomplished by two flank muscles 
which serve the same function as the mam- 



malian diaphragm: to enlarge the coelom 
and cause air to be "sucked" into the lungs. 
To accomplish expiration, the turtle uses 
paired expiratory muscles which enclose the 
viscera. Air is forced out of the lungs by the 
contraction of the expiratory muscles which 
press the viscera against the lungs. This ac- 
tion is assisted by pulling in the legs and 
neck, which further decreases the size of the 
body cavity. 

Many aquatic turtles probably carry on 
respiration to some extent by taking water 
into the cloaca and the accessor}' bladders 
and then forcing it out through the cloacal 
opening. Thus these structures may serve 
as supplementary respiratory organs (com- 
pare with sea cucumber, and nymph of 
dragonfly). It has also been sugested that in 
the aquatic forms certain areas of the skin 
may be modified for respiration. 

Urogenital system 

Excretion is carried on by the two kid- 

neys. Their secretions pass through the 
ureters into the cloaca (Fig. 277), are stored 
in the urinary bladder, and then make their 
exit through the clccal opening. This is 
often called the anr?, but the anus properly 
refers to the open'^g of the digestive tract; 
therefore, in those forms with a cloaca, this 
term should apply to the opening of the 
intestine into the cloaca. 

The sexes are separate. The male organs 
are a pair of testes and a pair of vasa 
deferentia through which the sperms pass to 
the grooved copulatory organ or erectile 
penis attached to the ventral wall of the 
cloaca. It should be noted that the reptiles 
are the first vertebrates in which there is a 
penis. The female organs are a pair of 
ovaries and a pair of oviducts; the latter 
open into the cloaca. The sperms are in- 
jected into the female by a sexual act 
(copulation), usually preceded by courtship 

Turtles are oviparous. The eggs which are 
white, round or oval, and covered by a more 




Urinary bladder 
Vas deferens 



Bladder opening 

Accessory bladder 

Penis groove 

Figure 277. Cloaca and urogenital organs of a turtle, ventral view. (After Gegenbaur.) 



or less hardened shell, are laid in the holes 
dug by the female, in soil or decaying vege- 
tation, in which heat aids in incubation. 

Nervous system 

The brain is more highly developed than 
in the amphibians. The cerebral hemis- 
pheres are larger, and a distinction can be 
made between the superficial gray layer and 
the central white medulla. The cerebellum 
is also larger, indicating an increase in the 
power of coordinating movements. There 
are 12 pairs of cranial nerves. 

Sense organs 

The eye is small. It has a round pupil and 
an iris which is usually dark in terrestrial 
forms, but often colored in aquatic turtles. 
The sense of hearing is not well developed, 
but the turtle responds readily to vibrations 

through the skin, so it is easily frightened by 
noises. The sense of smell enables the turtle 
to distinguish between various kinds of food 
both in and out of the water. The skin 
over many parts of the body is very sensitive 
to touch. 



Turtles live on land, in fresh water, or in 
the sea. The word turtle is often applied to 
semiaquatic species; tortoise, mainly or en- 
tirely, to land species; and terrapin to certain 
species that are edible and sold in markets. 
Most of the land and fresh-water turtles hi- 
bernate in the earth during the winter, but 
in warmer countries they "sleep" ( estiva te) 
during the hotter months. 

Some of the more interesting types of tur- 

Snapping turtle 

Painted turtle 

Figure 278. Common American turtles. The painted turtle is common in ponds. The snap- 
ping turtle is less protected by shell than some turtles; it is well named for it is said that it 
will snap as soon as hatched. 

ties are as follows. The snapping turtle 
(Fig. 278) is famous for its strong jaws and 
vicious bite. The musk turtle Sternotherus 
emits a disagreeable odor when molested or 
captured. The painted turtle (Fig. 278) is 
brilliantly colored. The diamondback ter- 
rapin Malaclemys (Fig. 279) is famous as 

food for man. The plastron of the box 
turtle (Fig. 281) is hinged transversely near 
the center so that the shell can be closed 
completely when the animal is in danger. 
The gopher tortoise Gopherus lives in bur- 
rows in dry sandy areas of the southeastern 
United States. Some of the giant tortoises, 



Figure 279. Malademys, the diamondback ter 
rapin. It derives its common name from the mark- 
ings on its shell. One of the most famous of all 
turtles as food for man. (Courtesy of Shedd Aqua- 
rium, Chicago.) 

Testudo (Fig. 274), which are found on the 
Galapagos Islands, have been known to 
weigh over 500 pounds and are probably 
over 200 years old. 

Sea turtles inhabit tropical and semitrop- 
ical seas and come to land only to lay their 
eggs on sandy beaches. Their limbs are 
modified as paddles for swimming. The 
leatherback turtle Dermochelys (Fig. 274) 
is the largest of all living turtles, sometimes 
attaining a weight well over 1500 pounds. 
It has a leathery covering over the shell in- 
stead of horny shields. Soft-shelled turtles 
(Fig. 280) also have shells that are leathery 
and without shields. 

Sphenodon, a living fossil 

Sphenodon is the sole surviving species 
(Fig. 283) of the order to which it belongs. 
Numerous skeletal characteristics are like 

Figure 280. Amyda, the soft-shelled turtle. Length of shell of adult about one foot. Accord- 
ing to Ditmars, a large specimen can amputate a man's finger. Note the leathery integument 
which is not divided into horny scutes. (Courtesy of N.Y. Zoological Society.) 

those possessed by some of the oldest fossil 
reptiles, and the ancestors of living reptiles 
were apparently much like this queer relic 
of past ages. Sphenodon is now restricted 
to some small islands in the Bay of Plenty 

in New Zealand; and because it is now pro- 
tected, it is thriving with an estimated 
5000 on Stephen Island alone. It is about 
two feet long and resembles a lizard in form. 
It lives in burrows, is nocturnal, and feeds 



Figure 281. The western box turtle Terrapene. The under part of the shell is hinged and 
encloses the animal as though it were in a box. This is a protective mechanism. The turtle is 
known to live to the ripe old age of 123 years. (Courtesy of American Museum of Natural 

on other live animals. One of its most strik- 
ing peculiarities, shared with many lizards, 
is the presence of a parietal organ or parietal 
eye in the roof of the cranium— a structure 

Figure 282. The green turtle Chelonia. A marine 
species that has been so much hunted for food that 
it may be in danger of extinction. Its fat is green 
in color. The frontlimbs are flippers used for swim- 
ming, and its hindlimbs are used as a steering ap- 
paratus and as kickers. It weighs about 400 pounds. 
(Courtesy of American Museum of Natural History.) 

with a retina and other characters resem- 
bling a true eye in juveniles, but it is vestigial 
in adults, and scarcely visible. 



The lizards usually have an 
body and 4 well-developed limbs for run- 
ning, clinging, climbing, or digging. Some, 

for example the glass snakes, have no limbs 
or only vestiges. The tail is generally long; it 
is easily broken off, but in many a new organ 
is soon regenerated which does not possess 

Figure 283. Tuatara [Sphenodon punctatus) at 
the entrance of its burrow. It has characters of the 
early ancestral reptiles. A relic of a remote past ("a 
living fossil"), now found only on islands near New 
Zealand. Length about two feet. (Photo by Blanch- 
ard. (Courtesy of National Geographic Societv.) 



vertebrae. The fact that the tail breaks off 
easily is of survival value, for the predator 
is attracted to and eats the wiggling tail, 
while the animal escapes. The skin of the 
lizard is usually covered with small scales. 

Geckos inhabit all the warmer parts 
of the globe; they are harmless and 
usually nocturnal. Many have specialized 
lamellae under the toes, which enable them 
to climb over trees, rocks, walls, and ceilings. 

Figure 284. An interesting lizard, the coal skink Eumeces. It illustrates autotomy in the 
vertebrates; when the touched skink sheds its tail, the would be captor stops to eat the tail 
while the skink escapes. (Photo courtesy of J.F. Nist and published by permission of The Amer- 
ican Biology Teacher.) 

The American chameleon is common in the 
southeastern United States and in Cuba. 
The common iguana, Iguana, reaches a 
length of 6 feet. It inhabits tropical Amer- 
ica and is a favorite article of food. The 
horned "toads" (Fig. 286) occur in the 
western United States and in Mexico. They 
live in hot, dry regions, many of them in- 
habiting deserts. They are ovoviviparous or 

The flying dragon is a species whose 
sides are expanded into thin membranes 
supported by false ribs. These membranes 
-enable the lizard to glide from tree to tree 
and are folded when not in use. A number 
of different kinds of lizards are called 
chameleons, but the 75 species of true 
chameleons all live in Africa, Madagascar, 
Arabia, and India. One of the features that 
have made the chameleons famous is the 

power to change colors rapidly. Worm liz- 
ards are limbless, burrowing lizards resem- 
bling worms in appearance. Only one 
species, the Florida worm lizard, Rhineura 
fioridana, occurs in the United States; it is 
restricted to the Florida peninsula. Some- 
what similar lizards are the "glass snakes" 
(Fig. 274) in the United States and Mexico. 
These have no limbs and move as most 
snakes do by lateral undulations. They can 
be distinguished from true snakes by the 
presence of movable eyelids and ear open- 
ings. Their name is due to the extreme brit- 
tleness of the long tail. Another species 
called the "blindworm" {Anguis) inhabits 
Europe, western Asia, and Algeria. It looks 
like a large, brightly colored worm, but it is 
not blind since it has well-developed eyes. 

Swifts and skinks are types of lizards that 
live in North America; many species of 



Figure 285. The American chameleon or green anole (Anolis). Anoles have great power to 
change their color (green to dark brown) and they are often sold for pets. About 6 inches long. 
(Courtesy of N.Y. Zoological Society.) 

Figure 286. A horned "toad" {Phrynosoma) , not a toad, but a common lizard in the arid 
western and southwestern states. Natural size, 6 inches long. (Courtesy of the American Museum 
of Natural History.) 

lizards are known only from the Old World. 
The largest of all the lizards is the dragon 
lizard of Komodo {Varanus) which lives on 
some of the small islands in the Dutch East 
Indies. The natives of the island of Komodo 
claimed that dragons existed on the island; 
and, in 1914, these "dragons" were dis- 
covered to be the largest living lizards. They 
reach a length of 9 feet and a weight of over 
250 pounds; they are ferocious reptiles able 
to capture wild pigs and other animals on 

which they feed. They readily lose their 
ferocity and become quite tame in captivity. 


Snakes resemble lizards in many of their 
anatomic features. They differ from them in 
at least 4 respects: (1) the right and left 
halves of the lower jaw are not firmly united, 
but are connected by an elastic ligament, 
(2) there is no pectoral girdle, (3) the uri- 



nary bladder is absent; and (4) the brain 
case is closed anteriorly. 

Snakes are covered with scales; those on 
the head are usually so regular as to be of 
importance in classification (Fig. 287B,C). 
On the ventral surface in front of the cloacal 
opening is a single row of broad scales called 
abdominal scutes, to which the ends of the 
ribs are attached. The outer horny layer of 
the skin is shed a number of times during 

the year. Appendages are entirely absent 
except in a few species like the python, 
which possesses a pair of short spurlike pro- 
jections, one on each side of the cloacal 
opening— vestiges of the hindlimbs (Fig. 
287A). The eyelids are fused over the eyes, 
but there is a transparent portion which 
allows the animal to see. When the skin is 
being shed, the snake is partially blind. 
There is no tympanic membrane, and 



Figure 287. A, vestigial hindlimb and girdle bones of the python. The skeletons of nearly all 
snakes are without limbs but the pythons are among the exceptions. These remnants of hind- 
limbs suggest that the ancestors of snakes traveled on legs. B, scales on anterior end of the 
hognose snake or puff adder. C, scales on anterior end of black snake or blue racer. (A after 
photo, courtesy of Chicago Natural History Museum; B, C, after photo, courtesy of General 
Biological Supply House.) 

there is much doubt regarding the existence 
of a sense of hearing. The tongue is a slen- 
der, deeply notched, protrusible structure 
that can be thrust out even when the mouth 
is closed because of the presence of grooves 
in the jaws. It serves as an auxiliary olfac- 
tory organ, carrying odorous particles to the 
paired organs of Jacobson in the roof of the 
mouth. The prevalent idea that the tongue 
can inflict an injury is erroneous. Further- 
more, there appears to be no good evidence 
that the tongue is sensitive to vibrations. 
The teeth are sharp and curve inward (Fig. 
288). They are adapted to prevent food 
from slipping forward, once swallowing has 
commenced. In the venomous snakes certain 

teeth are grooved or tubular and serve to 
conduct venom into any animal bitten. 

Snakes do not chew their food but swal- 
low it whole. They can eat animals much 
larger than their own bodies (Fig. 290). 
Some of the structural adaptations making 
this possible are: (1) the lower jaw joins 
with the skull very loosely, by means of two 
slender bones (quadrates); (2) furthermore, 
the lower jaw can spread at the anterior 
midpoint, allowing for lateral expansion; 
and (3) the bones of the palate are movable. 

Movement on land is accompanied by 
several types of motion, but the two prin- 
cipal ones are: lateral undulations of the 
body and the shifting of the abdominal 




Venom (poison) gland 

Opening of fong 

Constrictor muscle 



Mucous membran 


Figure 288. Rattlesnake drawing, showing the hollow teeth (fangs), venom gland, venom 
duct, and muscles used in forcing poison into the victim's flesh, as is done with a hypodermic 
needle. Note the pit between the nostril and eye, which is characteristic of pit vipers. (After 
drawing, courtesy of American Museum of Natural History.) 

scutes forward in alternate sections of the 
body. The body is drawn forward in the 
latter method by pressing the rough pos- 
terior edges of the abdominal scutes against 
the substratum. Most snakes cannot move 
fonvard efficiently on a smooth surface. All 
species are able to swim, and this, of course, 
is the normal method of locomotion of the 
aquatic forms. 

The majority of snakes are oviparous, but 
some are ovoviviparous, like the garter snake 
which brings forth its young alive. The idea 
that they swallow their young in order to 
protect them and then spew them out again 
when danger has passed is one of the com- 
mon snake fallacies. 

The tropics, perhaps, are more plentifully 
supplied with snakes than the temperate 
zones; and snakes are found in many places 
not inhabited by lizards. Madagascar seems 
to be the only large country in warm and 
temperate latitudes not inhabited by dan- 
gerous snakes. As in the other groups of 
vertebrates, the snakes are found in almost 
every kind of habitat; some species live in 
salt water, others in fresh water, some are 
arboreal, and many live underground. 

Only 5 of the 10 families of Serpentes oc- 
cur in North America. With a few excep- 

tions those described below are found in the 
United States. 

Blind snakes 

Two species of these small burrowing 
reptiles (genus Leptotyphlops) occur in the 
United States. They burrow long tunnels in 
the earth and feed on worms and insect 

Other snakes 

Pythons (Fig. 291) and boas (Fig. 290) 
live almost exclusively upon birds and mam- 
mals which they squeeze to death in their 
coils. None of them is venomous and only 
a few are large enough to be dangerous to 
man. There is only one boa constrictor. It 
is a native of the tropical parts of the Amer- 
icas and reaches a length of 18 feet. Boa 
constrictors are readily tamed in captivity 
and therefore preferred by snake "charmers." 

The common garter snake (Fig. 292) of 
eastern North America is the most abundant 
of our harmless snakes. It feeds largely on 
frogs, toads, fishes, and earthworms. The 
young are born alive, usually in August. The 
common water snake Natrix is semiaquatic 



... -i- 


Figure 289. Rattlesnake. A radiograph showing the absence of hmbs, hmb girdles, and sternum, 
but the numerous vertebrae and ribs are much alike in structure. Two rattles are visible at the 
posterior end of the body. (Courtesy of Armed Forces Institute of Pathology, Washington 25, D.C.) 

but is not a water moccasin. The black snake 
(Coluber) is a slender long-tailed snake 
which reaches a length of 6 feet. West of 
the Mississippi it gives way to a subspecies 
called the blue racer and to another species, 
the red racer, in Texas. Contrar}' to popular 
belief, this reptile does not attack snakes 
larger than itself, has no power to squeeze 
its prey to death, and is unable to hypnotize 
birds and squirrels. King snakes (Fig. 293) 
are of various sizes; they are constrictors and 
have received their common name because 
they prey on other snakes. The scarlet or 

coral king snake resembles the venomous 
coral snake in color. King snakes are im- 
mune to pit viper venom, but not to coral 
snake venom; hence they do not hesitate to 
attack rattlesnakes, water moccasins, and 
copperheads. The milk snake derives its 
name from the erroneous supposition that 
it steals milk from cows. The hog-nosed 
snakes, Heterodon (Fig. 287B), are popu- 
larly known as puflf adders, spreading vipers, 
or blowsnakes. They are nonvenomous, 
though they fiercely intimidate and also 
play possum. 



Figure 290. Boa constrictor in action. As this picture proves, the snake really deserves its 
name. It captured the deer, which it squeezed to death in its coils. Arrow indicates the point on 
the deer to which it had been swallowed when the photographer started preparations for taking 
the picture. Fear of man caused the snake to disgorge as much of the deer as possible before 
the shutter clicked. This boa is a native of tropical America; it has a length of from 10 to 15 feet. 
(Courtesy of James M. Keller and Clark Zeek.) 

Venomous (poisonous) reptiles will be 
considered in a later section. 


Crocodilians are lizardlike in form, but 
the jaws are extended into a long snout. 

The nostrils are at the end of the snout and 
the eyes protrude from the head so that the 
crocodilians can float at the surface with 
only these parts above water. The skin is 
thick and leathery, covered with horny 
epidermal scales, and with dorsal and some- 
times ventral bony plates somewhat like 



Figure 291. Indian python [Python) in its natural habitat. This is one of the world's largest 
snakes; it reaches a length of 25 feet. (Courtesy of N.Y. Zoological Society.) 

those in the shell of the turtles. The nostrils 
and ears are provided with valves and are 
closed when the animal is under water. 

The limbs are well developed. There are 
5 digits on the forelimbs and 4 more or less 
webbed digits on the hindlimbs. The tail is 
a laterally compressed swimming organ. The 
"anus" is a longitudinal slit. Two pairs of 
musk glands are present— one on the throat 
and one in the cloaca. 

Only 21 species of living crocodilians are 

known. One belongs to the family Gaviali- 
dae, 13 are included in the family Croco- 
dylidae, and 7 are placed in the Alliga- 
toridae. The American crocodile (Fig. 294) 
is an inhabitant of Florida, Mexico, and 
Central and South America. The African 
crocodile is one of the few man-eating 
species. Formerly it was held sacred by the 
Egyptians, and many specimens were pre- 
served as mummies. There are two species of 
the genus Alligator; the American alligator 

Figure 292. Common garter snake of eastern North America {Thamnophis sirtalis) with 
young; the latter are "born ahve," as many as 30 at one time. Many have the erroneous idea 
that the young garter snakes remain with the parent. Length of adult about 3 feet. (Courtesy 
of N.Y. Zoological Society.) 

Figure 293. Arizona king snake {Lampwpeltis) , a constrictor. Called "king" because it cap- 
tures and kills other snakes, including poisonous species. Color black with white or yellowish 
bands. (Courtesy of N.Y. Zoological Society.) 




Figure 294. The American crocodile. Note pointed snout, laterally compressed tail, webbed 
hindfeet, 5 toes in front and 4 behind, and claws on 3 inner digits. This is the largest known 
crocodile, reaching a length of 23 feet. (Courtesy of N.Y. Zoological Society.) 

Figure 295. The American alligator. Snout blunt, not pointed as in the crocodile. Natural 
size up to 16 feet long. (Courtesy of N.Y. Zoological Society.) 

(Fig. 295) inhabits the southeastern part 
of the United States; and the Chinese alH- 
gator is found only in China. 

Venomous (poisonous) reptiles 

Very few reptiles are poisonous. All turtles 
are nonvenomous. Only one species of lizard 
and 19 species of poisonous snakes live in 
the United States. 

Gila monsters 

This poisonous "beaded" lizard (Fig. 296) 
inhabits parts of Arizona, Utah, Nevada, and 

New Mexico. It is black and conspicuously 
spotted with pink or orange. A large speci- 
men measures about two feet. The bite is 
fatal to small animals and dangerous to man. 
The venom of the Gila monster is as strong 
a poison as that of some of the venomous 
snakes, but the mechanism for injecting it 
into the body of an animal is less effi- 

Venomous snakes 

Among the venomous snakes that it seems 
desirable to mention, besides those that oc- 
cur in the United States, are the sea snakes 



Figure 296. Gila monster of the American southwest, the only poisonous lizard in the United 
States. It is shown feeding on eggs, a common food. It is beautifully colored with black and 
orange patches; it has poison glands in the lower jaw, and grooved teeth which carry venom 
into an animal that is bitten. (Courtesy of N.Y. Zoological Society.) 

and the cobras. The sea snakes are true sea 
serpents (Fig. 274). They inhabit the Indian 
Ocean and the western tropical Pacific; and 
one species occurs along the western coast of 
tropical America. They reach a length of 
from 3 to 8 feet or more, and all are venom- 
ous. The tail and sometimes the body is 
laterally compressed — an adaptation for 
swimming. The venom of sea snakes is so 
deadly that their prey, which consists of 
fish, are quickly benumbed by it. Laboratory 
tests have indicated that one species of sea 
snakes has venom more potent than that of 

The cobra-de-capello, Naja naja, of India, 
China, and the Malay Archipelago is very 
vicious; when disturbed it raises the anterior 
part of the body from the ground, spreads 
its hood with a hiss, and strikes. In India 
the bare-legged natives are killed in large 
numbers by cobras (Fig. 297) and other 
snakes; for example, each year 7,000 to 
12,000 are reported killed by snake bites, 
most of them probably the bites of kraits, a 
species related to the cobra. 

Only 19 species of dangerously poisonous 
snakes occur in the United States: the 
harlequin snake, the Arizona coral snake, 
the copperhead, the water moccasin or cot- 

tonmouth, and 15 species of rattlesnakes.* 

If the pupil of the eye is round and the 
snake is ringed with red, black, and yellow, 
and if the red rings are bordered by yellow 
rings, the species may be a venomous coral 
snake. If the pupil is vertical and there is a 
pit between the eye and nostril on each side 
of the head, the snake is a poisonous rattle- 
snake, moccasin, or copperhead. All other 
snakes that live in the United States are 
harmless. Very few people in this country 
die as the result of a snake bite. According 
to the World Health Organization study, 
there are only about 300 to 400 deaths 
per year in North America from snake bite. 

The harlequin or coral snake, Micrurus 
fulvius (Fig. 298), of the southeastern 
United States is dangerous, but man is 
rarely bitten by it. 

The water moccasin (Fig. 299) occurs in 
the swamps of the Atlantic Coast south of 
North Carolina, and in the Mississippi Val- 
ley from southern Illinois and Indiana south- 
ward. The length of an average specimen is 
4 feet. 

The copperhead snake (Fig. 300) occurs 
from southern Massachusetts to northern 

* There are also 14 subspecies of rattlesnakes. 



_s^ '- r*8«liiv.^;2 



■ "^ ' • 

Figure 297. The ringhals, a South African cobra {Hemachatus haemachatus) , with its neck 
spread, ready to strike and inject venom which may cause death in a few minutes. (Courtesy of 
American Museum of Natural Histon,'.) 

Florida and west to Texas. An average speci- 
men measures about IVz feet. 

The rattlesnake (Fig. 301) is easily dis- 
tinguished by the rattle at the end of the 
tail in the adult. This consists of a number 
or horny, bell-shaped segments loosely held 
together. Before striking, the rattlesnake 
often vibrates the end of the tail rapidly, 
producing a sort of buzzing noise, which to 
the wise serves as a warning. The venom is 

secreted by a pair of glands on each side 
of the head above the jaws (Fig. 288) . These 
glands open by venom ducts into the fangs. 
The poison fangs are pierced by a canal 
which opens near the end. The venom 
glands are surrounded by muscles that con- 
tract to squeeze the poison out of them 
through the fangs and into the animal 
bitten. There are several small fangs lying 
just behind the functional ones; these are 



Figure 298. The coral snake, a very poisonous 
snake, which hves in southern United States and 
the tropical countries. It is the only representative 
of the cobra family in North America. It is about 
three feet long. (Courtesy of N.Y. Zoological So- 
ciety. ) 

held in resewe to replace those that are lost 
in struggles with prey or are normally shed. 
Rattlesnakes are most abundant both in re- 
gard to the number of species and the num- 

ber of individuals in the deserts of the south- 
western United States, but almost every part 
of this country is inhabited by one or more 

The rattlesnake is one of the so-called pit 
vipers because it has a pit between the eye 
and nostril on each side of the head. In this 
pit is located the pit organ w^hich consists 
of highly vascular tissue and many nerve 
endings. Experiments prove that this is a 
heat-sensitive organ; a rattlesnake can detect 
the movement of a moderately warm body 
passing its head at a distance of several feet. 
Obviously this sense is very useful to an 
animal which lives to a considerable extent 
on warm-blooded rodents. 

Rattlesnakes and other pit vipers usually 
strike from an S-shaped position of the body. 
Unless the venom is injected directly into a 
blood vessel, it usually travels slowly. The 
best first-aid measures to take in the case of a 
snake bite are: (1) apply a ligature or 
tourniquet a few inches above the bite— a 
rubber garter, handkerchief, cord, or even a 
shoestring will do; about every 15 minutes 

Figure 299. Water moccasin (cotton-mouth) snake about to strike. Its bite is occasionally 
fatal to man. Note the thick body, slender neck, and whitish mouth; it has a pit in front of 
the eye. It lives in or near water. (Courtesy of N.Y. Zoological Society.) 



Figure 300. The copperhead {Agki.^lrudon}. Its common name comes from the fact that the 
top of the head is copper-colored. Natural size 33 inches long. (Courtesy of N.Y. Zoological 
Society. ) 

release for one minute, then retighten; (2) 
make a cross-cut incision, each cut Vi inch 
in length and V4 inch in depth at each fang 
mark; (3) apply suction by mouth, suction 
cup, or other device; (4) have the patient 
lie down and keep quiet; (5) seek a phy- 
sician and have him inject antivenom as 
soon as possible. A person suffering from 
snake bite should not run or get overheated, 
take whiskey, inject potassium permanga- 
nate into the wound, or cauterize the site of 
the bite with hot irons, strong acids, or any- 
thing of a similar nature. 

Contrary to what many people think, fear 
of snakes is not inborn in children but is 
due entirely to conditioning by their elders. 

Fossil reptiles 

The reptiles now living are only a rem- 
nant of the great hordes that populated the 
earth during the Mesozoic Era. During this 

Age of Reptiles, a period of about 130 mil- 
lion years, enormous dinosaurs roamed the 
land, ichthyosaurs dominated the sea, and 
pterosaurs ruled the air. Today, compara- 
tively puny representatives of only 4 of the 
16 known orders of reptiles survive. 

Dinosaur means terrible lizard. The dino- 
saurs probably lived in swamps and on the 
uplands; remains have been found in most 
continents. Some species measured over 85 
feet in length. Both herbivorous and carniv- 
orous forms existed. Brontosaurus (Fig. 302) 
was herbivorous and about 75 feet long; 
remains have been found in Wyoming and 
Colorado. Stegosaurus reached a length of 
about 28 feet and was also herbivorous. It 
possessed huge triangular plates along the 
back. Remains have been discovered in Wy- 
oming and Colorado. Protoceratops (Fig. 
303) was a small, hornless, herbivorous 
dinosaur only about 6 feet long; fossils were 
discovered in the deserts of Mongolia. 



Figure 301. The timber or banded rattlesnake {Crotalus horridus). A dangerously poisonous 
species of the east and middle west; usually 3 or 4 feet long. (Courtesy of N.Y. Zoological Society.) 

Figure 302. A dinosaur (Brontosaurus) with man drawn to the same scale. This giant reptile 
probably reached the maximum size possible for land animals; it is believed to have fed on soft 
lush vegetation. (After Mavor.) 

Ichthyosaurs (Fig. 304) were fish-eating 
aquatic reptiles. Their bodies were admir- 
ably adapted for life in the water; they have 
been called the "whales" of the Mesozoic 
Era. The remains of ichthyosaurs occur in 
North America, Europe, Asia, Africa, and 
Australia. They showed a high degree of 
specialization for marine life. 

Pterosaurs, flying reptiles, had forelimbs 
modified for flight. They resembled birds 
in certain skeletal characters, but differed 
from them in others. Pteranodon is the 
largest form known; it had a skull two feet 
long and a wing spread of more than 25 feet. 
Teeth were absent, and the tail was short. 


The food of reptiles consists of both ani- 
mals and plants. The animals eaten belong 
to just about all classes, including the Rep- 
tilia. Many of the snakes live almost entirely 
upon birds and mammals. Frogs and fish are 
favorite articles of food. Most of the smaller 
species of reptiles feed upon worms and 
insects. In general it may be stated that 
reptiles do very little damage by destroying 
animals and plants for food, but are often of 
considerable benefit since they kill large 
numbers of obnoxious insects and destruc- 



2 . % ''*^- ■ 

Figure 303. Protoceratops (an armored dinosaur) with its eggs. The discovery of its eggs 
proved that some dinosaurs at least were hatched from eggs hke turtles; furthermore, it is unusual 
in that all stages of growth from the egg to the adult are represented by fossils. (Courtesy of 
American Museum of Natural History.) 

tive rodents. The U.S. Department of Agri- 
culture estimates that the value of the larger 
snakes to the farmer is somewhere between 
$50 and $75. 

The turtles and tortoises rank first as food 
for man. Especially worthy of mention are 
the green turtle, the diamondback terrapin, 
and the soft-shelled turtle. Certain lizards 
such as the iguana of tropical America form 
a valuable addition to the food supply in 
various localities. The flesh of the rattlesnake 
is said to have a distinctly agreeable flavor. 
There is a fair and growing market for 
canned rattlesnake meat. 

Skins of the lizards, snakes, and croco- 
dilians are used rather extensively for the 
manufacture of articles that need to com- 
bine beauty of surface with durability. The 
alligators in this country have decreased so 
rapidly because of the value of their hides 
that they will be of no great economic im- 
portance unless they are consistently pro- 
tected or grown on farms. Tortoise shell, 

especially that procured from the horny 
covering of the carapace of the hawksbill 
turtle and some others, is used for manu- 
facture of combs and ornaments of various 

As previously stated, the poisonous snakes 
of the United States are of very little danger 
to man. In tropical countries, especially 
India, venomous snakes cause a larger death 
rate than that due to any other group of 
animals. The Gila monster (Fig. 296), 
which is one of two poisonous lizards and 
the only one inhabiting the United States, 
attacks man only when handled carelessly 
and rarely inflicts a fatal wound. 


(For reference purposes only) 

Class Reptilia. Reptiles are cold-blooded 
vertebrates covered with horny scales or plates; 



ju,, ,J».ii.Ji 


Figure 304. Ichthyosaurus, a. fishlike reptile, and young. This fossil reptile had a fish-shaped 
body, porpoiselike snout, short neck, and dorsal and caudal fins. The limbs were modified into 
paddles, a remarkable adaptation for swimming. (Courtesy of American Museum of Natural 

their digits are usually provided with claws; the 
majorits- of them possess functional legs; and 
they breathe by lungs. 

Bogert estimates that there are about 7000 
or more species of living reptiles which may be 
grouped into 4 orders: (1) the Chelonia (Tes- 
tudinata), containing about 275 species of 
turtles, terrapins, and tortoises; (2) the Rhyn- 
chocephalia, represented by a single New Zea- 
land species; (3) The Squamata, containing 
about 6700 species of lizards and snakes; and 
(4) the Crocodilia, containing 25 species of 
crocodiles, gavials, alligators, and caimans. 

Order 1. Chelonia ( Testudinata ) . Turtles, 
terrapins, and tortoises. Body en- 
cased in bony capsule; jaws with- 
out teeth. 
Family 1. Chelydridae. Snapping tur- 
tles (Fig. 278). Exs. Chely- 

dra serpentina, snapping tur- 
tle, and musk turtles, 

Family 2. Testudinidae. Tortoises and 
most turtles. Ex. Chrysemys 
picta, painted turtle (Fig. 

Family 3. Cheloniidae. Sea turtles. Ex. 
Chelonia mvdas, green turtle 
(Fig. 282).' 

Family 4. Dermochelidae. Leatherback 
turtle. Ex. Dermochelys 
coriacea, leatherback turtle 
(Fig. 274). 

Family 5. Trionychidae. Soft-shelled 

turtles. Ex. Amyda spinijera, 

soft-shelled turtle (Fig. 


Order 2. Rhynchocephalia. One genus of 

New Zealand lizardlike reptiles. 



Vertebrae biconcave, often con- 
taining remains of notochord; 
parietal organ present. Ex. 
Sphenodon punctatus (Fig. 283). 
Order 3. Squamata. Lizards and snakes. 
Reptiles usually with horny ep- 
idermal scales; vertebrae usually 
procoelus; quadrate bones mov- 
Suborder 1. Sauria. Lizards. Cloacal open- 
ing transverse; paired copu- 
latory organs; usually well- 
developed limbs; rami of 
lower jaw united. 
Family L Gekkonidae. Geckos (Fig. 
274). Ex. Tarentola mauren- 
tanica, wall gecko. 
Family 2. Iguanidae. New World liz- 
ards. Ex. Anolis carolinensis, 
American "chameleon" or 
green anole (Fig. 285). 
Family 3. Agamidae. Old World liz- 
ards. Exs. Draco volans, 
"flying" lizard (dragon), and 
Agama (Fig. 274). 
Family 4. Chamaeleonidae. Chame- 
leons. Ex. Chamaeleo chC' 
maeleon, true Chameleon 
(Fig. 274). 
Family 5. Lacertidae. Typical Old 
World lizards. Ex. Lacerta 
viridis, green lizard. 
Family 6. Scincidae, Skinks. Ex. Eu- 
meces antracinus (Fig. 284). 
Family 7. Amphisbaenidae. Worm liz- 
ards. Ex. Rhineura floridana, 
Florida worm lizard. 
Family 8. Helodermatidae. Beaded liz- 
ards. Ex. Heloderma suspec- 
tum, Gila monster (Fig. 
Family 9. Anguidae. Old and New 
World lizards. Ex. Ophisau- 
rus ventralis, "glass snake" 
(Fig. 274). 
Suborder 2. Serpentes. Snakes. Elon- 
gated; no limbs; cloacal open- 
ing transverse; copulatory or- 
gans paired; without movable 
eyelids, tympanic cavity, 
urinary bladder, and pectoral 

arch; rami of lower jaw con- 
nected by ligament. 
Family I. Leptotyphlopidae. Blind 
snakes. Ex. Leptotyphlops 
dulcis, Texas blind snake. 
Family 2. Boidae. Pythons and boas. 
Ex. Boa constrictor, boa con- 
strictor (Fig. 290). 
Family 3. Colubridae. Harmless 

snakes. Ex. Thamnophis 
sirtalis, garter snake (Fig. 
Family 4. Crotalidae. Pit vipers. Exs. 
Crotalus horridus, timber 
rattlesnake (Fig. 301), 
Agkistrodon contortrix, cop- 
perhead and Agkistrodon 
piscivorus water moccasin. 
Order 4. Crocodilia. Crocodiles, alligators, 
gavials, and caimans. Vertebrae 
procoelous; nostrils paired, at end 
of snout; cloacal opening longi- 
Family L Gavialidae. Gavials. Ex. 
Gavialis gangeticus, Indian 
Family 2. Alligatoridae. Alligators and 
caimans. Ex. Alligator mis- 
sissippiensis, American alli- 
gator (Fig. 295). 
Family 3. Crocodylidae. Crocodiles. 
Ex. Crocodylus acutus (Fig. 


(See also Chapter 27) 

Ashley, L.M. Laboratory Anatomy of the Tur- 
tle. W.C. Brown, Dubuque, Iowa, 1955. 

Bogert, CM., and del Campo, R.M. The Gila 
Monster and Its Allies. Am. Mus. of Nat. 
Hist. Bull. 109:1-238, 1956. 

Carr, Archie. Handbook of Turtles. (Com- 
stock) Cornell Univ. Press, Ithaca, N.Y., 

Colbert, E.H. T