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Marine Biological Laboratory 

o....,.A ^r, 28, 1952 

Accession No 66567 

Q^^^ g^ Appleton, Centui:^^, Crofts, Inc. 

i'-ew York City 


Edited by Dwight E. Minnich 


\S> 7 U. , 




Associate Professor of Zoology 
University of Michigan 



Copyright, 1952, by 

All rights reserved. This book, or parts 
thereof, must not be reproduced in any 
form without permission of the publisher. 




This book is the culmination of more than twenty years of teaching intro- 
ductory zoology at the college and university level. During this period it has 
become increasingly evident to me that in order to understand the life sciences, 
students need, more than anything else, a broad underlying principle to cling 
to. In biology we have that in the story of organic evolution. Just as in the 
physical sciences, where the relationship of the elements in the periodic table 
provides a central core around which all other facts must revolve, so in biology 
evolution offers a great unifying principle to which all lesser principles can be 
related. This magnificent idea cannot be relegated to an isolated chapter; it 
must be the central theme, permeating the entire book, if it is to become a part 
of the student's thinking as it should. Thus organic evolution is the central 
theme of this work. 

Every attempt has been made to integrate the seven parts of the book into 
what amounts to an historical narration of animal life. My aim has been to 
point out the salient points in zoology without diluting the subject matter; to 
present important, and often complex, material in a manner that will at the 
same time fascinate the general education student and challenge the embryo 
professional zoologist. How well this dual function has been accomplished, 
students of the future will decide. 

After a brief discussion of the position zoology occupies as a science, there 
follows in Part I a discussion of the scientist and his methods. It is important 
to have the student understand the methods of science and recognize that 
zoology uses the same disciplines employed in other sciences. 

The story of evolution begins to unfold in Part II, beginning with a survey 
of the early history of the earth and the nature of the physical world. It seems 
logical to start with the physical setting in which life took its inception. A 
possible explanation of the origin of the first living thing is offered, and its 
properties are examined. 

In Part III we speculate upon the transition from single cells to many cells 
and discuss some of the problems arising out of organization into the more 
complex forms. The organized animal is considered in relation to its environ- 


Built upon this foundation concerning the nature of animals and their 
environments, Part IV traces the long evolutionary sequence from one-celled 
forms to man. Here animals are described both morphologically and physiologi- 
cally, with special emphasis upon their origins. The full weight of the evolu- 
tionary idea is given to the chordates in Chapter 13, concluding with a brief 
history of early man. 

With this background the student is ready for a rather careful study of the 
organ systems of man, to which Part V is devoted. Each chapter is introduced 
by a brief resume of the organ system under scrutiny as it appears in the 
various animal groups. This is done to fix firmly the origin of the various sys- 
tems and to demonstrate that they are not peculiar to man. Because of the 
importance of the human species, considerable attention has been devoted to 
this section which, in some respects, will be the most valuable to students. 

The continuity of life is taken up in Part VI, beginning with the reproduction 
of cells and carrying on through the continuity of the individual and of the 
race. Considerable attention is given to modern concepts in both embryology 
and genetics. 

The book concludes in Part VII with a return to the discussion of organic 
evolution — its meaning, theories, and mechanisms. A word of speculation about 
the future of mankind seems a fitting finale for a work of tliis kind. 

The material presented in these pages is derived from many sources, not only 
from published books and research papers but from first-hand observation in 
many instances. Wherever possible, living or preserved materials have been 
used in making the drawings. 

I am indebted not only to those who have received specific recognition in the 
list of Acknowledgments in the back of the book, but also to former teachers 
and associates who have influenced my thinking regarding many topics con- 
tained in this book. I should like particularly to mention Professor D. E. Min- 
. nich, who has gone over the entire book critically and has given advice and 
suggestions of inestimable value. Professors Norman E. Kemp, Paul A. Wright, 
Drs. Frank Hooper, James F. Hogg, and Stanley P. Wyatt have examined sev- 
eral chapters and have made many helpful suggestions. The attractiveness of 
the drawings is due to the skill of Doris Stirratt Garlock, whose patience in 
interpreting my ideas knew no bounds. Some of the drawings, taken from my 
Atlas in General Zoology, were originally done by Ohvia Jensen IngersoU. The 
chapter headings and some of the plates were done by Raymond Jansma. Much 
of the beauty of the photographs is due to the expert ability and many hours of 
hard labor by Herbert Weinert, who gave unstintingly of his time to obtain the 
best possible pictures. To him and Fred Anderegg, director of the Photographic 
Services of the University of Michigan, I express my sincere appreciation. 


Annette Van der Schalie and Ursula Freimarck together read and typed the 
entire manuscript. 

Finally I wish to express my deep appreciation to members of my family, 
especially my wife, for not only helping in the preparation of the glossary and 
index but more particularly for their kind understanding during the inception 
and gestation of this book. 

A. M. E. 

Ann Arbor, Michigan 



Part I 

1. Zoology as a Science 3 

Part II 

2. Early History of Life 21 

3. Units of Life — Cells 56 

Part III 

4. From Single Cells to Many Cells 65 

Part IV 

5. The Animal and Its Environment 87 

6. Orderliness among Animals 105 

7. Monocellular Animals — The Protozoa 108 

8. The Sponges and the Two-Layered Animals 136 

9. The Three-Layered Animals 160 

10. The Tube-within-a-Tube Body Plan 174 

11. Animals with Jointed Feet — The Arthropods 200 

12. Aberrant Animals — The Mollusks and Echinoderms 252 

13. The Animal Climax— The Chordates 278 




Part V 

14. Housing — Skin and Its Derivatives 365 

15. Support and Movement 374 

16. Coordination 392 

17. The Digestive System 451 

18. The Breathing Svstem 475 

19. The Transportation System 490 

20. MetaboHsm of Foods and Disposal of Wastes 518 

21. Reproduction 529 

Part VI 

22. The Continuity of Cells 551 

23. Development of the Individual 560 

24. Continuity of the Race 581 

Part VII 

25. Evolution — Past and Present . . .' 623 

26. Theories and Mechanism of Evolution 649 





INDEX 703 


Zoology as a Science 




If values were placed on all the things 
that go to make up the universe, the great- 
est, undoubtedly, would be assigned to life 
and living things, particularly the life that 
resides in one special animal, man. It is dif- 
ficult for us to interpret relative values in 
any other way because we are men, and 
nothing is more important to us than man 
himself. Since man is but one of many living 
things, tlie understanding of life itself be- 
comes the pinnacle of all goals. No prob- 
lems transcend this one, either in impor- 
tance or in complexity. Life in all of its 
diversity poses so many fascinating prob- 
lems that the scientist, grasping at one after 
the other, is only now beginning to see his 

way through this intricate maze of apparent 
confusion. Viewing the great profusion and 
variety of animal and plant life around him, 
it is small wonder he has been slow in bring- 
ing a semblance of order out of this chaos. 
Little by little, bit by bit, he has been 
able to assemble innumerable facts about 
living things, until today these apparently 
unrelated fragments of knowledge are be- 
ginning to fall into an integrated picture of 
the story of life on the earth. So far, great 
gaps exist in this pictvire, but from a dis- 
tance the basic pattern is discernible. It will 
be a long time before the entire panorama 
is completed, if it ever is, but in the mean- 
time, it seems important that the over-all 


pattern be presented in such a manner that 
it can become a part of the philosophy of 
the non-scientist as well as that of the scien- 
tist. That is the purpose of this book. 

Like all scientific knowledge, zoology has 
become more and more specialized into 
many compartments, such as anatomy 
(study of gross structure), physiology 
(study of function), embryology (study of 
early development ) , histology ( study of tis- 
sues), and many others. In seeking a broad 

of the spirit of science, and that he will take 
away with him something which may be 
integrated into his own philosophy of life. 


Zoology is the science of animal life. Two 
words in this definition require further ex- 
planation before the definition can be under- 
stood. First a consideration must be given 
to the word science, and second to the word 

Fig. 1-1. A course in general zoology merely gives the beginning student a panoramic view 
of the field by taking brief glances at its separate disciplines. 

view of the entire field it is necessary to 
select only the salient parts of each of these 
segments and fit them into a unified whole. 
The task set before us, then, may be com- 
pared with a professor escorting a student 
down the long corridor of zoology, allowing 
him only a short stay before each portal of 
entrance to the specialized knowledge 
within (Fig. 1-1). From these bits of infor- 
mation and with the aid of the interpreta- 
tion given by the professor, it is hoped that 
the student will be impressed with this uni- 
fied picture, that he will absorb something 

life. Of the two the latter is by far the more 
difficult to define, if indeed that is possible 
at all. Let us try to ascertain what science 

Many attempts have been made to define 
science, and, in general, there is a certain 
amount of agreement among all of the def- 
initions. Science is usually considered to be 
organized knowledge, either the knowledge 
itself or the process of gathering such knowl- 
edge. Obviously, there is a difference be- 
tween the knowledge itself and the process 
which leads to its accumulation. The appar- 


ent discrepancy here disappears when the 
various sciences are considered. For exam- 
ple, the science of human anatomy is indeed 
organized knowledge, because over the 
past centuries all of the parts of the body 
have been described in ever greater detail 
and it is highly unlikely that a great deal 
new can be added to our present knowledge 
in this field. Therefore, the anatomist thinks 
of his science as organized knowledge per 
se. The physiologist, on the other hand, is 
working in a field of biology that is con- 
stantly changing; from one month to the 
next new facts are being brought to light, 
so that at no time can he say his is a nearly 
completed volume of knowledge. He is apt 
to think of his science as the process by 
which organized knowledge is assembled. 
His is a dynamic definition of science, 
whereas the anatomist's is a static one. 

It is apparent, then, that as knowledge in 
a certain field of science comes to a point 
where there is very little left to be added, 
it can be considered as organized knowl- 
edge. During the process of gathering this 
knowledge, that is, during the early days of 
a new science, it must be considered as the 
process by which organized knowledge is 
assembled. It is a matter of time; some day 
perhaps all science will be the static type, 
although this is highly unlikely. 

In general, people appreciate the static 
type of science because it is the certain sci- 
ence. They like to be certain of the answers 
with no guesswork. Today we are in the 
midst of many new sciences, all of them 
dynamic, all of them crying out for more 
and better answers to problems involved in 
the business of living. Very few of them are 
certain; most of them are solving problems 
that change almost from day to day. This 
makes many people very uneasy, even to 
the point of wishing the scientist would stop 
his headlong plunge into the world of the 
unknown. In our present state he will never 
stop, nor should he. Whether his efforts are 
good or bad are not for him to decide; it is 
not what he learns that is either good or 

bad, it is the use of that knowledge that 
determines ultimately its value to society. 
The scientist should not be curbed in his 
inquiry into the unknown; rather society 
should take stock of its own interrelation- 
ships, so that what is discovered by the sci- 
entist can always be put to ultimate good 
in alleviating the burdens of the worka- 
day world, freeing man from pestilence and 
disease, and giving him a more enjoyable 
world in which to live. This is a real goal 
and one to which the scientist has contrib- 
uted mightily in the past, and can con- 
tribute to in the future. 

Just how did the scientific approach to 
the study of problems get underway, or has 
it always been a part of man's way of life? 
It is said that we live in a scientific age, a 
statement which would indicate that science 
is a recent thing. Just when did it have its 
inception and by whom and where? Per- 
haps a brief historical sketch would help 
answer some of these questions. From this 
history we may see how the scientist works. 


From the time man first organized into 
groups or societies he has had his problems 
either solved or greatly influenced by cer- 
tain members of his group who were placed 
in a position of authority, and whose word 
was absolute, even to the point of life and 
death. Conclusions were based on emotions, 
hunches, and superstition, but rarely, if ever, 
on observed facts. This cannot be relegated 
entirely to ancient times, since even today 
there are countries in the world where simi- 
lar policies are followed. In this modern 
world such tactics have led to much blood- 
shed and may continue to do so. 

Historically, we need not go back any 
farther than Aristotle (384-322 b.c.) to find 
the faint beginnings of a new way of solving 
problems, indeed, a complete new way of 
life. Aristotle gave us the inductive method 
of reasoning, which means that generaliza- 


tions are made from the facts observed, rate; one is certain that he was observing 
Many facts are collected on the topic in the viscera of some lower mammal when he 
question and after these are all assembled, wrote it. His ideas concerning the function 
certain conclusions are drawn. The counter- of the various parts of the body are rather 
part of this method is the deductive method amusing to us today. For example, he 
of reasoning, wherein the generalization is thought that the brain produced mucus and 
made and the facts gathered to fit the gen- was cold while the spinal cord was hot, 
eralization. Most of our great advances in that the heart was the seat of intelligence, 
science have come from the utilization of and that food "cooked" in the digestive tract, 
the inductive rather than the deductive Aristotle was convinced that animals 
method of reasoning. Even when the latter evolved from lower forms, culminating in 
method is employed certain basic facts, fre- man; thus he laid the foundation for the 
quently mathematical, are used as a starting theory of evolution, which was to gain a 
point. While Aristotle was undoubtedly in- foothold over 2,000 years later. He de- 
fluential in introducing the inductive method scribed 520 species of animals that have 
of reasoning to the world, even he was de- since been identified. He erected a crude 
pendent on considerable information that method of classifying animals, altliough his 
had accumulated during the centuries be- system was not accepted by subsequent tax- 
fore his time. It cannot be emphasized too onomists. 

strongly that no man, however great, stands Using Aristotle's contribution toward ini- 

out entirely alone. He always depends to tiating the scientific method of approach to 

some extent at least on what preceded him. the solution of problems, some progress was 

Aristotle was a man of considerable in- made by his followers. But the road was 

fluence in his day and his stature grew difficult and progress was extremely 

tremendously for centuries after his death; slow. Of the many men who wrote during 

the student of today could hardly miss his the first few hundred years following Aris- 

name in almost any field of learning, totle, Galen, who lived in the second cen- 

His greatest contribution, perhaps, was tury a.d., was the most outstanding; he 

his method of approaching problems. Not made some rather remarkable contributions 

that he was right in all of his observations, to the anatomical studies of the human 

for he made many mistakes, but it was his body (Fig. 1-2). Aristotle's influence prob- 

constant insistence on making careful ob- ably stimulated Galen to make some accu- 

servations, first hand, and recording them rate observations on the brain and heart 

accurately that was important. To be sure, particularly. He was denied the use of hu- 

he did rely on accounts by his friends or man bodies for dissection, and his writings 

earlier writers for the description of forms indicate that he used the ape and goat for 

he had never seen. For example, he wrote his studies. 

that there were no neck bones in the lion Galen was a voluminous writer, having 

but merely a fused mass extending from the completed 256 treatises during his lifetime, 

thorax to the skull. It is obvious that he had most of which were medical in character, 

not seen a lion's skeleton or he would never Of these his most outstanding were written 

have written this account. However, where on human anatomy. Galen lived at the close 

he made his own observations, such as his of the Age of Classical Culture, just before 

description of the development of the chick, the Middle Ages, in which all learning was 

he was meticulously accurate. The reader is at a standstill. It was fortunate, for the hun- 

impressed by his account of the breeding of dreds of generations of medical men who 

sharks. On the other hand, his description followed, that Galen did produce work of 

of the internal human anatomy is inaccu- such high grade, because it formed the ba- 

Fig. 1-2. This is a sketch of the human skeleton that appears in Galen's book, On Anatomical Preparations. By 
comparing it with a modern cJrawing some discrepancies will be noted. Note the shape of the neck vertebrae 
in the right-hand figure and observe that the coccyx (tailbone) is one piece and peculiarly shaped. 



sis for medical knowledge for the next 1,200 
years. His book, On Anatomical Prepara- 
tions, was the standard medical text during 
this long period, perhaps the longest "run" 
any textbook has ever enjoyed. 

In the next twelve centuries authority as 
a source of all information reached its peak. 
This was very well illustrated in the study 
of medicine. The professor of anatomy, for 
example, sat before his class with a large 
volume of Galen's text before him, and a 
human body, when it was available, dis- 
played on the dissecting bench some 
distance below. A barber or two huddled 
over the body, exposing the various parts, 
as the professor read about them from the 
text. The students looked on and absorbed 
what they could from this crude demonstra- 
tion. The unquestioning faith in authority 
of this period was revealed when some por- 
tion of the human body did not agree with 
the description in Galen; the body was al- 
ways considered at fault, not the text. Such 
mistakes were common, since Galen's dis- 
sections were based on animals other than 
man. This was typical of the time, even in 
learned circles, and this profound respect 
for authority persisted until a few bold men 
dared strike out to see and learn for them- 
selves some centuries later. 

Among the many men who had the abil- 
ity and courage to fight blind ignorance was 
Vesalius, who was born in Brussels ( 1514), 
trained for medicine in Paris, and finally 
became professor of medicine at Padua in 
Italy at 22 years of age. During his early 
medical school days he was a devoted fol- 
lower of Galen's teachings. But Vesalius was 
able to secure human bodies for dissection, 
and as he probed more deeply into the 
details of human anatomy he discovered 
Galen's errors. Being an independent 
thinker, he soon decided that he must write 
down his own observations concerning hu- 
man anatomy without reference to the work 
of anyone else. He secured the services of 
some of the best artists of his day to illus- 

trate the book that he felt the urge to write. 
Some recently discovered wood-cuts made 
by Calcar, a student of Titian, were found 
to be as beautiful as when first used in 
Vesalius' book. On the Structure of the Hu- 
man Body (Figs. 1-3 and 4). Because he 
was at variance with Galen, Vesalius soon 
became the subject of much adverse criti- 
cism, which finally forced him to leave his 
professorial post to become court physician 
for Emperor Charles V. In this position he 
apparently was not happy, because some 
years later he left on a pilgrimage to Jeru- 
salem from which he never returned. 

The greatest contribution this energetic 
man made to the world was his break away 
from authority and his reestablishment of 
the old Aristotelian ideal of pursuing the 
answer to problems by direct observations. 
This revolt from authority was furthered by 
others after Vesalius, but it received its 
greatest impetus from William Harvey 
( 1578 ) , a British physician, who gave us 
the first experimental approach to biological 
problems. Educated at Padua, Vesalius' old 
school, Harvey became interested in the 
blood vascular system, and went to work 
eagerly after he left the university, to learn 
more about this perplexing problem. Up to 
his time the heart was thought to be purely 
passive and non-muscular; the blood was 
supposed to flow into the heart causing it 
to expand suddenly, which accounted for 
its audible beat against the thorax. The 
blood somehow picked up that intangible 
"spiritus vitalis" while in the heart, then 
passed to the liver where the food was 
changed into blood, thus nourishing the 

Harvey demonstrated that the heart was 
a muscular organ and that its contractions 
were responsible for propelling the blood 
through the arteries. He experimented with 
animals and studied normal and abnormal 
humans for twenty years before he was cer- 
tain that the blood flowed away from the 
heart in the arteries, and returned to the 

Fig. 1-3. This is a woodcut taken from Vesalius' book, The Structure of the Human Body. It depicts Vesalius 
himself giving one of his public demonstrations of the internal anatomy of the human body. 

Fig. 1-4. This is another woodcut taken from Vesalius' book, showing the superficial muscles 
of the back. This is clone with remarkable accuracy. Nofe the background, which undoubtedly 
was the contribution made by the artist Calcar. 



heart in the veins (Fig. 1-5). His most im- 
portant contribution was to prove mathemat- 
ically that the blood must circulate through 
the body in this fashion. He went about 
demonstrating his theory, much the same as 
it would be done today in a modern re- 
search laboratory. He computed that at 
each beat, the human heart delivered about 
two ounces of blood; beating at 65 times 
per minute, in an hour it would force about 
50 pounds of blood through itself. He rea- 
soned that such a tremendous quantity of 
blood simply had to stay within the body 
and circulate from one part to the other. He 
was able to work out the pulmonary circu- 
lation cjuite satisfactorily, but he was forced 
to guess at the existence of body capillaries 
because, lacking a microscope, he never 
saw them. Truly Harvey was a great scien- 
tist, and his courage put physiology on its 
first step toward the profound science it is 
today. He brought experimentation based 
on mathematics into the study of biology 
for the first time, and has been looked upon 
as the founder of modern physiology. 

The values of direct observation and ex- 
perimentation gradually became obvious to 
the intelligent world in the years following 
Harvey's initial steps. All through the nine- 
teenth century brilliant men added their 
influence to the growing infant science of 
physiology until it gained full stature at the 
turn of the twentieth century, and it has 
been growing steadily up to the present. 
During the nineteenth century most biolog- 
ical work was morphological, that is, the 
study of form and structure of animals; 
today physiological problems are receiving 
most of the attention. Biologists are prima- 
rily concerned with the way animals func- 
tion. In order to find answers they have 
been forced to rely on the sister sciences of 
physics and chemistry, for it has become 
increasingly obvious that in the last analysis 
these sciences are most likely to give us the 
solution to our most profound problems. 

Using these four men, Aristotle, Galen, 
Vesalius, and Harvey, as examples spaced 

Fig. 1-5. These are two of the original sketches made by 
Harvey in his little book describing the circulation of 
the biood. The figures demonstrate the valves in the 
veins that permit the blood to flovt^ only toward the 

over nearly 2,000 years, it is possible to see 
how the science of zoology took root and 
grew to what it is today. Men could have 
been selected from other sciences to dem- 
onstrate the same evolution in a way of 
thinking. In any case, it is obvious that sci- 
ence was slow in developing during its first 
2,000 years. It has grown tremendously 
since then, however, particularly during the 
past 200 years, and in this century almost 
explosively. There seems to be no indication 
of any abatement in its growth at the pres- 
ent time, and if properly guided science will 
certainly help to make this world of ours an 
almost miraculously beautiful one in which 
to live. 


Before we discuss the methods used by 
the scientist, let us consider for a moment 
what science has done for people of the 
world, what it is doing today, and what it 
could or might do in the future. We can use 
one example, world population in human 
beings. Approximately 250 years ago there 
was a world population of some 600 millions; 
that number has increased fourfold up to 
the present time and is now increasing at 
an unprecedented figure. This means that 
within the relatively short period of 250 



years there has been a tremendous increase 
in population compared to the preceding 
period of approximately 500,000 years which 
produced the number of people alive in 
1700. What has been responsible for this 
sudden burst of reproductive powers in 
man? Certainly there has been no physical 
evolution in man himself in so short a time. 
It is generally agreed that food is the limit- 
ing factor in the growth of any population, 
be it fish or man. Populations always en- 
croach upon the food supply, just as closely 
as they can without widespread starvation. 
This increase in population, then, is due to 
increased food production; increased food 
production has come about through the ap- 
plication of the scientific method to food 
production problems. This might be all 
changed again by a single important dis- 
covery, for example, an economical metliod 
of producing food from inorganic sources, 
such as sugar from carbon dioxide and wa- 
ter. A discovery such as this would change 
the entire food problem of the world over- 
night. Feeding the half-starved world of 
today is within the realm of possibility now; 
it is not being done for reasons that are be- 
yond the realm of science, at least for the 

In colonial days the average life span was 
under 40 years for a man; today it is well 
over 60. It is not that men are any better 
physically today than they were then. It is 
due almost entirely to the progress made by 
science in understanding the cause and pre- 
vention of infectious diseases. All of the ad- 
vancements that have been made in medi- 
cine have been accomplished through the 
agency of science. Before the scientific 
method was inculcated into medicine it was 
largely superstition and mysticism. Since 
the discovery of the Germ Theory of Dis- 
ease, aseptic surgery, and anesthesia, many 
of man's ills have been partially or wholly 
conquered. There is no doubt that many of 
the infectious as well as organic diseases 
that plague man today will eventually be 
eliminated from civilized societies. If man 

would follow the scientific approach to the 
solution of his peacetime problems as av- 
idly as he employs them in the preparation 
of war machinery, this world would soon be 
a near-perfect place to live. Unfortunately, 
the hope that he will do this seems rather 
remote at the present time. 

It seems clear that the application of the 
scientific method to the solution of man's 
physical betterment has been good and bad. 
It has lifted many burdens from his shoul- 
ders by simplifying the work essential for 
his physical needs; it has extended his aver- 
age life span also, but, at the same time, has 
provided him with deadly weapons, such as 
the atomic and hydrogen bombs. He has for 
the first time an instrument within his grasp 
that can annihilate the whole of the civil- 
ized world as we know it. Such a situation 
in an uneasy world certainly is not good 
when considered from the point of view of 
survival of a race. Perhaps the application 
of the scientific method to man's social ills 
might have some of the success it has had in 
the alleviation of his physical ills. 

With all of the wonders that science has 
produced, it cannot answer all of our prob- 
lems. We have found no way to measure 
love, beauty, or the faith people have in 
God. By common agreement the scientist 
deals only with the things he can measure, 
either with his senses directly, or with 
instruments that magnify their sensitivity, 
such as the telescope and microscope. The 
ultimate purposes and goals of life itself are 
not within the scope of science, and must 
be left to religion and philosophy. 


Having considered some of the accom- 
plishments of science, we should now find 
out just how the scientific method operates. 
It follows a series of rather definite stages 
in the solution of a problem. 

1. Statement of the problem. In so far as 
it is possible, the problem should be clearly 
stated, that is, the investigator should have 



a rather clear-cut idea of his objective. Usu- 
ally, this is relatively easy, although the 
final answer as a result of the applica- 
tion of the scientific method may not be as 
clear as the initial problem indicated. For 
example, the problem might be: what 
causes cancer? After many years of work by 
thousands of scientists, there is as yet 
no clear-cut answer. An apparently simple 
problem may become very complex once it 
is pursued for a time. In fact, this is the 
usual experience of most scientists in the 
search for the solutions to problems. 

2. Hypothesis. Once the problem is 
stated, some kind of supposition or guess 
as to the answer should be formulated. This 
is the hypothesis or guessed theory. For 
example, thousands of various chemicals 
are frequently tested with respect to their 
ability to destroy bacteria in the bodies of 
animals. Continued testing is based on the 
theory that since some chemicals have been 
effective for the purpose of bacterial de- 
struction, others may also be effective. This 
is rather remote evidence, for various chem- 
icals are likely to react differently. How- 
ever, there is always the chance that a new 
chemical may react in some new way or 
may even supersede the reaction of some 
known chemical in eventual effectiveness. 

3. Gathering facts from observations or 
experimentation. Facts must be accumu- 
lated, based on observations with the 
senses or extension of the senses made pos- 
sible by the use of instruments such as the 
microscope to increase the ability to see 
small things and the telescope to observe 
distant bodies. These facts must be gath- 
ered painstakingly and sometimes over long 
periods of time. The impressions received 
through one sense should be checked 
against those received through other senses, 
in short, all possible information must be 
gathered from all sources. In collecting 
these facts the scientist must not be influ- 
enced in the slightest by his own opinions. 
Facts must be collected without bias, and 
records must be accurately kept. 

Some problems require observation alone, 
as, for example, the measuring of the orbit 
of the moon. It would be impossible to do 
other than to collect data on the activities 
of the moon over a long period of time. 
Other problems lend themselves to experi- 
mentation, that is, through alteration of 
the natural course of events it is possible to 
make observations over a short period of 
time, under controlled conditions. When- 
ever experimentation is possible, results are 
obtained much more rapidly than in those 
fields where observations must be taken on 
naturally occurring events. For example, 
the way in which color blindness is in- 
herited in man can be detected by studying 
several generations of families over a period 
of 50 or 100 years, whereas the underlying 
mechanism can be demonstrated in a month 
or two by studying fruit flies bred in the 

4. Compiling and interpreting the data. 
Once the observations and experiments 
have been completed, the data must be 
arranged and coordinated. Frequently, it 
is impossible to note trends or to determine 
any noticeable effects while the observa- 
tions are being made; it is only after ar- 
rangement and coordination into units that 
the real results can be detected. This sys- 
tematization involves the construction of 
tables, charts, and graphs. Often mathe- 
matics will be required before true 
relationships can be stated. 

5. Conclusion. The conclusion is prob- 
ably the most important part of the sci- 
entific method, and it is the part where 
great caution must be exercised. Facts are 
the only basis for interpretation. Conclu- 
sions must be drawn logically and should 
be as nearly absolute as possible. These are 
most reliable when based on mathematics. 
Caution must also be used in drawing con- 
clusions with absolute certainty, since 
nothing is absolutely final to the scientist. 
Science is relative and subject to change as 
more knowledge is attained. As new facts 
are discovered in the chemical and physi- 



cal worlds, biology is subjected to change. 
This uncertainty that runs through all sci- 
entific thinking is often very annoying to 
people employed in pursuits which to them 
possess more certainty. 

In order to illustrate the scientific method 
in operation, an example is included here: 

1. Statement of the problem. Is vitamin 
Bi necessary in the nutrition of a rat? 

2. Hypothesis. Since vitamin Bi is es- 
sential in the nutrition of other mammals, 
it is also essential in the nutrition of the rat. 

In order to solve this problem, it will be 

absence of growth, or decrease in growth 
are used to measure effect. 

The rats are separated into two groups 
of equal number, say five in each group. If 
the sexes are evenly divided, the males and 
females should be distributed evenly be- 
tween the two groups. While it is unlikely 
that sex differences affect the nutrition of 
young rats, it is well to control all possible 
factors. To the one group, known as the 
experimental group, should be fed a care- 
fully prepared diet that includes all food 
essential to the growth of a rat, with the 



^ I90 



_ 160 

5 I30-J 


(ail weights dv/era< 

(a\\ weights averaged) 
6i added 

30 35 40 46 50 55 60 







Fig. 1-6. A graph showing the effect of the lack of vitamin Bi on the normal growth of rats. 
The average weight of the control rats increased throughout the growth period, while the 
rats on the experimental diet (lacking Bi) failed to grow after 5 days. When the vitamin 
was restored to the diet they grew normally. 

necessary to obtain several rats, preferably 
litter mates from a pure-bred stock, since 
uniformity among the experimental animals 
is most important. Litter mates are brothers 
and sisters, therefore more alike than un- 
related rats, and should give the same or 
nearly the same response to treatment. It is 
highly desirable that only one factor vary, 
and that one should be the factor under 
observation. From past experience it has 
been found that young rats are more re- 
sponsive to dietary differences than older 
ones, and for that reason half-grown rats 
should be used. The increase in growth, 

exception of vitamin Bi. To the other group, 
the control group, should be fed the same 
diet in equal proportions, but with vitamin 
Bi added. 

3. Gathering facts from observations 
and experimentation. The rats in both 
groups must be weighed carefully at the 
beginning of the experiment. Normal 
growth charts for rats should be consulted 
in order that growth in the control group 
can be determined as normal. Each rat 
must be weighed every day and accurate 
records kept until the end of the experi- 
ment. Food and water must be abundant at 



all times. Temperature, humidity, air cur- 
rents, and all other environmental factors 
must be controlled in the room where the 
rats are kept. Observations of any abnor- 
malities among the rats must be recorded. 
These abnormalities would include infec- 
tion or any other factor that might influ- 
ence the results. 

4. Co7npiling and interpreting the data. 
The data must then be assembled in the 
form of charts, tables, and graphs (Fig. 
1-6). The rats which received no vitamin 
Bi failed to grow normally, whereas those 

rate of growth within each group; also, the 
poorest control rat was only slightly better 
than the best of the experimental group. 
However, when averaged out and plotted 
as in Fig. 1-7, the results are clear-cut. If 
one studied only two rats and by chance 
got rats No. 5 ( control ) and No. 2 ( experi- 
mental), the results might be inconclusive; 
hence the need for an adequate number 
of animals. This variation among animals 
can be measured mathematically and the 
significance of the variation computed very 
accurately. This is the field of biometry ( ap- 



^ 190 
• 160-1 

2 130 



Control Rat w?5 .^txperimental^at N^2 

E)cperimental Rat N^^ 

B, added 

30 35 40 4B 50 55 60 





80 85 

Fig. 1-7. Similar to Fig. 1-6 except the growth is recorded for specific control (#3 and #5) 

and experimental (#2 and #4) rats only. 

receiving this substance were healthy nor- 
mal rats. When the vitamin was added to 
the diet of the experimental animals, an 
almost immediate positive response was 
noted in their growth. It may be concluded, 
therefore, that vitamin Bi was necessary for 
the normal growth of the experimental 

More can be learned about this experi- 
ment if more data are studied. For exam- 
ple, how much variation was there in the 
growth rate of the rats? The two extremes 
in both the control and the experimental 
animals are plotted in Fig. 1-7. It is obvious 
that there is considerable variation in the 

plication of statistics to biological prob- 
lems ) and is veiy important in determining 
the answers to many experimental problems. 
5. Conclusions. It can be concluded that, 
within the limits of this experiment, vita- 
min Bi is necessary for normal growth in 
rats. Such a statement leaves room for fur- 
ther research which might prove that some 
substance other than vitamin Bi is the 
controlling factor. For example, one species 
of bacteria, growing in the intestine of the 
rat, might inhibit the use of vitamin Bi, 
whereas another species might actually 
produce the vitamin. Such a thing is pos- 
sible, but until it is proved by further ex- 



perimentation, the conclusions drawn from 
this experiment hold. 

From these data a generalization can be 
made concerning the use of Vitamin Bi 
by the rat. If, for example, all mammals, 
indeed all animals, required Vitamin Bi, it 
would then be possible to formulate a 
theory concerning this substance and nutri- 
tion in animals. Usually theories are applied 
to broad concepts such as the existence of 
atoms, molecules, gravity, and evolution. A 
theory is a tentative or probable explana- 
tion of a problem; once a theory is formu- 
lated, it must be tested in all possible ways, 
for if it fails to explain all of the subsequent 
findings, it must be discarded or altered. 
Many theories set forth by scientists have 
later found their way to the waste basket. 
If a theory continues to explain the facts 
after rigorous testing, it becomes a law. In 
zoology, "life from life" is a law. 

A scientist either has acquired or innately 
possesses the spirit of inquiry; without this 
trait he would never be fired by the un- 
known and would, therefore, never be 
inclined to investigate that which was not 
already well understood. All first-rate sci- 
entists are endowed with this trait to a 
marked degree. A good example of a man 
who was imbued with the spirit of inquiry 
throughout his life was Pasteur, who could 
never allow a problem to rest until he had 
attained an acceptable answer to it. As a 
result he probably revealed more impor- 
tant information about disease and the cure 
for human suffering than any other man. 
The scientist must constantly have an open 
mind but he must also have a very critical 
mind. Not only must he criticize the work 
of others, but what is far more difficult, he 
must measure his own work by the same 
yardstick. It is only through checking and 
rechecking one another's work that sci- 
entists have been able to arrive at present- 
day knowledge. Scientists are their own 
severest critics and so they should be. Con- 
clusions must be carefully drawn and must 
always allow for future discoveries which 

may alter the apparent facts today. The sole 
interest of the scientist lies in an explana- 
tion of the physical world about him and 
the life on it. To the pursuit of this task 
he is dedicated with unswerving devotion. 


There are many facets to the field of 
biology, each of which has become very 
specialized today. The oldest field is con- 
cerned with the structure and form of ani- 
mals, and is termed morphology. It has 
several subdivisions. Anatomy has to do 
with gross anatomical structures that can 
be studied with the naked eye. Histology is 
the study of the microscopic architecture 
of organs, and cytology is the study of cell 
structure; both involve the use of the micro- 
scope. Another branch of morphology is 
embryology, the study of the development 
of an animal. 

The study of animals in respect to their 
proper classification is known as taxonomy. 
The field which concentrates on the dis- 
tribution of animals geographically is zoo- 
geography, and the study of fossil remains 
is paleontology. 

Physiology is the study of the manner 
in which animals function; this phase of 
biology is more recent and occupies the 
attention of a great many people in all 
branches of biology today including agri- 
culture and medicine. It is probably the 
most fruitful of all in regard to the improve- 
ment of our physical shortcomings. Genetics 
is the study of the mechanics of inheritance. 
A study of the relation of animals to their 
environment is known as ecology. This is 
an important phase of biology because it 
provides the knowledge for the rehabilita- 
tion of stocks of wildlife. Naturally it in- 
volves plants as well as animals. The science 
of the mind alone resolves itself in the field 
of psychology and when the study includes 
the entire animal and its interrelationships, 
the field is known as sociology. 



The science of zoology is so vast and so 
specialized that a zoologist may spend his 
entire life effort on one small phase of the 
subject. For example, the entomologist con- 
centrates on insects and may be fairly famil- 
iar with a large number of them, but a dip- 
terologist is an expert on Hies, one order of 
insects. Likewise, a parasitologist may have 
a nodding acquaintance with many animal 
parasites, but a helminthologist is a special- 
ist on worms, and a 7nalariologist confines 
his work to causative organisms of malaria, 
which are protozoan parasites. The orni- 

thologist studies birds and their habits, 
whereas the mammalogist concentrates on 
mammals. It is obvious that there are many 
small segments of the science of zoology 
that occupy the attention of zoologists 
today, and it is from this concerted effort 
that our knowledge is going forward at a 
tremendous rate. The possibility for future 
benefits to mankind are almost limitless, 
providing man has sufficient intelligence to 
utilize this knowledge for the advancement 
of the human race rather than for its de- 


Life: Its Beginnings and Nature 





If we are to understand living things on 
the earth today, even in the most rudimen- 
tary way, we must have some understand- 
ing of the physical world in which they 
came into being. It must be remembered 
that the physical world was here first, and 
that all living things have sprung from it, 
using the materials and forces that are 
present in it. There can be no life without 
a surrounding world, an environment in 
which it resides. In a sense they must 
always be considered together, for the en- 
vironment can exist without the living 
organism, but the reverse is not true. Since 
the environment was here first, it is well to 
consider some of the fundamental charac- 
teristics of this environment, so that when 

we study life later it will have more 


The earth was born two or three billion 
years ago. Along with the other planets of 
our solar system, it was probably formed 
out of a very large, flattened, and highly 
rarefied mass of gas and dust that sur- 
rounded the sun and rotated about it. As 
the rotation proceeded, with the regions 
near the sun moving faster than the outer 
regions, whirling eddies must have formed 
in different parts of this tenuous nebula. 
Although most of the gas and dust probably 




escaped the solar system, the larger and 
denser of these whirlpools were able to 
coalesce by the forces of gravitation and to 
attract additional matter into them. After 
some millions of years, these planetary nu- 
clei had grown slowly and steadily by 
accretion into the solid planets we know 

We are concerned with the conditions of 
the earth in this early period that led to 
the formation of the first living things. 
What were the physical factors that pro- 
duced a setting in which such a delicately 
adjusted thing as life could have established 

At first a thin crust formed over the 
bottomless sea of molten rock; because of 
the tremendous heat, water could not exist 
at the surface, so a thick gaseous layer of 
superheated steam covered the entire 
earth. As the steam pushed upwards into 
the cooler regions, condensation occurred, 
causing dense fog and torrential rains. 
Eventually, after still further cooling, the 
rain drops penetrated the heat and reached 
the hot rocks below, only to evaporate as 
steam once more. Finally, however, with 
further cooling, water stayed in the de- 
pressions of the earth's extremely wrinkled 
surface, forming the infant oceans of the 
world. Hot rivers formed, of course, which 
were forever rushing to fill the oceans, 
carrying with them any minerals of the sub- 
stratum that would dissolve. These sub- 
stances were deposited in the young oceans, 
resulting in a constant increase in the 
chemical composition there. Water evapo- 
rated from their great surfaces, just as it 
does today, leaving the heavier salt par- 
ticles behind and thus continually raising 
their total salt content. That is why the 
ocean water of today is salty. The im- 
portance of these great bodies of water lies 
in the fact that undoubtedly life originated 
in them, sometime during this early stage. 
Here, then, we see a spinning new world, 
sufficiently far from the sun to be moder- 
ately warm in most of its parts and fixed 

in a never fluctuating orbit, which insures 
evenly spaced seasons. The surface is cov- 
ered with rocks, gravel, sand, great water 
masses, turbulent streams of cool to warm 
waters, but no soil. A thick homogeneous 
gas, rich in nitrogen and oxygen, envelops 
its surface, with clouds of water vapor lazily 
floating with the moving air currents. This 
is the setting in which life started; this is 
the world that gave birth to that dynamic 
something which, once initiated, extended 
into every part of that world. As it ex- 
tended it became more and more diversified. 
Finally it gave rise to human beings who 
can sit here now and consider how it all 
came about, a wonderful series of events, 
and certainly fascinating enough to stimu- 
late the curiosity within us. 


There are certain physical laws that 
influence all things, animate as well as 
inanimate, in the universe; therefore, it is 
well that we learn something about them, 
before we attempt to study life itself. 

All things in the universe are composed 
of matter, and, since living things are de- 
rived from material in the world, they are 
also composed of matter and therefore be- 
have as matter does. One of the character- 
istics of matter is that it occupies space 
and has mass. Mass may be defined as the 
amount of matter in a body which can be 
measured in terms of resistance to change 
of motion. This is more meaningful when 
it is considered in the light of attraction 
between bodies. Bodies attract one an- 
other according to their respective masses. 
For example, the earth has a greater attrac- 
tion (gravity) for a man than does the 
moon because the earth has greater mass. 
We have a convenient method of measuring 
this attractive force between bodies; we 
call it weight. Weight, of course, changes, 
depending on where it is taken. A person 
weighing 180 pounds on the earth would 



weigh 30 pounds on the moon, although his 
mass would be the same. Weight simply 
measures the pull of gravity. 

Animal bodies are constructed to com- 
pensate for the pull of gravity. For ex- 
ample, small animals, such as mice, have 
relatively light skeletons in comparison to 
their weights, whereas larger animals, such 
as the elephant, have much heavier skele- 
tons with respect to weight. This fact limits 
the size of animals, for should they go on 
increasing in bulk, they would reach a 
point where the skeleton alone could not 
bear its own weight. If life, as we know it, 
occurs on other planets, it too would show 
relation to weight. Animals on Jupiter, for 
example, would have to be constructed on 
an entirely different plan from those on the 
earth, because the pull of gravity is so much 
greater. They would probably be heavily 
boned animals and greatly flattened. 

One cannot mention the motion of matter 
without referring to another force that 
operates on bodies, namely, inertia. When 
a swift elevator starts up, one is con- 
scious of a sudden increase in weight; like- 
wise, when it comes to rest, one seems 
suddenly and momentarily lighter than 
usual. There may be a simultaneous pecul- 
iar feeling in the mid-section as the in- 
ternal organs respond to the effects of 
inertia. This force is the resistance of a 
body to change in its rate of motion. If 
standing still, it resists movement; if mov- 
ing at a certain rate of speed it resists any 
change in this rate. That is why we need 
low gear and good brakes on our cars — it 
requires more power to get started or to 
stop tlian to keep rolling. 

Most animals are little affected by inertia 
except when suddenly stopped, such as a 
bird flying into the side of a building. The 
sudden cessation of forward motion can be 
fatal to bird or man, as attested by car 
accidents. Another modern machine that 
brings the effects of inertia into promi- 
nence is the airplane. Pilots often "black 
out" because their forward motion is sud- 

denly changed, as in coming out of a power 
dive. The blood, being fluid, tends to fol- 
low the forward motion it has attained and 
is thus pulled away from the head, causing 
the "black out" or faint. The normal move- 

Fig. 2-1. Two cubes, one with dimensions l/lOth of the 
other. In respect to volume, the smaller has 10 times 
the surface area of the larger. 

ments of animals are little affected by in- 
ertia, and the problem arises in man only 
when he steps into one of his mechanical 
contrivances which carry his body faster 
than it was made to go. 

Surface phenomena 

The behavior of matter depends to a 
large extent on its surface area. Large 
bodies have smaller surface areas in respect 
to volume than do small bodies. A mouse 
has more surface area in respect to its vol- 
ume than does an elephant. This can easily 
be computed by simply measuring the 
surface area of a cube, say 10 inches on a 
side or 600 square inches, and then its 
volume, which is 1,000 cubic inches (Fig. 
2-1). This is a ratio of 0.6 square inch of 
surface to every cubic inch of volume. If 
a cube one inch on a side is cut out of 
the larger cube, and the surface computed 
in respect to the volume, it will be found 
that each inch cube will have 6 square 
inches of surface, a tenfold increase. Since 
chemical reactions take place at surfaces, it 
is obvious that the activity will be much 
greater as matter is divided into smaller 
and smaller particles. This is a very im- 
portant physical property, as we shall see 
when we discuss enzyme action and many 



other activities that go on within the animal 


Particles of matter, especially small par- 
ticles where the effect is more apparent, at- 
tract one another of the same kind and also 
have an attraction for other particles of a 
different sort. The first is given the name 
cohesion, and the latter adhesion. Particles 
of matter such as water tend to cling to- 
gether to the extent that they actually form 
a film at their surfaces. This gives the effect 
of a stretched membrane and is spoken of 
as surface tension. Water has a rather high 
surface tension, but mercury surpasses it 
by a considerable margin. Anyone knows 
that when mercury is dropped it breaks up 
into hundreds of small perfect spheres. 
Water gives a similar but lesser response 
when dropped on a dry, dusty surface. Rain 
drops are usually near-spheres. Liquids, 
when free of external forces, will assume 
the shape of a sphere because the cohesive 
forces of the particles of which it is made 
form a surface membrane. This fact makes 
it possible for certain species of insects to 
walk on the water (Fig. 2-2). Their bodies 
are li^ht and the weight is distributed over 
a large area, so that the cohesive force of 
the water is sufficiendy strong to keep the 
membrane intact. 

Fig. 2-2. Some insects, lii<e the water stridor, can walk 
on water because their evenly spread weight is not 
sufficiently great to break the surface tension. 

Adhesive forces are simultaneously at 
work with cohesive forces. These have no 
significant influence on large bodies, but 
on small particles they become very im- 
portant, just as important as gravity is 
to the larger bodies. Certain gaseous mole- 

cules, for example, will adhere to carbon 
particles so tenaciously that it requires high 
temperatures to remove them; this is the 
principle of the gas mask and certain puri- 
fication processes. It is important in bio- 
logical systems where enzymes, for ex- 
ample, adhere or adsorb (the process is 
called adsorption) to food particles, thus 
enhancing digestion. This physical property 
of matter manifests itself in many ways in 
the bodies of animals. 

Composition of matter 

Almost every school child knows that 
matter is made up of molecules, and some 
know that the molecules are composed of 
atoms. Any college student could hardly 
have missed newspaper discussions of the 
nature of atoms, how they are being in- 
vestigated, and particularly what effect 
this knowledge is having on our lives today. 
There is a great deal of excellent experi- 
mental proof for the existence of these small 
particles, some of which we should consider 


Matter exists in space in one of the three 
forms: solids, liquids, or gases. Water is a 
convenient example of matter. Below freez- 
ing it exists as ice, a solid; between freezing 
and boiling, it exists as water, a liquid; 
above boiling, it exists as a vapor or steam, 
a gas. In all of these states the substance is 
still water, its chemical composition un- 
changed. Whether it is in one state or 
another depends on the interrelationship of 
the molecules in space. In the solid they 
are close together and rather static in their 
position, although free to vibrate; in the 
liquid they are still close together, but not 
so close as in the solid and they are free to 
move within the liquid itself; in the gaseous 
state they are far apart and free to move. 
The particular state in which matter exists 
depends on the speed of movement of the 
individual molecules. In the sohd state they 
vibrate, but stay in fixed positions within 
the solid; in the liquid they move faster and 
are free to move about within the liquid. 



whereas in the gaseous state they attain 
high speeds, so high that they exceed the 
intermolecular attractions and separate 
from one another to become independent 
free-floating bodies. In the gaseous state 
they demonstrate their smallness because 
they then are invisible. The rate of molec- 
ular movement is reflected in the phe- 
nomena of heat and cold. Molecules move 
faster in hot bodies than in cold ones. When 
there is no movement, the body is as cold 
as it is possible to get, a condition called 
absolute zero. 

All specific chemical substances exhibit 
the same changes in state that water does, 
although they are not always so easy to 
observe. There are thousands of different 
kinds of matter, each composed of a spe- 
cific kind of molecule, such as oxygen, hy- 
drogen, or water. Since life is composed of 
matter, it must then also be composed of 
different kinds of molecules, and the prop- 
erties of these molecules must be reflected 
in the properties of living things. 

Molecules may be still further divided 
into atoms, which were until recently 
thought to be the ultimate irreducible par- 
ticles of matter. With the advent of "atom 
smashing," a new interpretation has been 
placed on the atom and its place in the uni- 
verse. Ignoring this recent information for 
the moment, chemists tell us that there are 
98 different kinds of atoms, called elements, 
each with definite distinguishing character- 
istics. At least five more have been pre- 
dicted although at present they are un- 
known. It is the various combinations of 
elements that make up all the thousands 
of chemical substances existing naturally or 
that can be produced in the laboratory. 
Water, for example, is made up of H^O, 
t\vo atoms of hydrogen (H) and one of 
oxygen (O). A more complex molecule is 
blood sugar, glucose CgHi^Og, composed of 
six atoms of carbon, twelve of hydrogen, 
and six of oxygen. It is obvious that with 
98 building units millions of combinations 
of atoms are conceivable. Many of these 

molecules appear in living things and it is 
necessary to know something about their 
individual behavior in order to have some 
understanding about their combined effect 
as it occurs in a cell, for example. Chemists 
have been studying these for a long time, 
and their knowledge is so significant to the 
study of animals that today zoologists are 
dependent to a large extent on this in- 
formation to aid them in solving some of 
their complex problems. 

The arrangement of atoms in a molecule 
gives to that molecule its properties. Sugar 
is sugar because of the arrangement of the 
atoms in its molecule. If any of the atoms 
are removed or even changed in their re- 
spective positions within the molecule, the 
substance is no longer the same; the prop- 
erties are different. For example, if the 
hydrogen and oxygen in the water molecule 
are separated, we no longer have water but 
two gases, neither of which acts like water 
in any way. When the molecules are all 
alike, we speak of the aggregate as a sub- 
stance; if, however, there are several dif- 
ferent kinds of molecules or substances 
present, we refer to the combined material 
as a mixture. A lump of sugar is a sub- 
stance; when it is placed in a cup of coffee, 
the result is a mixture. The sugar exhibits 
specific properties which are always the 
same, whereas a mixture displays variable 
properties. Living things are composed of 
mixtures and therefore respond as mixtures. 
Mixtures are much more difficult to under- 
stand than substances, and because life 
exists in a mixture, a very complex mixture, 
it likewise is difficult to understand. 

The nature of atoms 

In attempting to understand the material 
in which life resides, we are compelled to 
study the nature of the atom itself. In a 
study of this particle we must rely on the 
physicist, who has revealed a great deal of 
information in recent years concerning the 
constitution and behavior of the atom. The 
atom is composed of still smaller particles, 



units which have been weighed, counted, 
checked for speed, and measured for their 
electrical nature. The significance of atomic 

Fig. 2-3. The possible structure of the hydrogen atom. 
The proton with its positive charge lies at the center 
and the negatively charged electron revolves about it. 

structure came to the attention of everyone 
when physicists were able to manipulate 
atoms in such a manner as to have them 

would be invisible, but because of their 
terrific speed they would appear as a dim 
blur, giving a vague limit to the entire 
structure. In some atoms there would be 
other concentric rings within the outer shell 
and these would appear as hazy as the outer 
shell; they might be intertwined with one 
another. Only the speed of the electrons 
would make these limits apparent, because 
the electrons themselves seem to have very 
little, if any, mass. They are "intangible 
units of energy," whirling at unimaginable 
speeds, yet maintaining remarkable sta- 
bility. Atoms can be "smashed," as we all 
know, if unbelievable amounts of energy 
are directed at them. As a result of "smash- 
ing," the nature of the atom itself changes. 
The nucleus of the atom consists of a 
dense cluster of positively charged parti- 

Fig. 2-4. A hypothetical explanation of how helium is formed by the combination of two 

neutrons with two hydrogen atoms. 

release tremendous amounts of energy in 
the form of atomic explosions. 

One might think of the atom as a minia- 
ture solar system with its relatively large, 
heavy nucleus, the "sun," and its revolving 
electrons, the "planets" ( Fig. 2-3 ) . Reminis- 
cent of the universe, the most striking char- 
acteristic of an atom is the vast amount of 
space between the various particles of which 
it is composed. If we imagine the atom to 
be the size of a balloon 100 feet in diameter, 
it would appear as a hazy, transparent 
sphere. At the center would appear a 
nucleus, the only clearly visible part, about 
the size of a small marble. The electrons 

cles, protons, and uncharged particles, 
neutrons (Fig. 2-4). These are held 
together by some unexplained intra-nuclear 
force. The protons and neutrons make up 
almost all of the mass of the atom. For 
every positively charged particle (proton) 
in the nucleus there is an electron, which 
carries a negative charge, in one of the 
orbits. Thus the total atom is neutral, that 
is, it carries no apparent charge. It is inter- 
esting to note that all of the 98 elements 
are made up of these vmits, differing one 
from another only because of the relative 
number and arrangements of protons, neu- 
trons, and electrons. 



A specific atom behaves the way it does 
because of the number of protons and 
neutrons in its nucleus and the number of 
electrons in its orbits. While there is always 
the same number of protons in the nucleus 
as there are electrons in the orbits, there 
may be a varying number of neutrons pres- 
ent in any specific atom. Since the chemical 
characteristics of the atom are controlled 
by the electrons in the outer shell or orbit, 

occurring atom but they have different 
weights, and therefore can be identified or 
"tagged." Tagging atoms has made it pos- 
sible to trace various chemicals through the 
animal body. This has been very helpful in 
determining what happens to certain sub- 
stances in normal life processes. If the iso- 
tope happens to be radioactive, that is, if 
it happens to give off radiations that can 
be detected with a sensitive instnjment 







Fig. 2-5. A small portion of the Periodic Table. The number at the center (nucleus) indicates the 
atomic number. The electrons lie in the orbits, of which only two are shown here. The inner 
orbit requires only two electrons while the second needs eight, which is satisfied in neon. 
Of the ten elements indicated, hydrogen, carbon, nitrogen, and oxygen are important 
constituents of living material. 

and since the number of electrons is con- 
trolled by the number of protons at the 
center, any additional neutrons will be 
without effect on the chemical properties 
of the atom. The only difference will be in 
its weight. Physicists have found that they 
can add or knock out neutrons as well as 
electrons, and thus change the physical 
properties of the atom itself. When only the 
number of neutrons is changed the resulting 
atom is called an isotope; isotopes have the 
same chemical properties as the naturally 

(Geiger counter) or by some other means, 
then the problem of tracing the chemical 
becomes less difficult. We are gaining a 
great deal of knowledge today from this 
type of so-called "tracer research," and the 
future holds out much promise in tliis field 
of investigation. 

It would seem simple, then, to arrange 
all of the various elements in a series from 
1 to 98, according to the number of elec- 
trons ( or protons ) in each individual atom. 
This has been done and we call such an ar- 



rangement the periodic table. It is possible 
to diagram this atomic sequence ( Fig. 2-5 ) . 
The atomic number corresponds to the 
number of electrons in the orbits, or the 
number of protons in the nucleus, limited in 
range, of course, from 1 to 98. The atomic 
weights are arbitrary figures assigned to 
each atom, and they depend on the num- 
ber of protons and neutrons in the nucleus. 
Oxygen has been assigned the figure 16 
and others all vary in respect to it. Since a 
single atom cannot be weighed, these fig- 
ures must be based on measurements that 

starts a new outer orbit containing one elec- 
tron; thus, with its two inner electrons and 
the one in the outer orbit, it has three alto- 
gether, giving it the atomic number 3. 
Sodium (not shown in Fig. 2-5) is the first 
atom that has a third ring, with 11 electrons 
in all. Usually the inner ring must be com- 
pleted before the next one is formed. Fol- 
lowing this principle the numbers of pro- 
tons, neutrons, and electrons of nearly all 
98 elements have been determined. 

When the number of electrons in the 
outer orbit is less than half the total num- 


oxygzn a¥om 

2 ekctronft 


Fig. 2-6. A possible explanation for the formation of a molecule of water from two atoms of 
hydrogen and one of oxygen. Note that the hydrogen ion is a naked proton, and the 
oxygen ion is formed by the addition of two electrons. The union of the two kinds of 
ions produces the molecule of water. 

include a great many individual atoms, 
hence are not absolute figures. 

There are seven possible concentric rings 
of electrons among all of the atoms, the 
one closest to the nucleus containing two 
electrons, and each of those beyond hav- 
ing varying numbers. Hydrogen has one 
proton and one electron but contains no 
neutrons. It has the atomic number 1. 
Helium, the next in the series, has two elec- 
trons and, therefore, the atomic number is 
2. However, it has an atomic weight of four, 
because it also has two neutrons in its 
nucleus along with the two protons. Lithium 

ber it can hold, it may lose them, or if it 
has more than half, it may gain others to 
complete the ring. Any change in these 
numbers of electrons changes the electrical 
nature of the atom; if it gains electrons it 
becomes negative, and if it loses electrons 
it becomes positive. Whenever the atom is 
out of balance in respect to its electrons it 
is an ion; if it possesses an excess of elec- 
trons it is called an anion (because if 
placed in an electrical field it will move 
to the positive pole, the anode); if it has 
lost electrons it is known as a cation (be- 
cause it moves to the negative pole, the 



cathode). A solution containing ions will 
conduct an electric current and is called an 
electrolyte. Atoms will unite to form mole- 
cules depending on the number of electrons 
they can deliver or consume in their ionic 
state. For example, the hydrogen atom has 
one electron; when it loses this it becomes 
a hydrogen ion, a naked proton. The oxy- 
gen atom lacks two electrons in its outer 
shell; when these are gained it becomes an 
oxygen ion, an anion (Fig. 2-6). When the 
hydrogen and oxygen ions are present in 
the same system, they are brought together 
because "unlike charges attract"; since oxy- 
gen requires two electrons to complete its 
outer ring, two hydrogen atoms are re- 
quired to do the job. The result is a mole- 
cule of water, a stable compound, essential 
in all living things. Likewise, thousands of 
other molecules are formed from atoms, 
many of which play important roles in bio- 
logical systems. 

Water forms only a very few hydrogen 
and hydroxyl ions (H+ and OH"), that 
is, only a very small proportion of the total 
number of molecules break up into these 
ions. Since water is the main constituent of 
living material it might be expected that 
any increase or decrease in these ions would 
be detrimental to life. When acids break 
up into ions, large numbers of hydrogen 
ions are produced. That is why they are 
called acids. Likewise, bases produce large 
numbers of hydroxyl ions. Because they do 
produce such large numbers of these ions, 
neither strong acids nor bases are tolerated 
by living things. 

Of all the atoms in living things, carbon 
certainly is the most important. This may 
be due in part to its physical make-up. It 
possesses just one half the maximum num- 
ber of electrons in its outer shell, which 
means that it does not lose or gain others. 
It unites with a large variety of other atoms 
by simply sharing its electrons. This ar- 
rangement permits combination with other 
carbon atoms to produce long chains or 
rings which may then join up with a large 

variety of other atoms to produce immense 
molecules. It was undoubtedly this nature 
of carbon that made it the central atom 
around which life was built. We find it 
permeating all biological systems and play- 
ing important roles, not only in the con- 
struction of living materials, but also in 
storing and releasing energy which is 
essential in life processes. 

From the foregoing account we see that 
life is a complex system of protons, neu- 
trons, and electrons, combined into atoms 
and molecules, all committed by natural 
laws to follow specific patterns of behavior. 
Out of this have come all of the living 
things on the earth today, from amoeba to 
man. Our next step would logically be to 
find, if possible, when, where, and how the 
first living thing appeared on the earth. 


With the physical world settled down to 
a relative stable condition the stage was set 
for the beginning of this most remarkable 
drama, the inception and subsequent un- 
folding of life in all of its variety and com- 
plexity. The initial steps were almost im- 
perceptible, extending over hundreds of 
millions of years, but once underway, life 
gathered momentimi, spreading out in all 
directions over the extremely thin outer 
shell of the earth. This reirion, which ex- 
tends only a few feet above and fewer feet 
below the crust itself, has become thor- 
oughly inhabited except for small areas like 
tlie regions near the poles which possess 
such adverse conditions that life has never 
gained a foothold. 

Some people have speculated that life 
might have come to earth from some other 
planet in the form of spores (capsules ca- 
pable of withstanding unfavorable condi- 
tions) through interstellar space. This 
seems unlikely because of the intense heat 
it would have to endure en route. Bits of 
inorganic matter occasionally fall to earth 
in the form of meteorites; when they reach 

F!g. 2-7. A schematic representation of how life might have originated on the earth. 



Fig. 2-8. Pictures of viruses taken through an electron 
microscope. A. Tobacco-mosaic virus (X60,000). B. 
Bushy stunt virus from tomato plants (X85,000). Note 
the regular arrangement of the particles. C. Virus that 
attacks bacteria (bacteriophage) are the tiny club- 
shaped particles. The large white spheres are used 
as a means of measuring the size of the virus particles. 

our atmosphere they burn to incandescence. 
Life, as we know it, certainly could not 
withstand such high temperatures. It seems 
more within the realm of probability that 
life originated on this planet a long time 
ago. Geologists tell us that there was a time 
about 1 billion years ago when conditions 
were such that life could have started. 
Those conditions do not exist today, so it is 
unlikely that life is being generated now. 
Assuming this to be true, what are the most 
logical steps that could have taken place in 
its inception? From this point on, let us 
follow the speculations of Haldane ( a Brit- 

ish scientist) and Oparin (a Russian sci- 
entist), two men who have formulated a 
theory of the origin of life on earth. A 
sketch such as that shown in Fig. 2-7 might 
also be helpful. As plausible as this theory 
may seem, it is far from proven, and it is 
improbable that satisfactory proof will ever 
be forthcoming. 

All living things are composed of com- 
plex substances called organic compounds. 
They contain carbon atoms as the central 
core around which hydrogen and oxygen 
have accumulated in a precise manner 
to form such substances as ethvl alco- 



hoi (C2H5OH), and glucose (CoHi^Oc). 
Others have incorporated nitrogen into the 
molecule, in addition to carbon and oxygen, 
to form proteins, the complex compounds 
out of which living things are built. There- 
fore, conditions must have been right at this 
early period to bring about atomic combi- 
nations that produced these complex mole- 
cules, which later became the integral part 
of living material. There is reason to believe 
that this could have taken place because 
in the laboratory it has been possible, with 
the use of ultra-violet light, to bring about 
the conversion of simple substances such 
as methane (CH4) and cyanogen (CN), 
containing carbon, nitrogen, and hydrogen, 
into complex organic compounds. If, dur- 
ing the days of the young earth, more ultra- 
violet light reached its surface than today, 
great oceans of these complex organic mole- 
cules, like great pools of organic "soup," 
could have formed. 

These organic molecules could have 
formed aggregates that resembled proteins. 
These, once formed, must have found some 
way to organize themselves into repro- 
ducible entities. Such a condition is alto- 
gether possible and not pure speculation 
because we have reproducible protein par- 
ticles with us today, namely, viruses, these 
invisible (under a light microscope) mi- 
crobes so minute that they pass through the 
very small pores of a porcelain filter (Fig. 
2-8). They cause many plant and animal 
diseases, including such dread human 
maladies as poliomyelitis, yellow fever, and 
many others. Viruses are composed of pro- 
tein and very little else; furthermore, they 
reproduce themselves when placed in their 
normal environment, which is the internal 
parts of cells. They are able to organize 
the surrounding organic compounds into 
their own material and thus duplicate 

Assuming that the original protein mole- 
cules were like the familiar present-day 
viruses, they would be forced to consume 
other molecules in order to reproduce 

themselves just as the viruses do within 
cells. There would come a time, however, 
when all of the organic molecules in these 
oceans of molecular "soup" would be used 
up. Some time before this suicidal state was 
reached these particles must have evolved 
a method of utilizing simpler and simpler 
substances to construct their own frame- 
work. These would be substances such as 
carbon dioxide and water that were pres- 
ent in great abundance. Once this was 
achieved, they could continue indefinitely 
without depending on a supply of complex 
compounds for their subsistence. This is 
the way plants manufacture their products 

From non-living to living 

If viruses are considered to be alive, the 
gap between the living and non-living has 
been spanned. Although, in general, viruses 
exhibit characteristics of living things, they 
do possess one property that is not usually 
associated with life. When properly treated, 
at least one virus, tobacco mosaic virus, will 
form crystals and remain in a state of abso- 
lute inactivity for an indefinite period of 
time. Living things may hibernate, form 
resting cysts, or otherwise remain relatively 
inactive for periods of time, but such vital 
processes as taking in of oxygen and giving 
off of carbon dioxide are still observable 
although in much reduced amounts. The 
crystalline virus does not demonstrate these 
properties. Indeed, for all purposes it seems 
to belong to the inanimate world. Chemists 
usually consider any substance that crystal- 
lizes to be a relatively pure compound; it 
would appear, then, that the virus is a near- 
pure protein, nucleoprotein to be exact. 

When this apparently inanimate crys- 
tal is placed in the tobacco plant cell it 
bursts into activity, taking on all of the 
properties of living things. Is it possible 
that here is a form existing at the border- 
line between the living and non-living 
worlds? Perhaps so. At any rate, it gives 
us a possible clue as to how life originated. 



Even with this start it is a long trek to an 
animal cell, to the simple single-celled ani- 
mal living in stagnant water. 

Genes and gene strings 

All animals and plants are composed of 
cells, the details of which we shall go into 
later, but for the moment in order to con- 
tinue our story of the origin of life, it is 
necessary to know something about certain 
vital parts of these cells. The cell is com- 
posed of a limiting membrane, cytoplasm, 
and a nucleus (see Fig. 3-1). Within the 
nucleus are dark-staining bodies called 
chromosomes, which are made up of nucleo- 
protein molecules or aggregates of such 
molecules called genes (Fig. 2-9). These 
are capable of reproducing themselves 
precisely, obtaining material from the sur- 

Fig. 2-9. Chromosomes are composed primarily of 
nucleoprotein molecules which show up in these 
stained fruit-fly salivary gland cells as dark bands or 
discs. These molecules or aggregates of these mole- 
cules probably constitute the genes. 


«.2t;;*r'^*E£^s>*r,-. -. 

Fig. 2-10. A picture of a bacterium (Pseuc/omonas f/oo- 
rescens) taken with an electron microscope (X22,000). 
Note the long hair-like flagella that are used to 
propel the cell through the water. 

rounding cytoplasm for that purpose. In 
a sense, then, they resemble viruses, at 
least in being nucleoprotein in nature and 
being able to reproduce themselves. They 
also could be considered similar to the 
original protein molecules that formed in 
the early history of life. Here we see coun- 
terparts of these early substances in both 
the viruses that live at the expense of ani- 
mal and plant cells and in the genes which 
are an integral part of all cells. 

Let us consider these independent, self- 
sustaining protein molecules as free-living 
genes. They could then aggregate into long 
strino;s resembling chromosomes, but of 
course they would be merely free-floating 
gene strings or chromosomes. Sometime 
later, because of their physical nature they 
could gather about themselves a semi- 
liquid, semi-solid mLxture of substances 
which would set them apart from others 
in the same vicinity. Somehow a membrane 
could have formed, and a cell not unlike 
certain specialized cells alive today could 
have evolved. Some of the single-celled 
plants, such as the bacteria (Fig. 2-10) and 
certain blue-green algae, possess dispersed 
chromosomes without nuclear membranes. 
In fact, it is possible to select a whole series 
of algae which show succeeding stages 
from completely dispersed chromatin to 



those with the chromatin well centralized. 
The next step in this long competitive path 
would be the accumulation of more ma- 
terial about the free-living "nucleus" to 
make the cytoplasm, thereby forming the 
first cell as we know it today. 

From this brief discussion it should be 
obvious that it is quite impossible to de- 
termine just what the first living thing was, 
and this is exactly what one might expect. 
By tracing living things backwards, the 
inevitable end would be the inorganic 
world; precisely when and where and how 
the point of departure was established is 
still a controversial question and will prob- 
ably always be. Our biological studies have 
progressed far enough today to give us an 
inkling as to how it might have started, and 
with that we must go on to an examination 
of a few of the characteristics of living 
things that exist on the earth today. 


There are certain criteria which our 
study of living things has taught us to 
associate with them. No doubt exists as to 
whether or not a horse or rabbit is alive, 
but without some knowledge of a tree one 
might think that it is not alive, at least not 
in the sense the rabbit or horse is. Even 
the biologist becomes confused when he 
studies such things as viruses which fail 
to measure up in all respects to the living 
things he is accustomed to investigate. 
However, there are certain, rather well- 
defined, characteristics that pertain to liv- 
ing things alone, viruses excepted, which 
are not encountered in the inanimate world. 
1. Movement. Life is endowed with the 
ability to move, and by that is meant 
autonomous movement, the energy for 
which comes from within (Fig. 2-11). To 
be sure, water moves in a river, a stone 
may roll down a hill, a car moves, but all 
of this movement is due to forces acting 
from the outside. The water in the river 
bed flows, the stone rolls down hill, both 

because of the pull of gravity; the car 
moves because a force from the engine is 
applied to the wheels, causing a forward 
movement of the vehicle. This type of 
movement is quite different from that seen 
in the rabbit scurrying through the thicket 
with the dog close at its heels. In this case, 
both are expending energy to move in 
whatever direction each desires. Life im- 
plies movement, life is dynamic. From the 
gross movement of the entire organism to 
the activity going on within each cell of 
the body there is always change, always 
movement. Such movements are linked 
with living things and are not duplicated 
anywhere in the inanimate world. 

2. Irritability. All organisms exist in an 
external world which is spoken of as their 
environment. With this world they are all 
intimately associated, and it is impossible 
to think of the organism without its environ- 
ment. If living things are to profit by this 
association, they must at all times be aware 
of the nature of their immediate external 
world. For this reason they are endowed 
with the ability to sense its characteristics 
and respond to them. The response may be 
favorable, in which case the organism stays 
in the environment. On the other hand, it 
may be unfavorable and as a result the 
organism moves out of the environment, 
sometimes very rapidly as when a pin 
makes contact with the skin of a small 
boy (Fig. 2-12). Such responses play a 
large part in the survival and ultimate suc- 
cess of a race. 

An animal is equipped with an elaborate 
set of sense organs that keep it in constant 
contact with its external world; the func- 
tioning of these organs make the difference 
between life and death of the species. The 
eyes are sensitive to light, the ears to 
sounds, the nose to chemicals; all of these 
assist the animal to orient itself in its en- 
vironment, and to respond in such a way 
as to permit its continuance as an individual 
and as a race. 

3. Nutrition. In order that living things 

Fig. 2-11. This type of movement is characteristic of living things. 

Fig. 2-12. Living things are irritable. They respond to Fig. 2-13. Living things take in food from v»^hich they 
their environment in very definite ways. derive energy to carry on all their life processes. 

Fig. 2-14. Growth is a characteristic of all life. 

Fig. 2-15. All living things reproduce and the offspring 
resemble the parents in most respects. 



may carry on all of the processes that are 
essential for life they must have a constant 
source of energy (Fig. 2-13). They must 
also have building materials with which to 
construct more of their own substance if 
they are to grow or to replace parts that 
are worn out. The source of this energy 
and this building material in animals comes 
from the breaking down of large molecules 
into smaller ones and the delivery of these 
substances in utilizable form to all of the 
cells of the animal body. Animals must have 
food which usually exists as complex, in- 
soluble molecules. These are broken dowii 
in the digestive tract and delivered to the 
cells where they can be used as needed. 
This is nutrition, which is confined to the 
animate world only; it is closely linked with 

4. Metabolism. Once the utilizable build- 
ing materials and the energy-giving mole- 
cules are delivered to the cells, they un- 
dergo further changes depending on how 
they are used. If they are further degraded 
with large quantities of energy released, the 
process is referred to as catabolism, or de- 
structive metabolism; if they are used in 
building new or repairing old parts, the 
process is called anabolism, or constructive 
metabolism. This constant building up and 
tearing down, storing and consuming, is 
referred to as metabolism, the crux of life 

During the dynamic process of metabo- 
lism oxygen is constantly utilized to release 
energy, with the resulting production of 
carbon dioxide; this is called respiration. 
The removal of the waste products of me- 
tabolism, such as carbon dioxide and nitrog- 
enous wastes, is called excretion. These 
two processes are intimately linked with 
metabolism. All living things are charac- 
terized by metabolism, a process which is 
without duplication in the inanimate world. 

5. Growth. When the constructive forces 
exceed the destructive forces in an organ- 
ism, it increases in size, or, in other words, 
it grows (Fig. 2-14). This is typical of aU 

organisms, particularly during their early 
life. However, a stage is reached where 
there is a balance between these two forces, 
and this is referred to as maturity; at 
the time of maturity the organism merely 
holds its own, becoming neither larger nor 
smaller. As life continues, the anabolic 
processes fail to keep pace with the catabo- 
lic processes, and the whole bulk of the or- 
ganism loses ground and finally dies. This 
involves the processes of aging, the nature 
of which is understood only vaguely. 

Growth of living things is quite different 
from that of inanimate bodies. A crystal 
may grow in size by the addition of other 
similar crystals to its own bulk, but the 
pattern and the method of executing it are 
quite unlike that of a living cell. The crys- 
tal merely adds other crystals to its own 
external mass (accretion), like a mason 
adds bricks to a wall, whereas the cell takes 
its building materials within, and there 
makes them an integral part of its own 
structure ( intussusception ) . 

6. Reproduction. As a result of growth 
and increase in size, the or2;anism is able 
to duplicate itself (Fig. 2-15). It may do 
this by simple fission, that is, by dividing 
into two equal parts, or it may produce 
special reproductive cells, eggs and sperms, 
which subsequently unite to grow into a 
new organism similar in most respects to 
the parents. Duplication by the first process 
is called asexual reproduction, and by the 
latter, sexual reproduction. Reproduction 
by either of these methods is not shared 
by the inanimate world. 

One of the remarkable things that stems 
from reproduction is the continuity of pat- 
tern from generation to generation. Off- 
spring are endowed with structural and 
physiological characteristics that are exact 
duplicates of those found in the parents. 
The pup, colt, or child is expected to 
possess bodily form, and even personality 
traits, similar to those of its parents. This 
knowledge is so commonplace that it was 
taken for granted for centuries. The trans- 



mission of these characteristics from par- 
ents to offspring is known as heredity; just 
how this is done has stimulated biologists 
in recent years to search for the explana- 
tion at the level of molecules. The search 
has been fruitful, as we shall learn a little 


If the characteristics of living things are 
to be understood, it is necessaiy to study 
the physical material in which life resides. 
This material was observed by early biolo- 
gists and was given the name protoplasm 
by Purkinje over 100 years ago. The word 
means the first form (protos — first, plasma 
— form). If tiny pieces of any plant or 
animal are examined with a microscope, 
they will be found to be composed of cells, 
each of which contains protoplasm. There- 
fore, a study of this material is essential if 
much useful information is to be had about 
what life is and how it tvorks. We do not 
have the answer to the first of these ques- 
. tions at present, but some progress is being 
made on the second. If and when the an- 
swers do come, they will probably come 
from a study of protoplasm itself. 

Recalling our earlier discussion of the 
origin of life, it must be concluded that 
protoplasm is very complex. It is so deli- 
cately adjusted physico-chemically that any 
attempt to find out how it behaves or of what 
it is made means immediate loss of the very 
thing sought for. It can be handled only 
with utmost care without killing it. A dead 
animal no longer possesses protoplasm, be- 
cause by definition protoplasm encompasses 
that "something" called life. Biologists are 
not interested in "something," rather they 
are concerned with the underlying princi- 
ples that explain life. What protoplasm is 
composed of, how these materials are or- 
ganized in the cell and the body, and how 
they interact with one another to generate 
that which is referred to as life, are among 
the most fundamental questions which the 

biologist is seeking to answer. While the 
problem at the outset appears to be insur- 
mountable by its very nature, some progress 
has been made by employing the methods 
of the chemist and physicist. Today a great 
deal is known about protoplasm, but there 
is much more to be learned before a com- 
plete understanding of life can be had. 


The superficial examination of any living 
thing reveals a more or less homogeneous 
surface to the naked eye. If, however, small 
thin pieces are removed and placed under 
the microscope, an entirely different pic- 
ture is seen. The over-all pattern of tiny 
repeated units, cells, is the most striking 
impression that one gets. Now, if one of 
these cells is observed with the highest 
power of the microscope, some of the visi- 
ble features of protoplasm can be detected. 
It would not matter what cells we used for 
this study; they would all include proto- 
plasm, which would appear remarkably 
alike in all of them. This fact was learned 
by the early biologists, which convinced 
them that protoplasm formed the "physical 
basis of life," a statement with which we 
fully agree today. One never finds life re- 
siding anywhere but in protoplasm, with 
the possible exception of the controversial 

Since all cells are very similar in respect 
to their protoplasmic content, let us select 
a large animal cell for study. Amoeba, a sin- 
gle-celled animal, will do very well because 
it is huge as cells go and its parts can be seen 
very easily under an ordinary light micro- 
scope (Fig. 2-16). The general impression 
that one gets from observing this tiny ani- 
mal is that its protoplasm is grayish in color 
and is usually moving about within the 
limits of the organism. The cell crawls 
about by sending out projections, called 
pseudopods (false feet). Such a pseudopod 
forms by forcing out a clear watery lobe 
which is immediately followed by granular 




Fig. 2-16. A living amoeba, when viewed under the 
light microscope, gives us some clue as to the nature 
of protoplasm. Note the clear protoplasm that con- 
stitutes the pseudopods and the more or less opaque 
region that makes up the bulk of the cell. Tiny par- 
ticulate matter floats about in the body of the cell, 
conveying to it a gray color. 

material from within the cell. The most 
striking fact that one observes is that the 
entire mass of protoplasm is moving in 
what appears to be a more or less hap- 
hazard manner, even though the general 
flow is in the direction the organism is go- 
ing. Another important observation is that 
there appears to be many different kinds of 
particles suspended in this semi-fluid, semi- 
solid material. Some of the particles seem 
quite uniform in shape, whereas others are 
variable. Among them is the large flattened 
nucleus, several food vacuoles, and a clear 
pulsating vacuole, none of which will con- 
cern us now since they will be discussed 
later when we examine this little animal 
more thoroughly. Our concern now is with 
the material in which all of these are sus- 
pended, that is, the protoplasm. 

Our observations so far tell us very little 
about the nature of protoplasm, but we can 
try a few experiments on it. For example, 

we can drop a little alcohol or mercuric 
chloride on it to see what happens. All 
activity suddenly stops, and the entire cell 
becomes rigid, much like the white of an 
egg when cooked. It coagulates and be- 
comes slightly opaque. Nothing we can do 
will revive it, it has died, and in so far as 
we know now this is an irreversible reac- 
tion. It could then be given to the chemist 
who is able to give us a list of the elements 
and compounds of which it is composed. 
After this and many other experiments, we 
would still know very little about how the 
amoeba moved, how it reproduced, or how 
it carried on its metabolism. We need still 
more refined techniques if more is to be 
learned. Lacking these, we can only specu- 
late from this point on, in order to obtain 
a little better picture of what happens in 
this beautifully complex protoplasm. 

Let us suppose that we could magnify 
the protoplasm of an amoeba until we could 
observe its molecular structure. This is be- 
yond the power of the electron microscope, 
which can magnify 100,000 times. The mi- 
croscope we have in mind would also need 
to be designed to view living material, 
something the electron microscope cannot 
do. The most obvious characteristic, as we 
peer at this blob of protoplasm, would be 
the violent activity of molecules of all 
shapes and sizes. The most numerous would 
be the water molecules, which, because of 
their small size, would move faster than 
most molecules. Huge, slow moving mole- 
cules would be bound together, forming 
a continuous network of material that re- 
mained in one place most of the time, as 
an outer boundary of the cell marking it 
off from the outside world. This would be 
the plasma membrane, through which the 
water m.olecules pass freely in both direc- 
tions. Many other molecules pass through 
also, but some are stopped because of their 
large size, whereas others are stopped be- 
cause they possess electrical properties that 
prevent them from getting past the electri- 
cal barrier on the membrane. Oxygen mole- 



cules enter freely, uniting with larger mole- 
cules which suddenly break up into a great 
many smaller molecules, some of which 
immediately leave the scene through the 
plasma membrane. This is the union of 
oxygen and glucose, producing carbon di- 
oxide and water. There would be many 
other kinds of molecules, small and large, 
apparently floating about but in reality per- 
forming very specific functions which we 
do not understand at the present time. 

This mass of molecules in endless motion, 
constantly changing, adding new mole- 
cules, losing others, combining and separat- 
ing, is protoplasm. This dynamic activity 
constitutes the thing we call life. The mole- 
cules are no different from those in the in- 
animate world; they react in the same way. 
It is only when they come together in the 
combination to form protoplasm that they 
exhibit the characteristics we associate with 

Although much of the foregoing discus- 
sion is based on speculation, there is con- 
siderable evidence to demonstrate that per- 
haps the speculations are not too far from 
the truth. What can we learn about proto- 
plasm with ordinary laboratory equipment? 
Let us go a little further into its chemical 
and physical nature. 


If a mouse, butterfly, elephant, or a plant 
were analyzed in the chemist's crucible 
and the elements named, they would be 
remarkably similar in all of these widely 
different forms of life. The four principal 
elements are carbon (C), hydrogen (H), 
oxygen (O), and nitrogen (N). Carbon is 
the core element of the complex molecules 
found in protoplasm, probably because of 
its physical nature which was discussed 
earlier (p. 29). Additional elements regu- 
larly present in protoplasm are phosphorus 
(P), calcium (Ca), iron (Fe), potassium 
(K), sulfur (S), iodine (I), magnesium 

(Mg), sodium (Na), and chlorine (CI). 
There are many others existing only as 
traces but nevertheless essential for the nor- 
mal activities of protoplasm. 

Elements in protoplasm usually are found 
combined in the form of inorganic and or- 
ganic compounds. The former go to make 
up most of our world we live on, whereas 
the latter are always derived from living 
things. The inorganic compounds that are 
important in protoplasm are water, inor- 
ganic salts, and certain dissolved gases, 
such as oxygen, nitrogen, and carbon di- 
oxide. Important organic compounds are 
lipids (fats and related substances), car- 
bohydrates, and proteins. These we must 
consider in more detail. 

Inorganic compounds in protoplasm 

Water. Of all the inorganic compounds 
in protoplasm the most important is water, 
the substance in which all other materials 
are suspended and transported. Life with- 
out water would be impossible, because 
there would be no means of mixing and dis- 
persing the energy-yielding and building 
materials of protoplasm. Water is important 
in protoplasm because of its many unique 

Water dissolves more substances than 
most any other liquid. This property of 
water makes possible the mixing of a large 
variety of substances that would not dis- 
solve in any other liquid. Hence, the great 
complexity of protoplasm has come about 
because so many different substances could 
come together and mix freely in one com- 
mon medium. Furthermore, interactions oc- 
cur more readily between substances in a 
fluid condition where ample freedom of 
movement of the molecules is permitted. 
In a dry state, where substances are less 
free to mix with one another, chemical 
union would be greatly impeded. 

Water has a high capacity for holding 
heat. It is reluctant to take on or lose heat, 
a physical property that is very important 
in living things. Anyone living near large 



bodies of water is aware of their tempering 
effect on the chmate of the surrounding 
areas. The winters are not so cold and the 
summers not so warm as they are farther 
inland. The water warms up slowly and, 
likewise, cools slowly, conveying this dif- 
ference to the adjoining land. Since our 
bodies are primarily water (approximately 
65 per cent), we respond to heat much the 
same as water does. If this were not the 
case, it would be almost impossible for us 
to remain in the sun any length of time 
without becoming overheated; in fact, any 
place where the temperature differed 
greatly from that of our bodies could not 
be tolerated. The ability of the body to 
maintain an even temperatiire is a result 
of this property of water. 

Water possesses some interesting chemi- 
cal properties that make it an ideal medium 
in which life can be supported. Water has 
more ability than any other substance to 
dissociate molecules into their ions (see p. 
28). Salts such as sodium chloride (NaCl) 
ionize readily in water; others such as 
sugars and starches do not ionize at all. 
Water itself ionizes slightly forming hy- 
drogen (H+) and hydroxyl (OH") ions. 
However, the number of water molecules 
that form these ions is so small (1:555,000,- 
000) that water is a rather poor conductor 
of electricity. Both H and OH ions are 
extremely active and for this reason they 
seem to be tolerated only in very small 
numbers by protoplasm, that is, any in- 
crease of either over the other brings about 
prompt changes in the activity of proto- 
plasm, and any marked increase or decrease 
terminates life. By employing delicate in- 
struments capable of detecting the slightest 
increase or decrease of either one of these 
ions, it has been shown that protoplasm 
is approximately neutral all of the time, the 
numbers of each of these two ions being 
approximately the same. 

Salts. Many inorganic salts of sodium, po- 
tassium, calcium, and magnesium exist in 
protoplasm. Inorganic acids are also found 
in small concentrations. The role played by 

these various compounds will come out 
as we progress with this study. 

Gases. Because of their diffusing quali- 
ties, gases tend to enter protoplasm the 
same as they enter other material. There- 
fore, gases such as oxygen, nitrogen, and 
carbon dioxide found in the atmosphere are 
also present in protoplasm. Since the atmos- 
phere contains over 20 per cent of oxygen, 
considerable quantities of this gas are found 
in protoplasm. Oxygen, of course, is very 
important in the release of energy. Nitro- 
gen, on the other hand, is even more abun- 
dant, but because of its chemical inertness 
it takes part in no reactions that are impor- 
tant in metabolism. Carbon dioxide exists 
in small amounts both in the atmosphere 
and in protoplasm. It tends to accumulate 
in protoplasm because it is one of the end 
products of the oxidation of foods, but it 
is a waste product and is soon removed 
from the cell body. 

Organic constituents of protoplasm 

By definition organic compounds contain 
carbon. We have already spoken of the 
physical properties of carbon that make it 
an ideal element around which so many 
compounds can be built. The most signifi- 
cant organic compounds in protoplasm are 
carholn/drates, lipids, and proteins. 

Carbohydrates. Carbohydrates, such as 
sugars and starches, are commonly defined 
as compounds composed of carbon, hydro- 
gen, and oxygen in which the atoms are 
arranged in the ratio of 1C:2H:10. Not all 
carbohydrates fit this description exactly, 
but most of them do. Two examples of com- 
mon carbohydrates will illustrate the way 
the molecules are constructed. Glucose, the 
sugar in blood, has the formula CoHioOfj, 
and sucrose, common table sugar, has the 
formula C12H22O11. Note that both contain 
carbon, hydrogen, and oxygen, and that the 
C atoms numlDcr six or a multiple thereof, 
while the ratio of H to O is 2 to 1, just 
as it is in water. Also note that sucrose is 
simply two molecules of glucose, less one 
molecule of HoO. This is explained below. 



In the leaf of the plant, CO2 and HoO 
unite under the influence of the sun's rays 
and in the presence of a green plant pig- 
ment called chlorophyll, to form glucose in 
this manner: 

6 CO2 + 6 H2O -^ CeHisOe + 6 O2 
This chemical reaction is known as photo- 
synthesis. It is the starting point in the for- 
mation of all food substances. Glucose is 
known as a monosaccharide. By the simple 
subtraction of water (dehydration) from 
two glucose molecules a disaccharide, su- 
crose, is formed, thusly: 

2 (CeH.oOe) - H2O 

sucrose (cane sugar) 

If this is continued, large molecules can 
be built up. Such compounds are called 
polysaccharides, and are illustrated by 
starch in plants and glycogen in animals. 
Starch and glycogen do not dissolve readily 
in water and consequently are ideal storage 

The reverse process is simple also, for by 
merely adding HoO (hydrolysis), the large 
molecule falls apart into many molecules 
of glucose, the number depending on the 
original carbohydrate molecule. This is 
what happens during digestion in the ani- 
mal body, a process we are to discuss in 
some detail later. 

Glucose is the most important carbohy- 
drate in protoplasm because it is the sub- 
stance that unites with oxygen to release 
energy that is essential in the business of 

H H H H H H H 

living. We think of carbohydrates as energy 

Lipids. These include all of the fats, oils, 
and fat-like substances identified by a 
greasy texture. They are relatively insoluble 
in water, but soluble in hot alcohol, ether, 
and chloroform. With a few exceptions they 
all contain C, H, and O like carbohydrates, 
the chief difference being their low oxygen 
content. This is easilv illustrated bv exam- 
ining the formula of a common fat taken 
from beef tallow, C57H110O0. Note that 
there are only 6 oxygen atoms as compared 
to 57 carbon and 110 hydrogen atoms. This 
means that when fats burn, they require 
more oxygen than does glucose. It also 
means that the molecule can release more 
energy per gram when burned. Their in- 
solubility makes fats desirable material for 
the storing of energy. 

Like the starch molecule, a fat molecule 
is composed of simpler components that 
can be separated by hydrolysis. These are 
glycerol (CsHsOs) and fatty acids. A com- 
mon fatty acid is stearic acid with the 
formula (CH3(CHo)io • COOH). This is 
called an acid because the carboxyl group 
(GOOH) can liberate one hydrogen ion 
when dissolved in water. It can be written 
as in Equation (A) below. 

Going back to the plant leaf again, fatty 
acids are joined with glycerol to form fats 
of different kinds. This is done by the loss 
of water, as shown in Equation ( B ) below. 



HO— C— C— C— C— C— C— C— C— C— C— C— C— C— C— C— C— C— C— H 

stearic acid (from Vjeef fat) 

H H 

H— C— OH ^ 

' H— OOC • C17H.35 

H— C— COO- CnH3.0 


H— C— OH 

^ + 3 < 

H— OOC • CtHss 

^ H— C— COO • CnH35 

> -f 3 H2O 

H— C— OH , 

-H—OOC • CnHss 

H— C— COO- CnHss. 



1 molecule 

3 molecules 
stearic acid 

1 molecule 


3 molecules 



As in the case of starch, the reverse ac- 
tion, hydrolysis, takes place during diges- 
tion when water is added, splitting the fat 
into fatty acids and glycerol. 

There are other lipids called phospho- 
lipids and steroids, which have elements 
such as phosphorus in combination with 
one of the fatty acids. They resemble fats 
in many ways, but have other characteris- 
tics that tend to set them off in a group by 
themselves. Some of them play their role 
in the plasma membrane where they are 
responsible for the selective action of this 
delicate structure. They are also impor- 
tant in some of the intricate chemistry of 
the animal body which we shall touch on 

Proteins. In addition to C, H, and O, 
proteins contain the elements nitrogen ( N ) 
and sulfur (S); usually phosphorus (P) is 
also included. These are all combined into 
huge molecules, some composed of thou- 
sands of atoms. Like starch and fats, pro- 
tein molecules can be hydrolized to simpler 
components, called, in this case, amino 
acids. There may be hundreds of amino 
acid molecules in a single protein molecule 
but, when broken down, it yields only a 
few different kinds of amino acids. There 
are about 25 amino acids known, all of 
which are not usually found in any one kind 
of protein. The proportion of the different 
amino acids will depend on the nature of 
the original protein molecules. 

The atoms of the amino acids are ar- 
ranged in a definite manner so as to pro- 
duce two distinctly specific groups by 
which they can always be identified. They 
have the general formula 

H— C— NH2 


where R represents the main portion of the 
molecule. The remainder is found in vir- 
tually all amino acids. It will be noted that 

there is one group containing NHo, which is 
spoken of as the amino group; the other 
contains COOH, the carboxyl group, with 
which we are already familiar (p. 41). 
These two groups are responsible for the 
behavior of amino acids. It is obvious that 
the presence of the carboxyl group gives 
the molecule acidic properties, just as is 
true of any organic acid. Strangely enough, 
an amino acid can also act like a base due 
to the presence of the NH2 group. It re- 
sponds like a base by removing hydrogen 
ions, not by delivering hydroxyl ions. This 
is demonstrated by the following equa- 

R • NH, -f H+ + CI- ^ R • NH3 + Cl- 
in the presence of an acid, then, an amino 
acid acts like a base by absorbing or remov- 
ing the hydrogen ions. On the other hand, 
in the presence of a base it acts like an acid 
by delivering hydrogen ions from its car- 
boxyl groups, thus: 

R • COO- + H+ + Na+ -f OH" 
-» R- COO--H Na+ + HoO 

A substance that responds in this fashion is 
said to possess amphoteric properties. It 
therefore always has a tendency to bring 
a solution to neutrality, that is, to balance 
the number of hydrogen and hydroxyl ions. 
For this reason, amino acids tend to prevent 
too much fluctuation in the number of 
hydrogen and hydroxyl ions in a solution. 
Such a substance is spoken of as a buffer; 
amino acids are good buffers. 

Amino acids unite to form proteins in 
the plant leaf by the same process that 
starch and fats were produced, namely, by 
the loss of water. Likewise, in every cell 
of the animal body proteins must be assem- 
bled from the amino acids that come to it 
through the blood stream. Just how this is 
done has puzzled chemists for a long time, 
but it is thought to occur through the so- 
called peptide linkage. The following equa- 
tion will illustrate how this occurs: 

H H O H H 

\ I I! \ I II 

N— C— C N— C— C 










N— C— C 






N— C— C 




+ H,0 



In this manner the various amino acids 
are tied together to form molecules that 
range in size from several hundred to 
several thousand atoms. Their size can 
be estimated by the relative molecular 
weights. For example, glycine has a molec- 
ular weight of 75, whereas that of the 
oxygen-carrying protein of the crayfish's 
blood (hemocyanin) is over 5 million. Ob- 
viously, by combining the 25 amino acids 
in various ways, limitless numbers of dif- 
ferent proteins can be formed. That is why 
the proteins of every animal or plant difFer 
from those of every other animal or plant. 
It is a well-known fact that proteins of one 
species of animal cannot be exchanged with 
those of another. For example, it is impos- 
sible to transplant skin, let us say, from the 
back of a dog to the face of a man because 
the proteins that go to make up the skin 
are different in both man and dog. Think 
what startling surgery could be performed 
if this were possible! 

Proteins are the most characteristic and 
most abundant material (exclusive of wa- 
ter) in protoplasm. Besides forming the 
actual supporting structure of protoplasm, 
proteins afford excellent material in which 
the large variety of chemical reactions es- 
sential for life take place. 

Enzymes. Enzymes have been mentioned 
from time to time in the foregoing pages 
without an explanation of what they are; 
that must be clarified now. Enzymes are 
frequently spoken of as organic catalysts, 
which may be defined as substances that 

hasten a chemical reaction but are not 
themselves consumed by the reaction. The 
numerous enzymes present in all proto- 
plasm are responsible for the complex re- 
actions that go on so smoothly within every 
cell. Without enzymes protoplasm loses its 
ability to start and maintain the multitude 
of activities that go on within it. They are 
absolutely essential in the business of liv- 
ing but the details of their operation are 
only now beginning to be understood. 

A simple experiment will demonstrate 
how an enzyme works. A watery starch so- 
lution is placed in two test tubes, to one of 
which is added saliva, to the other, nothing. 
Any attack on the starch can be detected 
by using an appropriate test for maltose, 
one of the products of polysaccharide 
breakdown. Within a few minutes this 
sugar can be demonstrated in the tube con- 
taining saliva, whereas in the tube without 
it there will be no maltose for many hours. 
The enzyme ptyalin brings about this reac- 
tion without fuss or furor. The same break- 
down can be accomplished without the 
enzyme, but it requires drastic treatment 
with strong acids at high temperatures, 
conditions that could not be tolerated by 
protoplasm. It is obvious, then, that en- 
zymes can bring about a difficult chemical 
change at body temperatures and in a very 
short period of time. All of this is essential 
if the many reactions that go on in proto- 
plasm are to occur as quickly and smoothly 
as they do. 

In order that some appreciation may be 



had of the number and variety of enzymes 
present in protoplasm, consider for a mo- 
ment the burning of gkicose to CO2 and 
H2O. This can be done in a test tube, pro- 
viding external heat is applied, much more 
than could be tolerated in an animal body. 
However, this reaction proceeds speedily 
and without any marked temperature ele- 
vation in protoplasm. The impressive thing 
about this apparently simple process is that 
there are at least 25 steps involved in this 
oxidation, and probably a different enzyme 
is participating in each step. The glucose is 
broken down step by step, each enzyme 
contributing its part in the proper order. 
Furthermore, each enzyme involved in 
every step is specific for that step. It does 
that one job alone, and no other. Their 
specific action reminds one of union work- 
ers, each doing his own job and no other. 
Think of the myriad enzymes that must be 
present in protoplasm to make life possible. 

The analysis of over 30 enzymes up to 
the present indicates they are protein in 
nature. The first one, urease, was crystal- 
lized in 1926; many others have been pro- 
duced in pure form since. In general, an 
enzyme can accelerate a reaction in either 
direction, that is, its action is reversible. 
Experimentally, however, it is much easier 
to demonstrate the activity of enzymes that 
bring about exothermic reactions which lib- 
erate energy, than endothermic reactions 
which store energy. Theoretically, enzymes 
that break down a glucose molecule should 
be able to reconstruct the molecule, pro- 
viding energy is supplied from the outside. 
This may be true, but it is difficult to prove 
because an exothermic reaction is required 
to supply the energy needed for the recon- 
struction at the same time as the endother- 
mic reaction is going on. It is impossible 
to observe them simultaneously. These 
coupled reactions must always proceed syn- 
chronously and since they are locked up 
within the cell, they are extremely difficult 
to observe. 

The activity of enzymes is controlled by 

the movement of molecules just as all chem- 
ical reactions are. As the temperature rises 
from to 40 degrees Centigrade, enzyme 
action increases; with each 10 degree rise 
in temperature the action doubles, up to 
a critical temperature of about 40 degrees 
where all activity ceases. It is interesting 
to observe that cold-blooded animals 
(called poikilothermal, that is, those that 
cannot maintain a constant body tempera- 
ture, such as insects, frogs, and lizards ) are 
forced into hibernation because at reduced 
body temperatures enzymatic activity is 
slowed to a point where normal response 
is impossible. Warm-blooded animals 
(called monothermal, that is, birds and 
mammals ) are not bothered in this respect 
because their bodies maintain a constant 
temperature. It is not sheer coincidence 
that the body temperature of these animals 
happens to be the point of optimal activity 
of the body enzymes. 

Enzymes are particularly sensitive to hy- 
drogen and hydroxyl ions, as well as certain 
other specific ions such as calcium. Diges- 
tive enzymes do their best work in solutions 
with the proper number of hydrogen and 
hydroxyl ions. Ptyalin in the mouth acts 
best at or near neutrality, whereas pepsin 
in the stomach requires a strong acid solu- 
tion for optimal activity. Trypsin in the 
small intestine needs a slightly alkaline me- 
dium to do its best work. 

Enzvme chemistiy is an active field of 
research today and it is hoped that much 
more will be learned in the next few years 
about hoiv enzymes work. 

Coenzymes. Intimately linked with intra- 
cellular enzymes are certain simpler organic 
compounds which are essential in certain 
vital metabolic processes in protoplasm. 
These non-protein molecules are called co- 
enzymes. They are so associated with cer- 
tain enzymes that neither is effective without 
the other. Vitamin Bi is a coeiirzyme which 
is essential for the operation of several oxi- 
dizing enzymes in both plants and animals. 
Strangely enough, animals are unable to 



synthesize many of these coenzymes so they 
must receive them through their food sup- 
ply. The ultimate source of all of them 
apparently is plants. 


We have examined the chemical compo- 
sition of protoplasm and have found it com- 
posed of particulate matter in a vast array 
of sizes. These particles obey the same 
physical laws whether in or out of proto- 
plasm. They behave in particular ways 
when isolated from others of the same kind, 
or when in close association with those of 
similar structure. If each particle follows 
specific laws of behavior when among its 
own species it will behave differently when 
mixed with others of a different sort. Since 
protoplasm is made up of many kinds of 
molecules, it follows that the operating 
forces become extremely complex. In spite 
of this almost hopeless confusion each par- 
ticle seems to take its part in a definite pat- 
tern so that an orderly procession of reac- 
tions occurs. Let us examine some of these 
physical properties of protoplasm. 

Size of protoplasmic particles 

It was implied in an earlier chapter that 
the size of particles had a profound effect 
on their behavior. We should, therefore, 
have some appreciation of the relative mag- 
nitude of the innumerable particles of mat- 
ter existing in protoplasm. 

In order to speak with any degree of 
accuracy about the size of these tiny parti- 
cles it is necessary to apply some unit of 
measurement to them. Scientists throughout 
the world employ the metric system of 
measurement almost exclusively. Fractions 
of the meter, microns,* are used by the 
microscopist because these units are con- 
venient for measuring objects that fall 
within the range of the microscope. For 
example, red blood cells in man are about 









10,000 X 



prof ail 

100,0 oox 


. amino 'OcidS,ooox 

Fig. 2-17. Relative sizes of things that are of interest 
to the biologist. Magnifications greater than 100,000 
are useful to the physicist and chemist. 

* 1 meter = 1000 millimeters; 1 millimeter = 1000 microns; 1 micron = 1000 millimicrons. 



7 microns in diameter. Smaller objects, 
such as the particulate matter in protoplasm, 
are measured in millimicrons. It is possible 
to measure particulate matter . with the 
same degree of accuracy that can be at- 
tained in the macroscopic world. 

To demonstrate the relative sizes of ob- 
jects we may use a cell as the starting point. 
A large cell of your body, a cell lining 
your mouth, for example, would occupy a 
space about the size of a needle point 
(Fig. 2-17). If this were magnified ten 
times, you would see it quite easily with 
the naked eye but the parts would not be 
very well defined. Magnifying it another 
ten times (lOOX) brings the nucleus and 
cytoplasm into full view; even the nuclear 
structure can be made out. Another tenfold 
increase (lOOOX) shows the chromosomes 
in outline, but not in detail. A magnifica- 
tion of 100 to 3000 is the range in which the 
microscopist works with a light microscope. 
Any further increase in size must be viewed 
through the electron microscope, which 
operates much the same as the light micro- 
scope except electrons are used instead of 
light waves for a source of illumination. Of 
course, these cannot be seen directly with 
the eye because our eyes are sensitive only 
to light rays and not to electrons, but such 
objects can be photographed (Fig. 2-9). 
Furthermore, the treatment of the material 
is so drastic that living things cannot be 
studied under an electron microscope. In 
order to study any material it must be 
sliced into extremely thin sections (less 
than 1 micron ) . With this instrument, mag- 
nifications can go up to 100,000 diameters, 
which will reveal viruses and the larger 
molecules such as nucleoprotein molecules. 
The electron microscope allows us to see 
things in the molecular state. Beyond this, 
we must rely on methods familiar to the 
physicist to demonstrate the size and shape 
of particulate matter, methods that are be- 
yond the scope of this book. Physicists are 
able to measure the size, shape, and be- 
havior of molecules and atoms, and are now 

working on the nature of the components 
of the atoms themselves. For the present 
discussion it is only necessary for us to think 
in terms of the size of particles at the molec- 
ular level, because protoplasm is molecu- 

Colloids and crystalloids 

If a solid is ground to particles the size 
of dust, and these placed in water, they 
will form a murky fluid and after a time 
will settle to the bottom of the container. If 
the particles are ground still finer they will 
reach a size when they remain in suspen- 
sion and do not settle out even after a long 
time. These particles are then in the col- 
loidal state. Therefore, whether or not a 
substance exists as a colloid is merely a 
matter of size. Physicists have set an arbi- 
trary figure for colloids; they state that par- 
ticles ranging in size from 0.1 to 0.001 
micron are in the colloidal state. A colloidal 
system can often be observed with the 
naked eye. For example, if egg albumin, 
which is composed of large protein mole- 
cules, is placed in water the solution has 
an opalescent appearance. Light rays will 
strike the suspended particles and be scat- 
tered, rather than pass directly through as 
would be the case if the particles were 
smaller. We do not see the individual parti- 
cles, only the effect produced by scattered 
light. However, if the particles are larger 
than 0.1 micron in diameter, the light will 
be blocked altogether and the system will 
appear opaque, as it does in milk, for ex- 
ample. The larger particulate matter in milk 
will, of course, separate out (cream on the 
surface ) and is therefore not colloidal. 

The large size of colloidal particles also 
prevents them from passing through an ani- 
mal membrane. If egg albumin is placed 
in a loop of frog skin and submerged in 
water, very little, if any, of the albumin 
will be found in the water even after hours 
have elapsed. Furthermore, colloidal parti- 
cles move slowly when compared to smaller 
particles such as atoms or ions. This might 


be expected from our knowledge of the in concentration of bombardment that 

movement of objects that come within our causes the particle to move in the random 

experience. Physicists tell us that the move- fashion that is observed. This type of ac- 

ment of particles is dependent on the ab- tivity is essential in keeping the particulate 

sorption of heat; the higher the tempera- matter dispersed. 

ture, the faster the movement and the lower Other factors complicate the behavior of 

the temperature, the slower the movement, particles in solution. We learned earlier that 

Movement is governed by the size of the atoms and molecules may become ionized, 

particle; when the diameter of the particles that is, they may carry an electrical charge, 

is halved, the rate of movement is doubled, positive when electrons are short, and nega- 

Therefore, the huge lumbering molecules tive when electrons are in excess. We also 

in a colloidal system move slowly compared know that particles of the same charge 

to the tiny molecules of a salt solution. repel one another while those of unlike 

Particles that are smaller than 0.001 mi- charges attract. This fact has a profound 

cron in diameter are called crystalloids; effect on the behavior of particles in solu- 

sugar or table salt dissolved in water forms tion. 

a crystalloid solution. Such systems appear In a solution the dispersion medium is 

clear and transparent to the naked eye be- called the solvent, whereas the dispersed 

cause light passes directly through without particles are the solute. Because of the elec- 

being changed in any way by the tiny trical charges on the various particles of 

molecules of salt and sugar. Furthermore, the solute, some are attracted to one an- 

crystalloids pass readily through some mem- other, while others are kept apart. Fur- 

branes such as frog skin, and their individ- thermore, some of the molecules of the sol- 

ual particles move much faster than those vent are attracted to those of the solute, thus 

in the colloidal state. increasing their bulk. The strange thing 

Protoplasm contains numerous crystal- about the charge on a particle is that it may 
loidal and colloidal particles in the form of be stronger at one side or end than at the 
atoms, molecules, and molecular aggre- other; in other words, it can exhibit electri- 
gates. The particles remain evenly dis- cal polarity just as a magnet does. Because 
persed and do not respond to the pull of such substances as water, salts, and pro- 
gravity because of their continuous move- teins exhibit polarity they are called polar 
ment. Each particle is being bombarded by compounds. Fats and starches do not pos- 
others of its own kind as well as by those sess these properties, so are called non-polar 
of a different sort. This can be verified by compounds. This property of particles has 
observing even larger particles, such as cer- considerable bearing on their behavior in 
tain pollen grains, under the highest powers protoplasm. 

of a light microscope. They will be seen to Wlien the dispersed particles are molec- 

jostle about in a random manner, seeming ular aggregates of solid material they are 

to get nowhere. The apparent aimless mo- spoken of as suspensions; if the aggregates 

tion has been given the name Brownian are fluid they are referred to as emulsions. 

Movement. In an aqueous solution much of There may be a wide range in size of these 

the activity is due to the bombardment of particles from those that are so small as to 

water molecules which, of course, are much constitute a colloidal solution to those that 

smaller. It requires millions of hits of water are visible under the light microscope. A 

molecules to move the huge visible particles familiar emulsion is milk, in which droplets 

and these must be concentrated more on of fat are dispersed in a watery fluid of 

one side than the other if movement is to sugar, salts, and a soluble protein, 

occur in any one direction. It is the change It is essential to distinguish between the 



Fig. 2-18. Stable and unstable emulsions of water and oil (with a red dye) as seen under the microscope. A is the 

oiUin-water, C the water-in-oil, and B the unstable emulsion. 

two phases of suspensions and emulsions, 
that is, to differentiate between the dis- 
persed particles and the dispersing medium. 
The continuous phase refers to the latter, 
whereas the discontinuous phase identifies 
the former. Using milk again as an illustra- 
tion, the fat globules constitute the discon- 
tinuous phase while the fluid portion identi- 
fies the continuous phase. 

The nature of an emulsion can best be 
understood from a very simple experiment. 
If olive oil, a non-polar compound, is shaken 
up with water, a polar compound, an emul- 
sion forms in which the tiny oil droplets are 
dispersed throughout the water. If, how- 
ever, the emulsion is allowed to stand a few 
minutes, the oil will collect on the top of the 
water and there will no longer be two inter- 
mingling phases, merely two homogeneous 
fluids completely separated from one an- 
other. Such an emulsion is said to be unsta- 
ble. Now, if a small amount of soap or sol- 
uble protein is added to the two and shaken 
vigorously an emulsion will form, but this 
time it will remain for an indefinite time; 
tliis is a stable emulsion. The soap or pro- 
tein is known as a stabilizer. The reason 
why the stabilizer produces a stable emul- 
sion is that it is both a polar and a non-polar 
compound, and therefore tends to accumu- 
late at the surfaces between the oil and 
water, that is, it tries to find a place where 
the non-polar end of the molecules can rest 
in the oil (which is also non-polar), while 
the polar end can lie in the water (which 

is also polar). The stabilizer, when so ar- 
ranged, forms a thin protective film at the 
surfaces between the oil and water, pre- 
venting the oil droplets from coalescing. 
Thus they remain permanently separated. 

Phase reversal 

Watching an amoeba crawl leads one to 
believe that its protoplasm does not always 
have the same viscosity or fluidity, and care- 
ful experiments with a micro-dissection ap- 
paratus ( an instrument that makes cellular 
surgery possible) verifies this fact. The na- 
ture of the emulsion has some bearing on 
these constantly changing conditions within 
the protoplasm. 

Returning again to the oil-in-water emvil- 
sion experiment, we find that when soap is 
added the typical oil-in-water emulsion ap- 
pears ( Fig. 2-18A). If, however, a few drops 
of a calcium salt are added and the con- 
tainer shaken vigorously, an emulsion will 
again be established, but this time tiny wa- 
ter droplets will be surrounded by oil ( Fig. 
2-18C). In other words, water becomes the 
discontinuous or dispersed phase and oil 
the continuous phase. This makes a beauti- 
ful experiment, particularly if a fat-soluble 
red dye is added to the mixture. In the first 
case, red spheres appear in a clear back- 
ground of water; in the second, clear watery 
spheres shine out in a brilliant red back- 
ground. Just what has gone on to bring 
about these striking changes? 

Obviously, the stabilizer is responsible 




Fig. 2-19. Stable and unstable emulsions. >) is a stable emulsion of oil-in-water where the polar and non-polar 
ends of the stabilizer (sodium soap) are "satisfied"; C, likewise is a stable emulsion of water-in-oil where both 
the polar and non-polar ends of the stabilizer (calcium soap) are "satisfied." B is an unstable emulsion where 
the stabilizer (a mixture of calcium and sodium soaps) forms a film separating the water from oil. 

for the change. In the oil-in-water emulsion 
the stabilizer was soap, which is produced 
by the action of a sodium or potassium salt 
on a fatty acid. The resulting soap molecule 
is polar at the end bearing the sodium or 
potassium atom, the bulkier end, and non- 
polar at the end which is composed of the 
long chain of carbon and hydrogen atoms 
(Fig. 2-19A). Such a molecule may be con- 
sidered to be conical in shape; therefore, 
when lying side by side their combined ef- 
fect would form a film that would curve 
away from the thickened ends. In a water- 
in-oil emulsion, the heavy polar ends of the 
molecules would reside in the water while 
the lighter non-polar ends would lie in the 
oil. In this condition the soap molecules 
have satisfied both their polar or non-polar 
ends, tlius producing oil droplets in water. 
Once these are formed they tend to stay 
that way; hence an emulsion remains stable 

Now what took place when the calcium 
solution was added? It so happens that the 
calcium ion requires two chains of fatty 
acids in order to satisfy its electrical needs; 
therefore, the resulting calcium soap has 
two wings to it, shaped something like a V, 
both attached to the calcium ion (Fig. 2- 

19C). Again the long CH chains are non- 
polar and the calcium end is polar. When 
these molecules lie side by side they form 
a sheet or film that is curved in the direction 
opposite to the one formed by the sodium 
soap molecules. This time when in water 
and oil, the flared ends are emersed in oil 
and the calcium end is in water, thus cap- 
turing small droplets of water in oil. The 
phases have been completely reversed, and 
the response of such an emulsion is differ- 
ent from the former case. Such reversals 
happen continuously in protoplasm and ac- 
count for some of the activities displayed 
by this material. 

One might wonder what would happen if 
there was neither a preponderance of so- 
dium nor calcium soap molecules, but an 
equal number. Under ideal conditions a 
completely flat film should form, separating 
oil from water, as if the soap were not pres- 
ent. This can happen, but the molecules 
may also form layers that curve various 
ways, producing a continuous wavy film 
separating water from oil (Fig. 2-19B). In 
this condition the emulsion does not behave 
as the oil-in-water or the water-in-oil emul- 
sions. In both of these last two cases the 
colloidal particles move rather freely among 



one another and so the entire emulsion has 
a more or less fluid consistency ( Fig. 2-20 ) . 
In the half-way or critical condition the 
soap molecules act like those in a solid 
state, so that any change in shape of the 
emulsion must involve the rending and tear- 
ing of the soap film. In other words, the en- 
tire mass holds its shape; fragile though it 
is, it acts like a jelly. In protoplasm a sim- 



Fig. 2-20. A schematic explanation of how sois and gels 
form. In a sol the elongated molecules flow smoothly 
past one another in a more or less fluid state. In a 
gel a latticework effect is produced by the gelation 
of the emulsion. Such a physical arrangement of the 
particles produces a semi-solid material. 

ilar situation exists; here, the large elongate 
protein and polysaccharide molecules tend 
to interlock, forming a cotton-like meshwork 
in which the water and crystalloidal com- 
ponents are trapped. This is called gelation, 
and the resulting emulsion is a gel. The 
emulsion can quickly change from the gel 
to the fluid or sol condition under the vary- 
ing factors of the protoplasm itself. For ex- 
ample, such factors as hydrogen ion concen- 

tration or temperature directly determine 
the condition of the protoplasmic emulsion, 
and metabolic products constantly change, 
the condition of the emulsion changing 
with them. 

Myosin, the protein of muscle tissue, be- 
haves like a gel, which is probably respon- 
sible for its ability to contract. In such a gel 
the large interlinked molecules can bring 
about contraction by folding upon them- 
selves or upon other particles. The chemical 
and physical changes that go on in this proc- 
ess have been learned only recently, and 
they mark a milestone in the study of cellu- 
lar physiology. 

The protruding pseudopod of the amoeba 
is a result of a phase reversal from gel to 
sol; the clotting of blood is the opposite 
reaction, from sol to gel. Such changes are 
going on continually in protoplasm; it is 
hoped that the above discussion may give a 
little better understanding of how it occurs. 

Limiting membranes 

Protoplasm is confined within containers 
which are composed of molecules similar to 
those found in the rest of the emulsion, al- 
though they have different properties. They 
have the composition of a gel, that is, they 
are semi-rigid in order to contain the more 
fluid material within. Furthermore, they al- 
low certain substances to pass through them 
in both directions in order that material 
essential for life can enter and leave. These 
membranes are thus selectively permeable. 
Let us see, at least in part, how this selec- 
tive permeability is accomplished. 

The large protein and lipid molecules 
float freely unless they are forced to gel by 
the relative proportions of calcium and po- 
tassium ions present within the protoplasm 
and the outside fluid world. The propor- 
tions of these ions are just right in the nor- 
mal environment of a cell because they 
bring about the formation of the gelatinous 
membrane. Any change in the concentra- 
tions of these ions, either within or without, 
drastically affects the membrane. For exam- 



Na* ^ 


* glucose 

OO fot 


*^* cm'mo 

Fig. 2-21. A diagrammatic view of a cell showing how particles may enter and leave through the semi-permeable 
membrane. Glucose, amino acids, water, and some ions pass readily through the membrane due to its apparent 
porosity; fats enter by dissolving in the membrane itself. Ions with charges opposite to that of the membrane 
cannot pass through. Large protein molecules are denied entrance because of their large size. 

pie, if an amoeba is placed in a solution 
high in potassium and low in calcium, a 
new membrane will fail to form if a rent is 
made in it, whereas in a solution with the 
proper proportions of these ions a new 
membrane forms at once. If the concentra- 
tion is high in calcium and low in potassium, 
the entire organism gels, in other words, it 
loses all of its fluidity and becomes a con- 
gealed corpse. Therefore, a careful balance 
must be maintained between these two ions 
if a normal membrane is to form around the 
protoplasmic mass. 

Such a sheet of protein and fat molecules 
must possess some unique properties if it is 
to perform the necessary functions of keep- 
ing the protoplasm from disintegrating. This 
is accomplished by the physical nature of 
the membrane itself. The lipid and protein 
molecules cling together but between them 
are small openings where they do not fit 
quite snugly, and through these water and 

other small molecules may pass freely from 
one side to the other. Fat-like particles can 
dissolve in the lipid molecules forming the 
membrane, pass inside, then slowly move 
out of the other side to the interior of the 
cell. Larger protein molecules cannot pene- 
trate the membrane because the openings 
are too small to admit them (Fig. 2-21). 
Glucose and amino acid molecules pass 
through the openings readily but some of 
the ions, even though much smaller, are un- 
able to get through because of the electrical 
charge which they bear. The membrane is 
charged either positively or negatively de- 
pending on the surrounding conditions of 
the environment. If it is negative, such ions 
as chlorine and carbonate are repelled and 
cannot get by this electrical barrier. On the 
other hand, sodium and potassium ions are 
attracted to the membrane and pass through 
readily. A constant flow of these ions both 
ways satisfies the needs of the internal pro- 



i.h^iitdNait r i V l '>' i>' f I T I I 

. * • ' '^ ^ I JHM ■ . !■ .. I I 11 ■ . III ■■■ . ■ . ■ - ^ 1 . ' . ' J. ' . ' "^ ' .f'. ' . ' . 

••'•-•-'•'•-'•-'•'•'•-''-'•'-'•-•-•'■••''•"-'•'-'•••'■•'•'■"'''*''''■*■''■'■'''''■ ■'■ 

Fig. 2-22. Diffusion. A. Particles such as colored ions from a salt diffusing in water. B. Diffusion of particles through 

the cell membrane and within the cell itself. 

toplasm. Any excess flow one way or the 
other can be fatal. 

Diffusion and osmosis 

The constant strivmg of particles of all 
dimensions to reach an equilibrium or even 
distribution in a mixture of two or more 
different kinds is known as diffusion, a proc- 
ess that is constantly at work in protoplasm. 
Once the ions and other particles pass 
through the membrane of a cell, they diffuse 
throughout tlie protoplasmic mass until they 
have become evenly distributed ( Fig. 2-22 ) . 
However, when some of the particles sur- 
rounding a cell cannot penetrate the mem- 
brane, a different situation exists. Let us 
suppose the membrane is permeable to 
water, as most membranes are, but imper- 

meable to sucrose molecules. The sucrose 
molecules will constantly strive to intermin- 
gle with the water molecules on both sides 
of the membrane, but since they cannot pass 
through there will be an uneven distribu- 
tion of the water molecules on the two 
sides. The water and sucrose molecules will 
bombard the membrane with about equal 
hits on one side; corresponding hits will be 
made by the water molecules on the other 
side, but since there are only water mole- 
cules present one will pass thi'ough for every 
molecule of sucrose on the other side which 
cannot get through. The water molecules 
will be passing toward the sugar side more 
rapidly than they pass in the other direc- 
tion, thus building up a hydrostatic pressure 
on the sugar side ( Fig. 2-23 ) . This unequal 

sami-parmaobla mambrona 7/ 

Fig. 2-23. Osmosis. A. Movement of water molecules through an artificial semi-permeable membrane. The larger 
molecules cannot pass through. B. A similar situation in which the plasma membrane of the cell is the semi- 
permeable membrane. 



movement of water molecules through a 
semi-permeable membrane is called osmo- 

Osmosis can be easily demonstrated by 
placing a strong sugar solution in a bag of 
thin skin, such as frog skin, and tying it to 
a small-bore tube (Fig. 2-24). The water 
will pass into the bag, or toward the sugar, 
thus building up a pressure within the bag 
which will be registered by the rise of fluid 
in the small tube. If properly constructed, 
such an apparatus will demonstate a rise of 

significance of osmosis can be demonstrated 
with red blood cells. These tiny disks have 
a limiting semi-permeable membrane which 
encompasses a large variety of particles that 
constitute the internal protoplasm of the 
cell. Normally when these corpuscles float 
in the fluid of the blood, water passes in 
and out of the cell with equal speed, so the 
membrane is uninfluenced by the move- 
ment. Other particles pass in and out, but in 
so doing there is always an even distribu- 
tion on both sides of the membrane, that 

• ^ 








ar solution 


Fig. 2-24. An osmometer is made from the skin of a frog's foot into which a strong sugar 
solution is poured and a small-bore tube attached. A diagrammatic view of why the 
membrane is permeable to water and not sugar is shown on the left. 

the fluid column to many feet, the height 
being determined primarily by the effective 
semi-permeability of the bag. Usually such 
a preparation is not absolutely semi-perme- 
able; some sugar goes the other way, that 
is, after a time some sugar molecules will 
have wedged their way through the mem- 
brane to mingle with the uniforai water 
molecules outside. 

Both diffusion and osmosis are important 
physical processes in the movement of par- 
ticles, not only through the membranes but 
within the cells themselves. The practical 

is, for every particle going inside another 
comes out, so that the numbers, not neces- 
sarily the kinds, are approximately the same 
all of the time. Such a surrounding fluid is 
said to be isotonic to the corpuscles ( Fig. 2- 
25). If the corpuscles are now separated 
from the fluid portion of the blood and 
placed in distilled water, a very rapid and 
sudden change occurs. The water moves 
into the cell because the dispersed particles 
are greater (less water molecules) inside 
than out ( where there are more water mole- 
cules), so the water flows in, causing the 



membrane to swell and eventually burst 
(Fig. 2-25). The water in which the cells 
were placed is said to be hypotonic to the 
blood cells. Hypotonic solutions should not 
be injected into the blood stream of an ani- 
mal because the destruction of red cells, as 
well as others, could prove fatal. 

Again, if the cells were placed in a salt 
solution in which the numbers of dispersed 
particles were much greater than on the in- 
side of the cell, the direction of flow would 

ever cells are placed in any kind of solution, 
that the medium have the same number 
of particles, or, in other words, it must have 
the same osmotic pressure as the cells them- 
selves, if severe trouble is to be averted. 

Another illustration of the effect of os- 
motic pressure might be cited because of its 
practical application. Perhaps you have 
wondered why a person, floating upon vast 
quantities of water, must die at sea if he 
has no fresh water to drink. Sea water is 

salt soluVion 

distilled wotsr 

Solt solution > 



Fig. 2-25. The effect of salt solutions of various concentrations on the red blood cell. 

be in the opposite direction, namely, out of 
the cell, causing it to shrink to only a frac- 
tion of its normal size ( Fig. 2-25 ) . The rea- 
son here is the same as in the previous case. 
Such shrunken cells are called crenated 
cells, and such a concentrated salt solution 
is said to be hypertonic to the blood cells. 
If such a hypertonic solution were injected 
into the blood stream, it might also prove 
fatal because of the wanton destruction of 
blood cells. It is important, then, that when- 

heavfly laden with salts and its osmotic pres- 
sure is considerably above that of the blood 
and tissues of man and all other land ani- 
mals. If, then, he should take sea water 
into his stomach it would extract from his 
stomach the precious water that is already 
short, eventually filhng his stomach so that 
he would be forced to throw it up. This 
would leave his body with less water than 
it had before he swallowed the sea water. 
That is why drinking sea water can be fatal. 




This discussion of the physical and chem- 
ical properties of protoplasm may seem un- 
duly long and to belong more properly in 
a book of chemistry or physics. It is none 
the less basic to an understanding of life. 
Let us summarize briefly to emphasize this 
once more. 

We have seen that protoplasm consists 
of molecules, atoms, ions, and colloids no 
different in constitution from similar enti- 
ties existing in the inanimate world. How- 
ever, their intricate and complex association 
in protoplasm bestows upon them the prop- 
erties that we assign to living things. Al- 
though some of the properties of these par- 
ticles have been described, certainly not all 
of them have been, nor could they be, 
because they are not known. 

Many people have attributed to proto- 
plasm a vague sort of "something" called 
life, that suddenly leaves a living thing 
when it dies. However, attractive though 
this may be, it is no explanation, at least in 

the scientific sense. The reason whv we have 
difficulty in explaining life is because we do 
not know enough about the intricate work- 
ings of molecules, atoms, ions, and other 
particles that go to make up protoplasm. 
If we had sufficient knowledge about their 
behavior perhaps we could explain how 
protoplasm is put together and exactly how 
it works; indeed, we could explain life it- 
self. That is a far distant goal, but one wor- 
thy of the most intensive research. 

Can we define life? Perhaps we are some- 
what closer to a definition of life than we 
were at the beginning of this discussion 
some pages ago. We can make a few state- 
ments but they need to be shrouded in vague 
terms which we still do not quite under- 
stand. Certainly life involves motion and in- 
teraction of particulate matter at size levels 
from atoms to huge colloids. It includes 
ceaseless chemical change, with its concom- 
itant consumption or release of energy. 
Life means chemical and physical organiza- 
tion of pattern and design with a never 
ending trend toward greater complexity. 



Going back to the story of the origin of 
hfe, it will be remembered that protoplasm 
came into being very slowly and its organi- 
zation into cells must likewise have required 
a very long time. All cells, from the free- 
living one-celled Protozoa to the many- 
celled animals, are extremely complex, and 
certainly the result of hundreds of millions 
of years of evolution. All cells perform es- 
sentially the same functions whether they 
are deep in the muscles of an elephant or 
exist as independent individual units like 
the amoeba. Before we examine the general 
cell structure more carefully, a little of tlie 
historical background of cell studies may 
help us in our perspective of modern sci- 

Long ago, about 1665, an English biolo- 
gist by the name of Robert Hooke cut thin 
slices of cork and placed them under his 
newly fashioned microscope. He noted that 

this material was composed of numerous 
tiny compartments to which he assigned the 
name "cells," a name that has come down to 
the present time. He gave them this appel- 
lation because they reminded him of the cu- 
bicles in monasteries in which monks of his 
time lived. Today any beginning biology 
student can repeat Hooke's experiment and 
be rewarded with a much better visual 
image of cells, although he would probably 
lack some of the enthusiasm that compelled 
this inquiring man of the seventeenth cen- 
tury to make the discovery. 

Everyone who was sufficiently curious 
to follow the exciting hobby of looking 
through the newly invented microscope of 
this early period saw that all parts taken 
from living things were composed of these 
tiny "bladders," as Grew called them. Their 
first and only interest seemed to lie in the 
fact that animals and plants were made up 


cell uuall 



nuclear membrane 

'^ centre sphere - » 






plasnoa membrane 

Golgi apparatus 

Fig. 3-1. A generalized cell. 

of these tiny units, and they paid little or 
no attention to the internal parts of the cells 
for over one hundred years. By 1824, Dutro- 
chet in France had noted that plants grew 
by an increase in number and size of the 
cells of which they were composed. By 1831, 
Brown had seen and described the cen- 
trally located body within the cell which he 
called the nucleus. This, together with the 
increased use of the microscope, centered 
the attention of investigators on the internal 
parts of the cell. Purkinje saw the universal 
occurrence of protoplasm within cells and 
gave it the name which it bears today. 
About this time (1839), two Germans, 
Schleiden (a botanist) and Schwann (a 
zoologist ) advanced the Cell Theory, which 
was merely a concise statement of what had 
been learned by a great many men up to 
this point, namely, that all living things 
were composed of cells. 

The Cell Theory gave biologists for the 
first time a tangible theory upon which to 
base further studies. It meant to them what 
the molecular theory did to the chemist and 
physicist, and as a result, biology became 

more and more a study of cells rather than 
a study of the organism as a whole. Be- 
cause cells occupy the central core of stud- 
ies in embryology, reproduction, growth, 
heredity, and behavior, they have been 
given steadily increasing attention through 
the years by investigators of fundamental 
biology. Today there is more activity in this 
field than ever before. Once a thorough un- 
derstanding of the cell is acquired many of 
our most perplexing problems in biology 
will be closer to solution. 


While all plant and animal cells possess 
specific morphological features that identify 
them as particular kinds of cells, they are 
all fundamentally alike in certain respects 
(Fig. 3-1). All are composed of protoplasm 
bounded by a semi-permeable plasma mem- 
brane, and somewhere within the proto- 
plasm lies a nucleus, which is likewise 
bounded by a nuclear membrane. The pro- 
toplasm in which the nucleus floats is the 
cytoplasm. In addition to the plasma mem- 



brane, plant cells are enclosed by a cell 
wall, composed of a non-living carbohy- 
drate called cellulose which lends to the 
cell a definite, rigid shape. This cell wall is 
porous enough to allow the passage of sol- 
uble substances through it. In animal cells 
the cell wall is usually absent, the boundary 
therefore being the plasma membrane. 

Several characteristic structures are pres- 
ent in the cytoplasm of most but not all 
cells. Most animal cells possess a region 
near the nucleus known as the centre- 
sphere, inside of which two small bodies, 
the centrosomes (centrioles) are sometimes 
found. Both of these structures are absent 
in higher plant cells, although they are 
found among the lower plants, such as some 
of the algae. There is some doubt as to their 
function in the life of the cell. Small bodies 
called plastids are frequently present in the 
cytoplasm in both animal and plant cells. 
In the former they are usually colorless, 
whereas in the latter they may contain 
chlorophyll, the green pigment essential in 
photosynthesis. Fiber-like bodies floating 
in the cytoplasm are the mitochondria, 
which have recently been identified as bun- 
dles of enzymes important in the metabo- 
lism of the cell. The irregularly shaped 
Golgi apparatus is scattered through the 
cell or collected near the nucleus, very often 
in the region of the centrosphere. The func- 
tion of this structure is unknown, although 
it is thought to be associated with secretion 
in some cells. Vacuoles are spaces in the 
cytoplasm that usually contain gases, solids, 
or liquids. In addition to these more or less 
regularly occurring parts of the cell, there 
may be present also certain lifeless bodies 
floating haphazardly in the cytoplasm. They 
are referred to as cell inclusions. They may 
be stored starch or fat, or undigested bits of 
organisms which have been taken in as food. 
.The nucleus is the vital part of the cell 
because it contains the genes, carriers of 
heredity factors. Genes are localized in the 
chromatin granules that are visible under a 
light microscope. The chromatin is confined 

to discrete bodies called chromosomes, 
about which we shall learn a great deal 
more in a later chapter. The undifferenti- 
ated protoplasm of the nucleus is named 
nucleoplasm or nuclear sap. 

Although these are the essentials of most 
cells, some possess additional structures 
which are particularly designed to do spe- 
cific jobs. Furthermore, cells vary tremen- 
dously in shape, size, and special functions. 
Size. There is as much difference in the 
size of cells as there is between different 
animals, an elephant and a mouse, for ex- 
ample. It has been thought that certain 
species of spherical bacteria, which have a 
diameter of 1 micron or less, are the small- 
est cells. Cells must not be confused with 
viruses which are not organized into cells 
and which, in fact, normally live at the 
expense of cells. Bacteria can be seen only 
with the best high magnification micro- 
scopes. Recently they have been photo- 
graphed through the electron microscope 
which has made possible more detafled 
studies of their internal anatomy (Fig. 2- 
10). Most cells that compose the bodies of 
animals are considerably larger than the 
largest bacteria; on the average they are 
about 7 microns in diameter. The nerve 
cells are very long and tliin, particularly 
those reaching from the tip of the toe to the 
spinal cord in the back. In an elephant or 
giraffe these cells are 6 or 7 feet long. The 
largest animal cells are found among bird's 
eggs, in which what is popularly called the 
"yolk" is a single cell. The largest cell 
known is the yolk of the ostrich egg which 
reaches a diameter of 2 or more inches. 
The large quantity of stored food (yolk) in 
eggs is responsible for their great mass. 

Shape. The shape of cells depends a great 
deal on their function (Fig. 3-2). If they 
perform a tensile function, such as the cells 
found in tendons, they are long and thin 
because of the constant stretching action 
placed upon them. If they are conductive, 
as in the case of nerve cells, they must also 
be long and thin. On the other hand, red 



corpuscles are tiny discs, fitted by shape to 
float in the blood stream. Smooth muscle 
cells are spindle-shaped, which is the ideal 
shape for a cell that must shorten or con- 
tract. Cells that line the respiratory tract in 
our bodies are small cylinders with minute, 
vibratile, hair-like structures (cilia) on one 
end, which function in carrying the mucus 
along the tract by their whip-like action. 
Other cells, such as the amorphous white 
blood cells, resemble tiny bits of jelly that 
move by rolling along, taking in and de- 
stroying bacteria and other foreign par- 
ticles in the blood and tissues. 

Number. A glance through a microscope 
at a thin slice of tissue taken from any ani- 
mal will demonstrate the fact that there are 
a great many cells in even a small animal 
like the mouse or spider. The larger the 
animal, the more the cells, although swift- 
moving and very active animals such as 
insects and birds usually have smaller cells 
per unit volume than do sluggish, slow- 
moving creatures such as the salamander. 
There is no correlation between the size of 
the animal and the size of its cells. The 
larger animals simply have more cells. 
Every cubic millimeter of human blood 
contains about 5 million red blood cells, 
and the total number in the entire blood 
stream approximates 30 quadrillions. The 
human brain alone has billions of cells; the 
number in the whole body thus takes on 
astronomical figures. 


As previously noted, cells function much 
alike, irrespective of their situation. All 
have certain needs which are satisfied in 
much the same way. To be sure, certain 
complications arise when they are grouped 
together in great masses but that problem 
will be the subject of a later discussion. Let 
us now examine the processes common to 
all cells whether they be independent or- 
ganisms or members of large complexes. 

Fig. 3-2. Various kinds of cells. A. Red corpuscles. B. Flat 
(squamous) epithelial cell. C. Columnar ciliated epi- 
thelial cells. 0. Smooth muscle cells. E. Nerve cell 



oxyqen woter 



amino acid 

( g lucose ) 

-^ ammo acid 


cell repair 
"^and qrowth 



carbon dioxide 


■^ ammonia 

_^carbon dioxide 

oxyqen water 

Fig. 3-3. Schematic drawing showing how food is metabolized in a cell. 

Nutrition. In the broad sense, nutrition 
includes all activities that have to do with 
securing, consuming, and ultimately me- 
tabolizing food ( Fig. 3-3 ) . This is essential 
for cells because it is the only way they can 
obtain the energy with which to carry on 
their life processes (such as reproduction, 
motion, and generating heat). Plant cells 
require only simple inorganic material, 
whereas animal cells demand complex or- 
ganic compounds in addition to certain 
inorganic substances, such as sodium and 
iron. As was pointed out earlier, animals 
depend on the plants for much of their food 
directly and all of it viltimately. This differ- 
ence in food requirements between plants 
and animals has profoundly influenced the 
course each has pursued in evolution. The 
plant can remain serenely rooted in one 
spot throughout its lifetime, and under 
ordinary circumstances *all of its needs in 
the way of food are in its immediate sur- 
roundings. The essential inorganic salts and 
water are in the soil at its "feet," and the 
carbon dioxide and sunshine are in the air 
overhead. It is not so with the animals. 
They must pursue and apprehend complex 
organic diets, necessitating a much more 

highly specialized organism, endowed with 
sense organs, coordinating systems, and 
machinery for rapid movement such as 
skeletons and muscles. The food thev secure 
is not in a state for immediate utilization, 
but requires an elaborate digestive system 
to break the large insoluble molecules into 
smaller utilizable ones. This is the price ani- 
mals have had to pay for being unable to 
satisfy their needs from simple inorganic 
food sources. Perhaps there are otlier com- 
pensations for being an animal. For man, 
there is the joy of a rare steak smothered 
with mushrooms, for example. 

First of all, individual animal cells must 
receive their foods from the outside world, 
whether the cell is within the body of a 
complex animal or a single isolated cell. In 
the latter case, entire microorg-anisms are 
engulfed (Fig. 7-5), digestion proceeds 
within the cell body, and the products of 
digestion are absorbed into the surrounding 
protoplasm, ultimately to be utilized as a 
source of energy or for growth and repair. 
In the multicellular animal the individual 
cells receive their food in utilizable form so 
all they need to do is to use it. Each cell is 
bathed in a fluid (lymph in man) which 



contains all of the food requirements of the 
cell. Hence, the molecules need only trav- 
erse the membrane to gain entrance into the 
cell. Within the cell they are transported 
throughout the protoplasm by diffusion. 
Having reached this destination, the food 
is available for metabolism by the cell. 

Metabolism of food is the most important 
part of nutrition because it is the process 
by which energy is released from the 
energy-rich compounds and building ma- 
terials are incorporated into the structure 
of the cell. The business of providing the 
cells with food and removing the waste 
products is merely accessory to the real 
job of extracting energy from the food and 
building it into new protoplasm. During 
constructive metabolism (anabolism) the 
absorbed amino acids are built into the pro- 
tein framework of the cell's protoplasm. 
Those not needed for this purpose are 
deaminized (amino groups removed) and 
converted to glucose. Fats are stored and 
some of the glucose is converted to glyco- 
gen and stored. In the destructive metabolic 
( catabolic ) phase, the glucose is burned to 
carbon dioxide and water through a long 
series of steps producing many intenne- 
diary products. Fats are also oxidized and 
release large amounts of energy, leaving 
end products of carbon dioxide and water. 
Such energy is used to produce heat and 
movement, and to bring about the anabolic 
conversions just mentioned, for they are all 
endothermic reactions requiring energy be- 
fore they can take place. 

In order that oxidation can occur in the 
cell, oxygen must be supplied to it. This 
is abundantly furnished in the waters sur- 
rounding the single cells and it must like- 
wise be supplied in the fluids such as lymph 
surrounding the metazoan cell. Oxygen 
must be available at all times if the process 
of metabolism is to go forward normally. 
This gas, like food products, passes readily 
through the membrane of the cell by diffu- 
sion and mingles with the molecules in the 
protoplasm, ready to combine with glucose 

or with fats in order to release energy. 

As a result of these metabolic activities 
within the cell, certain waste products are 
formed which must be removed before they 
accumulate to toxic proportions. These 
wastes consist of carbon dioxide and water 
which result from the burning of glucose 
and fats, and nitrogen wastes including 
urea which results from the breakdown of 
nitrogen-containing compounds, principally 
amino acids. Some inorganic salts, such as 
phosphates and sulfates, accumulate as a 
result of the decomposition of phospho- 
lipids and certain sulfur-containing amino 
acids, and they too must be eliminated or 
excreted. Wastes leave the cell through 
the membrane by the same process that the 
food entered, and are deposited into the 
surrounding fluid in the case of metazoan 
cells ( many-celled animals ) or the envelop- 
ing water in the case of single cells. Any- 
thing that interferes with the elimination of 
these products affects the metabolism of the 
cell itself; in fact, the cell can survive only 
a short time if the waste products are forced 
to accumulate in any quantity. 

Reproduction. Another fundamental 
problem that the cell must solve is repro- 
duction. All isolated cells must duplicate 
themselves periodically if they are to in- 
crease their numbers. The ability to do this 
often determines whether or not a species 
will succeed as a race. The individual cells 
of a multicellular animal multiply rapidly 
during embryonic life and some do through- 
out life. Others are produced once and are 
never duplicated again. Just how this is 
accomplished at both the cellular and mul- 
ticellular level will be discussed in subse- 
quent chapters. 

We have seen then that cells, while 
highly variable in size and morphology, all 
have the same fundamental needs which 
must be cared for whether they are a part 
of a many-celled animal or live as isolated 
individuals. Let us now consider how these 
needs are satisfied when cells began to live 
in groups, that is, in a metazoan. 


The Organized Animal 



Once the fully organized cell had evolved 
on earth, it undoubtedly explored thou- 
sands of possibilities in structural patterns 
as well as environments in which to live. 
Some found niches in which they were 
"satisfied," where they have remained 
through the succeeding millions of years. 
We find them still occupying these same, 
or very similar, places today. They 
found no need for change, no need to 
search elsewhere for more favorable fea- 
tures in their environment. Others, how- 
ever, were forced into situations where 
they were subjected to a variety of en- 
vironments which compelled them to 
change or perish. It was from this group 
that we might expect to find not only new 

varieties of single cells but also some that 
banded themselves together in small groups 
for the sake of "better living," whatever 
that entails. 

The colonial idea proved advantageous 
for survival, and more and more cells were 
added until the mass became so great that 
changes became necessary to permit the 
continuing of the vital life functions. Sys- 
tems for surmounting these encroaching im- 
pairments of function were introduced in 
a diversity of form and structure; some 
proved efficient and allowed the animal to 
become still more complex, others must 
have been so poor that the organism 
changed no more or died out. From this 
long, tortuous path has come to us today 




the complex many-celled animal with all 
of its intricate machinery to do the jobs 
that were done so simply by the one-celled 
animals. The price has been a long and 
bitter struggle; whether or not it is worth 
while only man has sufficient intelligence to 

Assuming that during the millennia re- 
quired for the unfolding of animal life, 
some found satisfactory niches all along the 
way and remained essentially unchanged 
up to the present, it should be possible to 
find such representatives and to arrange 
them in order of their complexity, forming 
a continuous series from single cells to the 
most complex animals alive today. To be 
sure, we would expect to find gaps between 
groups and we would also expect that the 
animals we do find would not be exact 
duplicates of the originals. They too would 
undoubtedly have undergone some minor 
changes during this long period of time, 
even though they remained in a relatively 
unchanging environment. By erecting such 
a series the story of evolving animal life 
might become a little more clear. 

Most biologists agree that the Metazoa 
took their origin from some single-celled 
form. Surveying the thousands of species 
of single-celled animals ( Protozoa ) , it seems 
likely that the starting point could have 
been among those that bear flagella (Fig. 
4-1), some of which also contain the plant 
pigment chlorophyll and are, therefore, 
closely related to the plant world. Among 
this large and varied assemblage of Pro- 
tozoa are some that resemble one another 
very closely, except that they exist in 
groups of individuals which vary in number 
from one to several thousand. Starting with 
the single-celled Chlamydomonas, one can 
arrange a graded series where individuals 
differ only in the number of cells that cling 
together. Pandorina is composed of eight 
cells embedded in a spherical matrix of 
jelly-like material. Each of the cells is not 
greatly different from Chlamydomonas. 
The combined beating of their flagella 

causes the entire colony to roll along in 
a graceful manner. There is another form, 
Pleodorino, which is composed of many 
more cells, clustered in the shape of a hol- 
low sphere; aside from the increased num- 
ber of cells there is little difference between 
this one and Pandorina. We do note one 
rather interesting dissimilarity that will be 
discussed later but should be mentioned 
briefly now. Not all of the cells are the 
same size; some are smaller than others, and 
during reproduction the smaller ones are 
unable to produce new colonies, in other 
words, they are sterile. Therefore, in this 
form there seems to be two kinds of cells, 
reproductive cells and sterile or soma cells. 

A much larger aggregate is illustrated by 
Volvox, SL beautiful hollow spherical col- 
ony consisting of several thousand cells. 
Again these cells resemble Chlamydomonas 
in most respects, although there are tiny 
bridges between individuals which tend to 
lock them together more securely than the 
loose jelly of other forms. Most all of the 
cells are alike although there are some here 
and there that are larger and have a dif- 
ferent appearance. These are the reproduc- 
tive cells; all others are soma cells. A more 
careful observation will reveal that the re- 
productive cells are of several kinds. Some 
are bundles of tiny bodies, the sperm or 
male cells, whfle others are large ovoid egg 
cells. These special sex cells reproduce the 
colony by a sexual process, that is, the 
sperms are released into the water where 
they swim to and unite with the egg. This 
subsequently becomes a zygote, which 
overwinters in a heavy-walled case (Fig. 
4-1). Other reproductive cefls merely 
divide and move into the hollow of the 
sphere where they become small colonies, 
known as daughter colonies. These eventu- 
ally burst out, destroying the mother and 
becoming adult colonies themselves. 

Two striking events occurred in this 
gradual association of cells. First, simflar 
cells aggregated into a mass which appar- 
ently succeeded better, that is, there was 

Vol vox 
F!g. 4-1. A series of forms that illustrate the colonial theory of the origin of the Metazoo, 



strength in union. Secondly, division of 
labor was initiated among the cells, some 
becoming sterile and functioning only in 
locomotion and food-getting, whereas 
others retained the primitive condition of 
colony-reproducing. Some of the reproduc- 
tive cells became highly modified into eggs 
and sperms while others merely retained 
their primitive characteristics of reproduc- 
ing by simple fission. In other words, a dif- 
ferentiation of function took place among 
the cells of the aggregate, definitely mark- 
ing it off from the isolated single cells and 
at the same time creating the first step in 
the organization of a complex animal 
through the loss of the power of reproduc- 
tion by some of the cells. Once this step 
was taken, differentiation of the soma 
cells continued in various directions toward 
greater and greater complexity, and thus 
up the long trail to such highly intricate 
forms as man. 

This gradual advance in complexity 
might be compared to the evolution of our 
own society. The Protozoa may be com- 
pared to primitive man who lived alone and 
was compelled to obtain all of his own food, 
make his own clothing, and provide his own 
shelter. Existence by this crude means 
made chances for survival poor, and mor- 
tality high. Later, man associated himself 
with others in the common interest of sur- 
vival and of making the drudgery of life 
less grueling. The first groups were made 
up of the immediate family; they lived to- 
gether, hunted together, and made shelters 
together. In other words, they performed 
all the duties acting as a group rather than 
singly as heretofore. Food was easier to 
secure because they could surround and kill 
larger animals, their shelters could be more 
elaborate, and the burdens which fell upon 
each individual were not as great as when 
each lived alone. Such aggregation was 
continued to include larger groups until 
small villages were formed; with the in- 
creasing numbers of individuals partici- 
pating in mass efforts, less responsibility fell 

to each one, and what was more important, 
each shared in the results of the mass ef- 
forts. They aU lived better and longer. This 
has continued and finally developed into 
our modern civilization. There are places 
on the earth today where primitive peoples 
live just as they did many thousands of 
years ago. These people are unsuccessful, 
biologically speaking, because they have 
been unable to spread their kind over the 
world. Such is the criterion of biological 
success. Following this analogy, we can 
think of primitive society as resembling the 
single-celled animal and modern society as 
the complex metazoan, such as the mam- 
mal. As tlie cells began to aggregate into 
groups, individual cells specialized in par- 
ticular jobs, and the group as a whole be- 
came more complex. There are animals all 
along the evolutionary scale which repre- 
sent steps in increasing complexity. 

The next step toward a more complex 
animal is a simple metazoan with further 
differentiation among its soma cells. The 
body is now a simple sac composed of two 
layers of cells. Hydra is an excellent ex- 
ample, and while more will be learned 
about it in a later chapter, we may briefly 
examine its anatomy at this point in order 
to carry further the idea of increasing com- 

Hydra is made up of many cells, mostly 
soma cells, which are arranged in two lay- 
ers (Fig. 4-1). Some of the cells in the out- 
side layer (ectoderm) have differentiated 
into "nettle cells" for stinging purposes in 
defense or offense. Others are able to 
lengthen and shorten during locomotion 
and to convey impulses (neuromuscular 
cells). Still others have the ability to give 
rise to sperms and eggs, and to new indi- 
viduals by bud formation. Here, then, we 
see that the soma cells have differentiated 
into several kinds, while the sex cells re- 
main much like they were in Volvox. Divi- 
sion of labor has started among the soma 
cells which is the next step in the develop- 
ment of more complex animals. 


pavement epithelium 





qoblct cell 

basenoent membrane 

Columnar epithelium 

ciliated columnar epithelium 


cuboidal epithelium 

cornified layer- 

lumen of tubule 

bosemsnt membrane 

Fig. 4-2. Epithelial tissue. 

Stratified epithelium 



Fig. 4-3. Various kinds of glands that arise from epithelial tissue. A. A simple flask-shaped gland (alveolar) such 
OS that found in the skin of the frog used to secrete mucus. B. A compound alveolar gland such as that found 
in the salivary glands. C. A simple tubular gland such as that in the lining of the intestine. D. A compound 
tubular gland such as that found in the stomach lining. 

Once cells have established themselves in 
such intricate relationship as in Hydra, fur- 
ther differentiation is possible. That pos- 
sibility becomes a reality as we go higher 
in the animal series. The result is a number 
of different kinds of cells, each carrying on 
its own metabolism, but specialized for 
some particular function. The major types 
of such specialization are not great in num- 
ber but each type has many varieties. Let 
us examine these major forms of specializa- 


Division of labor among the soma cells 
spread throughout the Metazoa until a wide 
variety of cells was produced, each doing 
a specific job. Cells of the same kind grouped 
together in a continuous mass form a tissue. 
A particular kind of tissue is not necessarily 
limited to one region of an animal body, 
but usually is found in several different 
places, where it may or may not perform 
the same function. There are four major 
kinds of tissues, epithelial, sustentative, 
nervous, and contractile. Although tissues 
occur in all groups of animals, for simplicity 
let us consider only those found in a mam- 
mal such as man. 

Epithelial tissues. These are the surface 
tissues which cover and line not only the 

outside of the body but the cavities within 
as well. They are composed of closely 
fitting cells forming continuous membranes 
and with very little intercellular material 
binding them together. Since the jobs per- 
formed by epithelial tissues vary greatly, 
these tissues exhibit a wide variety of form 
(Fig. 4-2). The tissue is usually named ac- 
cording to the shape of its constituent cells, 
for example, squamous (flat), cuboidal 
(cubes), and columnar (columns or pil- 
lars ) . They may also be described in terms 
of accessory structures such as flagella, 
collars, or cilia. Finally, the tissue may be 
referred to as stratified if the cells have dif- 
ferent forms and lie several cells in thick- 


In addition to protection, epithelial cells 
that line cavities usually have the function 
of secretion, which is the production of spe- 
cial substances used by the organism in 
various ways. For example, the cells lining 
the digestive tract are mostly secretory in 
function. These cells form the secreting por- 
tion of glands whether the gland is single 
or many celled. In multicellular glands the 
secreting cells may lie beneath the sur- 
rounding surface forming simple tubes 
(tubular glands) or flask-shaped pockets 
(alveolar glands). Such tubes and pockets 
may be single structures (simple glands) 
or they may be grouped into aggregates 
(compound glands) (Fig. 4-3). 




haversian canal 

compact bone 




haversian canal 



Fig. 4-4. Various kinds of sustentative tissue. 



Sustentative tissues. These are the tissues 
that give the body form and support. They 
are composed of cells imbedded in a matrix, 
secreted by the cells and usually occupying 
more space than the cells themselves. The 
matrix may be composed of fluid, gelatinous 
material, long tough fibers, or hard miner- 
alized material. There are several different 
kinds of sustentative tissue, all dependent 
on the type of matrix ( Fig. 4-4 ) . 

The tough ligaments that fasten the 
bones tosether and the tendons that con- 
nect the muscles to the bones are composed 
mostly of tough fibers forming a matrix 
about the cells which produce them. Also, 
many of the internal organs of the body 
are laced together by sheets of similar tissue 
called mesenteries. In this type of tissue the 
fibers lie at random with no particular 
arrangement, which results in a thin layer 
of tissue that is soft and pliable, yet tough. 
A similar type makes up most of the deeper 
portions of the skin lying below the super- 
ficial epithelium. It is this sustentative 
tissue in its protective covering that gives 
the skin the qualities essential for an ade- 
quate body covering. 

Animals, particularly land forms, require 
a very rigid skeleton to support their mas- 
sive weights. This is provided by bone and 
cartilage, types of sustentative tissue that 
are composed of large quantities of matrix 
secreted by isolated cells. In the case of 
cartilage the matrix is a spongy semi-solid 
mass in which cells are embedded in tiny 
cavities (lacunae). The cells are usually 
single, although as they divide there may be 
as many as four in one cavity before they 
finally separate. Cartilage is excellent ma- 
terial to resist shock; therefore, it is found 
between bones such as the vertebrae. It also 
provides ideal support for the tip of the 
nose and the external ear where retention of 
shape and pliability are essential. Bone, on 
the other hand, consists of a mineralized 
matrix (calcium carbonate and phosphate) 
which is very rigid, imparting an element of 
solidarity to the entire structure. In this 

case also, the matrix is formed from cells 
embedded in tiny spaces (lacunae) within 
the matrix itself. These usually take on defi- 
nite patterns around blood vessels and 
nerves, called Haversian systems. All of 
the cells have access to a food supply from 
the blood system by means of tiny canals 
(canaliculi), for these cells are alive and 
must be nourished like any other cells 
(Fig. 4-4). 

Tissue in which fat is stored is often 
classified as sustentative tissue, primarily 
because there seems to be no other category 
for it. It performs no mechanical function 
other than to occupy space. The fat is 
stored within the cell itself and these cells 
are located under the skin and in the ab- 
dominal region as well as other well-known 
areas of the human body. During periods 
of starvation it is very scanty, but during 
good times it may be stored in quantities 
far beyond any usefulness to its owner, as 
attested by many overweight people. 

Contractile tissue. This tissue, called 
muscle tissue, has the ability to shorten, 
that is, to pull its two ends closer together. 
This apparently very simple action is re- 
sponsible for all of the movements of most 
organisms. Mviscle tissue consists of elon- 
gated cells or fibers whose internal parts 
consist of myofibrillae, tiny contractile 
fibrils, lying in a fluid protoplasm called 
sarcoplasm. There are three well-defined 
kinds of muscle tissue, visceral, skeletal, 
and cardiac, each of which differs in its 
appearance under the microscope (Fig. 

Visceral muscle is found in the walls of 
the digestive tract, and other places in the 
body which are not under voluntary nerv- 
ous control; this activity is primarily auto- 
matic, and is not under the influence of the 
will. The cells are spindle-shaped with cen- 
trally located flattened nuclei, and with 
myofibrillae running lengthwise in them. It 
is the shortening of the myofibrillae that 
pulls the two ends of the muscle cell closer 
toeether. Visceral muscle contracts and re- 


layer of circular muscle 
layer of lonqrtudinal muscle 


cross striations^ 
sorco plasm 


x-section of fiber 

smooth muscle 

striated muscle 

cross striations 

intercalated disc 

cardiac muscle 

Fig. 4-5. Types of contractile tissue. 



Fig. 4-6. Striated muscle is identified by the striking 
cross-striations that show under the microscope as 
seen in this photograph. The nuclei appear as black 
elongated elipses distributed along the edge of the 

laxes slowly, a behavior which is quite 
satisfactory for the kind of job it has to do. 
The skeletal muscles are usually attached 
to bones and they constitute the large mus- 
cles of the body. It is this muscle-bone com- 
bination that is responsible for the move- 
ment of the body as a whole. These muscle 
cells are peculiar in that they are not 
marked off by definite cell membranes, and 
a single skeletal muscle fiber is composed 
of many cells whose nuclei lie at regular 
intervals along the periphery of the fiber 
just under the surrounding membrane 
(sarcolemma). The fiber is called a syncy- 
tium, a name applied to any mass of proto- 
plasm which contains many nuclei without 
discrete cell membranes. Another marked 
difference between this muscle and the 
preceding is that there are evenly spaced 
dark and light transverse bands extending 
throughout the fiber. These striations iden- 
tify the tissue as striated muscle ( Fig. 4-6 ) . 

The skeletal fibers contract suddenly with 
considerable force, an essential feature in 
moving the body. They can contract rapidly 
again and again with only momentary rest 

Cardiac or heart muscle is characteristic 
of vertebrates and is not found in the heart 
of any of the lower forms. It differs from 
skeletal muscle in that all of the fibers 

axis cylinder 

x-secTion of axon 

Fig. 4-7. Nerve tissue as found in the spinal cord and 




are connected with one another so that the 
entire organ functions as a unit, that is, as 
a syncytium. This is an apparent advantage 
because the nature of its job requires almost 
continuous operation. Striations are present, 
but the nuclei are located deep within the 
fibers rather than at the surface as in the 
skeletal muscle fibers. Cardiac muscle cells 
are so closely connected with one another 
that a single nerve impulse sets them all 
contracting at once, thus executing a single 
powerful contraction. This is obviously very 
desirable in a pump such as the heart. 

Nerve tissue. This type of tissue is com- 
posed of neurons, special conducting cells 
that are found throughout the brains and 
nerve cords of all animals that possess a 
centralized nervous system ( Fig. 4-7 ) . The 
nerve cell is composed of a cell body, which 
contains the nucleus and surrounding cyto- 
plasm. Extending out from the cell body 
are threadlike fibers, consisting of numer- 
ous dendrites which normally convey im- 
pulses to the cell body and a single axon 
which usually conducts impulses away from 
the cell body. The cell body maintains the 
nutrition of the entire neuron, and if it is 
destroyed the fibers die. However, nerve 
fibers severed from their cell body will 
usually be replaced by new fibers growing 
out from the cell body. 

Cell bodies are concentrated into masses, 
the most conspicuous of which are in the 
brain and in the spinal cord; other masses 
called ganglia have special locations in the 
body. The nerves that we see on dissection 
are made up entirely of fibers, each of 
which is insulated by a fatty sheath, the 
myelinated sheath. These units go to make 
up the complex nervous system which we 
shall study in more detail in a later chapter. 




In order to perform specific functions, 
tissues must be in someway incorporated 

into organs, because by definition any struc- 
ture which performs a given function is an 
organ. Obviously, a single contractile cell 
could be an organ under this general defi- 
nition. However, in the usual, restricted 
sense, an organ is a group of tissues assem- 
bled for the purpose of performing a spe- 
cific function. The small intestine, for ex- 
ample, is an organ whose function is the 
digestion and absorption of food. It is com- 
posed of layers of different tissues — an 
outer layer of epithelial tissue covers the 
gut throughout its length; immediately in- 
side this are two layers of muscle tissue, 
then a layer of connective tissue, and finally 
a thin layer, one cell thick, of lining epithe- 
lium. All of these tissues perform specific 
jobs in bringing about the greater function 
of digestion and absorption of food. Even 
so, the small intestine is not adequate to 
complete the job of ingestion, digestion, 
absorption, and egestion as a single organ. 
This greater function involves a series of 
organs, the mouth, teeth, esophagus, stom- 
ach, small intestine, liver, pancreas, colon, 
and anus. In other words, the entire job is 
done by a system of organs. Likewise, cir- 
culation, breathing, excretion, and indeed 
all bodily functions are performed by dif- 
ferent organ systems. The combined activi- 
ties of all of the organ systems constitute 
an organism, or an individual. This can be 
relegated to the cellular level, as in the case 
of an amoeba in which all of the activities 
take place within a single cell. On the multi- 
cellular level, tissues, organs, and organ 
systems have been assembled to make up 
an organism which functions as a unit, just 
as the single cell functions as a unit. 


As cells became organized into groups 
they took on a definite relationship to one 
another, conveying to the resulting animal 
a particular shape that can be described in 
terms of symmetry. Symmetry refers to the 
arrangement of parts in relation to points. 



lon9itudinal sections 

tronsv«rsa saction 

Fig. 4-8. Orientation of an animal possessing bilateral symmetry. 



planes, and straight lines. In describing ani- extending from the tip of the nose to the 

mals a reference to planes describes most of tip of the tail, through the midline, and this 

them. A plane has length and breadth, but cut becomes a plane of symmetry (Fig. 

no depth, that is, it is two-dimensional. 4-8 ). Such a bisected pig now consists of two 

Therefore, dividing an animal by a plane mirror halves. A bilaterally symmetrical ani- 

results in two halves, each of which is a mal may also be cut transversely; such cuts 


R a d i a 




Fig. 4-9. Various kinds 

mirror image of the other. Note that each 
half is not a duplicate of the other because 
all of the parts are reversed. When only one 
plane can be drawn, the animal is said to 
be bilaterally symmetrical. This is the case 
with most animals. A pig, for example, can 
be divided into two halves by a single cut 

of symmetry in animals. 

are called transverse sections. Additional 
terms useful in orientation are the follow- 
ing: The back side is the dorsal side, while 
the opposite or belly side is the ventral 
side, the head is the anterior end, and the 
opposite end is the posterior end. The pig 
also has a left and right side. These terms 




Fig. 4-10. Different types of cavities in various animals. 

are important in describing the location of 
organs and other parts of the animal. 

A small number of animals, some Pro- 
tozoa and early embryos of higher forms, 
exhibit universal symmetry. In such animals 
it is possible to draw any number of planes 
passing through a central point which will 
divide the animal into equal halves. Obvi- 
ously such forms are spheres, such as Vol- 
vox (Fig. 4-9). 

In other animals it is possible to draw a 
number of planes through a central line or 
axis, dividing the animal body into equal 
halves. These animals are said to possess 
radial symmetry. Hydra has this type of 
symmetry. Radial symmetry is usually con- 
fined to lower animals, for such forms are 
more or less helpless in doing much about 
their surrounding world. These animals are 
usually sessile, though some move very 
feebly, and they must wait for their food 
to come within their grasp. In other words, 
unable to pursue and apprehend tiieir food, 
they are passive animals that survive only 
because of the richness of their food laden 
environment. As animals have become more 
complex, this type of symmetry becomes 
unsatisfactory and gives way to bilateral 
symmetry which provides the animal with 

a body plan that is conducive to further 
development both in size and complexity. 

Some animals, including many Protozoa, 
are without symmetry, that is, they are 
asymmetrical; there is no way of dividing 
them into halves by means of a plane. Ani- 
mals may be symmetrical externally but 
asymmetrical in regard to their internal 
organs. Man is a good example of this. Ex- 
ternally he is symmetrical, but his liver, 
stomach, spleen, and heart are asymmetri- 
cally arranged. Some fish such as the floun- 
der are asymmetrical as adults but sym- 
metrical during their immature stages. A 
very young flounder looks just like any 
other young fish, but as it matures one eye 
misirates through the head so that both 
of them come to lie on one side and the 
fish lies flat on the bottom on one side. This 
is an apparent advantage in survival for 
this peculiar animal. 

Body cavities. The formation of cavities 
is characteristic of the earliest metazoan 
forms. Such lowly forms as Volvox have a 
central cavity in which the young develop. 
Among the animals such as Hydra a more 
useful cavity has evolved, one which func- 
tions in the storing of food during digestion. 
Such a cavity is referred to as a coelenteron 



(Fig. 4-10). It functions both as a digestive 
cavity and as a space where food can be cir- 
culated to the Hning cells; therefore, it is 
referred to as a gastrovascular cavity as 
well. Such a cavity has but one opening 
through which food must pass upon enter- 
ing and undigested food upon leaving the 
body. The flatworms also possess such a 

As animals increase in complexity, the 
simple sac-like coelenteron of the lower 
forms is inadequate and becomes modified 
into an enteron with an additional opening, 
the anus, at the opposite end from the 
mouth. Thus food can follow a one-way 
path through the body, a more efficient 
arrangement, certainly, than the crude 
coelenteron of Hydra. In the earthworm 
and higher forms another cavity, the coe- 
lom or body cavity, appears, which lies be- 
tween the digestive tract ( enteron ) and the 
body wall. It is a convenient space into 
which wastes, sex cells, and some foods can 
be dumped, later to be eliminated or uti- 
lized. It also provides space for the internal 
organs which become much more compli- 
cated as animals grow more complex. The 
entire body cavity is lined with a thin sheet 
of tissue called peritoneum. Once the coe- 
lom had arisen in those early invertebrates 
somewhat like the earthworms, it appar- 
ently proved highly satisfactory in further- 
ing the development of animals, for it was 
retained through all higher groups of ani- 

Segmentation. Another feature that was 
introduced early among animals and re- 
tained throughout subsequent groups was 
the clinging together of individuals, form- 
ing long chains which resulted in seg- 
mented or metameric animals. Certain low 
worms divide by transverse fission but fail 
to separate until a large number of fissions 
have taken place, thus producing a long 
contiguous series of worms clinging to- 
gether head to tail ( Fig. 4-11). It is thought 
that some of these failed to separate at all, 
thereby producing a long wonu consisting 

• « 

Fig. 4-11. A possible explanation of the origin of 



of many parts very much alike. Once this 
happened, an integrating system could be 
devised which would then result in a seg- 
mented animal such as an earthworm. This 
may be the way segmentation came about. 
In any case, segmentation is a persistent 
character of many higher animals, includ- 
ing man. Lower forms such as the earth- 
worm show both internal and external seg- 
mentation, and most of its organs are 
duplicated in almost all of the segments. 
However, some organs, like the gonads, are 
confined to certain segments, and, in gen- 
eral, there is a concentration of nervous 
organs at the head end of the animal. 

Segmentation is not very obvious in such 
higher animals as man because it is ob- 
scured by the specialization of individual 
segments. Nevertheless, a glance at the 
skeleton shows that the basic plan is seg- 
mental (Fig. 15-2). The vertebrae and the 
ribs, while varying slightly in different parts 
of the body, are serially repeated and re- 
semble one another very closely. Segmenta- 
tion is clearly indicated in the early em- 
bryos of vertebrates, and during the first 
few weeks of development the human em- 
bryo shows segmentation which in principle 
is like that of the earthworm. As it grows 
older the clear-cut segments are obliter- 
ated by fusion and reorganization. 

With the gradual development of these 
various features, animals became organized 
into what they are today. Many new prob- 
lems arose with the ever increasing com- 
plexity of the total organism, and the con- 
sequences of these we shall consider a little 


As animals grew in bulk and complexity 
they were confronted more and more with 
the problems of transport and coordination. 
Such activities as respiration, nutrition, and 
excretion, which were simply performed 
when the cell was in constant contact with 

its fluid world, became difficult or im- 
possible when it was separated from this 
environment by even a few covering layers 
of cells. Such inner cells would have to de- 
pend on diffusion to carry oxygen and food 
to them and to remove wastes from them. 
At best this is a slow process and certainly 
not rapid enough to allow an animal to 
grow very big or become very active. 
Therefore, specific organ systems had to 
be evolved if animals were to grow in bulk 
and activity. 

In the following discussion we shall com- 
pare the activities of organisms at the cellu- 
lar and multicellular levels, pointing out the 
problems involved in becoming complex 
and indicating how the metazoan animal 
has solved them. We can use amoeba for 
the single-cell level, hydra for a simple 
metazoan, and man for the multicellular 

Respiration. Respiration is the taking in 
of oxygen and the eliminating of carbon 
dioxide. In the amoeba this is cared for very 
efficiently and simply by diffusion through 
the limiting membrane that envelops the 
cell (Fig. 4-12). Once in the cell, oxygen 
diffuses to where it is needed and the car- 
bon dioxide which results from the combi- 
nation of oxygen with food likewise makes 
its way to the cell or plasma membrane, 
passing out through it to the surrounding 
water. The only essential need is sufficient 
oxygen in the environment. 

Respiration is essentially the same in a 
simple metazoan such as hydra where each 
cell is in contact with its external world. 
Diffusion is adequate to take care of the 
respiratory needs of this simple animal. 

In a complex metazoan animal, respira- 
tion is, of course, the same, and oxygen and 
carbon dioxide exchange must take place in 
each cell. Since most of the cells lie deep 
within the organism there is no possible 
chance for gaseous exchange by diffusion 
with the external world, especially since 
the organism is covered with skin which is 
impervious to such gases. The metazoan 





Fig. 4-12. The problem of consuming food, digesting it, burning it, and excreting the waste products is essen- 
tially the same at all levels of animal organization. Here it is compared in the single-celled animal, amoeba, 
the simple metazoan, hydra, and in a highly complex form such as man. The problem is always reduced to the 
level of the cell and must be solved at that level in all animals. The letters EP mean end products and E means 


then is forced to develop a breathing sys- 
tem in combination with a transportation 
system, which would make it possible for 
oxvsen and carbon dioxide to ^et directly to 
and from each indi\'idual cell. In aquatic 
forms such as fish, the breathing organs are 
gills; in land forms such as man, they are 
lungs. Both must meet certain requirements 
if they are to function as breathing organs. 
They must be constructed so that oxygen in 
the surrounding medium (water or air) 
can pass readily into a transporting medium 
(blood) which will convey the gas to each 
cell in the body. This means that the blood 
must flow in thin-walled tubes ( capillaries ) 
very close to the external environment in 
order that the exchange can be readily 
accomplished. Microscopic examination of 
a gill filament or a lung sac will reveal that 
this situation is met beautifully. Blood flow- 
ing through thin-walled capillaries passes 
within two cells of the outside world ( Fig. 
18-19), which makes the gaseous exchange 
possible and simple. 

The transporting system delivers the oxy- 

gen to the cells of the body and collects the 
carbon dioxide from them. Here again the 
circulating blood must come in close prox- 
imity to every single cell of the body in 
order that at no time wfll the cell be short 
of oxygen or have a surplus of carbon di- 
oxide. When the blood reaches the cells it 
is no more than one cell away so that 
gaseous exchange can be accomplished 

Nutrition. At the cellular level the busi- 
ness of securing food, digesting it, absorb- 
ing it, and ultimately metabolizing it is also 
a relatively simple procedure. The amoeba 
surrounds its food, thus forming food vacu- 
oles which mio;ht be thouo;ht of as miniature 
intracellular "stomachs." Digestion is car- 
ried on in these tiny structures and the final 
end products (amino acids, glucose, fatty 
acids, and glycerol) are absorbed into the 
surrounding cytoplasm (Fig. 4-12). The 
food is burned and the locked up energy 
released to be utilized by the amoeba in the 
many ways that are essential for its life. 

The simple metazoan obtains and digests 



food by the cooperative effort of many cells. 
The food is captured by the tentacles and 
taken into the coelenteron where it can be 
retained until the linino; cells secrete en- 
zymes to digest it. The end products then 
can be absorbed directly. 

The cells of the complex metazoan are 
confronted with the same difficulty in ob- 
taining food as they were in receiving 
oxygen; therefore, something has to be 
done about providing space in the body 
where food can be satisfactorily digested 
and absorbed into a transporting medium 
that will deliver it to each cell of the body. 
The same transport system can be used that 
carries the gases in respiration. All that is 
needed is a place where food can be held 
sufficiently long so that digestive enzymes 
pouring into it will have time to break the 
complex insoluble molecules down into 
simpler soluble ones. Then proper facilities 
will be needed for absorption, that is, large 
surface areas, and so forth. 

These conditions are met in the digestive 
tract of all higher animals very satisfac- 
torily. They possess a portal of entry or 
mouth, which may or may not be armed 
with teeth for macerating food, and a long 
tube into which digestive glands empty 
their food-splitting enzymes. Undigested 
food leaves through the end of the tube, 
the anus. Once the food has reached the 
soluble stas[e it is absorbed into the blood 
and transported to each cell, which 
picks and chooses the particular energy- 
giving and constructive materials it needs 
to perform its own specific job in the organ- 
ism as a whole. 

Excretion. Closely linked with respira- 
tion and nutrition is the matter of getting 
rid of wastes, that is, excretion. In this dis- 
cussion we shall consider only the nitrog- 
enous wastes of metabolism as excretory 
products, although in the broad sense of 
the term excretion includes water and car- 
bon dioxide. At the cellular level, nitrog- 
enous wastes are also eliminated through 
the plasma membrane. At the multicellular 

level, excretory products, if not removed, 
accumulate very rapidly and soon reach a 
point where the cell cannot survive be- 
cause such products in quantity are toxic. 
Hence, an effective excretory system is de- 
manded in any animal where the cells are 
removed any distance from the surface. 

Again the simple metazoan gets rid of its 
nitrogenous wastes just as amoeba does, by 
simple diffusion. Where there are but two 
layers of cells, each in contact with the ex- 
ternal world, the problem of excretion is 
easily solved. 

In the complex metazoan, nitrogenous 
wastes such as urea are removed by the 
kidney, an organ designed so that all of 
the transporting medium must pass through 
it at regular intervals. By a process involv- 
ing filtration, selective reabsorption, and 
secretion (p. 524), wastes are removed 
from the blood and conveyed out of the 
body through a system of appropriate 
tubes. All metazoan animals above the 
simple two-layered animals possess such a 
system of excretory tubules. In the lower 
forms there are many units scattered among 
the cells so that fluid bathing these cells can 
find its way to one of these tubules and be 
relieved of its load of nitrogenous wastes. 
In higher forms the many excretory units 
become compactly arranged in a single 
organ, the kidney. 

Reproduction. Amoeba reproduces itself 
by simply splitting into two offspring, the 
most primitive type (fission) of reproduc- 
tion found in living things (Fig. 4-13). 
Whenever the amoeba reaches a certain 
size, it divides and continues growing. The 
rate at which it can increase its numbers 
when conditions are favorable appears to 
depend solely on the amount of food it can 
engulf and the rate at which it can build 
protoplasm. The daughter cells thus pro- 
duced are all essentially alike and, barring 
accidental death, live forever. 

Multicellular animals have nearly all 
given up simple fission as a means of in- 
creasing their numbers and have assigned 



Fig. 4-13. A single-celled animal can reproduce by simply dividing into two individuals. Higher forms (some of the 
Protozoa likev/ise) provide special cells, eggs and sperms, which unite and give rise to a new individual. 

the job of reproduction to special cells, 
eggs and sperms, which, with a few excep- 
tions, must unite to form new individuals. 
This device has the advantage of bringing 
two lines of protoplasm together which 
results in variation in the offspring, for each 
offspring possesses a combination of the 
characteristics of its parents and is there- 
fore different from either. Variation seems 
to have some advantage in survival of the 
species and probably has been important 
in the gradual evolution of complex forms. 
Specialized reproductive cells are pro- 
duced by special organs, the gonads. To 
insure the union of eggs and sperms spe- 
cial tubes are necessary to conduct these 
cells out of the body, and ultimate union is 
still more effectively assured in higher ani- 
mals by the development of copulatory 
organs. To insure greater survival, the off- 
spring of many of the higher animals are 
either retained within large egg shells or 

the body of the mother for various periods 
of their early development. All of this intri- 
cate machinery came into being because 
cells "insisted" on aggregating into large 

The penalty of organization. The organi- 
zation of cells into masses and the subse- 
quent specialization of different kinds have 
resulted in organisms that are able to 
penetrate a greater variety of environments 
because of their greater motility and in- 
tricately adjusted bodies. That means bio- 
logical success. Along with all of the bene- 
fits derived from specialization, however, 
there has been at least one rather severe 
penalty, and that is natural death. 

Recall that single cells reproducing by 
binary fission, barring accidental death, live 
on forever. The amoeba observed under 
your microscope has been alive since the 
dawn of life on the earth. Had death oc- 
curred along the road somewhere the one 



you see before you would not be there. 
Death put in its appearance when or^raniza- 
tion of cells occurred, that is, when some 
of the individual cells lost the power to 
reproduce, and hence were sacrificed in 
order that those that were able to repro- 
duce could continue. This seems to have 
been the penalty for organization and spe- 

It is difficult to understand why cells that 
are separate, free from other cells, may con- 
tinue living forever, whereas others that are 
bound together into a mass eventually die 
even though apparently all of their basic 
needs are satisfied. Perhaps during the 
process of evolution the organization was 
not quite perfect, that is, the individual 
cells were not completely cared for, or 
perhaps the whole organization slowed 
down after a certain period of time and 
could not keep pace with the demands of 
all the cells. This point has long intrigued 
biologists and has resulted in some very 
fruitful research. 

If it were possible to grow tissues away 
from the animal of which they are a part, it 
would be possible to determine whether or 
not such cells once released from their in- 
tended environment could survive like 

single isolated cells. This was first done in 
1907 by Ross G. Harrison, who grew em- 
bryonic tissues in flasks by feeding them 
special nutrients. Alexis Carrel, employing 
similar methods, kept embryonic chick 
heart tissues alive for over 30 years. At the 
end of this period of time, about three times 
the normal life span of a chicken heart that 
remained with its owner, the cells were ac- 
tive and appeared not to have changed at 
all. They apparently have the capacity to 
live forever, just as single-celled animals 
do. In other words, metazoan cells still 
retain their power of immortality. It is only 
when they become incorporated into a 
community of cells in the body that they 
undergo the changes which we associate 
with senescence. Something is not quite 
right in the animal body and causes a cell 
to fail. The delicate adjustment of youth 
and maturity is thrown out of tune so that 
eventually some cells do their job so poorly 
that the complete organism cannot main- 
tain life. It is not at all impossible that some 
day the exact reason for this lack of adjust- 
ment will be discovered and metazoan cells 
will regain their immortality. Imagine the 
social upheaval such a discovery would 



The Rise of Animal Life 




So far we have considered the origin of 
animal life in a physical world, centering 
our attention on the animal itself rather 
than the environment in which it thrives. 
Since the animal is an integral part of its 
environment, it is necessary to devote some 
attention to this relationship, which is the 
study of ecology. Our particular emphasis 
will be on animal ecology, although no 
ecological study can entirely ignore the 
role played by the plants. 

Great variations in the environments of 
the world are caused by such physical 

factors as light, temperature, and moisture, 
all of which have a profound effect upon 
the physical and physiological character- 
istics of animals. These physical factors not 
only determine the kinds of animals that 
are able to survive in certain regions but 
are also instrumental in building up associa- 
tions between animals and plants. Thus 
the problem in ecology is twofold: first, to 
consider the individual animal in terms of 
certain physical factors in its environment; 
second, to study the relationship between 
animals living together. 






Everyone is fully aware of his own sensi- 
tivity to change in temperature. We usually 
want our houses at a relatively constant 
temperature of about 25° C. and experience 
discomfort if it deviates a few degrees one 
way or the other. Our internal environment 
is even more critical — there a rise of a few 
degrees indicates sickness of a serious 
sort. What is true of man in this respect is 
equally true of all animals. When we con- 
sider that the temperatures known to us 
range from 273° C. below zero to several 
thousand degrees above zero, it is rather 
remarkable that life exists in that extremely 
narrow range of a few degrees above freez- 
ing to about 45° C. Even within these nar- 
row limits the physiological processes do 
their best work at an optimal point around 
the middle, on either side of which the rate 
of physiological reaction falls off. Animals 
tend to seek out a temperature that, at least 
most of the time, will permit their bodily 
activities to proceed at an optimal rate. 

Since animals are found in all parts of 
the earth except the polar regions, they 
must find ways of surviving extremes of 
temperature with the least amount of dis- 
comfort to themselves. Those living in 
colder regions either have a constant body 
temperature ( monothermal ) or else have 
developed a hardiness to cold that permits 
them to survive. The internal environment 
of the warm-blooded animals — birds and 
mammals — is constant and always main- 
tains the temperature at which physiologi- 
cal activities can proceed at an optimal 
rate. Cold-blooded animals ( poikilother- 
mal), on the other hand, vary their internal 
temperature and rate of reaction in accord- 
ance with the external environment. When 
the temperature drops, the animal becomes 
sluggish, even to the point of complete in- 
activity. Some can stand freezing for short 
periods of time. On a chilly morning in 

the fall of the year it is simple to capture 
a cold-blooded animal, from a common 
housefly to a rattlesnake, but the task be- 
comes more difficult on a hot summer day 
when the temperature approaches 100° F. 
Only at the higher temperature are all 
activities at their maximum. 

During cold seasons some mammals 
undergo a period of inactivity called hiber- 
nation, when their temperature drops and 
metabolic processes are reduced to a mini- 
mum. Hibernating rodents, such as the 
ground squirrel, pass into almost complete 
inactivity, their heart and breathing rates 
slowing down markedly. Indeed metabo- 
lism is just enough to keep the animal alive. 
The energy to maintain life is derived from 
stored fat, hence the fat bear in the fall and 
the lean bear in the spring of the year. 

Other animals survive periods of intense 
heat by going into an inactive state called 
aestivation. This is strikingly demonstrated 
by the African lungfish which lives in re- 
gions that are apt to dry up during the 
summer months (Fig. 5-1). With the ap- 
proach of hot weather and desiccation, the 
fish burrows in the mud and secretes a 
capsule in which it passes the warm dry 
months. When the temperature drops and 
moisture returns, it resumes its active life 
once more. 

Some cold-blooded animals put forth 
communal effort to prevent too great a drop 
in temperature. Bees, for example, become 
very active on cold winter days, beating 
their wings almost continuously. This keeps 
the temperature in the hive above freezing 
even though the outside temperature may 
be several degrees below zero. Snakes fre- 
quently aggregate in dens in the fall of 
the year for the apparent purpose of keep- 
ing warm. Even though they are cold- 
blooded, their temperature stays slightly 
above that of the external environment. 
Coiling about one another in large masses, 
the whole group stays a little warmer be- 
cause the individual heat loss is reduced. 

Keeping in mind that all living things are 


Fig. 5-1. The African lungfish (Protopterus) undergoes aestivation during periods of drought when the waters disap- 
pear from its normal habitat. If placed in a container filled with mud it will form its capsule and remam dor- 
mant for many months in this condition. Pictured here is such a fish being releosed from a can of mud. When 
placed in water the animal immediately breathes by means of its gills like any other fish. 



Active Staqe 

Fig. 5-2. Some Protozoa such as the human intestinal 
parasitic amoeba {Endomoeba histolytica) withstand 
periods outside the digestive tract where there is very 
little moisture. This it does by secreting a resistant 
cyst wall that prevents desiccation. It is in this stage 
that the parasite is transmitted from person to person. 

tuned to such a narrow temperature band, 
try to imagine what would happen if the 
earth's orbit shifted ever so sHghtly, just 
enough to change the average mean tem- 
perature on the earth by a paltry 100° C. 
up or down. All life would stop abruptly; 
every living thing would congeal either 
from freezing or cooking. Think how pre- 
carious our existence is, reckoned in astro- 
nomical terms. A slight celestial slip would 
mean the end of life as we know it. For- 
tunately that slip has not occurred in the 
past one or two billion years, and probably 
will not for a few billion more. 


^ We are already familiar with the impor- 
tance of water in relation to life processes 
(p. 40); here we need to consider it as an 
essential part of the environment. Getting 
the proper amount of water at the right 
time is one of the basic problems that con- 
fronts animals. This is sometimes very diffi- 
cult and, as a result, animals have devised 
various means for maintaining their water 
supply at a constant level. Although too 
much water is as detrimental to some 
animals as too little is to others, probably 
the greatest problem for most animals is 
the conservation of water. 

Even Protozoa have provided themselves 
with a method of withstanding desiccation. 
Amoeba, for example, forms a resistant cyst 
which is impervious to water loss (Fig. 

5-2). While beautifully housed in this 
tiny container the amoeba can withstand 
long periods of drouth without ill effects. 
The eggs of many metazoan animals such 
as Crustacea and rotifers are provided with 
a thick shell which resists drying. The eggs 
of many parasitic roundworms are likewise 
resistant to moisture loss. Some can be 
blown around in the dust for months and 
still become viable when picked up by the 
proper host. In fact, some even rely on this 
period of desiccation to disseminate the 

Some larger animals, desert turtles and 
lizards for example, never require water in 
the liquid state; they manage very well on 
that which is taken in with their food. 
Camels are notorious for their ability to 
work long periods without water. They can 
exist a week or more on dry food and if 
green plants are available it is not uncom- 
mon for them to go without water for a 
month. Jack rabbits, mountain goats, jump- 
ing mice, and other mammals living in 
arid regions are very well fitted to conserve 
their water intake, which is usually only 
that provided in the food. Most mammals, 
however, require a great abundance of 
water, especially those that perspire, such 
as man and the horse. 

Excessive moisture is fatal for some 
animals. The earthworm, for example, is 
driven from its burrows after heavy rains 
because it cannot get enough oxygen from 
the water. Even frogs may drown in spite 
of the fact that they are usually near water 
and require it in large quantities. High hu- 
midity often creates a favorable environ- 
ment for certain types of parasites which 
under normal amounts of moisture could 
not gain a foothold. 


Wave lengths extend from a fraction of 
a submicron (cosmic rays) to more than 
a thousand meters (Hertzian or radio 
waves), yet most animals are sensitive to 
ethereal vibrations that range only from 



about 0.38 micron (violet light) to 0.76 
micron ( red ) . This extremely narrow range 
includes the so-called visible spectrum to 
which our eyes are sensitive. Most animals 
seem to be sensitive to it also, but some 
animals are known to respond to wave 
lengths to which we are insensitive. On the 
other hand, we respond to wave lengths 
that some animals are unaware of. Our re- 
ceptors pick up only about 1/125 of the 
total range of ethreal vibrations which are 
constantly being showered on our bodies. 
Even though visible light is composed of 
such a small segment of this range, all ani- 
mals are profoundly affected by it. 

Light has a direct bearing on the orienta- 
tion of some animals. Moths, for example, 
fly toward a light, whereas pillbugs avoid 
bright light. The advantage to the animal 
in the former case is questionable, but in 
the latter it has a distinct advantage be- 
cause the pillbug breathes by means of gills 
and must seek out damp places. Dark 
places are more apt to be damp than 
brightly lighted areas. 

The reproductive cycle of some animals, 
particularly birds, is definitely influenced 
by light. If daylight is supplemented by 
artificial illumination the reproductive or- 
gans are stimulated to work longer, hence 
more eggs and young. Farmers have taken 
advantage of this fact by installing in their 
chicken houses lights which burn long after 
the sun goes down. Linked with this is the 
stimulus that causes migration in at least 
some birds. Bees are known to determine 
direction by the angle of the sun (see p. 

Some of the lower vertebrates, particu- 
larly fish and amphibians, have the ability 
to change color. They usually attempt to 
match the background upon which they 
are resting, obtaining the obvious advan- 
tage of camouflage ( Fig. 5-3 ) . This is done 
by condensing or spreading out the pig- 
ments that are confined to special skin cells 
called chromatophores. Experimentation 
has shownti that at least one mechanism in- 

». ' iff %. 

Fig. 5-3. A case of concealment by acquiring the color 
and position of the surrounding environment. Note 
how the upper part of this swamp eel (Fluta alba) 
resembles the surrounding eelgrass. 

volves the amount of light that enters the 
eyes of the animal. This is discussed more 
fully on p. 430. 

Chemical cycles 

The elements of which all organisms are 
composed come from the environment and 
return to the environment upon the death 
and subsequent decomposition of the or- 
ganism. There is, then, a constant cycle 
of the elements. An atom of carbon resid- 
ing in a protein molecule that goes to make 
up one of our muscle cells, let us say, may 
have been incorporated into any carbon- 
containing molecule of thousands of plants 
and animals before us, and will become a 
part of thousands of living things following 
us. It might be thought of as a kind of 
"reincarnation," so to speak, but not the va- 
riety that usually comes to mind when this 
word is mentioned. All elements found in 
protoplasm follow specific cycles, two of 



Fig. 5-4. The carbon cycle. 

which — the carbon and nitrogen cycles — 

will be discussed briefly. 

Carbon, being the core element of proto- 
plasm, is conspicuously present in all living 
things and, like all elements, follows a cyclic 
pattern ( Fig. 5-4 ) . Plants utilize the carbon 
in carbon dioxide to manufacture fats, car- 
bohydrates, and proteins, as well as many 

other essential food products. These foods 
are eaten, digested, and absorbed by ani- 
mals, and the carbon becomes a part of 
the body of the animal. During destructive 
metabolism carbohydrates are burned, re- 
leasing carbon dioxide into the air again. 
Similarly, carbon dioxide is released at 
night by plants as they oxidize carbohy- 

niTroqen in th^ air 

,-MOj used by 
plant to make 



Fig. 5-5. The nitrogen cycle. 



drates to obtain energy. It must be pointed 
out, however, that plants also produce 
carbon dioxide during the daytime, but 
because it is utilized immediately in the 
process of photosynthesis, its release is ob- 
scured. The burning of organic matter and 
decaying of dead plants and animals also 
release carbon dioxide into the air. Such is 
the extent of the carbon cycle. 

I I I 

i I 

The nitrogen cycle (Fig. 5-5) is some- 
what more complicated than the carbon 
cycle, primarily because plants cannot uti- 
lize atmospheric nitrogen. Nitrogen in the 
air must be converted first to nitrites 
(NO:-) and then to nitrates (NO3) before 
the plants can make use of it in producing 
proteins. This conversion is brought about 
by N-fixing bacteria in the nodules which 

I J 

COi+ HiO-^C6Hii06 






= ,,/////^^ 


i^l MI III ' ^^^^ JJJJJJ J^^p ^ 

PATS ^- H^O fo+fy ocids-^r^Mf ATS 
storogg I Q„j gycarol 

glycogen :: g*O^GLUcoS£ .^»<>starch 
"^^ / itiltrogcn 

ominoocids .^t!!iP proteins 

growth dnd 

Fig. 5-6. Solar energy is incorporated into the foods manufactured by plants. This energy is released from the foods 

by animals for their own use. 



protrude from the roots of certain legumi- 
nous plants, such as beans and clover. 
Again the plant proteins are consumed by 
the animals and converted into their own 
protein, or reduced to urea and lost from 
the body. Urea, as well as the dead body 
of an animal or plant, decomposes to form 


either atmospheric nitrogen or nitrates 
which are then used by the plants. Bacteria 
play an important part in the nitrogen 
cycle. If all bacteria suddenly disappeared 
from the earth, we would soon be short of 
nitrates and eventually of the basic materi- 
als that produce protoplasm. 


Fig. 5-7. Food chains. In each case the chain starts with the plant where photosynthesis produces the first food; the 
plants are eaten by herbivores, which are in turn eaten by a series of carnivores, in one ease ending with the 
large fish and the other with the leopard. 



Nutrition: food chains 

All animals with the exception of a few 
Protozoa depend ultimately on plants for 
their food. This is illustrated in Fig. 5-6, 
where we see that the plant manufactures 
fats, carbohydrates, and proteins, and the 
animal breaks these down for its own use. 
The plants, therefore, are continually build- 
ing up the organic world while animals are 
constantly tearing it down. It is apparently 
a well-established relationship, and one 
that we hope will endure for a long time 
to come. 

Energy passes from the plant where it 
has been stored from the sun to the animal 
that eats the plant. However, it does not 
always expend itself completely in a pas- 
sage that involves only two organisms. 
Often there are many intermediates which 
transfer the energy through a food chain, 
in which one animal after another is eaten 
until the energy can be released only by 
the death of the last animal in the chain. All 
of the food chains in a given community 
constitute a food cycle. Let us consider 
several food chains. 

In an abundantly populated fresh-water 
pond, plants and animals are constantly dy- 
ing, falling to the bottom, and decompos- 
ing. This disintegrating organic material 
forms a source of energy for the growth 
of bacteria. In addition many algae, simple 
plants, grow by the utilization of simpler 
substances, just as all plants do. These two 
then, bacteria and unicellular plants, form 
the basis of food for tiny organisms such 
as Protozoa. Small Protozoa are eaten by 
larger ones, these in turn are eaten by ro- 
tifers, then Crustacea, aquatic insects, and 
finally by fishes — first smaller fish, then 
larger ones. The latter either die or are 
eaten by fish-eating mammals such as mink, 
bear, or man. In the first case the chain 
ends with the death of the fish, in the 
second by the death of the mammal. A 
somewhat shorter pond cycle would be one 
starting with a snail eating a leafy plant, 

such as is depicted in Fig. 5-7. The snail 
is then eaten by a crustacean, the crusta- 
cean by a small fish, the small fish by a 
larger one. Here the food chain ends un- 
less, as before, the fish is eaten by bird or 

On land a food chain may follow a simi- 
lar pattern (Fig. 5-7). In this case the zebra 
feeds on plants, and is then eaten by a 
leopard. This may end the chain, providing 
the leopard dies a natural death, which is 
highly unlikely. As it grows older and loses 
some of its faculties, sooner or later it falls 
prey to another carnivore. This transfer of 
energy may go on almost endlessly. 


Some very interesting interrelationships 
between animals have been established, 
primarily on the basis of obtaining food, 
although some seem to have other purposes. 
Collectively these relationships are spoken 
of as symbiosis. They range all the way 
from a loose, more or less haphazard, as- 
sociation to a closely knit relationship in 
which the two or more animals are forced 
to live together. These interrelationships 
have fascinated bioloo;ists and should also 
be of particular interest to beginning zool- 
ogy students. 

Between individual animals 

Commensalism. This is a loose associa- 
tion of two animals in which one derives 
benefit from the combination while the 
other may or may not. There are many de- 
grees of such associations. For example, a 
flatworm, Bdelhira, can usually be found on 
the body of the king crab ( Fig. 5-8 ) . From 
this association the flatworm is able to pick 
up bits of food which are dispersed into the 
sea water as the crab tears up its prey. 
There seems to be no benefit to the crab 
from the association. 

Another interesting association is that of 
certain jellyfishes {Phtjsalia, Fig. 5-8) and 
several species of small fish. The fish live 



&••.»••»»►•/"• I , 

Fig. 5-8. Animals become associated rather intimately in many >vays. Here are some illustrations of some of these 


among the tentacles of the jellyfish which 
offer protection by their stinging cells. The 
benefit to the jellyfish comes from food 
brought by the fish during its sojourns in 
the surrounding waters. In all cases of com- 
mensalism the relationship is a loose one, 
and neither party is forced to live in co- 
operation with the other. 

Mutualism. A situation where animals 
live tofjether with benefit to both is called 
mutualism. There are all gradations of this 
association, from those who only casually 
meet and become associated to those that 
are always found together and, indeed, can- 
not live apart. One illustration of the tem- 
porary associations is that of hermit crabs 
and sea anemones (Fig. 5-8). In this case 
the sea anemone, attaching itself to the 
shell occupied by the crab, gets free trans- 
portation to areas which the crab finds at- 
tractive because of an abundance of food. 
In return for the ride the sea anemone acts 
as a camouflage, making the shell resemble 
the rest of the ocean floor. In addition, be- 
cause of its powerful battery of stinging 

cells, it functions as a line of defense 
against possible enemies of the crab. Some 
primitive chordates ( prochordates ) have 
adopted the same association with hermit 
crabs (Fig. 5-8). 

Another interesting association, where 
the relationship is more or less compulsory, 
is the case of the metazoan, hydra, and 
a unicellular plant, alga (Fig. 5-9). The 
algae live in the hydra's inner layer of 
cells (endoderm) where they carry on 
photosynthesis, releasing oxygen which is 
utilized by the hydra. The hydra in turn 
releases carbon dioxide which is used by 
the algae. While in nature this situation us- 
ually exists, it has been possible to separate 
them in the laboratory and each can sur- 
vive without the other. 

In some cases of muti^ialism the associa- 
tion of two animals is so intimate that 
neither can live without the other. The best 
illustration of this is among the termites, 
or white ants (Fig. 5-9). This association 
came to the attention of biologists when 
it was discovered that termites could sur- 


eododern? ectoderno Rrrrrrrr- 



•• M : - ,, 

wood pdrTicles 


Fig. 5-9. Two examples of mutualism: the hydra with its algae, and the termite with its intestinal Protozoa. Note 

that the first instance is between a plant and an animal. 

vive indefinitely on pure carbohydrate. 
They feed on wood alone, never receiving 
any observable nitrogen to build proto- 
plasm. Upon investigation it was found that 
great hordes of complex Protozoa inhabit 
the termite's intestines. If the termite was 
warmed up a bit the Protozoa died and 
such defaunated termites lived only a short 
time. Likewise, the Protozoa could not sur- 
vive outside the body of the termite. Ap- 
parently the Protozoa in some way provide 
the nitrogen essential for the termite. The 
termite, on the other hand, provides a good 
abundant home for the Protozoa. 

Parasitism. This is also a forced relation- 
ship between two animals, but it is a one- 

way proposition. The parasite lives at the 
expense of the host, taking all and giving 
nothing in return, possibly even causing 
injury to the host. An ideal parasite with- 
draws just enough nourishment from its 
host to maintain itself in good health and 
in reasonable numbers. If the parasite re- 
moves too much from its host, so that the 
latter becomes sick and dies, then the para- 
site too is destroyed. Many parasites reach 
a satisfactory balance with their host in 
which the latter merely contributes a home 
for the parasite and is not apparently in- 
jured by it. 

Parasitism should be distinguished from 
predation, in which one animal also lives 



at the expense of another. A predatory ani- 
mal feeds upon another by eating its entire 
body, frequently at one sitting. A parasite 
might just as surely destroy the host, but 
it does so in an entirely different manner. 
A cat kills and eats a mouse; the cat is liv- 
ing at the expense of the mouse. The cat 
has a tapeworm inside its intestine; the 
tapeworm is living at the expense of the 
cat. The first case is predation, the second, 
parasitism. "The difference between a pred- 
ator and a parasite is simply the difference 
between living upon capital and income, 
between the burglar and the black-mailer. 
The general results are the same although 
the methods employed are different." (El- 

Parasitism probably arose shortly after 
life originated on the earth. Some animals 
soon found that they could live to advan- 
tage either in or on the body of another. 
Perhaps at first the relationship was a per- 
fectly harmless one, something like com- 
mensalism, but as the association persisted 
the parasite became more and more de- 
pendent on the host for its existence. It 
modified its body both morphologically and 
physiologically in accordance with its para- 
sitic habit. In earlier relationships the para- 
site probably clung to the outside of its 
host, later going into the shallow cavities 
such as the mouth and cloaca. Some, of 
course, were inadvertently swallowed with 
the food and these after a time became 
adapted to life in the gut where all of their 
food requirements were provided for. Oth- 
ers that learned to live in the bodies of 
insects such as mosquitoes, also learned to 
thrive in the blood of vertebrates because 
they were dumped into that environment 
every time the mosquito feasted on a blood 

Through similar food chains parasites 
must have learned to live first in one host, 
then in another, and sometimes in a third, 
all in sequence, in order to complete their 
life cycle. This arrangement had the advan- 
tage of spreading the species but it also had 

the serious disadvantage of depending not 
only on three hosts but also on the necessity 
that the hosts be sequentially arranged in 
time and space. Should any one of the 
hosts die out, the parasite would likewise 
be destroyed because it could not com- 
plete its cycle. In fact, this is a most effec- 
tive way to control certain dangerous para- 
sites. Killing mosquitoes in order to control 
malaria is a familiar example. 

Parasites have been so intimately tied to 
their hosts for so many millions of years 
that ecologists today can often trace the 
history of certain species by comparing 
their parasites. In the subsequent chapters 
we shall study several different kinds of 
parasites as they occur in the various ani- 
mal groups. 

Between animals in communities 

Up to this point in our discussion, rela- 
tionships between individual animals have 
been considered. In addition to this special 
type of association, animals usually live in 
a much larger community in which a great 
many plants and animals are influenced by 
each other, in some instances more inti- 
mately than in others. 

Some biologists consider an entire plant- 
and-animal community as a unit. Such a 
"superorganism," that is, an organism in 
which "the whole is greater than the sum 
of its parts," is referred to as a biome. Just 
how far one can draw a parallel between 
a community of organisms and an individ- 
ual is a questionable. It begins to smack of 
anthropomorphism, which can be helpful 
in certain circumstances, ridiculous in oth- 
ers. Certainly it is well established that all 
life in a community is tightly woven to- 
gether, like the fibers of a spider's web, so 
that no part can be disturbed without dis- 
rupting the pattern. However, to compare 
the community arrangement to the intri- 
cate interrelationship of parts of the human 
body, for example, is stretching the point 
beyond recognition. 

Certain organisms live in specific habi- 



tats which are strangely similar no matter 
where they are found on the earth. Like- 
wise, groups of animals that require the 
same set of conditions are usually aggre- 
gated in one locality. The types of animals 
present in neighboring habitats change 
only slightly due to minor variations in 
certain physical conditions. For example, 
the oxygen content of the water in one lake 
is slightly different from that of neighbor- 
ing lakes. This tends to bring about the 
accumulation of animals that require just 
the amount of oxygen present in the lake, 
and those animals that require more or less 
are apt to seek water that satisfies their 
optimal needs. 

Sometimes the introduction of a species 
changes the community of animals, such as 
the inadvertent planting of carp minnows 
in the northern lakes of the Midwest. Fish- 
ermen driving into the lake country fre- 
quently obtain their minnow bait at some 
southern point where carp minnows are 
abundant. After the fishing is over the re- 
maining minnows are usually dumped into 
the lake where they propagate and flourish. 
Because of their feeding habits, fecundity, 
and general hardiness, they soon replace 
the more desirable game fish. Even under 
these conditions a balance will eventually 
be reestablished and once it is the animal 
community remains much the same for a 
long period of time, although not indefi- 
nitely, for there are always gradual environ- 
mental changes that necessarily affect the 
life in it. Another similar illustration is the 
case of the introduction of the English spar- 
row into the United States. 

In order to obtain some understanding of 
animal communities let us consider two 
diverse situations, one a fresh-water pond 
and the other a desert recjion. An examina- 
tion of these two communities may provide 
some appreciation of the complexity of 
ecological studies. 

A fresh-water pond. A pond is defined 
as a small body of fresh water, usually not 
more than two or three meters in depth, 

its temperature being approximately the 
same throughout (Fig. 5-10). Many ani- 
mals and plants live in such a limited en- 
vironment and even within its confines 
there are definite regions which support 
specific animals. The open water is largely 
devoid of both fish and plants, but the 
shores support a variety of animal and 
plant life, depending on the relative 
amounts of mud, sand, or rocks. 

If the bottom is muddy, many plants, 
such as water lilies, grow in profusion. In 
protected places around the edge there may 
be several varieties of fish, principally bass 
and pickerel. Crayfish and small fish may 
be seen darting here and there in search 
of food. Tadpoles can be found near the 
bottom. The water teems with tiny Crusta- 
cea and larvae of insects like midges ( small 
gnat-like flies in the adult stage), which 
form the basic food for young fish. By 
scooping up some of the mud in a fine 
mesh net, many other animals can be noted, 
including snails of various sizes and shapes 
and perhaps a few leeches. 

Many different kinds of flying insects 
make their home around the edge of the 
pond. The dragon fly (Fig. 11-31) and May 
fly nymphs can be found. An occasional 
diving beetle (Fig. 11-36) may be picked 
up. This is an interesting insect because it 
is so well adapted to aquatic life even 
though it must breathe air. It carries a film 
of air under its wings which acts as a reser- 
voir for underwater maneuvering. The hind 
legs are large and beautifully designed for 
swimming under water. 

The water boatman also carries a similar 
air film. Its long hind legs covered with 
hair, when in operation, remind one of a 
man rowing, and hence its name. Other 
insects such as the water strider (Fig. 2-2) 
and the whirligig beetle skim over the sur- 
face of the water, depressing but never 
breaking the surface film (see p. 24). 
Their food consists of air-borne insects that 
are blown out over the water and accident- 
ally fall on the surface. The whirligig beetle 



Fig. 5-10. A typical fresh-water pond. The association of plants and animals in such an environment is extremely 


possesses unique eyes which are divided 
into two parts; the lower part enables it 
to perceive objects in the water and the 
upper half gives it a clear view in the air. 
The pond may include a sandy shore 
where animals of a different kind live. 
Snails, different from those found on the 
mud bottom, crawl over the sand from 
which they remove the unicellular plants 
growing there. Frogs, toads, and turtles 
may live around the edge of the pond, and 
birds such as the redwinged blackbird may 
inhabit the vegetation along the shore. Al- 
though these animals do not live in the 
water, they do contribute to the combined 
interrelationships of the community. They 
seek at least some of their food in the water 
and when they die their bodies may fall 
into the water where they are eaten by ani- 
mals living there. 

It is obvious that there must exist many 
complex food chains in such a well-defined 
community. The chief occupation of each 
living thing is to nourish itself, a need that 
results in a severe struggle for existence. 
Rarely does an animal die a natural death, 
for the moment it wavers it is pounced 
upon and destroyed by another, thus be- 
coming a part of a long or short food chain. 
There is, however, a complete food cycle 
for the entire community which involves 
certain general groups of plants and ani- 
mals. The green plants always provide the 
beginning of such a food chain. In the case 
of the pond, the water plants extract their 
simple needs from the water and manufac- 
ture food which is consumed by plant feed- 
ers, such as the tiny Crustacea that feed on 
algae and the snails that eat larger plants. 
These animals are pursued and eaten by 



Fig. 5-11. Because of the exacting environmental conditions of a desert plain, there are few plants and animals. 

But even here there is an intricate relationship between them. 

a large variety of predators such as the 
predaceous diving beetles. Others, hke the 
midge larvae and May fly nymphs, are con- 
tent with the dead bodies of plants and ani- 
mals, and are known as scavengers. Finally, 
the organic matter that remains after all 
animals are through with it is decomposed 
by bacteria, known as decomposers, so that 
the inorganic compounds that are needed 
by the plants are restored to the water. 
Thus the cycle is completed. 

Seasonal changes occur in the pond com- 
munity and the animals living there must 
adjust to them. Usually there is an abun- 
dance of water in the pond during the 
spring of the year, but as fall approaches 
there may be very little water left. Con- 
sequently animals must migrate to other 
ponds or go into a resting stage until condi- 

tions improve. The sheet of ice which cov- 
ers the pond during the winter excludes 
most of the light, which results in less pho- 
tosynthesis, hence less oxygen. If animals 
are to survive the winter they must be able 
to get along on minimum quantities of oxy- 
gen as well as withstand low temperatures. 
Each animal living permanently in such a 
pond community is able to meet these situ- 
ations and is found year after year in the 
same locality. 

A desert plain. In contrast to the pond, 
the desert environment supports only those 
plants and animals that can survive on very 
little or no water, except that taken with 
the food. If we study the life existing on the 
plains of our own Southwest, we find a 
strange group of plants and animals, all of 
which are adapted in one way or another 



to life in a dry climate (Fig. 5-11). This 
region is not truly desert; it is about half- 
way between grassy plains and true desert. 
The vegetation is composed chiefly of cacti, 
yuccas, and other plants particularly well 
adapted to prevent loss of water through 
transpiration. They are protected from the 
marauding attacks of hungry and thirsty 
animals by their sharp stiff spines and tough 
outer coverings. 

Several species of lizards and rattlesnakes 
are permanent residents of this region. One 
interesting bird, the so-called road runner 
or chaparral cock, is commonly seen racing 
down the road ahead of your car; it seems 
to prefer using its powerful legs when pur- 
sued rather than its wings. The most num- 
erous mammals are the long-legged, long- 
eared, swift-running antelope jack rabbits, 
kangaroo rats, prairie dogs, and coyotes. 
The last-named feed on the others, all of 
which are vegetarians. Snakes and lizards, 
like the coyotes, are also carnivorous and 
feed on small mammals and some insects. 

Compared to the pond situation, the des- 
ert food cycle is rather simple, as would be 
expected where there is so little life. Like 
the pond food cycle, the desert cycle begins 
with the plants. These are preyed upon by 
the various herbivorous animals, which in 
turn are eaten by the carnivores, which 
probably also eat one another. Eventually 
death overtakes them and the elements of 
which each is composed return to the soil 
to be used again by the plants. 

There are many other communities of 
animals in a wide range of habitats. Some 
of these are the ocean, grassy plains, tun- 
dra, forest, and mountains. In all of these 
many niches have developed, inhabited by 
certain species of animals which are similar 
in respect to specific needs. Together they 
constitute the life of each community and 
the communities combined make up the 
complex life on the earth. 

Population densities 

Much can be gained from studying the 

interrelationship of life in a community. 
The study of populations alone has proved 
valuable not only in the control of insect 
pests and predators, in increased game and 
fish, and other redistributions of animal life, 
but it has also been very important in busi- 
ness and government, as for example in the 
formulation of insurance and retirement 
plans. During every census more and more 
information is gathered about people in 
order to learn what is happening to the 
status of our population. This makes it pos- 
sible to predict future trends and also sheds 
some light on what might be done to influ- 
ence the ultimate outcome. This is a hope- 
ful field of investigation and shows prom- 
ise of being of great value as time goes on. 

Population, measured in terms of rela- 
tive numbers and general distribution over 
the surface of the earth, indicates the bio- 
logical success or failure of any species of 
animal. One of the important factors in 
achieving success is the rate of reproduc- 
tion of a species. Here we find vast differ- 
ences among animals, from those like the 
whale that produces one offspring every 
few years, to the tapeworm that can pro- 
duce the incredible number of 100,000 per 
day. In general, the larger the animal the 
fewer the young, while the smaller the 
animal the greater the number of offspring. 
Even in this there are wide variations — 
rabbits, for example, can reproduce at such 
a rate as to overrun the entire earth in a 
matter of a few years. A single protozoan 
could fill all the oceans of the world in 
a few months if it were allowed to multi- 
ply at full speed and none of its offspring 
died. How successful these reproductive 
powers have been is indicated by a glance 
at some figures concerning numbers of ani- 
mals. A quart of sea water may contain 
over 1,000,000 microorganisms; an acre of 
fertile soil, over 13,000,000 animals. Grass- 
hopper eggs alone may exceed 200,000 per 
acre in heavy infestations. 

Although each species of animal has the 
potentiality for overrunning the earth, it 



never actually does. There are always con- 
trolling influences, such as competition for 
food, the ravages of the environment, in- 
fectious disease, and many others. Further- 
more, a single species may fluctuate widely 
from season to season. Grasshoppers may 
be very numerous one year, devouring all 
vegetation over large areas, whereas the 
next year there may be very few. Barring 
man's intervention, this may be caused 
by unseasonal weather during the young 
stages when the organism is sensitive to 
adverse conditions. Ruffed grouse may in- 
crease steadily for several years until they 
reach great numbers, then suddenly de- 
cline, much to the disgust of the hunter. 
Indeed, this rise and fall in the population 
of certain species is called rhythm, and it is 
so constant that it can be predicted. Some- 
times animals reach tremendous numbers, 
then go into a decline from which they 
never recover, and eventually become ex- 
tinct. The passenger pigeon is a good exam- 
ple. In other cases, like the American bison 
and the whooping crane, an attempt is 
being made to save them from extinction 
by the animal who nearly caused it in the 
first place, namely, man. 

We have seen that various controlling in- 
fluences keep any one species of animal 
from realizing its potentiality for spreading 
its kind over the earth. However, if the so- 
called reproductive pressure is allowed to 
exert itself ever so little, the results are 
often unfortunate. One example will suf- 
fice. The English sparrow was first intro- 
duced in Brooklyn, New York, in 1850 and 
1852 for the purpose of controlling certain 
insect pests that were destroying the shade 
trees of the city. In England the bird was 
desirable and because of its natiual ene- 
mies existed in modest numbers. In Amer- 
ica, however, it was free from its predators 
and the full powers of its reproductive 
capabilities came into play. Within a few 
years it became a pest. Instead of eating the 
insect pests, it fed on garden produce and 
cereal grains, man's own food, and in addi- 

tion destroyed other insect-eating birds. 
By 1886 it had spread to Salt Lake City and 
today its distribution is continent-wide. One 
interesting fact has been noted about the 
English sparrow: it no longer populates the 
streets of our cities as it did a veneration 
or two ago when the horse and buggy was 
a common means of transportation. Grass- 
hoppers and other insects clinging to the 
car radiator are a poor substitute for the 
horse droppings that dotted the streets 
some years past. 

Such mistakes as the one just described 
have been made on numerous occasions by 
man. Sometimes injurious animals are im- 
ported into new regions because they have 
escaped border inspections and have subse- 
quently become established, later to be- 
come serious pests. Great care is now taken 
to prevent this from happening. Many 
states have inspections on railroads and 
highways to keep any injurious pests out. 
Airplanes must be carefully inspected when 
they fly from one region to another, par- 
ticularly when the two are great distances 
apart. The danger of introducing certain 
disease-carrying insects, such as mosqui- 
toes, into a new environment is obvious. 

Man has intentionally introduced some 
animals to prey upon others with excellent 
success. One good illustration is that of the 
ladvbird beetle introduced from Australia 
a few years ago to destroy the cottony 
scale which was playing havoc with the 
citrus crops of California. This required the 
research efforts of an entomologist who 
studied the enemies of the cottony scale in 
its native Australia. The ladybird was fi- 
nally decided upon and when brought to 
this country proved very successful in par- 
tially controlling the pest. It has not, as yet, 
become a pest itself. 

Our economic zoologists are well aware 
of the great precaution that must be exer- 
cised in interrupting the delicate balance 
of life in any environment. For example, 
when the insecticide DDT was first avail- 
able for public consumption, many enthusi- 



astic laymen wanted it spread from air- 
planes over wide swamp areas in order to 
destroy mosquitoes as well as other insects 
that have mostly a nuisance value. If our 
practical zoologists had not prevented that 
procedure, the damage might have been 
so tremendous that it would have taken 
several generations and perhaps many mil- 
lions of dollars to repair. Think of the de- 
struction to honeybees as one example. 
Besides destroying them as a source of 

honey, there would be untold damage to 
orchards due to unpollenated flowers. 
Birds, amphibians, and reptiles feed largely 
on insects, to say nothing of the aquatic 
life that subsists on these very numerous 
little animals. We can tamper with the 
environment only in small areas for specific 
purposes; any large-scale operation must 
be carried out with utmost caution so that 
the balanced plan of the community of 
animals is not disturbed. 



Nearly all of the earth's thin crust is 
teeming with plant and animal life, each 
fitted by form and habit to its particular 
niche. Yet within this wide variety there 
exists a uniformity of structure and behav- 
ior that is recognized by anyone who has 
taken the time to make even casual obser- 
vations. Such facts as the shape of fast- 
swimming animals, the warm blood and fur 
of mammals, the feathers of birds, all point 
to a uniformity of design which indicates 
basic kinships that were early recognized 
by men interested in natural history. Fol- 
lowing man's innate tendency to catalog 
everything about him when it exists in 
sufficient numbers, animals were grouped 
according to similarities in structure, habit, 
and environment. 

The first schemes for classifying animals 
were based on convenience alone. Animals 
were simply put into groups in order to 
prevent confusion, the method of catalog- 
ing resembling the design of present-day 
telephone directories. This system served 
the single purpose of arranging animals so 
that one might conveniently find the name 
of any one of them. Animals were classified 
according to their environments: those liv- 
ing in water, in the earth, in the trees, or 
on the surface of the ground. For example, 
whales were at first classified as fish be- 
cause they lived in water and had a body 
plan resembling fish, in spite of the fact that 





Fig. 6-1. Carolus Linnaeus (1707-1778) is considered the 
father of taxonomy because he initiated a system of 
classifying plants and animals that is still in use 

they possessed hair and warm blood. The 
system served a useful purpose as long as 
the number of animals was not great, but 
with increasing knowledge it became cum- 
bersome and almost useless. Over a million 
animals are known today and to classify 
them in such a way would be a formidable 
and fruitless task. It became obvious that 
other factors must be selected as a founda- 
tion upon which a satisfactory system of 
classification could be built. Let us consider 
briefly the man who was responsible for 
our present system. 

Although John Ray (1627-1705), the 
English naturalist, has been considered the 
first true systematist, Carolus Linnaeus 
(1707-1778) is generally recognized as the 
father of taxonomy because he gave us the 
system of classification that is in current 
use today (Fig. 6-1). He was a Swedish 
physician who developed an interest in 
natural history that continued from child- 
hood throughout his lifetime. In his early 
youth he recognized the need for a better 
system of classification and soon set down 

the basic principles on which he later built 
a satisfactory method of cataloging plants 
and animals. Linnaeus had the insight to 
select important fundamental characters as 
bases for his classification. This was a for- 
tunate thought because not only did it give 
us a system which was workable and sound 
for an infinite number of additions, but it 
was also compatible with the doctrine of 
evolution, a theory Linnaeus himself did 
not subscribe to. He developed a branch- 
ing type of system, just as evolution is, so 
the two go hand in hand, not because of 
the foresight of the author but by sheer 

Linnaeus used such fundamental struc- 
tures as the skeleton, scales, hair, feathers, 
and so forth, in classifying the larger ani- 
mals; for the soft-bodied invertebrate types 
he used characters like the foot of the mol- 
lusk, the body segments and exoskeleton of 
the arthropods. All animals and plants in 
this system of classification were given two 
names, a generic (a noun) and a specific 
( an adjective ) name. This is now known as 
the binomial system of nomenclature. The 
generic name is comparable to our own 
family name, whereas the specific name is 
like our given name. Linnaeus decided that 
these names should be written in a lan- 
guage that would cause the least amount of 
international jealousy and therefore se- 
lected Latin. Animals that are most alike 
were placed in one species, such as sapiens, 
the specific name for all men alive today — 
there have been other species of men but 
they are all extinct. Likewise, man also 
belongs to the genus Homo; there have 
been other Homos but they, too, have been 
extinct many thousands of years. Under the 
Linnaean classification, therefore, man is 
known as Homo sapiens. 

Linnaeus grouped all the various genera 
(plural of genus) into larger groups which 
he called orders; while these animals re- 
sembled one another in certain respects, 
they differed much more than did the vari- 
ous species in the separate genera. He fur- 



ther grouped the orders into six classes, his 
largest category. Since his time, of course, 
a great many biologists have unearthed 
information about more and more animals 
so that it became necessary to enlarge his 
classification extensively. This was done by 
adding two more groups, namely, families 
(between genera and orders) and phyla 
(the largest group of all). Furthermore, 
each group has been subdivided again and 
again, so that we now have the following 
general categories: phylum, subphylum, 
class, subclass, order, suborder, family, sub- 
family, genus, subgenus, species, and sub- 
species. However, not all of these divisions 
are necessary in the classification of every 

The differences are less and less as the 
selection moves from the phylum to the 
species. For example, the differences be- 
tween the horse and the earthworm are 
various and striking; each belono;s to a 
separate phylum. The differences between 
the horse and the alligator, while many, are 

in different orders, families, genera, and 

In many cases, however, the problem of 
separating species is much more difficult. 
In fact, a point is reached where biologists 
are frequently in doubt as to whether or 
not a given animal is actually a separate 
species at all. This is as one would expect 
if the theory of evolution holds, namely, 
that animals have originated and are in- 
deed still originating from a common an- 
cestral stock. Undoubtedly new species are 
forming at the present time and will con- 
tinue to do so as long as there is life on 
earth. This problem occupied the attention 
of Charles Darwin, whose lifelong efforts 
culminated in his book. The Origin of Spe- 
cies, which conveys the answers to many 
questions concerning species differences. 

The following table illustrates the use of 
the Linnaean system in classifying man ac- 
cording to his distinguishing characteristics 
and in outlining the basis of his relationship 
to other animals: 

Phylum Chordata — notochord, gill slits, nerve cord 
Subphylum Vertebrata — backbone 

Class Mammalia — mammary glands, hair 
Subclass Eutheria — placenta 

Order Primates — superior nervous system 

Suborder Anthropoidea — flattened or cupped nails 
Family Hominidae — no tail or cheek pouches 
Genus Homo — manlike 

Species sapiens — present-day man 

not nearly as numerous as those between 
the horse and the earthworm; they belong 
to the same phylum but not to the same 
class. Thev show more differences than are 
observed between the horse and the dog, 
both mammals, belonging to the same class. 
The differences between the dog and the 
horse are sufficient, however, to place them 

Thus Homo sapiens includes all living 
men today. To distinguish between differ- 
ent colors and other characteristics of men, 
the subspecies is given. The scientific name 
then becomes trinomial, such as Homo 
sapiens africanus, which identifies a par- 
ticular race of living men, the African 




So far we have considered the physical 
world, the first living things, the develop- 
ment of Metazoa, the problems associated 
with organization on the multicellular level, 
and finally the animal and its environment. 
We shall now discuss in some detail the 
various kinds of animals that have arisen 
through the millions of years since the in- 
ception of life on earth. It has been a long 
steady trek, with millions of species gener- 
ating and few surviving. One should look 
with profound respect on any creature alive 
today because of the terrific struggle its an- 
cestors must have gone through in order 
to provide a body organization able to cope 

with the environment through the millennia 
and come up still managing to fit into its 
particular niche. The nature of the struggle 
and the path over which each species has 
passed will probably never be known, but 
as one of these species we have come to 
appreciate something of the magnitude of 
the problem even though much of it is still 
beyond our comprehension. 

We shall start with a discussion of the 
simplest animals, the Protozoa, then pass 
through the animal kingdom in the order in 
which we believe these animals appeared 
on the earth. You will note a gradual in- 
crease in complexity of structure and func- 
tion from the Protozoa to mammals. Always 
try to keep in mind the story that is being 




told, rather than the details concerning the 
individual animals. Of course you must 
know certain specific points about repre- 
sentative animals, but the whole picture 
is more important than the isolated facts, 
no matter how fascinating they may be. 


The Protozoa display the full potentiali- 
ties of protoplasm within the confines of a 
single cell. With a few exceptions, they are 
all unicellular, yet with so little they have 
done as much exploring with possible vari- 
ations in pattern and function as have the 
multicellular animals. As a consequence, we 
see a vast array of sizes, shapes, and habi- 
tats among some 30,000 different species 
of these tiny animals. They range in size 
from 3 to 15,000 microns and live in almost 
any environment, from soils to the red 
blood cells of vertebrates. 

Physiological and pharmacological re- 

Fig. 7-2. This is a photograph (photomicrograph) of a 
living amoeba taken through the light microscope. 
Note the highly granular nature of the protoplasm 
in the main body of the cell and the clear regions 
where the pseudopods are forming. 

Fig. 7-1. This ciliated protozoan, Tetrahymena geleii, has 
many of the nutritional requirements of higher ani- 
mals, including mammals, and for that reason has 
become valuable in research. The study of micro- 
organisms has become an important field of research 
where the fundamental workings of protoplasm are 
under investigation. 

search has and will continue to employ 
Protozoa for experimentation to find out 
about the effect of various substances, in- 
cluding poisons, on the animal cell. Tetra- 
hipnena geleii (Fig. 7-1) is a particularly 
valuable laboratory animal for many kinds 
of protozoan research. Before an under- 
standing can be had of the more complex 
Metazoa it is essential that more knowledge 
be accumulated concerning the single- 
celled forms. 

In order to understand the Protozoa as 
a group let us first study two representa- 
tives in detail, and later in more cursory 
fashion a few additional forms to gain some 
notion of diversity. We have selected 
amoeba as a simple form and Paramecium 
as one of the most complex protozoans. 
Both of these are ubiquitous in their distri- 
bution and because they are easily handled 
have been classic material for biology 
classes for many years. 



retractinq pseudopodium 
plasma membrane 

food vacuole 

controctile vacuole 


temporary anterior end 

Fig. 7-3. Amoeba with internal anatomy shown in detail. 


The common amoeba, Amoeba proteus 
(Fig. 7-2), spends its life today, just as it 
did many millions of years ago, in an 
aquatic environment from which it receives 
all of its nourishment and into which it 
deposits all of its wastes. A food-laden 
world lies within its ability to apprehend 
and devour. Its continual search for food 
keeps it on the move, crawling over vege- 
tation on the bottom of ponds and streams. 
It resembles an amorphous blob of nearly 
transparent jelly and its only outward mani- 
festation of animal affiliation is its move- 
ment, which consists of an interesting and 
bizarre method which seems to be confined 
almost exclusively to this tiny animal. 


Amoeba moves about by means of minute 
projections of its protoplasm, called pseudo- 
pods (pseudopodia — false feet). They are 
formed by what at first seems to be a 
very simple process — the fluid protoplasm 
merely flows out into an apparently weak- 
ened region in the outside layer, called the 
ectoplasm. Further observations indicate 
that the process is not a simple one. For 
example, pseudopodial formations seem to 
be "intentional" because they develop only 
in order to approach food or to move away 
from danger. Evidently there is some type 

of coordinating mechanism located within 
the cell body which makes this possible. 

The first indication of pseudopod forma- 
tion is activity in the protoplasm at a par- 
ticular point (Fig. 7-3). The ectoplasm 
then flows out into this region. Simultane- 
ously, the temporary posterior end gives 
up its position, and that protoplasm moves 
forward, filling the region left by the proto- 
plasm which is actively forming the new 
pseudopod. Pseudopods form vertically and 
anteriorly, that is, in the direction of the 
cell's general progress. The amoeba is able to 
"step over" particles which are not food and 
to engulf those which are a part of its diet. 

The inner portion of the cytoplasm 
(known as endoplasm) consists of a very 
fluid inner region called the plasmasol, and 
a more viscous region just outside the fluid 
region, the plasmagel. These two may 
quickly change from one consistency to 
the other (phase reversal, p. 48) and that 
is what happens when the amoeba "walks." 
Precisely, the plasmasol flows in the direc- 
tion in which the pseudopod is to form, and 
changes to plasmagel as it spreads out at 
the tip. The flow of plasmasol is continual 
as long as the cell moves in that particular 
direction. Most descriptions of this process 
point to the fact that the pseudopods form 
in several directions at one time. There fol- 
lows an apparent "tug-of-war" until finally 
the pseudopods on one side or the other 



accumulate the bulk of the protoplasm, 
causing the others to retract and follow the 
cell body in a specific direction. 

Ingestion of food 

The activity of amoeba is about the same 
during locomotion and in food-getting, with 
one or two minor exceptions. When it ap- 
proaches a motionless particle of food, such 
as an immotile alga, the pseudopods are 
spread around and over the plant cell in 
close proximity until they meet, forming a 
food vacuole. If the food is active, as in the 
case of a small protozoan, the amoeba 
spreads its pseudopods over a larger area 
in order to capture the organism first before 
closing in on it to form the vacuole (Fig. 

Once the food vacuole is within the cell 
body, digestion begins, much the same as 
it begins in man's stomach when a morsel of 
food is swallowed. The surrounding proto- 
plasm secretes enzymes into the vacuole 
where they proceed to digest the captured 
food. This interesting process can be ob- 
served under the microscope. A captured 
protozoan, for example, slows down in its 
activity and finally ceases movement alto- 
gether. It then begins to disintegrate and 
after a time there is very little left of the 
original organism, except the undigested 
parts which, incidentally, are left behind 
as the amoeba moves on its way. The di- 
gested food passes into the protoplasm 
where it is metabolized. This is the source 
of energy which enables the amoeba to 
keep continuously on the move and which 
also provides the materials from which it 
is able to grow and reproduce. 

Respiration and excretion 

The exchange of oxygen and carbon di- 
oxide is a simple business in amoeba. Oxy- 
gen moves into the protoplasm of the cell 
by diffusion whenever the concentration of 
the gas is greater outside than inside. Like- 
wise carbon dioxide diffuses out of the cell 
into the surrounding water when it forms 

as a result of metabolism within the organ- 
ism. Since this little creature demands an 
abundance of oxygen continuously, you 
would not expect to find it in stagnant 
water where there is little or no oxygen. 

As a result of the metabolism of nitrogen 
containing compounds (amino acids), poi- 
sonous wastes tend to accumulate in the 
cell. These are removed by diffusion to the 
outside fluid world also. They are not al- 
lowed to accumulate because of their toxic 

There is a prominent organelle (little 
organ) located variously in the cytoplasm 
of the amoeba which requires some expla- 
nation. It is the contractile vacuole. As the 
amoeba is watched, a clear, spherical area 
forms, which, while small at first, soon 
grows to maximum size, then suddenly dis- 
appears. It forms and disappears at regular 
intervals. Those who have studied this care- 
fullv believe that the contractile vacuole 
probably functions only as a device for get- 
ting rid of excess water that accumulates 
inside the cell. In other words, it acts like 
a bailer who works to get rid of the water 
that is constantly flowing into a leaking 
boat. Because of the hypertonicity of the 
amoeba, water is constantly flowing into 
its protoplasm. If it were not removed the 
little animal would soon become water- 
logged. Furthermore, if the concentration 
of dissolved substances in the surrounding 
water is increased (hypertonic), as it is in 
sea water or in a solution with high salt 
content, the contractile vacuole disappears. 
If the marine amoeba, which has no con- 
tractile vacuole, is placed in fresh water, 
vacuoles form very soon. All evidence 
points, therefore, to the fact that the con- 
tractile vacuole functions merely as a hy- 
drostatic organelle, a mechanism that con- 
trols the flow of water out of the amoeba. 


Amoeba, like all other living things, re- 
produces itself. It does this in the simplest 
way, namely, by dividing into two equal 



Fig. 7-4. Amoeba undergoing binary fission. 

parts, a process known as binary fission 
(Fig. 7-4). After the cell has grown to a 
certain size, it rounds up into a ball. The 
nucleus divides first, then the entire cell 
cleaves into two parts which are usually 
spoken of as "daughter cells" (they are 
called daughter cells perhaps because they 
give rise to other cells; "son cells" would 
be unable to do this). The amoeba seems 
to have to reach a certain size before di- 
viding; for if a small piece of cytoplasm is 
cut off periodically as the amoeba grows, 
never allowing the animal to reach the 
proper size, it will not divide. 

Amoeba is "immortal," as was pointed out 
in an earlier section (p. 83). If death 
occurs, it comes only through accident. Oc- 
casionally its watery environment may dry 
up, leaving it to desiccate and die if no 
provisions were made for it to exist through 
periods of adverse conditions. When such 
a times comes, the amoeba (there seems 
to be some question about A. proteiis form- 
ing a cyst although other amoebas do) 
secretes an impervious outer covering 
called a cyst (Fig. 5-2), which allows life 
to continue at a very low ebb until it is 
once more submerged in water. The cyst 
then splits open and the amoeba resumes 
its active life. This provision has made it 
possible for the animal to survive long peri- 
ods of unfavorable conditions and has been 
an important factor in its survival. 


Another innate quality of amoeba which 
makes possible its survival is its responsive- 
ness to changes in its external world. Its 
response is usually such as to protect itself 
from harm or to lead it to a rich food area. 
This is spoken of as its behavior. Although 
gradual changes in intensity of light elicit 
very little or no response, intense, sudden 
light causes it to send out pseudopods in 
such a way as to withdraw and move away 
from the light source (Fig. 7-5). Likewise, 
if a concentrated salt solution is placed near 
the amoeba, a definite response is noted. 

injurious chamitol* 




*'walkft " 

sida vi«w 

Fig. 7-5. Amoeba responding to various conditions in its environment. 



Fig. 7-6. Paramecium has been studied perhaps more 
than any other protozoan. It is large (100-200 mi- 
crons in length) and very easily cultured In the 

If it is floating free in the water and one of 
its pseiidopods contacts a surface, it at once 
adheres to the substratum. However, if a 
sharp point is pressed into its protoplasm 
at the surface, an immediate avoiding reac- 
tion follows. Amoeba thrives best at room 
temperature. If it is subjected to lower tem- 
peratures, all of its activities slow down and 
cease altogether as the freezing point is 
reached. Activity also ceases if the tempera- 
ture is raised to approximately 30° C. 

In general, then, the behavior of amoeba 
is geared to its needs and is responsible for 
its survival. If more were known about the 
amoeba's response to its external world, 
problems which arise in the more special- 
ized animals would be easier to solve. 


Not all Protozoa are as simple as amoeba. 
A study may now be made of a protozoan 
which is perhaps one of the most compli- 
cated of all single cells — paramecium, the 
"slipper animalcule" (Fig. 7-6). This form 
has been experimented upon and studied 
as much, if not more, than amoeba and a 

great deal of fundamental biological infor- 
mation has been derived from this source. 
As one might guess, the animal is shaped 
like a slipper, pointed at the posterior end 
and blunt at the anterior end (Fig. 7-7). 
A groove extends throughout most of its 
length and the mouth or cytostome (cell 
mouth) is formed in the groove on the 
ventral side, about two-thirds back from 
the anterior end. Paramecium has an outer 
covering, the pellicle, which is sufficiently 
rigid to maintain a constant shape. Careful 
examination with excellent optical equip- 
ment shows that the covering is made of 
minute hexagonal plates, and that the mid- 
dle of each plate is perforated by a central 
opening through which a tiny "hair-like" 
cilium (plural, cilia) passes. The animal 
moves by the combined rhythmic beating 
of these cilia. At the junction of the plates 
are other tiny holes through which threads 
are thrust when the animal is disturbed. 
The threads originate from small bodies ly- 
ing just beneath the pellicle, called tricho- 
cysts. They are apparently used in defense 
and perhaps also in attaching the animal to 
detritus in the water. Adding a small 
amount of acetic acid to the water near the 
Paramecium discharges them. 


The power stroke of the cilia is diagonal, 
so that the animal turns on its long axis. 
Since the cilia in the oral groove are larger 
and beat with more vigor, the anterior end 
describes a circle and causes the animal to 
swim in a spiral manner (Fig. 7-8). When 
the posterior end is stationary, the long axis 
of the body describes a cone (Fig. 7-9). 
When the animal is confronted with an 
obstacle, the cilia reverse their effective 
beat so that the cell moves backward a 
short distance, turns slightly, then moves 
forward again. If it meets the obstacle 
again, the process is repeated until the ani- 
mal passes around and goes on its way 
(Fig. 7-10). This is known as an avoiding 



In order to discover how the ciHa operate 
in such perfect coordination, biologists have 
made careful studies of the mechanism in- 
volved. It has been found that the cilia 
are attached to one another by tiny fibrils 

ments. Clearly something similar to the 
nervous system in multicellular animals ex- 
ists in Paramecium, which makes it possible 
for this tiny cell to carry on in such a com- 
plicated manner. 

contractile vacuole 
food vacuole 


oral qroove 







canal of c. vacuole 


Fig. 7-7. Paramecium with internal parts shown in detail. 

just beneath the pellicle. The fibrils con- 
centrate at a focal point in the region of 
the gullet where the protoplasm must form 
the equivalent of a miniature "brain." If 
this is destroyed experimentally, the cilia 
fail to beat in a coordinated manner, and 
the animal loses all control of its move- 

Ingesting of food 

A simple experiment can be performed 
to demonstrate how paramecium feeds. 
Some yeast cells that have been heavily 
stained with Congo red ( a dye ) are placed 
on a glass slide containing a drop of para- 



mecia. By studying the region of the gullet, 
one can see the cilia beat in such a way as to 
pass the yeast particles along its oral groove 
and down into the gullet. There they are 

Fig. 7-8. Path taken by Paramecium when moving 
freely through the water. 

Fig. 7-9. Movement of Paramecium when attachecJ at 
posterior end. 

rounded up into a mass which finally 
pinches off into the cytoplasm as a food 
vacuole. Once in the cytoplasm the yeast 
cells remain deep red for a time, but gradu- 
ally begin to turn blue as they approach the 
anterior end. This means that the contents 
of the food vacuole are alkaline at first 
(Congo red is red in alkali, blue in acid), 
just as is the case in the mouth of man. As 
digestion proceeds, the vacuoles become 
acid as indicated by the blue color, remi- 
niscent of the condition found in the hu- 
man stomach. The digested material passes 
through the wall of the vacuole and out 
into the protoplasm where it is metabolized, 
the same process that was noted for 
amoeba. Finally the undigested portions 
left in the vacuole pass through a tiny 
opening to the outside, the cytopyge, which 
is equivalent to an anus in higher animals. 
Some species of ciliates are able to re- 
ceive nourishment from dissolved organic 
matter in the medium. In fact, one species, 
Tetrahijmena geleii (Fig. 7-1), a small par- 
amecium-like ciliate, grows in a culture 
medium entirely free from bacteria or other 
microorganisms. This tiny animal has about 
the same food requirements as higher ani- 
mals, including man himself. Recent experi- 
ments have shown that to grow it requires 
a diet containing amino acids, sugar, salts, 
and vitamins. This seems to indicate that 
even a single-celled animal maintains met- 
abolic processes almost as intricate as 

Respiration and excretion 

Respiration and the excretion of nitrog- 
enous wastes take place in paramecium 
much the same as in amoeba. The contrac- 
tile or pulsating vacuoles which lie at either 
end of the cell contract alternately at about 
15-second intervals. Several radiating ca- 
nals empty into each contractile vacviole, 
being most obvious when the vacuole is 
nearly empty. Each vacuole discharges its 
contents to the outside through a minute 
pore in the pellicle. The rate of contraction 

responds io on 
aKctric current 

^ ^ ^ 
■< — 

Fig. 7-10. Paramecium responding to various conditions in its environment. 

Fig. 7-1 1 . Conjugation and asexual reproduction by binary fission in parameciur 



varies with temperature, activity of the ani- 
mal, and the concentration of salts in the 
surrounding medium. As in the amoeba, the 
contractile vacuole functions normally as a 
bailer to rid the animal of excess water 
constantly entering the cell. 


Paramecium is very sensitive to the rela- 
tive acidity and alkalinity of its environ- 
ment (Fig. 7-10). It responds positively by 
going toward an acid environment, even 
though the acid may be of sufficient 
strength to destroy it. If a drop of weak 
acid, for example, acetic acid, is placed on 
a slide containing paramecia, they will 
move toward the acid region and remain 
there. If they get away from this region 
and approach a less acid surrounding, the 
paramecia give the avoiding reaction, and 
thus return to the more acid medium. This 
sensitivity is probably owing to the fact 
that, in large numbers, paramecia give off 
carbon dioxide in such quantity that the 
water in the immediate vicinity becomes 
weakly acid. 

Paramecium selects a temperature opti- 
mal for its activities, usually around 25° C 
(Fig. 7-10). If given a choice, it seeks this 
temperature. If placed in an electrical field 
of direct current, it responds in a very defi- 
nite manner, always orienting itself witli 
respect to the flow of the current (Fig. 
7-10). It moves toward the negative pole, 
indicating that externally it is positively 


Paramecium, as in the case of amoeba, 
maintains its numbers by dividing trans- 
versely across its long axis (Fig. 7-12). The 
first sign of division is a change in shape of 
the nuclei. Paramecium has two nuclei, a 
large macronucleus and a small micronu- 
cleus; it is not quite clear how these differ 
in regard to function. When the nuclei have 
divided, a second gullet and two more con- 
tractile vacuoles form. Other structures are 

F!g. 7-12. Paramecium divides by binary fission as 
shown in this photograph of stained specimens. 

also duplicated before the two daughter 
cells separate. After a growth period they 
are ready to divide again. Under optimum 
conditions division occurs about every six 
to twelve hours. If division occurred three 
times a day and all individuals survived, 
their bodies would fill all the oceans of the 
world within a month. Under natural con- 
ditions they very soon cease dividing, be- 
cause of accumulation of waste products, 
lack of food, low temperatures, desiccation, 
or falling prey to other aquatic animals. 

When placed in a suitable culture, para- 
mecia periodically undergo a sexual process 
called conjugation (Figs. 7-11 and 7-13). 
Just why they do this is not clearly under- 
stood, but it can be induced by reduc- 
ing the bacterial food supply. During the 
process two individuals interchange micro- 
nuclear material, which means that genetic 
factors are involved. This has the effect of 
sexual reproduction, although there is no 
increase in numbers, as the term repro- 
duction would imply. Actually conjugation 
seems to be unnecessary for division. Pro- 
fessor L. L. Woodruff, some years ago, sep- 
arated paramecia after each fission so that 
conjugation was impossible. He followed 
through 15,000 generations over a period of 



Fig. 7-13. Conjugating paramecia which have been 
killed and stained. Note the large macronuclei and 
the smaller micronuclei. 

25 years without conjugation ever occur- 
ring. Just how important conjugation is to 
the success of the race can only be conjec- 

Research has recently shown that within 
a single species there are so-called "mating 
types," which are distinguished by their 
conjugation reactions. When known mating 
types are mixed in a culture, the animals 
form large clumps. They seem to become 
covered with a sticky substance which 
causes them to adhere to one another. After 
an hour or so they pair off, rather securely 
attached at their oral grooves (Fig. 7-11). 
Very shortly the micronuclei in both ani- 
mals divide twice in rapid succession; at 
the same time the macronucleus starts to 
disintegrate and disappear. Three of the 
four nuclei in each cell degenerate, leaving 
a single one which immediately divides 
into a lar^e immotile micronucleus and a 

small motile one. The latter then moves 
across a kind of protoplasmic bridge be- 
tween the two animals and fuses with the 
immotile micronucleus of the opposite ani- 
mal, a process resembling fertilization in 
higher forms. The animals then separate. 
Three successive divisions of the fused nu- 
cleus follow, producing eight nuclei, four of 
which grow into macronuclei, three degen- 
erate, and one remains as the micronucleus. 
The paramecia and their micronuclei then 
divide twice, eventually producing four par- 
amecia from each ex-conjugant. This is 
followed by ordinary fission (Fig. 7-11) un- 
til conjugation is once more induced. 

Recently several different groups of "mat- 
ing types" have been discovered by Jen- 
nings and his co-workers. Individuals from 
different groups do not conjugate, nor do 
individuals from the same type within the 
group. They mate only with other types of 
their same group. From these studies much 
is learned about the mechanics of inherit- 
ance, an important phase of genetics which 
will be discussed in the chapter on that sub- 


This brief introduction to two different 
Protozoa gives us a basis for considering the 
importance of the group as a whole in re- 
spect to numbers, variety, and classification. 
Over 30,000 Protozoa have been described 
as distinct species, and in numbers of indi- 
viduals they exceed all other animals. They 
live in water of all kinds, in soil and dust, in 
and on the bodies of plants and animals. 
Some of them cause the most destructive 
diseases known to man, malaria, for exam- 
ple. Most Protozoa are free-swimming, al- 
though some are sessile; most live singly, 
some form colonies. Many form a source of 
food for aquatic animals such as fish, but 
they are of little value to man, except a few 
which are useful in sewage treatment. They 
are such a large and varied group of animals 
that some biologists have considered plac- 


ing them in a group larger than the phylum, chlorophyll. Because it is able to live on the 

that is, a subkingdom. simple elements that plants utilize, and at 

The Protozoa are divided into four the same time possesses certain animal char- 
classes, based on their means of locomotion, acteristics of behavior, euglena is thought 
In the probable chronological order of their to be intermediate between the plant and 
appearance in the evolution of life on earth, the animal world, and is frequently re- 
they are: class Mastigophora — those that f erred to as a plant-animal type, 
move by means of flagella; class Sarcodina The anterior end of euglena is usually 
or Rhizopoda — those that employ pseudo- more blunt than the posterior end, in fact, 
pods; class Sporozoa — those that have no the latter is pointed in some species. There 
clearly defined method of locomotion; fi- is a gullet at the anterior end, the walls of 
nally, class Ciliophora — those that move by which give ri-se to the fiagellum, and lying 
means of cilia. The student is already famil- near the gullet is the contractile vacuole. 
iar with representatives of two of these In the immediate region of the gullet is the 
classes, the Sarcodina (amoeba) and the stigma or eye-spot, a conspicuous red dot 
Ciliophora (paramecium). Undoubtedly which apparently functions in aiding the 
there was some overlapping, and there is no cell to find the proper light intensity for 
assurance as to which types actually pre- photosynthesis. In order to manufacture its 
ceded which others, except in a general way. own food, euglena must receive the proper 
For our purposes the order above will be amount of light, hence the significance of 
followed, that is, we shall consider the Mas- the stigma. Euglena can, however, live in 
tigophora as the most primitive and the the dark providing nutrient materials are 
Ciliophora as the most complex. present in the surrounding medium, in 

which case it absorbs its food directly. 

Class Mastigophora Under favorable conditions euglenas are fre- 

This is a widely diverse group of Proto- quently found in such numbers as to pro- 
zoa in which some members are colored duce a green scum at the surface of the 
and live independently like plants, whereas water. As some species grow old they pro- 
others are colorless and require food from duce a red pigment called haematochrome 
the outside, such as parasites living in the in their cytoplasm. If this happens when 
intestinal tract of termites. they exist in great numbers, a visible red 

Colored flagellates. Although there are layer appears on the surface of the water, 

wide divergences in structure and habitat This has sometimes given rise to the name 

of flagellates, a brief description of Euglena "bloody pools." 

(Fig. 7-14) will help in understanding the Euglena divides longitudinally, that is, in 

group as a whole. an anterio-posterior direction, splitting the 

Anatomically, euglena is quite different cell into two equal parts (Fig. 7-14). In a 
from amoeba. It has a rather definite gen- rapidly growing culture, cells can be ob- 
eral shape, which is something like a spin- served in all stages of division. Because this 
die, although it is sufficiently elastic to protozoan can be grown in sterile cultures, 
be able to undergo animal-like movements that is, free from all other microorganisms, 
when confined to a small space (Fig. 7-14). it has been used in experimental work in 
It moves by means of a single, hair-like, an effort to determine its basic nutritional 
vibratile fiagellum ( plural, ftagella ) which, needs. It is an example of the increasing use 
when active, pulls the organism through the of Protozoa as experimental animals in fun- 
water in a spiral path. In its cytoplasm damental biological research, 
euglena bears bodies known as chloroplasts, Other colored flagellates. All kinds of 
which contain the green plant pigment, fresh water as well as the oceans are teem- 



animal like 

Ploot-oniinal type 

ir> presence of light. 

Nutrients ore 
necessary in 
the dark. 

F!g. 7-14. Euglena in detail and undergoing some of its life processes. 



Noctiluca Oyrodinium 

Goniodomo Gymnodiniurn 

Fig. 7-15. Various types of marine flagellates. 

ing with free-living flagellates. The dino- 
flagellates, for example, live in the ocean 
for the most part and constitute a large por- 
tion of the diet for small Crustacea and 
other animals. Some members, such as Noc- 
tiluca ( Fig. 7-15 ) , possess luminescent prop- 
erties which cause them to glisten in the 
dark when the surface of the water is dis- 
turbed. This is a particularly attractive 
sight in the wake of a boat. Another inter- 
esting dinoflagellate, Gijmnodinium brevis 
(Fig. 7-15), has appeared several times dur- 
ing the past hundred years along the Flor- 
ida coast in extremely large numbers (50,- 
000,000 per liter — a normal count is about 
100,000 for all kinds of protozoans). Fur- 
thermore, this protozoan apparently se- 
cretes a by-product which is lethal to all 
other kinds of animal life in the vicinity. In 
1947 half a billion fish were destroyed along 
the Florida coast, presumably by this toxin. 

Fresh water, particularly that containing 
a considerable amount of organic decompo- 
sition, supports a large variety of flagellates 
(Fig. 7-16). Many of them play a very im- 
portant function in providing food for 
aquatic animals, especially during their 
early life when their mouths are so small 
that no other food but a protozoan could be 
ingested. Without these tiny animals there 
would be no fish in many of our lakes and 
streams. Some, like Haematococcus pliivia- 
Jis ( Fig. 7-16), are bright green in color and 
can reach unbelievable numbers in small 
pools. Like euglena, they produce haemato- 
chrome at certain times of the year, impart- 
ino; a reddish color to the water. In some 
Alpine passes they have been responsible 
for the so-called "bloody snow," a familiar 
sight to mountain climbers. 

Colorless flagellates. Some colorless 
forms, such as Peranetna and Chilo^nonas 




Hemotococcus Peramma 

Fig. 7-16. Various types of fresh-water flagellates. 


(Fig. 7-16), live in stagnant water where 
they feed upon bacteria or other smaller 
Protozoa. Although in its normal environ- 
ment Chilomonas seems to live on a com- 
plex diet, it can be grown in a test tube on 
a diet consisting of ammonia, as a source 
of nitrogen, and carbon dioxide, as a car- 
bon source. Here apparently is an organism 
that in nature lives much like an animal but 
in the laboratory can be forced to live like 
a plant, or even more simply, since it does 
not require nitrogen in the form of nitrates. 
This may mean that Chilomonas has a full 
set of enzymes to utilize very simple food 
sources for the construction of its proto- 
plasm, but since it is not forced to use them 
normally, has taken up the animal type of 
nutrition. This would mean that this small 
organism had a very simple beginning and 
has not changed much through millions of 
years of evolution. 

Most colorless flagellates live as single 

Fig. 7-17. This is a colonial colorless flagellate, Synura 
uvella, which conveys bad odors to water supplies 
by the release of aromatic oils when its body de- 
composes. This is a plastic model. 



cells, although some live in colonies, such 
as Symira (Fig. 7-17) and Codosiga (Fig. 
7-16). The latter form possesses a collar at 
the anterior end of each cell from which the 
flagellum emerges. With this single excep- 
tion, collared cells such as these are found 
only among the sponges. Food is brought to 
the walls of the cell by the flagellum, then 
taken into the cell body to form a food vac- 

Parasitic flagellates. There is a wide va- 
riety of flagellates that have made their way 
into the body cavities and the blood streams 
of almost every group of animals. They are 
particularly common in the blood of verte- 
brates, including man. The most common 
offenders in this respect are the trypano- 
somes (Fig. 7-18), tiny leaf -like, elongated 
cells. A single flagellum, which lies at the 
outer edge of the membrane, undulates as 
the organism is propelled through the body 
fluids of the host. Frequently the parasite is 
transmitted from one vertebrate to another 
by means of an intermediate host, either by 
a blood-sucking insect or some other arthro- 
pod. Some of the diseases caused by this 
group of parasites are African sleeping sick- 
ness and kala azar, an oriental disease 
caused by Leishmania donovani. 

Kala azar has had devastating effects on 
the populations of North China, various 
parts of India, the Sudan, and South Amer- 
ica. It has occurred primarily in the past, al- 
though even today it is rampant in many 
sections of the world. With the advent of 
knowledge concerning epidemiology, diag- 
nosis, and treatment, many of the evil ef- 
fects of this disease have been greatly 

Leishmania is a tiny ( 2-4 microns ) ovoid 
parasite which attacks the cells of almost 
all of the tissues of the body, particularly 
the large cells of the tissues lining the circu- 
latory system, both blood and lymphatic. 
Upon entering the cells it multiplies (Fig. 
7-19) until eventually the host cell bursts 
and the released parasites attack other cells. 
Some enter the blood stream where thev are 

Fig. 7-18. These tiny leaf-like trypanosomes live in 
the blood of vertebrates and other animals. This is a 
blood smear showing the parasites among the red 
blood cells. 

picked up by the intermediate host, the 
sand fly (Phlebotomiis) . In the gut of this 
insect they become flagellated and change 
considerably in shape. When the insect 
bites another person, some time after it has 
received the parasite, the flagellated fonns 
are injected directly into the blood where 
they attack the lining cells of the blood ves- 
sels and the cycle is complete. Like most 
blood-sucking insects, the sand fly intro- 
duces a small amount of saliva, which has 
an anticoagulating effect on the blood. If 
this were not the case, a blood clot would 
shortly interrupt the anticipated meal. 
Therefore, the parasite is inadvertently in- 
troduced along with the saliva, through no 
"intent" on the part of the sand fly. 

The disease runs its course in a matter of 
months or several years, frequently ending 
in death. It has been shown that the flies 
also bite dogs, which in turn act as reser- 
voirs for the disease. So the problem of 
eradication consists not only of preventa- 
tive measures and treatment of infected 
persons, but also control of the dog popu- 
lation of any affected community. The most 



staqe ottackinq 

"o/ flaqellate staqe 

Fig. 7-19. Life cycle of a \.Biihmania causing kala azar. 

common method of prevention is the de- found that sleeping sickness, common 

struction of the breeding places of the fly among the populations of certain parts of 

as well as the fly itself. equatorial Africa, was also caused by a try- 

A trypanosome is the cause of nagana, panosome. The fact that the tsetse fly car- 

a disease of domestic animals. It has been ried the infection was long known, even 




before the cause was discovered. By 1909 
it was established that the tsetse fly was 
not only a mechanical vector but an inter- 
mediate host in which the parasite went 
through a definite part of its life cycle. 

The trypanosome is sucked up into the 
gut of the fly during its blood meal ( Fig. 7- 
20). Here it undergoes some changes in 
morphology and eventually makes its way 
into the salivary glands, a common proce- 
dure among parasites of blood-sucking in- 
sects. Some time later, if the fly bites 
another person, the parasite is injected 
alone with the saliva. It remains free in the 
blood for a time but finally makes its way 
into the fluid surrounding the brain and 
cord. In this stage the metabolic products of 
the parasite have a paralyzing effect on the 
person, eventually causing "sleep" from 
which he usually never awakens. 

Vast regions of Africa are denied man be- 
cause of the ravages of this disease. Like so 
many diseases, the parasite is ineffective 

against the local wild animals, which, how- 
ever, act as reservoirs, always keeping the 
parasite circulating in goodly numbers. For 
this reason the wholesale destruction of the 
tsetse fly is the only satisfactory control 
measure; this is not at all impossible now 
with the recent discovery of such effective 
insecticides as DDT. 

The future development of a large fertile 
area of Africa must await the control of this 
disease. This has become an urgent need in 
the face of an expanding world population. 
It was once thoucrht that the white man 
could not thrive in the tropics, but data col- 
lected during the past war, when large 
numbers of white men lived in tropical 
countries for several years, have disproved 
this conjecture. There is no reason why he 
cannot be as successful in the tropics as else- 
where, once we gain control of the tropical 
diseases to which the white man is very re- 
ceptive. Much of the backwardness of other 
races living in the tropics is due to the mur- 




Actinopbrys sol 

Fig. 7-21. Various types of fresh-water sarcodinids. 



Fig. 7-22. This amoeba-like protozoan {Arcella) carries 
around a "house" into which it may retreat when 
its life is endangered. The shell is brown in color 
and, because the opening through which the animal 
passes lies in the center, it resembles a doughnut. 
Note the long pseudopods protruding out from the 
shell. This is a plastic model. 

derous attack of parasites of all kinds, rather 
than any fundamental inability of the peo- 
ple themselves to thrive. 

Class Sarcodina 

Although members of this class resemble 
the amoeba to some extent, there is wide 
variation in form and structure in the group 
as a whole. Among the fresh-water forms 
there are those, such as Diffliigia (Fig. 7- 
21), that build houses for themselves. This 
tiny animal gathers grains of sand and ce- 
ments them together to form a pear-shaped 
outer covering into which it may retreat 
when in danger. PJagiophrys and Arcella 
( Figs. 7-21 and 7-22), likewise, build houses 
for themselves, but in this case they are se- 
creted by the animals. When observed 
through the microscope, the shell of Arcella 
resembles a doughnut. Corresponding to the 
hole in the doughnut is the opening through 

which the amoeboid form passes as it re- 
tracts or extends itself from the shell. 

Another fresh-water sarcodinid of inter- 
est is the "sun animalcule," Actinophnjs sol 
(Fig. 7-21), which resembles a miniature 
sun when it is floating in the water. The 
radiating, ray-like pseudopods seem to have 
a paralyzing effect upon other Protozoa, 
such as euglena, which serve as a food 
source. There are many related species of 
this spectacular protozoan. Together they 
constitute the order Heliozoa, a group that 
is commonly found in the oceans of the 
world. One, Oxnerella (Fig. 7-23), is a par- 
ticularly beautiful heliozoan. 

A large group of forms closely resembling 
Heliozoa form the order Radiolaria. These 
are also distributed throusihout the oceans 
of the world and float near the surface of 
the water. Most of them possess a siliceous 
skeleton which sinks to the ocean floor 
when the animal dies, forming a thick, 
mucky layer called "radiolarian ooze." This 
is particularly extensive in the Pacific and 
Indian Oceans. Skeletons of these animals 
are also found in rocks and have been used 
by geologists in learning about the history 
of the earth. 

Marine sarcodinids that have even greater 
significance to the geologists are found in 
the order Foraminifera, which secrete shells 
of almost pure calcium carbonate. While 
some, such as Boderia (Fig 7-23), possess 
extremely thin, delicate outer coverings of 
this substance, most of them secrete a heavy 
shell, for example, Discorhis and Peneroplis 
(Fig. 7-23). These many-chambered, snail- 
like shells, are produced as the animals grow 
larger and are perforated with tiny holes 
through which the fine pseudopods project. 
When they die their skeletons, like those of 
the Radiolarians, form a "globigerina ooze" 
( named after the most dominant form, Glo- 
bigerina). This eventually becomes chalk 
many hundreds of feet thick. The Cliffs of 
Dover are an outstanding example of this 
phenomenon. Wherever this chalk appears 
on land, one can be certain that it was once 







Oxnerellq Pencroplis 

Fig. 7-23. Various types of marine sarcodinids. 

at the bottom of the sea. While the globig- 
erina ooze covers much of the ocean floor 
and chalk has formed in some places, par- 
ticularly along the shorelines, there has usu- 
ally been a mixture of other deposits. Glo- 
bigerina deposits appear in certain strata 
of the earth's surface and have a definite 
relation to the formation of petroleum. 
Therefore, knowledge of foraminiferans has 
been useful to geologists in predicting the 
location of oil deposits. 

Parasitic amoebae. Some amoebae, like 
some flagellates, have become adapted to 
life in the body cavities of many different 
animals, including man. Of the half dozen 
or so amoebae that inhabit the various cavi- 
ties of man, only one, Endameba histoly- 
tica, causes any great harm. This amoeba is 
responsible for the well-known amoebic 
dysentery. Though not common in the pop- 
ulation as a whole, it became an important 
disease among our armed forces fighting in 

the tropics in the past war. Native villages 
were often infected 100 per cent, providing 
a rich source of parasites for spreading the 
infection to newcomers. 

The parasites are transmitted from per- 
son to person by contaminated food and 
water (Fig. 7-24). There may also be an 
indirect transfer by way of flies and other 
insects that pick up the infective stages on 
their feet and proboscis, carrying them di- 
rectly to food and water. Hence, the obvious 
method of control is to instigate sanitation 
in respect to human excreta and to destroy 
the flies. Both of these measures can easily 
be accomplished in civilized communities, 
but not among primitive peoples where 
both the knowledge and the facilities are 

Some notorious outbreaks of amoebic dys- 
entery have occurred in the United States 
which have been traced directly to faulty 
plumbing. Studies among groups in re- 



Fig. 7-24. Life cycle of the dysentery amoeba (Endameba histolyfica). Infective cysts are carried on the hands of 
food handlers and thus transmitted directly to uninfected people on uncooked food. The trophozoites emerge 
from the cysts in the intestine where they multiply and feed on the tissues and blood of the host, thus producing 
serious illness. 

tarded areas of this country show an infec- 
tion rate as high as 23 per cent, although 
for the nation at large it is around 5-10 per 
cent. Varying degrees of success have been 
achieved with a great many substances in 
attempts to combat the disease. Of the anti- 
biotics, penicillin and Chloromycetin have 
recently proven the most effective in exper- 
imental animals. 

Class Sporozoa 

Members of this group possess no appar- 
ent means of locomotion and thev lack con- 

tractile vacuoles. They reproduce asexually 
by multiple fission, and they are all para- 
sites. At some stage in their complex life 
cycle they produce sex cells, macro- and 
microgametes, which fuse in the formation 
of zygotes. The asexual stage produces the 
sexual stage, which in turn gives rise to the 
asexual phase, thus completing the cycle. 
This alternation of sexual and asexual gen- 
erations is spoken of as metagenesis. We 
will have occasion to study this biological 
phenomenon in metazoan animals a little 
later. One or both of the asexual and sexual 



phases are spent in the body of a plant or 
an animal, and throuo;h transfer from one 
to the other the cycle is kept going. Many 
diseases are caused by this group of Proto- 
zoa, the most significant of which is malaria. 

The malarial parasites (Plasmodium vi- 
vax is the most common ) infect large num- 
bers of warm-blooded vertebrates besides 
man. In fact, the life cycle was worked out 
oriejinally on birds by Ronald Ross in 1898. 
The widespread occurrence of the disease 
in human populations is indicated by the 
fact that over 300 million people are in- 
fected all the time. It has been estimated 
that 3 million die of malaria each year — 
over half of all the deaths in the world. 
This certainly places it in the number one 
position among deadly diseases. These fig- 
ures come as a surprise to most Americans 
because we now have the disease under 
control, although a hundred years ago it 
was responsible for a great many deaths in 
the South. During World War II it once 
again became a very important health prob- 
lem for men in the tropics, and the large 
number of men continuing to suffer from 
the disease attests to the fact that we were 
not altogether successful in our preventive 

Two factors are necessary for the propa- 
gation of malaria, a large population of the 
appropriate species of Anopheles mosquito 
and infected humans (Fig. 7-25). Only fe- 
male mosquitoes bite. In order to become 
infective the female anopheles must bite a 
person suffering from the disease and with- 
draw blood that contains the parasite in 
a particular stage called the gametocyte. 
There are two kinds of gametocytes, male 
and female, both of which must be taken 
into the stomach of the mosquito, where 
each type of cell undergoes certain modifi- 
cations. One remains pretty much as it 
is, producing a single large macrogamete, 
whereas the other produces 6 or 8 smaller 
motile, threadlike cells or microgametes. 
The macro- and microgametes unite in pairs 
to form zygotes which are able to bore 

through the stomach wall under their own 
power. In the outer part of the stomach 
wall each zygote multiplies many times, 
producing a great many tiny infective sporo- 
zoites. These swollen zygotes protrude from 
the outside walls of the mosquito's gut like 
tiny beads. They puzzled Ross when he first 
saw them, and one can imagine his surprise 
when he squeezed them and saw thousands 
of spindle-shaped bodies emerge. Normally 
they burst into the blood which fills the 
space between the gut and the body wall, 
and via the blood the sporozoites make 
their way into the salivary glands. With 
each bite of the mosquito from this time on 
sporozoites are injected into the blood of 
the next host. 

After entering man's blood stream the 
sporozoites seem to disappear for a few 
days. This fact has puzzled biologists for 
many years until the recent discovery that 
they undergo their early multiplication 
stages in various tissues of the body, notably 
certain cells of the liver. At any rate, within 
ten days some of the parasites are in the 
blood stream, each entering a red blood 
cell where it grows and multiplies asexu- 
ally. After a remarkably regular period of 
time — namely, 48 hours in P. vivax, the 
most common form of malaria — the in- 
fected red cells burst, each releasing 10-20 
tiny oval bodies called merozoites. These 
immediately enter other cells, and so the 
infection keeps increasing in intensity. 
When there is a sufficient number of in- 
fected cells, the person suffers alternate 
chills and fever with the bursting of the red 
cells at 48-hour intervals. The symptoms be- 
come more intense for the next two weeks 
when either the person is unable to combat 
the infection and dies, or he is able to and 
lives, although intermittent chills and fever 
continue for a lono; time, sometimes for sev- 
eral years. During this time some of the 
merozoites become modified into gameto- 
cytes. If some of these are taken up by the 
mosquito with its blood meal, they pass to 
the stomach and thus complete the cycle. 

36 4o hour6 

Fig. 7-25. Life cycle of the malarial organism, Plasmodium vivax. 



Fig. 7-26. Some ciliates are voracious carnivores. Dicfinium, for example, is able to engulf a Paramecium much 

larger than itself. 

The treatment for malaria has an in- 
teresting history. About 1640 a countess 
visiting in Peru became ill with malaria and 
when given extracts from the bark of a tree, 
since named cinchona in her honor, she re- 
covered. She was so impressed with the 
drug that she brought some back to Europe 

where it was shown to be extremely effec- 
tive in the treatment of malaria. From that 
time to the present quinine, the effective 
drug in cinchona bark, has been used as a 
specific treatment for malaria. It has prob- 
ably saved more lives and relieved more 
suffering than any other drug ever discov- 


Stintor Stylonicbia T«+t*obyi»)«oo Caixbcsium 

Fig. 7-27. Various types of fresh-water ciliates. 



Fig. 7-28. A stalked-ciliate, Vort'icella, in various stages 
of its life history. The two animals to the left are 
undergoing division while on the extreme right is 
the free-swimming stage which enables the species 
to spread to new environments. The fine contractile 
thread which can be seen in the stalk makes it pos- 
sible for them to suddenly become coiled as some 
of the organisms here. This is a plastic model. 

ered by man, the recent antibiotics and sul- 
fas included. During the past war our sup- 
plies of quinine were cut off, so we had to 
rely on substitutes such as atabrine and 
plasmochin. The problem of synthesizing 
quinine went ahead during the war years 
with unrelenting vigor until finally it was 
successfully accomplished by the war's end. 
At present, however, natural sources are 
used since they are cheaper than the synthe- 
tic product. A constant search is made for a 
new specific for malaria and some success is 
reported from time to time. Perhaps the 
ideal one will be found in the future. 

Class Ciliophora 

The members of this class are distin- 
guished by the possession of cilia and two 
or more nuclei. These characteristics were 
observed in paramecium, which is a repre- 
sentative of this group. The class has been 
subdivided according to the arrangement 

of the cilia into four orders, the more in- 
terestino; members of three of these will be 
described briefly. 

Those belonging to the order Holotricha 
possess evenly distributed cilia over most of 
their body. Paramecium is typical. 

Didinitim is an interesting member of 
this group because of its carnivorous habits. 
It is oval-shaped, with two bands of cilia 
encircling the anterior and posterior regions 
(Fig. 7-26). Protruding from the ante- 
rior end is a formidable-looking proboscis 
which is an effective organelle for impaling 
paramecia prior to engulfing them. The 
magnitude of this feat can be realized by 
imagining a man eating a full-grown horse 
at one sitting. This is only one illustration 
of the voraciousness of these carnivorous 
ciliates which abound in almost any stag- 
nant water. 

The order Spirotricha includes a large 
variety of diverse ciliates, one of which, 
Spirostomiim, is a veritable giant among 
the Protozoa. This cell reaches a length of 
3 millimeters and can easily be seen with 

Fig. 7-29. Anton leeuwenhoek (1632-1723) was the first 
man to see and describe many Protozoa as well as 
other microorganisms. He was not trained in science, 
but his devotion to the disciplines of the field places 
him among the foremost scientists of his day. 



the naked eye. In fact, when first observed, 
it might be mistaken for a tiny worm be- 
cause of its apparent crawHng movements. 
Another large form, Stentor (Fig. 7-27), 
vase-shaped and colored a beautiful green- 
ish blue, is a spectacular sight for the mi- 

This order also includes several species 
in which the long cilia are fused into stiff 
bristle-like org-anelles called cirri. Cells of 
this type are flattened dorso-ventrally and 
seem to use their cirri in "walking" along 
the substratmn. One of these is Stylotiichia 
(Fig. 7-27), which is about the size of 
Paramecium but whose actions are quite 
different. It moves along in a jerky fashion, 
darting forward and backward, and some- 
times crawling along on the bottom. Pro- 
fessor C. V. Taylor, working with a closely 
related ciliate, Etiplotes, some years ago, 
was interested in how this cell controlled 
the cirri in locomotion. With the use of a 
delicate dissecting instrument he was able 
to cut the tiny fibrils that connect each of 
the cirri with the others. Such an operated 
animal lost control of its cirri and was un- 
able to coordinate its movements sufficiently 
to move in any one direction. Thus he dis- 
covered that even this tiny animal possesses 
some kind of coordinating system resem- 
bling the nervous system of higher forms. 

Ciliates belonging to the order Peritricha 
have their cilia conspicuously arranged in 
the anterior region. Most of these forms 
are vase-shaped and many are stalked. 
Common examples are Carchesium (Fig. 
7-27) and Vorticella (Fig. 7-28). Of the 
two, the latter is more common and is 
familiar to anyone who has persisted in ex- 
amining stagnant water under the micro- 
scope. It was seen and described for the 
first time by Anton Leeuwenhoek in Hol- 
land during the seventeenth century (Fig. 
7-29). Vorticella is usually attached to the 
substratum by means of its contractile stalk. 
When disturbed or sometimes for no ap- 
parent reason, it suddenly contracts and 
at the same time the cilia around the 

Fig. 7-30. Plastic model of a suctorian {Ephelota 

funnel-shaped mouth disappear and the 
entire cell rounds up into a ball. Shortly, it 
emerges again and starts its oral (mouth) 
cilia beating so that food particles floating 
by are wafted into its mouth. 

A peculiar group of Protozoa, the Stic- 
toria (Fig. 7-30), are usually considered as 
belonging to Ciliophora because they 
possess cilia during the young stages, al- 
though they are absent in the adults. In 
their place these animals possess long and 
numerous "tentacles" which are used in 
capturing food (other Protozoa). Like 
Vorticella, the suctorian has stalks and is 
sessile most of its life. 

From this cursory survey of the Protozoa 
one is impressed with the tremendous diver- 
sity of form and habits of life that are avail- 
able to a group of animals even though con- 
fined to one cell. Think of the much greater 
opportunity for diversity when cells are 
aggregated into masses. Our course now 
is to study representatives of succeeding 
phyla where each is more complex than tlie 
preceding, finally terminating with the most 
complex of all animals, man. This great and 
wondrous story should be followed with 
keen interest because it is the way life 
came to where it is today on this earth. 





The sponges constitute the phyhnn 
Porifera, which means "pore bearer," the 
presence of pores being one of the charac- 
teristics of the group. This rather unique 
group of very simple animals is not in the 
direct line of ascent to higher forms and is 
sometimes placed in a separate subking- 
dom, the Parazoa. Sponges possess flagel- 
lated collared cells which resemble some of 
the protozoan forms (Fig. 7-16), indicating 
that they may not be far removed from the 
one-celled group. Digestion is wholly intra- 

cellular and, except for simple epithelia, 
there is no arrangement of cells that can be 
considered tissue. For this reason sponges 
are usually considered to be of the cellular 
grade of organization. 

Living sponges have a gelatinous tex- 
ture, quite different from the familiar bath 
sponge sold on the market. Being sessile, 
they are easily mistaken for plants. Sponges 
assume many shapes, vary in height from 1 
millimeter to more than a meter, and are 
usually drab in color. Some forms, however, 
take on shades of red, yellow, blue, black, 
or green, the last being caused by the green 


Fig. 8-1. A common marine sponge (S/con) which is 
about 1 inch long. The individuals grow in clusters 
and are very commonly found clinging to debris in 
the ocean. 

A B 

Fig. 8-2. Two deep-sea sponges. Their skeletons are com- 
posed of siliceous (glass) spicules. A. Venus' flower 
basket (Euplectella). 6. The glass rope sponge (Hya- 

Fig. 8-3. A fresh-water sponge collected in a Michigan stream; it measures about 14 inches long and is a green- 
gray In color due to the presence of unicellular algae growing in the body cells. 






Fig. 8-4. Anatomy and life history of a simple sponge. 

alga, Chlorella, living in the body cells. 
Their most common habitat is some feet 
below the tidal zone of the sea ( upper and 
lower limits of the tide) (Fig. 8-1), al- 
though some, like the glass sponge (Fig. 
8-2), live as much as three and a half miles 
below the surface of the ocean. Fresh-water 
forms are also known, some of these attach- 
ing themselves to twigs and rocks in the 
streams (Fig. 8-3). 

Morphology of a simple sponge 

In order to have some knowledcre of the 
morphology of the sponge, it is best to 
discuss a simple sponge, Leucosolenia, as 
an example. It lives beneath the low tide 
level in the sea and consists of many slender 
upright tubes which are joined at their 
bases in a many-branched common tube 
(Fig. 8-4). The upright portions are thin- 



walled sacs perforated with hundreds of 
microscopic holes (incurrent pores) and 
one large opening, the osculum (excurrent 
canal) at the upper tip. The cavity of the 
sac is called the spongocoel. Its wall is 
made up of an outer epidermis or skin-like 
layer of flat cells, and an inner continu- 
ous layer of the flagellated collared cells 
( choanocytes ) . A third jelly-like layer, the 
mesenchyme, lies between these two, and 
in this layer several kinds of amoeba-like 
cells, called amoebocytes, are present. This 
layer also contains the skeleton formed by 
spicules, which resemble crystals. In the 
glass sponge the spicules consist of siHceous 
material and in horny sponges, of fibers 
and spongin. In most forms, however, they 
are composed of calcium carbonate. A 
combination of spicules and fibers also 

Scattered among the ordinary epidermal 
cefls are the tubular pore cells, each with 
a central canal or pore. A pore cell, together 
with "helpers" surrounding it, controls the 
flow of water into the sponge. The vigorous 
beating of flagella on the collared cells 
lining the spongocoel causes water to move 
in through the pores and out through the 
osculum. This movement causes a constant 
stream of water, heavfly laden with micro- 
scopic organisms, to pass within reach of 
the choanocytes. The manner of beating of 
the flagella in the collared cells propels tiny 
food particles to the cell body which en- 
gulfs it, forming food vacuoles much like 
amoeba. Any food not needed by the 
choanocytes is passed to the amoebocytes 
for further distribution. Waste products are 
borne out through the osculum in this same 
current of water. 

Sponges in general 

Sponges reproduce asexually by means of 
internal and external buds, as well as sex- 
ually by means of eggs and sperms. Buds 
form on the outside of the sponge and 
sometimes move away, but as often remain 
a part of the parent sponge. During unfa- 

vorable conditions, as in drought or cold 
winters, sponges develop internal buds, 
called gemmules, which are merely masses 
of cells with a hard outer covering. They 
drop to the bottom of the stream or sea 
during these adverse conditions and grow 
into sponges the next season when circum- 
stances are again favorable. 

Some sponges are monoecious (of one 
household), that is, both sexes are present 
in one animal; others are dioecious ( of two 
households), the sexes being found in sep- 
arate animals. There are no special sex 
organs, and the sperms and eggs simply 
develop from certain of the amoebocytes in 
the middle layer or mesenchyme. Fertiliza- 
tion occurs in situ (in place) and is fol- 
lowed by rapid division until a blastula is 
formed with manv flacrellated cells (future 
choanocytes). The flagella are directed in- 
ward toward the central cavity, the blasto- 
coel. A hole appears later and the embryo 
turns inside out, bringing the internal 
flaeellated cells to the outer surface. In this 
stage, called the amphiblastula, it leaves 
the mother sponge through the osculum 
and swims around for a few days, finally 
settlins and attaching itself to a rock or 
some other solid object where it grows into 
a sponge ( Fig. 8-4 ) . The larger cells of the 
larva overgrow the flarellated cells and com- 
pletely surround them. The cell layers of 
the sponge seem to be the reverse of those 
found in higher forms and for that reason 
are not comparable to them. 

Some very interesting cases of commen- 
salism are found among the sponges. They 
occasionally strike up a friendly relation- 
ship with animals such as crustaceans, 
worms, and mollusks. Many animals make 
the channels of the sponges their home; 
even fish as long as 5 inches have been 
found swimming about inside of sponges. 
Certain species of crabs enlist the aid of 
the sponge in camouflage by placing small 
pieces of the sponge on their backs until 
they become attached. Thus the crab is 
amply hidden from its would-be food and 






^ ■ %^ Sycon 

Fig. 8-5. Various types of canal systems among sponges. 

from its enemies. Sponges can become a 
hindrance in oyster beds where they cover 
the oysters and compete for the food 


Sponges have canal systems which vary 
in complexity (Fig. 8-5). The simplest, the 
Ascon type found in Leucosolenia, is 
merely a thin-walled sac. The first step in 
increasing complexity occurs in the Sycon 
type, which has two kinds of canals, incur- 
rent and radial, the latter being lined with 
choanocytes. The most complicated of all is 
the Leucon type, which possesses a vast 
array of multiple tubes and chambers, with 
choanocytes lining only certain restricted 
chambers. Leucon sponges may reach more 
than a meter in diameter. 

Sponges have been used by man for 
cleaning purposes from earliest times. The 
part used in the bath sponge is the skeleton, 
composed largely of spongin, a protein. It 
has many fine fibers which, through capil- 
larity, have remarkable water-holding prop- 
erties. Sponges for commercial use are 
found most frequently in the warm seas 
such as the Mediterranean, the Gulf of 
Mexico, and the Gulf of Florida. In prepa- 
ration for market, sponges are pried loose 
from rocks by divers or dredges, and the 
living portion is allowed to dry and decay. 
They 'are then beaten, washed, and finally 
bleached. Needless to say, the bath sponge 
has considerable commercial value. In re- 
cent years it has even become necessary to 
cultivate it in order to prevent it from be- 
comins extinct as the result of the constant 
ravages of man. Two million pounds of 
sponges are taken annually, even though 
the sponge must compete with its rubber 
and plastic imitations. 

In review, we may think of the sponges 
as "blind alley" animals that came to their 
present state millions of years ago and have 
"stood still," failing to evolve any higher on 
the animal scale. They are sufficiently well 
adapted to their environment to carry on in 
their "primitive way and they probably will 
continue for millions of years to come. 
Since they are not on the direct line of 
ascent to higher forms, we must leave them 
in their isolated position without further 
reference to them and pass on to the next 



group which has much more significance in 
our story of the rise of animal Hfe on earth. 


The first true metazoan animals to have 
their somatic cells organized into definite 
tissues are the two-layered, sac-like coelen- 
terates. Greater differentiation of somatic 
cells and well-established division of labor 
make this form considerably more complex 
than the sponges. 

Coelenterates possess special epithelial 
cells, called cnidoblasts, which produce the 
nematocysts used for offense and defense. 
In many coelenterates there are two types 
of individuals, the polyp, representing the 
asexual phase of the life cycle, and the 
medusa, the sexual phase. In their life his- 
tories each generation successively gives 
rise to the alternate type, a phenomenon 
called metagenesis. The asexual polyp is 
tubular in shape, with a mouth at one end, 
surrounded by tentacles richly supplied 
with nematocysts. The other end of the 
tube is closed and forms an attachment 
organ, the foot. These animals are usually 
sessile or nearly so. The free-swimming 
sexual medusa is a delicate transparent 
animal, shaped like an umbrella. Around 
the periphery or edge of the umbrella are 
tentacles, which are also heavily fortified 
with nematocysts. Both the polyp and the 
medusa have primitive muscle fibers which 
make movement possible. 

The coelenterates are water-inhabiting 
animals, most of them marine. One form, 
hydra, has invaded fresh water and is very 
successful, being found in nearly all the 
ponds and streams of the world. The large 
marine jellyfishes and the sea anemones 
are members of this group in which the 
asexual generation has been greatly re- 
duced. In both the sea anemones and their 
close relatives, the corals, the asexual forms 
have been completely lost. 

With the exception of some jellyfish, 
which may annoy bathers by the stings of 

their nematocysts, and the coral animals, 
which are used for jewelry, the group as a 
whole has little direct significance to man. 
There are three classes of coelenterates 
(Hydrozoa, Scyphozoa, Anthozoa) whose 
characteristics can best be understood by 
studying two representatives. Hydra and 
Obelia, of the class Hydrozoa. Members of 
the other two classes are discussed briefly 
later in the chapter. 


This tiny animal may be found cling- 
ing to underwater vegetation in nearly any 
fresh-water pond, lake, or stream ( Fig. 8-6 ) . 
At some periods of the year they may be so 
numerous in swift-moving streams as to im- 
part a gray color to the rocks to which they 
are attached. Hydra moves about very little, 
hence it must seek its food by means of 
its long tentacles. Though the column of the 
body may be less than half an inch in 
length, the tentacles in some species may 
reach 10 to 12 inches. The cylindrical body 
may extend so that it is many times longer 
than its diameter or it may contract into 
a pear-shaped, compact body with the 
tentacles resembling tiny stumps. The hol- 
low tentacles surround a raised conical hy- 
postorne, in the center of which is the 
mouth. The mouth opens into the digestive 
cavity, the coelenteron or gastrovascular 
cavity, which continues into the tentacles 
( Fig. 8-7 ) . At certain times of the year small 
swellings, the sex organs, appear on the 
external walls. At other times or, in some 
cases, at the same time, small buds form 
on the sides which grow into tiny hydras, 
and finally detach themselves to grow into 
adults. Ciliate Protozoa are found crawling 
over the surface of the body of most hydras. 
Just what their relationship is to the hydra 
is not known. Perhaps they play the same 
part for hydra that a flea does for a dog. 
It might be expected that in the evolu- 
tion of Metazoa the primitive body organ- 
ization would be very much as it is in 
hydra. When cells group themselves to- 












•-1 :..; 
1 { ; : 



Fig. 8-6. Hydra often "hangs" from the water surface. A portion of the body wall is magnified here in order to 
distinguish its cellular structure. Note the variety of cells in both the epidermis and gastrodermis. 

gether, there is a need for some cells to 
protect the entire group from the outside 
world, others to locate food, and still others 
to detect the possibility of danger. This is 
observed in hydra. The outer layer, the 
ectoderm, sometimes called the epidermis, 
is made up of cells which form a protective 
covering. Some of these cells have an inner 
contractile portion which enables them to 
serve as muscle cells. Finally, scattered 
among the cells of the epidennis are spe- 

cialized sensory cells. The inner layer of 
cells, the endoderm or gastrodermis, pro- 
vides all other cells with nourishment. 
These tall, glandular cells secrete the en- 
zymes which digest food that is brought into 
the coelenteron (Fig. 8-6). These cells also 
possess muscle fibers at their bases which 
run at right angles to those in the epidermis. 
It is by the combined action of these muscle 
cells with those of the ectoderm that hydra 
is able to contract and extend itself. Lying 


volvcnt typa 

copsul* odhasive 


(undischarged nemotocysts) 


Fia 8-7 Hydra feeding on a small crustacean. One tenia 

charged nematocysts in detail. 

cle is enlarged to show the undischarged and the dis 



Fig. 8-8. Hydra somersaulting. 

between these two layers, the ectoderm and 
the endoderm, is a thin, non-celkilar layer, 
the mesogloea, which lends support and 
holds all the cells together. Lying embedded 
in both the ectoderm and endoderm are the 
nerve cells, which are connected by minute 
fibrils to the sensory and muscular cells. 
They function in coordinating the activity 
of all the cells. The nerve cells form a net- 
work, called the nerve net, which connects 
all parts of the animal, although there is no 
centrally located mass which could in any 
way be compared to the brain of higher 
forms. External stimuli initiate impulses in 
the sensory cells which are conveyed to 
the nerve net and through it to the con- 
tractile fibrils. This is the simplest form of 
a central nervous system, but it contains the 
fundamental elements of which all higher 
nervous systems are built. 

An interesting and intricate part of 
hydra's response mechanism is the cnido- 
blast, with its contained nematocyst. 
Cnidoblasts are located in nests or "batter- 
ies" along the tentacles, for the most part, 
although they may occur over the entire 
body with the exception of the foot, or 
basal disc, on which the hydra "walks." 
Nematocysts are derived from the interstitial 
cells and are usually arranged with one 
large and several small ones in each bat- 
tery. There are four different kinds of 
nematocysts, each having a different use. 
The largest and most conspicuous type is 
the penetrating or stinging nematocyst, 
which upon discharge pierces the body of 
small crustacean or other aquatic animals 
that happen to touch the tentacles (Fig. 
8-7). This nematocyst contains a hollow. 

coiled thread which everts through the 
trigger-like action of the cnidocil, a slight 
projecting bristle, when it is touched or 
stimulated in some other way. It is ejected 
with such force that it penetrates the soft 
and even some hard parts of the prey, in- 
jecting a small amount of poison which has 
a paralyzing effect on the victim. Once 
paralysis sets in, the tentacles move in a 
manner that draws the prey into the mouth 
and thence into the coelenteron. Other types 
of nematocysts function in a mechanical 
rather than chemical manner. When dis- 
charged, some wrap their threads about a 
portion of the attacked animal and hold 
it securely. Others fasten themselves to a 
portion of the substratum and by contrac- 
tions of the tentacles make possible a slow, 
somersaulting type of locomotion (Fig. 

A second method by which hydra moves 
from place to place in search of food is to 
"shuffle" along on its basal disc by means of 
special cells located in this region. Some 
species are able to secrete a bubble of air at 
the basal disc, which carries them to the 
surface where they float upside down. In 
this position it is not uncommon for the 
tentacles to stretch out into thin threads 
as much as 10 or 12 inches in length. 

Most of the actions of hydra are related 
to food-getting. A hungry hydra responds 
readily when a small crustacean or worm 
comes within reach of its tentacles. If meat 
juices are placed in the surrounding water, 
it responds by increased extension and 
contraction of its entire body. On the other 
hand, a well-fed hydra responds very little, 
if at all, to the presence of food. 



Fig. 8-9. Regeneration in hydra. 

The presence of food in the coelenteron 
stimulates the gland cells to secrete diges- 
tive enzymes into the cavity where the soft 
parts of the ingested animal are partially 
digested (extracellular digestion). The 
hard outer coverings are indigestible and 
are eventually regurgitated through the 
mouth. Thus a single opening functions 
both as a mouth for the entrance of food 
and as an anus for the exit of undio-ested 
food. The breakdown of food molecules is 
probably not as thorough as in higher 
forms, for it has been observed that many 
of the endoderm cells take in particles of 
food by an amoeboid process. Food vacu- 
oles are formed and intracellular dio;estion 
proceeds, the same as in the Protozoa. This 
might be expected, since this animal is not 
so far removed from its protozoan an- 
cestors. The cells of the epidermis and other 
portions of the body receive their food 
supply from the endodermal cells. 

Hydra is sensitive to light and seeks out a 
suitable illumination. The intensity sought 
is usually that in which food is most likely 
to be found. The colorless hydra seeks a 
lower intensity of light than its green rela- 
tive (Hydra viridis). The latter has tiny 
green algae in its endodermal cells, which 
require more light for photosynthesis ( Fig. 
5-9). Hydra prefers cool, clear water and 
seeks it out. If exposed to various concen- 
trations of injurious chemicals, it avoids 
each with regularity. If unfavorable condi- 
tions are forced upon it, such as desiccation, 
it undergoes a series of regressive changes 

called "depression." The tentacles and 
body begin to disintegrate and this con- 
tinues until the animal is destroyed. Under 
favorable conditions, however, it may re- 
cover at almost any stage in its disintegra- 

In a suitable environment, hydra repro- 
duces asexually by forming one or more 
buds along the body wall where cells are 
congested with a surplus of stored food 
(Fig. 8-10). This is accomplished by a pro- 
liferation of the cells pushing a part of the 
body wall outward. Small blunt tentacles 
develop and a mouth breaks through. Food 
obtained by the parent hydra circulates 
into the coelenteron of the bud during its 
formation, thus providing means for rapid 
growth. Eventually the bud constricts at 
the point where it joins the parent and di- 
vorces itself from the latter to carry on its 
own existence. Sometimes several buds 
form simultaneously, indeed, buds may 
have buds upon themselves. This is very 
close to a colonial form, such as obelia, an- 
other hydrozoan, which will be considered 
a little later. 

Since buds form so readilv from almost 
any part of its body wall, it should follow 
that hydra could perhaps be made to repro- 
duce itself experimentally by simply cutting 
off small pieces of the body. This idea 
apparently occurred to a Swiss naturalist, 
Abraham Trembley, around the middle of 
the eighteenth century ( 1744 ) . He did just 
that and gave us our first experiments on 
regeneration in animals. Trembley was em- 


Fig. 8-10. Life cycle of hydra. One hydra is cot transversely through a bud and testis. 


ployed as a tutor for a distinguished French 
family where one of his duties was to teach 
natural history to the young men of the 
family. He entertained these boys by 
cutting hydras, as well as other coelenter- 
ates, into small pieces and watching them 
regenerate into adult forms (Fig. 8-9). 

By splitting the hypostome region Trem- 
bley was able to obtain a double-headed 
animal. He also found that by passing a 
needle with a knotted thread throuo"h the 
mouth and the basal disc, he could com- 
pletely invert the two layers; in other 
words, he turned the hydra inside out. 
Then, instead of inverting; back to its origi- 
nal condition, the animal simply moved its 
epidermal cells in between the endodermal 
cells so that both layers of cells found 
their original location. Since Trembley's 
time, a great deal of work has been done on 
regeneration, not only of hydra but also of 
many other animals. In general, if pieces of 
hydras are grafted together in various posi- 
tions, the parts retain the characteristics 
which they originally possessed. It has been 
shown, for example, if the mouth region of 
one animal is grafted to the basal disc of 
another, the development of tentacles 
is induced. In other words, an anterior end 
develops so that the resulting animal has 
two "heads" and no "foot." 

An understanding of regeneration among 
these simple Metazoa has considerable 
significance. Experiments demonstrate that, 
as the animal scale is ascended from simple 
to complex, this ability to replace lost parts 
or regenerate whole bodies is gradually 
lost. Hydra can replace its entire body from 
a fragment. In animals slightly more com- 
plex than hydra this ability is confined to 
the replacement of a part. Finally in ani- 
mals as complex as man all power of re- 
generating parts has been lost, and the only 
remnant of this endowment left is the 
ability to heal or close over a wound. The 
significance of this comes closer when it is 
realized that this is the basis of all plastic 
surgery, which is playing a more and more 


Fig. 8-11. A hydra with many testes. The fourth testis 
from the anterior end is nearly mature. Note the 
nipple-like tip from which the sperm will be dis- 
charged. This is a stained specimen. 

significant role in the lives of people where 
exposure to serious injury is so common. 

Hydra also reproduces sexually by the 
production of eggs and sperms (Fig. 8-10). 
Usually both ovaries and testes are formed 



■-•<- \. 


Fig. 8-12. Only the posterior portion is shown of this 
hydra bearing several ovaries. Several eggs have al- 
ready matured and have dropped off leaving only 
the "cups" where they were attached. A newly formed 
ovary, evident at the anterior end, is in the process 
of producing an egg. This is a stained specimen. 

although as a rule not 

on the same animal 

at the same time. In formation of the sperm 

interstitial cells lying in the ectoderm 

undergo a series of divisions, forming a 
protuberance which gradually grows large 
as the sperms mature. A sexually mature 
hydra may have several "ripe" testes along 
its walls which resemble miniature mam- 
mary glands, "nipples" and all (Fig. 8-11). 
An opening appears in the "nipple" end of 
the gland and the mature sperm cells swim 
out into the surrounding water in search of 
a mature egg found on another individual. 
The sperms live and remain active for a 
day or two. 

The eggs develop from interstitial cells 
also, the difference being that only one 
egg is formed in an ovary, whereas thou- 
sands of sperm cells are produced in a 
single testis. Several eggs usually form at 
the same time on one hydra, giving it an 
unusual appearance (Fig. 8-12). The eggs 
at first resemble large amoeboid bodies. As 
they mature they become spherical in 
shape, resting on the outer wall of the 
hydra attached to a cup-like depression in 
the epidermis. 

After fertilization, cleavage is immediate 
and continues until a hollow blastula forms. 
An outer resistant shell then develops and 
simultaneously the cavity (blastocoel) is 
filled with cells from the lining. The young 
embryo then drops off and lies quietly until 
favorable conditions eventually arise, when 
it emerges as a very small hydra with blunt 
tentacles. Sexual reproduction usually oc- 
curs in the fall of the year and seems to be 
a safeguard for passing the winter months 
because the young embryo resides in a cap- 
sule which resists adverse temperatures. 
Temperature is apparently the controlling 
factor because sexual organs can be in- 
duced in hydra at any time by reducing the 

Hydra possesses no medusa stage and 
hence does not exhibit metagenesis, which 
is common among most Hydrozoa. The 
next representative is a more typical ex- 
ample of this class and is discussed pri- 
marily for that reason. 

medusa bud 


lonqitudinal section 
of qonanqium 


asexual polyp 

Fig. 8-13. Life cycle of Obelia. 




This tiny colonial coelenterate may be 
found attached to seaweed and other ob- 
jects lying in the clear water of tide pools 
and below low tide level to a depth of 
several fathoms. It is attached to the sub- 
stratum by means of a horizontal branching 
basal portion. 

An obelia colony begins as a single polyp 
which by budding and the subsequent 
clinging together of the buds forms a col- 
ony (Fig. 8-13). The process is reminiscent 
of the bud-upon-a-bud condition observed 
in hydra. The tissues and gastrovascular 
cavity are thus continuous throughout the 
colony. There are two types of polyps in 
an obelia colony, the feeding polyp, or 
hydranth, and the less common reproduc- 
tive polyp, or gonangium. The hydranth is 
not greatly different from hydra except that 
it possesses solid instead of hollow tentacles 
and it is surrounded by a tough, horny 

outer covering called the perisarc, which 
invests the entire colony. The transparent 
vase-like portion of the perisarc surround- 
ing a hydranth is called the hydrotheca. 
The cellular portion just beneath the peri- 
sarc is known as the coenosarc. After food 
has been captured and partly digested by 
an individual hydranth, it is carried through 
the gastrovascular cavity by the beating 
flagella which line the cavity. Thus all 
polyps share in the good fortune of any 
one. Digestion is finally completed intra- 
cellularly in the lining cells. 

Obelia reproduces asexually by forming 
buds either on the horizontal parent stalk 
or on the upright stalk. Sexual reproduction 
occurs in a second type of individual, the 
gonangia (singular, gonangium), which 
have no tentacles and no mouth. This cylin- 
drical polyp is covered by the transparent 
gonotheca, a continuation of the perisarc. 
Each gonangium contains a central stalk, 
the blastostyle, upon which are borne small 

batteries of 
nematocysts on 


tentacular bulb 





— radial canal 

^ covity 


— manubrium 


circular cana] 

r- o lA i~ «,»-^„=-4 ♦« cl,ow mtornal structure. Also the tip of one tentacle is enlarged. 

Fig. 8-14. Gonionemos sectioned to snow iniernai siru».i«i«s. i- 



medusa buds. As the buds mature they ap- 
proach the distal end of the blastostyle, 
finally leaving the gonotheca through a 
terminal opening and swimming away. 
These free-living medusae are the familiar 
jellyfish, which, since they are able to move 
about, spread the species to new areas. 

Medusae develop gonads which produce 
eggs and sperms that unite in the sea water. 
The resulting embryo, the planula, swims 
about for a time but eventually settles to 
the bottom, becomes attached to the sub- 
stratum, and grows into an asexual colony, 
the polyp thus completing the cycle. This 
species is an excellent illustration of meta- 

The medusa of obelia appears to be quite 
different from the polyp, but basically they 
resemble each other closely. The former has 
a central hanging manubrium, located on 
the concave side. At the center of the manu- 
brium is the mouth, which opens into four 
radial canals, continuing into the circular 
ring canal in the margin of the bell. This 
constitutes the coelenteron and is equivalent 
to the same organ in hydra or the polyp of 
obelia. The space between the epidermis 
and the coelenteron is filled with the rather 
extensive gelatinous mesogloea. The medusa 
of obelia is microscopic and these struc- 
tures can best be seen in larger jellyfish 
such as Gonionemiis, which is commonly 
used for laboratory studies (Fig. 8-14). 

Obelia illustrates the beginning of divi- 
sion of labor among the polyps, but this 
is carried to a greater degree of efficiency 
among some of the other Hydrozoa. For 
example, in Hydractinia (Fig. 8-22), a 
colonial form that is often found on the 
shells of hermit crabs, some of the polyps 
gather food, others reproduce the species, 
and still others protect the colony by use 
of large batteries of nematocysts. This is 
one step higher than obelia. Polymorphism 
(many shapes), the name applied to this 
type of division of labor, is carried even 
farther in Phijsalia, the Portuguese man-of- 
war (Fig. 8-15), which comprises at least 

four types of individuals. In addition to the 
various types of individuals found in the 
Hydractinia colony, Physalia also has a type 
that forms a gas-filled float which supports 
the colony on the surface of the sea as it 
is borne here and there by the wind and 
currents in a never-ending search for food. 
The tentacles bear unusually large nemato- 
cysts which are occasionally a menace to 
bathers if they happen to become entangled 
in them. This can easily happen in some 
species of Physalia, since the tentacles trail 
as much as 50 feet beneath and behind the 
float. Physalia has little difficulty in paralyz- 
ing a fish several inches in length and even- 
tually consuming it. The many types of 
individuals in this colony illustrate poly- 
morphism in its most advanced form among 
the coelenterates. 

Other coelenterates 

The second class of coelenterates, the 
Scyphozoa, includes most of the larger 
jellyfishes (Fig. 8-16) which either have a 
reduced polyp stage or lack it altogether. 
Aurelia (Fig. 8-22), one of the most com- 
mon examples, is found in great numbers 
up and down the coasts of the United 
States. Its rather flattened umbrella is 
fringed with small tentacles that are inter- 
rupted at eight equally spaced spots where 
a sense organ is located. The rhythmic con- 
tractions of the circular muscle in the bell 
are responsible for the graceful movement 
of this beautiful creature. Four long oral 
lobes, which are located on the short manu- 
brium, are heavily armed with nematocysts 
and function like tentacles in capturing 
food and directing it into the mouth. The 
coelenteron is divided into four large gastric 
pouches from which radiate smaller canals 
that connect with the ring canal at the 
periphery of the bell. The gonads, lying in 
the gastric pouches, form four horseshoe- 
shaped bodies when viewed from the 
aboral (opposite the mouth) side, and con- 
stitute a ready mark of identification. 

The sense organs, consisting of eye-spots 

Fig. 8-15. Portuguese man-of-war {Physalia) stinging a fish. Note the large gas bag which functions os a 
float and sail combined. The numerous tentacles perform all of the functions of the animal. Some are 
heavily armed with nematocysts which are effective weapons against other animals, as this photograph 
shows. Note dead fish. 



Fig. 8-16. A giant jellyfish (Cyanea capillafa) common on our eastern seaboard. The dome is flattened during 
relaxation; during its power stroke it becomes more oval in shape. 

(sensitive to light) and statocysts (sensi- 
tive to gravity), are located at the edge 
of the bell where the nerve net is some- 
what centralized. The statocysts are hollow 
spheres containing small calcareous gran- 
ules which, as they tumble about in the 
cyst, stimulate nerve endings and indicate 
to the animal its relation to the rest of the 
world. In other words, it functions as an 
organ of equilibrium, much the same as 
the semicircular canals in our ears. This is 
the first appearance of such an organ in the 
animal kingdom. 

Sometimes, following a strong in-shore 
breeze, thousands of large jellyfish are 
swept up on the beaches to perish. Al- 
though they are thought to be very jelly- 
like, they do maintain their shape out of 

water and seem almost semi-solid. They 
also retain some of their delicate beauty. 
When they are swimming in the open sea, 
they provide a sight that, once seen, is not 
easily forgotten. 

Representatives of the next class, Antho- 
zoa, are characterized by a heavy body, 
supported by numerous septa, transverse 
sheets of tough tissue. Members of this 
group possess no medusa stage, eggs and 
sperms being produced within the body 
and discharged into the surrounding sea. 
The most common representatives are the 
sea anemones and corals; others less com- 
mon are the horny, black, and soft corals, 
the sea pens, and sea pansies. They com- 
pare favorably with the jellyfish in respect 
to beauty and numbers. Both the Atlantic 

Fig. 8-17. A common Pacific Coast sea anemone {Cribrina). These feed on small fish which they capture with the 

aid of their powerful nematocysts. 

Fig. 8-18. A common sea anemone {Mefridium) with many tiny tentacles which it uses to catch minute organisms 

instead of large animals as is common for most sea anemones. 



Fig. 8-19. A giant sea anemone (Condy/ocf/s gigonfea) found near Bermuda. The tentacles are highly colored. 

and Pacific Coasts, extending from Alaska 
and Maine to Southern California and 
Florida, abound with anthozoans (Figs. 
8-17, 18, 19, 20). Some are found in the 
polar regions, some at great depths, but 
they are most numerous in shallow waters 
in the warmer seas. 

The sea anemone usually remains in one 
place for a long period of time; some have 
been observed to live for years in a small 
depression in rock just below low tide level. 
It can move slowly on its pedal disc and 
some of the smaller ones are able to swim 
by beating their tentacles. It feeds on any 
unsuspecting crustacean, mollusk, or even 
fish that comes within reach of its tentacles. 
Once the prey is paralyzed by the nemato- 
cysts it is taken into the coelenteron, and di- 
gestion goes on much the same as in other 
coelenterates. The sea anemone, in spite of 
its tough outer covering, is preyed upon 
by a variety of animals such as fish, starfish, 
and Crustacea. When in danger it can re- 

tract its tentacles, fold them inside the 
body, and contract its entire body until it 
is nearly flat against the substratum. In this 
condition it is very difficult to remove, in 
fact, the body is often torn apart before its 
grasp is released. Although it usually re- 
produces sexually, occasionally an anemone 
is found undergoing fission, either longi- 
tudinally or transversely. 

Other interesting anthozoans, resem- 
bling the sea anemones in many respects, 
are the corals ( Fig. 8-20 ) . These are usually 
very small and live in stony cups, of specific 
design, made by the limy secretions from 
the base. The colonies lie in close proximity 
and after thousands of generations, produce 
the massive corals, bits of which are fre- 
quently seen reposing as mantelpiece orna- 
ments in many homes. Corals live in vvami 
waters for the most part, although there is 
one species, Astrangia, living as far north 
as Maine. They abound in many tropical 
seas of the world, particularly in the Coral 



Fig. 8-20. A typical coral reef with the abouncJing life associated with it. The fish are strikingly colored to match 
the background of corals, sponges, and other marine life. The tentacles of a large sea anemone can be seen in 
the lower left. The corals are numerous and highly varied in size, shape, and color. 

Sea. Our overseas forces in the past war 
became familiar with the corals around the 
Philippines and Australia. 

Perhaps the most interesting thing about 
coral is its ability to form reefs, some of 
which reach many miles in length, like the 
Great Barrier Reef of Australia, for exam- 
ple, which extends over 1,100 miles in 
length. There are three kinds of coral reefs, 
depending on how they were formed ( Fig. 
8-21). The fringing reef lies along the 
shoreline of an island or mainland and 
usually extends up to a quarter of a mile 

out into the sea. Boats approaching such 
shores are in great danger, particularly in 
rough weather. Sometimes, as the result of 
a shifting shoreline, a lagoon appears be- 
tween the main reef and the shore; this type 
is called a barrier reef. The atoll is perhaps 
the most unique of the three kinds of reefs. 
It is a rim of coral taking on varying shapes, 
usually a completely enclosed circle. These 
have always fascinated biologists. One of 
the greatest among them was Charles Dar- 
win who gave an explanation of how tiiey 
formed, a theory which is still considered 

Fig. 8-21. Various types of coral reefs. 




)\J U ( ( r 


VV^' I 'i ' J 'M^rC/ 







\ \XVaI\A» 1 • • 


V^aM'! \ llL . 





'Gn'J'n^ 5-<\^ 






— - 






Fig. 8-22. A hypothetical representation of how the various relationships among the coelenterates might have come 




fundamentally sound. He thought the pe- 
culiar formations started out as a fringing 
reef around an island, but due to shifts in 
the earth's surface the island gradually be- 
gan to sink. The rate at which it submerged 
was about as fast as the corals were able to 
secrete lime and keep themselves in the 
tidal zone. By keeping pace with the sink- 
ing island, the corals built the fringing reefs 
sufficiently high to catch vegetation and 
support growth of plants while the central 
portion gradually became submerged be- 
low the water's surface. This then produced 
a rim of coral, inscribins; the outline of the 
old island and producing the strangely- 
shaped atolls seen in tropical seas today. 


A possible explanation for the many dif- 
ferent life cycles among the coelenterates 
might be that the original primitive coelen- 
terate was much like the phinula larva, 
which is found rather consistently in all 
of the groups, with the notable exception 
of hydra. The planula is a ball of cells that 
might be compared to Volvox except that 
the former is solid and the latter is hollow. 
It would not be difficult to understand how 
the early planula-like coelenterate might 
become temporarily attached to the bot- 
tom of the ocean and invaginate, to form 
the hydra-like polyp (Fig. 8-22: 1 to 5). 
The formation of the sex organs would be 
similar to those found in hydra (Fig. 8-22: 
6), and the life history would also be the 
same, except that the zygote would develop 

into a planula which would eventually settle 
down and grow into the polyp. 

From the hydra-like type could be pro- 
duced a form resembling Htjdractinia ( Fig. 
8-22: 7), where the sex organs develop into 
separate bodies, although they are not de- 
tached from the polyp. The next logical 
step would be an obelia-like form in which 
the medusae detach themselves from the 
polyp and become the free-swimming sex- 
ual stage of the animal (Fig. 8-22: 8). This 
affords an opportunity for the species to 
spread both by means of the planula and the 
medusa stages. The next variation mis;ht be 
a continued emphasis of the medusa and a 
de-emphasis of the polyp stage, as is seen in 
Aiirclia ( Fig. 8-22 : 9 ) . This can be carried 
still further until the polyp stage disappears 
altogether, which is the case with many of 
the large jellyfish (Fig. 8-22: 10). This is 
merely a possible explanation for the great 
variety of forms found in this phylum of 

In review we have found the coelenter- 
ates a widely diverse group of animals that 
have explored many possibilities of form, 
structure, and habitat, remaining all the 
while within the limitations of two body 
layers, ectoderm and endoderm. Animals 
could have gone no further in complexity 
had they remained within the limitations 
imposed by these two body layers. The next 
group surmounted this difficulty by the in- 
troduction of a third body layer which 
resulted in a modification in the entire body 
plan of the group. Let us see how this was 



Quite different from the symmetrical 
beauty of hydra, the jellyfishes, and the sea 
anemones, are the drab representatives of 
the next group of animals, the flat worms. 
Their flattened and elongated bodies ac- 
count for the name of the phylum to which 
they belong, Platyhelminthes (from the 
Greek, platij — flat). Just as the two-layered 
animals showed distinct advantages over 
the unicellular animals, so the flatworms 
demonstrate a higher form of life than was 
observed among the coelenterates. 

The most important single morphological 
structure acquired by this group is a third 
body layer, the mesoderm. It is only with 
the advent of this additional layer that ani- 
mals were able to reach higher levels of 
complexity. Most of the intricate and mas- 
sive structures of not only platyhelminthes 
but all higher animals have been derived 
from this layer. We see a forerunner of the 

mesoderm in the mesogloea found in the 
coelenterates, although in this group it 
never became a distinct layer. Once the 
mesoderm was established in the flatworms 
it was retained by all subsequent groups of 

As a result of the introduction of a third 
body layer other modifications were possi- 
ble. One of the most obvious of these was 
a change from radial to bilateral symmetry. 
This meant the acquisition of head and tail 
ends, dorsal and ventral sides, and left and 
right sides. Localization of the sensory sys- 
tem in the head region was initiated in 
these forms, signifying a definite step to- 
ward centralization of the nervous system. 
Moreover, the animal now moved in one 
direction to seek food rather than acquir- 
ing its meal in a passive manner as was true 
of the coelenterates. All of these changes 
resulted in a much more complex animal, 




one that was well on its way toward higher 

Of the three classes of the phylum Plat\'- 
helminthes only the class Turbellaria in- 
cludes free-living animals. Members of this 
group may be found among the rocks in 
cool streams or ponds, or upon the shady 
side of submerged plants. Turbellarians are 
carnivorous, feeding on small animals, 
either living or dead. There are also marine 
forms in tliis group which sometimes live 
in the intestines of sea urchins and other 
forms of ocean life. 

The other two classes of the phylum are 
the Trematoda, or flukes, and the Cestoidea, 
or tapeworms. All members of these two 
classes are parasitic and will be described 
further in a later section, but first let us 
examine the free-living turbellarians. 


The study of tripoblastic, or three-lay- 
ered, animals may well begin with planaria, 
a common inhabitant of North American 
streams. It seeks shelter in darkened, se- 
cluded spots and comes out at night to 
move around in the cool waters in search 
of food. Planaria is flattened dorsal-ven- 
trally and is darkly pigmented. It is cov- 
ered with cilia, which enable it to glide 
along the substratum over a mucous path 
(Fig. 9-1) secreted by glands on the ventral 
surface of the body. By use of a muscle 
layer developed in the mesoderm, planaria 
can crawl in true worm-like fashion. Cer- 
tain muscle groups produce a twisting mo- 
tion so that it sometimes appears to raise 
its head and look about. Although it may 
appear that planaria can see when this oc- 
curs, actually its two large eye-spots form 
no image and are only sensitive to varying 
intensities of light. 

Unlike most heads, that of planaria has 
no mouth, for the mouth is located on the 
ventral side of the body near the middle. 
It opens into a muscular pharynx, which 
lies in a sheath extending anteriorly, and 


Fig. 9-1. Planaria crawling and feeding. 

when planaria is hungry and in search of 
food, it often protrudes its pharynx and 
moves about with it thus extended. When a 
small piece of meat is tossed into the water, 
hungry planarians attach themselves to it. 
A digestive fluid pours out from the phar- 
ynx to aid in the disintegration of the meat. 
The partially digested food is then taken 
into the digestive tract where digestion is 
completed. Planaria's chief food consists of 
small Crustacea. In this case the epidermal 
slime glands secrete a sticky substance 
which is sprayed over the victim, rendering 
it helpless. It is usual for planaria to grip 
its prey with the head region first and then 
attach its muscular pharynx to the food, bits 



^%7 m2 

Fig. 9-2. Planaria stained so as to differentiate the 
digestive tract. Note its ramifications into every part 
of the animal. The protrusible pharynx is clearly 
shovt^n in this photo. 

of which are drawn into the mouth by a 
sucking action. 

The digestive system of planaria is sac- 

opening for the entrance of food and the 
exit of waste materials (Fig. 9-2). In some 
of the platyhelminthes, particularly the par- 
asitic members, the sac is merely a straight 
and unbranched tube, but in planaria it 
branches into three distinct parts to form a 
tri-clad intestine. Each part, in turn, rami- 
fies into many smaller branches which sup- 
ply food directly to the various regions of 
the body (Fig. 9-3). Large thin-walled, 
unciliated cells line the gut and secrete di- 
gestive juices which carry on extracellular 
digestion. In addition, the cells lining the 
intestine are able to ingest solid food by 
means of pseudopods and digest it intra- 
cellularly, as in the case of hydra. 

Between the ectoderm and the endoderm 
is a mass of large star-shaped mesodermal 
cells, the parenchyma. It is possible for 
food substances to pass not only from the 
gut into the linins; cells, but also from the 
parenchyma into the lining cells. Thus, 
when planaria cannot find food, it may con- 
sume certain organs in the parenchyma by 
passing them into tlie intestinal cells where 
they are digested. This enables planaria to 
go without food for quite a long time, 
through gradual reduction in size. In one 
species of planaria it was found that dur- 
ing starvation the absorption and digestion 
of its internal organs occurred in regular 
order. First the reproductive organs disap- 
peared, leaving the animal reduced to sex- 
ual immaturity; next the parenchyma, the 
2ut, and the muscles were consumed in that 
order. The nervous system remained es- 
sentially intact so that the animal appeared 
as a weird form with the bulk of its re- 
maining body in the head region. On feed- 
ing these starved forms, all the lost parts 
regenerated to normal size. 

Special excretory organs appear for the 
first time in a metazoan among the flat- 
worms. In planaria this system consists of 
a pair of branching tubes running down 
each side of the body. The main tubes or 
canals divide into small branches, each of 

like, similar to hydra, with but a single which finally ends blindly in a single flame 



diqcsf iv« +rac+ 

diqestivjc tract 


bcrve cord' cndoderrw 

circular muscles 


Fig. 9-3. A dorsal and sectioned view of planaria to demonstrate the systems, particularly the digestive. 

cell (Fig. 9-4). These cells have long cilia 
which extend into the lumen of the canal 
and it is their flickering motion that sug- 
gests the name. The movement of the beat- 
ing cilia carries nitrogenous wastes into the 
lumen of the tube and to the exterior 
throu2;h a number of excretory pores. Prob- 
ably this system's chief function is in regu- 

lating the water balance, because most of 
the nitrogenous wastes are lost through the 
endodermal cells. Because the tubes of this 
system are a primitive type of nephridium 
(kidney), they are called protonephridia. 
The nervous system of the planarian is 
simple (Fig. 9-4), yet it is strikingly more 
advanced than the primitive nerve net 




Fig. 9-4. Planaria in dorsal view and cross-section, 
showing the excretory system on the left and the 
nervous system on the right. Upper left indicates side 
view of how cut was made. 

of the coelenterates. Perhaps most unique is 
the concentration of the nervous tissue in 
the head region below the eyes. The nerve 
cell bodies are contained in two masses of 
nervous tissue, the cerebral ganglion, com- 
monly referred to as the brain. From this 
concentrated point two longitudinal nerve 
cords pass posteriorly and two short nerves 
extend anteriorly to connect with the eyes. 

Along the two longitudinal cords are many 
transverse nerves, which are distributed to 
the internal structures of the body. 

The eyes of planaria are found on the 
dorsal surface where they appear as two 
dark spots (Fig. 9-5). There is no lens, as 
such, although the ectoderm over the eye 
is without pigment so that light can pass 
through to reach the sensory cells below, 
which connect with the brain. Without a 
lens no image is possible, but the eye is 
sensitive to varying intensities of light and 
the animal withdraws from bright light and 
seeks out moderate illumination. Other 
sensory cells protrude from the surface of 
the body and act as receptors for register- 
ing changes in the flow of the water or 
other variations in the surroundings. 

Although it is evident that the nervous 
system of the Turbellaria is still very sim- 
ple, the increase of special sensory cells, 
their grouping into such an organ as the 
simple eye-spot, and the aggregation of 
nerve cells into the cerebral ganglion, are 
the beginnings of a definite central nervous 

Fig. 9-5. Head of planaria {Euplanaria or 
highly magnified. Note the "crossed" eyes 
two ear-like extensions of the head region 
tactile sense organs are located. 

and the 
in which 



Of the various systems, the planarian 
reproductive system shows the greatest ad- 
vancement over that of the coelenterates 
(Fig. 9-6). In the sexually mature worm, 
male and female reproductive systems are 
present in each individual, a condition 
known as monoecious (hermaphroditic). 
Both ovaries and testes develop from the 
cells of the parenchyma. The numerous 
testes are rounded bodies which lie along 
botli sides of the body. They give rise to the 
spermatozoa, or sperm cells, which are con- 
veyed through small ducts, the vasa effer- 
entia, to a larger tube, the vas deferens, or 
sperm duct, running the length of the body 
on each side. The two seminal vesicles ter- 
minate in a pear-shaped copulatory organ 
or penis. At rest the copulatory organ opens 
into the genital atrium, which leads into 
the genital pore, the external opening 
through which the penis is thrust during 

The two ovaries of the female reproduc- 
tive system are found near the anterior end 
of the body; these produce the ova. The 
yolk glands, which give rise to the yolk and 
shell of the egg, are found along the ovi- 
ducts. The two oviducts lie parallel to the 
nerve cords and join before entering the 
atrium. The seminal receptacle, a sac for 
storing sperm, also opens into the atrium, 
very near the external opening. The genital 
atrium, therefore, receives the openings of 
both the female and the male organs. 

At the start of copulation the ventral sur- 
faces of the two animals come together, so 
that the openings of the genital atria are 
opposite one another. The penis of each is 
extended into the genital opening of the 
other and the sperm cells are exchanged. 
At the time of copulation the ova are also 
ripe. To prevent self-fertilization, the geni- 
tal area has been elaborated. The penis, 
when extruded and dilated, completely fills 
the atrium and thus blocks the openings 
into the oviducts, so that neither can the 
ova escape nor sperm cells enter the ovi- 
duct, but sperm can be deposited into the 


yolk qland 

spcrro duct. 

soronol rcceptacla 
copulatory onqao 

qenital atriuo)_ 

qeoital pore 

seminal vesicle 

Fig. 9-6. Planaria in dorsal view and cross-section show- 
ing the reproductive system. Upper right indicates 
side view of how cut was made. 

seminal receptacles. At the completion of 
copulation the penes are withdrawn and the 
sperm cells then are able to enter the ovi- 
duct. The ova are fertilized in the oviduct 
and, as they move down toward the atrium, 
the products of the yolk glands are dis- 
charged. The mature fertilized egg is re- 
leased from the atrium through the genital 
pore, and the capsule-like shell becomes 
attached by a stalk to submerged objects. 
The egg cases may undergo a rest period 
before growing into young planaria. 

Planaria also reproduces asexually by 
means of transverse fission. Indeed, this is 
much the more common method of repro- 



Fig. 9-7. Regeneration in planaria. 

duction. The worm constricts itself in two 
and the parts which are missing after the 
fission has taken place are then regener- 
ated. Planaria is an excellent animal for 
regeneration experiments. For example, if 
the head of planaria is split and if the parts 
are kept separated for a short period of 
time, a double-headed monster is formed 
(Fig. 9-7). Should the animal be cut trans- 
versely into two separate pieces, two little 
planaria will result. An animal can be cut 
into as many as six pieces and each will 
give rise to a miniature worm one-sixth the 
size of the original. The significance of re- 
generation was pointed out in an earlier 


Although members of the class Turbel- 
laria are, for the most part, free-living, some 
live on the exterior of other animals and 
others are true parasites living in the in- 
testines of mollusks and various echino- 
derms, such as the sea cucumber. Economi- 
cally, they are of little importance. 

The small primitive marine forms of 
the order Acoela, have a mouth but no di- 
gestive system, the food being digested by 
the endodermal cells. Convoluta, an animal 
that lives on sandy ocean shores, is a good 
example of this order. As it matures, algae 
enter and inhabit its parenchymous tissue, 
giving it a green color. BdeUtira, referred 
to earlier (p. 95), is also a member of this 

Members of the order Rhabdocoelida are 
turbellarians with a straight tubular gut. In 
certain forms the gut has lateral pouches, 
but not as highly branched as that of pla- 

naria. These turbellarians are commonly 
found in fresh water, but because of their 
small size are not as easily studied as 
planaria. The largest of these forms is 
Mesostoma, which, like members of the 
order Tricladida, has its mouth on the 
ventral surface. The smaller rhabdocoeles 
have their mouths in the anterior region, 
ventral to the brain. Some of these animals 
have such remarkable powers of reproduc- 
tion by fission that individuals form long 
chains which remain t02;ether for some time 
before separating. This was mentioned ear- 
lier as a possible explanation of the origin 
of segmentation. 

Members of the order Polycladida are 
marine forms, usually small, although some 
attain a leno;th of 6 inches. This order in- 
eludes a few rare species found only in iso- 
lated places, such as the Gulf of Naples. 
The digestive system is well branched and 
the body is unusually flattened. In some of 
the simpler forms, the mouth is centrally 
located and the pharynx, which is funnel- 
like in structure, can be extended from the 
mouth to enclose food. One difference be- 
tween this order and the others is that 
development is not direct, but must pass 
through a rounded, ciliated larval form 
which possesses projecting arms. As the ani- 
mal grows, it loses the cilia and the arms, 
develops a crawling movement, and be- 
comes considerably lengthened and flat- 


The trematodes, commonly called flukes, 
are characteristically flat like all platyhel- 
minthes, but their gut is reduced in com- 
plexity. Because of their parasitic life they 




sperm duct excretory pore 
jtestis I 

ora\ sucker 




cercaha redia Sporocyst 

Fig. 9-8. Life cycle of Opisttiorchis sinensis, the human liver fluke. 

have lost their ciha and have developed 
hold-fast organs called suckers. They have 
no apparent sense organs, but the nervous 
and excretory systems are well developed. 
In general, the entire anatomy is adapted 
to a parasitic mode of life. 

All grades of parasitism are found in this 
group, from those that live on the outside of 

their hosts to those that inhabit the internal 
cavities. The life history of parasites that 
cling to the outside of animals such as fast- 
moving fish is relatively simple, its main 
characteristic being the development of 
hold-fast organs or suckers. The case of the 
internal parasite is an entirely different one. 
There is no great problem of staying put. 



Fig. 9-9. A stained adult human liver fluke (Opisfhorch/s 
sinensis). Compare this photograph with the drawing 
in Fig. 9-8 in order to make out the parts. 

hence the suckers are small, but in order 
to complete its life cycle an enormously 
prolific reproductive system has been de- 
veloped. Not only is the production of large 
numbers of potential offspring necessary, 
but also various kinds of larval stages that 
are able to pass through several hosts, all 
of which are instrumental in spreading the 
parasite far and wide. Let us consider two 
examples of flukes that infect man, one 
that lives in the liver {Opisthorchis sinen- 
sis) and another that lives in the blood 
( Schistosonm haematobium ) . 

The life cycle of the human liver fluke 
can serve as a typical example of most 
related flukes that are so prevalent in wild 
and domestic animals ( Fig. 9-8 ) . It involves 
two intermediate hosts which harbor the 
larval stages of the parasite, and, of course, 
one final host in which the adult lives. The 
human liver fluke infects 75-100 per cent 
of the people in certain parts of China, 
Japan, and Korea, constituting a real health 
problem in these regions. This situation 
should be alleviated with the advent of 
improved sanitation and a better educa- 
tional program. 

The adult fluke (Fig. 9-9) lives in the 
small bile ducts of the liver, where toxic 
products excreted by the flukes and the 
subsequent mechanical occlusions of the 
ducts may cause serious damage. For a 
heavily infested individual, this may even- 
tually develop into cirrhosis, together with 
complicating infectious disease which usu- 
ally terminates his life. The adult fluke is 
about three-fourths of an inch in length 
and has two suckers, one at the anterior 
end, another about one-third of the way 
from the posterior end. It feeds on blood 
which is drawn in through the anterior 

Eggs laid by the adult pass through the 
bile duct into the gut and eventually pass 
out of the body in the feces. Because of the 
oriental habit of using human excrement 
(night soil) as fertilizer in the rice paddies, 
the eggs usually get into water. Unlike most 



F!g. 9-10. Life history of a typical human blood fluke. 

flukes, the egg does not hatch into the larval 
stage known as the miracidium until it is 
eaten by a certain species of snail of tlie 
genus Bijthinia. Inside the snail the egg 
hatches, releasing the miracidium which 
makes its way into the tissues of the snail 
and develops into a sporocyst. The next 
stage, the redia, develops inside the sporo- 
cyst. Both the sporocyst and the redia 
stages make possible a tremendous increase 
in numbers by asexual reproduction. The 
redia finally develops cercariae within its 
walls, which make their way out of the snail 
into the surrounding water where they 
swim about by means of their large vibratile 
tails. The cercaria then becomes attached 
to the next host, one of several different fish 
and, after losing its tail, bores its way into 
the flesh of the fish. It rounds itself into a 
ball and produces a cyst wall; in response 
to the parasite, the fish secretes another 
wall around the invader. Here it lies until 
the raw fish is eaten by man in whose gut 
the cyst wall is digested away, releasing the 
young worm which makes its way up the 
bile duct and finally into the smaller tubes 
of the liver where it grows to maturity. 

The control of the disease is obviously 
very simple, destroy the snails or cook the 
fish, either of which interrupts the cycle 
and kills the parasite. 

Some of the most important flukes are 
the blood-inhabitino; schistosomes such as 
Schistosoma haematobium (Fig. 9-10). Un- 
like Opisthorchis, these worms are dioe- 
cious, that is, there are two separate sexes. 
They are long slender worms, beautifully 
adapted for living in the small blood ves- 
sels. A strange relationship exists between 
the males and females; the male holds the 
extremely slender female in a groove on his 
ventral side, from which she ventures forth 
during the business of laying eggs. Her 
slender thread-like body is ideally adapted 
to fit in the tiny blood vessels of the intesti- 
nal wall or over the bladder where she lays 
her eggs. The eggs have a single sharp spine 
by which passively they work their way 
through the wall into the cavity of the in- 
testine or bladder where they are voided 
with the urine or feces. 

Aeain through the use of human excre- 
ment for fertilizer, the eggs usually find 
their way into water. They hatch into mira- 



Fig. 9-n. A blood fluke normally infecting water birds occasionally enters human sicin, causing "swimmer's itch' 

(schistosome dermititis). 

cidia which penetrate the tissues of snails, 
and follow stages of development similar 
to those of Opistliorchis with minor varia- 
tions. Instead of encysting on a fish, the 
cercaria burrows through the skin of a per- 
son who is unfortunate enough to be in the 
vicinity and makes its way into the vascular 
system. It passes through the heart, lungs, 
and liver, eventually maturing in the blood 
vessels ( veins ) that drain the intestines and 
bladder. Here it grows rapidly, feeding on 
blood, and when sexually mature lays its 
eggs, thus completing the cycle. 

The several species of blood flukes infest 
the populations of tropical America, many 
parts of Africa, and the Orient, particularly 
China. In some irrigated regions the infec- 
tion nms as high as 90 per cent among the 
adult males who are constantly in contact 
with the water, hence exposed to the cer- 
cariae. Treatment consists of giving large 
doses of antimony compounds and, if the 
patient can stand the treatment, he can be 
cured. Like all of the complex parasites, 

blood flukes can be controlled by removing 
the intermediate host. 

Water birds such as ducks, terns, and 
Sulls, have their own varieties of blood 
flukes which apparently cause them no par- 
ticular harm. However, if this type of cer- 
caria cannot find its proper final host, it 
does penetrate the skin of any person who 
happens to be near, causing a severe itch- 
ing which has been called schistosome der- 
mititis, or just "swimmer's itch" (Fig. 9-11). 
The cercariae apparently are not able to 
penetrate the tough thick mammalian skin, 
but in their attempts to do so enter it and 
cause intense irritation. There are several 
different species of cercariae that follow 
this pattern. Some are found on the sandy 
bathing beaches in the lake regions of the 
North Central states, especially Michigan 
and Minnesota, where they sometimes be- 
come such a nuisance that bathing is actu- 
ally prevented, much to the disgust and 
economic loss of resort owners. Elaborate 
methods of treating the beaches with cop- 



per sulfate in order to destroy the snails 
have been developed and this control has 
had reasonable success. 


These are the tapeworms, a group of 
parasites that the layman has long errone- 
ously associated with lean hungry adoles- 
cents. The worm gets its name from its 
long ribbon-like appearance, a feature that 
is common to this large and varied group, 
members of which infect almost all, if not 
all, vertebrate animals. 

The tapeworm is, perhaps, the most de- 
generate of all animals, a condition indicat- 
ing that the association with its host is one 
of long standino;. At the same time- it is 
beautifully adapted to its specialized envi- 
ronment, the vertebrate grut. It is indeed 
the supreme parasite among parasites. It is 
provided with excellent hold-fast organs to 
keep it in place in the gut of the host ( Fig. 
9-12). All nourishment is received from the 
■contents of the gut or from the gut wall di- 
rectly, so the animal has not bothered to 
retain even a semblance of a digestive tract. 
Its nervous and excretory systems are very 
rudimentary, and its ability to move has 
been reduced to very feeble contractions. 
However, it has evolved an extremely elab- 
orate and prolific reproductive system, an 
essential feature in its survival since the 
possibility for any individual egg to reach 
maturity is very small. Although it has de- 
generated in other respects, it has gone all 
out in this one phase of its life, and meas- 
ured in terms of biological success, the shift 
in emphasis has apparently been satisfac- 

The common beef tapeworm (Taenia 
saginata) of man is a typical example of 
this group (Fig. 9-13). It consists of two 
principal parts, the head or scolex, and the 
proglottids, sectional pieces attaching to 
one another, and growing larger as tliey 
proceed posteriorlv. The scolex possesses 
hold-fast organs which make it possible for 

Fig. 9-12. The scolex of the dog tapeworm (Taenia 
pisiformis). Note the sharp hooks and sucking discs 
used as attachment organs. 

the worm to maintain its position in the gut. 
The proglottids, which are budded off from 
the region just back of the head called the 
neck, matrn-e as they move progressively 
posteriorly. The younger proglottids are 
tlierefore anterior to the older. The mature 
proglottids, gorged with eggs containing 
young embryo worms, break away from the 
worm and pass out of the body in the feces. 
Because of the close association of cattle 
and their keepers in certain parts of the 
country, it is not unusual for the eggs to 
be picked up by grazing cattle. Once in the 
gut of this host the egg membranes and 
shell are digested away and the young six- 
hooked embryo (hexacanth) emerges. It 
soon bores its way through the gut wall into 
a blood vessel where it floats to the muscles, 
particularly heart and jaw muscles. Here 
it develops into a bladder and a tiny in- 
verted tapeworm scolex grows from the 
wall of the bladder. Beef so infected is 
said to contain "bladder worms" and is usu- 
ally unmarketable. If such beef is poorly 
cooked and then eaten by humans the blad- 
der worms "hatch." The tiny scoleces evert 
and become attached to the soft intestinal 
wall where thev immediately begin bud- 
ding off proglottids. 


adult topevoorrn 


Fig. 9-13. Life history of the beef tapeworm {Taenis saginata) of man. 

An examination of the proglottid demon- 
strates the fact that it is almost a complete 
individual itself. Indeed, the tapeworm is 
sometimes considered a colony in which 
each proglottid is an individual, much like 
the buds in hydra or the polyps in Obelia. 

Besides rudimentary nervous and excre- 
tory systems it possesses male and female 
sex organs, which are capable of produc- 
ing prodigious numbers of sperms and 
eggs. The testes, numerous and scattered 
throughout the proglottid, are connected 



through fine tubules to the sperm duct 
which opens to the outside through the 
genital pore. The paired ovaries produce 
eggs which pass through a small duct ( ovi- 
duct) where they receive sperm from an- 
other proglottid, or another worm, via the 
vagina. Here they also obtain the food ma- 
terial called yolk from the yolk gland, while 
the shell gland secretes material for form- 
ing the shell. The fertile eggs then are de- 
posited into the uterus which eventually 
becomes greatly distended as the eggs be- 
gin to develop crowding all other structures 
out of place. Such a gravid proglottid (full 
of developing embryos ) breaks off and fol- 
lows the cycle indicated above. 

Meat inspections in this country, together 
with sanitation among cattle raisers, have 
greatly reduced the incidence of this para- 
site. In fact, it has become so difficult to 
obtain this human parasite that biological 
supply houses usually have standing orders 
for them. Control of tapeworms is very 
simple: merely cook the suspected meat. 
There is a similar tapeworm in pork ( Taenia 
solium) which is also becoming scarce. 

A few decades ago during the lumber- 
jack days in this country, particularly in 

Minnesota and Michigan, the fish were 
heavily infected with a tapeworm larva 
that was transmitted to man through his 
habit of eating raw fish. The worm, Diphijl- 
lobothritwi latum, was apparently intro- 
duced by immigrants from the Baltic re- 
gion of Europe where the infection was 
known for centuries. The life history in- 
volves two intermediate hosts, a small crus- 
tacean and a fish. The crustacean receives 
the infection by eating tapeworm eggs, the 
fish (pickerel or pike) eats the crustacean, 
and finally man gets the worm by eating 
the uncooked fish. This is a giant among 
tapeworms, having been known to reach 
a length of 60 feet. 

In review, we have seen that with the 
advent of the mesoderm and with it several 
important organ systems, the flatworms far 
outstripped the coelenterates in complex- 
ity. They are, however, still small creatures 
and relatively simple when compared to 
a mammal, in other words, still further im- 
portant changes must have taken place in 
subsequent groups of animals. We shall see 
what additions are made in the next group 
of tubular worms. 



To this point we have considered ani- 
mals of such a degree of complexity as to 
provide for only small bodies, from micro- 
scopic dimensions to several inches at most. 
Since there are animals of great size in the 
world, the ideas exploited thus far must not 
have been adequate for the development of 
such large bodies. Something new, then, 
must have been added. Undoubtedly, many 
different ideas were tried in reaching the 
present great complexity to which the high- 
est animals have attained. By far the great- 
est majority of these ideas were not suc- 
cessful and were discarded. Some proved 
satisfactory and these are the ones that are 

incorporated in the bodies of successful ani- 
mals in the world today. Success, biologi- 
cally speaking, means spreading the species 
over the surface of the earth: the English 
sparrow is a success in America today, 
whereas the now extinct passenger pigeon 
is a biological failure. The new ideas that 
appear in the next group of animals should 
be studied to see why they have been re- 
tained and how they have pushed the whole 
mass of living things one step higher on the 
evolutionary scale. 

One of the first things that needed atten- 
tion was the digestive tract, which in the 
coelenterates and flatworms is merely a sac, 




with or without ramifications. All the food 
makes its way into such a digestive tract 
through the mouth, and all undigested food 
comes out the same way. This is a very 
awkward method of handling such an im- 
portant function. A distinct improvement 
would be a tube running throughout the 
body with the mouth at one end, and a cor- 
responding opening at the opposite end to 
allow undigested food to pass out of the 
body. In this way food could be constantly 
taken in at one end, and progressively di- 
gested as it passes backwards, a kind of as- 
sembly line method. This would mean the 
development of a "tube-within-a-tube" body 

Another great need was a method of dis- 
tributing the digested food to all the cells 
of the body. So far this had been done by 
simple absorption and diffusion. To be sure, 
certain provisions had been made to facil- 
itate this process, but at best, such things as 
the diverticula in the gut of planaria, could 
suffice for only a very small animal. Such 
primitive devices for distribution could not 
supply all the cells fast enough to make it 
possible for the animal to reach any great 
size or to move with any speed. A system 
was required which would carry an ample 
supply of not only food, but also oxygen to 
burn it, to every cell. This could be done 
only with some sort of conveyor belt system. 
Since digested foods are soluble in water, 
the system must be made up of a circulating 
fluid, a series of tubes to confine and lead it 
to every cell, and some means of keeping 
it flowing continuously. Only with the de- 
velopment of such a mechanism could ani- 
mals climb any higher in this scale. 

The first of these important steps was 
taken by the animals found in the phylum 
Nemathelminthes, the roundworms, and the 
second step among an obscure group of ani- 
mals, the Nemertinea. These two groups 
will be studied from this point of view, and 
in this order, although in most other re- 
spects the nemertines are more primitive 
than the nematodes. 


In numbers of animals the nematodes, 
which is the name applied to most members 
of the phylum Nemathelminthes, perhaps 
exceed all others, with the possible excep- 
tion of the arthropods and Protozoa. They 
were once thought to be primarily parasitic. 
These members first came to the attention 
of biologists because they were responsible 
for some of the more serious diseases both 
in plants and animals. However, it is now 
known that there are equally as many, if not 
more, that are free-living. A spadeful of al- 
luvial soil contains literally millions of them. 
A drop of water, taken from nearly any stag- 
nant pond or the sea, would reveal many of 
them. Their characteristic whipping move- 
ment identifies them to even the casual 
observer. Most of the nematodes are very 
small, almost microscopic, although there 
are a few — the "horsehair worms," for ex- 
ample — that may reach a length of 1 yard. 
Some of the ascarid parasites of horses may 
reach a length of 10 to 12 inches. 

The most distinct improvement in this 
group over the preceding is the complete 
digestive tract, mouth to anus (Fig. 10-1). 
This is a slender tube, without pockets, run- 
ning throughout the body length. Digested 
food is absorbed directly through the gut 
wall and diffuses into a fluid which sur- 
rounds the digestive tract, thence to the 
body cells. Here again, the animal depends 
on diffusion to take care of the important 
matter of getting food and oxygen to the 
cells, and wastes away from them. This fact, 
among other things, is probably responsible 
for the small size of these animals. 

A pair of tubes run internally along each 
side of the body, forming excretory canals, 
but they lack any cells comparable to the 
flame ceUs found in planaria. The two tubes 
unite into a single one, which opens ven- 
trally to the outside through a minute ex- 
cretory pore. Another feature of the nema- 
todes is a complex nervous system which 
consists of several nerves extending the 




muscle cell 

mrw cord 

Fig. 10-1. The body plan of the roundworms includes two tubes: one, the gut, within the other, the body wall. This 
figure shows the anterior end of the worm with its three teeth surrounding the mouth, and a cross-section taken 
anterior to the gonads. 

length of the body and terminating ante- 
riorly in numerous ganglia. 

The body wall contains a thick muscular 
component, separated into four banks of 
muscle cells extending lengthwise, and so 
attached that the animal can flex its body 
only in a dorsal-ventral manner, a rather in- 
effective method of locomotion in the water. 
In fluids of high viscosity or in soil, how- 
ever, it is more effective. 

The rather elaborate reproductive system 
lies free in a fluid-filled cavity between the 
body and gut wall. Since the sexes are sepa- 
rate, only one set of organs is found in each 
animal. Females, which are usually larger 
than the males, possess two ovaries in the 
shape of long coiled tubes. The two ovaries 
continue into two oviducts, which enlarge 
to form two uteri (singular, uterus). These 
join to form a single short vagina, which 
opens externally on the ventral side in the 
anterior portion of the body. The mature 

eggs are stored in the uteri. In the male, 
sperms are produced in a long coiled tube, 
the single testis, which joins the vas deferens 
and then becomes the seminal vesicle, the 
storage place for the sperm. A pair of bris- 
tles at the posterior end aid in conducting 
the sperms from the male to the female dur- 
ing copulation. The opening of the male re- 
productive system is close to the posterior 
end of the animal near the base of the bris- 

Parasitic nematodes. Because of the eco- 
nomic significance of the parasitic nema- 
todes we will discuss representative forms, 
particularly those that attack man. Although 
over 50 different species are human para- 
sites, a still greater number affect man in- 
directly by their ruthless destruction of his 
domestic plants and animals. They invade 
almost every organ of the body, their dam- 
age depending on the kind and number of 
individuals. Like most parasites, the nema- 



todes tend to remain with a specific host, 
although they are a httle more careless in 
this regard than some. Occasionally they 
attack a variety of hosts and may produce a 
serious disease when they enter a new one. 
For example, a hog may be riddled with 
Trichmella with no apparent damage, 
whereas a man with a similar infection is apt 
to die because he is the "accidental host" 
while the hog is the normal host. The hog 
has had trichinella in its tissues so long 
that it has built up some resistance to the 
parasite. Since man gets the parasite only 
occasionally, he has not developed any re- 
sistance. Let us consider several common 
roundworm parasites. 

Ascaris liimhricoides (Fig. 10-2) is a 
common intestinal parasite of many domes- 
tic animals as well as man himself. It is an 
excellent example of the usual life cycle of 
parasitic roundworms, although there are 
wide modifications, as will be seen in trich- 
inella, for example. It is not infrequently 
found in the digestive tract of children, 
since they are apt to get ascaris eggs on 
their hands from the soil and transfer them 
to the mouth ( Fig. 10-3 ) . The embryonated 
eggs pass through the stomach to the intes- 
tine where they hatch into tiny worms ( 0.2- 
0.3 mm. long). These bore through the 
intestinal wall into the lymph, then the 
capillaries, and finally the general circula- 
tion. They are carried through the heart to 
the lungs where they grow somewhat in 
size. Eventually the larvae break through 
into the air sacs, migrate up the trachea, 
and are swallowed, arriving in the intestine 
for the second time. Here they mature, cop- 
ulate, and lay eggs which pass out of the 
host with the feces. The eggs may be picked 
up directly by another host, or they may be- 
come desiccated and blow around in the 
dust to be engulfed at some later stage. 

In general, small numbers of ascarids are 
relatively harmless, but large numbers can 
cause serious illness. Sometimes they wan- 
der away from their usual haunts: they 
may crowd into the appendix or perforate 

Fig. 10-2. One of the largest round worm parasites 
found in the intestine of both man and the pig as 
well as other animals is Ascaris /ombr/coides. The 
male is slightly smaller than the female and it pos- 
sesses a curved posterior end. The female is about 
25 cm. long. 

the gut wall, causing peritonitis; they may 
even get into the nasal chambers, obstruct- 
ing the air passages when full grown. When 
large numbers of larvae move through the 
lung tissue, they are apt to leave lesions 
which may give pneumonia an opportunity 
to gain a foothold. Appropriate vermifuges 
can be used to remove this parasite. 

The adult ascaris probably maintains its 
place in the intestine by active movements, 
since it does not possess an attachment or- 
gan such as the flukes and tapeworms do. 
It feeds on the food in the gut by a pump- 
ing action of its bulb-like pharynx. To keep 
from being digested by the enzymes se- 
creted by tiie host, ascaris, like all intestinal 
parasites, is protected by a tough cuticle, 
through which probably is secreted sub- 
stances that counteract the hydrolizing 

■A:,\x:.\'-\\\\^r..'^.:r.>.\--' .•.•.\\'':.\\\i::-- :.•'•:.:: :-.-y---'^ •■•-■■•.• •..•.••:.•.• •:..•■ 

Fig. 10-3. A schematic representation of the life cycle of Ascaris /ombricoides as it occurs in humans. 



Fiq 10-4 Life cycle of the common hookworm (Necafor americanus). The adult worms are shown attached to the 
lining of the intestine. The fertilized eggs begin development while still in the digestive tract. They pass out with 
the feces and eventually hatch in the soil where they lie in wait for their next host. 

power of the enzymes. Oxygen must be ob- 
tained from carbohydrate breakdown within 
its own body, since there is very httle oxy- 
gen in the gut. The only hope for survival 
is to produce a great many eggs, which it 
does most effectively. A large female has 
been known to contain 27,000,000 eggs, 
200,000 of which she lays every day. As in 
all parasites, the chance for any one egg to 

produce a mature worm is extremely re- 
mote, but by this colossal effort to bring 
forth potential offspring ascaris has been 
very successful in the world, as attested by 
its universal occurrence. 

A notorious relative of ascaris is the hook- 
worm {Nccator americanus), which is di- 
rectly responsible for untold misery and 
indirectly for the death of millions of peo- 


normal hosts- piq and rat 

infected pork S crops and rofs 

cyst in 
piq or rat muscle 

poorly cooked pork 
conta\n>nq cysts 

accidental host - man 

larva in blood vessel 

Fig. 10-5. Life cycle of trichina (Trichinella spiralis). 

pie throughout tropical and subtropical re- 
gions of the world. People in certain com- 
munities of our southeastern states are 
heavily infested which, in part at least, is 
responsible for their "poor white trash." It 
is little wonder that they are lazy and shift- 
less when their intestinal walls are teeming 

with hookworms sapping their strength. 
Children tend to be retarded both physi- 
cally and mentally which, coupled with ex- 
treme poverty and ignorance, dooms them 
to a life filled with misery, frustration, and 

The adult hookworm differs from ascaris 



in that the mouth is provided with teeth so 
it can cHng to the soft mucosal lining of the 
intestine from which it withdraws its food, 
blood (Fig. 10-4). Fertile eggs pass out 
with the feces of the host and are deposited 
on the ground where they hatch into larval 
worms. After a brief growth period the lar- 
vae are ready for their next host. They gain 
entrance by holding on to the foot or any 
other part of the body of the host, boring 
in, and finally getting into the blood stream. 
They then follow the same path described 
for ascaris, eventually reaching the intes- 

Preventive measures are so simple that 
one wonders why there are any cases of 
hookworm at all. Wearing shoes prevents 
the worms from getting into the host; 
proper methods of disposing of human ex- 
creta would also stop the infection very 
swiftly. Both of these methods have been 
tried with reasonable success but the inci- 
dence of hookworm disease is still much too 
high in this country. Perhaps the most im- 
portant factor is poverty; if all people were 
gainfully employed and had an adequate ed- 
ucation this disease would be completely 
eradicated. World-wide measures could 
stamp it out altogether, but such suggestions 
are only wishful thinking at the present 

Another roundworm parasite that is of 
grave importance to man is trichina ( Trichi- 
nella spiralis), a worm whose normal hosts 
are the pig and rat, although it has been 
found in other vertebrates as well. Man is 
an accidental host and is therefore perhaps 
even more severely affected by the parasite. 
The life cycle of trichinella varies somewhat 
from the two preceding examples of round- 
worm parasites (Fig. 10-5). The common 
source of human infection is through the 
muscle of the pig, which harbors trichina in 
its infective stage, small cysts containing 
larvae (Fig. 10-6). If these are eaten, 
uncooked, the tiny worms ( 1 mm. long ) 
emerge in the intestine where they mature 
and copulate. The very tiny worms depos- 

F!g. 10-6. Larval Trichinella cysts in the muscles of a rat. 

ited by the female bore through the in- 
testinal wall into the blood stream and dis- 
tribute themselves through the muscles of 
the body, attacking particularly those of the 
tongue, eyes, diaphragm, and ribs. It is this 
migratory period that is dangerous because, 
in addition to the mechanical injury that 
millions of worms can inflict, there is also 
the likelihood of bacterial infections. The 
disease at this stage is characterized by 
high fever, intense muscular pains, and 
frequently partial paralysis. A sufficient 
amount of infected meat can cause death at 
this time, but if the infection has been 
light enough not to cause permanent dam- 
age to nervous and muscular tissue, the 
symptoms will subside and the person re- 
cover. Infections are far more common than 
records indicate. For example, a large Mid- 
western hospital reported that 27 per cent 
of its autopsies showed positive trichinosis, 
although none of the deaths was directly 
caused by that disease. 

Preventive measures are even simpler 
than for hookworm: merely cook pork. 
There is no treatment for the disease once 


«? ^ 


Fig. 10-7. A. The microfilariae that cause the 
disease elephantiasis live in the blood of man 
and can be seen in blood smears taken at 
certain times of the day. The tiny worm is 
clearly visible in this picture and its relative 
size can be determined by comparing it to 
the small irregular objects which are the white 


B. A case of elephantiasis. (From Smith and 
Gault, Essentials of Pathology, 1938, D. Apple- 
ton-Century Company.) 



an infection has gotten under way. It is 
also well to remember that meat sold on the 
market is not inspected for trichina, pri- 
marily because it is too difficult to detect 
light infections. One poorly cooked ("pink") 
pork chop can contain billons of worms, 
which are adequate to kill a person. It is 
true that there are fewer and fewer cases of 
trichinosis reported, probably because the 
practice of feeding meat scraps to hogs is 
less prevalent and also because a general 
war on rats has cut down the rat-hog cycle 
which normally keeps the worms going. 

There are numerous parasitic nematodes 
that cause bizarre diseases in the tropics, 
diseases which are normally known only to 
parasitologists and medical men who are 
experts in the field of tropical medicine. 
However, during World War II the tropics 
became the battleground for many Ameri- 
can men and consequently tropical diseases 
suddenly loomed as a significant health 
problem. Among the numerous roundworm 
parasites the one that causes elephantiasis 
( Wiichereria hancrofti ) is perhaps the most 
important. The life cycle of this worm dif- 
fers from that of other nematodes in that it 
requires two hosts. The larvae, called micro- 
filariae, circulate in the blood of the in- 
fected person (Fig. 10-7). An interesting 
adaptation is that these tiny worms come 
out in the peripheral blood vessels in the 
evening, a time when the mosquitoes which 
are the carriers (intermediate host) are 
active. The mosquito picks up the micro- 
filaria with the blood as it feeds; inside the 
mosquito the worm grows and eventually 
makes its way out through the proboscis 
of the host. During the biting process the 
microfilaria slips off the proboscis onto the 
skin of the next host and immediately bores 
its way in. Once in the blood stream of 
the final host it moves into the lymph 
glands where it becomes mature. The 
worms become so numerous that they can 
effectively clog the lymph passages, which 
results in huge swellings, usually in the ex- 

tremities. A leg may grow to weigh 100 
pounds, hence the name elephantiasis. 

Preventive measures consist in keeping 
from being bitten by infected mosquitoes. 
Light infections are not serious because 
eventually the body forms new lymph chan- 
nels so the swelling is reduced to normal. 
The danger lies in continual infection where 
the same individual is bitten again and 
again by infected mosquitoes. 


The second step to higher complexity, 
namely, the development of a circulatory 
system, first appears in the phylum Nemer- 
tinea, representatives of which are some- 
times called "band worms" because of their 
long ciliated flat bodies. The nemertines 
live primarily in the ocean where they crawl 
among the rocks. They are highly colored 
and greatly elongated, sometimes measur- 
ing as much as 80 feet in length. If cap- 
tured, their bodies stretch so that they often 
break into two parts under their own 
weight, but regeneration occurs as readily 
as it does in planaria. They are also able to 
break their bodies into many parts, a proc- 
ess called autotomy (self-cutting), which 
is not an uncommon characteristic among 

In addition to a complete digestive tract, 
the nemertine possesses a very primitive 
circulatory system (Fig. 10-8). It consists of 
three blood vessels that run throughout the 
length of the body, but which do not break 
up into tiny capillaries as in higher animals. 
In these forms oxygen and food still diffuse 
some distance through fluid before arriving 
at the cells. Although still inefficient, this 
method is a considerable improvement over 
that found in lower forms, where diffu- 
sion of digested material and oxygen from 
sources of intake to cell must pass a greater 
distance. The circulating fluid ( blood ) con- 
tains red cells in some species, much like 
those in higher animals. The red color is 


lotcral blood vessel 



dorsal blood vessel 

nerve cord 

lo+erol blood vessel 

ig. 10-8. A longitudinal and cross-seCion of a ne.ertine showing the t„be-wi.hin-a-tube body p.on 
^ and also the beginnings of a circulatory system. 



due to hemoglobin, which makes up the 
major portion of the material found in these 
cells. The affinity of this pigment for oxygen 
makes it useful in an oxygen-carrying ca- 

Another deficiency of the nemertine cir- 
culatory system is the lack of a pumping 
station. The only propulsive force for the 
blood is furnished by the contractions of 
the animal as it swims. This massaging; ef- 
fect causes the blood to flow along in the 
vessels. These deficiencies of the blood 
system are veiy great, which is perhaps 
one reason why these animals have never 
achieved greater success. But imperfect as 
it is, it is a great step forward over the more 
primitive arrangements for distribution. 


Not only are the features initiated in ear- 
lier phyla further developed in the phylum 
Annelida, but new features are also intro- 
duced. Two principal ones are segmenta- 
tion, or metamerism, and the formation of 
a body cavity, the coelom. These two in- 
novations have made it possible for animals 
to grow larger bodies and to develop more 
complicated neuro-muscular mechanisms, 
thus permitting greater and more diversi- 
fied activity. 

The most obvious difference between an 
annelid and lower forms is its segments. 
These are serially similar parts, conspicuous 
both on the outside and the inside of the 
body. Internal organs are repeated in every 
segment, each part resembling all others 
in most respects. There are modifications, 
however, in certain body regions, as we 
shall see. There is an intricate connection 
between the segments, so that the animal is 
a coordinated whole. Segmentation is re- 
tained in higher forms such as the arthro- 
pods and the chordates, and is therefore a 
successful idea that has contributed toward 
the greater complexity of animal bodies. 
Just how segmentation came about is con- 
jectural. It may have resulted simply from 


individuals dividing asexually by transverse 
fission, remaining attached, and eventually 
becoming integrated into a coordinated 
whole. Phenomena leading to support this 
explanation can be found among some of 
the flatworms (see p. 79). 

The members of the phylum Annelida are 
mostly free-swimming marine worms. They 
abound in the oceans and live near the 
shore and hide in the sand in burrows dur- 
ing their quiet periods. Some biologists 
believe that the annelids are intermediate 
between the lowest protozoan forms and 
the highest vertebrate animals; in other 
words, they represent the halfway mark up 
the long path of evolution. Since they form 
the basis for further development, it is nec- 
essary to examine this group of animals 
carefully, which we shall do by studying 
two representatives, the sandworm (Nean- 
thes), and the common earthworm (Liim- 
briciis). Of these Neanthes is more typical 
because it is aquatic and possesses more 
of the characteristics of the phylum. The 
earthworm, on the other hand, is terrestrial 
and in many respects is quite unlike most 


Neanthes (formerly known as Nereis), 
the common "sandworm" or "clamworm" 
(Fig. 10-9), lives in shallow water on the 
sandy shores of most oceans of the world, 
where it is found in small burrows with its 
head and tentacles protruding slightly. 
When small animals venture too close, the 
worm suddenly everts its heavily armed 
proboscis, seizes its prey, and drags it into 
the burrow to be devoured. When at rest 
the worm actually "stands" in its burrow 
and undergoes a constant undulating move- 
ment, creating a current of water that flows 
in and out for breathing. The worm leaves 
its burrow during the breeding season but 
only rarely does it leave otherwise. 

The segments of Neanthes are conspicu- 
ous externally, especially because each one 
possesses a pair of laterally placed paddles, 



Fig. 10-9. The "sandworm" (Neanthes) is a common inhabitant of our Atlantic Coastal waters. It lives in a bur- 
row in mud or sand from which its head and tentacles protrude. Small animals passing near enough are snatched 
and devoured. During the breeding season worms leave their burrows and congregate in great numbers near 
the surface of the sea. 

called parapods, which function like oars 
on a boat to propel the animal through 
the water. In addition, snake-like undula- 
tions of the body aid in swimming. Paired 
bunches of bristles (setae), located in the 
parapodia, hold the animal in its burrow, 
should an outside force attempt to remove 
it. The head is a distinct structure well 
provided with sense organs in the form of 
four eyes, and two tentacles which appear 
to be tactile in function (Fig. 10-10). 
There is a protrusible pharynx which ter- 

minates in a pair of fierce-looking jaws. The 
sturdy muscular body is covered with a 
cuticle which takes on an iridescent sheen 
in the sunlight. One is impressed by its 
unique beauty as it glides through the 
water with its graceful undulations. 

The tube-within-a-tube body plan is con- 
spicuously evident when one studies Nean- 
thes internally (Fig. 10-11). The internal 
organs are serially repeated in each seg- 
ment with the exception of the first and 
the last. The gut, a straight tube passing 

pharynx extended 

pharynx retracted 

Fig. 10-10. Side views of the head of Neanthes showing the pharynx extended and refracted. 
















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from mouth to anus, is surrounded by a around in the dark. The centrahzation of 

sheet of tissue, the peritoneum, which forms the nervous system, initiated in planaria, is 

the walls of the coelom, a very important carried much further in Neanthes. Not only 

cavity introduced among the annelids for has the nervous tissue concentrated into 

the first time and referred to earlier. The two large masses, but also each segment has 

coelomic cavity in the annelids is decidedly a similar enlarged ganglion. With this or- 

different from the body cavity of the round- ganization the animal has a well-developed 

worms which had no lining whatever. The means of coordination, a far cry from the 

coelom functions in nutrition, since a large nerve net of hydra. 

portion of the food and waste products dif- If Neanthes is placed in a dish of sea 
fuse into the coelomic fluid and through it water, it keeps up violent swimming move- 
reach their ultimate destination. ments, but if a test tube is placed in the 

The circulatory system of Neanthes is far container the worm backs into it almost 
superior to the one found in tlie nemertines. instantly and quiets down. This is presum- 
Tiny capillaries now allow a rapid ex- ably due to the tactile stimulation from the 
change of oxygen and carbon dioxide, as tube wall, reminiscent of the burrow. Move- 
well as food products. Furthermore, circu- ments during the breeding period also indi- 
lation is maintained by contraction of the cate a highly developed neuromotor mecha- 
large blood vessels themselves, rather than nism. 

by the body as a whole. Here, then, is seen In the nights of late July and August, 

for the first time a contractile portion of the Neanthes gather in great numbers along the 

system which serves as a heart to keep the eastern coast of the United States for the 

blood circulating continuously. The blood, purpose of shedding their eggs and sperms, 

like that of the nemertines, contains hemo- By illuminating a small area of an oppro- 

o-lobin althoueh it is not confined in cells, priate ocean surface, thousands of worms 

but is free in the plasma. may be seen at this time churning the 

Neanthes receives its oxygen and elimi- water by swimming at a rapid, erratic rate, 
nates its excess carbon dioxide through the and swirling in apparent frenzy. Suddenly 
thin walls of the parapods. The frequent the females seem to split open along their 
waving movement of these organs facili- sides, discharging their eggs into the sea 
tates rapid gas exchange. Each segment has water like a white cloud. The males, which 
a pair of small coiled tubules with an inter- are smaller than the females, shed their 
nal opening, the nephrostome, which resem- sperm in a similar manner. Both sexes die 
bles a tiny ciliated funnel, and an external shortly after extruding their gonadal prod- 
opening, the nephridiopore, at the opposite ucts. By dipping up some of this water it 
end, which perforates the body wall. The is possible to observe the early stages in 
nephrostomes lie in the segment anterior to the embryology of this animal. The eggs 
the tubule and the nephridiopores. They undergo segmentation and develop into 
take in coelomic fluid from which nitrog- free-swimming ciliated larvae, called troch- 
enous wastes are extracted in the tubule ophores, which promptly settle to the bottom 
and excreted through the nephridiopore. and metamorphose into young Neanthes. 

This excretory system is considerably more 

1 ^1 X i- 1 • 1 Relatives of Neanthes 

complex than the one noted m planaria, 

although fundamentally it is similar. There are many close relatives of Nean- 

Neanthes shows more varied and specific thes living along the ocean shores, although 

responses to the external world than are some are found at great depths. They range 

found in lower forms. Its four eyes and from miscroscopic fomis to those that reach 

sensitive tentacles aid the worm in getting 10 feet in length. Some are active swimmers 



and live in the open ocean catching their 
prey in flight, whereas others such as the 
sea mouse (Fig. 10-12) crawl over the 
ocean floor. Many construct burrows out of 
mucus, such as Chaetopterus (Fig. 10-13); 
others bore into rocks to provide a home for 
themselves. Some are highly colored, such 
as the peacock worm (Fig. 10-14) which 
could easily be mistaken for a flower. 

Many of these worms have spectacular 
breeding habits. One, the palolo worm 
(Eunice viridis) of Samoa and Fiji, spawns 
in a most remarkably regular and peculiar 
manner. On the first day of the last quarter 
of the October-November moon, the pos- 
terior portion of the worm, heavily laden 
with eggs or sperms, breaks off from the 

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Fig. 10-T2. Aphrodite {AphrodHa hastata), the sea 
mouse, from ventral view which definitely establishes 
it as an annelid. From the dorsal side it resembles a 
furry animal, hence its name. It is about 12 cm. long. 

Fig. 10-13. This annelid (C/iaefopferus) lives in a tube 
secreted by its own body. The appendages are used 
as paddles to keep the water circulating through its 
tube, thus bringing in oxygen and small animals 
upon which it feeds. It Is luminescent, which seems 
strange since there is no opportunity for any other 
animal to appreciate its beauty. This female speci- 
men, which is removed from its burrow, is about 15 
cm. long. 

parent worm and swims to the surface 
where the gonadal products are discharged. 
The surface of the sea is milky white with 
the great numbers of these cells. Natives, 
who are familiar with the exact time of 
spawning, collect these worms when they 
are about to spawn and feast on them. 

The earthworm 

No discussion of the phylum Annelida is 
complete without a study of the lowly 
earthworm, spurned by the squeamish and 
cherished by the fisherman and robin. It 
seems striking; that this creature, which is 
so helpless when removed from its burrow, 
has been able to spread its kind over nearly 





( * 




Fig. 10-14. This annelid, called the peacock worm (Fu- 
distyl'ia), builds its long tube in sand among the 
rocks. The miniature head, along with many bril- 
liantly colored gills, protrudes from the end of the 
tube. The gills function in breathing and food gath- 
ering, since they are covered with a sticky mucus in 
which minute sea animals become enmeshed and are 
then drawn to the mouth by means of cilia. Peacock 
worms in clusters resemble a bunch of flowers; their 
real identity becomes apparent when they suddenly 
withdraw the tentacles into the tube. 

the entire surface of the earth. This is even 
more surprising when it is known that most 
of its relatives are aquatic forms. It appar- 
ently deserted its ancestral watery environ- 
ment and blazed a trail into a terrestrial 
habitat of semi-solid soils. In this transition 
it lost certain of its ancestral parts and ac- 
quired others. There are few species of 
earthworms, while there are many species 
of aquatic forms. 

The industrious robin searching for 
earthworms is a common sight, but other 
animals, such as moles, amphibians, small 
snakes, and fish, also feed upon this form. 
Some species of earthworms have been 
found to be intermediate hosts for such 
parasites as gapeworm and tapeworm of 
fowls, and lungworms of pigs. Hog influ- 
enza is caused by the mutual action of a 

virus and a bacterium contracted when 
lungworms borne by the earthworm invade 
the respiratory tract. They are highly bene- 
ficial to man by constantly tunneling the 
soil, thus permitting a greater circulation of 
water and air. Charles Darwin noted that 
their castings on fertile soil amounted to as 
much as 18 tons per acre per year. This 
constant elevation of subsoils to the surface 
tends to cover rocks and gravel, thus mak- 
ing the topsoil more tillable. In this sense, 
too, the earthworms benefit man substan- 

External anatomy. The large species, 
Ltimbriciis terrestris (Fig. 10-15), is a good 
example of an earthwonu. Its most con- 
spicuous external characteristic is its seg- 
mentation, resembling Neanthes in this re- 
spect. Mere vestiges of the parapods remain 
in the form of four pairs of very short 
setae located on each segment. These are 
used for traction in locomotion and their 
effectiveness in this capacity is easily ascer- 

Fig. 10-15. The common earthworm (Iwmbricus terres- 
tris). This specimen had just projected the head end 
forward and had contracted the posterior half of its 
body in its typical crawling movements. Note the 



tained by catching a worm that has its 
posterior end partly within its burrow. A 
slow steady pull removes it, whereas a sud- 
den one leaves the intruder with only a por- 
tion of the worm in his hand, the other end 
securely held in the burrow by the stiff 

There are no conspicuous sense organs on 
the head end, like the eyes or tentacles of 
Neanthes. Indeed, the animal seems to lack 
a head, though it does have a protruding 
"lip," the prostomium, which covers the 

The saddle-shaped clitelhim rests on the 
dorsal side about one-third of the way from 
the anterior end. There are also several 
openings which can be seen by careful in- 
spection. Most noticeable are those of the 
sperm ducts, which open on the fifteenth 
segment. The fourteenth segment bears the 
smaller openings of the oviducts, and each 
segment except the first three and the last 
bears a pair of nephridiopores. Finally, the 
four openings which lead into the seminal 
receptacles are located in the grooves be- 
tween segments 9 and 10, and 10 and 11. 

Internal structures. A section of the body 
wall (Fig. 10-16) shows the outer thin, 
tough cuticle, which serves as a protective 
layer for the tall columnar epithelial cells 
composing the bulk of the epidermis. 
Among these latter are scattered sensory 
cells, sensitive to light, touch, and chemical 
stimulation. Other cells dispersed among 
the epithelial cells are the mucus-secreting 
cells responsible for the slimy condition of 
the skin, which is essential both for respira- 
tion and locomotion. Beneath the epidermis 
lie two layers of muscle, the outer circular 
and the inner longitudinal. These function 
in locomotion. Lying beneath the muscle 
layers and lining the coelom is the peri- 
toneum. The digestive tract is as it is in 
Neanthes, making the tube-within-a-tube 
plan conspicuous. By removing the dorsal 
wall throughout the anterior half, the inter- 
nal anatomy can be studied. The segmen- 
tation which is so striking externally is just 

as conspicuous from the inside. Membra- 
nous partitions, septa, which wall off each 
segment, are perforated by the gut, nerve 
cord, blood vessels, and nephridia. 

Beginning at the anterior end, the diges- 
tive tract starts with the mouth, which 
opens almost immediately into the large 
muscular pharynx; this latter organ is used 
as a kind of pump to draw food into the 
mouth. Following the pharynx is the esoph- 
agus, which opens into the crop, a storage 
sac. This in turn leads into the gizzard, 
which functions in the grinding of food, 
much the same as a similar ortran in the 
chicken. The remainder of the gut is a long 
tube, the intestine. This organ bears a fold 
along its dorsal side, the typhlosole, which 
increases the surface of the 2ut without 
increasing the volume of the animal. A 
straight tube, from mouth to anus, suffices 
for small animals, but larger animals re- 
quire a tube with still greater surface area, 
both for digestion and absorption of food. 

Various gland cells are located through- 
out the digestive epithelium. Some produce 
digestive enzymes, while the secretion of 
others lubricates as well and thus facilitates 
the movement of food. Lateral to the esoph- 
agus and attached to it are three pairs of 
calciferous glands, which function in se- 
creting calcium carbonate for neutralizing 
any acid soil that may be taken in with the 
food. The epithelial lining of the gut se- 
cretes fat-splitting, carbohydrate-splitting, 
and protein-splitting enzymes. The gut is 
surrounded by chlorogogen cells which are 
derived from the peritoneum and probably 
function in the elimination of wastes from 
the blood. It is believed that fat is also 
stored in these cells. 

Food for the earthworm consists of leaves 
and any other available organic matter, 
even bits of meat. Much soil is taken in with 
the food and used later in the gizzard for 
grinding the food in preparation for diges- 
tion. Food is temporarily stored in the crop 
before it passes into the gizzard, where it 
is ground to a fine mass. It then passes on 


longitudindl muscles 
eireulor muscks 

photoreceptor eel* 

nerve cord 

Fig. 10-16. Longitudinal and cross-section of the earthworm. 




dorsol blood 

ventrol blood 

dorsal vassal 



narva cord 


to nepbridium 

to body woll 
lotar'al vassal Isasmant'ol vessel 
Subneurol vessel from body woll 

Fig. 10-17. A schematic view of the circulatory system of the earthworm. In the lower portion, one segment has 

been greatly enlarged in order to show the course of the blood. 

to the intestine where digestion and absorp- 
tion are carried on. Undigested material 
passes out through the anus. Earthworms 
deposit their "castings" on the surface of 
the ground near tlieir burrows. When mil- 
hons of worms continue this process for 
centuries in the same areas, the result is 
a constant inverting of the soil reminiscent 
of plowing in agriculture. The castings of 
the worms also greatly enrich the soil. 

The circulatory system (Fig. 10-17) of 
the earthworm is similar to that of Neanthes. 
However, there is an improvement in the 
pumping system in the form of five pairs of 
"hearts" which surround the esophagus and 
connect the dorsal with the ventral blood 
vessel. In addition to the peristaltic waves 
that move the blood forward in the dorsal 
blood vessel, the "hearts" send it by a rhyth- 

mic contraction of their walls to the ventral 
blood vessel with considerable force. There 
are five principal blood vessels in the earth- 
worm which convey blood to all parts of 
the body. The dorsal and ventral blood 
vessels are the main vessels that carry blood 
to and from the "hearts." The laterals, lo- 
cated on each side of the nerve cord, 
receive blood from the ventral vessel and 
carry it to the subneural vessel via the 
nerve cord. The blood then passes into the 
segmental vessels which convey it up to 
the dorsal blood vessel, picking up blood 
from both the nephridia and the body wall 
on the way. Short blood vessels extend from 
the dorsal vessel and convey blood to and 
from the intestine; blood also enters the 
intestine from the ventral vessel. A careful 
study of Fig. 10-17 shows the plan of the 






Fig. ]0-18. A schematic side view of one segment of the earthworm showing the details of the 
nephridium. The nephrostomes have been greatly enlarged. 

system, namely, five principal vessels 
running the length of the worm, with nu- 
merous cross-connections and branches 
supplying blood to all parts of the body. 
These are the first animals to possess a com- 
plete circulatory system. All animals above 
this group also have well developed circu- 
latory systems. 

The blood of the earthworm contains 
many white blood cells (leucocytes) float- 
ing in the fluid plasma. The respiratory 
pigment is hemoglobin, which is carried 
free in the plasma, not in corpuscles. The 
gas exchange takes place where the capil- 
laries come close to tiie surface of the epi- 
dermis. For this reason the surface of the 
animal must be moist at all times, for a dry 
membrane will not allow the gaseous ex- 
change to take place. 

The arrangement of the nephridia is 
similar to that in Neanthes. The nephridium 
consists of a small ciliated funnel, the 
nephrostome, which opens into a tiny coiled 
tubule (Fig. 10-18). This penetrates the 
septum of the next segment, where it coils, 
gradually becoming larger and finally ex- 

panding into a bladder-like sac before 
opening to the outside through the ne- 
phridiopore. Nephridia are found in all of 
the segments except the first three and the 
last. The actual excretory process is carried 
on by the beating of the cilia of the nephro- 
stome and the lining of the tubule, which 
causes waste products from the coelom to 
enter the tubule and be discharged from 
the body. Waste materials in the blood are 
picked up by the glandular portion of the 
tubule and excreted directly. The chlorogo- 
gen cells may also aid in this process. 

Behavior. As might be expected, the be- 
havior and hence the nervous system are 
more complex in the earthworm than in the 
lower types. The center of the nervous sys- 
tem is a bilobed "brain," located in the 
anterior region dorsal to the digestive tract 
(Fig. 10-16). The circumpharyngeal con- 
nectives connect to the ventral nerve cord 
which consists of a series of ganglia much 
the same as in Neanthes. There are a few 
nerve fibers extending into the prostomium, 
suggesting that this organ is probably sensi- 
tive to touch. 



Research on the nervous system of the 
earthworm has revealed that it possesses 
the components of reflex arcs very similar 
to those of man ( Fig. 10-16). Impulses come 
in from the external world through the 
sense organs on afferent, or sensory, fibers, 
pass to association neurons, and from these 
to efferent, or motor, fibers, which run out 
to muscles or glands. Thus in a form as low 
as the earthworm, there is an intricate nerv- 
ous mechanism which enables the animal to 
carry out complex operations. 

It is of interest to biologists as well as 
psychologists to know when the nervous 
systems of animals become complex enough 
to permit the storage of response patterns 
which we call memory. In other words, how 
far up the tree of animal life must we go 
to find animals that can profit by ex- 
perience, or learn? One experimentalist 
(Swartz, 1929) found that an earthworm 
which at first would enter either branch of a 
Y-shaped tube could learn, after several 
hundred trials, to avoid one branch of the 
tube if an electrode were placed in it and 
the worm received a shock each time it 
entered that branch. This is perhaps the 
first animal so far considered that can profit 
by past experience. 

There are no obvious sense organs pres- 
ent on the earthworm's body but micro- 
scopic examination of the epidermis reveals 
several kinds of sensory cells scattered 
among the ordinary epithelial cells which 
connect directly to the nervous system and 
function as sense organs (Fig. 10-16). Such 
cells are located in the parts of the body 
that are most likely to come in contact with 
the environment: the prostomium, the por- 
tion of largest diameter in each segment, 
and the mouth cavity. Some of these cells 
are specialized for light reception, while 
others respond to chemical and tactile 

The earthworm responds readily to light. 
If removed from the burrow, the worm 
becomes very active and immediately at- 
tempts to get away from the bright light 

and crawl into a crevice or burrow. If 
sought for at night with a flashlight, as is 
the habit of those who search for the so- 
called "night crawlers," the moment the 
light strikes, the worm retracts into its 
burrow with almost lightning-like speed, so 
fast that one must be very agile to catch it. 
It appears, then, that light is readily per- 
ceived and interpreted. 

If an earthworm is experimentally placed 
near any volatile chemical, such as acetic 
acid, it responds violently and moves in the 
opposite direction. Likewise, it moves to- 
ward bits of meat or decaying vegetation, 
which, of course, are its food. In this case 
the sense organs appear to be located in the 
mouth cavity. 

Since the earthworm is dependent on 
being able to find its way around under- 
ground in completely dark passages, it 
must rely on the sense of touch perhaps 
more than any other. The tactile end organs 
( Fig. 10-16) are bundles of modified epithe- 
lial cells with tiny protruding hair-like 
bodies that are in contact with anything 
that touches the body wall. They probably 
also function as a sort of hearing device, 
since any vibration on the earth near the 
burrow, as in the case of footsteps, causes 
the worm to respond readily. They do not 
respond to air-borne vibrations which af- 
fect our ears. 

The earthworm is sensitive to tempera- 
ture; it avoids cold and hot areas and seeks 
out moderate temperatures. In its natural 
environment it lies near the surface in its 
vertical burrow with the "head" uppermost 
when the temperature is moderate, reced- 
ing if it is too cold or too hot. In winter 
it burrows down below the frost line and 
remains inactive throughout the cold sea- 
son; it does likewise when the soil becomes 
dry during a drought. It cannot tolerate 
desiccation and therefore responds posi- 
tively toward moisture, but only to a certain 
point, since it cannot withstand immersion 
in water for a long period of time. After a 
rain it is common to see on sidewalks earth- 



worms which have been "drowned" out 
of their burrows. The apparently fantastic 
stories of earthworms falhng from the skies 
during storms are perhaps founded on the 
same basis as the stories of frogs and other 
animals having "rained." This usually fol- 
lows a tornado where masses of water have 
been carried into the air, resembling a 
water spout in the ocean, and these animals 
are taken along in the water and released 
many miles away. 

The bristle-like setae are the earthworm's 
chief organs of locomotion. While the ani- 
mal is in its burrow the setae in the pos- 
terior end project out and are imbedded in 
the wall of the burrow. This is done by 
contraction of the muscles at the base of the 
setae. Those at the anterior end then relax 
and the circular muscles contract, thus 
elongating the worm. The anterior setae 
then secure their end of the animal and 
the longitudinal muscles contract, bringing 
the posterior end forward. In such fash- 
ion the animal moves through its burrow. 
Since the setae are located on the ventral 
sides as well as the lateral walls, the animal 
is able to crawl slowly over a surface when 
removed from the burrow. If the anterior 
segments containing the brain are removed, 
the worm seems to move in a normal 
fashion, which means that the nerve centers 
for crawling movements are located in the 
ganglia and not centered in the brain. 

Reproduction. In order to survive in its 
terrestrial habitat, the earthworm has been 
forced to undergo some drastic adaptations 
in its reproductive system. It will be re- 
called that most annelids discharge their 
sex products into water where union of the 
eggs and sperms is purely fortuitous. On 
land, obviously, some other means must be 
provided to bring about this union and to 
insure adequate conditions for the develop- 
ing embryo. A most unique method has 
been devised for this purpose. In the first 
place, the sexes, which are separate in other 
annelids, are united in the earthworm, that 
is, it is monoecious, or hermaphroditic. This 

has the advantage of making it unnecessary 
for worms of different sexes to unite; any 
two worms can exchange sperms. This is 
important because the chances of animals 
meeting are less than would be the case in 
water where seasonal aggregations occur. 
The ovaries and testes are located in the 
anterior end of the worm where ducts pro- 
vide the proper exit for eggs and sperms 
(Fig. 10-19). There are two pairs of tiny 
testes located in the tenth and eleventh seg- 
ments, surrounded by large sac-like bodies, 
the seminal vesicles, which are storehouses 
for the sperm cells. Funnels direct the 
sperm into the sperm ducts which open to 
the outside on the fifteenth segment. A pair 
of ovaries cling to the posterior wall of the 
septum in segment 13, and small funnels 
catch the eggs and direct them into a sac 
where they are temporarily stored. Even- 
tually the eggs pass to the outside through 
the oviduct in sesi;ment 14. 

The process of exchanging sperms occurs 
at night, usually following a rain. Two 
worms become attached along their ventral 
sides, as shown in Fig. 10-19; this usually 
occurs while the posterior ends of the 
worms remain in the burrows. Some, how- 
ever, crawl some distance away from the 
burrow until contact is made with another 
worm. A slimy material is then secreted 
mutually by the worms which, as it hard- 
ens, encases both animals together in a 
temporary sheath. Small tubular passage- 
ways form between the sheath and the body 
walls, thus providing a pathway for the 
sperms, which are forced from each worm 
along these channels until they reach the 
seminal receptacles of the other worm. 
After this exchange of sperm cells the ani- 
mals separate. 

At some later period when the eggs are 
mature, the clitellum secretes a mucous 
sheath which slips forward like a tight 
sweater over a man's head. In the vicinity 
of the fourteenth segment eggs are forced 
into the mucous ring, and as it slips over 
the ninth and tenth segments sperms pass 

seminal receptode fertilization 


Fig. 10-19. Copulating earthworms, showing the various steps In the fertilization of the eggs and subsequent 

cocoon formation. 



out of the seminal receptacles and unite 
with the eggs. After the mucous ring slips 
over the anterior end, the ends contract, 
forming a closed capsule, or cocoon, full of 
fertilized eggs, which is left behind in the 
burrow. There is no larval stage and the 
eggs hatch into small earthworms which 
penetrate the wall of the mucous capsule 
and begin to shift for themselves. 

Relatives of the earthworm 

Earthworms have many relatives which 
range in size from microscopic forms to 
species which may reach 10 feet in length. 
There are well over 2,000 species and most 
of them are smaller in size than the com- 
mon earthworm. Although nearly all live in 
damp soil, some are found in fresh or 
polluted waters. One form, Tuhifex, is en- 
couraged to grow in filter beds of sewage 
disposal plants in order to keep the filter 
open. They are considered very valuable 
for this purpose and specimens are often 
shipped to new filters to start the "culture" 
going. The common blood worm, which is 
a species of Tubifex, is found in tvibes at 
the bottom of fresh-water ponds where it 
feeds on the muck and perhaps aids in the 
purification of such waters when they are 
polluted. Another small form, Enchytraeus, 
is sold in pet shops as a source of food for 
small fish. 

Certain of the small forms, such as 
Chaetogaster, reproduce asexually by trans- 
verse fission and sometimes several cling 
together, resulting in a chain of individuals. 
In some species this method has apparently 
replaced the sexual method altogether. 


After a swim in the old swimming hole, 
boys often find small black leeches that 
cling tenaciously to the skin. When re- 
moved, they leave a stream of blood flow- 
ing from the wound. It is also common for 
the fisherman to see a large ( 12 inches 
long) leech, Haemopsis grandis, swimming 

in beautiful undulating movements near 
the surface of the water. Another leech, 
Hirudo medicinalis, was once cultured in 
Europe for the specific purpose of blood- 
letting when that practice was in vogue. It 
is interesting to note that during the last 
century, and before, it was considered 
beneficial to remove blood in certain ill- 
nesses. Today the procedure is the reverse, 
as is indicated by the many blood banks 
over the country. 

In some regions, particularly the tropics, 
leeches live in watering places where large 
vertebrates come to drink. They attach 
themselves to the buccal cavity, sometimes 
in such numbers as to cause serious injury 
to horses as well as other animals, including 
man. Some leeches live on land and are 
occasionally so numerous that they are 
a serious hazard to human beings. Army 
commanders have been known to provide 
their men with leech-proof stockings in 
order to get through such infested areas. 

The leech has many of the annelid 
characteristics, but it lacks setae, and it pos- 
sesses copulatory organs, which other an- 
nelids lack. What appears to be segmenta- 
tion externally does not correspond with the 
actual segments, for there are fewer seg- 
ments internally than appear from the out- 
side. The body has remarkable powers of 
extensibility and contractibility, enabling it 
to move like the measuring worm. The 
hold-fast organs are two suckers, one on 
each end, which are used both in locomo- 
tion and in feeding. In the center of the 
anterior sucker is the mouth, which is 
usually provided with three small cutting 
teeth that inflict a wound when the leech 
is feeding upon its victim. The anus is 
located in the center of the posterior sucker. 
The digestive tract is sacculated so that it 
can retain a large meal of blood. Appar- 
ently this is provided because meals are 
usually few and far between, some leeches 
being able to live a year between feedings. 

When securing a blood meal the leech 
becomes attached to the skin, which it 



pierces with its teeth. An enzyme ( hirudin ) 
is secreted by the sahvary glands, which 
prevents the blood from clotting. Blood 
is sucked by the pumping action of the 
pharynx and stored in the large sacculated 
crop to be passed on into the intestine for 
digestion a little at a time. 

In review, we have seen in this chapter 
the advent of characteristics of key impor- 
tance to the development of more complex 
animal bodies. The acquisition of a coelom 

and segmentation, together with the "tube- 
within-a-tube" body plan, have laid the 
foundation for higher forms. Perhaps the 
features that have appeared so far in 
the animal kingdom are more important than 
any that follow, because later groups, while 
adding some new characteristics, actually 
become complex by elaborating features 
that are already established in the annelids. 
Let us keep these fundamental character- 
istics in mind as we examine the next 
groups of animals. 



The body plan of the arthropods is suf- 
ficiently plastic and flexible to permit mem- 
bers of the phylum to fit into most of tlie 
niches of the earth. Although the arthro- 
pods resemble the annelids in many re- 
spects, they show important innovations. 
One of these is the hard outer skeleton, the 

that the muscles can be attached to the 
inside of this material and function more ef- 
ficiently in moving the body. The exoskele- 
ton supports the entire animal, much like 
the framework of an airplane or automobile. 
One can well realize the difficulty of trying 
to fasten the engine of an airplane to a soft 

exoskeleton, which functions as a rigid coat fuselage. For the same reason the plan of 
of armor. It is as if the cuticle of the annelid the exoskeleton as an over-all superstruc- 
had become thick and rigid. This means ture is apparently a very good one. It is 



composed of a protein substance, chitin, plan to that of annelids, is considerably 
which is impregnated with more or less more centralized. There are fewer ganglia, 
lime, depending on the species. Some and more independence of all parts of the 
arthropods, like the lobster, have a high body. This increased integration has re- 
percentage of lime in the exoskeleton, suited in an animal that is swifter, more 
whereas the insects have smaller amounts, agile, and better able to cope with its en- 
The greater the lime content, the harder the vironment. 

skeleton. For the land dweller, exposed to The coelom is much reduced in size, 

rapid desiccation, a waterproof outer cover- being replaced to a large extent by a sys- 

ing becomes essential. Since the exoskeleton tem of blood spaces called the haemocoel. 

of these air breathers is practically water- There are certain modifications in other sys- 

proof, one may assume that this condition tems also, but these will be discussed in the 

made it possible for the animal to invade various groups, as they occur, 
land. The arthropods include such a wide va- 

One rather serious disadvantage to this riety of forms that it seems as though they 

external plate of armor is concerned with had explored nearly all the possibilities, 

growth. As the animal increases in size, this There are more species in this group of 

suit becomes tighter and tighter, until animals than in all others put together, in 

finally the animal must rid itself of the old fact, several times as many. Furthermore, 

suit and secrete a new one. During the time the number of individuals exceeds all other 

of transition, however, the arthropod finds Metazoa. The arthropods literally encom- 

itself in a very precarious situation. Having pass the earth; they invade the soil, the 

shed the old suit, it must wait for its body water, the air, the frigid polar zones, and the 

size to enlarge and for the new skeleton to torrid equatorial latitudes. Since they feed 

harden before its muscle can again be elfec- on the same foods as man, they have be- 

tive. This procedure may take several hours come his most serious competitor, indeed, 

and during this time the body is very soft it has been said that the main struggle for 

and nearly immovable. The animal is, survival today is between the arthropods, 

therefore, unprotected at such times and particularly the insects, and man. This 

takes special precaution to conceal itself might be true if man does not destroy him- 

from its enemies. The process of shedding self first by his own cunning. Insects are 

the skeleton and growing a new one is carriers of some of the most serious diseases 

called molting. which affect man and his domestic animals, 

Another important difference between and although some species are beneficial, 

the annelids and the arthropods is the pres- it is doubtful that the benefit to man of 

ence in the latter of feet with joints. This some outweighs the damage done by the 

conspicuous character gives the phylum its others, 
name. The feet and legs of animals with a 
hard exoskeleton need joints for the pur- 
pose of movement. In the arthropods many 
appendages have lost theii' original loco- 
motor function and have become radically The phylum Arthropoda is divided into 

modified into organs of defense, offense, several classes (Fig. 11-1), of which only 

and even sense organs. The exoskeleton and four warrant discussion in an introductory 

the jointed appendages thus have distinct text. The first is a very small group, the 

advantages and have contributed much to Onychophora, composed of about 70 spe- 

the success of this group. cies, all of which bear the name of Peri- 

The nervous system, while similar in patus. These small animals are as interest- 


annelid-like ancestor 

Fig. 11-1. Classes of arthropods. 



ing as they are rare, being found only 
occasionally in widely separated places — in 
Africa, Australia, Asia, and the two Ameri- 
cas. Because they are limited to widely 
scattered, isolated places, they are thought 
to have come from the annelid-arthropod 
stock at a very early period in biological 
history, subsequently dying out in the areas 
between. Peripatus lives in the dense, tropi- 
cal forests, in damp places under logs and 
other objects found on the forest floor. The 
inexperienced observer would mistake it for 
a caterpillar because of its long, many- 
appendaged soft body with anteriorly pro- 
truding antennae (Fig. 11-2). Like anne- 
lids, Peripatus has a pair of appendages 
for nearly every segment. These have 
clawed feet but they lack joints, hence un- 
like the typical arthropod appendage. The 
exoskeleton is also very thin, like the cuticle 
of annelids and very unlike the usual ar- 

The excretory system is definitely anne- 
lid-like, while the nervous system is even 
more primitive than that found among the 
annelids. The occurrence of cilia in the 
reproductive tubules is also an annelid 
character, since such structures occur 
nowhere else among the arthropods. On 
the other hand, its circulatory system and 
coelom are similar to those found in other 
arthropods. The respiratory system re- 
sembles that of the arthropods but is suf- 
ficiently different to be thought of as having 
arisen independently. 

It is apparent that Peripatus is, in many 
ways, intermediate between annelids and 
arthropods. This evidence seems to point 
unmistakably to the annelids as the arthro- 
pod ancestor. Of course, Peripatus is not 
identical with the early arthropod, since it 
must have gone through changes itself dur- 
ing this very long period of time. However, 
it seems to have changed but little by com- 
parison to other present-day members of 
the phylum, and it seems safe to say that it 
probably resembles the early arthropod 
more than any other living form. 

(Photo by Ralph Buchsbaum) 

Fig. 11-2. Peripafus is a rare animal that shows both 
annelid and arthropod characteristics and for that 
reason is of considerable interest to zoologists. 

Peripatus is thus a most remarkable ani- 
mal. Some zoologists now place it in a 
separate phylum. It is a pity that it cannot 
be studied in the beginning zoology labora- 
tory, but it is scarce, hence costly. 


The Crustacea make up a large and suc- 
cessful class of arthropods. They live on 
land, in fresh water (Fig. 11-3) and in 
the ocean (Fig. 11-4). They include such 
common forms as fresh-water fairy shrimps 
and crayfish (Fig. 11-5), seashore crabs 
(Fig. 11-6), lobsters, and barnacles (Fig 
11-7). The group is of considerable eco- 
nomic significance to man. It is a valuable 
source of food, as demonstrated by the fact 
that around $10 million worth of shrimp, 
crab, and lobster are marketed annually in 
this country alone. The large leg muscles 
and the choice abdominal muscles of the 

Fig. 11-3. Fresh-water and terrestrial crostacea. Although primarily aquatic, the pillbug lives on land. 

Coe&« Bornael* 

Fig. 11-4. Marine Crustacea. These vary widely, from the sessile barnacle to the burrowing fiddler crab. 

Fig. 11-5. The common crayfish (Cambarus) found over most of the United States. They live in ponds and streams 
where they feed on small fish or any other animal, dead or alive. Note the great area of the expanded tail 
(oropods and telson). This is very effective in locomotion. When disturbed, the animal contracts its tail power- 
fully, thus sending it darting backwards. The crayfish is much like its marine cousin, the lobster. 

A B 

Fig n-6. The large edible blue crab of the Atlantic Coast (Ca//.nectes). A. This crab is agile even on the sandy 
beach. It is moving rapidly toward the ocean in its typical side gait. When out of the water .t .s extremely pug- 
nacious. B. Ventral view of the same animal, showing the large egg mass which is carried about until the 
young hatch. 

Fig. 11-7. (Upper picture) A rock-encrusting acorn barnacle (Ba/anus tinfinnabu/um). Its arthropod characteristic is 
indicatecJ only by the jointed appendages which protrude between the valves of its shell. These are used to strain 
the water for tiny sea animals that make up the diet of this strange arthropod. 

(Lower picture) The goose barnacle (Lepas anotifera) attached to a lobster pot buoy. The long stalks make it 
possible for them to sample larger areas in search of food than is possible for the stationary acorn barnacle. 



lobster and shrimp are the parts usually 
eaten. The fresh-water crayfish (Fig. 11-5) 
and other Crustacea are generally not eaten 
in this country, although in certain parts of 
the world they are considered excellent 
food. In addition, the Crustacea play an 
important role in the food chain of fishes 
(see p. 95). There is a stage in the de- 
velopment of all fish when they must feed 
on some form of Crustacea; this may be 
the larval stage of some larger form, such 
as the crayfish, or it may be a minute adult 
crustacean, such as cyclops or daphnia 
(Fig. 11-3). If it were not for these small 
animals, fish would never get through the 
early part of their life. Some Crustacea, on 
the other hand, act as intermediate hosts for 
certain dangerous parasites of man, such 
as the human lung fluke, Paragonimus 
westermani. On the whole, the group is 
important both economically and biologi- 

Let us consider a representative member 
of the Crustacea in some detail. The lobster 
and crayfish are excellent examples and be- 
cause of their universal distribution are 
readily available for study. 

The lobster and the crayfish 

These two closely related animals live in 
different environments, the lobster in the 
sea and the crayfish in fresh water, yet their 
bodies have much the same appearance. In 
fact, a description for one also fits the other 
fairly well. This discussion is confined 
largely to the lobster, although frequent 
references will be made to the crayfish. 

The lobster is a bottom dweller, spending 
its life seeking food, which consists of other 
Crustacea, small fish, and mollusks. It feeds 
as a scavenger whenever it finds dead ani- 
mals on the ocean floor. It is a secretive ani- 
mal, hiding by day in any cavern that 
affords enough space for its body, and 
sallying forth at night, crawling forward 
along the sea bottom, alert to any stimuli 
that may mean food is in the vicinity. If in 
danger of attack by other predators, it can 

swim backward with darting speed by 
powerful strokes of its abdomen. In so 
doing, it stirs up the mud at the bottom, 
thus forming a "smoke screen," which con- 
fuses the intruder and allows the lobster 
an opportunity to gain distance to a safe 
place. The crayfish employs the same meth- 
ods in defending itself, and it seeks the 
same type of food in much the same 

Structure. The animal is enclosed in a 
chitinous exoskeleton, containing consider- 
able quantities of lime and sclerotin which 
make the skeleton rather heavy and bulky, 
but a very excellent armor plate (Fig. 
11-8). Since the animal is suspended in 
water, most of its weight is taken care of 
by buoyancy, and it is still a very agile 
creature. The chitin thins out at the joints, 
allowing maximum flexibility. The anterior 
portion of the body is covered by the cara- 
pace, and each posterior abdominal seg- 
ment by an arched dorsal tergum, two 
lateral pleura, and a ventral sternum. Tiny 
holes perforate the entire skeleton, being 
particularly numerous in the appendages 
and tail region. Set into these are bristles, 
which make the animal extremely sensitive 
to its surrounding world through tactile 

The appendages of the lobster or crayfish 
demonstrate a very interesting series of 
adaptations and modifications for a particu- 
lar mode of life (Fig. 11-9). There are nine- 
teen pairs of appendages in all, one pair on 
each segment. The antennules and antennae 
are modified for tactile and chemical stimu- 
lation; the mandibles, or jaws, for chewing; 
the next five, maxillae and maxillipeds, 
chiefly for food manipulation; the next pair, 
the enormous chelipeds, for grasping food 
and for defense; the next four for walking; 
and the last six for swimming and various 
other functions. All of these appendages, 
with their varietv of form and function, 
come originally from a simple appendage 
with a single function, namely, locomotion 
(Fig. 11-10). 


external anteno-\>entral view 

Fig. 11-8. Lobster, ventral view. A part of the carapace on the right side is cut away so that the underlying 

appendages can be seen. 

ti """i""i" '""'"'iiimm 




Fig. 11-9. Lobster appendages. Most of the right appendages have been removed for comparative purposes. Each 
retains parts of the original larval appendage, but here it is drastically modified to perform specific functions. 
This is an illustration of serial homologies among the invertebrates. 



The primitive appendage, one which ap- 
pears in the early embryology of all Crus- 
tacea, is said to be biramous, because it 
branches into two parts. It persists in its 
primitive condition in the swimmerets, lo- 
cated on the underside of the abdomen. 
The single basal portion, the protopod, is 
attached to the body and two branches ex- 
tend from it, the endopod, toward the 
median line and the exopod away from it. 
The original function of such an appendage 

exopod has been greatly reduced or lost 

Structures that have a common origin, 
such as in the case of these appendages, are 
said to be homologous, and when they are 
on the same animal they are said to be 
serially homologous. This introduces the 
principle of homology which is illustrated 
throughout the animal kingdom and is very 
important in determining animal relation- 
ships. Homologous structures have a com- 




carrying eggs 

Fig. 11-10. Young lobster, showing the undifferentiated appendages, together with the ap- 
pendages as they will appear in the adult animal. Note how they are modified for specific 
functions. In the young form the appendages are designed primarily for locomotion, in the 
adult they take on a variety of functions. 

was locomotion ( swimming ) , and all of the 
primitive Crustacea, such as the fossil trilo- 
bites, possessed just this type and nothing 
more. Through the ages it became modified 
in a most versatile manner in all of the 
appendages except the swimmerets. For 
example, the antennules and antennae are 
receptors for tactile and chemical stimuli 
and resemble the swimmerets very little. 
One of the most radical departures is the 
mandible, or jaw, where there is almost no 
hint of its progenitor. Among the walking 
legs, as well as other appendages, the 

mon embryological origin, therefore when 
two animals show such structures, even 
though they may not have the same func- 
tion in the adult form, the animals are 
known to be closely related. The more 
nearly the structures are alike the closer the 
relationship, which means tliat they came 
from a common ancestor. We shall discuss 
this topic again in the last chapter of this 

Upon cutting through the body wall, 
among the first and most obvious structures 
noted are the muscles, particularly in the 



skle viev) voitti ri 
diqcsfive qlanc) 

unnary bladder 
excretory orqan 

opbibaliDic arVcry 



-ven+ral tboracic arfery 
-diqcs+ive qland 

ncnxz cord 

an+enocry ar+ery 
bGDafic arteries 

pericardial ca\)ity 

ovidud-opeoiDq-andvwIkirx] Icq 

sternal arlcry 

sepjinal reccpfocle 

s|3erm duct immature 

sperm duct opemn(]-5ifowolkinq I 

ventrql abdominal arhzry 

dorsal abdominal arfery. 



.side view with 
diqestive qland imact 

Fig. n-n. Internal anatomy of the lobster, both male and female. 

abdominal region. The muscles are at- 
tached to the internal side of the exoskele- 
ton, where they are always in pairs, and 
oppose one another in action. One muscle 
pulls an appendage or any other part of 
the body in one direction, whereas the 
other pulls it back again. The principle is 

followed by all higher animals with skele- 
tons, including man. 

Digestive system. The chelipeds catch 
and crush the food, then pass it to the 
mouth with the aid of the maxillae and 
maxillipeds (Fig. 11-8). The mandibles 
break the food up still further, before it is 



swallowed through the short esophagus and 
taken into the large cardiac stomach, which 
functions more as a storehouse for food 
than as a digestive organ (Fig. 11-11). At 
the posterior end of this organ is the gastric 
mill, which is composed of three small 
tooth-like bodies called ossicles, two in a 
lateral position and one in the mid-dorsal 
line. These fit tightly together and the 
grinding movement is brought about by 
muscles attached to the outside of the 
stomach. Before food can pass on into the 
second stomach, called the pyloric stomach, 
it must pass through the gastric mill, which 
grinds all the food fine and renders it 
digestible. In addition, the two stomachs 
are separated by filtering "hairs" which act 
as strainers, allowing only fine particles to 
pass through. Foods that cannot pass 
through, such as parts of skeletons, are 
regurgitated through the mouth. Once the 
food is in the pyloric stomach some of it 
passes into the two large multi-lobed di- 
gestive glands. The enzymes necessary for 
the complete digestion of the food are 
secreted by these glands. Digestion occurs 
for the most part in the upper end of the 
intestine, although some apparently goes 
on in the digestive glands themselves. Ab- 
sorption also takes place here, and in the 
intestine. All undigested food in the intes- 
tine passes out through the anus as feces. 

Circulatory system. The circulatory sys- 
tem of the lobster varies markedly from that 
of the annelids. Instead of several pairs 
of hearts, the lobster possesses a single 
pulsating vesicle that lies on the dorsal 
part of the body, surrounded by a thin mem- 
brane and cavity, the pericardium and 
pericardial cavity, respectively (Fig. 11- 
12 ) . The heart has three pairs of tiny valved 
openings, called ostia, which allow the 
blood to enter from the pericardial cavity. 
Seven arteries lead away from the heart and 
convey the blood to all parts of the body. 
From this point on, the system differs 
even more radically from that of the earth- 
worm. The blood leaves the tiny arterioles 

and passes out into spaces, called sinuses, 
where it bathes the tissues. This type of 
system is called an open blood system, in 
contrast to the closed system of the earth- 
worm. Once the blood leaves the tissues, it 
seeps into the ventral portion of the cepha- 
lothorax, then through a set of afferent 
vessels (to the gills) where gas exchanges 
take place. It then makes its way to the 
pericardial chamber of the heart, through 
efferent vessels (away from the gills), 
thence through the ostia, and out into the 
body again. Valves located in the walls of 
the ostia permit the blood to pass through 
the ostia in one direction only, namely 
into the heart. 

The blood contains colorless leucocytes 
as well as a respiratory pigment, hemo- 
cyanin, which has a slight bluish color when 
oxygenated and serves the same oxygen- 
carrying function as the hemoglobin does in 
other forms. The blood has remarkable 
clotting properties. If an appendage is re- 
moved forcibly, there is hardly any notice- 
able loss of blood, the clot forming almost 
at once, and filling the large opening. 

Breathing system. When the lateral walls 
of the carapace are cut away, large feather- 
like delicate gills are exposed. These are the 
breathing organs of the animal. While the 
crayfish lies quietly in running water, 
the water moves over the gills without any 
help from the animal. However, when the 
demand for oxygen is greater or when the 
oxygen content of the water is low, a spe- 
cial modification of the second maxilla, the 
gill bailer ( scaphognathite ) , waves up and 
down, like a gondolier sculling a boat, 
causing the water to flow over the gills 
in a posterior-anterior direction. This can 
easily be demonstrated by placing a cray- 
fish in a shallow white pan and allowing 
a small amount of India ink to be placed 
near the posterior end. Great clouds of the 
carbon particles will issue from the anterior 
end, indicating a flow of water. Since the 
openings of the excretory organs, which 
lie at the base of the antennae, are in the 

orrows indicate course of Wood 

Fig. 11-12. Cross-section and partial side view of the lobster to show the internal organs, circulatory system in 




younq iobsfer 

Fig. 11-13. Life history of the lobster. 

path of this outgoing water current, the 
waste products from these organs are also 
carried away. 

Excretory system. Nitrogenous wastes are 
withdrawn from the blood and body fluids 
by a pair of kidneys, called the green 
glands because of their color. They are 
located beneath the antennae in the body 
and consist of a glandular portion and a 

bladder (Fig. 11-11). The bladder stores 
waste material until it is released through 
the opening at the base of the antennae, 
already referred to. 

Reproductive system. The gonads diflFer 
slightly in shape in the lobster and the 
crayfish; they are longer and thinner in the 
former and fused into a three-lobed gland 
in the latter (Fig. 11-11). Sperm cells are 



Fig. 11-14. A female crayfish carrying her eggs attached to the swimmerets. The eggs are about ready to hatch 
because close observation will reveal the embryos just under the shell of the egg. As a matter of fact, these did 
hatch the following day. 

produced in a tubular testis and pass 
through the vas deferens to an opening at 
the base of the fifth ( considering the cheh- 
peds as the first walking legs) walking legs. 
An ejaculatory duct toward the end of the 
tube aids in removing sperm during mating. 
The first pair of swimmerets is modified in 
the male to form a copulatory organ for 
transfer of the sperm to the seminal recep- 
tacle on the ventral side of the female. 
The ovaries of the female produce eggs 
which pass through a straight oviduct to 
the opening at the base of the third walking 
legs. Her first pair of swimmerets is rudi- 

At the start of copulation the male lobster 
grasps the chelipeds of the female with his 
pincer-like appendages and turns her over 
on her dorsal side (Fig. 11-13). The two 
ventral sides then come together. The first 
pair of swimmerets of the male is placed 
near the opening of the seminal receptacle 

of the female and the sperms are dis- 
charged. The female receives the sperms in 
packets or spermatophores. 

The female lobster lays eggs only once 
in two years, usually at the time when the 
water has reached its summer temperatures 
(from about mid-June to September 1st). 
The male, however, is unable to discern the 
sexual condition of the female and copula- 
tion often occurs long before the matura- 
tion of eggs in the ovary. In such cases the 
sperm cells, which are stored in the seminal 
receptacle of the female, must maintain 
their vitality for a long time, very often for 
several months. Eggs are fertilized after 
their ejection from the oviducts. As the eggs 
are extruded, a mucous material is pro- 
duced by glands in the swimmerets which 
glues the eggs to the many bristles on these 
appendages. The eggs are thus carried by 
the female from ten to eleven months (see 
Fig. 11-14 where a crayfish is carrying her 


nearly hatched eggs) and usually do not increasing reluctance, until but three re- 
hatch until the summer following that in main. Then it must be stimulated rather 
which they were laid. This condition has drastically before it will leave these behind, 
resulted in much confusion concerning the It seems to sense the danger of being leg- 
laying of eggs by the lobster. less, even though it will later acquire a new 

When the lobster hatches as a free-swim- set through regeneration. The value of au- 

ming larva, the period of fosterage is over totomy is obvious; it enables the animal to 

for the female lobster. By fanning move- escape with the loss of a single appendage, 

ments of the swimmerets the young are whereas without this ability it might not 

driven away from the body of the mother. escape at all. The break always occurs at 

Young lobsters, however, tend to keep to- a certain place near the base of the leg. 

gether in a cluster. There is a constriction of the wall at this 

Growth. Molting is an important process point so that the loss of the appendage is 

for the crayfish and lobster, as it is for all followed by only a minor loss of blood, 
of the arthropods. This is necessary in order The lost parts are replaced by the slow 

to provide for increase in the animal's size, regeneration of a new appendage. This oc- 

although a small amount of growth can oc- curs more readily in young animals than in 

cur under the rigid exoskeleton. Just before older ones, although the ability to regener- 

the molting process begins, some of the ate seems to remain throughout life. Per- 

lime is withdrawn from the exoskeleton, haps due to the greater specialization of 

softening it somewhat. Simultaneously, a these animals regeneration is limited to the 

new skeleton is secreted beneath it, arising appendages and to the eyes, and thus much 

from the epithelium, called the hypodermis. less pronounced than in the lower forms of 

The muscles and body bulk then shrink a animals. This is as might be expected, since 

little and the old skeleton splits on the the greater the integration of parts, the 

dorsal side, between the abdomen and more dependent each one is on the other, 

carapace. The animal backs out slowly, and the less possibility there is for any 

leaving behind a replica of itself, complete part to replace the whole animal. The abil- 

in all details, except for the actual body, ity to regenerate is reduced still further in 

This shedding is so thorough that even the the vertebrates. 

facets of the eyes, and the lining of a part The nervous system. In contrast to the 
of the gut are included. Once the old skele- rather meager sensory equipment of flat- 
ton is cast off, the animal grows rapidly for worms and annelids, the lobster and cray- 
a period of time as a result of taking in a fish are well supplied with sense organs. If 
great deal of water. For several days dur- a small bit of liver is dropped in one end 
ing the hardening period the animal re- of an aquarium containing a crayfish, very 
mains in hiding, and it becomes aggressive shortly the animal will orient itself in the 
again only after its skeleton is well hard- proper direction and move directly toward 
ened. the food. The soluble parts of meat dif- 

Another interesting characteristic of fuse through the water and touch tiny hairs 

many crustaceans, associated with growth on the chelipeds, antennules, antennae, and 

and reproduction, is autotomy, the power mouth parts, causing impulses to pass to 

to throw off appendages at will. The fiddler the central nervous system. This sensitivity 

crab affords a most striking example of this to chemicals is equivalent to man's senses 

phenomenon. If one of its posterior legs is of taste and smell. 

held with a forceps, it will drop it off The animal's ability to move about satis- 

readily (Fig. 11-15). As other legs are factorily in total darkness indicates that it 

pinched, they too are dropped but with must have senses that guide it under such 

Fig 11-15. (Upper picture) A "heod-on" view of a fiddler crab (l/co), showing the great difference in the size of 
the first pair of legs. The large claw is present only in the male and he brandishes it in a peculiar manner in 
ttie presence of a female who has no such adornment. This peculiar activity is probably responsible for the 
name. These animals live in burrows near the water's edge where they go to feed on any dead and decayinq 
matter. ' " 

(Lower picture) The same crab is shown after it has been stimulated to release its appendages (autotomy) 
Each appendage was grasped with a pair of forceps and squeezed until the crab threw it off. It released its first 
appendages readily, but no amount of pinching would force it to throw off the last three. It apparently was 
aware of the necessity of keeping a few legs. 



Fig. 11-16. Nervous system of the lobster, dorsal view. The eye and statocyst have been greatly enlarged and 
simplified in order to show their manner of function. The eye is further divided into two parts: the upper por- 
tion shows how it functions in the dark; the lower portion, how it functions in the light. 

conditions. The sensory structures involved 
here are the tactile hairs which are stimu- 
lated whenever any part of the body comes 
in contact with an object. Tactile hairs are 
particularly abundant on the chelipeds, at 
the end of the telson, on mouth parts, and 
underneath the body. They may also be 
sensitive to vibrations in the water, thus 
providing the animal with a "hearing" 
mechanism which functions somewhat like 
the ears of vertebrates. 

The animal is continually responsive to 
gravity, even when swimming freely, which 
means that it must have organs of equi- 

librium. These organs are the statocysts, 
located at the base of the antennules. They 
consist of small cuticular sacs, containing 
tiny grains of sand, which are glued to 
small sensory hairs (Fig. 11-16). The nerve 
fibers from these sensory hairs join to form 
a large nerve leading to the brain. The 
proof of the function of these statocysts can 
be determined experimentally. During the 
molt the cuticular lining of the statocysts, 
together with their grains of sand, are lost, 
so that the crayfish must replace its sand 
grains (statoliths) after each molt. Shortly 
after molting, the crayfish buries its head 


in the sand and thus fills the statocysts parts of objects that come within its vision, 

through its opening on the dorsal side of The image is thus a mosaic in which the 

the antennule. A bioloo;ist workins; with a sligrhtest movement is readilv detected. This 

marine shrimp, Valeomonetes, allowed the is true, however, only in bright light when 

animal to molt in a clean aquarium, then the pigmented walls of each ommatidium 

supplied iron filings which it promptly are spread over the entire cylinder. In dim 

added to its statocysts. When the animal light the pigment recedes toward the two 

was placed in a magnetic field, it oriented ends of the cylinders, so that the rays of 

itself with respect to the pull of gravity and light pass readily from one ommatidium to 

the pull of the magnet. The special function another. The animal then sees the image 

of the statocysts is their ability to respond superimposed, so that movement is not eas- 

more promptly to the pull of gravity than ily discerned. The image may be more dis- 

would be the case if only the tiny hairs tinct in reduced light, however, than when 

were present. seen as a mosaic pattern (Fig. 11-16). Thus 

The visual mechanism has become very the animal has a means of seeing under 

complex in the arthropods. While arthro- varying light conditions, 

pod eyes vary considerably, a description It is obvious that the Crustacea are well 

for the crayfish eye will convey a general supplied with sense organs that make the 

idea for the entire phylum. The eyes are animals aware of the outside world. How 

located on long movable stalks and can be they interpret the incoming sensations and 

protruded or retracted in order to improve respond to them depends on the develop- 

the vantage point of the observer. The eye ment of the nervous system, 

itself differs markedly from any vertebrate The crayfish and lobster possess a bilobed 

eye (Fig. 11-16). It has an outer rounded brain, relatively large when compared to 

transparent surface called the cornea, that found in lower forms. The brain lies 

which is divided into at least 2,000 tiny between the eyes and is connected with the 

sections, or facets. The facets merely indi- ventral nerve cord by means of a pair of 

cate the outer limits of the units or omma- circumesophageal connectives, very similar 

tidia, which make up the eye of the cray- to the arrangement found in the earthworm 

fish. Each ommatidium is composed of the (Fig. 11-11). There is a large subesopha- 

outer facet, which functions as a lens, a geal ganglion, made up of six fused ganglia, 

series of cells below which make up a sensi- followed by a series of ganglia, one for each 

tive portion, the retinula, and heavily pig- segment. Large nerves extend out from the 

mented regions at the outer and inner ends ganglia to the appendages and to other 

of this cylinder (Fig. 11-16). The sensory parts of the segment. 

cells of each retinula terminate in nerve In addition to its well-developed nervous 
fibers, which lead directly to the brain, and control the crustacean also possesses hor- 
the fibers from all the retinulae form a mones, chemical regulators that are pro- 
nerve equivalent to the optic nerve in ver- duced by one part of the body and affect 
tebrates. other parts. This has recently been demon- 
The eye functions as a very efiicient strated in connection with the chromato- 
organ for photoreception. Light falling phores, located beneath the epidermis, 
upon tlie lens of a given ommatidium at the which move to the surface and may either 
proper angle is focused on the sensitive spread out or contract into a tiny mass. The 
region below and stimulates the sensory alternate expanding and contracting by 
cells. Since the ommatidia are long cylin- thousands of these bodies have the com- 
ders, the animal sees tiny points (one for posite effect of changing the color of the 
each ommatidium stimulated) which are entire animal. In some invertebrates this 



is so well developed that an animal may 
match its background almost perfectly ( see 
p. 430). It was once thought that the chro- 
matophores were controlled by the nervous 
system, but it is now known that they are 
controlled by the secretions from a tiny 
gland, the sinus gland, located in the eye 
stalk. If it is removed, the spread of pig- 
ment granules in the chromatophores is af- 
fected so that the crustacean fails to show 
normal color change as a result. If the sinus 
gland is replaced in the body of the crus- 
tacean, the normal action of the chromato- 
phores is restored. The same hormone also 
regulates the molt and affects the deposi- 
tion of calcium salts in the exoskeleton; oth- 
ers affect heart action and carbohydrate 
metabolism. Research reveals that hor- 
mones are produced by the central nervous 
system as well as the sinus gland. 


Stories concerning the ravages of insects 
are as old as man himself; such stories 
found their way into the earliest writings 
including the Bible. This was primarily be- 
cause of the competition existing between 
man and the insects, for the insects con- 
sume the food man intended for his own 
use. They have caused devastating famine 
in many parts of the world, and even today 
they torment man as well as other animals. 
A trip through a boggy or swampy region 
on a humid summer evening will bring any- 
one to the sudden realization tliat the in- 
sects appear to have a rather secure place 
even in our modern civilization. Despite 
constant war upon them, they still persist 
and the cost of keeping insects under con- 
trol is nearly one and a half billion dollars 
each year. A traveler who crosses a state 
border is often subjected to inspection in 
order to determine whether or not he is 
carrying any insect which might add to the 
long list of "bed fellows" that the state 
already harbors. It has been truly said that 

man's big battle today is being fought 
against the six-legged little beast which, if 
left unhampered, could soon overwhelm 
man and his civilization. 

There are over 700,000 species, several 
times more than all other species of animals 
put together. How is it, that these small 
animals have so outstripped all other forms 
of life? First of all, their rigid exoskeleton 
has enabled them to invade the air and 
support themselves outside of water. This 
waxy covering prevents desiccation, an es- 
sential feature for an animal that divorces 
itself completely from an aquatic existence. 
They have become so successful in their air 
environment that they have conquered all 
possible regions — the only invertebrates 
that have taken to the air. They burrow in 
the ground and have returned to both fresh 
and brackish waters, but avoid the sea. 
They live on and in the bodies of plants and 
animals, becoming in some instances seri- 
ous parasites. They suck the juices of plants 
and the blood of animals, and often feed 
upon other species of their own group. This 
is nature's way of maintaining a balance 
among animals. This has been hailed and 
encouraged by man as a method of biologi- 
cal control. For example, certain ladybird 
beetles are grown by the million and 
planted on citrus fruit trees that are in- 
fested by the destructive cottony cushion 
scale insect. The beetles feed upon the 
pests, thus keeping them under control. 

The wino-s of the insects have made it 
possible for them to travel long distances, 
thus not only increasing their ability to find 
food but also to spread themselves to new 
areas where they might thrive more suc- 
cessfully. Fortunately, present-day insects 
have never attained any great size, al- 
though certain fossil forms did reach a wing 
spread of more than 2 feet. Most present day 
species range between one-eighth to one- 
and-a-half inches in length. There is a South 
American beetle which is about 5 inches 
long and some tropical moths have a wing 

orrhropod ancestor 










Womoptera Wemiptera Wymenoptero 


Fig. 11-17. The common orders of insects. 







Fig. 11-18. External ventral view of the grasshopper. 

spread of 9 inches. Insects as large as cats us study in some detail two representatives 
and dogs would increase man's problems the grasshopper and honeybee, 

Entomologists (specialists on insects) 
have grouped this vast array of insects into 
about 26 different orders, the more impor- 
tant of which are shown in Fig. 11-17. Let 

The grasshopper 

A study of this representative insect will 
throw some light on the reasons why the 
insects have reached the pinnacle of the 



Fig. 11-19. The lubber grasshopper (Romelea micropfera), common in our Southern states, demonstrates insect parts 
clearly and with a minimum number of modifications. It is commonly used in zoology classes. This is a male. 

invertebrate world. The grasshopper is se- 
lected because of its relatively large size 
and because it is well known to everyone. 
Furthermore, it shows certain primitive in- 
sect characteristics that make it easier to 
understand than other members of the 
group. Its many species are world-wide in 
distribution, living in and feeding on grass 
or any other available leafy vegetation. It 
sometimes increases in such numbers that 
it becomes a serious pest, great hordes de- 
vouring almost everything of plant origin, 
except wood, that lies in its path. Grass- 
hoppers have been known to stop trains 
from climbing grades because their crushed 
bodies caused the wheels to slip on the rails, 
and to cause cars to skid on the roads at 
places where the insects cross, as their 
bodies are crushed beneath the tires. Corn 

fields through which they pass are sheared 
to the ground and left in desolation. 

Structure. Externally, like all insects, the 
grasshopper is divided into three parts, the 
movable head, the thorax, and the abdo- 
men (Figs. 11-18, 11-19). There is a con- 
siderable amount of fusion of segments 
when compared to the crayfish. For exam- 
ple, the head appears as a single structure, 
but it is made up of six segments. Likewise, 
the thorax is composed of three segments, 
and there is a variable number of segments 
in the abdominal region, usually eleven. A 
pair of legs is attached to each of the three 
thoracic segments, and a pair of wings to 
each of the last two. The legs have several 
parts which named from the body outward 
are the coxa, trochanter, femur, tibia, and 
tarsus. The hind legs are long and well 




Fig. 11-20. Grasshopper head and mouth parts. 


developed for jumping, whereas the other 
four are used in walking. The outer wings 
are leathery and rigid, serving as protective 
covers for the more membranous under- 
wings. When the insect is at rest, the under- 
wings, which are the propelling wings dur- 
ing flight, are neatly folded under the outer 





The compound eyes of the grasshopper 
are securely integrated into the head skele- 
ton, but in other respects resemble those 
of the crayfish (Fig. 11-20). In addition, 
three small simple eyes, or ocelli, are lo- 
cated between the compound eyes. The 
ocelli function perhaps only in detecting 
light and dark, which seems unnecessary 
because the large eyes are so sensitive to 
varying light intensities. The single pair of 
antennae vary in length in different species 
of grasshoppers and function as tactile as 
well as olfactory organs. Although most of 
the head is encased in a solid epicranium, 
the several mouth parts can be traced back 
to modifications in the crayfish appendage 
plan. There is a broad upper lip, the la- 
brum, which is attached beneath the clyp- 
eus. A pair of lateral, dark colored mandi- 
bles oppose one another in chewing in such 
a way as to make it convenient for the 
animal to bite the edge of a leaf without 
turning its head. Lying outside the mandi- 
bles are the maxillae, which are composed 
of several parts, called palpi ( singular, pal- 
pus), and used in manipulating the food 
as it enters the mouth. The lower lip, the 
labium, possesses two small palpi, resem- 
bling the larger ones attached to the maxil- 
lae. Lying in the center of all these parts 
is the tongue, or hypopharynx. Together, 
these make an efficient chewing mechanism 
for handling the kind of food that is eaten 
by the grasshopper. 

Digestive system. As food is taken in, it 
is copiously mixed with colorless saliva, 
secreted by several salivary glands. The 
food moves through the esophagus to the 
crop where it is stored, until it passes into 
and through the gizzard where it is ground 






hind qut. 




Sperm receptacle 

Fig. 11-21. Internal anatomy of the grasshopper, dorsaS 


to a fine consistency (Fig. 11-21). The food 
then enters the stomach where digestion 
occurs by the action of enzymes, which are 
secreted by eight double digestive glands 
or caeca. Finally the digested food passes 
into a large and then a small intestine. 
Small excretory tubules, the Malpighian 
tubules, empty into the anterior end of the 
large intestine. The gut opens into the rec- 



turn, and then to the outside through the 

Circulatory system. The body cavity of 
the grasshopper is the haemocoel, or blood 
cavity, not the coelom, as was the case in 
the earthworm. This is formed by a contin- 
ued expansion of the vascular system until 
the entire coelom is obliterated, with the 
exception of the cavities of the gonads. The 
cavity is filled with a colorless blood which 
contains only leucocytes. Since the blood 
does not function as a conveyor of oxygen, 
it has no oxygen-carrying pigment as the 
blood of most other animals has. The blood 
is kept in motion by the action of the dorsal 
tube-like heart, composed of a number of 
chambers into which small ostia open ( Fig. 
11-21). The heart is surrounded by a peri- 
cardial sinus, which holds the blood before 
it enters the heart, much the same as in the 
crayfish. As the heart beats, blood moves 
forward and out into the haemocoel again. 
The heart has no occasion to be as active 
an organ as it is in many other animals, 
since respiration is carried on in another 

Breathing system. The respiratory system 
of insects is unique in the animal world. It 
seems strange that it should have appeared 
in this one group of animals and nowhere 
else. Air, with its oxygen, is carried directly 
to the cells through a system of tubules 
called trachea (Fig. 11-22). This very com- 
plex system consists of tiny tubes which 
must remain distended so that the air can 
pass freely in and out of them. Small chitin- 
ous spiral threads give support to the tu- 
bules and control their diameter. There are 
several openings into the trachea along the 
thoracic and abdominal walls; these are 
called spiracles. A valve covers the opening 
so that the spiracle can be opened and 
closed during the breathing process (Fig. 
11-22). Leading in from the spiracles, the 
tubules become smaller and smaller until 
they are as small as capillaries and lie di- 
rectly against the cells, supplying them 
with oxygen and carrying away the ex- 

creted carbon dioxide. The grasshopper 
also possesses several large air sacs which 
may be contracted to aid the movement of 
air through the many small tubules. The 
grasshopper contracts and enlarges its 
body, particularly the abdominal region to 
facilitate the air flow and the anterior spi- 
racles open and close alternately with the 
posterior spiracles, so that the air makes 
a one-way passage. 

The nervous system. While the grass- 
hopper has many of tlie sense organs that 
the crayfish possesses, it lives in an air en- 
vironment and therefore needs somewhat 
different methods of maintaining contact 
with its outside world. For example, the 
eyes and the tactile hairs which cover the 
various parts of its body resemble those of 
the crayfish, and it has organs of chemi- 
cal sense on its antennae and mouth parts. 
However, the grasshopper and many other 
insects have the means of making and re- 
ceiving sound vibrations. In the grasshop- 
per the organ for hearing is on the first 
segment of the abdomen. In several re- 
spects this resembles the ears of higher 
vertebrates, in that it is composed of a 
stretched membrane, the tympanum, to 
which is attached a slender process that is 
connected to a nerve. The animal makes its 
characteristic clacking sound by rubbing 
its roughened hind tibias against a wing 

As a result of the fusion of three head 
segments, the grasshopper possesses a 
rather large brain. Nerves extend directly 
from the brain to the eyes, the antennae, 
and to a ganglionated cord running through 
the body. The first ganglion in tlie cord, 
the subesophageal ganglion, formed by two 
great nerves or connectives proceeding from 
the brain around the esophagus, is fol- 
lowed by three large ganglia in the thorax 
and five in the abdomen, where some fu- 
sion has gone on. Nerves go out to all parts 
of each segment and to the legs and wings, 
and there is also a fine network of nerves 
beneath the epidermis. The nervous system 





. epKcn«rvc 


.subftsophageol ^an^lion 

1st thoracic gan9llon 

.2nd thoracic ganglion 

.3rd thoracic ganglion 


■ obdeminot 



Fig. 11-22. Dorsal view of the respiratory and nervous systems of the grasshopper. Trachea and a few muscle 
fibers arc drawn in detail at the left; the spiracle and end of the trachea to the right. 




Fig. 11-23. Grasshopper depositing its eggs, and the 
larvae hatching some time later. 

is somewhat better developed than that 
of the crayfish, as is indicated in both its 
structure and function. 

Excretorij system. The excretory organs, 
the Malpighian tubules, have already been 
mentioned in connection v^ith the digestive 
tract. Insects are the only animals whose 
excretory glands (kidneys) open directly 
into the intestine (Fig. 11-21). The long, 
coiled tubules lying in the haemocoel are 
bathed in blood so that suspended nitrog- 
enous wastes are easily removed. Uric acid 
has been found to be the end product of 
nitrogenous metabolism in insects. 

Reproductive system. On late August 
days it is common to see grasshoppers cop- 
ulating. Some days after fertilization tlie 
female grasshopper lays her eggs (Fig. 
11-23). She uses her powerful pointed ovi- 
positors for digging a hole in the soil where 
the eggs are deposited, together with a 
mucous substance which cements them into 
a packette, known as a pod. She lays about 
20 eggs at a time, and may deposit as 
many as ten pods before death overtakes 
her some days after the egg laying is ac- 
complished. The eggs begin to develop as 
soon as they are laid and become well- 
formed embryos before cold weather sets 
in. The embryos then undergo a rest period, 
the diapause, in which they pass the winter. 
In the warm spring days, development is 
resumed and the embryos hatch in early 
summer as nymphs (Fig. 11-23), which re- 
semble the adult grasshopper without 
wings. Nymphs undergo several molts dur- 
ing their subsequent rapid growth, the 

wings appearing and becoming longer at 
each shedding period. It is during this 
period of rapid growth, when their bodies 
demand so much food, that the animals are 
so destructive to crops. The summer is 
spent leisurely, feeding and growing, until 
the breeding season begins. 

The sex organs of the grasshopper consist 
of a pair of testes and ovaries (Fig. 11-21). 
The testes are made up of several small 
tubules or follicles, which are joined to the 
seminal vesicle by means of the vas defe- 
rens; the latter then join to the ejaculatory 
duct and copulatory organ, the penis. A 
pair of accessory glands secrete a fluid in 
which the sperms are suspended. The large 
ovaries are composed of several egg tubes 
in which the eggs develop; two oviducts ex- 
tend from the ovaries and join to form the 
vagina. A pair of accessory glands is also 
present in the female, which contributes to 
the eggs during their formation. 

The honeybee 

The honeybee has been associated with 
man as a domestic animal for many thou- 
sands of years and stories about this ani- 
mal have found their way into the writings 
of poets and historians as well as natural- 
ists from very early times. Aristotle de- 
scribed the parthenogenetic development 
of the drone bee, even though he had no 
microscope witli which to verify his state- 
ments. The bee offers excellent material 
for the study of social behavior in lower 
animals and is a remarkable illustration of 
adaptation. The more scientists study the 
social life of this little animal, the more 
remarkable does it appear. 

Bees have learned to live in colonies 
where all members work for the community 
in a rigid caste system. There is one queen 
which lays all of the eggs for the colony, 
a number of drones (males), one of which 
fertilizes each newborn queen, and thou- 
sands of sterile females, called workers, 
which do all the work of the colony (Fig. 



fertilized eqqs 

unfertilized eqq 

eqq larva pupa 

Fig. 11-24. Life history of the honeybee. 

11-24). All is sacrificed, even life itself, to 
the welfare of the colony. 

Anatomically, the bee differs from the 
grasshopper in many ways, although funda- 
mentally their bodies are alike. Since its 
diet consists of both fluids and solids, the 
mouth parts of the bee are modified for 

sucking as well as for chewing. The ovi- 
positors of the females have been modified 
into an organ of defense, the sting. The 
appendages which serve as walking legs are 
adapted for carrying pollen and for a vari- 
ety of other highly specialized but essential 
functions in addition to locomotion. 



eye brush 

antenna cleaner. 


Fig. 11-25. Ventral view of the honeybee to show modification of the legs for carrying pollen. In the upper right 
corner a small portion of the fore and hind wings are drawn to show how the wings may be hooked together 
during flight. 

longitudinal muscles' 
vertical muscles 


The external surface of the bee is cov- 
ered with "hairs." Those over the eyes are 
straight and unbranched, whereas those 
over the remainder of the body are 
branched, thus affording a place for pollen 
to cling. During a visit to a flower, the bee 
gathers some pollen with its mandibles and 
moistens it with honey. Pollen is also ob- 
tained from the action of the pollen brushes 
on the front two pairs of legs which clean 
the anterior portion of the body (Fig. 
11-25). These brushes pass the sticky pollen 
mass back to the middle legs which rub 
it upon the pollen combs of the hind legs. 
The pollen is then rubbed from the right 
hind leg onto the left and from the left onto 
the right, and thus carried to the pollen 
packer, which is composed of two parts, the 
auricle and the pecten. Once the pollen has 
reached this position, the tarsus is flexed 
on the tibia, packing the pollen from the 
bottom into the pollen basket (Fig. 11-25), 
A great quantity of pollen may be collected 
in this way so that when the bees fly home 
in the late afternoon the huge balls of pol- 
len cause the hind legs to dangle much 
lower than when they are not so loaded. 

The anterior pair of legs has two cleaning 
mechanisms, an eye brush and an antenna 
cleaner, which the bee uses to remove pol- 
len from these organs. The antenna cleaner 
is composed of a velum, a small flexible 
projection from the tibia, and a crescent- 
shaped depression on the proximal end of 
the tarsus, lined with short bristles. The 
antenna is brought into this depression and 
pulled through several times to clean it. In 
addition to the pollen brush on the middle 
pair of legs, there is also a spur, which is 
used in picking and transferring wax in the 
process of comb-building. 

The bee has two pairs of delicate mem- 
branous wings which can operate either 
separately or locked together by means of 
a row of hooks that fasten into a groove in 
the posterior margin of the forewing (Fig. 
11-25). During a straight flight, where 
speed is essential, the wings are locked to- 


wing process 

Fig. 11-26. Cross-section of the bee showing how the 
wings function ciuring flight. 

gether and the bee flies as if it had only 
two wings. The question of flight in insects 
has been a puzzling one, not only to biolo- 
gists but to engineers as well. The latter 
claim that, according to aerodynamics, a 
bee cannot fly! Aside from speed, flight in 
insects exceeds anything the engineer has 
been able to devise so far, yet aircraft de- 
signers have so far been unable to apply the 
principle employed by these little animals. 
Flight in insects is brought about in a pe- 
culiar manner, namely, not by wing mus- 
cles, as in the case of birds, but by powerful 
muscles which cause the thorax to vibrate 
and this in turn forces the wings to flap up 
and down. In Fig. 11-26 this is illustrated. 
As the anterior-posterior thoracic muscles 
contract, the dorsal wall of the thorax (ter- 









sensory hair 

Fig. 11-27. The head of the bee, showing the mouth parts. An antenna has been enlarged in cross-section to show 

the details of the sensory end organs. 

gum) is forced upward. This pulls the dor- 
sal basal edge of the wing upward, causing 
the wing as a whole to be forced downward 
with considerable force; the body wall acts 
as a fulcrum. The upstroke is accomplished 
by the sudden contraction of the dorsal- 
ventral muscles in a similar manner. Thus 
the wings move up and down by the throb- 
bing of the thorax and not by any effort on 
the part of the wing itself. Its pitch can be 
altered so that the bee can hover, fly for- 
ward, or fly backward with ease, something 
man has had great difficulty in duplicating 
mechanically. In the heHcopter this has 
been accomplished to a certain degree. 
Bees are capable of long flights, sometimes 
as long as 10 miles, although usually much 
shorter distances are covered in their rou- 
tine work. 

Another interesting modification of the 
bee's appendages is the sting. This is found 
only in the females, workers and queen, be- 
cause it is homologous to the ovipositor. The 
organ has become a complicated apparatus, 
retaining the muscle system which made it 
possible for the grasshopper to deposit its 
eggs in very hard soil. In the sting these 
muscles enable the bee to force sharp- 
grooved darts into the tough skin of an 
intruder. A pair of feelers on either side of 
the darts "selects" the spot where the sting 
is to be released. This prevents the bee 
from stinging inert bodies. Lying between 
the upper ends of the darts is the poison sac 
which is in contact by means of ducts with 
an alkaline and an acid gland. During 
the stinging procedure the poison sac is 
squeezed, and its product is forced into the 


subcsophaqeal qanqlion , ^ , 

"^ ^ ^ ' honey storwocn 

digestive stomach 

r^olpigbian tubules 

onherior thoracic 
gang I ion 

jostenor thoracic 

rectal glands, 
bind guf 

digestive and nervous s/stems of the honey bee 

Fig. 11-28. Side view showing some of the internal anatomy of the bee. 

wound made by the darts. This substance 
causes much of the pain and swelHng asso- 
ciated with the sting of the bee. Once a 
worker stings, its darts become firmly fixed 
in the skin of the recipient so that the entire 
apparatus and sometimes other internal or- 
gans are torn out when the bee leaves. A 
day or two later this results in the death of 
the bee. Queens use their sting in battle 
with other queens, but are able to use it 
over and over again without the injury 
which results to workers. 

The diet of the bee consists chiefly of 
nectar from flowers, which is essentially a 
solution of sugar. Although the mouth parts 
noted in the grasshopper are also found in 
the bee, they are greatly modified for suck- 
ing liquids (Fig. 11-27). The mandibles are 
much like those of a grasshopper and are 
used in wax manipulation and comb-build- 
ing. The maxillae and labium, together 
with their palps, however, are extended and 
grooved on the inside so that when they are 
brought together they form a tube or pro- 

boscis. The greatly elongated tongue (hy- 
popharynx ) , which lies in the groove made 
by these mouth parts, acts as a pump. When 
the bee feeds on colored honey it is pos- 
sible to observe the food make its way along 
the tongue with considerable speed until 
it disappears in the mouth, thence moves 
to the honey stomach, which is a crop for 
storage (Fig. 11-28). Between the crop and 
the true stomach is a small valve, controlled 
by the bee, which makes it possible for 
the bee to take as much nectar as it needs 
for use in its own digestive tract. The rest 
is regurgitated into a wax cell in the hive 
for storage as honey. 

The nectar undergoes chemical change 
while in the crop. The most significant dif- 
ference noted is a reduction from tlie di- 
saccharide, sucrose, to the monosaccharide, 
glucose, and the possible addition of other 
substances in small quantities. This watery 
substance is placed in the open comb cells 
of the hive, and allowed to undergo evap- 
oration to remove a large portion of the 


water. When this is complete the resultant sometimes reared in the same cells, and the 

honey is covered and sealed by a thin layer old cocoon cases can be found in them ly- 

of wax. Evaporation is hastened by "air ing one on the other. 

conditioning," which is brought about by The respiratory, excretory, and nervous 
certain workers detailed to keep the air in systems are much the same as those already 
constant motion by beating their wings, studied for the grasshopper. Reproduction 
After a particularly busy day, v^^hen large in the bee is somewhat different, however, 
amounts of nectar have been brought in, and a discussion of this system is necessary, 
the hives can be heard "singing," due to the The male or drone bees possess two kidney- 
intense activity of the ventilator bees. This shaped testes, from which a pair of vasa 
also provides a constant flow of fresh air deferentia conduct sperms to the large sem- 
through the colony, which probably con- inal vesicles. The vesicles, as well as a pair 
tributes to the general health of the colony. of large accessory glands, enter a single 
As previously mentioned, this same lively ejaculatory duct, all of which then connect 
beating of wings is also used as a means with the large, complicated copulatory or- 
for raising and maintaining the temperature gan, the penis. The major part of the ab- 
above freezing during the winter. In this dominal cavity of the queen is filled with a 
case die activity contributes heat from the pair of large ovaries, which resemble those 
burning of sugar in the bodies of the bees of the grasshopper. Oviducts connect with 
themselves. a single vagina. A dorsal evagination of the 

While the nectar supplies carbohydrates vaginal wall results in a sac called the 

for the bee's diet, nitrogen is provided in spermatheca, the storage place for sperms 

the form of pollen. The method of collect- received from the male during copulation, 
ing and transporting pollen to the colony Soon after the advent of spring the queen 

has already been described. The pollen, or lays many eggs. This brings about such a 

"bee bread," is packed in cells for future great increase in the number of bees that 

use in feeding the larvae as well as the adult the hive is soon overcrowded and a change 

bees during the winter. Bees also collect in arrangement becomes necessary. The 

"bee glue," or propolis, which is pitch found workers build new queen cells and a new 

around the base of buds. It is used as a queen is produced, who promptly proceeds 

varnish for mending and stopping up cracks to drive the old queen out. The latter gath- 

in the hive to make the hive as tight against ers a substantial portion of the workers and 

the elements as possible. "swarms" to a new home. Such a large mass 

The wax glands, located on the ventral of bees may be observed clinging to a tree 

side of the abdomen, secrete the material branch, where they rest until scouts find 

for building the wax cells of the comb. Cells a new home. The queen who is left behind 

of various sizes are constructed for specific kills any other queen larvae that may be 

purposes. The smallest are the worker cells, developing and flies out on her "nuptial 

which are used to rear the workers and also flight." At this time she copulates with one 

to store pollen. The drone cells are some- of the drones and receives sufficient sperm 

what larger than the worker cells and are in her spermatheca to last the rest of her 

used not only to rear the drones but may life, which may be as long as 15 years. A 

also be used for the storage of honey. Usu- small "sperm pump" makes it possible for 

ally, however, the honey cells slope upward her to fertilize or withhold sperm from 

to prevent the honey from being lost by eggs. Since she allows only three or four 

running out. The largest and most elaborate sperms to be deposited on the micropyle 

cells are those provided for the rearing of (small opening at one end of the egg) of 

the queen. Various generations of bees are an egg as it passes out of the oviduct, she 



Circle dance 

Wagqinq dance 

Fig. 11-29. Behavior of the bee during its circle and wagging dance. 

may lay over a million fertilized eggs in her 
lifetime. Such eggs develop into females, 
workers or queens. But unfertilized eggs 
also develop and these become males or 

The eggs hatch in three days into tiny 
larvae which are fed at first on a rich se- 
cretion from the queen's pharyngeal glands, 
the "royal jelly." Later the workers and 
drones are fed on honey and pollen, but the 
queen larva is retained on the royal jelly 
diet and thus becomes large and fertile. 
When full grown, the larvae are enclosed in 
a cell and pass into the inactive pupa stage, 
to metamorphose into the adult bee. Within 
two to three weeks, depending on the caste 
that is produced, the adult emerges by cut- 
ting its way out of the cell. The cell is then 
cleaned and another egg laid in it, starting 
the process over again. The number of bees 
produced depends to a large extent on the 
available pollen and nectar. When bees are 
raised commercially, the hives are always 
placed near a good source of food, so that 
the colonies grow fast and the honey pro- 
duced is as much as 100 pounds per hive 
per season. At the close of the season, most 
of the colonies are destroyed, only a few 
being carried over to start new colonies the 
following spring. 

Implemented by their remarkable sense 
organs, bees exhibit highly complicated be- 

havior. Their eyes are much like those of 
crayfish, with about 4,900 ommatidia in the 
eye of the queen, 6,300 in the worker, and 
13,000 in the drone. Bees distino;uish the 
colors of our spectrum, except red, which 
they confuse with black. They can also de- 
tect ultra violet. They have an excellent 
sense of smell which is made possible by 
some 1,600 (queen), 2,400 (worker), or 
18,900 (drone) sensory endings on tlie an- 
tennae (Fig. 11-27). This aids bees in find- 
ing their way about with exceeding preci- 
sion. Coupled with these well-developed 
sense organs is the size of the brain, which 
is much larger proportionately than for 
other invertebrates. 

When bees return to the hive after finding 
a rich source of nectar or pollen they go 
through a kind of dance in which they walk 
forward, wagging their abdomens rapidly, 
then circling and repeating the process ( Fig. 
11-29). At other times they simply walk in 
circles. Von Frisch, the brilliant Austrian 
zoologist who first worked out this amazing 
behavior, called these the "wagging" dance 
and the "circling" dance, respectively. The 
dances are closely followed by other bees in 
the hive, who then set out and very shortly 
are able to find the source of food. Soon 
a large number of bees is taking nectar and 
pollen from the spot. By a series of experi- 
ments with moving the source of artificial 



Fig. 11-30. The rat flea, carrier of bubonic plague. 

food for bees, von Frisch discovered that 
the circling dance meant that the source 
was less than 100 meters away, whereas the 
wagging dance meant that it was beyond 
that distance. Direction is determined by 
the position of the sun. If a bee is caught 
and then released very soon, it finds its way 
home without any difficulty. If, however, it 
is caught and placed in a dark box for two 
hours and then released, it will fly along the 
path that it would have taken when the sun 
was in the position it had been two hours 
earlier. Thus the bee is able to measure the 
angle of the sun and use it as a guide in 
returning home. It is able to do this even 
on cloudy days or when it sees only a small 
portion of the sky. Hence this little animal 
seems to possess powers of response never 
dreamed of in lower animals. Perhaps there 
are many more, equally as extraordinary, 
awaiting our discovery. 

Insects unlimited 

Insects are so widespread and numerous 
and touch upon man's life in so many ways 
that no one is entirely free from their influ- 

ence, from the housewife who fights the 
ubiquitous fly and cockroach to the flea- 
and louse-bitten beggar who is constantly 
struggling to rid himself of these pests. On 
the other hand, there are those who operate 
a million dollar industry and profit by the 
labors of the honeybee or the silkworm. 
Moreover, there are vast fertile areas of tlie 
globe that are denied occupancy by man 
because of the presence of insects which 
carry deadly diseases. The competition that 
is going on between man and these tiny 
beasts is quiet and not too apparent to 
the ordinary person, but it is a deadly battle 
and it is not at all certain that man will al- 
ways be the victor as he is today. He might 
better employ his efforts to fight this enemy, 
rather than to fight his fellow man. 

Perhaps the most benefit derived from 
insects is from their work as pollinators of 
flowers. Many trees would bear no fruit if 
it were not for various insects, and the same 
is true of such crops as clover and figs. Shel- 
lac is made from a secretion produced by 
certain lac insects in India; others produce 
a dye called cochineal. The cocoon of the 
silkworm is unwound and spun into silk 
thread used to make the fine silk cloth fa- 
miliar to everyone. 

On the debit side are those insects which 
carry disease, such as the mosquito (ma- 
laria, yellow fever, filariasis), the body 
louse (typhus), and the flea (Fig. 11-30) 
(bubonic plague). In the past and still to- 
day these diseases are the cause of a vast 
amount of human misery. Bubonic plague 
alone wiped out from one-half to three- 
fourths of the population in vast areas of 
the world several centuries ago. Today ap- 
proximately one-sixth of the population of 
the world is made wretched by malaria. 
With the eradication of the mosquito alone, 
much suffering would cease. 

Domestic animals also are harassed 
throughout their lives by numerous insects. 
The botfly causes most serious damage to 
the stomachs of horses, while the ox warble 
fly larvae bore holes in the hides of cattle 



Fig. 11-31. The dragonfly (Aescha) is noted for its excellent powers of flight and its enormous eyes. It captures 
mosquitoes and other insects while in full flight. Its two large eyes may possess as many as 30,000 ommatidia, 
and for that reason are probably excellent photoreceptors. Its antennae are rudimentary and are probably of 
little importance to the animal. 

causing them distress, as well as making the 
hide valueless for leather. Finally, there 
are the millions of gnats, flies, mosquitoes, 
and bugs that can be rated merely as having 
a high-grade nuisance value, but do no 
special harm. 

In addition to transmitting important hu- 
man diseases, insects attack man's food and 
either destroy or actually consume it. Cereal 
grains both in the storage bins and in the 

fields are injured or destroyed by various 
types of insects. Clothes, furs, and uphol- 
stered furniture are eaten by the clothes 
moth. Not only the furniture, but the house 
itself can be tunneled and destroyed by 

Modifications in form and function 

Although all insects, with few exceptions, 
have body parts similar to those of the grass- 



Fig. n-32. The larval dragonfly is equipped with mouth 
parts that are adapted for catching other insects in 
the water, where it lives until it becomes an adult. 
The upper picture shows the parts thrust out in the 
striking position; in the lower picture they are re- 
tracted where they are held except when in use. 

hopper and bee, there are wide modifica- 
tions in these parts in different species. 
Starting at the anterior end of the insect 
and working posteriorly, some of the modifi- 
cations are as follows: The antennae may 
be very short, as in the dragonfly (Fig. 11- 
31) or they may be very long, as in the 
long-horned grasshoppers, in each case per- 
forming a specific function that requires the 
^articular type of antennae in question. 
The eyes may be extremely large, as in the 
dragonfly (Fig. 11-31), where they detect 
the flying mosquitoes which the airplane- 
like insect pursues. Or they may be absent, 
as in the termites which work in the dark. 
The mouth parts vary even more widely 
than the differences between the grasshop- 
per and the bee would indicate. The larval 
dragonfly has a formidable weapon for 
catching its prey (Fig. 11-32). The butterfly 

has a long tube which is carried in a coil 
under its "chin" when not in use, but, when 
stretched out during the process of taking 
nectar from a flower, it may be as long as 
the animal itself. The cicada possesses a 
stiff beak which is used in penetrating plant 
tissues to obtain the juices on which it 
feeds (Fig. 11-33). The deerfly has fierce, 
biting mouth parts which make a deep inci- 
sion in the skin when it obtains a meal. In 
fact, when disturbed, it often departs with 
a small fragment of the skin between its 
mandibles. There are also the thin dart-like 
mandibles of the mosquito which can pierce 
the skin very delicately and withdraw its 
meal of blood, at the same time injecting a 
small amount of saliva to prevent the blood 
from clotting. The mouth parts of all insects 
are homologous, yet witness the variety of 
functions they perform. 

The thorax bears two pairs of wings and 
three pairs of legs, all of which are variously 
modified in different insects. The wings are 
formed as thin sacs, by evaginations from 
the thoracic wall, through which trachea 
make their way. Eventually the sacs col- 
lapse and the walls unite and harden, the 
"veins" being formed by the trachea. Some 

Fig. 11-33. The mouth parts of the cicada (Alagicicado) 
are modified to form a stiff beak, which it uses in 
piercing twigs in order to obtain the sap, its chief 
source of food. Note the three simple eyes; the one 
directed anteriorly is particularly conspicuous. 



Fig. 11-34. The wings of insects are highly modified. 
Those of the butterflies and moths are covered with 
scales. Some moths, such as this one, possess long 
scales that resemble fur. Note the long feather-like 
antennae and the heavily pigmented eyes. 

insects, such as the moth (Fig. 11-34), have 
wings which are large and covered with 
scales, while others, like the beetle (Fig. 
11-35), have hard anterior wings (elytra) 
that fold over and protect the soft posterior 
membranous wings. In the flies the wings 
have been reduced to two — the anterior 
pair only. The posterior pair has been re- 
duced to two short stumps called halteres, 
which serve as sensory organs to maintain 
balance during flight. 

The legs may all be the same size and 
used for walking or running, or they may be 
modified for jumping, as in the case of the 

Fig. 11-35. The forewings of beetles are hard and with- 
out veins, and the hindwings are membranous. This 
is clearly demonstrated in the familiar potato beetle 
(Leptinotarsa decemlineafa) which is destructive to 
potatoes as well as other crops throughout the United 
States and Europe. 

grasshopper. Other modifications include 
the paddle-like feet for swimming, in 
aquatic forms (Fig. 11-36), the digging legs 
for excavating, as in the mole cricket, and 
the pincer-like legs for grasping prey, as in 
the praying mantis (Fig. 11-37). Certain 
insects have large, hairy surfaces on their 
long legs which make it possible for them to 
walk on water, where they bend the surface 
tension without breaking it (Fig. 2-2). 
Some legs are modified for making a sound, 
as in the case of the cricket, in which the 
posterior legs are rubbed against the wings 
to make the characteristic chirp of this in- 

Fig. 11-36. Aquatic insects are modified in many ways for life in the water. The predaceous diving beetle (Dytiscos) 
shown here has its hind legs fringed with hair-like bristles which serve to increase the effectiveness of these 
appendages when used in swimming. As a result, it is an excellent swimmer. The insect also stores reserve air 
under its wings for use while submerged. 



Fig. 11-37. The praying mantis (Sfagmomanf/s Carolina) 
has its anterior legs modified into grasping append- 
ages. It gets its name because of its manner of pos- 
ture while waiting for some unwary insect to ap- 
proach within striking distance. It is one of the larger 
insects, reaching a length of 4 inches in some of the 
larger specimens. 

sect (Fig. 11-38). The cicada has special 
organs to make its shrill song (Fig. 11-39). 

The chief modification in the abdominal 
region is the ovipositors of the female. 
These have been described for the bee and 
the grasshopper. In some insects, however, 
the ovipositor is developed in a most ex- 
traordinary fashion. Thus in the Ichneumon 
fly, it is several times as long as the body 
and can drill a hole in wood an inch or more 
deep. Since this insect lays its eggs in the 
body of the larval wood beetle, such an ap- 
paratus is essential. 

The over-all color of insects varies as 
much as it does in birds, from the brilliant 
iridescent green Japanese beetle and highly 
colored butterflies to the inconspicuous drab 
color of the housefly and the camouflaged 
walking stick. The colors are either in the 

exoskeleton or they are produced by differ- 
ential interference of light impinging on 
regular minute depressions and elevations 
in the cuticula. Some insects resemble other 
insects or parts of their environment. One 
species of fly, for example, resembles and 
even acts like a bee, thereby taking advan- 
tage of the protection of the bee's sting, 
even though it has none itself. This is called 
mimicry. The mimic takes advantage of the 
weapon carried by other insects, simply by 
resembling it in both coloration and in ac- 
tion, and is thus able to discourage its nor- 
mal enemies. Other insects, such as the 
walking stick, resemble the twigs and leaves 
of the bush upon which they rest to such 
a great extent that they are not easily seen 
and when discovered even become stiff like 
a leaf petiole. 

There are numerous modifications in the 
respiratory systems. Although most of the 
insects breathe air, some, such as caddis fly 
larvae, receive their oxygen by means of 
thin gills and can get along perfectly well 
under water. These gflls are not, however, 
in any way homologous to the gills of the 
Crustacea. It is clear that the insects be- 
came air-breathing arthropods and that 
only a few have secondarily gone back into 
the water during their larval Hfe. It is also 
interesting to note that they have taken 
only to fresh water and not to salt water. 
This would be expected, since the osmotic 
pressure of the tissues of insects is quite 
different from that of the sea today, al- 
though it may have been the same when the 
insects, or their progenitors, left their ma- 
rine life long ago. It must be mentioned, 
however, that some vertebrates, such as tur- 
tles, seals, and whales were able to over- 
come this difficulty and returned to the sea. 
Finally, some insect larvae can survive in 
the mud at the bottom of bodies of water 
where oxygen is absent. They receive their 
oxygen by breaking down organic matter 
there, just as many anaerobic bacteria ( bac- 
teria that live without free oxygen) do. 

The digestive systems of insects vary con- 

Fig. 11-38. In some insects the appendages are utilized in making sounds. The common field crici<et produces its 
familiar chirp by rubbing its posterior legs against its wings. This is a female; note its long ovipositors. 

Fig. 11-39. The cicada produces its shrill song (only the males sing) by a specially designed sound-making appa- 
ratus. It consists of two sets of vibrating plates located on the ventral side of the abdomen. The right one can 
be seen opposite the tibia of the posterior leg in this photograph. 



siderably, depending on their type of diet. 
Furthermore, the diet may differ widely 
during the larval and adult stages : the but- 
terfly, for instance, feeds on leafy vegeta- 
tion as a larva, and on nectar as an adult. 
Feeding may be confined to certain stages 
of the life cycle and absent in others. Such 
insects as the May flies and fishflies feed 
only as larval forms, the adults living but 
a few days during which time food is un- 
necessary. The adult stage is devoted to 
mating and egg-laying. Among mosquitoes, 
the males mate and die, never feeding at 
any time on blood, whereas the females of 
some species must obtain a blood meal be- 
fore her eggs will mature. She has a vora- 
cious appetite and is able to take a meal 
of blood equal to several times her body 

Some insects, such as certain species of 
termites, feed entirely on wood, a carbo- 
hydrate, apparently never taking in any 
nitrogen. They are able to exist on this diet 
because of the action of the Protozoa ( flag- 
ellates ) which inhabit their intestinal tract. 
Neither the Protozoa nor the tennites can 
survive without the other, a case of perfect 
mutualism, as was pointed out earlier ( Fig. 

Some insects, like the adult dragonfly, are 
carnivores (meat eaters), others, like the 
grasshopper, are herbivores (vegetable eat- 
ers), and still others, like the cockroach, are 
omnivores (both meat and vegetable eat- 
ers). However, almost any one of them 
can be forced to change its usual dietary 
habits when it is confronted with starvation. 

The sense organs and nervous systems 
have become greatly modified among the 
insects. The central nervous system of some 
of the lower insects does not differ greatly 
from tliat found in the earthworm, while in 
others, like the honeybee, there has been 
a great deal of fusion of ganglia and an ap- 
parently higher or more closely knit coor- 
dination of parts developed. 

Insects have developed better means of 
communication than is found among the 

lower forms. Their ability to produce and to 
hear sounds has already been described 
in some forms. Another means of communi- 
cation is illustrated by the firefly, which is 
able to produce a light which seems to bring 
the sexes together at the mating season. 
How this light is produced is an interesting 
problem, and one which biologists have 
studied for a lono; time. It will be discussed 
under the topic of bioluminescence. Most 
insects are remarkably sensitive to chemi- 
cals, especially in the air, greatly exceeding 
man in this respect. Some leave a faint scent 
which is detected by other members of the 
species, usually of the opposite sex. It is a 
common schoolroom experiment to place 
a female Cecropia moth, as she emerges in 
the spring, on the inside of a window screen. 
Very shortly a great many males, detecting 
her presence either by odors or by the pro- 
duction of sound, will collect on the outside 
of the screen. The male mosquito has a very 
large feathery antenna with which he can 
detect aerial vibrations coming from the 
female as much as a quarter of a mile away. 
The social insects represent a very high 
development of the nervous system, per- 
haps the highest in the invertebrates. There 
is a long series of gradations from the soli- 
tary insects to those which aggregate during 
hibernation or migration, like some grass- 
hoppers and the monarch butterfly. Out of 
something like this gregarious behavior may 
have come the parental care in guarding 
the eggs and later the young. From such 
species have come the ti-ue social forms 
which live together in lar2;e numbers and 
have developed various castes, as already 
described in the honeybee. The change in 
the various castes has been fundamental 
because it involves hormonal changes 
which in turn alter the anatomy of the 
caste, as, for example, the development of 
the large mandibles of certain of the sol- 
diers among some species of termites. In 
this group, in addition to the soldiers which 
protect the colony, there are the workers, 
the males, and the queen. In some ant colo- 


nies there are as many as 27 different types 
of individuals, not all present at once but 
occurring at some time during the history 
of the colony. Each type performs certain 
duties and fits into tlie harmonious opera- 
tion of this complex venture. 

Ants seem to have followed the food hab- 
its of man, at least in their methods of pro- 
viding a food supply. The more primitive 
ants merely feed as carnivores, but pastoral 
ants take aphids into their nests and feed 
them, gathering the honeydew (an excre- 
tory product) from them in repayment for 
their efl^orts. The harvester ants show fur- 
ther development by carrying cereal grains 
into their burrow to supply them with food 
during the winter. Finally, the most ad- 
vanced are those ants which gather a cer- 
tain species of fungus and plant it in under- 
ground gardens, tending it even to the point 
of adding humus as a fertilizer. Such be- 
havior represents a very complex interrela- 
tionship of instincts, to say the least. 

Experiments seem to indicate that insect 
behavior consists entirely of instincts built 
up thi'ough millions of generations. Instincts 
are inherited behavior patterns. They are 
presumably inherited like other traits, and 
subject to the laws of natural selection. 
In order to speak of intelligence among in- 
sects, there would have to be evidence not 

Fig. 11-40. Some insects, such as certain braconid 
flies (not a fly but a relative of the honeybee), lay 
their eggs inside the body of other insect larvae as 
show^n here. The large sphinx-moth caterpillar in this 
case supplies the food for the developing fly larvae, 
which finally break through the skin and spin their 
white cocoons. The attack usually means death to the 

only of memory but of ability to choose, 
and while there is a little of the former in 
some species, the latter has never been 
observed. Complex integrated patterns of 
instincts which make an animal fit perfectly 
into its environment are oftentimes mis- 
taken for intelligence, but careful analysis 
will show that such an interpretation is not 

In the reproduction of insects, fertiliza- 
tion is internal. Most insects lay eggs ( ovip- 
arous) and deposit them in a place where 

Fig. 11-41. Insects lay their eggs in a variety of places. The katytid (Microcenfrum) digs a deep hole in the ground 

where she deposits her eggs. Note the long ovipositors. 



Fig. 11-42. Insects vary a great deal in the length of 
time they remain in the different stages in their life 
history. The periodical cicada (Tibieina septendecim) 
is interesting because it is thought to remain as a 
larva in the ground for seventeen years, hence its 
name "seventeen-year locust" (locusts are grasshop- 
pers). Most cicadas are larvae a much shorter period 
of time. Here is the case of one clinging to the bark 
of a tree after the adult emerged. Note the slit along 
the dorsal side through which it made its way. 

development of the larvae is most apt to 
succeed. This may be in the body of another 
insect (Fig. 11-40), the tissues of a plant, 
the ground (Fig. 11-41), or the water. In 
some species the eggs hatch very soon, as 
in the housefly, while in others many weeks 
or even months are required (Fig. 11-42). 
Certain flies and all of the aphids bring 
forth active young ( ovoviviparous ) . 

The eggs of some insects develop with- 
out fertilization by a sperm. Such reproduc- 
tion is called parthenogenesis, or unisexual 
reproduction. This has already been ob- 
served in bees, but it is also commonly 
found among the aphids, where the females 
lay eggs all through the summer months 
which hatch only into females. As fall ap- 
proaches, the eggs produce both males and 
females. Fertilization then occurs and the 
resulting eggs remain over winter and hatch 
into females again in the spring. This proc- 
ess seems to de-emphasize the importance 
of males, and one begins to wonder why 
males are necessary at all! However, they 
do bring in the possibility of variation 
which is impossible with only one sex. Par- 

thenogenesis is thus a regressive step, and 
genetically is more akin to the asexual 
budding that was observed among the coe- 
lenterates. Obviously this is a step back- 
wards in evolution. 

There is a wide variety among different 
species of insects with respect to the num- 
ber of offspring produced by one individual. 
Some of the viviparous flies, for example, 
produce only a few offspring, whereas the 
queen bee may lay a million eggs in 
her lifetime. Under optimum conditions 
the housefly, if unchecked, could increase in 
one summer to such proportions as to cover 
the earth completely, for it goes through its 
entire life cycle in eight days if the tempera- 
ture is high enough (80-90° F.). 

The manner of development from egg to 
adult is widely variable among many ani- 
mals and is particularly striking among 
the insects. The term metamorphosis, which 
means change in form, is applied to any 
animal that undergoes more or less marked 
changes of form between the time of hatch- 
ing and of reaching the adult state. A few 
primitive insects merely increase in size 
after hatching, showing no metamorphosis 
(Fig. 11-43). Others, such as the grasshop- 
per, hatch into a nymph, which resembles 
the adult fairly closely except for the wings 
which are acquired much later ( Fig. 11-44). 
Such change is known as gradual metamor- 
phosis. Some, such as the dragonfly, hatch 
into a naiad (Fig. 11-32), which resembles 
the adult to some extent, but not as much 
as the nymph resembles the adult grass- 
hopper. This type of change is called in- 
complete metamorphosis. In the case of the 
housefly or June beetle, the larval stage 
does not resemble the adult in any way 
(Fig. 11-45); the larva is worm-like and 
usually its diet varies radically from that of 
the adult. Moreover, between the larval and 
the adult stage there is a "resting" stage, 
known as the pupa, during which time the 
larval body is transformed into the adult 
body. This type of change is known as 
complete metamorphosis. Other aspects of 


fire brats 
bristle tails 

sprinq toils 


aphids , lice 

termites, buqs 



draqon flies 
damsel flies 

may -flies 

stone -flies 


bees , ants 

wasps, flies, fleas 

butterflies, moths 

Fig. 11-43. Various types of metamorphosis, together with the names of some of the insects that undergo each type. 



Fig. 11-44. Insects show wide variation in the manner in which they cJevelop from egg to the adult. The grass- 
hopper shows one type, namely, gradual metamorphosis. Note that the young resemble the parents in most 
respects when hatched. As they grow, however, they acquire adult structures such as functional wings. 

metamorphosis as it appears in other ani- 
mals will be discussed in a later chapter. 

It has recently been learned that insects 
as well as Crustacea possess certain hor- 
mones that influence development and 
probably profoundly affect other phases of 
their lives. It has been demonstrated that 
a gland, the corpus allatum, lying behind 
the brain, is honnonal in function. When it 
is removed from the bug Rhodnius, molting 
does not occur. If the gland is transplanted 
to other distantly related insects, it is still 
effective, hence the substance secreted is 
apparently non-specific. 


There are several other groups of less im- 
portant arthropods which will be considered 
briefly: the spiders, ticks, and scorpions 
( Arachnoidea ) ; the centipedes (Chilo- 
poda ) ; and the millipedes ( Diplopoda ) , Of 

Fig. 11-45. Insects such as beetles undergo complete 
metamorphosis during their development. The larva 
shows no resemblance to the adult and frequently 
lives in quite a different environment. This is a 
larva, "grub worm," of the June beetle which ma- 
tures in the soil, feeding on underground vegetation. 
Note the nine dark spiracles. 



Fig. 11-46. American tarantula (Eurypelma), one of the 
largest of all spiders. They are harmless if properly 
handled and live to a ripe old age in captivity. This 
specimen is 26 years old (estimated) and has been 
in the possession of Mr. Robert Baird for 22 years. 

The dorsal view is shown in upper picture. Note 
the eight pairs of legs and the "hairy" body. 

In the lower picture, the ventral view, the large 
powerful nippers are seen which are used in sting- 
ing the prey. The spinnerets can be seen on the tip of 
the abdomen. The silk glands lie just inside the 
abdominal cavity and the secretion is forced out 
through the tiny openings, solidifying into thread 
when it contacts the air. 

The food of this spider is other arthropods, usually 
insects, although it will kill a small bird and feed 
on it. Its sting is relatively harmless to man. 

these the first group is the most important. 

The acquired fear of spiders and scor- 
pions is almost as characteristic among peo- 
ple as is the fear of snakes. In either group 
of animals, however, only a small number 
is actually harmful to man; most of them 
benefit him in one way or another. The 
spiders, for example, feed almost entirely 
on insects, many of which are pests to man- 
kind; their efforts in keeping the insect pop- 
ulation down is probably considerable. The 
spider's persistence has become legendary 
through the story of Robert Bruce and the 

Passing through the woods or sometimes 
in the open in the late summer, all of us 
have no doubt had to brush cobwebs from 
our brows. These are made by the famous 
"ballooning spiders," which are recently 
hatched spiders that seek some high place 
from where they can float, clinging to their 
tiny thread. Even in the most feeble breeze 
these tiny spiders float great distances, 
sometimes far out to sea. It is their method 
of dispersal, which makes it possible for the 
species to find new and fertile places to 
hunt for food. Others build very delicate 
webs in which flying insects are captiu'ed 
and sucked dry by the owner of the web. 
Watching a spider spin such a web is in- 
deed a fascinating adventure. Other spiders 
chase and catch their prey; still others leap 
upon it and kill it by piercing the body with 
their sharp nippers (Fig. 11-46) and inject- 
ing a small quantity of poison that simply 
paralyzes the insect, until it is consumed by 
the spider. Such a paralyzed insect may stay 
in a fresh condition for a long time — a kind 
of room-temperature refrigeration. Some of 
the tarantulas may feed voraciously when 
food is available, but when it is scarce they 
may go for months without food and remain 
in good health. Although most spiders live 
only a year or so, tarantulas have been kept 
in captivity for as long as 26 years. 

The bite of most spiders is harmless, even 
the bite of the large tarantula being no 

Fig. 11-47. A female black widow spider {Lacfrodectus macfans) with her egg case. 


iw*sj ■- 

Fig. 11-48. The common scorpion (Ve/ovis) of the Southwest is an arachnid like the spider. It hides by day and 
hunts spiders and insects at night. The sting at the tip of the tail is an effective weapon against predators and is 
useful in capturing prey. It sucks the body fluids from its prey. Its sting is painful to humans but not fatal. 




Fig. 11-49. The king crab {Limulus), or horseshoe 
crab, is a cJistant relative of the spiders and scor- 
pions and is not a crustacean at all. It is a 
"living fossil" whose close relatives have all be- 
come extinct many millions of years ago. 

Note the leaf-like flaps just back of the legs. 
These are the book gills, so called because when 
in use the flaps wave in the water like the pages 
of a book. The snails {Crepidula) find it advan- 
tageous to "hitch hike" on the crab, thus afford- 
ing them a much more extensive feeding area 
than they could ever attain under their own 
power. When out of water note the track it makes 
in the sand. While clumsy on land Limulus moves 
effectively along the ocean floor where it shovels 
in the mud searching for worms of various kinds 
that make up its diet. 

more serious than a bee sting. There is one 
species, however, which is very common 
in the United States, particularly in Cali- 
fornia, which can cause serious illness and 
death in some cases. This is the black 
widow, Lactrodectiis mactans (Fig. 11-47), 
which is three-fourths of an inch long, and 
glistening black. On the ventral side is a 
bright red hour-glass shaped figure, which 
is a positive means of identification. It lives 
normally in piles of rocks, lumber, and more 
recently around buildings, particularly ga- 
rages. It has been known to cling to the 
underside of automobiles, thus beine trans- 
ported to all parts of the country. The bite 
causes severe abdominal spasms and gen- 
eral restlessness, and the mortality is about 
5 per cent. An anti-venom has been de- 
veloped which protects the victim from the 

more serious effects of the toxin. In spite of 
the relatively small amount of harm done 
by this one species of the group in our 
country, spiders are generally set upon and 
killed by the ordinary person. This is unfor- 
tunate, for these friends, rather than ene- 
mies, of man should be protected. 

The life history of the spider is rather 
unique in some respects. The male is always 
smaller than the female and in some cases, 
such as the black widow, he is hardly rec- 
ognizable because of his proportionately 
minute size. He spins a web upon which he 
deposits his sperms in a mass, which is then 
picked up by his specially formed front ap- 
pendages and carried while searching for 
a female. Once he has found a mate, he 
usually performs a rather weird kind of 
dance and then deposits the sperm bundle 



Fig. 11-50. Centipedes have numerous jointed legs. 
Some tropical forms are nearly a foot long and can 
inflict a painful wound in a man. This is a smaller 
form common in the U. S. 


into her genital pore. Sometimes the female 
proceeds to devour her unsuspecting mate, 
but such is not always the case, and he may 
succeed in escaping. She then spins a tiny 
ball in which the eggs are laid and often 
carries it around with her. It is with con- 
siderable difficulty that this ball of eggs can 
be removed from the female spider. She 
takes it with her everywhere and guards 
it very closely. But after the young have 
hatched, they are on their own. 

The scorpion (Fig. 11-48) is an elongated 
relative of the spider, with large, fierce- 
looking pincers held out in front. The long 
abdomen also terminates in a sharp-pointed 
sting which inflicts an irritating wound on 
man and a fatal one for the insects and spi- 
ders which make up its primary diet. It is 
active at night and hides by day under 
logs and rocks. The "matins; dance " of the 
scorpions has been described in detail by 
many observers and is a very interesting 

The horseshoe or king crab (Fig. 11-49) 
another arthropod, is of interest because it 
belongs to a very ancient group and is 
therefore sometimes referred to as a livins; 
fossil. It inhabits our Atlantic coastal waters 
from Maine to Central America and its molt 
is a common sight, if not the animal itself. 
For some strange reason, this surviving spe- 
cies of the arthropods which lived long ago 
(Cambrian period) has been able to con- 
tinue down to tlie present, while its millions 
of relatives have become extinct. 

The many-legged centipedes (Fig. 11- 
50) are commonly found under stones and 
logs where they remain inactive during the 
day. At night, however, they move swiftly 
about in search of their favorite food, earth- 
worms and insects. They possess a pair of 
poison claws on the first segment which are 
effective instruments in securing prey. Some 
of tlie tropical centipedes reach a length of 
10 inches, and their bite, while not danger- 
ous to man, is certainly painful. In temper- 
ate zones the most common is the house 



Fig. 11-51. The millipede (Spirobo/us) possesses two pairs of jointed appendages on most of its segments. These 
numerous legs move in a rhythmic manner as can be seen from this photograph. Millipedes live in decaying 
vegetation upon which they feed. 

centipede, which is not only harmless to 
man but actually beneficial because it feeds 
exclusively on some of his prime enemies, 
the insects. 

Millipedes (Fig. 11-51) occupy habitats 
similar to those of the centipedes and re- 
semble them in having many legs, except 
that they possess many more as the name 
implies. Some have over 100 segments with 
two pairs of legs on each. When the milli- 
pede crawls the legs move in a wave-like 
manner, the wave seemingly moves in a 
posterior-anterior direction. They are slow, 
crawling creatures, not at all like the centi- 
pedes. They feed on plants and decaying 
organic matter. When in danger some spe- 
cies roll into a ball, whereas others secrete 
an offensive fluid which serves them well as 
a protection against their enemies. 

In summary we have seen that the ar- 
thropods have acquired a body plan so 

beautifully designed that it has permitted 
the group to penetrate almost every avail- 
able type of land and fresh-water environ- 
ment, and to become the most successful of 
all animals alive on earth today. The body 
plan limits the group in only one respect, 
that of size. All arthropods (with few ex- 
ceptions) are small animals and to produce 
successful larger forms an entirely different 
design had to be evolved. This is beauti- 
fully accomplished in the chordates, the 
last group to occupy our attention. How- 
ever, before going on with this very impor- 
tant group we must discuss two peculiar 
groups of animals that appear to represent 
digressions from the phylogenetic sequence, 
the mollusks and echinoderms. Their aber- 
rant body plans, though strange when com- 
pared to others studied so far, have been 
sufficiently satisfactory to permit them to 
spread their kind over much of the earth's 
surface, both in the water and on land. 



All of the animals considered so far have PHYLUM MOLLUSCA 
followed a series of rather logical steps, in 

which increasingly complex physiological The soft-bodied animals that compose 

needs have been satisfied by the develop- the phylum Mollusca include the snail and 

ment of new parts supplementing the basic clam, which are familiar to nearly every- 

plan of the lower phyla. Two very large one, as well as the lesser known squid, 

groups of animals, the mollusks and the chiton, and octopus. These animals are 

echinoderms, have solved these needs in a scattered through the oceans and fresh 

manner quite different from the other waters of the world, their large fleshy 

groups. Since they have not followed the bodies providing an abundant source of 

trend that has been obvious from amoeba food for man and other animals. There are 

through the arthropods, they are known as over 70,000 species of mollusks which vary 

aberrant animals. Both groups are biologi- rather widely in external appearance but 

cally successful: not only are there several have similar basic body plans. Perhaps the 

thousand species of mollusks and echino- most notable and striking thing about the 

derms, but they are spread over a great part mollusks is the lack of segmentation. Even 

of the earth. the chitons (Fig. 12-1), which appear ex- 



ternally to be segmented, show no true 
metamerism when the internal anatomy is 
studied. It seems rather strange that this 
group of animals ignored or failed to ac- 
quire a body arrangement which is so suc- 
cessful among the annelids and arthropods 
and becomes even more so anions the 

In considering the ancestors of the mol- 
lusks it is necessary to go back a very long 
way, into prehistoric times. Fossil records 
indicate that mollusks were present in some 
of the earliest rocks and have continued, 
uninterrupted, to the present time. How- 
ever, even in the rocks there is little evi- 
dence to determine their ancestry. It is 
generally believed that they were derived 
from "worm-like" ancestors, although these 
were not the annelids known today. Larval 
studies indicate that there is a close rela- 
tionship to the annelids, but that relation- 
ship must go far back since the larval stage 
of the mollusk, the trochophore, does not 
show segmentation in the mesoderm (Fig. 

12-2). This means that the annelids and 
mollusks split off and went their separate 
ways before segmentation was introduced. 
The trochophore larva is common to both 

Fig. 12-1. Chitons are primitive mollusks and probably 
resemble the ancient forms that gave rise to our 
modern mollusks. Note the eight overlapping shells 
on the dorsal side. Chitons live among the rocks on the 
seashore and are active at night. 




Fig. 12-2. The ancestral trochophore larva from which 
both the mollusca and annelida are thought to have 
been derived. 

annelid and mollusk and remarkably simi- 
lar in both. The coelom is formed by a de- 
pression in the mass of mesoderm, which 
arises from a single cell. 

The molluscan body plan has certain 
characteristic featm-es that appear con- 
sistently in all of the species in the group. 
One of these is a muscular organ, the foot, 
an organ which serves for several tvpes of 
locomotion. The snail and chiton crawl on it, 
the clam digs a wedge-shaped path with 
it and also walks on it, while the squid uses 
it to capture prey as well as to crawl over 
the ocean floor (Fig. 12-3). Another new 
character is the mantle, which is an enve- 
lope of tissue covering the entire animal. 
The mantle gives rise to the shell, common 
in so many members of this group. The 
original shell appears in the larva as a 
product of the mantle epithelium and 
gradually expands as the animal grows. 

Those mollusks that possess shells use 
them as an abode which is readily available 







Fig. 12-3. Modifications in the body plan of various kinds of mollusks. 

for a retreat in case of danger. Although a 
clam is usually safe in its tightly closed 
shell, it is nevertheless preyed upon by the 
starfish, which has the ability to open the 
shell and devour the soft body parts (Fig. 
12-24). Some members of the phylum have 
no shells, such as the slug and octopus. 
They are protected only by their coloration, 
their habits, or the ability of some to dis- 
charge a cloud of inky material into the 
water which totally obscures them. 

The digestive tract is tubular, much the 
same as in the annelids, although it is coiled 
in various ways in the different groups of 
the mollusks (Fig. 12-3). Many of these 
animals are provided with a peculiar rasp- 
ing tongue, the radula, which is found no- 
where else in the animal kingdom. It is used 
in loosening algae from surfaces, and in 
tearing bits of plants loose as the animal 
feeds. The radula is a long ribbon of tough 
tissue, to which many sharp teeth are 
attached. Muscles are arranged so as to 

pull the radula back and forth over a pro- 
jection which is thrust out through the 
mouth while feeding. It is an interesting 
and clever device to facilitate feeding. 

The clam 

The fresh-water clam, although differ- 
ing in some respects from other molluscan 
forms, is a familiar representative of the 
entire phylum. It is a bilaterally symmetri- 
cal, "headless" animal, enclosed in a double 
shell, usually found partly buried in the 
sand of lakes or streams. By means of its 
hatchet-shaped, muscular foot, which pro- 
trudes from the shell, it is able to plow 
slowly along, feeding on microscopic forms 
of Hfe. 

When the clam "walks" the foot is thrust 
forward between the two valves of the 
shell. This permits blood to flow into the 
many sinuses of the foot, causing it to swell 
and thus form an anchor. As the retractor 
muscles contract, the clam is drawn for- 

Fig. 12-4. Locomotion of a clam. 


groin of sotxl 


montle epithelium 
groin oF sond 

Fig. 12-5. Artificial production of pearls. 

ward an inch or so. The blood then is 
forced out of the foot so that it thins down 
again and can be withdrawn from the sand. 
The process is repeated with each step 
(Fig. 12-4) and a wedge-shaped path is 
left behind. 

If a clam is molested, its foot is hastily 
withdrawn into the shell by the anterior 
and posterior retractor muscles, and the 
valves are slowly and tightly shut by two 
powerful muscles, the anterior and pos- 
terior adductors. This is the only means 
the clam has of barring its door from in- 
truders. To attempt to pull the valves of 
the shell open is a nearly hopeless task, 
unless a thin-bladed knife is first inserted 
thrcfiigh the edge of the shell to sever the 
large adductor muscles. The starfish, how- 
ever, has a novel way of opening the valves. 
It circumvents the clam, attaches its tube 
feet to the two valves of the shell, and 
exerts a steady pull. The pull is resisted by 
the clam for some time, but finally the mus- 
cles are exhausted and begin to relax ( Fig. 

The two valves of the clam are hinged 
dorsally by a ligament, which can be ob- 
served when the adductor muscles are cut. 
The shell itself is usually oval in shape, with 
a blunt anterior end. Along the dorsal sur- 

face is the umbo, a bulbous structure which 
is the oldest part of the shell. From it ap- 
pear the concentric lines of growth, indi- 
cating successive stages of development. 

The outer layer of the shell, the periostra- 
cum, is produced first, then the prismatic 
layer, and finally the innermost part, the 
pearly layer. The periostracum is rough 
and can resist the weak acids produced 
by the dissolved carbon dioxide in the 
water. The prismatic layer, which gives 
strength to the shell, is produced from 
crystals of calcium carbonate lying perpen- 
dicular to the outer layer. The pearly layer, 
the portion that interests the shell collec- 
tor, is also composed of calcium carbonate 
crystals that are arranged parallel with the 
shell, resulting in an extremely smooth iri- 
descent layer. The mantle deposits this 
layer over any irregularities that occur, 
either in the shell or loose particles that 
may lodge in the mantle itself. This is the 
origin of pearls. Foreign bodies, such as 
grains of sand or the eggs of certain para- 
sitic worms, sometimes become attached to 
the mantle or lodged between the mantle 
and the shell. In such a case, layer after 
layer of calcium carbonate (pearl) is se- 
creted over the particle, eventually result- 
ing in a pearl. The Japanese produce pearls 



nznal pore 

qcniial ponz 




bbial palps 






posterior retractor 
posterior adductor 

posterior qanqlioD 

pcurrerT^ siphon 



side vievs) 

cutdqe of shell 

anterior aorta 


openinq to kidney 

cut edge of mantle 

excur rent siphon 




dorsal vicW 

posrerior adductor 

posterior retractor 

Fig. 12-6. The internal anatomy of the clam shown from the side and dorsal views. 

arificially by inserting glass beads into the 
mantles of clams or oysters. After several 
years the pearls can be removed and sold 
on the market. These are true pearls arti- 
ficially produced (Fig. 12-5). 

Once the valves of the clam are opened, a 
soft body enveloped in a mantle is exposed. 
The mantle simply consists of two thin 

sheets of tissue, or lobes. The posterior free 
ends are muscular, and come together to 
form the ventral incurrent and the dorsal 
excurrent siphons, which permit water to 
move in and out by ciliary action of the 
inner mantle cavity (Fig. 12-6). Each side 
of the mantle adheres to the inner nacreous 
surface of the two valves. At these points of 



pericardial cavi 







cffererrj- qill 





.cpibroncbiat space 





Fig. 12-7. Cross-section of the clam through the region of the heart. 

adhesion, the pallial line is formed on the 
shell. The heavy muscular foot lies directly 
beneath the mantle and extends anteriorly 
from the mid-portion of the body. Just 
posterior to the foot and beneath the man- 
tle are two pairs of gills. The dorsal portion 
of the body, directly above the muscular 
foot, contains the internal organs of the 
animal and is called the visceral mass. 
The four plate-like gills are attached 

from a point between the siphons to the 
region just opposite the umbo ( Figs. 12-6, 
12-7 ) . They hang freely beween the mantle 
and the visceral mass. Each gill is made up 
of two plates, the lamellae, which are held 
together by bridges of tissue. The cavity 
between the lamellae is divided into sepa- 
rate water tubes. The lamellae are thrown 
into vertical folds called gill bars and are 
reinforced bv chitinous rods. In addition 



horizontal rows of ciliated pores, or ostia, 
perforate the lamellae through which water 
enters the gill. The water tubes lead to a 
dorsally situated supra-branchial chamber 
that continues to the posterior portion of the 
gill and opens into the excuiTent siphon. 
Blood from the veins circulate through tiny 
vessels within the gill to be aerated before 
returning to the heart. In this manner the 
constant stream of water flowing through 
the gills supplies the animal with oxygen. 
The beating cilia of the gills and the 

carried through the short esophagus to the 
dorsally located, sac-like stomach. A pair 
of digestive glands joins the stomach 
through ducts. Digestion occurs both in 
the stomach and in the glands themselves. 
In some species of clams the crystalline 
style, a gelatinous rod resembling a pouch 
or caecum of the stomach, secretes a starch- 
digesting enzyme. The intestine, leading 
from the stomach, coils several times 
through the visceral mass, much of which 
is the yellow-colored, branched gonad, be- 

Fig. 12-8. Schematic drawing of the circulatory system of the clam. 

mantle draw water and food into the man- 
tle cavity through the incurrent siphon. The 
siphon opening serves to strain out all but 
very minute food particles such as algae, 
Protozoa, and bits of debris. Mucus se- 
creted by the gills catches these particles 
which are borne anteriorly by cilia to two 
pairs of triangular, ciliated labial palps. 
Here a separation takes place (Fig. 12-6), 
the edible particles being carried into a 
groove between the palps and then to the 
mouth, and the debris passing out through 
the excurrent siphon. In the buccal cavity 
more mucus is secreted and the food is 

fore it turns dorsally to pass through the 
pericardial cavity and the heart. Absorp- 
tion takes place throughout the length of 
the intestine, particularly in the portion of 
the rectum which passes through the ven- 
tricle of the heart. The typhlosole is a 
longitudinal fold in the rectum, a structure 
very similar to that found in the intestine of 
the earthworm. Posteriorly, the intestine 
opens through the anus, located within the 
excurrent siphon, where the feces are 
carried away with the out-going current of 
water (Fig. 12-6). 

The heart, lying in the dorsal pericardial 


cavity, forces the blood through the circu- 
latory system of the clam (Figs. 12-7, 12-8). 
The ventricle, which is joined by two later- 
ally situated auricles, pumps the blood for- 
ward into an anterior aorta, supplying the 
muscular foot and viscera, and posteriorly, 
through the posterior aorta, supplying the 
rectum and the mantle. Blood from those 
parts of the body supplied by the aortas, 
with the exception of the mantle, is re- 
turned through a vein to the nephridia, or 

to absorption through diffusion. This is 
particularly true in the region of the foot 
where blood sinuses are numerous. 

Two U-shaped kidneys lie ventral to the 
pericardial cavity (Figs. 12-6, 12-7). These 
function in the removal of wastes from the 
blood and other fluid of the pericardial cav- 
ity. Each is a tubular organ, folded upon 
itself and divided into glandular and 
bladder-like parts. A ciliated opening from 
the pericardial chamber into the glandu- 

visccral ganglia 



— cerebral 


pedal gonglici 

dorsal view 

Fig. 12-9. Anterio-dorsal schematic drawing of the nervous system of the clam. 

kidneys, for the elimination of waste prod- 
ucts. It then moves to the gills to pick up 
oxygen and eliminate carbon dioxide. Oxy- 
genated blood is returned from the gills on 
each side of the clam to the corresponding 
auricle. The mantle also returns oxygenated 
blood to the auricles. Unlike the circulatory 
system of some other animals, some of the 
arteries and veins of the clam are not 
joined by capillaries, but end in sinuses, 
vsdthout cellular Hning. Food and oxygen 
carried directly to these sinuses can pass into 
intercellular spaces and are not restricted 

lar portion drains this region, while the 
bladder region opens into the path of the 
excurrent water, thus carrying wastes out 
of the body. 

Three pairs of ganglia and their connect- 
ing nerve cords constitute the nervous sys- 
tem of the fresh-water clam (Fig. 12-9). 
Each pair of ganglia controls the body 
region in which it is located: the anterior 
or cerebropleural ganglia on either side of 
the mouth; the pedal ganglia in the foot; 
and the posterior or visceral ganglia ven- 
tral to the posterior adductor muscle. Each 



Ji # vooi^ dams 


F!g. 12-10. Life cycle of the clam. 

pair of ganglia is connected by a commis- 
sure and by two connectives, the cerebro- 
visceral and cerebropedal to the other 
gangHonic pairs. Small nerves extend from 
the ganglia to the body surface and the 
muscles. Although the clam is a highly spe- 
cialized animal in many respects, its nerv- 
ous system is comparatively primitive. First 
of all, there is little evidence of a brain. 
Furthermore, the connectives that surround 
the esophagus are highly reminiscent of the 
circumpharyngeal connectives of the earth- 
worm. The sensory apparatus is limited to 
sensory cells on the siphon margins, tactile 
organs on the mantle, and some areas which 
are believed to be sensitive to chemicals. 
Most of these structures resemble similar 
parts of lower type animals. Clams are 
slow, sluggish animals and the sensory sys- 
tem required is relatively simple. 

Fresh- water clams, for the most part, are 
dioecious, but some are hermaphroditic. 
The ovaries and testes, yellow in color and 
surrounding the intestine, constitute much 

of the visceral mass (Fig. 12-6). The 
sperms of the male are liberated from the 
testes through the genital pore, just ventral 
to the aperture of the bladder portion of the 
kidney. From here they are carried through 
the body to be discharged through the ex- 
current siphon. As water is carried into 
the incurrent siphon of the female it may, 
purely by chance, carry sperm cells with 
it. These then enter the suprabranchial 
chamber of the gills where the ova dis- 
charged from the ovary await fertilization. 
After fertilization, the eggs are drawn into 
the water tubes of the gills and attached to 
them by mucus. While the tubes are carry- 
ing eggs they become enlarged and are 
called brood chambers. After a period of 
development, the zygotes become small 
larval glochidia (singular, glochidium), 
complete with two valves and a larval thread 
(Fig. 12-10). Many species develop a 
hooked valve. At this stage they are dis- 
charged into the water through the excur- 
rent siphon of the female where they may 


Fig. 12-11. The knobbed whelk (Busycon corico) laying its string of egg capsules. This large whelk inhabits the 
eastern coast of the United States. This one was taken at Woods Hole, Mass. 

float through the currents or sink to the 
bottom. There is no free movement of 
the glochidium at this time, other than the 
opening and closing of its valves. Hooked 
forms try to attach themselves to any fish 
with which they may come in contact, 
whereas the hookless forms grasp the gills 
of fishes by means of their valves. In time, 
the epithelium of the fish encases the 
glochidium in a cyst-like case. During this 
period the young clam is entirely para- 
sitic, receiving nourishment from its host 
through absorption. After the adult organs 
have developed, the glochidium bursts out 
of the cyst and sinks to the bottom as an 
independent free-living animal. 

Most pelecypods (from the class name 
— Pelecypoda) are bottom dwellers, some 
species of clams even burrow far down into 
the sand and push their siphons up into the 
water. Other forms, such as the oyster, are 
permanently attached to rocks or similar 
objects beneath the water. The shipworm 
(Toredo navalis) has such a slender shell 

that it can burrow into the wood of ships 
and wharves, where it does extensive dam- 
age. The scallop can swim freely in the 
water by flapping its shells together. 

Other mollusks 

Members of the class Amphineura are 
the most primitive forms among the mol- 
lusks. In this group are the chitons, which 
most nearly resemble the probable worm- 
like ancestors of the phylum (Fig. 12-1). 
Their dorsal covering of eight calcareous 
plates has led some biologists to believe 
that this may be the remnant of segmenta- 
tion. The ocean-inhabiting chitons attach 
themselves so securely to rocks that it is 
almost impossible to pry them loose. If they 
are dislodged they promptly curl up into a 
ball. Much like the annelids, chitons live 
under rocks. They are principally "vegetar- 
ians," feeding on various kinds of marine 

In most forms the sexes are separate. 
One investigator, Grave, found that the 



Fig. 12-12. Gastropoda. Two oyster drills (t/roso/pinx) and one periwinkle {Liftorina) are crawling over a rock en- 
crusted with barnacles (Balanus). These marine snails are very common on our coasts. 

sexual activity of one species of chiton was 
influenced by moonlight. It is generally be- 
lieved that periods of moonlight are pre- 
ferred by this animal because the tides are 
low, a condition most favorable for success- 
ful spawning. Coloration in chitons varies, 
ranging from tvirquoise and slaty blue to 
gray and white. Most of the Amphineura 
are "sby" animals and tend to avoid day- 
light, although there are some species with 
furry mantles that do sally forth in the 

The class Scaphopoda includes several 
"headless" species, the best known being 
Dentalkim, the "elephant's tooth." It has a 
muscular foot which is modified for bur- 
rowing into the sand and is therefore quite 
sharply pointed. Its elongated body is en- 
cased in a tapering shell. Unlike most mol- 

lusks, this animal bears no gills and the 
mantle alone takes care of respiration. 
Some of the most interesting and varied 
forms of the mollusks belong to the class 
Gastropoda. They range in size from mi- 
croscopic forms to the large whelks (Fig. 
12-11). Although most members of this 
class possess some kind of shell, forms 
that are entirely without a shell also ap- 
pear. Some have adapted themselves to 
terrestrial life as well as the usual aquatic 
habitat. The most common form is the 
snail, an animal familiar to almost every- 
one (Fig. 12-12). The snail is often ob- 
served wending its way slowly on the 
leaf of a water plant or along a sandy- 
bottomed pool. It moves by gliding over a 
secreted mucus path, using its flattened 
muscular foot which forms the ventral sur- 


face of its body. The movement of the snail 
closely resembles the gliding movement of 
planaria. In some species the foot is actu- 
ally ciliated to aid the gliding motion; in 
others movement occurs by rhythmic mus- 
cular contraction of the foot. The land snail 
(Fig. 12-13) has a definite head which 
bears two pairs of tentacles, one short pair, 
supposedly the center of the sense of smell, 
and a longer pair with a simple eye at the 
tip of each. In water forms the eyes are 
situated at the base of the tentacles. 

Judging from its coiled shell, one would 
expect the snail to have an asymmetrical 
body. This is only partly true, however, for 
the head and the elongated flattened foot 
are bilaterally symmetrical, whereas the 
remainder of the body, which composes the 
visceral mass, is asymmetrical, parts of 
the digestive and circulatory systems being 
coiled. The shell of the gastropods is uni- 
valved, that is, one piece, but the single 
valve may vary in shape from tiny flat- 
tened spirals to long spindle shapes or even 
turban or slipper-like forms. 

Originally snails were aquatic forms, but 
as some species migrated to land one of the 
two gills was lost and the mantle was 
gradually modified until one fold of it 
appeared as a primitive lung. These ani- 
mals, possessing one lung and one remain- 
ing gill, belong to the order Pulmonata. 
The more primitive marine forms that have 
retained the two gills are members of the 
order Prosbranchiata, while the fresh-water 
forms with the right gill remaining are in 
the order Opisthobranchiata. Some of the 
land snails have returned to fresh water, 
but the lung has remained. Such forms 
must come to the surface of the water occa- 
sionally for a supply of oxygen, particu- 
larly during hot weather. 

Snails occur in nearly all parts of the 
world, with the possible exception of ex- 
tremely cold climates, although one species, 
Vitrina glacialis, is found living high in the 
snow-covered Alps. Some of the fresh-water 
snails, such as species of Lymnaea and 

Fig. 12-13. Land snails such as this one {Helix aspersa) 
have a part of their mantle modified into an air 
breathing organ so they are at home on land. In the 
top picture of the dorsal view, the eyes appear at 
the tips of the two large tentacles. The ventral view 
of the snail crawling up a glass plate is shown in 
the bottom picture. Note the second pair of smaller 
tentacles and the mouth at the anterior end. 

Helisonm, are able to survive for several 
weeks in cakes of ice, providing they are 
frozen gradually. Movements of these spe- 
cies may be observed through the ice. For 
the most part, water snails are active 
throughout the four seasons of the year. 
Land snails, on the other hand, are active 
only during the warmer parts of the year 
and are most active at night or immediately 
following a light rain. As cold weather ap- 
proaches they seek a protected place for 
hibernation. During this period, a mem- 
brane is formed, covering the aperture of 
the shell to protect the animal. 

In general, snails are harmful to man. 
The herbivorous land snail, for example, 
often damages vegetation considerably. 
Some forms serve as intermediate hosts for 
parasitic flatworms, while others are para- 



Fig. 12-14. The nudibranch gastropods are without gills 
and breathe through their skin, which is often thrown 
up into papilla-like structures that function somewhat 
as gills do. Those pictured here (Hermissenda crassicor- 
nis) are yellow-green in color and are about an inch 

The two specimens in the top picture are feeding on 

sites themselves. In former times snails 
had some value as a source of food, but this 
is negligible today. 

The slugs are gastropods that do not bear 
shells. The nudibranch, prosaically called 
the sea slug, does have a shell during the 
larval stage but the adult form appears to 
be a snail without a shell (Figs. 12-14, 12- 
15). The name slug was apparently given 
to the lifeless preserved laboratory speci- 
men, which takes on a dingy collapsed ap- 
pearance after it has been exposed to light 
and preservative. When observed in its 
natural habitat on rocky coasts, however, 
these colorful animals are found to be most 
inappropriately named. 

Although the gastropods were of eco- 
nomic significance to ancient man, the 
pelecypods serve modern man to the great- 
est extent. Clam chowder, sauteed scallops, 
and oyster cocktails have become favorite 
forms of sea food all over the world. The 
shells of the bivalved animals are also used 
by man. Most cherished are pearls, the rare 
jewels secreted by the mantle of the sta- 
tionary fresh-water clams and pearl oysters. 
The bits of shell that are cut and polished 
into buttons are products of fresh-water 
bivalves, the clams. 

Members of the class Cephalopoda are 
the most highly organized of the mollusks 
and include the largest species of the in- 
vertebrate animals. The head region, as the 
name implies, is large and well developed, 
unlike most of the preceding groups. Most 
forms of this class bear two large complex 
eyes, resembling the eyes of vertebrates. 
Some have continuous shells, such as the 
shell of the Natitilm (Fig. 12-16), a mem- 
ber of the group immortalized by Oliver 
Wendell Holmes in his poem, "The Cham- 
bered Nautilus." In others, such as the 

hydroids which they are able to do without discharg- 
ing the nematocysts. In fact they incorporate the 
stinging cells into their own body to be used some- 
time later for their own defense. Just how they do 
this is unknown. 

Another specimen, pictured below, is laying its long 
strings of eggs. 


Fig. 12-15. This is another beautifully colored naked gastropod {Phyllaplysia taylori) found in the oceans. Note the 

delicate lines and the two tentacles that protrude like horns. 

cuttlefish (Sepia) and the squid, the shell 
is located internally. In addition to a gen- 
erally large, fleshy body, the cephalopods 
usually have long muscular arms, or 
tentacles, which are modified portions of 
the foot. 

Fig. 12-16. The chambered nautilus, a cephalopod mol- 
lusk, lives in the deep waters (1800 ft.) of the South 
Pacific. Its shell, shown here, is made up of many 
chambers, the last and most spacious being occupied 
by the animal. 

One cephalopod, the squid, is not only 
unique in organization but also in the wide 
range of sizes in which it occurs (Fig. 12- 
17). Different species of squids vary from 
miniature animals 1 inch long to giant 
forms of 18 feet, or twice that length when 
the arms are stretched out. Fossil remains 
show that the squid was one of the most 
prominent animals during prehistoric 
times. The tapering body of this mollusk, 
which suggests an arrowhead or rocket, 
enables it to shoot through the water with 
lightning-like speed, either forward or 
backward, changing its direction simply by 
directing its ventral siphon toward or away 
from the anterior arms (Fig. 12-18). This 
and the medusa are the only animals to use 
jet propulsion in locomotion. The squid is 
also equipped with fins to aid in swimming 
and directing its course through the water. 

Even though the squid is a rapid swim- 



Fig. 12-17. Members of the class Cephalopoda are conspicuous by their many tentacles or arms and their lorge 
complex eyes. The squid (Loligo) is a common member of the class. This one is resting on the bottom of an 
aquarium. Note the siphon just below the eye which controls the squid's direction of movement. Also note the 
contracted chromatophores which cause the animal to appear light in color. 

mer, it is often pursued by large fish and 
by some whales. When it is hard pressed 
by its enemy, it can resort to another 
method of defense. Near the base of the 
siphon is a sac filled with inky fluid which 
can be discharged into the excurrent si- 

phon, thus spreading a cloud of murky water 
which obscures the vision of the enemy and 
hides the squid. Here again this animal has 
employed a defensive device which man 
has only recently used in warfare. Other 
interesting adaptations of deep sea squids 


-^- backward 

Fig. 12-18. Locomotion in the squid. In order to go forward the siphon is directed backwards; to go backward the 

siphon is swung around so that it directs the water forward. 

F.g. 12-19. Cepha opoda The octopus or devilfish (Ocfopus) is completely naked and has eight very long powerful 
arms, well provided suckers, which aid the animal in crawling over rocks in search of crabs which a^r'ts 
mam source of food. Its heavy horny jaws are used in cracking the shells of crabs and the radula tears the 
flesh m tiny bits that are eaten. Most octopi are small and relatively harmless, although they continue to main- 
tain their diabolical reputation. o 7 

In the top picture the animal is resting. Note the large siphon which is effective in swimming 
complete\^\rrl"r" " '^'"""'"^ '" '^^ ^""""^ P'"'"'^- ^"'^ *^°* '* '* ""'v ^^''9^*^Y -"ore awake than when 



Fig. 12-20. The common starfish {Asterias), Mice most starfish, is found along rocky shores crawling over the hard 
surfaces in search of mollusks, particularly clams, which constitute its main food. This one is crawling over the 
shells of clams, many of which are empty because their soft bodies were sacrificed to satisfy the hunger of this 
and several other starfish. 

swim, but it more commonly crawls over 
rocks on the bottom of the ocean. 

are the luminescent organs, the value of 
which is not entirely known. It may be to 
attract food to the animal or to keep 
enemies away. 

Stories of the dangers of the devilfish, or 
octopus (Fig. 12-19), may be considered 
practically fictional, at least in respect to 
the grasp of its tentacles being deadly 
to man. Actually the octopus is harmless to 
man, with the possible exception of the 
giant devilfish, which reaches a length of 
28 feet. The bulbous and flexible body of 
the octopus possesses muscular tentacles 
"that are well armed with suckers. The ani- 
mal usually lurks in shady underwater cav- 
erns awaiting its prey, which it seizes by 
extending the tentacles. A siphon, similar 
to that of the squid, enables the octopus to 


The second group of higher invertebrate 
animals that possess remarkable and unique 
characteristics is the phylum Echinoder- 
mata. Members of this group deviate from 
the direct line of ascent to higher forms 
even more than the mollusks and occupy 
their own isolated position in the animal 
kingdom. Although they have acquired the 
complex organs of the higher types, they 
have reverted to radial symmetry, a pre- 
dominant characteristic of the coelenter- 
ates. For this reason they were once classi- 
fied with the coelenterates but, because of 


ractol eaoco 


pyloric stomach 

cordioc stomoch 

apo't'k caecum 


Fig. 12-21. Starfish cut in such a manner as to show the internal anatomy. 

their calcareous spiny exoskeleton and com- 
plex water vascular system as well as other 
more advanced systems, they have been 
placed in a separate and much more ad- 
vanced group. 

The echinoderms are marine animals, 
mostly free-living but slow-moving. Some 
are permanently attached forms living at 
the bottom of the sea; others are commonly 
found along the seashore, in the sand or on 
rocks. Very often starfish, which are the 
most common members of the group, in- 
vade oyster beds and cause a great deal of 
damage because these choice morsels are 
among their chief sources of food. At one 
time the damage to commercial oyster beds 
was so great that the problem was indeed 

menacing and costly. Men working in the 
ovster beds tried in vain to remove the star- 
fish and destroy them by cutting them in 
two. This was no solution to the problem 
for, like lower forms of animals, the starfish 
has tremendous powers of regeneration. 
Thus, instead of being destroyed by these 
measures, the starfish were actually in- 
creasing; in place of one starfish, the two 
pieces grew into two new individuals. 

The echinoderms are characterized 
chiefly by their spiny outer covering, the 
spines varying from those of microscopic 
size to the large movable spines found on 
such animals as the sea urchin. They re- 
semble the next phylum, the chordates, by 
the presence of a mesodermal endoskele- 



Fig. 12-22. A close-up of the tube feet of the deep sea red starfish (Hippasteria phrygiana) to show the nature of 
the sucking disc at the tip and the circular muscles in the tube that aid in its action. 

ton. In addition, the larvae of some forms 
resemble some chordate larvae (Fig. 

The starfish 

Starfish, found in abundance along most 
seacoasts, vary greatly in size from tiny 
species about one-half inch in diameter to 
the giant starfish, which measures about 
18 inches. The common starfish, Asterias 
vulgaris ( Fig. 12-20 ) , is found chiefly upon 
rocky seashores and bottoms where mol- 
lusks, its main food, are also most abun- 
dant. Starfish resemble the conventional 
five-pointed star pattern, the five radiating 
arms rising from a central disc. Unlike the 
higher invertebrate fonns already studied, 
the starfish is headless, similar to some mol- 
lusks. It is able to move itself in any direc- 
tion that one of the five rays may point. 
However, it usually moves forward with 
two particular arms, namely, the bivium, 
which consists of the two arms adjacent to 
the madreporite, a sieve-like structure 

through which water enters (Fig. 12-21). 
The upper portion of the body, the aboral 
surface, is covered with spines, and be- 
tween these projections are gills, or 
papulae, which function as respiratory 
organs for the animal. Along the bases of 
the spines are small pincerlike structures, 
the pedicellariae, which serve to keep the 
body free from foreign material. Because 
the animal is so well armored with various 
types of sharp projections, it is little won- 
der that it is not chosen as food by other 
animals. The oral side, or under surface, on 
which the mouth is centrally located, serves 
two main purposes, locomotion and food 

An outstanding feature of the echino- 
derms is the appearance of a unique device, 
the ambulacral system, consisting of two 
rows of tube feet which extend from the 
mouth down the oral side of each of the 
five rays (Fig. 12-22). The tube feet enable 
the animal to move slowly over rocks or 
along the ocean floor, to twist and turn its 


Fig. 12-23. Regeneration in a starfish. It required six weeks for this starfish to accomplish the regeneration of the 

two arms seen here. 

body, and to capture food. If a single foot 
is examined, the portion that protrudes ex- 
ternally from the oral surface is found to be 
an elongated tube. Internally, at the oppo- 
site end, a bulbous structure, the ampulla, 
joins a central tube, the radial canal, which 
extends up the ray of the animal to join a 
central circular tube, the ring canal. A 
short tube, the stone canal, is joined to the 
ring canal and runs up to the dorsal surface 
of the disc, opening externally into the 
madreporite plate (Fig. 12-21). 

Water enters through the madreporite 
plate, passes into the stone canal, then 
the ring canal, and to the ampullae. At the 
margin of the ring canal are located the 
Tiedemann bodies which produce the amoe- 
bocytes found in the fluid of the ambulacral 
system. When the ampulla contracts, the 
fluid is forced into the tube foot, which is 
thus elongated. If the sucker-like tip (Fig. 
12-22) of the foot touches and attaches to 

an object, the muscular wall of the foot con- 
tracts, forcing the fluid back into the am- 
pulla, thus causing the foot to be shortened. 
Since the foot adheres to the object it has 
touched, the shortenino; of the foot draws 
the body forward. In this manner the star- 
fish is able to move. It also uses this mech- 
anism to obtain food, but instead of the 
alternating "push-pull" system of locomo- 
tion, a steady contraction of the tube foot 
is exerted to produce a constant pull. 

The endoskeleton, which is produced by 
the mesoderm, is a calcareous framework 
composed of many ossicles, most of which 
are arranged in a definite pattern. Even 
though the starfish has this strong endo- 
skeleton, it is capable of autotomy. Thus it 
can break off an arm and readily regenerate 
the lost part, in a very short time (Fig. 
12-23 ) . An arm may live briefly after it has 
broken off the central disc, but it does not 
usually regenerate a new animal unless a 



portion of the disc has been removed with 
it. Experiments have been tried in which 
all five arms were removed from the disk, 
and in some cases the disc was able to re- 
generate the five rays. 

The ossicles of the endoskeleton are 
joined together by a network of connective 
tissue and muscle fibers. Lying within the 
skeleton and extending through all portions 
of the body is the coelom. It contains the 
internal organs and a lymphlike fluid which 
carries free amoebocytes, thus resembling 
the fluid of the ambulacral system. In cer- 
tain regions the coelom comes close to the 
external epidermis which forms a tiny 
finger-like extension, and in these structures, 
called dermal branchiae, the respiratory 
exchange of gases takes place. The amoe- 
bocytes gather waste materials and escape 
from the body through these same bran- 

When feeding, the starfish seizes the vic- 
tim with its arms and secures its grasp by 
attaching the tube feet (Fig. 12-24). The 
sac-like stomach, which consists of a large 
lower portion, the cardiac stomach, and the 
smaller upper region, the pyloric stomach, 
is then everted through the mouth. If the 
captured animal is small enough, the 
stomach may completely surround it. Re- 
tractor muscles in the arms, just below the 
digestive glands, draw the everted stomach 
back into the body to complete digestion. 
If the animal is large it is digested in por- 
tions while the stomach remains everted. 
The digestive juices flow from the pyloric 
region of the stomach and the hepatic caeca 
(paired digestive glands of each ray) until 
the remaining food is small enough to be 
withdrawn into the pyloric stomach. Very 
often partially digested food enters the 
hepatic caeca, as well as other portions of 
the digestive system, and absorption takes 
place in these various organs. The digested 
food passes into the coelomic fluid where it 
is distributed. Attached to the dorsal por- 
tion of the pyloric stomach is a short in- 
testine with rudimentary rectal caeca and 

a small anal opening on the aboral surface 
of the disc. 

The circulatory system of the starfish is 
reduced to such an extent that it can 
scarcely be called a circulatory system at 
all. There are vessels encircling the mouth 
and extending down into each ray, but they 
are too inadequate to transfer the digested 
material to all parts of the body. Instead, 
the fluid of the coelomic cavity trans- 
ports the food to various parts of the body. 

The nervous system of the starfish shows 
the same radial symmetry seen in the other 
parts of the body and is, in general, simple. 
It consists of a nerve ring surrounding the 
mouth, giving off five branches, one to each 
arm, called radial nerves (Fig. 12-21). Two 
other systems lie internally, one on the oral 
side and another near the aboral side. Each 
part of the nervous system seems to function 
independently. The starfish has only a few 
sense organs. An eye and a tentacle are 
located at the tip of each ray, and the pedi- 
cellaria function as dermal sense organs. 

Experiments show that the nervous sys- 
tem of the starfish is sufficiently organized 
to exhibit definite responses. A hungry star- 
fish, placed in a pan of sea water containing 
bits of pulverized mollusk, will move to- 
ward the food. This is a distinctly positive 
chemical response. 

Through its eyes the starfish reacts posi- 
tively to light. This response is best shown 
by removing the eyes of four rays, allowing 
one to remain. With but one eye, the star- 
fish will continue to react positively to light, 
but if the remaining eye is removed, orien- 
tation to light is lost. 

Professor H. S. Jennings tried memory ex- 
periments on starfish to see whether the 
animal is able to learn. He found that, after 
subjecting a starfish to 180 lessons over a 
period of eighteen days, it could be trained 
to use a particular arm to right itself after 
it had been turned over on its aboral side. 
After a lapse of one week, however, only 
one of the many animals tested remem- 
bered its training. 

Fig. 12-24. Starfish frequently feed on clams. In the top picture the starfish is huddled over the clam. In the bot- 
tom picture the starfish has been turned up on one side. Note the long tube feet attached to the two valves of 
the clam. Their continued pull eventually weakens the clam so that it gapes open, allowing the starfish to con- 
sume its soft body. 





Fig. 12-25. Life cycle of the starfish. 

Sexes of the starfish are separate. The re- 
productive system consists of five paired 
gonads, lying close to the hepatic caeca and 
attached in the ano;les between the arms 
where their external openings are located 
(Fig. 12-21). Ova and sperm cells are dis- 
charged into the sea water, where fertiliza- 
tion occurs ( Fig. 12-25 ) . The larva, or bipin- 
naria, is at first bilaterally symmetrical; 
later, as the pentagonal shape of the adult 
form appears, radial symmetry becomes evi- 

dent. Larval forms are partially ciliated and 
free-swimming. After a period of swimming 
near the surface of the water, sometimes for 
several weeks, the larva finally drops to the 
bottom of tlie sea, where it undergoes meta- 
morphosis into the adult starfish. 

Other echinoderms 

While the starfish is the best-known 
echinoderm, there are other forms belong- 
incr to different classes that show some in- 

Fig. 12-26. Various kinds of echinoderms. 


Fig. 12-27. A basket star (Gorgonacephalus articus) taken in 420 feet of water. The five principal arms are sub- 
divided into a great many smaller branches. This is a ventral view. 

teresting adaptations. The sea cucumber 
(Fig. 12-26), for example, is quite unlike 
the starfish in its general appearance, al- 
though its fundamental structure is similar. 
These animals are like cucumbers with a 
fringe of tentacles on one end. While their 
habitats vary widely, a common species of 
our Atlantic coastal waters lives in the mud, 
just below low tide, exposing only its tenta- 
cles above the muddy bottom. It will serve 
as an example. 

The surface of the body seems to be 
devoid of calcareous plates, but micro- 
scopic examination reveals tiny plates em- 
bedded in the soft tissue of the body wall. 
The branched tentacles are located at the 
anterior end, surrounding the mouth; the 
anus is at the opposite end of the animal. 
There are five rows of tube feet, the two 
dorsal ones functioning as respiratory and 
tactile organs, while the three remaining 
rows en the ventral side aid in locomotion, 

much the same as those of the starfish. The 
animal is able to crawl in worm-like fashion, 
by contracting the rather heavy muscles 
which make up the body wall. It feeds by 
allowing the tentacles to become covered 
with detritis from the muddy ocean bottom 
and then pushing them, one at a time, into 
the mouth. Organic material is separated 
from the mud and carried into the digestive 
tract where it is digested. 

Respiration is carried on by means of a 
pair of respiratory trees which extend from 
the lower end of the digestive tract anteri- 
orly in the body cavity. Water is taken into 
the cloaca through the anus and circulated 
through these ramifying tubes. It is likely 
that excretory wastes find their way to the 
outside through these organs as well. 

The sea cucumber has a nervous system 
equivalent to that of the starfish. The ani- 
mal's sensitivity to light is easily demon- 
strated. If it is suddenly placed in a bright 



Fig. 12-28. The sea urchins are covered with long spines 
which aid the long, slender tube feet in moving slowly 
over the ocean floor. By means of a set of five sharp 
teeth they are able to tear vegetative and decaying 
matter to tiny bits which they consume. The aboral 
and oral views of a young sea urchin (Arbacia) are 
shown here. 

light, it will contract at once. If given a 
choice, it will seek out moderate illumina- 

This animal possesses remarkable powers 
of autotomy and regeneration of lost parts 
and, in fact, even employs this behavior as 
a mechanism of defense. If disturbed by an 
intruder who persists any length of time, 
the sea cucumber suddenly contracts its 
muscular walls until considerable pressure 
is built up within. Then it splits open, al- 
most explosively, near the anus, everting 
the respiratory trees which secrete a mu- 
cous fluid that becomes stringy and tough 
when it contacts sea water. The unfortunate 
enemy, usually a lobster, thus becomes 
hopelessly enmeshed in this mass of threads 
so that it is no longer concerned with the 

sea cucumber as a prospective meal. The 
sea cucumber is able to break the trees 
loose at their base and regenerate a com- 
plete set within a short time. 

Another interesting group of echinoderms 
includes the brittle stars and the basket 
stars (Fig. 12-27). Both forms possess small 
discs and long, slender, motile arms, the 
arms of the basket star being branched to 

Fig. 12-29. Sand dollars are close relatives of the sea 
urchins but are much flattened and the spines are 
much smaller. Both the tube feet ^nd spines make 
movement possible in the sand where they live. They 
feed on the organic matter that is present in the sand. 

The typical five-arm arrangement of the openings 
on the aboral side through which the tube feet pass 
is shown in the top picture. These dorsal tube feet 
are long and are modified for breathing. 

The oral surface in the bottom picture shows the 
mouth opening in the middle and the anal opening 
near the edge. 


form a kind of basket. The brittle stars, 
characterized by five long, serpentine arms, 
can move more rapidly than any other 
echinoderm. Their tube feet are few in 
number and are used primarily as touch 
receptors rather than for locomotion. These 
animals do not "mind" losing an arm; mild 
stimulation can cause an arm to be snapped 
off immediately. The rapid regeneration of 
a new member makes this a valuable means 
of escape for these animals. 

The sea urchin (Fig. 12-28) and sand 
dollar (Fig. 12-29) are also seashore oddi- 
ties. A common species of the former is 
usually purple in color and often found ly- 
ing in the small pockets of rocks, in homes 
that may be occupied for many years. It has 
remarkably long spines that are used in 
locomotion as well as in securing prey. 
The tube feet which cover the rounded 
body are also used in locomotion. An 
interesting organ, characteristic of this 
animal, is Aristotle's lantern, a compli- 
cated arrangement of teeth in the mouth 
that is used for picking food apart. Even 
small fish are captured and torn to pieces 
by this effective instrument. In most other 
details the sea urchin resembles the starfish. 
The sand dollar is extremely flattened dor- 
sal-ventrally, so that it resembles a flat disc. 

Otherwise, it possesses the organs common 
to other forms already described. 

The least known of all the echinoderms is 
the sea lily, or feather star. It is composed 
of five greatly branched arms and found 
only in very deep water, attached to the 
bottom by means of a stalk. Food is carried 
down to the mouth by cilia contained in 
the ambulacral grooves. Its tube feet re- 
semble tentacles and are probably sensory 
in function. There are relatively few species 
in this class today, although ancient rocks 
show that there were once a great many 
more, a fact which indicates that they are 
probably on their way to extinction. 

With the echinoderms and mollusks we 
have concluded the study of the inverte- 
brate animals, although some of the primi- 
tive members of the next group are without 
vertebrae or backbones and may rightly be 
considered invertebrates. For the next few 
chapters we shall be concerned with the 
last phylum, the chordates, which is the 
most important of all groups not only be- 
cause it is a very successful phylum but 
because it includes man and nearly all of his 
domestic animals, and that alone is sufficient 
reason for a careful examination of the 
group as a whole. 



The last and most diversified group of 
animals is the phylum Chordata, to which 
man himself belongs (Fig. 13-1). These 
animals have struck off on a new line of 
development which has resulted in maxi- 
mum size and adaptability. Not only are the 
chordates the largest animals in existence 
today, but they have adapted themselves 
to more modes of existence than any other 
group, including the arthropods. They are 
found in the sea, in fresh water, in the air, 
and on all parts of the land from the poles 
to the equator. They range in size from 

spend periods of low temperature in a rela- 
tively inactive condition. Since fish remain 
in the water where the temperature does 
not vary greatly, they have no need for a 
temperature-regulating mechanism. 

All chordates possess at some time in 
their life cycle three characteristics which 
are not found among the invertebrates 
(Fig. 13-2). The first is a dorsal tubu- 
lar nerve cord, which varies from a more 
or less undifferentiated tube extending 
through the entire length of the body of the 
lower chordates, to a shorter, highly dif- 

the tiniest fish to the great whales, which ferentiated tube, with a greatly enlarged 

anterior portion, the brain, in the higher 
forms. In some chordates the nerve cord 
is proportionately about the same as in the 
invertebrates, while in others, such as man, 
it assumes greater prominence, both in size 
and importance. In invertebrates the nerve 
cord is solid, but in all chordates it is tubu- 
lar or hollow. 

reach a length of nearly 100 feet and a 
weight of 100 tons and more (Fig. 13-55). 
In order to penetrate the cold climates and 
remain active, the birds and mammals 
maintain a constant temperature (homo- 
thermal ) . Those animals without a constant 
temperature ( poikilothermal ) , such as the 
amphibians and reptiles, are forced to 




Fig. 13-1. The phylum Chordoto is composed of widely diverse animals as indicated by representatives from each 

of the many groups. 





Fig. 13-2. Comparative study of the vertebrate and invertebrate. 

A second characteristic of the chordates 
is the presence of an internal supporting 
rod, or skeleton, the notochord. This may 
be thought of as a precursor to the vertebral 
column in vertebrates, but it must not be 
considered identical. Although the noto- 
chord is found in the embryos of all verte- 
brates, it persists only in the adults of the 
most primitive. The notochord is made up 
of a gelatinous matrix, surrounded by a 
tough, outer sheath, which is inadequate 

to support a large animal in water, much 
less on land. In all higher forms, therefore, 
it is replaced by the more rigid vertebral 
column. As a change from an aquatic to 
a terrestrial environment took place, pro- 
vision for support had to be even more 
elaborate, for tlie weight of the body for- 
merly supported by buoyancy in the water 
now had to be borne entirely by the skele- 

The third charateristic is the presence of 



Fig. 13-3. Acorn worms (Dolichoglossus kowalevskyi) from the sand flats of Cape Cod. Note the long proboscis for 

burrowing in the sand and mud. 

pharyngeal gill slits. It is obvious that adult 
land animals have no gill slits, but during 
embryological development gill slits do ap- 
pear at some stage. The structures which 
originally produced functional gill arches 
in fish, produced other structures in higher 
forms, such as the sound-making apparatus 
(larynx) and the sound-receiving appara- 
tus (middle ear bones) (Fig. 25-11). These 
fitted the animal better for a terrestrial ex- 
istence and gave it a greater chance of suc- 
cess. As already noted, the arthropods have 
also been able to divorce themselves from 
water and have likewise developed a new 
means of communication by employing old 
structures to perform new duties. Legs and 
wings are employed in making sound; an- 
tennae and legs in receiving sound. 

However, the chordates are set off from 
all other animals by the possession of these 
three characteristics noted and these must 
have been important in contributing to the 
success of this most important of all groups 
of animals. Let us consider some members 
of this phylum. 


Scientists have been perplexed about the 
origin of the chordates and have been un- 
able to determine which lower forms gave 
rise to this last and perhaps most special- 
ized group. Fossil remains have provided 
us with a great deal of information about 
other animals, but man's digging into the 
earth has failed thus far to reveal any sub- 
stantial remnants of the early chordates. 
The reason for this is that these soft-bodied 
animals did not remain intact sufficiently 
long to become fossilized. In spite of the 
lack of evidence concerning the early pro- 
genitors of the chordates, there has been a 
great deal of speculation as to their origin. 

The acorn or tongue worms ( Fig. 13-3 ) , 
which are considered by many zoologists 
to be very low chordates, were at first clas- 
sified among the worms. Although they 
have the three cardinal characteristics that 
identify them as chordates, they resemble 
the annelids more closely than any of the 
great variety of chordate forms. At first 






acorn worm 

Fig. 13-4. A study of the larval stages of the acorn worm and echinoderms has lent support to the idea 

that both came from a common ancestor. 

glance it thus seems plausible to conclude 
that some annelid forerunner might have 
given rise to the vertebrates. There are, 
however, a great many anatomical features 
in the annelid that are impossible to corre- 
late even with the acorn worm. For in- 
stance, the annelid has a ventral instead of 
a dorsal nerve cord, and it has no gill slits, 
and no notochord. In addition there is noth- 

ing in the embryology that would lead one 
to believe they are related. Embryological 
development repeats the history of the race 
and thus indicates similarities of even dis- 
tantly related forms. 

Some biologists look with favor on the 
theory that the chordate ancestor stemmed 
from the same stalk that gave rise to the 
echinoderms. Remarkable similarities have 


two- layered 


echinoderms- chordate line ' 

I ,^ I ^cinnelid - arthropod - mollusk 
/7\\ \ / .rr7T>. line 


-^ ^^ / / fla+worm line \ 





Fig. 13-5. The coelenterates probably gave rise to the echinoderm-chordate line, the flatworm line, and the annelid- 

arthropod-mollusk line. 



been found in comparing the embryos of 
the echinoderm (bipinnaria) with that of 
the acorn worm (tornaria) (Fig. 13-4). In 
fact, they are so similar that it is very dif- 
ficult to distinguish between them. They 
are simple, bilaterally symmetrical, free- 
swimming forms. The bipinnaria sits down 
and develops radial symmetry to become an 
echinoderm, whereas the tornaria grows 
into the acorn worm. Still further back it 
is generally believed now that the coelen- 
terate type gave rise to three great groups: 
the echinoderm-chordate stock, the flat- 
worm stock, and the annelid-arthropod- 
mollusk stock (Fig. 13-5). 


The acorn worm (sub-phylum Hemi- 
chordata ) ( Fig. 13-3 ) is here considered 
as the first member of the phylum Chor- 
data, although some zoologists believe it 
so divergent as to warrant a phylum by 
itself. This earthworm-like animal lives 
buried in the mud, using a protrusible pro- 
boscis to move about as it feeds on organic 
matter. It fulfills the required characteris- 
tics of chordates by the presence of numer- 
ous gill slits, a dorsal as well as a ventral 
nerve cord, and a small anteriorly located 
notochord. There are only a few (60) spe- 
cies in the world, but individuals are rather 
common on both the Atlantic and Pacific 
coasts of the United States. Although they 
have no apparent economic significance to 
man, they are of interest to the zoologist. 

The tunicates, or sea squirts (sub-phy- 
lum Urochordata ) (Fig. 13-6), are also 
grouped with the chordates, although after 
looking at the adult form one would 
scarcely expect them to be classified here. 
Commonly attached to rocks along the sea- 
shores, they live by forcing water in and 
out of their sac-like bodies through siphons, 
resembling the clam in this respect. The 
water passes into a large perforated phar- 
ynx which strains out tlie tiny food particles 
that are carried into the digestive tract. 

Gills line the many openings in the pharynx 
wall, but aside from this one chordate char- 
acteristic, it appears to have no claim to 
membership among the chordates. 

However, a careful look at the larval 
form demonstrates at once its true chordate 
relationships, for the larva possesses a noto- 
chord and dorsal tubular nerve cord, in 
addition to the gill slits. As an embryo, the 
animal is tadpole-shaped and swims ac- 
tively in the sea water ( Fig. 13-6 ) . Late in 
embryonic life, however, it settles on a rock 
and metamorphoses into the sessile adult, 
which is a degenerate form compared to 
the active, free-swimming, fish-like chor- 
date from which it came. Tunicates are 
very numerous in the oceans of the world 
and range from microscopic size to more 
than 12 inches in diameter. They may live 
in shallow or deep water and are commonly 
found by the bather who is sufficiently curi- 
ous to examine the rocks along the coast. 
The group as a whole has no economic 

There is another tiny animal (2 inches 
long ) that cannot be mistaken for anything 
but a primitive chordate and, moreover, it 
possesses body structures that force us to 
believe that some such form might have 
given rise to the vertebrates. This animal is 
known as lancelet, or amphioxus ( sub-phy- 
lum Cephalochordata ) (Fig. 13-7). It is 
an ocean dweller, found in relatively few 
though widely separated regions, and 
reaches such numbers along a part of the 
shore of China that it is utilized as a source 
of food. Not only does it possess the three 
chordate characteristics exhibited by the 
two preceding groups, but it also has a 
body plan that closely resembles that of 
the vertebrates. 

Amphioxus has a general shape not un- 
like that of a slender fish, with two longi- 
tudinal folds of skin extending throughout 
most of its length, which may be forerun- 
ners of appendages (Fig. 13-8). Its noto- 
chord functions as a semi-rigid supporting 
internal skeleton, extending from one end 

nerve cord 

phorynqeol qill Sli1 





Fig. 13-6. Tunicate, 

The larval stage in the upper figure and the adult in the lower figure 



of the animal to the other. The muscles are 
segmentally arranged and by their rhyth- 
mic contractions make possible lateral un- 
dulations of the body used in swimming. 
Immediately above the notochord is the 
hollow nerve cord which tenninates anteri- 
orly in a light-sensitive end organ, the eye- 
spot. Numerous gills function in breathing. 
Its digestive and circulatory systems are 
relatively simple and add nothing to what 
we have already seen among the inverte- 
brates. In fundamental plan, however, these 
organ systems, like so many other features 
of this little animal, show great similarity 
to the vertebrates and thus point to the pos- 
sibility that the vertebrates may well have 
come from a form not greatly unlike it. 


It was not until the chordates somehow 
acquired a rigid internal skeleton that they 
became important. There were other fac- 
tors, of course, but certainly if the group 
was to advance it needed a substantial in- 
ternal support upon which a body could 
be built that would succeed not only in an 
aquatic environment but also on land. This 
was accomplished in the development of 
the vertebral column, or backbone. Let us 
examine a few typical examples of this 
highly successful group of the vertebrates. 

The first members of the sub-phylum 
Vertebrata that show the beginnings of a 
backbone are the cyclostomes, the "round- 
mouthed" eels (class Cyclostomata — 
round mouth). Typical representatives of 
this class are the lampreys. They have no 
appendages and no jaws, only a circular 
mouth lined with denticles, small tooth-like 
structures that aid in clinging to prey. 
When the lamprey seizes a bony fish, its 
usual prey, it first attaches itself with the 
sucking mouth and then proceeds to re- 
move small bits of tissue with its rasping 
tongue. If the point of attachment happens 
to be in the abdominal region a perforation 

Fig. 13-7. Amphioxus (Branc/iiosfoma californiensis), 
partly emerged from its burrow along the California 

is made through the body wall and the 
internal organs injured so severely that 
the fish usually dies shortly. However, if the 
injury occurs on the dorsal side over the 
large muscles, the effect is usually not fatal. 
The common sea lamprey, Petromyzon, has 
invaded rivers and streams where it has 
become a formidable foe of fish populations 
(Fig. 13-9). These ravages have been par- 
ticularly severe in the Great Lakes region 
where in many areas commercial fishing has 
all but ceased on this account. Efforts are 
being undertaken to destroy them during 
their nesting period, which takes place in 
small streams. Thus far, however, little prog- 
ress has been made against them. 

Internally as well as externally the lam- 
prey shows its lowly origin (Fig. 13-10). It 
retains a notochord similar to amphioxus, 
but also has the beginnings of a spinal 
column and other internal skeletal parts 
which, however, are composed of cartilage. 
There is a well-developed brain, together 
with an olfactory organ, a pair of poorly 





central canal 

cior6al aorta 

hyperbranchial qroovje 

bQpaTic vein 

qill bar 
qill slit 


ventral aorta 



Fig. 13-8. Amphioxus in longitudinal and cross-sections. 



Fig. 13-9. Sea lampreys {Pefromyzon marinus), attacking a fresh-water fish. Note the scar from a previous injury 

on the dorsal side just above the fore-fins. 

developed eyes, and a pair of simple semi- 
circular canals on each side of die head, 
used in balancing. It has five pairs of gills 
— less than amphioxus, but more than the 
common bony fish. Internally, also, the 
complexity of its body structures far ex- 
ceeds that of amphioxus in all respects. 

Studying fossil records in an effort to 
determine whether any cyclostome-like 
forms occurred in the past, paleontologists 
have been rather successful. A group of 
animals called ostracoderms (Fig. 13-11) 
that lived about 400 million years ago (Si- 
lurian Period), resembled the present-day in these animals today. This appears to be 
cyclostomes in many respects. They devel- true, since the early fossil remains present 

only hindered rapid progress, and perhaps 
this was a factor in its disappearance. At 
one time, the cartilage of the lamprey skele- 
ton, as well as that of the sharks and skates, 
which came later, was considered a precur- 
sor to bone and therefore a more primitive 
condition. More recently this has been in- 
terpreted as a degenerate condition. It is 
now thought that the cvclostomes and 
sharks probably descended from forms that 
possessed not only internal skeletons of 
bone but also heavy outside bony plates, 
which degenerated into the cartilage found 

oped heavy armor plates on their external 
surfaces, possessed a ventral mouth, and 
were without appendages. Their heavy exo- 
skeletons were essential to survive the on- 
slaughts of their invertebrate enemies, the 
water scorpions ( eurypterids ) . It is pretty 
well agreed now that later descendants of 
the ostracoderms lost their plates as these do very little free swimming in the water 
enemies disappeared. With the develop- and get along satisfactorily with a broad fin 
ment of jaws, it became possible for them in the tail region and a dorsal fin to aid in 
to pursue and capture their prey. Under locomotion. The even more primitive am- 
these circmnstances a heavy exoskeleton phioxus manages to steer itself with paral- 

so many forms of bone or a hardened bone- 
like substance. 


The lower forms such as the cyclostomes 

oral hood cartilaqe 


-olfactory orqan 

spinal cord 

dorsal aorta 


pharynqeal qill slit 



Fig. 13-10. Cyclostome in partial longitudinal section. 



lei ventral folds, a dorsal fold, and a tail 
fold which is perhaps extensive enough to 
be called a fin. In order to be more maneu- 
verable in their search for food, animals 
gradually developed more elaborate ap- 
pendages. Shark-like fossil remains of forms 
possessing many paired fins (Fig. 13-12) 
seem to indicate that they "had not quite 
decided" how many pairs of appendages 
were of the greatest utility, and, according 
to Romer, only later did they settle down 
to the orthodox two pairs, the pectoral 

Fig. 13-11. The oldest vertebrates are the ostracoderms 
shown in the upper figure. They had many features 
of modern cyclostomes shown in the lower figure. 

(chest region) and pelvic (hip region). 
From these two pairs of fins, which became 
so prominent in the early sharks and all 
later fishes, evolved the appendages of land 
forms (Fig. 13-13). 

The prehensile jaws of the early primi- 
tive fish were another important acquisi- 
tion, making it possible for them to become 
free swimmers and predators, searching out 
and capturing their prey. This, of course, 
went hand in hand with the evolution of 
better appendages to aid in swimming; 
both were essential if the animals were ever 
to become very important, and, what is 
more, be able to get out of the water and 
onto the land. A clue to the development 
of the jaws can be found from a study of 
the shark's gill arches. These differ but little 
from the jaw itself, and in fact, they are 
so much alike in this animal, as well as in 
many fossil forms, that it is generally 
agreed that the jaws have developed from 

the first gill arch. As will be shown later, 
other important organs also develop from 
these same primitive gill arches. 

The teeth found in the shark's jaw occur 
in never ending rows and show a remark- 
able similarity to its scales. It is thus clear 
that the scales in the region of the mouth 
opening merely enlarged and became the 
teeth of the shark. These teeth simply grow 

Fig. 13-12. Primitive sharks had many paired append- 
ages extending throughout the length of the body, as 
shown in the above figure. The two conventional pairs 
(pelvic and pectoral) appeared in later forms and in 
all present-day vertebrates. 

over the edge of the mouth and are con- 
tinuously shed as they wear out. Later it 
will be shown that all teeth are modified 
scales, including those of man (Fig. 14-4). 
The sharks and their close relatives, the 
rays, cannot match the success of the bony 
fish when it comes to number and variety 
of forais. There is, however, some diversi- 
fication in body form among the group, 
which becomes obvious when the ray is 
studied. It is greatly flattened, with enor- 
mously developed pectoral fins that look 
more like wings (hence the name, sea bat) 
as they undulate in the sea (Fig. 13-14). 
The tail is drawn out to a long whip-like 
structure, which, in the sting rays, bears a 
spine at the tip. When annoyed, the ray 
can inflict a painful wound. Some of the 
rays have gone so far as to produce another 
form of energy in considerable quantity 
and employ it as a mechanism of defense, 
namely, electricity, found in the electric 

Fig. 13-13. Primitive vertebrates, according to some zoologists, had fin folds much like amphioxus (upper 
figure). Portions of these folds were retained in the pectoral and pelvic regions to become the paired 
appendages; other parts became the other fins (middle and lower fish figures). Coming out on land 
necessitated greater development of the appendages, as is seen among the amphibians, reptiles, and 
mammals. At first they dragged their bodies over the ground, later the appendages supported the 
body above the ground, which made it possible for the animal to move more swiftly. This reaches its 
peak in such cursorial animals as the horse. 



V f' ♦S* 







Fig. 13-14. Sea bat (ray) lying on the deck of the A/botross. Note its huge pectoral fins that give it the appearance 

of flying when it swims. 

ray. Modified muscles are so constructed 
as to produce an electrical potential suf- 
ficiently high to stun lower forms and to 
cause a good deal of pain in larger animals. 
Shocking devices are not confined to the 
rays alone, but are also found among a few 
higher fish, namely, several species of eels 
and catfishes. The mechanism of this device 
has not been completely worked out. 


The cartilaginous fishes, the sharks and 
rays (class Chondrichthyes ) , developed 
appendages and true jaws as important ad- 
juncts to success in the water. Once these 
became established as a permanent part 
of the anatomy of aquatic vertebrates, a 
great deal of "experimentation" apparently 
ensued. The result was the modern bony 
fishes (class Osteichthyes ) which have 
gone "all out" in exploring possible body 

shapes, sizes, and colors that best suit them 
for their particular aquatic niches. They 
range from ordinary fish such as the com- 
mon perch (Fig. 13-17) to the vicious gar- 
pike (Fig. 13-15) and bizarre sea horse 
(Fig. 13-16). Obviously they have been 
highly successful, for they have penetrated 
virtually every aquatic environment. They 
are found in the oceans of the world — from 
the surface to great depths, where they have 
attained the most weird shapes and have de- 
veloped extraordinary luminescent organs. 
In fresh water they are found in swift mov- 
ing streams as well as stagnant pools. Some, 
such as the salmon and eel, can survive 
satisfactorily in either fresh or salt water 
and migrate seasonally from one environ- 
ment to the other in connection with their 
breeding cycle. 

Structurally, the bony fish are similar to 
the sharks with a few minor exceptions. For 
example, they have reduced the number of 



V. -i.'^, f^iiOX. -M 

Fig. 13-15. The long-nosed gar {Lepidosteus osseus), a 
vicious carnivore which feeds on other fish. This fish 
inhabits the Great Lakes and most of the streams 
of the Mississippi Valley. 

gills to four pairs and have covered them 
with a thin bony cover, the opercukim 
(Fig. 13-17). Their bodies are covered with 
large overlapping scales arranged like the 
shingles on a house. The fins, while in gen- 
eral of the prototype, are highly variable 
both in position and in size among the 
different groups of fish. There is nothing 
strikingly different about their internal 
anatomy with the possible exception of the 
swim, or air bladder, which occupies a large 
part of the body cavity in many species 
(Fig. 13-17). We shall discuss its origin 
later. Its function is to regulate the buoy- 
ancy of the body. As the fish moves to dif- 
ferent depths tlie gases (COo, N, and Oo) 
increase or decrease in the swim bladder 
automatically, adjusting the specific grav- 
ity of the fish to the corresponding depth, 
but if a fish is suddenly pulled from great 
depths to the surface the expanding blad- 
der may force the stomach out of the 

Most present-day fish possess bony skele- 
tons, a very ancient character. Finally, com- 
pared to the ancient bony fish, present-day 
forms show a tendency toward reduction of 
the massive head bones and toward a re- 
duction in the number of bones generally 
through fusion. There are never more, and 
frequently fewer, bones in later fishes. 

The retention of hard internal skeletons 
made it possible for the fish to begin their 
long migration onto land, to a new type of 

life outside of water. Although this move- 
ment began with the fish, it was not com- 
pletely accomplished until the advent of 
the reptiles, many millions of years later. 
The hard bones made it possible for ap- 
pendages to become sufficiently strong to 
support a body in the air, a feat which the 
degenerate cartilaginous skeleton of the 
sharks could never have accomplished. 

Among the ancient fish there were some 
that had a fleshy portion, or "lobe," which 
extended some distance out into the fin. 
This contained certain skeletal elements 
that have been found to correspond di- 
rectly with similar bony elements in the 
appendages of true land forms, even to the 
appendages of man himself. Descendants 
of these fish undoubtedly were able to 
migrate onto land at a later time to give 
rise to the great array of land vertebrates. 
"Lobe-finned" fish were long thought to be 

Fig. 13-16. The sea horse (Hippocampus kuda) swims in 
a vertical position by means of its dorsal fin. Note 
its prehensile tail, used to cling to vegetation. The 
male has a pouch under the tail where the eggs are 
brooded until they hatch. 



external nares 

olfactory bulb 
optic lobe 



external nares 



truncus arteriosus 



sinus veoosus 

pericardiol chamber 


qall bladder 

bile duct 

pectoral tin 

pyloric caecum 



pelvic fin 





uroqenital pore 
urinary bladder 

dorsal fin 

anal fin 
lateral line. 

caudal fin. 

internal anatomy dorsal external view 

F!g. 13-17. Teleost fish in longitudinal section and dorsal view. 

extinct until a fisherman off the coast of 
South Africa hauled one in (Latimeria) in 
1939. Unfortunately the true value of the 
find was not discovered until the body was 
destroyed, although the skin was mounted 
(Fig. 13-18). Relatives of the "lobe-finned" 
fish are not uncommon today. The lungfish, 
for example, inhabits certain tropical parts 

of the earth where frequent droughts occur. 
Since this form followed a different path of 
evolution, it does not possess well-devel- 
oped appendages and, in spite of the fact 
that it has lungs, probably did not give 
rise to the land forms. 

One other absolute essential had to be 
achieved if fish were to live outside of 



■H kAi^tUOiti/kiilAMlMS i **<■ 

Fig. 13-18. Ancient lobe-finned fish (lofimeria) found off the coast of Africa in 1939, supposedly extinct for 

millions of years. 

water, and that was some means of utilizing 
the oxygen of the air. The "lobe-finned" fish 
of the past, as well as the lungfish of today, 
accomplished this rather satisfactorily. Ap- 
parently these fish lived in regions where 
nearly all the water dried up for extended 
periods during the year, and in order to 
survive such arid periods, they found it 
necessary to come to the surface of drying 
pools and take in air, since there was little 
oxygen in the water. These animals devel- 
oped a pair of sac-like lungs from the ven- 
tral side of the pharynx which allowed 
them to gulp air during periods when their 
gills were useless. This, it must be remem- 
bered, is a primitive condition. Its counter- 
part is found in present-day fishes in the 
form of a swim bladder which functions as 
a hydrostatic organ rather than a lung, since 
these fish have no need for a lung-like struc- 
ture at any time during their lives. It is 
therefore easy to see that once this lung- 
like structure developed among the "lobe- 
finned" fish, it was utilized on land and 
there eventually became the complex organ 
that is found in such animals as the birds 
and mammals of today (Fig. 13-19). 

Thus two features, the bony appendage 
and the lung, made it possible, for animals 
to attempt the greatest of all transitions — 
from the water onto land. 


Of all the changes that have occurred in 
animals during their long evolution to pres- 
ent-day forms, one of the most intriguing 
is the fishes' forsaking of their aquatic life 
for life on land. According to Romer this 
was the "result of a happy accident." They 
would hardly have left the water in search 
of food, since during these times most ani- 
mals were aquatic except a few insects, and 
fish would hardly leave a food-laden world 
for one almost devoid of food. They had 
already supplied themselves with a means 
of breathing air, so this could not have been 
the cause. Romer reasons that, if drought 
periods were too extensive, those fish which 
could breathe air and walk about on the 
land were able to move to other ponds, and 
survive. Thus the appendages and lungs 
aided them in finding water rather than 


Fig. 13-19. History of lungs and swim bladder. 



Fig. 13-20. The common tiger salamander (Ambysfoma tigrinum) normally undergoes a typical amphibian metamor- 
phosis resulting in the adult shown in Fig. 13-21. One variety of this species living in western North America, 
particularly in the southwest, becomes sexually mature while still a larva and never reaches the adult stage. 
The specimen on the extreme left is a young larva, while the one next to it is a sexually mature larva. This is 
as far as development proceeds in nature. Several specimens similar to this one were placed in water with a high 
level of iodine. During the next few weeks the "adult larvae" metamorphosed to typical adults as the next two 
pictures show. 

leaving it. However, during these excur- 
sions some may have found abundant food 
near the water's edge, whereas others which 
could not stand drought may have found 
it more profitable to wander from pond to 
pond in search of food. Again, it does not 
take a great stretch of the imagination to 
see how some might have found other 
members of their own group to feed upon, 
while others might have changed to a her- 
bivorous diet ( as some we know did ) , since 
vegetation was abundant. From such be- 
ginnings the great variety of life among 
land vertebrates appears to have devel- 

There were undoubtedly numerous un- 
successful attempts by many groups of the 
fishes to make the transition onto land. 
The ancestors of the "lobe-finned" fish ap- 
parently were successful, and gave rise to 
the amphibians which include our present- 
day frogs and salamanders. As the name 
amphibian implies, these animals live both 
in and out of the water. Their larval stages 
are always spent in an aquatic environ- 
ment, but the adults of most species are 
able to live out of water, although they usu- 
ally do not venture far from moist sur- 

The life history of the frog is common 



Fig. 13-21. This normally occurring adult tiger salamander is in no way different from the one "artificially" pro- 
duced in Fig. 13-20. 

knowledge to every school child (Fig. 
13-22). He knows that frogs deposit their 
eggs in the water much the same as fish do, 
that the eggs hatch into tiny fish-like tad- 
poles which breathe by means of gills, that 
during the weeks and months that follow 
the tadpole eventually loses its tail and 
develops lungs and jumping legs, which en- 
able it to move onto land. It took the fish 
many million years to accomplish what the 
tadpole now repeats in a few weeks. 

The amphibians are tied to water in vary- 
ing degrees. Some species have tried to 
divorce themselves completely from the 
water, as, for example, the South American 
toad, whose eggs brood in fluid-filled sacs 
upon its back. Other species, such as the 
mud-puppy, spend their entire life in the 
water and cannot be forced to leave it. A 

curious intermediate is a variety of tiger 
salamander (Ambijstoma tigrinum) which 
normally spends its entire life in the larval 
body form, but which, if fed thyroid extract 
or high levels of iodine, can be made to 
lose its gills, develop lungs, and come out 
on land just as its relatives do (Fig. 13-20). 
This tiger salamander larva was thought to 
be a different species from the usual adult 
(Fig. 13-21) and was called the axolotl. 
It would seem that while the axolotl has 
descended from forms that attained the 
adult state, it "preferred" to retain its larval 
body form, perhaps because of more abun- 
dant food or for some other reason. Among 
the amphibians, then, there are those which 
attempt to leave the water altogether and 
those which tend never to leave it. This is 
exactly what would be expected if evolu- 



Fig. 13-22. Life history of the frog. 

Hon has occurred as has been described on 
these pages. 

The frog: the halfway vertebrate 

If numbers be taken as a criterion for 
success, the amphibians were much more 
successful at an earher time than now. At 
that time they did give rise to very success- 
ful groups of animals, the reptiles, birds, 
and mammals. The amphibians seem to 
have reached the halfway mark between the 
aquatic and the land forms and for that 
reason they show some very interesting 
intermediate structures. To study a bird 
or a mammal without reference to the frog 
would be like studying the present-day 
government of the United States without 
recourse to the struggle for independence. 
Understanding of a mammal can only come 
from a historical approach to the whole 

problem, which means that it is essential 
to examine an intermediate type. There is 
no better form to use for such a study than 
a representative amphibian, and the frog 
lends itself especially well for several rea- 
sons. First, aside from its well-developed, 
atypical, jumping legs, it possesses most of 
the typical ancestral amphibian character- 
istics. Secondly, it occurs universally, which 
makes it an inexpensive form for study. 
Lastly, it is of such a size that it is easily 
handled in the laboratory by students; a 
larger or smaller animal offers some difficult 
problems in this respect. A thorough knowl- 
edge of the "halfway" animal at this point 
provides the background for a better un- 
derstanding of the mammal. 

Life history (Fig. 13-22). One of the first 
harbingers of spring is the familiar croaking 
of the frogs. The one heard most frequently 



Fig. 13-23. The common leopard frog (Rana pipiens). 

is the leopard frog, Rana pipiens (Fig. 
13-23), found in shallow ponds and 
streams. The small, green tree toad, Hyla 
versicolor (Fig. 13-24), emits its musical 
sounds from wooded areas near ponds, 
while the bullfrog, Rana cafesbiana (Fig. 
13-25), gives forth its resounding bellows 
from larger bodies of water. While these 
"delightful" sounds are very much wel- 
comed by everyone, they have more impor- 
tant meaning to the frogs. The male frogs 
usually emerge from hibernation first and 
begin the croaking in order to attract the 
females who follow some days later. At this 
time of the year, the eggs in the body of 
the female are fully mature; as a matter of 
fact, they have been fully ripe for many 
months, waiting the coming of the breeding 
season. The male mounts and clasps the 

female with his front legs, grasping her just 
back of her front legs and pressing the 
small swollen parts of each thumb ( nuptial 
pads) against her breast. This process is 
called amplexus (Fig. 13-26), and is a kind 
of copulation. As the female lays her eggs 
the male discharges a milky fluid containing 
sperms, thus fertilizing them. Very shortly 
thereafter, the gelatinous matrix surround- 
ing the eggs swells, causing them to adhere 
to twigs or any other underwater debris 
(Fig. 13-27). The egg mass resembles 
cooked tapioca, and the eggs themselves are 
at first black above and white below, a 
characteristic which may possibly offer 
some degree of protection during the early 
stages of development. 

If an egg is viewed under the microscope, 
one can see it divide, once, twice, three 

Fig. 13-24. A tree toad (Hy/a versicolor) croaking. Note the greatly extended vocal sacs which aid in producing 

its shrill sound. 

Fig. 13-25. The great bullfrog (Rana cafesbiana), which 
may reach a length of a foot over-all. The large ear- 
drum identifies this as a male. 

Fig. 13-26. A pair of toads in amplexus. 



Fig. 13-27. A woodland pond with egg masses of the woodfrog (Rana sylvatica). 

times, and so on, producing furrows along 
one side, then elongating, and finally devel- 
oping a small tail. Sometime later a tiny 
tadpole emerges from the jelly mass. By 
means of a pair of suckers under the mouth, 
the tadpole remains attached to the mass 
for a few hours while it is undergoing fur- 
ther development. Presently, however, it 
begins to swim about and can be seen 
to feed by a scraping movement of its 
mouth as it moves along a leaf of a water 
plant. In this stage it breathes by means of 
gills just as fish do and as its ancestors did. 
It is a vegetarian, feeding exclusively on 
algae and other plant life. 

After some months or years, depending 
on the species of frog, the tadpole rather 
suddenly begins to develop miniature hind 
legs while its tail becomes shorter. At the 
same time its mouth grows larger and wider, 

and its digestive tract shortens. It gradually 
seeks shallower water and occasionally 
comes to the surface for air as its lungs de- 
velop. These trips for air become more 
frequent until finally the frog hops away 
from the water, sans tail and strictly 
carnivorous, a common sight to everyone. 
Thus, in a brief period it has reenacted the 
entire race history of this long and arduous 
migration out of the water onto land, a most 
remarkable feat! 

The summer months are spent in search 
of food which consists of insects, spiders, 
earthworms and even tadpoles and other 
smaller frogs. These are sought in damp 
places, usually near the water, although 
some species such as the leopard frog ven- 
ture considerable distances from the water 
in search of food. Eggs develop rapidly 
within the ovaries of the female, so that by 



midsummer they are fully mature. Ample 
amounts of fat are stored in special organs 
called fat bodies before the frogs go into 
hibernation at the approach of winter. 
Frogs are cold-blooded, as are turtles and 
snakes, and usually spend the winter buried 
in the mud at the bottom of ponds and 
streams. As the temperature drops, their 
body processes slow down simultaneously 
until the heart is beating very slowly and 
all metabolism is reduced to the lowest 
possible rate necessary to maintain life. In 
this state of inactivity the food demands are 
very slight, so that stored food carries 
the frog along quite adequately through 
the winter months. As the temperature rises 
in the spring, frogs soon become active and 
enter at once into the breeding period. 

Each female frog lays from several hun- 
dred to several thousand eggs depending 
on her age. Of these eggs, only a very 
few, perhaps none, become mature frogs. 
Enough manage to come through to ma- 
turity, however, to maintain the race, 
although with the onslaught by birds, tur- 
tles, snakes, fish, and man it is amazing that 
this little animal does survive and one won- 
ders if it will continue to do so. One of the 
greatest demands for its body is by begin- 
ning zoology classes to verify points dis- 
cussed in this book. 

The frog body plan. Frogs range in size 
from the tiny cricket frog (Psetidacris), 
about an inch long, to the bullfrog which 
may be a foot over-all. In general, their 
features are so similar that, aside from col- 
oration and habits, a description for one fits 
them all. When a study is made of a living 
frog, the moist, slippery skin is at once con- 
spicuous even if the frog is kept away from 
water for some time. This is due to tiny 
mucus glands in the skin which constantly 
pour out their fluids to keep it wet. Like the 
earthworm, the frog receives considerable 
amounts of oxygen through its skin and 
must therefore have a moist skin. The slip- 
pery skin also cuts down friction when the 
frog is swimming through the water. 

Other distinctive features include the 
large eyes which when touched are pulled 
down into the head (though actually they 
bulge into the mouth cavity). The protrud- 
ing eyes permit the frog to come to the sur- 
face of the water and see without exposing 
the rest of its body, a definite protection 
against possible enemies. Lying just back of 
the eyes are the large eardrums which are 
a part of the hearing mechanism. Above 
the tip of the nose are the nostrils which 
have valves that can be opened and closed 
at will. These function in breathing. The 
mouth is a very large one, and is kept shut 
all of the time except when the frog feeds. 
At the posterior end is the anus, which is 
the terminal opening of the cloaca. 

The front legs are turned in, "pigeon- 
toed," and there is a swelling ( nuptial pad ) 
on the inside disiit of each front foot of the 
male, already referred to in the process of 
mating. These legs function in breaking 
the fall after a jump, as well as in support- 
ing the anterior portion of the body. The 
long muscular hind legs are beautifully 
adapted for jumping. When the frog is at 
rest on land they are kept along side or par- 
tially under the body in the jumping posi- 
tion, but in the water they are customarily 
left dangling behind. When surprised on 
land, the frog suddenly straightens out its 
legs, throwing the body forward several 
feet. This process can be repeated in rapid 
succession, so that it requires an agile pur- 
suer to overtake the little animal. These 
are the principal external features that are 
noted in a cursory examination. 

Outer covering. Like all vertebrates, the 
skin of the frog consists of an outer thin 
epidermis and a thicker underlayer, the 
dermis. The outer layer, which is shed 
periodically, is made up of flat cells. The 
dermis contains many glands which provide 
the mucus for keeping the skin moist. 
Some species have, in addition, smaller 
glands in this region which secrete a sub- 
stance that is offensive to animals that 
might feed upon it. The dermis is also heav- 



ily vascularized for its function in breath- 


The dermal scales of the fish are notice- 
ably absent among the modern amphibians, 
although fossil remains indicate that their 
ancestors were well covered with scales. 
Scales offer excellent protection from attack 
and it seems sti-ange that the amphibians 
have given up this apparently valuable aid 
in self-preservation. It must be remem- 
bered, however, that in present-day species 
the skin is an "accessory" lung and very im- 
portant in respiration. Only because of this 
condition is it possible for the amphibian to 
spend its quiet periods in an environment 
where breathing with lungs is impossible. 

The locomotor organs. When the loose 
skin of the frog is removed, a set of muscles 
is exposed that far surpasses anything pos- 
sessed by fish. The most conspicuous differ- 
ence noted between the froo; and fish is the 
remarkable development of the muscles 
that operate the appendages, which are re- 
sponsible for the agile jumping movement 
on land. Although many of the muscles ap- 
proximate the position and seem to func- 
tion much the same as similar muscles in 
man, most comparative anatomists agree 
tliat only a very few of them are identical. 
Apparently they are derived from similar 
muscle masses of ancestral forms but prob- 
ably followed different lines of evolution. 
The muscles of the frog are named in Fig. 
13-28 and should be studied in terms of 
their function in locomotion and not from a 
comparative point of view. 

Muscles are contractile tissues which 
function much like rubber bands. They are 
always under slight tension in life, even 
when relaxed. They differ from the rubber 
band analogy in that they have the power 
to contract violently when stimulated. The 
contractile portion is the fleshy or "belly" 
part of the muscle which is attached to the 
bones by tendons. The latter are fibrous, 
tough, and non-contractile. A muscle is 
identified by noting its origin, which is the 
end that moves less when contraction oc- 

curs, and its insertion, the end which moves 
more. Muscles are attached to the bones in 
many positions, and they vary in size from 
the tiny muscles that close the eyelids to the 
large muscle that extends the leg. It is the 
great variety both in the muscles themselves 
and their points of attachment to the bones 
that make possible all of the many move- 
ments made by the frog. When such a sys- 
tem is carried to higher animals, man, for 
example, it is evident that there must be a 
great many muscles to carr\' out the many 
and complicated movements of which such 
a form is capable. 

The supporting structure. Although the 
internal skeleton of the frog is made of bone 
and in many respects resembles that of 
man, in other respects it must be considered 
as the skeleton of a "specialized" verte- 
brate ratlier than a "generalized" form be- 
cause it differs so markedly from primitive 
vertebrates (Fig. 13-29). Perhaps its most 
conspicuous loss it that of ribs and a tail 
which the majority of primitive vertebrates 
possess, but which the fiog, for some rea- 
son, has lost. Furthermore, its body is much 
foreshortened with the loss of many verte- 
brae. Most vertebrates have from 20 to 30 
vertebrae, whereas the frog has only nine. 

The appendages are attached to the ver- 
tebral column by means of girdles, the pec- 
toral in front, the pelvic behind. These are 
c^uite generalized and hence much like 
those in most other vertebrates. The pecto- 
ral girdle consists of three principal pairs of 
bones attached to a series of midventral 
bones called the sternum. The scapulae are 
located on the dorsal side of the trunk (the 
flat extension is called the suprascapula) 
and this structure is similar to the human 
shoulder blade. It joins ventrally with the 
clavicle and coracoid which in turn fuse 
to the sternum. The clavicle (collar bone) 
is well developed in man but the coracoid is 
only a small "bump," fused to the scapula. 
The pelvic girdle is composed of three pairs 
of bones, which in the adult are fused into 
a single structure. The long, flat, anteriorly 

hricaps faworis 

odduetor mognus. 
gracilis mojor. 

gracilis minor 

fibiolis anKicus 
axHnsor cruris 

tibiolit posticus 

f«ndon of Achillas 

Fig. 13-28. Frog muscles, dorsal and ventral views. 


extarnol nans 



Fig. 13-29. Frog skeleton, dorsal and ventral views. 



occipital # >t ■ ■f'^pt^ , 
,. ..1 V oorietol 



Fig. 13-30. Comparative study of the sl<ulls of frog and man. 

directed ilium joins posteriorly with the 
ischium and ventrally with the pubis (the 
latter remains as cartilage ) to form each half 
of this girdle. 

The front and hind legs of the frog are 
homologous, that is, they are very similar, 
possessing approximately the same bones 
although of somewhat different proportions. 
A single bone, the humerus, which fits into 
a cavity ( glenoid fossa ) of the pectoral gir- 
dle, forms the top of the front leg; this is 
followed by a pair of bones, the radius 
and ulna, which are fused together in the 
frog but separate in most other vertebrates. 
The wrist is composed of several bones 
called carpals; these are followed by the 
metacarpals and phalanges of the digits. 
The posterior appendage likewise has a sin- 
gle bone, in this case called the femur, 
wliich fits into a socket (acetabulum) in 
tlie pelvic girdle; this bone is followed by a 
pair of bones, the tibia and fibula, which 
again are fused in the frog. The tarsals are 
next, and two of these are enlarged to add 
a joint in the hind legs, thus facilitating 
jumping. Following the tarsals are the meta- 
tarsals and finally the phalanges. The bones 
of these appendages have remarkably simi- 
lar counterparts in tlie human skeleton. 

The anterior end of the spinal column 
articulates with the base of the skull. This 

skull is no longer a primitive and general- 
ized t}'pe. Fusion of the many bones found 
in fishes has taken place to such an extent 
that some of them have been entirely lost in 
the long evolution to the amphibian type 
of skull. Above the level of sharks there are 
two types of bones present in all skulls: re- 
placement bone, which is that bone replac- 
ing cartilage as the individual develops, and 
dermal bone, produced from the dermis. 
The frog skull is made up almost entirely of 
dermal bones, and the only replacement 
bones present are those immediately sur- 
rounding the brain. 

The skull has been used to trace the ori- 
gin of the amphibians as well as other types 
of animals. It has generally been thought 
that amphibians have given rise directly to 
the mammals; this idea is based on the fact 
that both mammalian and amphibian skulls 
possess two condyles ( bony projections ) on 
either side of the large opening, the fora- 
men magnum, through which the cord 
passes at the base of skull. Recently, how- 
ever, it has been discovered that primitive 
amphibians, like fish, have but one condyle. 
This simply means that both mammals and 
amphibians have followed similar paths in 
their evolution. 

In addition to that portion of tlie skull 
which protects the brain, there are the parts 



oculomotor nerve 3 
trochlear nerve 4 


Spinal nerve I 

externol nares 

olfactory nerve I 

abducens nerve 


triqeminal nerve 5 and 
facial nerve 7 

auditory nerve 8 

' •■ , 

qlossopbarynqeol nerve 9 ^^^^ 
vaqus nersfe 10 

pHoctory lobe 
lens of eye 

cerebrvjrn (telencephalon) 
optic nerve 2 


optic lobe 

„ \ 
cerebel lum (metencephalon) 

semicircvlar canal 

noedulla oblonqota 
(njyelencepba I on) 

Fig. 13-31. Frog brain and cranial nerves, dorsal view. 

which make up the food-getting and breath- 
ing mechanism. The jaws of the frog are 
made up of three bones and resemble the 
gill arches of tlie shark from which they 
were derived. In man these bones have 
fused into a single bone, the mandible 
(Fig. 13-30). Behind the jaws and lying be- 
tween them is the tongue, which possesses a 
rather good support, the hyoid apparatus. 

The hyoid apparatus, like the jaw, has been 
derived from the gill arches. Other gill 
arches have been modified into supporting 
structures for the larynx, the sound-making 
apparatus. Here is seen the method con- 
stantly used in evolution, that is, the forma- 
tion of new structures from old ones which 
no loncrer function in their original manner. 
Nervous system. The nervous system of 



any animal begins with the end organs 
which receive stimulations from the out- 
side world. These sense organs are well de- 
veloped in the frog and are very similar 
to those in man. To begin with, the nose 
bears a pair of nostrils with valves on them 
so that air may be taken in intermittently. 
The nostrils open into small but well-devel- 
oped nasal passages which are lined with 
sensory cells that join the olfactory nerve 
(Fig. 13-31). Tests indicate that the frog 
does rather well in identifying its odorifer- 
ous world and makes use of this sense in 
orienting itself in its environment. 

To the frog, as well as other animals, the 
eye is one of the most important organs of 
sense. It differs from the human eye in minor 
details only. For example, the lids will not 
cover the eye completely; to close it all the 
way, the frog must pull the eye down into 
its socket. The typical six muscles for mov- 
ing the eye in all directions are present, 
just as is the case in all higher vertebrates, 
and these will be studied later. The lens is 
fixed in place so that the focus cannot 
be changed as in man by altering its own 
shape, or as in the fish by moving back 
and forth. Therefore, the frog sees clearly 
only at one distance, and it is near-sighted 
in air and far-sighted under water. The 
rods and cones of the retina, which are the 
parts of the eye sensitive to light, are scat- 
tered, rather than concentrated in one spot 
as in man. Consequently, the frog probably 
does not see as distinctly as higher forms 
do. Due to the position of its eyes the frog 
does not possess stereoscopic vision and 
therefore cannot see depth. Although the 
frog eye is considerably inferior to that of 
mammals, it appears adequate to the needs 
of the animal. 

The conspicuous eardrum of the frog is 
exposed to the outside world, whereas 
in higher forms it is buried deep inside the 
head. Lying beneath the drum is a cavity 
in which a single bone, the columella ( one 
end of which is homologous with the stapes 
of higher animals), extends from the thin 

eardrum to a tiny bit of sensory tissue 
which is stimulated by the vibrations as 
they are passed to it, through first the drum 
and then the bone. Because of the rather 
primitive nature of the auditory organ, the 
frog probably hears most notes at the same 
pitch, that is, while it might hear a thud or 
a chirp, they would both sound the same. 
The organs of equilibrium (semicircular 
canals) are similar to those of both lower 
(shark) and higher (man) forms. 

The frog possesses a lateral line system 
only during the tadpole stage. All fish have 
such a row of sacs extending along each 
side of the body which are sensitive to vi- 
brations and movement of the water. It is 
interesting to note that in the evolution onto 
land these structures were lost, and cer- 
tainly they were not sufficiently sensitive to 
detect similar movements in air. No higher 
animals possess any organs that resemble 
the lateral line system of fish and the tad- 

With the exception of a few major modi- 
fications there is remarkable similarity be- 
tween the brain of the frog and man. The 
brain is proportionately much larger in 
man and the spinal cord sends out three 
times as many nerves. The foreshortened 
body of the frog accounts for the fact that 
there are so few spinal nerves. When tlie 
brain and cord are dissected out and viewed 
as a unit, the brain seems to be no more 
than a slightly expanded anterior end of the 
cord (Fig. 13-32). Starting at the base of 
the brain and progressing forward, the five 
parts of the brain can be seen. 

The first enlarged portion is the medulla 
oblongata ( myelencephalon — 5 ) which 
gives rise to most of the cranial nerves ( Fig. 
13-31). These have to do with most of the 
automatic functions of the body, just as 
they do in man. A slight projection which 
runs transversely across the medulla is the 
cerebellum ( metencephalon — 4) which is 
much smaller in the frog than in most other 
vertebrates. This may be owing to its func- 
tion in muscular coordination, which is 

Fig. 13-32. Ventre/ view of the frog nervous system. 








Fig. 13-33. Oral cavity of the frog, showing the openings of various tubes that enter it. 

poorly developed in amphibians. Animals 
that move in three dimensions, such as fish 
and birds, have proportionately larger cere- 

The most conspicuous objects of the en- 
tire brain are the optic lobes (outgrowths 
of the mesencephalon — 3) in which nerves 
from the eyes terminate. These lobes seem 
to function in inhibition of spinal cord re- 
flexes, rather than as the centers for sight. 
The small projection just anterior to the 
optic lobes ( diencephalon — 2 ) is the epiph- 
ysis, an organ of doubtful function. On 
the ventral side of this same region is a 
tube-like stalk which terminates in an 
enlargement, the pituitary (hypophysis) 
( Fig. 13-32 ) , a very important gland of in- 
ternal secretion about which more will 
be learned later. 

The anterior part of the brain (telen- 

cephalon — 1) is only poorly developed in 
the frog. It is composed of a pair of lobes 
which are partly divided transversely. The 
two anterior parts are the olfactory lobes 
to which the olfactory nerves are attached. 
The posterior parts of these lobes make up 
the cerebral hemispheres, the functions of 
which are not clear. In fact, when this por- 
tion is removed the animal responds, in a 
near-normal fashion when various stimuli 
are applied (Fig. 16-15). This is one of the 
greatest differences between the frog and 
man, for in man many important sensations 
occur in the conspicuous cerebral hemi- 

The frog has only ten cranial nerves, but 
reptiles, birds, and mammals possess well- 
developed eleventh and twelfth cranial 
nerves. There is some evidence that primi- 
tive amphibians, too, had these additional 













nerves, but strangely enough, the frog seems 
to have lost them somewhere along the way. 
The spinal nerves that pass to the legs are 
grouped together in two plexi, the anterior 
brachial plexus and the posterior sciatic 
plexus (Fig. 13-32). From these regions the 
nerves spread out again and pass to all parts 
of the appendages. The larger nerves are in 
the regions of the legs, as one might expect, 
since it is there that more of the messages 
must travel. 

Digestive system. This system starts with 
a disproportionately large mouth which 
when fully open can enclose a body one- 
fourth the size of the frog itself (Fig. 13- 
33). The jaws are feebly armed with a top 
row of teeth and a few on the front edge of 
the palate. The teeth function only in hold- 
ing prey and are incapable of crushing or 
chewing. The large protrusible tongue lies 
on the Hoor of the mouth with the two 
pointed tips directed down the throat. It 
is attached in a peculiar manner, the ante- 
rior end being fastened just inside the lower 
jaw (Fig. 13-34). When in use, the mouth 
is opened wide and the tongue is flipped 
out with lightning-like speed, so fast, in 
fact, that it has no difficulty in capturing 
agile insects, since the sticky mucus se- 
creted by mouth glands makes a good adhe- 
sive agent. There are no digestive enzymes 
in the saliva and hence no digestion occurs 
in the mouth. 

Openings into the mouth cavity are 
those of the eustachian tubes which con- 
nect with the cavity under the ear drums, 
the internal nares, which connect with the 
nostrils, and tlie esophagus which leads ab- 
ruptly into the large U-shaped stomach. 
The back part of the mouth, the pharynx, 
is lined with cilia which beat continuously 
and help carry food down to the stomach. 
The stomach is merely a portion of the di- 
gestive tract which is enlarged for storage 
of food, and the frog has occasion, indeed, 
to use a sac of such ample proportions. 
Some digestion takes place in the stomach, 
much as in the stomach of man. The lower 

extremity is marked by a constriction, the 
pyloric sphincter (a band of circular mus- 
cles which, when contracted, closes the 
opening). The stomach is followed in turn 
by the small intestine which receives the 
pancreatic juice and bile from a single duct 
(Fig. 13-34). The pancreas is a long, light- 
colored, ribbon-like organ that lies between 
the stomach and the first part of the intes- 
tine. The liver is composed of three lobes, 
two large lateral lobes and one smaller 
median lobe. The gall bladder usually lies 
dorsal to the smaller lobe, and the bile 
duct passes from it through the substance 
of the pancreas on its way to the intestine, 
picking up the pancreatic duct along the 
way. The gall bladder is green in color be- 
cause of the bile which it contains. 

The small intestine of the herbivorous 
tadpole is very much longer than that of the 
carnivorous adult frog, a distinction that 
generally separates animals that feed on 
vegetation from those that feed on meat. 
Digestion takes place much faster where 
meat is the principal diet and therefore a 
shorter gut is sufficient. On the other hand, 
the cellulose found in plant tissue requires 
a longer period to digest; hence a longer 
gut is necessary in animals that are vegetar- 
ians. The small intestine of tlie frog opens 
directly into the short expanded large intes- 
tine which soon constricts down to the 
rectum and then opens into the cloaca 
(sewer). Here the genital and urinary 
ducts also empty. Undigested food de- 
posited in this region is soon voided to the 
outside through the anus. The cloaca is 
found among reptiles, birds, and low mam- 
mals, but among all higher mammals the 
urogenital and digestive tubes have sepa- 
rate openings to the outside of the body. 

Circulatory system. Just as in the animals 
already studied, the circulatory system in 
the frog must transport food and oxygen 
to the cells of the body and waste products 
away from them. In fish, the heart is a 
simple pump in which all of the blood is 
carried through a single circuit; the blood 



focorotid arch 
to systemic arch 
carotid arch 

systemic arcb 

^ arch 

to pulmocutoneous arch 

lonqitudinol valve 
semilunon valve 

entrance of pulmonary veins 
entrance of sinus veno3u5 

ouhculo-ventricular valve 

Fig. 13-35. Ventral view of the frog's heart, sectioned to show chambers and valves. 

coming to the heart has lost its oxygen and 
must be sped on its way to the oxygen- 
replenishing station, the gills. The function 
of the heart of the fish is simply to keep 
this mass of blood in motion. In the air- 
breathing frog, however, a new complica- 
tion arises: there are two circuits of the 
blood. One circuit carries blood rich in 
oxygen to the tissues, and then brings the 
"used" blood, poor in oxygen, back to the 
heart, while tlie other carries this depleted 
blood to the lungs and brings it back after 
oxygenation to the heart. These two circuits 
must be kept separate in order to do an 
efficient job. Since the amphibians were the 
animals to take the first step onto land, 
they were the first ones to begin the solu- 
tion of this complex problem, and are in- 
teresting to study because they give some 
information as to how this development 
and evolution all came about. The higher 
reptiles (crocodiles), birds, and mammals 
have solved the problem very nicely by 

producing two complete hearts, but the 
amphibians seem to have been unable to 
make the complete transition, and have 
gotten along these millions of years with a 
rather crude system. 

The blood coming from all parts of the 
bodv first enters the sac-like sinus venosus 
and then the right auricle; simultaneously 
blood rich in oxygen enters the left auricle 
from the lungs. It would seem that upon 
syncronous contraction of the two auricles 
the blood would be badly mixed and that 
all blood leaving the heart could be mixed 
blood. This is not the case, however, be- 
cause of the anatomical arrangement of the 
ventricle. By studying Fig. 13-35, it is seen 
that the opening of the right auricle leads 
into the ventricle to the left of the opening 
from the left auricle and then delivers its 
blood nearer the exit, the conus arteriosis. 
When the blood from the rio;ht auricle flows 
into the ventricle it is just a little ahead of 
the blood from the lungs. When the ven- 

aortic arch 


occipito- vertebral ^ 



—external carotid artery 
internal carotid artery 

/ '^ ^ ^ 
carotid arch 
systemic arcif ^ 
pulmocutaneous arch 
cutaneous artery 

pulmon ory artery 

ic artery 

qastric artery 

„. coeliac artery 
penteric ortery 

I / pnncreas 

splenic ortery 



sciatic artery 

Fig. 13-36. Ventral view of the frog arterial syster 

external juqulor vein 

innominaTe veirr 

subclavian vein 

hepatic vein 
posterior vena cava 

hepatic portal vein 

fat body 
qenital vein 

renal vein 

renal portal vein 

ventral abdominal vein 

femoral vein 
sciatic vein 

:anterior vena cava 
sinus venosus 
^pulmonory vein 

qastric veins 

intestinal vein 

mesenteric vein 
splenic vein 

Fig. 13-37. Ventral view of the frog venous system. 


tricle contracts, the "used" blood passes 13-37). The blood from the hmd legs has 
out first. To further aid the separation of an alternate course in getting back to the 
the blood there is a longitudinal valve lo- heart. It may pass via the kidneys through 
cated in the conns arteriosus which tends the renal portal system, or via the liver 
to direct the first blood into the pulmonary through the hepatic portal system. A portal 
arteries and the later blood into the sys- system is a system of veins which starts and 
temic arches and head regions where it ends in capillaries. The frog, like other 
should go because it is richer in oxygen, lower vertebrates, has two such systems. 
There is some mixing of the blood poor in whereas man and the higher vertebrates 
oxygen with that rich in oxygen in this sys- have retained only the hepatic portal sys- 
tem, and it is not as efficient as that found tem. It can be seen that this system is most 
among the higher forms where separation is important in carrying the blood heavily 
complete. laden with food to the liver where it can be 
Arteries. There are three pairs of large stored and otherwise processed. If it were 
arteries leaving the heart: the pulmocuta- not for this short circuit much of the gen- 
neous which goes to the lungs and skin, eral circulation would be bogged down 
the systemic arches which join and become with sugar and other food products. The 
the dorsal aorta, and the carotids which go two precavas and the single post cava veins 
to the head and neck regions (Fig. 13-36). enter the sac-like sinus venosus through 
Each of these vessels divides many times three openings before proceeding on to the 
until a network of capillaries is formed, right auricle. Blood coming from the lungs 
and these networks supply all portions of in the pulmonary veins empties into the left 
the body with oxygen and food. auricle, thence into the ventricle where it 

Capillaries. The arteries terminate when joins the blood from the sinus venosus. 
their walls become one cell layer in thick- Blood. The plasma of the frog's blood is 

ness. Through vessels of this diameter very similar to higher as well as lower 
blood cells can only pass single file, and forms, but the cells that float in it are some- 
these tiny, thin-walled tubes are the capil- what different. The red cells are large, oval, 
laries. There are a great many capillaries nucleated cells, but at some seasons of the 
in all of the tissues of the frog body just as year many of the cells are without nuclei as 
there are in the tissues of man, and it is dif- are the red blood cells of mammals. There 
ficult to injure a portion of the skin any- are several types of white cells which vary 
where without breaking one of these tiny somewhat from those found in human 
vessels. These are the most important tubes blood. The blood also contains small spin- 
of the entire vascular system because it is die cells which are concerned with blood 
through the capillary walls that food and clotting. 

oxygen can get to the cells. The passing of Breathing system. The tadpole breathes 
the corpuscles through the capillaries 'can by means of gills much the same as fish do. 
be easily observed under the microscope. As it metamorphoses into the adult frog it 
The larger vessels in such a preparation are gradually loses its gills and develops a pair 
the arterioles and venules, which can be of lungs. The larynx, formed from the car- 
distinguished from each other by the fact tilages that were used earlier to support the 
that the blood flows in spurts in the arteri- gill arches, is located at the point of junc- 
oles and only gently in the venules. tion with the mouth cavity, and contains 

Veins. After leaving the capillaries, the the vocal cords which, when vibrated, pro- 
vessels become veins which carry the blood duce the characteristic sounds of the frog, 
back to the heart. These grow larger and sounds which vary with the different spe- 
f ewer as they approach the heart ( Fig. cies and are used as means of identification. 




va&a af Ferenfid . 
adrenol glond _ 

ranol vein 



Fig. 13-38. Male frog urinogenital system, showing the kidney and testis enlarged and in cross-section. 

The air passes into the mouth cavity 
through the nostrils where it is actually 
swallowed into the trachea and lungs 
through the glottis, a slit-like opening in 
the rigid circular larynx (Fig. 13-34). The 
trachea, into which the glottis opens, is 
very short and immediately branches into 
the two thin-walled, sac-like lungs. These 
are very inefficient organs of respiration 
when compared to those found in mam- 
mals. In fact, they are so inefficient that the 
animal must rely to some extent upon the 
skin to supplement the lungs in obtaining 
sufficient oxygen. 

The process of breathing in the frog 
differs considerably from that in man be- 
cause of the lack of both ribs and a dia- 

phragm. The air is brought into the mouth 
through the nostrils by the sudden lowering 
of the floor of the mouth. The valves in the 
nostrils are then closed and the floor of the 
mouth raised which causes the air to be 
swallowed into the lungs. However, much 
of the respiration takes place in the mouth 
alone, for only now and then is the air 
taken into tlie lungs. Apparently, respira- 
tion can take place through the lining of 
the mouth, as well as the lungs and skin. 
When the animal is quiet and the water is 
cold it can remain submerged for long pe- 
riods of time, as through the winter months, 
receiving all of its oxygen and giving off 
all of its carbon dioxide through the skin. 
Exact measurements show that actually 



more carbon dioxide is given off tlirougli 
tlie skin tfian tfirough the lungs. The frog 
has tlius made the transition to land, but, as 
indicated by its respiratory machinery, the 
adaptation to its new mode of life is far 
from perfect. 

Excretory system. The excretory system 
of the frog is essentially the same as in in- 
vertebrates, such as the earthworm or lob- 
ster, as far as the individual units are 
concerned. It is made up of a great many 
nephridia massed together into a pair of or- 
gans, the kidneys. Urinary wastes, urea, 
salts, and so forth, are withdrawn from the 
blood as they pass through the kidney. The 
blood coming forward from the posterior 
parts of the body passes to the kidney and as 
it does the vessels break up into tiny masses 
(glomeruli) in tlie renal corpuscles (Fig. 
13-38). As the blood passes tlirough the 
glomeruli the urinary products are removed 
in a manner similar to that in man (p. 
525). They pass down a long coiled tubule 
and finally reach a larger duct, the uro- 
genital duct, in the male (Wolffian duct). 
The corresponding duct in the female car- 
ries urine only. The urine is deposited in 
the urinary bladder which in turn opens 
into the cloaca. Urinary wastes and feces, 
as well as the genital products all pass to 
the outside through a single opening, the 

Reproductive system. The male: The sex 
organs of the male are the yellowish testes 
located ventral and anterior to the kidneys 
( Fig. 13-38 ) . They hang in a sheet-like bit 
of tissue, the mesorchium, through which 
tiny tubules, the vasa eflFerentia, pass on 
their way from the testes to the kidneys. 
Upon entering the kidney, the vasa efferen- 
tia connect with the uriniferous tubules 
which are connected to the renal corpus- 
cles. Therefore, the tubules carry both 
sperms from the testis and urine from the 
renal corpuscles. The two products flow to 
the lateral edge of the kidney where they 
are poured into a larger tube, the urogeni- 
tal duct, which eventually deposits its 

sperm load into the sperm sac and its urine 
into the bladder. In reviewing this peculiar 
situation it might be said that the testis has 
more or less "taken over" a portion of the 
original urinary system in order that the 
sperm cells might be carried conveniently 
to the outside of the body. This may be 
true, because lower forms such as the cyclo- 
stomes have no ducts to convey their prod- 
ucts to the outside of the body, whereas 
higher forms such as man have separate 
ducts for removing urine and sex cells from 
the body. 

The female: Between breeding seasons, 
the ovaries are tiny, wrinkled organs lying 
in the same position on the kidneys as the 
testes do in the male. Sometime in the 
summer months when the food is abundant 
the residual eggs lying in the walls of the 
ovaries begin to grow; and continue at a 
rapid pace until the ovaries are tremendous 
in size, almost filling the body cavity. The 
eggs develop in tiny pockets in the wall of 
the hollow ovary (Fig. 13-39) and when 
the breeding season approaches the mature 
eggs burst out in the body cavity. Here they 
are swept along by the united effort of cilia 
which line nearly all the walls. Their goal is 
the ostium, the tiny anterior opening of the 
long coiled oviduct. All of the cilia beat in 
such a manner as to direct the eggs to the 
ostium alone. Once inside the opening, the 
eggs make their way single file through 
the long oviduct which is also lined with 
cilia. During their passage they accumulate 
a jelly-like substance on their exteriors which 
swells rapidly the moment the eggs become 
immersed in water. 

The oviducts are much longer and more 
convoluted during the breeding season 
than between seasons. Near their posterior 
end just before they join the cloaca they en- 
large into thin-walled sacs, the uteri. Here 
the eggs are stored until amplexus occurs, 
at which time they are laid. Male frogs of 
some species possess rudimentary oviducts, 
just as male mammals possess rudimentary 
mammary glands. There is a time in the 




posterior vena cava 

odrenol glond 

Fig. 13-39. Female frog orinogenital system, showing the kidney and ovary enlarged and in cross-section. 

early development of the animal when the 
sex is not determined, so "to be certain," 
both organs are produced; later only one 
becomes functional. 


The amphibians were forced to spend 
their embryonic life in the water and were 
able to leave water only as adults. The rep- 
tiles moved one step farther in the long 
trek to complete terrestrial existence. They 
spend no part of their life in the water 
unless they choose to do so. In order to 
accomplish this feat, radical changes were 
required in the physical provisions for their 

early development. Means were provided 
whereby the early embryos could exist in a 
fluid environment, and this medium was 
supplied with sufficient nourishment to 
carry the embryo through the stages equiv- 
alent to the tadpole stage among amphib- 
ians. Enough food was stored in the egg so 
that the oncoming young one might be well 
along in its development when it emerged 
on its own, and, as a result, be able to care 
for itself, eat the adult diet, and move about 
under its own power on land. For this rea- 
son, reptilian eggs are large, with great 
quantities of stored food in the form of yolk 
and albumin ( Fig. 13-40 ) . The young rep- 
tile starts its life on the top of a large yolk 
mass in tlie egg, and eventually it incor- 



porates the entire yolk mass into its intes- 
tine and uses the stored food for growth. 

During this time it is floating in a fluid 
environment, reminiscent of its amphibian 
ancestors. A large, fluid-filled sac, called the 

Fig. 13-40. In order for vertebrates to completely divorce 
themselves from water they needed some means of 
caring for their young during their early develop- 
ment. This was accomplished with the evolution of 
the land egg. 

amnion, develops around the embryo which 
not only provides a fluid environment, but 
also protects the developing embryo from 
injury and desiccation. Shortly after the 
formation of the amnion, the allantois de- 
velops from the posterior end of the em- 
bryo. This enveloping membrane receives 
discarded material, including carbon diox- 
ide, from the embryo. It lies very close to 
the porous, rigid outer shell so that a gase- 
ous exchange can readily take place. There- 
fore, in addition to being an organ of ex- 
cretion, the allantois acts as a temporary 
respiratory organ during embryonic life. 
The young reptile need not be immersed in 
water at any time during its life, and thus 
the first true land animal has been evolved. 
This animal can seek out any environment 
it wishes without regard to water beyond 
its metabolic needs. This was probably the 
greatest step forward in conquering the 

These changes in the developing embryo 
were undoubtedly the greatest ones that 

took place in the reptile, although other 
changes also occurred that made it better 
suited for a terrestrial existence. Since an 
aquatic environment was no longer essen- 
tial, internal fertilization became a neces- 
sity to protect the delicate reproductive 
cells. Therefore, efficient copulatory organs 
developed from the floor of the cloaca of 
the male, insuring a direct transfer of the 
sperm into the genital tract of the female. 
In addition, reptilian legs became longer 
and were usually more ventrally located, 
making it possible for them to support the 
body completely off the ground, a feat 
which tlie amphibian had not accomplished 
(Fig. 13-13). The heart also began to form 
a complete partition in the ventricle, pro- 
ducing the beginnings of a four-chambered 
heart and thereby separating the pulmo- 
nary and systemic blood to make a much 
more eflFective circulatory system. The 
greater endurance and strength of reptiles 
reflects the effectiveness of this change. 

Early reptiles 

This group of animals has had a long and 
luxuriant history. Living forms, such as the 
turtles, crocodiles, lizards, and snakes, are 
relatively insignificant animals on the earth 
today. However, there was a time during 
the Mesozoic Era, the so-called Age of 
Reptiles, when this group dominated all 
animal life on the earth and reached such 
peaks that perhaps no other animal, not 
even mammals, will ever attain. Reptiles 
may be said to be the most successful ani- 
mals that have thus far existed, man not 

Sometime in the distant past there must 
have been forms stemming from the am- 
phibians that gradually took on reptilian 
characteristics. Such animals have been 
found in fossil remains and are called the 
Stem Reptiles. Among tliese is Seijmouria, 
which was probably the first animal that 
began to show what is now known as rep- 
tilian characteristics. Setjmouria was found 
near the small Texas town of Seymour. This 

Fig. 13-41. Primitive reptiles gave rise to all higher groups of animals. The reptiles as a group have 
been the most successful of all vertebrates, as indicated by their great numbers and variety of form 
in Mesozoic time and by the fact that they gave rise to the birds and mammals. 



Fig. 13-42. Dinosaurs often left footprints in soft mud that later became buried with fine sand, leaving almost per- 
fect impressions of the feet of these ancient animals. The sedimentary rock in which this one was found sepa- 
rated, so that both the mold (right) and the cast can be seen. 

discovery is a missing link, bearing resem- 
blances to both amphibians and reptiles. It, 
or forms like it, probably gave rise to the 
great variety of reptiles known to have 
lived during the Mesozoic Era and indeed, 
to birds and mammals as well ( Fig. 13-41 ) . 
The Age of Reptiles lasted over 100 million 
years. By comparison it is estimated that 
man is no older tlian a million years at 

The dinosaurs 

The saga of the dinosaurs presents one of 

the most amazing stories ever told, a story 
which has unfolded through a careful study 
of fossil remains over a long period of time. 
Dinosaurs had their meager beginnings in 
the Triassic Period when they were small 
unimportant animals. Evolving in both size 
and numbers during the Jurassic and Cre- 
taceous periods they reached the pinnacle 
of walking land vertebrates near the close 
of the Mesozoic Era, finally disappearing 
from the earth never to appear again. They 
ranged in size from the barnyard fowl to 
the largest of all land animals. 



Many of the dinosaurs developed the 
bipedal method of locomotion, that is, they 
rose on their hind legs when in haste and 
propelled themselves entirely by these two 
appendages. This idea proved successful in 
some of the largest flesh-eaters, which had 
powerful hind legs but only short anterior 
appendages (Fig. 13-41). Bipedal locomo- 
tion in four-footed animals is, then, very 

Footprints left in various parts of the 
world by these ancient animals have given 
paleontologists some interesting evidence 
as to the nature of the reptiles that made 
them (Fig. 13-42). For example, in the 
old mud flats of the Connecticut Valley 
there are those that resemble bird foot- 
prints of today, and hence were first 
thought to be those of giant birds. One in- 
teresting small dinosaur was the "ostrich 
dinosaur," which was toothless and pos- 
sessed a hornlike bill resembling that of a 
bird. It had long hind legs, indicating its 
great ability to run. Scientists have debated 
its possible habits; one suggestion made is 
that the animal probably fed upon the eggs 
of other dinosaurs. With such a diet, it 
would be understandable why the animal 
had no teeth. Long legs would be its means 
of escape once it was detected by the owner 
of the eggs. One report, according to 
Romer, states that a crushed skull of the 
ostrich dinosaur was found near the nest- 
ing grounds of the horned dinosaurs; if the 
complete story were known, it might be a 
case where the owners caught the thief 
practicing his trade. 

The flesh-eaters grew to enormous size 
and the largest one unearthed, Tyranno- 
saiirus ( tyrant reptfle ) , reached a height of 
19 feet (Fig. 13-41). It must have been 
an awesome creature in those prehistoric 
times, perhaps feared by all living crea- 
tures. Its skull was over 4 feet in length and 
the jaws were armed with a formidable set 
of teeth, which must have functioned well 
in rending and tearing other animals to bits. 
It possessed massive hind legs and small 

front ones. It is thought that its chief source 
of food was the giant amphibian forms 
which existed at the same time. 

The great amphibious dinosaurs were 
vegetarians and grew to great lengths and 
heights but did not become as massive as 
the flesh-eaters. Brontosanrus and Diplo- 
dociis grew to the largest size and are the 
ones most commonly displayed in museums. 
They reached a length of 85 feet and a 
weight of 40 tons or more. They walked 
on all four feet and could look over a three- 
story building. Their powerful legs were 
placed in such a position as to carry their 
body evenly balanced. The neck and tafl 
were very long, in fact, they seem to bal- 
ance one another on opposite ends of the 
trunk. The head was much too small for 
the size of the animal, and it is difficult to 
see how it could house a brain sufficiently 
large to govern such a massive hulk. 
Furthermore, the jaws were so small and 
weak that the animal must have been 
forced to eat continuously to maintain itself. 
The dorsally-placed nostrils have led sci- 
entists to conclude that the animal was 
amphibious and probably remained sub- 
merged most of the time with only the 
nostrils protruding above the surface of 
the water for breathing air. The tremendous 
burden would thus be partially buoyed up, 
relieving the legs from bearing the entire 

In the hip region, the spinal column 
supported an enlargement several times the 
size of the brain. Apparently impulses re- 
ceived by the brain from the sense organs 
were sent down to the large posterior 
ganglion which operated the posterior legs 
and perhaps the rear portion of the body. 
The strange anatomy of this great beast 
inspired the late Bert L. Taylor of the 
Chicago Tribune to write the following 
poem : 

Behold the mighty dinosaur. 
Famous in prehistoric lore. 
Not only for his power and strength 
But for his intellectual length. 



You will observe by these remains 
The creature had two sets of brains — 
One in his head (the usual place), 
The other at his spinal base. 
Thus he could reason a priori 
As well as o posteriori. 
No problem bothered him a bit 
He made both head and tail of it. 
So wise was he, so wise and solemn. 
Each thought filled just a spinal column. 
If one brain found the pressure strong 
It passed a few ideas along. 
. If something slipped his forward mind 
'Twas rescued by the one behind. 
And if in error he was caught 
He had a saving afterthought. 
As he thought twice before he spoke 
He had no judgment to revoke. 
Thus he could think without congestion 
Upon both sides of every question. 
Oh, gaze upon this model beast. 
Defunct ten million years at least. 

Horned dinosaurs such as Triceratops 
were the last of the large dinosaurs (Fig. 
13-41). The body was relatively bare but 
the head was heavily armed with bony or- 
gans of defense. Two great horns protruded 
anteriorly over the eyes and another over 
the ridge of the nose. An enormous flare 
of bone extended out from the back of the 
neck which probably functioned admir- 
ably in preventing an injurious blow to this 
vulnerable region. 

It has been a question why these great 
animals became extinct by the end of the 
Cretaceous Period. There are many an- 
swers, but possibly a combination of many 
factors was responsible for their extinc- 
tion. Since the flesh-eaters depended on the 
plant feeders for food, a gradual extinction 
of the latter meant annihilation of the 
former as well. Geological changes going 
on at that time indicate that the land was 
gradually rising, culminating in tlie forma- 
tion of the Rocky Mountains in this country. 
This meant not only less water and conse- 
quently fewer swamps where these animals 
lived, but also cooler climates and perhaps 
much less vegetation. With the declining 
food supply the great herbivores starved to 
death, taking the carnivores with them. 

Thus ended the reign of the greatest group 
of animals that have ever lived on the earth 
thus far. The future may brings others but 
it seems highly unlikely that they will reach 
such size as these mighty beasts. 

Modern reptiles 

From tlie time of Seymoiiria certain rep- 
tiles have continued down to the present 
time. In some regions they are rather nu- 
merous, but in comparison to their glorious 
past they are mere remnants. At least six- 
teen orders of reptiles have lived on the 
earth; today only four remain (Fig. 13-41). 
Of these, one is nearly extinct but it does 
include a species, Sphenodon, more com- 
monly known as Tuatara (Fig. 13-43), 
which is of considerable interest because it 
carries in its body many anatomical fea- 
tures definitely identifying it with the 
earliest of reptiles. Tuatara has appropri- 
ately been called a "living fossil." Many of 
its characteristics show a definite relation- 
ship to the stem reptiles as well as to mod- 
ern reptiles. It is always interesting to 
speculate why such isolated members of a 
once flourishing group were able to survive 
down to the present day when all its rela- 
tives are long since extinct. In the case of 
Tuatara, its location is probably responsi- 
ble; the reptile lives in New Zealand where 
it has had few, if any, natural enemies. 
Tuatara living in other parts of the world 
were set upon and apparently destroyed by 
the aggressive, more agile mammals. Thus 
it can be said that isolation has saved one 
species of animal. Since its environment has 
remained unchanged, Tuatara, itself, has 
changed but little through the past 200 
million years. 

The surviving members of the great rul- 
ing reptiles, the dinosaurs, are the croco- 
diles, the caimans of the Amazon, the 
gavials of the Ganges and the alligators of 
today (Fig. 13-44). Although feeble in size 
and small in numbers compared to the 
dinosaurs, some do reach a length of 30 
feet. They inhabit the large rivers of the 



Fig. 13-43. Tuatara (Sp/ienodon) from New Zealand. A "living fossil.' 

world and are often hunted for their 
valuable hides. These animals show some 
anatomical features which place them 
among the highest reptiles. For example, 
they have a nearly completely divided 
ventricle in the heart, resulting in a four- 
chambered heart like that of birds and 
mammals. They also possess the mammalian 
characteristic of a nearly complete dia- 
phragm, which is a muscular separation be- 
tween the chest and abdominal cavities. 

Another interesting characteristic of these 
large reptiles is that their scales do not 
overlap, but instead consist of plates placed 
upon dermal bones. The resulting very 
heavy protective armor is resistant to 
almost any attack of modern animals. 

The turtles 

These are the most odd looking of all 
reptiles (Fig. 13-45). If they were extinct 
man would regard them with wonder, but 

Fig. 13-44. American alligator (A//igofor mhsissippiensis). These grow to 16 feet in length. 



Fig. 13-45. The turtle is one of the strange animals of 
the >vorld, yet it is so commonplace that it goes un- 
noticed. It is completely protected by the shell which 
has the disadvantage of limiting the movements of 
the appendages. 

The above picture is that of the box turtle {Terra- 
pene ornata) and the lower picture is that of the 
huge green turtle {Chelonia mydas). While the former 
gets around well on land, the latter is particularly 
adapted for life in the water. Note how its anterior 
appendages are adapted for swimming. 

because they are so commonplace we think 
Httle about them. Turtles have existed a 
long time on the earth, since the earliest 
ones were contemporaries of the most 
primitive dinosaurs. The great reptiles 
came, and passed on again to extinction, 
but the turtles have persisted. Even with 
the advent of mammals the conservative 
turtle, concealed in its protective armor, 
has maintained itself and, who knows, may 
survive long after the mammals, including 
man, have passed out of existence. - 

The turtle shell is a combination of struc- 
tures identifiable in other reptiles. It is 

composed of horny scutes similar to the 
ordinary reptilian scales, with bony plates 
lying underneath which are fused firmly 
dorsally to the internal skeleton, including 
the clavicles, ribs and vertebral processes. 
A similar plate, the plastron, completes the 
shell on the ventral side. The combined 
"box" affords a first-rate exoskeleton into 
which the animal can withdraw almost 
completely to shield itself from the out- 
side world. Such a rigid outcovering has 
limited its movements to a large extent. By 
a paddle-like motion of the four append- 
ao;es it slides along on its ventral side. It 
moves very slowly and awkwardly so that 
when it is in danger it merely stops and 
"pulls into its shell" and outwaits its 
would-be predator. Body muscles have 
pretty much degenerated but the leg mus- 
cles are well developed. Breathing is ac- 
complished by a pumping action of the 
neck and leg muscles. 

Turtles live both on land (tortoises) and 
in the water. Like many other groups of 
land animals they have returned to the 
water and have so modified their bodies 
that they are well adapted to an aquatic 
existence. The great sea turtles, such as'*' 
the hawksbill and the green (Fig. 13-45), 
have their appendages modified into flip- 
pers. They are never seen on land except 
during the egg-laying season. Others, such 
as the common snapping and painted tur- 
tles, are usually found in water but are 
also frequently seen on land near bodies of 
water. Still others, like the high-shelled 
tortoises, live in certain parts of the world 
where they have no enemies and grow to 
enormous sizes, often weighing 600 pounds. 
These desert forms feed on vegetation 
alone and rarely if ever take any water. 
Members of this group have penetrated all 
habitats from the sea to the desert and have 
prospered through millions of years. 

The snakes and lizards 

Perhaps the most despised of all animals 
alive today are the snakes and lizards, not 
because they are particularly harmful to 



Fig. 13-46. One of the largest lizards alive today, Iguana iguana, reaches a length of 6 feet. It inhabits tropical 


man or because he inherits a fear of them, 
but because he is taught to be afraid of 
them, particularly snakes. The group as a 
whole does little harm to man or his 
domestic animals, and what harm is done is 
offset by its creditable deeds. 

Both lizards and snakes are covered with 
scales which they shed periodically. 

The lizards are four-legged animals and 
exemplify the typical modern reptile, that 
is, they show the least amount of modifica- 
tion in body form of any of the reptiles. A 
good example of the group is the Iguana 
(Fig. 13-46). The group contains rather 
bizarre types, among them the horned toad 
(Fig. 13-47) and the tree-dwelling chame- 
leon. The latter possesses a prehensile tail 
and feet that have three fused toes oppos- 
ing the other two, an ideal adaptation for 
arboreal life. It also has the ability to 

change its color rapidly, a characteristic not 
confined to reptiles alone by any means. 
However, this lizard does show a greater 
extreme in color change than most other 
animals. It also has a protrusible tongue 
almost equal to its body in length, a con- 
venient tool for catching insects. The only 
poisonous lizard is the Gila monster (Fig. 
13-47), a highly colored, sluggish, plump 
creature found in various desert regions of 
the world. It uses its venom in killing small 
animals which make up its diet. Its bite is 
rarely, if ever, fatal to man. The venom 
flows into the wound from the base of the 
teeth as a result of chewing action; this is 
a far less efficient mechanism than that of 
the snakes. 

The lowly snake is forever pursued and 
killed by mankind the world around. It is 
feared and hunted because some members 



Fig. 13-47. These two reptiles inhabit the Southwestern United States. The horned toad (Phrynosoma) is not a toad 
at all (top). The gila monster {Heloderma suspectum) is the only poisonous lizard in the United States (bottom). 

such as the cobra are deadly. There is httle 
reason for man to wreak his vengeance on 
every helpless snake that crosses his patli 
and yet he does so with unrelenting fury. 
Once he has made the kill, he often proudly 
displays the mutilated body of a creature 
that could never do him any harm but, if 
allowed to live, could do him considerable 
good by its constant pursuit of insects and 
small rodents which are its diet. In the 

United States, all snakes except four are 
helpful rather than harmful. 

With the exception of pythons and boas, 
snakes are without limbs. They move in an 
undulating fashion much the same as fish 
swim. The posterior edge of tlie ventral 
scales is loose, thus allowing these scales to 
make a firm contact with the ground, which, 
in turn, permits the animal to move forward 
but not backward. Their eyes are large and 



without lids. Their sound-recording -organs 
are superior to those of amphibians. The 
snake possesses a long forked tongue which 
jDasses in and out through a notch in the 
upper lip, a conspicuous habit when it is 
investigating new territoiy. The tongue has 
sensory functions in tracking down prey 
(Fig. 13-48). 

The snake's mouth is equipped with 
sharp teeth that curve inward and are well 
adapted for holding its victim ( Fig. 13-49 ) ; 
any struggling movement of the prey tends 
to force the creature further down the 
throat of the snake. Since all of the snake's 
diet consists of whole animals, another con- 
venient adaptation associated with food 
taking is the enormous potential size of the 
mouth, which can be stretched to accom- 

modate an animal several times its own 
diameter (Fig. 13-50). A python has been 
known to swallow a full-grown hog. The 
reason for this great distention lies in the 




poison duct 
opening oP 



Fig. 13-48. Head of rattlesnake with the right cheek dis- 
sected away in order to show the poison sac and 
other parts. 

Fig. 13-49. The skull of a rattlesnake. Note how loosely the jaw bones are attached to the skull. This, together with 
the lack of fusion of the jaw bones in front, makes it possible for the snake to swallow an animal several times 
its own diameter. One of the fangs has been enlarged in the lower picture to show its hollow construction. It 
resembles an inoculating needle. 



Fig. 13-50. This is a series of photos showing a 
common garter snake swallowing a frog. It wil- 
lingly attacks a large leopard frog but after get- 
ting one leg rather well swallowed and being 
confronted with the remainder of the animal, it 
apparently saw the futility of its efforts and re- 
leased its prey shortly after this picture was taken 
(top left). It then tackled a smaller frog and this 
time started swallowing its prey head first, which 
proved to be quite satisfactory as the next three 
pictures indicate. Note how the expansible jaws 
accommodate the frog, which is twice the diam- 
eter of the snake's head. 

fact that the lower jaw articulates with the 
skull very loosely by means of two slender 
bones; furthermore, the lower jaw can 
spread at tlie anterior midpoint, allowing 
a lateral expansion ( Fig. 13-49 ) . With such 
a loose jaw arrangement, it is possible for 
the snake to spread its mouth to an extraor- 
dinary degree (Fig. 13-50). 

The snakes have received their bad repu- 
tation from the poisonous members of the 
group. Just how these creatures evolved 
this deadly offensive and defensive mech- 
anism is hard to say. Many different kinds 
of tooth formations have been produced for 
inoculating the poison into the wound 
made by the sharp teeth. In some forms, 
such as the rattlesnake, the fangs possess 
a hollow tube through which the venom is 

injected, as with a hypodermic needle ( Fig. 
13-49). Others, such as the cobra, have 
deep-grooved fangs which allow the poison 
to enter at the base of the tooth and exit 
near the tip so that, upon striking, it would 
be deep within the wound. The poison 
glands are located above the angle of the 
jaw and empty their venom into a flap of 
skin at the base of the fangs (Fig. 13-48). 
Normally the fangs lie against the roof of 
the mouth pointing down the throat, but 
when the head is thrown back they are 
forced forward until they protrude at right 
angles to the roof of the mouth. The head is 
then thrown forward with a sudden lunge, 
about two-thirds the length of the animal, 
striking the prey and penetrating the skin. 
The compression of the victim's skin against 



the loose skin at the base of the fangs 
causes the poison to be inoculated into the 
wounds. Once the ^'enom has entered the 
blood stream, its effect is very rapid, caus- 
ing a small rodent to become paralyzed in 
a matter of a few seconds. The venom acts 
by destroying the red blood cells. Anti- 
venoms ha\'e been prepared for all of the 
common poisonous snakes and have proved 
valuable in alleviating the effects of their 
bite. A normal person rarely dies from the 
bite of a poisonous snake, although he may 
be very sick. Of the four types of poisonous 
snakes in the United States, the most com- 
mon are the rattlesnakes; the others are the 
water moccasin (cotton mouth), the cop- 
perhead (Fig. 13-51) and the beautifully 
banded coral snake. The last, although poi- 
sonous, is quite different from the other 
three in that it has no fangs but resembles 
the Gila monster in its method of imparting 


It is a far cry from the lowly tiu'tle and 
slithering snake to the most decorated of all 
animals, the birds, yet with few exceptions 
the birds are little different from the rep- 
tiles (Fig. 13-52). An early biologist has 
called them "glorified reptiles," which de- 
scribes them very well. They have mastered 
the air, as is obvious to the hunter who 
matches his marksman's skill against the 
abilities of the wary mallard to avoid being 
hit. In conquering the air several body 
modifications have been necessary. The 
drastic need for a lighter body framework 
was met by lighter, hollow bones, and a 
lessening of the weight of the outer cover- 
ing (Fig. 13-53). 

The scales of the reptiles have probably 
given rise to the feathers of birds, and 
actually there is little difference between 
them except in weight and textiu-e. 
Their general arrangement is very similar. 
The bird still carries scales on its legs and 
sometimes around the base of the beak. Be- 

Fig. 13-51. The copperhead {Agkistrodon mokasen) is 
one of our venomous snakes. Note the vertically slit 
pupil and the "pit" just in front of the eye. These 
two features characterize most poisonous snakes. 

cause of their loose arrangement, feathers 
act as excellent insulating material, an ex- 
tremely important need for birds who often 
fly in regions of very low temperatures. The 
wings are the modified anterior legs of the 
reptiles in which the three fingers have 
fused at the tip and the space between has 
been spanned by a sheet of skin covered 
with feathers. The wing spread is due prin- 
cipally to the large quill feathers at the 
outer edges of the wings. The breast bone 
is greatly over-developed. It is known as a 
keel and functions as an anchor for the 
powerful breast muscles which are used to 
give the power stroke in flight. 

Other structural modifications have been 
essential for the flying animal. In order to 
keep active during all times of the year in 
temperate and arctic regions, it was neces- 
sary to maintain the body at a constant 
temperature which, in birds, is slightly 
higher than in man. This, of course, meant 
greater expenditure of energy, thus requir- 
ing a better circulatory system. Many of the 
higher reptiles possess four-chambered 
hearts (alligators and crocodiles), whereas 
among birds it is the universal rule. The 
hearts of birds are large and very well 
developed, an essential factor in maintain- 
ing a rapid circulation when so much 
energy is consumed in flight. 

Fig. 13-52. All birds resemble one another anatomically, differing only in minor details. Their body parts are 
adapted to fulfill certain functions that are associated with their particular mode of life. For example, the beak 
in these four birds is modified for food-getting. Both the eagle (top right) and the owl (top left) are predators, 
usually feeding on small mammals. The beak is well suited for tearing prey. The woodpecker (bottom left) has a 
sturdy beak used for drilling holes in tree trunks in search of insect larvae, which it feeds upon. The young duck 
(bottom right) has a beak adapted for straining the water and retaining the small plant and animal life that 
forms its food. 



In order to secure its food and avoid its 
enemies the bird is compelled to rely on its 
keen eyesight. Its large eyes are protected 
by bony capsules which apparently prevent 
the pressure of passing air from forcing the 
eyes back into the head. The brain also is 
considerably larger than in reptiles. How- 
ever, the difference in size is owing prima- 
rily to those parts having to do with sharp 
vision, as well as balance and muscular 
coordination, functions essential to flight. 

The birds in general exhibit some rather 
fascinating habits which are not always 
well understood, as, for example, the return 
of a pair of purple martins each year to a 
particular bird house on precisely the same 
day, the building of a specific type of nest 
by the young bird without ever having 
done it before, or, and still more perplex- 
ing, the migrations of many species cover- 
ing vast distances, often over great bodies 
of water. Many observations have been 
made, demonstrating that birds do these 
feats with remarkable regularity, and all 
efforts up to the present to discover the 
sense involved have been unsuccessful. 

It has been thought that birds return to 
their nesting grounds, and in many species 
to the same nesting site, year after year at 
particular times because they are able to 
measure the exact angle of the sun. This is 
logical when one recollects what the bees, 
using similar devices, are able to do. Crows 
taken several hundred miles from their 
nesting groimds and released will return 
( almost to a bird ) within the period of time 
predicted, which has been based on the 
"cruising" speed of the bird. One might 
suspect that the bird would recognize land- 
marks along the route and use them to 
guide it back again. The birds have been 
kept in covered cages or actually placed on 
turntables and swung continuously through- 
out the trip, but even then they will return 
in the expected time. Some have guessed it 
to be simply a matter of chance — the birds 
merely fly at random and finally stumble 
upon their way home. That this is not true 

F!g. 13-53. Skeleton of the domestic fowl. The skeletal 
design is adapted to accommodate flight and bipedal 
locomotion. The large keel provides a secure point of 
attachment for the powerful wing muscles. 

has been shown by many experiments. For 
instance, if some birds are taken twice as 
far from home as others and released simul- 
taneously, the birds farthest away should 
require four times as long to get back by 
the wandering technique. This is not the 
case, however, because they arrive in about 
one half the time that it would take them if 
they flew on in a random manner. This 
homing instinct, present to some extent in 
other animals, is remarkably developed 
among the birds, but its explanation is still 
a mystery. 

Why or how birds are able to follow such 
varying paths in their migrations is as 
mysterious as their homing instincts. Every- 
one is conscious of the fall migration of 
great flocks of birds all heading south. Of 
course, a possible explanation is immedi- 
ately obvious: once the breeding season is 
over and the young are able to care for 
themselves, the approaching winter with its 
accompanying food shortages and severe 
cold might be reason enough for heading 



southward to regions where food is abun- 
dant and the dimate more comfortable. 
Most humans would certainly enjoy such an 
arrangement. All efforts to show that the 
migration south is started by a sudden drop 
in temperature have been to no avail. The 
duck hunter will attest to the fact tliat his 
quarry will often leave the swamps during 
a spell of warm weather, whereas at other 
times, even in the face of an unseasonal 
snow flurry, they will not budge from the 
swamps in which they were reared. 

Experiments have shown rather definitely 
that migrations start at remarkably regular 
periods which can be determined only by 
the amount of light received by the birds. It 
has been found, for example, that birds will 
start to lay eggs even in the winter time if 
the length of daylight is increased artificially. 
The farmer has profited from this informa- 
tion by turning on lights in the chicken 
houses, thus stimulating the hens to lay 
more eggs. Light shining on the retina 
of the chicken apparently stimulates the 
pituitary gland (a ductless gland which 
secretes a number of hormones ) , which then 
activates the ovary to produce eggs out of 
season. It hardly seems fair, does it? 

The paths taken by some birds in their 
migrations are truly amazing and that of 
the American golden plover is especially 
so. It nests in June on the northwestern 
shores of Canada above the Arctic Circle; 
shortly after the young birds leave the nest, 
the adults desert them and start across 
Canada, flying eastward to Labrador where 
they arrive in early autumn. From there 
they take a direct course south, flying out 
over the Atlantic Ocean until they reach 
Venezuela; they then fly more leisurely on 
to the region of Paraguay where they over- 
winter. Another remarkable point is that 
the young birds follow a month later over 
approximately the same path, a total dis- 
tance of over 6,000 miles, with no experi- 
enced birds to guide them. The answer to 
this mystery will be exciting, if and when 
it is found. 

The first birds 

It would be amiss not to mention some of 
the ancient birds that have been unearthed 
with the other animal fossils. As has already 
been mentioned, the birds apparently stem 
from reptflian ancestors (Fig. 13-41), a fact 
that should reveal fruitful intermediate 
types, animals that are neither reptile nor 
bird but a combination of both. Arcfiaeop- 
teryx, an ancient bird about the size of a 
crow, was found in limestone about 75 
years ago in Germany. Strangely enough, in 
all of the subsequent diggings no other 
remains have been discovered. These re- 
mains, however, were sufficiently intact to 
describe the bird rather well, even the 
feathers being preserved. Aside from them, 
however, the other parts of the animal ap- 
pear very similar to a small dinosaur. The 
wings were weak and the finger tips bore 
claws. The beak was lined with teeth and 
the skeleton was made up of heavy bones, 
in contrast to the light, hollow bones of 
modern birds. The fact that the bones were 
heavy and that the keel was poorly de- 
veloped indicates the bird was a poor flyer. 
In fact, if it could have been observed at 
that time it undoubtedly would have been 
seen to glide from branch to branch of trees 
much the same as the flying squirrel does 

Some of the marine rocks of Kansas re- 
veal later birds. One, the "fishbird" (Ich- 
thyornis), shows considerable advances 
over Archaeopteryx in that it possessed a 
large keel bone, indicating excellent powers 
of flight. It did retain a toothed beak, how- 
ever. Aside from this feature it probably 
was not greatly different from the modern 

Some giant wingless birds 

There are several species of wingless 
birds on earth today as there apparently 
were over a long period of geologic time 
also (Fig. 13-54). These birds are all much 
alike; they are usually very large, some 



Fig. 13-54. The ostrich is one of the largest flightless birds 

they can be used as 

reaching as much as 12 feet in height, with 
powerful legs, small heads and rudimentary 
wings which are useless for flight. They 
depend on their fleetness for security. An 
interesting fact is that all modern flisfhtless 
birds live in regions where there are no 
carnivores, that is, no members of the cat 
or wolf tribes. Examples are the ostrich of 
Africa, the cassowary and emu of Australia, 
the rhea of South America, and the kiwi of 
New Zealand. Romer has suggested that 
these birds once were able to fly but be- 
cause there were no terrestrial predators 
that might harm them they simply ceased 
flying and remained on the ground all of 
the time, thus conserving their energies for 
the important job of obtaining food. One 
reason that birds probably took to the air 
in the first place was to be better able to 
flee from their enemies. If the enemies are 
removed, then the need for flight is no 
longer present; hence, why fly? 

. Its powerful legs carry it swiftly over the ground, and 
organs of defense. 


The dominant and most complex animals 
on the earth today are the mammals. What 
was their origin and what characteristics 
have tliey been able to accumulate that 
have caused them to outstrip all others, at 
least temporarily? They are certainly as 
varied in size, shape, and habitats as any 
animals that have ever lived ( Fig. 13-55 ) . 
They seem to have invaded every possible 
portion of the earth, water, and air, and are 
reasonably successful in all environments, 
just how long they can retain the zenith 
where they now find themselves is a prob- 
lem for speculation. If they follow the pat- 
tern of former dominant types they will 
eventually decline, to be succeeded by 
some other form which is today, perhaps, 
an insignificant animal. 

According to the best estimates, the 



Whole -105 fact long 


giraffe- 20f««f bigb ckpbont- 1 1 feat high 


Fig. 13-55. Relative sizes of a few mammals. 

origin of mammals took place approxi- 
mately 150 million years ago (Fig. 13-41). 
These early forms were neitlier reptiles nor 
mammals as we know them, but probably 
intermediate in character, with resem- 
blances to both. It is generally agreed that 
mammals had their beginnings in some 
reptilian type and the first ones were prob- 
ably more lizard-like than mammal-like 
(Fig. 13-41). One of the earliest animals 
that showed mammalian features was 
Cynognathtis ("dog-jawed"), in which sev- 
eral skeletal improvements over the typical 
reptile are observed. The legs had assumed 
a more ventral position so that they not 
only lifted the body farther off the ground 
but in addition were capable of forward 
and backward motion, resulting in more 
forward speed. Thus, instead of crawling 
as its reptilian ancestors did, it was able to 
carry itself a considerable distance above 
the ground and undoubtedly travel much 
faster. This characteristic has culminated 
in such modern animals as the horse and 

The skull also had certain modifications, 
such as the more posterior position of the 
entrance of the internal nostrils into the 
mouth, for example. Being warm-blooded 
now, it was necessary for the animal to 
breathe continuously and rapidly so that 
sufficient oxygen could be supplied to the 
tissues to maintain the high temperature. In 

order that breathing might not be inter- 
rupted during feeding, the internal nostrils 
opened into the mouth farther back and 
were walled off by a plate, the palate. 
These characters are present in this primi- 
tive mammal. Furthermore, the teeth show 
the beginnings of a typical mammalian 
pattern, consisting of the front nippers ( in- 
cisors) and the large fang-like canines, 
followed by the shearing molars. Reptiles 
have no such dental pattern. 

These primitive mammals were present 
on the earth long before the coming of the 
great dinosaurs but they were small and in- 
conspicuous, and probably kept in hiding 
throughout the reign of these great beasts. 
During this long period of perhaps 15 mil- 
lion years they were a source of food and, 
consequently, were forced to live "by their 
wits," which gave them an opportvmity to 
try out many devious plans for a better 
body as an aid to survival. They remained 
small and insignificant until the dinosaurs 
met their end. The world was then left free 
for any animal that was ready to take over. 
The mammals apparently "saw their chance 
and took it." Even then, while the mamma- 
lian characteristics in general persisted, 
many "ideas" were evolved that produced 
animals not able to survive very long — the 
mammoth, saber-tooth tiger and the giant 
sloth, for example. The more intelligent and 
versatile did survive, however, and gave 


rise to the present mammalian population nourishment from the fluids of the mother, 

of the world. This was done by a temporary fusion of 

The presence of hair in place of scales the allantois and other membranes to 
was an accessory structure necessary in a the uterine wall of the mother, forming 
warm-blooded animal. Sweat glands aided a placenta (p. 533). At first the placenta 
in regulating body temperature, which is must have been a very primitive affair and 
nearly constant as distinguished from other only partially satisfactory in performing 
animals, except birds, which have tempera- this important function of nourishing the 
tures varying with the external environ- young. Consequently, the young were born 
ment. In addition, the heart was a double in a very immature state and needed extra- 
one: one for circulation through the body, uterine care. This was furnished by the use 
the other for circulation through the lungs, of a belly pouch, called a marsupium, in 

One of the more important characteris- which the young could not only be pro- 
tics that has contributed to the success of tected from the cold but also could receive 
mammals is their method of caring for nourishment from the mammary glands 
their genital products and subsequent off- through nipples located inside the marsu- 
spring. All reptiles lay large, yolk-packed pium ( Fig. 13-56 ) . Later in their evolution, 
eggs, whereas mammals typically have very more and more time was spent in the uterus 
small eggs which are fertilized internally in and less in the marsupium until the latter 
the female and go through a great part of was finally discarded as unnecessary. This 
their early life inside the body of the is a possible explanation of how the pres- 
mother. Although internal hatching of eggs ent mammalian reproductive system came 
is not uncommon in lower forms (reptiles into being; it is recapitulated in the mam- 
and fish ) , receiving nourishment from the mals of today from the duckbill to man. 
mother during the process of development In certain parts of the world such as 
is only rarely found ( for example, placental Australia, which were isolated during the 
shark) in the animal kingdom. These early time the mammals were evolving (Eo- 
mammals apparently found that it was bet- cene), there survives to this day two dif- 
ter to have fewer offspring and retain them ferent types of primitive mammals which 
longer within the body in order to give might help to bear out the narrative of the 
them greater protection while allowing preceding paragraph. They belong to the 
them, at the same time, to develop to a group known as Monotremes, which means 
more advanced state, than to lay the eggs "one opening," so named because both the 
as the reptiles did and rely on chance for urogenital and intestinal tracts open into 
subsequent maturity. Furthermore, since a common cavity, the cloaca (a reptilian 
mammals are so much more highly organ- character ) , with only one external opening, 
ized, longer time was needed to develop the The spiny anteater (Echidna) and the 
young to a stage where it could care for well-known platypus or duckbill (Ornitho- 
itself . The transition took place very slowly rhynchis. Fig. 13-57 ) are the two examples 
and over a long period of time. Even that remind us of what must have hap- 
though fossil remains offer little real proof pened many millions of years ago when the 
about such things, it is reasonably certain earliest mammals were struggling with this 
that this process of caring for the young problem of caring for tlieir offspring. These 
within the mother started very early in the animals lay large yolked eggs like the rep- 
evolution of mammals. tiles and birds and they possess bills like 
If the embryos were to remain within the the latter. They are partially warm-blooded 
mother, it was necessary first of all that and the duckbill incubates its two eggs 
some means be devised for them to receive ( Fig. 13-56 ) . When the eggs hatch the 

Fig. 13-56. Evolution of the care of the young in mammals, from the duckbill to man. 



Fig. 13-57. Platypus (Ornifhorhynchus anatinus), an egg- 
laying mammal. It possesses both reptilian and mam- 
malian characteristics. 

young are nourished for a time from milk 
secreted by two rows of glands along the 
belly side of the mother. There are no teats 
and the young merely lap up the secreted 
milk. It is noteworthy to mention again that 
these archaic animals have survived up to 
the present time due to their isolation. Had 
they been forced to compete with modern 
mammals they would long since have van- 
ished from the earth. Isolation may be 
either by actual land or water barriers or 
by environments in which there is little 
competition. It is indeed fortimate that such 
animals have been preserved for us to 
study, making possible more logical con- 
jectures as to just how present animals 
came about. 

The next step on the road toward "true" 
placental mammals would be the interme- 
diate types, namely, the marsupials. There 
are many of these interesting animals, in- 
cludins the kan2;aroo of Australia and the 
opossum of North America (Fig. 13-58). 
These do not lay eggs like the monotremes 
but retain them in the uterus where a rudi- 
mentary placenta forms. Since this organ 
is inadequate to maintain the embryos for 
any great period of time the young are 
born in a very immature state ( Fig. 13-56 ) . 
A 200 pound kangaroo for example may 
give birth to offspring no longer than 2 
inches. Following birth, the embryos make 

their way, or are placed, in the marsupium 
where they grasp and practically swallow 
the nipples of the mammary glands to 
which they cling during what is equivalent 
to later embryonic life of a more advanced 
mammal (Fig. 13-59). After a time they 
are able to face the world and they come 
out of the pouch to feed for themselves, 
retreating to it only in case of danger. Al- 
though the marsupials give us a clue to the 
past history of mammals, their direct line 
probably diverged from the reptilian mam- 
mal stock very early in geologic times. 

The placental mammals 

The next step in the evolution of the 
mammals must have been the formation 
of a well-developed placenta, one that was 
adequate to care for the developing off- 
spring within its mother for the period of 
time necessary to develop the complex 
structures essential for success once it was 
born. This organ increased in size and ef- 
ficiency until it was capable of receiving 
sufficient food and oxygen from the blood 
stream of the mother to allow for advanced 
development of the offspring (Fig. 13-59). 
Since, in these animals, so much of the 
nourishment is obtained from the mother, 
eggs with large amounts of yolk were un- 
necessary. Reduction of the yolk is reflected 
in present-day mammals where the egg is 
very small (0.5 mm. across), whether it 
be that of an elephant or a mouse. 

Along with tlie development of a success- 
ful reproductive system, the placental 
mammals showed further refinement in an- 
cestral systems. For example, the brain 
became an even more conspicuous part of 
the central nervous system and its functions 
were much more precise. The coordination 
and precision of operation of all the parts 
of the body were brought to an all-time 
high in these animals. This combination of 
improvement on systems inherited from 
their reptilian ancestors has been responsi- 
ble for the success of mammals. 

Fig. 13-58. Marsupials. A female kangaroo (top), showing the young in the pouch or marsupium. Below, a mother 

opossum with two half-grown offspring. 



The important mammalian groups 

The most primitive modern mammals, 
that is, the ones that resemble ancestral 
forms most closely, are the insect feeders 
( Insectivora ) . While there are some fairly 
large, highly specialized forms such as the 
moles in this group, the most typical are 
the tiny shrews. These mouse-like creatures 
are probably similar to the stock that gave 
rise to higher mammals including man, 
himself. Their mouths are armed with nee- 
dle-like teeth which aid in securing their 
specialized diet of insects. They are ex- 
tremely active animals, in fact, so active 
that it requires a volume of food equal to 

Fig. 13-59. The young of the opossum (top picture) are 
born in a very immature stage and must continue 
their development in the marsupium. Here they are 
shown clinging to the teats in the pouch. The young 
of higher mammals are well developed at birth. This 
calf (bottom picture) is able to stand and even run 
alongside its mother, though only a few minutes old. 

Fig. 13-60. One of the smaller carnivores, the raccoon 
(Procyon lotor). Its diet consists of many things, 
among them crayfish, which is the goal of the pres- 
ent search. 

tlieir body weight each day to satisfy theii' 
needs. Most of them live in burrows and 
are such secretive animals that they are sel- 
dom seen by the casual observer; only the 
biologist armed with cleverly devised traps 
is able to capture and study them. It is this 
secretive habit, reminiscent of their ances- 
tral cousins, that made it possible for them 
to survive up to the present. They are as 
isolated in their burrows as if they were 
separated by impassable water or mountain 
barriers. Some, however, live in trees much 
the same as the ancient forms that gave 
rise to the tree-loving primates (Fig. 

The next group significant in the evolu- 
tion of mammals are the Hesh-eaters, the 
carnivores (Carnivora) (Figs. 13-60, 13-61, 
13-62). These are very successful today, as 
shown by the numbers and kinds of such 
animals as cats, dogs, weasels, bears, civets, 
and the marine forms such as the walruses 
and seals. All our present dogs and wolves 
are supposed to have arisen from one form, 
Cynodictis, which resembled a weasel as 
much as a dos;. The carnivores have teeth 
well adapted to the rending and tearing 
of flesh. The large canines readily tear 
through tough skin and the shearing molars 
cut the flesh into pieces sufficiently small 
to be swallowed. Furthermore, since meat 
is easily digested, the alimentary canals are 

Fig. 13-61. A few of the larger carnivores. 

Fig. 13-62. Marine carnivores. Seals (P/ioca vltulina) off the coast of Maine (top). Elephant seals 
(/Vlacror/iinus) off the coast of Southern California (bottom). 



Fig. 13-63. The African rhinoceros (Rhinoceros). The thick 
armor-like hide and the two snout "horns" (not true 
horns) provide this great beast with ample protection 
against its enemies. 

short when compared to the plant feeders 

flippers which are effectively used in loco- 
motion. They swim in a fish-like manner by 
undulating motions of the body. Curiously 
enough, the tail has dwindled to a useless 
structure and the two posterior legs have 
taken over its function. 

Seals come out on land, usually specific 
isolated islands, during the breeding season 
(Fig. 13-62). The males arrive first, and 
when the females appear they gather as 
many as they can about them and jealously 
protect them within the family circle. 
Should a female stray some distance from 
the circle she is rudely retrieved. Males 
battle ferociously for the females through- 
out the breeding season and at its end they 
go back into the sea, rather badly battered. 

In contrast to the carnivores are the her- 

(Fig. 17-7). In general, the carnivores are bivores, represented today by the domestic 

active and sometimes vicious animals, some horse and cow. The horse possesses an odd 

hunting in packs like the wolves and others number of toes ( Perissodactyla ) and the 

leading more or less solitary lives like the cow an even number of toes ( Artiodactyla ) 

big cats (Fig. 13-61). Man has made or "cloven" hoof. In their evolution, the 

friends with at least two members of the former came first and reached large sizes, 

group many centuries ago— the domestic as illustrated by the giant rhinoceroses 

cat and the dog. Although he has taken a 
hand in changing the body form of cats by 
selective breeding, his greatest efforts and 
success have been with dogs. How success- 

that attained a shoulder height of 18 feet. 
Those alive today are doomed to extinction 
in spite of tlie fact that man has been able 
to save the horse by domestication. Mem- 

ful he has been with his handiwork is well bers of this group have their weight borne 

illustrated by the many kinds of "man's best 

Some very interesting and bizarre carni- 
vores are those that have taken to the water 
in a serious way, with the result that they 
have all but lost their ability to locomote 
on land. These are represented by the seals, 
sea lions, walruses, and others (Fig. 13-62). 
They are relatively helpless on land but in 
the water they compare favorably with the 
best of die true aquatic forms, including the 
fish. Having gone out on land long enough 
to acquire the intelligence and cunning of 
the mammals before returning to their orig- 
inal environment, they should perhaps offer 
considerable competition for the stupid 
fishes. These animals are dog-like in many 
respects. Their appendages have become 

on three toes in the case of the rhinoceros 
(Fig. 13-63) or on one toe in the case of 
the horse. The even-toed herbivores have 
four toes, of which two bear the main 
weight of the animal; this is illustrated by 
cattle, sheep, goats, pigs, deer, elk and 
many others. These have dominated the 
previous group both in the past and pres- 
ent. The present-day forms are the rumi- 
nants or "cud-chewers." Their stomachs 
have several compartments, a condition 
which permits large amounts of hastily 
acquired food to be temporarily stored and 
then brought back into the mouth at a 
later period to be properly chewed at the 
animal's leisure. This is a most comfortable 
and effective adaptation as well, since the 
the animals might feed voraciously during 



certain periods of the day when grazing 
would be less dangerous and then retire 
to secluded spots to finish the job of chew- 
ing. The teeth of both the odd- and even- 
toed animals are well adapted for cropping 
grass and grinding it to the proper con- 
sistency for digestion. 

The boy who attends the circus is forever 
in awe of the slow moving, thick-skinned 
elephant ( Proboscidea ) with its nose, in 
the form of a proboscis or trunk, touching 
the ground ( Fig. 13-64 ) . This handy organ 
is responsible for the unusually short neck 
of the elephant because it performs the 
function of securing its grassy diet from the 
ground or any other place it may be found. 
It is also useful in obtaining water. Actu- 
ally, it is a prolongation of the upper lip 
including the nostrils, a structure which has 
become highly muscular and very power- 
ful. The teeth have undergone several 
changes, the most striking of which is the 
formation of the great tusks, which are 
over-developed incisor teeth and, inciden- 
tally, excellent weapons for offense. These 
tusks are sought by the hunter because 
there is a good market for ivory. 

There have been many elephants in the 
past which were known as mammoths and 
mastodons and which apparently have only 
recently died out ( 15,000 years ago ) ( Fig. 

Fig. 13-64. The Indian elephant (E/ep/ios indica) with 
its upper lip and nose drown out into a trunk, a 
remarkable prehensile organ. 

Fig. 13-65. Two rodents that have become popular lab- 
oratory animals. The white rat (top), a domesticated 
variety of the Norway rat (Roffos norvegicos) and 
the guinea pig, a domesticated variety of the South 
American cavia (Covia porcellus). 

25-2 ) . In Siberia, within the past few years, 
mammoths have been taken from glacier 
ice in which they were completely pre- 
served, skin, flesh, and all. Such bodies have 
been found partially eaten by wolves, and 
in some cases man has fed upon these 
ancient remains. 

Some relatives of the elephants went into 
the sea and became adapted for an aquatic 
existence. These are the Sirenia, which in- 
clude such mammals as the dugong of the 
Indian Ocean, the sea cows formerly from 
Bering Straits but now practically extinct, 
and the manatee from the Atlantic Ocean 
in the tropics. These ugly, stupid beasts 
feed on the abundant marine vegetation 
along the shores but are unable to come 
ashore themselves because they lack pos- 
terior appendages and the front ones are 
adapted for swimming only. The Sirenia 
are not related to the whales; fossil records 
indicate that their closest relatives seem 
to be the Proboscidea, in spite of the vast 
difference in their external appearance and 
way of life. 

The most successful of all the mammals 
are the rodents ( Rodentia ) , the "gnawers." 
These include many small mammals com- 
monly known to everyone: squirrels, chip- 



munks, beavers, porcupines, guinea pigs 
(Fig. 13-65) and even the pestiferous rats 
(Fig. 13-65) and mice. They must have 
been highly successful as a group because 
all the families alive today have been in 
existence a very long time, and none has 
become extinct as is the case v/ith most 
other groups. They are characterized by 
their large chisel-like incisor teeth which 
are self-sharpening and which are kept al- 
most incessantly active. The incisor teeth 
grow continuously and if not worn down 
would soon become so long that the animal 
could not close its mouth. If, as often hap- 
pens, a member of this group loses one of 
the incisors, the tooth opposing it will then 
have nothing against which to grind, and 
continues to grow, passing through the an- 
terior portion of the skull. Since the tooth 
is curved, as it grows longer it tends to form 
a circle. As the movement of the jaws be- 
comes restricted such an animal is in danger 
of starving to death. In some cases squirrels 
with one or more teeth grown into com- 
plete circles have been reduced to feeding 
on the soft food found in garbage. 

Most rodents are small today, altliough 
at one time there were giant beavers which 
grew to the size of a small bear ( Fig. 25-2 ) . 
Rodents have a high degree of intelligence 
and are usually secretive animals, many liv- 
ing in burrows. One, the beaver, is almost 
human in its ability to build dams. It selects 
a place in a small stream, which is just 
what any engineer would do if he were 
building a dam. Beavers cut trees in such 
a way that they fall in the exact spot that 
will do the most good; they entwine the 
branches so that the debris coming down 
the river will catch there and make the dam 
water-tight. When they have finished the 
job, a first-rate dirt dam is the result. In 
some regions where conservation laws have 
made it possible for them to come back, 
they have become almost pests because of 
their habit of damming every small stream 
in large areas, thus flooding all the fields 
and roads in the surrounding country. 

The bats (Chiroptera) have conquered 
the air. To be sure, other mammals such as 
the flying squirrel are able to soar from tree 
to tree but they have not approached the 
bat in any sense as a flying creature. The 
bat competes very well with the birds; in 
fact, it can perform some feats that birds 
cannot. The wing is merely a modified hand 
with the fingers greatly extended and cov- 
ered with skin. Many species of bats live in 
deep caves, such as the Carlsbad Caverns 
in this country, where they are forced to fly 
in complete darkness. Just how these ani- 
mals could avoid objects as small as tiny 
wires strung across a room that is totally 
dark has puzzled biologists for a long time. 
Recently, however, with the aid of delicate 
electronic equipment, it has been discov- 
ered that the bat, instead of being a silent 
animal as all had thought, does emit bursts 
of high frequency sound waves during 
flight. These have a frequency of about 
50,000 vibrations per second and are there- 
fore beyond the range of the human ear, 
the upper limit of which rarely exceeds 16,- 
000 vibrations per second. When these high 
frequency sounds made by the bat strike 
an object, no matter how small, they 
bounce back to its ears; this happens at 
such speed that the animal can respond 
soon enough to avoid obstacles without the 
use of its eyes. Man now uses this same 
principle in detecting distant objects but 
instead of sound waves he uses radio waves. 
Thus the bat has had in operation for ages 
the first radar system. 

Bats usually feed on insects, although 
there are the so-called "flying foxes" which 
are herbivorous. A small number are blood 
suckers, though not as gruesome as current 
stories and moving pictures would imply. 

The whales (Fig. 13-66) and porpoises 
(Cetacea) are probably the most special- 
ized of all mammals. They are descendants 
of land carnivores which have returned to 
the sea, and become so successful in this 
environment that they are found in all of 
the oceans of the world. The story of how 



they happened to return to an aquatic en- 
vironment is lost because fossil records are 
very scanty. They probably were flesh-eat- 
ing mammals much like the present-day 
otters, fishers, and minks, which, returning 
to the water in search of prey, gradually 
became better and better adapted to ma- 
rine life. 

Whales have few anatomical features re- 
maining that are reminiscent of their land 
life. They have lost their posterior limbs, al- 
though they still have useless remnants left 
buried deep in their bodies, and their 
anterior appendages have modified into ef- 
ficient flippers. They are lacking hair com- 
pletely in the adult except around the 
mouth region in some species. Their nostrils 
have moved back to the tops of their heads, 
which facilitates breathing at the water 
surface without exposing the head. A tre- 
mendous tafl is the principal organ of loco- 
motion; it undulates horizontally rather 
than vertically as in the case of fish. 

Even though the first whales were prob- 
ably all carnivorous, many subsequent spe- 
cies became herbivorous. All whales today 
are divided into two groups : the whalebone 
whales and the toothed whales. The latter 
have retained their teeth, which are used 
in crushing fish and squids, their chief 
source of food. The more interesting whale- 
bone whales have evolved a sieve-like struc- 
ture called whalebone which hangs from 
the roof of the mouth and is used to strain 
small marine organisms. Whalebone is ac- 
tually modified skin, including the hair 
which makes up the strainer. The sperm 
whale, also a whalebone whale, has pros- 
pered on this diet of minute marine life, 
and some specimens reach a weight of 150 
tons, which exceeds that of any other ani- 
mal that has ever lived, dinosaurs not ex- 
cepted. Oddly enough, they reach this great 
size by feeding on some of the smallest 
plants and animals. It has been said that 
the reason why these animals can grow so 
fast and so large is that all of the energy 
received from the food is saved and goes to 

Fig. 13-66. A small beached whole called the beaked 
whale {Mesoplodon). Note the scratches in the hide 
as a result of being pounded on the rocks. 

form tissue rather than being lost in keep- 
ing the animal warm. There is practically 
no heat loss at all from these great bodies 
because they are enveloped in a thick layer 
of blubber just beneath the skin which acts 
as an insvilator against the cold water in 
which they live. Another problem that is 
difficult to understand is how this animal 
can dive to great depths and remain under 
water for 30-45 minutes without being 
crushed and getting the "bends" (see p. 
488 ) , a disease that humans get under simi- 
lar circumstances. It apparently has oxygen 
reserves and other devices for satisfying its 
needs under these rigorous conditions. The 
whale is truly a remarkable animal and 
many aspects of its life will probably re- 
main a secret for a long time, since it can- 
not be brought into the laboratory and 
studied like other animals. 

The primates 

This, the last group of mammals, is the 
most important of all because we belong 
to it. Other members of the group are the 
great apes, the circus monkey, the fierce- 
looking baboon, the lemurs, and the wide- 
eyed tarsiers, all coming from an arboreal 
insectivore ancestor (Fig. 13-67). It is dif- 
ficult to name any especially striking char- 
acteristics that set the primates off from 
all others, though there are several minor 


Fig 13-67. The primates apparently rose from tlie arboreal insectivores. Some groups retained tree 
habitats, whereas others descended to the ground where they became at home, among them man. 



variations which distinguish them as a 
group. First of all, most of them possess a 
brain and central nervous system that is 
developed far and above that of all others. 
It must be said, however, that some of the 
lowest primates are probably not as intel- 
ligent as some of the brightest carnivores. 
But the group as a whole can be character- 
ized as having very large brains, the cor- 
rollary being, of course, that they possess 
the most integrated and most coordinated 
bodies of any animals living or extinct. This 
implies excellent sense organs, particularly 
those of sight, because in order for the 
brain to perform its function it must receive 
accurate impressions. 

The lower mammals have depended to a 
large extent on their sense of smell to ori- 
ent themselves, for their vision at best is not 
very good. But the primates, having taken 
to the trees, needed good vision to detect 
their enemies on the ground and this was 
more important to them than a keen sense 
of smell. Arboreal life thus appears to have 
been one of the reasons for the develop- 
ment of the excellent visual organ of pri- 
mates. Since the animal could see better, 
it probably had a greater desire to examine 
things more closely which, in turn, would 
be correlated with further brain develop- 
ment. One other very important aspect of 
this course of events is that arboreal life 
had a profound effect on the skeleton. 

As the appendages became well adapted 
for life in the trees, both the anterior and 
posterior digits became modified for grasp- 
ing limbs. The great toes and thumbs op- 
posed the other four digits and the claw 
of the carnivores flattened out into nails. 
The legs became relatively short, so that 
the arms seemed to be unusually long. The 
appendages became well developed for 
supporting the body weight and for carry- 
ing it with considerable speed from limb 
to limb through the tree tops. When at rest, 
the hands were free to handle objects and 
to bring them close to the keen eyes for 
closer observation. Freeing the front ap- 

pendages from the burden of supporting 
the body weight and development of the 
prehensile hand have probably both been 
responsible to a large extent for the ad- 
vance in the primate brain. This superior 
brain is undoubtedly tlie one reason why 
these animals dominate all others today. 

Other minor characteristics of the pri- 
mates are the tooth pattern and skull struc- 
ture. The early primates were apparently 
omnivorous just as most primates are today, 
and a generalized mammalian tooth pattern 
is characteristic of all of them. They possess 
two incisors, two premolars and four molars 
in each half jaw, making 32 in all, consider- 
ably less than that possessed by the primi- 
tive mammals. The jaw is short, so that a 
relatively short face results; this is obvious 
when the faces of a monkey and a dog are 
compared (Fig. 15-3). Furthermore, the 
large nasal chambers essential to lower ani- 
mals which depend upon smell are much 
reduced in primates with a corresponding 
reduction in this sense. Since the brain has 
grown a great deal in size, it has risen over 
the face, bringing the latter into a more 
vertical position. Along with this change 
has come a gradual shifting in the position 
of the eyes from the lateral position occu- 
pied in lower forms to a frontal position. 
This has resulted in over-lapping images, 
producing stereoscopic vision, essential to 
tree-dwellers for judging distances accu- 
rately, and to men for driving a car. The 
great advantage of superimposed images is 
easily demonstrated by viewing an object at 
a distance first with both eyes and then 
with only one. The sense of depth is lost 
with monocular vision. 

Primates are rather shy, avoiding other 
animals that might be dangerous and pre- 
ferring tlie seclusion of dense foliaged for- 
ests to open areas. The more primitive ones 
remain in the safety of the tree tops most of 
the time. The more advanced forms, such 
as the chimpanzee and gorilla, have de- 
scended to live on the ground, but even 
these retreat to the trees on occasion. 



Fig. 13-68. Lemurs are primitive primates that are found 
only in isolated parts of the world. They are stupid, 
sluggish animals. This is the ring-tailed lemur, lemur 

The mother primate gives birth to single 
offspring as a rule, and multiple births are 
rare. This is to be expected in an arboreal 
animal, since a single young one is about all 
she could manage in the tree tops. She 

lavishes meticulous care and protection on 
her offspring, keeping it close to her chest 
at all times during the first few months of 
its life, close to the food supply. Primates 
possess only two mammae or breasts al- 
though occasionally even in humans several 
pairs may appear, located in the same posi- 
tion as on the lower mammals which pro- 
duce multiple offspring (Fig. 25-16). The 
ungulates, such as horses and cattle, for ex- 
ample, also produce a single offspring at 
a time but in this case the mammae are 
in the pelvic region. Dogs, on the other 
hand, produce numerous young at one time 
and consequently multiple mammary 
glands are necessary for all to get their 
share of the milk. The development and 
location of the mammae seem to be cor- 
related with the number of offspring and 
the method of caring for them. 

The various primates 

The lemurs. The lethargic lemurs are not 
i^reatly removed from the insectivores ( Fig. 
13-67). They are slow moving, sluggish, 
stupid animals quite unlike the other pri- 
mates. Their primitive nature is indicated 
by the fact that their eyes still lie in such 
a lateral position that their images do not 
overlap (Fig. 13-68). They live in the tree 
tops where they move about so conserva- 
tively and cautiously that they are rarely 
seen by enemies and consequently have 
been able to survive in Madagascar up to 
the present time. They would not have 
lasted long if they had been forced to com- 
pete with their aggressive cousins. 

Tarsier. The East Indies are the home of 
the only hopping primate, which is not 
much bigger than a rat (Fig. 13-69). Its 
mark of distinction lies in the fact that 
zoologists consider it intermediate between 
the lemurs and the monkeys. Its rat-like tail 
and long legs are well adapted for leaping, 
which it performs with great agility. The 
swollen tips of its digits are useful in catch- 
ing limbs of trees as it forages for insects 
during its nocturnal sorties. The extremely 



Fig. 13-69. Tarsier {Tarsius tarsler) is a rat-sized hopping primate. Note its tremendous eyes and the padded finger 

tips which aid in grasping twigs. 

large eyes are directed forward and prob- 
ably permit stereoscopic vision. 

The characteristics of this peculiar little 
animal place it directly between the mon- 
keys and lemurs, and this fact has caused 
zoologists to wonder if it might be the 
branch from which man sprung. Fossil re- 
mains indicate that in the Eocene period 
there were a great many tarsioids, contem- 
poraries of the lemurs, and it may well be 
that man descended from this group. 

Anthropoids. The highest group of all 
primates are the anthropoids, the man-like 
primates. These include the monkeys, the 
great apes, and man. While visiting the zoo 
many will resent the thought that these 
animals are our closest relatives, but after 
viewing them for any length of time only 
the most doubting are unconvinced. It is 

important to point out, however, that man 
did not descend from monkeys, nor is the 
monkey a degenerate man. Both had sepa- 
rate beginnings a long time ago and have 
been and are traveling along separate paths 
in their evolution (Fig. 13-67). The mon- 
key may become more man-like and the 
opposite may happen, though it is unlikely. 
But the monkey will never become man as 
we know him today, any more than the 
tiger will become a lion or vice versa. 

The monkeys. These creatures, endowed 
with unlimited curiosity, are man-like both 
in physical characteristics and in attitudes. 
They are primarily arboreal, although they 
are able to get along on the ground. They 
walk on all fours, but when at rest usually 
sit down on their haunches, thus freeing 
their hands for the job of manipulating 



Fig. 13-70. This spicier monkey (Afe/es panrscus), a rep- 
resentative of the New World Monkeys, possesses a 
handy adaptation, namely, its prehensile tail which 
functions as a fifth appendage. 

food or any other object that strikes their 
fancy. Their large and forward-placed eyes 
are probably as keen as those of humans 
and in appearance they certainly resemble 
them, even to the expression of emotions. 

The monkeys are divided into two 
groups, New World forms of Central and 
South America and the Old World forms 
of Asia, Africa, and Europe (Fig. 13-67). 
Two members of the New World monkeys 
are of interest because of their specializa- 
tion : the spider monkey, with its prehensile 
tail (Fig. 13-70) which functions as a fifth 
hand, and the howler monkey, which has 
a remarkable voice made possible by modi- 
fications of the throat into large, bony reso- 
nating chambers. Each has specialized in 
its own peculiar way and these variations 
set them off from all other members of the 

The Old World monkeys exhibit a wide 
variety of form and habits, from the sacred 
langur of India to the highly colorful 
ground-dwelling mandrill (Fig. 13-71). 
Most of them are tree-dwelling, although 
the baboon lives on the ground entirely. 
The baboon is of interest because some 
have highly colored callosities (buttocks) 
and dog-like snouts. When on the ground 
they walk on all fours. The baboon is as 

firmly committed to life on the ground as 
the spider monkey is to life in the trees. It 
would be interesting to know how these 
closely related animals came to adopt such 
distinct habitats. When in the course of 
events did man's precursor leave the trees 
and come down to the ground? Had he 
come with the baboons he would need 
shoes on both hands and feet today; had he 
stayed in the trees much longer he would 
never have been able to come down be- 
cause his body would have been so modi- 
fied that it would be unwieldy on the 
ground. He must have made the shift be- 
fore his legs got too short or his arms too 

The man-like great apes. These predomi- 
nantly large primates, the gibbon, orangu- 
tan, chimpanzee, and gorilla, separated 
very early from the common stem that also 
produced the monkeys, probably about the 
same time a branch separated off on its 
long course toward man. This appears to 

Fig. 13-71. The mandrill (Mandr'illus sphinx) is a color- 
ful member of the Old World Monkeys. It is noted 
for its dog-like face and its highly colored cheeks. 



have taken place at least 20 million years 
ago. Some think that among the great apes 
the gibbons do not appear to be very close 
relatives of man, whereas others believe 
they are more closely related than any of 
the rest. Certainly the skull and general 
trunk proportions are man-like. However, 
the gibbon has become so completely 
adapted to arboreal life that its arms are 
disproportionately long and the fingers are 
modified into hook-like structures for grasp- 
ing limbs, the thumb being reduced to a 
tiny protuberance (Fig. 13-72). Its long 
arms allow it to brachiate through the trees 
with great speed, that is, to move hand 
over hand along branches in a swift, grace- 
ful manner. Even in a cage, excited gibbons 
are a spectacle to observe. 

Brachiation in the gibbon may have some 
bearing on the achievement of upright pos- 
ture in man. The gibbon hangs in a perpen- 
dicular position, with the legs suspended 
and the back vertical, in other words, in a 
rnan-like position. It is possible that ances- 
tral arboreal forms began to assume this 
position at an early date but before speciali- 
zation had gone as far as it has in the case 
with the gibbons, and that at this point they 
descended to the ground and started their 
terrestrial existence. The gibbon stayed in 
the trees and became further adapted to 
arboreal life. Just why man's precursor, and 
the baboons too, for that matter, came out 
of the trees is a matter of speculation. It 
may have been due to a wasting away of 
the forested areas, in which case adaptation 
to life on the surface of the earth had to 
be made in order to survive. 

The orangutan, another tree-dweller, 
lives exclusively in Borneo and Sumatra 
and is more restricted in its range than any 
of the group. Its name means "man of the 
forest," which certainly describes this cum- 
bersome beast. In direct contrast to the 
swift-moving gibbons, the orangutan moves 
cautiously and very deliberately among 
the tree branches. It is well adapted to life 
in the trees, with its long arms, powerful 

Fig. 13-72. The long-armed gibbon {Hylobates lar) is 
strictly an arboreal primate, being able to travel 
more rapidly through the tree tops than many ani- 
mals can travel on the ground. This specimen has her 
nursing baby held securely between her legs. 

enough to handle its great weight easily 
and the hooked hands with very small 
thumbs like the gibbon. On the ground its 
small legs serve poorly as locomotor organs. 
Its brain case has a capacity of about 500 
cc, which is considerably more than that 
of the gibbons but less than that of the 
gorilla. In the latter, the brain case has 
been known to reach a capacity of well 
over 600 cc. The orangutan's eyes are close 
together and its general facial appearance 
resembles tliat of many people. 

The closest relatives of man are probably 
the great African apes, the gorilla, and 
chimpanzees. There seems to have been a 
tendency among the primates during Mio- 
cene times to try their luck at terrestrial 
existence. These apes have gone part of the 
way. Although they are at home among the 
trees, particularly the chimpanzee which 
brachiates very well even today, they still 
spend a good deal of their time on the 
ground. The gorilla even seems to prefer 



Fig. 13-73. The gorilla {Gorilla gorilla) is the largest and 
probably the most intelligent of all the great apes. 
This is a young male that has just been enticed into 
new quarters which it Is thoroughly investigating. 

the ground, retreating to the trees only to 
sleep. Man, of course, is the only one that 
has succeeded in making the complete 
transition from arboreal to ground life. 

The chimpanzee possesses a disposition 
more favorable to captivity and for that 
reason a great deal more is known about its 
behavior than that of any of the other 
great apes. In size it matches man very 
well, being about 5 feet in height and 
weighing about 150 pounds when full 
grown. When young it is easily handled, 
and is a very affectionate, playful, and 
extremely curious animal. It seems to ap- 
proach the human type of intelligence as 
illustrated by its ability to solve problems. 
Experiments, notably those by Yerkes, have 
shown that the "chimp" will solve simple 
problems much like a small child. For ex- 
ample, if food is placed just out of its reach 
and several boxes are placed near by, the 

chimp will pile the boxes in order to obtain 
the food. It has also been found that, if 
given two bamboo poles which fit together, 
it will so place the two parts in order to 
reach a desired object. This requires a kind 
of intelligence far greater than that pos- 
sessed by the smartest carnivores. 

The gorilla is perhaps more intelligent 
than the chimpanzee, but because of its 
sullen and individualistic disposition it will 
not tolerate training or testing of any sort 
(Fig. 13-73). A two-year-old gorilla re- 
sponds very similarly to a child of about 
the same age or a little older. Soon, how- 
ever, it develops a morose disposition and 
cannot be trusted for close association with 
man. In size the gorilla exceeds all other 
primates, and an old male may reach a 
height of 6 feet and a weight of 500-600 
pounds. Its massive torso and long, very 
powerful arms make it a formidable beast 
in a battle with almost any other animal. 
Because of its herbivorous diet the gorilla 
is not a predator, and for that reason does 
not get into much trouble. Since it is shy, it 
retreats into the dense forest and will not 
fight unless provoked. The gorilla walks 
rather well on the ground but frequently 
reverts to all fours, especially when in a 
hurry. Its short sturdy legs and man-like 
feet support its weight well, although it is 
not a swift runner. 

The next step— man 

The great apes apparently had a greater 
day during the Miocene and Pliocene than 
they have had since. They lived over vast 
areas of Africa, China, and even large parts 
of Europe. One of these, Dryopithecus, 
ranged over all of these regions and, it is 
thought, may have been the stem from 
which the present-day great apes and man 
were derived. It seems that this animal was 
well adapted to tree life, being able to 
brachiate expertly, although its arms did 
not reach the length of those of the gibbon. 
It may be that these fomis descended to 
the ground in the upright position they had 



achieved from their habit of brachiating, 
and became the terrestrial forms, including 
man, of today. However, there is a great 
deal of doubt by most paleontologists on 
this matter at the present time. 

Fossil remains of early man have been 
few and only fragmentary to date, a 
fact which makes the reconstruction of 
man's evolution a very difficult one. How- 
ever, certain important discoveries have 
been made which give some clue as to his 
origin. The most primitive skull of a man- 
ape is that found in South Africa, called 
Australopithecus, the "southern ape." The 
several skeletons or parts of skeletons that 
have been found were in Pleistocene de- 
posits, the period in which man made his 
long slow evolution to present-day types. 
This fossil ape skull shows characteristics 
of both the great apes of today and of man, 
which is what would be expected in a "miss- 
ing link." Its brain case has a capacity of 
over 700 cc, which exceeds that of the larg- 
est gorillas today, but is still a long way 
from that of the next higher fossil man, 
Pithecanthropus, with a capacity of about 
900 cc. Its teeth and brain development 
certainly could have belonged to a form 
that was heading toward modern man. 

Pithecanthropus has stirred up more con- 
troversy since its discovery than any other 
fossil man. This is understandable since its 
discoverer, Eugene Dubois, a Dutch Army 
officer, set out to find the missing link be- 
tween man and the apes, probably because 
of the intense controversy that had been 
stirred up by Darwin's Descent of Man, 
which was at the time in the minds of 
everyone. It is most remarkable that a man 
should set out to accomplish such a diffi- 
cult task and actually carry out his promise. 
Otliers searched furiously for the next few 
years, but it was not until 1936 that other 
similar skeletons were discovered. The orig- 
inal material, which consisted of a skull 
cap, three teeth, and a femur bone, was 
discovered in the banks of the Solo River 
in Java in 1892. Dubois called these finds 

Pithecanthropus erectus, the "erect man" 
(Fig. 13-74). His claim was that this was 
an intermediate form, half ape, half man, 
which may well have been the case, be- 
cause other skeletons tend to bear out these 
assumptions. The man had a very low brow 
and the size of his brain case was about 900 
cc, a much greater capacity than that of 
the great apes though smaller than any 
modern man ( average about 1500 cc. ) . The 
tooth pattern is definitely human, and it is 
interesting to note that the wisdom teeth 
show no signs of disappearing as is com- 
mon in modern man. He probably lived in 
the neighborhood of 500,000 years ago. 

A close relative of Pitheca7ithropus is 
Sinanthropus, commonly known as the Pe- 
king man because the remains were found 
in a cave some 30 miles from Peking, China. 
These middle Pleistocene deposits stimu- 
lated Dr. Davidson Black in the 1920's to 
investigate them for the possibility of hu- 
man remains. He found first only a tooth 
here, but during the next decade over 30 
individuals were unearthed, so that the 
evidence is very complete concerning this 
primitive man. In most respects, Sinanthro- 
pus is very similar to Pithecanthropus, ex- 
cept for the fact tliat the brain case is 
somewhat larger on the average (915-1200 
cc. ) . The lower jaw still maintains a gap for 
the fitting of the upper canines, a definite 
simian character. The leg bones indicate an 
upright posture. He apparently had a cul- 
ture which was far above anything the apes 
would be likely to have. He used fire and 
made stone tools. There is evidence, accord- 
ing to Romer, that he was cannibalistic, 
based on the fact that so many skulls are 
present in the deposits, each one crushed 
at the base. This might mean that the 
bodies were eaten and the brain removed 
for the same purpose, a horrible but prob- 
ably not uncommon custom for early man. 

The Neanderthal man was the first fossil 
man discovered ( 1856 ) and many of his 
remains since have been uncovered, indi- 
cating that he roamed over parts of Asia, 







Fig. 13-74. Evolution of man. 


Africa, and Europe about 100,000 years Whether or not he gave rise to Homo 
ago. Physically he was a rather short man, sapiens is questionable. There is evidence 
not exceeding 5 feet 4 inches (the females to indicate that he was wiped out rather 
were several inches shorter ) , but very pow- suddenly by another race, probably Homo 
erfully built ( Fig. 13-74 ) . His upper arms sapiens. If this was the case, then the latter 
and femurs were short and the latter were must have been derived from another stock 
curved forward so that in posture he prob- as yet not unearthed. The discovery in 1912 
ably walked with his knees bent slightly of a very old skull in Sussex, England, may 
forward. Furthermore, his massive head throw some light on the question, although 
was set somewhat forward over his thick it is still a subject of much argumentation, 
neck and barrel chest, so that he was prob- This man is known as the Piltdown or 
ably quite gorilla-like in appearance. His "dawn" man, named after the Manor where 
brain case capacity was 1550 cc, larger it was found. From the position of the de- 
than the average for modern man, al- posits, it would appear that this man lived 
though it had different proportions. The as a contemporary of Pithecanthropus or 
forepart of the brain case, the seat of higher even before, certainly early Pleistocene or 
intelligence, was rather small, the larger late Pliocene. The skull shows certain ape- 
portion being in the back of the head. His like characteristics in the lower jaw, but the 
skull was heavy, with large protruding shape of the brain case as well as other 
brows, and the massive jaws bore teeth features definitely place it on the direct line 
much more substantially built than those toward Homo sapiens. It is possible, as 
of modem man. The mandible was chinless Romer postulates, that two lines of men 
just as among the apes; a chin is a more were in existence at this early period: one, 
recent development. Pithecaiithropns, evolving to the Neander- 

He had some type of culture which in- thai man, and the other, the Piltdown man, 

eluded religious rites, because he buried evolving to Homo sapiens. 

his dead — probably the important reason Whenever Homo sapiens made his ap- 

why so many remains have been found. He pearance he split off into many races which 

had learned to shape and use stone imple- spread over the entire world. One of the 

ments in the daily business of securing outstanding of these is the Cro-Magnon, 

food, and these, together with bones of the who lived in Europe 15,000 to 40,000 years 

animals he hunted, are found in Neander- ago (Fig. 13-74). Most of the remains, 

thai deposits. about one hundred skeletons, have been 

There is always great interest not only found in France, and the records afford a 

among anthropologists but also among lay- rather complete story of the culture as 

men concerning the story of the origin of well as the anatomy of this man. He was 

modern man. Which of these early men tall, many males reaching 6 feet in height, 

gave rise to Homo sapiens? Was the Ne- although the females were considerably 

anderthal man the immediate forerunner shorter. His skull was long and massively 

of Homo sapiens? There has been a great built with the tremendous capacity of 1700 

deal of discussion over this point and, like cc, which is greater than that of any race 

all problems under controversial fire, there today. Furthermore, the shape was defi- 

are probably several possibilities, the real nitely modern in that the forehead was high 

answer perhaps lying in a compromise. It and the back part rounded as in modern 

must be admitted certainly that Nean- man. None of the ape-like characteristics 

derthal showed considerable advance observed in Pithecanthropus are evident in 

over Sinanthropus and Pithecanthropus, this man. The mandible terminated anteri- 



orly in a pronounced chin and the face was 
short and broad with the eyes set far apart. 
Undoubtedly, he was a handsome fellow. 

In culture these men far surpassed their 
predecessors. They made their hunting 
weapons skillfully, producing the spear and 
axe, and they may have even devised the 
bow and arrow. They apparently were su- 
perior hunters, as judged by the contents 
of their caves, which are strewn with the 
bones of large animals they had killed and 
feasted upon. They probably clothed them- 
selves with skins of animals to protect their 
hairless bodies from the elements. They 
evidently possessed great skill in making 
and mixing paints, and in depicting the life 
as it existed in their time, for they have left 
behind on the walls of many caves in Spain 
and southern France, magnificent paintings 
showing many of the animals which they 
hunted. These are portrayed in a most real- 
istic manner, even to the natural colors 
which have resisted the ravages of time. 
From these paintings a great deal has been 
learned about the fauna of these areas, 
much of which has since become extinct. 
The remains of these men would indicate 
that they were superior both mentally and 
physically to any races alive toda,y. What, 
then, has happened to them? Some think 
their descendants still live in Europe today. 
If so, are they degenerate types? Is Homo 
sapiens actually on the decline? 

Knowledge of early Homo sapiens in 
other parts of the world is not as complete 
as in Europe, although remains have been 
unearthed elsewhere that give an inkling as 
to the origin of some present-day races. One 
might expect to find some early Negroid 
types in Africa, the home of the Negro. An 
early skull from the Sahara does show 
Negroid characteristics. As this part of the 
world becomes more modernized the dig- 
gings of various sorts which always accom- 
pany the process will perhaps throw more 
light on this problem. Undoubtedly, rich 
deposits exist in parts of Asia which when 
unearthed will reveal an interesting story 

concerning the origin of the Mongoloids 
about whom little or nothing is known 

Apparently, Homo sapiens invaded 
America comparatively recently, because 
no remains of very early man have been 
found. The primates that were here in the 
early Tertiary times became extinct and no 
others appeared until modern man made 
his way from Asia across Bering Straits. It 
is highly probable that this is the path he 
took in populating the Americas, because 
geologists have shown that a drop in the 
sea level of 100 feet would leave a land 
connection between Asia and North Amer- 
ica. Such rising and falling of the sea took 
place near the end of the great glacial pe- 
riod. For many thousands of years it was 
possible for these people, filtering gradu- 
ally over this narrow neck of land, to mi- 
grate southward to the semi-tropical and 
tropical regions of the Americas. 

Races of Homo sapiens 

It hardly seems possible that all men 
from the African pigmy to the towering 
Swede are of the same species, yet by our 
definition of species this is true. The fact 
that all members of the human race alive 
today will interbreed is one of the most im- 
portant criteria for a species. When an 
attempt is made to differentiate between 
races (varieties of mankind) the matter of 
criteria to be used for distinction is a monu- 
mental problem. It has never been possible 
to use national boundaries or even lan- 
guages to differentiate races. For centuries 
man has been on the move constantly, rov- 
ing from place to place; today his wan- 
derings are even greater than they have 
ever been, due to improved transportation. 
However, some limitation has been placed 
upon him by boundaries laid down by both 
emigration and immigration laws; if it were 
not for these the intermingling would be 
such that in a few centuries it would be 
more difficult to distinguish races than it is 



The criteria that have been used to dis- 
tinguish the various races are such points 
as stature, hair, face proportions, skull 
shape, complexion, eye and skin color, and 
blood groups. Although there is no clear- 
cut line of demarcation between many indi- 
viduals due to interbreeding, there are cer- 
tain racial characteristics present that make 
it possible to set up a tentative classifica- 
tion. Using the simple basis of color for 
comparison, there are three major groups: 
the Negroids (black), the Caucasoids 
(white) and the Mongoloids (yellow). 

The Negroids. These are the darkest of all 
people, and include the Negroes, pygmies, 
Bushmen, and Hottentots. Aside from the 
heavy pigmentation in their hairless skin 
which gives them the dark color, their hair 
is black and has a kinky texture much like 
wool, their heads are long, their noses are 
flat, and their lips are thick and curved out- 
wards from the mouth. This group includes 
the African Negroes as well as those of tlie 
many Pacific islands from New Guinea to 
the Fiji Islands and elsewhere. They range 
in stature from the tall, powerful Zulus to 
the tiny African pygmies (Negritos). The 
group as a whole has done well both in their 
natural environments and when trans- 
planted to other parts of the world. 

Special mention should be made of the 
natives of Australia because they possess 
certain bizarre characteristics reminiscent 
of the animals on this isolated island. They 
are a very primitive people both in anatom- 
ical features and in culture. There are only 
about 60,000 of them left today and these 
are located along the northern coasts of 
Australia. Physically they are of a "low" 
type, exhibiting many features, such as 
heavy brow ridges, which remind one of the 
Neanderthal man. Although they have the 
broad nose, skull, and jaws of the Negro, 
they do possess characters that approach 
those found among white races, namely, 
greater hairiness, lighter skin color, wavy 
rather than kinky hair, and only moderately 
swollen lips. It has been suggested that 

since this race of colored people seems to be 
neither Negroid nor white, perhaps they 
made their way to Australia from Asia 
where the main human stock evolved at 
a time when the human race had reached 
the stage in evolution which thsy represent 
today. Due to isolation, they remained 
pretty much the same up to the present 
time, along with other animals living under 
similar conditions on this great island. If 
this idea is correct, we can observe today 
a "living fossil" of man. 

The Caucasoids. This group includes a 
wide range of people, in fact, about all of 
those that are neither Negroid nor Mongol- 
oid. In general, they possess characteristics 
that are consistent within wide limits. For 
example, they have skin color ranging from 
white to dark brown, eye color from blue 
to dark brown, and hair color from light 
blond to dark brown, the hair itself being 
straight or wavy but never kinky nor wooly. 
This group includes the most aggressive 
people of the world, representatives of 
which have inhabited Europe, the Ameri- 
cas, and parts of Asia. Some of the major 
types belonging to this race are listed 

1. Mediterranean. These are dark-eyed, 
dark-skinned, long-headed, straight-haired 
people of slender build, inhabiting the re- 
gions along the Mediterranean from Spain 
to India. They are the native Indians of 
India; those who pushed farther eastward 
to the East Indies are known today as the 
Polynesians. They have expanded still more 
to practically all of the Pacific Islands from 
New Zealand to Hawaii. They are a sturdy, 
hardy stock that has been able to travel 
great distances to inhabit these widely sep- 
arated islands. 

2. Aintis. In northeast Asia and particu- 
larly in a small section of Japan these long- 
headed, hairy, and dark complexioned peo- 
ple have established themselves. They are 
geographically isolated and represent as 
near a "pure" race as can be found. 

3. Nordics. These are tall and slender 



people with typically long heads. They 
have ruddy complexions with blond straight 
or wavy hair and blue eyes. They are found 
in the Scandinavian Peninsula and the East 
Baltic shores, as well as in Great Britain 
and the Low Countries. It is interesting to 
note that during and previous to World 
War I Hitler's regime was encouraging the 
propagation of more Nordics who actually 
make up only a small part of the population 
of Germany. The name later was appropri- 
ately changed to Aryan, which refers to cul- 
ture rather than to race. The Celts, who 
reside today in Ireland, Scotland, and 
Wales, are not true Nordics. 

4. Alpines. These are short, stocky, round- 
headed people with brown hair and eyes 
and a skin that ranges from white to olive 
in color. These are the first "broad heads" 
which, according to anthropologists, are a 
more recent race of people. They are sup- 
posed to have appeared in relatively recent 
times and have dominated the long heads. 
They are most common in central Europe 

Mongoloids. Much of Eastern Asia and 
the Americas were originally populated 
with members of this race which includes 
the Chinese, Japanese, Eskimos, and Ameri- 
can Indians. They are characterized by 
coarse, black straight hair, and skin that has 
varying shades of yellow and brown. Hair 
is confined to the head where it is abundant, 
whereas the face and the rest of the body 
are relatively naked. The head is round and 
the face broad with high cheek bones and a 
small nose. Many possess slanting eye aper- 
tures in which a peculiar fold of skin covers 
the upper eyelid; this is a specialized fea- 
ture, the function of which is not clear. 
They are usually rather short and stockily 

These people have thrived both in Asia 
and the Americas, although the North Amer- 
ican Indian has not fared as well as his 
oriental cousins. Indians that made their 
way south into Central and South America 
have maintained themselves but those that 

stayed north have not done so well since 
the infiltration of Europeans. Most of them 
were little concerned about culture al- 
though some, such as the Aztecs and Incas, 
did reach a rather high degree of civiliza- 
tion until it was interrupted by the white 
man. The nomadic Indian could hardly de- 
velop a culture when he was always on the 


The Eskimos, while definitely Mongol- 
oid, are quite different from the Indian. 
They have narrow heads which might indi- 
cate some earlier admixture from one of the 
long-headed races. Their northern habitats 
caused them to build up a particular cul- 
ture and they were a successful group until 
the coming of the white man who intro- 
duced, along with his good will and his re- 
ligion, his own infectious diseases, all of 
which the Eskimo would have been better 
off without. 

Mans present status. One might think 
that with all laws of evolution operative 
through these past 500,000 or more years 
present-day man might be a "super" human 
being, an animal physically perfect in all 
respects and geared beautifully to his envi- 
ronment. This, in fact, is far from the truth. 
The number of people with defective vision 
is a glaring example. The occasional inad- 
equacies of the various organ systems is 
confirming evidence that they do not always 
function as they were intended, at least 
under present treatment which itself may 
be at fault. One is no farther from the state 
of health of the American people than he is 
from his radio; to listen would convince the 
less astute that these past one-half million 
years have delivered upon the world a spe- 
cies of animal that cannot possibly cope 
with his own surroundings. His environ- 
ment has become so outrageous that he 
must constantly concoct and devise supple- 
mentary ingredients to his normal diet in 
order to keep his body functioning. This 
problem is far more economic than biolog- 
ical, and aside from differential birth rates 
and the possibility of self-annihilation the 



human race today might go on for a long 

In spite of all of his apparent caducity, 
man has done rather well, biologically 
speaking. He has spread himself over a very 
large portion of the globe and has reached 
well over two billion in numbers, not a 
large figure, to be sure, when one considers 
that there are more bacteria in a quart of 
sour milk! He has managed himself rather 
well in most respects; he plans for his own 
food and shelter as well as other comforts 
of life. He has, by concerted effort, been 
able to allow himself some leisure time from 
the endless task of providing the bare ne- 
cessities of life. He has used this time crea- 
tively, thus improving not only his immedi- 
ate environment but also his relation to it. 
What is more important, he has written 
down the information he has acquired so 
that his knowledge can be passed on to oth- 
ers. Man can learn in his own lifetime more 
than experience could bring him in 100 or 
perhaps 1,000 lifetimes. This has been the 
real secret of man's skyrocket ascent to his 
present position in the world, at least 
the civilized world. 

There has been little, if any, improve- 
ment in our brain since the Cro-Magnon 
man, and during the intervening 50,000 
years progress toward civilization as we 

know it has been extremely slow. Only dur- 
ing the last 5,000 years, and particularly the 
past 300 years, has outstanding progress 
taken place. Why was man so slow in rising 
as a social animal, and why, when he 
started, did he rise so rapidly? It was un- 
doubtedly due to the fact that he acquired 
the ability to put down in writing what he 
had learned so that those who followed 
could profit by his experience. Once this 
idea took root it flourished and with it the 
progress of mankind. Information is accu- 
mulating today at a staggering rate. Most 
scientists have great difficulty reading the 
literature that has been and is accumulatins; 
even in their own restricted field, to say 
nothing of that in the cognate sciences. It is 
doubtful if a person working 24 hours a day 
could read the titles alone of scientific pa- 
pers that appear continuously. One of the 
big problems today is to condense this in- 
formation so that any one person can have 
some understanding of it all. If the present 
rate continues, all the university buildings 
on all campuses will be filled with literature 
in another 100 years! 

It is well worth while, then, to study 
rather carefully this animal that has made 
such stellar progress in the past few hun- 
dred years, and whose primary mark of 
distinction is a huge brain. 


Organ Systems of Man 



MAN: A TYPICAL MAMMAL largest or the smallest mammal, nor is he 

highly specialized when compared to the 

The gradual evolution of animals has whale, for example. In fact, he is a rather 

been discussed in the preceding chapters mediocre mammal, being poorly endowed 

and considerable time has been spent on with organs of offense and defense. His 

each group so that a working knowledge puny, flat finger nails and short canine teeth 

of their anatomy and physiology was cov- are no match for the claw and tooth of the 

ered. In studying the last group of animals, tiger or lion. His hide is not thick, like that 

we have selected man as a typical mammal, of the elephant or whale, and it is com- 

A study of the rat, cat, guinea pig, or dog pletely unprotected by hair, the normal 

would afford no more information than that coat for most mammals. He has no horny 

of the human body and for the readers of outgrowths for defense, like the ungulates, 

this book certainly no more interest. and even his locomotor appendages are 

Man is quite a typical mammal, unusual only fairly effective in getting him out of 

only in the size of his brain. He is not the danger. 



Man is no longer at home in the trees but It requires only a slight knowledge of the 

has taken up life on the ground and with subject to realize that most of the informa- 

it the bipedal method of locomotion, a tion about the functioning of the human 

method far from new since the great camiv- body can come only through such experi- 

orous dinosaurs also employed it. Other ments. How would anyone suffering from 

present-day bipeds such as the ostrich can appendicitis, for example, particularly an 

easily outstrip him in cursorial travel. He is antivivisectionist ( the self-styled name ap- 

poorly fitted for life in the water where his plied to these people ) , appreciate having 

appendages are not well adapted for loco- a doctor who has had no experience what- 

motion. He can submerge for only very short ever on lower animals attempt to remove 

periods without coming up for air, and in the offending organ? If the antivivisection- 

cold water his survival time is very short, ist lived up to his code, he should rightly 

He has, however, one crucial organ that refuse any medication which stemmed from 
accounts for most of his success, his well- a study of lower animals. However, it is 
developed brain. This organ, by its intricate highly probable that if he is taken ill he will 
disposition of nerve impulses, has made it proceed with all haste to obtain the best 
possible for man to compensate for all of his available skill, no matter how it was ac- 
physical deficiencies. With it he has been quired. Such groups today are only a gen- 
able, through the power of speech, to com- eral nuisance, although they frequently 
municate with his fellows and later to put cause the loss of valuable research time of 
words down in writing. Over a long period prominent biologists who must stoop to the 
of time this type of specialization has finally task of defending themselves. Much more 
"paid off" because man today is the domi- could be said on the topic but perhaps 
nant species on the earth. at this point it would be well for the student 

In order to understand man it has been to draw from his own experiences and form 

necessary to study other forms of animal his own conclusions about this matter, 
life. Man does not lend himself well to ex- We shall now turn to a discussion of the 

perimentation for obvious reasons and, fur- organ systems of man, as illustrative of ver- 

thermore, he grows too slowly to permit tebrate organ systems in general. Each sys- 

studying succeeding generations. He can- tem will be discussed at some length, 

not be kept under the controlled conditions together with a brief account of similar 

that are possible with rats in a cage. How- structures in other animals from the lowest 

ever, most of our information about his to the highest. Although some attention has 

functioning has come through the careful been paid to these topics in the discussion 

study of lower animals. If it were not for of each animal group, it is well to review 

these experiments our knowledge would be tlie information briefly before each system 

very meager. This brings up an interesting of man is studied. With this approach, per- 

and important point concerning attitudes of haps the structures and their functions as 

the species, Homo sapiens. found in the human body may be better 

There are small isolated groups of people understood, 
who oppose any animal experimentation 

( primarily experiments on dogs, cats, and OUTER COVERING-THE SKIN 

other pet species), sincerely, perhaps, or 

stirred by some ulterior motive such as pub- As we have seen in the preceding chapter, 

licity, for example. In any case, they are a all animals are provided with an exterior 

small but usually active group who are covering that functions as a barrier against 

constantly stumping for legislation which the outside world. This is extremely simple 

would curtail present-day experimentation, in the lower invertebrates, but becomes 




Sffboecous ^land ^ 
arcctor muscle. 

bulb of hair 


sw«at 9lond 

Fig. 14-1. Human skin in cross-section. Gland ancJ hair development are shown in several stages (top left). 

much more complicated in higher forms; as, 
for example, in the vertebrate skin with all 
of its derivatives. 

In these animals the skin forms a contin- 
uous unbroken sheath which protects the 
internal structures from the environment. 
In addition to being tough itself, it is often 

fortified with scales, feathers, or hair for 
added protection against physical injury by 
predaceous enemies. Moreover, it is refrac- 
tive to bacteria and other microorganisms 
that could be injurious if allowed to enter 
the internal environment. It is impervious to 
most harmful chemicals and is important in 



regulating the water content of the body, 
functioning differently in aquatic and ter- 
restrial forms. Heat regulation is also con- 
trolled by the skin indirectly through water 
loss, as well as by various insulators such as 
hair and feathers. The penetrating effect 
of light is regulated by pigmentation of the 
skin. Among some vertebrates the skin takes 
part in respiration (for example, the frog). 
These many functions of the skin signify its 

It is necessary to examine the skin micro- 
scopically if one is to understand how it 

sloughed off in mammals and many of the 
lower vertebrates. Everyone is familiar with 
the loss of these cells in unexposed parts of 
the body such as back of the ears and be- 
tween the toes. They are particularly notice- 
able in the hair, where they resemble flaky 
"scales" and do not have an opportunity to 
escape readily. These dead cells are spoken 
of as dandruff and often erroneously as- 
signed a pathological condition, particularly 
by certain business establishments whose 
chief concern is to sell a product that will 
clear up this "malady." The corneum is per- 

F g. 14-2. Homologous digital tips, claws, nails, and hoof. 

performs its many and sundry jobs, and we 
will take human skin as our example. It is 
usually divided into two parts, the outer, 
thinner epidermis and the inner, thicker 
dermis (Fig. 14-1). The epidermis is com- 
posed of an outermost non-living covering, 
the corneum, which is the part in immediate 
contact with the outside world; lying be- 
neath it is a layer of epithelial tissue which 
is composed of actively growing cells. As 
the cells grow they move towards the out- 
side, die, and eventually become the cor- 
neum. These dead cells are constantly being 

forated by many tiny holes through which 
sweat passes from glands that lie deep be- 
low. The corneum becomes very thick on 
the soles of the feet and the palms of 
the hands (calloused), especially in people 
who perform heavy labor requiring the use 
of these appendages. Another interesting 
characteristic of the corneum of these areas 
is the formation of friction ridges. It is the 
presence of these in tlie hand and foot which 
causes fingerprints and footprints. These 
friction ridges apparently have come 
through a long evolutionary history, being 


originally digital pads of four-footed ani- ranged as to offer the least resistance to for- 

mals. When mammals took to the trees, ward motion. 

the pads developed into transverse ridges Feathers resemble scales in their over- 
which functioned in increasing the friction lapping arrangement, although otherwise 
between the hand and the branch, thus pre- the likeness is not so obvious. They are 
venting slipping. In man the ridges are gen- much lighter in construction, possessing 
erally arranged in whorls, although they are numerous tiny filaments that offer resist- 
transverse to the long axis of the fingers for ance to the passage of air through them, 
the most part. The designs appear to be in- The wing feathers of birds will allow air to 
finite in numbers and never seem to be pass one way but not the other— a beauti- 
repeated on the tip of any digit, either on ful example of adaptation to flight, 
the hand of one individual or on any other Teeth. Teeth are also epidermal out- 
individual. This has provided a convenient growths, having a common origin with 
means of identification because it positively scales (Fig. 14-4), particularly those of the 
distinguishes one person from another, and shark (placoid). Since the mouth is lined 
its primary use today is in criminal investi- with ectoderm (the germ layer that gives 
gations. rise to the epidermis ) , we might expect that 

The actively growing layer of the epi- it (the mouth) could be equipped with any 

dermis (stratum germinativum ) produces structure that could come from ectoderm, 

many structures which on the one side are The scales in sharks enlarge and grow over 

sunk deep into the dermis and on the other the edge of the jaw, producing teeth. Hu- 

side are an important part of the external man teeth come likewise from ectoderm 

covering. The scales of fish and reptiles, and fit into cups or sockets provided for 

the feathers of birds, and the hairs of mam- them in the jaw. 

mals have such relationships (Fig. 14-1). Hair. These tiny projections from the 
Although these all have similar origins, in skin of mammals perform a protective func- 
the final adult stage they are quite different tion against both physical injury and heat 
both in structure and function. Digital tips loss. The numerous hairs tend to provide a 
in various vertebrates, such as the carnivore dead air space just above the skin which 
claw, the ungulate hoof, and the primate prevents heat loss much like insulation ma- 
nail, are likwise produced from this region terials in a house. Feathers also act as heat 
of the epidermis (Fig. 14-2). They are all insulators, a fact readily observed on cold 
homologous, since each has the same origin days when birds ruffle up their feathers to 
but performs a different function. improve the insulating properties of their 

integument. On a hot day, a bird keeps its 

Derivatives of the epidermis feathers close to its body to allow as much 

Scales and feathers. Both of these outer heat to escape as possible, 

coverings have a common origin ( Fig. 14-3 ) Mammals, with the exception of the whale 

and they are much alike both anatomically and man, are covered with a thick coat 

and functionally. Scales are found princi- of hair. Man has lost most of his hair, prob- 

pally among the fishes and reptiles, although ably because he evolved in a warm climate, 

birds have them on their legs and some evi- Today it is present only in the pubic regions, 

dence of scales appears among the mam- under the arms, and on the face and head, 

mals (for example, tail of rat and beaver). The facial adornment is a male secondary 

Structurally, they resemble overlapping or sexual characteristic because it is not found 

abutting plates that offer considerable re- in the female. The rest of the body is usu- 

sistance to outside mechanical injury. When ally covered with very tiny hairs which are 

overlapping like shingles, they are so ar- vestigial, for they perform no function in 



stratyiT) . 


Fig. 14-3. Stages in the development of reptilian scales and feathers, showing their common origin. 




Fig. 14-4. Stages in the development of scales and teeth, showing their common origin (top). 
A photograph of shark's skin showing the tiny scales. Note their tooth-like nature 


modern man. This loss of body covering tected skin, as most people know. Continual 

carries with it the obvious disadvantage of exposure to bright sunlight produces a 

rapid heat loss in a cold climate and man dark, tough skin, even in white people — 

has thus been forced to clothe himself with a far cry from the complexion recommended 

an artificial covering. The adornment fea- by beauty experts. Yet would-be beauties 

ture of clothing cannot be overlooked, for will often expose their bodies too long and 

even in hot climates aboriginals frequently too often so that the damaging rays of the 

cover themselves, particularly if the cloth sun produce the type of skin that is consid- 

is highly colored. ered undesirable. 

Hairs are arranged in definite patterns Glands. Near the base of each hair is a 

in the various regions of the body of man as tiny sebaceous gland which secretes an oily 

well as other mammals. They are not per- substance designed to keep both the hair 

pendicular to the skin surface but slant, and the skin in a soft, pliable condition, 

usually in a specific direction. For example. These glands, like other skin glands, come 

in a dog the hair slants away from the mid- from the epidermis, although they are 

dorsal line, usually in the direction of the buried deep in the dermis (Fig. 14-1). The 

pull of gravity, and this probably helps in tiny, much coiled, sweat glands are likewise 

shedding water. In man the direction of the found deep in the dermis where they func- 

hair slant in the back region is the same as tion in extracting water from the blood and 

in dogs. On the arms and legs it also follows tissues and spreading it over the surface 

a common pattern but on the top of the head of the skin for the purpose of cooling the 

it sometimes forms whorls or "cowlicks," body (Fig. 14-1). The resulting evapora- 

which, strangely enough, are specifically tion reduces the temperature of the skin 

inherited from generation to generation. and thereby aids in the regulation of the 

Attached to the base of each hair are tiny temperature of the entire body. This is very 
muscle fibers which, when contracted, cause important to a mammal, although in many 
the hair to stand on end, producing "goose species these glands are localized in small 
flesh" (Fig. 14-1). These muscles are under areas, which are quite different in different 
the influence of the autonomic nervous sys- mammals. The cow, for instance, has them 
tem and are therefore beyond voluntary confined to its nose, whereas others such as 
control. When one is frightened or some- the horse and man have them distributed 
times under other emotional stress, the hair rather generally over the body. In man they 
can be seen to stand on end, particularly are concentrated in tlie palms of the hands, 
along the spine. The common statement, soles of the feet, and under the arms. 
"Chills run up my back," has a physiological Another very interesting skin gland 
foundation. A similar reaction can be ob- found only among mammals and fully de- 
served in a dog and cat when in the presence veloped only in females is the milk or rmim- 
of potential enemies. marij gland, a name that is linked with the 

Pigment granules lying in the lower epi- group. These are modified sebaceous glands 

dermal layers of the human skin give it and are confined to areas most convenient 

color ranging from no color at all, as in the for suckling the young mammal, which 

abnormal albinos, to the dense pigmenta- they supply with a complete food during its 

tion in black people. Sunlight has a decided early post-embryonic life, a sort of continu- 

effect, not only on increasing the amount of ation of umbilical feeding, 
pigment (tanning), but also on increasing 
the thickness of the epidermis itself. The 

purpose of such a response is protection This layer of the skin is much thicker 

because ultraviolet light damages unpro- than the epidermis and is composed of 


tough connective tissue (Fig. 14-1). The under the influence of the autonomic nerv- 
nature of this layer becomes apparent if one ous system and may be influenced in some 
recalls the various kinds of leather that are people by emotions such as in blushing 
employed by man for thousands of pur- (vasodilation). Also, the dermis contains 
poses. It is interesting to note that man uses many nerve endings which are receptors for 
the skins of other animals to supplement his heat, cold, pressure, touch, and pain. With 
own. Fat has a tendency to store itself in the exception of the pain nerve endings, 
the deeper parts of the dermis and becomes they are specially designed end organs for 
localized in characteristic regions of the special stimuli. Since the skin is in contact 
body familiar to everyone. In unusual with the outer world all of the time, a great 
cases, the stored fat can weigh as much as deal of information comes to the brain from 
the remainder of tlie body, a character it. Pain, for example, is a very uncomfortable 
which is admired by some races of people sensation, ideally designed to make the ani- 
and abhorred by others. The disti-ibution mal do something about the situation, if 
of this layer of fat is characteristic of the possible. This sensation is responsible for 
sex. For example, in the female the layer is preservation of the individual, for without 
thicker, giving the skin a more velvety it great areas of the body might be de- 
touch and, the body contours are smoother stroyed without the organism being aware 
curves with the underlying muscles less of it. A few people have no nerve endings 
pronounced than is the case in males, in their skin and hence feel no pain; as a 
Women are therefore better insulated than consequence, they can be seriously burned 
men, although they seem to suffer more or injured in other ways before becoming 
from the cold, perhaps due to their scanty conscious of the danger, 
artificial covering, a mere fraction of that 

with which the male burdens himself. Thus we see that most animals possess 

Many tiny bundles of blood vessels pro- some sort of protective covering. They also 

ject up from below into the dermis where need some internal support, particularly the 

they function in bringing the blood close to larger forms and those animals tliat live on 

the surface for cooling the body when it is land. Let us study the way vertebrates, and 

too warm. They contract when the body is man in particular, have solved this prob- 

cold (vasoconstriction), preventing the lem. 
blood from coming to the surface. They are 



Animals have devised many forms of tial support but are highly protective as 
support for their bodies. The various struc- well. Vertebrates, on the other hand, have 
tures not only hold the body together but adopted an internal skeleton designed pri- 
also, in many cases, have an important pro- marily for support. It affords very little 
tective function. To be sure, such single- protection to the soft external parts of the 
celled animals as amoeba exist completely animal, although it provides excellent pro- 
naked, with no protective covering what- tection for such vital organs as the brain, 
ever. Their close relatives, however, such as heart, and lungs. 

Difflugia (Fig. 15-1), secrete a substance Let us consider the human skeleton as an 

which collects tiny siliceous particles example of a vertebrate skeleton, 
(sand) and cements them together to pro- 
vide an enclosure into which they may THE 
draw themselves when hard pressed. Para- 
mecium possesses a semi-rigid pellicle 

which gives it some external support so that The skeleton of man is similar to that of 

its body maintains a relatively constant other mammals, almost bone for bone, but 

shape. Sponges produce minute angular certain parts are emphasized more or less 

spicules which afford a rather rigid skele- than similar parts in other mammals. This 

ton, and many coral animals secrete sub- is because of the upright position his body 

stantial external skeletons. Larger inverte- has taken. All animals that have taken to 

brates, such as arthropods and mollusks, bipedal locomotion, the dinosaurs, the birds, 

provide themselves with hard outer cover- and the kangaroo, for example, have shifted 

ings which not only lend the body substan- their body weight so that some parts of the 



bony andoftK<(eVor> 


Fig. 15-1. Representatives of some animal groups showing the way they have solved the problem of 







■ moxilla 
mand'ibk . 

■ ciovick 



.thoracic vertebroe. 

-lumbar vcrt-ebra*. 


-coccyx _ 


ischium ~ 














Fig. 15-2. The human skeleton, front and back views. 



skeleton must bear a greater portion of 
the burden than other parts. In quadrupeds 
the spinal column and legs resemble a sus- 
pension bridge where the column functions 
as tlie bridge itself and the legs as the sup- 
porting piers at either end. In man and 
other bipeds the body is elevated at one 
end until it is in a vertical position which 
requires more secure footings at the base. 
This is essentially how a tall building such 
as the Empire State Building in New York 

The axial skeleton 

The skeleton is usually divided into two 
general parts: the axial region comprising 
the skull, the column, and the ribs, and the 
appendicular region which includes the ap- 
pendages and their girdles ( Fig. 15-2 ) . All 
of the bones are so securely tied together 
with ligaments that they are torn apart 
only under great strain. In spite of this 
seemingly well-built frame, it is often badly 

Fig. 15-3. Comparative profiles of the dog, monkey, and man to show the relative shift in the 
facial angle (see text). Because of the increasing size of man's brain, which has grown 
over his shortened jaws, his facial angle has increased over that of lower forms. 

is constructed — merely a bridge stood on 
end with elaborate footings. In the human 
skeleton, this shift in weight has also neces- 
sitated more rigid connection between the 
supporting vertebral column, and the pelvic 
girdle to which posterior supporting ap- 
pendages are attached. This appears to 
function in a fairly satisfactory manner, 
although if one may judge by the number 
of middle-aged people suffering from a 
"sacroiliac" (the development of a faulty 
union between the column and pelvic 
girdle), it is clear that the arrangement is 
not as good as it might be. 

mutilated in accidents as a result of our 
modern means of transportation. 

The skull. The skull shows considerable 
modification vv^hen compared to that of 
lower vertebrates. The greater emphasis on 
the cranial case compared to the mandible 
is obvious when the heads of the dog, 
monkey, and man, for example, are placed 
side by side (Fig. 15-3). As the brain has 
grown forward over the shortened jaws, it 
is easy to see how the facial angle (angle 
between a line drawn along the forehead 
and one from the base of the nose to the 
foramen magnum) has increased and how 


the human face has been formed ( Fig. ventrally instead of posteriorly as is the case 
X5-3). with most mammals. The large opening 
The human skull is made up of 28 bones, through which it passes is called the fora- 
22 of which are joined by jagged-edged men magnum. Since the skull is precari- 
sutures (Fig. 13-30). The other 6 are the ously perched on the tip of the spinal col- 
tiny ossicles of the ears. The bone which umn it might be expected that the cord could 
supports the tongue and larynx, the hyoid, be broken at this point rather easily, and 
is loosely connected with the skull. Although such is indeed the case. A severe blow at the 
most of the skull is heavy, solid bone, cer- base of the neck will snap the cord at the 
tain portions contain cavities. These are point where it enters the skull. This vulner- 
remnants of chambers that formerly had able spot is taken advantage of by man in 
specific functions but which apparently getting rid of his incorrigible fellows, by 
have lost these and perform no known func- hanging. Other openings into the brain case 
tion today. For example, the three sinuses in are the foramina for numerous small blood 
the anterior and middle portion of the skull vessels and for the cranial nerves, including 
once served the sense of smell but do not do the optic nerves at the base of the orbits, 
so now. There is a pair of maxillary sinuses At birth, several bones from the brain 
in the cheek region, a pair of frontal sinuses case have not come together ( sutured ) , so 
over the eyes, and a single sphenoid sinus that five spaces are left without bony cover- 
in the posterior part of the nasal cham- ing. These are called fontanelles (little 
ber. They all have small ducts which drain fountains — so named because they rise and 
into the nasal chambers but the arrange- fall with each heart beat). This lack of 
ment is such that drainage is not good, es- suturing before birth plays a very impor- 
pecially when the membranes are swollen tant function in the birth process, for the 
with a cold. Under such conditions the head of the child undergoes severe squeez- 
large surface area of the sinus membranes ing while passing through the birth canal 
becomes infected, causing the so-called and needs to change its shape to fit the 
sinus trouble which is often difficult to narrow passage. Were the skull hard, the 
treat satisfactorily. Another spongy bone, difficult process of being born might be 
the mastoid, lying behind the external ear even more difficult or impossible. The head 
may also become infected via the eusta- of the newborn child is very plastic and 
chian tube and the middle ear. Such an can be molded into almost any shape, 
infection can reach the brain because the Flathead Indians took advantage of this 
mastoid is separated from it only by very fact by placing a board on the head of the 
thin bone. Surgery, in which a portion of newborn, thus causing the forehead to have 
the bone is removed, is one of the methods a peculiar flat appearance in the adult. As 
of clearing up such infections. the child grows, the fontanelles gradually 
The brain is exposed to the outside wall close, leaving five jagged lines at the junc- 
in only one place, and that is in the nasal tures. The age of a skull can be told by 
chamber. The floor of the brain case, where the clearness of these lines. They are faint 
the olfactory nerves leave the brain and or absent in old skulls, 
pass down into the nasal chamber, is called Injuries to the skull have been common 
the cribiform plate. It is a piece of bone throughout man's history. Early skulls often 
perforated with many small openings show evidences not only of natural injuries 
through which the nerves pass and through but also of apparent deliberate removal 
which, unfortunately, nasal infections can of small portions. Such drillings (called 
reach the brain. trephining) seemingly had some religious 
The cord enters the human brain case significance, but the remarkable thing 



about them is that the patients often re- 
covered, as revealed by the smooth edges 
of the opening, indicating that the bone 
healed. Similar operations are performed 
today for entirely different reasons and 
with much more satisfactory results. 

The spinal column. When a comparison is 
made between the spinal column of man 
and almost any of the other mammals, cer- 
tain striking differences are noted (Fig. 
15-4). These result from the upright pos- 
ture man has assumed. In the dog, for 
example, the column forms a smooth arch 
between the two pairs of legs; in man, on 
the other hand, it forms a sigmoid or 
S-shaped curve. This serves an important 
purpose in an upright animal. With the 
head resting on the top end of the column, 
a rigid, straight rod would afford very little 
resilience whereas a curved column would 
spring gently, thus cushioning the jolts that 
are conducted through the legs from the 
feet as they come in contact with the 
ground. The curved spine of man is ad- 
mirably designed to give the head a smooth 
ride. If the pliable spine of a growing child 
is subject to undue stress, it may ultimately 
affect the development of the adult skele- 
ton. Much of our posture is dependent on 
the spine and there is much emphasis to- 
day on the desirability of good posture. 
While this is highly desirable, it is not a 
guarantee of good health. Good health 
is due to a great many things and cannot be 
guaranteed by any such simple formula. 

The spinal column is composed of 33 
articulating vertebrae of rather irregular 
sizes from the neck to the pelvis and they 
fit snugly together. They are securely laced 
together by many ligaments, so that the 
column as a whole is a beautiful piece of 
engineering. This is essential because the 
column houses the very delicate spinal cord 
which, if injured even only slightly, may 
cause dire effects in the operation of the 
appendages as well as other parts of the 
body. The column is more flexible in some 
regions than in others. For example, the 

Fig. 15-4. The spinal column is the axial support of 
vertebrates and is subject to considerable variation 
among the different groups, depending on the stress 
and strain put upon it. In the quadraped, such as 
the dog, the column functions like a bridge v^rith the 
two supports at either end. When the support is 
shifted to the two posterior appendages, such as in 
the ape, a more secure attachment must be affected 
between the column and the pelvic girdle. This is car- 
ried further in man, where we see a huge pelvic 
girdle, since the posterior appendages must provide 
the only means of support and locomotion. 



opcntnq for 
spinal cord 

openiriQ for 
spinal mrvz 

Fig. 15-5. The spinal column is composed of interlocking 
vertebrae that, taken together, form a sturdy, flex- 
ible support for the entire body. The large openings 
in the vertebrae form a bony canal in which the 
delicate spinal cord is housed. Between the vertebrae 
are paired openings through which the spinal nerves 

vertebrae of the thoracic region are rela- 
tively immovable whereas those in the 
lower back and neck region have consid- 
erable amplitude of movement. This ar- 
rangement allows for a large variety of 
movements of the trunk, as evidenced by 
the ballet dancer in action. 

Pairs of small openings between the 
vertebrae provide exits for the spinal nerves 
(Fig. 15-5). Each vertebra has a large 
cylindrical passagev/ay, and these taken 
together form the neural canal which 
houses the nerve cord (Fig. 15-5). Five of 
the lower sacral vertebrae are fused into a 
solid bone, the sacrum, which joins the ilia 
(singular — ilium) on the dorsal side, thus 
securely attaching the pelvic girdle to tiie 
spine. It needs to be a broad, secure attach- 
ment because the whole upper body pivots 

at this point and the stress is considerable. 
Unfortunately, the joint is not bone-to- 
bone but via ligaments, and when it par- 
tially gives way under unusual strain much 
distress is caused. 

The spinal column terminates in sev- 
eral (5 to 12) tiny, useless vertebrae, col- 
lectively known as the coccyx. In many ver- 
tebrates they give support to a functional 
tail, but in man they are mere vestiges of 
the past. Undoubtedly, far back in man's 
early history, long before he was man, he 
had a tail. It must be remembered that the 
presence or absence of a tail means nothing 
from an evolutionary point of view. The 
bear and guinea pig are without tails, yet 
they are no more related to each other than 
either is to man. 

The ribs. The 12 ribs are attached to the 
transverse processes and the centra on the 
column side and 10 of them to the sternum 
directly or indirectly on the ventral side. 
These together form the thoracic basket, a 
convenient enclosure for the vital organs 
located in the chest region. It is interesting 
to note that the number of ribs is not 
always 12. The millions of chest x-rays taken 
of soldiers in the last war brought to light 
the fact that tliere is considerable variation, 
ranging from 11 to 13, the latter number 
beino; the most common variation. Inciden- 
tally, the gorilla also possesses 13 ribs. 

The appendicular skeleton 

The remainder of the skeleton, consisting 
of the appendages and their supports or 
girdles, is called the appendicular skeleton. 
The pectoral girdle to which the arms are 
attached is located in the anterior region. 
It consists of two clavicles (collar bones) 
and two scapulas (shoulder blades); taken 
together they form a triangular brace with 
the arm hanging at the apex. Clavicles are 
rudimentary or absent in most mammals, 
but in the primates they are large, func- 
tional bones. This difference is owing to 
life in the trees, where brachiation was re- 
sponsible for the development not only of 



the clavicle but also the nerves and muscles 
of the arms which make them such useful 
appendages today. While the clavicle is 
firmly attached to the sternum on the front, 
the scapula has no secure attachment and 
is loosely slung over the thoracic basket by 
means of muscles and ligaments. This ar- 
rangement permits a great deal of move- 
ment in which the shoulders can be freely 
rolled over the ribs. The anterior append- 
ages have much more freedom of move- 
ment than the posterior appendages, whose 
primary function is locomotion. 

The upper arm, the humerus, fits into a 
crude socket made by the union of the 
scapula and clavicle called the glenoid 
fossa. The humerus is held in place by liga- 
ments at its upper end, but since the attach- 
ment is none too secure, under certain 
stresses it may be forced out of the socket, 
resulting in a dislocation. Such stretched 
ligaments allow dislocation more readily 
under similar subsequent stresses. The ad- 
vantage of this junction lies in its loose 
arrangement which allows more freedom of 
movement for the arm. For example, the 
arm may be turned in a complete circle as 
well as rotated in the socket. A dog, on the 
other hand, could not possibly perform such 
a feat, for the arrangement of the bones in 
its pectoral girdle is much more rigid. 

The two forearm bones, the radius and 
ulna, form a combination whereby hinge 
action as well as partial rotation can take 
place. This means that the forearm can be 
flexed (bent on itself) in a straight pull or 
it can twist through 180 degrees. The num- 
ber of times one performs these movements 
each day is almost unlimited. At the wrist 
another hinge is produced by the end of the 
radius and the carpals, the small wrist 
bones. Actually this is as much a universal 
joint as it is a hinge, with the result that the 
hand can move in all directions with equal 
facility. The hand with its large, opposable 
thumb is a primitive but most useful instru- 
ment and it is hard to imagine life as it is 
lived today without it. 

The pelvic girdle is the most specialized 
part of the entire skeleton. A quadruped, 
running on all fours, does not require as 
secure an attachment to the column as does 
a biped, whose pelvis has become corre- 
spondingly modified. However, in the case 
of man the pelvis has become not only an 
excellent support for the entire body but it 
has also broadened and flared out so that 
it functions as a support for the organs of 
the abdominal cavity. This again is a satis- 
factory method of handling the pendent 
viscera of tlie upright animal. 

The pelvis is composed of three pairs of 
fused bones: the large, flat and cupped ilia 
(singular — ilium), the ischia (singular — 
ischium, the bones used in sitting), and the 
pubic bones which complete the girdle in 
front. The fused vertebrae of the sacrum 
form a complete circle at the back, leaving 
a large opening through which all mammal 
offspring must pass in tlie process of birth. 
The urinary and digestive tracts pass 
through here also. The dimensions of this 
opening are one of the clues used in deter- 
mining the sex of a skeleton. Not only is the 
opening larger in females but, in addition, 
the attachment of the pubic bones is not so 
broad. Both features are essential to allow 
such a large object as a fetus to pass 
through. The ilia also flare outward more 
abruptly in the female than in the male; this 
changes the position of the legs somewhat 
so that the method of walking and running 
differs in the two sexes. The familiar female 
, "waddle" is a result of skeletal arrange- 
ment, not any intention on her part. For 
the same reason it is highly unlikely that a 
woman wfll ever run the 100-yard dash in 
10 seconds. 

The femur is the longest bone in the 
skeleton. Its proximal end (end nearer the 
body) is a pronounced ball which lies at 
an angle to the rest of the bone and which 
fits into a deep socket in the pelvis called 
the acetabulum. This is a much more secure 
arrangement than the one in the shoulder 
region, although it does not have equiva- 



lent freedom of movement. For this reason 
the hip joint is not nearly so apt to dislocate 
as the shoulder joint. The leg can circum- 
scribe a narrow cone but not a wide one 
like the arm. The leg is primarily concerned 
with the business of carrying the body for- 
ward in progression and consequently is 
constructed to function essentially in a for- 
ward and backward motion. 

At the distal end (end farther from the 
body) the femur flattens out, forming a 
hinge with one of the two lower leg bones, 
the tibia or shin bone. The other lower leg 
bone is the fibula, which is smaller and lies 
on the outside of the leg. Together with the 
tibia it affords a point of contact, in tvirn, 
with one of the two large ankle bones. The 
other forms the heel. These two, together 
with the metatarsals and phalanges, form 
the foot. This part of the skeleton is man's 
contact with the ground and is a very im- 
portant part of his anatomy. When some- 
thing goes wrong here he is practically 

There are two arches in the foot, longi- 
tudinal and transverse, which are primarily 
supported by stretched tendons that come 
from muscles in the lower leg. Being always 
under tension, they possess a resilience that 
puts a "spring in one's step" and they also 
take away the shock from sudden contact 
with the substratum. Flat feet may be 
caused by undue stress such as comes from 
overweight or they may be inherited. Such 
dislocation of the bones of the feet may 
cause considerable pain and make normal 
walking difficult. 

We have considered in some detail the 
arrangement of the structural units of the 
vertebrate skeleton. Let us now examine 
the composition of these units. 

The composition of bone 

If a long bone like a femur is cut in cross- 
section, it will be found to be hollow with 
a soft spongy material, the marrow, occupy- 
ing the cavity (Fig. 4-4). The outer por- 

tion is very hard and resists breaking. The 
tubular nature of the bone makes it even 
stronger than a solid piece of equal weight; 
to understand this, one has only to compare 
solid and tubular rods of steel with respect 
to strength where bending and twisting is 
concerned. The hard part of bone is com- 
posed of calcium carbonate, or lime, and 
potassium phosphate, as well as an organic 
matrix which resembles cartilage. This can 
easily be demonstrated by placing the bone 
in an acid solution which dissolves out the 
minerals, leaving the matrix. Although the 
bone still retains its original shape it is very 
soft and pliable and as such could certainly 
be of no use to an animal. On the other 
hand, the organic matrix can be removed 
by heating the bone for some time so that 
only the minerals are left. Such a bone also 
retains its original shape but if disturbed 
crumbles into ashes. Again a bone of this 
composition would be of no use to an ani- 
mal. Minerals and matrix taken together, 
then, are necessary to produce satisfac- 
tory material of which to construct skeletal 

Bone growth 

It is obvious that the bones of a child, 
while fully formed and quite solid, must in- 
crease both in length and diameter as 
growth occurs. This is accomplished by a 
rather elaborate bone-destroying and bone- 
building process going on within the bone 
itself. The bone is covered on the outside 
by a thin cellular membrane, the perios- 
teum, which has to do with the increase in 
the diameter of the bone. At the ends, 
called the epiphyses (singular — epiphysis), 
there is also active cellular growth which 
causes the increase in length. As bone is 
produced by both periosteum and epiphys- 
eal cells, a simultaneous bone destruction 
is going on within the marrow cavity. In 
other words, as the bone cells produce bone 
on the outside and at the ends of the bone, 
similar cells are destroying bone on the 


Fig. 15-6. Various ways in which animals from representative groups have solved the problem of 




inside. Thus the bone gradually becomes 
longer and increases in diameter. 

Although bone may seem dead, it is far 
from it, as was pointed out earlier (p. 72). 
The Haversian system (Fig. 4-4) consists 
of a canal in the center containing blood 
vessels and a nerve, surrounded by concen- 
tric rings of bony matrix, and between them 
scattered tiny spaces, lacunae, filled with 
the bone cells. Very tiny tubes (canaliculi) 
connect the bone cells with one another and 
the central canal, and it is through these 
canals that the cells are nourished and kept 
alive. These bone cells secrete the bony 
matrix in which they are entombed. It is as 
if a mason were to surround himself with 
a concrete wall of his own building and 
thus be enclosed in a chamber which he 
could never leave, but in which he would 
be kept alive by small portals through 
which nourishment could be supplied. 

Ability to move 

Nearly all animals from amoeba to man 
have the ability to locomote, and the few 
which lack this are still able to move some 
parts of their body (Fig. 15-6). Amoeba 
moves by a complex sol-gel reversal mech- 
anism which causes the pseudopodia to ex- 
tend and retract. In addition to being able 
to move its body in a worm-hke manner, 
Euglena has a contractile flagellum which 
propels it through the water. Paramecium is 
provided with numerous cilia that beat in 
unison to bring about its erratic move- 
ments. Hydra is the first animal with cells 
that contain muscle fibers which contract 
along an axis. It is the combined action of 
the many neuromuscular cells that makes it 
possible for this animal to contract and to 
extend itself in its movements. Once this 
type of movement, that is, muscular contrac- 
tion, had appeared in animals, it persisted 
through all subsequent forms. We shall, 
therefore, spend some time in studying mus- 
cles and their operation, and again man will 
serve our purpose as well as any other 


One of the most striking characteristics 
of animals is movement. Since they are 
voracious feeders they must be on the move 
most of the time in search of food, and, 
movement is thus imperative to their con- 
tinued existence. Among all but the Proto- 
zoa and perhaps a few others, contracting 
muscles are responsible for movement, not 
only of the body as a whole and its external 
appendages, but the internal organs as 
well, such as the organs of digestion and 
circulation. It is not surprising, therefore, 
that a man's body has more than 600 sepa- 
rate muscles. 

The way muscles work 

The muscle responds like a rubber band; 
it can do only one positive thing and that is 
contract. When it is not contracted it is said 
to be relaxed. The function of a muscle, 
then, is to pull two objects closer together. 
This means that there must be muscles 
which pull bones in one direction and those 
which pull the same bones back again ( Fig. 
15-7). Muscles working against one another 
are said to be antagonists. For example, by 
contraction of the large muscle in the front 
of the upper leg the bent leg straightens, as 
in kicking a ball. Once the leg is straight it 
must be bent again before another step or 
kick can be executed, and several large 
muscles on the back side of the leg carry 
out this movement. To be sure, there is no 
complete relaxation of one set of muscles 
during the contraction of their antagonists. 
Both contract some, the resultant action 
depending on how much each contracts. 
When bones are bent on one another the 
action is spoken of as flexion; when they 
are straightened out the action is described 
as extension. The example of kicking is a 
case of extension and flexion of the leg 
bones. Likewise, the closing of the hand is 
flexion; the reverse or opening of the hand 
is extension. Although there are many other 
types of muscle action, antagonistic ac- 




biceps brochi 

Fig. 15-7. Muscle action in the human arm and leg. In the upper left figure, the triceps brachii muscle contracts in 
extending or straightening the arm while the biceps brachii relaxes. In the lower left figure, the opposite action 
occurs, that is, the triceps brachii relaxes and the biceps brachii contracts. This flexes or bends the arm as in 
lifting. To rise on the toes as in walking the large gastrocnemius contracts (right). 

tion is the most common in the animal 
body. Antagonist muscles are not equally 
matched as to strength. For example, the 
muscle which raises the jaw is stronger than 
the one that lowers it. Hence, when bodi 
contract violently as they do in convulsions 
the jaw is closed tightly (lockjaw). 

Muscles vary considerably in size and 
shape, some being long and fusiform, 
whereas others are thin and flat (Fig. 
15-8). Most of them have a fleshy middle 
or belly part and two tapering ends which 
terminate in round or flat cords called 
tendons. Tendons consist of tough, fibrous 
tissue that attaches the muscle to the bone. 
The two ends of the muscle are identified 
by the amount of movement that takes 
place in the bones to which the tendons are 
attached. The end which moves the bone 
the greater distance is called the insertion; 
the end which moves the bone the shorter 
distance is the origin. Thus the biceps 
brachii muscle (Fig. 15-7) has its origin 
on the point of the scapula and its insertion 

on the radius because the latter bone 
moves the greater distance when contrac- 
tion occurs. 

Tendons act like cables, attaching a mus- 
cle to a bone sometimes at a considerable 
distance from the muscle. This is a very 
convenient arrangement because it makes 
possible the location of muscles some dis- 
tance from the point where action must 
occur. For example, the muscles that sup- 
port and operate the foot are located in 
the lower leg. If one feels the calf of his 
leg while standing, tlie tenseness of the 
muscles in supporting the body weight is 
clearly apparent. The large tendon at the 
heel, the tendon of Achilles (Fig. 15-7), is 
like a steel cable when one is standing, par- 
ticularly if he is on his toes. If the large calf 
muscles that are necessary in operating the 
foot were located in the foot itself the latter 
appendage would reach astounding propor- 
tions. Furthermore, the foot would function 
poorly as compared to the slim-ankled in- 
strument that is man's, or better, woman's 





.pccVorolis major 
inFrospinofos — 

rhomboidaus — — 
-sarroHis ouf «rior 

lafissimusdorsi — 

-cxfarnal oblique 

-rectus obdom'mus 

gluteus moximus 


■ quodriceps femoris 


biceps famoris- 

.tibiolis anterior 

-peroneus longus 

tendon oF Achilles 



Fig. 15-8. The human musculature, front and bock views. 



proud possession. This trend to longer ten- 
dons and concentration of the muscle's ac- 
tion farther from the muscle is most beauti- 
fully illustrated by the leg of the deer. Its 
lower leg is little more than skin, bones and 
tendons, yet all the power of the strong leg 
muscles is transmitted efficiently to the tiny 
digits that contact the ground. 

Muscle structure 

Muscle is the principal part of the meat 
that is bought at the market and it usually 
makes up about 40-50 per cent of the body 
weight of large animals. Viewed with the 
naked eye, muscle is seen encased in a 
sheath of connective tissue which often 
glistens. The more expensive cuts of meat 
have bits of fat rippling tlirough the muscle 
tissue; this simply means that the animal 
was fat and its obesity extended to its 
muscles. The color of the muscle may vary 
with the nature of tlie fibers, and with age; 
young mammals such as calves have lighter 
muscle tissue than older beef. Finally, mus- 
cles of the viscera possess a different tex- 
ture than those of the skeleton. 

By sectioning the various muscles of the 
body the real nature of the muscle can be 
studied. Cuts anywhere through the diges- 
tive tract will show smooth or involuntary 
muscle, the structure of which was de- 
scribed earlier (p. 72, Fig. 4-5). In man, 
smooth muscles are located in organs of di- 
gestion and in the skin, as well as in other 
places. They have to do with those move- 
ments which are not directly under volun- 
tary control, such as peristaltic movements 
of the digestive tract. These muscles are 
slow to respond to stimuli and the response 
that eventually occurs is of long duration. 
For example, certain pains arising in the 
abdominal region may be caused by the 
formation of gas in various parts of the in- 
testines. When the peristaltic wave pro- 
duces undue stretching of the gut the 
pain begins slowly, gradually increasing its 
intensity and finally passing away. This co- 
incides with the contraction of the smooth 

muscle. If one pricks the intestine of a frog 
with a sharp needle it may take from 1 to 10 
seconds before any reaction is noted, but 
once contraction starts it lasts for a minute 
or two, clearly demonstrating the charac- 
teristic of smooth muscle action. 

The microscopic anatomy of skeletal 
muscle was described earlier (p. 74, Fio-. 
4-5 ) , so here we need to mention only some 
of its characteristics. Within each muscle 
fiber lie numerous fibrils (tiny fibers) sus- 
pended in the more fluid protoplasm, the 
sapcoplasm. Differences in the relative 
amounts of sarcoplasm and fibrils make a 
difference in the appearance of voluntary 
muscle tissue. Muscle fibers that contain a 
great many fibrils and relatively little sarco- 
plasm are light in color and when the pro- 
portion is reversed the muscles are dark. In 
birds such as ducks, where sustained flight 
for long periods of time is essential, the 
breast muscle fibers contain more sarco- 
plasm and are therefore red, whereas the 
breast muscles of the domestic chicken 
which flies only short distances, if at all, 
are white. This is also true of such birds as 
grouse which fly in short bursts but never 
for extended periods. It seems that bird 
muscles designed for sustained activity are 
red, whereas those that contract for only 
short periods are white. 

Cardiac muscle, described in an earlier 
section (p. 74, Fig. 4-5), functions as a 
unit because of the nature of its cells. As a 
result of its sustained action, it is dark in 
color, as one might expect. 

Muscle action 

Even though one is not aware of it, the 
muscles of the body, both voluntary and 
involuntary, are under constant mild con- 
traction. This is essential, for one thing, to 
keep the blood vessels sufficiently small to 
maintain adequate blood pressure. This 
contraction can be observed when a bullet 
pierces a muscle. The bullet makes a round 
hole on its way through, but the resulting 
aperture is a slit, because of the slight 




Fig. 15-9. Muscle action can be studied by attaching an 
isolated muscle, such as the gastrocnemius muscle 
of the frog, to a lever which can scratch a line of 
its path on a smoked moving drum (kymograph). 
When the muscle is electrically stimulated, the nature 
of the contraction can be recorded on the smoked 

pull of the muscles. Considerable energy 
is utilized in maintaining this continuous 
contraction and, as in all muscle contrac- 
tions, a large portion of it is released in the 
form of heat. This heat helps to keep a 
constant body temperature. 

Muscles normally contract as a result of 
impulses coming to them through nerves. 
However, an isolated muscle can be made 
to contract if stimulated directly by an 
electrical current, even though all the 
nerves have been destroyed. The nature of 
the contraction can be studied by attaching 
the muscle to a recording device (Fig. 
15-9) and noting its action following stimu- 
lation. When the muscle first receives a 
very brief stimulus there is no visible evi- 
dence of anything happening. This period 
is known as the latent period (Fig. 15-10), 
and lasts about 0.01 second in the fros; mus- 
cle. Contraction then begins and continues 


I f 
latent period 

0.0\ second 

Fig. 15-10. This record was made when the frog gastroc- 
nemius muscle contracted and relaxed, using a re- 
cording device as shown in Fig. 15-9. Note the time 
required for each event to occur. 

for 0.04 second. This is immediately fol- 
lowed by a relaxation period that lasts 0.05 
second during which time there is a 
chemical readjustment taking place in the 
muscle (discussed below). If successive 
stimuli are increased in their frequency 
there will come a time when the contrac- 
tions will be superimposed upon one an- 
other until there is a sustained contraction 
which is greater than any derived from 
single stimuli (Fig. 15-11). This is called 
tetanus, and is what usually happens in 
most muscular contractions, however short. 
If a stimulus is given to an isolated frog 
heart muscle, contraction occurs, provided 

Fig. 15-11. This record shows that by applying stimuli 
to a skeletal muscle with gradually increasing fre- 
quency, contractions merge until there is a sustained 
contraction called tetanus. The contraction is stronger 
in tetanus than in the single contractions. 



the stimulus is sufficient to initiate a re- 
sponse. No matter how much the stimulus 
is increased, the resulting contraction re- 
mains the same. This fact has led to the 
establishment of the so-called "all or none" 
principle, which means simply that if the 
heart muscle contracts at all, it will do so 
to its greatest extent. The question arises as 
to whether or not this applies to striated 
muscle. Obviously such muscles contract in 
graded amounts because one can contract 
any of his muscles as much or as little as he 
likes. Here the principle does not apply to 
whole muscle, but to individual fibers or 
to motor units ( about 100 fibers ) . Although 
there still seems to be some question about 
it, the available evidence points to the 
fact that motor units do obey the "all or 
none" principle. Hence, the force with 
which a muscle contracts depends on how 
many motor units are stimulated. A mild 
contraction would result when only a very 
few were stimulated; a maximal contrac- 
tion, when all of the units received a 

Just how muscles contract is still an un- 
solved mystery, although a great deal is 
known about the chemical and physical 
changes that take place. The movement of 
a human body does not differ from the 
movement of a car alono; a street with 
respect to the basic requirements. Both 
require energy to accomplish the feat and 
that energy comes from oxidation, a process 
with which we are already familiar. Muscles 
require oxygen indirectly in burning a 
series of energy-rich organic compounds. 
It was once thought to be a rather simple 
process, because when the leg muscle of 
a frog was stimulated continuously lac- 
tic acid accumulated, which subsequently 
burned to carbon dioxide and water. Since 
glycogen simultaneously disappeared from 
the muscle, it was considered to be the 
source of energy. Someone, not satisfied 
with this simple answer, discovered that 
after stopping the formation of lactic acid 
from glycogen (using the specific poison, 

iodoacetic acid), the muscle continued to 
contract. It was also found that the muscle, 
if denied oxygen, would contract with just 
as much force as in the presence of an 
abundance of the gas. 

From where, then, did the energy come? 
Since there was no glycogen breakdown, 
there could be no lactic acid to burn to COo 
and HoO. This meant, of course, that hid- 
den in the muscle were some other sub- 
stances that released energy in a manner 
resembling that of oxidation. A diligent 
search revealed the presence of an organic 
phosphate, adenosine triphosphate (ATP 
for short), which is formed through oxida- 
tion and which changes suddenly to phos- 
phoric acid and another compound with 
the release of large quantities of energy. 
This is done anaerobically, that is, without 
oxygen. Located in tlie muscle fibrils is an- 
other substance called myosin, which is 
known to consist of long protein molecules, 
and it is thought that the actual shorten- 
ing of the fiber is due to a folding or con- 
traction of these myosin molecules. The 
energy for such an action is obtained from 
the adenosine triphosphate breakdown. 
There seems, then, to be a series of reac- 
tions, a chain reaction, that makes the con- 
traction of a muscle possible. The sub- 
stances involved have been enumerated but 
perhaps their roles may be made clearer if 
we put their reactions in the form of equa- 
tions, similar to those used in expressing 
chemical reactions: 

Contraction Phase 

Adenosine triphosphate -^ phosphoric acid 
+ adenosine diphosphate + energy (A) 

Relaxation or Recovery Phase 

Glycogen -^ lactic acid (20%) -^ CO2 + 

H2O + energy (B) 
(B) energy + phosphoric acid -f adenosine 

diphosphate -^ adenosine triphosphate 
(B) energy + lactic acid (80%) -> glycogen 
(B) energy -^ heat (body heat) 

From this it is seen that during contrac- 
tion the adenosine triphosphate breaks 



down to form phosphoric acid and adeno- 
sine diphosphate, releasing energy in a sud- 
den but controlled manner. There is no 
oxygen involved in this reaction, which ac- 
counts for the fact that a man can run a 
hundred yards without taking a breath. 
When the adenosine triphosphate has been 
expended, no further contraction can occur 
without its recovery. Such an exhausted 
person must remain quiet undergoing rapid 
respiration to supply sufficient oxygen to 
allow the next reactions to proceed. This 
involves glycogen breakdown to lactic acid 
(a rearrangement of the molecules) and 
the subsequent oxidation of the latter sub- 
stance to CO2 and HoO. This last step re- 
quires large quantities of oxygen, hence 
the deep breathing after severe exercise ( or 
during, if prolonged). The energy released 
from this reaction is utilized in three ways: 
part of it is utilized in restoring adenosine 
triphosphate, part of it to convert 80 per 
cent of the lactic acid back into glycogen, 
and the remaining part is converted into 
heat that keeps the body warm. It will be 
observed that the entire chain reaction re- 
sults in the most economical method of 
obtaining the greatest possible energy from 
the stored food products. It means that the 
animal body is unusually efficient, about 40 
per cent of the available energy being re- 
leased in the form of work, 60 per cent as 
heat. This is a very satisfactory figure when 
one considers that the best internal com- 
bustion engines rarely exceed 25 per cent. 
Returning to the runner, the reason why 
he could run the entire hundred vards with- 
out taking a breath was that his ATP 
was being used up, but when he terminated 
the run he was forced to remain quiet and 
breathe deeply for some time. During the 
run he was building up an oxygen debt, 
which he "paid back" during the heavy 
breathing period at the termination of the 
race. The obvious advantage of such a 
mechanism is that a muscle is ready to con- 
tract with all of its force on a moment's 
notice. It can contract until its reservoir of 

high-energy phosphate is exhausted; then it 
must stop and wait until the blood brings 
sufficient oxygen to restore its glycogen and 
ATP to the original unspent condition. This 
can be compared to a toy gun which oper- 
ates with a spring; once it is shot the spring 
must be tightened before it will shoot 

One of the great complaints of human 
beings is fatigue; mankind would never for- 
get the scientist who could discover a way 
of preventing its regular and persistent 
occurrence. From the foregoing discussion 
it is quite obvious that fatigue involves 
the accumulation of lactic acid and the ex- 
haustion of glycogen and ATP in the mus- 
cles, although this is not the entire story 
because in an intact animal fatigue is pro- 
nounced before there is an appreciable 
amount of lactic acid present in the mus- 
cles. Experiments have demonstrated that 
the site most susceptible to fatigue is the 
junction between the muscle and nerve, 
and not these organs themselves. 

The limitations for work are set by the 
ability of the body to restore exhausted 
organic compounds in the muscles; this de- 
pends indirectly on the functioning of the 
respiratory, circulatory, and excretory sys- 
tems which, in turn, the muscles must de- 
pend on to receive their quota of burning 
material (sugar and oxygen) and to carry 
away their accumulated wastes (urea and 
CO2 ). There are great individual differences 
among human beings for this capacity. 
Some get along well with very little sleep 
— Edison was such an example — whereas 
others require eight hours or more per day. 

Muscles can be developed to consider- 
able size and strength if they are constantly 
put to difficult tasks. By lifting heavy 
weights each day tlie muscles of the entire 
body will grow disproportionately large 
and will function very well in lifting heavy 
objects. If this is to be the life work of the 
individual it is wise to have such a set of 
muscles, just as it is wise for the man who 
handles a shovel all day long to have thick 



calluses on his palms. It seems a bit ridicu- it hardly seems necessary in our modern 

lous, however, for an office worker to de- living to make what is equivalent to a draft 

velop a set of muscles that would make it horse out of a person who is going to have 

possible for him to lift tremendous burdens no occasion to use his great strength. It is 

when his most muscle-provoking task each like using a ten-ton truck to carry a loaf o£ 

day is gliding a pen over a piece of paper, bread home from the store. 
Although a strong body is highly desirable, 



The business of coordination is obviously 
a fundamental problem from the very be- 
ginning, because even the tiniest single- 
celled animals have some method of co- 
ordinating their separate parts. Amoeba 
must decide which way it will throw out 
its pseudopods in order to move in a certain 

like nerve cells are seen in planaria, where 
there is a well-defined anterior brain with 
two large lateral nerve cords running poste- 
riorly. Being a larger animal composed of 
more cells, and a much more complicated 
animal, a more elaborate coordinating 
mechanism is essential. The idea of central- 

direction. Euplotes (Fig. 16-1) shows an ization in the coordinating mechanism is 
advanced degree of specialization because continued and elaborated through the in- 
it is able to control the rhythmic beating of vertebrate and vertebrate groups as animals 
its cilia so that the direction of progression become larger and more complicated. The 
can be changed suddenly. However, Meta- gro\\i:h and organization of the individual 
zoa such as hydra have a coordinating may be hkened to the expansion of a tele- 
mechanism in the form of a simple arrange- phone system as the small monohippic vil- 
ment of nerve cells, the nerve net. The first lage grows to a great city. As the latter in- 
steps toward a centralization of these net- . creases in size, the system becomes more 




Fig. 16-T. Animals from Protozoa to man have solved the problem of coordination in many y^ays. 



and more intricate until a telephone system 
like that of New York City is about as diffi- 
cult to understand as the nervous system of 
a grasshopper or a man. 

Among some of the higher invertebrates 
and all the vertebrates the network of tiny 
fibers connecting all parts of the animal has 
apparently proven inadequate, because a 
supplementary system has evolved, namely, 
an endocrine system. In this system an 
entirely different principle is employed; in- 
stead of impulses passing over tiny fibers, 
specific chemicals produced by special 
glands are released into the blood and cir- 
culate to other parts of the body where they 
produce a specific effect. This method has 
proven very satisfactory for certain types of 
responses, as will be pointed out a little 
later in this chapter. 

In handling this very complex problem 
of coordination we shall use man as our 
example. We shall begin with a discussion 
of stimuli from the external world and the 
internal environment as received by the 
sense organs and other receptors, then pro- 
ceed to the nervous system which is the 
intermediary for the transmission and inter- 
pretation of the stimuli, and conclude with 
the eflFectors — muscles and glands — which 
give the response. 


The receptors in the human body consist 
of specialized end organs located in stra- 
tegic regions and are highly sensitive to 
certain kinds of stimuli. Conspicuous sense 
organs such as the eye and ear are familiar 
to everyone; others, such as the tiny recep- 
tors located in the muscles and other parts 
of the internal body, are not so well known 
but are just as important in the proper co- 
ordination of the organism. 

Skin receptors 

It might be expected that the outer cov- 
ering of the body would be highly sensitive 
to the environment around it, and this is 

true, from the lowly planarian to man him- 
self. A pin prick in the skin almost any- 
where over the surface of the body results 
in a pain sensation; this fact indicates that 
these nerve endings are very numerous and 
widespread. The same is true of the nerve 
endings for touch, pressure, heat, and cold. 
A thin section of the human skin will reveal 
tiny, oval-shaped tactile corpuscles from 
which nerves lead inward. Any pressure 
brought to bear on them causes impulses 
to be discharged from the specialized cells 
within the corpuscle which travel along 
nerve fibers to the central nervous system. 
Other kinds of sensory end organs which 
respond to pressure stimuli over larger 
areas are located in the deeper skin and in 
many internal organs. Free nerve endings 
which register pain terminate in the epi- 
thelium within the internal organs as well 
as in the skin. The endings ramify and come 
into contact with nearly every cell, which 
explains why pain sensations are felt even 
if only a small area is stimulated, such as 
in pricking with a pin. 

By marking off specified areas on the skin 
and using a stiff bristle as a stimulus the 
appropriate receptors can be located, and 
they will be found to be quite unevenly dis- 
tributed over the body. It is difficult, for 
example, to distinguish two points one-half 
inch apart in the middle of the back, 
whereas on the tip of the finger or tongue 
a distance of one-sixteenth of an inch is 
perceptible. Likewise, if metal pointed in- 
struments (styluses) are used, the hot and 
cold end organs can be detected. There are 
more cold spots than hot spots and that is 
why, for instance, one shivers at first if sud- 
denly exposed to a hot shower. When all of 
the end organs are stimulated simultane- 
ously, as would be the case in the above 
situation, the total response is that of cold- 
ness at first because there are more of the 
cold than hot spots. Later, the proper in- 
terpretation of the stimulation is recog- 
nized. Pressure end organs can be found by 
applying a blunt metal stylus having the 



same temperature as the skin to various 
regions. Other sensory nerve endings are 
located in the tendons and muscles which 
respond to tension placed on these tendons 
and muscles. These are important in bal- 
ance and will be discussed under that topic 


All of the receptors of the skin have to 
do with identifying energy changes that oc- 
cur at or very near the body. In addition, 
there are chemo-receptors that identify sub- 

composite sensation which is called taste. 
The difference in the "taste" of hot and 
cold foods is due to stimuli other than those 
which are caused by dissolved chemicals. 
The sense of smell is also important to taste, 
as anyone with a bad cold is well aware. 
The end organs of taste are called taste 
buds and are distributed over the surface 
of the tongue, laryngeal region, and parts of 
the roof of the mouth ( Fig. 16-2 ) . They are 
oval-shaped bodies made up of several cells 
which terminate in a slender sensory proc- 
ess on tlie end toward the mouth cavity. 

olfactory tract. 

olfactory bulb. 
olPdc^ory nerv* 

taste troct 

tbngue with, 
haste areos 

Fig. 16-2. The end organs for chemo-reception 
and on the tongue (taste). They are shown 
ways that conduct the impulses to the brain. 

stances dissolved in the saliva of the mouth, 
giving one the sense of taste, and chemicals 
dissolved in the mucus of the nasal cham- 
bers, imparting the sense of smell. In the 
latter case, the organism is made aware of 
changes in its environment some distance 
away. The sense of smell is, in this respect, 
like the senses of hearing and seeing which 
extend perception to great distances. 

Taste. The so-called sense of taste is actu- 
ally a combination of stimuli coming from 
the mouth cavity. Stimuli from the end or- 
gans of touch, heat, and cold located in 
various parts of the mouth cavity give a 

taste bud 

are located in the nasal chambers (smell) 
here in detail together with the nerve path- 
There are four kinds of taste sensations 
and consequently there are four different 
kinds of taste buds, each with a rather spe- 
cific distribution on the tongue and other 
mouth parts (Fig. 16-2). The taste buds 
registering bitter are located at the base of 
the tongue, salt and sweet on the tip and 
sour along the edges. These can all be 
identified both microscopically and experi- 
mentally. No matter how these buds are 
stimulated the resultant sensation is always 
sweet, sour, salt, or bitter. Some chemicals 
stimulate two kinds of taste buds, but in 
each case the taste bud responds as it 



should according to its predetermined func- 
tion. There are some classes of substances 
which have a consistent taste, for example, 
acidic substances usually taste sour, basic 
substances bitter. The threshold (a stimu- 
lus that is just sufficiently strong to elicit a 
response) is very low for the sense of both 
taste and smell. For example, it is possible 
to taste quinine in concentrations of one 
part in two million, and much greater dilu- 
tions of odorous substances can be smelled. 

Smell. The olfactory end organs which 
are responsible for the sense of smell are 
located in the nasal membranes, and it is 
through these organs that gaseous chemical 
stimuli (odors) are received. While in man 
the receptive area in the two nasal chambers 
is only about 10 square centimeters, in most 
mammals it is much more extensive. It will 
be recalled that the sense of smell is far 
more important to ground dwellers than to 
those that live in trees, where keen vision 
is of more value. It is not surprising, there- 
fore, that when the primates took to the 
trees the sense of smell diminished and in 
the present primate is very poorly devel- 
oped. The dog, on the other hand, receives 
a great deal of information about the world 
through his nose. This sense is so keen that 
the dog can pick up the odor of an animal 
that has passed over a trail some hours 
before. Most game dogs rely more on their 
sense of smell than on sight except at close 
range, and, in fact, many bird dogs are very 

The olfactory cells give rise to fibers 
(Fig. 16-2) which coalesce, after passing 
through the cribiform plate (p. 378), to 
form the olfactory bulb, which becomes 
a large nerve leading to the brain. The 
location of the olfactory end organs is such 
as to protect them from the desiccating ef- 
fects of incoming currents of air during 
respiration, and the nasal passages are kept 
continuously moist in order that the incom- 
ing odors may dissolve in the fluid bathing 
them. A dry olfactory end organ cannot 

be stimulated any more than can a dry 

It may be necessary for large quantities 
of air to pass over the receptors before 
stimulation is possible. As more air passes 
over them more of tlie chemical in gaseous 
form becomes dissolved in the fluid of the 
nose, thus increasing the concentration to 
a point where the threshold is exceeded. 
This accounts for the constant sniffing of 
the dog, or man, too, on occasion, to bring 
more air into contact with the nasal epi- 

Our knowledge of the sense of smell is 
very limited, as indicated by the fact that 
no satisfactory system of classification of 
odors has yet been set up. Odors are still 
referred to by the name of the aromatic 
substance in question, and there are nearly 
as many names for odors as there are aro- 
matic chemicals. It seems highly improb- 
able that there is an infinite number of 
kinds of olfactory nerve endings, although 
when they become fatigued to one odor 
they seem to respond with normal vigor to 
another. Different chemicals apparently do 
not stimulate the same nerve endings. In 
general, the nerve endings fatigue readily, 
and it is a familiar experience that an odor 
which is very strong when one first enters a 
room soon fades away until it is unnoticed, 
not because the chemical in the air has 
diminished in quantity but because the 
olfactory end organs fail to be stimvilated 
beyond a certain brief period. Just why this 
is so is not clearly understood at present. 


This is the most perfect of the distance 
receptors. It provides us the means of keep- 
ing aware of our environment at small or 
great distances, whether reading this page 
or looking at the stars. Not only is this sense 
the most important for man but for all pri- 
mates, as well, and most of their informa- 
tion concerning their environment comes 
through this sense. 

Fig. 16-3. A few of the various kinds of photoreceptors that are found in representative animals. 



eye muscle 

suspensory liqarwent 

ciliary muscle 

Fig. 16-4. A longitudinal section of the human eye to show its internal structure. 

Nearly all animals from the simplest to 
the most complex are sensitive to light. Not 
that they all are sensitive to the same wave 
lengths that are recorded by the human 
eye, but nearly all have evolved some sort 
of receptor which is sensitive to light. It has 
been proved, for example, that the bee sees 
shorter light waves than we can see. Dogs, 
on the other hand, appear to be color-blind. 
It is thus abundantly clear that different 
animals see, hear, smell, and so forth, quite 
differently from man. Indeed one must 
carefully avoid an anthropomorphic atti- 
tude with respect to all behavior. 

Euglena orients itself with respect to 
light by means of the stigma, and without 
this photo-sensitive organelle it would be 
unable to seek proper illumination for 
photosynthesis (Fig. 16-3). The delicate 
jellyfish {Gonionemus) possesses photore- 
ceptors that help orient it to light in the 
ocean. Planaria has rudimentary eyes which 
produce no images but detect direction. 
Neanthes has a set of four eyes which may 
be superior to those of planaria. The lobster 
has excellent compound eyes, the details of 
which were discussed earlier. The octopus, 
as well as some of its relatives, possesses a 
remarkably perfect eye. Strangely enough, 
although it has evolved along an entirely 

different path, it resembles the vertebrate 
eye very closely in most respects. This is 
one of the very interesting cases of conver- 
gent evolution, where very similar organs 
have evolved along two entirely different 
routes (p. 660). 

The vertebrate eye. Although there are 
some minor differences in the eyes of vari- 
ous vertebrates, the human eye will serve 
as representative of the group in our dis- 
cussion (Fig. 16-4). It is first necessary 
to consider briefly the way light behaves 
before an understanding can be had of the 
function of the various parts of the eye, par- 
ticularly the lens. 

Light travels in straight lines at a speed 
of 186,000 miles per second in air, but it 
travels at different rates in other media such 
as water, glass, and transparent tissues such 
as the lens. Therefore, when light passes 
from one medium to another it bends (re- 
fracts). It is a familiar fact to those who 
have observed it that when a stick lies at 
an angle partially in water, it appears bent. 
Actually, of course, the light is coming to 
the eye through two different media, water 
and air, and at different speeds, hence the 
bending at the juncture of the two media. 
Light coming through glass is bent in a 
similar fashion, and when the glass is 



Fig. 16-5. A comparison of the camera and the human eye. Note the likenesses and differences. (For names 

of parts of the eye see Fig. 16-4.) 

shaped so that it is uniformly curved on 
both sides ( a convex lens ) the rays of liejht 
are bent toward one another so that they 
come to a point, or focus, as it is called. Just 
how light passes through the lens depends 
on the angle at which it strikes the surface. 
The amount of bending; increases with the 
increase in angle between the hght ray and 
the surface of the glass. Therefore, a highly 
curved surface will bring the rays to focus 
at a very short distance (focal length), 
whereas a more flattened surface will bring 
them together at a greater distance. De- 
pending on conditions, therefore, a lens is 
said to have a long or short focal length. 
For example, in the objectives of the com- 
pound microscope the low power lens has 
a focal length of 16 mm., whereas the high 
power lens has one of only 4 mm. The lens 
with the shorter focal length has the greater 
curvature and consequently magnifies the 
greater, also. 

Light coming from an object on one side 
of a convex lens passes through and comes 
to focus on the other side at the focal length 
of the lens. The light comes from an infinite 
number of points on the object, and passes 

through the lens with the result that the 
image is completely reversed. With this 
knowledge of the working of the convex 
lens we can better understand how the eye 

The remarkable similarity of a simple 
camera to the eye will help us to under- 
stand how we see (Fig. 16-5). Both have 
convex lenses which bring the rays of light 
to focus upon a sensitive plate, the film in 
the camera and the retina in the eye. The 
amount of light entering the chamber is 
controlled by the iris diaphragm in both 
cases. The housing is a lightproof case 
which allows the rays of light to pass 
through unobstructed; in the camera it is 
made of an adjustable tube or collapsible 
bellows and in the eye it is composed of a 
tough outer covering, the sclerotic coat and 
a highly pigmented inner lining, the cho- 
roid coat (Fig. 16-4). In the eye the space 
behind the lens is filled with a semi-liquid 
substance, the vitreous humor, and the cav- 
ity in front of the lens is occupied by the 
aqueous (watery) humor. These maintain 
pressure within the eye and keep it from 





pigment choroid 
cell cell 

circular muscles con'Yracfed 

closeup vision 
lens thicte 

Fig. 16-6. Accommodation is accomplished by changing 
light entering the eye is controlled by the iris diaph 

There are some very important differ- 
ences between the camera and the eye. 
First of all, the film and the retina function 
differently. Once a picture is taken the film 
is used up, that is, it must be replaced by 
another, whereas in the retina a continuous 
series of pictures can be recorded without 
exhaustion of the sensitive cells. The retina 
might be compared to the film in a movie 
camera which is constantly being replaced. 
Secondly, the method of bringing objects 
at different distances into focus on the sen- 
sitive plate operates differently, although 
the end result is the same. In the camera 
the lens maintains one shape and must be 
moved forward and backward until the 
image is in focus on tlie film. In the human 
eye, however, the lens itself changes its 

radial muscles contracted 

the thickness and curvature of the lens. The amount of 
ragm. The image recording mechanism is the retina. 

shape, becoming more curved for close ob- 
jects and less curved for distant objects. 
This change in shape of the lens is called 
accommodation. Aside from the lack of a 
shutter in the eye, unless the eyelids could 
be so considered, there are no other differ- 
ences. It is unfortunate in many ways that 
the human eye cannot be focused like a 
camera, for if it could many people would 
be relieved of placing glass lenses in front 
of their eyes, some all their lives, others 
during most of the "down hill" years. 

In the eye, the job of bringing the light 
rays to a focus upon the retina is done by 
the cornea; only the slight differences that 
are needed to produce a sharp picture are 
accomplished by the lens itself. This slight 
variation, however, makes the difference 


between clear, sharp vision and poor, fuzzy light) and cones (sensitive to colors) (Fig. 
sight. Accommodation is effected by the ac- 16-6 ) . The cones are crowded around a 
tion of the ciliary muscle, located near the central region, the fovea centralis, where 
point of attachment of the suspensory liga- visual acuity is most pronounced. Else- 
ment which supports the lens (Fig. 16-6). where in the retina the cones are mixed 
When this muscle contracts the tension of with the rods and vision is not so clear. In 
the ligament is relaxed and the lens be- order to see clearly, it is necessary to look 
comes more curved, that is, thicker, which directly at the object so that the image falls 
brings near objects into focus. There is also on the fovea — all other vision is peripheral 
a change of internal pressures in the eye- and is less clear. This is readily demon- 
ball that influence the change in lens curva- strated by attempting to determine detail 
ture. When the muscle relaxes the ligament while looking to one side of an object, 
tightens and the pressures are shifted so The rods are more sensitive to light than 
that the lens flattens out, causing distant the cones and detection of weak light 
objects to come to focus on the retina, sources is best made when looking to one 
Therefore, the ciliary muscle is active only side of the source. Fliers search for beams 
when one is looking at close objects ( under at night with their peripheral rather than 
30 feet) which is the reason why the eyes foveal vision. Careful observation by scien- 
can be rested by looking out the window at tists has shown that the immediate stimulus 
a distant object. is probably chemical, much the same as 

The amount of hght entering the eye is with the senses of smell and taste. The rod 

controlled by a sheet of circular muscular cells of the retina contain a purplish red 

tissue, the iris, which contains the pigment pigment, visual purple ( rhodopsin ) , which 

granules responsible for eye color. Both breaks down into a protein and a substance 

radial and circular muscles are present in called retinene when exposed to light. A 

the iris and it is the antagonistic action of further degradation occurs, producing vita- 

these muscles that do the job of enlarging min A, a famfliar accessory food. The chain 

or constricting the opening, the pupfl. They of chemical events, thus initiated, leads to 

are under the control of the autonomic that physico-chemical condition wliich is 

nervous system and therefore beyond vol- the nervous impulse and which is propa- 

untary control. When the eye is exposed to gated along an optic nerve fiber to the 

bright light the circular muscles contract, brain. 

constricting the pupil, whereas in dim light Since vitamin A is produced upon visual 

the radial muscles contract, causing dfla- purple breakdown, it must also be essential 

tion of the pupil. Thus a delicate arrange- in its formation. People who have a defi- 

ment is provided to project just the right ciency of this vitamin suffer from "night 

amount of light on the retina to obtain the blindness," that is, they cannot see in dim 

best possible picture reception of the exter- light. A person who upon entering a mov- 

nal world. ing picture theater in the afternoon cannot 

Exactly how hght rays are transformed see the individual sitting next to him within 

into the nerve impulses that pass over the fifteen minutes had best increase the 

optic nerve to the brain is only poorly un- amount of vitamin A in his diet, 

derstood at the present time. The conver- Visual purple is derived from the rods 

sion from light energy to nerve energy takes only. It has recently been discovered that 

place in the retina, a very delicate and com- the cones in some reptiles and birds each 

plex structure. It is composed of numerous contain one of four different pigments: red, 

cells, some of which are sensitive to light, orange, yellow, and white. The pigment is 

These are the rods (sensitive to white in the form of a globule resting on the tip 






far sigh ted 



Fig. 16-7. Common human eye defects and how they are corrected with lenses (spectacles). 

of each cone. All light passing through the 
globule is filtered out except that of the 
particular globule. The various colored 
cones are scattered through the retina so 
that color images are possible. It is prob- 
ably the reduction or the lack of certain 
cones that is responsible for color-blind- 
ness. It is interesting to note that chickens 
have mostly cones with very few rods in 
their retinas and therefore do not see well 
in dim light, accounting for the fact that 
they go to roost with the setting sun. 

Eye defects. The inability of many hu- 
man eyes to produce clear images under 
usual conditions is evidenced by the 
large number of people wearing glasses. 
Perhaps the number of defects is no larger 
today than formerly, but the demands for 
clear vision in modern society are far 
greater than ever before, so that greater 
effort to correct these defects has been 
made. It is highly important that in driving 
an automobile the driver should have good 
vision. Like other defects in present-day 
civilization, congenitally poor vision is fos- 
tered and passed on to succeeding genera- 
tions. In primitive man such a defect would 
have often prevented its owner from grow- 
ing to adulthood, thus eliminating the de- 
fect before it got a "foothold." 

The most common defects of the eye are 
caused by the inability of the focusing 
mechanism (cornea and lens) to form an 
image on the retina in a clear form. If the 
rays come to focus in front of the retina, 
the person suffers from myopia, or near- 
sightedness; if they come to focus behind 
the retina, the defect is know as hyperopia 
or farsightedness (Fig. 16-7). In order to 
correct these conditions it is only necessary 
to place in front of the eye a lens ground 
so that it bends the rays of light just 
enough to compensate for the defective 
focusing mechanism. In the case of near- 
sightedness a biconcave lens is needed, 
while in farsightedness a biconvex lens will 
bring about the proper correction. When 
either the cornea or lens is irregular in its 
curvature the defect is called astigmatism. 
This can be corrected by using lens that are 
ground in such a manner as to compensate 
for these variations in curvature. 

Sometimes, as a result of disease or for 
other reasons, either or both the lens and 
the cornea may become fogged over so that 
vision is dimmed or completely obliterated; 
this defect is referred to as cataract. If the 
lens becomes fogged, it can be removed 
and a substitute lens placed in front of the 
eye either in the form of conventional 



glasses or of contact lenses which fit tightly 
against the eyeball. If the cornea clouds 
over it can be replaced by one from a nor- 
mal eye, restoring the vision to normal. This 
is an extremely delicate operation but one 
that is becoming more and more common. 
People often "will" their corneas to others 
at the time of death; this custom is preva- 
lent enough now that cornea "banks" have 
been established. 


Hearing is another sense that records 
stimuli coming from a distance. This is a 
convenient supplement to vision in that 
sound travels around corners and in the 
dark, two very important adjuncts to the 
problem of orientation. Nearly all groups 
of animals have some means of receiving 
water or air vibrations (Fig. 16-8). Under- 
water forms such as fish receive vibrations 
in the water and can, therefore, hear, al- 
though they possess none of the apparatus 
found in higher vertebrates that is used to 
receive and amplify sound. They do possess 
the inner ear structures that are essential 
for hearing. Apparently they can receive 
only vibrations set up in their own bodies. 
Air breathers, both vertebrate and inverte- 
brate, have not only solved the problem of 
receiving air vibrations but also of setting 
up such vibrations. The katydid, for exam- 
ple, produces its chirp by rubbing its wings 
together, and the sound waves are received 
by special "ears" located on the tibias of 
the front legs. This form of communication 
serves in bringing the sexes together in 
mating and may also be important in noti- 
fying other members of the species of fer- 
tile food sources. 

When the vertebrates evolved onto land 
they, too, started to solve the problem of 
sound making and sound receiving in air, 
although fish not only received sounds in 
water but some even made sounds. The 
frog has a crude ear and emits a very simple 
sound. It is interesting to note that in pro- 
viding for this change old structures, the 

gill arches, which now served no special 
function, were employed in making the new 
organs. For example, the tips of the jaws 
and the hyoid arch which evolved earlier 
from gill arches became the ear bones, and 
some of the remaining gill arches became 
the larynx (voice box) as well as other 
elements of the upper respiratory system 
(Fig. 25-11). In man, the production of 
sound has probably been carried to its 
greatest perfection, birds excepted, al- 
though the sound receiving apparatus of 
some other vertebrates, such as the dog, 
has a greater range than the human ear. 

Intimately associated with and usually 
considered as a part of the ear in the ver- 
tebrates is another organ that is physiologi- 
cally quite separate from the sense of hear- 
ing. This is the organ of equilibrium, which 
is a receptor for changes in conditions of 
balance and rotation. In the invertebrates 
these two organs are quite unrelated. Or- 
gans of equilibrium are found in many 
invertebrates, from the jellyfish to the cray- 

Among the lower vertebrates such as the 
sharks and skates both the hearing organ 
and the organ of balance are connected to 
the brain through the eighth cranial or audi- 
tory nerve. It is generally believed that 
hearing came later in evolution and merely 
took over a part of the eighth cranial nerve 
which had initially functioned only in con- 
veying impulses from the organ of equi- 
librium. A discussion of the organ of equi- 
librium will be found in a later section 
(p. 408). 

The human ear is composed of three por- 
tions, the outer, or pinna, the middle, which 
connects with the throat by means of the 
eustachian tube, and the inner, where the 
cochlea (sound receptor) and semicircular 
canals are located (Fig. 16-9). The outer 
ear is merely a skin-covered, cartilaginous 
projection from the head, designed to catch 
and concentrate sound waves. In some ani- 
mals, such as the mule and rabbit, this can 
be moved in several directions to help de- 

Fig. 16-8. Some animals and the manner in which they produce and receive vibrations in the air and 




auditory nerve 


eustachian tube 

Fig. 16-9. The human ear dissected in order to show the various parts that have to do v/lth receiving sound and 

the part that is concerned with balance. 

termine the source of the sound. In such 
forms where the appendages are especially 
enlarged, they also function as temperature 
regulating mechanisms, for their great size 
and profuse vascularization provides an 
ideal apparatus for cooling the blood. In 
manj the function of the pinnae is dubious, 
since auditory acuity is no greater in those 
with generous pinnae than in those who 
have small external ears. Furthermore, 
movement of the ears is limited to a privi- 
leged few, even though everyone possesses 
a full set of muscles to accomplish the feat 
(Fig. 25-14). The external ear surrounds 
the opening which leads through the audi- 
tory canal and terminates at the tympanum, 
or eardrum. The walls of the canal are sup- 
plied with glands that produce wax which 
discourages small creatures such as insects 
from entering. 

The middle ear consists of a chamber 
connected to the pharynx by the eustachian 
tube, an old gill pouch remnant. Bridging 
across this air chamber is a series of three 
bones (ear ossicles) which conduct vibra- 
tions of the tympanum to the cochlea. Al- 

though the eustachian tube is advantageous 
in equalizing the pressure on both sides of 
the eardrum, it does have a disadvantage 
in that microorganisms in the mouth can 
make their way through this tube and infect 
the middle ear region. Such infections can 
be dangerous, sometimes leading to deaf- 
ness. The tiny bones are named the ham- 
mer, anvil, and stirrup ( malleus, incus, and 
stapes, respectively) because of their 
shapes (Fig. 16-9). Together they produce 
a lever arm which diminishes the amplitude 
of the tympanic vibrations but at the same 
time intensifies them. The hammer is at- 
tached to the eardrum and the anvil, while 
the stirrup, attached to the anvil at its op- 
posite end, fits into the oval window of the 
cochlea. Vibrations conducted through the 
chain of bones are conveyed to the liquid- 
filled cochlea. Another membrane-covered 
opening, the round window, allows the fluid 
to vibrate freely, without being lost from 
the closed chambers. Since liquids do not 
compress, the vibrations retain all of their 
vigor until they are delivered to the sensory 
cells which generate impulses that are 



-62 vibrations 
per second 


hair cells 

Jt^ -basilar membrane 

organ of Corti 

Fig. 16-10. The cochlea uncoiled and cut in cross-section to show the organ of Corti in detail. 
The parallel lines to the left indicate the wave lengths of the various notes and the ap- 
proximate position in the cochlea where the organ of Corti picks them up. 

transmitted to the brain over the auditory which is divided into three chambers, all 

nerve. filled with fluid. The middle chamber con- 

The highly complex cochlea can best be tains the organ of Corti (named after its 

studied by uncoiling and cutting across it discoverer) which is the most important 

in order to examine its internal structures part of the vibration-receiving mechanism. 

(Fig. 16-10). It is a long tapering tube It consists of a basilar membrane composed 



of tightly stretched connective tissue fibers 
which are longer at the top of the spiral and 
shorter at the larger or lower end. Above 
this membrane are the so-called hair cells 
which send out nerve fibers that make up 
a part of the auditory nerve. Overlying the 
hair cells is the tectorial membrane, a thin 
sheet of tissue, which lies very close to the 
tiny hairs projecting from the hair cells. 
There are other structures too which, how- 
ever, need not be considered in this discus- 
sion. How air vibrations are converted into 
impulses that give the sensation called 
hearing is the next problem. 

Sound perception. The fluid in the canals 
of the cochlea is set into vibration by the 
stirrup entering the oval window. It seems 
probable, although no one has seen it, that 
these vibrations set the basilar membrane 
into sympathetic undulations which cause 
the hair cells to touch the tectorial mem- 
brane, and that such touch stimuli set up 
impulses in the nerve fibers of the hair cells. 
Since the basilar membrane is shorter and 
tighter at the bottom, or largest, portion 
of the cochlea (which seems quite con- 
trary to reason), it probably responds to 
the higher notes whereas the hair cells at 
the top of the spiral are stimulated by the 
low notes. This can be borne out be experi- 
mentation. If an experimental animal is ex- 
posed to sound of high pitch and consid- 
erable amplitude (loudness) for a long 
period of time, the hair cells in the lower 
end of the cochlea are destroyed and the 
animal is deaf over this range. Other seg- 
ments of the organ of Corti can be de- 
stroyed in a similar manner by appropriate 
frequencies. The same phenomenon has 
been observed in people suffering from the 
so-called "Boiler Makers" disease where the 
constant din of a trip hammer over many 
years finally destroys that portion of the 
organ which records the same pitch as that 
of the hammer. It seems, then, that vibra- 
tions coming in stimulate various parts of 
the organ of Corti according to the fre- 
quency of the vibration (pitch). When vi- 

brations of various pitches come in simul- 
taneously the corresponding segments must 
be stimulated. This is quite remarkable 
when it is recalled how many different 
tones can be distinguished when listening 
to a symphony, for example. Certainly the 
vertebrate ear is one of the most amazingly 
complex organs found in any living thing. 

Hearing defects. The human ear can de- 
tect low and high tones ranging from about 
20 to 16,000 vibrations per second. A 
young child can hear even greater ranges, 
and some animals such as the dog and bat 
can hear vibrations considerably beyond 
that detected by the human ear. In man, the 
range diminishes in the upper limits with 
advancing years. 

Deafness is usually caused either by 
faulty transmission of sound through the 
middle ear or by some difficulty in the 
cochlea, rarely by any deficiency in the cen- 
tral nervous system. One of the common 
causes of deafness is middle ear infections. 
Infectious bacteria can make tlieir way up 
the eustachian tube to the middle ear 
where they can cause damage to the ear 
bones or drum. Such infections can often 
be cleared up by puncturing the eardrum 
to allow the pus to drain out through the 
ear canal, thus preventing severe damage 
to the ear bones and drum. Repeated infec- 
tions usually result in some impairment of 
hearing. With the advent of antibiotics such 
infections are not as common in children 
as they once were and the coming genera- 
tions should suffer less from deafness. 

With the refinement of electronic equip- 
ment, hearing aids have gradually been 
perfected to a point where they are very 
useful to those who have imperfections in 
the transmitting portion of their ears. Ob- 
viously defects in the cochlea cannot be 
compensated for by hearing aids. These in- 
struments merely amplify the tones so that 
sluggish or imperfect ear bones will, by 
sheer force of the vibrations, pick up and 
transmit them to the inner ear. The wear- 
ing of hearing aids should cause one no 



more concern than the wearing of glasses 
and it is gratifying to see these devices 
gradually being accepted socially. 

Sense of balance 

The sense of body position, both static 
and dynamic, is very important to an ani- 
mal. The significance of this statement can 
be observed by watching the movements of 
an animal in which the balancing organs 
have been destroyed or by observing a per- 
son who has defective organs of equilib- 
rium. In either case, when the eyes are 
closed there is loss of control in maintaining 
normal body position. What, then, are the 
organs that control balance and body posi- 
tion and movement? 

There are three vastly different organs 
which have to do with tlie sense of balance 
and movement. The eyes, when functioning 
as indicated in the preceding paragraph, 
operate continually in this capacity. A sec- 
ond very important organ of equilibrium 
is a portion of the inner ear, the non-acous- 
tic labyrinth, while the third organ is the 
system of proprioceptors, which are tiny 
sensory endings located in the muscles and 
tendons. Let us consider the non-acoustic 
labyrinth and proprioceptors a little further. 
The non-acoustic labyrinth is composed 
of three semicircular canals and two small 
chambers, the utriculus and the sacculus 
(Fig. 16-11). While experimental evidence 
now shows clearly that even fishes hear, the 
cochlea is developed only in the higher ver- 
tebrates. In all vertebrates, moreover, the 
inner ear includes the organ of equilibrium. 
The parts of tlie non-acoustic labyrinth 
operate differently. The semicircular canals 
function when change in rate or direction 
of movement of the head occurs, and the 
utriculus and sacculus have to do with the 
position of the head. 

The three canals on each side are semi- 
circular tubes arranged so that each is at 
right angles to the others (Fig. 16-11); the 
importance of this arrangement becomes 
apparent when it is considered that no mat- 

ter how the head is moved two of the canals 
(one in each ear) function. They are fluid- 
filled and each has a small swelling called 
the ampulla which houses a tuft of hair 
cells that are sensitive to the movement of 
the fluid. Any acceleration or deceleration 
of head movement causes the fluid to flow 
in definite directions in the canals, thus 
stimulating the hair cells to send impulses 
to the brain which results in the sensation 
of movement. When this occurs in a hori- 
zontal plane — the one people are accus- 
tomed to — no particularly unpleasant sen- 
sation results. However, if the head is 
moved in a vertical plane, such as in the 
abnormal movements of flying or riding in 
an elevator, some very unpleasant sensa- 
tions are experienced; in fact, they may 
become so distasteful that nausea occurs, 
as in seasickness. Just why such movements 
should affect the stomach and bring about 
disagreeable feelings is not very clear. For- 
tunately, one can usually become accus- 
tomed to such movements so that eventu- 
ally there is no more response to them than 
to horizontal movement. 

Head position, that is, static position, is 
determined by the utriculus and sacculus. 
These two chambers are also filled with a 
fluid, and, in addition, each contains a tiny 
lime pebble (the otolith) attached to the 
sensory hair cells. Since the otolith is free 
to move in the chamber, when the head 
changes its position the pull of gravity shifts 
the tiny weight so that the hairs are bent 
in synchrony with its movement (Fig. 
16-11). This is very similar to the operation 
of the statocyst in the crayfish (p. 218). It 
is the perception of this movement that 
makes a person conscious of the position of 
the body with respect to gravity. 

Not only is it important to register the 
position and movement of the body as a 
whole but, if coordination of all the com- 
plex movements is to be had, there must be 
some way of bringing about this interrela- 
tionship. This is done by the hundreds of 
proprioceptors, tiny sensory endings lo- 

bcod upright 

endolympbotic spocc 
pcrilyrwpbatic space 

b«ad +iltecl- right 

Fig. 16-11. The non-acoustic labyrinth with the sacculus and one of the semicircular canals cut in 
cross-section to show their internal structures. The lower figures indicate schematically how the 
otoliths aid in setting up stimuli coordinated with head movements. 



cated in each muscle and tendon. These 
are sensitive to changes in tension. As 
the various muscles contract, propriocep- 
tors send out impulses which eventu- 
ally bring about an intei-play of various or 
all muscles and tendons to perform a co- 
ordinated act. This goes on without con- 
scious knowledge and functions without 
vision, since, for example, a person is able 
to play a piano in the dark. It is not only 
necessary to know that a muscle is contract- 
ing but also to know where the appendage 
is at all times. In such a simple action as 
grasping a fork and bringing some food to 
the mouth it is necessary to contract the 
proper muscles to extend the arm. Hex the 
lingers on the fork, and further flex the arm 
in order that the fork be brought to the 
mouth. The contraction must be of just the 
right amount or the fork will over- or under- 
shoot its target with resulting catastrophe. 
It is the proprioceptors that regulate the 
amount of contraction to bring this act to 
fruition. In a way, the muscles can be con- 
sidered sense organs because sensory im- 
pulses are coming from them almost as 
rapidly as motor impulses are going to 
them, though the cells involved in the two 
cases are different. The tendons are also 
abundantly supplied with proprioceptor 
end organs but they receive no motor 
nerves and cannot contract . 

It is because of the proprioceptors that 
one can judge weight. If a 24-pound weight 
is compared with a 25-pound weight, it is 
difficult to distinguish between them. There 
is no difficulty, however, in deciding that 
a 5-pound object weighs more than a 4- 
pound weight. In other words, one can de- 
termine the relative differences in weight 
and the heavier the objects the greater must 
be the difference to be discernible. For ex- 
ample, in the illustration above, there is 
a one-pound difference between the two 
lighter objects which is a 20 per cent dif- 
ference; in the heavier weights, although 
there is also a difference of one pound, the 
relative difference is 4 per cent. People 

show considerable variation in their ability 
to detect this difference and this skill prob- 
ably plays an important role in manual dex- 
terity. Some people have great difficulty in 
using their hands effectively whereas others 
are very proficient in this regard. In select- 
ing a life work such ability should be taken 
into consideration, and certain kinds of 
vocational aptitude tests are designed to 
determine this ability. 

Now that we have learned something 
about how stimuli are received we must 
consider next the portion of the coordinat- 
ing mechanism that adjusts these incoming 
impulses. This is the central nervous system 
with all of its ramifying nerves. 


The nervous system 

When an animal is dissected, the brain 
together with its many ramifying nerves 
is most conspicuous even to the beginning 
anatomist. Early morphologists, however, 
thought the brain was an inert part of the 
body and could not assign a function to it. 
Today the anatomy of the nervous system 
is well known, although the last word is 
far from being written concerning its func- 

In its evolution it might be expected that 
the nervous system originated from the ex- 
ternal part of the body because it is this 
part of the organism that contacts the out- 
side world. Embryology and the history of 
the animal groups both bear this out. As 
animals became more complex, certain cells 
in the outside layer became specialized to 
receive and transmit stimuli. These cells 
later confined their entire attention to the 
job of receiving stimuli and thus became 
the receptors or sense organs such as those 
of the eye and the ear. Transmission of the 
impulses was left to other cells which com- 
bined to form the sensory nerves. These, in 
turn, carried the impulses to a central sta- 
tion, the brain and spinal cord, where 
adjustment and interpretation took place. 



mo^or naupon 


sensory neuron 

Fig. 16-12. Neurons, motor and sensory. A myelinated nerve fiber is also shown in cross-section. The two neurons 

constitute a reflex arc. 

Another set, the motor nerves, carried the 
adjusted impulse to the glands and mus- 
cles where the final response was executed. 
With such a system it was possible for an 
animal to reach great size and complexity 
and still be intricately coordinated. The 
analogy of the centralized telephone system 
may be recalled to good advantage at this 

It may seem surprising to learn that this 
very complex system is composed of only 
one general kind of cell, the neuron. Here 
the telephone analogy breaks down, be- 
cause, while the organization may be simi- 
lar, the nature of the wire itself is vastly 
different from the neurons which go to 
make up the entire nervous system of ani- 

The neuron. Although all cells may con- 
duct an impulse within themselves, the 

neuron has developed this characteristic to 
an advanced stage. Anatomically the neu- 
ron is divided into three parts: the cell 
body, or cyton, containing the nucleus, the 
dendrites, which consist of numerous proto- 
plasmic outgrowths from the cell body, and 
the axon, a single, much longer extension 
terminating in a brush-like filament (Fig. 
16-12). The cyton resembles many other 
cells in appearance and the dendrites 
merely extend its surface of contact. The 
axons are usually covered with a fat-con- 
taining myelin sheath which functions like 
an electrical insulator. The axons are usu- 
ally long and an extreme example is illus- 
trated by the giraffe, where they extend 
from the tip of the toe to the back, a dis- 
tance of 6 feet or more. Many axons are 
bound together to form the nerve trunk, 
spoken of as the "nerve" by anatomists, 



which is covered by a tough sheet of tissue. 
This is remarkably similar to a telephone 
cable where each wire is insulated from all 
the others. 

There are several major kinds of neurons 
located in specific parts of the nervous sys- 
tem. In the brain and cord they are highly 
specialized and occur only in certain loca- 
tions. These neurons are called association 
neurons, and they function in connecting 
various parts of the brain and cord. Those 
that conduct impulses from the distal parts 
of the body to the brain or cord are called 
sensory or afferent neurons and those that 
conduct the impulses away from the brain 
and cord are called motor or efferent neu- 
rons. Some nerve trunks are composed 
entirely of one or tlie other, in which case 
they are called sensory or motor nerves. 
Most, however, carry both kinds of fibers 
and are called mixed nerves. In the verte- 
brates the cytons for the sensory neurons lie 
on each spinal nerve in a swelling, the dor- 
sal root ganglion, just outside the cord, 
whereas those for the motor neurons lie 
within the cord itself in a region that is 
grayish in color (Fig. 16-16). 

In order for a nerve impulse to complete 
its circuit, it must pass over more than one 
neuron; in fact, a great many are probably 
involved even in the most simple action. 
Neurons are not directly connected with 
each other but come in close association 
only. The region or area where the dendrite 
of one neuron is in close proximity with the 
axon of another is known as the synapse 
(Fig. 16-12). This is a very important part 
of the nervous system because it is here 
that a selection is made as to whether or 
not an impulse is permitted to pass on to 
the next neuron. The impulse can travel 
both ways within a neuron but where the 
neurons are in a series, as they always are, 
the impulse travels toward the cell body on 
the dendrites and away from it on the axon. 
The synapse, therefore, acts like a traffic 
signal on a one-way street. 

Nature of the nerve impulse. Up to the 

present time, attempts to solve the nature 
of the nerve impulse have been made only 
with peripheral nerves, that is, those out- 
side the cord and brain. Very little progress 
has been made toward an understanding 
of how they work within the brain itself, 
though there is no reason to doubt that in 
a general way the functioning is similar. 
However, our notions of how conscious- 
ness, reasoning, memory, thought, and so 
forth, are carried on is purely in the con- 
jectural stage today. Perhaps an under- 
standing will be reached some time; if and 
when that happens an understanding of life 
itself will undoubtedly be had. One can 
only conjecture to what extent the human 
brain may be able to comprehend its own 

The simplest way to study the nerve im- 
pulse is to observe the action of a muscle 
to which it is attached. The classic setup 
for such study is the sciatic nerve of the 
frog attached to the large gastrocnemius 
(calf) muscle and a mechanical device for 
recording the contraction of the muscle 
( Fig. 15-9 ) . Various stimuli can be used to 
stimulate the nerve but an electrical one 
is the best and most convenient. When- 
ever the nerve is stimulated the muscle 
twitches, indicating that some change set 
up in the nerve has traveled along the nerve 
to the muscle, causing it to contract. This 
change is called the nerve impulse. 

If a special instrument designed to detect 
minute electrical currents is placed on a 
nerve over which an impulse passes, there 
will be a definite response, indicating that 
the impulse has an electrical aspect (Fig. 
16-13 ) . If it were merely electricity, it would 
travel with the speed of electricity, namely, 
186,000 miles per second. An early experi- 
ment, however, demonstrated that it trav- 
eled at a much slower speed, 30 meters, or 
about 100 feet, per second in the frog and 
only four times that rate in man. The nature 
of stimulation bears no relation to the speed 
of transmission and whether the nerve is 
stimulated with heat, pressure, or electric- 



ity the impulse travels at exactly the same 
speed. Furthermore, once the impulse 
is started it continues with equal vigor 
throughout its course, unlike electricity 
which as it travels along a wire gradually 
diminishes in intensity the farther it goes. 

The fact that the nervous impulse con- 
tinues throughout its course with equal 
vigor indicates that something is added to 
it as it travels. It might be compared to a 
path of inflammable material where each 
portion ignites the succeeding part so that 
the entire trail burns with equal intensity. 
A minimum amount of heat must be sup- 
plied to initiate ignition, but once ignited 
the flame burns with equal vigor from that 
point forward. Furthermore, any excess 
heat beyond the minimum necessary to ig- 
nite the material will not change the situa- 
tion. It is likewise with a neuron, once 
stimulated, the impulse travels with equal 
intensity throughout the course of the cell. 
The neuron, like the muscle cell, obeys the 
"all or none" principle. In other words, if 
a given stimulus elicits an impulse, the im- 
pulse starts and continues throughout its 
course with full vigor. No matter how the 
minimum stimulus is altered, the impulse 
travels with its full force or it does not 
travel at all. 

Once an impulse passes over a nerve fiber 
there is a short period when the nerve is in- 
capable of transmitting a second impulse, 
that is, it refuses to accept any further stim- 
ulus. This is known as the refractory period 
and is very short in most nerve fibers, last- 
ing 0.001 to 0.005 of a second. It means that 
some reorganization is essential before the 
nerve fiber can once again be stimulated. 
This, in turn, is due to the physical make-up 
of the nerve fiber itself. The outside mem- 
brane of the nerve fiber is positively charged 
while the inside is negatively charged ( Fig. 
16-14), and is therefore said to be polarized. 
This condition is maintained untfl an im- 
pulse passes along which brings about a 
chemical change, resulting in the mixing of 
the charged ions through the outer perme- 





Fig. 16-13. Electrical changes occur in a nerve when an 
impulse passes over it {A to E) as indicated by the 
deflection of the galvanometer needle. Similar changes 
occur in a contracting muscle. 



able membrane, and the membrane becom- 
ing neutral in the region of the impulse. It is 
known that the change is chemical as well 
as electrical because the nerve fiber con- 
sumes oxygen and gives off carbon dioxide, 
respiring just as all cells do. The neutral 
condition is coincident with the refractory 
period and therefore lasts a very short time. 
Impulses pass along a nerve in rapid succes- 
sion and it is highly unlikely that a single 
impulse brings about a specific action. Im- 
pulses come in "bursts" like bullets from a 
machine gun 

cells to muscle cells? Careful and extensive 
microscopic examination has failed to show 
any protoplasmic connection either between 
nerve cells or between nerve cells and mus- 
cle cells. How, then, does an impulse pass 
from one unit to another? The best answer 
today is that the gap is traversed by chemi- 
cal means. A specific chemical is formed at 
the termination of a neuron which stimu- 
lates the dendrites of the next neuron or a 
muscle fiber. This will be discussed a little 
more fully under the autonomic system. 
Divisions of the nervous system. In an at- 


I — I 

Fig. 16-14. The nerve is normally polarized, being positively charged on the outside and 
negatively charged on the inside. This polarity is lost coincident v»^ith the nerve impulse 
(indicated in black). 

If a sensory nerve fiber is stimulated in its 
middle, a sensation results which we refer 
to the usual sense organ of origin. Such sen- 
sations in the case of pain are called referred 
pain. For example, if one sits for a consider- 
able period of time with the legs crossed, 
he finds that upon rising the crossed leg 
fails to function properly and prickly sensa- 
tions seem to come from the toes. The sci- 
atic nerve ( large leg nerve ) was compressed 
in the middle during that time so the stimu- 
lation really came from this region, although 
the sensation seems to originate in the toes. 

So far, consideration has been given only 
to the transmission of the impulses within 
the neuron itself. How does the impulse 
travel between nerve cells and from nerve 

tempt to understand some of the more sim- 
ple reactions effected through the nervous 
system, it is useful to divide it into parts, 
all of which, however, are very interdepend- 
ent. The central nervous system is com- 
posed of the brain and cord while the 
peripheral system is made up of the nerves 
which connect the brain and cord to all 
parts of the body. Impulses enter the cen- 
tral nervous system through the afferent 
fibers of the peripheral nerves. Interpreting 
the incoming messages and subsequently 
dispatching them is the function of the 
brain, the cord acting primarily as a relay. 
The impulses leave the brain and cord for 
the muscles and glands on the efferent fibers. 
The simplest type of such action involving 

Fig. 16-15. The frog has had the cerebrum removed by a cut just back of the eyes. The cord 
is intact. Acetic acid has just been brushed on the thigh of the right leg (top left). The 
right leg flexes and the toes scratch the region which is irritated by the acid (top right). 
When the posterior part of the body is cut away from the same frog, reflex activity is 
intact in the remaining part. Here acetic acid is brushed on the right fore leg (bottom 
left). A moment later that leg flexes first then extends violently; the left leg does likewise, 
indicating the cross-reflexes operate also when only a small part of the animal is intact 
(bottom right). 



dorsal fissure 
M t 
vwhite matter 

or qiand 


Fig. 16-16. A schematic view of the spinal cord in cross-section. The arrangement of the nerve cells is indicated on 

the left and the simple reflex pathway on the right. 

the peripheral and central nervous system 
is called a reflex. 

The cord and reflex action. Every person 
has experienced the operation of a simple 
reflex in his own body, as, for example, the 
quick withdrawal of his bare foot when it 
encounters a sharp object. It is difficult 

to determine whether the foot was removed 
after or just prior to the sensation of pain. 
A simple experiment with a frog can show 
tliat the reaction could have taken place 
without the sensation of pain. If the brain is 
removed and the skin irritated with acetic 
acid, the frog will withdraw its foot by ap- 



propriate leg movements (Fig. 16-15). The 
stimulus can be repeated again and again 
but the response will always be the same. 
Remember, there is no brain to interpret 
the message, yet the response is just as ef- 
fective as if the brain had been intact. 
Similarly, people with certain regions of 
their cord injured feel no pain though they 
respond to the stimulus. It is apparent in 
these cases that the impulse from the stimu- 
lus must have traveled only to the cord, 
returning from it to the appropriate mus- 
cles for bringing about that response. Var- 
ious portions of a frog's cord can be de- 
stroyed but the reflex will persist as long 
as the region where the sensory and motor 
nerves join the cord is intact. The complete 
action, then, must take place in a very local- 
ized resfion. 

In order to understand how a reflex can 
take place, a little information about the 
anatomy of the cord is in order. A spinal 
nerve bifurcates as it approaches the cord 
to enter on the dorsal side as the dorsal root 
(sensory) and on the ventral side as the 
ventral root (motor) (Fig. 16-16). The dor- 
sal root bears the dorsal root ganglion, in 
which the cell bodies of the sensory neurons 
are located. The cell bodies of the motor 
nerves are located in the ventral gray mat- 
ter of the cord (called the ventral horn). 
The function of these roots can be definitely 
identified by cutting them and artificially 
stimulating the stumps. When the ventral 
root is cut, no impulses reach the muscle 
from the cord, but when the cut end on the 
muscle side is stimulated a response will be 
evoked. Likewise, by severing the dorsal 
root no impulses will reach the cord, but 
when the stump on the cord side is stimu- 
lated a response will take place. The dorsal 
root thus contains only sensorv fibers and 
the ventral root only motor fibers. The spi- 
nal nerve, which is a union of the two, con- 
tains both, of course. 

A simple reflex begins with an impulse 
coming from a sense organ and traveling 
over an afferent nerve fiber to the cord via 

the dorsal root (Fig. 16-16). Here it comes 
in close contact ( synapse ) with one or more 
association neurons. The impulse is then 
dispatched to the proper muscle or gland 
over an efferent nerve fiber via the ventral 
root. With few exceptions reflexes involve 
more than the three neurons just described. 
The incoming sensory neuron usually con- 
nects with more than one motor neuron 
through association neurons in the cord. 
Thus the impulse may not only elicit the 
simple response but may also pass along 
the cord to stimulate other efferent neurons 
and in turn cause a large group of muscles 
to contract. Indeed, this is the manner in 
which it usually happens. Wlien applied 
to man the simple reflex accounts for many 
of our daily movements. These actions do 
not have to be learned, they are innate — 
one is born with them. 

Not only are different levels of the cord 
involved but also neurons on the opposite 
side of the cord. There is a crossing over of 
the association neurons so that an impulse 
may travel to both sides of the body ( Fig. 
16-17). If, for example, acetic acid is placed 
on the ventral side of a frog whose brain is 
destroyed, the leg on that particular side 
will attempt to remove the irritating acid 
while the leg on the opposite side will ex- 
tend (Fig. 16-15). Presently, the impulse 
will spread to the front legs so that eventu- 
ally all of the legs are moving in such a 
manner as to rid the animal of the offend- 
ing substance, and the body also begins to 
make mass movements as it would in at- 
tempting to crawl away from the irritation. 
To be sure, the first pathway is simple, but 
as the stimulus spreads many other path- 
ways are involved until the whole animal 
is thrown into movement. Under these con- 
ditions the higher centers also are involved 
(Fig. 16-17). 

In man, when the various parts of the 
brain are included, the impulse becomes a 
part of consciousness and one is aware of 
the reflex. When one steps on something 
sharp, for example, his foot may be with- 

Fig. 16-17. A series of sketches showing the nerve pathways in the cord and brain. The 
lower and middle figures show those where only the cord is involved while the upper 
figure indicates the pathways to and from the brain. 


drawn through simple reflex, but very that the speed of the nerve impulse would 

shortly certain parts of the brain are in- never be measured, and yet, just six years 

volved, even to the cortex where reasoning later, another scientist measured it very ac- 

and memory are possible. He may even con- curately; today any beginning student in 

sider. the advisability of wearing shoes, or physiology can duplicate the experiment 

the injury may be sufficiendy severe that with good precision. Such statements of 

he will remember the event for a long time, finality are dangerous, especially for a 

It seems that the impulses usually travel scientist, 

over certain paths but these can be altered Nerve impulses come to the brain from 

on a moment's notice and new paths are all over the body on afferent nerve fibers 

then followed. In other words, many con- like messages coming to the admiral of a 

ditions can determine the pathway over fleet. Just as the admiral must make deci- 

which an impulse may travel. There are sions diat will be sent by messages to var- 

many choices; which it chooses depends on ions parts of the fleet in order to accom- 

a large variety of circumstances. plish a certain goal, likewise, decisions are 

An illustration of how the reflex may be made in the brain and impulses are sent 

interrupted may be seen in the sneeze reflex, out over efferent nerve fibers to various 

Sneezing is caused by a stimulus originating parts of the body in order that certain 

in the nasal chambers; its ultimate fruition actions can be executed. Decisions are 

is an explosive expulsion of air through made for the most part in a routine manner, 

these chambers to dislodge the irritation, based on inherited principles or on experi- 

If, when the sneeze sensation begins, a sec- ence gained through having made simflar 

ond stimulation is set up by producing pres- decisions before. Most of the decisions are 

sure on the upper lip, the first pathway wfll made by the brain without breaking through 

be blocked. In other words, a new set of to consciousness, so that one is not aware 

conditions blocked the original reflex path- of most of its activity. This great center is 

way. Neurologists believed at one time that indispensable to the harmonious working 

certain pathways were set up not only in of the entire body. Aberrations in the brain, 

the cord but in the brain as well, and that so slight that they cannot be detected as 

each pathway was traversed by the impulse any physical change, produce such stark 

in exactly the same manner, thus making it changes in personality and emotional sta- 

"deeper" until it was well established. There bility that our society is forced to build 

seems to be no evidence for this idea and it special institutions such as prisons and asy- 

is now well known that the impulse may lums to house people so afflicted, 

travel over different pathways and may For a better understanding of the human 

never take the same course twice. Further- brain it is best to start with a lower verte- 

more, pathways may be employed by differ- brate type brain, which is relatively simple 

ent reflexes at different times. Which spe- in structure, and follow through to higher 

cific pathways are followed, and why, is one forms. In die discussion of invertebrates it 

of the most important problems in neurol- was shown that the obvious location for the 

ogy today. brain is in the animal's anterior end, which 

Brain. This part of the nervous system is is the part that arrives in any new environ- 
the most complex and, of course, the most ment first. Among vertebrates the brain like- 
difficult to understand. Only poorly under- wise has its beginnings and its subsequent 
stood today, the brain will, perhaps, never development in the anterior end of the or- 
be adequately comprehended, but to accept ganism, and as we progress from fishes to 
such an attitude would be highly unscien- mammals (Fig. 16-18) it becomes increas- 
tific. A great German scientist once said ingly prominent. 



Fig. 16-18. The relative development of the various parts of the brain of representative vertebrates. 

The three major regions of the brain, 
fore-, mid-, and hindbrain, are clearly 
marked off very early in the embryonic de- 
velopment of every vertebrate (Fig. 16-19). 
These three regions soon subdivide into five 
regions. The forebrain becomes the telen- 
cephalon and diencephalon, the midbrain 
remains undivided and is known as the mes- 
encephalon, and the hindbrain divides to 
form the metencephalon and myelencepha- 

lon. Each of these becomes modified and 
develops other parts as the brain becomes 
more complex in higher animals. In the hu- 
man brain the cerebrum comes from the 
telencephalon, the posterior lobe of the 
pituitary (an endocrine gland) and the 
optic chiasma from the diencephalon, 
the corpora quadrigemina from the mesen- 
cephalon, the cerebellum and pons from 
the metencephalon, and the medulla from 

kind brain 





myclcDccpbalon ' meseDcepbalon' 

metencephalon JSncepbalon metiocephalpn 



Fig. 16-19. The vertebrate brain begins as a hollov/ sac vt^hich subsequently divides first into 
three distinct regions and later into the many parts shown (left). The embryology of the 
human brain parallels that of other vertebrates as shovi^n by the series of figures to the 

Fig. 16-20. The relative sizes of the various parts of the brains of different vertebrates vary according 
to the need. Animals that fly or sv/im need better muscular coordination than do those on land, 
hence their cerebellums are proportionately large. Likewise a well-developed cerebrum is essential to 
the success of mammals, hence this part of their brain is best developed. 



the myelencephalon. There is a clear rela- 
tionship between the size of a particular 
part of the brain of an animal and its im- 
portance in the life of tlie animal. In the 
lower vertebrates, such as fish and frogs, 
the olfactory lobes are large, because this is 
probably the most important sense these 
animals possess, whereas in birds where the 
sense of smell is only poorly developed this 
part of the brain is proportionately small 
(Fig. 16-20). In the lowest vertebrates the 
cerebrum is non-existent, or almost so, 
whereas in the birds it begins to dominate 
the anterior end of the brain and in mam- 
mals it overgrows all other parts to become 
the most prominent part. In man, this trend 
is carried to the most extreme point of de- 

Emphasis of various parts of the brain 
seems to be associated with the kind of life 
its owner leads. For example, birds and fish, 
which move in three dimensions, require 
especially good muscular coordination to 
balance their bodies, and since this nervous 
center is located in the cerebellum, this 
portion of the brain is well developed ( Fig. 
16-20). Reptiles and land dwelling mam- 
mals (mouse and man), on the other hand, 
have a rather poor sense of balance because 
they move in two dimensions for the most 
part and do not require the coordination 
that fish and birds do for balancing their 
bodies in a fluid or gaseous environment. 

The cord takes care of the simplest activ- 
ities in the organism and its anterior end, 
the medulla, still retains that function to a 
high degree even in man. It is here that the 
reflexes for such basic activities as respira- 
tion and heart action center. The cerebel- 
lum functions in muscular coordination but 
the cerebrum, the last region to evolve, is 
the center of such highly complex activi- 
ties as thought and reasoning. 

As the vertebrate brain has evolved there 
was a gradual shift of function from the 
lower part, the brain stem, to the higher 
part, the cortex. This shift can be demon- 
strated experimentally. When the cerebrum 

of a frog is removed its normal activities 
are influenced only slightly. It jumps nor- 
mally when stimulated and it can swim in 
a perfectly normal fashion (Fig. 16-21). 
Even a "decerebrated" reptile shows very 
little concern about its loss. Such an opera- 
tion on a bird or mammal, however, brings 
about striking changes. The ability to loco- 
mote is destroyed and all actions which re- 
quire considerable muscular coordination 
are lost. This simply means that the higher 
vertebrates have shifted their nerve centers 
from the lower brain stem to the cerebral 

This shift has given these higher forms 
much greater plasticity in the control of 
their muscle coordination. For example, 
when certain muscles in man have lost their 
nervous connection with the brain in the 
disease known as facial paralysis, functional 
cranial nerves passing to relatively weak 
and less useful muscles can be transplanted 
to the larger more important muscles and 
eventually become functional through long 
retraining. In other words, impulses can be 
sent to a muscle via a wholly new nerve 
with the result that the muscle can eventu- 
ally respond in its usual manner. In man, 
the cortex so completely dominates the 
body that it is physiologically possible by 
intense effort to learn to move muscles 
in almost any manner. This is well illus- 
trated by the many human feats performed, 
activities that could never be executed by 
other animals because their nervous sys- 
tems are constructed in such a way as to 
make it impossible. This shift of control 
to the cortex accounts for man's great versa- 
tility and is one of the major reasons for his 
success. It has also made possible the devel- 
opment of our type of society. 

As the cerebrum increased in size in rep- 
tiles, birds, and mammals, it became neces- 
sary to increase the surface area whfle 
retaining a reasonable volume. This has 
been done by the formation of wrinkles 
or convolutions. Starting in the lower mam- 
mals with only very few convolutions, the 

Fig. 16-21. The anterior portion of the head, which includes the cerebrum, has been cut away in this 
frog. When placed in water it swims in a normal fashion, indicating that this portion of the 
brain has little, if any, function In performing this act. 



'!> \^>\V^^,-;/'' JW:\1\\'11, r:.-^^:Q!ig^ I J Iv.;'--. • , • . • > ;5!ivV?9e{vS i • • 


Fig. 16-22. Localized brain areas are concerned with special functions. 

number and extent of these increases as In general, the size of the brain is an indi- 

the brain increases in size, reaching a maxi- cation of intelhgence, although there are 

mum in man. Naturally, as the cortex has some notable exceptions, for instance, the 

increased in size, the entire brain has brain of both the whale and elephant weighs 

increased with respect to the cord. The rela- more than that of man. Dogs, gorillas, and 

tive weights of these two structures for sev- men of approximately the same body 

eral common animals are shown in the fol- weights have brain weights of about 140, 

lowing figures: 450, and 1350 grams, respectively. Within 

Ratio of limits of normal variation in a single species. 

Weight Weight brain size does not indicate intelligence. 

of Cord of Brain p^^ example, woman generally possesses a 

^^^ ^-^ ^'^ brain which averages about 100 grams less 

j^pj^j^gy I Q ]^5Q than man, but what man would be so brave 

Man 1.0 55.0 as to imply that she is less intelligent! Like- 


wise, there is a ■wide variation in the size of pathways can sometimes be broken up and 

brains of different individuals of the same new, more compatible ones be established, 

sex, just as there is in stature and body A more drastic treatment is actually to cut 

weights. Famous brains have been pre- across the pathways in the prefrontal lobe 

served and weighed, out of morbid curiosity of the cerebrum. It is too early to predict 

or for scientific purposes, and it has been how effective this type of treatment will be. 

shown that a variation of 800 grams exists The medical profession is learning that 

between brains of apparent equal intelli- more and more of the common ailments of 

gence. So, within limits, brain size alone is mankind are due to an overactive cerebral 

not a criterion of intelligence. Intelligence cortex rather than actual organic disease; 

is probably due to such things as the num- that is, people imagine they are sick, and 

ber of cell bodies or the number of associa- this can be carried so far that actual symp- 

tion neurons in the cortex of the cerebrum toms from heart disease to stomach ulcers 

which, in turn, is an indication of the num- are present. This has resulted in a new field 

ber of pathways over which impulses may of medical research known as psychosoma- 

travel. tic medicine. With proper treatment, certain 

The two cerebral hemispheres are each neurotic tendencies can be overcome and 

divided into five lobes and the nerve cells the person miraculously "cured." This type 

associated with various functions are local- of medical research is commanding increas- 

ized in these lobes ( Fig. 16-22 ) . The center ing attention and holds hope for a different 

for sight is located in the occipital or pos- approach to the study of certain diseases, 
terior lobe. Areas for smell, hearing, and 

taste are located in the lateral or temporal '^^^ autonomic nervous system 

lobe, while centers for muscle movements A great many activities of the body such 

are centered in the anterior or frontal lobe, as peristaltic movement in the intestines. 

Skin sensations lie in the parietal lobe. The breathing, and heart action go on unnoticed 

fifth lobe ( insula ) lies beneath the frontal and without voluntary control. These are all 

and parietal lobes and cannot be seen from under tlie influence of the so-called auto- 

the surface view. Large areas of the cortex nomic nervous system. The name implies 

are spoken of as "silent areas" because it that the system is a completely automatic 

appears that injuries to tliese regions result one, which is not the case, but it has become 

in no particular loss of sensory or motor fixed by usage. 

function. These are the great unknown areas Anatomically, part of the autonomic nerv- 

of the brain which have stimulated so much ous system can be seen as two rows of gan- 

research in recent years. glia ( lateral sympathetic ganglia ) lying on 

The complex life human beings live in each side of the spinal column in the tho- 
modern society has produced extremely in- racic and abdominal cavities. The ganglia 
tricate nerve pathways in the cerebral cor- are secured to the spinal nerves by means 
tex, and in a so-called normal person these of two short connectives, a white and a 
pathways are so arranged that he gets along gray ramus ( Fig. 16-23 ) . Distally they ex- 
well under normal conditions. During times tend as small nerve fibers to the various 
of great stress, these pathways sometimes organs of the chest and abdominal regions, 
become so warped as to disqualify the indi- The incoming ( afferent ) fibers pass through 
vidual for life in society. As a result, he the dorsal root just the same as the volun- 
must be confined to an institution. It has tary nerve fibers, and the cell bodies are 
been found that by subjecting such a per- located in the dorsal root ganglion. The 
son to severe shock by means of insulin, notable difference between the two systems 
electricity, and other agents, these aberrant is that the efferent fibers of the autonomic 






Fig. 16-23. A schematic drawing of the nerve pathways Involving the autonomic nervous system. Impulses arising 
from a temperature sense organ in the skin can stimulate sweating. Also impulses arising from the stomach can 
cause some action in other parts of the viscera. 

system have a chain of at least two neurons 
between the central nervous system and the 
organ innervated, whereas in the voluntary 
system there is but one. The former are also 
without a myelin sheath. 

The first autonomic cell bodies of the 
efferent neurons lie in the lateral portion of 
the gray matter of the cord, while the sec- 
ond neuron cell bodies lie outside of the 
cord in the ganglia already mentioned; 
sometimes the second ganglia lie directly 
on the organ itself some distance from the 
sympathetic chain. The second neuron may 
return to the ventral root and pass along 
with the spinal nerve fibers to such parts of 
the body as the blood vessels of the skin 
and sweat glands which are under the con- 
trol of the autonomic nervous system. Others 
may pass through to a large ganglion ( col- 
lateral sympathetic ganglion) outside the 
sympathetic chain where synapsis occurs 

with the second neuron. Fibers leading 
from the central nervous system to tlie gan- 
glia outside the cord are called pregan- 
glionic fibers, those leaving the ganglion, 
the postganglionic fibers. Such a system of 
fibers makes it possible for stimuli to come 
from the internal organs and to effect a re- 
sponse elsewhere in the body. For example, 
an impulse can come from the stomach 
(Fig. 16-23), causing some action to occur 
in the duodenum, initiating a peristaltic 
wave, perhaps. Moreover, pain impulses 
might come from the stomach which, by the 
intermingling of the two systems (central 
and autonomic), project through to the 
cerebral cortex where they are registered as 
an uncomfortable feeling about which some- 
thing might be done. Likewise, impulses 
might arise in the temperature end organs 
of a too warm skin and pass through the 
circuits (Fig. 16-23) giving rise to efferent 








r lumber 











Fig. 16-24. A schematic drawing of the sympathetic and parasympathetic nervous systems, separated 
for clarity, to show how they function. Note that each organ is supplied with nerves thot cause it 
to be activated or inhibited. 



impulses that cause the sweat glands to 
pour their secretion over the skin, and thus 
cool the body. Thousands of these pathways 
are continually carrying messages to and 
from parts of the body, most of them un- 
known to the owner. 

The autonomic nervous system is divided 
into two rather distinct parts: the thora- 
columbar (sympathetic) which is com- 
posed of the double chain of ganglia already 
noted; and the craniosacral (parasympa- 
thetic) which originates in the posterior 
portion of the brain (midbrain and me- 
dulla) and the sacral region (Fig. 16-24). 
The thoracolumbar system has short pre- 
ganglionic fibers and generally long post- 
ganglionic ones. The opposite is true of the 
craniosacral system, and in fact, the ganglia 
lie in some cases, such as the heart, em- 
bedded within the organ itself. These two 
systems send nerve fibers to all of the organs 
operating involuntarily so that each has a 
double innervation. However, the two sys- 
tems produce opposite effects. For example, 
if the nerve from the thoracolumbar system 
to the heart is stimulated, the beat is accele- 
rated, but if the nerve from the craniosacral 
system is stimulated the beat is slowed 
down. Stimulation of one nerve may cause 
excitation in one organ and inhibition in 
another, thus stimulation of the vagus ( cra- 
niosacral ) accelerates the heart but inhibits 
the stomach. The value of such a mechanism 
is obvious. It is the interaction of these two 
systems that regulates the flow of blood to 
various parts of the body, differing with 
each condition in which the animal finds 
itself. It also causes the pupil of the eye to 
dilate or constrict, depending on the amount 
of light that is needed for vision. These 
and hundreds of other routine jobs the 
body does quietly and efficiently and en- 
tirely without the knowledge of the owner. 
This system has certainly taken the drudg- 
ery out of operating the body, and has left 
for the higher centers the job of getting the 
whole organism in a position to obtain food 
or to do the many other things that are es- 

sential for life. If the central nervous system 
were burdened with the job of operating 
this machinery, little else could be done. It 
would be like requiring the President of the 
country to see that the proper amount of 
water flows through a certain aqueduct in 
New York City. 

Biologists have been bothered by the 
problem as to why impulses arriving in an 
organ via either part of the autonomic sys- 
tem cause acceleration or inhibition even 
though the nerves appear to be identical, 
and what the mechanism is that differenti- 
ates between them. Some ingenious experi- 
ments have been performed to give an 
answer to these perplexing questions. It has 
been shown that a chemical (neurohumor) 
is secreted at the point of juncture between 
the nerve endings and the organ innervated. 
Furthermore, the chemical is different for 
the two divisions of the autonomic system, 
which is what one might expect if the 
action is the opposite. Stimulation of the 
craniosacral system, for example, produces 
acetylcholine at the nerve endings, and 
stimulation of the thoracolumbar produces 
sympathin. Thus, in order to complete the 
mission of delivering a message to a muscle 
or gland, a physical and a chemical action 
must take place. A substance known as 
choline esterase is present, which counter- 
acts the neurohumor and thus prevents 
cumulative effects. The action of sympathin 
is much like that of adrenalin, the secre- 
tion from the medullary portion of the 
adrenal glands (endocrine) and its action 
is antagonistic to acetylcholine. It has no 
inhibitor and must be destroyed by oxida- 
tion some time after it is formed. 

The discovery of specific neurohumors 
in the autonomic nervous system that 
bridge the gap between nerve endings and 
muscle has naturally led scientists to postu- 
late that perhaps the same mechanism 
causes the bridging of the gaps between 
neurons in the central nervous system. Im- 
pulses passing to the synapse differ both in 
frequency and duration from those leaving 



• - 





sweot gland 

digestive gland 

Fig. 16-25. Two different types of glands and how they function. Sweat glands merely extract the substance from 
the blood, concentrate it, and secrete it. Digestive glands, on the other hand, must manufacture the enzyme from 
row materials supplied from the blood, and then secrete the finished product. In both instances energy is 

the synapse, which would indicate that 
sometliing happens to them as they pass 
through the gap. Not only is there a delay 
in the transmission of the impulse as it 
passes through the synapse but something 
happens in this junction which allows the 
impulse to cross it in only one direction. 
These characteristics of the synapse may 
indicate that the neurohumoral explanation 
is the correct one, although there is more 
work to be done on the problem before this 
can be stated positively. 


The principal effector organs of higher 
animals are the muscles and glands, al- 
though there are other types of effectors in 
use throughout the living world. For ex- 

ample, there are chromatophores, found in 
a large group of vertebrates and inverte- 
brates, that are responsible for rapid color 
change in the skin. Some animals possess 
luminous organs which radiate light when 
stimulated and still a few others possess 
electric organs that generate electrical dis- 
charges under suitable conditions. Each of 
these effectors will be discussed briefly with 
the exception of the muscles which have 
been treated rather thoroughly in Chapter 


Skin glands have already been discussed 
( Chapter 14 ) but there are other glands in 
the body which are stimulated directly by 
the nervous system. Just how a gland cell 
secretes is not well understood. Its job is 
to extract the product of secretion from the 


blood or lymph on one side and discharge dramatic. Such fish, when moved experi- 
it on the other into a lumen or tube that mentally from an aquarium with a black 
conducts the product to its proper place, bottom, over which they become very dark, 
either to the outside of the body, in tlie case to one of white sand, will shortly become a 
of sweat glands, or into the digestive tract, very light color. Furthermore, some, like 
in the case of digestive glands ( Fig. 16-25 ) . the flounder, will actually produce the mot- 
In such glands as sweat and tear glands no tied effect of colored stones on the ocean 
special substances are synthesized; their floor. Lizards will change from the color of 
secretion is merely extracted directly from the bark on the tree trunk to the intense 
the blood. However, in the case of many green of the leaf. The protective value of 
other glands, the digestive and endocrine such a mechanism is obvious, 
glands for example, while the raw materials This color change is accomplished by 
are extracted from the blood stream, the the movement of pigment either in the 
final product is synthesized within the cell effector end organ in the skin or closely 
itself. Just how this is accomphshed is un- associated with it. In vertebrates, the pig- 
known, ment is confined to single cells which are 

Glands respond to stimuli from the nerv- scattered throughout the skin of the animal, 
ous system and most of them secrete only When light falls on tlie eyes of the fish, 
when stimulated. Some endocrine glands, stimuli are sent to the chromatophores 
however, apparently secrete continuously, which adjust the amount of pigment that 
although the rate may be affected by nerv- spreads out on the surface to obtain just 
ous excitation or by hormones from other the right shade of color to match the back- 
endocrine glands. There is considerable ground. Blind fish remain one color no 
energy utilized during glandular activity, matter what the background. On a light 
Thus the sweat glands produce a fluid with background, the normal response of each 
a salt content considerably higher than that chromatophore is to concentrate the pig- 
of the blood, and energy is required to in- ment granules of the cell into compact "pin 
crease this concentration just as it would points." On a dark background the same 
be if heat were used to evaporate an equal pigment granules spread out at the surface, 
amount of dilute salt water to a similar con- darkening the entire area, 
centration (Fig. 16-25). Careful measure- Although most of the pigment cells in 
ments of gland cells demonstrate that some animals are primarily under the influence 
respire at a higher rate than any other cells of the nervous system, some, such as those 
of the body, even those of the heart. of Crustacea (Fig. 16-26) and Amphibia 

(Fig. 16-27), are controlled by hormones, 

Chromatophores although the initial stimulus is via the eyes. 

Although color change is not found in Pieces of frog skin, for example, can be 
human beings other than the gradual tan- stimulated to change color simply by dip- 
ning of the skin as a result of exposure to ping them in solutions containing specific 
sunlight, many lower animals are equipped hormones. However, under normal condi- 
with very efficient and often spectacular tions, stimuli from the eyes excite the en- 
color-changing apparatus. In some animals docrine glands whose secretion causes the 
such as the squid (Fig. 12-17) the variety chromatophores to respond. It resembles a 
and rapidity of change in color is almost chain reaction, 
fantastic. It can change from pearly white 
to intense black almost instantaneously. '° "*"' 

Others such as fish and reptiles change This is often spoken of as "cold light" 

more slowly but the final product is equally because very little heat is emitted and for 

Fig. 16-26. The shrimp, Grongon, is hidden because of its ability to match its background. The chromatophores 
bring about a mottling effect which resembles the sand particles. In addition, it half buries itself to bring about 
still further concealment. 

Fig. 16-27. The leopard frog changes from a light color where the spots are very prominent to a darker 
color where the spots are obscured. The activity of the chromatophores which are responsible for 
these varying colors is shown magnified in the lower figures which correspond with the frog above 
each one. The activity is hormone controlled. 



Fig. 16-28. This is a comb-jelly {Mnemiopsis leidyi) found along Cape Cod. The combs are luminescent and this 

photo was taken using their own light. 

that reason it is a highly efficient Hght. It 
has been said that if man could learn how to 
produce light as efficiently, a small boy could 
turn a generator that would light a moder- 
ately-sized city. Most of the energy in an 
electric light or any other kind of light is 
lost in the production of heat. 

It is interesting to note how widespread 
among living things is the ability to pro- 
duce light (Fig. 16-28). Indeed luminous 
species occur in all the major phyla, ex- 
cept the Platyhelminthes and the Nema- 
thelminthes. The sea is teeming with lumi- 
nescent bacteria, Protozoa, and a large 
variety of Metazoa. Luminous bacteria may 
cause a carcass or rotting log to glow in 
the forest on a dark night. They and other 
organisms may cause a brilliant display of 
light in the wake of a ship on the ocean at 
certain times of the year. The intermittent 
light of the common firefly has inspired the 
artistically inclined to describe it in verse 

and music. Deep sea forms as well as other 
animals are endowed with luminescent or- 
gans which provide light in an otherwise 
eternally dark world. 

Luminescent bacteria usually glow con- 
tinuously as long as they have sufficient 
oxygen, whereas the firefly flashes its light 
intermittently from an especially designed 
organ. Luminescence depends on the pres- 
ence of two substances, luciferase, an 
oxidative enzyme, and luciferin, an organic 
substance present in specific luminescent 
organs. If these two substances are ex- 
tracted from the animal and mixed in a test 
tube in the presence of free oxygen, lumi- 
nescence occurs. Stimulation of the lumi- 
nescent organs is apparently under the 
control of the animal that possesses it, at 
least in some instances. For example, the 
male firefly flashes its light during the mat- 
ing season, a fact which may have some- 
thing to do widi attracting the female. 



Electric organs 

Powerful electric discharges can be pro- 
duced by a few species of fish. They are 
"triggered" by stimuH coming from the 
nervous system. The electric eel and ray 
possess special "electric organs" that are 
composed of a great many modified muscle 
cells so arranged as to accumulate their in- 
dividual action currents and build up a 
considerable voltage. In some forms as 
much as 400 volts have been recorded, 
which is sufficient to stun or even kill a 
small fish. When a small light bulb is placed 
in the circuit, flashes have been recorded. 
Horses wading through shallow water 
where the large electric eel resides have 
been shocked enough to throw their riders. 

Muscles and glands occur in all species 
of the Metazoa. The additional effectors — 
pigmentary, luminescent, and electric — are 
much more restricted in their occurrence. 
They demonstrate how far animals may be 
able to modify certain parts of their bodies 
to perform very special functions. It is in- 
teresting to speculate how, through the mil- 
lions of years of evolution, these animals 
have been able to select such aberrant, 
though practical, modifications. 


An important adjunct to the nervous sys- 
tem in bringins; about coordination of the 
vastly complex animal body is the endo- 
crine system. This is made up of glands 
located in various regions of the body 
which secrete powerful organic compounds 
directly into the blood stream ( Fig. 16-29 ) . 
Their activity is manifest in other parts of 
die body. While the nervous system is re- 
sponsible for quick action, the endocrine 
system functions in bringing about the 
much slower reactions which may extend 
over some period of time. 

The glands which are known to be endo- 
crine in nature today were described by 

early anatomists. Thus, Galen, in the second 
century a.d., described the tiny pituitary 
gland of mammals, although he could 
assign no function to it. Indeed, it was not 
until nearly the end of the last century 
that actual experimentation began to bring 
to light the function of these mysterious 

The endocrine glands and their secretions 

The endocrines evolved after the nerv- 
ous system, so it might be expected that 
they would be found in the more highly 
specialized animals where the nervous sys- 
tem could not take care of the multitudi- 
nous jobs assigned to it. These glands are 
present in Crustacea and insects (Chapter 
11) and they may be important for coordi- 
nation or other functions in animals lower 
than the arthropods, but as yet their pres- 
ence has not been demonstrated. They are 
consistently found among the vertebrates, 
even in such low forms as the cyclostomes. 
The glands themselves are derived em- 
bryologically from various sources, and in 
their evolution have performed different 
functions. For example, the hormones that 
control chromatophore activity in the 
amphibia can exercise no comparable func- 
tion in birds and mammals because they 
have no chromatophores. Yet, the hormone 
is still present, as can readily be demon- 
strated by the injection into a frog of the 
proper extract. Undoubtedly, such re- 
cently acquired hormones as those that 
stimulate lactation in mammals are de- 
rived from similar hormones present in 
lower vertebrates where they perform a dif- 
ferent function. There has been a long, 
slow, biochemical evolution of these com- 
plex substances which have an intricate 
interrelationship in the higher animals 
today. This very complex relationship has 
been the subject of a tremendous amount of 
research during the past 50 years. 

The vertebrates have seven clearly recog- 
nized endocrine glands: the gonads, pan- 


hypo-normol- hyper- 






hyper - 

f exophttTolmic norr^l 



cretin normal 


w3 ^"^^^^''q) 

Fig. 16-29. The location of some of the endocrine glands and some of the results of their malfunctioning. 



creas, duodenum, thyroid, parathyroids, 
adrenals, and pituitary. Although they are 
referred to as ductless glands, the first 
two do have ducts, although the endocrine 
secretion does not leave the gland through 
the ducts but instead enters tlie blood 
stream directly. These two, together with 
the duodenum have two separate glandular 
functions. The gonads function in the pro- 
duction of eggs and sperms in addition to 
their endocrine function of producing hor- 
mones that are responsible for tlie sec- 
ondary sexual characteristics. The pancreas 
produces digestive enzymes in addition to 
insulin, and the duodenum has several 
functions besides producing secretin. More- 
over, a single gland such as the pituitary 
produces several hormones, each with a 
strikingly different function. Other endo- 
crine glands may be found, although this 
appears much less likely than the possibility 
of discovering new functions for the glands 
already known. 

From the very beginning of experimental 
work in endocrinology, as this phase of 
biology is called, the techniques of discov- 
ering the function of a suspected gland 
have been much the same. The gland is re- 
moved. If changes appear in the animal's 
normal function and if these can be re- 
versed by replacing the gland or its extract, 
then it is reasonably certain that the gland 
has an endocrine function. A more com- 
plete understanding can be had by using 
extracts, for if the gland has multiple 
functions, as in the case of the pituitary, 
various fractions of the extract can be used 
and these separate functions more clearly 
defined. Sometimes it is desirable to inject 
extracts into the normal animal to study 
the possible effects of oversupply of a hor- 
mone. In human beings, the occasional 
dysfunctioning of endocrine glands because 
of tumors or other abnormalities has given 
physicians and biologists abundant material 
to study and, in some cases, to conduct ex- 
periments. Results of experimentation on 
lower animals have also revealed a great 

deal of information that has been applied 
directly to the alleviation of many endo- 
crine aberrations in man. 

The ultimate goal in experimentation 
with endocrines is to find the exact chemi- 
cals that are involved and to be able to 
prepare them in tlie laboratory. Once the 
chemical structure of these substances is 
known, they can be synthesized and used in 
animals with deficiencies to restore normal 
conditions. This has been accomplished for 
a few hormones but there is still much to be 
learned. Before this book comes off the 
press a new endocrine gland may well have 
been discovered or a new function attrib- . 
uted to one already known. We will briefly 
survey the seven that are fairly well known. 

The duodenum 

Two British physiologists, Bayliss and 
Starling, found that the highly acidic food 
passing from the stomach into the duode- 
num stimulated the walls of the latter organ 
to produce a substance which circulated in 
the blood stream to the pancreas, causing 
it to secrete its products into the digestive 
tract. This substance they called secretin, 
designating it also by the name hormone, 
a name which has since come into general 
usage for this class of substances. We shall 
learn more about secretin in the chapter on 
digestion. With their pioneer work, the field 
of endocrinology was initiated. 


This compound gland has ducts and its 
most obvious function is that of producing 
digestive enzymes which are drained off to 
the duodenum through the pancreatic duct. 
The digestive function of the pancreas will 
be discussed in a later section; here we are 
concerned only with the portion of the pan- 
creas that produces the hormone insulin. 

The story of the discovery of insulin is 
one of the more fascinating sagas in the 
annals of biological science. As far back as 
1890 it was known that if the pancreas were 
removed from a dog, death followed in a 



few weeks. The salient point that came out 
in the early experiments was the appear- 
ance of large quantities of sugar ( glucose ) 
in the urine of such operated dogs. The 
two German workers who first performed 
these operations noticed that ants were at- 
tracted to the cages of these dogs and found 
that it was the sugar in the urine which was 
attracting them. This reminded them of 
human diabetes, a disease that had been 
known for centuries. A long series of experi- 
ments followed by workers all over the 
world, and it was soon proven conclusively 
that the small groups of cells in the pan- 
creas, called the Islets of Langerhans, pro- 
duced a hormone called insulin that was 
responsible for retaining and storing sugar 
in the body. If these islets were destroyed, 
as in the case of human diabetes, sugar no 
longer was retained in the liver and other 
tissues, but poured out through the kidneys 
into the urine and was lost from the body. 
This was a great discovery but what could 
be done about it now that the cause of this 
dread disease was known? 

The first step was to try to find a substi- 
tute for the non-functioning islets. Feeding 
the whole pancreas to depancreatized dogs 
I ailed to produce the slightest effect. The 
hormone was digested in the alimentary 
tract; consequently, it never got into the 
blood stream where it could be carried to 
the liver and tissues in which it would work. 
The next step was to inject an extract into 
the body, but because of the difficulty en- 
countered in getting a pure product, no 
satisfactory results were obtained for 20 
years. During this long period experi- 
menters were attempting to obtain a pure 
hormone from the whole glands of various 
animals, mostly domestic animals such as 
cattle, sheep, and hogs. 

It occurred to a young Canadian physi- 
cian. Dr. Frederick Banting, that perhaps 
the digestive enzymes produced by the 
pancreas destroyed the insulin before it 
could be extracted. This was later shown to 
be true. Banting reasoned that since the 

embryonic pancreas was known to produce 
the islet tissue before the enzymes ap- 
peared in the pancreas, if such glands were 
used, perhaps the hormone could be iso- 
lated in an active state. In 1922, he and three 
other men — Best, McLoed, and Collip — 
working together, set out to isolate the hor- 
mone. After a great deal of labor, they 
eventually prepared a product which 
caused no ill effects on the dogs when in- 
jected under their skin and which cured 
their diabetes. It was a short step to the 
treatment of humans, where success was 
immediate. The thousands of diabetics then 
had, for the first time, some means of stav- 
ing off an early death from a disease that 
had always been fatal. 

Insulin was produced in more concen- 
trated and more purified form during the 
ensuing years until today the product is 
responsible for the near-normal lives of 
hundreds of thousands of men, women, and 
children. Someday it may be possible t(> 
take the hormone by mouth, but at present 
it still must be injected under the skin at 
rather frequent intervals depending on the 
severity of the disease. 

The nature of diabetes. Without treat- 
ment a diabetic suffers from insatiable 
thirst, excessive urination, a gradual loss 
in weight, general body weakness, and 
finally a coma which terminates in death. 
During this course the sugar in the blood 
and urine is found, by measurement, to be 
abnormally high (as much as 8 per cent in 
the urine), the liver loses its glycogen and 
finally, in the coma state, acetone and par- 
tially degraded fats also appear in the 
blood and urine. Before death the acetone 
may reach such concentrations that it can 
be detected on the breath. All of these 
symptoms are immediately relieved with 
the administration of insulin. 

The first, most obvious function of insulin 
is to maintain normal carbohydrate metabo- 
lism in the body. For some reason, in the 
absence of insulin the liver fails to store 
glycogen and glucose is oxidized very 


poorly. Strangely enough, the abstinence of caring for both the gonadal prod- 
from carbohydrates in the diet does very nets (eggs and sperms) and the early 
little good in preventing any of the symp- embryo. Such a process becomes highly 
toms. In fact, it seems that without insulin complex in mammals where the young are 
the body mobilizes all of the sugar at its few in number and are cared for both 
disposal and discharges it from the body within the body of the mother long before 
via the urine. Even the amino acids are birth and for some time after they have 
deaminized at an abnormally high rate, so made their appearance in the outside 
that the sugar residue is added to the world. To aid in its functioning an intricate 
already heavily sugar-laden urine. Further- system of hormones, intricately adjusted to 
more, the fats are withdrawn from storage one another, has thus been evolved, 
and only partially oxidized, leaving the un- The gonads' primary function of pro- 
oxidized fractions in the blood and urine, ducing eggs and sperms will be discussed 
It seems tliat all the forces of the body are later in Chapter 21 and only such anatomy 
put forth to produce sugar which is then as is necessary for an understanding of their 
wastefully thrown away. Death is the in- endocrine function will be given at this 
evitable answer to such a course, unless time. 

insulin from an external source can inter- The testes. Located among the sperm- 

vene. producing tubules of the testes is a special 

In some diseases of the pancreas the type of tissue ( interstitial ) which produces 
islets are stimulated to produce more than a hormone, testosterone, that is secreted 
the normal amount of insulin. The results direcdy into the blood stream. This stimu- 
are the same as when a diabetic gives him- lates the production of the secondary sex- 
self too much insulin. The blood sugar is ual characteristics in all male vertebrates, 
dropped to such a low level that the brain characteristics which are associated with 
becomes irritable and finally the person maleness. They are the very obvious traits 
goes into the severe condition called in- which separate the male from the female 
sulin shock. Most diabetics are familiar both moq^hologically and physiologically, 
with the possibility of this condition and The comb and brilliant plumage of the 
accordingly carry sugar or some other cock, the thick neck and massive body of 
sugar-containing substance that can be the bull, and the beard of man are all sec- 
taken quickly to overcome the lowered ondary sexual characteristics. The absolute 
blood sugar level. Because of his liability to proof that these characteristics are associ- 
insulin shock or coma, either of which may ated with the testis can be demonstrated by 
render him unconscious, it is advisable that castration, or removal of these glands, an 
the diabetic carry among his possessions a ancient custom practiced by man not only 
card or tag identifying his disease, so that on his domestic animals but in some cases 
in event of collapse his condition will not also on his fellow man. The castrated cock 
be mistaken for some other malady or even shows none of the brilliant plumage of the 
drunkenness. . normal male; the steer is quite different 

both in its anatomy and behavior from the 

The gonads j^^H. ^\^q gelding has none of the fire nor 

The testes and ovaries are also compound cantankerousness of the stallion. In ancient 
glands whose primary function is the pro- times it was customary to castrate slaves, 
duction of sperms and eggs. In addition, producing eunuchs who would then be 
they have very important endocrine func- docile, subservient beasts of burden and 
tions which have developed in the evolu- trusted keepers of the harem. When it was 
tion of vertebrates primarily for the purpose desired to retain the soprano voice of a 



Fig. 16-30. The removal of the single ovary in birds results in the subsequent development of testes and male 
characteristics. In these photos the chickens on the extreme ends are female and male respectively. The animal 
in the middle had its ovary removed at seven w^eeks of age and shows a vt^ell-developed male comb. Autopsy 
showed two fully formed testes. 

particularly talented youth, castration did 
the trick, and thus choirs could be pro- 
duced with remarkable musical qualities. 
Occasionally, the accidental loss of the 
testes in a young boy has resulted in an 
adult with a high-pitched voice, lacking a 
beard, obese and with little of the ambition 
and the usual emotional characteristics 
associated with the male. Castration after 
maturity, however, seems to initiate few, if 
any, of these changes. 

The onset of interstitial tissue activity is 
associated with puberty when pubic hair, 
change of voice, and increased size of the 
genitalia occur. 

If testosterone is injected into a castrated 
animal or a testis is transplanted into some 
part of the body where it can grow and 
secrete testosterone into the blood stream, 
the male secondary sexual characters will be 
restored. Such injections given to a cas- 
trated female will cause it to develop 
masculine characteristics. A perfectly nor- 
mal egg-laying hen can be induced to be- 
come a functional father rooster with comb, 
wattles, and crow and all by castration fol- 
lowed by a series of injections of testos- 
terone. By removing the single ovary from 
a seven-weeks old chick a normal "rooster" 
will result (Fig. 16-30). This can be ac- 
complished in female birds because they 
possess a residual testis and no external 
genitalia. Sex reversal in mammals is 
limited to the secondary characteristics 

Occasionally the testes in mammals fail 
to descend normally into the scrotal sac as 
they should do during the last few weeks of 
gestation. This condition is known as crypt- 
orchidism and males in which it occurs are 
invariably sterile. If, however, the testes 
are brought down into the scrotum by 
surgery, they very soon become functional 
and produce viable sperm. A cryptorchid is 
perfectly virile in every way except fertility, 
that is, he possesses all of the normal char- 
acteristics of the male including sex drive. 
This is because his interstitial tissue is un- 
impaired, so that testosterone is produced 
in proper amounts to allow for normal 
development of his masculine character- 
istics. Upon microscopic examination, these 
testes will show perfectly normal interstitial 
tissue but degenerate non-functional sperm- 
producing tubules. Experiments show that 
if the normal testes of mammals are placed 
back into the body cavity or heated to the 
internal temperature of the animal the 
sperm tubules degenerate. Therefore, ster- 
ility of the cryptorchid is due to the higher 
temperature existing in the body as com- 
pared to the scrotal sac. This is difficult to 
correlate with the fact that the internal 
testes of birds are fertile and the tempera- 
ture is even higher than that of mammals. 
In the long evolution of mammals one fails 
to note the advantage of placing these or- 
gans, upon which the race depends for its 
perpetuation, in such a hazardous position 
when they would be much safer housed 



within the body cavity as are their counter- 
parts, the ovaries. 

Radiations of various kinds, such as 
x-rays, if given in large doses have a lethal 
effect upon the sperm-producing portion of 
the testis. In smaller doses the eflfect may be 
simply to alter the genes which ultimately 
might produce far greater casualties in the 
human race than mere sterility in a few 
individuals. At any rate, people who work 
around x-ray machines must guard them- 
selves carefully or they will become sterile, 
although none of their secondary character- 
istics will be affected. 

Chemically, testosterone is well known 
and is found to be similar to one of the 
female hormones. Other related testicular 
hormones are collectively called androgenic 
compounds and seem to be generally dis- 
tributed throughout the body. They are 
probably substances which are utilized in 
the production of testosterone or they are 
products of its breakdown, because they 
are found in the urine. 

After the testes had been associated with 
male vigor, the intriguing idea of trans- 
planting them or injecting their extracts 
into the body of a senile male caught the 
imagination of early biologists. Long ago, 
Brown-Sequard, a famous physiologist, in- 
jected himself with testicular extracts 
which he professed renewed his vigor. This 
initiated a long series of such experiments 
both on animals and on man himself. The 
results have all been disappointing, and 
today it is believed there are no beneficial 
effects from either the administration of 
testosterone or the grafting of testes. It can 
be concluded that testosterone does initi- 
ate and maintain the secondary sexual char- 
acteristics and probably contributes to sex- 
ual behavior and urge. In man, however, 
the latter function is so complexly inter- 
woven with psychological reactions that it 
is difficult to determine just how much 
effect it really has. 

The ovary. A far more complex battery of 
hormones is produced by the mammalian 

Fig. 16-31. A Graafian, follicle from a rat, sectioned to 
show the egg. 

female generative apparatus than by the 
male. This is due to the recently acquired 
though intricate mechanism of caring for 
the developing embryo, both before birth 
and immediately thereafter. These hor- 
mones are produced in the ovary, although 
others probably occur in various parts of 
the genital tract. 

At birth and throughout the early life of 
a female the potential eggs lie dormant in 
the outer region of the ovary. From puberty 
on they begin to grow. As an egg grows, 
there develops about it a fluid-filled space, 
and the entire structure is called a Graafian 
follicle (Fig. 16-31). These follicles pro- 
duce a hormone, estrogen, which is the 
counterpart of testosterone in the male. The 
influence of estrogen in the blood stimu- 
lates the onset of changes both in body con- 
tours and in the female organs which finally 
result in the mature human female. A 
similar situation occurs in all mammals. 
Before completion of maturity occurs, an- 
other hormone, progesterone, must be pro- 
duced by a special part of the ovary called 
the corpus luteum. After the rupture of the 
Graafian follicle and the liberation of an 
egg (Fig. 16-31), the cavity left fills with 
this tissue which, in turn, produces pro- 


' Id, 


^ corpus luVcum ^ 


--3 ,^-— 


Fig. 16-32. The events that occur during the menstrual cycle in man are graphically outlined. 

gesterone. Once this hormone is released 
the menstrual cycle is initiated in man. 

The menstrual cycle in man and other 
primates has its counterpart in the estrus or 
"heat" cycle in other mammals. It involves 
a distinct rhythmic cycle of sexual activity 
during some part of which the female is 
receptive to the male. Such a cycle is com- 
pleted every 14 days in guinea pigs, twice 
a year in dogs, and once every 5 days in 
rats. In humans and anthropoid apes the 
estrus cycle is complicated, involving a pe- 
riodic sloughing off of the highly vascular 

lining of the uterus every 28 days by men- 
struation. In other mammals the uterine lin- 
ing returns to the resting state with no 
sloughing off or bleeding. The word men- 
struation comes from the Latin mensis, 
which means month. The menstrual flow 
consists mostly of the epithelial lining of 
the uterus together with the incorporated 
gorged blood vessels. The course of events 
that lead up to this dramatic event are 
rather well known, but just tvhy it must 
occur, since it is not found in lower mam- 
mals, lacks an immediate explanation. 



Since we are dealing with a cycle, de- 
scription can start at any point in it (Fig. 
16-32). The termination of the menstrual 
flow may arbitrarily be taken as the start- 
ing point for this discussion. At this point 
a new Graafian follicle begins to form in 
the ovary and as it grows in size it produces 
more and more estrogen. Maturity is 
reached in about 12 to 18 days at which 
time a rupture occurs in its wall and the 
egg is released. This marks the end of estro- 
gen production from this structure but not 
of the hormone itself. The empty follicle is 
quickly converted into the corpus luteum, 
which continues to produce in increasing 
amounts the closely related hormone, pro- 
gesterone, from the 15th to the 26th day. 
These hormones, in addition to bringing on 
the changes already referred to at puberty, 
are thus also responsible for the rhythmic 
menstrual cycle. The production of both 
estrogen and progesterone have a stimulat- 
ing effect on the walls of the uterus, causing 
it to proliferate and to become highly 
vascular in preparation for the fertilized 
egg, if and when it makes its way into the 
uterine cavity. What happens from this 
point forward depends on whether or not 
the egg is fertilized. 

If the egg is not fertilized the corpus 
luteum suddenly retrogresses and proges- 
terone is reduced to zero during the 25th- 
27th days of the cycle. This results in the 
sloughing off of the uterine wall known as 
the menstrual flow, which continues over a 
period of 4-5 days. Another Graafian follicle 
then begins to grow and the cycle is started 
over again. All this is complicated by the 
pituitary hormones which will be discussed 
a little later. It might seem that a rather 
elaborate preparation is made each month 
for the event of pregnancy and that an un- 
necessary waste results when fertilization 
fails to occur. One speculates why some- 
thing a little less pretentious might not be 
satisfactory until it became certain that the 
great climax, fertilization, had taken place. 
But, even though there seems to be little 

justification in its complicated machinery, 
this is the way it is set up. 

If the egg is fertilized as it passes down 
the oviduct, the corpus luteum is retained 
and goes right on producing progesterone 
until just a few days before the end of 
gestation. The zygote is implanted in the 
uterine wall wherever it happens to touch. 
In fact, the highly vascular wall is so recep- 
tive to tiny particles that almost any small 
object is readily picked up by it at this 
time. The walls also produce mucus rich 
in glycogen, which probably acts as a 
source of energy for the early stages of the 
embryo until it gains a secure foothold and 
can withdraw nourishment through its pla- 
centa. With the developing stages of preg- 
nancy, progesterone continues to cause fur- 
ther accommodations of the uterine wall for 
the enlarging embryo. It also causes the 
mammary glands to increase in size, pre- 
vents any further Graafian follicles from 
forming, and inhibits uterine contractions. 
As the end of pregnancy approaches, the 
corpus luteum ceases to produce any more 
progesterone and the uterine wall, which 
has during this time become an accessory 
in producing the hormone, reduces its out- 
put. This precipitates changes which are 
similar to the beginning of menstruation, 
that is, in the absence of progesterone the 
uterine wall begins to degenerate, therefore 
becoming incapable of nourishing the fetus 
any longer. Furthermore, without the in- 
hibiting effect of progesterone, the muscles 
of the uterine wall begin powerful contrac- 
tions which eventually result in expelling 
the fetus. The production of milk by the 
mammary glands occurs after birth due to 
another hormone, lactogen, which is pro- 
duced by the pituitary gland and will also 
be discussed later. 

It takes some time after the birth of the 
offspring for the hormones to readjust them- 
selves and the menstrual cycle once again 
to reestablish itself. This usually does not 
occur until the amount of lactogen from the 
pituitary subsides, which means, of course, 



that the offspring has ceased to rely on the 
mammary secretion as its principal source 
of food. Once lactation ends altogether the 
menstrual cycle begins again. 

The thyroid 

The thyroid, together with the remaining 
glands, are purely endocrine in function. 
The thyroid has various shapes in different 
vertebrates but in man is bilobed and lies 
on either side and under the larynx (Fig. 
16-29). The two lobes are connected by a 
narrow strip of tissue, called the isthmus, 
passing across the trachea. The presence 
of the gland can be determined by merely 
feeling it with the fingers. 

This gland was seen, naturally, by early 
anatomists, and its importance suspected 
because they noted that in certain individ- 
uals it became enlarged, seeming even to 
cause their death. At the beginning of the 
Christian Era the Greek physicians pre- 
scribed the drinking of sea water as a cure 
for goiter (the term used for the swollen 
gland). Later, others gave their patients 
products of the sea, such as dried seaweed 
leaves, which undoubtedly gave some relief 
because all sea products are rich in iodine, 
the important ingredient in the production 
of the thyroid hormone, thyroxin. The exact 
function of the thyroid was not known until 
replacement experiments in 1885 demon- 
strated that the gland did produce a hor- 
mone. This was isolated in pure form in 
1916 and synthesized in 1927 by Haring- 
ton and Barger, two English investigators. 
When this substance is administered to an 
animal deprived of its thyroid, the animal 
remains perfectly normal in every respect. 
If it is denied such treatment, stark meta- 
bohc changes occur which, if prolonged, 
may terminate the life of the animal. What 
is the specific function of this gland? 

It is generally agreed that the thyroid 
gland secretes thyroxin, which controls the 
level of basal metabolism, that is, the rate 
of burning foods and formation of nitrog- 
enous wastes, as well as the degree of 

irritability. Thyroxin must be produced at 
a uniform rate in order that these important 
processes proceed at what is spoken of as 
a normal level. If more or less is produced, 
these processes accordingly increase or de- 
crease in speed with accompanying symp- 
toms that are very definite and easily 
recognized. A diseased thyroid merely pro- 
duces too little or too much of its secretion. 
Underactivity. When the gland fails to 
produce the proper amount of thyroxin the 
effects are somewhat more pronounced in 
a young animal than in an adult. For ex- 
ample, if the thyroids are removed from 
tadpoles or pups the animals will not ma- 
ture properly. The tadpoles will not meta- 
Inorphose into frogs and the pup will not 
mature into an adult dog. Likewise in 
human beings, if a child has a deficient 
thyroid he becomes a cretin. Such a child 
is small and badly formed, with pudgy, 
puffy skin and swollen tongue, and his men- 
tal development is at a rather complete 
standstill. If given thyroxin in the early 
stages of the disease, the child responds re- 
markably well and can grow into a normal 
adult. Obviously, if a human cretin is al- 
lowed to live for twenty years without treat- 
ment, thyroxin will do him little good be- 
cause his body tissues have completed their 
development and can be changed but little. 
If the thyroid becomes atrophied for 
some reason and fails to produce an ade- 
quate supply of thyroxin in the adult, a 
familiar disease known as myxedema re- 
sults. The obvious symptoms of the disease 
are general loss in vigor, reduction in men- 
tal activity, increase in weight, and a thick- 
ening of the skin to give it a puffy appear- 
ance. Less obvious symptoms are a drop in 
basal metabolism, the improper burning 
of food, sometimes as much as 40 per cent 
below normal, a slowing of the heart rate, 
and a lessening in the sex drive. It seems 
that the entire machinery of the body 
slows down. The administration of proper 
amounts of thyroxin or thyroid extracts 
restores the rate of metabolism to its nor- 



mal level, and subsequently all symptoms 
of the disease disappear. Sometimes in sur- 
gery too much of an overactive gland is 
removed and the patient may then find 
that he is suffering from myxedema and 
must take thyroxin all the rest of his life. 
Fortunately, the digestive enzymes have no 
effect on thyroid extract, thyroxin, or even 
the dried gland in contrast to insulin, a fact 
which permits administration by mouth, an 
important detail in the treatment of any 

Fifty years ago the presence of an un- 
sightly enlarged thyroid was very common- 
place in certain parts of the world. Sur- 
prisingly, these regions were rather well- 
defined and in them even the domestic ani- 
mals had goiters. An examination of the soil 
and water showed that there was a marked 
deficiency of iodine. Along with this dis- 
covery, the thyroid secretion was found 
to be remarkably rich in iodine; it was not 
difficult to fit the two together and con- 
clude that goiter appeared in regions where 
there was very little iodine available in the 
food products and water. These areas were 
spotted over the world. In the United States 
they are concentrated along the St. Law- 
rence River and Great Lakes regions. For 
example, in 1924, 36 per cent of the school 
children in Detroit showed incipient en- 
demic goiter, but within 7 years after the 
addition of potassium iodide to table salt 
the incidence had dropped to 3 per cent. 
In view of this experience, it has been pro- 
posed that the word salt be legally recog- 
nized in Michigan as iodized salt and that 
the sale of any other salt in food stores be 

The presence of a goiter does not neces- 
sarily mean that tlie gland is under- or over- 
active. It does mean, however, that there is 
some sort of disturbance in the thyroid 
output. It may be compensating for the 
lack of iodine and does this by producing 
more thyroid tissue in an effort to supply 
sufficient thyroxin to keep the body at a 
normal basal metabolic level. Such com- 

pensating action is not uncommon in other 
parts of the body, for example, an enlarged 
heart muscle. If the thyroid cannot main- 
tain a normal level of thyroxin, myxedema- 
tous symptoms may be evident. If it can 
supply the proper amount there are no 
symptoms of the disease, although the indi- 
vidual harbors the greatly enlarged gland 
on the front of his neck which is at least 
inconvenient, if not embarrassing. 

Overactivity. For some unknown reason, 
the thyroid sometimes begins sponta- 
neously to produce more thyroxin than the 
body needs, and this may be accompanied 
by a slight enlargement of the gland. It 
differs from the simple goiter described 
above because while it may not be en- 
larged, or only slightly so, its output is 
far greater. Obviously, abnormally high 
amounts of hormone in the blood stream 
increase the rate of burning foods (30 per 
cent or more) and speed up all tlie bodily 
activities. More food is consumed, yet there 
is a wasting away of the body. Profuse 
sweating occurs, the heart is overworked, 
and external heat cannot be tolerated. All 
this step-up produces a highly irritable and 
nervous individual who is continually on 
the move but accomplishing very little. The 
action is like running an automobile at top 
speed with the brakes set. It is clear that 
such activity will soon result in the de- 
struction of the organism itself. 

The control of such a condition, which is 
called exophthalmic (from the fact that it 
sometimes causes the eyes to bulge out of 
their sockets) or toxic goiter, is by destroy- 
ing a part of the cells that produce the 
hormone ( Fig. 16-33 ) . This is easily accom- 
plished by surgery and such operations are 
very successful. In this age of the atom a 
new method has been discovered which is 
sometimes employed when for some reason 
or other it is inadvisable to operate. It is the 
use of radioactive iodine. Iodine, when sub- 
jected to atomic radiations, becomes radio- 
active itself. Such a form is called an iso- 
tope. Since the thyroid picks up about 80 



Fig. 16-33. A case of hyperthyroidism. 

per cent of all the iodine taken into the 
body, it can be determined beforehand the 
exact amount that will be delivered to 
the gland shortly after swallowing isotopic 
iodine. Furthermore, radiations are known 
to destroy thyroid tissue; therefore, by feed- 
ing radioactive iodine "cocktails" to the pa- 
tient, a certain amount of success has been 
had in destroying a part of the gland. Its 
more satisfactory use, however, is in the 
treatment of cancer of the tliyroid. 

The parathyroids 

In the early studies of the thyroid much 
confusion resulted because in removing the 
gland the four tiny parathyroids embedded 
in the thyroid were inadvertently removed 
also (Fig. 16-29). The symptoms that fol- 
lowed this were not only the result of thy- 
roid deficiency but also of parathyroid de- 
ficiency. The small parathyroid glands were 
discovered in 1891 and many years later 
their true function was determined. 

If the parathyroids are removed from an 
animal, injected extracts of a hormone 
(parathormone) produced by the glands 
will keep that animal in good health. If no 
extract is given, the animal suffers from 
severe muscular tremors, cramps, and fi- 
nally convulsions. The composite symptoms 
are called tetanus, and without treatment 
the animal passes into a coma and death 
soon follows. The parathyroids or their ex- 
tracts are essential for life in mammals, in- 
cluding man. The value of the parathyroids 
seems to be in maintaining proper levels of 
calcium and phosphorus in the blood. 
When the glands are removed the blood 
calcium level falls rapidly, which correlates 
with the symptoms of the disease. Adminis- 
tration of calcium will prevent symptoms 
of parathyroid deficiency. If the glands pro- 
duce an overabundance of the hormone, the 
calcium level in the blood then rises too 
high and even the calcium of the bones is 
sacrificed, so that a weak, twisted skeleton 
is the result, rendering the unfortunate in- 
dividual a cripple. 

The adrenals 

The adrenals are located on the upper 
inner edge of each kidney, as one might 
guess from their name (Fig. 16-29). Their 
combined weight is no more than an ounce, 
and each is composed of two parts, an outer 
covering called the cortex and an inner 
dark-colored mass called the medulla. The 
gland is therefore a composite one and each 
part has a separate origin, the cortex com- 
ing from the mesodermal lining of the coe- 
lom whereas the medulla is derived from 
a part of the neural tube. One might expect 
structures of such different origins to pos- 
sess different functions and they do. 

The medulla. The medullary portion of 
the adrenal produces a single hormone 
called adrenalin, or sometimes adrenin or 
epinephrine. Adrenalin has been analyzed 
chemically and its formula determined. It 
has also been synthesized from sources 
other than adrenal glands. Related syn- 



thetic compounds such as ephedrin produce 
similar effects when administered to ani- 
mals. If the medullary portion of the 
adrenals is removed from an animal, death 
does not follow nor is the animal markedly 
affected by its loss. If injections of med- 
ullary extract are given to such an animal 
or one with intact adrenals characteristic 
changes occur rather rapidly. The heart ac- 
tion becomes stronger and the blood vessels 
to the skin and viscera constrict, sending 
most of the blood to the muscles, brain, and 
lungs. The hair "stands on end," the pupils 
dilate (wide-eyed), and the skin blanches. 
The spleen constricts, forcing its reserve of 
blood out into the general circulation, and 
simultaneously the blood's ability to clot is 
stepped up. More glycogen in the liver is 
converted to glucose, so that the total 
amount in the blood is definitely increased. 
This chain of events prepares the body for 
undue stress such as occurs in a fight or a 
sudden retreat. The body is made ready to 
function to the maximum of its ability in 
case a sudden burst of energy is needed. 
Provision against possible injury is afforded 
by the increased speed of blood coagula- 
tion. This whole series of effects is similar 
to excitation of the sympathetic nervous 
system. Thus, both the nervous system and 
the adrenal medulla play an important role 
in fear and ancrer. Knowledge of this fact 
has lead to the so-called "emergency theory 
of adrenal function." 

The cortex. The cortex of the adrenal is 
essential for life, although when even such 
a small portion as one-fifth of the total 
gland tissue is left, life is undisturbed. Its 
product or products are not as simple as 
adrenalin. They are numerous; in fact, over 
20 such compounds have been isolated in 
recent years. The first substance, isolated 
in 1930, was called cortin and was effective 
in treatment of people suffering from Addi- 
son's disease, which is the name identified 
with a deficiency of this portion of their 
adrenals. Since that time, many compounds 
have been produced, the best known and 

most effective being cortisone. It is interest- 
ing to note that all of these are steroid (fat- 
like) compounds which are closely related 
to the gonadal hormones, testosterone and 
progesterone, and, as will be seen below, 
produce some effects on the gonads as well 
as other parts of the body. 

If the cortex fails to function, marked 
changes occur which are fatal if uninter- 
rupted by treatment. The carbohydrate 
metabolism is greatly affected, as indicated 
by a drastic drop in blood sugar, because of 
the inability of the enzymes to convert the 
proper amounts of proteins to carbohy- 
drates and then to convert the latter to 
sugar in the liver. Salt (NaCl) is lost from 
the blood and tissues at a rapid rate which 
reduces the entire blood volume and with 
it the blood pressure. As Addison's disease 
progresses, the sexual functions fail due to 
an actual atrophy of the Graafian follicles 
and the seminiferous tubules. 

If, on the other hand, the gland becomes 
overactive as a result of irritation caused 
by a tumor, changes of a different kind 
occur. In males, the maleness is greatly en- 
hanced, accompanied by excessive hair 
growth. If it happens to a very young male 
child the sex organs may become fully ma- 
ture (except the testis) within the first or 
second year of life, and the hair, muscula- 
ture, and voice resemble that of an adult 
man. These are very rare cases, fortunately. 
In females, the situation is even worse. If 
the overactivity occurs in an adult woman 
the changes are all toward maleness; the 
beard grows (the bearded lady in the cir- 
cus ) , the body becomes more muscular, and 
the voice deepens. Even the female sex or- 
gans begin to atrophy and become non- 
functional. These effects of the cortical hor- 
mones are not well understood, in spite of 
the sudden burst of experimentation result- 
ing from the recent discovery that corti- 
sone, when given in controlled doses, has 
a beneficial effect on a large number of 
diseases, among them arthritis. Because the 
hormone's beneficial effects cover such a 

in+ersfitial cells 

Fig. 16-34. A schematic sketch showing the various hormones produced by the anterior lobe of the 

pituitary and the other endocrines affected by them 



wide variety of diseases, some biologists 
believe that we have an entirely new ap- 
proach to the study of organic disease. 
However, it is too early to draw any conclu- 
sions from these observations. 

The pituitary 

The last and perhaps the most complex 
of all the endocrine glands is the pituitary, 
or hypophysis. Located in approximately 
the middle of the head, it lies in a bony 
capsule and is attached to the base of the 
brain by a slender stalk, the infundibulum 
( Fig. 16-34 ) . Like the adrenals, the hypoph- 
ysis is a double gland, composed of two 
principal lobes: the anterior, which arises 
embryologically from an outpocketing of 
the roof of the pharynx; and the posterior, 
which originates as a solid outgrowth from 
the floor of the brain. The point of contact 
with the brain through the infundibulum 
is retained while all connections with the 
pharynx are lost very early in embryologi- 
cal development. The anterior lobe is the