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61059

Accession No.

^. n McGrsYZ-Hiil Book Co., Inc.

^'"'" ^y— ^ Ngvr York City —

Place,,

McGRAW-HILL PUBLICATIONS IN THE

ZOOLOGICAL SCIENCES

A. FRANIvLIN SHULL, Consulting Editor

PRINCIPLES OF ANIMAL BIOLOGY

Selected Titles From

McGRAW-HILL PUBLICATIONS IN THE
ZOOLOGICAL SCIENCES

A. Franklin Shull, Considtiyig Editor

Baitsell • Human Biology

Breland ■ Manual of Comparative Anatomy

Burlingame ■ Heredity and Social Problems

Chapinan ■ Animal Ecology

Clausen ■ Entomophagous Insects

Frost • General Entomology

Goldschmidt • Physiological Genetics

Graham ■ Forest Entomology

Haupt ■ Fundamentals of Biology

Hyman ■ The Invertebrates: Protozoa through Ctenophora

Johannsen and Butt ■ Embryology of Insects and Myriapods

Metcalf and Flint ■ Insect Life

Mitchell ■ General Physiology

Mitchell and Taylor • Laboratory Manual of General Physi-
ology

Pearse ■ Animal Ecology

Reed and Young ■ Laboratory Studies in Zoology

Riley and Johannsen ■ Medical Entomology

Rogers ■ Textbook of Comparative Physiology

Laboratory Outlines in Comparative Physiology

Senning ■ Laboratory Studies in Comparative Anatomy

Shull ■ Evolution

Heredity

Principles of Animal Biology
Shall, LaRue, and Ruthven ■ Principles of Animal Biology
Simpson and Roe ■ Quantitative Zoology
Snodgrass ■ Principles of Insect Morphology
Storer ■ General Zoology

Laboratory Manual for General Zoology
Van Cleave • Invertebrate Zoology
Welch • Limnology
Wieman ■ General Zoology

An Introduction to Vertebrate Embryology
Wolcott ■ Animal Biology

There are also the related series of McGraw-Hill Publications in
the Botanical Sciences, of which Ednuiml W. Sinnott is Consulting
Editor, and in the Agricultural Sciences, of which Leon J. Cole is
Consulting Editor.

^ ?z

PRINCIPLES OF

ANIMAL BIOLOGY

A. FRANKLIN SHULL

Professor of Zoology in the University of Michigan

WITH THE COLL-IBORATION OF

GEORGE R. LARUE

Professor of Zoology in the University of Michigan
AND

ALEXANDER G. RUTHVEN

President of the U niversity of Michigan

Sixth Edition
Second Impression

McGRAW-HILL BOOK COMPANY, Inc.

NEW YORK AND LONDON
1946

principles of animal biology

printed in the united states of AMERICA

parts thereof, may not be reproduced

in any form without permission of

the pubiishers.

To the Teachers of
ZOOLOGY

WHO HAVE BEHELD THEIR SUBJECT
OUTGROW A PEDAGOGICAL METHOD

PREFACE

The changes introduced in this sixth edition are more than usually
varied. While none can be regarded as radical, they affect in important
ways nearly every part of the general plan. The book is still devoted to
principles; indeed, the changes appear even to emphasize its devotion to
fundamental concepts.

If any one statement can be made which would characterize much of
the alteration now made, it is that the treatment of function has been
increased or clarified or thrown into relief by emphasis. Such changes
relate, among others, to enzymes, photosynthesis, oxidation, muscle
action (including cardiac), breathing movements, transfer of respiratory
gases, blood composition, the clotting process, kidney function, vitamins,
endocrines, the placenta, and reflex arcs. The authors have not hesitated
to give chemical formulas and reactions that beginning students need not
be expected to remember or reproduce, because these exact forms of
expression carry conviction concerning the precision of present knowledge
Avhich no more general statement can produce.

Greater clarity of exposition has been sought at many places by illus-
trations and slight changes of language or inclusion of features not
heretofore expressly described. Comparisons that were formerly illus-
trated by figures borrowed from research contributions are now in several
instances portrayed by simplified diagrams placed side by side with the
contrasts indicated. Among the phenomena thus treated are symmetry,
centralization and cephalization of the nervous system, endocrine secre-
tions, the hydroid metagenetic cycle, and the evolution of living things
in geological time. More explicit description is the method adopted for
the types of circulatory and excretory systems, for the operations of the
kidney, for the biogenetic law, and others.

Order and emphasis have occasionally been changed at the suggestion
of teachers elsewhere, even when the authors were not quite convinced
that the new method was an improvement but could see no objection to
it. Molecules and atoms have been introduced before protons, neutrons,
and electrons. The names of the phases of mitosis have been restored
in the belief that under the guidance of a good teacher cell division will
still be conceived as a continuous process. Biological terms have been
introduced in a number of places with the conviction that names some-
times clarify ideas, simply because terms must have definitions. Yet the
glossary is today shorter than in the early editions.

X PREFACE

In the treatment of genetics the simple phenomena have been described
in less space than formerly but, it is believed, with greater clarity.
Description of the mechanism in advance of its operation, a method
used with success in genetics courses, should contribute to this result.
The two linkages have been restored to the general text — at the request
of teachers and in conformity with the authors' preference. If the work
in genetics is to be shortened in any institution, this can still be done by
omitting the later parts of the chapter, for the topics are treated in the
order of their importance and desirability for beginning students. The
problems in genetics have been modified to call for precise (usually
numerical) answers, not for charts or discussions. There is no reduction
in the thought or organization required of the student in solving them;
he merely gives one specific part of his conclusion instead of all of it,
which should facilitate checking his accuracy.

Among the more general of the other changes should be mentioned the
addition of marine habitats to the chapter on ecology, a considerable
extension of the historical treatment in zoogeography, and an enlarge-
ment of the account of prehistoric man.

To compensate in part for the increase of space that many of the
foregoing revisions entail, omissions and condensation have been effected
elsewhere. The authors will be interested to learn whether the omissions
are missed.

One incidental consequence of these extensive revisions is the removal
of some distinct contrasts of literary style, which are seemingly unavoid-
able results of joint authorship. While the present style may not be
better, uniformity of style is surely to be desired. •

As usual, the authors' colleagues who use this book in an elementary
course have been generous with suggestions for improvement. Among
teachers in other institutions who have furnished ideas, special mention
should be made of Prof. Roy D. Shenefelt, whose recommendations could
have come only from a well-considered philosophy of teaching.

A. Franklin Shull.

Ann Arbor, Mich.
May, 1946.

RK*^^^'

-♦ g^ ^ CONTENTS

Page

Preface ix

Chapter

1. The Growth and Scope of Biology 1

2. Primary Organization of Living Matter 23

3. Some Fundamental Physics and Chemistry 30

4. The Functions of Protoplasm and Cells 39

5. Cell Division 55

6. From One Cell to Many Cells 64

7. Basic Organization of the Metazoa 77

8. Physical Support and Movement 87

9. Sources of Energy and Materials 100

10. Respiration and Release of Energy 113

11. Transportation System 122

12. Disposal of Wastes 133 .

13. Integration of Activities 140

14. Reproduction 159

15. The Breeding Behavior of Animals 177

16. Embryonic Development 193

17. Genetics 222

18. Principles of Taxonomy 244

19. The Groups of Animals 259

20. Animals and Their Environment 281

21. Geographic Distribution 307

22. Fossil Animals 325

23. Modification of Species 349

Glossary 369

Index 407

61059

PRINCIPLES OF ANIMAL BIOLOGY

CHAPTER 1
THE GROWTH AND SCOPE OF BIOLOGY

When knowledge can be classified and organized on some basis
which exists in nature, not just in the minds of men, the body of knowl-
edge so arranged is science, or a science if the field is limited. The devotees
of science seek to discover the natural principles which control the
phenomena they observe. The more fundamental the ascertained
principles are, the more significant the science is. The first step in dis-
covering principles is usually observation of the facts or phenomena
which require explanation. Very often the second step is experiment,
or interference with natural events, with possible alternative outcomes
in mind. Finally, there is the logical consideration of all facts to see
what relation exists among them.

When the phenomena studied are those of living things, the organized
knowledge is called biology. It is not distinct and separate from other
sciences, for all life processes are fundamentally physical and chemical.
Indeed, no science is a province unto itself, for the constitution of matter
and energy, with which they all deal, is everywhere the same. Each
division of the field of science emphasizes certain types of phenomena,
but the more the various fields can be intertwined the more fruitful
scientific work becomes.

The several sciences have also been interrelated as they developed
over the centuries. Let us see how biology has shared in the early stages
of this growth and what it has come to be in later times.

Ancient Civilizations. — Among primitive peoples knowledge and
superstition regarding life came chiefly from three sources: from their
wonder and awe at the phenomenon of death and customs relating to
the preservation of the bodies of the dead, from their fear of the great
wild beasts, and from their attempts to cure disease and heal injury.
The earliest known civilization is that of Babylon. Medical science,
which is the form that early biology most often took, made some progress
there. Clay models of various organs of the human body have been
preserved, and Babylonian writings show that two kinds of blood, light
and dark, were recognized. The heart, however, was regarded as the
seat of intelligence. In Egypt, another very old civilized country,

2 PRINCIPLES OF ANIMAL BIOLOGY

embalming of the dead led to a knowledge of anatomy, and an art of
healing based not upon superstition but upon observation was developed.
The Israelitic tribes borrowed their scientific knowledge from other
peoples and clothed it with a religious significance but added nothing to
the store of biological information. The other great peoples who might
have contributed to early biological knowledge were interested in other
branches of culture — the Hindus in mathematics, the Chinese in ethical
and social problems.

Early Greeks. — It was among the Greeks, therefore, that biology
received its first great impetus. The passion of these people for intel-
lectual inquiry was due partly to their innate qualities but in part to
the practical absence of powerful restrictive governmental and religious
organizations. The Ionic tribes, coming into contact with the cultivated
peoples of the East, through their colonies in Asia Minor, developed the
earliest natural philosophers. One of these was Thales (about 650-580
B.C.), who, though he left no writings, is reputed to have regarded water
as the source of all things, including life. Anaximander (about 611-
546 B.C.) entertained a theory of the origin of the universe from a vague
something which he called "apeiron," but his chief concern with biology
was his supposition that living things arose from mud. First, he thought,
came the lower animals and plants, and then human beings; but the
latter were in the form of fish and lived in water. Later these human
beings cast off their fish form and lived on land. This view of the origin
of living things was adopted by Diogenes of Apollonia (not the famous
cynic Diogenes), who conceived that the agent which brought forth
living things out of the earth was solar heat. Diogenes was the author
of the earliest known work on anatomy, fragments of which are still
preserved, and his ideas of human embrj^onic development give evidence
of being based on dissection.

Some of the more important of the remaining Greek natural phi-
losophers came from the colonies of the west. Xenophanes, who had
wandered to southern Italy, is chiefly noted for his discover}^ of fossils,
his recognition that they were animal remains, and his conclusion there-
from that in some cases what are now mountains were once under the
sea. He died about 490 b.c. Another western Greek was the braggart
Empedocles, in Sicily, who lived about the middle of the fifth century
before Christ. Among the many things which he boasted of doing, he
appears actually to have rid a neighboring town of malaria by draining
the district. On the theoretical side of his biology, he conceived living
things to have arisen out of the earth, plants having come first. Animals
arose in the same way, but in pieces. Separate limbs, trunks, etc., arose,
kept apart by the force of hate. When love triumphed, these members
joined in accidental manner. Some such combinations were malformed

THE GROWTH AND SCOPE OF BIOLOGY

monsters incapable of life; others, more fortunately constructed, survived
and gave rise to the animals of today. The blood he regarded as the seat
of intelligence, the eye he likened to a lamp, and respiration he thought to
occur partly through the skin.

Democritus. — More important for natural science than any of his
predecessors was Democritus (Fig. 1) who was born about 460 B.C.
Chaste in morals and temperate in habits, he lived to the ripe age of a
century. Curious about the world, Democritus spent his patrimony in
travel, then lectured for pay to avoid the serious Greek charge that he had
wasted it. His interests were exceedingly inclusive, and he is best known
for a materialistic ("atomic") theory
of the universe, some features of which
have a distinctly modern flavor.
While it was through his general
philosophy that he most influenced
subsequent thought, not a few strictly
biological concepts are found in his
writings. He distinguished types of
animals differing in the quality of their
blood, a basis of classification later
adopted by Aristotle. In embryonic
development, he supposed the external
organs arose first, the internal struc-
tures later. He knew that mules are
sterile and conceived an anatomical
reason for it. He regarded the brain as
the organ of thought, the first of the natural philosophers to do
so. In his more subtle theoretical ideas, Democritus was strictly
materialistic; even the soul was regarded as a material thing, consisting
of globules of fire which impart movement to the body. He represents
the climax and close of the first scientific period of Greek philosophy,
which was an era of search for purely natural causes.

Hippocrates. — A contemporary of Democritus was Hippocrates,
the Father of Medicine. What Hippocrates actually wrote is not cer-
tainly known. A collection of about a hundred works has been attributed
to him, but many of these were probably not his. His interest was
scarcely scientific, but rather in the healing of men; yet in one of the
works on diet in the collection is a reference to an attempt to classify
animals. While the study of medicine is biology, Hippocrates treated
it as an art; his descriptions of operations are models of clarity. The
social and moral responsibilities of physicians engaged his attention,
and a famous oath administered to medical graduates was based on his
teaching.

Fig. 1. — Democritus.

4 PRINCIPLES OF ANIMAL BIOLOGY

Aristotle. — A reaction set in against the materialistic conceptions
of Democritus and others. Philosophy came to be dominated by
Socrates, who was interested in ethics, and by Plato, who found true
reality in the world of abstract thought. The latter says expressly
that no true knowledge is to be attained through observations of the
senses. One leading philosopher who came under Plato's influence was
Aristotle (384-322 b.c.) (Fig. 2), the greatest of the early biologists, to
whom the essence of living things was their form. Everything that
happens, he taught, is due to a supreme intelligence, everything is done

Fig. 2.— Aristotle, 384-322 b.c. {From Heklcr, "Greek and Roman Portraits," G. P. Pul-

nam,'s Sons.)

for a purpose, and the primary purpose in nature is the development
of a higher form. As a result of this continuing purpose, there has been
an evolution from lower types to higher ones.

Despite his leaning to supernatural causes, Aristotle made some excel-
lent observations in biology and sought to organize them wherever possi-
ble. He classified animals according to their mode of life and their
structure and knew over five hundred kinds, all Greek; those from other
countries he knew only from descriptions. He insisted that the study of
anatomy should be comparative, which is a fruitful procedure at the
present time. The heart was regarded as the organ of the soul and
intelligence; here Aristotle drops behind his predecessor Democritus.
Digestion was to him a process of ''cooking." Nerves were confused with
tendons; the brain was thought to be cold and the spinal cord hot. Fleas

THE GROWTH AND SCOPE OF BIOLOGY 5

and mosquitoes were held to arise by spontaneous generation out of putre-
fying substances, while other insects originated through sexual reproduc-
tion. His descriptions of the embryonic development of animals, mostly
the chick and certain marine forms, are rather accurate. He devised an
ingenious scheme of heredity and regarded temperature as a sex-determin-
ing agent. He believed that the future of a man could be read from the
lines of his palms and that flat-footed people have treacherous disposi-
tions. Indeed, a curious mixture of truth, error, and superstition!

Aristotle's greatness in biology lay not so much in his discoveries as
in the fact that he devised a system of thought that dealt with the entire
realm of living things. He has long been credited with insisting upon
the inductive method, in accordance with which one first collects facts
and then draws conclusions based upon them. Other philosophers had
been prone to reach a conclusion first and then to decide what the facts
must be to accord with the adopted principle. Aristotle did more than
urge the inductive method, he used it — part of the time. In general, his
work in natural history followed this method. For his scheme of the
universe, however, he had not enough facts at his disposal, and here he
drew upon fancy. As a consequence, his concept of the cosmic system
had what modern biologists consider a serious fault in that it called for
the guidance of nature by an outside intelligence. Democritus had come
nearer than he to the modern scientific view in that he postulated a natural
necessity which determined the course of events; but Democritus had no
inclusive theory relating to living things in particular.

Pliny. — At the time of Aristotle's death, Greek culture was already
declining, so that the accomplishments of this naturalist-philosopher
represent the highest attainment of antiquity in most fields of science.
His successors and followers include Theophrastus, generally regarded as
the founder of botany, and a number of others by none of whom was any
notable advance made. Specilized phases of biology fared a little better,
particularly anatomical studies at Alexandria.

Rome did not advance far until a much later time. Her chief biolo-
gist of this period was Pliny (a.d. 23-79), who is best known through his
"Natural History" of 37 volumes. This work was a curious compilation
of all the stories of nature which the author was able to gather. Nothing
appears to have been rejected, so that fantastic fables abound, along with
reliable accounts of the habits of animals, their utility, the particulars
of cattle husbandry, etc. Pliny had recourse to two thousand books
in the preparation of his "Natural History," and for fifteen centuries
thereafter this work supplanted all of them in the popular mind as the
source of information regarding natural objects. The author did not,
however, add anything of importance to the store of knowledge by his
own observations.

6 PRINCIPLES OF ANIMAL BIOLOGY

Galen. — Rome, though succeeding to a dominant position in world
affairs, did not foster learning in scientific fields. Instead of an intellec-
tual revival during her period of prosperity, there was a notable decline.
Pliny lived in the midst of this decline. The last great biologist of
antiquity was Galen (131-210?), a physician living in Rome but of Greek
parentage. He dealt mostly with human anatomy and reveals a pro-
found admiration for the creator of so marvelous a mechanism. Every
organ had its use and was constructed on the plan best calculated to
serve that end. He was obliged to study these organs mostly in other
animals, for dissection of human bodies, once permirjsible, was in Galen's
time forbidden. When he describes the human hand, it is obvious that
the object before him is the hand of an ape. His errors are mostly
traceable to this necessity of using other animals.

His accomplishments are numerous, such as his proof that the arteries
and the left side of the heart contain blood, instead of air as others sup-
posed, and his inference that the arteries and veins must be connected.
He seems not to have been fully appreciated in his own time, yet Galen's
books were for many centuries thereafter the standard of reference.
They were used in the medical schools, where anatomy was taught from
the desk with little or no demonstration, and modern criticism has given
to him a high measure of praise.

The Dark Ages. — The thousand years and more following Galen's
time constitute the dark ages for biology as for other fields of learning.
Among the Arabs, who were dominant in the East, mathematics, astron-
omy, and chemistry made some advance, but writings in the field of
biology were mostly commentaries on the works of Aristotle and of Galen.
The division of the Roman Empire and the ravages of migratory peoples
in the West were not conducive to learning. Universities arose beginning
about the eleventh century, but these came to be controlled by religious
orders. The churchmen, finding a powerful ally in Aristotle's conception
of the earth as the center of the universe and his belief in a dominating
intelligence directing natural phenomena, turned the reverence in which
ancient philosophy was held to their own advantage. It took little guid-
ance from them to ensure that biological inquiry should consist merely
of commentaries on the writings of Aristotle, with no effort to ascertain
facts afresh. The views of the Greek natural philosopher were accepted
as correct even where simple observations could easily have proved them
wrong. The few books about animals which appeared in this era, aside
from the commentaries mentioned, contained only entertaining stories
and notes on the usefulness of animals to man.

To deliver biology from the dominance of Aristotle, it was necessary
to destroy his system of thought. Aristotle, as was pointed out earlier,
based his theory of a universal order on an outside intelligence which

THE GROWTH AND SCOPE OF BIOLOGY 7

directed the transformations of matter. This outside intelligence was
naturally not subject to inquiry, and it was this feature of the Aristotelian
doctrine which won to him the support of the conservatives of the Middle
Ages. The uprooting of this system of thought required time, and it was
not until the seventeenth century that other well-defined systems of
philosophy replaced it. In the meantime biology was struggling up
out of the inaction of the Middle Ages, through the period of the
Renaissance.

The Revival. — In the early part of the period of renewed interest in
learning, several works on natural history appeared, which showed they

Fig. 3. — Andreas Vesalius, 1514-1564. {From Garrison, "History of Medicine," W.B.

Saunders Company.)

were based in part upon observations made by their authors. The leader-
ship in the revival, as far as it concerned biology, was taken by Andreas
Vesalius (1514-1564) (Fig. 3), an anatomist. Born at Brussels, he went
to Paris at the age of eighteen to study medicine and there showed great
independence and force of will. After several years of practice he was
called to the University of Padua, in Italy, where everything was favor-
able to his work. In his teaching he first followed Galen but soon found
the latter incomplete and in places self-contradictory. He then realized
tha.t he must teach from his own observation and, to make this possible,
published two anatomical works which were masterpieces. His over-
throw of Galen infuriated conservative anatomists, including Vesalius's

PRINCIPLES OF ANIMAL BIOLOGY

revered teacher Sylvius, himself an anatomist of high reputation. Vesa-
lius was charged with all sorts of crimes, from being godless and sordid
to dissecting men alive. This persecution finally drove him to resign
his professorship, after which he was physician to Emperor Charles
V. Upon the succession of the less liberal Philip II, Vesalius found small
opportunity for creative work. He left the court and tried to regain
his old post at the university but died on a journey to Jerusalem before
the appointment was made. His ideas of anatomy, and particularly
of the functions of the organs, were not wholly correct. Some of them

were borrowed from Galen, whom
he still admired, and now seem
absurd. His great contribution
was his overthrow of authority
and his return to firsthand obser-
vation as the basis of knowledge.

Harvey and the Circulation of
the Blood. — One of the sharpest
reactions against the authority of
antiquity, and one of the most
hotly contested, was the recogni-
tion of the circulation of the blood.
Against the prevailing early view
that the arteries conveyed air,
Galen had held that they carried
blood; but he was never clear how
the arterial blood became converted
into venous blood, and in the veins
he definitely supposed the blood
to flow in both directions alternately. His views on this question
were still accepted in the sixteenth century.

The first recognition that the entire course of the blood is a circulation
is found in the works of William Harvey (1578-1G57) (Fig. 4), of England.
He proved that the wall of the heart is muscular and that its contraction
drives the blood forward into the arteries; in the old theory the heart
was regarded as passive. By a simple calculation he demonstrated that
the quantity of blood passing through the heart in a very short time
exceeded the weight of the whole body and reasoned that new blood could
not be produced at such a rate. He showed by the swelling of the veins
below a ligature, and by the point of exit of blood at a wound, that blood
flows toward the heart in veins and away from it in arteries. He con-
cluded as a logical necessity that there must be a connection between
arteries and veins, but without a microscope he could never visually
demonstrate the capillaries.

Fig. 4.— William Harvey, 1578-1657
{From Garrison, "History of Medicine.")

THE GROWTH AND SCOPE OF BIOLOGY

Besides correcting an ancient mistake, Harvey performed a service to
biology in making it an experimental science. While others before
Harvey had occasionally used experiments, he gave the method a strong
impetus. But while Harvey was modern in his method of solving
problems, at the same time his concept of life and its manifestations in
general was no more advanced than was that of Aristotle.

The Seventeenth and Eighteenth Centuries. — The two centuries
following Harvey mark a distinct phase in the development of biology.
The lethargy of the Middle Ages had been definitely cast off, and the
spirit of inquiry was again prevalent among intelligent people. Two

Fig. 5. — Two early microscopes. Left, that used by Robert Hooke; right, from eight-
eenth century. (From "Educational Focus," Bausch & Lomb Optical Co., and American
Museum, of Natural History.)

general concepts of natural phenomena arose, one of them mechanistic,
the other mystical ; and the history of biology ever since has been in part a
conflict between these two systems of thought, with the former steadily
gaining ground. The science of chemistry was coming to the aid of
biology by enabling physiology to seek for purely mechanistic explana-
tions of life processes. Following Harvey's proof of the circulation came
the dicovery of the lymphatic system of vessels carrying digested food
from the intestines to one of the larger veins. The nervous system was
more thoroughly studied, and the functions of the divisions of the brain
began to be understood. The contraction of muscles was explained by
fermentation — incorrectly, but it is significant that the role of chemistry
in living matter was recognized. However, the early advances were
mostly in the field of morpholog}'', the science of structure.

10 PRINCIPLES OF ANIMAL BIOLOGY

The Microscope. — One important aid to the mechanistic theory of
living matter was the invention of the compound microscope. The
refractive power of glass had long been known, and simple lenses had
come to be used in the sixteenth century for spectacles and as scientific
toys. The combination of two or more lenses in a tube to form a com-
pound microscope is generally attributed to Zacharias Jensen and is
said to have been first used about the year 1591. During the following
century considerable improvement of these instruments was effected. An
early microscopist, Robert Hooke (page 15), described the one at the

Fig. 6. — Marcello Malpighi, 1628-1694. {From Garrison, ''History of Medicine," after the

painting by Tabor, Royal Society.)

left in Fig. 5, while a moderately improved one is on the right. Almost
no further improvement was made thereafter for a century and a half.

The founder of microscopic anatomy was Marcello Malpighi (1628-
1694), of Italy (Fig. 6). He studied the lungs and observed the capil-
laries, thus confirming the theory that blood circulates through them.
He also examined various glands, the embryo of the chick, the structure
of the silkworm, and the tissues of plants. His work on plants was
extensive, and, with Nehemiah Grew (1628-1712) of England, he became
the founder of plant anatomy. Anton van Leeuwenhoek (1632-1723)
(Fig. 7), of Holland, stepped out from behind his dry goods and notion
counter often enoiigli to become one of the most skillful of the makers of
lenses; one of his lenses, still in existence, magnifies two hundred and
seventy times. He made these for his own use, never sold one, and never

THE GROWTH AND SCOPE OF BIOLOGY 11

loaned one. Everything that could be observed with a microscope
became an object of his study. The biological objects included were the
blood capillaries, red blood cells, spermatozoa (male germ cells), striated
muscle, the crystalline lens of the eye, the eggs of insects, and minute
organisms in pond water. Another Dutchman, Jan Swammerdam (1637-
1680), besides some work on gross anatomy, studied the minute anatomy
of insects and snails and the development of the eggs of various animals.
Microscopes existed in America in the seventeenth century, but no
important use of them in biology appears to have been recorded.

Fig. 7. — Anton van Leeuwenhoek, 1632-1723. {From Garrison, "History of Medicine.")

Classification of Animals and Plants. — One of the early trends away
from structure was the series of attempts to classify living things. Efforts
to systematize the listing and arrangement were made in very early times
by Plato and Aristotle. These were very simple; Aristotle mentions by
name only two ranks, which correspond roughly to the species and family
of our present classification. When the great geographic discoveries of
the sixteenth and seventeenth centuries were made, and many new
animals became known, such simple groupings were of little use. The
first classification worthy of note was that of John Ray (1627-1705),
in England. Ray's idea of the species was very similar to that of the
present time. He grouped similar species into a genus, but his genera
were much more inclusive than at present. Anatomical likeness was the
basis on which species were grouped together, though he allowed old

PRINCIPLES OF ANIMAL BIOLOGY

prejudice to prevail in some cases, as when he included the whales with
the fishes despite his knowledge that they more closely resemble the
mammals.

It was Carolus Linnaeus (1707-1778) (Fig. 8), however, who made the
greatest advance in classification. Of a Swedish family and trained to be
a physician, he yielded to his interest in natural history and was even-
tually named professor of botany in the University of Uppsala. He had a
passion for arranging all sorts of natural objects into groups on the basis
of like qualities. The choice of qualities to form the basis of this clas-
sification was sometimes arbitrary, especially in his earlier years, as

Fig. 8. — Carolus Linnaeus, 1707-1778, in Lapland dress at the ago of thirty.

of New York Botanical Garden.)

{Courtesy

when he classified plants according to the number of stamens and pistils
in their flowers. In later life he recognized that likeness in a single
character, in the absence of other similarities, was not a safe ground on
which to group organisms. He followed Ray at first in assuming that
species have now the characters with which they were created, and in
general he held to the "fixity" of species. Yet in his later writings he
(juestions whether the several species belonging to one genus ma}^ not
have evolved, l^y change, from a single origin in creation. One of
Linnaeus's greatest services was the introduction of two terms in the
name of a species — the first the name of the genus, the second that of the
species — a method which is used at the present time. It was fully
developed in his great work, the "Systema Naturae," in which all the

THE GROWTH AND SCOPE OF BIOLOGY 13

animals and plants which Linnaeus knew are described and named.
So accurate are the descriptions that many of his species are recognizable
today, and his names for them are still applied.

Foundations of Modem Biology. — Naturalists of a certain stamp have
always found the classification of objects a fascinating occupation, and
Linnaeus had many followers. For the most part they were less able than
he, and their labors often degenerated into an attempt to discover and
name as many species as possible. Because of this tendency, classifica-
tion suffered a degree of disrepute. Moreover, there were many other
features of living things to engage attention. Discoveries were made
and theories formulated in nearly all the fields of biology. The phys-
iology of sense organs and the nervous system was studied. Embryology,
the science of development of the individual, was greatly advanced.
The process of fertilization of eggs by spermatozoa came gradually to
be understood, and it was found that some eggs could develop without
the intervention of the male cells. The existence of sex in plants was
recognized, and some crosses were made to ascertain the course of
heredity. Mutilated animals were observed to regenerate their missing
parts. Comparisons of the structure of various animals foreshadowed
the comparative anatomy of the next century. The behavior of the
castes of social insects was studied, marking the beginning of animal
psychology. In the sister science of chemistry, the nature of oxygen
and carbon dioxide was discovered, and naturalists began to see their
relation to the respiration of animals. Vague ideas of change of species,
implying concepts of evolution, began to be put forth.

With this increase in the factual phase of biology, philosophy declined ;
and with the rising tendency to limit theory to what could be reasonably
supported by the ascertained facts, biology entered upon what may be
regarded as its modern period. This period corresponds roughly to the
nineteenth and twentieth centuries. It witnessed the rise of comparative
anatomy, the discovery of cells, the development of embryology and
cytology, the general acceptance of the evolution doctrine, the rapid
increase in the use of the experimental method, research in heredity, the
study of the general physiology of protoplasm, and specialization in
several of the narrower fields of biology.

Comparative Anatomy. — The earliest well-defined modern trend was
in the field of comparative anatomy. The founder of this branch of
biology was Georges Cuvier (1769-1832) (Fig. 9). Cuvier possessed a
natural interest in living things and, being a clever draughtsman, had
made pictures of many of the animals he studied. Some of these pictures,
exhibited in Paris, won him a professorship of comparative anatomy there.
His rise was rapid, and mmierous honors were bestowed upon him.
Cuvier's comparative anatomy differed from all previous brands in that

PRINCIPLES OF ANIMAL BIOLOGY

the standard of comparison was not man but the lower animals. He
had begun his biological career by studying marine animals; and, while ho
later went over almost wholly to the vertebrates, he never, as did the
medically trained anatomists before him, adopted man as the starting
point for comparison. Paleontology also traces its origin to Cuvier,
since his comparative studies were extended to fossils, especially to the
elephantlike forms, the mastodons.

It is curious that Cuvier, who was forcibly brought face to face with
the evolution theory, never saw fit to embrace it. His discoveries in
comparative anatomy are now regarded as indicating kinship of various

Fig. 9. — Georges Cuvier, 1769-1832. {From Locy, "Biology and Its Makers.")

animals, and the fossils he studied clearly demonstrate that living things
of successive ages were of very unlike kinds. Cuvier chose to explain
these successive types of beings by catastrophes, which destroyed all
life, and subsequent recreation of new kinds of beings. He was not
merely passive in rejecting the evolution doctrine but actively opposed
it. In a series of discussions participated in by him and Geoffroy St.
Hilaire before the French Academy of Sciences in 1830, his opposition was
repeatedly stated. Cuvier, who was an excellent debater and very
influential, was then generally held to have won this debate.

The Cell Theory. — The comparative method of study was applied to
smaller and smaller objects as rapidly as moans of doing so were available.
Further progress in the improvement of the microscope (such as the first
production of achromatic lenses about 1827), after a period of nearly

THE GROWTH AND SCOPE OF BIOLOGY

a century and a half in which Uttle change took place in these instruments,
led to the discovery of the universal occurrence of cells. The credit for
this discovery belongs to no one person. Hooke had seen the boxlike
cavities in cork in 1665, and Malpighi observed those of other plant tissues
in 1670. Lamarck and Mirbel taught, early in the nineteenth century,
that plants and animals are composed of "cellular tissue." The nucleus
was sporadically seen and in 1833 recognized by Brown as of regular
occurrence in plants. His observation was verified by Schleiden, and
Schwann (Fig. 10) extended it to animals. The universal occurrence of
cells in living things was recognized by Dutrochet and Purkinje (Fig. 11),

Fig. 10. Fig. 11.

Fig. 10. — Theodor Schwann, 1810-1882. Fig. 11. — Johannes Evangelista Purkinje,

1787-1869. {Both from Garrison, " History of Medicine.")

and a formal statement of that universality was published by Schwann in
1839. Knowledge of the nature of cells was gradually accumulated
through the work of various biologists, culminating in the convincing
proof by Max Schultze, about 1861, that the essential feature of living
things is the jellylike substance called protoplasm, which was at first
regarded as merely incidental.

This knowledge of cells had a profound influence upon further
advances in morphological biology. The study of tissues, begun several
decades before, now became a study of like cells grouped together.
Embryology was pushed back to the very beginning of development,
to the egg cell, and the so-called germ layers (of cells) in the embryo of
the chick were discovered. Unfortunately, knowledge of the minute
structure of cells was not sufficient until much later to influence physiolog-

PRINCIPLES OF ANIMAL BIOLOGY

ieal work appreciably. The theoretical and natural history phases of
biology also went on quite unaffected, for the time, by cell discoveries.

Modem Physiology. — Physiological investigations were much more
dependent upon the advances being made in animal chemistry than upon
cell studies. Knowledge of the composition of all sorts of animal struc-
tures was strengthening the belief that life is a group of chemical phe-
nomena. Studies of function necessarily made use of the experimental
method, which once more became one of the most valuable tools of
biology. One of the leaders of this period in physiology studied the

Fro. 12. — Jean Baptiste Lamarck, 1744-1829. (From Locy, " Biolosjij and Its Makers"

and Thornton, " British Plants." }

processes of nutrition (particularly the role of the liver), the production
of sugar in animal bodies and the influence of the central nervous system
upon this process, the secretion of the pancreas, and the effects of poisons.
Another studicxl sense perception and the function of different kinds of
nerve cells, while a third worked on reflex actions. But all this was done
without particular reference to cells. It was t)nly much later that the
physiology of the cell was recognized as lying at the foundation of all
physiology.

Evolution. — Another of the great developments of the nineteenth
century which occurred quite without reference to the knowledge of cells
was the growth of the evolution doctrine. The idea of evolution, or
change of species, was briefly and crudely stated or suggested in the writ-
ings of the early Greeks, Empedocles in particular. Linnaeus, in the

THE GROWTH AND SCOPE OF BIOLOGY

eighteenth century, betrayed a sUght loaning to t,he possibihty of evohi-
tion in his later writings when he conceived that the species belonging to
the same genus might have had a common origin. His contemporary,
I3utfon, speculated more openly upon the origin of the various life forms
and was unwilling to accept the notion of independent creations. It was
not until the time of Lamarck (Fig. 12), however, that any general theory
of evolution was proposed. Lamarck observed the great variation exhil/-
ited by animals and conceived that it was due to the effects of use or disuse
of the various organs by the animals. He supposed that the changes thus
induced were inherited, thus becoming permanent — a view that has been

Fig. 13. — Charles Darwin, 1809-1882. (From University Magazine.

Leonard Darwin.)

Photograph by

abandoned by most biologists since then. These views of Lamarck were
expressed most fully about 1809, at the beginning of what may be
regarded as the modern period in biology. As has been pointed out in an
earlier section, Cuvier opposed the evolution doctrine, notably in the
series of discussions in the French Academy of Science in 1830, and his
great personal influence determined the attitude of French biologists
toward the new doctrine.

It was in another land, therefore, that the chief modern development
of the evolution idea had its origin. To Charles Darwin (1809-1882)
(Fig. 13), of England, is due the credit of convincing the thinking world
that change of species has taken place throughout the whole history of
living things. This he did partly by marshalling such a mass of evidence
in favor of evolution that there was no rejecting it, partly by devising a

18 PRINCIPLES OF ANIMAL BIOLOGY

theory — natural selection — to account for it, so plausible that acceptance
of the fact of evolution was rendered easy. Within a few years of the
publication of Darwin's "Origin of Species" in 1859, the supporters of the
evolution idea far outnumbered its opponents in intellectual circles.
Naturalists everywhere were busy finding examples of apparent evolution
and striving to fit the observed facts into the natural selection theory.
The whole course of development of biology was modified by this prev-
alence of evolutionary speculation during the two or three decades after
1859.

Not all discussions of evolution were wholly speculative; some were
founded on detailed facts which were gained by hard labor. An example
is the expansion of work in comparative morphology in Germany. This
science became distinctly evolutionary; the comparisons were made with
an eye to kinship and became some of the most important of the evidences
of evolution. Embryology, too, profited by the idea of kinship of animal
forms and in turn furnished much of the evidence on which the evolution
theory is based. Only among the French, of the great intellectual
peoples, was the acceptance of the evolution doctrine long delayed; and
when the idea finally triumphed there, it was rather in the form proposed
by their countryman Lamarck (as a consequence of use and disuse) than
in the Darwinian form (as guided by natural selection).

Genetics. — In one respect in particular did enthusiasm for the
evolution theory overreach itself. Since evolution can consist only of
hereditary variations, it would be supposed that any information regard-
ing the phenomena of heredity would be promptly seized upon as of
importance to evolution. Darwin himself did strive to learn from
practical breeders and others what was known of these phenomena.
His feeling of their importance was not shared sufficiently by biologists
in general, so that when in 186G Gregor Mendel (Fig. 14), an Austrian
monk, published some experiments dealing with inheritance in garden
peas, they attracted no attention. Mendel's work lay unnoticed until
1900. By that time the ardor of the natural selectionists had cooled
enough that the futility of attempting to discover the course of evolution
by speculation alone was duly recognized. Realizing that in a knowledge
of heredity lay the best hope of explaining evolution, various biologists
had resumed the study of inheritance by means of experiments. Plants,
being simplest, yielded the first results, and in 1900 three European
botanists, working independently, publislunl at about the same time
accounts of their crosses, from which they derived the same conclusion as
Mendel had derived before them. Fortunately they also discovered
Mendel's old paper. These experiments were capable of being explained
in so simple a manner that a great impetus was given to the experim(;ntal
study of heredity. Hundreds of plants and animals have been shown to

THE GROWTH AND SCOPE OF BIOLOGY

follow the fundamental rule laid down by Mendel. His principles have
undergone some modification, as a result of the investigations of T. H.
Morgan and others, so that the known operations of heredity are no longer
so simple as Mendel's statement. Further complexities are still being
discovered, but with few exceptions they form a harmonious whole, and
genetics at the present time approaches more nearly the condition of an
exact science than any other division of biology.

Fig. 14. — Gregor Johann Mendel, 1822-1884. {From a photograph taken about 1880.
Reproduced from the report of the Royal Horticidtural Society Conference on Genetics, 1906,
by permission of the President and Council.)

Cytology. — The handmaiden of genetics in all this advance has been
the science of cytology, which deals with the very small structures of the
cell. Advance in this field beyond the stage to which Max Schultze
brought it has depended upon further improvement of the microscope,
the discovery of dyes or stains by which these minute objects could be
made more readily visible, and the invention of mechanical devices for
cutting cells into very thin sections. These improvements in technique
led early to an understanding of cell division (in the eighteen seventies)
and later of the ripening of the germ cells. While cytology has been
concerned with all sorts of cell structures, the chromosomes, minute
objects in the cell nucleus, have long been regarded as of chief importance.
It is the chromosomes that have allied cytology so closely with genetics,
for the machinery of heredity is found in the chromosomes. At first, in
this alliance of genetics and cytology, the latter took the lead. Chromo-

20 PRINCIPLES OF ANIMAL BIOLOGY

somes were observed (1880-1910) to behave in certain wa3\s before their
genetic significance was understood. Later the order of discovery was
reversed; the demonstrated workings of heredity required that the
chromosomes should operate in a certain manner, and in many cases their
behavior has been subsequently found to coincide with the theoretical
expectation.

General Physiology. — While stains, smears, section-cutting apparatus,
and improved microscopes have been the traditional tools of the cytolo-
gists, recent work in that field has dealt with living cells and has included
minute dissection of cells by means of ingenious devices which can be
operated under the microscope. This phase of cytology borders closely
upon general physiology, which deals with fundamental activities of
protoplasm. General physiology is concerned with chemical composition
and reactions of living matter, permeability, viscosity, colloid structure,
electrical charges, transformations of energy, etc., in an attempt to relate
these conditions or processes to the phenomena of life. The material
used in such studies is partly a host of one-celled organisms, partly the
eggs of various aquatic forms, and partly the specialized masses of cells,
or tissues, of higher animals. Although these cells differ much in
appearance and in their ultimate fate, they must do certain fundamental
things in common. It is in the province of general physiology to discover
these common processes. This development is comparatively recent,
and a large number of biologists at the present time are engaged in this
type of work.

Change in Content of Biology. — It will have been observed that
throughout the development of biology, from the early Greeks to the
present time, the bulk of what was known regarding living things con-
cerned their structure. This branch of biology is known as moryhology .
At first little else was known, and in the Middle Ages the continuity of
biology hung on the one thread of anatomy. Only gradually did the
functions of organs come to be of much interest, and William Harvey, in
the seventeenth century, is often regarded as the founder of -physiology.
At first a study of mechanics, physiology later became concerned with
the principles of organic chemistry. Attempts were made to apply
these principles not only to the workings of the organs of the adult but
to the processes of embryonic development. Embryology thus became
physiological as well as morphological, and modern work in embryology
is chiefly of the former kind.

Simultaneously with physiology there grew up the science of classifica-
tion, or taxonomy. At first, as developed by Linnaeus, classification was
arbitrary. Though similar animals were grouped together, their similar-
ity was not held to have any significance. A century later, when evolu-
tion was generally accepted, the basis of taxonomy came to be kinship.

THE GROWTH AND SCOPE OF BIOLOGY 21

Similar animals were grouped together because they were believed
to be related through common descent. Concepts of evolution and
hence of taxonomy were altered in quite recent times by increasing
knowledge of genetics which lies at the foundation of both of the sciences
just named.

These five sciences, morphology, physiology, taxonomy, evolution, and
genetics, are the main fundamental divisions of pure biology. Because
they are all concerned with living things, they necessarily overlap.
Evolution and genetics have much in common, as have both with taxon-
omy. Physiology and morphology are not wholly separable, since
function cannot exist apart from structure. Yet there is considerable
independence among them. It is possible to study morphology without
being concerned with the function of the structures involved. One may
study genetics without knowing or caring what bearing the discovered
facts have on evolution. Taxonomy may — and did for a century —
proceed without any relation to evolution, even though that kind of
taxonomy would be regarded now as without significance.

Composite Biological Sciences. — There are several divisions of biology,
however, which do not possess this degree of independence, but which are
only special phases or combinations of the five named above. One of
these is paleontology, the science of extinct animals. Paleontology is
only a specialized form of zoology, limited in its scope because it is con-
cerned only with fossil types, not with living animals. It deals largely
with morphology, chiefly of external features, though internal anatomy is
sometimes preserved in fossils. Taxonomy is quite feasible in paleon-
tology, since external form of fossils, taken in connection with similar
kinds of living animals, is sufficient to indicate probable kinship. Evolu-
tion is clearly shown by the differences between fossils of successive
geological periods. However, the physiological processes of extinct
animals can only be inferred from their structure, and knowledge of
genetics is impossible in the absence of detailed comparisons of parents
and offspring. Paleontology is thus a limited sort of zoology.

Ecology, which is a study of the relation of living things to the environ-
ment, is likewise a composite of the fundamental biological sciences.
The ecologist strives to discover in what ways organisms meet the condi-
tions imposed by the world around them. He learns in what situations
animals live, and why they are there. He studies the interplay of
processes within organisms and processes occurring outside. To some
extent this relation to the environment is purely structural ; very largely
it concerns function. So far as ecology concerns the organisms them-
selves, therefore, it is but a combination of morphology and physiology.
The other things with which ecology has chiefly to deal concern the
organization of the environment. This latter phase of ecologj^ is not

22 PRINCIPLES OF ANIMAL BIOLOGY

really biology at all, except as the environment of one animal is made up
of other living things ; but it is as essential to ecology as is a knowledge of
physics and chemistry in general physiology.

Somewhat related to ecology is the geographic distribution of animals,
or zoogeography. Ecology relates partly to local distribution of organisms,
as determined by environmental conditions. Zoogeography also involves
these questions of local distribution, since no species can live where the
conditions are not suitable, and wrong conditions constitute barriers to
distribution. However, no kind of animal is found in all the places on
the earth where conditions suitable for it exist. The absence of a species
from some regions entirely capable of supporting it is accounted for by
such things as the place where the group originated and the time of its
origin. These things are historical; ecology has nothing to do with them,
but they are an important part of zoogeography. The latter science is
therefore morphology and physiology, as far as the fitness of species to
occupy certain regions is concerned; and it is evolution and geolog}^
whenever absence from a given region is explained by the time or place
of origin of the species.

Too much emphasis should not, however, be placed upon the clearly
composite nature of these several biological sciences. All the divisions
of biology overlap to some extent; indeed, the unity of them all, which
makes them biology, would not exist but for such overlapping. Plants
share this unity with animals. There is a morphology, a physiology, a
taxonomy of plants. These sciences differ from the corresponding ones
for animals in the objects with which they deal, but not greatly in the
principles involved. Each of the other divisions of biology discussed
above relates to plants as well as to animals. It is traditional to separate
botany from zoology, but there is scarcely more difference between plants
and animals as they relate to one of these sciences than there is between
some of the more extreme animals.

References

LocY, W. A. Biology and Its Makers. Henry Holt & Company, Inc. (Especially

Chaps. I-IV, VI, VII, XI.)
LocY, W. A. Growth of Biology. Henry Holt & Company, Inc. (Particularly

Chaps. II, IV, IX, X.)
MiALL, L. C. The Early Naturalists, Their Lives and Work. The Macmillan

Company. (Sec. V, minute anatomists; Sec. VIII, part on Linnaeus.)
NordenskiOld, E. History of Biology, .\lfred A. Knopf, Inc. (Especially Chaps.

I, II, V, VII, VIII, XIV of Part 1. ^Fhe rest of the book will be better appre-
ciated after several advanced courses in biology.)
OsBOHN, H. F. From the Greeks to Darwin. The Macniilian Company. (Hi.story

of the evolution idea over the period indicated.) ^

Singer, C. Biology: History. Medicine, History of. Articles in Encyclopaedia

Britannica.

CHAPTER 2
PRIMARY ORGANIZATION OF LIVING MATTER

No feature of organisms has so many and such varied consequences
as the fact that they are composed of protoplasm which is usually
arranged in the form of cells. If a bit of animal tissue, cut thin, be
it from muscle, gland, skin, brain, or sense organ, is examined under
a microscope, it is found to be blocked off in small areas, all of which
resemble one another in certain respects and some of which ai-e alike in a
great many ways. These are the cells. We have seen (pages 14, 15) how
the existence of cells gradually became known, and how much this dis-
covery influenced work in different fields of biology. The authors of the
•ceil theory, as it was first formulated, were content to claim that all things
are composed of these units. Its immediate effect was therefore only on
the structural side of biology, as has already been related. Had the
theory developed no further, it would have continued to affect only
morphology. When, however, the chemical and physical composition of
the protoplasm was studied, and when the minute structure of the parts
of the cells began to yield to the microscope, it became apparent that the
existence of cells was highly important in physiology, heredity, and
evolution. A knowledge of cells therefore lays a foundation for much of
the rest of biology.

The Size of Cells. — It is surprising to find how much difference there
is among cells with respect to size. The radius within which the various
activities of cells must occur should be of some significance. Each cell
consists typically of a nucleus lying within a bit of protoplasm which is
the cell body or cytosome. Important reactions take place between
the different parts of the cell. Since the nearness of these parts to one
another must influence the ease with which they work together, the
size of the cell should be of some importance. Yet cells show very great
differences in this respect. Some bacteria are so small as to be almost
invisible even with a good microscope; somewhat larger are most tissue
cells, which are quite easily seen when thus magnified but cannot be seen
without such aid; but all these are topped by the egg yolks of the larger
birds, which are 2 or 3 inches in diameter. Nerve cells often have great
length, particularly those which extend from the spinal cord to the ends of
the extremities in man or the other large mammals, but are quite slender.
Sometimes these great differences in size fit the cells for their particular

PRINCIPLES OF ANIMAL BIOLOGY

functions, but in most cases no such explanation is known. When cells
that are presumably alike in their origin and function show great differ-
ences in volume, as when one unicellular animal (Paramecium, for
example) is several hundred times as large as another
of the same species (Fig. 15), it is probable that
differences in the environment have caused part, though
not all, of the contrast.

The size of cells bears no constant relation to the
size of the animals or plants in which they are found.
In very many kinds of animals, large individuals have
more, but not larger, cells than do small ones. In
others, the number of cells in each individual is
always the same, and in them large size is attained
only by the growth of each cell. In salamanders in
which, through some abnormal step in cell division,
the cells have extra chromosomes, the cells are larger
but the body is not: such animals simply have fewer
cells. Sluggish animals like frogs generally have larger
cells than active ones such as birds, and there is
presumably some important connection between these
facts.
Gross Shape. — The shape of cells is also very variable. Some cells,
owing to surface tension, are typically spherical; but that shape is
attained, even approximately, only in free cells, such as eggs and a iew
of the one-celled organisms. Cells take on other forms for various
reasons. Amoeba and other related protozoa may actively change their

Fig. 15. — Ex-
treme difference of
fsize in otherwise
similar cells; two
members of same
species of Para-
mecium, one 300
times as large as
the other.

Fig. 16. — Change of shape in amoeba. Half-ininuto interval between first and second,
five mjnutes between second and third. {CoiirUsy of Gcnenil Biological Supply House.)

shape by thrusting out portions of the body into fingerlike pseudopodia.
Such an animal is seldom of the same shape for any considerable time
(unless it goes into a "resting" state, in which it is apt to be nearly
spherical), and it may even be changing every instant (Fig. 16). Other
free-living cells, of more or less constant form, are kept constant by a wall
or pellicle that the cells themselves have secreted (Fig. 17). These

PRIMARY ORGANIZATION OF LIVING MATTER

pellicles may be flexible but firm, so that while the shape of the body
may become temporarily distorted it is characteristic of the species.

Cells that exist in groups usually have their form altered by the
mechanical pressure of the cells around them. When this pressure is
the only factor altering their shapes, the cells are irregular polyhedrons.
Other factors, such as unequal growth in different directions and perhaps
inequalities of surface tension, combine to produce cells of a great variety
of shapes. They may be box-shaped, as in plants; long cylinders, as in
voluntary muscle; greatly flattened cells with their largest sides polygons,
as in the outer layer of frog skin; somewhat flattened elliptical cells, as in
the blood of many animals; circular and flattened, as in human blood;
narrow and spindle-shaped, as in involuntary muscle; or finely branched,

'i«Aj ^"-~_-^-^^^==^

Fig. 17. Fig. 18.

Fig. 17. — Various forms of ciliated protozoa whose body shape is kept fairly constant
by a surrounding pelhcle. Though this shape may be altered by pressure, it is restored
when the pressure is removed. Cilia project from the surface.

Fig. 18. — Various forms of nuclei in cells. A, part of muscle cell with multiple ellip-
soidal nuclei; B, gland cell of butterfly with branching nucleus; C, marrow cell of rabbit with
ring nucleus; D, Epistylis with curved rodlike nucleus; E, Stentor with beaded nucleus;
F, Trachelocerca with distributed nucleus. (5, C, and F after Wilson, courtesy of The Mac-
Millan Company.)

as in pigment cells of the skin of frogs and salamanders, or bone and nerve
cells.

The Nucleus. — The most important part of a cell is its nucleus.
This body is ordinarily located somewhere near the middle of the cyto-
some but may be crowded to one side by other structures and may move
from one place to another. It is most often spherical, owing to the ten-
sion of the very thin film, or nuclear membrane, which surrounds it, but
other shapes may be impressed upon it or it may actively take other
forms. In long narrow cells the nucleus is generally elongated (Fig.
18A), and in flat cells it is disk-shaped. Physiologically very active
cells often have branched or lobed nuclei {B, C) ; and in certain unicellular
organisms the nuclei may be of odd shapes — ropelike, beaded, or broken
up into many small bits (D-F) — characteristic of the species but without
any known significance. The red cells of human blood are devoid of
nuclei, a condition generally held to be due to degeneration of the nuclei
which they possessed in young stages.

PRINCIPLES OF ANIMAL BIOLOGY

The importance of the nucleus derives from a substance known as chro-
matin which it contains. This substance, as will appear in later chapters,
exercises some control over physiological processes, development, and
heredity. It owes its name to the fact that it colors deeply in most
ordinary dyes such as are used by cytologists to make it conspicuous
enough for study. The chromatin is collected into a number of distinct
masses, the chromosomes, but these bodies are so diffuse in their structure
that they cannot usually be recognized as separate objects except at the
time of cell division. During the periods between cell divisions one com-
mon form in which chromosomes exist is that of distended bags, the walls
of which contain the chromatin itself, while the interior is filled with a

Vacuole

Nudearsap
Chromatin

Nucleolus-

Nuclear
membrane

Cell wall-

Cell
membrane

■Plasfid
Golgi body

-4 Cenlriole
— Cenfrosphere

Mihchondrla.

Cell
inclusion

Fig. 19. — Generalized cell.

semiliquid substance called the nuclear sap. The chromatin is thus
greatly thinned out, though quite irregularly so, for there are little knots
and branching strands of it thick enough to be seen when stained. Some
chromosomes are shown in Fig. 38 (page 59), gradually experiencing this
expansion at the end of cell division. When the chromosomes in this
distended form are packed closely together in a nucleus, it is usually quite
impossil)le to see the outlines of the chromosomes, but the kn-ots and
strands of thicker chromatin are visible, together gi^^ing the appearance of
a network (Fig. 19). In other cells the chromosomes appear to be in
the form of branched threads rather than bags, but the resulting appear-
ance of the nucleus is still that of a chromatin network whose spaces are
filled with nuclear sap.

Some nuclei contain, in addition to the sap and the network of
chromatin, a nucleolus. Two or more nucleoli may be present. They
are rounded bodies that stain readilv, but in a manner different from

PRIMARY ORGANIZATION OF LIVING MATTER 27

the chromatin. Nucleoh are therefore not to be confused with bunches
of chromatin, which have sometimes been called nucleoli. The nature
and function of the nucleolus, when it is present, are not understood.
Some biologists have regarded it as a waste product; others have held it
to be a reserve supply of materials used in cell division, since it dis-
appears during that process; and it has been regarded as a reserve food
supply for the nucleus.

The Cytosome. — The body of a cell is seldom uniform in composition
but includes a number of different structures. The more common ones
are here described, though very few cells have all of them. At the surface
there may be a definite cell wall which is lifeless, not composed of proto-
plasm but secreted by the cell. It is very common in plants, where it is
composed mostly of cellulose, one of the principal components of wood.
Some animal cells have such a lifeless covering, but in them it is often
made of other materials. Sometimes the cell is covered by a much
thinner and more flexible coat, the pellicle, as are the cells of Fig. 17.
Beneath the cell wall, or at the surface of the cell if there is no other cover,
a somewhat firmer layer which may be called the cell membrane is formed
out of the protoplasm itself in about the same way that water forms a
film at its surface.

Within the cytosome, plastids are common. In the higher plants
they are universal and are usually green. Some are of other colors,
as in fruits and flowers, and some are colorless. In animals, plastids are
found chiefly in certain classes of protozoa (one-celled animals) where
they are mostly colored.

Vacuoles are vesicles of liquid enclosed in the protoplasm. They
may be permanent or temporary. In the protozoa, temporary vacuoles
are common. They usually either enclose bodies of food in process of
digestion, in which case they are called food vacuoles, or disappear at
intervals by ejecting their liquid contents through the surface layer of
protoplasm into the surrounding medium. The latter kind is called a
pulsating or contractile vacuole. In some cells a centrosphere is found,
usually near the nucleus. It is a mass of somewhat differentiated proto-
plasm, containing a minute body that stains deeply, the centrosome
or centriole. When present, the centrosphere takes a conspicuous though
probably unimportant part in cell division, as described in another
chapter.

Structures known as mitochondria (Fig. 19) are found in many kinds
of cells, perhaps in all cells. They are of various shapes — rods, threads,
granules — and occur almost anywhere in the cytosome. Many conjectures
regarding their function have been made, but little is definitely known
regarding it. An object known as the Golgi apparatus, of various forms,
often a conspicuous network, occupies various positions, usually near the

28 PRINCIPLES OF ANIMAL BIOLOGY

nucleus and in some cases characteristically near the centrosphere. The
function of the Golgi apparatus is still unknown, though there is some
indication that it takes part in the process of secretion by gland cells.

Besides all the above structures which serve, or ma}^ serve, some func-
tion in the cell, and which may therefore be regarded as cell organs, there
are often lifeless matters enclosed in the protoplasm. These may be
grains of starch, or oil or fat globules, which the cell has produced and
which are stored as future food. Or the lifeless objects may be undigested
remains of organisms taken as food, or even objects picked up incidentalh'
along with food or otherwise. These nonliving objects may be spoken of
as cell inclusions.

Polarity. — Beside the differentiations described above, cells may
possess another type of organization which is termed polarity. One por-
tion is destined to perform certain functions, another portion othei-
functions, even when these portions are visibly alike. In a develop-
ing egg one part will become the nervous system and associated sense
organs, another part the digestive tract. In the ordinary course of
development these parts are not interchangeable. This evident arrange-
ment of parts, as shown l)y their future activities, is the phenomenon
which is called polarity. Examples of polarity are found in the eggs of
insects, in which one end of the egg, in some way different from the other
end, always becomes the head. Other cells than eggs are commonly
polarized. Thus, cells bearing cilia (hairlike projections) on one end are
polarized. So also is the connection (synapse) between nerve cells, since
nerve impulses travel over it in only one direction. Many gland cells
receive materials from the blood on one side and after working them o^-er
extrude the product into a chamber on the opposite side. When long
slender cells standing on end are crowded together to form a layer covering
the surface of some organ, the nuclei of the cells are usually near the lower
end. These are all polarized cells. In some cases the polarity is visible;
but, before the structures indicating the polai-ity were developed, there
was presumably an invisible difference in the proto])lasm. The nature
of this organization is not known, and there is much disagreement as to
whether it is inherent in the cells or is impressed on them by external
circumstances.

Structural Relation to Other Cells. — When cells are free-living and
independent, as in the protozoa, they may have little or no influence
upon one another. When they are aggregated into masses, as in the
multicellular animals, there is always the possibility that each cell may
be modified, and its activities guided, by the cells around it. Often such
interdependence must follow merely from the diffusion of fluids from cell
to cell, or from electric phenomena. In some cases, however, proto-
plasmic connections extend from one cell to another. These have been

PRIMARY ORGANIZATION OF LIVING MATTER

demonstrated in the skin of the salamander, are conspicuous in Volvox,
and have been described for many kinds of animal cells (Fig. 20). In
plants, cell bridges are usually present, the fine protoplasmic filaments
passing through minute pores in the cell walls. Presumably these
l)ridges are lines of communication between cells, but they are not

Fig. 20. — Iiiteicellular l)riclges; left, highly- thicketied human ei)itlieliiiiii; right, ik r. iiinuon.
{Courtesy of General Biological Supply Hous,.)

essential, since cells in contact with one another are capable of passing
litiuids or electric currents from one to another without such connections.

References

Shari', L. ^^^ An introduction to Cytology. 3d Ed. McGraw-Hill Book C*oni-

pany, Inc. (Chap. II; details of parts of cells, Chaps. III-VII.)

Wilson, E. B. The Cell in Development and Heredity. 3d Ed. The Macmillan

Company. (Chap. I.)

CHAPTER 3
SOME FUNDAMENTAL PHYSICS AND CHEMISTRY

In the activities of cells, great importance is to be attached to that
very fine, mostly invisible, structure which inheres in the chemical
composition and minute physical constitution of the protoplasm itself.
These features of protoplasm are appropriately discussed along with
the physiological processes which depend on them. Such processes
should next engage our attention. Since, however, an understanding of
this minute structure presupposes a knowledge of elementary chemistry
and physics, it is advisable to pause a moment to acquire some of the
more important ideas in that field.

Composition of Matter. — The physical substance of which objects
are composed is called matter. Matter exists in a number of different
forms called elements. An element is a svibstance possessing a character-
istic structure which is different from that of every other element and
which cannot be broken down into substances different from itself (that
is, into other elements) by ordinary chemical means. The stipulation
"ordinary chemical means" is intended to exclude radioactivity and
powerful electronic machines. Among the more common elements
entering into the composition of living things are carbon, nitrogen,
oxygen, and hydrogen.

The elements may exist by themselves, chemically separate from
other elements, as do oxygen and nitrogen in the air. More often they
exist in compounds; these are distinct substances, made up of two or
more elements, joined in definite proportions and with characteristic
internal structure. Carbon dioxide is a very stable compound, made
of carbon and oxygen, which is eliminated as a waste product by all
living things. Calcium carbonate, of which bones are largely built, is a
compound composed of three elements: calcium, carbtm, and oxygen.

Both elements and compoimds are divisible into molecules. These
are the smallest imits of a substance in which its characteristic chemical
structure is maintained. The molecules are likewise the smallest units
which exhibit the chemical properties of that substance. If a molecule
is divided or broken up, its parts no longer have those properties. The
elements which enter into a compound are present in each molecule in
the same proportion as in large masses of the substance. Each molecule
is exactly like every other molecule of the same substance, not only in

SOME FUNDAMENTAL PHYSICS AND CHEMISTRY 31

the quantity of its elements but also in their structural arrangement.
The molecules are completely separable from one another; in a solution
of sugar in water, the molecules of sugar float singly, and in air the
molecules of oxygen or of nitrogen are free from other molecules.

The molecules of many substances are in turn composed of atoms.
These are defined as the smallest divisions of matter that may exist,
either singly or in combination. Some molecules consist of only one
atom, as in the gas helium. In such substances there is no distinction
between molecule and atom. In oxygen, however, the molecule is
composed of two atoms. Here the atoms have properties veiy dif-
ferent from those of the molecules ; the atoms enter into chemical reactions
much more readily than do the molecules.

Protons, Neutrons, and Electrons. — Even the atoms are not the
ultimate units in the structure of matter, for they are made up of protons,
neutrons, and electrons. These entities may be spoken of as particles,
though they may be such only in a very special sense. The astounding
feature of these units is that they are the same in all kinds of matter.
The protons of an atom or molecule of oxygen are exactly like the protons
of chlorine. Similarly the neutrons are everywhere the same, in all
elements, and the electrons are the same in all.

The protons have mass, and each of them bears a positive electric
charge. This positive charge is a unit which is the same in all protons.
Neutrons have mass, practically identical with that of the protons, but
they carry no electric charge. Electrons are units of negative electric
charge; their mass is negligible. Atoms and molecules of all substances
are made up of these units. The mass (weight) of an atom is dependent
almost entirely on the protons and neutrons it contains, while its volume
is determined mostly by the electrons. These relations will be made
clear by an examination of the structure of the atom in several elements.

Structure of the Atom. — An atom of any substance consists of a
central nucleus, around which one or more electrons are distributed. The
nucleus of an atom contains one or more protons, and usually one or
more neutrons. Since the protons bear positive electric charges, the
nucleus of an atom is always positively charged. How great a charge
it carries depends on how many protons it contains. Both protons and
neutrons contribute to the mass of the nucleus, but only the protons
furnish the charge. This positive charge of the nucleus is balanced by
the negative charges of the surrounding electrons. There are as many
electrons around the nucleus as there are protons in it, so that the atom
is neutral.

Structure of the Elements. — With this knowledge of the fundamental
similarity of all matter let us return to the elements. The number of pro-
tons and neutrons in the nucleus varies considerablv, as does also the

PRINCIPLES OF ANIMAL BIOLOGY

Fig. 21. — Diagrams of atoms
of hydrogen (left) and helium
(right). The central black spot
is the nucleus; the concentric
circles mark oflf the shell of
negatively charged electrons.

number of electrons surrounding it. In hydrogen (H), which is the
simplest and lightest of the elements, the nucleus consists of just 1 proton,
no neutron, and the atom has just 1 electron (Fig. 21, left). The single
unit of positive charge furnished by the proton is neutralized by the
negative charge of the electron. Helium (He), has 2 protons and 2

neutrons in the nucleus, which is therefore
four times as heavy as the hydrogen
nucleus, but it bears only twice as great a
positive charge. To balance this positive
charge, there are 2 electrons in the atom
(Fig. 21, right). Carbon (C) has 6 protons
and 6 neutrons (in the nucleus) and 6
associated electrons; oxygen (O) has 8
protons and 8 neutrons in the nucleus, with
8 electrons'; while chlorine (CI) exists in
two forms, one of which has 17 protons and 18 neutrons, the other
17 protons and 20 neutrons in the nucleus, with 17 surrounding
electrons.

The details of these particular elements are not important to the
biologist, but the fact that they are composed of identical kinds of
units and that they differ only in the number and arrangement of these
units should be understood. Every element has a different number of
protons and electrons from every other element. From the lightest
element, hydrogen, which has 1
proton and 1 electron, to the one
long believed heaviest, uranium
(U), which has 92 protons and 92
electrons, there should be 92 ele-
ments. All but two or three of
these have been obtained in
chemical laboratories. News-
papers occasionally report the
discovery of one or more of the missing elements, which await confirma-
tion by other investigators. In the construction of the atomic bomb two
elements with 93 and 94 protons, respecti\'ely, were produced.

The chemical properties of an element, the ways in which it reacts
with other elements, are determined by the electrons surrounding the
nucleus. When these electrons are numerous, they are arranged in shells,
some near the nucleus, others farther away. The 2 electrons of lu^ium
(Fig. 21) constitute such a shell, and a similar inner shell of 2 is in all
elements heavier than helium. Outside this is a shell which may contain
from 1 to 8 electrons. Oxygen has 6 electrons in this outer shell, as
diagrammatically indicated in Fig. 22. When the number of electrons

Fig. 22.- -Atom of oxjgcn; two wajs of
representing its two shells of electrons.

SOME FUNDAMENTAL PHYSICS AND CHEMISTRY

is greater than 10, the additional ones are in a shell outside of a first
shell of 2 and a second shell of 8.

It is only the electrons of the outermost shell which enter into ordinary
chemical reactions. Different elements having the same number of
electrons, similarly placed, in this outermost shell tend to possess similar
properties and enter into similar reactions. A number of families of
elements are thus recognized whose properties are much alike, such as
the halogen family which includes fluorine (F), chlorine (CI), bromine
(Br), and iodine (I), in which there are 7 electrons in the outer shell — but
a different shell in each of these elements.

f ^

•

]/ -

Fig. 23. — Two atoms of oxygen combined to complete their outer shells of electrons.

Chemical Reactions. — Some elements react more easily than others.
The difference between them in this respect lies in their outermost shells
of electrons. -In each shell of an atom there is a maximum possible
number of electrons. An element which has this maximum number
of electrons in its outer shell does not react I'eadily; the inert gases, such
as helium used in balloons and neon in electric signs, are in this state.
Most elements, however, have less than the maximum number of electrons
in the outer shell, and it is easy for such elements either to complete that
shell or to lose the electrons which are already in it. Because of this ease
of reaction, two atoms of the same element sometimes join to complete
their outer shells. Oxygen, as already stated, has six out of a possible
eight in its second (outer) shell. If one atom shares two of its electrons
with the other atom, and in turn accepts two electrons from the latter,
each has a complete shell of eight electrons (four of them in common)
and the two atoms are combined (Fig. 23). A molecule of oxygen is thus

PRINCIPLES OF ANIMAL BIOLOGY

formed. In chloi'ine, which has seven of the possible eight electrons in its
outer shell, two atoms combine by sharing two electrons (one furnished
by each atom, Fig. 24), thus making a molecule of chlorine.

Two atoms of different elements may combine, for the same reason,
and thus a compound is produced. Sodium (Na), for example, has just
one electron in its outer (third) shell, which it readily gives up to any
other atom capable of accepting it. Chlorine, as just explained, has
seven in its outer shell and readily accepts an electron from an outside
source. The two atoms perform these easy reactions by combining;
they form a new substance, sodium chloride (NaCl).

Valence. — The number of electrons which an atom readily gives up
or acquires constitutes its valence. Sodium has a valence of one, since

Fig. 24. — Two atoms of chlorine combined to complete their outer shells of elections.

it easily loses but one electron. Magnesium easily loses two electrons,
because that is the number in its outer shell, and its yalence is two.
These valences must be matched when compounds are formed. Thus,
while one atom of chlorine (whose valence is one) matches one of sodium,
it requires two atoms of chlorine to take up the two extra electrons of
magnesium and form magnesium chloride (MgCU).

Ions. — When a sodium atom gives up its one outermost electron to
some other atom, its electric balance is disturbed. It has lost one unit
of negative electric charge; hence the net charge of the remainder is
positive. Such a positively charged body is no longer the element
sodium, it is not even an atom; it is instead an ion (Na+). Similarly,
when a chlorine atom acquires one extra electron (which is, of course,
negative), its electric balance is disturbed and it becomes negative. It
is no longer chlorine, no longc i- an atom, but a chloride ion (Cl~). An

SOME FUNDAMENTAL PHYSICS AND CHEMISTRY

Na+

ci-

ion may be defined as part of a molecule, consisting of one or more atoms
with an electric charge. Ions are either positive or negative, depending
on whether the atom has lost or gained electrons in producing them.

When sodium and chlorine combine to form sodium chloride, which
is common table salt, a crystal of the salt is supposed to have the lattice
structure shown in Fig. 25. There is no sodium in the crystal, no chlo-
rine, but only sodium ions and chloride ions. There are not really any
sodium chloride molecules, since each chloride ion (observe the central
white one in the figure) is surrounded by six sodium ions at equal dis-
tances, and each sodium ion is surrounded by six chloride ions at eqi'.al
distances. One cannot say which
negative ion neutralizes a given
positive one, so that no specific
pair of ions can be said to form a
molecule. A molecule can hardly
be said to exist in a sodium chlo-
ride crystal, but only positive and
negative ions.

Radicals. — In all the above
examples, the units of chemical
reactions have been atoms of ele-
ments or ions derived from them
by transfer of electrons. Very
often such reaction units are
formed of two or more different
elements. Sulfur (S) and oxygen, for example, may unite in the
proportion of one of the former to four of the latter. In this propor-
tion, however, their electric charges are not balanced, and the group bears
two units of negative electric charge — that is, two extra electrons. They
constitute a negative ion. In this form they act as a unit in combining
with atoms which have lost electrons (positive ions). Potassium (K)
may unite with them, but it takes two potassium ions to balance them,
and K2SO4 (potassium sulfate) is formed. A group of atoms acting as a
unit, as do the sulfur and oxygen (SO4") in this example, is called a radical.
Other groups of atoms (radicals) are positively charged (as NH4+),
forming positive ions.

Acids, Bases, and Salts. — When a hydrogen atom (see Fig. 21) gives
up its electron, only its nucleus remains. This nucleus is a proton and is
positively charged: it may also be called a hydrogen ion (H+). Certain
substances in water readily yield up these protons to other substances,
and they possess certain properties as a consequence. They have a sour
taste, color litmus paper red, and do a number of other characteristic
things. SuTJStances which readily donate hydrogen ions (protons) are

Fig. 25. — Crystal of sodium chloride,
showing lattice arrangement of sodium ions
and chloride ions.

3() PHINCIFLES OF ANIMAL BIOLOGY

called acids. Other substances which easily accept protons are called
bases. They do so through the formation of negative ions consisting of
oxygen and hydrogen (0H~), known as hydroxyl ions. Bases in solution
have the properties of lye, are said to be alkaline, and are recognized by
the blue color they confer on litmus.

It will be observed that the characteristic positive ions (H+) of acids
and the characteristic negative ions (0H~) of bases together contain the
components of ordinary water (H2()). Now water is an exceedingly
stable compound. It is to be expected, therefore, that when an acid
and a base are brought together in a solution the above ions will promptly
unite to form water. This they do. But what becomes of the other
radicals that belong to the acid and the base? They also combine in the
sense that sodium ions (Na+) and chloride ions (Cl~") combine to produce
sodium chloride. What they produce depends on what the other
radicals of the acid and base were, but in any ease the product is called a
salt. A salt is defined as a substance which produces, or is a combination
of, positive and negative ions other than H+ and 0H~.

If the acid used was hydrochloric (HCl) and the base was sodium
hydroxide (NaOH), the solution containing the former would contain
hydrogen ions (H+) and chloride ions (Cl~), while the latter in solution
would consist of sodium ions (Na+) and hydroxyl ions (0H~). When
these two solutions are mixed, the hydrogen ions (H+) and hydroxyl
ions (0H~) promptly unite to form water. The ions of the other two
kinds, Na+ and Cl~, do not actually unite, but they form a solution of
sodium chloride. If such a solution is dried up, crystals of sodium chlo-
ride having the lattice structure shown in Fig. 25 are formed. The
sodium chloride is a salt.

If sulfuric acid (II2SO4), in which there are hydrogen ions (11+) and
sulfate ions (S04=), is mixed with potassium hydroxide (KOH). in which
there are potassium ions (K+) and hydroxyl ions (()H~), water is again
formed by the H+ and 0H~ ions. This leaves the potassium ions (K+)
and sulfate ions (S()4=°) to form potassium sulfate (K2SO4). The potas-
sium sulfate is likewise a salt.

Salts may be obtained in other -ways than by mixing acids and bases.
Mixing two salts gives rise to two other diffei-ent salts. Thus, if a
solution of soduun chloride is mixed with a solution of potassium sulfate,
the combined solution contains two kinds of positive ions (Na+ and K+)
and two kinds of negative ions {C\- and SOr). While the ions do not
join in solution, it is just as correct to regard the solution as containing
potassium chloride (KCl) and sodium sulfate (Na2S04) as the original
two.

Electrolytes. — Ions, because of their charges, are able to carry an
electric current when they are free to move. The sodium and chloride

SOME FUNDAMENTAL PHYSICS AND CHEMISTRY 37

ions in a crystal of common salt are too rigidly held to move, but if the
crystal is dissolved in water they are free. If into different parts of such
a solution wires from the two poles of a l^attery are placed, a current of
electricity is carried through the solution from one pole of the battery
to the other (Fig. 26). The positive ions (Na+) go toward the negative
pole and, by taking up electrons from it, becomes ordinary neutral
sodium (Xa). Removal of electrons from the negative pole reduces the
negative charge conferred upon it by the battery and sets up a current
in the wire. The negative ions (Cl~) pass in like manner to the positive
pole, where they deposit their surplus
electrons on that pole, forming neutral
chlorine (CI). Sodium is thus col-
lected about one pole of the battery,
where it reacts with the water; chlo-
rine collects about the other pole and
escapes as a gas. Decomposition of

a substance in this manner is known t. „^ t.- ^ . , ■ .

biG. 2b. — Diagram of electrolysis of

as electrolysis. In the metal mdus- sodium chloride in solution. Chloride

tries this process is used to separate \'^l\^^'''' *° *^^ "^^*' ^°'^'"'" '""'' *°
certain metals from their ores. Sub-
stances which, like sodium chloride, form ions in solution and are thus
capable of carrying a current are called electrolytes. Most of the salts
are good electrolytes.

Energy. — Energy is the capacity to do work, that is, to produce
change. The arrangement of the electrons and protons in an atom
involves energy. Changing that arrangement either requires that
energy be expended upon the change or releases energy no longer needed
in the new arrangement. Both types of change are exceedingly common.
Of the common elements about us near the earth's surface, oxygen is by
far the most abundant, making up nearly half of the total. It is also
very common in living things. Since oxygen is a fairly active element,
some of the most frequent chemical reactions are the combinations of
oxygen with other substances. These changes are called oxidation.
The rusting of iron and the burning of wood or coal are examples. An
important feature of oxidation is that it releases energy. Use is made
of this fact in industry, when the energy of steam engines or electric
current is furnished by burning coal, and in plants and animals whose
activities depend on energy obtained by oxidizing food. The energy
which is tied up in the composition of chemical substances, whether foods
or any other, is called potential energy. When converted by a chemical
reaction into the energy of heat or of movement, it becomes kinetic energy.

Applications to Biology. — The examples used in this chapter to illus-
trate chemical principles have been taken mostly from inorganic chemis-

38 PRINCIPLES OF ANIMAL BIOLOGY

try because of their simplicity. The examples therefore need hardly be
remembered if the ideas they represent are mastered. The principles
have been kept at a minimum but should suffice for a fair understanding
of the simpler operations of protoplasm. Living things are essentially
chemical and physical laboratories, with this distinction, that the chemical
substances are not limited to a few reagent bottles on the shelves nor
the physical apparatus to a few resistance boxes and potentiometers in
the cabinets; instead these things constitute most of the building itself.
Changes are going on in them everywhere and all the time. It is of
these chemical and physical processes that life consists. As explained
in other parts of this book, the common physiological processes of
digestion and respiration are chemical reactions and physical phenomena
that are fairly well understood. Not so well known but assuredly
chemical and physical are muscular contraction and elimination of wastes.
Even growth, the development of the embryo or young stages, and the
conduction of impulses by nerves must be largely physicochemical.

It is important to know, in connection with all these life processes,
that substances react as they do because of their electronic structure.
This structure is, in most protoplasmic substances, enormously com-
plicated by radicals of complex design. Their reactions and structure
are for this reason not easy to discover, but there is every reason to
assume that their physiological behavior is quite as dependent upon their
architecture as are the reactions of the simplest inorganic compound.
Valence determines the proportions of different substances which will
unite in protoplasm as certainly as in the salts. Electric phenomena
result from electronic reactions in living things just as in batteries.
Energy, one of the most important requirements of animals and plants,
flows from chemical combination as abundantly and as certainly in
protoplasm as in a test tube or an engine. It seems likely that life
consists entirely of physical and chemical changes.

With this equipment of elementary knowledge in a pair of sister
sciences, and an understanding of the extent to which these sciences
underlie all knowledge of biology, we may notv return to the operations
of cells.

References

Partington, J. R. A Textbook of Inorganic Chcnustry. 5tli Ed. The Macmillan

Company. (Pp. 428-430; 446-453; 466-473.)
Smith, A. W. The Elements of Physics. 4th Ed. McGraw-Hill Book Company,

Inc. (Chaps. 60, 61 ; structure of atom, nuclear physics.)
TiMM, J. A. An Introduction to Chemistry. McGraw-Hill Book Company, Inc.

CHAPTER 4
THE FUNCTIONS OF PROTOPLASM AND CELLS

The living substance whose functions we are to study differs from
nonhving matter in certain characteristic ways. It has certain types of
chemical structure, not easily defined, but not duplicated in inorganic
bodies. It is arranged in unit masses, the cells, which are usually
recognizable by their form and such nearly universal features as the
nucleus. This living matter moves spontaneously, that is, from causes
arising within itself. It grows by taking up new material throughout
its interior, not just by additions on the outside. It is irritable; that is
it responds in some way to changes in the environment, or changes
within itself, which are great enough to act as stimuli. And finally,
individual living things are capable of producing other individuals of
their own particular kind; that is, they reproduce.

These statements are not intended as a definition of life, or of living
things, because there are exceptions to them, or situations in which the
criteria could not be practically applied. They are meant merely to
indicate the general types of functions which must be examined in a
survey of life activities.

Protoplasm is not a chemical compound, the structure of Avhich may
be expressed by a chemical formula, but is an elaborate mixture of
chemical compounds in water. A bit of protoplasm large enough to
analyze, from any source, always yields carbon, hydrogen, nitrogen,
oxygen, phosphorus, sulfur, sodium, potassium, magnesium, calcium, iron,
and chlorine. Additional elements that frequently occur in such analyses
are aluminum, silicon, manganese, copper, fluorine, bromine, and iodine.
Naming these elements tells very little, however, concerning protoplasm,
since it does not suggest the manner in which the elements are combined,
and it is the compounds, not the elements, that are of real importance.
These compounds in protoplasm are of a variety of kinds, which are
partly organic (produced in living things) and partly inorganic. The
latter are described first; they are water and the various salts.

Water and Salts. — Water is the most abundant constituent of proto-
plasm, making on the average about 80 per cent of the total mass. The
properties and activities of protoplasm are quite as dependent upon the
remarkable properties of water as upon the properties of its other con-
stituents. Some of these properties of water are its power to absorb or
give off great quantities of heat without changing much in temperature,

40 FRINCIFLES OF ANIMAL BIOLOGY

its capacity to dissolve many ditferent substances, and the free movement
which it permits in the ions of salts dissolved in it. These features of
water enter into so many of the living processes that life without water,
if it could exist at all, would have to be of a very different sort from any
that is known.

Dissolved in this water of protoplasm are the salts. The commonest
ones have sodium and calcium as their positive ions, but potassium, mag-
nesium, iron, and manganese are also present in this positively charged
state. The negative ions are the chloride ion and the radicals known as
carbonate, nitrate, sulfate, and phosphate. These ions of salts dissolved
in water give protoplasm certain electrical properties. Inorganic salt
make up about 1 per cent of average protoplasm.

The Organic Compounds. — There are three principal classes of
organic compoimds, the carbohydrates, lipids, and proteins. The carbo-
hydrates are the sugars, starches, celluloses of plant walls, glycogens or
animal starches, and some others. They constitute less than 1 per cent
of most protoplasm but are important out of proportion to their quantity.
They are composed of onty three elements: carbon, hydrogen, and
oxygen. The hydrogen and oxygen are always in the ratio of 2:1; that
is, there are twice as many atoms of the former as of the latter, just as
in water. In most carbohydrates the carbon atoms are in multiples of
six. A simple sugar has only six carbon atoms and is known as a mono-
saccharide. Glucose, one of the most common of them, is present in
nearly all cells. Other simple sugars are fructose (fruit sugar) and
galactose. The formula of all these simple sugars is CeHigOe, but there
are differences between them in internal ai-rangement. When two
molecules of a monosaccharide are combined into one (with loss of
water) the combination is a disaccharide. Sucrose (CioHooOh), the
ordinary cane or beet sugar of table use, maltose (malt sugar), and
lactose (milk sugar) are of this type. When many molecules of simple
sugar are combined (with more loss of water), a polysaccharide is pro-
duced. The starches (of plants), glycogens (animal starches), and
celluloses (of cell walls) are of this kind. The polysaccharides are prac-
tically insoluble in water, so that the starches and glycogens are excellent
food-storage forms. None of the carbohydrates forms ions when dis-
solved; hence they play no role in electrical i)hen()mena. They contain
a gi'eat deal of potential energy, which may be released by oxidation.

either reservoirs of stored energy are the lipids. The physical
properties of these substances are very characteristic, including the non-
evaporating grease spots which they make and their insolubility in
water. This insolubility is what makes them good storage products.
The lipids constitute about 3 per cent of oi-dinary protoplasm, though
stored lipids may be many times that fra(;tion of an animal's body.

THE FUNCTIONS OF PROTOPLASM AND CELLS 41

Among the lipids are the true fats, such as butter fat, oHve oil, and
the fat of beef or pork. True fats are composed entirely of carbon,
hydrogen, and oxygen, with the proportion of oxygen much lower than
in carbohj^drates. The natural fats have large molecules — around 50
atoms of carbon, double as many of hydrogen — but only 6 atoms of
oxygen. They are a combination of 1 molecule of glycerol (commonly
called glycerin) with 3 molecules of fatty acid. There are a number of
different fatty acids characteristic of different fats, some of them used
commercially in water emulsions to produce the brushless kinds of
shaving cream.

In other types of lipids there may be more than the three elements
which true fats contain. Lecithin, which includes phosphoric acid and
another substance in place of one of the fatty acid molecules, is abundant
in egg yolk and is probably present in all cells as part of the proto-
plasmic structure. Cholesterol, which is foiuid in bile and is a source
of gallstones, consists only of carbon, hydrogen, and oxygen, but the
carbon in it forms a "skeleton" j^n rings instead of straight chains as in
the fats.

Most significant of the organic compounds are the proteins, because
it is they that make one kind of living thing so sharply and definitely
different from others. Aside from water, they are the most abundant
substances in protoplasm — about 15 per cent of the total mass. Proteins
are especially characteristic of lean meat (muscle) but are distributed
through all cells. They do not diffuse readily through other substances
but alloAV some, though not all, other substances to diffuse readily
through them. Chemically they are compounds of the amino acids, a
group of 25 different organic acids. A generalized formula of amino
acids is R— CH(NH2)-C00H, in which R stands for the "body" of the
molecule, different in each of the 25 acids. The rest of the formula
applies to all of them. The COOH makes them organic acids, the NH2
makes them amino acids. In the simplest amino acid, glycine, R is
simply an atom of hydrogen, H; in the next simplest, alanine, R is the
radical CHg. These amino acid molecules may be joined with one
another, as carbohydrate molecules are joined, with the loss of a molecule
of water at each junction. The more complex of these combinations
are the proteins. The molecules of proteins are relatively huge, con-
taining hundreds or even thousands of atoms. With such large molecules,
which may include varying proportions of most of the amino acids, and
frequently carbohydrates or lipids, there may be an enormous number
of kinds of proteins.

Enzymes. — Many chemical reactions are greatly hastened by the
presence of certain chemical substances which do not enter into the
reaction in a definitive way. Hydrogen peroxide (H2O2) is stable enough

42 PRINCIPLES OF ANIMAL BIOLOGY

to last for months in a bottle; but if a pinch of manganese cUoxide (Mn02)
is added, the extra oxygen of the peroxide comes away so rapidly as to
produce a froth. The manganese dioxide acts as a catalyst, which is the
name applied to inorganic accelerating agents. Now, many living tissues
are constantly producing hydrogen peroxide, but it is promptly decom-
posed. Something in the cells does what manganese dioxide does in the
bottle. That something is called catalase. It is one of many organic
accelerators called enzymes.

For the first time in 1926 an enzyme was isolated, and now some 30
of them have been purified. All of these are apparently proteins or
protein compounds. Some of them work in the cells; others, as the
digestive enzymes, are extruded from the cells and do their work outside.
They work best at temperatures of 30 to 40°C., are inhibited by tempera-
tures around 50°, and destroyed by prolonged exposure to this tempera-
ture. Each enzyme accelerates some particular reaction, and all cells
possess a wido variety of these agents. Theoretically an enzyme may
accelerate a reversible reaction in either direction, and the direction is
dependent on other conditions. Actually, however, the other conditions
in living things are usually such that the enzyme works only one way.
Som.e enzymes ordinarily break down substances (for example, the
digestive enzymes); others build up materials into more complex sub-
stances. The destructive type may be extracted and work in about the
same way under artificial conditions. Those of the constructive class,
however, seldom work outside of cells. Perhaps protoplasm could be
manufactured in the lal^oratory if constructive enzymes worked as Avell
in test tubes as the analytical or destructive ones do.

Physical Structure of Protoplasm. — No matter how smooth and
structureless protoplasm may look to be in a microscope, it is far from

homogeneous. In general, it consists
of particles of various sizes, mostly
very minute, distributed through a
supporting liquid substance. In the
terms of physical chemistry, proto-
plasm is a ''system" consisting of two
"phases," of which the particles are
'^gQS:C);#BlQ%MlC the "dispersed" phase and the sup-
,, ^ ^^. , , porting liquid is the "continuous"

i'lG. 27. — Diagram of an emulsion, i i-

illustrating the physical structure of a phasc. In SO far as thc dispersed
very common kind of protofjiasm. particles are liciuid and large enough

to be visible in a microscope, such
a mixture is an emulsion (Fig. 27). If the particles are submicro-
scopic in size and liquid, as they usually are, the mixture is an emulsoid.
Material in such a finely divided state is also said to be colloidal, or,

THE FUNCTIONS OF PROTOPLASM AND CELLS 43

though somewhat improperly, such substances are called colloids. The
existence of invisible particles may be detected and they may be counted
with the ultramicroscope against a dark background. Some of them
may be photographed by means of the electron microscope. Even the
fine particles are mostly larger than molecules and so may be composed
of more than one substance. Their composition cannot be precisely
known, but they must be relatively insoluble in water in order to main-
tain themselves as particles. There are indications that the particles
are surrounded by a lipoid film, which may have something to do with
their insolubility in water.

This whole structure is, of course, permeated with water, and there
are always salts, and usually sugars, in solution. The particles of these
dissolved substances, being either ions or single molecules, are much
smaller than the dispersed emulsoid particles and confer very different
properties on the protoplasm.

Diffusion and Osmosis. — The molecules and ions of a substance in
solution engage in continual spontaneous movement. So do the mole-
cules of the water or other liquid in which the substance is dissolved.
The particles bombard one another and the walls of the containing
vessel if there is one. The direction of movement of individual particles
is entirely impredictable. Yet if a substance is more concentrated in
one part of a solution than in another, the particles spread more
from the place of high to the place of low concentration than in the
opposite direction. The spontaneous random movement of the particles
in a solution is known as diffusion, and it tends to equalize the concen-
tration in all parts. Protoplasm is the scene of constant shifts of this
kind. The elimination of the waste product carbon dioxide is effected
by diffusion from a place of high concentration in a cell or tissue to a
place of low concentration in the surrounding air or water. The entrance
of oxygen into the cell is dependent on the same principle. Rapid
entrance of water into single-celled animals, requiring its elimination by
pulsating vacuoles, is practically simple diffusion. There are many
situations where an important physiological process is merely diffusion.

There are places, however, in which the diffusion of different sub-
stances is quite unequal. The membrane of a cell — not the dead wall
or the secreted pellicle, but the outer film of protoplasm itself — exercises
a selective influence on the passage of substances through it. Some
substances pass through it readily, others slowly, still others practically
not at all. The membrane is said to be semipermeable. The exchange
of particles between two solutions on opposite sides of a semipermeable
membrane is known as os77iosis. In general, the gases (carbon dioxide
and oxygen) and water pass through a cell membrane rapidly. Simple
sugars (glucose), the amino acids (components of proteins), and glycerol

•i-i

PRINCIPLES OF ANIMAL BIOLOGY

and fatty acids (components of fats) pass through slowly. The ions of
inorganic salts, and the disaccharides (sucrose, etc., page 40) penetrate
the membrane very slowly, and the proteins, polysaccharides, and fats
practically not at all. For some of these substances the inability to
traverse the membrane is explained by the large size of their particles.
For the ions of salts it is probal)ly their electric charges which keep them
out. The cell membrane itself has a charge, usually negative, which
repels ions of like charge; and since the oppositely charged ions cannot

part company, both are excluded. There are
probably other reasons, not yet understood, for
the retardation of passage of particles through
membranes.

The result of osmosis is easily illustrated by
tying a piece of bladder tightly over a thistle
tube, filling the tube with sugar solution, and
immersing the expanded end in a dish of piu-e
water (Fig. 28). After a short time it is found
that the sugar solution in the tube has risen to a
higher level, but that it is not so concentrated
as at first. Water has obviously passed through
the bladder into the sugar solution. A little,
but not much, of the sugar has also found its
way through the membrane into the Avater.
The molecules of water are in constant motion,
striking the walls of the dish, the membrane, and
other molecules of water. Their impacts against
the membrane drive some of them through.
Now the water inside the thistle tube is also in
motion, and some of its molecules pass out into
the water of the larger vessel. But there are
fewer molecules in a given volume of the sugar
solution because the sugar molecules take some
of the space, and their movement is less vigorous
owing to hindrance by the sugar. Hence fewer
molecules of water get out /of the thistle tube than would do so if the
sugar were not there. Water is thus passing through the membi-ane
in both directions, but more of it goes toward the sugai- solution than
away from it. The sugar solution thus rises in the tul)e but becomes
more dilute.

Surface Phenomena. — An important consequence of the colloidal
structure of protoplasm is the enormous surface exposed by the dis-
persed particles. Extremely finely divided ])articles present a greater
surface relative to their volume than do larger particles. This great

Fig. 28. — Diagram of
apparatus used to illus-
trate osmosis. T , inverted
thistle tube covered with
animal membrane and con-
taining a solution of sugar
in water; V, vessel of
water.

THE FUNCTIONS OF PROTOPLASM AND CELLS 45

surface increases the rate of chemical and physical activity at every face
of contact between the two phases of the system. These activities have
been called surface 'phenomena. Some surface phenomena are surface
tension, adsorption, and various electrical phenomena.

Surface tension is exemplified by the film at the surface of water, the
external membranes of cells, the membrane of the nucleus, and the films
that surround vacuoles. A considerable pull is exerted by these films.

Extremely finely divided solids or those with extremely fine pores tend
to condense on their surfaces anj^ gases or vapors or other substances
with which they are in contact. Such substances are said to be adsorbed.
The thin films of these adsorbed substances are held so tenaciously that
great pressures are required for their removal. A gas mask removes
gases from the air because of the great adsorptive power of charcoal, and
the clarification of sirups and sugars is accomplished by making use of
the adsorption of coloring matter by bone black. Certain properties of
living matter are best explained on the basis of adsorption.

Electrical properties are conferred on protoplasm by its ionized salts.
Ions are capable, as explained in Chap. 3, of conducting electricity but
in protoplasm are more important because they are probably adsorbed
upon the surfaces of the colloidal particles. These particles thereb}^
acquire an electric charge. Through the interior of the cytosome the
particles appear to carry positive charges, but in the nuclear sap they are
negative. The surface of a cell as a whole seems, as stated before, to be
negatively charged. The occurrence of like charges on the interior
particles causes mutual repulsion and is probably the chief reason why
these particles do not adhere to one another. If they did adhere, the
protoplasm would coagulate or harden.

Changes in Viscosity. — Viscosity is the resistance which the particles
of a substance otfer to movement upon one another. The viscosity of
light liquids like water or gasoline is low, while that of thick sirup — or
still more so of solids — is high. When a bit of fresh meat is subjected
to pressure while still warm, even if it be from an organ which like the
liver has no conspicuous fibers in it, it appears to be highly viscous. The
resistance is offered mostly, however, by the cell membranes. These are
firm enough, like well-filled bags of wheat, to tend to preserve the shape
of the cells. The interior protoplasm of a cell, at least of those which
have been studied in this respect, turns out to be quite liquid. In one
kind of cell the protoplasm is only about ten times as viscous as water
and only about one one-hundredth as viscous as ordinary glj^cerin.

This fluid state is probably maintained, as indicated in the preceding
section, by the like electric charges on the colloidal particles in the proto-
plasm, causing these particles to repel one another. The viscosity
changes frequently, however, for reasons not yet understood. Such

46 PRINCIPLES OF ANIMAL BIOLOGY

changes occur regularly during cell division, the protoplasm being firmer
at the beginning of division, more liquid (less viscous) later on.

Metabolism. — The protoplasm of a cell carries on all the general
processes of any living body. Within it occurs a multitude of complex
chemical reactions by which the protoplasm maintains and renews itself
and produces more protoplasm. Protoplasm digests food and for this
process secretes various chemical substances. When food is broken down
into simpler substances during digestion, it is absorbed and built up into
the living substances itself or perhaps is combined with oxygen for the pro-
duction of heat and motion. Protoplasm also respires, gets rid of waste
materials by the process of excretion, grows, is capable of movement,
and responds to changes in external conditions, or exhibits irritability.
The chemical processes involved in all these activities of protoplasm are
included under the term metabolism.

Metabolism may be defined as the sum of all the chemical and physical
processes carried on within the protoplasm. It consists of two phases,
namely, the constructive phase or anahoUsm and the destructive phase
or catabolism. Anabolism includes all the processes concerned in the
growth and repair, or upbuilding, of protoplasm. It includes all processes
by which substances are transformed into reserves of food. Catabolism
includes all those processes opposed to anabolism. These are the proc-
esses by which protoplasm is broken down and the waste products
eliminated. Both anabolism and catabolism are continous processes.
As long as anabolic processes are in excess of catabolic processes, growth
occurs; but when catabolic processes are in excess a diminution in size
takes place.

So far as metabolism of animals relates to food, it pursues the following
cycle in the economy of living things collectively. Organic food is first
made out of inorganic matter through the process of photosynthesis in
plants. These organic substances become the food of animals which
arc unable to subsist on inorganic food. The animals digest these foods,
and from the simpler digestive products build up their protoplasm
through the process of assimilation. To supply the energy required for
all this work the animal must secure oxygen by respiration. Waste
materials produced along the way are eliminated by excretion, and useful
products accessory to the general processes are elaborated by secretion.
One of the products of the food cycle is commonly growth. All these
processes are part of metabolism; they are described in the next seven
so(;tions.

Photosynthesis. — Tlu; things which i)hints may take in are water
and salts from tiic soil, and oxygen and carbon dioxide (COo) from the
air (or water, in the case of aquatic plants). The fii-st three of these
are utilized in about the same way in plants as in animals. The carbon

THE FUNCTIONS OF PROTOPLASM AND CELLS

dioxide and some of the water, however, are put to a totally different use.
Carbon dioxide is a by-product of the burning of coal or wood or the
decay of dead animals and plants or of anything else composed partly of
carbon. It is constantly' being thrown off as a waste product by animals
and by plants, except as they use it in the process about to be described.
Plants absorb the carbon dioxide into their leaves or other green parts
and there combine it with water to form one of the simple sugars, glucose.
The final results of this reaction are indicated by the equation

6CO2 + 6H0O + energy^CeHijOe + 6O2

Tn words this means that six molecules of carbon dioxide and six of
water are decomposed and their parts recombined to form one molecule
of glucose and six molecules of oxygen. The energy
expended in bringing about this change comes from
sunlight, hence the process is called photosynthesis,
literally construction by light. In most plants
production of glucose can occur only in the pres-
ence of chlorophyll, the green substance in their
plastids, and certain enzymes. The energy of the
sun in this reaction appears not to affect the car-
bon dioxide directly, but to decompose the water.
The hj'drogen set free from the water is picked up
by other substances which then, without any aid
from light, proceed to attack the carbon dioxide.
The oxygen that is liberated is not produced
directly by the decomposition of the original raw
materials; it comes from a peroxide which is an
intermediate product. That oxygen is liberated
may be demonstrated by an experiment with water
plants. In such an experiment the cut ends of a
Avater plant, as Elodea, are inserted in a test tube
filled with water, the plant and tube are immersed
in water, and the tube is inverted (Fig. 29).
When the plants are placed in sunlight, bubbles of gas escape from their
cut ends and collect in the tube. Suitable tests show the gas to be
oxygen.

Photosynthesis is not absolutely limited to plants, for there are some
simple animals which contain chlorophyll, and in these glucose is pro-
duced in the same way as in plants. Nor are chlorophyll and light
always necessary for the production of glucose, since some colorless
organisms are capable of doing this in darkness.

Plant Products as Food of Animal Cells. — Inasmuch as most animals
are incapable of producing carbohydrates directly from inorganic com-

FiG. 29. — Method
of collecting oxygen
produced by the
aquatic plant Elodea
during photosynthesis.
The oxygen rises from
the plant into the
closed end of the test
tube.

48 PRINCIPLES OF ANIMAL BIOLOGY

pounds or the simple elements, they must get them from plants. Plants
store any excess of carbohydrates above their immediate needs, in some
insoluble form, usually starch or some similar substance. Animals, from
the simplest one-celled ones up to the most complicated, use these stores
of plant starch for food. Out of these plant carbohydrates the charac-
teristic components of animal protoplasm are made. Glucose is to be
had by merely breaking down the starch. Glucose can be converted,
mostly by rearrangement, into glycerol and fatty acids; from these, fats
may be formed.

For one of the essential parts of animal protoplasm, however, the
plant starches will not suffice; that is the highly important class of
proteins. Animals in general cannot make proteins out of inorganics
substances. Only a few can make proteins out of carbohydrates. There
is something lacking in the physiology of most animals which prevents
them from making this particular synthesis. The missing thing is
probably an enzyme or a set of enzymes. Animals must therefore get
their proteins, as well as their carbohydrates, either directly or indirectly
from plants. They may obtain these proteins from other animals, as
the carnivorous animals almost exclusively do, l^ut these other animals
must get the proteins ultimately from plants. •

Conversion of Food. — Very little of the food which animals take can
be utilized at once for its ultimate object, unless water and oxygen be
considered food. Most of the food has to be worked over in some way.
Glucose and other equally simple sugars are ready to use, but these
constitute only a very small fraction of the food of animals. One of the
chief reasons why other foods cannot be used at once is that they are
not soluble. The starches, lipids, and proteins must all be converted
into some form that will diffuse through protoplasm. This conversion
is effected in the process of digestion.

Digestion is essentially the same process everywhere but will be con-
sidered here chiefly as it occurs within cells rather than in the cavities of
large organs like the stomach. Unicellular animals take in small organ-
isms and surround them with a droplet of water containing one or more
enzymes, thus forming a food vacuole. All such animals can produce
enzymes that will digest proteins, many can digest starches, most of
them can digest fats. Proteins are dismembered to yield their amino
acids; fats are split up into glycerol and fatty acids; starches are con-
verted into simple sugai's. The final products named in each case are
all soluble in water and can diffuse through protoplasm.

In this soluble form they pass to every part of the cell, or from cell
to cell. Oxidation of them may occur if energy is needed. The deriva-
tion of energy from oxidation of glucose is represented by an equation

THE FUNCTIONS OF PROTOPLASM AND CELLS 49

which is just the reverse of that by which ghicose is formed in photo-
synthesis, namely,

CeHizOe + 6()2->6H20 + 6(X^2 + energy

This equation says, in words, that one molecule of glucose and six
molecules of oxygen are recombined (in combustion) to form six molecules
of water and six molecules of carbon dioxide, with the release of energy.
Even some of the transitory steps involved in this reaction are reversals
of those occurring in photosynthesis.

If new protoplasmic structure is required, the soluble products of
digestion are available for this purpose. If the digested foods are in
excess of the requirements for these two purposes, they may be stored;
but in this case they must be rendered insoluble again, for otherwise they
could not be retained. If carbohydrates are to be stored in animals, the
glucose is commonly converted into animal starch or glycogen. Glycerol
and fatty acids are again converted into fats, although the fats are likely
to be of different kinds from those which were taken as food. The pro-
duction of these insoluble storage products is done by enzymes, and the
same enzyme may work in both directions, that is, either break down
substances (starches, for example) or build them up.

Little is known about the construction of new protoplasm out of
digested foods. The name assimilation is given to the process, and it
seems certain that enzymes are engaged in the work, but of its nature
we are mostly ignorant.

Respiration.— To provide energy or new protoplasm, all living things
require oxygen. Land animals and plants get it from the air, submerged
aquatic ones from the oxygen which is dissolved in water. There
are, however, some kinds of animals and plants that normally live in
situations devoid of oxygen, and some of these organisms would die
if brought into contact with free oxygen. Such organisms require
oxygen in their metabolism, but they secure it from compounds in
which it occurs.

The combination of oxygen with protoplasm and foods results finally
in the formation of water and carbon dioxide, as indicated by the equation
in the preceding section. The carbon dioxide must be eliminated. The
absorption of oxygen and the elimination of carbon dioxide are together
called respiration.

In simple animals and plants, dissolved oxygen diffuses directly
through the surface of the organism into the protoplasm. Thence by
diffusion and protoplasmic currents it is carried to all parts of the cell.
In many small multicellular animals and plants with few layers of cells the
oxygen may readily diffuse through the intervening cells to those which

50 PRINCIPLES OF ANIMAL BIOLOGY

lie deeper. In larger organisms, however, a transport system is required,
as discussed in Chap. 11.

Excretion. — Metabolism results in the formation of various gases,
water, and other compounds, which are of no value in the body or would
be harmful if allowed to accumulate. The process of their elimination
is called excretion. Gases resulting from metabolism are eliminated
along with carbon dioxide in respiration. Other waste substances pass
through the cell membranes to the exterior, or in some of the protozoa
they are collected by the contractile vacuoles, along with excess water,
and voided through the outlets of these organs. In higher animals
excretions are taken up by the blood and lymph, from which they are
then separated by special organs.

Secret;;ion. — All cells produce certain chemical compounds which may
be used in the processes going on within the cell or in cavities adjoin-
ing the cells. Such products are called secretions. They differ from,
excretions in that they are used in performing some function. Many of
the secretions which are discharged from the cells are first stored in the
cells as granules, which finally break out of the cell and then become
gaseous or liquid. Other secretions produced as liquids within the cell
diffuse out and escape as rapidly as formed, are absorbed by other cells,
or are carried in the blood stream. Such secretions may perform their
functions at a considerable distance from the cells where thej' were
elaborated. Secretions are very diverse in their uses. Some aid in
digestion, others give protection because of their odor or because oi
poisonous properties, some serve as lubricating material, others oxidize
readily with the production of light as in fireflies.

Growth. — Growth is caused by the conversion of foods into proto-
plasm at a more rapid rate than protoplasm is being broken down.
Increase in the size of cells may not be wholly due to increase in the
quantity of protoplasm. Fat cells increase in size because of the depo-
sition of globules of fat, a process which may be continued until there is
much more fat than protoplasm. In plant cells and certain animal cells
volume may be increased by the imbibition of water which may be
stored in vacuoles. In such extreme cases as those mentioned, the
quantity of protoplasm may be actually decreased, although the cell
may be larger.

Reproduction. — Reproduction, or the formation of new individuals,
is likewise characteristic of living beings. In unicellular organisms, and
only in these, reproduction is equivalent to cell division. In higher
organisms, reproduction usually involves the formation of special cells,
the germ cells, which by their division, with rearrangement of the result-
ing cells, give rise to new organisms. Here reproduction involves cell

THE FUNCTIONS OF PROTOPLASM AND CELLS

division too. Cell division is described in Chap. 5, reproduction in
Chap. 14.

Fig. 31. — Fibrillar structure of
cilium of Stylonychia. {From Del-
linger in Journal of Morphology.)

I'lG. 30. - Locomotion in an amoeb;i with sevcM :il psoudopodia, which rest on the substratuni
only at their tips. (From Dellinger in Journal of Experimental Zoology.)

Protoplasmic Movement. — One of the attributes of living organisms
usually distinguishing them from nonliving matter, is the power of
independent motion. Most animals at some stage in their existence,
many plants of the lower orders, and the
swarm spores of other low plants are
motile. Higher plants are not capable of
locomotion, but within their cells the
protoplasm may undergo movement.

In many cells the protoplasm frequently travels as if in channels,
particle following particle, carrying plastids, food vacuoles, and cell

inclusions along with it. When
an amoeba (a one-celled animal)
moves, it thrusts out one or more
lobelike processes, called pseudo-
podia. Then the body is pulled
forward or flows forward. Some-
times there is only one pseudo-
podium, and the amoeba just
flows along. In other kinds of
amoeba there are several pseudo-
podia at one time, and only their
tips touch the substratum, in
which case the animal may almost
be said to walk (Fig. 30). A
pseudopodium is extended appar-
ently because of a local increase
of viscosity in the outer layer of
protoplasm at some part of the
cell, carrying with it a slight contraction which forces the protoplasm else-
where to protrude; but how the change in viscosity is effected is not clear.

Fig. 32. — Form of cilium during strokes;
forcible stroke at left, return stroke at right.
Numbers show successive positions, indicate
direction of movement.

PRINCIPLES OF ANIMAL BIOLOGY

Many of the simple unicellular animals and some of the multi-
cellular ones perform movements by means of cilia or flagella. The
cilia are minute hairlike projections capable of rapid vibration.
Each cilium has an elastic outer layer containing one or more con-
tractile threads within it, as in Fig. 31. Contraction of the threads
on one side bends the cilium in that direction, and elasticity of the sheath
causes it to return.

In the vigorous stroke of a cilium, it is extended and moderateh'
stiff, so as to catch much liquid; on the return stroke it bends limply
nearer the surface of the cell (Fig. 32). Neighboring cilia usually beat
in unison or in waves.

Flagella differ from cilia chiefly in their greater length and are few
in number (usually one to eight per cell). Sometimes the flagellum is

surrounded by a vaselike collar (Fig. 33).
Flagella may beat regularly in one plane, as do
cilia, or they may have a rotary motion. The
whole flagellum may move, or only the free end of
it. The flagellum is composed of an elastic per-
ipheral layer within which are several contractile
threads (Fig. 34), and the movement is due to
'^^s the contraction of these threads. Flagella give
a motile cell a jerky erratic movement; cilia cause
it to glide.

Fig. 33.— Portion of ReSDOnses to Stimuli. — A characteristic pro-

cross section of the sponge ,• 1- • J, ■ -i 1 •!•, J 1 i

Grantia. cc, collared cells perty ol livmg matter is its ability to respond to
of endodenn; ect, ecto- stimuli. A stimulus is anv influence of sufficient

derm; fl, flagellum of col- • i , \ • , i

lared cell; mes, mesogioea; magnitude to cause a change m protoplasm.

sp, spicule (portion only), ^j-^g stimulating agent may be external to the
organisms, such as changes in light, temperature, chemical substances,
sound, pressure, or electric current; or it may originate within, through
osmosis, electric charges, chemical substances, pressure, or nerve impulses.
To be a stimulus, the modification must have a certain degree of
suddenness. A very gradual change in the intensity of light may have
no observable effect, while a sudden change of the same amount produces
a marked reaction.

Responses are of very different sorts. Muscle cells and others con-
tract; gland cells produce secretions. Pigment cells in the skin of a
frog, which are highly branched and have their pigment distributed
throughout all parts of the cell when at rest, contract their pigment into
a small compact mass in response to light, thereby changing the animal's
color. Streaming of protoplasm in plant cells stops in response to an
electric current. A chemical substance in the retina of the eye of

THE FUNCTIONS OF PROTOPLASM AND CELLS

vertebrate animals is decomposed by light. The electric organs of
certain fishes produce a series of discharges.

The nature of the response is determined by the nature of the respond-
ing protoplasm, not by the kind of stimulus. A muscle cell contracts,
whether the stimulus be chemical or electrical. A gland cell secretes,
and its product is always the same, regardless of what started its
activity.

The extent of a response is, in general, rather definitely fixed for any
given cell. If the cell responds at all, it does so to its full capacity. An
organ made of many cells may respond in various
degrees, depending on whether few or many of its
component cells join in the response. How many
cells respond depends on the intensity of the stimulus.
Each individual cell, however, follows the all-or-none
rule of acting either at its maximum capacity or not
at all.

What Is Living Matter? — The characteristics of
living matter enumerated in the opening paragraph of
this chapter do not constitute a criterion which would
enable even an expert to say in a specific instance
whether a bit of matter were alive. Application of
the rules would occasionally be futile. The chemical
composition of recently killed protoplasm would, on
analysis, be indistinguishable from that of living proto-
plasm; but something intangible would be gone from
it. Spontaneous movement and change of shape may
occur in a drop of liquid, under certain circumstances,
because of changes in the surface film. Moreover,
living things in the form of resting spores exhibit no
detectable movements over long periods of time. A crystal may be made
to convert part or all of itself into a flock of smaller crystals, in a way
that would be hard to exclude in a definition of reproduction. Finally,
metals respond to things in the environment, such as a magnet or
electric potential.

A definition of life which lists the ordinary activities or conditions of
Uving things is feasible; but it could not be used practically for a
complete classification of all objects into two categories, living and
nonliving.

A B

Fig. 34.— Fla-
gellura ifl) of Eu-
glena, showing
(right) contractile
threads within it.
(B after Dellinger
in Journal of
Morphology.)

References

Heilbrunn, L. V. An Outline of General Physiology. W. B. Saunders Company.
Marsland, D. Principles of Modern Biology. Henry Holt and Company.

54 PRINCIPLES OF ANIMAL BIOLOGY

Mitchell, P. H. Textbook of General Physiology. McGraw-Hill Book Company,
Inc. 3d Ed. (Chap. VII, the chemistry and physiology of proteins, lipids, and
carbohydrates; Chap. VIII, the salts; Chap. IX, water and electrolytes.)

Rogers, C. G. Textbook of Comparative Physiology. McGraw-Hill Book Com-
pany, Inc. 2d Ed. (Chap. Ill, diffusion and osmosis; Chaps. IV and V, struc-
ture and properties of protoplasm.)

Verworn, M. General Physiology. The Macmillan Company. (Part II of
Chap. II, contrast of living and lifeless.)

CHAPTERS
CELL DIVISION

When cells .were first discovered, and even after it became fairly cer-
tain that all organisms were composed of them, no one appreciated how
fundamentally the cells Avere involved in the constitution of living things.
They were thought, for example, to be of secondary origin; that is,
animals and plants were believed to possess a formative or nutritive
substance without any particular organization or structure, and out
of this the cells were supposed to be formed. While all organisms were
found to contain cells, it was not thought that these cells had any neces-
sary function in the production of new cells out of the formative material.
Gradually, however, the idea gained ground that the origin of new cells
occurred by division of old cells, a doctrine which in 1855 was expressed
by the famous pathologist Virchow in the words omnis cellula e cellula —
all cells from cells. While the origin of cells from cells was thus early
recognized, the mechanism by which cells originated from other cells
was not known until twenty or thirty years later. It was not until
1873 that the common method of cell division — resolution of the chroma-
tin into distinct separate bodies and the formation of a spindlelike
mechanism manipulating these bodies — was discovered. The same
method was soon witnessed in a variety of plants and animals and is
now found to be nearly universal. To this method of cell division the
names mitosis and karyokinesis are applied. The latter is the more
descriptive, but the former is more often used.

Interphase. — A cell not in division is said to be in interphase. In
such a cell the chromatin is so diffuse as to present the appearance of a
network (Fig. 35 A). Actually, in most cells, this chromatin exists in
a number of distinct portions, the chromosomes; but the threadlike form
which these chromosomes take in most animals makes it impossible to
distinguish them. In a few organisms (some grasses among them) the
chromosomes are more condensed and are separately visible even in
the interphase. In some special tissues, such as the salivarj^ glands of
flies, the chromosomes are greatly enlarged and are more easily recog-
nizable in interphase than in any cell division. The chromosomes of
these glands also have a pattern by which they can be distinguished;
and every nucleus has a set of chromosomes identical in pattern with
those of any other nucleus. The individuality of the chromosomes which

PRINCIPLES OF ANIMAL BIOLOGY

is so evident in these glands undoubtedly exists elsewhere. One indi-
cation of this is found in animals which have different shapes and sizes
of chromosomes. At every division there is the same number of chromo-
somes of a given shape and size, which could hardly be true unless the
chromosomes maintained their identity in the intervening interphase.
Also, chromosomes may be broken up by X rays, and reconstituted in
new sizes and shapes, and these new chromosome forms appear again
after cell division and in later generations. Obviously chromosomes

Fig. 35. — ^Mitotic cell division. A, cell not in division; B, centrioles move apart; C,
distinct chromosomes formed; D, nuclear membrane dissolved, spindle completed; E, F,
equatorial plate, side and end view, with chromosomes duplicated; G, H, chromosomes
move apart; /, /, division of cytosome and construction of new nuclei. (A, interphase;
B-D, prophase; E, F, metaphase; G-I, anaphase; J, telophase.)

maintain their individuality in the interphase, even though it cannot
be observed.

Prophase. — Mitosis is nearly enough alike in most cells to make
possible a general account of the process. Starting with a cell in inter-
phase, in which the centriole is already divided into two parts, one of
the early signs of division is the condensation of the chromatin into
distinct threads tangled about in the nucleus (Fig. 35/?). In whatever
way the chromosomes (page 26) are spread out through the nucleus,
they now contract into smaller compass, usually in the form of slender
strings or ribbons. The parts of the centriole' separate and move toward
opposite sides of the nucleus. Sometimes between them a few threadlike

CELL DIVISION ' 57

lines are stretched, and around each one radiating Hnes may develop,
giving the appearance of a star. The contraction of the chromatin
continues, and before the process is more than well under way the chro-
mosomes are distinguishable as separate bands or ropes (C). The
entire scattered chromatin of the interphase nucleus is now collected
into these conspicuous bodies. Though the chromosomes have been
separate bodies all the time, it can now be seen for the first time that they
are distinct. While this change in the chromatin has been taking place,
the nucleolus, if one was present, has disappeared. The centrioles have
moved around to opposite sides of the nucleus, and very distinct threads
from them appear to be pushing against or even into the nucleus. The
membrane of the nucleus then dissolves away, leaving the chromosomes
free in the general protoplasm. Some of the threads from each centriole
quickly pass through the space formerly occupied by the nucleus and
connect with the other centriole, establishing a complete spindle between
them. Other threads go only halfway and end at the chromosomes.
The chromosomes shorten still further and thicken to form definite
bodies, often of very different shapes and sizes within the same cell.
The chromosomes are placed where the nucleus was, without any par-
ticular arrangement. The changes so far described, including stages
B to D in the figure, are collectively called the prophase, though the
plural form would be more accurate.

Metaphase. — The chromosomes then move, probably are drawn, into
a flat group across the middle of the spindle. In this position they
form what is called the equatorial plate. Seen from the side of the
spindle they appear as in E, but viewed from one of the centrioles they
are as in F. This stage of mitosis is called the metaphase. It is of very
brief duration, so that it appears less often in preparations than the
other stages do. Either in the metaphase or at some earlier time,
the chromosomes become double structures. This doubling is usually
described as a division, but it may equally well be conceived as a dupli-
cation, that is, the formation of a second chromosome just like the
original. It is not important to decide at this point which of these
methods is employed, since in either case two identical chromosomes
exist where only one of that kind existed before. The chromosomes
are shown thus duplicated in E, less clearly so in F because of the direction
from which they are viewed.

An important feature of this division is that the two chromosomes
produced from one are, in all significant features, identical with each
other and with the original chromosome which produced them. To
understand this fact one must know that the chromosomes have a
longitudinal pattern. They contain different substances at different
points in their length. A longitudinal division of the chromosome

58 PRINCIPLES OF ANIMAL BIOLOGY

divides all the different components, so that the resulting two chromo-
somes have the same pattern as the original one.

Anaphase. — From their position in the equatorial plate the two
chromosomes, formed from one, move or are drawn toward opposite
ends of the spindle. This stage is known as the ana-phase. The shapes
of the chromosomes often indicate that they are being pulled. Thus, in
Fig. ?>bG, the long chromosomes could be given their V shape by being
pulled from their middle points toward the centrioles. Moreover, some
of the so-called fibers extending out from the centrioles may often be
seen to attach to the chromosomes at these points. Consequently, the
fibers are often thought of as pulling the duplicated chromosomes apart.
Whether they actually pull or not is uncertain. The fibers may be only
lines of flow in the protoplasm, that is, courses along which the fluid
protoplasm is moving. Whatever causes this flow could drag the chromo-
somes along. If the middle parts of the long chromosomes were caught
in this current, the characteristic V form of such chromosomes would
still result.

Whatever the cause of their movement, one chromosome of each
pair of duplicates goes to each end of the spindle {H). Here they collect
in two close groups (/), ready to form two new nuclei. In the meantime
the cytosome narrows between the retreating groups of chromosomes
(//, 7) and finally constricts in a sharp furrow (/) which eventually cuts
the cell completely in two (J). In many cells, about this time, the
centriole divides in two, as if in preparation for the next division (7), so
that during the whole ensuing interphase the centriole is double.

This separation of the daughter chromosomes has as important a
consequence as does the longitudinal duplication of each one. ' The
chromosomes are of different kinds; they contain different things. Each
cell possesses a complete set of the different kinds of chromosomes. The
accurate separation of the sister chromosomes, one going to each pole
of the spindle in the anaphase, insures that the two daughter cells will
likewise have a complete set of chromosomes. All the cells of a multi-
cellular animal thus have identical chromosomes in them.

Telophase. — The remainder of the process of cell division consists of
th(^, restoration of the chromosomes to the diffuse state in whicli they
existed before division began, and the disappearance of all remnants of
the divisi(m apparatus from the cytosome. The chromosomes become
diffuse either by becoming filled and distended with a fluid or by spinning
out their chromatin into fine, perhaps branching, threads, as explained
on page 26. Some particulars of this process are given later. By either
method the chromatin comes to be scattered in irregular knots or strands,
giving the appearance of a network. A membrane is formed about the
whole grou]) of chromosomes (./) and the reconstruction of the nuclei

CELL DIVISION

approaches completion. During these changes the new nucleus may
rotate considerably in the cytosome, as it is shown to have done in the
illustration. In the figure (J) the two cells are shown in different stages
of the reconstruction process. This is done merely to illustrate the
steps involved, for as a rule they transform at about the same speed and
are at all times in about the same stage.

A B C

Fig. 36. — Chromosomes of various shapes and sizes shown just before they are arranged
across the middle of the spindle. A, oogonium of the beetle Dytiscus (from Dehaisieux
in La Cellule); B, spermatogonium of arrow worm Sagitta (from Bar das in La Cellule);
C, egg of hellbender (from B. G. Smith in Journal of Morphology and Physiology) .

The principal other features of the reconstruction are the loss of the
remaining spindle fibers in the cytosome and the formation of a nucleolus
if there was one prior to division (J) . When these steps have been taken,
two new cells of smaller size, essentially identical with one another, have
been produced from one older cell.

Variations Relating to Chromosomes. — While the foregoing account
represents a fairly typical mitosis, there are many variations in the
process. The number of chromosomes differs greatly in different species.

Fig. 37. Fig. 38.

Fig. 37. — Splitting of the chromosomes before the equatorial plate stage; peritoneum
of the salamander Ambystoma. (From Parmenter in Journal of Morphology.)

Fig. 38. — Reconstruction of nuclei through imbibition of liquid by the chromosomes
to form vesicles. A and B, early and late stages of vesiculation in the egg of the sea urchin,
in which the vesicles fuse. (From Danchakoff in Journal of Morphology.)

In the parasitic worm Ascaris megalocephala each cell has 4 chromosomes;
in the vinegar fly Drosophila melanogaster the number is 8; and man
has 48 chromosomes. Most of the numbers from 4 to 60 are found in
one or more species, and there are some numbers above and below these
limits. The number differs in the two sexes in some animals, being
usually more numerous in the female when there is such a difference.

PRINCIPLES OF ANIMAL BIOLOGY

The chromosomes differ greatly in size in different organisms, and
often in the same cells. Two sizes of chromosomes are shown in Fig. 35,
and further differences are represented in Fig. 36.

The time of duplication or splitting of the chromosomes varies con-
siderably. In some cells, as in Fig. 35, the chromosomes do not duplicate

Fig. 39. Fig. 40.

Fig. 39. — Vesiculation of chromosomes by formation of protoplasmic film around each
chromosome; A early, B late stage. The vesicles do not fuse.

Fig. 40. — Interphase nucleus of the hellbender, showing the chromosomes distinct and
separate as vesicles, in which, however, the chromatin is very diffuse. {From B. G. Smith
in Journal of Morphology and Physiology.)

themselves until they are in the metaphase. In others they are doubled
while still in the long ropelike stage before taking their places on the
middle of the spindle (Fig. 37), that is, in the prophase.

Fig. 41. Fig. 42.

Fig. 41. — Mitosis without centrioles in a cell of the root tip of the hyacinth. {From
Dahlgren and Kepner, "Principles of Animal Histology.")

Fig. 42. — Dividing cell with conspicuous spindle in whitefish embryo. (.Courtesy of
General Biological Supply House.)

The expansion of the chromosomes to form new nuclei at the close
of division differs in different animals and plants. In some species there
is a very plain formation of vesicles by the accumulation of liquid \\ithin
each chromosome (Fig. 38.4). Then the vesicles fuse to form one largo
vesicle {B), though it is still quite likely that the chromosomes maintain

CELL DIVISION

their individuality in and at the wall of this vesicle. In other organisms

a film of protoplasm forms around each chromosome,

and within the vacuole so created the chromatin spreads

out in diffuse form (Fig. 39). In the example used in

this figure these vacuoles do not fuse but remain

separate. A third way of rendering the chromosomes

indefinite in appearance, hinted at in the preceding

account and earlier on page 26, is to have their chromatin

spin out into fine threads, often branching, without the

formation of vesicles in or around them. This method

is combined with vesicle formation in the generalized

illustration Fig. 35 (/, left). Occasionally, even when

chromosome vesicles are formed, they are distinguishable

as separate objects during the interphase (Fig. 40).

Variations in Spindle and Cytosome. — A striking
variation in the spindle is the lack of any centrioles in
the cells of flowering plants (Fig. 41). In animal cells ^ ,^ ^ ^

. ^ \ o / Fig. 43. — Intra-

they may be very minute but are usually present. The nuclear spindle in
rest of the spindle, that is, the fibers and radiating lines 1,^®, piotozoon

•^ ' ' ° Euglypha. {From

about the centrioles, may or may not be conspicuous. Wilson," The Cell,"
In Fig. 42 the spindle fibers and the rays around the ^^^^^ Schevnakoff.)
centrioles are very conspicuous. But in the very fiat cells in the outer

layer of the skin of salamanders there is
little or no sign of a spindle, even though
the chromosomes are sharply defined.

The place where the spindle forms is
different in different organisms. In Fig.
35 it is shown forming outside, but near,
the nucleus. This is its usual origin. But
in certain protozoa and some multicellular
animals it forms wdthin the nucleus. In
^f^^^^^^mlKi^M^^ such animals the spindle may be well

developed and the chromosomes arranged
on it, or the chromosomes may even be
moving toward the ends of the spindle
(Fig. 43), before the nuclear membrane
disappears.

With respect to the cytosome, the prin-
,,,...,, .p cipal variation is the way in which the two

the beginmng of the process, {rrom ^ -^

Dahigren and Kepner, "Principles cells produced by division are separated
of Animal Histologyn f^^^ ^^^ another in plants as compared

with animals. Instead of dividing by means of a furrow around the
cell, plant cells form a group of nodules on the middle of the spindle

Fig. 44. — Formation of the cell
plate in a dividing cell of the root
tip of the hyacinth. The thicken-
ings on the fibers of the spindle are

PRINCIPLES OF ANIMAL BIOLOGY

(Fig. 44). These lumps increase in size until they coalesce into a plate,
which forms a new wall dividing the cell into two.

Amitosis. — Amitosis is a type of cell division which involves no com-
plicated visible mechanism. The word means, literally, not mitosis.
Many supposed examples of amitosis are merely distorted forms of
mitosis, the distortion being due either to faulty preparation or to natural
degenerative changes in the cells. Preparations of cells have in some
cases been so defective that cell division was at first regarded as amitotic,
but better technique revealed some of the features of mitosis. Also, in
certain degenerate animals it appears that the process itself has become
so modified that even the most perfect preparations of dividing cells
resemble amitotic division very closely.

Fig. 45. — Amitotic division of the nuclei in the follicle cells of the cricket's ovary.
Various stages of nuclear division are shown. (From Conklin.)

Confusion has arisen from the fact that the nucleus of a cell may
divide without any subsequent division of the cell body. This division
is often called amitosis of the nucleus, but it is not amitotic cell division.
Follicle cells in the cricket's ovary (Fig. 45) show nuclear division of
this sort.

Genetic Significance of Mitosis. — The longitudinal duplication of the
chromosomos and the equal distribution of sister chromosomes to the
cells in division, to which attention has been called, has a greater sig-
nificance than has yet been indicated. The chromosomes contain the
units of heredity, which are called genes (Chapter 17). It is these genes,
more than anything else, which are arranged in longitudinal pattern in
the chromosomes. In the division of the chromosomes, the greatest
imi)ortance attaches to the duplication of the genes. The necessity of
distributing a complete set of cihromosomes to each cell rests on the

CELL DIVISION 63

necessity of having a complete set of genes in each cell. Incidentally
it is the genes in the chromosomes which make the word duplication
preferable to division in describing the formation of two chromosomes
from one, for the genes may be single protein molecules. As such they
could not be divided and retain their identity; they could, however, be
duplicated.

How the cells in different parts of a multicellular animal become and
do different things when they contain identical chromosomes and genes
is a question which must be postponed until embryonic development is
studied. The even greater importance of genes and chromosomes in
reproductive cells, and a different type of cell division which manipulates
the genes in germ cells, must likewise await the discussion of embryology.

References

Calkins, G. N. Biology of the Protozoa. Lea & Febiger. (Pp. 208-245, types of
division in unicellular organisms.)

Dahlgren, U., and W. A. Kepner. A Textbook of the Principles of Animal His-
tology. The Macmillan Company. (Chap. V.)

MiNCHiN, E. A. An Introduction to the Study of the Protozoa. E. J. Arnold & Son,
Ltd. (Chap. VII.)

Sharp, L. W. An Introduction to Cytology. 3d Ed. McGraw-Hill Book Com-
pany, Inc. (Chap. VIII.)

Wilson, E. B. The Cell in Development and Heredity. 3d Ed. The Macmillan
Company. (Chap. II.)

CHAPTER 6
FROM ONE CELL TO MANY CELLS

Knowledge of the structure, function and multiplication of single
cells should pave the way for an understanding of the more intricate
structure, function, and interrelations of the complex animals or metazoa.
As a step toward such an understanding it will be useful to reflect upon
some of the consequences of the differences between the complex and
the simple.

Insight into the nature of multicellular organisms would be furnished
by some certain knowledge of how they became multicellular. It seems
clear that living things have not always existed in the highly complicated
form that many of them now show. There must have been an origin of
complex beings from simpler ones. This conclusion is often couched in
the statement that multicellular organisms must have arisen from
unicellular ones, but it would be somewhat safer, as we shall see, not to
imply that cells were involved in the change. Some biologists hold that
a step comparable to a change from one cell to many was made before
these living things had arrived at a genuinely cellular constitution. But
whatever the origin of multicellular organisms was, if we knew that origin
we should have an important clue to some of their other characteristics.

Relation of Parts to the Whole. — Two schools of thought have arisen
concerning the relation between multicellular animals and the cells of
which they are composed. One school has held that the whole is the
sum of its parts; hence that many-celled organisms are what their cells
make them. If cells of a certain structure and certain capacities are
assumed, any body composed of them will have the combined structures
and ca])acities of those cells. The other school has regarded the whole
as superior to its parts. A living thing is a whole first of all; its parts are
secondary. Animals and plants are not determined by the cells com-
posing them. Instead, they impress upon their cells certain properties
because the parts of the given whole must have those properties. Tlio
former view is the more easily understood of the two, though probably
only because in the physical and industrial world about us we see many
examples of construction of wholes, such as buildings, out of units, such
as bricks, whose properties are predetermined and do not change, just
as bricks do not change when they are set in a wall. We are not accus-

FROM ONE CELL TO MANY CELLS 65

tomed to building materials whose nature depends on the kind of struc-
ture to which they contribute.

Two Contrasted Theories of Multicellular Origin. — In consequence of
these two views of the relation of parts to wholes, two general theories
of the origin of multicellular organisms have been entertained. Accord-
ing to one theory, parts have joined to make wholes; cells have joined
to make many-celled bodies. According to the other theory, wholes have
been divided into parts. Organisms became complex, then divided into
cells whose qualities were dictated by the nature of the whole from which
they were produced.

Which of these theories contains the greater element of truth it is
impossible to say. As applied to the origin of metazoa, both have
received ardent support from biologists. Both have certain physiological
facts in their favor. On the one hand, as a purely logical deduction, it is
obvious that the function of an organ is the sum of the things which its
component cells do. But that deduction means nothing if the single
cells are doing things which are dictated by the whole. On the other
hand, it is known from the development of embryos that cells become
certain structures because they occupy a certain place among their
fellows. But there is no certainty that this is in any sense a consequence
of a property of w^holeness in the embryo. The two theories must be
left, therefore, with the mere statement of their import, without any
attempt to judge between them.

When, however, one considers the step-by-step consequences of the
possible evolution of higher organisms by the one or the other of these
general methods, the two concepts rest on different planes. Biologists
have usually held that, in the evolution of any line of descent, many
branches of the group have arisen, some of which have advanced farther
than others. If all of these branches could be collected, they could be
arranged in such an order as to give at least a hint of the steps by which
the evolution of the most advanced branches had reached their ultimate
condition. The less advanced types might, of course, become extinct
and so destroy the evidence of the successive stages, and in actual
evolution it is certain that such extinction has often occurred. On the
chance, however, that some of them have survived, biologists have fre-
quently sought among existing relatively simple organisms approximate
representatives of the conditions through which the more complex ones
have gone in their evolution. The attempt to reconstruct lines of descent
by means of series of modern organisms must be done with caution, and
no very close correspondence between modern forms and ancestral types
can be expected.

In a reconstruction of the origin of the metazoa by means of a series
of modern organisms supposed to represent the evolutionary steps, the

PRINCIPLES OF ANIMAL BIOLOGY

two theories of the relation of parts to wholes fare very unequally. Only
a few modern representatives of the one type of change may be selected,
while very many are available to represent the other.

The Organismal Theory. — The organismal theory is that which treats
living things primarily as wholes, to which the parts are subordinate.
In accord with this theory, the evolution of complex organisms from
simple ones should start with an increase in complexity in some animal or
plant while it is still a single cell. Much differentiation in the structures

of the cytosome must have occurred. It would
be expected also that the nucleus would have
divided into many nuclei without corresponding
divisions of the cytosome. That is, a multi-
nucleate cell would have arisen out of a uninu-
cleate one. Protoplasm containing many nuclei
without separating cell membranes is known in
a number of animals and plants and is called a
syncytium. Voluntary muscle cells (page 95) in
the higher animals have many nuclei, and the
developing eggs of insects (Fig. 172) pass through
a stage in which there are many nuclei before
cell membranes begin to appear. One cannot,
however, think of these very complex metazoan
structures as remnants of an evolutionary stage
which most of the other metazoa have passed.
To have any possible significance as representa-
tive of a step in evolution, the syncytium should
be some rather simple organism. Vaucheria
(Fig. 46), one of the simple plants, is syncytial,
and there are several other plants. Good ex-
amples are lacking among animals. The organismal theory is thus
not well supported by living representatives of the stages for which
it calls, though this lack can hardly be regarded as a fatal objection to
the theory.

The Colonial Theory. — If one regards organisms as the sum of their
component parts, the natural supposition is that multicellular animals
and plants arose through some form of colony formation. Cells multi-
plied by division and then, instead of falling apart as they do among the
protozoa, they clung together in groups. Such colonies could be formed
before any of the cells became any more greatly differentiated than the
single cell had been. The differentiation and increase in complexity
could then follow in a succession of steps. The multicellular condition
comes first, the complexity later, rev(n-sing the order expected from
the organismal concept. This way of deriving the metazoa has the

Fig. 46. — Vaucheria, a
simple plant illustrating
a syncytium or multinu-
cleate cell. {From Sharp,
"Introduction to Cytology.")

FROM ONE CELL TO MANY CELLS

advantage — if advantage it be — of being capable of illustration by
organisms now living. The series of types used to illustrate it must
still show considerable gaps, and the representation is sure to be only
approximate; but the imagination can easily fill the vacant
places. Let us consider what these representative living
organisms may be.

Types of Colonies. — The adherence of the two cells
produced by division should require no more explanation
than the physical connection and the mode of separation
seen in mitosis in multicellular animals. The fact that
protozoan cells should regularly separate is quite as remark-
able as that metazoan cells should regularly cling together.
Protozoan species in which the cells remain attached exist
in colonies. Sometimes no more than two cells adhere;
sometimes the number is thousands. The manner of
adherence varies. An envelope of jelly may help hold
the cells together, or they may be joined by stalks, or
the cells may cling to one another merely by small areas of
contact.

Colonies take various forms. In Ceratium (Fig. 47), the cells are in
a single row, making what is called a linear colony. This type is rare in
animals but common in the simple plants (algae), in which cylindrical
cells are set end to end in long fine filaments. In some species the cells
do not touch one another but are joined by branching stalks (Fig. 48),

Fig. 47.—
A linear
colony, Cera-
tium cande-
labrum.

Fig. 48. Fig. 49.

Fig. 48. — Codosiga cymosa Kent. A, treelike colony; B, individual cell in detail.
Fig. 49. — A gregaloid colony, Microgromia socialis. {From Calkins, "The Protozoa,"
The Macm,illan Com-pany.)

forming a treelike or dendritic colony. These branching colonies are of
many degrees of complexity, from those in which two cells fork off from a
single common stalk to ones in which the stalks branch and rebranch
and end in hundreds of cells. These organisms are all aquatic. The
branched stalks and cells may be quite exposed to the water, so that
currents of water pass freely among them, or they may be imbedded.

68 PRINCIPLES OF ANIMAL BIOLOGY

stalks and all, in a mass of jelly. Such colonies may be as large as
walnuts, or even baseballs.

A third type of colony is the gregaloid, in which the cells are irregularly
placed in a mass of jelly. These cells may be loosely arranged and in

Fig. 50. Fig. 51.

Fig. 50. — A gregaloid colony, Proterospongia haeckeli. {From Hegner's "College
Zoology," The Macmillan Company.)

Fig. 51. — Pandorina morum, a spheroid colony.

contact with one another by means of fine processes branching out from
them (Fig. 49), or they may be quite separate with only the jelly to hold
them together (Fig. 50).

Somewhat more compact and more regularly arranged are the spheroid
colonies. In these there is usually a mass of jelly nearly spherical in

A B

Fig. 52. Fig. 5:3.

Fig. 52. — A spheroid colony, Eudorini elegans. A, adult colony, X475; B, daugh-
ter colony, X7.30. {From West after Goehd.)

Fig. 53. — Anthophysa vegetans. Spheroid colonies arranged on a branching stalk,
thus combining two typos of colonies. {After Kent.)

shape, in which cells are imbedded in a layer near the surface, but none
is in the center. The cells may be actuall\' in contact, or nearly so
(Fig. 51), especially in young colonies (Fig. 52/i), or widely separated,
as in most such forms when older (Fig. 52A).

FROM ONE CELL TO MANY CELLS

In some organisms two of these types of colonies may be combined.
The cells may be in globular masses (spheroid type), though not iml^edded
in jelly, and several of these masses joined by a branching stalk (Fig. 53).

Choice of Colony to Illustrate Metazoan Origin. — If it is assumed, in
tracing probable lines of descent, that the colonial theory is correct,
which of these colonial types is most likely to represent the early evolution
of the metazoa? The massive compact form of most of the metazoa
suggests that the linear and dendritic colonies may be left out of consider-
ation. Of the other two types, each has something in its favor. The
fact most favoring the gregaloid colony is that in one of
the best known organisms of that kind, Proterospongia
(Fig. 50), each cell at the surface bears a delicate proto-
plasmic collar around its one flagellum. Such a collar,
surrounding a flagellum, is found on certain internal
cells of the sponges (Fig. 33, page 52), which constitute
one of the simplest groups of metazoa. Some biologists
have inferred from these collared cells that the earliest
metazoa may have been in some degree spongelike and
that they came from colonies somewhat like present-day
gregaloid colonies.

The spheroid type of colony is favored by its greater
abundance at the present time. Most of the spheroid
colonies consist of cells bearing flagella, and many
students of protozoa have held that the flagellate forms
are the most primitive of the single-celled animals,
which is another pair of facts in favor of the spheroid
colony. Furthermore, the spheroid colonies lead directly
to other forms that may, as we shall see, be used to illustrate later steps
in the evolution process.

This reasoning may not be correct, biit many biologists in the past
have followed it and concluded that the metazoa probably arose from
a single-celled organism, bearing some resemblance to modern flagellates
(Fig. 54), through the formation of colonies.

The First Differentiation. — In all the colonies described, the cells of
one group are all alike, at least potentially. In Proterospongia (Fig. 50)
they may seem to be of two kinds, since the cells in the interior of the
jelly mass do not have collars. This is not a real difference, however,
for the cells take turns coming to the surface, where they feed, and while
at the surface develop a collar and flagellum, which they lose when they
retreat to the interior.

Now, the chief distinguishing mark of the metazoa is that their cells
are not all alike. In the evolution of the multicellular organisms there
must have been a differentiation of the adhering cells into two or more

Fig. 54 .—
Chlamydomonas,
illustrating a
primitive type of
organism from
which colonies
and later met-
azoa may have
arisen.

PRINCIPLES OF ANIMAL BIOLOGY

kinds, if the colonial theory of origin is correct — or a differentiation of the
parts of the cell which later became distinct cells, if the organismal
theory is correct. Following only the colonial origin, what differentiation
shall we expect?

A B

Fig. 55. — Pleodorina illinoisensis , consisting of 28 reproductive and 4 sterile cells. A,
young organism; B, reproductive stage. The sterile cells may be regarded as the beginning
of a soma.

If we are to draw our answer to this question from the animals and
plants that live at present, we should look for those in which there has
been only one differentiation — in which, as a consequence, there are only
two kinds of cells. The only organisms which exhibit a single differentia-
tion among their cells are those in which some cells have lost the power of
reproduction, while others retain it. Pleodorina is an example. In one

Fig. 50. Fig. 57.

Fig. 56. — Pleodorina californica, with small sterile cells almost as numerous as large
reproductive ones.

Fig. 57. — Volvox wcismannia, with 10 reproductive cells and thousands of sterile cells.
{From Powers, in Transactions of American Microscopical Society.)

of its forms (Fig. 55), which may be only a variety of Eudorina elegans
(Fig. 52), it consists of 32 cells in a jelly matrix. Four of these cells,
placed at that side which moves foremost as the organism swims, are
smaller than the rest. These 4 cells are sterile, while the remaining 28
may reproduce. Any of the 28 larger cells may divide to form a group of
32 cells which escape from the jelly and lead an independent existence

FROM ONE CELL TO MANY CELLS 71

or form special cells which reproduce the group in another way; but none
of the 4 small cells can do this. In another species (Fig. 56) the cells are
more numerous, and the sterile and reproductive cells are more nearly
equal in number; but they are again of two sizes, the smaller ones being
sterile. Volvox (Fig. 57) is another, though much larger, form in which
there are sterile and reproductive cells; but here the sterile cells greatly
outnumber the reproductive. The two Pleodorinas and Volvox, taken in
the order in which they are used here, show an increasing number of the
sterile Cells.

The existence of such forms as these suggests that the earliest differ-
entiation between the cells of a colony, on its way to becoming a met-
azoon, was the loss of reproductive powers by some of the cells. The
group of sterile cells in these organisms corresponds to the soma, or body,
as contrasted with the germ cells, or reproductive cells, of the metazoa.

Further Differentiation. — In the organisms just studied, all the sterile
cells are alike in structure and function, except in Volvox, in which the
cells on the front side, as the organism swims, differ
slightly in color and the size of certain of their structures
from those on the rear side. This is quite at variance with
the higher metazoa, in which the cells of the soma are of
very many markedly different kinds. There is no way of
knowing which of the manj^ types of somatic cells originated
earliest; hence no clue as to what kind of modern animal Fig. 58. —

we should look for to illustrate that step. The best we ^^ '^senemi
can do, if we are to pursue this plan of choosing present- form of body,
day representatives, is to select some animal in which the ^'
differentiations among the somatic cells are not too numerous. A
suitable form is the fresh-water Hydra, in which half a dozen kinds of
somatic cells are found. A brief description of the body as a whole must
precede the study of these cells.

The form of Hydra is essentially cylindrical (Fig. 58) when extended
and more or less globular when contracted. Ordinarily the body is
attached 'by one end, the foot, to a solid object. At the tip of the free
end of the body the mouth is located. Near the mouth is a circlet of
long contractile tentacles which have arisen from the body by an out-
pushing of the body wall. By means of the tentacles Hydra captures and
thrusts into its mouth minute aquatic animals. The conical eminence
between the mouth and the tentacles is the hypostome.

The body of Hydra is hollow (Fig. 59), the interior space being a
digestive cavity. Its wall is composed of two layers of cells, the outer
known as the ectoderm, the inner as the endoderm. The endoderm cells
are all essentially alike, being tall and slender and bearing flagella. Their
function is the digestion of food. The ectoderm has differentiated into

PRINCIPLES OF ANIMAL BIOLOGY

Fig. 59.
gramniatic

-Hydra, dia-
representa-

several kinds of cells. The bulk of that layer is made up of nearly cubical

cells called the epithelial cells. Some of these epithelial cells, at the side

toward the endoderm, are drawn out into long slender processes which

serve both to contract, like muscles, and to convey impulses, like nerves.

They are accordingly called neuromuscular cells
(Fig. 60). Between the bases of the epithelial
cells are numerous smaller rounded cells which,
from their location, are named subepithelial cells.
These give rise, at intervals, to very specialized
cells, the cnidohlasts, which travel toward the
surface of the ectoderm and produce within them-
selves a threadlike stinging apparatus called a
nematocyst. As the nematocysts are consumed
in attacking other animals or in defending the
Hydra, other cnidoblasts migrate to the surface
and produce new stinging threads. At the foot
of the animal the epithelial cells have the ability
to produce a sticky substance by which the body
is made fast to other objects and may therefore
be called gland cells.

Hydra has also reproductive cells, which are
included in the ectoderm layer and which at inter-
vals develop into the mature cells, eggs and

spermatozoa. The former, which are the female cells, raise the ectoderm

into a rounded lump called the ovary (Fig. 59ov) ; the latter, the male

cells, elevate the ectoderm into a conical mound called the testis (ts).

Hydra also reproduces by buds (Fig. 596 1, 62), into which all the various

body cells in the region of the bud enter.
It is thus apparent that Hydra, like

Pleodorina and Volvox of the preceding

section, possesses germ (reproductive) and

somatic (sterile) cells. The existence of a

budding process in Hydra, by virtue of

which the somatic cells may share in the

production of new individuals, does not

alter the fundamental contrast between one

class of cells which retain the typical mode

of reproduction and another class of cells

which have lost that power. Unliko> Pleodorina and A'olvox, however,

Hydra has not stoi)ped with this one differentiation. It has gone farther

and differentiated its somatic cells into five or six different kinds.

Parallel between Foregoing Series and Individual Development. —

Some biologists have favored tiie foregoing series of colonial i)i'otozoa

tion of a lengthwise sec-
tion, bi, b2, buds in dif-
ferent stages of growth;
ec, ectoderm; en, endo-
derm; /, foot; gvc, gas-
trovascular or digestive
cavity or coelenteron;
TO, mouth; ov, ovary; t,
tentacle; ts, testis.

Fig. go. — Neuromuscular cell
II.Nilra. (From Schneider.)

FROM ONE CELL TO MANY CELLS

and simple metazoa as representative of the course of evolution of the
metazoa because it finds a parallel in the development of the individual
among the metazoa. Whether this
parallel has any particular significance,
or is of interest only as part of the
historical development of evolution
theory, is uncertain, but the comparison
is interesting.

Individual development begins with
a single cell, the egg, which is com-
parable to the supposed protozoan
ancestor of the metazoa. This egg
divides repeatedly (Fig. 61//-/F) to
form a group of cells, which may be
likened to the protozoan colony. As
the division of the egg proceeds farther,
it yields a hollow ball of cells, the
hlastula (V, VI), which has a form very
much like that of Pleodorina and
Volvox. It will be recalled that in
these organisms the cells are all near
the surface, no cells being at the middle
of the jelly. The next step in develop-
ment is the indentation of one side of the
blastula to form a two-layered embryo,
the gastrula (Fig. 62A, B). When a
diagram of Hydra is placed beside
a diagram of a gastrula (B and C,
Fig. 62), they are seen to be built

on the same general plan — that of a two-layered sac open to the exterior
at one end. At a stage quite as early as these, some animals show the

Fiu. 61. — Early metazoan develop-
ment. I, undivided egg; II-IV,
successive segmentation stages; V,
blastula, exterior view; VI, blastula in
section to show hollow interior or
blastocoele. (From Wilder, ''History
of the Human Body," Henry Holt and
Company, Inc.)

-<i-bp

Fig. 62. — Gastrula compared with Hydra. A, beginning of gastrula formation; B,
completed gastrula; C, diagram of Hydra; ap, animal pole; ar, archenteron; b, blastocoele;
bp, blastopore; ec, ectoderm; en, endoderm; g, gastrovascular cavity; m, mouth.

distinction between germ and somatic cells (Fig. 63), just as Pleodorina,
Volvox, and Hydra do. The germ cells are usually larger than the somatic

74 PRINCIPLES OF ANIMAL BIOLOGY

cells, when they can be distinguished at all, and sometimes contain granules
of a peculiar sort. Finally, to complete the comparison, development of
the embryo need be followed only a short way to ol^serve differentiation
of the somatic cells into at least as many kinds as Hydra possesses.

A Conclusion, and Caution in Adopting It. — The principle of using
embryonic development to discover the course of evolution is known as
the biogenetic law. According to this generalization, the development of
an individual repeats the history of its race. This law is seriouly ques-
tioned by many biologists and vigorously opposed by some. Also, the
^■r^f^;:y,-;^!-s'yf"'£r-<:S:^\:-f/:m:::M use of scrics of modern organisms to illustrate

what may have taken place in evolution must
])e made, if at all, with great care. Both of
these comparisons have been made, however,
by l)iologists in the past, using the organisms
referred to in the preceding pages. The con-
clusion to which they lead is that metazoa
have arisen through (1) the adherence of pro-
FiG. 63. — Posterior end tozoan cclls to fomi a colony, (2) the loss of
of developing insect egg. reproductive powers by some of the cells of
ductive cells, all others this colony, and (3) the differentiation of these
somatic. sterile cells into a number of kinds. These are

the fundamental steps; the details of cell structure and the general form
of the colony are immaterial.

This conclusion, it will be observed, accepts the colonial and rejects
the organismal theory. It rather favors spheroid colonies over the
gregaloid type because the modern organisms availal)le for a series of
representative types are spherical, and because the blastula of embryonic
development is a hollow ball. Many biologists hesitate to recognize
these reasons, and reference to them here is in no sense a pronouncement
in favor of the mode of origin of the metazoa which they appear to
indicate. Nevertheless, that origin is not improbable. And even if the
scheme of evolution described should be far from correct, a consideration
of it has led to an understanding of the relation of parts to wholes and a
glimpse of some of the situations which many-celled organisms have to
meet.

What Is a Colony, What an Individual? — When any change is effected
by a number of graduated steps, as the origin of metazoa from simpler
organisms must have been, it is difficult to say just wluui any stage that
may be named is reached. When, for example, has a metazoon been
evolved out of a protozoon? How far must the change go to be recog-
nized as having reached that goal? No matter what process led to
the metazoa, the answer to this question must be a matter of definitions.
If the organismal theory is correct, was the animal with numerous

FROM ONE CELL TO MANY CELLS 75

nuclei but no cell membranes around them a metazoon? If not, was
it a metazoon as soon as the cell membranes were formed? If not
then, was it a metazoon after some differentiation among those cells
had occurred? If the colonial theory is correct, was the first group of
adhering cells a metazoon or only a colony of single-celled animals?
Would a group of a thousand cells be a metazoon, while a group of
four was only a colony of protozoa? If number makes no difference,
would differentiation among the cells constitute the mark of a metazoon?

Whatever the event that marks the advent of a metazoon, the
organism that has experienced that event is an individual. Without
that characteristic, it is a protozoon or a colony of protozoa, depending on
the nature of the origin of the metazoa. Biologists have differed in their
definition of the individual. To some, a group of cells that shows any
differentiation becomes a metazoan individual. Since in actual cases
when only one type of differentiation exists it is that between reproductive
and sterile cells, as in Pleodorina, defining the multicellular individual as
any group of cells in w^hich differentiation exists is equivalent to saying
that the individual is any group in which sterile cells are set apart from
reproductive cells. Other biologists have insisted that a group of cells is
not an individual unless its sterile cells are differentiated into several
kinds, as in Hydra. Under the former definition Pleodorina and Volvox
are individuals; under the latter they are colonies of unicellular organisms
exhibiting division of labor, since some reproduce and others do not.

The distinction between reproductive and sterile cells is more funda-
mental than the distinctions among several kinds of sterile cells. In this
respect the former definition has the advantage. It is also preferable for
the reason that the criterion of individuality is, according to it, always the
same thing — loss of the capacity to reproduce by some of the adherent
cells — while under the latter definition the criterion of the individual
would presumably be a different distinction between sterile cells in every
line of descent. But definitions are arbitrary, and there is no tribunal
except usage which can choose among them.

Further Organization. — Beyond the stage at which they are barely
"entitled to be called metazoa, most of the higher animals have gone long
distances. They have increased the number of their cells so that even
a moderate-sized animal contains literally billions of these units. With
increase in size, they have usually developed a framework or shell of some
sort wdiich provides protection or aids locomotion. Special devices are
created for the providing of food and the elimination of waste materials.
With large volume, they have had to provide means of communication by
which substances may be quickly transported from one part to another.
Structures of different sorts capable of effecting movement have arisen.
Unified control and the harmonious working together of the various parts

76 PRINCIPLES OF ANIMAL BIOLOGY

have been provided in different ways. So multifarious are these char-
acteristic developments that a group of chapters, immediately following,
must be devoted to them.

References

Lankester, E. R., editor. A Treatise on Zoology. A. & C. Black, Ltd. (Vol. 1,

Fascicle I, Introduction.)
Sharp, L. W. An Introduction to Cytology. 3d Ed. McGraw-Hill Book Company,

Inc. (Pp. 20-24, 435-43G, for Organisnial Theory.)

CHAPTER 7
BASIC ORGANIZATION OF THE METAZOA

Beyond the evolutionary stages traced in the last chapter, ending
with the differentiation of the somatic cells into a number of kinds,
the metazoa have gone various ways in great groups. Within each group
there is much in common, both in structure and in physiology; but
between groups there are many differences. Scarcely anything is com-
mon to them all. There are a few features, however, that are character-
istic of several or many of the great groups. Some of the more important
of these frequent structural conditions should be passed in review.

Symmetry. — Symmetry is an arrangement of parts in relation to
planes, straight lines or points. A point is a position in space; it has
no dimension or size. A straight line is the shortest distance between
two points; it has only one dimension, length. A plane is, in common
words, a flat surface; more precisely it is a geometric figure of two dimen-
sions— length and breadth but no thickness — such that if any two
points in it be connected by a straight line that line is everywhere within
the figure.

Symmetry is defined as a correspondence in shape or arrangement of
parts on opposite sides of a dividing line or plane, such that if the portion
on one side were viewed in a mirror it would appear identical with the
part on the other side. A symmetrical surface is divided into the corre-
sponding parts by a straight line; solid (three-dimensional) objects,
including animals, are divided into their equivalent parts by a plane.
The plane which divides a body into its corresponding halves is called the
plane of symmetry. Objects have difi'erent types of symmetry (Fig. 64)
depending on the number of planes of symmetry which may be passed
through them. If only one such plane is possible, the symmetry is
bilateral. Most animals (including all the higher ones) are bilaterally
symmetrical. They possess anterior and posterior ends which differ,
right and left sides which are alike except for the reversed order, and a
dorsal side (at or toward the back) and a ventral side (literally per-
taining to the belly, hence opposite to the dorsal side). The plane of
symmetry passes through the two ends, through the dorsal and ventral
surfaces, and between the right and left halves.

Some animals possess a number of planes of symmetry. If these
planes all have a certain straight line in common, that line is the axis

PRINCIPLES OF ANIMAL BIOLOGY

of symmetry. An axis is a line around which something rotates, or
around which things are placed. The planes of symmetry may be
thought of as rotating on the axis of symmetry. Symmetry of this sort
is known as radial. In one of the major groups of animals (Fig. 65) the

PLANE OF
-BILATERAL SYMMETRY

CENTER OF-
UNIVERSAL SYMMETRY

AXIS OF
RADIAL SYMMETRY

-PLANE OF
31LATERAL SYMMETRY

ASYMMETRY
Fig. 64. — Types of symmetry illustrated by familiar objects.

bodies regularly possess radial symmetry. The arms or tentacles of the
animals of that group limit the number of planes that divide them
symmetrically; in practice the symmetry is called radial if there are two
or more such planes all having the axis line in common.

Sometimes there are many planes of symmetry having, not a line,
but a point in common. Symmetry is then said to be universal, and
the common point is the center of symmetry. In a sphere, any plane
that passes through the center is a plane of symmetry. Not many
animals have a spherical form, but Eudorina (Fig. 52) approaches it.

An object may possess symmetry of two types. A football, for
example, has radial symmetry around its long axis, but bilateral sym-
metry in relation to the plane halfway between its ends. Some cells
have approximately that form, as do also some protozoan colonies.

BASIC ORGANIZATION OF THE METAZOA

In general, animals which move rapidly or are capable of well coordi-
nated movements are bilateral. The radial animals are usually slow
movers and frequently are attached to fixed objects. Universally sym-
metrical animals are aquatic and progress with a rolling movement.

Asymmetry. — Any object which cannot be divided into corresponding
halves by any plane is said to be asymmetrical. Many of the protozoa
are made asymmetrical by a groove running spirally part way round the
body. The coiled shell of a snail is asymmetrical.

Fig. 65. — ^Various coelenterates, showing their radial symn^etry. A, sea anemone;
B, group of coral polyps; C, the medusa, Mitrocoma cirrata, ventral view. D, polyp of the
hydroid, Perigonimus serpens. {A and B after Jordan, Kellogg, and Heath; C after Mayer;
D after Allman.)

Many animals which are externally symmetrical may have their
internal structures arranged on an asymmetrical plan or on a plan of
symmetry different from the external plan. Examples are the heart,
stomach, and other parts of the alimentary tract and the lobes of the
liver in man, which are arranged asymmetrically. Many animals which
exhibit asymmetry in certain of their adult organs are symmetrical in
early stages of development. The flatfishes (halibut, floimder, and sole)
which have two eyes placed on one side of the head, are in their early
embryos bilaterally symmetrical, but one eye migrates through the head
to its new^ position.

Metamerism. — Animals exhibiting metamerism are composed of a
linear series of body segments fundamentally alike in structure. These
units are called somites or metameres, and animals so constructed are
said to be metameric. In simple metameric animals the somites closely
resemble one another in size, form, and the arrangement of organs. In
no animal, however, are all somites entirely alike because some of them
have become specialized and perform special duties.

The common earthworm (Figs. 135, 137) is a metameric animal.
It is composed of a series of ringlike somites outwardly much alike.
The limits of the somites are marked on the outside by grooves, and on the
interior by the septa (cross partitions) which lie immediately under the
grooves. The segmental arrangement extends to both external and
internal structures and involves organs of locomotion and excretion,
muscles, blood vessels, and the nervous system. The sexual organs also
have a segmental arrangement, although they are limited to a few somites.

PRINCIPLES OF ANIMAL BIOLOGY

Certain other organs are repeated in only a few segments, but in general
the earthworm's structure is that of a metameric animal.

In complex animals the metameric arrangement has often become
obscured through fusion of somites, loss of organs, and centralization.
The primitive arrangement, however, is readily seen in the embryos of
such animals. Thus the embryos of the vertebrates generally reveal a
well-marked metamerism in certain organs (the muscles, for example),
in which this arrangement is later partly or completely lost. Not all
metamerism has been lost even in the adult vertebrates, however, for
it may be seen in the vertebrae and ribs (Fig. 79), spinal nerves and
ganglia (Fig. 117), and branches of the dorsal artery.

Body Cavities. — Most of the higher animals have a cavity of some sort
in their bodies, but these cavities are of several kinds. In Hydra (Fig. 59)

— VERTEBRA

KIDNEY

ONODUCT
GONAD
MESENTERY
PERITONEUM

COELOM

ENTERON
(INTESTINE)

Fig. 66. — Relations of body cavities (enteron and coelom). At left, the earthworm; at

right, cross section of a vertebrate animal.

there is but one cavity, which is open at one end, the mouth, and closed
at the other end. A cavity so constructed is called a coelenteron, though
in Hydra, in recognition of its function of digestion and its assumption of
some of the tasks of a blood system, it is often named the gastrovascular
cavity. Flatworms also have a coelenteron. Undigested food must, in
such animals, be ejected through the mouth.

In most of the metazoa there are two cavities. One is in the digestive
tract, the other lies between the digestive organs and the body wall.
The digestive cavity in most complex animals is open at both ends and
to distinguish it from the closed sac in Hydra is known as the enteron.
The space between the digestive organs and the body wall is the coelom.
These relations are shown diagrammatically for the earthworm in Fig. 66
(left). In vertebrate animals (right) the cavities are in the same relative
position; the coelom appears to be filled with many organs. These,
however, arc merely pushed into it from the outside. Since some animals

BASIC ORGANIZATION OF THE METAZOA

Fig. 67. — Connective tissue,
consisting of cells, matrix, and
fibers.

(the lobster, for example) have irregular spaces among their organs, filled
with body fluids, there is sometimes difficulty in deciding whether a
cavity is a coelom or not. In general, the coelom must be lined by a
definite layer of cells, the peritoneum, which is lacking around the spaces
in the lobster, and the principal reproductive organs {gonads) are sus-
pended from its walls.

Tissues. — In practically all metazoa in which the several kinds
of somatic cells are very numerous, those of any one kind are grouped
together, not necessarily all in one place but
usually in a number of places. In Hydra
(Fig. 59), as we have seen, all endoderm
cells together form a continuous layer con-
stituting the inner part of the body wall.
The epithelial cells are similarly placed
together in the ectoderm. The secreting
cells of the foot are together in a small
group. The other somatic cells of Hydra
are not conspicuously grouped, since the
subepithelial cells and the cnidoblasts derived from them do not form a
continuous layer.

In most metazoa the somatic cells of any given sort are more con-
spicuously assembled in layers or masses than in Hydra. Such groups
or masses of like cells are called tissues. A tissue may be defined as a
number of cells of the same kind forming a continuous mass. Ordinarily
they perform some function in common, but it is not necessary to know
their function to consider them a tissue. Tissues may be classified on
the basis of both structure and function. In the vertebrate animals
these classes are sustentative, epithelial, contractile, nervous, vascular, and
reproductive.

Sustentative Tissues. — The sustentative tissues are primarily those
which support. The typical sustentative tissue is ordinary connective
tissue which binds the skin to the flesh beneath or holds the muscles
of the thigh together in a mass or helps suspend the intestine from
the body wall. It contains scattered cells (Fig. 67), but the serviceable
part is made of things secreted by the cells. These things are a gelati-
nous matrix, or ground substance, and large numbers of tough fibers
imbedded in the matrix. It is the latter that give connective tissue its
strength.

Certain connective tissues of very great strength are given special
names. The ligaments binding bones together at the joints, and the
tendons joining muscles to the bones which they move, are examples.
The essential fea,tures of connective tissue — cells, matrix, fibers — are
present in both, but the fibers far outbalance the other parts.

PRINCIPLES OF ANIMAL BIOLOGY

Cartilage and bone are likewise specialized forms of sustentative
tissue. They are alike in having their cells more or less scattered in a
substance, the matrix, which the cells have secreted. In cartilage the
cells are entirely separate from one another, though often placed in pairs,
trios, or quartets (Fig. 68) resulting from recent divisions of an earlier
cell. The matrix is firm or pliable, contains much gelatin, and is used as a
buffer to absorb shock or in places requiring flexibility. In bone the
cells possess numerous slender projections, some of which, probably, are
always in contact with similar projections from other cells (Fig. 69). The
hard bony material of the matrix, consisting largely of calcium carbonate

Cart.

W-W"'r^

vies

if^'m

^■<«-*

Fig. 68. — Sections through cartilage. A, development of cartilage (top) from meson-
chyme (bottom); B, hyaUne cartilage. {From Lewis-Stohr, '"Textbook of Histology,"
The Blakiston Company.)

and calcium phosphate, is secreted by these cells; consequently there are
always spaces in the bone for the cells and their slender processes.

Fatty or adipose tissue is regarded as sustenfative, but rather because
of its original similarity to connective tissue than from any mechanical
function which it may serve. The cells are numerous and closely packed,
not scattered as in other sustentative tissues. The fat itself is in globules
of small or large size contained within the cells. It is reserve food; hence
fatty tissue fluctuates greatly in \^olume, depending on the state of
nutrition of the organism. Favorite places for the deposit of fat are in
the abdominal wall and beneath the skin at many other places.

In many embryos, and in the adult of certain lower animals, such as
the flatworms, there is a tissue known as mesenchyme, which should be
included with the sustentative tissue, though chiefly because of its struc-

BASIC ORGANIZATION OF THE METAZOA

tural resemblance to some of the supporting tissues. It is a very loose
tissue whose cells are irregular, often star-shaped. These cells are not

closely packed but touch one another
only by their corners or the tips of
their projections (Fig. 70). Con-
siderable space is thus left among the

Fig. 69. Fig. 70.

Fig. 69. — Section through bone, showing the stellate spaces in the matrix occupied by
cells, and at left part of the space occupied by a blood vessel. (From Hill, "Manual of
Histology and Organography,'' W. B. Saunders Company.)

Fig. 70. — Mesenchyme from umbilical cord. {From Hill, "Manual of Histology and
Organography," W. B. Saunders Company.)

Thi

cells, which is filled with some more or less liquid substance,
spongy structure is everywhere characteristic of mesenchyme.

Epithelial Tissue. — An epithelium is a layer of cells covering some
surface, either the outside of an organ or the lining of the wall of a cavity.

•^^

Fig. 71. — Types of epithelium. A, columnar; B, cubical; C and D, squamous (side
and surface views, respectively); E, ciliated; F, flagellate; G, collared; H, stratified; vac,
vacuole.

The endoderm and ectoderm of Hydra, already described, are epithelia.
Others likely to be observed in laboratory studies are the outer layer
(hypodermis) of the body wall of the earthworm, the lining of the intestine

PRINCIPLES OF ANIMAL BIOLOGY

of any animal, the peritoneum which covers the intestine and Hnes the
abdominal cavity (coelom) of vertebrate animals, the outer layer (epi-
dermis) of the skin, and the inner or secreting layer of any gland.

An epithelium is designated cubical, columnar, or squamous, according
to the shape of its component cells (Fig. 71A-D), the last term meaning
flat and tilelike ; ciliated, flagellated, or collared, if the free ends of the cells
bear any of the structures indicated by these words (E-G) ; and stratified,
if the layer is several cells thick and the cells at different levels have
different shapes (H).

Fig. 72. — Types of secreting surfaces and glands. A, scattered gland cells (two
goblet cells containing secretion in the darkly stippled goblets) ; B, gland cell enlarged and
dropped below general level; C, group of secreting cells dropped slightly below the general
level; D, a simple multicellular gland; E, alveolar gland with neck; F, tubular gland; G,
compound alveolar gland; H, compound tubular gland; I, lumen; m, mouth; n, neck; v,
acini. Secreting portions of the glands are stippled.

Epithelia on the outer surfaces of organs are usually in some degree
protective. When they line a cavity, they often have the function of
secretion. The lining membrane of the intestine in vertebrate animals
is secretory, and in all glands the secreting portion is epithelium. If a
gland consists of a single cell, that cell is in an epithelial layer (Fig. 72A,
B). If the gland is multicellular, its secreting cells may dip below the
general level of the surface, but still it is part of the epithelium (C, D).
When the secreting cells thus indented form a channel of nearly uni-
form diameter, the gland is said to be tubular; if the deepest portion is

BASIC ORGANIZATION OF THE METAZOA 85

expanded like a flask, the gland is alveolar. Such an indented epithelium
may branch, that is, form subsidiary indentations {G, H), and then the
gland is termed compound, as contrasted with simple glands in which the
tube is not branched. Nearly all glands, in the higher animals at least,
have other tissues, including blood vessels, collected around or spread
among the epithelial part; but in every case it is the epithelium that
does the actual secreting.

The Other Tissues. — The two types of tissues described in the
preceding sections are distinguished largely on structural grounds, while
the functions performed by different samples of them may be quite
unlike. The remaining tissues of those listed on page 81 are, however,
highly specialized for specific functions. They are so much more impor-
tant in connection with those functions than with respect to their struc-
ture that descriptions of them are deferred to later chapters. Contractile
tissue includes mainly the voluntary and involuntary muscles; nervous
tissue comprises all the nerve, brain, and ganglion cells; vascular tissue
includes the blood and the more fluid parts of the blood-producing organs
(red marrow, spleen); and reproductive tissue consists of the germ cells
and their forerunners.

Organs and Systems. — An organ, generally speaking, is any structure
which performs a given function. In this general sense, a single cell
may be an organ, as in the case of single secreting cells scattered through
an epithelium. Usually, however, cells that do a certain thing are
grouped. Thus the secreting cells of Hydra which provide the adhesive
substance that holds the animal fast to other objects are all located on
the foot. Also, the stinging cells of Hydra show a tendency to be col-
lected in patches, particularly on the tentacles. Where such patches are
sharply marked off, as the glandular foot of Hydra, each group could be
considered an organ.

Some biologists, however, reserve the term organ for a collection of
tissues acting together to perform some function. The stomach of a
vertebrate animal is a suitable example. The inner epithelium, just one
cell thick, does the secreting of the digestive fluid or fluids. Outside this
layer is a connective tissue layer rich in blood vessels and lymph spaces
by which the materials for secretion are brought in and the digested foods
are carried away. Covering this layer are two layers of muscles, running
in different directions and together serving to churn up the contents of
the stomach and mix them with the digestive fluids. The several tissues
are structurally unlike, but each contributes in some way to the digestion
of the food. The stomach is thus an organ in this more restricted sense.

When a number of organs are occupied with different phases of a
complicated general process, they constitute a system of organs. The
mouth, esophagus, stomach, intestine, and several glands associated

86 PRINCIPLES OF ANIMAL BIOLOGY

with these organs are all concerned in some way with digestion. They
constitute the digestive system. The heart, arteries, veins, and capil-
laries propel or convey the blood and so make up the circulatory system.
In like manner the brain, spinal cord, ganglia, and nerves compose the
nervous system. The term system is sometimes applied to a group of
organs of a single kind, when these are the only organs concerned with
that function. Thus, as will be explained in a later chapter, the excretory
organs of some of the simple animals (the earthworm, for example)
are all alike, but there are many of them. There is no objection to
speaking of these organs collectively as a system; but in all the more
complex animals the systems are everywhere made up of unlike parts,
each contributing a different portion of the general process.

References

Dahlgren, U., and W. A. Kepner. A Textbook of Principles of Animal Histology.

The Macmillan Company. (Chap. VI, epithelium; Chap. VII, supporting and

connective tissue.)
KiNGSLEY, J. S. Textbook of Vertebrate Zoology, Part I. Henry Holt & Company,

Inc. (Pp. 9-16 for tissues.)
Storer, T. I. General Zoology. McGraw-Hill Book Company, Inc. (Pp. 17, 51-58

for tissues; Chap. 4 for organs and systems.)

CHAPTER 8
PHYSICAL SUPPORT AND MOVEMENT

In many animals the characteristic activities could be performed only
in the presence of hard parts which may collectively be termed the
skeleton. A skeleton is any more or less firm framework on or within
which the softer fleshy parts of the body are placed. The services per-
formed by the skeleton are chiefly of three types: (1) it provides support
for soft organs whose relations to one another could not otherwise be
maintained; (2) it protects delicate structures; and (3) it furnishes a
mechanism through which different types of movement may be executed.
Skeletons are widespread, from the protozoa to the largest mammals.
Such prevalence is testimony to their usefulness; yet some large groups
of animals (fiatworms, roundworms) and some members of other groups
(jellyfishes) get along without them.

Support Furnished by Skeleton. — It is not practicable to separate
mere mechanical support from protection in many cases, though an
attempt will be made to choose examples
where this may be done at least in principle.
Sponges of all kinds possess narrow channels,
lined in places by collared cells (Fig. 33,
page 52) which take in food. Currents of
water are constantly maintained in these
channels by the flagella of the collared cells, Fig. 7.3. — e i e m e n t s of

and it is essential that the passages be XTLttZX'o.S'T,^.
prevented from collapsing. While conceiva- spicules of different types.
bly the canals might be kept open by cells of ^^''"^ "^"^ ^^"''' '"^''^ Hertwig.)
firm consistency, they actually are kept open by means of a skeleton.
In the so-called bath sponges, this skeleton is a network of horny material;
in other kinds the skeleton is made of numerous limy or siliceous rods or
variously shaped objects called spicules (Fig. 73).

Fresh-water mussels and marine clams bear on the outside of their
bodies a bivalve shell, consisting of two saucerlike pieces hinged together
at one edge and opening like a book. Between the edges of these pieces,
at certain places, water must enter and leave by fixed routes in order to
bring the animal its food and oxygen and remove its wastes. The actual
channels for the water are formed by the fleshy parts of the mussel, but
these fleshy parts must be kept in their proper positions. In many of

88 PRINCIPLES OF ANIMAL BIOLOGY

the mussels they are too soft and deHcate to do so unaided, and it is the
shell which holds them in place.

The importance of the skeleton is closely related to size of body and
the place where the animal lives. A large animal may exist in the sea
and, because the body is of about the same density as the surrounding
water, be buoyed up in such a way as to allow its parts to function.
Cuttlefishes, for example, lead active lives in marine waters but washed
up on shore are helpless and shapeless. On land, however, even moder-
ate-sized mammals, because the medium around them, the air, is so much
lighter than themselves, would be unal^le to maintain the physical rela-
tions of their parts to one another sufficiently to enable them to function
if they were made of mere protoplasm. Some form of mechanical sup-
port other than a skeleton might have been evolved; but large size with-
out such support, along with physiologies of the general sort exhibited
by modern land animals, would have been out of the question.

Skeletons and Protection. — Nearly every skeleton may be regarded
as a source of protection, though often there is little definite information
to show what injuries might result in the absence of the skeleton. Those
sponges which have a skeleton of limy spicules generally bristle all over
with long shafts projecting from the surface cells (Fig. 74). How much
they are thus protected from predatory animals can only be
conjectured. In some marine animals known as hydroids,
having the general structure of Hydra but existing in
branching colonies, there is a horny tubular-sheath covering
the various branches and main stem of the colony. This
skeleton enables the hydroids to stand out more or less
firmly instead of being lashed against other objects by the
Fig. 74.— waves. In insects, crayfishes, spiders, and their allies
simp e i\^Qy.Q jg Q^ skeleton of a horny substance known as chitin

sponge. -^

{From Heo- which covers the entire l^ody on the outside. This does
^Zooloav " The ^^^ protect them from predatory animals, since members
Macmillan of this group, particularly the insects, are abundantly
ompany.) eaten by other animals; but it must serve to ward off
mechanical injuries of other kinds. The limy wall, or test, of sea urchins
and the shells of clams are presumably likewise protective structures. In
the vertebrate animals some of the most delicate and vital organs are
within bony cases — the brain within the skull, the spinal cord in a canal
running through the backbone, the heart within the framework of the
chest, and such sense organs as the cars and eyes either imbedded in
solid bone or set in among projecting ridges or other prominences.

Skeletons which serve only the functions of supi)ort and protection
may often be rigid one-piece structures. Some of the protozoa have a
solid limy shell sm-rounding the whole cell, and corals rest in limy cups

PHYSICAL SUPPORT AND MOVEMENT

which they have secreted. Most skeletons serving other functions are
either flexible or jointed.

Function of Hard Parts in Movement. — Only occasionally are the
hard parts of much service in movement among the simpler animals.
One of the best examples of such use
is the earthworm, which is provided
with a number of spines, or setae,
projecting from the body in each
segment except a few at the ends.
These setae are operated by muscles
attached to their inner ends and
sloping off in different directions
(Fig. 75), like the ribbons of a May-
pole, to the body wall. When the
worm crawls forward, the outer end
of the seta is tilted backward, so
as to catch the soil, and in crawling
backward or holding fast in the worm's burrow the seta points forward.

Sea urchins also have movable hard parts, which, however, are not
precisely a part of the locomotor equipment. The fleshy parts are
enclosed in a round shell, or test, the surface of which is studded, porcu-
pinelike, with a host of spines (Fig.

76). These spines are capable of cr-lki M^iB—:-f

movement in any direction and, when ^^?Q|/ Vf^l

the animal is thrust over on its side

Fig. 75. — Seta and muscles in the
earthworm, drawn from a longitudinal
section anterior to the clitellum; cm,
circular muscles, and Im, longitudinal
mucles.

.A B

Fig. 76. Fig. 77.

Fig. 76. — A sea urchin, covered with a test and spines. {From Haupt ," F iindamentals
of Biology.")

Fig. 77. — Relation of muscle to hard parts in appendages of insect and man. A, leg
of insect; B, leg of man; /, femur; fs, skeleton of foot; i, insertion of muscle; m, nmscle;
o, origin of muscle; ta, tendo-Achilles; ti, tibia. {A after Berlese; B after Hesse and Dofiein.)

or back, may give it an irregular motion that helps it right itself.
But the main movement is effected by fleshy tubes ending in suckers.

The fullest use of skeletal parts for movement is found in the insects
and their allies and in the vertebrate animals. In both groups the hard
parts are joined by curved surfaces, which permit free movement of one

PRINCIPLES OF ANIMAL BIOLOGY

upon another. Sometimes these curved surfaces are such as to permit
movement only in one plane, as in a hinge, while other joints allow a
rotary motion. The skeleton of insects and that of vertebrates differ,
however, in one important respect. In the insects it is on the outside,
covering all the fleshy parts, and here is known as an exoskeleton. In
vertebrate animals the skeleton is on the inside, everywhere covered by
flesh, hence of a type called an endoskeleton. The muscles which operate
the movable parts must work from the inside in the former but from the
outside in the latter (Fig. 77).

Skeleton of Vertebrates. — To illustrate the main features of a typical
skeleton, that of the vertebrate animal is chosen. This skeleton is
composed of bones and cartilages united partly by ligaments, is covered

TRUNK.

Fig. 78. — Regions of the vertebrate skeleton (cat). {From Jayne, "Mammalian Anatomy.")

by the soft parts of the body, and is supplied with blood vessels and
nerves. It may conveniently be divided into regions as indicated in
Fig. 78. On more fundamental anatomical grounds it is also subdivided
into the axial and the appendicular skeleton. The former lies in the
longitudinal axis of the body, and to it the latter is appended; hence the
names.

Axial Skeleton. — The axial skeleton (Fig. 79) is made up of the skull,
hyoid apparatus, vertebral column, ribs, and sternum. The skull furnishes
a case for the brain, capsules for the organs of hearing and smell, and
orbits for the eyes. It also includes the bones of the jaws. To it is
attached the hyoid apparatus which is a bony or cartilaginous support
for the base of the tongue.

The vertebral column is a jointed structure composed of a number
(different in different species) of vertebrae placed end to end. Together
they form a tube enclosing the s])inal cord, and their outer surfaces
form attachments for ligaments and muscles. The vertebral column

PHYSICAL SUPPORT AND MOVEMENT

is structurally differentiated into five regions, the cervical, thoracic,
lumbar, sacral, and caudal (see Fig. 79). The plan of a vertebra is shown
in Fig. 80. It is composed of a heavy ventral portion, the centrum, from
which arises a bony arch, the neural arch. The latter encloses the neural
canal which is occupied by the spinal cord. From the sides of the arch
two transverse processes project, and from the apex of the arch arises the

VERTEBRAL COLUMN.

Lumbar

Sacral.

Fig. 79. — Axial skeleton of the cat. (From Jayne, ''Mammalian Anatomy.")

neural spine. One pair of articular processes or zygapophyses projects
anteriorly and another posteriorly from the sides of the arch. The
relations of the anterior and posterior zygapophyses and the articular
faces of the centra of adjoining vertebrae are made clear in Fig. 80 (right).
The forms of the vertebrae in different regions of the vertebral column
are very different, as shown in Fig. 79. In the thoracic region of an

Fig. 80. — Diagram of a typical vertebra viewed from in front or behind and from the
left side: az, anterior zygapophysis; c, centrum; /, intervertebral foramen through which
nerves and blood vessels pass; ic, intervertebral cartilage; na, neural arch; nc, neural canal;
ns, neural spine; pz, posterior zygapophysis; tp, transverse process; z, zygapophysis.

animal having ribs the vertebrae have faces for the articulation of the ribs.
In the sacral region the vertebrae in some animals are considerably thick-
ened without great change in form, while in others they are much flattened
and more or less fused into a platelike structure, the sacrum. In the
sacral vertebrae the neural canal is reduced in size and in the caudal
vertebrae it is entirely absent. The spinal cord does not pass into the
latter region.

PRINCIPLES OF ANIMAL BIOLOGY

Vertebrae articulate with each other chiefly by means of the centra.
The articular surfaces of the centra may be concave or convex. Com-
monly one of the surfaces of a centrum is concave and the other convex,
the convex surface of one vertebra fitting into the concavity of the next.
But in some vertebrae both surfaces are concave and the space between
the centra is filled with a lens-shaped pad of cartilage. Biconcave verte-
brae are called amphicoelous (amphi = both and koilos = hollow). In

the concavoconvex type of vertebra,
if the concavity is directed toward
the head, the vertebra is said to be
procoelous, but opisthocoelous if the
concavity is directed posteriorly.
These types of vertebrae are illus-
trated in Fig. 81.

Ribs are usually attached to the
vertebrae in such a manner that they
can be moved. Some of the hinder-
most ribs are free at their ventral
ends, while others are connected to
the sternum or breast bone more or
less directly by means of cartilage.
The sternum is a bony or cartilaginous
structure which lies in the median
ventral part of the thorax. The
number of pairs of ribs varies in different species, being 12 in man.

Parts of Appendicular Skeleton. — The appendicular skeleton consists
of the shoulder or pectoral girdle, the hip or pelvic girdle, and the fore
and hind limbs. The generalized plan of the girdles and limbs of animals
higher than the fishes is shown diagrammatically in Fig. 82. In these
appendicular skeletons each of the girdles is composed of three pairs
of bones which are similarly arranged in the two gii-dles. Each side
of the pectoral girdle is composed of a flat bone, the scapula, or shoulder
blade, directed dorsally, a coracoid bone connecting the scapula and the
sternum (the latter not shown), and a clavicle which in some vertebrates
also connects the scapula and the sternum. There may be a cartilage,
the precoracoid, affixed to the posterior edge of the clavicle. A cavity, the
glenoid fossa, located at the junction of scapula and coracoid, serves as
the surface of attachment of the fore limb. Each side of the pelvic
girdle consists of an ilium, ischium, and pubis. These three bones in a
generalized skeleton are arranged similai-ly to the bones of the pectoral
girdle. The cavity at the junction of the three bones is the acetabulum.
In it is seated the head of the femur (thigh bone).

The bones of the arm and leg or fore and hind limbs are arranged

A ' \ u \ I c

Fig. 81. — Three types of vertebrae.
Only the centra and lateral processes are
shown. Upper end is anterior. A, pro-
coelous; B, opisthocoelous; C, amphi-
coelous.

PHYSICAL SUPPORT AND MOVEMENT

according to the same plan and may be compared bone for bone, humerus
with femur, radius and ulna with tibia and fibula, respectively, carpal
(wrist) bones with tarsal (ankle) bones, metacarpals with metatarsals
(body of hand and foot, respectively) and phalanges (bones of the digits)
of the hand with those of the foot. Vertebrates with primitive limbs have
five digits on fore and hind feet, but the limbs of specialized animals
have undergone more or less extensive modifications from the original
five-fingered and five-toed plan. In them usually the number of digits
has been reduced.

int

cn.Z

mtts.tjj

,»/

mtts.S

Ein

Fig. 82. — Diagrams of generalized fore {A) and hind (B) limbs with limb girdles:
acth, acetabulum; CL, clavicle; en. 1, en. 2, centralia; COR, coracoid; dst. 1-5, distal row
of carpals and tarsals; FE, femur; FI, fibula; fi, fibulare; gl, glenoid fossa; I-V, digits;
HU, humerus; IL, ilium; int, intermedium; IS, ischium; mtcp. 1-5, metacarpals; mtts. 1-5,
metatarsals; ph, phalanges; p.cor, precoracoid; PU, pubis; RA, radius; ra, radiale; SCP,
scapula; TI, tibia; ti, tibiale; UL, ulna; ul, ulnare. (From Parker and Haswell, "Textbook
of Zoology.")

The Motive Power. — The movement of structures in the higher
animals, whether these structures contain parts of the skeleton or not,
is all effected by muscles. Protoplasm in general has the power of
contracting, and in the protozoa there are motile structures, the cilia and
flagella, which have already been described (page 51). The muscles are,
however, much more specialized than any of these.

In general, the muscles are arranged in opposing pairs or sets. In
the earthworm, in which crawling is effected by alternate contraction
and expansion of the length of the animal, there is one set of muscles
running lengthwise, another passing circularly around the body. With
the front end of the worm holding to the soil with its sloping setae, a wave
of contraction of the lengthwise muscles draws up the rest of the body.

PRINCIPLES OF ANIMAL BIOLOGY

Then the circular muscles contract, while the longitudinal ones relax.
Since the body cavity (coelom) is filled with a fluid and cannot reduce its
volume, contraction of the circular muscles forces the body to elongate,
thus pushing the front end forward to take a new hold upon the soil.
The setae, as previously explained (page 89), are tilted forward or back-
Avard by opposing muscles. In vertebrate animals, bones are moved by
muscles and tendons placed on opposite sides of the bones at or near the
joints. The arrangement at the knee joint in man is shown in Fig. 83.
The flexor muscle bends the joint, the extensor straightens it. When

NSERTION

ORIGIN
Fig. 83. — Diagram of knee joint in man, illustrating opposed muscles.

one of these muscles contracts, the other must relax if movement is to
be produced. If both contract the leg is merely made tense. The area
of attachment of the less movable end of the muscle (usually that nearest
the body) is called the origin of the muscle, that of the more movable
end its insertion. In such boneless movable parts as the eyelids and lips,
one set of muscles, operating to pull radially away from the openings
which these structures surround, is opposed by circular bands of muscles
which close the openings. The stomach and intestine of vertebrate
animals possess longitudinal and circular muscles which operate much
as do those of the earthworm. Everywhere muscle is opposed by muscle.

Fig. 84. — Smooth-muscle cells.

The necessity of this arrangement arises from the fact that, while muscle
contracts vigorously, its expansion is entirely passive. It can force move-
ment in one direction but can only permit it in the opposite direction.

Muscle. — Muscles constitute the contractile tissue referred to in the
preceding chapter (page 81). They are nearly always plates or bundles
of cells, not single cells. Three types of muscle cells in vertebrate
animals may be recognized, known respectively as smooth, striated, and
cardiac.

Smooth muscle is composed of cells each of which is provided with a
single nucleus. The cytosome contains well-marked longitudinal fibrils.
These cells (Fig. 84) have the form of slender spindles with unbranched
tips or in certain organs the tips may be branched. They are found in

PHYSICAL SUPPORT AND MOVEMENT

the walls of the digestive tract, urinary bladder, gall bladder, arteries
and veins, and in certain glands and their ducts.

Striated muscle differs greatly in its structure from smooth muscle.
For one thing, it has many nuclei in each cell. The cells of an embryo
from which striated muscle cells develop have only one nucleus apiece,
but after a time the nucleus divides a number of times without an
accompanying division of the cell body. Many nuclei are thus present
in the muscle cells of the adult. The striated muscle cell is roughly
cylindrical in form and usually very long. It is covered by a firm mem-
branous sheath, the sarcolemma. Within this is the rather liquid proto-

"P-j-

m-'^

A B C

Fig. 85. — General appearance of striated muscle. A, part of a muscle fiber of a frog;
B, part of a fiber teased out to show myofibrils; dh, darli bands; lb, light bands;/, myofibril;
n, nucleus; s, sarcolemma; C, a myofibril, diagrammatic; dh, dark band; Ih, light band with
a thin band of dark material dividing it into two portions. (A and B from Parker and
Haswell, " Textbook of Zoology.")

plasm called the sarco'plasm. Imbedded in the sarcoplasm, and forming
a large part of the bvilk of the cell, are numerous slender strands, the
contractile myofibrils (Fig. S5B,f). Each myofibril consists of alternate
segments of different substances, light and dim in appearance. In the
muscle cell these myofibrils extend parallel to each other and to the long
axis of the cell and are so aligned that the dim segments are side by side,
and light segments are side by side. Collectively they give the whole
cell the appearance of being marked by light and dark transverse bands
(Fig. 85A). These are the marks to which the term "striated" refers.
Little is known of the chemical or physical properties of the substances
in the light and dim bands, but when they are examined with polarized
light it is found that the dark substance is doubly refractive.

PRINCIPLES OF ANIMAL BIOLOGY

Cardiac muscle is found only in the heart of vertebrate animals. It
contains fibrils somewhat resembling those of striated muscle, and has
cross striations which these fibrils confer on it. However, the strands
of heart tissue interconnect in a network, and there is little or no blocking
off of the protoplasm into cells. The heart is thus practically a large
syncytium (page 66).

The actions of the three kinds of muscle are very different. Smooth
muscle is capable of only relatively slow movement. It is not directly
subject to the will, hence is sometimes called involuntary muscle; but
this is not a distinctive designation, since the heart is also free from

CONTRACTION

RELAXATION

RECOVERY

o
g

LU
Q.

^ \

ELECTRIC POTENTIA

Fig. 86. — Curve illustrating the course of a single muscle twitch.

conscious control, and even striated muscle sometimes acts involuntarily.
Striated muscle acts very strongly and very rapidly; and since its move-
ment is regularly initiated by act of will, it is known as voluntary muscle.
Heart muscle acts without control of the will, as do other vital organs.
Its rhythmic action can be maintained for long periods after removal of
the organ from the body, as determined by a mechanism to be described
in a later chapter. Because of its syncytial nature, waves of stimulation
pass rapidly over the whole heart, and the organ tends to act as a single
unit.

Muscle Contraction. — In the living animal, contraction is stimulated
only by nerve impulses, though in laboratory experiments artificial
stimuli can be given. A single nerve cell may govern only a few muscle
cells, or as many as 150. The group of muscle cells controlled by one
nerve fiber constitutes a motor unit. It is characteristic of motor units
that, if they (contract at all, they do so to their fullest capacity, in accord-
ance with the all-or-none law already stated (page 53). Since muscles
are made up of many motor units, some contracting, others usually not,
an entire muscle may experience many degrees of contraction. How
many motor units act depends on the intensity of the nerve stimulus, a
strong stimulus activating many of them, a \veak stimulus few.

A single stimulus to a striated muscle results in a single quick twitch
of the muscle. If tlic muscle is attached to a movable pointer, which

PHYSICAL SUPPORT AND MOVEMENT

traces a line on smoked paper on a revolving drum, the single twitch is
recorded by a curve of characteristic form (Fig. 86). The twitch as a
whole lasts about 0.1 second in the frog. It takes a very short time
(0.01 second) for the muscle to start to contract. This brief period of
inaction is known as the latent period; by the time it is ended the change

Fig. 87. — Curves of jcontraction of muscle in response to repeated stimuli. Rate of
stimulation is slow at the bottom, but gradually increases toward the top. {From Howell,
"Textbook of Physiology," W. B. Saunders Company.)

of electric potential which is the sign of stimulation has usually reached
a peak and subsided. Then the muscle contracts for about 0.04 second,
and the succeeding relaxation lasts about 0.05 second. Following the
tw'itch there is a period of recovery lasting a number of seconds in which
the muscle returns to its previous condition. If stimuli are applied
repeatedly before the recovery is complete, the muscle shows fatigue

98 PRINCIPLES OF ANIMAL BIOLOGY

and its responses are weaker. Smooth muscle, as in the intestine, reacts
much more slowly, the contraction lasting about 20 seconds. The relax-
ation of any muscle is purely passive; the ends of the muscle fibers do
not push.

Single twitches are not, however, the commonly observed type of
muscle action. During ordinary contraction, nerve impulses are deliv-
ered in rapid succession, beginning, say, at 4 or 5 per second and increas-
ing in frequency to 40 or 50 per second. These rapidly repeated stimuli
may be shown experimentally to be the most effective method of getting
strong and sustained contraction. The nature of the contraction result-
ing from stimuli repeated at different rates is shown in Fig. 87. In the
lowest curve the stimuli were given at a slow rate, and after each one
the muscle relaxed almost to its former state. But when the stimuli
were given more and more rapidly, as in the remaining curves of the
figure, complete relaxation did not have time to occur between them,
and the total contraction gradually increased.

In striated muscle the cells act separately and do not communicate
stimuli to surrounding cells. In smooth muscle, however, stimulation
at one point may lead to a wave of contraction passing over a whole
sheet of muscular tissue, showing that the stimulus is communicated
from cell to cell.

The efficiency of muscle, that is, the ratio of work done to energy
consumed, is rather high. For a single twitch, including the recovery
period following, this ratio is about 50 per cent. For sustained contrac-
tion, however, the efficiency is much less — around 25 per cent.

Chemistry of Muscle Contraction. — Just what happens in a striated
muscle when it contracts is only partially understood. It is the myo-
fibrils that do the contracting, but the important thing to know is the
set of physical or chemical conditions which cause them to shorten.
Clues have been furnished by chemical analysis of fatigued muscle.
Most of the glycogen, which in rested muscle amounts to about 3 per
cent of the weight, has disappeared in fatigue, as has also much of the
oxygen. At the same time the inorganic phosphates (produced out of
organic phosphates) have considerably increased; so also has carbon
dioxide. If imder experimental conditions oxygen is excluded there is
also an increase of lactic acid. How the glycogen is lost is known;
combining with water, it is converted into glucose and lactic acid.
Something must also have been oxidized to account for the increased
carbon dioxide. Under ordinary conditions the lactic acid does not
persist, for part of it is oxidized to obtain energy with which the rest
of the lactic acid is reconverted to glycogen. Formerly it was thought
that the breaking down of glycogen or the oxidation of one of its products
furnished the energy for muscle contraction; yet conversion of glycogen

PHYSICAL SUPPORT AND MOVEMENT 99

may be prevented by certain poisons, and the muscle still be able to
contract. It seems necessary to conclude that the energy comes from
decomposition of organic phosphates; such phosphates are known to
release energy with almost explosive speed when they are decomposed.
The organic phosphates must be reconstituted, ready for the next con-
traction, and the energy for this reconstitution comes from oxidations.
The oxidations are thus accessory phenomena; instead of furnishing the
energ}^ for the contraction itself, they provide for the restoration of the
phosphates, and the latter on decomposition furnish the energy for
contraction.

The mechanism of the contraction itself is probably the sudden
folding of long protein molecules arranged lengthwise in the myofibrils.
Since the most abundant protein in muscle is myosin, this may be the
responsible agent. Myosin extracted from muscle exercises a strong
catalytic action on the decomposition of organic phosphates, and this
action may be a part of the contraction process.

A muscle in which there is no more organic phosphate nor glycogen,
and in which much lactic acid has been accumulated, is incapable of
contraction; it is "fatigued." In living animals as distinguished from
laboratory preparations, however, the common source of fatigue is
not in the muscle itself, but between the muscle fibers and the nerve
which delivers the commands to contract. Some substance there, at
the junction of nerve with muscle, experiences a change in response
to repeated stimulation such that it no longer transmits the stimulus
or does so more weakly. The nerve fiber still conducts, and the muscle
is still able to contract. The nature of the failure of the junction is not
known.

References

Carlson, A. J., and V. Johnson. The Machinery of the Body. University of
Chicago Press. (Pp. 345-360.)

Mitchell, P. H. A Textbook of General Physiology. McGraw-Hill Book Com-
pany, Inc. 3d Ed. (Chap. II, muscle contraction.)

CHAPTER 9
SOURCES OF ENERGY AND MATERIALS

Muscular action and the other activities of an organism entail destruc-
tion of living substance, which must be steadily replaced. In growing
animals, not only are repairs necessary, but provision must be made
for new construction. The general source of material for growth and
replacement is food. How this material is utilized in single cells has
already been described; how it is transformed in multicellular animals is
now our concern.

Since most food is not in a form that can be transported through
protoplasm, it must usually be converted in some way. In large part
the conversion consists of making it soluble. But even some soluble
foods are unable to pass through tissues, because of the selective action
of protoplasm which will receive some substances and not others. The
conversion is accomplished by the process of digestion which, in multi-
cellular animals, is carried on in some sort of digestive system.

The Locus of Digestion. — In the protozoa digestion is an intracellular
process. Amoeba engulfs food by flowing around it at any part of the
cell. Paramecium takes the food in at a particular place, through a
permanent gullet. In either case the food is surrounded by a droplet
of liquid, which is acid in reaction at first, and presumably enzymes are
secreted into this fluid. The food vacuole thus formed is the digestive
apparatus. These features of protozoan digestion were described earlier
but are repeated here in the first two parts of Fig. 88 for contrast. Among
the multicellular animals, sponges retain the intracellular type of diges-
tion. Through the channels and cavities which are characteristic of
sponges, water flows, kept in motion by the flagella of collar-bearing
cells in some of the channels (Fig. 33). From the water the collared
cells seize organisms, after the manner of Amoeba, and digest them.
Products of this digestion are passed on to other cells by diffusion or
osmosis, so that nutrition in sponges is on a cooperative basis; but just
as in protozoa, digestion is done within the cells.

In all metazoa other than sponges digestion is performed partly, even
chiefly, in cavities of organs — -surrounded by cells, but not in cells. The
process is at least bcgvm in these cavities, and in the higher animals is
almost completed there. The more complicated types of food are
rendered quite simple before they leave these cavities. Some foods are

100

SOURCES OF ENERGY AND MATERIALS

101

rendered completely soluble and immediately ready to enter into the
metabolism of protoplasm. Other foods leave the digestive cavities
lacking still one or two of the simplifjdng steps which are necessary.
The cells which receive these incompletely digested foods finish the
process themselves. Indeed, all cells which use these kinds of foods in
their metabolism must have the power of taking these last digestive
steps. Thus some of the primitive digestive activities characteristic of
protozoa are not lost b}'' any active cells in any organism.

FOOD VACUOLES

COELENTERON

AMOEBA

PARAMECIUM

HYDRA

\PHARYNX

GIZZARD,

.MOUTH

NTESTINE

ANUS'

BUCCAL
CAVITY

.MOUTH

EARTHWORM

CAECUM

^-^S' LARGE

ccT,K.?-~0"^ INTESTINE

INTESTINE
GALL BLADDER

ANUS,

::p'

RECTUM"

SALIVARY/
GLAND

MAMMAL

Fig. 88. — Diagrams of several types of digestive systems in metazoa, compared with

protozoa.

Simple Digestive Systems. — ^The simplest system in which digestion
occurs in a cavity is that known as a coelenteron. Hydra (Fig. 88) has
such a system. A coelenteron has only one opening to the outside,
usually called the mouth, although besides taking in food that opening
must also be the place of exit of undigested matter. The coelenteron
of Hydra is in the main a simple sac, though it is branched into the
ring of tentacles near the free end of the body. A less diagrammatic
representation of Hydra's coelenteron is given in Fig. 59, where it is
labeled the gastrovascular cavity and the cells forming its wall are the
endoderm. Flatworms also have a coelenteron. In some of them (Fig.
89, above) it is as simple as in Hydra, but the simplicity is not primitive;

102

PRINCIPLES OF ANIMAL BIOLOGY

MOUTH-*^"^PHARYNX

Fig. 89. — Digestive system
(coelenteron) of a rhabdocoele
flatworm (above) and a triclad
turbellarian.

it is a result of degeneracy. Other flatworms have a three-branched
coelenteron, each part of which is extensively branched (Fig. 89, below).
As animals rise in the scale of complexity the digestive system becomes
a tube open at both ends. One end is the mouth, which ingests food,
the other end the anus through which undigested, mostly indigestible,

matter is ejected. In the course of the
tube it is differentiated into organs. In
the earthworm (Fig. 88), following the
mouth, there is a short buccal cavity, a
-pharynx with strong muscular walls, an
esophagus^ a croy in which food may be
stored, a gizzard with thick muscular
walls and a chitinous lining by means of
which food may be finelj^ ground, and an
intestine with secreting and absorptive
cells. An internal ridge, the typhlosole,
formed by an infolding of the dorsal wall
of the intestine (Fig. 66), gives increased
surface. About the exterior surface of
the intestine is a layer of brown cells, the
chloragogen cells, which have been thought to serve as a digestive gland,
possibly as a liver.

Digestive Systems in the Vertebrates. — In the vertebrates the diges-
tive system reaches its highest development. Here it consists not only
of an alimentary canal, subdivided into regions, but also of highly
developed glands which produce digestive secretions. A diagram repre-
senting vertebrates in general fairly well, but more particularly the
mammals, is at the bottom of Fig. 88. The system in the frog is slightly
more simple (Fig. 90, left). In the mouth the upper jaw bears teeth
which serve to hold the prey when caught. Attached to the anterior
portion of the floor of the mouth is a prehensile tongue which is provided
with many glands that produce a sticky secretion. The buccal cavity or
mouth cavity leads backward into the short broad esophagus through a
distensible opening, the pharynx. The esophagus leads into the muscular
stomach which in the frog, as in most vertebrates, is a curved organ
usually lying somewhat to one side of the middle line. The walls of
both the esophagus and stomach are provided with highly developed
glands which secrete digestive solutions. The stomach opens into the
small intestine through a muscle-encircled passage, the pylorus. The
small intestine of vertebrates is usuall}^ subdivided into three portions
named, respectively, the duodenum, jejunum, and ileum. Of these the
duodenum and ileum alone are recognized in the frog. These regions
as a rule merge imperceptibly into one another, yet each shows certain

SOURCES OF ENERGY AND MATERIALS

103

characteristic structural features and each occupies a certain portion of
the intestine. The duodenum receives the secretions of two large diges-
tive glands, the liver and the pancreas. In the frog the secretions of
these two glands are discharged through the common hile duct into the
middle region of the duodenum. A reservoir, the gall bladder, attached
to the liver and connected with the bile duct, serves as a storage place
for the hile, one of the secretions of the liver. The small intestine is
connected at its posterior end with the large intestine which in the frog
is subdivided into two portions, namely, the rectum and the cloaca. The
term cloaca is used to designate that portion of the large intestine
which is used as a common passage for undigested materials from the

Fig. 90. — Digestive systems of the frog (left) and nlan, somewhat simplified.

alimentary tract, for urine, and for reproductive cells from the urino-
genital system. It occurs in a few mammals and in most other verte-
brates. The large intestine opens to the exterior by means of the anus.

The human digestive system differs little enough from that of the
frog that the illustration in Fig. 90, right, should suffice without further
description. That figure, however, omits the mouth and its associated
salivary glands, and the small intestine is greatly shortened.

It is worthy of note that the intestine of the frog is relatively short.
This condition is found in flesh-eating animals in general. Animals which
feed entirely or largely upon vegetable food have long intestinal tracts
and frequently have a large caecum (a blind pouch) at the junction of
the small and large intestines. The rabbit and muskrat have a large
caecum with a vermiform appendix at its end; the chicken and dove have
two caeca. In man the caecum is small, rudimentary, with a vermiform

104 PRINCIPLES OF ANIMAL BIOLOGY

appendix. Highly specialized modifications of the stomach occur in
ruminants (animals which chew the cud) and in seed-eating birds.

Digestion in Man. — Inasmuch as the digestive process as it occurs in
man has been much more intensively studied than in any other animal,
the discussion of digestion which follows will be based on the human
system. In the mouth, food is broken up, during which process the three
pairs of salivary glands pour out their secretion (saliva) which is mixed
with the food. The saliva contains an enzyme, ptyalin, which is al^le to
transform starch, particularly cooked starch, into certain sugars. The
breakdown of starch occurs by degrees, the intermediate products being
various dextrins, but in no case does the digestion in the mouth go
farther than to maltose, which is not one of the simple sugars. It is still
a disaccharide (page 40) and not readily diffusible through protoplasm.
Ordinarily, because of the short sojourn of the food in the mouth, little
starch digestion actually takes place there ; and since ptyalin acts only in
an alkaline medium, its action is stopped by the acid of the stomach
when the food reaches that organ.

In the stomach, the food is acted upon by the secretion of the gastric
glands which are small branched or simple tubular glands located in the
inner layer of the stomach. The movement of the muscles of the
stomach mixes the food with the gastric secretion, which contains hydro-
chloric acid and two important enzymes, pepsin and rennin. The hydro-
chloric acid affords a suitable medium for the action of the enzymes
and incidentally stops the action of the ptyalin descending from the
mouth. The rennin coagulates milk, a fact made use of in cheese fac-
tories where a preparation of rennin made from calves' stomachs is used
to separate the curd from the whey. Pepsin as it comes from the gastric
glands is in an inactive state in which it is called pepsinogen. Pep-
sinogen is activated (converted into pepsin) by the hydrochloric acid,
which is secreted in a concentration of about 0.4 to 0.5 per cent. Pepsin
acts only on proteins, converting them to peptones and proteoses,
which are also proteins but simpler than most proteins taken as food.
Ordinary fats are not acted upon in the stomach.

Absorption of foods in the stomach is negligible. Alcohol is absorbed
there, which may account for its quick action on mental and other
physiological processes.

Secretin. — When the acid stomach contents are ejected through the
pylorus, the acid acts upon a substance in the lining epithelium of the
duodenum and changes this substance to secretin. The secretin is
absorbed by the blood and is carried to the pancreas and liver which are
thereby stimulated to secrete their fluids. Secretin belongs to a class of
activators known as hormones. Normally, the pancreas and liver are
also controlled in part by nerve impulses. Nevertheless, these glands dis-

SOURCES OF ENERGY AND MATERIALS 105

charge their secretions even after the nerves which innervate them
are cut.

The Pancreatic Juice. — The pancreas produces a thin watery secre-
tion containing three enzymes, which act upon proteins, carbohydrates,
and fats, respectively. The protein-spHtting enzyme is inactive when
it emerges from the pancreatic duct and is then known as trypsinogen;
but, when it comes in contact with the duodenal surface, it is quickly
rendered active. The conversion of trypsinogen is initiated by the
enzyme enter okinase , produced in the lining of the duodenum. This
enzyme acts upon the inactive trypsinogen, changing it to the active form
called trypsin. The trj^psin splits proteins, proteoses, and peptones from
the stomach into simpler and simpler compounds. The end products of
protein digestion are amino acids (page 41) and several other compounds.
Trypsin works in alkaline, neutral, or even acid media. It completes the
work begvin by the pepsin and works more rapidly and breaks up the
protein more completely than does the pepsin.

The carbohydrate-splitting enzyme of the pancreas is amylopsin.
Unlike trypsinogen, it requires no activation. It converts starches, dex-
trins, and complex sugars (with the aid of so-called inverting enzymes)
into simple sugars (glucose and others), which are in condition to be
absorbed.

The fat-splitting enzyme of the pancreatic juice is steapsin. Steapsin
splits fats into glycerol (glycerin) and one or more fatty acids (page 41)..
These substances are soluble and are absorbed in this condition.

The Secretion of the Liver. — Bile, the secretion of the liver, contains
no enzyme. It contains water, bile salts, and certain excretory materials.
The discharge of bile is stimulated, as explained above, by the hormone
secretin in the same manner as is the secretion of pancreatic juice. Bile is
ordinarily stored in the gall bladder until the partially digested acid food
is ejected by spurts from the stomach, but it has been shown in some
animals that such a temporary storage place is not essential to the
proper production and ejection of the bile. Each ejection of food into the
intestine stimulates a flow of bile through the bile duct. The bile salts
break up the fats into very fine droplets, thus greatly increasing the
surface through which the fat-splitting enzyme may attack them. If
the bile is artificially prevented from entering the intestine, a large
share of the ingested fat is not digested and may be recovered in the
feces.

The Intestinal Secretion. — The secretion of the small intestine is
produced in small tubular glands which are local evaginations of the lining
layer. This secretion consists of enterokinase, erepsin, several other
enzymes, and secretin. Enterokinase, as stated above, converts inactive
trypsinogen into active trypsin. Erepsin is a protein-splitting enzyme

106 PRINCIPLES OF ANIMAL BIOLOGY

which, although unable to digest the original proteins, attacks the pep-
tones which result from digestion in the stomach, reducing them to
amino acids. It thus supplements the action of trypsin. The other
enzymes convert maltose and the dextrins (resulting from the operation
of ptyalin and amylopsin upon starches) into glucose and other simple
sugars.

Secretin, as indicated above in connection with the stimulation of the
pancreas, is not an enzyme but a hormone. It exists in the wall of the
duodenum as prosecretin which is stable and does not affect the pancreas.
The acid from the gastric juice mixed with the food coming from the stom-
ach changes the prosecretin into secretin which is absorbed and carried
by the blood to the pancreas and the liver, which are thereby stimulated
to secrete pancreatic juice and bile, respectively.

Digestion in the Large Intestine. — The large intestine produces no
enzyme. Water and some of the products of digestion are absorbed
here. Bacteria flourish in the large intestine. Many of these attack
proteins, while others attack the cellulose of plant cells and perhaps so
break it down that some sugars are recovered from it. Bacteria which
attack proteins are not numerous, however, when the products of protein
digestion are removed with normal rapidity. Bacteria may also supply
an important vitamin, as is indicated later.

Absorption. — In the more complex animals absorption occurs along
the portions of the alimentary tract. In such simple animals as Hydra
all the endodermal cells are bathed in the products of digestion or carry
on digestion in themselves, and through these cells absorption takes place.
Some of this material not used by the endoderm is passed on by diffusion
to the ectodermal cells. In animals with a circulatory system the sim-
pler substances pass through the absorbing cells directly into the blood
stream.

In man, as stated earlier, there is little absorption in the stomach.
Most of it occurs in the small intestine, whose inner surface is enormously
enlarged by the fingerlike protrusions called viUi (Fig. 91). Amino acids
and simple sugars are absorbed directly into the blood, which carries them
through the liver before delivering them to the general circulation.
Glycerol and the fatty acids are absorbed, but in the process are at least
partly reconverted into fats. Since fats are insoluble, they exist in the
form of droplets and are delivered thus, not to the blood, but to the lymph
vessels. However, since the lymph vessels empty into the blood stream
(in the left shoulder, page 131), the entrance of fat into the blood is
merely delayed.

While absorption by the intestinal wall is partly simple diffusion,
some selection is practiced by the absorbing cells, so that certain sub-
stances are passed readily, others are retarded or rejected. This selecti\'e

SOURCES OF ENERGY AND AI ATE RIALS

107

action may even send substances against the diffusion gradient — that is,
cause them to go from places of lower to places of higher concentration.
Storage of Food. — Carbohydrates, in the form of glucose or other
simple sugars, are ordinarily present in the blood to the extent of less than
0.1 per cent. After a meal they may increase perceptibly, but when they
rise above 0.14 per cent they begin to be excreted by the kidneys and are
lost. Protoplasm contains some glucose, mostly in combination with
other compounds, and to that extent carbohydrates contribute to the

CAVITY OF INTESTINE

villus-

capilla^^/ network
in villus

artery-

vein

l^mph vessel

peritoneum-

FiG. 91. — Diagram of .section through wall of small intestine, showing two villi and their
enclosed blood and lymph vessels. {From Stover, "General Zoology")

architecture of the living substance. Much carbohydrate material is
stored in the form of glycogen, which is made up of many molecules of
simple sugars combined. The main storehouse of glycogen is the hver,
from which it is withdrawn when current supplies are inadequate.

Lipids enter into the construction of protoplasm, particularly at the
surfaces of cells, where they play an important role in determining perme-
ability of the cell membrane. Since the need of these materials is con-
tinuous, while the supply from digested food is intermittent, lipids must
be stored. The ones so deposited are chiefly fats. All cells store them to
some extent, but connective tissues between skin and muscles and among

108 PRINCIPLES OF ANIMAL BIOLOGY

the muscles, and the mesenteries of the intestine and other organs, are
particularly devoted to this function.

Storage of indiffusible substances such as glycogen or fats necessitates
redigestion of them when they are to be used; consequently enzymes for
carbohydrate and fat digestion must be produced or producible in all
cells which store these products.

Proteins are not stored in animals, as carbohydrates and fats are
stored. The supply of protein foods must therefore be rather steady;
that is, they should be included in the diet almost daily. Amino acids
enter the blood after the digestion of protein foods and are taken up
by the cells which require them. When the diet is deficient in proteins,
requirements of amino acids in vital situations are supplied only by
breaking down body proteins elsewhere, as happens in starvation.

Energy Requirements. — Any balanced diet must provide two things,
energy and materials. Energy is measured in the units known as
calories, one calorie being the amount of heat necessaiy to raise the
temperature of a kilogram of water 1°C. Each gram of a carbohydrate
or protein food utilized in metabolism yields about 4 calories, a gram
of fat about 9 calories. A relaxed, fasting human body of average size
and shape, in prone position, requires about 1600 calories daily. More
than half of this energy goes to maintaining the body temperature.
The rest is expended by the vital organs such as the heart and the
muscles performing breathing movements. If food is taken, so that
muscles of the digestive tract are also active, the daily energy require-
ment is about 1800 calories. For sedentary workers leading normal lives
it rises to about 2400 calories, while manual laborers need 3000 to 5000
calories, depending on how hard and long they work. If an average
person consumes much more energy than is proper to his mode of life
and occupation, he may have an overactive thyroid gland or a fever.
If the energy consumption is much less than normal, the cause may be
a deficient thyroid or pituitary or adrenal gland, or low nutrition.

So far as mere quantity of energy is concerned, it may be obtained
from any of the types of food. Pligh protein diet requires more work
of the kidneys because of the increased nitrogenous wastes, but the
kidneys are capable of much more than an average load if they are
healthy. An excess of fat is objecticmable chiefly because fats do not
oxidize very completely unless carbohydrates are being oxidized at the
same time. To some extent the human body can alter the proportion
of the different kinds of compounds derived from its food, for amino acids
can be converted to glucose, and carbohydrates to fat; but there is little
conversion of fat to carbohydrate, and only the simpler amino acids can
be made from nonprotein foods.

If the food currently taken does not provide the required energy.

SOURCES OF ENERGY AND MATERIALS 109

stored foods are consumed. The carbohydrates (glycogen of the Uver and
muscles) are used first. Fats are used simultaneously with the carbo-
hydrates but usually last until after the carbohydrates are exhausted.
Then the materials of the protoplasm itself are used, first those of the
less essential organs, then of the brain, spinal cord, and heart. Death
usually follows quickly upon such extreme starvation.

Materials Required. — Besides furnishing energy, food must also pro-
vide materials with which to build protoplasm and such secreted products
as the hard parts of bone and teeth. One of the most urgently required
materials is water — 2000 cc. a day in an average person. Certain salts
must be regularly supplied, since about 30 grams are lost per day, mostly
in urine and sweat. Most ordinary foods contain about the right pro-
portion of the various salts, though vegetable foods are deficient in
sodium chloride (NaCl). This is the reason for the common use of
table salt. Any one sweating profusely because of heavy labor in hot
places must usually drink salt water to avoid muscular spasms.

There are certain minerals which are necessary. The ones most
likely to be poorly represented in the diet are iron, calcium, and iodine.
The hemoglobin of red blood cells requires iron, and this is adequately
provided in liver, meats in general, eggs, and many vegetables and fruits.
Calcium is needed for bone and teeth, and is obtained from milk, cereals,
peas and beans. Iodine is necessary for the hormone of the thyroid
gland. It is abundant in sea foods; and in inland communities health
authorities often require that potassium iodide be introduced into table
salt. Other minerals, including copper, zinc, manganese, and cobalt, are
essential for the production of important enzymes, but the amounts
needed are exceedingly small and natural diets usually contain enough
of them.

For construction of protoplasm proteins are steadily required — a
minimum of 50 grams a day for an average adult person. A variety
of amino acids is necessary, and since only a few of the simplest ones
can be synthesized from other substances, the others must be included
in the diet. Foods which supply all the necessary amino acids are the
proteins of eggs and lean meat, the glutenin of wheat, and the lactalbumin
of milk and cheese. Most other protein foods lack, or include too small
quantities of, certain amino acids. Some fat is also required; for though
most of the fatty acids can be synthesized from carbohydrates, the ones
which the human body can not synthesize are quite essential, and these
must be received ready-made.

Vitamins. — One group of required specific substances deserves sepa-
rate treatment. It has long been known that a diet consisting of purified
proteins, carbohydrates, and fats leads to serious trouble. Natural foods
evidently contain something that does not occur in the purified foods.

PRINCIPLES OF ANIMAL BIOLOGY

These essential substances were given the collective name of vitamins
before anything was known of their identity. These substances, in
small quantities, are needed for healthy activity or growth. If any of
them is lacking, or present in too small amount, a deficiency disease results.
The disease is specific for each of the vitamins.

The earliest known and recognized of the deficiency diseases was
scurvy. Before the end of the sixteenth century an officer of the English

Fig. 92. — The need of vitamin A. Upper two dogs show xerophthahuia caused by
deficiency of vitamin A. Lower figure, one of same dogs after 10-day treatment with cod-
liver oil. {From Steenbock, Nelson, and Hart in American Journal of Physioloyy.)

navy observed the bruised skin, bleeding gums, and general anemia of
his crew after they had been many months at sea and fresh foods had
been exhausted, and he discovered that these symptoms could be com-
pletely prevented l)y giving his men a small amount of lime juice daily.
The essential feature^ of the lime juice was long designated vitamin C,
though its nature was unknown. In 1933 this vitamin was separated
out in pure form, and was found to be ascorbic acid, of the chemical
formula CeHgOe. It is abundant in citrus fruits (oranges, lemons, limes,
grapefruit), many other fruits, tomatoes, and many vegetables. Diets

SOURCES OF ENERGY AND MATERIALS 111

which inchide raw plant food are generally adequate, but cooking in
vessels exposed to air usually destroys much of the antiscorbutic effect.

Vitamin A, itself colorless, can be split off, in the human body, from
the yellow pigment carotene found in carrots and many yellow and green
vegetables. Its formula is C20H30O. Severe lack of it in the diet leads
to a dry, ulcerated condition of the cornea of the eye known as xeroph-
thalmia (Fig. 92). Milder deficiencies cause abnormalities of epithelial
membranes and retard growth. Vitamin A is also used by the retina
of the eye in the synthesis of visual purple, one of the light-sensitive
pigments, and was administered during the war to night-flying pilots to
improve their vision. Being soluble in fats (as are two other vitamins,
D and E), vitamin A is obtainable in liver oils and in such foods as milk,
butter, and egg yolk. Manufactured butter substitutes are usually
fortified by the addition of this vitamin.

What was originally called vitamin B eventually proved to be a
collection of different substances, enough alike to be hard to separate,
and occurring mostly in the same natural foods. This group, consisting
of seven or more vitamins, is now known as the B complex. Only the
more important of these can be mentioned here. Lack of thiamin (Bi)
causes polyneuritis, which in man is usually named beriberi. This
disease involves degeneration of the nerves, causing progressive paralysis.
Along with paralysis go retarded growth and loss of appetite and vigor.
Intravenous injection of Bi into polyneuritic animals restores normal
muscular movement in as short a time as one hour. The formula of
thiamin is C12H16N4SO. One of its sources in food is in cereals, especially
the outer seed coats. For this reason polished rice, in which the seed
coats are removed, and highly refined wheat flours (as contrasted with
whole wheat) are poor in thiamin. It is common practice now to add
thiamin in the manufacture of white flour. Other natural sources of
thiamin are meats, especially pork, and yeast.

A second member of the B complex is riboflavin (C17H20N4O6), called
also B2. It is found in the same foods as Bi and the other vitamins of
this group. Lack of it induces a predisposition to cataract, loss of
weight, and scaliness of skin around the ears and mouth.

Closely associated with the other B vitamins is niacin (C6H5NO2), or
nicotinic acid. Lack of it is the principal cause of pellagra, which is
characterized by dermatitis (eruption of the skin) and diarrhea. As a
pellagra preventive, niacin has come to be called vitamin P-P. The
disease is still common in southeastern United States, where corn,
molasses and meat are the staple diet. Niacin is manufactured and is
available to prevent pellagra, but is not yet in sufficiently wide use. The
dermatitis feature of pellagra may be due to lack of Be, or pyridoxin,
which is frequently absent from the pellagra-producing diet.

112 PRINCIPLES OF ANIMAL BIOLOGY

Rickets, the imperfect growth of bones and teeth, is caused by a
deficiency of vitamin D. This substance is now known to be calciferol
(C28H44O). It is produced from a closely related substance, ergosterol,
regularly present in the skin, by ultraviolet radiation. In summer time
the conversion of ergosterol to calciferol is usually adequate in most
regions, but in winter it is often advisable to supply vitamin D artificially.
The common foods containing it are butter, milk, and the oils of liver and
other animal tissues. So well understood are the preventive properties
of these foods, or the manufactured vitamin, that rickets, once a common
disease, is seldom observed in most communities.

Reproductive disturbances in some animals are caused by lack of
vitamin E, a-tocopherol (C29H50O2). In its absence female rats do not
retain the embryos in the uterus, and male rats do not produce functional
spermatozoa. No such effects have yet been shown in man. Vitamin E
occurs widely in plant and animal oils, particularly in the germ of wheat.

Failure of coagulation of the blood may be caused by lack of vitamin
K, whose formula is C31H46O2. In its absence the body does not pro-
duce enough prothrombase, from which the clotting enzyme is produced
at wounds. Vitamin K is regularly administered before child-birth, with
a considerable decrease in mortality from bleeding in both the newborn
children and their mothers. Natural food sources of the vitamin are
leafy vegetables; it is prepared commercially from alfalfa.

Vitamin P, not yet identified chemically, is closely related to ascorbic
acid (C) and is involved in scurvylike weakness of the walls of blood
capillaries. Its status is still unsettled.

The necessary amounts of vitamins are so small (0.01 gram or less
daily) that they cannot be regarded as sources of energy. They must
be in some way essential in protoplasmic structure. Three of the
vitamins, thiamin, riboflavin, and the antipellagra factor, are known to
enter the composition of important oxidative enzymes; that is, they
furnish the nonprotein part of the enzymes. What other structural
contributions the vitamins make is not known.

The need of vitamins in food differs greatly in different animals.
Rats, for example, need no ascorbic acid in their diet, since they syn-
thesize it in their metabolism; rats never have scurvy. Man can get
along with little or no thiamin in his diet; but bacteria in his large
intestine must then supply it. As stated above, man probably does not
require vitamin E, or else produces it in normal metabolism.

References
Carlson, A. J., and V. Johnson. The Machinery of the Body. University of

Chicago Press. (Chap. VII.)
Mitchell, P. H. A Textbook of Ceneral Physiology. 3d Kd. McGraw-Hill Book

Company, Inc. (Chap. XVIII, digestion; Chap. XXI, respiration; pp. 745-772,

vitamins.) ,

CHAPTER 10
RESPIRATION AND RELEASE OF ENERGY

The total requirements of energy and the general source of it in the
food have already been discussed in connection with nutrition. How
energy is released from food is a separate problem.

Derivation of Energy. — Ultimately most energy comes from sun-
light. Many plants and a few of the simplest animals have chlorophyll,
which utilizes solar energy to make sugars. In these sugars, energy is
bound up in chemical structure. As sugars are converted into starches,
or fats, or proteins, by combining them with other substances, still further
energy is stored in these higher products. When plants are devoured by
animals, the latter take possession of this potential or stored energy.
So it is that all energy of life is traceable to sunlight. Indeed, most other
energy in the world comes from the same source. Coal and oils used for
fuel got their energy from ancient sunlight. Even the energy of water-
falls came from the same source, for it was the energy of the sun which
lifted the water to its higher level. About the only energy expended on
the earth which is not traceable to sunlight is that of the tides.

Animals derive some of their energy directly from the sun, for sunlight
is one of the most potent of health-giving agencies. In the main, how-
ever, they obtain it from food, and for this they are directly or indirectly
dependent on plants. To get energy from foods, it is necessary that the
latter be chemically decomposed. The foods must be changed into
simpler substances whose content of potential energy is smaller. In
general, complex substances with large molecules have more energy
bound up in their constitution than do simple substances with small
molecules. Nearly all chemical reactions which split up molecules into
smaller and simpler ones may therefore be depended on to release a
certain amount of energy. Proteins, carbohydrates, and fats, on being
decomposed, even in the process of digestion, liberate energy.

There is, however, one type of energy-yielding chemical reaction which
is so much more abundant than any other that it is common practice to
speak of energy as coming from that source. That type of reaction is
oxidation (page 37), the union of oxygen with other elements. The
commonest of these unions is that of oxygen with carbon, because carbon
is abundant in all the classes of organic compounds — in proteins, but
especially in carbohydrates and fats. Carbon dioxide, a very stable

113

114 PRINCIPLES OF ANIMAL BIOLOGY

compound which ties up very httle potential energy, is a product of these
oxidations, so that the amount of carbon dioxide which an animal pro-
duces is often taken as an indication of the quantity of energy it uses.

Respiration. — How is all the oxygen for these oxidations obtained?
There is not enough of it in the substances to be oxidized. The common
carbohydrates contain only about half enough oxygen to oxidize their
own carbon, even if all their oxygen were available — which it is not — for
that purpose. Fats, the other main source of energy, have even less
oxygen than the carbohydrates. The oxygen must therefore be intro-
duced from external sources. For land animals that source is the air,
about one-fifth of which is oxygen. Aquatic animals of most kinds
secure the oxygen which is dissolved in the water about them.

The obtaining of oxygen is included in the process known as respira-
tion. In small animals — unicellular and small multicellular ones — oxygen
is absorbed more or less directly by the cells that use it. In the larger
animals, those in which most of the cells are too far away from the surface
to rely on this simple diffusion, respiration is a double process. That is,
the oxygen must first be got into their bodies, a process known as external
respiration, and then be conveyed to the cells where it is ultimately used.
Its absorption by these cells, often far within the organism, is called
internal respiration. In the protozoa, external and internal respiration
are merged into a single process, to which neither name may be properly
applied.

Whether an animal must have any special devices to carry on its
external respiration depends on its oxygen requirement in relation to its
surface. A large animal has much less surface relative to its volume than
a small one has ; hence, in general, the larger animals must have structures
which greatly increase their absorptive surfaces. Warm-blooded animals
consume much more oxygen than do cold-blooded ones, and active
animals much more than sluggish ones. Even as large an animal as the
earthworm, which is cold-blooded and not very active, is able to absorb
enough oxygen through its general surface. Many smaller animals,
however, because they are active, require some sort of respiratory organ
for their external respiration.

Types of Respiratory System. — Probably the earliest external respira-
tory organs devclopcxl in animals were gills. These may be employed by
aquatic animals, and by aerial animals having some way of keeping them
moist, for oxygen cannot bo absorbed through dry surfaces. A gill,
like any other respiratory organ, must furnish a large surface, since the
amount of oxygen taken in increases with increase of surface. It may
consist of branching or treelike projections (Fig. 93), or of bunches of
fine tubes, or of clusters of flat plates, or of numerous ridges or fingerlike
projections, or of sievelike sheets through which water passes. In

RESPIRATION AND RELEASE OF ENERGY

115

every such organ the first essential is an increased surface, and the
different forms of gill merely represent various ways of attaining that end.
Among animals that use gills are fishes, some salamanders, crayfishes,
clams, some marine worms, and young stages of many insects.

Lungs are internal cavities into
which air is drawn for absorption
of its oxygen. Notwithstanding
their internal location, lungs are
organs of external respiration,
since the bulk of the oxygen they
absorb is not used for energy
rel6ase in the cells of the lungs
themselves but is passed on to
other cells of the organism.

The lung in lower amphibians
is a baglike organ with a large
central cavity (Fig. 94a); but
in higher amphibians it becomes more complex since its inner surface
is thrown up into corrugations with cross corrugations forming boxlike
spaces (b, c). These corrugations increase the respiratory surface.
In higher vertebrates the lung (d) is entirely subdivided into minute
air spaces which are in indirect connection with one another through

Fig. 93. — External gills of the amphibian,
Epicrium glutinosum. {From Wiedersheim
after Sarasin.)

BRONCHIOLE
WITH ALVEOLI

Fig. 94. — Diagrams of types of lungs, a, amphibian lung with plain surface; b, amphib-
ian lung with low folds making simple alveoli; c, amphibian lung with higher folds which
are themselves folded making more numerous alveoli; d, human lung.

large tubes, the bronchi, and their branches, the bronchioles. The
bronchi unite in a single large tube, the trachea, which is present in
the higher vertebrates, but absent in some of the lower forms, as
the frog. The trachea opens into the mouth through a slitlike glottis.
The trachea and bronchi have cartilage rings in their walls, so they

116

PRINCIPLES OF ANIMAL BIOLOGY

do not collapse. The bronchioles end in expanded chambers, the
alveoli, which are in close contact with blood capillaries. The aggregate
interior surface of the alveoli in man (Fig. 94d) is more than 1000 square
feet or about fifty times as great as the general surface of the body.

In most insects, air is taken in by tracheae. These are tubes opening
at the surface of the body at various points. The tracheae branch, tree-
fashion, in such a way as to reach all parts of the body (Fig. 95). No

part of any insect tissue is more than a
few cells away from the nearest tracheal
branch. Formerly it was thought that
air pulsed back and forth, into and out
of these tracheae. It is nOw known for
some insects, however, that air goes in at
certain tracheae, out at others, thus
implying a circulation of the air. The
tracheae are connected with one another
by branches, so that such a circulation is
possible.

The young stages of May flies, dragon
flies, and some beetles live in the water,
yet respiration is carried on by tracheae.
Instead of opening at the surface of the
body, such tracheae begin in fine closed
branches which spread out in external
gills (flat plates or tubes), from which
they receive their oxj^gen by absorption.
Such gills richly supplied with tracheae
are known as tracheal gills.

Breathing Movements. — Whatever
mechanism an animal possesses for the
absorption of oxygen, it is necessary that
there be a continuous supply of oxygen
to absorb. An animal that lives fully
exposed but attached to some object in
swiftly flowing water usually requires no
special device to ensure that supply. But one that lives in still water
and i-emains motionless soon absorbs all the neighboring oxygen; and
since oxygen diffuses only very slowly through water, the supply is not
([uickly renewed. Fishes swim about; but since the gills are under a
protective plate (the operculum) at each side just behind the head, mere
moving about does not suffice. Renewal of the oxygen supply next to
the gills is effected by taking water into the mouth and then pumping it
out through clefts among the gills. The opercula are raised to allow

Fig. 95. — Tracheal system of an
insect, a, antenna; b, brain; I, leg;
n, nerve cord; p, palpus; s, spiracle;
st, spiracular branch; t, chief
tracheal trunk; v, ventral branch;
vs, visceral branch. (From Folsom,
"Entomology," after Kolbe.)

RESPIRATION AND RELEASE OF ENERGY 117

the water to pass out but settle back immediately after, so as to prevent
water from entering there. The action is repeated, and a pulsating
current of water is kept up. Lobsters have a fanlike structure at one
edge of the gill chamber, and by its movement a continuous stream of
water is kept flowing over the gills.

Land animals have various devices acting to the same end. Insects
expand their chitinous exoskeleton by muscular movement, and air
rushes in: the skeleton collapses, and the air is forced out. Valves at the
entrances of the tracheae determine which ones shall receive air. In
general the air chambers or passages have, of themselves, no power of
either expansion or contraction ; they are manipulated by something else.
The lungs in man are expanded at all times, to fill the cavity of the
thorax, merely by the air pressure within them. If the chest expands,
more air is forced in from the outside to equalize the pressure. In
inspiration, the volume of the chest is increased by two means: (1)
raising the ribs, and (2) lowering the diaphragm. The ribs are movably
joined to the vertebral column, from which they slope downward both
laterally and forward. The muscles between the ribs contract, so that
all ribs are lifted, the lowest ones most of all. Since the ribs slope down-
ward, elevating them pushes them outward (sidewise and to the front),
thus enlarging the chest in both directions. The diaphragm, a muscular
sheet across the bottom of the thorax, is convex like an inverted bowl.
When its muscles contract, the diaphragm is flattened, thus further
increasing the size of the chest cavity. Air pressure in the lungs is thus
reduced, hence air is forced in to restore an equilibrium. In expiration,
the rib muscles relax, and the ribs drop, largely by their own weight.
Both width and depth of the thorax are thus decreased. When the
muscles of the diaphragm relax, tension of the muscles of the abdominal
wall presses the viscera up against it and the diaphragm rises. With
the accompanying decrease in the size of the thorax, air is forced out
of the lungs.

All such movements designed to ensure a continuous supply of oxj^gen,
whether in air or water, are termed breathing movements. To supply
the right amount of air, these movements must vary in vigor as the
animal's activities change. In man, the rate of breathing is controlled
by a nerve center in the medulla, posterior division of the brain. The
action of this center depends on the amount of carbon dioxide in the
blood. If muscular activity increases, much more carbon dioxide enters
the blood from the tissues; this extra quantity stimulates the respiratory
center in the medulla, and breathing becomes more rapid. Panting is
an extreme response to such stimulation. If the breath is voluntarily
"held" for a short time, carbon dioxide accumulates in the blood to
such an extent that restoration of breathing is forced. No will power

'118 PRINCIPLES OF ANIMAL BIOLOGY

can resist the urgent demand of the respiratory center that breathing
be resumed.

Mechanism of Oxygen Collection. — It has already been stated
that oxygen does not spread through dry surfaces. This is because
the movement of oxygen in entering an organism is a process of diffusion,
which can occur freely only when the oxygen is in solution. Aquatic
animals, except a few air-breathing types like whales and other swimming
mammals, never meet oxygen except in solution. When air comes in
direct contact with an animal, its oxygen cannot enter unless it is first
dissolved. All that is necessary is to have the surfaces moist; oxygen
dissolves in the film of moisture, then passes readily inward through
the membranes. Lungs and tracheae have no difficulty in maintaining
this moisture, since they possess internal cavities in which there can
be little evaporation. Land animals with gills, however, must either
live in places that are perpetually moist, such as swamps, or must prevent
evaporation in some way. Land-dwelling crayfishes protect their gills
from drying by means of chitinous flaps of the exoskeleton (page
90) and have the habit of burrowing in the soil until moisture is
reached.

The passage of oxygen through moist membranes depends on the same
principle as that which causes water to flow down hill, or winds to blow
from areas of high atmospheric pressure to those of low pressure. Oxygen
goes from places of high oxygen pressure to those of lower pressure. This
pressure is not entirely a matter of quantity, for a small amount of oxygen
dissolved in a certain volume of water may exist at a greater pressure than
does a greater amount in the same volume of air. When oxygen enters
the gills of an aquatic salamander, it is because the oxygen in the water is
at greater pressure than is the oxygen in the gills. In a land animal with
liuigs, the oxygen in the air in the lungs is at higher pressure than in the
tissue of the lungs. In the human lungs the air in the remote alveoli,
being diluted with waste products there, exhibits an oxygen pressure
somewhat lower than the oxygen pressure of open air; and yet it is nearly
three times as great as the oxygen pressure in the tissues of the lungs;
hence the transfer to the tissue. From the cells lining the alveoli of the
lungs it is a very small step to the blood, for the capillaries are closely
applied to the alveoli. Oxygen enters the plasma, the liquid portion of
the blood, again in response to a pressure gradient : pressure is lower in the
plasma. Pressure is constantl}^ kept lower in the plasma, because the red
blood cells contain a protein which takes up (]uantities of oxygen in
chemical combination. Moreover, the blood is circulating; blood that
has absorbed oxygen is continually l)eing replaced by blood that has
little of it. So a perpetual transfer oi (jxygen to the blood is set up in the
lungs.

RESPIRATION AND RELEASE OF ENERGY 119

Internal Respiration. — When the oxygen is finally presented to the
tissues or cells in which it is to be consumed, its introduction to those cells
is again dependent on relative pressures. Oxygen is at higher pressure
in the plasma of the blood than in the adjoining tissue cells, which have
used their oxygen. As the plasma gives up its oxygen to the cells its
oxygen pressure is lowered; and in response to this reduction, oxygen is
released from chemical combination in the red cells, and is dissolved in the
plasma. The plasma thus maintains a higher oxygen pressure as long
as there is oxygen in loose combination in the red cells; and before the red
cells have lost all their loosely combined oxygen, the blood has passed
on and been replaced by fresh blood which has not yet been called upon
to give up its oxygen. So there is a continual diffusion of oxygen from
the blood to the tissue cells. The transfer is very rapid, for the oxygen
pressure in the blood is reduced by half in one second of time. The cells
nearest the capillaries pass some of their oxygen on to cells farther away,
again in response to differences in pressure but aided by a fluid (see next
chapter) bathing the cells, and no cell is very far from the nearest blood
vessel.

Respiration Also an Excretory Process. — While we are not yet ready
to discuss the general phenomenon of removal of Avastes, it should be
pointed out in passing that certain wastes are removed in respiration.
These wastes are carbon dioxide and a small amount of water. Carbon
dioxide results from the very abundant oxidation going on everywhere
in living things. It leaves the tissues where it is produced because its
pressure is higher than in the near-by blood plasma. The resulting
increase of pressure in the plasma causes the chief protein of the red cells
to combine with carbon dioxide. Delivered by the blood to the lungs,
the carbon dioxide is at greater pressure in the blood than in the air of the
lungs; hence the plasma gives up carbon dioxide to the air on the other
side of the two thin walls w^hich separate blood and air, and red cells
yield more carbon dioxide to the plasma. Since the blood moves on, no
equilibrium can be reached; always carbon dioxide passes from blood to
air in the lungs. This elimination of carbon dioxide is regarded as part
of respiration, even though it is also excretion. Excretion in general is
treated in another chapter.

Release of Energy. — Energy for all sorts of work in living things is
obtained, as stated earlier, by combustion of foods. These substances
are literally burned, just as coal is burned in a boiler, with the difference
that combustion in living things is carried on at relatively low tempera-
tures. The reason for the ability of animals to burn their fuel without
great heat lies in their possession of enzymes. The burning is simple
oxidation, and the enzymes serve to bring oxygen and the foods together
in chemical reaction. One of the chief functions of respiration is to

120 PRINCIPLES OF ANIMAL BIOLOGY

furnish oxygen, just as one of the principal ends of digestion is to provide
foods, for this reciprocal reaction whose object is the release of energy.

Carbohydrates require less oxygen from outside sources for their
combustion, because they furnish some of their own. The carbon of the
sugar molecules unites with the oxygen which the same molecules con-
tain and with oxygen of respiration. Carbon dioxide, the end product
of this combustion, contains little stored energy. Most of the energy
residing in the sugar is thus liberated.

Fats, which are also primarily fuels, are burned in the same way;
but since they contain relatively little oxygen, more oxygen of respira-
tion is required for their combustion. Again carbon dioxide is the
energy-poor end product. As stated in the preceding chapter, fats are
not readily burned unless carbohydrates are being oxidized at the same
time; the reason for this connection is not known.

Proteins, which are primarily material for construction, may also be
burned. To some extent they are utilized as a normal source of energy,
but in times of starvation this use is stepped up markedly. Since
proteins are not stored to any extent in animals, combustion of them is
at the expense of the body tissues. Animals literally burn themselves
at such times. Part of the living organism is being destroyed to main-
tain the rest of it. Proteins are intermediate l:>etween fats and carbohy-
drates in the amount of outside oxygen they require for their oxidation.

Heat. — One of the important uses to which energy is put in some
animals is the development of heat. This heat comes mostly from
oxidations occurring in muscle. If the amount of heat is regulated in
some way, so that a fairly constant temperature is maintained, an
especially advantageous situation is produced. Many physiological
processes bear a time relation to one another, and the speed of most such
processes is accelerated by high temperatures and retarded by low ones.
If the speeds of various processes are not equally affected, a change of
temperature destroys a nice adjustment among them. Hence a con-
stant temperature is an advantage.

Many invertebrate animals have no heat regulation; and, when their
muscular movements are slight, as in clams and snails, their temperatures
are almost identical with that of othcn* things around them. Such
animals are said to be cold-blooded. Among the vertebrates, the fishes,
amphibia, and reptiles are all regarded as cold-blooded because their
temperatures rise and fall with changes in external temperature; but some,
perhaps most, of them have temperatures somewhat above that external
to them.

The higher mammals, including man, are warm-blooded (as are also
the birds) and have very marked regulation of temperatiu'e. The tem-
perature of the human body in health seldom rises much above 38° or falls

RESPIRATION AND RELEASE OF ENERGY 121

much below 37°C. Regulation works in both directions. When the
internal temperature falls to a certain degree, shivering is caused, and
heat is produced by the additional muscular movement. When the
temperature rises too far, there are several ways of checking it. Rapid
breathing serves to cool the lungs, and with them the whole body. More
blood flows to the skin; hence there is greater loss of heat by radiation.
And in man and horses, but not so much in many other mammals, sweat
exudes upon the surface, where its evaporation serves to lower the tem-
perature. In the dog there are no sweat glands except on the nose and
on the foot pads. In this animal rapid ventilation of the lungs in pant-
ing is the chief source of control; whatever cooling is caused by evapora-
tion occurs in the open mouth and on the lolling tongue.

Regulation of temperature is governed by a nerve center in the
thalamus of the brain. When this center is warmed, the nerves going
to the blood vessels in the skin cause the latter to enlarge, and the sweat
glands are stimulated to excrete. On cooling the nerve center, these
actions are reversed, and muscle tension is increased, all of which leads
to a rise of temperature.

References

Carlson, A. J., and V. Johnson. The Machinery of the Body. The University

of Chicago Press. (Chap. VI.)
Mitchell, P. H. Textbook of General Physiology. 3d Ed. McGraw-Hill Book

Company, Inc. (Chap. XXI.)

CHAPTER 11
TRANSPORTATION SYSTEM

Only in small animals can oxygen be taken in, digested food distri-
buted, and carbon dioxide and other wastes eliminated by mere diffusion.
In large animals the distances are too great for these slow-moving pro-
cesses. In such animals there must be a system of transportation con-
necting all parts of the body. This communication is furnished b}^ the
circulatory system.

Open and Closed Circulatory Systems. — In crayfishes, insects, and
their allies there is a heart which forces blood into a small number of
major blood vessels. These vessels or their branches open into small or
great spaces among the ceils and organs, so that the blood comes into
contact with the tissues directly. Food is carried to the cells, and wastes
are removed, by direct contact. From the intercellular spaces the blood
is passed through the gills, and finally returns to the heart. Circulation
in such an open system must be slow because of the resistance offered by
the tissues.

Any system of fluid communication must, like that of the crayfish,
reach the cells rather directly. To retain this necessary direct contact
and at the same time speed up the circulation, the vertebrate animals have
evolved two separate yet cooperating systems: (1) a blood system in which
there are smooth, closed tubular vessels in which the flow is very rapid,
and (2) a lymph system in which movement is slow but the cells are
reached directly. These systems are connected, and the fluid in the latter
is derived largely from the former.

The Blood System. — A closed blood system consists of a set of tubes
which branch so extensively as to bring all parts of the body very near to
the circulating liquid. The blood is propelled through these tubes by a
contractile organ, th(i heart. In some animals the walls of the blood
vessels are contractile, and waves of contraction pass along them in
the direction of circulation. When these vessels arc especially large, and
when their contraction is more mai'ked than those; of other vessels, as are
those at the sides of the esophagus in the earthworm, they may properly
be called hearts. In the higher animals, vessels conducting blood away
from the heart are called arteries; those returning it to the heart are veins;
and the fine tubes leading from the arteries to the veins are called capil-
laries. The arteries have strong walls capable of withstanding consider-

122

TRANSPORTATION SYSTEM

123

able pressure, and they are firm enough to stand open even when empty
of blood. The veins are not called upon to endure such pressures as are
the arteries; their walls are comparatively thin and collapsible. More-
over, in the veins there are at invervals valves, consisting of membranous
flaps directed forward (in the direction of flow), which close and stop
the blood if it starts at any time to flow backward (Fig. 96).
The capillaries are of various sizes, the smallest ones
being just large enough to allow the blood cells to pass
along single file. They have very thin walls, only one cell
thick. Being thin, they are collapsible, and at times of rest,
when the circulation is slow, many of them are closed.

Blood is kept coursing through these vessels by the
motive power of the heart. Any muscular activity is apt
to exert pressure on near-by veins, and this in conjunction
with the valves in the veins helps to keep the blood mov-
ing; but the heart action is the main source of power.

Chambers of the Heart and Course of Circulation. —
The hearts of various vertebrates have two, three, or four
chambers, and the course of the circulation is in part
related to this feature of heart structure. A diagram of
the circulator}^ system in the dogfish, an animal with a
two-chambered heart, is shown in Fig. 97. This diagram indicates that
the blood of animals with gills and a two-chambered heart passes from the
ventricle of the heart through the gills and then forward to the head or
backward through the dorsal aorta to the organs of the body, where it
passes through capillaries and returns to the auricle of the heart by means
of the veins.

TO HFAn DORSAL AORTA

,H°^"° ^ I ) } I

Fig. 96.—
Vein slit open
to show
valves.
Course of
blood is
upward.

GILLS-

'llllllll

VEIN

VENTRAL AORTA

BODY AND
ORGANS

Fig. 97. — Simplified diagram of the circulatory system of the dogfish.

Except for the fact that the blood in the arteries is distributed to
different organs, from each of which it returns independently to the veins,
the blood of a fish covers only one circuit. It passes through two sets of
capillaries, one in the gills and another in the head or some body organ or
tissue, and goes to the heart only once in each circuit. This course is a
consequence of the two-chambered construction of the heart.

In animals with lungs and a heart of more than two chambers the
circulatory system is more complicated. The heart of amphibians and

124

PRINCIPLES OF ANIMAL BIOLOGY

reptiles, except crocodilians, has three chambers in place of two as in the
heart of fishes (Fig. 97), and the heart of mammals, birds, and croco-
dilians has four chambers. The four-chambered heart is composed of
two halves, right and left. Each half is made up of two chambers, a
thin-walled auricle and a thick-walled muscular ventricle. There is no
passage between the two halves of the heart but there is a broad passage
guarded by valves connecting each auricle with the ventricle of the same
side. The relations of the parts of a four-chambered heart may be
understood from Fig. 98.

The circulation in such an animal is a double one. Beginning at the
left ventricle (see Fig. 99 for the human scheme), the blood is driven
into the large artery which, with its divisions, leads to the body in
general, including the head. In these parts the arteries divide into

capillaries, which are collected again into
veins. The veins gather into two large veins
which enter the heart by the right auricle.
The circuit just described from left ventricle
through the body to right auricle, is called
the systemic circulation. The blood now goes
from the right auricle, through valves, to the
right ventricle, thence is forced to the lungs.
After passing through the capillaries of the
lungs it returns by a large vein to the left
auricle of the heart, thence to the left ventricle.
The circuit through the lungs is called the
'pulmonary circulation. In a complete circula-
tion, therefore, the blood passes through the
heart twice, once through the left side, once
through the right. The blood has no alter-
native in this course, except that in the
systemic circulation it may go to any one of a number of parts of the
head, trunk, extremities, or abdominal organs. When it has gone
through the systemic circuit, it has no choice but to go to the lungs.

The doubleness of this circulation is a consequence of the four-cham-
bered heart, that is, of its complete separation into right and left halves.
In animals with a three-chambered heart, as in a frog, this distinctness
does not prevail, for while there are two auricles there is but a single
ventricle. There is therefore some mixing of the l)lood in the ventricle;
but the structure of the ventricle with its deep recesses and the operation
of valves in the principal artery are such that the mixing of venous and
arterial blood is partially prevented.

In general, when the heart has four chambers, the blood passes
through only one set of capillaries in each circuit. There is only one set

Fig. 98. — Diagram of a
four-chambeied heart. LA,
left auricle; RA, right auricle;
LV, left ventricle; RV, right
ventricle; L\, vessel from
lungs; L2, vessel to lungs; S\,
vessel to system; Si, vessel
from system.

TRANSPORTATION SYSTEM

125

Fig. 99. — Diagram of human circulation: a, aorta; ca, celiac artery; ch, capillaries of
head; ci, capillaries of intestine; clu, capillaries of lungs; civ, capillaries of liver; fv, femoral
vein; hv, hepatic vein; ia, iliac artery; I, lacteals (intestinal lymphatics); lea, left carotid
artery; Ijv, left internal jugular vein; Isa, left subclavian artery; Isv, left subclavian vein;
ly, lymphatic capillaries; pa, pulmonary arteries; par, portal vein; pv, pulmonary veins;
rca, right carotid artery; rjv, right internal jugular vein; rid, right lymphatic duct; rsa,
right subclavian artery; s, subclavian vein; S7na, superior mesenteric artery; td, thoracic
duct; vci, vena cava inferior; vcs, vena cava superior.

126 PRINCIPLES OF ANIMAL BIOLOGY

in the pulmonary circulation, and for the bulk of the blood there is only
one in the systemic course. There are, however, certain exceptions.
The blood which traverses the stomach, intestines, pancreas, and spleen
collects into a vein (Fig. 99 'por) leading to the liver; in the liver it passes
through a second set of capillaries, then enters the large vein returning
to the heart. A circuit beginning and ending in capillaries is known as
a 'portal system, and that going from the abdominal viscera to the liver is
the hepatic portal system. Fishes and amphibia have a portal system
leading to the kidneys also, but that is lacking in man and mammals in
general.

It has been estimated that about IY2 pei" cent of the weight of the
human body is blood. From the amount ejected from the heart at
each beat, it may be calculated that the speed of the blood is such that
an entire circulation, both systemic and pulmonary, requires on the
average only about 23 seconds.

Composition of the Blood. — The blood consists of a liquid known as
the plasma and a number of kinds of cells or cell derivatives. The

A BCD

Fig. 100. — Formed elements of human blood. A, red corpuscle; B, C, two forms of white

cell; D, platelets.

plasma floats the cells, and in addition carries a number of kinds of
substances in solution. Among these substances are some temporary
ones such as the products of digestion (glucose, amino acids, neutral fats,
glycerol, fatty acids), waste materials (urea, uric acid), the respiratory
gases (oxygen and carbon dioxide), hormones (secretin and others), and
various enzymes, which are introduced and removed at certain places in
the system. Other substances are permanent. Of these, proteins make
up about 7 per cent of the weight of the plasma; one of the proteins is
fibrinogen which features prominently in the clotting of the blood.
Inorganic salts are about 1 per cent of the weight of the plasma; an
important one is a bicarbonate which carries carbon dioxide in its negative
ions (HCOs"). Finally, there are antibodies which the tissues of the
body have produced in reaction to and protection against foreign proteins,
including disease-producing organisms.

The visible objects in the blood are of three general kinds: (1) red
cells, (2) white cells, and (3) platelets (Fig. 100). The red cells are flat
disks, circular in form and thin in the center in man and most of the
other mammals, but elliptical in other vertebrates. There are about 25
trillion (25 million million) red cells in an average human being. The

TRANSPORTATION SYSTEM 127

human red cell has no nucleus when in the blood, but in its develop-
mental stages in the red marrow of the bones, by which it is produced,
it has a nucleus. Red cells contain an important protein substance
known as hemoglobin, which gives the cells their red color. From the
rate at which hemoglobin is disintegrated in the liver, -it is estimated that
at least 5 per cent of the red corpuscles are destroyed every day. In
other words, more than 10 million of them disappear every second. Hence
there must be a rapid replacement of them by the marrow.

The white cells are of half a dozen kinds. Two-thirds of them belong
to one type having an irregularly lobed or even divided nucleus (Fig.
lOOB), the power of movement like Amoeba, and the ability to engulf
bacteria. These cells may creep out of the capillaries, through small
crevices between the cells of the capillary walls (Fig. 101). They emerge
from the capillaries in great numbers at the site of an infection, to
engulf the infecting organisms. In their battle with the bacteria many
of the white cells are killed, and their bodies make up a large part of the

(CD

Fig. 101. — Successive stages in the emergence of a white blood cell from a capillary.

pus which collects in an abscess. White cells of this kind originate in bone
marrow. The next most numerous kind, about one-fourth of the total,
originate in lymphoid tissue (Ij^mph glands, spleen). The remaining
types are recognized by different staining reactions as well as by their
size and nuclear structure; some of these devour bacteria, others do not,
but their functions are not well understood. All kinds of white cells
together number about 30 to 40 billions in an average human being.

The platelets are not cells, but pieces of cells. They come from
certain large cells in the bone marrow by fragmentation. They dis-
integrate so rapidly when the blood leaves the capillaries that it is
difficult to count them. By special techniques it has been estimated
that there must be from one to three trillion of them in a human being.
Only the mammals are certainly known to have them. Their disinte-
gration on leaving the blood vessels yields a substance which is important
in the clotting of the blood.

Regulation of Heart Beat. — Because the heart is histologically practi-
cally a unit, it beats also as a unit. It is one of the best organs with
which to demonstrate the all-or-none principle, because of this unity

128

PRINCIPLES OF ANIMAL BIOLOGY

and the constant vigor of its contraction. Several other features of its
beating are of the utmost importance.

The heart has a long refractory period. Any muscle, after it has con-
tracted, will refuse to respond to a subsequent stimulus until a certain
time has elapsed. This interval of rest, known as the refractory period,
is exceedingly short (0.005 second) in skeletal muscle, but very long in
the heart. This prevents the heart from responding to any abnormal
nervous condition by remaining continuously contracted. It contracts
once, then must wait an appreciable time, during which it relaxes,
before it can contract again.

Contraction of the heart is initiated by a mass of rather embryonic

tissue located in the right auricle, near
the point where the great veins enter.
This tissue is known as the sinus node
(Fig. 102). When this node is stimu-
lated, the right auricle starts to con-
tract, and a wave of contraction spreads
to the left' auricle. This wave is mo-
mentarily blocked at the margins of the
ventricles but is carried over to them by
another node located on the partition
between the two auricles, a bundle of
whose tissue is distributed through the
ventricle walls.

The sinus node is the "pacemaker"
of the heart. It responds to an increase
of carbon dioxide in the blood by caus-
ing the heart to beat faster. An in-
crease of temperature, acting through
the sinus node, also leads to faster
beating. For both of these reasons, exercise accelerates the circulation
of the blood.

The pacemaker is in turn partly regulated by nerves. A pair of
accelerator nerves comes to it from the spinal cord in the chest region
and a pair of inhibitor nerves from the medulla of the brain. The
inhibitors are working constantly, exerting a continual drag on the heart.
Against this braking effect the accelerators act to variable degree.
Excitement and various reflexes (page 146) stimulate heart beat through
the nervous control of the sinus node.

Blood Pressure. — Tli(> pressure of the blood against the walls of the
vessels is greatest in the arteries near the heart, declines moderately
in the more distant arterial branches, diops markedly in the minute
arterioles and capillaries, then declines slightly in the veins (Fig. 103).

Fig. 102. — Pacemaker of human
heart, the sinus node (SN). AVN,
auriculoventricular node, with its
extension in auriculoventricular bun-
dles (AVB). V, valves between left
auricle and left ventricle.

TRANSPORTATION SYSTEM

129

In the veins next to the heart it is on the average less than atmospheric
pressure; that is, a "suction" is present there when the auricles relax.

The high pressure in the arteries is necessary to drive the blood
through the capillaries where the resistance is great. It is also needed
to send the blood above the pumping organ, as to the head in man.
Pressure drops in the capillaries because of the great increase in the
aggregate cross section of these numerous vessels, but there must still
be a small pressure beyond the capillaries to push the blood (against
gravity in much of the system) on to the heart.

Blood pressure is elevated if heart action is accelerated, also if resist-
ance in the vessels is increased. This resistance depends on the diameter

120-

■

5->

■«.^

:2 90-

(L)

o
E

"tl
<

_- \ / ^^ 1 ^\ i* j._ ^

<- y ems — • *

I-

• E 60-

q: so-

)

o
o

<---

—Arteries >■

Course of Circulation

Fig. 103. — Curve showing decrease of blood pressure in course of circulation in man.

of the vessels, which is under the control of a nerve center in the medulla.
A sUght increase in the carbon dioxide in the blood stimulates this
center, the vessel walls contract, and pressure is raised. However, in an
active muscle, where the excess carbon dioxide is being produced, there
is an opposite effect, a local dilation of the vessels, perhaps a response to
higher acidity caused by the extra carbon dioxide or extra lactic acid.
The net result is a shunting of the blood to the active organ where it is
needed.

A special situation in the great artery from the left ventricle regulates
the heart beat by stimulating it (through a nerve) when the pressure in
the artery falls, depressing the heart when this pressure rises. Other
stimuli are associated with these, but they all work together to check
activity when it becomes too great, stimulate it when it lags. Highly
adaptive controls thus depend upon automatic responses of organs to
stimuli which the organs themselves Kelp to create.

130 PRINCIPLES OF ANIMAL BIOLOGY

Coagulation. — One property possessed by blood, as a protection for
its own operations and the life of the organism, is its power to clot.
When blood vessels of small size are broken, the gap may be stopped by
the coagulation of the blood, thus preventing loss of excessive amounts of
blood. The clot consists of a tangled mass of threads of a substance
known as fibrin, in which are trapped multitudes of red corpuscles. The
fibrin is produced from fibrinogen, already mentioned as an important
protein component of the plasma. Conversion of fibrinogen into fibrin
is accomplished by the enzyme thromhase. This enzyme cannot exist in
the blood during normal circulation, but its forerunner, called prothrom-
base (page 112), is regularly present. The conversion of pi^othrombase
into thrombase is induced by a substance known as thromboplastin which
is liberated partly from the damaged tissue cells at a wound, partly
from the blood platelets which promptly disintegrate in exposed blood.
The chain of reactions here described in reverse quickly leads to the
precipitation of the fibrin network. Some other things are necessary to
that chain. Calcium ions must be present, and clotting may be pre-
vented in shed blood by precipitating its calcium with an oxalate or
citrate. Vitamin K (page 112) also aids coagulation. Clotting can
be artificially checked in surgical operations by injecting something
{heparin, for example, extracted from liver and muscle) wiiich inactivates
thrombase. People afflicted with hemophilia have a very slow coagula-
tion and bleed a long time from minor wounds. One feature of their
blood is the slowness with which blood platelets disintegrate, so that
production of thromboplastin is delayed, but there must be other factors.

The fibrin network traps most of the blood cells, and as it contracts
it squeezes out a clear yellowish liquid, the serum, which is nearly identi-
cal with the plasma minus its fibrinogen.

Lymph and the Lymphatic System. — As a means of fluid communica-
tion between all parts of an animal, the blood system alone is not quite
sufficient. The blood as a complete entity is confined to the blood vessels,
and diffusion of substances held in it, even from the capillaries, is too
slow to meet all needs. Moreover, the diffusion of water itself from the
capillaries must be a one-way movement because of the pressure of the
blood. Some of these inadequacies of the blood system are overcome
by the second of the great networks of vessels (page 122), the lymph
system.

Because of the considerable pressures which are maintained in the
blood, there is a tendency for any of its components to escape if the}^ can
do so. The capillaries, with their thin walls, are the only place where this
is possible. The liquid part, the plasma, filters out rather readily, passing
into the spaces (Fig. 104) among the tissue cells. Some of the dissolved
parts of the plasma (chiefly proteins) are held back by the walls of the

TRANSPORTATION SYSTEM

131

foIoFoTolfo)^

CAPILLARY

Fig. 104. — Diagram showing
lymph spaces adjoining capil-
lary and among cells.

' capillaries, as happens in osmosis, and some other things may be added to
it by a sort of secretion as it passes through those walls. The white
corpuscles may crawl between the cells and escape (Fig. 101), and now and
then a red cell may also pass out. The fluid which escapes from the
capillaries is thus very little different from blood minus its red corpuscles
and minus about two-thirds of its proteins.
It is called lymph.

The lymph carries with it most of the
blood substances which can be dissolved in
water, including most of the digested foods
and a small amount of oxygen. It bathes
the cells, which take any of the substances
that are required. These cells also lose to
the lymph any of their soluble \A»stes,
principally carbon dioxide and urea. There
is some diffusion of the various substances
directly through the protoplasm of the cells,
so that lymph is not the sole means of communication between the blood
capillaries and the surrounding tissues.

Lymph cannot continue exuding from the capillaries unless it is some-
how removed, and it cannot return to the blood vessels from which it
came, because of the blood pressure. Instead, it is drained off by another
set of vessels known as the lymphatic system. Very small
lymph capillaries pass among the cells everywhere, and the
lymph moves into them, mostly by diffusion, though
minute solid particles are somehow able to get into them.
These capillaries collect into larger vessels, which even-
tually empty into a vein. In man there are two main
lymphatic trunks, one which receives lymph from the entire
lower portion of the body below the chest and from the left
side above that level, the other from the right side of the
chest and head and the right arm (Fig. 105). These large
vessels empty into certain veins, one at the base of the
neck, the other in the left shoulder (Fig. 99td, rid). The
lymph is thus returned to the blood system from which it
came. In the course of the lymph capillaries there are
valves (Fig. 106) which prevent backward flow, and there
are valves at the two points where the main lymph ducts
enter the veins. While these valves, together with pressure exerted
by muscles, help maintain the flow of the lymph, the main cause
of movement is the pressure of the blood behind it, and that is
furnished by the heart. Because the source of pressure is distant and
the resistance is great, the flow of lymph is sluggish. It takes an hour

Fig. 105.
Very unequal
portions of
human body
supplied by
the two main
lymphatic
systems.

132

PRINCIPLES OF ANIMAL BIOLOGY

or more to flow from the leg to the vein in the shoulder, a.s compared
with less than a minute for the blood to make a complete circuit from
heart to heart.

Interrupting the lymph vessels are numerous enlargements made of
connective tissue, called lymph nodes, which filter out or otherwise remove

the solid particles in the lymph. In these nodes
one of the kinds of white blood cells (Fig. lOOC)
is created. In the nodes any bacteria which
escape destruction by white corpuscles at the
seat of infection are apt to be destroyed, and
nodes are often swollen during an infection.

Connected with the lymph vessels is a set of
tul:)es Avhich originate in the walls of the small
intestine. These are the lacteals (Fig. 990,
which are part of the lymphatic system. They
extend into the minute fingerlike projections
(the villi, Fig. 91) in the walls of the intestine
and are especially useful in absorbing digested
fats. These lacteals collect into larger vessels
and finally merge with the lymph vessels of the lower part of the body,
at a point shortly below the lowest rib. Their contents are thus
disgorged into the left one of the veins which receive lymph.

Fig. 106. — Lymph cap-
illary, diagram of short
segment above, photo-
graph of single valve be-
low. {Photograph by cour-
tesy of General Biological
Supply House.)

References

Carlson, A. J., and V. Johnson. The Machinery of the Body. University of

Chicago Press. (Chaps. III-V.)
Mitchell, P. H. General Physiology. 3d Ed. McGraw-Hill Book Company, Inc.

(Chap. XIX, chemistry of blood and lymph; Chap. XX, the circulation.)
Rogers, C. G. Textbook of Comparative Physiology. 2d Ed. McGraw-Hill

Book Company, Inc. (Deals largely with invertebrate animals: Chaps. X and

XI, the blood; Chap. XII, coagulation; Chap. XIII, circulation; Chap. XIV,

heart action.)

CHAPTER 12
DISPOSAL OF WASTES

Substances which cannot be built up into protoplasm, or do not
yield energy when decomposed, or do not act as vehicles for important
substances, or do not stimulate cells to activity can be of little use to
animals. Such substances must be eliminated if they are incidentally
acquired, as are the indigestible parts of various foods, or if they are
produced as a consequence of physiological processes. Indigestible
portions of objects taken in as food are removed as feces by the digestive
tract itself. Those which result from the life processes are thrown off
by the general process of excretion. It is only the latter group, the wastes
which originate within the organism, that are dealt with in this chapter.

Origin of Wastes. — Since oxidation (page 37) is the main source
of energy in living things, some of the principal wastes result from that
process. Carbon is abundant in all protoplasm and in all the classes of
organic foods (proteins, carbohydrates, lipids). Oxidation of these
things results therefore in quantities of carbon dioxide (CO2). This sub-
stance, as previously explained, is very stable and contains very little
potential energy, besides being toxic in large quantities; hence it is waste
matter. Water must be taken in as a vehicle for other substances, but
in larger quantities than can be retained; the excess is waste. Destruc-
tion of proteins, whether those of protoplasm or unutilized food, must
yield some nitrogenous wastes, the principal one being urea. There are
minor substances of many kinds, but these three — carbon dioxide, water,
and urea — form the bulk of the material that has to be removed.

Gaseous Wastes. — The removal of carbon dioxide has already been
mentioned (page 119) as part of the process of respiration. Cells accumu-
late quantities of this substance as a result of their own oxidations and in
man usually contain it at a pressure equivalent to about one-fifteenth of
an atmosphere, or more. Since this pressure is double the pressure of
the same substance in the blood of the capillaries, carbon dioxide diffuses
from the cells into the blood. In the lungs, the pressure of the carbon
dioxide in the blood is distinctly greater than in the air of the lungs;
hence diffusion is outward. Gills operate in the same way as lungs, but
the differences in pressure are smaller; hence the rate of elimination of
carbon dioxide is slower.

Small quantities of other gases, especially those arising from bacterial
action in the intestine, or from defective digestion, are also removed by

133

134

PRINCIPLES OF ANIMAL BIOLOGY

the lungs. Considerable water (about one-tenth of the total water loss
of the human body at rest) is also there removed in the form of vapor.
Excretion through the lungs, therefore, involves only gaseous wastes.

Water and Urea. — Urea is a solid substance ; hence by most organisms
it can be excreted only in solution. Many other substances besides urea
contain nitrogen and are produced by decomposition of proteins, but

nearly all of them are solids that
require to be eliminated in dissolved
form. As just stated, only about a
tenth of the excess water taken in by
man can be removed as vapor, so that
the bulk must leave as a liquid.
These two groups of wastes may thus
be removed by a single operation.
The urea and the other nitrogen-con-
taining substances are dissolved in
water, and all are eliminated to-
gether. The amount of these wastes
is much greater than that of all other
wastes combined, and their removal

Fig. 107. — Portion of a protonephridial sjstem from the tapeworm Taenia crassicollis.
f, flame cell; n, nucleus of excretory tubule; tu, excretory tubule. {From Hesse and Doflein
after Bugge.)

Fig. 108. — Flame cell of a protonephridium of a flatworm: ci, cilia within funnel-
shaped cavity of flame cell; n, nucleus. {From Hesse and Doflein after Lang.)

is the chief task of what is called the excretory system of the multicellular
animals. The excretory system is often aided by the skin, and there are
other minor ways of removing water.

Excretory Systems of Invertebrate Animals. — The excretory system
varies considerably in different animals. In tlui flatworms and some
others it consists of protonephridia, which are fine tubes rising in flame
cells and discharging to the exterior. A portion of such a system is shown
in Fig. 107, and the structure of a flame cell in Fig. 108. The flame cell is

DISPOSAL OF WASTES

135

capillaries
around tubules -

bladder

blood
vessels

somewhat stellate or irregular in shape, hollowed out to form a funnel-
shaped cavity within itself. A number of long, slender cilia (the ' ' flame ' ' )
take their origin from the body of the cell and hang freely into the funnel-
shaped cavity. In life, the cilia beat continuously and by their beating
cause currents in the liquid wliich is excreted into the funnel by the cell.

Nephridia. — In the annelid worms each segment or somite (with some
exceptions) is provided with a pair of more or less coiled tubes, the
nephridia, which have a ciliated opening, the funnel or nephrostome,
which projects through the septum into the cavity of the somite ahead.
There it opens directly into the body cavity or coelom. The other end of
the coiled tube is connected to the body
wall where it has an opening to the exterior,
through the nephridiopore (Fig. 109).
Through much of its course this tube is
surrounded by a network of capillaries, a
feature of the excretory organ of all the
higher animals. In its operation, the
nephridium takes in fluid from the coelom
through the nephrostome. This fluid con-
tains wastes exuded into it by the various
tissues, but it also contains some usable
substances, one of them being glucose. As
the fluid passes along the tube, the glucose
and other useful substances are absorbed by the tubule walls and are
carried away in the capillaries to be used elsewhere. Excess water is
also thus reabsorbed into the blood, and the fluid finally ejected at the
nephridiopore is highly concentrated.

Kidneys. — In embryos of the higher animals the excretory system
starts in a form which is comparable to a row of nephridia in the earth-
worm. It consists of a series of uriniferous tubules, a pair in each segment,
the inner ends of which open into the coelom. The outer ends, instead
of opening to the outside independently, all empty into a pair of tubes,
one on each side, and these open to the exterior. In the course of develop-
ment the coelomic openings, with a small portion of the tube, are closed
off. Minute networks of blood capillaries are pushed into the sides of the
tubules near the coelomic ends, and in the adult organ the tubule ends
at that point. The tubule wall has grown almost completely around the
invading group of capillaries, to form a double-walled cup through the
open interior of which a blood vessel passes. This cup and the blood
vessels in it are together known as the renal corpuscle (Fig. 110). The
^\alls of the cup are Bowmaii's capsule, and the contained blood vessels
are the glomerulus. The renal corpuscles with the uriniferous tubules
are the essential excretory units in the vertebrate animals generally.

nephridiopore

Fig. 109. — Nephridium of
earthworm. (From Storer, "Gen-
eral Zoology.")

136

PRINCIPLES OF ANIMAL BIOLOGY

In the lower vertebrates (up to the amphibians) much of this embryonic
state is retained in the adult, particularly the repetition of the tubules
in a serial arrangement. In the higher vertebrates the segmental

arrangement is completely lost in the gross
form of the system. Yet in all of them the
uriniferous tul)ule with its renal corpuscle is
the functional unit.

The adult kidney in the frog, in cross sec-
tion, is arranged as in Fig. 111. The renal
corpuscles are located toward the ventral side.
The uriniferous tubules from them pass up-
ward, downward, and upward again, with
many convolutions, and empty into collecting
tubules, a number of which traverse the
kidney near the dorsal surface. The collect-
ing tubules begin in Bidder's canal, which
extends along the median (inner) edge of the
kidney, and end in the ureter, which extends
along the lateral edge of the kidney, and then
on to the cloaca and bladder. At the ventral
side are nephrostomes, remnants of the
embryonic openings into the coelom, but end-
ing blindly in the adult. An important addi-
tional feature of the kidney is the abundant
supply of blood vessels ; the tubules are every-
where in close contact with capillaries.
The corresponding system in man is shown in Fig. 112. The kidney
is bean-shaped, with the ureter emerging from the "eye" of the bean

Fig. 110. — Structures
from vertebrate kidney, dia-
grammatic. A, renal cor-
puscle; B and C, cross-sec-
tions of uriniferous tubules at
different levels; av, afferent
vessel; be, Bowman's capsule;
cap, capillary; cil, cilia (found
in amphibia, not man) ; ev,
efferent vessel; gl, glomer-
ulus; ut, neck of uriniferous
tubule.

Fig. 111. — Diagrammatic representation of a cross section of the kidne.\- of a frog.
B, Bidder's canal; C, collecting tubule; D, dorsal, L, lateral margin of kidney; AI, renal
corpuscle; A'^, neijhrostoinc; 7', uriniferous tubule; U, ureter; V, renal portal vein. (Modi-
fied f 10771 Holmes, " Biol 00 U of the Froy.")

and discharging below into the bladder. A copious blood supply is
furnished by l)ranches of the main artery and veiij. Inside the kidney
are typical uriniferous tubules. Their renal corpuscles are massed toward

DISPOSAL OF WASTES

137

the convex outer surface of the organ (Fig. 113). From there the course
of the tuliules is in general two convohited stretches, with a more or less
straight-limbed loop between them. The collecting tubules into which
they empty converge toward the branches of the ureter, in pyramid-
shaped groups. The ureters empty into the bladder, and this discharges
through the urethra.

Excretion by the Kidney. — The elimination of w^aste by the kidney
involves two general processes: (1) filtration of a great deal of liquid under
pressure from the blood in the glomerulus into the tubule at the renal
corpuscle, and (2) resorption of the greater part of this liquid by the
uriniferous tubules in the rest of their course. The liquid forced out
of the glomerulus, through the inner wall of Bowman's capsule into the

capsule

'A^\ illl'i, -^^cc^^^y^ coniaining

-URETHRA

Fig. 112. — Excretory system in man.

renal corpuscles

pyramid of medulla
with collecting
tubules

renal artery
renal vein
pelvis of kidney

ureler

Fig. 113. — Human kidney, bisected.
{From Storer, "Ge7ieral Zoology.")

tubule, consists of water, urea, glucose, amino acids, and the salts of
the blood plasma, in about the same proportion as these things exist in the
blood. The proteins of the blood, however, are not allowed to pass ; nor
are the other colloidal substances, such as the lipids, nor the blood cells.
These are all retained in the blood vessels. The amount of fluid thus
filtering into the tubule is about 1 per cent of the liquid of the blood
passing through the glomerulus.

Then the resorption of much of this material occurs as the liquid
passes along the tubule. The glucose in it is taken back into the blood
capillaries, unless there is already too much glucose in the blood. The
salts are also partially resorbed, not necessarily in equal fractions, but in
proportion to the need of them in the blood. Amino acids return in like
manner to the blood; so also does about 99 per cent of the water. What
remains in the tubule is therefore a rather concentrated solution of the
waste substances, mostly urea and uric acid. This liquid is the urine.
About 1500 cc. of it leaves the kidneys daily in an average adult person

138

PRINCIPLES OF ANIMAL BIOLOGY

under average conditions. Urine consists of about 96 per cent water, 2
per cent urea, 0.5 per cent uric acid, and 1.5 per cent inorganic salts.

A small amount of waste material may be added to the forming urine
in the tubules, by excretory action of the cells of the tubules; but this
addition is unimportant in relation to the amount filtering in at the
renal corpuscle.

The Skin as Excretory Organ. — Excretion in the skin is done by the
sweat glands, of which there are about two millions in man. These
glands are of the simple tubular type (page 84), the deeper portion of
the tube being closely coiled, while the outer part forms a duct which
empties on the surface. Around the coiled bottom is a network of

capillaries (Fig. 114). The amount of sweat excreted
varies greatly with the temperature and the amount
of muscular exertion; in mild weather and with
moderate or slight exercise, about 600 cc. may be
produced in a day, but five times that amount is not
uncommon in hot weather and with great exertion.

Sweat is much more dilute than urine, about 99
per cent of it being water. Of its solids, sodium
chloride is the most important. Urea is not very
abundant; at the minimum production of sweat (600
cc. per day) only about 1.5 per cent of the total
urea is lost through the skin in man. Other soluble
wastes, of the same kinds as are eliminated by the
kidneys, are found in the sweat, but in much smaller
amounts. Since the sweat evaporates as rapidly as
it is formed under ordinary conditions, these solids
dry on the surface of the skin. As is pointed out on
page 121, in connection with heat regulation, many
mammals have only a few sweat glands, or none at
all. In them the kidneys bear the whole burden of
The sweat glands even in man are not an important
Their chief service is regulation of temperature.

The liver shares in the excretion of urea.

Fig. 114.— Hu-
man skin, dissected
to show sweat gland.
At left, complete
gland, much coiled
at bottom. At
lower right, network
of capillaries from
the midst of which
the coiled portion of
another gland has
been removed.

urea elimination,
excretory device.

Other Means of Excretion,
since it helps convert protein wastes into urea. When proteins are
broken down, ammonium salts are among the products. These salts
are converted into urea partly in the liver, but the actual excretion is
elsewhere. The liver performs, however., a primary act of excretion in
the removal of the hemoglobin of worn-out red l)lo<)d corpuscles. The
bile pigments are produced from this hemoglobin and are eliminated with
the bile into the intestine, where they eventually pass out with the feces.
Cholesterol is another waste substance excreted by the liver and elimi-
nated into the intestine with the bile. '

DISPOSAL OF WASTES 139

Other glands producing liquid secretions have some chance of casting
out soluble wastes. Thus in the saliva there are traces of urea; but since
most of the saliva is retained within the body, the occurrence of urea in it
hardly amounts to excretion. Drugs injected into the veins can often
be tasted owing to a similar exci'etion of them in the salivary glands. The
wall of the large intestine is able to excrete small amounts of unusual
foreign substances occurring in the blood or of ordinary substances when
present in excessive amounts, as calcium and magnesium sometimes are.
These substances are removed from the intestine with the feces.

None of these other excretory organs is important as a substitute for
'the kidneys; not even all of them combined could take over the job
of the kidneys. Fortunately the kidneys have a wide margin of safety,
for a kidney and a half may be removed and the necessary excretion still
go on. There is no recovery, however, from overdestruction of kidney
tissue, for the renal tubules do not regenerate.

Some organisms, principally plants, excrete wastes by simply render-
ing them insoluble and then retaining them within or between the cells.
Insoluble substances can do no harm and, when they are not abundant,
are not greatly in the way. Among animals, sea urchins are said to
store insoluble excretions.

References

Baitsell, G. a. Human Biology. McGraw-Hill Book Company, Inc. (Chap. VI.)
Carlson, A. J., and V. Johnson. The Machinery of the Body. University of
Chicago Press.

CHAPTER 13
INTEGRATION OF ACTIVITIES

When many different operations are performed by the same machine,
it is essential that they bear some definite relation to one another. Living
organisms are subject to the same necessity. Their processes must dove-
tail into one another. When unusual exertion increases consumption of
energy and output of carbon dioxide, it would be disastrous were the
circulation not speeded up to provide oxygen and remove wastes. When
the circulation is accelerated, it would be inefficient not to hasten the
breathing movements to introduce more oxygen. In the digestive system
it would be wasteful to have saliva, bile, and other digestive fluids
secreted all the time, yet they must be produced when foods require
digestion. If in warm-blooded animals the temperature increases above
the most favorable point,* it is important that the sweat glands of the skin
or the breathing movement act to stop the rise. Even so simple an act as
walking involves so many muscles that cooperation among the several
units is necessary. The various organs cannot simply be wound up and,
clocklike, run at the same speed, thereby ensuring proper timing, for
many activities are carried on in response to external conditions and these
change at irregular intervals.

Some means of coordination is necessary. Animals in general have
contrived two devices — one nervous, the other chemical — to serve this
end. The former has assumed the larger burden, but both are essential.
While it has been necessary, in describing the action of the heart, the
respiratory movements, and the production of digestive fluids, to refer
to the controls which keep these processes in tune with the rest of the
organism and with the environment, it is desirable now to examine the
mechanisms of control more specifically.

Rise of the Nervous System. — The advantage or necessity of a
nervous system is attested by its very general presence in widely different
animals. Only a few groups are without it. It is made up of specialized
types of cells, whose arrangement in the body exhibits an increasing
complexity as other anatomical features become more complicated.
Animals which have simple systems of other kinds have, in general,
simple nervous systems.

The simplest form of nervous system is that of Hydra. The cells
which are specialized for conduction in this animal ha\e long, slender

140

INTEGRATION OF ACTIVITIES

141

projections, usually branching (Fig. 115) and joining one another to
form a network. The spread of these cells through the ectoderm is
fairly uniform, though they are slightly more abundant at the foot and
among the bases of the tentacles and around the mouth. Hydra's close
relatives, the jelly fishes, have a ring of nerve cells around the edge of
their cuplike bodies, with a loose network over the remainder.

Animals successively higher than the jellyfishes show a progressive
tendency to collect their nerve cells into masses or strands. In the
flatworms there is a mass of them, which may be called a ganglion, in
the anterior region (Fig. 116), and from this mass two long strands or
cords pass back on either side of the body. From both the ganglion

Fig. 115. — Nervous mechanism of Hydra. The long fibrils in the background are the
contractile parts of neuromuscular cells lying in the mesogloea. {From Schneider.)

and the cords slender threads called nerves extend to all parts of the
organism.

Invertebrate animals above the flatworms generally have two longi-
tudinal nerve cords, but these are usually joined into a single cord in
which the two components are still easily recognizable. In the earth-
worm (Fig. 116) these cords separate in the anterior region, pass upward
around the digestive tract in the form of a collar, and become enlarged
above the tract to form the bilobed brain. The rest of the double cord
in the earthworm is swollen into a moderate ganglion in each segment,
and from this ganglion two pairs of nerves emerge. The ganglia of the
main nerve cords are much larger in the crayfish (Fig. 116) and its allies,
with the larger ganglia located toward the front.

The tendency to mass the nerve tissue in a head region is carried
much farther in vertebrate animals. In them there is always a dis-
tinctly enlarged brain. In the frog it is moderately larger than the cord
behind it, which in the vertebrates is known as the spinal cord. The

142

PRINCIPLES OF ANIMAL BIOLOGY

relative size of the brain increases up through the vertebrate group,
reaching its maximum in man, whose brain includes more nerve tissue
than all the rest of his nervous system together.

There is thus a tendency, in the animal scale, for complexity in
general to be accompanied by a massing or centralization of the nerve
tissue, and to emphasize this massing in the head region. The sug-
gestion is near that somehow a concentrated system is better fitted to
serve as a mechanism of control of a complex body than is a diffuse
system. Additional reasons for reaching this conclusion will appear as
the arrangement of cells in the larger masses of the system are examined.

HYDRA

FLATWORM EARTHWOI?M CRAYFISH FROG MAN

Fig. 116. — Diagrams of nervous systems illustrating centralization and massing in the

head region.

The large masses of the nervous system, particularly the brain and
spinal cord, constitute the central nervous system. The position of the
central system in the body, and its structure, constitute fundamental
differences between vertebrate and invertebrate animals. In the inverte-
brates the nerve cord is below the digestive tract, in the vertebrates
above it. The cord is a double one (or there are two separate cords) in
the invertebrates, single in the vertebrates. Finally, the cords are
solid in invertebrates, hollow in vertebrates (resulting fi-om the system's
embryonic origin as a groove in the ectoderm which is pinched off below
as a tul)e).

Peripheral Nervous System. — The nerves which pass out fiom the
central system and branch to all parts of the organism are collectively
called the peripheral nervous system. Of the principal nerves, a number
(10 in amphibia, 12 in the higher animals) arise from the brain within
the cranium; these are called cranial nerves. From the spinal cord there

INTEGRATION OF ACTIVITIES

143

emerge, between the vertebrae, pairs of spinal nerves (31 of these in man).
Each of the spinal nerves arises from the cord by two roots, a dorsal
and a ventral, which join in a single nerve trunk a short distance from
the cord (Fig. 121). The dorsal root includes a ganglion which contains

a host of nerve-cell bodies. The
relation of the peripheral to the
central system in the frog is illus-
trated in Fig. 117.

A special part of the peripheral
system is known as the autonomic

T^\^JXandX

cer

Fig. 117 Fig. 118

Fig. 117. — Nervous system of frog, ventral view. I-X, cranial nerves; a, autonomic
system; cer, cerebrum; n, nasal sac; op, optic lobe; spc, spinal cord. (After Wiedersheim.)

Fig. 118. — Diagram of a typical neuron, ax, axon; d, dendrite; ms, medullary sheath;
mu, muscle; n, node; ne, nerve endings; nu, nucleus of cell of neurilemma.

nervous system because of its control, entirely free of the will, of many
vital functions. It consists visibly of a pair of ganglionated cords on
either side of the vertebral column, lying exposed in the body cavity.
The ganglia are connected with the spinal cord by nerve fibers passing
through the ventral roots of spinal nerves. In the extreme anterior and
posterior parts of the autonomic system, however, there are nerve fibers
which pass directly from the central nervous system to the organs con-
trolled without connections in centrally placed ganglia.

Unit of Structure of Nervous System. — The unit of structure of the
nervous system is the neuron. The neuron is a cell possessing a number

144

PRINCIPLES OF ANIMAL BIOLOGY

of fine projections which sometimes extend to great lengths. The cell is
compact in the embryo like most other cells, and the processes can be
seen to grow out from it, passing among other cells and dodging obstacles,
until they reach the organ to whose action they are to be related. These
projections are of two kinds, distinguished from one another not by struc-
ture but by their normal functioning. Those which normally conduct
impulses toward the body of the neuron are called dendrites; those which
convey impulses from the body of the neuron are axons. Figure 118
diagrammatically represents the parts of a typical neuron, and three
very different kinds of neurons are sho^vn in Fig. 119.

These cells, which are strictly speaking the only constituents of the
nervous system, are bound together by connective tissue, and the masses
thus formed are supplied with blood vessels.

Fig. 119. — Three kinds of nerve cells. A, from ventral horn of spinal cord of an ox;
B, from cortex of cerebrum of a cat; C, Purkinje cell from cerebellum of a cat; d, dendrite;
neu, axon; nu, nucleus; ntic, nucleolus. {B and C from Golgi preparations.)

Functional Unit. — In the operations of a nervous system, the func-
tional unit is a group of neurons called a reflex arc. These neurons are
so related to one another that, following a stimulus or excitation, they
induce some sort of action. One end of the arc is in some tissue or
organ capable of receiving a stimulus, the middle of it is in the central
nervous system or an associated ganglion, and the other end of the arc
is in a tissue or organ capable of responding, such as a muscle or gland.
The arc consists of at least two neurons-, usually more. Leading from
the sense organ is a nerve fiber (neuron) which, on lacing stimulated,
conducts an impulse toward the central nervous system. This neuron
is called an afferent fiber, the name meaning literally "bearing toward" —
that is, toward the central system. It is also appropriately called a
receptor neuron; very commonly, also, it is called a sensory neuron,
though the result of the impulse it carries is not always sensation. The
opposite end of the reflex arc consists of a neuron whose tip is applied

INTEGRATION OF ACTIVITIES

145

to a muscle, or gland, or some organ capable of responding to a stimulus.
This neuron carries the impulse away from the central nervous system,
hence is designated an efferent fiber. It is also called an effector neuron,
often a motor neuron though the action produced may be something else
than movement.

If the reflex arc consists only of an afferent and an efferent fiber, these
two neurons are in contact with one another by a minute surface known
as a synapse. The axon of the afferent touches a dendrite of the efferent,
and the surface of contact is the synapse. An arc of this simple two-
neuron type is represented above, at the right, in Fig. 120. The afferent

X ;

'ASSOCIATION

-SPINAL COED
■''0^' AFFERENT

■^^

::i^Ass

RECEPTOR SENSE
Oi3GAN

ASSOCIATION

■--EFFERENT

GLAND
> •

I <

Fig. 120. — Diagram of simple reflex arcs in the vertebrate nervous system.

neuron enters the spinal cord through the dorsal root of a spinal nerve,
in Avhose ganglion the body of the neuron lies. Within the spinal cord
the axon synapses with the dendrite of another cell whose body lies
within the cord. The axon of the latter cell passes out through the
ventral root of the spinal nerve, and its tip is applied to the responding
organ (muscle in the diagram).

Most reflex arcs consist of more than two neurons. The extra ones
are interpolated between the receptor and effector neurons. These con-
necting neurons are kno\vn as intermediate or association neurons. The
spinal cord is the seat of vast numbers of them. The association fibers
are especially useful in carrying the arc over considerable stretches of
the central system. In the lower right half of Fig. 120 is a reflex arc

146

PRINCIPLES OF ANIMAL BIOLOGY

whose afferent fiber enters the cord by one spinal nerve, while the efferent
fiber leaves it by way of the nerve next below. The lower level is
reached by an association neuron between the receptor and effector.
This same receptor is represented as connected also with an effector
neuron on the opposite side of the spinal cord. A second association
fiber establishes this connection. Some association neurons take the arc
through the brain, across a number of cells, and back down the spinal
cord again. Many arcs much more complicated than these exist. In
all cases the first neuron in the chain is an afferent, the last one an
efferent. All the contacts between any of the neurons are synapses,
axon touching dendrite.

The response to a stimulus carried over a reflex ai'c is called a reflex
action. Many of these actions are inherited. The vital organs in the
chest and abdomen are controlled by innate reflexes, as are also the con-

FiG. 121. — Chain of efferent neurons in human- autonomic system, in chest region, n, the
neurons; g, gangUon. Dotted lines represent neurons of ordinary spinal reflex arc.

traction and dilation of blood vessels and the action of sweat glands.
Other reflexes are learned — "conditioned" is the usual descriptive term
applied to them. Habitual movements of all sorts are conditioned
reflexes.

Functions of Autonomic System. — The reflexes for which the auto-
nomic nervous system is responsible are of such vital importance and
are related to one another in so remarkable a manner as to call for
separate description. Attention will be directed only to the efferent
fibers of the reflex arcs, because it is their control of the vital organs
with which we will be concerned. The system in man is the one used
for illustration.

The neurons of the autonomic system lack a myelin sheath. Between
the central system and the organ innervated there are always at least
two, and often only two, neurons. In the chest region the bod}^ of the
first neuron of such a chain is in the lateral part of the Il-shaped gray
matter of the cord (Fig. 121), and its axon passes out through the ventral
root of one of the spinal nerves. It leaves that root, however, close to
the cord and enters a special ganglion. Here the first neuron terminates,
its axon synapsing with the dendrite of the second neuron of the chain.

INTEGRATION OF ACTIVITIES

147

This second neuron may then join the mixed spinal nerve at the same
level of the cord, or pass up or down to nerves at other levels, in which
it goes out to the organ which it controls.

The autonomic system is divided functionally into two major regions.
One centers in the middle portion of the spinal cord (chest and small

Fig. 122. — Autonomic nervous system of man, in part, showing double innervation of
each organ and the action of each nerve. Organs on the left, iris of eye, rectum, and
bladder; on the right, heart, stomach, and small intestine. Small circles are ganglia.

of back) and may be called the thoracolumbar system. The other has its
center partly in the brain, partly in the lower end of the spinal cord, and is
called the craniosacral system (Fig. 122). The chain of neurons described
above belongs to the thoracolumbar. The ganglia of the craniosacral
system lie in general much farther from the spinal cord, sometimes
actually in the organ that is controlled.

148 PRINCIPLES OF ANIMAL BIOLOGY

Each organ governed by the autonomic system is innervated twice,
one nerve coming to it from the thoracolumbar system, one from the
craniosacral. One of these nerves is an activator, the other a depressor.
Each organ is thus accelerated by one of the major divisions of the
autonomic system, inhibited by the other; but neither division is exclu-
sively excitatory or wholly inhibitory, each division exciting some
organs, depressing others. The thoracolumbar system accelerates the
heart but inhibits movement of stomach and intestine. The iris of the
eye is constricted by the craniosacral, dilated by the thoracolumbar.

The excitation or inhibition is apparently accomplished by producing
a chemical substance, and the organ responds to this substance. Accord-
ing to current theory, all the nerves belonging to the craniosacral system
produce the same substance, which is probably acetylcholine. In like
manner, the thoracolumbar nerves produce one substance which has
been called sympathin. Acetylcholine inhibits the heart, increases
stomach movement and secretion, contracts the rectum and urinary
bladder, dilates the vessels of the salivary glands, and constricts the iris
of the eye. Sympathin produces the opposite reaction in each of these
organs.

Nerve Impulse. — The impulse which is carried along a neuron like
that in Fig. 118 travels at a speed of about 120 meters per second in mam-
mals, about one-fourth of that velocity in a frog. The rate is in some way
related to the presence or absence of a sheath around the branches of the
cell, and to the structure of that sheath if one is present. The axon
of the cell in Fig. 118 is surrounded by a white layer of noncellular fatty
substance known as the myelin (medullary) sheath, which is divided into
segments by irregularly placed nodes. Not all neurons possess such a
sheath. Those of the autonomic system do not, and in them the impulse
travels much more slowly — only 10 or 12 meters per second. Among
myelinated nerve fibers, those with the longer segments of myelin between
nodes conduct, in general, more rapidly than those with short segments of
the sheath. There is some reason from experiment to believe that the
impulse jumps from node to node; the longer the segments between nodes,
therefore, the faster the impulse travels.

According to present view, the nerve impulse is a surface phenomenon.
The membrane of a nerve fiber — not the cellular covering or neurilemma
and not the myelin sheath, but the outer film of the nerve cell itself — is
charged positively on the outside, negatively on the inside. The charges
are really borne by ions, which are located on opposite sides of the some-
what impermeable membrane. This membrane keeps them apart and so
prevents them from neutralizing one another (Fig. 123). The impermea-
bility prevents neutralizing, and the separation of the ions in turn is
supposed to hel]) keep up the impermeability. If, now, something (a

INTEGRATION OF ACTIVITIES

149

stimulus of some sort) destroys the impermeability of the membrane at
one point the polarization there is lost; the ions get together and neutralize
one another. Such neutralization could then proceed to adjoining parts
of the nerve fiber as rapidly as the impermeability is lost. No material
thing moves along the nerve, but a wave of neutralization and permea-
bility proceeds at considerable speed.

Waves of some sort pass over other organs, as over the heart when it
contracts, over skeletal muscle, and over glands. It seems likely that

+ +

+ + + + 'f ++ + + + + +

FiG. 123. — Propagation of nerve impulse, a wave of permeability associated with neutraliza-
tion of positive and negative ions. Dotted lines, permeable membrane.

in all these structures essentially the same changes in polarization of
surface membranes are taking place.

Initiation of and Response to Nerve Impulses. — Though the impulses
carried by all nerves are the same, no matter where they begin or end,
the things that start them and the actions they induce are quite different.
The impulse is initiated by a receptor of some kind, that is, a specialized
nerve ending which is exceptionally sensitive to some one sort of stimulus.
In the retina of the eye the receptors (rods and cones) are sensitive to

Fig. 124. — Various receptors: left to right, rod and cone of retina of eye, taste bud of tongue,
olfactory cells of nasal lining, and cold, touch, and pain endings in skin.

light, the taste buds of the tongue and the olfactory cells in the lining of
the nose (Fig. 124) are sensitive to chemical substances. Certain nerve
endings in the skin are sensitive to cold, others to touch, still others lead
to pain; the several kinds are structurally different from one another.
These receptors are not interchangeable, each does its own work, no other.
If a cold spot on the hand is stimulated in some other way than by low
temperature — mechanically, for example — the sensation is still that of
coldness.

150 PRINCIPLES OF ANIMAL BIOLOGY

The response which a nerve impulse eUcits depends on the nature of
the structure to which it is dehvered. An impulse delivered to a motor
unit in a muscle causes contraction; an exactly identical impulse carried to
a gland causes secretion. It is probable that in each instance a chemical
substance is produced at the nerve ending, and that it is this substance
rather than the nerve impulse itself which really stimulates the responding
organ. At least that is true of responses of some of the internal or
visceral organs.

An impulse from one of the sensory endings in the skin leads to a cer-
tain center in the brain, and the appropriate sensation is there produced.
The nerve fibers from the retina go to one region of the brain, neurons
from the olfactory area in the nose go to another, fibers from the pain
endings in the skin lead to a third. These regions of the brain are indi-
cated more fully later; the important point nov: is that for each activity
there is a special kind of receptor, located at a particular place or places,
and a certain organ or region of the nervous system where the appropriate
response is given. The nerve impulse which goes from the place of
stimulation to the place of response is everywhere the same.

Direction of Impulse. — When a neuron is stimulated at its receptor
ending, the impulse thus started travels toward the other end; there is no
place else to go. Experimentally, however, and sometimes in special
situations naturally, a neuron may be stimulated in the middle of the
length of its axon or dendrite. When this happens, impulses travel in
both directions to the limits of the neuron itself; but in one of the direc-
tions it goes no farther than the end of that particular neuron. The
difference lies in the synapses at the ends . of the axon and dendrite.
Each synapse is a one-way conductor. An impulse can go over it
from axon to dendrite but never from dendrite to axon. This is the
reason why nerve impulses alwa3\s go in one direction over such a chain
of neurons. As stated above, when a neuron is stimulated somewhere in
its middle, the impulse moves in both directions from that point to both
ends of that neuron. In the "forward" direction, arriving at the termi-
nus of the axon, it goes over to the dendrite of the next neuron and
continues the propagation, Ijocause the synapse there permits passage in
that direction. But in the "backward" direction the impulse is blocked
when it reaches the tip of the dendrite because the synapse will not carry
it over to the adjoining axon.

What gives the synapse this power of distinguishing direction? While
the answer to this question is not certainly kno\\'n, a possibility is sug-
gested l)y what is kno^vn of responses to stimuli in general. We are
familiar with the control of su(!h organs as the heart by a double innerva-
tion, one nerve acting to stimulate, the other nerve to inhibit. Each
nei've pi-esumably produces a chemical substance to which the oigan

INTEGRATION OF ACTIVITIES

161

directly responds. It is not unlikely that an impulse arriving at a
synapse from an axon produces an activating substance so that the wave
is initiated anew in the adjoining dendrite, while an impulse going back-
ward over a dendrite to a synapse produces an inhibiting substance so
that further propagation is prevented.

Fig. 125. — Functional areas of human cerebrum. Above, lateral surface from left
side. Below, median surface viewed from left. The olfactory area, because it is dis-
continuous, is dotted. All boundaries are only approximate.

Localization in Brain. — It is more difficult to ascertain the function of
different parts of the brain than to determine the role of nerves, because
those parts cannot be isolated and experimented upon wholly separately.
Knowledge of the regions where different brain functions are performed
comes from destruction of certain areas in laboratory animals, artificial
stimulation of brain areas in anesthetized animals, the consequences of

152 PRINCIPLES OF ANIMAL BIOLOGY

lesions due to accident or disease in man, and, recently, the study of
"action potentials," which mark the path of nerve impulses from the
point of stimulation to their center. The latter method is particularly
useful in locating functional areas in the cerebrum. To understand
what follows, it is necessary to know the general structure of the brain.

As it originates in the embryo, the central nervous system is a tube,
wider in front where the brain develops (page 208), narrower behind
in the spinal cord. The brain tube enlarges moderately in three regions
known as the fore-, mid-, and hindbrain. This tubular structure remains
in the adult as the "brain stem," but the forebrain expands enormously
upward, laterally, and backward, to form the cerebrum (divided into two
hemispheres), while the hindbrain develops the cerebellum. Behind the
latter is the medulla oblongata, which is usually counted a part of the
brain but is really the somewhat enlarged anterior end of the spinal cord.

The cerebrum has a gray surface layer, the cortex — gray because
of the cell bodies which it contains — which in man and the mammals
generally is greatly increased in extent by folds and furrows. It is
the cortex which has been the subject of much of the localization study,
because it is the seat of those psychic qualities which tend to distinguish
man from the beasts. By the methods outlined above, the functions of
various parts of the cerebral cortex have been found to be roughly as
portrayed in Fig. 125. The best established of the areas there shown
are the motor area and the area of skin sensation which together form a
transverse band halfway between the front and rear, the areas for hearing
at the sides, and that for vision at the extreme posterior part. The rest
of the cerebrum is largely given over to what may be termed associations,
some of the particular forms of which are indicated in the illustration.
The association areas deal with integration of individual sensations into
a whole. The cortex is not responsible for pain except to localize it,
and it is not concerned with any viscer^il sensations such as hunger and
thirst. Pain is a function of the thalamus, in the stem region of the
forel)rain.

The cerebellum serves to coordinate muscular actions. Destruction
of it results in irregular, jerky, fumbling, or reeling movement, or in
thick slurred speech. The middle portion influences muscles of the
trunk, neck, and head; each side of the cerebellum acts on muscles of the
same side of the body, but there is not much other known localization.

The more important functions of the medulla in controlling the heart
and digestive canal, the contraction and dilation of blood vessels, and the
movements in breathing have already been described in this and earlier
chapters.

Chemical Regulation. — The control of vital actions by the medulla
is exercised partly at the behest of accumulated carbon dioxide. It has

INTEGRATION OF ACTIVITIES

153

PINEAL
PITUITARY

THYROID
PARATHYROIDS

■THYMUS

been necessary in earlier chapters to point out some of the initiatory-
actions of this substance which may be here recalled. Increased con-
centration of carbon dioxide in the blood causes centers in the medulla
to increase breathing movements and to contract the blood vessels.
Here the effect is produced through the nervous system. Sometimes
carbon dioxide may act directly, without mediation of nerves, as when
it stimulates stronger heartbeat by direct action on the sinus node,
and almost directly when, perhaps by increasing acidity, it locally
causes dilation of blood vessels. There is thus an important chemical
regulation of muscle action, partly through, partly independent of, the
nervous system. Coagulation of the blood is also initiated by chemical
substances liberated from disintegrating platelets and injured tissue cells,
in conjunction with certain substances in the blood plasma. There are
some physical agents, also, which exercise regulatory control either
directly or through the nervous system.
Thus slightly higher temperature of the
blood, warming the thalamus of the fore-
brain, starts activity of the sweat glands,
which lowers the temperature; and
higher blood pressure in the great
arteries, acting through nerves, slows
down the heartbeat. And, finally,
greater warmth of the blood, influencing
the sinus node directly, not through
nerves, accelerates heart action. All
these influences have been discussed
before.

Besides these chemical and physical
agents, which are all part and parcel of
the general physiological mechanism of
the higher animals and which mostly
serve other ends besides regulation,
there is a group of chemical substances
which have no other known function
than to exercise control over something.
These substances are know^n as hor-
mones. In general they are produced at
one place, but stimulate action at another, to which they have been
carried by the blood. One of the earliest of these substances to be
discovered was secretin, whose action in stimulating the pancreas and
liver has been described (page 104).

While it is possible that most tissues produce substances that have
some influence elsewhere, the marked and well-known instances of

•ADRENALS

PANCREAS

OVARIES
(in female)

-TESTES
(in mole)

Fig. 126. — Location of endocrine
glands in human body. Dotted lines
represent kidneys (above) and ovi-
ducts and uterus (below) to show
positions of glands.

154 PRINCIPLES OF ANIMAL BIOLOGY

hormone action are those exhibited by certain definite glands. These
gland.s do not have ducts, or, if they do, the hormone is not ejected
through the duct. All hormones diffuse directly into the blood. Such
ductless glands are known as endocrine glands, and the hormones are also
called endocrine secretions. The best-known hormonal actions are
those of man, so the account here given must draw heavily upon the facts
ascertained for human endocrine glands. The names and locations of
most of those which are known or believed to be endocrine are shown in
Fig. 12G.

Endocrine Glands and Their Work. — One qf the best-known hor-
mones is that of the thyroid gland, a bilobed structure lying beneath and
beside the trachea in the neck in man. Its hormone, called thyroxin,
has been isolated and has the formula C15H11O4NI4. The direct effect
of thyroxin is to increase the rate of metabolism. Deficiency of this
hormone in children or young animals retards their development. If
this influence starts early enough it leads to cretinism, in which body and
limbs are dwarfed and distorted, and mental development is arrested.
Some regions of the Avorld have little iodine in the soil, hence little in
crops, and the inhabitants are finable to produce adequate thyroxin,
which includes that element. Cretins were common in such regions
until public health measures, such as the requirement that potassium
iodide (KI) be added to table salt, were adopted. Deficiency of thyroxin
in adults often causes endemic goiter, a swollen condition of the thyroid
caused by an accumulation of a colloid fluid in the capsules of the gland.
A more serious effect of deficient thyroid is myxedema, with its low
metabolism, a state of lethargy, and puffed skin. Excessive thyroxin
commonly causes exophthalmic goiter, with its increased metabolism,
high blood pressure, and protruding eyeballs; removal of part of the
thyroid, the proportion depending on how much the metabolic rate has
been raised, is one of the cures.

Closely associated with the thyroid (imbedded in it in man) are the
parathyroids. There are four of these bean-shaped bodies in the human
thyroid. Separate experimentation with them has been hindered by
their position. Their primary effect is upon calcium and phosphorus
metabolism, and the calcium deposit in bones is reduced when the para-
thyroids are deficient. Complete removal of the glands causes violent
muscular convulsions.

The adrenal glands rest on the kidneys (above them in man). They
consist of a central part or medulla, which arises in the embryo as an
outgiowth of the nervous S3^stem, and an outer part or cortex, which
(;omes from the lining of the coolom. The two parts produce different
hormones, that from the cortex being the more critically important.
About one-fifth of the cortex suffices for normal processes, but if the whole

INTEGRATION OF ACTIVITIES 155

cortex is removed pro»stration and death soon follow. Deficiency of its
hormone interferes with carbohydrate metabolism, and the blood loses
most of its glucose. Sodium chloride is also lost from the plasma, the
osmotic properties of the blood are changed, and so the volume of blood
is diminished and blood pressure falls. Development of reproductive
cells is also stopped, and Addison's disease is partly caused by a cortical
defect. Many substances have been extracted from the cortex, the
potent ones all being chemically related to one another. The name
cortin has been given to the active principle, but it has not been
identified or isolated.

The adrenal medulla produces the well-known adrenalin (C9H13O3N).
This hormone has been synthesized artificially. Its effect is to strengthen
and accelerate heartbeat, increase the glucose in the blood, whiten the
skin, dilate the pupils of the eyes, and erect the hair. In general its
action is the same as that of the thoracolumbar part of the autonomic
nervous system. One theory of adrenalin is that it is a stand-by for
emergencies. By its control of glucose in the blood, it has been supposed
to increase muscular power and resist fatigue. In fear and rage and great
excitement, adrenalin is increased, and the body is supposed to be able
to perform feats under such emotions which it could not normally do.

The pancreas, though a digestive gland whose digestive secretion
flows through a duct, also produces a secretion which must diffuse out
to the blood. This secretion is called insulin. It is produced in certain
groups of cells, the islands of Langerhans, which in the embryo were
budded off from the digestive tubules but which lose all connection with
the duct. The function of insulin is to control sugar metabolism.
Failure of the supply of this hormone causes the disease known as
diabetes mellitus, excess of sugar in the blood and hence its presence
in the urine. The disease may be relieved by administering insulin
extracted from other animals, but it has to be injected into the blood
vessels, not taken by mouth, for insulin is destroyed by the digestive
enzymes. Also, its ^effect lasts only a few hours, hence it must be used
frequently.

The pituitary gland, at the base of the brain, consists of two parts.
The anterior lobe is derived in the embryo from the roof of the pharynx,
the posterior lobe from the floor of the brain. The connection with the
pharynx is lost in the adult, but that with the brain persists. The ante-
rior lobe produces a variety of hormones, one affecting growth, several
affecting the sex organs, others acting on the thyroid, adrenal cortex, and
mammary glands. Because of this multiple activity, particularly in
control of other endocrine glands, the anterior pituitary is sometimes
spoken of as the "master gland." The growth hormone was first isolated
in 1944 as a pure protein. Oversuppiy of this hormone produces giants —

156 PRINCIPLES OF ANIMAL BIOLOGY

8- or 9-foot stature with disproportionately long limbs. Too little of it
produces midgets, with disproportionately short limbs. The hormones
related to the sex organs and mammary glands are to be described in a
succeeding section. The hormones affecting the thyroid and adrenal
cortex have not been isolated ; but in an animal whose pituitary has been
removed these glands experience degenerative changes; and when
additional pituitary extract is injected, the thyroid and adrenal cortex
are enlarged.

The posterior lobe of the pituitary produces at least two substances,
one of which stimulates contraction of the uterus in the reproductive
system, the other constricts the smaller arteries and so raises blood pres-
sure. Neither of these substances has been isolated. Injury to the
posterior lobe also deranges the uriniferous tubules of the kidneys, so
that they no longer resorb the great quantities of water from the filtrate
entering through Bowman's capsule. A large volume of dilute urine is
produced under these circumstances.

The primary reproductive organs, ovaries and testes, produce hor-
mones which are responsible for the development of the secondary sex
characters, such as the beard and baritone voice in man, long tail feathers
in cocks, and the contrasted features of the females. They also govern
the mating behavior, and determine parental instincts. The principal
hormone in the male is testosterone (C19H30O2), isolated as a crystalline
compound. It is produced by the interstitial cells of the testis, not by the
germ cells nor the tubules which produce germ cells. The corresponding
hormone of the ovary (sometimes called estrogen though the name has
varied) is produced by the follicles, blastulalike spheres of cells surround-
ing the mature eggs.

Other hormones may be produced by the pineal body above the
brain, which regulates the speed of sexual development, and the thyrnus
in the upper part of the chest, which is in some way related to sex develop-
ment and appears to control the production of the hard shell on birtl
eggs. Both of these organs are present in children, but the former degen-
erates into a fibrous structure and the latter disappears in youth.

Reproductive Cycle. — The influence of the pituitary on other endo-
crine glands, mentioned al)Ove, hints at interrelations much more exten-
sive. Presumably not all the interrelations between the glands are
known, but one group of them has received considerable attention because
of its bearing upon medical practice. This is the group of glands and
other secreting structures which control the reproductive cycle in female
mammals.

These females show a rhythmical change in their behavior, in that
periods of sexual excitement occur at regular intervals, separated by
periods of apathy. This rhythm of behavior depends on an alterna-

INTEGRATION OF ACTIVITIES 157

tion of production and disappearance of certain hormones; to understand
these, it is necessary to know the operations of the female reproductive
system. The following account is limited to the mammals.

The female reproductive cells, in different stages, are contained
in the ovary. Each cell is surrounded by liquid enclosed in a layer
of cells known as the follicle. The cells (one or more at a time) ripen
with considerable regularity, every 5 days in the rat, each 28 days in
man, tmce a year in the dog. In the maturing of a cell the follicle grows
and approaches the surface of the ovary (Fig. 127). The follicle is there
ruptured, and the egg escapes into the open end of the oviduct. The cells
of the broken follicle become converted into a yellowish mass called the
corpus luteum, while the egg moves down the oviduct. If the animal
has mated, spermatozoa may have moved through the uterus and into
the oviducts, and the egg may be fertilized there. If it is not fertilized,

Fig. 127. — Human ovary to show follicles and corpora lutea. At left, surface view,
with two follicles of different ages protruding. At right, section showing cla, two degener-
ating corpora lutea of different ages; civ, fresh corpus luteum;/, follicles; o, ovum.

the egg disintegrates or passes out to the exterior. If it is fertilized, it
sinks later into the wall of the uterus and proceeds to form an embryo.
To receive the fertilized egg, the wall of the uterus must become thickened,
glandular, and supplied with an extra amount of blood. This prepara-
tion is all wasted if the egg is not fertilized, for then the uterine wall
recedes to its "resting" condition. The corpus luteum degenerates
(in about 2 weeks in man) if the egg is not implanted in the uterus but
continues throughout pregnancy if implantation occurs.

What governs all these events, to ensure that they occur in the proper
relation to one another? In general, it is an interplay of hormones from
the reproductive organs and the pituitary gland, one gland stimulating
the other and then being inhibited when its product increases to a certain
concentration. The pituitary, by means of a hormone, stimulates the
growth of the egg follicle; the follicle then produces a hormone which
induces the thickening of the uterus just described. When the follicle
is ruptured, its hormone is no longer produced, but another hormone is
produced by its successor, the corpus luteum, which continues the
preparation of the uterus. No other follicle is growing in the meantime,
for the hormones of the follicle and corpus luteum inhibit the pituitary, so

158 PRINCIPLES OF ANIMAL BIOLOGY

that no follicle-stimulating hormone is forthcoming. If the egg is not
implanted in the uterine wall, the corpus luteum degenerates, and its
hormone is no longer produced. The thickening of the uterus conse-
quently disappears, and the pituitary is relieved of its inhibition. The
latter gland therefore begins to produce its follicle-stimulating hormone,
and the cycle is started all over again.

Why the corpus luteum persists if the egg is implanted is not entirely
clear, but its hormone is essential to the continued development of the
embryo, and the pituitary gland is in some way responsible for its
persistence. Some have supposed that a hormone from the placenta
guides the pituitary in this particular function, but this is not established.
Increase in the size of the mammary glands during pregnancy, with
their secretion of milk at birth, is also caused by a hormone of the pitui-
tary, but the persistent corpus luteum seems to be the mentor of the
pituitary in this control.

The cycle in other vertebrate animals is likewise controlled by hor-
mones, but, since their young are developed outside the mother's body
and are not nourished with milk after birth, much of the complexity of
the reproductive rhythm is wanting in them. In the amphibia, the repro-
ductive cycle is an annual one. Eggs ripen during the winter and are
laid in early spring. During the summer the ovaries are small flabby
organs, in which the oocytes gradually increase in size into the fall,
but normally none is liberated until the next spring. If, however, an
extract of the anterior lobe of the pituitary gland is injected into one of
these animals in the fall, eggs are released from the ovary in three or four
days.

References

Carlson, A. J., and V. Johnson. The Machinery of the Body. The University of
Chicago Press. (Pp. 360-533.)

Corner, G. W. The Hormones in Human Reproduction. Princeton University
Press.

Mitchell, P. H. A Textbook of General Physiology. 2d Ed. McGraw-Hill Book
Company, Inc. (Chap. IV, reflexes; Chap. V, correlating action of nervous sys-
tem; Chap. VI, receptors.)

Rogers, C. G. A Textbook of Comparative Physiology. McGraw-Hill Book Com-
pany, Inc. ' (Includes many invertebrates: Chap. XXVIII, nervous system;
Chap. XXV, hormones.)

Sherrington, C. S. The Integrative Action of the Nervous System. Charles
Scribner's Sons. (Chap. I, simple reflexes.)

CHAPTER 14
REPRODUCTION

A new animal or plant comes into existence only by the transfor-
mation of some part of a previously existing organism. While repro-
duction must have been understood for man and his domesticated animals
from time immemorial, it is not so long since it was popularly believed
thei-e were other ways whereby new individuals could arise. Among the
ancient Greeks it was common belief that leaves could be converted into
fish or birds, mud into frogs, dead flesh into bees. In the Middle Ages
barnacles were thought to be transmuted fruit of a tree, and to give rise
in turn to geese. As these notions were abandoned, the idea was trans-
ferred to the smaller organisms which improved microscopes were
revealing. It was only comparatively recently that the view that
bacteria arose de novo from nonliving matter was given up. The sup-
posed origin of living things from nonliving matter was called abiogenesis
or spontaneous generation. While in the evolution of life there must
once have been a beginning of the living out of the lifeless, it is not
likely that such changes are happening now. Certainly there is no pro-
duction, from nonliving substance, of new individuals belonging to
recognized present-day species of animals or plants.

Increase in numbers of individuals, or replacement of losses, is pro-
vided for by a variety of reproductive methods which fall into two
general categories, namely, sexual and asexual reproduction. Sexual
reproduction as a rule involves two parents and the union of two germ
cells, or of two cells of some kind, or of two nuclei of different cells.
Asexual or nonsexual reproduction includes all forms of reproduction
not involving germ cells or any of the unions just named.

Sexual Reproduction. — Sexual reproduction is a well-nigh universal
method of reproduction. It is employed by representatives of every
great group of animals and by many of them to the exclusion of the
asexual method. It is also used by the plants, except the bacteria.

In one of its very common forms, sexual reproduction is the union of
two cells to form a single cell, the zygote, which by its subsequent divisions
produces a new individual (in the metazoa) or a new series of individuals
(in the protozoa). Not all cells are capable of uniting in this way, and
cells which are capable of this act are called gametes. Certain gametes
are relatively large, contain a considerable amount of nutritive material,

159

160

PRINCIPLES OF ANIMAL BIOLOGY

and are nonmotile; these are called ova (singular, ovum), or eggs. Other
gametes are minute, often a very small fraction of the size of the ova of
the same species. These are poorly supplied with nutritive material,
have a very small cytosome, and usually are motile; they are kno^\^l as
spermatozoa (singular, spermatozoon). The individuals in which eggs
develop are females, and those in which spermatozoa develop are males.
Sexual Reproduction in Metazoa. — In metazoa the germ cells (ova
and spermatozoa) are the only cells which retain the power of uniting
to initiate the development of a new metazoan individual. All other
cells have completely lost this power. As the time for sexual repro-
duction draws near, the germ cells undergo a certain process of develop-

A B

Fig. 128. — Sperm cell and ovum. A, spermatozoon of rabbit; B, fertilized ovum of
Nereis with two polar bodies, ph. (B from Wilson, " The Cell.")

ment or of preparation for the sexual act. This preparatory process is
described in detail in Chap. 16, but its essentials may be stated here.
In the ovum it consists in the main of two cell divisions by which three
or four cells are produced. Of these cells one is much larger than the
others, and its nucleus has one-half the usual number of chromosomes.
The small cells are called polar bodies and are nonfunctional. In the
sperm cell the process does not differ essentially from that in the ovum,
except that it results regularly in the formation of four relatively small
cells of about equal size, all of which are usually functional. Like the
eggs they have half the usual number of chromosomes. The male germ
cells must then be transformed, by a striking change of shape, into
spermatozoa. A sperm cell and an ovum with polar bodies are illus-
trated in Fig. 128.

When mature spermatozoa and eggs of the same or closely related
species are brought together, the actively motile spermatozoa meet and

REPRODUCTION

161

penetrate the eggs. Usually but one sperm cell can enter an egg. After
its entrance other spermatozoa are excluded, either by a change in the
surface of the egg or by some other mechanism. The spermatozoan
nucleus and egg nucleus arrange themselves
side by side; and, as the zygote begins to
divide in development, the chromosomes
of the two nuclei mingle in such a way that
their separate sources are as a rule com-
pletely obscured. A new cell has arisen
from two cells, and out of it comes a new
individual derived from two parents.

Sexual Reproduction in Protozoa. — In
some of the protozoa, sexual reproduction
involves union between two cells that are
alike, which are accordingly known as iso-
gametes (Fig. 129). In other unicellular
organisms the cells that unite are neces-
sarily of different kinds and are then

known as anisogametes. In Eudorina elegans the difference is one of
size; fusion is always between a large cell and a small one (Fig. 130).
These might at first seem comparable to the egg and spermatozoon of

Fig. 129. — Isogamy in Heteromita
lens. {After Kent.)

Fig. 130. — Reproduction in Eudorina elegans Ehrenberg. A, adult colony X 475; B,
daughter colony produced by division of one of the cells of such a colony as in yl, X 730;
C-E, development of spermatozoa from a mother cell; F, separate spermatozoa. {From
West after Goelel.)

metazoa, but both the large and the small gametes in Eudorina have
flagella and are therefore motile. In Volvox and Pleodorina there arc
likewise differences in size, and the large cells are nonmotile. Still, the
parallel between these large cells and the eggs of metazoa is not complete.

162

PRINCIPLES OF ANIMAL BIOLOGY

because in Volvox and Pleodorina the reduction in the number of chro-
mosomes occurs, not just before the cells are ready for reproduction, but
a long time earlier. Indeed, all the cells of these organisms have the half
number of chromosomes; only the zygote from which they spring has
the full number. In Pandorina morum (Fig. L31) there is a curious com-
bination of isogamy and anisogamy; it has reproductive cells of two sizes,
and union may occur between two small ones or between a large and a
small one, but not between two large ones.

In the foregoing examples, union of gametes is a fusion of whole
cells. In some of the ciliated protozoa, however, it is only the nuclei
of the cells which fuse. In the species in which this occurs, there are

Fig. 1.31. — Reproduction in Pandorina morum Borg. A, vegetative colony; B, asexual
reproduetion; C, gametes (y) ; D-E, union of gametes to form zygote {z) ; F-H, development
of zygote. {From West after Pringsheim.)

two nuclei in each individual, a large one called a macronucleus and a
small one, or micronucleus. Only the micronuclei are involved in the
union, and it is justifiable to regard these nuclei, rather than the whole
cells, as the gametes. To effect this union, the cells must come together
temporarily and make an exchange of nuclei. Temporary union of
two protozoan individuals for exchange of nuclei is called conjugation.
Since the process is rather complicated, it is best illustrated by a specific
example, for which Paramecium is selected.

At the time of conjugation (Fig. 132.1) two individual paramecia
come together with their oral surfaces in contact. They are held in
this position for a time because of the sti(;kiness of the protoplasm
on those siu'faces. While they continue to swim about, internal changes
in the micronucleus and macronucleus of each individual take place.
The micronucleus of each Paramecium divides by mitosis {B, C, D),

REPRODUCTION

163

and then each half divides again. Thus each micronucleiis gives rise to
four micronuclei (Fig. 132E'). Of these micronuclei, three undergo
degeneration, and the one remaining in each Paramecium divides again
into two parts, usually of unequal size (F). The smaller micronucleus
of each individual now passes over into the other individual {G), while

FIRST MATURATION DIVISION OF MICRONUCLEUS

SECOND AND THIRD
DIVISION OF MICRONUCLEUS

THREE SOMATIC DIVISIONS OF rERTILlZE.D NUCLEUS

FERTIUZATIOM

TWO CONSECUTIVE DIVISIONS
GIVING FOUR NORMAL CELLS

Fig. 132. — Diagram of the process of conjugation in Paramecium. The reference to
maturation in the figure will be clear only after a perusal of the section on oogenesis in
Chap. 16. {From Calkins, "Biology of the Protozoa.")

the larger one is retained. The two pieces, one derived from each indi-
vidual, now fuse to make the fusion micronucleus {H). During these
stages of the process the macronucleus has been undergoing fragmentation
and sooner or later its parts degenerate completely. Soon after the
exchange of micronuclei the individuals separate and the process of

164

PRINCIPLES OF ANIMAL BIOLOGY

conjugation itself is completed. Fusion of the micronuclei, however,
initiates a series of changes covering a long period. These processes in
one of the exconjugants are essentially as follows. The fusion micro-
nucleus divides three times (Fig. 132/-M), resulting in the formation
of eight micronuclei. Of these, four enlarge and become macronuclei,
while the other four remain micronuclei. The exconjugant then divides
twice (N-P), each new individual receiving one micronucleus and one
macronucleus. After a growth period each cell divides by fission (page
1G9) in the ordinary manner and at intervals of 16 to 24 hours thereafter
for a considerable period, when again conjugation usually occurs. The
part of this process which corresponds to fertilization is the exchange of
micronuclei and the formation of a new nucleus from the two parts.

■*» ^•

■ft **'

". ^

i3»

Fig. 133. — Conjugating strains of paramecia: at left, single strain, no conjugation;
middle, two strains mixed ; right, clumps sorted out, mostly into pairs. {From Wichterman
in Turtox News.)

The repeated divisions of the cells following conjugation arc to be likened
to segmentation of the fertilized ovum of the metazoa.

A most interesting fact is that there are diffei'ent strains of Para-
mecium, so organized physiologically that members of the same strain
will not conjugate with one another, but all of them will conjugate with
those of certain other strains. When members of two strains which
will conjugate are mixed, they first form large clumps (Fig. 133). These
aggregations slowly disintegrate and after a few hours are sorted out,
mostly into pairs. Some biologists have been tempted to regard this
distinction between .strains as sex, despite the difficulty of deciding
which of two conjugating strains is female, which male. Since each
member of a pair receives a micronucleus from the other, they would
seem rather to be hermaphrodites (see page 166).

Parthenogenesis. — In an earlier paragraph it was said that sexual
reproduction usually involves two parents and the fusion of two germ

REPRODUCTION 165

cells. It is not uncommon, however, to find species of invertebrates
among which, for considerable periods of time, no males can be found.
The females produce eggs which develop into new individuals like the
parent, although fertilization by spermatozoa does not occur, since no
males are present. By their origin and division and nuclear changes
the cells giving rise to new individuals are ova; hence the method is
regarded as a sexual one. Development of an egg without fertilization
is known as parthenogenesis. There are many animals which employ
parthenogenesis. Some insects which do so are the plant lice, or aphids,
and many ants, bees, and wasps. The method has also been observed
in a few moths, a few of the scale insects, and commonly among the
flower-inhabiting insects known as thrips.

The females of many parthenogenetic species produce, for a number
of generations, only females. At intervals, frequently in the fall, males
are also produced which fertilize the eggs. These zygotes usually differ
from the unfertilized eggs in being provided with hard shells and in being
resistant to the rigors of a winter season. The fertilized eggs hatch in
the spring into parthenogenetic females which repeat the cycle as out-
lined. Many species of aphids and of the lower Crustacea have cycles
of this type. In certain insects the bisexual reproductive phase is
apparently entirely omitted, and reproduction is exclusively partheno-
genetic. Thus the black flower thrips Anthothrips niger, the brown
chrysanthemum aphid Macrosiphum sanhorni, many species of scale
insects, and some gall-producing and parasitic insects never produce
males. In the ants, bees, and wasps, both males and females are usually
produced. The female lays both fertilized and unfertilized eggs, in some
way controlling fertilization of the eggs by the release or retention of
spermatozoa stored in the seminal receptacles. Among bees the males
(drones) are derived from unfertilized eggs, the females (queens and
workers) from fertilized eggs.

Fertilization, where it occurs, has a dual function, that of (1) stimu-
lating the egg to develop, and (2) introducing the hereditary properties
of the male parent. In parthenogenesis there is only one parent; hence
no paternal qualities can be transmitted, and the eggs are able for
some reason to start development without any stimulus from a
spermatozoon.

Parthenogenetic development has been induced in the eggs of a
number of animals which ordinarily require fertilization. The methods
have been various. Bathing the eggs with weak solutions of chemical
substances, shaking them vigorously in a bottle, heating them, or pricking
them with a fine needle, all have started division in certain eggs. Most
of the individual animals whose development was started in this arti-
ficial way have died in early stages, but a few frog eggs pricked with a

166 PRINCIPLES OF ANIMAL BIOLOGY

needle and moth eggs raised to a high temperature have yielded adult
offspring.

Paedogenesis. — Although sexual reproduction is usually carried on
only by adults, this is not always the case, for there are certain species
whose members have the remarkable power of reproducing sexually while
they are in the larval ^ state. This reproduction by a larval animal
is called -paedogenesis. Paedogenesis may be either parthenogenetic or
bisexual.

Parthenogenetic paedogenesis occurs in certain species of flies. The
larvae in these species (Fig. 134) produce ova which develop by partheno-
genesis into larvae before the oviducts are present. The latter generation
of larvae escapes from the parent larva by rupture of the body wall. This
results in the death of the parent. Several generations may be produced

in this fashion; then the larvae of one
generation pupate and emerge as normal
adult male and female flies.

Fig. 134. — Paedogenesis in the Paedogenesis of the bisexual type

fly Miastor. The parent, itself a -iU iii lii/ii.

larva, contains a number of larval OCCUrS m the Wcll-knOWn axolotl Amhy-

ofTspring. {From Folsom after stoTYia Hgrinum, ov tiger Salamander.

Pagenstecher.) tt i j. • j-i- au* • i

Under certam conditions this animal
attains sexual maturity and breeds while it is still in the larval form
having gills. In some of the Mexican lakes this is said to be the
usual occurrence, while in Kansas and Nebraska it is rare, and in many
localities it probably does not occur at all.

Hermaphroditism. — Most animals — a very great majority of the
metazoa — possess either male or female organs of reproduction but not
both. Species which have the sexes thus separate are said to be
dioecious (living in two houses), while those species whose individuals
produce both eggs and spermatozoa are called monoecious (living in one
house). Individuals with both male and female organs are said to be
hermaphrodites.'^ Two common species of Hydra are hermaphroditic,
as are most of the flatworms, most snails, and some roundworms. In
many monoecious species the spermatozoa are produced first and later
the ova, but in some species this condition is reversed. By developing the
sexual products at different times, cross-fertilization, that is, fertilization
of eggs by spermatozoa from another individual, is assured. In the earth-
worm, eggs and spermatozoa are produced in the same individual and

' A larva is a young independent individual which differs from the adult in the
possession of organs not possessed by the adult, or in lacking certain organs which arc
present in the adult (for example, a frog tad{K)le).

"^ The word monoecious is also applied to individuals, and is then synonymous
with hermaphrodite; but the corresponding word dioecious cannot well be applied
to individuals.

REPRODUCTION

1G7

at the same time. Cross-fertilization is assured in this case by the
arrangement of the generative organs and by the method of mating. In
mating, the bodies of two worms are closely applied by their ventral
surfaces, the heads pointing in opposite directions and the thickened band
or clitellum of each worm approximately opposite segments 7 to 12 of the

Fig. 135. — Copulation of earthworms. (Courtesy of General Biological Supply House.)

other worm (Fig. 135). In this position each worm secretes a slime tube
(Fig. 136) which sheathes its body. Spermatozoa are discharged into
the space between the slime tube and the body of the worm, are carried
backward within the slime tube by the muscular contractions of the body,
and finally are picked up by the seminal receptacles of the other member

Fig. 136.-

-Slime tube and cocoon of earthworm: above, in process of formation; below,
after slipping off the worm. (After Foot.)

of the pair. A cocoon is secreted around each worm, and eggs are laid
in it. The cocoon with the eggs in it is then slipped off over the head
end, along with the slime tube, and spermatozoa are discharged into it
as it passes the seminal receptacles (see page 168). Fertilization occurs
in the cocoon.

168

PRINCIPLES OF ANIMAL BIOLOGY

To work in this manner, the ducts discharging the germ cells must be
in front of the clitellum, by which the cocoon is secreted. Their arrange-
ment is shown in Fig. 137. The male organs are two pairs of testes,
three pairs of seminal vesicles, and one pair of vasa dcferentia. Male
germ cells are originated by the first of these organs, are developed in the
second, and are discharged thrqugh the third. The same worm also
possesses a set of female reproductive organs consisting of one pair
each of ovaries, ovisacs, and oviducts and two pairs of seminal receptacles.
The eggs, after leaving the ovaries, are held temporarily in the ovisacs
and then discharged through the oviducts. The seminal receptacles
receive spermatozoa from another worm and hold them until a cocoon
passes by their openings.

Fig. 137. — Reproductive organs of the earthworm, schematic representation of the
side view: IX— XV, numbers of somites; cm, circular muscles; ep, epithelium; /, funnel
of vas deferens; Im, longitudinal muscles; ov, ovary; ovd, oviduct; ovs, ovisac; rs, recep-
taculum seminis; ts, testis; vd, vas deferens; vs, vesicula seminalis; vsb, base of vesicula
seminalis; 9, opening of oviduct; cf, opening of vas deferens. {Modified from Hesse.)

While in the earthworm and in some other hermaphroditic species
an elaborate mechanism ensures cross-fertilization, in other hermaph-
roditic species no such devices exist and, indeed, self-fertilization (fertili-
zation of eggs by spermatozoa of the same individual) is well known either
as a regular or occasional occurrence. Some plants as wheat and beans
regularly self-fertilize. Other plants as the violet produce some flowers
which are regularly cross-fertilized and others which can only be self-
fertilized. Among parasitic flatworms (tapeworms and flukes) and
among snails both cross- and self-fertilization have been observed.

As stated in an earlier section, Paramecium is to be regarded as
hermaphroditic. One individual conjugates with another for exchange
of micronuclei. Besides this, at intervals there is, without conjugation, a
reorganization of the nuclei of a single individual which results in rein-
vigoration, but which seems not to correspond to self-fertilization since
there is no fusion of nuclei.

REPRODUCTION

169

Asexual Reproduction: Fission. — Fission is a common reproductive
method among the protozoa, and occurs less commonly among the
metazoa. The essentials of fission are that the parent cell or the parent
body (if a metazoon) be divided into approximately equal parts, each of
which grows and regenerates the misvsing parts and thus comes to resemble
the parent. The parent disappears as an individual and two new indi-
viduals take its place. The plane of fission may be longitudinal or

r-x

\ <

14-

4 %

t. "i

Fig. 138. — Successive stages in the fission of a single Paramecium. (Courtesy of Ralph
Wichterman and General Biological Supply House.)

transverse. Transverse fission, the more common type, is illustrated in
Fig. 138, which shows, step by step, the division of Paramecium cauda-
tum. Structures which extend across the plane of fission are divided,
and the missing portion regenerated. Other structures go with that
portion in which they are located before fission, and corresponding
structures arise anew in the other portion. Thus in a Paramecium with
two contractile vacuoles, one placed anteriorly, the other posteriorly,
one vacuole goes to each new individual and a second vacuole arises anew
in each, usually before division is completed, as in the figure. In forms
which have both macro- and micronuclei, both nuclei elongaste and finally

170

PRINCIPLES OF ANIMAL BIOLOGY

divide, a half going to each new individual. After the separation into two
individuals, regeneration is completed and each individual grows in size.
As stated on page 164, fission occurs every 16 to 24 hours in a healthy line
of paramecia.

In the reproduction of certain parasitic protozoa the nucleus of a
large cell may divide many times without the division of the cytosome.
Later the cytosome divides, not by successive equal fissions but by
many simultaneous divisions, into as many pieces as there are nuclei,
thus forming a number of small cells at the same moment. This process
is sometimes called multiple fission and sometimes sporulation. It
occurs regularly in the complicated life history of the organism of malaria.

Budding. — When an organism divides unequally, the reproduction is
termed budding. The larger portion may be regarded as the parent,
the smaller one as the offspring. Usually, also, there is a definite protru-

■^^mfM,^

Fig. 139. — Gernmule of fresh-water sponge (left), and young sponge recently emerged from
gemmule: os, osculuni; sp, spicule. {Gemmule after Hesse and Doflein.)

sion of the bud, which is small at first but grows larger. The bud usually
develops organs similar to those of the parent and either becomes inde-
pendent of, or remains attached to, the parent. Budding is a rare
reproductive process among the protozoa but is common in certain
groups of the metazoa.

In the metazoa the budding may be either internal or external. In
the former, the buds are formed somewhere within the body substance ;
in the latter, they are on the surface.

Internal Budding. — In fresh-water sponges, masses of cells collect
in the jellylike middle layer of the body wall. Hundreds of cells are in
each mass, and around them is a horny layer which often contains many
spicules. Such a reproductive body is called a gemmule (Fig. 139, left).
The gemmules are not shed, but when the parent's body disintegrates at
the end of the season, they are left exposed on the log or stone to which the
sponge was attached. They may remain there, or they may be trans-
ported considerable distances by water currents or perhaps by the feet
or beaks of birds.

With the return of favorable (conditions the bud enclosed within the
outer coating. of the gemmule begins to develop. There is an opening at

REPRODUCTION

171

one side of the geramule (above in the figure), which is plugged shut
during the resting stage. This plug is removed by the developing sponge,
which then creeps out. It is greatly distorted while crawling out, for the
aperture may be so small as to permit the sponge to pass only several cells
abreast. Once out, however, it quickly takes on the form of a sponge

Fig. 140. — Diagram of bryozoan with statoblasts (s) ; also photograph of animals and (at

right) a statoblast.

(Fig. 139, right). Gemmules allow sponges to live through winter and
permit them to be carried to other bodies of water.

In the Bryozoa, or moss animals, the internal buds are called stato-
blasts. They appear at first as white or yellow spots along a stalk which
joins the stomach of the animal to its body wall (Fig. 140). The oldest
statoblasts are next to the stomach. In forming them a mass of cells

Fig. 141. — Statoblasts of several Bryozoa: a and b, two views of that of Cristatella;
c, Pectinatella; d, Lophopus; e and/, floating and sessile types, respectively, of Plumatella;
g, J'redericella. (a and b from Sedgwick after Allen; c-g, from Ward's Natural Science
Bidletin.)

comes to be enclosed in a horny cover consisting of two valves, like two
cymbals pressed together (Fig. 141a, b); or they may be of other shapes
(e-g). These statoblasts escape by the degeneration of the body or
some part of it. Some possess floats so that currents of water carry
them, and some have hooks which tend to hold them fast to fixed objects.
Some germinate late in the summer of the year in which they are pro-
duced; others remain undeveloped over winter. They endure long

172

PRINCIPLES OF ANIMAL BIOLOGY

freezing with impunity, but complete drying for a few days usually kills
them. When they germinate, the two valves are forced apart but may
remain attached to the growing animal for a long time.

External Budding. — In external budding, the body wall is pushed
out at some point and develops the characteristic features of the animal.

Fig. 142. — The hydroid, Bougainvillea ramosa, portion of a colony at left; medusa at right:

mb, medusa bud; p, polyp. {After Allman.)

In some species the bud is eventually pinched off, as it is in Hydra (Fig.
58, page 71). This is doubtless the original method. In other species
the buds remain attached, and colonies are produced, as is common in the
hydralike animals called hydroids. A typical one of these, Bougainvillea
ramosa (Fig. 142), forms a colony with a much-branched coenosarc (inte-
rior cellular portion) bearing at the ends of the branches flowerlike

Fig. 143. — Diagram of structure of polyp (left) and medusa, with imaginary intermediate
form between; the plan of structure is the same.

zooids, called polyps or hydraiilhs. Each polyp is provided with a
hypostomc, a conical projection at the distal end, around which is a
circlet of lentades. The coenosarc is surrounded b> the pcrisarc, a tough,
lifeless cuticle secreted by the cells of the coenosarc. The colony arises
from a branched rootlike structure, the hydrorhiza, which is attached
to a solid body such as a rock or log. This colony is produced by budding
without a separation of the l)uds from the parent. From the stalks
of many of the polyps, medusae (jellyfishes) are formed by budding.

REPRODUCTION

173

Medusae are bell-shaped individuals (right in figure) which after maturity
become separated from the colony and swim freely in the water by means
of rhythmic contractions of the bell. Each medusa produces eggs or
spermatozoa. The fertilized egg develops into a ciliated free-swimming
embryo which eventually attaches itself by one end to a rock and develops
into a polyp. This polyp puts forth buds and thus a new colony is
formed. Though polyps and medusae are so different in gross form as to
have been regarded as different species before the production of one
by the other was known, yet the general plan of their bodies is the same.
In Fig. 143, by turning the polyp upside down, and introducing an
imaginary form between, the scheme of structure is showm to be ahke

OVARY

-MEDUSAE

TESTIS

HYPOSTOME
GONOTHECA
MEDUSA BUD

Fig. 144. — Diagram of life cycle of Obelia, illustrating metagenesis.

in both. The cavity or enteron does not always enter the tentacles;
in some polyps and medusae the tentacles are solid chains of cells.

Obelia forms a colony somewhat resembling Bougainvillea. In
Obelia, however, the medusa buds are produced by budding from the
gtalks of certain individuals (blastostyles) which, unlike polyps, have no
tentacles. Each blastostyle is enclosed in a swollen chitinous sheath,
the gonotheca. Blastostyle, attached medusa buds, and gonotheca
together are often designated the gonangium. Obelia is thus composed
of three types of individuals, two of which are sessile and incapable of
sexual reproduction, w^hile the other is a sexual free-swimming form
(Fig. 144).

174

PRINCIPLES OF ANIMAL BIOLOGY

Species which, hke Obelia, exhibit several forms of body are said to be
polymo7-phic (hterally of many forms) ._ In ObeHa, as in many other
hydroids, polymorphism is accompanied in the life cycle by an alter-
nation of asexual and sexual reproduction. The medusae, which are
of separate sexes, produce eggs and spermatozoa. The fertilized egg,
after fertilization, produces a larva or planula (Fig. 144). This settles

yC?y7 eumatophore

Swimmino
~~5e//

Sensory
^dividual

ophore

I'lti. 145. — Diagram of a siphonophore colony composed of six kinds of individuals.

fied from Fleischmann.)

(Modi-

down, grows into a polyp which buds off other polyps. The colony
thus formed also buds off gonangia, whose contained blastostyles bud
off medusae. These medusae are set free, and the cycle starts o\-er again.
In this cycle the medusae reproduce sexually; all other reproduction in
it is budding, that is, asexual. It should be noted that the sexual indi-
viduals have a very different structure from the asexual ones. Such a
combination of polymorphism with sexual and asexual reproduction is
called metagenesis.

REPRODUCTION

175

Extreme Polymorphism. — Remarkable examples of colony formation
and metagenesis, accompanied by division of labor among the types of
individuals that reproduce asexually, occur among' the marine animals
known as siphonophores, which have a structural similarity to Hydra.
The siphonophores are free-swimming colonies of varying complexity.
Each colony (Fig. 145) consists of a common tube of coenosarc which
bears at one end a pneumatophore

■ z i

or float and along its length zooids
of various forms. The float is
the expanded end of the coeno-
sarcal tube. It generally con-
tains gas and serves to support
the colony which hangs freely in
the water. Near the float is a
group of swimming bells {nedo-
calyces) which resemble medusae
and whose function it is to propel
the colony through the water by
their alternate contraction and
expansion. At intervals l:)elow
the swimming bells occur bracts,
or covering scales ; feeding polyps
which ingest the prey and digest
it for the entire colony; sensory
polyps which in some species at
least also serve as digestive or-
gans; tentacles (defensive and
offensive individuals) provided
with nematocysts (page 72) ; and
gonopJiores (reproductive zooids)
with or without bells. A first
examination of a siphonophore
might lead to the conclusion that
it is a complex individual with
half a dozen kinds of organs. By
a careful study of selected forms,
however, and by means of a
comparison of these with such forms as Obelia, it may be determined
that most of the structures which in a siphonophore resemble and
function as organs are really much modified individuals, either polyp
or medusa (Figs. 143, 144). In certain species the bracts contain
remains of radial canals which are characteristic of medusae. The
bracts, swimming bells, and gonophores are constructed on a medusoid

Fig. 146. — Physalia, the Portuguese man-
of-war, drawn from live animal floating on the
surface of the sea. cr, crest; p, polyp; pn,
pneumatophore; t, tentacle. {From Parker
and Haswell, " Textbook of Zoology,'^ after
Huxley.)

176 -PRINCIPLES OF ANIMAL BIOLOGY

plan, while the feeding polyps, sensory polyps, and tentacles are con-
structed on the polyp plan. In a few species the gonophores may
separate from the colony, as do the medusae in typical hydroids, but
usually they remain attached.

The Portuguese man-of-war Physalia (Fig. 146) differs from the
generalized form described above in possessing a float which sits high
above the water and serves as a sail. It has no swimming bells or bracts.

Origin of Colony Formation. — Among the metazoa the formation of
colonies, the integral union of individuals of the same species, occurs
only in those groups which employ an asexual mode of reproduction such
as budding or fission. Animals which employ the sexual method of
reproduction alone do not form colonies. Colony formation, especially
when it involves polymorphism and division of labor, may have made
for greater efficiency in the performance of certain functions, but it should
not be considered that efficiency is a goal toward which species have
striven. It seems rather to have been an accident made possible by the
existence of an asexual method of reproduction and to have been due
to a failure of the mechanism by which budding or fission is normally
completed.

Limits of Asexual Reproduction. — ^Asexual reproduction occurs only
among the lower forms of life. It never occurs among vertebrate animals,
and there are a number of great groups of invertebrate animals which
never employ it. Even in those groups in which it occurs, there are
many species which never use it. Nevertheless, asexual reproduction is
very widespread. Because its mechanism is less complicated than that
of sexual reproduction and because it is employed chiefly by animals of
simple structure, it is regarded as the primitive method of reproduction.
Animals must have reproduced asexually for ages before even the simplest
arrangement for reproductive cooperation of two cells or individuals
arose.

References

Hegner, R. W. The Germ Cell Cycle in Animals. The Macmillan Company.

(Especially (!liaps. I and 11.)
LocY, W. A. liiology and Its Makers. Henry Holt & Company, Inc. (Abiogenesis,

pp. 277-293.)
LoEB, J. Artifi('ial Parthenogenesis and Fertilization. Chicago University Press.

(Chap. I, history of attempts to initiate development artificially.)
MiNCiiiN, E. A. Introduction to the Study of the Protozoa. E. J. Arnold & Son,

Ltd. (Chap. VII, fission; Chap. VIII, conjugation and sex.)
Thomson, J. Akthur. The Study of Animal Life. John Murray. (Chap. XIV.)
Wilson, E. B. The Cell in Development and Heredity. 3d Ed. The Macmillan

Company. (Chap. III.)

CHAPTER 15
THE BREEDING BEHAVIOR OF ANIMALS

Reproduction in which both sexes are involved is dependent upon the
uniting of the germ cells, proper conditions for the development of
the fertilized egg, and conditions suitable for the development of the
immature animal. The parents often do more or less to ensure these
events and conditions, to guard against accident to the immature off-
spring, and to help it over the period of its own helplessness. These
services of the parents are habitual and are known collectively as breed-
ing behavior.

Breeding behavior in the animal kingdom is exceedingly varied.
There are two apparent reasons for this variety. First, different forms
have different modes of life, and the breeding habits must be suited to the
manner of living if they are to accomplish their purpose. Second, the
increasing complexity attained in the higher forms of life apparently
necessitates in them a longer period of prenatal development. At least,
the development before birth or hatching is longer in the complex forms
than in the simpler ones. The differences in behavior are not characters
that distinguish large groups of related animals from one another, for
within these groups there is considerable dissimilarity in breeding habits.
Even closely related tree frogs, for example, may employ very different
means of assisting the processes of reproduction and development.
Because of this diversity no attempt will be made to describe in detail
the various breeding habits of animals, but rather to classify and sum-
marize and to introduce just enough detail to illustrate in concrete
manner the several types of breeding behavior.

Urinogenital Systems. — Since some features of the breeding habits
of animals are dependent upon the structure of their reproductive organs,
these must first be examined. In vertebrate animals the reproductive
and excretory systems are intimately connected and together they com-
prise the urinogenital system. The excretory system of the frog has
already been described (page 13G). In both sexes of the frog the gonads
(meaning testes or ovaries) develop ventrally to the kidneys and here they
hang suspended in sacs of peritoneum. This relation is most plainly seen
in the male and in young females whose ovaries have not yet become
voluminous.

The oviducts are coiled tubes passing by the ovaries (Fig. 147, left).
Each oviduct takes its origin in a ciliated funnel which lies near the

177

178

PRINCIPLES OF ANIMAL BIOLOGY

heart and at the extreme anterior end of the coelom or body cavity. The
posterior end of each oviduct is transformed into a thin-walled distensible
bag, the uterus, which is connected by means of a narrow passage with the
cloaca, in the same region as the opening of the ureter. The walls of the
uterus and the ureter become united side by side in their lower courses,
but their cavities remain distinct. Eggs are released into the body
cavity by ruptures in the peritoneum covering the ovaries. They are
carried forward to the funnels of the oviducts by the general body
movements, assisted by pressure of the fore arms of the clasping male.

Fig. 147.— Uriuogenital system of female (left) and male frog. Kidney at left in male
is in surface view, that at right dissected to show internal tubes. A, anus; BT, Bidder's
tube; CL, cloaca; CT, collecting tubule; CV, postcaval vein; F, funnel of oviduct; FB, fat
bodies; K, kidney; LI, large intestine; MD, Muellerian duct; OV, ovary; OVD, oviduct;
SI, small intestine; T, testes; UB, urinary bladder; UR, ureter; UT, uterus; VE, vasa
efferentia.

Currents are also produced by the strong beating of the cilia which line
the funnels of the oviducts. These currents sweep the eggs and other
matter into the open funnels and down the oviducts. The remainder
of the path to the exterior is indicated by the structure and arrangement
of the organs.

In the male frog (Fig. 147, right) the testes are connected to the
kidneys by means of fine ducts, the vasa efferentia. These fine ducts
penetrate into the kidney and open into a longitudinal canal (Bidder^s
canal), which is a long tube running lengthwise of each kidney near its
median border. Bidder's canal is connected with the lu'eter by means
of a series of collecting tubules into which the uriniferous tubules (page
135) also open. Spermatozoa in the frog must therefore pass through
the vasa efferentia, Bidder's canal, the collecting tubules, the ureter.

THE BREEDING BEHAVIOR OF ANIMALS

179

and the cloaca on their way to the exterior. In some species of frogs,
the lower end of the ureter in the male may be expanded into a seminal
vesicle in which spermatozoa are stored until they are emitted at the time
of breeding.

A comparison of the reproductive systems of male and female frogs
reveals that in the male the reproductive organs are more intimately
connected with the excretory organs than in the female. In reptiles and
birds, the genital system, especially in the male, is mare distinct from
the excretory system, though in
both of these groups, as in the
frogs, both excretory and genital
systems discharge into the cloaca.

In most mammals, the genital
and excretory systems open to the
exterior through a common open-
ing which Is separate from the anal
opening. That is, there is no
cloaca. In the female, the funnel
of the oviduct is close to the ovary
but is not connected with it. The
oviduct opens into the uterus in
which the young are retained and
nourished until birth. The form
of the uterus differs in the different
groups of mammals. That illus-
trated in Fig. 148 is common
among the carnivores, rodents, and
others which bring forth young in
litters. The uterus is connected
to the exterior by the vagina which
is the copulation passage. The
urinary bladder, which belongs to
the excretory system, is connected
to the lower portion of the vagina
by means of the urethra. In the
male the testes are connected by
means of the vasa deferentia (singular, vas deferens) with the urethra,
which extends from the urinary bladder through the penis.

Methods of Ensuring Fertilization. — In chronological order, the first
event of the breeding process in bisexual animals is fertilization of the
germ cells. From the nature of their reproductive systems it might be
expected that this event would occur differently in hermaphroditic
animals and those with the sexes separate, for in hermaphrodites self-

Fig. 148. — ^Urinogenital system of a fe-
male mammal having a bicornuate uterus,
somewhat schematic, bl, urinary bladder;
A;, kidney; od, oviduct; ov, ovary; sug, uri-
nogenital sinus; ur, ureter; ut, uterus; ng,
vagina; *, position of embryos. {Modified
from Wiedersheiyn.)

180

PRINCIPLES OF ANIMAL BIOLOGY

fertilization is conceivable. This expectation is not usually realized,
however. Relatively few animals are hermaphroditic, among them being
some sponges, Hydra and a few animals similar to it, worms, and snails.
Though hermaphroditism is often described in various kinds of vertebrate
animals, the condition so named is usually merely the existence of egglike
and spermlike cells in the same gonad. Since usually only one, if either,

o—.

Fig. 149. — Genital organs of a hermaphi-odite animal, a common land snail Polygyra
albolabris (Say). Note that some of the organs are characteristic of a male, others of a
female. 1, atrium; 2, penis; 3, prepuce; 4, vagina; 5, spermatheca; 6, vas deferens; 7, free
oviduct; 8, uterus; 9, spermatic duct; 10, talon; 11, hermaphroditic duct; 12, hcrmaphioditic
gland; 13, penis retractor; 14, albumen gland.

of these kinds ever reaches maturity and since appropriate ducts for
leading off both kinds of cells are not often present, such animals are not
functional hermaphi-odites at all. The reproductive system of a really
hermaphroditic animal, a snail, is shown in Fig. 149.

Most hermaphroditic animals have some way of avoiding self-fertiliza-
tion. In some of them, though l)oth kinds of germ cells are produced,
eggs predominate in some individuals, spermatozoa in others, and mere
chance favors cross-fertilization. In other animals, the two kinds of

THE BREEDING BEHAVIOR OF ANIMALS

181

germ cells are produced at different times, eggs first in some, spermatozoa
first in others. Obviously there can be no self-fertilization under these
circumstances. In still others, the mating habits prevent self-fertiliza-
tion, as has been described for the earthworm in the preceding chapter.
Some of the roundworms, however, and Sacculina, which is a parasite
attached to crabs, fertilize their own eggs regularly. No special act is
necessary to bring eggs and spermatozoa together in these self-fertilizing
forms, since they mingle freely within the body. Sometimes self-fertiliza-
tion may occur accidentally, as in Hydra whose sperms are shed into the
water, where they penetrate eggs still located in the ovaries. Since the
spermatozoa find the eggs largely by chance, they may reach either an
egg in the same individual or one of another individual.

Fertilization in Dioecious Species. — In many aquatic animals with
separate sexes the sexual elements, or at least the spermatozoa, are
simply discharged into the water and the germ cells come together by
chance. Thus in the jellyfishes the spermatozoa are liberated into the
water and may or may not happen to meet the eggs, which are retained
in the ovaries of the females of some species just as in Hydra. In other
animals there is congregation of the sexes at the breeding time, and the
eggs and spermatozoa are liberated in
proximity. Starfishes and sea urchins
periodically congregate in this manner.
This close association of the sexes un-
doubtedly greatly favors the meeting of
the germ cells but still leaves to chance
an important role, and many eggs are
never fertilized. The hellbender (a sala-
mander) is a form that congregates with
its fellows at the breeding season. In
certain other salamanders the male
deposits the spermatozoa in a naked,
nearly spherical mass resting on a
gelatinous stalk which is attached to a
leaf or some other object in the water.
This structure, including the stalk, is
called a spermatopho)-e (Fig. 150). The mass of spermatozoa at its top is
subsequently removed by the female with the lips of the cloaca, and the
eggs are fertilized within her body.

This last way of bringing eggs and spermatozoa together can be
adopted only by animals that fertilize their eggs internally. In frogs
and toads, fertilization occurs outside the body, and in these forms special
behavior is designed to bring the sex cells together. In addition to
congregating at the breeding season, the males practice clasping. The

Fig. 150. — Spermatophore of No-
tophthalmus viridescens viridescens
(Raf.), the common newt of eastern
North America. The stalk is a clear
gelatinous substance; the apical mass
(dotted in the figure) is a snowy-
white mass of seminal fluid contain-
ing spermatozoa. {After B. G.
Smith.)

182

PRINCIPLES OF ANIMAL BIOLOGY

male grasps the female, with his forelegs around her body (Fig. 151),
and pours out the fluid containing the spermatozoa as she lays her eggs
in the water. One of the salamanders, Notophthalmus viridescens, shows
a curious combination of methods, the value of which is obscure ; the male
first clasps the female, but instead of pouring out the spermatozoa into
the water, he then deposits them in spermatophores, from which the
female takes them into her cloaca as just described.

In many other animals the spermatozoa are introduced into the
body of the female by direct act of the male, a process known as copula-
tion. Fertilization then occurs internally. This method is employed

Fici. 151. — Clasping in a species of toad, Bnfo typhonius (Linnaeus). The small individual
is the male, the larger the female. {Photograph by A. G. Ruthven.)

by some parasitic worms, snails, fishes, and amphibia, and by all insects,
reptiles, birds, and mammals.

Place of Development. — From the methods of ensuring fertilization,
it will be seen that the eggs may be fertilized either before or after they
are laid. That is, fertilization is either internal or external. When
fertilization is internal the eggs may be retained for a long time after
fertilization, or they may be laid very soon thereafter. Whatever period
of time the eggs remain in the organs of the female after fertilization is
utilized in development, so that the embryo may be far advanced before
it is separated from the mother, or it may have attained only an early
stage of development, or development may scarcely have started. Thus,
in most of the insects and in all the birds the eggs are laid soon after
fertilization. In these cases only a few divisions of the egg, or of its
nucleus, have taken place at the time of oviposition, or it may not have
divided even once. On the contrary, development may proceed until a

THE BREEDING BEHAVIOR OF ANIMALS

183

-Ovi

Ov_

well-formed embryo is produced, and then the eggs are laid; this occurs
in some of the salamanders. Usually, if the eggs undergo more than a
few cleavages while within the mother, they remain until a rather late
larval stage, or until the form of the adult is attained. Some insjects,
some snakes, and the true mammals are of the last-named type.

Source of Nourishment of the Embryo. — Animals that lay their eggs
are said to be oviparous; the eggs may be laid before fertilization, or, if
after fertilization, while the embryos are still incapable of existence out-
side the egg membranes. Animals that retain the embryos until with
proper care they are capable of independent existence are designated
viviparous. Of viviparous species there are two general types. In one
of these, the eggs are large and laden
with yolk, from which the embryo
derives its nourishment, just as in
oviparous animals. The mother
serves, in such cases, chiefly as a
nest in which the eggs may develop.
Viviparous animals in which prac-
tically the whole nourishment of the
young lis furnished by the egg itself
are said to be ovoviviparous. Some
reptiles are ovoviviparous (Fig. 152),
the embryos being held in the oviduct
of the mother until they are far
advanced but receiving the food from
the egg. The second type of vivipa-
rous animal is that in which the
nutrition of the embryo is obtained
from the mother, whose reproductive
system is then of the general type
represented in Fig. 148. The embryo,
resting in the uterus, has as intimate a relation with the mother's blood
vessels as do the mother's own tissues. Blood vessels of the embryo
extend out through the umbilical cord, and branch profusely at the end
in a highly vascular structure known as the placenta (Fig. 153, left).
The placenta is furnished partly by the embryo, partly by the uterus of
the mother. In it the blood of the mother and that of the embryo, while
never joining in the same vessels, are separated only by the thin walls of
their respective capillaries. In the human placenta the connection is
even closer, for the walls of the maternal vessels become eroded away, so
that the blood comes to lie in large sinuses, resembling the open blood
spaces of the crayfish or insect circulatory system (page 122). In this
lake of maternal blood the capillaries of the fetal system (branches of the

Fig. 152. — Urinogenital system of a
lizard. B, bladder; CI, cloaca; K, kid-
ney; 0, ovary; Ov, oviduct; Ow^, cloaca!
opening of oviduct; Ov^, abdominal open-
ing of oviducts; R, rectuni. The lizards
are oviparous or ovoviviparous.

184

PRINCIPLES OF ANIMAL BIOLOGY

umbilical vessels) are bathed, as shown by the diagram at the right in
Fig. 153. The physiological operation of the two blood systems is
precisely like that of blood and adjoining tissue. Digested food and
oxygen in the maternal blood are transferred to the fetal blood, because
they are at higher pressure in the former. Accumulated urea and carbon
dioxide go in the opposite direction because they are at greater pressure
in the embryo. The fetus is thus being fed, and its wastes removed,
as efficiently as if it were a rapidly growing tissue in the mother's own
body. No blood cells are transferred in either direction, however; the
exchange is entirely a process of diffusion and osmosis.

Forms in which the embryo is connected with the maternal uterus
by a placenta are spoken of as truly viviparous. Hydra and some of

MATERNAL

LAKE OF MA
TEPNALJLOOp

WALLOP
UTERUS
(PART)

FETAL
BLOOD
VESSELS

UMBILICAL

ARTERY AND

VEIN,

Fig. 153. — Position of fetus in uterus (left), with its attachment by umbilical cord and
placenta to uterine wall: F, fetal placenta; 0, opening of oviduct; <S', maternal placenta.
Rectangle shows approximate location of dissection of human placenta at right. {Left
after Kingsley, " Vertebrate Zoology," Henry Holt and Company, Inc.)

the jellyfishes, among aquatic animals, exhibit something like viviparity,
since only the spermatozoa are shed into the water. The spermatozoa
in these forms find the eggs, largely by chance, while the eggs are still in
the maternal ovary and penetrate the eggs in that situation, and the fer-
tilized eggs develop there for a time. In these cases the eggs are large
and presumably contain much of the necessary nourishment.

Intermediate between ovoviviparous and vivipai-ous forms are those
in which the young develops for a considerable time in the egg and later
becomes attached to the body of the mother. C^ertain sharks (Fig. 154)
exemplify this intermediate condition. The expanded end of the yolk
sac becomes attached to the wall of the uterus, terming an organ like
the placenta of mammals. The young receive nourishment through
it from the mother.

THE BREEDING BEHAVIOR OF ANIMALS

185

No Evolutionary Sequence. — Since some of these types of breeding
behavior are plainly much more specialized than others, one might be
tempted to suppose that they exhibit some sort of evolutionary sequence.
That is, it might be thought that the simpler habits would be employed by
the more primitive groups of animals, while the complicated methods
would be adopted by the higher forms. Such appears not to be the case,
however, either as to assurance of fertilization or as to place of develop-
ment. Thus, copulation, which is a specialized habit, is employed by
some parasitic worms, some snails, the insects, reptiles, birds, and mam-
mals. These groups are so diverse in structure that it is impossible to
regard them as all primitive or all
highly developed. Furthermore,
most of the fishes and amphibia
use either external fertilization with
clasping or internal fertilization
without clasping, while some mem-
bers of each of these groups employ
copulation. In general, the same
breeding habits may occur in
animals of widely different groups,
and animals of the same group often
have very different habits. The
principal generalization concerning
fertilization is that among aquatic
or amphibious forms the habit pre-
vails of depositing the spermatozoa
and eggs freely in the water or
in immediate proximity to each
other, or of depositing the sperma-
tozoa so that they can be secured
later by the female; while in
the groups composed mostly of land forms the habit of introducing them
into the body of the female predominates. The latter method is essen-
tial to most land forms, since air is fatal to the delicate sexual cells,
whereas in aquatic forms the eggs (at least after fertilization) can endure
the water for a prolonged period.

In the method of bearing the young, also, there is no evolutionary
sequence. Oviparity and viviparity are found in the vertebrates and the
invertebrates. Certain conditions of reproduction itself, however, make
one generalization possible. The forms in which the eggs are fertilized
outside the body of the mother are necessarily oviparous; and it is only
among forms mth internal fertilization that viviparity, ovoviviparity,
and the laying of fertilized eggs can occur. As a result, viviparity,

Fig. 154. — Embryo sharks of a vivip-
arous species, Mustelus mustelus (Lin-
naeus), attached to the wall of the uterus,
which is here dissected open. {After
Fowler.)

186

PRINCIPLES OF ANIMAL BIOLOGY

ovoviviparity, and the laying of fertilized eggs prevail among land
forms, where protection against evaporation of the eggs is necessary; and
the habit of laying eggs before fertilization is mostly found among the
aquatic species and the amphil^ious forms which lay their eggs in water.
Care of Fertilized Eggs. — Among oviparous species the methods of
caring for the fertilized eggs are almost endlessly varied in their details.

FxG. 155. — Nest of the Australian brush turkey, consisting of Utter in which the eggs
are buried to be hatched by the heat of tlie decomposing vegetable debris. The nest is the
heap of debris in the lower half of the photograph. {Photograph by E. R. Sanborn, loaned
by the New York Zoological Society.)

There are many animals which give no care whatever to the eggs. This
is particularly true of aquatic species which pour the eggs and sperma-
tozoa freely into the water to come together by chance. The starfishes
and sea urchins and many other marine animals exhibit this lack of
parental care. Other forms merely put the eggs in places where develop-
ment is facilitated. Thus toads and certain salamanders which live
on land in the adult stage lay the eggs in the water. Aquatic turtles
come to land to lay eggs in the warm sand which hastens their develop-

THE BREEDING BEHAVIOR OF ANIMALS

187

ment. Digger wasps, ichneumon flies, and certain other insects deposit
their eggs in various places and provision them with Hving or dead animal
food. Birds of one group, the megapodes, lay the eggs in a pile of
decaying vegetation, the decomposition of which liberates heat that aids
in development (Fig. 155). Again, many animals build nests. These
nests may be very simple in construction. In the fishes, for example,
many species merely hollow out a small area on the bottom of the stream
by pulling out the pebbles and heaping them up on the downstream side
of the nest. The eggs, when laid, drop into this hollow and among the
loose stones. Birds build nests of a great variety of forms, from the loose
collection of grass or straw put on the ground by the killdeer, or the

Fig. 156. — Blue-tailed skink, Eumeces fasciatus (Linnaeus), with eggs. This lizard
buries its eggs (the white mass in the middle foreground) in decaying wood and stays with
them until hatched. The curved white streak to the left of the center of the picture is the
tail (blue in life) of the parent, and a part of the striped body can be seen to the right of the
center. {Photograph by A. G. Ruthven.)

insecure litter of twigs set in the branches of trees by the mourning
dove, to the elaborate hanging basket of the orioles. Still other forms
enclose their eggs in cases, as was pointed out for the earthworm in the
preceding chapter and as is true also of the leeches and some insects,
snails, and spiders.

Among the nest-building forms the habit of caring for the eggs has
usually been developed ; that is, one or both of the parents in many species
remain with the eggs until they are hatched. The habit of remaining
with the eggs may ensure incubation, or the elevation of the temperature
to a point at which development will proceed. Incubation by the parents
is necessary in most birds and is an aid in some other animals. Remain-
ing with the eggs does not, however, necessarily imply incubation. For
example the common skink is a "cold-blooded" animal which remains
with the eggs (Fig. 156). Its temperature is so nearly that of the sur-

188

PRINCIPLES OF ANIMAL BIOLOGY

rounding air that the development of the eggs can scarcely be affected
by the presence of the parent. Some other species apparently incubate
the eggs to a small extent. The python, for example, coils about its

Fig. 157. iio. i5<s.

Fig. 157. — Hyla fuhrmanni Peracca, a South American tree frog that has the habit of
carrying the eggs on the back. The female carries the eggs. (Photograph by A. G.
Ruthven.)

Fig. 158. — A marsupial frog, Gastrotheca monticola Barbour and Noble, from Peru.
The opening of the pouch and a protruding egg may be seen in the lumbar region. (Photo-
graph by G. K. Noble.)

eggs, and as the temperature within its coils is a few degrees above that
of the surrounding atmosphere, development is thereby probably some-
what accelerated.

The habit of carrying the eggs attached to the body is found in several

groups, among both nest-building forms and
others that build no nests. Thus, the female
crayfish carries her eggs attached to the swim-
inerets under her abdomen, where she waves
them back and forth. The movement of the
eggs increases aeration, which is perhaps neces-
sary. Fresh-water mussels keep their eggs in
the chambers of the gills of the female, where
they are furnished oxygen by the water that is
constantly passing through the gills. In spiders
the silken egg case mentioned earlier is often
carried about by the mother. Certain frogs
(Fig. 157) and insects bear the eggs glued to the
back of one sex or the other. In other frogs
the eggs are attached to the belly, or the egg
masses are wrajiped around the hind legs of the
male or are held in the vocal sacs. One frog, the
marsupial frog (Fig. 158), has a pouch formed of a fold of the skin on the
back in which the eggs are carried. This habit is again found in the pipe-
fish and sea horse (Fig. 159) which carry the eggs in a ventral pouch.^

Fig. 159. — Hippo-
campus, the sea horse,
male specimen showing
brood pouch: hr. ap,
branchial aperture; brd.
p, brood poucli; df, dorsal
fin; op, opening of brood
pouch; pet. f. pectoral fin.

THE BREEDING BEHAVIOR OF ANIMALS

189

Eggs thus carried in pouches may perhaps receive oxygen from the parent,
but little is known on this subject. Either the male or female may carry
the eggs, but usually only one sex does this in any given species.

Care of the Young after Birth or Hatching: Birth Stages. — After
birth in viviparous forms and after hatching in oviparous species, the

Fig. 160. — The black swamp wallaby. The young are born in a very immature stage
and are carried in a pouch (marsupium) on the ventral side of the mother. {Photograph
loaned by the New York Zoological Society.)

young may or may not require protection and assistance in getting
food. This is partly dependent upon the stage of development which the
offspring has attained at the time of birth, but not entirely so.

The animal may leave the egg complete in all its parts and needing
only the growth of the body and the maturity of the sex cells to attain
the climax of its development. Among these forms the young may

190

PRINCIPLES OF ANIMAL BIOLOGY

receive little or no parental care or they may be fed and cared for for
many weeks or even months. Among the reptiles, for example, the
young are left to their own devices as soon as they hatch or are born.
Most fishes and invertebrates also throw off all parental solicitude after
their offspring leave the eggs. Most birds, on the contrary, must feed
and protect their young for a period of days or weeks; and mammals care
for their offspring for weeks or years. In these cases, how long the
young must receive aid depends on how far they develop before birth.

Fig. 161. — Recently hatched young of the chimney swift, Chaetura pelagica (Linnaeus),
left, and spotted sandpiper, Adiius macularia (Linnaeus), right. These are examples,
respectively, of altricial and precocial birds.

There are great differences in birth stages even in the same group. Thus
among mammals the marsupials (opossums and kangaroos) give birth
to young in a very immature state and carry them in a pouch (Fig. IGO)
until they are well formed; mice are born blind, hairless, and very helpless;
rabbits are born blind but covered with hair; and guinea pigs are born in
such an advanced stage that they are very shortly independent of the
mother. Among birds are to be distinguished altricial and precocial
forms (Fig. 161), the former usually, although not always, hatched blind
and practically without feathers, thus requiring longer parental care; the
latter covered with down and with the eyes open, requiring shorter care.

THE BREEDING BEHAVIOR OF ANIMALS

l91

The common song birds are all altricial, while domestic fowls, partridges,
most wading birds, and the various ducks are precocial.

There are also animals which escape from the egg so early that they
lack important organs and must undergo extensive changes to attain
the adult form. Or they may possess
organs which they must lose before *they
become adults. Young animals, leading a
separate existence but lacking certain organs
of the adult or possessing organs not found
in the adult, are known as larvae. The
offspring of jellyfishes emerge from the ovary
of the mother, where in some kinds as stated
earlier the eggs are fertilized, as a simple
ball of cells, almost at the beginning of
development. They receive no care what-
ever thereafter. The embryos of sponges
escape at a stage almost as early, as the
jellyfishes. The developing embryos of
starfishes, sea urchins and their allies (Fig.
162), and marine worms are also capable of
free-swimming existence at a very early
stage. In the frogs and toads the tadpole is a larval form (Fig. 163), but
it hatches at a much later developmental stage than do the larvae of the
several preceding examples.

Early development, may be direct or indirect. In direct develop-
ment the embryo develops directly toward the sexually mature condition,
the organs being outlined and developed one after the other. In indirect

Fig. 162. — Free-swimming
larva of the holothurian Syn-
apta, leading an active inde-
pendent existence at a very
early stage of embryonic de-
velopment.

Fig. 163. — Tadpole of frog, illustrating a larval form. Organs are present that are lacking
in the adult, and some organs are missing which the adult possesses.

development, on the contrary, organs belonging only to the immature
stages and for that reason called larval organs are first formed and later
destroyed. Thus the caterpillars (larval stage) of butterflies are dis-
tinguished from the adult not only by the absence of wings and compound
eyes but also by the presence of anal feet and spinning glands which
are absent in the adult butterfly; and tadpoles of toads and frogs (Fig.

192 PRINCIPLES OF ANIMAL BIOLOGY

163) are distinguished from the adult frog not only by the absence of lungs
and legs but also by the presence of gills and tail. The transformation
by which the larval organs disappear and the missing organs are con-
structed is kno^vn as metamorphosis. The more numerous the larval
organs the more pronounced the metamorphosis becomes. This phe-
nomenon is further described in Chap. 16.

Relation of Birth Stages to Parental Care. — That birth at an early
stage of development necessitates parental care would seem at first con-
templation to be obvious. That is not usually true, however, except
for the animals of common daily observation. It cannot be said for
animals in general that the stage of development at birth determines
the amount of parental care necessary, for many of the lower invertebrates
with incomplete larvae and many fishes which have very immature young
give no care to the offspring, while other invertebrates with feeble young
(for example, the ants) carefully guard and feed them. But it is note-
worthy that, where no care is exercised, the young born in early stages are
usually those of aquatic or amphibious forms, while the young of ter-
restrial forms are mostly born in relatively advanced stages or receive
parental care. Furthermore, while many aquatic forms give some atten-
tion to the young, it is among the terrestrial forms that the greatest
development in the habit of caring for the offspring is found. It may
thus be concluded that, when aquatic animals, or amphibious forms
with aquatic young, deposit the eggs or young in suitable habitats, they
have done much to facilitate postembryonic development, but that
land forms must usually give birth to young in an advanced stage of
development or exercise parental care in proportion to the helplessness
of the offspring.

CHAPTER 16
EMBRYONIC DEVELOPMENT

The minimum accomplishment of the reproductive processes is the
formation of germ cells. With the aid of breeding behavior these germ
cells are brought together in a favorable environment, where they are
gradually converted into new organisms. Into this period of transfor-
mation of the fertilized egg into an active independent being is crowded a
multitude of changes — analyses, reconstructions, rearrangements, growth,
and differentiations — which constitute embryonic development. Embry-
ology may properly treat of many of the things already described as
breeding habit or reproduction; but there is left for examination in this
chapter the whole series of structural changes and the chains of physi-
ological events which lead to the formation of the new individual. The
story may begin with the reorganization of the reproductive or germ cells.

Maturation of the Germ Cells. — The germ cells in a very young
animal may remain for a long time in a relatively undifferentiated con-
dition. Often it cannot even be stated whether they will become eggs
or spermatozoa, yet in most animals, despite theu' lack of recognition
marks, they are irrevocably destined to become the one or the other.
During this time they divide frequently by ordinary mitosis, thereby
multiplying in number. In this apparently unspecialized condition the
reproductive cells are called, in a male animal, spermatogonia (singular,
spermatogonium), in a female, oogonia.

As the time of reproduction approaches, the spermatogonia and
oogonia undergo a series of remarkable changes called spermatogenesis and
oogenesis, respectively. These changes consist typically of two rapidly
succeeding cell divisions, in one of which the number of chromosomes is
reduced to half. There are many variations in the process in different
species, but the fundamental features are the same for nearly all the
higher animals.

Spermatogenesis. — As soon as the spermatogonia reach the end of
their multiplication period, that is, as soon as they have divided by
ordinary mitosis for the last time, the cells are known as primary sperma-
tocytes. The history of these cells in their further development is illus-
trated in Fig. 164, to which constant reference should be made throughout
the following account.

During all of their history up to this time, the germ cells contain the
same number of chromosomes as any other cells of the body. That

193

194

PRINCIPLES OF ANIMAL BIOLOGY

number, barring differences in the sexes, is constant for the species.
In an animal descended from two parents, these chromosomes, with cer-
tain exceptions that may for the present be ignored, come in equal num-
bers from the father and the mother. Half of the chromosomes in any
cell may therefore 'be designated paternal, the other half maternal.
These chromosomes may look precisely alike and may in fact be exactly

^ermatogonia.

Primary
Spermatocyte.

Dyads.

Secondary
Spermatocyte^

Tetrads
Reduction

Spermatozoa

Primary
Oocyte

Isf Polar Body.

fSecondary
Oocyte

Dyads.

^ad Polar Body.

Mature tqq.

Pert/I/zed Egg.

First Ckai/age.

Fig. 1G4. — Spermatogenesis and oogenesis diagramniatically represented. The black
chromosomes may be assumed to be of paternal origin, the white ones maternal.

alike; the terms paternal and maternal refer only to their source, not to
their nature.

The spermatocytes grow considerably in volume, and at the same
time their chromosomes come together in pairs. Each pair is composed
of one paternal and one maternal chromosome. The pairing is not a
purely fortuitous occurrence, for each paternal chromosome meets a
particular maternal chromosome. As a result of this union of the
chromosomes there are, of course, half as many pairs as there Avere
chromosomes before.

EMBRYONIC DEVELOPMENT

195

While the chromosomes have been coming together, they may also
have become duplicated; that is, each chromosome is in some way con-
verted into two. Each pair thus comes to consist of four half chromo-
somes, and the quadruple body formed is called a tetrad. Owing to
its origin, two of the parts of each tetrad are maternal, the other two
paternal.

The Divisions in Spermatogenesis. — In the two divisions that follow,
the tetrads are divided in two planes, first into double bodies called
dyads, next into their single components. A spindle is formed on which
the tetrads take their place. How the tetrads are divided depends
on the way they are placed on the spindle. In some animals the tetrad
may be turned so that its maternal half faces one end of the spindle,
the paternal half the other end. In other animals the maternal and
paternal halves of the tetrad may be turned toward the sides of the
spindle (Fig. 165). In either posi-
tion the tetrad is cut in two in such
a way that the two parts facing an
end of the spindle go to that end in
the cell division. In Fig. 164 it is
assumed that the tetrads were so
placed that the maternal half was
separated from the paternal half.
It is a matter of chance, however,
whether the paternal half is turned
toward one end of the spindle or
toward the other. It may happen,
therefore, that all the paternal dyads go into one cell and all the maternal
dyads into the other or, as in the figure, part into one cell and part
into the other. The cells produced by this division are called secondary
spermatocytes.

It 'is important to note that in the division just described no chromo-
some has divided. The tetrads have divided, but merely by the sepa-
ration of the two chromosomes which had previously come together.
Such a division is called a reduction division, or meiosis;^ it never occurs
in divisions of somatic cells.

The secondary spermatocytes now divide by a mitosis in which the
dyads are divided into two components. The resulting cells are called
spermatids. A given spermatid may contain only paternal chromosomes,
or only maternal, or both paternal and maternal in any proportion.
The number of these chromosomes is only half that of the original
spermatogonium.

^ The term meiosis is sometimes applied to the whole process of spermatogenesis
and oogenesis, including both divisions.

Fig. 165. — The t'^^-o possible positions
of a tetrad on the spindle of the first
division in spermatogenesis, and the kinds
of cells resulting from them.

196

PRINCIPLES OF ANIMAL BIOLOGY

By a transformation in shape, the spermatid becomes a mature sfer-
Tnatozoon. This cell consists usually of a head and a whiplike tail, but
the forms are very different in different animals (Fig. 1G6). The chromo-
somes are all contained in the head, the tail being merely a motile organ.

Oogenesis. — The ripening of the female germ cells is in most respects
similar to that of the male. The early germ cells or oogonia undergo a
period of multiplication in which they divide by ordinary mitosis.
Eventually this ordinary division ceases, and the cells are ready to initiate

Fig. 166. — Different forms of .spermatozoa: A, badger; B, slieldrake; C, sturgeon;
D, flycatcher; E, opossum; F, lobster; G, crustacean Polyphemus; H, crab Droniia; /, crab
Porcellana; J , crustacean Ethusa. (A-D after Ballowitz; F after Her rick; G after Zacharias;
H-J after Grobben. From Wilsori, " The Cell in Development and Heredity." Courtesy of
The Macmillan Company.)

the maturation process. They are now known as primary oocytcfi.
These oocytes grow rapidly to many times their original volume, the
growth being much greater than in the male.

During growth the chromosomes meet in pairs, each pair, as in the
male, being composed of one maternal and one i)aternal chromosome.
Each chromosome may divide or be duplicated as they come together, so
that the pair presents a quadruple body, the tetrad.

Divisions in Oogenesis. — These tetrads are divided in the remainder
of the process, first into dyads, next into their single components, in a
manner strictly comparable to the divisions in the male. When a sjjindle
is formed for the first division, it appears not in the center but near the

EMBRYONIC DEVELOPMENT

197

surface and is placed approximately perpendicular to the surface. The
tetrads take their place on this spindle, again with their maternal and
paternal halves either toward the ends of the spindle or toward its sides
(Fig. 167). What kinds of dyads go into the two daughter cells depends

Fig. 167. — Two possible positions of tetrad on spindle of first division in oogenesis, and

the kinds of cells resulting from them.

on which of these two positions the tetrads take. In Fig. 164 the tetrads
are assumed to have been turned with their maternal half toward one
end of the spindle, the paternal half toward the other, so that the first
division was a reduction division. Each dyad formed is thus either
wholly maternal or wholly paternal, although of the
dyads in a given cell some may be paternal, some
maternal.

The two cells are of very unequal size. One
contains nearly all the protoplasm of the primary
oocyte, the other very little indeed. The disparity
between them is much greater than Fig. 164 indicates;
the correct sizes for one animal are shown in Fig. 168.
The larger cell is named the seco7idary oocyte. The
smaller cell is never functional and is called the
first polar body or first polocyte; it eventually
degenerates.

In most animals only the secondary oocyte undergoes further division.
In some species the first polar body also divides, and, to complete the
comparison with the male, this occasional division is represented in Fig.
164, but the resulting two polar bodies are not functional.

Fig. 168.— Star-
fish egg with
polar body above.
(Courtesy of General
Biological Supply
House.)

198

PRINCIPLES OF ANIMAL BIOLOGY

The division of the secondary oocyte involves the division of the
dyads into their halves. The division of the cytosome is again very
unequal, so that one small cell, the second polar body or second polocyte,
and one large cell are produced. The large cell, unlike the final cells
in the male, does not undergo any change of shape ; its maturation is fin-
ished when the second division is completed, and it is therefore a mature

egg-

Comparison of Oogenesis and Spermatogenesis. — Comparison of the

maturation of spermatozoa with that of eggs reveals that with respect to

Fig. 169. — Homolecithal egg of the sand worm Nereis. C, cytosome; /, fat droplets;
m, egg membrane; ri, nucleus; nl, nucleolus; y, yolk spheres. (After Wilson. Courtesy oj
The Macmillan Company.)

the chromosomes the two processes are parallel. The chromosomes unite
in pairs and are often at the same time duplicated so as to produce tetrads.
Two rapidly succeeding divisions divide the tetrads into dyads and
then single chromosomes.

The final cells contain half as many chromosomes as did the reproduc-
tive cell before the process began. These chromosomes may be paternal,
or maternal, or paternal and maternal mixed in any pro{)ortion.

The striking feature in which the processes differ in the two sexes
concerns the cytosome. In the female the divisions are very unequal,
so that from each original cell there are produced not four functional
cells as in the male but only one functional cell and two or three degener-
ate ones.

EMBRYONIC DEVELOPMENT

199

The Eggs. — The eggs of animals are typically spherical or nearly so.
Often, however, one diameter is much greater than the others, or the
egg may be elongated and curved, as in many insects. Internally the
substance of the egg is in some way differentiated so that opposite
sides or poles do different things. One
side is known as the animal pole, the
opposite side as the vegetative pole.

The food, or yolk, stored in an egg may
be very meager and is in such instances
rather uniformly distributed through the ^
protoplasm. Sea urchins, marine worms
(Fig. 169), and mammals have such eggs.
In fishes, reptiles, and birds, and less

Fig. 170. Fia. 171.

Fig. 170. — Generalized egg of telolecithal type, a, animal pole; c, cytosome; m,
second spindle in oogenesis; p, first polar body; s, spermatozoon; v, vegetative pole; y, yolk
crowded toward vegetative pole.

Fig. 171. — Centrolecithal egg of the fly Musca, in longitudinal section, cy, cytosome;
em, egg membrane; m, micropyle; 7i, egg and spermatozoan nuclei; pb, three polar bodies; y,
yolk. (From Korschelt and Heider, after Henking and Blockmann. Courtesy of The Mac-
■millnn Company.)

strikingly so in frogs, the yolk is crowded toward the vegetative
pole, so that most o*f the protoplasm is at the animal pole (Fig. 170).
In insects the yolk is in the central part, with a principal layer of
protoplasm at the surface (Fig. 171). Eggs with little yolk are said to be
alecithal or, from the uniform distribution of the yolk, homolecithal. Eggs
with much yolk aggregated toward the vegetative pole are telolecithal;
those with the yolk in central position, the protoplasm in a surface
layer, are centrolecithal.

200 PRINCIPLES OF ANIMAL BIOLOGY

Eggs are very often enclosed in a membrane or shell, particularly
among species that lay their eggs on land where evaporation must be
retarded. These envelopes may be of a chitinous nature, as among
insects, or composed of keratin which resembles chitin, or they may be
impregnated with calcium salts. The shell of the egg of the domestic
fowl is composed of three layers. The inner layer is composed of limy
particles with conical faces pointing inward. These particles do not
fit closely, and air may pass between them. Outside this layer is a com-
pact sheet of calcareous strands which also permits the slow passage of
gases. On the outer surface of the shell is a third layer, the cuticle,
which appears to be structureless except that it is penetrated by pores.
Within the shell is a membrane consisting of two layers of fibers crossing
one another in various directions. The envelope as a whole is calculated
to prevent excessive evaporation, and yet it permits the passage of gases
necessary for the respiration of the egg and embryo. Indeed, air begins
to penetrate the shell soon after the egg is laid and accumulates in a space
between the two layers of the membrane within the shell at the large end
of the egg.

Time and Mechanism of Fertilization. — Eggs and spermatozoa are
brought together in fertilization by breeding behavior or some sort of
affinity, as described in the preceding chapters. The time of their
union, particularly in relation to the stage of oogenesis, is very variable.

In Ascaris megalocephala, a roundworm parasitic in the intestine of
the horse, the spermatozoon enters the oocyte about the time of the
formation of the spindle of the first division. It remains in the oocyte
during the succeeding divisions. In the frog, rabbit, and some others
the spermatozoon enters after the first polocyte is formed but before
the second. In the sea urchin the spermatozoon does not enter until
after both divisions.

In eggs having a shell at the time of fertilization, there is an opening
through which the spermatozoon enters (Fig. 171m). In naked eggs,,
the spermatozoon may enter anywhere. Usually only one male cell
penetrates an egg. Some change of a chemical or physical nature takes
place in the protoplasm of the egg when a spermatozoon unites with it,
such that no other spermatozoa can be drawn in. When by accident
two or more spermatozoa gain entrance at the same time, al')normalities
of development are likely to occur. However, in some animals numei'ous
spermatozoa regularly enter the egg; but the imcleus of only one of them
unites with the egg nucleus.

Cleavage. — Shortly after fertilization, within a time measured by
minutes or hours in most animals, the fertilized egg begins to divide.
This division, which is repeated in rapid succession until the egg is con-
verted into many cells, is called cleavage or segmentation. In the follow-

EMBRYONIC DEVELOPMENT

201

ing account of cleavage the egg may be likened to the earth with its two
poles, so that a plane passing through the animal and vegetative poles
may be spoken of as meridional, other planes aii equatorial or parallel
to the plane of the equator.

In alecithal eggs the early cleavage is very regular (Fig. 172, above).
The first cleavage plane is meridional, passing through both animal and
vegetative poles and dividing the egg into two approximately equal cells.

ALECITHAL
or
HOMOLECITHAL'
(SEA CUCUMBER)

MILDLY

TELOLECITHAL

(FROG)

STRONGLY

TELOLECITHAL

(BIRD)

CENTRO-

LECITHAL
(INSECT)

Fig. 172. — Cleavage of eggs, in relation to the amount and distribution of the yolk in them.

The second cleavage is also meridional and perpendicular to the first
plane; four cells are thereby produced. The third cleavage is nearly
equatorial, resulting in eight cells.

After the third cleavage there are two or more cleavage planes at the
same time. The fourth cleavage passes through two planes, both of
them meridional, and perpendicular to one another. The 16 cells thus
formed then divide into 32, and so on. Up to the 32-cell stage, in such
an egg, the divisions usually take place at the same time in all the cells;
but irregularities occur later, and some cells divide earlier and more
rapidly than others. By this cleavage the single cell (fertilized egg) is
converted into hundreds of cells forming a nearly spherical mass, with a

202

PRINCIPLES OF ANIMAL BIOLOGY

liquid-filled cavity in the interior. The whole embryo is now designated
a hlastula, the cavity within it the hlastocoele.

In telolecithal eggs, cleavage is considerably modified. In general,
the third cleavage is elevated toward the animal pole, so that the upper
quartet of cells is smaller than the lower. Also the divisions occur
earlier and require less time near the animal pole than at the vegetative
pole, with the consequence that the smallness of the upper cells is accen-
tuated. In some way connected with this difference between the poles,
the blastocoele is eccentric in position, nearer the animal pole. All
these features are sho^vn in the frog cleavage (Fig. 172, second row).

In fishes, reptiles, and birds there is so little protoplasm in the yolk-
laden vegetative part of the egg that no cleavage occurs there at any stage.
Only the cap of protoplasm above the yolk segments and the blastocoele
is bounded by a layer of cells above and by undivided yolk below (Fig.
172, third row). In the bird egg in the figure the animal pole is in the
center of the first three illustrations, but at the top in the fourth.

Fig. 173. — Cleavage in arrowworm Sagitta, showing x body (x), which identifies the germ

cells.

In insects, cleavage is limited to the surface of the egg, where most
of its protoplasm is located. A layer of cells is formed there (Fig. 172,
below), while in the interior of the egg is undivided yolk. There is no
hollow interior corresponding to the blastocoele at this stage in the insect
egg.

First Differentiation during Cleavage. — Later stages of embryonic
development are replete with diffenniiiations of cells. Far in ad\'an(;e
of them is a most important differentiation, that between sterile cells
which go to form the body {somatic cells) and those which retain their
reproductive powers and give rise to the germ cells. In some animals
this distinction arises during cleavage, ev(Mi in Acry early cleavage.

In the arrowworm Sagitta the egg contains a small object, the x body,
which in the first six divisions goes undivided into one of the cells (Fig.
173). Thus in the 64-cell stage only 1 coll contains an x body. This is
the forerunner of all the germ cells, the other 63 are somatic cells. After
the sixth cleavage, the x body divides at each cell division, and every
germ cell contains it.

EMBRYONIC DEVELOPMENT

203

In Ascaris megalocephala (page 200) the first distinguishing mark of
somatic cells is their early division. In the second cleavage of the
fertilized egg, one cell divides earlier than the other. Thus in Fig.
174A, B the left cell is ahead of the right in division, and it gives rise in
later cleavages only to somatic (sterile) cells. The cell which lags
behind gives rise to both somatic and germ cells. As the 4 cells derived
from this cleavage begin to divide to produce 8 cells, a second mark of
somatic cells becomes evident (Fig. 174C). The middle portion of each
of their chromosomes breaks up into many small pieces, which continue

c D.

Fig. 174. — Cleavage of the fertilized egg in Ascaris megalocephala, showing distinction
of somatic and germ cells. A, second cleavage, in which cell on left, in more advanced
stage of division, is somatic; B, later stage of second cleavage, with cells in same relative
positions and same relative states of advancement; C, third cleavage, with chromosomes of
three cells (somatic) fragmenting and losing their ends, those of the fourth remaining
intact; Z), fourth cleavage, with chromosomes fragmented in six cells (one hidden), becom-
ing fragmented in one (middle right), and remaining intact in one (upper right). All germ
cells are descended from the last-named cell. (Schematized from account by Fogg in Journal
of Morphology and Physiology.)

as chromosomes, while the ends of the original long chromosomes are
thrown off into the surrounding protoplasm where they degenerate.
Three of the 4 cells lose chromatin in this way, and all these give rise
later only to somatic cells, while the one which retains its chromosomes
intact (upper right in C) produces both germ and somatic cells. In
each of the next two cleavages, in one of the cells that had retained
whole chromosomes, these chromosomes break up into small fragments
and lose their ends in the cytosome (D). Thus at the 32-cell stage
there is only 1 cell with long chromosomes like those of the fertilized egg.
In subsequent divisions of this cell there is no further loss of chromatin,
and all its descendants become germ cells. The other 31 cells have
fragmented chromosomes, and all their descendants are somatic cells.

204

PRINCIPLES OF ANIMAL BIOLOGY

In insects the germ cells usually either are larger (Fig. 175) or contain
certain granules not found in somatic cells. In vertebrate animals the
distinction between somatic and germ cells is not recognizable until a
much later stage. In the embryos of a number of forms the germ cells
are found as large cells in the lining of the digestive tract (Fig. 176),
whence they migrate up through the mesentery and out to the place

where the gonads subsequently develop.
Whether germ and somatic cells have
existed as distinct entities through the
earlier embryonic stages is not known.

Gastrulation. — When the blastula is
well formed, it is converted into a two-
layered embryo. The process by which
this conversion is effected, already briefly
outlined in C'hap. G, is called gastrulation.
The simplest form of invagination takes
place in those animals whose eggs have a
small amount of yolk evenly distributed,
that is, in alecithal or homolecithal eggs.
In these the vegetative side of the
blastula becomes flattened, then in-
turned (invaginated) (Fig. 177, above).
The invagination proceeds until the
inturned cells are in contact with the
opposite side of the blastula wall. The
embryo now has two layers of cells, an
outer or ectoderm and an inner or etido-
dcrm. The blastocoele has been obliter-
ated, but a new cavity, the archcntcron,
lies within the endoderm. This cavity
has been pushed in from the outside,
with which it is still connected by a
small opening called the blastopore. The two-layered embryo of this
stage is known as a gastrula. The endoderm of the gastrula becomes
the lining layer of cells of the digestive tract of the adult.

The blastula produced from a mildly telolecithal egg could not well
be invaginated directly from the vegetative side by flattening and
infolding, because the layer of cells there is so thick. In such a blastula
the invagination begins about midway between the ar\imal and vegetative
poles, where the cell layei- is thinner (Fig. 177, below). It is mostly the
cells above the blastopore which are invaginated, though there is some
withdrawal of the whole yolk-laden mass of lower cells into the interior.
The end result is, as in the alecithal embryo, a two-layered gastrula.

Fig. 175. — Early recognition of
germ cells (gc) in the development of
the egg of the fly Miastor, showing
also the cleavage cells (cl) at the
periphery and the yolk globules (y).
(After Hegner in Journal of Alor-
phology.)

EMBRYONIC DEVELOPMENT 205

In the frog, whose gastrulation is of this type, the invagination appears
from the exterior as in Fig. 178. The cells are inturned along a short
crescent-shaped line, which becomes extended into a marked U, and
finally completes a circle which diminishes in size to a mere pore as the
yolk-filled cells are withdrawn inside.

Gastrulation in strongly telolecithal embryos, like those of birds,
reptiles and most fishes, is so modified as to require an interpretation
o[ events too difficult for presentation here. In insects there is an
infolding which is usually called gastrulation, but the tissue turned in

Fig. 176. — Earliest recognition of the distinction between somatic and germ cells in a
vertebrate animal. Diagram of cross section of the body of the embryo, showing germ cells
in the endoderm of the intestine and their path of migration (shown by arrows) to the site
of the reproductive organs. How much earlier than this stage the somatic cells have lost
their reproductive powers is not known, c, coelom; en, endoderm of intestine; gc, germ
cells; ge, germinal epithelium which later covers the gonads and from which the germ cells
issue; i, intestine; m, myotome, or muscle segment; ms, mesentery; nc, neural crest, from
which nerves and ganglia develop; nd, notochord, forerunner of the backbone; s, spinal cord.

becomes not just the lining of the digestive tract but the whole internal
structure of the body. These two types are omitted from the com-
parisons in Fig. 177.

Mesoderm Formation. — At the end of gastrulation at least two
layers of cells, ectoderm and endoderm, are present. In most multi-
cellular animals a third layer, the mesoderm, if not already present is
soon formed between these two. In the fishlike amphioxus, a classical
form in biology, the upper portions of the endoderm (Fig. 179) are
turned outward in the form of grooves, shown dotted in cross section
in the illustration {A). The edges of each groove meet and fuse, and

206

PRINCIPLES OF ANIMAL BIOLOGY

the groove now in the form of a tube is completely separated from the
endoderm {B). The two tubes thus formed are the mesoderm, and the
slender openings in them constitute the body cavity, or coelom. In later
stages of development the tubes expand, as in C, shown black. One
side becomes a thin layer of cells applied to the digestive tract, while
the other side lines the inside of the ectoderm.

ECTODERM-

ALECITHAL

BLASTOCOELE

ENDODERM
APCHENTERON

MILDLY
TELOLECITHAL

■BLASTULA GASTRULA'

Fig. 177. — Gastrulation of embryos, in relation to the quantity and distribution of yolk in

them.

In the frog the mesoderm is formed simultaneously with the endo-
derm— indeed, almost before endoderm — <iuring gastrulation. The cells
which turn in over the dorsal rim of the curved blastopore, in its middle
portion, form the mesoderm directly (Fig. 180). A band of these cells
migrates forward from the blastopore, above the archenteron, to the
front end of the future embryo. In late stages of gastrulation, cells

^.«s

'':<^.

Fig. 178. — Gastrulation in, frog, external view.

invaginated at the lower margin of the (now circular) blastopore also
contribute to the mesoderm. The endoderm below the main sheet of
mesoderm is in the form of an open trough. The upper rims of this
trough (a) were originally continuous with the edges of the mesoderm,
but they break loose and curve up under the mesoderm. They meet
at the top, to enclose a tube which becomes the intestinal tract.

Subsequent Development of the Vertebrates. — The three layers of
cells, ectoderm, endoderm, and mesoderm, are often called gerin layers.
They are so designated because certain organs are normally derived

EMBRYONIC DEVELOPMENT

207

from each one, so that the layers may be thought of ais containing the
germs of those organs. They are not irrevocably destined to form these
organs, for, as we shall see later, their fate may be experimentally altered
in a variety of ways.

From the ectoderm ordinarily arise the epidermis of the skin, reptilian
(but not fish) scales, feathers, hair, nails and claws, the nervous system
including nerves and their endings, and some glands which discharge at
the surface. From the endoderm comes the lining of almost the whole
digestive tract and of all the organs which branch off from it, such ai.
the lungs, liver, and pancreas, and of the thyroid gland which, though
wholly separate in the adult, is an out-pocketing of the digestive tract

cc-

en —

vm
Fig. 179. — Mesoderm formation in the amphioxus, in cross section. A, evagination of
ridges (dotted) at upper lateral regions of endoderm; B, these ridges pinched off as tubes;
C, mesoderm (black) expanded so as almost to surround the digestive tract; c, coelom; dg,
digestive tract; dm, dermatome; ec, ectoderm; en, endoderm; m, mesoderm; mc, myocoele;
mp, mesodermal pouch; mt, myotome; n, neural plate; nd, notochord; nt, neural tube; soin,
somatic layer of mesoderm; spl, splanchnic layer of mesoderm covering the digestive tract;
vm, ventral mesentery. (A and B after Hatschek.)

in the embryo. From the mesoderm are derived muscle, bone, connec
tive tissue, blood vessels, and the thick inner layer of the skin.

The development of the several organs from the ectoderm and endo-
derm is in its early stages a bending or folding of these layers, which \6
called invagination or evagination according as the sheets of cells are
bent into, or out from, some enclosed space. The following account
of their origin is limited to the vertebrate animals.

The Early Embryo. — Several of the chief systems of organs are laid
down at a very early time. One of the first changes visible externally is
the appearance of two prominent ridges, close together, along the
dorsal side of the future embryo. These extend lengthwise and are
roughly parallel except at the anterior end where they diverge from
one another (Fig. 181). In a cross section of the frog these ridges
appear as in Fig. 182, nf. They are the neural folds, the beginning
of the central nervous system. Where these folds are near one
another, the spinal cord develops; the divergent folds in front form

208

PRINCIPLES OF ANIMAL BIOLOGY

the brain. These ridges approach one another and fuse along their
upper surfaces (Fig. 18 IB), cutting off a tube beneath the ectoderm.
In longitudinal vertical section at this time, the nervous system appears
as in Fig. 183.

Fig. 180. — Mesoderm formation in frog. First three figures, median sections; last
figure, cross section. The hne-shaded cells on the outside, as they turn in, become the
mesoderm, a, edges of trough of archenteron.

Beneath the nervous system a cylindrical rod of cells, the notochord,
is formed out of the middle portion of the inturned mesoderm. Around

it later is formed the backbone.
The digestive tract has been pres-
ent, as the archenteron, ever
since gastrulation took place. At
first it is usually enlarged in front
and narrowed behind. These parts
correspond roughly, in the frog, to
the stomach and intestine. Poster-
iorly the intestine opens to the
outside through the anus, which in
some animals is the same opening
as the blastopore but in others a passage produced anew aftei- the
blastoi)or(i has closed.

As indicated earlier (page 20()) and in Fig. 182, the mesoderm is early

Fig. 181. — Neural folds of frog em-
bryo. A, folds still separate, brain above,
spinal cord below; B, folds fused, produc-
ing neuial tube beneath surface.

EMBRYONIC DEVELOPMENT

209

divided into two layers, one applied to the inside of the ectoderm, the
other covering the endoderm. The peritoneum, which occupies approxi-
mately the corresponding positions in later stages, is derived from these
layers. Above the endoderm, between it and the notochord, two layers
of the mesoderm approach one another and form the mesentery (Fig. 176,
page 205) which later suspends the digestive tract in a trough of peri-
toneum. In the longitudinal section (Fig. 183) the mesoderm is not
represented above the digestive tract, since the section passes exactly
through the median plane. But below the intestine the mesoderm occurs,
divided into its two characteristic layers.

Fig. 182. — Cross section of the early embryo of a frog, diagrammatic, c, coelom; dig,
digestive tract; ec, ectoderm; en, endoderm; ms, mesoderm; nc, neural crest; nd, notochord;
nf, neural fold; ng, neural groove.

Anterior Digestive Tract. — The gill pouches, represented as seen from
above in Fig. 184, are evaginations of the endoderm in the sides of the
pharynx, or anterior part of the gut. Typically there are five of these
protrusions on each side, but some of them are often rudimentary, or
two of them may be nearly combined, so that the number frequently
appears to be less. Successive stages in the evagination of the gill
pouches are shown in A, B, C. They finally reach the ectoderm, with
which they fuse. In fishes and usually in amphibians the ectoderm
and endoderm both break open at the point of fusion, so that the pharynx
is open to the outside. These openings are the gill clefts. They serve
as channels for the passage of water, which enters at the mouth (not
shown in the figure since it is at a lower level). The course of the water

210

PRINCIPLES OF ANIMAL BIOLOGY

is indicated in the figure by arrows. In the fishes and in at least the
young stages of amphibians, gills (organs of respiration) are developed
upon the tissue {gill bars) between the gill clefts.

In the higher vertebrates the gill pouches do not open to the outside
at all or do so only temporarily. They are to be regarded as to some
extent vestigial organs, an inheritance of an ancestral condition in which
functional gills were present. However, some of them are regularly
converted during embryonic development into other functional or non-

Fig. 183. — Longitudinal section of the early embryo of a frog, diagrammatic, an, anus;
br, brain; c, coelom; ec, ectoderm; en, endoderm; int, intestine; li, liver; ms, mesoderm;
nd, notochord; sp, spinal cord; st, stomach.

functional organs. Thus the first pouch becomes part of the Eustachian
tube and middle ear. Certain of the bars share in the production of the
tonsils, the thymus, and the parathyroid glands.

The mouth starts as an invagination of the ectoderm from the outside,
as in Fig. 185m. For a time it is separated from the rest of the digestive
system by a membrane composed of an outer layer of ectoderm and
an inner layer of endoderm. This membrane later breaks, and part
of the fore end of the gut is incorporated in the mouth cavity. That
part of the mouth derived from the external invagination is of course
lined with ectoderm.

Outgrowths of the Digestive Tract. — The liver appears at an early
stage as an evagination from the lower side of the intestine just behind the
stomach. In the frog the liver is present shortly after the fusion of the
neural folds (see Fig. 183, li). An early indication of the liver is also
shown in Fig. 185, li. This pouch grows in extent and soon becomes
branched. One branch at the posterior side of the liver forms the gall
bladder (gb). The rest are bound together by mesodermal tissue which
collects about them, forming part of the body of the liver. The undivided

EMBRYONIC DEVELOPMENT

211

basal portion of the original pouch remains as the bile duct (bd), through
which the secretions of the liver are conveyed into the intestine. During
all this development the liver has been covered by the layer of peritoneum
(mesodermal) which invests the entire digestive tract. The adult liver
is thus covered by peritoneum and suspended by mesenteries formed from
the same layers of mesoderm.

The pancreas originates from two pouches evaginated from the intes-
tine (Fig. 185). One arises from the dorsal side of the intestine nearly
opposite the liver (dp) ; the other springs from the angle between the liver

Fig. 184. — Diagrams showing the early development of some of the organs of verte-
brate animals, as seen in section from above. The stages here shown are not contempora-
neous in all cases. A, B, C, successive stages; au, auditory vesicle; br, brain; cr, crystalline
lens; ec, ectoderm; en, endoderm; eu, Eustachian tube; gb, gill bar; gp, gill pouch; gs, gill
slit or cleft; me, endodermal portion of mouth; olf, olfactory pit; opn, optic nerve; ops, optic
stalk; ph, pharynx; ret, retina; sp, spinal cord; ty, tympanum or middle ear. Arrows in C
denote current of water through mouth, pharynx, and gill slits.

and the intestine (vp). The two pouches intertwine their branches to
form the pancreas, which is likewise invested with peritoneum.

The lungs take their origin from a protrusion from the ventral side
of the gut some distance in front of the stomach (Fig. 185, Ig). This
pouch is at first single (Fig. 186A), but soon divides into two parts
(B, C, D). As these grow in size they become branched. The undivided
stalk of the lung rudiment is the trachea, the two principal branches are
the bronchi, and the finer divisions are the air passages and alveoli
within the lungs. Mesoderm is constantly pushed before the growing
lung rudiments, so that the adult lungs are invested with a peritoneum.
Other mesodermal tissue is incorporated in the lungs among the air pas-
sages, where blood vessels are abundantly developed.

It should be borne in mind that Fig. 185 is diagrammatic and does
not represent a condition prevailing at any one time in embryonic
development. For the sake of compactness, organs have been shown
in the same figure in stages which do not occur simultaneously.

212

PRINCIPLES OF ANIMAL BIOLOGY

Nervous System. — It has already been pointed out, and shown in
Figs. 182 and 183, that the early central nervous system is a tube formed
by the fusion of two folds or ridges of the ectoderm. This tube is wide
in the anterior region, where it
forms the brain, and narrow pos-
teriorly, where it produces the
spinal cord. The thickening and
folding of the walls of this tube,

Fig. 185. — Diagram representing the development of .some of the organs of vertebrate
animals, early and later stages. The figures are a trifle to one side of the median plane.
The stages shown are not necessarily contemporaneous, an, anus; bd, bile duct; hr, brain;
c, coelom; dp, dorsal rudiment of pancreas; gh, gall bladder; int, intestine; Ig, lung; li, liver;
m, mouth; ms, mesoderm; nd, notochord; pc, pericardial chamber; rt, root of tongue; sp,
spinal cord; st, stomach; t, tongue; vp, ventral rudiment of pancreas.

especially in the formation of the lobes and cavities of the brain, are
very complicated processes.

The nerves extending from the spinal cord take their origin in part
from the neural creds. These crests are masses of cells budded off from
the inner surface of the ectoderm at or near the region of the neural folds,
as indicated in the cross section of the frog (Fig. 182, nc) and in Fig. 17(5.

As was pointed out in Chap. 13, the large nerves arising from the
spinal cord are connected with the cord by two roots. The dorsal root is
composed of afferent fibers and the ventral root of efferent fibers. The
dorsal root is enlarged to form a ganglion.

The dorsal ganglion is in each nerve developed from one of the neural

EMBRYONIC DEVELOPMENT

213

crests. The dorsal root is completed bj'' processes of nerve cells growing
inward from the neural crest and entering the dorsal part of the spinal
cord, and by other processes growing outward from the same cells in the
neural crest toward the periphery of the body, forming the afferent part of
the spinal nerve. The ventral root fibers grow out from the ventral part
of the spinal cord and join the fibers of the dorsal root at a point beyond
the ganglion. The nerve fibers from these two roots remain distinct from
one another but are enclosed in the same connective tissue coverings.

Sense Organs. — The principal sense organs are developed either as
outgrowths from the central nervous system or as ingrowths, chiefly from
the ectoderm, which come secondarily into connection with the nervous

Fig. 186. — Successive stages in the development of the lungs. The esophagus is
shown in A, B, and C, but not in D. As the lungs grow the mesoderm is pushed before them
and thus comes to invest the adult lungs and to make part of the lung tissue, br, bronchus;
es, esophagus; I, lung; m, mesoderm; tr, trachea.

system, or by a combination of these two modes of origin. The eye
begins as an evagination from the side of the brain (Fig. 184^). This
protrusion elongates and at the same time expands at its outer end into
a hollow bulb. The bulblike expansion flattens on its outer side and is
then invaginated to form a double-walled cup resembling a gastrula (Fig.
18-1:5, C). The inner layer of this cup becomes the visual part of the
retina, and the basal stalk on which the cup rests is the optic nerve. When
the outgrowth from the brain comes near the ectoderm, the latter thickens
and later invaginates, finally pinching off a rounded mass of cells {B, C).
This mass becomes the crystalline lens of the eye. The ectoderm at the
point where the lens was formed becomes transparent and with additions
from the mesoderm in most vertebrates forms the cornea. The rest
of the eye, including its muscles, is derived from the mesoderm.

The ear begins its development in the surface ectoderm, not, as does

214 PRINCIPLES OF ANIMAL BIOLOGY

the eye, from the central nervous system. A patch of ectoderm on each
side of the head region thickens and then invaginates (Fig. 184A),
producing a pear-shaped vesicle. The vesicle is pinched off from the
ectoderm and comes to lie within. It changes its shape, producing the
characteristic semicircular canals and the (sometimes) coiled body of
the inner division of the ear. Nerve cells growing out from the ganglion
of the eighth (auditory) nerve join the vesicle with the brain.

The middle ear, which contains the bones of the ear, is derived at
least in part from the first gill pouch (Fig. 184A, B, C). In the distal
portion of this pouch the ear bones are developed out of mesoderm, while
its connection with the digestive system, as already stated, forms the
Eustachian tube which connects the middle ear with the pharynx.

The olfactory organ, like the ear, is at first a patch of thickened ecto-
derm on each side of the head far to the front. This ectoderm invaginates
(Fig. 184, olf), but unlike the ear the pit thus formed does not close; it
remains open to the outside as the nostril. The pit enlarges and protrudes
inward to meet the front end of the digestive tract just behind the ecto-
dermal part of the mouth. An opening is subsequently formed at this
point of contact, and the nostril is thus connected with the deeper portion
of the mouth cavity. Only certain parts of the ectoderm that forms the
olfactory cavity become sensory. From these parts nerve processes
grow toward the brain, thus forming the olfactory nerve.

Metamorphosis. — Besides the usual course of development, which is
in large measure the same for all vertebrate animals, some members of
that group undergo an additional series of changes called metamorphosis.
Animals that metamorphose are born or hatched with one or more
organs which they will not possess as adults, or lacking organs that will
be developed before they become adult. It is the process of losing the
larval organs and of gaining the missing adult organs which is called
metamorphosis.

The transformation of a tadpole into a frog (Fig. 187) or toad is the
classical example. The readily visible changes are the degeneration
of the so-called "sucker" or attaching organ beneath the head; the
development of the legs; and the absorption of the tail, the material of
which is probably used elsewhere for growth. The external gills, hidden
under a fold of skin called the operculum, disappear early, to be replaced
by internal gills which are developed on the endodermal lining of the gill
slits. The internal gills are lost later, and their function served by lungs,
which have all the while been developing. The jaws are provided with a
horny armature, serving as teeth, but these are shed and the mouth
increases greatly in size. The intestine, from the early tadpole stage a
\(iiy long and much coiled tube, is greatly shortened.

Other kinds of animals undergo metamorphosis, notably among the

EMBRYONIC DEVELOPMENT

215

insects. In some kinds the changes are very small from stage to stage,
as in the bugs (Fig. 188). These sucking insects shed their skins periodi-
cally as they grow, and at each change they are a little more like the adult.
At the very beginning, however, they are easily recognized as bugs.
Such a series of changes is scarcely metamorphosis at all but is usually

Fig. 187. — Metamorphosis of frog. 1-4, growth of gills; 5-6, covering of gills by
operculum, degeneration of sucker; 7—10, growth of legs (9 shows greatly elongated intes-
tine); 11, fore legs pushed through operculum; 12-14, degeneration of tail. {Rearranged
from Newman, "Outlines of General Zoology,^' by perm.ission of The Macmillan Company.)

called incomplete metamorphosis. Contrasted with this gradual trans-
formation is the very marked one which flies, butterflies, bees, and beetles
experience. In the flies (Fig. 189) the larva is a legless wormlike animal
called a maggot. This changes, in a very brief operation, into a quiescent
nonfeeding form, the pupa. After a definite (usually short) time there
bursts from the pupa shell the adult insect. The development of the

216

PRINCIPLES OF ANIMAL BIOLOGY

adult occurs gradually enough within the pupa, but the emergence of
the fly is sudden. These marked and more sudden modifications make up
complete metamorphosis.

Fig. 188. — Incomplete metamorphosis of a bug. The most easily recognized change is the
gradual growth of the wings. {From Frost, "General Entomology.")

Problems of Development. — The question naturally arises, how are all
these complicated developmental changes brought about? This is the
general problem which experimental embryologists have set themselves.
Some progress in solving it has been made, but much remains to be done.
The knowledge already gained has to do with such questions as why the
embryo is placed in a given position in the seemingly indifferent material
of the fertilized egg; how a structure is stimulated to develop, and how it
is guided so as to acquire its characteristic form; the time at which the fate
of any bit of tissue is settled, and whether the decision at that time is
final or revocable; whether development is a sorting out and the loss of
capacities, or a gain of new ones; the importance of the mere position of a

Fig. 189.

-Complete metamorphosis of a fly. The successive stages shown are the larva,
pupa and adult. {From Frost, "General Entortiolofjy.")

piece of tissue in the embryo, in deciding what it shall become; whether
a tissue is passively moved about l)y some force, or actively assists in the
change; whether the agencies which direct development reside within
the colls, or impose their control from the outside; and the duration of any
influence in relation to the period within which it can normally be efTec-

EMBRYONIC DEVELOPMENT 217

tive. Some of these topics will be considered in connection with concrete
illustrations.

Orientation of Embryo. — The higher animals are all bilaterally
symmetrical; yet they all develop from an egg which is apparently radial.
The animal pole is differentiated from the vegetative, and it is clear why
development should commence in the animal portion. But so far as can
be seen in an unfertilized egg, the head of the embryo might be turned
toward any point in the circumference of the circle of which the animal
pole is the center. What decides the position which it
actually does take?

In the frog, the median plane of the future animal
is fixed by the point of entrance of the spermatozoon
in fertilization (Fig. 190). The first cleavage of the
egg passes through that point and also thi'ough the
animal pole and vegetative pole. Up to the time of Fig^~790 Sec-

fertilization, any plane passing through the two poles tion through frog's
may become the plane of symmetry. In some of the |^J^ cleavag!J!Thow-
salamanders either the first or the second cleavage ing at right the
plane may become the median plane, and the entrance T!^ ^ ^ J'o + ^^^-.T!!^
of the spermatozoon has nothing to do with fixing the (Modified from
positions of these planes. In fishes, sea urchins, '^ " ^^
and some other animals there is no connection between the early
cleavages and later symmetry, and in them it is unknown how the median
plane is determined.

After the position of the embryo is fixed, all later questions of orienta-
tion are settled in relation to it. When, by artificial methods, a second
embryo is made to develop at the surface of the same egg, it is roughly
parallel with the first, with its head pointing in the same direction. A
patch of ectoderm in which gills would normally develop at its anterior
margin may be cut out, turned halfway around, and made to grow in
place. The gills still grow in the anterior portion, but this was originally
the posterior part. Also, if the regenerating stumps of cutoff arm and leg
rudiments be removed and their positions exchanged in transplantation,
the anterior one becomes an arm, the posterior one a leg, which is the
reverse of their normal fate.

Some biologists have suggested that a gradient of some sort is set
up at the first orientation of the embryo. Perhaps a chemical substance
occurs in gradually less and less concentration from front to back, or a
physical phenomenon becomes less and intense in that direction, and
the position of structures is governed by this gradient. Little is known,
however, that would establish this supposition.

Principle of Determination. — Another important question is why dif-
ferent parts of an embryo produce different structures. In the majority

218 PRINCIPLES OF ANIMAL BIOLOGY

of animals there is no fundamental difference between the cells of different
regions. For example, the cells of a sea urchin embryo, in the two- or
four- or eight-cell stage, may be separated from one another, and each
becomes a complete, though small, larva. If left in contact with the
other cells, each cell would have produced only certain parts of a single
animal, but it obviously has the capacity to produce all of it. In a few
animals, however, the cells are in some respect different, for, if the cleav-
age cells are separated, each one gives rise only to a fraction of a larva.
Animals of the former type are said to have indetermininate development,
the latter kind determinate development.

The cells of indeterminate embryos take on their specific destinies
at a mut;h later time. This has been most completely shown for some
of the salamanders. If, at a time shortly after gastrulation begins, bits
of tissue are transposed, a group of cells that would normally become
nervous system exchanging places with a group that would become epi-
dermis, the fate of each is altered. The would-be part of a nervous
system becomes epidermis, the prospective epidermis becomes nervous
system. The exchange of regenerating stumps of fore- and hind limbs,
described in the preceding section, is a similar example. The interchange
of bits of tissue may be made between different species ^vith equal success.
One such interspecific exchange was effected between species differing
in color, one very light-colored, the other quite dark. The cells retained
their color characters but produced strange organs. In one experiment,
presumptive brain cells of a dark species were transplanted to the region
on a light species where gills develop. Now these species differ not only
in color; their gills are of different shapes. The transplanted dark cells,
while being converted into gills instead of brain, produced gills of the
form characteristic of the dark species. The general fate of the cells
may be altered, but their specific performance within the general field
remains unchanged.

In all these examples the fate of the transplanted tissues had not
yet been determined. For each of them, however, there comes a time
after which such reversals of fate are no longer possible. After that time,
transplanted parts become what they would have become in their original
situation. If, for example, a patch of ectoderm including a portion of the
neural folds (a stage shortly after the end of gastrulation) is placed on the
side of the body, it Ijecomes nervous system despite its strange location.
Something has hapi)ened to these cells during the process of gastrulation
wliich has deprived them of the capacity to respond to their position in
the embryo and has fixed their fate regardless of location. An area of
such determined ectoderm may oven be cut out of the embryo and culti-
vated by itself in a suitable salt solution, and it still develops the sort of
organ (nervous system, for example) which it was destined to become.

EMBRYONIC DEVELOPMENT

2H:

Organizers. — What induces this change in a tissue, destroying its
apparent independence of action, and forcing it into a single further
course? It is often some influence coming from other cells near it. In
salamander embryos, the cells which roll over the dorsal lip of the blasto-
pore and become the notochord and mesoderm (Fig. 180) exert such an
influence. It is because of them that neural folds are produced in the
ectoderm above the notochord. The mesoderm cells possess that power
of inducing nervous system even before they are invaginated into the
gastrula. This is beautifully shown by an experiment. If some cells
are removed from the dorsal rim of the blastopore, before they are invagi-
nated, and are inserted among the ectoderm cells of another embryo, at a
place where only epidermis w'ould ordi-
narily develop, they sink below the surface
and are covered over by the ectoderm.
From that ectoderm an additional nervous
system is formed, so that the embr^^o has
two nervous systems (Fig. 191). The
transplanted cells would, in their own
embryo, have been invaginated to form
mesoderm and would have induced a
nervous system in the ectoderm above
them. That same influence they exerted
on the strange ectoderm beneath which
they were planted.

In a similar way, the eye stalk pro-
truding from the side of the brain (Fig.
184), as it approaches the outer ectoderm,
stimulates that layer to thicken and
invaginate to form the crystalline lens of
the eye. In some animals the ectoderm
forms a sort of lens without such stimulus, as when the eye stalk is cut
off; but the lens is seldom normal unless the optic stalk comes near it.

Something issues from the prospective mesoderm and the eye stalk,
in the above examples, which causes the ectoderm to develop a certain
structure. This something, whatever it is, has been called an organizer.

An important question arises, whether embryonic development is
conducted by a series of such organizers, produced in succession in dif-
ferent structures. May one organizer ensure the development of a cer-
tain organ, and then a different organizer arise in that organ that would
stimulate a third organ, and so on? Some slight indications of such
chains may be found, but they are not general. The eye stalk often
stimulates a lens, and the lens then helps to bring about the invagination
of the optic cup to form the retina. A few other such chains of influences

Fig 191. — Development of
nervous system in response to trans-
planted cells. Left, neural fold of
salamander, Triton, developing in
its normal situation. Right, op-
posite side of same embryo, witli
additional neural fold produced
because cells from the dorsal lip of
the blastopore of another embrjo
were transplanted in that region.
The transplanted cells were from a
lighter colored species and form the
pale streak in the middle. {Modi-
fied from Spemann.)

220 PRINCIPLES OF ANIMAL BIOLOGY

are known. In general, however, the events of early embryonic develop-
ment appear to be more or less independent, though working in harmony.
Probably they are helped to keep in the proper order of time by organizers
that successively arise.

Nature of Organizers. — These organizers are not specific, not effec-
tive merely in their own species, since transplants between species show
about the same consequences as those within species. This fact has
encouraged a search for the nature of such influences, for the same ones
must be fairly general and widespread. Almost certainly organizers are
chemical substances. A number of organic acids have been shown to
induce certain differentiations. Among them are several of the fatty
acids, nucleic acid, and adenylic acid from muscle. There is some
indication that the sterols (higher alcohols) have inductive powers.
Glycogen is probably in some way connected with the power of induction
in salamanders, for while the cells are being rolled over the rim of the
blastopore during gastrulation (which is about the time at which these
cells first acquire the power to induce nervous system), they rapidly
lose their glycogen. What an organizer does to stimulate development,
what happens between the stimulus and the response in differentiation,
is unknown.

The power of an organizer to induce a certain event usually lasts much
longer than there is any need of it in ordinary development. Thus,
notochord and mesoderm, taken from embryos in which nervous systems
have long since been irrevocably established above them, are still capable
of stimulating secondary nervous systems in younger embryos into which
they are transplanted. The power of the tissues to respond to organizers
is, however, not so persistent. Usually they must be stimulated at about
a certain time, or they cannot respond at all.

In general, it may be said that the inherent properties of the tissues
to respond by developing arc more important than the stimuli received
from organizers.

References

Bailey, F. R., and A. M. Miller. Textbook of Embryology. William Wood &

Company. (Chaps. I-VI.)
Hegner, 11. W. The Germ Cell Cycle in Animals. The Macniillan Company.

(Chaps. I and II.)
Holmes, S. J. The Biology of the Frog. The Macmillan Conii)aiiy. (Chap. V.)
Kellicott, W. E. a Textbook of General Embryology. Henry Holt & Company,

Inc. (('haps. VI-VIII, cleavage to formation of germ layers.)
Morgan, T. H. The Development of the Frog's Egg. The Macmillan Company.

(Chap. V, early development.)
Morgan, T. H. Experimental Embryology. Columbia University Press. (Chap.

XV, fate decided before cleavage; ("laps. XVII and XVIII, partial embryos;

Chap. XIX, fate determined by position.)

EMBRYONIC DEVELOPMENT 221

Prentiss, C. W., and L. B. Arey. A Laboratory Manual and Textbook of Embry-
ology. W. B. Saunders Company. (Chaps. I and II.)

Spemann, H. Embryonic Development and Induction. Yale University Press.

WiEMAN, H. L. An Introduction to Vertebrate Embryology. McGraw-Hill Book
Company, Inc. (Chaps. Ill and IV, early development of the amphioxus, frog,
and chick.)

CHAPTER 17
GENETICS

The object of the study of embryology, as outHned in the preceding
chapter, is to discover how animals become what they are. Mere obser-
vation shows by what steps the development proceeds, and experiments
have revealed some of the physiological principles underlying these
events. Embryology discloses these things satisfactorily for the major
features of structure which are essentially alike in whole large groups
of animals — satisfactorily, that is, if one does not require to know the
fundamental causes of the different types and steps of development.

There are, however, many minor features of organization which are
just as definitely fixed parts of animals as their digestive and nervous
systems are, but which are different in different individuals. Color of
eye, shape of hair, dimples, stature, complexion, and talents are different
in different people, yet all through their embryonic development it is
quite settled what these characters are going to be. No embryologist
could tell what the outcome would be in any of these traits in the adult,
but the die would have been cast before cleavage of the egg had begun.

These individual differences furnish another way of learning the rules
governing the development of characteristics. This method consists
of crossing individuals having different traits and observing the occur-
rence of these different traits among the descendants. This is the method
of genetics. It would be impossible to use it to discover much about
the structures with which embryology deals, for there is no difference
between individuals ^vith respect to the major features. Nevertheless
when the mode of inheritance of minor characters has been discovered,
it may be taken as certain that the inheritance of the major features
follows the same scheme. Genetics uses minor features to discover the
principles of heredity, with the conviction that the same principles apply
to the major features as well.

Genetics has the further advantage, in the study of origins, that it
reveals more fundamental causes. While embryology, when it employs
experiment, may reveal physiological processes (;ausing the developmental
changes, genetics lays bare to some degree the causes of the physiological
processes. It is today one of the biologist's most })otent tools in delving
into the fundamental nature of living things. Embryology is an aid
because it reveals some of the visible mechanism of heredity, particularly

222

GENETICS 223

in the maturation of the germ cells, but the crossing of unlike individuals
demonstrates the nature of much that is invisible.

Modem Genetics. — The story of the Austrian monk Gregor Mendel
as the leading figure in the beginnings of modern genetics has been
recounted in the opening chapter (page 18). Before going into the
details of hereditary transmission, it will be profitable to indicate briefly
wherein his ideas of heredity differed from those which preceded him;
for it must be remembered that Mendel was not the first student of
heredity. Many before him had tried to solve its mysteries, and the
mere fact of resemblance between parents and offspring, or even between
more distant relatives, had been recognized from time immemorial.

One of the chief distinctions of the Mendelian system was the recogni-
tion that offspring do not necessarily inherit any particular character
of either parent. Not only do the offspring not have to show such a
character in themselves, they may even be quite incapable of transmitting
it to subsequent generations. Prior to Mendel's time there had been a
prevalent suspicion that any character which appeared in one or more
individuals in a given line of descent might be expected at some future
time to appear in any branch of their posterity. No one of the descend-
ants was to be regarded as free from the possibility of that character's
recurrence. According to this old notion, if in a given line of descent
of horses there had once been a chestnut animal, there was a distinct
expectation that some time or other the chestnut character would
reappear in some individual of any branch of the descending family.
According to the IMendelian scheme, it is now clear that this color may
be bred entirely out of the descendants. It is almost certain to be
bred out of some branches of the general relationship and may be lost
to all of them; and chestnut is no more likely to occur after such elimina-
tion than it is in a line which never had a chestnut ancestor. Later
we shall see why this is true.

Another distinctive feature of Mendel's contribution to knowledge
of heredity was his discovery that characters may be transmitted quite
independently of one another. Wing length is one character, eye color
another, body color a third, and so on, each having its own inheritance.
Because of their separateness, such characters have been spoken of as
"unit" characters. Some degree of detachment of traits was, of course,
popularly implied when it was pointed out that a child had its mother's
eyes, its father's lips, and perhaps its grandfather's wavy hair. But
complete scattering of one individual's characters in succeeding genera-
tions was not previously thought to take place — certainly not as a regular
occurrence. Before Mendel's time there was a strong tendency to think
of heredity in terms of the totality of characters exhibited by an indi-
vidual; by Mendel himself emphasis was put upon the single characters.

224 PRINCIPLES OF ANIMAL BIOLOGY

Heredity, Mendel concluded, juggles characters, not individuals; it
deals with traits, not ancestors and descendants. The complete inde-
pendence which he supposed characters to have is illustrated by the peas
which he studied. He found that shape of pod, color of seed, height of
stem, etc., were entirely free to go to the various offspring without refer-
ence to the other characters. Thus there arose different combinations
of the characters in different plants. One would have constricted pods,
green seeds, and tall stems; another inflated pods, green seeds, and dwarf
stems; a third constricted pods, yellow seeds, and dwarf stems; and so on.
This freedom of assortment proved later, in heredity in general, to be
less than Mendel supposed, but it is very ^\idespread.

Mechanism of Heredity. — How heredity operates will be more
easily understood if its mechanism is known. Inherited characters are
represented in the cells of an organism by minute bodies called genes.
These genes are located in the chromosomes and are demonstrated in
some animals and plants to be in a row, from one end of the chromosome
to the other. There are two genes representing each character in each
cell, one of them derived from the mother, the other from the father.
These two genes must, from their source, be in two different chromo-
somes, one of which has come from the individual's mother, the other
from its father. The genes in one of these chromosomes all relate to
the same characters as do the genes in the other chromosome. Two of
the chromosomes in the cells of the vinegar fly Drosophila are diagram-
matically sho^vn in Fig. 192. Two chromosomes having corresponding
genes, as these do, are said to be homologous with one another (see
page 252 for homology). The genes in them are likemse homologous;
the gene for yellow body is homologous with the gene for gray body,
white eye with red eye, complete eye with bar eye, and so on.

All the chromosomes in a cell are members of such homologous pairs.
One chromosome of each pair has come from the mother, the other from
the father. The two homologous chromosomes come together in a
pair in the oocytes and spermatocytes early in the maturation of the
germ cells, as in Fig. 164. In the reduction division they are separated
again, one going to each of the cells produced by that division. Since
the genes are in the chromosomes, the two homologous genes of every
pair part company at the reduction division, one gene going to each of
the cells produced. At the end of maturation in the male, each sper-
matozoon contains one gene of every pair, never both of any of them.
In the female, each mature egg contains one gene of every kind, ncncr
both. Polar bodies receive their share of the genes, but these genes are
lost as the polar bodies degenerate.

As a result of the reduction division, therefore, the mature germ cells
have a single set of genes, one of every kind. Body cells, on the con-

GENETICS

225

trary, have a double set, two genes for each character. When egg and
spermatozoon unite in fertihzation, the zygote receives a double set of
genes, and these are handed on as a double set to all the cells of the
body of the individual produced from that zygote. With this under-
standing of the mechanism of heredity we may now turn to some con-
crete examples of its operation.

Yell 01^ Body
Nhife £ye

I Oroi/ Body
Red Eye

Norma/Win^

^NonmalU/inq

Verm f a on
E.ye

MiniaturQ
Nino

RedEye
Miniature

Rudimen-'
fan^Ninq

Forked
Brisf/es

Co/n/JsfeEi^e

Rud I men-
fa r^l^in^

Forked
Bnisiles

Ban £y6

Fig. 192. — Diagrams of two homologous chromosomes of the vinegar fly Drosophila.
Some of the genes are represented, and are in their proper order through the length of the
chromosome. Homologous genes are located at the same level in the two chromosomes.

Simple Inheritance. — Among guinea pigs there are different color
varieties which breed true so long as animals of the same color are mated
Avdth one another. One of these true-breeding strains is black (Fig. 193),
another one is albino or wdiite, from the absence of all of the ordinary
pigments in skin and hair and the iris of the eyes. If a black animal is
mated with a white one, the offspring are all black. This result is
described by saying that black is dominant, white recessive. The hybrid

226

PRINCIPLES OF ANIMAL BIOLOGY

generation is known as the Fi generation (abbreviated from the words
first filial). The white coat is not lost in the Fi animals, however, for
when they are mated together they produce an F2 (second filial) gener-
ation consisting of some blacks and some whites. In a large collection

Fig. 193. — Black and white guinea pigs, with smooth coats. (Courtesy ofProfessor W. E.

Castle and the Harvard University Press.)

of such F2 families the black animals are found to make up about three-
fourths of the total number, the wliites about one-fourth.

These results are explained by the diagram in Fig. 194, where the
genes involved are symbolized by letters — the white gene by w, the
black gene by W. The two letters under each parent are its genetic

formula, the single letter under these

White

Eggs'

Black X
WW

F, V/w (Block)

w>=^— ^ w

Sperm

the formula of the germ cells of that
parent. The Fi generation has the
formula Ww, and the animals are
black because one gene W is capable
of producing black pigment just as
well as two IF's are. That ability
of one gene to do the work of two is
what is called dominance; W has
that ability, w does not.

When the reduction division
occurs in Fi animals, two kinds of
germ cells are produced because the
two genes are different. Some eggs
and spermatozoa contain W, some
contain w; and the numbers of the
two kinds are about equal. When two kinds of eggs, equally numerous,
are fertilized at random by two kinds of spermatozoa, equally numerous,
four combinations result, also about equally numerous. These combi-
nations are WW, Ww, wW, and ww, as shown in the F2 line of the figure.
The first three of these are black, the last one white — hence the 3:1
ratio of blacks and whites. The two middle formulas are identical, and

MWW /4Ww MwW A WW

Black Black Black White

Fig. 194. — Inheritance of black and
white color in guinea pigs: W, gene for
black; w, gene for white.

GENETICS 227

would ordinarily be written the same way; they are written in opposite
orders here merely to show that the gene which came from the egg in
one came from the spermatozoon in the other.

Choice of the letters W and w to represent this particular pair of
genes is in accord with a generally accepted convention that the name
of the newer character should suggest the symbol. Without much doubt
there were colored guinea pigs before there were white ones, hence white
is the newer color. In accord "with another convention the small letter
is used for the recessive gene, the capital for the dominant.

To describe other types of matings, it is desirable to provide names
for certain of the genetically different types of individuals. An organism
whose two genes for any particular character are alike (TFTl^ or ww) is
called a homozygote; one whose genes are different (Ww) is a heterozygote.
The same animal may be, and usually is, homozygous for some genes,
heterozygous for others.

F. Black X White Fj Black x Black

Ww WW

Eggs _3li;ir= W Sperm Eggs

erm

B.C.

)iWw /iww
Black White

WW /^Ww
Black Black

Fig. 195 Fig. 196

Fig. 195. — Backcross of a heterozygous black guinea pig with a white animal.
Fig. 196. — Backcross of a heterozygous black guinea pig with a homozygous black
animal.

Backcross. — Not always do e.xperiments proceed from an Fi gener-
ation to an F2. A very useful kind of cross is that between an Fi animal
and another like one of its parents. Such a cross is a backcross. Essen-
tially it is a mating of a heterozygote with a homozygote. Such a cross
might well be made between a heterozygous Fi black guinea pig and a
white one exactly like the white parent. Figure 195 shows what happens
when that is done. The heterozj^gous parent produces two kinds of eggs,
in equal numbers, the white parent only one kind of spermatozoon (w).
Consequently there are two kinds of offspring, heterozygous black (Ww)
and white (ww) in equal numbers.

The backcross may also be made betw^een an Fi and the black parental
type, as in Fig. 196. There are two kinds of offspring as before, with
respect to their formulas; but they all look alike (black). The difference
between these two backcrosses is that one was made to the recessive
parental type, the other to the dominant type. The former cross is

228

PRINCIPLES OF ANIMAL BIOLOGY

made often, the latter seldom because its two kinds of offspring cannot
be distinguished.

Two Pairs of Characters. — Since every animal possesses probably
thousands of different kinds of genes, any mating between individuals
serves as a test of the mode of inheritance of any or all characters in
which the two individuals differ. The experimenter may center his
attention on as many or as few of these as he wishes. For most purposes,
the smaller the number of characters studied simultaneously the better,
for the interpretations are clearer. No more than two pairs of characters
will be used in this book. For an example, Ave may add another pair of
characters in guinea pigs to the black-w^hite contrast already presented.

Ordinary guinea pigs have smooth coats of hair, since the individual
hairs all slope in the same general direction in any part of the skin.

Fig. 197. — Two guinea pigs with rough coats. The hairs are in many places arranged
in whorls, sloping away from the central point. {Courtesy of Professor W. E. Castle and the
Harvard University Press.)

One variety, however, has a rough coat because, at a number of places
on the body, the hairs slope outward in all directions from a central
point like the radiating spokes of a wheel. These hairs push against
other hairs sloping in other directions, producing an unkempt appearance
(Fig. 197). Rough and smooth coat could be used in a single-pair cross,
in which case rough would appear in Fx, and the F2 would be three-
fourths rough, one-fourth smooth. That is, rough is dominant.

The two pairs of characters, hair slope and color, can be combined
in four ways, namely, rough black, rough white (these two in Fig. 197),
smooth black, and smooth white (Fig. 193). To test the inheritance of
the two pairs of characters simultaneously, the animals crossed must
differ in both of them. Suppose one of the original parents is rough
black, the other smooth white. The Fi generation is rough black, since
these are the two dominant characters. When these hybrids, which are
heterozygous for both pairs of genes, produce their germ cells, the genes
of each pair separate from one another in the reduction division and go
to different cells. The two pairs undergo this separation independently,

GENETICS

229

for they are in different pairs of chromosomes. As a result of this
independent distribution, four kinds of germ cells are produced, RW,
Riv, rW, and rw. There are these four kinds of eggs, about equally
numerous, and the same four kinds of spermatozoa, equally numerous.
In fertilization, random unions take place between each kind of egg and
each kind of spermatozoon — 16 combinations all told.

Rough Black x Smooth White

RRWW rrww

RW rw

Eggs'

F; RrWw fRough Black)

RW RW
Rw Rw

rW rW

rw rw

Sperm

RW-RW

Rw-RW

rWRW

rwRW

RW-Rw

Rw-Rw

rW-Rw

rw-Rw

RWtW

RwtW

rW-rW

rw-rW

RWrw

Rw-rw

rW-rw

rw-rw

Fig. 198. — Inheritance of two pairs of independent characters in guinea pigs, black
and white color, rough and smooth coat. The points on the backs of the animals indicate
rough coat.

To write these 16 combinations in the F2 generation without omission
or duplication, it is convenient to use the Punnett square, so named
from the English geneticist who devised it. Such a square is included
in Fig. 198, which explains this cross. Each egg formula is written four
times, in one of the columns of four spaces down the chart. They are
put there to be fertilized by the four different kinds of spermatozoa.

230 PRINCIPLES OF ANIMAL BIOLOGY

Then each sperm formula is written four times in one of the rows of
spaces across the chart. In each space are the genes found in one of the
sixteen kinds of F2 animals. They are written in the chart with the genes
from the egg separated by a dot from the genes from the spermatozoon;
but in other situations it is preferable to write the two genes of one pair
together, followed by the genes of the other pair. Some of the sixteen
formulas are identical Avith others, but they have been arrived at in
sixteen different ways.

It remains only to indicate the appearance of the guinea pigs having
these genes. The little figures accompanying the gene formulas are
intended to do this. Nine of the sixteen are rough black, three rough
white, three smooth black, and one smooth white. It should be remem-
bered that these numbers are a ratio, 9:3:3:1, not absolute numbers.
They are so many sixteenths of the total number in F2. In a single
litter the least frecjuent kind (smooth white, the double recessive) could
easily be missing.

Fig. 199. — Gray and ebony body and long and vestigial wings in Drosophila, combined in

the four possible ways.

A Two -pair Backcross. — ^As a basis of judgment of certain phenomena
to be described later, a backcross involving two independent pairs of
characters will be useful. The characters chosen for illustration are the
color of the body and the shape and size of the wngs of the fly Drosophila.
The body is normally of a brownish gray, but there is a very dark variety
known as ebony. The wings are ordinarily long and lie flat over the
back of the fly when at rest; but in one variation of them, called vestigial,
the wings are small and crumpled and project obliciuely outward from
the body. The vestigial wing is useless for flight; flies with such wings
merely crawl or jump.

The four combinations into which these characters may enter are
shown in Fig. 199. Suppose that the cross be made between a gray long-
winged fly and an ebony vestigial- winged one. The Fi generation is
gray and long-winged, for these are the dominant characters of the two
pairs. If these Fi flies, which are heterozygous for both pairs of genes,
are mated with ebony vestigial flies, which are necessarily homozygous
for the two recessive genes, all four of the kinds of flies illustrated in
Fig. 199 are produced. Moreover, they are about equaUy numerous;

GENETICS

231

about one-fourth of the backcross family are of each of these kinds.
Figure 200 gives the explanation. The four kinds of eggs produced by
the Fi flies are about equally numerous because the two pairs of genes
are distributed at random in the reduction divisions of their germ cells.
Whatever ratio exists among these eggs must also prevail among the
backcross offspring produced from them — hence the equal numbers of
the four kinds of flies in that generation.

Eggs

Gray
Long

EEW

Gray
Lons

EeVv

EV
Ev
eV

Ebony
Vestigial

eevv

Ebony
Vestigial

eevv

ev Soerm

J^EeW i^Eevv J^eeVv J^eevv

Gray
Long

Gray
Vestigial

Ebony
Long

Ebony
Vestigia

Fig. 200. — Inheritance of two pairs of characters in a mating between a double hetero-
zygote and a double recessive. The characters are gray and ebony body and long and
vestigial wing in Drosophila.

Interactions of Genes. — The two pairs of genes studied in guinea
pigs, and the two in Drosophila, appear to be entirely independent of
each other in the production of their characters. An animal with gene
W is black, regardless of the slope of its hair; and one whose formula is
rr is smooth, no matter whether it is white or black. A long-winged
fly may be either ebony or gray, and an ebony fly either long- or vestigial-
^vinged. Very often, however, the action of one gene is modified by
some other specific gene if they are both present in the same individual.
A striking example is found in the combs of fowls. When a pea-combed
fowl (Fig. 201, upper left) is crossed with a single-combed one (lower
right), their offspring are pea-combed, and the F2 generation is three-
fourths pea and one-fourth single. These results indicate that pea comb

232

PRINCIPLES OF ANIMAL BIOLOGY

and single comb differ in just one pair of genes, with pea dominant. In
like manner it is shown that rose comb (upper right) is dominant over
single and differs from single by just one pair of genes.

What should be expected, then, if fowls showing the two dominant
characters pea and rose, respectively, are crossed? No one could pre-
dict the result; it has to be determined by experiment. The hybrid
proved to have a large rounded comb overhanging the base of the beak,
as in the center of Fig. 201. From its shape this comb is called walnut.
A clue to the nature of this remarkable character is obtained by breeding
some of the Fi walnut fowls together. They yield four kinds of offspring

Fig. 201. — Interaction of genes for combs in fowls. The gene for pea comb (upper
left) interacts with that for rose comb (upper right) to produce walnut comb (center). Two
of these Fi walnut-combed fowls, bred together, produce four types of offspring. Single
comb (lower right) is produced when^ neither the pea gene nor the rose gene is present.
{Rearranged from Punnett, " Mendelism." Courtesy of The Macmillan Company.)

walnut, pea, rose, and single (lower row, Fig. 201). Very significantly,
the ratio of these four types is 9 :3 :3 : 1 in the order named and pictured.
This ratio indicates that two pairs of genes are involved. The pea-
combed fowl must have had the formula PPrr, the rose-combed one
ppRR, in which P is the gene for pea comb, p for no pea, R iov rose
comb, r for no rose. Single, which is "no pea" and "no i-ose," is pprr.
The student is encouraged to work out the gene explanation for the
Fi and F2 generations; any individual possessing both dominant genes
P and R will have a walnut coml). Whatever effect these genes have,
singly, on the physiology of comb development, together they interact
to produce a very different effect.

Many other examples of interaction of genes belonging to different
pairs have been discovered. Sometimes the relation is such that one of
the genes in question cannot produce a visible result unless the other

GENETICS 233

gene is present. Sometimes one gene suppresses the action of a gene of
some other pair. Sometimes two genes, neither of which produces any-
thing detectable by itself, combine to produce a visible result when they
occur together in the same animal. When the interactions are between
dominant genes, they result in Fo ratios which are some modification
of the fundamental ratio 9:3:3:1. This ratio is changed because two
or more of the classes of individuals appear alike. To describe details
of such interactions would go beyond the scope of a first study. The
complexity is considerably increased by interactions among three, four,
or five different genes. So many examples of combined actions have
been found that it seems probable that they are universal. That is,
every gene probably interacts with some — even many, or all — other genes.
The phenomena of heredity can be very complicated.

#1% • %■

iw ^ i.»#

' ' , / I I'

9 k\4lA

Fig. 202. — The divisions of the male germ cells of the bug Anasa: a, polar view of
equatorial plate of first division; all the chroniatic bodies are double except one and there-
fore represent 21 chromosomes, the somatic number; b, second division in side view, not all
of the chromosomes shown; the single chromosome of a is going undivided to the lower pole;
c and d, polar view of the two anaphase groups of the second division; 11 chromosomes go
into one spermatid (female-producing), 10 into the other (male-producing). (After Wilson
in Journal of Experimental Zoology.)

Inheritance of Sex. — A special genetic situation exists in the dis-
tinction between the sexes, for in a large number of animals and some
plants the chromosomes of the male and female are in some respect
unequal. Either one sex has one more chromosome than the other, or one
of its chromosomes is larger than the corresponding chromosome of the
other, or certain corresponding chromosomes are of different shapes.
When the number of chromosomes is different, most species of animals
have more in the female than in the male. An example of this condition
is found in a species of bug whose chromosomes are shown in Fig. 202.
The male has 21 chromosomes, the female 22. The figure show\s the
reduction division of the spermatocytes of the male. At the left (a)
are the pairs of chromosomes, mostly so closely united that their double
nature is not revealed. At the bottom of a, outside the circle of other

234 PRINCIPLES OF ANIMAL BIOLOGY

chromosomes, is the odd chromosome not paired with any other. This
unmated chromosome is called an X chromosome. In the division
which follows (b), the paired chromosomes separate (not all of them are
shown), while the X chromosome goes undivided to one end of the
spindle (the lower end in Fig. 202) . The two cells thus formed (c and d)
have 10 and 11 chromosomes, respectively. These two numbers of
chromosomes go into the final spermatozoa, so that there are two kinds
of spermatozoa, one with 11 chromosomes (including an X), the other
with 10 (without an X).

Now, the female of this species has 22 chromosomes, two of which
are X chromosomes identical in composition with the one X of the male.
Her eggs ripen in typical fashion, and every egg has 11 chromosomes,
including one X. When an egg is fertilized by a spermatozoon contain-
ing an X chromosome, the fertilized egg has 22 chromosomes, two of
which are X's, and it develops into a female. If an egg is fertilized by a
spermatozoon without an X chromosome, the fertilized egg has only one
X (21 chromosomes all told), and it becomes a male.

Whether there is a definite gene, or perhaps several genes, for sex
in the X chromosome is not yet certain. They are in any case not the
sole determiners of sex, for in Drosophila the other chromosomes contain
genes modifying sex.

Sex -linkage. — If, in species in which the two sexes have unlike chro-
mosome groups, there are genes for other characters in the chromosomes
that are chiefly associated with sex, it is obvious that these characters will
be difi^erently inherited in the males and females. When the female has
two X chromosomes, and the male only one X chromosome without any
mate, any genes contained in the X chromosome will come to the male
from only one parent (his mother), while the female will receive them
from both parents. Furthermore, such genes even if recessive will
produce their character in the male, because there is no other gene of the
same pair to be dominant over it. The same situation exists in species in
which the nujnhcr of chromosomes is the same in both sexes, but the shape
or physiological properties of one of them are different. In such species
the male possesses what is called a Y chromosome corresponding to one
of the X's of the female; that is, the male is XY, the female XX. The Y
chromosome possesses few known genes and with respect to most charac-
ters might as well be absent.

Characters whose genes are in the X chromosome are said to be sex-
linked. How sex-linked characters are inherited is shown in Fig. 203
which illustrates Drosophila in which the males have the XY constitution.
The Y chromosome in this fly is shaped somewhat like a letter J. The
character involved is white eye as contrasted with red. In the first
cross (left) the female is white-eyed (ww), the male red-eyed (IF). The

GENETICS

235

female produces only one kind of egg {w); but the male, because of his
Y chromosome which lacks any gene of this pair, produces two kinds of
spermatozoa, one having the X chromosome (with w), the other the Y
chromosome. The two combinations of eggs and spermatozoa produce
the two sexes, respectively, of the Fi generation. The males of this
generation are white-eyed because there is no red gene (W) to dominate
over their white gene. In the F2 generation, as the figure shows, there
are four combinations, two of which are red, two white. The marks of
sex-linkage in tliis cross are (1) that the Fi generation is of two kinds.

Fig. 203. — Sex linkage of eye color in Drosophila. Left, white-eyed female X red-eyed
male. Right, the reciprocal cross. (Modified from Morgan, Sturtevant, Muller, and Bridges,
"Mechanism of Mendelian Heredity," Henry Holt and Company, Inc.)

instead of only the dominant type, and (2) that the F2 ratio of dominant
to recessive is 1:1 instead of 3:1.

If the cross is made with the red eyes in the female and white eyes in
the male, the results shown are as indicated at the right in Fig. 203. Tht
Yi males get their eye color gene from their mother as before but now are
red-eyed, as are also the heterozygous Fi females. In the F2 generation
there are again four combinations. Three of these are red-eyed; hence
the F2 ratio is 3 red: 1 white. However, the white-eyed F2 f^ies are all
males. This last feature is the only sign, when the cross is made this
way, that the character being studied is sex-linked.

Any animal or plant whose sex is determined by chromosomes, and in
which, as a consequence of this chromosome relation, the male produces
two kinds of spermatozoa, may be expected to show sex-linkage of the
kind just i"'istrated. Man is one of these animals. A modified form of

236

PRINCIPLES OF ANIMAL BIOLOGY

this same phenomenon is found in birds, butterflies, and moths, for in
these groups the sex-determining chromosomes are so arranged that the
female produces two kinds of eggs and the male only one kind of sperma-
tozoon. The distribution of the sex-linked genes in these animals is
precisely .like that in Drosophila except that the sexes are reversed.
What is true of the male in Drosophila is true of the female in birds, for
example. An opportunity to work out the situation in birds is afforded
by one of the problems at the end of the chapter.

Autosomal Linkage. — The chromosomes other than X and Y are
known as autosomes. When two genes for different characters are located

in the same autosome, they have a
strong tendency to remain together for
a while, going to the same germ cells.
How many successive generations they
stay together depends on how far apart
the genes are in the chromosome.
The chromosomes break more or less
at random and homologous chromo-
somes recombine their pieces in new
ways. If the breakage occurs between
two pairs of genes, the genes enter into
new combinations. The genes which
had been going to the same germ cells
now go to different germ cells. Natu-
rally the farther apart they are, the
more often the breaks occur between
them.

Linkage operates to distort the
expected ratios of different kinds of
individuals. This effect is illustrated
in Fig. 204. The two pairs of genes
involved are v (vestigial wing) con-
trasted with V (long wing), and h
(black body) as against B (gray). The chromosome composition of the
two flies is shown at the top of the illustration. The chromosomes in
their respective germ cells are pictiu*ed between the parents, and the Fi
female fly below. This Fi fly is hetei'ozygous for both color and wing
length and affords an opportunity to discover the breakage of the chromo-
somes. It produces four kinds of eggs, as shown at its right. The first
two of these {Bv and hV) are produced if the two pairs of genes are not
separated; and the genes are near enough together so that this happens
in about 83 per cent of all cells. In the other 17 per cent the two pairs of
genes are separated by breakage of the chromosomes, resulting in the

Fig. 204. — Linkage of body color
and wing length in Drosophila. Left,
above, gray vestigial-winged male;
right, black long-winged female.
{From Morgan, "Physical Basis of
Heredity," J. B. Lippincott Company.)

GENETICS 237

other two kinds of eggs (bv and BV). The Fi female is represented as
mated to a black vestigial male, whose spermatozoa are necessarily hv.
These spermatozoa fertilize the four kinds of eggs and produce four kinds
of offspring which should be in the same proportion as the kinds of eggs.
The first two kinds (gray vestigial and black long) together make up
about 83 per cent of the family just as the Bv and hV eggs made 83 per
cent of the eggs. The other two classes (black vestigial and gray long),
coming from eggs containing broken and recombined chromosomes, con-
stitute about 17 per cent. If these two pair of genes had been in different
pairs of chromosomes and so had been independent of one another, the
last generation would have exhibited a 1:1:1:1 ratio, each kind making
about one-fourth of the total, as in the two pairs of characters in Fig. 200.
The distorted ratio is the evidence that the genes are all in one pair of
chromosomes.

Mendel's Law; Mendelian Heredity. — Gregor Mendel never stated
his discoveries in the form of a concise principle, but this has been done
by others since. Heredity as Mendel conceived it differed in two impor-
tant respects from heredity as understood by his predecessors. A state-
ment of these two differences is commonly spoken of as Mendel's law.
Using present terminology, one might state this law as follows. The
genes of any pair separate from each other in the production of the germ cells,
so that each germ cell receives only one of them; and the distribution of each
pair of genes to the germ cells is independent of the distribution of other pairs.
The separation of genes of the same pair is effected by the reduction
division in maturation. Independence of the genes of different pairs
exists when the pairs of genes are in different pairs of chromosomes,
since these pairs of chromosomes are independently placed on the spindle
of the reduction division. As is indicated in the preceding section, this
latter condition is not always met. Many pairs of genes are in the same
pair of chromosomes. Autosomal linkage, which results from this
association, is very common. Such linkage is a violation of the second
part of Mendel's law. Apparently Mendel never witnessed this relation
between any two pairs of genes.

Despite the fact that Mendel's law as stated does not provide for
linkage, all the phenomena so far described are still regarded as belonging
to Mendelian heredity. The concept of Mendelism has been widened to
include them. Any heredity is now considered Mendelian if it is depend-
ent on chromosomes. Most heredity is so dependent. Yet in some plants
the plastids go over directly from one generation to the next, and what-
ever color characters these plastids determine are independent of chromo-
somal genes. Heredity of plastid colors in such plants is not Mendelian.
Possibly, even probably, there are some other structural units which are
transmitted directly like plastids.

238 PRINCIPLES OF ANIMAL BIOLOGY

Since the inheritance of Hnked characters is still called Mendelian,
it would be better if the statement of Mendel's law could also be liberal-
ized. A better formulation would be: The fundamental units of heredity
are distributed hy means of the chromosomes. This would exclude' plastids.
Also, to understand the law it would be necessary to know a good deal
about chromosomes.

The Nature of Genes. — It is practically certain that the genes are
chemical substances and that it is through their chemical properties
that they control the development of the characters they represent.
Presumably they are protein in nature. One reason for considering them
protein is that the chromosomes give protein reactions, and the genes
make up a fraction of the chromosomes. Moreover, genes are highly
specific in their action; that is, they do certain definite things with con-
siderable precision, and not other things. Highly specific reactions are
characteristic of proteins in general, which would help to explain the
functioning of genes if these be protein.

Moreover, genes are subject to change. Although any mechanism of
heredity must have some degree of permanence — otherwise there would
be no heredity — genes do not remain forever the same. One of the genes
for red eye in Drosophila changed, and the eye color was then brown.
A gene for gray body color in the same species changed, and yellow body
resulted. A gene for uniform color in mice changed, and the mice in
succeeding generations were spotted. Changes of this sort are known
as mutations. They must be chemical changes of the genes, which would
be not only possible but probable if the genes were proteins. The chemi-
cal structure of proteins is very complex, and occasional permanent
change is more likely in complex substances than in simple ones.

It is a current concept that a gene may be a single protein molecule.
One reason for so believing is the suddenness with which gene mutations
occur. If a gene were composed of several molecules, any change in
chemical structure would presumably, just as a matter of chance, affect
only one of them. The argument is that, with a number of molecules to
change, mutation might tend to be a gradual process. With only one
molecule, any structural change must affect the whole gene at once.

Practical Applications. — Knowledge of heredity has been used for
centuries to improve the economic situation of the human race. The
classical field in which that has been done is the in^provement of crops and
farm animals. The knowledge upon which this improvement rested
was, until comparatively recent times, little more than a knowledge that
heredity existed. Its laws have been fairly well understood by breeders
only in the present century, but by the year 1900 most of the develop-
ment of domestic races had already been accomplished. The reason for
this great success of the early breeders is that their method was practically

GENETICS 239

the same as it is at present. That method is selection. Those animals
and plants which were most valuable were selected for breeding, in the
belief that their good qualities would be transmitted. If even only a few
of these characteristics were inherited, long-continued selection would
result in great improvement.

The discovery of Mendel's principles thus found mankind already in
possession of very valuable varieties of animals and plants. Man had
attained this result without knowing very much about how he did it.
Improvement has, of course, gone on since then. It is now considerably
plainer why certain results are obtained, and these results often come
more quickly. Among the important domestic animals, poultry have
probably yielded more to the newer Mendelian knowledge than any
others. Considerably less has been done with pigs and sheep, and little
has been revealed about Mendelian behavior of the valuable characters of
cattle and horses. Undoubtedly the cost of experimenting with these
larger animals and the long time involved, when one generation requires
several years, are responsible for the lag of knowledge concerning their
heredity.

Plants have revealed more of their hereditary constitution, partly
because they are inexpensive to rear, partly perhaps because they are
of simpler composition. The most important feature of most crops is
yield, which is inherited, since varieties differ greatly in this respect. The
principal factor contributing to yield which is being studied now more
successfully than a generation ago is resistance to disease. The various
bacterial and fungous diseases of the grains and fruits are receiving con-
centrated attention at most of the experiment stations, and the results
attained are very gratifying.

Room exists for improvement of man himself, through the elimination
or diminution of some of his defects. Every system of organs and every
sense organ exhibits hereditary deficiencies in some individuals, such as
feeble-mindedness, fragility of bones, a tendency to bleed, cataract of
the eyes, atrophy of muscles, and baldness. Some of these defects are
more important than others, but there is not one which the human race
would not choose to banish if it could. The only method is to avoid
reproduction by individuals possessing genes for the undesirable qualities.
With respect to most defects, this avoidance must be voluntary, and it is
uncertain how seriously men and women take their responsibilities.
Some of the more serious defects, such as feeble-mindedness and epilepsy,
are, however, frequently dealt with by law. At present 29 states of
the United States have laws designed to prevent people afflicted with
these infirmities from rearing families.

Theoretically, man should be able to improve himself by favoring
those qualities, talents of various sorts, which it is particularly desirable

240 PRINCIPLES OF ANIMAL BIOLOGY

to possess. Unfortunately, too little is known of the heredity of these
traits to raise the hope that such improvement is imminent. No one as
yet knows the formula for the production of genius at will.

Problems

1. A rose-combed fowl (Fig. 201, upper right) mated with a single-combed fowl
produces only rose-combed offspring. If many of these offspring are mated together
and produce an aggregate of 64 fowls, how many of the latter should be rose-combed?

2. Tall peas are dominant over dwarf peas. What would be the appearance of
a plant heterozygous for tall and dwarf? If such a heterozygote were self-fertilized
and produced 30 dwarf offspring, how many tall offspring should it yield?

3. Mating a red-eyed and a pink-eyed fly yields red-eyed offspring. If one of
these red-eyed offspring is mated with its pink parent, and they produce 60 offspring,
how many of these should be red-eyed?

4. Brown color in mice is dominant over albinism. In a given cross between a
brown mouse and an albino, 6 of the offspring were brown, 5 albino. What was the
formula of the original brown parent?

5. A long-winged fruit fly mated with one having vestigial wings (a recessive
character) produced 28 long-winged and 23 vestigial offspring. What were the
formulas of the parents? Of their long-winged offspring? Of their vestigial-winged
offspring?

6. Snapdragons with bilaterally symmetrical flowers, crossed with plants with
radial flowers, produce only bilateral Fi. If an Fi plant is self-fertilized, what is the
chance that one of its offspring selected at random will be radial?

7. Shepherd's-purse with triangular seed capsule is dominant over the variety
with spindle-shaped seed capsule. If a homozygous triangular is pollinated from a
heterozygous triangular, and 20 offspring are obtained from them, how many of these
should have spindle-shaped capsules?

8. Starchy grain is dominant over sugary grain in corn. If, in a cross between
these types, 58 of the progeny are sugary, how many of the progeny should be starchy?

9. A certain white-fruited squash, self-fertilized, produced some white and some
yellow offspring. If there were 21 yellows, how many white would be expected?

10. Short hair is dominant over long hair in guinea pigs. A short-haired guinea
pig, one of whose parents was long-haired, was mated with a long-haired animal.
If, blindfolded, you selected one of their litter from the cage, what is the chance you
would get a long-haired animal?

11. The offspring of a brown mouse and an albino are all brown. If the hetero-
zygous brown mice are mated together and produce 80 offspring, how many of these
should be albino? How many of the brown ones should be heterozygous? How
could you tell which browns were heterozygous?

12. If gray color in an animal mutates to yellow, and in crosses between stocks
of gray and yellow the offspring are yellow, what (according to accepted conventions)
would be the symbol for the gray gene? For the yellow?

13. If an animal having the formula Cc produces 100 eggs, how many of these
eggs should luive the formula C? How many c? How many Cc?

14. A family consisting of 17 red-eyed and 15 purple-eyed flies probably came from
a mating of parents whose formulas were P and (Fill the blanks properly.)

15. Applying the conventions relating to choice of symbols for genes, make a
number of matings between trotting horses Pp X Pp, and obtain 24 foals. How
many of these should be pacers?

GENETICS 241

16. If two parents which haVe the same visible characters produce some offspring
wliich are hke the parents, some different, write the formulas of the parents using any
symbols you choose.

17. In squashes, white fruit is dominant over yellow. From a certain cross
between a white- and a yellow-fruited plant, 54 white and 59 yellow offspring were
obtained. What were the formulas of the parents, if squashes were primitively yellow
like pumpkins?

18. One flower of a white-fruited squash plant A is pollinated from another white-
fruited plant B, and both white and yellow progeny are produced. Another flower
of plant A is pollinated from a yellow-fruited plant and produces 44 offspring. How
many of these should be white?

19. A third flower of plant A in problem 18 is self-fertiUzed and produces 44
offspring. How many of these should be white?

20. Two gray female mice are mated with a black male. In several litters the
first female produces 12 gray and 10 black offspring, the second female 19 gray.
What are the formulas of the two females? Use your knowledge of wild mice in
determining part of your answer.

21. Pink eye in mice is recessive to the wild-type dark eye color. From a certain
mating between two dark-eyed mice some dark- and some pink-eyed mice are obtained.
The male is then mated with a pink-eyed female, and they produce, in several litters,
20 offspring. How many of these should be pink-eyed?

22. Uniform or self-color in mice is dominant over spotting. A self-colored mouse
is mated with a spotted mouse, and their self-colored offspring are mated together.
All the offspring of these crosses are mated to spotted mice. Assuming all matings
to be successful, and the resulting litters of equal size, what fraction of the mice from
the last matings should be spotted?

23. Mating a red-eyed fly with curved wings and a claret-eyed fly with straight
wings yields an Fi all red-eyed and straight-winged. If the Fi flies are bred together
and produce 96 offspring, how many of these should be claret-eyed and straight-winged?

24. Frizzled feathers in fowls are turned up at the end, smooth plumage lies down
flat. Pea and single combs are illustrated in Fig. 201. If a cross between single
smooth and pea frizzled yields pea frizzled, and if these are mated together and pro-
duce in the aggregate 48 fowls, how many of these should be single smooth? How
many pea frizzled?

25. Self-colored rats (color distributed over the body) are dominant over hooded
(color only on head, rest of body white). Albino rat is recessive to gray. Crossing
a homozygous gray hooded rat with an albino having a pair of genes for self-color
(which, of course, cannot show in an albino) would produce what kind of offspring
in Fi? If the Fi animals were bred together and produced 80 offspring, how many
of these should be albino? How many gray hooded?

26. Two walnut-combed fowls, mated together, produce 9 walnut-combed and
3 pea-combed ofifspring, and no others. Assuming that no class of offspring is missing
because of the small numbers, what were the formulas of the parents?

27. In cattle, black (B) is dominant over yellow (fe), and polled (P) (hornless) is
dominant over horned (p). If several homozj^gous black horned cows are mated with
homozygous yellow hornless bulls, what will be the appearance of their offspring?
If these offspring are mated with one another, and in a number of such matings 9
yellow polled animals are produced, how many black polled ones would be expected?
How many yellow horned?

28. If a homozygous red mule-footed pig (toes grown together) is mated with a
homozygous black normal-toed pig, their offspring are black and mule-footed. If

242 PRINCIPLES OF ANIMAL BIOLOGY

the Fi animals are crossed with red normal-toed ones and produce 80 offspring, how
many of these should be red and normal-toed?

29. Black is dominant over white in sheep, and in certain breeds horns are domi-
nant in males but recessive in females. A homozygous black hornless ewe of one of
these breeds is mated with a homozygous white horned ram. If their offspring is
female, what will be its appearance? If male, what appearance? If a number of
Fi males and females from such parents are mated together, and produce 32 offspring,
equally divided between the sexes, how many of these will be black horned females?
How many white horned males?

30. Red eye (B) is dominant over brown (6) in Drosophila, and pigmented ocelli
(WO) dominant over white ocelli (wo). A certain brown-eyed fly with pigmented
ocelli is mated with one having red eyes and white ocelli, and some of their offspring
have brown eyes and white ocelli. What are the formulas of the parents?

31. In Drosophila, gray body is dominant over ebony, and straight wing dominant
over curved. A certain gray-bodied curved-winged female is mated to a gray straight-
winged male, and they produce some ebony curved offspring. Out of a total of
40 offspring, how many should be ebony straight? How many gray curved?

32. Each cell of the muscles of a certain male bug contains 27 chromosomes. How
many chromosomes in its spermatogonia? How many in its mature spermatozoa?
How many chromosomes in the body cells of the female of the same species? How
many in her mature eggs? How many in fertilized eggs?

33. Can a male Drosophila be homozygous for a sex-Unked character? From
which parent does a male Drosophila receive his sex-linked genes? To which sex
among his offspring does he transmit his sex-linked characters? If a gene were
located in his Y chromosome, to what offspring would he transmit it?

34. Color blindness is a sex-linked recessive, and sex in man is determined essen-
tially as in Drosophila. A girl of normal vision whose father was color-blind marries
a color-blind man. What is the chance that their first child will be color-blind?

35. A woman of normal vision, whose father was color-blind, marries a man of
normal vision whose maternal grandfather was color-blind. Among their three
daughters how many should be color-blind?

36. A. color-blind boy's parents and grandparents all had normal vision. What
was the formula of his maternal grandfather? Of his mother? Of his maternal
grandmother?

37. Yellow body (y) in Drosophila is a sex-linked character recessive to gray
body ( F). A certain gray female mated with an unknown male produced some yellow
and some gray offspring of both sexes. What was the formula of the original female?
What was the appearance of the male to which she was mated?

38. A female fruit fly with sable body (s) is mated with a male having gray body
(S). Their daughters are gray, their sons sable. In what cliromosomes are the
genes S and s?

39. A barred rock hen mated with a black cock produced black daughters and
barred sons. Using B and b to represent the genes, give the formulas of the two
parents.

40. The genes for purple eye (normally red) and curved wings (normally flat) in
Drosophila are in the same pair of chromosomes, and the normal red eye and flat
wings are dominant. A homozygous purple-eyed flat-winged fly is crossed with a
homozygous red-eyed curved-winged fly. One of their daughters is mated with a
purple curved male. What kinds of offspring will they produce, and in what pro-
portions, assuming that 21 per cent of the pairs of chromosomes break between the
genes for eye color and wing shape?

GENETICS 243

41. The character known as speck (s), a spot near the base of the wing in Dro-
sophila, is recessive to no speck (S), and plexus (p), a tangled patch of wing veins, is
recessive to no plexus (P). A doubly heterozygous no-plexus no-speck female (PpSs)
is mated with a plexus speck male (ppss). Of their 200 offspring, 10 are plexus
no-speck. How do you account for the smallness of this number? How many of the
offspring should be plexus speck?

42. In four-o'clocks the red flower color is not wholly dominant over white, so that
heterozygous flowers (Pr) are pink. What would be the appearance of the offspring
of a self-fertilized pink-flowered plant? If the progeny of this plant numbered 100,
how many red ones should there be?

43. If a pink-flowered four-o'clock is pollinated from a red one, and they produce
84 offspring, how many of these should be red?

44. In shorthorn cattle, the hybrid between red and white is roan (having white
hairs and red hairs intermingled). What would be the nature of the offspring of a
roan and a white animal? The offspring of a roan and a red animal?

46. In Drosophila, cinnabar eye is recessive to red, stripe (a mark down the back)
is recessive to no stripe, and bent wing recessive to straight wing. A cinnabar stripe
bent fly is mated with a homozygous red no-stripe straight (wild-type) fly, and their
offspring crossed with cinnabar stripe bent flies. Of 288 offspring from this latter
cross, how many should be red stripe straight?

46. If two Fi flies from Problem 45 are mated together, and among their r2 off-
spring there are 36 cinnabar no-stripe straight-winged individuals, how many wild-
type flies would be expected in the F2 generation? How many red stripe bent flies?

References

Bateson, W. Mendel's Principles of Heredity. Cambridge University Press.

(Part II: biography of Mendel, translations of his papers.)
Shull, a. F. Heredity. 3d Ed. McGraw-Hill Book Company, Inc. (Chaps.

VII-XIII on simple heredity; rest of book deals with more complex inheritance

and emphasizes applications to human affairs.)
SiNNOTT, E. W., and L. C. Dunn. Principles of Genetics. 3d Ed. McGraw-Hill

Book Company, Inc. (Chaps. Ill and IV for Mendel's laws.)

.^^.

CHAPTER 18
PRINCIPLES OF TAXONOMY

Objects of all kinds that have ever interested civilized man have been
classified by him as soon as they became numerous enough to show simi-
larities amid differences. Animals have not escaped this human pro-
pensity for cataloguing. Classification was not necessary when chiefly
the large, conspicuous animals were known, and when travel and com-
munication between regions was so meager that each naturalist knew
only the beasts of his own land. But as knowledge enlarged through
travel, and as microscopes increased the range of size of animals that
could be observed, the method of describing animals and their habits
and modes of life singly, without reference to other animals, became
cumbersome. It was then that classification began.

The classification of living things is known as taxonomy (from the
Greek taxis arrangement and nomos law), which means literally an orderly
arrangement. Both animals and plants are classified, and the principle
on which their grouping is based is the same in both; but the schemes
adopted for these two great kingdoms are somewhat different. Tax-
onomy of animals is often called systematic zoology, that of plants sys-
tematic botany. Only the plan adopted for zoology is considered in
this book.

Conceptions of Taxonomy. — An orderly arrangement of objects or
facts presupposes a system of classification. The same objects or facts
can usually be classified in different ways by the use of different charac-
ters, qualities, or relations as a basis. What qualities are chosen to form
the basis of classification depends on the importance attached to those
qualities. If their importance is not known, the classification depends on
the purpose or bent of mind of the classifier. It thus happened that
in the early taxonomy of animals there were likely to be various schemes
of classification, because no settled convictions existed regarding the sig-
nificance of such grouping. Some of the first schemes arc described
below, but it may be pointed out in advance that all but one of the sys-
tems of classification that have ever been in use have been essentially
devices to save confusion. Things were put upon shelves, figuratively,
and labeled and catalogued. As long as prevention of confusion was
the chief aim, classification might be artificial and arbitrary. The one
exception to this arbitrary basis of arrangement is found in the system of

244

PRINCIPLES OF TAXONOMY 245

classification that prevails at the present time. The modern system
serves two purposes instead of but one. It has fitted admirably the
modern evolution doctrine, according to which species of animals are
related to one another through common descent. Classification may
now afford the convenience that was desired in the earliest attempts at
organization and at the same time express the kinship which the evolution
doctrine implies. It is rather by accident than by design that the
modern system is both a convenience and an expression of the course of
evolution, because the author of it did not subscribe to the evolution
doctrine. The system of classification is a branching one, and evolution
results in a branching scheme of kinship. When the evolution idea was
adopted, therefore, it was easy to adapt the branching classification to
the portrayal of evolution. The scheme had the further advantage of
being capable of expansion; the successive branchings could be as numer-
ous as was required in any line of descent. A classification which
expresses evolutionary development is called a genetic or natural sys-
tem— genetic because ancestries are involved, natural because the basis
of it exists in nature, not just in the minds of men.

Ray and Linnaeus in Taxonomy. — It has been said that John Ray
(1627-1705), an Englishman, was the first true systematist. Ray pro-
posed a dichotomous systematic table of the animal kingdom, that is,
a system which branched by twos. He used anatomical likenesses as the
basis on which animals were grouped, and the soundness of his judgment
of these characters is shown by the fact that several of his groups are still
recognized as natural ones. It is Carolus Linnaeus (Fig. 205), 1707-
1778, however, who is considered to be the real founder of classification.
Linnaeus's most important work was the "Systema Naturae," which
appeared in 12 editions between 1735 and 1768 and, after his death, in a
thirteenth, edited by Gmelin. In this work Linnaeus completed a classi-
fication which Ray had established in part, giving names to important
groups that Ray had left without appellations and describing animals in
language which, unlike many of the writings of his time, could not be
misunderstood. Linnaeus also had the courage to defy prejudice in such
details as removing the whales from the group of fishes, to which Ray also
knew they did not belong, and placing them with the terrestrial hairy
animals called mammals. For, in the Linnaean classification, structural
characters, rather than habits or external forms, were used as a basis.
Six classes were employed, four of them vertebrate (borrowed from Ray)
and two invertebrate. These classes were divided into orders, the orders
into genera, and the genera into species. The lesser groups were usually
much more inclusive than the groups now given these same ranks. Thus,
a Linnaean genus occasionally includes three or four orders, as these
groups are now reckoned. Moreover, the genus often contained animals

246

PRINCIPLES OF ANIMAL BIOLOGY

now placed in widely separated categories. One genus was erected to
include certain sea cucumbers, a worm, a colonial jellyfish, and several
primitive near vertebrates ; some of these are now placed near the bottom,
others near the top, of the animal scale.

Later Temporary Systems of Classification. — Following Linnaeus,
many naturalists concerned themselves with systematic zoology. Some
of them adopted the Linnaean system in general but altered it to suit their
tastes, sometimes improving it but quite as often not. Others invented
new classifications. Georges Cuvier (1769-1832) established four major

Fig. 205. — Carolus Linnaeus,

1707-1778, at the age of forty.
Botanical Garden.)

{Courtesy of New York

groups, called branches, which he divided into classes, 19 in number;
and some parts of his classification remained in vogue in his own country
(France) for three-quarters of a century. De Blainville (1777-1850) in
several instances happily discovered the structural characters that were
of genuine importance in distinguishing natural groups. He proposed a
classification involving three subkingdoms, distinguished by the arrange-
ment of their parts about a center or axis. These subkingdoms weie the
Artiomorphes, having a bilateral form like the majority of animals; the
Adinomorphes, with a radiate form like a starfish; and Heteromorphes,
animals having an irregular form (chiefly protozoa and sponges).
Lamarck (1744-1829) devised a classification based upon nervous sensi-
bility and proposed three principal groups: the apathetic animals, those
without nervous systems or apparent sensation among the invertebrates;

PRINCIPLES OF TAXONOMY 247

the sensitive animals, also among the invertebrates; and the intelligent
animals corresponding to the vertebrates. Oken (1779-1851), who was a
philosopher rather than a naturalist, advocated simultaneously at least
two classifications, which were equally worthless. One divided animals
into groups according to their systems of organs, as intestinal, muscular,
sexual, respiratory, vascular, etc. His other classification was based
on the senses. Thus, there were the Dermatozoa (literally, skin or touch
animals), by which he meant the invertebrates; the Glossozoa (literally,
tongue animals), the fishes; the Rhinozoa (nose animals) which included
the reptiles; the Otozoa (ear animals), or the birds; and another class,
which appears to have been called interchangeably the Ophthalmozoa
(eye animals) or Thricozoa (hair animals), the mammals. It would be
hard to name a set of distinctions less applicable as classification marks
than most of these, but Oken did not engage in practical matters. Then
there was a host of minor systematists the value of whose labors was
diminished by attempts to force their classifications into some numerical
system, as, for example, those who held that the number of orders in
each class should be the same as the number of families in each order,
or the number of genera in each family. The favored number was five
in some classifications, less often three, four, or seven.

These early modes of arrangement of animals have been described not
for any value that may attach to them as classifications but to form a
background for the one system that has survived. It should be obvious,
from the brief statements made, that most of the plans used were totally
unsuited to the requirements which later developments of zoology would
have imposed upon them. The system of Linnaeus, however, was hap-
pily capable of being adapted to the demands of the tenets of evolution,
and it alone has persisted to the present time.

The Linnaean System. — That the Linnaean system was rapidly
adopted in advance of the general acceptance of the evolution idea is
doubtless due largely to the fact that it introduced a sharply defined
grouping, a definite terminology, and brief, clear diagnoses. It also
permitted early naturalists to group those forms that resembled each
other, which would be a natural tendency in any classifier. And then,
as stated earlier, came the added advantage that it equally well per-
mitted the classification of forms according to their relationships. As
stated above, Linnaeus recognized groups of four different values — the
class, the order, the genus (plural, genera), and the species (plural,
species). To these categories have been added the phylum (plural,
phyla) and subphylum (assemblies greater than the class), the subclass, the
suborder, the family, the subfamily, the subgenus, the subspecies, and
others. Of these additional groups the phylurn and family are now
generally accepted, and every classification includes a named group of

248 PRINCIPLES OF ANIMAL BIOLOGY

each of these ranks. So regular is this practice that if there were only
one kind of animal in a phylum, it would probably be assigned also to a
named class, an order, and a family, as well as a genus and a species.
The other ranks named are used for some groups or by some naturalists.
The rank of recognized categories may be expressed as follows:

Phylum. Example, Chordata (the chordates)

Subphylum. Example, Vertebrata (the vertebrates)
Class. Example, Mammalia (the mammals)

Subclass. P]xample, Eutheria (the placental mammals)
Order. E>xample, Rode7itia (the rodents)

Suborder. Example, Sciuromorpha (the squirrellike rodents)

Family. Example, Sciuridae (the flying squirrels, marmots, squirrels,
chipmimks)
Subfamily. Example, Sciurinae (marmots, squirrels, chipmunks)
Genus. Example, Sciurus (the arboreal squirrels)

Subgenus. Example, Tamiasciurus (the red squirrels)

Species. P^xample, hudsonicus (the Hudsonian red squirrel)
Subspecies. Example, loqiiax (the southern Hudsonian
red squirrel)

In some grovips "divisions" or '^sections" are recognized by authors,
but these categories have no definite place in the system; that is, they
may be introduced to mark off a group of genera, an assemblage of orders,
etc.

The Linnaean system designates the species by two Latin or latinized
names, the generic name, a noun, and the specific name, usually an adjec-
tive. Thus Natrix is the generic name of a group of water snakes, and
Matrix rhomhifera and Natrix sipedon are two species of water snakes.
This is known as the binomial system of nomenclature. When subspecies
are recognized, three names are used — the generic, the specific, and the
subspecific — thus: Thamnophis sirtalis parictalis. Subspecies must
usually have somewhat separate geographic ranges, but they grade into
the neighboring subspecies at their common l:)oundaries. The term
variety, sometimes carelessly used synonymously with subspecies, often
means only a genetically different type of individual not having geo-
graphic separation, for which the word phase is a preferable designation.
Thus, the cinnamon individuals that occur not infrequently throughout
the range of the black bear, Euarctos americanus, to which species it
Ix'longs, may be called a phase or variety. Such varieties are not ordi-
narily named in the Linnaean scheme. However, the taxonomic rank
of variety may be assignc^l to divisions smaliei- than subspecies, and in
one group, the ants (family Formicidae), the systematists regularly
recognize and designate divisions smaller than sul).spe('ies by name, using
four names for each variety (for example, Camponotus hcrculeanus
ligniperdus noveboracensis, the northern carpenter ant).

PRINCIPLES OF TAXONOMY 249

Rules of Nomenclature. — The binomial and trinomial systems of
nomenclature have been of great convenience to naturalists. Before
their adoption, common names were in use in the scientific world and led
to much confusion, the same animals being known by different names and
different animals by the same name. To make certain that each animal
shall have but one scientific name and that no two animals shall have the
same name, rules of nomenclature have been proposed at different times
for the purpose of determining which name shall prevail when several
have been or are likely to be inadvertently proposed for the same form.
Linnaeus seems to have appreciated the necessity for rules and to have
proposed a set. These rules were not sufficient, and several other codes
have been proposed, the more important of which are the British Associa-
tion Code, the American Ornithological Union Code, the Code of the German
Zoological Society, and the Code of the International Zoological Congress.
The code now almost universally in use is the International Code of Zoo-
logical Nomenclature, adopted by the International 2^oological Congress
and governed through a Commission on Nomenclature created in 1898.

The International Code. — Some of the essential features of the Inter-
national Code are as follows. The first name proposed for a genus or
species prevails on the condition that it was published and accompanied
by an adequate description, definition, or indication, and that the author
has appUed the principles of binomial nomenclature. This is the so-called
law of priority. Duplicate names which have to be rejected because not
prior are called synonyms. The tenth edition of the "Systema Naturae"
of Linnaeus is the basis of the nomenclature. Names given earlier and
not used in that edition are not recognized. The author of a genus or
species is the person who first publishes the name in connection with a
definition, indication, or description, and his name in full or abbreviated is
given with the name; thus, Bascanion anthonyi Stejneger. In citations
the generic name of an animal is written with a capital letter, the specific
and subspecific name mth initial small letter. The name of the author
follows the specific name (or subspecific name if there is one) without
intervening punctuation. If a species is transferred to a genus other
than the one under which it was first described, or if the name of a genus
is changed, the author's name is included in parentheses. For example,
Bascanion anthonyi Stejneger should now be written Coluber anthonyi
(Stejneger), the generic name of this snake having been changed. It is
common practice now for the author of a species to designate one par-
ticular specimen as the type of the species, and to indicate the museum
or other collection in which it is placed. If the species is later divided,
the original name goes to that part of it which includes the type specimen.
Also the specimen can be inspected in case of doubt regarding the identity
of the species. One species constitutes the type of the genus. This

250 PRINCIPLES OF ANIMAL BIOLOGY

decides, in case the genus is later divided into two genera, which group
shall receive the original name. One genus constitutes the type of the
subfamily (when a subfamily exists), and one genus forms the type of
the family. The type is indicated by the describer or, if not indicated
by him, is fixed by another author. No two genera in the whole animal
kingdom may have the same name — a rule still occasionally violated
because the interested taxonomists have not proposed corrected names.
The name of a subfamily is formed by adding the ending -inae and the
name of a family by adding -idae to the root of the name of the type
genus. For example, Colubrinae and Colubridae are the subfamily and
family of snakes of which Coluber is the type genus. Names of sub-
families are accented on next to the last syllable, family names on the
third syllable from the end.

The Basis of Classification. — Early systematists largely employed
superficial characters to differentiate and classify animals, and their
classifications were thus largely artificial and served principally as con-
venient methods of arrangement, description, and cataloguing. Since
the time of the development of the theory of descent with modifications
by Lamarck (1809) and Darwin (1859), as stated in an earlier section,
there has been an attempt to base the classification on relationships.
Very nearly related animals are put into the same species. They are
related because they descend from a common ancestry. The common
ancestry could not in most cases have been very ancient, otherwise
evolution within the group would have occurred and the species would
have been split into two or more species. Species that are much alike are
included in one genus, being thus marked off from the species of another
genus. The similarity of the species of a genus is held to indicate kin-
ship ; but since there is greater diversity among the individuals of a genus
than among the members of a species, the common stock from which the
species of a genus have sprung must have existed at an earlier time, in
order that evolution could bring about the degree of divergence now
observed. In like manner, a family is made up of genera which resemble
one another more than they resemble other genera, and their likeness is
again a sign of affinity. But to account for the greater difference
between the extreme individuals belonging to a family, evolution must
have had more time; that is, the common source of the members of a
family must have antedated the common source of the individuals of a
genus. Orders, classes, and phyla are similarly regarded as having
sprung from successively more remote ancestors, the time differences
being necessary to allow for the differences in the amount of evolution.
This statement is, however, only in a general way correct. Since
evolution has probably not proceeded at the same rate at all periods
or in all branches of the animal kingdom at any one time, the time rela-

PRINCIPLES OF TAXONOMY 251

tions of the groups of high or low rank must not be too rigidly assigned.
Thus certain genera in which evolution has been slow are probablj^
much older than some families in which evolution has been rapid. The
genus Lingula (a burrowing marine brachiopod found between tide lines)
has evolved very little. The modern animals differ only slightly from
fossil Lingula of Ordovician time, estimated by some to be 400,000,000
years old. This is an extreme instance of slow evolution: Lingula is
probably the oldest living genus. Many families, even orders, and some
classes must be younger than that. It is not improbable, also, that
some genera are quite as old as the families which include them; but in
no case can they be older. Furthermore, different groups are classified
by taxonomists of different temperaments, so that groups of a given
nominal rank may be much more inclusive (and hence older) in one
branch of the animal kingdom than in another. On the whole, neverthe-

FiG. 206. — Analogous structures; legs of several animals. A, kangaroo; B, crayfish; C,
honeybee. {C from Met calf and Flint, "Destructive and Useful Insects.")

less, the groups of higher rank have sprung from ancestry more remote
than that of the groups of lower rank.

Judging Kinship. — The means of recognizing the kinship implied in
classification permit some differences of opinion. It is recognized that
likeness in structural characters is the chief clue to affinities. However,
similarity in one or several structures unaccompanied by the similarity of
all parts is to be distrusted, since animals widely separated and dissimilar
in most characters may have certain other features in common. Thus,
the coots, phalaropes, and grebes among birds have lobate feet but, as
indicated by other features, they are not closely related ; that is, the lobes
on their feet are analogous, meaning that they serve the same function.
Analogy is mdespread in the animal kingdom, since the same activities
must be carried on by animals of very different structure. Locomotion,
for example, is effected by legs of vei;y different kinds. The legs of a
kangaroo, a crayfish, and a honeybee (Fig. 206) are analogous, but their
structure is unlike. The skeleton is within the flesh in the first of these
but on the outside in the other two, and the materials of the skeleton
are different. The crayfish and the bee, though alike in the position of
the skeleton, differ in the number and character of the segments of the
leg. Another case of analogous structures is that of lungs and gills

252 PRINCIPLES OF ANIMAL BIOLOGY

(Fig. 207). Both are used for absorbing oxygen but are wholly different
in structure.

The foregoing analogous organs are so unlike in structure that no one
would be led to classify together the animals that possess them. Not
always, however, are the structural differences so obvious externally. A
whale swims by means of paddles and a flattened tail which greatly
resemble fins, and the early naturalists regarded whales as fishes. Yet
the whale is a warm-blooded air-breathing animal with a four-chambered
heart and some hair on the skin and has also the other characters of
mammals, while the fishes are cold-blooded and aquatic, and have a
two-chambered heart and scales in the skin. A close resemblance is also
exhibited by certain lizards (Amphisbaenidae) to a group of snakes
(Typhlopidae), because the former are blind and legless and have a short
tail. These external similarities have apparently arisen in evolution

A B

Fig. 207.— Analogous structures; respiratory organs: A, gills of salamander; B, lung of

frog. {From Wienian, "General Zoology.")

independently of one another and for that reason are not an indication of
kinship.

Homology. — In judging of kinship by means of structural similarities,
therefore, care must be taken to employ only those structures that
have had similar origins in evolution. It is sometimes difficult to deter-
mine now whether similar structures in two groups of animals arose in
evolution in the same way, or have converged for some reason from
originally distinct beginnings. In general, if two or more groups of
animals have one or a few structures in common while all others are
different, it is safer to assume that the common structures arose inde-
pendently, or at least that their recent evolutionary developments have
been independent, and that the groups are therefoi'e not closely related.
The lobate feet of the several groups of birds mentioned above fall in
this category. If, however, a gi-eat many features of two groups of
animals are closely similar, the probability is that such similarities could
only have come from similar or identical origins in evolution. The work
of the taxonomist therefore becomes, in large measure, the recognition of
those characters in different animals whose similarities are due to com-
mon evolutionary origin.

PRINCIPLES OF TAXONOMY

253

Structures that arise in the same way in evolution are said to be
homologous with one another or to exhibit homology. Homology means
similarity of origin in evolution. Unfortunately for the taxonomist the
early evolution of the structures on which his classification is based took
place in many instances millions of years ago. How can he ascertain,
under these circumstances, whether the evolution of structures in two
animals was similar or not? The answer to this question must usually
be arrived at indirectly.

Homology Judged from Adult Structure. — The most reliable means
of judging of similarity of evolution in two groups would be fossil mem-
bers of those groups, if fossils could be obtained in sufficient numbers to
establish a fairly complete history extending far back into their ancestry.
Some such histories are given in Chap. 22. In most families of animals,

WALKING

FLYING

SWIMMING DIGGING HANDLING

salamander

crocodile

carpals
meiacarpals

phalanges
mole

man

bird bat whale

Fig. 208. — Homology in the bones of the fore Umbs of vertebrates. Numbers 1-5 refer to
digits, from thumb to Uttle finger. {From Storer, "General Zoology.")

however, good fossil series are wanting, and the taxonomist must rely
on what can be discovered from the living animals of today. In clear
cases adult structure is sufficient, but only where many features are
alike in the animals in question. A classical case of homology, judg-
ment of which could safely rest on adult structure alone, is that which
exists among the forelimbs of vertebrate animals (arms, wings, forefeet,
etc.. Fig. 208). Although the external forms of these forelimbs differ
greatly in birds, seals, horses, whales, bats, and man, their skeletons are
found to correspond very closely, bone for bone, at most points. It is
believed that so many similarities could not be the result of accident or of
convergence from originally distinct sources and that the likenesses are
a sign of similar evolutionary origins. The nervous systems of vertebrate
animals are equally good examples. The parts of the brain in fishes,
amphibia, reptiles, birds, and mammals have a very obvious correspond-
ence, and the origin and distribution of the cranial nerves are very similar
in all of them. It is scarcely conceivable that these nervous systems could

254

PRINCIPLES OF ANIMAL BIOLOGY

be alike in so many respects unless their evolutionary histories were
largely the same.

Homology Ascertained from Embryonic Development. — Somewhat
better evidence of homology than is afforded by adult structure can often
be obtained from a knowledge of embryonic development. As was
pointed out in Chap. 16, corresponding structures in vertebrate animals
arise in essentially the same way in the embryo. The nervous system of
one vertebrate begins with ridges that are much like those of another
vertebrate embryo. The eye of a bird develops as does the eye of a frog.
The early ear also is about the same, whether found in a reptile or a

/7s/7 Salamander Tortoise Chick

Hocf Calf Rabbit Man

Fig. 209. — Homology of embryonic form, and particularly of gill clefts and bars, in verte-
brates. {From Haupt, after Romanes, "Darwin and After Darwin.")

mammal. This similarity of the first appearance of embryonic structiu"es
occurs even when the adult organs are strikingly diffei'ent. The arm of a
man and the wing of a bird are different from one another in the adult
condition, especially in the hands; but in the embryo the earliest limb
l)uds are almost identical. An even greater difference exists between the
adult fore- and hind limbs of a bird. When compared, bone for bone,
there is scarcely a point at which there is not a distinct difference. Yet
the wing and leg could be interchanged in the early embryo, and few
observers would detect the substitution. Even the general form of the
whole embryo is similar in the several classes of vert(^brates (Fig. 209).
This illustration also shows the common origin of gill clefts and gill bars,
and their presence in the embryos of reptiles, birds, and mammals which

PRINCIPLES OF TAXONOMY 255

do not have gills in the adult. Thus, on the whole, animals whose
adult structure is similar resemble each other even more closely in
embryonic stages. Similarity of embryos is particularly useful in taxon-
omy in those instances in which the adult animals, though closely related,
have become so changed as to lose all similarity. An example of this
kind is found in the parasite Sacculina described in Chap. 23. Biolo-
gists believe that similarity of structures in the embryo can be due only
to similarity of the evolution of those structures ; and because resemblance
in the embryo sometimes remains after adult similarity has been dimin-
ished or destroyed, embryonic development is frequently better evidence
of homology than is adult structure of the same animals.

The only known phenomenon which could preserve the similarities
possessed by different animals is heredity. The likenesses of present-
day animals must therefore be inherited from like animals of the past.
Since it is scarcely conceivable that two identical organisms ever could
have arisen independently of one another, inheritance from like ancestors
must ultimately be inheritance from the same ancestors. Animals of
different modern groups are held to possess like features in both adult
and embryo because of this descent from a common source. This is the
argument upon which the taxonomist relies when he classifies animals
on the basis of supposed homologies.

Biogenetic Law. — The evident dependence of homology upon a com-
mon descent led, in the last century, to a conception comprised under
the term biogenetic law, sometimes called by the more expressive and
less committal name recapitulation theory. According to this law or
theory (already stated page 74), the embryonic or other early stages of
individual animals of today represent the condition of successive ancestors
of these animals. That is, early developmental conditions represent
very remote ancestors, later embryonic stages represent more recent
ancestors. Some biologists held that the early embryonic stages are like
the adult ancestors; others believed merely that the embryonic stages
of the present are like the embryonic stages of the ancestors.

If this law were capable of rigid application, it would be easy to trace
the evolutionary history of a race simply by studying the development of
its individuals. In some cases this simple precedure is almost feasible.
A series of fossil cephalopods (allies of the cuttlefishes) is a case in point.
The fossil remains of these animals indicate that, in their racial history,
their shells were at first provided with straight partitions, later with
partitions whose edges were bent, crooked, and finally lobed in a very
complicated manner (Fig. 210). Since in the fossils both the young and
old stages of each individual shell are preserved, it is possible to compare
the individual development with the racial development. When this is
done, it appears that the individuals of the highly complex types passed

256

PRINCIPLES OF ANIMAL BIOLOGY

through very similar stages, in which the partitions were first straight,
then bent, crooked, and finally complicated.

Another suggestive and perhaps significant individual development
is that of the decapod Crustacea (lobsters, prawns, shrimps). The shrimp
Penaeus hatches as a nauplius, and goes through several increasingly
complex forms (Fig. 211), the last immature one being the so-called my sis

Badrites

Anarcesfes

Pronoritcs

Ceraiites

Phylloceras

Fig. 210. — Biogenetic law illustrated by fossil cephalopods. Edges of partitions of
shells start nearly straight and become increasingly crooked, in both evolution of group and
development of individual ammonites (like Phylloceras, E). {From Storer, "General
Zoology.")

stage. The appendages through all this development are two-branched.
In the adult shrimp, however, the outer one of these branches on the five
pairs of trunk appendages is considerably reduced. The lobster has
shortened its individual development and hatches as a mysis which has
two branches on all appendages; but the five pairs of walking legs have
lost the outer branch completely in the adult. The support which these

Nauplius Protozoea 2oea Mysis , ADULT

P'iG. 211. — Larval stages and the adult of the shrimp Penaeus, jjeihaps illustrating
biogenetic law. Numbcis refer to successive appendages. (From Storer, "General
Zoology.")

decapods give to the biogenetic law lies principally in the fact that there
exists a present-day animal called Mysis (from which the larvae of other
forms take their name) in which two well-developed branches persist
on the trunk appendages of the adult. The lol)ster and shrimp thus
pass through a developmental stage which resembles a supposedly more
primitive animal.

In most animals embryonic develoi)ment has undergone many
changes, so that steps in development no longer represent accurately

PRINCIPLES OF TAXONOMY 257

the steps in the evolution of their ancestors. That is, the biogenetic
law is less generally applicable than it was formerly supposed to be.
However, many important facts of evolution, of limited scope, have been
discovered by an appeal to this law. A case in which the recapitulation
theory is presumably correct is in the development of gill pouches in all
the vertebrate animals. Gills are never developed in the reptiles, birds,
and mammals ; but gill pouches are formed in the embryo, and these may
actually open temporarily to the outside as gill clefts, between which are
the gill bars upon which gills are developed in fishes and amphibia. The
production of gill pouches and bars in the higher vertebrates as well as in
the lower, besides indicating a common ancestry of all these animals,
points to the conclusion that the ancestor was an aquatic animal that
respired by means of gills.

Practical Taxonomy. — The foregoing scheme of genetic classification
is a goal toward which taxonomists in general strive. Application of it
is attended with some difficulties. One obstacle is that before a satisfac-
tory classification of even a small group can be made the species in it
must be known. Judgment of kinships rests largely on a comparison of
structures, and the characters of each species have an influence on one's
judgment of the relationship among other species. Omission of some
species tends to modify judgments concerning the whole group. Since
there are usually many species in a family, or even a genus, the task of
discovering and describing them is no small one. This work has been
going on a long time, yet many species are still unknown. Every year
many new species are described — few in the groups of large, conspicuous
animals, but many in those less generally observed. Because of this
still waiting task of describing species, many taxonomists, particularly
in the past, have devoted their energies chiefly to naming and putting on
record the newly discovered forms. They have had to concern them-
selves with kinship to the extent of putting species in the right genera,
etc., but they have conceived their main task to be filling out the record.
More and more, however, the genetic classification will have to be their
aim.

The large number of species in existence is also a difficulty. Among
well over a million, possibly over two million, species no one person can
be expert on any considerable fraction. Each taxonomist must limit
himself to one group, perhaps an order, often only a family. Names are
given to these specialists according to the phyla or classes in which they
have competence. An entomologist deals with insects, though he is never
an expert in all the orders ; a protozoologist studies the unicellular animals ;
an ornithologist knows birds, a herpetologist reptiles or amphibia or both,
a mammalogist mammals, etc.

The other difficulties are mostly those which inhere in the animals

258 PRINCIPLES OF ANIMAL BIOLOGY

studied. To know which characters best indicate kinship is the chief
problem. In the higher ranks of the classification, those qualities which
are constantly associated with one another are presumably best. Thus
feathers are constantly associated with wings, a beak, claws, a four-
chambered heart, and warm blood. These are the marks of one class, the
birds. This principle may be pushed down to the lower ranks, the orders
and families, but in less marked degree. When it is used for genera and
species it is still valid but often difficult to apply. For species the uncer-
tainties of its application are so great that some systematists have
advocated abandoning it in favor of some more or less arbitrary scheme.

Relations of Taxonomy. — Classification has wide connections with
nearly all other phases of biology. In a practical manner every biologist
has occasional or frequent use for the technical knowledge of the systema-
tist, and this requirement is not a purely formal one. Many investiga-
tions whose principal aim is entirely apart from classification must,
nevertheless, constantly use the data of taxonomy. Thus the zoogeog-
rapher, as will be apparent in Chap. 21, is not primarily interested
in classification ; but in order to discover the principles which have guided
migration or determined extinction in the past, he must be thoroughly-
conversant with the taxonomy of the group whose distribution he studies.
The paleontologist also requires a knowledge of classification not only
of extinct forms but of their living relatives. The Work of the physiolo-
gist frequently involves the question of relationship, as does that also
of the ecologist. Indeed, every biological field is in very close connection
with taxonomy.

This intimate relation is not one-sided, for each of the phases of
biology contributes to a knowledge of classification. Distribution and
fossil forms supply information where morphology fails or may refute
conclusions based on morphology alone. Physiological facts must be
taken into account in explaining the formation of species. Ecological
relations must be understood if certain taxonomic questions are to be
correctly answered. In practice, this close relation between taxonomy
and the other phases of biology is not always observed, but all of them
suffer from its neglect.

References

Gill, T. Systematic Zoology: Its Progress and Purpose. Annual Report of Smith-
sonian Institution, 1907. (Pp. 449-471 for history of taxonomy.)

CHAPTER 19
THE GROUPS OF ANIMALS

In applying the principles of taxonomy systematic workers have
often disagreed. This is inevitable because of the many judgments which
must be made from meager evidence. When groups of facts seem to
point to different conclusions, biologists may and frequently do weigh
the conflicting data differently. Various schemes of classification have
therefore arisen, all of them agreeing in many major features, differing
from one another in less fundamental respects. The one here given may
not be the best, but it is in common use.

The principal groups of animals are given, with brief descriptions
and some well-known examples. The definitions are necessarily incom-
plete and are often not sufficient to distinguish all the members of one
group from those of another. They will serve, however, to give a general
concept of classification and a bird's-eye view of the animal kingdom.

Phylum 1. Protozoa. — These are single-celled animals, mostly of
microscopic size, though some are visible to the unaided eye. Some
species are colonial, but in these the cells are usually all potentially alike;
that is, ^here is no differentiation among the attached cells to form tissues
or organs. Protozoa live in very varied situations but usually require
moisture. Many of them live in the soil. They are exceedingly com-
mon in ponds, streams, lakes, and oceans and may be attached to solid
objects, be buried in mud or debris, or swim freely in the water. Many
of them are parasitic in other animals. Some of the parasitic ones cause
disease, as malaria, dysentery, and African sleeping sickness in man.
Some protozoa live in other animals in a relation that is beneficial to the
host as well as to the guests. A most remarkable example of mutual
benefit is that received and conferred by certain protozoa in the digestive
tracts of termites. These insects, whose food is wood, would be quite
unable to digest the cellulose without the aid of the guest protozoa.

Untold numbers of protozoa live in the sea, and lived there ages ago.
The great limestone beds, chalk cliffs, and quartzite and flint deposits
are made up of shells of ancient protozoa. Noctiluca is a marine proto-
zoon which is responsible for some of the remarkable phosphorescence
observable in disturbeji waters at night.

There are three principal modes of locomotion. Some protozoa
thrust out pseudopodia, projections of their protoplasm, and then flow

259

260

PRINCIPLES OF ANIMAL BIOLOGY

into them. This is characteristic of the class to which Amoeba (Fig.
212) belongs. Protozoa of this type have no constant characteristic
form but are always changing. Others have at one end of the cell one
or two long whiplike flagella whose lashing or sometimes wavelike motion
propels the organism through the water. Euglena (Fig. 213) is one of
these. Still others have the body covered by hundreds of cilia, short

Fig. 212. — Amoeba.

Fig. 213.
Euglena.

Fig. 214.—
Paramecium.

Fig. 215. — Podophrya,
one of the Suctoria.

hairlike projections whose beating drives the body along, as in Para-
mecium (Fig. 214). Some protozoa, particularly the parasitic ones, have
no locomotor structures. The classification of protozoa follows.

SUBPHYLUM I. Plasmodroma. Protozoa that never have cilia in any stage.

Class I. Mastigophora. Protozoa with flagella, which serve for locomotion
or for taking food. Euglena. (Figs. 34, 47, 48, 50, 51, 52, 53, 54, 129, 130, 131, 213.)

Subclass I. Phytomastigina
Order 1. Chrysomonadina
Order 2. Cryptomonadina
Order 3. Dinoflagellata
Order 4. Eiiglenoidina
Order 5. Phytomonadina

Subclass II. Zoomastigina
Order 1. Protomonadina
Order 2. Polymastigina
Order 3. Hypermastigina
Order 4. Distomatina
Order 5. Cystoflagellata

Class II. Rhizopoda. Protozoa with pseudopodia or other changeable processes.
Amoeba. (Figs. IG, 30, 43, 49, 212.)

Order 1.

Amoebina

Order 4.

Foraminifera

Order 2.

Ehizomastigina

Order 5.

Radiolaria

Order 3.

Heliozoa

Order G.

ISlycL'tozoa

Class III. Sporozoa. Parasitic; Protozoa, usually without motile organs or
mouth, reproducing by spores. Malarial organism.

Subclass I. Telosporidia
Order 1. Coccidiomorpha
Order 2. Gregarinida

Subclass IT. Neosporidia
Order 1. Cnidosporidia
Order 2. Sitrcosporidia
Order 3. Haplosporidia

THE GROUPS OF ANIMALS

261

SUBPHYLUM II. CiLlOPHORA. Protozoa having cilia in some stage.

Class I. Ciliata. Ciliophora with cilia throughout life. Paramecium. (Figs.
]5. 132, 138, 214.)

Order 1. Holotricha
Order 2. Heterotricha
Order 3. OHgotricha

Order 4. Hypotricha
Order 5. Peritricha

Class II. Suctoria. Ciliophora with cilia in young stages, tentacles in adult.
(Fig. 215.)

Phylum 2. Porifera. — The sponges are roughly radial in form and
always diploblastic (two-layered), though many wandering cells are
found in a jellylike substance between the layers. The body wall is
always penetrated by many pores, which give the phylum its name.
These pores lead to chambers within, which may be single cavities extend-
ing from outside to inside, or may branch or connect with other cavities
in a complex system. The final opening through which the water leaves

Fig. 216. — Elements of sponge skele-
tons. 1, spongin; 2-7, spicules.

Fig. 217.1
A sponge.

the body is called the osculum, and there may be many of these oscula.
Some of the chambers are lined by collared cells (Fig. 33, page 52). The
collared cells also possess flagella, by means of which a current of water is
kept up continuously in the same direction. Food organisms and oxygen
are brought, and wastes are carried away, by these currents. The
collared cells seize the food, digest it, and pass along much of the nutrition
to the other parts of the organism.

The sponges all possess a skeleton, which in some consists of a host of
limy or siliceous spicules, in others of a network of horny (spongin)
threads (Fig. 216). It is this latter horny skeleton which makes the
ordinary bath sponge.

Members of this phylum are all sessile; that is, they are attached to
other objects and do not move about. About a hundred and fifty species
live in fresh water, where they sprawl in irregular form over twigs or
logs. It is these fresh-water forms that reproduce by gemmules (page
170). The bulk of the phylum is marine, and they are found all over the
world.

Courtesy of General Biological Supply House.

262 PRINCIPLES OF ANIMAL BIOLOGY

A curious feature of the development of sponges is their "inside-out"
gastrulation. It is the cihated cells of the blastula that are invaginated
and form the endoderm, whereas other gastrulas, if ciliated at all, regu-
larly bear the cilia on the outside. Sponges also have remarkable powers
of regeneration. Their bodies may be crushed, the separated cells sifted
through a bolting-cloth net upon a surface under water, and there the
cells gradually collect into lumps from which new sponges grow.

In the irregular, spreading, fresh-water and bath sponges, there has
been some debate as to what constitutes the individual sponge. One
concept is that each osculum is the center of an individual, and that
the mass called a sponge is a colony. The boundaries of the individuals
would then necessarily be indefinite, since all the oscula are parts of one
system of canals.

There are three classes of sponges:

Class I. Calcarea. Sponges with spicules composed of calcium carbonate, mon-
axon or tetraxon in form. (Figs. 74, 139, 217.) /

Order 1. Homocoela Order 2. Heterocoela

Class II. Hexactinellida. Sponges with spicules composed of silicon, triaxon in
form.

Class III. Demospongiae. Sponges with spicules composed of silicon, not triaxon
in form, or skeleton composed of spongin, or with skeleton of both spicules and
spongin.

Order 1. Tetraxonida Order 3. Keratosa

Order 2. Monaxonida

Phylum 3. Coelenterata. — This phylum includes Hydra, the
hydroids, jellyfish, sea anemones, and corals (Figs. 218, 219). Its
members are radial in form and are all diploblastic.
They possess a coelenteron (page 101), a cavity with only
one opening, the mouth. There is no other body cavity.
They have tentacles, and in the ectoderm are stinging
cells used for offense and defense. Their nervous system
is very diffuse, consisting of a network of scattered cells.
While such a system provides for related actions through-
out the body, the coordination is often imperfect and

Fig. 218. ,, ,

-Hydra, rather slow.

with buds. There are in general two forms of body: (1) the polyp,

Carolina Bio- which is typically tubular and elongated with tentacles
Logical Supply arouud One end, and (2) the medusa or ic^llvlish, which
is ordinarily compressed into a hemisphere or flat
disk with tentacles around the edge. Polyp and medusa are really
built on the same fundamental plan, as is readily understood if the
mouth and the center of the convex surface of the medusa be imagined

THE GROUPS OF ANIMALS

263

drawn apart so that the body is a long cyUnder hke a polyp (also see
Fig. 143). The medusa is regularly free-swimming, though because it is
produced by budding from the polyp form it remains in some species
attached to its parent. The polyp is usually sessile, though sometimes,
as in Hydra, it may become detached from one object and loop along
to a new situation where it again glues itself fast.

A BCD

Fig. 219. — Various coelenterates: A, Gonionemus; B, Aurelia; C, sea anemone; D, coral.
{A-C from Carolina Biological Supply Co.; D from Wolcott, "Animal Biology.'')

Colony formation is common. Most of the hydroids are branching
colonies. The corals have massive stony skeletons which in the aggregate
may form reefs and atolls or other islands. The sea pens are colonies
resembling a quill pen, with the pointed end thrust into the sand. Many
of the colonial types are gorgeously colored and are responsible for some of
the brilliance of tropical seas. The siphonophores (Figs. 145, 146, pages
174, 175) are free-swimming colonies.

Coelenterates exhibit a great deal of polymorphism. The polyp
and medusa have already been mentioned as generalized types. Each
may be considerably modified in different species and
modified in several different ways in the same species.
In the hydroids the medusa shows more variation than
the polyp. It is free-living in some species, perma-
nently attached to the hydroid colony in others. When
attached, it may suffer considerable reduction; that is,
it does not develop the full medusoid structure, which
-would be useless to an inactive individual. Sometimes
the reduction of the medusa is so great that practically
only the gonads are left (Fig. 220). Then the medusa
looks like a reproductive organ belonging to a colony
of polyps.

Much more marked polymorphism is found in the siphonophores
(page 174). In them there are usually several kinds of structures which
betray, sometimes in vague but often in unmistakable ways, their
medusoid architecture and several other kinds which, in development or
adult anatomy, are more or less like the polyp.

These polymorphic species often show that type of alternation of
generations which is kno^vn as metagenesis (page 174). One or more

Fig. 220.— Hy-
droid, with re-
duced medusae.
{Courtesy of Caro-
lina Biological
Supply Co.)

264 PRINCIPLES OF ANIMAL BIOLOGY

kinds of individuals reproduce by budding (asexually), another kind by
eggs and spermatozoa.

In some groups (Scyphozoa, Fig. 219B) only the medusoid generation
exists, and in them its structure is different (see table of characterizations
below).

Corals are the skeletons of two kinds of coelenterates, the Hydrocoral-
linae and the Madreporaria (see below), the latter being the more com-
mon. Aside from their use as ornaments, corals are of interest because of
the long debate concerning the origin of coral reefs and atolls. The theo-
ries of their origin differ largely in whether the sea bottom on which they
grew was assumed to be subsiding, stationary, or rising.

Class I. Hydrozoa. Coelenterates without stomodaeum and mesenteries; sexual
cells discharged to the exterior; life history including hydroid form, or medusa (with
velum), or both hydroid and medusa in same species. Polyps (including Hydra), a
few corals, small jellyfishes. (Figs. 58, 59, 65A, 142, 144, 145, 146, 218, 219^, 220.)

Order 1.

Anthomedusae

Order 4.

Narcomedusae

Order 2.

Leptomedusae

Order 5.

Hydrocorallinae

Order 3.

Trachymedusae

Order 6.

Siphonophora

Class II. Scyphozoa. Coelenterates with only the jellyfish, not hydroid form;
velum lacking; notches at margin of umbrella. Larger jellyfishes. (Fig. 219-B.)

Order 1. Stauromedusae Order 3. Cubomedusae

Order 2. Peromedusae Order 4. Discomedusae

Class III. Anthozoa. Coelenterates without medusoid forms, with well-developed
stomodaeum and mesenteries. Sea anemones, most corals. (Figs. 65A, B, 219C, D.)

Subclass I.

Alcyonaria

Order 1.

Stolonifera

Order 3.

Gorgonacea

Order 2.

Alcyonacea

Order 4.

Pennatulacea

StJB CLASS II

Zoantharia

Order 1.

Edwardsiidea

Order 4.

Zoanthidea

Order 2.

Actiniaria

Order 5.

Antipathidea

Order 3.

Madreporaria

Order 6.

Cerianthidea

Phylum 4. Platyhelminthes. — This phylum includes the planarians
(Fig. 221), the flukes (Fig. 222), and the tapeworms (Fig. 223). The
name of the phylum comes from the generally flat form of the bod}'-, and
its members are commonly called flatworms even when the body is not
flat. The body is bilaterally symmetrical, the only phylum so far men-
tioned to possess this form. The animals are triploblastic, the third
layer being mesenchyme (page 82), which makes up the bulk of the body.
The digestive tract is a coelenteron (page 80), opening only at the mouth,
and there is no other body cavity. Parasitic forms may, however, lack
the digestive tract completely. The free-living species have cilia
on the epidermis, but the parasitic ones lack them. The excretory sys-
tem is of the protonephridial type (page 134) ending in flame cells.

THE GROUPS OF ANIMALS

265

The planarias, which are free-hving, Hve under stones or logs in fresh
water. They have remarkable powers of regeneration, and have been
used by many investigators to study the physiology of development and
growth. The theory of gradients (page 217) in embryonic development
originally grew out of studies on planarias.

The flukes are parasitic. Some of them are external' parasites, as on
the gills of fishes or other aquatic animals. Others — and these are the
menacing ones — are internal parasites. Some of the latter pass through
very complicated life cycles, in which the successive generations are
totally different in form. Usually these different types of individuals
must live in different hosts, one of which is a snail, the others being
usually arthropods (Phylum 9, below) and vertebrate animals. One such
life cycle involves four dift'erent hosts, following one another in a certain

Fig. 221. — Planaria.

Cvari/
tMdef

Fig. 222. — A fluke.
{From Van Cleave.)

Fig. 223.— a tape-
worm.

order. Sometimes the host, of any of the several successive general
types, must be a particular species — a certain species of snail, a specific
arthropod, a definite vertebrate species; in other trematodes there is a
choice of species for host, but usually only a very limited one. Some
degeneration (loss of eyespots, reduction of sense organs and nervous
system) has been permitted by the parasitic mode of life, but the repro-
ductive system is highly developed and specialized.

The tapeworms are parasitic in the digestive tracts of vertebrate
animals. They consist of chains of rectangular individuals budded off
from a small "head" which is attached to the intestinal wall of the host.
There is no digestive tract, and no use for one since all food is absorbed
already digested by the host. Longitudinal nerves and longitudinal
excretory tubes pass along the margins of the "tape," common to all the
individuals in it; but each individual has its own highly developed repro-
ductive system which makes up most of the substance of the animal.
Man gets his commonest tapeworms from insufficiently cooked pork;
thorough cooking is the best guarantee against infection.

266

PRINCIPLES OF ANIMAL BIOLOGY

Class I. Turbellaria. Free-living flatworms with ciliated epidermis. Planaria.
(Figs. 89, 221.)

Subclass I. Rhabdocoelida
Subclass II. Tricladida

Subclass III. Polycladida

Class II. Trematoda. Parasitic flatworms without cilia but with a hardened
ectoderm, usually parasitic and with attaching suckers. Flukes. (Fig. 222.)

Subclass I. Monogenea

Subclass II. Digenea

Class III. Cestoda. Parasitic flatworms with the body diff'erentiated into a
scolex, an enlargement usually provided with suckers and sometimes with hooks, and a
chain of similar structures (proglottides), the whole being usually regarded as a colony.
Tapeworms. (Fig. 223.)

Phylum 5. Nemathelminthes. — These are elongated, bilaterally
symmetrical animals, commonly called round- or threadworms. They

A B

Fig. 224. — Important Nemathelminthes: A, Trichinella encysted in muscle; B, hook-
worm. (A from photograph by General Biological Supply House; B from Rivas, "Human
Parasitology," W. B. Saunders Company.)

are triploblastic, and there is a "coelom" in the middle tissue layer.
The digestive tract, unlike that of the two preceding phyla, is not a
coelenteron, for it opens at both ends. There are no cilia on any part of
the body. The sexes are separate; that is, some individuals are males,
some females, none hermaphroditic.

This is probably one of the richest phyla in numbers of species, but
its species are not proportionately well-known. Most of the members
of this group are free-living, and they are found in all sorts of situations,
in water or soil. Some infest plant tissues. Others are parasitic in
animals. The dread human disease called trichinosis is caused by round-

THE GROUPS OF ANIMALS 267

worms which are introduced in insufficiently cooked pork. The pigs get
it by eating meat refuse or infested rats. The larvae get into the lym-
phatic vessels or bore out through the intestinal wall and enter the mus-
cles, where they become encysted (Fig. 224A). Government inspection
of meats is carried out in a few countries, but in some of those with the
most rigid inspection the incidence of trichinosis is high. The reason is
the habit of eating rare pork in those countries. Thorough cooking is
the safest preventive; once the larvae are on their way to the muscles there
is no cure. Members of another family of roundworms may cause ele-
phantiasis by clogging the lymph passages.

The hookworm (Fig. 2245) of the southern states is also a member of
this phylum. The larvae develop in moist soil. From there they enter
the body through the skin of the feet, get into the blood, and thus reach
the lungs and intestines. By feeding upon the blood and causing bleed-
ing through an inhibition of clotting they produce an anaemic condition.
Injury to the lungs predisposes the victim also to tuberculosis. The
shiftlessness of the ''poor whites" in the South is attributed in part to
hookworm disease. An important feature of preventive measures is
proper disposal of human feces, so as to prevent pollution of the soil, thus
stopping further infection. Curative treatments are also available for
those already diseased.

Phylum 6. Echinodermata. — Members of this phylum are radially
symmetrical in the main, though usually some small feature is eccentri-
cally placed so as to introduce slight bilaterality. Usually there are five
rays, but the number may be very much greater. The skeleton consists
of limy plates, either firmly joined into a globular shell or more loosely
aggregated in the body wall so as to be readily movable on one another.
There is a distinct coelom. Many echinoderms possess a peculiar method
of locomotion by means of tube feet. These are hollow muscular tubes,
ending in suckers and filled with water by which they are operated. The
tube feet may be thrust out long distances by pressure on the contained
water, attached to fixed objects by the suckers, then contracted, pulling
the whole animal slowly along. Locomotion is more rapid in the brittle
stars, since the slender arms of these animals can be bent rapidly and pro-
vide a sort of walking or running movement. Some of the feather stars
are sessile^ being attached by a jointed stalk to the bottom. All members
of this phylum are marine.

Starfishes (Fig. 225 A) have arms usually well marked off from the
body disk. The brittle stars (B) have this distinction of arms from the
body disk especially clearly marked. The name brittle star comes from
the animals' practice of breaking off injured arms, which thereupon
regenerate.

Sand dollars (D) have a nearly smooth margin, without division into

268

PRINCIPLES OF ANIMAL BIOLOGY

arms. The sea urchins (C) are globular and without arms. Sea cucum-
bers {E) have no arms, but around the mouth is a series of branched
tentacles. The arms of the feather stars are branched like a feather, and
the branches are featherlike.

The starfishes have the peculiarity of digesting their food outside the
body. They prey upon clams, forcing the valves of the shell open by a
steady pull with the tube feet. The stomach is thrust out through the
mouth, pushed between the separated valves, and wrapped around the
exposed parts of the clam, which is then slowly digested. Oyster beds
suffer considerably from these attacks. The other kinds of echinoderms
take their food inside the body.

A curious habit is that of the sea cucumbers, of eviscerating them-
selves when irritated. If they are attacked, the body wall contracts so
vigorously that it bursts, and a part (or even all) of the intestine is forced

A B C D E

Fig. 225. — Various echinoderms: left to right, starfish, brittle star, sea urchin, sand dollar,
sea cucumber. {Courtesy of Carolina Biological Supply Co.)

out, along with the branching respiratory organs that are attached to
the cloaca. The tangled mass of viscera may so hinder (or perhaps
appease) the enemy as to stop the attack. During a brief resting period
the missing internal organs of the sea cucumber are regenerated.

Echinoderms are invaluable subjects in experimental laboratories
because of the abundance of their eggs and the ease with which they may
be obtained. Hundreds of studies of cytology, physiology of fertiliza-
tion, and embryology have been made on the eggs of starfishes and sea
urchins, and sometimes the other groups of echinoderms.

The relationships of echinoderms to the other phyla have been much
debated because there is little clear evidence of them. Adult anatomy
is entirely different from that of any other animals, and conclusions
drawn from developmental stages have been various. There is less basis
for establishing kinships of echinoderms than of almost any other group.

Class I. Asteroidea. Free-livinfi;, typically peiitamerous echinodoniis with wide
arms moderately marked off from disk and with ambulacral grooves. Starfishes.
(Fig. 2251.)

Class II. Ophiuroidea. Free-living, typically pentameroiis echinoderms with
slender arms sharply marked off from disk and no ambulacral grooves. Brittle stars.
(Fig. 225B.)

THE GROUPS OF ANIMALS 269

Class III. Echinoidea. Free-living, pentamerous echinoderms without arms;
test composed of calcareous plates bearing movable spines. Sea urchins, sand dollars.
(Figs. 76, 225C, D.)

Class IV. Holothurioidea. Free-living, elongated, soft-bodied echinoderms with
muscular body wall and tentacles around mouth. Sea cucumbers. (Fig. 225E.)

Class V. Crinoidea. Sessile echinoderms with five arms generally branched with
pinnules, aboral pole usually with cirri, sometimes with jointed stalk for attachment
to substratum. Feather stars, sea lilies.

Phylum 7. Annelida. — These are the true worms, as distinguished
from Phyhim 4 and Phyhmi 5 whose members are called flatworms and
roundworms. The annelids are triploblastic, bilaterally symmetrical
animals, with elongated body divided into segments. The segmentation
is internal as well as external, for thin membranes divide up the body
cavity or coelom. Corresponding with these segments, many of the
internal organs are repeated in most of the segments, while some are
repeated in only a few of the segments. The excretory organs, which
are nephridia (page 135), occur in most segments; the nervous system,
which is chiefly a long cord near the ventral side, typically has a ganglion
and nerves in most of the segments; and the main blood vessels give off
branches in each segment. Spiny projections or setae are common aids
to locomotion.

Some of the annelids are hermaphroditic but do not fertilize their
own eggs. Some of them (Fig. 226) also reproduce by budding or unequal
fission. Many of them have remarkable powers of regeneration if cut
into pieces. Some curious results are obtained by cutting off the head
end of an earthworm; at certain levels the head structures are regenerated,
while if cut at other levels a tail is developed in place of a head.

Despite the large size which many annelids attain, some of the larger
ones respire only through the general body surface. Some others, no
larger, have branched or filamentous gills which greatly increase the area
through which oxygen is absorbed.

Some theoretical interest attaches to the larval stage of many marine
annelids, which is known as a trochophore. It is pear-shaped or nearly
spherical, with a circle of cilia around its equator. Similar larvae are
found among the clams and snails, and adult rotifers may have roughly
the same shape. Many biologists have considered that some relation-
ship among these phyla is indicated by the trochophore larva or trocho-
phorelike adult form.

Among the services to man performed by annelids may be mentioned
the comminution and constant overturning of the soil by earthworms.
These animals eat the soil, for whatever organic matter it may contain,
and eject it from their digestive tracts. In making their burrows,
much soil is brought to the surface from below. The burrows also leave
the soil porous. Some annelids are also used for human food, notably

270

PRINCIPLES OF ANIMAL BIOLOGY

the palolo of Samoa and other Pacific islands. These worms burrow
in the coral reefs, and swarm in the open water in huge numbers just
before the last quarter of the moon in October and November. They are
captured in quantity by the natives at that time.

Fig. 226. — Autolytus, a
marine worm.

Fig. 227. — Aeolosoma.
a fresh-water worm.

Fig. 228.— a leech. (From

Leunis.)

Some of the annelida live in the soil (earthworms), many live in fresh
water (Figs. 227, 228), and many are marine. Some of the leeches (Fig.
228) attach themselves to vertebrate animals by means of suckers and
feed upon the blood of their host. Chaetopterus (Fig. 229) lives in a

Fig. 229. — Chaetopterus. {Courtesy of Carolina Biological Supply Co.)

U-shaped parchmentlike tube in the sand under marine waters. The
tube is open at both ends, and circulation of the water in it is maintained
by flat appendages at the sides of the body.

Class I. Archiannelida. Marine Annelida with no setae or parapodia.
Class II. Chaetopoda. Annelida with setae and a perivisceral coelom; marine,
fresh-water, or terrestrial in habitat. Earthworms.

Subclass I. Polychaeta. With many setae. Marine worms. (Figs. 226,

229.)

Order 1. Phanerocephala

Subclass II. Oligochaeta.
worms. (Figs. 135, 136, 227.)

Order 1. Microdrili

Order 2.
With few setae.

Cryptocephala

Fresh-water and terrestrial

Order 2. Macrodrili

Class III. Hirudinea. Annelida without setae, and with anterior and posterior

suckers. Leeches. (Fig. 228.)

Phylum 8. MoUusca. — This group includes clams, snails, and cuttle-
fishes. Their structure is so diverse that the phylum is difficult to
define. Mollusks are triploblastic, unsegmented, and bilaterally sym-
metrical, though their symmetry is disturbed by a secondary spiral
winding in some kinds. They have a coelom of restricted extent and
usually possess a shell. The name moUusk refers to their soft bodies.

A structure called the foot is characteristic of the phylum but varies
greatly in form. In the chitons (Fig. 230) it is like the sole of a shoe. In

THE GROUPS OF ANIMALS

271

the clams and mussels (Fig. 231) the foot is a wedge which plows through
the sand or mud. In the snails (Fig. 232) it is flat, and the animal creeps
along on it, usually by rapid wavelike muscular contractions, but some-
times by means of cilia. The foot of a snail may secrete a mucous
substance along which the animal creeps; a vertical
roadway may thus be erected directly through the water
without any support except at one end. The foot of the
squids (Fig. 233), cuttlefishes, and nautili is transformed
into a circle of arms bearing suckers.

The shell consists of two valves in the clams, oysters,
and mussels; is spirally wound in the snails; is a row of
movable plates in the chitons; is entirely embedded in the
flesh in the cuttlefishes, squids, and most slugs; and is
entirely lacking in certain marine mollusks called nudi-
branchs, which bear some resemblance to snails, and in a
few species of several other groups.

The sexes are usually separate, but one class of snails is hermaphro-
ditic, as are also some members of other classes. Among the latter, self-
fertilization may occur, or two animals may mate.

The mollusks began as marine animals, then began to invade fresh
water, and finally the land. Only the snails have gone on land, however.

Fig. 230.
— Chiton.
(.Courtesy of
Carolina Bio-
logical Supply
Co.)

Fig. 231. — A clam.

Fig. 232. — A snail.

Fig. 233.— a squid.

and only the snails and clams into fresh water. The group as a whole
has always been a successful one. It has maintained its abundance
throughout geological time and is as well represented by number of species
now as it ever has been. The evolution of mollusks has in general led to

a reduction of the shell and the growth of the
mantle over it, though one class has escaped
these changes.

Man has an economic interest in mollusks
in several ways. Oysters and clams are im-
portant articles of food, the former being
extensively cultivated. Pearls are made up of layers of nacre secreted
around some irritating foreign object by the epithelial cells of clams.
Such objects are deliberately inserted by pearl raisers, and pearls of any
desired shape may be obtained. The same substance, nacre, on the
inside of clam shells constitutes mother of pearl, which is used for buttons

Fig. 234.— Teredo, the
ship worm. {Courtesy of
Carolina Biological Supply
Co.)

272 PRINCIPLES OF ANIMAL BIOLOGY

and knife handles. On the debit side of the ledger, the shipworm Teedo
(Fig. 234, a mollusk, not a worm) bores' into wharves and shipping and
does considerable damage.

Class I. Amphineura. Mollusca with obvious bilateral symmetry, sometimes an
eight-parted calcareous shell and several pairs of gills. (Fig. 230.)

Order 1. Polyplacophora Order 2. Aplacophora

Class II. Gastropoda. MoUusks with a head and with bilateral symmetry usually
obscured by a spiral shell of one piece. Snails. (Fig. 232.)

Subclass I. Streptoneura Subclass II. Euthyneura

Order 1. Aspidobranchia Order 1. Opisthobranchia

Order 2. Pectinibranchia , Order 2. Pulmonata

Class III. Scaphopoda. Mollusca with conical tubular shell and mantle.
Class IV. Pelecypoda. MoUusks without a head, with bilateral symmetry, a
shell of two lateral valves and a mantle of two lobes. Clams, mussels. (Fig. 231.)

Order 1. Protobranchia Order 3. Eulamellibranchia

Order 2. Filibranchia Order 4. Septibranchia

Class V. Cephalopoda. MoUusks with distinct bilateral symmetry and a foot
bearing eyes and divided into arms usually with suckers. Cuttlefishes, octopods.
(Fig. 233.)

Order 1. Tetrabranchia Order 2. Dibranehia

Phylum 9. Arthropoda. — Members of this phylum have jointed
bodies and jointed legs. Their skeletons are composed of a horny mate-

FiG. 235. — A centipede. Fig. 236.— A beetle.

(From Hegner, "College Zoology," The Macmillnn Company.)

rial on the outside of the body. This horny shell is burst and shed at
intervals, and replaced by a new skeleton beneath, as the animal grows.
Examples of arthropods are crayfishes, shrimps, centipedes (Fig. 235),
insects (Fig. 23(5), and spiders (Fig. 237). They are triploblastic and
))ilaterally symmetrical. The blood system includes sinuses, which are
merely spaces among the organs, into which the arteries open. The
coclom is much reduced in size.

The number of differcmt kinds of arthropods is almost unbelievably
great. More known species belong to this phylum than to all other phyla
combined. About half a million have been described, but the number

THE GROUPS OF ANIMALS 273

must be several times as great. The insects furnish a greater share of
these than all other arthropods together.

Arthropods are found in practically all situations that support life — in
fresh and salt water, in mud, burrowing in soil, on the surface of the earth
where they feed on animal or plant food, flying in the air, boring in trees
or herbaceous plants, and parasitic in or on animals.

While most arthropods go through a fairly direct development, such
that they are readily recognized at all stages even by the uninitiated,
some of them, including many insects, have a striking metamorphosis
involving larva, pupa, and adult. In the larva there are groups of cells
forming the rudiments of the adult organs. These persist through the
pupa, but the rest of the larval organization disintegrates into a milky
mass which is doubtless partly used as nutrition for the growing adult
structures.

Fig. 237. — A slider. Fig. 238. — A crab. Fig. 239.— A crayfish.

(Courtesy of Carolijia (From. Van Cleave.)
Biological Supply Co.)

Among the interesting features of arthropods is the social organization
among some of the insects. The bees, ants, wasps, and termites have
structurally different types of individuals which are also distinguished as
social castes, sometimes in a very complicated system.

Worthy of note with respect to reproduction in the phylum is the
rather frequent occurrence of parthenogenesis. In some of the smaller
Crustacea there is diploid parthenogenesis, in which the eggs do not
experience a reduction division. Such parthenogenesis may be repeated
for many generations but is usually interspersed with bisexual reproduc-
tion at intervals. In bees and many other insects there is haploid parthe-
nogenesis, meaning that the egg which develops without fertilization has
undergone chromosome reduction. The haploid individuals thus pro-
duced are regularly males.

Many members of this phylum are of economic importance to man.
Lobsters, crabs (Fig. 238), in some regions crayfishes (Fig. 239), and
shrimps are used as food, and bees collect honey in domestication. Small
aquatic forms are common food for game fishes. Insects often pollinate
flowers and are important to certain seed crops and fruits (figs). The
silkworm moth is a valuable adjunct to the textile industry. Many
species are injurious. They may destroy fruit or grain crops or shade

274

PRINCIPLES OF ANIMAL BIOLOGY

trees or carry disease-producing organisms (mosquitoes, housefiies).

Some of the insects are parasitic in domestic animals, and the mites may

attack the skin of man ("jiggers"), poultry,
or cattle. Barnacles injure bottoms of ships.
Relationship of the arthropods to the
annelids has often been suggested, largely
because of the segmentation of the body into a
longitudinal chain of metameres. In support
of this idea is brought the annelidlike Peri-
patus (class Onychophora, below). Peripatus
has a body superficially like a worm and has
segmentally arranged nephridia; but it has
tracheae like insects, sinuses in the circulatory

system, and no coelom. If Peripatus is a primitive form, which may be

doubted, its value as a connecting link is considerable.

Fig. 240. — A scorpion
(left) and king crab. {From
Carolina Biological Supply

Co.)

Class I. Crustacea. Arthropods

breathing by means of gills,

antennae. Crayfishes, crabs, and shrimps.

(Figs. 238,

239, 299, 300.)

Subclass I. Branchiopoda

Order 1. Phyllopoda

Order 2.

Cladocera

Subclass II. Ostracoda

Subclass III. Copepoda

Subclass IV. Cirripedia

SxjBCLAss V. Malacostraca

Order 1. Nebaliacea

Order 6.

Isopoda

Order 2. Anaspidacea

Order 7.

Amphipoda

Order 3. Mysidacea

Order 8.

Euphausiacea

Order 4. Cumacea

Order 9.

Decapoda

Order 5. Tanaidacea

Order 10.

Stomatopoda

two pairs of

Class II. Onychophora. Primitive air breathing arthropods with tracheae and
nephridia. Peripatus.

Class III. Myriapoda. Arthropods with tracheae, one pair of antennae, and
many similar legs. Centipedes and millipedes. (Fig. 235.)

Order 1. Pauropoda Order 3. Chilopoda

Order 2. Diplopoda Order 4. Symphyla

Class IV.

Insecta. Arthropods with tracheae, one

pair of antenn

airs of legs.

Insects. (Figs. 95, 203,

204,

236, 303.)

Order

Aptera

Order 11.

Hemiptera

Order

Ephemerida

Order 12.

Neiiroptera

Order

Odonata

Order 13.

Mecoptera

Order

Plecoptera

Order 14.

Trichoptera

Order

Isoptera

Order 15.

Lepidoptera

Order

Corrodentia

Order 16.

Diptera

Order

Mallophaga

Order 17.

Siphonaptera

Order

Thysanoptcra

Order 18.

Coleoptera

Order

Euplexoptera

Order 19.

Hymenoptera

Order 10.

Orthoptera

THE GROUPS OF ANIMALS

275

Class V. Arachnida. Arthropods with tracheae, book lungs or book gills and no
antennae. Spiders, mites, scorpions, king crabs. (Fig. 240.)

Order 1.

Araneida

Order 6.

Palpigradi

Order 2.

Scorpionidea

Order 7.

Solifugae

Order 3.

Phalangidea

Order 8.

C'hernetidia

Order 4.

Acarina

Order 9.

Xiphosura

Order 5.

Pedipalpi

Order 10.

Eurypterida

Invertebrate Groups of Uncertain Position. — Certain groups of inver-
tebrates have not been assigned a definite relation to other groups.
Opinion differs so widely as to their affinities that they may well be
kept out of the classification for the present.

Mesozoa. Parasites apparently intermediate between the protozoa and metazoa.
Not improbably degenerate relatives of the flatworms. , ;,».

Nemertinea. Terrestrial, fresh-water, and marine animals resembling flatworms
but with a proboscis, blood-vascular system, and alimentary canal with two openings.

Nematomorpha. Long threadlike animals with the body cavity lined with
epithelium, a pharyngeal nerve ring, and a single ventral nerve cord.

Acanthocephala. Parasitic worms with spiny proboscis, a complex reproductive
system, and no alimentary canal. (Fig. 241.)

Fig. 242. — Arrowworm, Sagitta.

Fig. 241. — Echinorhynchus, one of the
Acanthocephala.

Chaetognatha. Marine invertebrates with a distinct coelom, alimentary canal,
nervous system, and two eyes. Arrowworm. (Fig. 242.)

Ctenophora. Triploblastic animals; symmetry partly radial, partly bilateral;
eight rows of vibratile plates radially arranged. Sea walnuts or comb jellies. (Fig.
243.)

Fig. 243.

ctenophore.
Cleave.)

{From Van

Fig. 244. — A rotifer.
{From Whitney.)

Rotifera. Invertebrates with a head provided with cilia, usually a cylindrical or
conical body often with a shell-like covering, and a tail or foot, bifurcated at the
tip where it is provided with a cement gland. (Fig. 244.)

Bryozoa. Mostly colonial invertebrates resembling hydroids in form, with dis-
tinct coelom, and with digestive tract bent in the form of a letter U. (Fig. 140.)

Phoronidea. A single genus of wormlike animals having tentacles and living in
membranous tubes in the sand.

276

PRINCIPLES OF ANIMAL BIOLOGY

Brachiopoda. Marine tentaculate animals with a calcareous shell, composed of
two unequal valves, a dorsal and a ventral. (Fig. 245.)
Gephyrea. Wormlike animals of doubtful affinities.

Fig. 245. — A brachiopod. Left, the shell; right, the animal.

Phylum 10. Chordata. — The animals of this phykim have at some
stage a skeletal axis called the notochord, gill slits in the embryo or adult,
and a nerve cord dorsal to the alimentary canal. (In preceding phyla
when a nerve cord is present it is ventral to the alimentary tract.) This

Fig. 246. — Balanoglossus. {From Carolina
Biological Supply Co.)

Fig. 247. — A tunicate. {From Carolina
Biological Supply Co.)

phylum includes a number of degenerate animals such as Balanoglossus
(Fig. 246) and the tunicates (Fig. 247) which must be included here
because of the presence of the notochord in the embryo. It also includes
the amphioxus (Fig. 248), a fishlike animal in which the notochord is the

^■.'J?:y* \^v>:9W:fi:m^

Fig. 248. — Amphioxus.

Fig. 249.^ — Lamprey. {From Carolina
Biological Supply Co.)

permanent skeletal axis. The remaining chordates are called vertebrates
from the fact that the notochord becomes invested with cartilage which
is segmented to form a vertebral column. In some lower forms the carti-
laginous vertebrae and the notochord which they surround persist
throughout the life of the animal, but in the higher forms the cartilage is
replaced by bone and the notochord disappears.

Fig. 250. — Hagfish. {From Carolina Bio-
logical Supply Co.)

Fig. 25 L — A shark. {From Carolina Bio-
logical Supply Co.)

At the bottom of the vertebrate scale are the lampreys (Fig. 249) and
hagfishes (Fig. 250) which are eellike in form but have no jaws and no
lateral fins. The skeleton is made of cartilage only. Some of the lam-
preys inhabit fresh water and lay their eggs in nests made by pulling up
stones from the bottoms of brooks. Next above these in the scale are
the sharks (Fig. 251), skates (Fig. 252), and rays, whose skeletons are also
cartilaginous but which have jaws. Their skin is armored with a type of
scale having a tooth or spine mounted on a flat base. The dried and proc-

THE GROUPS OF ANIMALS

277

essed skin Avith these scales forms the leatherhke natural shagreen of
certain costume accessories.

Above these are five major groups, the true fishes, amphibia, reptiles,
birds, and mammals. Differences among these are found in the hard
parts of the skin, the form of the limbs, the structure of the heart, and
the means of respiration.

Fig. 252. — Skate.

253. — A fish. {From Carolina Bio-
logical Supply Co.)

The fishes (Fig. 253) are aquatic, and respire by means of gills. The
skin usually bears scales, but these are not toothed like the scales of
sharks. The skeleton is at least partly of bone. Locomotion is effected
by fins (and the bending of the body), and the heart consists of but two
chambers (one auricle and one ventricle).

Fiu. 254. — A saltiniander.

Fig. 255.— a frog.

The amphibia are the salamanders (Fig. 254), toads and frogs (Fig.
255), and certain legless forms called caecilians. Their skin is smooth,
nearly always devoid of scales, thovigh some fossil amphibia were heavily
armored. They are nearly all aquatic at least in young stages, and some
of them throughout their lives. Division of their habitats between water

Fig. 256. — A lizard. {From Carolina Bio-
logical Supply Co.)

Fig. 257. — A turtle. {From Carolina Bio-
logical Supply Co.)

and land is what gives the class its name amphiliia. The heart is three-
chambered — two auricles, one ventricle. Though the amphibia are of
less value to man than are the fishes, frogs' legs are a table delicacy, toads
devour many insects, and most orders have contributed material for
important biological and medical investigations.

Reptiles include lizards (Fig. 256), snakes, alligators, turtles (Fig.
257), and such fossil forms as dinosaurs. Their skin contains scales or

278 PRINCIPLES OF ANIMAL BIOLOGY

hard plates. They are cold-blooded in common with the fishes and
amphibia but unlike the following two classes. They have no gills in any
stage. The heart is three-chambered (approximately four-chambered in
crocodiles, in which the ventricle is partially divided). Some snakes are
poisonous, but most of them are beneficial to man (as are also the lizards)
because they devour noxious animals. Some turtles are used for food.

The birds are characterized by feathers, which grow from pits in the
skin, forelimbs adapted to flight, a four-chambered heart, warm blood
(warmer than that of the next class, mammals), and a beak with horny
covering but no teeth. The bones of the skeleton are extensively fused,
particularly in the wings. The body is made light for its size by large
air spaces, variously placed, some of them extending into the cavities of
certain bones. These spaces connect with the lungs, but their walls are
not made of lung tissue, though doubtless they do effect some exchange of
oxygen and carbon dioxide.

Mammals are mostly quadrupeds. The skin is covered with hair
— very sparsely in some. They breathe air even when they inhabit water.
The heart is four-chambered, the blood warm. The red cells of the blood
are devoid of nuclei except while they develop in the marrow. There is
a muscular sheet or diaphragm between the thorax and the abdomen,
important in breathing. The young are usually developed in the uterus
of the female — a few lay eggs — and are nourished with milk from the
mammary glands after birth. The most primitive mammals, the egg
layers, live in Australia and neighboring islands. The marsupials, which
give birth to their young in a very early stage and carry them for a long
time in a pouch, are next most primitive. They live in the Australian
region, in South America, and one kind (opossum) in North America.

SUBPHYLUM I. Enteropneusta. Wormlike chordates of somewhat doubtful
systematic position. (Fig. 246.)

Order 1. Balanoglossida Order 2. Cephalodiscida

SUBPHYLUM II. TUNICATA. Saclike marine animals with a cuticular outer covering
known as a tunic or test. Tunicates. (Fig. 247.)

Order 1. Ascidiacea Order 3. Larvacea

Order 2. Thaliacea

SUBPHYLUM III. Cephalochorda. Fishlikc chordates with a permanent noto-
chord composed of vacuolated cells. Amphioxus. (Fig. 248.)

SUBPHYLUM IV. VertebratA, Chordates in which the notocliord cither persists
or becomes invested by cartilage, segmented, to form a vertebral colunui, or disap-
pears, the vertebral column being made up of bony segments.

Class I. Cyclostomata. l']ellike vertebrates without functional jaws or lateral
appendages. Lampreys and hagfishes. (Figs. 249, 250.)

Subclass I. Myxinoidea Subclass II. Petromyzontia

THE GROUPS OF ANIMALS 279

Class 11. Elasmobranchii. Fishlike vertebrates without air bladder, with jaws,
and with a cartilaginous skeleton and placoid scales. Sharks, rays, and skates.
(Figs. 251, 252.)

Subclass I. Selachii

Order 1. Squali Order 2. Raji

Subclass II. Holocephali

Class III. Pisces. Aquatic, cold-blooded vertebrates breathing by means of gills,
with air bladder, a two-chambered heart, and usualjy a dermal exoskeleton of scales.
Fishes. (Figs. 159, 253.)

Subclass I. Teleostomi. Fishes with a skeleton consisting wholly or par-
tially of bone, breathing by means of gills. True fishes.

Order 1. Crossopterygii Order 3. Holostei

Order 2. Chondrostei Order 4. Teleostei

Subclass II. Dipnoi. Fishes with a skeleton of cartilage and bone, and air
bladder functioning as a lung. Lungfishes.

Class IV. Amphibia. Cold-blooded vertebrates breathing by means of gills in
some stage, skin usually not covered with scales, heart of three chambers. Sala-
manders, toads, and frogs. (Figs. 93, 151, 157, 158, 163, 187, 254, 255.)

Order 1. Caudata Order 3. Apoda

Order 2. Salientia

Class V. Reptilia. Cold-blooded vertebrates usually covered with scales, breath-
ing throughout life by means of lungs. Lizards, snakes, crocodilians, turtles. (Figs.
156, 256, 257, 279, 281.)

Order 1. Testudinata Order 3. Crocodilini

Order 2. Rhynchocephalia Order 4. Squamata

Class VI. Aves. Warm-blooded vertebrates with the body covered with
feathers, with the forelimbs usually modified as wings, and a heart of four chambers.
Birds. (Figs. 161, 201.)

Archaeornithes
Neornithes
Struthioniformes. Ostriches.
Rheiformes. Rheas.
Casuariiformes. Cassowaries, emus.
Apterygiformes. Kiwis.
Tinamiformes. Tinamous.
Sphenisciformes. Penguins.
Gaviiformes. Loons.
Colymbiformes. Grebes.
Procellariiformes. Albatrosses, petrels.
Pelecaniformes. Pelicans, frigate birds.
Ciconiiformes. Herons, storks.
Anseriformes. Ducks, geese, swans.
Falconiformes. Vultures, hawks, falcons.
Galliformes. Pheasants, grouse, turkeys.
Gruiformes. Cranes, rails.
Charadriiformes. Shore birds, gulls, auks.

Subclass I.

Subclass II.

Order 1.

Order 2.

Order 3.

Order 4.

Order 5.

Order 6.

Order 7.

Order 8.

Order 9.

Order 10.

Order 11.

Order 12.

Order 13.

Order 14.

Order 15.

Order 16.

280

PRINCIPLES OF ANIMAL BIOLOGY

Order 17. C-olumbiformes. Pigeons, doves, sand grouse.

Order 18. Psittaciformes. Parrots, macaws.

Order 19. Cuculiformes. Cuckoos, plantain eaters.

Order 20. Strigiformes. Owls.

Order 21. C'aprimulgifornies. Goatsuckers, oil birds.

Order 22. Micropodiformes. Swifts, hummingbirds.

Order 23. C'oraciiformes. Kingfishers, hornbills.

Order 24. Piciformes. Toucans, woodpeckers.

Order 25. - Passeriformes. Broadbills, ovenbirds, lyrel)irds, songi)irds.

Class VII. Mammalia. Warm-blooded animals which are covered with hair at
some stage, suckle the young, and have a diaphragm between thorax and abdomen.
Mammals. (Figs. 92, IgO, 193, 197, 272, 294.)

SXJBCLASS I. PrOTOTHERIA.

Order 1. Monotremata
Subclass II. Metathkria

Order 1. Marsupialia
Subclass III. P]utheria.

Order 1. Insectivora

Order 2. Dermoptera

Order 3. C'hiroptera

Order 4. Carnivora

Order 5. Pinnipedia

Order 6. Menotyphla

Order 7. Rodentia

Order 8. Lagomorpha

Order 9. Primates — With nails

Egg-laying mammals. Monotremes.
Pouched mammals. Marsupials. (Fig. 157. >

Viviparous mammals.
Order 10.
Order 11.
Order 12.
Order 13.
Order 14.
Order 15.
Order 16.
Order 17.
Order 18.

With
claws

True mammals.
Artiodactyla
Perissodactyla
Proboscidea
Hyracoidea > With
Xenarthra \ hoofs
Pholidota
Tubulidentata
Sirenia

Cetacea

}

\(iuatic

References

CoMSTOCK, J. H. A Manual for the Study of Insects. Comstock Publishing Com-
pany.

Pratt, H. S. A Maimal of Common Invertebrate Animals. A. C. McClurg &
Company.

Storer, Tracy I. General Zoology. McGraw-Hill Book Company, Inc. For fuller
account of various groups, also biological principles.

CHAPTER 20
ANIMALS AND THEIR ENVIRONMENT

Ecology deals with the relations of organisms to the environment. It
has to do primarily with those relations to environment which determine
the organism's characteristics, its success, its mode of life, and its distribu-
tion. Ecology is also concerned with the environment itself. Since the
things to which animals and plants are sensitive in the world about them
are not everywhere the same, any organization which the environment
may possess is of importance to living things. This organization is some-
times very intricate, and many an ecological study has been directed
toward an understanding of the environmental system, mthout imme-
diate reference to any particular organism.

The environmental relations of organisms may be approached from
two different points of view: (1) that of the individual or single species,
in which case ecology comes very near to a limited physiology, and (2)
that of groups of species living in the same general situations and forming
what are called associations or communities. These two points of view
are successively adopted in this chapter.

Temperature. — Each kind of animal is capable of carrying on its
metabolism only within a certain range of temperatures. At some point
within this range, usually above the middle but sometimes below, the
physiological processes work best. For most animals the lower limit is
slightly above freezing, while the upper limit is usually below 45°C. Fish
eggs develop best a few degrees above freezing, birds' eggs at about 40°C.
Some animals possess remarkable powers of adjusting themselves to tem-
peratures outside their usual range. Thus some of the pj-otozoa which
die when raised ^\dthin a short time to a temperature of 23°C. will endure
70°C. if the temperature is raised very gradually.

Since temperature varies irregularly on the earth's surface from the
equator to the poles, with elevation above sea level, seasonally, and as
between day and night, it is obvious that animals must be so located that
their permissible temperatures are present and that their limits are not
overstepped. Ordinarily, species with a low optimum temperature must
live in temperate or cooler zones, those Avith a high optimum temperature
in tropical regions. The factor of dormancy also enters into the deter-
mination of geographic position. Most animals become torpid at suffi-
ciently low temperatures, and some endure actual freezing. Many of

281

282 PRINCIPLES OF ANIMAL BIOLOGY

them, however, cannot be dormant for any great length of time and still
live. Such species have to live in tropical regions.

Some animals avoid the dangers of extreme temperatures by special-
ized habits. Certain wasps which dig in sand dunes cannot endure for
long the high temperature at the surface of the sand on sunny days.
They survive these temperatures by digging vigorously for a few seconds,
then flying about a few inches above the sand where the air is much cooler,
then returning to their digging for a brief period.

The regulation of body temperature by birds and mammals has
already been described (page 120). This physiological feature enables
animals of these classes to range widely so far as temperature is con-
cerned. Among cold-blooded animals there is occasionally the ability
to regulate temperatures in groups. Honeybees can do this in colonies,
though each individual bee cannot. A certain amount of heat is liberated
in their metabolism; and if this is conserved in masses of bees, the tem-
perature may be considerably raised. Temperatures in their hives are
much higher than those outside in winter.

One curious relation to temperature is the acceleration of metabolism
by fluctuating as compared with constant temperatures. Grasshopper
eggs develop much more rapidly at their optimum temperature if that
temperature has been interrupted by a cold period. The acceleration is
greater if the interruption by low temperature comes early than if it
comes late in development. Eggs are laid by these insects in late summer
and fall, over a period of many weeks. It would be expected that those
laid early would be the first to hatch the following spring ; but all the eggs
hatch about the same time. Those laid late in the fall enter the winter
in an early embryonic stage but are accelerated enough more in the spring
to enable them to overtake their older companions and emerge at the
same time. This is an important reaction to temperature, for if any of
the young grasshoppers emerged much later in spring or summer they
would miss the most favorable period of the year for passing through
their immature nymphal stages.

Some anilnals change their reactions to other stimuli with changes of
temperature. Thus one of the leaf-boring beetles studied by Chapman
goes toward the light at high temperature and takes to flight if mechani-
cally disturbed, but avoids light at low temperature and draws up its
legs and falls if disturbed. These beetles live on a plant which grows in
water, and during the warm part of the day they are out on the leaves.
If their reactions were reversed and they were disturbed at this time, they
would fall into the water, but instead they fly away. Disturbance during
the cool part of the day, when they are hiding at the center of the plant,
merely causes them to fall into the recesses at the bases of the leaves.

The structure of an animal sometimes depends on the temperature.

ANIMALS AND THEIR ENVIRONMENT 283

Among the aphids or plant lice some individuals have wings, other do
not, and it has been sho^vn that temperature helps to determine whether
wings develop. No general rule can be given for the control of wing
production, since different strains respond differently, even within the
same species. In one strain maximum wing production is obtained if
the parents are reared continuously at low temperatures. Since the
aphids generally alternate between two host plants during a season, and
since ^\ings are the easiest means of effecting their migrations, it is impor-
tant to them that uings develop in at least some individuals at the right
time. Another insect that responds developmentally to temperature
is the vinegar fly, Drosophila. One of its mutant varieties has vestigial
wings (Fig. 204, page 236) which are quite useless for flight. At very
high temperatures, however, the wings of this variety are nearly normal.
This response happens to be of very little use to the flies for two reasons :
first, the temperature which induces full wing development is so high
that it is otherwise detrimental and flies seldom meet exactly that
temperature, and, second, inability to fly is not this mutant's worst
handicap, since it is physiologically weak and never matures so rapidly or
in so large numbers as do the normal flies of the species. Color in butter-
flies is likemse known to be affected by temperature, and it seems certain
that the differences between the northern and southern varieties of a
species are sometimes thus determined.

Genetic and evolutionary effects of temperature are known in a few
organisms. Mutations in Drosophila have been produced by heat in
experiments by Goldschmidt and others. The amount of separation and
recombination by characters in this fly due to breakage and reconstitu-
tion of chromosomes by exchange of pieces (page 236) is increased by
high temperature. And Seller has found that whether a given sex-
determining chromosome in a certain moth goes into the polar body or
remains in the egg at the meiotic division depends partly on the tem-
perature. The effect is such that more females are produced at high
temperatures.

Light. — The most obviously important influence of light upon the
ecology of animals is its effect upon green plants upon which the animals
feed. These plants are dependent on photosynthesis for their own
nutrition and can maintain themselves only where sufficient light is
present. Animals that live in caves must therefore subsist on plants
that do not carry on photosynthesis or on other animals whose food
chains do not end in green plants. In moderately deep lakes, as is pointed
out later, green plants are limited to the surface water, if floating, and
to a strip along the shore, if rooted (Fig. 258). Animals dependent
on such plants for food must spend part of their time in the regions
indicated.

284

PRINCIPLES OF ANIMAL BIOLOGY

Another influence that may be indirectly important for animals is
the effect of daily duration of light upon the reproductive processes of
plants. Many plants mil come to flower only if they are exposed to a
certain number of hours of light each day. . A certain range of duration
is always permissible. If the daylight period is longer or shorter than
this required range of hours, the plant may grow vegetativel}'^ even more
vigorously than usual but will not bloom. Unless a plant has some
satisfactory asexual method of propagation, it cannot maintain itself
in a region not affording the right duration of daylight. Probably no
plant whose required amount of daylight is yet known is the sole food

l"'iu. 258. — Shore vegetation of lake, which is too deep elsewhere for rooted plants. {Photo-
graph by F. C. Gates.)

of any species of animal, but the possibility exists that the range of some
animal is thus limited by the length of day.

Of structural changes induced in animals b.y light, the most significant
ecologically is probably the production of wings in aphids. In some
sti-ains of aphids, light has an e\c\\ more important influence than has
temperature, the effect of which is described above. In one of these
strains, alternating light and darkness caused nearly every individual to
be winged, provided the dark period was at least 12 hours long. Shorter
periods of darkness, including continuous light, made most of the aphids
of this strain wingless. Since temperate regions in summcn- do not have
12-hour nights, wing production must be considerably cui'tailed in such
strains. Other strains respond differently to light, some of them directly
reversing the behavior just described. The importance of wings in
the migration of these insects from one host plant to another has already
been mentioned.

ANIMALS AND THEIR ENVIRONMENT 285

The color of flatfishes, certain shrimps, and some other animals
changes to correspond to the background on which they rest. When
on a dark background, the pigment diffuses so as to fill the cells that
contain it and in the aggregate makes the animal dark. When on a
light background, the pigment collects into small knots, leaving much
of the surface exposed; hence the animal is pale. These changes may be
a concealing adaptation helping the animals to escape enemies.

]Many animals respond to light with changes of behavior, some of
which are of ecological significance. Isopods, the "pill bugs" or "sow
bugs" that live under boards or stones or in other dark places, are driven
into these places by their negative reaction to light. Such situations
are generally moist, which is necessary for an animal which, like the pill
bugs, respires by means of gills. Most other Crustacea live in water, but
some of the pill bugs have taken to land and have done so by utilizing
damp places. Their crevices also doubtless give them some useful
protection.

Some animals change their response to light according to certain
other conditions. A species of thrips, a minute flower-inhabiting insect,
crawls away from the soui'ce of light when it is quiet but is positive to
light when mechanically disturl^ed. Under ordinary conditions these
reactions drive the insect into the flower (a clover head, for example)
^vhere its food is; but if the flower is vigorously shaken, as by a grazing
animal, it crawls out. Probably their lives are often saved by this
behavior.

A more complicated adaptation involving response to light is exhibited
by a parasitic copepod (crustacean) named Lernaeopoda. This animal
is free-living in its larval stage but must attach itself to the gills or some
other part of the brook trout to complete its development. During
the day the larval copepod, because it is positive to light, swims near the
surface of the water, but at night it sinks to the bottom because it is
heavier than water. The brook trout likewise swims near the surface
in the daytime, either in response to light or in deliberate search for food
organisms which are located there, but at night settles to the bottom
because of its high specific gravity. Day and night, therefore, fish and
copepod are brought together — an arrangement highly satisfactory for
the parasite but not so advantageous for the host.

Moisture. — All organisms contain in their protoplasm a certain
amount of water, usually a very large amount. Without it they are
unable to function as living things. Many of them are so constructed
as to be unable to maintain this required water without living directly
in water. Probably no animal can endure complete desiccation, though
there are some that can exist for a long time in situations regarded as dry.
Protozoa may secrete a thick wall (cyst) and lie in dry hollows (former

286 PRINCIPLES OF ANIMAL BIOLOGY

ponds) or be blown about by the wind. Eggs of Crustacea and rotifers,
similarly covered with heavy shells, may likewise be dried without all
being killed. One family of rotifers may be dried in the adult stage, as
may also certain roundworms. Earthworms burrow deeper in the soil
as moisture disappears near the surface, and eventually they roll up in
balls to conserve their moisture.

Excess of moisture is often as injurious as dearth of it. Soil organisms
may be drowned in wet seasons because the air is driven from the soil
by water, and they are unable to obtain their required oxygen from
water. The sugai'-beet root louse suffers most damage from excess
moisture at the time of hatching from the egg and at the periodic shedding
of the skin as it grows. So much damage is done at these times that the
louse multiplies in dry soil more than fifteen times as fast as in soil
moistened from below, and nearly thirty times as fast as in soil moistened
by water falling from above.

Insects that suck the sap of plants are more or less independent of
moisture in the air around them, as long as their host plants can maintain
themselves. Indeed, in such animals the water may be regarded as a
waste material to be eliminated. The white fly, common on many
greenhouse plants, ejects water from its rectum in frequent bubbles
that burst and spray over the surrounding leaf surfaces at considerable
distances. Aphids are similarly supplied with excess water.

Among the higher animals, the water requirements differ enormously.
Mammals that lose much water through sweat (man, horse) or con-
siderable excretion of urine or milk (cattle) must make good the loss by
drinking. Most mammals are included in one or more of these categories,
but some manage to get along with very little water except that taken
with their food. Camels are the classical illustration of the ability to
do without water, since they can subsist a week with only dry food, and
if they are fed green plants they can avoid other water for a month or
more. Mountain goats, prong-horn antelopes, mule deer, jack rabbits,
gazelles, jumping mice, and some of the ground squirrels are said to use
only the water that is eaten with other food. Such animals are peculiarly
fitted for regions where there are few or no bodies of water.

One important ecological function of water in the protoplasm of
animals is its modification of the effect of temperature. Relatively
dry protoplasm endures high temperatures — even above that of boiling —
without coagulation, and low temperature without freezing. It is not
necessary that the water be actually removed from protoplasm to pro-
duce this effect, but merely that the amount of free water be reduced.
Thus, in the pupa of the polyphemus moth, which is covered with a thick
horny coat, there is little actual evaporation of the water, but as winter

ANIMALS AND THEIR ENVIRONMENT 287

approaches more of the water is adsorbed on the colloidal (page 42)
particles in the pupal liquids, leaving less water free. As a consequence
of this condition, the pupa endures winter freezing for months.

Nutrition. — With very few exceptions, all of which are among the
protozoa, animals are ultimately dependent on plants for their food. The
green plants provide carbohydrates by photosynthesis, and a few micro-
organisms; including those forming nodules on the roots of clover and
other legumes, can utilize the nitrogen of the air to produce nitrites and
nitrates. Out of these primary substances animals can make any com-
pounds they require, but plants have to make the start.

The manner of taking foods from plants is very variable. Many
insects or their larvae eat the leaves or suck sap from the leaves, stems,
or roots. Some eat the wood, though it is quite possible that fungi or
other organisms growing on the wood or in the burrows form part of their
food. Bees get carbohydrates (honey) from the flowers and proteins
from pollen. Many animals grow on decaying logs or other plant matter,
but it is likely that the microorganisms which are always present in
such places constitute the actual food. Of the animals that do not feed
directly upon plants but upon other animals, the larger ones usually,
and the small ones often, kill their prey and eat its flesh. The larvae
of the clothes moth eat hair or wool. Some insects live in the excrement
of animals, but here again it is probably the microorganisms that furnish
the food.

A very special way of obtaining nutrition is through parasitism.
The host is usually not killed — -at least until the parasite is past its para-
sitic stage — but contributes some of its substance to the parasite. The
flukes and tapeworms are regularly parasitic, as are some of the round-
worms and some insects. Parasites show a tendency to be degenerate,
which they can afford to be, since in their protected situations and with
their food often digested (page 265) before they receive it many of the
usual organs are unnecessary. The advantages of parasitism accrue only
to the parasite, none to the host.

Contrasted with this is the relation known as symbiosis, which is an
association of two species with mutual benefit. A very striking example
of symbiosis, in which food appears to be at least part of the advantage
gained by both species, is that existing between termites (the so-called
white "ants") and certain protozoa harbored in their intestines. The
protozoa are so abundant that in some instances they weigh as much
as the termite itself. The termites are wood-eating insects, and their
normal food is cellulose. They are not themselves, however, able to
digest the cellulose. This is done for them by the protozoa (page 259).
These protozoa may be removed from the intestine artificially by high

288 PRINCIPLED OF ANIMAL BIOLOGY

temperature or increatse of oxygen or starvation, and aftei* that the
termites are no longer capable of .subsisting on wood. Also, the protozoa
are unable to survive outside the termites.

The amount of food and frequency of taking it vary greatly in different
animals. A certain protozoon can swallow another protozoon ten
times its own bidk, digest it in two hours, and be ready for anotlier;
while some insect larvae may eat a hundred times their own weight daily.
Cold-blooded vertebrates, on the contrary, subsist on small quantities
eaten at long intervals. Certain birds may go without food for four or
five weeks, a lobster for months. Some insects do all their eating in the
immature stages and take no food when adult; certain butterflies and
May flies are examples. Male rotifers get all their food by eating done
a generation in advance; for they take no food after hatching, all their
nutrition coming from material stored in the eggs from which they hatch.

►Structural characteristics in a few animals are determined or modified
by their food. In the honeybee, for example, any fertilized egg may
tlevelop into a (jueen bee; l)ut to attain that end the lar\^ae must be fed
on "royal jelly," which is predigested pollen jjrepared for and given
to them by the workers. Other similar lar\'ae denied this food become
workers. A certain predatoi\y bug acquires a yello^^' color l\v eating
potato beetles, and the potato beetle gets the pigment from its food
plant. The dependence of the (jueen bee on its food has an important
ecological bearing, l>ut no such significance is known for the other exam-
ples given.

How serious a prol^lem food is in the ecology of a species depends on its
food tolerance. An animal can live only where its food is obtainable, and
it can be very successful onl.v if its food is rather abundant, iiut some
animals are omnivorous, being capable of eating a wide variety of other
organisms, while others are very specialized. AIan>' insect lar\'ae li^'e
characteristically only on certain plants, numerous aphids are limited
to two hosts (usually one at a time), and certain parasites are found
only in on(^ kind of animal in each (or some) of their stages. Such animals
lead a precarious existence unless their food is abundant or widespread,
or both.

Maintenance of Wumbers. — There are other factors wliich enter into
the lives of animals that help to determine their success of their distribu-
tion. Among them are altitude, as in mountainous regions, which affects
temperature and density of the atmosphere, and pressure, as of the water
in dee[) s(ias or lakes oi- of the aii' on mountains. The four discussed
above are, however, among the most important, and they will suffice to
illustrate the ecological situation of animals.

Each species of animal has a certain capacity to maintain itself, and
this capacity is matched against an environment made up of all the

ANIMALS AND THEIR ENVIRON .\rENT 289

factors that influence the hfe of the animals. If the net result of all
these elements favors the species, it is successful.

An important part of the success of a species is the numl)er of indi-
viduals it is prepared to pit against any unfavorable features of the
environment. This number depends first of all on the rate of repro-
duction. In this activity, animals differ greatly. The larger mammals
produce as a rule only one at a birth, and the period of development is
long, so that successive offspring are separated by wide intervals of time.
Rate of reproduction is slow in such animals. Contrast with them the
small mammals. A mouse produces half a dozen at a litter, and several
litters in a year, at which rate only a few years would he rec^uired for
the descendants of one pair to overrun the earth. A shad may lay
100,000 eggs in a year, a tapeworm 100,000 eggs per day. A protozoon
could, in seven years, produce a mass of protoplasm ten thousand times
as large as the earth. One aphid could in a single summer gi\'e rise to
500 thousand million million descendants. Punnett has calculated that
a female rotifer^-which is parthenogenetic, lays 50 eggs, and requires
only two days to reach maturitj^ — would be able to produce in a single
year, if all its potential offspring survived, a mass of rotifers large enough
to (ill the whole known universe and leave some over.

In bisexual animals, the sex ratio is significant in the maintenance
of numbers, since the number of offspring is determined primarily by
the females. A species with many females has an advantage over one
with few. A short life history also favors large numbers, because there
w ill be more generations in a given time.

Every species having great powers of reproduction is subject to
enormous destruction. This is proved by the fact that it does not, in
the long run, increase in numbers. Indeed, it may actually decrease.
The rotifer for which the foregoing calculation was made, once an abun-
dant object of biological experimentation, seems now to elude collection
altogether; and the passenger pigeon, exceedingly abundant over most
of eastern North America only a few decades ago, is now extinct. What
keeps a species in check is not easily ascertained. Accidents reduce
numbers to some extent, while predatory animals, disease, parasites, lack
of food, and unfavorable climatic or other physical conditions must
account for other extensive losses. The efficienc}^ of a species in over-
coming these obstacles determines its success. Rapid increase is not
always a sign of efficiency, for species which become especially abundant
in one season or over a period of several years must usually suffer a
reverse later; and there are circumstances (such as exhaustion of their
food) in which the greater the increase the more severe the following
decline. Greater safety lies in a steady maintenance of numbers. This
principle is illustrated by one of the most successful of birds, the English

290 PRINCIPLES OF ANIMAL BIOLOGY

sparrow. A census of this species in the north central states over a five-
year period showed a minimum of 9 pairs and a maximum of 13 pairs
per 100 acres of land. In the northeastern states it was almost equally
steady at a lower level — 3 to 7 pairs per 100 acres. The number of eggs
laid by the English sparrow is such that, starting with the normal
number of pairs, about 260 individuals could have been produced in each
100 acres in one year. But in the long run the numbers did not increase
at all, and at no time mthin the five years were the sparrows excessively
abundant. Casual observation indicates that this stability is common
over longer periods.

When some unusual event removes from the environment of a species
one of its chief limiting factors, the number of individuals may increase
enormously. Some of the best examples are found in the annals of eco-
nomic entomology. An insect plant pest imported into a new region
without the parasites which kept it in check at home may experience a
remarkable outbreak. The end of such "explosions" has, in economic
entomology, usually been brought about by introduction of the appro-
priate parasites. How they might end in the absence of help from
man is problematical. The pest might exterminate its only food plant,
resistant strains of the food plant might be developed through selection,
or some other parasite might find the newcomer a suitable host.

Animal Communities. — Though the foregoing discussion deals mostly
with single species in relation to their environment, more ecological infor-
mation is often obtainable by a study of animal communities. A
community consists of all the species living in one general situation.
In a broad way, it is found that the species making up a community
tend to be the same in many localities of the same kind. As will be
seen later, similar ponds over a wide area have in part the same species
in them; lakes of like size and depth not too far from each other are apt
to contain many of the same species. These species are held together in
a community by their requirement of practically the same set of environ-
mental factors. Organisms requiring a given range of temperature,
moisture, oxygen, and light herd together where these features are to be
found. The constitution of communities is not rigid, for no two situations
are exactly alike. One lake may have slightly more oxygen or lower
temperature or clearer water than another. The difference may cause
the communities of the two lakes to differ in certain species, tliough they
are alike in most. Occasionally also two communities will differ in their
component species by the mere accident that one or two species have been
introduced into one but not into the other.

Sometimes species are held together by some very specific relation
between them This relation may involve merely the nutrition of one
of the species. Many plant-eating insects favor, or are practically

ANIMALS AND THEIR ENVIRONMENT 291

limited to, a single species of plant: for example, an aphid that lives
almost solely on the chrysanthemum. Carnivorous animals are less com-
monly or less rigidly limited; lady beetles nearly always feed on aphids,
but accept a number of species, and can eat other small insects, such as
thrips. They also devour insect eggs.

A highly specialized interspecific relation is parasitism, which has
already been mentioned as one means of securing nutrition. It is
referred to here again as an example of interspecific relations, because
of the great lengths to which life cycles of parasites have sometimes gone
in affecting other species.

In simple cases a parasite has only one host. The trematode Gyro-
dactylus is parasitic on the skin and gills of the goldfish. When it
reproduces, the offspring become attached to the same or another gold-
fish. The liver fluke, however, employs two hosts. Its egg-producing
stage is spent in the liver of the sheep, or certain other large mammals,
but the offspring developed from these eggs must find a snail — any one
of a number of genera. In the snail it undergoes a series of developmental
changes, after which in a larval form it emerges from the snail and either
floats in the water or becomes attached to grass. Here it is drunk or
eaten by a sheep (or cow, or man) and the cycle is repeated.

A parasite in the human lung passes through three hosts in its cycle.
Escaping in the sputum into water, it enters a snail. Then at a certain
stage of its development it emerges into the water again, and penetrates
the body of a crustacean. If the crustacean is eaten raw, as is the cray-
fish by people in Japan or sometimes shrimps in America, the human
host is reentered and the cycle is concluded. And finally, the trematode
Alaria passes through four hosts. From a carnivorous mammal, often
a dog or a member of the mink family, it goes through a snail, then a frog,
next a mouse or some other small mammal, and thence to a dog or
other mammal which eats the mouse.

Ecological Succession. — No community of organisms is in a stable
condition. It is to be expected that the component species will vary
in relative abundance seasonally and from year to year. Occasionally
a species seems to disappear, perhaps to return later, and other species
may be added from some outside source. While these frequent changes
are of interest, they are far surpassed in importance by the alterations
known as succession. Ecological succession is an orderly sequence of
substitutions of species in a community. Certain species increase in
numbers, become perhaps dominant members of the group, then decline or
even disappear. Other species rise in succession, enjoy dominance for a
time, and then recede. Were this succession a purely random change, it
would have little more meaning than do the irregular seasonal and
sporadic fluctuations mentioned above. But in ecological succession, the

292

PRINCIPLES OF ANIMAL BIOLOGY

species in any given type of habitat tend to follow one another in a cei-tain
order. This order of change of species is correlated with the order of
change in the environment and in general is a change from instability
toward a condition of equilibrium.

Plant communities of certain kinds have advantages for a study of
succession, because in them the remains of earlier species are preserved.
Thus, when peat is dug from bogs for fuel, successive layers of the mate-
rial are well enough preserved to indicate what plants produced them
(Fig. 259). The general order in such places seems to be aquatic plants,
sedges, grasses, bog shrubs or alders, bog trees (larches), dry-ground

Fk;. 259. — Section through peat bed. The type.s of phmts that produeed it at the
(iiffeieiit levels can be determined from the remains. {From Weaver and Clements, "Plant
Ecology r)

forest. Successions starting on bare land begin with herbs and pass
through shrubs to forests.

Animal successions are less easily ascertained and less simply described,
l^rief cycles may be demonstrated, such as the succession of protozoan
types in laboratory cultures. These cultures are at first dominated by
flagellate protozoa, then several types of free-swimming ciliates (nearly
always in a given order), then the stalked ciliate \'orticella, and finally
the amoebalike species. Following these protozoa come the simple
plants or algae. Another .succession much longer than the above, but
short as compared with the plant series described, is that \\'hich is started
by the wood-boring beetles that live in the trunks of living oak trees and
gradually kill them. Larvae of another family, the long-horned beetles,
enter the dying trees, utilizing the burrows of their predecessors. Dai-k-
ling beetles come next and leave the bark separated from the wood by

animal;s and their environment

293

decaying material. Click beetles follow, bringing with them the wood-
rotting fvmgi and bacteria. The trunk falls, and decay is gradually
completed by various microorganisms. In the late stages of decay,
spiders, small salamanders, and various other animals may use the log
for shelter.

The longer successions of animals are too complicated for description
here. The number of changes and the number of species belonging to the

i: .{■:.. \-:^:--;-r-y ■'■.■:■.::■ ■'rr^-'r:::-,-.-:::':.-;;.i)-i

■:,^r?;#^

^' . * * * * - * * - • 1 * ■ • . • - ' - . m . . ,..

Fig. 260. — The filling of a lake wth peat deposits; the succession of land plants is also
shown. {After Dachnowski in Bulletin of Geological Survey of Ohio.)

successive communities are too great for simple summarizing. However,
out of later sections of this chapter, in which various aquatic habitats
are described, animal successions can be derived if the history of lakes
is kept in mind. In general, lakes are being ftUed with washed-in
soil and the remains of vegetation growing in them. They are generally
converted into swamps or bogs, and finally dry land (Fig. 260). Under
these circumstances, the animal communities characteristic of these
situations may be placed one after another and together present a rough
indication of the animal succession involved.

294 PRINCIPLES OF ANIMAL BIOLOGY

Fresh-water Habitats. — Of the many situations occupied by animals,
the aquatic habitats have many advantages for purposes of ecological
illustration. They are amenable to inexpensive study and have accord-
ingly been thoroughly explored. They present a considerable variety of
physical features and so accommodate very diverse communities. More-
over, they possess an organization which for orderliness is not easily
matched in any terrestrial habitat. This organization depends on
general principles which render aquatic situations capable of significant
classification.

Besides the properties of water which make it an important constitu-
ent of protoplasm (see page 39), an excellent heat reservoir, and a solvent
of gases, salts, and other chemical substances, it has certain character-
istics which pertain to it in the bulk. These qualities become the quali-
ties of the various bodies of water. It may be turbid or clear, which
greatly affects the penetration of light. It has considerable weight;
hence objects located at great depths are subjected to high pressures.
And lastly, being liquid, it is highly mobile and is subject to waves, con-
vection currents, and horizontal currents. Most organisms living in
water are influenced by one or more of these features, some organisms by
all of them.

The animals in water occupy different situations which are character-
istic of different species. Some live on the bottom, others are free in the
water and independent of the bottom. The latter include species that
float passively or, if they swim, do so in an aimless, undirected fashion;
and other species which, like fishes, swim actively and steer themselves in
given directions. The other characteristics of aquatic animals are best
described in connection with their several habitats.

Ponds. — Ponds are shallow — usually not over 2 or 3 meters in depth —
and heat from the sun penetrates through all the water. The tempera-
ture is consequently nearly uniform from surface to bottom, though
shading or resistance to currents by vegetation may cause differences
of 5°C. between different parts of a pond. Although the water may
be stirred completely by wind, wave action is so slight as to cause no
important mechanical disturbance. The important gases are almost
uniformly distributed through pond water; bright light and abundant
algae may increase the oxygen content through photosynthesis, and, when
crowded with animals, the water may contain excess carbon dioxide.
Abundant rains dilute the chemical content and increase the turbidity
temporarily.

Whether the bottom of a pond is covered with vegetation depends
on its depth and tiu'bidity; even when the water is fairly clear, there are
few plants beyond 3 meters in depth. In most ponds, however, this
permits vegetation throughout their area (Fig. 261). These plants

ANIMALS AND THEIR ENVIRONMENT

295

furnish additional surface to which aquatic animals may cling. One of
the chief characteristics of pond life is that it must be prepared to dry up.
Typical pond organisms are those which spend part of their life cycles out

Fig. 261. — Ponds; large one above with vegetation practically throughout, snaall one
below with heavy vegetation completely concealing the water. {Photographs by F. C.
Gates.)

of water, either m a resting condition or actively in the air or on land.
The larger ponds are permanent, and the organisms in these approach
those of lakes in their characteristics. Ponds are temporary bodies also
from a long-range point of view sinccj as explained above, they are being

296

PRINCIPLES OF ANIMAL BIOLOGY

()

gradually filled in and converted into dry land; but that fact is of n
importance in relation to the community of organisms existing in them
at any one moment.

Animals of ponds are of too many kinds to l^e named with any com-
pleteness, but very characteristic ones are many protozoa; the fairy
shrimp (particularly in temporary ponds) ; the immature stages of May
flies, dragonflies, stone flies, and midges; mites or water spiders; snails
and small bivalve mollusks; and often frogs, toads, and salamanders.
Fish are uncommon; and occasionally muskrats build their houses in and
over the water. Visitors are ducks, grebes, and other wading birds
AA'hieh feed upon the pond animals. Pond animals must produce many
offspring, for the environmental toll is especially heavy.

Lakes. — Lakes differ from ponds chiefly in size, but this difference
carries Avith it profound changes in all the principal factors of environ-
ment— light, temperature, and dis-
solved gases, with their effect upon
nutrition. Lakes vary so much in
their qualities, depending largely on
size and geographic position, that what
is said here will be limited chiefly to
those of moderate size in the temperate
zones. Two-thirds of the light falling
upon a lake is absorbed by the flrst
meter of water, and almost none pene-
trates farther than 3 or 4 meters. The
bottoms of most lakes are therefore in
total darkness. The heat received
from the sun and from contact with
warm air in summer aftects only the
surface water. The water near the
A\'ind, and a layer of water of nearly
down as far as wave action I'eaches.

0
5

10
15

? 20

S 25
0.30

a
35

8 10 12 14 16

18 20 2?
Degrees Centigrade

Fig. 262. — A thermocline; curve
of temperature at different depths in a
typical lake in summer.

surface is stirred up by the
uniform temperature extends
In a lake of moderate size this surface layer is apt to have a temperature
around 2()°C'. in late summer, and to be 10 or 12 meters in thickness.
Below this depth the water becomes rapidly coldei' with increasing
depth, as shown in Fig. 2G2. This layer of rapidly falling temperature
is known as the thermocline, and in the lake represented in the figure it
extends from about 11 meters to about 20 meters in depth. Ji(>low the
thermocline the water continues to become colder at lower depths, but
at a very much slower rate. Since the warm water above the thermocline
is less dense than the cold water below it, there is practically no inter-
mingling, and the water lielow is rather completely cut off from an>' com-
munication with th(» world above.

ANIMALS AND THEIR ENVlliONMENT

297

In the fall the surface water becomes cooler, and the thermocline
gradually disappears. When the surface water is of about the same
temperature (and hence density) as the bottom water, the wind is capable
of stirring the water from surface to bottom. Then the bottom water,
which is held captive during the summer, may escape to the surface.
In winter the surface water usually freezes to a very slight depth, but
the bottom never freezes. Indeed, the bottom may be only 2. or 3°
colder than in summer. The sheet of ice, if one forms, prevents wind
action; and besides, Avater at or near 0°C\ is not so heavy as that around
4°. Consequently there is no intermingling of surface and bottom water
during \nnter. In spring, however, as the surface water warms, there is

■r4

Fig. 263. — Lake shore kept bare of vegetation by wave action. (Photograph by F. C

Gates.)

again a complete circulation of the water under the influence of the wind.
As summer advances, the surface water is heated, and the thermocline is
again produced.

Thus, twice a year, spring and fall, the water of moderate-sized lakes
in the temperate zones circulates freelj' from surface to bottom; but at
other times there is effective stratihcation, and surface and bottom waters
do not mix. These facts have an important bearing upon the general
ecological features of lakes. A lake is divided into regions whose proper-
ties are distinctly different not only in temperature but in light, gas con-
tent, and mechanical agents.

The region above the thermocline in summer is relatively warm, is
well lighted near the surface, is subject to mechanical disturbance by
waves, has no fixed objects to which organisms may be attached except
near the shores, is well svipplied with oxygen from the air (supplemented
by that coming from green organisms carrjdng on photosynthesis), and

298 PRINCIPLES OF ANIMAL BIOLOGY

is poor in carbon dioxide (a condition like^vise accentuated by any
photosynthesis going on there). The temperature of the shallow water
along the shore is likely to fluctuate greatly between day and night,
especially on the side of the lake toward the prevailing wind and among
vegetation, where there is little agitation; but out in the open water in
the middle of the lake temperature is much more nearly constant. Shore
regions exposed to the wind are subject to vigorous wave action which
usually prevents vegetation from gaining a foothold (Fig. 263).

Below the thermocline the water is always cold, often varying only
3 or 4° throughout the year. It is always dark. There is no wave
action, and almost the only mechanical disturbance is that occasioned by
the complete circulation of the water in spring and fall. A solid sub-
stratum is available for attachment. There is very little oxygen, some-
times none at all, for whatever oxygen is brought in by the spring and
fall overturn of the water is consumed by decay of dead organisms that
fall to the bottom, and there can be no photosynthesis in this dark region.
Carbon dioxide is always abundant, likewise because of the decomposition
of organic matter, except temporarily at the times of the spring and fall
overturn.

The Organisms of Lakes. — It is obvious that the conditions described
above have much to do with the types of organisms inhabiting lakes,
and that different parts of a lake will have very different kinds. Plants
can as a rule occupy only about 3 meters of the depth of a lake, owing
to deficiency of light below that level. For plants springing from the
soil, this means that they are limited to a narrow strip along the shore
(Fig. 258). While a pond may have vegetation throughout, most lakes
have plants over only 10 to 30 per cent of their area. This difference
between ponds and lakes is indicated in Fig. 264. Since many animals
depend on these plants, the abundance of the latter is important in the
general ecology. Of the many animals found in such situations it is
possible to mention only a few. In the shore region with the plants
are usually snails and immature caddis flies, midges, IVIay flies, and
dragon flies. Where there is little or no vegetation because of waves,
there are often mussels and young insects \vith flattened bodies and
clinging habits (certain May flies). In the open water of the middle of
the lake are sometimes floating plants, chiefly algae, so abundant as to
reduce very materially the amount of light that enters the water. With
the algae, and often feeding upon them, are many small animals, chiefly
Crustacea, protozoa, rotifers, and mites, abundant in numl)ers but not
usually of many kinds in any one lake.

On the bottom of a lake, below the thermocline, are found those
organisms requiring no light and little or no oxygen. Characteristic
examples are the minute plants known as diatoms, some of the annelid

ANIMALS AND THEIR ENVIRONMENT

299

worms, small bivalve mollusks, and the larvae of midges and of the
mosqiiitolike Corethra. Such a place would not seem favorable to much
life, yet Juday has found these animals make up a mass of over three
hundred pounds per acre on the bottom of a typical lake.

The free-living population of a lake is subject to considerable fluctua-
tion in amount and distribution. There is a daily variation in distribu-
tion caused by the reactions of these animals to light. Since most of
them are positive to light, they accumulate at the surface during the
day and settle away from the surface at night. Reference has already
been made to this reaction in one of the parasitic Crustacea (page 285).
There is also great variation in the seasonal abundance of floating species.
The algae generally have one maximum each year, occurring in midsum-
mer, as have also certain protozoa. The diatoms, however, regularly

Po/vD CommoNz if,^ Conditions

Submerged aquafK regefafion
Floating " •>

Fig. 264.-

' Emergent

-Section through pond and lake, showing contrast in extent of vegetation due
to difference in depth. (From Chapman, "Animal Ecology.^')

have two maxima, in spring and fall, respectively. The animals which
feed upon these minute plants are naturally influenced by this seasonal
fluctuation.

It will be observed that there is much overlapping in the general
kinds of organisms living in lakes and ponds, respectively. This is
largely due to the fact that the vegetated strip along the shore of a
lake is not very different from many ponds. The most characteristic
difference between lakes and ponds is in the swimming organisms. , Fishes
are common in lakes, but there are few ponds that contain them, and then
only certain species. Comparison of ponds of different ages shows that
the older the ponds the fewer the fish they harbor. In some regions the
amphipod crustacean Gammarus appears also to be a distinguishing
mark of lakes as compared with ponds.

Streams. — Water in motion has characteristics, as the habitat of
animals, not possessed by standing water. The mechanical disturbance
which it offers is very considerable in young streams (that is, those
whose slope is steep) but much less in old streams. Soil may be carried

300 PRINCIPLES OF ANIMAL BIOLOGY

in suspension; hence light penetration is periodically very low. Tempera-
ture is nearly uniform at various locations in a stream at any one time,
but its variation seasonally for the stream as a whole is often extreme.
The oxygen content of s^viftly flowing water is generally near the satura-
tion point, and most of the characteristic brook animals cannot be rearee.
in a concentration of oxygen much below that level.

Since the chief feature of streams, as distinguished from* lakes, is the
movement of their water, consideration of their animals will here be
limited to those whose currents are strong. This is the condition in
most small streams or brooks. In such streams, animals have to be able
to maintain their position; with the exception of the minute floaters,
they cannot as a rule allow themselves to be carried along by the current
and still be successful. One method of holding their places is to be
attached to fixed objects. That is a feasible method in general, since
animals do not have to travel in search of food, for it comes to them.
In very swift water, one finds the larvae of the black fly, Avhich hold fast
by adhesive organs at the posterior end, while their appendages are so
construct;gd as to strain minute organisms out of the water that flows
through them. Some of the caddis fly larvae spin nets on stones or other
objects in rapids; they cling to the net, which also serves to catch food as
the water goes through it. The other most abundant insects in brooks
appear to be the larvae of midges, which live on or in the bottom, and May
flies and stone flies of clinging varieties. In other animal groups there
are snails, flatworms (planarians), amphipod Crustacea, and mites.
Some algae form incrustations on rocks and other objects. In the same
brooks, but in the quieter water, are miller's-thumbs (fish) lurking under
overhanging banks, catfishes which lie close to the bottom, and darters
(fish) which are strong sudmmers. The larger rivers contain larger
animals, but they differ less from lake inhabitants.

The rate of reproduction of brook dwellers must be high, since the
risks of loss are large. An individual that loses its station, if dependent
on attachment, is not likely to become reattached until it reaches slow-
water, and there the conditions are not usually favorable. A single pair
of midges, producing four generations a year, have a potentiality of
nearly eight million descendants, but on the average only two are pro-
duced and live, in each generation, to do what their progenitors did.

Marine Conditions. — The oceans are so huge and are subject to so
many variable influences in their various parts that no .simple descrip-
tion of their environmental organization is possible. Their waters are a
little heavier because of the salt content, hence offer more support to
animal bodies than fresh water does. Deep water is at very high pres-
sure, but this feature is of little significance to any animals except those
fishes which have a closed swim bladder which is compressible. \'ery

ANIMALS AND THEIR ENVIRONMENT

301

strong mechanical disturbance may be caused by waves and tides, and
currents such as the Gulf Stream and the Labrador Current create
special conditions of temperature and distribution. Temperature of
surface waters varies little during the year in the open oceans (from 7
to 12°C. in one situation), but considerably in stagnant seas and bays
(3 to 18° in the Baltic Sea). Deep water is always cold. Light of
sufficient intensity to aid plant growth penetrates the first 30 to 100
meters of water but is detectable at 200 to 600 meters. Concentration
of salt is increased ° by evaporation in the tropics, and diminished in
summer in polar areas by the melting of ice. Ocean waters are also
diluted by rivers, but these streams are more important for the materials

METERS

I2°C.

-'-'^^i^fS\

200

SWIMMERS

•^x.

-y^Jmj^^X

AND

<5^
0^

--■ >- :^^^W CONTINENTAL
-o^^^^ SHELF

400

FLOATERS

-'— -j^^^F Depf hs are exagger-
— — -'^^^f o'^^<i' in relation +0
~-^-^^f horizontal scale

600

8°C.

z~~^^ Temperatures and light in relation
Tsrl'^m ■fo depth are very variable and
^^-^M only approximote

800

Fig. 265. — Vertical section through portion of ocean near the shore. Bottom fauna includes
animals which are able to move briefly but must periodically come to rest.

they bring in from the land. The Atlantic and Arctic Oceans receive
by far the greatest contribution from rivers, while the southeastern
Pacific receives the least. The solubility of oxygen in marine waters is
about 20 per cent less than in fresh water, and cold water (either salt
or fresh) dissolves more than warmer water does. Deep waters, which
are cold and which are replenished by a circulation from the polar
regions, therefore have a good oxygen supply.

Ocean Bottom. — A great majority of marine animals live on, in, or
near the bottom (Fig. 265). Near the shores the bottom is lighted;
here it is that life is most abundant, and all groups of marine animals
are found in this relatively shallow water (200 meters or less). The
stock of animals in these coastal waters is generally regarded as having
produced all the water-breathing aquatic forms, whether marine or
fresh-water. Below the low-tide level there is abundant plant life if the

302 PRINCIPLES OF ANIMAL BIOLOGY

bottom is one in which roots can take hold (clayey or sandy), and a
rich animal population finds shelter, support, and oxygen among the
plants. Where plants are lacking merely because shifting of the sand
prevents their rooting, numerous animals (clams, worms, sea cucumbers,
crabs) burrow in the bottom material and feed on remains of seaweeds and
animals. Gravel bottom is practically without life, because movements
of the pebbles in strong wave action destroy living things. Animals of
this coastal area depend for food on the organic matter (largely dead
bodies, including plant remains) brought in by rivers or produced in the
coastal area itself.

Between low and high tide there are fewer animals, yet some are
able to endure the twice-a-day uncovering, exposure to the heat of the
sun in summer and temperature extremes in winter, and dilution of the

Fig. 266. — A burrowing animal between tide lines; the clam Scrobicularia.

water by rains. The clam Scrobicularia (Fig. 2G6) burrows in the sand
and with its long inhalent siphon explores the surface around it for
food-bearing water.

The ocean bottom below the effective penetration of light is less well
populated. The organic remnants which serve as food here are the
decaying bodies of swimming and floating animals and plants which
settle down from above. The ooze thus formed has a pasty consistency.
The most abundant bottom animals of the deep sea are sea cucumbers.
Others are Crustacea (amphipods and isopods), hydroids, sponges, clams,
and worms. In general, deep-sea animals are smaller than their relatives
near the surface. Also they may be more delicately constructed (fragile
skeletons, thinner shells) liecause there is little motion of the water.

Animals of the Open Ocean. — Organisms of open water either swim
or passiv-ely float. Floating life must have some way of reducing its
specific gravity, since protoplasm itself is heavier even than salt water.
One way is to take up much water, without the salt, into the tissues.
Other ways are to develop fat, or gas chambers like those of the siphono-

ANIMALS AND THEIR ENVIRONMENT 303

phores or the smm bladders of fishes. The air-breathing whales, seals,
and turtles are floated by their lungs. Animals having no floating
mechanism must actively swim, if they are to avoid settling on the
bottom; among vertebrate animals only the powerful sharks and a few
bony fishes without swim bladders are capable of the incessant exertion
necessary to prevent sinking.

Fewer groups of animals are represented in the open ocean than on
the bottom. There are no sponges, no brachiopods, no bryozoans.
Hydroids and other sessile coelenterates are missing, and there are few
echinoderms (except larval stages), few worms, few clams and snails.
The bulk of the sudmming animals (90 per cent) are copepod crustaceans.

Ocean currents either come to an end by spreading out and slowing
down to zero (Gulf Stream), or they form a closed circuit. The meeting
of warm (Gulf Stream) and cold (Labrador) currents of the terminating
type causes great mortality of organisms, and adds to the organic detritus
used by bottom forms. The larger closed circuits take a year (North
Atlantic) or two (South Atlantic) to bring their organisms back to any
starting point. In the eddy enclosed by such a circuit there are often
accumulations of seaweeds (Sargassum), perhaps torn loose by hurri-
canes, and in these weeds is a characteristic animal community (certain
fishes, crabs, shrimps, hydroids). An eddy of this sort is known as a
Sargasso Sea, and each of the great oceans except the polar ones has one
or more of them.

Coral Reefs. — Coral reefs are built up from the bottom in tropical
seas by two different groups of coelenterates, aided by a number of
other lime-depositing organisms. They may be developed along the
shore line (fringing reefs), out some distance leaving lagoons between
them and the shore (barrier reefs), or at any distance from the mainland
in the form of a ring or horseshoe (atolls). Various theories to account
for reefs, beginning with those of Charles Darwin, have been proposed.
The theories postulate the type of habitat in which corals will grow, the
possible rise or fall of the land, differences in exposure to the open ocean,
and long-time changes in the water level of oceans; but none of the
theories is entirely satisfactory. About these reefs there are character-
istic communities of other kinds of animals.

Geographic Areas in the Oceans. — Swimming and floating organisms
requiring moderate or relatively high temperatures are limited to their
respective oceans, being cut off from other oceans by the continents
which the}^ cannot pass around. Yet the animals of the Atlantic have
a considerable likeness to those of the Indian and Pacific Oceans. In
the coi^epods, even some of the species are identical. This likeness
presumably resulted from the connection between the two areas across
Central America in Tertiary time.

304 PRINCIPLES OF ANIMAL BIOLOGY

The colder ocean waters, north and south, have fewer species of
animals, but more individuals in a given volume, than do tropical
regions. There is a striking similarity of arctic and antarctic animals,
the same genera and even species occurring in l)oth oceans. This is
presumably accounted for by the fact that there is a connection between
them in the cold deep waters of the intervening ocean, which is kept
cold by a north-and-south circulation of surface and bottom waters.
Another possible explanation is that northern and southern species have
evolved independently under the guidance of similar conditions.

Soil. — Different types of land environment represent different stages
in the evolution and concjuest of the earth. Starting with bare rock,
the succession is roughly rubble, bare sand, sparse grass and other vege-
tation, herbs, shrubs, and trees. The soil may thus be in a variety of
conditions, since it develops by weathering and by the action and con-
tributions of the vegetation. In texture it may range from very fine
particles, as in clay, to coarse stones, as in gravel. In a good loam suit-
able for plant growth, about half of the bulk of the soil is made up of
spaces between the particles, and these spaces are occupied about equally
by air and water. About 10 per cent of the solid matter is derived from
plants; the rest is mineral.

The temperature of the soil varies most at the surface and is nearly
constant below a depth of 1 meter. Surface temperature fluctuates
much more if the ground is bare than if it is covered by vegetation. In
very cold regions the soil may freeze so deep in winter that it is never
thawed out in summer; nevertheless, vegetation may gi-ow above this
perpetual ice.

Water may be held loosely in the larger spaces between soil particles,
in which case it tends to drain away by its own weight, or it may be
retained by capillary action between the fine particles. Even "dry"
soil has some moisture adsorbed on the small particles. Silt and clay
retain much more moisture than does sand or humus.

The importance of the soil as an ecological unit may easily be under-
estimated, unless it is remembered that most animals spend at least part
of their life cycle in the soil. Some animals spend their whole lives
there, such as earthworms, some roundworms, and protozoa. Some
live in the soil during one stage, such as the grub of May beetles or the
pupae of many other insects. Others make their homes in the soil but
spend much of their time on or above its surface, as ants and termites.
Burrowing in the soil is the common mode of life of moles and shrews,
while homes are built in the ground by many other vei'tebrate animals
(ground squirrels, ground hogs, mice, etc.). In numbers of individuals,
the I'oundworms are the most abundant group, reaching as many as
half a billion per acre.

ANIMALS AND THEIR ENVIRONMENT 305

Most soil animals are near the surface, not deeper than 5 or 6 inches
during the active season. Many species migrate downward annually
to avoid frost and return in the spring. Earthworms have been found as
deep as 6 feet, where they went to find moisture in dry seasons. Species
which merely make their nests in the ground often go rather deep —
gophers 2 feet, termites 5 feet, ants 9 feet, and the prairie dog as deep
as 14 feet.

As a special type of soil environment may be mentioned sand dunes.
The chief physical characteristic of dunes is the extreme variation of
their temperature and moisture. Even in moderately moist regions,
rain water drains out of sand quickly; and in the heat of midday the
temperature may rise to 50 or 60°C. The hottest part of a sand dune,
when the sun has been shining upon it, is directly at the surface. The
air a few inches above it and the sand at a depth of several inches are
cooler. Certain wasps which dig burrows are among the most character-
istic dune animals; and with them are certain other insects parasitic
upon the wasps. Many other animals are occasional visitors but have
no particular dependence on dime conditions.

Associations in Vegetated Areas. — When vegetation has taken hold
in the soil in abundance, the physical conditions are modified in several
important ways. Sunshine is intercepted, thereby reducing the fluctu-
ations of the temperature of both soil and air. The diminution of light
by trees is much greater than that by shrubs or herbs, and the reduction
by pine trees is much more than by larches or elms. In one forest it
was found that the maximum daily temperature was 5 or 6° lower, and
the daily minimum an equal amount higher, than in a near-by cutover
area. Evaporation of water from the trees is one of the ways in which
temperature is lowered. Some of this reduction of temperature is, how-
ever, nullified by stoppage of the wind by trees, so that open spots sur-
rounded by forest may, when the sun shines long upon them, become
warmer than they would if there were no trees. General evaporation is
also reduced by this retardation of the wind, beech-maple forest exercising
a much greater control than cottonwood, for example.

Introduction of vegetation modifies the characteristics of an area in
very many ways, depending on what plants are present. As a conse-
quence the animals become ex(?eedingly varied. The nature of an animal
association is determined largely by the plant association. Insects
which feed upon the leaves of plants often utilize only one or a few species.
Those which produce galls on leaves are commonly limited to one species
of plant. Wood-boring and bark insects prefer certain trees. Soil is
altered differently by different plants, and root-feeding animals usually
specialize in certain roots. Rotten logs in various stages of decay con-
tribute to the variety of situations. The general effect of vegetation

306 PRINCIPLES OF ANIMAL BIOLOGY

on temperature, light, and humidity, described above, introduces much
diversity. As a consequence of this heterogeneity, it is impossible to
regard vegetated areas as single ecological units. They consist of a
number of types mingled with one another. Attempts have been made
to classify them on the basis of predominant types of plants, but in an
elementary discussion it is not practicable to follow any of these schemes
through.

References

Chapman, R. N. Animal Ecology. 2d Ed. McGraw-Hill Book Company, Inc.
(Especially Chaps. XV, XVI, XVII on aquatic habitats.)

Elton, C. E. Animal Ecology. Sidgwick & Jackson, Ltd.

Hesse, R., W. C. Allee, and K. P. Schmidt. Ecological Animal Geography. John
Wiley & Sons, Inc.

Pearse, a. S. Animal Ecology. 2d Ed. McGraw-Hill Book Company, Inc.
(Chap. IV, biological factors in ecology.)

Semper, Karl. Animal Life. D. Appleton-Century Company, Inc. (Old, and
lacks the modern organization of ecology but discusses, chapter by chapter,
influence of food, light, temperature, water and air currents, etc., upon organisms.)

Shelford, V. E. Animal Commimities in Temperate America. University of
Chicago Press. (Chap. XV, general discussion.)

Weaver, J. E., and F. E. Clements. Plant Ecology. McGraw-Hill Book Com-
pany, Inc.

CHAPTER 21
GEOGRAPHIC DISTRIBUTION

The locations of species on the earth have been determined by two
general sets of factors, the ecological and the historical. Animals must
live in situations which are at least moderately favorable to them, but
they are able to occupy suitable areas only if these are within reach.
Many excellent sites are not occupied because they are far away, and
there is no adequate means of transport. Moreover, most animals can-
not be assumed to have purpose, and they cannot have knowledge of
the conditions of life in other places. Accordingly, if they find new
locations it must be as a result of normal activities, including some events
which must be regarded as accidental.

The purely local distribution of species, which depends on ecological
factors, has been discussed in the preceding chapter. While it will be
necessary to point out relations to the environment in this chapter also,
only such relations as bear on the history of distribution will be included.
Let us see how animals have come to be where they are.

Interplay of Two Evolutions. — While present distribution of living
things has often been used to prove that evolution has occurred, an
understanding of zoogeography is most easily attained by reversing the
arguments. If it be assumed that evolution has taken place, many
peculiarities of distribution have a natural explanation.

There are two of these evolutions, independent of each other in their
origins, but with intert^vining results. One is the evolution of species
of animals and plants, the other the evolution of the earth on which
they live. New species have arisen out of older species, ever since life
began. A group of individuals becomes different from their fellows,
through mutation and recombination of genes and other events, and a
new species is started. Usually the new species finds or at any rate
occupies an area somewhat different from that of the other species. In
time it gives rise to further new species, which take up their special
locations. As more and more new species arise, there is a cleavage
among them; some of them are much more alike, but differ strikingly
from another group within which the species are rather similar. Genera
are thus produced. As species change still more and more, there is
cleavage among the genera, and families arise. Continued change of
species results in divisions of higher ranks, the orders, classes and phyla —

307

308 PRINCIPLES OF ANIMAL BIOLOGY

all of which was described in the chapter on taxonomy. The whole
evolution process is a change of species, carrying with it necessarily the
changes of genera and higher categories.

An important feature of this process, as it relates to geographic dis-
tribution, is that new species have been arising all the time since living
things first existed. New species are originating at this very moment,
and may be expected to continue to come into existence in the indefinite
future. Also important is the fact that new species have taken their
origin everywhere, in all parts of the earth which support life. Time
and place thus enter in an important way into all questions of present-
day distribution. The range occupied by each species becomes a center
of dispersal from which its descendants tend to spread, and these centers
have existed all over the earth and through long periods of time.

Starting much earlier than the evolution of life was the development
of the earth. With the early stages of this process we are not concerned;
but those parts of the earth's evolution which were contemporaneous
with the evolution of living things are very important in geographic dis-
tributiom The changes which affected distribution have been largely
the rise and fall or other shifts of position of the land, and changes of
climate. The question of permanence of continents is an important one.
Most zoogeographers have held that the continents have always been, in
general, about where they now are; but there is another theory, that of
continental drift, according to which continents, floating on the plastic
interior of the earth, have moved horizontally. A common example of
this drift is the alleged separation of Africa and South America. Many
European and some American geologists have supported the drift theory,
but distribution of animals has seemed to most biologists to call for more
nearly permanent continents.

Regardless of the general position of continents, their shapes have
changed. Many areas of dry land teem with fossils belonging to classes
which are strictly marine. Such areas must once have been under the
sea. Michigan, for example, contains many extinct corals, though it is
now hundreds of miles from any salt water. Even high mountains have
arisen out of the ocean. Land has also sunk, and areas which were the
shores of an ocean have become its bottom. Broken shore lines are a
common result of the sinking of hilly or eroded land.

Changes of climate have also been frequent. Michigan and most
neighboring areas have been under glaciers more than once. At the
other extreme, more northern regions have been tropical, as indicated
by luxuriant plant growth preserved as fossils. Humid areas have
become dry, swamps have become dry plains, forests have been con-
verted into grasslands. These changes must have affected the distri-
bution of animals profoundly.

GEOGRAPHIC DISTRIBUTION 309

The timing of the changes of species and the changes of the earth
must have had important consequences for hving things. When a
group of animals experienced the genetic changes which might lead to
the formation of a new species, any changes of the land or climate
occurring at the same time and in the same region could spell the differ-
ence between survival and destruction of the new group. When a region
of the earth was undergoing a physical (perhaps climatic) change, any
genetic change going on in a few individuals could decide whether any
members of their species would survive the changes of environment.
For the sake of emphasis these changes are described as sudden and
radical; actually they have been very gradual. Interplay of the physical
forces of the environment and the genetic forces of animal or plant life
must have been crucial in the guidance of evolution, and in the deter-
mination of the location of resulting species and higher groups. Let us
turn to some of the facts of distribution, to see how they fit into the
general scheme just outlined.

Position of Ranges. — It is easily observed that species, families,
orders, etc., have their characteristic places on the earth. With the
exception of closely interdependent species, such as parasite and host,
probably no two species have exactly the same range. The musk ox is
arctic; the nine-banded armadillo ranges from Texas to South America;
the North American alligator exists only in the extreme southeastern
part of the United States. Among such vastly different groups, widely
separated ranges do not occasion any comment. Within a single genus,
however, the several species have their distinct areas. For example, in
the genus of spadefoot toads, Scaphiopus couchii extends from Texas to
Arizona, and into northern Mexico, including Lower California; S. ham-
mondii ranges from Montana to Mexico and west to the Pacific states;
8. holbrookii holbrookii occurs along the Atlantic from Massachusetts
to Florida, and west to Louisiana, Texas and Arkansas; S. holbrookii
albus is only in the Florida Keys, or possibly also the extreme tip of
Florida; and S. hurterii is found only in Texas.

The location of a species range depends primarily on where the
species started. The present range must usually be around or near the
point of origin. Looking backward, one sees the ''center of dispersal"
of a species as some point in or near its present range. Most species
have not lived long enough to have traveled far. Very old species,
however, especially those which survive in only a few individuals, may
be far from their places of origin. Such old and nearly extinct species
may usually be recognized as such because no closely related species are
anywhere near them. Species consisting of few individuals because they
are very young are, on the contrary, surrounded by very similar types.
Remnants of very old species are spoken of as relicts. Examples are the

310 PRINCIPLES OF ANIMAL BIOLOGY

several species of Nautilus, sole survivors of a once flourishing family
(the tetrabranchiate cephalopods), now found only at places in the
Pacific and Indian Oceans.

Size of Range. — Equally striking are the different sizes of ranges
occupied by the various forms. When groups of high and low taxonomic
rank are compared, as orders with genera, inequalities are to be expected.
One simple reason is that the higher groups are made up of a number
of lower ones. When those of the same rank occupy very unecjual areas,
an explanation is not always easy. Particularly important in the theory
of distribution are unequal ranges of species. Some ranges are very
small. One species of ant is found only in the Garden of the Gods in
Colorado, another species occupies much of North America. Kirtland's
Warbler, not including its migration routes, exists only as a few indi-
viduals in a very limited area, while the American Robin numbers
millions of individuals and covers a continent. Among plants, a species
of Oenothera includes only 500 to 1000 individuals and is known only
in a mountain range in southern New Mexico. One of the spadefoot
toads, already mentioned, occurs only in the islands off Florida and per-
haps at the extreme tip on the mainland, while another species of the
same genus has a range a thousand miles wide.

When there appears to be no difference in the tolerance, rate of
reproduction, or means of locomotion of two species, a tempting expla-
nation is a difference in age. This is thought to be the reason for the
very unequal ranges of four species of tree frogs (genus Hyla). Hyla
versicolor is found from southern Canada to the Gulf states, and west to
a line between Montana and central Texas; H. squirella extends from
Virginia to Florida, west to Texas and Indiana; H. gratiosa from South
Carolina to Florida and Mississippi; and H. evittala only along the
Potomac and York Rivers in Virginia and New Jersey. The species
believed to be the younger have the smaller ranges, and the explanation
may be simply the shorter time they have had to spread.

This idea has been developed as the "Age and Area" hypothesis,
and has been applied more to plants than to animals. In accord with
it is the fact that on the average groups of higher taxonomic rank (orders,
for example) occupy areas larger than those belonging to groups of
lower rank (genera, let us say). In general, the higher groups are older,
and have had longer time to disperse. Some paleontological support for
it is also claimed, for when the ages of taxonomic groups can be judged
from the geological periods which furnish their earliest known fossils, the
older ones again have the larger average ranges.

There are known exceptions to the rule, however, and probably
many which are not known. Two species of shophord's-purse differ in
the number of chromosomes in their cells, one having just twice as many

GEOGRAPHIC DISTRIBUTION 311

as the other, and it is fairly certain that the one with the larger number
sprang from the other by a doubling of the chromosomes. This is a
weh known method of origin of species in plants, and must apply to this
example. However, the species with the double number of chromosomes
(which must be the younger one) ranges much more widely than the one
with the smaller number. One species is simply much more successful
than the other.

Continuity of Range. — Because a group of animals starts at some
point, from which its members tend to spread until barriers are reached
on all sides, ranges are expected to be continuous unless something
happens to break them up. Taxonomic groups of as high rank as
families and orders have usually been developing long enough for that
"something" to take place. Ranges of such groups are large, and living
conditions may change sufficiently to extinguish the animals across the
middle of the area, thus dividing it in two. The camel family, for
example, is represented by the true camels in Asia and Africa, and by
the llama and its relatives in South America, wdth the great land gap
of Europe and North America between. Fossil camels, however, are
found in the area now vacant. The genus Alligator is composed of two
species, one in central China, the other in southeastern United States.
Extinct relatives of the alligators once ranged widely in North America
and Europe, shoAnng how the modern range became discontinuous.

When the range of a species is found to be discontinuous, which is
rare, the reason is not easily found. The skink, Lciolopisma laterale, is
found in southeastern United States, in China, and in certain of the
southern Japanese islands. Why is it not in the areas between? Only
if the species is an exceedingly old one would it be hkely that destruc-
tion of its members over a large portion of its former range could have
occurred. So improbable is the division of a species range by extinction
that every example of it raises the question whether the species may not
have developed independently in two places. Such an occurrence is not
impossible. j\Iany mutations are known to be produced repeatedly, and
among random recombinations of genes the same combinations could
occur anywhere. If environment of a certain type tends to preserve
certain genetic combinations, similar environments in two areas could
guide evolution in the same direction. Such double origin of a species
would not be a violation of the taxonomic concept that all members of a
species are descendants of common ancestors, for the two groups from
which the species arose would necessarily be much alike, both having
come from the same ancestry. The common ancestry of a duplicated
species would thus be simply pushed farther back. Nevertheless, this
dual origin of a species is so unlikely that it is not to be lightly assumed
as an explanation of discontinuity.

312

PRINCIPLES OF ANIMAL BIOLOGY

Physical Conditions of Ranges. — Lest the ecological factors be I'or-
gotten in the study of historical phenomena, it should be observed how
different are the conditions obtaining in different ranges. A striking
illustration of this is found in the distribution of vegetation. The general
vegetation areas of North America are shown in Fig. 267. Coniferous

J''i(j. 2G7. — Geuerul vegetation areas of North America. {From, liurlingarnc, Heath, Martin
and Peirce, "General Bioloyy,'" Henry Holt and Comjmnij, Inc. Prepared by A. G. Vestal.)

and deciduous foj'csts are sepai-ated l)y pliysical conditions, largely tem-
perature, and they in turn determine the location of many animals. The
eastern deciduous forests are the home of the opossum, gray fox, fox
s(iuirrel, cardinal bii-d, (-arolina ^\'ren, and yellow-breasted chat. The
northern coniferous forests shelter the snowshoe rabbit, pine martin,

GEOGRAPHIC DISTRIBUTION

313

northern jumping mouse, three-toed woodpecker, and spruce grouse.
The open treeless areas of the west are inhabited by the prong-horn
antelope, bison, ground squirrels and many others. So nearly are many
of these animals limited to one type of vegetation area that it is difficult

iGlaucomys Sabrinus
IGlaucoinys Volana

Fig. 268. — Ranges of the North American flying squirrels, Glaucomys volaiis and G. sa-
brinus. {After Howell, "North A7nerica?i Fauna," No. 44.)

to avoid concluding that the conditions prevailing in such areas are
paramount in their lives. Sometimes the maps of ranges of forest
animals and of prairie animals appear to overlap, as if the vegetation
were not of great importance. In one such case, however, the appear-

314

PRINCIPLES OF ANIMAL BIOLOGY

ance of mixture was occasioned by the fact that a series of somewhat
parallel streams with trees along their courses were separated by strips
of grassland. The forest animals were along the streams, the prairie

35"

35 to 40 Inches.
More than 40

Fk;. 2G9. — Annual rtiinfall in the lower part of the state of Mieliigan. This illustrates
the differences in physical conditions which may prevail even in relatively small areas.
(After C. F. Schneider, Publication 9, Michigan Geological (iml Biological Survey.)

animals between them; but on a map of moderate scale they appeared
to be together.

An actual example of species definitely related to I'orests is the genus
of North American flying sciuirrels. As shown in Fig. 2(58, Glaucomys

GEOGRAPHIC DISTRIBUTION

315

volans is in general limited to the deciduous forests, while the range of
G. sabrinus approximates that of the coniferous forests. Limitations of
these species to forests is mostly caused by their feeding on nuts and
seeds, to a lesser extent by their habit of ''flight." Two animals that
do not pay much attention to vegetation areas are Rana pipiens, the
common leopard frog, which occurs all over North America east of the
Sierra Nevada range; and the raccoon, Procyon lotor, which lives in
deciduous forest areas and the prairie-plains region as well. The leopard

Fig. 270. — Proximity of ranges of three varieties of one species of garter snake. West
of the Mississippi and in Mexico, Thamnophis sauritus proximus; in Florida, Thamnophis
sauritus sackeni; north of Florida, Thamnophis sauritus sauritus. {Modified from Ruthven.)

frog ignores forests because of its semiaquatic habits, the raccoon because
of its tolerance of various conditions.

Besides vegetation, important physical conditions bearing on the dis-
tribution of animals are temperature and rainfall. Even in a limited
area the amount of rainfall differs greatly, as shown in the map of
Michigan in Fig. 269. *

These several factors are sufficient to illustrate that geographic dis-
tribution is not wholly a historical development. Ecology and the time
and place of origin of species have worked together.

316

PRINCIPLES OF ANIMAL BIOLOGY

sauritus

sacken

proximus

sauritus

Proximity of Related Forms. — If species originate from other species
it would be expected that a very young species would still be near its
progenitor. It would not have had time to travel very far. If the
youth of species and the sources from which they have sprung be judged
from the similarity between species, this expectation is in general realized.
Those species of a genus, or those subspecies, which are most nearly
alike are found geographically near one another. An example is found
in a group of garter snakes known as ribbon snakes. The forms in

question are three subspecies of one
species, Thamnophis sauritus. One sub-
species, called proximus, occupies a range
west of the Mississippi River and along
the east coast of Mexico (Fig. 270);
another, named sackeni, is in Florida and
on the Gulf Coast east of the Mississippi ;
the third, sauritus, is east of the Mississippi
and north of Florida. The three ranges
are practically in contact wdth one
another; at any rate they are not sepa-
rated by ranges of other garter snakes.
Other species of garter snakes are at a
distance.

The earliest of these subspecies, as
judged from their characteristics, appears
to be proximus; from it sackeni and
sauritus must have sprung. The order
in which the latter two forms arose is in
doubt. The possibilities are portrayed
by Fig. 271; proximus may have given
rise to sackeni and sauritus separately, or
it may have produced one of them (either
one), and this in turn produced the other.
The principle that nearly related (that is, similar) animals are geo-
graphically near one another is illustrated also in the higher taxonomic
categories. The genera of mammals east of the Rocky Mountains in
the United States have more similarities among themselves, and the
genera of the Pacific coast area more mutual likenesses, than do the
eastern genera to the western genera. The principle holds even for
continents. The animals of one continent are usually more alike than
they^are like those of any other continent. Moreover, the faunas of
neighboring continents are more alike than are those of more distant
continents. The animals of North America and Eurasia are particularly
good examples of this phenomenon. The similarity of these two faunas

proximus

sackeni

sauritus

sQckeni

proximus

Fig. 271. — Three pcssible ori-
gins of subspecies sauritus and
sackeni from proximus, in a garter-
snake species. {After Ruthven.)

GEOGRAPHIC DISTRIBUTION 317

is believed to have been increased by a land connection between them
across the Bering Strait and the adjoining Arctic Ocean, which would
have permitted migration between them up to (geologically) compara-
tively recent times.

In all these instances the argument is that the similar animals have
had more recent common ancestors, and there has been less time to
migrate far away. The effect of a barrier, for example, the Rocky
Mountains helping to keep eastern and western mammals apart, is
merely to push back the time of the common ancestors of the less similar
types, and so make their dissimilarities greater.

Normal Migration. — So important in the explanation of these pecu-
liarities of distribution are the abilities of the members of species to
spread, and the time they have had at their disposal to attain their
present locations, that the means by which they have become dispersed
should be examined. By far the most important method is what may
be called their normal migration. This is best seen in freely moving
terrestrial forms. The individual seeks food or shelter, avoids enemies,
seeks a mate. How rapidly it moves depends on its powers of loco-
motion. Whether it goes alone or in flocks or herds depends on little
understood psychology. These activities lead inevitably to the occu-
pation of more territory, unless barriers forbid, and by a young species
barriers are not as a rule reached very soon.

This spread by normal migration is ordinarily very gradual. Under
special circumstances, however, it may be greatly accelerated for a time.
The potato beetle, Leptinotarsa decemlineata, was long restricted to the
Rocky Mountains and the plains east, as far as western Kansas and
Nebraska. It could go no farther because its natural food, a wild species
of Solanum, did not exist east of that area. As the western part of the
Mississippi valley became settled, the range of the cultivated potato
(Solanum tuberosum) extended farther and farther west, until between
1845 and 1850 it reached the range of Leptinotarsa. The beetle found
the new Solanum a suitable food, so the eastern barrier was removed.
In about 20 years it had reached the Atlantic seaboard, where it stopped
until about 1918. Presumably in troop movements and shipment of
food supplies in the war, the beetle was carried to Europe, where it has
since existed despite efforts to eradicate it.

How effective normal migration may be in spreading species is indi-
cated by some computations. For the slow-moving earthworms, Gado iv
calculates that if one pair produces enough offspring to occupy one square
yard of soil in one year, their descendants in the time since the Ice Age
(perhaps 30,000 years) would have choked the earth. Again, if a human
family moved, gypsy fashion, only one day a week, and not more than
three miles, then it would wander 156 miles each year; and the Mongo-

318 PRINCIPLES OF ANIMAL BIOLOGY

lians, crossing Bering Strait, might at this rate have reached the Straits
of Magellan in 50 years.

Periodic Migration. — Not all the movements of animals are of the
slow, steady, progressive type just described. Many species move in
large numbers from one place to another at different times of the year
or at different times in their life history. The southward migration of
many birds in the fall and their return in the spring is an example of
seasonal migration. The great majority of bird species which may be
found in the course of a year at a given place in the middle of the north
temperate zone, for example, are seen there only at certain times. A
small number of species spend the summer there, building their nests
and rearing their young but disappearing southward in the fall. A still
smaller number are winter residents, some of which have come south
from a more northerly summer range. A much greater number are
migrants, going north in spring to their breeding range and returning
southward as cold weather approaches in the fall. What causes birds
to migrate is one of the great biological enigmas. ]\Iigration starts
before the situation where they spend the winter or summer becomes
unfavorable. In some species the migrating is done correctly by young
birds without previous experience and without guidance. Individual
birds have often been found to return to the same nesting place in suc-
cessive summers, but the way in which they are guided to the spot can
only be guessed. It has been suggested that endocrine secretions (page
154), particularly those of the pituitary and of the gonads, and the
duration of daylight may initiate migration, but how they could guide
it is not clear.

Some other animals migrate seasonally in search of food. When
the bison was abundant in the Avestern plains, it wandered in droves
north and south as grazing lands developed. The mule deer moves up
and down the mountains likewise in search of vegetation. In these
instances, however, there is no puzzle, for the animals move slowly, and
they wait until the new feeding grounds are needed and are available.
They do not anticipate events but direct their movements in relation to
what can be actually seen.

In a few animals the migration is not seasonal but occurs once each
direction in a lifetime. The fresh-water eel migrates at times .separated
by an interval of years. In its youth this animal ascends the rivei's
from the sea and lives there for years but does not breed ; upon reaching
maturity it returns to. the sea to breed. The Alaska salmon shows a
similar migratory habit.

Though periodic migration is important in the physiological cyQle
of individuals and in the economy of species, i( is not know u to have any
influence on species ranges. There is no known peculiarity of distribu-

GEOGRAPHIC DISTRIBUTION 319

tion anywhere which seems to demand periodic migration as its explana-
tion. The mere fact that the animals have to return from the place
to which they periodically travel nullifies any effect which such move-
ments might have on the size of the range. Migration would have to
be accompanied by some physiological change in order to extend the
area occupied.

Sporadic Migration. — Somewhat allied to periodic movements pei'-
haps are the sudden outbreaks or irruptions of a species that may occur,
during which the range is widely extended. The classic example is that
of the Lapland lemming, a small rodent. The migration of this species
has been described by Lyell as follows.

" Once or twice in a quarter of a century they appear in vast numbers, advanc-
ing along the ground and 'devouring every green thing.' Innumerable bands
march from Kolen, through Northland and Finmark, to the Western Ocean,
which they immediately enter; and after swimming about for some time, perish.
Other bands take their route through Swedish Lapland to the Bothnian Gulf,
where they are drowned in the same manner. They are followed in their journey
by bears, wolves and foxes, Avhich prey upon them incessantly. They generally
move in lines, which are about three feet from each other, and exactly parallel,
going directly forward through rivers and lakes ; and when they meet with stacks
of hay or corn gnawing their way through them instead of passing around."

Another case of sudden movements is afforded by Pallas's sand
grouse. This species inhabits the steppes of central Asia, extending
into northern China and the Kirghiz Steppes north of the Aral Sea in
the winter. At least since 1859 the bird has been in a restless and dis-
turbed state and great waves of individuals have moved out from the
normal range. In an irruption in 1859 some of them reached Poland,
Holland, and the British Isles. Another outbreak in 1863 apparently
involved thousands of individuals, and the birds reached Italy and the
Pyrenees in the south of Europe, Scandinavia and Archangel in the north,
and the British Isles and the Faroes in the west. Still another wave
occurred in 1888 and at this time flocks appeared in England, Scotland,
and Ireland. After each wave the species soon disappeared from the
invaded countries. The extinction may have been due to slaughter by
man; but while some of the invaders bred the first year, they were not
so well established that they could have reared young.

Such sporadic outbreaks are apparently of the same nature as those
which have been observed within the range of a species. An example
is the mouse plague of 1907-1908 in the Humboldt Valley, Nevada.
These mice (Microtus montanus), which live in scattered colonies in
swampy places, are not usually abundant enough to attract notice.
They produce half a dozen at a litter and four to six litters per year,
but ordinarily are kept in check. In the year named, however, some

320 PRINCIPLES OF ANIMAL BIOLOGY

element of control was removed, and the mice were produced in countless
thousands. On some ranches there were as many as 12,000 per acre.
Crops were destroyed, trees killed by injury to their roots, and banks
of drainage ditches were riddled with their burrows. Great armies of
mice moved on to new fields 5 miles or more from the point of first con-
centration. Then their hordes disappeared even more quickly than they
arose. In the course of three months they dropped to only 200 to 500
per acre. No satisfactory explanation of either their increase or their
disappearance was ever discovered.

Apparently sporadic migration, as these irruptive movements may
be termed, does not usually result in an extension of range, for the species
in the cases observed have not been able to maintain themselves in the
invaded regions. However, it is possible that at times such irruptions
have brought species into regions where conditions were favorable and
thus enlarged the inhabited area. Instances of widely discontinuous
range have sometimes been explained, whether correctly or not, by appeal
to sporadic migration.

Accidental Dispersal. — Discontinuous ranges have been more often
attributed to accidental dispersal than to sporadic migration. Animals
are sometimes carried on rafts or floating logs or are blown by the wind
beyond their normal range. Marine birds, such as the gannet, are occa-
sionally during storms blown inland from the Atlantic Ocean as far
west as Michigan, and a number of observers in the tropics have noted
terrestrial animals on floating logs and rafts in the rivers and even out
at sea. It has often been asserted that this method of dispersal is effica-
cious in extending the range. Islands may have received certain forms
by accident, but there are many difficulties in accounting for the entire
faunas of islands in this way. Some of these difficulties are (1) the
inability of some forms to survive a long sea voyage, (2) the fact that
many island forms, such as the giant tortoises, could not possibly be
carried on rafts or blown by the winds, (3) the necessity that in the
higher animals at least a pair of individuals or a pregnant female be
landed if the form is to be perpetuated, etc. But the greatest obstacle
to the acceptance of accidental dispersal as an eft'ective method of
extending ranges lies in the fact that actually observed cases of accidental
dissemination beyond the range of a form are very few and mostly open
to question. Possibly it may operate at rare intervals, for certain forms
and over short distances.

Man himself is responsible for the introduction of animals and plants
to new regions in a few instances that are well known. Sometimes it
was done by design, more often by accident as in the transport of rats in
ships. The animals carried by man have sometimes succeeded much
better in their new locations than in the original ones, witness the rabbit

GEOGRAPHIC DISTRIBUTION 321

in Australia, the cotton boll weevil in southern United States, and the
English sparrow in America.

World-wide Scheme of Distribution. — Having so far examined some
of the peculiarities of distribution, and the biological or geological
processes needed to explain them, we may now attempt to see how these
interlocking phenomena affect distribution on a large scale. One must
usually limit such a study to a single major group of animals because
of the different timing of evolutionary events in relation to changes in
the earth. Zoogeographers have proposed different groups for this pur-
pose. Mammals, snails, earthworms, birds, reptiles, insects, all have
been urged as suitable. We shall use mammals, primarily because the
different kinds are better known among nonbiologists, but partly because
they are large, and the world has been explored enough to discover the
location of most of them. They have one further advantage: their
evolution has been rapid and recent, so that the effects of changes of
the earth will be more readily discovered than in groups whose evolution
has been slow and protracted.

The bulk of the land area of the earth is in the northern hemisphere.
With the connection which must have existed across Bering Strait, this
land was formerly a continuous body. From this area there project
southward three great continental masses, South America, Africa, and
Australia. The last is believed to have been connected with Asia across
the Malay Archipelago prior to Jurassic time. South America, though
now connected with North America, is held to have been separated
from it in early Tertiary time. This is indicated by similarity of the
marine animals on the east and w^est coasts of Central America, as well
as by geological evidences.

Origin of Mammals. — Primitive mammals are believed to have arisen
first in the northern continents. This conclusion flows partly from
theory, since the great variations of environmental conditions character-
istic of huge land masses should have been able to act selectively on
almost any type of evolutionary change w^hich happened to occur in
living things. The northern continental mass as the place of mammalian
origin is supported, moreover, by the fact that the most primitive fossils
of the group have been found there, though it must also be said that
more explorations have been made in that area.

These primitive mammals, resembling our monotremes and mar-
supials more than true mammals, must have spread in all directions.
To the north barriers were soon reached, but to the south the three
great prongs of land provided ample room; and they had a geological
age or two in which to enter these.

Then the higher (true) mammals began to arise, also in the northern
land mass. They proved to be superior to their predecessors, that is,

322 PRINCIPLES OF ANIMAL BIOLOGY

more able to cope with the environment. This supposed superiority of
the later mammals has been demonstrated in modern times by the intro-
duction of northern true mammals into the southern areas, where they
began to replace the primitive forms already there. This has happened
very noticeably in Australia, where the dingo and rabbit were intro-
duced. Something like a general principle must be involved here, for
in other groups of animals northern forms have displaced southern ones
when they have been brought together. This has happened in the
case of birds (sparrow, starling, blackbird, and others) introduced into
Australia, the goldfish in Madagascar, European ants and earthworms
in all the southern continents.

The early mammals were thus driven out of the northern continents
which they first occupied. With Australia then joined to Asia, and
South America not yet separated from North America, they were free
to fill all the southern land masses. Then the sinking of the land cut
off Australia, so that the true mammals were not able to follow, and
that continent was and is the principal home of the marsupials and
monotremes. The severance of the Americas from each other checked
the southward migration of the higher mammals, so that primitive
types are relatively more common in South America. Restoration of
the land connection at Central America has, however, permitted many
of the true mammals to reach the southern continent. The traffic was
not all in one direction at the isthmus, since the opossums and armadillos
reached North America from the south over this restored land connection.

Primitiveness of Southern Faunas. — The scheme just outlined should
have caused the faunas of the southern continents to be on the average
more primitive than those of Eurasia and North America. For the
mammals of Australia and South America this has already been shown
to be true. To a less marked extent it is true also of Africa south of
the Sahara; for there is the primitive little deerlike chevrotain, and there
are the lemurs, the aardwolf, and the golden mole. In Madagascar is a
host of lemurs; and if other groups of animals are to be considered, that
island has the most primitive bird of the crane and rail group. Also
outside the mammalia, Australia has the most primitive termites, the
simplest insects of the butterfly-and-moth order, and some of the most
primitive bees. The most primitive land snails are in the southern
continents; indeed, the whole mollusk fauna of South America may be
characterized as primitive. The three surviving genera of lungfishes are
in the three southern continents, one genus in each. The lungfishes are
well represented by fossils in North America and Eurasia, and the three
living genera are plainly relicts.

Land Connections. — The connection and separation of land masses
postulated in the foregoing account mostly are supported by geological

GEOGRAPHIC DISTRIBUTION 323

evidence; that is, they have not been invented merely to explain animal
distribution. This is particularly true of the changes in Central America
or the Isthmus of Panama. These changes could be safely assumed on
geological evidence alone.

Zoogeographers have not hesitated, however, to assume former land
connections for which geology gives no support. Geologists have some-
times been the authors of such connections but have based them on the
facts of modern distribution. North America and Europe have been
assumed to be connected through a strip of land taking in Greenland,
arching north of the Atlantic, and joining Europe through the Scandi-
navian Peninsula and the British Isles. An antarctic land bridge con-
necting the tips of South America and Africa with Australia was proposed
by the British geologist Hutton to account for the large flightless birds in
those areas. This bridge has been adopted by many others since, but
it seems unnecessary, for the connection of the southern continents with
the northern land mass is adequate to account for the degree of similarity
of the animals. A land bridge has even been thro\\'n across the middle
of the Atlantic Ocean, from western Africa, say, to Brazil and the West
Indies. This bridge has been employed by many students of distribution
and is supported even now by reputable zoologists. The trend, how-
ever, has been away from extensive land bridges. They may have
existed, but some of them seem geologically so improbable that zoogeog-
raphers are seeking other explanations for similarities of faunas, or are
frankly leaving the facts unexplained rather than postulate the bridges.

Major Realms. — From the beginnings of zoogeography, many
attempts have been made to divide the earth into half a dozen or so
major realms which would have significance for all kinds of animals.
Birds were first used for such a division, then mammals. For these
two vertebrate groups the boundaries of the realms were somewhat
similar, and the authors of the schemes believed that other animals
would fit into the same divisions. Much of the work of zoogeography
has consisted of fitting groups of animals into the realms and modifying
the boundaries when necessary.

It has become increasingly clear, however, that different kinds of
animals do not observe the same distributional limits, and that theo-
retically they should not do so. Each group must be delimited by a
different scheme. New Guinea, with respect to its earthworms, belongs
with eastern Asia; but in its other animals it is Australian. The earth-
worms of Ceylon, on the contrary, are of Australian types, despite the
nearness of the island to Asia. Chile differs from the rest of South
America in its mollusks, fresh-water fishes and earthworms, but agrees
with other parts of the continent in its birds and mammals. It is true
that in highly isolated areas like the Hawaiian Islands, Madagascar, and

324 PRINCIPLES OF ANIMAL BIOLOGY

New Zealand, the barriers are such as to affect nearly all animal groups;
but they have done so to very unequal degrees.

One reason for the necessity of different distributional areas for the
different kinds of animals is the very different history of evolution of
each group. It makes a great difference whether, at the time of geologic
isolation of an area, the animals in it are evolving rapidly or are rather
stable. Madagascar, for example, is inhabited by mammals belonging
mostly to families found nowhere else, but by amphibia, reptiles and
insects belonging frequently even to the same genera as those of the
African mainland. Australia is peculiar as to its mammals, but much
like the Oriental realm (including southeastern Asia and some East
Indian islands) in its lizards, butterflies, and earthworms.

It seems clear now that progress in interpreting the distribution of
animals is to be made only by working out the history of each group
separately.

References

Gadow, Hans. The Wanderings of Animals. G. P. Putnam's Sons. (Chap. Ill,

the spreading of species; Chap. V, ancient geography inferred from distribution

and fossils.)
Hesse, R., W. C. Allee, and K. P. Schmidt. Ecological Animal Geography. John

Wiley & Sons, Inc. (Chap. VII.)
Rowan, William. The Riddle of Migration. The Williams & Wilkins Company.

(Chaps. II-IV, bird migration.)
ScHARFF, R. F. Distribution and Origin of Life in America. Archibald Constable

& Co., Ltd. (Chap. XII, fauna and flora of the Galapagos Islands.)
ScHARFF, R. F. The History of the European Fauna. Charles Scribner's Sons.

(Chap. II, general outline.)

CHAPTER 22
FOSSIL ANIMALS

Many of the fundamental problems which exist in connection with
living organisms may also be studied, and in some degree solved, with
reference to beings, now extinct, which lived on the earth in times past.
This biology of ancient life is termed paleontology. Paleontology may
be defined as the science of fossil organisms.

Fossils. — A fossil is any trace of prehistoric life. Most organisms
have left no trace because they were soft-bodied. Organisms mth hard
shells or skeletons had the best chance of being preserved, but even
these were screened by a fine sieve of circumstances and most were lost.
An animal whose bones are to be fossilized must usually be buried soon
after death to prevent the destructive action of oxygen, water, freezing
and thawing, and bacteria; and after it is seemingly safe the fossil is
subject to the risk of heat and pressure which would alter it beyond
recognition. Teeth are more likely to be preserved than bones, because
they are highly resistant; teeth of mastodons are often saved when the
bones of the same individuals have disintegrated.

A fossil need not l^e any part of an organism. It may be only an
impression, a track, or even a burrow. A dinosaur walking on clay, not
too hard or too soft, has left its footprints to the present time. A leaf
leaves an imprint in the silt in which it is buried, and this impression
is a fossil.

Similar objects buried only several thousand years ago are not
regarded as fossils; that is a matter of definition. Fortunately not
many objects belonging to the border line of prehistory are found, so
that little difficulty arises from the stipulation that a fossil be prehistoric.

How Fossils Are Preserved. — Some animals in cold regions are pre-
served in the flesh. That happened to numerous woolly mammoths in
Siberia (Fig. 272). They fell into crevasses in the ice, were covered
■with snow, and at the very low temperatures were quickly frozen. Even
the undigested food in their stomachs is recognizable in some of them.
These bodies have been frozen for probably 20,000 years. Some frozen
mammoths have been found in Alaska also, but only fragments of the
flesh were preserved. Other preservatives of flesh are oil in petroleum
lands (Poland, Galicia) and the acids of peat bogs. Human bodies have
retained their flesh, thoroughly dried and therefore resistant to bacteria,
in the dry southwestern parts of the United States.

325

326

PRINCIPLES OF ANIMAL BIOLOGY

Soft parts have sometimes been preserved merely as films of carbon,
which is the residue of the protoplasm. These films outline the body
perfectly, around the skeleton which retains more nearly its original
condition.

Entire insects in coniferous forests of the Oligocene epoch became
immersed in the sticky resin on the bark of the trees, which then hardened,
and may still show the delicate spines or the scales of the wings in butter-
flies as clearly as in the original.

More often only the hard parts are left — the tubes of corals (Fig. 273),
the shells of clams, the bones of vertebrate animals. Usually these hard

Hiiwij^iiWJ. '^'->"T»"—

I'lci. 272. — Muniinoth found fruzeu in .Sil)eiia in lUUl. Most of the fle.sh was still on
the body and intact. The skin is mounted in the museum of Leningrad in the posture in
which it was found. {From Lull, "Organic Evolution," courtesy of The Macmillan Company.)

parts must be buried before disintegration has proceeded far. They may
rest at the bottom of a lake, and be covered by silt carried in from the
land; they may lie on flood plains of streams and be buried under deposits
at times of high water; they may sink in the soft mud of bogs, be buried
in wind-blown dust, or covered with volcanic ash. Very often the
burying material hardens into rock by the cementing action of ground
water carrying minerals; this is particularly true of under- water deposits.
After such hardening, the shape of the buried object is usually main-
tained, regardless of what becomes of the material of which it is composed.
Sometimes the entire buried shell or bone is dissolved away by ground
water, which usually contains some carbonic acid (carbon dioxide in
solution). The cavity thus left is a mold. If this cavity is later filled

FOSSIL ANIMALS

327

by minerals deposited from the ground water, the mass thus formed
(called a cast) has the external shape of the original structure (Fig. 274).
Both molds and casts are fossils, though they include no part of any
living thing.

Fig. 273.

-Fossil chain coral, Halysites, found in Michigan. (From specimen in the
Museum of Geology, University of Michigan.)

Mud in which tracks were made hardened as it dried, and was resistant
enough to keep its shape while new material was washed over it in the
next freshet. New and old deposits hardened into rock, and the two
slabs were readily separable at the level of the tracks. One slab bears
molds, the other casts (Fig. 275).

SedirrCervtl

Fig. 274. — Diagram illustrating molds and casts. Horizontal shading represents sedi-
mentary deposits, vertical shading the material subsequently filled in. a, mold of a shell
which has been dissolved away by ground water; b, cast formed by subsequent filling in of
the cavity of a; c, mold of a shell whose interior was filled with sediment; d, cast produced
by filling the mold represented in c. {From Schuchert, "Historical Geology," courtesy of John
Wiley & Sons, Inc.)

Many bones and shells were dissolved away and replaced piecemeal.
That is, the most soluble parts were removed first and replaced by the
least soluble minerals which the ground water then carried. Less soluble
portions were removed later, and replaced by minerals then prevalent.
Different parts of the original bone are thus replaced by different minerals,

328 PRINCIPLES OF ANIMAL BIOLOGY

so that even the minute anatomy is preserved. Such objects are said
to be 'petrified (the process being called petrifaction).

Large collections of fossils are sometimes found at ancient water
holes, where animals congregated and died in periods of drought, or in
asphalt pools where they were trapped and were probably attacked by
predators which also were caught in the mire. The great collection
of fossil bears, lions, saber-toothed tigers, horses, elephants, antelopes,
and vultures at Rancho La Brea near Los Angeles was caught in a pit
of tar. Caves are likewise the sites of numerous such collections. For
the most part, however, fossil forms occur singly or in small groups,
where they are discovered during excavations for buildings, by mine
operations, or other accidental means.

Fig. 275. — Natural casts of dinosaur tracks and rain imprints. {From Schuchert, "Histori-
cal Geology," courtesy of John Wiley & Sons, Inc.)

Paleontology Relates Two Evolutions. — Like zoogeography, paleon-
tology treats of the interrelations of two evolutions, the evolution of the
earth, and the evolution of living things. According to either of two
prevalent theories of the origin of the earth, this planet was in some way
derived from the sun, and went through a period of great heat. It is
only the earth's history in the later cool period that concerns us in the
study of fossils. Many of the superficial parts of the earth's crust are
in strata of different kinds of rock. Obviously, where these strata are
undisturbed, the lower ones were deposited first and are the oldest. In
many places the strata have been compressed sidewise, and forced to
rise in arches. With further lateral pressure, the arch may break, and
the strata of one slope be shifted over the strata of the other slope. At

FOSSIL ANIMALS 329

the bottom of the overriding portion, an older stratum is above a younger
one. Often this disturbance is readily recognized, but not always.

Considerable help in recognizing disturbed strata is given by the
fossils they contain. While the earth's crust was changing, plants and
animals were also evolving. Animals of one period were distinctly
different from those of another. So characteristic of a given period
are certain kinds of animals that the fossils are known as index fossils.
Good index fossils must be abundant and widely distributed over the
earth, and large enough not to be overlooked. Occurrence of an index
fossil in a stratum at one place is not, however, a complete guarantee
that any other stratum containing such fossils was contemporaneous
with the first. These animals had to have a certain type of environ-
ment, and there are reasons to believe that similar environments occurred
in different areas at different times in the earth's history. For example,
the "red beds," made red by the oxidation of iron under certain climatic
conditions, occur in the Conemaugh formation in Pennsylvania and
West Virginia, and in the Wichita formation of mid-continental United
States; but according to other evidence the Wichita is much younger
than the Conemaugh.

While there are other ways of correlating rock strata of different
regions, the changes in types of animals occurring simultaneously
with changes in the earth are among the most reliable of the means
of identification.

Divisions of Geological History. — Geologists use a classification of
the earth's history which serves much the same purpose as does taxonomy
for zoologists. The classification is known as the geological time scale.
Major revolutions of the earth's crust caused elevation of great mountain
systems, erosion on a grand and extremely rapid scale, and redeposit of
the eroded material elseAvhere. As a result of these great changes, layers
of the earth's crust having very different characteristics and containing
very different fossils lie next to one another. These contrasts, known
as unconformities, are used to divide geological time into five great eras.
Within each of these eras the land of continents sank in large areas
so that the sea invaded the land, then rose again to push the oceans back.
On the basis of such changes, each era is divided into periods. Minor
and local changes of the same general type are used to divide the periods
into epochs.

All the rocks belonging to a period constitute a system, those of an
epoch make a series, while smaller divisions than the epochs have their
rock formations. These terms are not generally used in this book, but
are constantly met in geological works.

The accompanying table gives the geological time scale as far as the
terms are needed in an elementary study of biology.

330

PRINCIPLES OF ANIMAL BIOLOGY
Geological Time Scale

Eras

Periods

Epochs

Dominant life

Tertiary-

Recent

Age of man

Cenozoic

Pleistocene

Pliocene

Miocene

Oligocene

Eocene

Age of mammals and modern floras

Mcsozoic

Cretaceous
Comanchean
Jurassic
Triassic

Age of reptiles

Permian

Pennsylvanian'

Mississippian'

Age of amphibians and lycopods

Paleozoic

Devonian
Silurian

Age of fishes

Ordovician
Cambrian

Age of higher (shelled) invertebrates

Proterozoic

Keweenawan
Huronian

Age of primitive marine invertebrates

Archeozoic

Algoman
Timiskaming
Laurentian
Keewatin

Age of supposed unicellular life

1 Together constituting the Carboniferous (coal age).

Usually only a small part of this scale is represented in exposed
strata at one place. One of the more extensive exposures of the crust
is in the walls of the Grand Canyon in northern Arizona. The Colorado
River at its bottom is cutting its way through granite at the rate of
perhaps an inch in a century. Just above it in the slopes are Archeozoic
rocks; the rim, a mile above, is Permian. Between these are mostly
undisturbed strata in the order of the scale.

How old the strata are in years may be computed from the trauvs-
formation of radioactive substances. The element uranium is being
constantly transmuted into other simpler elements, the chain ending in
lead and helium. The rate of change is constant, and independent of
surrounding conditions. Where uranium is present, the amount of it
and the amount of lead are ascertained as accurately as possible. If it be

FOSSIL ANIMALS

331

assumed that all the lead came from uranium, and that none of the lead
has been removed, the time required for the transformation can be
computed. On this basis, one computation gave certain Permian strata
an age of about 220,000,000 years, late Cambrian 400,000,000 years,
and several pre-Cambrian formations ages ranging from 900,000,000 to
1,800,000,000 years. The age of the earth itself has been variously
estimated by the same method to be 3 to 6 billion years.

Fig. 276. — Cambrian brachiopods (left) and trilobite. {From Hussey, " Historical

Geology.")

Change of Animal Life. — How animals have changed during the
millions of years since life began can be indicated here only in a general
way. Most of the Archeozoic rocks are igneous (once molten), hence
could not bear fossils. Unicellular organisms are supposed to have
existed then, but there is little evidence of them. In the Proterozoic,
which witnessed two periods of glaciation at a number of places (Utah,
Canada), there are limy deposits undoubtedly produced by unicellular

Fig. 277. — Ordovician graptolite (left) and snail. {From Hussey, " Historical Geology.")

plants. Sponges, spicules, and a jellyfish which have been reported are
still somewhat in doubt.

Cambrian presents a great outburst of animal life of many different
kinds. Most characteristic and most abundant were the brachiopods
and trilobites (Fig. 276). Some shale in British Columbia contains
marvelously preserved jeUyfishes, sea cucumbers, siliceous (glassy)
sponges, annelid worms, and Crustacea. There were so many kinds of
Cambrian animals that the preceding era must have included many ; but

332

PRINCIPLES OF ANIMAL BIOLOGY

the long period of erosion between Proterozoic and Paleozoic destroyed
whatever fossils there were.

Trilobites were even more common in the Ordovician, and brachiopods
continued abundant but mostly with shells of lime instead of horn.
With them in this period were graptolites, snails (Fig. 277), and others.
The first vertebrate animals, the armored ostracoderm fishes, are found
in Ordovician but must have existed long before. The following Silurian
preserved few fossil fishes, but they must have been present, for that

group blossomed out extensivel}^ in the
Devonian; these two periods are known
as the age of fishes (see time scale).
Among the invertebrates of these periods
were the brachiopods (now at their peak) ,
trilobites (now on the decline), corals,
snails, siliceous sponges, cup corals (Fig.
278), and the scorpionlike eurypterids.

In Mississippian time the crinoids
(stalked echinoderms) reached their cli-
max (some of the best-preserved ones in
Iowa), and declined greatly in the next
period. Clams are preserved in Pennsyl-
vanian with their actual shells; before
this period the shells had dissolved away
and the fossils were only casts. The
latter period also had many insects, some
of them giants having a wing spread of
over two feet, also a number of amphibia
chiefly of the armored type. The succeed-
ing Permian had many of these armored
amphibia, but was chiefly distinguished
by its great variety of reptiles, some of
which had curious bony spines in a sail over the back (Fig. 279).

The most characteristic invertebrate animals of the Triassic period
were the ammonites, the most highly developed group of cephalopods
whose evolution is described in a later section. These animals continued
through the rest of the Mesozoic era but declined in the Cretaceous.
Other invertebrates of the IMesozoic were crinoids, squids, and Crustacea
(particularly crabs). The great evolution of the Mesozoic, however, was
in the group of reptiles. On the land were the dinosaurs, in the sea the
ichthyosaurs (looking like porpoises or sharks) and the four-paddled
plesiosaurs, in the air the pterosaurs. Dinosaurs often had curious rows
of dor,sal plates, as in the Jurassic Stegosaurus (Fig. 280), or shields and
spines as in the Cretaceous Tricoratops (Fig. 281). Some of them were

Fig. 278. — Fossil cup coral
found in Michigan. {From speci-
men in the Museum of Geoloyy,
University of Michigan.)

FOSSIL ANIMALS

333

of huge size, as the massive 75-foot Brontosaiirus and the 10-ton Stego-
saiirus. Other points of interest concerning the Mesozoic are that tmc
mammals were in existence, recognizable from their teeth and jaws, and
the first birds appeared.

.... . ••■'"" ...'Aiiv*" -vV,i.V*T«>:

"I ; 'I

•x:

tf^Va*

Fig. 279. — Permocaiboniferous reptile, Edaphosaurus cruciger; skeleton above,
restoration below. {From specimen in Museum of Geology, University of Michigan; restora-
tion by Prof. E. C. Case.)

The outstanding feature of the Cenozoic is the tremendous develop-
ment of the mammals, which rivaled that of the reptiles in the Mesozoic.
No brief account can do them justice. The primitive members of this
group are the marsupials, represented in North America by the opossum,
which is little changed now from its Eocene form. Contrasted with these
were huge forms (7 or 8 feet high) with bony protuberances on the head,

334

PRINCIPLES OF ANIMAL BIOLOGY

represented in Eocene but long since extinct. This varied assemblage
appears suddenly in the earliest Cenozoic deposits, indicating a long
evolution before that era. The evolution of two mammals whose
histories are most completely preserved is presented later in another
connection.

The purpose of the brief account in this section is to show the general
nature of the evolution of animals in relation to the evolution of the
earth's crust. So far as it relates to the vertebrate animals the story

Fig. 280. — Skeleton of the armored dinosaur Stegosaurus. {From Lull, "Organic Evo-
lution," courtesy of The Macmillan Company.)

is summarized by the diagram in Fig. 282. A similar chart for the more
numerous kinds of invertebrates would be too confused for our purpose.
Lines of Evolution. — Out of the wealth of fossil forms barel.y hinted
at above it is possible to select a few groups that show especially well
the step-by-step changes which animals have undergone. These gi'oups
are particularly instructive because the relative ages of their members
are not in doubt, and the differences between any two successive mem-
bers are so small as to leave no question that they possess genetic conti-
nuity. Such a scries of related forms is spoken of as a line of evolution.

FOSSIL ANIMALS

335

The lines of descent of modern elephants, horses, and cephalopods are
especially useful for illustration.

Evolution of Elephants. — The mastodon-elephant series shows a
larger number of obvious changes than either of the other series named.

Fig. 281. — Restoration of horned dinosaur Triceratops. {After Lull, from Schuchert,
"Historical Geology," John Wiley & Sons, Inc.)

Figure 283 will disclose the more striking steps of their evolution. The
earliest animal recognized as belonging to the elephant series, Moeri-
therium by name, was recovered from late Eocene and early Oligocene

Fig. 282. — Diagram of the fossil history of the major groups of vertebrate animals,
width of the bands indicates abundance and number of kinds.

The

deposits of northern Egypt. It was slightly over 3 feet in height. The
elephantine features are the high posterior portion of the skull (Fig. 283F')
composed of cancellate bone, that is, bone containing open spaces; the

336

PRINCIPLES OF ANIMAL BIOLOGY

elongation of the second pair of incisors in each jaw to form short tusks;
the indication of transverse ridges on the molar teeth (F) ; and the position
of the nasal openings some distance back of the tip of the upper jaw,

Fig. 283. — Evolution of the head and molar teetli of the mastodons and elephants.
The shuUs on the right are enclosed in the flesh in the form the latter is supposed to have
had. A, A', Elephas, Pleistocene; B, Stegodon, Pliocene; C, C, Mastodon, Pleistocene;
D, D', Triloi^hodon, Miocene; E, E', Palaeoinastodoii, Oligocene; F, F' , Moeritherium,
Eocene. {Frorn Lull, "Organic Evolution," courtesy of The Macmillan Company.)

indicating probably a prehensile upper lip. There were 3G teeth, and the

neck was long enough to enable the animal to put its head to the ground.

Palaeomastodon, which lived in Egypt and India, dates from early

Oligocene time. It was of somewhat larger size, the posterior part of

FOSSIL ANIMALS

33:

the .skull was distinctly higher {E') with a greater development of
cancellate bone, and the neck was somewhat shortened. The upper
incisors of the second pair were more elongated as tusks; the lower
second incisors were present, but not enlarged; while all other incisors
and the canines had disappeared. The molar teeth {E) resembled those
of Moeritherium but were larger. The lower jaw was considerably
elongated, and the number of permanent teeth was reduced to 26. The
nasal openings had receded until they were just in front of the eyes,
which is believed to indicate the existence of a short proboscis extending
at least to the tips of the tusks.

Trilophodon, a great migrant and consequently widespread over
several continents in Miocene time, Avas a huge animal, nearly as large
as modern Indian elephants. The tusks were considerably longer (-D')-
The molar teeth were large and greatly
reduced in number, so that only two were
present at any one time on each side of
each jaw. The surface of these teeth
bore a somewhat larger number of trans-
verse crests {D) than were present in the
earlier forms. The lower jaw was enor-
mously elongated, so that it projected as
far forward as the tusks. There was a
considerable development of cancellate
bone in the skull, to which the supporting
muscles of the neck were attached. The
long lower jaw, which was not continued
in later forms, has led paleontologists to
conclude that Trilophodon was not in the
direct line of descent, but that it was an
offshoot. Dinotherium (Fig. 286, upper left), a contemporary of
Trilophodon but with a strongly recurved lower jaw, is likewise
regarded as a lateral branch.

The mastodons were somewhat larger than Trilophodon, being about
the size of the Indian elephant. The tusks {€') were much elongated
(9 feet or more), but the lower jaw was greatly shortened and the lower
incisor teeth were reduced or wanting. The molar teeth (Figs. 283C,
284) were scarcely more complex than earlier forms and numbered 18 to
20 in the permanent set. They were still crushing teeth, and the food
must have been tender twigs and succulent plants; indeed, remains of
such objects have been found in the region of the stomach of some of
the fossil mastodons.

Apparently arising from the primitive mastodons was Stegodon,
knoAvn only from Asiatic Pliocene. Its molar teeth (Fig. 283 i?) had

Fig. 284. — Mastodon tooth,
showing the enormous cusps on the
upper surface. {From a California
specimen in the Museum of Geology,
University of Michigan.)

338

PRINCIPLES OF ANIMAL BIOLOGY

distinct transverse ridges, though not many of them, and its lower jaw
was short.

The extinct elephants known as mammoths belong to Pleistocene
time, while from them or directly from Stegodon have arisen two kinds
still living, the Indian and the African elephant. The gross features of
the elephants are their size, short neck, long proboscis, and heavy tusks.
The skull is very high and short (Fig. 283A'), due chiefly to the develop-
ment of cancellate bone. As in the earlier forms, the high skull affords
the necessary leverage for the muscles that support the weight of the
tusks. The molar teeth are distinctly grinding teeth (Fig. 283A ; see
also Fig. 285). Each tooth bears a number of transverse ridges, about
10 in the African elephant and 24 or more in the Indian species. These

A B

Fig. 285. — Tooth of mammoth (Elephas) from the Pleistocene, showing the flat grinding
surface and the numerous plates of enamel bound together by cement. A, side view;
B, surface view. {From specimen discovered at Ridgeivay , Michigan, in 1912, and preserved
in the Museum of Geology, University of Michigan.)

ridges are worn down by the chewing of harsh food, so that the upper
surface displays the cross sections of a number of flattened tubular
plates of enamel enclosing dentine and bound together by cement.
While the tusks (incisors) are of two sets, one following the other like
milk and 'permanent teeth of other mammals, the grinders succeed one
another in continuous fashion. As the molar teeth that appear first
wear down they move forward in the jaw and are replaced by others
from behind. Three permanent molars may thus successively appear
on each side of each jaw, but the wearing and movement are slow, so
that the interval between the appearance of the second molar and that
of the third may be 30 years. The total number of permanent teeth,
including the tusks, is 14.

Correlated with the nature of the teeth of the elephants are their food
and chewing habits. The an(;estral forms whose molars bore prominent
elevations lived on twigs and tender herbage which they crushed in
mastication, but the mammoths with their flattened tooth surfaces

FOSSIL ANIMALS

339

devoured grasses, sedges, and other harsh vegetation which they ground
with lateral motion of the teeth upon one another.

Fig. 286. — Restorations of heads of fossil elephantlike animals. Upper center, Moeri-
therium; below it, Palaeomastodon; upper right, Trilophodon; upper left, Dinotherium;
lower right, Mastodon; lower left, Elephas. {From models prepared by Ward's Natural
Science Establishment.)

The appearance of the heads of the series of elephantlike animals is
imagined to have been as shown in Fig. 286. The ears are suggested
by those of modern elephants, the proboscis by modern elephants and
the position of the nasal openings, as
already indicated. The general form of
the head and tusks is, of course, accurate.

Evolution of the Horse. — Most of the
development of the line of descent of the
horse took place in North America.
Eohippus, a lower Eocene form, is the
first member of the series recognizable as
ancestral to the horse, though it may also
be regarded as approximately represent-
ing an ancestor of the tapirs and the
rhinoceroses. It stood about 12 inches
high and had a short head and neck (Fig. 287). The hind foot
had three well-developed functional toes. On the outer side was a splint
bone representing an additional toe, and on the inner side a rudiment

Fig. 287.— Restoration of
Eohippus. {From, model prepared
by Ward's Natural Science Ls-
tablishment.)

340

PRINCIPLES OF ANIMAL BIOLOGY

of still another. Many living vertebrates have five digits on each hand
or foot, and there is anatomical and embryological evidence that primitive
vertebrates in general had five digits. These are numbered from the
inside outward, the thumb or great toe being first, the little finger or
little toe last. In the hind foot of Eohippus the functional toes are the
second, third, and fourth, while the fifth is reduced to a splint bone

ah c

Fig. 288. — Fossil teeth of ancient horselike animals, a, tooth of Eohippus with the
root broken; b, tooth of Mesohippus; c, tooth of Merychippus. {Photographed from
specimens in the Zoological Laboratory of the University of Michigan.)

and the first is rudimentary. The forefoot had four functional digits,
the first being wholly wanting, though some old figures erroneously
include one. In the ancestors of the horse the first digits seem to have
disappeared first, followed by the fifth. The teeth of Eohippus were
short of crown and relatively long of root. The upper surface bore

several conical cusps which, howevei-,
showed some sign of fusing to form
transverse crests (Fig. 288). The skull
(Fig. 289) was small, the lower jaw com-
paratively short, and the orbit Avas placed
well over the teeth, making the face
relatively short. Orohippus, which lived
in middle Eocene time, resembled Eohip-
pus closely but lacked the splint bone of
the forefoot (Fig. 290, left).
Mesohippus, an Oligocene form, was about 18 inches high. It had
only three digits on each foot (Fig. 290), but on the outer side of the
forefoot was a splint bone representing an extra toe (the fifth). Of the
three well-developed toes, the middle one (third) was in each foot dis-
tinctly larger than the others. The skull (Fig. 291), except for its
increase in size, had not changed materially. The cro\\Tis of the molar
teeth were still low (Fig. 288) and were tuberculate, that is, provided
with cusps on the upper surface, but the cusps were more distinctly
united into ridges or crests. Miohippus, a little later in Oligocene, was
somewhat larger, but otherwise much like Mesohippus (Fig. 290).

Fig. 289.— Skull of Eohippus,
about ^io natural size. (From
model prepared by Ward's Natural
Science Establishment.)

FOSSIL ANIMALS

341

In Merychippiis, a INIiocene animal, the feet were all three-toed (Fig.
290), vestiges of the fifth toe being present in some specimens and wanting
in others. The lateral toes, however, were high above the ground; the

Fig. 290. — Fore feet of fossil horselike animals; from left to right, Orohippus, Meso-
hippus, Miohippus, Meryehippus, Pliohippus. Of each type there are represented the
bones and the restoration in the fiesh. (From, models prepared by Ward's Natural Science
Establishment.)

entire weight of the body was borne upon the middle (third) toe. The
permanent molar teeth had moderately high crowns, and the upper sur-
face was worn down to a flat grinding surface marked by sharp ridges of
enamel set among dentine and cement (Fig. 288). Meryehippus was
evidently a grazing animal, whereas its predecessors must have fed upon
succulent herbage which was crushed, not ground. The skull was

Fig. 291. — Skull of Mesohippus, about J^fo natural .size. (From photograph of specimen
in Museum of Geology, University of Michigan.)

enlarged (Fig. 292), and the lower jaw was heavier in evident relation to
the change of the teeth. The orbit of the eye occupied a more posterior
position relative to the teeth, making the face longer. The orbit was
also completely closed behind by a bar of bone which in the earlier

342

PRINCIPLES OF ANIMAL BIOLOGY

forms was merely a process projecting down from above. The body had
increased to a height of 3 or 4 feet.

Phohippus (Phocene) was not appreciably larger than the preceding
member of the series but the two lateral toes had disappeared (Fig. 290),
except as long splint bones. Pliohippus was thus the first one-toed
horse. The teeth were moderately long-crowned and possessed grinding
surfaces. The body stood about 48 inches high.

The fossil horses of Pleistocene time were so nearly like the living
forms as to be included with the latter in the &ame genus (Equus). The
recent animals are 60 inches or more in height and weigh many hundreds
of pounds. Each foot has but one toe. Two lateral toes are evidenced

Fig. 292. — Skull of Merychippus, about ^{q natural size. {From model prepared by

Ward's Natural Science Establishment.)

by splint bones, and in rare cases a reversionary horse is born with exter-
nally visible digits articulated with one of these splints on each forefoot.
The teeth are long and columnar and grow continuously during early
and middle life, during which time the wear at the upper surface approxi-
mately equals the growth. The grinding surface is worn fiat, except
that the enamel resists the abrasion more successfully than do the dentine
and cement, so that the enamel forms sharp cutting ridges. The posi-
tion of these ridges changes somewhat as the tooth is worn to different
levels and the pattern of the upper surface is indicative, in a general
way, of the age of the animal. Later in life growth of the teeth prac-
tically ceases, and then the teeth may become quite short. The face is
relatively longer than in the ancestral forms, since the eye is set well
back of the teeth and the brain case has not been relatively enlai-god.

FOSSIL ANIMALS

343

Evolution of the Cephalopods. — ^An excellent fossil record among the
invertebrates has been established for the tetrabranchiate (four-gilled)
cephalopods (Mollusca), already used to illustrate the biogenetic law
(page 255). This branch of the cephalopods is represented today by
Nautilus, which lives in a coiled shell, externally resembling a snail shell.
The animal lives in only a small portion of the shell near the aperture.
The rest of the shell is divided by partitions into a number of chambers,
from which the animal is excluded except for a small stalk that extends
back through all of them. These partitions, or septa, represent the
positions occupied by the animal earlier in its life. As the body grows.

Fig. 293. — Diagrams of sutures of cephalopods, slightly more than half shown,
orthocone; B, nautiloid; C, goniatite; D, ceratite; E, ammonite.

it moves periodically forward into the wider part of the shell and secretes
a partition behind itself each time it moves.

Tetrabranchiate cephalopods have been found as fossils in Cambrian
rocks. They became fairly abundant in early Ordovician time. At
that time, unlike the modern Nautilus, their shells were straight cones
(orthocones) . All later forms appear to have descended from these
orthocones.

The course of evolution was as follows. The shell soon began to
bend and in many forms became closely coiled in flat spiral form (Fig.
210) like the shell of some snails. Owing to their resemblance to Nautilus
these animals are called nautiloids. They were very abundant in Silurian
time. Up to this period the septa across the shell were flat and saucer-
like, and the sutures, the lines of junction of the septa with the wall of the
shell, were nearly straight or only slightly curved. Later the septa

344 PRINCIPLES OF ANPMAL BIOLOGY

became bent in various Avays, at least at their edges, so that the sutures
were curved or anguhir (see Fig. 293). Forms whose sutures were of
this curved and angular form are called goniatites, and they were al)un-
dant in the Carboniferous period. These were to a large extent super-
seded in Triassic time by other genera, still tightly coiled but with sutures
thrown into a number of regular curves and sawteeth, which may be
described as "crooked." These forms with crooked sutures are known
as ceratites, from a very common genus keratites. And finally, in the
forms known as ammonites, the sutures became finely crimped in a com-
pound fashion, often producing exquisite foliaceous patterns. Fossil
ammonites are most abundant in the Jurassic to Cretaceous strata.

Though there were many irregidarities and overlappings in the series
of tetrabranchiate cephalopods, the fossils show on the whole clear evi-
dence of progress from a straight shell to one tightly coiled, and from
nearly straight sutures to sutures that were bent, angular, crooked, and
finely lobed.

Prehistoric Man. — The human line of evolution is not comi)lete
enough to offer as an example of such lines, but it has an extraordinar^y
appeal to the modern representatives of it. Man is one of the ortler
of primates, other members of which are the lemurs, monkeys, and
manlike apes. There is some reason to believe that the primates evolved
from the insectivores, the group to which moles and shrews belong.
If a series of fossils were available to show human evolution, it should,
assuming our surmises to be correct, lead from the insectivore t,ype,
through forms resembling lemurs, monkeys, and apes. The later stages
of this series would be especially useful as connecting the apes with man.
Unfortunately, not man.y primate fossils have been foimd. The probable
reason for the lack of fossils is that the primates have been tree-dwellers.
Dead individuals would have dropped to the groiuid, and forested areas
offer little chance for burial under either wind- or water-borne material.
Fossils of man himself were not preserved in numbers until burial customs
arose. As a result of these customs, more fossil men are known than
fossil apes. Kinship of man and the apes must therefore be judged
largely from homologies. Paleontology can begin to h(>lp only after
considerable divergence has occiu-red. Nevertheless, the earliest man-
like fossils show unmistakable leanings toward the ape structure in
certain respects.

One of the most primitive of the fossils appearing to connect man
with the apes, a form usuall}^ regarded as belonging to middle Pleistocene
tim(;, is Pilhccanthropus crectus, uncovered in some excavations in 1891
hi Java by a Dutch army surgeon. A femur, parts of the skull, and
several teeth were in the original find, and parts of several skulls and
jaws and additional teeth have been added from near-by locations since.

FOSSIL ANIMALS

345

The craniul capacity is about 900 cc., \v}u(!li is intermediate betwecm apes
(600 cc.) and men of today. The straight femur indicates erect posture,
since quadrupeds have doubly curved thigh bones. The heavy brow
ridges, rounded chin, and protruding face are all apelike.

Also of middle Pleistocene time are a number of skulls and a few
leg bones M^hich were found in a cavern south of Peking, China, in 1928
and later explorations. Their massive brow ridges, low foreheads, and
round chins are apelike, the average 1000-cc. cranial capacity inter-
mediate, the straight femur human. Along with the remnants in this

Fig. 294. — Restorations of prehistoric men. Left, Pithecanthropus erectus; middle,
Homo ncanderlhalensis, modeled on the Chapelle-aux-Saints skull; right Cro-Magnon man
modeled on type skull of the race. {From original busts by Prof. J. H. McGregor.)

cave were crude flint implements, and charred bones of animals which
indicated that Peking man was a hunter and knew the use of fire.

Piltdown man, so called from Piltdown common in Sussex, south of
London, where it was found, might on the basis of associated fossils be
assigned a slightly earlier time than the preceding ones, but had charac-
teristics which are indicative, in part, of a later period. The find includes
parts of two skulls and some loose teeth. Very much like man of the
present were the cranial capacity of 1400 cubic centimeters, which is as
large as many European skulls now, and the poor development of brow
ridges. Like apes were the considerable thickness of the skull bones,
the broad low nose, and the receding chin. The skull is a mixture of
advanced and primitive features.

Neanderthal man, so named because the first-described specimens
came from a cave in the Neander Valley near Dlisseldorf, Germany,

346

PRINCIPLES OF ANIMAL BIOLOGY

invaded western Europe in the warm period before the last of the great
Scandinavian glaciers. Many skulls and nearly complete skeletons have
been found in caves in a number of countries. These men were seldom
as tall as b}4, feet, but were powerfully built. The cranial capacity was
1300 to 1600 cc, brow ridges were heavy, chin usually receding (though
some had a small prominence). A restoration of Neanderthal man, with
two other types here described, is shown in Fig. 294. The measure-
ments of the skull are correct, but the thickness of the skin and under-
lying connective tissue at various places, and the amount of hair, can
only be conjectured. Well-designed flint tools (Fig. 295) were their

main weapons, scarcely adequate
to kill the cave bear, * mammoth,
reindeer, and bison whose bones
are found in the caves, so they may
have used traps, pitfalls, and prob-
ably stones. Neanderthal men
were not good housekeepers, for
debris was allowed to accumulate.
To this untidy habit and their
burial customs we owe our very ex-
tensive knowledge of the anatomy
and culture of this early human
type.

Toward the end of the last
glacial epoch (late Pleistocene)
Neanderthal man disappeared from
Europe and was followed by the
Cro-Magnon race. Probably it
was a forcible displacement. The
name Cro-Magnon comes from
the cave in which the earliest-
discovered skeletons of this type
were buried. From these individuals it would be said that Cro-
Magnon man was tall (6 feet or more), that his face was broad and flat
(from prominent cheekbones), that his forehead was high (hence he was
probably as intelligent as men of today), and that he was strongly built.
But men elsewhere in southern Europe, who must presumably be assigned
to any prevalent "type" of that time and region, were not all so tall,
often had protruding faces, and even sloping foreheads. Thus there
were tribes of Cro-Magnon man, just as there are tribes of American
Indians, who are at the same time still Indians. The burials of these
people wore evidently conducted ceremonially. Bodies were placed in
ai'tificial positions, or were shrouded in garments of shells, or were

295. — Neiiiiderthal flints; point
scraper below. {From Hussey,

Fig
above,
" Historical Geology.")

FOSSIL ANIMALS

347

painted, or community tombs were walled all around with certain bones,
as the shoulder blades and jaws of mammoths. Flint tools were brought
to perfection, but horn, bone, and ivory were also used for that purpose
as being more easily worked. Sewing was done with bone awl and
needle (Fig. 296). The bow and arrow had been invented, and these
with the spear, thrown from a short holder which remained in the hand,
were the principal weapons. Art had a considerable development, and
pictures of animals were cut (Fig. 297) or painted on the walls of caves.

Fig. 296. — Cro-Magnon tools of bone; needle above, harpoon point below.

These murals also indicate the existence of witch doctors whose bizarre
masks are there pictured, and of dome-shaped dwellings presumably
made of skins stretched over a framework of wood.

Then new people began to appear from the East, from the plains of
Persia or farther north. These newcomers migrated north of the
Mediterranean, or south of it and across to southern Europe, or along
the sea itself to the Atlantic Ocean and thence to the British Isles.
They did not destroy the Cro-Magnons of southern Europe, but mixed
with them, or by-passed and surrounded
them. In southern France, elsewhere in
Europe, and in the Canary Islands there
are still people whose measurements are
nearly identical with the Cro-Magnons
of the first-found cave, and these are
believed to be practically unaltered
descendants of the Cro-Magnon race.
With the coming of this eastern tide
Cro-Magnon art declined, and the
implement worker's skill deteriorated.
But the Asiatic invaders had their
culture, which included weaving of nets and baskets and, far more
important, agriculture. In their Persian home they had learned to raise
plants and animals for food — a step which made possible a tremendous
increase in the number of people in a given area.

Continued migration from Asia, and evolutionary developments
within Europe itself, led to the races and cultures that have succeeded
one another to the present time. Since the white people of North
America are the descendants of European immigrants, the history of

Fig. 297. — Cro-Magnon engrav-
ing of the woolly mammoth on the
wall of a cave in France.

348 PRINCIPLES OF ANIMAL BIOLOGY

man given alcove is the history of the bulk of people of the western
continent also.

Man in America. — The American Indians are so plainly Mongoloids
that they must have come from Asia; and the means of travel available
to these people almost guarantees that they crossed the Bering Strait,
which could have been dry. The Asiatics most like the American
Indians are not the Chinese, but the more generalized people of central
Asia, Tibet, or the East Indies. Migrations of these people extended to
Patagonia on the south, and to the Atlantic seaboard, long before white
men came to America. The Eskimos of the arctic region are more nearly
like the Chinese and Siberians, and probably are the latest immigrants.

Important discoveries of arrow points with fossil bison in New
Mexico in 1927 were followed in rapid succession by other revelations
of culture in relation to such extinct animals as horses, camels, masto-
dons, and ground sloths. The making of pottery, an art which for some
reason Cro-Magnon man never developed, has entered extensively into
the later history of culture in America. The New Mexico points were
interpi-eted as belonging to the late Ice Age, or perhaps 25,000 years
ago, so the migration must have occurred earlier.

References

BouLE, ]Marcp:llix. Fossil Man. Oliver & Boyd. (Chap. IV, Pithec'anthro})u.s;

pp. 147-157, Heidelberg man; pp. 157-175, Piltdown man; Chap. VII, Neander-
thal man; pp. 281-289, Cro-Magnon man.)
HowELLS, W. Mankind So Far. Doubleday, Doran & Company, Inc.
HussEY, R. C. Historical Geology. McGraw-Hill Book Company, Inc.
Lull, R. S. Organic Involution. 1929 Ed. The Macmilian Company. (Chap.

XXVI, cephalopods; Chap. XXX, dinosaurs; Cliap. XXXV, elephants; Chap.

XXXVI, horses; Chap. XXXVII, camels. Book lacks modern viewpoint on

factors of evolution.)
Matthew, W. D. The p] volution of the Horse. SitpplciiHnt to American Mustniin

Journal, January, 1903.
OsBORN, H. F. Men of the Old Stone Age. Charles Scrilmer's Sons. (Pp. 72-84,

Pithecanthropus; 9.5-102, Heidelberg man; 130-144, Piltdown man; 214-244,

Neanderthal man; 289-303, Cro-Magnon man.)

CHAPTER 23
MODIFICATION OF SPECIES

At many places in the preceding chapters tlie assumption has been
made that the kinds of hving things on the earth haxe clianged over
periods of time. In Chap. G possible ways of deriving multicellular
organisms from unicellular ones were postulated, in the belief that the
complex life of today could not always have existed. In describing the
varied breeding habits of animals (pages 185-186), it was assumed that
animals had evolved, but it was pointed out that the evolution of their
habits had not closely followed their structural evolution. In the classi-
fication of animals the basis of grouping is the supposed kinship of the
various species, due to descent from common ancestors and ascertained
from homology (pages 250^). The environmental relations of animals
were shown (page 283) to involve questions of evolution, since it was shown
that temperature could produce permanent modification of races. All
through the discussion of geographic distribution (Chap. 21) changes in
species were assumed to have occurred, in order to explain the position,
size, continuity, and proximity of ranges, and the differences between
southern and northern continents. And, finally, fossil animals (pages
331-348) were regarded as giving positive evidence not only of evolution
but of the direction which some evolutionary changes have taken. These
frequent references to evolution in advance of its separate discussion
indicate how intimately the idea of change of species is woven into the
entire fabric of biology. It would have been impossible to discuss these
phenomena adecjuately without relating them to evolution. Without
repetition of the facts and discussions already presented, it is left to
this chapter to summarize briefly with additions the reasons for believing
such changes to have occurred, and the methods by which they may have
been brought about.

Evidences of Evolution. — One of the most compelling reasons for
assuming evolution is the existence of many similarities among species
of animals and plants. Some of these similarities have already been
detailed in the chapter on classification. To the homologies there
described may be added that shown by the membranous labyrinths of
the inner ears of vertebrate animals (Fig. 298). Each has a series of
three semicircular canals set in different planes and attached to a central
sac; but in each group of vertebrate animals there are characteristic

349

350

PRINCIPLES OF ANIMAL BIOLOGY

differences that make it possible to recognize the group of animals by
the labyrinth alone. The embryos of animals also show homologies.
Every college course in embryology is a recognition of the existence of
types of development; for the laboratory studies, based on one or two
animals, are used to exemplify most of the classes in a phylum. The
homology of embryos is more spectacular when it is discovered in species
that are not alike in the adult. This situation is more likely to arise in

Fig. 298. — Membranous labyrinths of inner ear of various vertebrates. Each consists
of a saccular portion from which three semiciicular canals arise. A, of a fish; B, of a frog;
C, of a reptile; D, of a bird. (^Modified from Retzius.)

parasitic animals, since adult parasites are frequently very degenerate.
An excellent example is a parasite, Sacculina, found attached to the under-
side of the abdomen of common crabs (Fig. 299). Sacculina, in the adult
stage, is a rounded pulpy mass with practically no definite structure,
except a host of rootlike processes which extend throughout the crab's
body and absorb the body fluids. The embryo, however, is a three-
cornered little animal with jointed legs which clearly marks Sacculina as
one of the Crustacea. It is, in fact, one of the barnacles, a group in which
adult structure is usually quite complicated (Fig. 300).

MODIFICATION OF SPECIES

351

Similarities in physiological properties are quite as abundant as are
likenesses of structure. The enzymes of digestion are in general very
much alike in different vertebrate animals. As a rule, protein-spJitting
enzymes are produced and used in corresponding organs in different
^'ertebrates. Nervous and hormone control are in most respects alike.
Even the composition of the blood shows close similarity between animals
whose structures are alike; the hemoglobin (page 127) has nearly the same
crystalline chai-acters, and the serum has almost the same chemical com-
position as shown by precipitin tests. In using this precipitin reaction
an animal is rendered immune to, let us say, sheep blood by repeated
injection of sheep blood into its veins. This immune blood then pro-

FiG. 299. — Sacculina, parasitic on crabs. A, young Sacculina, shortly after hatching.
B, young animal shown attached to its host, the crab. The projection at the anterior end
has penetrated the chitinous ventral wall of the abdomen of the crab, only a small piece of
the chitin being shown. C, adult Sacculina (s), consisting of a pulpy mass on the under
side of the crab's abdomen, and a host of branching processes in the host's body. A and
B greatly but unequally magnified, C reduced.

duces a white precipitate when blood of a sheep or of an animal very
similar to sheep is mixed with it, but not when blood of a very different
sort of animal is mixed with it. The precipitate is formed only in
response to blood of a given chemical composition, and similar composi-
tion has been found almost solely in the blood of animals that are
structurally similar.

The argument from all these similarities, already advanced on page 255,
is that only heredity — hence common ancestry — could account for them.
But if two species of animals have come from a common source, any
differences between them — and there always are differences — must have
arisen 'since the time of the common ancestors. Emphasis is now to be
put on these differences, for their origin constitutes evolution.

The other principal evidences of evolution are derived from fossils,

352 PRINCIPLES OF ANIMAL HlOLOdY

from distribution of present-day organisms over the earth, and from
observation of the process. The rather complete series of fossil animals
leading up to modern horses and elephants, and the series of cephalopods
ending with extinction, as described in the preceding chapter, need no
comment as indications of evolution. To them may be added an
immense amount of less complete data of fossils, all of which point to
the same conclusion, namely, that species and larger groups of animals
and plants have changed. Geographic distribution, as repeatedly shown
in Chap. 21, likewise requires the assumption of evolution
to be intelligible. It should not be necessary to comment
further upon it here. The observational evidence will
be referred to later.

Evolution a Change of Species. — Though evolution has
effected a separation of groups of high rank (orders, classes,
phyla) from one another, it has accomplished this result
entirely by modification of species. There is no such thing
as single wide cleavages that at once produce cA^en families
or genera out of single common stocks. The divergence is
everywhere a slow accumulation of small differences such
as characterize species or varieties. When life originated, , , '"^' ,■.

_ _ o 7 Adult free-hv-

assuming that it did so only once, there was at first only ing barnacle of
one species of organism. When a change occurred in a part ^^^ ^''•ti'* t^ u
of this group, all experience indicates that the difference of its shell re-
could have been no greater than that now existing between moved.
species — or more probably varieties. When further changes occurred, it
is not likely that altogether the same changes took place in both varieties,
so that each of them gradually bi-oke up into two unlike sets of varieties
or species. The two varieties produced by the first modification may
thus have given rise to two species, later to two genera. By fiu'ther
change of species, each group of species pursuing a course somewhat
different from the other, these two genera may be supposed to have been
transformed into families. Still further changes in species within the
families shovdd have resulted in the degree of difference now held appi'o-
jjriate to orders. By continued change of species, the orders may have
diverged from one another enough to be regarded as classes and finally
to have attained the rank of phyla. The coiu'se of evolution has been
not to create phyla and then to proceed to split tliem up into groups of
lower ranks, ending in species and varieties; it has ratlier gone in the
opposite direction, beginning with species and by repeated ciianges of
species gi-adually converting them into groups of higher rank. The pro-
blem of evolution thus becomes that of the origin of species.

The Nature of Species. To understand evolution it is necessary,
therefore, to know how species arc constituted. A species may be

MODIFICATION OF SPECIES 353

thought of as a group of individuals most of which have^most of their
inherited characteristics in common. Characteristics due to environ-
ment and differing in individuals solel}^ because of different environ-
mental influence are not considered. The difficult}^ in applying the
foregoing idea lies in the word ''most," for there is much disagreement
among taxonomists as to how much it should include. Probably no
species that can be recognized as a species anywhere in the world has
all of its individuals alike in all hereditary qualities. It would be possible
to assemble groups of individuals alike in all their genes (page 224), but
such assemblages would be much smaller than the ones now recognized
as species. To insist that species be entirely homogeneous would simply
multiply the number of species and would solve no problem either of
evolution or of classification. In practice, therefore, some heterogeneity
is admitted. As far as taxonomists agree on the grouping, all individuals
of a species have certain qualities in common; these qualities are held to
characterize the species. Beyond this general heritage, there are other
characters each of which is present in some individuals, but none in all
of them. A certain amount of variation thus exists among the indi-
viduals of every species. Some of this variation is nearly always visible
or otherwise capable of detection; but some of it is not seen, since it
consists of recessive genes scattered through the population. These
recessive genes, unless very numerous, are present more often in hetei'o-
zygotes than in homozygotes (page 227) and do not greatly affect the
species visibly; but they are a potential source of visible qualities in later
generations.

Species do not as a rule cross with other species, though there are
many exceptions. Also, species tend to occupy different ai-eas from other
species. These are marks which help the taxonomist to recognize species
as distinct, and their intersterility is an important agent in making them
distinct.

Origin of the Differences among Individuals. — What is*the source
of the minority of qualities in which the individuals of a species may
differ? Since a species is ordinarily descended from a single individual,
it would be expected, unless the ancestor had been an extremely heterozy-
gous organism, that its descendants would possess practically the same
genes throughout. The existence of a number of genes which are not
alike in all individuals indicates that some of the genes have changed in
some individuals. Such changes of genes are the mutations already
referred to (page 238) in the discussion of genetics.

Mutations are not merely inventions to explain the variation within
species; the visible changes due to them have been witnessed again and
again in many animals and plants. Some of the first changes to be called
mutations were observed by Hugo de Vries, one of the rediscoverers of

354

PRINCIPLES OF ANIMAL BIOLOGY

Mendel's law (page 18), in the evening primrose Oenothera lamarckiana
before the year 1890. Since that time, individuals of this species and
others of the same genus have continued to produce offspring unlike
themselves in some permanent way. Not a year passes without the
production of one or more new forms. Some of them represent changes

Fig. 301. — Mutation in Oenothera involving the length of the seed capsule. The two
specimens at the left arc Oenothera reynoldsii mutation dehilis, a foiin which gives lise by
mutation to the form represented by the two figures at the right, Oenothera reynoldsii
mutation bilonga. {Photograph by Prof. H. H . Bartlett.)

in the seed capsules (Fig. 301), others the whole habit of growth. Some
mutations are detectible only in the adult plant, others in the young stage
known as the rosette (Fig. 302). The alterations arising in Oenothera
are not the simplest examples of evolutionary change, for it has been
found that most of them are due not to simple changes of genes but to
rearrangement of large fragments of the chromosomes and ]-egrouping of
whole chromosomes that adhere to one another. Probably such changes

MODIFICATION OF SPECIES

355

should not be called mutations, but the name has been applied to
them.

]\Iodifications that are due to changes of single genes — and hence are
true mutations — have, however, been abundantly witnessed in other
organisms. Over a thousand alterations have occurred in pedigreed
strains of the vinegar fly Drosophila melanogaster, and many of these

< S C/77 >

'! •■ -^^at^^^tiliiff-

Fig. 302. — Mutation in Oenothera involving the rosettes, or young plants. Below
(8 and 9), Oenothera pratincola; above (3 and 4), Oenothera pratincola mutation nummularia,
a mutant of the preceding form. {Photograph by Prof. H. H. Bartlett.)

are presumabl}^ changes in single genes. The first of these mutations to
be discovered was a change from red eye to white in one fly in the labora-
tory of Prof. T. H. Morgan in the year 1910. Since then there has been
almost a continuous procession of mutations, affecting eyes, wings, body
color, bristles, legs, antennae, and physiological properties (Fig. 303).
Most of these mutants have been bred so that the mode of inheritance of
their new characters was ascertained, and most of them turned out to be

350

PRINCIPLES OF ANIMAL BIOLOGY

recessive to the wild-type characters from which they sprang. Smaller
numbers of mutations liave been obser\^e(i to occur in other species of
flies, and in wasps among insects; in mice, rats, rabbits, and guinea pigs
among mammals; and in corn, barley, peas, and morning-glories among
plants. So freely have these and other organisms mutated that the bulk
of evolution may reasonably be assumed to follow from just such changes.
True mutations may be supplemented by the breakage or duplication of
chromosomes, but changes of this nature cannot be emphasized in an
elementary discussion.

Causes of Mutation. — What causes mutations to occur under natural
conditions is still unknown. The genes are almost certainlj^ chemical,
and it is likely that they are fundamentally protein. If these surmises
are correct, mutations should be chemical modifications and of the sort

Fig. 303. — Mutations in the vinegar flj' Drosophila melanogaster . A, normal wing; B,
beaded ■wing; C, notch wing; D, vestigial wing; E, miniature wing; F, club wing; G, rudi-
mentary wing; H, truncate wing; I, normal red eye; /, bar eye; K, eyeless; L, white eye.
(C from Morgan; D and L original; the rest from Morgan, Sturtevant, Muller, and Bridges,
courtesy of Henry Holt and Company, Inc.)

that proteins are capable of undergoing. A century and a ciuarter ago
Lamarck (page 17), who was the first naturalist to propound a compre-
hensive theory of evolution, held that species changed in indirect response
to the environment, effected thi'ough use and disuse. Lamarck knew
nothing of the single character changes now called mutations; but, were
his idea correct, it would mean that mutations are caused by environ-
mental action. As Lamarck conceived the changes to occur, they con-
stituted inheritance of acquired characters. For the individuals were
supposed to be modified by the activity of the animals themselves which
led to such things as overdevelopment of the muscles or stretching of the
legs or neck. These changes were induced only in the soma or body at
first, but he believed that the body was then capable of influencing the
offspring in like manner. In the light of present knowledge, this influence
of the body on the offspring would have to take the form of causing
mutations in the germ cells while still in the body. It seems whollj^
unlikely that any such influence can bo exerted. The organization of

MODIFICATION OF SPECIES 357

animals appears to offer no possible mechanism whereby an altered body
can produce in the germ cells within it any modification such that
offspring developing from them would have the same alteration. More-
over, though many experimental attempts to produce such changes
have been made, no satisfactory evidence of their success has ever been
adduced.

It seems necessary, then, to exclude somatic influence from the list of
possible causes of mutation. When mutations began to arise under
observation in experimental cultures, it was further observed that there
Avas no apparent difference between the environment of the one mutant
individual and all the rest. It was long supposed, therefore, that the
cause of mutation was an unknoAvn something within the animal, possibly
in some way connected with its physiological processes. In recent times,
however, it has been found that certain environmental agents are not
stopped by the body but reach the germ cells directly. They may not
influence the body in any detectable way yet produce modifications in the
germ cells. The most potent of these kno\\Ti agents is X rays, and the
most responsive organism is Drosophila. Hundreds of alterations have
appeared in the offspring when the parents were exposed to the rays.
Some of the modifications are visible structural changes; more of them
have physiological effects. How much natural mutation may be due to
such radiation is in doubt. Though there is always a certain amount of
radiation from the earth, it appears much too feeble to account for the
mutation that has occurred in laboratory cultures. Heat is the principal
other agent that has been found to produce mutations, and again Droso-
phila is the subject. While some parts of the earth have as high tempera-
ture as was employed in these experiments, the temperate zones, where
most of the thousand mutations of Drosophila have arisen in laboratories,
are not among them.

On the whole, while it must now be recognized that external agents
may produce mutations b}^ direct action on the germ cells, the chief
agents have not yet been discovered; and the possibility of wholly
internal agents has not been exhausted.

Hybridization. — Given a number of genes in which various members of
a species are different, an important other source of variation is at hand.
If individuals having different genes are capable of crossing, as they
nearly always are ^dthin a single species, the genes may be combined in
different ways. How many recombinations may be produced depends
only on the contrasting genes. If there are only 20 spots in the chromo-
somes at each of which, somewhere in the population, two different genes
exist, it is possible to have over a million different kinds of individuals.
Most species presumably have more than 20 mutant genes floating about
in scattered members, and for each additional mutation the number of

358 PRINCIPLES OF ANIMAL BIOLOGY

possible combinations is doubled. The importance of this sort of varia-
bility in evolution can scarcely be overestimated. A species that is
confronted by a number of environmental situations may easily be in a
position to take advantage of several of them. Its success would thereby
be enhanced.

The variability that is due to combinations of genes in different ways
is changed in its nature when genes of different pairs interact with one
another to produce a character not like that produced by either one
alone. An example is given on page 232 and in Fig. 201, where walnut
comb is produced in fowls by a combination of the genes for pea and rose
comb. The genes for brown and scarlet eye in Drosophila produce
together a nearly white eye. Many such interactions between genes are
known. It is indeed doubtful whether any gene fails to interact with
those of other pairs in some way. Such interactions do not increase the
number of different kinds of individuals which may result from recombi-
nation of genes, but they do introduce unpredictable qualities into the
species. This feature may likewise be highly important to a species in a
variable environment.

The hybridization referred to alcove h merely that occurring between
slightly unlike individuals within the species. Whether hybridization
occurs between two species or not depends partly on whether their
chromosomes are similar and equally numerous. If the species has the
same number of chromosomes and if the genes in them are in large
measure alike, crossing is usually possible. The normal pairing of the
chromosomes in the preparatory stages of germ cells depends on these two
things. If the numbers of chromosomes are not equal, odd single chromo-
somes are left over from this pairing. And if corresponding genes do not
exist in l)oth species, the chromosomes do not unite readily. Many
abnormalities result from these situations. The majority of species
crosses fail to produce offspring, or the offspring are partially or wholly
sterile. It seems unlikely, therefore, that any considerable part of evolu-
tion is due to hybridization between species.

The Direction of Evolution. — Evolution has taken by no means all
of the courses that were theoretically open to it. Even if life originated
only once, and even though the million or two species now probably in
existence is a good round number of end products, this degree of differen-
tiation is much less than might conceivably have occurred. The actual
divergence of lines of descent has been considerably curtailed. What the
other possibilities were that have not been realized, why certain species
were produced and not others, why certain spcnnes that were produced
survived and not others are problems to which we must now turn. Their
solution is largely speculative but impoi'tant.

The first element entering into the direction of evolution is the charac-

MODIFICATION OF SPECIES 359

ter of the mutations with which it starts. Some students of evohition
have assumed that mutations are of every conceivable sort, just as a
needle thrown on the floor may eventually, if thrown often enough,
point in every horizontal direction. This seems an unreasonable assump-
tion because, if genes are chemical in their nature, they should be no more
free to enter into unlimited reactions than other substances are. Chem-
ical substances are restricted to a certain range of reactions by the
structure of their molecules. Furthermore, in organisms which, like
Drosophila, have produced the greatest numbers of observed mutations,
there is not so much variety among the mutations as a purely random
determination of them should produce. They are too much alike, and
some of them occur too often, to be the result of chance alone. It is
more likely that each gene is capable of mutating in certain ways, and
only in those ways. If this is correct, a species can evolve along any
line which its possible mutations provide, but along no other. From
these possible lines something has to choose.

With respect to the combinations of genes that result from hybrid-
ization within the species, chance probably plays an important role in
the early stages of differentiation. When certain genes are present in a
population in given numbers of individuals, certain combinations of
genes are expected to occur in calculable proportions of individuals.
Almost certainly, however, the expected proportion is never exactly
realized. The accidental meeting and pairing of individuals will usually
result in some small deviation from the expected result. A gene that
ought theoretically to occur in 25 per cent of the individuals may easily
happen to be in 28 per cent or only 22 per cent solely through chance.
Should the deviation from expectation in the next generation happen to
be in the same direction, the difference is accentuated. Different parts
of a range may thus come to be inhabited by groups of individuals which,
while still belonging to the same species, nevertheless have their genes in
different proportions. These groups may look essentially alike, especially
if the genes in which they differ are recessive and exist mostly in heter-
ozygotes; but their potentialities for the future are distinctly different.
Such differences tend to be preserved by lack of random mating. No
individual travels the whole range of its species, so that it mates with one
of its neighbors. When the genes become numerous enough to produce
many homozygotes, or if they are or become dominant, the two groups of
individuals show noticeable differences.

It is believed that varieties of a species may arise and come to occupy
different parts of the range, entirely as a result of random wandering and
the accidental union and fortuitous survival of certain gene combinations.
Possibly even a divergence great enough to mark two separate species
may take place in this purely random manner. Beyond this degree of

360

PRINCIPLES OF ANIMAL BIOLOGY

differentiation pi-ohably other factors enter. The most important of
such factors is beheved to be natural selection.

Charles Darwin and the Natural Selection Idea. — Though Charles
Darwin is often popularly credited with introducing the evolution doc-
trine, that is not correct, since, as shown in Chap. 1, the idea of evolution
was already old in Darwin's time. His real contribution was the theory
of natural selection. This theory made evolution seem so reasonable
that opposition to evolution itself from intelligent people quickly fell
away. From this fact, and from the confusion which exists between
natural selection and evolution in Darwin's o\^^l writings, has no doubt

come the popular misconception of
Darwin's share in promulgating the
evolution idea.

The development of the natui-al
selection concept in Darwin's mind
is one of the fascinating romances of
biological science. Darwin had
come under the spell of the great
English geologist Sir Charles Lyell
(Fig. 304), one of whose principal
teachings was that geological proc-
esses of the past were essentially
the same as those in progress now.
Thi;; doctrine, which has been called
iiniformitarianism, means specifi-
cally that erosion, warping of the
earth's crust, rise and fall of the
land, volcanic action, etc., had been
periods of time just as they are
occurring now. By means of these present-day processes and no others,
Lyell attempted to explain the development of earth features. Darwin
was impi'essed with this method and was inclined to apply it to living
things as well. When, therefore, from 1831 to 1830 he was privileged to
accompany as naturalist an expedition that was traveling around the
world on the ship Beagle, he was already in a frame of mintl to reflect
present occurrences back into the past to see what they might explain.

It was not until after his return from this voyage, however, that the
idea of natural selection occurred to him. As he himself says, he got it
from a book by Malthus, "Essay on Poi)ulation," in which it was pointed
out that human populations tended to increase rapidly, thus leading to a
struggle for existence. Darwin quickly saw in this situation a means of
modifying species of other organisms; for if individuals varied, and if they
were competing with one another, any advantage possessed by certain

-Sir Charles Lyell, 1 797-1 S75.

continually occurring over

long

MODIFICATION OF SPECIES

301

types of individuals would tend to preserve them while less favored ones
would either suddenly or gradually disappear. If the favorable qualities
were hereditary, as he apparently assumed they would be, the result
would be the formation of a new species.

For 20 years Darwin collected facts that seemed to bear on the
possible correctness of this natural selection, but he published nothing.
Only a few friends, including Lyell and the botanist Joseph Hooker, with
whom he frequentl}' discussed his views, knew what conclusions he was
reaching. Then a curious coincidence induced him to put a synopsis of
his work into print. Alfred Russel Wallace, a young naturalist then in
the Orient, sent to Darwin a sketch of a theory of which he desired
Darwin's opinion. To the latter's
surprise, this theory proved to be
none other than the theory of
natural selection, or survival of
the fittest; and, as Wallace after-
wards related, he too had first
got the idea from reading the
work of Malthus, "Essay on
Population." At first Darwin
was inclined to withhold his own
manuscript and allow that of
Wallace to be published. But
since Wallace's idea was admit-
tedly a sudden one, in favor of
which he had collected no facts
whatever, whereas Darwin had
long been gathering data relati^-e
to it, DarA\an's friends protested.
It was finally arranged to present
extracts from both Darwin's and
AVallace's manuscripts simultaneously to the Linnaean Society of London,
which was done in 1858. Darwin's theory was developed at length in
"The Origin of Species" in 1859. The book was written in language
intelligible to the average reader without biological training. Further-
more, the time was ripe for such an advance. These facts, coupled with
championship b}': T. H. Huxley (Fig. 305), who carried the evolution idea
to the general public in lectures and popular articles, Avon a quick victory
for the new doctrine. The history of the evolution idea in the last 60 or
70 years has been the accumulation of new facts in support of it, the
development of theories to account for it, the grouping of animals on the
basis of the relationship implied in evolution, and the application of
corollaries of evolution to other branches of biology.

Fig. 305. — Thomas Henry Huxley, 1825-
1895.

362 PRINCIPLES OF ANIMAL BIOLOGY

Operation of Natural Selection. — How natural selection is believed
to work may be best illustrated from the standpoint of the genes. In any
species in which a certain gene is becoming either more or less common,
evolution is occurring. Even if the gene in question is recessive and
even if it occurs only in heterozygotes so as never to produce a visible
effect, if this gene is present in a gradually increasing or decreasing num-
ber of individuals, the species is evolving. Now most genes produced by
mutation are recessive. They cannot at first affect the visible or phys-
iological properties of the individuals, for these organisms are hetero-
zygous. During this early period, pure chance must be responsible for
the fluctuation of the prevalence of the new gene, as described above
(page 359). Many a new gene is lost before it can become established.
Most new genes are thus lost. Evolution would be going on at a tremen-
dous rate in some species if even a majority of mutations succeeded. Out
of the large number that occur only a few happen to become numerous
enough to begin to show their effects in homozygotes. Then they are
on trial. They may confer some advantage on their possessors, such as
longer life, more rapid growth, or greater strength. If the advantage
is one that enables them to leave more descendants, that gene tends to
become more prevalent. If on the contrary the new gene is harmful, in
Guch a way that its possessor leaves fewer descendants, it is checked in
any increase which it might otherwise enjoy. Mere harmfulness cannot
eradicate a gene altogether if it is recessive, for it may continue to exist in
heterozygotes beyond the reach of natural selection; but a very harmful
gene cannot become much more abundant than the level at which hetero-
zygotes begin to meet and mate, thus producing homozygotes. A less
harmful gene may become slightly more abundant than this, so that some
homozygotes appear; but it cannot replace the alternative gene which is
superior to it. A neutral gene, one conferring neither advantage nor
disadvantage, is at the mercy of chance. As pointed out before (page
359), local races within a species may come into existence in different
parts of a range by this accidental method.

A considerable degree of variability may thus exist in any species.
Partly it is observable, as in the distinguishable local races ; but much of i't
is hidden in heterozygotes. At any particvilar time it is to be expected
that a species will exhibit approximately that part of its genetic composi-
tion which is most favorable; and ''favorable" means conducive to large
numbers of descendants. This most advantageous group of genes would
be expected to show, because, if they were not expressed, natural selection
would gradually bring them to expression. If, under these circumstances,
the environment were to change in some respect, so that certain genes
increased and their alternates decreased in value, without much question
the species would change toward the genetic make-up that had acquired

MODIFICATION OF SPECIES 363

enhanced usefulness. Such changes would necessarily be slow; hence the
alteration of the environment would have to be at least semipermanent to
accomplish any important modification of the species exposed to it.
Even a hundred thousand generations might well prove too short for
any important change of a species under the selective action of the
environment.

Among groups of organisms that differ in more than the mere propor-
tion of certain genes, selection should work more effectively. Of the
varieties that arise by chance within a species, one or more may well fit the
environmental situation better than others. Unless these varieties are
kept in their separate areas by different physiological responses to features
of the environment, which is probably not often true, the favored variety
or varieties should gain the ascendancy. They might or might not
crowd out their fellow varieties ; but even if they were only more abundant
in individuals, they should have a greater share in determining any later
evolutionary changes. In like manner, species must differ in their
capacity to propagate, and the more capable ones should increase in
numbers. Genera, families, orders, all higher ranks must be subject to
this action of natural selection, but the action is always on the individuals
that compose them.

Adaptation. — The guidance of evolution by natural selection should
result in a considerable degree of fitness for the environment. If individ-
uals and species are preserved in proportion to their ability to succeed,
their success should grow with the passage of generations. The fact that
natural selection offers a general explanation of adaptation is one of the
chief reasons for the rapid acceptance of Darwin's theory among biol-
ogists. For adaptation is very widespread, and some of it is very remark-
able. So abundant is it, and so marvelous are parts of it, that many
naturalists have come to feel that adaptation is the outstanding feature
of life requiring an explanation.

It would be easy, however, to overemphasize its frequency, its degree,
and its necessity. Most species are not so well adapted to their situ-
ation as they conceivably could be, but they get along. Lack of satis-
factory adaptation in certain species or larger groups seems to be proved
by extinction. Moreover, obvious adaptation, as among taxonomic
groups, is found most markedly in the groups of higher rank. Classes,
orders, families are marked off from one another by such things as wings,
gills, armor plate, webbed feet, and quills which perform definite functions
in the lives of the individuals and often help to determine where they shall
live. Such structures are highly adaptive. In lower ranks, however,
this adaptiveness is much less common. Most genera of the same family
do not make any particular use of the characters that distinguish them
from one another, though there are exception?. Among species of the

364 PRINCIPLES OF ANIMAL BIOLOGY

same genus, almost never do the distinguishing characters seem to l)e of
2k,ny particular vakie to the individuals possessing them. This lack or
infrequency of adaptiveness of the so-called species and genus characters
is one of the principal reasons for adopting the view, just described,
that varieties and species may become separated from one another by
accidental changes in genetic composition, while natural selection does not
exert its most powerful influence until some degree of differentiation has
been attained.

Pointing out the adaptations of animals has been one of the favorite
pastimes of naturalists. Books and articles on natural history are full
of examples, and recitation of the marvelous fitness of organisms to some
special niche in the environment never fails to excite wonder. The
several decades following the publication of Darwin's "Origin of Species"
were marked by inordinate attention to the features of living things that
enable them to cope with the environment, for to explain the develop-
ment of any character through natural selection it was only necessary
to find a use for it. The things most often regarded as making for suc-
cess were ability to secure food, escape enemies, resist conditions of the
physical environment, and attract the opposite sex. The supposed uses
of spots and spines, colors and habits, to attain these ends were exceeded
in marvelousness only by the ingenuity of the naturalists in devising
them. In this period, what are probably the things of greatest impor-
tance, the physiological qualities, were relegated to minor roles. Com-
paratively little attention was given, for example, to resistance to disease
and exceptional fertility. Either of these should influence the number
of descendants more than most of the structural characters whose origins
were sought. Plasticity, or the capacity of either an individual or a
species to adjust itself to many types of environment, must be highly
important but was seldom considered then. These mistakes of the early
followers of Darwin led to a reaction against the natural selection theory
over the end of the last century, but the doctrine has emerged again with a
very different type of support, based on knowledge of mutations, the laws
of heredity, and the mathematics of chance.

It should be pointed out that adaptation is a quality of an organism
as a whole. While in some instances one feature of an organism stands
out as supremely important, so that other characters all yield to it in
determining success, in most living things fitness is composed of many
things. The success of an individual is a product of them all. An animal
has only one life to lose or preserve. If a frog perishes in the tadpole
stage because it has not the requisite power to withstand desiccation, it
cannot be preserved in the adult stage by any special agility in escaping
from enemies. Likewise, an animal gives rise to only one set of descend-
ants. If these are few because the animal's life is short, they cannot be

MODIFICATION OF SPECIES 365

numerous because it lays eggs rapidly. It is the totality of qualities,
some favorable, some unfavorable, that determines success, and it is on
this total product that selection acts.

Another point requiring emphasis is that, from the evolutionary
standpoint, a successful species or individual is one that leaves many
descendants. No quality is of any particular advantage to a species
unless it entails numerous posterity. Long life may seem to be an advan-
tage; but if it is merely a prolongation of activity after the reproductive
period is over, the species gains nothing by it. Rapid growth is a
good sign physiologically; but if it is expressed only in somatic tissue
and does not result in more germ cells or more embryos or does not in
some way enlarge families, it is useless as an element in the security of
the species. On the whole, also, it is the far distant progeny, rather
than the near generations, which are most important. A species so
constituted that in its present environment it succeeds moderately but
safely, but will in a much later environment thrive exceptionally, is
more influential upon evolution than is a species which is exceedingly
abundant now but dwindles in later time. These statements are, of
course, merely definitions ; it is not possible to apply them and say which
present species are going to be successful later.

Isolation. — Many biologists have always believed that an important
part of the divergence of species from one another is due to some sort of
isolation. Attention was early called to the supposed effects of geo-
graphic isolation, as of terrestrial organisms living on an island. The
species in such an isolated .region are mostly different from those of the
nearest other land areas. It is easy to see how, with different mutations
happening to arise among island forms, and probably with a different
sort of environment acting selectively upon them, there should be a
gradual divergence of the two groups. Taxonomists, moreover, have
generally held that the classification implies much more isolation than
geographic features provide. Since species have presumably split up
into varieties, which are free to cross with one another as far as they
meet in the same area, and since by further divergence varieties are
believed to advance to the rank of species, it might be supposed that
hybridization between species would continue indefinitely. Now hybridi-
zation should operate to remove the distinctions between species. How,
then, have arisen the generally rather sharp lines between species? For
there are relatively few intermediate individuals that might be regarded
as species hybrids.

The nature of the answer to this question is indicated by the discovery
that most species are not fully fertile with other species, even with those
most like them. While there are many exceptions, especially in plants,
they are in a small minority. Some species, exen within the same

366 PRINCIPLES OF ANIMAL BIOLOGY

genus, cannot be crossed; that is, they cannot or do not produce hybrid
offspring. Other species may be crossed, but the hybrid offspring are of
low fertihty or even completely sterile. Some species, if crossed, produce
offspring only of the female sex, and these, since they are not partheno-
genetic, cannot give rise to a new type.

What causes this sterility between species or in their hybrids is only
partially known. Difference in numbers of chromosomes is one obvious
cause, since there can be no complete pairing and meiotic division (page
195) of the chromosomes in a hybrid unless the chromosomes match.
Rearrangement of the genes in the chromosomes, such as turning one
segment of a chromosome end about, has a similar effect ; for in a hybrid
having two chromosomes alike in genes but differing in their arrangement
the pairing of the chromosomes is not normal. Many other chromosome
changes may occur. When an individual has two chromosomes of the
same sort in each pair, even if both are aberrant, it may behave normally;
and a group of such individuals may constitute a species. But when
they attempt to cross with individuals having chromosomes differently
constituted, abnormalities arise. Species are just as effectively isolated
by such chromosome changes as they would be if separated by a thousand
miles of ocean. Indeed, it is probable that separation of species from
one another is often rendered complete by such chromosome aberrations.

Evolution of Domesticated Races. — One of the arguments used by
Darwin in favor of natural selection is the fact that animals and plants
under control by man have experienced enormous modifications. A very
few years of selection by man have produced observable changes in
cultivated plants, and herd records show similar though less striking
changes in domesticated animals. The method is selection. The l:)reeder
preserves individuals most nearly approaching his ideals in the belief
that they will transmit the desirable qualities, and sometimes they do.
Darwin concluded that all that was necessary to accomplish a similar
result in wild species would be some selecting agency to replace the
breeder. That selecting agency could not be endowed with reason or
foresight, but highly adaptive modifications could, he believed, be pro-
duced by selective action of the environment itself. The method, as
conceived now, has already been outlined.

The written histories of domestic breeds do not go back far enough
to show the source from which any of our principal types of animals
came. Very early records show animals already in man's control, but
not much information about them is given. The sources of the various
animals have been conjectured from the qualities of breeds today and
the characteristics of wild species, but nothing is certain. The breeds of
poultry are believed to be descended from two mid sources, the jungle
fowl and the Malay fowl, both of the Orient. Egg-laying (juahties

MODIFICATION OF SPECIES 367

are thought to have come mostly from the former, while table birds
have inherited more from the latter. The various breeds of pigs are
all regarded as descendants of two wild boar species, one from Europe
and Africa, the other from India. Dogs probably have a somewhat
greater variety of wild ancestry, since their characteristics indicate con-
tributions from the timber wolf of Russia, the jackal of Europe, the
coj^ote of North America, and the dingo of Australia. Sea island cotton
is probably derived from two wild species, upland cotton from at least
three. Corn has an obvious relative in wild teosinte, but it is likely that
other species of grasses are also ancestral to it.

All this modification of breeds is evolution of a sort. That Darwin
was justified in concluding from it that selection has been likewise the
guiding factor in nature, some biologists have doubted. For domestic
breeds exhibit one important quality which is uncommon in natural
species; they are generally interfertile. The several kinds of dogs differ
from one another structurally quite as much as wild species do; but they
can be crossed, while wild species usually cannot be. It has often been
argued that if selection were responsible for species formation in nature,
these species should be as fertile with one another as are domestic varie-
ties. This criticism overlooks one difference between the selecting agents.
Man is vitally interested in maintaining interfertility of" his stocks, for
his method requires that he cross them. If sterility had arisen between
individuals, because of chromosome aberrations or for any other reason,
those individuals would have been rejected. In nature, such individuals
would have survived if lucky and if otherwise fit. By keeping his stocks
fertile among themselves, and by crossing them frequently, man has
speeded up the process of change far beyond any rate that might have
occurred naturally. Man's goals have also been very different from
those to which natural selection leads. But in no other important respect
have the two processes been unlike.

Evolution of Man. — The fossil evidence of man's origin was briefly
outlined in the preceding chapter. Whether there has been any impor-
tant evolution in man since he attained the capacities of Cro-Magnon
man, for example, is uncertain. There is no historical evidence of such
change. It is often said that man has made no progress in physical or
mental qualities in the last 10,000 years. This statement may be true,
but there is no way to know. It would be expected that there had been
some evolution during that time. Man is extraordinarily heterozygous,
and there is much hybridization between stocks. Presumably also muta-
tions arise in man. Unless all individuals survive, and all are equally
fertile, it is difficult to see how evolution can fail to occur. Whether
that evolution is progress upward or not is another matter.

Since man has guided the evolution of his flocks and herds, it would

3G8 PRINCIPLES OF ANIMAL BIOLOGY

seem entirely possible^ that he .shouhl guides liis ()^\'n. The .science of
eugenics aims at impr()\'ement of the race by such methods. Assuming
that man can judge correctly which of his qualities are most desirable
and that he can subordinate his emotions to his reason, there is no appar-
ent obstacle to progress as far as his present genes and future mutations
make possible. How great this progress may be it is futile to estimate,
for no one knows what new qualities may arise through interaction of
genes already in existence, and certainly no one can guess what genes
will mutate or how. Predictions regarding man's future evolution are
accordingly meaningless.

References

Darwin, Charles. The Origin of Species. D. Appleton-C'ontury ("ompany, Inc.

(Chap. XIV, recapituhition.)
Darwin, Charles. Variation of Animals and Phmts under Domestication. D.

Appleton-Century Company, Inc. (Introduction, a general outline of argument

for natural selection.)
DoBZHANSKY, T. Clcnetics and the Origin of Species. Columbia University Press.
Ford, E. B. Mendelism and Evolution. Dial Press (Lincoln MacVeagh), Inc.

(Chap. IV, evolution through the selection of mutations.)
Haldane, J. B. S. The Causes of Evolution. Harper & Brothers. (Chap. V, the

nature of adaptation.)
Lull, R. S. Organic Evolution. 1929 Ed. The Macmillan Company. (Chaps.

XIX-XXIV, various types of adaptations; book lacks modern viewpoint on

factors of evolution.)
Morgan, T. H. The Scientific Basis of Evolution. W. W. Norton <fe Company,

Inc. (Chap. V, adaptation and natural selection; Chap. VI, nuitation.)
Newman, H. H. Readings in Evolution, Genetics and Eugenics. University of

Chicago Press. (Chap. XVII, criticism of natural selection.)
Scott, W. B. The Theory of Involution. The Macmillan Company. (Chap. I\',

evidence from paleontology.)
Shull, a. F. Evolution. McGraw-Hill Book Company, Inc.

GLOSSARY

Pronunciations are indicated in tlie glossary as far as possible without the aid of

diacritical marks, but the following symbols have been necessary:
H = the German ch;
N = the French nasal n;

u = the French u, pronounced by shaping the lips for sounding long oo and the
tongue for long ee.

Abiogenesis {ab' i o jen' e sis). The origin of living things from nonliving matter;
same as spontaneous generation.

Absorption. The imbiding of a liquid by osmotic or capillar}^ action.

Acanthocephala (a kan' tho scj' a la). A group of parasitic wormlike animals some-
times included with the Nemathelminthes. For definition see Chap. 19.

Acetabulum {as' c tab' u lum). The socket on either side of the pelvic girdle for the
head of the femur.

Acetylcholine. A substance produced by nerve endings of the craniosacral sj'stem
and serving to stimulate certain organs, to inhibit others.

Acid. A substance which readily gives up hj'drogen ions, H+.

Actinomorphes (ak' tin o mor' Jeez). A group of animals in Blainville's early classifi-
cation; animals with radiating parts, such as the starfish.

Adaptation. Fitness for the environment. In a concrete sense, an adaptive struc-
ture, habit, or function.

Adductor. One of the large muscles attached to the valves of a mussel shell, or the
corresponding muscle of a glochidium ; also, one of nvmierous muscles in other
animals which draw a structure toward the median axis.

Adipose. Pertaining to fat.

Adrenal. One of two or more ductless glands in close relation with the kidneys in
most vertebrates.

Adrenalin {ad ren' al in). A hormone produced by the adrenal medulla.

Adsorption. The adherence of molecules of gases or dissolved substances to the
surfaces of other bodies.

Aeolosoma (e' o lo so' ma). A genus of worms, phylum Annelida, subclass Oligo-
chaeta.

Afferent. Leading toward; said of nerve fibers which conduct impulses toward the
central nervous system.

Aganides {ag' a ni' deez). A genus of extinct cephalopods with bent sutures of the
goniatite form.

Agkistrodon piscivorus (ag Ms' tro don pis siv' o rus). A species of snake, the cotton-
mouth moccasin.

Alanin {al' an in). A very simple amino acid.

Alecithal (a les' i thai). Containing httle or no yolk; said of certain eggs.

Altricial (al Irish' al). Hatched in a weak, helpless condition; said of certain birds.

Alveolar gland {al ve' o lev). A gland in which the lumen is inflated at certain points.

Alveolus (al ve' o Ivs). One of the microscopic air chambers to which the bronchioles
lead in lungs.

Amblycorypha {atji' bli kor' i fa). A genus of katydids.

369

370 PRINCIPLES OF ANIMAL BIOLOGY

Ambystoma {am. his' to ma). A genus of salamanders. A. maculatum, A. tigrinum,
common species.

Amino acid {am' i no). One of a number of organic acids containing the NHj radical
and having certain chemical properties. These acids enter into the composition
of all proteins and are produced by the splitting of proteins.

Amitosis {a' mi to' sis). Cell division not involving the formation of chromosomes
or a spindle.

Ammonite {am' mo nite). An extinct cephalopod having a coiled shell and com-
plicated foliaceous sutures; so called from the genus Ammonites.

Amoeba {a me' ha). A genus of one-celled animals, a protozoon of the class Rhizop-
oda.

Amphiaster {am' fi as' tcr). The figure produced by two asters and the connecting
spindle in a dividing cell.

Amphibia. A class of Vertebrata embracing the frogs, toads, salamanders, and some
others. For definition see Chap. 19.

Amphicoelous {am' fi see' lus). Having both ends of the centrum concave; said of
vertebrae.

Amphineura {arn' fi nu' ra). A class of Mollusca, the members of which are bilater-
ally synunetrical, have a shell of eight pieces or no shell at all, and many pairs of
gills. Chiton is an example.

Amphioxus. A primitive fishlike animal belonging to the subphylum Cephalochorda
of the Chordata.

Amphiuma {am' fi u' ma). A genus of salamanders.

Amylopsin (am' i lop' sin). A starch-digesting enzyme produced by the pancreas.

Anabolism. The aggregate of constructive processes comprised in metabolism.

Analogous {an al' o gus). Similar in function.

Anaphase {an' a faze). Any stage of cell division during the passage of the chromo-
somes from the middle to the ends of the spindle.

Anatomy. The science which treats of the structure of animals and plants as revealed
by dissection. It more commonly deals with the grosser features, but the finest
details of strvicture are not excluded.

Anaximander {an aks' i man der). A Greek physical philosopher and mathematician,
pupil of Thales, who lived about 611-547 b.c.

Animal pole. That part of an egg in which the protoplasm is concentrated (in eggs
with much yolk), and which in most animals produces the nervous system, sense
organs, etc. Other features may also characterize the animal pole.

Anisogamete {an' i so gam' eet). One of two unlike cells which fuse in reproduction.

Annelida {an neV i da). The phylum of animals comprising the segmented worms.
For definition see Chap. 19.

Anodonta. A genus of fresh-water mussels.

Antenna {an ten' na) {pi., antennae). One of a pair of jointed appendages project-
ing forward from the head of an insect or crustacean.

Anthophysa (a?i' tho fi' za). A genus of colonial flagellate Protozoa whose cells are
borne in radiating masses on a branching stalk.

Anthothrips niger {an' Iho Ihrips ni' jer). A species of insect of the order Thysan-
optera, commonly called thrips.

Anthozoa {an' tho zo' a). A class of X'oelenterata, comprising the sea anemones and
most of the corals. They have no mcdusoid form in the life cycle.

Anus {a' nus). The posterior oix-ning of the digestive tract.

Apoda {ap' o da). An order of Amphibia comprising the legless forms called
caecilians.

GLOSSARY , 371

Appendicular skeleton. The bones of the Hmbs and their attaching girdles in

vertebrates.
Arachnida (a rak' ni da). A class of Arthropoda comprising the spiders, scorpions,

and mites. For definition see Chap. 19.
Archaeozoic {ar' he o zo' ik). Of the earliest geological era; the oldest known rocks

are of this era.
Archenteron {ark en' ter on). The cavity within the endoderm of a gastrula. It

communicates with the exterior.
Archiannelida {ar' ki an nel' i da). A class of primitive marine worms (Annelida)

without setae.
Aristotle {ar' is tot' I). The most famous of the Greek naturalist philosophers, who

lived 384-322 b.c.
Armadillo. An armored mammal of the order Edentata, which includes also the

sloths and anteaters.
Arteriole. One of the smaller branches of an artery, leading to capillaries.
Artery. A blood vessel conducting blood from the heart.
Arthropoda {ar throp' o da). A phylum of animals, including the insects, Crustacea,

centipedes, etc. For definition see Chap. 19.
Articulate. To join; said of bones.

Artiomorphes {ar' ti o mor' feez). A group of animals in Blainville's early classifica-
tion; it comprised the animals whose bodies are bilaterally symmetrical.
Ascaris (as' ka ris). A genvis of roundworms (Nemathelminthes) parasitic in various

animals. A. megalocephala {meg' a lo scf a la), parasitic in the intestine of the

horse.
Ascorbic acid. Vitamin C, the preventive of scurvy.
Asexual. Not involving germ cells or fusion of nuclei; said of reproduction, or of an

individual employing such a mode of reproduction.
Assimilation. The conversion of digested foods and other raw materials into proto-
plasmic substances.
Association neuron. A nerve cell within the central nervous system, which helps to

connect an afferent with an efferent neuron.
Aster. The starlike figure composed of a centriole and the radiating lines about it;

or the centriole may be lacking.
Asteroidea {as' ie roi' de a). A class of Echinodermata comprising the starfishes.

For definition see Chap. 19.
Astral rays. The radiating lines surrounding a centriole in a dividing cell.
Asymmetry. Absence of any kind of symmetry.
Atoll {at' ol, or a toV). A ring- or horseshoe-shaped coral island.
Atom. A unit of a chemical element, composed of one or more protons and electrons,

arid usually neutrons.
Auditory. Pertaining to hearing; applied to the nerve of hearing and the sensory part

of the inner ear.
Auricle. The anterior chamber of the heart in fishes, and one of the two anterior

chambers in higher vertebrates.
Autonomic nervous system. A system of ganglia and nerve fibers, comprising two

mutually antagonistic groups, which center in specific parts of the central nervous

system and regulate the involuntary responses of the heart, blood vessels, diges-
tive tract, glands, and pupil of the eye.
Autosome {aw' to some). Any chromosome not closely associated with sex, that is,

not an X or Y chromosome.
Aves {a'veez). A class of vertebrate animals comprising the birds.

372 PRINCIPLES OF ANIMAL BIOLOGY

Avoiding reaction. The behavior by which Paramecium avoids obstacles of various
kinds. It consists of stopping, moving backward, turning through an angle away
from the oral groove, and starting forward in a new direction.

Axial skeleton. The skull, vertebral column, ribs, sternum, and hyoid apparatus of
vertebrates.

Axolotl {aks' 0 lot'I). The larval form of the tiger salamander Ambysioma tigrinum
which reproduces while in the larval state.

Axon {aks' one). A projection from a nerve cell which ordinarily conducts impulses
away from the body of the cell.

Backcross. A cross between an Fi individual and one of its parent types.
Balanoglossus. A genus of wormlike animals doubtfully included in the phylum

Chordata.
Bascanion. A genus of snakes, including the black snake or blue racer.
Base. A substance giving rise to free hydroxyl ions, 0H~, and thereby accepting

hydrogen ions, H"*".
B complex. A group of related vitamins found in meats, seed coats of cereals, yeast,

etc., including thiamin, riboflavin, niacin, and pyridoxin.
Biconcave. Having the centrum hollow both in front and behind; said of vertebrae.
Bidder's canal. A longitudinal tube near the median border of the kidney of certain

Amphibia; into it the collecting tubules open.
Bilateral symmetry. An arrangement of the parts of an object or animal body such

that the halves on opposite sides of a certain plane (only one in number) are

mirrored images of each other.
Bile. The fluid secreted by the liver in vertebrates.

Bile duct. The tube through which bile is di.scharged into the intestine.
Binomial. Consisting of two names or terms. Applied to the system of nomencla-
ture by which each species is given two names, one for the genus, the other for

the species.
Biogenetic law. The doctrine that animals in their embryonic development repeat

the evolutionary history of the race.
Biology. The science of life and of living things, whether plants or animals.
Bladder. A membranous sac in which urine is stored.

Blainville, Henri Marie Ducrotay de (5/aN veel'). French naturalist, 1777-1850.
Blastocoele (bias' to seel). The hollow interior of a blastula.
Blastopore. The opening through which the archenteron of an early embryo (gas-

trula) communicates with the exterior.
Blastostyle. In hydroids, a nontentaculate individual which produces medusae.
Blastula {bias' tu La). An early developmental stage, consisting of a hollow ball of

cells.
Blood platelet. One of the formed components of the blood, produced by fragmenta-
tion of certain cells.
Book gill. See book lung.
Book lung. A respiratory organ composed of flat sheets joined together like pages of

a book, found in spiders.
Bougainvillea ramosa {boo' gin viV le a). A species of marine hydroid.
Bowman's capsule. The expanded end of a kidney tubule, in which a glomerulus is

located.
Brachiopoda {brak' i op' o da). A group of marine animals of uncertain rank or

relationship. They have a bivalve shell, the two halves of which are unequal.

Sometimes placed in a phylum with the Bryozoa and Phoronidea.
Bract. One of the covering (protective?) members of a siphonophore colony.

GLOSSARY 373

Bradypus {brad' i pus). A genus of sloths.

Bronchiole (brong' ki ole). One of the smaller branches of the bronchi, air tubes in
the lungs.

Bronchus (brotig' kus) (pi., brouchij. One of the two main branches of the trachea
in many vertebrates.

Brown, Robert, British botanist, 1773-1858.

Bryozoa {bri' o zo' a). A group of marine and fresh-water animals of uncertain rank
and relationships, mostly colonial, bearing tentacles, and commonly known
as moss animals. Sometimes placed in a phylum with the Phoronidea and
Brachiopoda.

Buccal cavity {bitk' k'l). The most anterior division of the digestive tract of an earth-
worm. Also the mouth cavity of other animals.

Budding. Division of an organism into unequal parts in reproduction.

Buflfon, Comte de {bufo^'). French naturalist, 1707-1788.

Bufo. A genus of toads.

Byssus. A thread attached near the adductor muscle of a glochidium ; or a bunch of
threads attached to the foot of certain clams.

Caecilian. One of a group of legless, wormlike Amphibia of the order Apoda.

Caecum (see' kum). The blind pouch at the beginning of the large intestine.

Calcarea. A class of sponges (Porifera) whose skeletons are composed of spicules
of calcium carbonate.

Calciferol [kal sif er ole). Vitamin D, the preventive of rickets.

Calorie (kaV o ri). The quantity of heat required to raise the temperature of a kilo-
gram of water 1°C.; this is a large calorie, equal to 1000 small calories.

Cambrian (kam' bri an). Of the earliest Paleozoic time.

Camponotus. A genus of ants.

Canaliculus (kan' a lik' u lus). One of numerous minute channels radiating from
each lacuna in the matrix of bone, in which slender processes of the bone cells
are located.

Cancellate {kan^ set late). Composed of a number of chambers separated by parti-
tions; said of spongy bone.

Canine (ka' nine). A tooth located laterally to the incisors.

Capillary. One of numerous small vessels conveying blood through the tissues from
arteries to veins or from one vein to another.

Carapace. The hard bony covering of a turtle; also the chitinous or calcareous cover-
ing of the cephalothorax of a crayfish or lobster.

Carbohydrate. Any one of a class of organic substances, embracing the starches,
sugars, cellulose, etc., which are composed of carbon, hydrogen, and oxygen,
with the number of atoms of hydrogen and oxygen regularly in the ratio
of 2:1.

Carboniferous. The geological age to which the principal known coal beds belong;
succeeding the Devonian, it is a combination of Mississippian and Pennsylvanian.

Carchesium (kar ke' zi %im). A genus of colonial ciliated Protozoa, resembling
Vorticella.

Cardiac. Pertaining to or near the heart.

Carnivore. Technically, a mammal of the order Carnivora, including the cats, dogs,
and seals. In a popular sense, any flesh-eating animal.

Carnivorous. Flesh-eating.

Carotene. A yellow pigment found in carrots and many green or yellow vegetables;
a source of vitamin A.

Carpal. One of a number of bones in the wrist in vertebrates.

374 PRINCIPLES OF ANIMAL BIOLOGY

Carpometacarpus (kar' po met a kar' pus). A compound bone in the wing of a bird,

formed by the union of several of the metacarpals and carpals.
Cartilage. A flexible, somewhat translucent tissue composed of cells imbedded in a

matrix, found on the ends of bones at joints and in other situations.
Cast. A mass of rock formed within a cavity, as the cavity of a shell or of a mold

formerly occupied by an animal.
Catabolism {ka tab' o liz'm). The aggregate of destructive processes comprised in

metabolism.
Catalase. An enzyme which liberates oxygen from hydrogen peroxide.
Catalyst {kat' a list). A substance which brings about a reaction but is not consumed

in that reaction. It probably often participates in the reaction but is promptly

reformed.
Caudal. Belonging to the tail.

Caudata. Aii order of Amphibia comprising forms with tails (salamanders, newts).
Cell. A mass of protoplasm containing a nucleus or nuclear material.
Cell doctrine. See cell theory.

Cell inclusions. Nonliving objects enclosed in cells.

Cell membrane. A thin sheet either of differentiated protoplasm, or of some sub-
stance produced by protoplasm, surrounding a cell.
Cell theory. The theory that all animals and plants are composed of similar units

of structure called cells. The theory is now so well established as to be inore

properly called the cell doctrine, and other features concerning physiology,

development, etc., may be included in it.
Cellulose (sel' u lose). The substance, one of the carbohydrates, of which the cell

walls of plants are commonly composed.
Cell wall. A nonliving structure secreted by a cell around itself. It is commonly

composed of cellulose or chitin.
Cement. A binding material in the composition of teeth.
Cenozoic (se' no zo' ik). Pertaining to the most recent geological era.
Central nervous system. The brain and spinal cord.
Centriole. A minute body in the center of a centrosphere, and located at the end of

the spindle of many dividing cells.
Centrolecithal {sen' tro les' i thai). Having the yolk in a central position, surrounded

by protoplasm at the surface; said of eggs.
Centrosome {sen' tro some). A minute body often present in a cell, usually near the

nucleus in a centrosphere, related in some way to the process of cell division.

By many writers the name is used interchangeably with centriole.
Centrosphere {sen' tro sfeer). A differentiated portion of the cytosome of a cell,

usually near the nucleus, and typically containing a centrosome or centriole.
Centrum. The massive portion of a vertebra ventral to the neural canal in which

the spinal cord rests.
Cephalochorda {sef a lo kor' da). A subphylum of Chordata, comprising the species

of Amphioxus. For definition see Chap. 19.
Cephalopod {sef a lo pod). One of the group Cephalopoda, to which the cuttlefishes,

squids, and nautili belong.
Cephalopoda {scf a lop' o da). A class of Mollusca, comprising the octopi, squids,

cuttlefishes, and nautili, animals in which the foot is developed into a headlike

structiu-e with eyes and a circle of arms.
Cephalothorax {scf a lo tho' raks). A fused head and thorax, found in crayfishes and

their allies.
Ceratite {ser' a tite). An extinct cephalopod having a coiled shell and crooked

sutures; named from the genus Ceratites.

GLOSSARY 375

Ceratites (scr' a ti' teez). A genus of extinct cephalopods with crooked sutures; the

common name ceratite is derived from this genus.
Ceratium candelabrum (se ra' shi um can' de la' brum). A species of protozoon which

forms linear colonies.
Cerebellum. A division of the brain of vertebrates developed on the dorsal side

anterior to the medulla.
Cerebrum, The anterior division of the brain in vertebrates. In man it forms the

greater part of the brain but is smaller in other vertebrates.
Cervical. Pertaining to the neck.
Cestoda. A class of Platyhelminthes, comprising the tapeworms. For definition see

Chap. 19.
Chaetogaster (ke' to gas' ter). A genus of worms, phylum Annelida, subclass Oligo-

chaeta.
Chaetognatha {he tog' na tha). A group of marine animals of uncertain kinship, repre-
sented chiefly by the arrowworm Sagitta.
Chaetopoda {ke top' o da). A class of worms (Annelida) provided with setae, to which

the earthworm and sandworm belong.
Cheloniidae {kel' o ni' i dee). A family of turtles.
Chelydidae {ke lid' i dee). A family of turtles.
Chelydridae {ke lid' ri dee). A family of turtles.
Chitin {ki' tin). A horny substance forming the outside skeleton of insects and many

other animal parts.
Chiton [ki' ton). A genus of primitive mollusks, having a shell of several pieces.
Chloragogen cells {klo' ra go' jen). The cells of the outer layer of the intestine of the

earthworm.
Chlorophyll. The green substance in chloroplasts through whose agency photosyn-
thesis occurs.
Chloroplast. A green plastid.
Cholesterol {ko les' ter ol). A substance, one of the solid alcohols, found in many

animal tissues.
Chordata {kor da' ta). A phylum of animals including the vertebrates and a few

others. For definition see Chap. 19.
Chromatin {kro' ma tin). The deeply staining substance of the nucleus of a cell.
Chromoplast. One of several kinds of colored structures or organs found in many

plant and some animal cells.
Chromosome. One of the rodlike or rounded bodies into which the chromatin of a

nucleus is resolved at the time of cell division.
Chrysemys {kris' e juis). A genus of turtles.
Ciliate. A class of the protozoa, in which both young and adult stages are provided

with cilia.
Ciliophora {siV i of o ra). A subphylum of protozoa, members of which are covered

with a pellicle, have a fixed mouth, and are usually covered with cilia; example,

Paramecium.
Cilium. A minute hairhke motile structure occurring on the surface of certain cells.
Circular canal. A channel passing around a medusa near its margin.
Circulation. The movement of the blood through a system of vessels.
Circumpharyngeal connectives {ser' kum fa rin' je al) . Nerve cords in the earth-
worm connecting the brain with the ventral nerve cord; so called because they

pass around the anterior end of the pharynx.
Citellus tridecimlineatus {si tel' lus tri des' im lin' e a' tus). A species of ground

squirrel.
Class. A subdivision of a phylum ; a group of higher rank than the order.

370 PRINCIPLES OF ANIMAL BIOLOGY

Clavicle. The collar bone in man. One of the bones of the ventral part of tlie

pectoral girdle in vertebrates in general.
Cleavage. The division or segmentation of an egg.

Clitellum. A thickened glandular band encircling the body of an earthworm.
Cloaca {klo a' ka). A common passageway through which the intestine, kidneys,

and sexual organs discharge their products in some fishes, in amphibia, reptiles,

and birds, and in a few mammals.
Cnidoblast (?//' do blast). A cell containing a nematocyst or stinging thread in Hydra

or other Coelenterata.
Coagulation. Hardening; specifically, the clotting of the blood.
Cocoon. A case in which eggs are stored and in which frequently the larvae are

developed; also a silky covering around the pupa.
Codosiga iko' do si' go). A genus of flagellate Protozoa having a collar around the

flagellum.
Coelenterata {se Icn' ter a' ta). The phylum to which Hydra, the hydroids, jelly-
fishes, and siphonophores belong. For definition see Chap. 19.
Coelenteron (se len' ter on). A cavity in forms like Hydra which have only one body

cavity. It serves the digestive and circulatory functions and is therefore also

called the gastrovascular cavity. It has only one opening.
Coelom (see' lome). The true body cavity, a cavity within the mesoderm on the w^alls

of which the principal reproductive organs are located.
Coenosarc {se' no sark). The celhilar living part of a hydroid, as distinguished from

the j)erisarc.
Collared epithelium. Epithelivim each of whose cells bears a collar.
Collecting tubule. One of a number of tubes leading across the kidney of the frog

from Bidder's canal to the ureter.
Colloid (koV lord). A mixture in which particles invisible through a microscope but

greater in size than molecules are held in suspension in a liquid.
Colloidal (kol loi' dal). Contained in a liquid in aggregations submicroscopic in size

but greater than molecules.
Colony. A group of individuals of the same species organicall}- connected with each

other.
Coluber. A genus of snakes.
Columnar epithelium. Epithelium in which the cells have one dimension distinctly

greater than the others, that dimension being vertical to the surface covered by

the epithelium.
Comanchean {ko man' che an). Pertaining to Mesozoic time prior to the Cretaceous;

formerly called lower Cretaceous.
Common bile duct. The tube leading from the liver to the small intestine and serv-
ing to convey bile to the small intestine.
Compound. A substance produced by two or more elements in combination.
Compound gland. A branching gland.
Conemaugh iko' ne maw). A rock formation of eastern United States, belonging to

Permocarboniferous time.
Coniferous. Cone-bearing (as pine or cypress trees).
Conjugation. The meeting of two cells for exchaiige of nuclear material or (by exten-

.-^ioii of meaning) for complete fusion.
Connective tissue. .\ tissue composed of cells and ('(MtMiii other material protluccd

by the cells, which in its simple foiin binds organs and other tissues togeth(M'. in

a broader sense it includes such modified tissues as cartilage, bone, tendon, and

liganiejits.
Contractile tissue. .\ny tissue cajjable of acti\(' contraction: as muscle.

GLOSSARY 377

Contractile vacuole. A vacuole whose contents ;ir(' pon'odicallj- ejected to the outside

of the cell in which it is contained.
Copepod (ko' pe pod). Any one of a group of small Crustacea.
Copulation. The act of introducing spermatozoa into the body of the female.
Coracoid {kor' a koid). A bone of the ventral part of the pectoral girdle of vertebrate

animals; a distinct bone in the bony fishes, amphibia, reptiles, birds, and lowest

mammals, but fused with the scapula in t'he higher mammals.
Cornea. The transparent bulging membrane at the front of the eye.
Corpus luteum [pi., corpora lutea). A mass of cells invading the space in an ovary

from which an ovum has escaped.
Corpuscle. One of the cells of the blood.
Cortex. The layer of gray matter which covers the cerebrum and dips into its

folds. Also, an outer layer on various other organs, as the kidney or adrenal

body.
Cranial nerve. One of 10 or 12 pairs of nerves arising from the central nervous system

within the skull.
Craniosacral system. That part of the autonomic nervous system which centers in

the brain and posterior portion of the spinal cord. Each organ controlled by the

autonomic system is innervated once from it.
Cretaceous. Pertaining to the late Mesozoic time ; so named from the chalk deposits

characteristic of it.
Cretinism. A developmental deficiency caused by inadequacy of the hormone

thyroxin.
Crinoidea {kri noi' de a). A class of Echinodermata, including the feather stars and

sea lilies. For definition see Chap. 19.
Crocodilini (krok' o di W ni). An order of Reptilia comprising the alligators and

crocodiles and their allies.
Cro-Magnon {kro man yon'). A rather highly developed race of men preceding the

principal races of today. It dwelt, as far as known, in Western Europe.
Crop. In the earthworm, an enlargement of the digestive tract behind the esophagus

and in front of the gizzard. In birds, an enlargement of the esophagus for the

temporary storage of food.
Crustacea. A class of arthropods including the lobsters, crabs, water fleas, barnacles,

etc. For definition see Chap. 19.
Crystalline lens. A rounded, transparent, refractive body situated behind the pupil

of the eye.
Ctenophora {te nof o ra). A group of animals of uncertain rank including the comb

jellies and sea walnuts. For definition see Chap. 19.
Cubical epithelium. Epithelium in which the height and width of the cells are about

equal.
Cuvier, Georges {ku vyay'). French naturalist, founder of comparative anatomy,

1769-1832.
Cyclostomata {si' klo sto' ma ta). A class of Vertebrata having an eellike form, a

cartilaginous skeleton, no jaws, and no lateral fins; lampreys and hagfishes.
Cytology. The science which deals with the structure of cells.
Cytoplasm. The protoplasm of a cell exclusive of the nucleus.
Cytosome. The body of a cell as distinguished from its nucleus.

Darwin, Charles. Celebrated English naturalist, founder of the doctrine of natural
selection, author of several works on evolution. Lived 1809-1882.

Deciduous. Falling off at maturity or at the end of a season ; said of the leaves of trees
which fall periodically. Applied also to trees whose leaves fall periodically.

378 PRINCIPLES OF ANIMAL BIOLOGY

Deficiency disease. Any disease resulting from the lack or scarcity of some specific

substance in the diet.
Democritus {de mok' ri tus). Greek philosopher, known for his atomic theory, who

lived about 460-357 b.c.
Demospongiae {de' mo spun' ji ee). A class of Porifera (sponges). For definition see

Chap. 19.
Dendrite. A projection from a nerve cell which ordinarily conducts impulses toward

the body of the cell.
Dendritic. Treehke.
Denticulate. Finely notched or toothed.

Dentine. The dense bony substance composing the bulk of mammalian teeth.
Dermatozoa {der' ma to zo' a). A group of animals (literally, the skin or touch

animals) in Oken's early classification. It comprised the invertebrates.
Dero. A genus of worms, phylum Annelida, subclass Oligochaeta.
Determinate. Leading infallibly to a given end result from a given beginning; said

of development in which each cleavage cell produces a certain structure and

nothing else, regardless of experimental interference.
Devonian (de vo' ni an). Of middle Paleozoic age, next following the Silurian.
Dextrin. Any one of several related carbohydrates derived by hydrolysis from starch,

among them being erythodextrin, achroodextrin, and maltodextrin.
Diaphragm {di' afram). A partition; specifically, the partition between the thorax

and abdomen of a mammal.
Diffusion. The spreading of the molecules of one substance among those of another.
Digestion. The conversion of food into soluble substances which may diffuse through

protoplasm.
Dinosaur {di' no sawr). One of an order of extinct reptiles of Mesozoic time, mostly

of large size.
Dinotherium {di' no the' ri um). An extinct elephantlike animal from the Miocene.
Dioecious {di ee' shus). Having the male and female organs in separate individuals;

said of species.
Diogenes {di oj' e neez). Greek natural philosopher of the fifth century before Christ,

born at ApoUonia.
Diploblastic. Composed of two layers of cells.
Diploid {dip' laid). Double; specifically, the double number of chromosomes found

in the somatic cells, and in germ cells before meiosis, in bisexual animals. Cf.

haploid.
Dipnoi {dip' no i). A subclass of Pisces, fishes with an air bladder functioning as a

lung; the lungfishes.
Disaccharide {di sak' a ride). A carbohydrate whose molecule can be split into two

molecules of simple sugar (monosaccharide).
Dominant. Receiving expression when only one determining gene is present, and

in the presence of the gene for a contrasted recessive character; said of inherited

characters that are exhibited by heterozygotes.
Dorsal. Pertaining to the back; hence, usually, upper.
Dorsal aorta. A large artery formed, in fishes, by the union of vessels coming from

the gills, and passing backward in the dorsal region.
Dorsal root. The dorsal one of two roots by which a spinal nerve is connected with

the spinal cord. Its fibers are sensory in function.
Drosophila {dro sof i la). A genus of flies, of which the vinegar fly (D. melanogaster,

met' a no gas' ter) is a common species.
Duodenum {du' o de' num). The first of three divisions of the small intestine.
Dutrochet, Rene Joachim Henri {dii' tro' shay'). French physiologist, 1776-1847.

GLOSSARY 379

Dyad. A double body formed by the division of a tetrad into two parts. Its two
parts may be derived from the same chromosome or from different chromosomes.

Echinoderm (e ki' no derm). One of the Echinodermata.

Echinodermata (e ki' no der' ma to). The phylum of animals including the starfishes,

sea urchins, sea cucumbers, brittle stars, etc. For definition see Chap. 19.
Echinoidea (ek' i noi' de a). A class of Echinodermata, comprising the sea urchins,

sand dollars, and heart urchins. For definition see Chap. 19.
Echinorhynchus (e ki' no ring' kus). An Acanthocephalan worm.
Ecology {e koV oji). The branch of biology dealing with the relation of animals or

plants to their environment.
Ectoderm. The outer layer of cells of a gastrula, or the representative of this layer

in later stages.
Ectosarc. The outer layer of protoplasm in cells in which the outer and inner proto-
plasm differ distinctly in structure, as in Amoeba.
Edaphosaurus (e daf o saw' rus). An extinct lizardlike reptile bearing a spiny fin

on its back, from Permocarboniferous rocks of North America.
Effector. A structure specialized for some specific response; also the nerve carrying

impulses to such a structure.
Efferent. Leading from; said of nerve fibers which conduct impulses away from the

central nervous system.
Elasmobranchii (e laz' mo brang' ki i). A class of Vertebrata comprising the sharks,

skates, rays, torpedoes, and chimaeras. For definition see Chap. 19.
Electrolysis (e lek troV i sis). Decomposition of an ionized substance in solution by

passing an electric current through the solution.
Electrolyte. A substance which, because it ionizes, is in solution capable of conduct-
ing an electric current and of being decomposed by the current.
Electron. A unit of negative electric charge entering into the composition of atoms.
Element. One of the approximately 90 primary forms in which matter exists.
Elephas {eV e fas). A genus of animals including living elephants and their fossil

relatives of Pleistocene time.
Elodea {eV o de' a). A genus of aquatic plants.
Embryo. An undeveloped animal while still in the egg membrane or in the maternal

uterus.
Embryology. The science which deals with the development of the embryo, or

young stages, of animals or plants.
Embryonic. Pertaining to an embryo.
Empedocles {em ped' o kleez). Greek philosopher and poet, born in Sicily. Lived

about 490-430 B.C.
Emulsion. A mixture of two liquids or semiliquid substances, neither one soluble

in the other, the one being in the form of separate droplets suspended in the

other.
Emulsoid. A mixture consisting of a liquid in which are distributed particles of a

liquid or semisolid substance which are exceedingly minute yet larger than

molecules.
Emys (e' mis). A genus of turtles of the family Testudinidae.
Enamel. The very hard, polished calcareous substance forming the surface layer or

internal plates in the teeth of mammals.
Endocrine secretion. A secretion which must leave the gland by diffusion, not

through a duct.
Endoderm. The inner layer of cells of a gastrula, or the representative of this layer

in later stages.

380 PRINCIPLES OF ANIMAL BIOLOGY

Endosarc. The inner mass of protoplasm in cells in which the outer and inner proto-
plasm differ in structure.
Endoskeleton. A skeleton within the fleshy parts, as in vertebrate animals.
Energy. The capacity for performing work. It is kinetic when employed in pro-

dvicing motion or heat, potential when stored in chemical combination.
Enterokinase {en' ter o ki' nase). An enzyme produced in the small intestine and

serving to convert trypsinogen into trypsin.
Enteron. A digestive system open at both ends.
Enteropneusta {en' te rop nu' sta). A subphylum of Chordata, wormlike animals,

of which Balanoglossus and Cephalodiscus are representatives.
Entomology. The zoology of insects.
Enzyme {en' zime). An organic substance which brings about a chemical reaction

but is not consumed bj- that reaction. Probably it participates in the reaction

but is promptly restored.
Eocene (e' o seen). Of the earliest Cenozoic and Tertiary time.
Eohippus {e' o hip' pus). The earliest known ancestor of the horse, an extinct animal

of Eocene time.
Epidermis. The outer of the two principal layers of the skin. Also an outer layer of

cells in general.
Epistylis (rp' i sti' lis). A genus of colonial ciliated Protozoa, resembling Vorticella.

E. flavicans {flav' i kanz), one of the species.
Epithelial. Pertaining to an epithelium; as epithelial tissues or structures.
Epithelium. A layer of cells at the surface of a tissue or organ, or lining a cavity.
Epoch. One of the divisions of a period in the geological time scale.
Equation division. A division in which chromosomes are duplicated, producing two

equal cells ; said of one of the divisions of germ cells as contrasted with the other

or meiotic division.
Equatorial. In the plane of a great circle halfway between the poles; said of a cleavage

plane of an egg. Also, in a middle position in other objects.
Equatorial plate. The flattened group of chromosomes on the middle of the spindle

of a dividing cell. Also, the plane which they approximately occupy.
Equus {e' kwus). A genus of animals including the living horse and some of its fossil

relatives of Pliocene and Pleistocene time.
Era. One of the five major divisions of geological time.
Erepsin (e rep' sin). A proteolytic enzyme produced in the small intestine.
Ergosterol {er goa' ter ol). A substance, chemically a solid alcohol, obtained from

ergot, a fungus. On irradiation with ultraviolet it possesses strong antirachitic

properties.
Erosion. The wearing away (of rocks) through the action of water and other agencies.
Esophagus (e sof a gr(/.s). In the earthworm, a narrow passage leading from the

pharynx to the crop. In vertebrates, the passage between the pharj-nx and the

stomach.
Estrogen. A hormone or grouj) of liornioiics produced by the follicles of tlie human

ovary; several other names have been applied to it.
Euarctos (// ark' lose). A genus of bears, including the western black l)ear.
Eudorina elegans {u' do ri' nn). A species of colonial chlorophyll-bearing organism

whose cells are imbedded in a spherical jell^ylike mass.
Euglena ('/ gle' na). A genus of green flagellate Protozoa.
Eustachian tube {u sta' ki an). A passage between the pharynx and the tympanum

or middle ear.
Eutheria {n the' ri a). A subclass of Mammalia comprising tlic A-iviparous mammals.
Eutrephoceras in' t re fox' er ns). .\ genius of extinct ceiilialo])ods rcsemhiing Nautilus.

GLOSSARY 381

Evagination. The folding of a layer of cells outward from an enclosed cavity.
Evolution. The gradual or sudden change of animals or plants through successive

generations.
Evolve. To change; to undergo evolution.

Excretion. The elimination of waste substances. Also a substance excreted.
Exhalent. Breathing out; applied to one of the siphons of a clam or mussel.
Exophthalmic goiter {cks' of thai' mik). A disease resulting from overactivity of the

thyroid gland, and having as one of its symptoms the bulging of the ej-es.
Exoskeleton. A skeleton on the outside of the body, as in the arthropods.
Extensor. A muscle whose contraction straightens a joint.
External respiration. The passage of oxygen from the surrounding air or water to

the blood.

Fi (ef unin'). An individual or generation of individuals resulting from the crossing

of two unlike parents. An abbreviation of the words first filial.
Fi {ef too'). An individual or generation of individuals resulting from the mating of

two Fi individuals as parents. An abbreviation of the words second filial.
Family. A taxonomic group of higher rank than the genus but below the order.
Fat. A compound of glycerol and one or more fatty acids.
Fatty acid. An organic acid entering into the composition of fats.
Fauna. Collectively, the animals of a given region or of a given period of time.
Femur. The single bone of the thigh in vertebrates above the fishes.
Feral. Plscaped from domestication. Also, sometimes, wild.
Fertilization. The union of an egg with a spermatozoon, a process requisite, in the

higher animals, to the development of the egg.
Fetus (/(' tus). The embryo of a mammal while still in the uterus.
Fibril. One of the longitudinal contractile threads of a voluntary muscle cell.
Fibrin. A substance in blood which forms much of the clot on escape from the

vessels.
Fibrinogen (fi. brin' ojen). A soluble protein carried in blood plasma, from which

the insoluble fibrin of a clot is formed.
Fibula (fib' u la). The outer one of two bones in the lower leg of vertebrates except

the fishes.
Fission. The division of an organism into two approximately equal parts ; or, simply,

division.
Flagellate iflaj' el late). Possessing flagella. As a noun, a flagellate protozoon.
Flagellum [pi., flagella). A long whiplike motile projection from a cell.
Flame cell. A cell having a hollow interior in which a bunch of vibratile cilia are

located, forming part of a protonephridium.
Flexor. A muscle whose contraction bends a joint.
Fluke. Any one of several species of trematode worms.
Follicle. A layer of cells surrounding some object, as an ovum in an ovary.
Foot. The basal muscular part of a clam or snail, variously modified in many other

mollusks. Also the terminal part of a leg, the base of Hydra, etc.
Foraminal aperture (/o ram' i nal). In a sponge gemmule, the opening in the shell

through which the young sponge escapes when it begins to develop.
Formation. The rocks belonging to one of the minor divisions (lower than epoch)

of geological time.
Fossil. The remains, or other indication, of a prehistoric animal or plant.
Fructose. A simple sugar (monosaccharide) found in fruit juices, and one of the

products (with glucose) obtained by breaking down sucrose (cane sugar); same

as levulose.

382 PRINCIPLES OF ANIMAL BIOLOGY

Funiculus {fu nik' u lus). A muscular strand which draws the body of a br^yozoan

into a U shape.
Furcula (fur' ku la). The wishbone of a bird, consisting of the fused clavicles of the

two sides.

Galactose (ga lak' lose). A simple sugar (monosaccharide) obtainable by breaking
down lactose, or lipids of the brain.

Galen. Famous Greek physician and anatomist, born about a.d. 130. His writings
were long the highest authority in medical science.

Gall bladder. A pouch in which the bile secreted by the liver is stored.

Gamete (gajn' eet). A germ cell, or other cell which fuses with a second cell in repro-
duction.

Gametogenesis (ga me' to jen' e sis). The ripening of germ cells.

Ganglion (gang' gli on) (pi., ganglia). A mass of nerve cell bodies, usually forming
a thickening in the course of a nerve.

Gastric. Pertaining to the stomach.

Gastrocnemius (gas' trok ne' mi us). A large muscle in the calf of the leg in verte-
brate animals.

Gastropoda [gas Irop' o da). A class of Mollusca including snails and slugs, mollusks
whose bilateral symmetry is often obscured by a coiled bodj' and shell.

Gastrotheca. A genus of frogs.

Gastrovascular. Serving the functions of digestion and circulation.

Gastrovascular cavity. See coelenteron.

Gastrula {gas' tru la). An early developmental stage, formed from a blastula, com-
monly by the invagination of the vegetative pole of the latter. The gastrula
consists of two layers of cells (ectoderm and endoderm) surrounding a cavity
which communicates with the exterior.

Gastrulation. The formation of a two-layered embryo from a blastula, by invagina-
tion of the vegetative pole, by delamination, or otherwise.

Gemmule. A group of cells forming a reproductive body in fresh-water sponges.

Gene. Something in a germ cell or other cell which is responsible for a hereditary
characteristic.

Generic (je ner' ic). Pertaining to a genus.

Genetics. The science of heredity, variation, sex determination, and related phe-
nomena.

Genital. Concerned with reproduction.

Genus (je' nus) (pL, genera, jen' e ra). A group of species having so many structural
features alike that they must be regarded as having sprung from common ances-
try; a group of lower rank than the family.

Geoflfroy Saint-Hilaire, Etienne (zho frwa' sa^ ie lair'). French naturalist, 1772-
1844.

Gephyrea (je fi' re a). A group of wormlike aninuils of doubtful rank and relation-
ships. Thej^ have sometimes been referred to the Annelida.

Germ cell. A cell capable of reproduction, or of sharing in reproduction, as con-
trasted with the somatic or body cells which are sterile; or, more strictly, a repro-
ductive cell that has undergone, or will undergo, or whose cell descendants will
. undergo, oogenesis or s{)orniatogenesis before partici])ating in reproduction.

Germ layers. The embryonic layers, ectoderm, endoderm, and mesoderm; so called
because, in a sense, each one contains the "germ" of certain adult structures.

Gill. A structure having a surface enlarged usually by branching or folding, which
serves a respiratory fimction.

Gill bar. The tissue between two gill clefts.

GLOSSARY 383

Gill cleft. One of several openings from the pharynx to the sides of the neck or head

of a vertebrate embryo or adult; derived from a gill pouch. Also called gill slit.
Gill plate. The thickened patch of ectoderm in an embryo from which gills are

developed.
Gill pouch. One of several evaginations from the sides of the anterior part of the

digestive tract in the embryos of vertebrate animals. In some animals they

break open to the outside, becoming gill clefts.
Gizzard. In the earthworm, a thick-walled portion of the alimentary tract behind

the crop. In birds, the posterior muscular division of the stomach.
Gland. An organ whose function is the secretion of something to be used in, or ejected

from, the body.
Glaucomys. A genus of flying squirrels.

Glenoid fossa. The cavity into which the head of the humerus fits.
Glochidium (glo kid' i um). The young stage of a mussel, which becomes temporarily

attached to fishes.
Glomerulus {glo mer' u lus). A coil of blood capillaries at the end of each tubule in

the kidney of a vertebrate animal.
Glossozoa {glo' so zo' a). A group of animals (literally, tongue animals) in Oken's

early classification. It comprised the fishes.
Glottis. A slitlike opening in the larynx at the anterior end of the trachea in

vertebrates.
Glucose. Grape sugar.
Glycerol {glis' er ole). An alcohol entering into the composition of fats and having

the empirical formula C3H!i(OH)3. Same as glycerin.
Glycine {glV seen). The simplest of the amino acids.
Glycogen {gW ko jen) . Animal starch ; a common form of stored carbohydrate food

in animal tissues.
Gmelin, Johann {gma' lin). German botanist, 1748-1804.
Golgi apparatus {gole' jee). A structure of various shapes, often a network, and of

unknown function, found in many cells, usually near the nucleus.
Gomphoceran {gam fas' er an) . Any extinct cephalopod resembling Gomphoceras,

whose shell was short and wide.
Gonad {go' nad). An organ in which germ cells (either oogonia or spermatogonia)

are produced or lodged.
Gonangium {go nan' ji um). A reproductive structure in a hydroid, consisting of a

blastostyle, its attached medusa buds, and a gonotheca.
Goniatite {go' ni a tile). An extinct cephalopod having a coiled shell and bent or

angular sutures; so named from the genus Goniatites {go' ni a ti' teez).
Gonionemus {go' ni o ne' mu-s). A genus of jellyfishes.
Gonium. A genus of colonial flagellate organisms in which the cells are arranged in a

flattened plate.
Gonophore. One of the reproductive members of a siphonophore colony. Also, a

medusa or medusalike member of a hydroid.
Gonotheca. A transparent sheath forming the outer part of a gonangium of a

hydroid.
Gopherus {go' fer us). A genus of turtles.
Grantia. A genus of calcareous marine sponges.

Graptemys {grap' te 7nis). A genus of turtles of the familj' Testudinidae.
Graptolite {grap' to lite). An extinct group of colonial hydroid coelenterates of

Cambrian and Devonian time.
Graze. To eat grass or similar herbage.
Gregaloid. Loosely adhering in an irregular mass.

384 PRINCIPLES OF ANIMAL BIOLOGY

Grew, Nehemiah. English botanist, 1641-1712.

Gullet. A tube leading from the posterior end of the oral groove in Paramecium to
the interior of the cell.

Habitat. The kind of place in which an organism lives.

Halogen. One of a family of chemical elements including chlorine, iodine, bromine,

and fluorine.
Halysites {hal' i si' teez). A genus of extinct chain corals.
Haploid. Single; referring to the reduced number of chromosomes in the mature

germ cells of bisexual animals. Cf. diploid.
Harvey, William. English physician and physiologist, 1578-1657.
Hemoglobin. A reddish protein contained in the red blood cells.
Heparin {hep' a rin). A substance extracted from liver, carbohydrate in nature with

amino and phosphate components, used to prevent clotting of blood.
Hepatic portal system. The portal system leading from the stomach, intestine,

pancreas, and spleen to the liver.
Herbivorous. Plant-eating.
Heredity. The occurrence, in organisms, of any qualities due to the nature of the

protoplasm of which they are made.
Hermaphrodite. An organism possessing both male and female organs. Also

(adjective), possessing the organs of both sexes.
Herpetology [her pe toV oji). The zoology of reptiles and Amphibia.
Heteromita lens. A species of flagellate protozoon.

Heteromorphes {het' er o mor' Jeez). A group of animals in Blainville's early classifi-
cation; animals of irregular form, mainly sponges and Protozoa.
Heterozygote. An organism to which its two parents have contributed unlike genes

with respect to some inherited character, and which in turn produces two kinds

of germ cells with respect to that character.
Heterozygous. Of the nature of a heterozygote.
Hexactinellida {heks ak ti neV It da). A class of Porifera (sponges) whose spicules

are composed of silica.
Hipparion {hip pa' ri on). An extinct horselike animal of ^Miocene and Pliocene

time in North America and Europe.
Hippocampus {hip' po kam' pus). A genus of fishes of bizarre form resembling in

part tlie head of a horse.
Hippocrates {hip pok' ra teez). Greek physician. Father of Medicine, 460-359 (?) B.r.
Hirudinea {hi' ru din' e a). A class of Annelida comprising the leeches. For defini-
tion see ( 'hap. 19.
Holothurioidea {ho' lo thu' ri oi' de a). A class of Echinodermata, comprising the

sea cucumbers. For definition see Chap. 19.
Homo. The genus of animals comprising man.
Homolecithal ihn' mo Ics' i thai). Having little yolk, nearly evenly distributed; said

of ("figs.

Homologous iho inol' o giis). Originating in the same wa\' in evolution; said of organs

or structures.
Homology. Siniilaiit y of origin in evolution; api)lied to organs that arise in the

same way.
Homozygote. An organism whose two parents contributed to it similar genes for

some inherited character, and whose germ cells are therefore all alike with respect

to that character.
Homozygous {ho' ino zi' gi(s). Of the Jialure of a homozygote.
Hooke, Robert. English natural philosopher and mathematician, 1635-1703.

GLOSSARY 385

Hooker, Sir Joseph Dalton. English botanist, 1817-1911.

Hormone [hor' mone). A secreted substance which stimulates activity in an

organ.
Humerus. The single bone of the upper arm in Amphibia and the higher vertebrates.
Huxley, Thomas. English biologist and lecturer, 1825-1895.

Hybrid. The offspring of two parents unlike one another in some heritable character.
Hybridization. The process of crossing animals having unlike heritable characters,

thereby producing animals possessing genes for the traits of both parents.
Hydra. A small tubular fresh-water animal with tentacles and stinging organs,

belonging to the phylum C'oelenterata.
Hydranth. A hydralike tentacle-bearing member of a hydroid colony.
Hydroid. A colonial coelenterate, the individuals of which resemble the hydra in

certain respects.
Hydrorhiza {hi' dro ri' za). That part of a hydroid colony which is attached to the

substratum.
Hydrotheca. The tough transparent sheath surrounding a hydranth of a hydroid;

an expansion of the perisarc.
Hydroxy! ion [hi droks' il). The radical 0H~ produced in solutions of bases.
Hydrozoa. A class of Coelenterata, including Hydra, the hydroids, jelly fishes, and

some corals. For definition see Chap. 19.
Hyla. A genus of tree frogs.
Hyoid. A bone or group of bones or cartilages located at the base of the tongue or

in a corresponding situation.
Hypodermis. An external layer of cells beneath a secreted cuticle, as in the earth-
worm and in insects and Crustacea.
Hypohippus. An extinct horselike animal of Miocene time in North America.
Hypostome. A projection from the center of the circle of tentacles in a hydra or

one of the hydroids. It is perforated by the mouth.

Ileum {iV e um). The last and usually longest of three divisions of the small intestine.
Ilium {iV i um) {pL, ilia). The dorsal bone of the pelvic girdle in Amphibia and the

higher vertebrates.
Incisor. One of the front cutting teeth of a mammal.

Incubation. The warming of eggs, resulting in acceleration of their development.
Indeterminate. Not leading necessarily to a given end result from a given beginning:

said of development in which cleavage cells may, if disturbed, produce some

structure other than that which they would have produced without interference.
Ingestion. The taking in of food.
Inhalent. Breathing in; applied to one of the siphons of clams and mussels, to

certain pores of sponges, and to other passages.
Innominate bone. The single bone formed by the fusion of three bones of the pelvic

girdle in man. This name is not usually applied in the case of other vertebrates,

though fusion of the bones of the girdle commonly occurs.
Insecta. A class of Arthropoda having one pair of antennae, three pairs of legs, and

tracheae for respiration; the insects.
Insectivore. Technically, a mammal of the order Insectivora, including the moles,

shrews, and hedgehogs. In a popular sense, any insect-eating animal.
Insertion. The place of attachment of the distal end of a muscle or its tendon.
Insulin (m' su lin). An endocrine secretion produced by the islands of Langerhans

in the pancreas; its function is control of sugar metabolism.
Internal respiration. The transfer of oxygen from the blood to the surrounding cells;

true respiration.

386 PRINCIPLES OF ANIMAL BIOLOGY

Interphase. The stage in the cycle of a cell in which it is not dividing: the so-called
"resting" stage.

Interstitial cells. The cells of a testis which lie between the seminiferous tubules.

Intracellular. Within a cell.

Invagination. The folding of a layer of cells inward into a cavity.

Ion. An atom or group of atoms bearing an electric charge.

Ischium {is' ki um) {pi., ischia). The posterior of two ventrally located bones of the
pelvic girdle of vertebrate animals above "the fishes.

Islands of Langerhans. Groups of isolated cells in the pancreas, which produce
insulin.

Isogamete {i' so gam' eet). One of two gametes of equal size which fuse in reproduc-
tion.

Isogamy {i sog' a mi). Fusion of like gametes in reproduction.

Isolation. In evolution, the inability of species to cross with one another.

Jejunum. The second of three divisions of the small intestine.

Jensen, Zacharias. Dutch inventor of the microscope about 1591.

Jugular vein. A large vein returning blood from the head.

Jurassic. Of middle Mesozoic age; named from rocks in the Jura mountains.

Karyokinesis {ka' ri o ki ne' sis). Same as mitosis.

Kidney. The chief organ for the excretion of nitrogenous wastes in most vertebrates.

Also an excretory organ in certain other animals.
Kinostemidae {ki' no stcr' ni dee). A family of turtles.

Labial palp. One of two pairs of flattened organs beside the mouth of mussels.

Labyrinth. The inner ear of vertebrates.

Lacteal (lak' tc al). One of the minute vessels leading from the intestine to the

• lymph ducts.
Lactose. Milk sugar, a disaccharide found in the milk of mammals.
Lacuna. A space in the matrix of bone which contains in life a bone cell.
Lamarck, de, Jean Baptiste, etc. Celebrated French naturalist and proponent of

evolution, 1744-1829.
Lamella. A layer.

Lamprey. An eellike animal of the class Cyclostomata.
Lampsilis {lamp' si lis). A genus of fresh-water mussels.
Large intestine. The enlarged portion of the digestive tract following the small

intestine.
Larva. A free-living developmental stage of an animal in which certain adult

organs are still lacking or in which organs are present that are lacking in the

adult.
Lateral fold. One of two ridges of skin along the back of certain species of frogs,

extending lengthwise at either side.
Lecithin {Ics' i thin). One of a number of lipoid substances common in egg yolk,

nerve tissue, and other kinds of cells.
Leeuwenhoek, van, Anton {lay' ven hook). Dutch naturalist and microscopist, 1632-

1723.
Leiolopisma {li' o lo piz' ma). A genus of skinks (lizards).
Lemming. A rodent of the family Muridae, to which the rats, mice, and muskrats

belong.
Lepas anatifera {le' pas an' a tif cr a). A species of barnacle (subclass Cirripedia

of the Crustacea). The goose barnacle.

GLOSSARY 387

Leptinotarsa {lep' tin o tar' sa). A genus of leaf-eating beetles to which the common

potato beetle belongs.
Leptodactylus (lep' to dak' ti lus). A genus of frogs.
Lernaeopoda {ler' ne op' o da). A copepod (Crustacea) parasitic on the gills of certain

fishes.
Leucocyte. A white blood cell.
Linear. Arranged in a line or row.

Lingula. A genus of brachiopods, a group of uncertain relationships.
Linkage. The occurrence of the genes for two or more hereditary characters in the

same pair of chromosomes.
Linnaeus, Carolus {lin ne' us). Swedish botanist and naturalist, author of the

binomial system of nomenclature and an artificial classification of animals and

plants, 1707-1778.
Lipid {Up' id). Any organic compound of the class including true fats, and the

compounds of fats with other substances such as phosphates and sugars.
Liver. A gland which secretes bile and other substances.

Loxoceras {loks os' er as). A genus of extinct cephalopods of the orthocone type.
Lumbar. Pertaining to the loins, the region of the back posterior to the ribs.
Lumbricus terrestris {lum bri' kus). A species of earthworm.
Lung. A respiratory organ in the vertebrates.
Lycopod (W ko pod). A club moss.
Lyell, Sir Charles. British geologist, 1797-1875.

Lymph. A clear fluid containing colorless cells found in lymph vessels. It is essen-
tially blood without its red cells and with much less of the proteins.
Lymphatic system. A system of vessels conveying lymph in vertebrates.
Lymph capillaries. The smaller vessels of the lymphatic system.
Lymph node. A connective tissue enlargement in a lymph vessel, which removes solid

materials from the lymph and produces one kind of white blood cell.

Macronucleus. The large nucleus in a cell or organism having two nuclei of unequal

size.
Macrosiphum sanborni (niak' ro si' fum). A species of insect, one of the plant lice,

living on chrysanthemum plants.
Malpighi, Marcello {mahl pee' gee). Italian anatomist, founder of microscopic

anatomy, 1628-1694.
Malthus, Thomas Robert. English political economist, author (1803) of Essay on.

Population, who hved 1766-1834.
Maltose. Malt sugar.
Mammal. One of the Mammalia.
Mammalia. A class of vertebrates having hairy bodies, producing young within the

body of the mother, and nourishing the young after birth with milk secreted by

the mother.
Mammalogy. The zoology of mammals.
Mammoth. An elephantlike animal of prehistoric times.

Mantle. A sheet of tissue, typically quite thin, which secretes the shell in mollusks.
Mantle fibers. Fibers lying about the periphery of the spindle of a dividing cell

and extending from the centrioles to the chromosomes.
Manubrium {ma nu' bri xim). A projection from the center of the subumbrella of a

medusa, corresponding to the hypostome of a hydranth, and bearing the mouth

at its end.
Marginal bone. One of a ring of bones around the margin of the carapace of a

turtle.

388 PRINCIPLES OF ANIMAL BIOLOGY

Marsupial. A mammal having a pouch in which the young are carried (for example,

the opossum and the kangaroo). As an adjective, possessing a pouch; as the

marsupial frog.
Mastigophora {mas' ti gof o ra). A class of protozoa, characterized by flagella.
Mastodon {mas' to don). An extinct genus of elephantlike animals of Pliocene and

Pleistocene time.
Maternal. Pertaining to or derived from the mother.
Matrix. The noncellular material in which the cells of bone and cartilage are

imbedded.
Matter. That of which any physical object is composed; anything which can occupy

space.
Maturation. A process which germ cells undergo before they become functional,

consisting of one or two cell divisions; if of two divisions, the chromosomes remain

imduplicated in one of them.
Medulla oblongata. The enlargement of the anterior end of the spinal cord in verte-
brates, commonly regarded as the posterior division of the brain.
Medusa (pZ., medusae). A jellyfish; the free-swimming member of many hydroid

species.
Megapode {meg' a pode). A bird of the family Megapodiidae, the mound birds and

jungle fowls.
Meiosis {mi o' sis). Separation of maternal from paternal chromosomes in oogenesis

or spermatogenesis. Also, according to some, the two divisions of oogenesis or

spermatogenesis.
Mendel, Gregor. Austrian monk and plant breeder, founder of modern movement

in genetics, and author of Mendel's law of heredity. Lived 1822-1884.
Mendel's law. The law that genes for inherited characters separate from one

another and recombine in various ways in the germ cells.
Meridional {me rid' i o nal). Passing through the animal and vegetative poles; said

of certain cleavage planes of an egg.
Merychippus imer' i kip' pus). An extinct horselike animal of Miocene time.
Mesenchyme {mes' en kime). A tissue composed of cells of irregular shape, loosely

joined in a network with extensive meshes.
Mesentery {mes' en ter i). A double sheet of tissue, continuous with the peritoneum,

which supports an organ (such as the intestine) from the body wall.
Mesoderm. A layer of cells between the ectoderm and endoderm.
Mesohippus. An extinct animal of Oligocene time, ancestral to the horse.
Mesozoa. A group of degenerate animals of uncertain rank and relationship, onco

regarded as intermediate between protozoa and metazoa; hence the name.
Mesozoic. Pertaining to the geological era between the Paleozoic and Cenozoic,

or the age of reptiles.
Metabolism {me tab' o liz'tn). The sum total of the chemical processes going on in

protoplasm.
Metacarpal. One of the bones forming the body of tli(^ liand or forefoot in vertolnates.
Metagenesis. The occurrence of two or more forms of individual in the same sj)ecies,

one of which reproduces sexually and one or more asexually.
Metamere. See somite.
Metamerism {me tam' er iz'ni). The condition of being divided into a number of

similar metameres or somites.
Metamorphosis {met' a mor' fo sis). The transformation of a larva into an adult.
Metaphase. That stage of cell division in which the chromosomes are in the equa-
torial plate. The chromosomes are typically (hii)li('ate(l in this stage.
Metatarsal. One of the bones forming the body of tlu' iliindi loot of vertebrates.

GLOSSARY 389

Metatheria. A subclass of mammals including the marsupials or pouched mammals.
Metazoon. An animal composed of many cells. Although the term contrasts an

animal with the protozoa, it is not a name of any taxonomic group of animals.
Miastor. A genus of fhes; the larvae are often paedogenetic.
Microgromia socialis. A species of protozoon which forms a gregaloid colony in a

gelatinous supporting substance.
Micronucleus. The smaller nucleus in a cell or organism having two nuclei of unequal

size.
Micropyle. A small hole in the shell of an egg through which the spermatozoon enters

in fertilization.
Microstomum {mi Aros' to mum). A genus of rhabdocoele flatworms.
Microtus. A genus of field mice.

Miocene. Belonging to middle Tertiary time; succeeding the Oligocene.
Miohippus {mi' o hip' pus). An extinct horselike animal from the Oligocene.
Mirbel, Charles Francois {mccr hcV). French botanist, 1776-1854.
Mississippian. The hfth period of the Paleozoic era, following the Devonian and

preceding the Pennsylvanian.
Mitochondria {mi' to kon' dri a). Objects of unknown function and of various shapes

(threadlike, rod-shaped, or granular) found in the cytosome of many cells.
Mitosis. Cell division involving the formation of chromosomes, spindle fibers, etc.

Also called karyokinesis.
Moeritherium {me' ri the' ri um). An extinct animal from the Eocene of Egypt,

probably an early ancestor of the elephants.
Molar. One of the grinding teeth of a mammal, back of the incisors and canines.
Mold. A cavity in a rock representing the form of an animal or plant or other

object whose remains formerly occupied the cavity.
Molecule. Usually a group of atoms behaving as a imit of the substance which they

compose. It is the smallest particle which possesses the chemical nature of the

substance.
Mollusca. The phylum of animals including the clams, snails, cuttlefishes, etc.

For definition see Chap. 19.
Mollusk. One of the ^Nlollusca.
Monoecious {mo nee' shus). Having the organs of both sexes in the same individual

which is thus a hermaphrodite; said of species.
Monosaccharide {mon' o .sak' a ride). A simple sugar; one which cannot be broken

down into simpler sugars.
Monotreme. One of the Monotremata (Prototheria) ; an egg-laying mammal having

a cloaca.
Morgan, T. H. Leading American geneticist, 1866-1945.

Morphology. The branch of biolog,y which deals with the structure of living things.
Motor. Pertaining to movement; applied to a neuron which conveys impulses result-
ing in muscular movement, glandular action, and the like.
Motor root. The ventral one of two roots by which a spinal nerve is connected with

the spinal cord. So called because its fibers have a motor function.
Motor unit. The group of muscle cells innervated by a single nerve fiber.
Muellerian duct {mul le' ri an). A tube formed in the embryo of most vertebrate

animals, becoming the oviduct in the female and degenerating (with few excep-
tions) in the male.
Muscle. An aggregation of contractile cells.
Mustelus mustelus {mus te' lus). A species of shark.
Mutation. A heritable modification arising in an organism.
Myelin. A fatty substance forming a sheath around many nerve fibers.

390 PRINCIPLES OF ANIMAL BIOLOGY

Myofibril (mi' o ji' hril). One of the contractile threads in a voluntary muscle cell.
Myosin {mi' o sin). A common protein in muscle.

Myotome. One of the segments into which certain muscles are divided.
Myriapoda (meer' i ap' o da). A class of Arthropoda having tracheae, one pair of

antennae, and many unspecialized legs; centipedes and millipedes.
Mysis. A genus of Crustacea having all appendages two-branched; also a larval

stage of other Crustacea in which all appendages are two-branched.
Myxedema (miks' e de' ma). A disease whose symptoms are puffy tissues, reduced

metabolism, and mental depression, caused by deficient thyroid action.

Nacre. The pearly substance secreted by mollusks upon their shell or other objects.

Nais. A genus of fresh-water worms, phylum Annelida, subclass Oligochaeta.

Nasal pit. The ectodermal depression in an embryo which forms much of the nostril.

Natrix. A genus of snakes. N. rhombifera, N. sipedon, two of the species.

Natural history. A descriptive account of things in nature, particularly animals and
plants, though the term is sometimes used to include minerals, rocks, climate,
etc.

Natural selection. The survival of the fittest individuals or the fittest species in a
variable population.

Nauplius {naiv' pli ris). The earliest larval stage of shrimps, barnacles, and some
other Crustacea.

Nautiloid. One of the extinct cephalopods resembling, Nautilus.

Nautilus. An animal belonging to the Cephalopoda, living in a coiled shell divided
into chambers.

Neanderthal man (no ahn' der tahl). A primitive man whose remains have been
found in various places in Europe.

Necator. The genus of roundworms to which the hookworm belongs.

Nectocalyx {nek' to ka' liks) {pi., nectocalyces, nek' to ka' li seez). One of the swim-
ming members of a siphonophore colony.

Necturus. A genus of salamanders; the mud puppy.

Nemathelminthes {nem' a thel min' theez). The phylum of roundworms and their
allies. For definition see Chap. 19.

Nematocyst {nem' a to sisi). One of the stinging bodies of Hydra and other coelen-
terates.

Nematode {nem' a tode). Any roundworm of the class Nematoda, phylum Nemathel-
minthes.

Nematomorpha {nem' a to mar' fa). A group of wormlike animals of uncertain
affinities. They have usually been doubtfully included in the Nemathelminthes.
For definition see Chap. 19.

Nemertinea {nem' er tin' e a). A group of wormlike animnls of uncertain relation-
ships. They are regarded by some as a class of Platyhelminthes. For definition
see Chap. 19.

Nephridiopore {ne frid' i o pore). The external opening of a nephridium.

Nephridium {ne frid' i um). An excretory organ of certain invertebrate animals
(worms, mollusks, etc.), approximately corresponding in function to the kidney
of vertebrates. It is commonly a coiled tube, as in the earthworm.

Nephrostome (ne/' ro stome). The opening at the inner end of a nephridium as in
the earthworm. Also an opening (originally like that in the earthworm) con-
necting the coelom with the blood vessels of the kidney in certain Amphibia.

Nereis, A genus of marine worms, phylum Annelida.

Nerve. A bundle of axons or dendrites of nerve cells or of both axons and dendrites.

GLOSSARY 391

Nervous tissue. Tissue capable of transmitting impulses; as the tissues of the brain,

spinal cord, and nervos.
Net knot. A thickened portion of the chromatin of a cell nucleus.
Neural arch. That part of a vertebra above the centrum and neural canal.
Neural canal. The opening in a vertebra through which the spinal cord extends.
Neural crest. One of a number of groups of cells at the sides of the brain and spinal

cord of an embryo, from which ganglia and nerves are developed.
Neural fold. One of the ridges of ectoderm forming the earliest development of the

nervous system.
Neural groove. An elongated depression between the neural folds of an embryo.
Neural spine. A projection rising from the middle of the neural arch of a vertebra.
Neural tube. The tube formed beneath the ectoderm by the union of the neural

folds along their crests.
Neurilemma. The thin cellular covering of a nerve fiber.
Neuromuscular. Combining the functions of contraction and the transmission of

impulses.
Neuron {nu' rone). A nerve cell.
Neutron. A particle, like a proton but without electric charge, entering into the

composition of the nuclei of most atoms.
Niacin (nz' a sin). The antipellagra vitamin, part of the B complex.
Nicotinic acid {nik' o tin' ik). Same as niacin.

Nomenclature {no' men kla' lure). A system of naming; terminology.
Nostril. One of the external openings of the nasal chamber.
Notochord (no' to kord). A cylindrical rod of cells beneath the nervous system of an

embryo (adult of some animals). It is the forerunner of the spinal column of the

vertebrate animals.
Notophthalmus {no' tof thai' mus). A genus of salamanders.
Nuchal plate {nu' kal). In turtles, the median plate of the carapace at the anterior

end.
Nuclear membrane. A thin film of protoplasm surrounding the nucleus of a cell.
Nuclear sap. The liquid forming the bulk of the nucleus of a cell.
Nucleolus {nu kle' o lus). A small, usually rounded body found in the nuclei of many

cells, which is of different chemical composition from the rest of the nucleus.

Its function is uncertain.
Nucleus. A highly refractive, deeply staining body of specialized protoplasm found

within nearly all cells.
Nudibranch {nu' di brank). One of a group of marine moUusks.

Obelia. A genus of hydroids, or colonial hydralike animals of the phylum Coelen-

terata.
Octopus. A genus of devilfishes (moUusks) having eight arms.

Oenothera (e' no the' ra). A genus of plants to which the evening primroses belong.
Oken, Lorenz. German naturalist and transcendentalist philosopher, 1779-1851.
Olfactory. Pertaining to the sense of smell.

Oligocene. Of early Tertiary time, between Eocene and Miocene.
Oligochaeta {ol' i go ke' to). A subclass of Chaetopoda (Annelida), including chiefly

terrestrial and fresh-water worms with relatively few setae which do not rest on

fleshy outgrowths but project directly from the body wall. The earthworm is an

example.
Onychophora {on' i kof o ra). A class of primitive Arthropoda having tracheae and

one pair of antennae. Peripatus is an example.

392 PRINCIPLES OF ANIMAL BIOLOGY

Oocyte (o' o site). A female germ cell subsequent to the initiation of oogenesis and
prior to the second division. An oocyte is designated primary during the growth
period and prior to the first division; secondary after the first division and before
the second.

Oogenesis (o' o jen' e sis). The series of changes imdergone by female germ cells
in preparation for reproduction.

Oogonium (o' o go' ni urn). One of the early germ cells of a female animal, prior to
the beginning of oogenesis.

Operculum (o per' ku lum). A fold of skin covering the gills and gill clefts in some
amphibian larvae; also a similar covering of the gills in fishes.

Ophiuroidea {o' fi u roi' de a) . A class of Echinodermata, comprising the brittle
stars. For definition see Chap. 19.

Ophthalmozoa {of thai' mo zo' a). A group of animals (literally, eye animals) in
Oken's early classification. The term was synonymous with Thricozoa and
comprised the mammals.

Opisthocoelous (o pis tho see' lus). Having the centrum concave behind and convex
in front; said of vertebrae.

Optic nerve. The nerve of sight.

Oral groove. The spiral depression on one side of Paramecium, leading to the gullet.

Order. A group of animals forming a subdivision of a class, and being composed
of one or more families.

Ordovician. Of early Paleozoic time, succeeding the Cambrian.

Organ. A group of cells or tissues performing some specific function.

Organism. A living being, whether plant or animal.

Organismal theory. The theory that parts of an organism owe their nature to the
nature of the whole.

Organizer. A substance which controls some feature of embryonic development.

Origin. The place of attachment of the proximal end of a muscle.

Ornithology. The zoology of birds.

Orohippus. One of the earliest known ancestors of the horse, an animal of Eocene
time in North America.

Orthoceras (or thos' er as). A genus of extinct cephalopods of the orthocone type.

Orthocone. One of the early cephalopods that lived in a straight shell.

Osculum. An opening through which water leaves the passages of a sponge.

Osmosis. The diffusion of a substance through a membrane in response to unequal
distribution of that substance on opposite sides of the membrane.

Osmotic pressure. Objectively defined, the pressure that will just prevent diffusion
of a solvent into a solution when the two are separated by a semipermeable mem-
brane. Also, the pressure due to the greater kinetic energy of the molecules of a
solvent on one side of a semipermeable membrane than on the other, due to the
presence of a solute on the side exhibiting the lesser kinetic energy of the solvent.

Otozoa. .\ group of animals (literally ear animals) in Oken's early classification.
It comprised the birds.

Ovary. The organ in which the immature germ cells of a female animal are
lodged.

Oviduct. A tube through which the eggs of a female animal leave the body.

Oviparity. The condition of being oviparous.

Oviparous (o vip' a rus). Egg-laying.

Oviposition. The laying of eggs.

Ovisac. A chamber for the storage of eggs. b(>ing in some cases a lateral pouch of
the oviduct, as in the earthworm.

Ovoviviparity {o' vo viv' i par' i ti). The condition of being ovoviviparous.

GLOSSARY 393

Ovoviviparous (o' vo vi vip' a rus). Producing young from eggs that are retained in
the oviduct during their development, but without attachment to the oviduct,
and wholly from nutrition stored in the egg.

Ovum. An egg; a relatively large passive cell which, in preparation for reproduction,
has undergone one or two unequal divisions.

Oxidation. The chemical process of combining with oxygen.

Paedogenesis (pr' do jen' c sis). Sexual maturity in an animal otherwise immature;
the capability possessed by some species of reproducing wliile in the larval con-
dition.

Palaeomastodon (pa' le o mas' to don). A genus of extinct animals belonging to the
clej)hant ancestry, found in the Oligocene of Egypt and India.

Paleontology. The science wliich treats of prehistoric life on the earth, now repre-
sented by fossils.

Paleozoic (pa' le o zo' ik). Pertaining to the geological era prior to the Mesozoic,
when amphibia, fishes, and the higher shell-bearing invertebrates were the domi-
nant forms.

Pancreas. A gland which secretes a fluid containing several digestive enzymes and
discharges into the intestine.

Pandorina. A genus of colonial flagellate organisms in which the cells are held in
a spheroidal jellylike mass. P. monim (trio' rum) is one of the species.

Paramecium. A genus of ciliated protozoa.

Parasite. An animal which lives in or on another species of animal (its host), at the
expense of the latter.

Parasitism. The condition of being a parasite.

Parathyroid. One of a pair (or two pairs) of small ductless glands closely associated
with the thyroid.

Parietal bone. One of a pair of bones on the posterior upper part of the skull of
vertebrate animals.

Parthenogenesis (par' the no jen' e sis). The development of an egg without fertiU-
zation.

Parthenogonidia (par' the no go nid' i a). The asexually reproducing cells of Volvox.

Paternal. Pertaining to or derived from the father.

Pectoral girdle. A group of connected bones serving to attach the bones of the fore-
limbs of vertebrate animals to the rest of the skeleton.

Peking man. An early hvmian type somewhat resembling the Piltdown and Neander-
thal types, found in China.

Pelecypoda (pel' e sip' o da). A class of Mollusca having bivalve shells and a bilobed
mantle; the clams and mussels.

Pellagra (pel la' gra). A condition of malnutrition accompanied by eruption of the
skin.

Pellicle. A thin skin or film on the surface of a cell.

Pelvic girdle. A group of bones serving to join the bones of the hind limbs of verte-
brate animals to the rest of the skeleton.

Penis. The copulatory organ in the male of many animals.

Pennsylvanian. The sixth period of the Paleozoic era, following the Mississippian
and preceding the Permian.

Pentadactyl (pen' ta dak' til). Having five fingers or toes.

Pepsin. An enzyme of the stomach of vertebrate animals, whose function is digestion
of many kinds of protein. '

Pepsinogen (pep sin' n jen). An inactive substance from which the enzyme pepsin is
derived.

394 PRINCIPLES OF ANIMAL BIOLOGY

Period. One of the divisions of an era in the geological time scale.

Periodic. Occurring at rather regular intervals; said of migration which depends

on the seasons or on the age of the migrating animals.
Peripheral nervous system. In general, the nerves, collectively ; the nervous system

aside from the brain and spinal cord or other central cord.
Perisarc. The tough sheath surrovmding the stalk and branches of a hydroid.
Peritoneum {per' i to ne' urn). A sheet of cells covering the viscera and lining the

body cavity in many animals.
Permeable. Permitting the passage of both liquids and dissolved substances.
Permian. Belonging to the close of the Paleozoic era.
Petrifaction. The piecemeal substitution of mineral matter for the body substance

of dead animals or plants.
Phalanx {fa' lanks) {pL, phalanges, fa Ian' jeez). Any one of the bones of the fingers

or toes in vertebrate animals.
Pharynx {far' inks). In an earthworm, the thick-walled portion of the digestive

tract just posterior to the buccal pouch and in front of the esophagus. In verte-
brates, the portion of the digestive tract at the back of the mouth, into which the

gill clefts open.
Phoronidea {fo' ro nid' e a). A small group of marine animals, of which Phoronis is

the onl}'^ genus, of uncertain relationship to other animals. Sometimes placed in

a phylum with the Bryozoa and Brachiopoda.
Photosynthesis. The construction of glucose from carbon dioxide and water by

the energy of sunlight in the presence of chlorophyll.
Phylum. One of a dozen or more major groups into which the animal kingdom is

divided; in general, the largest group of which it can be said that the members

are related.
Physalia. A very complex colonial coelenterate, one of the siphonophores.
Physiology. The branch of biology which deals with the functions of animals and

plants, and the processes going on in them.
Piltdown. A locality in Sussex, England, near wliich primitive human fossils have

been found.
Pineal body {jnn' e al). A structure on the dorsal side of the brain in vertebrate

animals. Because of its similarity, in development, to the embryonic stages of an

eye, it is often called the pineal eye and is believed by many to be a vestigial

sense organ.
Pisces {pis' seez). A class of vertebrate animals including the fishes. For definition

see Chap. 19.
Pithecanthropus {pilh' e kan thro' pus). An extinct apelike and manlike animal

believed to be closely related to the early ancestry of man.
Pituitary {pi tu' i ta ri). A glandular organ beneath the brain composed in part of

nervous tissue.
Placenta. A vascular tissue dovetailing into the wall of the uteiiis on one side and

(connected with the umbilical cord on the other, thus forming an intimate nutritive

connection betweeen the embryo and the mother in viviparous animals.
Planaria. A genus of flatworms, phyhmi Platyhelminthes.
Planula {plan' u la). A ciliated larva consisting of a solid elliposidal mass of cells,

dov(^loped from the fertilized egg of a medusa or similar organism.
Plasma. The liquid part of the blood.
Plasmodroma. A subphylum of protozoa devoid of cilia.
Plastid. One of several kinds of protoplasmic bodies in cells, like the green bodies

in {)lant cells, which are centers of chemical activity.
Plastron. The flat plate of bones on the ventral side of a turtle.

GLOSSARY 395

Platelet, See blood platelet.

Plato. A Greek pliilosopher, pupil of Socrates and teacher of Aristotle. Lived

about 427-347 b.c.
Platyhelminthes {plat' i hel min' theez). The phylum of flatworms. For definition

see Chap. 19.
Pleistocene {plise' to seen). Belonging to the epoch following Pliocene in the Tertiary.
Pleodorina (pie' o do ri' no). A minute spherical organism composed of cells of two

sizes embedded in a jellylike substance. P. californica {kaU i jor' ni ka), with

numerous small cells; P. illinoisensis {iV li noi zen' sis), with four small cells.
Plethodon {pleth' o don). A genus of salamanders.
Pliny {plin' i). Roman naturahst (a.d. 23-79) and author of works on natural

history.
Pliocene. Pertaining to the epoch of Tertiary time following Miocene.
Pliohippus. An extinct animal of Pliocene time, closely resemljling the horse.
Plumatella. A group of fresh-water bryozoa.
Pneumatophore {nu' ma to fore'). A capsule enclosing gas, serving to float a siphono-

phore colony.
Podophrya (po dof ri a). A protozoon belonging to the class Suctoria.
Polar body. A small nonfunctional cell, one of the two cells produced Ijy each divi-
sion in oogenesis.
Polarity. The condition of exhibiting or possessing different properties in different

parts; the condition of a cell in wliich the protoplasm is unlike in different parts

of the cell.
Pole. A differentiated part or extremity, as of an egg, or of the spindle of a dividing

cell.
Poloc3rte. The small cell produced at either of the divisions of oocytes in oogenesis.

Same as polar body.
Polychaeta (poU i ke' la). A subclass of Chaetopoda (Annehda) including those

marine j^^orms having numerous setae borne on fleshy outgrowths at the sides of

the somites. Nereis, the sandworm, is an example.
Polymorphic. Having a variety of forms.

Polymorphism. The existence of two or more kinds of individuals within a species.
Polyneuritis. A disease due to vitamin Bi (thiamin) deficiency.
Polyorchis (poZ' i or' kis). A genus of jelly fishes.
Polyp. One of the feeding individuals of a hydroid or coral colony or simple related

form.
Polysaccharide {poV i sak' a ride). A carbohydrate whose molecule can be split into

many molecules of simple sugar (monosaccharide).
Porcellio. A genus of sowbugs (Isopoda, Crustacea).

Porifera {po rif er a). The phylum of animals comprising the sponges. For defini-
tion see Chap. 19.
Portal system. A blood vessel or group of vessels beginning and ending in capillaries.
Postcava. A large vein leading to the heart from behind or below.
Poterioceras {po te' ri os' er as). A genus of extinct cephalopods of the gomphoceran

type.
F*recipitin {pre sip' i tin). A substance which produces a precipitate when two blood

sera are mixed.
Precocial. Able to run about as soon as hatched; said of certain birds.
Precoracoid. A ventrally situated bone or cartilage of the pectoral girdle in Amphibia

and some reptiles.
Primary. For application to spermatocytes, see spermatocyte. For application to

oocytes, see oocyte.

396 PRINCIPLES OF ANIMAL BIOLOGY

Primate. A mammal of the order including man and the apelike animals.

Priority, law of. The rule that the name first given a species along with a description

is the one that shall be accepted when different names have been applied to the

same species.
Proboscis {pro bos' sis). The trunk of an elephant, consisting of the elongated nose

and upper lip. Also a fleshy projection of other sorts.
Procoelous {piv see' lus). Having the anterior end of the centrum concave, the

posterior end convex; said of vertebrae.
Procyon (pro' si on). The genus of Carnivora to which the raccoon belongs.
Proglottis (pi., proglottides, pro glot' ti deez). One of the individuals in a chain of a

tapeworm.
Prophase. Any early stage of mitotic cell division, prior to the equatorial plate.
Prosecretin (pro' se kre' tin). A substance in the walls of the small intestine from

which secretin is produced.
Prostomium. A rounded projection overhanging the mouth of an earthworm.
Protein. One of many organic substances, compounds of amino acids, which therefore

contain carbon, hydrogen, nitrogen, and oxygen and often other elements. The

molecules are large and very complex. Lean meat and egg albumen contain

quantities of proteins.
Proterospongia haeckeli {pro' ter o spun' ji a hek' el i). A species of protozoon which

forms gregaloid colonies.
Proterozoic. Belonging to the era preceding the Paleozoic.
Proteus. A genus of salamanders.
Prothrombase. A substance from which an enzyme of clotting (of blood) is

produced.
Proton, A particle bearing a positive electric charge entering into the composition

of the nuclei of atoms.
Protonephridium. A primitive excretory organ consisting of flame cells and con-
necting tubes. «
Protoplasm. The living matter of which animals and plants are essentially composed.
Prototheria. A subclass of Mammalia, including the egg-laying mammals such as

the duckbill Ornithorhynchus and the spiny anteater Ecliidna.
Protozoa. One-celled animals. The phylum comprising the one-celled animals,

including colonial forms in which the cells of the colony are, at least potentially,

all alike.
Protozoology. The zoology of the protozoa.

Pseudemys (su' de mis). A genus of turtles of the family Testudinidae.
Pseudopodium (su' do po' di U7n) (pi., pseudopodia). A blunt Hngerlike projection

thrust out by Amoeba and other rhizopods.
Ptarmigan (tar' mi gan). Any one of several species of birds related to the grouse

and partridges.
Ptyalin (ti' a lin). The starch-digesting enzyme of the saliva.
Pubis (pL, pubes, pu' hc.ez). The anterior one of two ventrally ])la('(Hl bones in the

pelvic girdle of vertebrate animals abovi; th(> fishes.
Pulmonary circulation. The circiUation of the blood through the lungs, as distin-
guished I'roiii that, through the body in general (syst(>niic).
Pulsating vacuole. Same as contractile vacuole.

Pupa. A quiescent stage in lh(> development of an insect, just before the adult con-
dition is reached.
Purkinje, Jan Evangelista (poor keen' ya). Bolicniian physiologist in the University

of Prague, 1 787-1 Xf)9.
Pus. A collection of white cells at a wound or i)lace of infection.

GLOSSARY 397

Pylorus (pi W rus). The opening from the stomach to the intestine.
Pyridoxin. "N^itamin Be, the antidermatitis vitamin.

Quadrate. One of the bones of the skull; in birds and reptiles and bony fishes, the
bone from wliich the lower jaw is suspended.

Race. A group of individuals having certain characteristics in common because of

common ancestry.
Radial canal. One of four tubes extending from the middle to the margin of a medusa.
Radial symmetry. An arrangement of the parts of an object or organism such that

it is capable of being divided into halves that are mirrored images of one another,

by two or more planes all of which pass through a common longitudianl axis.
Radiating canal. One of a series of collecting channels surrounding the pulsating

^■acuoles of Paramecium and similar protozoa.
Radical. A group of atoms behaving as a unit in reactions.
Radio-ulna. The fused radius and ulna of frogs and toads.
Radius. The bone of the lower arm located on the thumb side in Ampliibia and

the higher vertebrates.
Rana. A genus of frogs. R. cantabrigensis, the wood frog; R. catesbeiana, bidl

frog; R. clamitans, green frog; R. palustris, pickerel frog; R. pipiens, leopard frog.
Range. The area occupied by a species or larger taxonomic group of animals or

plants.
Ray, John. English naturaHst, 1627-1705.
Reaction. Any response of an animal to a stimulus; also any chemical change taking

place in a substance, particularly a change involving some other substance as well.
Recapitulation theory. See biogenetic law.
Receptor. An organ which is especially sensitive to certain stimuh and serves to

initiate impulses in nerve fibers.
Recessive. Not being produced when the gene for a contrasted dominant character is

also present; said of inherited characters that are not developed in heterozygotes.
Reciprocal. Involving the same types of individuals, but with the sexes reversed;

said of two crosses, in one of wliich the female possesses the same characteristic

as does the male in the other cross.
Rectum. The terminal portion of the large intestine in the higher vertebrates. In

vertebrates with a cloaca, the term is sometimes applied to the part of the large

intestine anterior to the cloaca.
Reduction. Cell division in which chromosomes are not duplicated but merely

separated from one another after having previously come together in pairs, as

occurs in one of the two divisions in the ripening of most germ cells.
Reflex. Same as reflex action.
Reflex action. An action performed as a result of an impulse which passes over a

reflex arc.
Reflex arc. A group of two or more neurons, one of them afferent, another efferent,

so connected as to be able to transmit impulses resulting in reflex actions.
Regeneration. The production of lost parts by organisms.
Relict. A li\ing remnant of an otherwise extinct group of organisms.
Renal corpuscle. One of numerous bodies in the kidneys of vertebrate animals, each

composed of the expanded end of a kidney tubule (Bowman's capsule) and an

enclosed knot of blood capillaries (glomerulus).
Rennin. An enzyme i)roduced by the gastric glands and having the property of

coagulating milk.
Reproduction. Tlie formation of new individuals among organisms.

398 PRINCIPLES OF ANIMAL BIOLOGY

Reptilia. A class of vertebrate animals including the snakes, lizards, crocodiles,

turtles, and some others. For definition see Chap. 19.
Respiration. The gaseous metabolism of protoplasm, including elimination of carbon

dioxide, usually absorption of oxygen, and, according to some physiologists, the

chemical reactions which consume oxygen or produce carbon dioxide.
Retina {ret' i no). The sensitive inner layer of the eye of vertebrates and some other

animals.
Retractile. Capable of being withdrawn.
Rhabdocoele {rab' do seel). A flatworm (Platyhelminthes) of the order Rhabdo-

coelida.
Rhinozoa (ri' no zo' a). A group of animals (literally, nose animals) in Oken's early

classification. It comprised the reptiles.
Rhizopoda (n zop' o da). A class of Protozoa having a form that is changeable

through the production of pseudopodia; example. Amoeba.
Rhynchocephalia (ring' ko se fa' U a) . An order of Reptiha, comprising only one

living form, Sphenodon, of the New Zealand region.
Riboflavin (ri' bo fla' vin). Vitamin B2, the preventive of scaliness of skin, tendency

to cataract, etc.
Rodent. A gnawing mammal, a member of the order Rodentia (rats, mice, squirrels,

etc.).
Rodentia. The order of mammals including the rodents (rats, mice, squirrels, etc.).
Rotifera (ro tif er a). A group of animals (the rotifers) usually regarded as a separate

phylum, but of uncertain position in the animal kingdom. For definition see

Chap. 19.

Sacculina (sak' ku W na). A degenerate crustacean, related to the barnacles, para-
sitic on crabs.

Sacral. Pertaining to the sacrum, the region between the hips.

Sacrum. A group of vertebrae, more or less fused, in the region between the hips.

Sagitta (so jit' ta). A marine animal of small size, sometimes called the arrowworm,
but not a true worm at all. Its relationship to other animals is obscure.

Salientia (sa' li en' shi a). An order of Amphibia including the tailless forms (frogs,
toads).

Saliva. The fluid secreted by the salivary glands about the mouth.

Salivary. Pertaining to saliva, the fluid secreted into the mouth in mammals.

Salt. A compound, other than an acid or base, which in solution produces ions.

Sarcolemma. The membrane surrotmding a striated muscle cell.

Sarcoplasm. The protoplasm of a striated muscle cell, as distinguished from the
enclosed myofibrils.

Sargasso sea. A great eddy in an ocean, enclosing masses of seaweeds; with capital-
ized initials the name may be limited to the eddy of the North Atlantic Ocean.

Sargassum. A genus of seaweeds.

Scaphiopus (ska fi' 0 pus). A genus of spadefoot toads.

Scaphites (.skaf i' ieez). A genus of extinct cepliMlopods of the ammonitic form.

Scaphopoda (skaf op' 0 da). A class of MoUusca in which the shell and mantle are
tubular, as in Dentalium.

Scapula. The shoulder blade; a bone of the pectoral girdle, located on or near the
dorsal side of the body.

Schleiden, Matthias (shli' den). German botanist, 1804-1881.

Schultze, Max (shooW sa). German biologist and anatomist, 1825-1874.

Schwann, Theodor (shvahn). German physiologist and anatomist, 1810-1882.

GLOSSARY 399

Sciuridae (si u' ri dee). The family of rodents including the flying squirrels, squirrels,

marmots, and chipmunks.
Sciurinae (si' u ri' nee). The subfamily of Sciuridae comprising the marmots,

squirrels, and cliipmunks.
Sciuromorpha (si' u ro mor' fa). The suborder of rodents comprising the squirrellike

forms.
Sciurus (si u' rus). The genus including the arboreal squirrels.
Scolex. The enlarged attaching organ from which are budded off the proglottides

of a tapeworm chain.
Scyphozoa (si' fo zo' a). A class of Coelenterata, jellyfishes of large size which have

no hydroid form in the Ufa cycle.
Secondary. For application to spermatocytes, see spermatocyte. For application

to oocytes, see oocyte.
Secretin (se kre' tin). A substance produced in the small intestine and serving to

stimulate secretion by the pancreas and liver.
Secretion. The act of producing from the blood or other fluids or substances in the

protoplasm some new material to be used in metabolism or otherwise. Also the

new substance thus formed.
Segmentation. Same as cleavage.
Self-fertilize. To fertilize the eggs of an individual by spermatozoa of the same

individual.
Semicircular canal. One of several curved tubes forming part of the inner division

of the ear in vertebrates.
Seminal receptacle. An organ in a female animal for the reception and storage of

spermatozoa from the male.
Seminal vesicle. One of several bodies closely connected with the testes in the earth-
worm, in which a large part of the development of the spermatozoa takes place.

Also, an enlargement in the vas deferens or similar duct in which spermatozoa

may be stored in various animals.
Semipermeable membrane. A membrane which allows some substances to pass

through it, but retards or excludes others.
Sensory. Pertaining to sensation; applied to a neuron which transmits an impulse

resulting in sensation, or by extension to any other receiving neuron whether

concerned with sensation or not.
Septum. A partition.

Series. The rocks, collectively, which belong to a geological epoch.
Serum. The yellowish fluid which escapes from a blood clot; it is approximately

the plasma without any fibrinogen.
Sessile. Attached directly, as distinguished from stalked. Sometimes, also,

attached, as distinguished from free-Uving.
Seta (pi. setae, se' tee). A spine; specifically, one of the spines projecting from the

somites of an earthworm and used for locomotion.
Sex-linked. Associated with sex; said of hereditary characters the genes for which

are in the X chromosomes associated with sex.
Sexual. Involving the production of true germ cells, or the fusion of nuclei; said

of reproduction, or of an individual employing such a mode of reproduction.
Shoal. A shallow place in a body of water; also a sandbank or bar which makes the

water shallow.
Silurian. Of middle Paleozoic time, between Ordovician and Devonian.
Sinus node. A mass of rather undifferentiated tissue in the right auricle of the heart

which receives stimuU and initiates the heart beat.

400 PRINCIPLES OF ANIMAL BIOLOGY

Siphon. A passageway for currents of water; as the clefts l)etweeii the lialvcs of

the mantle of mussels where the edges do not meet, or the tulie on tlie Aeiitnil

side of a scjuid or cuttlefish.
Siphonophora {si' fo nof o ra). An order of Hydrozoa (C'oelenterata), the members

of wliich form highly polymorphic colonies. Example, Physalia, the Portuguese

man-of-war.
Siphonops {si' fo nops). A genus of caecilians (Apoda, Amphibia).
Siren. A genus of salamanders.
Skeleton. A framework of hard parts serving for support, protection, or movement,

or a combination of these functions, in animals.
Slime tube. A sheath of mucous material secreted on the surface of an earthworm

at the time of mating.
Small intestine. That part of the intestine of vertebrates immediately following

the stomach, as distinguished from the large intestine.
Smooth muscle. Muscle composed of nonstriated, uninucleate, spindle-shaped cells.

It is common in the intestine, bladder, and glands of vertebrates.
Socrates {sok' ra teez). Greek philosopher who lived about 470-399 b.c.
Solanum (so la' num). A genus of plants including the common potato, nightshade,

and many others.
Solution. A liquid containing another substance in the form of particles not greater

than molecules in size.
Soma. The body, as contrasted with the germ cells.
Somatic. Pertaining to the body; when applied to cells, referring to the sterile bodj-

cells in contrast to the germ cells which are reproductive.
Somite. One of the segments into which the body of a worm or arthropod or othei-

segmented animal is divided.
Species {pL, species). A group of animals or plants so nearly alike that, in general.

they might have sprung from the same parents. (The term is rather arbitrarily

used, however.)
Specific. Pertaining to a species.

Sperm. One of the male germ cells in an animal or plant ; also called sperm cell.
Spermary. See testis.
Spermatheca. See seminal receptacle.
Spermatid. One of the two cells formed by the second division in spermatogenesis.

By transformation in shape the spermatids become mature spermatozoa.
Spermatocyte {sper' ma to site'). A male germ cell between the beginning of sperma-
togenesis and the second division in that process. A spermatocyte is called

primary during the growth period and prior to the first division; secondary aft(>r

the first division but ])rior to the second.
Spermatogenesis {sper' iim to jen' e sife). The ripening of male germ cells.
Spermatogonium {sper' nia to go' ni um)^ {pi, spermatogonia!. One of the early

germ cclis of a male animal, prior to the Ix'giiuiing of spermatog(>nesis.
Spermatophore {sper' ma to fore'). A mass of spermatozoa, sometimes resting upon

a stalk or being otherwise attached, as in some salamanders.
Spermatozoon {sper' ma to zo' on) {pL, spermatozoa). The male germ cell in animals.
Sphenodon (sfen' o don). A genus of reptiles of tlie order Rhynchocephalia. Only

on(; living species is known.
Spheroid. Of nearly s])lierical shape.
Spicule. A body of various shapes commonly of calcareous or siliceous material,

forming part of the skeleton of a sponge.
Spinal cord. That part of the central nervous system of vertebrate animals lying

behind the brain and largely enclosed in a chaiuicl in the vertebrae.

GLOSSARY 401

Spinal nerve. One of the paired nerves arising by two roots from the spinal cord.
Spindle. A group of structures resembling threads, in the form of a spindle, formed

in the cytoplasm of a cell during mitosis.
Spiracle. In frog tadpoles, an opening through which water passes out of the gill

(•hanil)er on one side. In insects, one of a inunber of openings on the sides of

the body tlu'ough wliich air is introduced to or ejected from the tracheae.
Spireme {spi' reem). The coiled or tangled thread formed by the chromatin of a cell

prior to division.
Spirostomum {spi ros' to nmm). A genu^ of ciliated protozoa.
Splint. .\ bone at either side of the foot of the horse and some of its relatives, being

the remnant of a lost toe.
Spongilla. A genus of fresh-water sponges.

Spongin. The horny material of the skeleton of the bath sponges.
Spontaneous generation. Same as abiogenesis.
Sporadic. Occurring at irregular intervals, often without apparent reason; said of

migration of animals.
Spore. One of a great variety of reproductive cells usually having protective cover-
ings. Often the term is limited to asexual reproductive cells. The word is often

compounded with qualifying prefixes or preceded by quahfying adjectives.
Sporozoa. A class of protozoa, parasites usually without locomotor organs or mouth.
Sporulation. The formation of spores; sometimes applied to multiple division of the

nucleus followed by fragmentation of the cj'tosome, which occurs in the spore

formation of certain species.
Squamata (skwa ma' to). An order of reptiles to wliich the snakes, lizards, and

cliameleons belong.
Squamosal. A bone of the posterolateral region of the skull of vertebrates. In the

mammals it suspends the lower jaw, but not in the other vertebrates.
Squamous epithelium {skwa' ■rnus). Epithelium whose cells are low and flat.
Statoblast (skit' o blast). A gemmulelike bodj' by means of wlrich many Bryozoa

reproduce asexually.
Steapsin {ste ap' sin). The fat-splitting enzyme of the pancreatic fluid.
Stegodon (steg' o don). A genus of extinct animals, related to the elephants, from

the Pliocene of southern Asia.
Stegosaurus {steg' o saw' rus). A genus of dinosaurs bearing rows of plates set verti-
cally on the back, belonging to Jurassic and Comanchean time.
Stejneger, Leonhard {sti' ne ger). A living American herpetologist.
Stentor. A genus of ciliated protozoa.
Sternum. The breastbone; present in most vertebrates except fishes and some

reptiles.
Stimulus. A change in the environment or some internal condition wliich produces a

reaction in an organism.
Stomach. An enlargement in the anterior part of the digestive tract of many animals;

certain phases of the digestion of food occur there.
Storeria. A genus of snakes. S. occipitomaculata {ok sip' i to mak' u la' ta); S.

dekayi {de kay' i).
Stratified. Arranged in strata or layers; said of epithelia, geological deposits, etc.
Stratum (pL, strata). A laj^er; specifically, a layer of sedimentary rock.
Stratum corneum. The thin outermost layer of cells in the skin of certain animals

(as the frog).
Striated muscle. Muscle composed of cylindrical, cross-banded, multinucleate cells

(except in the heart). Skeletal muscles in vertebrates are of this kind.
Striation. A stripe; as the crosslines of voluntary muscle cells.

402 PRINCIPLES OF ANIMAL BIOLOGY

Stylonychia {sti' lo nik' i a). A genus of ciliated protozoa.

Subepithelial cells. In Hydra, rounded cells lodged among the epithelial cells, often

near the base of the latter.
Sucker. An attaching organ beneath the head of a frog tadpole ; a similar organ on

the scolex of a tapeworm colony; also the attacliing organ of leeches.
Sucrose. Common table sugar, a disaccharide derived from cane or beets.
Suctoria. A class of ciliated protozoa which bear no cilia when adult, but have tybe-

like tentacles.
Surface phenomena. A group of physical and chemical phenomena characteristic

of surfaces (of cells, particles, fine pores, etc.)
Sustentative {sus ten' ta tiv). Supporting; applied to connective tissue and other

supporting tissues.
Suture. The line of junction between a septum of a cephalopod shell and the outer

wall of the shell. Also the immovable joint between two flattened bones, as

those of the skull.
Swammerdam, Jan {swahm' me?- dahm). Dutch naturalist, anatomist, and ento-
mologist, 1637-1680.
Sweat gland. One of the excretory organs of the skin.
Sylvius, Jacques Dubois. French anatomist, 1478-1555.
Symbiosis {sim' hi o' sis). The association of two species of animals for their mutual

benefit.
Symbiotic. Of the nature of symbiosis.
Symmetry. The state of being symmetrical, or capable of being divided by a hne or

plane into two parts which are mirrored images of each other.
Sympathin. A substance produced by nerve endings of the thoracolumbar sj^stem

and serving to inhibit certain organs, stimulate others.
Synapse (sin aps'). The point of contact of two neurons.
Synapsis (sin ap' sis). The pairing of maternal with paternal chromosomes earlj- in

the maturation of the germ cells.
Synapta. A genus of sea cucumbers.
Syncytium (sin sish' i urn). An undivided mass of protoplasm containing several or

many nuclei.
Synonym (sin' o nim). A taxonomic name which is rejected because it is a duplicate.
Synura. A genus of colonial flagellate protozoa.

System. A collection of organs concerned with the same general function, as diges-
tion. Also, the rocks, collectively, which belong to a geological period.
Systematic botany. See taxonomy.
Systematic zoology. See taxonomy.
Systemic circulation. The circulation of the blood through the body in general, as

distinguished from that through the lungs or limgs and skin (pulmonary or

pulmocutaneous) .

Tadpole. The larva of a frog, or certain other animals.

Tail. A slender posterior appendage. In a spermatozoon, the whiplike propelling

organ behind the head and mid-piece.
Tamiasciurus (ta' mi a .sr n' rus). The subgenus of the genus Sciurus including the

red squirrels. Sciurus (Tamiasciurus) hudsonicus loquax {hud son' i kus lo'-

kwaks), the southern Hudsonian red squirrel.
Tarsal. One of a number of bones in the ankle of most vertebrate animals.
Tarsometatarsus (tar' so met' a tar' sus). A compound bone in the leg of a l)ini,

formed of several of the metatarsals and tarsals.
Taxonomy. The science of the classification of animals or plants.

GLOSSARY 403

Teleostomi {te' le os' to mi). A subclass of Pisces comprising the true fishes. They

have a skeleton partly or wholly of bone and respire by means of gills.
Telolecithal {teV o les' i thai). Containing much yolk, crowded toward the vegetative

pole; said of eggs.
Telophase {tel' o faze). The final phase of mitotic cell division, in which the nuclei

are reconstructed.
Tentacle. One of a number of armlike projections from hydroids, Bryozoa, Nautilus,

and other animals. Also one of certain elongated individuals of a siphonophore

colony.
Termite. One of an order of insects called "white ants," but not really ants.
Terrapene iter' a pee' nee). A genus of turtles of the family Testudinidae.
Terrigenous {ter rij' e nus). Derived from the land; as applied to lake bottoms,

composed of material washed in from the land, as distinguished from material of

organic origin.
Tertiary {ter' shi a' ri). The single period of Cenozoic time.
Test. A hard outer covering, capsule, or shell; as of a sea urchin.
Testis. The organ in which the male germ cells are lodged and developed.
Testosterone {les ios' ter one). A hormone produced by the interstitial cells of the

testis; it controls development of secondary sexual characters and sex behavior.
Testudinata {tes tu' di na' ta). An order of reptiles, comprising the turtles.
Testudinidae {les' tu din' i dee). A family of turtles.
Tetrad. A quadruple body formed, during the growth period in the ripening of

germ cells, from the union of two chromosomes which at the same time become

duphcated.
Thales {Iha' leez). Greek philosopher and astronomer who lived about 640-546 B.C.
Thamnophis {Iham' no fis) . A genus of garter snakes. T. butleri {but'leri); T.

proximus {proks' i runs); T. sackeni {sak' en i); T. sauritus {saw ri' tus).
Theophrastus {the' o fras' tus) . Greek philosopher, founder of botany, who lived

about 372-287 b.c.
Thermocline. A layer of water in a lake in which the temperature falls at least 1°C.

for each additional meter of depth.
Thiamin {thi' a min). Vitamin Bi, the preventive of polyneuritis or beriberi.
Thoracic. Pertaining to the thorax or chest.
Thoracolumbar system. That part of the autonomic nervous system which centers

in the middle portion of the spinal cord. Each organ controlled by the auto-
nomic system is innervated once from it.
Thorax. A middle portion of the body of many animals, between head and abdomen.
Thricozoa {Ihrik' o zo' a). A class of animals (hair animals) in Oken's early classifi-
cation. It comprised the mammals which Oken also called Ophthalmozoa.
Thrombase. An enzyme which brings about the conversion of fibrinogen into fibrin

in the clotting of the blood.
Thromboplastin. A substance which converts prothrombase into thrombase in the

clotting of the blood ; it is found in blood platelets and many cells.
Thymus. A ductless gland located near the gill clefts, or in the neck, or in the anterior

part of the thorax in vaiious vertebrates.
Thyroid. A ductless gland located in the ventral part of the pharynx.
Thyroxin {thi roks' in). The hormone of the thyroid gland.
Tibia. The inner one of the two bones in the lower leg of vertebrates, except the

fishes.
Tibiofibula. The fused tibia and fibula of some Amphibia.
Tibiotarsus. A compound bone in the leg of a bird, formed of the tibia and certain

of the tarsal bones.

404 PRINCIPLES OF ANIAfAL BIOLOGY

Tissue. A group of cells of similar structure forming a contiiuious mass or

layer.
a-tocopherol (aV Ja to kof er ole). Vitamin E, the antisterility vitamin of rats.
Tonsil. A glandular organ at the side of the throat.
Trachea {tra' ke a). The tube conveying air to and from the lungs in vertebrates.

Also an air tube in insects and some other invertebrates.
Tracheal gills. Threadlike or leaflike projections in which tracheae have their l)egii^-

ning in certain aquatic insect larvae.
Trachelocerca {tra' ke lo ser' ka). A genus of ciliated protozoa.
Transverse process. One of a pair of projections at the sides of a vertebra in most

vertebrate animals.
Trematoda {trem' a to' da). A class of Platyhelminthes, parasitic flatworms with

suckers and without cilia.
Triassic. Of the earliest Mesozoic time.
Triceratops {tri ser' a tops)^ A genus of three-horned dinosaurs of late Cretaceous

time in western North America.
Trichinella (trik' i neV la). A genus of parasitic roundworms, the cause of the disease

tricliinosis.
Triclad. Having the digestive tract divided into three branches; said of an order of

flatworms.
Trilobite (tri' lo bite). A primitive crustacean of Paleozoic time, having the body

partially divided by longitudinal grooves into three lobes.
Trilophodon {tri lof o don). An extinct genus of animals from the Miocene of several

continents; related to the elephants.
Trionychidae (tri' o nik' i dee). A family of turtles.

Triploblastic (trip' lo bias' tik). Composed of three fundamental layers of cells.
Triturus. A genus of salamanders.
Trochophore. A form of free-swimming larva characteristic of many worms, mol-

lusks, and rotifers.
Trypsin. A protein-splitting enzyme produced by the pancreas.
Trypsinogen (trip sin' o jen). The inactive substance from which the enzyme trypsin

is produced.
Tube feet. Tubular protusions from the arms of echinoderms, which serve as organs

of locomotion.
Tubercula pubertatis (tu ber' ku la pii' ber ta' lis). Two thick glandular ridges on the

clitellum of an earthworm near the ventral surface.
Tuberculate. Bearing cusps or conical prominences; said of teeth.
Tubular gland. A gland whose lumen is of about uniform bore throughout.
Tunicata (tu' ni ka' ta). A subphylum of Chordata, including the sea squirts, s(>a

pork, salpas, etc. For definition see Chap. 19.
Turbellaria (tur' bcl la' ri a). A class of Platyhelminthes, ciliated flatworms leading

a free existence.
Type. In systematic zoology, an individual or group which is formally held to Im>

typical of the species or larger group to which it belongs; as, the type specimen of

a species, the type species of a genus, or the type geims of a family.
Typhlosole (tif lo sole). A ridge resulting from the infolding of the dorsal intestinal

wall of the earthworm.

Ulna. The bone of the little-finger side of the forearm in Anipliibia and tlu^ higher

vertebrates.
Umbilical cord. A ropelike cord in which blood vessels pass bet\v«!en an embryo and

the placenta in viviparous mammals.

GLOSSARY 405

Unconformity. A sharp contrast, often a lack of parallelism, between adjoining rock
strata, cansod by a long period of erosion.

Uniformitarianism. The doctrine that geological processes of the past were similar
to those of the present time.

Unisexual. Involving but one sex, the female; applied to parthenogenetic reproduc-
tion.

Unit character. A hereditary trait that behaves as a unit in transmission, being
capable of inheritance independently of other luiit characters.

Universal symmetry. An arrangement of the parts of an object or organism such
that it is capalsle of being divided into symmetrical halves by an infinite number
of planes passing in any direction through a central point.

Urea (u re' a). A substance, C0(NH-))2, produced by the decomposition of proteins
and some other substance in organisms.

Ureter (it re' ter). A tube conducting urine awaj' from the kidney.

Urethra {u re' thra). The duct by wliich urine is discharged from the bladder.

Urinary bladder. A bag in which urine is stored.

Urine. The liquid waste excreted bj^ kidneys.

Uriniferous tubule. One of the many coiled tubes making up the bulk of the kidney
in vertebrates.

Urinogenital system. A group of organs concerned with both excretion and reproduc-
tion in vertebrates.

Uterus («' te rus). A modified portion of the oviduct in which the eggs undergo at
least part of their development. Strictly the term uterus is apphcable only in
animals in which the developing embryo becomes attached to the wall of the
organ.

Vacuole. A region within a cell occupied by a liquid other than protoplasm, usually
water with various substances in solution.

vVagina. The passage leading from the uterus to the exterior in many animals.

Valence. A measure of the mmiber of other elements or radicals with which a given
element or radical may combine; it is determined by the number of electrons in
the outer layer.

Variety. In taxonomy, a division of a species; a group of individuals within a species
tliat differ in some minor respect from the rest of the species.

Vascular tissue. Blood or lymph, or the more liquid parts of blood-producing organs.

Vas deferens (vas' def er enz) (pi., vasa deferentia, vas' a def er en' shi a). A duct
conveying spermatozoa from the testis to the exterior.

Vas efferens {vas'' ef fer enz) {pi., vasa efferentia, vas' a ef fer en' shi a). One of a
number of minute tubes leading away from a testis, serving to convey the sper-
matozoa. They lead into a larger tube called in many cases the vas deferens.

Vaucheria {vaw ke' ri a). A multinucleate fresh-water alga.

Vegetative. Concerned with nutrition. When applied to an egg, meaning that side
near wliich the yolk is accumulated (vegetative pole).

Vein. A vessel conveying toward the heart blood which has already traversed capil-
laries since leaving the heart.

Ventral. Literally, pertaining to the belly; hence, usually, lower.

Ventricle. The posterior chamber of the heart in fishes, amphibia, and some reptiles,
and one of the two posterior chambers in higher vertebrates. Its function is the
propulsion of the blood through the main arteries and connecting vessels.

Vermiform appendix. A narrow blind pouch forming a prolongation of the caecum.

Vertebrata. A subphylum of the phylum Chordata, comprising the backboned
animals. For definition see Chap. 19.

406 PRINCIPLES OF ANIMAL BIOLOGY

Vertebrate, adj. Possessing a backbone, n. An animal having a backbone.

Vesalius {ve sa' li us). Belgian anatomist and court physician, 1514-1564.

Villus {pi., villi). One of the fingerlike projections from the inner surface of the small

intestine.
Virchow, Rudolf {veer' no). German physiologist and pathologist, 1821-1902.
Visceral. Pertaining to the viscera, or organs contained in some large cavity of the

body ; applied in the vertebrates chiefly to the organs of the abdomen, in clams to

the digestive organs and glands above the foot.
Viscosity. The resistance offered by a substance to the relative movement of its

molecules.
Visual purple. A light-sensitive pigment in the retina.
Vitamin. One of several substances common in leafy vegetables, animal fats, and

elsewhere, which are necessary for specific aspects of metabohsm in animals.
Viviparity {viv' i par' i ti). The condition of being viviparous.
Viviparous {vi vip' a rus). Producing young from eggs that are retained in the uterus

of the mother, with the aid of nutrition derived from the mother through a

placenta and umbilical cord.
Volvox. A small spherical organism composed of flagellated green cells embedded in

jelly, in a single layer around a liquid interior. Sometimes regarded as an animal,

though more properly included among plants.
Vorticella. A cihated protozoon attached to a contractile stalk.

Wallace, Alfred Russel. EngUsh naturalist, 1823-1913.

X body. An object in the cytosome of some of the early cleavage cells of Sagitta,

which marks the germ cells.
X chromosome. A chromosome closely associated with the determination of sex. In

many animals the female has two of them, the male only one.
Xenophanes (ze nof a neez). Greek philosopher who lived about 570-480 b.c.
Xerophthalmia {ze' rof thai' mi a). A dry, lusterless condition of the eyeball.

Y chromosome. A chromosome possessed only by the males of many species. It
behaves in spermatogenesis much as if it were homologous with the X
chromosome.

Yolk plug. The remnant of the vegetative cells last to be drawn into the interior of a
gastrula in certain embryos.

Zoogeography. The branch of zoology treating of the geographical distribution of

animals.
Zooid. One of the members of a hydroid or siphonophore colony. Often, in a

restricted sense, a particular kind of individual, as a hydranth.
Zoology. The science of animals.
Zygapophysis {zi' ga pof i sis). One of four short projections, two in front and two

behind, extending from the upper portion of a vertebra. Those of the posterior

pair articulate with the anterior pair of the vertebra next behind.
Zygote, A cell or individual produced by the fusion of two cells or their nuclei in

the process of sexual reproduction.

CORRELATED LIST OF VISUAL AIDS

The following list of 16-mm. motion pictures and 35-mm. filmstrips
can be used to supplement some of the material in this book. These
visual aids can be obtained from the producer or distributor shown with
each title. (The addresses of these producers or distributors are listed
at the end of the bibliography.) In many cases these visual aids can
also be procured from your local film library or local film distributor.

The running time (min) of the film and whether it is silent (si) or
sound (sd), filmstrip (FS) or color (C) are listed with each title. All
those not listed as color are black and white.

Each film has been listed only once, usually in the first chapter to
which it is applicable. However, in many cases it can be used advan-
tageously in several of the other chapters.

CHAPTER I. THE GROWTH AND SCOPE OF BIOLOGY

Eyes of Science (Bausch & Lomb 45min si). — Shows Galileo with his
early telescope; Leeuwenhoek and his simple microscope; and today's
lenses and tubes that have given scientists the "super eye."

Marvels of the Microscope (Gut lOmin sd). — Microscopic studies of
water plants and minute forms of animal hfe.

Unseen Worlds (Ganz lOmin sd). — Explains the intricacies of the
newly developed electron microscope.

CHAPTER HI. SOME FUNDAMENTAL PHYSICS AND CHEMISTRY

Chemical Reactions (Brandon 20min sd). — Explains the composition
of an atom; relationship between nucleus and electrons; chemical reac-
tions.

Electrons (EBF 11 min sd). — Shows phenomena associated with con-
duction of electricity in liquids, gases, and vacuums.

Molecular Theory of Matter (EBF 11 min sd). — Molecular hypothesis
illustrated by animation; behavior of molecules in various conditions;
Brownian movement.

CHAPTER IV. THE FUNCTIONS OF PROTOPLASM AND CELLS

Protoplasm — the Beginning of Life (Bray ISmin si). — Protoplasm
shown in characteristic motion in one-celled and many-celled hosts.

Living Cell (EBF 15min si). — Shows single-celled organisms and many-
celled organisms under the microscope.

407

408 PRINCIPLES OF ANIMAL BIOLOGY

Green Plant (EBF ISmin si). — ^Shows that living things are dependent
for food upon the green plant; the latter's processes of foodmaking and
growth are diagramed.

Nitrogen Cycle (EBF 15min si). — Shows how nitrogen compounds
serve as a key to the transfer of energy in nature and how animals depend
upon plants for food.

CHAPTER VIII. PHYSICAL SUPPORT AND MOVEMENT

Body Framework (EBF 15min si). — Function of skeleton; structure,
chemical composition, growth and repair of bones; main types of joints.

Muscles (EBF 15min si). — The structure and use of muscles are
presented.

CHAPTER IX. SOURCES OF ENERGY AND MATERIALS

Digestion (EBF 15min si).— Covers complete digestive tract.

Digestion of Foods (EBF lOmin sd). — A summary of the digestive
process; show^s relation of circulatory and nervous systems to the diges-
tive process.

Alimentary Tract (EBF llmin sd). — Treats in detail motihty phe-
nomena of the gastrointestinal tract by means of actual photography.

CHAPTER X. RESPIRATION AND RELEASE OF ENERGY

Breathing (EBF 15min si). — Explains action of diaphragm, breathing,
and lung structure and function.

Mechanisms of Breathing (EBF lOmin sd). — The breathing mecha-
nism in operation.

CHAPTER XL TRANSPORTATION SYSTEM

Blood (EBF 12min si). — Illustrates the separation of plasma from
]>l()()d cells, protein and salts from plasma, etc.

Circulation (EBF ISmin si). — Traces circulatory system; ('omi)ares
liuman heart with that of the frog.

Control of Small Blood Vessels (Lutz 2()min si). — Illustrates both the
sti-uctural and physiological features of the arterioles, precapillaries and
capillaries.

Heart and Circulation (l^^^BF lOmin sd). — Detailed explanation of the
mechanics of the pulmonary and systemic systems.

CHAPTER XII. DISPOSAL OF WASTES

Work of the Kidney (l*]IU<' liinin sd). ^Detailed exposition of the
kidneys and their functions.

CORRELATED LIST OF VISUAL AIDS 409

CHAPTER XIII. INTEGRATION OF ACTIVITIES

Reactions in Plants and Animals (Harvard llmin sd). — Characterizes
the concepts of stimiikis and reaction and presents a study of different
types of reactions in plants and animals.

Nervous System (Brandon 150min si). — Study of development of
nei'vous system with special sections on development of early nervous
systems; reflex actions; spinal cord; the brain; conditioned reflexes and
l)ehavior.

Nervous System (EBF lOmin sd). — Shows structure of the nervous
system; nerve impulse.

CHAPTER XIV. REPRODUCTION

How Animal Life Begins (NYU llmin sd). — The fundamentals of
reproduction in the rabbit; cell growth involved in animal reproduction
is used to illustrate human reproductive processes.

In the Beginning (USD A 17min sd). — Prologue to life, shows ovula-
tion, fertilization, and early development of mammalian egg.

Reproduction among Mammals (EBF lOmin sd). — Presents the story
of embryology, using the domestic pig.

Reproduction in Plants and Lower Animals (B&H 15min si). — Shows
the process of fertilization, conjugation, and mitosis as well as reproduc-
tion by budding.

CHAPTER XV. BREEDING BEHAVIOR OF ANIMALS

Development of Bird Embryo (EBF 15min si). — Shows development
from early stages to hatching.

Frog (EBF lOmin sd). — Portrays life cycle of the frog and develop-
ment of the embryo.

Salamanders and Their Young (Rvitgers 15min si). — Shows the habi-
tat and l^reeding of salamanders.

Snapping Turtle (EBF llmin sd). — Presents complete hfe story of
this reptile.

Insects : Their Growth and Structure (USDA FS). — Shows types and
kinds of insects; external anatomy; internal anatomy; and life cycles.

CHAPTER XVII. GENETICS
Heredity (EBF lOmin sd). — ]\Iendelian laws of inheritance presented.

CHAPTER XIX. GROUPS OF ANIMALS

Animal Life (Harvard lOmin sd). — A review of the main types of ani-
mals: protozoans, sponges, coelenterates, echinoderms, worms, molluscs,
crustaceans, insects, and vertebrates.

410 PRINCIPLES OF ANIMAL BIOLOGY

Parade of Invertebrates I-IV (Rutgers lOmin si C). — In four reels;
shows numerous types of invertebrates.

Microscopic Animal Life (EBF 15min si). — Shows four single-celled
animals and one multicellular animal.

Marine Communities (Rutgers 15min si C). — Shows how many types
of undersea life associate in communities.

One-celled Animals (EBF 15min si). — An excellent study of the
protozoa.

CHAPTER XXII. FOSSIL ANIMALS

Lost World (EBF 15min si). — Exhibition in motion of extinct pre-
historic animals recreated in full-scale, hfelike models.

History of Horse in North America (Cal 20min sd). — Demonstrates
advancement of the horse in fifty million years.

Evolution (Gut 30min sd). — Presentation of theories of the origin and
development of the earth and its living inhabitants; prehistoric animals.

Monkey into Man (NYU 20min sd). — A study of monkey Ufe showing
family and social life and variation in brain power among them; compares
most intelligent of apes and man with a brief review of man's develop-
ment from the primitive stages to modern evolution.

Fingers and Thumbs (NYU 20 min sd). — Traces the development of
man's hands; evolution from earhest form of life to the ape is clearly
depicted as well as the actual development of the hand in the ape family.

SOURCES OF FILMS LISTED ABOVE

Bausch & Lomb Optical Co., 635 St. Paul St., Rochester, N.Y.

B&H — Bell & Howell Company, 1801 Larchmont Ave., Chicago.

Brandon Films, Incorporated, 1600 Broadway, New York 19

Bray Pictures Corporation, 729 Seventh Ave., New York

Cal — University of California, Extension Division, Department of Visual

Instruction, 301 California Hall, Berkeley, Calif.
EBF — Encyclopaedia Britannica Films, 1841 Broadway, New York 17
Ganz, Wilham J. Company, 40 E. 49th St., New York
Gut— Gutlohn, Walter O. Inc., 25 W. 45th St., New York 19
Harvard Film Service, School of Education, Lawrence Hall No. 4, Cam-
bridge 38, Mass.
Lutz, Brenton R., 088 Boylston St., Boston, Mass.
NYU — New York University Film Library, Washington Square, New

York
Rutgers University, Box 78, New lirunswick, N.J.

USDA — U. S. Department of Agriculture, Motion Pictui-c Division,
Washington, D.C.

INDEX

Boldface numbers refer to pages bearing illustrations of items indexed.

Abiogenesis, 159
Absorption, 106

of food, 104
Acanthocephala, 275
Accidental dispersal, 320
AcetabuliuTi, 92, 93
Acetylcholine, 148
Acids, 35, 36
Actinomorphes, 246

Adaptation, as quality of whole organism,
364

resulting from natural selection, 363-
365

taxonomic ranks showing, 363, 364
Adipose tissue, 82
Adrenal glands, 153, 154, 155
Adrenalin, 155
Adsorption, 45
Afferent neuron, 144, 145
"Age and Area" hypothesis, 310
Age of earth, 331

in geological periods, 331
Alecithal egg, 199

cleavage of, 201
AU-or-none rule, 53
Altricial birds, 190
Alveolar gland, 84, 85
Alveoli, 114, 115
Amino acids, 41
Amitosis, 62
Ammonite, 332

sutures of, 343
Amoeba, 24, 51, 260

food vacuole of, 101
Amphibia, 277, 279

evolution of, 335
Amphicoelous vertebrae, 92
Amphineura, 271, 272
Amphioxus, 276
Amylopsin, 105
Anabolism, 46

Analogy, 251, 252

Anaphase, 56, 58

Anatomy, comparative, 13-14

Anaximander, 2

Animal communities, 290-291

relation of, to vegetation, 305, 306
Animal pole, 199
Anisogamete, 161
Annehda, 269-270
Anthophysa, spheroid and dendritic

colony of, 68
Anthozoa, 263, 264
Anus, of earthworm, 101, 102

in embryo, 208, 210, 212

in vertebrate, 103
Apathetic animals (Lamarck), 246
Appendicular skeleton, 90, 92, 93
Arachnida, 273, 274, 275
Archenteron, 73, 204, 206
Archeozoic, 330
Archiannelida, 270
Aristotle, 3, 4, 5, 6
Arms, homology of, 253
Arteries, 122, 123, 129
Arterioles, 129
Arthropoda, 272-275
Artiomorphes, 246
Ascaris, chromosomes in, 59

cleavage of, 203
Ascorbic acid, 110
Asexual reproduction, 159, 169-174, 176

of Paramecium, 169
Assimilation, 46. 49
Association neuron, 145, 146
Associations, 290-291

in vegetated areas, 305, 306
Asteroidea, 268
Asymmetry, 78, 79
Atolls, 303
Atom, 31-33

"Atomic" theory of the universe, 3
Auricle, 123, 124
Auriculoventricular bundles, 128

411

412

rRlNClFLEH OF ANIMAL BIOLOGY

Auriculoventricular node, 128

Autonomic n(>rvou« system, 143, 146, 147

Autosomal luikage, 236, 237

Autosome, 236

Aves, 279

Axial skeleton, 90, 91

Axon, 143, 144

Backcross, 227

two-pair, 230, 231
Balanoglossus, 276
Barnacle, 300

homology of, with Sacculina, 350, 361
Bases, 35, 3(i
B complex, 111

Beagh\ Darwin's voyage on the, 360
Beetle, 272
Beriberi, 111
Bidder's canal, 136, 178
Bilateral symmetry, 77, 78
Bile, 103, 105
Bile duct, 103

iji embryo, 211, 212
Biogenetic law. 74, 255, 256, 257
Biology, defined, 1

history of, 1-20

scope of, 20-22
Birds, 278

cleavage of, 201

evolution of, 335

migration of, 318
Birth stages, 189-192

relation of, to parental can', 192
Bladder, 136, 137, 178, 179, 183
Blainville, Henri de, taxonomy, 246
Blastocoele, 73, 201, 202, 206
Blastopore, 73, 204, 206
Blastostyle, 173
Blastula, 73, 201, 202, 206
Blood, circulation of, S, 9

coagulation of, 130

composition of, 126

pressure, 128, 129
Blood system, 122-126

of dogfish, 123
Body cavities, 80, 81
Bone, 83

Bougainvillea, 172
Bowman's capsule, 135, 136, 137

Brachiopoda, 276, 331
Bract, 174, 175
Brain, 141

in embryo, 208, 210, 211, 212
Breathing, 116, 117
Breeding behavior, 177-192
Brittle star, 268
Bronchi, 115

in embryo, 211
Bronchioles, 115
Brush turkey, nest of, 186, 187
Bryozoa, 275

reproduction of, 171
Buccal cavity, in earthworm, 101

in frog, 102

in mammal, 101, 102
Budding, 170-174

external, 172, 173

Caecum, 103

Calcarea, 262

Calciferol, 112

Cambrian, 330, 331, 335

Camel family, discontinuity of range of

311
Capillaries, 122, 123, 129
Carbohydrates, 40

storage of, 107
Carbon dioxide, as waste, 133
Carboniferous, 330
Cardiac muscle, 96
Caroten(>, 111
Caipals, 93
Cartilnge, 82
Cast, a fossil, 327
Catnbolism, 4(i
Catalase. 42
Catalyst, 42
Cell division, 55-()3
Cell inclusions, 26, 28
Cell membrane, 26, 27
Cells, 23-29

generalized, 26

relation of, to other cells, 28, 29

shape of, 24, 25

size of, 23, 24
Cell theory, 14-16
Cell wail, 26, 27
Cenozoic, 330, 335

INDEX

413

CciitiT of dispersal, 309
Centipede, 272
Central nervous system, 142
Centriole, 26, 27, 60
Centrolecithal egg, 199

cleavage of, 201
Centrosome, 27
Centrosphere, 26, 27
Centrum, 91
Cephalochorda, 276, 278
Cephalopoda, 271, 272

biogenetic law in, 256

evolution of, 343, 344
Ceratite, 344

suture of, 343
Ceratium, linear colony in, 67
Cerebellum, 152
Cerebrum, cortex of, 152

localization in, 151
Cestoda, 265, 266
Chaetognatha, 275
Chaetopoda, 270
Chaetopterus, 270
Chain coral, fossil, 327
C-hance, directing evolution, 359, 362
Chemistry, 30-38

of living things, 37, 38
C^hitin, 88
Chiton, 271

Chlamydomonas, primitive protozoon, 69
Chlorine, atom of, 34

molecule of, 34
Chlorophyll, 47
Cholesterol, excreted. 138
Chordata, 276-280
Chromatin, 26
Chromosomes, 19, 26, 55, 56, 57, 58, 59

division of, 59, 60, 63

doubling number of, 311

variable size of, 59, 60

vesiculation of, 59, 60
Cilia, 28, 51, 52
Ciliata, 261

Ciliated epithelium, 84
Ciliophora, 261
Circular canal, 172, 173
Circulatory system, 122-132

human, 125

open and closed, 122
Clam, 271
Clasping, by frog, 181, 182

Class, 245, 248
Classification, 11-13
Clavicle, 92, 93
Clevage, 194, 200, 201, 202

relation of, to yolk, 201
Climate, changes of, in relation to distri-
bution, 308
Cloaca, 103, 136, 178, 179, 183
Cnidoblasts, in Hydra, 72
Coagulation, of blood, 130

failure of, 112
Codes of nomenclature, 249
Codosiga, dendritic colony in, 67
Coelenterata, 262-264

symmetry ot, 79
Coelenteron, 80, 101, 262
Coelom, 80, 81, 206, 206, 209, 210, 212
Coenosarc, 172, 173
Collared cells, in sponge, 52
Collared epithelium, 84
Collecting tubule, 136, 178
Colon, 103

Colonial theory, 66, 67, 74
Colony, defined, 74. 75

types of, 67-69
Colony formation, origin of, 176
Columnar epithelium, 84
Comanchean, 330, 335
Communities, 290-291
Complete metamorphosis, 215, 216
Compound, 30
Compound gland, 84, 85
Conjugation, 162, 163, 164
Connective tissue, 81
Continents, interconnections of, 322, 323

permanence of, 308
Contractile tissue, 81, 85
Copulation, 182

in earthworm, 167
Coracoid, 92, 93
Coral, 263, 264

fossil, 332
Coral reefs, 303
Cornea, in embryo, 213
Corpus luteum, 157
Cortex, adrenal, 154, 155, 156

cerebral, 152
Cortin, 155
Crab, 238

Cranial nerves, 142, 143
Craniosacral nervous system, 147

414

PRINCIPLES OF ANIMAL BIOLOGY

Crayfish. 273

nervous system of, 141, 142
Cretaceous, 330, 335
Cretinism, 154
Crinoidea, 269
Cro-Magnon man, 345, 346, 367

cave engraving by, 347

tools used by, 347
Crop, of earthworm, 101, 102
Cross-fertilization, 180, 181
Crustacea, 273, 274
Crystalline lens, in embryo, 211, 213
Ctenophora, 275
Cubical epithelium, 84
Cuvier, Georges, 14

taxonomy by, 246
Cyclostomata, 276, 278
Cytology, 19, 20
Cytosome, 23, 27

division of, 61

Dark Ages, 6-7
Darwin, Charles, 17, 18

natural-selection theory of, 360, 361,
364
Deficiency diseases, 110
Democritus, 3, 4, 5
Demospongiae, 262
Dendrite, 143, 144
Dendritic colony, 67, 68
Dermatozoa, 247

Desiccation, resistance to, 285, 286
Determinate development, 218
Determination, principle of, 217-220
Development, place of, 182, 183
Devonian, 330, 335

De Vries, Hugo, study of mutation by, 353
Diaphragm, 117
Differentiation, leading to metazoa,

69-72
Diffusion, 43
Digestion, 48

human, 104-106

locus of, 100, 101
Digestive systems, 101, 102, 103
Dinosaurs, 332, 334, 335
Dinotherium, 337, 339
Dioecious, 166
Dioecism and fertilization, 181-182

Diogenes of Apollonia, 2

Diploblastic, 262

Disaccharide, 40

Distribution, as evidence of evolution,

352
Division, taxonomic, 248
Domestic animals, evolution of, 366, 367
Dominant, 225
Dormancy, 281, 282
Dorsal aorta, 123
Drosophila, chromosomes in, 59

heredity in, 230, 234, 235, 236

mutation in, 355, 356
Duodenum, 102

Dutrochet, discovery of cells by, 15
Dyads, 194, 195, 197

Ear, development of, 214

in embryo, 213
Earthworm, body cavities in, 80

digestive system of, 101

metameric, 79

nervovis system of, 141, 142

reproduction of, 166-168

seta in, 89
Echinodermata, 267-269
Echinoidea, 268, 269
Echinorhynchus, 275
Ecological succession, 291-293

in peat bed, 292
Ecology, 21, 22, 281-306
Ectoderm, 204, 206, 207, 208, 209, 210,
211

of Hydra, 71, 72, 73

organs from, 207
Edaphosaurus, 333
Eel, migration of, 318
Effector, 145
Efferent neuron, 145, 146
Egg, 199, 200

of Hydra, 72

mature, 198
Eggs, care of, 186-189
Elasmobranchii, 276, 277, 279
Electrolysis, 37
Electrolytes, 36, 37
Electron, 31
Element, 30, 31-33
Elephant, evolution of, 335-339

INDEX

415

Elephas, 336, 339

tooth of, 338
Embryo, orientation of, 217

source of nourishment of, 183
Embryonic development, 73, 193-220

problems of, 216-220
Empedocles, 2, 16

Endocrine glands, human, 153, 1.54-158
Endoderm, 101, 204, 206, 207, 208, 209,
210, 211

of Hydra, 71, 72, 73

organs from, 207
Endoskeleton, 90
Energy, 37

derived from oxidation, 48, 49

release of, 113, 119, 120

requirements of, 108

source of, 100-108, 113
Enterokinase, 105
Enteron, 80, 172
Enteropneusta, 276, 278
Entomologist, 257
Environment, 281-306
Enzymes, 41, 42
Eocene, 330, 335
Eophippus, 339

skull of, 340

tooth of, 340
Epithelial cells, in Hydra, 72
Epithelial tissue, 81, 83
Epoch, in paleontology, 329, 330
Equation division, 194
Equatorial cleavage, 201
Equatorial plate, 66, 57
Equus, 342

Era, in paleontology, 329, 330
Erepsin, 105
Ergosterol, 112
Esophagus, of earthworm, 101, 102

of frog, 102, 103

of mammal, 101

of man, 103
Estrogen, 156
Eudorina, reproduction of, 161

spheroid colony in, 68, 70

symmetry of, 78
Euglena, 260

flagellum of, 53
Eustachian tube, 211

origin of, 214
Evagination, 207

Evolution, 16-18, 349-368

of breeding habits, 185

a change of species, 307, 352

direction of, 358-365

evidences of, 349-352

human, 367, 368

lines of, 334-348

relation of, to distribution, 307-309

shown by fossils, 331-348
Excretion, 46, 50

by kidney, 137
Excretory system, 133-139

human, 137

of invertebrates, 134. 135
Exoskeleton, 90
Extensor muscle, 94
External respiration, 114
Extinction, 289
Eye, in embryo, 213

F,, 226

Fo, 226

Family, 247, 248, 250

Fats, 41

Feather star, 267

Femur, 93

Fertilization, 163, 165, 167, 168

entrance of sperm in, 217

methods of ensuring, 179-182

time and method of, 200
Fetus, 184
Fibrin, 130
Fibrinogen, 126, 130
Fibula, 93
Fishes, 277

evolution of, 335
Fission, 169

multiple, 170
Flagella, 52, 53
Flagellated epithelium, 84
Flame cell, 134

Jlatworm, nervous system of, 141, 142
Flexor muscle, 94

Flints, used by Neanderthal man, 346
Fluke. 265, 266
Flying squirrels, distribution of, 313, 314,

315
Follicle, 156-158
Food storage, 107

4ilJ

rUINCIFLES OF ANIMAL BIOLOGY

Food vacuole, 101
Foot, of Hydra, 71, 72
Formations, in paleontology, 329
Fossils, 325-348

as evidence of evolution, 351, 352

index, 329

nature of, 326, 327

preservation of, 325-328

tracks, 327, 328
Fowls, heredity of combs of, 231, 232
Fresh-water habitats, 294-300
Frog, 277

cleavage in. 201

digestive system of, 102, 103

metamorphosis of, 214. 215

nervous system of, 141, 142

tadpole, 191

Galen, 6-8

Gall bladder, 101, 103

Gamete, 159, 173

Ganglion, 141, 146, 147, 212

Garter snakes, distribution of, 315, 316

Gastropoda, 271, 272

Gastrovascular cavity, 80, 101

of Hydra, 73
Gastruia, 73, 204, 206
Gastrulation, 204, 205, 206

of sponges, 262
Gemnuile, of sponge, 170
Genes, 62, 224

choice of symbols for, 227

interaction of, 231, 232

nature of, 238
Cienetics, 18-21, 222-243

history of modern, 223, 224

n)echanism of, 224, 225

practical applications of, 238-240

problems, 240-243

of sex, 233, 234
Genus, 245, 248
Geoffroy St. Hilaire, 14
Geographic distribution, 307-324

affected by kinship, 315, 316, 317

as evidence of evolution, 352

major r(>alms of, 323, 324

•world-wide scheme of, 321-324
(ieol()gic;il time; scale, 330
( Jephyrea, 276

Germ cells, 71

early marks of. 203, 204, 205

in insect egg, 73

maturation of, 193-198

origin of, 205
CJerm layers, 206, 207
Gill, 114, 115

analogy of, 251, 252
Gill bar, 210, 211
Gill cleft, 209, 211
Gill pouch, 209, 211
Gizzard, of earthworm, 101, 102
Glands, digestive, 102, 103

in frog tongue, 102

types of, 84
Glaucomys, distribution of, 313, 314, 31. j
Glenoid fossa, 92, 93
Glomerulus, 135, 136, 137
Glossozoa, 247
Glottis, 115
Golgi body, 26, 27, 28
Gonad, 80, 81

migration stimulated by, 318
Gonangium, 173
Goniatite, 344

suture of, 343
Gonionenuis, 263
Gonoduct, 80
Gonophofe, 174, 175
Gonotheca, 173
Grant ia, collared cells in, 52
Graptolite, 331
Gregaloid colony, 67, 68
Grew, Nehemiah, 10
Grov.'th, 46, 50
Guinea pigs, heredity in, 226, 228, 229

Haglish, 276
Haly^ites, fossil, 327
Harvey, William, 8, 9. 20
Heart, 122, 123, 128

four-chambered, 124

regulation of beat of, 127, 128

two-chambered, 123
Heat, as cause of nuitation, 357

produced by oxidation, 120

regulation of, 120, 121
Ih^lium atom, 32
Hemoglobin, 127

INDEX

417

Hemophilia, 130

Heparin, used to prevent blood clotting,

130
Hepatic- portal sj^stem, 126
Hermaphrodite reproductive system. 180
Hermaphroditism, 166-168
Herpetologist, 257
Heteromorphes, 246
Heterozygote, 227
Hexactinellida, 262
Hippocampus, brood pouch of, 188
Hippocrates, 3
Hirudinea, 270
Holothurioidea, 268, 269
Homolecithal egg, 198

cleavage of, 201
Homologous chromosomes, 224, 225
Homology, in adult structure, 263

in blood composition, 351

in embryonic structure, 254, 255

as evidence of kinship, 252-255, 349-
351

in physiological characters, 351
Homozygote, 227

Honeybee, influence of food on, 288
Hooke, Robert, 10

and microscope, 9
Hookworm, 266, 267
Hormones, 104, 153-158
Horse, evolution of, 339-342
Human evolution, 335
Humerus, 93
Huxlev, T. H., spread of evolution theory

by, 361
Hybridization, as source of evolution,

357, 358
Hydra, 71, 72, 73, 75, 262

coelenteron in, 101

nervous system of, 140, 141, 142

reproduction of, 172
Hydranth, 172, 173
Hydrochloric acid, in stomach, 104
Hydrogen atom, 32
Hydroid, 263

Hydrorhiza, 172, 173 »

Hydroxyl ion, 36
Hydrozoa, 264
Hyla, distribution of, 310
Hyoid, 90
Hypostome, 172, 173

Hvdra, 71 , 72

Ileum, 102
Ilium, 92, 93

Incomplete metamorphosis, 215, 216
Incubation, 187

Indeterminate development, 218
Index fossils, 329
Inductive method, 5
Insect, cleavage in, 201
Insecta, 272, 274
Insertion, of muscle, 94
Insulin, 155

Intelligent animals (I^amarck's classi-
fication), 247
Intercellular bridges, 28, 29
Internal respiration, 114, 119
International code, 249, 250
Interphase, 55, 56
Intersterility, specific, 353, 365, 366
Interstitial cells, 156
Intestine, 80, 101, 102

in embryo, 205, 207, 209, 210, 212

of flatworra, 102

of frog, 102, 103

large, digestion in, 106

of man, 103

secretion of, 105
Invagination, 204, 207
Involuntary muscle, 96
Ions, 34-36
Ischium, 92, 93
Islands of Langerhans, 155
Isogamete, 161
Isogamy, 161
Isolation, geographic, 365

reproductive, 365, 366

Jejixnum, 102
Jensen, Zacharias, 10
Jurassic, 330

Kar^'okinesis, 55-62

Kidney, 80, 135, 136, 137, 178, 179, 183

Kinetic energy, 37

King crab. 274

418

PRINCIPLES OF ANIMAL BIOLOGY

Lab5^rinth, membranous, homology of,

349, 350
Lacteals, 132
Lakes, 296-299

filling of, 293

organisms of, 298-299

vegetation in, 284, 297
Lamarck, Jean Baptiste, 15, 16, 17, 18

taxonomy by, 246
Lamprey, 276
Land bridges, 323
Larva, 191, 215, 216

defined, 166
Law of priority, 249
Leech, 270

I-eeuwenhoek, Anton van, 10, 11
Leg, analogy of, 251

of insect compared with that of man, 89
Leiolopisma, discontinuity of range of,

311
Lemming, migration of, 319
Lepas, 352

Leptinotarsa, distribution of, 317
Life, defined, 53
Life history, of parasites, 291
Light, affecting color, 285

ecological relations of, 283-285

effect of, on reproduction, 284

modifying structure, 284

reactions to, 285

relation of, to photosynthesis, 283
Limb skeletons, 92, 93
Linear colony, 67
Linkage, autosomal, 236, 237

sex, 234, 236, 236
Linnaean classification, 247, 248
Linnaeus, Carolus, 12, 16, 20

taxonomy by, 245, 246
Lipids, 40, 41

storage of, 107
Liver, 101, 103, 105

in embryo, 210, 212

excretion by, 138
Living matter defined, 53
Lizard, 277
Lungs, 115

analogy of, 251, 2E2

in embryo, 211, 212

Lyell, Sir Charles, influence of, on Dar-
win, 360
Lymph, 130, 131
Lymph capillaries, 131, 132
Lymph nodes, 132
Lymph system, 122, 125, 130, 131, 132

Macronucleus, 162, 163

Madagascar, animal distribution on, 322

isolation of, 323
Major realms of distribution, 323, 324
MaJpighi, Marcello, 10, 15
Malthus, Thomas Robert, influence of, on

Darwin and Wallace, 360, 361
Mammalia. 278, 280
Mammalogist, 257
Mammals, digestive system of, 101

evolution of, 335

origin of, 321, 322

primitiveness of, in southern conti-
nents, 322
Mammary glands, 155
Mammoth, fossil, 326
Man, fossil, 344-348
Marine habitats, 300-303

geographic areas in, 303, 304

on ocean bottom, 301, 302
clam on, 302

relation of, to depth, 301
Marsupial frog, egg pouch of, 188
Mastigophora, 260
Mastodon, 336, 339

tooth of, 337
Material requirements, 109
Matrix, of sustentative tissue, 81
Matter, 30

Maturation, of germ cells, 193-198
Medulla oblongata, 152
Medullary sheath, 143
Medusa, 172, 173, 174, 262, 263
Meiosis, 195
Mendel, Gregor, 18, 19, 223, 224

law of, 237-239
Mendolian heredity, 237-239
Meridional cleavage, 201
Merychippus, 341

feet of, 341

skull of, 342
Mesenchyme, 82, 83

INDEX

419

Mesentery, 80, 205

Mesoderm, in embryo, 205, 206, 207, 208,
209, 210, 211, 212

organs from, 207
Mesohippus, 340

feet of, 341

skull of, 341

tooth of, 340
Mesozoa, 275
Mesozoic, 330, 335
Metabolism, 46-50
Metacarpals, 93
Metagenesis, 173, 174
Metamere, 79, 80
Metamerism, 79
Metamorphosis, 192, 214, 215
Metaphase, 56, 57, 60
Metatarsals, 93
Metazoa, evolution of, 64-76

sexual reproduction in, 160, 161
Metazoan individual, as distinguished

from colon.y, 74, 75
Microgromia, gregaloid colony in, 67
Micronucleus, 162, 163
Micropyle, 199
Microscope, 9, 10-11, 14, 19
Microtus, outbreaks of, 319, 320
Migration, normal, 317

periodic, 318 *

sporadic, 319, 320
Miocene, 330, 335
Mirbel, 15

Mississippian, 330, 332, 335
Mitochondria, 26, 27
Mitosis, 55-62

genetic significance of, 62, 63
Moeritherium, 335, 336, 337, 339
Moisture, ecological relations of, 285-287

effect of, on reactions, 286
Mold, a fossil, 326, 327
Molecule, 30
Mollusca, 270-272
Monoecious, 166
Monosaccharide, 40
Morgan, T. H., 19

genetic studies of, 355
Morphology, 20, 22
Motor neuron, 145
Motor unit, 96
Mouth, of earthworm, 101

of flatworm, 102

Mouth, of frog, 102, 103

of Hydra, 71, 72

of mammal, 101
Muscle, 93, 94, 95

cardiac, 94, 96

chemistry of contraction of, 98, 99

contraction of, 96-99

efficiency of, 98

fatigue of, 96, 99

involuntary, 96

opposing sets of, 93, 94

smooth, 94

striated, 94, 95

voluntary, 96
Muscle twitch, diagram of, 96
Mutation, 238, 353-357

causes of, 356, 357

direction of, 359
Myehn sheath, 143, 146, 148
Myofibril, 95
Myosin, 99
Myotome, 205
Myriapoda, 272, 274
Mysis, 256

Natural selection, 360-363

criterion of advantage in, 362, 365
Nauplius, 256
Nautiloid, suture of, 343
Neanderthal man, 345, 346

flints used by, 346
Nectocalyx, 174, 175
Nemathelminthes, 266, 267
Nematocyst, of Hydra, 72
Nematomorpha, 275
Nemertinea, 275
Nephridiopore, 135
Nephridium, 135
Nephrostome, 135, 136
Nerve, 141, 146
Nerve impulse, 148, 149, 150
Nervous system, 140-152

autonomic, 143

central, 142

diagrams of, 142

in embryo, 207, 208, 212

induced by notochord, 219

peripheral, 142, 143

rise of, 140-142

420

I'RINCIPLES OF ANIMAL BIOLOGY

Nervoufs tissue, 81, 85

Nests, 186, 187

Neural arch, 91

Neural canal, 91

Neural crest, 205, 209, 212

Neural folds, 207, 208, 209, 212

Neural groove, 209

Neural spine, 91

Neurilemma, 143, 148

Neuromuscular cells, in Hydra, 72

Neuron, 143, 144

Neutron, 31

Niacin, 111

Nicotinic acid. 111

Nomenclature, rules of, 249

Nostril, 214

Notochord, 205, 208, 209, 210, 212

as an organizer, 219
Nuclear membrane, 25, 26
Nuclear sap, 26
Nucleolus, 26
Nucleus, 23, 26

membrane of, 25

reconstruction of, 59, 60, 61

shape of, 25
Nutrition, cause of structural change, 288

dependence of animals on plants for,
287

in ecological relations, 287-288

Order, 245, 248

Ordovician, 330, 332, 335

Organ, 85

Organic compounds, 40-42

Organismal theory, 66, 74

Organizers, 219, 220

Origin, of muscle, 94

"Origin of Species," 18

Ornithologist, 257

Orthocone, suture of, 343

Osculum, 261

Osmosis, 43, 44

Ostracoderms, 332

dtozoa, 247

Ovary, 153, 156, 157, 177, 178, 179, 183

of earthworm, 168

of Hydra, 72
Oviduct, 177, 178, 179, 183

of earthworm, 168
Oviparous, 183, 185
Ovisac, of earthworm, 168
Ovoviviparous, 183-185
Ovum, 160
Oxidation, 37

as source of energy, 48, 49
Oxygen, atom of, 32, 33

mechanism of collection of, 118

molecule of, 33

Obelia, reproduction of, 173, 175
Ocean currents, 301, 303
Oenothera, mutation in, 354, 355
Offspring, care of, 189, 190
Oken, Lorenz, 247
Olfactory organ, 214
Oligocene, 330, 335
Oligochaeta, 270
Onychophora, 274
Oocyte, 194, 196, 197
Oogenesis, 193, 194, 196-198
Oogonium, 59, 193, 194
Open ocean, 302, 303
Operculum, 116

of tadpole, 214
Ophiurodica, 268
Ophthalmozoa, 247
()pistho('0(>l()us vertebrae, 92
Optic nerve, 211, 213

Paedogenesis, 166
Paleomastodon, 336, 339
Paleontology, 14, 21, 325-348

and interrelated evolutions, 328, 329
Paleozoic, 330, 335

Pallas's sand grouse, migration of, 319
Palolo, 270
Pancreas, 103, 105, 153, 155

in embryo, 211, 212
Pancreatic juice, 105
Pandorina, reproduction of, 162

spheroid colony in, 68
J^aramecium, 260, 261

conjugating strains of, 164

fission in, 169

food vacuole in, 101

reproduction of, 162, 163, 164, 1()8

size of, 24
I'arasitisni, 287, 290, 291
Parathyroids, 163, 154

INDEX

421

I'MToiital rare, relation of, to birtli

stages, 192
Parthenogenosis, 164-166

artificial, 165
Parts, relation of, to whole, 64, 65
Pectoral girdle, 92, 93
Peking man, 345
Pelecypoda, 271, 272
Pellagra, 111
Pellicle, 25, 27
Pelvic girdle, 92, 93
Penaeus, biogenetic law in, 256
Penis, 179, 180
Ponnsylvanian, 330, 332, 335
Pepsin, 104
Pepsinogen, 104

Period, in paleontology, 329, 330
Peripheral nervous system, 142, 143
Perisarc, 172, 173
Peritoneum, 80, 81
Permian, 330, 332, 335
Petrifaction, 327, 328
Phalanges, 93

Pharynx, of earthworm, 101, 102
of flatworm, 102
of frog, 102
Phoronidea, 275
Photosynthesis, 46, 47
Phylum, 247, 248
Physalia, 175, 176
Phvsics, 30-38

Physiology, 16, 20, 22, 39-53
Piltdown man, 345
Pineal body, 153, 156
Pisces, 277, 279
Pithecanthropus, 344, 345
Pituitary, 153, 155, 157

migration stimulated by. 318
Placenta, 183, 184, 185
Planaria, 265, 266
Plant communities, 292
Plants, as food for animals, 47, 48
Planula, 173, 174
Plasma, 126
Plasmodroma, 260
Plastid, 26, 27

Platelets, blood, 126, 127, 130
Plato, 4

Platyhelminthes, 264-266
Pleistocene, 330, 335

Pleodorina, gamet(>s in, 161, 162

sterile cells in, 70, 71-73, 75
Pliny, 5, 6
Pliocene, 330, 335
Pliohippiis, 342

feet of, 341
Pneumatophore, 174, 175
Podophrya, 260
Polar body, 160, 194, 197, 198
Polarity, 28
Polocyte, 197, 198
Polychaeta, 270
Polymorphism, 174, 175

in coelenterates, 263
Polyneuritis, 111
Polyp, 172, 173, 262
Polysaccharide, 40
Ponds, 294-296

vegetation in, 294, 295
Porifera, 261, 262

Portal system, 126

Portuguese man-of-war, 175, 176

Potato beetle, distribution of, 317

Potential energy, 37

Precocial birds, 190

Precoracoid, 92, 93

Prehistoric man, 344-348
American, 348

Primates, evolution of, 335

Priority, law of, 249

Precocious vertebrae, 92

Prophase, 56, 57

Prosecretin, 106

Proteins, 40, 41

Proterospongia, gregaloid colony in, 68,
69

Proterozoic, 330

Prothrombase, 112, 130

Proton, 31

Protonephridium, 134

Protoplasm, movement of, 51-53
organization of, 23-29
structure of, 42, 43

Protozoa, 259-262

essential to termites, 287, 288

Protozoologist, 257

Pseudopodia, 24, 51

Ptyalin, 104

Pubis, 92, 93

Pulmonary circulation, 124

Pupa, 215, 216

422

PRINCIPLES OF ANIMAL BIOLOGY

Purkinje, Johannes, 15
Pus, 127
Pylorus, 102
Pyridoxin, 111

Races, domestic, 366, 367
Radial canal, 175
Radial symmetry, 78

in coelenterates, 79
Radicals, 35
Radius, 93
Rainfall, distribution of, in Michigan,

314, 315
Ranges, continuity of, 311

physical conditions of, 312, 313, 314,
315

position of, 309

size of, 310
Ray, John, 11

taxonomy by, 245
Reactions, chemical, 33, 34
Recapitulation theory, 255, 256, 257
Recent, in geological time, 330, 335
Receptor, 144, 145, 146, 149
Recessive, 225
Rectum, 103

Red corpuscles, 126, 127, 130
Reduction division, 194, 195
Reefs, 303
Reflex action, 146
Reflex arc, 144, 145, 146
Regeneration, of flatworms, 265

of sponges, 262
Regulation, chemical, 152-158
Relicts, 309

Renal corpuscle, 135, 136
Renal portal system, 126
Rennin, 104
Reproduction, 50, 159-176

asexual, 159, 169-174, 176

rate of, 288-290

sexual, 159-168
Reproductive cycle, mammalian, 156-158
Reproductive tissue, 81, 85
Reptiles, evolution of, 335
Reptilia, 277, 279
Respiration, 46, 49, 114-119

as an excretory process, 119
Respiratory systems, 114-116

Responses to stimuli, 52, 53
Retina, 213
Rhinozoa, 247
Rhizopoda, 260
Riboflavin, 111
Ribs, 90
Rickets, 112
Rotifera, 275

Sacculina, homology, 350, 351
Sacrum, 91
Sagitta, 275

X body in, 202
Salamander, 277
Saliva, 104

and excretion, 139
Salivary gland, 101
Salmon, migration of, 318
Salts, 35, 36

of protoplasm, 39, 40
Sand dollar, 267, 268
Sarcolemma, 95
Sarcoplasm, 95
Sargasso Sea, 303
Scaphiopus, distribution of, 309
Scaphopoda, 272
Scapula, 92, 93
Schultze, Max, 15, 19
Schwann, Theodor, 15
Science, definition of, 1
Scorpion, 274
Scyphozoa, 263, 264
Sea urchin, 89, 268
Secretin, 104
Secretion, 46, 50
Section, taxonomic, 248
Segmentation, 73, 194, 200, 201, 202
Selection, natural, 360-363

criterion of advantage in, 362, 305
Self-fertilization, 180, 181
Seminal receptacle, of earthworm, 168
Seminal vesicle, 179

of earthworm, 168
Semipermeable membrane, 43
Sense organs, in embryo, 213
Sensitive animals (Lamarck's classifica-
tion), 247
Sensory neuron, 144

INDEX

423

Series, in paleontology, 329

Serum, 130

Seta, in earthworm, 89

Sex, inheritance of, 233, 234

Sex-linkage, 234, 235, 236

Sexual reproduction, 159-168

Shark, 276

Shell, egg, 200

Shrimp, biogenetic law in, 256

Silurian, 330, 335

Simple gland, 84, 85

Sinus node, 128

Siphonophore, 174, 175, 263

Skate, 277

Skeleton, 87-93

Skin, excretion by, 138

Skink, care of eggs by, 187

Skull, 90

Slime tube, of earthworm, 167

Snail, 271

fossil, 331

reproductive system of, 180
Socrates, 4

Sodium chloride crystal, 35
Soil, ecological relations of, 304, 305
Soma, origin of, 70, 71
Somatic cells, origin of, 202, 205
Somite, 79
Species, 245, 248

intersterility of, 353

nature of, 352, 353
Spermatids, 194, 195
Spermatocytes, 193, 194, 195

primary, 193, 194

secondary, 194, 195
Spermatogenesis, 193, 194, 195, 198
Spermatogonium, 59, 193, 194
Spermatophore, 181
Spermatozoa, 160, 194, 196

in Hydra, 72
Spheroid colony, 68, 69
Spicules, 87, 261
Spider, 273
Spinal cord, 141, 146

in embryo, 205, 210, 211, 212
Spinal nerves, 143, 212, 213
Spindle, 56, 57, 60, 61

location of, 61
Sponges, 88, 261, 262

collared cells in, 52

Sponges, reproduction of, 170

spicules in, 87
Spongin, 261

Spontaneous generation, 159
Sporozoa, 260
Sporulation, 170
Squamous epithelium, 84
Squid, 271
Starfish, 267, 268
Statoblast, of Bryozoa, 171
Steapsin, 105
Stegodon, 336, 337
Stegosaurus, 334
Sternum, 90

Stimuli, responses to, 52, 53
Stomach, 101

in embryo, 210, 212

of frog, 102, 103

of man, 103
Stratified epithelium, 84
Streams, 299, 300
Subclass, 247, 248
Subepithelial cells, in Hydra, 72
Subfamily, 247, 248, 250
Subgenus, 247, 248
Suborder, 247, 248
Subphylum, 247, 248
Subspecies, 247, 248
Succession, ecological, 291-293

in peat bed, 292
Suctoria, 261

Surface phenomena, 44, 45
Sustentative tissue, 81
Sutures, of cephalopod, 343
Swammerdam, Jan, 11
Sweat gland, 138
Sylvius, 8

Symbiosis, 287, 288
Symmetry, 77, 78, 79
Sympathin, 148
Synapse, 28, 145, 150, 151
Synapta, larva of, 191
Syncytium, 66
Synonym, 249
System, 85

in paleontology, 329
"Systema Naturae," 12, 245, 249
Systematic botany, 244-247
Systematic zoology, 244-247
Systemic circulation, 124

424

PRINCIPLES OF ANIMAL BIOLOGY

Tapeworm, 265, 266

Tarsals, 93

•Taxonomic groups, 259-280

Taxonomic ranks, adaptation in, 363, 364

Taxonomy, 20, 21

evolutionary basis of, 250, 251

history of, 244-247

practical, 257, 258

principles of, S44-258

relation of, to other biology, 258
Telolecithal egg, 199

cleavage of, 201
^Pelophase, 56, 58, 59
'J'empeiature, affecting reactions, 282

causing mutation, 283

effect of, on animals, 281-2o3

modifying structure, 283
Tentacles, 172, 173, 174, 175

of Hydra, 71, 72
Teredo, 271
Termites, dependence of, on proto;:oa,

287, 288
Tertiary, 330
Testis, 153, 156, 178

of earthworm, 168

of Hydra, 72
Testosterone, 156
Tetrads, 194, 195, 197
Thales, 2

Thamnophis, distribution of, 315, 316
Theophrastus, 5
Thermocline, 296, 297, 298
Thiamin, 111

Thoracolumbar nervous system, 147
Thricozoa, 247
Thrombase, 130
Throml)oplastin, 130
Tliynnis, 153, 156
lliyroid, 153, 154, 155, 156
Thyroxin, 154
Tibia, 93
Tissues, 81-85
a-tocopherol, 112
Tongue, of frog, 102
Trachea, 115, 116

in embryo, 21 1
Tracheal gills, IKi
Tracks, fossil, 327, 328
Transplantation, embryonic. 218

Transverse process of ^■ertebl•;^, 91

Tree frf)g, carrying eggs. 188

Trematoda. 265, 266

Triassic, 330, 335

Triceratops, 332, 335

Trichinella, 266, 267

Trilobites. 331, 332

Trilophodon, 336, 337, 339

Trochophore, 269

Trypsin, 105

Trypsinogen. 105

Tubular gland, 84

Tunicata, 276, 278

Turbellaria, 265, 266

Turtle, 277

Tympanum, in embr^vo, 211

Type, taxonomic, 249, 250

Typhlosole, earthworm, 102

Ulna, 93

Umbilical vessels, 184

Unconformity, 329

Uniformitarianism, 360

Unit characters, 223

Universal symmetry. 78

Urea, 133, 134

Ureter, 136, 137, 178, 179

Urethra, 137, 179

Urine, 137

Uriniferous tubule, 135, 136

TTrinogenital system, 177-179

Uterus, 178, 179, 180, 184, 185

Vacuole, 26, 27
Vagina, 179, 180
Valence, 3*1

Vasa deferent ia, 179, 180
Vasa (>fferentia, 178
of earthworm, 168
Vascular tissue, 81. 85
Vaucheria, as a syiic\'tiuin, 66
Vegetation areas, 312
Vegetative pole, 199
Veins, 122. 123, 129
Ventricle, 123, 124
Vermiform appendix, 103
Vertebra, 80, 91

INDEX

425

Vertebral eoliunn, 90

divisions of, 91
Vertebrata, 278-280

evolution of, 335
Vesalius, Andreas, 7, 8
Villus, 106, 107
Viscosity, changes in, 45
Visual purple, 1 10
Vitamin K, 130
Vitamins, 109-112
Viviparous, 183, 184, 185
Voluntary muscle, 96
Vol vox, gametes in, 161, 162

sterile cells in, 70, 71-73, 75

Wastes, water, 134
Wat(>r of protoplasm, 39, 40
White corpuscles, 126, 127
\\'liole, relation of, to parts, 64, 65

X body, Sagitta, 202

X chromosome, 233, 234, 235

Xenophanes, 2

Xerophthalmia, 110

X rays, as cause of mutation, 357

Wallaby, pouch of, 189

Wallace, Alfred Russel, natural-selection

doctrine of, 361
Wastes, defined, 133

gaseous, 133

origin of, 133

Y chromosonic. 234, 235

Zoogeography. 22, 307-324
major realms in. 323, 324
Zygapophyses, 91
Zygote, 159, 173

:i;:^!:::!;'!jnf!fii

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