MARINE BIOLOGICAL LABORATORY.

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McGRAW-HILL PUBLICATIONS IN THE

ZOOLOGICAL SCIENCES

A. FRANKLIN SHULL, Consulting Editor

GENERAL ZOOLOGY

Selected Titles From

McGRAW-HILL PUBLICATIONS IN THE

ZOOLOGICAL SCIENCES

A. Franklin Shull, Consulting Editor

Chapman Animal Ecology
Goldschmidt ■ Physiological Genetics
Graham • Forest Entomology
Haupt • Fundamentals of Biology
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
Senning ■ Laboratory Studies in Comparative Anatomy
Shull ■ Evolution
Heredity
Shull, LaRue, and Ruthven ■ Principles of Animal Biology
Snodgrass • Principles of Insect Morphology
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 Edmund W. Sinnottis Consulting
Editor, and in the Agricultural Sciences, of which Leon J. Cole is
Consulting Editor.

GENERAL ZOOLOGY

BY

H. L. WIEMAN

Professor of Zoology, University of Cincinnati

Third Edition

McGRAW-HILL BOOK COMPANY, Inc.

NEW YORK AND LONDON
1938

Copyright, 1925, 1927, 1938, by the
McGraw-Hill Book Company, Inc.

PRINTED IN THE UNITED STATES OF AMERICA

All rights reserved. This book, or

parts thereof, may not be reproduced

in any form without permission of

the publishers.

THE MAPLE PRESS COMPANY, YORK, PA.

PREFACE TO THE THIRD EDITION

With the exception of perhaps a dozen pages in which only
minor changes have been made, the preparation of the present
edition has entailed the complete rewriting of the text of the
preceding edition. This has been necessary for the accomplish-
ment of desired changes in emphasis and in the order and arrange-
ment of subject matter. Some new material on the frog has
been added with the idea of correlating the subject matter of
the text more closely with that of the laboratory program and,
as a result, the frog assumes a position of importance in the
early chapters of the book. This emphasis on the frog is in
keeping with the common practice of using the frog as the sub-
ject of laboratory study at the very beginning of the introductory
course in zoology. The primary reasons for selecting the frog
for this purpose are that it is a common animal, large enough for
the inexperienced student to study with profit, and also that it
displays easily recognizable resemblances to the human body
in both structure and function. In addition, the frog and allied
forms have played an important part in the hands of investiga-
tors in the development of the science of biology.

Another departure from the preceding editions consists in the
attempt to avoid a cleavage between structure and function in
the descriptions of individual organs and organ systems, by
combining the discussion of structure with that of function, as
far as possible. The aim, of course, is to emphasize the essential
unity of structure and function. Thus metabolism is discussed
in connection with the structure of the individual organs con-
cerned rather than pigeonholed in a separate chapter. Similarly,
examples of different tissues are described as they are encountered
in the study of gross anatomy rather than under a separate head-
ing at the beginning of the book, where they have but little
meaning for the student.

The chapter on endocrines has been completely reorganized
and brought up to date. A more logical order has been followed
in the treatment of the topics of heredity, evolution, and adapta-

vi PREFACE TO THE THIRD EDITION

tion. A short chapter on environment and distribution has been
added. The final chapter on systematic zoology has been con-
siderably enlarged. A glossary has been included and also a list
of references for further reading. Some fifty additional illus-
trations have been inserted.

The author is grateful to his colleagues, Dr. C. K. Weichert
and Dr. W. A. Dreyer, for helpful suggestions and criticisms,
and to his wife for invaluable aid in preparing the index.

H. L. WlEMAN.

University of Cincinnati,
May, 1938.

PREFACE TO THE FIRST EDITION

The present volume is the product of some years' experience
in developing an introductory course in zoology designed to
meet the needs of general college students and at the same time
to satisfy the technical requirements of various groups of pre-pro-
fessional students, especially those preparing for medicine.
The foundation for such a course must be laid in the actual study
of animal forms in the laboratory but the limitation in time pre-
cludes the presentation and demonstration of many important
and interesting aspects of the subject by the usual laboratory
methods. For this reason the author, sharing the experience of
others, has found it advisable to devote two or three lecture or
recitation periods weekly to the enlargement and rounding out
of the student's knowledge of the subject by class-room discus-
sions of those phases of zoology not adequately dealt with in the
laboratory. The problem of a suitable text for this work has
been met by the compilation of the contents of this book, which is
a rather condensed account of some of the outstanding facts and
principles of zoology, selected and arranged to serve the student
as a guide; the form of the text having been kept as flexible as
possible to permit of expansion or extension in whatever direc-
tion the instructor may see fit. The references listed at the end
of each chapter are easily available works which the author has
been in the custom of assigning for collateral reading.

The book begins with a consideration of such general topics
as morphology and physiology, the protoplasmic doctrine, the
cell doctrine, taxonomy and adaptation; followed by an outline
of organology illustrated by examples from common laboratory
animals, considered largely from the morphological side. Then
follows a section dealing primarily with the functional side of
the animal organism, centering in metabolism. Next come the
main facts of ontogenesis, followed in turn by a discussion of
phylogenesis, evolution and heredity. To cover this ground
takes the greater part of a year, the remainder of which is devoted
to a general survey of the animal kingdom, as outlined in the final
chapter, paying particular attention to life histories and the sys-
tematic side of zoology, which the author believes should form

vii

viii PREFACE TO THE FIRST EDITION

an integral part of the course. Such a systematic survey can
best be undertaken toward the end of the year when the stu-
dent through his laboratory work has obtained sufficient familiar-
ity with animal forms and taxonomy to make it worth while.
Incidentally there is perhaps no better way to review or sum-
marize the course and to bring out vividly the applications of
biological principles. The author realizes that principles are
based upon facts and that logically facts should come first; the
plan of presentation followed here is a compromise which is suc-
cessful with beginning students and which at the same times does
not violate standards of scientific method.

In the laboratory the endeavor is made to have the student
obtain a substantial knowledge of a relatively few animal forms as
whole organisms rather than a smattering acquaintance with
parts of many. Selection of animal forms for laboratory study is
a matter of judgment guided by experience. The author has
found it best to begin with a relatively large animal, like the frog,
then turning to Protozoa and working up the scale. This general
program is broken into from time to time with special demonstra-
tions, collecting trips and visits to the zoological garden. For
the laboratory work mimeographed directions are used similar
to those used in the work in other places.

Naturally it is not always possible to correlate lecture and
laboratory work but the class-room instructor constantly bears
in mind what the student is doing in the laboratory and, so far
as possible, makes full use of the student's laboratory experience
in dealing with the subject under discussion. Free use of special
demonstrations helps to obviate this difficulty.

In the compilation of the material for this book the author has
made use of many sources which are acknowledged in the text and
in the lists of references at the ends of chapters. The majority
of the illustrations are new drawings based upon the author's
material or figures of others; the remainder have been taken
directly from other works with the permission of the publishers
whose courteous cooperation is duly appreciated. The author
is also indebted to his wife for valuable assistance of a miscel-
laneous sort at each stage of the book's progress; and to Dr.
C. V. Piper for helpful editorial suggestions.

Cincinnati, O. H. L. Wieman.

June, 1925.

*o\

CONTENTS

Page

Preface to the Third Edition v

Preface to the First Edition vii

CHAPTER I
The Study of Zoology 1

CHAPTER II
The Animal Organism 26

CHAPTER III
Integument 42

CHAPTER IV
Endoskeleton and Voluntary Muscle 54

CHAPTER V
Alimentation 85

CHAPTER VI
Circulation and Respiration 120

CHAPTER VII
Excretion 144

CHAPTER VIII
Means and Methods of Reproduction 153

CHAPTER IX
The Nervous System 165

CHAPTER X
The Endocrine System 205

CHAPTER XI
Cell Division and Gametogenesis 219

ix

49471

x CONTENTS

Page

CHAPTER XII
Ontogeny 237

CHAPTER XIII
Heredity 262

CHAPTER XIV
Evolution 290

CHAPTER XV
Adaptation 324

CHAPTER XVI
Environment and Distribution 359

CHAPTER XVII
The Animal Kingdom 367

References 459

Glossary 463

Index 477

GENERAL ZOOLOGY

CHAPTER I
THE STUDY OF ZOOLOGY

Biology, a word compounded of two Greek nouns, /3tos (bios),
life, and \oyos (logos), discourse, is believed to have been used for
the first time as a scientific term by Gottfried Reinhold Tre-
viranus (1776-1837) in his work entitled "Biologie; oder die
Philosophie der lebenden Natur" (Biology; or the philosophy of
living nature), published between 1802 and 1805. It was
adopted by the French naturalist, Jean Baptiste Lamarck
(1744-1829), and has since spread into general use. Biology is
the science of organisms, the study of living things. Since living
things for the most part fall into two general categories — animals
and plants — the two major fields of biology are concerned with
these two natural groups. Zoology, from faov (zoon), animal,
and X070S (logos), discourse, deals with the animal side of biology,
while botany, from fioTavq (botane), plant, has to do with the
study of plants.

In zoology a knowledge of the nature of life is sought by
studying animal organisms at rest and in various stages of
activity, under natural and controlled conditions. A knowledge
of the structure and function of the animal body is highly impor-
tant in reaching a scientific view of the nature of life because the
collective phenomena constituting what we call life are only
manifested in association with the material substance of the
organic body. Matter without life is common enough, but life
without matter is unknown to science. As a first step in seeking
a solution of the problem of life and explaining it in scientific
terms a careful study of the organic body in all of its structural
and functional details is necessary.

Approach. — The common aim of all biological studies is an
understanding of the origin and nature of life. The biologist

1

2 GENERAL ZOOLOGY

attempts to reach such an understanding by assuming, as a work-
ing hypothesis, that life had its origin in natural rather than
supernatural causes and that it is therefore a phenomenon sub-
ject to the laws and principles of the basic sciences of physics and
chemistry. The ultimate goal of the scientific searcher — the
explanation of living phenomena in terms of scientific phe-
nomena— is still far from achievement; in fact it may be impos-
sible of achievement for the reason that life may involve some
factor or principle outside the realm of chemistry and physics.
However that may be, the broad, general, and practical lines of
approach to the study of the problem of life are relatively few in
number and may be summarized as follows :

1. The study of the physical and chemical properties of proto-
plasm, the living matter of which organisms are composed and
with which life is always associated. Such work falls largely in
the province of the biochemist who deals with protoplasm as a
substance to be analyzed by appropriate chemical laboratory
methods.

2. The study of the morphology or the form and structure of
animals and plants. This is commonly subdivided into:

a. Gross anatomy, which is morphological study in its
broad aspects, carried on by the dissection of the bodies of
animals. In gross anatomical studies the analysis of structure is
made with the unaided eye, i.e., without the use of a magnifying

lens.

b. Microscopic anatomy or histology, the study of the more
minute structure of tissues and organs, in which special prepara-
tions are made for examination and study under the microscope.

c. Cytology, the study of the finest structural details of
cells, the morphological units of which tissues and organs are
composed; the microscopic study of protoplasm as distinguished
from histology, which is primarily concerned with the study
of the structure and arrangement of cells.

d. Embryology, the study of the development of an
organism; the study of embryogeny. In embryology the
methods of dissection and microscopic study are both used.

3. Physiology, the study of the functions of organs considered
separately and in relation to the organism as a whole. It deals
with the process of waste and repair in living things, food, and the
sources and transformation of energy. It also has to do with the

THE STUDY OF ZOOLOGY 3

underlying causes of animal behavior. In its method physiology-
is largely experimental.

4. Ecology, the study of the relations of an animal or plant to
its environment both animate and inanimate. It is funda-
mentally physiological in character but involves also a considera-
tion of morphological features of animals and plants as well as
attention to such factors as temperature, moisture, light, pres-
sure etc., which make up the nonliving environment.

5. The study of evolution, the process through which, there is
much reason to believe, the great wealth and diversity of life and
its distribution in time and space have been brought about.
Evolution is a central principle in biology which holds that the
present condition of life on the earth has been arrived at through
changes extending over long periods of time, and that in general
higher forms of life have evolved from lower forms, through
natural processes. The fields of study devoted principally to the
topic of evolution are:

a. Paleontology, the study of prehistoric forms of life, a
record of whose existence has been preserved in the form of fossils
embedded in the earth. It deals with the distribution of organ-
isms in time.

6. Zoogeography, the study of the distribution of animals in
space.

c. Genetics, the study of the origin of the individual
organism as distinguished from evolution in which the problem
is the origin of kinds of organisms. Genetics is the science of
heredity.

6. Taxonomy or systematic zoology, the science of classification
of organisms, which is concerned with naming organisms and
indicating as far as possible the relationship between different
kinds.

Structure and Function. — In the various methods of approach-
ing the subject of biology, structure of the organism and function
form the background of the study. Structure and function
represent two aspects of the organization of the living body. A
general distinction is sometimes made between structure and
function in which structural details are regarded as representing
the static side of organization, in the sense that the actual form
and structure of the components of the animal body stand for the
visible machinery by means of which functions are carried out;

4 GENERAL ZOOLOGY

while function, since it is concerned fundamentally with the pro-
duction of motion, is considered dynamic in nature. Such
distinction is of value in a general way only, since matter, and
particularly living matter, is constantly changing and therefore
never absolutely static. In other words, living objects like non-
living objects exist not only in a space relationship but also in a
pattern of time. The organism therefore should not be regarded
as matter (structure) plus motion (function) but rather as
matter in motion, or as structure in function. It is not necessary
to decide whether structure causes function or function causes
structure, for neither exists without the other. Structure and
function are complementary.

Though structure and function considered separately have no
real meaning, for purely practical reasons the study of organisms
is pursued along two general lines, morphological and physio-
logical. Ideally, these should be carried out simultaneously or
as nearly so as possible, but for the beginning student it seems
best to begin by acquiring a sound knowledge of morphology as a
background for physiology. In such preliminary morphological
studies, questions as to the possible or probable functional
significance of structural relationships constantly arise. Such
questions should be raised even though a satisfactory answer may
not be forthcoming until a later time. Morphological detail is
meaningless without functional interpretation, nevertheless in
most cases a preliminary mastery of morphology facilitates the
attack on physiology and is the method followed in this book.

Morphology and physiology are merely convenient terms for
classifying two fairly distinct groups of findings and the assign-
ment of a field of study under one head or the other is always
subject to some limitation. Thus embryology is ordinarily con-
sidered a morphological subject because its study consists to a
great extent in the examination and study in considerable detail
of the form and structure of embryos at different stages of devel-
opment ; but since this is done for the purpose of piecing together
a history of the process of development, an embryological study
that does not give due weight to the physiological aspects of
development becomes a series of lifeless descriptions of embryo
anatomy.

By means of gross microscopic dissections of an animal, com-
bined with carefully planned laboratory experiments to demon-

THE STUDY OF ZOOLOGY 5

strate functional features, a detailed knowledge of its anatomy
and physiology may be obtained; yet a far from satisfactory
conception of the animal as an organism is gained unless labora-
tory observations are supplemented by studying the animal
in its natural state in the field and thus noting its relationships to
other organisms and to its environment generally. Ecological
field studies often supply the key to morphological and functional
adaptations because adaptations are adjustments to environ-
mental conditions. Since different animals occupy different
stations or levels in the living world, a complex interlocking
environmental relationship has been built up, a knowledge of
which is necessary in order to gain a full understanding of any
given organism. But even after exhausting these avenues of
approach, the problem of the organism still presents difficulties
which can be removed — and not completely at that — only by
applying all the refinements of technique of the sciences of
physics and chemistry. The problem of the living thing is
beset with difficulties many and diverse because of the presence
in it of so many variable factors.

The Protoplasm Doctrine. — From the point of view of the
biologist protoplasm is the living substance of which the bodies of
animals and plants are composed. The protoplasm doctrine
of life assumes that life is a development of protoplasm or that
life results from a certain physical and chemical combination of
matter such as occurs in protoplasm; that protoplasm is life.
This doctrine has not been proved — it is merely a working
hypothesis that has been more serviceable than any other
hypothesis in extending our knowledge of the nature of the
processes that go on in the living body. As a working
hypothesis it will be adhered to so long as its application yields
results.

Physical Properties of Protoplasm. — All of the material of the
animal body is not living matter. Certain parts of the body such
as the hard covering of an insect or parts of the bone of the
vertebrate skeleton are products of the activity of the proto-
plasm; i.e., they are formed through the agency of living tissue
and serve definite functions but are not, strictly speaking, alive.
Such substances can scarcely be regarded as typical protoplasm.
Rather than call such substances dead material they may be
regarded as inert substances subject to the control of active

6 GENERAL ZOOLOGY

protoplasm and which form part of the picture of the anatomy of
some animals.

A small mass of living protoplasm viewed through the micro-
scope appears as a faintly grayish, jellylike substance in which
granules are suspended. Sometimes a streaming or flowing
movement of materials can be seen. Protoplasm is heavier than
water and somewhat more refractive to light.

Protoplasm has a semifluid consistency which is known as a
colloidal state of matter. This means that some of its constit-
uents at least, instead of being in the form of a true solution,
consist of suspensions of molecular aggregates dispersed in a
medium. The medium in this case is largely water. In a true
solution these aggregates of molecules would separate as indi-
vidual molecules which in turn might dissociate into ions. The
molecular aggregates are therefore much larger than the particles
making up the solute in an ordinary solution. Their size varies
roughly between 0.0001 and 0.000001 mm. in diameter. In
protoplasm there seem to be three physical systems, viz., true
solutions, emulsions, and gels, of which the latter two are colloidal
systems.

The colloidal state is common enough in nonliving nature
where the components may be solid, liquid, or gaseous, in various
combinations. Thus smoke is a colloidal mixture of solid in a
gas; foam, of gas in a liquid; fog, of liquid in a gas, etc. Proto-
plasm seems to be largely an emulsion, i.e., the components con-
sist of liquid phases, such for example as is produced when oil is
shaken up in water. In an oil-and-water mixture the components
usually separate in the course of time, if undisturbed, unless some
other substance is added that serves to keep one liquid phase
dispersed in the other. This is accomplished in the preparation
of mayonnaise dressing by adding yolk of egg to an emulsion of
oil and water (vinegar). The yolk of the egg serves to preserve
the colloidal state of the oil in the aqueous medium by acting as a
binder. The physical state of protoplasm is roughly approxi-
mated by the physical state of mayonnaise dressing.

Substances in the colloidal state diffuse slowly or not at all
through animal membranes, which partially explains why living
protoplasm does not dissolve in water. Colloids within limits
have the power of changing from a fluid or sol state to a more
solid or gel state and back again.

THE STUDY OF ZOOLOGY 7

From the purely physical side, protoplasm resembles sub-
stances like glue or gelatin rather than crystalline substances
like cane sugar or sodium chloride. When living cells are sub-
jected to dissection under the microscope with extremely fine
needles, the more solid portions (gels) can be drawn out into
thin threads having considerable tenacity. The viscous prop-
erties of protoplasm can also be demonstrated by centrifuging
living cells. Under the pull of the centrifugal force the heavier
components are dragged through the cell and become arranged in
layers according to density. If the centrifugal force is great
enough, the cells can be pulled into dumbbell shapes, the proto-
plasm between the heavier and lighter half being drawn out into
a thin strand, which may finally snap.

The mere touch of a needle to a living cell may cause liquefac-
tion or the reverse change, gelation, to occur with extreme
rapidity. The reversibility from solid to liquid, or liquid to
solid, is one of the striking features of protoplasm and one which
may be regarded as a characteristic of the living colloid.

Composition of Protoplasm. — Protoplasm is not a definite
chemical compound in the technical sense. It is rather a chem-
ical complex of substances, the percentage composition of which
is not the same for all kinds of protoplasm. The same kinds of
substances occur in all forms of protoplasm, but the proportions
and the actual composition of the different constituents vary.
The most abundant constituent is water, which accounts for
from about 75 per cent to more than 90 per cent of the total
weight of the protoplasm. Water is utilized as a solvent and as
such plays an important part in bringing about chemical reac-
tions. It is also required to maintain the colloidal state. In
some marine forms as in jellyfishes, water forms 99.8 per cent of
the body weight. The human body contains about 65 per cent of
water, the weight of the bones lowering the average. Inorganic
salts such as sulphates, chlorides, phosphates, and carbonates of
sodium, potassium, magnesium, and calcium, and small amounts
of iron make up about 5 per cent of the whole. Small amounts
of iodine, manganese, copper, zinc, barium, and silicon are present
in varying amounts. Of the remaining constituents, viz., pro-
teins, carbohydrates, fats and extractives, which are organic in
nature, protein seems to be the most important and forms about
15 per cent of the human body. Protein is a very complex

8 GENERAL ZOOLOGY

substance which has never been synthesized. Chemical analysis
shows that it contains carbon, hydrogen, nitrogen, oxygen, sul-
phur, phosphorus, and sometimes iron. Fats and carbohydrates
contain carbon, hydrogen, and oxygen, but in different propor-
tions and combined in different ways. Starches and sugars are
examples of carbohydrates. Extractives include substances like
urea, CO(NH2)2, creatinine, C4H7N3O, inosite, C6H6(OH)6, all of
which are soluble in water by means of which they can be
extracted from coagulated protoplasm.

The substances making up protoplasm are determined by
chemical analysis, which gives but little more than the percentage
composition of these constituents. Such an analysis shows that
protoplasm is made of chemical elements of common occurrence
in inorganic nature and also that there is no chemical element
peculiar to protoplasm. Such a simple analysis throws little
light on how the chemical elements are combined. For example,
the amount of water in a sample of protoplasm is determined by
driving off the water by means of heat; the difference in weight
before and after representing the percentage of water. Thus
by this method mammalian meat is found to contain 76 parts by
weight of water. If the same piece of tissue is reduced to ash by
incineration, the weight of the residue represents the total of
inorganic salts which in this case is about 1 per cent of the whole.
Other methods show that this same kind of tissue is composed of
21.5 per cent of nitrogenous material and 1.5 per cent of fat.
The real problem is to explain how these various components are
combined in protoplasm. Certainly the inorganic salts and
organic substances are not present as such in living matter but
are combined in a chemical complex along with the water in a very
intricate manner. If we are to look upon protoplasm as a form
of physicochemical system, it differs from other systems in its
internal atomic or molecular organization. In other words, the
difference between protoplasm and other chemical combinations
is not in the chemical elements concerned but in the manner in
which they are combined chemically and physically. A random
mixture of constituents in the proportions and concentrations
as determined by a chemical analysis of protoplasm would not
produce a system having the properties of living matter. In
order to produce a living system, what is required in addition is a
knowledge of the structure or arrangement and the physical state

THE STUDY OF ZOOLOGY 9

of the constituents, both of which are necessary to provide a
basis for the orderly interactions that characterize living matter.

Metabolism. — An organism maintains itself by taking in
substances from the environment and at the same time excreting
other substances, most of which are of no value to it. The
energy generated in an organism and utilized in various ways is,
in the case of animals, obtained from food. In the body the food
is transformed; some of it is built up into protoplasm and the
remainder undergoes oxidation with the release of energy. What
are called waste products are the end products of chemical
reactions and are of no further use to the body as sources of
energy. The organism functions as a transformer of energy and
remains alive only as long as it is capable of carrying out this
function. Metabolism is a collective term which includes all
the chemical and physical changes involved in the transformation
of food, the elimination of water, and the functional activity
of all the organs of the body. In general it consists of a twofold
process of disintegration, or catabolism, and reintegration, or
anabolism. In disintegration, complex chemical compounds are
broken down, in the course of which energy is released and waste
substances, poor in energy content, are produced. This process
may take place in certain food substances before they are incor-
porated as a part of living protoplasm and it also occurs in proto-
plasm itself. The reintegrative phase of metabolism is the
constructive process which makes good the losses accompanying
disintegration of protoplasmic constituents. The two general
processes are taking place continuously in the living organism.
When metabolism ceases, death ensues.

The principal difference in the metabolism of animals and
pigmented plants lies in the fact that animals require food of an
initial complexity for the support of life while the colored plant is
capable of manufacturing food from raw materials of relatively
simple composition. Chlorophyll-bearing plants in the presence
of sunlight are capable of utilizing water and carbon dioxide to
synthesize starch and other energy-containing substances from
them. The carbon dioxide comes from the air, the water
from the soil. Protein is synthesized from substances in the
soil. Nitrogen for the use of protein building comes largely from
nitrates in the soil, which also supplies other necessary elements
including sulphur and phosphorus. The process of synthesizing

10 GENERAL ZOOLOGY

starches under the influence of light is called photosynthesis.
Only plants possessing chlorophyll or other similar substances are
capable of carrying out this function. In photosynthesis, carbon
dioxide and water are absorbed by the plant and oxygen is given
off. In animal respiration, oxygen is absorbed and carbon
dioxide and water given off. There is thus a reciprocal relation
between animals and plants in the use of these substances.
Similarly, other waste products of animal metabolism can be
utilized by the plants in synthesizing proteins, after such nitrog-
enous waste products have been acted on by bacteria present in
the soil.

Photosynthesis does not occur at night or in the dark. Under
these conditions oxygen is taken in and carbon dioxide is given
off, just as in animals, regardless of light conditions. Respira-
tion of oxygen, then, is a characteristic of metabolism in all
organisms. Photosynthesis is a process which accompanies
respiration in plants, taking place when light is available. Dur-
ing photosynthesis, respiration is overshadowed by the construc-
tive activities of the plant tissues. Properly speaking, oxygen
is not a food, if one defines food as a fuel or a means of repair.
Oxygen is a necessary element in processes of oxidation which
accompany all energy-releasing reactions of the body.

Movement. — Nonliving things as a rule move under the influ-
ence of gravity. Occasionally, as in a volcanic eruption or an
explosion of dynamite or similar substances, nonliving objects
may be moved by the action of forces generated by processes of
combustion. The power of controlled or independent movement
exhibited by animals results from internal processes of combus-
tion or oxidation. The energy liberated is utilized in producing
movement. Movement is one of the outstanding character-
istics distinguishing animals from plants. In plants movement
is largely limited to changing positions of the foliage in response
to sunlight. It is true of course that in some plants, as in
Mimosa, the sensitive plant, whose leaves fold up in response to
touch, movements of parts resemble similar movements in
animals; but on the whole movement has been much more highly
perfected in animals than in plants.

Irritability. — Animals respond when subjected to disturbing
influences. The disturbing influence or stimulus may be pro-
duced by impact, heat, light, sound, electricity, chemical action,

THE STUDY OF ZOOLOGY 11

etc. Irritability refers to this ability to respond to stimuli of
various sorts. Knowledge of the external world is gained from
influences which stimulate part or all of the sensory mechanism
of an animal. Movements are the result of stimulation of the
motor apparatus by nervous impulses acting on muscle tissue.
The flow of digestive juice may be invoked by the chemical
stimulation of the walls of the digestive canal by food substances.
The entire mechanism for the coordination of functional activ-
ities in different parts of the body depends ultimately on the
irritability of protoplasm to certain stimuli.

Growth. — Organisms subsist on materials taken from the
environment. During metabolism a certain portion of the
absorbed food products is built up into protoplasm by what is
known as intussusception, which means the deposition of new
particles of material among those embodying the living sub-
stance. Protoplasm is constantly growing from material
supplied from the outside but incorporated from the inside.
This occurs after undergoing preliminary changes, in chemical
and physical form, called digestion. Food in the process of
digestion is changed into a form in which it can pass through
surfaces of cells and be assimilated. Only a fraction of the
digested food is required for anabolic processes. Most of it is
oxidized without ever becoming an actual part of living tissue.
The immediate cause of increase in size of an organism is the
multiplication of individual cells by a process of cell division.
When the adult stage is reached, cell division ceases in the body
generally. The growth process characterizing the preadult
period results from an excess of anabolic over catabolic processes,
accompanied by cell multiplication.

Size and Form. — Another peculiarity of living things is that
each kind has a determinate size and form, which is reproduced
as a rule generation after generation within rather narrow limits
of variation. Lifeless objects may be of almost any size and
form as, for example, water may be in the form of a rain drop or
a lake; a stone may be a pebble or a mountain. The distinctive
size and form of any given species are due to factors that are
rooted in the organization of the protoplasm of that particular
species.

Reproduction. — Living things reproduce their own kind. In
Protozoa, unicellular animals, reproduction consists in the

12 GENERAL ZOOLOGY

division of the entire animal into halves, each of which
then grows to the original size. In multicellular animals, this
same simple method may take place, but only in lower forms.
The method of reproduction common in all forms above the
Protozoa is far more complicated and its discussion must be
postponed until a later period. Suffice it to say, that the ele-
ments of the reproductive process are an egg cell, produced by
the female and a sperm cell produced by the male. These unite
in fertilization and out of this relatively simple beginning the
organism gradually develops into the adult form. Each stage of
development is alive. The different stages represent the organ-
ism in stages of transformation. Under some conditions the
egg alone may reproduce the adult animal, showing that at least
in some forms the potency for reproduction is possessed by the
egg alone.

Vitalism and Mechanism. — The list of attributes of proto-
plasm as outlined above constitute a definition of an organism.
Can these attributes be explained in terms of the protoplasm
doctrine or is it necessary to assume the presence of some super-
natural, vital factor in addition to known chemical and physical
factors? Formerly, the idea that the living body is presided over
or directed by some kind of vital force or energy such as the
"soul" of Descartes or the "entelechy" of Driesch met with
more general acceptance than at the present time. The pro-
ponents of such a view are usually spoken of as "vitalists," and
their doctrine is known as vitalism. According to the vitalists, a
living thing is protoplasm plus some unknown something that is
absent in nonliving objects. The unknown thing is called the
vital factor, without which life cannot exist. The objection to
such a view from the scientist's standpoint is that it bars the way
to investigation. If investigators of living matter must always
admit the presence of a factor which cannot be seen, felt,
weighed or measured, that is to say, a factor that cannot be con-
trolled, his attempt to solve the riddle of life in scientific terms
is doomed to failure from the start. Such a view discourages
investigation and creates an atmosphere of hopelessness which is
not conducive to success. It does not help but actually hinders
further investigation into the problem. For this and other
reasons many biologists are inclined to support the protoplasm
doctrine which is a mechanistic theory of life to the extent that it

THE STUDY OF ZOOLOGY 13

attempts to explain the phenomena of life in terms of chemistry
and physics. The rejection of the vitalistic hypothesis by
biologists generally must not be attributed to narrowness on
their part, but simply to the lack of satisfactory evidence to
support the vitalistic doctrine. Since the vital factor does not
exist, scientifically speaking, the scientist is compelled to do
without it, since he can deal only with those facts and factors
in the problems of protoplasm that are amenable to scientific
analysis and measurement. The main support of the mecha-
nistic interpretation, from a practical point of view, is the fact that
its application has actually overthrown hypothetical vitalistic
explanations of many phases of metabolism and replaced them
with understandable explanations in terms of chemistry and
physics. Such a view recognizes a common background for all
natural phenomena and assumes that living and lifeless objects
are forms of matter and energy. On this basis the organism is a
physicochemical system which owes its peculiar properties to its
physicochemical make-up, and which differs only in its internal
arrangement from known nonliving physicochemical systems.

Cells. — Protoplasm exists in the form of small structural units
more or less distinctly marked off from one another, and known as
cells. In the Protozoa, the entire body of the animal is a single
cell; but in all higher forms, the Metazoa, the body is composed
of many cells, structurally and functionally differentiated. In
1838, Mathias J. Schleiden, a botanist, and Theodor Schwann, a
zoologist, each published observations on the structure of plant
and animal tissues which led to the formulation of what is known
as the cell theory. The central idea of this theory — that the
bodies of animals and plants are composed of cells and the
products of cells — was undoubtedly held by R. J. H. Dutrochet
as early as 1824, but a clear description of the cell nucleus by
Robert Brown in 1831 enabled both Schleiden and Schwann to
distinguish between cells and other structures, and led them to a
more accurate conception of the cellular structure of organisms.
Schwann in 1839 published a very precise account of cell struc-
ture in a variety of animals which surpassed in accuracy and
completeness that of any of his predecessors. The protoplasm
doctrine is a much later development and not until 1861 was it
clearly understood that cells are really masses of protoplasm and
that protoplasm is essentially similar in all organisms. This was

14 GENERAL ZOOLOGY

a logical conclusion that united the cell theory and the protoplasm
doctrine, and provided a material basis for a scientific theory of
life. Protoplasm is a living substance and this substance has a
cellular structure.

A cell may be defined as a small mass of protoplasm containing
one or more nuclei. The cytoplasm or cytosome forms the body
of the cell in which the nucleus is embedded. This means that
the protoplasm of the cell is made up of two regions, nuclear and
cytoplasmic. These two regions are not merely topographical
distinctions, but they actually mark the boundaries of two
different kinds of protoplasm which have distinct chemical
attributes. It also happens that under certain conditions the
boundaries between cells are not sharply marked off, forming
what is known as syncytial tissue structure; therefore, the
significance of cells does not depend so much upon their morpho-
logical distinctness as upon the fact that each cellular region is
made up of a nuclear and a cytoplasmic component. The cell is
something more than a building block with sharply defined
boundaries. It is a functional unit which functions with or
without cell boundaries.

Cells do not conform to a standard shape. Free cells, such as
egg cells, particularly those deposited in water, are often spherical
in form. Tissue cells generally, owing to the pressure of the
surrounding cells, take on some other form. Under conditions of
relatively uniform pressure, as in the liver, cells are poly-
hedrons with approximately equal faces. At the outer surface of
the body, cells tend to become flattened and thin, as in the surface
layer of the frog's epidermis. Nerve cells have a round or
irregular region in which the nucleus is located and from which
slender processes extend, in some cases for several feet. Muscle
cells are usually spindle-shaped. Pigment cells have irregular
branching processes extending in all directions. In general some
correlation exists between the shape of the cells and its function
(Figs. 1, 2, and 3).

The nucleus is usually a centrally located body whose shape
conforms with the general shape of the cell. Thus, in a spherical
cell the nucleus is rounded, whereas in a columnar cell the
nucleus is elongated in the main axis of the cell. In the tissue
cells of the metazoan animals, as a rule, there is but one nucleus
in a single cell. In syncytial tissue, where cell boundaries are

THE STUDY OF ZOOLOGY

15

lacking, a cytoplasmic mass may contain numerous nuclei, all of
which are alike. Protozoa often show two kinds of nuclei in a
single cell. Thus in Paramecium (Fig. 2) there is a small
spherical micronucleus embedded in the side of the large kidney-
shaped macronucleus. In some species there are two or more
micronuclei. By microdissection it can be demonstrated that the
nucleus is bounded by a definite membrane. Some cells, such as
mammalian red blood corpuscles in the blood stream, lack a

Fig. 1. — Cells. A, three cells from the outer layer of the skin of the sala-
mander one of which (d) is dividing, c, chromosomes; o, yolk globules; n,
nucleus; B, blood cell of the salamander, nucleated. C, human red blood cells,
non-nucleated. X 900.

nucleus though each corpuscle is a nucleated cell at an earlier
stage of its development. Properly speaking, such a corpuscle
is only part of a cell and its life in the blood stream is of short
duration (about ten days in the case of man).

The nucleus contains an important substance called chromatin
which, unless the cell is dividing, appears in fixed and stained
sections as flocculent masses of irregular outline. In the course
of cell division the chromatin is transformed into very distinct
bodies called chromosomes, each of which then divides in half, so
that both daughter cells receive the same number and kinds of
chromosomes. Chromosomes are important in connection with

16

GENERAL ZOOLOGY

YiG. 2. — Cells. A, two pigment cells from the skin of the salamander. B.
Paramecium, a ciliated protozoan, a single-celled animal, c, contractile vacuole,
expanding; e, contractile vacuole, expanded; f, food vacuoles; g, gullet, a primi-
tive alimentary tract; M, macronucleus; n, micronucleus. X 300. The con-
tractile vacuoles are filled with cell fluid which is emptied on the outside when
the vacuole contracts, thus maintaining an intracellular circulation. Food
vacuoles are simple organs of digestion.

Fig. 3. — Endothelial cell from a tissue culture of the chick embryo, grown in
a culture medium on a glass cover-slip inverted over a hollow-ground slide;
stained to show mitochondria, m. X 1080. {From a photo by W. H. Lewis.)

THE STUDY OF ZOOLOGY 17

heredity. A detailed account of cell division and the behavior
of the chromosomes is reserved for a later chapter. A nucleolus
is also usually present in a nucleus. It is as a rule rounded in
shape and stains differently from the chromatin. Its function is
unknown. The remainder of the nucleus is composed of karyo-
ly?nph which serves as a matrix in which the chromatin and
nucleolus are suspended. This ground substance is semifluid in
character.

In the cytoplasm the principal formed elements common to all
cells are the mitochondria and the Golgi material (Figs. 3 and 4).
The mitochondria are minute objects, usually
rod-shaped, that occur in large numbers in the
cytoplasm. The Golgi material is usually in
the form of a closed network of fine threads
or canals which may extend throughout the
cytoplasm or be confined to a more restricted
area. Both mitochondria and the Golgi
material can be seen in living cells. Materi-
als such as fat, oil globules, yolk, glycogen,
etc., occur in varying amounts as inclusions in ganglion cell of the
the cytoplasm of cells. The ground substance rabbit s.howi"f Golgi

J l ° material in the form

of the cytoplasm in which these structures and of a network in the
materials are suspended or dispersed is opti- J^^J "entra^ob-
cally homogeneous. Cilia (short hairlike proc- ject is the nucleus.
esses), flagella (long whip-shaped structures), (After Golgi.)
and membranellae (delicate fin-shaped membranes) may develop
as cytoplasmic structures at the surface of some cells. In general
there is a greater degree of variability in differentiation and form
in the cytoplasm than in the nucleus.

The complex nature and orderliness of the chemical reactions
that take place in the living cell seem to call for some sort of
structural protein framework within the cell by means of which
the cytoplasm is subdivided and supported. Such a structural
framework has been demonstrated in the following manner:
Fresh cells such as those of the liver of the rabbit, are frozen in
liquid air at — 30°C. and dried under partial vacuum for 12
hours, and then ground to a flour in a mortar. If the ground up
powder is then treated with salt solutions all of the soluble protein
in the form of globulins can be extracted. The solid residue from
which the extraction is made when examined under the micro-

18

GENERAL ZOOLOGY

scope is found to consist of fragments of liver tissue in which the
typical outlines of liver cells can be seen (Fig. 5). These
fragments are also protein in nature. Thus in cells from which
all of the soluble protein has been extracted there still remains
what may be considered a structural protein upon which the
morphology of the cell depends. The structural protein is
colloidal, i.e., its demonstration depends upon the fact that it is
less soluble than the other protein constituents.

Cells as a rule are microscopic in size. Cells of macroscopic
proportions are the result of special conditions. Thus the

Fig. 5. — Section of the liver of rabbit frozen in liquid air, dried at — 30°C,
extracted for 48 hours with salt solution, 48 hours with distilled water, fixed in
formalin and stained. Cell form and protoplasmic structure is still intact and
some remains of nuclear content may be seen. X 300. {After R. R. Bensley
and N. L. Hoerr.)

unfertilized egg of the domestic fowl is a cell which is approxi-
mately the size of the yolk of the laid egg. (The white of the egg
and the hard shell are protective coverings.) In this case the
large size of the cell is accounted for by the yolk material stored
in the cytoplasm of the egg cell. The yolk-free cytoplasm and
the nucleus are of microscopic volume. The large size of eggs
generally is due to the presence of yolk material in the cytoplasm.
The common unit of linear measurement employed in microscopy
is the micrometer or micron, abbreviated as p, (Greek letter n),
which is a length of 0.000001 meter. Human red blood cells
measure 7.5 to 8.5 /j, in diameter; some leucocytes about 10 /x; a
skeletal muscle fiber, which is really a multinucleated cell, may
be 1 in. long and 50 /j. in width.

In the years following the publication of the cell theory there
was an almost total lack of understanding as to how cells arise.

THE STUDY OF ZOOLOGY 19

The first step toward the final solution of this question was taken
by Carl von Nageli, who in 1844 stated, as a result of his studies,
that cells always come from preexisting cells and that they do
not arise from a generative matrix of some sort, as do crystals
from a liquid. Cells come from preexisting cells by a process of
cell division, but the discovery of the details of the process came
to light slowly because of their complicated nature. In cell
reproduction the division process is initiated in the nucleus,
where the transformation of the chromatin into chromosomes
takes place and where all of the intricacies of the division mecha-
nism seem to center. The importance of the nucleus in cell
division was recognized by Eduard Strasburger, a botanist, and
in 1879 led him to state that a nucleus always comes from a
preexisting nucleus, a conclusion confirmed in 1882 by W. Flem-
ming in studies of animal cells. It is largely due to the work of
these two men that the complicated details of cell division were
worked out. The term karyokinesis means division of the
nucleus and refers to the changes undergone by the nucleus in
the process of cell division. Mitosis is a synonym for karyo-
kinesis. A fuller discussion of mitosis is reserved for Chap. XI,
and it need only be mentioned at this point that, as a rule,
division of the nucleus is accompanied or followed by division
of the cytoplasm, resulting finally in the production of two cells
of equal size, each of which in the course of time grows to the
size of the parent cell. The division of the cytoplasm is called
cytokinesis.

The development of an animal from an egg is usually preceded
by the fertilization of the egg by a spermatozoon. One of the
earliest observations of the entrance of the spermatozoon into the
egg was made in the case of the frog's egg by Newport in 1854,
but it was not until 1861 that it was clearly shown by the com-
parative anatomist Karl Gegenbaur that the egg is really a cell.
The same conclusion regarding the spermatozoon was reached
in 1865 by Schweigger-Seidel and La Vallette St. George. In
fertilization the essential feature is the union of the egg nucleus
with a nucleus derived from the spermatozoon, so that the
fertilized egg is really a cell with a nucleus of biparental origin.
These demonstrations of the cellular character of the egg and
spermatozoon have been verified many times since and have
tended to emphasize the importance of the cell in developmental

20 GENERAL ZOOLOGY

processes. Since the egg and spermatozoon must contain all the
hereditary qualities derived from the parents the problem of
heredity centers in explaining how an adult organism develops
from a cell.

Unity in Organisms.— In Protozoa, the individual animal is
bounded by a single plasma membrane which is comparable to
the cell membrane of a metazoan cell. Within the bounds of this
membrane there may be one or more nuclei. If a cell be denned
as a small mass of protoplasm containing one or more nuclei,
then a protozoan is a cell, and the individual organism and the
cell are one and the same. The cell in this case is organized as an
individual animal. In Metazoa, the animal body is made up of
many cells or, the metazoan body may be said to be more or less
subdivided by many plasma membranes, represented by the
cell walls. Schwann believed that each cell is, within certain
limits, an individual, an independent whole; that the vital
phenomena of one cell are repeated, entirely or in part, in all
other cells of the body, and that the individual cells are not
arranged side by side as a mere aggregate, but that they operate
to produce a harmonious whole. "The whole organism sub-
sists only by means of the reciprocal action of the single elemen-
tary units." This idea is in keeping with the fact that the
organization of a single cell is capable of maintaining itself in
the case of Protozoa; that the metazoan animal starts from a cell
and develops as new cells are produced ; and that the individuality
or unity of the cell as represented in the fertilized egg may be
repeated in each cell formed from the egg. From this point of
view the cell is the unit of organization. In Protozoa, the unity
of the cell and the unity of the individual organism coincide;
in Metazoa the unity of the organism is made up of a number of
cell units, cooperating in some way to produce the whole. In
either case the cell would be the unit of organization.

Others have maintained that the fact that the protozoan is a
cell should not be permitted to overshadow the equally pertinent
fact that the protozoan is also an organism, but an organism
whose size is so small that it is possible for it to be contained
within a single plasma membrane. The organization of a
protozoan is the organization of an individual animal that happens
to conform in a general way to a definition of a metazoan cell.
If in the case of the metazoan we also regard the organism as the

THE STUDY OF ZOOLOGY 21

unit in organization, the individual cells composing it lose their
significance as primary units of organization. Sachs recognized
this long ago when he said (in 1865) that "cell formation is a
phenomenon, very general it is true, in organic life but still
only of secondary significance ; at all events it is merely one of the
numerous expressions of the formative forces which reside in all
matter, in the highest degree, however, in organic substance."

The view that the organism and not the cell is the unit of
organization in living nature is now generally accepted and is
known as the organismal theory. On this basis the important
difference between protozoan and metazoan animals is not
primarily in the number of cells but in the difference in the
organization of the protoplasm in the two cases, which in the
final analysis means difference in the chemical and physical
constitution of the two kinds of protoplasm.

The problem of cell versus organism in determining the location
of organic unity is, of course, a problem only in metazoan animals,
for the simple reason that in protozoans the cell and the organ-
ism are identical. The criteria for determining unity are
primarily physiological. It is a common observation that reac-
tions to stimuli are adaptive, i.e., reactions tend to preserve the
condition of wholeness in the organism. Isolated groups of cells
of metazoan animals as a rule do not survive, or if they do survive,
under natural conditions, they tend to organize themselves along
the lines of the organism from which they came, rather than as
cell units.

Evolution. — There exists abundant evidence for the belief
that the present population of the earth, and the condition of the
earth itself, are the results of a process of change or evolution
that has extended over enormous periods of time. The only
alternative to such an explanation is some form of Special
Creation, which raises more problems than it solves. The
theory of evolution implies that animals and plants were not
always as they are now, but that they have descended from more
primitive forms of life through the action of slow processes of
divergence which for the most part have been adaptive in
character.

Evolution has been the guiding thought of biology for more
than half a century, thanks largely to the painstaking work of the
great English naturalist, Charles Darwin, whose publications,

22

GENERAL ZOOLOGY

beginning with the "Origin of Species" in 1859, rank as the
most notable contributions to biological literature of the nine-
teenth century, and perhaps of all time. The nature and extent
of the evidence upon which the theory of evolution rests will
be considered more in detail in later chapters.

Taxonomy. — The science of classification is known as taxon-
omy. The theory of evolution teaches that all living things had

Individual Cah
•SPECIES, domeshca
-GENUS, Fetis
.-FAMILY, Felidae

-ORDER, Carn'ivora
...CLASS, Mammalia
... SUB -PHYL UM, Verlebrala
..:PHYLUM, Chordaia

...ANIMAL KINGDOM
.PLANT KINGDOM

.-L/V/N6 WORLD

pIG e_ — Diagram to show the tree-like form assumed by a natural system of
classification, as illustrated by the classification of the house cat, Felis domestica.

an origin in some simple form or forms of protoplasm. From
these hypothetical ancestral organisms there first arose, pre-
sumably, the simpler forms of plant and animal life, and from
these in turn the higher forms. Thus it follows that a relationship
of varying degree exists between all forms of life — even animals
and plants being connected by simple unicellular forms, some
of which combine both animal and plant characteristics.
Lamarck was the first to express this relationship diagrammati-
cally by means of a tree whose trunk divides almost at once into
two main stems, one for the plant kingdom and one for the animal

THE STUDY OF ZOOLOGY 23

kingdom, each stem in turn sending out branches that represent
the subdivisions of plants and animals (Fig. 6). Such a tree
would show the relationships between organisms in a very exact
manner if knowledge of the evolutionary processes were complete,
but since this knowledge is at its best rather fragmentary,
the construction of the tree of life is a difficult matter and subject
to continual alteration.

Descent. — When it is held that the higher animals are derived
or descended from animals simpler in form and organization, it
is not meant that the lower animals living today are the ancestors
of the higher ones. Thus, it would be incorrect to speak of
monkeys or apes as the ancestors of Man, although it is true
that Man is more closely related to them than to any other
animals. The fact is that Man and monkey probably had some
common ancestor from which monkeys were evolved on one hand
and Man on the other — the common ancestor having in the mean-
time disappeared from the living fauna. Since the same princi-
ple applies to other groups of animals and plants, it follows that
the part of the genealogical tree that represents the living popula-
tion of the earth includes only the twigs — the trunk and its
branches, representing the connecting links between living forms,
having dropped out and become extinct.

Natural Affinities. — Evidence of relationship between organ-
isms may be obtained from a study of living animals and plants.
The great Swedish naturalist, Linnaeus (1707-1778), who
founded the modern system of classification, was not an evolu-
tionist, but he was able nevertheless to classify animals and
plants with a remarkable degree of accuracy by carefully noting
their resemblances and differences. The point is, however, that
these resemblances, or natural affinities, as Darwin called them,
find their rational explanation in the theory of evolution, with the
result that the application of the evolution principle has had a
clarifying effect on the problems of classification.

Species. — In the modern scheme of classification the species
constitutes the basic group of individuals. A species may be
simply defined as the offspring of similar parents. The members
of similar species resemble one another because they are
descended from common ancestors. Species have become more
or less sharply differentiated from all other species by the dis-
appearance of intermediate forms. Owing to variation, the

24 GENERAL ZOOLOGY

individuals composing a species are never identical, and the
individual differences which constitute these variations make it
difficult to define the limits of a species. Such gradations among
or between species are exactly what would be expected on an
evolution basis; while, on the other hand, variations have been a
stumbling block to upholders of the doctrine of special creation,
who with Linnaeus believed that species did not develop gradu-
ally but were made outright, and that all species were therefore
fixed. At present, the idea that species are fixed is no longer
tenable because of the vagueness of the boundaries between
them, but the employment of the term "species" is useful in
describing the basic unit in the system of classification.

Scheme of Classification. — Species are arranged in groups of
higher order called genera. A number of similar species consti-
tutes a genus in much the same way that a number of similar
individuals makes up a species. The characteristics which distin-
guish one genus from another are more deep-seated and funda-
mental than those that distinguish species. Related genera, in
turn, are combined into families, families into orders, orders into
classes, and classes into phyla. The phylum, then, is the largest
group in either the plant or animal kingdom. Each group is
sometimes made up of subgroups, such as subphylum, subclass,
etc. ; and occasionally entirely new terms are introduced to meet
the needs of classification. The custom of employing Latin or
Greek derivatives in the technical naming of these groups is now
universally followed and makes for clearness in identification.

Binomial System. — Every known kind of animal and plant has
a scientific name, consisting of the name of its genus, capitalized,
followed by the specific name in small letters. Thus, the name of
the house cat is Felis domestica; a common species of frog (Leopard
frog) Rana pipiens; Man, Homo sapiens. This method of naming
was formulated by Linnaeus and is known as the binomial
system of nomenclature. It is also customary to follow the
scientific name with name of the author who originally proposed

it.

Trinomial System. — It sometimes happens that the difference
between two kinds of animals is so slight as to scarcely warrant
placing them in separate species, and yet requires some practical
method of classification. In such cases the intraspecies dis-
tinction is made by adding a subspecies name to the name of the

THE STUDY OF ZOOLOGY 25

genus and species; thus the mountain salamander is known as
Desmognathus fuscus carolinensis and the closely related brook
salamander as the Desmognathus fuscus fuscus. These two forms
are varieties of the same species, and so far as one knows, are not
connected by intermediate forms at the present time.

Animal Phyla. — The animal kingdom is composed of 17
animal phyla, of which the names of those commonly used in
laboratory study, with examples of each, are as follows:

Protozoa, unicellular animals, microscopic in size such as Amoeba and
Paramecium.

Porifera, sponges.

Coelenterata, hydra, jelly fishes.

Ctenophora, sea walnut, comb jelly.

Platyhelminthes, flatworms, tapeworms.

Nemathelminthes, roundworms or threadworms.

Trochelminthes, rotifers.

Annelida, segmented worms, such as the earthworm.

Arthropoda, crayfish, crabs, insects, spiders.

Mollusca, snails, clams, oysters.

Echinodermata, starfishes, sea urchins.

Chordata, animals with a notochord, of which the most important sub-
division is the Vertebrata, animals with a backbone.

The classification of animals is considered in some detail in the
final chapter (XVIII), which should be consulted whenever a
new or unfamiliar animal is mentioned in the text. Since a very
definite part of the program of an introductory course in zoology
is a mastery of the fundamental facts of animal taxonomy, the
student is expected to learn the names of all the animal phyla and
their principal subdivisions and to acquire more detailed knowl-
edge of those phyla, of which examples are studied in the labora-
tory. This knowledge can best be acquired by a process of
gradual absorption, learning the important facts about a single
phylum, one at a time, until the entire system has been mastered.

A-

CHAPTER II
THE ANIMAL ORGANISM

The fundamental biological functions of animal organisms
are the same in all forms of animal life. The principal differences
between different kinds of animals lie in the manner in which
these functions are carried out and in the degree of morpho-
logical differentiation that accompanies functional differentiation.
Thus the Protozoa, the lowest phylum in the animal kingdom,
consist of animals in which the body is a single cell, free-living, or
united with other similar cells to form a colony. Protozoa are
capable of maintaining and reproducing themselves, in a favor-
able environment, with as much success as any higher form of
life. In the ascending scale of animal life one finds that the
organism takes on an ever-increasing structural and functional
differentiation of its parts that is absent or at best only very
feebly expressed in Protozoa. The elaboration of structural and
functional features found in higher forms is an inevitable accom-
paniment of their greater size. It is inconceivable that a frog
could exist as a frog in the form of a single cell.

The varying degree of structural differentiation found in
different kinds of animals lends itself to an analysis in terms of
cells, the common though variable structural units of which their
bodies are composed. In forms above the Protozoa — Metazoa
or multicellular animals — larger morphological units composed
of groups of cells constitute what are called tissues and organs. A
tissue is a group of histologically similar cells, such as the cells
forming the outer layer of the frog's skin. An organ is composed
of different kinds of tissues, characterized by a distinctive
histological structure, and capable under proper conditions of
performing one or more distinct functions in the animal body.
Tissues and organs are the visible evidence of differentiation in
the bodies of metazoan animals.

The Frog. — A study of the biology of an individual animal
might begin with any animal as the subject matter. Logically,

26

THE ANIMAL ORGANISM

27

perhaps, one should start with the Protozoa and work up the
scale, but practically more rapid and satisfactory progress is made
if the beginner starts with a larger animal and one with which he
is to a certain extent familiar, such as the leopard frog or grass
frog, Rana pipiens, or any other species of frog. The leopard
frog is classified as a member of the phylum Chordata, the
subphylum Vertebrata, the class Amphibia, the order Salientia,
the family Ranidae, the genus Rana, and the species pipiens. It
is therefore a member of the highest phylum, Chordata, and also
of the subphylum Vertebrata to which Man also belongs. It is
an example of a vertebrate animal, which refers specifically to the
fact that it has a vertebral column or backbone.

Fig. 7. — Rana pipiens, sketched from life.

The leopard frog is found in nature living near ponds, lakes, and
streams of fresh water of the North American continent east of
the Pacific slope. The winter months are spent in hibernation,
during which active feeding ceases and all bodily activities are
reduced to a minimum. Several varieties of the species differing
from one another in color pattern are known. In the variety
illustrated in Fig. 7 the dorsal side of the head and body, and
the upper sides of the legs, are marked by large brownish spots,
bordered by narrow light iridescent green edges, on a background
of lighter brown. The sizes of the spots and the pattern vary in
different races. The throat, the ventral side of the body and the
inner sides of the legs are whitish and unmottled.

The skin of the frog is smooth and is rendered slippery by
mucus, a viscid secretion poured out on its surface by skin
glands. The skin is held loosely to underlying parts by sub-

28

GENERAL ZOOLOGY

cutaneous connective tissue, divided into inner and outer layers by-
large lymph spaces. These spaces are readily seen when the skin
is removed from the frog. A thickened ridge in the skin extends
back from the eye on either side of the body to form a lateral
dermal fold or plica. In addition to mucus-secreting glands,
which are very numerous, the skin also contains smaller numbers
of poison glands, located principally in the skin of the dorsal body

surface, lateral dermal plicae and in
the skin of the hindlegs. The poison
is in the form of a whitish fluid. It
is not so toxic as the secretions of
the poison glands of toads.

In a sitting posture the body rests
on the folded hindlegs and the ex-
tended forelegs. From this position
the sudden extension of the hindlegs
sends the animal through the air in a
long leap. This is its usual form of
locomotion on land. In water the
swimming strokes are also provided
by the hindlegs. The hindfoot has
five elongated toes connected by
interdigital membranes which increase
the surface of the foot and add power
to the swimming stroke. Each

Fig. 8.— Palmar surfaces of feet forefoot has four toes and a very
of Rana pipiens. A, right fore- n-i

foot of female; B, right forefoot rudimentary thumb. A padlike
of male; C, hindfoot. enlargement on the proximal segment

of the first digit is present in the male and is practically the only
external anatomical distinction between the sexes (Fig. 8).

The eyes are prominent structures at the sides of the head.
Each eye has an immovable upper eyelid in the form of a thick
fold of the integument. The lower lid is well developed and can
be moved upward, covering the eye. When this happens the
eyeball is withdrawn into the socket by a special muscle, the
retractor bulbi. The upper part of the lower lid is thin and
transparent and is called the nictitating membrane. When the
lower lid is withdrawn, the nictitating membrane folds inside
on the lower thicker part of the lid. On the forehead between
the eyes an unpigmented spot, the brow spot, can usually be seen.

THE ANIMAL ORGANISM

29

This spot shows the location, beneath it, of the pineal organ,
which is a dorsal outgrowth of the diencephalon of the brain. In
the embryos of some amphibians and reptiles the tip of the pineal
organ enlarges to form an eyelike structure beneath the integu-
ment. The pineal organ with its distal enlargement is regarded
as a vestigial organ which in modern amphibians has lost its
original function and much of its structure. On either side
of the head behind the eye is the tympanic membrane or eardrum,
a flattened circular area flush with the surface of the head.

A live frog at rest exhibits pulsating movements in the region
of the throat which are concerned with respiration. On the

Fig. 9. — Two views of the oral cavity of Rana pipiens. A, tongue in place in
floor of mouth; B, tongue pulled forward, e.t., Eustachian tube; g, glottis; i.n.,
internal naris; t, tongue; v.s., vocal sac openings; v.t., vomerine teeth.

dorsal side of the snout not far from the tip are located the two
external nares, leading to respiratory and olfactory passages
which open into the anterior end of the oral or buccal cavity. If
the frog's mouth is held open (Fig. 9) several structures can be
seen. In the roof of the mouth on either side near the anterior
end is a small opening, the internal naris, and between the two
nares is a pair of white vomerine teeth. Small bluntly pointed
teeth are also found in the upper jaw which forms a frame for the
roof of the mouth (Fig. 9, A) . On either side, near the angle formed
by the articulation of the upper and lower jaws, is the opening
of the Eustachian tube, which connects with the tympanic cavity or
middle ear. The tympanic cavity is closed to the outside by
the tympanic membrane. In the mid-line between the two
openings of the Eustachian tubes is the constricted opening into
the esophagus. The most conspicuous structure in the floor

30 GENERAL ZOOLOGY

of the mouth is the tongue with its free, forked end pointed back
toward the esophagus. Between the spread tips of the tongue
is a slightly raised circular area with a longitudinal slit, the
glottis, which opens into the larynx. On either side of the floor
of the mouth in the male is a pit leading to a vocal sac, which is a
small pocket of the oral epithelium that can be expanded with
air. The lower jaw is without teeth. When the mouth closes
the lower jaw meets the upper jaw in a groove, the sulcus mar-
ginalis, which lies inside of the tooth-bearing margin of the
upper jaw. The tuberculum prelinguale is a rounded conical
prominence at the tip of the lower jaw.

Live insects and worms form the principal diet of frogs. In

capturing insects, the tongue, coated
with an adhesive secretion is extended
(Fig. 10), forked end foremost, and
then retracted with the prey sticking
to it. The nature of the teeth restricts
their function largely to grasping and
holding. The food moistened by the
saliva is swallowed, passing down the
Fig. 10.— Showing tongue of wide esophagus to the stomach and

intestine where it is digested and ab-
sorbed. If a longitudinal slit is made in the abdominal region of
a frog, the cut passes through the skin and a thin layer of muscle.
The skin can be easily separated from the underlying muscle,
which is the real supporting element of the abdominal wall.
To expose the underlying parts between the forelegs it is necessary
to cut through the sternum which is part of the skeleton. Like-
wise the pubic portion of the pelvic girdle between the hind limbs
must be severed to follow the termination of the alimentary
canal. The cavity thus exposed is the pleuroperitoneal cavity
or coelom, in which lie the lungs, liver, pancreas, spleen, alimen-
tary tract, kidneys, and reproductive organs (Fig. 11). The
heart, it will be noted, is in a sac, the pericardium, which is really
a subdivision of the coelomic cavity. The wall of the coelom is
lined by a thin epithelium, the peritoneum or parietal peritoneum,
which is continued over the organs, in whole or in part, as an
investing membrane, the visceral peritoneum. The sac surround-
ing the heart, the pericardium, represents the parietal peritoneum
in that region. The small intestine is attached to the dorsal

THE ANIMAL ORGANISM

31

part of the body cavity by a thin membrane, the mesentery, which
provides a pathway for blood vessels and nerves to the intestine.
The mesentery really consists of two thin sheets, right and left,
continuous at the point of insertion in the body wall with the
parietal peritoneum, and surrounding the intestine at the free
edge, where it forms the visceral peritoneum, or serous membrane.

Fig. 11. — Rana pipiens, male; ventral half of abdominal wall and parts of
pectoral and pelvic girdles removed to show internal anatomy. B, bladder; C,
colon (large intestine); Cl, cloaca; D, duodenum; G, gall bladder; H, heart;
I, ileum; K, kidney; L, liver; Lu, lung; O, oviduct; S, stomach; T, testis.

The visceral peritoneum, the mesentery, and the parietal peri-
toneum are therefore different parts of a continuous epithelium.
The lungs, liver, and pancreas have a similar relation to the
peritoneum. The kidneys, on the other hand, are retroperitoneal
— behind the peritoneum — which refers to the fact that the
parietal peritoneum is fastened against the ventral faces and
edges only of the kidneys. The dorsal surfaces of the kidneys
are attached directly to the body wall. The term mesentery, in

32

GENERAL ZOOLOGY

strict usage, applies to the portion of the peritoneum between
the intestine and the body wall, but it is often used for the
suspensory or supporting membrane of other viscera.

The form of the entire alimentary tract can be seen if the liver

and other overlying parts are re-
moved, as in Fig. 12. The posterior
continuation of the broad, ciliated,
buccal cavity is the pharynx, which
is also ciliated, and leads in turn to
the esophagus, a wide muscular tube,
lined by a ciliated epithelium and
creased by longitudinal folds. Like
the mouth, the esophagus is provided
with mucous glands. The esophageal
glands, located near the lower end of
the esophagus, secrete pepsin. The
demarcation between the esophagus
and stomach is indicated externally
by a constriction, but on the whole
it is not very distinctly marked. If
unpalatable food reaches the stom-
ach, it may be regurgitated by a
reversed peristaltic action which
turns the stomach inside out, some-
times causing it to bulge outside of
the mouth. The soft glandular lin-

Fig. 12.— Alimentary tract of ing of the stomach secretes pepsin
Rana pipiens. B, bladder; BD, ancj hydrochloric acid, in addition to

bile duct; C, colon (large intes- . „ . . ,. ,.

tine) ; Ca, cardiac end of stomach; mucin. Pepsin is a digestive enzyme
Cl, cloaca; D, duodenum; I, which in the presence of hydrochloric

ileum; L, lung; La, larynx; M, . . • r j mi

mesentery; Ob, esophagus; P, acid acts upon protein food. Ihe

pylorus; Sp, spleen. curved stomach lies with its convex

surface to the left and is attached to the body wall by the
mesogaster and to the liver and small intestine by the gastro-
hepatoduodenal ligament. These attaching structures are exam-
ples of mesenteries.

The lower end of the stomach leads to the small intestine, a
sharp constriction at the pylorus marking the junction. The
first part of the small intestine makes a sharp bend at the pylorus,
turns forward and parallels the course of the stomach. This is

THE ANIMAL ORGANISM

33

the duodenum. In the loop between the duodenum and the
stomach lies the pancreas. At the top of the loop the duodenum
passes over into the ileum, which turns back and follows a coiled
path to the large intestine, from which it is marked off by a con-
striction. The spleen, a small globular structure, reddish in
color, is attached to the mesentery near the upper end of the large
intestine. It is really a part of the circulatory system. The
liver consists of three main lobes from which bile is collected by a
system of hepatic ducts, which unite
to form a common bile duct. The
latter passes through the tissue of
the pancreas and opens into the
middle region of the duodenum.
Since the bile duct also collects the
secretions of the pancreas, the lower
part of the duct serves as a hepato-
pancreatic duct. The gall bladder is
an enlargement on one or more of the
hepatic ducts where bile is stored,
until liberated from time to time
into the bile duct (Fig. 13).

The lining of the small intestine is duodenum

Fig. 13.

— Rana Pipiens. B,

C, cystic duct; D,

, G, gall bladder; H,

folded into longitudinal and trans- hepatic duct; L, liver; P, pan-

, . , , , creas; S, stomach.

verse ridges, which increase the

absorptive surface and delay the passage of food. Digestion of
protein started in the stomach by pepsin is continued in the
intestine by trypsin, one of the constituents of the pancreatic
secretion. Amylopsin or amylase, and lipase, also produced in the
pancreas, act on starches and fats, respectively. Amylase con-
verts starches into sugars and lipase changes fats into fatty acids
and glycerin. The bile has no digestive function but facilitates
fat digestion. The liver is not primarily an organ of digestion.
It serves to store sugar absorbed from the intestine, in the form of
glycogen or animal starch, and also is concerned in the storage
and utilization of fat and in the conversion of certain nitrogenous
metabolic products of the blood into urea, which is excreted
through the kidneys. Fibrinogen, from which the fibrin of clotted
blood is formed, is produced in the liver.

The large intestine opens into a chamber, the cloaca, which in
turn opens to the outside. The cloaca also receives the openings

34 GENERAL ZOOLOGY

of reproductive and urogenital systems and the mouth of the
bladder. The large intestine serves to store fecal matter until
it can be voided.

The frog obtains oxygen by absorption through the skin, the
surface of the buccal cavity, and the lining of the lungs. If the
alimentary canal is dissected out as shown in Fig. 12, the lungs
are seen as two conical sacs connected to an oblong structure, the
larynx, attached to the ventral surface of the wall of the pharynx.
The glottis, a slitlike opening, seen in the floor of the pharynx
behind the tongue when the mouth is opened, leads to the larynx.
From the internal surface of the lung, septa project inward and
divide the lung cavity into alveoli, all of which open into the
central, undivided cavity of the lung (Fig. 14). Bands of muscle

Ai

Fig. 14. — Cross section of lung of Rana pipiens. A, arteries; V, veins.

are found on the inner, free edges of the septa, from which thinner
strands of muscle extend through the septa to the wall of the
lung, which likewise contains scattered muscle tissue fibers.
The contraction of this muscle tissue under certain conditions
collapses the lung and almost completely empties it of air. The
cells at the free edges of the septa are ciliated and among the
ciliated cells are found mucus-secreting goblet cells. Elsewhere
the alveoli are covered by a flattened layer of cells. The septa
are richly supplied with blood vessels, through which oxygen is
absorbed from the air in the lungs and from which carbon dioxide
is given off.

A living frog displays two kinds of respiratory movements in the
throat region: (1) a regular series of shallow pulsations, and (2)
a deeper lowering and raising of the throat repeated several times.
During the period of shallow respiratory movements, the external
nares are open, the glottis closed, and air is drawn in and out of
the buccal cavity through the nasal passages. The ventilation

THE ANIMAL ORGANISM

35

of the buccopharyngeal region is accompanied by the absorption
of oxygen through the lining of the mouth. When these shallow
movements are interrupted by a deep lowering of the throat, an
increased volume of air is drawn in ; but since before the comple-
tion of this movement the external nares are closed and the
glottis opened, air passes from the lungs into the buccal cavity.
The vigorous contraction of the throat, which now follows,
forces the mixed air back into the lungs. One or more repetitions
of these deeper respiratory movements of the throat muscles
cause the air to surge back and forth from the lungs to the mouth,
after which the glottis closes, the
nares open, and the shallow respira-
tion is resumed. The closure of the
external nares is effected by a slight
forward movement of the lower jaw
which wedges the tuberculum pre-
linguale of the lower jaw between
the two elements forming the tip of
the upper jaw, the premaxillary
bones, spreading them and closing
off the nares.

It is known that oxygen is
absorbed through the skin of the
frog throughout the entire year.
This rate of oxygen absorption
through both lungs and skin reaches its maximum during the
spawning season. It is interesting that a greater amount of
carbon dioxide is released from the skin than from the lungs.
During the winter, with a general reduction of respiratory
activity, the amount of carbon dioxide given off is at its lowest
level. The respiratory function of the skin is highly important
in the frog. A damp or moist environment is necessary for a
frog, since the skin cannot prevent the loss of body fluids, in a
dry warm atmosphere.

The vocal cords are folds in the lateral walls of the larynx form-
ing thickened lips, lying parallel to the edges of the glottis. They
can be seen best by laying open the larynx with a cut through
the ventral wall (Fig. 15). In producing sounds, air is forced
back and forth from the lungs to the buccal cavity, thus causing
the vocal cords to vibrate. Since sounds can be produced with-

Fig. 15. — View of interior of right
half of larynx of Rana pipiens. g,
margin of glottis; l, lung;vc, vocal
cord, attached at its ends and along
its length to the wall of the larynx.

36 GENERAL ZOOLOGY

out taking in air from the outside, frogs can call under water. In
the male the volume of sound is increased by the vocal sacs,
which are merely extensions of the floor of the mouth at each
angle of the jaw, capable of dilation when filled with air and
which act as resonators. They are lacking in the female.

The frog has a well-developed blood-vascular system, con-
sisting of a heart, arteries, veins, and capillaries, through which
the blood is circulated. The heart is a pump which furnishes the
motive power, by means of which the fluid blood is propelled
through the system. The blood is forced from the heart through
arteries which eventually break up into thin-walled capillaries
from which the blood is returned to the heart by veins. There
are two vascular circuits in the frog: (1) a pulmonary circuit
connecting the heart and the lungs, and (2) a general systemic
circuit connecting the heart with all other parts of the body.
The blood itself is made up of a fluid plasma and several different
kinds of cellular corpuscles suspended in the plasma. It func-
tions as a vehicle for the conveyance to the cells of the body of
nutritive material, absorbed from the alimentary canal, and of
oxygen, absorbed from the lungs, skin, and buccal cavity. It
also absorbs from the tissues generally, products of metabolism,
including carbon dioxide, which are excreted through the kidneys,
lungs, and skin.

The lymphatic system is related to the blood circulatory system
both structurally and functionally. It consists of large and
small spaces, between and within body tissues, all connected, and
drained into the blood system by means of four lymph hearts.
Lymph also enters the blood stream through ciliated openings in
the ventral surface of the kidneys which lead from the coelomic
cavity to the renal veins. The subcutaneous lymph spaces are
large and abundant, as a result of which the skin seems to be
loosely attached to the underlying muscles. A large lymph
space within the body is known as the cisterna magna which
occupies most of the space above the dorsal side of the body
cavity. The anterior pair of lymph hearts pumps lymph into
the vertebral veins and the posterior pair pumps it into the
transverse iliac veins. The general direction of flow of the lymph
is from the tissues toward the veins, which in turn lead toward
the heart. The lymph supplements the action of the blood in
absorbing excretory products from the tissues.

THE ANIMAL ORGANISM

37

The skeleton of the frog consists of bones which serve as a
supporting framework for the body. Bodily movement is
brought about by the contractions of muscles which are located
external to the skeleton. The body
musculature is composed of numerous
distinct bundles of muscle tissue each of
which is known as a muscle, whose action
is under the control of the central nervous
system. Joints between skeletal ele-
ments make possible the movement of
different portions of the body. The
extension or flexion of a segment of a
limb, for example, is brought about
through the action of two antagonistic
sets of muscles extending over the joint,
the contraction of one set producing an
effect opposite to that of the other. The
muscles are attached to the bones by
means of tendons and it is usually the
tendon from the distal end of the muscle
that extends over the joint to be inserted
on the bone. A muscle produces move-
ment by contraction, which draws its
ends, and any parts attached to them,
toward each other. Relaxation of the
muscle is passive and does not produce a
pushing effect.

The frog has a central nervous system fig. 16— Dorsal view of
consisting of the brain, enclosed in the cDentral nervous sys4tem °f

a • 7 j Rana catesbeiana. A, audi-

Cranium Of the skull, and a spinal COrd tory nerve; C, cerebellum;

lying in the channel formed by the neural gH- ce£,ebrai hemisphere;

J & J E, eye; F, filum terminate;

arches of the vertebrae of the backbone, i.e., internal ear; M, me-
Connected with the brain are ten pairs d,uflla; N* nasal cavity: .?•

. olfactory nerve; Op, optic

of cranial nerves distributed mainly to nerve; O.L., optic lobe; P,
various parts of the head. The tenth Pinf \ organ; S.C spinal

^ m cord; 1,2,7,8,9, spinal nerves.

cranial nerve, the vagus, has unportant

connections with the viscera. There are also ten pairs of spinal
nerves connected with the spinal cord which are distributed to the
trunk and limbs. The chain ganglia of the sympathetic nervous
system consist of a pair of nerve trunks which extend from within

-7,8,9

38

GENERAL ZOOLOGY

the cranium posteriorly along the dorsal surface of the body cavity.
These trunks are connected by means of rami communicantes
with the spinal nerves and certain of the cranial nerves. Enlarge-
ments on the trunks are known as ganglia. Nerves extending
from the chain ganglia are distributed among the viscera and the
walls of blood vessels. The regulation of the heart beat, the
secretion of digestive fluids, the contraction of the nonstriated

Fig. 17. — Female reproductive system of Rana pipiens, ventral view, right
ovary removed. A, adrenal gland; B, bladder; Cl, cloaca; K, kidney; LI, large
intestine; O, oviduct; Os, ostium of oviduct; Ov, ovary; U, uterus; Ur, ureter.

muscles in the walls of the alimentary tract, and other functional
activities of the viscera are controlled through the agency of the
sympathetic nervous system and the closely related parasym-
pathetic nervous system (Chap. IX). All of these activities are
involuntary and autonomic. The central nervous system, with
its peripheral connections, furnishes a mechanism for controlling
and coordinating bodily activities of all sorts (Fig. 16).

Reproduction in the frog results from the fertilization of an
egg. The reproductive organs of the female consist of a pair

THE ANIMAL ORGANISM

39

of ovaries, which are attached by mesenteries to the body wall
at the level of the anterior end of the kidneys. Each ovary
has the appearance of a mass of black and white beads, which are
developing eggs. During the breeding season the ovaries
become so large as to exceed the rest of the viscera in volume.
The younger eggs or ova are small and white in color. As they
grow older, they become black at the animal pole and white at
the opposite, vegetative or vegetal pole, where most of the yolk is
concentrated. When mature, the eggs
are released by ruptures in the wall of
the ovaries and, propelled by the action
of cilia located on the surface of the
liver, pericardium, and peritoneum,
eventually find their way into the ovi-
ducts. Each oviduct is provided with
a wide mouth or ostium at its anterior
end, located near the base of the lung,
through which the eggs enter. As they
pass through the coils of the oviducts,
the eggs are covered with three layers
of an albuminous secretion of the walls
of the oviducts. At the lower end of
each oviduct, just before it opens into
the cloaca, an enlargement, the uterus,
serves as a storage place for mature eggs tive system of Rana pipiens,
before they are discharged through the ™t»'B ^SiJbJ^;

cloaca (Fig. 17). F, fat body; K, kidney;

The reproductive organs of the male ^^^Ur!^?.' ^^
consist of two yellow, bean-shaped testes,

attached by mesenteries to the body wall between the anterior
ends of the kidneys. Spermatozoa pass from each testis by way
of a small number of vasa efferentia into the corresponding kidney,
from which they leave by the ureter, a duct passing from the outer
border of the kidney to the cloaca. The ureter in the male thus
serves a double function in that it provides a passageway to the
cloaca for both the urine from the kidney and the spermatozoa
from the testis. In the male oviducts are present but reduced in
size. They are not functional (Fig. 18).

The reproductive organs reach the height of their development
in the early spring when spawning occurs. The eggs are fertilized

40 GENERAL ZOOLOGY

in the water by a single spermatozoon entering each egg. The
jelly surrounding each egg swells to several times its original
diameter and serves as a protective covering for the developing
embryo. When the embryo reaches the length of six or seven
millimeters it hatches by wiggling out of the gelatinous capsule.
This takes place about two weeks after fertilization, depending
upon the temperature of the water, a low temperature prolonging
the time. The embryo, now a larva, has a blunt head and a short
median fin, but no limbs or even a mouth. At either side of the
head are rudiments of external gills. A mouth soon forms from
an indentation (invagination) in the outside layer of cells covering
the ventral surface of the head, communicating with the anterior
end of the pharynx. The larva then begins to feed on water
plants and grows rapidly. The external gills become enclosed
in a fold of the skin, called the operculum, which forms a respira-
tory chamber, opening on the left side by a spiracle. With the
formation of the operculum, the external gills are resorbed and
are replaced by internal gills, which develop on the edges of gill
slits located in the wall of the pharynx on either side. The
forelegs develop inside of the operculum and at first are not
visible externally. The hind limbs appear later at the base of the
tail.

At about the end of the third month of larval development
metamorphosis takes place. Metamorphosis involves profound
changes, both external and internal. Briefly, these consist in the
completion of the development of the lungs with accompanying
changes in the circulatory system ; an enlargement of the stomach
and liver and a shortening of the intestine; the resorption of the
operculum and the liberation of the forelimbs; disappearance
of the gills and closure of the gill slits; and finally the resorption
of the tail. During the larval period the mouth is provided with
horny jaws which are cast off with the molting of the skin
accompanying metamorphosis. During the larval period the
diet is largely plant material. After metamorphosis, the frog
becomes more carnivorous in its feeding habits. Correlated with
this there occurs a shortening of the alimentary tract, in keeping
with well-known fact that flesh eaters have relatively shorter
digestive tracts than plant eaters.

Hibernation. — Following the breeding season and throughout
the summer the frog is an active feeder, storing up a reserve

THE ANIMAL ORGANISM 41

supply of energy to carry it through the winter. Late in the
autumn the frog buries itself in mud below the frost line and
passes the cold months in a state of suspended animation.
Respiration is reduced to a minimum and is confined to the skin.
The body temperature drops until it is only slightly above the
temperature of the ground. Metabolic activity is displayed in
the activity involved in producing a circulation of the blood and
in the development of the eggs and spermatozoa. When the
frog emerges from hibernation in the spring it still possesses
sufficient reserves of energy to complete spawning before resum-
ing feeding.

The foregoing is a summarized statement of the general plan
of the structural and of the functional activities of the frog.
In the following chapters the discussion of organization is con-
tinued by comparing organ systems of selected animals, both
structurally and functionally, using the frog as the basic form,
for the purpose of obtaining a bird's-eye view of the conditions
found in as many different types of animals as can be conven-
iently studied. It must be remembered, however, that the organ-
ism as a whole is the real unit in living nature and that the
comparison of organ systems is a comparison of parts of the
organism unit. The alternative to such a plan is to consider only
entire organisms, a procedure that is entirely sound but one
which limits the scope, if the time available for study is to be
considered. However, a basis for the appreciation of the com-
parison of organ systems is provided by the laboratory work in
which a number of selected forms are studied as whole organisms.
With the laboratory experience as a background, the comparative
study of organ systems serves to bridge the gaps left by the
laboratory programs and offers a practical method of securing a
general survey of the field in the shortest time.

CHAPTER III
INTEGUMENT

Integument or skin is the covering of the body. Its form,
thickness and physical consistency, in all cases, provide some
degree of protection from mechanical injury. Combined with
this purely passive function of the skin is a nervous function
maintained by sensory-nerve endings located just below the
surface of the body. Since the entire nervous system evolved
from the primitive integument or ectoderm, the nervous function
of the skin is one of its primary functions. Respiration, the
absorption of gaseous oxygen and the evolution of carbon dioxide,
is another primary function of the integument and one that
persists in the integument of many animals. The protective
function of the integument, provided by its toughness or texture,
is further extended in some cases by the presence of glands
capable of secreting poisonous, slimy, or malodorous substances.
Sebaceous glands, producing oil, and sudoriparous glands, pro-
ducing sweat, are found in the skin of some animals.

Integument of the Frog. — The outer layer of the integument
of the frog is known as the epidermis. Beneath this is the
corium or dermis. These two layers occur in the skin of all
vertebrates.

Epidermis. — The epidermis consists of two regions: the
stratum corneum at the surface, and the stratum germinativum
below, both of which are epithelial in structure (Fig. 19). An
epithelium may be defined as one or more layers of cells, com-
pactly arranged, covering an external or internal surface of the
body of an animal or derived from such a surface. The stratum
corneum, an example of a simple squamous epithelium, is com-
posed of broad flattened cells of a horny texture, and serves
as the outermost protecting layer of the skin. From time to
time this layer is shed as a whole, a process known as molting or
ecdysis. The stratum germinativum is composed of several
layers of cells, forming a stratified epithelium. The innermost or

42

INTEGUMENT

43

basal layer of this epithelium is composed of columnar cells,
while the cells of the upper layers are polygonal in outline,

Fig. 19. — Section of skin of Rana pipiens, showing a mucous gland. C,
stratum compactum; Co, stratum corneum; Ch, chromatophores; E, epidermis;
S, stratum spongiosum.

becoming definitely flattened next to the stratum corneum.
When the stratum corneum is lost at molting, its place is taken
by the outermost layer of the
stratum germinativum, and new
cells, formed by cell divisions in
the basal layer, are pushed up
from below. All of the cells of the
epidermis originate in the basal
layer. A peculiar histological
characteristic of the cells of the
stratum germinativum is the pres-
ence of delicate processes, called
intercellular bridges, connecting
adjacent cells (Fig. 20).

Pigment cells or chromatophores °[ Rana Vipicns:y ^hXl magnified,

° . showing intercellular bridges. C,

OCCUr in Small numbers in the stratum corneum; G, stratum

epidermis. Cutaneous innerva- germinativum.
tion is supplied by nerves terminating in the lower layers of the
epidermis. The epidermis is pierced by ducts leading from glands
located in the corium.

Fig. 20. — Section of epidermis

44 GENERAL ZOOLOGY

Corium. — The histological structure of the corium is entirely-
different from that of the epidermis. It is composed of two
layers: an outer stratum spongiosum and an inner stratum com-
pactum, both containing a large amount of connective tissue
fibers. These fibers are of two kinds: white fibers, which are
inelastic and unbranched ; and yellow fibers, which are elastic and
branched. The fibers of the spongiosum layer form a more or
less open network, in which are found blood vessels, lymph spaces,
nerves, and glands. At various points this layer is elevated to
form dermal papillae, each of which contains a touch corpuscle
composed of a conical group of flattened cells supplied with sensory
nerve endings. Most of the chromatophores lie in the upper
layer of the corium.

The fibers of the stratum compactum appear in sections as
a wavy layer of compact tissue, traversed at intervals by vertical
bands extending upward into the stratum spongiosum and the
epidermis. The fibers are both branched and unbranched.
Here and there are nonstriated muscle fibers whose contractions
move the skin. Nerves and blood vessels also occur.

Beneath the stratum compactum is a layer of loose connective
tissue, the subcutaneous connective tissue, which is divided into
an inner and outer layer by large lymph spaces. The lymph
spaces are separated by septa joining the two layers of subcuta-
neous tissue. This tissue attaches the skin to the underlying parts.

In general, connective tissue is characterized by the presence
of a large amount of intercellular material, which in the case of
the corium is fibrous. Connective tissue cells are found in and
among the fibers, but the fibers themselves lie outside of cells.
We shall see later that bone and cartilage are also forms of con-
nective tissue, but in these cases the intercellular material is
represented by the matrix of bone or cartilage, in which the
cells are embedded.

Glands.— The cutaneous glands of the frog are of the simple
alveolar type. This means that each gland consists of a globular
alveolus, whose wall is composed of a single layer of cells, con-
nected with a short straight duct whose wall is also a single layer
of cells (Fig. 19). The duct opens on the surface of the skin,
but the alveolus is located in the stratum spongiosum. However,
both alveolus and duct originate as a downgrowth from the
basal layer of the epidermis and are therefore epithelial structures.

INTEGUMENT 45

Microscopic sections stained with haematoxylin (blue stain)
and eosin (pink stain) show two types of glands differing both in
structure and in the staining reactions of the cytoplasm of the
alveolar cells (Fig. 21). In one, the mucous type, the cytoplasm
is in the form of a network and takes the blue stain; in the
other, the poison type, the cytoplasm is granular and stains a
deep pink. The size and shape of the cells of both types vary
with the activity of the gland. Cells filled with secretion enlarge
until they practically fill the cavity of the alveolus. As the
secretion is discharged, the cells shrink to a low cubical form.
Each type of gland is surrounded by a muscular and connective
tissue tunic, the contractions of the muscles
aiding the discharge of the secretion.

Respiration. — The skin of the frog plays
an important part in aerating the blood,
supplementing the respiratory functions of
the oral epithelium and the lungs. The
cutaneous blood vessels are separated from
the surface of the body by at least the
thickness of the epidermis, so that oxygen FlG 2i.— Section of a
absorbed from air or water must diffuse small immature poison
through the epidermis and the walls of g an
blood capillaries in order to reach the blood. Carbon dioxide
given off from the blood passes from the blood in the reverse
direction. In this respiratory exchange, therefore, the gases pass
through several layers of cells, though the total distance is not
very great.

Water. — Water passes in either direction, to or from the blood
stream, through the frog's skin. Experiments indicate that
the rate of diffusion of water under pressure through the skin is
much greater from without than from within. Water thus passes
in more readily than out. The limited control of water loss
through the skin makes it necessary for the frog to live in a moist
environment. In a dry, warm room a frog will die overnight as
a result of the loss of water.

Human Skin. — Though built on the same general lines as frog's
skin, human skin differs in a number of structural and functional
features. Human skin is much thicker and tougher, gives rise to
different types of glands, and is richly supplied with a variety of
specialized nerve endings. It is impervious to water and does

46

GENERAL ZOOLOGY

not function in respiratory processes. It may develop hair
(Fig. 22).

The human epidermis is composed of many layers of cells,
morphologically differentiated into distinct regions which,
beginning with the lowest or innermost layer, are as follows: (1)
stratum germinativum, (2) stratum granulosum, (3) stratum
lucidum, and (4) stratum corneum. The basal layer of the

CO

Fig. 22. — Vertical section of human skin, semidiagrammatic. b, basal layer
of epidermis; c, stratum corneum, the outer layer of epidermis; co, corium; d,
duct of sweat gland; e, epidermis; g, stratum granulosum; ge, stratum germi-
nativum; h, hair; l, stratum lucidum; s, sweat gland; be, sebaceous gland, open-
ing into hair follicle.

stratum germinativum produces all of the epidermis and from it
also develop hair follicles and hair, sweat and oil glands, mam-
mary glands, and nails. It is composed of five or six layers of
cells, graded in shape from a columnar outline in the basal layer
to a flattened polygonal form in the outer layers. The stratum
granulosum is about two cell layers in thickness and gets its
name from the fact that the cytoplasm of the cells in this layer
has undergone a granular degeneration, the product being known
as keratohy aline granules. The condition of the cells of the

INTEGUMENT 47

stratum lucidum represents an advance in degenerative changes
in which the keratohyaline granules of the granulosum cells are
converted into a substance called eleidin, which gives the stratum
lucidum a glassy appearance in sections. The stratum corneum
is composed of numerous layers of dead, dry, squamous cells
that are constantly rubbed off piecemeal. The loss of cells at
the surface of the skin is made good by the production of new
cells in the basal layer of the epidermis, whence they spread to
the outside. The various layers of the epidermis, above the
basal layer, represent progressive stages in cell degeneration.

The corium of human skin is made up of white and elastic
connective tissue fibers, blood vessels, nerves, and glands pro-
jecting into it from the epidermal layer. The stratum subcuta-
neum lies beneath the corium to which it is firmly attached. It is
composed of loose, fibrous connective tissue, with numerous
fat cells and is connected below by connective tissue with muscle
or with the periosteum of bone. Human skin is more firmly
anchored to underlying structures than is the skin of the frog.

There are no chromatophores in human skin. The color of the
skin is due to pigment granules in and among the lowest layers of
the epidermis. A few granules are also found in the corium.

There are two general types of glands in human skin, sweat
glands and oil glands. Sweat glands, epidermal in origin, are
long unbranched tubes, extending from the surface of the epi-
dermis into the deep part of the corium or into the subcutaneous
tissue, each tube terminating in a coil. Ordinarily, the secretory
cells, located in the coiled portion of the gland, secrete an oily
fluid for the lubrication of the skin, but under nervous stimulation
the secretion becomes more watery and by evaporation serves to
cool the surface of the body. Sebaceous glands, also derived
from the epidermis, are branched or unbranched glands located
in the upper layer of the corium and usually attached to the
sheath of a hair. On the margin of the lip they occur independ-
ently of hair. The secretion consists of fat and cell debris. In
its production the gland cells fill with fat and then break down,
the fat and cell debris forming a semifluid material. The
mammary glands, which may be regarded as modified sweat
glands, are made up of a branching system of ducts terminating
in alveoli, i.e., rounded vesicles, which extend from their point
of origin in the epidermis, at the nipple, into the corium and

48

GENERAL ZOOLOGY

subcutaneous tissue of the breast. In the secretion of milk, the
free end of the gland cell ruptures and discharges fatty droplets
along with cell substance. The empty cells then fill up. Few
gland cells are cast off. The ceruminous (wax) glands of the
external auditory meatus are also modified sweat glands.

Nails and Claws. — A cross section of a finger through the
nail shows that the nail is composed of a layer of cornified cells
overlying a stratum germinativum (Fig. 23). The corium of the
nail bed consists of fibrous and elastic connective tissue running
vertically from the periosteum of the bone (phalanx of the finger)
to the stratum germinativum, and also of connective tissue
fibers running the length of the finger. The cornified layer of the
nail represents modified epidermal cells. In the embryo this

A B

Fig. 23. — The nail of a human fetus of 10 weeks. A, dorsal view; B, longitu-
dinal section; b, nail bed; e, eponychium; h, hyponychium; 1, lunula; n, nail; s,
nail sulcus. (After Kollman.)

layer is covered by the eponychium, which in the adult is reduced
to the thin layer at the base of the nail, continuous with stratum
corneum of the contiguous skin. The nail is produced from the
stratum germinativum at the root of the nail and as far forward
as the boundary of the lunula, the crescentic white area at the
base of the nail. Claws are similar modifications of the epidermal
layer.

Hair and Feathers. — In the embryo, the development of a hair
begins as a solid downgrowth of the basal layer of the epidermis,
terminating in a bulb, which becomes indented from below.
The indented region is occupied by cells from the corium, form-
ing the papilla which in later stages is provided with blood vessels.
The central part of the column above the papilla separates from
the peripheral cells to form a core, from which the shaft of the
hair develops (Fig. 24). The peripheral cells of the hair column
form the sheath of the hair. The sebaceous glands develop from

INTEGUMENT

49

the cells of the hair sheath. Later, a connective tissue sheath is
formed from the corium around the lower half of the hair follicle.
The arrectores pile-rum are bundles of nonstriated muscle extend-
ing from the fibrous tissue of the corium to the connective tissue
sheath of the follicle. Their contractions raise the hair on end.
The hair grows by additions of cells from the bulb. The shaft
of the hair is formed of cornified cells derived from stratum
germinativum of the bulb which, as they are pushed out, adhere

B

Fig. 24. — Diagrams showing the development of hair. A, the formation of
the hair column from the stratum germinativum of the epidermis; B, the hair
at birth, b, hair bulb; c, hair column; d, corium; e, epitrichium; h, hair shaft;
h.c, hair canal; i.e., inner layers of epidermis; m, muscle (arrector pili) ; oe.,
outer layer of epidermis; p, hair papilla; s.g., rudiment of sebaceous gland.

in the form of the hair. The feathers of birds, though somewhat
more complicated in structure than hairs, are also composed of
modified epidermal cells and develop in a somewhat similar
manner.

Scales. — The scaly covering of reptiles, such as snakes, is
composed of the stratum corneum of the epidermis, which is shed
as a whole when the animal molts. On the other hand, the
scales of fishes are primarily subepidermal structures. Of these
the most primitive type is represented by the place-id scale of the
elasmobranchs (sharks). Such a scale consists of a flattened
base of dentine from the center of the external surface of which
a spine projects. As the bony basal plate and its spine are laid

50 GENERAL ZOOLOGY

clown in the corium, the overlying epidermis secretes a hard
enamel covering upon the dentinal base and spine. The rough
"sandpaper" surface of the shark's skin is thus due to numerous
sharp spines, each projecting outward from a basal plate. The
exposed, enamel-covered spines are like so many short sharp-
pointed teeth on the surface of the body, the basal plates remain-
ing embedded in the corium below and the epidermal layer above.
This type of scale is thought to be the forerunner of vertebrate
teeth and dermal bone (Fig. 25).

The scales of teleost fishes, such as the perch or bass, consist of
bony plates developed in the dermis and overlaid by the epi-
dermis. Scales of this type may be hard and bony or soft and
flexible. The epidermis takes no part in their development.

A B

Fig. 25. — Placoid scale. A, view from above; B, side view, s, spine; p, basal

plate.

Horns, Hoofs, Antlers. — The horns and hoofs of sheep, goats,
and cattle represent cornifications of the epidermis and resemble
nails and claws in their general composition. Usually horns are
not shed, a notable exception being the horns of the prong-horn
antelope, Antilocapra americana, of Western U. S. Antlers are
outgrowths of the frontal bones of the skull and are at first
covered with skin which may persist throughout life, as in the
giraffe, but which in forms like deer, elk, etc. becomes worn off,
exposing the bone. The antlers are shed each year, the succeed-
ing ones displaying a greater number of tines, which thus serve
as an index of age.

Invertebrate Integument. — The outermost layer of the integu-
ment of some invertebrate animals is cellular, as in the flatworm,
Planocera, in which it consists of short columnar epithelial cells,
one cell layer in thickness, and provided with cilia on the outer
surface (Fig. 26). In many other invertebrates a typical condi-
tion consists in the presence of a noncellular structure, the cuticle
or cuticula, at the surface of the body. The cuticle is a substance

INTEGUMENT

51

Fig. 26.—
Ciliated cells of
the epidermis

secreted by the underlying epidermal cells, varying in different
animals in its thickness, texture, and composition. It is pierced
by openings of small size through which secretions of glands are
discharged. Other openings are occupied by sense organs of
touch, taste or smell connected with nerve endings
coming in from below.

The cuticle of the earthworm is a very thin but
tough layer which, if stripped off and examined
under the microscope, shows a crisscross arrange-
ment of fibrils which give it the appearance of a
piece of thin-woven cloth (Fig. 27). Numerous
small openings provide a passage for secretions of of the turbel-
unicellular mucus glands. The fibrillar structure of clra^inqJalna.
the cuticle may be the result of stresses and strains The darkly
set up in the cuticle during its formation. In both fhaTdites^H-
the flatworm and the earthworm the integument creted by the

, , <> ,. epidermis.

has a respiratory function.

In the group Arthropoda, which includes animals like the
lobster, crayfish, spiders, insects, etc., the cuticular layer develops
into a stiff armor. Joints between segments of the body and
between segments of appendages such as legs or antennae, are
provided by a softer cuticle connecting adjacent segments. The
soft cuticle is protected by a telescoping of the hard cuticle of one
segment over the end of the adjoining one. In
insects the cuticle is composed of an organic sub-
stance called chitin which is not only hard and
tough but also resistant to the action of acids and
alkalis. In the crayfish the cuticle consists of an
organic material impregnated with lime salts.
Beneath the cuticle are the epidermal cells which
secrete it. The cuticle of the arthropods not
, only serves as a protective outer covering but
of cuticle stripped also acts as an exoskeleton to which the body
worm an Car " muscles are attached. Figure 28 shows a section
of the integument of the lobster. The cuticle,
above, is laid down in stratified layers. The epidermal cells
immediately below this layer are large columnar cells con-
taining vertical fibrils. On the left side of the figure, the fibrils
of the epithelial cells seem to be continuous with fibrils con-
nected with muscle cells below. The muscles are attached to the

52

GENERAL ZOOLOGY

cuticle through the agency of epidermal cells. The epidermal
layer lacks a basement membrane, and the lateral walls of the
cells are poorly defined.

conn, t
nu.

mvs.c.

Fig. 28. — Portion of the new integument of a lobster, Homarus, as seen in
section. The cuticle is shown above in stratified layers, conn.t.nu., connec-
tive-tissue nucleus; bl.c, blood cells; mus.c, muscle cells. (From Dahlgren and
Kepner, Principles of Animal Histology, The Macmillan Company. By
permission.)

Animals with a hardened cuticle undergo periodic molting
or ecdysis, during which the cuticle splits in definite places and
the animal crawls out. The new cuticle is then secreted, and
while it is still soft and elastic, the animal grows in size. A
soft-shell crab is one that has recently shed its cuticle.

INTEGUMENT 53

The shell of molluscs such as the snail, clam, and oyster is
also the product of the activity of cells at the surface of the body.
The secretion consists of organic matter, conchiolin, richly
impregnated with calcium carbonate. The shell increases in
size by additions at its edge and, unlike the cuticle of arthropods,
is not shed.

The hard integument of the starfish and sea urchin consists
of calcareous plates developed in the subepidermal region of the
skin. The epidermis is a thin simple epithelium overlying
the plates except where points of the plates project. These
plates act as an armor protecting the internal organs and con-
stitute an exoskeleton though differing in origin and development
from the exoskeleton of the arthropods.

General. — A comparison of the integument of invertebrates
with the integument of vertebrates brings out the following
points: (1) In both groups an epidermal layer of cells, repre-
senting the primitive covering of the body, is present. Among
invertebrates, the epidermis may secrete a cuticle of variable
hardness and chemical composition which serves as a protective
covering and also provides points of attachment for body muscles
and other internal organs. Among vertebrates, on the other
hand, the surface layer of the body remains cellular, though the
cells may be highly transformed, as in the outer layer of the shell
of turtle which is epidermal in origin. In snakes the hardened
outer layer of the skin is the stratum corneum of the epidermis.
(2) The subepidermal layer of the invertebrate skin is as a rule
poorly developed. An exception is found in Echinodermata,
such as the starfish, in which the hard calcareous plates of the
exoskeleton develop below the epidermis. The corium of
vertebrates is a well-developed layer that provides the skin with
its tough and at the same time flexible character. Leather is the
chemically treated corium of hides. In those vertebrates in
which the skin takes on the form and consistency of an exo-
skeleton, such as in many fishes, turtles, alligators, crocodiles,
and some mammals (armadillo), the condition is the result of the
development of scales or bony plates in the corium.

CHAPTER IV

ENDOSKELETON AND VOLUNTARY MUSCLE

Many invertebrate animals are soft-bodied and lack a hard,
rigid supporting or protecting skeletal structure.
If a skeleton is present, it is in the form of an
exoskeleton enveloping the body. An internal
supporting hard framework is absent. The
invertebrate skeleton is integumentary in both
origin and location. In sharp contrast, all verte-
brates possess an internal skeleton regardless of
the fact that in some cases an exoskeleton may
also be present. The vertebrate type of skeleton
is called an endoskeleton because it is situated
internal to the body muscles. In the inverte-
brate type of skeleton the muscles are internal to
the skeleton (Fig. 29).

The endoskeleton is composed of cartilage or
bone, or a combination of both, and in its
development it is closely associated with the
development of the voluntary musculature of the
body. Such a skeleton provides an internal
framework to which the body muscles are
attached. Joints between the parts of the
skeleton permit movement of the parts upon or
about one another, while the movement itself is
produced by the contractions of the attached
29_Reia- muscles. Skeleton and muscle are intimately
tion of muscle to related functional as well as structural compo-

hard parts in the „ , -i 1 r i_ 1

leg of an insect, nents of a system responsible lor body move-
(AfterBeriesejrom ments, controlled through nervous connections

Skull, LaRue and . , ,

Ruthven; Animal by the central nervous system.
Biology.) two general regions are recognized in the

endoskeleton of vertebrates: (1) the axial skeleton, which includes
the skull and vertebral column or backbone, and (2) the appen-
ds

ENDOSKELETON AND VOLUNTARY MUSCLE

55

dicular skeleton, made up of the skeleton of the limb girdles and
the skeleton of the paired appendages, such as the paired fins of
fishes or the arms and legs of terrestrial forms. Ribs and the

pre max if ferry
nasal
maxillary
-fronfo -parietal

pterygoid

angu/otre
Squamosal

quadrato-jugal
exoccipjfal

auditory capsule
transverse process

sacrum

urostyte

Fig. 30. — Axial skeleton of Rana catesbeiana, dorsal view.

skeleton of unpaired fins if present belong to the axial skeleton.
The sternum or breast bone seems to be a development of the
anterior limb girdle and therefore a part of the appendicular
skeleton.

56 GENERAL ZOOLOGY

Cranium of the Frog. — The skull is composed of two parts:
(1) the cranium, which encloses the brain, the inner ears, and
olfactory organs, and (2) the visceral skeleton, made up of the
upper and lower jaws and the hyoid apparatus. The cranium
of the frog occupies the central region of the skull. Beginning
at the posterior end are two bones, a right and left exoccipital
bone, between which is a large opening, the foramen magnum,
through which the spinal cord passes from the cranium to the
neural canal of the vertebral column (Fig. 31). Each exoccipital
bone, on its posterior face has a rounded projection, the occipital
condyle, one on each side of the foramen magnum. The two

frarf/o-parfefoU^^ exoccipital condyles articulate with the

columella $£~\ 1 ^*\/^$rpro6t'c atlas or first vertebra. At the
quadratoH^^X^^'^^^tiuamosal side and just in front of each

9q\{//r „,„,. occipital condyle is a foramen for

the passage of the ninth and
tenth cranial nerves.

Fig. 31. — Posterior view of skull of The prootic bones, which form

Rana catesbeiana. ^ auditory capsuleS) are short

cylindrical bones at the sides and in front of the exoccipital
bones. On the lower lateral surface of each prootic bone near
the outer edge is an opening, the foramen ovale, which faces the
middle ear (Fig. 31). The foramen ovale is closed by a cartilage
which in turn articulates with the broad, inner end of the colu-
mella, the principal ossicle of the middle ear. From the broad
base, the columella extends outward as a slender, slightly curved
rod to the tympanic membrane to which it is attached. Vibra-
tions of the tympanic membrane are transmitted through the
columella and the cartilage at its inner expanded end to the inner
ear. At the front and toward the ventral side of each prootic
bone is an opening serving as a passageway for the fifth, sixth, and
seventh cranial nerves.

The frontoparietal bones, one on either side, extend forward
from the exoccipital bones to form the greater part of the roof of
the portion of the cranium enclosing the brain (Fig. 30). These
two bones are firmly united in the mid-line by a median suture.
From the side of each a wing extends downward to form a portion
of the side wall of the cranium. The greater part of the floor
of the cranium is formed by the unpaired parasphenoid bone.
This bone consists of a transverse process spanning the region

ENDOSKELETON AND VOLUNTARY MUSCLE

57

beneath and between the prootic bones, from the center of which
a narrow longitudinal tongue extends forward (Fig. 32).

The anterior end of the cranial cavity is closed by the cylin-
drical sphenethmoid bone which is overlapped above by the
frontoparietals and below by the anterior end of the parasphenoid.
In front of the sphenethmoid is the nasal cavity, which is divided
by a median partition. The side walls of the cranium between
the sphenethmoid in front and the prootic bones behind, and the
frontoparietals above and the parasphenoid below, are formed of
cartilage. The large spaces on either side of the frontoparietal
bones are the orbits of the eyes.

pre maxillary
men to- Meckel ictn

vomer
palatine

dentate

sphen - ethmoid
paras ph e no/ d

squamosa/

angulare

pterygoid

exoccipi+al

columella

Fig. 32. — Ventral view of skull of Rana catesbeiana.

The paired nasal bones, lying just in front of the frontoparietals,
meet in the mid-line, overlying the cartilaginous nasal capsules.
From each nasal bone a narrow process extends laterally to the
upper jaw. Below the nasal capsules and forming the floor
of the cranium in front of the sphenethmoid bone are the paired
vomer bones. Each vomer bone bears a tooth on its ventral face.

Suspensorium of the Frog's Skull. — Three pairs of bones, the
squamosals, pterygoids, and palatines, attach the jaws to the
cranium and constitute the suspensory apparatus of the skull.
The squamosal bone can be seen best in a side view of the skull
(Fig. 33). It consists of a narrow stem extending from the
angle of the jaw diagonally upward to a shorter cross piece, which
is articulated at its upper end to the prootic bone and whose
lower end curves downward toward the upper jaw. The ptery-

58

GENERAL ZOOLOGY

goid can be seen in a ventral view of the skull (Fig. 32). Begin-
ning at the angle of the jaw, it curves forward beneath the stem
of the squamosal to the upper jaw at its middle where it is also
connected to the lower end of the cross piece of the squamosal by
cartilage. From the center of this arched portion of the ptery-
goid bone an arm projects medially to the outer end of the prootic
bone. Each palatine bone extends from the anterior end of the

exoccipifal
pterygoid-

squamosal

fronhpariefo/^

sphenethmoid
■nasal

parte faL
iemporal/

angulare

-pre-

maxil/ary

=^~- S> menfo-
dentale Meckelian

frontal

■superior
maxillary

Fig.

occipital
mastoid process
inferior maxillary

33. — Side view of skull Rana catesbeiana and of human skull, reduced in
size for purpose of comparison.

sphenethmoid outward to the upper jaw, directly beneath the
lateral processes of the nasal bones.

Jaws of the Frog. — The upper jaw is composed of three pairs of
bones. In front are the two premaxillary bones, short segments,
from each of which a short facial process curves upward and back
toward the external nares. A maxillary bone forms the long
segment, on either side, extending from the premaxillary in front
to the quadratojugal bone behind. The quadratojugal bone
articulates with the squamosal and pterygoid bones at its
posterior end. Teeth are borne by the premaxillary and maxil-
lary bones (Fig. 33).

ENDOSKELETON AND VOLUNTARY MUSCLE 59

At the tip of each half of the lower jaw is a short segment, the
mentomeckelian bone, directly beneath the corresponding pre-
maxillary bone in the upper jaw. The mentomeckelian bone
continues backward to the angle of the jaw as Meckel's cartilage,
passing between two overlapping bones, the dentate in front and
outside of the cartilage, and the angulare, inside of and below the
cartilage. The posterior half of the cartilage runs in a groove
on the upper surface of the angulare, widening out to form an
articulating surface at the end of the jaw. In prepared skeletons,
the cartilage is often absent, but the groove in the angulare can be
clearly seen. There are no teeth in
the lower jaw. The coronoid process
of the angulare offers a point of attach-
ment for muscles that close the jaw.

The tips of both upper and lower | §|f J .alp

jaws are movable. When the tip of
the lower jaw is thrust upward, the

premaxillaries are raised, pushing the ss^ £/ \j| w PI R

pref acial processes of the latter against
the sides of the external nares, closing

them. Fig. 34.— Hyoid apparatus of

HyOld Skeleton. — The frog in the Rana catesbeiana, ventral view.

,11 , • • i i -ji •ii a.c, anterior cornu; a. p., an-

tadpole stage is provided with gills terior process. al p f alarpy pro.
and gill clefts, perforating the pharynx cess; p.i.p. posterior lateral
on either side. When metamorphosis process; **" thyroid procesa-
occurs, the gills disappear and the branchial skeleton supporting
the gills becomes converted into the hyoid skeleton of the adult,
most of which is cartilaginous (Fig. 34). The body of the hyoid
is a flat plate of cartilage, roughly quadrilateral in shape, located
in the floor of the mouth cavity. The anterior cornua of the hyoid
are a pair of slender rods curving backward and upward, one to
each prootic bone. The alary processes extend laterally, one on
each side just behind the corresponding anterior cornu. The
posterior lateral processes extend from the body behind and at the
sides. Between them are the ossified thyroid processes which
enclose and support the larynx.

Chondrocranium. — The primitive vertebrate skull was cartila-
ginous. In higher vertebrates parts of this chondrocranium are
ossified or replaced by membrane bone. In the frog much of the
cartilaginous cranium remains. The exoccipitals, prootics, and

60 GENERAL ZOOLOGY

sphenethmoid of the cranium are cartilage bones, by which is
meant that they are first laid down in cartilage, and later con-
verted into bone. The remaining cranial bones, the fronto-
parietal, parasphenoid, nasal, and palatine bones are membrane
bones, i.e., they develop directly out of connective tissue and are
not preformed in cartilage. If they are carefully removed, the
cartilaginous cranium is found beneath them enclosing the brain
completely except for several openings in the roof.

It has already been pointed out that the core or axis of the
lower jaw is formed by Meckel's cartilage. The upper jaw like-
wise is built on a cartilaginous basis continuous with the chon-
drocranium. Excepting the quadratojugal and mentomeckelian
bones, the components of the suspensory apparatus and the
jaws are membrane bones.

Some of the general and more obvious differences between
the human skull and that of the frog are shown in Fig. 33. In the
human skull there has been an enormous increase in size of the
cranium as compared with the frog. The frontoparietal region
of the frog is flat; in the human skull it is highly elevated and
convex. The human cranium is also relatively wider and more
elongated posteriorly above the occipital region. The human
jaws are relatively shorter than those of the frog. The human
skull is completely ossified. The human face results from the
increased size of the cranium accompanied by a relative and
actual reduction in visceral skeleton.

Vertebral Column. — The vertebral column or backbone of the
frog consists of a series of nine vertebrae, all of which are based on
a common structural plan, followed by a tenth, the rodlike
urostyle. The urostyle represents the caudal vertebrae of the
frog tadpole which become fused into a single bone during the
process of metamorphosis. Each of the first nine vertebrae,
except the first and ninth, consists of (1) a centrum, a solid ventral
portion, concave in front and convex behind, and (2) a neural
arch, forming the dorsal half of the vertebra (Fig. 35). The
space between the arch and the centrum is occupied by the
spinal cord. On the dorsal side of the arch in the mid-line is a
projection, the neural spine. From either side of the neural arch
a transverse process extends laterally. In a fresh specimen each
transverse process bears at its outer end a short cartilaginous
segment, which represents a rudimentary rib. Ribs are there-

ENDOSKELETON AND VOLUNTARY MUSCLE

61

fore practically absent in the frog. Extending forward from the
anterior edge of each neural arch is a pair of spoon-shaped
articulating surfaces, the anterior zygapophyses, which face
upward and inward. A pair of posterior zygapophyses, projecting
backward from the neural arch, faces downward and out, fitting
over the anterior zygapophyses of the following vertebra. These
articulations between the vertebra at the level of the neural
arch together with the ball and socket union between the centra
permit a limited amount of movement in the body axis.

The vertebrae are held together by means of hyaline cartilage
between the ends of the centra and by ligaments extending along
the sides of the centra and between the neural arches and between

NS.

A B

Fig. 35. — Fourth vertebra of Rana catesbeiana. A, posterior view; B, right
side view, az, anterior zygapophysis; c, centrum; na, neural arch; ns, neural
spine; pz, posterior zygapophysis; t, transverse process.

the neural spines. The intervertebral foramina are spaces
between adjacent neural arches through which the spinal nerves
emerge.

The first vertebra, or atlas, lacks transverse processes and
anterior zygapophyses. In place of the latter there is on either
side of the centrum a deep concave surface which articulates with
an occipital condyle of the exoccipital bone of the cranium. The
ninth vertebra lacks posterior zygapophyses. A pair of rounded
knobs, which project from the posterior face of the centrum,
articulates with a pair of cavities on the anterior face of the
urostyle. The transverse processes of the ninth vertebra are
well developed and are attached to the ilia of the pelvic girdle.
The urostyle has a small vertebral canal into which the spinal
cord extends. The last pair of spinal nerves passes out through
small openings near the anterior end.

In the higher vertebrates the vertebral column is differentiated
more completely into regions than is the case in the frog. Thus
the cervical region, which in the frog is represented by the single

62 GENERAL ZOOLOGY

atlas vertebra, in higher forms consists of a number of vertebrae
characterized by a reduction in the size of ribs or by the complete
absence of ribs. In the higher forms the cervical region is
followed by the thoracic region, provided with well-developed
ribs; the lumbar region, usually without ribs; the sacral region,
made up of several vertebrae, fused together and firmly united
to the pelvic girdle; and the caudal or tail region (Fig. 36). The
caudal region is sometimes reduced or modified as in the case of
the pygostyle of birds (Fig. 44), or the coccyx of man (Fig. 63).

Fig. 36. — Regions of the vertebrate skeleton (cat). (From Jayne, Mammalian
Anatomy, J. B. Lippincott & Co. By permission.)

Unpaired Appendages. — The dorsal, caudal, and anal fins of
fishes and the continuous median fin of larval amphibians, such as
the tadpole of the frog, are examples of unpaired appendages.
They are located in the midplane of the body, and in fishes are
supported by cartilaginous or bony spines. In amphibians
skeletal elements are lacking. Median fins, by extending the
lateral surface of the trunk and tail, enlarge the pushing surface
of the trunk and tail and thus increase the efficiency of the
swimming strokes.

Notochord. — In the lowest vertebrates, the Cyclostomata,
there is a rudimentary vertebral column with poorly developed
neural arches. The centra of the vertebrae are absent and in
their place is a long unsegmented cylindrical rod, the notochord,
which serves as a body axis. The notochord develops in all
vertebrate embryos (Fig. 37). In fishes, vertebrae are devel-
oped, but the notochord is not completely replaced even in the

ENDOSKELETON AND VOLUNTARY MUSCLE

63

adult stage and can be recognized as a thin thread extending
through the centra of the vertebrae and as rounded masses
between the concave ends of the centra, so that the actual form
of the notochord resembles a string of beads (Fig. 37, B). In
vertebrates above fishes the notochord is more and more com-
pletely replaced by the centra of the vertebrae until in mammals
it vanishes completely in the adult. The notochord may be

NT

Fig. 37. — A, the notochord (in solid black) as seen in a longitudinal section
of the larval salamander in which it represents the axial skeleton. The neural
tube (nt) lying above consists of the brain and spinal cord. B, diagram of a
median view of the spinal cord (sc) and developing centra (dc) split lengthwise.
The notochord (in black) is partially replaced by the enveloping centra which
give it a beaded appearance that remains a permanent condition in fishes. C, a
diagrammatic side view of two completely developed vertebrae of a mammal,
showing the intervertebral foramina through which nerves and blood vessels pass.
The nerves and blood vessels are omitted in the drawing. D, vertebra from the
anterior side with the spinal cord shown in section in the vertebral canal, a,
neural arch; c, centrum; s, neural spine; sc, spinal cord; T, transverse process;
az, anterior zygapophysis; pz, posterior zygapophysis.

regarded as the primitive skeletal axis of vertebrates which in
the higher vertebrates is replaced by the vertebral column.

Ribs. — There are two kinds of vertebrate ribs: (1) hemal ribs,
occurring in fishes and some amphibians and reptiles, and (2)
pleural ribs, which may occur in all vertebrate groups. In the
frog there are no hemal ribs. Pleural ribs are represented by
the small projections on the ends of the transverse processes of the
vertebrae. In birds and mammals only pleural ribs are present.

64

GENERAL ZOOLOGY

In fishes the hemal rib extends from the side of a centrum of a
vertebra into the muscle of the body wall. In the caudal region
the ventral ends of the ribs meet in the mid-line to form a hemal
arch, enclosing blood vessels (Fig. 38). A caudal vertebra in
such a case is provided with a neural arch above the centrum
and a hemal arch below.

The pleural ribs of man are restricted to the thoracic region
of the body. The first seven vertebrae are the vertebrae of the
neck or cervical region. The next twelve vertebrae belong to

Fig. 38. — Posterior (A) and side view (B) of a caudal vertebra of a teleost fish,
c, centrum; ha, hemal arch; na, neural arch.

the thorax and it is to the thoracic vertebrae that the dorsal
ends of the ribs are articulated at two points, viz., the head and
tubercle (Fig. 39). The head or capitulum, at the dorsal end of
the rib articulates with a facet on the centrum of the vertebra
called the parapophysis. The facet on the centrum may be
entirely on a single vertebra, as in Fig. 39, or it may be shared
by two adjoining vertebrae. The tubercle or tuberculum of the
rib articulates with the diapophysis, a facet in the transverse
process of the vertebra. The neck of the rib is the region
between the head and the tubercle. The first ten pairs of ribs
are connected at their ventral ends by means of costal cartilages

ENDOSKELETON AND VOLUNTARY MUSCLE

65

to the sternum or breast bone. The eleventh and twelfth pairs
of ribs are known as floating ribs. In many birds and some
reptiles a thoracic rib may bear an uncinate process directed
backward and overlapping the rib behind, thus reinforcing the
thoracic framework (Fig. 44).

Fig. 39. — A, sternum of Man showing the manner in which the ribs are
attached by costal cartilages, c. e, ensiform process; g, gladiolus or body; M,
manubrium. B, view of a thoracic vertebra from the right side with the dorsal
end of the corresponding rib rotated so as to show its posterior aspect and the
manner in which it articulates with the vertebra, d, diapophysis; h, capitular
head of rib; p, parapophysis; t, tubercular head of rib.

Pectoral Girdle and Sternum. — There is a difference of opinion
as to whether the sternum evolved in connection with the
anterior limb girdle or from ribs. In the frog the sternum is an
integral part of the pectoral girdle, the combined elements form-
ing a bony structure almost completely encircling the body at the
level of the forelimbs. On each side of the body, the girdle is

66

GENERAL ZOOLOGY

composed of three bones, the scapula, clavicle, and coracoid,
which meet at the shoulder. The scapula extends dorsally from

prcmaxillary
mento-Meckelian
vomer
palatine
dentary
Sphen- ethmoid
parasphenoid
Squamosal
anguloire
pterygoid
exoccipilal
Suprascapula
scapula
clavicle
glenoid- fossa
- coracoid
sternum
xiphiiternum

'hum.

CCtrpals,

uro style

acetoibulum.

-pubis

ischium

calcaneum

Fig. 40. — Skeleton of Rana catesbeiana, ventral view. A lateral view of the
left half of the pelvic girdle is shown on the right.

the shoulder and at its distal margin is attached to the broad and
flat cartilaginous swprascapula, most of which is calcified. The

ENDOSKELETON AND VOLUNTARY MUSCLE

67

clavicle in front and the coracoid behind extend from the shoulder
medially and slightly ventrally to the epicoracoid cartilages in the
mid-line (Fig. 40). The glenoid fossa is a shallow cavity at the
junction of the scapula and coracoid, most of it being located on
the posterior side of the scapula. This cavity receives the head
of the humerus of the arm. The clavicle joins the scapula at the
acromium of the latter, directly in front of the glenoid fossa.

The calcined edges of the epicoracoid cartilages, closely united
in the mid-line between the median ends of the clavicles and

4Lm.

Fig. 41. — Ventral view of pectoral girdle (A) and lateral view of left half of
pelvic girdle (B) of Rana catesbeiana. a, acetabulum; c, clavicle; co, coracoid;
G, glenoid fossa; h, humerus; i, ilium; is, ischium; o, omosternum; p, pubis;
s, scapula; 88, suprascapula; st, sternum; x, xiphisternum. The epicoracoid
cartilages make up the narrow region compressed between the median edges of
the coracoid bones.

coracoids, represent the middle section of the sternum. The
remainder of the sternum is composed of the omosternum in front
and the sternum behind (Fig. 41). At the anterior end of the
omosternum is an expanded cartilaginous segment, the epister-
num. A somewhat larger cartilaginous disk strongly notched,
the xiphisternum, is attached to the posterior end of the sternum.
In man the sternum is of more importance in connection with
the ribs than with the pectoral girdle. The suprascapula is
absent and the coracoid is reduced to the coracoid process of the
scapula. The clavicle extends from the acromium of the scapula
to the anterior end of the sternum. The glenoid cavity is
located entirely on the scapula (Fig. 42). The human sternum

68

GENERAL ZOOLOGY

is an unpaired structure consisting of the manubrium in front
(above), the gladiolus in the middle, and the ensiform process
behind (below). To the gladiolus are attached the intercostal
cartilages which link it with the ventral ends of the first ten pairs
of ribs. Its only contacts with the pectoral girdle are through
the clavicles and these are of minor importance as far as support-
ing the sternum is concerned (Fig. 39).

Pelvic Girdle. — The pelvic girdle is much like the pectoral
girdle, with which it may be compared part for part. Thus, the

pubis is on the ventral side in the same
relative position as the clavicle; the ischium,
lying posteriorly to the pubis, corresponds
to the coracoid; and the ilium, extending
dorsally, to the scapula (Fig. 41). The
pelvic girdle, however, differs from the
pectoral girdle in its relation to the axial
skeleton in all forms above the fishes. In
fishes, both girdles are free of the vertebral
Fig. 42.— Left human column, but in the higher groups the pelvic
scapula from the front. o;irdle is firmly united to the sacrum on each

a, acromium process; c, ° .

coracoid process; g, side by a sacroiliac ankylosis, a fusion ol the
glenoid cavity. sacrum and ilium. This, together with the

fact that the pubic portions may meet in the midventral line,
makes the pelvis a firm, rigid structure. This is especially
important in bipeds like birds and man, where the entire weight
of the trunk rests upon the pelvis. In man, the three elements
of each side of the pelvis, though separate in the embryo, are
fused in the adult into a single bone, the os innominatum.

Paired Appendages. — The anterior and posterior pairs of fins
of fishes are relatively simple structures, used in maintaining
balance and to a certain extent in swimming. They may be
classified as organs of locomotion, though the important strokes
in swimming are produced in the fish by the contractions of the
body muscles, which push the trunk and tail against the water.
The skeleton of the fin consists of cartilaginous or bony rods,
which support the tough membrane making up the rest of the
fin. Save for differences in details and some cases of special
adaptation, the paired fins are uniform in shape and structure,
in keeping with the uniform nature of the fluid medium, water,
in which they live. The paired appendages of the terrestrial

ENDOSKELETON AND VOLUNTARY MUSCLE 69

vertebrates, though based on a common plan of structure, show
a wide variety of structural forms, in conformity with the fact
that they may be adapted for crawling, walking, running,
swimming, or flying.

Forelimb of the Frog. — The fore- and hindlimbs of the frog
are made up of similar parts based on the same structural pattern.
The proximal element of the skeleton of the upper arm is an
elongated bone, the humerus, whose head articulates with the
glenoid cavity of the pectoral girdle to form the shoulder joint
(Fig. 40). The deltoid ridge is a narrow elevation running from
the head of the humerus to its middle. The lower end of the
humerus terminates in a large rounded articulating surface, which
fits into a depression in the olecranon of the forearm to form the
elbow joint. • The skeleton of the forearm is a single unit, the
radioulna, formed by the incomplete fusion of two bones,
the radius and ulna. The olecranon is a prolongation of the ulnar
portion of the radioulna and lies on the postaxial (away from the
head) side of the limb. The lower end of the radioulna is
definitely bifurcated into a preaxial radial component, in front,
and a postaxial ulnar component, behind. The bones of the
wrist or carpus consist of six small irregularly shaped bones,
known as car pals, arranged in two transverse rows. Beginning
on the preaxial side, the proximal row is made up of the centrale,
the radiale and, the ulnare; in the distal row are the first carpal, the
second carpal, and the outer carpal, the latter representing a
fusion of three bones. The skeleton of the palm or metacarpus,
which adjoins the wrist, is composed of five cylindrical bones of
which the preaxial one is greatly reduced in size, especially in the
female. Only four digits or fingers are present and these corre-
spond to digits II, III, IV, and V of the human hand, the thumb
(I) being absent. The bones of the fingers are called phalanges, of
which there are two in digits II and III and three in digits IV
and V.

Hindlimb of the Frog. — The skeleton of the upper leg is the
femur, which corresponds with the humerus of the forelimb. It is
a cylindrical bone in the form of a slightly reversed curve. Its
rounded head fits into the acetabulum of the pelvic girdle to form
the hip joint. Its distal end is enlarged to form an articulating
surface with the tibiofibula of the lower leg or crus. The tibio-
fibula like the radioulna is a fusion of two bones. The double

70 GENERAL ZOOLOGY

origin of the tibiofibula is indicated by deep grooves in either
end of the shaft, the tibial component being preaxial and the
fibular postaxial. The ankle or tarsus of the frog is highly
modified. Adjoining the distal end of the tibiofibula are two
long bones, united at either end but separate in the middle,
called the tibiale or astragalus and the fibiale or calcaneum,
preaxial and postaxial, respectively. Beyond these are two or
three smaller tarsal bones, the largest of which represents the
fusion of two bones. There are five metatarsal bones, correspond-
ing to the five metacarpal bones. There are five toes of which
digits I and II contain two -phalanges each, digits III and V three
each, and digit IV, four. On the preaxial side of the first digit
is the prehallux or calcar, composed of one or two parts, and
regarded by some as a rudimentary digit.

Limbs in General. — The similarity in the structural plan of the
fore- and hindlimbs of the frog is also true of the limbs of other
vertebrates. The limbless condition of snakes is secondary and
is the result of loss of limbs. All vertebrates above fishes are
believed to have had limbs at some time. If one wishes to get
some idea of what the primitive vertebrate limb was like, one
should study an amphibian in which the limbs are not highly
developed. The frog does not serve for this purpose because
of the obvious structural and functional specialization of its
limbs. Instead, one should turn to a tailed amphibian, such as
Amby stoma microstomum, which has relatively simple limbs. Its
locomotion consists of slow crawling movements in which the
body is raised just high enough to clear the ground, the tail
dragging. The limbs move principally in a horizontal plane, the
entire volar or plantar surface of the foot touching the ground
at each step. This type of foot posture is known as plantigrade.
In more rapid movements the body is thrown into snakelike,
side-to-side undulations by contractions of the trunk and tail
muscles, which accompany the movements of the legs. The
undulatory movements resemble the swimming movements of
these animals, except that in swimming the legs are not used.

The proximal element of the limb skeleton of the salamander is
a single bone, the humerus in the forelimbs and the femur in the
hindlimb. The radius and ulna of the forelimb are separate
bones as are also the tibia and fibula of the crus. The carpal and
tarsal bones are relatively unspecialized and vary from seven to

ENDOSKELETON AND VOLUNTARY MUSCLE

71

nine in number. The foot is provided with four or five digits
and seems to be derived from a pentadactyl type. Such a limb
is regarded as a fairly close approximation to the primitive type
of vertebrate limb from which the limbs of all vertebrates have
been derived.

Departures from this primitive amphibian type of limb are
numerous in the higher vertebrates. Thus in the frog the radius
and ulna are fused and, likewise, the tibia and fibula. In other
animals, one component in the forearm or leg may be distinct
and the other greatly reduced. An extreme example of this

Fig. 43. — Foot postures. A, plantigrade, bear; B, digitigrade, hyena; C,
unguligrade, horse. (A and B after Lull, Organic Evolution, C after Pander and
D'Alton.)

among mammals is found in the horse, in which the fibula is
distinct but reduced in size to a thin rod. There is a reduction
of parts through loss as well as diminution in the more distal
elements of the limb of the horse. Thus metacarpal III is the
principal element of the metacarpus, metacarpals II and IV being
reduced to splints, while metacarpal V is missing. A similar
condition is found in the metatarsus. Of the digits only digit III
is present. The horse walks on the nail (hoof) of its middle
finger or toe. This type of foot posture is unguligrade (Fig. 43).

Dogs and cats are digitigrade, i.e., they walk on their toes.
Strangely enough, the human limb has retained a number of
primitive features, such as distinct radius and ulna in the fore-
arm, and distinct tibia and fibula in the leg, though the fibula is

72

GENERAL ZOOLOGY

much reduced; a pentadactyl hand and foot, and a plantigrade
foot posture. Of the larger mammals, bears alone have planti-
grade feet.

The skeleton of the limbs of birds in the course of evolution
has undergone considerable modification (Fig. 44). In the wing

Fig. 44. — Skeleton of trunk and appendages of domestic fowl. 1, cervical rib;
2, ilium; 3, ischium; 4, pubis; 5, pygostyle; 6, ilio-sciatic foramen; 7, uncinate
process of thoracic rib; 8, humerus; 9, ulna; 10, radius; 11, ulnar carpal; 12, meta-
carpal of ulnar digit; 13, coracoid; 14, sternum; 15, metasternum; 16, hypoclei-
dium; 17, clavicle; 18, femur; 19, fibula; 20, tibiotarsus; 21, tarsometatarsus; 22,
metatarsus of first digit. I, II, III, IV, V, digits. The digits of the fore limb
should be labeled II, III and IV instead of III, IV and V respectively.

of the domestic fowl, ulna and radius are both present, the ulna
being the larger. Carpals and metacarpals are reduced by
fusion. Of the metacarpals, II, III, and IV can be identified, and
adjoining these are the corresponding rudimentary digits. The
proximal element of the skeleton of the hindlimb is the femur.
The next segment consists of a very much reduced fibula and a
long cylindrical iibiotarsal bone, formed by fusion of the tibia

ENDOSKELETON AND VOLUNTARY MUSCLE

73

with proximal tarsals. Distal to the tibiotarsal is another
cylindrical bone, the tarsometatarsal, formed by fusion of distal
tarsals with metatarsals. As a result the ankle joint in birds is
intertarsal instead of between the tarsals and the tibia and
fibula. This condition also occurs in reptiles. Four toes are
present in the fowl, digit V being absent.

Histology of Cartilage and Bone. — Bone and cartilage are
forms of connective tissue. Cartilage is composed of cells
embedded in a matrix of solid material that is a mixture of
collagen (a substance also found in white connective tissue
fibers), chondroitin- sulphuric acid in a combined form, and
albuminoid substances. Hyaline cartilage, which is translucent

Fig. 45. — Cartilage, larval salamander. X 300.

and pale gray in color, forms the articulating surfaces of bones, the
nasal cartilages, the costal (rib) cartilages, and the rings of the
trachea (windpipe) and bronchial tubes. Elastic cartilage is
yellowish in color, owing to the presence of yellow elastic fibers.
It occurs in the external ear of mammals. Fibro cartilage contains
in its matrix dense connective tissue composed mainly of white
fibers. In man it is found in the disks between the centra of the
vertebrae. The matrix of cartilage is produced by the cells
embedded in it (Fig. 45).

Bone resembles cartilage in the topographical and functional
relations of cells and matrix. The matrix of bone in the early
stages of its histogenesis is soft and at that time is composed of
collagen, osseomucoid, and usually some connective tissue fibers.
The bone hardens as a result of deposition of inorganic salts, of
which the most abundant is calcium phosphate. Cartilage bone
is bone that passes through a cartilage stage in its development.
Membrane bone, on the other hand, is formed directly from con-

74

GENERAL ZOOLOGY

nective tissue without passing through a cartilaginous stage.
The greater part of the skeleton is laid down in cartilage, which,
except in those forms in which the skeleton is permanently
cartilaginous, is replaced by bone tissue formed about the
cartilage as the latter is resorbed. In the frog some of the primi-
tive cartilaginous skeleton survives in the adult, though the
greater part of it is converted into bone. Membrane bones, such
as those of the face, the flat bones of the skull, parts of the jaw,
etc., may be formed regardless of whether the original carti-

Fig. 46. — Cross section of compact bone from the shaft of the humerus show-
ing the ground substance in white. The large black areas are occupied by blood
vessels, the bone cells lying in the smaller spaces arranged in concentric circles.
X 150. (From Schafer, Textbook of Microscopic Anatomy, Longmans, Green &
Co., after Sharpy. By permission.)

laginous skeleton becomes ossified or not. The calcified cartilage
of sharks is almost as hard as bone.

The nature of the process of histogenesis is the same for both
cartilage and membrane bone. The preliminary formation of
cartilage in the formation of cartilage bone probably signifies the
existence of a primitive ancestral cartilaginous skeleton in which
the homologues of the membrane bones were not represented.
Therefore, membrane bone would seem to be a later addition to
the skeleton.

A cross section of a decalcified human long bone (Fig. 46)
shows large black spaces, Haversian canals, surrounded by
concentric circles of smaller spaces, lacunae, with fine radiating
lines connecting the circles. The Haversian canals contain blood

ENDOSKELETON AND VOLUNTARY MUSCLE 75

vessels and some nervous tissue, which enter from the periosteum
covering the bone. The lacunae enclose bone cells from which
processes extend into the fine radiating canals.

Skeletal Muscle. — If the skin is removed from the body of a
frog, the superficial skeletal muscles are exposed. Muscles are
usually attached by one or both ends to bones by tendons, which
are composed of white connective tissue fibers. Fasciae are
slightly elastic connective tissue membranes covering muscles,
and uniting with the tendons at the ends of the muscles. The
origin of a muscle is the end that remains stationary, or relatively
so, when the muscle contracts. The insertion is the end that
moves during contraction. Since movements are brought about
by the contractions of muscles, the flexion or extension of a body
part at a joint is the result of the actions of two sets of antago-
nistic muscles known as flexors and extensors, respectively.
Adduction or abduction refers to movements of the arms or legs
toward or away from the mid-line of the body, or in the case of
fingers or toes, toward or away from the principal axis of the arm
or leg. A rotator muscle causes a part to rotate about its main
axis. In connection with the jaws a levator muscle raises the
lower jaw and a depressor lowers it. The description of the
muscles of the bullfrog, Rana catesbeiana, in the following
paragraphs, is confined to those muscles that can be exposed
by the removal of the skin and fasciae and by a small amount of
dissection as illustrated in Figs. 47 and 48.

Muscles of the Head and Trunk. — Beginning at the ventral
side of the head, the submaxillaris muscle, forming the floor of the
mouth, can be seen. Its fibers run transversely between the
rami of the lower jaw. It is important in swallowing and
respiratory movements. If one-half of the submaxillaris is
removed, three other muscles are exposed: (1) the submentalis,
a very small muscle joining the tips of the dentary bones that
aids in closing the external nostrils by drawing together the two
halves of the lower jaw and thus raising the premaxillary bones;
(2) the hyoglossus and (3) the geniohyoideus, two ribbon-shaped
muscles extending from the submentalis posteriorly. The outer
one, the geniohyoideus, arises from the lower jaw and is inserted
on the hyoid bone. The inner one, the hyoglossus, has its origin
in the hyoid bone, runs forward uniting with the muscle of the
opposite side and passes into the tongue at the anterior end of

76 GENERAL ZOOLOGY

the mouth, where it turns back and continues to the tip of the
tongue.

The following muscles are found on the ventral and lateral
walls of the trunk. The deltoideus, arising from the clavicle,
precoracoid, omosternum, and scapula, is inserted on the deltoid
ridge of the humerus. Its contraction draws the limb forward.
The sternoradialis arises in the mid-line from the omosternum and
epicoracoid, passes outward, piercing the distal portion of the
deltoid, and is inserted on the radioulna. It corresponds to the
human biceps muscle and its contraction flexes the forearm.
The recti abdominis muscles of each side meet in mid-line in a tendi-
nous union, called the linea alba. Each rectus muscle is also
traversed by four or five transverse inscriptiones tendineae, which
are tendinous insertions in the muscle. As each rectus muscle
passes forward from its origin in the pubis, it widens and at the
second inscriptio tendinea from the rear divides into two parts : a
median portion continuing straight forward to the xiphisternum
and beyond; a lateral portion branching off to the shoulder to
form the abdominal component of the pectoralis muscle. The
continuation of the median portion beyond the xiphisternum
forms the sternohyoideus, which is inserted on the hyoid bone.
The pectoralis muscle consists of three parts; of which the
sternalis anterior, in front, and the sternalis posterior, just behind,
both have a broad origin from the epicoracoids, sternum, and
xiphisternum, while the abdominalis portion arises principally in
the rectus abdominis. All three portions of the pectoralis
muscle are inserted in the humerus. If the pectoral muscle is
removed (right side of Fig. 47), the coracohumeralis is exposed.
This is a narrow muscle having its origin in the coracoid near the
sternum and its insertion on the middle of the humerus. Pos-
terior to the coracohumeralis a small portion may be seen of the
obliquus internus (or transversus) muscle, which forms the inner
layer of the abdominal wall. Its origin is principally in the
ilium and transverse processes of the last six vertebrae. Its
points of insertion are various, such as the xiphisternum, coracoid,
esophagus, pericardium, and the linea alba. The obliquus
externus, whose fibers run almost at right angles to those of the
obliquus internus, forms the outer wall of the abdominal cavity
except ventrally where it is overlaid by the recti abdominalis
and pectoral muscles.

ENDOSKELETON AND VOLUNTARY MUSCLE

77

Fig. 47. — Muscles of Rana catesbeiana, ventral side, ab, adductor brevis; al, adductor
longus; am, adductor magnus; ch, coracohumeralis; d, deltoideus; ec, extensor cruris; g,
gastrocnemius; gh, geniohyoideus; hg, hyoglossus; m, semimembranosus; oe, obliquus
externus; oi, obliquus internus; p, pectoralis; pe, pectineus; ra, rectus abdominis; rima,
rectus internus major; rimi, rectus internus minor; ru, radioulna; s, sartorius; sm, submaxil-
laris; smt, submen talis; sr, sternoradialis; st, semitendinosus; t, tendon of sternoradialis;
ta, tibialis anticus; tf, tibiofibula; tp, tibialis posticus; trb, triceps bracbii; vi, vastus internus.

78 GENERAL ZOOLOGY

The triceps brachii is an important muscle of the upper arm
whose contractions extend the forearm. A portion of it may be
seen in Fig. 47 at the posterior side of the upper arm. It arises
by three heads: (1) from the scapula and from the capsule of the
glenoid cavity; (2) from the proximal half of the inner surface of
the humerus; and (3) from the outer surface of the humerus. It
is inserted by a single tendon on the radioulna. The muscles
of the forearm consist of extensors and flexors of the wrist and
fingers, a description of which will be omitted.

Muscles of the Head and Trunk, Dorsal Side. — A portion of
the temporalis muscle can be seen between the orbit and the
tympanum (Fig. 48). This muscle has its principal origin in
the prootic bone, from which it passes out beneath the squamosal
and over the pterygoid, between the latter and the maxillary
and quadratojugal to its insertion in the coronoid process of the
angulare. A portion of its fibers also come from the anterior
border of the tympanic ring and from the squamosal bone. It is
an important levator muscle of the lower jaw. The depressor
maxillae inferioris, overlapping the temporalis posteriorly, arises
(1) from the dorsal fascia and (2) from the squamosal and the
posterior and inferior border of the tympanic ring; it is inserted
in the posterior angle of the lower jaw. Its action depresses
the jaw and opens the mouth. The infraspinatus muscle,
arising from the suprascapula, and the latissimus dorsi, from
the dorsal fascia, are both inserted by a common tendon in the
outer surface of the deltoid ridge of the humerus. The longissi-
mus dorsi arises from the urostyle and is inserted in the spinous
processes of the first five vertebrae and in the lateral processes of
the first six vertebrae. The coccygeosacralis arises from the
anterior lateral surface of the urostyle and is inserted in
the neural arch and transverse process of the last vertebra. The
coccygeoiliacus arises from nearly the entire length of the urostyle,
and is inserted in the anterior two-thirds of the ilium. The
obliquus externus, a portion of which may be seen in the dorsal
view, has its origin in the scapula and the dorsal fascia, and its
insertion on the xiphisternum and in the rectus abdominis on the
ventral surface of the body.

Muscles of the Thigh, Ventral Side. — The sartorius, a long
thin muscle traversing the mid ventral region of the thigh, arises
from the posterior end of the ilium and is inserted in the head
of the tibia, just beyond the knee joint. Its contraction pulls

ENDOSKELETON AND VOLUNTARY MUSCLE

79

Fig. 48. — Muscles of Rana catesbeiana, dorsal side, b, biceps; ci, coccygeo-
iliacus; cs, coccygeosacralis; d, deltoideus; dm, depressor maxillae; ed, extensor
digitorum; g, gastrocnemius; gl, gluteus; i, infraspinatus; ip, iliopsoas; Id, latissi-
mus dorsi; lgd, longissimus dorsi; oe, obliquus externus; p, pyriformis; per,
peroneus; ra, rectus femoris anticus; ri, rectus internus minor; sm, semimem-
branosus; t, temporalis; ta, tibialis anticus; trb, triceps brachii; trf, triceps
femoris; ve, vastus externus.

80 GENERAL ZOOLOGY

the hindlimb forward in a ventral direction. The rectus internus
major {gracilis major) is a large muscle occupying most of the
inner half of the ventral surface of the thigh, marked by an
oblique tendinous insertion across its middle. It arises from the
posterior border of the ischium and is inserted at two points in
the head of the tibiofibula. Its contraction draws the thigh
backward and may flex or extend the crus, depending upon the
degree of bend in the knee joint at the time of contraction. Thus,
if the crus is partially extended, contraction of the rectus major
internus will extend it still more. If the crus is at an angle of
less than a right angle, contraction of the muscle will flex the
crus. The rectus internus minor (gracilis minor) is a long narrow
muscle at the inner edge of rectus internus major, having its
origin in the ischium and its insertion in one of the tendons of the
rectus internus major. The actions of the two internal recti
muscles are similar (Fig. 47).

Between the sartorius and the rectus internus major a portion
of the adductor magnus can be seen (left side of Fig. 47). If the
sartorius and rectus muscles are removed, the adductor magnus
is fully exposed (right side of Fig. 47). Just distal to its origin
in a tendon attached to the pubis and ischium, this muscle
divides into two parts, between which passes the tendon of one
of the heads of the semitendinosus muscle. Distally the two
parts of the adductor magnus reunite and join with a small slip
from the semitendinosus. The adductor magnus is inserted in
the distal end of the femur and it acts either as an abductor or
adductor, depending upon the position of the thigh at the begin-
ning of contraction. If the centers of its origin and insertion lie
in a line behind the head of the femur, contraction of the muscle
adducts the thigh, i.e., draws it back to the mid-line. If this
line lies in front of the head of the femur, contraction draws the
thigh forward, i.e., away from the mid-line, and ventrally.
The semitendinosus, located to the inner side of the adductor
magnus, arises by two distinct heads from the pubis, the anterior
head dividing the adductor magnus near the head of the latter.
Its insertion is a tendon which, with that of the sartorius, forms
a connective tissue arch joined with the fascia of the crus, beneath
which the tendons of the internal recti muscles pass.

The adductor longus arises from the ventral part of the ilium
and is inserted with the adductor magnus in the middle of the

ENDOSKELETON AND VOLUNTARY MUSCLE 81

femur. Between the upper ends of the adductor longus and
the adductor magnus can be seen two small muscles, the adductor
brevis and the pectineus, both of which originate in the pubis and
are inserted in the proximal half of the femur.

Muscles of the Thigh, Dorsal Side. — The vastus externus which
can be seen at the outer surface of the thigh in the ventral view,
is a part of the triceps femoris which can be better understood
from the dorsal view (Fig. 48). The triceps femoris forms the
front of the thigh and arises by three heads : (1) the vastus internus,
from the outer covering of the acetabulum at the junction of the
ischium and pubis; (2) the rectus femoris anticus, from the ventral
side of the middle of the ilium; and (3) the vastus externus, from
the crest of the ilium. The fibers of the rectus femoris anticus
join those of the other two components at the middle of the
thigh, forming a single muscle that is inserted by a single tendon
passing over the knee joint to the proximal end of the tibio-
fibula. Its action extends the cms and pulls the whole leg for-
ward. The semimembranosus is a large muscle occupying the
inner half of the dorsal surface of the thigh, arising from the
posterior margin of the ilium and, passing under an arch formed
by the tendon of origin of the gastrocnemius muscle, ends in a
tendon inserted on the dorsal surface of the tibiofibula. It is
divided obliquely into anterior and posterior halves by a tendi-
nous insertion. This muscle adducts the thigh and may extend
or flex the crus, depending upon whether the crus is in an extended
or flexed position when the muscle contracts.

The pyriformis is a narrow muscle originating at the tip of
the urostyle and is inserted on the inner surface of the femur.
The iliopsoas extends from the ilium to the outer surface of the
femur. The gluteus arises from the posterior two-thirds of the
ilium and is inserted on the femur near the head. The biceps
or iliofibularis can be seen near the lower ends of the semimem-
branosus and the vastus externus. Its tendon of origin passes
from the ilium between the pyriformis and the iliopsoas, under
and behind the origin of the vastus externus. It has two points
of insertion: one on the inner surface of the femur and the other
on the dorsal surface of the femur and on the tibiofibula.

Muscles of the Crus. — The gastrocnemius muscle forms the
calf of the leg or crus. One of its two heads arises from a tendi-
nous arch extending from the femur to the tibiofibula and the

82

GENERAL ZOOLOGY

other from the tendon of the triceps. It terminates in a strong
tendon, the tendo Achillis, which after passing over the ankle
joint spreads out in a connective tissue sheet over the plantar
surface of the foot. Its contraction flexes the crus and extends
the foot. The tibialis anticus (Fig. 47) lying at the front of the
leg, originates on the distal end of the femur, its ligament passing

B

Fig. 49. — Fibrils (myofibrils) of the wing muscle of the wasp. A, contracted
fibril showing a reduction in the size of the isotropic bands and a corresponding
increase in the anisotropic bands; B, a stretched fibril with its anisotropic bands
constricted at the line of Hensen; C, an uncontracted fibril, k, membrane of
Krause forming the boundary of a sarcomere; Y, isotropic substance; s, aniso-
tropic substance; h, Hensen's line. (Adapted from Schafer, Essentials of Histology,
Longmans, Green & Co. By permission.)

beneath the tendon of the triceps. This muscle divides at about
the middle of the leg, the inner half being inserted on the astrag-
alus and the outer half on the calcaneum. It extends the foot
and flexes the tarsus. The peroneus muscle, seen in a dorsal
view of the leg (Fig. 48) between the tibalis anticus and the
gastrocnemius, originates in a tendon attached to the distal
end of the femur and ligaments of the knee joint; it is inserted

ENDOSKELETON AND VOLUNTARY MUSCLE

83

in the end of the tibiofibula and in the calcaneum. It extends
the leg and may either flex or extend the foot. The extensor
cruris, seen in the ventral view of the leg, arises from the inner
condyle of the femur and is inserted on the outer (anterior)
surface of the tibiofibula. It extends the leg. The tibialis
posticus lies between the gastrocnemius and the tibiofibula.
It arises from the entire length of the tibiofibula and is inserted
on the astragalus. It flexes or extends the foot (Fig. 47) .

Histology of Skeletal Muscle. — A skeletal muscle is made up of
numerous muscle fibers, separated from one
another by thin sheets of connective tissue,
continuous with connective tissue enveloping
the muscle. The muscle fiber may be regarded
as an elongated cell, tapered at the ends, and
containing many nuclei. A vertebrate muscle
fiber may be 3^ in. or more in length. Some-
times, as in the frog's tongue, the fibers are
branched. In mammals the nuclei occupy a
peripheral position in the fiber, directly inside
of the sarcolemma, except toward the ends
where they may be centrally located. The
sarcolemma is the wall of the fiber. In the
frog the nuclei are scattered through the fiber.
The cytoplasm of the muscle fiber contains
numerous striated myofibrils, or sarcostyles,
occurring in groups known as muscle columns,
so arranged that the dark parts of one fibril are
opposite those of adjacent fibrils, with the
result that the muscle fiber as a whole has a striated appearance.
For this reason skeletal muscle is sometimes called striated or
striped muscle. The muscle columns are separated from each
other by clear sarcoplasm. A clear hyaline material also separates
individual myofibrils. The structure of a myofibril is shown in
Fig. 49. Under polarized light, the dark bands are anisotropic
(doubly refractive) and the light bands are isotropic (singly
refractive) . Each anisotropic band is crossed by a median mem-
brane, Hensen's line. In the same way each isotropic band is
divided by Krause's membrane. Krause's membrane is thought to
be a continuous membrane extending from one myofibril to
another, dividing the muscle fiber into disklike segments called

Fig. 50. — Section of
skeletal muscle of lar-
val salamander.

84 GENERAL ZOOLOGY

sarcomeres. Thus a single sarcomere would include the parts of
each myofibril, within two adjacent membranes of Krause, lying
at the same level. Usually, however, the longitudinal cohesion is
greater than the lateral with the result that some striated muscle
fibers cannot be made to cleave transversely into disks. Figure
50 illustrates the appearance of a longitudinal section of skeletal
muscle of a larval salamander.

Action of Skeletal Muscle.— Skeletal muscle is voluntary in
its action, i.e., its action is under the control of the central
nervous system, the brain and spinal cord. This is in sharp
contrast to the involuntary activity of the plain or unstriated
muscle cells of the walls of the alimentary tract and blood
vessels. Cardiac muscle, though striated, is also involuntary
in its action.

CHAPTER V
ALIMENTATION

The energy-producing substances of an animal's diet consist
of chemically complex compounds, viz., proteins, carbohydrates,
and fats, each of which undergoes digestion before being absorbed.
Water, simple sugars, and some inorganic salts, such as sodium
chloride, taken with food do not require digestion. In the higher
forms, the organs of digestion include, in addition to the ali-
mentary canal in which digestion and absorption take place, a
number of glands, such as the oral (salivary) glands, the pancreas,
and the liver, though the last in vertebrates does not produce a
digestive enzyme.

Gastrovascular Cavity. — A simple kind of alimentary tract is
that of the common Hydra, a tube-shaped animal found in fresh
water, its length in different species varying from 2 to 60 mm.
The closed end of the tubular body is attached by a basal disk
to the substratum, which is usually a plant; the opposite free end
is open and serves as a mouth. The mouth is surrounded by
from six to ten long, narrow, tubular tentacles. The body wall
consists of two well-defined cellular layers, an outer ectoderm and
an inner endoderm, with some indication of a third layer between
them. The third layer, known as the mesoglea, is noncellular and
of jellylike consistency. Since only two embryonic germ layers,
the ectoderm and endoderm, are developed, the animal is said
to be diploblastic. Each tentacle likewise consists of an inner
layer of endoderm and an outer layer of ectoderm. The cavity
of the tube, the gastrovascular cavity, is the digestive cavity
(Fig. 51).

Food captured by the tentacles passes through the mouth into
the gastrovascular cavity, where it is acted upon by a secretion of
the cells lining the cavity. Some of the food is digested in this
way; and some of it apparently is engulfed by the endodermal
cells to undergo intracellular digestion. The products of extra-
cellular digestion are absorbed by the endoderm and any undi-

85

86

GENERAL ZOOLOGY

gested remains are cast out through the mouth. From the
endoderm, absorbed food is distributed to the ectoderm by
cell-to-cell transfer. The simple digestive tube of Hydra may be
regarded as an early step in the evolution of the digestive tract of

Fig. 51. Fig. 52.

Fig. 51. — A, Hydra, expanded, diagrammatic; B, longitudinal section of body
wall, ec, ectoderm; en, endoderm; g, gastro vascular cavity, shown in black;
me, mesoglea; m, mouth; t, tentacles.

Fig, 52. — Digestive and nervous system of a flatworm, Euplanaria; the diges-
tive system shown in solid black, g, ganglionic mass; i, intestine; p, pharynx.

higher forms. A more complete description of the histology of
the hydra is given on page 389.

The flatworm, Eupla?iaria, may be utilized to illustrate a
modification of the same type of canal (Fig. 52). This animal is
also a fresh-water form, of common occurrence in ponds. Its
body measures about 12 mm. in length, and in shape it is flat-
tened and elongated, being blunt at its anterior end and pointed

ALIMENTATION 87

posteriorly. A ciliated layer of ectoderm covers the outside, and
an inner layer of endoderm lines an alimentary tract. Between
the two are muscle tissue and other organs derived from a third
germ layer, the mesoderm. Euplanaria is, therefore, triplo-
blastic. The opening into the alimentary tract is on the ventral
surface of the body near the middle through which a muscular
tube, the pharynx, can be thrust like a proboscis. Inside the
body the pharynx connects with an intestine consisting of three
main trunks, one forward and two backward, from each of which
numerous smaller branches, ending blindly, penetrate among the
tissues. Digestion, both intracellular and extracellular, takes
place in the intestine, which also serves to distribute the products
to the tissues by means of its collateral branches.

OP

ov

M SG y U

Fig. 53. — Internal anatomy of a grasshopper. A, anus; C, crop; CM, caecum;
E, eggs; G, supresophageal ganglion; H, heart; 1, intestine; L, labium; M,
mouth; O, esophagus; OP, ovipositor; OV, oviduct; R, rectum; S, stomach; SG,
salivary gland; U, Malpighian tubules; V, ventral nerve cord. (After Packard,
Textbook of Entomology.)

Complete Alimentary Canal. — The digestive organs of an
insect like the grasshopper display a considerable advance in
complexity over the preceding forms, and have many features in
common with the vertebrate type of system. The alimentary
canal of the grasshopper is a tube open at both ends, beginning
with a mouth and terminating in an anus; a type that is known as
a complete alimentary canal (Fig. 53). The tube lies in a cavity
of the body, the hemocoel, which is filled with blood. The mouth
is provided with organs of mastication : a labrum, or upper lip ; a
labium or lower lip ; and between them a pair of mandibles and a
pair of maxillae. The mandibles are the principal chewing
organs, and both they and the maxillae operate by a side-to-side
movement, instead of up and down as with the jaws of man.
The maxillae aid the lips in manipulating the food so that it can

88 GENERAL ZOOLOGY

be chewed by the mandibles. The mouth leads to a narrow
esophagus extending dorsally to about the center of the head,
where it turns posteriorly to dilate into the crop. The lat-
ter is lined with a cuticular membrane armed with toothlike
projections for completing the process of mastication begun in
the mouth. Alongside the crop are branched salivary glands
whose ducts run forward to empty into the mouth. Next comes
the stomach surrounded by a set of six to eight double cone-
shaped pouches, known as caeca, which secrete a digestive fluid.
The posterior limit of the stomach is marked by a large number of
Malpighian tubules, excretory in function, which enter the diges-
tive tube at the junction of the stomach with the intestine. The
latter passes back to terminate at the tip of the abdomen in an
anal opening. The rectum is an enlargement in the intestine
near the anus where fecal matter accumulates until excreted.

Digestion and absorption in the grasshopper take place in the
stomach and intestine, whence the products of digestion pass into
the body fluid filling the hemocoel. The body fluid and the blood
in insects is the same, and is pumped by the heart to all parts
of the body, supplying the tissues in this way with nutrition and
at the same time ridding them of the waste products of metabo-
lism. Undigested food and excretory products are expelled
through the anus.

Vertebrate Alimentary System.— The alimentary canal of
vertebrates begins with a mouth, opening into an oral cavity,
continues as a tube, regionally differentiated, and terminates in a
rectum that opens into a cloaca, as in the frog, or directly to the
outside, as in the majority of mammals.

Oral Cavity. — The lining of the oral cavity as well as part of the
rectum is ectodermal in origin, i.e., it is derived from the primitive
outer covering of the vertebrate embryo. The lining of the
remainder of the alimentary canal is formed of endoderm, which
for the present may be described as an internal germ layer.
Briefly, the oral cavity develops as an invagination of the head
ectoderm, producing an indentation, that deepens until it comes
in contact with the wall of the pharynx. The ectoderm of
the oral cavity fuses with the endoderm of the pharynx, after
which the fused layers break down to form a passageway. The
posterior opening of the alimentary canal is formed in a similar
way. From this it follows that the lining of the oral cavity

ALIMENTATION

N't

a

■

Fig. 54. — Maxillary teeth of
Rana pipiens, shown in normal
position, attached above to bone,
below which transparent gum tissue
is shown.

differs in its embryological origin from the lining of the pharynx,
though in the frog or in the adult of any air-breathing vertebrate
it is impossible to establish any
sharp histological difference at the
line of fusion of ectoderm and
endoderm. In fishes the pharyn-
geal region is definitely distin-
guished from the oral cavity because
it is provided with gill clefts and
gills. In the frog gill clefts are
present in the embryo and tadpole,
but all of them except the first dis-
appear during metamorphosis.
Since the persisting first gill cleft is
converted into the cavity of the middle ear, connecting with the
pharynx by the Eustachian tube, the location of the opening of

the Eustachian tube gives a rough idea
of the anterior limit of the pharynx.
Otherwise in the frog and in all higher
vertebrates there is no simple way of
establishing the line of demarcation
between the oral cavity and pharynx.
The oral cavity may be provided with
teeth, a tongue and the openings of
oral glands.

Teeth. — True teeth, such as human
teeth, are hard bony structures found
in the oral cavity of most vertebrates,
notable exceptions being birds and some
reptiles (turtles). The teeth of the frog
consist of a single row attached to the
inner edge of the premaxillary and
maxillary bones of the upper jaw and a
small group in the ventral surface of
each vomerine bone. The tooth con-
sists of a conical bony core v of dentine,
whose root is attached to the bone of the
jaw by a cement substance and whose
exposed crown is covered with enamel thickened at the apex
(Fig. 54). The enamel in turn is covered with the cuticula dentis,

Fig. 55. — Diagrammatic
longitudinal section of human
tooth. B, bone of jaw; C,
cement; D, dentine; E, en-
amel; G, gum; P, pulp cavity.

90

GENERAL ZOOLOGY

a colorless membrane, highly resistant to the action of chemical
reagents. The pulp cavity is a space in the dentine, open at the
base of the root, and rilled with blood vessels and odontoblasts
(cells that produce dentine). Human teeth have the same
general structure except that a cuticula dentis is absent, and the
enamel is more evenly distributed over the entire crown (Fig. 55).
The pulp also contains nerve endings. Each human tooth is
inserted in a socket or alveolus, the dentine of the root of the tooth

Fig. 56.

Upper teeth of the dog. c, canine; i, incisor; p, premolar; m, molar.
The carnasial or flesh-cutting tooth is the fourth premolar.

being attached to the bone of the jaw by a cement substance.
The dentine of the vertebrate tooth is formed of mesoderm, the
same embryonic germ layer that gives rise to bones, while the
enamel is a secretion of ectodermal cells of the oral cavity.

The vertebrate tooth evolved apparently from what is known
as a placoid scale, such as is found in the skin and in a modified
form, in margins of the jaws of the Elasmobranchii (sharks).
The placoid scale consists of a curved spine projecting from the
outer surface of a flattened base of dentine embedded in the skin
(Fig. 25). The spine, which extends beyond the surface of the
skin is covered with an enamel-like substance of ectodermal origin
and is also provided with a pulplike cavity, open below. Near

ALIMENTATION

91

the jaws in sharks the scales pass by gradations into the teeth.
Presumably teeth originated as placoid scales covering the sur-
face of the body as well as the margins of the jaws, a condition
that has persisted in sharks, accompanied by a certain amount of
specialization in the jaw scales; while in higher forms the scales

Fig. 57. — Side-view of the upper teeth of the sea-lion showing the peg-like char-
acter of all of them.

have become purely oral structures that have gradually taken
on the form of typical vertebrate teeth.

Primitive vertebrate teeth, if one is to judge from their struc-
ture and from the manner in which they are used by the shark,
were prehensile in function. That is to say, they were used for
grasping, tearing and holding, and not for chewing. In the case
of the frog the teeth are poorly adapted for anything but holding.
The similarity in form and structure of the frog's teeth illustrates

C p M

Fig. 58. — Side view of the upper teeth of the horse, z, incisors; c, canine; p, pre-
molars; m, molars.

what is known as homodont dentition. The teeth of mammals, on
the other hand, are with few exceptions heterodont, and may be
differentiated or specialized in four ways: (1) as incisors, located
at the front of the jaws and used for cutting or grasping; (2) as
canines for grasping or tearing; (3) as premolars for shearing;
or (4) as molars for grinding (Figs. 56, 57 and 58).

92

GENERAL ZOOLOGY

Mammals belonging to the order Carnivora have strong,
recurved canine teeth, poorly developed incisors, and high
crowned premolars and molars. In such animals the jaws have
practically no side play, a condition which reaches its highest
development in cats, in which the cheek teeth have a purely
shearing action on food. In dogs, the premolars are shearing
teeth, but the molars are adapted for grinding or crushing (Fig.
56). Omnivorous bears, such as the common black bear, lack
shearing teeth but do have well-developed grinders. Carrion-

Fig. 59. — Side and ventral view of the upper teeth of the ox.

molars.

p, premolars; M,

eating forms have blunter teeth, while fish eaters such as the seal
have teeth that are all pointed and prehensile in function (Fig. 57).
Some of the plant-eating animals belonging to the Ungulata,
hoofed animals, illustrate a high degree of specialization which
contrasts strongly with the conditions found in the teeth of
typical Carnivora. Incisors may be present in both jaws and
adapted for seizing or cutting as in the horse; but in ruminants,
such as the ox, they are absent in the upper jaw, which, instead
of teeth, is provided with a tough pad against which the incisors
of the lower jaw bite (Fig. 59). The tusks of the elephant
are highly modified incisors. Canine teeth as a rule are not well

ALIMENTATION 93

developed among ungulates unless used for defense or for digging
as in swine. The premolars and molars of the ox represent
a typical condition in which these teeth are adapted solely for
grinding. In such animals the jaw is articulated so as to permit
considerable side play, a motion that is familiar to anyone who
has watched a cow chewing its cud.

Human teeth of the permanent dentition consist of four
incisors, two canines, four premolars and six molars in each jaw.
The incisors are spade-shaped with a cutting edge and occupy the
front of the jaws. The canines are conical and bluntly pointed.
The premolars and molars have flattened but not smooth biting
surfaces that are adapted for grinding. Human teeth are
adapted for an omnivorous diet.

Tongue. — The tongue of fishes is simply a fold in the floor
of the mouth incapable of movement since it has no intrinsic
muscles. In some fishes it bears teeth. Above fishes the tongue
becomes a muscular organ appearing in a wide variety of morpho-
logical forms. The frog's tongue is attached at the margin of the
lower jaw and its free end when at rest is folded back on the floor
of the mouth. It is used in capturing food. In snakes the tongue
is used as a tactile sense organ. The tongue of mammals,
whales excepted, is a highly muscular organ capable of a variety
of movements. Its dorsal surface may, as in cats, develop
cornified, filiform papillae producing a rasping surface. The
tongue also may contain mucous glands and at its posterior end,
gustatory organs.

Oral Glands. — Glands are lacking in the oral cavity of most
aquatic vertebrates. In air-breathing vertebrates glands are
present in a variety of forms. Such glands produce a secretion
that is poured into the oral cavity by means of ducts leading
from the glands. The principal oral glands of the frogs are
the intermaxillary glands, located chiefly between the pre-
maxillary bones and the nasal capsule and opening in the fore
part of the roof of the mouth by about 25 ducts. The secretion
of this gland gives the tongue its adhesive properties.

The lining of the oral cavity consists of a ciliated columnar
epithelium, containing goblet cells, which produce a mucous
secretion.

Mammals, in addition to numerous mucous glands located in
the oral epithelium, are provided with three kinds of salivary

94

GENERAL ZOOLOGY

glands, viz., the -parotid, the sublingual, and the submaxillary
glands (Fig. 60). The parotid gland lies beneath the skin of the
side of the head near the ear and its duct (Stenson's duct) enters
the oral cavity by piercing the cheek near the molars of the
upper jaw. The sublingual gland lies between the tongue and
the margin of the lower jaw and opens by several ducts in the
floor of the oral cavity. The submaxillary gland lies inside the
lower jaw and its duct (Wharton's duct) has its outlet near
the lower incisor teeth. All of these glands are paired.

The secretions of the salivary glands vary. In man the sub-
maxillary and sublingual glands
produce most of the mucin, the
substance that gives saliva its ropy,
mucilaginous character; whereas
the secretion of the parotid is more
watery and contains ptyalin in
larger amounts than the other two.
Ptyalin is a digestive enzyme that
acts upon starchy food.

Pharynx. — The pharynx, so far as
alimentation is concerned, is merely
a passage connecting the oral cavity
with the esophagus. Its impor-
tance is in connection with respira-
FiG.eo.-Diagram showing loca- tion> discussed in the following

tion of the salivary glands in Man. chapter.

L, sublingual; s, submaxillary; p, rr ? rr<L v

parotid; t, tongue. Esophagus.— The esophagus IS

for the most part a tube connect-
ing the pharynx with the cardiac end of the stomach. In the
frog it is neither sharply marked off from the pharynx in front
nor from the stomach behind. A cross section of the frog's
esophagus shows the four layers that are typical of the wall of
the vertebrate alimentary tract. These layers, beginning at the
outside, are (1) serous membrane or visceral peritoneum; (2) the
tunica muscularis, consisting of an outer longitudinal layer and
inner circular layer of plain muscle, the longitudinal layer thin-
ning out in the stomach region; (3) the submucosa, composed of
connective tissue, nerves, blood and lymphatic vessels; and (4)
the mucosa, an epithelium, supported by connective tissue
(Fig. 61). Between the submucosa and mucosa of the stomach

ALIMENTATION

95

and intestine there is another muscular layer, the muscularis
mucosae, which is composed of an inner circular and an outer
longitudinal layer. Both are very thin. The inner surface of
the esophagus is thrown into longitudinal folds. The mucosa
consists of columnar goblet cells
among which ciliated cells are
found similar to those lining the
oral cavity. The frog's esopha-
gus is provided with tubular
glands that secrete pepsin, which,
however, does not function until
it reaches the stomach.

The muscle tissue found in the
walls of the vertebrate alimen-
tary tract and also in the walls of
blood vessels consists of mono-
nucleated, spindle-shaped cells
(Fig. 62), without the striations
found in skeletal and cardiac
muscle. It is known as plain or
nonstriated muscle and, as
already noted, is involuntary in

its action. Fig. 61. — Diagrammatic cross sec-

The innervation of the esopha- $on of. half of1 lower **>&**** of

c nana pipiens. 1, serous membrane or

gUS as Well as Of the remainder visceral peritoneum; 2a, longitudinal

of the alimentary tract is from muscle; f ' drcular T8C!,e; 3' sub_

J mucosa; 4, mucosa; g, glands.

branches of the sympathetic nerv-
ous system and the vagus nerve, that penetrate the tunica
muscularis to form the myenteric plexus between the two layers
of muscle and the submucosal plexus in the submucosa.

Fig. 62. — Plain muscle cells of the muscularis layer of intestine of Rana pipiens.

The crop or ingluvies of birds is a differentiated, saclike
expansion on one side of the esophagus serving as a receptale for
food. Sometimes its wall forms a secretion that moistens the
food or starts its digestion. In pigeons the crop, consisting of a

96

GENERAL ZOOLOGY

Fig. 63. — Median section of the human body showing the alimentary canal
and other viscera diagrammatically disposed; bone is shown in solid black. 1,
external nares; 2, mouth; 3, nasal passage; 4, pharynx; 5, tongue; 6, epiglottis;
7, larynx; 8, trachea; 9, esophagus; 10, sternum; 11, pleural cavity; 12, lung;
13, heart; 14, diaphragm; 15, abdominal cavity; 16, duodenum; 17, liver; 18,
stomach; 19, large intestine; 20, jejuno-ileum; 21, vermiform appendix; 22, kid-
ney; 23, ureter; 24, bladder; 25, urethra; 26, anus, 27, coccyx; 28, sacrum; 29,
neural spine of first lumbar vertebra; 30, centrum of first thoracic vertebra; 31,
spinal cord; 32, cerebellum; 33, cerebrum; 34, pituitary body.

ALIMENTATION

97

median and two lateral pouches, produces during breeding
season a thick fluid, called pigeon's milk, that is fed to the young.
The human esophagus is a tube of uniform diameter lined with
a stratified epithelium which receives the openings of two types
of glands, producing secretions having the properties of mucin
and serving apparently for lubrication. The esophagus joins
the stomach at the cardiac aperture (Fig. 63).

Fig. 64. — Stomach region of the domestic fowl, cut open. D, duodenum;
G, gizzard, showing thickness of muscular wall; L, cornified lining of gizzard;
deeply creased; O, esophagus; P, proventriculus, showing rounded elevations of
its soft glandular lining.

Stomach. — The stomach of fishes and lower vertebrates is
generally little more than a dilatation in the digestive tract,
serving as a food reservoir in which a certain amount of digestion
may take place. The stomach of the frog is widest at its anterior
(cardiac) end, where it bends slightly to the left and then con-
tinues to the right in almost a straight line to the narrow pyloric
end. Its union with the duodenum, at the pylorus, is marked by
a well-defined constriction, where the circular layer of the mus-

98

GENERAL ZOOLOGY

cularis is well developed. The mucosa receives the openings of
tubular glands of two general types, cardiac and pyloric, of which
the cardiac glands possess the greater number of granular cells.
Experiments indicate that the pepsin content of the stomach wall
is greatest in the cardiac portion, from which it is inferred that
the pepsin is formed by the granular cells. Hydrochloric acid
is produced in the stomach and gives its contents an acid reaction.
In a bird such as the domestic fowl the stomach consists of
two distinct regions: an anterior glandular part, the proven-
triculus, and a highly muscular posterior part, the gizzard (Fig.
64). The glands of the proventriculus produce a digestive

Fig. 65. — Diagram of ruminant stomach, the dotted line showing the course
of the food, a, abomasum; oe, esophagus; p, pylorus; ps, psalterium (omasum,
manyplies); rt, reticulum (honeycomb); ru, rumen (paunch). {From Kingsley,
Comparative Anatomy of Vertebrates, P. Blakistons Son and Company. By
permission.)

secretion that is mixed with the food before it enters the gizzard
where it is ground up. In the absence of teeth the gizzard of
grain-eating birds serves as an organ of mastication, to which
end it is adapted by having thick muscular walls lined with a
tough, horny membrane. The process of trituration is facilitated
in such birds by small pebbles or other gritty material, swallowed
with the food. The gizzard is less well developed in purely
carnivorous birds, whose food is of a softer consistency.

The stomach of a ruminant, like the ox, also shows rather
striking departures from the common type of vertebrate stomach.
In this case the stomach region consists of four parts; (1) the
rumen or paunch; (2) the reticulum or honeycomb; (3) the psalter-
ium; and (4) the abomasum. Of these, the first two divisions
seem to be modifications of the lower end of the esophagus in

ALIMENTATION

99

which food is stored. Thus in feeding, vegetation is cropped by
the incisors of the lower jaw and swallowed unmasticated into the
rumen and reticulum. When the animal stops grazing, it seeks
a quiet spot and begins to chew its cud. In this process food
is regurgitated in small amounts from the reticulum to the
mouth and then thoroughly ground up by the premolar and
molar teeth. When the masticated food is swallowed, it passes

DC

SF

Fig. 66. — Human alimentary tract, diagrammatic, ac, ascending colon;
bd, bile duct, extending from the liver to the duodenum; c, caecum; d, beginning
of duodenum; dc, descending colon; gb, gall bladder; l, liver, turned up so as to
show its under surface; o, end of esophagus; p, pancreas, whose duct connects
with the duodenum near the end of the bile duct; r, rectum; s, stomach; sf, sig-
moid flexure of descending colon; si, small intestine; tc, transverse colon; va,
vermiform appendix.

into the psalterium and abomasum where it undergoes gastric
digestion (Fig. 65).

The human stomach is sharply marked off from the esophagus
at the cardiac orifice and from the intestine at the pylorus (Fig.
66). Between the cardiac and pyloric regions it is enlarged
to form the fundus, which gives the stomach the shape of a
pouch with a greater and lesser curvature. The actual shape
of the human stomach varies considerably in different individuals
and in the same individual at different times. When empty,

100

GENERAL ZOOLOGY

2a

its walls are collapsed and after a meal its outline is continually
changed by waves of contraction, beginning at about the middle
and extending to the pylorus. The epithelium of the mucosa
consists of a single layer of cells, some of which are mucus-
producing goblet cells. As in the frog, the surface of the mucosa
is pitted with the mouths of glands whose epithelium is con-
tinuous with that of the surface. The glands of the fundus are
believed to secrete pepsin and hydrochloric acid. No specific

m function is attached to the glands

of the cardiac and pyloric regions.
The tunica muscularis of the
stomach is composed of a thin
outer longitudinal layer and a
,2b thicker inner circular layer. What
is sometimes referred to as a third
and innermost layer, whose fibers
run obliquely, is really a modifica-
tion of a portion of the circular
layer, resulting from the twisting
of the stomach in the course of its

Fig. 67. — Diagrammatic cross sec- development. At the pylorus the

tion of the small intestine of Rana circular layer Js enlarged to form
jnpiens. 1, serous membrane or . .

visceral peritoneum; 2a, longitudinal a sphincter muscle which acts as a

muscle; 2b, circular muscle; 3, sub- i)yloric valve
mucosa; 4, mucosa; m, mesentery by

which the intestine is attached to the Intestine. — The superficial basis
bodywalL for the division of the intestine

into large and small portions rests on a difference in diameter.
More important differences are found in the function and struc-
ture of the mucosa of the two regions. The small intestine is
primarily concerned with digestion and absorption, and the large
intestine with absorption alone, principally the absorption of
water. The duodenum is the first segment of the small intestine.
In the frog it extends from the pylorus to the first bend in the
intestine and receives the bile duct about midway. The distinc-
tion between the duodenum and the remainder of the small
intestine, the ileum, or the jejuno-ileum, is largely based on
histological structure. The liver and pancreas pour their secre-
tions into the duodenum and these secretions play important
parts in intestinal digestion. In the frog the bile duct collects
the secretion of the pancreas and thus serves as a common bile

ALIMENTATION

101

and pancreatic duct. In the higher vertebrates the small
intestine also produces digestive agents. This apparently is
not true of the frog.

The mucosa of the small intestine of the frog is arranged in
irregular folds, but there are no villi or tubular glands such as are
found in mammals. The epithelium of
the mucosa consists of columnar cells,
some of which are goblet cells. The
structure of the large intestine is much
the same. In the rectum the folds of
the mucosa are longitudinal (Fig. 67).

In man the mucosa of both the large
and small intestine is provided with
numerous tubular glands. In the small
intestine the mucosa between the face of mucosa of small intes-
mouths of the glands is raised in thin *?* ,of man" G\ ,intest>nal

° gland opening on the surface

folds Or finger-shaped processes Called between the bases of the villi;

villi, which are important in absorbing v' vlIlus"
the products of digestion (Fig. 68). A curious example of a sim-
plification of a villus structure is seen in the spiral valve of the
intestine of the dogfish (Fig. 69). This valve consists of a
rather wide, thin, spiral fold of the mucosa which slows the
movement of the contents of the intestine and also increases

Fig. 69.- — Spiral valve of Raia. The arrow indicates the direction in which the
food passes. {Modified after Mayer.)

the absorbing surface of the relatively short intestine. In the
higher vertebrates an increase in digestive capacity and absorb-
ing area is achieved by an increase in the length of the intes-
tine as well as by villiform elevations of the mucosa. Also,
a correlation exists between the character of the food and the
length of the intestine. The frog tadpole feeds on plants grow-
ing in the water and has an alimentary canal relatively and

102 GENERAL ZOOLOGY

actually much longer than that of the adult frog, which is
mainly carnivorous. The length of the alimentary canal of a
ruminant mammal is from 20 to 28 times the length of the body
while that of a meat-eating form is only five to six times the
body length. This may be understood if it is remembered that
plant eaters require a larger volume of food than carnivorous
forms, because the bulkier vegetable food such as grass, hay,
clover, etc., contains less nutriment and more of difficultly
digestible material than an equal volume of meat.

The large intestine of the frog is simply the enlarged continua-
tion of the small intestine. In mammals the small intestine
joins the large intestine at right angles in such a way as to leave
a blind end of the large intestine at one side of the union (Fig. 66).
At this junction is a circular valve, the ileocaecal valve. The
blind end of the large intestine, called the caecum, is highly varia-
ble in form and size, being best developed in herbivorous forms
and least in carnivorous ones. Thus in the horse it measures
about four feet in length and has a capacity of seven or eight
gallons. In the cat it is practically absent. In man the caecum
measures about 2% in. in length and Z% in. in width. The
vermiform appendix is a diverticulum of the caecum about 3 in.
long and about % in. wide. The human large intestine is known
as the colon. From its point of union with the small intestine
in the lower right abdominal (pelvic) region, the colon passes
anteriorly as the ascending colon, continues to the left beneath
the anterior wall of the abdomen as the transverse colon, and then
turns posteriorly as the descending colon, which takes an S-shaped
turn, the sigmoid flexure, before joining the rectum which termi-
nates at the anus (Fig. 66). The longitudinal layer of the tunica
muscularis of the colon is arranged in three equidistant longi-
tudinal flat bundles or bands between which the longitudinal
layer is reduced to a thin sheet. These bands, known as taeniae,
are shorter than the internal layers of the colon, and as a result,
the colon is divided into pockets, called haustra, by transverse
folds or plicae semilunares.

Liver. — The liver is a large gland whose duct, the bile duct,
empties into the duodenum. The tributaries to the bile duct are
the bile capillaries, which course between the liver cells and send
lateral branches into the cells. Bile is a fluid secreted by the
liver cells into the bile capillaries, which unite to form larger

ALIMENTATION 103

vessels leading to the bile duct. The gall bladder is a dilatation
of the bile duct in which bile is stored. Bile consists of water,
salts, pigment, bile acids, lipoidal substances such as cholesterin
and lecithin, neutral fats and soaps, a nucleo-albumin, which
gives bile a mucilaginous consistency, and traces of urea. It is
partly excretory in nature but is also important in the digestion
and absorption of fats. It has an alkaline reaction and is
secreted continuously, and in those forms in which a gall bladder
is present it is discharged intermittently into the duodenum.
The liver, as already noted in an earlier chapter, in addition to
secreting bile, performs other important functions in metabolism,
such as the storage of sugar and the storage of fats, the produc-
tion of urea and the formation of fibrinogen. In its develop-
ment the liver is an outgrowth of the endoderm, forming the
lining of the embryonic intestine. The liver is formed of the
distal part of this outgrowth and the bile duct represents
the proximal part.

Pancreas. — The pancreas is also a digestive gland. It con-
tains two sorts of secretory cells: (1) alveolar cells, whose secretion
is removed by ducts, and (2) islet cells having no connection
with ducts and whose secretion is removed by the blood stream.
The alveoli are spherical or tubular arrangements of cells sur-
rounding a lumen, drained by the ducts. The epithelium of the
ducts is simply the continuation of the glandular epithelium.
The islet cells are isolated groups of cells scattered between
alveoli at irregular intervals. The secretion produced by the
alveolar cells is conveyed by ducts to one or more large trunks
terminating in the duodenum at or near the mouth of the bile
duct. In the frog the pancreatic ducts all open into the bile
duct before the latter reaches the duodenum. In man the
pancreatic duct joins the bile duct near the intestine, thus
forming a short common hepatopancreatic duct, or there may be
in a certain percentage of cases an additional pancreatic outlet
into the duodenum. The pancreatic fluid produced by the
alveolar cells of the pancreas contains important digestive agents,
or inactive forms of these agents, that act upon proteins, fats,
and carbohydrates in the alimentary canal (Fig. 70).

Digestion. — The absorption of nutritive substances from the
alimentary canal is preceded by a process of digestion which
renders food soluble, since only liquids can pass through the

104

GENERAL ZOOLOGY

wall of the alimentary canal into the blood or lymph streams.
Liquefaction is produced by breaking down the complex organic
food substances into simpler compounds; but a chemical trans-
formation is also undergone by a soluble substance like cane
sugar. The diet of a frog or man consists of organic and inorganic
substances. Of the latter, water is the principal one taken alone.
Inorganic salts are of course present in meat, milk, vegetables
bread, etc., but we ordinarily think of these articles of food as
organic in nature. Since in the process of digestion inorganic
salts are released from organic combination and absorbed or

Fig. 70. — Section of an island of Langerhans surrounded by glandular alveoli
of pancreas; diagrammatic, a, alveoli of glands drained by pancreatic duct;
b, blood vessel; i, island of Langerhans (islet cells), having no connection with
ducts. (After Stiihr.)

secreted without further change, digestion is primarily concerned
with the changes undergone by the organic constituents of food.
The organic foods, protein, carbohydrate, and fat, are distinctly
different chemical substances, yet digestion in the case of each
is brought about by the same sort of chemical process, known as
hydrolysis, the essential feature of which is a preliminary union
with water and a subsequent splitting of the combination into
simpler products. An example of a relatively simple form of
hydrolytic cleavage may be illustrated by the breaking down of
a molecule of a polysaccharide, such as malt sugar, into two
molecules of a monosaccharide, dextrose:

C12H22O11 + H20 ■

malt sugar water

2C6H12O6

dextrose

Digestion is not always completed by a single hydrolytic cleavage,

ALIMENTATION 105

the number of cleavages involved depending upon the composi-
tion of the food. What happens in the case of complex sub-
stances such as proteins, is a continuous separation of soluble
fractions from a residue that becomes more and more soluble as
its chemical composition is simplified, until the original complex
molecule is converted entirely into simpler and more soluble
molecules.

Enzymes. — The hydrolytic cleavages of foodstuffs in the
alimentary canal are brought about by organic catalytic agents
known as enzymes. A catalytic agent is a substance whose
presence hastens chemical reaction. It does not initiate the
reaction but greatly accelerates the rate of reaction. Thus, to
use a common example, hydrogen and oxygen which do not
combine in any appreciable degree at ordinary temperatures to
form water, may be made to do so if exposed to spongy platinum.
Since the catalytic agent, platinum, does not appear in the
product of the reaction, a very small amount is capable of pro-
ducing an infinite change. Similarly, digestive enzymes bring
about changes in food without becoming a part of the product
of digestion.

Enzymes are organic catalytic agents produced by cells from
which they may be extracted by water, salt solutions, or by
glycerin. From such an extract the enzyme may be obtained
in a fairly pure state by precipitation with an excess of alcohol.
The action of enzymes is destroyed by temperatures between
60 and 100°C. and greatly retarded by temperatures near 0°C.
Enzymes are specific in their reaction; thus proteolytic enzymes
act only on proteins, fat-splitting enzymes only on fat, etc.
Enzymes are reversible in their reaction, the direction of the
reaction being determined by whether or not the products of the
reaction are removed. If the products are removed as rapidly
as formed, the reaction may go to completion in one direction.
This is what happens in all probability in the alimentary canal,
where the products may be absorbed as soon as formed. If the
products are allowed to accumulate, the reaction comes to
equilibrium, under which conditions there is as much tendency
for the reaction to go in one direction as the other, and thus,
for all practical purposes, to come to a halt.

It has been shown that an enzyme may be secreted by the cell
in an inactive form, to which the general term zymogen is applied.

106 GENERAL ZOOLOGY

Zymogen has been identified with certain granules in the cyto-
plasm of the cell producing it. To bring about its normal reac-
tion, the zymogen must be activated by some agent encountered
after it leaves the cell. Thus trypsinogen, the inactive form of
trypsin, is a product of the pancreas that is converted into
active trypsin by enter vkinase, an organic substance secreted by
the small intestine.

The enzymes concerned in the digestion of protein, starches,
sugars, and fats all act by hydrolysis. Other kinds of enzymes
are: (1) coagulating enzymes, such as the rennin of the stomach
of young mammals, that produces the coagulation of casein in
milk; (2) oxidizing enzymes, that bring about oxidizing processes
in the body; and (3) deaminizing enzymes, that produce the
separation of the NH2 group from amino acids, of which an
example is the formation of ammonia and lactic acid from alanine
(CH3CH(NH2)-COOH).

Enzymes are sensitive to the hydrogen-ion content of the
medium. Thus an enzyme normally acting in a neutral or
alkafine medium becomes inactive in the presence of acid.

Digestive Enzymes. — The enzymes concerned in digestion and
nutrition in the human body include the following:

1. Proteolytic enzymes, acting on proteins.

a. Pepsin, secreted by the glands of the stomach, acts in
an acid medium ; it converts proteins into peptones and proteoses.
(In the frog pepsin is also secreted by the esophagus, but becomes
active only after passing into the stomach.)

b. Trypsin, secreted in an inactive form as trypsinogen by
the pancreas and activated by the enterokinase in the intestine;
it acts in an alkaline, neutral, or slightly acid medium; converts
proteins into polypeptides.

c. Erepsin, secreted principally by the mucosa of the small
intestine and in small amounts by the pancreas. It also is
found in the tissues generally. It acts in an alkaline or neutral
medium and splits the products formed by pepsin and trypsin
into amino acids. (In the frog no erepsin is produced by the
small intestine.)

2. Diastatic enzymes, act on starches in an alkaline or neutral
medium.

a. Ptyalin, secreted by the salivary glands, converts
starch into sugar (maltose).

ALIMENTATION 107

b. Amylase, secreted by the pancreas, converts starch into
sugar (maltose). Ptyalin and amylase have a similar action on
starch.

c. Glycogenase, produced in the liver, converts glycogen to
dextrose. It is also found in muscle.

3. Inverting enzymes, secreted by the small intestine princi-
pally, act in an alkaline or neutral medium ; they convert double
sugars (disaccharides) into simple sugars (monosaccharides).

a. Sucrase secreted by the small intestine; converts cane
sugar to dextrose and levulose.

b. Maltase, secreted by the small intestine, salivary glands,
and pancreas, converts maltose to dextrose.

c. Lactase, secreted by the small intestine, converts lactose
(milk sugar) into dextrose and galactose.

4. Lipolytic enzyme, acting on fat in an alkaline or neutral
medium.

a. Lipase, secreted by the pancreas, acts in conjunction
with bile to split neutral fats into fatty acids and glycerin, in the
small intestine.

5. Coagulating enzyme.

a. Rennin, secreted by the stomach in an inactive form,
prorennin, which is activated by the acid of the stomach, converts
the casein of milk into an insoluble protein, the curd, which is then
acted upon by pepsin.

Foodstuffs. — The useful constituents of food consist of the
following foodstuffs: water, inorganic salts, proteins, fats, and
carbohydrates. These substances are foods in the sense that
they yield energy or serve as the raw material for the replace-
ment of the losses involved in metabolism. The living state is
accompanied by continuous physical and chemical changes in the
cells of the body, as a result of which, energy, largely in the form
of heat, is evolved along with carbon dioxide, water, inorganic
salts, and other excretory products.

Calorimetry. — By means of a respiration calorimeter it is
possible to demonstrate that the energy of heat and work is
obtained from the potential energy of the food. The potential
energy of foodstuffs can be determined by burning or oxidizing
them in a calorimeter, which is an apparatus by means of which
the heat liberated during oxidation may be accurately measured.
Naturally only organic foodstuffs are involved in this considera-

108 GENERAL ZOOLOGY

tion since the inorganic constituents of the diet are incapable of
further oxidation. Organic foodstuffs contain potential energy
in a chemical form in an amount equivalent to the amount of
energy expended in the original synthesis of the foodstuff from
simple elements, just as a bent spring possesses potential energy
in proportion to the amount of work expended in bending it.
If one end of the spring is released, it straightens out and in
doing so its potential energy is converted into kinetic energy.
In a similar way, when a complex organic substance, such as
protein or carbohydrate, is oxidized, the complex spatial arrange-
ment of the atoms in the molecule is destroyed in the formation
of simpler molecules, in which process energy is released. Thus
a molecule of carbohydrate possesses potential energy as long
as the atoms of carbon, hydrogen, and oxygen maintain a certain
structural and spatial relation to each other in the molecule.
When this relation is disturbed by an oxidation process, the
structure collapses, energy is liberated in the form of heat, and
the carbon, hydrogen and oxygen emerge from the reaction as
carbon dioxide and water.

The energy value of foods is measured in calories. A small
calorie or gram-calorie is the amount of heat necessary to raise
the temperature of 1 gm. of water 1°C. By means of the
calorimeter it has been determined that, on the average, 1 gm.
of fat yields 9,300 gram-calories, 1 gm. of carbohydrate 4,100
gram-calories, and 1 gm. of protein 5,778 gram-calories. Except
for protein these values represent the actual energy values of
these substances to the body. Owing to the fact that protein
is not completely oxidized in the body (1 gm. of protein yielding
3^ gm. of urea), to obtain the calorific value of 1 gm. of protein,
it is necessary to subtract the energy value of ]^ gm. of urea,
which is 841 calories, from 5,778 calories, leaving 4,937 calories.
Very likely this value is too high since all of the nitrogen of
protein metabolism is not eliminated as urea. If allowance is
made for nitrogen losses in forms other than urea, the actual
calorific value of 1 gm. of protein is reduced to 4,100 gram-calories.

The energy requirements of a man can be determined by
means of a respiration calorimeter. In its most complete and
elaborate form the respiration calorimeter is a small room in
which an individual may remain for long periods in comfort, and
so arranged that the amount of oxygen consumed from the air

ALIMENTATION 109

supply can be determined as well as the quantity of carbon
dioxide produced. It also permits the measurement of the
amount of heat produced by the subject which, when supple-
mented by analysis of the urine and feces, gives the total carbon
and nitrogen excretion along with the heat loss. Since the
amount of protein, fat, and carbohydrate oxidized in the body
may be determined from the carbon and nitrogen excretion, the
amount of foodstuffs necessary to meet these requirements can
be calculated; and since there is a close correspondence in the
estimated and actual food requirements, a satisfactory proof is
reached that the energy released as heat, or as heat and work,
is derived from the potential energy of the food stuffs eaten.

The basal metabolism refers to the amount of heat produced
when the body is at rest. On the average this amounts to about
1,600 calories for a man and about 1,400 calories for a woman,
during a 24-hour period. If the subject in the calorimeter
performs active work on a bicycle ergometer, the total energy
produced as heat and muscular work may be increased to over
5,000 calories. The dietary requirements vary in proportion
to the total energy requirements.

Foods. — Animals use food closely allied in chemical composi-
tion to animal tissues. Foodstuffs, except water, cane sugar,
and sodium chloride, are not ordinarily used as separate articles
of diet. Any single article of food, such as rice or meat, may
contain all the foodstuffs, yet rice is thought of as a carbohydrate
food and meat as protein food, because of the relative pre-
dominance of carbohydrate in rice and of protein in meat-
Protein exists in various forms, such as the myosin of meat,
casein of milk, gluten of bread, and albumin of eggs. Carbo-
hydrate foods, such as rice or whole-wheat flour, contain a high
percentage of starch. Carbohydrate in the form of sugar is
obtained principally from fruits or in a pure form, usually as cane
sugar. A variable amount of fat is found in meat, fruit, and
vegetables. In diets this is usually supplemented by additional
fat in the form of butter or vegetable oils. Water and inorganic
salts are constituents of all foods.

Vitamins. — In addition to protein, fat, carbohydrate, water,
and inorganic salts the animal body requires minute amounts of
organic substances known as vitamins. A number — at least
six — have been identified, though not completely isolated in a

110 GENERAL ZOOLOGY

pure state, so that their chemical formulas are not known with
any degree of certainty, at least in all cases. Their importance
as essential constituents of a diet has been established beyond
all question. They occur in milk, fruit, fresh vegetables, meat,
and in the outer kernels of grain. Vitamins exist in such small
quantities in foods that they are negligible as sources of energy.
They are not enzymes. Their precise role is unknown, but there
is some resemblance between them and hormones. Some authors
designate them as exhormones to indicate their origin outside the
body in contrast to the hormones of the endocrine glands which
are formed within the body.

At the present time, five vitamins, known as A, B, C, D, and
E, are recognized, with some evidence to indicate that vitamin
B is a combination of two or more vitamins.

Vitamin A is fat-soluble and promotes growth. Rats on a
vitamin A-deficient diet show in addition to stoppage of growth
and loss of weight, inflammatory conditions in the conjunctiva
and the cornea of the eye, as well as other effects. Cooking does
not destroy it in fruits and vegetables.

Vitamin B or B i is water-soluble and its absence in food pro-
duces polyneuritis or beriberi. Fowls fed on polished rice (rice in
which the reddish outside layer has been removed) develop in the
course of a few days a weakness in the legs caused by a nervous
degeneration which is known as polyneuritis. Such fowls soon
die unless the diet is changed, or rice polishings added to it.
Beriberi, a disease common among rice-eating peoples, shows
similar symptoms and seems to be due to similar causes. In
this case the vitamin involved is located in the cortex of the grain.
It also occurs in yeast, milk, leafy vegetables, and fruit.

Vitamin C is alcohol-soluble and is antiscorbutic. Absence of
vitamin C in the diet produces scurvy, a disease manifested by
bleeding of the gums, tooth pulp, mucous membrane, and skin.
It was particularly prevalent among sailors in the days of sailing
vessels when voyages too frequently outlasted the available
supply of fresh vegetables and meat. Scurvy results in bodily
weakness and terminates in death unless checked by feeding
vegetables and meat or by giving the juice of lemons, limes or
oranges. Ascorbic acid, C6H806, has the antiscorbutic potency
of lemon juice, of which it is one of the constituents.

Vitamin D is fat-soluble and antirachitic. Rickets, a disease
in which the bones and teeth are defective, may be prevented by

ALIMENTATION 111

administering cod-liver oil, which is high in vitamin D. Since
rickets may also be controlled by ultraviolet radiation, it is
thought that radiation liberates vitamin D from substances in the
tissues that might be regarded as a kind of 'provitamin. This
substance is ergosterol, an unsaturated fat. The active vitamin
occurs in abundance in fish oils, butter fat, whole milk, and yolk
of eggs. Administration of irradiated ergosterol shows the same
antirachitic effects as the vitamin D of fish oils. Vitamin D is
involved in regulating the concentrations of phosphorus and
calcium in the blood and is concerned with the calcification of
bones and teeth.

Vitamin E is fat-soluble and is necessary for fertility in repro-
duction. It is found in meat, cereal, lettuce, liver, and egg yolk.
Its omission from a diet results in sterility.

Vitamin G or B2 consists of two substances: (1) lavine, a
yellow pigment, which seems to be essential for growth, and (2) a
portion called B6 that prevents and cures a dietary skin disease
in rats somewhat similar to pellagra in man. These vitamins
occur in yeast, whole cereals, milk, vegetables, and fruits.

Salivary Digestion. — In the frog, food is swallowed without
mastication and salivary digestion is entirely lacking. In man
and most mammals, the saliva contains a starch-splitting enzyme,
ptyalin, secreted principally by the parotid gland. The flow
of saliva is a result of reflex stimulation of secretory nerves.
Thus the sight and odor of food cause a flow of saliva before the
food enters the mouth. In the mouth the taste of food also
stimulates salivary secretion. The action of saliva upon starch
may be demonstrated in a test tube, or by holding a small quan-
tity of boiled potato in the mouth for a short time. After a
few minutes sugar is found in the solution. The conversion of
starch into sugar is a step-by-step reaction, in which maltose is
split off leaving residues that become progressively simpler in
chemical structure until entirely converted into sugar. This
may be illustrated as follows:

, maltose
Starchy maltose

erythrodextrin^ , maltose

achroodextrin^ ™altose

dextrin /

maltose

112 GENERAL ZOOLOGY

Chewing thoroughly mixes saliva with the food, but swallowing
takes place before all the action of ptyalin can be completed in
the mouth. The reaction of saliva is neutral or slightly acid,
but its activity is stopped by concentrations of hydrochloric acid
as low as 0.003 per cent and also by strong alkali. The gastric
juice contains enough acid (0.50 per cent of HCL) to stop the
action of ptyalin on food in the stomach ; nevertheless the action
of ptyalin may continue in the stomach because the food may
remain undisturbed in the fundus for an hour or more and
remain unmixed with acid. If a rat is fed with food of different
colors, the stomach removed, frozen, and sectioned, it is found
that the food is deposited in concentric layers, the first layer
against the wall and each later layer inside the preceding one.
Since time is required for the complete mixing of the food with
gastric juice, ptyalin might well continue its action particularly
on food swallowed toward the end of the meal.

Gastric Digestion. — The acidity of pure gastric juice, is about
0.5 per cent, while the acidity of the contents of -the stomach
during digestion is about 0.2 per cent. The drop in acidity during
digestion is due to neutralization and dilution by the stomach
contents and also apparently by regulated regurgitation of the
alkaline contents of the duodenum during gastric digestion.

Gastric juice is secreted by the stomach in response to the
stimulations of nerve endings of taste and odor, produced during
the chewing and swallowing of food. This constitutes the
appetite secretion. In addition there is evidence of a chemical
stimulation of the gastric glands brought about by the action of
food on the stomach mucosa, producing a substance, gastric
secretin, which is absorbed by the blood and carried to the
gastric glands, where it causes secretion of the gastric juice.

The principal enzyme of the stomach, pepsin, acts only in an
acid medium, upon proteins, reducing them to simple and more
soluble forms known as peptones. The curdling of milk brought
about by rennin, the coagulating enzyme of the gastric juice,
takes place in two steps: (1) the conversion of casein into para-
casein, and (2) the reaction of the paracasein with calcium salts
of the milk to form the curd, which is an insoluble protein. The
latter then undergoes proteolytic digestion by pepsin. Digestion
of protein is begun in the stomach and carried to completion in
the intestine.

ALIMENTATION 113

Gastric digestion is facilitated by movements of the pyloric half
of the stomach. During these movements, which consist of
rhythmic muscular contractions beginning at about the middle
of the stomach and extending to the pylorus, the fundus remains
quiescent. The stomach cavity is entirely shut off from the rest
of the alimentary canal by the contracted sphincter muscles at
the cardia and pylorus, except at certain intervals when the
pyloric sphincter relaxes and allows a portion of the stomach
contents to be pushed out into the duodenum. That portion
of the food subjected to the rhythmic contractions of the stomach
is gradually converted into a liquid mass known as chyme; and
it is thought that the liquefaction of the food may be one of the
factors involved in causing the relaxation of the pyloric sphincter,
since observations have been recorded indicating that when
liquid food alone is taken, it leaves the stomach in a few minutes.
In other words, the pyloric sphincter remains contracted as long
as the pyloric contents are solid, but relaxes when subjected to
the pressure of chyme of a certain degree of fluidity. It has also
been suggested that the chemical reaction of the chyme may be a
factor, but agreement is lacking on this point. After the expul-
sion of a quantity of the chyme, a corresponding quantity of food
is pushed out of the fundus, presumably by the tonic contractions
of the fundic muscles.

Digestion in the Small Intestine. — In the intestine, the chyme
from the stomach is subjected to the action of enzymes contained
in the pancreatic juice and also enzymes produced by the small
intestine itself. The alkaline secretion from the pancreas con-
tributes four enzymes: two proteolytic enzymes, trypsin and
erepsin; an amylolytic enzyme, amylase; and a lipolytic enzyme,
lipase. From the intestinal mucosa comes a proteolytic enzyme,
erepsin, identical with the erepsin of the pancreatic fluid; various
inverting enzymes; and enterokinase, which activates trypsin-
ogen. All of these enzymes act in an alkaline or neutral or
faintly acid medium.

When the acid chyme from the stomach is discharged into the
duodenum the pancreatic juice begins to flow into the duodenum.
A substance, pancreatic secretin, is formed by the action of acid
on the duodenal mucosa and is carried by the blood stream to the
pancreas, where it causes active secretion. It has a similar effect
on the liver, causing the secretion of bile. Pancreatic secretin,

114 GENERAL ZOOLOGY

like gastric secretin, is an example of a hormone, a chemical
agent that stimulates the activity of organs other than the one
in which it is produced. The chemical control of pancreatic
secretion provides an automatic mechanism for regulating the
secretion of pancreatic juice.

The peptones formed by the gastric digestion of proteins are
hydrolyzed by trypsin and erepsin into amino acids, the final
product of protein digestion.

Amylase is similar to ptyalin in its action on starches. Thus
the amylolytic action by ptyalin begun in the mouth and con-
tinued in the stomach until interrupted by hydrochloric acid, is
renewed and carried to completion by amylase in the intestine.
Maltose, the product of starch digestion is then converted by
maltase into dextrose. Most of the maltase is produced in the
intestine, but saliva also contains small amounts, so that some
inverting action on maltose may take place before the food reaches
the intestine. Cane sugar, taken in a pure form with food is
converted in the intestine by sucrase into dextrose and levulose.
Milk sugar or lactose is changed by lactase into dextrose and
galactose. Dextrose, levulose, and galactose are all monosacchar-
ides and represent the end products of carbohydrate digestion.

It is usually agreed that the digestion of fats takes place in the
small intestine. The lipolytic enzyme involved is lipase, pro-
duced by the pancreas. Lipase is capable of hydrolyzing or
saponifying neutral fats. Its action on beef fat, tristearin, may
be illustrated as follows:

C3H5(C17H35COO)3 + 3H20 -> C3H5(OH)3 + 3C17H35COOH

tristearin water glycerin stearic acid

Since this reaction takes place in an alkaline medium, the free
acid is at once neutralized to form a soap, sodium stearate, as
follows :

C17H35COOH + NaOH -> C17H35COONa + H20

stearic acid sodium hydroxide sodium stearate water

Or both steps may perhaps be combined in a single reaction as
follows :

C3H5(C17H35COO)3 + 3NaOH -> C3H5(OH)3 + 3C17H35COONa

tristearin sodium hydroxide glycerin sodium stearate

ALIMENTATION 115

Glycerin and a salt of a fatty acid, or soap, constitute the final
products of the digestion of fats.

Since lipase acts more rapidly and effectively when bile is
present, it is thought that bile acids alone or with lecithin (a
constituent of bile) functions as a coenzyme, which increases
lipolytic action.

Gastric Absorption. — Relatively little absorption of substances
takes place in the stomach. Water is not absorbed to any
appreciable extent. One experiment with a dog showed that of
500 cc. of water given through the mouth 495 cc. entered the
duodenum within a half an hour. Alcohol, on the other hand,
is absorbed readily by the stomach. Sugars and peptones are
not absorbed.

Absorption in the Small Intestine. — The products of digestion
are absorbed principally in the small intestine. In this connec-
tion it is necessary to refer again to the structure of the intestinal
villi. The mucous lining of the human small intestine throughout
the greater part of its length is raised into permanent transverse
folds, the valvulae conniventes. Both on the valvulae and
between them the mucosa is in the form of closely packed cylin-
drical or club-shaped villi. In the duodenum villi are plate-
shaped. Each villus is covered with columnar epithelium,
beneath which is an incomplete layer of plain muscle. The core
of the villus is formed of loose connective tissue, with a centrally
located lymph capillary or lacteal, which communicates with
larger lymphatic vessels in the submucosa. Peripheral to the
lacteal is a network of blood vessels (Fig. 71).

Absorption and Metabolism of Protein. — The products of
protein digestion, amino acids, are absorbed through the epi-
thelium of the intestinal mucosa into the blood stream and carried
by the blood to different parts of the body. They are stored
temporarily, principally in the liver and muscles. Experiments
show that with excessive feeding some of the products of protein
digestion may enter the lymph stream. Apparently there is no
circulating protein to provide the tissues with their protein
requirements. Instead it is believed that this function is served
by amino acids in the blood, each tissue using what is required to
build up its own form of protein. Amino acids that are not used
for tissue-building may undergo deaminization. This consists in
splitting off a portion of the molecule as urea, leaving a non-

116

GENERAL ZOOLOGY

nitrogenous residue which can be readily oxidized. Deaminiza-
tion occurs principally in the liver, though there is evidence that
it also occurs in the tissues generally. Urea, which has the

/NH2
formula C=0, is the chief end product of nitrogen metabolism

N\NH2
in the body. The role of the products of protein digestion is

m.

m.m.
s.m.

m&km *

m.pX

A B C

Fig. 71. — A, diagram of the blood vessels of the small intestine; the arteries
appear as coarse black lines; the capillaries as fine ones, and the veins are shaded.
(After Mall.) B, diagram of the lymphatic vessels. (After Mall.) C, diagram
of the nerves. (After Cajal.) The layers of the intestine are m, mucosa; m.m.,
muscularis mucosa; s.m., sub-mucosa; cm., circular muscle; i.e., intermuscular
connective tissue; l.m., longitudinal muscle; s, serosa; c.l., central lymphatic; n,
nodule; s.pl., submucous plexus; m.pl., myenteric plexus. (From Stohr, Text-
book of Histology, by Lewis. P. Blakiston's Son and Company. By permission.)

therefore twofold: (1) to build tissue, and (2) to provide fuel.
Of the two the first is the more important. There is evidence
that the liver is involved in the formation or in the mechanism
of regulation of the blood proteins, especially fibrinogen.

It is possible to keep a dog in nitrogen equilibrium (in which
intake and outgo of nitrogen are equal) and even with a plus
balance of nitrogen if it is fed on certain amino acids without
fats or carbohydrates. This means, of course, that some part

ALIMENTATION 117

of the amino acids can act as a source of heat or energy. Some
of the non-nitrogenous product of deaminization may be con-
verted to sugar and glycogen and then into fat, though most of
the body fat comes from fat taken as such in the food and from
carbohydrate.

Absorption and Metabolism of Carbohydrates. — Since carbo-
hydrate food is taken mainly in the form of starch, the principal
end product of carbohydrate digestion is dextrose. This passes
through the walls of the villi into the blood capillaries, whence it
is carried by the blood of the portal vein to the liver. In the
liver a certain amount of sugar is removed and converted into
glycogen, an insoluble starchlike substance, which is stored in the
liver cells, leaving about 0.1 per cent of sugar in the blood. As
sugar in the form of glucose is used up in the general circulation
and tissues, glycogen is converted back into dextrose by the
enzyme glycogenase, thus maintaining a fairly constant sugar
content in the blood. Sugar may be absorbed in a form other
than glucose, in which case it is converted into glycogen by the
liver and released as glucose in the blood.

The islet cells of the pancreas produce a hormone, insulin,
which plays an important part in sugar metabolism. Sugar is
used in the body (1) to provide a source of energy for cells and
for muscular work; (2) by oxidation to supply heat; and (3)
excessive amounts may be converted into fat and stored as such.
Energy is obtained from sugar by oxidation which may occur in
the circulation or in the tissues, but in order for oxidation to take
place, insulin must be present. If insulin is kept out of the
general circulation in an experimental animal by removing the
entire pancreas, the percentage of sugar rises, indicating that
the normal amount of sugar is not being used. If the pancreatic
ducts are ligated, all of the cells of the pancreas except the islet
undergo atrophy. In such an animal the amount of the sugar
in the blood remains normal. Thus the rise of sugar in the blood
in the first case must have been due to the absence of something
produced by the islet cells.

In the disease, diabetes mellitus, practically all the carbohydrate
taken as food may be lost in the urine in the form of sugar and
even if no carbohydrate is eaten, sugar, derived from stored-up
glycogen, continues to be eliminated in the urine. That this
condition is due to lack of insulin is indicated by the fact (1)

118

GENERAL ZOOLOGY

that in such cases lesions have been found in some or all of the
islet tissue of the pancreas, and (2) that the administration of
insulin, a preparation containing the active principle of the islet
cells, causes a return to normal sugar metabolism.

Absorption and Metabolism of Fats. — The end products of fat
digestion are glycerin and soaps. These are absorbed as such
but are apparently at once combined
to form fats in the outer zone of the
epithelium of the mucosa because fat
droplets can be demonstrated in
these cells in sections of tissue taken
from an animal during absorption
(Fig. 72) . In absorption it is believed
that the bile salts or other constitu-
ents of the bile dissolve the fatty
acids or their soaps, thus enabling

Fig. 72. Fig. 73.

Fig. 72. — Intestinal epithelium of frog showing spherules of fat (in black) as
they appear during absorption. {After Krehl.)

Fig. 73. — Diagram of the larger veins and lymphatic vessels of intestine.
H, hepatic vein; hp, hepatic portal vein; i, intestine; l, liver; m, mesentery; p,
postcava; s, superior cava; td, thoracic duct, which conveys lymph to the sub-
clavian vein.

them to pass into the cells. Thus the bile is not only important in
the digestion of fats but also in their absorption. Most of the
synthesized fat passes from the epithelium of the mucosa into the
lacteals, then into the larger intestinal lymphatics, the thoracic
lymph duct, and finally into the blood stream at the point where
the thoracic duct joins the right subclavian vein near the heart.
During absorption the lymphatics of the intestine are distended
with fat, which gives them a milky appearance. Fat in the form
found in the lymphatics is known as chyle. A smaller quantity
of fat enters the blood capillaries of the mucosa and thus passes
to the liver through the portal vein, before entering the general

ALIMENTATION 119

circulation. Since liver cells show an accumulation of fat during
absorption, it follows that some of the fat is taken out of the
blood by the liver (Fig. 73). Chyle is removed from the lacteals
by contractions of the musculature of the villi.

Fat is used by the tissues for heat production, the final products
of oxidation being carbon dioxide and water. Fats like carbo-
hydrates are energy producers. An excess may be stored in the
body as adipose tissue, a reserve which may be drawn upon
when needed.

Digestion and Absorption in the Large Intestine. — The
mammalian large intestine does not produce specific digestive
enzymes. The same is also true of the frog. The secretion of
the human large intestine consists of mucus and is alkaline in
reaction. However, since the contents received by the large
intestine from the small intestine contain some undigested food
together with enzymes and since the large intestine retains its
contents for a long time, digestion and absorption continue
there as in the small intestine. The outstanding function of the
large intestine is the absorption of water as a result of which
the contents are dried to the consistency of the feces.

Bacterial Digestion. — The intestine of man as well as that of
other animals normally contains large numbers of bacteria. In
the small intestine they cause fermentation of carbohydrates,
which results in the production of carbon dioxide, water, alcohol,
and acetic acid. These substances are probably absorbed and
used by the body, but they could be used equally well if absorbed
as unfermented sugar. In the large intestine the same thing
may take place, but usually bacterial action in this part of the
digestive tract is on undigested fragments of protein, causing
putrefaction. Autointoxication results from the absorption in
the large intestine of toxic substances formed in the process of
putrefaction.

CHAPTER VI
CIRCULATION AND RESPIRATION

A circulatory system consists of a fluid, blood or lymph, driven
by the contractions of one or more hearts to all parts of the body,
usually through tubular blood vessels. If blood vessels are
absent or poorly developed, as in insects, the blood shortly
after leaving the heart flows through the tissues unconfined by
vessels. The heart is a highly specialized region of a blood vessel.
In the frog there is a single heart for moving the blood and a
number of lymph hearts which maintain a flow of lymph from the
tissues to the veins of the blood system. Lymph hearts are
absent in mammals. The circulatory system serves to transport
to the tissues nutritive substances, absorbed from the alimentary
canal, and oxygen, absorbed from air or water by the respiratory
organs. The blood collects from the tissue products of metab-
olism such as carbon dioxide, water, and various other substances,
which are eventually eliminated. In warm-blooded animals
the blood is important in controlling and regulating body temper-
ature. The importance of the circulatory system in connection
with these functions is not the same for all animals. Thus in
insects, air containing oxygen is carried to blood-bathed tissues
and carbon dioxide from them, by special tubes that function as
respiratory organs (p. 426), the blood forming only a short link
in the final phase of oxygen transport and the initial phase of
carbon dioxide transport. Blood is a fluid tissue composed of a
liquid plasma in which cells or cell-like bodies called corpuscles
are suspended. Lymph has a somewhat similar composition.

Blood Vessels of Vertebrates. — Arteries are vessels that carry
blood away from the heart. Veins carry blood toward the heart.
The small arteries or arterioles terminate in still smaller vessels
known as capillaries, which soon reunite to form veins. The
arterial and venous vessels are connected centrally by the heart
and peripherally by the capillaries, the whole providing a cir-
cular pathway through which the blood moves. Sometimes a

120

CIRCULATION AND RESPIRATION

121

capillary network is intercalated in the arterial system, as in
the gill capillaries of a fish, to and from which blood is carried by
arteries. A similar situation may also occur in the venous cir-
culation. Thus the hepatic portal vein arises from capillaries
in the intestine and terminates in capillaries in the liver, forming
what is known as a portal circulation. Since the major functions
of the blood are carried out in the capillary bed, arteries and veins
serve principally as conduits for conveying blood to and from
the capillaries. In the capillaries the important exchanges take
place between blood and tissues
upon which metabolism depends.
The heart, the walls of the larger
arteries and veins, as well as
all other parts of the vertebrate
body except some epidermal struc-
tures are provided with a capillary
circulation.

Arteries. — A cross section of a
large artery shows three regions:
the intima, the media, and the ex-
terna (Fig. 74). The intima con-
sists of a flattened layer of cells
known as endothelium, lining the
vessel. External to the endothe-
lium, the intima contains a layer
of white and elastic fibers forming
a meshwork elongated in the direction of the length of the vessel.
The media consists of a thick layer of circular, nonstriated or plain
muscle fibers, spread through wide-meshed elastic tissue. The
externa, the outer covering of the artery, is composed of some-
what denser connective tissue, part of which is elastic.

Veins.- — The wall of a vein is thinner than that of a correspond-
ing artery, but otherwise the plan of structure is the same for
both. In larger veins the media is reduced or absent. Unless
distended with blood, veins appear collapsed or wrinkled in
sections. At the same level veins appear larger than correspond-
ing arteries in sections because the arteries contract markedly when
treated with a killing fluid such as is used in preparing sections.

Capillaries. — As arteries approach capillaries, the outer layers
of the artery are lost, until only the endothelial layer with a few

Fig. 74. — Section of wall of
medium sized artery of cat. E,
externa; I, intima; M, media.

122

GENERAL ZOOLOGY

scattered muscle cells remain. These endothelial tubes are not
the true capillaries, the latter having an even more simplified
structure, the details of which are not completely known.
According to some descriptions, the capillary is a noncellular
membrane or 'perithelium which contains scattered contractile
cells and nerve endings. The diameter of the smallest capillaries
is so reduced that blood corpuscles are distorted in their passage
through them. The wall of the capillary is therefore extremely
thin which would seem to be of significance in connection with

rapid exchanges by diffusion between blood
and tissue.

Heart of the Frog. — The pericardium, the
sac enclosing the heart, bears the same
relation to the heart as the peritoneum
bears to the intestine. The pericardium is
really a double-walled sac, whose inner or
visceral wall is closely applied to the heart
and separated from the outer or parietal
wall by a space filled with pericardial fluid.
The two walls of the pericardium are con-
tinuous about the heart where the large
vessels are joined to it. The heart is an
organ composed almost entirely of muscle.
Cardiac muscle is striated and the structure
of the myofibrils is similar to that found in
skeletal muscle. Unlike skeletal muscle, whose fibers are usually
straight, cardiac muscle fibers form a branching network (Fig. 75).
The nuclei are located in the central axes of the fibers. In human
cardiac muscle the fibers are crossed by irregular intercalated
disks, located about midway between adjacent nuclei, which gives
the appearance of a cellular structure. Though striated, cardiac
muscle is involuntary in function.

The frog's heart is composed of a single ventricle and two
auricles or atria. Viewed from the ventral side, the triangular
ventricle forms the posterior half of the heart and is marked off
from the atrial region in front by the coronary sulcus. The
ventral face of the atrial region is traversed by the broad truncus
arteriosus arising from the right anterior border of the ventricle
and dividing into right and left branches at the anterior edge of
the heart. On the dorsal side of the heart is a triangular sac, the

Fig. 75. — Cardiac
muscle tissue of mammal.
C, intercalated disks.

CIRCULATION AND RESPIRATION

123

sinus venosus, which communicates with the right atrium. The
right and left atria are completely separated by an interatrial
septum. The right atrium is larger than the left (Fig. 76).

The cavity of the ventricle is a single chamber but its thick
muscular wall is indented by deep depressions which form
numerous alcoves. The atria, on the other hand, have smooth
thin walls. The blood which enters the right atrium from the
sinus venosus is deoxygenated blood from the systemic circulation.
Backflow of blood into the sinus venosus when the heart con-

A.V.C.

A B

Fig. 76. — Heart of Rana catesbeiana. A, dissection from ventral side; B,
dorsal view, a.v.c, anterior vena cava; a.v.v., atrioventricular valve (dorsal);
B.C., bulbus cordis (proximal segment of truncus arteriosus); i.a.s., interatrial
septum; l.a., left atrium; l.v., longitudinal valve; P.C., postcava; p.v., pulmonary
veins; r.a., right atrium; s.v., sinus venosus; T.I., truncus impar (distal segment
of truncus arteriosus) ; v, ventricle.

tracts is prevented by a pair of transverse lips or valves guarding
the sinu-atrial aperture. The left atrium receives oxygenated
blood from the lungs brought to it by the 'pulmonary veins, which
enter the atrium at an acute angle so that when the atrium con-
tracts, the mouth of the venous aperture is closed. Both atria
open into the ventricle through the atrioventricular aperture,
which, however, is divided into right and left portions by the
interatrial septum (Fig. 76). The atrioventricular aperture is
guarded by four valves, viz., a large flap on the dorsal edge, a
similar one on the ventral edge, and two smaller valves, one at
the right and one at the left edge of the aperture. The free

124

GENERAL ZOOLOGY

edges of the valves are prevented from being pushed back into
the atria by small fibers, known as chordae tendineae, extending
from the edges of the valves to the wall of the ventricle.

The truncus arteriosus takes its origin from the anterior margin
of the right side of the ventral wall of the ventricle and then
follows a diagonal course across the atria from right to left. It is
composed of two regions: a proximal segment, the bulbus cordis,

CAQ

Fig. 77. — Dissection of truncus arteriosus of Rana catesbeiana from ventral
side. a. a., aortic arch; c.a., carotid arch; c.ao., cavum aorticum of bulbus
cordis; c.p., cavum pulmocutaneum of bulbus cordis; l.v., longitudinal valve;
p.a., pulmonary arch; s.i., septum interaorticum; 3, valve.

marked off by a transverse constriction from a distal segment, the
truncus impar, which almost immediately bifurcates into right
and left trunks. Each of these trunks separates into three
branches, known as arterial arches, of which the anterior one
is the common carotid artery, the next, the aorta, and the posterior
one the pulmocutaneous artery. Each common carotid artery
divides almost at once into internal and external carotid arteries
which supply the head. Each aortic arch curves outward and
dorsally to join its fellow of the opposite side at about the level
of the sixth vertebra, to form the dorsal aorta. Each pulmo-

CIRCULATION AND RESPIRATION

125

cutaneous artery divides into (1) a great cutaneous artery, which is
distributed mainly in the skin of the dorsal and lateral regions,
and (2) a pulmonary artery to a lung. One division of the great
cutaneous artery, the auricularis, forms anastomoses with other
arteries, such as the occipital and carotid (Fig. 81).

If the truncus arteriosus is opened by a ventral incision from
its point of origin to the bifurcation, its bulbar region is found to
contain a number of valves, of which the largest is the septum
bulbi or longitudinal valve (Fig.
77). The septum bulbi is at-
tached at its posterior end to
the ventral wall of the proximal
end of the bulbus, but as one
traces the valve forward, its
point of attachment spirals
sharply to the left through an
arc of 180 degrees to the dorsal
wall of the bulbus, which it
follows in a slight curve from
left to right to the beginning of
the truncus impar where it is
united dorsally and laterally to
the truncal walls. Since the
ventral edge of the septum is ssection proxir j end of

free throughout the greater part truncus arteriosus of Rana catesbeiana,
r •■ i ,i i i j- ■ with walls spread apart, d, dorsal

of its length, a complete divi- valve. LVm left valve. R v ( right valve
sion of the bulbar channel into

two can only occur when the free edge of the septum is pressed
tightly against the ventral wall of the bulbus. The attached
edge of the septum is broad and the free edge is thin and movable.
The two channels formed in the bulbus by the septum are known
as the cavum aorticum, on the right, and the cavumpulmocutaneum,
on the left.

At the posterior end of the septum bulbi, where it is attached
to the ventral and lateral wall of the bulbus, are three valves: (1)
a left ventral valve, closely applied to the right lateral face of the
septum; (2) a right ventral valve, attached to the right wall
of the bulbus; and (3) a dorsal valve, very small and lying between
the first two. These valves are connective tissue flaps, whose
free edges are attached to the walls of the bulbus by chordae

126

GENERAL ZOOLOGY

C.A

tendineae. Another set of three valves is found at the anterior
end of the septum, as follows: (1) a large one, valve 3, on the left
side of the septum, beyond which is an opening leading to the
pulmocutaneous arteries; (2) another one, called valve 2, lying
ventral and to the right of valve 3; and (3) a cup-shaped valve
formed by the anterior end of the septum bulbi and known as
valve 1 (Figs. 78 and 79).

The truncus impar is divided into dorsal and ventral chan-
nels by the septum principale, which,
taking its origin from the anterior
edge of the septum bulbi, divides the
cup-shaped valve of the latter into
dorsal and ventral portions, valves la
and 16. The dorsal channel, lead-
ing from the cavum pulmocutaneum
of the bulbus, divides with the trun-
cal arms of the truncus impar into
right and left pulmocutaneous arches.
The ventral channel of the truncus
impar is divided into right and left
portions by the septum inter aorticum,
Fig. 79.— Dissection of distal which is at right angles to the septum

end of truncus arteriosus of Rana principale. The two channels thllS
catesbeiana. a. a., aortic arch;

c.a., carotid arch; c.p., cavum formed are the beginnings of the
pulmocutaneum; L.v.iongitu- aortic arches. The carotid arches

dinal valve of cordis bulbus; p. a.,

pulmonary arch; s.i., septum originate as two small openings in the
interaorticum; s.p. septum prm- septum interaorticum, leading from

cipale; la, lb, 3, valves. (Valve *■ . ~.

2, removed with the ventral wall the right aortic branch of the

of the truncus, is not shown.) truncus. Blood leaving the cavum
aorticum of the bulbus enters the aortic and carotid arches.

The sinus venosus, situated on the dorsal' side of the heart,
receives a right and left anterior vena cava (precava) in front, and
an unpaired postcava behind. The blood brought to the sinus
venosus is for the most part deoxygenated or venous blood,
coming from the capillaries of all parts of the body except the
lungs. Since the sinus venosus empties into the right atrium,
the blood of the right atrium is venous in character. On the other
hand, blood returned from the lungs to the heart by the pul-
monary veins is usually oxygenated blood, and this blood enters

CIRCULATION AND RESPIRATION 127

the left atrium. The plan of the blood circulatory system of the
frog provides for two circuits, a general systemic circulation and a
pulmonary circuit, but these two circuits are not completely
separated because they both make use of a single ventricle in
passing through the heart.

Action of the Frog's Heart. — The study of the living heart
shows that each atrium and the ventricle contract and dilate in a
regular sequence. Contraction of a part of the heart is known
as systole, and the dilatation which follows is diastole. The order
of sequence of contraction is right atrium, left atrium, ventricle.
Immediately after the ventricular systole, the bulbus cordis
swells and then contracts to its normal size. Since both atria
contract before the ventricle, the latter becomes filled with
venous blood from the right atrium and oxygenated blood from the
left. However, owing to the spongy internal structure of the
ventricle and to the fact that ventricular systole follows almost
immediately after the contraction of the left atrium, there is
probably little mingling of the two kinds of blood in the ventricle.
When the ventricle contracts, the blood is forced into the bulbus
cordis, the atrioventricular valves preventing backflow of blood
into the atria. The first blood to leave the ventricle is the
venous blood since it occupies a position on the right side of the
heart near the entrance to the bulbus cordis. This venous
blood seems to enter the cavum pulmocutaneum and thence
flows into the pulmocutaneous arteries for two reasons, viz., (1)
the relatively slight peripheral resistance in the pulmocutaneous
trunks as compared with the pressure in the carotid and systemic
arches, and (2) the contraction of the bulbus brings the free edge
of the bulbar septum in contact with the bulbar wall, closing the
cavum pulmocutaneum immediately after it is filled. Backflow
into the bulbus is prevented by the anterior bulbar valves. The
opening into the pulmocutaneous arteries is guarded by valve
16 on the dorsal side of the septum bulbi and by valve 3 which
lies opposite on the wall of the bulbus. The aortic opening is
guarded by valve la, on the ventral side of the septum bulbi,
and by the large valve 2, which is attached to the ventral wall
of the bulbus. In each aortic arch, just before turning poste-
riorly, are semilunar valves. Each carotid artery is provided
with a carotid gland at the point of bifurcation into internal and

128 GENERAL ZOOLOGY

external carotid arteries. The carotid gland is a spongy struc-
ture containing a network of small blood vessels through which
the blood of the carotid artery passes.

There is still a difference of opinion as to the function of the
valves in the bulbus. The present account is based on the mor-
phological relations and also on experimental data. If India ink
is injected into the postcava, the sinus venosus or the right
atrium, much of the ink may appear in the aortic and carotid
arches, but on the average, less in amount than in the pulmo-
cutaneous trunks. Thus it would seem that there is a con-
siderable separation in the bulbus of the two kinds of atrial blood
but that the separation is not absolute. Such experiments sup-
port the idea that most of the blood from the right atrium enters
the pulmocutaneous arches and that most of the blood from the
left atrium enters the aortic and carotid vessels. It would also
seem that the last blood to leave the heart, the least venous blood,
enters the carotid system since the small openings of the carotid
arteries are beyond the larger entrances to the aortic trunks.

This interpretation of the action of the frog's heart can have
a functional significance only on the assumption that the blood
returned to the heart from the lungs has a higher oxygen content
than the blood in the right atrium. This point must be con-
sidered because the frog obtains a considerable portion of oxygen
through buccal and cutaneous respiration. It would seem to be
fairly obvious that the internal structure of the lung provides a
mechanism for the absorption by the blood of oxygen from the
alveolar air and for the release of carbon dioxide from the blood
into the cavity of the lung. The pulmonary artery runs along
the outer surface of the lung to the tip, giving off at right angles
lateral branches forming a rich capillary network in the alveolar
walls (Fig. 14). The pulmonary vein arising from this capillary
network courses along the inner surface of the lung from the tip
to the base. On the other hand, the great cutaneous veins from
the skin of the dorsal and lateral regions of the body contain
oxygenated blood which passes into the venous stream, but the
volume of this contribution to the venous stream is relatively
small, compared to the volume of the venous stream as a whole.
Therefore, it would appear that the venous blood brought to the
right atrium contains less oxygen than the blood entering the left
atrium directly from the lungs.

CIRCULATION AND RESPIRATION

129

The Blood Vascular System of the Frog. — The arterial system
is made up of the arterial trunks leaving the heart and their
subdivisions, which go to all parts of the body (Fig. 81). The
carotid arteries supply the head. The remainder of the body
except the lungs and some parts of the skin are supplied by the
aortic arches and the dorsal aorta. The pulmocutaneous arteries
lead to the lungs and certain regions of the skin. The arched
form of these arterial trunks is a survival of a fish type of bran-
chial circulation that is present in the frog tadpole.

In a fish such as the dogfish shark, a common marine form,
the sinus venosus receives all the venous trunks of the body and

Fiu. 80. — The circulatory system of the dogfish, a, atrium or auricle; a.b.a.,
afferent branchial arteries; a.c, anterior cardinal vein; b.c, branchial clefts; c.a.,
carotid artery; c.c.v., common cardial vein; cl.a., caudal artery; cl.v., caudal
vein; co.a., coeliac artery; d.a., dorsal aorta; e.b.a., efferent branchial arteries;
h.p.v., hepatic portal vein; h.v., hepatic vein; il.a., iliac artery; il.v., iliac vein; k,
kidney; I, liver; l.v., lateral vein; m.a., mesenteric artery; p, pancreas; p.c, post-
cardinal vein; r.a., renal artery; r.p.v., renal portal vein; r.v., renal veins; s, stom-
ach; su.a., subclavian artery; su.v., subclavian vein; s.v., sinus venosus; v.
ventricle; v.a., ventral aorta. (Modified from Parker and Haswell.)

pours its blood into a single atrium (Fig. 80). From the atrium
the blood passes into a single ventricle. Thus all of the blood in
the dogfish heart is venous blood. Leading forward from the
ventricle is a short truncus arteriosus which divides right and
left into afferent branchial arteries leading to the gills. After
passing through the wide capillaries of the gills, during which the
blood absorbs oxygen from water passing over the gills and gives
off carbon dioxide to the water, the oxygenated blood is collected
dorsally by efferent branchial arteries which unite to form a
dorsal aorta that proceeds posteriorly to the tip of the tail. The
carotid arteries are given off from anterior efferent arteries to the
head. The arrangement of the branchial vessels is such that
all of the blood leaving the heart passes through the gills. The

130

GENERAL ZOOLOGY

gill capillaries are thus intercalated in the arterial system. In
the frog a similar condition exists in the tadpole, but with the

development of the lungs and
other changes accompanying
metamorphosis, the gills are
resorbed, and the branchial
arteries become the three pairs
of arterial arches of the adult
frog (Fig. 81).

Blood is returned to the sinus
venosus of the frog's heart
through three large venous
trunks, viz., a right and left
precava or anterior cava, and a
single postcava. The anterior
cavae drain blood from the
capillaries of the head and the
anterior part of the body.
The postcava has its roots in
the segmental renal veins which
collect blood from the kidneys.
The principal venous trunks of
each hindleg are the sciatic and
femoral veins. At the juncture
of the leg with the body each
femoral vein bifurcates to form
a pelvic vein and an external iliac
vein. The two pelvic veins
unite in the mid-line to form
the anterior abdominal vein
which proceeds forward in the
mid-line of the abdominal wall
to join one of the branches of
the hepatic portal vein. The
external iliac vein unites with
the sciatic to form a renal portal
vein, which terminates in capil-
laries in a kidney. Each kidney receives oxygenated blood from
segmental renal arteries, given off by the dorsal aorta, and it also
receives venous blood from the renal portal vein. The blood is

Fig. 81. — Diagram of the circulatory
system of the frog from the ventral side.
a, anterior abdominal vein; a. a., aortic
arch; c.a., common carotid artery; Co.,
coeliac artery; d.a. ; dorsal aorta; f, femo-
ral vein; il, iliac artery; k, kidney; l,
liver; lu, lung; p, pancreas; p. a., pulmo-
nary arch; p.c, postcava; p.e., pelvic
vein; pk, precaval vein; p.v., pulmonary
vein; r.a., renal artery; r.p., renal portal
vein; r.v., renal vein; a, sinus venosus;
sc, sciatic vein; st, stomach.

CIRCULATION AND RESPIRATION 131

drained from the kidneys by the renal veins. The renal portal
veins and their tributaries begin in capillaries in the hindlegs and
end in capillaries in the kidneys. A portion, therefore, of all the
venous blood from the hindlegs must pass through kidneys before
reaching the heart.

The hepatic portal system is made up of the hepatic portal vein,
which originates in the capillaries of the stomach, intestine,
spleen and pancreas and ends in capillaries in the lobes of the
liver. The anterior abdominal vein, draining parts of the hind-
legs, divides into two branches just before entering the liver.
One of these branches usually joins a branch of the hepatic portal
vein. Before it enters the liver, the anterior abdominal vein
receives a small vein, the cardiac vein, from the bulbus cordis.

Human Heart. — The human heart consists of four chambers:
two atria, or auricles, and two ventricles, making possible a com-
pletely double circulation (Fig. 82). A sinus venosus is not
present in the adult. In the fish the heart cavities contain only
venous blood; in the frog the right atrium contains venous blood,
the left atrium arterial blood, while the ventricle contains both
kinds imperfectly separated; in the mammal the right side of the
heart carries only venous blood while the left side carries arterial.
Each atrium is connected with the ventricle of the same side by
an opening guarded by valves which allow the blood to pass
from atrium to ventricle, but not in the reverse direction. The
ventricles have much thicker walls than the atria and the left
ventricle is much stouter than the right. The inner walls of the
atria are relatively smooth, while those of the ventricle are raised
into thick muscular ridges. The atrioventricular valves are
thin, tough flaps, three {tricuspid) on the right and two (bicuspid)
on the left side of the heart. The free edges of the valves are
held in place by thin tendinous threads (chordae tendineae)
attached by thick muscular pillars to the walls (Fig. 82). The
contraction of the muscular pillars exerts a steady tension on the
tendons, keeping the valves closed during ventricular contraction.
Venous blood is returned from the head and anterior parts of the
body by a single superior vena cava that enters the right atrium;
from the posterior part of the body by the inferior vena cava
(postcava) whose entrance into the right atrium is guarded by a
large naplike valve (Eustachian valve). The large veins of the
extremities are provided with cuplike valves that allow the

132

GENERAL ZOOLOGY

blood to flow toward the heart but not away from it (Fig. 83).
From the right atrium venous blood passes into the right ven-
tricle, whence it is pumped through the pulmonary aorta to the
lungs. Backflow is prevented by a set of three semilunar valves
located in the aorta a little beyond the ventricle. The pure,
aerated blood is returned from the lungs by the pulmonary veins

LC

LS

PV

RA

TV

Fig. 82. — Human heart dissected from the ventral side, diagrammatic, a,
aorta; bv, bicuspid valve; db, Ductus Botalli, a solid cord, which in the embryo
is a blood vessel connecting the pulmonary artery with the aorta; i, innominate
artery; ic, inferior cava (postcava) ; la, left auricle; lc, left carotid artery; ls,
left subclavian vein; pa, pulmonary aorta which divides into right and left pul-
monary arteries; pv, pulmonary veins; ka, right auricle; sc, superior cava; tv, tri-
cuspid valve.

to the left atrium, from which it passes through the atrioven-
tricular aperture into the left ventricle. The latter then pumps
it through the systemic aorta to all parts of the body. The
systemic aorta, like the pulmonary aorta, contains three semi-
lunar valves that prevent backflow. Mammals have a hepatic
portal circulation, but no renal portal system, except in embryonic
stages.

CIRCULATION AND RESPIRATION

133

Lymphatic System. — Among invertebrate animals the blood,
lymph, and body fluids are in many cases the same fluid but in
vertebrates lymph is distinct from blood and a portion of its
circulatory pathway is composed of lymphatic vessels, which are
distinct from blood vessels. Lymphatic vessels are best devel-
oped in mammals. In the frog the lymphatic system is charac-
terized by numbers of large communicating lymph spaces,
especially in the subcutaneous region. The cisterna magna, one
of the largest lymph spaces, is formed
in the subvertebral region, as a large
sac in the dorsal wall of the body
cavity. Lymph spaces are also found
within and between the organs, the
larger spaces being lined with flat-
tened endothelial cells.

There are two pairs of lymph hearts
in the frog. The two anterior hearts
are located on the posterior side of
the transverse processes of the third
vertebra, each heart connecting with
a vertebral vein, which in turn joins
an internal jugular vein. The two
posterior hearts lie one on either side
of the tip of the urostyle, each heart
connecting with the transverse iliac
vein, which is a vein joining the sci-
atic and femoral veins near the hip
joint. Each heart is provided with a pair of semilunar valves
at its venous opening. The lymph hearts pulsate rhythmically
and slowly pump the lymph into the blood. There is no relation
between the beating of the heart and the pulsations of the
lymph hearts. The action of the lymph hearts on opposite sides
of the body is not correlated. Another connection between the
lymphatic system and the blood stream is through ciliated open-
ings (nephrostomes) from the body cavity into some of the
tubules opening on the ventral surface of the kidney. The rela-
tion of these tubules to the blood system will be discussed in
connection with the kidneys in the following chapter. The
lymph entering the nephrostomes comes from the body cavity.

Human Lymphatic System. — In the human body there are
well-developed lymphatic vessels, which originate in lymph

Fig. 83. — Diagram to show the
position of the cup-like valves in
an opened vein spread out flat.
The arrow shows the direction of
blood flow.

134 GENERAL ZOOLOGY

capillaries in practically all parts of the body and end in openings
into the large veins near the heart. The lymphatic vessels differ
from blood vessels in two respects: (1) Their walls are more
delicate in structure and (2) the free passage of the vessels may
be interrupted from time to time by lymph glands, which are
nodules of various sizes, composed largely of lymphocytes
enclosed in capsules, through which the lymph slowly niters.
The flow of the lymph, as in the frog, is from peripheral parts
toward the heart. Since lymph hearts are absent in the human
body, the movement of the lymph is brought about principally
by movements of the surrounding parts exerting pressure on the
thin-walled lymph vessels. Other factors are involved, such as
the low pressure in the large veins near the heart, which would
encourage flow toward the heart. Lymphatic vessels of the
extremities are provided with valves, similar to those in the long
veins, which allow the lymph to be squeezed past them toward
the heart, but not in the reverse direction (Figs. 84 and 85).

Blood. — The blood of vertebrates consists of a fluid, the plasma,
in which various kinds of cells or cell-like bodies, known as
corpuscles, are suspended. The principal constituent of plasma
is water, in which fats, sugar, and proteins are held in colloidal
solution or suspension, along with a number of inorganic salts.
In addition the plasma contains various decomposition products
of metabolism, formed in various parts of the body. It also
contains various endocrine substances, a discussion of which is set
forth in Chap. X.

The principal proteins of the blood are serum albumin, serum
globulin, and fibrinogen. The source of these proteins is probably
from end products of protein digestion (amino acids) in the
alimentary canal that are absorbed into the blood stream. Of
these proteins, fibrinogen is of special interest because under
certain conditions it forms a solid fibrous substance known as
fibrin which entangles the blood corpuscles to form a blood clot.
The fluid left after fibrin has been formed from plasma is called
serum. Normally blood does not clot in the vessels, but clotting
may occur in the vessels from the presence of air or foreign bodies
produced as a result of injury or infection. There are a number
of factors involved in the clotting process, not all of which are
understood. That a certain amount of calcium is necessary for
clotting can be demonstrated by adding oxalic acid to the blood,

CIRCULATION AND RESPIRATION

135

which precipitates the calcium and prevents coagulation. Blood
drawn into an oil-coated vessel may be kept from clotting partic-
ularly if kept at a temperature near 0°C. Blood heated to near
100°C. loses its power to clot, owing apparently to the destruc-
tion of an enzyme, thrombin, which is necessary in addition to

Fig. 84. — Diagram of a lymph gland, a, afferent lymph vessels; c, capsule,
from which traveculae extend inward; e, efferent lymph vessels; n, nodule, com-
posed mostly of lymphocytes. (Modified after Stohr.)

calcium to produce clotting. However, if a calcium-free solution
of fibrinogen is brought into reaction with a calcium-free solution
of thrombin, a clot is formed, the fibrin of which is practically
free of calcium. From this it would seem that calcium is not
essential to the process of clotting after the thrombin is once

Fig. 85. — Silver nitrate preparation of a lymphatic vessel from a rabbit's
mesentery, showing the boundaries of the endothelial cells and a bulging just
beyond a valve. (From Stohr' s Textbook of Histology, by Lewis. P. Blakiston's
Son and Company. By permission.)

formed and that, therefore, the role of the calcium is in the pro-
duction of thrombin. Thrombogen, the inactive form of thrombin,
is thought to occur in blood plasma and tissues.

Corpuscles of the Frog's Blood. — Three kinds of corpuscles
can be distinguished in the frog's blood, viz., red corpuscles or

136

GENERAL ZOOLOGY

erythrocytes, white corpuscles or leucocytes, and spindle cells (Fig.
86). The red corpuscle is an elliptical disk measuring about
22.3 by 15.7m, with a rounded edge and a bulged center in which
a single nucleus is located. The red color of the blood is due to
the presence of a chromoprotein known as hemoglobin, con-
tained in the cytoplasm of the erythrocyte. However, if a thin
film of freshly drawn blood is examined under the microscope,
the hemoglobin appears yellow rather than red.

Leucocytes lack hemoglobin and are variable in form and size.
Under the microscope, the outline of the leucocyte slowly changes
in the manner of an amoeba. The cytoplasm of the living cells

i

fr

*

scles o
C, spindle cells. (After Jordan.)

c

Fig. 86. — Blood corpuscles of Rana pipiens. A, erythrocytes; B, leucocytes;

is colorless, and staining brings out the presence of granules of
various kinds.

The nucleus may be single and rounded or irregular in outline.
In some cells the irregularity seems to lead to subdivision into
several nuclei (larger cell of Fig. 86, B). The amoeboid move-
ments of the leucocytes endow them with independent loco-
motion, which enables them to pass through capillaries, into
surrounding tissues, without apparently rupturing the walls.
They may actually leave the tissues and pass into the alimentary
tract. Red cells may also leave through the capillary walls,
though less frequently than leucocytes.

The spindle cells, the third type of blood corpuscle found in the
frog, are colorless spindle-shaped cells about half the size of
the erythrocytes. They are said to develop from small leuco-
cytes and in the spring transform into erythrocytes. However,
they may be found as a normal constituent of frog's blood at all
times of the year. The fact that spindle cells collect in masses in

CIRCULATION AND RESPIRATION

137

clotting blood is believed to indicate that they play a role in
clotting similar to that of the blood platelets of mammalian
blood, described below.

Human Blood Corpuscles. — The erythrocytes of human blood
are non-nucleated, biconcave circular disks (Fig. 87). They
average about 5,000,000 per cubic millimeter of blood. Nucle-

Fig. 87. — Group of red corpuscles and two leucocytes (l) as seen in a fresh human

blood preparation. X 900.

ated erythrocytes are present in the blood stream of the human
embryo but not in later life under normal conditions (Fig. 88).
Erythroblasts, the cells which give rise to erythrocytes, are found
during fetal life in the liver and the spleen, and in postnatal life,
mainly in the red bone marrow. The erythrocyte of the adult
is a cell that has lost its nucleus. Though incapable of amoeboid
movement, erythrocytes do leave
the blood stream under certain
conditions, through the capillary
walls. The duration of an eryth-
rocyte in the blood stream is
relatively short. Some determi-
nations indicate that in the rabbit
the life of a red blood corpuscle is
about 8}i days; in the dog about
16 days. Their destruction and
removal from the blood stream
occur in different ways. They
may be engulfed by phagocytes of the spleen or hemolymph
glands, or they may first break down into small pieces which are
taken up by certain leucocytes or by the endothelial cells of the
liver. The bile pigments, formed principally by the liver, are
derived from the hemoglobin of the erythrocytes.

The leucocytes are nucleated and lack hemoglobin. The
average number in a cubic millimeter of blood is 7,000. They

Fig. 88. — The development of red
corpuscles in cat embryo, a, succes-
sive stages in the development of
erythroblasts, b, the extrusion of the
nucleus. (From Stohr, Textbook of
Histology, by Lewis. P. Blakiston's
Son and Company. By permission.)

138 GENERAL ZOOLOGY

may be divided into three classes: (1) Lymphocytes, forming about
23 per cent of the total; (2) large mononuclear leucocytes, 1 to
3 per cent; and (3) polymorphonuclear leucocytes, the remainder.
The principal basis for this classification is the shape of the
nucleus. In lymphocytes the nucleus is spherical and relatively
large, the cytoplasm forming a narrow rim about it. Small
lymphocytes measure from 4 to 7.5m in diameter; large ones are
twice this size. The mononuclear lymphocytes have a bean-
shaped nucleus and may measure 20/z in diameter. They are
phagocytic. The polymorphonuclear leucocyte has a nucleus
consisting of a number of lobes, all connected by narrow strands.
Polymorphonuclear leucocytes measure from 8 to 10m in diameter.
^ They are further subdivided according to the
(§ ®<SJ staining reaction of their cytoplasmic granules
into basophiles, eosinophiles, and neutrophiles.
All display amoeboid movement, particularly the
neutrophiles, which are the most abundant form
Fig. 89.— Blood of leucocytes in the blood. They also emigrate

coarpeu8creSide(Froem from the bl°°d VeSSels m0re readily than other

Stohr's Textbook of types of leucocytes.

SSnS The blood platelets of human blood are non-

and Company, nucleated fragments of the pseudopodia of the

By permission.) ^^ ^^ q{ the b(me marrow They have &

circular or irregular outline, measuring from 2 to 4m in diameter,
and have centrally located granules and clear borders. They
occur only in mammalian blood. When blood clots, fibrin is
deposited in slender threads that radiate from disintegrating
platelets; from this it is assumed that the platelets supply the
clotting enzyme, thrombin, and accordingly, blood platelets are
sometimes spoken of as thrombocytes. A similar function has been
attributed to the spindle cells of the frog's blood (Fig. 89).

Functions of the Blood.— Blood is the nutritive stream that
supplies the cells of the body with their requirements. The
nutritive qualities of the blood are derived from digested food
absorbed from the alimentary canal. The end products of
digestion are: (1) sugar; (2) fat as such or in the form of its
cleavage products, glycerin and fatty acids; and (3) amino acids.
The sugar is absorbed by the capillary blood stream of the
intestinal lining and is normally present in the blood plasma in a
certain amount. Glycerin and fatty acids are synthesized into
fat as they pass through the intestinal mucosa whence the fat

CIRCULATION AND RESPIRATION 139

is taken up mainly by the lymphatic system. Eventually this
fat finds way into the blood and is found as a normal constituent
of the plasma. Some fat is directly absorbed from the intestine
by the blood. Amino acids, the end products of protein digestion,
are absorbed as such by the blood from the intestine and are also
normal constituents of plasma. Those portions of sugar, fat, and
amino acid stored in the liver and muscles are later released into
the blood stream. The blood also receives water and inorganic
salts from the alimentary canal. It also carries oxygen and
endocrines. In the case of the frog and certain other animals, a
considerable amount of water may be taken through the skin
into the blood. The blood also transports wastage.

Respiration. — In vertebrate animals and in some invertebrates
too, the blood supplies the tissues with molecular oxygen. The
source of oxygen is the air. The latter enters the body at the
respiratory surface, which may be a gill, lung, or the body
surface, from which it is then taken up, in the case of vertebrates,
by the hemoglobin of the red corpuscles. Hemoglobin has a
peculiar affinity for uniting with oxygen under certain conditions.
If exposed to air, 100 cc. of blood plasma will, like an equal
volume of water, absorb 0.56 cc. of oxygen at 20°C, at sea level.
The amount of oxygen absorbed depends upon the partial pres-
sure of the oxygen in the air, which is roughly one-fifth of the total
atmospheric pressure. If the atmospheric pressure is increased,
more oxygen will be absorbed; if decreased less. On the other
hand, if defibrinated blood (blood from which fibrinogen has been
removed, leaving corpuscles and serum) is exposed to air, it will
absorb practically as much oxygen as it can be made to absorb
under any higher pressure. Thus if the partial pressure of the
oxygen is doubled, only as much additional oxygen is absorbed
as the serum can take up, and this amount is very small. If the
partial pressure of oxygen is reduced one-half, only a small
amount is given off, and that comes from the serum. Since
most of the oxygen of the blood absorbed under these conditions
is unaffected by such pressure changes, it must be combined
chemically in some way with something in the corpuscles. It
has been shown that the substance in the corpuscles is the
hemoglobin of the erythrocytes.

Oxyhemoglobin, the compound formed by oxygen and hemo-
globin, is not stable at low pressures, for if the oxygen pressure
to which it is exposed falls below 25 mm. of mercury, it dissociates

140 GENERAL ZOOLOGY

completely and suddenly, all of the oxygen being given off. The
pressure of oxygen in saturated arterial blood is equivalent to the
pressure of 75 mm. of mercury, and since the oxygen tension
of the tissues is zero, oxygen is readily given off to the tissues as
the blood circulates through the systemic capillaries. Returning
to the lungs with its oxygen tension reduced to 37.6 mm., the
blood, in passing through the pulmonary capillaries, readily
absorbs oxygen from the alveolar air, which has an oxygen
tension of about 100 mm.

As a result of these properties the blood can absorb oxygen
from the lungs and give it off to the tissues with great rapidity.
At the same time, the absorption of oxygen is independent of
small variations in atmospheric pressure.

The blood collects waste products of metabolism from the
tissues and these leave the body through an excretory organ, such
as the kidney, through skin as sweat (in mammals), and also
through respiratory surfaces of the body and through the
alimentary canal.

Carbon dioxide is an important gaseous waste product of cells.
It is about 20 times as soluble in blood plasma as in oxygen. In
the blood, part of it is held in solution in the plasma and the
remainder in two forms of chemical combination: (1) in the form
of sodium carbonate, and (2) in combination with blood proteins,
including hemoglobin, forming a compound similar to oxyhemo-
globin in the readiness with which it dissociates. The tension of
carbon dioxide in the arterial blood entering the capillaries is
about 35 mm., while that of the lymph and tissues is about 50 to
70 mm., so that there is a ready movement of carbon dioxide
from the tissues to the blood. The carbon dioxide tension
of the blood when it reaches the lungs is 42.6 mm. while that of
alveolar air is 35 mm., as a result of which carbon dioxide leaves
the blood.

The remaining excretory products are mostly in the form of
solids in solution. In the dissolved state these substances are
carried from the tissues by the blood to various excretory sur-
faces, the details of which will be discussed in connection with
the general function of excretion.

Temperature Regulation. — About 90 per cent of the plasma of
vertebrate blood is water. Some of the water that is excreted is
water absorbed as such and some of it is metabolic water formed

CIRCULATION AND RESPIRATION 141

as a result of chemical reactions involved in metabolism. Water
has greater solvent properties than any other substance, which is
at least one of the reasons why it is important in cells. In
mammals particularly, one of the functions of the blood is in
connection with the regulation of body temperature, since,
except under conditions of hibernation, normal metabolism
requires that the body temperature be kept within rather narrow
limits. A complicated mechanism is found in these animals
for the maintenance of a fairly constant body temperature, only
the broad outlines of which can be considered here. In the
human body, a dilatation of peripheral blood vessels will increase
loss of heat by radiation. Likewise, the activation of sweat
glands covers the skin with perspiration, largely water, the
subsequent evaporation of which appreciably lowers the body
temperature. Contraction of the peripheral blood vessels tends
to conserve body heat. Animals like dogs, when overheated,
depend principally upon evaporation of water from the surface
of the lungs. The same is true of birds.

Lymph. — Lymph consists of plasma, similar to blood plasma,
and relatively few corpuscles, of which lymphocytes are the most
abundant. Red corpuscles and polymorphonuclear leucocytes
are usually absent. Strictly speaking lymph is the fluid con-
tained in lymphatic vessels, though it differs but slightly from
the body fluid found in spaces between the organs and body
wall, or from the so-called tissue fluids occurring in microscopic
spaces between cells. Except in the case of the contents of the
lymphatics of the small intestine, which during digestion become
gorged with fat, the lymph generally may be considered a part
of the drainage system of the body, charged with excretory
products, which eventually reach the blood system. Its move-
ment is centripetal and thus comparable with the movement of
the venous blood. Since the lymphatic vessels begin as capil-
laries in the tissues, the source of the fluid component of the
lymph is from the blood.

In the human body, most of the lymphatic vessels are tribu-
taries of the thoracic duct, which begins below the diaphragm by
the confluence of the lymphatic vessels from the legs, abdomen,
and viscera. It then passes anteriorly along the ventral side of
the vertebral column, receiving branches from the thorax, left
arm, and left side of the head, and empties into the left sub-

142

GENERAL ZOOLOGY

clavian vein near the heart. The right arm and shoulder, and
the right side of the head are drained by a much smaller lymphatic
trunk which opens into the right subclavian vein. Lymph nodes
lie along the course of the lymphatic vessels, particularly those
draining the neck, abdomen, axilla and groin. As the lymph
niters through the nodes, lymphocytes from the nodes are added
to the lymph stream. The nodes also act as checks to the spread

of infection along the lymphatic
channels.

From this it should be clear that
the lymphatic and blood systems
are merely parts of a general cir-
culatory system. Each organ
receives its primary blood supply
from an artery. Most of this fluid
leaves the organ by means of cap-
illaries which unite to form veins.
Some of it, however, mainly
plasma, diffuses through the capil-
laries into tissue spaces and thence
into lymph capillaries. Thus the
lymph originates as an overflow of
plasma from blood capillaries and
accumulates cells as it moves
along, in part, by immigration
from the capillaries, but mostly
from the lymph nodes. Since the
cells are bathed by the lymph
(tissue fluid), the lymph contrib-
utes nutritive material from the blood to the cells and at the same
time takes from the cells metabolic products, part of which are
carried away by the lymph and part by the venous blood.

Hemolymph Glands. — These glands resemble lymph glands,
except that they have blood vessels in place of lymphatic vessels.
Blood filters through the cells of the gland in much the same
way as lymph filters through a lymph gland. During its passage
through the gland blood loses red corpuscles, engulfed by phago-
cytic leucocytes, and gains lymphocytes, formed in the gland.
Hemolymph glands occur in certain parts of the abdomen,
thorax, and neck of mammals.

Fig. 90. — Diagram of vascular con-
nections in spleen of rat. a, arteriole;
c, capillaries passing directly to veins-
c.s., conducting sinus, walls con-
tracted; pc, peripheral capillaries (no
sinuses) ; s, sinus, walls expanded by
blood; v, vein; arrows indicate direc-
tion of blood flow. {After Knisely.)

CIRCULATION AND RESPIRATION 143

Spleen. — The spleen occurs as a distinct organ in all of the
vertebrates except the Cyclostomata. In the frog it develops in
the mesentery of the intestine; in man in the mesentery of the
stomach. It has a rich blood supply but no lymphatic vessels,
in which it resembles a hemolymph gland. The capillary cir-
culation is peculiar in that in addition to the usual capillaries
connecting arteries and veins there is also a series of venous
sinuses in parallel with the capillaries. Thus arterial blood may
reach the veins either through the capillaries or through the
sinuses. The sinuses therefore serve as a by-pass to the capil-
laries, in which blood may accumulate in large quantities under
certain conditions and be diverted from the general circulation
(Fig. 90). In man the cells of the splenic tissue, known as the
spleen pulp, consist of erythrocytes, lymphocytes, and phago-
cytic splenic cells resembling leucocytes.

The precise function of the spleen is unknown. It has long
been thought to have to do with the normal destruction of red
corpuscles, but agreement on this point is lacking. After a meal
the spleen enlarges, reaches a maximum in five hours and then
slowly contracts. The presence of venous sinuses accounts for
the changes in volume, since these are brought about by the
storage and release of blood from the sinuses.

CHAPTER VII
EXCRETION

Excretions are substances formed in the course of metabolism
of cells. Such substances are in the nature of end products of
chemical changes in the tissues and are useless as further sources
of energy to the animal body. Not only are they useless but
actually detrimental if allowed to remain in the body. A secre-
tion differs from an excretion in that the secretion is of some
definite use in metabolism, whereas the excretion is not. Gastric
juice is a secretion of the glands in the stomach; urine is an
excretion of the kidneys. Obviously undigested food, and
inorganic salts and nitrogenous products of putrefaction absorbed
from the alimentary canal, are not excretions. In man the major
part of the solid excretions in solution, and certain substances
occurring largely as by-products of digestion, absorbed from
the alimentary canal by the blood, are excreted by the kidneys.
On the other hand, the bile pigments, which are derived from
broken-down erythrocytes and which are true excretory products,
leave the body by way of the alimentary canal. Carbon dioxide,
a gaseous excretory product, is eliminated by the respiratory
organs — gills, lungs, or the surface of the body. In man some
of the constituents of perspiration are excretory in character.
Excretory substances are formed by and in the living tissues,
from which they are collected by the blood and eliminated by
various organs. A number of organs take part in excretion, but
the kidney is unique in that the absorption of excretory products
from the blood and the subsequent excretion of these products
are its sole functions. The discussion of excretion therefore
centers about the structure and function of the kidney.

Protonephridia. — One of the important functions of the blood
is the collection or absorption of products of metabolism from the
cells of the body and their transport to the excretory organ,
which removes them from the blood stream. In some forms
lacking a blood-circulating system, excretory products collect in

144

EXCRETION

145

the fluid of the body cavity from which they are removed
directly by the kidney. In the group of flat worms (Platy-
helminthes), in which neither a blood-circulatory system nor a
coelomic cavity is present, a very primitive type of excretory
organ is found consisting of numerous capillaries whose blind
ends are formed of single cells, known as flame cells (Fig. 91).
The flame cell is provided with irregular projections extending
into the surrounding tissues and bears a bundle of cilia extending
into the cavity of the capillary. The flickering movement of
the cilia is like that of a flame. Leaving the flame cells, the
capillaries unite to form large tubes,
which eventually drain into one of two
longitudinal ducts that open separately
or by a single pore on the dorsal side of
the posterior end of the body. Excre-
tory products dissolved in water are
absorbed from the surrounding tissues
through the flame cells and the walls of
the tubules, and are propelled through
the tubes to the outside by the vibra-
tions of the cilia. Such organs con-
stitute a -protonephridial system or a
water-vascular system.

Malpighian Tubules. — The hemocoel
of insects is a poorly developed body -;%£*-£-*-.
cavity filled with blood. The Malpigh-
ian tubules of the grasshopper are a clump of fine hairlike tubules
attached to the alimentary canal at the anterior end of the intes-
tine, from which they extend into the hemocoel where they are
bathed in blood (Fig. 53). They are blind at their free ends
and open at their attached ends into the intestine. These tubes
absorb nitrogenous and other excretory products from the blood
and deposit them in the intestine, whence they pass out of the
body with the feces. These products consist of urea, uric acid,
urates of calcium, sodium and ammonia, as well as oxalates,
carbonates and phosphates of calcium and potassium.

Nephridia. — The nephridial tubules of the earthworm are
involved in connection with development of a coelomic cavity
and a blood-vascular system, both of which are present in the
earthworm. The earthworm is a segmented animal, which

Fig. 91. — Flame cell of a
protonephridium of a flat-
worm, ci, cilia within the
funnel-shaped cavity of flame-

146

GENERAL ZOOLOGY

means that its body is made up of a linear series of more or less
similar structural units called segments. The annulated form
of the body is an external indication of segmentation. As a
result, the body cavity instead of being a continuous space is
subdivided by transverse septa, called dissepimenta, extending
from the body wall to the intestine, in the planes of the annular

Fig. 92. — Stereogram showing the relations of organs in posterior segments
of earthworm. The anterior end is to the right. A piece of the intestine has
been removed and two dissepiments have been cut. Blood vessels are shown
in black. The blood moves toward the anterior end (right) in the dorsal vessel
and posteriorly in the subintestinal one. d, dissepiment; n, nerve cord; nt,
nephridial tubule; t, typhlosole, a median longitudinal fold hanging from the dor-
sal wall of the intestine. (After McGregor and Calkins.)

grooves (Fig. 92). Thus each body segment has its own body
cavity. In each of these cavities, except the first three and the
last one, are two nephridial tubules, one on each side of the body.
Five regions can be distinguished in each tubule, as follows: (1) a
ciliated nephrostome, (2) a ciliated neck, (3) a coiled narrow tubular
part, (4) a wide glandular portion, and (5) an ejaculatory duct
opening to the outside by a nephridiopore (Fig. 93). The
nephrostome, provided with a slit-shaped opening, projects
through the anterior wall of the compartment in which the

EXCRETION

147

remainder of the tube lies, into the preceding compartment.
Thus the nephrostome is in one segment and the nephridiopore
in the following. Since the body cavities are filled with a
colorless fluid or lymph containing corpuscles, excretory products
dissolved in this fluid or in the form of solid particles can be
drawn in by ciliary action through the nephrostome and eventually
expelled to the outside through the nephridiopore. However,

since the tubules as a whole are
richly supplied with blood ves-
sels, excretory material is prob-
ably also absorbed directly from
the blood through the walls of
the tubule.

Kidney of the Frog.— The
kidneys of the frog lie against the

Sffife*

Fig. 93. Fig. 94.

Fig. 93. — Nephridium of earthworm, schematic, neph, nephrostome; se,
portion of septum; 1, neck and long narrow coiled part of tubule; 2, 3, 4, 5,
includes two wide portions connected by narrow tube; 6, ejaculatory duct; 7,
nephridiopore. (From Hesse and Doflein after Maziarski.)

Fig. 94. — Section of nephrostome of Rana pipiens. o, opening into nephro-
stome from ventral surface of kidney; v, vein, with contained blood corpuscles, c.

dorsal wall of the posterior half of the body cavity, one on either
side of the mid-line (Fig. 17). The ventral surface of the kidneys
and their edges only are covered by the peritoneum, the dorsal
surface being in direct contact with the body wall. The kidneys
are therefore retroperitoneal. Each is flattened and oblong in
shape and reddish in color. The yellow longitudinal band on the
ventral face of the kidney is an adrenal body, which has nothing
to do with the excretory function of the kidney. From the outer
edge of each kidney a ureter passes back to open into the cloaca
on its dorsal side near the mid-line. The functional unit of the
kidney is the uriniferous or renal tubule, each of which bears a

148

GENERAL ZOOLOGY

certain general resemblance to the nephridial tubule of the
earthworm, in that both are segmental structures and both have
nephrostomal connections with the body cavity, though in the
frog this relation to the coelom is best shown in the embryo.
The uriniferous tubules of the frog's kidney lack nephrostomes;
but nephrostomes, no longer connected with tubules, are found
as ciliated funnel-shaped openings on the ventral surface of the
frog's kidney. These nephrostomes, strangely enough, have
become secondarily connected with renal veins into which they

Fig. 95. — Diagram of glomerulus within capsule of uriniferous tubule, highly
magnified, a, afferent artery; e, efferent artery; t, tubule. The arrows show
the direction of blood flow.

draw body fluid from the coelom. This relation is found only in
Salientia (Fig. 94). In other Amphibia and in the embryos of
Salientia, the nephrostomes are connected with the renal tubules.
The blind nephrostomal end of the uriniferous tubule of the
frog is provided with a double-walled capsule, enclosing a net-
work of blood vessels, known as a glomerulus, in close contact
with the inner wall of the capsule (Fig. 95). The outer wall of
the capsule continues as a tubule, which after following a twisted
course, during which it forms two loops, opens into a collecting
duct, running transversely across the kidney, near the dorsal
surface, from the inner margin to the ureter. The first loop of

EXCRETION 149

the tubule, lying in the dorsal region of the kidney is generally
referred to as the 'proximal convoluted tubule and the second loop,
the distal convoluted tubule. Bidder's canal is a longitudinal duct
lying within the inner margin of the kidney roughly parallel
to the ureter on the outer margin. The collecting ducts extend
from Bidder's canal to the ureter. There are many collecting
ducts and each duct has attached to it a number of uriniferous
tubules. The capsular ends of the tubules are located in the
ventral region of the kidney (Fig. 96).

The walls of the capsule are composed of thin flat cells which
gradually pass over into a short columnar
form in the tube proper. The first part of
the tubule, next to the capsule, is the neck,
so-called because its diameter is smaller
than the rest of the tubule. The inner faces
of the cells of the neck are provided with
long cilia. Shorter cilia are also found in
the cells of the tubule a short distance
beyond the neck.

The arterial blood supply to each kidney Fig. 96.— Uriniferous

„ „ ... r^, tubule and collecting tu-

comes from four to six renal arteries. 1 nese bule of frog-s kidney, c,
enter the ventral surface of the kidney and collecting tubule; d, dis-

. i ,i ii i i ii_ tal convoluted tubule; G,

send branches to both the capsular and the giomeruius; p, proximal
tubular portions of the renal tubules. Each convoluted tubule.

. . ~ , , rr , (After Nussbaum.)

glomerulus has an afferent and efferent

artery. The kidneys also receive blood from the renal portal
veins, which enter the dorsal surface of the kidney and
form capillary networks about the tubules. The efferent
arteries from the glomeruli join the capillaries from the renal
tubules, from which the venous blood is drained by branches of
the renal veins. The glomeruli therefore receive blood only
from the arteries, but the tubular parts of the renal tubules
receive blood both from the renal arteries and from the renal
portal veins; mainly from the latter.

Excretory material is removed (1) by the capsule from the
blood as it circulates through the glomerulus, and (2) by the
remainder of the tubule as the blood flows through the capillary
network about it. Opinion differs as to the manner in which the
tubule functions, but subsequent observations tend to confirm the
general conclusions reached by Nussbaum as the result of experi-

150 GENERAL ZOOLOGY

ments made in 1875. Using Rana esculenta, Nussbaum cut off
the circulation to the glomeruli by completely ligating the renal
arteries. If a solution of a dye, indigo-carmine, is then injected
into the venous system, the color is later found only in the
loop of the tubule (proximal convoluted part) near the dorsal
surface of the kidney, the other segments of the tubule remaining
free of color. Practically no water is excreted. This shows that
dyes are absorbed from the blood by the portion of the tubule
supplied by the renal portal circulation. If a 10 per cent solution
of urea is injected into the venous system instead of dye, the
bladder becomes filled with urine in the course of two or three
hours. This experiment shows that the uriniferous tubule,
deprived of its glomerulus, is able to excrete urine under the
stimulus of urea in the blood. Further, more recent work indi-
cates that the urine is concentrated by the resorption of water
in the distal convoluted tubule. Thus it would seem that the
fluid absorbed from the glomeruli, largely water, as it flows down
the tubule receives the specific excretion of the proximal con-
voluted region, the mixture being further modified by the resorp-
tion of water as it passes through the distal convoluted region.

Most of the nitrogen leaves the body in the form of urea, which
is largely formed in the liver. Urine also contains inorganic salts
such as sulphates, chlorides and phosphates of sodium, potassium,
calcium and magnesium, as well as other substances in small
quantities. There is evidence to show that in addition to most
of the water, the glomeruli also excrete albumin, sugar, and
inorganic salts.

The Bladder of the Frog. — The ureters open close together in
the dorsal wall of the cloaca. The bladder arises as an evagina-
tion of the ventral wall of the cloaca, opposite the mouths of the
ureters. In its fully developed state the bladder is attached to
the cloaca at its point of origin by a narrow pedicle, beyond
which it expands into a thin-walled, bilobed distensible vesicle
(Fig. 17). It is lined with a stratified epithelium and contains
plain muscle in its wall. The external opening of the cloaca is
closed by a sphincter muscle. Urine passes from the ureters into
the cloaca and collects in the bladder, from which it is expelled
rather abruptly by the contraction of the muscles of the body wall.

The Human Kidney.— The human kidneys (Fig. 97) are
bean-shaped retroperitoneal organs, attached to the dorsal body

EXCRETION

151

wall, opposite the last thoracic and the first two lumbar vertebrae.
Each kidney is enclosed in a thin fibrous capsule, surrounded by
loose fibrous tissue normally containing considerable fat. A
ureter leaves each kidney from the concavity on the median side
and proceeds to the bladder, a large saclike vesicle lying in the

Fig. 97. Fig. 98.

Fig. 97. — Excretory organ of Man, diagrammatic, a, aorta; b, urinary blad-
der; k, kidney (left); p, postcava; ra, renal artery (left); rv, renal vein (left); u,
ureter (left), extends from the pelvis of kidney to the base of bladder; ua, urethra,
a canal to the outside.

Fig. 98. — A. Diagram to show the relations between the blood vessels and
the nephridial tubules of the human kidney. A, artery; c, capsule (glomerulus)
1 to 4, parts of tubule; t, straight collecting tubule which leads to the pelvis of
the kidney; v, vein. The arrows indicate the direction of blood flow. B, section
of the thick portion of a loop of Henle to show its relation to the blood capillary
(b). (Modified from Stohr.)

lower part of the abdominal cavity against the abdominal wall.
A single duct, the urethra, leads from the bladder to the outside.
A single renal artery brings blood to the kidney and a single renal
vein drains it. There is no renal portal circulation. In the
kidney the renal artery breaks up into smaller branches, some of
which end in ordinary capillaries while others form glomeruli in

152 GENERAL ZOOLOGY

the capsules of uriniferous tubules, before breaking up into capil-
laries about the loops of the tubules. Thus the blood supply to
the tubule is entirely arterial. The tubule is somewhat more
complicated in form and structure than in the case of the frog and
consists of the following well-defined regions: (1) a capsule
enclosing a glomerulus; (2) a proximal convoluted portion; (3) a
U-shaped part composed of straight descending and ascending
limbs of Henle's loop; and (4) the distal convoluted portion (Fig.
98). The distal convoluted portion is joined by an arched
collecting tubule to a straight collecting tube, which leads to a
renal sinus or renal pelvis, a chamber in the concave region of
the kidney, drained by the ureter. The form of the tubule is
such that the distal and proximal convoluted portions are very
close to the capsule. The efferent arteries, after leaving the
glomerulus, are distributed among the tubules, forming a capil-
lary network leading to venous channels which eventually unite
to form the renal vein.

Judging from experimental observations in rabbits, the
mammalian uriniferous tubule functions in much the same way
as the tubule of the frog's kidney. In the rabbit, excretion of
solids in solution occurs in the proximal convoluted segment and
in the descending limb of Henle's loop, and resorption in the
ascending limb and in the distal convoluted segment. From the
glomerulus water principally is excreted. It might also be added
that in certain fishes, Sygnathus and Hippocampus, the urinif-
erous tubules lack glomeruli, yet urine is excreted containing
dissolved substances in different concentrations than in the
blood. All of this strengthens the idea that certain portions of
the uriniferous tubule, exclusive of the capsule, excrete solids
in solution and that other portions of the tubule modify the
concentration of the solution by resorbing water.

It should be clear from what has been said that the organ of
excretion, whether it be gill, lung, skin, or kidney, is merely a
means of ridding the body of waste matter. The circulatory
fluid transports excretory substances, formed by the body cells
and to a certain extent in the circulatory fluid itself, conveying
them to the excretory organ. The excretory organ is the elimi-
nator. In the absence of a circulatory system, the excretory
organ absorbs the metabolic products in a more direct manner
from the cells.

CHAPTER VIII
MEANS AND METHODS OF REPRODUCTION

The reproduction of an individual organism begins with the
separation from the parent of a part of its body, which is of course
alive and continues to live as it progresses through developmental
stages to the adult condition. Reproduction is a method of con-
tinuing life rather than one of creating life. It results in the
production of generations of new individuals which, however,
are derived from the preceding generations through living
material. Single-celled animals, Protozoa, reproduce by the
division of the entire animal. This also occurs in some of the
lower metazoan animals. In all of the Metazoa, regardless of
the occurrence or not of reproduction by simple division of the
animal as a whole, there is a form of reproduction in which the
development of a new individual starts in an egg that is
usually fertilized by a spermatozoon. Both egg and sperma-
tozoon are cells, so that the morphology of the elements out of
which the new individual is produced is quite different from the
morphology of the adult animal. The organism in all trans-
formations undergone from the egg stage to the adult stage is a
living thing. Life does not start at the hatching of the frog
tadpole or at the birth of a human being. Life started in the far
distant past and has continued in those kinds of organisms
still alive at the present time. Extinct species are forms which
for one reason or another ceased to produce descendants.

The organs of reproduction are the ovaries and testes, also
known as gonads. In many invertebrates both ovary and testis
are present in the same individual, in which case the animal is
said to be monoecious or hermaphroditic. Other invertebrates
and most vertebrates are dioecious, which means that two sexes
are present and that ovaries are present only in the female and
testes in the male. Sometimes, as in some of the Mollusca,
there is a single gonad, the ovotestis, one portion of which produces
eggs, the other spermatozoa; or the ovotestis may at one time

153

154

GENERAL ZOOLOGY

function as an ovary, at another time as a testis, a condition
known as dichogamy. Ova or eggs are produced in the ovary
and spermatozoa or sperm cells in the testis. Ova and sperma-
tozoa are spoken of generally as germ cells or gametes. In the
method of reproduction known as amphigony, a spermatozoon
(in some cases several) enters the egg and forms an oosperm or
fertilized egg or zygote, which develops into a new individual
animal. The details of the process of fertilization vary, but its
essential feature is the union of a nucleus derived from the
sperm cell with the egg nucleus. If more than one sperm cell

enters the egg, as for example in
the hen's egg, only one sperm
nucleus combines with the egg
nucleus. Fertilization may take
place before or after the egg
leaves the body of the female.
In parthenogenesis, the egg de-
velops without the intervention
of the spermatozoon. Some eggs
develop either parthenogenetic-
ally or in response to fertiliza-
tion. Usually an egg requiring
fertilization for its development
Fig. 99.— a, fission in Hydra by does not develop parthenoge-
longitudinai division. (After Koditz.) netically, under normal condi-

B, fission in Planaria. (After Child.) . ■" . ...

tions. An oviparous animal is
one in which the young are hatched from eggs laid by the female.
A viviparous animal gives birth to its young. Both methods
of producing young occur in both invertebrates and vertebrates.
Agamic Reproduction. — Some animals reproduce by fission,
which consists in a division of the entire body into two parts of
approximately the same size. Since no gametes or germ cells
are involved, fission is an agamic or asexual form of reproduction.
Another form of this type of reproduction is budding, which differs
from fission in that the process starts with the formation of a
bud on the surface of the body. The bud enlarges, differentiates,
and finally detaches itself as a small fully formed new individual.
Both fission and budding occur in Hydra. Fission takes place
by a longitudinal splitting, beginning at the distal end and
extending down through the basal disk, or by a transverse con-

MEANS AND METHODS OF REPRODUCTION

155

striction across the middle of the body, the result in either case
producing eventually two complete but smaller animals (Fig. 99).
Budding in Hydra starts with the formation of a bulge in the
body wall which takes on a cylindrical form with a circlet of
tentacles developing at the free end of the cylinder. When
fully grown, the bud is released as a new, free individual (Fig.

219). Euplanaria is another com-
mon laboratory animal in which
fission occurs (Fig. 99).

Gamic Reproduction. — Gamic
or sexual reproduction is repro-

M-4

Fig. 100. Fig. 101.

Fig. 100. — Section of mature ovum, dn, degenerating nuclei of interstitial
cells contributing to formation of ovum; e, ectoderm; i, interstitial cells; m,
mesoglea; n, nucleus of ovum; o, ovum. (After Tannreuther.)

Fig. 101. — Section of ovary of Hydra, showing developing ovum.

duction from an egg whether fertilized or not. It occurs in all
metazoans, even in those that reproduce by fission or budding.
Thus in Hydra, the gonads, which are not present at all times,
develop as localized proliferations of the interstitial cells of the
ectoderm. The spermaries or testes, containing numerous sper-
matozoa, form as round protuberances near the oral end, while
the ovaries, somewhat similar in shape, are formed near the aboral
end (Fig. 221). Within each ovary a single cell enlarges to form
an egg by the rather unusual method of engulfing surrounding
interstitial cells (Figs. 100 and 101). Thus a single cell becomes
an egg at the expense of neighboring cells. Hydra is hermaph-
roditic, but since the ovary and testis of the same animal

156 GENERAL ZOOLOGY

usually do not mature at the same time, the egg in most cases is
fertilized by a spermatozoon from another animal. In this way
the occurrence of self-fertilization is reduced to a minimum.
When fully developed, the walls of the spermaries rupture,
releasing free-swimming spermatozoa into the water. When the
egg is mature, the wall of the ovary is partially ruptured, thus
exposing a portion of the free surface of the egg, through which
a spermatozoon may enter. The fertilized egg remains attached
to the wall through the early stages of development, after which
it is released and completes its development as an independent
organism. In Hydra and other invertebrate animals of a rela-
tively simple structure, the release of the germ cells from the
gonads is accomplished by the rupture of the enclosing body
wall, but in more complex animals, in which the gonads are
located internally, ducts and other accessory structures are
present for bringing the germ cells together or for conveying them
to the outside of the body, either before or after fertilization.
The female genital tract consists usually of a pair of oviducts,
one for each ovary, through which the eggs pass to the outside
and which, as in the frog, also provide the eggs with egg envelopes
or capsules. In viviparous animals a portion of the oviduct is
modified into a uterus in which the embryo develops until birth.
The so-called uterus of the frog is merely an enlarged storage
region in the oviduct in which eggs accumulate. In a true uterus
nutrition is supplied to the embryo by the walls of the uterus.
The vas deferens of the male corresponds to the oviduct of the
female. It conveys spermatozoa from the testis to the outside.
In the male there may also be a penis, an intromittent organ for
introducing sperm cells into the genital tract of the female in
those cases in which the eggs are fertilized internally. Internal
fertilization occurs in all viviparous animals and in many ovip-
arous ones as well. So far as development is concerned, the
difference between oviparous and viviparous forms consists in
the fact that in the former embryonic development takes place
after the egg is laid, and in the latter embryonic development is
completed before birth.

Among segmented worms, Annulata, and also among Verte-
brata, there is a close relationship between the accessory parts of
the reproductive system and the tubules and ureter of the excre-
tory organs, both in embryonic development and in the adult

MEANS AND METHODS OF REPRODUCTION

157

state. Thus the vas deferens of vertebrates is a modified
excretory duct (Wolffian duct) and some of the tubules of the
testis are modified uriniferous tubules.

Reproduction in the Earthworm. — The manner in which repro-
duction takes place in the earthworm illustrates the process in a
hermaphroditic animal, somewhat higher in the scale than Hydra.
The testes of the earthworm consist of two pairs of organs lying

N

10

11

13

14

15

Fig. 102. — Diagram of the reproductive organs of earthworm, n, nerve cord;
o, ovary; od, oviduct; sd, sperm duct (vas deferens); sr, sperm receptacles; sv,
sperm vesicle, with part of the roof removed to show the testes, T, and the funnels
of the sperm ducts.

in the body cavity, one pair on the posterior side of the anterior
wall of segment 10, and the other pair in a similar position on the
anterior wall of segment 11. These same segments each contain
a pair of ciliated funnel-shaped openings of the sperm ducts or
vasa deferentia. Two ducts of the same side unite posteriorly
to form a single duct or vas deferens, which opens externally on
segment 15. Three pairs of seminal vesicles, communicating
with each other, extend over segments 9 to 15. Those belonging
to segments 10 and 11 enclose the testes and the mouths of the
sperm ducts. When the sperm cells reach a certain stage of

158

GENERAL ZOOLOGY

103. — Copulation i
foetidus. (After Foot.)

development, they are liberated from the testes and complete
their development in the confines of the seminal vesicles. At the
proper time they escape from the seminal vesicles through the
four mouths of the sperm ducts (Fig. 102).

The ovaries of the earthworm are two in number, situated one
on each side of the mid-line on the posterior surface of the anterior

wall of segment 13. As the eggs
mature, they burst from the
ovaries and are conveyed to the
outside through a pair of oviducts,
each of which begins in a funnel-
n Hdodriius shaped opening located on the
posterior wall of segment 13 and
terminates in an opening on the outside of segment 14.

The seminal receptacles or spermathecae are two pairs of sacs
in segments 9 and 10. At the reproductive period these sacs are
filled with sperm from another worm in the manner to be
described presently. Later the sperm cells are emitted to
fertilize the eggs.

According to the observations of Grove on the common earth-
worm, Lumbricus terrestris, two
worms oppose each other in a
head-to-tail position in such a
manner that segments 9 to 1 1 of
one worm are opposite the
ventral surface of the clitellum
of the other. The clitellum is
an enlarged region of the body
composed of segments 32 to
37. At these points the worms become firmly bound together
by bands of mucus secreted by the epidermal cells which
soon hardens into a thin membrane (Fig. 103). There is
also a slighter attachment between segment 15 of one and
segment 26 of the other. A slime tube, composed of mucus,
is then secreted about each worm, save at the ventral surfaces in
intimate contact between segments 9 and 37. Seminal fluid
containing spermatozoa then issues from the openings of the
vasa deferentia in segment 15 and passes back through tempo-
rary grooves beneath the slime tube in the body wall to the clitel-
lum. Emission of seminal fluid takes place in each worm

Fig. 104. — The formation of a slime
tube by a single worm at some period
after copulation, c, cocoon formed
about the clitellum; t, slime tube.
(After Foot and Strobell.)

MEANS AND METHODS OF REPRODUCTION 159

though not always at the same time. The spermatozoa gradually
accumulate between the clitellum and the lateral surface of
segments 9 and 10, and then pass into the spermathecae. What
takes place next is uncertain so far as the earthworm is con-
cerned. In a related species, Eisenia foetida, the Brandling
worm, according to Grove and Cowley, the worms separate
after the spermathecae are filled and each worm forms a slime
tube extending from segment 7 to some point beyond the clitel-
lum; the portion enclosing the clitellum forming the cocoon
(Fig. 104). Eggs are then discharged from the oviducts and pass
back to the cocoon. It remains to be determined whether the
spermatozoa also pass back to the cocoon while the slime tube is
still in place or whether they are pressed
into the cocoon when the worm wrig-
gles out backward from the slime tube.
At any rate, the eggs are fertilized in the
cocoon, whose ends later contract to form
a closed capsule as the worm withdraws. FlG loT^pearance of

Apparently the Supply of Sperm is not slime tube after shedding.

exhausted in fertilizing the first batch of

eggs, since single worms may form a number of slime tubes, with
cocoons, a number of times after copulation. Each of these later
slime tubes is shed after eggs and sperm have been deposited in
the cocoon (Fig. 105).

Reproduction in the Frog. — The frog is an example of an
oviparous vertebrate whose eggs are fertilized externally. The
testes of the frog are a pair of yellow ovoid bodies, each attached
by a mesorchium to the dorsal body wall at the anterior border
of the kidneys. Spermatozoa, formed in the testis, leave through
a number of fine ducts, the vasa efferentia, extending from each
testis to the inner border of the kidney. The vasa efferentia
enter the kidney and open into Bidder's canal (Fig. 106), from
which the passageway for the spermatozoa is continued through
the collecting tubes of the kidney to the ureter. Near the
termination of the ureter in the cloaca is an enlargement, the
seminal vesicle, where the spermatozoa are stored until amplexus,
the sexual embrace, takes place, when they are discharged
through the cloaca. The ureter and some of the collecting ducts
of the kidney in the male thus serve as genito-urinary passages.
In the female these parts are urinary passages only.

160 GENERAL ZOOLOGY

The ovaries are a pair of tabulated organs, each of which is
attached by a mesovarium to the dorsal body wall just in front
of the kidney. The paired oviducts or Mullerian ducts are
long convoluted tubes, each of which is provided with an opening
or ostium at its narrow anterior end and a widened region or
uterus near its posterior end just before it enters the cloaca.
Since the ostia of the oviducts are located in the region of the
pericardium, near the base of the lungs, they are separated by
T ct some distance from the ovaries. When

\ / /*ZIM3\ ^ne e*=§s f°rmed m the ovary are mature,

/^^^^^^\ they escape into the body cavity through

I • ' ::^^m^^~a\ ^ne ruPture °f the walls of the egg follicles

IX.V.V.^3^^^^IL^----K and are carried by ciliary movement over

\i0^\Mr^}/ the ciliated surface of the liver, body

|\S37 wall, and pericardium to the ostia where

WW/ they are engulfed. In the upper parts of

the oviducts the eggs receive a coating of

^ -UR thin albumen and then pass into the

uterus where they are stored until
Fig. 106. — Diagram of spawned. Oviducts are developed in

urogenital organs of male both maleg d femaleg but m the former
frog, left side, bd, Bidder s /

duct; ct, collecting tubules they become reduced in size and remain

of kidney, which also receive f urxctionless (FigS. 17 and 18).
unniferous tubules, not i •

shown; e, vasa efferentia, Fat Body. — At the anterior end of each

through which sperm from f the gonadg 0f both gexeg of the frog is
testis reach Bidders duct; ° , °

k, kidney, shown in section the fat body, a conspicuous mass of fatty
t, testes; ur, ureter. tissue drawn out into finger-shaped lobes

(Fig. 18). Its development is closely associated with the develop-
ment of the gonad, in consequence of which the fat bodies are
thought to represent degenerated segments of the embryonic
rudiment from which the gonads develop. In temperate zones
spawning takes place in the first warm days of spring that signal-
ize the end of hibernation and the resumption of more active life.
The fat bodies retain their full size throughout the period of hiber-
nation but just before the beginning of the breeding season they
undergo a reduction in fat content that continues until spawning
is completed. The fat bodies thus serve to meet the energy
requirements of the developing germ cells and of the bodily activi-
ties of the frog before active feeding is resumed. The fat may be
drawn upon to meet metabolic requirements at other times, but

MEANS AND METHODS OF REPRODUCTION 161

the greatest demand arises in connection with breeding activities.
Once these are completed, fat begins to accumulate in the fat
bodies until by autumn they are restored to the maximum size.

Amplexus. — In amplexus the male embraces the female from
the dorsal side, clamping its forelegs about the female just
behind the latter's forelegs, firmly gripping the body of the
female with the enlarged sexual pad of the first digit of each
hand. Captured female frogs may lay eggs in the laboratory in
the absence of males, but under natural conditions amplexus
acts as a stimulus to spawning. Amplexus takes place in the
water. Under its stimulus the female sheds its eggs and as the
latter pass from the cloaca, the male emits sperm over them.
The amplectant posture persists for three or four days until all
the eggs are shed.

When the eggs leave the female cloaca, the thickness of the
capsule of the surrounding jelly is less than one-half the diameter
of the egg. A single spermatozoon now enters each egg, after
which the capsule absorbs water and swells to two or three times
its original thickness. At this time it can be readily seen that
the capsule is really composed of three layers. The gelatinous
capsule is comparable to the white of the hen's egg and serves as
a protective covering. The jelly also serves another function in
retaining heat absorbed from sunlight. The jelly seems to
transmit the shorter rays but checks the loss by radiation of the
longer heat waves from the egg.

Intrauterine Development. — The young of viviparous ani-
mals, whether invertebrates or vertebrates, are brought forth in
a more or less fully developed state. What happens in the
majority of cases, among invertebrates and among vertebrates
below Mammalia, is that the large-yolked egg after fertilization
remains in a specialized portion of the oviduct, known as the
uterus, and develop there instead of outside the body. Thus in
viviparous fishes and snakes, the yolk of the egg seems to be the
main source of nutrition for the embryo, though an organic
attachment of the egg to the wall of the uterus may occur in
some cases, as in Mustelus, a common viviparous shark (Fig. 107).
All mammals are viviparous with the exception of the lowest
group, the Prototheria, which are oviparous. The uterus of
viviparous mammals is a modified region of the oviduct whose
walls are highly vascular and come into close enough contact

162 GENERAL ZOOLOGY

with the egg, and later the tissues of the embryo, to supply the
latter with nutrition and oxygen and to remove carbon dioxide
and other waste products of metabolism. The eggs of such
mammals are small in size and contain an insufficient amount of
stored nutrition in the form of yolk to carry development beyond
the very earliest stages.

Both the tissues of the uterus and the tissues of the embryo
participate in the formation of a placenta, which is an organ that

40 "i in..

Fig. 107. — Embryo sharks of a viviparous species, Mustelus mustelus, attached
to the walls of the uterus. The expanded end of each umbilical cord forms a
placenta-like structure, resembling that of mammals, through which the young
are nourished. (From Shull, LaRue, and Ruthven, Animal Biology, after Fowler.)

develops during the period of gestation. The placenta is thus a
composite organ which differs in detail in different mammals.
The maternal and embryonic blood streams are never confluent
in the placenta, but they are brought close enough together to
permit the diffusion of gases and substances in solution through
the intervening tissue. The mammalian egg is released from
the ovary by the rupture of the ovarian wall and enters the
oviduct, in the upper part of which fertilization takes place. The
fertilized egg begins to develop as it passes down the oviduct to
the uterus where it becomes attached more or less firmly to the
lining of the uterus. The placenta then begins to develop and
with it the development of an umbilical cord through which blood

MEANS AND METHODS OF REPRODUCTION

163

vessels of embryonic origin pass from the embryo to the placenta.
In some mammals, the pig for example, there is at birth a clean
separation of the embryonic from the uterine portion of the
placenta; in other mammals, as in Man, some of the uterine
portion is shed along with the entire embryonic portion, the
whole being known as the afterbirth. The uterine tissues, lost at
birth or parturition, are soon restored by the uterus. The

uv

Fig. 108. — -Diagram of gravid human uterus, anterior wall removed, a, amni-
otic cavity, filled with fluid in which the foetus (late embryo) rests; ft, Fallopian
tube (oviduct); u, uterus; uc, umbilical cord, containing foetal arteries and veins
through which blood circulates to and from the placenta, pumped by the foetal
heart; uv, uterine vessels, carrying maternal blood to and from the placenta; v,
vagina. The shaded portions of the uterus are shed at birth.

young are expelled by the contractions of the muscular walls of
the uterus. In Man, the navel is the place on the abdomen where
the umbilical cord was attached during intrauterine development
(Fig. 108).

General. — The organs of reproduction, the ovary and the
testis, as has been said before, are those producing germ cells,
ova and spermatozoa, which, when united, are capable of repro-
ducing a new animal. Both egg and sperm are merely detached
portions of the parent body, possessing the capacity, when
brought together under proper conditions, of developing a new
individual. In parthenogenesis the ovum alone can do this.

164 GENERAL ZOOLOGY

Though a part of the body, the germ cells remain distinct from
body cells, and when favorable conditions arise for the union of
ovum with sperm, they proceed forthwith to display their remark-
able developmental energy. Any other cell, such as muscle or
liver cell, does not have this reproductive power, yet each, like
the germ cell, has its origin in the fertilized egg. In becoming
specialized body cells, it seems that the tissue cells lose the repro-
ductive capacity, which must have been latent in them at the
beginning. Germ cells, on the other hand, retain it because they
do not differentiate into tissue cells. It would follow that once
a cell has developed into a tissue cell, it can never afterward
function as a germ cell. This is true in most animals, there
being some apparent exceptions among invertebrates like the
Coelenterata, where the germ cells develop from time to time from
differentiated endoderm or ectoderm. Obviously, in such cases
differentiation of body tissues even in the adult does not proceed
so far that a return to an unspecialized state is impossible.

CHAPTER IX
THE NERVOUS SYSTEM

The nervous system is concerned with the perception of stimuli
by receptors, the coordination of stimuli, and the conduction of
nervous impulses to effectors. Receptors may be simple, unmodi-
fied nerve endings, such as occur in the lower layers of human
epidermis, or nerve endings, sometimes highly specialized and
intimately related with nonnervous structures, the whole form-
ing a sense organ, such as the eye or the ear. Responses to
sensory stimulation are manifested in movements of muscles or
secretory activity of glands, both of which are the usual effectors.
The coordination of the incoming sensory impulses and the
responses is the function of nerve centers, such as the brain and
the spinal cord. It should be noted that a chemical method of
coordination also exists, examples of which were discussed in
connection with the secretion of digestive fluids and of which
further examples will be considered in the following chapter,
dealing with the endocrine organs. Sensation is something
produced in the central nervous system, dependent upon the
nature of the connections of the nerve fibers carrying the afferent
impulses. Afferent nerve fibers normally are stimulated by
their receptors, but if a given afferent nerve is stimulated at any
point the sensation is always the same. We know the nature of
sensations in animals other than Man only by comparing their
responses with our own, under similar conditions. Since sensa-
tions are entirely subjective, there must be considerable variation
in both quantity and quality of sensations produced by the same
sensory stimulation in different individuals.

Reduced to simple terms, the functions of the nervous system
and sense organs are: (1) to bring about in the organism appro-
priate responses to external stimuli, and (2) to coordinate the
functional activity of different parts of the body by nerve path-
ways. Irritability and conductivity, two properties manifested
by all forms of protoplasm, are highly developed in nervous tissue,
and all nervous phenomena are based primarily on these two

165

166 GENERAL ZOOLOGY

fundamental properties. Since Protozoa respond to the same
stimuli that activate the nervous system of Metazoa, it is clear
that the action of the nervous system as a receptor-effector and
coordinating mechanism involves nothing new in principle.
Nerve cells may be regarded as cells that have become especially
sensitive to stimulation of various sorts and that have also
developed the capacity to conduct or transmit nervous impulses
at a very rapid rate. A stimulus may be defined as any dis-
turbing influence. A nerve impulse results from action of the
stimulus. The impulse travels along the nerves by conduc-
tion, which is essentially a series or chain of chemical reactions
running through the nerve, accompanied by the production of
extremely small amounts of heat and carbon dioxide.

Neuron. — A neuron is a nerve cell. It is the anatomical and
to a certain extent the functional unit of the nervous system.
Neurons vary considerably in both size and shape in different
animals and in different parts of the same animal, but in all
cases the neuron consists of at least two regions: (1) a nucleated
cell body, and (2) processes extending from the cell body. In
higher forms the processes can usually be classified both on a
structural and on a functional basis into two groups: (1) dendrons
or dendrites, consisting of one or more processes, often branched,
that convey nervous impulses toward the cell body; and (2) a
single axon, that transmits impulses away from the cell body.
Since the neuron as a whole under normal conditions carries
impulses in one direction only, it is said to be polarized. Den-
drons may be lacking in some neurons, and in others a single
dendron may be present and so similar to the axon in form and
structure as to be almost indistinguishable anatomically. The
drawing of the neuron from the human brain, shown in Fig. 109,
illustrates a highly complicated type. The cell body is the pear-
shaped region in the center of the figure. , The nucleus cannot be
seen because the preparation only shows the entire nerve cell in
silhouette. A number of dendrons with collateral branches are
attached to the cell body, above and at the sides. The single
axon leaves the cell body below and then divides into collateral
branches, the main axis of the axon extending downward to join
a bundle of nerve fibers. The term "nerve" refers to a bundle
of axons or dendrons, or both, bound together by a connective
tissue sheath. Axons or dendrons are also known as nerve

THE NERVOUS SYSTEM

167

Fig. 109. — A nerve cell from the cerebral cortex, a, basal dendrons; b, apical
dendron; c, collaterals of axon; e, axon; p, apical dendrons ending in branches
near the surface of the brain. (From Schafer, Textbook of Microscopic Anatomy,
Longmans, Green & Company, after Cajal, By permission.)

168

GENERAL ZOOLOGY

N.

_M

.AX

fibers. An afferent or sensory nerve is one composed of dendritic
fibers and one that conducts impulses from the surface of the
body, or from some outlying point, to a centrally located nerve
center, such as the brain or spinal cord. An efferent or motor

nerve is made up of axons and
transmits impulses in the opposite
directions. Efferent nerves are
called motor nerves because their
stimulation commonly produces
muscular contraction, which in
turn produces movement of the
body as a whole or in part. Ex-
perimentally the direction of the
impulse through a nerve may be
changed, though normally the direc-
tion is from dendron to cell body
to axon.

A ganglion is a mass of nervous
tissue consisting largely of nerve
cell bodies. In higher forms, nerve
cell bodies are confined to ganglia,
special nervous epithelia, and nerve
centers such as the brain and spinal
cord. Different levels of the brain
and spinal cord are connected by
nerve tracts, composed of nerve
fibers, which are comparable to the
peripheral nerves connecting the

Fig. 110. — Diagram of a verte- .,,

brate motor neuron with a medul- central nervous system with vanous

lated axon, a, axon; ax, axis partg of the body. Many of the
cylinder; b, cell body; d, dendrons; , , , .

E, motor end organ in contact with connecting pathways of the brain

muscle cells; M, myelin sheath; n, an(J Spmal cord Consist of neurons
nucleus of sheath cell; s, one of the ... , ,

cells forming a sheath enclosing the lying entirely within the central
myelin. nervous system.

A medullated nerve fiber is one provided with a sheath of white
fatty material known as myelin. The axis cylinders of the spinal
nerves of vertebrates in addition are provided with a neurilemma
composed of delicate sheath cells surrounding the myelin (Fig.
110). Medullated fibers of the brain and spinal cord do not have
sheath cells. Nerve fibers of the sympathetic system, to be

THE NERVOUS SYSTEM

169

discussed later, lack myelin but are provided with sheath cells.
Finally, nerve terminations may consist of axis cylinders (naked
axons) only.

The entire nervous system, including the sensory portions of
the sense organs, develops from the ectoderm, with the possible
exception of taste cells, which in some cases of vertebrates at
least are said to be endodermal in origin. An ectodermal origin
of the nervous system is what one would expect since the ecto-
derm is the primitive outer covering of the body and therefore the

A B

Fig. 111. — Nervous mechanism of Hydra. A, epitheliomuscular cell from ecto-
derm; B, ectodermal nerve plexus. The long fibrils in the background are the
contractile parts of epitheliomuscular cells. They lie in the mesoglea. {From
Schneider.)

portion of the body through which the environment is sensed.
Neurons apparently are ectodermal cells that have gradually
taken on a nervous function, accompanied by structural differ-
entiation from other ectodermal cells.

Diffuse Nervous System. — A primitive form of nervous system
is found in Hydra and related forms, consisting of nerve cells
with branching processes, located in the ectoderm. The cells
are arranged in a loose network, showing no marked tendency to
be concentrated at any points to form nerve centers or ganglia.
The nearest approach to centralization is about the mouth
(hypostome) where the nerve cells are more numerous than
elsewhere. The arrangement of the nervous system of Hydra
suggests a crude mechanism for receiving stimuli and correlating

170 GENERAL ZOOLOGY

responses, such as one might expect in the early stages of the
evolution of the nervous system. The system has remained at
its site of origin in the surface layer of the body (Fig. 111).

Primitive Ganglionic Nervous System. — The nervous system
of a flatworm, such as Euplanaria, illustrates an early step in the
process of centralization of a portion of the nervous mechanism.
This has been brought about by the development of a pair of
cephalic ganglia, in the anterior end of the animal, from which a
pair of nerve cords extends posteriorly to the tip of the body.
Both the ganglia and the nerve cords are completely covered
over by the integument. From both ganglia and nerve cords,
nerve fibers extend to various parts of the body. The anterior
end of the body is provided with special sense organs, such as
eyes, tactile organs, and organs of chemical sense. The concen-
tration of sense organs at the anterior end is correlated with the
presence there of the cephalic ganglion, the general result of
which is to convert this region of the body into a "head end."
When Euplanaria explores new territory, the part of the body
that leads the way is the head end, which is the part best fitted
with means of sensing the environment. Thus the cephalic
ganglion may be regarded as a kind of brain that serves as a
center for correlating the sensory and motor mechanism. As a
result the head end assumes a position of dominance or control
over the activities of the rest of the body. That the head end
of Euplanaria is dominant in a physiological sense has been
demonstrated by experiments that show that the rate of meta-
bolism is higher in the head end than elsewhere. The evolution
of a head end, i.e., cephalization, is bound up with the evolution
of the nervous system and sense organs (Fig. 52).

Ganglionic Nervous System. — A further elaboration of the
type of nervous system found in flatworms occurs in a large num-
ber of invertebrates such as segmented worms (Annelida), and
crabs, spiders and insects (Arthropoda). In these forms the
general functional significance of the ganglia is the same as in the
flatworms, but the arrangement and specific forms of the ganglia
are influenced by a morphological factor, segmentation, that is
absent in flatworms. A segmented worm, such as an earthworm,
differs from a flatworm, among other things, in that the body of
the earthworm is composed of a linear series of parts, called
segments or metameres, of which mention was made in connection

THE NERVOUS SYSTEM 171

with the discussion of the excretory and reproductive systems of
this form. Save for those at the anterior and posterior ends, the
segments of the earthworm are practically alike. Some light on
the significance of segmentation may be obtained from the
manner in which segments originate. The first step consists in
the constriction of the unsegmented embryo into anterior and
posterior parts, of which the latter represents the posterior seg-
ment of the fully developed worm. New segments are budded
from the posterior border of the anterior part until the full
number is formed, which in Lumbricis terrestris is between 140
and 150. If segmentation be regarded as a sort of imperfect
fission, a segmented animal really represents a string of individ-
uals united end to end, an idea that is born out by the fact that
each segment is provided with similar organs. Actually, of
course, the segments are parts of a whole in which the indi-
viduality or independence of the segments is lost or obscured.
That segmentation among annelids is a relatively primitive char-
acter is indicated by the fact that segments are strikingly similar.
In an insect, such as the grasshopper, the primitive segmentation
is obscured by a regrouping to form larger body divisions such as
head, thorax, and abdomen. The type of segmentation found in
annelids is called homonymous or homodynamous segmentation;
that in insects heteronymous segmentation. Within limits, the
greater the difference between the segments of the body of an
animal, the less independence is retained by individual segments.
Thus if an earthworm is cut in two through the middle, the
anterior half is capable of living by itself and is able to regenerate
some of the missing parts. If an insect is cut in two, both parts

die.

The nervous system of the earthworm consists of two general
regions, (1) a fused pair of relatively large superior ganglia
located in segment 3 on the dorsal side of the pharynx and, (2)
ventral ganglionated nerve cord extending backward from segment
4 through the length of the body (Fig. 112). The ventral nerve
cord really consists of a series of ganglia, one to a body segment,
joined together by a double nerve cord, partially fused. The
superior ganglia are connected with the anterior ganglion of the
ventral chain by a nerve cord on either side of the pharynx.
The pharynx at this point is thus encircled by a nervous ring.
The superior ganglion innervates the anterior three segments

172

GENERAL ZOOLOGY

and the prostomium, an incomplete segment overhanging the
mouth. Its large size is correlated with the higher development
of the sensory field at the anterior or head end of the animal as
compared with other portions of the body. The cephalic sensory
field in some annelids, such as Nereis, a marine polychaete worm,
is more highly developed by the presence of eyes, palps, etc.
(Fig. 242). The ventral nerve cord with its ganglionic enlarge-
ments is primarily a segmental structure, in which correlation
between segments is effected by longitudinal connections
between the ventral ganglia (Fig. 113).

A pair of superior ganglia is a constant feature of the nervous
system of invertebrate animals provided with a ganglionic
nervous system. Where the segmentation is of the heteronymous

Fig. 112. — The nervous system of earthworm, diagrammatic, c, nerve com-
missure encircling pharynx; i, inferior ganglion; s, superior ganglion.

type, such as occurs in insects, the ganglia of the ventral chain
become reduced in number and varied in size, in accordance with
the extent of the modification of the primary segmentation.
Thus in the grasshopper the head contains a superior ganglion
(supraesophageal ganglion) and an infraesophageal ganglion,
connected by nerve cords on either side of the esophagus. The
infraesophageal ganglion is the anterior unit of the ventral nerve
cord which is provided with three ganglia in the thorax and five in
the abdomen (Fig. 53). The cord connecting these ganglia is
double in the thorax and single in the abdomen. The total
number of ganglia in the ventral chain is considerably less than
the number of primary body segments of the region supplied by
the ganglion. In such insects there is then a definite tendency
toward centralization of nervous control. In other arthropods,
such as certain crabs, this tendency is carried even farther, the
entire ventral chain being represented by a single large ganglion.

THE NERVOUS SYSTEM 173

Vertebrate Nervous System. — The central nervous system of
vertebrates, consisting of the brain and spinal cord, is a hollow

1 i t

Fig. 113. — Portion of ventral cord of the earthworm with two ganglia and six
pairs of lateral nerves. The sensory nerves end in terminal branches which con-
nect with the dendrites of the motor and coordinating nerve cells of the cord.
{From Calkins, Biology, Henry Holt & Company, Inc., after Retzius. By permission.)

structure that develops from an embryonic neural tube. The
embryonic primordium of the vertebrate nervous system consists

174

GENERAL ZOOLOGY

of a portion of the ectoderm covering the dorsal side of the
embryo which is called the neural plate. In most vertebrates
the first step in the conversion of the neural plate into a neural
tube consists in the elevation of the lateral boundaries of the
neural plate into approximately parallel folds or ridges. The
crests of these ridges grow toward each other, meet, and fuse
in the mid-line, thus forming a tube which soon separates from
the under side of the reunited ectoderm (Fig. 114). A slight
variation of this occurs in some of the lower vertebrates (cyclo-
stomes, teleosts, and ganoids) in that the neural tube is derived

D E F

Fig. 114. — Three stages in the development of the nervous system of the
salamander, Ambystoma. A, dorsal view of the embryo with open neural folds;
B, edges of neural folds meeting in the mid-line; C, neural tube completely formed;
D, cross section of open neural folds; E, cross section of closing neural folds; F,
cross section of spinal cord of C, showing the cord as a tube pinched off from the
ectoderm which has reunited above, fb, forebrain; hb, hindbrain; mb, mid-
brain; sc, spinal cord.

from a thickened keel formed in the mid-line of the neural plate on
the under side. This keel acquires a lumen and becomes sepa-
rated from the ectoderm above. In all cases, then, a neural tube
is formed in vertebrates, and this neural tube develops into the
brain and spinal cord. The neural tube also gives rise to all the
neurons of the body except those of the olfactory nerve and some
elements of the ganglia of cranial nerves V, VII, VIII, IX and X,
that come from the ectoderm.

The walls of the anterior end of the tube thicken and differ-
entiate into the embryonic fore-, mid-, and hindbrain, while the
posterior portion of the tube becomes the spinal cord. The
cavity of the neural tube in the region of the brain is represented
by the ventricles and in the spinal cord by the central canal.
Both ventricles and spinal canal are filled with cerebrospinal

THE NERVOUS SYSTEM

175

fluid, which resembles lymph but contains more water. Cross
sections of fresh brain and cord show that they are composed
of white and gray matter. The gray matter is made up largely
of nerve-cell bodies, the white matter of myelinated or medul-
lated nerve fibers.

Brain and Spinal Cord. — The three primary divisions of the
embryonic vertebrate brain give rise to five regions of the adult
brain as follows: (1) the telencephalon, composed of paired lobes,
the cerebral hemispheres, are dorsolateral outgrowths of the fore-

Fig. 115. — Diagram of vertebrate brain. Above, dorsal view showing ventri-
cles; below, median view of right half of brain, d, diencephalon; f, foramen of
Monro; h, hypophysis attached to infundibulum, i; mt, metencephalon; my,
myelencephalon; o, olfactory tracts; ol, optic lobes of mesencephalon; p, pineal
body; t, telencephalon; 1 to 4, ventricles.

brain; (2) the diencephalon, or twixt brain, develops from the
remainder of the forebrain; (3) the mesencephalon, whose walls
form the optic lobes, is the fully developed midbrain; (4) the
metencephalon or cerebellum, arises as a dorsal, unpaired out-
growth of the anterior end of the hindbrain; and (5) the myel-
encephalon or medulla oblongata forms from the remainder of the
hindbrain (Fig. 115). The cerebellum of the frog is poorly devel-
oped and appears as a transverse ridge behind the optic lobes.
In higher vertebrates there is a tendency for the cerebral hemi-
spheres and the cerebellum to increase in size. In the adult
vertebrate, the brain is enclosed in the cranium of the skull, and
the spinal cord in the vertebral canal formed by the neural arches
of the vertebrae.

176 GENERAL ZOOLOGY

The cavities of the cerebral hemispheres constitute the first
and second ventricles, each of which is connected by a narrow
passage, the foramen of Monro, with the third ventricle, which is
the cavity of the diencephalon. A narrow channel, called the
iter or aqueduct, extends through the mesencephalon, connecting
the third ventricle with the fourth ventricle lying in the medulla.
The roof of the fourth ventricle is very thin and is easily torn in
dissecting the brain. The cavity beneath it is broad and shallow.
In lower vertebrates the iter is expanded dorsally to form the
mesocoel in the region of the mesencephalon.

Pituitary Gland. — The pituitary gland of the adult vertebrate
is composed of two general regions, known as the anterior and
posterior lobes. The anterior lobe, or hypophysis, develops as an
evagination of the dorsal part of the embryonic oral cavity. The
posterior lobe is formed from an evagination of the floor of the
diencephalon known as the infundibulum. In all vertebrates
except some of the cyclostomes the hypophysis loses its connec-
tion with the oral cavity and unites with the infundibulum, which
remains connected with the brain throughout life. Thus the
anterior lobe of the pituitary gland is derived from the oral
epithelium and the posterior lobe from the neural tube. The
intermediate lobe of the pituitary gland is proliferated from cells
of the anterior lobe and forms a more or less distinct part between
the anterior and posterior lobes.

Parietal Organ and Pinealis. — Two structures, varying con-
siderably in different vertebrates, develop from the roof of the
diencephalon. The more anterior of these, the parietal organ, is
best represented in the lower vertebrates and lizards, in some of
which it is provided with a lens and retina, though it has not been
established that visual function is present. In other vertebrates,
such as amphibia and mammals, the parietal organ is more
rudimentary or lacking. The pinealis is a slender outgrowth in
the mid-line, posterior to the parietal organ. In some of the
lower vertebrates its structure is like that of an eye, but in the
higher forms it takes on a glandular character. In the develop-
ment of the pinealis of the frog, the distal end is cut off to form the
brow spot. Apparently the parietal organ and the pinealis
represent degenerated median unpaired eyes.

Meninges.— The brain and spinal cord of the frog are sur-
rounded by two connective tissue membranes called the meninges.

THE NERVOUS SYSTEM 111

Of these, the inner vascular layer corresponds to the pia mater
and arachnoid membrane of higher forms. The outer tougher
layer, the dura mater, consists of two sheets separated by an
interdural space filled with lymph. The roof of the diencephalon
in front of the pinealis, and also the roof of the medulla, is folded
in by the invasion of blood vessels of the pia mater to form the
anterior and posterior choroid plexuses, respectively. Such
choroid plexuses occur in vertebrates generally and are probably
the source of the cerebrospinal fluid, which fills the cavities of the
brain and spinal cord. In man this fluid may leave the brain
through minute passage ways in the roof of the medulla and join
the fluid in the meningial space about the pia mater, thus pro-
viding a means for circulating the fluid. The cerebrospinal fluid
is to be regarded as part of the circulatory mechanism of the
brain and cord. In addition it also serves as a protective
cushion of fluid.

Cranial Nerves. — The brain is connected with different parts
of the head and adjacent regions by means of cranial nerves.
In Amphibia and lower vertebrates there are 10 pairs of such
nerves (Fig. 116), while the higher vertebrates (reptiles, birds, and
mammals) have two additional pairs, making 12 in all. Some
of the cranial nerves are composed of sensory fibers only, some of
motor, and others of both sensory and motor. These nerves are
known by name or by number, as follows:

I. Olfactory nerve, sensory in function, arising as axons of
sensory cells located in the olfactory epithelium of the nose and
terminating in the olfactory lobe of the telencephalon.

II. Optic nerve, sensory in function, arising in the retina of the
eye, from which it extends across the ventral surface of the
diencephalon in front of the pituitary gland to the optic lobes.
In all vertebrates except mammals there is a crossing over of the
fibers of the optic nerve at the optic chiasma, those from the
left eye going to the right side of the brain and vice versa. In
mammals the crossing is incomplete, some fibers remaining in
the side of origin.

III. Oculomotor nerve, motor in function, arising from the ven-
tral surface of the mesencephalon and supplying four of the
extrinsic eye muscles: the superior, inferior, and internal recti
and the inferior oblique muscles. A branch also goes to the
ciliary ganglion of the eye.

178

GENERAL ZOOLOGY

cer

Lopt.

S.&X

Fig 116.— Nervous system of frog showing cranial and spinal nerves and
sympathetic system, ventral view. I to X, cranial nerves; cer cerebrum; Lopt
optic lobe; n, nasal sac; s, sympathetic system; spc, spinal cord. (Prom btiull,
LaRue, and Ruthven, Animal Biology, after Wiedersheim.)

THE NERVOUS SYSTEM 179

IV. Trochlear nerve, also motor, arising from the dorsal surface
of the hinder margin of the mesencephalon and passing to the
superior oblique muscle of the eye.

V. Trigeminal nerve, a large nerve arising from the antero-
lateral angle of the medulla and dividing in the higher verte-
brates into three main trunks : the ophthalmic, the maxillary, and
the mandibular. The first two are sensory in function and supply
the skin in the region of the eye, the roof of the mouth, the teeth
of the upper jaw, etc., while the mandibular branch, composed
of both sensory and motor fibers is distributed to the jaw muscles,
the tongue, teeth of the lower jaw, and adjacent parts. Near the
point where it leaves the brain the trigeminal nerve has a semi-
lunar (Gasserian) ganglion.

VI. Abducens nerve, a motor nerve arising from the ventral
surface of the medulla and innervating the external rectus muscle
of the eye.

VII. Facial nerve, a mixed motor and sensory nerve, arising
from the medulla posterior to the fifth nerve. In the lower
vertebrates, it has two ganglia, the geniculate and the lateralis,
but the latter in lung-breathing forms may disappear or unite
with the geniculate or with the semilunar ganglion of nerve V.

VIII. Auditory nerve, sensory in function, consists in mammals
of a vestibular branch from the utriculus and a cochlear branch
from the cochlea, each from a separate ganglion, which unite to
form a single trunk to the medulla.

IX. Glossopharyngeal nerve, a mixed motor and sensory nerve,
extends from the medulla to the pharynx and tongue principally.
It has a large petrosal ganglion near its junction with the brain.

X. Vagus nerve, also a mixed motor and sensory nerve, extend-
ing from the medulla to the esophagus, stomach, heart and other
viscera. The jugular ganglion and, in gill breathers, a lateralis
ganglion, occur near its origin. In higher forms the intestinal
branch of the vagus has a nodose ganglion.

XI. Spinal accessory nerve, a motor nerve from the medulla,
innervates the muscles of the pectoral region.

XII. Hypoglossal nerve, also a motor nerve from the medulla,
is distributed in the muscles of the neck and tongue.

Long after the 12 cranial nerves were described and named,
another pair was discovered at the anterior end of the brain.
This nerve which should really be first in the series is known as

180 GENERAL ZOOLOGY

the nervus terminalis. It has been found in all vertebrates but
is easily overlooked because of its inconspicuous size which
accounts for its remaining undiscovered until a relatively recent
time. It extends from the brain near the olfactory nerve, to
the olfactory epithelium. It is provided with a ganglion and is
undoubtedly a sensory nerve, but nothing definite is known of
its function.

The nerve cell bodies of the cranial motor nerves are located in
the brain. The nerves themselves are composed of axons from
their centrally located cell bodies. There is less uniformity in
the location of the cell bodies of the sensory nerves. The cell
bodies of the olfactory nerves lie in the olfactory epithelium,
which morphologically is at the surface of the body. The olfac-
tory nerves are composed of bundles of axons extending from the
olfactory epithelium to the brain. The nerve cell bodies of the
optic nerve are located in the retina of the eye from which axons
pass to the brain. The sensory components of the fifth, seventh,
eighth, ninth, and tenth nerves arise from cell bodies in the
ganglia of these nerves. From the ganglia dendrons extend to
the periphery and axons toward the brain.

Spinal Nerves. — The spinal cord is connected with peripheral
regions of the body, below the level of the head, by paired spinal
nerves. These pass from the cord through openings between the
vertebrae. The total number of spinal nerves varies in different
animals — 10 pairs in the frog, 31 in Man, 42 in the horse, etc.
Each spinal nerve is attached to the cord by two roots, a dorsal
root provided with a ganglion and a ventral root without a ganglion.
The ganglion is composed largely of nerve cell bodies whose axons
pass into the cord and whose dendrons pass out to the periphery,
as fibers from the dorsal root. The ventral root is com-
posed of axons whose cell bodies lie in the cord. At their points
of attachment to the cord each root splits up into smaller strands
spread fanwise along the cord. The dorsal root, except in some
of the lower vertebrates, is purely sensory in function. In the
exceptional cases, efferent fibers arising from nerve cells within
the cord pass out through the dorsal roots along with the sensory
fibers. The ventral root in all cases is purely motor. The two
roots unite to form a spinal nerve which almost at once divides
into three divisions: (1) a ramus dorsalis to the skin and muscles of
the dorsal region of the body; (2) a ramus ventralis to similar

THE NERVOUS SYSTEM

181

structures in the sides and ventral parts of the body; and (3) a
ramus communicans, penetrating the coelomic cavity, but
remaining retroperitoneal, just lateral to the attachment of the
mesentery, where it forms connections with the autonomic
nervous system (Fig. 117). The rami communicantes may be
double, in which case one is a white ramus and the other gray.

Fig. 117. — Diagram showing relations between spinal cord, spinal nerve and
ganglia of autonomic nervous system, c.g., chain ganglia; g, spinal ganglion;
g.m., gray matter of spinal cord; g.r., gray ramus communicans; m.r., motor
root of spinal nerve; pe.g., peripheral ganglion; pr.g., prevertebral ganglion;
r.d., ramus dorsalis; R.v., ramus ventralis; s.r., sensory root of spinal nerve;
w.m., white matter of spinal cord; w.r., white ramus communicans; broken lines
represent postganglionic fibers.

This difference is due to the fact that the fibers of the white ramus
are medullated and those of the gray are nonmedullated. Both
kinds of rami are not present in all spinal nerves. Thus in Man
each spinal nerve has a gray ramus and nearly all except the
cervical spinal nerves have, in addition, a white ramus
communicans.

The functional significance of the dorsal and ventral roots of
the spinal nerves of a mammal can be demonstrated by experi-

182 GENERAL ZOOLOGY

ments. If a dog is anesthetized and the ventral roots of the
spinal nerves supplying the forelimb are cut, the animal loses
control of the muscles supplied by these nerves, though sensitivity
to pain in the skin of the limb remains. If the dorsal roots are
cut instead of the ventral, sensation in the limb is lost, but
ability to move the muscles remains. If both dorsal and ventral
roots of the limb nerves are cut, stimulation by an electric current
of the distal end of the ventral root causes contractions of
muscles in the limb, while stimulation of the central end of the
cut ventral root produces no effect. Nor is any effect produced
in the limb by stimulating the distal end of the cut dorsal root;
but stimulation of the central end of the same root causes
symptoms of pain. The dorsal root is therefore sensory in func-
tion and the ventral root motor.

Autonomic Nervous System. — The autonomic nervous system
consists of (1) a pair of ganglionated nerve cords, one on each side
of the mid-line of the dorsal wall of the body cavity and extending
anteriorly into the cervical region; (2) a number of outlying gang-
lia, some of which are included within organs; and (3) nerve fibers
forming connections between the ganglia and between the ganglia
and the central nervous system. In Man the chain ganglia are
composed of 3 cervical, 12 thoracic, 4 lumbar, and 4 sacral
ganglia, though variations in these numbers are not uncommon.
The white rami communicantes consist of both afferent and
efferent fibers derived from dorsal and ventral spinal nerve roots,
and extending from spinal nerves to the autonomic ganglia; some
to the chain ganglia and others directly to more distal ganglia.
Of these there is a thoracolumbar group from the first or second
thoracic nerves to the second or third lumbar nerves and a
sacral group from two or three of the sacral nerves. There is also
a cranial group from cranial nerves III, VII, IX, and X. The
thoracolumbar group connects with the chain ganglia and with
the coeliac, superior, and inferior mesenteric ganglia. The sacral
and cranial groups have no connections with the chain ganglia
but pass directly to various organs and tissues. Nerve fibers
arising in the central nervous system and passing to chain
ganglia or to other ganglia of the autonomic nervous system are
called preganglionic fibers because their terminations connect
with other ganglia, whose fibers (postganglionic) innervate the
tissue. Gray rami communicantes connect the chain ganglia

THE NERVOUS SYSTEM 183

to each of the spinal nerves. Association cords, made up of both
white and gray fibers, connect each set of chain ganglia in a
longitudinal direction.

The autonomic nervous system is composed of two parts; the
sympathetic or ortho 'sympathetic and the parasympathetic divisions
(Fig. 118). The sympathetic division includes all of the chain
ganglia, the coeliac, superior, and inferior mesenteric ganglia, the
rami communicantes of the thoracolumbar group of spinal
nerves, and their preganglionic and postganglionic continuations.
The parasympathetic division is made up of the visceral branches
of two or three of the sacral spinal nerves and visceral branches
of cranial nerves III, VII, IX, and X, all of which pass directly
to certain structures such as the ciliary ganglion of the eye, the
cardiac ganglion of the heart, the myenteric plexuses of the gastro-
intestinal tract, etc., where they connect with postganglionic
fibers that in turn innervate the tissue. Thus in both the
sympathetic and parasympathetic divisions, the preganglionic
fibers originate in the brain or cord and the postganglionic fibers
in chain or peripheral ganglia. These are the efferent pathways.
Afferent pathways to the central nervous system are provided
by fibers from spinal-ganglion neurons that accompany the pre-
ganglionic fibers. The white rami communicantes are composed
of efferent and afferent fibers originating in the spinal cord or the
spinal ganglion. The fibers of the gray rami originate in chain
ganglia of the autonomic system.

The autonomic system is concerned with the coordination and
control of involuntary functions such as the beating of the heart
and activities of the viscera in general, including blood vessels.
With the exception of pain, fullness of the bladder or rectum,
most of these activities do not enter consciousness to any extent.
Actually the autonomic system is not completely independent
of the central nervous system since all of its preganglionic fibers
begin in the cord or brain and its afferent pathways end there.
It is autonomic in the sense that most of its functional activity
takes place without interference from the central nervous system.

Reflex Action. — A reflex action is an immediate involuntary
response to a sensory stimulus. Such responses are invariable
and can be predicted as a consequence of applying a definite
stimulus at a certain point. The simplest kind of reflex arc
accounting for such a result would consist of a sensory neuron

184

GENERAL ZOOLOGY

leading to a motor neuron, passing in turn to the effector.
Actually at least one or more intermediate neurons, between the

Oculomotor nerve

midbrain

medulla

ciliary 4.

■pheno palatine g.

b maxillary g.

bmaxillary.
blingual glands.

t-iZsmall intestine
'.adrenal

' large intestine

'■ kidney bladder
reproductive organs

Fig. 118. — Diagram showing relations between central nervous system and
autonomic nervous system. Postganglionic fibers of sympathetic system are
represented by broken lines; those of the parasympathetic system by dotted
lines. The short broken lines passing to the left of the chain ganglia represent
postganglionic fibers passing to sweat glands and to arrector pili muscles. (After
Meyer and Gottlieb.)

principal afferent and efferent neurons, are involved as well.
An example of a simple type of reflex action is the knee jerk.

THE NERVOUS SYSTEM 185

If the knees are crossed or if the leg is allowed to dangle freely
from a chair or table, a tap on the patellar ligament, just below
the knee cap, causes the foot to be jerked forward by the con-
traction of the quadriceps femoris muscle of the thigh. The
peripheral nerve connections involved are the second and third
lumbar spinal nerves, which provide the afferent pathways from
the skin and ligament to the spinal cord, and also the efferent
pathways from the spinal cord to the thigh muscle. In addition
there are central pathways consisting of intermediate neurons in
the spinal cord that form an integral part of the nervous circuit.
The entire nervous pathway along which an impulse is trans-
mitted to and from the central nervous system is a reflex arc.
In the cord or brain a sensory neuron may connect with several
motor neurons and its stimulation may thus bring about response
in a number of effectors. Intermediate neurons connecting
sensory and motor neurons of the same side are called association
fibers; if they connect sensory neurons of one side with motor
neurons on the opposite side, they are called commissural fibers.
Thus through commissural and association pathways sensory
impulses may affect motor neurons of the same or opposite side,
or of different levels. Reflexes occur not only in the central
nervous system but also in the peripheral ganglia of the auto-
nomic system. The reflex is thus the functional unit common to
all parts of the nervous system.

Synapse. — The axon of a neuron may terminate in an effector,
such as muscle, or it may connect with the dendrites of another
neuron. Whether the connection between two neurons is merely
one of contact or actual fusion is a question about which much
has been written. The term synapse refers to the functional
connection between two neurons by means of which the impulse
is transferred from one to the other.

Functional Significance of the Central Nervous System. — The
cell bodies of the sensory neurons are located in the spinal ganglia,
in the ganglia of the cranial nerves, in the olfactory epithelium,
and in the retina of the eye. The sensory fibers make connections
through intermediate neurons with motor neurons. The brain
and spinal cord are composed of tracts or groups of neurons, all
of the fibers of a tract usually having similar functions. In sim-
ple reflexes, the cord alone may be involved in distributing sen-
sory impulses to motor neurons. In other cases the sensory

186 GENERAL ZOOLOGY

impulses may be carried to the brain by ascending tracts and
back by descending tracts to spinal motor neurons, the nature
of the response being altered or controlled as a result of this
special routing. Responses, under such conditions show the
effect of past experience, as for example in conditioned reflexes.
Shining a light into the eye causes a contraction of the pupil, but
ringing a bell does not. If a bell is rung each time light is
directed into the eye, after a while ringing the bell will cause the
pupil to contract without the stimulus of the light. The spinal
cord is composed principally of relatively simple reflex arcs and
longitudinal tracts that connect these arcs with the brain.

Consciousness seems to be a function principally of the cere-
brum as a whole. The reactions of a person to a pin prick may
be the same when he is asleep as when he is awake, but when
asleep the stimulus is not perceived. Intelligence, memory,
interpretation of sensations are all general functions of the cere-
brum. Localized areas, such as the "motor areas," are con-
cerned with the control of individual muscle movements. Thus,
if the area of the cerebrum controlling the movement of the foot
and leg muscles is stimulated, the response is a coordinated
movement, entirely unlike the uncoordinated movements
produced by directly stimulating the peripheral nerves supplying
these parts. There are many such "centers" in the cerebrum
and also in other parts of the brain. The location of such "cen-
ters" in the brain is the principal difference between the brain
and the cord.

Sense Organs. — Sense organs are specialized in their sus-
ceptibility to different kinds of stimulation and are classified
accordingly. Since the matter of sensation also enters into the
problem, more is known of human sense organs than of those of
other animals, where one is limited to studying the responses — ■
muscular or glandular activity — and where one can know but
little of the sensation experienced by the animal. The best we
can do is to compare their responses with our own under similar
conditions. Similarity in the structure of the sense organs of
vertebrates is fair presumptive evidence of similar function and
of similar, though not necessarily identical, sensations. A classi-
fication of sense organs for animals generally is patterned on a
classification of human sense organs. These include the follow-
ing types of receptors: (1) tactile, (2) pain, (3) warmth, (4) cold,

THE NERVOUS SYSTEM

187

-cu.

(5) proprioceptive, (6) chemical (taste and smell), (7) sound, (8)
light, and (9) gravitation (equilibrium).

Tactile Organs. — The sensitivity to touch that characterizes
the integument of animals generally is brought about by the
presence in the integument of tactile
organs developed in connection with
nerve endings. The tactile organ itself
is usually composed of nonnervous
tissue. Among invertebrates the com-
monest type of tactile organ is the
tactile hair, examples of which are found
in arthropods (Fig. 119). Such a tactile
hair consists of a core formed of epider-
mal cells, covered by a thin cuticle, the
distal end of which tapers to a delicate
point. The base of the tactile hair is in
contact with a nerve ending so that any
movement of the hair serves as a stimu-
lus for the nerve ending. In mammals,
including man, the hair follicles are sup-
plied with sensory nerve endings that
are stimulated by movements of hairs.
Where hairs are lacking, tactile corpus-
cles of Meissner serve as end organs of
touch in the skin. These are elliptical
structures composed usually of several
nerve fibers enclosed by a capsule of
nonnervous cells and are located in the
subcutaneous tissue (Fig. 120). They

r,v.end.__

Fig. 119. — Tactile end-or-
gan of a nerve fiber in the
tactile hair of a shrimp, Palac-

monetes. cu., cuticle; hyp.,

are numerous in the finger tips, the hypodermis, which is invagi-
soles and palms, the nipples of the J*- •* ^'"l"

mammary glands, the lips, and Other the hair; nv.end., nerve ending
u • r„ „ „„~„ , ^f +1-.,-. ol^i.-. in the lower part of the hair.

hair-free areas of the skin. _ _ (From Dahlgvren and Kepner,

Pain Receptors. — Sensation of pain Principles of Animal Histoi
can be aroused everywhere in the
human skin by pricking with a sharp
needle. The receptors for cutaneous pain are thought to be
naked axis cylinders of sensory nerves distributed in the lower
layers of the epidermis. The nerves involved are sensory com-
ponents of cranial and spinal nerves. Some internal structures

ogy, copyright, The Macmillan
Company. By permission.)

188

GENERAL ZOOLOGY

lack pain receptors. Thus surgical operations involving cutting
through the skull and removing parts of the brain, can be per-
formed under local anesthesia without causing pain. In such
cases the anesthetic produces insensibility in the scalp but not in
the deeper structures. Cutting or cauterizing the intestine causes
no pain, though pain is produced in the peritoneum by the disten-
tion of intestine by gas or by the excessive contractions of the
muscle in the intestinal wall. In general, since pain is a symptom
of danger threatening life, the adaptive significance
of the presence of pain receptors throughout the skin
can be understood.

Warmth and Cold Receptors. — If the human skin
is explored with warm or cold thin metal rods, it is
found that some spots respond with a sensation of
warmth and others with a sensation of cold. Evi-
dence has been produced to show that the receptors
for warmth and cold consist of two distinct types of
receptors, viz., the corpuscles of Ruffini for warmth
and the corpuscles of Krause for cold. Human skin
is composed of a great number of very small sensory
areas, each of which is especially related to one or
more of the sensations of touch, warmth, cold, and
pain. Areas concerned with one sensation are
everywhere mingled with areas concerned with
others. It has been estimated that there exist in
the skin of the trunk and limbs 30,000 warmth spots,
25,000 cold spots and 500,000 touch spots, with pain
spots present everywhere.
Proprioceptors. — Corpuscles of Pacini, ovoid laminated bodies
with a nervous core, resemble in their general structure the
corpuscles of Meissner. They are found near joints and liga-
ments, and are thought to be receptors for proprioceptive sensa-
tions caused by movements of one bone upon another, such as
accompany voluntary muscular movements. Other types of
receptors, such as muscle and tendon spindles, composed of
spirally coiled nerve endings, are also concerned in proprioceptive
sense. Neuromuscular spindles must not be confused with the
end plates of motor nerves which terminate in muscle fibers.
The latter cause the muscle to contract when the motor nerve, of
which they are the terminations, is stimulated. When the

Fig. 120 —
C o r p vis cl e of
Meissner, dia-
grammatic.

THE NERVOUS SYSTEM

189

muscle contracts, the muscle spindle is stimulated and produces
eventually the sensation of movement. In other words, the
muscle and tendon spindles are the terminations of sensory nerves
and the motor end plates of motor nerves (Figs. 121 and 122).

Taste Receptors. — A sense of taste is probably present in all
animals, but since the end organs of both taste and smell are
activated by chemical stimuli, it is often difficult in lower animals
to differentiate between them. Thus, if a piece of crushed clam
be placed near a burrow of Nereis virens, a marine annelid, the
animal responds by thrusting its anterior
end out of the burrow toward the food
which it seizes, pulls back into the
burrow, and devours. The animal is
evidently stimulated by the meat juices,
but whether by taste or smell or a com-
bination of both, it is difficult to decide.
Insects are provided with organs of taste
located on the mouth parts and the
antennae and in some cases at least on
the legs. The taste receptors of verte-
brates are known as taste buds (Fig. 123).
Each consists of a number of taste cells
provided with short bristles at their outer
ends and a number of supporting cells,
the whole forming a spherical body
buried in the surrounding tissue except
for a small area at the surface containing
a pore. The outer ends of the taste cells
converge at the pore. In fishes taste buds occur in the walls of
the pharynx, on the gills, on the outside of the body, and in some
(catfish) in the tail and the barbels. In higher vertebrates they
are restricted to the oral cavity, where they are found in the
tongue, soft palate, and epiglottis. The base of each taste cell
is in contact with a nerve ending from which the sensory impulse
is conveyed to the brain. In mammals the nerves involved in
the sense of taste are a branch (chorda tympani) of the seventh
cranial nerve and the lingual branch of the ninth.

Smell Receptors. — Olfactory organs are stimulated by chemical
action, but the amount of the stimulating agent required is very
much less than in the case of the taste receptors. Smell has been

Fig

121. — Pacinian corpus-
cle, diagrammatic.

190

GENERAL ZOOLOGY

aptly described as "taste at a distance." It has been established
that organs of smell occur in insects and in other invertebrates
such as snails. In the case of insects the antennae and other
parts of the body bear olfactory pits in which the receptors lie.
In vertebrates, except Cyclostomata, the olfactory organ con-
sists of paired sensory areas, which in fishes lie at the bottom of

Fig. 122. — A, neuromuscular spindle. B, neurotendinous spindle; C, motor
end plate, m, muscle; n, nerve; t, tendon. (After Huber and De Witt.)

ectodermal pits opening to the outside by nostrils, but having no
connection with the pharynx. In air-breathing vertebrates these
pits are continued backward as nasal passages, opening into the
pharynx by internal nares. This condition exists in the frog and
in Man. The sensory epithelium lining the passages consists of
olfactory cells and supporting cells, the former bearing bristles
at their outer ends, while their inner ends continue backward as
nerve fibers to form the olfactory nerve. Since olfactory cells
arise and remain in the ectoderm, the olfactory epithelium is

THE NERVOUS SYSTEM

191

said to be the most primitive nervous epithelium in the body
(Fig. 124).

Light Receptors. — Sensitivity to light is a characteristic of
protoplasm exhibited in its simplest form by organisms being
attracted or repelled by light. Phototaxis, as this reaction is
called, occurs even in forms that have no special organs for
perceiving light stimuli, such as many Protozoa and some
Metazoa, such as Hydra. Where special receptors exist, these
in lower forms are often merely mechanisms for distinguishing

£gTc/t<

if

Fig. 123. Fig. 124.

Fig. 123. — Section of a taste bud, diagrammatic. Four taste cells are shown
with their outer ends converging toward the pit opening on the surface above.
The inner ends of the cells are in contact with nerves entering the base of the bud.
The pale cells within the bud are supporting cells.

Fig. 124. — Section of human olfactory epithelium, diagrammatic. The slender
outer ends of the olfactory cells are ciliated. The process leaving the base of
each olfactory cell is a nerve fiber.

between degrees of light. Such animals "see" only in the sense
that they are capable of distinguishing light from dark. An
example of such an eye is found in Planaria gonocephala. In
this form the paired eyes are located in the head region, one on
either side of the cephalic ganglion. Each eye consists of a cup-
shaped structure lined with pigment, into which project the
processes of 20 or more visual cells, each process being somewhat
thickened at its distal end to form a rhabdome (Fig. 125).
Proximally the visual cells are continued as fibers that form the
optic nerves extending to the cephalic ganglion. The eye is
sunk in the tissues and covered by a transparent epithelium,
continuous with the ectodermal body covering. Light passes
through the transparent epithelium and through the cell bodies

192

GENERAL ZOOLOGY

of the visual cells until it strikes the rhabdomes, which are the
receptors of the eye. The nervous impulse set up by the light
stimulus then travels back through the visual cells to the central
nervous system.

Compound Eye. — Invertebrates show a
large variety of eyes which seem to have
image-forming possibilities. Of these the
most striking is the compound eye of
arthropods. In the crayfish and other
crustaceans these eyes are spherical in
shape and are mounted on the ends of
movable stalks. In an insect such as the

ft

~+ 1*3 "£? fit &

vis. r.

Fig. 125.

Fig. 125. — Axial section of the eye of Planaria gonocephala. vis. c, visual
cells; vis. r., visual rods or rhabdomes; nv. /., centripetal fibers of visual cells;
pg. c, pigment cells. (From Dahlgren and Kepner, Principles of Animal Histology,
copyright, The Macmillan Company, after R. Hesse. By permission.)

Fig. 126. — Longitudinal section of a single ommatidium of a roach, Periplaneta
orientalis. b. m., basement membrane; cu., cuticle divided into corneal areas;
rt. c, retinal cells; rh., rhabdome or cell organ of light perception; I., lens; I.e.,
lens cells; c. c, corneal cells; sup. c, supporting cells; nv. f., nerve fiber. (From
Dahlgren and Kepner, Principles of Animal Histology, copyright, The Macmillan
Company, after R. Hesse. By permission.)

grasshopper the eye forms a prominent rounded elevation on
cither side of the head. The outer surface of this type of eye
consists of a transparent cornea divided into thousands of poly-
gonal facets. In the crayfish the facets are quadrilateral in
shape and in the grasshopper they are hexagonal. Each facet is

THE NERVOUS SYSTEM 193

the base of a narrow cone-shaped structure, the ommatidium,
whose inwardly directed apex rests against a hemispherical
basement membrane. Each ommatidium contains a pear-
shaped lens supported by cells beneath the cornea. The remain-
der of the ommatidium is usually made up of seven elongated
retinal cells, extending from the lens to the basement membrane.
The surface of each retinal cell facing the cavity of the ommatid-
ium is provided with rows of transverse plates or rods, which
constitute a rhabdome. Since the plates or rods of the rhabdome
project at right angles to the axis of each retinal cell, they are in a
position to intercept light entering the ommatidium through the
cornea. The lower or inner ends of the retinal cells are pro-
longed into nerve fibers to form an optic nerve leading to the
central nervous system. Pigment cells occur between the cornea
and the outer edges of the lens and also about the retinal cells so
that the amount of light entering the ommatidium may be
controlled (Fig. 126).

The compound eye is provided with all the elements necessary
to produce an image of a mosaic type, each ommatidium in the
proper location with reference to the object forming a portion of
the total image. The number of ommatidia affected depends
upon the degree to which the ommatidia are sheathed by pigment
cells. When the pigment is extended about the ommatidia,
only those ommatidia will be stimulated whose axes are in line
with the source of light. When the pigment is withdrawn a
larger number of ommatidia will be affected by a single source of
light. Such an eye would seem to be especially adapted for
recording a moving object, since a change in position of the
object would affect in turn a larger number of ommatidia than
would be the case if the object were stationary.

Human Eye. — The eyes of vertebrates are definitely image-
forming organs, conforming to a common structural plan, though
differing in details. The general plan of structure can be illus-
trated by the human eye. The shape of the human eye is roughly
that of a sphere whose curvature in the region of the transparent
part in front is somewhat greater than elsewhere (Fig. 127).
The outer layer of the eye is made up of the transparent cornea
in front and the tough, opaque sclera or sclerotic coat, forming
the remainder. The visible portion of the sclera is the white of
the eye. Beneath the sclera lies the vascular choroid layer,

194

GENERAL ZOOLOGY

beneath which in turn lies the retina. The retina is composed
of an inner nervous layer and a thin pigmented layer in close
contact with the choroid. Neither the retina nor the choroid
layer extends in front beyond the line of junction of the cornea
and sclera. The lens is a doubly convex disk, composed of
transparent cells, enclosed in a thin tough capsule, held in place
by suspensory ligaments attached to the sclera along the line of
the forward edge of the choroid coat. The iris is a pigmented
muscular diaphragm with an opening, the pupil, in its center.

Fig. 127. — Diagram of a vertical section of human eye and eyelids, a, eye-
lash; b, lid; c, conjunctiva, a thin membrane continuous with the lining of the
eyelid; d, cornea; e, anterior chamber filled with aqueous humor; /, iris; g, lens;
h, muscles to ligament suspending lens; i, suspensory ligament of lens; k, chamber
filled with vitreous humor; I, retina; m, blind spot; n, fovea centralis; o, optic
nerve; p, choroid coat; q, sclera; r, superior rectus muscle of eyeball; s, inferior
rectus muscle.

It lies in front of the lens and is attached at its outer edge to the
ciliary process, which is a circle of plain musculature inserted in
the sclera between the corneosclerotic juncture and the choroid
layer. The lens divides the eye into an anterior region con-
taining a liquid, the aqueous humor, and a posterior region filled
with a jellylike vitreous humor. These two fluids keep the eye
distended. The iris divides the anterior region into anterior and
posterior chambers which are connected by the pupillary opening.
Retina. — The arrangement of the nervous elements of the
retina is highly complicated and only the barest outlines of their
histology will be considered. The light receptors of the retina
are the rods and cones, which are cells lying in the outer zone of
the sensory part of the retina and pointing toward the pigment
layer of the retina. Under certain conditions processes from the

THE NERVOUS SYSTEM

195

pigment cells extend inward between the rods and cones. From
the inner ends of the rods and cones processes extend to inter-
mediate neurons which in turn form synapses with ganglion cells
lying at the innermost surface of the retina (Fig. 128). Axons
from ganglion cells all over the inner surface of the retina con-
verge at the blind spot to form the optic nerve, which extends from
the eyeball to the brain. The blind spot is not in the direct line
of vision, being toward the nasal side of the center of the eye.

Fig. 128. — Diagram of human retina, a, cone cell; b, membrana limitans
externa; c, rod cell; d, stellate ganglion cell; e, bipolar cell;/, amacrine ganglion
cell (without axons); g, multipolar ganglion cell; h, nerve-fiber layer, at inner
surface of retina; i, subepithelial ganglion cell. (After Stohr.)

In a small area of the retina lying in the optical axis of the eye,
rods are absent and the other retinal layers are greatly reduced.
The slight depression at this spot is known as the fovea centralis,
which is the point of most acute vision. In the fovea centralis
cones alone are present. In other parts of the retina the rods
far outnumber the cones.

Light Perception. — Light enters the eye through the cornea,
which converges the rays, and then passes through the lens,
where the rays are further converged and brought together at
the focal point of the lens system. Before reaching the rods and

196 GENERAL ZOOLOGY

cones, which are the percipient elements of the eye, it is neces-
sary for the light to traverse the various layers of the retina lying
between them and the inner surface of the retina. The rods
and cones having been stimulated by the light, the nervous
impulse set up passes in the reverse direction from the rods and
cones to the fibers forming the optic nerve which convey the
impulse to the brain. The cones are concerned in both chromatic
and achromatic vision, while the rods function only in achromatic
vision. Thus colors are perceived by the cones alone.

Accommodation. — The human eye resembles a photographic
camera in many respects. The curved cornea, distended by aque-
ous humor, and the lens, provide a means for converging light
rays, as in a camera, and the sensitive retina corresponds to the
film or plate of the camera. Rays of light entering the eye from
a luminous object, or from an object reflecting light, are brought
to a focus at a point behind the lens, beyond which an image is
formed on the retina. The human eye possesses a range of vision
that makes it possible to see objects miles away or close at hand.
The power of accommodation for objects at varying distances is
due to the fact that the focal length of the lens can be changed.
In the eye of some molluscs and in the eye of fishes, accommoda-
tion is brought about by altering the distance between the lens
and the retina, as in a camera, where the lens is moved away
from the film for near objects and in opposite direction for more
distant objects. In these cases the focal length of the lens is
fixed. In the human eye, the same end is achieved by changing
the curvature of the lens rather than by moving the lens as a
whole. Thus in viewing near objects the curvature of the lens is
greater than when more distant objects are looked at. Increas-
ing the curvature of the lens shortens the length of the focus, and
decreasing the curvature lengthens it. According to the gener-
ally accepted explanation, this adjustment is brought about by
the contraction of the ciliary muscles. When the eye is relaxed
or unaccommodated, the suspensory ligament and the capsule
containing the lens are taut, and the front surface of the lens is
flattened, thus reducing its curvature, with the result that distant
objects are seen without effort. In viewing near objects the lens
becomes more convex as a result of the contraction of the ciliary
muscles which pulls forward the region of attachment of the
suspensory ligament to the eyeball, thus easing the tension on

THE NERVOUS SYSTEM 197

the ligament. The lens then thickens and increases its convexity
until checked by the tension of the capsule. The response of the
lens to changes in tension of the capsule and ligaments is due to
its elasticity. Loss of elasticity is a natural accompaniment of
old age and results in a loss of accommodation.

Acuity of vision also depends upon the amount of light enter-
ing the eye. This is controlled by the size of the pupil, which is
contracted in a bright light and enlarged in a dim light. The
"color" of the eye is determined by the amount and distribution
of the pigment in the iris. Dark-brown and black eyes have
pigment in the outer surface, through the stroma, and in the
inner surface of the iris; light-brown, gray, and green eyes have
less pigment in the outer surface; while blue eyes have pigment
only in the inner surface of the iris. The pigment is the same
substance in all cases.

Eye Muscles. — By means of six extrinsic eye muscles, the eye-
ball of vertebrates, including that of Man, can be moved about
in the orbit in all directions through a considerable arc. The
internal and external recti muscles move the eye from side to side ;
the superior and inferior recti muscles move it up and down;
and the superior and inferior oblique muscles rotate it. These
muscles have their origin in the wall of the orbit and are inserted
on the sclerotic surface of the eyeball. They are innervated by
cranial nerves III, IV, and VI. In the frog there are two addi-
tional eye muscles, the retractor bulbi, which pulls the eyeball
into the orbit, and the levator bulbi, which raises the eye.

Visual Purple. — The change undergone by the rods and
cones through which the nervous impulse is set up is generally
believed to be photochemical in nature. By this is meant that
light generates chemical changes in the rods or cones, similar in
a general way to the effect of light on a photographic plate or
film. This view is based on the fact that the outer segments of
the rods contain a reddish pigment, called visual purple or
rhodopsin that is bleached by light. This pigment is distinct
from the pigment of the pigmented layer of the retina, though
this layer seems to play some part in restoring the visual purple
in the rods following its destruction by light. It would seem,
however, that visual purple is not essential to vision since it is
absent in the cones, which means that it is absent from the
fovea centralis, the region of greatest visual acuity. It has been

198 GENERAL ZOOLOGY

suggested that the visual purple increases the irritability of the
rods in dim lights. The domestic fowl, the pigeon, and some
reptiles lack visual purple. It is present in the eye of the frog.

Sound Receptors of Invertebrates. — Among invertebrates the
only group in which organs of hearing are known with some
degree of certainty are the insects. Since some insects are pro-
vided with structures for producing sounds, it is reasonable to
assume that they also have means of perceiving sounds. Thus
grasshoppers produce a sound by rubbing the femur of the last
pair of legs against the anterior wings. (The femur is the large
segment of the leg near the body.) In grasshoppers, the organs
of hearing are thought to be the tympanal organs, located one on
either side of the body where the abdomen joins the thorax.
The tympanal organ consists of a thin chitinous membrane,
stretched over a shallow cavity and provided with nerve endings.
Sound waves impinging upon the membrane cause it to vibrate
and stimulate the nerve ending. Similar organs are found on
the anterior legs of crickets, katydids, and ants.

Gravitational Receptors of Invertebrates.— Gravitational
receptors provide an animal with a means of maintaining its
balance or equilibrium. Organs of this sort, known as stato-
cysts or lithocysts, are found in Crustacea, such as the crayfish,
lobster, shrimp, etc. The statocyst is a sac, located in the basal
segment of each antennule, open to the outside, and lined with
chitin. Projecting from the inner surface of the sac are sensory
hairs among which are found a few grains of sand, called stato-
liths. Changes in position of the body displace the statoliths
among the hairs, the differential stimulation of which gives the
animal a sense of its position in space. This interpretation of the
function of the statocyst is borne out by the fact that when
the animal molts — a process in which the entire chitinous covering
of the body, including the lining of the statocyst and the con-
tained statoliths, is lost — the animal for a time lacks full power
for maintaining itself right side up. The most convincing evi-
dence regarding the function of the organ comes from an experi-
ment in which shrimps newly molted and therefore without
statoliths were placed in water containing iron filings. In the
absence of sand grains to replace the discarded statoliths, the
experimental animals placed iron filings in the statocysts. When
an electromagnet of sufficient strength was brought near, the

THE NERVOUS SYSTEM 199

shrimp oriented itself with reference to the resultant of the
lines of force of the magnet and the pull of gravity as it normally
would to gravity alone. From this it would appear that under
natural conditions the normal position of the animal is determined
by the stimulation of certain sensory hairs in the statocyst by
the weight of the statoliths. In the experiment, when these par-
ticular hairs are stimulated, so far as the animal is concerned, the
resulting orientation is normal even though the animal is tilted
at a considerable angle.

Inner Ear. — Among vertebrates the inner ear serves both as
organ of hearing and as an organ of equilibration. It is present
in all vertebrates. Vertebrates higher than fishes also have a
middle ear, in addition to which mammals have an external ear.
The middle ear and external ear are part of the sound-conducting
apparatus employed in hearing.

The inner ear arises as an invagination of the ectoderm in the
region of the embryonic hindbrain to form the otic sac, which
later becomes embedded in the cartilage or bone of the skull and
completely separated from the outside except in the case of
elasmobranchs. Two regions develop in the otic sac; (1) the
utriculus, bearing three semicircular canals, and (2) the sacculus,
which in fishes is a rounded sac with a short appendage, the
lagena (Fig. 129 A). In the frog the lagena is short, but near it
another outgrowth, the basilar papilla, is formed. In the higher
vertebrates the lagena and the basilar papilla of the sacculus
enlarge to form the scala media or cochlear duct which in mammals
becomes a spirally coiled tube (Fig. 129B). The utriculus with
its canals is concerned with equilibration, while the sacculus, the
lagena, and its derivatives are concerned with hearing. Fishes
are practically deaf so far as discrimination of pitch is concerned,
which may be correlated with the slight development of the
lagena. Goldfish respond to vibrations under water. By
removing parts of the inner ear it has been shown that the
utriculus is sensitive to vibration rates of 43 to 488 per second,
and the sacculolagena to rates of 688 to 2,752 per second.

The utriculus and sacculus are connected by a sacculo-utricular
canal. The endolymphatic duct is a slender tube, closed at its
distal end, arising from the sacculo-utricular canal. In elasmo-
branchs it is lacking, but the cavity of the inner ear remains
connected with the outside by a slender duct, the invagination

200

GENERAL ZOOLOGY

canal, through which the inner ear is filled with sea water, which
serves the same function as the endolymph of higher forms.
The invagination duct is attached to the sacculo-utricular canal.
The various parts of the inner ear constitute what is known as
the membranous labyrinth. It is filled with endolymph contain-
ing otoconia, which are microscopic crystals of calcium carbonate.
The "ear stones" of fishes are really large otoliths. Areas of
sensory epithelia consisting of large cells with long hairs, known

ED

A B

Fig. 129. — A, diagram of membranous labyrinth of lower vertebrate. B,
membranous labyrinth of human embryo, 30 cm. in length, c, cochlea (scala
media); cn, cochlear nerve; ed, endolymphatic duct; l, lagena; s, sacculus; u,
utriculus; vn, vestibular nerve. (B after Streeter.)

as cristae acusticae, are located in the enlarged end (ampulla) of
each semicircular canal, while other areas of cells with shorter
hairs, called maculae acusticae, are found in the utriculus, sac-
culus, and scala media. In mammals, including Man, the bony
labyrinth is the casing of bone closely following the outlines of
the membranous labyrinth, but separated from the latter more or
less completely by a perilymphatic space filled with lymphlike
perilymphatic fluid. In the region of the spiral cochlea the
perilymphatic space does not completely surround the scala
media. Instead it is divided into two channels, one above and

THE NERVOUS SYSTEM

201

one below the scala media (Fig. 130), the two channels being
connected with each other only at the distal end of the scala
media. The upper channel is the scala vestibuli and the lower
one the scala tympani. The terms "upper" and " lower" are
used with reference to the cochlea, the apex of the spiral being
"above" and the base "below." The bony labyrinth is pierced

Fig. 130. — A, diagram of mammalian ear;
B, cross section of one of coils of cochlea.
an, auditory nerve; cn, cochlear nerve; d,
perilymphatic duct; e, Eustachian tube; em,
external meatus; g, spiral ganglion; i, incus;
M, scala media; ma, malleus; p, pharynx; r,
foramen rotunda; s, sacculus; st, stapes; t,
scala tympani; tc, tympanic cavity; tm, tym-
panic membrane; u, utriculus; v, scala vesti-
buli; vn, vestibular nerve. (Modified from
Kingsley, Comparative Anatomy of Vertebrates.
P. Blakiston's Son and Company. By permission.)

by two openings toward the middle ear: (1) the fenestra rotunda,
located at the base of the cochlea where the scala tympani
terminates, and closed by a membrane; and (2) the fenestra
ovalis, in the region (vestibulum) of the sacculus, and closed by a
bone of the middle ear, the stapes (Fig. 130A). The vestibulum
is an enlarged perilymphatic space about the sacculus. The only
passageway from the vestibulum to the fenestra rotunda is

202 GENERAL ZOOLOGY

through the length of the scala vestibuli to the apex of the
cochlea and then down through the scala tympani.

Scala Media. — The sound receptors are located in the scala
media or cochlear duct, which in man is triangular in cross sec-
tion (Fig. 1305). On its upper side the scala media is separated
from the scala vestibuli by the thin vestibular membrane, also
known as Reissner's membrane. The outer side of the scala
media is firmly attached to the wall of the bony labyrinth by the
spiral ligament. The floor of the scala media is formed by the
basilar membrane which, with a narrow bony shelf extending
from the axis of the bony cochlea, separates the scala media from
the scala tympani. The organ of Corti, resting on the basilar
membrane, is a complicated structure provided with sensory hair
cells, which are the sound receptors through which nerve fibers
of the cochlear branch of the auditory nerve are stimulated.

Middle Ear. — The middle ear is a chamber derived from the
first gill cleft of fishes. It is connected with the pharynx by
the Eustachian tube, by means of which air pressure within the
chamber is kept constant. Its inner wall is in contact with the
bony labyrinth at the fenestra ovalis and the fenestra rotunda.
Its outer wall is provided with a circular tympanic membrane,
which in the frog is visible externally at the side of the head.
From the tympanic membrane a chain of small bones extends
across the cavity to the fenestra ovalis. In the frog these con-
sist of a rod-shaped columella, extending from the center of the
tympanic membrane to a small cartilage lying in the fenestra
ovalis. In man three bones are present : (1) the malleus, attached
by an arm to the tympanic membrane; (2) the incus, extending
from the malleus to (3) the stapes, a stirrup-shaped bone fitting
into the fenestra ovalis. A middle-ear cavity is absent in snakes.

External Ear. — Mammals alone have external ears. The
external ear consists of (1) the pinna, supported by cartilages
and provided with muscles by means of which it may be moved,
and (2) the auditory meatus, a canal leading to the tympanic
membrane. The external ear increases the effectiveness of the
ear by collecting and directing sound waves against the tympanic
membrane.

Auditory Function. — Sound vibrations are transmitted from
the tympanic membrane across the middle ear by the ear bones
to the fenestra ovalis, where they are taken up by the perilymph

THE NERVOUS SYSTEM 203

in the vestibulum. Vibrations thus set up in the perilymph
travel up the scala vestibuli to the apex of the cochlea, and then
down the scala tympani to the fenestra rotunda, whose mem-
brane moves in and out with the vibrations. In passing the
scala media the vibrations in the perilymph are transmitted
through the walls of the scala media, eventually stimulating the
hair cells of the organ of Corti. Whether the vibrations reach
the hair cells through the vestibular membrane and endolymph
or directly through the basilar membrane remains an open
question.

Equilibration. — The semicircular canals serve as organs of
equilibration. Every movement of the head causes the endo-
lymph and the contained otoconia to stimulate the cristae
acusticae in the semicircular canals from which impulses are car-
ried to the brain by the vestibular branch of the eighth cranial
nerve. Since the three canals lie in planes at right angles to each
other, movement in any direction is certain to affect one of them.
The action of the fluid on the hairs can be illustrated by a simple
experiment. If a glass of water is given a sudden rotary twist
the glass moves, but the water tends to lag behind, so that if the
glass had contained fine hairs extending from its sides into the
water, these hairs would have been bent or otherwise disturbed
by the water. In a similar way, when the head moves, the
endolymph in the canal lying in the plane of motion tends to
remain stationary while the sensitive hairs are dragged through
it and stimulated. Thus means are afforded for recognizing the
direction and components of any motion. At the same time
sensory cells in other parts of the utriculus and sacculus are
stimulated by the endolymph.

Integrative Action of the Nervous System. — The first appear-
ance of the nervous system in the lower invertebrates, as has
been already pointed out, does not involve the creation of a new
functional activity so much as a specialization of the functions of
excitation and transmission, which are common properties of all
forms of life. Protozoa are sensitive to the same stimuli that
affect higher forms, yet they lack a nervous system. The
importance of the nervous system becomes more and more pro-
nounced with increase in size and complexity of the animal body,
because of the greater dependence on it as a means for providing
rapid excitation and transmission. The significance of the ever-

204 GENERAL ZOOLOGY

growing complexity of the central nervous system as the animal
scale is ascended, lies in its function of integration. A large
animal without an adequate nervous system would be merely a
mass of protoplasm incapable of finely coordinated activities.
The nervous system, with its ramifications connecting various
parts of the body with each other and with the brain and spinal
cord, provides a mechanism for coordination that brings about
an integrative effect, so that activities of widely separated parts
of the body function as parts of a whole.

CHAPTER X
THE ENDOCRINE SYSTEM

The internal secretion of an organ includes all substances
formed in the organ as a result of metabolic activity and removed
from the organ by the blood stream. In the case of glands pro-
vided with ducts there is an external secretion removed from the
organ through the ducts. Thus the liver takes from the blood
certain substances which, after undergoing various transforma-
tions, are secreted as bile through the bile duct into the intestine
and utilized in part in the digestion of fats. At the same time
the liver takes other substances from the blood and transforms
them into glycogen and urea, which return as sugar and urea to
the blood stream. Bile is the external secretion of the liver,
while sugar and urea are its internal secretions. Similarly, the
pancreas produces a substance that is necessary for the utiliza-
tion, by oxidation, of sugar, but this substance instead of being
poured through the pancreatic duct into the intestine along with
the pancreatic juice — its external secretion — is absorbed by the
blood as an internal secretion.

Endocrines or hormones are internal secretions, characterized
by their capacity to stimulate metabolic activity in other organs
or to influence the metabolism of the body as a whole in a rather
specific manner. It should be clear, however, that while hor-
mones are internal secretions, all internal secretions are not
hormones. Urea is an internal secretion of the liver but is not
a hormone, since it really represents one of the end products of
nitrogen metabolism that is removed from the circulation by the
kidneys.

Endocrines have been studied principally in vertebrate animals,
particularly amphibians, birds, and mammals. In these forms it
has been shown that normal metabolism of the body as a whole
depends upon contributions to the blood stream of specific sub-
stances, endocrines, from various organs. Since the effects are
produced through chemical means, the endocrine system repre-
sents a chemical method of coordinating and controlling metab-

205

206 GENERAL ZOOLOGY

olism. The fact that the various endocrine organs have definite
functional relations with each other makes it impossible to define
the functions of one type of endocrine organ without considering
the functions of all of them. This interlocking relationship
complicates the problem of understanding the system as a whole,
since new discoveries may make it necessary to correct and restate
what were seemingly well-founded conclusions. While this
might be said to be true of all scientific work, it is particularly
true of endocrinology because of the intense activity in this field
of study at the present time. For this reason only a general out-
line of the subject will be presented in these pages, the discussion
being confined to the known effects produced by the ductless
glands individually and collectively. These glands include the
pituitary, thyroid, parathyroid, thymus, adrenal, pancreas, and
gonads. The pineal gland is also classified as a ductless gland,
but since little is known of its function, it will not be considered
here.

Anterior Pituitary Gland. — The pituitary gland, attached to
the floor of the diencephalon, consists of two principal regions:
(1) the anterior lobe, or hypophysis, derived from the embryonic
oral epithelium, and (2) the posterior lobe, or infundibulum,
derived from the floor of the forebrain. A third region, the
intermediate lobe (Fig. 131), a band of tissue between the first
two, is derived from the hypophysis. That the anterior lobe of
the pituitary plays a dominant role in the endocrine complex is
indicated by the fact that it produces at least five distinct
endocrines or hormones, as follows: (1) somatotropic or growth-
stimulating; (2) gonadotropic, or gonad-stimulating; (3) thyro-
tropic or thyroid-stimulating; (4) lactogenic, or milk-producing;
and (5) adrenotropic, having a relation with the cortex of the
adrenal gland. Reciprocal functional relations exist between
the anterior pituitary lobe and the organs affected by its hormones.

Somatotropic Hormone. — The somatotropic hormone is neces-
sary for the normal growth and development of the body, its
absence or reduction resulting in some form of dwarfism. This
has been demonstrated experimentally by causing an arrest of the
growth of bones and other body tissues of mice and rats as a
result of removing the anterior pituitary lobe.

Overproduction of the growth hormone of the anterior pituitary
lobe in children produces gigantism and in adult life, acromegaly.

THE ENDOCRINE SYSTEM

207

Gigantism is caused by an overstimulation of the growth of bones,
particularly the long bones, during childhood, before the epiphyses
(cartilaginous zones near the ends of the long bones) have
ossified. A recent authenticated example of gigantism is that
of a boy 19 years old, weighing 435 lb. and measuring 8 ft.,
6 in. in height and still growing. Acromegaly also involves bone
development, but since it arises in adult life, after the length
of the bones has been fixed, the result is an increase in breadth
of the bones, particularly noticeable in the bones of the face,
hands, and feet. In both gigantism and acromegaly, post-
mortem examinations show hypertrophy (enlargement) of the

Fig. 131. — Cut surface of right half of pituitary gland and floor of diencephalon
of rat, semi-diagrammatic, a, anterior lobe; i, intermediate lobe; i.s., infundibu-
lar stalk; p, posterior lobe; 3, third ventricle. {From a preparation by L. B.
Hobson.)

anterior pituitary lobe. Gigantic rats and acromegalic dogs have
been produced experimentally by the injection of extracts of the
anterior pituitary lobe.

Go?iadotropic Hormone. — One of the results of removing the
anterior pituitary lobe of young mammals is the suppression of
gonad development. Conversely, implantation of anterior lobe
tissue under the skin or in the muscle of young rats causes a
precocious development of the gonads. The ovary of the frog,
Rana pipiens, is quiescent and does not produce mature eggs
except during the breeding season, which occurs in the spring.
However, ovulation can be induced at other times by implanting
fresh frog pituitary tissue. As a result mature eggs may be
obtained in the middle of winter. The testes of the male may
be stimulated to produce mature spermatozoa by similar treat-
ment. In fact, all the sexual responses, including amplexus, may

208 GENERAL ZOOLOGY

be evoked out of season by implanting anterior lobe tissue.
Ovulation may be produced by anterior lobe tissue from either
male or female frogs.

Thyrotropic Hormone. — There is a close functional relationship
also between the anterior pituitary lobe and the thyroid gland.
If the anterior lobe of the frog tadpole is removed, metamorphosis
fails to occur. The thyroid gland of such animals is highly
atrophic, a condition that can be corrected by transplanting
anterior pituitary tissue or injecting pituitary extract. In either
case the thyroid gland is restored to normal condition and
metamorphosis follows. Metamorphosis seems to require the
presence of a normal thyroid gland in the tadpole. Thus removal
of the thyroid gland in tadpoles prevents metamorphosis; feeding
thyroid or thyroid extract to such tadpoles results in a completion
of metamorphosis; and feeding thyroid gland to normal tadpoles
hastens metamorphosis. The thyroid hormone is the immediate
cause of metamorphosis, but the thyrotropic hormone of the
anterior pituitary is necessary for its normal functioning.

Lactogenic Hormone. — An extract has been obtained from the
anterior pituitary gland, which when injected into female
rabbits, dogs, or pigs, causes a secretion of milk in the mammary
glands, without increasing the amount of the mammary tissue.
It is interesting that such extracts obtained from beef or sheep
pituitary glands also stimulate the formation of "pigeon milk"
in the crop glands of male or female pigeons. The crop glands
of pigeons normally produce pigeon milk only when young are
being reared.

Adrenotropic Hormone. — One of the effects of removing the
anterior pituitary from rats is a degenerative change in the
cortex (outer layer) of the adrenal glands, which can be restored
by the implantation of pituitary tissue. An extract of the
anterior pituitary lobe has been made that is high in adrenotropic
activity. The relation of the adrenotropic hormone to the
adrenal gland seems to be similar to the relation of the thyrotropic
hormone to the thyroid. That is to say, both glands depend upon
hormones from the anterior pituitary lobe for the performance
of their normal functions.

In addition to these five well-known anterior pituitary hormones
there is also evidence of endocrine relationship between the
anterior pituitary lobe and the islet cells of the pancreas or

THE ENDOCRINE SYSTEM 209

their secretion, and therefore with carbohydrate metabolism.
These results will be discussed in connection with the pancreas.
Since removal of the anterior pituitary lobe also causes atrophy
in the parathyroid glands, the presence of a parathyrotropic
hormone is indicated.

Posterior Pituitary Gland. — Two substances, pitressin and pito-
cin, have been extracted from posterior lobe tissue, but since
neither has been obtained entirely free of the other, there is
some overlapping of the effects produced by them. Pitressin
has effects on the heart, blood vessels, respiration, kidneys, and
intestine. Pitocin causes the contraction of the muscles of the
uterus. Pitressin produces at first a fall and then a prolonged
rise in pulse rate, oxygen consumption, and heart output. An
irregular quickening of the respiratory rate is a secondary
effect of the circulatory disturbance. On the intestine, pitressin
produces a muscular constriction. Pitressin has an antidiuretic
effect, i.e., it decreases the volume of urine. This may be the
result of the resorption of water by the kidney tubules, or the
retention of water by the tissues. Such results are obtained with
patients having diabetes insipidus or with normal individuals,
who have previously taken water by mouth. On the other
hand, diuresis (secretion of urine) may be increased by pitressin
in animals with a low urine volume. Pitressin thus has both
a diuretic and antidiuretic action, depending upon the condition
of the subject. The action of pitocin on the musculature of the
uterus is more potent during the later stages of pregnancy and is
sometimes used to induce parturition.

The Intermediate Lobe of the Pituitary. — A hormone called
intermedin obtained from the intermediate lobe of the pituitary
gland causes chromatophores (pigment-bearing cells) to expand.
This can be demonstrated by producing a darkening of the skin
of frogs as a result of injecting an extract of the intermediate
lobe. In man there is evidence that intermedin has an anti-
diuretic action on patients suffering from diabetes insipidus,
similar to the action of pitressin.

Thyroid Gland. — The thyroid gland of the frog consists of two
lobes, completely separated, lying one on each side of the hyoid
apparatus between the posterolateral and thyrohyoid processes.
In man it consists of two lobes, joined usually by a narrow isth-
mus, the whole encircling the front and side of the trachea just

210

GENERAL ZOOLOGY

below the larynx (Fig. 132). Histologically the thyroid gland is
composed of a large number of follicles of a more or less spherical
shape and filled with a viscous secretion known as colloid, in
which the thyroid hormone is dissolved (Fig. 133). The thyroid
gland has a very rich supply of blood vessels.

In the frog, removal of the thyroid glands of the tadpole pre-
vents metamorphosis. In mammals, removal of the thyroid

of the young results in a lowered
rate of metabolism and a re-
tarded physical, mental, and
sexual development. When

Fig. 132. — Diagrammatic view of Fig. 133. — Section of human thy-

human thyroid gland in position against roid gland. B. blood vessels; f, folli-
anterior end of trachea. cles filled with colloid. (After Stohr.)

thyroidectomy is practiced on adult mammals, the main effect
is lowered metabolic rate, manifested by a lowered heat produc-
tion. Impaired functions and retarded growth can be restored
to normal by implanting or feeding thyroid tissue.

The active principle of the internal secretion of the thyroid
gland is thyroxine, whose formula is C15H11O4NI4. This com-
pound has been synthesized and the synthetic substance has the
same physiological action as natural thyroxine. Thyroxine fed
to tadpoles results in a rapid loss of weight and the completion of
metamorphosis in from four to five days. Thyroxine may be
used effectively as a substitute for thyroid gland in cases of
thyroid deficiency, natural or experimental.

THE ENDOCRINE SYSTEM 211

Hypothyroidism or functional insufficiency of thyroid activity
produces three pathological conditions that have been known for
a long time, viz., endemic goiter, cretinism, and myxedema. The
thyroid gland is enlarged in endemic goiter, but the enlargement
is due to an abnormal increase in number of the thyroid cells
(hyperplasia) to compensate for a subnormal secretion of effective
thyroid hormone; the diminished amount of effective thyroid
hormone being the result of insufficient iodine in the food and
water. Endemic goiter can be controlled by adding iodine in the
form of iodides to food and water. Cretinism is a long-known
pathological condition in human beings, marked by atrophy
(degeneration) of the thyroid gland and retardation of growth
in all parts of the body, including the nervous system and the
reproductive organs. Myxedema results from a marked thyroid
deficiency in adults and is characterized by reduced metabolism,
and general ill health, mental and physical. Usually the skin is
thick and dry and the hair falls out. Both cretinism and
myxedema respond favorably to the administration of thyroid
gland or thyroxine if continued throughout life.1

Hyperthyroidism refers to an increased functional activity of
the thyroid gland, which, in exceptional cases only, may result in
an enlargement, also called a goiter. In Graves' disease or
exophthalmic goiter, metabolism, nervous irritability, rate of
heart beat, and blood pressure are increased. The skin is moist
and flaccid. Surgical removal of a part of the thyroid gland may
correct the condition, though not always since nervous factors
seem to be involved in addition to hyperthyroidism.

In general, the thyroid hormone serves to stimulate metabolic
processes by maintaining the rate of oxidation at a certain level.
Increased thyroid activity raises the level, and decreased activity
lowers it. Removal of the thyroid, therefore, has some effect on
every organ in the body. However, the functional activity of
the thyroid seems to require for its full accomplishment the
presence of the thyrotropic hormone from the anterior pituitary

lobe.

The colloid content of the follicles of the thyroid gland varies
in amount under different functional states. If anterior pituitary
lobe is implanted in a normal animal, the contents of the follicles

JFor a different view regarding the cause of goiter see H. M. Jones, The
Cause of Goitre, Chicago, 1937.

212 GENERAL ZOOLOGY

disappear, the follicles shrink in size and the height of the
follicle cells increases. Thus under the stress of pituitary
stimulation the follicles are emptied and the additional secretion
produced by the stimulated follicle cells is so rapidly removed
from the glands that the follicles remain empty. This is also the
picture one finds in cases of hyperthyroidism. On the other
hand, in cases of endemic goiter (hypothyroidism) the follicle
cells are low in height and the follicles are filled with colloid,
which, however, is without the potency of the normal colloid
because of insufficient iodine. In a normal individual, the
colloid and the thyroid hormone contained in it seem to accumu-
late in the follicles under normal conditions of thyroid activity
to produce a reserve supply of hormone. When the gland is
stimulated, the colloid is resorbed by the follicle cells and passes
into the blood. The height of the cells forming the walls of the
follicles is a direct index of the activity of the gland.

Parathyroid Gland. — In man the parathyroid glands consist
usually of two pairs of nodules attached to, or embedded in, the
dorsal surface of the thyroid. They consist of corded masses of
polygonal cells, with very little connective tissue. The follicles
so characteristic of the thyroid gland are lacking in the para-
thyroid. Complete removal of the parathyroids in dogs is
followed by death in four days, with the development of tetany
(spasmodic muscular contraction) and a fall of calcium in the
blood. Injection of calcium salts causes the tetany to disappear
and partially restores the animal to normal conditions. Extracts
of parathyroid gland when injected produce results similar
to calcium salts. It is therefore concluded that the parathyroids
produce a hormone which regulates calcium metabolism and
maintains a normal calcium content in the blood. Since removal
of the anterior pituitary lobe also produces disturbances in
calcium metabolism, there is some evidence that a parathyro-
tropic hormone is produced by the pituitary which has a relation
to the parathyroid similar to the relation of the thyrotropic
hormone to the thyroid.

Thymus Gland. — The thymus gland of mammals extends down
from the lower throat region into the thorax, overlying the
pericardium. Like the thyroid and parathyroid glands, it is also
a derivative of the embryonic pharynx. It increases in size until
puberty, after which it undergoes atrophic changes so that in the

THE ENDOCRINE SYSTEM 213

adult it is reduced to fatty and fibrous strands. Histologically
the thymus is quite different from the thyroid and parathyroid
glands and shows a general resemblance to a lymph gland. It is
distinguished by the presence in the medullary region of peculiar
structures known as thymic or Hassall's corpuscles, consisting of
rounded groups of degenerated cells bounded by flattened
epithelial cells, whose function is unknown. The development
and fate of the thymus would seem to imply some relation with
the gonads. However, while removal of the gonads results in a
hypertrophy of the thymus and thus seems to remove an inhibi-
tion to its continued growth, removal of the thymus in young
guinea pigs has no effect on general body growth and fails to
cause hypertrophy of the gonads. Feeding thymus to frog
tadpoles stimulates growth and causes them to grow to unusual
size before metamorphosis. Pigeons with defective thymus
glands lay eggs deficient in shell, a condition that can be corrected
by feeding dried thymus. Much uncertainty exists regarding
the exact role of the thymus in the endocrine complex. The
most recent work indicates that administration of an extract of
thymus increases fertility in adult rats and that when given
continuously through successive generations it produces marked
precocity in growth and development.

Adrenal Glands. — The adrenal glands of mammals are paired
and located near, or in contact with, the kidneys, with which,
however, they seem to have no functional relation. In man, each
adrenal is a triangular-shaped organ capping the anterior end
of the kidney (Fig. 135). In the rat they are spherical in shape
and are attached to the dorsal body wall just in front of the
kidneys. Two general regions are recognized: a central, medul-
lary region, and an outer, cortical region. These two regions
differ in their embryological origins and also in their functions.
In the frog (Fig. 134) the adrenal gland consists of a long narrow,
orange-colored band embedded in the ventral face of each
kidney. The cortical and medullary cells can be recognized, but
they are intermingled.

The cells of the medullary region of the adrenal gland are
stained by salts of chromium and are therefore referred to as
chromaffin or chromophile cells. The staining reaction is due to
the presence of a substance known as epinephrine or adrenalin,
which has been prepared in a pure state and used in medicine

214

GENERAL ZOOLOGY

many years. Its chemical structure is known and it has been
prepared synthetically. Its formula is C6H3(OH)2CHOHCH2-
NHCH3. The principal action of epinephrine is on tissues
innervated by the sympathetic nervous system. Its frequent
use as a styptic (astringent) in minor surgery to control hemor-
rhage depends on its capacity to constrict arterioles. The vaso-
constriction is, however, followed by a vasodilation, which makes
the use of epinephrine as a styptic of questionable value. Sub-
cutaneous injection of epinephrine also produces an increase of

Fig. 134. — Adrenal gland of frog in Fig. 135. — Human adrenal gland

position on ventral face of kidney. and kidney, somewhat diagrammatic.

a, adrenal; ao, aorta; p, postcava; k, a, adrenal; b, adrenal blood vessels; k,

kidney. kidney.

sugar in the blood (hyperglycemia) and loss of sugar through the
kidneys (glycosuria), indicating that it stimulates glycogenolysis
(conversion of glycogen into sugar) in the liver and elsewhere.
It has been shown that in emotional excitement, such as is pro-
duced by fear or anger, there is an increased secretion of adrenalin
which likewise increases the sugar in the blood. This has an
adaptive significance since increased sugar means a ready source
of energy. Secretion of epinephrine may be produced in an
experimental animal by stimulating the splanchnic nerves to the
adrenal glands. Injection of epinephrine in frogs causes con-
traction of chromatophores, an effect opposite to that produced
by the hormone of the intermediate lobe of the pituitary gland.

THE ENDOCRINE SYSTEM 215

An animal can get along without the medulla of the adrenal
but not without the cortex. This is shown by the fact that while
death follows the complete removal of the adrenal glands, death
does not occur if only the medulla is destroyed. Death can be
prevented in an animal whose adrenals have been removed if an
extract of the cortex is given by injection. The extract of the
cortex is known as cortin. Cortin has been used successfully
in treating Addison's disease, which is an affection of the adrenal
cortex. It is believed by some that the essential function of
cortin is the regulation and maintenance of a normal circulating
volume of fluid within the vascular system. In a dog whose
adrenals have been removed the depletion of the volume of
circulatory fluids seems to be due to a loss of sodium salts and
water. The administration of sodium salts to such an animal
has effects similar to those produced by cortin in restoring normal
salt metabolism and circulatory volume, temporarily at least.
Ascorbic acid, the crystalline form of vitamin C has been obtained
from the adrenal cortex. It is generally believed that the high
concentration of vitamin C in the adrenal cortex is the result of
storage rather than of local synthesis.

The difference in properties of epinephrine and cortin are
correlated with a difference in embryonic origin of the medulla
and cortex. The chromaffin cells, which produce epinephrine,
seem to be modified sympathetic nerve cells which retain close
anatomical relations with the sympathetic nerves innervating the
medulla of the gland of the adult. Isolated masses of chromaffin
cells also occur in connection with sympathetic ganglia in various
parts of the human body. In sharks, the chromaffin tissue exists
in the form of numerous distinct organs, completely independent
of what corresponds to cortical tissue of higher forms. In the
frog, as already mentioned, chromaffin and cortical cells are
intermingled. In mammals the cortical tissue is derived from
what are known as mesenchyme cells, which enclose the chro-
maffin cells and sympathetic cells to form a single organ.

Pancreas. — The islet cells of the pancreas produce an internal
secretion that is also important in the metabolism of sugar.
Complete removal of the pancreas in a dog produces glycosuria,
and an increased production of urine and of urea, accompanied by
pronounced thirst and hunger. Acetone is also present in the
urine. These symptoms are the same as those shown by human

216 GENERAL ZOOLOGY

patients suffering from diabetes mellitus. That the symptoms
in the case of the depancreatized dog are due to the loss of the
islet cells is shown by the fact that the destruction of the alveolar
cells alone does not produce them. This can be done by tying
off the pancreatic ducts, which causes the degeneration of the
alveoli but does not affect the islet cells. Then, too, there is the
additional observation that lesions in the islet cells have been
found in cases of diabetes mellitus.

Insulin is an extract prepared from islet cells which when
injected into diabetic dogs or diabetic patients alleviates the
symptoms. Injection of too much insulin into a normal rabbit
produces a rapid fall in blood sugar to about 0.045 per cent, when
convulsive seizures occur. These promptly disappear if glucose
is injected. Obviously the function of insulin is concerned with
the metabolism of sugar, though how it is brought about remains
an open question. In general terms, insulin has the property of
making dextrose available to tissues so that it may be oxidized.

Glycosuria and hyperglycemia can be produced in normal
animals by injecting extracts of the anterior pituitary gland.
The same symptoms often accompany acromegaly. Therefore
the anterior pituitary gland produces a hormone which has the
power to increase the amount of sugar in the blood. When
insulin is absent, as in pancreatic diabetes, the sugar continues to
rise in the blood as a result of the continued action of this hor-
mone of the anterior pituitary which is known as the diabetogenic
hormone. If the anterior pituitary is removed from a depan-
creatized clog, glycosuria and hyperglycemia are greatly reduced
and the animal may survive for months, at any rate much longer
than if its anterior lobe had remained in place. In a sense, then,
the diabetogenic hormone is antagonistic to insulin, and this
might also be said to be true of epinephrine and of thyroxine.
The diabetogenic hormone is antagonistic to insulin in that the
former tends to increase the sugar in the blood by promoting
glycogenosis, while the latter is active in consuming sugar.
The action of epinephrine and thyroxine resembles that of the
diabetogenic hormone in their power to increase the sugar in
the blood.

Gonads. — Internal secretions of the gonads are necessary for
the full development of secondary sexual characters, which are
characters aside from the gonads that distinguish males and

THE ENDOCRINE SYSTEM 217

females, such as the comb, wattles, and spurs in the cock, or
beard, voice, body size, and strength in a man. Removal of the
gonads in young animals prevents or alters the full development
of such characters in the adult. There is, however, no general
rule for predicting the results of gonadectomy. Removing the
testis of a young Brown Leghorn cock interferes with the com-
plete development of the male type of plumage and of the head
furnishings. Otherwise such birds, known as "capons," are
large in size, have spurs, and look like cocks, though they lack the
pugnacity and temperament of the normal male bird. The result
of removing the ovary of the Brown Leghorn early in life is com-
plicated by conditions peculiar to birds. In birds only the left
ovary is functional, the right gonad being very rudimentary.
If the functional ovary of the Brown Leghorn is completely
removed, in a certain percentage of cases the rudimentary right
gonad is changed into a testis, with the result that the spayed
bird later develops all the pugnacious qualities and sexual
instincts of a normal male. Such birds, however, retain their
smaller size and develop a male plumage that is succeeded by a
permanent typical hen plumage. The female in this case has the
potentiality of both maleness and femaleness, the outcome
in the normal cases being determined by the development of the
left gonad and the suppression of the right.

In mammals extracts have been made from various parts of the
ovary and their properties studied. Theelin has been made from
the follicles in which the eggs develop. After an egg is dis-
charged from a follicle, the cavity becomes filled with cells,
forming what is called a corpus luteum, from which another
hormone, progestin, has been extracted. Theelin has also been
obtained during pregnancy from the placenta. It is also present
in the blood of the female from puberty on. When injected into
immature female rats, it causes the uterus and vagina to become
mature. Theelin partially stimulates the proliferation of the
uterine mucosa in anticipation of the implanting of the fertilized
egg; and progestin holds the functioning of the ovary in abey-
ance until the birth of the young and also augments the action of
theelin on the uterus. The production of both theelin and
progestin is controlled by the gonadotropic hormone of the
anterior pituitary. The latter is necessary for the normal
growth of the follicles and the production of corpora lutea.

218 GENERAL ZOOLOGY

A hormone has been prepared from the testis which stimulates
the growth of the accessory reproductive organs of the male.
As in the case of the ovarian hormones, its secretion is under the
control of the anterior pituitary.

General Significance of the Endocrine System. — The secretions
of the endocrine organs furnish a chemical mechanism for the
functional coordination of various parts of the body. The other
general mechanism for this purpose is the nervous system with its
nervous pathways, along which impulses are conducted to and
from all parts of the body. Of the two, the chemical method is
perhaps older phylogenetically, but practically it is difficult in the
final analysis to draw a sharp line between them because the
functional activity of the nervous system rests upon a chemical
basis too. In the nervous system, however, the paths along
which nervous impulses travel are separated and insulated from
each other, while in the endocrine system the means of transport-
ing the exciting agents is the blood, which is a common vehicle for
all of them. In the nervous system some of the pathways have
become specialized to perform a single function, as for example
the optic nerve; in the endocrine system, specific substances are
all mingled in the blood stream, where they may augment,
counteract, or stimulate each other before reaching their func-
tional destinations. Both systems, however, are closely related
functionally and each loses its individuality in the correlation
of the body as a whole.

CHAPTER XI
CELL DIVISION AND GAMETOGENESIS

The cell seems to be the smallest or simplest aggregate of
matter capable of maintaining itself alone, or in combination with
other cells, as a living thing over a period of time, and capable also
of reproducing itself. A basic fact in the organization of the cell
seems to be that it is always composed of two kinds of protoplasm,
nuclear and cytoplasmic. In metazoans cell boundaries may be
indistinct or lacking, but every cell region is provided with a
nucleus. It is true, of course, that bacteria lack a morphological
nucleus, but they do contain nuclear material, even though it is
not segregated inside of a nuclear membrane. Mammalian
erythrocytes are not nucleated, but these structures are really
enucleated cells that survive for a relatively short time and are
incapable of reproducing themselves. Since it has also been
shown that under experimental conditions neither nucleus nor
cytoplasm can survive long in the absence of the other, it appears
that the chemicophysical system represented by the nucleus and
cytoplasm combined as in a cell is the minimum for the produc-
tion and continuation of the living state.

It is a dictum of biology that cells come only from preexisting
cells by a process of cell division; and, on the other hand it is also
said that the first forms of life, or the first cells, to appear on the
earth were produced from nonliving matter (abiogenesis). No
one can say what these first forms of life were like, or to what
extent they resembled the simplest kinds of living things known
today. Whatever may be the answer to that problem, it has
been conclusively shown that known present-day species of living
things come from preexisting species, or that the cells of which
they are composed come from preexisting cells. Reproduction of
organisms depends upon the reproduction of cells, which in turn
is the result of cell division. Cell division is thus a very impor-
tant phase of biological activity (Fig. 136).

The cells of the body of the metazoan animal fall into two
general groups, somatic cells and germ cells, although these two

219

220

GENERAL ZOOLOGY

groups are not distinguishable at all times, as in the case of some
of the lower invertebrates. Somatic cells comprise the bulk
of the body and include all cells except the reproductive cells,
which are the germ cells. In the higher forms these two groups
can always be recognized, often from very early stages in develop-
ment. In lower forms, Hydra for example, germ cells are indis-
tinguishable from somatic cells until the formation of the ovary
and testis.

Somatic Mitosis. — The common method of cell division,
known as mitosis, consists of a series of complicated changes
which for convenience has been arranged in progressive stages as
follows :

Resting Stage. — When a cell is not dividing, it is said to be in
the resting stage, though as a matter of fact it may at the time be

Fig. 136. — Mitotic figures in cells of larval salamander as they appear in an
ordinary section of the skin. X 900.

actively engaged in the performance of its functional role in the
body economy and be resting only in the sense that it is not
dividing. Except in the basal layer of the epidermis, somatic
mitoses are rare in the adult stage of vertebrates. For the study
of somatic cell divisions embryonic stages are preferable since
these abound in cell divisions. An embryonic vertebrate cell,
say, of the skin or of the liver, consists of a cytoplasmic region in
which usually a single nucleus is embedded. Cytoplasm and
nucleus are each bounded by a membrane. In the ordinary
fixed and stained section, the nucleus contains irregular faintly
stained clumps of chromatin and usually one or more rounded
bodies called nucleoli. . If the cells are undergoing rapid cell
division, a structure known as the centrosome may be seen in the
cytoplasm, close to the nuclear membrane. The centrosome is a
rounded area composed of compact cytoplasm, in the center of
which are two rounded bodies called centrioles. Centrosomes and

CELL DIVISION AND GAMETOGENESIS 221

centrioles are usually not visible in sections of adult tissues.
During rapid cell divisions they continue from one cell generation
to the next, but there is evidence that they may be formed
de novo from the cytoplasm.

Division Stages. — The following description is in the nature
of a generalized account of the mitotic process as it occurs in
somatic cells. There are many differences in detail in mitosis in
different animals, which can be only touched upon here. The
preparatory steps leading to cell division make up the prophase
of the process, which is initiated apparently by the separation
of the centrioles and the formation of a mitotic spindle between
them (Fig. 137). This is often accompanied by the formation of
astral radiations extending in all directions from the centrosomes.
The chromatin of the nucleus meanwhile condenses into a number
of thin threads, corresponding to the number of chromosomes
characteristic of the species. In some forms the chromatin
threads seem at first to be connected in a continuous thread,
known as a spireme, which later breaks up into chromosomes ; in
others the chromatin threads appear separate from the beginning
of their formation. In either case a definite number of chromo-
somes forms from the chromatin of the resting nucleus, regardless
of whether a continuous spireme is formed or not. Chromosomes
are frequently bent in V shapes, as shown in Fig. 137. The
details of mitosis can be followed more readily in the latter type
of chromosome, which for this reason has been used in the
illustration.

As the chromosomes assume form, the centrioles move farther
apart with an accompanying enlargement of the spindle which
gradually shifts to the center of the cell. The nuclear membrane
then gradually dissolves, releasing the chromosomes, which
quickly assume places in the equatorial plane of the spindle. In
some forms, the chromosomes appear as undivided rods, or each
may show a longitudinal split that foreshadows the ensuing
division, and which may appear in the chromosome while it is
still within the nucleus. In the former case the undivided
chromosomes become split shortly after reaching the spindle.
The nucleolus melts away with the dissolution of the nuclear
membrane and takes no part in the division process. When the
chromosomes have become arranged in the equatorial plane of
the spindle, the metaphase stage of mitosis has been reached

222

GENERAL ZOOLOGY

(Fig. 137, D, E, and /O . In the case of the V-shaped chromosomes,
shown in the illustration, the chromosomes are arranged with the

Fig. 137. — Diagram of karyokinesis. A, resting stage, two centrosomes in
cytoplasm near nucleus; B, early prophase, spireme stage; C, later prophase,
spireme segmented into eight chromosomes; D, polar view of chromosomes at
metaphase; E, side view of spindle and chromosomes at metaphase, each chromo-
some undivided; F, metaphase, each chromosome shows a lengthwise split;
G, side view of a single chromosome dividing, early anaphase; H, anaphase;
I, telophase; J, two complete daughter cells.

apices of the V's pointed toward the spindle to which they
become attached.

In the next stage, the anaphase, each chromosome, beginning
at the apex of the V, is split longitudinally into halves, the result
being the formation of two daughter groups of chromosomes,

CELL DIVISION AND GAMETOGENESIS 223

equivalent in every respect. The appearance of the mitotic
figure at this point suggests that each half chromosome is being
drawn toward a pole of the spindle, but nothing final is known
as to the mechanics of the process. In favorable material,
during the anaphase, each chromosome seems to be attached to a
fiber extending from the chromosome to the pole of the spindle
and known as a chromosome fiber or a half-spindle fiber. At
this same stage, as the ends of the chromosome separate, one or
two fibers stretch between the separated ends. These fibers are
called the interzonal fibers. There is some evidence to show that
chromosomal and interzonal fibers are formed by the chromo-
somes after they reach the spindle. The central or axial part of
the spindle seems to be formed as a result of activity in the
centrosomes. In endeavoring to understand the mechanics of
mitosis it is necessary to remember that the separation of the
chromosome into two halves is begun, in some cases at least,
before the chromosome reaches the spindle. Thus there may be
a repelling force operating to separate the chromosome halves,
begun in the nucleus and continuing on the spindle. Some
students of mitosis believe that both a "pulling" and a "push-
ing" factor are concerned in the process.

The anaphase includes stages following the metaphase up to the
point where each of the two daughter groups of chromosomes
forms a dense mass about each pole of the spindle. When this
point is reached, mitosis is in the telophase or final phase of
division. In the late anaphase or early telophase, the centriole
at each pole of the spindle divides in two. A constriction in the
cytoplasm in a plane at right angles to the spindle begins to
appear at this time and gradually cuts the cell in two. The
spindle fibers and astral radiations then fade and a typical
nucleus forms in the region of the telophase chromosomes. The
chromosomes become diffuse and a nucleolus reappears.

The most impressive feature of mitosis is the exact lengthwise
division of each individual chromosome into two equal parts,
resulting in each daughter cell receiving not only an equivalent
quantity of chromosome material but also an equivalent quality,
since each chromosome possesses qualities or characteristics that
distinguish it from other chromosomes. It is true, of course, that
the cytoplasm is also divided in equal amounts in mitosis, but in
this case only a mass division takes place. The achromatic

224 GENERAL ZOOLOGY

structures involved in mitosis are the spindle, the centrosomes,
centrioles, and astral radiations, all of which together with the
chromosomes may be involved in a final explanation of the
mechanics of the process. Since tissues or cells ordinarily used
for the study of mitosis consist of material that has undergone
special treatment with reagents and stains prior to mounting on
glass slides, it has been claimed that some of the structures seen
in such preparations are possibly artifacts caused by coagulation
or precipitation by reagents. These objections have been
answered by demonstrating that chromosomes as well as a
spindle and centrosome areas can actually be seen in living cells
under proper conditions. When living cells undergoing mitosis
are dissected, the spindle appears as a hyaline material to which
the chromosomes adhere as jellylike masses. The spindle is not
an artifact though the fibers may be. The centrosome areas
are more viscous than the surrounding cytoplasm of dividing
cells, showing that they too are actual structures in the cell.

Division of somatic cells occurs throughout the developmental
stage of Metazoa. In the adult animal, cell division is confined
to certain cells whose function is to replace loss. Thus the outer
layer of the human epidermis is being continually rubbed off and
replaced by cells pushing up from the deeper layers of the
epidermis, in the lowest of which, cell reproduction takes place
throughout life. The red blood corpuscles of mammals also have
a short life in the circulation and are replaced by cells of the bone
marrow. Mitotic divisions may occur sporadically among other
tissue cells. It is said that the nerve cells of adult mammals
never undergo cell division. The number of chromosomes
appearing in somatic mitosis is a constant one for the species,
although there may be a difference, also constant, between the
sexes of the same species. This point will be discussed in con-
nection with the germ cells.

Amitosis. — Though simple and more direct than the mitotic
method of cell division, amitosis is a much rarer form of cell
division. There has been much controversy about its occurrence
and nature, which need not be entered into here. Amitosis has
been described in various tissues under normal conditions, and in
degenerating and pathological tissues, from which it seems clear
that it is not the ordinary method of cell division. Figures of cells
dividing amitotically show each constituent of the cell, beginning

CELL DIVISION AND GAMETOGENESIS

225

with the nucleus, dividing in two by simple constriction without
the accompaniment of chromosome and spindle formation

(Fig. 138).

Gametogenesis. — Reproduction in Protozoa may consist
simply in the division of the animal as a whole into two parts,
each of which then grows to the size of the original. In sexual
reproduction among Metazoa, the egg and sperm (spermatozoon)
are cells that have the power, when united in fertilization, of
developing into a new organism by cell division, accompanied by
growth and differentiation. Some Protozoa also produce germ
cells and exhibit a primitive form of sexual reproduction. The
asexual reproduction that occurs in some Metazoa does not
involve the production of germ cells.

Fig. 138. — Amitosis in nurse cells of ovary of the potato beetle, Leptinotarsa

signaticollis.

All of the cells of the metazoan body develop from the fertilized
egg. That means that both somatic and germ cells have a com-
mon origin in the fertilized egg. The essential fact in fertiliza-
tion, to be considered more in detail later, is the formation of a
single cell, the fertilized egg, from the fusion of the egg nucleus
with a nucleus brought in by the sperm. The number of chromo-
somes, the zygotic or diploid number, occurring in the somatic
cells of the animal is thus determined at fertilization. The egg
then undergoes cell division (cleavage) by the mitotic method, as
described in the preceding pages, the cells or blastomeres thus
produced containing the potencies of both somatic and germ
cells. Then, after a number of cell generations have been
produced, certain cells, called primordial germ cells, may be
distinguished from the somatic cells. There is evidence in
many animals that the appearance of the primordial germ cells
marks an important and final differentiation of germ-cell material
from somatic. In higher forms, it is generally thought that the
functional germ cells are the descendants of the primordial germ

226

GENERAL ZOOLOGY

cells and, as a corollary, that once the somatic cells have become
differentiated as such, they never form germ cells at any sub-
sequent period.

Gametogenesis is the production of gametes, or eggs and
spermatozoa, from primordial germ cells. Oogenesis includes all

Spermatogenesis GAMETOGENESIS Oogenesis

Primordial Germ Cell
(Diploid chromosome number)

Division Period

Grow+h Period,
Synapsis

.st.Mat.Div..
Chromosomes re
duced+ohaploid
number

Spermatocyte II

Spermatid

#) (B (B C3

2nd. Mah Division
(quantitative)

V

I

/

06cy+e]I
1st. Polar Body

3rd.4th.RB

2nd. PB.
Ootid

Cleavage

5permato2oa
Fig. 139. — Diagram of gametogenesis, fertilization and cleavage.

stages involved in the development of eggs, and spermatogenesis
is the process of producing spermatozoa from primordial germ
cells. Oogenesis and spermatogenesis are essentially alike but
differ in certain details.

Oogenesis. — In the female rapid cell division of the primordial
germ cells leads to the production of a large number of oogonia,
only a few of which are shown in Fig. 139. This period of cell
multiplication is known as the division period, during which the

CELL DIVISION AND GAMETOGENESIS

227

rate of mitotic division is so rapid that practically no growth
occurs between divisions. The growth period that follows the
division period is characterized by cessation of mitosis and the
enlargement of the oogonia by the storage of yolk in the cyto-
plasm. By this means oogonia are converted into primary
oocytes. In insects (Fig. 140) some of the oogonia are converted
into nurse cells and supply the growing oocytes with nutriment
that enters the oocyte through the nutritive string, a temporary
process of the oocyte. In Hydra, as mentioned in an earlier

Fig. 140. — Longitudinal section of an ovariole showing how nutrition is sup-
plied to the egg of the potato beetle, Leptinotarsa signaticollis, during the growth
period, nc, nurse cells; ns, nutritive string; o, oocytes; fc, follicle cells.

connection (Chap. VIII) the oocyte grows by engulfing sur-
rounding cells, some of which may have been potential oocytes.
A similar engulfment of cells by the oocyte occurs in the growth
period of the oocyte of flatworms. In vertebrates, the oocyte
derives its nutrition from the blood vessels supplying the ovary.
The fully developed oocyte is a cell of large size, in some cases, as
in birds, of macroscopic proportions. Thus the yellow or yolk
of the hen's eggs is the same size as the primary oocyte at the
end of the growth period.

The oocyte at the end of the growth period now enters what is
called the maturation period, characterized by two maturation

228 GENERAL ZOOLOGY

divisions. During the first maturation division the oocyte
divides into two cells of very unequal sizes, the larger called the
secondary oocyte and the smaller the first polar body or polocyte.
In the second maturation division the secondary oocyte divides
into two cells, again of unequal size, the larger of which is the
ootid and the smaller the second polar body. At the same time
the first polar body may divide to form the third and fourth polar
bodies. As a result of the two maturation divisions a primary
oocyte may produce four cells: a single ootid, which is relatively
very large and is the mature ovum, and three small polar bodies.
The polar bodies may remain for a time attached to the egg
at the point where they are extruded, but sooner or later they are
lost and play no further part in development of the egg. It is
generally supposed that the polar bodies have the same poten-
tialities as ova but lack sufficient cytoplasm and yolk for develop-
ment. This view is supported by experiments in which it was
found that (in certain flatworms) abnormally large first polar
bodies, when fertilized, develop to an early larval stage. Devel-
opment up to a certain point is possible in this case because the
polocyte contains a certain amount of yolk. In most animals it
is very small and consists of little more than a nucleus (Fig.
141). '

Spermatogenesis. — In the male, the division period produces a
large number of spermatogonia. These then pass through a
growth period, but the primary spermatocytes formed as a result
are much smaller in size than the corresponding primary oocytes
of oogenesis (Fig. 139), though greater in number. In the first
maturation division each primary spermatocyte produces two
secondary spermatocytes of equal size. Likewise, in the second
maturation division each secondary spermatocyte divides to
form two spermatids of equal size. Therefore each primary
spermatocyte produces four spermatids. Each spermatid now
undergoes a further development, called spermiogenesis, as a
result of which it is transformed into a spermatozoon. In this
transformation the nucleus of the spermatid is crowded into
what is called the head of the spermatozoon, while the remaining
parts of the spermatozoon, called the middle piece and tail, are
formed of the cytoplasm. The tail is an organ of locomotion by
means of which the spermatozoon propels itself through fluid.
The spermatozoa of some animals lack a tail and are nonmotile.

CELL DIVISION AND GAMETOGENESIS

229

Comparing oogenesis with spermatogenesis, the main differ-
ences are: (1) a more intensive growth period in oogenesis; (2) a
larger total number of spermatozoa than of ova; and (3) the
completion of oogenesis at the ootid stage.

Chromosomes in Gametogenesis. — What is perhaps the most
striking feature of gametogenesis, viz., the behavior of the
chromosomes, may now be considered. Assuming that the

Fig. 141. — Four stages in the formation of the first polar body in the egg of
Planocera inquilina. d\ sperm nucleus. (From Patterson and Wieman.)

zygotic number of chromosomes of the fertilized egg is 8, both
primordial germ cells and somatic cells exhibit 8 chromosomes
at each mitotic division. So far as the somatic cells are concerned
this condition prevails in all subsequent mitotic divisions. It
also holds for the division period of gametogenesis, as indicated
in the diagram (Fig. 139). During the growth period of the
nucleus of the developing oocyte or spermatocyte, the chromo-
somes lose their identity in the resting nucleus. At the end
of the prophases of the first maturation divisions, however,

230 GENERAL ZOOLOGY

instead of the expected 8 chromosomes, only 4 appear, but each
of the 4 is a double chromosome. This pairing of the chromo-
somes is known as synapsis and takes place in the prophase
of the first maturation division.

Meiosis. — Owing to constant differences in form and size, it
is possible to arrange the chromosomes of the presynaptic period,
or of the cleavage period, or of any somatic cell, into two series
of homologous chromosomes. In the hypothetical case repre-
sented in Fig. 139, the two series consist of 4 chromosomes each.
During the prophase of the first maturation division these pair off
according to form, size, or other characteristics to form double
or bivalent chromosomes. As shown in Fig. 139, during the first
maturation division, the members of the synaptic pairs undergo
disjunction and pass as whole chromosomes to opposite poles of
the spindle, so that each secondary spermatocyte or each second-
ary oocyte and the first polar body, receive 4 chromosomes. The
reduction of the chromosome number from the diploid number
8 to the haploid number 4 is known as meiosis. It is also spoken
of as a qualitative division. In the second maturation division,
which follows, each of the four chromosomes divides in the usual
way, with the result that each spermatid and each ootid and
second polar body receive 4 chromosomes. The second division
then is a quantitative division, such as takes place in any somatic
mitosis.

If, as appears to be the case in some animals, the first matura-
tion division is a quantitative one, each component of each
synaptic pair being divided longitudinally, the second maturation
division becomes a reduction division; but whether reduction
occurs in the first or second maturation division, the final result is
the same, viz., the ootid, polocytes, and spermatids all receive
the haploid number of chromosomes. The diploid or zygotic
number is restored at fertilization by the union of the nuclei
of the ootid and the spermatozoon. Since the cells of the
embryo, and later the adult, are produced by cell division from
the fertilized egg, it follows that each cell in the body is provided
with the zygotic number of chromosomes, except the germ cells
of the postreduction stages. It should also be clear that the
zygotic chromosome complex consists of a maternal and a
paternal series, which separate in meiosis. These facts are of
utmost importance in understanding the mechanism of heredity,

CELL DIVISION AND GAMETOGENESIS 231

in which connection they will be referred to again in a later
chapter.

Sex Chromosomes. — In the hypothetical example used to
illustrate mitosis and gametogenesis, the zygotic number of
chromosomes was assumed to be 8. Though constant for any

%M • ' 'I

'•

*• a mi, h ••/

"9

fit ••«••••
|M«tMt»

* f

m
m

£» HltMIMf

• !••

Fig. 142. — Chromosomes of the squash bug, Anasa tristis. a, polar view of
equatorial plate, first spermatocyte; b, side view of anaphase, second spermato-
cyte; the heterochromosome (h) passes undivided to one pole of spindle; c and
d show the results of the second spermatocyte division, ten chromosomes going
to one cell (c) and eleven to the other (d), so that two classes of spermatozoa
result; e, polar view of spermatogonium showing twenty-one chromosomes;
/, diploid chromosome complex of male arranged in pairs according to size, and
forming a biparental series; g, polar view of equatorial plate of oogonium show-
ing twenty-two chromosomes; h, diploid complex of female, paired according to
size. (From Wilson, Journal of Experimental Zoology.)

given species, the number may actually be from 2 to 100 or jnore.
Thus in the nematode worm Ascaris megalocephala univalens the
zygotic number is 2; in the frog, Rana pipiens, 26; in man, 48;
etc. In some cases the number may be different in the male and
female of the same species, an example of which may now be
considered. Anasa tristis is the common squash bug. In this
form the zygotic number of the female is 22, while that of the

232 GENERAL ZOOLOGY

male is 21. During oogenesis the number is reduced to 11 in
one of the maturation divisions, with the result that the ootid
remains with 1 1 chromosomes, the gametic number of the female.
The zygotic chromosomes of the female therefore consist of 11
synaptic pairs which undergo disjunction in one of the maturation
divisions. In spermatogenesis, the prophases of the first
maturation division show 11 chromosomes, 10 of which are
synaptic pairs and the eleventh a single chromosome, known
as the odd chromosome or the sex chromosome. It should be
noted that the homologue of this chromosome is paired in the
female zygotic series (Fig. 142). In the first maturation division,
in the male, the 10 paired chromosomes undergo disjunction
and the odd chromosome divides quantitatively, with the result
that each secondary spermatocyte receives 11 chromosomes, one
of which is an odd chromosome. In the second maturation
division, the odd chromosome passes undivided to one pole of the
spindle, while the remaining 10 chromosomes divide quantita-
tively. This results in the production of two kinds of spermatids
(and later two kinds of spermatozoa), one containing 10 chromo-
somes and the other 11. Since the ootids all contain 11 chro-
mosomes, a male is produced when a sperm containing 10
chromosomes fertilizes an egg (10 + 11 = 21), and a female
when a sperm containing 11 chromosomes fertilizes an egg
(11 + 11 = 22). In Anasa tristis, the presence or absence of
the odd chromosome apparently determines the zygotic sex, and
it is for this reason called the sex chromosome. It is also known
as the X chromosome or as the hetero chromosome. The remaining
chromosomes, which are equally represented in the two sexes are
known as autosomes.

An inspection of the spermatogonial chromosomes (Fig. 142)
shows that the diploid complex of the male can be arranged into
a series of 10 synaptic pairs of autosomes plus one X chromosome,
which lacks a synaptic mate. The oogonia of the female consists
of 10 synaptic pairs of autosomes, corresponding to those of the
male, plus two X chromosomes. The difference in the zygotic
chromosomes of the two sexes is the difference in the number of
X chromosomes, of which the male has one and the female two.

X and Y Chromosomes. — Inequality of some sort in the chro-
mosomes of the male and of the female seems to prevail in
practically all animals. Frequently, as in Anasa tristis, the

CELL DIVISION AND GAMETOGENESIS 233

inequality consists of a difference in number of the X chromo-
somes, but in other cases it may consist in differences in size of
one or more of the sex chromosomes. An example of such a
condition occurs in the fruit fly Drosophila melanog aster , the
zygotic number of which is 8 in both sexes (Fig. 143). In the
female these 8 chromosomes are found to be made up of 3 pairs
of autosomes plus 2 X chromosomes, and in the male of 3 pairs of
autosomes plus one X and one Y chromosome, which is larger
than X and is also hook-shaped. The difference in the chromo-
somes of the male and the female then becomes the difference
between X and Y, which is a difference in form and size. At the
end of the maturation divisions of the oocytes, each ootid receives
3 autosomes and one X chromosome. In the male two kinds of

s A * B

ix a

Fig. 143. — The diploid series of chromosomes of Drosophila melanogaster. A,

female; B, male. (After Morgan.)

spermatids are produced, one receiving 3 autosomes plus one
X chromosome and the other 3 autosomes plus one Y chromo-
some; resulting in the production of two kinds of sperm. An
egg (3 + X) fertilized by. a sperm carrying an X chromosome
(3 -f- X) develops into a female (6 + 2X) ; fertilized by a
sperm carrying a Y chromosome (3 + Y), it becomes a male
(6 + X + Y). A similar condition prevails in man where the
zygotic number is 48. In the female these 48 chromosomes are
made up of 23 pairs of autosomes plus two X chromosomes, and
in the male of 23 pairs of autosomes plus one X and one Y
chromosome. X and Y combinations sometimes are found in
reversed order so far as the sex is concerned, as in birds where the
female has the XY combination and the male the XX.

Sex Determination. — The term zygotic sex refers to the sex,
male or female, established at fertilization, presumably as a
result of certain chromosome combinations. The definitive sex is
the sex that finally develops. Under normal conditions it would

234 GENERAL ZOOLOGY

appear that the zygotic sex is the same as the definitive sex.
However, it has been conclusively shown that under abnormal or
under experimental conditions, the zygotic sex may be altered, or
completely changed. That amounts to saying that an animal
determined as a female at fertilization may be changed to a
definitive male or vice versa. Thus when the functional ovary
of a Brown Leghorn is removed early in life, the rudimentary
right gonad may develop into a testis and the bird takes on most
of the characteristics of the male. This seems to indicate that
the female in this case is heterozygous for sex, i.e., that it has the
potencies of both male and female sex organization. This is not
true of the cock, where removal of the testes does not produce a
female, but a kind of neutral condition known as the capon. In
the female the presence of the normal ovary and, as a result, the
presence of hormones secreted by the ovary, exercise a restraining
influence on the development of inherent male potencies. Hens
in their old age sometimes crow like cocks, a condition that also
seems to be due to the removal of active ovarian tissue through
atrophy. There is at least one authentic case on record of a hen,
after having laid eggs that produced normal chicks, changing into
a cock that became a functional male in all respects even to the
point of "fathering" two chicks. In this case postmortem
examinations showed that the ovary had been destroyed by
tuberculosis.

There is nothing specific in the X or Y chromosome as sex
determiners. Neither is endowed with what might be called
exclusive qualities of maleness or femaleness. Some observations
indicate that the result of normal XX or XY combinations seems
to rest on the relative proportion of autosome to sex chromosome
material. Thus the presence of XX or XY in Drosophila does
not necessarily produce a female or male, respectively. Studies
of flies in which abnormal chromosomal combinations had been
formed as a result of nondisjunction in the maturation divisions,
shows that flies with 2 X chromosomes but with 3 each of the
autosomes, making a total of 11, are not females but intergrades
between males and females. The tendency toward maleness in
such cases would seem to be due to the fact that there is less X
chromosome material in proportion to autosomes than is present
in normal females. In other words, sex factors exist in the
autosomes as well as in the "sex" chromosomes.

CELL DIVISION AND GAMETOGENESIS

235

In pigeons, it has been found that the female-producing egg
is not only larger than the male-producing egg but also contains
a greater amount of chemical potential energy. Ordinarily
pigeons lay eggs in clutches of two, one of which on the average
develops into a male and the other into a female. It has been
discovered that if eggs are removed from the nest as fast as laid,
the female is stimulated to lay a greater total number of eggs
than normally and that as a result the percentage of female-
producing eggs is increased. Presumably the overstimulation
caused by enforced egg-laying activity led to the production of
eggs of greater metabolic capacity. The normal sex ratio of the
offspring is changed by changing the metabolic rate of the
mother.

Delaying fertilization in frog eggs for three or four days
increases the number of males. Frog tadpoles reared at a
temperature of 32°C. produce a pre-
ponderance of males. These and
many other experiments might be
cited to illustrate the fact that the
zygotic sex may be changed by factors
that in some way disturb the metabolic
balance that normally accompanies, or
is correlated with, a certain combina-
tion of chromosomes.

Chromatin Diminution. — As a gen-
eral rule, the zygotic number of
chromosomes, established at fertiliza-
tion, continues to be the number found
in the somatic cells of the embryo and later of the adult, and
also the number occurring in the presynaptic stages of the
germ cells. An exception to this generalization is the phe-
nomenon of chromatin diminution, as it occurs in certain species
of nematode worms belonging to the genus Ascaris. In Ascaris
megalocephala bivalens, the zygotic number of chromosomes is 4.
In the first and second cleavages of the fertilized egg, each of these
large V- or U-shaped chromosomes is divided longitudinally
in the usual way, so that each of the four cells receives 4 chromo-
somes. In the late prophase stages of the next cell division
(third cleavage) in each of three cells the ends of the chromosomes
are cast off into the cytoplasm and the chromosomes that actually

S.C.

Fig. 144. — Third cleavage of
egg of Ascaris megalocephala,
semidiagrammatic. s.c, stem
cell.

236 GENERAL ZOOLOGY

appear later on the spindle are numerous small spherical bodies
formed from the middle portions of the chromosomes. These
divide as chromosomes and appear in succeeding cells descended
from the three. Thus as a result of the process of chromatin
diminution, the ends of the chromosomes are eliminated and the
remainder of each chromosome breaks up into numerous smaller
fragments. In the fourth cell, called the stem cell, the chromo-
somes appear in their usual form and are divided in the usual
manner. In the fourth and fifth cleavage, chromatin diminution
again takes place in one of the descendants of the stem cell.
As a result, there are established two kinds of cells with regard to
chromatin content, diminished and undiminished. In Ascaris
the somatic cells are derived from embryonic cells in which
chromatin diminution has occurred, and the germ cells from
undiminished cells (Fig. 144).

CHAPTER XII
ONTOGENY

Ontogeny is the development of the individual; phylogeny is the
development of the race. Since the history of a race is composed
of a number of individual life histories, phylogeny and ontogeny
are merely different aspects of the same process, viz., develop-
ment. The ovum and sperm are the basic elements from which
the individual develops, while individuals in turn constitute the
elements of the race. Ontogeny is the subject matter of embry-
ology, the study of the development of the individual. In the
study of organic evolution interest is focused on the origin and
history of the larger unit, the race.

Germ Cells. — The egg and spermatozoon are alive by virtue of
the fact that they are derived from living cells. Reproduction
of an individual animal from germ cells is not a creation of life,
but a continuation of life in the form of an individual animal
body. New individuals are alive because they are produced from
living germ cells. The egg is a relatively large cell, whose bulk
is due to the presence in the cytoplasm of a greater or lesser
amount of yolk stored there during the growth period of oogene-
sis; the amount of yolk being correlated with the conditions
under which embryonic development takes place. Thus eggs
scantily supplied with yolk, such as a human egg (Fig. 145),
receive nutriment from maternal tissues during intrauterine
development. Other eggs poor in yolk, such as the starfish egg,
develop rapidly to a free-living larval stage capable of gaining
food from the environment, which is sea water. Large-yolked
eggs usually develop outside the body of the mother, the yolk
serving as the source of nourishment. Exceptions to this are
found in viviparous sharks and snakes, whose large-yolked eggs
develop in the uterus. Eggs that are laid are provided with a
shell of some sort to protect the developing embryo. The shell
is not a part of the egg proper, but is secreted about the egg as it
descends the genital tract. Physiologically the egg, before it is
stimulated to develop by the sperm or by other means, is a cell

237

238

GENERAL ZOOLOGY

whose metabolism has almost come to equilibrium. Eggs requir-
ing fertilization die unless fertilized.

The spermatozoon is a very small fraction of the size of the
egg, even if the egg has but little yolk. In sharp contrast with
the immobility of the egg, the sperm is usually capable of ener-

/
Memh

-o

Fig. 145. — A nearly ripe human ovum in the living condition. The ovum
is surrounded by follicle cells [FC) inside of which is the clear membrane (Memb)
and within this is the ovum proper containing yolk granules (F) and a nucleus
(N) embedded in a clear mass of cytoplasm. Magnified 500. B, two human
spermatozoa drawn to about the same scale of magnification. (From Conklin,
Heredity and Environment, Princeton University Press. After 0. Hertwig; B after
G. Retzius. By permission.)

getic movements by means of its flagellum or tail. In some forms,
such as nematode worms and crustaceans, a tail is lacking and the
sperm is incapable of rapid locomotion (Fig. 146). Spermatozoa
vary greatly in structure. In motile forms, the sperm consists of
two general regions, (1) the head, which proceeds first in locomo-
tion and which contains the chromatin, and (2) the tail, which is a
stout flagellum, sometimes provided with a fin. Between the
head and tail, a neck or middle-piece region may be recognized.

ONTOGENY

239

Fertilization. — If mature sea urchin or starfish eggs are placed
in a shallow glass dish under a microscope and a drop of sea water
containing ripe sperm from a male is added, one can readily see
that almost at once each egg is surrounded by a vibrating fringe
of spermatozoa, each of which seems to be doing its utmost to gain
entrance into the egg (Fig. 147). The spermatozoa reach the egg

Fig. 146. — Types of spermatozoa. A, salamander. {After Ballowitz.) B,
annelid (Nereis). (After Lillie.) C, guinea pig. (After M eves.) D, bird (Phyl-
lopneuste). (After Ballowitz.) E, sturgeon. (After Ballowitz.) F , bat. (After
Ballowitz.) G, turbellarian (Castrada). (After Luther.) H, crustacean (Pin-
notheres). (After Koltzoff.) I, crustacean (Homarus). (After Herrick.) J,
nematode (^.scan's). (After Scheben.) a, apical body; n, nucleus; r, refractive
body. (From Sharp, Introduction to Cytology.)

by swimming movements executed by whiplike contractions of
the tail, but these mechanical movements do not completely
account for the results. Thus it has been shown that ripe eggs
of a number of marine forms, when placed in sea water,
give off a substance called fertilizin, which forms a necessary
link in the fertilization process; for if this substance is removed
from the eggs by repeated washings in sea water, the washed eggs
are not fertilized when brought in contact with normal active

240 GENERAL ZOOLOGY

spermatozoa. Fertilizin seems to sensitize the sperm and as a
result of this the sperm is capable of inducing reactions in the
egg, leading to engulfment of the sperm when the latter touches
the egg. Penetration of the egg by the spermatozoon is not a
mechanical boring process.

Entrance of the Sperm. — The ootid is the final stage of oogene-
sis, but in some cases the formation of the polocytes, which marks
the completion of the maturation divisions, is not accomplished
until after the egg (oocyte) has left the ovary. There is no
general rule regarding the stage of development reached by the
egg before it is laid or fertilized. In some species the egg is ready
for impregnation or insemination by the sperm at the primary

A B C

Fig. 147.- — Entrance of the spermatozoon into the egg of the starfish, Asterias.
(After Fol.) A, a single spermatozoon (s) has penetrated the jelly (j) and
become attached to the fertilization (entrance) cone (c). B and C, succeeding
stages in the engulfing of the spermatozoon, m, fertilization membrane.

oocyte stage, in which case the sperm enters before the polocytes
are formed. In fact, unless the sperm does enter such an egg,
no maturation divisions take place. In other cases, the sperm
enters at the secondary oocyte stage, and in still others in the
ootid stage.

Egg of Nereis. — The reactions of the egg that follow or result
from the contact of the sperm with the egg are well illustrated
in the case of the marine annelid, Nereis. When the egg of
Nereis is deposited in the water, it is in the primary oocyte stage,
neither polar body having been formed. The nucleus occupies
a central position and the cytoplasm contains numerous yolk and
oil globules. The egg is bounded by a thin vitelline membrane
which is its only covering when it is laid. The spermatozoon
consists of a flattened head, middle piece, and tail. The apex
of the head is provided with a short spike, called the perfora-
torium, by means of which the sperm attaches itself to the egg

ONTOGENY

241

membrane. The instant a sperm strikes the vitelline membrane,
a jellylike substance begins to flow out from the cortical region
of the egg and continues to flow until the egg is surrounded by a
thick gelatinous envelope, except at the point where the sperm is
attached to the egg. At this point there is a funnel-shaped
depression in the jelly (Fig. 148). The jelly must not be con-
fused with fertilizin, which is soluble in sea water and is given
off from the egg independently. As the single sperm adheres to

Fig. 148. — The egg of Nereis 12 minutes after insemination; the secretion
of the jelly is completed, the walls of the emptied alveoli of the cortical layer
appearing as radiating lines across the perivitelline space. The spermatozoon
lies in the funnel-shaped depression in the jelly; the egg having formed a fertili-
zation cone crossing the perivitelline space and touching the membrane beneath
the spermatozoon, re, fertilization cone; J, jelly; M, egg membrane; s, sperma-
tozoon. (After Lillie.)

the egg by means of the perforatorium, the unsuccessful or
supernumerary spermatozoa are pushed away from the vitelline
membrane by the outflow of jelly. The jelly comes from alveoli
in the cortical zone of the egg directly beneath the vitelline
membrane and after the discharge of the jelly, a -perivitelline
space appears, traversed by delicate radiating strands. The
impact of the sperm with the egg produces a small puncture
in the vitelline membrane and evokes the formation of the
fertilization cone. The fertilization cone arises in the cytoplasm
opposite the point of attachment of the sperm and pushes out

242

GENERAL ZOOLOGY

across the perivitelline space until it touches the vitelline mem-
brane.

The next step is the retraction of the cone, as a result of which
the sperm head is pulled into the egg, leaving the middle piece
and tail outside. As the sperm head is drawn through the hole
in the vitelline membrane, it is stretched into an elongated
shape (Fig. 149), and as it is pulled away from the egg membrane

. .^:::-:^M: l^2$?'M ^■'4)-)^2

e f

Fig. 149. — a, spermatozoon of Nereis, h, head; m, middle piece; p, perforator-
ium; t, tail, b-f, penetration of the spermatozoon into the egg. b, 37 minutes
after insemination. The perforatorium is imbedded in the fertilization or
entrance cone (closely stippled) which has withdrawn from the surface mem-
brane. The tail of the spermatozoon is not shown, c, d, e, stages from eggs
killed 48 minutes after insemination, showing the cone sinking into the egg
and drawing the sperm head with it. f, 54 minutes after insemination, the
sperm head is entirely within the egg, leaving the middle piece (m) outside.
(After F. R. Lillie.)

deeper into the interior of the egg, it gradually becomes vesicular
(Fig. 150). In the meantime the first and second polar bodies
are formed and the primary oocyte is converted into an ootid.
The nucleus of the ootid and the nucleus derived from the head of
the sperm, the sperm nucleus, unite to form the fertilization
nucleus. A spindle then forms, the nuclear membrane dissolves,
and the egg is ready to undergo the first cleavage. The chromo-
somes of the first cleavage spindle consist of those contributed
by the egg nucleus and of those contributed by the sperm.

ONTOGENY

243

Egg of the Frog. — The freshly laid egg of Rana pipiens is
spherical in form and measures about 2 mm. in diameter; it is
bounded by a thin membrane, which in turn is surrounded by the

PB

•-^ •--%•-■->--";, .^'/^C :'.' ft

•SN

Fig. 150. — Penetration of the spermatozoon in the egg of Nereis, a, same
stage as Fig. 149f, showing first maturation spindle; b, 54 minutes after insemina-
tion, showing rotation of sperm head and cone, and the origin of the sperm aster
from the pole of the nucleus opposite the cone; c, 67 minutes after insemination
showing the sperm nucleus between the unequal centrosomes and asters of the
sperm amphiaster; d, later stage after insemination showing the fused egg and
sperm nucleus lying between the cleavage centers which seem to be a further
development of the sperm amphiaster. ec, entrance cone; Mi, M2, first and
second maturation spindle; pbi, first polar body; sn, sperm nucleus. (After
F. R. Lillie.)

jellylike egg capsule. More than half of the egg is colored by a
dark pigment located in the outer portion of the cytoplasm,
leaving a much smaller, cream-colored area on the opposite side.

244 GENERAL ZOOLOGY

The color of the dark portion fades out gradually at its edge
toward the light area. The cream color of the light area is the
color of the yolk seen through the egg membrane. Since the
yolk is concentrated in the light-colored or vegetal half of the egg,
the dark or animal half is relatively free of yolk. The egg is
radially symmetrical, its principal axis being a line connecting
the animal pole, a point in the center of the dark area, with the
vegetal pole, a point in the opposite half in the center of the light
area (Fig. 151, A).

When the egg is released from the ovary, it is in the primary
oocyte stage, the nucleus lying in the animal half of the egg near
the animal pole. The first polocyte is formed after the egg
reaches the oviduct and then, without an intervening resting
stage, the mitotic spindle of the second maturation division
develops as far as metaphase, beyond which under normal condi-
tions it does not proceed until the egg is fertilized. The first
polocyte is extruded near the animal pole. Just before the
formation of the spindle, the pigment thins out in the region of
the animal pole to form what is known as the fovea of the egg.
The fovea is also marked by a slight flattening in the surface
of the egg.

Fertilization of the Frog's Egg. — During amplexus, eggs and
sperm are deposited in the water simultaneously and the eggs
are at once fertilized. In this process a single spermatozoon
makes its way through the jelly of the capsule and penetrates
the egg at a point about 40 degrees from the animal pole. The
entrance of the sperm acts as a stimulus for the secretion of fluid
from the egg which raises a fertilization or vitelline membrane
from the egg surface and creates a perivitelline space within
which the egg orients itself with the dark animal pole up. A
second result of the entrance of the sperm is the formation of a
gray crescentic area on the surface of the egg in the equatorial
region opposite the point of penetration (Fig. 151). A third
result is the completion of the second maturation division and
the formation of an egg nucleus.

Usually the entire spermatozoon enters the egg, but the tail
quickly disintegrates and plays no further part in fertilization.
As the head and middle piece move through the egg, with the
middle piece foremost, the penetration path is marked by a trail
of pigment, which enables one to determine that the entrance

ONTOGENY

245

path lies in a plane of a meridian of the egg that bisects the gray
crescent. As a result the egg now has a bilateral symmetry.
The entrance path also determines the bilaterality of the future
embryo, since in the majority of cases the plane of the entrance
path coincides with the median plane of the embryo.

The middle piece of the sperm as it moves through the egg
forms a centrosome which soon divides into two. The egg and

&*&.--,

^

B

Fig. 151. — A. Unfertilized frog's egg, drawn without the capsule. B. Fertil-
ized frog's egg showing gray crescent (c). C. Section of frog's egg in the plane
of the first cleavage, showing the bisected gray crescent (c) ; s, penetration path
of spermatozoon. D. Surface view of frog's egg during first cleavage. (C and
D after Schultze.)

sperm nucleus come together near the center of the egg; a spindle
develops between the centrosomes; chromosomes are formed in
each nucleus; the nuclear membranes dissolve and the egg is
ready for the first cleavage. The diploid number of chromo-
somes of Rana pipiens is 26, of which 13 are furnished by the egg
nucleus and 13 by the nucleus derived from the head of the sperm.
Mature, unfertilized eggs of the frog can be made to develop
by pricking them with a needle. Since under these conditions
no additional chromosomes are brought into the egg by a sperm-
atozoon, one would expect embryos developing from such eggs

246 GENERAL ZOOLOGY

to have the haploid number of chromosomes, if it may be taken
for granted that the second maturation division is completed.
As a matter of fact, many of the larvae produced this way are
haploid in chromosome number; but in addition others are
diploid and a few triploid and tetraploid. How chromosome
numbers greater than the haploid are produced in these embryos
is a matter of speculation, but it is interesting that only diploid
animals metamorphose into frogs of both sexes. It would seem
that in the frog the diploid complement of chromosomes is neces-
sary for normal development.

Cleavage.- — Cleavage is a period in development following
fertilization, marked by rapid mitotic cell divisions, in each of
which the diploid number of chromosomes is present. An excep-
tion to this so far as the chromosomes are concerned is in cases of
chromatin diminution, such as occurs in Ascaris, referred to in
a previous chapter; but in the majority of animals it is generally
believed that the diploid number of chromosomes appears on each
cleavage spindle. This seems to be the case in the frog. The
cells produced in the course of cleavage are known as blastomeres.
Some of these produce primordial germ cells, which later partici-
pate in gametogenesis, and the remainder become somatic cells.

The manner in which cleavage takes place depends upon the
quantity and distribution of the yolk in the egg. The frog's egg
is called a telolecithal type of egg because the yolk is concentrated
about one pole of the egg. The first cleavage plane (the plane
separating the first two blastomeres) passes through the principal
axis of the egg, and therefore through a meridional plane, and
bisects the gray crescent. The furrow separating the blastomeres
begins in the animal pole and gradually cuts through to the
vegetal pole, forming two complete cells adhering to each other
on their adjacent faces (Fig. 152). Since the entire egg is
divided, the cleavage is spoken of as holoblastic. In contrast to
this, in a meroblastic cleavage, such as takes place in the hen's
egg, cleavage is confined to a small yolk-free area at the animal
pole, the yolk remaining undivided. In the frog the first cleavage
plane in the majority of cases coincides with the median plane
of the future embryo. In such cases the first two blastomeres
represent the right and left sides of the embryo. The second
cleavage plane is also meridional, at right angles to the first. The
third cleavage plane passes about 60 degrees from the animal

ONTOGENY

247

pole, cutting the planes of the first two at right angles and for
that reason is called equatorial. The 8 blastomeres formed at the
end of the third cleavage consist then of an upper quartet of cells
about the animal pole smaller in size than the four cells of the
lower half of the egg. From this point on the rate of cleavage is
retarded in cells located in the vegetative region of the egg.

Fig. 152. — Cleavage in the frog's egg. 1,2-cell stage, 1st cleavage plane bisect-
ing gray crescent, c. 2. 8-cell stage. 3. 8-cell passing into 16-cell stage. 4.
32-cell stage. 5-6. Later cleavage stages. 7. Section of egg in early stage of
blastocoel, b, or segmentation cavity; approximate position of gray crescent
material shown by stippling. 8. Section of later blastula; i, point where invag-
ination takes place.

Since the dark pigmented cells formed of animal-pole material
divide more rapidly, they soon become smaller in size than
those containing yolk. The cleavage thus becomes unequally
holoblastic.

Blastula. — A section of a developing egg at the end of the third
cleavage shows that the inner surfaces of the 8 blastomeres are
rounded off so as to produce a space or cavity between them.
This space is called the segmentation cavity or blastocoel and marks

248 GENERAL ZOOLOGY

the beginning of the blastula stage in development. The walls
of the blastula bounding the blastocoel become several cell layers
in thickness as cleavage progresses. The cells forming the roof
and sides of the blastula are derived from animal pole cells and
are accordingly smaller than the yolk-laden cells forming the
floor of the blastula. The blastocoel becomes filled with fluid
absorbed from the outside or produced by metabolism of the
cells of the blastula (Fig. 152, 7, 8).

Gastrulation. — Cleavage at first merely produces a number of
cells in the dark and light areas, respectively, so that for a time
the distribution of pigment is similar to that of the egg before
cleavage. Gradually the pigmented area encroaches on the
light area as the result of an overgrowth of the small pigmented
cells. The advancing edge of the pigmented cells, known as the
germ ring, slowly covers the vegetative cells, until the latter are
completely hidden, except in a very small area, known as the
yolk plug. At the beginning of the overgrowth a short horizontal
notch forms at the margin of the germ ring in the region of the
gray crescent and gradually extends around the entire margin of
the ring. Thus the advancing margin of the germ ring is marked
by a shallow groove which is more pronounced at its point of
origin. The germ ring may now be considered as a blastopore
whose dorsal lip lies in the region of the gray crescent. At the
dorsal lip an active invagination of cells takes place principally
from the animal-pole region, though yolk-laden cells are also
lifted from the floor of the blastocoel. As the germ ring is
reduced in size, the invagination becomes active throughout its
extent, but only in the region of the dorsal lip is a large cavity,
called the gastrocoel or archenteron, formed (Fig. 153). As this
cavity pushes into the interior of the gastrula, the blastocoel is
gradually obliterated. Externally the cells forming the yolk
plug are finally covered up by the complete closure of the blasto-
pore and gastrulation is completed. The region of the blastopore
may now be referred to as the posterior end of the embryo.

Germ Layers. — The gastrocoel, produced largely as a result of
the invagination of the dorsal-lip region of the blastopore, lies in
the animal half of the egg. Its roof and side walls are derived
mainly from animal-pole cells and its floor is made up of yolk
cells. These cells represent the endoderm of the embryo except
for some cells lying in the mid-line of the roof which separate to

ONTOGENY

249

form the rod-shaped notochord. The cells covering the outside
of the gastrula may now be called ectoderm. The rudiment of
the notochord thus extends from the blastopore region forward
under the ectoderm and above the roof of the endodermal

;>gc

Fig. 153. — Schematized diagrams of gastrulation in the frog. A. optical
section of blastula from left side showing the location of the gray crescent, gc,
and the edge of the germ ring, gr, encroaching upon the yolk. B. beginning
of gastrulation showing the hypothetical composition of the gray crescent sub-
stance, according to various interpretations. C. showing downward move-
ment of the dorsal lip of the blastopore, dl, and the appearance of the ventral
lip, vl, on the opposite side of the gastrula. D. formation of gastrocoel, separa-
tion of notochord from endoderm. b, blastocoel; dl, dorsal lip; ec, ectoderm; en,
endoderm; g, gastrocoel (archenteron) ; gc, gray crescent; gr, lower edge of germ
ring; m, mesoderm of ventral lip; no, notochord; np, neural plate; vl, ventral lip;
yc, yolk cells; yp, yolk plug. Curved arrows outside figures; clockwise show
movement of dorsal lip; counter-clockwise show rotation of embryo. Other
arrows indicate direction of cell movements.

cavity. The third germ layer, the mesoderm, originates from
cells lying between the ectoderm and endoderm, just within the
lip of the early blastopore; and as the blastopore closes, meso-
derm is formed throughout the extent of the blastopore. From
this region mesoderm grows forward on either side of the noto-

250 GENERAL ZOOLOGY

chord between the ectoderm and endoderm. On either side
of the notochord the mesoderm becomes segmented to form a
row of boxlike structures called somites; but lateral to each
series of somites, the mesoderm extends as a sheet of cells between
the ectoderm and endoderm. Later this sheet splits into two
layers, one applied to the endoderm and the other to the ecto-
derm, the space between the two forming the coelom or body
cavity. The somites later are transformed into muscle and
connective tissue.

The ectoderm of the frog produces the epidermis of the skin,
cutaneous glands, the entire nervous system, the lens of the eye,
the lining of the oral and cloacal cavities, the enamel of the
teeth, and some muscle. In other vertebrates the ectoderm also
produces mammary glands, sweat glands, hair, horns, hoofs,
nails, scales (of reptiles and birds), and feathers. The meso-
derm, in addition to producing most of the muscle tissue, forms
the corium of the skin, connective tissue, blood and lymph
vessels, most of the organs of the excretory and reproductive sys-
tems, peritoneum, and skeleton, including the dentine of the
teeth. The endoderm forms the inner layer of the alimentary
canal, the epithelium of the liver, the pancreas, thyroid and
thymus glands, and the respiratory tract. It also forms the
lining of the Eustachian tube and middle ear.

Neural Tube. — At the end of gastrulation, the ectoderm over
the entire surface is divided into an outer and inner layer of
cells. As the blastopore is closing, a groove, the neural groove,
is formed in the ectoderm, extending forward for about 90 degrees
from the blastopore. This groove is flanked on either side by
low ridges in the ectoderm called lateral neural folds, which are
connected anteriorly by a curved transverse fold, the whole form-
ing a continuous low wall enclosing the medullary plate in front
and at the sides. The inner layer of the ectoderm of the medul-
lary plate is several cell layers in thickness, and a similar thicken-
ing of the inner ectodermal layer is present in the outer slope of
each of the lateral neural folds. These lateral thickenings of the
inner layer of ectoderm, known as neural crests, lie one on either
side of the medullary plate, to which each is connected along its
inner edge (Fig. 154). The neural folds increase in height, grow
toward each other, and fuse along their edges, thus converting
the medullary plate into a neural tube. In the formation of the

ONTOGENY

251

tube, the neural crests are carried up with the neural folds but
are not included in the neural tube. After the fusion of the
neural folds, the outer layer of ectoderm becomes continuous over
the neural tube which is thus cut off from the outside. The
closure starts in the region of the future hindbrain and then
proceeds forward and back, the anterior end being the last to
close. Posteriorly the neural tube connects with the blastopore
by the neurenteric canal, which later disappears. The neural
crests at first are continuous strands of cells one on each side of
the neural tube. Later these strands break up into segments
from which develop spinal and parts of cranial nerve ganglia.

m%.

m.p.

Fig. 154. — Semidiagrammatic cross sections illustrating four stages in the
development of the neural tube and spinal ganglia of the frog, e, endoderm; g,
spinal ganglion; m, mesoderm; m.g., medullary groove; m.p., medullary plate,
n, notochord; n.c, neural crest; n.t., neural tube; o.e., outer ectoderm. Inner
layer of ectoderm and structures derived from it are shown in solid black.

The neural tube is the rudiment of both the brain and spinal
cord. Sensory axons arising in the neural-crest ganglia grow back
to the neural tube to form sensory roots of cranial and spinal
nerves. The motor roots of both cranial and spinal nerves
originate from nerve cells located in the neural tube. The
anterior half of the neural tube becomes constricted into three
regions known as the forebrain, midbrain, and hindbrain, which
in turn give rise to the five divisions of the adult brain. The
ventricles of the brain are derived from the cavity of the neural
tube. In the spinal cord this cavity is the central spinal canal.
The sympathetic nervous system develops from cells that
migrate from the spinal ganglia. Cranial portions of the neural
crests in addition to forming ganglionic cells also produce con-
siderable quantities of nonnervous mesenchyme which later give

252 GENERAL ZOOLOGY

rise to connective tissue structures. Figure 155 is a longitudinal
section of an early frog embryo.

Ganglia. — Placodes are thickened areas in the inner layer
of the head ectoderm which give rise to sensory epithelia and to
varying portions of the ganglia of certain cranial nerves. Thus
the ganglia of cranial nerves V, VII, IX, and X are formed of
placodal cells as well as of cells derived from the neural crest.
The ganglion of the auditory nerve (VIII) is formed entirely
from placodal material. The ganglia of spinal nerves, as already
noted, are derived entirely from the neural crests. The ganglia

mb

Fig. 155. — Diagrammatic longitudinal section of early frog embryo, a, anus;
b, blastopore; e, ectoderm; en, endoderm; fb, forebrain; h, hypophysis; hb, hind-
brain; m (arrow), mouth; m, mesoderm; mb, midbrain; n, notochord; nc, neu-
renteric canal; p, pharynx; r, rectum; sc, spinal cord; y, yolk.

of the sympathetic and parasympathetic systems are formed
later from cells that migrate out from the central nervous system.
Notochord. — The notochord develops from cells forming part
of the roof of the archenteron, derived by invagination from the
dorsal-lip region of the blastopore. These cells, at first insepa-
rable from those forming the lining of the archenteron, soon
become organized or differentiated into a rod-shaped form lying
directly beneath the medullary plate and, later, the neural tube.
This rod of cells is the rudiment of the notochord which histo-
logically is classified as a form of connective tissue. It becomes
completely separated from the endodermal cells of the arch-
enteron. The notochord is developed in all animals belonging
to the phylum Chordata, which includes the subphyla Enterop-
neusta, Tunicata, Cephalochorda, and Vertebrata. The noto-
chord evolved apparently as a skeletal axis and serves as such

ONTOGENY 253

in the embryonic or adult stages of all chordates. In some of
them such as Cephalochorda and some vertebrates (Cyclostomata)
it persists in the adult animal. In most vertebrates, however,
including the frog, the embryonic notochord is replaced by the
centra of the vertebra, developed from tissue derived from the
somites. As pointed out in an earlier chapter, in many fishes the
notochord persists in the adult in spaces between the ends of
vertebrae and to a slighter degree in the axial portions of the
centra. In cyclostomes it persists in the adult in a fully devel-
oped condition as a long, flexible, tough rod, tapered at the ends,
extending from about the middle of the brain to the opposite
end of the body and lying directly below the central nervous
system. It functions primarily as a supporting structure, is
nonnervous in character, and should not be confused with the
neural tube (Fig. 155).

Alimentary Canal. — The cells lining the archenteron after the
separation of the notochord from its roof, represents the definitive
endoderm of the frog and later forms the lining or mucosa of the
alimentary canal, except that of the mouth and of the cloaca.
Since the lungs, liver, and pancreas, as well as the thyroid and
thymus glands, originate as outgrowths of the embryonic ali-
mentary canal, all of these structures are endodermal in origin.
The remaining parts of the wall of the alimentary canal, viz.,
submucosa, muscularis, and serous membrane (visceral peri-
toneum) are derived from the lateral unsegmented mesoderm
that grows down between the endoderm and ectoderm. About
two weeks after fertilization, the tadpole hatches, i.e., it leaves
the egg capsule and swims about. By this time the animal has
an elongated form with a short tail but no mouth or anal opening.
On the underside of the head is a curved sucker, by means of
which the tadpole attaches itself to solid objects. The mouth is
formed by an invagination of the ectoderm just in front of the
sucker to form a pit which deepens until it touches and fuses
with the endoderm; the fused layers shortly after become per-
forate. An anal opening is formed in a similar way at the pos-
terior end of the body. With the formation of the mouth, the
sucker begins to atrophy and eventually disappears. Until a
mouth is formed the embryo is dependent for nutrition upon yolk
stored in the large endodermal cells, particularly those forming
the floor of the alimentary canal. After the mouth is formed, the

254

GENERAL ZOOLOGY

yolk rapidly disappears and the tadpoles gain sustenance from
food taken in at the mouth. The alimentary canal lengthens
and becomes spirally coiled. A peculiar larval feature of the
mouth is the development of horny sheaths on the jaws which
function as rasping organs. The lips are also provided with horny
papillae which aid in obtaining food (Figs. 156 and 157).

Fig. 156. — A, B, and C, three stages in the development of mouth, suckers
and gills of Rana arvalis. D, side view of stage B. e, eye; g, gill; m, mouth;
n, nasal pit; s, sucker. (After Lieberkind.)

Branchial Structures. — In addition to the notochord, another
important distinguishing feature of the phylum Chordata is the
presence of gill clefts and of gills (in aquatic forms) in the pharyn-
geal region. At the time of hatching the rudiments of the exter-
nal gills can be seen on either side of the body just behind the
head as a pair of small conical projections. Later these are
joined by a third rudiment on each side. All three rudiments
grow rapidly, though the third remains smaller than the other
two, until they form a tufted mass on either side of the body
(Fig. 156). Each gill develops finger-shaped lobes and is supplied
with blood vessels, much as in a fish. Meanwhile four gill clefts
develop on each side, one in front of the first pair of gills, one
behind the last pair, one between the first and second and

ONTOGENY

255

Fig. 157. — Larval stage of Rana
arvalis, ventral view, in which the
suckers (d.s.) are degenerating and the
mouth is provided with horny jaws.
{After Lieberkind.)

another between the second and third. The clefts are narrow

slits connecting the pharynx with the outside of the body. The

gill slits develop from gill

pouches. These are lateral out-

pocketings of the pharynx, sepa-
rated from each other by gill

arches. A total of six pouches

are formed, of which the first and

last remain rudimentary. Each

of the remaining four on each

side enlarges until the endoder-

mal lining comes in contact with

the ectoderm when the tissue

breaks down to form a vertical

slit. The external gills now become enclosed in a branchial

chamber by the operculum, which is formed of two folds of

skin, one on each side, from the region in front and above the

first pair of gills. These two folds grow
backward over the gills, meet, and fuse
with each other ventrally and with the
body wall posteriorly, forming a common
chamber communicating with the outside
by a small opening, the spiracle, on the left
-6 side. The external gills are now resorbed
and replaced by internal gills developed
from the borders of the gill clefts. The
internal gills are aerated by water taken
in the mouth and passed through the gill
clefts into the branchial chamber and out
through the spiracle. The gills and skin
Fig. 158. Larval stage are ^^g principal organs of respiration

of Rana arvalis, ventral .

view, showing gills (g) during the larval period (approximately
still visible beyond oPer- three months). The lungs develop as

culum on left side. On

the right side the gills are outgrowths from the floor of the pharynx
enclosed in the bran- ancj are ready to take over the respiratory

chial chamber formed by

the operculum. (After function by the end of the larval period

Lieberkind.) (Fiff 158)

Paired Appendages. — During the larval period the caudal
region of the tadpole increases in size and is provided with a well-
developed vertical fin. The development of the legs begins at

256

GENERAL ZOOLOGY

about the end of the first month. Both pairs of legs are formed
from the body wall, but the forelegs develop within the branchial
chamber which makes them invisible externally until metamor-
phosis sets in. The hindlegs develop from the lateral body wall
at the base of the tail, and their development can be readily
followed from the beginning. Actually the forelegs start to
develop before the hindlegs.

Olfactory Organs. — The olfactory passages develop from a
pair of ectodermal pits on either side of the head just above the
oral invagination of the early larva (Fig. 156). These pits
deepen and eventually connect with the pharynx to form the

internal nares. The olfactory
nerve develops from cells lining
the olfactory passages. These
cells are the olfactory cells, from
which axons extend back to the
forebrain to form the olfactory
nerve. The passages them-

Fig. 159. — Cross sections illustrating , , , „ .

two stages in the early development of selves are enlarged to form the

the inner ear of Rana sylvatica. a.c, nasal Cavities.

alimentary canal; o.p., otic pit; o.s., ,-, ,-,-,, . , ,

otic sac, developed from the pit and Ear.— the inner ear develops

which in later stages forms the mem- at about the same time as an
branous labyrinth. (After Pollister and . ■ ■, • , i • i ,.

Moore.) °^lc Pl* m the inner ectoderm ot

the skin at the level of the
hindbrain (Fig. 159). The pit becomes cut off from the surface
and later differentiates into the membranous labyrinth. The
middle ear develops later through an enlargement of the vestigial
first (hyomandibular) gill pouch. This gill pouch, which in
fishes is provided with a cleft opening to the outside and serves
as a respiratory passage, in the frog never breaks through to the
outside. It may therefore be regarded as an incomplete gill
cleft. In the frog as the gill pouch enlarges, it encroaches on the
surface until its outer wall is reduced to a thin membrane, the
tympanic membrane.

The enlargement forming the tympanic cavity is connected
with the gill pouch by a narrow channel, which is the Eustachian
tube of the adult frog. The columella of the middle ear is derived
from the dorsal wall of the tympanic cavity from which it
becomes separated by excavation to extend freely from the

ONTOGENY

257

o.v.

o.c.

I

tympanic membrane to unite with a small cartilage plugging
the foramen ovale.

Eye. — The optic vesicles are paired evaginations of the ventro-
lateral walls of the forebrain which grow out toward the ectoderm.
Each optic vesicle becomes constricted at its point of attach-
ment to the brain to form the optic stalk. As the vesicle
approaches the ectoderm, its outer surface invaginates to form a
double-walled optic cup (Fig. 160). The invagination is contin-
ued along the ventral side of the
cup to form the choroid fissure.
In the meanwhile a thickening in
the ectoderm opposite the mouth
of the cup is pinched off the inner
surface of the ectoderm to form
the lens. The choroid fissure now
closes down to the optic stalk
where an opening persists, through
which blood vessels and nerve
fibers enter the optic cup. When
the choroid fissure is closed, the
optic cup is a double-walled hemi-
sphere whose cavity is largely

occupied by the lens (Fig. 160). Fig. 160.— Cross sections illustrat-
The ectoderm Overlying the lens ing four stages in the development
. . , j. ,r of the eye of Rana sylvatica. 1,

becomes transparent to form the optic vesicle stage. 2 optic cup

OUter layer of the cornea. The stage; 3, formation of the lens; 4,
, - . , , . . . c , separation of lens from ectoderm.

inner layer of the latter is formed e> cornea. h lens; 0.c., 0Ptic cup;
of mesodermal cells (mesenchyme) . o.s., optic stalk; o.v., optic vesicle;

„,, . . , , - , r, retina (inner layer of optic cup).

The ins develops from mesoderm {After PoUister and Moore.)
at the edge of the optic cup and

also perhaps from ectoderm derived from the cup. The inner
layer of the optic cup becomes thicker than the outer layer early
in its formation. It gives rise to the sensory part of the retina in
which the rods and cones develop. The outer layer of the optic
cup develops into the pigmented layer of the retina. The
choroid and sclerotic coats of the eye are derived from mesoderm
formed over the pigmented layer of the retina. The fibers
forming the optic nerve arise from cells lying in the inner surface
of the retinal layer of the optic cup. This passes out of the eye

258 GENERAL ZOOLOGY

through the lower (inner) end of the choroid fissure and then
along the ventral side of the optic stalk to the brain. The
optic stalk itself gradually extends around the optic nerve to
produce a sheath for the latter.

Metamorphosis. — The tadpole of Rana pipiens undergoes
metamorphosis, i.e., transformation into a frog, toward the end
of the third month of development. As the time nears, the
tadpole frequently comes to the surface and gulps air, which is
drawn into the lungs. The principal changes occurring during
metamorphosis are: (1) the final stages in the development of
the lungs, accompanied by the atrophy of the gills and the
closure of the gill slits; (2) the freeing of the forelimbs by the
rupture of the operculum, which disappears, and a marked
lengthening of the hindlimbs; (3) the enlargement of the stomach
and the shortening of the intestine; (4) the shedding of the larval
epidermis together with the horny jaws and the labial papillae;
and (5) the resorption of the tail.

Law of Biogenesis. — The embryos of higher vertebrates often
display certain features, mainly anatomical, which seem to
represent some sort of recapitulation of structures found in lower
forms. Thus the structure and functions of the gills of the
tadpole and the plan of branchial circulation have their proto-
types in the corresponding organs of the fish. From this it is
not to be inferred that these organs are structurally identical,
because they are not; but the similarity in the general structural
plan of these organs in the two cases is very marked and the
function so far as the gills are concerned is the same in both.
These resemblances may not seem unusual since both the fish
and the tadpole live in an aquatic habitat and for that reason
require somewhat similar respiratory organs. However, the
fact that the embryos of the higher air-breathing vertebrates,
such as reptiles, birds, and mammals, which have no aquatic
larval period in development, also pass through stages in which
the branchial region resembles that of the fish in its general
anatomy, calls for some other explanation than that of environ-
mental requirements.

To take the chick embryo of 72 hours' incubation for an
example (Fig. 161), the heart lies in an anteroventral position in
the body cavity and pumps the blood forward through a ventral
aorta (truncus arteriosus), from which it passes right and left

ONTOGENY

259

dorsally through aortic arches to a pair of dorsal aortae. The
latter convey the blood to all parts of the body and to the yolk
sac, from which it is returned to the heart by cardinal veins
and vitelline veins (from the yolk sac). The yolk-sac circulation

Fig. 161. — Circulatory system of chick embryo of 72 hours incubation;
embryonic membranes not shown. Starting at the ventricle (v) the blood
is pumped forward through the ventral aorta to the aortic arches of which the
four of the right side are shown (in solid black). The gill clefts lie between the
arches. Above, the aortic arches unite to form a dorsal aorta each of which
sends a carotid artery (right one shown) to the head. Posteriorly, the aortae
unite to form the dorsal aorta (da) which in the middle of the trunk region gives
off a right and left vitelline artery (va) carrying blood to the yolk capillary
circulation to be aerated and charged with food from the yolk. The blood
returns to the embryo by right and left vitelline veins (w) which unite and join
the sinus venosus of the heart. Blood is collected from the embryo by right and
left anterior and posterior cardinal veins (ac and pc) which form on each side a
common cardinal (c), also joining the sinus venosus. The mingled venous
blood from the cardinal veins and the aerated blood from the vitelline veins
passes from the sinus venosus to the atrium (a) and then to the ventricle, d,
diencephalon; lb, leg bud; m, mesencephalon; my, myelencephalon; o, otic
vesicle; ol, olfactory pit; s, somite (muscle); t, telencephalon; wb, wing bud.

serves to aerate the blood and to supply the embryo with nutri-
tion. The blood in passing through the gill arches is not aerated,
since that is done in the yolk sac at this stage of development.
The resemblance of the aortic arches to the afferent and efferent
branchial arteries of the fish or of the tadpole is merely ana-

260 GENERAL ZOOLOGY

tomical and not complete at that, because no gill capillaries are
present. Since the gill clefts of the chick later on disappear,
accompanied by changes in the branchial circulation, it would
seem that the only reason for their development at all in the chick
is that the chick in its development is merely following the path
along which its evolution from lower forms took place. The
fact that the chick develops gill clefts and aortic arches seems to
mean that the chick is descended from ancestors in which these
structures had a functional significance. Hence the appearance
in an embryo of a higher form of morphological relationships
characteristic of lower forms is taken as an indication of rela-
tionship between the higher and lower forms. A more accurate
picture of this relationship is obtained if corresponding embryonic
stages are compared. Thus the resemblance in the branchial
region is much closer between the chick embryo and the fish
embryo than between the chick embryo and the adult fish.

No animal in its development repeats every step in its racial
history or phylogeny — many are slurred over and omitted, and
new ones interspersed among the old — so that a general conclu-
sion must be limited to the statement that ontogeny is, to a
certain extent only, a repetition of phylogeny. This generaliza-
tion, known as the law of bioge?iesis, seems to hold throughout
the animal kingdom and has been useful in many cases in throw-
ing light upon phylogenic relationships.

Homology. — A practical difficulty in applying the biogenetic
law as a criterion for determining the origin of embryonic con-
ditions is that in the embryo old (paling enetic) characteristics
are intermingled with new (cenogenetic) ones, so that an embry-
onic structure is never a perfect recapitulation of an ancestral
structure. Thus the circulation of the vertebrate embryo only
approximates the fish type of circulation; but the inference is
implied that the fish type of circulatory system is near to the
primitive type from which the circulatory systems of the higher
vertebrates have evolved. The wings and legs of birds and the
limbs of quadrupeds and of man all develop from the same sort
of embryonic limb rudiment, which in its inception is an out-
growth of the body wall. The development of vertebrate limb
buds may be said to be practically the same for all vertebrates
up to a certain point, beyond which the path of development
diverges in various directions. Vertebrate limbs are said to be

ONTOGENY 261

homologous, by which is meant that they conform to a general
structural type because they were differentiated by evolution
from the same or corresponding part of an ancestral organism.
Similarity of development of parts is taken as an indication of
similarity in origin. Analogy refers to similarity in function.
The wings of a bird and the arms of a man are homologous but
not analogous ; the eyes of a squid and those of man are analogous
but not homologous; while the arms of an ape and the arms of
man are both homologous and analogous.

CHAPTER XIII

HEREDITY

A fertilized frog's egg develops into a frog, rather than into
a salamander or some other kind of animal, because it is endowed
with certain inherited qualities, which under proper environ-
mental conditions bring about development. Development is
accompanied or caused by an interaction of internal factors,
present in the egg, with external factors present in the environ-
ment. A certain sort of environment is required for the develop-
ment of the frog's egg, but since the eggs of other species may
develop in the same pool along with those of the frog, the factors
determining the difference in results in each case must be internal
rather than external. There is specificity in different kinds of
eggs just as there is specificity in the animals developing from
them. Each kind of egg is characterized by its own particular
kind of developmental potencies which, however, are realized
only when provided with the proper environment. The same
environment may evoke the potencies of different kinds of eggs;
but the same kinds of eggs require the same kind of environment
for their development.

In the intrauterine development of mammals there is a
similar relation between the egg and environment, the environ-
ment in such cases being the uterine tissue surrounding the egg.
The uterus is the means through which the metabolic needs of
the developing egg, and later the embryo, are met; and thus
provides an environment in which the potentialities of the
hereditary factors present in the egg can be expressed in the form
of developmental activity. Theoretically, therefore, a mam-
malian egg should develop equally well in the proper sort of
artificial culture medium outside the body; and, as a matter of
fact, this has been accomplished with rabbit ova through early
cleavage stages. As in the frog, the mammalian offspring
inherits from its parents only those qualities that are repre-
sented by factors of some sort in the fertilized egg.

262

HEREDITY 263

The fact that in parthenogenesis the egg develops without the
intervention of a sperm shows that the egg alone may possess all
of the internal factors for development and for heredity. How-
ever, since it is common knowledge that paternal characters are
inherited in cases where the egg requires fertilization, it is clear
that hereditary factors are contributed by the sperm as well as
by the egg. The problem of heredity centers in explaining the
nature and location of herditary potencies of the germ cells and,
if possible, how the hereditary characters of the adult are con-
trolled or produced by them.

We are accustomed to think of heredity as the cause or reason
for the reproduction in children of characters similar to those of
their parents; but while it is true that the reproduction of similari-
ties is due to heredity, it is also true that characters in children
differing from those of the parents are also the result of heredity.
Thus the fact that blue eyes as well as brown eyes may be
inherited from brown-eyed parents, means that blue eyes and
brown eyes, though classified as dissimilar hereditary conditions,
are really genetically related, and that the production of blue
eyes from brown-eyed parents is a result of this relationship.
Heredity may be defined as the production in successive genera-
tions of conditions not necessarily similar or identical, whose
specific character is determined by factors located in the germ
cells. The term hereditary factor or gene may be defined as a
substance or a condition in the germ cells that determines the
development of a hereditary character in the adult. Each
hereditary character presumably is represented by a factor or set
of factors in the germ cells.

Chromosome Theory of Heredity. — The germinal factors of
heredity are generally thought to be located in the chromosomes
or chromatin rather than in the cytoplasm of the germ cells for a
number of reasons, some of which may be considered. The
basic argument for this view is that, since the chromatin of the
fertilized egg is the only substance contributed in practically
equal amounts by the parents to the fertilized egg, and since in
the long-run offspring inherit equally from the parents, it follows
that the chromatin must be the hereditary substance. The
cytoplasm of the fertilized egg is entirely maternal in origin,
save for the slight addition to its bulk made in those cases where
more than the head of the sperm enters the egg; yet the heredity

264 GENERAL ZOOLOGY

of the animal developing from the egg is on the average as much
paternal as maternal. The chromosome theory of heredity finds
additional support in the fact that the biparental character of
the nuclei of the cells derived from the cleavage nucleus pro-
vides an equal opportunity for both maternal and paternal genes
to produce a biparental hereditary effect.

The disjunction of homologous chromosomes that occurs in the
reduction division of gametogenesis may be interpreted in this
connection as the mechanism which is directly responsible for
the constancy in the total number of chromosomes, since without
the reduction there would be an ever increasing number pro-
duced at each fertilization. In cases where disjunction fails to
occur, or is incomplete, it can be shown that definite hereditary
effects are produced. Change in the ratio of autosomes to sex
chromosomes produces a definite effect in the characters of the
offspring, because of a change in the normal number and relations
of factors or genes. A further reason for assuming that the fac-
tors of hereditary characters are located in the chromosomes is
that such an assumption provides the only known explanation
of the results of animal and plant breeding, some of which will be
discussed presently. The science of genetics is in fact postulated
on this assumption.

Cytoplasm in Heredity. — The chromosome theory of heredity
relegates the cytoplasm to a secondary role in development.
To students of heredity, the cytoplasm is rather superfluous
except as a source of nutrition or as a background for gene action.
The facts warrant the assumption that genes or factors account
for the inheritance of those characters that serve as material for
genetic study; but these characters, such as eye color, body
color, hair color, shape of legs and wings, etc., are largely super-
ficial characters and genetic studies as a rule fail to deal with
more fundamental characters such as those determining body
axes or body plan. The cytoplasm of the egg can be shown in
many cases to possess a certain amount of Organization that
foreshadows axial relationships in the later embryo, and this
must be taken into account in a complete explanation of heredity.
It will be recalled in this connection that in the frog's egg the
entrance of the sperm results in the formation of the gray crescent
and that in the majority of cases the future plane of bilateral
symmetry of the embryo is established by the entrance plane of

HEREDITY

265

the sperm. The cytoplasm of the egg responds to the stimulus
produced by the entrance of the sperm by a rearrangement of
its substance to produce the gray crescent, which is the first
indication of bilateral symmetry. Certainly the cytoplasm is
organized and this organization is inherited, but for reasons to be
considered later it is difficult to express the basis of this organiza-
tion in terms of hereditary factors. In general terms it might be
said that the cytoplasm of the egg is concerned with the deter-
mination of the groundwork on which the chromosomal factors
operate. It is to be noted that there is no contribution from the
sperm comparable to the cytoplasm of the egg. The latter is a
purely maternal contribution to the development of the embryo.
The conclusion is justified that there is in the egg cytoplasm a

©—©—>(§

©—©—(§

Fig. 162. — Diagram to illustrate Weismann's germ track, e, fertilized egg;

g, germ cell; s, soma.

specific and highly organized substance, which with the chromo-
somes constitutes the material basis of development and heredity.
Inheritance of Acquired Characters. — In many animals the
primordial germ cells are segregated from the somatic cells at a
relatively early stage in ontogeny. Both germ cells and somatic
cells are derived from the fertilized egg, but the germ cells retain
the reproductive potency of the egg while the somatic cells lose
it, presumably because the latter become differentiated into the
various tissues of the body. It is generally believed that in
higher forms the germ cells of the individual come only from the
primordial germ cells and not from somatic cells. The reproduc-
tive potencies are therefore retained by the germ cells simply
because they do not undergo differentiation into tissue cells.
The somatic cells die with the individual, but descendants of the
germ cells live on in following generations forming a line of
germinal continuity or germ track (Fig. 162) connecting genera-

266 GENERAL ZOOLOGY

tions. This idea is the basis of the theory of the continuity of
the germ plasm which was developed vigorously by the German
zoologist, August Weismann (1834-1914). According to this
view, the developmental and hereditary qualities of the germ cells
are derived from antecedent germ cells, the somatic cells of each
generation serving in a purely vegetative capacity as a means of
protection and of supplying the metabolic needs of the germ
cells, but contributing nothing to the hereditary qualities of the
germ cells. The facts of development support this idea in many
cases of animal embryogeny but not in all.

From the Weismannian point of view, on purely a priori
grounds, one should expect that acquired characters (i.e., pecu-
liarities acquired by the somatic cells through special training or
experience) would not be inherited, because there is no conceiv-
able way in which changes in somatic cells, caused by environ-
mental factors, could induce changes in the germ cells of such a
nature that the latter would reproduce the somatic condition in
the absence of the original cause. This does not mean that genes
cannot be disturbed by external causes acting directly on the
germ cells or indirectly through the somatic cells, but that a
specific somatic effect, produced by external causes, does not
produce a corresponding hereditary effect. One difficulty in
dealing with this question in a limited space is that the term
"acquired characters" includes a large category of conditions
that are not all comparable. Every one knows that if a man
loses his legs as the result of an accident, his children are not born
legless. All such mutilations are not inherited. Neither is there
any ground for the belief that maternal impressions produce
specific effects on the unborn human young. The uterus is a
place in which the fertilized egg develops — the kind of individual
developing from that egg depends upon the kind of parents that
produced it or more accurately upon the kind of ancestral germ
cells preceding it. Experimentally it has been impossible to
produce an inherited defect following an alteration in the somatic
tissue. Observation and experiment over a wide field fail to
demonstrate satisfactorily the inheritance of an acquired condi-
tion. On the other hand, had no characters been acquired by
primitive protoplasm, there would have been no evolution, and
consequently no great variety in life. A partial answer to this
question is that the hereditary material of the germ cells does

HEREDITY 267

change — though the causes may be obscure — that environment
also changes; and that either or both kinds of changes may affect
development over a long period of time. Many experiments,
some of which are described in the following paragraphs show
that the genes can be affected by external causes.

Direct Action of External Factors on Germ Cells. — Stockard
and Papanicolaou have succeeded in producing defects in the
eyes and skeleton of the limbs in guinea pigs whose parents had
been subjected to the inhalation of alcohol fumes. Since the
defects were inherited in succeeding generations without further
treatment with alcohol, it is clear that the germ cells of the
treated parents must have been affected by the alcohol. How-
ever, since the treated parents showed no ill effects from the
alcohol, the effect on their germ cells must have been direct
rather than through the somatic cells. There is in this case no
inheritance of acquired characters because the defects of the
offspring were not present in the somatic tissues of the treated
parents.

Muller and others have produced hereditary effects by treating
fruit flies (Drosophila) with X rays. Here also the germ cells
were affected directly and the genes were sufficiently modified to
produce hereditary changes in the characters of the offspring.

Parallel Induction. — It is conceivable that under certain
conditions both germ cells and somatic cells might be altered by
environmental or external causes and result in hereditary effects.
Such a result is known as -parallel induction, which may be
illustrated by the experiments of Guyer and Smith. These
investigators injected into the blood stream of fowls the pulped
lenses of rabbit eyes, for the purpose of producing an antilens
substance in the fowl's blood. The serum from such an immun-
ized fowl would therefore possess antilens properties. When
such antilens serum was injected into the blood vessels of preg-
nant rabbits, the offspring showed a number of eye defects of
which the most common were opaque lenses, small eyes, and
abnormally rotated eyes. The defects, though not confined to
the lens of the eye, were inherited through the female lines and
occasionally through the male. These results are regarded as
due in the first instance to the simultaneous effect of the antilens
serum, circulating in the blood of the pregnant mother, on the
eyes and germ cells of the developing embryos.

268 GENERAL ZOOLOGY

Somewhat similar eye defects were produced by Little and
Bagg on young mice whose mothers were exposed to X rays
during pregnancy. The defects in this case were also inherited.

It is probably significant that the same parts, viz., the eyes, and
to a certain extent the brain, were affected by such different
harmful agents as alcohol, X rays, and antilens substance. Thus
in all cases the head region of the embryo seemed most sensitive
to these destructive agencies, which in view of the fact that the
work of C. M. Child and others has demonstrated that the head
end of an embryo is the region most susceptible to the action of
harmful agents, such as potassium cyanide, suggests that the
results obtained were due to a general effect on the most sus-
ceptible part of the body. Since alcohol, antilens serum, and
X rays, under the conditions of the experiments, all seem to
produce similar results in mammals, particularly in regard to the
eye, the idea of specificity of effect, so far as the antilens serum is
concerned, is rather difficult to maintain. The skeleton defects
noted especially by Stockard and Papanicolaou may well be
secondary results following the primary injury to the region
of highest metabolism, viz., the head.

Germ Track. — The failure of efforts to produce changes in the
hereditary material or germ plasm through the somatic cells is in
keeping with the idea of a germ track or lineage of germ cells
connecting generations of individual animals, more or less distinct
from somatic cells. Somatic cells undoubtedly influence the
germ cells physiologically but not genetically, so far as known.
Weismann regarded the germ track as a device for conserving the
hereditary potencies of the chromosomes, but since somatic cells
receive the same assortment of chromosomes as the germ cells,
he believed that the chromosomes of the somatic cells during
cleavage and differentiation underwent a kind of differential treat-
ment, which he considered to be the cause or basis of differentia-
tion. Something of this sort is indicated in the process of
chromatin diminution of Ascaris, but since this is not of general
occurrence in other species, it cannot be accepted as the general
cause of differentiation. If it is generally true that as a result of
cleavage both somatic and germ cells receive the same kind and
number of chromosomes present in the fertilized egg, every
somatic cell receives the same complement of genes from the
egg. Differentiation of somatic cells presumably results from an

HEREDITY 269

interaction of the genes in the chromosomes with the cytoplasm
and other environmental factors. The genes of the germ cells,
because they are shielded in some way from these influences,
fail to enter into differentiating processes and thus retain their
full potencies as reproductive cells. It is true, of course, that in
many animals, notably Coelenterata and Platyhelminthes, the
evidence of a germ track from embryology is very unsatisfactory.
And it is also true that in plants germ cells may be produced
from meristematic cells, which are embryonic plant cells, also
present in growing regions of mature plants. However, if the
general thesis is true that the genes are the important hereditary
material, and that they are located in the chromosomes, it
might be maintained that even in these exceptional cases — since
every cell in the body possesses the full complement of genes — any
cell might become a germ cell. Why this does not ever happen to
a definitive somatic cell of higher animals is another matter; but
it is probably due to the irreversible nature of the differentiation
accompanying normal development, an irreversibility that seems
to increase with ascent in the animal scale.

Mendelian Heredity. — In 1866, Gregor Mendel, an Augus-
tinian monk, published the result of experiments with the common
garden pea conducted in the monastery at Briinn in what was
then Austria. His work remained practically unnoticed until
1900 when conclusions similar to his were reached independently
by three men, De Vries, Correns, and Tschermak. For this
reason Mendel's work does not figure in the literature until about
thirty-five years after it was published. His work is important
because it demonstrated experimentally the fundamental laws
governing the distribution of hereditary characters in offspring,
and because it complements in an extraordinary way the chromo-
some theory of heredity, which was evolved independently of
Mendel's work. The chromosome theory deals with the internal
mechanism of heredity, while Mendel's work showed the exter-
nally visible results of the operation of this mechanism. A
definite step in the direction of a more complete understanding
of the working of the mechanism of heredity was made when it
was realized that the results of Mendel's experiments and
many others since can be readily explained by assuming that the
genes for so-called Mendelian characters are located in the
chromosomes.

270 GENERAL ZOOLOGY

Mendel's experiments dealt with the heredity of what are
known as allelomorphic conditions of a hereditary trait or char-
acter. The traits studied by Mendel were such things as stature,
color of seed, contour of seed, etc., each of which existed in
allelomorphic or alternative forms. By this is meant that in the
case of stature, for example, some plants were tall, others dwarf,
but none intermediate ; seeds were either yellow or green in color,
smooth or wrinkled, etc. Mendel tested the heredity of these
allelomorphic conditions of the same character by crossbreeding
them and then breeding the hybrids and their progeny. In a
typical experiment Mendel found that when he crossed a plant
having yellow-colored seed coat with one having green-colored
seed coat, the hybrid or first filial generation (Fi) produced only
yellow-colored seeds. In the next generation (^2), produced by
the Fi, both yellow- and green-seeded plants were obtained in
the proportion of 3 yellows to 1 green. The green, when inbred,
gave only green from this point on. Of the yellows, one-third
proved in subsequent breeding to be pure yellow, while the
remainder behaved like the Fh producing 3 yellows to 1 green.
In this experiment, yellow is said to be the dominant character
and green the recessive.

Purity of Gametes. — Mendel explained the results of this experi-
ment by assuming that the gametes are pure with regard to the
genes or factors responsible for producing yellow or green color
in the seeds. A single gamete carries a yellow gene or green
gene, but never both. In the first part of the experiment when
yellow and green genes are brought together in the fertilized
egg or zygote of the Fh yellow is dominant to green. In the
formation of the gametes of the F\, the genes for yellow and green
are segregated into different germ cells (law of segregation), with
the result that two kinds of gametes are formed, one bearing the
yellow gene and the other the green gene. Since this is assumed
to occur in both male and female gametes, the F2 results from the
chance combinations of two kinds of male and female gametes,
as illustrated in the following diagram:

Parents Gametes Fi Zygotes Fi Gametes F2 Zygotes
Yellow YY -^Y Y Yy -> Y y

= YY : 2Yy : yy

Green yy >y y Yy

HEREDITY 271

From this it can be seen that of the four possible combinations,
there is one chance for a pure or homozygous yellow, one for a pure
or homozygous green and two for a mixed yellow and green or
heterozygous combination. A homozygote or homozygous individ-
ual is one that has received from its parents like genes for a
given character, and a heterozygote one that has received unlike
genes for a character. The gametes produced by an individual
that is homozygous for a given character will be alike with regard
to the genes for the character. On the other hand, an individual
that is heterozygous for a given character will produce two
numerically equal classes of gametes with regard to the genes.

Independent Assortment of Genes. — It seems clear, then, that
the results of crossing a single pair of allelomorphic conditions
find a rational explanation in the assumption that the two kinds
of genes of the Fh and of similar hybrids, are segregated in differ-
ent gametes, according to what Mendel called the law of segrega-
tion. Mendel determined further that when more than two
pairs of allelomorphic conditions are hybridized simultaneously,
each allelomorphic pair follows the law of segregation, each
allelomorphic pair of genes being assorted independently of other
pairs. If a plant bearing peas that are both yellow in color and
smooth in contour is crossed with one whose seeds are green and
wrinkled, the seeds of the Fi plants are yellow and smooth.
Yellow, as before is dominant to green, and smooth is dominant
to wrinkled. If these plants are inbred, the F2 from them are
produced in a ratio of 9 yellow smooth : 3 green smooth : 3 yellow
wrinkled: 1 green wrinkled. All the yellows taken together are
to the greens as 3:1, all the smooths taken together are to the
wrinkled as 3 :1 ; but some of the yellows are wrinkled and some of
the greens are smooth. In other words, while the results for
each pair of allelomorphic characters are in accord with the law
of segregation, a recombination of characters has been brought
about as a result of an independent assortment of the genes in the
formation of the gametes of the Fi. That the actual composition
of the F2 in this experiment is in accord with the principles of
segregation and independent assortment is shown in the accom-
panying diagram (Fig. 163). If one arranges the Fi gametes in
a horizontal and vertical series, along two sides of a square, the
points of intersection of imaginary lines from any pair of gametes
will give the gene composition of the resulting zygote.

272

GENERAL ZOOLOGY

Chromosome Basis of Mendelian Inheritance. — A knowledge
of the principles of segregation and independent assortment
enables one to predict the result of hybridizing one or more pairs
of allelomorphic conditions. The operation of both of these
principles can be understood if one assumes that the genes for
hereditary characters are located in the chromosomes, since the
disjunction of homologous chromosomes in the reduction division
of maturation meets all of the requirements of a mechanism to
bring about segregation and independent assortment of genes.

Gameies YS

YS

Ys

ys

ys

©

®

®

®

®

®

©

<D

©

®

©

®

©

®

©

®

Ys

ys

ys

Fig. 163. — Diagram showing composition of the F2 in a dihybrid cross. The
light circles represent yellow smooth peas; the heavy circles green smooth; the
light irregular circles yellow wrinkled; the heavy irregular circles green wrinkled.
YSYS (yellow smooth) is an extracted dominant; ysys (green wrinkled) an
extracted recessive; these with YsYs and ySyS are homozygous; the remainder
are heterozygous. Summary: Nine yellow smooth; three yellow wrinkled;
three green smooth; one green wrinkled.

In the diagram (Fig. 164) this mechanism of segregation is illus-
trated by using a single pair of homologous chromosomes (synap-
tic mates), each of which carry a gene for character A or its
allelomorph a. To illustrate the law of independent assortment
for two pairs of characters, it would be necessary to use two
pairs of chromosomes, since there must be as many pairs of
chromosomes as there are independently assortable pairs of
characters. Thus, if we are dealing with two pairs of characters,
A, a, and B, b, the zygotic composition of the Fi would be AaBb.
In the formation of the gametes of the Fu A separates from a, and

HEREDITY

273

B from b in the reduction division of the chromosomes carrying
them. But since the synaptic mates, Aa and Bb, may arrange
themselves on the spindle independently of one another, there is
the possibility of four kinds of cells resulting from the division,
depending on whether or not chromosome carrying A goes to the
same pole of the spindle as the chromosome carrying B. Thus
one result of the reduction division would be a separation of A a,
Bb combination into A, B and a, b; the only other into A, b and
a, B; yielding in all four possible gene combinations. If three
pairs of characters are hybridized and if they are independently

Homozygous parents

Reduction division (only one class of
gametes from each parent)

t) (Dif) (D

Fi zygotes (all alike)

Reduction division (two classes of gam-
etes from each F 1 zygote)

F2 zygotes (one homozygous dominant:
two heterozygous: one homozygous
recessive)

Fig. 164. — The behavior of a single pair of chromosomes in a Mendelian
cross. Only the reduction division of maturation is shown. A, dominant;
a, recessive.

assorted, three pairs of chromosomes are required to explain the
breeding results. In such a case, if the third pair of characters
be represented by C, c, there would be the possibility of eight
different kinds of Fi gametes, as follows: ABC, abc, AbC, aBc,
ABc, abC, Abc, and aBC. From this it should not be inferred
that a chromosome carries only one gene. A chromosome may
carry many genes but only one of an allelomorphic pair.

Dominance. — In Mendel's experiments one character of an
allelomorphic pair is dominant and is always expressed in
heterozygous individuals to the complete exclusion of the reces-
sive character. In later work and with other material it has
been found that dominance is often incomplete, and the question
as to whether or not one character is dominant to its allelomorph
is a matter of no theoretical importance to the principle of

274 GENERAL ZOOLOGY

segregation. Often the Fi shows a condition intermediate
between the parental characters and sometimes an entirely new
character is formed. Correns, one of the rediscoverers of
Mendel's laws, showed that when the white variety of the four-o'-
clock, Mirabilis jalapa, is crossed with the red variety, the Fi is
pink. Inbred the pink-flowered plants produce in the F2 a ratio
of 1 red: 2 pink: 1 white, a modification of the 3 : 1 ratio, resulting
from the fact that the heterozygotes are all pink. If a black
Andalusian fowl is crossed with a white-splashed-with-black fowl,
the Fi is slaty blue. These inbred produce in the F2 on the aver-
age a ratio of 1 black: 2 blue: 1 white-splashed-with-black. In
cases of incomplete dominance the heterozygotes are distinguish-
able externally from the homozygotes.

Universality of Mendelian Heredity. — The basic principles
brought to light by Mendel's experiments have been found to
apply to the heredity of animals and plants generally, which
means that all heredity, except cytoplasmic heredity, is Men-
delian heredity. On this basis it becomes possible to analyze the
heredity of an organism in terms of genes, which on the whole
remain remarkably stable from generation to generation as a
result of their immunity from the effects of disturbing environ-
mental conditions of various sorts. They may, however, be
affected by certain external influences under experimental condi-
tions, as has been shown, and they may also change spontaneously
to produce what are called mutations, some examples of which
will be discussed presently.

A common human trait whose inheritance follows along the
usual Mendelian lines is eye color. Difference in eye color is due
to difference in amount and distribution of pigment in the iris
and not to difference in the color of the pigment. In dark-brown
and black eyes the pigment is present in the outer and inner sur-
face of the iris and in the region (stroma) between the two
surfaces. Light-brown, gray, and green eyes have less pigment
in the outer surface. Blue eyes have pigment only in the inner
surface of the iris. These various grades of eye color represent
allelomorphic conditions. Thus brown is dominant to blue;
so that if one parent has homozygous brown eyes and the other
blue, the children will all have heterozygous brown eyes. If one
parent has heterozygous brown eyes and the other blue, there is
one chance out of two for blue eyes in the children. If both

HEREDITY 275

parents have heterozygous brown eyes, there is one chance out of
four for blue-eyed children.

Sex Inheritance. — It has been pointed out in a preceding
chapter (XI) that there is a correlation between the sex of the
individual and the kind and number of sex chromosomes present.
In Anasa tristis, the female condition is accompanied by the
presence of 2 X chromosomes in the body cells, while the cells of
the male possess only 1 X chromosome. It was also brought out
that in other animals where the number of chromosomes in the
two sexes is alike there are differences in form, size, or behavior of
certain members of the chromosome complexes of the two sexes.
It can therefore be accepted that under normal conditions there
is a constant relation between the determination of sex and the
kind and number of sex chromosomes present in the fertilized
egg. Sex may be regarded as an inherited condition whose
genes are located, in part at least, in the sex chromosomes. In
dichogamous animals the sex of the individual may be male at
one time and female at another.

In the fruit fly, Drosophila, 2 X chromosomes in an individual
do not necessarily establish femaleness if there are irregularities
in the number of autosomes. In Drosophila the zygotic or
diploid complex is normally composed of 4 pairs of chromosomes
consisting of a pair of sex chromosomes, known as group I, and
3 pairs of autosomes, known as groups II, III, and IV, in order of
decreasing size. The difference between the male and female
diploid groups is that group I in the male is composed of an X
and a Y chromosome, while in the female it consists of 2 X
chromosomes. The autosome groups II, III, and IV are the
same in both sexes. Therefore under normal conditions in the
female the ratio of X chromosomes to sets of autosomes is 2:2
(2 X chromosomes to 2 of each of autosomes II, III, and IV).
In the male the normal ratio is 1 : 2. A fly having a ratio of 3 : 3
is still a female; but one having a ratio of 2:3 is a sex intergrade,
combining qualities of both sexes.

From such facts it would appear that the genes for sex are
present in autosomes as well as in the sex chromosomes and that
maleness or femaleness depends upon the proportions of these two
kinds of chromosomes. Since, however, under normal conditions
the fertilized egg contains a double set of autosomes, the sex of
the individual developing from such an egg is determined by

276 GENERAL ZOOLOGY

whether, in addition to the autosomes, 1 or 2 X chromosomes are
present. Under normal conditions, if only 1 X chromosome is
present, there is also present a Y chromosome. Such an egg
develops into a male. If 2 X chromosomes are present, the
presence of a Y chromosome is excluded, and such an egg develops
into a female.

Sex-linked Inheritance. — The results obtained by Mendel in
his experiments with peas were the same in the F\ and F2 genera-
tions regardless of the parental origin of the allelomorphic
characters. In the yellow-green cross, for example, yellow
gene may be supplied by the egg and green by the pollen, or
vice versa — the result is the same. The genes for these charac-
ters are common to both sexes and do not seem to be related in
any way with the genes for sex. A sex-linked character, on the
other hand, is one whose genes are linked in some way with the
genes for sex. The eye color of wild Drosophila is red. Flies
with white eyes appeared in cultures grown in the laboratory.
White eye color is an example of what is known as a mutation,
i.e., a sudden and spontaneous variation, the cause of which is
unknown. That the cause of the mutation is due to a change in
the chromosomes or genes is indicated by the fact that the charac-
ter is inherited. White-eyed flies breed white-eyed flies. The
gene for white eye seems to be the result of a change in the gene
for red eye, which is its allelomorph. If a normal red-eyed
female of Drosophila is bred with a white-eyed male, the Fi
generation is red-eyed, males and females alike. If males and
females of the F\ are bred, there are produced in the F2 red-eyed
females, red- and white-eyed males, but no white-eyed females.
If the reciprocal cross is made by starting with a white-eyed
female and a red-eyed male, the F\ consists of red-eyed females
and white-eyed males. These produce in the F2 red- and white-
eyed males and females in equal numbers. The fact that the
results differ, depending upon the parental origin of the charac-
ters, suggests that there is some connection between the genes
for sex and those of eye color.

A simple explanation for the results may be reached by making
the following assumptions: (1) that red eye is dominant to its
allelomorph, white eye; (2) that the genes for both are located
in the X chromosome; (3) that 2 X chromosomes produce a
female ; and (4) that the Y chromosome does not carry a gene for

HEREDITY

277

Fig. 165. — Red-eyed female by white-eyed males, Drosophila melanog aster .
The factors for these characters are carried by the X chromosomes. Y chromo-
some shown with a knob. In this diagram red is indicated by the symbol W
and white by w. The history of the chromosomes is shown in the middle of the
diagram. {From Morgan, Physical Basis of Heredity, J. B. Lipvincott Company.
By permission.)

278

GENERAL ZOOLOGY

r\ s^i

Fig. 166. — White-eyed female by red-eyed male, Drosophila melanogaster.
This is the reciprocal of the cross shown in Fig. 165. {From Morgan, Physical
Basis of Heredity, J. B. Lippincott Company. By permission.)

HEREDITY

279

eye color. In the diagrammatic explanation of the results of
crossing red-eye and white condition shown in Figs. 165 and 166,
only the distribution of the X and Y chromosomes is considered,
since the autosomes are not involved in the explanation and are
common to both males and females.

Color blindness in man is inherited in a similar way. That
color blindness is sex-linked accounts for the well-known fact
that color blindness is more common in men than in women;
since, according to the explanation, color blindness occurs in

Eyes

Chromosomes
XO IK Parents

•| Xi I Gametes

X i 9 J

9

d

K ?

IX i

Fi

Gametes

<Q> <Q> <D>
9 9 *

m m m xo F2

9 ? 6 6

Fig. 167. — Diagram of the inheritance of color blindness through the male.
A color-blind male (here black) transmits his defect to his grandsons only.
The corresponding distribution of the sex chromosomes is shown on the right,
the one carrying the factor for color blindness being black. The Y chromosome is
shown as an O. (From Conklin, Heredity and Environment, Princeton University
Press, after Morgan. By permission.)

women only when both X chromosomes carry the gene for color
blindness. Heterozygous women are not color-blind, because
color blindness is recessive to normal vision. In the male, on
the other hand, a single gene in the X chromosome produces
color blindness. The Y chromosome is not concerned in color
blindness. As shown in the diagram (Fig. 167), the children of
a color-blind father and normal mother are not color-blind. In
the next generation, one-half of the sons of Fi daughters may be
color-blind, if mating is with a normal male. If such heterozy-
gous F\ daughters were mated with color-blind males, one-half of
the daughters would be color-blind as well as one-half of the

280

GENERAL ZOOLOGY

sons. Figure 168 shows the results of the mating of a color-
blind mother and a normal father.

Crossing Over. — Many other sex-linked characters have been
discovered and their heredity has been intensively studied in
Drosophila. One should expect that if two sex-linked characters
are present in the same animal, these two characters would
remain associated in a single individual in subsequent generations,
since according to the theory, the genes for both characters are

Eyes

Chromosomes

X X Parents

6 9

X 0 6 ,

l XI ^ I Gametes

XX?

<o>

<u>

6

2lXl 2X§

9 <y

Fi

X X ?'

X © <J

J

Gametes

M . XX XO

? 9 &

X9 ^j

Fig. 168. — Diagram of the inheritance of color blindness through the female.
A color-blind female transmits the defect to all her sons, to half of her grand"
daughters and to half of her grandsons. Corresponding distribution of sex
chromosomes on the right. (From Conklin, Heredity and Environment, Princeton
University Press, after Morgan. By permission.)

located in the same chromosome. This, however, is only par-
tially true, as may be illustrated by the following experiment.
Yellow wing is a sex-linked character which, when crossed with
its dominant allelomorph, gray wing, exhibits the same heredi-
tary relationship as when white eye color is crossed with red eye.
Its gene like that of white eye must therefore be located on the
X chromosome, and the same must be true for the gene for gray
wing. If a female having both white eyes and yellow wings is
crossed with a male having red eyes and gray wings, the males of
the F\ have yellow wings and white eyes, and the females have
gray wings and red eyes. Breeding the Fi flies produces in the
Fi four classes as follows:

HEREDITY 281

1. Yellow wings and white eyes, male and females)

2. Gray wings and red eyes, males and females ) per C

3. Yellow wings and red eyes, males and females {

4. Gray wings and white eyes, males and females / ^er Ce

In the first and second classes, which make up 99 per cent of the
total of the F2, the characters have remained in their original
combination or linkage, a result that should have applied to all
of the F2 flies, had the genes for yellow wing, white eye and
gray wing, red eye remained in their original locations in their
respective chromosomes in all cases. But since in 1 per cent of
the total F2 flies, represented by classes 3 and 4, new combina-
tions of characters are present, the conclusion would seem to be
that an exchange of parts of chromosomes must have taken
place, provided, of course, the original premise that chromosomes
are the bearers of these genes is true. In Fig. 169, which por-
trays these results, the first pair of flies represent the parent
generation, the next pair, the Fh and the last two groups the
F2. It will be noted that in the formation of the gametes of the
Fi female the assumption is made that an interchange of genes
between the 2 X chromosomes has taken place, giving the possi-
bility of four kinds of eggs instead of the expected two. Such
a recombination of genes is known as crossing over and is thought
to take place as a result of a twisting of the X chromosomes about
each other during synapsis, with a subsequent reciprocal trans-
location or formation of composite chromosomes, as illustrated
in Fig. 170. The result is 2 chromosomes, equivalent in size
but differing from the originals in their gene content. If one
assumes that such a recombination of genes occurred in 1 per
cent of the cases, the experimental results are explained. Cross-
ing over does not take place in the male. To detect and deter-
mine the amount of crossing over between two characters one
must cross a heterozygous female with a recessive male.

Crossing over of non-sex-linked characters has also been
studied in Drosophila. The genes for such characters are borne
on the autosomes and therefore presumably are inherited from
both parents regardless of sex. The wild Drosophila has a gray
body and long wings. Black body and vestigial wings occur as
mutations. If a female having a gray body and long wings is
mated with a male having a black body and vestigial wings, the

282

GENERAL ZOOLOGY

ffl

o

c?

Q

p

w

If

r

•k

■J

<~>

<J

J

<»j

eff

Fig. 169. — Diagram to illustrate the result of crossing a white-eyed yellow-
winged female with a red-eyed gray-winged male. Y, gray wing; y, yellow
wing; W, red eye; w, white eye. Y chromosome knobbed. {From Morgan,
Physical Basis of Heredity, J . B. Lippincott Company. By permission.)

HEREDITY

283

Fi flies all have gray bodies and long wings. Since the reciprocal
cross gives the same results, these characters are not sex-linked.
Gray body is dominant to black, and long to vestigial wings.
If a heterozygous gray-long female is mated with a recessive
(homozygous) black-vestigial male, the following four classes of
flies are obtained:

83 per cent

17 per cent

1. Black body, vestigial wings, males and females)

2. Gray body, long wings, males and females /

3. Black body, long wings, males and females )

4. Gray body, vestigial wings, males and females J

The first two classes show the characters in their original com-
binations which held for 83 per cent of the progeny. The new
combinations, present in classes
3 and 4 and representing 17 per |y
cent of the total, are the result
of crossing over of genes in the
formation of the germ cells of
the heterozygous female parent.
That crossing over takes place
only in the female is shown by
the fact that if the reciprocal
cross is made (black-vestigial
female X heterozygous gray-
long male), only the first two
classes are obtained, viz., gray-
long and black-vestigial. Limi-
tation of crossing over to the female, though true of Drosophila,
does not hold generally, since many forms exhibit crossing over
in both sexes.

Chromosome Maps. — The percentage of crossovers differs in
the two examples just cited. Other characters whether sex-
linked or not show different crossover percentages, but the same
percentage under the same conditions is obtained from the same
pair of characters. This has been interpreted to mean that the
genes for different characters occupy different positions along the
length of the chromosome and that the nearer together the genes
are, the less the chance of their being separated in the crossover
process. The percentage of crossovers obtained in any given
case then becomes a measure of the distance separating the genes

&

1 2 3

Fig. 170. — Diagram illustrating recom-
bination of characters in crossing over.

284 GENERAL ZOOLOGY

concerned. The strength of linkage tending to keep genes in the
same chromosome is in inverse proportion to the distance sepa-
rating the genes. What these distances actually are is unknown
— the conclusions drawn from crossover data merely indicating
relative locations of genes in a linear order.

Exhaustive breeding experiments have demonstrated that in
Drosophila four linkage groups occur, and only four, which,
fortunately for the theory correspond with the four pairs of
chromosomes. Characters belonging to different linkage groups
are independently assorted in breeding experiments because their
genes are located in different chromosomes. Of the four groups,
the sex-linked group may be assumed to have its genes in the sex
chromosomes, the locations being determined by the crossover
percentages obtained when the genes are tested in combinations
of two and two. The assignment of the genes of the non-sex-
linked groups to the autosomes is more difficult, since there is no
simple way of determining why a character should be assigned to
any particular autosome, except that there may be a correlation
in the size of the linkage groups and the size of the chromosomes.
A more accurate basis for answering this question is found in
what is known as translocation or transport of a part of one
chromosome to another, aside from that occurring in crossing
over. This sometimes occurs spontaneously, but may be
induced by heat and X rays. The alterations produced in the
linkage groups, when correlated with the cytological conditions,
enable one to arrive at a fairly accurate conclusion as to the
chromosomal distribution of the linkage groups. Figure 171
illustrates such a chromosome map for Drosophila. The
numerals opposite each character indicate the relative distances
of the genes from the tops of the chromosomes. Similar maps
have been prepared for other forms, including plants.

The Gene and Development. — The gene concept provides an
internal mechanism governing the inheritance of characters that
distinguish members of the same species. In the fertilized eggs
these genes are present in a biparental series, the entire com-
plement of which is distributed to every somatic cell by quan-
titative mitotic divisions. Since the nucleus of each body cell
contains all the genes for the heredity of the entire body, the
actual differentiation of the body cells would seem to be the result
of an interaction between genes and environmental factors,

HEREDITY

285

CHROMOSOME I

CHROMOSOME!! CHROMOSOME!!! CHROMOSOMES

o.o-. ,

0.3-.\\
0.6

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3.0;

3.1'
5.5-

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notch facet
abhormal
echinus
= =;- bifid
ruby

13.1

16.1

20.0
21.0

43.0-
44.4-

53.5-
54.5
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57.0
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lethal 7

broad
^-prune

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club

cut ,
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~b<o.\~ -miniature
37.5..1 --'/dusky
18.0 furrowed

Sable,
garnet

Small-wing
rudimentary

-forked
-bar

-small-eue
•fused °

clef*

-2.0
0.0
1.0

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9.0-

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13.0-

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22.5-

28.0-
29.0-

33.0-
35.5-

44.0

4£5

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48.5-

58.0-
61.0-

65.0-
66.5-

61. 0'

10.0-
11.0'

13.5-
75.0

11.0-

85.0
88.0

95.0- -

91.5.

985

100-0

101.0

103.0-4-

104.5-1

telegraph

star ,
anstaless

truncate

gull .

pink-winq
streak

cream-b

flipper
aacns

ski
squat-

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rough
beaded

95.4. .claret

-minute

Fig. 171. — Chromosome map

Introduction to Cytology

balloon

of Drosophila

melanogaster .
after Morgan.)

(From Sharp,

280 GENERAL ZOOLOGY

represented largely by the cytoplasm. If the cytoplasm pro-
vides a necessary condition for the realization of the hereditary
potencies of the genes, it becomes highly important in heredity.
There is always a cytoplasmic inheritance from the egg, but its
analysis into specific factors is difficult because its organization is
not expressed in easily recognized morphological forms. The
organization of the egg cytoplasm is a property of the hyaline
ground substance of the egg, rather than the visible formed
structures such as yolk, fat, mitochondria, etc. Unfortunately
for purposes of microscopic analysis, the ground substance of the
cytoplasm is optically homogeneous and contains no units com-
parable to chromosomes. Nevertheless, it is organized and an
understanding of this organization is essential to an under-
standing of the action of genes in development. An animal
inherits a certain protoplasmic organization permeating cyto-
plasm as well as nucleus; the final outcome of development
being a result of the reaction of the organized plasm with the
environment.

Some biologists regard the gene as the ultimate unit of life.
It is thought to be protein in nature, capable of reproducing itself
and carrying on the usual metabolic activities ordinarily asso-
ciated with life. The cell is a medium in which its properties
can be manifested. Within recent years it has been possible to
obtain in a pure crystalline form the virus causing mosaic disease
in tobacco plants. This virus is also protein in nature and is
capable of reproducing itself. It has been suggested that the
virus and the gene are possibly of the same general nature and
that they represent a simple form of living system. If so, the
first form of life evolved from nonliving nature may have been
a system of this sort. The thought is suggestive in that it aids
in partially bridging the gap between nonliving chemicophysical
systems and typical cellular living systems.

Human Heredity. — Inheritance in man obeys the same laws
of heredity that hold for other forms of life. Eye color, hair
color, morphological peculiarities and physiological traits of
various sorts are inherited. From the standpoint of their value
to human society the inheritance of some human traits are far
more important than others. The science of the improvement
of the human race by better breeding, or as Francis Galton, the
author of the term, put it, "the science which deals with all

HEREDITY 287

influences that improve the inborn qualities of a race," is known
as eugenics. The argument of the eugenicist is simply this: If
man has succeeded through artificial selection or controlled
breeding in improving domestic animals and plants, it should
also be possible by similar means to improve the inborn qualities
of the human race, since man is no exception to the laws govern-
ing the heredity of other animals. A few of the characters of
obvious importance to the human race are discussed in the
following paragraphs.

Mental Ability. — There is good evidence that mental ability, or
the lack of it, is inherited; but it is difficult to measure it in
absolute units, such as can be applied to characters like stature
or weight. Education and training do not create mental
ability so much as provide opportunity for its development and
expression. Environment plays an important part in providing
suitable opportunity for the fullest realization of mental ability.
Therefore, the actual accomplishments of two persons of equal
mental ability may differ because of differences in opportunity
for mental development. On the other hand, environment
alone, however favorable, cannot make up for low-grade ability,
for the simple reason that heredity in such cases establishes
fixed limits to accomplishment. Thus, in the case of Charles
Darwin, the author of the theory of evolution by natural selec-
tion, we have an example of marked mental ability, exhibited
consistently through a number of generations as a result of
heredity. Darwin's own accomplishment was of a very high
order and marks him as one of the greatest biologists. His
paternal grandfather, Erasmus Darwin, was a physician, natu-
ralist, and poet. He was interested in the problem of evolution,
to which he made interesting contributions and to other subjects
as well. His maternal grandfather, Josiah Wedgwood, was the
originator of Wedgwood pottery. His own father was a suc-
cessful physician and his sons have achieved eminence in science.
It is true, of course, that Darwin was favored in the enjoyment
of desirable environment, which enabled him to pursue his work
in relative comfort, though he was an invalid throughout the
greater part of his life. It is therefore possible that his books
on evolution might never have been written had he been poor and
his time crowded with the duties of earning a livelihood. On
the other hand, he might have spent his life in invalid ease, had

288 GENERAL ZOOLOGY

he not inherited the ability and the urge to make the most of
his opportunities.

In sharp contrast to cases of inherited mental ability, there are
numerous records of marked mental defectiveness, which in the
extreme conditions becomes feeble-mindedness or idiocy. The
feeble-minded breed feeble-minded. At best they are helpless in
competing with normal individuals, and at worst they are
depraved and irresponsible. Criminal tendencies of a common
sort, often associated with feeble-mindedness, form a prominent
feature of the so-called Juke family of New York, whose ancestry
has been traced as far back as 1772. In five generations of 1,200
individuals, including those who married into it, and who were of
the same social stratum, only 20 learned a trade, and of these,
10 received their instruction in a state prison.

Disease. — In order that a disease caused by a microorganism
may develop, infection by a specific organism must take place.
Such diseases are not inherited. That children of tuberculous
parents often develop tuberculosis is due in part to an inherited
physiological resistance of a low order, which makes infection
more likely than in children of sound healthy parents. A
healthy individual may carry the germs of tuberculosis in his
mouth, respiratory passages, including the lungs, and yet not
develop symptoms of the disease. Disease in parents resulting
from infection by pathogenic organisms does not affect their
germ cells in such a way as to produce the same disease in their
children, in the absence of infection through the usual channels.
Theoretically, therefore, and for the most part practically too,
it is possible by proper precautions against infection and by
providing proper food and living conditions, to rear such chil-
dren to healthy adults.

Syphilis, another disease whose control is important for
human society, is also caused by an organism, though the state-
ment is frequently heard that it is hereditary. Practically the
latter is true, to the extent that the children of syphilitic mothers
almost invariably are or become syphilitic. But here again it is
not the germ cells that are responsible but the environment,
because the opportunities for infection either before, during, or
after birth are so great that the offspring rarely escapes.

Environment. — Attempts to improve the human race by
means that fail to take the environment into consideration will

HEREDITY 289

not meet with much success. It could be maintained that
Darwin owed much of his accomplishment to his education, his
surroundings, and his private means, the last permitting him
to pursue his life-long studies unhampered by the necessity of
earning a living. Likewise it might be truly said that a large
part of the misfortunes of the Juke family could be laid to the
miserable environment in which they lived. But admitting
that environment plays a large part in the achievement of
success in life, it must not be forgotten that all the evidence
points to the fact that heredity plays a greater part in one's
destiny because it fixes limitations. All men are not born equal
in a biological sense. A Juke would never make a Darwin.

Program of Eugenics. — In animal breeding man establishes
an arbitrary standard, in the selection of which the animal has
no part. In human breeding such a procedure is impossible
except to the extent of legally preventing reproduction of the
least desirable members of human society. A eugenics program
at the present time can have no particular individual in mind
as an ideal type for whose attainment human breeding should
be controlled and regulated, but it can aim at the general adop-
tion of a policy that will gradually eliminate the hereditarily
unfit. Practical steps toward this end seem to be:

1. The promotion of eugenic marriages, i.e., marriages between
persons of sound mental and physical stock.

2. The forbidding of marriages between persons suffering
from diseases such as gonorrhea and syphilis.

3. The prevention of reproduction among the mentally and
socially unfit. At the present time (1938) some 27 states of the
Union legalize the sterilization of social offenders of low men-
tality by the performance of simple surgical operations. In the
male, vasectomy is performed by removing a small section of each
vas deferens, thus preventing the egress of spermatozoa. In the
female, by a similar operation known as tubectomy, a small piece
of each Fallopian tube (oviduct) is excised, so that eggs cannot
reach the uterus or be fertilized.

The ultimate adoption of a eugenics program rests largely upon
education. Ignorance of facts forms the bulwark of resistance
to such a program, which on mature consideration proves to be
merely an honest attempt to raise the general physical and mental
level of society as a whole and with it the individual happiness
of its members.

CHAPTER XIV
EVOLUTION

The different kinds of animals and plants, their number and
distribution, are all thought to be the results of natural causes
collectively known as evolution. Evolution is a process of
change that affects everything in the universe. All matter,
living and nonliving, is changing continuously. In biology,
evolution implies descent in living things; that the living animal
and plant population is descended from a more primitive popula-
tion; that higher forms have evolved from lower forms. This
does not mean that the lower forms living today are in direct line
of ancestry of higher forms, but that they have departed to a
lesser degree from ancestral types common to both. Evolution
is in direct conflict with the idea that the various forms of life
known today were created as such in the beginning of the living
world. The theory of evolution teaches that life evolved from
nonliving matter, as a result of a universal process of change in
matter, guided or controlled to a certain extent by environment,
and that the first living things were relatively simple in form and
structure and in all probability quite unlike the living things of
the present time. Possibly the first living matter was not cellular
at all but something of the nature of a so-called living virus, such
as is the cause of mosaic disease in tobacco plants, referred to in
the previous chapter. Such hypothetical primitive forms of life
are thought to have gradually given rise to the various types of
life known today. Diversity in living things is thus the result
primarily of a fundamental law or condition of change.

From the broad view of evolution not only are all animals
genetically related, but animals and plants as well, since the
differences between the lowest animals and the lowest plants are
bridged more or less by certain intermediate forms that combine
animal and plant qualities. The community of origin is recog-
nized in the employment of the term Protista to include all
unicellular forms, both plant and animal. Protista, presumably,

290

EVOLUTION 291

represent living forms that have deviated least from the first
living things to be evolved from nonliving matter.

The evidence supporting the doctrine of evolution consists of
a body of facts derived from morphology, embryology, physi-
ology, ecology, and paleontology, for which evolution seems to
be the most plausible explanation. This evidence is largely
circumstantial but so convincing as to meet with acceptance
among biologists and others who give the matter serious thought.
At the same time the truth of the evidence is not experimentally
demonstrable because it has not yet been possible to change one
species into another, i.e., to bring about experimentally the
evolution of a lower form into a higher one. Failure to demon-
strate species change experimentally detracts very little from
the value of the principle of evolution as a practical means of
interpreting biological phenomena, inasmuch as an evolutionary
interpretation gives meaning to otherwise inexplicable facts of
biology and is the basis of the scientific organization of the
subject.

Variation. — Animals are classified into different species largely
on a basis of morphological characters. A species is composed of
organisms that resemble each other more closely than they do
other organisms. It is a group of similar individuals. Similar
species form a genus, similar genera a family, and so on, up to
the -phylum, which is the largest subdivision of either the plant
or animal kingdom. The species is the basic unit in classifica-
tion, but actually it is a variable unit because its members,
though closely resembling each other, are rarely identical. This
variability is the basis of evolution and because of it there is
often difficulty in deciding whether extreme variants belong to
the same or to different species. This is a familiar experience
of any one who has collected sea shells and then arranged the
shells of each species into a graded series according to color or
structural differences. Often the extremes of such a series are so
different that had none of the intermediate forms been found,
there might have been some question as to whether they really
belonged to the same species or not. Such lability in species
characters suggest that a species is not a permanently fixed unit
and, as a corollary, that evolution or change has taken place in
the past and may be taking place in the present as a result of the
instability of the species unit. Difficulties arising in connection

292 GENERAL ZOOLOGY

with the classification of similar animals in the same or different
species result sometimes in arbitrary decisions, which must be
changed later when additional specimens become available for
study. On a basis of special creation, every species should be
distinct from every other species, without wide variation within a
species and without overlapping between species. On the other
hand, variability within and among species is a natural predis-
posing cause or accompaniment of an evolutionary process. To
the degree, then, to which plasticity can be demonstrated as a
property of species, presumptive evidence of an evolutionary
process may be said to exist.

The mutant conditions appearing in Drosophila and used as a
basis for numerous breeding experiments are examples of vari-
ations of a greater degree than normal. Little or nothing is
known as to the cause of mutations, but their occurrence pro-
vides a basis for further evolutionary change. Since mutations
occur naturally, the condition cannot be attributed to the
abnormal laboratory conditions under which the flies are bred.
A fuller discussion of the subject of variations will be considered
presently in connection with the work of Darwin and De Vries.

Homology. — The concept of homology in parts or organs of
different animals implies a common evolutionary origin of cor-
responding parts. A study of the comparative anatomy of any
animal group, such as the vertebrates, reveals a basic pattern of
structure occurring throughout the group, modified in various
ways in the different subgroups, but always referable to a single
structural type. Thus the skeleton of the forelimb of the bird,
bat, whale, ox, horse, and man show the same parts of a common
type of structure, though in some cases the parts are distorted
almost beyond recognition (Fig. 172). The hand region of the
bird consists only of rudiments of three digits and three meta-
carpal elements. In the bat the digits are enormously elongated,
exceeding in length the remainder of the forelimb skeleton. In
the whale the forelimb is greatly shortened in all its parts to
form a paddlelike structure. In the ox there is a reduction of
parts in the hand region to two fused metatarsals and two digits.
This reduction is carried even farther in the horse. Curiously
enough the human forelimb is the least modified of the lot.
Such limbs are said to be homologous because they are believed
to be derived by evolution from a common type of ancestral

EVOLUTION

293

limb, such as that represented by the forelimb of Amphibia.
In the case of vertebrate limbs and also other so-called homol-
ogous structures, the facts of comparative anatomy do not, of
course, prove evolutionary relationship any more, perhaps, than
a collection of automobiles including all types from the earliest
one-cylinder motor to the latest multicylinder type proves that
there is a genetic relationship between them. However, in
the case of biology there is another line of evidence from embry-

A B C

Fig. 172. — Skeleton of the right forelimb of several vertebrates to show the
fundamental similarity of structure. A, bird (raven); B, bat; C, whale; D,
ox; E, horse; F, man. c, carpals; cm, carpometatarsal; h, humerus; m, meta-
carpals; r, radius; u, ulna; I to v, digits. (Modified from Scott, The Theory of
Evolution, copyright, The Macmillan Company. By permission.)

ology which warrants an evolutionary interpretation of facts of
comparative anatomy and which justifies the use of the term
homology.

Development. — The law of biogenesis holds that an animal
recapitulates to a certain extent the history of its race. Actually
ontogeny presents such a mixture of old (palingenetic) and new
(cenogenetic) characters that the facts of development in any
given case must be interpreted with proper care. Nevertheless
there are cases where a knowledge of embryogeny has been useful
in determining the relationship between two or more animals.
Thus the subphylum Tunicata of the phylum Chordata includes

294 GENERAL ZOOLOGY

animals that in the adult state resemble molluscs rather than
chordates. A sessile tunicate, such as Ciona, is tubular in form
and is provided with an incurrent and excurrent siphon through
which water passes in and out as in a clam. There is no trace of
a notochord and the central nervous system is represented by a
ganglion lying between the two siphons (Fig. 264). About the
only definite chordate character recognizable in the adult animal
is the presence in the pharynx of gill clefts, but even these are
highly specialized when compared with the gill clefts of other
chordates. The question as to the position of the group Tunicata
in the classification is settled by the presence in the motile, free-
swimming, tailed larval stage of a well-developed dorsal
neural tube and of a notochord lying between the neural tube
and the alimentary canal. In metamorphosis the tail is lost,
and with it the notochord, while the central nervous system is
reduced to a ganglion embedded in the body wall between the
two body apertures.

Aside from such special instances where embryology sheds light
on problems of classification and therefore on problems of evolu-
tionary origins, there is in all of the Metazoa a common plan of
development followed by all of them. Thus the egg, a cell, is
the starting point in metazoan embryogeny. Cleavage, a period
of cell division following fertilization of the egg, occurs in all
Metazoa, and leads in turn to the formation of the blastula,
gastrula, and differentiation of the germ layers. Coelenterates
go very little beyond the development of two germ layers.
Hydra in its adult stage is but little more than a tube of ecto-
derm, lined with endoderm, with just the beginning of a trace of
a third germ layer, represented by the mesoglea, between them.
It might be said that the development of Hydra is arrested at the
two-germ-layer stage. Higher forms, on the other hand, pass
through and beyond the diploblastic condition and add a third
layer, the mesoderm; the three germ layers serve as the point of
departure for the differentiation of the tissues and organs of the
embryo and later of the adult. Gastrulation is a fundamental
process in the development of all Metazoa.

One of the characters distinguishing Vertebrata from other
subphyla of the Chordata is the presence of the vertebral column,
which in its embryonic origin and general development is the
same in all of them. Since similarity in embryonic origin of the

EVOLUTION

295

vertebral column is taken to mean evolution from a common
ancestral structure, embryogeny becomes a criterion for homol-
ogy. The embryonic record in a given case is regarded as an
evolutionary record, but one that is often blurred and incomplete,
and must therefore be interpreted with great care. The embryos
of all vertebrates develop pharyngeal pouches, which in many
cases fuse with the ectoderm and break through to form gill
clefts. These are visible externally in early human embryos as

Fig. 173. — Human embryo of about thirty days in age. a, arm bud; c,
gill cleft (hyomandibular) ; l, leg bud; m, mesencephalon; mt, metencephalon;
my, myelencephalon; o, olfactory pit; T, telencephalon; u, umbilical cord. (After
Keibel and Mall, Human Embryology, J. B. Lippincott Company.)

grooves (Fig. 173), but no actual clefts are formed. Nor are
gills developed. Obviously these incomplete gill clefts cannot be
compared with the gill clefts of a fish which function as
passageways for water, but they may be compared with a corre-
sponding stage in the fish embryo. On this basis they may be
also compared with the gill clefts of the embryo of the chick, of a
reptile, or of an amphibian. The only plausible explanation of
gill clefts in the human embryo would seem to be an evolution
from an ancester whose embryo possessed gill clefts. In the
fish the embryonic gill clefts continue developing, together with
gills, to form a functional respiratory organ. In man or the chick

21)6

GENERAL ZOOLOGY

the gill clefts never become functional parts of a respiratory
system. Long before the adult stage is reached they disappear
or, as in the case of the first gill cleft, enter into the forma-
tion of entirely different structures. Embryonic rudiments
in higher forms must always be compared with embryonic
rudiments of lower forms and not with the corresponding organs
of the adults of the latter. Facts of development of this sort are
none the less significant as evidences of evolutionary relationships.
The homology assumed to exist between various types of
vertebrate limbs, previously referred to, on a basis of the com-
parative morphology of the adult limbs, is supported by develop-
ment to the extent that in each case the limb arises from a

Fig. 174. — Domestic pig with its wild-boar ancestor. {After Romanes, Darwin
and after Darwinism, Open Court Publishing Company.)

similar rudiment. Vertebrate limb rudiments are strikingly
alike, particularly those of a single group, such as the mammals.
Soon, however, each type of limb develops along its own par-
ticular lines into the type characterizing the adult. Thus the
human limb does not pass through stages found in adults of
lower mammals — instead it develops from a limb-bud stage,
common to all of them, into a human limb. The origin of the
limb in all cases from the same sort of rudiment is interpreted as
indicating a common evolutionary origin. Enough of the com-
mon plan of structure survives in the adults to make the com-
munity of origin recognizable. The facts of comparative
anatomy find their explanation in the facts of development.

Artificial Selection. — It has been pointed out that the members
of the same species are not identical. Animal and plant breeders

EVOLUTION 297

have known for a long time that by careful selection, certain
chance differences or variations in a species of animal or plant
can be perpetuated. The criterion by which the breeder decides
which variations are desirable is in most cases utility. Thus the
breeder of horses for running races selects for sires stallions that
have established good records themselves or that are descended
from good racing stock. Improvements in animals from the
standpoint of their usefulness as food as in the case of the pig
(Fig. 174) have been brought about as the result of perpetuating

Fig. 175. — Columba livia, the ancestor of 150 or more varieties of domestic
pigeons. (After Whitman, Carnegie Inst. Pub.)

desirable qualities, that arose originally as chance variations.
In the case of the fancy breeds of dogs, characters other than
utilitarian ones, have become the standards of the various breeds.
In general the various breeds and varieties of our domestic
animals and plants have been developed by artificially controlled
breeding from relatively few primitive natural or wild species.
Anyone unacquainted with the history of the various breeds of
domestic pigeons would say that the breeds were at least as dis-
tinct as different species; yet, as Darwin first pointed out, they
have in all probability been derived by selective breeding from a
single wild species, Columba livia, the rock pigeon (Fig. 175).
A common breed, the fantail, possesses something like 40 tail
feathers, whereas the ordinary pigeon has but 16. In all prob-

298

GENERAL ZOOLOGY

ability the former was derived from the latter by taking advan-
tage of chance variations in tail feathers and selecting for breeding

Fig. 176. — A few of the domestic varieties of pigeons. A, English Fantail;
thirty to forty tail feathers, small feet. B, English Pouter; short beak, upright
posture, inflated crop. C, English Carrier; elongated beak, neck, and body,
corrunculated skin surrounding the eyes. D, short-faced English Tumbler;
small beak, feet, and body; tumbling habit (tumbles backward, involuntarily,
during flight) . {Redrawn from Darwin, Animals and Plants under Domestication.
D. Appleton-Century Company. By permission.)

only those birds showing abnormally large numbers, until the
present fantail was produced (Fig. 176). The same principle has
been followed in developing the various other breeds.

EVOLUTION 299

Wild species maintain species standards even in the face of
promiscuous cross-breeding. Breeds developed by artificial
selection can be maintained at a constant level only by breeding
from selected stock. Promiscuous or uncontrolled breeding
results in loss of breed characters and a tendency to revert to
the unimproved natural or wild type. Among pigeons an occa-
sional rock type will appear in an otherwise standard brood.
Such tendencies to revert to primitive types indicate that the
distinctive breed characters are not firmly fixed and that they
would not in all probability survive long in the wild state.
Indeed if domestic breeds are allowed to run wild they soon lose
their distinctive breed characters and tend to become similar to,
though not necessarily like, the original wild forms from which
they were derived.

The results achieved by artificial selection in animal and plant
breeding show that the protoplasm of a species has a certain
amount of plasticity or capacity for modification, which can be
directed or controlled to a certain extent. Heredity is a con-
servative mechanism— it tends to keep species constant. Evolu-
tion is possible because the heredity mechanism permits a certain
amount of variability to appear in the offspring of similar parents.
Variability provides a background for evolutionary change.
Experience has shown that there are limits to the use to which
variations can be put in selective breeding, as will be pointed out
in later pages.

Serum Tests. — The accuracy of the evolutionary relationships
of the members of a phylum or of any of its subdivisions, as
portrayed in the taxonomic system on the basis largely of mor-
phological data, may be put to a further test by means of certain
blood reactions. If human blood serum, i.e., blood minus cor-
puscles and fibrinogen, is injected in small quantities at repeated
intervals into the blood vessels of a rabbit, there are developed
after a while in the blood of the rabbit antibodies which, when
brought into contact with normal human blood serum, cause the
precipitation of the human blood proteins. Serum from the
blood of such an immunized rabbit may be called antihuman
serum. Blood sera of man, chimpanzee, gorilla, and monkeys,
at a certain dilution, all produce precipitates when mixed with
antihuman serum. At a higher dilution no precipitation is
obtained with monkey sera; and at still higher dilution only with

300 GENERAL ZOOLOGY

chimpanzee and human sera. If the sera are diluted still more,
a point is reached when the antihuman serum reacts only with
human serum. Thus on a basis of the susceptibility of the blood
proteins to precipitation by the antihuman bodies, the chim-
panzee ranks next to man, the gorilla next to the chimpanzee and
the monkeys farthest from man, all of which is in keeping with
the morphological findings. In general the precipitin tests
confirm the generally held view that apes are more closely
related to man than monkeys.

Similar tests show that lizards and snakes are more closely
related to each other than to turtles, which in turn are more
closely related to crocodiles. When reptilian sera and bird sera
are tested, birds show a closer relationship to crocodiles. In
general when the serum reactions of two animals are tested with
the antiserum of a third, that animal whose serum in the higher
dilution produces a precipitate with the antiserum of the third is
more closely related to the third. From such experiments the
important fact emerges that the degree of relationship as ascer-
tained by ordinary methods of classification is borne out by
blood reactions. In some cases the precipitin tests have estab-
lished relationships between species when the usual morphological
criteria have failed or proved uncertain.

Blood Groups. — Within a species, the blood of different
individuals may be classified into groups, depending upon the
conditions under which the serum of one individual will cause the
agglutination or clumping of the red blood corpuscles of another.
An understanding of the individual differences in properties of
human blood has become important in connection with blood
transfusions, because agglutination leads to the destruction of
the injected corpuscles and thus makes the transfusion ineffective.
When the blood of one individual is transferred into the blood
system of another, in some cases agglutination occurs and in other
cases not. Since agglutination is to be avoided, studies have
been made leading to the classification of human beings into
four blood groups known as 0, A, B, and A B. The designation
of the groups depends upon the presence or absence of one or
both of two substances A and B, located in the red corpuscles
and which are necessary for agglutination. Group 0 contains
neither A nor B in its corpuscles. Group AB has both. The
principle underlying the agglutination reaction is simply this:

EVOLUTION 301

The serum of a given group or type will agglutinate whichever of
A or B is absent in the red corpuscles of its own group. Serum
of a group 0 individual will agglutinate red cells of A, B, or AB
individuals; A serum will agglutinate B or AB corpuscles; B
serum will agglutinate A or AB corpuscles; while AB serum will
not agglutinate corpuscles of any of the other types. An AB
type is a universal recipient; an 0 type, a universal donor.

Similar blood groups have been discovered in lower mammals,
but only in the apes do the groups correspond with those of man;
thus adding one more link in the closeness of the relationship
between man and the apes. The blood-group characteristic is
permanent throughout life and is also inherited. If a mother
belongs to the 0 group and her child to the B group, the father
could be of B or A B but not 0 or A group.

Paleontology. — Paleontology deals with the distribution of
animals in time. Its material consists of remains of hard parts,
such as bone or shell, or imprints of bodies and tracks made in
soft mud, later hardened and preserved. A fossilized bone is a
bone whose organic constituents have been completely replaced
by mineral matter without destroying the original form of the
bone. Bones do not survive long if left exposed to the weather
on the surface of the ground. Fossilization resulted from the
embedding of dead animals in silt and soil at the bottoms of
streams, lakes, and oceans, where slow chemical changes trans-
formed the harder parts into minerals. It therefore is very sel-
dom that the entire bodies of extinct animals are preserved. It
is true, of course, that complete skeletons of prehistoric men
and of various animals have been found in limestone caverns,
and that there are unusual instances where insects have been
found in amber and where whole bodies of mammoths have been
exposed by the erosion of the covering layers; but these are in the
nature of exceptions rather than the rule. The value of paleon-
tology, as evidence for evolution, ranks high because fossils
definitely prove that animals (and plants, too) have actually
changed and that when the fossils are arranged in proper chrono-
logical order, they form a more or less progressive series from
low to high forms of life.

The data of paleontology must be interpreted in terms of age
as well as in terms of comparative morphology. A paleontologist
must be a good comparative anatomist and at the same time

302 GENERAL ZOOLOGY

have a good working knowledge of geology in order to arrive at a
correct estimate of the age of any given fossil-bearing geological
stratum. Fossils are found in what is known as sedimentary rock,
produced by the solidification of fine particles eroded from the
earth's surface and eventually washed into lakes, streams, and
oceans. It is possible to measure the amount of erosion at the
present time over a given surface of land, from which the time
required to form a layer of a certain thickness can be estimated.
Thus the thickness of a geological formation gives an indication
of the approximate time required for its formation. If all of the
different kinds of strata occurring in all parts of the world could
be piled on top of one another, the total thickness of the super-
imposed layers would be about 100 miles. Knowing the approxi-
mate rate of formation of the series, the total time for its forma-
tion may be computed. All of the components of the geological
series are not found in one place because more recent rocks are
formed in valleys from material eroded from materials of more
ancient layers, which at the time constituted the hills and moun-
tains. When all of the strata of sedimentary rocks are arranged
in order of their formation, the age of contained fossil remains
must correspond to the age of the strata in which they are found.
Another method of determining the age of the earth depends
upon measuring the rate of the decomposition of radioactive
ores contained in igneous rocks, from which the amount of time
required to form the decomposition products occurring in a given
sample can be estimated. Igneous rocks have been formed at
various times in the history of the earth, but since they contain
no fossils, their age in terms of geological eras and periods must
be determined largely from their relations to fossiliferous strata.
The total elapsed time since the beginning of the formation of
sedimentary rocks, based on a combination of purely geological
data and measurements of the decomposition of radioactive
ores, is about 1,800 million years.

A plan of the distribution of life in time is shown in Table 1,
in which the latest time is at the top and the earliest at
the bottom. "Eras" are intervals separated from one another
by marked changes in the earth's plant and animal life. "Peri-
ods" are distinguished by geological changes principally.
"Epochs," which are subdivisions of periods, are not shown.
The figure opposite each era is the duration of time for the era

EVOLUTION

303

in millions of years. The percentage following this figure is the
percentage of the whole of geological time. The bracketed
figures give time durations of the periods. The table is based
on a total of 1,800 millions of years of geological time.

Table 1.-

—The Distribution of Life in Time

Eras and periods

Time

Fauna and flora

Psychozoic era

Recent period

Postglacial

Historical time
Cro-Magnon man

Cenozoic Era

Quaternary period
Tertiary period

60 — 3 per cent

(1)
(59)

Age of mammals

Neanderthal man, Sinanthropus

man
Primates, birds

Mesozoic Era

Cretaceous period
Jurassic period
Triassic period

155 — 9 per cent

(75)
(40)
(40)

Age of reptiles

Archaic mammals, flowering
plants
Archaeopteryx, insects
Cycads (fernlike palms), reptiles

Paleozoic Era

Permian period
Carboniferous period

410— 23 per cent

(30)
(90)

(50)
(40)

(HO)
(90)

Age of amphibia, primitive

reptiles
Ferns and other early plants

Devonian period
Silurian period

Age of fishes
Early land plants

Ordovician period
Cambrian period

Ostracoderms, bryozoans, crin-
oids

Cephalopods, brachiopods,
corals, trilobites (crustacean-
like arthropods)

Proterozoic era
Archeozoic era
Azoic era

1,175 — 65 per cent

Simple forms of life
Indirect evidence of life
No evidence of life

It will be noted from Table 1 that prior to the Cambrian
period only the more primitive forms of life have been found.
Thus the sedimentary rock of the Azoic era is entirely without
fossils. During the Archeozoic and Proterozoic eras life was
beginning, but in all a total of something over 1,000 millions of
years passed before the record shows the existence of inverte-

304 GENERAL ZOOLOGY

brates in any numbers. The lack of fossils in these early rocks
may be due in part to the fact that the conditions for fossilization
preclude the possibility of the preservation of many forms, partic-
ularly soft-bodied forms. This, of course, holds true for all
succeeding strata and accounts for gaps in our knowledge of the
ancient history of soft-bodied progenitors of modern groups of
animals. Beginning with the Paleozoic rocks the material
becomes richer in fossil content and reveals the gradual emer-
gence of higher forms as time goes on. The predominant forms
of the Paleozoic era are higher invertebrates, fishes, amphibians,
and toward the end of the era, primitive reptiles. The Mesozoic
era is spoken of as the age of reptiles, because during this period
they achieved a greater size and abundance than during any
time before or since. During this era also birds and mammals
made their first appearance. The Cenozoic era is characterized
by a fuller development of mammals and the Psychozoic by the
later stages in the evolution of man.

On an evolutionary basis, the relationships of the various
animal groups can be represented graphically in the form of a tree,
whose terminal twigs alone represent the living fauna. The rest
of the tree structure represents the paths leading to the evolution
of the living forms. All of the tree except the twigs therefore
represents generations of ancestral forms that have become
extinct as species. The proof that such ancestral forms actually
existed in the past is furnished by paleontology. But since many
ancestral forms have left no record, and since the record of those
preserved is often fragmentary, the task of reconstructing
ancestral series of any single species with any degree of comple-
tion is a very difficult one. Nevertheless, this is being accom-
plished in an ever-increasing number of instances, of which one
of the best known is the case of the ancient history of the horse.

Evolution of the Horse. — The earliest known ancestor of the
horse lived in the Eocene (epoch) of the Cenozoic era of North
America and is known as Hyracotherium or Eohippus. It had
four toes and a rudiment of digit I on its forefeet, and three toes
and a splint of digit V on its hindfeet. In later strata of the
Eocene were found the remains of Protorohippus, somewhat
larger than Hyracotherium, with four toes on the forefeet and
three on the hindfeet. Mesohippus, the size of a sheep and
found in the Oligocene epoch, had three toes and the splint of

EVOLUTION

305

306 GENERAL ZOOLOGY

digit V on the forefeet, and three on the hindfeet; all the toes of
all feet touched the ground. Protohippus, about the size of a
donkey, lived in the Miocene epoch and had one large toe (III)
and two smaller ones (II and IV) on each foot; only the larger
one touched the ground. The earliest remains of the modern
horse Equus date from the Pleistocene epoch. It had a large,
well-developed, single digit and two very rudimentary splints
on each foot. This fossil series demonstrates the changes under-
gone in the morphology of the body of the horse, particularly
the steps by which the single-digited foot of the modern horse
has been arrived at. The teeth also show interesting transitions
(Fig. 177) . The evidence indicates that the horse was not created
as such in the beginning, but arrived at its present state after a
series of changes extending over millions of years from Hyraco-
therium, prior to which other forms, not completely known, push
the ancestry of the horse still farther into the past. As for the
horse, so for other forms of life.

Evolution of Man. — The evidence for evolution in man is
similar to the evidence for the evolution of other animals. If
the principle of evolution is accepted for the animal kingdom
generally, it must also be accepted for man, since he is part of
the phyletic series of animals. The structure of the human body
is that of a mammal, without doubt the most intelligent mammal,
but neither more nor less than a mammal. Taxonomically,
man belongs to the Class Mammalia, Order Primates, Suborder
Anthropoidea, Family Hominidae, Genus Homo, and Species
sapiens. The oldest known member of the human race, the
Peking man or Sinanthropus pekinensis, lived in the early
Pleistocene epoch at the onset of the Great Ice Age in the northern
hemisphere, which occurred between 500,000 and 1,000,000
years ago. The site in which the remains were found was a
limestone cave, located 40 miles southwest of Peiping, China,
and filled with sand and debris, along with the remains of other
mammals, such as the rhinoceros, elephant, saber-toothed tiger,
horse, etc., all dating from early Pleistocene times. The first
complete cranium was found in 1929 and since then parts of
at least 11 individuals have come to fight. The skull is apelike
with heavy brow ridges and receding forehead (Fig. 178). The
lower jaw is heavy and without a chin. The Peking man could
build a fire and make tools. The Java man, Pithecanthropus

EVOLUTION

307

erectus, was discovered near Trinil, Java, in 1891 (Fig. 179). The
remains consisted of a skull, a jaw bone, a few teeth, and a femur,
from which it has been possible to reconstruct the entire skull.
The straight femur indicated that he walked erect. The Java
man is dated at a little less than a million years in age. The
Piltdown man, Eoanthropus dawsoni, found in 1911 at Piltdown,
England, lived about 500,000 years ago. His cranium in front
was broader and steeper than that of the Java man, and his
brain was nearly as large as that of modern man. The Heidel-
berg man, represented by a jaw bone found near Heidelberg,
Germany, in 1907, was contemporary with the Piltdown man.

Fig. 178. — Cranium of Sinanthropus pekinensis.

(After Black.)

s.o. p., supraorbital process.

The Neanderthal man, Homo neanderthalensis, is a type of man
prevalent in Europe about 200,000 years ago. He is named from
a skull found in the Neander Valley, in 1856, though a similar
skull had been unearthed in Gibraltar, in 1848. Parts of other
skeletons discovered since provide a very complete series of bones
of this particular species of prehistoric man. Neanderthal
man, who is thought to be descended from Heidelberg man, may
have become extinct during the latter part of the Great Ice Age
when the glaciers covering half of Europe were beginning to
retreat; though some anthropologists believe that he survived
and that Australoid and negroid races are descended from him.
Another opinion holds that modern human races sprang from
men originating in Asia about 40,000 years ago and represented in
Europe by the Cro-Magnon Man of the Paleolithic or Old Stone

308

GENERAL ZOOLOGY

Age. A considerable amount of material of the Cro-Magnon
man is available for study, and this material shows that human
morphology has changed very little during the last 40,000 years.

Fig. 179. — Prehistoric men. A, Java Man, Pithecanthropus erectus, lived
approximately 1,000,000 years ago. Portion below irregular line restored. B,
Piltdown Man, Eoanthropus dawsoni, 400,000 to 500,000 years old. C, Neander-
thal Man, Homo neanderthalensis, 200,000 years old; probably an offshoot not in
the direct line of human ancestry. D, Cro-Magnon Man, Homo sapiens, 40,000
years old. (After Lull, The Evolution of Man, Yale University Press.)

The evidence from paleontology indicates that the older pre-
historic men were apelike and that, in all probability, men and
apes sprang from a common ancestor. Many of the species of

EVOLUTION 309

apelike men became extinct and left no living descendants. Such
failures have been recorded time and again in other lines of
descent and are therefore not unusual. Finally in the human
lines, the Neanderthal and the Cro-Magnon types were evolved
and one or both survived and left living descendants. The
apelike skulls of prehistoric men, characterized by prominent
orbital ridges over the eyes, sloping foreheads, and heavy jaws,
are present to a certain extent in modern primitive races of men.
The close resemblance between the human skeleton and the
skeleton of the gorilla may be noted in Fig. 180. Other apelike
characters, such as ears pointed above, and the presence of
unusual quantities of hair about the ears and over the entire
body, are also seen in some modern men. There seems to be a
closer resemblance between the young of man and the adult ape
than between the young of man and adult man. In the human
infant the great toe stands out and the foot is almost prehensile ;
the abdomen protrudes, the arms are longer in proportion to the
legs; and the grip in the hands is so great that if a three- weeks
old infant is permitted to grasp a stick, it can be lifted from the
ground by raising the stick.

Man's advance over other animals has been due to the exercise
and development of ingenuity and intelligence rather than
physical strength, although the human body as a machine is
probably the most efficient mechanism known. The high degree
of adaptability of the human race, shown by its ability to live in
any climate and to move about over land, water, or through the
air is in keeping with the relatively unspecialized morphology
of the human body. In comparison with man, most mammals
show a greater degree of structural adaptation to special habitats,
as for example, the form of the body of the whale for an aquatic
habitat, the forefeet of the mole for digging, or the hoofs of the
ungulates for locomotion over a hard substratum, all of which
in each case restrict the animal in varying degrees to a certain
environment. Man has supplemented his rather meager natural
equipment for obtaining food and for providing protection from
destructive factors in the environment, by learning to use fire and
tools, to manufacture weapons and clothing, to build suitable
shelter, and to store food. These with the ability to speak,
and later to write, are the manifestations of an intellectual
development brought into being with the human brain. The

310

GENERAL ZOOLOGY

superiority of the human brain as a mechanism through which
action can be suited to requirement, distinguishes man from all

Fig. 180. — Skeletons of Man and Gorilla. {From Lull, Organic Evolution, copy-
right, The Macmillan Company. By permission.)

other animals. Superior intelligence enables man to control
his environment to the extent that he is able to withstand its

EVOLUTION 311

onslaughts and to bend to his own uses phenomena exhibited by
it. Man's superiority over other animals rests upon his greater
success in controlling the forces of environment.

Pre-Darwinian Theories. — The essential features of evolution
as a process were understood by the ancient Greek philosophers,
but prior to the time of Charles Darwin theories advanced to
explain the process were too feebly supported by facts to carry
much conviction. Several of Darwin's immediate predecessors
contributed ideas which were important in the later development
of the subject. Comte de Buffon (1707-1788), supported the
teaching of the Greek philosopher Aristotle (384-322 B.C.),
that species form a single ascending series from the lowest to
the highest, and that higher forms evolved from lower ones.
He emphasized the importance of variations as the basis of
evolutionary change and believed that the larger differences
between species resulted from the building up of smaller differ-
ences or variations between the members of the same species,
under the direction or guidance of environment. He accepted the
principle of the inheritance of acquired characters, a principle
that was stressed by his successors, particularly Lamarck.
Buffon pointed to the results achieved by animal and plant
breeders in developing new varieties from a common wild stock
as an example of the effect of environmental conditions or stimuli
on the plastic nature of organisms. This idea was also supported
by Erasmus Darwin (1731-1803), poet and physician, and the
grandfather of Charles Darwin. In connection with the question
of the effects produced by environment, Erasmus Darwin
emphasized the importance of the inheritance of functional
responses of the organism to external stimuli.

The French biologist, Jean Baptiste Lamarck (1744-1829),
discarded the idea of a single evolutionary series from the lowest
to the highest forms and formulated instead the modern concept
of the origin of all forms of life from a common stock by diverging
lines of ascent, similar to the formation of limbs and smaller
branches from the trunk of a tree. Thus the treelike arrange-
ment of the scheme of classification was originated by Lamarck.
In his later years Lamarck was a staunch evolutionist, yet he
failed to produce a completely worked-out theory to explain the
process. His ideas may be summarized as follows: (1) The
environment has a direct effect in modifying plants and an

312 GENERAL ZOOLOGY

indirect effect, through functional responses, in modifying
animals. (2) Organisms develop new structures and parts as a
result of need or necessity, i.e., new parts evolve to meet certain
requirements. (3) Use or exercise develops an organ, while dis-
use results in atrophy. (4) All characteristics or changes,
whether caused by environmental effects, habits, use or disuse,
are inherited. Lamarck's emphasis and elaboration of the prin-
ciple of the inheritance of acquired characters have resulted in
this principle being often referred to as the Lamarckian factor
in evolution, though as a matter of fact the idea was also held by
a number of his predecessors.

Darwin. — Appointed naturalist of a British naval surveying
expedition, Charles Darwin, an Englishman, at the age of 22 sailed
from Plymouth on H.M.S. Beagle for a round-the-world voyage
that lasted five years. Most of this time (nearly four years)
was spent in coasting the Eastern and Western shores of South
America and it was in South America that Darwin first became
interested in evolution. There is every reason to believe that
Darwin left England firm in the conviction that species were
created separately and that he was converted from that view
by " certain facts in the distribution of organic beings inhabiting
South America, and in the geological relations of the present to
the past inhabitants of that continent." Darwin is outstanding
in the field of evolution because he was the first properly to
evaluate the data of morphology, embryology, and paleontology,
and to utilize the derived principles in the elaboration of an
explanation of evolution that anyone could understand. Save
for minor details, that have proved to be incorrect, Darwin's
theory of evolution has withstood the tremendous surge of
adverse criticism with remarkable success, all of which is a
tribute to the fundamental soundness of his work. Darwin
neither affirmed nor denied the possibility of the inheritance of
acquired characters. On the other hand, he appreciated the
greater importance of congenital variations and heredity in
evolution, and stressed them accordingly. Alfred Russel
Wallace, Herbert Spencer, Patrick Mathew, and others had also
recognized the importance of these factors, but none achieved
Darwin's success in welding these factors into a constructive
theory. Darwin published the results of his studies in a series

EVOLUTION 313

of books of which the first, "Origin of Species," appeared in
1859, over 20 years after his return to England.

Natural Selection. — In 1838 Thomas Malthus published an
article on the Law of Population to the effect that since man
reproduces in a geometrical ratio, the world would soon be over-
populated were it not for checks such as war, famine, disease,
etc., which keep down the total numbers. Darwin's theory of
natural selection is the doctrine of Malthus applied to the whole
animal and vegetable kingdom. The argument of the theory of
natural selection may be outlined as follows :

1. Overproduction. — Animals and plants tend to reproduce
more offspring than can possibly grow to maturity.

2. Struggle for Existence. — Overproduction brings about a
struggle for existence, which is largely a struggle for food.

3. Variation and Heredity. — Offspring of the same parents vary
from each other and from the parents; and variations are
inherited.

4. Natural Selection or Survival of the Fittest. — As a result of
propositions 2 and 3, the fittest in each generation survive and,
in consequence of the gradual accumulation of favorable varia-
tions growing out of a slow process of natural selection, a gradual
change in species is brought about.

Overproduction. — That living things reproduce in a geometrical
ratio is a self-evident fact that will be admitted by everyone. If
an annual plant produced only two seeds — and there is no plant
so unproductive as this — in 20 years there would be a million
plants. There are numerous cases of a single fish producing
millions of eggs in one year. Darwin pointed out that even the
slow-breeding elephant, which, beginning at 30 years of age and
ending at 90 brings forth 6 young on the average, would at the
end of 740 to 750 years produce nearly 19,000,000 elephants. If
human breeding were to continue unchecked for say 1,000 years,
there would not be standing room on the earth for the progeny.

Struggle for Existence. — An important limiting factor con-
trolling the total population of animals and plants from year to
year is food. Doubtless other factors, difficult to evaluate, are
also involved. However, since the food supply, under natural
conditions, remains practically constant from year to year, a
definite limit is set to the numbers of animals and plants that can

314 GENERAL ZOOLOGY

be supported in a given area. Darwin, however, used the term
struggle for existence in a larger sense to include such things as
the dependence of one organism on another and the success in
leaving offspring. In times of scarcity, two animals of the same
species may be truly said to struggle with each other to get food
and live. On the other hand, a plant on the edge of a desert
struggles for life against dryness. Mistletoe is parasitic on cer-
tain trees and therefore dependent upon them, but cannot be
said to struggle with the tree host, since too many parasites on
the same tree would kill it. But two seedling mistletoes on the
same branch may be said to struggle with each other. Since
mistletoe seeds are disseminated by birds, the mistletoe plant
is dependent on birds. The struggle for existence is a struggle
against various factors of environment, including other organisms.

Survival of the Fittest. — Darwin believed that "variation is
generally related to the conditions of life to which each species
has been exposed during several successive generations," and
"that changed conditions act in two ways, directly on the whole
organization or on certain parts alone, and indirectly through
the reproductive system." Variations, favorable or otherwise,
are inherited. Darwin was uncertain as to what extent changed
conditions or food, climate, etc., acted in a definite manner to
produce variations, but he believed that "the effects have been
greater than can be proved by clear evidence." Success in the
struggle for existence over a period of time is heightened or
diminished according to whether favorable or unfavorable char-
acters are inherited. The fittest of each generation survive as a
result of the inheritance of favorable variations.

Changes in environment alter the conditions determining
fitness. Thus the food supply of a large area, while averaging
constantly from year to year, may not be uniform throughout the
entire area, with the result that animals in the pursuit of food
migrate from famine zones to regions of plenty. Such immigra-
tion of a foreign species introduces a competitive factor for the
native species that in extreme cases might cause its extinction,
unless necessary readjustments in "fitness" were accomplished.
As Darwin notes, "when a variation is of the slightest use to
any being, we cannot tell how much to attribute to the accumula-
tive action of natural selection and how much to the definite
action of the conditions of life."

EVOLUTION 315

Geographic changes such as are known to have occurred in the
past have been potent factors in modifying the struggle for
existence and therefore the characters of the survivors. When
islands are separated from continents, the problem of living
becomes changed for those left on the island and may result in
the development or preservation of unique characters. Thus
the peculiar fauna of Australia is attributed to the fact that
Australia was cut off from Asia at the beginning of Cenozoic
times. In this case isolation fostered the preservation of egg-
laying mammals comparable to those alive in times immediately
preceeding the Cenozoic era. Such Australian mammals pre-
sumably failed to survive in other parts of the world because of
inability to meet competition of more favorably endowed forms.
Changes of consequence to the inhabitants occur when a con-
tinent is cut in two, or when two continents or smaller bodies of
land become joined by an isthmus. The formation of mountain
ranges by eruptions in the earth's surface results in barriers as
effective as water for some animals and may bring in its wake
changes in climate. When new conditions of living arise,
animals must meet them or perish. Granted that the proper
variations materialize at the proper time to enable living things
to overcome new obstacles to existence, the fittest of each
generation will in the course of time come to differ so much from
the original stock as to be distinct species. Evolution results
from the survival of the fittest which in turn results from the
accumulation of favorable variations in each generation.

Variation. — Darwin's stand on the question of the inheritance
of acquired characters was uncertain, but the acceptance of this
principle is not vital to the theory of natural selection. Natural
selection is the "preservation of favorable individual differences
and variations, and the destruction of those which are injurious."
Darwin offered no explanation for variability — variability is
axiomatic. The minimum amount of variation between two
human individuals is found in identical twins, which are twins
that have developed from a single egg. Identical twins are of
the same sex and show a striking correspondence in physical
and mental traits. Fraternal twins, each of which develop from
a single egg, may be as unlike as any brothers or sisters born at
different times. Variation among individuals of the same species,
developing from different eggs, is a fact many times verified.

316

GENERAL ZOOLOGY

Number of
Individ-
uals

180

160

140

120

100

80

CO

40

20

0

*

»%

1

-

i

i

•

i

■

i

i
i

I
1

I

/
1

1
1

t

i

i

1

1

i

i

1
1

I

1

1

1

1

1

1

1

i
i

i

t
1

1

t

4

1

1
1

**

■^ .

_•

15G 159 162 165

168 171 174 177 180 183 186 189 192 195 198

Height in Centimeters

Fig. 181. — Frequency polygon and curve showing variation in height of
1,000 college students of ages 18 to 25. Modal class 174 centimeters; average
height 175.33 centimeters. (From Castle, Genetics and Eugenics, Harvard
University Press. By permission.)

EVOLUTION 317

s

Two general kinds of variations are recognized: (1) continuous
or fluctuating or Darwinian variation, and (2) discontinuous or
sport variation or mutation. Fluctuating variation may be
illustrated by stature. If the individual heights of 1,000 college
men are grouped into classes, and the results plotted on right-
angled coordinates, a graph such as shown in Fig. 181 is obtained.
A class contains all the individuals within certain limits of height.
Thus the first class (from the left in the figure) contains all whose
height is from 155 to 157 cm., the next class, those who measure
from 158 to 160 cm., etc. The size or frequency of each class is
measured in a vertical direction on the ordinate, and the value
of each class is recorded on the horizontal base line or abscissa.
The height class containing the largest number of individuals is
the mode, in this case, class 174. The average height, 175.33 cm.,
is somewhat greater than the modal height, because, as compared
to the modal height, there are more taller individuals than
shorter. The curve produced by connecting the height polygons
is a smooth curve showing gradations between classes. This is a
general type of distribution found in fluctuating variations.

A discontinuous variation in stature would be one falling out-
side the normal limits of variation and not connected with the
normal variation range by intermediate conditions. Thus, if the
group measured had included individuals of, say, 150 or 210 cm.
in height, their position in the group would be outside the limits
of the range of the rest of the group, to which they would not be
related or connected by intermediate groups.

Darwin attached more importance to fluctuating variation as
the basis of evolution because he believed discontinuous varia-
tions occurred too rarely in nature to be of any considerable value
in bringing about the origin of new species. To illustrate his
interpretation of the action of natural selection, if we assume that
the plus variations of any given character, i.e., variations greater
than the mode, tend to improve the equipment of an animal in
the struggle for existence, there is a premium on "plus" char-
acters which makes them more valuable than "minus" charac-
ters. If plus individuals reproduce more of their own kind and
less of undesirable kinds, i.e., if the variations are inherited, the
minus variants would as a result of the struggle for existence
tend to be reproduced in smaller and smaller numbers, so that the
mode of the variation graph would in the course of time gradually

318

GENERAL ZOOLOGY

shift in a plus direction (Fig. 182). Thus, through the partial or
complete elimination of minus individuals in each generation,
the variation of Graph 1, representing the original distribution
of the character, would shift in a plus direction to position 2,
position 2, then to 3, and finally to 4, which represents a variation
range entirely beyond the upper limit of the original. The
character would have undergone a complete change as a result
of the natural selection of the more favorably endowed individ-
uals in each generation.

If the same argument held for the sum total of the variable
parts, there would result in later generations an entirely different
sort of animal from the original one. According to Darwin,

2 3

Fig. 182. — The hypothetical effect of selection on a single character.

the origin of a new species results from such a gradual accumula-
tion of slight variations under the guidance of natural selection.

On the other hand, discontinuous variations, provided they are
favorable, would hasten the process of change, since a discon-
tinuous variation represents a greater difference from the mode
than the most extreme fluctuating variation. Darwin under-
stood this but refused to attach much importance to discon-
tinuous variations as a basis for species change, because he
believed they occurred too infrequently to be of selective value.

Darwin assumed that all variations are inherited, which
of course simplified his explanation of the action of natural
selection. It is now known that many fluctuating or continuous
variations are not inherited and while this fact does not destroy
Darwin's theory, it limits the action of natural selection to the
role of an eliminating agent rather than that of a creative factor
in the evolution of new species — natural selection determines
whether or not a character shall survive after it has appeared;
but the causes leading to its appearance are independent of

EVOLUTION

319

natural selection. The work of De Vries has been important in
establishing this point of view.

The Mutation Theory. — In 1886 Hugo de Vries, a Dutch
botanist, found a number of evening primroses, Oenothera
lamarckiana, growing wild in an abandoned potato patch at
Hilversum, Holland. The peculiar feature about these plants
was that in addition to the usual fluctuating variants there were
some discontinuous variants which De Vries called mutations.
On transplanting some of the typical lamarckiana plants to a
garden where he could keep them under observation, he found
that in the course of 7 generations, involving a total of some
50,000 plants, 5 or 6 different mutations, totaling about 800
plants, were produced from the parent stock (Table 2). Since

Table 2. — An Eight-generation Pedigree Culture of Lamarck's

Evening Primrose

Genera-
tion

Gigas

Albida

Oblonga

Rubri-
nervis

Lamarck-
iana

Nanella

Lata

Scintil-
lans

1

9

2

15,000

5

5

3

1

10,000

3

3

4

1

15

176

8

14,000

60

73

1

5

25

135

20

8,000

49

142

6

6

11

29

3

1,800

9

5

1

7

9

3,000

11

8

5

1

1,700

21

1

The giant mutant was obtained only once, but all the others, in at least three different
generations, from lamarckiana parents. (From Castle, "Genetics and Eugenics," Harvard
University Press.)

the characters of the mutations were markedly different from
those of the typical Oenothera lamarckiana that produced them,
and since the mutant characters were inherited, De Vries believed
that he was witnessing the formation of new species at one single
step.

De Vries was so impressed with the importance of mutation as
the method of evolution that he discounted Darwin's view of the
gradual accumulation of fluctuating variations under the guidance
of natural selection in the building up of new species. De Vries
believed that fluctuating variations are of no importance in this
connection because they are not inherited. Mutations, accord-
ing to De Vries, are due to changes of unknown cause in the germ

320 GENERAL ZOOLOGY

plasm, the hereditary substance of the germ cells, while fluctuat-
ing or Darwinian variations are due to variations in the somatic
cells caused by environmental conditions.

The work of De Vries gave great impetus to the search for
mutations and their study in animals and plants. The sport
variations of Darwin, which correspond to mutations, have been
observed in many forms, though not in great abundance, nor
under the conditions found in Oenothera. The mutations
appearing spontaneously and also produced by radiation in
Drosophila, black sheep in an otherwise white flock, albinos
(individuals lacking pigment in the skin, hair, and eyes), horn-
less cattle, mule-footed pigs, six-fingered hands and six-toed feet
in man, are all common examples of mutations. It has been
established that these conditions are hereditary, which means
that they are germinal rather than somatic in origin. Many
mutations are abnormal in nature, but it is possible that some
of the more favorable characters might have survival value and
serve as a basis for species change. So far as Darwin's theory
is concerned, the occurrence of mutations would hasten the
process of evolution for the reason that mutation provides abrupt
and quick changes in place of the slower changes resulting from
the accumulation of smaller fluctuating variations.

It is rather curious that the mutations in Oenothera, discovered
by De Vries and others since, differ in their origin from mutations
in other forms. According to some investigators, Oenothera
lamarckiana is not a pure stock but some sort of complex
hybrid whose heterozygous character is shown from time to time
by the production of so-called mutations. It cannot be an
ordinary Mendelian hybrid because the mutations are not pro-
duced in Mendelian ratios. In some cases it has been shown
that mutations breed true, not because they are true (new)
species but because they are heterozygotes that do not produce
viable homozygous offspring. In another case, that of Oenothera
gigas, distinguished by its large size, the mutant condition is
accompanied by a doubling of the chromosomes. Even though
mutation may be an important factor in evolution, the mutations
of the primrose cannot be regarded as incipient species, unless,
as some maintain, hybridism plays a part in the evolution of
species. Though mutations occur in many forms in the absence
of any visible changes in the chromosomes, it is generally thought

EVOLUTION 321

that mutations result from minute invisible changes in the
chromosomes or genes.

Efficacy of Selection. — Selection, natural or artificial, of itself
cannot bring about an enlargement of variability. In self-
fertilized plants both egg and pollen gametes develop in the same
parent individual. A parent individual, homozygous for all
genetic factors, should presumably produce descendants in which
the same genetic factors are reproduced. Johannsen found that
when the progeny of single beans of a common self-fertilizable
garden variety, of different weights, were raised, pure lines could
be sorted out by selection (Fig. 183), each pure line varying
about its own mode. A pure line was defined as "the descendants
from a single homozygous organism exclusively propagating by
self-fertilization." Selection within a pure line failed to shift
the mode of the variation curve in either a plus or a minus direc-
tion. The only result of selection in the general population was
the isolation of pure lines, which were beans of similar germinal
constitution. The difference between pure lines rests upon a
hereditary basis; the variations within pure lines upon non-
heritable, environmental factors. Selection within a pure line
failed to produce larger or smaller beans because such size
differences were the result of environmental factors. New
types of variants within a pure line can only be produced by
mutation.

The term phenotype, first used by Johannsen, is now generally
applied to a group of individuals having similar external features.
A genotype includes individuals having the same germinal com-
position. Put in another way, the genotype refers to the funda-
mental genetic constitution of an organism, including all the
genes; while the phenotype includes all of the characters appear-
ing in the individual, regardless of genes, which though present,
do not produce visible effects. The general population of beans,
or the phenotype, is made up of a number of genotypes. In an
ordinary monohybrid Mendelian cross involving a dominant and
a recessive character, the 3 :1 ratio of individuals in the F% is com-
posed of two phenotypes, one showing the dominant trait and the
other the recessive. The dominant individuals (comprising a
single phenotype) consist of two genotypes, one homozygous and
the other heterozygous. The recessives are homozygous and of
the same genotype.

322

GENERAL ZOOLOGY

Tests of the pure-line idea by other workers seem to show that
in the protozoan, Difflugia, selection within a pure line may be
effective in increasing the number of spines on the shell, the
number of teeth on the mouth of the shell, and the dimensions

Pure Line

Fig. 183. — Diagram to illustrate five pure lines derived from a population of
beans assorted according to weight. Tubes containing beans of the same weight
are placed in the same vertical view. {From Walter, Genetics, copyright, The
Macmillan Company, after Johannsen. By permission.)

of the shell as a whole. The differences in these results and
those of Johannsen may be due to a difference in the organisms
as to the frequency of variation. It would seem that if an
organism is in a period of active mutation, selection of maximal
individuals will extend the variation range at its maximal end.
The outcome of selection, in any given case, all boils down to

EVOLUTION 323

whether or not the organism is relatively quiescent or active in
producing mutations.

Present Status of Darwin's Theory. — Darwin explained
evolution by the gradual accumulation of small variations under
the direction of natural selection. De Vries emphasized muta-
tions because they were hereditary and also because they repre-
sented larger steps in evolution. For a while many geneticists
favored the mutation idea and made use of mutations in the study
of heredity. Now as a result of intensive study it has been
learned that small mutations are more numerous than large ones,
which in fact wipes out the distinction between continuous and
discontinuous variation. If mutations occur in small steps as
well as in large, we return once more to Darwin's view that the
origin of species involves the accumulation of small heritable
variations. It may be fairly said that on the whole the work of
three-quarters of a century since the publication of Darwin's
"Origin of Species" has tended to support and verify his theory
of evolution by the natural selection of heritable characters.
Natural selection offers the only scientific explanation of the
coordinated adaptation detectable in living things.

CHAPTER XV
ADAPTATION

Every animal seems to be fitted at least moderately well to its
environment. A certain amount of harmony, between the
anatomical structure and functions of organisms and the environ-
ment, is evident to even the casual observer. The common
grass frog, Rana pipiens, for example, possesses structural and
functional features that can be understood if interpreted with
reference to an amphibious habitat. Its well-developed hind-
limbs, provided with webbed toes, and its shorter forelimbs
enable it to move about in water or on land with equal facility.
It has lungs with which it can obtain oxygen by breathing air;
but in addition a considerable amount of oxygen can also be
absorbed through the skin. It can remain for a long time under
water. Its protective coloration renders it inconspicuous. The
young develop from eggs laid in the water. The tadpoles upon
hatching are capable of living entirely in water until sufficient
time has elapsed for organs to develop to the point where the
animals can live on land as well. The adults can survive a con-
siderable range of temperatures. A frog may be frozen in a
block of ice and, when the ice is thawed, resume its normal
activities none the worse for the change. The frog is more or
less specialized for the conditions under which it lives, but this
specialization in turn imposes limitations. Thus a frog requires
a moist environment. Because its skin cannot prevent a loss of
water in a dry atmosphere, a frog left overnight in a warm dry
room may be dead in the morning, its body shrunken to a fraction
of its normal size.

There are various views about the origin and significance of
adaptation. The old traditional view of adaptation, such as was
held by Georges Cuvier (1769-1832), the greatest comparative
anatomist of his time, looked upon adaptation as a universal
property of living matter. The adaptive relation of organisms to
environment is an expression of a plan of nature ruled by Provi-
dence. It is an a priori established harmony in living things

324

ADAPTATION 325

and between them and their environment. It is the work of
Providence whose foresight extends to every minute detail of
creation. Cuvier actively combated all attempts to explain
life as the result of evolutionary processes and this attitude is
reflected in his conception of adaptation. Such a dogmatic
view of life finds little support in scientific circles today.

Lamarck also believed in the reality of adaptation, but his
interpretation was entirely different from Cuvier's. These men
were contemporaries and on opposite sides on the question of
evolution. Lamarck emphasized the importance of environ-
ment in shaping the development of adaptive features. To him
the organism is plastic with reference to environmental influences,
and is always responding to environmental stimuli, the stimulus
and response gradually bringing about a state of adaptation in
the organism. Environment induces needs, needs induce habits,
and the use and disuse of organs play prominent parts in giving
directions to adaptation. Useful organs improve with use,
useless ones degenerate and finally disappear. According to
Lamarck, adaptation is an a posteriori fact. Organs are created
by function. Lamarck assumed further that the effects of use
and disuse are inherited, the inheritance of acquired characters
becoming the immediate cause of the evolution of new species
from old. He offered no proof of the inheritance of acquired
characters, i.e., characters acquired by the organism as the
result of special training, education, or experience directly due to
environmental influences; and proof is still lacking, yet the idea
that organisms are molded more or less by environmental factors
is one that today is given serious consideration in the problem
of adaptation and evolution. The main value of Lamarck's
writings on this subject lies in calling attention to the probable
importance of environment in developing adaptive conditions in
the organism.

Charles Darwin, in building up his theory of evolution by nat-
ural selection, used variations as the basis of adaptation. Those
individuals whose variations best fit them for the struggle for
existence survive, the others perish. The survival of the fittest
is the survival of the best adapted. Adaptation is recognized as
a real condition. The adaptation begins as a chance variation,
the environment determining whether or not the variation will
survive.- Darwin did not throw any light on the origin and

326 GENERAL ZOOLOGY

source of variation. He accepted variations as facts and
analyzed the manner in which they are continued or eliminated.

A more modern point of view (Cuenot) maintains that varia-
tions arise independently of any utilitarian value, but after their
appearance they may or may not become useful. Organisms
are preadapted. In the struggle for existence the new variations
become useful if the bearer can find an "empty place" in nature
where its new variations are of real service in the struggle for
existence. The bearer of the new variations is fully adapted.
There is no slow process of "fitting" into the new environment.
In other words, the organism does not develop characters adapted
to its environment, but chooses or finds an environment that is
compatible with whatever equipment it has. The skin of the
frog is adapted to a moist environment; but the nature of the
skin also acts as an effective bar to living in a dry atmosphere.
The nature of the skin makes it necessary for the frog to live in a
moist environment.

Some deny that structure has any adaptive meaning. For
example, most aquatic birds have webbed toes, but the moor hen
(Gallinula chloropus), a bird of distinctly aquatic habits, has toes
without webs. There are also some terrestrial birds with inter-
digital membranes. Many other cases might be cited in which
well-developed structures seem to be without adaptive value.
How can these apparent exceptions be reconciled to what appear
to be adaptive structures in other forms? In biology as in other
sciences no law has an absolute and uniform value. "The laws
are the common resultant of a large set of particular facts not
identical, but statistically compensatory" (Caullery). The
webbed toes of aquatic birds occur in sufficient frequency to be of
statistical value and are certainly correlated with aquatic life,
regardless of exceptions. "It is not likely that pure chance had
resulted in such a statistical uniformity of conformation."

What, then, is the relation of adaptation to. environment?
Lamarck's idea of direct effect of the environment lacks support
and cannot be accepted in its original form. On the other hand,
the environment does play a part in the development of certain
characters in some animals. An often cited example is that of the
European cave salamander, Proteus anguinus. Under natural
conditions in the dark, the body is pale, but if kept in the light,
the pigment appears. The basis for pigment formation is present

ADAPTATION 327

in the skin, but it requires light for its development. In surface-
dwelling salamanders, pigment develops in the skin even in the
dark. Therefore, something in the skin of Proteus was lost as a
result, presumably, of residence in caves. The eyes of Proteus
under cave conditions develop to a certain stage, stop, and then
degenerate. If Proteus is reared in an alternating combination
of white and red light, the eyes develop almost to a normal state.

The American blind salamander, Typhhtriton spelaeus, spends
its larval (tadpole) period near the mouths of caves and during
this period has functional eyes. During metamorphosis (when
the gills are lost and other changes take place) the animal
migrates into caves and as a result, the eyelids become partially
fused and the rods and cones, the sensory cells of the retina,
degenerate completely. That the degenerative effect on the eyes
is the result of cave life is indicated by the fact that the eyes fail
to degenerate when the animals are raised in the light. In the
case of both Proteus and Typhlotriton the reactions of the animals
to external conditions must be due to the cumulative effect of
cave life on the ancestors, induced by darkness. It does not seem
to be a direct and gradual effect as Lamarck pictured it, but a
complex and indirect effect involving the loss of factors that are
necessary for the full development of the pigmentation of the skin
and the complete development of the eyes in the absence of light.

It should be clear from what has been said that there is a
difference of opinion as to the significance of adaptation. Assum-
ing that structural adaptation is a fact and not an illusion, it is
entirely reasonable to admit that in some cases preadaptation
has played an important part in bringing about what are called
adaptive relations in organisms ; but it must also be admitted that
environmental influences have also played a part in modifying
preadapted structures and, in some cases at least, have been a
potent influence in shaping the course of adaptation inde-
pendently of a preadapted origin. That an interplay of internal
protoplasmic factors and external environmental stimuli, though
not clearly understood and difficult to demonstrate, should exist,
is a logical conclusion; but to translate this interplay into terms
of structural adaptation is another matter. It must be borne in
mind that environment determines the conditions under which
organisms live and that organisms must be adapted to these
conditions.

328 GENERAL ZOOLOGY

An animal must be able to meet the conditions of environment
at any given time and to meet new conditions when the environ-
ment changes. The alternative is extinction. Man is inde-
pendent of his environment only to the extent that he can control
it by profiting by experience, a faculty that has been slowly
acquired. Adaptations center about certain activities which are
common to all forms of animal life. These are:

1. Preservation of self, which consists in gaining sustenance
from the environment and in the protection of self from destruc-
tive agents both animate and inanimate.

2. Preservation of race, which depends on each kind of orga-
nism reproducing its own kind in sufficient numbers to assure the
continued existence of the race.

The examples of adaption discussed in the following paragraphs
have been chosen largely because of their obviously adaptive
nature. They illustrate states of fitness of a structure or function
for the accomplishment of certain ends. All of them center in
the two main activities of animals: self-preservation and race
preservation.

Weapons.— Structures, such as claws, teeth, horns, etc., are
familiar examples of weapons, some of which may be used in
capturing food. An unusual weapon of highly specialized type
is the sting of the honeybee, Apis mellifica. The sting, located
in the sting chamber at the tip of the abdomen, consists of right
and left basal arms which curve toward each other in the mid-
line and then continue as a pair of lancets or darts, each of which
is barbed near the end (Fig. 184). The sheath, lying above the
darts also consists of two arms, paralleling the course of the arms
of the darts. The two arms of the sheath meet in the mid-line
to form a median bulb, beyond which the sheath tapers to form
a slender shaft. The sheath is hollowed on the side next to the
darts, the channel between the sheath and the darts thus forming
a poison canal, which in the region of the bulb expands into a large
cavity. The darts can be moved separately or together along
the lateral edges of the sheath. A large poison sac opens into the
enlarged region of the sheath. Arising from the free end of the
sac is a narrow tubular gland, which consists of two highly coiled
branches. The liquid produced by this gland is principally formic
acid, and the gland is known as the acid gland. In addition
there is an alkaline gland opening into the base of the sac. The

ADAPTATION

329

secretions produced by these two types of glands form the
"poison" of the bee's sting. The two substances are mixed
in the bulb at the base of the sting and, when the sting is used, are
forced through the channel formed by the shaft of the sheath and
the two darts, by a pump located within the bulb. The darts
are moved along the edges of
the sheath by the contraction
of abdominal muscles.

The damage to a victim of a
bee's sting is caused by the
action of the poisonous fluid
injected into the wound. The
effect of this poison on insects
like houseflies and blowflies is
fatal; on man it produces a very
painful result. There is some
question as to which of the con-
stituents of the poison sac is the
effective toxic agent. If the
sting remains in the wound,
the poison sac and the adjacent
organs in the tip of the bee's
abdomen are torn out with the
sting and the bee loses its life.
The adaptive value of the sting
might be questioned, since its

&. , ' , ,, Fig. 184.— Sting of the honeybee,

use frequently causes the death Apis memficat dissected, d, serrated

Of the User; but this objection darts; V, feelers provided with tactile

, sense organs; m, muscles, attached at the

disappears when one under- lower edge to the body wall, whose

Stands that the activities of the contractions force the darts into the

. f. , wound; p. s., poison sac with a duct lead-

members oi the colony ol bees ing to the darts; Pi0#i poison gland; s,

center about the welfare of the sheath into which the darts can be

. ,, ,, . ,. . , withdrawn. Alkaline gland not shown.

colony rather than the individ-
ual. The honeybee is a social insect, and the protective adapta-
tions in such organizations are directed toward safeguarding the
social unit, the colony, rather than the individual. In such a
colony, worker bees, which are abortive females, and the queen,
the sexually active female, have stings. The drones or males have
no stings. The workers use the sting indiscriminately on enemies
of all kinds; the queen reserves her sting for use on other queens.

330 GENERAL ZOOLOGY

The queen's sting is larger than that of the worker, with fewer and
smaller barbs on the darts. The poison sac is large.

It is well established that the sting of the honeybee is a modi-
fied ovipositor and is an example of a complete change in function
of an organ. An ovipositor is a structure developed at the end
of the abdomen of the female of many insects. It is used for
making holes in leaves or in stems, in which the eggs are laid.
Grasshoppers make holes in the ground for the eggs. Among
the Hymenoptera, the group to which the honeybee belongs, it is
only forms like bees and wasps that are provided with stings.
The females of other Hymenoptera have ovipositors which closely
resemble those of such insects as the katydids, crickets, cicadas,
and the grasshoppers. Figure 185 shows the ovipositor of a

common grasshopper. Since the queen
honeybee deposits her eggs in wax cells
prepared by the worker, it can be under-
stood why the ovipositor should no longer
function as such. That it develops into
Fig. 186.— Abdomen Tf a piercing organ is in keeping with the
the grasshopper showing the fact that the parasitic Hymenoptera use
ovipositor, o. the ovipositor t0 insert eggS into the

bodies, eggs, or nests of other insects. The absence of a sting
in drones is understandable since the forerunner of the sting
was primarily a female organ.

The fangs of a rattlesnake are another example of a weapon,
quite different from the sting of the bee in structure and develop-
ment and yet functionally of the same general significance. The
timber rattlesnake, Crotalus horridus, is provided with a pair of
erectile fangs surrounded by a fleshy sheath and located in
the front of the upper jaw. Each fang is hinged at its base to the
maxillary portion of the jaw, so that it can be folded back into the
sheath when the mouth is closed. The fang is a hollow tooth,
the channel extending from the base of the tooth to a point near
the sharp end. At the base of the tooth, the channel connects
with the duct of a poison gland, located at the side of the upper
jaw (Fig. 186). In striking, the fangs are buried in the victim
and the poison is injected into the wound. A rattlesnake does
not always coil and rattle before it strikes, although the power
of the blow is undoubtedly increased when delivered from the
coiled position. The poison of the timber rattlesnake is highly

ADAPTATION 331

potent and kills a small animal in a few minutes. It may also
cause the death of a human being. Rattlesnakes are not immune
to their own poison. In captivity they sometimes bite them-
selves and die as a result. On the other hand, the rattlesnake
suffers no ill effects from eating prey killed by its venom. The
rattle consists of a series of cornified shells on the end of the
tail which produce a shrill note when vibrated. This "warning
signal" is a protective adaptation for the snake, since its use in
most cases serves to keep its enemies at a distance. The rattle
itself is harmless and receives an additional segment after each
molt. Since molting occurs at irregular intervals the number of
rattles is not an exact indication of the snake's age.

The skin secretion of some amphibians is highly poisonous in
nature. Various species of Bufo (toads), which are otherwise
defenseless, produce in their skin highly toxic substances, which
render them inedible. One of these substances is bufotalin,
which has the formula C34H46O10. Another is bufogin, C18H2404.
Both these substances when tested on experimental animals have
an action similar to digitalis. They stimulate the heart muscle,
eventually stopping its action. The secretions of the parotoid
glands of the skin in the region of the ear and the side of the body
have been found to contain adrenalin, which in other animals
is a secretion of the adrenal gland. The secretions of a tropical
toad, Dendrobates, is said to be used by the Indians of Colombia
as a source of poison for their arrows.

The electric organ of certain fishes is a rather remarkable
example of an organ that is capable of producing an electric
current, which in the case of the electric skate, Torpedo, or the
electric eel, Gymnotes, is great enough to be considered of value
as a means of protection. The electric organ is built on the
general principle of a voltaic pile. A voltaic pile consists of a
column of alternating disks of dissimilar metals, such as copper
and zinc, separated by pieces of cloth or paper moistened with
dilute acid. When the top and bottom disks, or any two neigh-
boring disks, are connected by a wire, an electric current flows
from the zinc disk to the copper disk. In Torpedo marmoraia, a
Mediterannean skate, the electric organ consists of rows of
electroplaxes (Fig. 187) forming a considerable mass of tissue on
either side of the head. Each electroplax consists of an inner
striated layer covered above and below by a gelatinous layer.

332

GENERAL ZOOLOGY

Fig. 186. — Head of the rattlesnake, based on a dissection to show the poison
apparatus, f, hollow fang, shown in vertical section, is hinged above to the
maxillary bone of the skull, so that it can be folded back when the mouth is
closed; p.s., poison sac, a modified salivary gland, whose duct conveys the
secretion to the fang; t, tongue, which is harmless and serves principally as a
tactile sense organ. As in the bee, the damage or injury is caused by the highly
toxic venom injected into the wound.

Fig. 187. — Electric organs of Torpedo marmorata. A, dorsal view of entire
animal, the upper surface being partially dissected away to show the electric
organs, o, and the brain with its electric lobes, el, from which nerves pass to
the electric organs. B, Transverse section through the entire body at the level
of the electric lobes of the brain, showing the electric organs composed of flat
disks, electroplaxes, arranged in vertical columns. The polarity of the current
generated in the organ is indicated by plus and minus signs. (Redrawn from
Dahlgren, Carnegie Inst. Pub., after Fritsch.)

ADAPTATION 333

Nerves are distributed principally to the upper layer. The
electroplax, which represents the electric unit in the organ, seems
to be modified striated muscle tissue. The discharge is brought
about by nervous stimulation, the quantity of the discharge
varying with the size of the organ and its state of fatigue. If a
small Torpedo is grasped by the thumb and fingers in the upper
and lower surface of the body, respectively, a slight pressure
brings forth a distinct shock, which produces a tingling sensation
in the hand. Electric organs are relatively rare among animals.
If the electroplax be regarded as a modified muscle fiber, the
relationship between the nervous stimulation and the electrical
discharge can be understood when it is remembered that the
contraction of a muscle occurs through nervous stimulation and
that the contraction is accompanied by minute electrical dis-
charges. In the case of the electroplax, the conditions seem to
be reversed to the extent that the major result of nervous stimula-
tion is the production of an electric current rather than motion.
The outstanding features of the electric organ can thus be
explained as the result of a change in the functional response of
what in other animals is a contractile tissue.

Protective Coloration. — A general harmony in the coloration
of an animal and its surroundings is a common condition in many
animals. The coloration of the animal is rarely an exact duplica-
tion of a part of its natural surroundings; often it is merely a
neutral tint which blends with the surroundings when the animal
is at rest. Thus the fur of the rabbit, Sylvilagus floridanus, is a
shade of gray which makes the animal inconspicuous against
almost any natural background. This same color tone is often
found in many small and some large fur-bearing animals. In
other cases there is a closer correspondence between the coloration
of the animal and its surroundings. Among amphibians for
example, tree frogs, such as Hyla andersonii, which are found in
leafy trees, are bright green. Hyla arenicolor, another species, is
found on rocks and is gray. Rana pipie7is, the grass frog, is
greenish in color with spots and stripes which adapt it to a
meadow environment. The value of interpretation of coloration
comes into question when forms with conspicuous color patterns
are found living side by side with inconspicuous forms. Thus
Amby stoma maculatum, a common salamander which has a black
body with brilliant yellow spots, lives in the same habitat with

334 GENERAL ZOOLOGY

A. jeffersonianum which lacks the spots. If one adopts the view
that in the case of Hyla the green color is protective, one must
also admit that in other cases the coloration is of no particular
protective significance. That does not necessarily destroy the
value of protective coloration to some animals.

Arctic mammals such as the polar bear, Thalarctos maritimus,
and some other arctic mammals, are white in conformity with the
general character of the background. Green-colored birds
abound in the tropics but are rare elsewhere. The flounder,
Rhomboidichthys podas, a bottom-feeding fish, is white under-
neath and mottled above. The color pattern of the upper surface
can be changed to harmonize with the sea bottom to such an
extent as to render the fish practically invisible from above.
Some remarkable examples of protective coloration are the
seasonal variations such as are found in the ptarmigan, Lagopus
lagopus, the otter, Lutra canadensis, the northern fox, Alopex
lagopus, and the varying hare, Lepus americanus, whose color in
each case is white in winter and varying shades of brown, or gray
during the rest of the year.

Among birds, as a rule the male has far more gorgeous and
more conspicuous plumage than the female. The plumage of
the male may not have any special significance, but the role of
the inconspicuous coloration of the female as a protective adapta-
tion during the breeding season seems evident. The young of
either sex are protectively colored to an equal degree until sexual
maturity when the characteristic adult types of plumage develop.
The immediate cause of the two types is an internal secretion
produced by the ovary in the female and by the testes in the
male. The effect of this secretion in the plumage of the female
is the production of a protective type of plumage; in the male
the testicular substance seems to produce a result without
adaptive meaning.

Mimicry. — The protective value of coloration is greatly
enhanced when the animal possesses in addition a bodily struc-
ture bearing a close resemblance to surrounding objects. Such a
combination of color and structural resemblances is called
mimicry, of which the best examples are found among insects.
The dead-leaf butterfly, Kallima inachis, an Asiatic species,
when at rest with the wings brought together in a vertical plane
over its body, looks remarkably like a dead leaf not only because

ADAPTATION 335

of the brown color of the underside of its wings but also for the
even more striking reproduction there of minute details of leaf
structure in the form of stalk, mid-rib, and veins. Even worm
holes are present. The gaudy color of the upper side of the
wings is also significant. Rendered conspicuous by these colors
during flight, the insect practically drops out of the field of vision
when it alights and changes into a dead leaf (Fig. 188) . Similarly,
Phyllium, the green-leaf insect of South America, is pro-

Fig. 188. — Kallima inachis, the Indian leaf butterfly. A, at rest with its wings

folded; B, with the wings spread.

tected by a broad leaflike body and a bright-green color (Fig. 189).
The common American insect, Diapheromera femorata, the walk-
ing stick, gets its name from its resemblance to a branched twig,
its long greenish-gray body and slender legs making it almost
indistinguishable from the plant structure (Fig. 190). Mimicry
sometimes is found between two animals, a harmless or defense-
less one deriving a certain advantage from its resemblance to a
noxious one. Bees and wasps which have dangerous weapons in
the form of stings are mimicked by a whole host of harmless flies
and moths.

336

GENERAL ZOOLOGY

A very common example of this type of mimicry is the case
of the resemblance of the viceroy butterfly, Basilarchia archippus,
to the monarch butterfly, Danaus menippe (Fig. 191). Both are
rather large butterflies of a warm-brown color, with black lines.

-*%?

Fig. 189. Fig. 190.

Fig. 189. — Phyllium, the green-leaf insect, a South American form. (From
Jordan and Kellogg, Evolution and Animal Life, D. Appleton-C'entury Company.
By permission.)

Fig. 190. — The walking stick, Diapheromera, on a twig. (From Jordan and
Kellogg, Evolution and Animal Life, D. Appleton-Century Company. By per-
mission.)

The monarch butterfly is distasteful to birds, and therefore
escapes, to a certain extent, from being eaten by birds. The
viceroy, on the other hand, is edible but is in all probability
frequently unmolested because of its resemblance to the inedible
monarch. In these and other cases of mimicry the condition

ADAPTATION

337

is not to be regarded as a conscious act on the part of the imitator.
The steps by which the resemblance has been brought about are
unknown, but their history is doubtless similar to that of other
adaptations.

Warning Adaptations. — Many animals provided with stings,
poison glands, or other effective means of defense, such as inedi-

Fig. 191. — Mimicry.

A, the monarch butterfly, Danaus menippe; B, the viceroy
butterfly, Basilarchia archippus.

bility, are often conspicuously marked by what is known as
warning coloration. The orange- and black-banded coloration
of hornets and wasps are examples of this. Some brightly
colored caterpillars are avoided by birds because they are dis-
tasteful. The distinctive black and white of the fur of the skunk
is associated, in the minds of the experienced, with its powerful
scent glands. Poisonous amphibians and snakes are sometimes
gaudily colored or distinctively marked, but not always. Den-

338 GENERAL ZOOLOGY

drobates, the tropical toad whose secretions are used for arrow
poison, is bright green or pink, spotted with a dark color. On
the other hand, one of the most poisonous toads, Bufo marinus,
found widely distributed in the American tropics is drab-colored.
The rattlesnake, whose color pattern is well denned but not
particularly conspicuous, is provided with a warning mechanism
in the shape of its rattle. The copperhead snake, Agkistrodon
■mokasen, colored an inconspicuous yellowish brown, is also
poisonous but lacks a rattle. It would seem to be entirely lacking
in any sort of warning adaptation.

Warning adaptations may or may not be present in animals
that are provided with some noxious means of defense. Where
present, warning adaptations are to be regarded as primarily
for the protection and benefit of the possessor rather than its
enemies. While it is true of course that the lack of a warning
rattle makes the copperhead snake in some ways more dangerous
to man than the rattlesnake, a harmful animal is benefited by
announcing its presence, because it may thereby be relieved of
the necessity of risking battle. One or two experiments prob-
ably teach a bird to avoid unpalatable insects, with the result
that the latter as a whole are benefited, though some are killed.
This interpretation is supported by experiments showing that
hungry animals discriminate between palatable caterpillars and
other varieties.

Signals. — It is thought that signals may play a part in the
protective mechanism of some animals, though there is consider-
able difference of opinion among biologists on this point. The
American antelope, Antilocarpa americana, an inhabitant of open
country in Western North America, has white hairs on the rump
which when spread flash a signal. The "cottontail" of the
rabbit Sylvilagus floridanus, which is so conspicuous when the
animal is running, is supposed by some to serve as a beacon for
the young to enable them to follow the parent in escaping from
danger.

Feint.— Some animals, particularly insects and spiders, when
touched or held in the hand, become motionless. The death-
feigning posture is held for a short time and then the animal
resumes its usual activity. Several species of toads when
suddenly disturbed respond by bending back the head and twist-
ing both pairs of legs over the back of the body, leaving the

ADAPTATION 339

animal resting on its belly on the ground. In this position the
animal remains completely quiet, the deathlike immobility
being heightened by the closing of the eyes and a slowing down
of respiratory movements. The newly hatched young of the
common tern, Sterna hirundo hirundo, can be handled without
exciting fear, but later when about a third grown and not yet
able to fly, the same birds will feign death by lying perfectly quiet
in the grass when one approaches them. If a bird is picked up,
it maintains its lifeless attitude even when feathers are pulled out.
Its patient indifference to such treatment does not last long,
however, and comes to an end abruptly, with the bird struggling
frantically to escape. This death feint is not shown by terns
after they have learned to fly. Feigning a broken wing or
lameness is a device employed by a number of ground-nesting
birds during the breeding season to direct the attention of
intruders away from the young or the nest. The tactics of the
opossum, Didelphis virginiana, in shamming death when uncov-
ered in its nest is an example of the same general sort of behavior-
ism that is displayed by a number of other smaller mammals.
Feigning death or injury is widespread in different groups of
animals. The tonic immobility produced in many animals may
merely be the result of fear and have no especial adaptive signifi-
cance. They may be "scared stiff." If the temporary immo-
bility aids in concealing the animal, as in the case of young terns,
there results a certain benefit which may be considered adaptive.
The simulation of bodily injury by adult birds would certainly
seem to be directed toward protecting the young.

Animal Associations. — Certain kinds of animals are ordinarily
found in groups of individuals. Such animals are gregarious—
they live in flocks or herds. The horse, antelope, elk, buffalo,
elephant, etc., illustrate this mode of existence. The herd is
made up of individuals of the same species. The leader of the
herd, in the case of horses, is a male and the same is true of other
groups. In the buffalo (bison) herd the leader is a female.
Sometimes two or more species are associated as in the presence of
flocks of white herons with elephants, or of the ostrich with the
zebra and gnu. All domestic animals except the cat are gregari-
ous in habit. Herding in mammals is confined almost entirely
to herbivorous forms. Such animals subsist on plant food and
require a relatively large amount which must be gathered from

340

GENERAL ZOOLOGY

more or less open country. The usual natural defenses of such
animals are speed and kicking ability, to which herding has come
to be an additional measure of protection. Carnivorous animals
such as the wolf occasionally hunt in packs, but the pack is
seldom large and its organization is purely temporary in character.
Wolves hunt in packs only when food is scarce. The members
of the pack are competing against each other to obtain food. In
a herd of antelope the benefit of association comes from the
increase in power to sense danger. Instead of depending on one
set of sense organs each member of the herd receives the benefit

Worker Queen Drone

Fig 192. — Honeybee. X 2. {From Phillips, U. S. Dept. Agr. Farmers' Bulletin

447.)

of the sense organs of every other member in perceiving danger.
Escape depends upon individual effort. In some cases of
gregarious animals, however, the herd acts as a unit in defending
its members. A herd of musk oxen held at bay by dogs forms
a circle, with the calves in the center, the adults having their
heads toward the outside. The protective value of herding in
such cases consists in multiplying the individual weapons of
defense by the number in the herd.

Communalism. — In herding, all members of the group are alike
except for differences in sex. By communalism is meant a social
condition in which a species of animals lives in a group of a
permanent character, distinguished by a marked division of labor
among the members, and often accompanied by a physical
differentiation of the members. Such forms of social organiza-
tion have reached the highest degree of perfection among the
social insects, such as the honeybee and the ant.

ADAPTATION 341

A honeybee colony can be understood by regarding it as a
compound organism, in which the various and specialized activ-
ities of the individual members are to be interpreted in terms of
the colony as a unit. A colony normally consists of a single
queen bee, the mother of the colony, thousands of sexually
undeveloped females, called workers, and, during part of the
year, some hundreds of drones or males. The queen alone is
capable of laying eggs. The workers, though genetically
females, have no part in reproduction. They build the wax
comb, gather food, keep the hive clean, feed the young, ventilate
the hive — in short take care of the entire colony. The function
of the drone is to inseminate the queen, a single insemination
providing the queen with sufficient spermatozoa to last her life-
time. These three forms differ in size and structure (Fig. 192),
the worker being the smallest of the three. Under natural con-
ditions, the colony inhabits a hollow tree or similar cavity, but it
thrives equally well in an artificial hive. The comb of the hive
is formed of wax secreted by the workers. It is slab-shaped and
consists of a double layer of tubular cells, hexagonal in cross sec-
tion and open at their outer ends (Fig. 193). The cells of the two
sides of the comb are separated by a septum running through the
center of the comb parallel to its two faces. The cells are pitched
at a slight angle, the bottom of the cell being slightly lower than
the opening. They serve as brood chambers for developing bees
and as storage places for pollen and honey. Those used for
rearing workers are about } 4 in. in width, and those for rearing
drones and storing honey measure about 34 in- across. These
make up the majority of the cells. At certain times, the worker
bees enlarge some worker cells for rearing queens (Fig. 194).

With the coming of warm weather in the spring, the queen
begins to lay eggs in the worker cells, one egg to a cell. The eggs
develop into white grubs or larvae which are fed by the workers.
At the end of the larval period the grub completely fills the cell,
which is then capped by the workers. During the quiescent
stage that follows the larva is gradually transformed into a pupa
and then into an imago, or adult stage, after which it emerges as
a worker bee. The first activities of the workers are inside the
hive where they engage in secreting wax, caring for the new
brood of young (developing in the meantime from eggs laid by
the queen) and in cleaning the hive. Later they join the outside

342

GENERAL ZOOLOGY

ttteOOOQ?

Fig. 193. — Structure of the comb of the honeybee, Apis mellifica. a, vertical
section at top of comb; b, vertical section showing transition from worker to
drone cells; c, horizontal section at side of comb showing end bar of frame;
d, horizontal section of worker brood cells; e, diagram showing transition cells.
Natural size. (From Phillips, U. S. Dept. Agr., Farmers1 Bulletin 447.)

ADAPTATION

343

force in collecting food. The varied activities of the workers
under normal conditions follow each other in a regular sequence.
This process goes on until the hive is nearly full of bees and stored
food, when the queen begins to lay eggs in the drone cells pre-
pared by the workers. Several hundred drones develop from
these eggs. The queen then lays eggs in four or five specially
prepared queen cells. The royal larvae are provided with food
composed of a rich mixture of digested honey and pollen mixed
with glandular secretions. When the queen larvae have com-

S^Q^jfNi

Fig. 195.

Fig. 194.

Fig. 194. — Queen cells. Natural size.

Fig. 195. — Stages in the development of the honeybee, o, egg; b. young
larva; c, old larva; d, pupa. X 3. (From Phillips, U. S. Dept. Agr., Farmers'
Bulletin 447.)

pleted their development, the queen cells are capped and the
colony is now ready for swarming (Fig. 195).

The first swarm from the colony consists of the original queen
and hundreds of workers which pour out of the hive and fly away
in a dense mass and finally settle down in a new location. What
ordinarily happens at this time under artificial conditions is
that the beekeeper gathers the swarm and places it in a prepared
hive. Under these conditions, the bees start in and gradually
build up the conditions of a typical hive.

Meanwhile in the old hive, from which the swarming took
place, activity goes on as usual until a queen emerges from one of

344 GENERAL ZOOLOGY

the queen cells. If the colony is not too populous, the first queen
to emerge tears down the other queen cells and kills the develop-
ing queens. If a second swarm is imminent, the workers see to it
that the other queen cells are protected. In about a week after
emerging, the first queen flies from the hive and mates with one
of the drones in the air. Mating usually occurs but once, after
which the mating drone dies. The mated queen returns to
the hive and after about two days starts laying eggs, repeating
the routine of her predecessor. The spermatozoa received
from the drone are stored in the spermatheca, which is a small sac
opening into the oviduct. As the eggs pass the opening of the
spermatheca, spermatozoa may be released or not. Fertilized
eggs, those which have been penetrated by spermatozoa, develop
either into workers or queens depending upon the kind of food
given during the larval period. If a virgin queen is lost on her
flight, or if the colony for any reason becomes queenless, it may
happen that a worker will start to lay eggs, though normally
workers do not lay eggs. Such eggs develop into drones, just as
the unfertilized eggs laid by the queen do under normal condi-
tions. Queens that have exhausted their supply of spermatozoa
lay only drone eggs. The average number of eggs laid daily
during the breeding season by a single queen is about 900. The
maximum on any single day may reach 2,000.

Life in a colony of honeybees is ordered along rather definite
lines. The function of the queen is to lay eggs, of the drones to
produce spermatozoa for fertilizing the eggs, while to the workers
falls the burden of caring for the entire colony, including the
rearing of the young and the building and guarding of the nest.
If the colony as a whole be compared with an organism in the
ordinary sense, the queen and drones represent the germ cells
of the organism, and the workers the somatic or body cells,
which carry on the routine metabolic activities of the body.

The length of life of the queen is bound up with her ability to
lay eggs for the reason that when she stops laying she is super-
seded by a young queen reared by the workers. The queen lives
on the average from three to four years. Drones make their
appearance in a hive after a considerable number of workers have
been produced and conditions become crowded. Drones begin
to fly about a week after emerging. The drone mating with a
young queen in the nuptial flight dies after copulation. Other

ADAPTATION 345

drones may return to the hive with the queen and remain there.
Drones may be present in a colony up to the end of the honey
flow, after which they are carried off by the workers to some dis-
tance from the hive, 100 ft. or more, and left to die. The life of a
drone thus extends over a variable number of weeks. The life
of the worker is determined by the amount of work it does.
When food is abundant, it is actively employed in collecting it
and under these conditions lives for about six weeks. With less
work the life span is lengthened. The bees which survive the
winter are those which emerged in the early autumn. In their
case the length of life extends over several months.

There are also many varieties of such colonial organization
among ants, such as Atta, a genus of leaf-cutting ants found in
tropical America. A typical colony is made up of a queen, some
drones, and numerous sterile females, which in this group are
found in three types, known as the maxims, mediums, and minims.
As in the case of the honeybee, the function of the queen is to
lay eggs, the drones to supply spermatozoa for fertilizing the
eggs, and of the workers to rear the young and to provide for the
colony generally. The maxims are large in size and have power-
ful biting jaws with which they defend the nest. The mediums
are smaller and their duties are largely domestic. They bring into
the nest small pieces bitten out of a leaf which are handed over
to other mediums who thoroughly chew the leaf tissue. The
triturated tissue is then taken over by the minims who form it
into balls on which a certain fungus (Rozites gongylophora) is
cultivated. The fungus is used as food by the entire colony.

Commensalism. — Commensal animals, literally, are those that
eat at the same table. In biology, commensalism refers to an
association between two different species of animals partaking
of the same food. The benefit of such an association in many
known cases seems to be one-sided rather than mutual. A com-
mon example of commensalism is found in the association of
suckfishes, members of the genus Remora, and sharks. The
suckfish has a slender body about 18 in. in length, provided with
a peculiar sucking disk on the top of the head, by means of
which it attaches itself to the side of the shark's body (Fig. 196).
The shark carries the suckfish about and apparently suffers no
harm from its passenger; but neither does it benefit. The suck-
fish, on the other hand, probably picks up scraps of the shark's

346 GENERAL ZOOLOGY

food. It will leave the shark to seize a baited hook and is
frequently caught in that manner. All the benefit of the associ-
ation seems to be in favor of the suckfish. Sycotypus canalicula-
tus is a large, marine, gasteropod mollusc, common on the New
England coast. In the respiratory chamber of this animal one
frequently finds one or more flatworms, Planocera inquilina,
about x/i in. in length. They can be readily picked off the surface
of the respiratory chamber and so far as is known cause no injury
to the host. The benefit is again one-sided and in favor of the
flatworm, which gains food and shelter by the association and
gives nothing in return. Commensal associations occur fre-
quently and may represent a step toward a parasitic association,

Fig. 196. — Remora, the suckfish. The dorsal fin is modified into a sucking disk
by which the fish attaches itself to a shark. {After Jordan and Kellogg.)

in which the host definitely pays for the entertainment of the
guest.

Symbiosis. — The living together of two different kinds of
organisms may develop to the point where the union becomes
very close without either suffering harm from the presence of the
other. This is known as symbiosis. Usually there is a mutual
benefit from the association. The hermit crab, Eupagurus, is
always found inhabiting an empty gasteropod shell, its abdomen
having become modified and slightly crowded to conform with
the spiral passage of the shell. In feeding or walking, only the
head and thorax protrude from the mouth of the shell. It can
retract into the shell and block the opening with an enlarged
claw (Fig. 197). When the shell becomes too small, the crab
seeks a larger one, sometimes killing and removing the original
inhabitant. In some species the surface of the shell is covered
with sea anemones or hydroid polyps which serve to conceal the
crab and also to protect it from fishes, because the polyps are
armed with stinging cells that make them unpalatable. When
the crab feeds, the polyp shares the food, capturing fragments as
they float upward or bending over the edge of the shell to reach
them. Evidently each animal benefits by the association. If

ADAPTATION

347

the polyps are removed from the shell, the crab makes an effort
to replace them, showing that the association between the crab
and the polyp is by no means an accidental one. This tendency
for disguisement is shown by other species of crabs. The spider
crab, Libinia emarginata, is often provided with a covering of
sea weed, sponges, hydroids, etc. Kept in aquaria, spider crabs
have been observed to tear off bits of sea weed with their claws
and fasten them on their backs among the stiff hairs which

Fig. 197. — Hermit crab and sea anemones. Usually the gasteropod shell
inhabited by the crab is completely covered by sea anemones, but only two of
the latter are represented for the sake of clearness.

occur there. The transplanted plants and animals often grow
and provide an effective covering.

An example of a closer symbiotic relationship is found in the
green hydra, Chlorohydra viridissima, which owes its green color
to the presence of microscopic green plants embedded among its
cells. The plants utilize the carbon dioxide given off by the
animal cells and in return yield oxygen which is readily absorbed
by the cells. The same sort of relations is found in some Protozoa
that contain numbers of unicellular plants.

A rare symbiotic relationship between a mammal and algae is
illustrated by conditions found in the hair of the two-toed sloth,
Choloepus didactylus, a South American form. This animal is
largely arboreal in its habits, moving about in the trees, sus-
pended in an upside-down position from the branches by its

348 GENERAL ZOOLOGY

strongly curved claws. It assumes other postures when climb-
ing vertical stems or when asleep. It is a good swimmer but is
incapable of assuming a standing position on the ground. All
of its movements are extremely slow, and active means of defense
seem to be entirely lacking; its fur and hide and its habit of
rolling up into a tight ball serve as its principal protection. This
passive means of defense is heightened by the presence of algae
in the fur. This is made possible by the structure of the hairs,
which are distinctly fluted, with four ridges and three grooves
on each side, the grooves offering a lodging place for the algae.
The algae, which are said to be always present in the long hairs,
change color under different degrees of moisture. When the fur
is dry, the algae produce a dirty brown color pattern; when wet,
the* pattern turns green. Such a protective coloration would
seem to be of value against enemies flying overhead. There is
no reason to suppose that there is a mutual respiratory benefit
such as is present in Chlorohydra and algae.

A common symbiotic condition exists in the alimentary canal
of vertebrates in the presence there of bacteria, which appear to
be not only harmless but actually beneficial to the host to the
extent that they aid in digestion of the host's food. The bac-
teria benefit by being provided with food and protection.

Parasitism. — When the association between two different
kinds of organisms becomes such that one of them, the parasite,
receives food and protection from the other, the host, without
giving anything in return, the condition is known as parasitism.
The effect on the host is usually detrimental, and if fatal, the
death of the host as a rule takes place only after the parasite has
developed to the point where the host is no longer needed. Judg-
ing from the great frequency of the condition, parasitism must
be a very successful way of living and reproducing, though often
it is the most complicated. The adjustments necessitated by
this mode of existence have resulted in remarkable states of
structural and functional adaptations on the part of the parasite,
producing in many cases a distinct specificity in the host of
hosts invaded by the parasites. Parasites may occur in prac-
tically any organ of the host's body, but the most common
location is the alimentary canal. Taenia solium, the pork tape-
worm, is found as an adult in the human intestines where it
reaches a length of 4 to 10 ft. The head or scolex (Fig. 231) is

ADAPTATION 349

very small and is provided with hooks and suckers by means of
which it attaches itself to the intestinal wall. The body behind
the head consists of a series of segments or proglottids. The
parasite has no mouth or alimentary canal of its own, the host's
food being absorbed directly through the body wall. The more
posterior proglottids become filled with eggs, which are probably
fertilized by spermatozoa from the same proglottid. As the
proglottids become laden with embryos, they break off and pass
out of the alimentary canal of the host. If the proglottids are
eaten by a pig, the embryos are released and bore through the
walls of the pig's alimentary canal, whence they make their
way to voluntary muscles where they encyst. Here the embryo
develops into an oval cysticercus, or bladder worm, and remains
as a cyst until the infected muscle is eaten (Fig. 233). When
the wall of the cyst is dissolved by the human gastric juice, the
scolex is protruded from the inverted position occupied in the
cyst, in the manner of a finger of a glove. The scolex then
attaches itself to the intestine, and with the development of
proglottids the bladder worm is transformed into the adult.
From this it is seen that in the usual life history of the pork tape-
worm two hosts are involved. Neither host is killed by the
presence of the parasite in the ordinary course of events.

In other cases of parasitism the host is killed as a result of the
parasitic association. Thus, the chalcid flies deposit their eggs
in the bodies of caterpillars. The eggs hatch into larvae which
devour the caterpillar. Similarly the ichneumon flies lay their
eggs in caterpillars and other insects, which are destroyed by the
developing parasitic larvae.

Regeneration. — Many animals have the power to replace or
regenerate new parts if old ones are lost. Ewplanaria is a genus
of the free-living flatworms occurring in fresh-water ponds. If
such an animal is cut transversely, the head end will regenerate a
new tail and the tail end a new head. If a crab loses a leg, a new
one is regenerated. The same is true of the cockroach before
the final molt. If one or more arms are torn from a starfish,
they are replaced by new ones (Figs. 198 and 199). Regeneration,
which enables an animal to survive the loss of considerable por-
tions of the body, is common among lower invertebrate animals.
Regeneration of lost parts is present in the larval stages of lower
vertebrates. Thus the limbs of amphibian tadpoles if cut off are

350

GENERAL ZOOLOGY

replaced. If a limb bud is removed from a mammalian embryo,
regeneration does not occur. Regeneration in the adults of
vertebrates is confined almost entirely to wound healing. This

Fig. 198. — Regeneration in Euplanaria. A, regeneration of the anterior and
posterior ends following transverse section. B, regeneration in piece taken
from the middle. C, regeneration of new head on piece from head region. D,
regeneration of a larger head piece. (After Morgan.)

Fig. 199. — Regeneration in the starfish, Asterias. In A a single arm is regenerat-
ing, in B four arms. Both are young animals.

also has a protective significance since wounds caused by injuries
of whatever sort, unless promptly closed, may lead to serious
results not only because of the primarily traumatic effect but

ADAPTATION 351

more particularly because the broken integument opens the way
for invasions of bacteria.

Immunity. — By slowly increasing the dosage, the tolerance
of the body for certain poisons can be gradually increased. An
animal can acquire an immunity to bacterial diseases by having
the disease itself. In either case there is a functional response
to the action of the toxic substance of the pathogenic agent
which results in an improvement of the defense mechanism of
the body. The usual explanation of what happens during the
recovery from a bacterial infection is that substances known as
antitoxins are formed and that these substances counteract the
effects of the bacterial toxins, with the result that the patient
for some time afterward is immune to further attack by the
particular organism concerned. Immunity against smallpox,
for example, may be established by having the disease or by sub-
mitting to vaccination with virus of a mild form of the disease.
In other cases, the practice of giving antitoxin as a prophylactic
or preventive measure is based upon the fact that an animal such
as a rabbit, guinea pig, or horse, if inoculated with disease-pro-
ducing organisms, reacts by producing a substance, antitoxin,
which tends to neutralize the injurious effect of the organisms.
Serum, the clear fluid obtained from clotted blood taken from
such an animal, contains the antitoxin, which if injected into
another animal or a human being, is capable of establishing
immunity in the latter against the particular pathogenic organism
involved. Immunity reactions and the power to regenerate lost
parts are excellent examples of purely physiological adaptation.

Temperature. — The rate of metabolism varies with the temper-
ature of the animal body, within a range that is not the same for
all forms of life. The so-called cold-blooded animals have a body
temperature that is slightly higher than that of the surrounding
medium. Fluctuations in body temperatures follow fluctuations
in outside temperature, a condition described by the term poikilo-
thermous. All invertebrate animals and the lower vertebrates,
including fishes, amphibians, and reptiles, with the possible
exception of the turtle, are poikilothermous. Such animals lack a
mechanism for maintaining a constant body temperature and a
constant rate of metabolism. Some kinds of goldfish can be
frozen stiff in icy water and be recovered unharmed if the warm-
ing is done gradually. In the "frozen" state the temperature of

352 GENERAL ZOOLOGY

the body is only a little above 0°C. The integument of such
animals is without any insulating mechanism for preventing loss
of heat.

Birds and mammals are warm-blooded or homoiothermous.
In such animals the range of body temperature compatible with
living is much more limited and must be maintained in the
neighborhood of 39°C. for birds and 37°C. for mammals, except
in a hibernating animal. Most homoiothermous animals are
provided with feathers or hair which, in addition to serving as a
protection from mechanical injury, also aid in conserving body
heat. Secretion of sweat and evaporation of water from the
lungs are important means for preventing overheating. Feathers,
heavy fur, or hide constitute good protections against cold.
Aquatic mammals, such as whales, are provided with a layer of
fat or blubber beneath the skin. A thick hide is also effective
as a protection from the heat of the sun, as in the case of the
hide of the elephant and rhinoceros. The human body in its
natural state is poorly adapted for living in cold climates. The
problem has been met by the use of artificial covering in the form
of clothing.

Hibernation. — Many of the invertebrate animals such as
snails, crustaceans, insects, spiders, and myriapods undergo
hibernation. Among vertebrates hibernation also occurs during
the winter in most amphibians, and in terrestrial reptiles such as
lizards, tortoises, and snakes, and also in some mammals, par-
ticularly many rodents and some carnivores. Hibernation in
mammals occurs in holes, hollow trees, caves, dens, etc., where
the animal is protected from cold and enemies. During hiberna-
tion the animal is in a deep torpor resulting from a greatly lowered
rate of metabolism. In some cases there are records to show
that the hibernating animal awakes from time to time to take
food from a supply previously stored in the nest. The time
spent in hibernation varies roughly from three to six months,
depending upon the kind of animal and also upon the latitude
of its habitat.

The decline in metabolic rate of a hibernating animal is indi-
cated by the rates of respiration and heart beat, body tempera-
ture, and loss in weight. The respiratory rate of a species of
ground squirrel, Citellus tridecemlineatus, during hibernation
ranges from 3^ to 4 per minute as compared with 100 to 200 per

ADAPTATION 353

minute under conditions of normal activity. The heart beat dur-
ing hibernation averages 17.4 beats per minute as compared with
200 to 300 per minute under normal conditions. The body
temperature, which under normal conditions varies from 32 to
41°C, during hibernation may drop as low as 2°C. The loss of
body weight may total 40 per cent; it occurs principally in
stored fat.

When the hibernating animal awakes, the resumption of nor-
mal activities is gradual, but the rise in body temperature is
abrupt. Observations on the European dormouse record a rise
from 13.5 to 35.7°C. in 1 hour and in the case of a bat, a rise from
17 to 34°C. in 15 minutes.

Among warm-blooded animals hibernation occurs only in some
mammals and never in birds. It is relatively common in amphib-
ians and reptiles and also occurs in some fishes (lungfishes).
The lack of a delicately adjusted temperature-regulating mecha-
nism in the lower vertebrates fits them for the hibernating habit ;
in fact, if a frog is chilled in cold water, it takes on the character-
istics of a hibernating animal. In birds and mammals, the
normal range of fluctuation of body temperature is rather
limited, particularly in forms that do not hibernate. In man a
few degrees' rise in body temperature produces a fever condition
with deleterious results, while a subnormal temperature beyond
a very limited range interferes with metabolism. Hibernating
mammals, on the other hand, have a temperature-regulating
mechanism that maintains a fairly constant value under condi-
tions of normal activity but which ceases to function when the
animal hibernates. In such animals the fluctuation of body
temperature under nonhibernating conditions, 32 to 41°C, is
much greater than in the case of man. The hibernating habit of
certain mammals is possible because of the absence of a rigid
temperature-regulating mechanism which in turn may be a sur-
vival from cold-blooded ancestors.

Light. — Animals are sensitive to light and react to it by mov-
ing toward or away from it, or by developing anatomical features
of an adaptive significance. A light-sensitive organ, such as the
red eyespot of the protozoan, Euglena viridissima, by increasing
the light absorption makes the region of the body in which it is
located more sensitive to light than other regions. The various
types of eyes found in different groups of animals, climaxed by

354 GENERAL ZOOLOGY

the highly complex vertebrate eye, all represent specialized end
organs for the perception of light stimulation. In the lower forms
these organs do not form images of external objects but serve
simply as light receptors which distinguish intensities of light.
The image-forming function of eyes of higher forms is an acquisi-
tion that came with the evolution of the lens and accompanying
structures, which in the case of the vertebrate eye is a cameralike
organ capable of projecting images of external objects on a sensi-
tive retina.

There is undoubtedly a relation between light and the forma-
tion of pigment in the skin, but there are difficulties involved in
reaching a clearcut conclusion regarding the relationship, as has
been indicated in the discussion of the pigment of cave sala-
manders. In addition to the facts brought out in that discussion,
it might be noted that light, in some cases at least, is necessary
for the complete development of pigment in the skin. Thus, if
frogs or young salamanders are allowed to develop in the dark,
the formation of pigment is retarded. Flounders are normally
pigmented above and are white on the undersurface of the
body, which ordinarily is not exposed to the direct action
of light. If young flounders are allowed to develop in tanks
illuminated from below, it has been found that pigment develops
on the undersurface as well as above. Human races native to
tropical regions have deeply pigmented skin and dark-brown
or black-colored eyes, which certainly act as protective adapta-
tions against the intense illumination of the sun. The dark eye
of the Eskimos similarly is a protection from the intense reflec-
tion of a snow-covered environment. White-skinned races
developed in regions of less intense light. Skin color seems to
be a purely environmental phenomenon, not necessarily corre-
lated with other traits such as intellectual vigor.

Moisture. — Metabolism requires that the concentration of
the body fluids be maintained at a certain value. If an undue
amount of water is lost through the integument or in other ways,
death follows. Aquatic and semiaquatic animals, such as
fishes and amphibians, are so highly adapted to a moist environ-
ment that they cannot live for any length of time in a medium of
dry air because under these conditions there is no check on the
loss of water through the integument. Such uncontrollable loss
does not occur in terrestrial animals because of a difference in the

ADAPTATION 355

nature of the integument. Animals living in a desert must be
adapted to an environment in which there is a scarcity of water.
Many of the smaller desert forms of mice and lizards never take
water in the free state and are able to derive sufficient amounts
from their food to meet their needs. The stomach of the camel is
provided with water reservoirs in the form of small flask-shaped
cavities, each with a constricted mouth. When the stomach is
filled with water, the muscles at the mouths of the cavities relax
automatically, allowing the cavities to be filled. Water is
absorbed from the stomach but not from the reservoirs. As the
demand for water arises it is released from the reservoirs into the
stomach.

Pressure.- — Animals are adapted to conditions of atmospheric
or hydrostatic pressure under which they live. This is strikingly
brought out when the animal is subjected to a considerable
change in pressure conditions. Deep-sea fishes brought to the
surface from depths of 10,000 to 16,000 ft. are unable to survive
the transition for any length of time. At high altitudes bleed-
ing at the nose may occur in human beings owing to the
diminished air pressure. Since there is less oxygen, per unit
volume, in high altitudes than at lower ones, the reduced amount
of oxygen taken with each lungful of air also causes respiratory
distress.

Adaptations for Race Preservation. — In many animals there is
no such thing as parental care of the young. Marine forms, such
as the starfish, sea urchin, and many fishes, deposit the eggs in
water, where they are fertilized, after which all parental responsi-
bility ceases. The development of starfish is so rapid that in
24 hours it has advanced to the stage where free-swimming
larvae are produced which are fully able to care for themselves.
In fishes developing outside of the body, the egg is provided with
a quantity of yolk which serves as a source of nutrition until the
young fish is able to shift for itself, but even here the rate of
development is very rapid. Among other fishes and among
amphibians the eggs are deposited with some attempt at conceal-
ment, so that they do not become too easy prey, but on the whole
the parental supervision and protection are rather scanty. Natu-
rally, an enormous number of eggs and embryos are destroyed,
but since the rate of reproduction is so great in such cases — a
cod is said to produce 6 million eggs — there is under ordinary

356 GENERAL ZOOLOGY

conditions but little danger of the extermination of any given
species. Most birds protect and feed their young as long as the
young are weak and unable to care for themselves. Bird nests
are in most cases temporary homes, built primarily for the pro-
tection of the young, and therefore occupied only during the
breeding season.

Among other animals the protection, care, and rearing of
the young seem to be the principal business of life. The
meticulous care which the honeybee bestows upon the young in
all stages of development is an example. The food of the devel-
oping larvae is not only collected but is partially digested by the
workers before being administered to the young. The character
of the food is changed to suit the change in requirements of
the larva at different stages of its development. The care
and the guardianship exercised by the worker do not cease until
the young reaches its fully developed condition when it is relin-
quished with a few final touches. The remarkable part of it
all is that the progeny are not the offspring of the workers, but of
the queen. Parental care is exercised only by the foster parents.

The method employed by the digger wasp of the genus 4m-
mophila for providing the young with food during the larval period
has some points of seeming refinement about it that merit
notice. The food, consisting of caterpillars which the wasp over-
comes by stinging, is sealed in the burrow with the wasp's eggs.
A discriminating surgical skill has been ascribed to the wasp
because in many cases the caterpillar is not killed by the sting
but only paralyzed, and this has been interpreted as an adapta-
tion to keep the food alive until the larvae of the wasp are ready
to feed upon it. Unfortunately for this idea it has been shown
that in cases where the wasp has killed the caterpillars, the
larvae feed upon the dead bodies with as much satisfaction as
upon the paralyzed ones. It seems to be largely a matter of
chance whether the stinging of the caterpillar causes death or
paralysis, and so far as the larvae are concerned it makes no
difference whether the food provided is dead or kept in live
storage.

One of the most curious habits in guarding and rearing the
young is found in a species of marine catfish, the gaff-topsail
catfish, Felichthys felts, a subtropical form common along the
South Atlantic coast of the United States, the male of which car-

ADAPTATION 357

ries the eggs in its mouth until they hatch. The number of eggs
found in a single individual male varies from 2 to 55 in observed
cases. The eggs are held loosely in the mouth and are subjected
to effective aeration by the water streaming over them and out
through the gill clefts at either side of the mouth. So far as
known the male does not feed during the time the eggs and the
developing young are in its mouth, which may be 70 days.

The actual time required for the development of mammals
to the adult state is considerably longer than for other animals.
In general, mammals that depend primarily upon speed for escape
from danger do not have helpless or unprotected young, and in
such cases either the young are able to move rapidly as soon as
born, or they are brought forth in some secluded spot. Thus, the
young of the ox or deer are capable of running or walking shortly
after birth. Of course, the young are dependent upon the
mother's milk for food — as is the case with all mammals — but
in times of danger they are capable of fleeing with the mother. In
the opossum and also in the kangaroo, the young are born in
an extremely immature condition but are at once transferred into
a large pouch formed by a fold of the skin of the abdomen in
which they are carried (Fig. 200). According to the observations
of Hartman, in the case of the opossum the newly born animal
reaches the pouch by its own efforts. In the pouch the young
grasps the nipple of the mammary gland, from which milk is
forced into the mouth by mammary muscles, strangulation being
prevented by the prolongation of the larynx into the naso-
pharynx. Even after the young kangaroo runs about, it often
retreats to the pouch. Rodents and carnivores also bear their
young in a relatively immature state of development. Newly
born rabbits or kittens are blind and remain so for days, during
which time they are dependent on the parent for both food and
protection. In each case the young are brought forth in a
secluded place.

• A consideration of adaptations emphasizes the fact that living
demands an adjustment between the organism and its environ-
ment. Without its environment the organism does not exist.
It has been said that "evolution is no more than adaptation of
organisms to environment" (Osborn). The debate as to the
origin and method of development of adaptation is of minor
consideration in comparison with the importance of the fact of

358

GENERAL ZOOLOGY

adaptation. As to their general character, adaptations "are
primarily for the good of the species; they are beneficial to indi-
viduals only so far as these individuals are essential to the welfare

Fig. 200. — Black swamp-wallaby of Australia carrying young in the marsupium.
{From Shall, La Rue, and Ruthven, Animal Biology.)

of the species; and they often are injurious or destructive to the
individual, but since the welfare of the species is usually identical
with that of the constituent individuals, the destructive or
injurious effect is not obvious unless the good of the species
demands the sacrifice of individuals" (W. K. Brooks).

CHAPTER XVI
ENVIRONMENT AND DISTRIBUTION

The environment of an animal includes biological factors,
mainly breeding and feeding conditions, as well as physical fac-
tors, such as temperature, moisture, light, altitude, etc., all of
which are reflected in its adaptations. Darwin, in discussing
the nature of the struggle for existence, illustrated the compli-
cated interplay of biological factors involved in the production of
red clover (Trifolium repens). The fertilization of red clover
depends upon the visits of bumblebees, which in their search
for nectar unwittingly carry pollen from one flower to another.
The number of bumblebees in any district is controlled in part
by the number of field mice, which destroy the combs and nests
of the bees. The number of field mice in turn is regulated by
the number of cats or other enemies of the field mice. Hence the
number of cats may become an important factor in determining
the size of the clover crop. Every living thing is beset with
biological hazards which it must surmount in order to survive.
The background for the interplay of biological forces is the
physical environment, some aspects of which will be briefly
summarized in the following paragraphs. The two major
physical environments are water and land.

Water. — The most obvious adaptive features found in animals
living submersed are means of obtaining sufficient oxygen for
respiration and means of locomotion in a fluid medium. The
source of oxygen available for respiration is air dissolved in the
water and oxygen generated by aquatic chlorophyll-bearing
plants. The water of a well-aerated stream contains from 9 to
10 cc. of oxygen per liter, which is about 3^o of the amount of
oxygen in an equal volume of air. However, the amount of
oxygen available to aquatic animals may be greater than this
if the water contains green plants. Aquatic animals are adjusted
or adapted physiologically to a relatively low supply of oxygen
because they are poikilothermous, which eliminates the necessity
of maintaining a body temperature higher than that of the water.

359

360 GENERAL ZOOLOGY

The organ, by means of which oxygen is absorbed from the
water and carbon dioxide is excreted into it, is some sort of gill,
the fundamental structure of which is the same for both inverte-
brate and vertebrate aquatic animals. Very small forms lack
gills and are capable of respiring through the body surface.
Aquatic mammals, such as the whale, seal, porpoise, and manatee,
have taken to an aquatic existence secondarily and obtain oxygen
from the air above the water. The same is true of aquatic
reptiles and some aquatic insects.

Locomotion is simplified for an aquatic animal because the
density of water is great enough to support partially or completely
the body weight. Movement is accomplished by pushing against
the water, and resistance to forward motion is reduced by the
shape of the body. It is interesting that aquatic mammals have
evolved a fish-shaped body with degenerated limbs.

Land. — Terrestrial animals are enveloped by air from which
oxygen is absorbed by means of lungs, the buccal cavity, the
surface of the body, or by a tracheal system. Locomotion over
the ground is accomplished as a rule by means of limbs, which
in many cases raise the body clear of the ground. The weight of
the body, whether carried or dragged, is a factor that does not
figure in aquatic locomotion. There is therefore a correspond-
ingly greater energy requirement involved in terrestrial as
compared with aquatic locomotion which is reflected in the
greater oxygen (and food) requirement of the terrestrial animals
as a whole. Both oxygen and food requirements are increased
for flying animals since the density of air is very much less than
that of the body. The requirements for flying are met by a
heightened basal metabolism and, particularly in the larger
flying forms such as birds, by a lightening of the body. The
form of the body also is such as to reduce friction to a minimum
during flight. The streamlining of both fishes and birds has the
same significance.

Animal Communities. — Animals whose needs are met by
similar conditions, or who have become adapted to similar
conditions, form an animal community. The members of the
same community, though often unlike and belonging to widely
separated taxonomic groups, are capable of living together
because their responses to the environment are similar. Less
specialized and therefore less rigidly adapted forms may live in

ENVIRONMENT AND DISTRIBUTION 361

two or more communities. The classification of communities is
primarily a classification of physical environments of which there
are three subdivisions: (1) that of the ocean (halobios); (2) that
of the fresh water (limnobios) ; and (3) that of the land (geobios).

Ocean Communities. — Animals found in the ocean are capable
of living in an aqueous medium containing certain concentra-
tions of inorganic salts, oxygen, and some organic matter, as well
as plant life. Some marine animals are so completely adapted
to the salinity of the ocean that they die when placed in fresh
water. The ocean offers a variety of aquatic environments
differing in depth, salinity, oxygen content, light, temperature,
etc.

Littoral Animals. — The water lapping the shore of the ocean
forms the littoral zone. Owing to its slight depth, it has the
greatest amount of light and wave action and is subject to
maximal variations in temperature. Since the ocean tides
vary from a few inches to many feet, great changes in depth of
the shore water are common. When the tide is out, many
marine animals are uncovered for hours. The community of
shore animals includes coelenterates, echinoderms, worms,
arthropods, and molluscs. At low tide some of these lie buried
in sand, while others such as oysters, barnacles, some echino-
derms, and corals remain exposed to the air.

Animals of the open ocean may be divided into three groups,
viz., (1) those of shallow water, (2) those living at the surface,
and (3) those at great depths.

Shallow-water Animals. — Shallow water includes depths of
450 to 500 ft. At these depths in addition to those animals
found in the littoral zone, free-swimming stages of littoral
animals occur. Such forms are never uncovered by the tide, are
less subject to wave action, and are adapted to a greater pressure.

Pelagic Animals.— This group includes the free-swimming
forms such as fishes, and also floating forms such as jelly-
fishes, small Crustacea, many larval forms, and Protozoa. The
transparency of the bodies of some of these plankton animals
makes them almost invisible. The Portuguese man-of-war is
a common floating type provided with a large gas-filled sac
(pneumatophore) .

Animals of Great Depths. — Animals living at great depths of
the ocean, 5,000 to 15,000 ft., bathybic animals, are adapted to

362 GENERAL ZOOLOGY

enormous pressures, low temperature, and varying degrees of
darkness. Fishes taken from a depth of 1 mile show no visible
structural adaptation to temperature, which is not surprising
since fishes are poikilothermous and even shallow-water forms
can withstand freezing temperatures. As to pressure, some
withstand the transition from a depth of 1 mile to the surface
without disintegrating. Adaptations to darkness, however,
can be recognized and these take two general forms: (1) the
development of long attenuated, highly sensitive tentacles, or
(2) the development of luminescent organs. The most abundant
forms of deep-sea life are shrimps, prawns, and related crus-
taceans, particularly copepods. Many are luminescent.

Fresh-water Animals. — Fresh-water communities are classi-
fied according to the form of the body of fresh water, viz., pond,
lake, or stream. Fresh-water environment differs from the
ocean in a greater variability in temperature, lower salt content,
and greater liability to contamination arising from erosion.
Many of the common marine forms such as echinoderms, bar-
nacles, and oysters are not found in fresh water. There are
fresh-water sponges, some coelenterates (Hydrozoa), and a few
species of polychaete worms. The common fresh-water forms
are fishes, amphibians, oligochaete worms, leeches, arthropods,
molluscs, and protozoa. These, as in the case of marine forms,
may be divided into swimmers, floaters, and bottom dwellers.

Land Communities. — Terrestrial animals may be divided into
two general classes: (1) those that live in the soil or in caves, and
(2) those that live above ground.

Subterranean Animals. — The habitats of subterranean animals
resemble aquatic habitats in that sudden changes in temperature
and moisture do not occur. Oxygen is less abundant than on the
surface. Common soil-dwelling forms include protozoans, earth-
worms, arthropods, amphibians, reptiles, rodents, and moles.
Darwin found earthworms at a depth of 6 ft.; the burrows of
prairie dogs may reach a depth of 8 or 9 ft.; but the majority of
soil animals are found nearer the surface. Temperature and
moisture are remarkably constant throughout the year in caves.
The common terrestrial cave dwellers include spiders, insects,
and salamanders. Owing to the uniformity of temperature and
moisture in caves, hibernation is rare among permanent cave
dwellers. Cave animals in general have degenerated eyes and
pigment, and highly developed tactile sense organs. Since caves

ENVIRONMENT AND DISTRIBUTION 363

may contain pools of water or streams, the cave population is
increased by a number of aquatic forms.

Surface Animals. — Many animals having their habitat primar-
ily on the surface may at times enter the soil, and likewise some
burrowing forms may come to the surface. The surface habitat,
in sharp contrast to the subterranean, exposes animals to wide
variations in temperature, light, and humidity. Surface animals
may be divided into three general classes: (1) those living on the
ground, (2) those associated with plants, and (3) those able to
fly or glide through the air.

Ecological Succession. — The physical environment, par-
ticularly of land and fresh-water forms, is constantly changing
and this change for the most part takes place in an orderly
manner. Lakes change to ponds, ponds to marshes, and marshes
to dry land, on which forests grow, or which becomes a treeless
prairie. Each change in physical environment is accompanied
by changes in the animal communities affected. Aquatic
communities in the usual course of events are succeeded by land
communities. Ecology is concerned with the study of all factors
involved in the determination of animal communities. It
consists largely in the study of the reactions of animals to environ-
mental conditions which result eventually in a balance or equilib-
rium between the individual members of a community and their
environment. Actually such balances or equilibria are unstable
and change with conditions. Then too, major geographical
changes, such as are known to have occurred in the past, may
completely destroy the balances established in the usual course
of ecological successions.

Distribution. — Zoogeography is the study of the geophysical
distribution of animals. As might be expected, such studies
show that representatives of the same group of animals are found
living under similar conditions in different parts of the world,
ecological relations being what they are. However, a review of
the natural, geographical distribution of animals in modern
times shows areas devoid of animals that are capable of living in
those areas. Thus the introduction into Australia of the rabbit
by man has resulted in the rabbit becoming established com-
pletely as a member of the Australian fauna. Similarly the horse
introduced into the American continent by Spaniards has been
able to thrive in the plains of North and South America. Many
animals show a discontinuous distribution in that members of the

364

GENERAL ZOOLOGY

ENVIRONMENT AND DISTRIBUTION

365

366 GENERAL ZOOLOGY

same group have their natural habitats in widely separated parts
of the world. Thus the tapir is found in Central and South
America and in Southern Asia; and alligators occur in central
China and Southeastern United States. Why have many ani-
mals failed to occupy certain regions favorable for their support
and why are other groups found in widely separated regions?

In answering these questions, in addition to the usual factors
operating in relatively stable communities and in ordinary
ecological succession, the effects of major geographical changes,
dealt with in geology and paleontology, must be considered.
Paleontology demonstrates that animal groups often originated
in localities far removed from the present habitats of the living
descendants. Fossils show that members of the camel family
formerly ranged North America as well as Asia, a fact that can
be readily understood when it is pointed out that these two
continents were formerly connected where Bering Strait now
separates them (Figs. 201 and 202). Ancestral camels became
extinct in North America but survived in Asia, whence they
spread to Africa. In South America they are represented by the
llama. The horse is not indigenous to North America. It arose
there, spread to the Old World, but became extinct in the place
of its origin during the Quaternary period. In the Old World it
survived and gave rise to modern horses. Paleontology discloses
the wandering of many animals from place to place, in all prob-
ability in search of food and shelter, until the paths of these
various migrations have become a complicated network, which
has been interpreted in relatively few cases by fossil remains.

Discontinuous distribution in many cases is the result of the
creation of barriers isolating animal communities in whole or in
part in various ways. The peculiar fauna of Australia is undoubt-
edly due to the fact that this continent has been separated from
other continents from the beginning of Cenozoic times, with the
result that its egg-laying mammals are entirely different from
mammals found in continental Asia. Accordingly, prior to the
coming of civilized man much of its fauna remained mesozoic in
character. Apparently the higher types of mammals never
evolved in Australia. In addition to water, other barriers
such as mountains, hills, deserts, temperature, climate, etc., have
been equally effective in bringing about a checker-board dis-
tribution of many animals of similar species.

CHAPTER XVII
THE ANIMAL KINGDOM

The animal kingdom may be subdivided into 17 phyla, only
about 10 or 12 of which are ordinarily studied by the beginning
student. The present account includes all the phyla and usually
the major subdivisions, but detailed descriptions are confined
to animals commonly studied in the laboratory. Since there is
still uncertainty as to the proper systematic position of some
animals, the grouping followed here must be regarded as a
tentative one and not necessarily the ultimate arrangement. In
citing examples of animals typical of a phylum or its subdivisions,
there is given either the scientific name, consisting of the name
of the genus followed by that of the species, or the generic name
alone. A common name is mentioned also where a common name
is in use.*

PHYLUM 1— PROTOZOA

Knowledge of the existence of the group of animals now known
as Protozoa dates from 1674, when they were observed and
described by the Dutch microscopist, van Leeuwenhoek, whose
pioneer work with the microscope led to the discovery of many
organisms of minute size. The term Protozoa was applied to
the group by Goldfus in 1820. Protozoa (first or primitive
animals) may be defined as single-celled animals living singly,
or in colonies in which the members are attached to one another.
The apparent simplicity of their organization is largely a matter
of minuteness in size of the individual organisms. Visible
evidence of differentiation consists in the presence of organelles,
which to all intents and purposes are functionally comparable to

* In the preparation of this chapter numerous sources were drawn upon,
particularly the following: H. S. Pratt: "A Manual of the Common Inverte-
brate Animals," Philadelphia; H. S. Pratt: "A Manual of Land and Fresh-
water Vertebrate Animals," Philadelphia; H. H. Wilder: "The Pedigree of
the Human Race," New York; C. L. Metcalf and W. P. Flint: "Destructive
and Useful Insects," New York.

367

368 GENERAL ZOOLOGY

the organs of metazoans. Examples of such structures, not all
of which are present in every protozoan, are the following: one or
more contractile vacuoles, by means of which a rudimentary circu-
lation is maintained; a mouth or cytostome, where food enters; a
number of food vacuoles, where food is digested; an eyespot that is
sensitive to light; and organs of locomotion, such as pseudopodia,
cilia, and flagella. Protozoa occur practically everywhere — in
water, soil, air, and in or on the bodies of other animals. In the
air they are carried in an encysted form that prevents desiccation
and permits them to be distributed by air currents. As para-
sites they cause a number of diseases in man and other animals.
SUBPHYLUM 1. PLASMODROMA. Includes Protozoa hav-
ing pseudopodia, or flagella, or entirely without organs of
locomotion. A pseudopodium is a finger-shaped or filamen-
tous extrusion of the cytoplasm, usually of a temporary nature.
A flagellum is a permanent, threadlike, and vibratile process of
the cytoplasm.
CLASS I. MASTIGOPHORA. Protozoa whose motile organs
consist of one or more flagella, which by whipping movements
draw or propel the animal through water.
SUBCLASS 1. PHYTOMASTIGOPHORA. Plantlike flag-
ellates.

Examples: Volvox globator, a colonial form (Fig. 203);
Euglena viridis, a common pond form; Ceratium, an armored
form, found in fresh and salt water; Noctiluca, a luminescent
marine form (Fig. 204).

Euglena viridis has a fusiform body, colored green by masses
of chlorophyll (chromatophores) and is provided with a long
flagellum at its anterior end. The body is enclosed in an
elastic pellicle which permits contractile movements but other-
wise holds the body in a more or less definite shape. Near the
anterior end of the body is a light-sensitive organ, the eyespot,
or stigma, which contains red pigment. The nucleus is single,
oval in shape, and centrally located. The presence of chloro-
phyll indicates that photosynthesis is possible, but there is also
a gullet or cytopharynx, a small depression at the base of the
flagellum, through which food particles are probably taken.
Thus Euglena seems to utilize the nutritional methods both of
plants (holophytic nutrition) and of animals (holozoic nutrition).
It is also thought that Euglena may to a certain extent be

THE ANIMAL KINGDOM

369

Fig. 203 — Volvox globator, containing developing germ cells, G. Vol vox ia
made up of thousands of flagellated cells arranged in a gelatinous matrix in the
form of a hollow sphere. The individual cells are connected by cytoplasmic
extensions which give the sphere a reticulate appearance. Daughter colonies
develop within the cavity of the sphere.

Fig. 204. — A, Euglena viridis, a flagellate, showing the striated cuticle, c,
chromatophores; e, eyespot; p, pyrenoid body. (After Bourne.) B, Trypano-
soma lewisi, from rat's blood, b, blepharoplast, a nuclear structure. (After
Laveran and Mesnil.) C, Codosiga, a choanoflagellate. (After Kent.) D,
Noctiluca miliaris, a cystoflagellate. (After Cienkowski.) E, Ceratium cornu-
tum, having an armor of cellulose plates. (After Stein.)

370 GENERAL ZOOLOGY

saprophytic, i.e., capable of living on dead or decaying organic
material. The cytoplasm consists of a clear outer layer, the
ectosarc, and an inner region, the endosarc, in which lie
the nucleus, chromatophores, and contractile vacuoles. The
nature and action of the contractile vacuole are considered in
connection with Amoeba, in which it can be more readily
studied (p. 372). During encystment Euglena secretes a thick
gelatinous envelope about itself and assumes a spherical form.
Euglena may reproduce (1) in the free state by longitudinal
fission; (2) by conjugation, a method described in detail in
connection with paramecium (p. 382); and (3) during encyst-
ment by binary or multiple fission.

SUBCLASS 2. ZOOMASTIGOPHORA. Animal-like flag-
ellates, lacking chromatophores and living as solitary or
colonial forms under holozoic, saprozoic, or parasitic conditions.
Examples: Trypanosoma gambiense, a blood parasite, the
cause of sleeping sickness among human beings in Western
and Central Africa. It is transmitted by the bite of a tsetse
fly, Glossina palpalis. T. lewisi, another species, is trans-
mitted to rats by fleas. Codosiga, a colonial form, each member
of the colony being provided with a collarlike ridge surrounding
the base of the flagellum (Fig. 204).

CLASS II. SARCODINA. Protozoa whose body lacks a firm
pellicle, in consequence of which change of body shape is
possible. Some have shells.

SUBCLASS 1. RHIZOPODA. Creeping forms with root-
like PSEUDOPODIA.

Order 1. Amoebida. Naked forms found in fresh and salt
water, soil, and as parasites in the alimentary canal of Metazoa.
Contractile vacuoles are present except in salt-water forms.

Examples: Amoeba proteus, a common fresh-water form;
Endamoeba histolytica, parasitic in the human colon, causing
dysentery and ulcers in the liver.

Amoeba proteus, examined on a slide under the microscope,
appears as an irregular, jellylike mass of protoplasm, slowly
changing its outline by projecting pseudopodia (Fig. 205).
Its size varies from about 127 to 340 fj, in diameter. The
action of the pseudopodia in bringing about movements can
best be understood if the amoeba is viewed from the side
instead of from above. To do this, the amoeba is placed in

THE ANIMAL KINGDOM

371

water in a narrow glass chamber which is mounted on the
stage of a microscope, tilted into a horizontal position. In
this position the bottom of the chamber is horizontal and the
amoeba can then be watched as it moves over the bottom to
the observer's right or left. Under these conditions it can

Fig. 205. — Amoeba proteus with pseudopodia extended. EC, ectoplasm;
en, endoplasm; f.v., food vacuole; c.v., contractile vacuole, expanded; n,
nucleus.

be determined that the amoeba moves by extending and
attaching a pseudopodium to the bottom and then drawing the
rest of the body toward the attached pseudopodium (Fig. 206).
The nucleus has a rounded outline but is not unalterably
fixed in position in the cytoplasm. Two general regions are

Fig. 206. — Side view of Amoeba proteus moving over a solid substratum by
attaching the extended pseudopodia and then drawing itself forward. (Adapted
from Dellinger, Jour. Exp. Zool.)

recognized in the cytoplasm: an inner portion, the endosarc
or endoplasm, and an outer bounding layer, the ectosarc or
ectoplasm. The endoplasm, in which the nucleus is located,
has a granular appearance caused by the presence of embedded
particles of various sizes, some of them food particles, others

372 GENERAL ZOOLOGY

crystals and grains of sand. The ectoplasm is flexible and
transparent but somewhat tougher and denser than the
endoplasm. The larger granules in the endoplasm represent
food substances that have been engulfed and that are in the
process of digestion. The spaces occupied by the food, the
food vacuoles, are temporary digestive organs which disappear
when the food is digested. Near the surface of the body is a
clear circular area, which disappears and reappears at regular
intervals. This is the contractile vacuole. Careful observa-
tion shows that when the vacuole contracts, the contents are
expelled through the ectoplasm. The vacuole then fills from
the confluence of streams of fluid drawn from the cytoplasm,
and accumulating at the site of the contractile vacuole. The
fluid is water containing small amounts of metabolic products
dissolved in it. Water in regulated amounts probably enters
the amoeba through the surface, though this cannot be demon-
strated by direct observation. The vacuole seems to be the
focal point where metabolic products dissolved in water
accumulate before being expelled.

There are no special respiratory organs in Protozoa for
absorbing molecular oxygen and giving off carbon dioxide; such
respiratory changes are functions of the surface of the body.
In this respect the amoeba behaves in the same way as a cell
in the body of a metazoan, with the difference that in the latter
oxygen is absorbed from the blood and carbon dioxide is given
off into the blood, while in the amoeba, the water in which the
animal lives supplies it directly with oxygen and absorbs carbon
dioxide from it. A nervous system and sense organs are
lacking in amoeba, yet the animal reacts to stimulation of
touch, light, heat, etc., by movements toward or away from
the source of stimulation, thus indicating that irritability
and contractility are present as properties of the animal as a
whole. There is no indication of differentiation in the cyto-
plasm to account for the conduction of nervous impulses or
the contractions that characterize the active animal. The
semifluid character of the animal precludes the development of
rigid structural features. Even in the case of the contractile
vacuole, which has the appearance of stability, microscopic
examination fails to reveal any structure that might account
for its functional activity.

THE ANIMAL KINGDOM

373

There is no mouth through which food passes into the interior
of the animal. Food is obtained by means of the pseudopodia,
the method used varying with the type of food. In securing
nonmotile food such as certain plants (desmids) and encysted
protozoans, the pseudopodia surround the object in a tight
embrace and gradually engulf it. In attacking a larger object
such as a mass of bacterial glea, two pseudopodia surround a
portion of the mass and gradually pinch it off. As a rule
rapidly moving objects cannot be captured, though motile
Protozoa such as Paramecium or Chilomonas (Fig. 207) may
be secured when they are relatively quiet. Thus a Paramecium

Fig. 207. — Amoeba proleus capturing Chilomonas para, another protozoan, by
means of its pseudopodia. Sketched at successive stages. (After Kepner and
Taliaferro, Biol. Bull.)

may be cornered by an amoeba against a solid object and
trapped in a pocket formed by two pseudopodia, or the amoeba
may send pseudopodia about the Paramecium and confine it to
an enclosed space before disturbing it. Sometimes the amoeba
advances upon a Paramecium and holds it fast by gripping it
with the ends of the pseudopodia, after which a portion of the
prey is "bitten" off (Fig. 208). Smaller objects are engulfed
entirely. Within the body of the amoeba the food is broken
up into small particles and digested in the food vacuoles.
Undigested remains are cast out through the body wall. It
may be inferred from this that the taking of food is not a
haphazard process, for the amoeba does not engulf everything
that it happens to meet; this means that it is capable of dis-
tinguishing between food and inedible objects.

374

GENERAL ZOOLOGY

An amoeba is protected to a certain extent by its ectoplasm,
which has the characteristics of a thin elastic membrane; but
since it often overcomes its enemies, if small enough, by
engulfing them, securing food and gaining protection are in
such cases accomplished by the same operation.

Amoeba proteus reproduces by simple fission, during which
an intranuclear spindle and chromosomes are formed. Accord-
ing to some observations the nuclear membrane persists

.*•:•.*■ .-.. . v .1 ■ m *>v ... • ■ . '.. ' . •■■*..■ f

u

£/

ill

B

Fig. 208. — Reactions of Amoeba proteus to food. A, ingesting a portion of
nonmotile bacterial glea; B, ingesting a Paramecium, which has been constricted
into a dumbbell shape. (After Kepner and Whitlock.)

throughout the mitotic process, surrounding the spindle.

During the telophase it constricts into two parts, within each

of which nuclear reorganization then follows.
Order 2. Proteomyxa. Protozoa with filamentous and often

branching pseudopodia.

Example: Nuclearia simplex, common on Spirogyra and

other fresh-water plants.
Order 3. Testacea. Body enclosed in a single-chambered shell

provided with a single opening through which the pseudopodia

can be thrust.

Example: Difflugia urceolata, a fresh-water form (Fig. 209).

THE ANIMAL KINGDOM 375

Order 4. Foraminifera. Body enclosed in a single or many-
chambered shell of siliceous or calcareous material, through
the numerous pores of which delicate pseudopodia extend.

Example: Globigerina bulloides, a marine form, both pelagic
and bathybic down to depths of 18,000 ft. Chalk beds are
composed largely of the discarded shells of this species.

Order 5. Mycetozoa. Large amoeboid forms, multinucleated,
with numerous contractile vacuoles and often colored red,
orange, yellow, or green. Reproduce by spores and regarded
as border-line forms between animals and plants.

Example: Mucilago spongiosa and other slime molds, found
on decaying wood. Some colonies may be several inches in
diameter.

SUBCLASS 2. ACTINOPODA. Floating forms with radi-
ating UNBRANCHED PSEUDOPODIA.

Order 1. Heliozoa. A spherical body with fine raylike pseudo-
podia and often a siliceous skeleton. Cytoplasm is divided
into an outer cortex and an inner medulla, the latter containing
one or more nuclei. Mostly fresh-water forms.

Example: Actinospherimn eichhomi, common in fresh water
(Fig. 209, C).

Order 2. Radiolaria. Marine forms, usually with a siliceous
skeleton. The spherical body is divided by a perforated
chitinous central capsule into an extracapsular cortex and an
intra capsular medulla containing one or more nuclei.
Example: Acanthometra elastica (Fig. 209, D).

CLASS III. SPOROZOA. Endoparasites in the cells and
tissues of many animals. No vacuoles. Body is covered by a
thick pellicle through which food is absorbed. Reproduction
by spores.

SUBCLASS 1. TELOSPORIDIA. Intracellular parasites
whose life history terminates in spores.

Order 1. Gregarinina. Intestinal parasites of arthropods and
annelids.

Example : Gregarina blattarum, found in the digestive tract of
roaches. The body of the gregarine {trophozoite stage) is
divided by a transverse partition into an anterior protomerite
and a posterior nucleated deutomerite (Fig. 210). The epimer-
ite is an additional segment in front of the protomerite which
is well defined during only a part of the life cycle. In repro-

376

GENERAL ZOOLOGY

duction two trophozoites, also known as gametocytes, unite and
become encysted (pseudoconjugation) within a single envelope,

Fig. 209. — A, Amoeba polypodia, dividing. (After Schulze.) B, Difflugia
urceolata, having a shell composed of grains of sand held together by chitin.
(After Leidy.) C, Actinospherium eichhorni, a multinucleated heliozoan without
a skeleton; the oblong objects in the medullary region are food particles; two
contractile vacuoles are shown in the cortex. (After Hertwig.) D, Acanthom-
etra elastica, a radiolarian with a spiny skeleton; the central capsule contains a
large number of small rounded nuclei. (After Hertwig.) (A, C, and D redrawn
from Hertwig, Manual of Zoology, by Kingsley, Henry Holt & Company.)

the gametocyst. Each conjugant then forms gametes, those
from one individual uniting with those from the other to form

THE ANIMAL KINGDOM

377

f< ■'•]

A B C

Fig. 210. — Gregarines. A, Gregarina blatlarum, showing a chain of two indi-
viduals. (After Cuenot.) B, Corycella armata. (After Leg er .) C, Stylorhynchus
longicollis. (After Schneider.) e, epimerite by means of which the parasite is
attached to tissues of the host; p, protomerite; d, deutomerite.

5 6 ' 8

Fig. 211. — Life history of Coccidium schubergi, parasitic in the intestinal
epithelium of the centipede, Lithobius forficatus. (After Schaudinn.) Cysts
(8) swallowed with food are dissolved by digestive fluid. Each cyst contains
four spores each of which in turn contains two sporozoites. The sporozoite
attacks the intestinal cell (1), enlarges to form a schizont (3) which undergoes a
rapid nuclear division eventually forming merozoites (4). The latter reinfect
other cells, repeating the cycle. Sooner or later schizogony is replaced by spo-
rogony which leads to the formation of microgametocytes (2) and macrogam-
etocytes (5). Figure 6 shows a macrogamete surrounded by a number of
microgametes. The fertilized macrogamete becomes encysted, forming four
sporoblasts (7) each of which develops two sporozoites (8). The latter remain
encysted until favorable conditions arise for their development in another host.
e, intestinal cell; z, sporozoite.

378 GENERAL ZOOLOGY

a large number of zygotes. Each zygote divides to form a large
number of sporozoites, which penetrate the epithelial cells of the
host's intestine and grow into trophozoites. Only a single
host is necessary for the completion of the life cycle. The
trophozoite eventually emerges from the epithelial cells but
remains attached for a time by the epimerite. The latter
disappears when the parasite becomes free in the intestine.
Order 2. Coccidiomorpha. Parasitic in many invertebrates and
vertebrates.

Examples: Coccidium schubergi, an intestinal parasite of the
centipede (Fig. 211); Plasmodium vivax, a blood parasite caus-
ing tertian malaria in man. The sexual phase of the life cycle
is passed in the body of the female of a species of mosquito
belonging to the genus Anopheles. An asexual phase, includ-
ing preparatory steps for the sexual phase, is passed in the
human blood stream (Fig. 212).

The salivary glands of a mosquito capable of infection carry
sporozoites, which are spindle-shaped cells, 10 to 12 n in length.
These are introduced into the wound when the mosquito
bites a human being. In the human blood stream the parasite
bores into a red blood corpuscle, where it takes on an amoeboid
shape known as the trophozoite stage. On reaching its full
growth, the trophozoite undergoes segmentation or schizogony,
as it is called, to form spores or merozoites, which are liberated
in the blood stream by the rupture of the corpuscle, about 48
hours after infection. The free merozoites attack fresh cor-
puscles and the cycle is repeated; each time the corpuscles
break down, the chill characteristic of malaria occurs. After
several generations of merozoites have been produced, two
kinds of gametocytes are formed, macrogametocytes and
microgametocytes. The factors determining the development
of the merozoites into gametocytes instead of trophozoites are
unknown. For further development and complete differentia-
tion, the gametocytes must pass into the stomach of the
mosquito, a transfer readily brought about when a mosquito
bites a patient containing them. The macrogametocyte
undergoes certain nuclear changes somewhat akin to polar-
body formation and is thus transformed into a macrogamete.
Each microgametocyte, on the other hand, produces from six
to eight whiplike microgametes. A single microgamete enters

THE ANIMAL KINGDOM

379

a macrogamete, with which it fuses to form the zygote. The
zygote in the course of 24 hours is transformed into an active

Some may be taken
into the stomach
of the Ulosquito when
it bites

luman

Development of
Parasite in

6

InSalivary Development
-n Gland
f\ offflosq.

in the

\

\ J /^ Body

my Cavity°fm°sci
i/

mosquito

><»

'•RARy

Fiu. 212. — Diagram illustrating the life history of malarial parasite. 1,
red blood corpuscle; 2 to 7, schizogony; o, 6, c, a', b' , c' , development of gameto-
cytes; d, zygote; e, ookinete; /, g, h, development of oocyst; i, liberation of
sporozoites; k, section of salivary gland. (From Doane, Insects and Disease,
Henry Holt & Company. By permission.)

ookinete, which bores into the stomach wall, in the outer
layers of which it forms the oocyst. The latter grows in size
and then divides to form sporoblasts, each of which, in turn,

380

GENERAL ZOOLOGY

forms a number of sporozoites. The oocyst bursts, liberating
the sporozoites in the body cavity, whence they find their way
via the body fluids to the salivary glands (Fig. 212).

Tertian malaria is so called because the chill comes at the
end of 48 hours, i.e., on the third day. Other forms of malaria
have a different incubation period and are caused in each case
by a different species of parasite. The life
histories are all similar to that of P. vivax
given above, though differing in details.
SUBCLASS 2. CNIDOSPORIDIA. Amoe-
boid multinucleated forms, undergoing
continuous spore formation. Spores charac-
terized by thread capsules.

Example: Myxobolus lintoni, occurring in
the subcutaneous tissue of the carp.
SUBCLASS 3. ACNIDOSPORIDIA. Para-
sitic in invertebrates and vertebrates.
Reproduction continued through vegetative
life.

Example: Sarcocystis miescheriana, para-
sitic in the muscles of the pig (Fig. 213).
SUBPHYLUM 2. CILIOPHORA. Protozoa

PROVIDED WITH CILIA THROUGHOUT LIFE OR

IN early stages. Cilia are short processes
resembling hairs, located on the surface of the
body (Fig. 2, B).
CLASS I. CILIATA. Ciliated protozoans
with a firm pellicle. A macronucleus and one
or more micronuclei are present, except in
parasitic species. A mouth (cytostome) and
gullet (cytopharynx) are usually present,
along with food vacuoles and contractile vacuoles. The ecto-
plasm or ectosarc may contain trichocysts, which are small sacs
containing poisonous fluid that is discharged, with the sacs, for
purposes of offense or defense. In the water the trichocysts
are converted into long, thin threads. The cytoplasm may
also contain contractile fibers called myonemes. Cirri and
membranelles may be present. A cirrus is formed by the fusion
of a small tuft of cilia; a membranelle by the fusion of two or
more transverse rows of cilia. Cirri are used in creeping over

Fig. 213. — Sarco-
cystis miescheriana,
from the pig's dia-
phragm. The or-
ganism, enclosed in
a cyst, has divided
into numerous al-
veoli, each contain-
ing a number of
spores some of which
are shown free where
the cyst has been
cut open. (After
Mam.)

THE ANIMAL KINGDOM

381

the substratum. Membranelles are vibratile membranes
found in the region of the cystostome.
SUBCLASS 1. PROCTOCILIATA. Parasitic in the large
intestine of a number of amphibians. No cytostome; a single
type of nucleus; reproduce by binary fission.

Example: Opalina ranarum, intestinal parasites of frogs and
toads.

Fig. 214. — Diagram of the nuclear changes in Paramecium aurelia during
conjugation. A, union of conjugants; B, degeneration of macronuclei and first
division of micronuclei; C, second division of micronuclei; D, degeneration of
seven of the eight micronuclei in each conjugant; E, each conjugant with a single
micronucleus, which in F has divided into a stationary and micronucleus; G,
each conjugant with a synkaryon formed by the fusion of the migratory nucleus
of one with the stationary nucleus of the other conjugant; H, first reconstruction
division of the synkaryon to form two micronuclei (takes place in each of the
conjugants which now separate) ; I, second reconstruction division of the micro-
nuclei; J, two micronuclei transformed into two macronuclei; K, division of two
micronuclei and division of cell; L, two complete new individuals. {After
Woodruff, Foundations of Biology, copyright, The Macmillan Company. By
permission.)

SUBCLASS 2. EUCILIATA. Macronucleus and one or

MORE MICRONUCLEI PRESENT AND USUALLY A CYTOSTOME.

Order 1. Holotrichida. Cilia rather uniform in size and dis-
tribution, and usually arranged spirally in parallel lines. The
longest cilia are about the mouth. Trichocysts are common.

Example: Paramecium aurelia, found in fresh water has a
cytostome, gullet, and cytopyge; contractile and food vacuoles;
a large macronucleus and two small micronuclei. If samples

382 GENERAL ZOOLOGY

from a culture of this protozoan are kept under continuous
observation, it is found that at fairly regular intervals binary
fission takes place and a single individual divides into two
daughter cells. Each of the latter grows and in about 10 hours
attains the size-limit characteristic of the species, when division
again occurs. A large number of binary fissions may take place
after this fashion; but sooner or later, under ordinary condi-
tions in a laboratory culture, an entirely different sort of
phenomenon, known as conjugation, occurs. In this process
two individuals touch, at first in front, and then along the
entire surface of one side, so that the cytostomes come together;
the macronucleus swells and breaks up, the fragments even-
tually dissolving; and the micronuclei by two successive divi-
sions produce eight nuclei in each conjugant. Seven of these
nuclei disintegrate while the eighth divides, forming a stationary
micronucleus and a migratory micronucleus. Each migratory
micronucleus then passes into the opposite cell and fuses with
the stationary micronucleus to form a synkaryon or fertilization
nucleus. The conjugants now separate and in each the
synkaryon divides twice, producing four micronuclei, two of
which are transformed into macronuclei. The two remaining
micronuclei each divide again, accompanied by a division of the
cell, so that two complete individuals, each provided with a
macronucleus and two micronuclei are derived from each of the
conjugants (Fig. 214).

Woodruff has shown that if the medium in which isolated
paramecia are living is kept fresh by constant changing, con-
jugation does not occur for thousands of generations (12,000, in
the period from 1907 to 1921). It was found, however, that
every 40 or 50 generations the macronucleus degenerates and is
replaced by chromatin from the micronucleus, a process known
as endomixis. The nuclear changes, as may be seen from the
figure, are similar to those occurring in conjugation, except
that there is no exchange of nuclear material between two
individuals; i.e., reciprocal fertilization does not occur (Fig. 215).
The common feature of both conjugation and endomixis, viz.,
the periodic replacement of the macronucleus with material
from the micronucleus, would seem to have some significance as
a means of rejuvenescence. However, it is generally believed,
in view of experiments with other species of Protozoa, that

THE ANIMAL KINGDOM

383

the prevention of death in Protozoa is due less to the environ-
ment than to the nature of the organism — in the case of
Paramecium, to the process of endomixis.

Order 2. Heterotrichida. Membranelles about the oral zone.
Cilia uniform over the rest of the body.

Example: Stentor, common in fresh water (Fig. 216, A).

Order 3. Oligotrichida. Cilia limited almost entirely to the
oral zone.

Fig. 215. — Diagram of the nuclear changes in Paramecium aurelia during
endomixis. A, typical nuclear condition; B, degeneration of macronucleus
and first division of micronuclei; C, second division of micronuleci; D, degenera-
tion of six of the eight micronuclei; E, cell division; F, first reconstruction division
of micronuclei; G, second reconstruction division; H, transformation of two
micronuclei into two maeronuclei; I, division of micronuclei and cell division;
J, two complete new individuals. (After Woodruff, Foundation of Biology,
copyright. The Macmillan Company. By permission.)

Example: Halteria, a fresh-water form that moves in leaps
by means of cirri.
Order 4. Hypotrichida. A flattened body, as a rule with cilia,
cirri, and membranelles on the ventral surface.
Example: Stylonychia (Fig. 216, D).
Order 5. Peritrichida. Cylindrical or cup-shaped body, usually
free of cilia except in the adoral zone; and usually provided with
a contractile stalk.

Examples: Vorticella (Fig. 216, C); Carchesium, a branched
colonial form.

384

GENERAL ZOOLOGY

CLASS II. SUCTORIA. Sessile protozoans without cilia
except in early stage of development. They are parasitic
forms provided with tentacles for piercing or sucking.

Example : Tokophrya (Fig. 216, B) . One species is frequently
found attached to fresh-water copepods, which are small
crustaceans.

B

D

Fig. 216.

-A, Stentor. B, Tokophrya. {After Hertwig.) C, Vorticella.
Stylonychia mytilus. {After Stein.) N, macronucleus.

D,

METAZOA

The group Metazoa includes all animals above Protozoa and
therefore has the rank of a subkingdom. On this basis Protozoa
is both a subkingdom and a phylum. The general features that
distinguish Metazoa from Protozoa are as follows:

1. The body of the Metazoan is composed of many cells that
may be divided into two general classes: somatic cells and germ
cells.

2. The somatic cells are differentiated into tissues and organs,
in which there is specialization of structure and function.

THE ANIMAL KINGDOM 385

3. The germ cells are the reproductive cells, which in many
forms are segregated from the somatic cells early in ontogeny.

4. Though asexual reproduction by fission or budding occurs,
there is always sexual reproduction from a fertilized egg or, less
commonly, an unfertilized egg.

5. The developing egg undergoes cleavage; the cells or blas-
tomeres thus formed adhere to one another to produce a multi-
cellular complex.

6. At least two germ layers develop : an ectoderm, forming the
external covering, and an endoderm, lining the alimentary canal
and its outgrowths. Between these, in the majority of meta-
zoans, a third germ layer, the mesoderm, is formed from which
muscles, vascular and other tissues and organs develop.

The distinction between Metazoa and Protozoa is not sharp
since colonial protozoans, such as Volvox, consist of groups of
different kinds of cells, organically connected with one another.
Colonial Protozoa to a certain extent bridge the gap between
solitary Protozoa and Metazoa, but in the latter there is a greater
degree of interdependence among the cells of the individual
organism than there is between the individual members of a
protozoan colony.

PHYLUM 2— PORIFERA

Porifera (pore bearers) or sponges are lowly organized Metazoa
that do not seem to lie in the direct line of ancestry of the higher
forms. They are sessile, aquatic animals, most of which live in
the sea. The body of the simplest sponges (Ascon type) is
tubular in form and is attached at its closed basal end to the
substratum. The free end is provided with an opening, the
osmium. The thin walls are pierced with smaller openings
called pores (Fig. 217A). The principal cavity of the tube, the
gastral cavity, is lined with flagellated collar cells (Fig. 218), known
as choanocytes, which bear a striking resemblance to certain
flagellated protozoans such as Codosiga. The food of sponges
consists of small animals and plants and organic matter in the
water, which with its contents is drawn in through the pores by
the action of the flagella of choanocytes into the gastral cavity,
where the food is ingested and digested by the choanocytes, the
water leaving the gastral cavity by the osculum. In the Sycon
type of sponge the gastral cavity consists of numerous outpocket-

386

GENERAL ZOOLOGY

ings of the main cavity, called flagellated chambers or ampullae,
which alone contain choanocytes. Water enters the ampullae
through pores and passes into the central cavity, now called the
cloacal cavity, from which it leaves by the osculum (Fig. 217B).
The Leucon type of sponge (Fig. 217C) results from a separation
of the ampullae from both external and cloacal surfaces by an
increase of mesodermal tissue, the ampullae retaining their
connections with both surfaces by means of narrow canals.
Both the incurrent canals (the original pores), leading to the
ampullae, and the excurrent canals, leading away from them to
the cloacal cavity, may be enlarged to form subdermal and sub-

A B c

Fig. 217. — Diagrams of three structural types of sponges. A, Ascon type; B,
Sycon type; C, Leucon type. The dermal epithelium is indicated in light line,
the gastral epithelium in heavy black. Since the outer layer of the sponge larva
becomes the inner layer of the adult, there is some difficulty in applying the
terms ectoderm and endoderm. As it is, the dermal epithelium corresponds in
position to the ectoderm of other forms and the gastral epithelium to the endo-
derm. c, cloacal cavity; f, flagellated chamber; g, gastral cavity; o, osculum;
p, incurrent pores; s, subdermal cavity.

cloacal spaces. In any case the choanocytes are confined to the
ampullae.

The outer layer of the body of the sponge is composed of a
single layer of flattened cells. The gastral cavity and the
ampullae are lined with choanocytes. The remaining cavities
are lined with a smooth epithelium. The middle layer, or meso-
derm, lying between the dermal layer and the gastral layer or the
derivatives of the latter, varies in thickness and contains a variety
of structures, some of which are skeletal parts, such as the hard
calcareous or siliceous spicules, or the softer horny fibers of
spongin. Both spicules and spongin are present in many sponges,
though some contain neither. Some spicules are narrow rods,
pointed or rounded at the ends, others are tri- and tetra-actinal

THE ANIMAL KINGDOM

387

and various other shapes. The cells of the mesoderm or more
properly, the mesenchyme, consist of (1) scleroblasts, which secrete
the spicules, and spongioblasts, which produce the spongin;
(2) stellate connective tissue cells; (3) myocytes, which are con-
tractile and are found at the pores and osculum, where they
function as sphincters; and (4) archeocytes. The last are
amoeboid cells that share with the choanocytes the ingestion
and digestion of food and also give rise to germ cells and to

Fig. 218. — Portion of cross section of Grantia, a Sycon type of sponge, cc,
choanocytes; ect, dermal epithelium; fl, flagellum of choanocyte; mes, mesoglea;
sp, portion of spicule. (From Shull, LaRue, and Ruthven, Animal Biology.)

gemmules. Reproduction occurs asexually by budding, sexually
from fertilized eggs and by the formation of gemmules. Gem-
mule formation, which is more common in fresh-water sponges,
seems to be an adaptation for surviving low temperature or lack
of water. This is accomplished, in the case of the fresh-water
sponge during summer and fall by the formation of capsules of
archeocytes in the mesenchyme. The gemmules, composed of
encapsulated archeocytes, fall to the bottom if the sponge dies
and remain there until the following spring, when they develop
into sponges. All fresh-water sponges do not die in the winter,
even though gemmules are formed.

388 GENERAL ZOOLOGY

Separate sexes occur in some species of sponges; others are
hermaphroditic. The fertilized egg develops into a two-layered,
ciliated, motile larva. The gastral cavity and its derivatives
develop from the outer layer of the larva; and the dermal layer
and mesenchyme come from the inner layer of the larva. Thus
there is in sponges what is called an inversion of the germ layers,
since in other metazoa the outer layer of the gastrula gives rise to
ectoderm and the inner layer to endoderm. At metamorphosis
the ciliated sponge larva becomes attached to the substratum
and is transformed into a sponge.

Sponges lack the usual organ systems found in higher Metazoa.
There is no nervous system. The flagella of the collar cells beat
independently and not in unison, as do the cilia of ciliated epi-
thelia of other metazoans. The currents created in the water
by the flagella bring in food and oxygen and remove waste.
There is no other circulatory or excretory system. The myocytes
located about the pores and the osculum represent a very primi-
tive form of contractile tissue which combines the properties of
nerve and muscle, since it responds normally to direct stimula-
tion. In keeping with their low degree of organization, sponges
display a remarkable power of regeneration. If sponges, of
certain species, are crushed to a pulp and pressed through fine
bolting cloth, the dissociated cells collect in small masses, some
of which eventually develop into normal sponges.
CLASS I. CALCAREA. Marine sponges, small in size,

tubular in shape, solitary or colonial, with calcareous spicules.
Example : Grantia canadensis, a common species of the Sycon

type.
CLASS II. NONCALCAREA. Marine and fresh-water

forms with siliceous spicules or spongin, or both, or neither.
Examples: Euplectella aspergillum, Venus flower basket, a

"glass" sponge; Spongilla lacustris, a common fresh- water

sponge; Hippospongia gossypina, an American species of

commercial sponge.

PHYLUM 3— COELENTERATA

There are two morphological types among the Coelenterata:
(1) the polyp or hydroid form, which is sessile, and (2) the jellyfish
or medusoid form, which is free-swimming. The polyp type is
illustrated by Hydra, whose body is a double-walled, tubular sac

THE ANIMAL KINGDOM

389

provided with a fringe of usually six tentacles around its open,
free oral end. Its body wall consists of two layers of cells: the
outer one, ectoderm, and the inner one, endoderm. Between
these two layers is a thin noncellular, supporting tissue, the
mesoglea. The endoderm lines the gastrovascular cavity and the
tentacles. Thus each tentacle is a double-walled tube, whose
cavity, reduced in size, is a continuation of the gastrovascular
cavity (Fig. 219A). The medusa type is illustrated in a simple
form by Gonionemus, the convex side of whose bell-shaped body,
the exumbrella, corresponds to the attached blind end of the

Fig 219. — Diagrams for comparing the polyp type, A, with the medusa
type, B. 1, ectoderm; 2, endoderm; b, bud; g, gastrovascular cavity; m, mouth;
o, ovary; t, testis; v, velum. Mesoglea is shown in solid black.

polyp; while the open end of a tube, the manubrium, leading
from the concave surface of the bell, corresponds to the oral
end of the polyp (Fig. 219B). In jellyfishes, which are large
medusoids, the mesoglea is a thick jellylike layer containing cells
that have immigrated from the ectoderm and endoderm. Both
polyp and medusa types are radially symmetrical. The term
" Coelenterata " (hollo w-intestined) refers to the presence of the
gastrovascular cavity in these animals.

The ectoderm of Hydra is composed of epitheliomuscular cells,
interstitial cells, and nerve cells. The epitheliomuscular cell of
the ectoderm has the form of a short column whose inner end is
drawn out at right angles to the main axis of the cell and contains

390

GENERAL ZOOLOGY

contractile fibers. These fibers run parallel to the long axis of
the hydra, so that when they contract, the body of the hydra
shortens. The interstitial cells produce egg and sperm cells and
also give rise to nematoblasts, which are stinging cells capable of
discharging a threadlike tube, the nematocyst, and usually a

nem

Fig. 220. — Nematocysts of Hydra before and after discharge, cnc, cnidocil;
nem, nematocyst; nu, nucleus of nematoblast; t, threadlike tube. (From Dahlgren
and Kepner, Principles of Animal Histology, copyright, The Macmillan Company,
after Schneider. By permission.)

poisonous fluid. Discharge of nematocysts can be produced in
a living hydra on a slide in water by the addition of dilute acetic
acid or methyl green. The function of the cnidocil, a short
projection at the free end of nematocyst, is unknown. Once a
thread has been discharged the nematoblast from which it came
disappears and a new one takes its place from the interstitial

THE ANIMAL KINGDOM 391

cells. The thread of a penetrant type of nematocyst is capable of
piercing the integument of small animals. In the volvent type
the thread forms a coil about objects when discharged. In a
third type, the glutinant type, the discharged thread adheres to
objects by means of a sticky secretion. All three types are
probably used in capturing food (Fig. 220).

The nerve cells of Hydra are in the form of a loose network
extending throughout the ectoderm and also in the endoderm.
In medusoid forms there is a nerve ring in the outer rim of the
umbrella, which also contains sense organs (simple eyes or pig-
ment spots, with or without a lens).

Hydra attaches itself by a basal disk to the substratum by
means of an adhesive substance secreted by the ectodermal
cells. In Hydra oligaetis, these cells, under certain conditions,
secrete a gas, which, confined in a bubble of mucus, serves as
a float from which the hydra hangs downward in the water
(Fig. 221).

The mouth of the hydra is a star-shaped opening in a rounded
elevation, the hypostome, about which the tentacles are arranged.
The tentacles with the aid of the nematocysts capture food and
bring it to the mouth. The single layer of cells composing the
endoderm is thicker than the ectoderm. Its principal cells are
epitheliomuscular cells and gland cells. Most of the epithelio-
muscular cells of the endoderm bear several flagella on their
free surfaces. They are digestive in function, ingesting and
digesting food after the manner of an amoeba (intracellular
digestion). The gland cells are also flagellated but are without
contractile fibers at their bases. They secrete a digestive fluid,
by means of which food is digested in the gastrovascular cavity
(extracellular digestion). Undigested material is ejected through
the mouth. The peristomal (about the mouth) gland cells and the
gland cells of the basal-disk region differ in appearance from
the remainder and may have special functions. Interstitial cells
are also found in small numbers in the endoderm near the mes-
oglea. A few nerve cells also occur in the endoderm.

The contractile fibers of the endodermal epitheliomuscular
cells run transversely to the main body axis of the hydra and thus
form a circular muscular band. This, with the longitudinal band
formed by the fibers of the ectodermal epitheliomuscular cells,
accounts for the active and varied movements of the hydra-

392

GENERAL ZOOLOGY

Hydra attaches itself by means of the basal disk, but it can move
by a creeping motion of the disk. It also moves by a head-over-

g.b.

Fig. 221. — Hydra oligactis, semidiagrammatic longitudinal section. B, bud;
B.D. basal disk; E, ectoderm; En, endoderm; e.c, epitheliomuscular cells of
endoderm; G, gastrovascular cavity; g.b., gas bubble; g.c, gland cells; H, hypo-
stome; M, mesoglea (solid black line); Mo, mouth; N, nematocysts; O, ovary;
T, testis; Te, tentacle. (After Kepner.)

heels movement by arching the body until the ends of the tenta-
cles can grasp the substratum and then, releasing the basal disk,

THE ANIMAL KINGDOM

393

swinging the latter to a new position for attachment. Letting
go the hold by the tentacles, the hydra rises to an upright posi-
tion. In strong contrast with sponges, the hydra is capable of
quick energetic movements of its body and tentacles. As
already noted, it may in some cases detach itself and float about
by means of a mucus-enclosed gas bubble formed at its base.

Reproduction in Hydra, referred to in an earlier connection
(p. 155), may be asexual (budding or fission), or sexual. The
germ cells of Hydra and the lower coelenterates are said to arise
from interstitial cells of the ectoderm. In the higher coelen-

Fig. 222. — Obelia. g, gonosomes and gonotheca; h, hydranth and hydrotheca.

terates they come from similar cells in the endoderm. In the
face of these facts, it is difficult to hold that a line of germinal
continuity or a germ track of cells as distinguished from the
somatic cells, exists in these animals, unless one assumes that the
interstitial cells are undifferentiated cells that have retained
the reproductive potencies of the egg. In Hydra the testis
develops as a swelling in the ectoderm of the body wall just below
the tentacles; the ovary develops as a similar swelling near the
base (Fig. 221). A single testis develops numerous spermatozoa;
a single ovary produces a single egg. The egg is fertilized in the
ovary and undergoes development as far as gastrulation within
the ovarian epithelium, after which it escapes and grows into an

394

GENERAL ZOOLOGY

adult hydra. In many of the Hydrozoa there is metagenesis
in the life cycle, i.e., an alternation of polyp with medusoid
generations. The polyp, developing from fertilized egg, repro-
duces asexually by budding the medusae, which are male and
female.

Fig. 223. — Physalia, the Portuguese man-of-war, a pelagic colonial hydrozoan.
cr, crest; p, polyp; pn, pneumatophore. (From Parker and Haswell, Textbook of
Zoology, copyright, The Macmillan Company. By permission.)

Regeneration is marked in Hydra. Grafting of parts can also
be experimentally accomplished without difficulty.
CLASS I. HYDROZOA. Radially symmetrical, sessile, polyp
forms, and free-swimming sexual medusae; both sometimes
occurring in the life history of a single species. Without going
into the intricacies of classification, these different conditions
may be illustrated by the following:

THE ANIMAL KINGDOM

395

Chlorohydra viridissima, also known as Hydra viridis, the
common fresh-water hydra, colored green by algae in the
endodermal cells, has a polyp stage only. Gonionemus mur-
bachi, a marine, craspedote medusa (i.e., having a velum extend-
ing inward from the edge of the subumbrella), has practically

Fig. 224. — Aurelia, a jellyfish, and stages in the life history. A, vertical
section of adult; B, vertical section of gastrula; C, polypiform larva with eight
tentacles; D, scyphistoma with sixteen tentacles, in beginning of strobilation;
E, strobila; F, ephyra, which develops into a jellyfish, g, gastral cavity; go,
gonad; m, mouth; o, oral lobe; t, tentacles. {After Leuckart-Nitsche wall chart.)

a medusa stage only, its polyp stage being minute and of short
duration. Obelia dichotoma, a marine form, has a definite
alternation of generations, the colonial polyp producing sexual
buds (gonosomes) that develop into free-swimming male and
female medusae. The fertilized egg develops into a polyp
(Fig. 222). Physalia pelagica, the Portuguese man-of-war is

396

GENERAL ZOOLOGY

a free-swimming colonial form, the individual, highly poly-
morphic members of which are in communication with one
another by means of the common gastrovascular cavity. The
colony is attached to a float (pneumatophore) containing gas
that can be released through a pore, and later regenerated,
enabling the animal to drop below the surface and rise again.
There is an alternation of generations (Fig. 223).

Fig. 225. — Dissection of Metridium marginatum, a sea anemone, showing
the internal structure. 1, 2, 3, and 4, primary, secondary, tertiary, and quater-
nary mesenteries extending inward from the body wall, only the primary reaching
the gullet, g. The gullet opens into a common basal gastrovascular cavity,
o, ostia, pores through which water passes from one chamber to the other through
the primary mesenteries, r, reproductive organs; s, scyphonoglyphe, a ciliated
groove in either side of the gullet; t, tentacles. (Modified from Lineville and
Kelly, Textbook in General Zoology, Ginn and Company.)

CLASS II. SCYPHOZOA. Jellyfishes. Radially symmet-
rical, usually with an alternation of generations, although the
medusoid or the hydroid generation alone may be present in
some. In general, the medusoid stage is more prominent in
the group than the polyp. The latter, known as the scy-
phistoma, differs from Hydra in the following points: (1) the
attachment of its aboral end in a cup; (2) the presence of four
endodermal mesenteries projecting into the gastrovascular
cavity ; (3) the possession of an ectodermal gullet. The medusae
are acraspedote (lacking a velum), and are produced from the
scyphistoma by terminal budding (strobilation) .

THE ANIMAL KINGDOM 397

Example: Aurelia, a large jellyfish, common on the Atlantic
coast (Fig. 224).
CLASS III. ANTHOZOA. Sea anemones and corals.
The body is cylindrical and attached at the aboral end by the
foot or pedal disk. The mouth is oval, giving the animal a
biradial symmetry, and is surrounded by from six to several
hundred tentacles. An ectodermal gullet leads from the mouth
to the gastrovascular cavity, which is subdivided by six or
more longitudinal mesenteries composed of mesoglea and
endoderm. The ectoderm secretes, in corals, a skeleton of
calcium carbonate, and in other forms a hornlike substance
called ceratine. All are marine and dioecious.

Examples: Metridium marginatum, a sea anemone (Fig. 225) ;
Epizoanthus amerieanus, a sea anemone often attached to a
hermit crab; Porites porites, a common West Indian coral.

PHYLUM 4— CTENOPHORA

These beautiful marine animals are sometimes included in the
Phylum Coelenterata, which they resemble very closely (Figs.
226 and 227). The biradially symmetrical body is almost trans-
parent and may be round, oval, or ribbonlike in outline. Its
outer surface is soft and bears eight longitudinal rows of combs
whose teeth are composed of transverse plates of fused cilia.
Ctenophore means "comb bearer." In many, branched retractile
tentacles arise from pits near the aboral pole (Fig. 227). They are
provided with adhesive cells, which are used in capturing prey.
The slit-shaped mouth opens into a widened stomach region, to
which is connected a system of tubes opening to the outside by
minute pores. This complicated gastrovascular space is lined
with endoderm. Between the latter and the ectoderm is a thick
jellylike mesenchyme, which differs from the mesoglea of coelen-
terates in that it represents a true third germ layer. It also
contains muscle fibers of mesodermal origin. The animals,
therefore, are triploblastic.

Lying in a pit at the aboral pole is a statocyst.

Ctenophores are hermaphroditic, and pass through a compli-
cated metamorphosis before reaching the adult stage. Paedo-
genesis, reproduction in the larval stage, occurs in some species.
CLASS I. TENTACULATA. Characterized by a pair of long

tentacles or a pair of oral lobes which, however, may appear

only in the larva.

398

GENERAL ZOOLOGY

Fig. 226. — Mnemiopsis, a ctenophore with lobes in place of tentacles. A,
side view; B, aboral side; C, oral side, showing the mouth as a small opening in
the center.

N-4

B

M

Fig. 227. — A diagrammatic dissection of Hormiphora, a ctenophore; the
ectoderm is dotted, the endoderm striated, and the mesoderm solid black, i,
infundibulum, the stomach region, which is connected with eight meridional
canals one of which is shown beneath the more central row of combs. The infun-
dibulum also gives off two stomodaeal canals and two tentacular canals, one of
each being shown in the figure. The stomodaeal canal is close to the stomo-
daeum, the tube leading from the mouth (m) to the infundibulum. p, excretory
pore; s, statocyst; t, tentacle, the one on the left, in longitudinal section, shows
the muscular core by which the tentacle may be retracted into the sheath.
{Based on Parker and Haswell.) B, adhesive cell from a tentacle. The convex
surface is sticky, and the coiled filament acts as a spring to prevent the cell being
pulled out when it is attached to prey. The spiral thread is attached at its base
to the muscular axis of the tentacle, n, nucleus. (After Hertwig and Chun.)

THE ANIMAL KINGDOM 399

Examples: Pleurobrachia pileus, sea comb; Cestus veneris, a
flattened, ribbonlike form; Mnemiopsis leidyi, the sea walnut
(Fig. 226).
CLASS II. NUDA. Tentacles and oral lobes absent.
Example: Beroe ovata.

PHYLUM 5— PLATYHELMINTHES

Platyhelminthes (flatworms) are the first forms in which a
definite "head end" is present. The process of cephalization
as pointed out in a preceding chapter (p. 170) involves the con-
centration of sense organs at the anterior end of the animal and
a corresponding concentration of nervous control mediated
through cephalic nerve ganglia. Usually the body in cross
section is an oval, flattened dorsoventrally. Flatworms occur
in water, moist earth, and as parasites in animals and plants.
The body is triploblastic, unsegmented and covered by a ciliated
epithelium or by an unciliated cuticle. An alimentary canal
may be present but not a body cavity, the space between the
alimentary canal and the body wall being filled with a loose
connective tissue known as parenchyma, of mesodermal origin.
The alimentary canal like that of coelenterates is usually a blind
tube lacking an anal opening. The excretory system consists
of numerous flame cells connected with tubes opening by single
or paired excretory pores to the outside of the body (Fig. 91).
There is no blood circulatory system, nor a respiratory system.
The nervous system consists of a pair of cephalic ganglia from
which nerves pass to various parts of the body. Reproduction
is sexual, but fission and budding are also common. Her-
maphroditism is the rule. The ovaries and testes are well-
developed internal organs provided with ducts leading to the
outside.

CLASS I. TURBELLARIA. Ciliated, free-living forms, living
in fresh or salt water, or in moist soil. In most cases the cili-
ated glandular epidermis contains rhabdites, very small rodlike
bodies, produced by the epidermal gland cells or by the
parenchyma. They are discharged in the slimy secretion of
the ectoderm. Functional nettle cells, nematoblasts, are
sometimes found, but these have been shown to have been
acquired from coelenterates taken as food. Adhesive suckers
are present in some. The mouth is an opening at the end of a

400 GENERAL ZOOLOGY

pharynx, which usually can be thrust from the mouth as a
movable muscular proboscis (Fig. 228).

SUBCLASS 1. ACOELA. A mouth, with or without a probos-
cis, is present, but the remainder of the alimentary canal is
absent. Eyes are usually lacking. There is a statocyst in
the dorsal surface over the cephalic ganglia.

Example: Childia spinosa, marine form, 1.4 mm. in length.

SUBCLASS 2. COELATA. An intestine is present.

Order 1. Rhabdocoelida. The intestine is an unbranched tube,
in some cases, slightly lobulated. Found in marine and fresh
water and on land (Fig. 228, A and B). Some reproduce
asexually by terminal budding (Fig. 228, B).

Examples: Dalyellia, a fresh-water American form; Micro-
stomum, both fresh- and salt-water genus.

Order 2. Tricladida. Commonly known as planarians and
characterized chiefly by an intestine with three main branches,
one extending forward and two backward from the pharynx.
Each main trunk gives off diverticula with separate branches.
A pair of eyes are usually present (p. 192) in the head end.
On either side in front of the eyes is a sensitive lobe or auricle.
Example: Euplanaria maculata, a fresh- water form (Fig. 228,

C).

Order 3. Polycladida. The intestine has many branches
ramifying to all parts of the body. Eyes, otocysts, tentacles,
and tactile organs are well developed (Fig. 228D).

Example: Planocera inquilina, found as a commensal form
in the branchial chamber of a large marine snail, Busycon.

CLASS II. TREMATODES. Flukes. Exclusively parasitic.
Cilia are absent, or present only in larval stages. Usually the
mouth is subterminal and the intestine is bifurcated. Hooks
and suckers are found in ectoparasites for attachment to the host,
but suckers only in endoparasites. The name of the class,
Trematodes, refers to the presence of suckers (trema, hole).
Eyespots occur in ectoparasites, but only in the larva of
endoparasites.

Order 1. Monogenea. For the most part are ectoparasites on
aquatic animals, but some change to endoparasitism. There is
but one host. The organs for attachment, at the ends of the
body, are well developed. Some have small pores.

THE ANIMAL KINGDOM

401

Fig. 228. — Types of Turbellarians. (Based on von Graff.) A and B, Rhabdo-
coeles; C, triclad; D, polyclad. The alimentary canal is represented in solid
black. B shows strobilation or division taking place in a chain of four individ-
uals of Microstomum. Division begins in the alimentary canal by the formation
of septa which reach to the body wall. The chain really consists of sixteen
partially formed zooids. b, bursa copulatrix; g, genital aperture; m, mouth; n,
nervous system; o, ovary; p, penis; r, receptaculum seminis; s, vesicula seminalis;
t, testis; u, uterus; v, vagina; y, yolk glands; in C and D, the testis and its ducts
are shown on the left side of the animal and the ovary and oviduct on the right.

402

GENERAL ZOOLOGY

Examples:' Tristoma coccineum, parasitic on the gills of the
swordfish; Poly stoma integerrimum, as a larva, parasitizes the

gills of the frog tadpole, but when
the gills are absorbed at meta-
morphosis, it passes into the
pharynx, through the alimentary
canal to the urinary bladder,
where it is found in the adult
frog as an internal parasite.
Order 2. Digenea. Endoparasites
living in two or more hosts, to
which they attach themselves by
one or two median suckers, of
which the anterior one is the
mouth, and the second one, when
present, is for attachment only
(Fig. 229).

Example : Fasciola hepatica, the
liver fluke, lives as an adult in the
liver of sheep, cattle, man, and
other animals, where sexual
reproduction takes place. The
young embryos pass down the
bile duct into the intestine, and
out of the sheep's body into
water, where they develop into a
ciliated miracidium larva. Its
intermediate host is a water snail
of the genus Limnaea, whose tis-
sues it enters by boring, and
inside of which it forms a sporo-
cyst. Eggs contained in the
sporocyst develop parthenoge-
netically into rediae, which leave
the sporocyst and enter the tis-
sues. A number of parthenoge-
netic redia generations may be
produced, followed, finally, by different larvae known as
cercariae. These resemble the adult, except that they have
a tail, which serves as an organ of locomotion until the animal

Fig. 229. — Fasciola hepatica, dia-
grammatic from the ventral side.
The digestive tract in solid black is
shown only in the left of the figure.
The reproductive system is com-
plete only in the right side of the
figure, d, sperm duct; e, excre-
tory pore and a small portion of the
terminal excretory tubes; m, mouth;
o, ovary; p, penis; s, shell gland;
t, testis; u, uterus; v, vagina; y,
yolk glands. The ventral sucker
is indicated by a circle below the
penis.

THE ANIMAL KINGDOM

403

encysts on a water plant, when the tail is lost. Sheep are
infected by eating plants bearing cysts, the contents of which
find their way to the liver, where the young animals grow to
maturity. Liver rot, the disease produced by this parasite, is
often fatal (Fig. 230).

Fig. 230. Fig. 231.

Fig. 230. — Stages in the development of Fasciola hepatica. A, miracidium ;
B, sporocyst, with rediae developing internally; C, rediae with second generation
of rediae and cercariae; D, free cercaria; E, encysted cercaria. (From Van Cleave,
Invertebrate Zoology, after Tho?nas.)

Fig. 231. — A, Taenia solium, the pork tapeworm, s, scolex. B, scolex,
magnified. (After Leuckart-Nitsche wall chart.)

CLASS III. CESTOIDEA. Tapeworms. Endoparasites with
two body regions: (1) the scolex, or head, containing hooks or
suckers for attachment to the host; (2) the strobila, which is a
series of segments, called proglottid^, each of which is provided
with both male and female reproductive organs. A mature
proglottid is practically filled by the enlarged gonads. Tape-
worms inhabit the intestine of vertebrates and live as larvae in
the tissues of another animal used as food by the principal host.

404 GENERAL ZOOLOGY

There is no digestive tract, nutrition being absorbed through
the body wall. The outer covering is a nonciliated cuticle
capable of resisting the action of digestive juices (Fig. 231).

Example: Taenia solium, the pork tapeworm, is found as an
adult in the human intestine, where it reaches a length of from
4 to 10 meters. It attaches itself by means of hooks and suck-
ers. A small six-hooked larva develops from the fertilized egg
and is expelled. The larva must then enter the stomach of the
pig, where it bores through the stomach wall, and finds its way
to the skeletal muscles. Here the larva develops into an oval
cysticercus (9 by 5 mm.) and remains as a cyst until the infected
tissue is eaten by man. In the cavity of the cysticercus the
scolex is developed in an inverted position, and when the cyst
wall is dissolved by the gastric juice, the scolex is protruded like
the finger of a glove. In the intestine the scolex attaches itself
and develops proglottids from its posterior end (Figs. 231, 232,
and 233). Taenia saginata, the beef tapeworm, has a similar
life history, except that the intermediate host is the ox. It is
the commonest tapeworm in the United States.

PHYLUM 6— NEMERTEA

Nemertea (from Nemertes, one of the sea nymphs of Greek
mythology) is the name of a group of ciliated, unsegmented
worms, mostly marine, often included in the Phylum Platyhel-
minthes. The body is wormlike, ovoid, or circular in cross
section and varies in length in different species from a few milli-
meters to 6 meters or more (30 meters in the case of Lineus
longissimus) . The alimentary canal, beginning with a mouth
on the ventral surface near the anterior end, is a pouched tube,
extending the length of the body and terminating in an anal
opening. There is no body cavity, the space between the intestine
and the body wall being occupied by a gelatinous parenchyma.
A simple blood circulatory system is represented by two or more
longitudinal vessels, connected with each other and with blood
sinuses in the tissues. At the anterior end is a four-lobed cephalic
ganglion above the alimentary canal, from which nerves extend to
various parts of the body. The excretory system consists of
flame cells, connected by tubules with two longitudinal canals
opening on the surface of the body. Numerous simple eyes,
ocelli, occur, sometimes along the sides of the body. Some also

THE ANIMAL KINGDOM

405

have auditory vesicles. The body wall contains several layers of
circular and longitudinal muscles. They are dioecious as a rule,
though a small number are hermaphroditic.

Fig. 232. — Diagram of a proglottid, showing organs of reproduction (Based
on Leuckart.) d, sperm duct; e, excretory duct; n, nerve cord; o, ovary; s,
shell gland; t, testis; u, uterus; v, vagina; y, yolk gland.

One of the striking features of nemerteans is the proboscis,
a muscular organ that can be extended from a pocket in front of
the mouth to a distance almost as great as the length of the
body. It seems to be primarily a tactile organ, though the fact
that it is armed with dartlike
stylets indicates an offensive
or protective function also.
Another feature is the contrac-
tility of the body. A specimen
of Cerebratulus lacteus, capable
of extending itself to 5 meters,
can shorten to less than a meter.
Many are brightly colored.

Most nemerteans are nonpar-
asitic.

Examples: Cerebratulus lac-
teus, a marine form, found on
the New England coast in the sand near low-water mark.
Length of body is from 2 to 6 m.; width, 25 mm. Prostoma
rubrum, breeds in fresh-water aquaria; reddish in color and
about 18 mm. in length. Carcinonemertes carcinophila, para-
sitic on the gills and eggs of crabs.

Fig. 233. — A, cysticercus (bladder-
worm) with inverted scolex. B, cysti-
cercus with everted scolex. (After
Leuckart.)

406 GENERAL ZOOLOGY

PHYLUM 7— NEMATHELMINTHES

Nemathelminthes (threadworms or roundworms) are non-
ciliated, unsegmented worms circular in cross section, elongated
and threadlike in form; some are free-living in water or moist
earth and others parasitic in animals and plants. They lack
paired appendages, though bristles, hairs, and suckers may be
present. Most are dioecious.

CLASS I. NEMATODA. A smooth firm cuticula, overlying a
softer subcuticula, forms the body covering. The mouth is
at the dorsal side. The alimentary canal is a slightly differ-
entiated tube lying loosely in a body cavity and provided with
an anal opening near the posterior end of the body. The
body cavity is a hemocoel and lacks a peritoneum. The sub-
cuticula on each side projects into the body cavity, forming two
longitudinal ridges (lateral lines) each of which contains
an excretory tubule connected with the body cavity by a
ciliated funnel or nephrostome. The two excretory tubules
open to the outside by a single pore on the ventral side behind
the mouth. A dorsal and ventral band of the form of longi-
tudinal muscle fibers bulges into the body cavity. Each band
of muscle is divided into right and left portions by a longi-
tudinal nerve trunk. There are no circular muscle fibers.
The nervous system consists of a ganglionated ring encircling
the esophagus, and of a number of longitudinal nerves. Simple
eyes and sensory papillae are present in some. Most nematodes
are parasitic and dioecious. A few are viviparous. They
are very widespread and the number of species has been esti-
mated at over 100,000.

Examples: Trichinella spiralis, the cause of trichinosis, lives
as an adult (male 1.5 mm. in length, female 3 to 4 mm.) in the
small intestine of man, the pig, rat, dog, and mouse, where they
reproduce sexually. The female after copulation penetrates
the mucosa and gives birth to from 1,500 to 10,000 young.
The latter migrate via the blood and lymph to the muscles of the
thorax, neck, and jaw, in which they become encysted, thereby
injuring the muscles to such an extent that death of the host
may ensue. Man is infected by eating undercooked pork con-
taining cysts from which the young worms are liberated by the
action of the gastric juice (Fig. 234).

THE ANIMAL KINGDOM

407

Necator americanus, the American hookworm, is found in the
intestine of man and the gorilla, where it lives on the blood of its
host, after first making a wound with its cutting lips and teeth.
The adult male is 9 mm. in length, and the female 11 mm.
After copulation, the female deposits numerous eggs, which do
not complete development until discharged with the feces.
Development then proceeds, the embryo molting twice and
remaining inside of the loosened skin after the second molt.
There are two general modes of infection: (1) through the mouth,
in which case the larva passes directly to the intestine; (2)

Fig. 234. Fig. 235.

Fig. 234. — Trichinella spiralis, encysted in the muscle of the pig. (After
Leuckart.)

Fig. 235. — Enlarged view of the dorsal side of the anterior end of the hook-
worm Necator americanus, showing the quadrangular shaped mouth opening
through which two of the flat, platelike teeth can be seen. (After Stiles.)

through the skin of the feet and hands, from which it is carried
by the blood to the lungs. Considerable damage is then caused
by the animal boring through the lungs, heart, trachea, etc.,
to the intestine (Fig. 235).

The hookworm is a serious menace in the South, especially in
localities where precautions are not taken to avoid soil and
water pollution.

Ascaris lumbricoides is an intestinal parasite, especially com-
mon in children. The male averages about 20 mm. in length by
3 mm. in diameter, and the female 30 by 5 mm. The eggs pass
out with the feces and develop directly to the larval stages in
water or moist soil. Infection is by mouth from water, soil, and
the skin of raw fruits (Fig. 236). The larvae bore through the
walls of the intestine of the host, migrate through the lungs,
liver, or heart, and then return to the intestine where they
complete their development.

408

GENERAL ZOOLOGY

CLASS II. NEMATOMORPHA. Hairworms. In the adult
state they resemble thick horsehairs. The integument consists
of a thick cuticula, beneath which is a single layer of epidermal
cells (hypodermis). The mouth and esophagus are closed in the
adult state in some species. The body cavity is lined with a
peritoneum and is provided with dorsal and ventral mesen-

Fig. 236. Fig. 237.

Fig. 236. — Ascaris lumbricoides. {After Beneden.) A, male three-fourths
natural size; B, female, three-fourths natural size; c, head, enlarged, ventral side
showing excretory pore; d, head, front view, three lobes surrounding the mouth;
e, posterior end of the body of male greatly enlarged showing penial setae.

Fig. 237. — Acanthocephalid, male. (After Leuckart.) b, bursa; g, cement
glands; l, lemnisci, two saccular organs of unknown function; li, ligament; n,
nerve ganglion; p, proboscis and sheath; pe, penis; t, testes.

teries. A pair of eyes and a number of tactile bristles are pres-
ent. The animals are dioecious and the fertilized eggs are
deposited in the water, where the larvae develop and later
parasitize aquatic insect larvae. If the host is eaten by a
predaceous insect, or if the host dies, the larva may enter the
body of a grasshopper or other insect. In either case, develop-
ment is completed in the second host, from which the adult
escapes through the body wall and eventually reaches water.

THE ANIMAL KINGDOM 409

Example : Gordius robustus, a common horsehair worm, about
28 cm. long and 1 mm. wide; found in fresh water.
CLASS III. ACANTHOCEPHALA. There are three body
regions: (1) the proboscis, armed with hooks, (2) the neck,
(3) the trunk. The proboscis contains a ganglion from which
two nerve cords pass backward. There are no special sense
organs. There is a body cavity, but no alimentary canal. A
pair of nephridia opens into the reproductive duct. These
animals are parasitic in the intestine of vertebrates, attaching
themselves by the hooked proboscis. The larval stage occurs
in another host. Infection is by mouth from water contain-
ing the intermediate host (Fig. 237).

Examples: Echinorhynchus gadi, parasitic in the cod and
other fishes; Macranthorhynchus hirudinaceus, parasitic in the
intestine of the pig; larval stage in beetle grubs.

PHYLUM 8— TROCHELMINTHES

Trochelminthes (wheelworms) are very small aquatic animals,
often microscopic in size, unsegmented, and with cilia, if present,
confined to the anterior end or to the ventral surface of the body.
CLASS I. ROTATORIA. Rotatoria (rotators), or rotifers
(wheel bearers), the best known members of the phylum, are
extremely small aquatic animals, discovered by Leeuwenhoek in
1703. A common species, Hydatina senta, measures only 0.6
mm. in length. Other species are even smaller. The body of
the rotifer is composed of three regions: head, trunk, and a
postanal tail or foot, depending upon whether the animal is free-
swimming or fixed. Free-swimming forms may attach them-
selves temporarily by means of cement secreted by a gland in the
tail (Fig. 238). The cuticle covering the body is secreted by the
epidermis (hypodermis). The head is provided with a ciliated
disk {corona), in the center or ventral edge of which is the mouth.
The cilia serve as organs of locomotion and also direct food into
the mouth. The alimentary canal is a straight or slightly
curved differentiated tube provided with an anal opening.
The muscular pharynx (mastax) contains on its inner surface
hard jaws (trophi). A pair of excretory tubes, connected with
flame cells, opens into the hinder end of the intestine, on either
side. Sometimes they open into a pulsating bladder, which in
turn empties into the intestine (Fig. 238, v). The central nerv-

410

GENERAL ZOOLOGY

ous system consists of a cephalic (cerebral) ganglion from which
nerves extend to the periphery. There are one or two dorsal
antennae, located over the ganglion, and two lateral antennae,
one on each side of the trunk. One or more rudimentary eye-
spots are usually present. They are dioecious, though there

are a few species in which males are
unknown. Both oviparous and
viviparous types of reproduction
occur.

The life history of a typical rotifer
is interesting. There are two kinds
of eggs: (1) a thick-shelled "winter"
or "resting" egg, which can survive
drought or cold; and (2) a thin-
shelled "summer" egg. There are
two types of females, externally
alike, but differing in the kinds of
eggs they produce. Amictic females
produce eggs that cannot be ferti-
lized. Midic females produce eggs
that may develop parthenogenet-
ically but are also capable of fertili-
zation. The fertilized winter eggs
develop into amictic females.
These in turn produce eggs that
develop parthenogenetically into
females, both mictic and amictic.
The eggs of mictic females develop
parthenogenetically into males,
which produce sperm that may
fertilize other eggs of mictic
females. Thus the same eggs that
produce males, if unfertilized, pro-
duce females if fertilized.
Whitney has shown that feeding Hydatina senta with
Polytoma, a colorless flagellated protozoan, results in the pro-
duction of a larger number of female-producing (amictic)
daughters; while feeding with Chlamydomonas, a green flagel-
late, increases the number of male-producing (mictic) daughters.
It will be noted that the effect of diet on sex potencies does not
appear until the second generation following special feeding.

Fig. 238. — Diagram of a Roti-
fer, a, anus, b, brain; d, ovi-
duct; df, dorsal feeler; f, flame
cell; g, cement gland, by the
secretion of which the animal
attaches itself temporarily; e,
eyespot; i, intestine; m, mouth;
n, nephridial tube; o, ovary; p,
pharynx, containing masticat-
ing organs; R, retractor muscles;
s, stomach; v, contractile vesicle
(bladder). (After Parker and
Has well.)

THE ANIMAL KINGDOM 411

Physiologically, rotifers have remarkable powers of resistance
to heat and drying. Some species when slowly dried on a
slide, secrete a protective covering that enables them to survive
lack of water and extremes of temperature. Such animals can
be revived if placed in water after a year or more of desiccation.
Under natural conditions this property enables them to survive
drought and unfavorable conditions of temperature.

Example: Hydatina senta, a fresh-water species.
CLASS II. GASTROTRICHA. Gastrotichs are common
aquatic forms but less well known than rotifers, from which
they differ principally in possessing cilia on the flattened ventral
surface of the body. The head is characteristically marked off
from the trunk by a narrow neck region. They are both dioe-
cious and monoecious. Some of them measure only 1.5 mm. in
length.

Example: Lepidoderma rhomboides, a fresh-water species.
CLASS III. KINORHYNCHA. Very small marine trochel-
minthes measuring from 0.18 to 1.0 mm. in length; lacking cilia
but provided with spines and bristles. Locomotion char-
acterized by a peculiar invagination and evagination of the
head. Sexes are separate.

Example: Trachydemus mainensis, found on tidal flats,
Maine coast.

PHYLUM 9— BRYOZOA

Bryozoa (moss animals) are small, usually colonial animals,
found on the surface of rocks, plants, and other objects in salt or
fresh water. Externally some of them bear a resemblance to
compound hydroids, but internally the resemblance ceases; for the
individual zooids have a body cavity and a complete alimentary
canal, bent so that the anus lies near the mouth (Fig. 239) . There
is usually a central nervous system between the mouth and the
anus, and in some a pair of nephridial tubes with flame cells. The
lophophore is a characteristic horseshoe-shaped ridge about
the mouth, bearing hollow, ciliated tentacles. The epidermis
(hypodermis) secretes a calcareous cuticle, the ectocyst, which
protects the soft parts of the animal. In some the tentacles can
be completely withdrawn into the ectocyst. These animals are
either monoecious or dioecious. They are very ancient forms of
life, fossils having been found in the Cambrian and all subsequent
formations. The great majority of the species is marine.

412

GENERAL ZOOLOGY

Examples : Plumatella princeps, common in American ponds
and streams; Bugula flabellata, Vineyard Sound; Cristatella
mucedo, in ponds; colony, an elongated jellylike mass, 3 to
25 cm. in length.

Fig. 239. — Flustra membranacea (after Nitsche), a single animal, a, anus; ek
ectocyst; en, entocyst; /, funiculus; g, ganglion; k, collar, which permits retrac-
tion; m, stomach, also dermomuscular sac; o, esophagus. (From Hertwig's
Manual of Zoology by Kingsley. Henry Holt & Company. After Claparede.)

PHYLUM 10— BRACHIOPODA

Brachiopods are characterized by a bivalve shell, which suggests
a relationship to a mollusc, such as a clam or oyster; but the valves
of the brachiopod are dorsal and ventral instead of right and left
as in molluscs. As a rule, the animal is attached by a muscular
stalk, the peduncle, which is a prolongation of the body, passing
out between the valves or through an opening in the ventral
valve (Fig. 240) . The animal is attached to the valves by means
of thin sheets of tissue called mantles. Within the space between
the dorsal and ventral mantles are the lophophores, a pair of hol-
low coiled tentacles attached at either side of the mouth. Each
lophophore has a ciliated groove, along one side of which are
small ciliated tentacles. The lophophore gives the name to the
group (Brachiopoda — arm-footed) and serves as a sensory-respira-
tory organ and also to direct food to the mouth. The mouth is a
simple opening into a digestive tube, differentiated into esophagus,

THE ANIMAL KINGDOM

413

stomach, and intestine. An anal opening may or may not be
present. There is a circulatory system consisting of a heart,
blood vessels, and lacunar spaces. The body cavity is repre-
sented by a space in each mantle, connected with the cavities of
the lophophores. Nephridial tubes, one or two pairs, connect
the body cavity with the space between the two mantles. Usually
dioecious, there is a trochophore larval stage resembling a similar
stage in annelids (p. 415). Like bryozoans, brachiopods are very
ancient animals, of which fossils are known from the Cambrian to

Lophophore
Dorsal valve

Gonad

Digest i ve gland

Sfomacb

Heart

Venfra/

Manile
Fig. 240. — Semidiagrammatic sagittal section of a brachiopod, Magellania
lenticularis. (From Van Cleave, Invertebrate Zoology, after Parker and Haswell.)

the present time. According to Pratt, there are about 2,500
fossil species and about 120 living species known at the present
time.

Examples: Terebratulina septentrionalis, off Cape Cod;
Laqueus calif ornicus, California coast.

PHYLUM 11— PHORONIDEA

The Phoronidea are a small group of animals of uncertain
systematic position but showing some resemblance to bryozoans
and brachiopods. They are marine worms living in chitinous
tubes within which the body can be completely withdrawn. A
lophophore, consisting of a horseshoe-shaped ridge, bearing ten-
tacles, is present at the anterior end of the body and can be thrust
out of the tubes. The tubes frequently form tangled masses but
do not communicate with one another. Each animal develops
from a fertilized egg and there is no asexual generation formed by
budding. The mouth, located in the center of the lophophore,

414 GENERAL ZOOLOGY

opens into a U-shaped digestive tube terminating in an anus, also
located in the center of the lophophore but separated from the
mouth by a small lobe, the epistome, overhanging the mouth.
The digestive tube is supported in a body cavity by longitudinal
mesenteries. The body cavity is divided by a transverse portion
into two parts, of which the anterior one is continuous with the
cavities in the epistome and the lophophore. There is a blood
circulatory system and also a nephridial system consisting of a
pair of tubes leading from the body cavity to the outside. The
nervous system consists of a ring about the anterior end of the
alimentary canal and nerves extending from it to the tentacle and
other parts of the body. The animals are monoecious. Devel-
opment includes a larval stage (actinotrocha) . There are only a
single genus and about 11 species (Pratt).

Example: Phoronis architecta, lives in a straight tube, about
12 cm. in length. Coast of North Carolina.

PHYLUM 12— CHAETOGNATHA

The Chaetognatha (bristle-jawed) are free-swimming, uncili-
ated, and unsegmented marine worms. The group gets its name
from a fringe of prehensile, hooklike bristles and one or two rows of
small spines on either side of the mouth. The digestive tract
is a straight tube lying in a body cavity to which it is attached by
dorsal and ventral mesenteries. An anus is present. The body
cavity is divided into three parts by transverse septa. Special
organs of circulation, excretion, and respiration are absent. A
large dorsal nervous ganglion is located anteriorly above the
mouth and a ventral ganglion under the intestine. There are a
pair of eyes and an unpaired olfactory organ. These animals are
monoecious and their development is interesting because the
coelomic cavity is formed by the longitudinal fusion of a pair of
diverticula of the lining of the archenteron, much as in annelids
and chordates.

Example: Sagitta elegans, the arrowworm, about 3 mm. in

length and common in the North Atlantic.

PHYLUM 13— ANNELIDA

The Annelida (ringed or annulated forms) are the segmented
worms, which are found in fresh and salt water and in the earth.
They differ from other worms in the fact that the body is made

THE ANIMAL KINGDOM 415

up of a number of similar (homoclynamous) segments or meta-
meres, united end to end. The body wall is covered on the out-
side by a cuticle, secreted by a single layer of underlying cells, the
epidermis (hypodermis). Beneath the epidermis are an outer
circular and an inner longitudinal layer of muscle. The inner
surface of the body wall encloses a coelomic space and is usually
lined by a peritoneum. As a rule, the coelomic cavity is sub-
divided by transverse septa (dissepimenta) into as many compart-
ments as there are complete body segments. Paired appendages,
when present, are not jointed. The subterminal mouth is over-
hung by a lobe, the prostomium, which represents the anterior end
of the animal. The prostomium may bear eyes, tentacles and
palps (tactile organs). The region about the mouth, the peri-
stomium, may bear tentacles, also known as cirri. The mouth
leads to a straight, differentiated digestive tract, sometimes
lobulated, terminating in an anus. A circulatory system, approxi-
mating a closed type, is present. The excretory system consists
of a segmentally arranged series of paired nephridial tubes,
described in the case of the earthworm in an earlier connection
(p. 146) . Likewise a description of the reproductive system of the
earthworm and the manner in which it functions has already been
given (p. 157). The central nervous system consists of a
circumpharyngeal ring, connecting ganglia above and below, the
latter joined with a ventral ganglionated nerve cord, extending
posteriorly the length of the body. Annelids are monoecious
(earthworms) and dioecious (marine worms). In the dioecious
annelids, the eggs, fertilized externally, develop into trochophore
larvae. The trochophore larva of the marine worm Polygordius
is unsegmented and is provided with an alimentary tract having a
right-angled bend in it. The exterior of the larva is at first
ciliated everywhere, but later the cilia become restricted to one or
more bands of epithelium, the ciliated bands, of which the preoral
band is shown in Fig. 241. The latter encircles the body, marking
off a prestomial area in front (above in the figure), which contains
an apical plate, beneath which is the rudiment of the supra-
esophageal ganglion. The larva also contains nephridial organs
opening to the outside. Most of the larva seems to represent the
head end {prosoma) of the fully developed worm. The segmented
trunk (metasoma) of the worm grows from the posterior edge of
the prosoma by the formation of segments one after the other.

416

GENERAL ZOOLOGY

In some annelids the head of the adult later increases in size by-
taking up trunk segments. In monoecious annelids such as the
earthworm, the egg develops directly into a worm, but the pro-
stomium shows such a close resemblance to the trochophore larva
that the two structures are thought to be homologous. The
trochophore larva is important in connection with the evolution
of the annelids, since it bears a rather striking resemblance to a
rotifer and an even closer resemblance to the larva of the Brachio-

A b c

Fig. 241. — Polygordius. A and B, two stages in the development of the
trochophore (trochosphere) larva which grows by the addition of segments to the
posterior end. C, dorsal view of the adult, a, anus; g, rudimentary supra-
esophageal ganglion; m, mouth; n, protonephridium, provided with flame cells;
T, tentacles. {After Hatschek and Fraipont.)

poda and the Phoronidea and also to the veliger larva of the

Mollusca (p. 433).

CLASS I. ARCHIANNELIDA. Primitive marine annelids.
Setae (locomotor bristles) absent or scanty. Head may bear
ciliated bands or a pair of tentacles. External segmentation
often indistinct. A trochophore larval stage occurs. All
dioecious.

Example: Chaetogordius canaliculatus, has setae on its
posterior segments. Common at Woods Hole.

CLASS II. POLYCHAETA. Mostly marine annelids, with a
few fresh-water species. They are free-swimming or tube

THE ANIMAL KINGDOM

417

dwellers. The body is distinctly segmented, the head with
sense organs being well developed in the free-swimming forms.
A characteristic structure of the latter is the parapodium. This
is a fleshy outgrowth of the body wall usually consisting of two

Fig. 242. Fig. 243.

Fig. 242. — A, Anterior end of Nereis, with proboscis extended. (After Ehlcrs.)
B, enlarged view of parapodium from posterior aspect. {After Quatrefages.)
a, acicula (large bristles embedded in parapodium); d, dorsal cirrus; h, head
(prostomium) bearing four simple eyes; J, jaw; n, notopodium; ne, neuropodium;
p, palp; pt, peristomal tentacles; T, tentacles; v, ventral neuropodium.

Fig. 243. — Diagram of alimentary canal of leech, a, anal opening; g, nerve
ganglia; h, head with eyespots; p, muscular pharynx; s, stomach; su, sucker.

lobes: a dorsal notopodium and a ventral neuropodium (Fig.
242), both of which may bear cirri and setae, and are supported
internally by heavy bristles called acicula. Parapodia are
reduced or lacking in the sedentary forms. They are used in
locomotion and in aerating the body by producing a flow of
water over it when the animal is in its burrow or in a tube.

418 GENERAL ZOOLOGY

Filamentous external gills are present in some. The pharynx
of the free-swimming predaceous forms is provided with a pair
of sharp, pointed chitinous jaws and may be protruded as a
proboscis in capturing food. The sedentary forms live in
tubes of a slimy material, secreted by the integument and
usually encrusted with bits of sand and shell. As a rule,
polychaet annelids are dioecious.

Examples: Nereis virens, the clamworm, a free-swimming
marine form about 25 cm. in length ; Nereis limnicola, a fresh-
water California species about 45 mm. in length ; Chaetopterus
pergamentaceus, a marine form, living in a U-shaped tube
buried in the sand.
CLASS III. OLIGOCHAETA. Fresh-water or terrestrial anne-
lids, lacking parapodia and cephalic appendages. The head
region consists of the prostomium, a lobe projecting dorsally
over the mouth, and the peristomium, containing the mouth.
The setae, mounted in pits in the body wall, are few in number
and may be absent. Aquatic forms may have gills.
Oligochaete worms are monoecious and development is direct,
there being no larval stage. Some also reproduce asexually by
transverse fission.

Examples : Enchytraeus albidus, milk white in color, and about
25 mm. in length. Used as food for aquarium fishes. Lum-
bricus terrestris, an earthworm, about 10 in. in length. An
Australian species reaches a length of 3 to 4 ft.

Earthworms are found in temperate and tropical countries
at depths down to 6 ft. They live in burrows in the soil and in
decaying vegetation, and obtain nourishment by passing soil
with organic matter through the alimentary canal. Burrows
are made by pushing the soil aside or by swallowing it. After
swallowing the earth, whether for food or burrowing, the earth-
worm returns to the surface and ejects the earth through the
anus. Charles Darwin, in his book "The Formation of
Vegetable Mold" (1881), calculated that the weight of the
castings of all earthworms present in an acre of soil during the
course of a year averaged between 7.56 to 18.12 tons. Earth-
worms are important agents in preparing the ground for the
growth of fibrous rooted plants and seedlings of all sorts.
Incidentally, earthworms play a part in the burial of stones and
parts of buildings.

THE ANIMAL KINGDOM 419

The mouth of the earthworm is without teeth. It leads to
a muscular pharynx, which seems to be the principal agent in
drawing food into the mouth. Food or earth passes from the
pharynx to a narrow esophagus into which the secretions of
two pairs of calciferous glands is discharged. This secretion
contains calcium carbonate and serves to neutralize the strongly
acid condition of the food. From the esophagus the food passes
to a muscular gizzard, where it is pulverized, and then into a
stomach-intestine, where it is digested and absorbed. The
digestive fluid contains proteolytic and lipolytic enzymes.
Earth and other indigestible material taken into the alimentary
canal are eliminated at the anus.

CLASS IV. GEPHYREA. Marine worms without parapodia;
unsegmented in the adult stage. The body cavity is undivided
and the nephridia are reduced to a single pair. A few setae
may be present. They are dioecious and there is a trochophore
stage in development.

Example: Thalassema melitta, found on the coast of North
Carolina.

CLASS V. HIRUDINEA. The Hirudinea, or leeches, lack
parapodia, tentacles, and setae, but have sucking disks: one
surrounding the mouth and another at the posterior end of
the body. The body is flattened dorsoventrally and is marked
externally by two or more grooves to a segment. The body
cavity is partially filled with a vacuolated parenchyma. In
some leeches there are three sharp chitinous plates in the
pharynx that are used in drawing blood. Others, lacking jaws,
can pierce the integument by means of a proboscis that can be
thrust out of the pharynx. The salivary glands of leeches pro-
duce a substance called hirudin, which prevents coagulation of
the blood. It acts apparently by preventing the action of
thrombin upon fibrinogen and may therefore be considered an
antithrombin.

In many leeches the crop and stomach are provided with
capacious pouches that become greatly distended after a meal.
The food consists principally of the blood or body fluids of
various animals. Leeches live in the water or in moist ground
and vegetation. They completely devour small aquatic
worms, insect larvae, etc.; and from larger animals, such as
fishes, amphibians, turtles, and mammals, they draw blood. A

420

GENERAL ZOOLOGY

full meal of blood may suffice for a year. Eyes and other sense
organs are present upon the head. Leeches are monoecious
and development is direct (Fig. 243).

Examples: Hirudo medicinalis, the medical leech, a European
form used in blood letting, is now found in ponds and streams
in this country. Macrodella decora is one of the larger fresh-
water species.

PHYLUM 14— ARTHROPODA

Arthropods, like annelids, are segmented externally, and the
arrangement 'of the nervous system, muscles, heart, and other
organs shows evidence of internal segmentation. On the other

1

Fig. 244. — Appendages of crayfish of left side, ventral view, three-fourths
natural size. 1, first antenna; 2, mandible; 3 and 4, first and second maxillae;
5, 6 and 7, maxillipeds; 8, pleopod; 9, first walking leg (biramous).

hand, arthropods differ from annelids in that (1) the segmentation
is heteronomous, which means that the primitive segments are
fused to form larger segments, such as the head, thorax, and abdo-
men; (2) the paired appendages are jointed; and (3) the body
cavity is almost eliminated. The possession of jointed appendages
is the most important feature and gives the name to the phylum ;
Arthropoda means jointed feet. These appendages were pri-
marily locomotor and sensory in function, functions which some of
them have retained, while others have evolved into (1) mandibles
and maxillae, chewing organs; (2) maxillipeds, intermediate
between legs and jaws; (3) pleopods, the swimmerets of Crustacea;
(4) spinnerets, the spinning organs of spiders; and (5) the ovipositor
of insects, etc. (Fig. 244).

The chitinous exoskeleton is a secretion of the epidermis and
serves as a protective covering to which muscles and other

THE ANIMAL KINGDOM 421

organs are attached. The alimentary canal is a complete tube,
highly differentiated into various regions. The muscular system
is well developed; all of the muscles are striated. The nervous
system is of the ganglionic type, like that of annelids, but as a result
of condensation, or concentration, the number of ganglia in the
ventral chain is reduced and often does not correspond in number
with the body segmentation. There are two types of eyes,
simple ocelli and compound eyes. Except in the small crustaceans,
there is a blood circulatory system, but the degree of development
of the vessels depends upon the character of the respiratory
system. Thus, vessels are practically absent in forms that
have a diffuse organ of respiration, like the tracheal system of
insects, but are well developed in gill breathers, like the
crustaceans. The heart is tubular and lies in the mid-dorsal
line. Both nephridia and Malpighian tubules occur as organs
of excretion.

The eggs as a rule contain a large amount of yolk, which is
enclosed in a thin layer of yolk-free cytoplasm. The union of the
male and female nuclei takes place in the center of the yolk.
Cleavage in large-yolked eggs is of the superficial type which
means that the yolk-free cytoplasm is organized into a blastoderm
by the immigration of cleavage nuclei arising in the yolk. Her-
maphroditism is rare. Alternation of amphigony with
parthenogenesis (heterogony) occurs. Sometimes parthenogenetic
reproduction in these cases takes place in the larval stage (paedo-
genesis). Metamorphosis is common.

Over 600,000 species of arthropods are known, which is five-
sixths of the total number of known species of animals.
CLASS I. CRUSTACEA. Mostly aquatic, breathe by means of
gills, have biramous appendages, and two pairs of antennae.
The presence of large amounts of calcium carbonate in the
exoskeleton makes it thick and hard, and gives the name to the
group. The first two body regions are usually fused into a
cephalothorax. Many of the smaller Crustacea are parasitic.
Barnacles are the only sessile forms.
SUBCLASS 1. ENTOMOSTRACA. These are small in size,
with a variable number of segments. Head, thorax, and abdo-
men are distinct in some.

Examples : Eubranchipus vernalis, the fairy shrimp ; Daphnia
pulex, a fresh-water cladoceran ; Temora longicornis, a copepod,

422

GENERAL ZOOLOGY

marine; Cyclops viridis, a fresh-water copepod; Eucypris
virens, a fresh-water ostracod; Balanus balanoides, the rock
barnacle.
SUBCLASS 2. MALACOSTRACA. Usually with 20 body
segments: head 5, thorax 8, and abdomen 7; and 19 pairs of
appendages: head 5, thorax 8, and abdomen 6. Head and
thorax generally combined in a cephalothorax.

Examples: Porcellio scaber, a sow bug, terrestrial;
Palaemonetes vulgaris, a common shrimp, marine; Homarus
americanus, American lobster; Cambarus bartoni, American
crayfish; Cancer irroratus, a common New England crab;

Libinia emarginata, a spider crab
(Fig. 245).
CLASS II. ARACHNOIDEA.
No antennae are present. There
are two body divisions, the
cephalothorax, with six paired
appendages, and the abdomen,
without appendages, except in the
horseshoe crab.
SUBCLASS 1. XIPHOSURA. A
horseshoe-shaped cephalothorax,
six platelike paired appendages
on the abdomen, and a spikelike
tail.

Example : Limulus polyphemus,
the American horseshoe crab.
SUBCLASS 2. ARACHNIDA. The cephalothorax bears six
pairs of appendages : the mandibles or chelicerae, the pedipalps,
and four pairs of walking legs. The last three pairs of abdom-
inal appendages of spiders are modified into spinnerets.
Mostly terrestrial and breathe by modified gills known as
lungs. Some have a tracheal system.

Examples: Diplocentus whitei, a scorpion of the South-
western United States; Argiope aurantia, an orb-weaving spider
(all spiders belong to the Arachnida); Phalangium opilio
(daddy-long legs). Has a tracheal system with a single pair of
spiracles. Does not spin a web. Sarcoptes scabiei, one of the
itch mites, causing itch in man and mange in pigs. Margaropus
annulatus, the Texas cattle tick. The bite of the tick inoculates
cattle with Babesia bigemina, a sporozoan which causes Texas

Fig. 245. — Libinia, a spider crab,
sketched devouring a sea urchin.

THE ANIMAL KINGDOM

423

fever. Trombicula irritans, chigger; Macrobiotics hufelandi,
a water bear, .7 mm. in length.

CLASS III. ONYCHOPHORA. The head bears a pair of sim-
ple eyes, a pair of segmented antennae, and a mouth with a pair
of hooked jaws. The wormlike trunk is unsegmented and
provided with numerous annulated legs (14 to 43 pairs). The
body cavity is hemocoel through
which the alimentary canal passes
as a straight tube. A tracheal
respiratory system and a nephrid-
ial excretory system are present.
There is an opening of a nephrid-
ium at the base of each leg. The
sexes are separate. In viviparous
forms the egg is poor in yolk and
is totally divided in cleavage.
The embryo in such cases is nour-
ished by the walls of the uterus,
forming a very primitive sort of
placenta. The animals are ter-
restrial and feed on insects and
other small forms. From the
point of view of evolution, they
are important as a connecting
link between annelids and
arthropods (Fig. 246).

Example: Peripatus eiseni, a
viviparous species; habitat, cen-
tral South America.

CLASS IV. MYRIAPODA. The
head bears a pair of segmented
antennae,

Fig. 246. — A, ventral view of the
head of Peripatus capensis, showing
the mouth surrounded by lips
raised into large white papillae.
B, dorsal view of entire animal.
a, antenna, p, oral papilla at the
base of which a gland ejects a
a pair of mandibles, sticky slime used in capturing food.
r .,, (After Leuckart-Nitsche wall chart.)

and one or two pairs of maxillae.

The trunk is segmented (11 to 173 segments) and each segment
has one or two pairs of legs with claws. The sexes are separate
and all are oviparous. Development is direct. There are four
orders, of which the Diplopoda (millipedes) and Chilopoda
(centipedes) are the more important.
Order 1. Diplopoda. A cylindrical body with usually two pairs
of legs to a segment. A single pair of maxillae. An anterior
genital pore.

424

GENERAL ZOOLOGY

Example: Julus virgatus, a common millipede.
Order 2. Chilopoda. A flattened body with one pair of legs to a
segment. Two pairs of maxillae. The first pair of legs are
maxillipeds, provided with poison glands. Reproductive
openings are posterior.

Example: Scolopendra morsitans, a common southern
centipede.

Man

.Max.—

La-''

Fig. 247. — A, chewing type of mouth parts as found in grasshopper; B,
piercing-sucking type as found in female mosquito, g, galea; h, hypopharynx
(a tonguelike prolongation of the floor of the mouth, attached to the inside wall
of the labium); l, labrum; La, labium; Lac, lacinia; Man, mandible; Max, maxilla;
p, palp. (After Metcalf and Flint: Destructive and Useful Insects.)

CLASS V. INSECTA. Insects. Insects are characterized by
three body divisions, head, thorax, and abdomen, and by three
pairs of legs. The head bears four pairs of appendages;
antennae, maxillae, mandibles, and lips or labia (labrum and
labium). The trunk is composed of three segments, prothorax,
mesothorax, and metathorax, each of which bears a pair of legs.
The mesothorax and metathorax may each bear in addition
a pair of wings. Respiration is carried on by means of a
tracheal system. Insects are dioecious and the reproductive
openings are at the end of the body. A metamorphosis in
development is common.

THE ANIMAL KINGDOM

425

The mouth parts conform to two general types, viz., biting
and sucking. In the biting type there is an upper lip or labrum
and a lower lip or labium, each of which is a chitinous plate.
The labium bears a pair of palps. Inserted laterally between
the lips are a pair of maxillae and a pair of mandibles (Fig. 247).
Each maxilla consists of a forked lacinia and a spoon-shaped
galea, and also bears a single palp. The palps are not only
tactile but also contain organs of taste and smell. Typical
mandibles are hard resistant plates of irregular outline and
edged with sharp points. Both maxillae and mandibles operate
with a side-to-side motion. The mandibles are the chewing
organs, the maxillae serving with
the lips to hold and manipulate the
food. In sucking insects such as
the mosquito the mouth parts are
shaped to form a slender beak
(Fig. 247B). The elongate max-
illae and mandibles lie in a sheath
formed by the labium, the labrum
being reduced to a small plate at
the upper side of the base of the
beak. There are variations of
these two types, combining ^Xnter.
features of both.

On the head there are usually two kinds of eyes, simple and
compound, and a pair of segmented antennae. The compound
eye has been described (p. 192). The simple eye or ocellus is
.much smaller and consists of a spheroidal mass of light-sensitive
cells covered externally by a lens-shaped thickening of the
integument.

Each leg, beginning at the proximal end, consists of five
parts: (1) a short coxa, (2) a short trochanter, (3) a long, broad
femur, (4) a slender tibia, and (5) a foot or tarsus, composed of
several segments (Fig. 248). The insect wing is without a fore-
runner in the preceding groups of animals. Since primitive
insects are wingless, the insect wing must be a structure that
originated in the insect group. The insect wing is a chitinous
plate, usually very thin and flexible, and reinforced by "veins"
composed of branches of the tracheal system. In beetles the
anterior wings are thickened and serve as a protective covering

Fig. 248. — Parts of metathora-
cic leg of grasshopper, c, coxa;
f, femur; ta, tarsus; ti, tibia; tr,

426

GENERAL ZOOLOGY

for the posterior pair when the insect is not flying. In butter-
flies and moths the wings are
covered with small scales. There
are two general views as to the
origin of insect wings: (1) that
they are modified gills, or (2)
that they have developed from
chitinous thoracic plates.

The tracheal system of respira-
tion, though present in certain
other arthropods, is best devel-
oped in insects. It consists of
tubes called tracheal tubes, which
beginning at openings (spiracles)
arranged in a row on each side of
the body penetrate to all parts of
the body, branching as they go
and terminating in trachioles in
the tissues. The wall of the
tracheal tube consists of a single
layer of cells lined with a chitinous
membrane. Under the micro-
scope the larger tubes have a
ringed appearance caused by
transverse folds in the chitinous
lining arranged in a close spiral that prevents collapse of the
tubes (Fig. 249). In flying insects, the tracheal tubes near the

oc. o

Fig. 249. — A, portion of trachea
of caterpillar with its branches, B,
C, D. a, peritracheal membrane;
b, nucleus. (From Packard, " Text
book of Entomology,1' The Macmil-
lan Company. By permission.)

s v

Fig. 250. — The distribution of tracheae and air sacs in a grasshopper. D, left
dorsal trachea; 0, left cephalic trachea; OC, ocular trachea; S, left stigmatal
trachea with stigmata (spiracles) ; V, ventral trachea. (After Packard, Textbook
of Entomology.)

spiracles are expanded into thin-walled air sacs, which are com-
pressed and expanded by contractions and expansions of the

THE ANIMAL KINGDOM

427

body wall (Fig. 250). Air enters and leaves through the spira-
cles. In the grasshopper it has been shown that the two
thoracic spiracles and the first two abdominal spiracles are
inspiratory and that the last six pairs of abdominal spiracles are
expiratory. Oxygen is carried directly to the tissues by the
tracheal system and carbon dioxide is removed by the same
system. The blood plays a minor role
in the respiratory exchange, but since the
tissues are bathed in blood, the latter
serves as a medium through which
oxygen passes from the trachioles to the
tissues and carbon dioxide in the opposite
direction.

Blood vessels, poorly developed in
insects, are practically absent except near
the heart. The heart of the grasshopper
is a delicate muscular tube lying directly
in the mid-line of the dorsal region of the
hemocoel. It is closed at its posterior
end, open at its anterior end, and per-
forated along its sides by apertures called
ostia, guarded by valves (Fig. 251).
Anteriorly, blood vessels extend a short
distance from the heart. When the heart

,i i i ,i , i sheet lying ventral to the

contracts, the ostia are closed so that the heart; h< arrangement of

Wave of Constriction, beginning at the fibers. {From Packard,
, j i r i r Textbook of Entomology,

posterior end and passing forward, forces copyright< The Macmillan

the blood OUt of the anterior end. As the Company, after Grober.

heart relaxes, blood reenters the heart v Permisslon->
through the ostia from the pericardium, a space about the
heart, incompletely separated from the rest of the hemocoel.
In some insects the heart reverses the direction of its beat at
regular intervals, propelling the blood first in one direction and
then in the other. The blood distributes nourishment to the
tissues and collects waste material other than carbon dioxide.
There are two general types of metamorphosis: complete
(holometabolous) and incomplete (hemimetabolous) . When
metamorphosis is complete, the egg hatches as a worm-shaped
larva, such as the maggot of flies, the grub of the beetle, or the
caterpillar of moths and butterflies. The larval period is

Fig. 251.— Part of
heart of a beetle, Dytis-
cus marginalis, showing
spiral arrangement of
fibers, c, closed valve;
e, open valve; a, muscu-
lar and connective tissue

428 GENERAL ZOOLOGY

characterized by voracious feeding, in the course of which
the larva passes through stages known as instars, separated by
periods of molting or ecdysis, the number varying in different
insects. Loss of the chitinous covering of the body permits
growth of the larva from the first to the last instar. The last
larval instar is succeeded by the stage of the pupa, during which
feeding ceases and a reorganization of the body takes place. In
many insects the pupal period is passed in the ground, in others
in a cocoon formed of silky fibers secreted by special glands. At
the end of the pupal period, which in extreme cases may
extend over a number of years, the insect emerges as an imago
or adult.

If the metamorphosis is incomplete, the larva or nymph
resembles the adult in its general morphology, except that it
lacks wings, as, for example, in the case of the larval grass-
hopper, where the nymph undergoes successive molts and
gradually takes on the adult form.

Of the many orders of insects those listed in the following
paragraphs include only the more common ones.

Order Collembola (Glue Bar). Springtails. The order is
named from the presence of a collophore, a ventral tube on the
first abdominal segment that is provided with a pair of eversi-
ble sacs, by means of which the insect adheres to smooth sur-
faces. These insects are wingless and are believed to have
descended from ancestors in which wings were never evolved.
The fourth abdominal segment usually bears a forked append-
age that folds under the body and is used as a spring in propel-
ling the insect. There is no metamorphosis in development.

Examples: Papirius maculosus, the spotted springtail;
Achorutes nivicola, the snow flea, which is remarkable in that
it is found in winter on snow.

Order Ephemeroptera. Mayflies. The delicate wings, tri-
angular in outline, are held vertically above the body when at
rest. The tip of the abdomen bears a pair of long slender cerci,
and in addition a median caudal filament. The adult mayfly
takes no food, the alimentary canal being inflated with air,
which gives buoyancy to the body. It lives only a day or two
as an adult. The eggs are laid in water, where the larval stage
is passed. Metamorphosis is incomplete. The entire life
history may extend over several years. So far as known, these

THE ANIMAL KINGDOM 429

are the only insects that undergo a molt after attaining func-
tional wings.

Example: Ephemera simulans, occurs in large numbers,
usually late in June in the Great Lakes region.

Order Orthoptera (Straight-winged). Crickets, grasshoppers,
roaches, katydids, etc. The wings, when well developed,
consist of a thickened anterior pair, straight and narrow in
outline, and a thin posterior pair, which when at rest is folded
in plaits under the anterior pair. The mouth parts are for
chewing. Metamorphosis is incomplete. Most of them feed
on vegetation and their destruction of plant life causes enor-
mous damage. Some, cockroaches, for example, are more
general feeders.

Examples: Gryllus domesticus, the house cricket, an old
world species that has been introduced into this country;
Melanopus differ entialis, a large grasshopper, common in
Eastern United States; Periplaneta americana, the American
cockroach; Diapheromera femorata, the walking stick, lacks
wings; Stagmomantis Carolina, the praying mantis, feeds
principally on other insects and therefore has a positive eco-
nomic value.

Order Odonata (odous, tooth). Dragonflies and damsel flies.
There are two pairs of membranous wings with marked veining,
the posterior pair as large as, or larger than, the anterior.
The mouth parts are for chewing and the metamorphosis is
incomplete. The eggs of American species are laid in water.
The maxillae and mandibles of the larva are provided with
sharp teeth. The labium of the larva is enlarged, jointed,
armed with hooks, and can be thrust out in front of the head to
seize prey.

Examples: Anax Junius, a common dragonfly; Agrion
maculatum, a common damsel fly.

Order Isoptera (Equal- winged). Termites. These are social
insects, comprising a number of castes, each caste including
males and females. They may be distinguished from ants,
which they greatly resemble, by the fact that the abdomen is
joined to the thorax by a broad connection. (In ants this con-
nection is slender.) There are usually two pairs of long narrow
wings which are shed by the adult sexual males and females
after the nuptial flight. The intestine of some termites contains

430 GENERAL ZOOLOGY

flagellated Protozoa that are capable of rendering digestible
the cellulose of wood eaten by the termites. It has been
shown that without the flagellates termites are unable to digest
cellulose. The mouth parts are for chewing, and meta-
morphosis is gradual. Some tropical species build nests 10 or
12 ft. in height. In the United States they live in mines in the
earth and in wood. They cause considerable damage to the
wooden structure of buildings.

Example: Reticulitermes flavipes, a common termite of North-
eastern United States.

Order Hemiptera (Half- winged) . Bugs. The proximal halves
of the anterior pair of wings are thick and the extremities,
which overlap, are very thin. The posterior wings are mem-
branous. The mouth parts are for piercing and sucking.
Metamorphosis is incomplete. Many have stink glands
located on the abdomen.

Examples: Anasa tristis, the squash bug; Lygaeus kalmii, the
milkweed bug; Lethocerus americanus, the giant water bug or
electric-light bug; Gerris remigis, the water strider; Cimex
ledularius, the bedbug; Blissus leucopterus, the chinch bug;
Arctocorixa alternata, the water boatman.

Order Homoptera (Same-winged). Cicadas, aphids, and gall
insects. There are usually two pairs of wings of uniform
thickness. Some are wingless and in one family (Coccidae)
the posterior pair of wings is reduced to a pair of club-shaped
halteres. The mouth parts are for piercing and sucking.
Metamorphosis is incomplete.

Examples: Tibicen linnei, the "dogday" cicada; Magicicada
septemdecim, the 17-year cicada, whose larval stage lasts 17
years; Aphis gossypi, the melon aphis, one of the plant lice;
Phylloxera vitifoliae, forms galls on grape leaves; Aspidiotus
perniciosus, the San Jose scale insect.

Order Anoplura. Lice. These are wingless parasitic insects
whose mouth parts are in the form of a tubular proboscis
provided with sharp stylets. Recurved hooks in the base of the
proboscis serve to anchor the proboscis after it has been inserted
in the skin of the host. Metamorphosis is incomplete.

Examples: Pediculus capitis, the head louse; Pediculus
corporis, the body louse or "cootie"; Phthirius pubis, the crab
louse.

THE ANIMAL KINGDOM 431

Order Coleoptera (Sheath-winged). Beetles. When at rest
the thickened anterior pair of wings, known as elytra, form
covers for the posterior membranous wings. Mouth parts are
for chewing, and metamorphosis is complete.

Examples: Dytiscus marginicollis, diving beetle; Coccinella
novemnotata, ladybird beetle; Photinus scintillans, firefly;
Leptinotarsa decemlineata, potato beetle; Calosoma scrutator, a
common ground beetle with conspicuous iridescent coloring;
Tenebrio molitor, mealworm beetle.

Order Lepidoptera (Scale- winged). Moths and butterflies.
The two pairs of membranous wings are covered with overlap-
ping scales. Scales also cover the body, legs, and appendages.
Mouth parts are for sucking. Metamorphosis is complete.
Usually moths have feathered or threadlike antennae and hold
the wings horizontally or wrapped around the body when at
rest. Most moths are nocturnal. The antennae of butterflies
and skippers as a rule are threadlike and knobbed at the ends.
At rest the wings are held together in a vertical position over the
body. They fly in the daytime.

Examples: Samia cecropia, the large cecropia moth; Actias
luna, luna moth; Protoparce quinquemaculata, a hawk moth,
whose large green larva is known as the tomato or tobacco
worm, depending upon which plant it is feeding; Porthetria
dispar, the gypsy moth; Bombyx mori, the silkworm moth;
Papilio polyxenes, common swallowtail butterfly; Pieris rapae,
cabbage butterfly; Epargyreus tityrus, silver-spotted skipper.

Order Diptera (Two-winged). Flies. The anterior wings are
large and membranous; the posterior wings are reduced to a
pair of knobbed threads, the halteres. Anterior wings are
sometimes lacking. Mouth parts are for sucking, and meta-
morphosis is complete.

Examples: Anopheles quadrimaculatus, a mosquito, the female
of which transmits malaria; Glossina palpalis, tsetse fly that
transmits African sleeping sickness ; Musca domestica, housefly ;
Braula caeca, bee louse, a wingless form parasitic on the thorax
of queens and drones of the honeybee; Melophagus ovinus, the
sheep tick, wingless.

Order Siphonaptera (Tube, Wingless). Fleas. Wingless insects
with the body compressed laterally; mouth parts for piercing
and sucking; legs for leaping. Metamorphosis is complete.

432 GENERAL ZOOLOGY

Examples : Pulex irritans, human flea ; Ctenocephalis felis, cat
flea; Ctenocephalis canis, dog flea; Xenopsylla cheopis, rat flea,
carries the germ of bubonic plague.
Order Hymenoptera (Membrane-winged). Bees, wasps, ants,
sawflies, and ichneumon flies. Two pairs of wings with
reduced venation; the anterior pair larger than the posterior.
Mouth parts are for chewing or for both chewing and sucking.
Ovipositor of the female may be modified into a sting, piercing
organ or saw. Complete metamorphosis.

Examples: A pis mellifica, honeybee; Bombus pennsylvanicus,
a bumblebee; Vespa maculata, a social wasp, builds large papier-
mache nests; Trypoxylon albitarsis, mud-dauber wasp; Vespa
maculifrons, yellow jacket; Monomorium pharaonis, red ant,
common about houses ; Megarhyssa, a genus of ichneumon flies,
the females of which drill through the wood of trees to parasitize
wood-boring insect larvae; Cimbex americana, a sawfly. The
ovipositor of the female is composed of two sharp plates, the
saws, flanked on either side by saw guides and is used in depos-
iting the eggs in the bark of maples, elms, and other trees.

PHYLUM 15— MOLLUSCA

Molluscs are soft-bodied, bilaterally symmetrical, unsegmented
animals, with only a remnant of a coelom, and usually enclosed
in a shell. In well-developed molluscs, like the snail, four body
parts can be distinguished: (1) the visceral sac, containing the
viscera and forming the bulk of the body. This is continuous
in front with (2) the head, bearing the mouth, tentacles, and eyes.
(3) The foot, the organ of locomotion, lies ventral to the visceral
sac. (4) The pallium, or mantle, is a dermal fold, forming
between it and the sac a mantle cavity which is provided with
gills or lungs, and receives the openings of the intestine, reproduc-
tive ducts and excretory organs. The outer surface of the
mantle secretes the shell (Fig. 252).

The nervous system, typically, consists of three pairs of ganglia
connected by cords: (1) The cerebral ganglia lie dorsal to the
esophagus and supply the sense organs of the head. (2) The
pedal ganglia lie in the foot, and supply statocysts and muscle.
(3) The visceral ganglia lie in the viscera, which they supply, and
also send nerves to the osphradia, organs of chemical sense
located in the mantle cavity.

THE ANIMAL KINGDOM

433

The mouth opens into a muscular pharynx, usually provided
with a radula, a toothed, chitinous ribbon, used for rasping.
Attached to the stomach is a large liver. Paired or single

Fig. 252. — Diagrams of three types of molluscs. A, Gastropod; B, Pelecy-
pod (clam), cross section; C, Cephalopod. a, alimentary tract; E, eye; f, foot;
g, gills; h, head; m, mantle cavity; s, shell; si, siphon; v, visceral mass; 1, cerebral
ganglion; 2, pedal ganglion; 3, visceral ganglion.

nephridial tubules drain the 'pericardium, which represents the
coelom, and open into the mantle cavity. The circulatory sys-
tem is composed of a heart with a ventricle and one or two aur-
icles, arteries, and veins. The
ventricle drives the blood
through the arteries to the
lacunar spaces in the tissues,
from which veins take it to the
kidneys and gills, and then to the
heart. The blood in the heart
is, therefore, always pure. Both
dioecious and hermaphroditic
forms occur. A veliger-larva
stage is common in development
(Fig. 253).

Molluscs are classified as
follows :
CLASS I. AMPHINEURA.

Fig. 253. — Veliger larva of Toredo.
{After Hatschek.) a, anus; AP, apical
plate bearing a cilium; m, mouth; n,
nephridial tube; s, shell. (Compare
with Fig. 241, A.)

The nervous system consists of an esophageal ring and four
longitudinal nerve cords. Those without a shell are wormlike
in appearance. A radula is usually present. All are marine.

434 GENERAL ZOOLOGY

Examples: Chaetopleura apiculata, rock sucker; Chaeto-
derma nitidulum, lacks a shell.
CLASS II. SCAPHOPODA. Have tubular shells. A radula is
present. Sexes are separate. All marine.

Example: Dcntalium entale, tooth shell.
CLASS III. GASTROPODA. Snails and slugs. Asymmetrical
body with four parts well developed; a creeping foot; an
unpaired mantle, nephridium, and gonad. The shell is usually
coiled spirally. A radula is present. Aquatic forms breathe
by gills, the land forms with lungs. The shell is reduced or
lacking in slugs.

Examples: Lymnaea palustris, a fresh-water snail; Polynices
duplicata, a marine snail (Fig. 254); Busycon canaliculatum,

Fig. 254. — Polynices, a marine snail with a highly developed mantle and foot.

F, foot; M, mantle; S, syphon.

a whelk, marine; Helix pomatia, French snail, edible; Limax
flavus, a land slug ; Dendronotus frondosus, a sea slug.

CLASS IV. PELECYPODA. Lack a head and cephalic append-
ages. Have a bivalve shell consisting of right and left halves;
with paired mantle, gills, nephridia, and gonads. Frequently
the hinder edges of the mantle are modified to form incurrent
and excurrent siphons. The sense organs are poorly developed.
Sexes are usually separate.

Examples: Anodonta grandis, a fresh- water clam; Ostrea
virginica, the American oyster; Venus mercaneria, the hard-
shelled or little-neck clam; My a arenaria, the soft-shelled or
long-necked clam (Fig. 255).

CLASS V. CEPHALOPODA. Active, carnivorous, marine
forms, including the largest and most highly organized molluscs.
As a rule, eight or ten tentacles surround the mouth which with
the siphon represent the foot region. A pair of sharp chitinous
jaws lie just back of the lips in the mouth. The siphon is an

THE ANIMAL KINGDOM 435

excurrent tube from the mantle cavity. The mantle is
unpaired. A shell may or may not be present. The highly
developed paired eyes strongly resemble those of vertebrates.
Statocysts and osphradia are also present. Except in Nautilus,
there is a glandular ink sac opening near the end of the rectum
that secretes a brown or black fluid. The ink is expelled from
the mantle cavity through the siphon and affords protection
by clouding the water. The sexes are separate.

Fig. 255. — Mya, the soft-shelled clam. E, excurrent opening of syphon; F, foot;
I, incurrent opening of syphon; M, mantle; S, syphon.

Examples: Loligo pealei, the common squid, its shell (cut-
tlebone) being embedded in the tissues; Octopus bairdi, a com-
mon devilfish (no shell); Architeuthis princeps, a squid, the
largest mollusc, having a body about 6 meters in length and
tentacles up to 10 to 12 meters long. Nautilus pompilius,
pearly nautilus, which has a shell divided into compartments.
It has about 90 tentacles about the mouth. Argonauta argo,
the paper nautilus, has a spiral shell without septa.

PHYLUM 16— ECHINODERMATA

The term Echinodermata (hedgehog-skinned) was originally
applied to sea urchins, for which it was very appropriate, but the
term is now used to include in addition to sea urchins all forms
related to them, such as starfishes, sea lilies, brittle stars, and sea
cucumbers, some of which lack spines and are not in the least
"hedgehog-skinned." Echinoderms are marine animals having a
radial symmetry, usually of a five-radiate plan. The larva is
bilaterally symmetrical, indicating that the radial symmetry of the
adult is secondary. Calcareous plates, with or without spines, are
developed in the mesoderm, and may form a hard exoskeleton.
An ambulacral water-vascular system is present. In the starfish
this system begins on the aboral surface with a porous plate, the

436

GENERAL ZOOLOGY

madreporite, through which water enters a stone canal, leading to a

ring canal encircling the mouth
(Fig. 256). From the ring
canal, five radial canals extend
into the arms, and from each of
these are given off at right
angles paired ambulacr at canals.
Each of these, in turn, joins a
tubular muscular sac, the
ambulacrum, expanded at one
end into an ampulla, and ter-
minating in a sucker disk at its
longer free end, which can be
extended or retracted through
system of the ambulacral groove on the

the starfish, diagrammatic. A, ampulla; oral Surface of each arm. The
AM, ambulacra; C, ring canal; M. , _ • £ii„j „*j.u „ a *j

madreporite; P, Polian bodies; R, system 1S filled Wlth a flmd>

radial canal; S, stone canal; T, Tiede- mostly water, but containing

mann bodies. lymph an(j Uood corpuscles

supplied by glands (Polian vesicles, Tiedemann bodies) attached
to the system at various points. The ambulacra are used in loco-

Fig. 257. — Asterias feeding on a clam. The valves of the clam are pulled
apart by the ambulacra and the stomach of the starfish is then everted about the
soft parts of the clam. The stomach juices of the starfish may aid in causing
the valves of the clams to open.

motion by extending and attaching the sucking disks to the sub-
stratum, a muscular contraction of the appendage then pulling

THE ANIMAL KINGDOM 437

the animal along. The ambulacra are also used in capturing and
holding prey (Fig. 257). They are important as tactile organs.
The fluid in the ambulacral system is moved by cilia on the inner
surface of the stone canal and by the contractions of muscles in
the walls of the ampullae and the ambulacral feet. In addition
to the ambulacral system there is also a blood circulatory system
in which blood is circulated by cilia lining the vessels.

The radial symmetry is impressed on all of the internal
organs. The body cavity is a complicated system of spaces, some
of which are cut off from the rest, and all containing a fluid
similar to that of the ambulacral system. The alimentary canal
is usually a complete tube and lies in the largest of these spaces.
The nervous system consists of (1) a superficial nerve ring around
the esophagus, with radial cords extending into the arms; (2)
a deeper oral ring and radiating nerves; (3) an apical system in
the aboral wall but not present in all echinoderms. Respiration
and excretion are carried on at the surface of the body, which
is usually ciliated. Some brittle stars reproduce asexually by
fission, but otherwise reproduction is sexual. The sexes are
usually separate. While the eggs normally require fertilization,
it has been found that they can be stimulated to develop by artifi-
cial means, artificial parthenogenesis; for this reason they have
been favored material for experimental work along these lines.
Ripe echinoderm eggs, when subjected to the effects of hypertonic
sea water, certain acids, temperature changes, or mechanical shock,
will develop without the intervention of a sperm.
CLASS I. CRINOIDEA, Sea lilies and feather stars.
Have a nonciliated, cup-shaped body, the calyx, usually
attached by a stalk on its aboral side. The oral surface is
pointed upward, and contains the mouth, and also the anal
opening. The arms, 5 or 10 in number, are usually branched
and feathery in appearance, and bear ambulacral grooves on the
aboral surface. Crinoids were more abundant in paleozoic
times than now, about 2,100 fossil species being known (Pratt).
Example: Hathrometra tenella, a feather star. Atlantic
Coast, at 150 to 3,000 ft.
CLASS II. ASTEROIDEA. Starfishes. Usually have five
arms, with open ambulacral grooves on the oral side, through
which the ambulacra are extended. Gastric pouches and
hepatic caeca extend into the arms. Spines, pincerlike

438

GENERAL ZOOLOGY

pedicellariae, and tubercles project on the surface. At the tip
of each arm is an eyespot. Starfish possess a remarkable
power of regenerating lost parts.

Fig. 258. — Asterias, a starfish, viewed from the aboral side. The pale circular
area to the left of the center is the madreporite. The ambulacral grooves through
which the ambulacra extend can be seen on two of the upturned arms.

Fig. 259. — A, Ophiura, a brittle star, aboral view; B, oral view of central disk;
C, oral side of portion of arm, magnified, showing ambulacra, am.

Example: Asterias forbesi, a common starfish, Cape Cod
region (Fig. 258).

THE ANIMAL KINGDOM

439

CLASS III. OPHIUROIDEA. Brittle stars. The arms are
sharply set off from the central disk and are without ambulacral
grooves. The ambulacra are tactile, respiratory and excretory
in function. The external surface is not ciliated. Hepatic
caeca are absent and the intestine lacks an anal opening.
Example: Ophiura robusta, a Cape Cod form (Fig. 259).

AM

Fig. 260. — A, Arbacia, a sea urchin, side view, am, ambulacra. B, oral view
showing mouth region surrounded by shorter ambulacra and blunter spines.

CLASS IV. ECHINOIDEA. Sea urchins and sand dollars.
A subglobular or disk-shaped body without arms. Usually,
five calcareous teeth project from the mouth. The calcareous
exoskeleton is well developed and is often provided with long
movable spines. The surface of the body is ciliated.

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.'§" ::':■ •..'■.■ ://■'%

&-:

■ '/.

'V ■ ■

it ■.;• ■ .- ■■ ■*. ,:e;J^

v.

Fig. 261. — A, Echinarachnius, a sand dollar, aboral view; B, oral view.

Examples: Arbacia punctulata, a sea urchin common on the
Atlantic coast (Fig. 260); Echinarachnius parna, the sand
dollar (Fig. 261).
CLASS V. HOLOTHURIOIDEA. An elongated, wormlike
shape, with a leathery integument, the exoskeleton being much
reduced. There is a partial bilateral symmetry. The
madreporite is usually internal, the fluid filling the ambulacral
system coming from the body cavity. The external surface is

440 GENERAL ZOOLOGY

not ciliated and lacks both spines and pedicelluriae. Regenera-
tion is well marked.

Examples: Holothuria marmorata, trepang, used as food by
the Chinese ; Cucumaria frondosa, sea cucumber of the Maine
coast; Thyone briareus a common Woods Hole species of sea
cucumber (Fig. 262) ; Leptosynapta inhaerens, a wormlike form,
without ambulacral feet. Atlantic Coast.

Fig. 262. — Thyone, a sea cucumber. The branched tentacles (at the right)
are alternately drawn into the mouth, wiped off and extended. Ambulacra are
present over the surface of the body.

PHYLUM 17— CHORDATA

The Chordata are characterized by (1) the presence at some
time in the life history of a notochord, a flexible fibrous elastic rod,
lying between the central nervous system and the digestive tract ;
(2) a tubular central nervous system lying dorsal to the digestive
tract; and (3) a pharynx with gill slits, which may or may not
function as repiratory organs in the embryo or in the adult.
SUBPHYLUM 1. ENTEROPNEUSTA. Unsegmented worm-
like animals with three body divisions: (1) proboscis, (2) collar,
and (3) trunk (Fig. 263). A blind tubular extension from the
pharynx into the proboscis is taken to represent the notochord.
The nervous system consists of (1) a dorsal tubular portion
lying in the collar, and (2) a layer of nerve fibers beneath the
entire ectoderm. The dorsal portion develops from a tube, as
in vertebrates. The mouth is ventral and in front of the collar.
The pharynx, perforated with gill slits, leads to the intestine,

THE ANIMAL KINGDOM 441

which terminates in an anus at the posterior end of the body.
Dorsal and ventral mesenteries support the intestine and divide
the coelomic cavity into two pouches. There are dorsal and
ventral blood vessels, a portion of the dorsal one being con-
tractile. The sexes are separate and there is a metamorphosis
in development. Usually a tornaria larva, resembling the
larvae of echinoderms, is present. In colonial forms, in addi-
tion to sexual reproduction, asexual reproduction by budding
also occurs. All are marine.

*EE3Xi5SpBBSBBBE3SB (I

y ff M C t

Fig. 263. — Diagram of the anterior end of Balanoglossus. b, branchial region
of alimentary tract with gill clefts; c, collar; d, dorsal blood vessel; dn, dorsal
nervous system; m, mouth; n, notochord consisting of a tube of cells connected
with the alimentary canal; o, esophagus; p, proboscis; t, trunk; v, ventral blood
vessel.

Examples: Balanoglossus aurantiacus, about 15 cm. long,
occurs in sand and mud of shallow water of the Carolina coast.
It lacks a tornaria larva. Dolichoglossus kowalevskyi, in the
sandflats, Atlantic Coast; Rhabdopleura sp., a deep-sea colonial
form without gill slits.
SUBPHYLUM 2. TUNICATA. The presence in many
tunicates of a mantle cavity, with incurrent and excurrent
siphons, gives them a superficial resemblance to molluscs
(Fig. 264). The mantle is covered by a tunic composed largely
of cellulose. The incurrent opening is the mouth, which leads to
the wide pharynx through whose gill slits the water passes
either directly to the outside, or into a peribranchial or cloacal
space, which opens to the outside by the excurrent syphon. In
the mid-ventral line of the pharynx is the endostyle, a glandular
ciliated groove, whose cilia move entangled food masses for-
ward to the ciliated peripharyngeal band, from which it passes
to the dorsal lamina, another ciliate tract in the mid-dorsal
line, whence it reaches the esophagus. The latter is followed
by a stomach and intestine, the intestine opening into the
cloacal cavity. The heart has the peculiar property of period-

442

GENERAL ZOOLOGY

ically reversing the direction of its beat, driving the blood toward
the gills for a certain interval and then driving it in the opposite
direction to the viscera, etc. The central nervous system
develops from a tube and is entirely dorsal to the alimentary
canal. In the adult it is represented by a simple dorsal

ganglion, located in the mantle near
the mouth. Close to it is the sub-
neural ganglion, thought by some to
be homologous with the hypophysis
of vertebrates. Tunicates are gener-
ally hermaphroditic. The tailed
larvae of the sessile tunicates show
unmistakable evidence of relationship
to Amphioxus and the lower verte-
brates. Some of these larval char-
acters are retained in the adult stage
by free-swimming forms. In some
colonial tunicates (Salpa) there is an
alternation of generations between a
solitary asexual form and a sexual
chain form. All tunicates are marine.
Examples : Oikopleura flabellum,
pelagic, retains larval characters;
Ciona intestinalis, the sea squirt,
. about 7 cm. in length, a common New
England form: Salpa democratica,
about 2 cm. in length, a free-swim-
ming form; Amaroucium stellatum,
known as sea pork, a colonial tuni-
cate common in the Cape Cod
region.
SUBPHYLUM 3. LEPTOCARDIA.
The body is fish-shaped but lacks a
clearly defined head (Fig. 265). The
mouth, encircled by numerous cirri, lacks jaws and is located
slightly behind and under the anterior end of the body. The
pharynx, into which the mouth opens, is provided with numerous
gill slits that connect externally with a branchial chamber, from
which water taken in through the mouth leaves by the atriopore.
The atriopore is a median, ventral opening at the posterior end

Fig. 264. — Diagram of a
Tunicate, one-half of the tunic
removed, a, anus; at, at-
rium; c, cloaca; e, excurrent
siphon; g, gonad whose duct
following the intestine opens
into the cloaca; h, heart; i,
incurrent siphon; in, intestine;
ng, nerve ganglion; o, esopha-
gus; p, pharynx; pv, perivis-
ceral cavity; s, stomach; t,
tunic. (Based on Leuckart-
Nitsche wall chart.)

THE ANIMAL KINGDOM

443

of the branchial chamber. Above the dorsal ends of the gill
slits are numerous paired nephridial tubes opening into the
branchial chamber. In the mid-dorsal line and also in the mid-
ventral line of the pharynx is a ciliated groove, thought to be
homologous with the endostyle of tunicates. Back of the
pharynx the intestine continues as a straight tube to an anal
opening near the posterior end of the animal, but to the left
of the mid-line. The liver is a diverticulum from the anterior
end of the intestine, extending forward to the right of the
pharynx. Its walls are said to produce a digestive fluid.
The digestive tract lies in a body cavity lined by coelomic

nt

7&

n

t an

is I J aP

Fig. 265. — Diagram of young Amphioxus shortly after metamorphosis.
a, atrium; an, anus, opening asymmetrically on the left side; ap, atropore; c,
oral cirri, surrounding mouth; e, eyespot; gs, gill septa; i, intestine; I, liver; n,
notochord; nt, neural tube; o, olfactory pit. (Based on Leuckart-Nitsche wall
chart.)

epithelium. The musculature of the trunk is conspicuously
segmented into V-shaped somites as in fishes.

The ventral surface of the body is flattened in the region of
the branchial chamber, with slightly projecting edges on each
side, known as metapleural folds. A dorsal fin runs the length
of the body and connects posteriorly with a caudal fin around
the tail. Ventrally the caudal fin is joined with the ventral
fin, which extends forward to the atriopore.

There is no heart, auditory organ, skull, or vertebral column.
The notochord is a well-developed rod, tapered at the ends,
lying between the neural tube above and the digestive tract
below, and extending the entire length of the body. The
anterior end of the neural tube is slightly enlarged into a region
corresponding to the brain of vertebrates. The remainder of
the neural tube is the spinal cord. A median olfactory
pit, located at the anterior end of the neural tube, opens
externally on the dorsal surface just back of the tip of the snout.
Nerves extend from the neural tube to all parts of the body.

444 GENERAL ZOOLOGY

The spinal cord gives off a pair of dorsal and ventral nerves to
each segment of the body, the dorsal and ventral nerves arising
independently from the cord. The blood circulatory system
consists of (1) a longitudinal vessel dorsal to the alimentary
canal and divided into right and left forks over the branchial
region; (2) a longitudinal vessel lying ventral to the alimentary
tract; and (3) lateral connections between the longitudinal
vessels, those passing between the gill clefts functioning as
branchial vessels. The blood is circulated by the contractions
of the anterior end of the ventral longitudinal vessel and parts
of other vessels. The blood moves forward in the ventral
vessel and backward in the dorsal one.

The sexes are separate. All are marine.

Example: Branchiostoma virginiae, one of the lancelets, also
known as Amphioxus, lies buried in the sand up to the mouth in
an upright position. About 5 cm. in length, it is found on the
southeastern Atlantic Coast. There are also numerous other
species.
SUBPHYLUM 4. VERTEBRATA. A notochord develops in the
embryo and, in some cases, persists as a rod-shaped structure in
the adult. In most of the fishes, including the sharks, and in
all higher vertebrates, it is partially or totally replaced by the
centra of the vertebral column. The skeleton of vertebrates is
an endoskeleton composed of cartilage or bone, or a combina-
tion of both. The integument consists of two well-defined
layers: the epidermis and the corium, supplemented by (1)
epidermal modifications such as the scales of reptiles, the feath-
ers of birds and the hair of mammals; and (2) modifications in
the corium, such as the scales of most fishes, and dermal bone of
certain reptiles and mammals. The body is fundamentally
bilaterally symmetrical and is made up of at least three body
parts: head, trunk, and tail (postanal region). There are
usually two pairs of locomotory appendages : the paired fins of
fishes and the fore- and hindlimbs of other forms. Paired
appendages are lacking in the lowest vertebrates (Cyclosto-
mata), some Amphibia (Apoda), snakes, and some lizards.
Dorsal, ventral, and caudal median unpaired fins are found in
aquatic adult and larval forms. The segmental character of
the vertebrate body is shown by the metameric arrangement
of the vertebrae, the nerves, and the body muscles. The body
cavity is a coelom in which the alimentary canal is attached by

THE ANIMAL KINGDOM

445

mesenteries (Fig. 266). The pharynx is concerned with
respiratory function (1) by serving as a respiratory surface, (2)
by developing gill slits and gills, or (3) by developing lungs.
The presence of gill slits in the pharynx is a constant feature of
the vertebrate embryo regardless of the type of respiration
found in the adult. The central nervous system is tubular and
lies entirely dorsal to the
alimentary canal. Its anterior
end is enlarged to form a brain
composed of five regions: (1)
telencephalon, (2) dienceph-
alon, (3) mesencephalon, (4)
metencephalon, and (5)
myelencephalon. The re-
mainder of the neural tube is
the spinal cord. The special
senses of taste, smell, sight,
hearing, and equilibration are
highly developed, though not
to the same degree in all
forms

system is of the closed type.
Hermaphroditism is rare.

CLASS I. CYCLOSTOMATA.
Jawless vertebrates with a
cylindrical body, lacking
paired appendages, ribs, scales, and true teeth. The notochord
persists as the axial skeleton throughout life (Fig. 267). Above
it lies the neural tube, supported by rudimentary cartilaginous
neural arches. The brain is enclosed in an imperfect skull,
which is firmly attached to a cartilaginous basketwork of carti-
lage supporting the branchial region. There is a median fin
supported by cartilaginous rays. The mouth is suctorial and
is provided with sharp cuticular spines, which with similar
spines on the end of the tongue are used as boring or rasping
organs. There is no pancreas, spleen, or swim bladder. The
olfactory organ is unpaired and median. The heart has one
ventricle and one atrium. All are destructive to fishes.

Order 1. Hyperotreta. Hag fishes. These are the most
primitive vertebrates and are practically parasitic in their
habits. There is a single cornified spine or tooth in the roof of

Fig. 266. — Diagram of a cross sec-
tion of vertebrate body, a, aorta; e,
mucosa, the endodermal lining of the
The blood circulatory alimentary tract; g, gonad; k, kidney;

me, mesentery supporting the alimen-
tary canal; mu, muscularis coats of
alimentary canal; n, notochord; n.t.,
neural tube; p.p., parietal peritoneum;
r, pleural rib; s, skin; s.m., body mus-
culature; v, vertebra; v.p., visceral
peritoneum.

446

GENERAL ZOOLOGY

the mouth which, with the rasping lingual teeth on the end of
the tongue, is used in boring into the bodies of fishes. There
are from 6 to 14 pairs of gills, which in some have but a single
orifice on each side of the body. The hypophysis connects
with the nasal sac in front and with the pharynx behind,
forming a channel through which water enters and passes out
through the gill slits behind. All are marine.

Examples: Myxine glutinosa, a North Atlantic species,
reaching a length of 2 ft., has six pairs of gills with a common

Fig. 267. — A, Petromyzon marinus. B, median section of anterior end of body.
b, brain; be, buccal cavity; dm, dorsal musculature; g, internal openings of gill
pouches; h, hypophysis; I, lingual cartilage; n, nasal opening; nc, notochord; o,
olfactory sac; oe., esophagus; of, oral funnel; pm, protractor muscle of tongue;
rm, retractor muscle of tongue; rt, respiratory tract; sc, spinal cord; t, tongue.

aperture on each side of the body. It is thought to be pro-
tandrous hermaphroditic (male and female alternately). The
pronephros of the embryo is retained in the adult. Bdellostoma
stouti, of the California coast, has 6 to 14 pairs of gills, each with
a separate opening to the outside. A peculiar fact is that the
number of gills may differ on the two sides of the body of the
same individual.
Order 2. Hyperoartia. Lampreys. The nasal sac opens
posteriorly into the hypophysis, but the latter does not com-
municate with the pharynx. The mouth is at the apex of a
cornified buccal funnel, armed with sharp cuticular teeth

THE ANIMAL KINGDOM 447

(Fig. 267). The tip of the tongue is also provided with similar
teeth. These animals attach themselves to fishes; and feed by
sucking the blood and shredded tissues from the rasped sur-
faces. There are seven pairs of gill pouches, opening separately
on the outside and communicating internally with a median
respiratory canal, located ventral to the esophagus and opening
from the mouth in front. When the animal is attached to prey
or to a stone, both inspiration and expiration of water take
place through the external gill openings.

Example: Petromyzon marinus, the sea lamprey, reaches a
length of 3 ft. and is found in the North Atlantic and also in
fresh water.

CLASS II. ELASMOBRANCHII. Sharks and rays. These
have a cartilaginous skeleton, in which the notochord is par-
tially replaced by the centra of the vertebrae; a skull with
jaws; a ventral subterminal mouth and ventral paired nostrils;
paired and median fins; a spiral valve in the intestine; and a
heterocercal tail (one in which the axis of the tail curves dorsally).
The skin is invested with dermal denticles or scales of the
placoid type. On the margins of the jaws the scales are
enlarged into teeth. The placoid scale is thought to be the
forerunner of the vertebrate tooth. The first gill cleft or
spiracle has only rudimentary gill filaments and with the mouth
serves as an incurrent respiratory passage. An operculum
covering the gill slits is usually lacking. There is no swim
bladder. All are marine.

Examples : Mustelus canis, a dogfish shark, 2 to 3 ft. in length ;
Carcharias taurus, the sandshark, 10 to 12 ft. long; Pristis
pectinatis, the sawfish of Florida and the Gulf of Mexico, 16 to
18 ft. long; Raji erinacea, a common ray or skate, 1 to 2 ft.
long. Rays and skates have gill slits on the ventral surface
of the body. The spiracle or first gill cleft through which water
enters is located on the dorsal surface just back of the eye.
Torpedo marmorata, an electric ray.

CLASS III. PISCES. True fishes. These have a more or
less ossified skeleton; a skull with membrane bones, and usually
distinct maxillary and premaxillary bones; paired nostrils;
median fins; and usually two sets of paired fins.

SUBCLASS 1. TELEOSTOMI. Cartilaginous or bony
skeleton. Breathe principally by gills, which are usually

448

GENERAL ZOOLOGY

covered by an operculum (Figs. 268 and 269). A swim bladder
is usually present.
Series 1. Ganoidei. Skeleton cartilaginous or bony, with
heterocercal or homocercal (outwardly symmetrical) tail fin.
Scales may be (1) the ganoid type (rhomboidal bony plates
covered with enamel or ganoin), (2) the cycloid type (bony
plates with evenly curved borders), (3) the ctenoid type (free
edge of scale spiny), or (4) the scute type (bony plate with
enameled spines and articulating with one another to form
a stiff armor. A swim bladder is present with an open duct
connecting with the pharynx or esophagus. There is a spiral
valve in the intestine.

Fig. 268. — Diagram of one side
of the mouth and pharynx of a
fish, split lengthwise, c, gill-
clefts, separated by gill arches; N,
nasal pit; o, esophagus; t, tongue.

Fig. 269. — Trutta irideus, the rainbow
trout, with the gill cover bent back to
show the gills. (After Jordan and Kel-
logg, Animal Life and Evolution.)

Order 1. Crossopterygii. Jointed pectoral fins; continuous
median fin; ganoid scales; and paired swim bladder. Mostly
extinct.

Example: Polypterus bichir, a living species found in the
River Nile.

Order 2. Chondrostei. Cartilaginous skeleton ; persistent noto-
chord; heterocercal tail; body naked or covered with bony
plates.

Examples: Acipenser fulvescens, lake sturgeon, length up to
6 ft.; Acipenser oxyrhynchus, common sturgeon found in the
North Atlantic and tributary streams, length up to 9 ft.;
Polyodon spathula, the spoon bill or paddle fish of the Missis-
sippi Valley, skin naked, length up to 6 ft.

Order 3. Holostei. Bony skeleton; ganoid or cycloid scales;
terminal mouth provided with teeth ; lunglike air bladder con-
nected with the esophagus.

THE ANIMAL KINGDOM 449

Examples: Lepisosteus osseus, common garpike, in fresh
water, length up to 4 ft.; Amia calva, fresh-water dogfish or
bowfin, length up to 2 ft.

Order 4. Teleostei. Bony skeleton; homocercal tail; cycloid
or ctenoid scales; gills covered by an operculum; intestine
without spiral valve. There is usually a swim bladder. The
majority of fishes belong to this order.

Examples: Coregonus clupeaformis, whitefish of northern
lakes; Oncorhyncus tschawytscha, Chinook or king salmon of
Pacific Coast; Trutta irideus, rainbow trout; Carpiodes carpio,
carp sucker of Ohio Valley southward; Ictalurus furcatus, chan-
nel catfish, has no scales; Ameiurus nebulosus, common bull-
head, no scales.

SUBCLASS 2. DIPNOI. Ltjngfishes. A paired or unpaired
swim bladder opening into the esophagus functions as a lung,
though gills are also present. The nasal passages are respira-
tory and connect with the pharynx. The notochord persists
as an unsegmented rod. The skeleton is partly ossified
cartilage. The pectoral fins are lobate and jointed; the scales
are cycloid. To a certain extent lungfishes represent a transi-
tion from fishes to amphibians. There are only a few living
species.

Examples: Neoceratodus forsteri, the Australian lungfish,
lives in brackish water and frequently comes to the surface to
breathe air; Protopterus annectans, one of three African species,
aestivates in the dry season by burrowing in the mud, where it
forms a slimy cocoon about itself and remains in a state of
suspended animation until the return of the rainy season;
Lepidosiren paradoxa, a South American species that also
aestivates.

CLASS IV. AMPHIBIA. Salamanders, frogs, and toads.
These are both terrestrial and aquatic animals, provided usually
with two pairs of pentadactyl limbs, lacking claws or nails on
the digits. Limbs and limb girdles are absent in the Apoda.
The integument is usually without scales. The eggs develop in
water or damp earth into swimming larvae (tadpoles), provided
with a tail and breathing by integumentary gills (Fig. 270).
The heart has a single ventricle and two atria except in the
case of lungless salamanders, in which there is a single atrium.
None is marine.

450 GENERAL ZOOLOGY

Order 1. Apoda. Blind burrowing forms, without limbs. The
skin contains small bony scales.

Example: Ichthyophis glutinosus, about 1 ft. in length.
Found in India and Ceylon.

Order 2. Caudata (Urodela). Salamanders and related
forms. An elongate body with a well-developed tail; usually
two pairs of limbs; teeth may be present on the maxillary and
premaxillary bones, the vomer, pterygoid, parasphenoid, and
mandible; gills are permanent in some, lost in others at meta-
morphosis. There is no tympanum.

Examples : Necturus maculosus, the mud puppy, is completely
aquatic and has permanent gill slits and external gills, and also

Fig. 270. — Ambystoma maculatum, with external gills which disappear at meta-
morphosis.

lungs. Length about 1 ft. Common in rivers and lakes of
Eastern United States. Cryptobranchus alleganiensis, the hell-
bender, has internal gills and lungs. Length about 1}4 ft.
It is aquatic and is common in Eastern United States. Amby-
stoma maculatum, the spotted salamander (black body with
large bright-yellow spots), is terrestrial but returns to water to
lay its eggs. The adult has lungs but no gills. Ambystoma
opacum, the marbled salamander, lays eggs on land in the fall.
Rains wash the eggs into water where larvae are hatched and
complete their development. Eurycea bislineata, a common
lungless salamander, has neither gills nor lungs in the adult
state. Siren lacertina, the mud eel, an aquatic form, breathes
by gills, and lacks hindlegs. Length about 2% ft. Found in
Southeastern United States. Proteus anguinus, a blind sala-
mander about 1 ft. in length, found in caves in Austria.
Order 3. Salientia (Anura). Frogs and toads. A tail is
absent, the caudal vertebrae being transformed into the uro-
style. The posterior pair of legs modified for leaping; a
tympanum is present, level with the surface of the head; males

THE ANIMAL KINGDOM 451

frequently have vocal sacs. There is usually a marked
metamorphosis.

Examples: Rana pipiens, the grass or leopard frog; Rana
catesbeiana, the bullfrog; Bufo americanus, a common toad.

CLASS V. REPTILIA. Reptiles. The skeleton of reptiles is
well ossified ; the body is covered by epidermal scales or ossified
dermal plates ; there are claws on the digits ; breathing by lungs ;
no gill respiration at any time in the life history; the heart has
two incompletely separated ventricles and two atria. Most
reptiles are oviparous, though viviparous forms also occur.
The egg in either case is large, resembling a bird's egg, and in the
oviparous forms the egg is laid on the ground. There is no
metamorphosis in development.

Order 1. Crocodilia. Alligators and crocodiles. The
teeth are set in sockets (thecodont); there are bony plates in the
skin; two pairs of legs, and a laterally flattened tail. A
tympanum is present. All are oviparous.

Examples: Alligator mississippiensis, the common American
alligator has a broad head, blunt snout and its length may
reach 15 ft. Crocodylus acutus, the American crocodile, has a
long head, pointed snout and reaches a length of 20 ft. Both
occur in Southeastern United States and tropical America.

Order 2. Lacertilia. Lizards. The body is elongate and is
covered with scales, which are periodically shed and renewed.
The teeth are either attached to the edge of the jaws (acrodont)
or laterally to the walls of a groove (pleurodont) in the jaw.
The tongue is usually well developed and is protractile. Most
lizards are oviparous.

Examples: Sceloporus undulatus, the common fence lizard;
Phrynosoma cornutum, the horned toad; Sphenodon punctatum,
a New Zealand lizard remarkable for the presence of a well-
developed pineal eye: Rhineura fioridana, the legless lizard
of Florida, about 8 or 9 in. in length.

Order 3. Serpentes. Snakes. The elongate body is limbless
and covered with scales, which are periodically shed and
renewed. The teeth are acrodont and the jaws are loosely
articulated to the skull. The tongue is long and forked.
There is no middle ear. The eyelids are not movable. Loco-
motion is brought about by the lateral bending of the body and

452 GENERAL ZOOLOGY

by movements of the ribs, which are attached at their lower
ends to the abdominal scales or scutes. Snakes are either
oviparous or viviparous.

Examples: Coluber constrictor, the common blacksnake;
Thamnophis sirtalis, the common garter snake; Agkistrodon
mokasen, the copperhead, poisonous; Crotalus horridus, the
common rattlesnake, poisonous.

Order 4. Testudinata. Turtles. The body is enclosed in a
dorsal carapace and a ventral plastron, each of which is usually
composed of epidermal plates, internally supported by bone.
Toothless jaws are covered with horny sheaths. The eyes have
upper and lower lids and a nictitating membrane. The
skin is usually covered with scales. The limbs are pentadactyl
with clawed toes. In marine turtles the limbs are modified into
flippers with reduced numbers of digits. Eggs of both aquatic
and terrestrial forms are deposited on land.

Examples: Chelydra serpentina, the snapping turtle;
Terrapene Carolina, the box turtle; Malaclemys centrata, the
diamond-back terrapin; Chrysemys picta, the painted turtle;
Amy da spinifera, the soft-shelled turtle, lacks horny plates or
scales and has fleshy lips; Eretmochelys imbricata, the tortoise-
shell turtle, marine; Caretta caretta, the loggerhead turtle,
is also marine.

CLASS VI. AVES. Birds. A modern bird is a homoiother-
mous animal, whose body is covered with feathers and scales,
whose anterior limbs are modified for flight, and whose jaws
lack teeth. The heart has two ventricles and two atria and the
circulation is completely double. The stomach consists of two
regions: a glandular proventriculus and a muscular gizzard.
There are usually two caeca at the junction of the ileum and
colon. The lungs are attached to the walls of the coelom and
are provided with long sacs that extend among the viscera
and into some of the bones. Sense organs are well developed.
A middle ear is present. The carpals of the forelimb are
reduced by fusion and the rudimentary digits correspond to
digits II, III, and IV of the pentadactyl hand. In the hind-
limb as a result of fusions, tibiotarsal and tarsometatarsal bones
are formed. The elements of the pelvis are fused to form a
rigid structure. The sternum of flying birds is provided with a
deep keel for the attachment of the wing (pectoral) muscles.

THE ANIMAL KINGDOM

453

The caudal vertebrae are fused to form a pygostyle. Birds are
oviparous. The egg is large and is incubated by the parents.
The left oviduct and left ovary only are functional.

That birds are undoubtedly descended from reptilian ancestors
is indicated by embryogeny and by the anatomy of extinct and
modern birds as well. Archaeopteryx (Fig. 271), a prehistoric

Fig. 271. — Archaeopteryx macura, restored. The fossil remains of this link
between reptiles and birds was found in the Jurassic of Bavaria. The jaws do
not have a beak but are provided with teeth. The tail consists of about twenty
separate vertebrae to which feathers are attached on either side. Each wing
has three clawed fingers. Feathers are well developed on the wings and tail but
the head and other parts of the body are naked. The bones are without air
sacs. (After Romanes, Darwin and after Darwinism, Open Court Publishing
Company.)

bird, had toothed jaws, a vertebrated tail (instead of a pygo-
style), and three clawed fingers on the wing, all of which are
reptilian rather than avian in character. In modern birds the
fingers are so reduced and modified as to be practically non-
existent, yet in the young of the hoactzin, a South American
bird, the forelimbs are provided with fingerlike claws which,
with the feet, are used in climbing about the branches of trees in
a lizardlike fashion before the bird learns to fly.

454 GENERAL ZOOLOGY

SUBCLASS 1. ARCHAEORNITHES. Extinct birds.
Example : A rchaeoptcryx.

SUBCLASS 2. NEORNITHES. Includes four orders of
extinct birds as well as all existing birds.

CLASS VII. MAMMALIA. Mammals. A mammal is a
homoiothermous animal, covered more or less with hair and
provided with mammary glands on the ventral surface of the
body which in the case of the female are used in suckling the
young. The teeth are thecodont and are of four kinds:
incisors, canines, premolars, and molars. Except in certain
aquatic mammals, an external ear, in addition to a middle and
inner ear, is present. The middle ear contains three ossicles:
malleus, incus, and stapes. A muscular diaphragm divides the
body cavity into a thoracic cavity containing the heart and
lungs in front and an abdominal cavity containing the rest of
the viscera behind. Each lung lies freely in a pleural cavity.
The heart consists of two ventricles and two atria, and the
circulation is completely double. The red corpuscles of the
blood lack nuclei.

SUBCLASS 1. PROTOTHERIA. Egg-laying mammals.
These have a single cloacal opening for both the urinogenital
and alimentary systems. The mammary glands have no
distinct nipples. The egg is large, contains much yolk, and
resembles the reptilian or avian egg. As in birds, the left
ovary and left oviduct only are functional.

Examples: Ornithorhynchus anatinus, the Australian duck-
bill or platypus, is about 20 in. in length. It has a duck-
like bill, webbed feet, and is covered with fur. Echidna
aculeata, the spiny ant eater, also found in Australia. These
with a few other species are the only representatives of the
entire subclass. They occur only in Australia and the neigh-
boring islands.

SUBCLASS 2. METATHERIA. Marsupials. These are
viviparous mammals whose young are born in an immature
state and are usually carried for a period after birth in a
marsupium or pouch, located on the abdomen of the mother.
A cloaca is absent and the urinogenital and anal openings are
distinct. The mammary glands are within the marsupium and
are provided with teats. The vagina is partially or completely
double.

THE ANIMAL KINGDOM 455

Examples: Didelphys virginiana, the Virginia opossum;
Macropus giganteus, the giant kangaroo of Australia.

SUBCLASS 3. EUTHERIA. Placental mammals. The
vagina is single and the embryo is nourished by a placenta
formed from the allantois of the embryo and the lining of the
uterus. The allantois is an embryonic membrane. Eutheria
are called placental mammals, but as a matter of fact a placenta
is also present in some marsupials.

Section A. Omnivorous Forms. Five digits on hand and foot ;
clavicle well developed.

Order 1. Insectivora. Typically, walking feet with dorsal sur-
face often scaly; first digit not opposable; radius and ulna
separate, but tibia and fibula often fused.

Examples: European hedgehog, shrews, and moles.

Order 2. Primates. Primitive dentition; first digit opposa-
ble on fore or hind limb or both ; hands and feet for grasping.
Large cranium; reduced jaws and nasal cavities; orbits directed
forward. Semierect or erect posture.
Examples: Monkeys, apes, and Man.

Order 3. Rodentia. Two chisel-shaped incisor teeth (four in
rabbits) in each jaw, modified for gnawing. Radius and ulna
separate; tibia and fibula some times fused. Clavicle absent
in guinea pigs.

Examples: Mice, rats, rabbits, guinea pigs, beavers, and
porcupines.

Order 4. Galeopithecoidea (Dermoptera). On each side a
lateral fold of skin including limbs and tail, when spread acts as
a parachute, enabling the animal to make long leaps. Primi-
tive dentition. Ulna, fibula, and clavicle rudimentary.

Example: Galeopithecus, the flying lemur of India and the
Malay Archipelago, the only representative of the order.

Order 5. Chiroptera. Greatly lengthened arm bones and fingers
supporting a membranous wing used in flying.
Example: Bats.

Order 6. Edentata. Incisors and occasionally all of the teeth
lacking.

Examples: Anteaters, sloths and armadillos.

Order 7. Tubulidentata. Body covered with bristlelike hairs;
five prismatic molars in each jaw; four toes in front, five
behind.

456 GENERAL ZOOLOGY

Example: Orycteropus, the South African aardvark, the only-
representative of the order.
Order 8. Pholidota. Body scaly with scattered hairs; teeth
lacking.

Example : Pangolin, the scaly anteater.
Section B. Carnivorous Forms.

Order 9. Carnivora. Strong recurved canine teeth; molars
more or less modified for cutting. Four- or five-toed feet which
may be plantigrade, semiplantigrade, or digitigrade; toes
usually clawed. Some are omnivorous and some may live
largely on a vegetable diet.

Examples: Wolves, dogs, cats, and bears.
Order 10. Pinnipedia. Pentadactyl webbed feet for swimming;
feed on fish.

Examples: Walrus and seals.
Order 11. Cetacea. Fore limbs paddlelike; hindlimbs lacking;
usually a dorsal fin; a caudal fin composed of two lobes or
flukes. There is no neck owing to fusion of the cervical
vertebrae. Rudimentary pelvic bones and, in some, vestiges
of the skeleton of the hindlimbs occur embedded in the flesh.
Skin may be entirely naked.

Examples : Porpoises, dolphins, and whales.
Section C. Herbivorous Forms. Terminal digits encased in

hoofs. Ungulata.
Order 12. Hyracoidea. Plantigrade, three toes in front and
four behind.

Example: Hyrax, the coney of the Bible. Most primitive
living ungulate, found in Western Asia and South Africa.
Order 13. Proboscidea. Pentadactyl; long proboscis with
nostrils at the tip; never more than two incisors (tusks) in each
jaw; no canine teeth; no clavicles.
Examples: Elephants.
Order 14. Sirenia. Pentadactyl, finlike forelimbs; hindlimbs
lacking. Skin naked or sparsely haired. Harmless aquatic
animals of huge size, feeding on seaweed and river grasses.

Examples: Manatee or seacow of tropical America and Africa;

Dugong of the Indian ocean.

Order 15. Artiodactyla. Even number of digits (four or two),

the axis of the foot passing between the third and fourth digits.

Examples : Swine, hippopotamus, deer, sheep, oxen, and camels.

THE ANIMAL KINGDOM 457

Order 16. Perissodactyla. Odd number of digits (five, three, or

one) the axis of the foot passing through the middle of the third

digit.

Examples: Horse, zebra, tapir, and rhinoceros.

Man. — The Primates, the order to which man belongs, are, from
the standpoint of general anatomy, the most primitive mammals
with the exception of the Insectivora, and for that reason occupy
a position among the lowest orders. When compared with
Primates, most other mammals show considerably more departure
from the original type, as for example, in the general form of the
body of the whale or manatee, the specialized fore arm of the bat
or the mole, the hoof of the ungulate, or the teeth of the rodent.
The primitive characters of the Primates are best seen in the
skeleton, which aside from the skull has retained the original
elements of the class with but little modification. Thus, in Man
the clavicle is present, both radius and ulna occur in the forearm
and the tibia and fibula in the leg; the arrangement of bones of
the wrist is primitive, the elements being distinct and showing
only slight modification. On the other hand, the ankle region is
modified as a result of the elongation of the foot which anatom-
ically is more highly specialized than the hand; the tail, highly
developed in some Primates, is absent in apes and Man, being
represented by the coccyx, composed of reduced caudal vertebrae
and not visible externally.

The important feature which makes the Primates a superior
group is the brain and this reaches its highest development in
Man. The enlargement of the brain has been accompanied by
modifications in the cranium and secondary modifications in
the face, such as the forward-directed orbits, the reduction of the
nasal region with a corresponding loss of olfactory sense, the
shortening of the jaws and the retreat of the teeth, the latter
causing the formation of the chin in the lower jaw. Very likely
the shortening of the face developed as the hand with its opposa-
ble thumb came to be used more and more to bring food to the
mouth, thus relieving the latter of seeking and grasping food.

Primates were, and some of them still are, essentially arboreal
animals. Man is a terrestrial animal, descended in all probability
from arboreal ancestors. The change from arboreal to terrestrial
habitat is thought for many reasons to have taken place in Cen-
tral or Southern Asia as a direct result of an increase in bulk too

458 GENERAL ZOOLOGY

great for arboreal life, or because of geological or geographical
changes which compelled the larger Primates to seek the ground
for shelter and food. Since Primates long before this event had
become widely scattered, only those living in the disturbed region
modified their mode of living, with the result that monkeys and
apes, the gorilla excepted, surviving in other parts of the world
are largely arboreal to this day. The subsequent progress of
the human race is undoubtedly the result of this change in
habitat, whatever the cause, since an arboreal existence has
rather limited possibilities for human development. If the
interpretation of the geological record is at all correct, speciali-
zation and increase in size without a corresponding development
of the brain have spelled disaster for many animals in the past.
Man is fortunate in that his specialization has been in the direction
of greater brain power while the rest of his body has retained a
certain flexibility by remaining relatively unspecialized, a combi-
nation which enables Man to make use of past experiences and
to suit action to requirement to a greater degree than any other
animal.

REFERENCES

The following list of references, suggested for collateral reading,
is divided into groups, arranged according to subject matter
under chapter headings.

Chapter I

Locy, W. A. : "Biology and Its Makers," Henry Holt & Company, New York.
Loeb, J.: "Dynamics of Living Matter," The Macmillan Company, New

York.
Mathews, A. P.: "Physiological Chemistry," William Wood & Co., New

York.
Needham, J.: "The Skeptical Biologist," W. W. Norton & Company, Inc.,

New York.
Nordenskiold, E. : "The History of Biology," Tudor Publishing Co., New

York.
Seifriz, W.: "Protoplasm," McGraw-Hill Book Company, Inc., New York.

Chapters II to IX, inclusive

Frog.

Dickerson, M. L.: "The Frog Book," Doubleday, Doran & Company, Inc.,
Garden City, N.Y.

Ecker, A.: "Anatomy of the Frog," Oxford University Press, New York.

Holmes, S. J.: "The Biology of the Frog," The Macmillan Company, New
York.

Morgan, T. H.: "The Development of the Frog's Egg," The Macmillan
Company, New York.

Noble, G. K.: "The Biology of the Amphibia," McGraw-Hill Book Com-
pany, Inc., New York.

Morphology.

Dahlgren, U., and W. A. Kepner: "A Text-book of the Principles of

Animal Histology," The Macmillan Company, New York.
Kuntz, A.: "The Autonomic Nervous System," Lea & Febiger, Philadelphia.
Maximov, A., and W. Bloom: "A Textbook of Histology," W. B.Saunders

Company, Philadelphia.
Parker, J. T., and W. A. Haswell: "Textbook of Zoology," The Macmillan

Company, New York.
Scott, G. G.: "The Microscopic Anatomy of Vertebrates," Lea & Febiger,

Philadelphia.
Wilder, H. H.: "History of the Human Body," Henry Holt & Company,

New York.

459

460 GENERAL ZOOLOGY

Physiology.

Carlson, A. J.: "The Control of Hunger in Health and Disease," The
University of Chicago Press, Chicago.

Crandall, L. A.: "An Introduction to Human Physiology," W. B. Saunders
Company, Philadelphia.

Howell, W. H. : "A Textbook of Physiology," W. B. Saunders Company,
Philadelphia.

Lusk, G. : "The Elements of the Science of Nutrition," W. B. Saunders Com-
pany, Philadelphia.

Mitchell, P. H.: "A Textbook of General Physiology for Colleges,"
McGraw-Hill Book Company, Inc., New York.

Sherman, H. C. : "The Vitamins," Chemical Catalog Company, Inc., New
York.

Chapter X

Allen, E.: "Sex and Internal Secretion," Williams & Wilkins Company,

Baltimore.
"Glandular Physiology and Therapy," American Medical Association,

Chicago.
Hoskins, R. G: "The Tides of Life," W. W. Norton & Company, Inc., New

York.
Schafer, E. A.: "Endocrine Organs," Longmans, Green & Company, New

York.

Chapter XI

Sharp, L. W. : "Introduction to Cytology," McGraw-Hill Book Company,

Inc., New York.
Wilson, E. B.: "The Cell in Development and Heredity," The Macmillan

Company, New York.

Chapter XII

Jenkinson, J. W.: "Vertebrate Embryology," Oxford University Press, New

York.
Kellicott, W. E.: "General Embryology," Henry Holt & Company, New

York.

: "Chordate Development," Henry Holt & Company, New York.

Lillie, F. R. : "Problems of Fertilization," The Macmillan Company, New

York.
Morgan, T. H.: "Experimental Embryology," Columbia University Press,

New York.

Chapter XIII

Babcock, E. B., and R. E. Claussen: "Genetics in Relation to Agriculture,"

McGraw-Hill Book Company, Inc., New York.
Guyer, M. F.: "Being Well Born," The Bobbs Merrill Company,

Indianapolis.
Kellicott, W. E.: "The Social Direction of Human Evolution," D.

Appleton-Century Company, Inc., New York.

REFERENCES 461

Morgan, T. H.: "The Physical Basis of Heredity," J. B. Lippincott Com-
pany, Philadelphia.

: "The Theory of the Gene," Yale University Press, New Haven.

Punnett, R. C: "Mendelism," The Macmillan Company, New York.

Sinnott, E. W., and L. C. Dunn: "Principles of Genetics," McGraw-Hill
Book Company, Inc., New York.

Shull, A. F.: "Heredity," McGraw-Hill Book Company, Inc., New York.

Chapter XIV

Darwin, Charles: "Origin of Species," London.

: "Diary of the Voyage of the Beagle," Cambridge University Press.

Holmes, S. J.: "The Evolution of Animal Intelligence," Henry Holt & Com-
pany, New York.

Lamarck, J. B.: "Zoological Philosophy" (transl. by H. Elliott), The Mac-
millan Company, New York.

Lull, R. S.: "Organic Evolution," The Macmillan Company, New York.

Osborn, H. F.: "From the Greeks to Darwin," Columbia University Press,
New York.

Pirrson, L. V., and C. Schuchert: "A Textbook of Geology, etc.," Vol. II,
John Wiley & Sons, Inc., New York.

Romanes, G. J.: "Darwin and After Darwin," Open Court Publishing Com-
pany, Chicago.

Romer, A. S. : "Man and the Vertebrates," University of Chicago Press,
Chicago.

Scott, W. B.: "The Theory of Evolution," The Macmillan Companv, New
York.

Shull, A. F.: "Evolution," McGraw-Hill Book Company, Inc., New York.

De Vries, H.: "Species and Varieties," Open Court Publishing Company,
Chicago.

Wilder, H. H.: "The Pedigree of the Human Race," Henry Holt & Com-
pany, New York.

Yale Sigma Xi Lectures 1916-1917. "The Evolution of the Earth and Its
Inhabitants," Yale University Press, New Haven.
-: 1921-1922. "The Evolution of Man," Yale University Press, New
Haven.

Chapters XV and XVI

Bailey, V.: "Animal Life in the Carlsbad Cavern," Williams & Wilkins
Company, Baltimore.

Borradaile, L. A.: "The Animal and Its Environment," Froude, Hodder &
Stoughton, London.

Brooks, W. K. : "Foundations of Zoology," The Macmillan Company, New
York.

Chapman, R. L.: "Animal Ecology," McGraw-Hill Book Company, Inc.,
New York.

Cuenot, L.: "L' Adaptation," Paris.

Henderson, L. J. : "The Fitness of the Environment," The Macmillan Com-
pany, New York.

462 GENERAL ZOOLOGY

Hesse, R.: "Ecological Animal Geography," John Wiley & Sons, Inc., New
York.

Kepner, W. A.: "Animals Looking into the Future," The Macmillan Com-
pany, New York.

Morgan, T. H.: "Evolution and Adaptation," Columbia University Press,
New York.

Pearse, A. S.: "Animal Ecology," McGraw-Hill Book Company, Inc., New
York.

Shelford, V. E. : "Laboratory and Field Ecology," Williams & Wilkins Com-
pany, Baltimore.

: "Animal Communities in Temperate North America," University of

Chicago Press, Chicago.

Snodgrass, R. E.: "Anatomy and Physiology of the Honey Bee," McGraw-
Hill Book Company, Inc., New York.

Wheeler, W. M.: "Ants, Their Structure, Development and Behavior,"
Columbia University Press, New York.

Chapter XVII

Beebe, C: "The Bird, Its Form and Function," Henry Holt & Company,
New York.

Harmer, S. F., and A. E. Shipley: " Cambridge Natural History," The Mac-
millan Company, New York.

Lankester, E. R.: "A Treatise on Zoology," A. & C. Black, Ltd., London.

Metcalf, C. L., and W. P. Flint: "Destructive and Useful Insects,"
McGraw-Hill Book Company, Inc., New York.

Parker, T. J., and W. A. Haswell: "Textbook of Zoology," The Macmillan
Company, New York.

Pratt, H. S.: "A Manual of the Common Invertebrate Animals, Exclusive
of Insects," P. Blakiston's Son & Co., Philadelphia.

: "A Manual of the Land and Freshwater Vertebrate Animals of the

U.S., Exclusive of Birds," P. Blakiston's Son & Co., Philadelphia.

Snodgrass, R, E.: "Principles of Insect Morphology," McGraw-Hill Book
Company, Inc., New York.

Ward, H. B., and G. C. Whipple: "Freshwater Biology," John Wiley &
Sons, Inc., New York.

Wilder, H. H.: "The Pedigree of the Human Race," Henry Holt & Com-
pany, New York.

Williams, S. H.: "The Living World," The Macmillan Company, New York.

GLOSSARY

Abiogenesis. The spontaneous generation of living things from nonliving
matter, a theory held but not experimentally demonstrated.

Aboral. A pole of the body opposite the mouth.

Absorption. The passage of a fluid into living cells by osmotic or capillary
action.

Achromatic. Uncolored. Free of color.

Adaptive. Having the quality of fitness; favorable to life processes.

Adoral. A point of the body near the mouth.

Agamic. Without gametes. Asexual.

Albinism. The absence of pigment in the skin, hair, feathers, eyes, etc.

Albumin. One of the blood proteins; found also in milk, muscle, and other
animal substances.

Albuminoid. A substance having many characteristics of true proteins.

Alimentary canal. Same as digestive tract.

Allelomorph. One of a pair of alternative characters in Mendelian inherit-
ance; said also of the genes which represent these characters in the
chromosomes.

Amino acid. An organic acid in which one or more of the nonacid hydro-
gen atoms is replaced by the NH2 group. Glycine (CH2NH2COOH);
Lysine (CH2NH2CH2CH2CH2CHNH2COOH).

Amitosis. Direct cell division, in which the formation of a spindle and
chromosomes is lacking.

Amoeboid. Having a flowing movement, such as occurs in Amoeba.

Amphigony. Reproduction from a fertilized egg. Sexual reproduction.

Anabolism. The constructive or reintegrative phase of metabolism.

Analogous. Similar in function.

Anastomosis. A joining or communication between vessels.

Anatomy. The structure of organisms as determined by dissections.

Animal pole. The yolk-free region of the egg.

Ankylosis. Abnormal immobility and consolidation of a joint.

Antenna. One of a pair of jointed appendages of the head of an insect,
or crustacean.

Anterior. Toward the head or front end of an animal.

Antitoxin. A complex compound, probably protein in nature, formed in the
blood serum and capable of neutralizing the effect of a specific poison or
toxin, especially such as is produced by pathogenic bacteria.

Anus. The terminal opening of the digestive tract.

Apical. Pertaining to or located at the apex.

Archenteron. The cavity of the gastrula; gastrocoel.

Arterial blood. Oxygenated blood. It may be carried by either arteries
or veins.

463

464 GENERAL ZOOLOGY

Articulate. To join; to unite by means of a joint.

Artificial parthenogenesis. The development of an egg following artifi-
cial stimulation without the intervention of a sperm.
Assimilation. The transformation of digested and other absorbed food

products into living substance, or into material that can be utilized by

living substance.
Asymmetry. An arrangement of parts incapable of being divided by a

plane into halves which are mirrored images of each other.
Atom. The smallest particle of a chemical element which can exist either

alone or in combination with other similar atoms or with atoms of

another element; the smallest particle of a chemical element which

enters into the composition of a molecule.
Autonomic, autonomous. Self-governing; said of the sympathetic and

parasympathetic nervous systems.
Axial. Pertaining to an axis. The backbone is the axial skeleton of

vertebrates.
Axon. Process of a nerve cell conducting impulses away from the cell body.
Bacteria. A group of microscopic plants, some of which are the causative

agents in the production of certain diseases. They exist in three general

forms as follows: cocci, which are spherical; bacilli, rod-shaped; and

spirilla, spiral filaments.
Barbel. A slender tactile process on the lips of certain fishes.
Biconcave. Hollow on each side or end.
Bilateral symmetry. An arrangement of parts capable of being divided

by a plane into right and left halves.
Binomial nomenclature. The method of designating organisms by two

names, one for the Genus and the other for the Species. Example:

Rana pipiens, the leopard frog.
Biogenesis. The generally accepted principle that all living things are

derived from living things; that only life reproduces life (see Abiogenesis).
Biogenetic law. In its modern form, the doctrine that an animal in its

development repeats to a certain extent the history of its race. Also

known as the law of recapitulation.
Biparental. Derived from two parents, male and female.
Biped. An animal which walks on two feet.
Blastocoel. The segmentation cavity within the blastula.
Blastomere. One of the cells formed in cleavage of the egg, fertilized or

parthenogenetic.
Blastopore. The opening of the archenteron or gastrocoel.
Blastula. The stage during cleavage, when the cells are arranged in the

form of a hollow sphere.
Buccal cavity. Mouth cavity.

Carbohydrate. Sugars, starches, and cellulose. Organic compounds con-
sisting of carbon, hydrogen, and oxygen, the hydrogen and oxygen being

in the same proportion as in a molecule of water (H20).
Carnivore. A mammal belonging to the order Carninora.
Carnivorous. Flesh-eating.
Catabolism. The disintegrative phase of metabolism.

GLOSSARY 465

Catalysis. The acceleration in rate of a chemical reaction by an agent

which itself remains unchanged.
Caudad. Toward the tail.
Caudal. Pertaining to the tail.

Cell. A small mass of protoplasm containing one or more nuclei.
Cell doctrine. That all organisms are composed of cells and the products

of cells; that the cell is a functional and structural unit of the organic

body.
Cellulose. A carbohydrate found in the walls of plant cells. Its occur-
rence in animal tissues is rare.
Centigrade thermometer. One in which 0° is the freezing point and 100°

the boiling point of water.
Centimeter. A unit of linear measurement in the metric system. Abbre-
viation: cm.; (2.54 centimeters = 1 inch).
Cephalad. Toward the head or anterior end.
Cephalic. Pertaining to the head.
Cercus. Either of a pair of appendages at the posterior end of the body of

arthropods.
Cervical. Pertaining to the neck.

Characters or characteristics. Physical, mental, and physiological traits.
Chlorophyll. An important plant pigment, concerned in photosynthesis.
Chondrocranium. A continuous cartilaginous structure incompletely

enclosing the brain. It occurs in the embryos of all vertebrates and

in the adult sharks and lampreys.
Chordate. An animal in which the notochord is or represents the skeletal

axis during some period of its life history. All vertebrates are chordates.
Chromatic. Pertaining to color.
Chromatin. The deeply staining substance of the nucleus which forms

into chromosomes when the cell divides.
Chromatophore. A cellular organ containing pigment.
Chyle. The fatty contents of the lacteals of the intestines.
Chyme. Partly digested food as it occurs in the stomach and intestine.
Cilia. Hair-like processes of Protozoa and ciliated epithelial cells.
Circulation. The movement of the blood or body fluid through a more or

less complete system of vessels.
Cirrus. A soft tentaclelike appendage.
Class. One of the principal subdivisions of a phylum in the system of

classification.
Cleavage. The period of cell division following fertilization, during which

the egg is converted into a blastula. Parthenogenetic eggs also

undergo cleavage.
Cloaca. A common cavity into which open the urogenital ducts and the

alimentary canal, in fishes, amphibia, reptiles, birds, and the lowest

group of mammals (Prototheria).
Cocoon. A case in which eggs are deposited and in which larvae may

develop; a protective covering of mucus secreted by the integument.
Coelenteron. A saclike body cavity in which digestion and absorption

of food take place. Same as gastrovascular cavity,

466 GENERAL ZOOLOGY

Coelom. The body cavity of the vertebrate type, lined with mesothelium

and containing the viscera attached by mesenteries. It is entirely

distinct from the digestive cavity of the alimentary canal.
Collagen. A gelatinlike protein.
Colloid. A state of matter consisting of a dispersed system of molecular

aggregates.
Colony. A group of individuals of the same species, often organically

connected, forming a unit of a higher order than the individual.
Commensalism. The association of two or more individuals of different

species, often for mutual benefit.
Concave. Curved inward.
Conjugation. A temporary union of two Protozoa during which an

exchange of nuclear material takes place.
Convex. Curved outward. Bulging out.
Copulation. The sexual act during which spermatozoa are transferred

from the male to the female; or the sexual act of two monoecious

animals.
Cortex. The outer region of a cell or organ.
Cranial. Pertaining to the cranium, the portion of the skull enclosing the

brain.
Cretinism. A condition characterized by subnormal mental and physical

development, caused by thyroid insufficiency.
Cuticle. The outer layer covering the surface of the organic body. It

may be a secretion product of underlying cells, as in many invertebrates,

or it may be composed of dead cells, as in the stratum corneum of human

skin.
Cytoplasm. The extranuclear part of a cell; the cytosome.
Cytozoic. Parasitic in a cell.

Darwinism. The explanation of evolution by the theory of Natural Selec-
tion as set forth by Charles Darwin.
Deaminization. The process in the body by which nitrogenous radicals

are removed from amino acids, thus liberating nonnitrogenous portions

capable of oxidation and energy production.
Differentiation. In embryogeny the transformation of blastomeres

into tissues and organs. In general, a change from homogeneity to

heterogeneity.
Digestion. The result of the action of digestive agents on food which

reduces food to a liquid condition capable of absorption and assimila-
tion by living cells.
Dioecious. A condition in which the male and female organs, testis and

ovary, are borne by different individuals.
Diploblastic. Having two germ layers.
Diploid. The unreduced number of chromosomes.
Distal. Remote from point of origin or attachment.
Diuresis. Free or excessive secretion of urine.
Dorsal. Pertaining to the back.
Ductless gland. A gland whose secretion is poured directly into the blood

stream. An endocrine gland.

GLOSSARY 467

Dynamic. Pertaining to change or process. Characterized by energy or
action.

Ecology. The study of the relation of organisms to their environment,
both animate and inanimate.

Ectoderm. The outer layer of the gastrula.

Ectoparasite. External parasite.

Egg. The female germ cell, either before or after fertilization.

Embryo. An animal in the early stages of its development before it is
liberated from the egg membranes.

Embryogeny. The formation of the embryo and the course of its develop-
ment. Embryogenesis. Embryology is the study of embryogeny.

Encystment. The formation of a protective covering about an organism,
particularly Protozoa.

Endocrine gland. See Ductless gland.

Endoderm. The germ layer forming the lining of the gastrula and bound-
ing the archenteron or gastrocoel; lining of the alimentary canal.

Endomixis. A nuclear reorganization occurring in Protozoa without
conjugation and therefore without synkaryon formation (fertilization).

Endoparasite. Internal parasite.

Endoskeleton. An internal living skeleton such as is present in
vertebrates.

Entelechy. A word adopted from Aristotle by Hans Driesch to desig-
nate the "vital force," which he believes is necessary as an addi-
tional factor outside the range of known forms of energy to explain life.
Entelechy and the "soul" of Descartes are practically the same con-
cept, except that Descartes entertained the fanciful notion that the
soul resided in the pineal gland, which is a dorsal outgrowth of the roof
of the forebrain and in all probability represents an ancestral eye.

Enzyme. An organic catalytic agent which accelerates chemical reaction
in the body.

Epigenesis. The idea that an organism in its development starts as a
relatively homogeneous initial plasma which becomes heterogeneous
as a result of the action of external factors upon it. The opposite of
preformation in development.

Epithelium. A layer of cells covering an external or internal surface of the
body.

Eugenics. The science of improving the inborn qualities of the human
race by better breeding.

Eustachian tube. A duct connecting the middle-ear cavity with the
pharynx. It is a survival of the first gill cleft of the fish.

Eutheria. A subclass including the viviparous mammals.

Evagination. The outgrowth of a pocket of cells from a surface.

Evolution. The doctrine that organisms of today are derived by descent
from those of the past; that organisms have changed from time to time;
that, in general, higher organisms are descended from lower ones.

Excretory. Pertaining to waste substances formed in metabolism.

Exoskeleton. The lifeless external cuticle forming the protective covering
and supporting framework of arthropods. An external skeleton.

468 GENERAL ZOOLOGY

External respiration. The passage of oxygen from the air or water into

the blood.
Extracellular. Outside of cells.

Factor. Any causative agent. Its effect is called response.
Family. A subdivision of an order in classification.
Fat. An organic compound consisting of carbon, hydrogen, and oxygen,

in the form of a glyceric ester of a fatty acid.
Fauna. The animal organisms occurring in a given region or place.
Feces. The excrement or undigested food residue discharged from the

alimentary canal.
Fermentation. The decomposition of organic compounds, usually

through the action of enzymes {ferments).
Fertilization. The culmination of the series of events following the

entrance of the sperm into an egg, ending in the union of a male and of

a female nucleus to form the first cleavage nucleus.
Fetus. The mammalian embryo in later stages of development when the

body regions are well defined.
Flagellum. A greatly enlarged cilium occurring in Protozoa, and fre-
quently as the tail of a spermatozoon.
Fluctuating variation. A relatively slight variation of somatic origin

which is not heritable.
Fossil. The remains of a prehistoric organism or its tracks, etc., found

usually embedded in the earth's crust.
Gamete. Male or female germ cell; sperm or egg.
Ganglion. A tissue mass composed principally of nerve-cell bodies.
Gastric. Pertaining to the stomach.
Gastrocoel. The cavity of the gastrula.
Gastrula. In embryogeny, the double-walled sac resulting from the

invagination of the single-layered blastula. The outer layer is ecto-
derm and the inner endoderm. Gastrulation is a fundamental process

in development.
Gel. A more or less rigid colloidal state.
Gene. The germinal representative of a character.
Genotype. The factor or gene complex of an organism or a group of

organisms. The germinal complex as contrasted with somatic.
Genus. A group of related species. A subdivision of a family in

classification.
Germ layer. One of the primary embryonic tissues, ectoderm, endoderm,

or mesoderm, from which the tissues and the organs of the adult develop.
Gestation. The period of development in viviparous animals between

fertilization and birth.
Gill. A platelike or filamentous appendage of an aquatic animal bathed

by water and serving as an organ of respiration.
Gill clefts. Paired openings leading from the sides of the pharynx

to the exterior. Present in all Chordates. In fishes gills are attached

to the septa separating the clefts.
Gland. An organ whose cells secrete one or more substances that may

be used by the organism (as in secretions of the glands of the alimentary

GLOSSARY 469

tract), or that may be excretory in character (as in the metabolic prod-
ucts excreted by the kidney).
Glomerulus. A complicated network in the course of an arteriole of the

kidney.
Glottis. A slitlike opening in the floor of the pharynx leading to the

respiratory tubes and the lungs.
Glycosuria. The presence of excessive amounts of sugar (dextrose) in

the urine.
Gonad. The organ in which germ cells develop. Ovary or testis.
Gustatory. Pertaining to the sense of taste.
Habitat. The natural abode of an organism; the kind of environment in

which it lives.
Haploid. The reduced number of chromosomes.
Hemocoel. A body cavity which contains blood. A type of body cavity

found in many invertebrates.
Herbivorous. Plant eating.
Heredity. The occurrence or production in offspring of parental traits

or characteristics; the transmission of genes through the germ cells.
Hermaphrodite. An organism provided with both male and female

gonads.
Heterocercal. Said of the type of fish tail in which the terminal part of

the vertebral column takes an upward bend, making the tail fin asym-
metrical, the ventral portion much smaller than the dorsal.
Heterogony. The alternation of amphigony with parthenogenesis.
Heterozygote. An organism which has received from its parents two

unlike genes for a given character and which, in turn, produces two

numerically equal classes of gametes with respect to the genes.
Homologous. Said of organs having a similar origin in evolution

though not necessarily a similar function.
Homoiothermal. Having a practically constant body temperature.
Homozygote. An organism which has received from its parents two like

genes for any given character. Its gametes are, therefore, all alike

with respect to these genes.
Hormone. An internal secretion of a gland which activates other organs,

or the body as a whole, in a specific manner.
Host. An organism which harbors a parasite.
Hyaline. Glassy, translucent.
Hybrid. The offspring of parents, differing from one another in at least

one heritable character.
Hydroid. Colonial coelenterate made up of individuals resembling Hydra.

A polyp.
Hydrolysis. A double chemical decomposition reaction into which water

enters.
Hydrostatic. Relating to the pressure and equilibrium of liquids.
Hyperglycemia. The presence of excessive amounts of sugar (dextrose)

in the blood.
Hypertonic. Having a higher osmotic pressure than normal, or than

another substance.

470 GENERAL ZOOLOGY

Hypothesis. A tentative supposition provisionally adopted for explaining
certain facts and serving as a guide for further investigation. A
stage in the development of a theory.

Hypotonic. Having a lower osmotic pressure than normal.

Immunity. The resistance of the body to infection by pathogenic organ-
isms, natural or acquired, through the production of antitoxins.

Infection. Implantation of disease, pathogenic organisms, or parasites
from without.

Insemination. The addition of sperm to eggs. The introduction of sperm
into a female.

Integrative action. Said of the function of the nervous system as a
coordinating mechanism for unifying bodily activities.

Intercellular. Between cells.

Internal respiration. The interchange of oxygen and carbon dioxide
between tissues and the circulatory fluid. True respiration.

Internal secretion. Usually the product of a ductless gland that is
absorbed by the blood. Hormone. Endocrine.

Intracellular. Within a cell.

Intussusception. Growth by the intercalation of substances throughout
the cells of an organism, as contrasted with growth by accretion-
deposits of particles on the outside — such as occurs in crystals.

Invagination. The ingrowth of a pocket of cells from a surface.

Invertebrate. An animal without a backbone.

Ion. An atom or group of atoms bearing an electrical charge. Hydrogen-
ion concentration refers to the number of free hydrogen ions in a solu-
tion, which determines whether the solution is "acid" or "basic" in
reaction.

Irritability. The power of protoplasm to respond to stimuli.

Isotonic. Having the same osmotic pressure.

Karyokinesis. The indirect method of cell division; mitosis.

Kinetic energy. Energy possessed by a body by virtue of motion.
Manifested by heat production in chemical reactions.

Lacteal. A lymph vessel of the intestine.

Lacuna. An intercellular space.

Lamellated. Arranged in layers.

Lamellibranchiate. Belonging to a group of molluscs having platelike
gills, such as the clam, oyster, mussel, etc.

Larva. A usually active stage in the development of an animal marked
by the presence of larval organs and by the absence of adult ones. An
inmature but free-living stage in development.

Lateral. Pertaining to the side.

Law. A statement of an order or relation of scientific facts which, so far
as known, is invariable.

Lesion. A wound or local degeneration.

Linear. Arranged in a line or row.

Lumbar. Pertaining to the region of the back posterior to the thorax.

Lumen. Cavity or passageway of a tubular structure.

Lymph. In vertebrates a circulatory fluid similar to blood but lacking red
corpuscles. Same as blood in many invertebrates.

GLOSSARY 471

Macroscopic. Large enough to be seen with the unaided eye.

Mammal. A vertebrate having hair and mammary glands.

Mandible. A biting mouth part of invertebrates. The lower jaw of

vertebrates.
Maternal. Pertaining to or derived from the female parent.
Matrix. The intercellular material of cartilage and bone. Intercellular

substance.
Maxilla. A mouth part of an invertebrate. The upper jaw of vertebrates.
Mechanism. The hypothesis supported by many facts that the phenomena

of life are inherent in the physical and chemical properties of the con-
stituents of protoplasm. It does not admit "vital force" as a factor in

explaining life processes.
Mendelism. A universal type of inheritance based on the fact that genes

of inherited characters separate and combine as units in the germ cells.
Mesoderm. The embryonic germ layer formed between the endoderm and

ectoderm.
Mesoglea. A noncellular layer lying between the ectoderm and endo-
derm of Hydra and related forms.
Metabolism. The chemical processes of protoplasm which are made up

of disintegrative and reintegrative phases, catabolism and anabolism,

respectively.
Metagenesis. The alternation of sexual and asexual generations in the

life history of an organism. The alternation of sexual medusa and

asexual polyp in Obelia.
Metamerism. The repetition of parts or segments in a linear series, as in

the segmentation of the earthworm, or the arrangement of vertebrae

in the vertebral column.
Metamorphosis. The more or less sudden change of the larva into the

adult. The transformation of a tadpole into a frog; or of a caterpillar

into a moth.
Metazoa. Animals whose bodies consist of more than one cell. All

animals above Protozoa.
Millimeter. One-tenth of a centimeter. Abbreviation: mm.
Mitosis. The ordinary form of cell division. Also called karyokinesis.
Molecule. A group of atoms. The smallest particle of a substance that

possesses the properties of the substance.
Monoecious. Having both ovary and testis in one individual.
Motor neuron and nerve. One carrying impulses away from the central

nervous system and causing muscular contraction or glandular activity.

Efferent neuron.
Mucosa. The layer of cells lining the digestive tract of vertebrates.
Mutation. A heritable variation of a discontinuous type caused by some

sort of change in the germplasm.
Myotome. One of the segments in the body musculature of vertebrates.
Natural selection. The natural process of eliminating the unfit. The

survival of the fittest in nature.
Nephridium. A tubular type of excretory organ such as occurs in the earth-
worm and which is the forerunner of the tubules of the vertebrate

kidney.

472 GENERAL ZOOLOGY

Nerve. A bundle of nerve-cell processes, axons, or dendrons, or both.

Neural canal. The cavity in the vertebral column containing the spinal
cord.

Neural tube. The embryonic ectodermal tube from which the brain and
spinal cord develop.

Neuron. A nerve cell, including cell body, axon, and dendron.

Nitrogen equilibrium. A condition in which the animal body is receiving
from food as much protein nitrogen as it is metabolizing and eliminating.

Notochord. A structure characterizing the phylum Chordata consisting
of a cylindrical rod lying ventral to the neural tube. In all vertebrates
except Cyclostomata it is replaced in varying degrees by the centra of
the vertebral column.

Nucleolus. A usually spherical body within the nucleus taking an acid
stain. Chromatin takes the basic stain.

Nucleus. One of the two principal components of a cell, usually occu-
pying a central position in the cytoplasm.

Olfactory. Pertaining to the sense of smell.

Ontogeny. The development of the individual.

Oocyte. The developmental stage of the egg during the growth period.

Oogenesis. The development of a mature egg from a primordial germ cell.

Oogonium. One of the products of division of the female primordial germ
cell.

Oosperm. A fertilized egg.

Ootid. The final product of oogenesis, the mature egg.

Order. A subdivision of a class in classification.

Organ. A tissue complex which performs a definite function.

Organelle. A protozoan organ; an organ within a cell.

Organic. Of or pertaining to organisms.

Organism. A living thing.

Organology. The study of organs.

Osmosis. The slow passage or diffusion of fluids through semipermeable
membranes. Osmotic pressure results from the difference in behavior
of solvent and solute with respect to the membrane, the latter not being
equally permeable to both. The surfaces of cells constitute the mem-
branes through which substances must pass in and out of cells, the
direction being determined by conditions within the cell with reference
to the surrounding medium.

Osseomucoid. A glycoprotein found in bone.

Ovary. The organ in which eggs develop.

Oviparous. Egg laying.

Ovum. Egg, female gamete.

Oxidation. The chemical combining of a substance with oxygen, partial
or complete. Combustion.

Paedogenesis. Reproduction in the larval state.

Paleontology. The science of prehistoric organisms.
Parasitism. An animal association in which one member, the parasite,
derives nourishment from the tissues of the other, the host, to the
detriment of the latter.

GLOSSARY 473

Parthenogenesis. Development of an egg without fertilization.

Partial pressure. The pressure exerted by one component of a mix-
ture of gases. The pressure of each component is proportional to its
quantity.

Parturition. Act of bringing forth young.

Paternal. Pertaining to, or derived from, the male parent.

Pathogenic. Disease-producing.

Pelagic. Said of the habitat of organisms living at or near the surface of
large bodies of water.

Pelvis. The posterior region of the abdomen in vertebrates. The pelvic
girdle. A region in the kidney.

Penis. An intromittent organ of the male by means of which spermatozoa
are transferred to the vagina of the female.

Pericardium. In vertebrates the peritoneal sac surrounding the heart.

Peristalsis. The rhythmic contraction of the intestinal wall.

Peritoneum. The membrane lining the coelom of vertebrates.

Phagocyte. A cell which ingests and destroys waste and harmful material,
bacteria, etc., in the body.

Pharynx. The region of the alimentary tract between the mouth and
esophagus.

Phenotype. The somatic complex of an organism or a group of organisms
regardless of the potential germinal possibilities.

Photosynthesis. The synthesis, or building up, of complex organic
compounds from relatively simple inorganic substances through the
agency of sunlight in the presence of pigments like chlorophyll. A
natural process in plants.

Phylogeny. The developmental history of the race.

Phylum. A main subdivision of a plant or animal kingdom.

Placenta. A composite maternal and fetal organ of the mammalian
uterus which serves to attach the embryo, to supply nourishment and
oxygen from the maternal blood, and to remove waste products. It is
shed in whole or part at birth.

Placoid scale. Consists of a rhombic basal plate of dentine (bone) from
the middle of which a spine projects. In the spine is a pulp cavity with
blood vessels. Characteristic of Elasmobranchs. Forerunner of verte-
brate tooth.

Plasma. The liquid part of the blood. Blood minus corpuscles.

Plexus. A union of several nerves to form a network.

Poikilothermal. Having a body temperature slightly higher than that
of the environment.

Polar body. One of the minute cells formed in the maturation divisions
of the egg.

Polarity. Differentiation at the two ends of an axis.

Polymorphism. Several forms. The existence of two or more types of
individuals in a species, as in the honeybee.

Polyp. A Hydralike coelenterate.

Potential energy. The energy a body possesses by virtue of its position.
A lifted weight has potential energy in proportion to the kinetic energy

474 GENERAL ZOOLOGY

expended in lifting it. An organic compound possesses potential
energy in a chemical form in proportion to the energy expended in
establishing a certain spatial relation between atoms in the molecule.
Preformation. In its original form, the idea that development consisted
in the unfolding of adult structures already preformed in the germ;
which, of course, can no longer be accepted. In a modified sense the
term can be applied to the organized ground substance of the egg and
to the orderly arrangement of genes in a chromosome.
Proboscis. A tubular process or prolongation of the head or oral region.
Protandry. A condition in monoecious animals in which the testis

becomes functionally mature before the ovary.
Protein. An organic compound of large molecules made up of carbon,
hydrogen, oxygen, nitrogen, and sulphur with sometimes phosphorus
and iron which upon hydrolysis yields amino acids. A very important
constituent of protoplasm and one which has not yet been synthesized.
Protoplasm. Living matter.

Prototheria. The lowest group of mammals, which are oviparous.
Protozoa. Unicellular animals.

Proximal. Next or nearest, as to a point of attachment.
PsETjDOPODiUM. A temporary fingerlike extension of the cytoplasm of

Amoeba and related forms.
Pulmonary. Pertaining to the lung.
Pupa. The quiescent stage following the larval period in an insect, during

which the adult organs are developed.
Pure line. All the offspring of a homozygous self-fertilized parent. A

group of individuals having an identical germinal constitution.
Putrefaction. The decomposition of proteins as brought about by

enzymes or bacteria.
Radial symmetry. Symmetry that is referable to a circle. Two or more
planes passing through a common axis will in each case produce halves
that are mirrored images of each other.
Receptor. A sensory-nerve ending or end organ.
Rectum. The terminal portion of the alimentary canal in vertebrates and

in many invertebrates.
Reducing agent. A chemical agent which causes a loss of oxygen in a

substance.
Reduction division. One of the two maturation divisions in gametogene-
sis during which synaptic chromosome mates separate, reducing the
number to one-half.
Refractive. Having power to turn from a direct course. Said of the effect

of certain substances on light.
Respiration. The absorption of gaseous oxygen and the excretion of

carbon dioxide by protoplasm.
Response. Any change in protoplasmic activity resulting from a stimulus.
Reticulum. A network.

Ruminant. A herbivorous animal that chews its cud.
Sacral. Pertaining to the region of the vertebrate axial skeleton to which
the pelvic girdle is attached.

GLOSSARY 475

Salivary glands. Glands whose secretion is discharged into the mouth
and usually has some digestive property.

Sebaceous glands. Oil-secreting glands present in hair follicles.

Secondary sexual characters. Traits other than the gonads that dis-
tinguish sexes.

Secretion. The substance produced by a gland. The activity of a gland.

Segmentation. See Metamerism.

Sensory. Pertaining to sensation. Said of a neuron carrying impulses
from the periphery toward the central nervous system.

Septum. Partition.

Serum. The clear fluid remaining after blood has clotted.

Sessile. Fixed; not free-swimming.

Sex-linked characters. Characters whose genes are linked with those
of sex.

Sol. A more or less fluid colloidal state.

Solute. The substance dissolved in a liquid.

Solvent. The liquid in which a substance is dissolved.

Somatic. Pertaining to the soma or body as distinguished from germ cells.

Species. A group of individuals derived from similar parents, or practically
alike in all important characters.

Sperm. Male gamete. Spermatozoon.

Spermatid. The male germ cell after the second maturation division but
before its transformation into a spermatozoon.

Spermatocyte. The developmental stage of the male germ cell during
the growth period.

Spermatogenesis. The development of a mature sperm from a primordial
germ cell.

Spermatogonium. One of the products of the division of the male pri-
mordial germ cell.

Spontaneous generation. Same as abiogenesis.

Static. Pertaining to a body at rest or in equilibrium.

Stimulus. Any disturbing influence producing a reaction in an organism.

Stratum. A layer.

Striation. Striping, such as the cross marks of skeletal muscle.

Symbiosis. The association of two species of organisms for mutual benefit.

Synkaryon. The fertilization nucleus formed by the union of male and
female germ nuclei.

Systematise A student of taxonomy. An expert in classification.

Tactile. Pertaining to the sense of touch.

Taxonomy. The science of classification.

Theory. A general principle serving as a plausible explanation of phe-
nomena, supported by facts and by relevancy of reasoning.

Thorax. In mammals, the anterior division of the coelom separated from
the abdomen by the diaphragm and containing the heart and the lungs.
In arthropods, the middle body region.

Thyroidectomy. The surgical removal of the thyroid gland.
Tissue. A group of histologically similar cells.
Traumatic. Due to a wound or injury.

LltRARYJSa

476 GENERAL ZOOLOGY

Triploblastic. Having three germ layers.

Umbilical cord. A ropelike connection between the mammalian embryo

and the placenta carrying embryonic blood vessels, by means of which

the nutritive and respiratory needs of the embryo are supplied. The

umbilicus or navel on the abdomen of the adult marks the point of

attachment of the cord.
Unguiculate. Provided with claws.
Ungulate. Provided with hoofs.
Unit character. A hereditary trait that maintains its integrity from

generation to generation.
Ureter. The duct leading from the kidneys to the cloaca, or to the urinary

bladder.
Urethra. The duct leading from the urinary bladder to the exterior.
Urine. The secretion of the kidney consisting of water and other catabolic

products.
Urinogenital. Pertaining to the excretory and reproductive systems.
Uterus. In mammals, a specialized portion of the oviduct or oviducts in

which the placenta forms and the embryo develops. In the frog, a

portion of the oviduct in which eggs are held before deposition.
Vacuole. A small cavity or space in a cell containing fluid or other

substances.
Vagina. The terminal portion of the female genital tract leading from the

uterus or oviducts to the exterior.
Vascular. Pertaining to blood vessels, or blood supply.
Vas deferens. A duct in the male leading from the testis to the exterior.
Vasoconstriction. The constriction of blood vessels brought about by

the contraction of muscles in the walls of the blood vessels.
Vegetal, vegetative. Nutritive. Said of the pole of the egg in which

the yolk is concentrated.
Venous blood. Deoxygenated blood.

Vertebrate. An animal with a backbone or vertebral column.
Viscera. Internal organs. In mammals, the contents of the abdomen.
Vitalism. The doctrine that living phenomena can only be explained by

assuming the presence in protoplasm of a "vital" factor of some sort in

addition to the factors made up of the chemical and physical properties

of protoplasm.
Viviparous. Reproduction by birth.
Voltaic pile. A vertical series of alternating disks of two dissimilar metals,

as zinc and copper, separated by disks of paper moistened with acid

water. When the top and bottom are connected by a wire, a current

of electricity is produced.
Zoom. One of the members of a hydroid colony.
Zygote. The fertilized egg, or the individual produced from the egg.

Exconjugants are also zygotes.

INDEX

Names of genera and species are printed in italics; names of higher ranks,
such as families, orders etc., in capitals and small capitals; all other terms
and proper names in roman type. Numbers refer to pages.

Abducens nerve, 179
Abomasum, 98
Absorption, 115

carbohydrate, 117

fat, 118

gastric, 115

large intestine, 119

protein, 115

small intestine, 115

ACANTHOCEPHALA, 409

Acanthometra, 375, 376
Accommodation, 196
Acetabulum, 67, 69
Achorutes, 428
Achroodextrin, 111
Acicula, 417
Acipenser, 448

ACNIDOSPORIDIA, 380

Acoela, 400

Acquired characters, 265, 266

Acromegaly, 206

Acromium, 67

Actinopoda, 375

Actinospherium, 375, 376

Adaptation, 324-358

Adductor brevis muscle, 81

Adductor longus muscle, 80

Adductor magnus muscle, 80

Adhesive cells, 398

Adrenal gland, 39, 147, 213, 214

Adrenalin, 213

Adrenotropic hormone, 208

Afterbirth, 163

Agamic reproduction, 154

Agglutination, 300

Agkistrodon, 338, 452
Albumin, 109

Alcohol, effect on germ cells, 267
Alimentary canal, 85

Euplanaria, 86

frog, 32, 95, 100, 102

grasshopper, 87

human, 96, 99

Hydra, 86, 392

vertebrate, 88
Alimentation, 85-119
Allelomorph, 270
Alligator, 451
Alopex, 334
Alveolar cells, 103
Amaroudum, 442
Ambystoma, 450

locomotion of, 70

coloration of, 333
Ambystoma maculatum, 450
Ambystoma microstomum, 70
Ambystoma opacum, 450
Ameiurus, 449
Amia, 449
Amictic female, 410
Amino acid, 114
Amitosis, 224, 225
Ammophila, 356
Amoeba, 370-376
Amoebida, 370
Amphibia, 449
Amphigony, 154
Amphineura, 433
Amphioxus, 443
Amplexus, 154, 161
Ampulla, 200, 436
Amylase, 33, 107

477

478

GENERAL ZOOLOGY

Amylopsin, 33
Anabolism, 9
Analogy, 261
Anasa tristis, 430

sex chromosomes, 231

sex inheritance, 275
Anax, 429
Angulare, 59
Animal associations, 339
Animal communities, 360-363
Animal distribution, 363-366
Animal kingdom, 367-476
Annelida, 414
Anodonta, 434
Anopheles, 378, 413
Anoplura, 430
Antelope, 338
Anthozoa, 397
Antibody, 299
Antilocarpa, 338
Antitoxin, 351
Antlers, 50
Ants, 345, 432
Anura, 450
Aorta, 124, 132
Aphid, 430
Aphis, 430
Apis mellifica, 432

sting of, 328, 329
Apoda, 450
Appendages, paired, 68, 255

unpaired, 62
Appendicular skeleton, 55
Appetite secretion, 112
Aqueous humor, 194
Arachnida, 422
Arachnoidea, 422
Arbacia, 439
Archaeopteryx, 453, 454
Archenteron, 248
Archeocytes, 387
Archiannelida, 416
Architeuthis, 435
Argiope, 422
Argonauta, 435
Aristotle, 311
Arrectores pilorum, 49
Arterial arches, 124

Arteries, 121-125
Arthropoda, 420
Artificial parthenogenesis, 437
Artiodactyla, 456
Ascaris, 407, 408

chromosomes in, 231

cleavage in, 235
Ascon sponge, 386
Ascorbic acid, 110
Aspidiotus, 430
Asterias, 436, 438

ASTEROIDEA, 437

Astragalus, 70

Atlas vertebra, 61

Atta, communalism of, 345

Auditory function, 202

Auditory ganglion, 178, 252

Auditory meatus, 202

Auditory nerve, 179

Aurelia, 395

Autonomic nervous system, 181-184

Aves, 452

Axial skeleton, 54

frog, 55

larval salamander, 63
Axis cylinder, 169

B

Babesia, 422

Bacterial digestion, 119

Bagg, H. J., 268

Balanoglossus, 441

Balanus, 422

Basal metabolism, 109

Basilar membrane, 202

Basilar papilla, 199

Basilarchia, mimicry of, 336, 337

Basophile leucocytes, 138

Bathybic animals, 361

Bdellostoma, 446

Bees, 432

Beetles, 431

heart of, 427
Beriberi, 110
Beroe, 399
Biceps muscle, 81
Bicuspid valve, 131

INDEX

479

Bidder's canal, 149
Bile, 33, 102, 103

capillaries, 102
Binomial nomenclature, 24
Biogenesis, 258, 293
Biology, 1
Birds, 452

Bivalent chromosomes, 230
Bladder, frog, 32, 39, 150

human, 151
Blastomeres, 246
Blastopore, 248
Blastula, 247
Blind spot, 195
Bliss us, 430
Blood, 134-138

frog corpuscles, 135, 136

human corpuscles, 137
Blood groups, 300
Blood platelets, 138
Blood vessels, 121-129

arteries, 121

capillaries, 121

endothelium, 121

of frog, 129

perithelium, 122

small intestine, 116

veins, 121

vertebrate, 120
Bombus, 432
Bone, 73, 74

Bones, vertebrate skeleton, 56-73
Bony labyrinth, 200
Brachiopoda, 412
Brain, 175

Branchial structures, 254
Branchiostoma, 444
Br aula, 431
Brooks, W. K., 358
Brown Leghorn, gonadectomy, 217,

234
Brown, Robert, 13
Brow spot, 28
Bryozoa, 411
Buffon, Comte de, 311
Bitfo, 331, 338, 451
Bufogin, 331
Bufotalin, 331

Bugs, 430
Bugula, 412
Bulbus cordis, 124
Busy con, 434
Butterflies, 431

Basilarchia, 336, 337

Danaus, 336-337

Kallima, 334-335

mimicry of, 334

C

Caecum, 102
Calcaneum, 70
Calcar, 70
Calcarea, 388
Calciferous glands, 419
Calories, 108
Calorimetry, 107
Calosoma, 431
Cambarus, 422
Camel, 366
Cancer, 422
Canine teeth, 91
Capillaries, bile, 102

blood, 121
Capon, 234
Carbohydrate, 7
Carbohydrate metabolism, 117
Carbon dioxide, 140
Carcharias, 447
Carchesium, 383
Carcinonemertes, 405
Cardiac glands, 98
Caretta, 452
Carnivora, 456
Carotid gland, 127
Carpus, frog, 69
Cartilage, 73

Meckel's, 59
Casein, 109
Cat skeleton, 62
Catabolism, 9
Catfish, 449

Felichthys, 356, 357
Caudata, 450
Cavum aorticum, 125
Cavum pulmocutaneum, 125

480

GENERAL ZOOLOGY

Cells, 13

adhesive, 398

alveolar, 103

blood, 135, 136

chromaffin, 213

chromophile, 213

ciliated, 51

division, 219

endothelial, 16

germ, 219, 237

goblet, 93

granular, 98

islet, 103, 104

mitosis in, 220

nerve, 167

pigment, 16, 43

plain muscle, 95

somatic, 219

spindle, 136

structure of, 14
Cement substance, 89
Centipedes, 423
Central nervous system, 184
Centrioles, 220
Centrosome, 220
Centrum, 60
Cephalization, 170
Cephalopoda, 434
Cephalothorax, 421
Ceratium, 368, 369
Cercaria, 402
Cerebellum, 175
Cerebral hemispheres, 175
Cerebratulus, 405
Cerebrospinal fluid, 177
Ceruminous glands, 48
Cestoidea, 403
Cestus, 399
Cetacea, 456
Chaetoderma, 434
Chaetognatha, 414
Chaetogordius, 416
Chaetopleura, 434
Chelydra, 452
Chick embryo, 259
Child, C. M., 268
Childia, 400
Chilopoda, 424

Chiroptera, 455
Chlorohydra, 395

symbiosis of, 347
Choanocytes, 385, 387
Choloepus, mimicry of, 347
Chondrocranium, 59
Chondrostei, 448
Chordae tendineae, 124
Chordata, 440-457
Choroid fissure, 257
Choroid layer, 193
Choroid plexus, 177
Chromaffin cells, 213, 215
Chromatin, 220

diminution of, 235, 236
Chromatophores, 43, 47, 368, 369
Chromophile cells, 213
Chromosomes, 15

Anasa tristis, 231

Ascaris, 231, 235

bivalent, 230

crossing over of, 280-283

diminution of, 235

disjunction of, 230

Drosophila, 233

in gametogenesis, 229

heterochromosome, 232

maps of, 283, 285

in meiosis, 230

in Mendelian heredity, 272, 273

number, 225, 230, 245

odd, 232

qualitative division of, 230

quantitative division of, 230

Rana, 231, 245

sex, 231, 232

theory of heredity, 263

X, 232, 234, 275, 276

Y, 232, 234, 275, 276
Chrysemys, 452
Chyle, 118
Chyme, 113
Cicada, 430
Cilia, 368

ClLIATA, 380
ClLIOPHORA, 380

Cimbex, 432
Ciinex, 430

INDEX

481

Ciona, 294, 442
Circulatory system, 120-143

dogfish, 129

frog, 129, 130
Cisterna magna, 36
Citellus, hibernation of, 352, 353
Classification, scheme of, 24
Clavicle, frog, 66
Claws, 48
Cleavage, 246

Ascaris, 235

frog, 247

holoblastic, 246

hydrolytic, 104

meroblastic, 246
Clitellum, 158
Cloaca, 33
Cnidocil, 390
Cnidosporidia, 380
Coagulating enzyme, 107

COCCIDIOMORPHA, 378

Coccidium, 377, 378
Coccinella, 431
Coccygeoiliacus muscle, 78
Coccygeosacralis muscle, 78
Coccyx, 62
Cochlear duct, 199
Cockroach, 429

regeneration in, 349
Codosiga, 369, 370
Coelata, 400
coelenterata, 388
Cold receptors, 188

COLEOPTERA, 431
COLLEMBOLA, 428

Colloidal state, 6

Colon, 102

Color blindness, 279, 280

Coluber, 452

Columella, 202, 256

Commensalism, 345, 346

Communalism, 340-345

Communities, animal, 360-362

Compound eye, 192

Cones, 194

Conjugation, Paramecium, 381

Contractile vacuole, 368

Copperhead, 338

Copulation, earthworm, 158
Coracohumeralis muscle, 76
Coregonus, 449
Corium, 44
Corocoid, 66
Coronoid process, 59, 67
Corpus luteum, 217
Corpuscles, 134

frog, 135

human, 137

Krause, 188

Pacini, 188

Ruffini, 188
Corti, organ of, 202
Cortin, 215
Crab, hermit, 347

regeneration of, 349
Cranial nerves, 177
Cranium, frog, 56

human, 58
Creatinine, 8
Cretaceous time, 365
Cretinism, 211
Cricket, 429
Crinoidea, 437
Cristae acusticae, 200
Cristatella, 412
Crocodile, 451
Crocodylus, 451
Cro-Magnon man, 307
Crossing over, 280-283
Crossopterygii, 448
Crotalus, 330, 452
Crus, 81
Crustacea, 421
Cryptobranchus, 450
Ctenocephalis, 432
Ctenoid scale, 448
Ctenophora, 397
Cucumaria, 440
Cuticle, 51
Cuticula dentis, 89
Cuvier, Georges, 324
Cycloid scale, 448
Cyclops, 422
Cyclostomata, 445
Cysticercus, 349
Cytokinesis, 19

482

GENERAL ZOOLOGY

Cytology, 12
Cytopharynx, 368
Cytoplasmic heredity, 264, 286
Cytosome, 14
Cytostome, 380, 381

D

Dalyellia, 400

Danaus, mimicry of, 336, 337

Darwin, Charles, 311, 312, 325

evolution theory of, 323
Darwin, Erasmus, 311
De Vries, H., 319, 320
Deaminization, 115, 116
Deltoideus muscle, 76
Dendrobates, 331
Dendronotus, 434
Dentale, 59
Dentalium, 434
Dentine, 49, 89
Dentition, 91

Depressor maxillae inferioris mus-
cle, 78
Dermoptera, 455
Dextrin, 111
Dextrose, 104
Diabetes mellitus, 117
Diastatic enzymes, 106
Diastole, 127
Diapheromera, 429

mimicry, 335
Diapophysis, 64
Dichogamy, 154
Didelphys, 455

feint of, 339
Diencephalon, 175
Difflugia, 322, 374, 376

DlGENEA, 402

Digestion, 103

bacterial, 119

extracellular, 85

gastric, 112, 113

intracellular, 85

large intestine, 119

salivary, 111

small intestine, 113, 114
Digestive enzymes, 106

Digitigrade, 71

Dioecious, 153

Diplocentrus, 422

Diplopoda, 423

Dipnoi, 449

Diptera, 431

Disease, inheritance of, 288

Disjunction, 230, 264

Dissepiment, 146, 415

Distribution, 359, 363, 366

Dogfish, circulation, 129

Dolichoglossus, 441

Dominance, 273

Drosophila, chromosomes of, 233

sex inheritance in, 275

sex-linked inheritance in, 277, 278,
282
Duodenum, frog, 33, 100

human, 100
Dutrochet, R.S.H., 13
Dytiscus, 431

E

Ear, external, 202

inner, 199, 256

middle, 202

stones in, 200
Earthworm, 418

calciferous glands, 419

clitellum, 158

cuticle, 51

nephridia, 147

nervous system, 171, 172

reproductive system, 157, 158
Ecdysis, 42, 52, 428
Echidna, 454
Echinarachnius, 439
echinodermata, 435
Echinoidea, 439
Echinorhynchus, 409
Ecology, 3, 363
Ectoderm, 250
Edentata, 455
Effectors, 165
Egg, frog, 247

human, 237

Hydra, 156

INDEX

483

Egg, Nereis, 240

starfish, 240
Elasmobranchii, 447
Electric organ, 331, 332

ray, 447
Electroplax, 331
Eleidin, 47
Embryo, chick, 259

frog, 252

human, 295

shark, 162
Enamel, 89
Enchytraeus, 418
Endamoeba, 370
Endemic goiter, 211
Endocrines, 205
Endoderm, 248, 253
Endolymph, 200
Endomixis, 282, 383
Endoskeleton, 54
Endothelium, 121
Ensiform process, 68
Entelechy, 12
Enterokinase, 106
Enteropneusta, 440
Environment, 288, 359
Enzymes, 105, 106

coagulating, 107

diastatic, 106

digestive, 106

inverting, 107

lipolytic, 107

proteolytic, 106
Eoanthropus dawsoni, 307
Eohippus, 304
Eosinophile, 138
Epargyreus, 431
Ephemera, 429
Ephemeroptera, 428
Epidermis, 42, 43
Epinephrine, 213
Episternum, 67
Epithelium, 42
Epizoanthus, 397
Equilibration, 203
Equus, 305
Erepsin, 106
Eretmochelys, 452

Ergosterol, 111
Erythrocyte, 137
Erythrodextrin, 111
Esophageal glands, 32
Esophagus, 94-96
Eubranchipus, 421
Euciliata, 381
Eucypris, 422
Eugenics, 287-289
Euglena, 368, 369

eyespot in, 353
Eupagurus, symbiosis of, 346, 347
Euplanaria, 86, 400
Euplectella, 388
Eurycea, 450
Eustachian tube, 29
Eutheria, 455
Evolution, 3, 21, 290

horse, 304, 305

man, 306
Excretion, 144
Exhormone, 110
Extensor cruris muscle, 83
External ear, 202
External iliac vein, 130
Extracellular digestion, 85
Eye, 257

accommodation of, 196

compound, 192

development of, 257

human, 193, 194

muscles of, 197

Planaria, 192
Eyespot, 368

F

Facial nerve, 179
Factor, 263
Fangs, 330
Fasciola, 402, 403
Fat, 7, 118
Fat body, 160
Feathers, 48
Feather stars, 437
Feeble-minded, 288
Feint, 338, 339
Felichthys, 356

484

GENERAL ZOOLOGY

Femur, 69
Fenestra ovalis, 201
Fenestra rotunda, 201
Fertilization, 239

frog, 244

Nereis, 242, 243
Fertilizin, 239
Fibiale, 70

Fibrinogen, 33, 116, 134
Fibula, 70
Fishes, 447
Flatworms, 399-405
Fleas, 431
Flemming, W., 19
Flies, 431
Flounder, 334
Flustra, 412
Food, 109
Foodstuff, 107, 109
Food vacuole, 368
Foramen magnum, 56
Foramen of Monro, 176
Foramen ovale, 56, 201
Foramen rotunda, 201
Foraminifera, 375
Forelimb, 69
Fossils, 302
Fovea centralis, 195
Fresh-water animals, 262
Frog, 449

adrenal body, 147

alimentary canal, 32, 33, 94-97

amplexus, 161

bladder, 32, 39, 150

blood, 135, 136

blood vascular system, 129, 130

brow spot, 28

chromatophores, 43

development, 243-252

ear, 256

fat body, 160

gill clefts, 254

gills, 254, 255

glands, 44, 45

glottis, 30

heart, 122, 127

hyoid skeleton, 59

Frog, kidney, 147

liver, 30, 31, 33

lung, 34

lymphatic system, 32, 36

metamorphosis, 40, 258

nephrostome, 147

nervous system, 178, 251

ovary, 38, 39

pancreas, 33

pleuroperitoneal cavity, 30

reproduction, 38, 159

skeleton, 56-60

skin, 27, 28, 43

teeth, 29, 89

testis, 39

urinogenital organs, 160

vocal cords, 35
Frontoparietal, 56

G

Galeopithecoidea, 455
Gall bladder, 33
Gall insect, 430
Gametes, 154

law of segregation, 270
Gametogenesis, 219, 225, 226
Ganglion, 168, 252
Ganglionic nervous system, 170
Ganoidei, 448
Ganoid scale, 448
Ganoin, 448
Garpike, 449
Gastric absorption, 115
Gastric digestion, 112-114
Gastric secretin, 112
Gastrocnemius muscle, 81
Gastrocoel, 248

Gastrohepatoduodenal ligament, 32
Gastropoda, 434
Gastrotricha, 411
Gastrovascular cavity, 85, 392
Gastrulation, 248, 249
Gegenbaur, Karl, 19
Gene, 263, 284, 286
Genetics, 3, 269-289
Geniohyoideus muscle, 75

INDEX

485

Genotype, 321
Geological map, 364, 365
Geological table, 303
Gephyrea, 419
Germ cells, 237, 267
Germ layers, 248
Germ ring, 248
Germ track, 265, 268
Gerris, 430
Gigantism, 206, 207
Gill, 254, 255

cleft of, 254

pouch of, 255
Gizzard, 98
Gland, 44
Glenoid fossa, 67
Globigerina, 375
Glomerulus, 148
Glossina, 370, 431
Glossopharygeal nerve, 179
Glottis, 30
Gluten, 109
Gluteus muscle, 81
Glycerin, 114, 115
Glycogenase, 107
Glycosuria, 216
Goblet cell, 93
Golgi material, 17
Gonad, 153, 216, 217
Gonadectomy, 217, 234
Gonadotropic hormone, 207
Gonionemus, 389, 395
Gordius, 409
Gorilla, skeleton, 310
Gram-calorie, 108
Grantia, 387, 388
Granular cells, 98
Grasshopper, 87, 330, 424, 429
Gravitational receptor, 198
Gray crescent, 244
Green hydra (Chlorohydra), 347
Gregarina, 375, 377
Gregarinina, 375
Growth, 11
Gryllus, 429
Guyer, M. F., 267
Gymnotes, 331

H

Hagfish, 445
Hair, 48, 49
Hairworm, 407
Half-spindle fibers, 223
Halteria, 383
Hare, 334
Hathrometra, 437
Haversian canal, 74
Heart, beetle, 427

frog, 122, 123

human, 131, 132

lymph, 36
Heidelberg man, 307
Heliozoa, 375
Helix, 434

Helodrilus, copulation, 158
Hemal rib, 63
Hemiptera, 430
Hemocoel, 87
Hemoglobin, 139
Hemolymph gland, 142
Henle's loop, 152
Hensen's line, 83
Hepatic duct, 33
Hepatic portal system, 131
Hepatopancreatic duct, 33
Heredity, 262-288
Hermaphroditic reproduction, 153
Hermit crab, symbiosis, 346 347
Heterochromosome, 232
Heterodont dentition, 91
Heterotrichida, 383
Heterozygote, 271
Hibernation, 40, 352, 352
Hindlimb, 69
Hippocampus, 152
Hippospongia, 388
Hirudin, 419

HlRTJDINEA, 419

Hirudo, 420
Histology, 2

bone, 73

cartilage, 73

corium, 44

epidermis, 42

skeletal muscle, 83

486

GENERAL ZOOLOGY

Hoactzin, 453
Holoblastic cleavage, 246
Holophytic nutrition, 368
Holostei, 448
Holothuria, 440

HOLOTHURIOIDEA, 439
HOLOTRICHIDA, 381

Holozoic nutrition, 368
Homodont dentition, 91
Homoiothermous, 352
Homology, 260, 292, 293
Homo neanderthalensis, 307
Homo sapiens, 305
Homoptera, 430
Homozygote, 271
Honeybee, 340

comb of, 342

development of, 343

organization of, 340

sting of, 328, 329
Hoof, 50
Hookworm, 407
Hormiphora, 398
Hormones, 114, 205-218
Horns, 50
Horse, distribution of, 366

evolution of, 306

spinal nerves of, 180

teeth of, 91
Humerus, 69
Hydatina, 86, 389, 391
Hydra, 86, 389, 392

egg of, 156

nematocysts of, 390

nervous system of, 169
Hydrolytic cleavage, 104
Hydrozoa, 394
Hyla, 333

Hymenoptera, 432
Hyoglossus muscle, 75
Hyoid apparatus, 59
Hyperoartia, 446
Hyperotreta, 445
Hyperthyroidism, 211
Hypoglossal nerve, 179
Hypophysis, 206

hormones of, 206-208
Hypostome, 391

Hypothyroidism, 211
Hypotrichida, 383
Hyracoidea, 456
Hyracotherium, 304

Ichneumon flies, 432

Ichthyophis, 450

Ictalurus, 449

Ileocaecal valve, 102

Ileum, 33

Iliofibulare muscle, 81

Iliopsoas muscle, 81

Ilium, 67

Immunity, 351

Incisor teeth, 91

Incus, 202

Inferior vena cava, 131

Infraspinatus muscle, 78

Infundibulum, pituitary gland, 206

Inheritance, acquired character, 265

color blindness, 279, 280

cytoplasmic, 286

disease, 288

feeble-mindedness, 288

mental ability, 287

sex, 275, 276
Inner ear, 199
Inorganic salts, 7
Inosite, 8
Insecta, 424
Insectivora, 455
Insulin, 117, 216

Integrative action of nervous sys-
tem, 203
Integument, 42

frog, 42

human, 45-48

invertebrate, 50-52
Intercellular bridges, 43
Intermaxillary glands, 93
Intermediate lobe, pituitary gland,

206, 209
Internal secretion, 205-218
Interzonal fibers, 223
Intestine, 32, 33
Intracellular digestion, 85

INDEX

487

Intrauterine development, 161
Inverting enzymes, 107
Iris, 194, 257
Irritability, 10
Islet cells, 103, 104

ISOPTERA, 429

Iter, of midbrain, 176

Jaws, 254

Johannsen, W., 322
Joints, 37
Jnliis, 424

K

Kallima, mimicry of, 334, 335
Kangaroo, 357
Karyokinesis, 19, 222
Karyolymph, 17
Keratohyaline granules, 46
Kidney, frog, 147
human, 150, 151

KlNORHYNCHA, 411

Knee jerk, 184
Krause's corpuscle, 188
Krause's membrane, 183

Labium, 87, 424
Labrum, 87, 424
Labyrinth, bony, 200

membranous, 200, 256
Lacbrtilia, 451
Lactase, 107

Lactogenic hormone, 208
Lagopus, 334

Lamarck, J. B., 1, 311, 325
Lamprey, 446
Laqueus, 413
Large intestine, 33

cat, 102

frog, 102

horse, 102

man, 102

Larva, 40

tadpole, 254-256

trochophore, 416

veliger, 433
Larynx, 32-35
Lateral dermal fold, 28
Latissimus dorsi muscle, 78
Lavine, 111

Law of biogenesis, 258, 293
Law of independent assortment, 270
Law of population, 313
Law of segregation, 270
Leech, 417

Leeuwenhoek, A. van, 367
Lens, 194, 257
Lepidoderma, 411
Lepidoptera, 431
Lepidosiren, 449
Lepisosteus, 449
Leptinotarsa, 431

amitosis, 225

egg, 227
Leptocardia, 442
Leptosynapta, 440
Lepus, 334
Lethocerus, 430
Leucocytes, 136-138
Leucon sponge, 386
Levator bulbi, 197
Libinia, 422

symbiosis of, 347
Lice, 430
Light, 353, 354

perception of, 195

reaction to, 353

receptors of, 191
Limax, 434
Limbs, 69, 70
Limulus, 422
Linea alba, 76
Linkage, 284
Linnaeus, C, 23
Lipase, 33, 107, 114
Lipolytic enzyme, 107
Lithobius, 377
Lithocyst, 198
Little, C. C, 268
Littoral animals, 361

488

GENERAL ZOOLOGY

Liver, 30, 31, 33, 102, 103

Liver fluke, 402

Liver rot, 403

Lizards, 451

Llama, 366

Loligo, 435

Longissimus dorsi muscle, 78

Longitudinal valve, 124, 125

Lophophore, 411

Lumbricus, 418

Lung, 34

Lungfish, 449

Lutra, 334

Lygaeus, 430

Lymnaea, 434

Lymph, 141

Lymph gland, 135

Lymph heart, 36, 133

Lymphatic system, 36, 133

vessels of, 116, 135
Lymphocytes, 138

M

M acranthorhynchiis, 409

Macrobiotus, 423

Macropus, 455

Maculae acusticae, 200

Madreporite, 436

Magellania, 413

Magicicada, 430

Malaclemys, 452

Malacostraca, 422

Malaria, 380

Malaria parasite, 379

Malleus, 202

Malpighian tubules, 87, 145, 420

Maltase, 107

Malthus, Thomas, 313

Maltose, 111

Malt sugar, 104

Mammalia, 454

Mammary gland, 47

Man, 305, 457

alimentary canal of, 96, 99

Cro-Magnon, 307-309

ear of, 199-203

evolution of, 306-311

Man, excretory organs of, 151

eye of, 194

heart of, 132

heredity of, 287-289

prehistoric, 304-309

skeleton of, 310
Mandible, 87
Manubrium, 68
Map, chromosome, 285

Cretaceous time, 365

Permian time, 364
Margaropus, 422
Marsupials, 454
Mastigophora, 368
Maxilla, 87
Maxillary bone, 58
Mayflies, 428
Mechanism, 12
Meckel's cartilage, 59
Medulla oblongata, 175
Medullated nerve, 168
Medusa, 389
Megarhyssa, 432
Meiosis, 230
Melanopas, 429
Melophagus, 431
Membrane bone, 73
Membranous labyrinth, 200, 256
Mendelian heredity, 269
Meninges, 176

Mental ability, inheritance, 287
Mentomeckelian, 59
Meroblastic cleavage, 246
Mesencephalon, 175
Mesentery, 31
Mesoderm, 249
Mesogaster, 32
Mesohippus, 304
Mesorchium, 159
Mesovarium, 160
Metabolism, 9

basal, 109

carbohydrate, 117

fat, 118

protein, 115
Metamorphosis, 40, 258, 427
Metatheria, 454
Metazoa, 384

INDEX

489

Metencephalon, 175
Metridium, 396, 397
Micrometer, 18
Micron, 18
Microstomum, 400
Mictic female, 410
Middle ear, 202, 256
Millipedes, 423
Mimicry, 334-337
Miracidium, 402
Mitochondria, 16, 17
Mitosis, 19, 220, 221
Mnemiopsis, 398, 399
Moisture, 354
Molar teeth, 91
Mollusca, 432
Molting, 42
Monoecious, 153

MONOGENEA, 400

Monomorium, 432
Morphology, 2, 4
Mosquito, 431
Moths, 431
Motor end plate, 190
Motor neuron, 168
Mouth, frog, 254

insect, 424
Mucilago, 375
Mucosa, 94, 101
Muller, H. J., 267
Musca, 431
Muscles, 75-84

cardiac, 122

eye, 197

head and trunk, of frog, 75-78

histology of, 83

leg, of frog, 81

plain, 95

thigh, of frog, 78-81
Mustelus, 447
Mutation theory, 319
Mya, 434, 435
Mycetozoa, 375
Myelencephalon, 175
Myenteric plexus, 95
Myocyte, 387
Myofibril, 82, 83
Myoneme, 380

Myriapoda, 423
Myxedema, 211
Myxine, 446
Myxobolus, 380

N

Nageli, Carl von, 19
Nails, 48
Nares, 29
Nasal bones, 57
Nasal cavity, 57
Natural affinities, 23
Natural selection, 313
Nautilus, 435
Navel, 163
Necator, 407
Necturus, 450
Nemathelminthes, 406
Nematoblast, 390
Nematocyst, 390
Nematoda, 406
Nematomorpha, 408
Nemertea, 404
Neoceratodus, 449
Neornithes, 454
Nephridia, 145
Nephridial tubule, 146
Nephridiopore, 146, 147
Nephrostome, 146, 147
Nereis, 24, 417, 418

fertilization of, 241, 243
Nerves, cranial, 177-180

spinal, 80
Nervous system, 165

autonomic, 181-184

central, 184

diffuse, 169

earthworm, 171-173

flatworm, 86

frog, 178

functional significance of, 185

ganglionic, 170

integrative action of, 203
Nervus terminalis, 180
Neural arch, 60
Neural spine, 60
Neural tube, 250, 251

490

GENERAL ZOOLOGY

Neurenteric canal, 251
Neuromuscular spindle, 188, 190
Newport, G., 19
Nictitating membrane, 28
Nitrogen equilibrium, 116
Noctiluca, 368

NoNCALCAREA, 388

Notochord, 62, 63, 249, 252, 440
Nuclearia, 374
Nucleolus, 17
Nuda, 399
Nutrition, 368

O

Obelia, 393, 395

Obliquus externus muscle, 76

Obliquus internus muscle, 76

Occipital condyle, 56

Octopus, 435

Oculomotor nerve, 177

Odd chromosome, 232

Odonata, 429

Odontoblast, 90

Oenothera lamarckiana, 319, 320

Oikopleura, 442

Oil gland, 47

Olecranon, 69

Olfactory epithelium, 191

Olfactory nerve, 177

Oligochaeta, 418

Oligotrichida, 383

Omasum, 98

Ommatidium, 192

Omosternum, 67

Onchorhynchus, 449

Ontogeny, 237

Onychophora, 423

Oocyte, 227, 228

Oogenesis, 226

Opalina, 381

Ophiura, 438, 439

Ophiuroidea, 439

Opossum, 339, 357

Optic cup, 257

Optic nerve, 177

Optic vesicle, 257

Oral cavity, frog, 29

mammal, 88
Oral glands, frog, 93

mammal, 93

man, 94
Organismal theory, 21
Ornithorhynchus, 454
Os innominatum, 68
Osphradium, 435
Ostrea, 434
Orthoptera, 429
Orthosympathetic nervous system,

183
Otic pit, 256
Otoconia, 200
Otter, 334
Ovary, 39, 155
Overproduction, 313
Oviduct, earthworm, 158

frog, 160

man, 238
Oviparous, 154
Ovipositor, 330
Ovotestis, 153
Ovum, frog, 160

Hydra, 155

man, 238
Oxygen, 139
Oxyhemoglobin, 139

Pacinian corpuscle, 188, 189
Paedogenesis, 421
Pain receptors, 187
Paired appendages, 68, 255
Palaemonetes, 422
Palatine, 57
Paleontology, 3, 301
Pancreas, 33, 103, 104, 215
Pancreatic secretin, 113
Papanicolaou, G. N., 267
Papilio, 431
Papirius, 428
Paracasein, 112
Parallel induction, 267
Paramecium, 381, 383
Parapodium, 417

INDEX

491

Parapophysis, 64

Parasitism, 348

Parasympathetic nervous system,

183
Parasphenoid, 56
Parathyroid gland, 212
Parenchyma, 399
Parietal organ, 176
Parotid gland, 94
Parotoid gland, 331
Parthenogenesis, 154, 437
Pectineus muscle, 81
Pectoral girdle, 65, 67
Pectoralis muscle, 76
Pediculus, 430
Pelagic animals, 361
Pelecypoda, 434
Pelvic girdle, 67, 68
Pelvic vein, 130
Pepsin, 32, 106
Perforatorium, 241
Pericardium, 30
Peripatus, 423
Perilymphatic fluid, 200
Periplaneta, 429
Perissodactyla, 457
Perithelium, 122
Peritrichida, 383
Permian time, 364
Peroneus muscle, 82
Petromyzon, 447
Phalanges, 69, 70
Phalangium, 422
Pharynx, 32
Phenotype, 321
Pholidota, 456
Phoronidea, 413
Phoronis, 414
Photiniis, 431
Photosynthesis, 10
Phrynosoma, 451
Phthirius, 430
Phyllium, 335
Phylloxera, 430
Physalia, 394, 395
Physiology, 2, 4
Phytomastigophora, 368
Pieris, 431

Pigeons, 235, 279, 298

Pigment, 354

Pineal organ, 29

Pinealis, 176

Pinna, 202

Pinnipedia, 456

Pisces, 447

Pithecanthropus erectus, 305

Pitocin, 209

Pitressin, 209

Pituitary gland, 176, 206, 207, 209

Placenta, 162, 423

Placodes, 252

Placoid scale, 49, 59, 90

Plain muscle, 95

Planaria, 192

Planocera, 50, 51, 400

commensalism of, 346

polar bodies of, 229
Plantigrade, 71
Plasma, 134
Plasmodium, 378
Plasmodroma, 368
Platyhelminthes, 399
Platypus, 454
Pleurobrachia, 399
Pleuroperitoneal cavity, 30
Plumatella, 412
Poikilothermous, 351
Poison gland, 28
Polar bear, 334
Polar body, 228, 229
Polocyte, 228

POLYCHAETA, 416
POLYCLADIDA, 400

Polygordius, 416
Polyneuritis, 110
Polynices, 434
Polyodon, 448
Polyp, 389
Polypterus, 448
Polystoma, 405
Porcellio, 422

PORIFERA, 385

Porthetria, 431

Posterior pituitary lobe, 206, 209
Potato beetle, 431
amitosis in, 225

492

GENERAL ZOOLOGY

Potato beetle, egg of, 227

Praying mantis, 429

Precava, 126

Precipitin tests, 300

Prehallux, 70

Prehistoric Man, 308

Premaxillary, 58

Premolar teeth, 91

Pressure, 355

Primates, 455

Pristis, 448

Proboscidea, 456

Proctociliata, 381

Progestin, 217

Proglottid, 349, 405

Prootic, 56

Proprioceptors, 188

Prorennin, 107

Prostoma, 405

Prostomium, 415

Protective coloration, 333, 334

Protein, 7

Proteolytic enzymes, 106

Proteomyxa, 374

Proteus, 326, 450

Protohippus, 305

Protonephridia, 144

Protoparce, 431

Protoplasm, 2, 5, 7

doctrine, 5
Protopterus, 449
Protorohippus, 304
Prototheria, 454
Protozoa, 367
Proventriculus, 98
Provitamin, 111
Psalterium, 98
Pseudoconjugation, 376
Pseudopodia, 368
Ptarmigan, 334
Pterygoid, 57, 58
Ptyalin, 94, 106, 112
Pulex, 432

Pulmocutaneous artery, 124
Pulmonary aorta, 132
Pupil, 194
Pure line, 321, 322
Putrefaction, 119

Pygostyle, 62
Pyloric glands, 98
Pyloric valve, 100
Pylorus, 32
Pyriformis muscle, 81

Q

Quadratojugal, 58
Qualitative division, 230
Quantitative division, 230

R

Rabbit, 33, 338

Race preservation, 355

Radiale, 69

Radiolaria, 375

Radioulna, 69

Radius, 70

Rata, 101

Raji, 447

Rami communicantes, 181

Rana, 450

(See also Frog)
Rattlesnake, 330, 332
Ray, 447

Receptors, 165, 187-189, 198
Rectum, 102

Rectus abdominis muscle, 76
Rectus femoris anticus muscle, 81
Rectus internus major muscle, 80
Rectus internus minor muscle, 80
Red corpuscles, 137
Redia, 402

Reflex action, 183, 184
Regeneration, 349, 350
Reissner's membrane, 202
Remora, 345, 346
Renal artery, 130
Renal portal vein, 130
Renal vein, 130
Rennin, 107
Reproduction, 163

agamic, 154

dioecious, 153

earthworm, 157

frog, 38, 159

INDEX

493

Reproduction, Hydra, 154

monoecious, 153

parthenogenetic, 154

Planaria, 154

Protozoa, 11
Reptiles, 451
Reptilia, 451
Respiration, 45, 139
Respiratory movements, frog, 34
Reticulitermes, 430
Reticulum, 98
Retina, frog, 257

human, 195

insect, 192
Retractor bulbi, 28, 197
Rhabdites, 51, 399
Rhabdocoelida, 400
Rhabdome, 191, 193
Rhabdopleura, 441
Rhineura, 451
Rhizopoda, 370
Rhodopsin, 197
Rhomboidichthys, 334
Rib, frog, 60

hemal, 63

pleural, 63
Rickets, 110
Roaches, 429

ommatidium of, 192
Rodentia, 455
Rods of retina, 194
Rotatoria, 409
Rotifers, 409, 419
Ruffini, corpuscle of, 188
Rumen, 98

S

Sacculus, 199

Sagitta, 414

St. George, La Vallette, 19

Salamanders, 449

color adaptation of, 326, 327, 333
Salienta, 450
Salivary digestion, 111
Salivary glands, 94
Salmon, 449
Salpa, 442

Samia, 431
Sand dollars, 439
Saprophytic nutrition, 370
Sarcocystis, 380
Sarcodina, 370
Sarcolemma, 83
Sarcomere, 84
Sarcoptes, 422
Sarcostyle, 83
Sartorius muscle, 78
Sawfish, 447
Sawflies, 432
Scala media, 199, 202
Scales, 49, 50, 90, 448
Scaphopoda, 434
Scapula, frog, 66

human, 67, 68
Sceloporus, 451
Schleiden, M. J., 13
Schwann, T., 13
Schweigger-Seidel, O., 19
Sclera, 193
Sclerotic coat, 257
Scolex, 404
Scolopendra, 424
Scute, 448
Scyphozoa, 396
Sea cucumber, 440
Sea lily, 437
Sea lion, teeth, 91
Sea pork, 442
Sea urchin, 439
Sebaceous gland, 47
Secondary sexual characters, 216
Sedimentary rock, 302
Segmentation, 420
Segmentation cavity, 247
Selection, artificial, 296-298

efficacy of, 321

hypothetical effect of, 318

natural, 313

pure line, 321, 322
Semimembranosus muscle, 81
Seminal receptacle, 158
Seminal vesicle, 157, 159
Semitendinosus muscle, 80
Sensation, 165
Sense organs, 186

494

GENERAL ZOOLOGY

Septum bulbi, 125

Septum interaorticum, 126

Septum principale, 126

Serous membrane, 31, 94

Serpentes, 451

Serum, 134

Serum albumin, 134

Serum globulin, 134

Serum tests, 299

Sex chromosomes, 231, 232

Sex determination, 233-235

Sex inheritance, 275

Sex-linked inheritance, 276-280

Shallow-water animals, 362

Sharks, 447

embryos, 162
Sheep tick, 431
Sigmoid flexure, 102
Signals, 338
Sinanthropus pekinensis, 305, 307

SlPHONAPTERA, 431

Siren, 450
Sirenia, 456
Size, 11
Skeleton, action, 84

appendicular, 55

axial, 54, 55

cat, 62

fowl, 72

frog, 66

gorilla, 310

human, 310
Skin, frog, 27

human, 45, 46

secretion of, by toad, 331
Skull, frog, 56-58

human, 58, 310
Slime tube, 158

Sloth, two-toed, symbiosis of, 347
Slugs, 434
Small intestine, absorption, 115

blood vessels of, 116

digestion in, 113, 114

frog, 32, 100

mucosa of, 101
Smell receptors, 189
Smith, E. A., 267

Snails, 434
Snakes, 451

adaptations, 338

tongue of, 93
Somatic mitosis, 220
Somatotropic hormone, 206
Somite, 250
Sound receptor, invertebrate, 198

vertebrate, 199
Species, 23, 290
Spermatocyte, 228
Spermatogenesis, 228
Spermatogonia, 228
Spermatozoa, 239
Sphenethmoid, 57
Sphenodon, 451
Spider crab, 347
Spinal accessory nerve, 179
Spinal cord, 175, 181
Spinal ganglia, 251
Spinal nerves, 180
Spindle cell, 136
Spiracle, 255, 426
Spiral valve, intestine, 101
Spireme, 221
Spleen, frog, 32, 33

rat, 142
Sponges, 386, 388
Spongilla, 388
Sporozoa, 375
Springtails, 428
Squamosal, 57
Squamous epithelium, 42
Stagomantis, 429
Stapes, 202
Starfish, 240, 436, 437

regeneration, 349, 350
Statocysts, 198, 435
Stearic acid, 114
Stenson's duct, 94
Stentor, 384
Sterna, 339
Sternalis muscles, 76
Sternohyoideus muscle, 76
Sternoradialis muscle, 76
' Sternum, frog, 65, 67

human, 65

INDEX

495

Stimulus, 166

Sting of honeybee, 328, 329
Stockard, C. R., 267
Stomach, fish, 97

fowl, 97

frog, 32

human, 99

ruminant, 98
Strasburger, E., 19
Stratified epithelium, 42
Stratum corneum, 42, 46
Stratum germinativum, 42, 46
Stratum granulosum, 46
Stratum lucidum, 46
Strobilation, 396
Structural protein, 17, 18
Structure, 3

Struggle for existence, 313
Stylonychia, 383, 384
Subclavian vein, 142
Sublingual gland, 94
Submaxillaris muscle, 75
Submaxillary gland, 94
Submentahs muscle, 75
Submucosa, 94
Submucosal plexus, 95
Subterranean animals, 362
Suckfish, 345
Sucrase, 107
Suctoria, 384
Superior vena cava, 131
Suprascapula, 66
Surface animals, 363
Survival of fittest, 314
Suspensorium, frog skull, 57
Sweat gland, 47
Sycon sponge, 386
Sycotypus, 345
Sygnathus, 152
Sylvilagus, 333, 338
Symbiosis, 346, 347
Sympathetic nervous system, 183
Synapse, 185
Synapsis, 230
Syphilis, 288
Systemic aorta, 132
Systole, 127

Tactile organs, 187
Taenia, 348, 349, 403, 404
Tapeworm, 348, 403
Tarsometarsal, 73
Taste bud, 191
Taste receptors, 189
Taxonomy, 3, 22
Teeth, frog, 29, 89

mammal, 89-93
Telencephalon, 175
Teleostei, 449
Teleostomi, 447
Telolecithal egg, 246
Telosporidia, 375
Temora, 421
Temperature, 351, 352

regulation of, 140
Temporalis muscle, 78
Tendo Achillis, 82
Tendon spindle, 188
Tenebrio, 431
Tentaculata, 397
Terebratulina, 413
Termites, 430
Tern, feint of, 339
Terrapene, 452
Terrestrial animals, 360, 363
Testacea, 374
Testis, earthworm, 157

frog, 39
Testudinata, 452
Thalarctos, 228
Thalassema, 419
Thamnophis, 452
Theelin, 217
Thoracic duct, 118, 141
Thrombin, 135
Thrombogen, 135
Thymus glands, 212
Thyone, 440

Thyroid gland, 209, 210
Thyrotropic hormone, 208
Thyroxine, 210
Tibia, 70
Tibiale, 70
Tibialis anticus muscle, 82

496

GENERAL ZOOLOGY

Tibialis posticus muscle, 83
Tibicen, 430
Tibiofibula, 69
Tibiotarsal, 72
Toad, 331, 338, 449
Tokophrya, 384
Tongue, 93
Torpedo, 331, 332, 447
Trachea, 210, 426
Tracheal system, 426
Trachydemus, 411
Transverse colon, 102
Transverse iliac vein, 36
Transverse process, 60
Trematodes, 400
Trepang, 440
Treviranus, G. R., 1
Triceps brachii muscle, 78
Triceps femoris muscle, 81
Trichinella, 406, 407
Trichocyst, 380
Tricladida, 400
Tricuspid valve, 131
Trigeminal nerve, 179
Trinomial nomenclature, 24
Tristearin, 114
Tristoma, 402
Trochelminthes, 409
Trochlear nerve, 179
Trochophore larva, 146
Trombicula, 423
Trout, 449

Truncus arteriosus, 124-126
Truncus impar, 124
Trutta, 448, 449
Trypanosoma, 369, 370
Trypoxylon, 432
Trypsin, 33, 106
Trypsinogen, 106
Tsetse fly, 431
Tubectomy, 289
Tuberculosis, 288
Tuberculum prelinguale, 30, 35

TUBULIDENTATA, 455

Tunica muscularis, 94
Ttjnicata, 441, 442
Turbellaria, 399, 401
Turtles, 452

Twixt brain, 175
Tympanic cavity, 29
Tympanic membrane, 29, 256
Typhlosole, 146
Typhlotriton, 327

U

Ulna, 70

Ulnare, 69

Umbilical cord, 162

Uncinate process, 65

Unguligrade, 71

Unity of the organism, 20

Urea, 8, 116

Ureter, 39

Urethra, 151

Urine, 117

Uriniferous tubule, 148, 149

Urinogenital organs, 160

Urodela, 450

Urostyle, 39

Uterus, 39, 163, 423

Utriculus, 199

Vacuoles, 368

Vagus nerve, 179

Valves, 100, 101, 124, 131, 133

Variation, 291, 315-317

Vas deferens, 157, 289

Vasectomy, 289

Vas efferens, 39, 159

Vastus externus muscle, 81

Vastus internus muscle, 81

Veins, 36, 121, 130, 133, 142

Veliger larva, 433

Ventral nerve cord, 173

Venus, 434

Vermiform appendix, 102

Vertebra, 61, 63, 64

Vertebral column, 60

Vertebrata, 444

Vespa, 432

Vestibulum, 201

Villus, 116

Virus, 290

INDEX

497

Visceral peritoneum, 30, 94
Visceral skeleton, 56
Vitalism, 12
Vitamins, 109-111
Vitelline membrane, 244
Vitreous humor, 194
Viviparous, 154, 162
Vocal cord, 35
Vocal sac, 30
Volvox, 368, 369
Vomerine teeth, 29
Vorticella, 383, 384

Wharton's duct, 94
Whitefish, 449
Wild boar, 296

X

X chromosome, 232, 234, 275, 276
Xenopsylla, 432
Xiphisternum, 67
Xiphosura, 422

W

Walking stick, 335, 429
Wallaby, 358
Warmth receptors, 188
Warning adaptations, 337, 338
Wasps, 356, 432

wing muscle, 92
Water, 45

environment, 359
Water vascular system, 436
Weapons, 328-332

Y chromosome, 232, 234, 275, 276
Yolk, 237
Yolk plug, 248

Z

Zoogeography, 3

ZOOMASTIGOPHORA, 370

Zygapophyses, 61
Zygote, 154
Zymogen, 105, 106