mdls and Man
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Kinsliips
or Animals and M
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By ANN H. MORGAN, Ph.D.
Mount Holyoke College
McGRAW-HILL BOOK COMPANY, Inc.
New York Toronto London
1955
KINSHIPS OF ANIMALS AND MAN
Copyright © 1955 by the McGraw-Hill Book Company, Inc.
Printed in the United States of America. All rights re-
served. This book, or parts thereof, may not be repro-
duced in any form without permission of the publishers.
Library of Congress Catalog Card Number 55-6859
To
Enzabeth Adams
Professor oi Zoology
m
Mount Holyoke College
Pref
ace
Both living and non-living things are composed of strikingly similar sub-
stances. The living ones are not only composed of like substances but all
living matter is put together in the same unique way. Kinships based on
these similarities form the central theme of this book. The author's experience
has indicated that the study of these relationships makes zoology more vital
and interesting to the student.
The first chapter, "Relationships of the Living World," presents the gen-
eral plan of the text and is its introduction. Part I, "The Foundation," tells
of matter and energy as they occur in plants and animals, and familiar
natural processes. It includes a discussion of atoms and molecules and
references to the newer knowledge concerning them. This leads logically to
the cell, as a sample of the complete organization of living matter and the
focus of a great wave of contemporary investigation.
Relationships are persistently evident in our world and universe. Many
of them are suggested in the Kinships of Animals and Man, and they are
emphasized in the special discussions of Part II, "Ecology." Among these
relationships are the competitions and unconscious cooperations of animals,
the associations of animals in communities, and photosynthesis, the most im-
portant food-making process in the world.
Protoplasm must have water wherever it is. The inside of the animal body
is wet. Structures and functions of tissues and organs are affected by the
fact of their dependence upon fluid. In Part III, "The Internal Environment
of the Body," the main systems of the animal body, invertebrate and vertebrate
are studied with respect to their basic similarities. It is well known that modern
medicine is the internal ecology of the human body.
Traces of history remain in animals and parts of animals showing the
broken story of their evolution. "The Evolution of Animals" is a series of
chapters which recount the great contribution of one or another group. Above
all, evolution appears as a story of the continuity of life.
Vi PREFACE
'.'Evolution and Conservation," or applied ecology, belong together in a
final summary. Conservation is already taking its place as a part of Evolution.
I am grateful to all those who have made this work first of all a pleasure
to me. There are more of them than I can name here.
First are my colleagues in the Department of Zoology in Mount Holyoke
College; Professors A. E. Adams, E. M. Boyd, J. W. Kingsbury, C. Smith,
I. B. Sprague and K. F. Stein. Dr. Adams has been generous beyond my
telling in giving her time, scholarship and keen critical sense.
Others have read parts of the book and given suggestions that have greatly
helped. These are: Dr. H. M. Allyn, former Dean of Mount Holyoke Col-
lege; Dr. E. P. Carr, Professor of Chemistry, Mount Holyoke College; Dr.
M. P. Cloud, Librarian of Peabody Museum, Yale University; Dr. E. T.
Eltinge, Associate Professor of Plant Science, Mount Holyoke College; Dr.
M. P. Grant, Professor of Zoology, Sarah Lawrence College; Dr. E. K. Moyer,
Associate Professor of Anatomy, Medical School, Boston University; Dr.
F. A. Saunders, Professor of Physics, Harvard University.
The original drawings except a very few of my own have been made by
Shirley P. Glaser, Biological Artist of the Peabody Museum of Yale Uni-
versity. I have been fortunate in having the benefit of her ability and ex-
perience. Other illustrations, exclusive of my own photographs, are used
through the courtesy of those whose names are written beneath them. The
generosity of many authors in particular is a cause of my warm thanks.
There are many others whose interest and good wishes I have appre-
ciated. Among them are Andrew Bihun of the National Audubon House;
J. P. Hughes of the W. B. Saunders Company; my sister, Christine M. Kenyon;
and my brother, Stanley D. Morgan.
Finally I give my hearty thanks to those who have helped to make my
manuscript into a book. At first, Helen L. Goodwin, and later, Irene Moss-
man have typed the revisions with exacting accuracy.
Fortunately, the index of Kinships has been made by a biologist who is
also a librarian. Dr. M. P. Cloud of the Peabody Museum at Yale University.
Ann H. Morgan
Contents
Preface v
1. Relationships of the Living World 1
Part I. THE FOUNDATION
2. Life Is a Concern of Matter and Energy 9
3. Living Matter and Cells 25
Part II. ECOLOGY
4. Plants Provide for Themselves and Animals 51
5. Animals and Their Environments 67
6. Mutual Relationships of Animals 91
Part III. THE INTERNAL ENVIRONMENT
OF THE BODY
7. Tissues 107
8. An Agent in Evolution — The Body Covering 126
9. Protection, Support, and Movement — Skeletons 135
10. Movement — Muscles 155
1 1 . Foods and Nutrition 168
12. Circulation and Transportation — Body Fluids 195
13. The Release of Energy — Respiration 224
14. The By-Products of Metabolism — Excretion . , 242
15. Chemical Regulation — Endocrine Glands 255
16. Conduction and Coordination — Nervous System 279
17. Responsiveness — The Sense Organs 309
18. Reproduction 331
Part IV. THE NEW INDIVIDUAL
19. Development 359
20. The Physical Basis of Heredity 388
vii
71142
Vlll CONTENTS
Part V. EVOLUTION OF ANIMALS
21. The Protozoans — Representatives of Unicellular Animals 425
22. Sponges — A Side Line of Evolution 454
23. Coelenterates — Simple Multicellular Animals 465
24. Ctenophores — Comb Jellies or Sea Walnuts 493
25. Flatworms — Vanguard of the Higher Animals 498
26. Roundworms — The Tubular Plan 519
27. An Aquatic Miscellany 533
28. Annelids — Pioneers in Segmentation 552
29. Arthropods — Crustaceans 572
30. Arthropods — Insects, Spiders, and Allies 589
31. MoUusks — Specialists in Security 630
32. Echinoderms — Forerunners of the Vertebrates 651
33. Introduction to the Vertebrates — Lower Chordates and Fishes . . 662
34. Amphibians — The Frog, An Example of the Vertebrates 681
35. Reptiles — First Land Vertebrates 713
36. Birds — Conquest of the Air 729
37. Mammals and Mankind 752
Part VI. EVOLUTION AND CONSERVATION
38. Organic Evolution — Conservation 777
Appendix 795
Scheme of Classification 795
The Plant Kingdom 795
The Animal Kingdom 796
Equivalent Measurements 799
Suggested Reading 801
Index 819
1
Relationsnips or
tne Living Worla
This book is about animals, those that are regularly called animals and
others, the human animals. The human ones descended from some now un-
known ancestors of the apes, developed language and mind with ideas and
became unique among all animals. It is about the relations of animals to one
another, and to the plants upon which they depend, to water, to the sun, and
to the earth about them. The organization and relationships inside and out-
side of animals are the keys to their existence. Inside, the secretion of a gland
in one part of the body is carried by the blood and stimulates the heart and
muscles in other parts. Outside, the seasons change, the woodchucks go into
their holes for the winter and the bobolinks fly south.
Like the sun and the atmosphere and the soil, living organisms — the wood-
pecker in its hole in the pear tree and the fisherman and the fish — are composed
of atoms and molecules. For every organism, life is a concern of matter and
energy. It is not that its substances are so unusual; it is the way they are put
together that makes living matter different from everything else.
Living matter occurs in cells. They are samples of its composition and
activity, units of the architecture of plants and animals, rosebush and man. To
the passing glance cells appear disarmingly simple although they are complex
far beyond our present understanding. In many-celled animals the bridge
over which all inheritable qualities pass to the next generation is in the con-
tent of two microscopic cells. By their union and the divisions which follow it,
the billions of cells in the body receive their quotas of inheritance.
Plants and animals are bound together in a multitude of ways and the same
fundamental processes of living are common to both. A cactus is nearer to
human kin than a stone; the starvation of corn is not as spectacular as the
starvation of cattle, but it also is a disaster.
1
2 RELATIONSHIPS OF THE LIVING WORLD Chap. 1
On every hand animals depend upon plants directly and indirectly, for food,
for shelter, even for decoration. Long before mankind made bouquets, the
bowerbirds of Australia scattered blossoms on their courting grounds. Green
plants carry on the great business of making the food that is essential to
themselves and to animals. In spite of the schemes for providing the world
with synthetic food, a cow will keep her mouth to the grass for some time to
come. Plants also profit from the animals; many of them, including large
numbers of fruit trees, do not produce seeds without insect pollination.
The two main ways to study animals are: with emphasis on their asso-
ciations in groups of other living organisms, and with emphasis on the indi-
vidual.
As associated organisms animals are considered among others of their own
kind or of different kinds in environments of soil, water, or air, within a
complex web of influences. The environment of the butterfly on the flower
includes the sun, the rotating earth, and the atmosphere as well as the flower
(Fig. 1.1). Ecology is the study of plants and animals in their home environ-
ments. It is discussed near the beginning and again at the end of this book,
Fig. 1.1. Escaping energy, the heat and light of the sun. Left, the sun in total
eclipse by the moon. The sun's corona of light streaming out great distances from
behind the darkened moon. Right, part of the profile of the sun showing its promi-
nences, great flames that extend hundreds of thousands of miles into space. All
processes of living are related, directly or indirectly, to the capture of the radiant
energy of the sun. (Courtesy, Mount Wilson and Palomar Observatories.)
but the relations of living organisms to their surroundings pervade all of its
chapters. The animal body itself is a portable environment; the lungs and the
heart carry on unique activities in their own special surroundings. The evolu-
tion of animals is a history of relationships.
Everybody has had experience with an animal in its home territory: clothes
moths in flannel, skunks along the byways, or robins on the lawn. Everyone
knows that plant lice suck up plant juices, that robins eat hugely of earth-
Chap. 1 RELATIONSHIPS OF THE LIVING WORLD 3
worms which in turn eat heavily of leaves and of soil rich in microscopic
plant cells.
The watery homes of .animals are exciting because they are relatively
primeval. Wade into the border waters of a lively pond and you look down
into a world in which animals are swimming, climbing, burrowing, eating
plants, eating one another, mating, laying eggs, floating, and doing nothing
but the basic business of living. The pond is affected by surrounding condi-
tions but its swarming population is primarily adjusted to an ancient world of
water. The tidal and surface waters of the sea contain populations which
dwarf those of fresh water, but ponds and seas bestow similar benefits in the
same great boon of water. Living substances must be wet. Life began in the
water and all plants and animals are still bound by their need of water, even
though many of them have moved into deserts. All plants and animals are sub-
ject to the chemical and physical features of their environments. The carbon
cycle begins when plants take carbon dioxide from the air and build it into
carbohydrate food. The atmospheric pressure in high and low places and the
amount of oxygen in the air or water continually affect animals.
Ecological relationships — the fish to the sea, the bird to the air — pervade
the evolution of animals and plants. They are apparent in a survey of the main
groups of animals arranged with respect to their structures and activities. They
also appear in special studies of certain animals as representative types,
such as those of the ameba, hydra, grasshopper, honeybee, and frog. In this
book each of these has been included with its own group of relatives instead
of being considered as an isolated creature; no plant or animal lives unto itself
alone. In the systems of the body and in their fundamental patterns animal
groups show resemblances and relationships. The circulatory systems of all
vertebrates are built on a similar ground plan. Except in protozoans and the
simplest of multicellular animals, kidneys are tubular organs closely associated
with circulating blood. From earthworm to man the body is a tube within a
tube; in invertebrates the nerve cord is on the ventral side of the body; in all
vertebrates it is on the dorsal side. Environment has been a sculptor. In envi-
ronment and outward form a whale is fishlike; in internal anatomy it is closer
to a squirrel.
Conservation is applied ecology. Not until good things are going or gone do
we appreciate what they used to be. A stream runs clean and cold and well-
fed trout cut through its currents. This home is right for them. No alterations
are needed. Presently an upstream paint shop is established, the waste warms
and poisons the water, supplies it with scum, bad smell, and gases that kill the
trout. The need of getting back the clean, cold water is urgent for whatever
fishes may still be alive. If the paint shop and the bad smell had not become a
part of their environment, there would be many more alive. Conservation of
our natural resources is growing daily more important. The kinships of ani-
4 RELATIONSHIPS OF THE LIVING WORLD Chap. 1
mals and man extend in every direction and include all living organisms and
the times and places which have made them and are still making them what
they are.
The second way of studying animals is with emphasis on the individual. It
is the study of the structure and function of tissues and organs, by examination
and experiment. Every animal has an internal environment inherited from its
ancestors through ages of evolution. Within the body all cells live in a watery
environment as truly as do animals in a pond. The amount of water is con-
tinually regulated; chemical conditions — acidity, alkalinity, enzyme, and hor-
mone actions are constantly balanced, unbalanced, and rebalanced; physical
conditions are changed; temperatures shift, and pressures vary. Every animal
body holds a special environment of which there is no duplicate and probably
nothing in existence that is at once so complex, delicate, and generally durable.
The release of energy in respiration, chemical regulation by the endocrine
glands, and the excretion of the by-products of metabolism especially empha-
size the balancing associated with these processes. As animals are examined,
it becomes more and more clear that there are not thousands of separate facts
to be learned, but a few associations and principles that apply to essentially
similar things.
The Fields and Subdivisions of Zoology. The science of biology includes all
living organisms. The term, actually meaning the science of life, Gr. bios, and
logos, discourse, is used commonly and loosely, often with little understanding
of its meaning. It may include only the plants and be called the biology of
plants; it often deals only with animals, the biology of animals. In either case
it is concerned with the general facts and principles of plant or animal life.
Zoology is the study of all aspects of animals, including their relations to
each other and their environments in time and space. Other associated sciences
are those particularly concerned with the environment, such as geology, physi-
ography, oceanography, and meteorology which is concerned with conditions
of the atmosphere. All of these are supported by physics, treating of the prop-
erties of matter, and by chemistry which deals with its constitution.
There are many subdivisions of zoology, the science of animals (including
man). The principal ones are the following:
Subdivisions of Zoology
Name Description
Anatomy Gross structure of the animal
Histology Function and microscopic structure of tissues and organs
Cytology Function and structure of the cell and its contents
Physiology Function of the whole animal, or of its parts
Embryology Development of the new individual
Chap. 1
Name
Genetics
Ecology
Taxonomy
Zoogeography
Paleontology
Sociology
Parasitology
Psychology
Zoology is
animals, such
Entomology
Ornithology
Protozoology
Herpetology
RELATIONSHIPS OF THE LIVING WORLD 5
Description
Science of heredity dealing with characteristics arising from
the behavior of genes
Relationships of animals to one another, to plants, and to
the environment; their home life
Classification of animals and its principles
Distribution of animals in space
Distribution of animals in time; fossils
Societies of animals and man
Study of animals that live and subsist upon other animals or
plants to their harm
Study of the mind
also divided into branches for the study of special groups of
as:
Insects
Birds
One-celled animals
Amphibians and reptiles
Parti
Tne Founaation
2
Lire Is a C
s a v^oncern
or Matter and Energy
We live in a universe of substance and force. Everything that we can discern
with our senses is either one or the other, matter or energy. So far as they are
known, matter and energy are always associated. They are in the grass beneath
our feet, the wind and the rain, our food and our use of food. Even a little
understanding of the character and relationships of matter and energy throws
light upon the lives of plants and animals; it may be the eyeshine of a cat in
the dark, the song of a wood thrush, the drip of sweat from the skin, the heat
of fever, the chnch of muscles.
Matter
Our bodies are composed of matter. It is all around us: books, plants, ani-
mals, sugar, smoke, gasoline, the earth, the planets, and the far-off galaxies,
each of them Uke the Milky Way of which our own solar system is a part.
What are these things? What is matter? A good deal has been learned about
its structure mostly during the last part of the nineteenth and the first part of
the twentieth centuries. In its analysis all the roads have led toward electricity.
But nobody knows what matter is because no one yet understands electricity.
All matter is composed of invisible atoms; there are millions of billions of
them in a drop of water, each one containing extraordinarily minute electrical
particles. The electrical nature of living matter has been known in one way or
another for a long time, but in recent years more and more evidences of it have
been discovered. The Italian anatomist, Luigi Galvani (1737-1789) was
observing a freshly killed frog hung from an iron fence by a copper wire
hooked under the sciatic nerve when he noticed that the muscles twitched
whenever the wind-blown legs touched the iron fence. Thus a century and a
half ago Galvani discovered that living matter conducts electricity and re-
9
10 THE FOUNDATION Part I
corded his observations in his essay, "Force of Electricity on the Motion of
Muscles." Less than half a century later it appeared that living tissues not only
conduct electricity but also produce it. Now rhythmically repeated waves of
electrical charges are received over wires connected with metal plates placed
against the human head, and the records of them are taken by recording mech-
anisms (Fig. 16.23). The existence of electrical brain waves is clearly estab-
lished.
Energy
Energy is the capacity for action, the ability to do. Expressions of it are the
jumping of fleas, the wriggling of a baby, the leap of a rabbit, the response of
a tear gland. Just as life is known only through matter, so energy is measured
only by its effect on matter, the size and the speed of a flea's jump.
Characteristics of Energy. Heat is the commonest form of energy. This is so
generally true that measurements of energy can be stated in units of heat. The
small calorie is the amount of heat required to raise one gram of water one
degree centigrade at sea level pressure of nearly 15 pounds per square inch.
Since the gram is too small to be a convenient unit, a large calorie has been
adopted for general use. It is the amount of heat required to raise one kilo-
gram (1000 grams or 2.2 pounds) of water one degree centigrade, also at sea
level pressure.
Potential and Kinetic Energy. Usually energy can be in two forms, potential
in the rabbit's readiness to jump, and kinetic in the actual jump. Atomic
energy is seemingly of a different sort.
Potential energy is that contained in any object because of its position or
shape or substance. Kinetic energy is that of motion. A fish hawk (osprey)
hovering aloft over a lake has potential energy of position. This becomes
kinetic energy as the hawk cuts downward to pick a fish from the water. The
wiry threads wound around the eggs of certain mayflies have potential energy
that becomes kinetic (Fig. 2.1). They are tightly coiled as long as the eggs are
in the body, but they spring loose and catch on plant stems as soon as the eggs
are laid in the water. Living cells hold potential energy of substances such as
fat which may be transformed into the energy of heat. In a more particular
sense the energy of substances is usually called chemical energy (Fig. 2.2).
Energy is either stored or liberated in all chemical reactions. A coal fire is a
chemical reaction in which chemical energy stored millions of years ago is
liberated from the coal:
coal -f O- (oxygen in the air) = CO2 (carbon dioxide gas) + energy (heat).
Catalyzers are aids in chemical reactions, hastening them without entering or
being affected by them. Many of them are known as enzymes or ferments and
each one acts upon particular substances and under certain conditions. The
Chap. 2
LIFE IS A CONCERN OF MATTER AND ENERGY
11
Fig. 2.1. Change from the potential energy of position to the kinetic energy of
motion in the threads of a mayfly egg, the size of a sand grain. Before the egg is laid
a wiry thread is coiled like a watch spring around each end of it; the energy in their
coils is potential. Mayflies strew their eggs on lakes and streams. As the eggs touch
the water the coils spring loose; in so doing their potential energy becomes kinetic.
The threads catch on submerged twigs and the eggs are suspended above the mud
that otherwise would smother them.
respiration of every living cell is a chemical reaction in which the chemical
energy in the cell's substance is transformed into the energy of activity and
heat.
Transformations of one kind of energy into another are constantly going on
about us. The radiant energy of the sun becomes that which is stored in the
simple sugars of green grass. Cows feed on grass and its stored energy is even-
tually transformed into milk for calves or babies.
Atomic Energy. The energy within the atom shows itself in qualities of
cohesion. It is liberated when under special conditions one kind of matter is
changed into another, e.g., nitrogen into oxygen. Such a change generally
occurs in atoms in which the particles in the nucleus are numerous. They may
be unbalanced for a long period and relatively unstable as in radium, uranium,
and thorium. Such atoms cannot hold themselves together and their radio-
activity is a long, continued breaking apart.
The beginnings of the knowledge of radioactivity moved rapidly. In 1895
Rontgen concluded that some active radiation emitted spontaneously from
12
THE FOUNDATION
Part I
Fig. 2.2. Chemical energy stored in a globule of fat. Fat cells from connective
tissue underlying the skin of a rat. Fat stained black. (After Maximow. Courtesy,
Gerard: Unresting Cells. New York, Harper & Bros., 1940.)
uranium had fogged photographic plates protected by light-tight envelopes.
He coined the word radioactive to describe the activity. In February of the
next year Henri Becquerel read a paper before the Academy of Science in
Paris in which he announced that compounds of uranium were able to affect a
plate through an envelope that was proof against light. The radiations were
called x-rays because they were not understood. Following Becquerel's dis-
covery his one-time student, Marie Curie, succeeded in isolating minute quan-
tities of two highly radioactive new elements from uranium minerals, to which
she gave the names polonium and radium. In 1899 Becquerel showed that the
rays from uranium could be separated into two types, alpha rays easily ab-
sorbed by a few sheets of paper, and beta rays able to penetrate thin alumi-
num. In 1900 Villard discovered still a third and more penetrating radiation
from uranium minerals, the gamma rays. By 1913 Rutherford and Soddy had
coordinated the various processes and proposed a theory that the nucleus of
the atom was spontaneously disintegrating. They suggested that the nuclear
disintegration was explosive and showed that during the process particles of
matter and energy were lost. Since that time the knowledge and use of atomic
energy have become important in many fields of biology; x-ray photographs
are routine items in medical practice; exposure to controlled quantities of
x-rays is a common treatment of cancer; Muller's experimental radiation of
fruit flies produced inheritable differences in generation after generation of
their offspring; and the use of radioactive tracers has opened a new era in
biological investigation.
Atomic energy is now a tool in world politics; perhaps it is more true that
Chap. 2 LIFE IS A CONCERN OF MATTER AND ENERGY 13
world politics is a tool of atomic energy. The most startling display of energy
that had ever been known to the world occurred on August 6, 1945, when an
atomic bomb exploded over Hiroshima, Japan, and uranium atoms (U-235)
broke apart and unloosed their extraordinary power.
Structure of Matter
The physical states of matter are more or less easily changed by conditions
about them. In shifting temperatures, the state of water may be a gas, fluid, or
solid, i.e., vapor, rain, and sleet in quick succession. The composition of mat-
ter is not thus easily changed, the elements and their compounds, the atoms
and molecules. Atoms are the incredibly minute, organized units of matter that
are the building blocks of elements.
An element is composed of one kind of atom for which it is named, oxygen,
carbon, calcium, and so on. One hundred elements are known, mainly dis-
covered in nature: certain radioactive ones have also been created experimen-
tally. The elements are distributed unevenly. Four of them, oxygen, carbon,
hydrogen, and nitrogen constitute 96 per cent of living matter; less than 20
make up 99 per cent of the atmosphere, the ocean, and the earth's crust.
Molecules are usually the units peculiar to an element or a compound.
Molecules of elements contain two or more atoms of the same kind. Molecules
of compounds have two or more different kinds of atoms. The molecule of
water has two atoms of hydrogen and one of oxygen (Fig. 2.3).
Molecules are continually attracted to one another by intermolecular force
that is electrical rather than gravitational. They are in constant motion, in a
random jumpy dance. They are too small to be visible and the dance cannot
be seen but can be felt as heat. When a substance is cold, e.g., ice, the dance
is slow; when hot, e.g., boiling water, the dance is extraordinarily rapid. Turn
an electrical current through a cold iron and the dance of the molecules is
changed from the slow to rapid rate. The motion never stops. The lounger in
Hydrogen
H
ooo
MOLECULES
Oxygen
Water
Fig. 2.3. Diagram of the formation of a molecule of water by the sharing of elec-
trons between two atoms of hydrogen and one of oxygen. Electrons are the particles
that take part in chemical reactions.
14 THE FOUNDATION Part I
Boston Common and the dead bench on which he sits both abound in speed-
ing molecules (Figs. 2.4 and 2.5).
Characteristics of Atoms. Nobody has seen the atoms. Their existence was
assumed by John Dalton (1766-1844) and it has been proved by patient,
skillful experimentation with radioactivity and other means.
Fig. 2.4. Molecules are continuously repelled
and attracted in a random jumpy dance. Those in
a thin gas move in free curves. Those in a fluid
or a solid are packed together as if in a crowded
hall. (Courtesy, Gerard: Unresting Cells. New
York, Harper & Bros., 1940.)
The relatively small center body or nucleus contains practically all of the
atom's mass. Electrically negative particles rotate around it. In comparison
with their size, they swing through space relatively as great as that in which
planets rotate about the sun (Fig. 2.6). The nucleus is composed of protons
carrying positive charges of electricity and neutrons that carry no charge. The
sum of their masses is the weight of the atom. The electrical charge of the
nucleus indirectly controls the nature and behavior of the atom. Atomic nuclei
are bound together by a force that was unimagined until experimental splitting
demonstrated its reality. As interdependence permeates living organisms, so
interdependence of parts is the keystone of the atoms that are the foundation
of living matter.
Within the space around the nucleus are particles called electrons, so light
that they are ignored in the computation of atomic weight. Each carries a
negative charge of electricity and spins like a coin that is spun upon a table
top. It is generally believed that electrons revolve around the nucleus, but
their spinning is independent of it. The number of electrons in an atom governs
its chemical properties. Electrons, for example, determine that one atom of
oxygen will unite with two atoms of hydrogen to form water (H^O).
Isotopes. Isotopes are different forms of atoms existing in the same element
(Fig. 2.6). They have nearly the same chemical properties but differ in the
number of neutrons in their nuclei. Since the weight of an atom is the sum of
the numbers of its protons and neutrons, the isotopes of an atom have differ-
ent atomic weights. For example, hydrogen has three known isotopes: hydro-
gen, atomic weight 1; deuterium (heavy hydrogen), atomic weight 2; tritium,
atomic weight 3. Isotopes that have few neutrons in their nuclei are called
light isotopes and those with the most neutrons heavy isotopes. In general the
heavy isotopes are less stable, since an excess of neutrons weakens the co-
Chap. 2 LIFE IS A CONCERN OF MATTER AND ENERGY 15
hesion of the nucleus. Those that do not readily change are called stable
isotopes; the radioactive isotopes give off nuclear energy. Isotopes have been
detected in nature and many radioactive ones have been made in laboratories.
The separation of isotopes is a means of exploring changes that take place
within the nuclei of atoms. One of the problems in dealing with isotopes is to
separate out the kind which is to be used. In some cases this is easy; in others
it is extremely difficult. In the distillation of water the vapor which first con-
denses is water containing the light isotope of hydrogen. Later the heavy water
Oxygen
Carbon
Hydrogen
Nitrogen
Misc.
Water
Proteins
Carbohydrates,
Lipoids, Minerals
Fig. 2.5. Top, Percentages of different kinds of atoms in the human body. In-
cluded under miscellaneous are, in order of decreasing amounts, calcium, phos-
phorus, potassium, sodium, sulfur, chlorine, magnesium, and iron. Bottom, Percent-
ages of different kinds of molecules in the human body. (Modified from Moment:
General Biology. New York, Appleton-Century-Crofts, 1950.)
16
THE FOUNDATION
Part I
containing a heavy isotope of hydrogen also distills. Isotopes of uranium are
not procured by any of the easier methods; skill, persistence, and elaborate
equipment are required.
Isotopes are also put to various uses, in war, in biological investigation, and
in medicine. The atomic bombs of the Second World War contained isotopes
of heavy atoms with unstable nuclei that flew apart establishing chain ex-
plosions of tremendous destruction. The political condition of the world has
established an association of isotopes and war. There is hope that this may
sometime give place to great constructive uses. To the world at large, atomic
bombs have almost hidden the importance of the radioactive isotopes that are
being used as tracers in living plants and animals.
Hydrogen atom
Deuterium atom
Fig. 2.6. Diagrams of the structural plan of the atom. As they
are at this date generally named the particles inside the nucleus
are: the protons ( + ) that carry positive charges and the neu-
trons (0) that carry no charges; the electrons outside the nucleus
bear negative charges. Hydrogen atoms have one proton and one
electron. Deuterium atom, an isotope of hydrogen (heavy hy-
drogen) consists of a nucleus with one proton and one neutron,
and a single electron moving around it. Helium atom, the
nucleus consisting of two protons and two neutrons, has two
electrons moving around it. Helium gas is used in dirigible bal-
loons.
Helium atom
Ions. Atoms may gain or lose electrons and are then known as ions. If elec-
trons are lost, the ion is positively charged; if they are gained, it is negatively
charged. Ions combine more readily than electrically neutral atoms. Water
facilitates the splitting of substances into ions. Living organisms are largely
water and many substances are present in them chiefly in a dissolved state. In
solution many of these dissolved substances split into simpler ones and ions
are formed (Fig. 2.7). When crystals of common salt (sodium chloride,
NaCl), a component of the blood of all animals, dissolve in water, the ions of
the sodium (Na+) already present in the lattice of the crystal are separated
by the attraction of the polar molecule of water. The crystal framework is thus
broken and the ions are free in the solution. Their formation in salt solution
is expressed by the formula, NaCl = Na+ -f CI-.
Because of the positive and negative charges of ions, the living body can
conduct electricity. When the opposite poles of a battery are placed in water,
Chap. 2 LIFE IS A CONCERN OF MATTER AND ENERGY 17
the sodium ions (Na+) are attracted toward the negative pole where they
acquire electrons and their positive charge is neutralized. The chlorine ions
(CI") are attracted toward.the positive pole, give up an electron and become
neutral atoms. The moving ions conduct an electrical current and thus estab-
lish a complete circuit. Any substance which thus ionizes in water is called an
electrolyte because of its ability to conduct electricity.
Fig. 2.7. Diagram of the ionization or dissolv-
ing of salt in water. When sodium chloride (salt)
is put into water the atoms Na (sodium) and CI
(chlorine) separate and become electrically
charged wandering atoms or ions, Na+, Cl~.
The movements of the sodium ions (-(-) and
chlorine ions ( — ) conduct an electrical current
in water. In general, water promotes the forma-
tion of ions and ions promote chemical reactions.
The properties of electrolytes depend upon the kind of ions which they
produce in a solution. On the basis of the simpler theory of electrolytes there
are three classes: acids, alkalis, and salts. The degree of acidity or alkalinity
of a compound depends upon the degree to which it ionizes in water, that is,
the degree to which the molecules yield positive hydrogen ions (H+) or nega-
tive hydroxyl ions (OH~) in the solution. Acids are electrolytes that as a
group form positively charged hydrogen ions, giving the acid its sour taste.
Hydrochloric acid ionizes in water:
HC1^H+ + C1-.
The alkalis or bases form negatively charged combinations of oxygen and
hydrogen, the hydroxyl ions, OH~, The alkali, sodium hydroxide, ionizes
thus:
NaOH^Na+ -f OR-.
Some compounds of protoplasm yield both H+ and OH~ in solution. The
third class of electrolytes is the salts whose ionization produces neither H+
nor OH~, Sodium chloride is an example:
NaCFNa+ + C\-.
Many of the important characteristics of cells, such as the permeability of
their membranes, their irritability or response, are associated with the existence
18
THE FOUNDATION
Part I
of electrolytes either within or outside them. The sensitiveness of the ani-
mal organism to hydrogen ions is apparent in scores of cases; in a large num-
ber of animals the control of respiration is through the hydrogen-ion concen-
tration of the blood. Hydrogen-ion concentration (symbol pH) of substances
in their surroundings is also of greatest importance to living organisms; the
range of many aquatic animals, certain protozoans, insects, and fishes is
limited by it; so is the range of earthworms.
Tracers. The use of radioactive isotopes as tracers for investigating life
processes is probably one of the most significant developments in modern bio-
logical work. Such a possibility had been recognized for some years but was
limited by the fact that all the work had to be done with heavy elements such
as lead, bismuth, and mercury. The isotopes chosen are labeled by exposure
to radiations from a radioactive element. After this treatment they give off
radiations for a longer or shorter period. The ease of this modern technique
is comparable to locating a white penny among ordinary copper ones. They
are introduced into plants and animals in various ways (Fig. 2.8). For exam-
ple. 2.8. The presence of radioactive tracers shown by radioautographs in slices
of tomato, especially in the seeds. The vine from which the tomatoes were taken
was grown in a solution containing radioactive zinc (Zn^^). This was taken up
throughout the plant and affected the photographic plates like light. (Courtesy,
P. R. Stout, University of California.)
Chap. 2 LIFE IS A CONCERN OF MATTER AND ENERGY 19
pie, in the body of a rabbit they may be carried in and out of organs, into cells
and perhaps out again.
The travels and destinations of such labeled isotopes are detected most com-
monly by the now familiar Geiger-MuUer counter. This apparatus detects and
amplifies each radioactive disintegration of an atom. The number and rate of
disintegrations are a measure of the amount of labeled material present. In
general the use of tracers is directed toward investigations of the constant
buildup and breakdown, and the come and go of chemically active molecules
in the living organism. In this way it has been learned that thyroxin, the
iodine-containing amino acid that is so important in the functioning of the
thyroid gland, is manufactured by muscle and in the intestine as well as in
the thyroid gland. Recent studies on the metabolism of rabbits by means of
radioactive isotopes have shown that radioactive phosphorus administered to
adult animals enters their bones and the enamel and dentine of their teeth.
This shows that such hard substances, deposited in early youth, do not stay
unchanged for a lifetime, but are continually exchanging material with the cir-
culating blood.
States of Matter
Molecules are continually affected by the attraction of their neighbor mole-
cules. Their relative sizes and the distances between them determine the
strength of their mutual attraction and the state of the substance in which they
are contained whether gas, liquid, or soUd (Fig. 2.9). Changes of matter from
one state to another involve a change in energy, usually the giving off or
absorption of heat.
In gas, the molecules are scattered away from each other; their movements
are rapid and disorderly and they take zigzag turns into their surroundings.
The volume of a gas is dependent upon temperature and pressure. The gas
spreads through all available space but is compressible because it does not
RELATIVE DENSITIES
MIXTURES
Gas
Liquid
Solid
B Solution
Suspension
o;.oi.?jo
Emulsion
Fig. 2.9. A. Diagrams showing the relative densities of molecules in a gas,
liquid, and solid. B. Diagrams of mixtures: solution thoroughly dissolved and
homogeneous; suspension with particles of one substance undissolved; emulsion
with very large undissolved droplets.
20
THE FOUNDATION
Part I
actually fill the space. Air is a gas and its density varies with the compression,
with the pressure and temperature of the atmosphere. In high places where
pressure is lessened, its molecules are relatively far apart and it may be too
"thin" in oxygen to be adequate for respiration. In liquids, the molecules are
closer together. In a solid, such as iron, the molecules are crowded together in
patterns. Solids have fixed shape and volume.
The behavior of water molecules is very exceptional. Down to 39° F. they
draw closer together; between 39° F. and 32° F. they move apart. Thus, ice
expands and floats, forming a protecting cover to the animals beneath it (Fig.
5.17).
Surface film. Surface films are composed of molecules that are attracted
only by those at and close to the area where one substance comes in contact
with another, such as water and air (Fig. 2.10). Molecules below the surface
are attracted equally from all directions. Surface film occurs on all bodies of
water and forms the boundary of such units as soap bubbles and raindrops. It
is important in the lives of many small aquatic animals. Certain insects, such
as the water striders, forage on the upper face of surface films that bend but
do not break with the pressure of their feet (Fig. 2.10). Snails glide over the
underface of the film and hydras are often buoyed up against it.
SURFACE FILM OF WATER
A o-o-o-o-o
/ \ / \ / \
o o o o
o o
o o
B
Fig. 2.10. The surface film of water. A, In surface film molecules of water are
attracted only by those at the surface or just below it. B, Molecules below the sur-
face are attracted evenly from all directions by other molecules. C, Hydras rest
against the surface film in the topmost water where oxygen is plentiful. D, Water-
striders skim over the surface film of quiet water and their feet make the dimples
that cast shadows on the brook bed.
Chap. 2
LIFE IS A CONCERN OF MATTER AND ENERGY
21
Mixtures of Substances
Mixtures of substances may be of different kinds and states, those of solids,
liquids, gases, or a solid and a gas (Fig. 2.9).
Solutions. These are homogeneous mixtures. We usually think of solutions
as aqueous since natural water is a solution containing dissolved air. Bubbles
of air leave water when it is heated, appearing just before it boils. When it is
freezing bubbles of air appear and are caged in the ice. Glass is also a ho-
mogeneous mixture, in spite of its hardness, a true solution.
Suspensions. The particles of at least one of the substances in a suspension
are larger than molecules and remain undissolved. One or several kinds of
substances, or different states of one or more of them may be suspended in
another substance. Suspensions include various types of colloids all of which
consist of one or more substances dispersed in another. There is no escape
from colloids. We consume them as food, breathe them as fog and smoke,
and are composed of them.
Colloids. These are gelatinous substances that include two or more com-
ponents: (1) a solid in a solid — the ruby glass of cathedral windows usually
containing metallic gold; (2) a solid in a liquid — sodium chloride (salt) in
water; (3) solid particles in a gas — blue cigarette smoke; (4) a liquid in a
solid — natural pearl, which is water in calcium carbonate (a secretion of
Ooo 1
.X=k-
>.^k>
Sol
Gel
Movement of ameba accompanied by
changes sol to gel and reverse
Fig. 2.11. Diagrams of the colloidal states, sol and gel. In the sol state the par-
ticles and droplets (white) move about freely in fluid. In the gel state the surfaces
of the droplets are in contact and the substance is jellylike. The protoplasm of an
active ameba constantly changes from sol to gel and reverse.
22
THE FOUNDATION
Part I
oysters), natural opal, water in silicates; (5) liquid in a liquid — gelatin in
water (gelatin may be a liquid or solid); (6) liquid in a gas — fog. Fog and
mist are actually solid particles in gas since the water molecules are gathered
on solid particles. It has been noted that at 6 a.m. the air over London may
be clear and at 9 a.m. there may be a dense fog. The onset of the fog is
largely due to the smoke that has provided particles on which the water
gathers.
The most important of all mixed substances is protoplasm. It is a colloid,
the most complicated, most studied, and still largely unknown one without
which life does not occur. This colloid varies in consistency; when it thickens
its droplets swell, come closer together, and become a gel; when it thins, the
droplets do not absorb water, are smaller and farther apart, and form a sol
(Fig. 2.11). Protoplasm is a reversible colloid that may change from sol to
gel and return. Such changes may be seen through the microscope in any
ameba. White of egg is a gel when heated but it will not return to a sol.
Emulsions. Although containing larger droplets than most colloids, emul-
sions are similar to them. Familiar emulsions are whole milk, egg yolk, and
mayonnaise dressing.
Diffusion and Osmosis
Diffusion is the movement of a gas or liquid from points of greater to those
of lesser concentration continued until an even distribution is achieved
throughout the available space (Fig. 2.12). Mice find the cheese from the
D
Water
Sugar
Diffusion
Osmosis
Permeability
Fig. 2.12. Diagrams illustrating diffusion. In simple diffusion (A and B), mole-
cules of sugar without any barrier become evenly distributed among the molecules
of water in consequence of the motion of both. In osmosis, the diffusion through a
semipermeable membrane (C and D), the molecules of water can pass through the
membrane in either direction. They continue to do so until their number is equal on
each side of the membrane. Thus, the level of the sugar solution is raised. The mole-
cules of sugar, imprisoned by their larger size, continue to hit against the membrane
in their random movements exerting the force called osmotic pressure. In the com-
plete permeability (E) both kinds of molecules pass through the membrane at the
same rate and the solutions have uniform content on each side.
Chap. 2 LIFE IS A CONCERN OF MATTER AND ENERGY 23
particles of it diffused in the air. Skunks have few enemies because of the
diffusion of their scent. The success of the great perfume industry is dependent
upon human responses to the diffusion of its products, the various perfumes.
Osmosis. The diffusion of water or of certain gases through membranes
that permit certain simpler molecules to pass, but not the more complex and
larger ones, is osmosis. A membrane which does this is said to be semi-
permeable.
Living cells are enclosed by semipermeable membranes containing sub-
microscopic pores through which certain molecules can pass and others
cannot. The rate of passage varies with the kind of membrane and the
material on the two sides of it. Such membranes regulate many functions of
the body such as the exchange of oxygen and carbon dioxide, the absorption
of food, and the constant come and go between cells and body fluids. Two
liquids that contain equal concentrations of dissolved substances are called
isotonic. When living mammalian blood cells are examined microscopically
they are usually immersed in a solution of 0.9 per cent NaCl in imitation of
the body fluids whose salt content is isotonic with the cell content.
An example of osmotic diffusion or osmosis through an artificial mem-
brane illustrates this principle (Fig. 2.12). The membrane is permeable to
molecules of sugar as well as water, but so much more so to the latter that
equal amounts of sugar and water on each side are never reached. Red blood
cells puff out like pillows (called laking of blood) if the salt content of the
plasma becomes too much reduced, that is, hypotonic. This is because mole-
cules of water enter them, establishing an equal concentration with the too
watery plasma (see Chap. 12). If the salt content of the plasma is too high,
i.e., hypertonic, the water is drawn out and the cells wrinkle.
Vacuole
Vacuole
Fig. 2.13. Brownian movement occurs in the contents of vacuoles of an ameba
(right) and of the green alga Closterium (left). With the high power of a micro-
scope the zigzag pathways of the larger particles can be traced. The Brownian
movement is due to bombardments of usually invisible particles striking unevenly
against the larger ones.
24 THE FOUNDATION Part 1
Brownian movement. This motion is an irregular agitation of particles ol
difTerent sizes. The molecules constantly jostle against relatively huge par-
ticles, striking them unevenly on one side or another. Many of them are very
small molecules and others are large molecules. The molecular motion is
invisible, but that of the larger particles is evident with the high power of
the microscope. The motion occurs in gases, fluids, and especially in colloids
including protoplasm. It is common in the vacuoles of algae and protozoans
(Fig. 2.13). It was discovered in 1827 by Robert Brown, an English botanist,
who saw the motion in a fluid in which pollen grains were suspended. Like
other diffusions, Brownian movement is an example of kinetic energy.
3
Living Matter and CelL
No one has ever found anything aHve apart from matter. We see the
evidences of matter in the protoplasm of every plant and animal: sunflowers
turn toward the sun; bees gather about nectar; the ticket line moves toward
the show. All of these beings are composed of matter uniquely organized in
protoplasm and active in an equally unique process of living. Protoplasm
reproduces itself; like produces like but never duplicates itself. A cat has
kittens, not squirrels. Her kittens grow and they have kittens, and so, on and
on, cats and kittens. None of them repeats its mother or father or grand-
parents but each one shows its origin.
Protoplasm occurs in cells. The cell is a sample of the complete basic
organization and activities of protoplasm. It becomes more and more evident
that nonliving and living states blend together since the most complex protein
molecules have certain characteristics of protoplasm. The submicroscopic
gene that carries hereditary qualities is believed to be a protein molecule
that, like a living organism, reproduces itself. Whether viruses are alive or
not is still debated; it appears however that they have many of the properties
of living matter and are very active. Protoplasm came into being in a very
remote time but even now in the nucleoproteins there may still be a twilight
zone of originating protoplasm.
Protoplasm
General Features. We seldom see naked protoplasm. Generally we see
and touch the dead remains of cells in the outer layer of skin, scales, feathers,
and hair. The softness of a kitten's fur is all due to dead cells. Most animals
shed such dead cells seasonally; human molting or shedding goes on the year
round. No plant or animal is entirely alive. Cells contain nonliving as well
as living structures; freckles are groups of cells holding lifeless pigment that
has been deposited within them. Protoplasm looks fundamentally similar
wherever it occurs. A dozen cells flecked from the lining of one's own mouth
25
26 THE FOUNDATION Part I
and a living ameba shifting its shape through the water on the same micro-
scopic slide can be seen to have many differences. Their differences are not
surprising, but that their respective protoplasm should look so much alike is
unforgettable.
Protoplasm is a glassy fluid jelly that suggests the white of an egg be-
sprinkled with translucent particles and globules of liquid whose sizes and
arrangement change, at one time forming an open network, at another crowded
together (Fig. 2.11). Even through the microscope protoplasm often appears
inert. It is never really so as long as it is alive and after that it ceases to exist.
Dead protoplasm is only the somehow disorganized remains of protoplasm
and a contradiction of its name.
Structure. Protoplasm consists of a watery solution (hyaloplasm) in which
salts and other substances are dissolved and in which solid and semisolid
bodies are suspended. Many of these are molecules, mainly proteins that are
invisible through ordinary microscopes; others are clearly visible droplets.
Water may pass into protoplasm, making it more liquid, or out of it leaving
it less so. Under osmotic pressure (Fig. 2.12) minute amounts of solution
pass in or out of the droplets by way of their surface films which play the
part of semipermeable membranes. The numbers and sizes of the suspended
bodies constitute a relatively enormous surface, all of it inviting to chemical
and physical changes (Fig. 3.1).
Protoplasm is an exceedingly complex colloid. At one time it may be as
fluid as water (sol state) and at another a jelly (gel state) depending upon
Fig. 3.1. In keeping with their colloidal nature, even minute particles in proto-
plasm present a relatively enormous surface to the molecules which continually
jostle them. (Courtesy, Gerard: Unresting Cells. New York, Harper & Bros.,
1940.)
Chap. 3 LIVING MATTER AND CELLS 27
conditions around it such as the degree of temperature, its chemical environ-
ment, and its age, or phase of life. The streams of protoplasm which pour
like water into the forming pseudopodia .of an ameba are in the sol state;
their borders are changeful, now sol, now gel. If the cell membrane is broken
slightly, a little of the sol will flow out and "set," thus healing the wound.
Chemical Characteristics. Protoplasm has substantially the same chemical
content in all plants and in the great procession of animals whether jellyfish,
redbird, or man. The four elements, oxygen, carbon, hydrogen, and nitrogen
make up 96 per cent of living matter. No element occurs in protoplasm
which is not also present in nonliving substance. It cannot be recalled too
often that it is not the content of protoplasm but the way it is put together
that is unique.
Water. The most abundant compound in active protoplasm is water, in
general terms of weight at least 75 per cent of it. A jellyfish may be 96 per
cent water, a paramecium 80 per cent. The gray matter often called the
"thinking part" of the adult human brain is at least 80 per cent water; in early
youth the percentage of water is still greater. The water content of a cell is
controlled by the living membrane which encloses it. Protoplasm has a water-
regulatory power which resembles that of gelatin in that it takes in water
and swells to a limited amount and no more. Water heats slowly and holds
its heat. Thus the temperature of an animal with its high water content
rises slowly and tends to hold its level. Water works toward a temperate
climate for protoplasm, whether it is in the body cells of a fish or surrounding
the fish in a stream. Certain very important changes in the water content
of their protoplasm make animals of low metabolism relatively cold-hardy,
such as the numberless cold-blooded ones, insects and others that withstand
temperatures of zero (F.) and far below. As winter approaches their proto-
plasm loses water, but this is only part of the cold hardening. The water
which remains is not all in the same state; it may be free or bound, more
of one than the other. Free water is water that contains truly dissolved
materials and acts as a dispersion medium for them. In both plants and ani-
mals it transports digested foods and waste products and forms a liquid
base for secretions. Bound water is held in a loose chemical combination
with other molecules. Ordinarily bound water does not freeze. Free water
freezes readily forming ice crystals, which because of their size and pressure
kill the protoplasm. Studies on the bound and free water in gelatin and egg-
white show that part of the water freezes when the temperature reaches
— 6°C. (21.2°F.) while what remained did not freeze even at — 50°C. Thus,
for the beetle that must endure a northern winter there are striking advantages
in having a content of bound water.
Chemical Activity. Water is the closest approach that we have to some-
thing which dissolves everything. This is the basis of its prominence in diverse
28 THE FOUNDATION Part I
metabolic processes, of its power to shape the earth's surface, and its efficiency
in the digestive tract, in the washtub, and in the factory. Chemical reactions
are hastened by any agent that finely divides a solid, and this happens when
water divides a lump of sugar. Living depends upon chemical reactions, both
continual and intermittent, all of them together making up the grand process
of metabolism, the chemical changes in which water is a constant attendant.
Water conducts electricity; when salt is added it does so much more readily.
Thus, protoplasm is an efficient conductor since a variety of salts occurs in
it and especially in body fluids, the latter being similar to sea water in their
salt content.
Atmospheric Gases. The gases of the atmosphere are soluble in water
and therefore in protoplasm. Nitrogen (No), abundant in the air (79%), is
always present in living cells but is chemically inactive; in pure form it does
not take part in metabolism although its compounds, e.g., proteins, do so.
On the other hand, oxygen, varyingly abundant in the atmospheric air (about
21%), takes an essential part in oxidation in the cells. Carbon dioxide,
usually 0.03 per cent in the air, is produced as a by-product of oxidation in
protoplasm. Although a by-product in the respiration of both plants and
animals, carbon dioxide is essential for photosynthesis in plants (Chap. 4),
and in small amounts for important functions in the respiration of animals.
Mineral Salts. Protoplasm doubtless came into existence in sea water
and mineral salts must have been included in it from the beginning. It con-
tains a variety of salts; sodium, potassium, calcium, and magnesium are the
chief positively charged ions, and chloride, carbonate, phosphate, and sulfate
are the common negatively charged ones. Mineral salts are important in
maintaining the osmotic balance between protoplasm and its environment,
in regulating the passage of water into and out of the cell. Calcium may take
part in the change of protoplasm from a sol to a gel state.
Organic Compounds. The most important difference between inorganic
and organic compounds is in the carbon content of the latter. This is so
universal that carbon is the one element with which organic chemistry deals.
Carbon is present in some inorganic compounds, but it is present in all or-
ganic ones. Virtually every organic substance will char if hot enough and
yield charcoal, that is, carbon. Roast pork and apples can be burned to char-
coal; chicken fat and chicken feathers make a lively fire.
Protoplasm contains many organic compounds which continually shift
through interactions with one another. The most abundant of these are car-
bohydrates, lipids or fatty substances, and proteins. They constitute the main
part of food and are included in the discussion of foods and digestion (Chap.
11), but their distribution and importance make many other allusions to
them essential. Certain fundamental facts about them may be appropriately
taken up here with protoplasm.
Chap. 3 LIVING MATTER AND CELLS 29
Carbohydrates. All protoplasm is believed to contain carbohydrates. Those
of one group (pentoses) are one of the main components of the chromatin
in the nuclei of all cells. Qther than that important role, carbohydrates are
not actually a part of protoplasm but are only contained in it. Their great
function is the immediate supply of energy, of which they are the chief
source for all living organisms.
The familiar carbohydrates are sugars and starches, the cellulose in the
walls of plant cells, pectin, and glycogen or animal starch stored in animal
cells (Fig. 3.2). Cellulose gives stiflfness to plant stems and forms most of
the fiber of cotton. Pectin, a carbohydrate of fruit, insures the stiffening of
jelly. Starch in plants and glycogen in animals are the reserve food supply
of the cells. They occur in the watery solution of protoplasm and the mole-
cules come and go through cell membranes (Fig. 3.3).
All carbohydrates contain only carbon, hydrogen, and oxygen. In forming
them, untold numbers of green plants capture the energy of the sun, the
source of energy for all living matter, and use this energy to combine carbon
dioxide with water, thus creating the energy-packed food, glucose, and the
by-product oxygen.
The simplest of the carbohydrates are sugars, all of them more or less
sweet. They include the simple sugars, pentoses with five and hexoses with
six carbon atoms (CoHjoOe), the latter including glucose (also called dex-
trose). This is an almost universal protoplasmic fuel. It is the form of sugar
present in human blood in which the essential blood-sugar content is about
0.1 per cent. One of the compound sugars (polysaccharides) is table sugar
(sucrose, Ci:.HooOii) from sugar cane and sugar beets. It is the commonest
sugar in the nectaries of flowers, easily tasted in violets and columbines.
Sucrose is produced by the union of a molecule of glucose with one of
. .,^vV-
Fig. 3.2. Glycogen (black) or animal starch in human liver cells. It is stored in
many kinds of cells but is most abundant in the liver and muscles. Soluble in water
and therefore in protoplasm it is a quickly available food. (Courtesy, Bremer and
Weatherford: Textbook of Histology, 6th ed. Philadelphia, The Blakiston Com-
pany, 1944.)
30 THE FOUNDATION Part I
fructose and the loss of a molecule of water — glucose (CiHu-Oo) + fructose
(C,iH,:.0(i) — HjO = sucrose (Ci^-H^i-On )• When it is hydrolyzed sucrose
gives one molecule of glucose and one of fructose.
Other compound sugars are starch, glycogen, and cellulose. These contain
units of simple sugars combined into large molecules. Starch is the common
storage form of carbohydrate in plant cells and glycogen or animal starch in
animal cells. The molecules of both are too large to go through the cell mem-
branes, but protoplasm can hydrolyze both and obtain glucose with its smaller
molecules.
Fats. Fatty substances take part in the composition of cell membranes
and therefore in their selective permeability (Fig. 3.3). In animals they
constitute the principal supply of food. They produce more energy per gram
than carbohydrates but oxidize more slowly and are less quickly accessible.
Fat persons get hungry just as soon as lean ones. Fats are the backlogs of
the fire of which carbohydrates are the kindling. Fats are abundant in animals
and by no means absent in plants. They may be in the cells, as in bacon,
or in the secretions that cells produce, as in cream, or in the wax of honey-
comb.
Fig. 3.3. Diagram of a cell membrane where there is continuous activity, con-
stant separation of what shall and shall not pass in and out of the cell. These
processes are discovered by chemical analysis. Here, the cell membrane is shown
cut so that its inner surface is at the left and its outer edge at the right. Lipoid
(fatty) particles are shaded, protein particles are white. Water channels (arrows)
permit water and other smaller molecules to pass. Larger molecules are blocked by
the small pores but those that are soluble in fats may enter the lipoid (shaded)
particles of the membrane, mix with their molecules and thus pass in or out of the
cell. (Courtesy, Gerard: Unresting Cells. New York, Harper & Bros., 1940.)
Chap. 3 LIVING MATTER AND CELLS 31
Fats resemble carbohydrates in being composed only of carbon, hydrogen,
and oxygen but differ in the proportions of each of these, the hydrogen atoms
being twice as numerous as. those of carbon and the amount of oxygen rela-
tively small. Fats are colloids, relatively insoluble in water. They liquefy at
various temperatures, oils at room temperature or lower, others near the
body temperature of the animals in which they occur. Those of snakes and
other cold-blooded animals liquefy at relatively low temperatures.
The complex phosphorus-containing fats (phospholipids) include lecithin,
abundant in egg yolk, in nerve tissue, in bile, and blood. The steroids, an-
other group of fatty substances, include cholesterol, well known in the bile
and gallstones. The male and female sex hormones are also related to these
fats. Certain vitamins are associated with them; the growth vitamin A and
vitamin D, which prevents rickets, occur especially in butter and cod-liver oil
and in green vegetables; the fertility vitamin E is in butterfat and lettuce.
Proteins. All protoplasm contains proteins. They are the keystones in its
organization and next to water its most abundant compound. Different pro-
teins occur in different kinds of cells. The proteins of every species of organ-
ism evidently differ from those of every other. The kinship of animals is
recorded in the proteins of their blood. Proteins in the blood of whales that
have lived in the sea for countless generations are more like those of their
relatives, the land mammals, than of their neighbor fishes. Proteins are
prominent in the nuclei of all cells. Chromatin, the chief physical basis of
heredity, is composed of nucleic acid and extraordinarily complex proteins.
The nuclei of the male and female sex cells together contain most of what
determines the inherited qualities of an offspring, maybe its chance to become
a codfish or a senator.
Proteins are the most complicated and various of all substances. They are
composed not only of carbon, hydrogen, and oxygen, like the carbohydrates
and fats, but include nitrogen, sulfur, phosphorus as well. Their molecules
are very large, often containing thousands of atoms, and are complex, and
variable like living matter itself. This means variety of structure and enables
protein to interact with many other substances and to share continually in
the metabolism without which life ceases.
Proteins are constructed of chains or groups of smaller molecules called
amino acids, the simplest of which is glycine (C1.H5O0N) which can be syn-
thesized in the body. Molecules of proteins are too large to enter cell mem-
branes, but those of amino acids go through them freely and form within
the cell the kind of proteins which are characteristic of it (Fig. 3.3). By
varied combinations of about thirty-odd amino acids, a variety of protein
molecules enormous beyond imagination is achieved. They not only differ
with every species but with every individual. This is shown in many ways,
such as the usual difficulty in skin grafting, even between nearly related
32 THE FOUNDATION Part I
persons as contrasted with its success between identical twins. The variety
of proteins is no less remarkable than their constancy. One remembers the
whales that after thousands of years in the ocean still have blood proteins
similar to their near kin on land. In the inheritances of plants and animals
proteins have not only kept their basic patterns for millions of generations,
but countless variations have been added, making their constancy all the
more remarkable.
Enzymes, Vitamins, and Hormones. These are associated with other sub-
jects that are discussed later, the first two with foods and digestion, the
hormones with endocrine glands. All known enzymes and many of the vita-
mins and hormones are proteins or intimately associated with proteins and
all are catalysts.
Enzymes are vital ^catalysts of living matter affecting the rate, and even
initiating chemical reactions of all cells. Their importance is realized in light
of the fact that they participate in the breaking down of proteins into amino
acids, of starch molecules into simple sugars, and of fats into fatty acids and
glycerol before any one of them can go through a cell membrane (Fig. 3.3).
Characteristics of Protoplasm
The physical basis of life is made of common materials largely composed
of a few of the most abundant substances in the earth and atmosphere, all
of them easily attainable. Its organization is in the highest degree complex,
a continuous series of reactions which follows a permanent general pattern
with details that are related to particular surroundings. It has its own char-
acteristic organization and punctuality, precision of arrangement, and inter-
dependence of parts. Plants and animals exist in multitudinous variety yet
they are fundamentally similar. They all have the capacity for the composite
of continual chemical changes called metabolism.
Protoplasm has a capacity to change and yet hold its stability: in its con-
tent of water, an almost universal solvent; in its abundance of proteins; in
its colloid structure, with variability in size and shape of particles allowing
large total areas of exposure to surrounding influences and subject to con-
tinuous movement. It is susceptible to external and internal influences and
consequent shifts in the phases sol and gel. It has rhythms and continuity of
income and outgo of materials, resulting in a balance maintained between
constructive and destructive changes.
Cells
Cells are the units of the architecture of plants and animals. A cell is a
bit of protoplasm containing a nucleus without which it cannot grow or re-
produce itself (Fig. 3.4). As long as it lives the cell constantly builds and
burns in the unceasing chemical changes of metabolism.
Chap. 3 LIVING MATTER AND CELLS 33
A cell is enclosed by thin protoplasmic layers forming a semipermeable
membrane. This membrane is the lifeguard of the cell. It is permeable to
certain dissolved substances but impermeable to others, a constant control
over what may enter or leave the cell. The plant cell produces on its outer
surface a definite wall that is not living, an important difference between it
and the animal cell.
Cells may live independently of others and if so each behaves like a com-
plete organism, as an ameba does. In multicellular animals each cell is con-
tinually affected by its relations with others, and by the behavior of the whole
cellular community comprising the animal of which it is a part. A cat consists
of billions of cells, yet when it springs on a mouse it moves as a single organ-
ism.
Origin and Importance of Cells. Every cell originates from a preexisting
one and in no other way. This is a complex process during which the new
cells receive equal amounts of this essential substance of a parent cell. Every
1P1 za,s-m.o^o-co.e
Ciar ora&t l-rx-
Cy to-piasin
PlasUci-
P'a.t ^loioTjules
Ceil wall
■Ceil
Vea.c-cLoie
hA.itoc'hio-rxS-tPi.m
Fig. 3.4. Diagram of body cell. Some of the parts are visible only after special
preparation and very high magnification. Plasmosome is another term for nuclet)]us.
The karyosome is a body of nuclear substance. Organoids such as the centrosome,
chondriosomes (mitochondria), Golgi bodies, and fibrillae are parts of the cell that
have particular functions. Plastids are characteristic of plant cells. There may be
many nonliving inclusions, e.g.. droplets of water; and granules of yolk: the yolk
of a hen's egg is loaded with these. The inner cell membrane, an extremely thin
layer of protoplasm, is ordinarily invisible. It is in close contact with the porous
outer cell membrane (or "wall"); m animal cells the inner and outer membranes
are together and commonly called the cell membrane. (Courtesy, Stiles; Individual
and Community Health. New York, The Blakiston Company, 1953.)
34 THE FOUNDATION Part I
multicellular animal begins its existence as a single cell which soon divides
into two. Each of these grows and divides into two, and thus in the majority
of the cells the repeated growing and dividing go on as long as the animal
increases in size, whether it is a flea or a cow. This reproduction of cells is
entirely independent of sex.
The characteristics of a many-celled animal are the expressions of its cells
acting together. A bird flies and its sensory cells react to light, gravity, and
air currents; its nerve cells carry messages to and from the brain; its muscle
cells contract; its body consumes more oxygen and releases more energy as
flight demands it. The responses of its cells are the links between the bird
and the world about it.
Structures and Functions. Interphase means that the cell is in a phase of
life between divisions. In this phase, also called the resting stage, the cell is
resting from division. It is not in any sense resting from respiration and
other routine metabolic processes. Certain structures are typical of animal
cells though all are not necessarily present in every kind (Fig. 3.4). Some
plant cells do not have an organized nucleus and the chromatin is naked in
the cytoplasm.
Nucleus. The nucleus is essential to the growth and reproduction of the
cell. It is usually clearly defined and sharply bounded by a thin, scarcely
visible membrane. It contains a foundation of nuclear sap in which definite
structures are suspended. In living cells the nuclear sap looks watery; in
prepared cells it often shrinks away leaving open spaces. With rare exceptions,
the nucleus alone contains chromatin, the physical basis of heredity and the
most remarkable substance of protoplasm. The delicate, darkly staining
threads, the chromonemata or color threads form a webby network in the
nuclear sap. They represent the future chromosomes. One or more minute
spherical bodies, the nucleoli, are often conspicuous during the interphase;
their substance disappears during cell division, much of it being incorporated
in one or more chromosomes.
The importance of the nuclei has been shown by removing them from
living cells and noting the results. An ameba can be cut in two so that only
one part contains a nucleus. After such treatment the part without the nucleus
will live for some days, will respire, digest its food, and move about but it
doe» not grow or reproduce. On the other hand the part containing the
nucleus grows, replaces the lost part, and finally divides as usual. All well-
established cells have nuclei at some time during their life history. The red
blood cells of man and other mammals have no nuclei when mature as they
usually are when in circulating blood. However, nuclei are always present
when the cells are first formed.
Cytoplasm. As already defined, the cytoplasm is all of the cell except the
nucleus. The ground substance of cytoplasm is a clear semifluid, the hyalo-
Chap. 3 LIVING MATTER AND CELLS 35
plasm (Fig. 3.5). In living cells it looks like white of egg; in stained ones it is
usually granular, sometimes with and sometimes without a delicate network
running through it.
The cytoplasm is enclosed by the protoplasmic semipermeable membrane,
mentioned earlier in this chapter as the lifeguard of the cell. It controls the
passage of everything that comes in or goes out of the cell, water, the respira-
tory gases, digested food, and other materials. Likewise it regulates the dis-
posal of waste substances from the cell.
Fig. 3.5. A bit of seemingly
homogeneous protoplasm in a
clear space in the living cell. Very
highly magnified it shows particles
such as protein molecules and
others that are jostled about by
molecules of water and other
smaller molecules. (Courtesy,
Gerard: Unresting Cells. New
York, Harper & Bros., 1940.)
The semipermeable membrane has submicroscopic holes through which
smaller molecules, such as those of water and amino acids can freely enter
or leave the cell. The passages are too small for the larger molecules. How-
ever, those that dissolve in fat merge with the fatty substances in the mem-
brane and pass between their molecules and into the cell (Fig. 3.3). Such
fat substances include alcohol, ether, and many organic compounds. Mole-
cules of these, among them alcohol and anesthetics, may enter in such numbers
that they clog the surfaces of the cells and slow down their normal activity.
Brain cells are especially rich in fat and take in alcohol or an anesthetic and
are strongly affected by them, while muscle and other kinds of cells may be
undisturbed. Thus, the cell membranes figure at the cocktail party as well as
in the hospital.
In both animal and plant cells, but more commonly in the latter, there may
be vacuoles, evidently surrounded by ultradelicate semipermeable membranes
and usually containing liquid.
The cytoplasm contains the organoids which reproduce themselves, thus
exhibiting one of the fundamental characteristics of living matter. It also
contains nonliving cell-inclusions (Fig. 3.4).
Organoids. The centrosome consists of a spherical mass of specialized
36
THE FOUNDATION
Part I
protoplasm called the centrosphere and at its center are either one or two
minute, deeply staining bodies, the centrioles. During the interphase of the
cell the centrosome is almost always located just outside the nuclear mem-
brane (Fig. 3.4). It plays an important part in cell division and at that time
divides into two parts from each of which rays extend stimulating a star.
Centrosomes have been found in practically all animal cells except nerve
cells, but are not present in those of higher plants. Chondriosomes (mito-
chondria) are threadlike or granular bodies (lipoproteins) scattered through
the cytoplasm, visible in specially treated cells and sometimes in living ones
(Fig. 3.4). It is generally agreed that they are physiologically important
although details of their function are unknown; in actively secreting cells
they increase in size and number. The Golgi substance is an irregular net-
work located near the nucleus, first discovered by Golgi, an Italian physician
(1898), in nerve cells and later found in almost all the cells of vertebrates
and in many invertebrates, especially in glands. Its nature continues to
be debated. Fibrillae are fine threads that extend in a definite direction in
the cell and may have a supporting, conducting, or contractile function (Fig.
3.6). CiHa and flagella are thin cytoplasmic processes extending from the
Fig. 3.6. Extremely minute fibrils stretched by a
microdissecting needle (black spot) pulling out one side
of the living cell (a malarial parasite, Plasmodium).
(From Seifriz. Courtesy, de Robertis: General Cytol-
ogy. Philadelphia, W. B. Saunders Co., 1949.)
surface of the cell and are used in locomotion or to create currents of fluid.
Flagella are relatively long; there are few of them to a cell and different
ones lash independently. One group of protozoans, the flagellates, are so called
because they swim by means of flagella. Cilia are short and there are many
on one cell. They move in unison, rhythmically. Paramecium is the most
familiar ciliated protozoan though there are many others. In multicellular
animals surfaces are often covered with ciliated cells: the lining of the human
trachea, the gills of clams, the gullet of a frog. Gills of fresh, as well as salt
water clams, are good material for the study of ciliary movement.
Nonliving Cell-inclusions. In animal cells the most abundant of these
is stored food: yolk granules and oil globules in eggs, glycogen in other cells
(Fig. 3.4). In gland cells the materials to be secreted are often held in the
Chap. 3 LIVING MATTER AND CELLS 37
cells as droplets or granules. Crystals, pigment, and droplets of water and
waste matter are common cell-inclusions.
Shapes and Sizes of Cells. The shape of a cell depends upon the viscosity
of its protoplasm, the pressure from other cells, and upon its function (Fig.
3.7).
Most cells are microscopic, with dimensions of a few thousandths of a
millimeter (1 mm. = 345 ot an inch). Certain nerve cells of man and other
large mammals have processes that extend from the cell bodies in the nerve
Cell wall
Cytoplasm
Nucleus
Cells have thickness
Cells are usually seen in slices
B
Columnar often with cilia
at one end
Thin plates of
lining cells
Cuboided
for covering
/^^^
Packed m cords
.'. .. '.l.i.i.i.i_L'|l'i' il ' ■ t^»i.d*«g^i^
"""'•"'••''"" 'flT'i'i ill" liti'i"''''"'"^
Elongated in the direction of the pull
Fig. 3.7. Shapes of cells. In a multicellular organism most of the cells are pressed
together, often flattened, or six- or eight-sided. It has been recently maintained that
packed cells are actually 14-sided. This is apparent only under special conditions
and observation. A. Diagram of a cell cut in section as cells are commonly studied.
B. The shapes of these cells, muscle and others, are correlated with their special
functions and also affected by crowding.
38 THE FOUNDATION Part I
cord along the whole length of the leg. The largest single cell is the unfer-
tilized egg, commonly called the yolk, of an ostrich's egg. The egg cells of
birds, reptiles, and amphibians are all large because of the yolk stored in
them. Relatively large or small body cells are characteristic of different
groups of animals. Cold-blooded amphibians with low metabolism have
larger body cells than warm-blooded birds and mammals whose body tem-
perature and metabolism are high. A horse has smaller cells than a salamander
and literally lives faster because it has a relatively greater cell surface exposed
to body fluids bringing in oxygen and food and taking away waste.
Differentiation of Cells. Diff'erentiation is a process of becoming different
and specialized. The skin of an embryo fish seems to be all alike; then scales
and glands develop in it. The possibility of difference was there, but it ap-
peared only under certain conditions. The epitheliomuscle cell of hydra has
become specialized for contractility at one end. Shapes and sizes of cells,
already mentioned, are results of differentiation. They are inherited patterns
brought out and also modified by the surroundings of successive generations
through the ages.
Polarity of Cells. Polarity of a cell is consistent — difference between
opposite regions. It is a special kind of differentiation as in the epithelio-
muscle cell of hydra, one end useful as lining or as a gland, the other end
muscular. Polarity is almost universal in cells as it is in all living organisms.
Among the diverse examples are nerve cells in which the impulse enters
at one end and passes out the other, and gland cells in which the secretion
collects and passes out through the membrane at one pole. The polarity of
plants and animals is well known by the differences in the opposite ends as
in a turnip, a rose bush, or a donkey.
Phases in the Life of the Cell
Every cell goes through two phases: the first includes its growth, metabo-
lism, and characteristic activity, such as secretion; the second includes metab-
olism and reproduction by division.
Interphase. The individual lifetime of the cell is known as the interphase.
It begins when the cell is produced by the division of a parent cell and lasts
until the cell itself divides or dies. The structure and general characteristics
of an animal cell have already been described and shown (Fig. 3.4). Further
mention of conditions in the nucleus should now be made. The nucleus con-
tains a tangle of threads of chromatin, the latter containing genes, the bear-
ers of hereditary traits. The chromatin threads are double, made up of two
slender strands, the chromonemata or colored threads in which lumps of
chromatin, the chromomeres, the probable locations of groups of genes, are
arranged irregularly. The two chromonemata are actually two future chromo-
somes lying so close together that the doubleness is difficult to discover.
Chap. 3 LIVING MATTER AND CELLS 39
Each pair of chromonemata was formerly a single thread (potential chromo-
some) with genes arranged along its whole length. As a thread doubles,
each gene makes a duplicate of itself out of materials lying close to it. As
a result of this a new string of genes, forming a new thread, lies close to
the old one and is identical with it, gene for gene, in every part (Fig. 3.8).
This creation of new genes, as pointed out by H. J. Muller (1947), "should
perhaps be regarded as the most remarkable process in nature; it consists
of the simultaneous creation, under the guidance of each gene, of a new
gene in its own image, lying next to itself and built out of materials lying
around it" (Fig. 3.8). Now having the layout of its future chromosomes,
each with its quota of genes, the nucleus is ready for reproduction.
Reproduction of Body Cells — Mitosis. Cell division usually includes that
of the nucleus and cell body. However, the nucleus may reproduce when the
cell body does not and a multinucleate cell results. The cause of cell division
is not understood. If it were, the cause of cancer would be known, since
that is a disease of too rapid and usually abnormal cell division.
Mitosis is the almost universal method of cellular reproduction. The only
significant exception is the variation of it called meiosis which occurs regu-
larly in the multiplication of sex cells. Mitosis is the precise rearrangement,
doubling, and separation of nuclear material by which two new nuclei are
formed that are quantitatively and qualitatively similar to each other and
to the nucleus from which they came. By means of it each daughter nucleus
receives an equal share of every substance which was in the parent nucleus.
It is a continuous process having four main stages; each stage has its own
characteristics but each merges into the one following (Fig. 3.8).
Prophase (Preparation). Features of the interphase gradually change.
The knotted chromonemata are more distinct with the members of each
pair clinging together. At first each pair forms an irregular open spiral.
Then the coil tightens, shortens, and is filled in with darkly staining sub-
stance finally forming a chromosome. At the same time the centrosome just
outside the nucleus is active. It divides, and, if the cell has two centrioles,
they move toward opposite poles of the nucleus. If there was but one centriole
during the interphase, it now divides and the two new ones move apart. In
either case the area between them contains lines of protoplasmic particles.
These form the mitotic spindle, a double cone that at first lies a little outside
the nuclear field and later extends directly across it. This region is now occu-
pied by the chromosomes among the lines of the spindle and directly between
its dynamic poles. The nucleolus may still be visible, but it looks soft as its
substance begins to diffuse, seeming to scatter.
Metaphase (Midway). The chromosomes are balanced midway between
the poles of the spindle (Fig. 3.8). Each one of the two chromonemata in a
chromosome has at exactly the same level a special point (centromere) of
40
THE FOUNDATION
Part I
y
egg (cell) membrane
3 sperm
chromosomes
3 egg chromosomes
I. Egg shortly offer fertilization
oecomes the first cell of the embryo.
2. The nucleus formed by the
coalescence of sperm and egg
nuclei. Interphase.
3. Soon after duplication of the
chromosome threads. Early prophase.
4. Chromosomes shorter and thicker.
Aster dissolving the nuclear membrane.
Later prophase.
5. Lines from centrosomes are
attached to each chromosome at a
given point. Early metaphase.
6. Lines of force from centrosomes
exert a pull that separates "sister
chromosomes'! Later metaphase.
7. Pulling apart of two identical
groups of chromosomes. Body of
cell dividing. Late anaphase.
8. Two separate cells, each with a
nucleus of tangled threads as in 2.
Interphase (Telophase omitted)
Chap. 3 LIVING MATTER AND CELLS 41
attachment to the spindle. This "owes its existence to a particular gene lying
at that point" (Muller, 1947). When it is at the center the chromosome is
V-shaped with the tip of the V in contact with a line of the spindle. Some-
times it is fairly near the end and the chromosome then hangs J -shaped on
the spindle, or, if very close to the end, it is rodlike. During the early part
of the metaphase the centromeres are apparently repelled from the poles
of the spindle and moving toward the equator they draw their chromosomes
with them. There, all the chromosomes become arranged exactly half-way
between the poles of the spindle at the midplane in an equatorial plate. The
chromosome and its duplicate are still in contact (Fig. 3.8),
Anaphase (Separation). Each chromosome and its duplicate begin to
separate always starting at the centromeres which are responsive to the forces
of the attraction of the spindle. The members of each pair of chromosomes
gradually draw apart until they become entirely separated and each one
moves toward the nearer pole. During this journey the centromere is always
in front, pointed toward the pole (Fig. 3.8). Late in the anaphase the
chromosomes are in two identical groups, one at each end of the spindle.
In each group the chromosomes of the parent cell with their genes are all
represented.
In animal cells the division of the cytoplasm starts from the outside and
Fig. 3.8. Diagrams showing changes in the nucleus during the reproduction of a
cell by mitotic division such as occurs in every cell of a growing body, or in parts of
the adult, except in the later divisions of maturing sex cells. /. Part of an egg
shortly after its fertilization. Three chromosomes (black) represent the inheritance
from the male parent, and three (in outline) the inheritance from the female
parent. The descendants of these six chromosomes occur in all the cells of the new
individual. The star-shaped centrosome is a center of force. 2. Interphase or "rest-
ing stage" with the chromosomes uncoiled in threads so ensnarled that individual
chromosomes cannot be identified except with great difficulty. 3. Early prophase.
Each thread has doubled and now consists of two identical strands, thickened by
means of the ultra-fine coiling of the strands. The centrosome has divided and there
are now two centers of force. 4. Late prophase. The chromosomes are mates,
shortened and lying side by side. Every one of the thousands of genes contained
in one is duplicated in the other. The centrosomes are moving to opposite sides of
the nucleus and the nuclear membrane is dissolving. 5. Early metaphase. Lines of
force from the centrosomes have become attached at given points (centromeres)
to the respective mates, called identical chromosomes. This has forced them into
positions on an equatorial plane half way between the centrosomes. 6 and 7. Late
metaphase and late anaphase. The apparent lines of force exert a pull on the
centromeres, thus separating the identical chromosomes, and drawing the respective
mates toward the opposite ends of the spindle formed by the lines of force.
8. Division completed. Interphase. (Telophase omitted.) The two identical groups
of chromosomes are pulled near to the centrosomes and ceil membranes separate
the cell body into halves. The fine coils of the chromosomes unwind in threads
similar to those in 2. This process occurs in the telophase stage not shown here.
With the attainment of two new cells in the interphase stage, the reproduction is
completed.
42
THE FOUNDATION
Part I
Fig. 3.9. Stages (metaphase and anaphase) in the mitosis of cells of a white fish
embryo. Microphotographs of stained and sectioned cells at an enlargement of
about 700 times. Note the lines of force that compose the spindle and radiate from
the centrosome in the metaphase, and the dimming of the spindle and the new
cell membranes in the anaphase. (Courtesy, General Biological Supply House, Inc.,
Chicago.)
the membrane separating the two new cells extends inward in a plane at right
angles to the spindle. In plant cells it starts from the center as a cell plate
and extends outward.
Telophase (Reconstruction). The chromosomes in each nuclear group
uncoil and lengthen into knotted chromonemata. The spindle and at the same
time the rays about the centriole disappear. If two centrioles are characteristic
of the interphase each centriole now divides; if not, each one remains single.
The nucleolus becomes visible again and the boundary of the nucleus regains
Chap 3 LIVING MATTER AND CELLS 43
its sharpness. The daughter cells are now complete growing cells in the inter-
phase stage.
The time required for the complete process of cell division varies greatly
with the kind of cell and the surrounding conditions, especially temperature.
A cell of a salamander's heart observed living in tissue culture completed
the process in two hours. The process may be much quicker.
Results of Mitosis. Two cells are formed that are identical with one
another in respect to every gene and every chromosome. This is accomplished
first by the doubling of the genes in the chromosomes, and then by the sepa-
ration of the chromosomes and their inclusion in the new nuclei. The re-
mainder of the cell may or may not be equally divided. In the growth of a
multicellular animal, whether hydra or man, mitosis is repeated thousands to
billions of times, and each time hereditary qualities originally received from
the parents and contained in the first cell are distributed equally to new cells.
In amitosis the nucleus simply constricts into an hourglass shape and then
separates into two parts without forming chromosomes. This is a very rare
arrangement which occurs only under unusual conditions, especially in de-
generating cells.
Reproduction of Sex Cells — Mitosis and Meiosis. Body cells reproduce
exclusively by mitosis. Germ or sex cells reproduce by mitosis and meiosis.
The reproduction of sex or germ cells in males and females includes an in-
crease in numbers from a few original germ cells, a reduction to half their
number of chromosomes, i.e., from the diploid to the haploid number, and
changes in the shape and size of the cells (Fig. 3.10). The all-important genes
inherited from the parents of the individual and present in the chromosomes
of his or her original germ cells are distributed so that each gamete (egg and
sperm cell) has an inheritance from its ancestors, even remote ones. The
process in the male is spermatogenesis, the history of the sperm cell from its
earliest stage to maturity, and in the female, oogenesis, the history of the egg
cell. There are differences in size and numbers of the mature sex cells in the
male and female, but the changes in their nuclei are essentially similar.
Spermatogenesis. The original primordial germ cells in the male divide
repeatedly by mitosis, gradually producing great numbers of extremely
minute, nearly spherical cells called spermatogonia. These have the diploid
(or body) number of chromosomes; half of them were in the male cell or
sperm and half in the female cell or egg when fertilization occurred.
Suppose, for example, that a primordial germ cell has six chromosomes,
three derived from each parent (Fig. 3.10). Such cells divide mitotically,
producing several generations of cells called spermatogonia, each one of
which contains six chromosomes. A change then occurs beginning with the
maturation or meiotic divisions. First the cells become relatively larger and
are called primary spermatocytes. In the prophase of the first meiotic division
44
THE FOUNDATION
Part I
Distribution of chromosomes in the developmeni of
sperm cells. Dork chromosomes = mole inheritance.
Light chromosomes = femole inheritance.
Body cell of fother
I.e. skin, muscle, etc
Germ cell destined to divide
and develop into sperm cells
Spermatogonium
Primary —
spermatocyte
A.B.MEIOTIC
divisions
Secondary
spermatocyte
MITOTIC
divisions
Cell enlarges
Similar chromosomes pair
(Synapsis)
Eacn chromosome duplicates
itself. Tetrads result. Tetrads
separate into pairs. Cell divides.
MEIOSIS
Tetrads separate into pairs.
Cell divides
— Sister chromosomes
separate.
Spermatids
Sperm
cells
Fig. 3.10. Diagrams showing the behavior of the chromosomes during (A) the
development of the sperm cell (spermatogenesis) and (B) the similar features in
the development of the egg cell (oogenesis). In each sex cell the process includes:
Chap. 3
LIVING MATTER AND CELLS
45
Distribution of chromosomes m the development of
egg cells. Light chromosomes = female inheritance.
Dark chromosomes = mole inheritance.
MITOTIC
divisions
Body cell of mother I.e.
skin, muscle, etc.
Germ cell destined to divide
and develop into eggs.
Oogonium
Fertilization
Cell enlarges
Similar chromosomes pair
(Synapsis)
Each chromosome duplicates
itself. Tetrads result.
Tetrads separate into pairs
A.B. MEIOTIC divisions
Cell divides, 3 pairs of
chromosomes in each
Cell divides
3 chromosomes
./a V
Mature
egg
Second polar body
B
Primory
Oocyte
Primary
Oocyte
Secondary
Oocyte
First
polar body
These cells
die
an increase in number of chromosomes by MITOSIS and a reduction in the number
of chromosomes by MEIOSIS. For simphcity six chromosomes are used here for
body cells. Cells of the human body have 48 chromosomes.
46 THE FOUNDATION Part I
the two chromosomes of each similar or homologous pair, one derived from
the male and one from the female parent, come together and lie parallel to
one another. This is called synapsis. Soon each chromosome duplicates itself
as in mitosis, so that there is a cluster of chromatids (potential chromosomes),
a quartet or tetrad in which two chromatids are of male and two of female
parental origin (Fig. 3.10). A spindle forms and in the metaphase the tetrads
become arranged on its equator. In the anaphase, the two chromatids of
female parental origin in the tetrad go to one pole of the spindle and the two
chromatids of male parental origin go to the other. Each of the resulting cells
is a secondary spermatocyte with three chromosomes, each of which contains
two chromatids. In these secondary spermatocytes a spindle soon forms for
the second meiotic division, and in the metaphase the two chromatids of
each chromosome separate and one goes to each pole. Each of the cells
(spermatids) that result contains three chromosomes. Some of the cells may
hold chromosomes entirely of male or entirely of female parental origin; some
may hold chromosomes of both origins. Meiosis is now completed, the
chromosome number being reduced by half, i.e., to the haploid number. The
rest of the process is a change in form. The nucleus becomes more compact
and the cell body relatively minute with a slender cytoplasmic tail or flagellum
that acts as a swimming organ. At its base is the bead-shaped middle piece
that holds the centrioles (Fig. 3.10). Thus, from each primary spermatocyte
four sperm cells (gametes) are formed. The foregoing process is usually com-
pleted before the sperm cells leave the testis.
Oogenesis. Fewer and larger sex cells (gametes) are produced in oogenesis.
Great numbers of oogonia result from divisions in the period of multiplication
(Fig. 3.10). Following this period certain of the oogonia become primary
oocytes which grow to be larger than the spermatocytes, the comparable stage
of the male germ cells. But they are similar to them in the behavior of the
chromosomes, in synapsis, tetrad formation, and the reduction of the number
of chromosomes in the first meiotic division. In this division, however, one
secondary oocyte receives practically all of the cytoplasm along with its three
chromosomes, while the other one, called the first polar body, has very little
cytoplasm with the same number of chromosomes. Likewise in the second
meiotic division, the large secondary oocyte divides unevenly. The bulk of the
cytoplasm surrounds the nucleus of the incipient egg (ootid or ovum) with its
three chromosomes. The little remaining cytoplasm and the nucleus contjain-
ing three chromosomes compose the second polar body, actually a rudimentary
egg. The first polar body goes through a division that parallels the second
meiotic one. Thus there are three polar bodies and the egg, each with three
chromosomes assorted as in spermatogenesis (Fig. 3.10). The polar bodies
with their loads of precious hereditary substance eventually degenerate and
come to nothing. The egg keeps its form and is enlarged by its supply of yolk.
Chap. 3 LIVING MATTER AND CELLS 47
In different species of animals the production of polar bodies may occur
inside or outside the ovary.
With the fusion of the nuclei of sperm and egg that occurs at fertilization,
the number of chromosomes is returned to six, that of the zygote, the first cell
of the new individual.
Part II
Ecology
4
Plants Provide lor Tneniselves
ana me Animals
The existence of the living world depends upon green plants since they
alone make the food that is essential both to themselves and the animals.
Through the long past animals became agile of movement, swimming, running,
or flying, developed keen senses, and became alert to their surroundings. Great
numbers of them fed upon plants, and as time went on many became carni-
vores and devoured their fellow animals. But none of them could make their
own food from the chemical elements about them. Human beings are no better
off than other animals. Although they have extraordinary capabilities, their
existence finally depends upon the carbohydrate foods, the sugars and starches
that green plants make by photosynthesis. After years of study it now seems
that photosynthesis may be understood, but to furnish the world with food
is another and probably much more difficult matter.
The meals of Eskimos are far removed from the cabbage patch, yet they
too originate in plants. Eskimos live on seal meat and fish and birds, but ulti-
mately all these are fed by the microscopic plants which swarm in the arctic
seas. The seals and the birds feed upon the fishes; big fishes eat little fishes
and both devour little copepods by billions; and finally copepods feed ex-
clusively upon microscopic plants, mainly diatoms (Fig. 4.1). Thus, the
substance of the Eskimo's diet is in origin mainly digested diatoms. For the
dweller farther south in America or Europe the food chain is different, usually
beginning with grass and ending with beef, or starting with diatoms and ending
with codfish. Grass can live without cattle and diatoms without codfishes but
no animals can exist without plants somewhere in their food story. Plants and
animals are fundamentally similar. A sunflower and a horse look strikingly
different; yet they are both living organisms existing basically in the same
way.
51
52
ECOLOGY
Part II
I diatoms ) > ( copepods | — ► (crustaceans]
Fig. 4.1. In their own way of living Eskimos are finally dependent for food upon
diatoms and other algae, the microscopic plants that crowd the surface waters of
the arctic seas. The dependence is indirect but sure, just as farther south human
dependence for beef steak is upon plants. (After Transeau and Tiffany: Textbook
of Botany. New York, Harper & Bros., 1940.)
Plant and Animal Relationships
Building Materials and Protection. Plants furnish building materials for
all animals from insects to man. Wasps bite off wood fibers for their paper
nests, a host of insects lives within burrows in stems and tree trunks. The
habits of land birds would be changed beyond recognition if those birds did
not perch and nest in trees, or nest and feed in grass and mosses. There is
scarcely a mammal, short of ocean-going whales and their kin, that does not
at some time take to plants for shelter. Hundreds of field mice live among the
grasses of empty-looking fields; the wildcat climbs a tree for a meal of young
birds; in South America trees furnish the bandstands for the howling monkeys
and the hammocks for sleeping sloths. In the noon heat of the tropics the
silent forest is populous with hiding animals.
With the main exceptions of beavers and man, mammals do not use wood
for building. Man is the great builder with plant fiber. From the time human
animals left their caves they began to make earthen and wooden houses and
long before that they must have used windbreaks of wood. The prehistoric
lake-dwellers lived in wooden houses raised on piles above the lakes, ideal for
safety as well as for fishing at home.
Throughout history plants have supplied humanity with wood for boats
and wagons, and fibers for ropes and cloth. In recent years the elegant and
versatile rayons and plastics have been produced mainly from plant products.
The existence of all this outfit of civilization hinges upon a microscopic struc-
Chap. 4 PLANTS PROVIDE FOR THEMSELVES AND THE ANIMALS 53
ture peculiar to plants, their strong cell walls composed of cellulose, or cellu-
lose impregnated with lignin if the tissue is woody.
The Plant Cell Wall. Plant cell walls have long provided heat and power
for humanity (Fig. 4.2). Whether lignified or not, cellulose burns rapidly in
combination with oxygen; its stored energy is released in the form of heat and
it is converted back to carbon dioxide and water. When cellulose is subjected
to heat and pressure for long periods of time it undergoes chemical changes;
hydrogen and oxygen are removed and solid carbon remains. This is what
happened in the ancient swamps and forests where peat, lignite, and coal were
formed, one or another product depending upon the material and the stage of
the carbonization. Coal exposed longer and under the right conditions becomes
graphite; exposed still further and properly conditioned, it crystallizes as pure
coal, or with extreme hardness as diamonds. The heated live coal of the open
Fig. 4.2. Typical plant cell. In plant cells the cytoplasm occupies a relatively
small space and the central part contains one or more large vacuoles filled with
watery solution containing many substances related to the life processes of the
plant. The vacuoles are separated from the protoplasm by an almost invisible semi-
permeable membrane (or tonoplast), a lively and important region of exchange of
substances. In contrast to animal cells those of plants have a prominent cell wall
strengthened by cellulose, made woody by lignin. (Courtesy, Rogers, Hubbell, and
Byers: Man and the Biological World, ed. 2. New York, McGraw-Hill Book Com-
pany, 1952.)
54
ECOLOGY
Part II
fire is "alive" in so far as it is freeing energy gathered from the sun and stored
in plant cells millions of years ago. Neither coal nor diamonds are modern
upstarts: both have long been important to humanity, in fires for the tempering
and molding of metals, in various techniques, and in tokens and jewelry.
Distribution. There are many ways in which plants depend upon animals.
Most animals can travel around freely; plants cannot. Plants are carried about
by the natural forces of air and water and by animals. Thus insects carry
pollen (male sex cells) and cross-pollinate the flowers as they seek nectar and
pollen in one after another (Fig. 4.3). Birds carry seeds across land and water
often to germinate safely in distant regions. Plants are directly dependent on
the content of the soil and animals fertilize this with their excretions and
disintegrating remains.
Photosynthesis
Green plants are, with exceptions such as nitrifying bacteria, the only self-
supporting organisms on the earth. They accumulate energy from the sun and
Pathway of male cell
to the
Pollen grains touch
stigmatic surface
Fig. 4.3. The parts of a typical flower. Insects visit flowers to gather nectar and
pollen. The nectaries are at the bases of the petals and many flower-visiting insects
must brush against the pollen-bearing anthers in order to reach the nectar. In mov-
ing around they transfer the pollen containing the male cells to the stigma of the
flower or other flowers of the same kind and thus bring about the fertilization of
the ovules (eggs).
Chap. 4 PLANTS PROVIDE FOR THEMSELVES AND THE ANIMALS
r
55
Fig. 4.4. Carolus Linnaeus (1707-78), the Swedish botanist, at age 25, in Lap-
land dress, holding his favorite, the twin-flower (Linnaea) and equipped with a col-
lecting kit for his Lapland journey. Linnaeus made one of the great contributions
to natural sciences, the two-word naming (binomial nomenclature) of plants in
1753 and of animals in 1758. His work made way for the natural arrangements of
living organisms. (Courtesy, Greene: Carolus Linnaeus. Philadelphia, Christopher
Sower Co., 1912.)
store it as chemical energy in carbohydrates (starches, sugars). The process
of photosynthesis or carbohydrate-making is the greatest chemical industry in
the world with the widest importance of all biochemical reactions. It is carried
on by all chlorophyll-bearing plants from microscopic algae to the largest
trees. Red and brown seaweeds and plants of various other colors contain
chlorophyll cloaked with pigments. Although the manufacture of food by land
plants is enormous, it is estimated that 90 per cent of the total is produced by
the large (seaweeds) and small algae of the ocean (Fig. 4.5). They constitute
the basic food supply of the great animal populations of the seas. In general,
the plants themselves use a good deal of the food which they produce. Much
of it is decomposed into water, carbon dioxide, and mineral salts by the decay
of leaves and plant bodies in water and on land, and is used over again by
the plants.
Materials and Conditions. The natural conditions for photosynthesis include
the presence of chlorophyll, the energy of sunlight or artificial light, water, and
56
ECOLOGY
Part II
Fig. 4.5. Common brown seaweeds that are great food producers. From left to
right, fan kelp, Laminaria: giant or vine kelp, Macrocystis; bladder wrack. Fucus;
ribbon kelp, Nereocystis. (Not drawn to scale.) Seaweeds constitute a large percent-
age of the basic food supply of the seas. On the rocks between the tides where they
abound they furnish food and holdfast for hosts of small animals.
carbon dioxide. The chlorophyll occurs in chloroplasts usually rounded green
bodies in the tissues of leaf and stem. It is a complex protein, in higher plants
consisting of two pigments, a blue-green one, chlorophyll a {Cr.-Mi-Or.NiMg)
and the less abundant yellow-green, chlorophyll b (C55H7oOGN4Mg). The
chemical content of chlorophyll is in many ways similar to that of the hemo-
globin of blood except that iron occurs in the latter instead of magnesium. In
the higher plants chlorophyll is almost always associated with yellow pigments,
the carotenoids, and the various xanthophylls related to carotene. Their func-
tion is not wholly known; if they are concerned with photosynthesis they are
far less important than chlorophyll. Carotene and xanthophyll are much more
stable; the rich yellow autumn colors of birch and elm leaves are exultant
witnesses that these colors endure after chlorophyll has broken down.
The Process. During photosynthesis the kinetic energy in light is changed
to the potential chemical energy of food. Carbon dioxide is mainly absorbed
from the atmosphere. It enters the leaf through the millions of pores or
stomata, diffuses through cell membranes in a dissolved state, and goes into
the chloroplasts (Fig. 4.8). Water enters chiefly through the roots. In the
presence of chlorophyll and with the aid of the energy of light, the carbon
Chap. 4 PLANTS PROVIDE FOR THEMSELVES AND THE ANIMALS 57
dioxide and water unite to form glucose (CoHn-Oe), the simple sugar from
which all the organic compounds of plants and animals are eventually derived.
The chlorophyll itself is not used up and is evidently a catalyzer that hastens
other chemical processes.
Green plants include the seed plants, and the mosses, ferns, the green algae,
and the lichens, many first named by Carolus Linnaeus in his two-name system
(Fig. 4.4) . As already noted, besides these there are other plants whose chloro-
phyll is blanketed with various colors, as in the deep red, yellow, or variegated
Coleiis often called foliage plants. The pigment of red and brown seaweeds also
effectively clothes the chlorophyll as does the brown cloak of the microscopic
diatoms of fresh and salt waters. Although the process of food-making in these
plants is not clearly worked out, it is certain that pigments other than green
ones take an important share in it. One investigator has observed that in red
seaweeds the light absorbed by red pigments is more efficient in photosynthesis
than that absorbed by the green of chlorophyll. The food product in blue-green
algae, for example, is not glucose but glycogen which is also found in fungi
(bacteria, molds, mushrooms, and rusts) and in the tissues of animals. The
tons of rockweed washed by the breakers on many headlands press home the
estimate that "90 per cent of the photosynthesis on earth is carried out, not
by green land plants, but by the multicolored sea algae" (Fig. 4.5).
Studies of Photosynthesis. In 1772 Joseph Priestley discovered that a plant
produced oxygen. He piped air into a glass jar from another jar in which a
mint plant was growing. Then he put a lighted candle in the empty jar and the
candle, being well supplied with oxygen from the plant, went on burning. Later
he took the candle out and put a mouse into the same jar. The mouse breathed
comfortably and Priestley wrote of it, "nor was it at all inconvenient to a
mouse which I put into it" (Fig. 4.6). In 1779 Jan Ingenhousz, a court phy-
sician to Empress Maria of Austria, observed that plants "corrected the bad
air" in which they were growing. He wrote of his observations, "I found that
this operation of the plants is more or less brisk in proportion to the clearness
of the day and the exposition of the plants." Julius R. von Mayer, who formu-
lated the principle of conservation of energy, first stated in 1845 the physical
function of photosynthesis as the conversion of light energy into chemical
energy. Photosynthesis is a subject of joint chemical and biological inquiry in
which new dscoveries are made from month to month, and sunlight has created
sugar from carbon dioxide and water.
Organization of a Green Plant
Essential Needs. Plants are light-seeking, light-directed organisms. They
have four essential needs, light, air, water, and certain minerals. The sun sheds
its energy in light and heat upon the earth. It creates currents in the water,
winds in the air, quickens the activity of water molecules that scatter as vapor.
58
ECOLOGY
In sunlight a green water plant
gave off bubbles (of oxygen).
Part II
Mouse could breathe
in closed jar. (Oxygen
supplied by plant )
PRIESTLEY'S DISCOVERY
Fig. 4.6. The chemist (England, 1733-1804), Joseph Priestley kept a plant grow-
ing within a glass jar connected with another jar in which he kept a mouse. The
mouse breathed on comfortably because the plant provided it with oxygen, a
product of its photosynthesis. (Data for figure from Memoirs of Joseph Priestley,
1:253. London, J. Johnson, 1806.)
and activates the photosynthesis of green plants. Thus the sun surrounds plants
with light and keeps air and water circulating about them. Plants may have
all of this without going after it as the majority of animals do. Light bathes
the whole plant from above or from one or more directions; the branches
reach out for light and the leaves take positions to receive it. Light does not
penetrate deeply into the tissues, but leaf surfaces are spread out and the
chlorophyll is always near to them (Fig. 4.8). The spread of maple leaves to
receive light is a marvel of efficient arrangement. The essentials for a green
plant's existence are in two layers of its environment. Light and air are above;
there the plant is green and its stem upstanding. Water and minerals are be-
lov/; there the plant is colorless and its roots are pliant.
The Individual. The plant has a particular form recognizable as character-
Chap. 4 PLANTS PROVIDE FOR THEMSELVES AND THE ANIMALS 59
istic of its species and of itself — the barrel cactus of the southwestern desert,
the American elm, the jack-in-the-pulpit. There is a strict division of labor in
the plan of the body; different parts perform particular functions such as
protection, support, and water transport (Fig. 4.7). The plant body has two
main regions, the shoot system of stem and leaves which is intimate with the
atmosphere and the root system which is correspondingly intimate with the
soil.
Stem. The stem or axis is a support and a highway. Its first function is the
raising of leaves to the light, of flowers upward for light and pollination, of
seeds in position for better dispersal. Its second function is the distribution of
water and nutrient solutions and gases throughout the plant. In most plants,
the stem is a cylinder that tapers at the top and gives off branches that are
Absorption
Water
Salts--
Oxygen
^^ >Respiration
Fig. 4.7. A diagram indicating the main structures and functions of a seed plant,
the bean. The first leaves (cotyledons or seed leaves) are richly stored with protein
and contribute only slightly to photosynthesis. (Courtesy, Woodruff and Baitsell:
Foundations of Biology, ed. 7. New York, The Macmillan Co., 1951.)
60 ECOLOGY Part II
ultimately continuous with the veins of the leaves. Stems vary in circumference:
the stem of a California redwood is thick enough for a car to drive through;
that of the young maidenhair fern has a hair's thickness. Stems are squat in
turnips and tall in royal palms.
The main layers of the stem are the cambium, and the phloem, and xylem,
the latter two named from the Greek words for bark and wood. Cambium is
the vital growing layer from which the other two layers originate, the xylem
from its inner and the phloem from its outer side. In tree trunks the wood is
composed of xylem and most of the bark of phloem. The xylem holds the
supporting tissue and tubes through which water and dissolved substances are
conducted from root to leaf. The phloem contains tubes through which manu-
factured foods are distributed especially from the leaves to regions of the
plant where they are stored or used. The epidermis covers the stem and is
continuous over the leaves and roots. Tons of water mixed with mineral
nutrients ascend from the soil and through the tubes of the xylem into the
veins of the leaves. Great quantities of food made in the leaves pass through
the veins and stem by way of the tubes of the phloem. The pattern of con-
duction in xylem and phloem is essentially the same whether in a buttercup or
an oak tree.
Sugar cane, potatoes which are underground stems, and tree trunks are
stems that have million-dollar values and high places in history. Except for the
plant stems that made his ships, Columbus would not have crossed the ocean
nor the Norsemen set foot upon American shores. A few plant stems made
the raft Kon-Tiki on which six men crossed the Pacific Ocean.
Leaf. A leaf is a thin blade, greener on the upper than the underside and
freely exposed to light and air. Continuous with its petiole or stem is the
stiffened vein or group of veins from which other more delicate ones branch
olT and hold the leaf outspread. The unique function of green leaves is photo-
synthesis. Water from the plant stem is conducted to the leaf, and carbohydrate
food from the leaf to the plant stem. There is great variety in the shapes of
leaves, but, whether they are simple or compound they all fit three types: the
rounded leaf like that of the nasturtium, the linear leaf like the grass blade,
and the cone-shaped one such as the elm leaf.
Microscopic openings of stomata occur in the otherwise waterproof epi-
dermis, especially on the lower side of the leaf (Fig. 4.8). Each opening is
between two specialized cells of the epidermis, called guard cells because
changes in their size and shape determine whether the stomata are open or
closed. Water enters through the root hairs and passes out mainly through the
open leaf-stomata and to some extent through the cuticle, in the process of
transpiration. Of the total quantity of water absorbed by the roots, as much as
98 per cent escapes by transpiration. Stomata also regulate the exchange of
gases between the air and leaf. If the leaf is well lighted they are open and
Chap. 4 PLANTS PROVIDE FOR THEMSELVES AND THE ANIMALS
61
Sun's energy
Palisade cell
Chloroplost —
Cuticle
Upper epidernnis
?'l I* gases, diffuse toother cells.
CO2 H2O C6H12O6
^"^MIM
spongy tissue
•Vein
Air and
fluid spaces
Lower epidermis
COz enters
with air
Guard cells turgid Stonnates
open in the normal daytime
condition
Excess O2 leaves during
sugar making
Excess water (HjO)
goes out as vapor
sroiAUR
Fig. 4.8. The leaf blade. The essential structures are: the upper and lower cover-
ing layers or epidermis; the cells of the palisade and spongy tissue containing the
chlorophyll that carries on photosynthesis; the veins that are the highways of trans-
portation between leaf and stem (the xylem ducts transport water and the phloem
carries food) . Each stoma is a breathing pore leading to the air spaces in the spongy
tissue. The guard cells on either side of the pore regulate its size according to the
moisture and the amount of oxygen and carbon dioxide exchanged.
photosynthesis is in full swing. The bean-shaped guard cells are then rotund
with stored sugar and water which the sugar has attracted by osmosis. Their
plumpness causes them to pull apart and thus to form an opening between
them; when they collapse the opening closes. Other conditions within or with-
out the leaf affect the guard cells, especially scarcity of water. The stomata
are then closed and what water there may be left in the leaf is kept from
passing out in transpiration.
Respiration occurs in all cells of the leaf as it does in the root, the stem and
other parts of the plant. Within the green leaf the upper layers of cells hold an
62
ECOLOGY
Part II
iff
:■:}
i1
^
"te
'n 1
\B.
f
^
^
y
B
Central cylinder
Cortex
Tubes and
growing cells
Moturing zone
Epidermis
root tiairs
Elongating zone
Growing point
Protective root cap
Air space
Soil particles
rmal cell, comparable to
outer skin layer of animals
Chap. 4 PLANTS PROVIDE FOR THEMSELVES AND THE ANIMALS 63
abundance of chlorophyll (Fig. 4.8). Here the leaf is greenest and the light
falling on it is strongest. These cells are the all important food-makers, the
links between the energy of the sun and the living world. The lower layers
contain spongy cells of odd shapes and hold less chlorophyll than those of the
upper layers. They are loosely packed in clusters with air spaces in between.
This region of the leaf provides for the income and circulation of gases and
the outgo of water. Extra water is also eliminated in droplets (guttation)
from openings at the tips of the veins of grasses, corn and many other plants.
In early morning the droplets hang in beautiful symmetry on the edges of the
leaves of strawberries and jewelweeds. During the day some water is lost from
the leaf and at night moisture in the air condenses on its cool surface. The
main supply of water is always from the root.
Root. The main functions of the root are the anchorage of the plant, the
absorption of water and mineral matter, the storage of manufactured food and
sometimes of chemicals, e.g., nicotine is produced in the roots of tobacco
plants and transported to the leaves. The spread of surface necessary for ab-
sorption also makes it an efficient anchor in the soil (Fig. 4.9). The root is
the extension of the stem and resembles it in having long tapering branches
and an essentially similar structure, although the pattern of the conducting
tubes is different. Although roots are various in size, form, and structure, they
have no such diversity as the leaves and stems, for conditions in the soil are
less variable than those in the air.
Of all the material which the root absorbs the most important is water. It
is a great part of the plant substance and as essential for the processes of living
as it is in animals. Absorption occurs exclusively in the microscopic root hairs
in the white terminal parts of roots, the ones whose injury in transplanting is
followed by the familiar wilting of the plant. Near the tip of each new root,
hairs are continually forming, a little farther back they are constantly dying.
The root hair is a single cell of the epidermis. It grows outward in a hairlike
projection that turns and twists about the particles which in any moist soil are
clothed in a thin capillary film of water (Fig. 4.9). The root hair is an osmotic
mechanism (Chap. 2, p. 22). Water and salts enter it but sugar does not pass
out. Although each root hair is virtually microscopic, their total area is a
marvel of expansiveness. In one species of grass the total length of root hairs
Fig. 4.9. A. The root system of a corn plant. (After Weaver.) B. Diagram of a
section of a root tip and its different zones. Cells of the root cap are worn off and
replaced by new ones from the growing zone above it. The force that pushes the
root through the soil is the lengthening of cells in the elongating zone. Epidermis,
root hairs, and the ducts of the food-transporting phloem and the water-transporting
xylem all develop in this zone. (After Woodruff & Baitsell: Foundations of Biology,
ed. 7. New York, The Macmillan Co., 1951.) C. Root hairs are branches of
epidermal cells. In every well-grown root billions of root hairs take in water from the
films of it that surround particles of soil wherever there is moisture on the ground.
64 ECOLOGY Part II
held within one cubic inch of soil has been estimated to be four-fifths of a mile.
Root pressure pushes sap to the top of the tallest trees. It acts under various
conditions, in trees of tropical rain forests where there is no evaporation from
the leaves, and in trees of temperate climates before the leaves appear in
spring. In some parts of our country the maple sugar season is the time of the
first great lift of sap from its winter storage in the roots of sugar maples. Root
pressure is all-important to plants. Details of the causes of it are complex and
not completely understood. Root hairs are the first actors in root pressure
because they carry on the absorption of water from the soil. About one-third
of the pressure is believed to be osmotic and two-thirds metabolic, that is, due
to respiration and other life processes.
Reproduction
Higher plants reproduce asexually and sexually. Some species reproduce
more often or exclusively in one way, some in the other. Young strawberry
plants develop from creeping stems which grow from the parent; grass plants
spread out many sprouts from older plants. The white potato of the dinner
table is a food-filled underground stem. When used for planting it is cut into
pieces each containing an "eye" or bud from which a new plant grows. In
most higher plants both methods of reproduction are common which is never
the case in higher animals. A strawberry plant buds forth a new plant; a cat
never buds off a kitten.
The root, leaf, and stem are concerned with the vegetative functions, the
intake of food and water, digestion, respiration, and asexual reproduction; the
flower with sexual reproduction. In higher plants sexual reproduction is more
important than asexual. Any bouquet of flowers — roses, orchids or butter-
cups— is a cluster of reproductive organs. Although sexual reproduction differs
greatly in detail in plants and animals, its essential features are the same.
Flower. The flower is the reproductive organ of the plant. The more or less
conspicuous parts are the sepals, petals, stamens, and pistil. The latter two are
directly and the others only indirectly concerned with the formation of male
and female sex cefls and their union in the process of fertilization. Flowers
differ greatly in the position and form of the parts and whether male and
female cells are borne on the same or different plants of a species. They are
often in the same flower as in the diagram (Fig. 4.3). The stamen consists of
the stalk supporting the anther and its pollen sacs. When it is mature, the
pollen sacs break open and liberate the pollen grains within each of which
there are two male sex cells. These are equivalent to the male sex cells (sperm)
of animals. The pistil (or pistils) usually consists of a central stalk with a
sticky tip, the stigma. At its base is the ovary containing the ovules, the
female sex cells equivalent to eggs. The union of the sex cells is brought about
in one way or another, such as by the locations of the parts, or by insects. The
Chap. 4 PLANTS PROVIDE FOR THEMSELVES AND THE ANIMALS 65
male cells come in contact with the stigma and make their way down through
the stalk of the pistil to the ovary. Finally one of them reaches the ovule and
enters it. The subsequent fusion of the male and female cells within the ovule
is fertilization. These are the essentials of the journey of the male cell and its
union with the female, with many complexities omitted and numbers of irregu-
larities unmentioned. The fact remains that the behavior and function of the
primary sex cells are strikingly the same in plants and animals.
Seed. The seed is an embryo plant which has developed from a fertilized
ovule. A fruit is a growth around one or more embryos (seeds) which protects
them and is a common means of their dispersal.
Similarities of Plants and Animals
1. Cells. Their basic material is protoplasm organized in cells.
2. Food. Their main food and chief sources of energy are carbohydrates —
starches and sugars. Amino acids, the "building blocks" of proteins, are essen-
tial to them. Water is a vital need.
3. Metabolism. The basic processes of respiration and of digestion and
assimilation are similar. Excess products of metabolism are mentioned below.
In the respiration of plants and animals oxygen enters the cells and unites
with carbohydrates, fats, and lastly with proteins. Oxidation, i.e., chemical
burning occurs. Chemical energy is released as activity and heat. Carbon
dioxide and water are formed.
During digestion food is changed to simpler chemical compounds. During
assimilation the digested food becomes part of a specific kind of protoplasm.
For example, food assimilated by the chromosomes in certain cells of an oak
tree acquires the characteristics of the appropriate substances in those chromo-
somes; food assimilated in the chromosomes of certain cells in a goat does
likewise.
In both plants and animals, certain excess by-products may be stored. Ex-
amples of these are digitalis in foxgloves, opium in poppies, calcium carbonate
in earthworms. The use of these, if any, to the producing organisms is not
clearly understood. Certain other by-products may be used; carbon dioxide by
green plants in photosynthesis, and by animals in small amounts as a stimulus
to breathing and as a control of the force of the heartbeat.
Differences between Plants and Animals
1. Locomotion. The majority of plants do not move from place to place.
The majority of animals move about freely.
2. Food. Green plants make carbohydrates by photosynthesis. Animals take
carbohydrates from plants. Plants are the chief makers of proteins which they
elaborate from amino acids. Animals take proteins from plants and other
animals.
66 ECOLOGY Part II
3. Metabolism. Even in higher plants the rate of metabolism is low. In
active respiration the temperature of plants may rise only slightly above their
environment. In higher animals the rate of metabolism is high. The temper-
ature of birds and mammals is usually much higher than that of their environ-
ment.
In the majority of animals, there are special organs of excretion by which
nitrogenous waste products of metabolism are eliminated. In plants, there are
no such organs. The only approach to an excretory product in plants is prob-
ably the excess by-products of metabolism such as opium (see similarities of
plants and animals). There are no excretory organs in plants.
4. Hormones. Plants produce relatively few hormones and these have
general effects, such as, growth of stem and growth of root. Animals produce
an elaborate and delicately adjusted series of interacting hormones which have
specific effects, such as, thickness of skin.
5. Responsiveness. In plants, the ordinary cells are variously responsive,
e.g., to light, to temperature, in some regions more than others. In animals,
special sensory cells are highly responsive to one or another kind of stimulus,
e.g., the rod cells and cone cells of the eye.
5
Animals and Tlieir Environments
Animals abound in great numbers. Thrust a stick into a large ant nest on a
July day and millions of ants pour out, many carrying white packages that
taken altogether contain myriads of their eggs and young ones. Sea birds
scarcely have room to sit on their eggs during the great gatherings of the breed-
ing season (Fig. 5.1). Populations of animals, except the human ones, seem
to stay about the same size, but those that have been carefully observed have
proved quite the opposite. The dips and peaks in the populations of one kind
of animal also affect others. In Labrador in a recent year the numbers of
field mice ran up to a peak and the hawks and snowy owls grew fat; in another
year they almost vanished becoming so scarce that the snowy owls flew down
to New England for better eating.
Animals enter every part of the earth except craters of active volcanoes and
places poisoned by civilization. They abound in the damp tropics. Microscopic
organisms crowd the surface waters of arctic seas, for cold water holds more
oxygen than warm water and food is abundant. On their journey into the
Antarctic members of the Robert Scott Expedition found emperor penguins
incubating their eggs, holding them on the tops of their feet in the dark of the
antarctic winter "with the temperature seventy degrees below frost and the
blizzards blowing."
The Numbers of Species. The term species is commonly used but difficult
to define. Animals of one species resemble one another, interbreed with one
another and do not usually interbreed with animals of other such groups. The
number of described species is still growing. For birds and mammals it may
for the present be nearly complete; for protozoans and insects it is far from
that. Frequent estimates suggest that only ten per cent of all insects is yet
accurately described. In 1946 the total number of known living species of ani-
mals was figured at about one million (Fig. 5.2),
Variety and Similarity. Large numbers of animals have basic similarities;
they also have many less fundamental differences. Likenesses and differences
67
68
ECOLOGY
Part II
->«.
— r—
v^'
» mm
^^^E.f
^^
^
'*
**
'''.
1
^* ,,
^
-
m^^' ^ "_i
■ -a^^5^^^ -
n
M
4^ -'w
¥ V
1
4
^;/4
I Fig. 5.1. Abundance. Gannets nesting on ledges of Bonaventure Island, off the
coast of the Gaspe Peninsula, on rocks as high as a 20-story building. A gannet is
about the size of a duck. (Courtesy, Allan D. Cruickshank, from National Audu-
bon Society.)
make classification possible. Animals may have two or four or more legs;
insects have six; spiders have eight; there are a hundred or more in millipedes.
The bones of the arms and legs of a man are arranged like the comparable
bones in the legs of a horse (Fig. 9.13), but in other ways the legs are differ-
ent. Such structures are correlated with the history of their environment,
human arms and legs with ancestors that climbed trees and the horse's legs
with ancestors that ranged the swamps and the plains. Various noses are
adapted to various functions in addition to smell; an elephant can give itself
a shower bath with its nose (Fig. 5.3).
Sizes of Animals and the Environment. Animals of a given species vary
relatively little in size. Size, proportions, and structure of the body and en-
vironment are mutually related. Water lifts and supports weight as air does
not; boats can anchor and float but airplanes cannot poise in the air without
special devices. Many animals can swim, but few can fly. Aquatic animals are
often larger than their terrestrial near-relatives and literally lean on the water
Chap. 5
ANIMALS AND THEIR ENVIRONMENTS
69
Fig. 5.2. Diagram showing the approximate number of living species of animals.
The grand total is often given as one million. Numbers differ greatly with the
methods and time of counts. New species of insects are being discovered even in
familiar places; probably only a fraction of all the tropical insects have been de-
scribed. (Courtesy, Hunter and Hunter: College Zoology. Philadelphia, W. B.
Saunders Co., 1949.)
for support. Giant grasshoppers are small compared to the largest lobsters,
their marine relatives. Blue whales, the largest living animals, are ten times as
long as elephants, more than twenty-five times heavier, and, like large ships,
are helpless when stranded (Fig. 5.4),
Only the smallest mammals burrow or live in grassy runways. The pigmy
shrews are very small, one of them, Microsorex hoyi winnemana, total length
with tail, 3.12 inches, is the smallest mammal known in North America. Noc-
turnal and mouse-like but more slender it travels comfortably in a runway
half an inch wide.
70
ECOLOGY
Part II
Fig. 5.3. Noses are adapted to many uses in addition to smell and breathing. A
ground mole bores its way wedging with its nose and digging with its feet; mice
and other rodents use their noses as wedges; anteaters probe into anthills. The noses
of elephants are general tools, for shower baths, hfting logs, and picking up nuts; a
pig's snout is a living plow.
Form, Symmetry and Segmentation. The symmetry of animals is the ar-
rangement of structures with respect to a point, a line or a plane. In radial
symmetry the structures are placed like the parts of a wheel in relation to its
center. In bilateral symmetry the right and left sides correspond to one another.
Symmetry is correlated with an animal's way of life, especially its lack of
locomotion or kind of locomotion. Hydras, corals, jellyfishes, and others are
radially symmetrical. Such animals move about slowly or are attached like
the corals. In sea anemones and starfish and their kin bilateral symmetry
appears within the radial; that is, the wheel or cylinder shows a division into
two parts. This is a persistence of the bilateral symmetry of their free-swim-
ming young. The majority of animals, and all the vertebrates, are bilaterally
symmetrical (Figs. 5.5, 5.6). They move about freely, often with great speed,
Fig. 5.4. The relative size of the blue whale (length, 90 to 100 feet), whale
shark, and giant squid. All of them live surrounded by the lifting capacity of
buoyant salt water. The ostrich and elephant receive no such support.
Chap. 5
ANIMALS AND THEIR ENVIRONMENTS
71
■^i-^llum
SPHERICAL
^-tMi0^
RADIAL
ASYMMETRICAL
BILATERAL
Fig. 5.5. Types of symmetry. Spherical, a protozoan (radiolarian) floats in water
that presses against it equally on all sides; radial, a sea anemone, its shape common
in animals that are attached for most of their lives; asymmetrical, in a snail that no
plane will divide into halves; bilateral, in a salamander, in animals that move about
freely, and are mainly symmetrical on each side of a plane extending the length of
the body.
and the brain and sense organs are always at the end that arrives first. Scarcely
any animal is perfectly symmetrical, whatever the type; all tailors know that
the human ones are a little one-sided.
Segmentation. The bodies of all animals from earthworm to man are
segmented, i.e., partitioned into sections that are joined together in a series.
The segmentation may be conspicuous inside and out, as it is in the earth-
worm; it may be mainly on the outside as in the abdomen of an insect; or
prominent in certain structures such as vertebrae and ribs. The arrangement
has the advantage of making parts of the body more independent of one
another; it is an insurance lessening the disaster of injury to the whole body.
If one or more segments are hurt, others can carry on. Segmentation gives
flexibility to long slender bodies such as those of worms. It allows great variety
by the modification of different segments for different functions, as in a lobster,
in which some segments bear swimmerets while others bear mouthparts and
eyes.
72
ECOLOGY
Part II
ORAL
lone or section
ABORAL
RADIAL
BILATERAL
Fig. 5.6. Axes, planes and regions in animal bodies.
Environments
Rhythms of Sun and Moon. The lives of all plants and animals are inter-
woven with the rhythms that originate outside the earth, their income of
energy from the sun, the changes of the tides, and shifts of climate. Patterns
of living change from hour to hour as the earth rotates on its axis in its journey
round the sun. Evening with its own ways comes to a countryside as it is
turned from the sun. If it is New England and early June, the wood thrushes
sing through the sunset and afterglow; the whippoorwills begin calling when
the hedges are black; the mosquitoes are enlivened by the subdued light and
the dampness. From moment to moment animals as well as plants respond
punctually and precisely to changes in light and atmosphere.
The gravitational attraction between the sun and the earth and the moon
and the earth constantly pulls upon these bodies, its strength varying with
their respective positions in their orbits. On land its effect is relatively slight
but upon the sea it is the basic cause of tides. Sun and moon both take part in
the changes of the tides, but the moon, being much nearer the earth, has the
stronger influence upon them. With many variations there are in general four
tides on every seashore, two high and two low ones in each period of 24
hours. The tide rises and water that has swept the ocean bottom floods over
the tide pools bringing additions to the already crowded communities of ani-
mals, some of them to eat, others to be eaten. Each little group is continually
changed by flooding and ebbing water. Everything that belongs to the sea
waits on the tides. Fishermen in harbors put out their seines for the fishes that
follow the rising tide. Great ocean steamers wait at their docks until the tide
rises.
The Sun, a Great Provider. The sun sustains life upon the earth, providing
living organisms with heat, light, the energy stored in food, and indirectly with
water. The sun is a great furnace of transmuting atoms, extraordinarily differ-
ent from the earth yet with a similar chemical content. According to certain
theories the earth originated from a torn-out piece of it. It is the source of
Chap. 5
ANIMALS AND THEIR ENVIRONMENTS
73
Fig. 5.7. Types of marine plankton, the great population of minute plants and
animals that live in the surface of the seas and includes the eggs and developing
young of the majority of marine animals. Top, the larva of the porcelain crab
like other plankton organisms is translucent and bears outgrowths that serve as
floats characteristic of animals of the plankton. (Photograph by D. P. Wilson,
Marine Biological Lab., Plymouth, England.) Bottom, the protozoan, Globigerina
biilloides. Enormous numbers of these live among the plankton in the surface
waters of the sea. Their chalky frames and fine spines dropping through the water
for millions of years have formed the globigerina ooze of many parts of the ocean
bottom. (After Murray and Hjort. Courtesy, Coker: This Great and Wide Sea.
Chapel Hill, N.C., Univ. of N. Carolina Press, 1947.)
practically all the energy on earth, excepting atomic energy. It is the prime
mover of the winds because it heats different places unevenly and this sets
currents of air in motion. As heat it lifts water by evaporation eventually to
form clouds and be distributed in rain. With its energy plants make the food
for which directly or indirectly all animals including man struggle unceasingly.
74 ECOLOGY Part II
Types of Environment. The Land. Terrestrial animals of various groups
are described briefly in Part 5.
The Sea. The greatest numbers of living organisms in the world are the
plankton that live in the surface waters of the sea. They are small, mainly
minute and microscopic plants and animals that drift with the currents. No
plants are so completely open to the energy of the sun. No mixed population
of animals is more uniformly short-lived and prolific. In no other place are
there, in season, such multitudes of floating eggs and swimming young (Figs.
5.7, 5.8).
The richest population in numbers and kinds of animals visible to the naked
eye lives between the tides and near the bottom out to depths of about 400
feet. Hosts of them are attached to rocks and seaweeds; or crawl and burrow
on the bottom (Fig. 5.9 and 5.10). Farther from shore are the larger free-
swimmers (nekton), the fishes; coastal waters are the main fishing grounds.
The deep water of the open sea from the surface well into its depths is the
home of the largest fishes, the giant squids, sea turtles, and the mammals,
porpoises, dolphins, and great whales. Except for the whalebone whales all
M^::
Fig. 5.8. Photograph of marine diatoms. Their beauty and variety are due to
their silicious shells. Diatoms of fresh waters are less various but equally beautiful
and important in the economy of their environments. (Courtesy, Paul B. Conger,
United States Museum, Washington, D.C.)
Chap. 5
ANIMALS AND THEIR ENVIRONMENTS
75
Fig. 5.9. Hosts of animals cling to the rocks and seaweeds between the tide lines.
Common rock barnacles (Balaniis balanoides) {above), and edible periwinkles
{Littorina litorea) {below). Periwinkles are about the size of cherries. In British
shore resorts "winkles" are roasted and sold like peanuts in America. (Photograph
by D. P. Wilson, Marine Biological Lab., Plymouth, England.)
of these live upon one another and the oflspring of one another (Fig. 5.11).
Salt water is a far better support and carrier than fresh water. The eggs of
marine animals float easily; those of fresh-water animals often drop to the
bottom, are attached to vegetation, or carried about by the parent. The young
ones climb, creep, and hold onto whatever comes their way.
Ponds and Lakes. Healthy ponds and the coves of lakes usually hold
goodly populations; in midsummer they teem with them (Figs. 5.12, 5.13).
Ponds are smaller than lakes. They are defined as bodies of water so shallow
that green plants can grow attached to the bottom even at the center. Lakes are
too broad and deep for this. Near the borders of ponds and the protected
shores of lakes the plants are food depots and shelters for invertebrates, snails,
climbing fingernail clams, innumerable crustaceans, and aquatic insects. There
are a few resident vertebrates, chiefly frogs and turtles. The plants have partly
or completely submerged stems — blue-blossomed pickerelweeds, arrowheads,
rushes, and waterlilies. All of their stems are coated with green algae and bac-
teria (Fig. 5.14). Yellow perch, bass, and pickerel come among them to
forage.
76
ECOLOGY
Part II
Fig. 5.10. With every high tide the tide pools and surrounding rocks are flooded
with water carrying milHons of little plants and animals that are fit for food. During
low tide the pool dwellers are busy consuming the meal. They are attached and
slow moving protozoans, bryozoans, barnacles, tunicates, and many mollusks often
along with a few crabs, starfishes, brittle stars, and sea urchins. (Courtesy, the
American Museum of Natural History.)
Chemical Conditions
Plants and animals are continually taking materials from their environments
and making them into their own bodies. Certain substances and conditions
must be present around them. Whether in arctic or tropic regions, in water or
on land, these essentials are: sufficient energy from the sun for the plants to
synthesize food, enough oxygen for respiration, enough water, the chemical
elements which take part in protoplasmic activities, and certain physical con-
ditions, such as temperature and pressure.
Chap. 5
ANIMALS AND THEIR ENVIRONMENTS
77
Fig. 5.11. Larger free swimmers (nekton) of the open coastal waters. Upper
left, dolphins, length up to 12 feet; North Atlantic sea turtle (loggerhead), 100
to 200 pounds. Center, swordfish, 250 to 400 pounds. Bottom, blue-fin tuna (or
marlin), up to 600 pounds. Not drawn to scale.
Carbon Cycle. Carbon, a main element in protoplasm and its products, is
available only in small amounts. Ordinary air contains about 0.035 per cent
of carbon dioxide by volume and only a quarter of this is carbon. From this
small amount, plants obtain all they use and in turn become the source of
carbon for all organisms. The sources of free carbon dioxide are plant and
animal respiration, decay of the bodies of plants and animals, and the release
from burning oil and coal. From all these sources it is automatically returned
to the atmosphere. The only way that it gets back to protoplasm is by green
plants.
Plants take carbon dioxide {CO 2) from the air and with the help of energy
from the sun during photosynthesis, produce the valuable food, carbohydrate.
When a carbohydrate unites with oxygen, the energy of action and heat and
carbon dioxide are set free, the latter in part a waste product respired into the
air. One branch of the cycle is thus complete. In another branch of the circuit,
carbon is built into the protoplasm. It is locked within the cells until they die,
decompose, and free it into the air to unite with oxygen as carbon dioxide
(Fig. 5.15).
Oxygen Cycle. Plants and animals take oxygen (Oo) from air or water in
78
ECOLOGY
Part II
im
Ibis
^m
\%::v\
am
BLUE-GREEN ALGAE
DESMIDS
DIATOMS
Arcella
Ceratium
PROTOZOANS
Cyclops
ROTIFER
young stage
(Nauplius)
Bosmina
CRUSTACEANS
Fig. 5.12. Important groups in fresh-water plankton. Blue-green algae, common
in lakes especially in hot weather, sometimes turn color and create "red water";
green algae (desmids) and diatoms, present the year round with spring and other
upswings of abundance; protozoans, few; rotifers, many; crustaceans, abundant,
creating the basic fish food.
respiration. They return it to the atmosphere in combination with carbon as
carbon dioxide and with hydrogen as water. In addition green plants release
oxygen in photosynthesis. In an aquarium properly arranged for plants and
animals, the output of carbon dioxide from respiration and of oxygen from
photosynthesis is balanced.
Nitrogen Cycle. The great reservoir of nitrogen in the atmosphere (78.03
per cent of volume) is an inactive associate of oxygen and carbon dioxide.
The nitrogen dissolved in bodies of water comes mainly from the atmosphere.
Its cycle is more complex than that of carbon because living organisms do not
release nitrogen in a form that green plants can use. It is released from animals
as nitrogenous waste such as urea (CO(NH2)2) and from decaying tissues
after death (Fig. 5.15). Saprophytic bacteria attack these and produce
ammonia. Other bacteria feed upon the ammonia, combine oxygen with it,
derive energy from the oxidation, and produce nitrites (NOo) — upon which
they feed. Still other bacteria (Nitrobacter) attack the nitrites and, through
anaerobic (without free oxygen) respiration, derive energy from them and
Chap. 5
ANIMALS AND THEIR ENVIRONMENTS
79
SR6LASER
Fig. 5.13. Stems and leaves of pond lilies are nurseries for hatching eggs and
young animals, mainly invertebrates. A, strings of jelly that shelter minute eggs
of midges. B, eggs: on the under side of a lily leaf: 1 , snail; 2, water mite; 3, caddis
fly; 4, whirligig beetle; 5, beetle (Donacia); 6, beetle, the waterpenny (Psephenus).
convert them into nitrates (NO3) — that are taken up by green plants, and
finally converted into the amino acids and proteins of green plants. Blue-green
algae are now known to fix nitrogen and the process may be even more
general than this. Many commercial fertilizers contain nitrates.
Nitrogen-fixing bacteria are able to fix free atmospheric nitrogen in nitrog-
enous compounds which can be used by green plants. Some of these bacteria
live in the soil, estimated at least two billion to a teaspoonful in garden soil;
others live in nodules on the roots of clover, peas, and beans. The value of
these plants in building up the nitrogen supply in the soil is recognized by
farmers who rotate crops of clover with corn in order to supply the soil with
nitrogen which corn exhausts. Denitrifying bacteria occur in some soils. These
reverse the nitrifying process and reduce nitrites to free nitrogen which is then
released into the atmosphere. This is the nitrogen that is compounded with
water and brought to the earth in an electrical storm. The bolts of lightning
fix the nitrogen as nitrites and nitrates that are brought to the earth by the rain.
Mineral Cycles. These include the time in which iron, phosphorus, or other
minerals are in the crust of the earth and in the body of a hving organism.
Calcium carbonate (CaCO:0 or lime is a good example for it is widely dis-
tributed in nature and an important component of bone. The developing
embryo of a mouse receives lime from its mother and after birth from its food,
notably milk. Lime is maintained in the body of the mouse, chiefly in its bones,
as long as it lives. Exactly the same storage of lime occurs in an elephant ex-
80
ECOLOGY
Part II
:S.P. 6WSER :■•..•
;.'.:;.*.•:■. •.^. .•;■>/ :»/.-.
Fig. 5.14. The web of feeding habits among the animals of pond and lake bor-
ders: frogs on immature insects, snails, small fishes, crustaceans; pickerel on
insects, fishes; turtle on tadpoles, frogs.
cept that a larger amount is involved and for a much longer time. Large
amounts of lime and other minerals are temporarily stored away in plants and
animals.
Water Cycle. The internal environment of the body is completely dependent
upon the come and go of water. It enters the body bearing traces of iron,
iodine, sulfur, or salt from the external environment. It leaves the body carry-
ing the wastes of metabolism that are records of protoplasmic activity. Water
rises in vapor from the sea and land, floats in the atmosphere as clouds, con-
denses, falls as rain, and runs down from the highlands to the sea again. Water
is a traveler. Like mineral matter it is taken into plants and animals but it
Chap 5 ANIMALS AND THEIR ENVIRONMENTS 81
never remains in them. Whether they are pine trees or cattle, living organisms
take in relatively large amounts of water that gradually filters completely
through their bodies.
Physical Environment
The chief physical influences upon plants and animals are gravity, pressure,
temperature, and light.
Gravity. Its weight, actually the earth's pull, greatly affects an animal. The
bridge-type of four-legged animal is a four-cornered support of the body
against the pull of the earth (Fig. 9.11). Birds are the master adjusters to the
force of gravity. No other animals approach them in lightness and strength,
due to the air-filled outpocketings of their lungs that extend into the bones,
their rapid elimination of waste products, and the lightness of feathers (Chap.
36).
Pressure. The medium in which animals live presses upon them continually
from every point, upon their forms, actions, and the amount of gases which
they hold.
The atmosphere of the earth is like a haystack (Fig. 5.16). At the bottom
or sea level its content is closely packed; the atoms of oxygen are near to-
gether. At sea level an animal, like every other object, carries 14.7 pounds of
atmospheric pressure on each square inch of the surface of its body, and this
pressure so evenly permeates its body that none is felt. At 20,000 feet (300
feet lower than Mt. McKinley) the same animal would be exposed to pressure
less than half that of sea level. In spite of their high oxygen demand in breath-
ing, birds fly through air of low oxygen content probably securing an adequate
supply because of the speed with which they drive into it. At 18,000 feet mules
in South America carry riders without great difficulty, and this is said to be
due to their frequent stops during which oxygen accumulates in their blood.
Anyone acquainted with them knows that mules have the same sagacity at sea
level where they also make frequent stops.
Water is about 775 times more dense than air and consequently heavier. It
is peculiar in that it becomes denser and heavier as it cools to a temperature
of 39.2° F. (4° C). When colder than that it is less dense and lighter, finally
floating as ice. Because of this the pond is covered with a blanket of ice below
which fishes can disport themselves in safety (Fig. 5.17).
The pressure upon an animal in water is the weight of a column of water
extending above a given area of its body plus the atmospheric pressure above.
The pressure on a fish in Lake Tahoe in California, over 6,000 feet above sea
level, is far less than that on a codfish in the Atlantic Ocean. At great depths
of the ocean the pressure is several tons per square inch. It does not crush the
animal because the fluids in its body are under the same pressure as the water
surrounding it. Pressure compresses gas which expands when deep-sea fish are
82
COz
ECOLOGY
Atmosphere
Part II
CO2
CO2
CO2
f
CO2
Photosynthesis C O2
Bacterial action
Decomposition
Respiration
of plants
\
Bacterial action
Decomposition
/Respii
of an
ration
imals
CO2
Air, water,
rocks, soil
CARBON CYCLE
Free nitrogen Ng is mode available
to plants and animals ifixed) by
certain bacteria. Also fixed by
lightning and washed to earth.
N2 fixing
soil bacteria
The processes from free nitrogen to protein
are carried on mainly in the ground
NITROGEN CYCLE
Chap. 5
ANIMALS AND THEIR ENVIRONMENTS
83
60,000 Ft
THE ATMOSPHERIC HAYSTACK
Fig. 5.16. Atmospheric pressure illustrated by stacked hay showing the weight
it would carry at various heights to 60,000 feet. The proportions of the gases in
the atmosphere do not change at different heights but their total amount does.
This is why the air is thin in high places.
brought to the surface, just as gas expands when a bottle of compressed fluid
pops. When deep-sea divers rise to the surface rapidly the pressure on the
nitrogen in the blood is released too quickly; it gathers in bubbles in their
muscles and joints producing a condition known as the bends (Fig. 5.18).
Temperature. Except for those that live in hot springs, plants and animals
can live only within a narrow range of temperature and can endure relatively
low temperatures better than high ones. Many tropical animals cannot bear
extreme exposure to the sun's heat. In zoological gardens ostriches, croco-
diles, and snakes have often been killed by heat. Birds have the advantage in
their cooling devices of air sacs and mammals of panting and sweating.
Wherever there are severe winters, animals resort to various ways of avoid-
ing or meeting them. Birds go to warmer regions or remain in the cold and
depend on heavy feeding to keep up their metabolism; many mammals, rab-
bits, foxes, and others are active but must have abundant food; other mam-
mals hibernate, put on layers of fat in the fall, and live at a kind of physio-
FiG. 5.15. Chemical cycles. The carbon cycle. Respiration of plants and animals
returns most of the carbon to the air as carbon dioxide. The storage of carbon in
coal and oil is an important exception to the general rule that the carbon used by
the green plant in photosynthesis returns to the air. Coal is largely carbon derived
from the cellulose of the trees about 250 or more million years ago. Carbon is
also captured in the calcium carbonate (CaCO^) of clams, crabs and others.
The nitrogen cycle is much more complex than that of carbon. The main reason
is that many organisms do not release nitrogen in a form that can be immediately
used by green plants. They can use it when it appears as certain inorganic salts,
particularly nitrates.
84
ECOLOGY
Part II
MIDSUMMER TEMPERATURE
A TYPICAL DEEP LAKE OF A TEMPERATE ZONE
Water surface
(Ti.e'F)
2 re
(69.8''F)
io'c
(SCF)
Epilimnion
29 Ft.
Wind-stirred, air-
mixed water.
Plenty of light.
Abundant plankton
Thermocline
Transition area, 65%
fall in temperature here.
45 Ft.
Hypolimnion
S-S^C .1 128 Ft.
Bottom
Still water.
Little or no light.
Maximum range of
temperature for year
about 40° F.
■ 1 1MB "■•'ill" r--' " ■•''■'•■■•■■•''■■•'-•-■-'••'■• •.'■■.:■••■••.••• •:■ i: :■.':■■■. .•...••- ■ . ■■. ■■ .. .-. t.- :•■■.:.■■ ■ '■ ■-•■.
B
V -*^
-«— ■
;|kN.
Epilimnion
:|
Thermocline
:'\ Hypolimnion
69.0°F
SO.O'F
41. CF
SUMMER
Layers as in A Wind blows
surface waters. Temperature
shift in thermocline.
v/yy/y/A .ce ^//////////a 32.2" f
37.2° F
39.2° F
TrWTTy^TTr' 39. 2 F
WINTER
Ice cover is a boon to population
beneath it. Plankton sinks with
heavy water.
39.2° F
•::':y^^T?.Ui.i.i!.t|ii|giii
39.2° F
39. 2° F
AUTUMN OVERTURN
Cold winds blow and chill surface
waters. Their temperature changes
fo 39.2° F. They fall and mix.
" ^ if ' "' ic ' '' '' ^j/^
39. 2° F
39. 2° F
39. 2° F
39. 2° F
SPRING OVERTURN
Ice melts. Surface water changes
to 39.2°F, IS heavier and falls,
mixes ond displaces lower waters.
Stir brings plankton to surface.
Chap. 5 ANIMALS AND THEIR ENVIRONMENTS 85
logical low gear for which little or no food is needed. In winter the water is
warmer than the air; frogs stay in muddy pond bottoms but do not drown
because they take in enough, oxygen through the skin for their lowered metab-
FiG. 5.18. Bubbles of nitrogen gas (black)
collect at the joints when a person, e.g., a deep-
sea diver, rises suddenly into greatly lowered
pressure.
olism. Insects go through a special cold-hardening, partly by loss of water and
the production of bound water which does not freeze except in extraordinarily
low temperatures. Earthworms burrow below the frost line and gather in clus-
ters conserving heat and moisture. Lady beetles spend the winter in companies
Fig. 5.17. A, midsummer temperature of a lake. Water contracts with cooling
and becomes heavier but only to 4° C. (39.2° P.). When warmer or colder than
this it becomes lighter.
Water takes its place in layers according to its weight which is dependent on
temperature.
B, sections of a lake showing the seasonal changes in temperature. Summer.
The light is stronger but the diatoms decrease probably because of inadequate
nourishment and perhaps of silica since the thermocline seems to bar the way to
chemical substances that might otherwise well up from the bottom. Autumn.
With the mixing of the water and disappearance of the thermocline there is an
upward diffusion of nutrient salts. Another increase of diatoms occurs, not so great
as in spring since the sunlight is weaker. Winter. The lake is covered with ice
which is water at its lightest and coldest. Spring. Light increases and with it an
increase of diatoms called the spring pulse, of great importance in the food supply
of all young animals.
86
ECOLOGY
Part II
Fig. 5.19. Social hibernation of ladybird beetles. With the first frosts the beetles
fly to the ground and then to trees searching for holes in which they gather by
hundreds. Animals that are solitary in summer may be social in winter. (Photo-
graph by Carl Welty.)
though they are solitary at other seasons (Fig. 5.19). Cold as well as sex
encourages sociability.
Light. Light is necessary for vision but there are other ways in which it con-
cerns animals. Like plants they are deeply affected by longer or shorter days.
This shows in their breeding seasons, in the migrations and seasonal changes
of color in birds, and in the color changes of snowshoe rabbits, and other
northern animals. In general, animals are responsive to light whether they have
light-perceptive organs or not, but lenses are present even in certain proto-
zoans. The majority of higher animals probably find their way chiefly by
vision, but by no means entirely.
The amount of Hght that enters water depends upon the direction of the
rays, which differs with the time of day and year, the amount and clearness of
the water through which the rays pass, and the intensity of the light. In rela-
tively clear water, one-third of the light is generally lost in about three feet
and three-quarters of it in 16 feet. At depths of 2,000 feet or more the ocean
is completely dark except for the luminescent animals, mainly fishes.
Biological Environment
The neighboring plants and animals compose an organism's biological envi-
ronment. Whether the organism is a crocus in a mountain meadow, a parasite
Chap. 5 ANIMALS AND THEIR ENVIRONMENTS 87
in human blood, a squash bug on the vine, or a citizen in the town, it is con-
cerned with a biological environment, human or otherwise. The animals of an
environment are roughly divided into producers of food and competitors in
the consumption of food. Some of the consumers are predators that rob and
kill.
Search for Food. Numerous and widely distributed animals are apt to live
on common foods. Rodents — squirrels, field mice, and rabbits — all abound in
great numbers; so do the shrubs, grasses, and clover which they eat. Grass-
hoppers and crickets live surrounded by grass and grain. At the height of their
season the only grass-eaters that compete with them in open fields are cattle
and sheep. During the great migrations of grasshoppers nothing stands in
their way (Chap. 30). Birds, small mammals — shrews, ground moles, and
chipmunks — commonly prey upon them. But their reproductive capacity is
so high that these predators do them the good turn of keeping the population
to a size which the space and food can support. Animals multiply greatly in
regions where they have few or no competitors for the particular food on
which they live. This is strikingly true of penguins in the Antarctic. The same
principle applies to nocturnal animals such as owls and skunks that hunt by
night when there is less competition.
Biological environments obviously depend on the chemical and physical
ones. Plant populations rely particularly upon water and temperature and
animals follow the plants. Animals abound at river mouths to which the river
brings rich organic deposits. Rivers and their valleys have always determined
the location of animals just as they have always determined locations for man-
kind.
Size of Food. Man is the only animal that can catch all sizes of animals,
from frogs to cattle, oysters to whales, and use them for food. He can eat
small, large, and medium-sized animals indiscriminately: an important control
to have over the environment. The scavengers — vultures (turkey buzzards),
lobsters, pigs, and chickens — approach mankind in the variety and sizes of
food which they appropriate. With the exception of parasites and scavengers,
other meat-eaters must deal with food that is adequate but not too large to be
manipulated. Fierceness and skill may take the place of size in capturing prey,
so may social behavior. Packs of wolves will attack a moose but a solitary
wolf seldom does so. Millions of South American army ants will set upon and
kill small mammals but no one of them could do it alone.
Food Relations. The food relations of a community are exceedingly com-
plex, changeful, and affected by factors in the immediate environment as well
as others far outside it. The complexity of the human food market is an exam-
ple with its many and remote causes of undersupply and oversupply and
resulting prices. The food relations between animals are expressed as food
chains, food webs, and pyramids of numbers (Fig. 5.20, 5.21 ). A food web is
88
ECOLOGY
Part II
dead animals
Food- web on Bear Island in the Arctic zone. (Sinriplified from
Elton.) The arrows are read as "eaten by/ e.g./bacteria — ►
protozoa" means bacteria are eaten by protozoa.
Fig. 5.20. In food webs the successive eaters are usually larger, e.g., insect,
ptarmigan, fox, but fierceness, cunning or group action may take the place of size,
e.g., in army ants, wolves, and wild dogs. (Reprinted from Readings In Ecology
by Ralph Buchsbaum, by permission of The University of Chicago Press. Copy-
right 1937.)
literally what eats which in a community of animals or of animals and plants.
Plant-eating animals are the basis of any community; they serve as food for
the small carnivores which are in turn eaten by the larger ones. Such a series
of food links is a food chain. In a pond bacteria and unicellular plants are the
Chap. 5
ANIMALS AND THEIR ENVIRONMENTS
89
basic supply. Beginning with them, smaller animals are eaten by larger ones,
protozoans by minute crustaceans and the fry of fishes, and these by aquatic
insects and so on to the large, fishes and turtles. If they die in the lake their
bodies are returned to the bacteria; if they are caught and taken elsewhere
they may become part of another food chain. In any long food chain, the
successive eaters are not only larger in size but fewer in number. There are
few sparrow hawks compared to the number of sparrows, few owls to the
number of field mice, one fox to dozens of rabbits.
In communities of animals there are many more small adults than there are
large ones (Fig. 5.21). What seems obvious is borne out, in broad fines, by
analyzing a definite area of a community, counting the animals of various sizes
and measuring the totals by bulk or weight. The result is a pyramid of num-
bers. Such a pyramid applies particularly to predatory animals. It shows that
smaller animals have a higher reproductive capacity than large ones and are
Hawk A
A Fish
CarnivA
/warblers
orous \
/Thrushes
Beetles \
/ + + -t-
+ + + + +\
/ Spiders
\
/ Carnivorous
Daphnia \
/ Beetles
Cyclops \
/ + + + + +
+ + + 4- + 4- + + + + \
/ Aphids
Protozoans \
/ ++++++++++
+ + + + + + + + + + + + + + + \
Open woods
MILLIONS OF INSECTS TO ONE HAWK
Pond
BILLIONS OF PROTOZOANS TO ONE FISH
Fig. 5.21. A pyramid of free living animals in one area. Plus signs express
abundance of types of animals. The smallest ones are most abundant. They supply
food to carnivores that are larger in size and fewer in number and these in turn
supply other carnivores that are still larger and fewer.
90 ECOLOGY Part II
generally the prey of larger ones, that there are great numbers of small animals
and relatively few large ones. This food situation is very complex. It clearly
involves sizes of food; it also includes feeding equipment such as cilia, teeth,
and claws, all sorts of locomotion, and kinds and extent of territory covered
in hunting food, as well as shifts in population due to cataclysms from the
action of weather and humanity. The food relations of animals, actually the
connections between the soil and the beefsteak, are exceedingly important to
human economy.
Protecrive Resemblance and Mimicry. Protective resemblances are charac-
teristics that seem to make life safer for animals in their own environments.
Such protection is a debatable subject which has much to be shown for it and
considerable against it. It is a pattern of colors that makes an animal unrecog-
nizable against its home background. A brown streaked sparrow is lost among
the twigs of a brush pile; katydids are as green as the leaves beneath them;
ground squirrels (gophers) and prairie chickens are streaked Hke prairie grass;
fishes that swim in and out between bright-colored corals are also brightly
colored. Polar bears are white. Snowshoe rabbits and weasels (white phase is
ermine) are brown during the short northern summer and white in winter.
There are vast numbers of animals whose coloration does conceal; there are
also many in which it does not. There are animals whose coloration seems to
have no significance in their survival. Throughout the Arctic there are two
color phases of arctic foxes, one of them is brown in summer and white in
winter; the other is grey or black in summer and blue or black in winter. Both
the blue and white phases interbreed and are common and successful in the
same areas of Greenland and Alaska.
Camouflage is the painting or screening of boats, buildings, other objects,
or persons so that they are lost to view in the background. It was first widely
used in World War I. Its principles were based upon those of protective
coloration suggested by a British zoologist, E. B. Poulton, and later developed
by an American artist, G. H. Thayer, and published in his finely illustrated
book. Concealing Coloration in the Animal Kingdom. The first of the princi-
ples is counter-shading, a generalization of the fact that in the great majority
of animals the back is dark and the underparts are pale. By painted models
Thayer showed that any object so colored is less conspicuous on being strongly
lighted from above and with dark reflection from below. Another principle is
related to the break-up of a familiar form such as that of a dark-colored bird
whose head is separated from its body by a white ring around the neck.
Colors of animals are often strikingly different in the two sexes, the males
usually the more brightly colored, especially in birds, fishes, and insects.
Sexual coloration is often associated with endocrine secretions and is men-
tioned further in connection with them (Chap. 15).
6
Mutual Relations nips or Animals
Whirligig beetles spin and turn in companies on the pond surface; a hundred
starlings swing into a treetop; swarms of gnats rise and fall in quiet air; men
and women join in a folk dance. These are all social beings, those of each
group sharing particular surroundings. Animals express their sociability by
being in the same place at the same time.
Two kinds of behavior, competition and natural cooperation, are character-
istic of sociability.
Competition and Cooperation
Competition occurs when there is a common demand on a limited supply.
A certain amount of it is stimulating and healthy. An unlimited competition
is dangerous to individuals and communities. Its basic cause is the overpro-
duction of animals, human or otherwise. During the spring breeding season
many small ponds are populated with toads and each female lays about 15,000
eggs in a clutch. Presently the water swarms with toad tadpoles. All these tad-
poles have insistent appetites for the algae of the green pond scum that over-
spreads the water. At the start there is an abundance of algae as well as tad-
poles but it thins out as the eating goes on. Then competition begins. Some of
the tadpoles manage to get food, but many of them starve. If they were fighting
animals, there would be conflicts along with the starvation. In all communities
plants and animals compete for such essentials as earth, water, food, warmth,
and light as well as for less necessary things. Competition is commonly accom-
panied by a struggle for power and dominance usually gained by one or a few
individuals.
Competition is usually keenest between those of the same species since they
have the same wants; two rabbits go for clover, but a sheep eats grass and a
cat eats birds. The overpopulation, sparsity of food, and starvation of individ-
uals that occur in nonhuman animals have been matched in human ones
throughout history. Competition is reduced by differences of diet: among
91
92
ECOLOGY
Part II
*^j>.~'V.*«««» jS^
Fig. 6.1. The overpopulation of rabbits in Australia, too many for the space and
food available, a prime cause of competition and ultimate destruction. This tele-
photo lens picture shows how rabbits denude the pastures and drink the water holes
dry. (Courtesy, Australian News and Information Bureau, New York.)
birds, as in seed-eaters and insect-eaters, among larvae of insects, e.g., tomato
worms and cabbage worms. The rabbits of Australia, a country almost without
predators, have repeatedly overpopulated the land, devastated vegetation, and
brought themselves to starvation (Fig. 6.1).
Cooperation, conscious or unconscious, is the behavior of plants or animals
which benefits the lives of those about them. Animals may produce a flourish-
ing population beneficial to all concerned. They easily pass this point however,
by multiplying to such an extent that they are hungry and sick for want of
food and space. Thus their cooperation may be turned to disoperation. Exam-
ples of cooperation are plentiful. In winter bees crowd together in clusters
within the hive and thus conserve the heat in their bodies. Northern musk oxen
stand close together, heads down, against attacking wolves; geese band to-
gether with outstretched necks to hiss their disturber. People join in applause
by clapping their hands together; tent caterpillars join in making their web
and mending it when it is torn; beavers work together on their winter lodge
and their food stores (Fig. 6.3).
Competition and cooperation are fundamental biological principles. Com-
petition has long been recognized as such, especially since Darwin based his
Theory of Natural Selection upon it. Although the importance of cooperation
had been suggested by certain European workers, its prevalence and the
J
Chap. 6
MUTUAL RELATIONSHIPS OF ANIMALS
93
Fig. 6.2. Cooperation. Tent caterpillars and their community web. The young
caterpillars spin a dragline of silk from the time they hatch. After a few days of
feeding and trial spinning they begin to work together constructing the nest, at
first a small night tent, then a larger one a foot and a half or more long. They
leave the tent in the day time and creep in single file to a feeding place leaving
a trail of silk behind them. (Photograph by Lynwood Chace. Courtesy, National
Audubon Society.)
soundness of the principle have been demonstrated in recent years by the
observation, experiments, and conclusions of the eminent American ecologist,
W. C. Allee and his co-workers (Suggested Reading, Chap. 6).
Varieties of Partnerships
Partnerships may occur between plants, between animals, or between plants
and animals.
Symbiosis. Living together is known as symbiosis. This is a general term
that includes all aspects of physiological and ecological association (Fig. 6.4).
It is often difficult to determine the exact nature of the relation between two
organisms that live together, whether it is a neutral aflfair or an advantage to
both partners. In either case, symbiosis would describe it. Commensalism,
mutualism, and parasitism are types of symbiosis.
94
ECOLOGY
Part II
Fig. 6.3. Cooperation. Beavers' lodge and winter food storage — a community
project. The lodge and passageway to the pond bottom are shown as if cut open
and the ice bound pond as if in section. The two beavers working below the water
line must frequently come up for air. (Courtesy, Hamilton: American Mammals.
New York, McGraw-Hill Book Co., Inc., 1939.)
CoMMENSALiSM. Meaning at the same table, commensalism was originally
applied only to sharing the same food. It is now used for neutral associations
which do not seem to affect either partner. A classic example is the sea
anemone that rides about on the shell of the hermit crab and thereby gains
wider range for forage, but does not eat the same kind of food as its host. Less
familiar is the mahout beetle that rides on the head of a worker termite and
takes bits of food as it is passed from one termite to another (Fig. 6.4).
Mutualism. A symbiosis that benefits each partner is mutualism. Honey-
bees and many flowering plants aid one another to the point of dependence.
Honeybees eat nothing but flower products. And as they collect the nectar
and pollen they distribute the latter, usually to flowers of the same kind be-
cause they grow together. Thus the bees cross-pollinate them. Many flowers
are so formed that they can be pollinated only by insects. In nature the yucca
lily (Spanish bayonet) and the yucca moth (Pronuba) are entirely dependent
upon each other (Fig. 6.5). The lily is pollinated by the moth, which thrusts
a blob of pollen onto the pistil. Thus she effects the fertilization of the ovules
and then lays her eggs in the ovary where the larvae can feed on the ovules.
The plant does not suffer, for more seeds develop than are eaten by the larvae
of the Pronuba. Yucca lilies are native to southern North America but are
cultivated farther north, since they are easily pollinated by hand.
Chap. 6 MUTUAL RELATIONSHIPS OF ANIMALS 95
Fig. 6.4. A minute beetle, Termitonicus mahout, that rides on the heads of the
workers of the termite, Velocitermes beebei, and takes bits of the food as it is
passed from one worker to another. An example of symbiosis, a general term that
includes a variety of partnerships. (Redrawn after Allee et ah: Principles of Ani-
mal Ecology. Philadelphia, W. B. Sanders Co., 1949.)
One of the most remarkable examples of mutualism is that between wood-
eating termites and certain species of protozoans. The protozoans live pro-
tected within the intestines of the termites and in turn actually digest their food
for them. Bits of the cellulose food are taken in by the protozoans and changed
to sugar (dextrose) which is squeezed back into the intestine and absorbed by
the tissues of the termite. Experiments have shown that termites cannot sur-
vive long without the protozoans unless they are given a diet other than cellu-
lose. On the basis of the evolutionary history of termites it is estimated that
these intestinal intimacies have existed for 150 million to 250 million years.
Parasitism. Another form of symbiosis in which an organism lives on or in
and at the expense of a larger plant or animal, called the host, is parasitism.
The parasitic mistletoe grows on a tree, commonly an oak. Animal parasites
are always small in comparison with their host and usually numerous. The
parasite obtains food, protection, or transport from its host, often all three of
these.
Parasitic animals are discussed in the chapters dealing with the groups to
which they belong. These are especially: Chapters 21, Protozoa (sporozoans,
e.g., malaria); 25, Flatworms (tapeworms, et al.); 26, Roundworms (trichi-
nae, hookworms, et al.); 28, Annelids (leeches, et al).
The relationship of parasitism costs the host its substance and the parasite
its independence. People who must have special food are restricted in their
travels; so are fleas and bedbugs.
The Host, a Living Habitat. Plants and animals have three major dwelling
places: terrestrial — on or in the earth's crust, aquatic — in fresh or salt water,
and on or in living organisms.
Parasites occupy living habitats. In them there are special places in which
various parasites thrive, such as the skin or the liver, just as different seashore
animals thrive in tide pools or in mucky sand.
Living habitats offer ready food and protection, within limits. Parasites must
96
ECOLOGY
Part II
anther
Yucca flower natural size
Sickle shaped jaw,
a pollen collector
Moth gathering pollen
from anther
Yucca lily
MOTH AND LILY, MUTUAL BENEFACTORS
Fig. 6.5. Mutualism, a partnership that benefits each member. The yucca lily,
Yucca filamentosa, whose stalks of white flowers grow four to six feet high in the
eastern and much higher in the western United States. When the female moth
visits a flower she thrusts her long ovipositor into the ovary and deposits an egg
beside each of the several ovules (eggs). Then she climbs to the tip of the pistil
and carrying pollen that she has collected from some other flower she pushes it
into the stalk incidentally making it possible for the transported male cells to
fertilize the ovules of the flower she is visiting. After fertilization the ovules
develop into seeds; some of them are eaten by the larvae of the moth but others
that are untouched propagate the plant.
hold their places, often against pressure, lack of oxygen, and the defenses of
their host. If parasites of digestive tracts did not have a protective immunity
to digestive fluids they themselves would be digested. Parasites must reproduce
and be distributed in such a way that the young ones can enter into new hosts
of the right type and at the right time. Trichinae, the minute worms resting in
the pig's muscle, must arrive still alive in a human stomach by way of a sand-
wich or a sausage. It is a great gamble, but not a rare feat for trichinae in the
United States (Fig. 6.7 and 26.5).
Development of a Food Habit. Parasitism is primarily a food habit and
parasites are mainly chronic predators. Typical free-living predators are larger
than their prey, kill it quickly, and devour it soon. A cat pounces upon a
mouse, and if hungry, kills and eats it at once. Cats, foxes, and hawks are
Chap. 6 MUTUAL RELATIONSHIPS OF ANIMALS 97
typical predators. Parasites are smaller than their host, feed upon its substance
persistently, and chronically weaken or gradually kill it. A field mouse can
supply blood to a moderate -population of lice without great injury. But an
excessively large population results in great competition among the lice and
the death of the mouse from loss of blood. Like tax collectors after more in-
come the lice must then find another mouse.
Development of Parasitic Living. In the early stages of parasitism the in-
cipient parasites visit their hosts only for meals. Blood-sucking leeches clamp
their suckers to the flesh, insert their jaws, suck blood until they are satiated,
and then drop off into the water. Such a meal supplies a leech with food for
several weeks. The blood-sucking mosquitoes, always female, spend even less
time on their hosts and simply take a firm stand on the skin while they suck
up the blood (Chap. 30). In certain species mosquitoes do not lay their
eggs until after they have had a blood meal. In laboratories where they are
reared they are allowed to bite a human victim whenever eggs are needed
for experiment. Such mosquitoes have taken a long step into parasitism
-is. ^ LARVA
^.j ^/ (CHieeiR)
EGG
o
ADULT
Fig. 6.6. Ectoparasites; examples of parasitic life on the external surface of the
body: fleas, lice, chiggers. Left, the common rat flea, Nosopsyllus fasciatus (after
Bishopp) : upper, the nonparasitic larva and pupa that live near the host, not upon
it; lower, the blood-sucking parasitic adult (female) that stays much of the time
feeding on its host, slips easily between the hairs, has great ability to spring on and
off its host, and is able to adopt a human one temporarily. Center, common chig-
ger, or jigger mite, harvest mite, Entrobicula alfredugesi. The exceedingly minute
six-legged parasitic larva that bores into the skin, liquefies the local tissue and
sucks up the fluid. After feeding the mite is no longer parasitic but drops to the
ground and develops the free living eight-legged stage, Chiggers are distributed
from New York to Minnesota and are pests in the southern states attacking all
land vertebrates including man. Right, human head louse, Pediculus humaniis. var.
capitis. Adult showing the claw and thumb that lock around the hairs. Lice are
highly adapted for clinging and blood sucking and do both throughout their life
history. {Left, courtesy, Matheson: Medical Entomology. Ithaca, N.Y., Comstock
Publishing Co., 1950. Center, courtesy, Stiles: Individual and Community Health.
New York, The Blakiston Company, 1953. Right, courtesy, Herrick: Household
Insects. New York, The Macmillan Co., 1916.)
98 ECOLOGY Part II
in being so seriously dependent upon the special diet of warm blood that the
species will die without it. Male mosquitoes do not show any such trend to-
ward the habit; they still drink fruit juices. Fleas and sucking lice represent
steps in increasing parasitism in the persistence with which they stay on their
host. Fleas stay on a dog most of the time; they also frequently jump off. Lice
stay on except by accident. Their claws lock onto the hairs of the mouse or
other host and they cling fast as fleas never do. Chiggers go still further. They
are the parasitic larvae of certain kinds of mites that actually burrow into the
skin (Fig. 6.6).
The parasites so far mentioned are a few of the great host of ectoparasites
that attack the outsides of animals and represent the earlier stages of para-
sitism. Endoparasites spend most of their lives inside the bodies of animals
and represent the extremes of adjustments to parasitic living (Fig. 6.7). The
easiest way for an endoparasite to enter an animal is by way of the mouth
along with food or drink. Other possible entrances are into the breathing
organs, the excretory ones, the reproductive organs, and through the skin.
Life Histories. Whatever their habit, animals go through various phases
during their life spans. The embryo of any animal is very different from the
adult; young animals may live in one environment and later move to a very
different one. Parasites often change from one host to another while in their
egg or larval phase of life. This is especially difficult for endoparasites which
have to take advantage of the habits as well as the structure of their second
hosts in order to enter them.
A parasitic animal may pass directly from one host to another of the same
Fig. 6.7. Endoparasites; phases in the life of two endoparasites in which parasit-
ism is highly developed. Left, trichina worms: Trichinella spirella, coiled and
dormant among muscle cells, an example of the phase of waiting, characteristic
of many endoparasites. Right, trypanosomes: Trypanosoma gambiense, a proto-
zoan blood parasite. (Fig. 21.10, trypanosomes and West African sleeping sick-
ness.) They reproduce in enormous numbers in the blood of man and in the big
game of Africa and are transmitted by the tsetse fly. The multiplicity of their
populations and dependence upon a second transmitting host are characteristic
of many endoparasites. (Courtesy, General Biological Supply House, Inc., Chi-
cago, 111.)
Chap. 6 MUTUAL RELATIONSHIPS OF ANIMALS 99
species, in which case it has a direct life history. Such parasites may live
through their entire lives in one host, producing eggs and larvae which in turn
live and reproduce in the sarne place. Many of them are usually carried out of
the body with waste from the intestine. They then await the chance of getting
into the mouth of another individual; this is the usual history of pinworms
{Enterobius vermicularis) , common parasites of children (Chap. 26). In con-
trast to such direct life cycles are the indirect ones of parasites with hosts that
belong to two or more different species. Larvae of these parasites develop to
a certain stage in one host, such as a sheep. But they cannot develop further
unless they are cast out of the sheep's intestine at the edge of a pond where
they can enter certain pond snails, their intermediate hosts. In the snails they
develop to a particular stage in which they leave the snails, swim about in the
water, and finally onto the wet grass around it. In this stage and in no other are
they able to infect another sheep when swallowed (Fig. 25.11). These para-
sites, called flukes, prove that gambling is a very ancient and enduring practice.
Certain important variations apply to both direct and indirect life histories
of parasites. Some species with direct life histories can live parasitically in
several related animals, such as sheep, cattle, and others that chew the cud;
while other species, such as the human hookworm, can live only in one type
of host. Parasites with indirect life histories spend part of their lives in an
intermediate host before they can pass to the definitive host in which they re-
produce. An example of indirect life history is that of the liver fluke of sheep;
the intermediate host is a snail in which the parasite is immature; the definitive
hosts are sheep in which the flukes reproduce.
Effects of Parasitic Life on Parasites. Parasitic animals have to contend with
many difficulties and risks. Such gamblers stake their lives on finding their
hosts and maintaining themselves upon or within them. They accomplish this
by an enormous production of sex cells, by the development, in many species,
of male and female organs in the same individual, making fertilization of the
eggs more certain, and by parthenogenesis, the production of young from un-
fertilized eggs. It has been calculated that the beef tapeworm of man produces
between 50 and 150 millions of eggs a year. American hookworms probably
release about 6 to 20 thousand eggs per day. Numbers are also increased by
asexual reproduction. In certain parasitic wasps one egg divides so as to produce
several embryos. The single-celled malarial parasite produces many new in-
dividuals by division. It has been estimated that these parasites (Plasmodium
vivax) can produce about 40 thousand parasites to every cubic millimeter of
the host's blood. Eventually parasites kill their host and destroy their own
welfare by overpopulation, just as too many gasoline stations kill a business.
Some Important Parasites of Man. Parasites occur in all the main groups
(phyla) of animals. Parasitic members of the phylum Chordata are extremely
rare, e.g., hagfishes and a few blood-sucking bats. Of the invertebrates, the
100
ECOLOGY
Part II
protozoans, roundworms, and flatworms are deeply committed to parasitism.
Among parasitic arthropods the insects are best known, such as fleas, lice, bed-
bugs. They are transmitters of disease-producing parasites and are themselves
in the earlier stages of parashism.
The life cycles of various parasites are described and figured in Part IV with
the groups to which they belong. The accompanying list shows the occurrence
of some important animal parasites of man (Table 6.1).
Table 6.1
Some Important Parasites of Man
Parasite
Means of
Disease in Human Host
Transmission
or Other Mammal
Spirochaetes
Ticks
Tick-borne relapsing
fever
Spirochaetes
Bodily contact
Syphilis
Protozoans
Eiidamoeba histolytica
Water
Amebic dysentery
{Chapter 21 )
Plasmodium
Anopheline
Malaria
(various species)
mosquitoes (female)
(various types)
Trypanosoma gambiense
Tsetse fly (Glossina)
African sleeping
sickness
Schistosoma mansoni
Water, snails
Bilharzia in about 50
et al.
per cent of popula-
Blood flukes
tion of Egypt, also in
Flatworms
other tropical coun-
tries
Inhabits intestine,
{Chapter 25)
Taenia saginata
Cattle,
Beef tapeworm
beef muscle
muscles
Taenia solium
Pork muscle
Inhabits intestine.
Pork tapeworm
muscles
A scaris
Soil, food, clothing
Ascariasis
A scar is lumbricoides
Enterobius vermicularis
Clothing
Enterobiasis
Pinworms
Necator americanus
Water, soil
Hookworm disease
Roundworms
{Chapter 26)
American hookworms
Wuchereria
Mosquitoes
Filariasis
(several genera)
Trichinella spiralis
Hogs, rats, et al.
Trichinosis
Trichinae
(in pork, sausage,
etc.)
Animal Communities
Organization of Groups. Aggregation is a general term for a group of
organisms of the same or different species, associated but not organized into
societies. Many of them are examples of natural cooperation and as such were
cited at the beginning of this chapter in connection with cooperation. Un-
organized groups like these were doubtless the beginnings of complex societies
such as those of ants and termites. Animals congregate because their environ-
Chap. 6
MUTUAL RELATIONSHIPS OF ANIMALS
101
t
I
^ ^
-
V
1-^
> •
^^H t-^ :
•~>.'
-ft.
IT*
EmL' k..^' ^-^El^ . 'mt .^<«
*; C'' *^^--'-A^'-
. .Ifca.^ . -t. . ..
•t"
•^
Fig. 6.8. Hundreds of white pelicans rising and thousands still to rise from a
preserve on Lake Washington, State of Washington. A typical aggregation of ani-
mals associated because something in the environment has beckoned them. (Photo-
graph by Hugh M. Halliday. Courtesy, National Audubon Society.)
ment drives or beckons: the cold of winter, the heat of summer, the dark that
starts the crows crowding into their roosts, the low tide that leaves new forage
for the gulls, the lakes kept as safe stopping places for migrating waterfowl
(Fig. 6.8). Animals are brought together by accident; starfishes, snails, and
others thrown on the beach enmeshed in seaweed. The spring gatherings of
frogs and toads and the shoals of spawning fish are aggregations stimulated by
climatic conditions and breeding habits.
Social Organization of Animals. Among invertebrates social organization
102 ECOLOGY Part II
reaches its highest development in the insects — termites, wasps, bees, and ants.
Their organization has a complexity comparable to that attained by vertebrates
but of an entirely dift'erent character. It is a strictly defined and inflexible
division of labor in which the various needs of the community are attended to
by individuals whose structures and functions mark them, with rare exceptions,
inescapably as members of particular castes with special work to do. Among
bees such specialists are the queen, the workers always females, and the male
drones (Chap. 30).
Organization of Vertebrate Groups. This is based upon three general prin-
ciples: the holding of territory, social hierarchies in which dominance and
power exist in a graded order from highest to lowest, and leadership-follower-
ship.
Territorial Rights. Birds take possession of a parcel of good habitat,
sing loudly from a prominent perch and defend it against trespass, driving off
members of their own or other species. American song sparrows sing special
proclamations of their ownership of territory and defend the mating and nest-
ing grounds by fighting. The willow wrens that migrate into England every
spring have a regular system of dividing up their usual territory into roughly
equal parts, and the males fight among themselves for their respective rights.
Social Hierarchies. Groups in which one individual dominates all the
others have been observed in birds, rats, cats, dogs, apes, and human groups.
A dominance known as peck right, observed in small flocks of domestic hens,
has been investigated mainly by Alice and his co-workers. In these flocks one
particular hen pecks any other hen without being pecked in return, that is, she
is dominant with peck right over the whole flock. Below her a small group of
hens peck those of lower social levels than themselves without receiving return
pecks. Below them again, similar levels occur down to the lowest level, the
hen which is pecked by all others yet does not peck back. During observations
each hen was tagged for identification by colored leg bands and other mark-
ings. Observations were taken with great care and repeated many times. The
dominance of a hen was generally first established by fights. Ailing hens and
those newly installed were in the low levels, and regular members which were
taken from the flock lost their positions by being absent. Similar social hier-
archies or grades of power exist among flocks of male fowls. Flocks of white-
throat sparrows represent social hierarchies similar to those of domestic fowls
but are less fixed.
Leadership and Followership. The leader of a group may or may not be
its dominant member. The leader is the individual that wiU not desert the
group in any emergency and that its members will follow. It is the experienced
"knowing" animal, not necessarily the largest or fastest. In herds of Scottish
red deer a stag is ordinarily the dominant member, but in crises the males
leave the group and a female assumes leadership. With real leadership the
Chap. 6
MUTUAL RELATIONSHIPS OF ANIMALS
103
Fig. 6.9. Relations of parents and young. Top left, termite, Hodotennes turke-
staniciis, king and queen beginning to dig the burrow that will lead to an elaborate
underground nest with thousands of occupants of which they will be the parents
(After G. Jacobson). Top right, male water bug, Pelostoma fiiimineum, with de-
veloping eggs glued to his back by the female. Such nursery-bearing males can be
found commonly in ponds during the summer. Bottom left, Koala, Australian
teddy bear. Female first carries the young one in a pouch like that of the kangaroo,
then on her back. Adults are about two feet long. Bottom right, male sea horse
(Hippocampus) with brood pouch in which the developing young are carried
(After Boulenger). (Termites courtesy, Wheeler: Social Life in the Insect World.
New York, Harcourt. Brace & Co., 1923. Waterbug courtesy, Morgan: Fieldbook
of Ponds and Streams. New York, G. P. Putnam's Sons, 1930. Seahorse courtesy,
Rand: The Chordates. Philadelphia, The Blakiston Co., 1950. Koala courtesy,
Young: The Life of Vertebrates. Oxford, Clarendon Press, 1950.)
104 ECOLOGY Part II
followers are dependent upon the leader, and the leader upon the followers in
a way which is not the case with the dominant animal or with the pseudo
leader which chance may place temporarily at the top. Interdependence be-
tween leader and followers is complete in the queen honeybee and the workers,
and is very marked in other social insects. Male bees are the least social mem-
bers of the hive. After the mating season, male deer separate from the rest of
the herd and forage for themselves. On the other hand, the females are accom-
panied by the young ones wherever they go. Many similar habits point to the
female as the deeply social influence in groups of animals.
The Family. Both parents may take part in rearing the young. The male
water bug carries the eggs stuck to his back until they hatch; the male sea-
horse has a brood pouch where the female deposits the eggs which he carries
until the young ones swim out into open water; male birds usually take their
turn at bringing food to the nest (Fig. 6.9). In general, however, the mother
and young relations are more stable and intimate, more truly social. Mother
and young have a comparatively long association in widely different types of
animals. Female spiders carry nurseries of spiderlings on their backs; cray-
fishes and lobsters swim about for many weeks with eggs and then young ones
hanging on their swimmerets; for days the female robin keeps close company
with her young ones, showing them what it is to listen for earthworms and
how to tackle them. A great company of young mammals are carried or trail
beside their mothers, young kangaroos or joeys, skunklets, bear cubs, and
fawns. They explore the surroundings from their shelter of maternal care.
They imitate the turns of their parents and gradually take part in the customs
of their kind. They are products of family associations, mothers, and some
times both parents, and young. Thus the family constitutes one of the bases,
though not the only one, from which society has sprung. Competition and co-
operation exist in the family as they do in other groups.
PART III
Tne Internal Environment or tlie Body
7
Ti
issues
In multicellular animals, cells live crowded together and constantly affected
by one another. Whether similar or different they cooperate closely in the
organization of the animal. Differentiation, the modification of certain parts
for certain functions, and cooperation are fundamental properties of their
structure and activity. The body of a flying bird and the body of the pilot of
an airplane are both great companies of cooperating cells.
Cells are associated in groups, the tissues and organs, and these in turn in
systems. The study of groups of cells is histology or microscopic anatomy.
Tissues, Organs, and Systems. Tissues are groups of similar cells with the
intercellular substances which they may produce. The substances may be of
hardly noticeable amount as in epithelium, or conspicuous as in bone, or fluid
as in blood.
An organ is an association of tissues all of which cooperate toward the per-
formance of one or more particular functions. The heart is an organ that con-
sists largely of muscle; it is covered and lined with epithelium; nervous tissue
acts in the control of its pulsation; and connective tissue holds the other tissues
together.
A system is a group of organs which collectively perform certain related
functions. The digestive system is concerned with intake of food, its prepara-
tion for absorption, and elimination of undigested waste substances.
The animal body, like the plant, is built of groups of cells that form tissues,
of tissues that form organs, of organs forming systems, and of systems that
compose the whole body.
The tissues are discussed in this chapter. The organs are included with their
respective systems.
Classes of Tissue
There are four types of tissues, grouped according to their structure and
function: epithelial, connective tissue including blood and supporting tissues,
muscular, and nervous tissue.
107
108
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
Epithelial Tissues
Epithelium covers the outer surfaces of the body, Hnes its cavities such as
lungs, alimentary canal, the coelom or body cavity, and the blood and lymph
vessels (Fig. 7.1). It forms glands and the essential parts of the sense organs
— sensory cells in the eye and nose.
Epithelium is an essential guardian of the integrity of the body. It has a
general and vital part in metabolism since all substances which take part in
metabolic activity must go through epithelial cells. All digested food is ab-
sorbed through epithelium, mostly in the small intestine. The amount of water
contained in an animal is controlled through epithelium, in the skin, alimen-
tary canal, and kidneys. In the liver and kidneys it takes part in excreting
waste substances. It secretes such varied products as oyster shells and pearls,
the chitinous cover of insects, the digestive fluids of all multicellular animals,
and the hormones of glands such as the thyroid and the pancreas. It is directly
the protection against all manner of mechanical and chemical injuries. It was
Fig. 7.1. Epithelial tissues through which all substances that take part in the
metabolism of multicellular animals must pass. A, simple flattened or squamus
epithelium from the surface of the mesentery of a guinea pig; B, lining of a small
vein of mesentery. Intercellular cement is darkened by the preparation. xl200.
(Courtesy, Nonidez and Windle: Textbook of Histology, ed. 2. New York,
McGraw-HiU Book Co., Inc., 1953.)
Chap. 7
TISSUES
109
Fig. 7.2. Cuboidal or low simple columnar epithelium: A, lining of a collecting
tubule in the kidney of a monkey; B, in the thyroid gland of a monkey. These
cells produce the thyroid secretion. xl200. (Courtesy. Nonidez and Windle:
Textbook of Histology, ed. 2. New York, McGraw-Hill Book Co., Inc., 1953.)
due to the epithelium on their bodies and in their kidneys that the animals of
ancient time could leave the sea and gradually become adjusted to living in
fresh water or on land. The epidermis or outer layers of human skin which is
formed of epithelial cells is in general about as thick as tissue paper. Yet a bit
of vinegar dropped on broken and unbroken skin are vividly different expe-
riences. In certain regions, the epidermis is many-layered, as on the palms of
m.
lil
Sgi^^ligg:^^^
Fig. 7.3. Columnar epithelium with motile cilia (c) lining the trachea of a
monkey. Mucous or goblet cells (g) secrete the mucus (m) that passes through
the membrane at one end of the cell and spreads over the inner surface of the
trachea. The delicate non-cellular basement membrane (b) separates the epi-
thelium from the loose connective tissue beneath. A lymphocyte ( 1 ) is migrating
through the epithelium. X1200. (Courtesy, Nonidez and Windle: Textbook of
Histology, ed. 2. New York, McGraw-HillBook Co., Inc., 1953.)
no
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
the hands, and thick and tough in the footpads of cats and dogs; the cells are
heavily cornified in fingernails, in the horns of cattle, in the hoofs of horses,
the outer shell of turtles, the hair of mammals, and feathers of birds. New
epitnelial cells are formed as others are worn out or injured. Otherwise we
should be walking records of our encounters — scrapes, burns, and pinches.
Regeneration is constantly going on in skin and its outgrowths of feathers and
hairs. The snake's skin comes off in one piece, the human skin in little frag-
ments; feathers are shed in late summer, human skin at any time.
Different Kinds of Epithelial Cells. These are classified according to their
shapes — flattened, cuboidal, columnar, and arrangement in single or multiple
layers, simple or stratified (Table 7.1). A single layer of simple flattened (or
squamous) epithelium lines blood and lymph vessels including the heart (Fig.
7.1). Cuboidal epithelium lines the ducts of glands (Fig. 7.2). The cells of
columnar epithelium are tall prisms or cylinders (Figs. 7.3, 7.4). They form
the lining of the small intestine where they secrete digestive juices and absorb
the digested food. All columnar cells have polarity, that is, are different at their
two ends. In ciliated columnar cells the polarity is conspicuous since they bear
a large number of cilia only on their free surfaces. Cilia beat with rapid
effective and slower recovery strokes, always bending in one direction. The
movements travel over the surface in waves which rapidly succeed each other
at regular intervals. This occurs in the lining of the human trachea with the
stronger stroke toward the mouth. Cilia on the gills and lips of clams wave
particles of food toward the mouth. In the oviducts of mammals they create
currents which move the eggs toward the uterine cavity (Fig. 18.13).
Stratified flattened epithelium of the skin is several layers thick; the outer
Table 7.1
Forms and Functions of Epithelium
Name
Form
Examples
1 Flattened
Mesentery of frog; in man, lining of capillary
\ Cuboidal
Lining salivary gland in insect; lining normal
Simple
(
thyroid of frog
/ Columnar
Lining food cavity of hydra; small intestine of
cat
Pharynx of frog; human trachea; gill of clam
Ciliated
Columnar
Stratified
Cells in layers, outer ones
flattened
Skin of frog and man
Name
Function
Examples
Glandular
Digestive secretion
Small intestine of mammals
Sensory
Response to vibration.
Lateral line organ in fishes, tadpoles; rod and
light, chemicals
cone cells of human eye; chemoreceptors in
jellyfishes
Germinal
Origin of sex cells
Seminiferous tubules in testes of frog, cat, man;
ovary in hydra, grasshopper
Chap. 7
TISSUES
111
Fig. 7.4. Simple columnar cells in a gland in the human uterus. Droplets of the
secretion have collected at the ends of the cells and are about to pass through the
membranes; other droplets are free of the cells in the cavity of the uterus. All the
epithelial cells have polarity, most striking in the ciliated and glandular ones.
Preparation by Dr. G. N. Papanicolaou. xl200. (Courtesy, Nonidez and Windle:
Textbook of Histology, ed. 2. New York, McGraw-Hill Book Co., Inc., 1953.)
ones are dead and horny (Fig. 8.2). They are constantly being worn away at
the surface and replaced in the deeper layers. Stratified epithelium is extremely
thick in the footpads of large carnivores — tigers, lions.
Connective Tissue, Including Blood and Supporting Tissues
Connective tissue contains a large amount of nonliving intercellular sub-
stance, fibers in connective tissues, tough resilient chondrin in cartilage, hard
rigid substance in bone, and the fluid plasma in blood (Fig. 7.5).
Connective tissue connects and binds together the tissues and organs of the
body. It seems ever present, penetrating into glands and muscles along with
the blood vessels, and binding nerve and muscle fibers into compact bundles.
If all other tissues were destroyed, the body with its organs would keep its
shape because of connective tissue. During dissection its whitish sticky strands
have to be pushed aside and torn. In beefsteak and roast beef such strands
display their tough and threadlike quality. Surface wounds are closed mainly
by connective tissue and scars of all kinds are chiefly composed of it.
Loose areolar or open tissue is the papery fastening which must be torn
as one skins any animal, especially birds and mammals. This most generalized
connective tissue supports and surrounds other tissues and is a living pack-
ing material in the body.
The substances which other tissues receive from the blood and lymph —
112
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
{Legend on facing page)
i
Chap. 7 TISSUES 113
oxygen, food, water — and the metabolic products which tissues pass on to
the blood and lymph must all go through connective tissues. Like epithelium
it is a screen through which substances pass to and fro. The characteristic
of the body known as its constitution is probably connected with properties
of the loose connective tissue. Abnormal growths such as tumors persist or
fail to develop, depending to some extent on the reactions of this tissue. In
its defensive response the phagocytic cells called macrophages (large eaters)
which originate in it are the main actors. These cells are scattered throughout
the body and are ordinarily quiet, but if properly stimulated, as by infection,
they become mobilized like an army, enlarged, and active.
The structure of loose areolar connective tissue is typical of all connective
tissue (Fig. 7.5). It is composed of: (1 ) cells, such as macrophages, fibroblasts
(associated with the formation of fibers); (2) nonliving collagenous white
and elastic yellow fibers; (3) a thin jellylike ground substance. Collagenous
fibers are so-called because they contain a protein, collagen, which on boiling
yields glue and gelatin. In areolar tissue they run in all directions, are very
flexible and resistant, but are not elastic. They are really bundles of very,
very fine cross-striated fibrils, but these are invisible except by special tech-
niques. Elastic fibers appear as single strands, branched and like rubber
bands; when a pull is released they return to their original length. Areolar
tissue pulls the skin into place after it has been pinched up from the back of
the hand, more quickly in a younger than an older person; it also surrounds
organs. Dense areolar tissue, the dermis of the skin, is the fibrous part of
leather.
In many ligaments and tendons collagenous fibers are predominant and
compactly arranged according to the strains put on them. They are densely
woven like felt in the sclerotic coat commonly called the white of the eye.
Connective tissues often contain very few collagenous white fibers and many
yellow elastic ones, the latter so abundant that the whole tissue is elastic.
This is the case in the nuchal ligament of grazing animals: a strap of ex-
FiG. 7.5. Connective and supporting tissues. Top, cross section through the human
tailor's or sartorius muscle showing how muscle cells are held together by a web
of interlacing strands of connective tissue, the white lines in most cross cuts of
meat. This muscle is the longest in the body originating on the hip, crossing the
thigh obliquely, extending down the leg, and attached to the inner side of the shin
bone. Bottom, microscopic structure of the loose areolar connective tissue of a
kitten, spread out and stained to show its parts. This tissue tears like paper as one
skins an animal, a tissue with many open spaces, c, non-living collagen (or pro-
tein) white; e, elastic yellow fibers; /, fibroblasts, the cells associated with produc-
tion of the fibers; /, lymphocytes; m, macrophages, the cells that consume bacteria
and foreign particles; m^, mast cells, function unknown. [Top, courtesy, Maximow
and Bloom: Textbook of Histology, ed. 6. Philadelphia, W. B. Sanders Co., 1952.
Bottom, courtesy, Nonidez and Windle: Textbook of Histology, ed. 2. New York,
McGraw-Hill Book Co., Inc., 1953.)
114
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
Atlas
Nuchal ligament
Stieet like extension
of ligament
Scapuio
Cartilage of
scapula
Humerus
Elbow
Radius
Fig. 7.6. Nuchal ligament of the horse, a strap of tough, yellow elastic fibers, often
called whetleather, highly developed in grazing animals.
tremely tough yellowish tissue, sometimes called whetleather, which extends
along the back of the neck (Fig. 7.6). In the larger arteries these nonliving
elastic fibers form a large part of the wall. In older animals they lose their
elasticity.
Certain connective tissue cells are storage places for fat. In adipose tissue
or fat each cell is so filled with the fat globule that the nucleus and cytoplasm
are pushed into a thin rim around it (Fig. 7.7). Fat enters and leaves the
cell in soluble form. Fat cells border the blood vessels, often great masses
of them in the mesentery of the human abdomen constituting the so-called
fatty apron. Blubber, the fat of whales, has long been a valuable source of
oil; for the whale it is a great insulation against cold as well as a store of food.
All insects contain more or less fat, especially caterpillars and various pupae.
The weight of full-grown larvae of honeybees is 65 per cent fat, due to rich
diet and no exercise.
Supporting Tissues
Cartilage and bone are living tissues with cells that produce the substances
giving these tissues strength and rigidity.
Chap. 7
TISSUES
115
Fig. 7.7. Development of adipose (fatty) tissue in the larynx of a newborn kitten;
c, blood capillaries; /, nucleus of developing fibers (cells); s, signet fat cell. A, in
a region in which fat droplets (white spots) have appeared in only one cell; B,
another region in which fat droplets almost fill the cells crowding the cytoplasm
and nucleus against the cell membrane so that the shape is like a signet ring. Cells
containing large amounts of fat are found in connective tissue almost everywhere
throughout the body. (Courtesy, Nonidez and Windle: Textbook of Histology,
ed. 2. New York, McGraw-Hill Book Co., Inc., 1953.)
Cartilage. The intercellular substance of cartilage is firm and gumlike.
Normally it contains no lime but with age may gather deposits of it. Hyaline,
glassy cartilage or gristle, occurs in the higher vertebrates in many regions,
such as the ventral ends of the ribs, the joints, end of the nose, the rings of
the trachea (Fig. 7.8). The cells are surrounded by their semitransparent
secretion in which there are no blood vessels. Yellow elastic cartilage contains
a network of elastic fibers and is more flexible and elastic than the hyaline
type (Fig. 7.9). It constitutes much of the external ear of mammals, such
as man, bats, donkeys. White fibrous cartilage composes the intervertebral
discs which act as cushions between the vertebrae (Fig. 7.10). Those of the
human body are subject to various disarrangements especially in the lumbar
region where there is most pressure upon them.
Bone. This is a supporting tissue composed of bone cells surrounded by
organic material, collagenous (protein) fibers, and inorganic salts (Fig. 7.11).
116
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
Fig. 7.8. Hyaline cartilage from the head of the thigh bone (femur) of a puppy.
The cells (chondrocytes) secrete the glassy substance surrounding them and from
which they have shrunken away. Nourishment filters through the cartilage to the
cells. (Courtesy, Nonidez and Windle: Textbook of Histology, ed. 2. New York,
McGraw-Hill Book Co., Inc., 1953.)
Fig. 7.9. Yellow elastic cartilage from a pig's ear; groups of hyaline cartilage
cells are isolated by the hyaline substance which holds a meshwork of the elastic
fibers of connective tissue. The springback of the human ear when pulled is due
to these fibers. (Courtesy, Nonidez and Windle: Textbook of Histology, ed. 2.
New York, McGraw-Hill Book Co., Inc., 1953.)
Chap. 7 TISSUES 117
These salts are largely calcium phosphate and calcium carbonate. Two types
of structure are found in most bones, compact bone and latticed or spongy
Fig. 7.10. The intervetebral disk or cushion be-
tween the vertebrae mainly composed of white
fibrous cartilage. A human vertebra seen from
above with part of the intervertebral disk adhering
to it. The outer side of the vertebra is down; in
life the hole contains the nerve cord. 1, rings of
fibers arranged in layers; 2, a small central body
of cartilage (nucleus pulposus). (Courtesy, 2"^'"' •5^
Elements of Anatomy, ed. 11. New York, Long-
mans, Green & Co., 1915.)
bone. The Haversian system is the unit of bony structure (Fig. 7.12). Its odd
name comes from that of Clapton Havers, an English anatomist, who de-
scribed the system in the 17th century. The unit is an irregularly cylindrical
structure with a central or Haversian canal containing nerves and blood
Fig. 7.11. Bone cells in a thin section of human thigh bone (femur) with bone
cells and their processes highly magnified; the naturally colorless nuclei have been
deeply stained. The bone cell lies in a minute cavity (lacuna) with its living
processes extending into extremely fine canals (canaliculi) which branch out in all
directions through the intercellular substance often connecting with those of other
cells. Materials pass through these to and from the cells, ultimately to blood ves-
sels. (Courtesy, Nonidez and Windle: Textbook of Histology, ed. 2. New York,
McGraw-Hill Book Co., Inc., 1953.)
118
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
Fig. 7.12. Microscopic structure shown in a cross section of human dried com-
pact bone, one complete unit of bony structure (Haversian system) and parts of
others. In life the central or Haversian canal (black) contains nerves, blood and
lymph vessels; the lacunae, also black, contain the bone cells. All nourishment and
oxygen come to the bone cells by way of the canals. Layers of bone surround each
canal like successive coverings of a cylinder. (Courtesy, Nonidez and Windle:
Textbook of Histology, ed. 2. New York, McGraw-Hill Book Co., Inc., 1953.)
and lymph vessels. The blood vessels of the Haversian canals are connected
with those in the marrow or with larger vessels entering and leaving the bone.
Thus, when young, even compact bone proves to be a living tissue through
which body fluids can circulate. Respiration occurs in bone cells and conse-
quently metabolism does also, the latter at a lower rate than in other tissues.
In the finest structure of the bone around the Haversian canal the fibers
are wound spirally and are thus made stronger as the fibers of rope are
strengthened by twisting. In spongy bone the "lattices" are like bridges which
increase the strength against blows and breakage. The intercellular substance
of bone acts as a storage for calcium and phosphorus. There is continual inter-
change of calcium between the blood and bones which keeps the calcium
content of the blood constant. Insufficient calcium and phosphorus cause
rickets, a softening of the bones. In small children this may be a cause of
bow-legs.
Bone marrow is a soft cellular tissue in the central cavity of long bones
and the spaces of spongy bones. There are two closely related kinds, the
yellow and red. The yellow marrow that fills the central cavity of long bones
I
Chap. 7 TISSUES 119
is chiefly fat. Red marrow occurs mainly at the ends of long bones. It con-
tains fewer fat cells and is characterized by the development of red blood
cells and granular white ones. Great numbers of these are continually passing
into the blood and a comparable number of worn-out cells is withdrawn.
This is an instance of the regulated economy of the body which breaks down
comparatively seldom.
Blood and Lymph
Blood and lymph, its supplemental fluid, are tissues comparable to connec-
tive tissue and the skeletal tissues, bone and cartilage, to which they are
related. As here described, there are four types of connective tissue in each
of which the cells are surrounded by abundant intercellular substance. In
ordinary connective tissue the substance is gelatinous; in cartilage, it is tough
and jellylike; in bone, hard; in blood and lymph, a liquid in which the cells
float freely.
As far as its origin and related tissues are concerned, the discussion of
blood should be included at this point. Instead it is given in Chapter 12,
Blood and Circulation, and is thus placed with the vessels that carry it through
the body.
Muscular Tissue
Muscle cells are so elongated that they are commonly called muscle fibers;
thus, the terms muscle cell and muscle fiber are used interchangeably. A mus-
cle fiber is living matter; a connective-tissue fiber is not. Muscle fibers, that is,
muscle cells, contain fibrils (myofibrils) within their cytoplasm; the shorten-
ing of these is the contraction or muscular action. Muscle cells are usually
in bundles held together by connective tissue. Muscle has a high degree of
contractility. This fundamental character of protoplasm is evident in the
movements of an ameba and the action of its contractile vacuole, as well as
in the movements of all other animals. Contraction of protoplasm is accom-
panied by chemical and physical changes.
Chemical Composition of Muscle. About three-fourths of muscle is water.
Of the remainder about four-fifths is protein; the other one-fifth includes car-
bohydrates and fats, nitrogenous substances (urea, creatine), lactic acid,
pigments, enzymes, and inorganic salts. The most abundant protein is myosin
which makes up most of the contractile myofibrils. The carbohydrate is largely
glycogen, the ready-to-use food stored in many tissues. When a muscle has
been excited and fatigued its store of glycogen disappears and an equivalent
amount of lactic acid takes its place. When the oxygen supply is renewed
and after oxidation occurs the lactic acid is reduced and a proportional
amount of heat results. Muscles contain a red pigment, muscle hemoglobin
or myoglobin, which has an even greater affinity for oxygen than has the
120 THE INTERNAL ENVIRONMENT OF THE BODY Part III
hemoglobin of blood cells. It is abundant in the "red" muscle of birds and
mammals and the heart muscle of all vertebrates.
Types of Muscle. There are two main types: smooth, unstriated, or invol-
untary; and striated, skeletal or voluntary. Cardiac (heart) muscle, although
striated, is involuntary and contracts rhythmically.
Smooth Muscle Cells. These spindle-shaped cells occur in sheets held
together by connective tissue (Fig. 7.13). They include muscles in blood
vessels, in the urinary bladder, in the bronchial tubes of the lungs, in the
alimentary canal, and in other structures not under voluntary control. The
contraction of the iris of the eye in bright light is due to the contraction of
smooth muscle. The contraction of smooth muscle causes goose flesh, the
erection of hairs on the arms resulting from fear or cold, and the vivid lift
of hairs on a cat's tail.
Striated or Skeletal Muscle. This is the muscle attached to the skele-
ton, the voluntary type that comprises the bulk of muscle in the body. Most
of the meat that we eat is voluntary muscle, cut in slices, actually cross-
sections, taken at right angles to the length of the muscle cells (Fig. 7.14).
Striated muscle differs from the smooth type in the size and shape of its cells.
The most conspicuous microscopic structures are the alternating light and
dark crossbands of the cells. Striated muscle fibers are regarded as giant
multinucleated cells. Some very long ones have about 100 peripheral nuclei.
Each muscle cell contains a bundle of contractile fibrillae. In insects probably
all muscle is more or less striated. Striations are prominent in the flight
muscles of the honeybee when spread thinly on a slide in their fresh condition.
ii^^^^^;^:;^mimMi:mm
Fig. 7.13. Smooth muscle. A, fibers (cells) from a frog's bladder; B, cross sec-
tion of smooth muscle from the bladder of a kitten; the muscle cells are held
together by connective tissue; the section misses the nuclei of many cells; C,
branching smooth muscle cells in the aorta of a dog. x 900. (Courtesy, Nonidez
and Windle: Textbook of Histology, ed. 2. New York, McGraw-Hill Book Co.,
Inc., 1953.)
Chap. 7
TISSUES
121
Fig. 7.14. Skeletal or striated muscle cells. A and B, in long section; C and D,
in cross section. Note the nuclei with large nucleoli. The differences in appear-
ance are due to different methods of preparation, an example of what often
happens to preserved material. (Courtesy, Nonidez and Windle: Textbook of
Histology, ed. 2. New York, McGraw-Hill Book Co., Inc. 1953.)
Red and White Muscle. The cells of dark red muscle (dark meat)
contain an extra amount of muscle hemoglobin (myoglobin), and abundant
cytoplasm. This muscle also has a large blood supply and is usually active
for long periods of time. Pale muscle fibers (white meat) contain less cyto-
plasm, less myoglobin, and have a smaller blood supply. The color of muscle
also varies with the animal; in birds, red and white; in rabbits, red and white;
in nearly all human muscles, a mixture of both types.
Cardiac Muscle. In all vertebrates the heart is composed of a network
of striated muscle fibers. They are unique in being branched and having
centrally placed nuclei and intercalated, or literally, inserted discs, that is,
dark bands that cross the fibers at irregular intervals whose function is not
known (Fig. 7.15).
Nervous Tissue
The functioning of nervous tissue is due to two properties of protoplasm:
irritability, the power to react to various chemical and physical stimuli, and
122
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
Fig. 7.15. Cardiac muscle. A and B, ventricle of a monkey's heart; C, from a
human heart. /, intercalated disks, the cross bands that are characteristic of heart
muscle; p, granules of pigment; v, blood capillaries carrying rich supply of blood.
(Courtesy, Nonidez and Windle: Textbook of Histology, ed. 2. New York,
McGraw-Hill Book Co., Inc., 1953.)
conductivity, the ability to transmit the reactions from one place to another.
The nerve cell or neuron is the structural unit of the nervous system. Its
striking feature is the extension of the cell body into processes. These in-
clude two types: the relatively short dendrites through which the changes
known as nerve impulses move toward the cell body, and a single process,
the axon, through which nerve impulses move away from the cell body
(Fig. 7.16). In different parts of the nervous system the cell bodies vary
widely in size and shape but all of them have certain characteristics in com-
mon. They have prominent nuclei, no centrosomes, fine fibrils which become
visible in the cytoplasm with special stains, the neurofibrils, and irregularly
shaped bodies, the Nissl or tigroid bodies. The state of the Nissl substance is
a sensitive indicator of the condition of the nerve cell. It is depleted in infec-
tions such as poliomyelitis, in intoxications, and exhaustion, and is reformed
during recovery from illness or during sleep. In all but the simplest animals,
such as hydra, the nerve-cell bodies exist only in ganglia and in the gray
matter of the brain and spinal cord.
Chap. 7 TISSUES 123
Nervous tissue is mentioned here because it is one of the four main types
of tissues. Since nerve cells are peculiarly related and interdependent as a
whole system, the general discussion of them is given with The Nervous Sys-
tem, Chapter 16.
Fig. 7.16. Nerve cell from the cerebral cortex or gray
matter of a rabbit. The axon gives off numerous branches
and then enters the white substance, within which it
extends a long distance. Only a small part of the axon is
shown in the drawing, a, axon; b, white substance; c,
collateral branches of axon; d, descending or apical
dendrite; p, its terminal branches at the outer surface
of the brain (After Ramon y Cajal. Courtesy, Maximow
and Bloom: Textbook of Histology, ed. 6. Philadelphia,
W. B. Saunders Co., 1952.)
Important Reactions in Tissues
Inflammation. The defense reaction of living tissues to an unfavorable
condition such as an infection is evidenced by inflammation. Its general results
are redness, swelling, heat, and pain at or near the site of the injury. The
region becomes congested and swollen by an accumulation of body fluids
and their associated cells. There is increased activity of these cells; this and
the greater supply of blood produce a local heat rise. The congestion with
pressure on the nerve endings results in soreness and pain.
There is an efficient cellular defense against inflammation. Cells which
produce antibodies or antitoxins and may be phagocytic are scattered every-
where in loose connective tissue and in the blood and lymph. In the loose
connective tissue there are many capillaries from which increased numbers
of leucocytes migrate to the inflamed areas (Fig. 7.18). The neutrophils
move in first and act quickly; monocytes enlarge and, along with the now
124
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
••;*•
>••«
H?
Fig. 7.17. Drawing of a leucocyte (neutrophil) at half-minute intervals showing
its ameboid movement, and the intake of bacteria (black dots). The nucleus
(black) is many-shaped. (From Best and Taylor: The Living Body, ed. 3. Copy-
righted by Henry Holt and Co. Reprinted with their permission.)
active macrophages (connective tissue), attack and take in the poisonous
alien matter. The ability of these cells to adjust themselves to a different situa-
tion is characteristic of protoplasm and a keystone in the body's defense
against injury. As the inflammation decreases, healing begins. Scar tissue
forms with new connective cells and white collagenous fibers. Some of the
macrophages remain in resting condition among the new connective tissue
Tilpty ipUnten
Dilated, congested
capUlarle* rnake
^*su.'pface pedL,
escaping ploLSma.
capillaries and
- veriules cau.^e»
sv^ttUing.
Fig. 7.18. Diagrams to show how leucocytes (neutrophils) migrate from small
congested blood vessels to combat bacteria introduced into the tissues by an injury.
(Courtesy, Ham, Histology, ed. 2. Philadelphia, J, B. Lippincott Co., 1953.)
Chap. 7 TISSUES 125
cells. In the walls of adjoining blood vessels, cells (endothelial) multiply
and form branches which extend into the scar tissue, their presence account-
ing for the "red scar." By this time the surface of the scar is covered by
epithelium. Contraction of the white fibers reduces the capillaries and the
"white scar" results.
Bruises. Such bruises as a black eye are produced by blunt objects which
crush blood capillaries and other tissues. The capillaries bleed; the hemoglobin
of the accumulated blood breaks down, causes the black and blue and later
the greenish colors.
Fever. There may be a general response to injury in a fever involving the
whole body. It results in an increase of metabolic activity and a consequent
rise in temperature. High temperature is a dependable sign that something
unusual is going on in the cells of the body.
Hypertrophy. The enlargement or hypertrophy of a particular region or
organ may be due to enlargement, i.e., hypertrophy of individual cells and/or
increased number of cells, i.e., hyperplasia. If one kidney has been removed,
the other usually enlarges with more cells and does extra work.
Atrophy. This is a degenerative process in which cells diminish in size
and number. It is sometimes due to lack of blood or nervous control. A com-
mon example is the degeneration in leg muscles following the destruction of
parts of the nerve cord in infantile paralysis.
8
An Agent or Evolution—
T lie Body Covering
Skin is a meeting place, the frontier between an animal and its surround-
ings, a region of come and go, of shutting in and out.
The body coverings of animals are strikingly different: tenuously delicate
in a jellyfish, tough enough to stop bullets in a rhinoceros. They include such
contrasts as the ectoplasm of an ameba, the ciliated pellicle of paramecium,
the simple slimy skin of earthworms, the thin skin of birds, the leathery
skin of mammals. The multiplicity of structures that have developed from
skin is a record of its many functions that usually help and sometimes hinder
animals that live surrounded by shifting climates and shifty neighbors. Skin
glands secrete the shells of oysters, the chitinous exoskeletons of grasshop-
pers, the scales of butterflies, the slippery mucus of fishes and frogs, the
watery sweat of mammals, and the oil that waterproofs the feathers of birds.
Cellular outgrowths of skin form the claws of owls and tigers, horns of cattle,
beaks of birds and turtles, and hair — bent and crinkled in the wool of sheep
and straight on a monkey. Although less significant than the kidneys, the sweat
glands are also excretory organs. Sweat is similar to very dilute urine; in man
it contains about 99 per cent water, about 0.08 per cent urea and some other
salts. Skin is more or less resistant to disease and to the entrance of bacteria
and parasites. The mucus secreted from the skin glands of fishes and the
cornified layers in the skin of land animals are among its defenses.
Pigment is deposited in skin cells making patterns — the spots on leopard
frogs, the stripes of zebras, which disguise their owners against the back-
ground of their homes. Certain cells of the skin are sensitive to touch, others
to temperature, to chemicals, some of them to light. Animals, human and
nonhuman, learn much about their surroundings through their skins.
126
i
Chap. 8
AN AGENT OF EVOLUTION THE BODY COVERING
127
General Structure of Skin
Skin consists of one or more layers of cells which cover the outside of the
body and make a sheath over the delicate tissues beneath. Thus the outer
layer of protoplasm that covers unicellular protozoans is not related to skin
except in function. In all multicellular animals the outermost covering is a
layer of epithelial cells, the epidermis. This is the only layer present in the
invertebrates, except the starfishes and their near kin (Fig. 8.1). In the
vertebrates there is also an underlying connective tissue layer, the dermis,
sometimes called leather skin, because when properly prepared it is leather
(Fig. 8.2).
Epidermis. The epidermis is composed of several layers of epithelial cells.
The inner ones next to the dermis form a growing zone (malpighian layer)
where new cells are constantly being formed and pushed outward by the
pressure for space. As this occurs they are gradually flattened and outspread
(Fig. 8.2). In fishes and other moist-skinned animals even the outermost
cells stay alive for considerable time, but in land animals they become dry
and lifeless. Amphibians and reptiles molt the old epidermis in one piece;
birds lose their old feathers; and mammals continually shed little fragments
of skin. The constant flecking off of the human scalp in dandruff must be
familiar to everybody, in advertisements if not otherwise. Epidermal cells
become horny by deposits of the protein called keratin (horn). Keratin is
prominent in land dwelling vertebrates, in hair and feathers, horns of cattle,
footpads of dogs, and hoofs of horses. The "horny hands of toil" are actual
facts.
Many glands originate in the epidermis although they usually enlarge and
Fig. 8.1. A section of the epidermis and cuticle of an earthworm highly magni-
fied. It shows four mucous cells in different stages of secretion, all swollen with
the mucus which has pushed the nuclei to the bottom of the cells. It finally pours
out through microscopic pores, one at the end of each cell, and spreads over the
cuticle (cm). Mucus keeps the surface of the body moist, makes skin respiration
possible, lubricates the skin and lines the burrow in which the worm lives. (Cour-
tesy, Dahlgren and Kepner: Principles of Histology. New York, The Macmillan
Co., 1908.)
128
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
Opening of duct
of sweat gland
Corneal
layer
•Malpighian
layer
-Hair
follicle
Sebaceous
gland
Erector
- muscle
of hair
—Hair
papilla
Nerve
"> Sections of coiled tubules of sweat gland
Fig. 8.2. Section of human skin showing the two layers, epidermis and dermis,
characteristic of all vertebrates. The outermost corneal layer of the epidermis com-
posed of the horny remains of cells is gradually shed in small bits and replaced by
new cells from the growing (Malpighian) layer beneath. Cells of this layer contain
the pigment that is responsible for dark complexion. As shown in this figure a hair
is a shaft of cells that arises from a layer of epidermal cells that form a narrow
pocket in the dermis from the bottom of which a core of cells grows upward and
forms the hair shaft. Sensory cells, nerves, and the erector muscle provide for the
sensitivity and movement of the hair, and sebaceous glands for the oil. (Courtesy,
Gardiner: Principles of General Biology. New York. The Macmillan Co., 1952.)
Chap. 8 AN AGENT OF EVOLUTION THE BODY COVERING 129
push down into the dermis (Fig. 8.2). Their great variety includes the
stinging cells of hydra, wax glands of honeybees, the mucous glands whose
secretion earthworms leave, behind them in shiny trails, and the mucous
glands that make the slipperiness of fishes. More familiar are the oil glands
of hair and the sweat glands whose products have become the symbol of
human toil, the lacrimal or tear glands, and the mammary glands which pro-
duce food for all young mammals. The activity of these glands is deeply
associated with human experiences. The epidermis has earned a high place
in human history; Sir Winston Churchill gave it two-thirds of Blood, Sweat
and Tears,
Dennis. The dermis is the inner and thicker layer of the skin, the one
where the prick of a needle first hurts (Fig. 8.2). The bulk of it is composed
of the crisscrossing fibers of connective tissues familiar in leather. Dermis
is a nutrient layer containing lymph and blood capillaries and fat cells, the
latter often extremely abundant. There are many nerve endings in it; the
autonomic (involuntary) nerves control the contraction and dilatation of
the capillaries and consequent paling or flushing of the skin. The dermis is
the scene of blushing. Heat regulations also occur there; blood may be spread
out and cooled in the dilated surface capillaries or driven into the warm
deeper parts of the body when they are contracted. The colors of frogs and
other lower vertebrates are mainly due to pigment-bearing cells (chromato-
phores) in the dermis. Epidermal structures, glands, and feather and hair
follicles project into the dermis where dermal structures such as blood ves-
sels, nerves, and smooth muscle are associated with them (Fig. 8.2).
Skin Derivatives
Such notable developments from the skin layers as horns, claws, nails, and
hoofs should be added to the scales, feathers, and hair already mentioned.
Teeth have a history of close association with the skin and in certain sharks
there are rows of them just outside as well as inside the mouth cavity. The
plates of whalebone that hang from the upper jaw of toothless whales are
composed of cornified epidermal cells.
Epidermal Glands. The epidermis contains glands. Lobsters, grasshoppers,
and every other arthropod are completely clothed in the secretion of their
epidermal glands. Natural pearls are epidermal secretions as are shells of
the giant clam {Tridacna gigas) weighing 300 pounds or more, often used
as basins for holy water.
Scales. The scale of an insect, a butterfly, or moth is a minute plate of
cuticle secreted by one or more epidermal cells. It is solely a secretion and
does not contain any cells. The "hairs" and spines of other invertebrates are
similar. In contrast to these, the scales of bony fishes and other vertebrates
are composed of cells that originate from groups of skin cells.
130
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
Certain members of each class of vertebrates bear scales except the
amphibians, and in them scales are unknown. Most fishes and all reptiles
are more or less covered with scales; birds have them on their legs; many
mammals bear them on their tails — mice, rats, ground moles, opossums,
beavers; and armadillos have them on their bodies and tails (Fig. 8.3).
Fishes and reptiles are the typically scaly animals. In the yellow perch, sal-
mon, and other bony fishes, the scales grow out from pockets of connective
tissue in the dermis and overlap one another like shingles. Fishes do not
molt and scales keep growing and wearing off as long as the animals live.
The scales of reptiles are formed by the thickening and hardening of the
cornified epidermis. Those of turdes lie flat over the bony plates beneath;
those of snakes partly overlap one another. Turtles never shed their scales
but each one increases in size as the animal grows. The cornified scaly epi-
dermis of snakes and lizards forms a complete armor that is shed in early
summer. It is then that reptiles appear most sleek and burnished in their
new skins.
Feathers. These are slender upgrowths from the dermis. A feather carries
the epidermis with it and at its base sinks into a depression or pit in the skin.
Feathers are cellular structures but only near the level of the skin do they
remain alive as the feather grows. Nearly all of the feather consists of cornified
walls of microscopic air spaces that once were living cells. Thus each feather
is an extraordinarily complex horny air trap, an insulation, whose light weight
is only a part of its great efficiency. The habits and successes of birds are
peculiarly bound up with their feathers. (See also Chap. 36.)
Hair. The most striking development of mammalian skin is hair, an in-
sulation as characteristic of mammals as the feathers of birds. Among the
very few almost hairless mammals are the armadillo, the hippopotamus with
a few bristles around the snout, elephants, and whales that are covered with
hair before birth but afterward have only a few bristles about the lips.
A hair is a shaft of purely epidermal cells which projects outward obliquely
from its bulb-shaped root that extends down into the dermis (Fig. 8.2),
Below the surface of the skin a hair is a column of rapidly multiplying cells;
Fig. 8.3. Hairs and overlapping scales on the tail of a rat, section of it magnified.
Chap. 8 AN AGENT OF EVOLUTION THE BODY COVERING 131
the outer ones form a pit or follicle sunk in the dermis; the inner ones de-
velop into the homy shaft which extends out as the hair. A minute papilla
of dermal cells containing blood capillaries and nerve endings projects into
a cup in the root and furnishes nourishment in this spot where growth is
very rapid. Sebaceous glands feed oil onto the hair, sometimes in super-
abundance. An involuntary muscle extends from near the base of the hair to
the epidermis. When this muscle contracts it pulls on the base of the hair and
makes it "stand up." In thickly furred animals this increases the insulating
power of the coat. Standing hair on the back of a dog's neck is a warning; on
human skin it is only "goose flesh," and no indication of danger to others,
meaning only that its owner is scared or chilly. It is too sparse to create any
insulation from the cold and is a sign of kinship to furred animals rather than
a protection. Above the skin a hair is composed of the dead and horny re-
mains of cells (Fig. 8.4). Pigment, most commonly black, is distributed along
the rod in varying degrees of abundance, causing the different shades of brown
and black hair. When the papilla of the hair does not supply materials for
pigment, the hairs are gray or white. Air vesicles are frequent in white hair;
it is an air trap, in a feeble way, like a white feather. Hairs are also like
feathers in being shed at regular intervals. Human hairs are among the ex-
ceptions in being shed irregularly; healthy human hairs of the head are esti-
mated to live a few years, eyelashes only a few months. A curly hair is slightly
flattened and shorter on one side than the other like a shaving; a straight hair
is a perfect cylinder.
Claws, Nails, and Hoofs. These are all structures of cornified skin (epi-
dermis) (Fig. 8.5). Their development is similar to that of hairs; they are
•■ \ .
1 :
; i
i
j'
i ':
] I
■
i
J
-,»
f
!■ ■
1
^
Fig. 8.4. Left, diagram of a human hair showing the characteristic shape of the
cuticular scales (F), colorless in all animals unless the hair has been dyed. Scales
composed of dead or cornified epithelial cells are arranged like shingles with their
free margins always directed toward the end of the hair. The main thread of the
hair (medulla, C, and cortex, D) consists of compressed remains of cells, through
which pigment is distributed. A, fusi or air vesicles; B, pigment granules; £, cu-
ticle. Center, sections of hairs from the human head showing the distribution of
pigment granules in hair of different colors. The color or absence of color depends
upon the hair's content of pigment and air. Loss of pigment makes the hair look
gray; when it contains much air, it is silvery white. A, cream buff; B, befza brown;
C, black; D white. Right, hairs from various mammals have characteristic scales;
hair of a star-nosed mole, percheron horse, sheep, and other. (Courtesy, Hausman,
Sclent. Monthly 59:195-202, 1944.)
132
THE INTERNAL ENVIRONMENT OF THE BODY
Unguis
,Subunguis
Unguis (noil) /^^^^>>^^
A. CARNIVORAN CLAW
(Cat)
Pad-
Subunguis
B. HUMAN NAIL
Unguis
Pad
Unguis /Subunguis
C. HORSES HOOF
Part III
-Pad
Subunguis
Unguis
C^ — Pad
Subunguis
Unguis /Subunguis
Pod
Fig. 8.5. Diagrams of claws, nails and hoofs seen in section and from beneath.
All of these are modified scales, an unguis or scale above and a subunguis or
cushion below. Thus, the front of a horse's hoof is a modified nail essentially
similar to the claw of a lizard or a human fingernail. ( Redrawn after Walter and
Sayles: Biology of the Vertebrates, ed. 2. New York, The Macmillan Co., 1949.)
actually fused hairs. Lizards, turtles, and birds have claws as do many mam-
mals, but nails belong solely to a few mammals. A claw fits like a hood over a
terminal joint and beneath it is a pad of softer tissue. A nail is a thin horny
plate growing on the upper side of the end of a finger or toe. The human
fingernail is like a broad flattened claw on the upper surface of the fingertip.
None of these structures is molted but broken nails are regenerated. The hoof
of a horse is a claw which has become a greatly thickened sheath for the
toe-tip.
Horns and Antlers. The horns of cattle, sheep, goats, and Old World ante-
lopes are outgrowths of bone covered by thick layers of cornified epidermis
and, like claws and nails, are tough and resistant to chemicals. Horns are not
shed and are never branched.
The antlers of deer, reindeer, moose, and elk are annual growths of bone.
Deer shed their antlers when they are about two years old and every year after
that. At first the bony outgrowth is covered with hairy skin, later the skin is
resorbed and the spike of bone breaks off. In the second year the antler de-
velops in the same way, is shed, and in each following year the process is
repeated with new branches added (Fig. 8.6). Growing antlers are said to be
"in the velvet" because their skin is thickly covered with short hairs. They are
hot and feverish to the touch due to the large blood supply and the almost
explosive expenditure of heat in their rapid growth. Giraffes, which are close
relatives of the deer family, do not shed their stubby antlers, that remain in the
AN AGENT OF EVOLUTION THE BODY COVERING
133
Feb. 2
%^^
March 20
June 22
Fig. 8.6. Antlers of male mule deer. A, usual annual growth: Feb. 2, March 20,
June 22. B, structure and shedding; diagrams of sections. 1, growing prong in the
velvet, i.e., covered with hairy skin; 2 and 3, skin worn off and antler shed; 4, 5, 6,
regrowth and mature condition in which the bone is bare. Each successive breed-
ing season is marked by new antlers; to a certain limit older animals have more
prongs. (A, redrawn from Hamilton: American Mammals. New York, McGraw-
Hill Book Co., 1939. B, redrawn from Walter and Sayles: Biology of the Verte-
brates, ed. 3. New York, The Macmillan Co., 1949.)
velvet stage throughout life (Fig. 8.7). Antlers of deer, reindeer, moose, and
elk are not composed of horn at any time.
Functions o£ Skin
Skin is a protection from heat and cold: by pigment in cells (frog); by
coverings of feathers (birds) and hair (mammals), with few apparent excep-
tions— whale, armadillo, et al.; by erection of feathers and hairs securing
greater insulation from cold because of the increase of air space between them;
by fat associated with the deep layer (dermis) — the blubber of whales and
other marine animals.
The amount of water in the body is regulated by the control of its entrance
through the skin (frog), resistance to its passage through the skin by chitinous
coverings (many insects) and by cornified layers and fat (mammals), by
scales (fishes and reptiles), by feathers and hair, by oil or wax glands (in birds
especially water birds, cockroaches, certain beetles, bees, ants, and aphids).
Skin resists the entrance of parasites and diseases by special thickened areas,
134
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
Fig. 8.7. Masai giraffe. Although giraffes belong to the deer family they never
shed their stubby antlers which are knobs of bone permanently in the velvet.
(Courtesy, New York Zoological Society.)
e.g., soles of the feet, pads, hoofs (man, elephant, horse), by scales (fishes,
reptiles, feet of birds), by feathers and hair, by secretions (mucus in fishes,
frogs and toads, mild poisons of hydras, caterpillars of gypsy moths).
The skin is a receptor of stimuli through sensory cells and nerve endings,
sensitive to touch, heat, cold, and pain.
Skin takes part in the heat regulation of the body: in mammals through
control of surface blood vessels, through evaporation of sweat from the body
surface (man, horse), by coverings of the body, i.e., by hair, or feathers.
Vitamin D is produced through irradiation or the exposure to sunshine of
oils in skin and on feathers and hairs. In licking their fur mammals secure
irradiated oil containing vitamin D involved in the metabolism of calcium and
phosphorus.
Sweat glands located in the skin excrete products of metabolism, such as
water, small amounts of urea, and certain salts.
In certain invertebrates (earthworms, planarians, et al.) the respiratory
gases pass through the skin.
9
Protection, Support, and
Movement — Skeletons
Skeletons provide protection and support. The advantage of having a skele-
ton is made most vivid by the animal which does not have one. Jellyfishes drift
and in calm seas can even swim. But let them be thrown on a sandy beach and,
having neither protection nor support, they flatten against the sand and dry to
papery wisps. All vertebrate animals have skeletons and the character of their
existence is inseparable from skeletons. Imagine a spirited horse without bones!
In their relations to their environments and their achievements of speed,
strength, and grace animals are greatly dependent upon an outer or an inner
frame.
General Functions
The skeleton determines the form of an animal. Contrast the long leg bones
of an ostrich and the lack of them in a snake; or the seven long vertebrae in
the neck of a girafi'e and the seven short ones in the neck of a man.
Bones are the living tools of the muscles. Watch the fingers striking piano
keys, or the legs taking part in defense when a donkey kicks, and in offense
when a cat springs upon a mouse.
The skeleton's oldest and most general function is protection. The shell is
a complete armor around a lobster; the boxlike cranium encases the human
brain. The red marrow that produces the vital blood cells of vertebrates
throughout adult life is housed within bones.
Skeletons are old in animal history. Even in early times the yielding proto-
plasm of the smallest animals was doubtless protected by shells and rodlets of
hardened secretion as radiolarians are now (Fig. 9.1). Tons of fossil deposits
that have been dredged from the sea bottoms testify to the abundance of such
microscopic skeletons in primeval seas. Fossil animals of other groups show
135
136 THE INTERNAL ENVIRONMENT OF THE BODY Part III
that there were successive ages when skeletons were enormously large and
heavy. Those of reptiles commonly weighed many tons. Even modern alli-
gators have such heavy ones that they can scarcely lift their bodies from the
ground.
During their evolution vertebrate skeletons have changed from ponderous
burdens to light jointed bones, adapted to muscular control. Of all the land
vertebrates, birds have the lightest skeletons, for their tubular bones contain
air cavities connected with the lungs. The frigate bird, a famous flier, has a
wing expanse of seven feet and weighs two pounds, but its skeleton weighs
only four ounces, less than its feathers.
Types of Skeletons
Skeletons are either exoskeletons, on the outside of the body, or endoskele-
tons, within the body.
Exoskeletons of invertebrates are composed entirely of nonliving material,
the secretion of cells usually deposited in layers (Fig. 9.2). The majority are
light in weight, except the shells of mollusks that are often heavy. The muscles
are attached on the inner surfaces of the shells (Fig. 9.5, crayfish).
Endoskeletons are composed of living cells with their products, such as the
limy substance of bone. They are located between muscles and connective
tissues, and the muscles are attached to their outer surfaces. Such skeletons
are unique to the great group of chordates presently described.
Skeletons of Invertebrates
In the vast assemblage of invertebrates there is an unending variety of
skeletons that fit their owners to live in thousands of niches, in water, on land,
Fig. 9.1. Skeletons of representative radiolarians of crystal transparency, beauty
and precision of pattern. A vast area of the ocean bottom is covered with ooze
mainly composed of these skeletons that have dropped downward and accumulated
through the ages. (Courtesy, Kudo: Protozoology, ed. 3. Springfield, 111., C. C
Thomas, 1947.)
Chap. 9 PROTECTION, SUPPORT, AND MOVEMENT SKELETONS 137
or in the air. These skeletons are calcareous (limy), silicious (glassy), and
chitinous (horny), or are combinations of these. Those of aquatic animals
often have flotation devices, cavities that contain air or gas, fat, and oil
droplets. In the larger groups of multicellular invertebrates there are three
general types of skeletons.
Permanent Skeletons. Clams, snails, and other mollusks have but one skele-
ton throughout life enlarging it as their bodies grow. Although the molluscan
shell is not called a skeleton it has the requirements of one. In clams the oldest
part of the shell is the hinge region from which larger and larger concentric
ridges show where new secretion has been added (Fig. 9.2). The swiftest
mollusks are the squids whose skeletons are completely hidden by a fleshy
mantle.
An exoskeleton may be a network of minute units, or a mosaic of closely
fitted plates. As the animal grows, the units are enlarged or new ones added.
Clam
Starfish , ossicles (black)
Sponge, spicules
Lobster
SKELETONS OF INVERTEBRATES
Fig. 9.2. Skeletons of invertebrates. Permanent: clam with lines showing the
additions to the shell throughout life; cut across the arm of a starfish showing the
limy ossicles (shaded) embedded in the flesh of the body wall. Left lower: spicules
of fresh-water sponge that form a net-like support in the body wall. Temporary:
lobster whose skeleton is periodically replaced by a new one as long as the animal's
growth continues.
138 THE INTERNAL ENVIRONMENT OF THE BODY Part III
These skeletal units are strikingly different, white limy ossicles in starfishes,
glassy spicules in fantastic shapes and netted fibers in sponges.
Temporary Skeletons. Such skeletons are shed and replaced throughout the
growing period of the animal. The peak achievements in invertebrate skeletons
are the jointed ones of insects and other arthropods that are shed and replaced
by larger ones as their owners grow (Fig. 9.2). A new shell is formed before
the old one is shed and while the new cover is still soft and pliable it stretches
enough to allow for another interval of growth (Fig. 9.3). Most insect skele-
tons are delicately wrought; those of moths and butterflies are covered with
scales many of these lined with extraordinarily fine grooves. At the other
extreme is that of the male Hercules beetle of tropical America, nearly five
inches long, with heavy headgear that occupies a third the length of its body.
Aquatic species are larger than the related land forms; crabs and lobsters have
the heaviest skeletons of the arthropods. Yet when lobsters are submerged in
Fig. 9.3. Dorsal shells (carapace) of the same crab before and after molting.
A, hard shell that was recently shed; B, larger new shell that stretched and is still
soft. Crab, Loxorhynchus grandis, Pacific Coast. (Courtesy, MacGinitie and Mac-
Ginitie: Natural History of Marine Animals. New York, McGraw-Hill Book Co.,
Inc., 1949.)
their native sea water they are so buoyed up by it that the tips of their claws
touch the rocks as lightly as if they were engaged in a ballet.
Joints. Joints are the places where adjacent parts of a skeleton join, often
closely fitted together. In lobsters and other arthropods the outer covering or
exoskeleton is continuous over them, yet it is so thin and pliable that the
joint bends easily. Joints are highly developed in the skeletons of insects and
vertebrates, two dominant groups of animals. Those of invertebrates began
as creases in the epidermis and cuticle such as are so clearly visible in earth-
worms. As an insect breathes, its abdomen rhythmically lengthens and shortens
at the telescopic joints. When air enters the body, the plates of the skeleton
move apart, stretching the soft membrane between them (Fig. 9.4). Alter-
Chap. 9 PROTECTION, SUPPORT, AND MOVEMENT SKELETONS 139
nately, as the muscles of the abdomen contract and air leaves the body, the
plates are drawn together with the edge of one overlapping the one behind it.
Insects and other arthropods also have hinge joints. The leg of a lobster or an
insect bends like a jackknife.
Changing Content of Skeletons
The content of skeletons is in part changeable, in part permanent. Their
composition depends upon the material brought by the blood to the cells which
produce the more rigid substance. What is brought depends upon the materials
Fig. 9.4. Joints of the arthropod skeleton.
A, telescopic joints in the abdomen of an insect
when outstretched; pieces of skeleton held to-
gether by muscles and skin; B, insect's leg held
straight and flexed showing the stretching and
folding of the soft skin around the joints. (A, re-
drawn after Guyer: Animal Biology. New York,
Harper & Bros., 1936. B, redrawn after Ross:
A Textbook of Entomology. New York, John
Wiley & Sons, 1948.)
in the animal's environment and the physiological pattern that the animal
inherits.
Calcium, occurring in limestone, soil, and water, is continually passed in and
out of animals, but during its sojourn in an animal's body it is mainly located
in the skeleton. Striking exceptions are horny structures and the chitinous
skeletons of insects. In its usual state, 16 per cent of a crab's shell is calcium;
when it is "soft," such a shell is but one per cent calcium. This is the only
time when the shell stretches.
The skeletons of primitive vertebrates are more or less cartilaginous; those
of vertebrate embryos are at first composed of cartilage, later mainly replaced
by bone. Cartilage is composed of connective tissue cells which produce a
more or less resilient gel.
The connective tissue cells which produce bone form two different materials:
minerals, chiefly calcium and phosphorus, and collagen, a protein. The colla-
gen fibers are arranged spirally in the mineral matter, binding it like wires in
140 THE INTERNAL ENVIRONMENT OF THE BODY Part III
concrete. The combination of the materials makes bone hard and resistant to
strain. Bone can support a greater weight than granite without being crushed.
Despite its great firmness, it is moderately flexible especially in young animals.
The flexibility of the human skull at birth is well known; even in an adult the
skull can stand some compression before it cracks. Bone may be deprived of
either mineral matter or collagen and yet keep its shape. Soaking in dilute
hydrochloric acid will remove the minerals; burning will remove the animal
substance (mainly protein) (Fig. 9.10). The proportion of calcium to living
matter varies with age, with the amount of vitamin D in the diet, and other
factors. The body's calcium supply is regulated by the parathyroid glands
that are located on either side of the thyroid gland (Fig. 15.1). Calcium also
indirectly controls the coordinated activity of muscles by slowing down the
transmission of nerve impulses to them. When there is an excess of impulses,
the secretion of the parathyroids circulating in the blood extracts calcium
from the supply in the bones. This, in turn, circulated in the blood, slows the
activity of nerves and muscles. On the other hand, if the body becomes
sluggish, the parathyroid secretion is diminished and less calcium is called
forth from the bones. Again, the parathyroids may be too active and may rob
the bones of their calcium and produce abnormal formations. Sometimes this
is deposited as kidney stones.
Discoveries by Tracers. The behavior of calcium and phosphorus in the
tissue of living bone has been observed by means of their isotopes used as
tracer substances. The movements of radioactive calcium and phosphorus are
detected by a sensitive instrument (Geiger counter) placed on the outside of
the body (Chap. 2). Radioactive calcium has been demonstrated in the bones
of mice 24 hours after its injection into the veins. Radioactive phosphorus was
immediately deposited in the teeth, in the ends of bones, and in the ring of
healing (callus) in a bone which had been fractured. Radioactive phosphorus
in the form of a solution of sodium phosphate has also been given to human
patients either by mouth or by injection into the veins and its movement in
the body and its behavior in the bone followed by the Geiger counter. Such
explorations are more and more frequently made in the treatment of broken
and diseased bones.
Skeletons of Vertebrates and Their Ancestors
Notochord and Vertebral Column. Vertebrates are named from the chain of
bones which composes the vertebral column, the oldest part of the skeleton
and the support to which their development and dominance are supremely
indebted (Fig. 9.6). "Having backbone" has long come to mean having
strength and resolution. With a flexible, dorsal, median backbone, and the
bilaterally symmetrical appendages which developed later, the vertebrates
gained agility first in water and then on land. They moved about more.
Chap. 9 PROTECTION, SUPPORT, AND MOVEMENT SKELETONS 141
traveled in different ways and to different places, and made all manner of new
relationships.
Long before any of this occurred, the ancestors of vertebrates had an in-
ternal axial support, the notochord, on the dorsal side of the body below the
nerve cord and above the digestive tube (Fig. 9.5). Following their ancestors
of millions of years past, every individual vertebrate, including man, has a
complete notochord at some time during its embryonic life. In amphioxus the
notochord persists through life; in the vertebrates it is replaced by cartilaginous
or bony vertebrae. The presence of the notochord at some period of life in all
vertebrates as well as in their nearer ancestors is the reason for the name of
the phylum Chordata, the group to which they all belong. The more limited
subphylum Vertebrata includes only the chordates that have vertebrae, lam-
preys, fishes, amphibians, reptiles, birds, and mammals, including man.
The notochord is a slender rod of turgid vacuolated cells held together so
tightly within two sheaths that the whole structure is stiffened like a sausage
and the substance itself resembles condensed jelly (Fig. 9.5). In mammals,
it is soon replaced by bone and cartilage except possibly for a small part of
the cartilaginous cushion (intervertebral disc) that persists between the verte-
brae. In fishes, remains of it persist through adult life. The conical cavity at
each end of a vertebra, familiar to us especially in salmon and tuna fish, was
once filled with notochordal cells.
Vertebrae. A vertebra is a ring of cartilage, in sharks and other lower fishes,
or of bone surrounding the nerve cord in higher vertebrates (Fig. 9.6). The
Dorsal
Dorsal
Ventral
CRAYFISH
exoskeleton
(shell)
— muscle
nerve cord
endoskeleton
( notochord )
Ventral
AMPHIOXUS
Fig. 9.5. A characteristic and important difference. Cross sections of an inverte-
brate (crayfish) with exoskeleton and ventral nerve cord; and a chordate (amphi-
oxus) with endoskeleton and dorsal hollow nerve cord.
142
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
body or centrum occupies the space previously filled by notochordal cells and
is so shaped that it fits closely to its neighboring centra or to the intervertebral
discs. Dorsal to the centrum is the neural arch; fitted closely together the
neural arches form the bony canal in which the nerve cord is enclosed. Each
vertebra has particular areas, knobs, and edges, the attachment places of
ligaments and tendons of muscles that bind one vertebra to another, as well
as surfaces where the centra are pressed against the intervertebral discs. The
thoracic vertebrae have special hollows where the ribs articulate.
The joints between the vertebrae have only limited freedom of motion, yet
Fig, 9.6. A section through articulated
human vertebrae, showing one of the
intervertebral disks that separate the suc-
cessive vertebrae; / and 2, ends of circu-
lar fibers; 3, central cushion of cartilage
(nucleus pulposus). (Courtesy, Quain's
Elements of Anatomy, ed. 1 1. New York,
Longmans, Green & Co., 1915.)
the backbone, like the spring from a curtain roll, can be bent backward, for-
ward, or sideways and swung back into place (Fig. 9.7). A cat's back can
take a high curve in a split second, and that of a bucking bronco outdoes the
cat in curves; it lifts a cowboy and is just as fast. A snake coils and twists; a
kitten sleeps in a ball; an owl rotates its head until it looks directly behind
itself; and human acrobats are close competitors, yet the vertebrae stay in their
places.
Joints. In endoskeletons the muscles and ligaments are fastened to the outer
surfaces of the cartilages and bones. Some joints are immovable, such as those
in the cranium, little noticed except in very young infants in which they have
not grown together. Among the familiar types of movable joints are (Fig. 9.8) :
(1) hinge joints, such as those that are worked hard in typewriting; (2) ball-
and-socket such as the hip joint in which the head of the femur fits into the
pelvic girdle, a joint that is highly important in tap dancing, as well as in
walking and sitting and rising; (3) rotating joints in which the radius of the
human forearm shifts on its axis across the ulna as when the hand turns a door-
knob; and (4) pivotal joints that rock one upon another, such as the im-
portant "yes and no" joints, in action as the skull rocks upon the first vertebra
(atlas) when we nod "yes"; the atlas revolves upon the vertebra behind it
(axis) when we shake our heads "no."
In every typical free-moving joint the ends of the bones are held together
by sheets of tough connective tissue, the ligaments that enclose the joint in a
1
Chap. 9
PROTECTION, SUPPORT, AND MOVEMENT SKELETONS
143
/"X.X'^^"
Fig. 9.7. The flexibility of the vertebral column: in a walking salamander which
swings from side to side like a fish; in a fighting cat that arches its back as easily
as a bucking bronco. A human "backbone" bends forward, backward, and side-
wise.
capsule (Fig. 9.8). The end of each bone is capped with cartilage and folds
of thin synovial membrane project into the capsule of the joint from the sides.
This membrane secretes the synovial fluid, a lubricator that is transparent
and viscid like the white of egg. When the synovial membrane of the knee
becomes inflamed, its excess secretion often accumulates as "water on the
knee."
Long Bones. The humerus of the arm or femur of the leg may be taken as
an example of the general structure of long bones (Fig. 9.9). The cellular
structure of bone is described in Chapter 7.
The tubular plan of long bones makes them much stronger than rods of the
same size and weight. Two arrangements of their bony tissue, the compact
bone mostly surrounding the hollow shaft and the spongy (cancellous) bone
at the ends, create strength and lightness at the same time. Spongy bone is a
network of plates laid down in lines running in the directions which best meet
the stress that falls upon the particular part, such as the weight borne by the
head of the femur (Fig. 9.8). It contains spaces filled with red bone marrow
in which the red and some of the white blood cells are formed (Chap. 7). An
important layer of connective tissue, the periosteum, surrounds all bones. It
144
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
Finger bone
Pholonx
Bock of hand
Metocorpus
Fingers benf
Typewriting Clinging
lUMSAR ^iUt
lUNATE
A9T1CUU"
CARTILAC-
iOlNt CAVITY
FC'/fA
rttAP UG/v-WENT
'- ACeTAfiULAR
FAI PAD
KANj/iSSE UG
OF ACflABUtUM
(NOTE. ARTERY OF
HEAD IIO- MAY it
DERIVED FROM MED.
FEM CIRCUMFLEX)
Fig. 9.8. Two important types of joints in the human body. Top, hinge joints:
finger flexed as in striking typewriter, in clinging. Bottom left, ball and socket
joint: the hip joint in which the head of the femur fits into a cup in the pelvic
girdle. Bottom right, a section through the hip joint showing the capsule and the
ligaments holding the head of the femur in place. The ligament that binds the
head of the femur in place is the strongest in the body and rarely is torn even
when the joint is dislocated. The section of the femur shows the smooth, very hard
compact bone (whitish band) and outside it except at the joints the thin perio-
stracum (black line) layer which is the growing zone of the bone. The network
of bony tissue called spongy bone because of the many holes is well developed
at the ends of long bones and its lines of strength here suggest the braces of a sus-
pension bridge. It contains the red marrow in which red blood cells and granular
leucocytes (white blood cells) originate. In life the center of the bone is occupied
by the fatty marrow, here a black space. (Hip joint drawings courtesy, Ciba Clini-
cal Symposia, Vol. 5, No. 2, 1953.)
Chap. 9 PROTECTION, SUPPORT, AND MOVEMENT SKELETONS 145
receives abundant nourishment through a network of blood vessels and is the
region that provides for increase in diameter in growing animals.
Arteries enter and veins leave the bones in an oblique direction and are
/synovial fluid
-Articular cartilage
^Jblood vessel
yrOMPACT BONE
<MARROW CAVITY
'articular ugament
Fig. 9.9. Structure of a long bone. Periosteum is the growth area. (Courtesy,
Rand: Chordate Anatomy. Philadelphia, The Blakiston Company, 1950.)
connected by capillaries within them. The abundance of blood vessels in bone
emphasizes the fact that its cells are living, that metabolism goes on within
them as elsewhere, and that in them food and oxygen are expended, and heat,
energy, and waste are produced. Bone cells constantly take up organic and
inorganic substances from the blood and release such substances into it.
A. Normal bone
B. Soaked in HCL
(hydrochloric acid)
C. Burned until collagen
fibers removed
Fig. 9.10. A, normal bone. B, bone with calcium dissolved out after which it
can be bent and twisted. C, bone burned until the organic matter, cells and fibers,
are destroyed after which it is brittle. In a baby one year old the proportion of
calcium to animal matter is about as 1:8; at eighty years it is commonly about
8:11.
146
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
The intimacy between bones and muscles is evident in the ridges and
grooves on the surface of dry bones, for these are special attachment places of
tendons. The latter are also attached to the large smooth areas of the pelvic
bones and shoulder blades (Fig. 9.16).
Main Divisions of the Vertebrate Skeleton. All vertebrate skeletons consist
of two basic divisions: the axial skeleton, composed of skull and vertebral
column, and the appendicular skeleton, the shoulder and hip girdles and their
appendages (Fig. 9.15). Their parts correspond in relative position and
structure; they can be homologized more or less completely in all vertebrates.
The Vertebrate Plan
Early History. The lobe-fin fishes, probably ancestors of the land verte-
brates, must have tugged their bodies across oozing mud from one pool to
another, pulling with their front fins and pushing with the hind ones. Untold
generations later, their successors also pulled and pushed their bodies but by
limbs that bent at the joints and had small spreading bones at the ends that
got a foothold upon the earth (Fig. 9.11). After many more generations, the
limbs were held closer to the body and bent, the front ones backward and the
hind ones forward. In all the four-limbed vertebrates that have succeeded
them from early times into the present, the elbow, meeting place of the
humerus with the radius and ulna, has pointed backward, and the knee, the
B
^^'iS'csoo.iAjilcooocriaU,,]!!!. „iil\, ^ ^
'<r^ooo
■.L.
~S^,H~, II,. f..,,^
'n^
Fig. 9.11. Diagrams illustrating the evolution of the limbs of the ancestors of
land vertebrates. A, front view of a probable early stage when the limbs projected
side wise and the body rested on the ground, an era when land vertebrates tugged
their bodies out of the water and through the muddy ooze. B, the body is lifted
from the ground and the limbs are bent outward at the knee joints. C, side view,
hypothetical condition; hind leg rotated so that the knee points forward; front leg
rotated backward so that the elbow points backward. D, side view, condition in
modern quadrupeds in which the radius crosses over the ulna when the forearm
rotates forward. E, front view of stage shown in D. (After DeBeer. Courtesy,
Walter and Sayles: Biology of the Vertebrates, ed. 3. New York, The Macmillan
Co., 1949.)
Chap. 9 PROTECTION, SUPPORT, AND MOVEMENT SKELETONS 147
meeting joint of the femur with the tibia and fibula, has pointed forward
(Figs. 9.11, 9.15).
The Bridge. The plan of the vertebrate body is like the layout of a single-
span bridge. The piers of the bridge are the front and hind limbs attached to
their respective girdles and the arched span is the backbone. This metaphor
drawn by D'Arcy Thompson has been developed effectively by W. K. Gregory
in The Bridge that Walks with photographs of skeletons of fossil and present-
day vertebrates that illustrate the theme (Fig. 9.12). In its long history the
bridge plan of the vertebrate skeleton has admitted hundreds of variations
without departing from its unique character and basic simplicity. It persists
under many guises and ways of making a living, in burrowing ground moles,
swimming muskrats, and climbing squirrels, in elephants that are sure-footed
and ponderous, deer that are light and agile, cats that hunt their prey, and
cattle that forage on grass.
Paired Appendages and Locomotion. Paired appendages attached to carti-
laginous or bony girdles are typical of vertebrates. The basic pattern of these
structures underlies great modifications, especially in amphibians and birds.
In this pattern the pelvic or hip girdle is attached directly to the axial skeleton,
the pectoral or shoulder girdle indirectly by muscles. Each girdle is formed of
Fig. 9.12. Skeletons of a giant Percheron horse and a Shetland pony, the latter
in grazing position. Both show the bridge-like plan of the vertebrate body, the
front and hind limbs and their girdles taking the place of supporting piers, and
the backbone that of a connecting span. The neck has been compared to the arm
of a steam shovel; in the pony the steam shovel is in action. (Skeleton mounted
by S. H. Chubb. Photograph, courtesy, American Museum of Natural History.)
148 THE INTERNAL ENVIRONMENT OF THE BODY Part III
three bones and the front and hind limbs likewise have three main bones
(Fig. 9.13). The same number and arrangement of bones occur typically in
the forefoot (or hand) as in the hind foot. Both are correlated with thevir uses
in swimming, running, flying, climbing, and burrowing. The feet of horses have
undergone striking modifications for running. In their wild state horses have
grazed over wide ranges of grassland and escaped their enemies by speed.
Their bodies are held relatively high by long slender legs. Through their evo-
lutionary history their toes have been reduced to one, the third, on each foot
and in readiness for flight they stand upon their hoofs, the nails of these single
toes (Fig. 9.12, 9.14).
The Human Skeleton
The human skeleton has no bones which are not represented by similar ones
in skeletons of other mammals. Nevertheless it has certain entirely unique
features: a round head, a chin, a broad chest, a triply curved backbone, and
most important, a bowl-shaped pelvis and an opposable thumb that fronts the
fingers (Fig. 9.15).
Backbone. The 34 vertebrae of the human backbone are arranged like the
stones in a tower with the smallest cervicals (7) at the top, next the stronger
thoracic ones (12) jointed to the ribs, then the heavy lumbars (5). Beyond
this broad base of the tower are the fused pelvic vertebrae (5) forming the
sacrum which helps support the weight of the body and, finally, there is the
coccyx, the fused vertebrae (5) which are the remnants of the tail.
A baby is born with a nearly straight backbone which gradually assumes its
Clavicle
CorQcoid
Scapulo
Humerus
Femur
Puhis
Radius
UIno
Corpols — /o^^c
Metacorpals
Phalanges
Ischium
-- Fibula
Torsals —f^^^
Metatarsals — /
Phalanges /
Fig. 9.13. Diagrams showing the basic patterns of the girdles and appendages
of vertebrates and the similarity of arrangements in the fore and hind limbs. A,
forelimb and pectoral girdle; B, hindlimb and pelvic girdle.
Chap. 9
PROTECTION, SUPPORT, AND MOVEMENT SKELETONS
149
Fig. 9.14. Skeleton of the running horse, Sysonby, mounted after photographic
studies from life. The versatility of the skeleton: the pillars of the bridge working
as springs. (Skeleton mounted by S. H. Chubb. Photograph, courtesy, American
Museum of Natural History.)
peculiarly human shape of three slight curves, two outward and one inward.
In the thoracic and pelvic regions the outward curves create shallow bays
filled respectively by the lungs and abdominal organs. The latter are suspended
by mesenteries attached to the wall of the inward curve or small of the back.
Thus there is a strain upon this part of the back even though the abdominal
organs rest mainly upon the pelvic girdle. In the upright human body the
weight of the organs comes only indirectly upon the front wall of the ab-
domen. In quadrupeds, the abdominal organs are strung more evenly along
the back, rest directly on the ventral body wall and scarcely at all upon the
pelvic girdle except in those that sit — cats, kangaroos, and others.
Ribs. Articulated to the thoracic vertebrae are the 12 pairs of ribs, 13 pairs
in about 6 per cent of persons. These with the sternum or breastbone form a
protecting basket for the heart and lungs. In the evolution of vertebrates the
number of ribs has gradually decreased. There are many more in reptiles than
in birds and more in lower than in higher mammals.
Pectoral and Pelvic Girdles, Arms and Legs. The human pectoral or shoul-
der girdle and the arms are carried about as passengers, important and active
to be sure, but not burden-bearers like the hip girdle and legs. The human
arms are legs freed from the former activities of legs and now engaged in every
kind of business. The size, structure, and attachments of their bones allow for
freedom of movement but not support.
150
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
Neck vertebro-
Collar bone —
Jaw bone
Sternum
Shoulder blade
Humerus —
UIno—
Radius
Cartilage of ribs
— Radius
Fig. 9.15. The human skeleton. There are seven vertebrae in the human neck
as there are in the neck of a mouse, giraffe and every other mammal. There are
twelve vertebrae in the thorax, five in the small of the back (lumbar), five fused
in the sacrum hidden by the hipbone, and four rudimentary ones forming the
coccyx or tail. Comparison of right and left arms will show that (left) the radius
is twisted around the ulna when the hand is rotated. In a frog's forelegs these
bones are permanently crossed; in most mammals they are permanently straight.
Power to rotate the forearm has provided man, monkeys, and other primates with
facility in the use of their hands. (Courtesy, Etkin: College Biology. New York,
Thomas Y. Crowell & Co., 1950.)
The broad thin shoulder blade (scapula) is anchored by muscles but not
attached to the axial skeleton (Figs. 9.15, 9.16, 9.17), At the shoulder the
scapula is joined by the collarbone (clavicle) extending to the breastbone
(sternum). The head of the humerus of the upper arm fits into a relatively
shallow cavity forming a ball-and-socket joint in the scapula that allows the
free motion of throwing a ball. When an arm is lifted the pectoral girdle is
Chap. 9 PROTECTION, SUPPORT, AND MOVEMENT SKELETONS
151
Stflrnoclgidomostoid i
/
Tropezius — ^
Temporol
Massefer
Extensors ': jl
of
fingers
Gluteus
moximus,
Tendons
Flexor of hond
Rectus obdominus
External oblique
ist ligoment
Broad fascio of leg
> Quodnceps
Biceps
Fig. 9.16. Principal muscles of the human body. The names and uses of the
muscles are given in Table 1. (Courtesy, Etkin: College Biology. New York,
Thomas Y. Crowell & Co., 1950.)
lifted. Human clavicles stand out like slender bridges from shoulder to sternum,
easily broken and dislocated. The whole shoulder girdle is turned and shifted
in playing a piano, washing windows, driving a car. Clavicles are often re-
duced or lacking, as in cats and some other mammals that run and pounce,
in horses that run, and in deer that leap.
The radius and ulna of the forearm, chiefly the ulna, articulate with the
humerus at the elbow in a hinge joint. The upper end of the ulna is called the
funny bone or crazy bone, because of the sharp pain which occurs when it is
struck. This is due to the stimulation of the ulnar nerve which passes over a
knob or condyle on the end of the humerus. At their opposite ends the bones
o'' the forearm are jointed to the short wrist bones (carpals). Most of them
152 THE INTERNAL ENVIRONMENT OF THE BODY Part III
are hinge-jointed and bound about by ligaments. The capacity of the hands to
turn palms up and palms down and to twist a screw driver and turn a door-
knob is due to the position of the radius on the thumb side. When the hand
is held palm up, the radius and ulna are parallel; when it is turned palm down,
the radius is twisted across the ulna (Fig. 9.15). In many vertebrates, except-
ing the primates, the radius and ulna are permanently crossed as in the frog,
and the front foot cannot be rotated. When a cat is washing her face, her paw
makes beautiful curves but never turns palm up.
Five metacarpals form the middle bridge between the wrist and the fingers
(Fig. 9.15). The five phalanges, thumb and fingers, play the chief role in the
remarkable activities of the hand. The thumb turned palmside to the fingers
has taken great part in the development of art and science, actually in the
whole of history. The power of the human hand is in its ability to do a large
number of things moderately well, to scratch and dig in the soil, to write
letters, and do hundreds of other things. A ground mole can scratch and dig
in the soil with its front feet doing it extraordinarily well, but it cannot do
anything else with them.
The pelvic girdle supports the trunk and, with the femurs firmly attached
to it, takes the first impact of all the jolts of locomotion. It is a shallow bowl
and in man bears the weight of the abdominal organs to a degree that is
uniquely human. As in all other mammals, the pelvic ring of bones of the
human female is the birth passage of the young.
Each side of the pelvic girdle is composed of three fused bones (ilium,
pubis, and ischium). Where they meet a deep cavity receives the head of the
femur in a ball-and-socket joint (Fig. 9.8), the hip joint, the most deeply set
and strongly bound with ligaments of any joint in the body. As the shoulder
girdle and arms are constructed for pliability, so the pelvic girdle and legs are
built for strength. The neck of each femur is an arch that thins with age and
becomes very easily broken.
In the leg the distal end of the femur articulates with the tibia and fibula
at the knee, a critical joint which is protected by an extra bone, the kneecap
(patella) (Fig. 9.15). The tibia and fibula are comparable to the bones of
the forearm but are far more rigid. Their distal ends articulate with the ankle
bones (tarsals), one of which forms the heel. These bones are bound so
tightly by ligaments that they are not allowed much movement; on the inner
side of the foot they are lifted up, and with the metatarsal bones take part in
forming the arch or instep. Actually this is a double arch, one across the foot
and the other running the length of it. The common flatfooted condition comes
about when the ligaments lose firmness and allow the tarsals to separate and
the metatarsals to drop down. Thus the foot loses its natural spring and lift.
The activities of human toes are slight as compared with those of the
fingers. The first cause of their limitations is that the great toe cannot separate
Chap. 9 PROTECTION, SUPPORT, AND MOVEMENT SKELETONS 153
off from the other toes and face about with its sole side toward them. It cannot
act like a thumb. Compared with the importance of toes in other mammals,
that of the human toes is lessening.
Skull. The human skull is a group of bones (22) that forms the house of
the brain. It holds most of the sense organs, the gateways to the brain, and
the entrance way for food. The skull is divided into the cranium, holding the
brain and the face with the eyes, nose, and ears arranged around the mouth.
In man the cranium is large in proportion to the face; in a frog the cranium is
relatively small and the face large. The uniquely human features of the skull
are the rounded dome of the cranium and the chin (Fig. 9.15) ; the latter was
not well developed in primitive man nor is it now in infants.
The 22 bones of the adult skull include a number that are fused together.
In the newborn infant even the main immovable joints of the cranium have
not closed and there are six spaces or fontanelles where the hard matter of the
bone has not been formed. At birth the edges of these bones overlap as the
baby's head is squeezed through the pelvic girdle. The skeleton of the human
face is comparatively light in weight because it is so full of cavities. The promi-
nent openings of eye sockets, nostrils, and mouth occupy a goodly area and
there are also extensive cavities (sinuses) within certain bones (frontal,
ethmoid, sphenoid, maxillary), all of which open by small passages into the
nasal chambers. Painful inflammation of the lining of the sinuses commonly
originates with colds and congestion in the nasal chambers and spreads
through the passageways that open into them.
Teeth. Teeth are actually outgrowths of the integument or skin tissues and
their ancestry goes back to the scalelike structures which develop about the
mouths of sharks and other fishes. They are discussed with the intake of food
and mechanical digestion, their main functions (Chap. 11).
Broken Bones and Dislocated Joints
These are common disorders of the skeleton (Fig. 9.17). Breaks or frac-
tures are either simple, in which the skin is unbroken, or compound, if jagged,
broken ends of bone protrude outside the flesh. With any fracture nerves and
blood vessels are broken and there is pain and bleeding, the latter often within
the flesh. In treating a break the bones are first put back into normal position.
This is known as reduction. As a broken bone heals bone-forming cells, mostly
from the newly formed fibrocartilage in which bone regenerates, gradually
grow into the area surrounding the break. Limy salts characteristic of bone are
deposited in an enlargement, a callus, that is later resorbed.
Sprains are due to the wrenching or twisting of ligaments that bind bones
together at a joint. Severe ones may tear the ligaments and even the periosteum
of the bone, but even moderate ones disturb nerves and blood vessels.
154
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
Fig. 9.17. Two common fractures. Top, break of the collar bone (clavicle),
arrows showing the force of breakage. Bottom, section of a hand showing a break
in the "wrist"; actually the end of the radius causes a displacement of the wrist
and disturbance of the joint. (Courtesy, Blakistons New Gould Medical Diction-
ary. Philadelphia, The Blakiston Company, 1949.)
10
Movement— Muscles
Partnerships of the Muscular System. The business of muscles is to pull;
they cannot push. Voluntary muscles in the arms and legs pull from attach-
ments to the skeleton; others such as most involuntary ones pull from fibrous
attachments. They are specialists in contraction. Skeletons are the frameworks
for the hundreds of bodily movements that we see in rabbits or butterflies,
bird or man. The nervous system regulates and controls movement that the
muscles accomplish with the skeleton as their essential tool (Fig. 10.1). The
human brain is helpless to express itself without the contraction of muscles of
the face, the eyes, hands, stomach; looking cheerful is a muscular exercise,
looking cranky is another in which arms, legs, and face take part. Breathing
and the circulation of the blood are completely dependent upon muscular
action. When the thoracic muscles are paralyzed by poliomyelitis, breathing
cannot go on without an iron lung to take the part of their contraction.
Compared with other tissues of the body, the activity of muscle demands a
large amount of food, but it also liberates a great deal of energy and the major
part of bodily heat. And heat is an important catalyst in chemical action,
contributing greatly to the more rapid metabolism that is characteristic of
warm-blooded animals.
Muscle constitutes a third to one-half the bulk of vertebrate animals as well
as a goodly proportion of it in bees, lobsters, and many other invertebrates.
Wherever they occur, muscles and skeleton contribute form as well as function
to the body, the pillarlike legs of elephants, the supple foreshoulders of all
the cat tribe. The greatest theme of sculpture has been the form and relation-
ships and the power of muscles in such figures as the sitting greyhound, resting
lion, flying Mercury, as well as those of kings, soldiers, and prophets. Actors
on any stage turn the meaning of comedy or tragedy by tricks of their muscles.
Without muscles television would be indeed a bleak monotony. All this is not
to mention the muscular contractions that control the vocal cords whereby
155
156
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
i
Fig. 10.1. Kittens falling and preparing to land. The grace and flexibility of their
muscles are unsurpassed. The nervous system controls the movements that the
muscles accomplish with the skeleton as their tool. (Photograph by Ylla. Courtesy,
Rapho-Guillumette Studio, New York, published in Cats, London, Harvill Press.)
I
Chap. 10 MOVEMENT MUSCLES 157
the world is filled with cackle, bark, speech, and song. These and other char-
acteristics of muscles are matters of great social and economic importance.
Muscle is meat, almost the sole food of carnivorous animals and also of high
value to man and other omnivorous ones.
Kinds of Muscle
There are two main types of muscle, distinguished by their activity and
appearance under the microscope. Involuntary or smooth muscle, the older
one in the history of muscle, is generally distributed in the invertebrates except
arthropods, and occurs in the hollow organs of vertebrates such as the stomach,
intestines, and arteries. The pupil of the eye becomes smaller when involuntary
circular muscles contract and narrow the iris. Hairs stand up when their erector
muscles contract from cold and other causes (Fig. 10.2).
Voluntary, skeletal, or striated muscles are the bulkier ones. Those of the
body wall and arms and legs contribute largely to the form of the body (Figs.
9.16, 10.3).
Cardiac or heart muscle, often named as a third type, is intermediate in
structure to striated muscle and in activity to smooth muscle.
Dermis
B
Fig. 10.2. An involuntary muscle, the hair muscle. Diagrams of sections of skin
showing the follicle or root of a hair with the muscle attached to the skin and the
root sheath. A, hair naturally leans at an angle when the muscle is relaxed. The
region about the hair is supplied with nerves associated with the muscle. B. under
certain conditions, such as cold and nervous shock the nerves stimulate the hair
muscles to contract pulling the hairs up straight and the skin into little hillocks,
"goose flesh."
158
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
Voluntary Muscle
Voluntary muscles are here presented before the simpler involuntary ones
because they are the ones we most often see and eat. They are characteristic
of animals with endoskeletons, that is, the backboned animals and their im-
mediate ancestors.
General Structure and Arrangement. Each voluntary or skeletal muscle
consists of bundles of slender cells. Each bundle is held together by very deli-
cate connective tissue and the whole muscle is also sheathed by connective
tissue, the white strands visible in roast beef and ham (Fig. 7.5). The older
the animal, the thicker and tougher these are. Blood and lymph capillaries
and nerves run throughout the muscle actually in touch with the muscle cells
(Figs. 10.5, 10.6, 10.7).
Although the form of muscles differs with their functions, most of them
are spindle shaped and the ends are drawn out to their points of attachment,
the origin and insertion. The origin is usually on a firmly fixed part of the
body; that of the biceps muscle which bends the arm is on the shoulder. Its
insertion is on the radius of the forearm, the bone to be moved, and the at-
Clavicle (collar bone)
Tendon of biceps
(origin)
Biceps muscle
Triceps muscle
Fig. 10.3. Voluntary muscle. The biceps muscle takes the main part in lifting
and bending the arm; the triceps acts in lowering and straightening the arm. These
and other voluntary muscles work in pairs independently, e.g., as the biceps
contracts, the triceps relaxes. The nicety of nervous control which is essential for
such synchronous action occurs in many regions and at the same time. The
shoulder joint adapted for flexibility should be compared with the hip joint adapted
for support. (Redrawn from Haggard: The Science of Health and Disease. New
York, Harper & Bros., 1927.)
Chap. 10 MOVEMENT MUSCLES 159
tachment is by a tough and very flexible but inelastic tendon (Fig. 10.3).
Like many muscles, the biceps and triceps of the arm work in opposition. The
biceps muscle contracts and, as the arm bends the triceps is stretched. The
triceps contracts; the arm straightens, and the biceps is stretched.
The great advantage of tendons is in their strength, considering the small
space they occupy. The cords on the back of the hand, each attached to a
finger bone, are the tendons of muscles that straighten the fingers. All of these
Myofibrils (l-2>i)
Nuclei of Muscle Fiber
Muscle Fiber Crushed
1 K' ''^^^iZZ Showing
MV/Wl/'fa ^'^^' Membrane (l/»)
Connective Tissue Cells
(Perimysium)
Fig. 10.4. A group of skeletal or striated
muscle cells commonly called fibers. Some of
the fibers are cut oflf to show them in cross
section. A skeletal muscle fiber is actually a
sort of super-cell containing many nuclei and
other cell elements, a highly specialized struc-
ture. (Courtesy, Gerard: The Body Functions.
New York, John Wiley & Sons, 1941.)
^jAo.-,. E>-r'o
muscles are located in the forearm and depend on the tendons to communicate
their pull (Fig. 9.16); if the muscles were near the fingers the back of the
hand would be a bulging pillow. By a similar arrangement in the leg, the calf
muscle (gastrocnemius) lifts the heel by its tendon of Achilles (Fig. 9.16).
Picture the tendon of Achilles omitted and the calf muscles moved to their
immediate place of business at the heel!
Conditions of Muscular Activity. Muscles contain an enormous number of
blood and lymph capillaries, the former apparently in contact with every mus-
cle cell (Fig. 10.7). The glycogen stored in muscle cells is a readily oxidized,
quickly available food. An extra amount of blood flows into muscles as soon
160
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
Fig. 10.5. Ending of a motor nerve cell fiber on a striated muscle cell, highly
magnified. A change sweeps over the fiber of the nerve cell to the junction with
the muscle fiber and there it acts upon the muscle stimulating it to contract. When
a nerve cell acts upon muscle the ends of the nerve fiber produce a minute amount
of chemical substance which stimulates the muscle to contract. This substance is
a neurohumor, a chemical link between stimulation and activity. (Courtesy, Gen-
eral Biological Supply House, Inc., Chicago, 111.)
as they go into action and, as they work, their great demand for oxygen is
answered by deeper breathing.
Stimulation. Contraction is due to a rapid succession of stimuli coming into
the muscle fibers from nerve cells (Fig. 10.5). If many fibers are stimulated
the contraction is strong; if few are stimulated it is weak. The strength of the
stimulation whether of one or several muscles depends originally upon stimuli
received through the eyes, ears, nose, and other sense organs. Making a home
run means that strong sensory stimuli, the sight of the opposing players, and
applause of the spectators, have been translated into motor stimuli and have
put millions of muscle cells into action. A muscle cell is stimulated, contracts,
relaxes, and recovers. These steps are gone through with great rapidity and
can be analyzed only because living muscle can be isolated and subjected to
experiment and observation.
The contraction of muscle is completely dependent upon receiving messages
via certain nerves. Muscles also send forth messages via certain other nerves.
It is their ability to do this that makes it possible for us to know that our feet
are on the floor.
Fatigue of Muscle. A muscle acts for a considerable period until it is
Chap. 10 MOVEMENT MUSCLES 161
fatigued. Fatigue is a loss of contractility, apparently from accumulation of
the waste products of metabolism. Symptoms of muscle fatigue are easily pro-
duced. Hold your arm out straight; at first it is steady, then it trembles, and
finally you cannot prevent its sinking down in exhaustion.
Tonus. This is the continuous partial contraction of muscle cells arising
Fig. 10.6. Sensory nerve fibers with their end plates spread upon the surface of
a fiber of an eye muscle. The sensory end plates can be stimulated by conditions
within the muscle and changes sweep over the sensory nerve fibers as they do over
the motor ones (Fig. 10.5). Muscles are supplied with both kinds of nerves. A
muscle can receive a message and can also send one. (Courtesy, Maximow and
Bloom: Textbook of Histology, ed. 6. Philadelphia, W. B. Saunders Co., 1952.)
from muscle sense of position. Sense of position is closely associated with
environment and habit. An aviator may lose his "sense of right side up." A
cat's feet feel for the floor or the ground surface to which they are accustomed
(Fig. 10.1 ). Tonus of skeletal muscles of the legs and trunk occurs in sitting,
standing, and walking. In general tonus does not require as much energy as
ordinary contraction.
.>u.-
Fig. 10.7. Capillaries surrounding skeletal muscle fibers in a dog's tongue. A,
longitudinal section; B, cross section. The abundance and intimacy of capillaries
with the muscle cells reveal an elaborate provision for the exchange of oxygen
and carbon dioxide and a rich supply of food. (Courtesy, Nonidez and Wind!e:
Textbook of Histology, ed. 2. New York, McGraw-Hill Book Co., Inc., 1953.)
162 THE INTERNAL ENVIRONMENT OF THE BODY Part III
Tetanus. When a frog's muscle is stimulated by a single electric shock, it
contracts and relaxes again within a tenth of a second. Usually muscles do not
move so fast. This is because practically all contractions of voluntary muscles
are tetanic, the results of rapidly repeated stimuli, which maintain their con-
traction. When you carry a box of eggs your arm holds them steadily, not by
jerks. In human skeletal muscle tetanus results from stimuli entering the mus-
cle cells at the rate of 40 to 60 per second. They are so close together that the
resulting reactions blend into one. This shift from jerk to blend resembles that
of moving pictures, the separate pictures are shifted so rapidly that they look
continuous. Muscles may tremble and pictures will vibrate when the respective
movements are not sure and rapid.
Production of Heat. Muscles are the greatest living heat-producers. Jumping
rope increases the body's outgo of heat but thinking (except as it may involve
the muscles) does not, much as it may seem to do so. The heat liberated by
muscular action is an extremely important catalyzer which hastens chemical
reactions throughout the body. Even the fluffing out of feathers and fur pro-
vides extra heat because of the contraction of involuntary muscles in the skin.
Honeybees can raise the temperature of their hives a few degrees by the mus-
cular exercise of vibrating their wings.
Muscular Action. The energy for muscular action is freed by oxidation of
food. Muscle cells hold a store of food, principally the carbohydrates, glyco-
gen made from the glucose brought to them by the blood. How does the
chemical energy in the food become the energy of motion in a particular kind
of muscle? Although this is only partly known, a great many things have been
learned about the minute structure of muscle and the chemical and physical
changes which occur in it. A great deal has been discovered through observa-
tion and experiment on living muscle, commonly the calf muscle of the leg
(gastrocnemius) removed from freshly killed frogs. Organic compounds, such
as adenosine triphosphate, and glycogen, which muscle contains, are ready to
break down and liberate energy whenever conditions allow it.
It was long ago discovered that during contraction muscles change their
shape but not their size. In one of his excellent experiments the naturalist, Jan
Swammerdam (1637-1680), placed a muscle in a container of water attached
to a fine capillary tube in which the water line was visible. Then he watched
the line while the muscle contracted and wrote, "I must confess that the drop
of water sinks so little that I can scarcely observe it." A recent and significant
observation of muscular activity is that it may take place in the entire absence
of oxygen and without producing carbon dioxide. This means that contraction
is not the usual oxidative process, but has not proved that contraction of mus-
cle is independent of oxidation.
Chemical changes occur during muscular action and recovery. One unit of
any muscular action consists of a latent period following stimulation, a con-
Chap. 10 MOVEMENT MUSCLES 163
traction and relaxation phase and recovery, all together termed a muscle
twitch. This is the reply to any one of the stimuli which come into muscle cells
in rapid succession during muscular action.
Contraction is accompanied by the explosive breakdown of an unstable
organic compound, phosphocreatine, into phosphate and creatine. The separa-
tion of the creatine and phosphate liberates the energy taking part in the con-
traction, plus some energy in heat.
Relaxation is also associated with a series of chemical changes. Through
the action of an enzyme in the muscle, glycogen breaks down, ultimately into
lactic acid. This energy takes part in reuniting phosphate and creatine into
their previous state as the unstable organic compound, phosphocreatine. The
muscle is then ready for another breakdown at the next contraction. The fore-
going series of changes does not require oxygen and constitutes the nonoxida-
tive or anaerobic phase of muscle action.
Anaerobic respiration of mammalian muscle is a chain of chemical reac-
tions during which the muscle uses glucose, which it derives from its store of
glycogen. These anaerobic reactions release the energy used by the muscle in
doing work. The process results in the by-product of lactic acid. Part of this is
eliminated by oxidation and the energy thus released rebuilds the remainder
of it into glucose and glycogen. The muscle is then ready to do more work. It
loses some of its store of glycogen with each contraction because the lactic
acid that is burned in oxidation turns into carbon dioxide and water which are
eliminated. Strenuous exercise may run up a debt of several quarts of oxygen.
Lactic acid accumulates, diffuses into the blood, and makes it acid and this
acidity is a demand for oxygen. Its increase in the blood, modified by the
buffering salts, stimulates the respiratory center of the brain which sends out
impulses that lead to vigorous breathing. Forced breathing continues until
enough oxygen has accumulated to burn the lactic acid and reinstate the glu-
cose. The blood is no longer unusually acid and ceases to stimulate forced
breathing.
Recovery occurs following the changes in the relaxation phase of the mus-
cle. One-fifth of the lactic acid previously produced in the anaerobic phase is
now oxidized, and water, carbon dioxide, and energy are released. Of the
energy thus freed part is heat and part becomes active in the resynthesis of the
remaining four-fifths of the lactic acid in glycogen. These changes constitute
the oxidative, the recovery or aerobic phase of the muscle action.
At first, it may seem as if there would be an advantage if oxygen came into
the chemical changes earlier. Muscular action however actually starts more
quickly because it does not. The blood is constantly bringing oxygen to the
muscles, but they collect no supply above their momentary use. There is no
extra oxygen to spend, on a sudden action like snatching away one's hand
when it touches a nettle or a hot iron. Although no supply of oxygen is ready.
164 THE INTERNAL ENVIRONMENT OF THE BODY Part III
there is a reserve of an organic compound (phosphocreatine) ready to break
down explosively and liberate energy at the instant the nervous impulses affect
the muscle.
These chemical changes are a part of the intricate workings of muscle. They
and others are going on in every animal motion that we see, the quick whirring
of the hummingbird's wings, or the movements of the bagpiper who at the
same time marches, blows into the bag, and fingers the keys for a Highland
fling.
Involuntary Muscle
Smooth muscles contract and relax slowly, skeletal ones rapidly; these
processes take several seconds in the former, less than one second in the latter.
Smooth muscles may hold a certain degree of contraction for a long time with-
out apparent fatigue and with great economy of energy. Smooth muscle cells
are spindle-shaped, each with a single nucleus and minute contractile fibrils
running lengthwise in the cell. None of them is cross-striped, hence the name
smooth muscle.
They are never attached to bone and rarely have tendons (Fig. 10.8). In
Fig. 10.8. Integumental or skin muscles
of a horse, by means of which the skin may
be "shuddered" and flies dislodged especially
on the neck and shoulders. Such muscles are
practically absent on the flanks. (Redrawn
from Walter and Sayles: Biology of the
Vertebrates, ed. 3. New York, The Macmil-
lanCo., 1949.)
the vertebrate body they occur mainly in the hollow organs of the body cavity,
the stomach, intestines, the urinary bladder, the uterus, also in the blood
vessels and the air passages of the lungs. In arteries the individual cells are
curved in circular layers around the tube; in the intestine they form circular
and also longitudinal layers. By the contraction and relaxation of circular
layers the intestine executes its peristaltic waves of contraction and relaxation,
bulges out in some places, squeezes in at others, shortens and lengthens much
as an earthworm does with the rhythmic deliberations characteristic of smooth
muscle.
In their control of skeletal muscle, nerve cells act through the long exten-
sions of the cell body; in smooth muscle whole autonomic nerve cells may be
present among the fibers. In addition to their stimulation by nerves, muscle
cells are also stimulated directly by movements of one another as waves of
contraction pass over them.
Smooth muscles are never bulky and conspicuous but their functions are
Chap. 10 MOVEMENT MUSCLES 165
dramatically important. Those of the uterus are responsible for birth. They
hold blood in the vessels at a regulated capacity, thus largely maintaining blood
pressure and the circulation of blood. Attacks of asthma are spasmodic con-
tractions of smooth muscles that under normal nervous control regulate the
amount of air in the bronchioles. Less serious but vivid in experience are the
contractions in the walls of the stomach that cause hunger pains.
Muscles of Some Familiar Invertebrates
Smooth muscles are located in the viscera and the body wall of many inver-
tebrates. Clams, mussels, and oysters can hold their shells closed for long
periods, some of them for days at a time. The shells of all bivalves are hinged,
and in the hinge is an elastic band which continually resists the closing of the
shells. This resistance is met by the tonic contraction of adductor muscles at-
tached at either end to the inner surface of the shells. The large adductor mus-
cles of the scallop (Pecten) are familiar as fried scallops. Experimental stimu-
lation of these muscles indicates that they contain certain rapidly contracting
muscle cells along with a majority of slowly contracting ones. This combina-
tion is ideal for the lively habits of scallops which, by clapping their shells
together and rapidly expelling the water between them, are able to skip out
for short distances through the water by a kind of jet propulsion. Involuntary
muscles with a very different function take part in the "blushing" of the squid.
When these handsome relatives of the devilfishes are excited, glimmering
flashes of pink and red shift over their bodies due to the movements of pig-
ment (in chromatophores) controlled by muscles.
The movements of the common earthworm are an easily observed example
of peristalsis, i.e., successive waves of contraction of the rings of smooth mus-
cle in the body wall. Close to these, layers of longitudinal muscles extend the
length of the worm. When the long ones contract, the fluid-filled body of the
worm shortens and bulges; when the circular muscles contract, they squeeze
the body to slenderness and drive the fluids forward and backward forcing it
to elongate.
Insects have the most complex muscular systems and most clearly striated
muscle of all invertebrates. The number of distinct muscles is very large, vary-
ing in different insects, but there are often over 2,000. In a dissection, muscle
is one of the most conspicuous tissues of the insect body. It is either colorless
and transparent, or yellowish white, often soft, almost gelatinous, notwith-
standing its efficiency.
Patterns of Vertebrate Locomotion
No other animals take such long journeys by sea and land as the vertebrates;
eels swimming down streams and half across the Atlantic; birds flying from
Alaska to the Argentine; and human populations moving to distant lands. All
166
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
these great travelers are aided by their bilateral symmetry, their light internal
skeletons, and their muscles.
In fishes the muscles of the body wall are usually divided into segments
(myomeres). They carry on the main work of locomotion; fins do not do the
heavy work. Fishes move by a sidewise undulation, a wavy motion with mus-
cles contracting first along one side of the body, then along the other (Fig.
10.9). They push their bodies with their tails giving the main drive. Spotted
Fig. 10.9. A fish swims by undulating movements and
pushes its body forward by pressing against the water,
successive waves of curvature traveling backward along
the trunk and tail. (Redrawn from Romer: The Verte-
brate Body. Philadelphia, W. B. Saunders Co., 1949.)
newts (Triturus) spend one period of their lives in water and another on land,
but they move with the wavy swing of the fishes all their lives (Fig. 34.2).
Land vertebrates have a different problem; they cannot push against the air
with their bodies for air is too yielding; they must push the ground or its
counterpart. Their movement on four legs from one place to another is the
great achievement of their voluntary muscles and partner bones and nerves
(Fig. 10.10). Man's movement on two legs is still more difficult and more
significant in that it has left the muscles of his hands and arms free to use
tools. Standing on two legs is a continued balance which requires that a large
number of muscles be kept in sustained contraction in reply to impulses that
recur because of the stimulation of the sensory receptors of position (proprio-
ceptors). In contrast to the healthy resilience of upright posture is the com-
plete limpness of muscles that follows poliomyelitis. The muscles are still nor-
mal but the motor nerve cell bodies in the spinal cord have been attacked by
the virus. Walking on two legs involves holding the body and head upright and
shifting the entire weight to the hind limbs, thus freeing the front ones. With
the start of walking the body falls forward, then one leg, say the left one, is
Chap. 10 MOVEMENT MUSCLES 167
flexed and thrust forward to catch the falling body. At the same time the calf
muscle of the right leg contracts and lifts the heel. The left foot is being placed
on the ground and for an instant both feet are on the ground. The weight of
the body is now shifted to the left leg and the right one is swung forward into
/4
Fig. 10.10. The same pattern of movement of the arms and legs of a man, and the
legs of a cat, an ancient inescapable habit.
a new position in front of the left. When the right foot is planted, the weight
of the body is shifted to that leg, and the pull of the muscles now lifts the left
heel. The left leg is then swung forward again in front of the right one. Thus
walking is like the movements of a pendulum repeated several thousand times
per day.
Infants begin their travels on all fours as quadrupeds continue to do through
life. Brisk walkers swing their arms and when they do so the right arm and the
left leg go forward at the same time in exactly the same pattern as in a walking
cat and with the same muscles operating (Fig. 10.10). We cannot walk in any
other way; neither can the cat. Inherited pattern of the movement of muscle is
as inescapable as the inheritance of its structure.
11
Foods ana Nutrition
Nature of Nutrition
Nutrition is a remarkable process by which the protoplasm of a cabbage
becomes rabbit, that of a fish becomes cat, and the proteins of lamb are trans-
formed into proteins of man. The processes of nutrition include: the physical
and chemical breakup of foods called digestion; the absorption by cells of the
foods simplified by digestion; and assimilation, by which the basic units of pro-
tein are interwoven into the particular pattern of proteins of the animal nour-
ished, and the simplified carbohydrates and fats stored to be available for
energy. All cells of the body and the chromosomes within them are nourished
in this way. Human chromosomes doubtless contain substances that originated
in beans and cattle, but they have lost their original characteristics and by
assimilation have become the protein peculiar to the chromosomes of man.
Nature of Foods
Foods are the substances that are taken into the body and used in its metab-
olism, in building protoplasm for growth and repair, and in liberating energy
to do work. Work includes all activity such as movement, responses of the
sense organs, and secretion of glands. Animal food consists of plants and ani-
mals and their products, such as sugar and milk. The essential substances are
proteins, carbohydrates, fats, vitamins, and very small amounts of certain
minerals (Fig. 11.1). Water, necessary for all organisms, is essential in the
process of nutrition.
Proteins. Since protein is constantly being broken down in the body, more
of it must be furnished for repair as well as for growth. When there is no pro-
tein in the food, the body burns its own protein. This happens in starvation.
Sixteen per cent of protein is nitrogen. The body must be kept in a nitrogen
balance, that is, as much nitrogen should be taken in as is excreted, and some-
times more, as during pregnancy, during growth, and after injury or illness.
Proteins are abundant in meat (muscle), cheese, eggs, peas, and beans. Their
168
Chap. 11
FOODS AND NUTRITION
169
Non-food I I
Foodstuff
Foodsfuff
FotQ
Carbohydrate
Protein
Mineral
Minerals
Calcium [
Phoiphorut ^^
Iron
Fig. 11.1. Fats, carbohydrates, proteins, and minerals are contained in most
food but in different proportions. (Reprinted from Food for Life edited by R. W.
Gerard, by permission of The University of Chicago Press. Copyright 1952.)
basic elements are carbon, hydrogen, oxygen, nitrogen, sulfur, and phos-
phorus. The protein molecule is made up of amino acids of which at least ten
are essential to life. The simplest one known is glycine (H5C2O2N). All
growth and repair of the body is dependent upon proteins, but they must be
thoroughly digested into amino acids before they can be used. Fish protein
does not repair the tissues of cats or increase the growth of kittens until it is
thoroughly disorganized from its previous character. The body stores no pro-
tein. But after the removal of its amino group (NH^) the remainder of an
amino acid may be converted into glucose and used as food or changed to
glycogen and stored in cells as starch is stored in a potato.
Carbohydrates. The familiar carbohydrates are starches and sugars. They
are made up of the elements carbon, hydrogen, and oxygen, with the hydro-
170 THE INTERNAL ENVIRONMENT OF THE BODY Part III
gen and oxygen usually in the same proportion to oxygen as in water (H^O).
Carbohydrates furnish a large share of the energy required for the regular
needs of living; they provide the energy for such routine processes as respira-
tion, circulation, digestion, and excretion — the metabolism of the body. Fats
also provide energy but they are chemically less quickly accessible for use.
During digestion carbohydrates are broken down into glucose, a simple sugar
(CeHi-Oo) which is distributed in the blood to the liver, muscles, and other
tissues throughout the body. It is converted into glycogen, commonly called
animal starch. This is readily reconverted to glucose for immediate use any-
where in the body. Human blood usually contains about 0.1 per cent glucose
ready for instant use.
Fats. Fat accumulates as pure fat, not mixed with water like protein and
carbohydrate, and when oxidized has a high heat output. Fat is a long-range
supply not ready for quick use like glycogen. Thus, the fat boy gets just as
hungry as the thin one.
Fats (or lipids) are simplified by hydrolysis, that is, by chemically splitting
up and taking in water, into glycerol (or glycerin) and fatty acids. The true
fats, liquid and solid, are combinations of glycerin and fatty acids; oleic acid
in butter (CisH.s40o) is an example. They all contain carbon and hydrogen,
with less oxygen than carbohydrates. All fats are greasy and are soluble in
organic liquids such as ether or benzene, rarely in water. Certain of them, such
as cod-liver oil, are liquid in ordinary temperatures; others, such as lard and
tallow, are solid. The wax produced in the human ear and beeswax are sub-
stances very like the fats. The sterols are complex waxlike compounds of a
different chemical nature. Cholesterol in the bile and calciferol (vitamin Do)
are such sterols; the male and female sex hormones and certain cancer-pro-
ducing compounds also belong to the steroid group. Compound fats such as
lecithin contain nitrogen and phosphorus in addition to the elements regularly
contained in fats. Lecithin occurs in almost all living cells; it is a major item
in the yolk of eggs.
Vitamins. Vitamins are compounds that are present in foods in small quan-
tities. They play an important part in human nutrition and probably in that of
all plants and animals. Vitamins are highly specific; for example, vitamin A
affects the cornea of the eye; others affect the hardening of bone (Fig. 1 1.3).
Some are soluble in fats, others in water; certain ones are destroyed by heat,
others are not. Human diet is apt to be deficient, especially in vitamin A, folic
acid, riboflavin, ascorbic acid, calciferol, and thiamine (Table 1 1.1). In nature
most of the vitamins are produced by plants. They are abundant in grasses,
and cats frequently bite off grass blades, apparently satisfying some kind of
hunger. Cats and other carnivores secure vitamins as they lick their fur and by
eating the fur and feathers of their prey. The old name "limey" for a British
sailor is indirectly connected with vitamins. In the days of sailing ships and
Table 11.1
A List of Important Vitamins and Their Characteristics
Selected from the 40 or more known vitamins or vitaminlike substances. Investigations
of vitamins are still in progress and new discoveries and revisions are constantly being
made.
Name
Important Sources
Physiological
Functions
Chief Results
of Deficiency
A Group
Plant form (carotene) in
Maintain health of
Dry cornea of eye
(fat soluble)
green leaves, carrots,
mucous membranes
(xerophthalmia), no
tomatoes; animal
and other epithelial
tear secretion
form in liver, milk,
tissues
Night blindness
egg yolk; both forms
Needed to regenerate
in eggs, milk and
visual purple in
butter
retina of eye
B Group
Whole grains of wheat.
Needed for carbohy-
Beriberi, a disease of
(water
rice, other cereals.
drate metabolism
the nervous system;
soluble)
beans, peas, green
Stimulates root growth
polyneuritis a nerv-
Thiamine
vegetables, egg yolk,
in plants
ous disability in
and lean meat
birds, stops growth
Made synthetically
Riboflavin
Green leaves, fruit,
Essential for growth;
Nervous disorders.
milk, eggs, liver
concerned with
stunted growth in
body's use of food
cattle and poultry;
scaly skin
Nicotinic acid
Green leaves, wheat
Essential to normal
Pellagra, a severe nerv-
or niacin
germ, lean meat, eggs.
functions of cells
ous disease in man
milk, yeast. Made
and monkeys
synthetically
Folic acid
Green vegetables, eggs.
Essential for growth
Anemia in man. Slow
yeast, liver
and formation of
growth and anemia
blood cells
in chicks and rats
Bl2
Egg yolk, fermentations
Essential for blood cell
Pernicious anemia.
of Streptomyces
formation by bone
caused by a change
(source similar to
marrow
in gastric secretion
that of penicillin).
so that B,2 is not
milk, fish, liver, meat
absorbed from the
digestive tract
C or ascorbic
Citrus fruits, tomatoes;
Maintains the health
Scurvy, bleeding in
acid
oil of fish livers
of capillary walls
mucous membranes.
Can be made
under skin, and into
synthetically
joints
D or anti-
Fish liver oils; exposure
Regulates m.etabolism
Rickets in young.
rachitic
of skin to ultraviolet
of calcium and
bones and teeth soft
radiation
phosphorus; needed
and often deformed;
for normal growth
severe bowlegs
and mineral content
of bones
E or anti-
Green leaves, wheat
Essential to rapid cell
Sterility in poultry and
sterility
germ, and cottonseed
division and growth
rats, death of em-
oils
in embryo
bryos
K or antihem-
Green leaves, spinach.
Essential to produc-
Bleeding
orrhagic
cabbage, also in
tion of prothrombin
certain bacteria of the
in liver, necessary
intestinal flora
for clotting of blood
171
172 THE INTERNAL ENVIRONMENT OF THE BODY Part Til
voyages that took a year or more, the great dread of sailors was scurvy, a dis-
ease caused by the long steady diet of dried and salted foods lacking in vita-
mins. When it was discovered that eating limes would prevent scurvy, no ship
went to sea without them. Sailors ate limes and unknowingly treated them-
selves to vitamin C (Table 11.1).
Vitamin research really began when it was discovered that animals needed
vitamins and could be used as subjects in experimentation with deficiency dis-
eases. The first clearcut results (Eijkman, 1893) were obtained upon chickens.
When they were fed on polished rice, the chickens developed a disease similar
to beriberi, common among human rice-eating populations (Fig. 11.2). As
soon as they were fed the previously cast off rice polishings they recovered
from the disease. As often happens, the wide significance of these results was
not recognized until some time later. By 1915, however, it was fully realized
that in addition to the regular foods, more than one vitamin was essential for
health. The discovery of vitamin A came about through attempts (1913-
1915) to use pure fats in the diets of experimental animals. It was observed
that for no apparent reason butterfat was far superior to other fats, such
as lard. When young rats were fed diets containing only lard, they were
stunted and had a scaly, infected condition of the eyes known as xeroph-
thalmia (Fig. 11.3). In contrast to this, when butter was substituted for lard
in the diet, the rats grew and remained healthy. Oleomargarine made from
vegetable oils has a food value identical with that of butter now that sufficient
vitamin A is added.
Minerals. Minerals required by the body are usually obtained with the food
or drinking water. Several such substances are essential to plants and animals,
but in minute quantities. These are called micronutrients and trace elements,
the latter not to be confused with radioactive tracer substances. Experimental
diets given to animals have revealed most that is known about the use of
micronutrients.
Types of Nutrition
There are three principal types of diet: herbivorous, carnivorous, and om-
nivorous. Herbivorous animals feed on vegetation. They include grazing cattle,
leaf-eating insects such as Japanese beetles, seed-eating birds, and rodents.
Carnivorous animals are flesh-eaters. Among the most voracious are the fresh-
water protozoans Didinium nasutum. When they are placed among a population
of paramecia, each one immediately attaches its trunklike proboscis onto a Para-
mecium which is speedily "swallowed" (Fig. 11.4). An individual Didinium
may devour paramecia until its own body splits open. Among other carnivores
no tiger can be more bloodthirsty than female mosquitoes and blood-sucking
leeches. Most fishes are typical carnivores; so are snakes, owls, and hawks.
The order Carnivora is a group of mammals that includes cats, tigers, dogs,
Chap. 1 1 FOODS AND NUTRITION 173
wolves, raccoons, and seals but the animals of this group have no monopoly
on the carnivorous diet.
Omnivorous animals feed on both vegetable and animal matter, dead or
alive. They include such scavengers as lobsters, domestic fowls, and man. The
O O O o
o o O O o o „
O O O O n o
O O o o o °
Rice grain in husk
Showing germ
B
Polished rice
Fig. 11.2. A, Pigeons: top, suffering from polyneuritis (beriberi) developed
as a result of a diet of polished rice, lacking thiamine (of the vitamin B complex);
bottom, the same bird after injection of thiamine resulting in a spectacular cure.
B, Diagrams of rice grain (seed), in natural condition, and polished rice, with
bran or husk and the germ removed as in the milling process. Thiamine is con-
fined almost entirely to the germ. Milled grains contain little or no thiamine. In its
absence an essential enzyme of the body fails to function and finally there is a
poisoning of the nervous system known as polyneuritis. (A, after Morse. Courtesy,
Heilbrunn: Outline of General Physiology, ed. 3. Philadelphia, W. B. Saunders
Co., 1952.)
174
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
Fig. 11.3. Left, Dog with vitamin A deficiency causing xerophthalmia, a dry-
ness of the eyeballs. Right, Same dog after treatment with cod liver oil for 10
days. From early times it has been known that night-blindness and xerophthalmia
are associated with sparsity of fat in the diet. In man the most sensitive test for lack
of vitamin A is loss of adaptation to darkness. The visual purple in the retina of
the eye is a derivative of vitamin A. (After Steenbock, Nelson, and Hart. Courtesy,
Bogert: Nutrition and Physical Fitness, ed. 3. Philadelphia, W. B. Saunders Co.,
1941.)
Table 11.2
Minerals and/or Trace Elements* Required by the Body of man and Other
Mammals
Name
Location and/or Chief Functions
Effects Caused
by Deficiency
Calcium
In bones, blood, teeth, nerves
Stimulates milk production
Rickets, nervous irritability
Chlorine
Activates enzymes, such as gastric juice
Loss of weight, loss of
Regulates osmotic pressure
water, digestive
disturbances
Fluorine
In enamel of teeth
Decay of teeth
Iodine
In thyroxin, a secretion of thyroid gland
Low basal metabolism,
nervous disturbances
Iron
In hemoglobin of the blood
Decreased hemoglobin
Magnesium
In bones, nerves, muscles, especially of the
Retarded growth, rapid or
heart
irregular heartbeat,
nervousness
Phosphorus
In bones, blood, teeth, muscles. Metabolism
Poor development of bones
of carbohydrates and proteins; activates
and teeth, rickets,
enzymes
retarded growth
Potassium
Important in muscle action, normal growth.
Poor muscular control.
osmotic pressure
irregular heartbeat
Silicon
In hair
Sodium
Important in regulation of osmotic pressure
Loss of weight, nervous
disorders
Sulfur
In proteins of the body
Retarded growth
* Authorities differ as to whether some of these substances are in small enough amounts
to be classed as trace elements.
Chap. 11
FOODS AND NUTRITION
175
0. I mm
Fig. 1 1 .4. Didinium nasutum, a microscopic animal, nevertheless a fierce car-
nivore. A large Paramecium is attacked by four small Didinia. The Paramecium
is torn in pieces and each attacker gets a piece. Or, one Didinium gets the whole
Paramecium and forces the others off while it swallows the victim. (Courtesy,
Mast, "Reactions of Didinium nasutum," Biological Bulletin 16:100, 1909.)
human diet includes living plants, living animals, oysters and others; freshly
killed animals; and decayed plant and animal tissues. In primitive cultures the
latter are inexpensive foods; in more highly cultured circles, decayed foods,
among them "high cheese" and mellowed venison are expensive.
Food Intake by Plants and Animals
Plants absorb food in solution. Water and salts enter the plant through the
root hairs whose delicate surfaces must be constantly moist (Fig. 4.9). Roots
turn toward water and stems toward light, but plants hunt only in these ways.
Excepting parasites, most land animals and many aquatic ones go from one
place to another after food, a continual prowl if they prey on other animals,
sometimes a long wandering if they feed on plants. The majority of animals
eat solid foods, microscopic particles taken into the food vacuole of an ameba,
a whole sheep into the stomach of a great python snake. But before any food
is absorbed it must be in solution.
Feeding Devices. Contraction of protoplasm always figures in the intake of
solid food. The protoplasm of an ameba contracts about a diatom. The lashing
ciUa of a paramecium or a rotifer create currents that bear processions of
microscopic food particles through their mouths and gullets (Fig. 11.5). Cilia
bring the food to the mouths of such aquatic animals as the sea anemones,
clams and oysters, and the swimming young of starfishes. Certain sizes and
shapes of particles are selected by the ciliary currents, often by means of
176
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
Pinch and snap-back
Dragonfly nympn
Incoming current
Rotifer
Soft lip and suction
Earthworm
Saw-edged tongue
Woodpecker
Long distance suction
Butterfly
Fig. 11.5. Animals gather food into their mouths by many devices. Circlets of
cilia create currents of water on which particles ride toward the mouth; earthworm,
a soft lip and ring of muscle grip the leaf and the suction of the pharynx pulls
it in; dragonfly nymph with underlip outstretched to grasp its prey; woodpecker
hammers its bill into the wood and saws with its tongue; butterfly extends its
proboscis to the nectar of the flower and sucks.
elaborate filters. Other animals grasp food within a ring of muscle located in
their fleshy lips, sucking it in as earthworms grip a wisp of leaves, and as in-
fant mammals suck milk from a nipple. Most of the vertebrates seize their food
with the aid of a beak or various kinds of teeth. Dogs lunge forward, clutch
meat with their teeth, and hold it against the ridges on the hard palate.
With each lap of its tongue a cat gathers up milk and throws it well back
into the gateway of its throat or, with strong strokes of its tongue, rasps the
flesh from a bone. With strokes like these but rougher and stronger lions clean
up the carcass of a zebra. A giraffe wraps its tongue around high-hanging
leaves and pulls them down to its grasp; a cow does the same with a bunch of
hay from the hayrack. Woodpeckers hunt over the bark of trees using their
tongues like bayonets to pierce the grubs (Fig. 11.5). Thus, by thousands of
Chap. 1 1 FOODS AND NUTRITION 177
devices, animals get their particular foods into their mouths, by pulling, push-
ing, cutting, and squeezing. Hunting and eating occupy most of the lifetime
of animals. Compare the winning and eating of food by all human beings.
Essentials of Digestion
Digestion is a series of physical and chemical changes by which food is
prepared for assimilation in protoplasm. Physically it is the breaking and mix-
ing of food; chemically it is the process of changing large organic molecules
into smaller ones through the action of hydrolyzing enzymes. Enzymes are not
only essential to digestion but to all other chemical activities of a living organ-
ism. In all multicellular animals and in many protozoans digestion occurs in a
cavity, a temporary one in the ameba, a sac in hydra, a tube in many inverte-
brates and in all vertebrates. Among the tools of digestion are beaks, teeth,
muscles, and secretions.
Digestive Cavities and Tlieir Accomplishments
Most of the multicellular animals contain relatively spacious digestive cavi-
ties (Fig. 11.6). In hydras, jellyfishes, corals, planarians, and others, it is a sac
with but one opening. In the great majority of animals it is a tube, the alimen-
tary canal, with extraordinary variations of structure and function. Some of
them are adapted to other uses besides those concerned with food, such as the
respiratory chamber in the intestine of the nymphs of dragonflies.
Successful developments in the alimentary canals of various animals are:
holding capacity, means of movement and physical breakup of food, means of
chemical breakup, extensive cell surface for absorption of digested food, and
means of eliminating undigested waste. Animals have to take their food and
drink when and where they find it and a capacious stomach to carry away as
much as possible is useful. The stomach of a yellow perch may hold fishes of
the catch of yesterday, of the day before, and of the day before that, each lot
in a different stage of slow digestion. Cows graze steadily through the summer
forenoon, swallowing grass into their storage stomachs and chewing it over at
their leisure as they rest under the trees in the afternoon (Fig. 11.14). An
arrangement like this might be a happy one for commuters who must rush
through breakfast and catch the train. The holding capacity of stomachs is a
social asset to termites, honey ants, and several other animals. The social and
political prominence of many persons has been frequently due to the elastic
capacity of their stomachs, and just as frequently they have come to grief
because of it. Within colonies of certain species of honey ants, the repletes,
continually overfed with honeydew, are useful to the community as living
storage tanks of food and drink. From time to time a hungry worker taps the
head of a replete which promptly spits a drop of honeydew into the waiting
mouth of the worker.
178
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
Digestive Cavities (black)
Adequate digestive area developed within limited size of body
Moutti
Tops of cells reocti
into sac
B
Branches and out
pouches from sac
Crop
Gizzard
Locol enlargements and
out pouches from tube
Stomoch
D
Local enlargement and tube
double the length of body
intestine in
watch-spring coil
Stomach
Local enlargements, coils
and branches (caeca)
Fig. 11.6. Diagrams of digestive cavities (black). They are examples of rel-
atively large capacity and area for secretory and absorptive cells contained within
limited body size: A, Hydra, a sac into which the ends of secretory cells are
extended; B, planarian, a sac with outpouchings that reach out through the body
and partly take the place of circulating blood; C, earthworm, local enlargements,
a common device for greater capacity; £>, snail, lengthening and doubling-back of
the digestive tube, mainly in mollusks; E, tadpole, the intestine, a watchspring coil;
F, bird, enlargements (crop, stomach, gizzard), coils and branches (caeca). The
foregoing are the chief patterns of the alimentary tract in multicellular animals;
no account has been taken of accessory glands, such as the liver and pancreas.
There are various ways of breaking up food physically and chemically,
especially the former: some of these are briefly mentioned or figured in this
chapter.
Human Digestion
In the Mouth. The mouth cavity is the vestibule of the digestive system, the
reception place of the food (Fig. 11.7). The teeth break it into pieces; the
smaller the bits, the more quickly digestive enzymes can diffuse through them.
In the meantime the alkaline saliva floods the mouth and pours over the food,
a shifting mass because it is held on the tongue, a gymnast that continually
Chap. 11 FOODS AND NUTRITION 179
ripples and tilts and explores every newcomer. No dry food is tasted until it is
well moistened since the sense organs of taste on the surface of the tongue
are stimulated only by substances in solution (Fig. 17.3). Saliva enters the
mouth more or less continually, except under nervous tensions — when a song
is to be sung and "the mouth goes dry." Saliva and mucus keep the mouth
well lubricated (Fig. 11.8). In a few mammals, including man, saliva contains
ptyalin (salivary amylase), a hydrolyzing enzyme, and a slippery substance.
NASAL CAvmr
PALATE
MOUTH
TCN6UB
JWSALPnAHVJK
Fig. 11.7. Diagram of the human alimen-
tary canal with the liver and pancreas. The
same devices for adequate area in limited
space as shown in Fig. 11.6. (After Morris.
Courtesy, Rand: The Chordates. Philadel-
phia, The Blakiston Co., 1950.)
mucus, both secreted by the cells of the salivary glands. Ptyalin splits the large
molecules of cooked starches into the smaller ones of sugar. In most mammals,
however, there is no chemical digestion in the mouth.
Function of the Teeth. Because teeth tell what an animal eats, they also tell
where it lives. The most specialized teeth belong to the mammals. According
to their function, they are divided into incisors for cutting and chiseling,
canines for grasping and tearing, premolars or grinders, and molars or
crushers. Squirrels, mice, and other rodents chisel with incisors and crush nuts
with molars. In horses, cattle, and other herbivorous animals except the
rodents, the front teeth, especially the canines, are reduced or absent and the
molars are well developed. In cats, dogs, and other carnivores the upper and
lower premolars slide on one another like scissors (Fig. 11.9). A cat grasps
meat with its canines, and tears the flesh with its premolars, hardly using the
weak molars at all.
180
THE INTERNAL ENVIRONMENT OF THE BODY
PHY
Part III
PAROTID
SALrVARY GLANO
OUCT OF PAROTID GLANO
OPEN'NG INTO MOUTH
SUBMAXILLARY
SALIVARY GLANO
3 6L
SUBLINGUAL
SALIVARY GLANO
Fig. 11.8. Salivary glands. Left, In man the salivary glands are under the con-
trol of the autonomic (involuntary) nervous system. The parotid unit is stimu-
lated indirectly by dry food and by acids. Substances in solution stimulate the
taste buds of the tongue, nervous impulses are sent to salivary centers in the
hind part of the brain, and are relayed by nerves to the salivary glands. Right,
In the honeybee the salivary glands are relatively enormous. There are two pairs
packed between the air sacs in the head and around pharynx (PHY) and the
brain (pharyngeal and cephalic) (IGL, 2GL), and another pair (thoracic) (3GL)
extends into the thorax and about the esophagus (OE). In bees the uses of saliva
are highly social: in royal jelly food for the young queen; mixed with honey;
mixed with the wax for the comb, and other materials. {Left, Courtesy, Mac-
Dougall and Hegner: Biology. New York, McGraw-Hill Book Co., 1943. Right,
Courtesy, Snodgrass: U. S. Bureau Entomology Technical Series Bull. No. 18.)
Carnivorous teeth
Dog
Herbivorous teeth
Cow
Fig. 11.9. Carnivorous teeth of a dog and herbivorous teeth of a cow. Car-
nivores have few cheek teeth and those shear like scissors. Dogs grip their food
with their stabbing upper canines and gulp it hurriedly. Herbivorous mammals
have full sets of cheek teeth with high crowns resistant to grinding. Cattle have
no canines and no upper incisors but clinch grass between their lower incisors and
a horny pad on the upper jaw.
Chap. 11 FOODS AND NUTRITION 181
As to teeth, at least, the human mouth is a middle-of-the-road type. Human
teeth like pigs' teeth are generalized and adapted to mixed diets. Although the
main kinds of teeth are moderately represented, none could be safely used to
nibble a cupboard door. There are 20 human milk or baby teeth, which
usually develop before three years of age; and ordinarily 32 teeth in the so-
called permanent set which begins to appear at about six years and finishes at
twenty-five (Fig. 11.10). Actually we have one-and-a-half complete sets of
teeth in a lifetime, the first set and a partial second one, since the molars of
the first are not shed like all the other milk teeth.
The jav/s of modern man are shorter than those of his early ancestors who
PERMANENT^
INCISORS
DEC IDUOUS— s; — ^-™
INCISORS. |- \y
SECOND
PERMANENT-
MOLAR.
PERMANENT-
PREMOLARS
PERMANENT-
CANINE.
DEaDUOUS
MOUliRS.
HRST
PERMANENT
MOLAR.
PERMANi=:^n'
INCIS0r^O.
Fig. 11.10. Human teeth, one and a half natural sets in a lifetime. The teeth
of a five-year-old child with portions of the jaws cut away to expose the roots of
the milk teeth and the partially developed permanent teeth. (Courtesy, Rand: The
Chordates. Philadelphia, The Blakiston Co., 1950.)
still had a fourth molar, now uncommon. Even the third molar or wisdom
tooth comes late and with difficulty and is little more than a nuisance. As a
result of the modern shortened jaws, the wisdom teeth often do not have
enough room, are crooked and out of position.
Swallowing. When food is about to be swallowed, the tongue is moved back-
ward and pressed up against the hard palate (Fig. 11.11). One swallows
quickly and momentarily stops breathing. In that instant the food, now almost
at the crossways in the pharynx, moves obliquely toward the esophagus.
It might go into the nose, back into the mouth, or into the windpipe
were it not so well prevented. But the soft palate is automatically pulled up,
closing the way to the nose, and the base of the tongue shuts off the mouth.
At the same time, the voice box, or larynx is pulled upward against its cover,
the epiglottis, and this shuts off the road down the windpipe. On the instant
that all the ways are closed, the throat muscles contract and the food is shot
182
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
MOUTH
SOFT PALATE
TRACHEA
Fig. 11.11. The passageways for air and food, showing the cross ways in the
pharynx that must be taken separately by each. When air has the road the trachea
(windpipe) is open and the esophagus is closed. When food has the road, the
epiglottis covers the trachea and the esophagus is open. (Reprinted from The
Machinery of the Human Body by Carlson and Johnson, by permission of The
University of Chicago Press. Copyright 1948.)
into the esophagus, always slippery with mucus. There circular muscles grip
it and urge it along the short passage to the stomach (Fig. 11.12). In the
upper part of the mammalian esophagus the rapid contractions of striated
muscles extend all the way to the stomach, an arrangement well adapted to
the amazing rapidity with which animals swallow their food, a long established
Wave of muscular contraction
grasps the bolus of food
Wave of relaxation opens
the tube before it
Contraction squeezes tube
behind it
Six seconds from mouth to stomach
•
1
#
Fig. 11.12. It takes about 6 seconds for a bolus of solid food to pass from
the mouth to the stomach. A wave of contraction follows a bolus of food; a wave
of relaxation opens the way in front of it.
I
Chap. 11 FOODS AND NUTRITION 183
eat and watch and run habit. But no matter how we try to hurry it, the human
esophagus never speeds up like that of a dog; on the contrary like an elevator,
it takes its own time. At man^' a modern table the primitive habit of eat and
watch and run continues.
The human esophagus is strictly a passageway. Usually we swallow down-
ward, but it is quite possible to swallow upward while standing on one's head.
Any acrobat can demonstrate this and every day horses and cows drink up-
ward at a sharp angle. Even if its esophagus rises perpendicularly to the milk,
this does not hinder a drinking weasel (Fig. 11.13).
Fig. 11.13. Weasel drinking milk with its esophagus at right angles to the milk.
(Courtesy, American Museum of Natural History, New York.)
In other animals the esophagus may be distended into a sac which holds the
extra food and acts as a waiting-room for gastric digestion. Cattle and other
ruminants have such temporary storage sacs: the largest one is the rumen
which in an average-sized steer has a capacity of about 30 gallons; the others
are the reticulum and omasum. In the market the lining of the reticulum is
known as honey-comb tripe (Fig. 11.14). After a period of eating and
swallowing into the rumen, cattle, sheep, deer, and other ruminants lie down
to chew their cuds. At that time contractions of the esophagus go into reverse
and bring one bolus after another of the slightly fermented grass to the mouth
where it is chewed and again swallowed, this time permanently.
The crops of birds, especially of domestic fowl, are lateral enlargements of
the esophagus. The chicken that goes to roost with a full crop sleeps on while
184
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
food entering.
.course of food after
/ final mostication
esophagus
2 reticulum
(tripe)
— 3 omasunn
4 obomasum
I rumen
"fermentation vot"
esophagus
diaphragm
3 omasum
"filter press"
2 reticulum
agitator
lining-tripe
4 abomasum
( true stomach )
Fig. 11.14. The pouches and stomach of a cow, a cud chewer or ruminant.
Three pouches, all enlargements of the esophagus, compose the rumen (capacity
120 or more liquid quarts), the reticulum, and the omasum; the true stomach is
called the abomasum. In the upper figure the course of the food after its first
swallowing is shown by long dashes and arrows. The second swallowing, after the
cud has been chewed, is indicated by short dashes and arrows. In the lower figure
the pouches and stomach are in their natural position.
its crop automatically delivers the corn and grass to the glandular and grind-
ing sections of its stomach (Fig. 36.15).
Function of the Stomach. The human stomach is a J-shaped enlargement of
the digestive tube with a muscular wall and a glandular lining. Its anterior end
closes by the contraction of a ring of muscle (cardiac valve) and its posterior
end by another ring (pyloric valve). Its muscular movements are con-
trolled by the nerves of the autonomic system; the vagus nerve, partly
parasympathetic, stimulates contractions, and the sympathetic nerve inhibits
Chap. 11 FOODS AND NUTRITION 185
them (Figs. 11.15, 11.16). While food is in the stomach it is stirred and
pressed by the contractions of the walls, and digestion of protein and some
fats is begun by the gastric juice. Nerves from the taste organs in the tongue
are associated with the vagus nerve, branches of which spread through the
stomach wall and carry impulses that start the secretion of the gastric juice
while food is still in the mouth.
Glands in the wall of the stomach produce the gastric juice containing
mucin, hydrochloric acid, and three digestive enzymes or ferments, pepsin,
rennin, and gastric lipase, of which pepsin is the most important. The esti-
mated 35,000,000 gastric glands formed by inpocketings of the stomach lining
Fig. 11.15. An x-ray photograph of the waves of contraction of the human
stomach. Such contractions work upon food and in the early stages of an empty
stomach cause hunger pangs. The stomach is here made visible by barium salts
recently swallowed in milk. Bits of intestine are similarly visible in the lower
part of the illustration. (Courtesy, Gerard: The Body Functions. New York, John
Wiley and Sons, 1941.)
186
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
show how the surface of an organ can be enormously increased without
enlarging the organ.
Of the various components of the gastric juice, mucin secreted by mucous
cells makes a slippery protective layer over the lining of the stomach, and the
hydrochloric acid kills bacteria and provides an acid medium without which
the gastric enzymes will not work. Enzymes work best in a particular acidity
or alkalinity, and if this changes, their action is hampered or stopped. The
Fig. 11.16. Arrangement for recording stomach contractions on a kymograph,
the revolving drum that carries a strip of smoked paper. The writing finger, at the
top, makes the sharp lifts that record contraction. The other finger records the
repeated pressures caused by breathing. (Courtesy, Gerard: The Body Functions.
New York, J. Wiley and Sons, 1941.)
enzyme, pepsin, reduces protein only to the stage of proteoses and peptones;
probably no amino acids are formed, and there is relatively little absorption
from the stomach (Table 11.3). The action on proteins is a preliminary hy-
drolysis, a splitting and absorption of water that expose the products to the
enzyme action that occurs after they enter the intestine. The enzyme, rennin,
is an aid to pepsin in that it coagulates milk and produces the soluble milk
protein, casinogen, upon which pepsin can act. Lipase is an enzyme that acts
on the finely emulsified fats of cream and egg yolk. The production of gastric
juice is first stimulated by food in the mouth, but when it reaches the stomach,
the flow is greatly increased. Experiments on animals have shown that partly
digested food stimulates the production of gastrin, a hormone said to be pro-
duced by cells of the lining of the pyloric end of the stomach and discharged
into the blood stream whenever such food comes in contact with these cells.
If an extract of pyloric lining is injected into an animal, its gastric glands
begin to secrete within a short time. In cross-circulation experiments a blood
Chap. 11
FOODS AND NIHKITION
Table 11.3
The Main Actions of Chemical Digestion
187
Location
SoKice
Digestive
Substances
of the
of the
Enzymes
Acted
Products
Process
Secretion
Present
Upon
.
Mouth
Salivary glands
Ptyalin
(salivary
amylase)
Cooked starch
Douhie sugars
Pepsin
Proteins
Proteoses and
peptones
Rennin
Milk proteins
Ciudied proteins
( intermediate
Stomach
Gastric glands
stage between
protein and
amino acids)
Gastric lipase
Finely emulsified
fats
Simpler fats
1. Trypsino-
Proteins or pep-
Peptones and
gen^'^ (con-
tones
amino acids
verted into
trypsin acts
upon pep-
tones)
Pancreas
2. Steapsin
(pancreatic
lipase)
Fats
Fatty acids and
glycerol
3. Amylopsin
Starches, intact or
Simple sugars
(pancreatic
partly digested
amylase)
Small intestine
Erepsin
Peptones
Amino acids
Lactase
Lactose (milk
sugar)
Simple sugars
(glucose)
Maltase
Malt sugar
Simple sugars
(glucose)
Intestinal
Sucrase
Sucrose
Simple sugars
glands
(glucose)
Enterokinase*
Trypsinogen*
(inactive)
Peptones
Trypsin* (active)
Amino acids
Lipase
Fats
Fatty acids and
glycerol
* Trypsinogen is an inactive enzyme produced by the pancreas. It passes into the intestine
in the pancreatic juice, is there acted upon by enterokinase produced in the intestinal wall,
and becomes the active enzyme trypsin.
vessel of one dog is connected by a rubber tube with the blood vessel of an-
other. After food given the first dog has arrived at the pylorus, the gastric
glands of the second dog begin to secrete gastric juice although that dog has
been given no food. This shows clearly that they are stimulated by a hormone
carried by the blood from one dog to the other.
The time that food remains in the stomach depends mostly upon its con-
ISS TMF INTFRNAL nNVIRONMFNT OF- Till BODY Part III
sistency. Fluids (alcohol is absorbed directly into the blood) leave almost
immediately, and solids last; carbohydrates, proteins, and fats leave the
stomach in that order. An ordinary mixed meal remains in the human stomach
from three to tour hours.
Observations and Experiments. The famous experiment which first
showed that food undergoes chemical changes during digestion was performed
by a pioneer in experimental zoology. Rene Reaumur ( I6(S3-1757). It proves
first of all that an inquiring mind can discover something with simple equip-
ment. Reaumur placed bits of meat in small, perforated metal tubes fastened
to threads and fed them to his falcon and some other pet animals. When he
recovered them from the stomachs, he found the meat partially dissolved.
Soon after that he fed bits of sponge to a chicken and later pulled them
forth drenched with gastric juice. He next discovered that meat would be
dissolved if dropped into an open dish of gastric juice. Years later the Rus-
sian physiologist, Pavlov (1849-1936), discovered that the gastric glands
were stimulated not only when a dog took food into its mouth, but when it
smelled food, or heard a bell which it associated with food.
Experiments with balloons have shown that hunger pangs are due to the
futile contractions of an empty stomach. The subject of such experiments
swallows a soft balloon which is then partially inflated and attached by a
tube to an apparatus which records the changes of pressure on the balloon.
Every time the stomach contracts it squeezes the balloon, causing a lift in
the writing point of the kymograph. At the same time the subject, who does
not see the record, presses a button because he feels a hunger pang (Fig.
11.16). As the experiment proceeds, the signal of the hunger pang and the
record of a squeeze on the balloon occur regularly at the same time. Con-
tractions during hunger do not seem to be different from ordinary ones
except that they are stronger. In moderate hunger the pangs are felt for a
time and then cease, like a recovery from frustration. Observations and photo-
graphs of the movements of the stomach are made by x-ray after a meal
containing some harmless substance, usually barium sulfate, that appears
opaque in the photographs (Fig. 11.15).
Function of the Small Intestine. Food comes into the intestine in jets of
fluid projected through the relaxed circular muscle that forms the pyloric
valve. In doing so, it is shifted from the highly acid environment of the
stomach into an alkaline environment in which different digestive enzymes
can work. The small intestine is the most important region of the digestive
tract, the one where food is treated by the versatile pancreatic and intestinal
juices (Fig. 11.17). It is lined with millions of motile villi through which
almost all absorption of food occurs (Fig. 11.19). The intestinal tract con-
sists of the small intestine, much longer (human, about 20 feet) if not as
large around as the large intestine. The first part of the small intestine is the
FOODS AND NUTRITION
189
Chap. 11
important region into which the ducts of the liver and pancreas open. This
U-shaped bend is the duodenum, an old-time name meaning 12, given it be-
cause in man it is about the length of 12 fingers. Except in the duodenal
region, the intestines are loosely attached to the dorsal body wall by mesentery
(Fig. 11.18).
Function of the Liver and the Gall Bladder. Among its many other activities,
the liver secretes the bile, vitally important in digestion although it contains
Thorocic
covity
Pancreos
Abdominol
cavity
Gall blodder
Bile duct
Poncreotic ducf
Fig. 11.17. Diagram showing the human liver and stomach beneath the di-
aphragm, and ducts from the liver, gall bladder and pancreas leading through one
opening into the intestine.
no enzymes. All the blood in the body passes through the liver several times
per hour; thus it maintains a content of about one-fifth of the body's total
blood supply.
Microscopic bile capillaries so permeate the liver that they are in contact
with every cell. These capillaries join to make the larger hepatic ducts, which
in turn form the main bile duct to the duodenum. The gall bladder, a tempo-
rary storage place for bile and part of a remarkable mechanism, branches
from the main duct. As it is produced bile passes into the bile duct, but is
ordinarily kept from passing into the intestine by the continued contraction
of a band of muscle that encircles the opening of the bile duct into the
intestine. When the duct is filled, the bile is forced back into the gall bladder
where it is stored until required, in the meantime becoming more concen-
trated by loss of water through the bladder wall. With the entrance of
food into the duodenum, the gall bladder contracts and discharges bile down
the duct whose circular (sphincter) muscle then relaxes, allowing the bile
to flow into the intestine. Although the gall bladder is controlled by the
parasympathetic part of the vagus nerves, experiments have proved that it
190
THE INTERNAL nNVIRONMFNT OF THE BODY
Part III
will contract after the nerves arc cut away. They have also shown that its
contraction is stimulated by a hormone, cholecystokinin, secreted by the
lining of the duodenum. Products of the digestion of fats in the intestine
seem to stimulate the production of this hormone much more than those
of proteins and carbohydrates. Bile contains organic salts. Some of these
are absorbed by the lining of the intestine, taken up by the blood and returned
to the liver. These bile salts and secretin, a hormone from the intestine, both
ARTERIES
VEINS
LYMPH NODE
LYMPHATICS
NERVES
SMALL INTESTINE
Fig. 11.18. The mesentery formed by a double layer of the peritoneum supports
the intestine; between its layers are the blood vessels, nerves and lymphatics that
supply the intestine. (Courtesy, Haggard: Science of Health and Disease. New
York, Harper & Bros., 1927.)
stimulate further bile-making in the liver. As digestion in the intestine is
completed, the production of secretin and cholecystokinin is reduced. With
this reduction the sphincter muscle at the exit end of the bile duct tightens.
This prevents the escape of bile which once more fills the bile duct and backs
into the gall bladder.
A gall bladder may be regularly present in one species of animal and
regularly absent in another nearly related one with a similar diet. It is lack-
ing in the white rat, horse, pocket gopher, and pigeon, but present in the
Norway rat, mouse, cow, striped gopher, chicken and duck, also in cats
and dogs. Experimental study of these animals has shown that the ones
without gall bladders have a relatively larger production of bile than the
others. The human gall bladder is often removed because of inflammation
and the formation of gallstones by accumulations largely of cholesterol, the
Chap. 11 FOODS AND NUTRITION 191
fatlike substance that the liver absorbs from the blood. Its removal does
not necessarily cause any digestive difficulties.
Functions of Bile. Bile produces no digestive enzyme but it performs
several functions in the intestine. It supplies organic salts (bile salts) which
are the emulsifying salts of fats. Bile salts serve as specific activators of
pancreatic lipase.
Important as bile is for the more efficient digestion of fats, it is still more
important for their absorption. If bile is prevented from entering the intestine,
a large proportion of the fatty acids passes out with the waste products in-
stead of being properly absorbed. This effect of the absence of the bile salts
has only recently been discovered. Bile salts unite with the fatty acids and
form compounds that pass into the lining cells of the intestine. Here the bile
salts are separated from the compounds, enter the blood capillaries, and are
carried to the liver where they are picked up by the liver cells and once more
go into the bile. The fatty acids that were freed from the bile salts combine
with glycerin (absorbed by the lining cells) to form neutral fat. The greater
part of this fat passes into the microscopic lymphatic vessels, the lacteals, in
the centers of the intestinal villi (Figs. 11.19, 11.20). It eventually enters the
blood by way of the lymph.
Bile salts make possible the absorption of the antihemorrhagic vitamin K,
which occurs in spinach, cabbage, and other green foods (Table 11.1). In
the treatment of obstructive jaundice, when the bile ducts may be clogged
by gallstones and no bile enters the intestine, the usual tendency toward
bleeding is countered by doses of bile salts. Chickens develop a hemorrhagic
disease if they do not get any grass or other green foods.
Functions of the Pancreas. The pancreas Hes between the stomach and
duodenum (Fig. 1 1.17). It is an irregularly shaped gland composed of groups
of lobules that make the surface look bubbly. In each lobule the cells are
arranged around a minute drainage tubule. These tubules unite with one
another and finally form the main pancreatic duct. This carries the secretion
to the intestine, emptying into it through a common opening with the bile
duct. Scattered through the pancreas are the entirely different glands called
the islands of Langerhans. Their hormone, insulin, is secreted directly into
the blood and is necessary for the utilization of sugar in the body, the safe-
guard against sugar diabetes (diabetes mellitus).
The pancreatic juice is a clear alkaline fluid secreted on an average of
about a liter (1.05 liquid quarts) per day. Its principal enzymes are: tryp-
sinogen, which, when converted into trypsin, carries the digestion of proteins
a step beyond that occurring in the stomach; amylopsin (pancreatic amylase)
which completes the digestion of starch begun in the mouth, and steapsin
(pancreatic lipase) which splits fats into fatty acids and glycerol. Pancreatic
secretion collected directly from the ducts has very little power to digest
192
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
LoctAol
Goblet cell
Crypt of
Lieberkijtin
UCOSO
usculoris mucosae
ubmucoso
Fig. 11.19. Diagrammatic view of a minute portion of the lining of the small
intestine showing the villi that are constantly dipped into the digesting food; two
of them are cut open to expose the blood and lymph vessels into which digested
food is absorbed. The continual springing up and down of the villi keeps the food
in motion. (Courtesy, Villee: Biology. The Human Approach, ed. 2. Philadelphia,
\V. B. Saunders Co., 1954.)
proteins. On the other hand, when it is mixed with intestinal juice containing
the enzyme enterokinase secreted by the glands in the lining of the intestine,
it immediately becomes potent.
As soon as food comes into the intestine, pancreatic juice begins to flow
into it. The reason for this was a puzzle for many years. The nerves leading
to the pancreas were cut but, in animals thus treated, the flow went on as
before. In 1903, the British physiologists Bayliss and Starling discovered
that the mysterious messenger was a fluid, the hormone, secretin. The partly
digested food from the stomach stimulates glands in the intestinal wall to
produce secretin. This is picked up by the blood, carried to the pancreas,
and immediately stimulates that gland to produce its digestive secretion.
The Completion of Digestion. The walls of the small intestine contain
minute glands which secrete additional enzymes that complete the digestion
of proteins and carbohydrates and fats (Table 11.3). Peptidases (erepsin and
others) complete the breakdown of protein into amino acids that can be
absorbed into cells everywhere. Sucrase, maltase, and lactase are enzymes
Chap. 11 FOODS AND NUTRITION 193
that act upon cane sugar, malt sugar, and milk sugar, respectively, turning
them into simpler sugars, such as glucose, which can be absorbed by ceils
of the body. The intestinal glands also secrete enterokinase which changes
inactive trypsinogen into active trypsin that simplifies proteins. An intestinal
lipase acts on fats. Glands in the intestinal wall also secrete a large amount
of mucus that lubricates the passage of the food.
Protein
apillary
Lymph vessel
sulphate
Fig. 11.20. The surroundings of a villus, one among millions, and names of
some substances absorbed. Large particles cannot penetrate the wall of the villus;
most others enter it and the blood capillary along with certain minerals. Fats
enter the lymph vessel and later pass into the blood.
Absorption of Food
Absorption of food occurs almost entirely in the walls of the small in-
testine. Only then is it actually inside the body. This wall has in superlative
degree three essentials for the absorption of food: means for keeping the
food in constant agitation, a great area of semipermeable membrane, and
blood and lymph to pick up and transport the absorbed food (Figs. 11.19,
11.20). The walls of the small intestine contract peristaltically, pushing the
content of the food through the lumen. They also constrict rhythmically,
sharply squeezing the tube into segments hke a chain of sausages and repeating
the process again and again, twenty times per minute or more (visible by
x-ray), each time pressing the food against the absorptive wall.
Especially in the duodenum the lining is thrown into circular folds and its
entire surface, folds and all, is covered with millions of villi all in more or
less continual, slightly tremulous motion. The shortening, lengthening, and
bending of the villi keep the digested food agitated and moving against the
194 Tin: INTl RNAL HNVIRONMENT OF THE BODY Part III
cells which are absorbing it. Between the villi arc the microscopic openings
of the intestinal glands. Each villus is covered with epithelial cells into which
the food is absorbed (Fig. 11.19). In the core of each one is an arteriole
and a venule with their connecting capillaries; the blood in these vessels
picks up the digested food (except the bulk of the fat) and transports it to
the liver. Also in the core is a minute lymph vessel, a lacteal, that ends
blindly at the tip, but at the other end is continuous with larger lymph ves-
sels. Lymph vessels from all over the body finally coalesce and form the
thoracic ducts which flow into large veins. Thus the fat, now a milky white
emulsion, ultimately reaches the blood.
There is no adequate theory to account for the assimilation of the vital
proteins in the various tissues from the collection of amino acids that is
assembled in the blood stream. It is known, however, that there is a rigid
selection and that a given cell, perhaps a muscle or nerve cell, always as-
similates particular amino acids. The changes of proteins and amino acids
within the cells have been revealed by tracing the paths of compounds con-
taining "labeled" isotopes of hydrogen, carbon, or nitrogen during metabolism.
Function of the Large Intestine (colon and rectum). Reabsorption of water
is the principal function of the large intestine in all mammals. It produces no
digestive enzymes and little or no food is absorbed in it, but it secretes a
large amount of mucus which acts as a lubricant. The indigestible matter is
gradually admitted from the small intestine with considerable water and
some unabsorbed secretions. An enormous population of bacteria is always
present. In no part of the alimentary canal are all bacteria destroyed.
At the junction of the small and large intestine there is a sac or caecum,
large in birds and other herbivorous animals and small or missing in carnivores
(Fig. 11.6). In many mammals, especially in monkeys and man, there is a
blind sac at the end of the caecum, the vestigial appendix (Fig. 11.7). In
the primitive monotremes, e.g., the duckbill platypus of Australia, the rectum
opens into the cloaca as it does in the frog, but in all other mammals it has
a separate external opening.
12
Circulation and Transportation—
Body Fluids
However dry the atmosphere may be outside, the cHmate inside an animal
is as wet as a rain forest. In the majority of many-celled animals the creation
of such an adequate internal environment is due to circulating fluids, primarily
the blood. It provides for the needs of cells no matter where they are located;
those in the roots of the hairs receive oxygen as freely as those in the lungs;
waste products are cleared from the bones as well as from the kidneys.
Water and Body Fluids
Water composes the largest part of all body fluids. In lower animals the
internal fluid is known as body fluid, in higher ones, as blood with its auxil-
iaries, tissue fluid and lymph. The high water content of their bodies has made
it possible for animals to travel long distances over parched lands. Even in
the desert a lizard is a colony of wet cells watered by streams of blood, an oasis
in the sands.
Balancing their water content, to keep the right amount of it in and out
and to make up for what is lost, is a universal problem of plants and animals.
Evaporation is prevented by thick skins, shells, and scales; undue loss from
excretion is prevented by controls of the sweat glands and kidneys; and loss
from various causes throughout the body is offset by shifts in the osmotic
pressure of membranes. Wherever animals live, in fresh water, salt water, or
on land, their body fluids are similar; all are salty. In marine invertebrates,
whether jellyfishes or horseshoe crabs, the body fluids are practically filtered
sea water. Even in fresh water and land animals, the saltiness of the body
fluids tells of their origin in ancient ancestors that lived in the sea.
195
196
THF INTF.RNAL F.NVIRONMENT OF TlIF BODY
Part 111
Hiinuin Blood
(General Composition. Blood is composed of fluid and cells. It is as much
a tissue as bone; in blood the substance between the cells is fluid; in bone it
is solid. When a tube is filled with blood and whirled in a centrifuge, the cells
are thrown down to the bottom of the tube. The blood is thus separated into
a mass of cells constituting about 45 per cent of the whole blood, and a
clear pale yellowish fluid, the plasma, composing the other 55 per cent (Fig.
12.1).
Plasma and lymph and tissue fluid are fluids which come and go, join
together, are separated, and join again, over and over, continually sharing
Human blood contains
Fluid
(plasma)
55%
<
r
Solids
(cells)
45%
<
^
— — — —
^^_ 1 1 ^^K .■■
J
About 1 white cell to
every 600 red cells
About 1 platelet to
every 18 red cells
Water
Proteins and
ottier substances
Salt and
other minerals
Cells
Fig. 12.1. Fluid and solids of human blood. (Redrawn and modified from Public
Affairs Pamphlet No. 145. New York, Public Affairs Committee, Inc., 1948.)
their contents (Fig. 12.2). Plasma is complex because via the tissue fluid it
receives contributions of every kind from all cells of the body (Table 12.1).
When blood is under sufficient pressure in the capillaries, the excess fluid
seeps through the walls and becomes the only fluid that is in actual direct
contact with cells. When there is greater pressure on the tissue fluid outside
than on the blood inside the capillaries, the excess fluid goes back into the
capillaries or the lymph vessels from whence it is ultimately returned to the
blood. Thus these fluids continually pass to and fro; taking food and other
substances to the cells; removing their useful secretions and waste products;
distributing the heat of their oxidations; keeping them wet; providing them
with necessary salts, acids, and gases; and guiding their behavior by hormones.
The life of all cells is dependent on the continuity of this environment, and
its delicately balanced content must not change unless it is altered specifically
and in a way useful to the whole animal. If the plasma does not contain
enough salt the osmotic pressure rises and water enters the blood corpuscles
Chap. 12 CIKCULAIION and transportation BODY FLUIDS 197
until they burst; if it contains too much salt they shrivel. Or, the salt content
of the plasma may be right but the proportion of other constituents may be
wrong. Without oxygen cells cannot liberate their energy and they die; with-
out sugar they starve. In a solution that contains potassium but no calcium,
muscle tissue twitches; with too much calcium it becomes inert.
Plasma, Its Content and Functions. Plasma maintains a content of about
90 per cent water that is constantly lost and replaced. Water is lost from the
lungs in amounts varying with the temperature and humidity of the air and
the rate and depth of breathing, from the kidneys in urine, and from the
Arteriol end of blood
capillary in which
pressure is high
Lymph capillary
helps to remove
excess tissue fluid
Venous end of blood
capillary In which
pressure is lower
Tissue fluid
among cells
Fig. 12.2. Diagram of the balancing of body fluids. When pressure in the
blood capillary is high fluid passes out of the blood through the capillary wall,
is dispersed among the cells, and becomes tissue fluid. When the pressure upon
the tissue fluid becomes high, the latter may enter the lymph capillary becoming
lymph or it may return to the blood capillaries to merge with the blood plasma.
Normally there is a constant balancing easily disturbed by slight chemical changes.
sweat glands in sweat. It is increased mainly by eating and drinking. There
is a more or less constant demand for water, since in animals there is no
special storage of water, as there is of fat. There are some exceptions to this.
Camels, like cattle, swallow their food into a pouch, to be recalled later for
leisurely chewing. They are also provided with water stored in water pockets
opening ofl the pouch. The great mass of fat in the Jiump also provides water
as well as energy.
Organic Substances. About 7 to 9 per cent of the plasma of human
blood consists of proteins. These take part in keeping the volume of blood
constant in the vessels, in giving thickness or viscosity to the blood, in holding
back too great seepage from the vessels, and in maintaining normal blood
pressure. Fibrinogen is unique among the proteins in its essential role in the
coagulation of blood. Serum globulin is associated with the development of
198 THE INTERNAL ENVIRONMENT OF THE BODY Part III
substances that create immunity to disease. Amino acids, the building stones
of protein, arc the materials tor building and repairing tissues.
Blood Sugar. Glucose, the fuel necessary for most cell metabolism, must
always be available in proper amounts. It is never excreted by the kidney
until the amount in the plasma is excessive, as it is in diabetic conditions
when the insulin from the pancreas fails to provide for its complete use in
the body. Uncomfortable and serious conditions follow if blood sugar falls
to half its normal amount. This drop occurs only in certain illnesses, because
normally the glycogen stored in large amounts in the liver is converted into
glucose as need arises. Among other organic substances in the plasma are
urea, uric acid, and fats absorbed through the walls of the intestine.
Inorganic Substances. Sodium chloride (table salt), the commonest salt
in the blood plasma, is continuously taken in with the usual diet and lost in
urine, in sweat, and the lachrymal fluid which keeps the eyes moist and is
known to everybody as tears. Salt hunger is persistent in all animals, espe-
cially those that live on plant diets, usually low in salt. Much was heard about
the salt hunger of East Indians at the time of their rebellion under Gandhi's
leadership against the British salt tax. Wild animals will take great risks in
order to reach a saltlick.
Calcium is also an essential substance for metabolism, for deposition in
bones, and coagulation of blood. The control of the amount of calcium in
Table 12.1
Important Constituents of Human Blood Plasma
Constituents
Role
Fate
Water necessary for life
Excreted by kidneys.
of all cells
lungs, sweat glands
Transports all sub-
Water
stances
(90 per cent of
plasma)
First importance in
maintaining blood
pressure and the con-
stancy of other com-
ponents
Proteins
Fibrinogen
Major role in clotting of
Some used when blood
(7 per cent of
blood
coagulates
plasma)
Albumin and
Associated with sticki-
Albumin sometimes ex-
globulin
ness or viscosity of
creted by kidneys, not
blood
normal
Globulin concerned with
differences of blood
groups
Nonprotein
Urea
Waste substances in
Excreted by kidneys
nitrogenous
Uric acid
transport to excretory
substances
Creatin
organs
Chap. 12 CIRCULATION AND TRANSPORTATION BODY FLUIDS
Table J 2.1 (Cont'd)
199
Constituents
Role
Fate
Amino acids
Food in transport to
cells
Building materials of
new protein
Some excreted by kid-
neys
Nonnitroge-
nous sub-
stances
Phosphatides
(fatty com-
pounds)
Food in transport
Important to cells in
coagulation of blood
Sugar (glucose)
Food in transport to
cells
Burned to carbon diox-
ide and water; energy
released
Stored in liver, muscles,
some excreted
Fat
Food in transport
Burned to carbon diox-
ide and water, excess
stored
Cholesterol
Quantities in nervous
tissue and adrenal
glands
Part excreted in bile
Lactates
(products of
sugar break-
down)
Associated with con-
traction of muscles
Burned to carbon diox-
ide and water; some
reconverted to glyco-
gen
Salts
(0.9 per cent of
plasma)
Compounds of so-
dium, potassium,
calcium, magne-
sium, and iron
Changes in concentra-
tion of various salts
result in profound
changes in activity of
cells
Calcium acts in coagu-
lation of blood
Iodine important to thy-
roid gland
If in excess they are ex-
creted
Special sub-
stances
Enzymes
Hormones
Regulation and coordi-
nation of activity of
cells, tissues, and
organs
If in excess hormones
appear in urine
Antibodies
Act on bacteria and
foreign proteins
May appear in excretions
Gases
Oxygen
Oxidation
Carried mainly in loose
combination with
hemoglobin in red
blood cells
Burned in oxidation
Carbon dioxide
Produced by oxidation
in cells
Carried in plasma
mainly as sodium bi-
carbonate
Exhaled
Nitrogen
An inert gas dissolved
in plasma
Diffuses into lungs and
exhaled
200 TUP INTFRNAL nNVIRONMFNT OF Tlir nODY Part III
the blood depends upon the proper amount of a hormone produced by the
parathyroid glands (Chap. \5). Potassium acts in opposition to calcium and
it is the balance between the two rather than the exact amount of either
one that is essential. In blood plasma there are about 20 milligrams of po-
tassium per 100 cubic millimeters of plasma but over 800 milligrams of
sodium; in muscle cells this proportion is reversed. If blood potassium rises
a little, muscle is stimulated; if it rises too much, the muscle is paralyzed. Its
amount is regulated partly by a hormone from the cortex of the adrenal gland.
Nervoii.s Control of Body Heat. The main control of body temperature is
in the hypothalamus, an ancient part of the floor of the brain. When this is
destroyed, the muscles are paralyzed and their ability to liberate heat is lost;
when it is stimulated, the muscles are activated and the body temperature
rises (Fig. 12.3).
Human Blood Cells
This description of blood cells is based chiefly upon human blood with
references to other vertebrates (Fig. 12.4).
There are two main kinds of blood cells, red and white ones (Fig. 12.5).
Erythrocytes, red cells or corpuscles are those whose cytoplasm is permeated
with nonliving hemoglobin. Mammalian red cells lose their nuclei as they
Fig. 12.3. Nervous control of body heat. Rabbit with left ear in normal con-
dition; the blood vessels are kept in a state of partial constriction by vasocon-
strictor nerves. The vasoconstrictor nerves to the right ear have been cut; the
vessels are dilated and the ear is unnaturally hot. (From Best and Taylor: The
Livinii Body. ed. 3. Copyrighted by Henry Holt and Co. Reprinted with their
permission. )
Chap. 12 CIRCULATION AND TRANSPORTATION BODY FLUIDS 201
become mature; in all other vertebrates they are retained. Leucocytes or white
cells are of several kinds; none of them has hemoglobin; all have nuclei. In
mammals blood also contains colorless bodies called platelets.
Red Cells. The red cells. (erythrocytes) of mammals (except camel, llama)
are biconcave discs (Fig. 12.4). It is estimated that 3000 human red cells
set in a line would make a row less than an inch long. The number in a cubic
millimeter of blood is calculated as four and a half million for women and five
million or more for men. It varies slightly during a 24-hour period, being
lowest in the early morning, and increasing through the day. In a healthy
person it is increased during exercise, at high altitudes, and with a rise in the
temperature of the environment.
In microscopic preparations and within the living capillaries red cells often
MA M M A L S.
MAN
WHALE
ELEPHANT ! MOUSE i HORSE
Fnusk deer cam el
# 1
#
# • -• i '•
0^ '
: ^ i
1 1 3200
1 i
1-3099
1-2745 1-4263 l-^eOO
1
1-12325
1 3123
- BIRDS.
HUMMING BIRD i PHEASANT 1 PIGEON
OSTRICH
Fig. 12.4. Relative sizes of red blood cells, all microscopic, of representative
vertebrates. The size of the red cells varies much in different classes of vertebrates;
that of the white blood cells, not shown here, is more uniform. Their extremely
minute size, coupled with relatively large surface exposure, is a key to the
efficiency of the mammalian red blood ceils in their intake and outgo of oxygen.
The absence of nuclei in all mature mammalian red cells allows extra room for
oxygen. (Courtesy, Guyer: Animal Biology, ed. 3. New York, Harper & Bros.,
1941.)
202
THF INTFRNAl. HNVIRONMrNT OF TUF BODY
Part III
A. Erythrocytes
(red cells)
ond platelets
_ 9
O 9 ^
V
Ci
A^
B. Life tiistory of o red cell. Immature
with nucleus (1,2, 3)- mature, front
ond side view (4,5) extruded nucleus
C. Diogrom of a
mature humon
red blood cell
cut m half
D. Leucocytes
Gronulocytes (1.2.3)
Agranulocytes (4.5,)
Neutrophils
Eosinophils Basophils
Monocyte
Lymphocytes
Fig. 12.5. Types of human blood cells in stained preparations. The cells and
their parts are different chemically and take different stains. Leucocytes D 1, 2,
and 3 are named for their reaction to dyes, e.g., 2, an eosinophil, takes the pink-
orange color of eosin. Granulocytes are named for the grainy appearance of their
protoplasm, and agranulocytes, D 4 and 5, for lack of graininess.
pile up in rolls, the rouleaux formation. Red cells are pliable and resilient,
and in blood circulating through the capillaries in the web of a frog's foot
or in a rabbit's ear, they can be seen twisting and turning at the sharp
junctions of the branches and then quickly regaining their shape (Fig. 12.6).
They are extremely sensitive to the content of fluids and to the surfaces
which they touch. Normally they are in a state of osmotic equilibrium with
the plasma. When water is added to the plasma, they absorb it, swell, and
lose their hemoglobin; when water evaporates from the plasma or salt is
added, they give up water, shrink, and appear spiny (or crenated). Physio-
logical salt solution (0.9 per cent sodium chloride) has the same osmotic
pressure as normal human plasma, which accounts for the fact that in this
fluid the red cells keep their natural shape.
Functions. Red cells function first as carriers of oxygen to cells; they also
carry carbon dioxide in much smaller amounts from the cells to the lungs or
to the gills of aquatic animals. Their color and power to carry oxygen is
due to hemoglobin which composes about a third of the content of each cell.
Hemoglobin is the combination of a pigment containing iron and a protein,
and is related to other blood pigments such as the bluish hemocyanin of clam
Chap. 12 CIRCULATION AND TRANSPORTATION BODY FLUIDS 203
blood that creates the blue-gray tinge of clam broth. Hematin, the most im-
portant pigment in the higher animals, is a near relative of the pigment of
chlorophyll, the substance in plants which can utilize energy from the sun.
In the lungs and in the- gills of aquatic animals where oxygen pressure is
high, hemoglobin combines with oxygen and forms oxyhemoglobin, an un-
stable combination which colors the blood bright red (Chap. 13). In the
Fig. 12.6. Photograph of blood vessels in ear of living rabbit. The picture
is taken through glass pressed against the skin with the camera focused into the
vessels. The red cells are moving through the capillaries in rolls (rouleaux), the
number of these probably increased by the pressure of the glass. The extreme thin-
ness of the capillary wall is an evidence of the ease with which certain cells pass
through it. (Courtesy, Bremer and Weatherford: Textbook of Histology, ed. 6.
Philadelphia, The Blakiston Co., 1944.)
tissues of the body the oxygen pressure is low and the oxygen separates from
the oxyhemoglobin leaving reduced hemoglobin which darkens the blood,
usually in the veins. Hemoglobin also takes part in transporting very small
amounts of carbon dioxide from the tissues to the lungs or gills.
With the aid of isotopes the life span of human red cells has been shown
to be about 120 days. They wear out and fragments of them are eaten by
macrophages (phagocytic cells) in many parts of the body, especially in the
spleen, bone marrow, and liver. In a healthy human adult about one million
red cells are thus destroyed per second and a comparable number of new
ones are added per day as the blood passes through the red marrow of the
bones.
Blood Counts. By diluting a small, measured quantity of blood and
spreading it upon a special ruled slide, the different kinds of blood cells can
2U4 Till INIIRNAI. INVIKONMI N I Ol llll HOOY Part III
he counted and their proportionate numbers determined. This is a routine
examination in many doctors' oflices and hospitals.
Ikkugularitii-s in Numbers of Red Cells. Polycythemia, an increased
number of red cells, accompanies conditions in which the body fluids are de-
creased. Rarely there may be an overproduction of red cells in the red mar-
row.
Anemia. In anemia the amount of hemoglobin is below normal; either
there is too little in the red cells or there are too few of them. There are many
causes and types of anemias.
Anemias are caused by malnutrition, excessive blood loss, or destruction
of cells due to: ( 1 ) lack of iron in the diet resulting in sparsity of hemoglobin
in red cells; (2) hemorrhages from wounds, ulcers, etc; (3) defects in the
cells or poisons (hemolytic anemias); (4) an inherited condition, the Rh
factor (Chap. 20).
Anemias are caused by defective formation of cells because of failure of
red cells to develop to maturity as in pernicious anemia; or damage to red
bone marrow due to certain chemical poisons, e.g., radium salts.
The effects of various articles of diet, especially liver, on the regeneration
of hemoglobin was first noted by Dr. G. H. Whipple at the University of
Rochester. In 1926 Dr. G. R. Minot and Dr. W. P. Murphy at Harvard
University suggested that liver might be of value in treating pernicious anemia.
Although this has not proved to be a cure, it has become a treatment which
has kept thousands of persons living efficiently as long as it is continued.
It is now known that the liver discharges into the blood a substance (vitamin
Bij) essential for the blood cell-making function of the bone marrow, and
that pernicious anemia is caused by lack of this antianemic substance (Table
11.1). It is originally produced by the reaction of a specific secretion of the
stomach upon some substance in meat. The secretion by the stomach being
the inside product, it is called the intrinsic factor in contrast to the substance
in meat, an outside or extrinsic factor. Together these result in the antianemic
material which is stored in the liver, whence it is taken up by the blood. It is
this that eventually reaches the red marrow of the bones and stimulates the
development of the red cells (Fig. 12.7).
White Cells. White cells (leucocytes) look white only when several are
banked together, otherwise they are colorless. They never contain hemoglobin,
are always nucleated, are more resistant to change than red cells and exist
in smaller numbers, in human blood — one to about 600 reds. They are older
in animal history than the red cells. Colorless nucleated cells occur in the
body fluids of planarians, annelids (earthworms, clamworms), insects, and
other arthropods. In mammals, some of them originate in the red bone
marrow and others in the lymphatic tissues (Fig. 12.8). In circulating blood
their number varies with the physiological changes in the body during the
Chap. 12 CIRCULATION AND TRANSPORTATION BODY FLUIDS 205
24-hour day and in different parts of the circulatory system. Some white cells
are phagocytic. Motion pictures show them consuming bacteria, overstuffing
themselves with anything alien or broken that they can manage; even the
human appetite cannot be- so overreaching. Many white cells destroy them-
selves by their consumption of bacteria and others wander through the walls
of the intestine and are swept out of the body.
Main Types. In structure there are two main types of white cells: granu-
locytes, those with specific granules in the cytoplasm and nuclei with lobes;
Fig. 12.7. Diagram of the general location of the substances responsible for the
development of the red blood cells in the red marrow of bone. The intrinsic factor
is a specific secretion of the stomach; the extrinsic factor is a substance in meat.
Together these pass to the liver via the blood, are stored there, and in combination
are gradually given off via the blood to the red marrow of the bones. (From
Best and Taylor: The Living Body, ed. 3. Copyrighted by Henry Holt and Co.
Reprinted with their permission.)
and nongranulocytes, those without specific granules in the cytoplasm and
with unlobed nuclei (Table 12.2, Fig. 12.5). White cells are very sensitive
in their reactions to chemical conditions both in the blood and outside
the body when they are treated with stains. They are classified according to
their reactions to the latter.
Granulocytes are of three types, whose names end in phil indicating their
love or affinity for the respective stains, eosinophils (the stain, eosin), baso-
phils (basic stains), and neutrophils (neutral stains). The group of non-
granulocytes contains the monocytes and the lymphocytes (Table 12.2 and
Fig. 12.5).
Functions. The neutrophils, lymphocytes, and monocytes, together with
the phagocytic cells of the connective tissue constitute one of the body's most
important defenses against poisons and invading organisms. All leucocytes
206
Till INXrRNAI. rNVIRONMFNT OF THE BOOY
Part III
Ri^ht vocaLl cord — -\|
Thyroid _ „
ca.rtila.ge T\J
Tra.che2k.-'-^
Fig. 12.8. The upper respiratory tract of a child showing half of the ring of
lymphatic tissue, the right tonsil, the adenoid, and lingual tonsil, at the back of
the throat. All of these are relatively large in children. In mammals some white
cells originate in red bone marrow and others in lymphatic tissue. (Courtesy,
Clendening: The Human Body. New York, Alfred A. Knopf, Inc., 1930.)
are to some extent motile and move about in other tissues as well as in
the blood, the neutrophils most actively of them all. They are easily ob-
served alive in microscopic preparations and when properly warmed, behave
like so many active amebas. Neutrophils, lymphocytes, and monocytes wan-
der among the cells of the body, rapidly working their way in and out
through capillary walls with scarcely any place barred to them (Fig. 12.9).
When on its way through a capillary wall, a neutrophil wedges itself between
the cells and quickly pushes them apart. Whenever neutrophils reach a place
where bacteria are present they at once proceed to engulf them, living up
to their name of phagocytes (cell eaters). Within the leucocytes the living
bacteria are killed by digestive fluids as live oysters are killed in the human
stomach.
Chap. 12
CIRCULATION AND TRANSPORTATION BODY FLUIDS
207
Table 12.2
Blood Cells
(Refer to Figure 12.5)
Kinds and A mounts of Cells
Role
Origin
Fate
Red blood cells or eryth-
Transport oxygen
Red marrow
Filtered out mainly
rocytes 4,500.000 to
and small amount
of bone
in spleen and
5,000.000 per cubic mil-
of carbon dioxide
liver
limeter
Life span figured
from tracer exper-
iments to be
about 120 days
White blood cells or leuco-
cytes
1. Granulocytes, with
granular cytoplasm
a. Neutrophils so named
Marked motility out-
Bone marrow.
May be filtered out
because they take neutral
side blood vessels
ribs, long
in spleen and live
stains— 60 to 70% of
and destroy bac-
bones
in spleen and liver
leucocytes
teria except
tubercular ones
b. Eosinophils
Migrate into tissue
Red marrow
Filtered from circu-
Take acid stains e.g., eosin
spaces of diges-
of bone
lating blood by
— 2 to 4% of leucocytes
tive and respira-
spleen and livef
tory tracts
Others may perish
Numbers increase in
outside the blood
disease caused by
vessels
parasites and al-
lergic conditions
c. Basophils
Function unknown
Red marrow
Uncertain
Take basic stains 0.5 to
No phagocytosis and
of bone
1.5% of leucocytes
motility feeble
2. Nongranulocytes
Phagocytes within
Uncertain
Most of them de-
a. Monocytes 5 to 10%
(?) and without
generate within
of leucocytes
the blood stream
the blood stream
b. Lymphocytes 20 to
Not phagocytic; oc-
Lymphatic tis-
Degenerate outside
30% of leucocytes
cur in blood and
sue and
the blood stream
lymph vessels, in
glands
tissue fluid and
probably function
difi'erently in each
place
-
Platelets estimated over
Seem to be essential
200,000 per cubic milli-
in clotting of blood
meter
208 THI-. INTIRNAL I;NV1R()N M IN I C)l I III HODV •'i'lt HI
AllliDUgh the lympluKytcs arc the greatest wanderers ol all blood cells,
little is known about their liinction. They work their way out through the
walls of blood vessels, lymph vessels, and the lining of the alimentary canal.
The tonsils and the appendix are loaded with them (Fig. 12.9). Great migra-
tions of lymphocytes accompany certain types of inllammation.
Variations in the Numbers. Certain normal physiological conditions,
among ihcm muscular exercise and pregnancy, cause an increase of leuco-
cytes. Quick shifts in temperature and in states of mind may result in their
immediate increase in the blood as if they had suddenly moved from the sides
of the blood vessels out into the currents (Fig. 12.10). There are also daily
rhythms with an afternoon rise, in order to determine what type of cell has
increased, it is necessary to make diiTercntial counts. Stained preparations
'■•
'• ■■ li^::^
Fig. 12.9. Drawings of a leucocyte (neutrophil) at one-half-minute intervals
to show its movement and ability to consume bacteria, represented by dots. Myriads
of such cells arc continually moving about in the body. (From Best and Taylor:
The Liviufy Body, ed. 3. Copyrighted by Henry Holt and Co. Reprinted with
their permission.)
of blood are examined and since different types of leucocytes stain differently,
it is easy to distinguish them. Several hundred leucocytes arc counted and the
various types are recorded separately. The percentage of each type is then
calculated.
Blood Platelets. The blood platelets (thrombocytes) are about one-quarter
the size of the red cells. They have no nuclei, seem to be fragments of certain
giant cells of the red bone marrow, and are usually clumped together. They
play an important part in the coagulation of blood, but beyond that their
function is unknown (Fig. 12.11, and Table 12.2).
Human Blood Groups. Whenever foreign proteins such as those of bac-
teria are taken into the blood stream of an animal, the cells of the body pro-
duce antibodies, counteracting substances that immunize foreign matters.
Antibodies are produced abundantly by the cells of the blood. The foreign
proteins that stimulate the production of antibodies are called antigens. "No
smoke without a fire" might be changed to "no antibody without an antigen."
Whenever one or another kind of antigen and antibody are brought to-
gether, a characteristic reaction occurs. If the antigen is a poison, the antibody
that neutralizes it is called an antitoxin. Foreign cells, such as bacteria or alien
blood cells, may get into the human blood stream by injection. Although
Chap. 12
CIRCULATION AND TRANSPORTATION — BODY FLUIDS
209
Numbers 15000
of cells
Time
Total
Leucocytes
Neutrophiles
Lymphocytes
Chylomicrons
(fat)
8 A.M.
Fig. 12.10. Increased number of leucocytes in the blood due to emotional dis-
turbance in afternoon after receiving a letter from fiance. Curves showing total
number of white blood cells, of neutrophils, of lymphocytes, and of fat particles
(chylomicrons), the last not concerned in the disturbance. (Data, courtesy
Smith: "The Absence of Digestive Leucocytosis," Folia Haemotologica, Leipzig,
1932.)
human blood, it may be the wrong type. In this case the red cells carry an
antigen (agglutinogen) which reacts with an antibody (agglutinin) already
present and the blood cells are agglutinated, i.e., stuck together in clumps
(Table 12.3).
In 1900 Karl Landsteiner, experimenting in a medical laboratory in Vienna,
discovered that when the blood cells of some persons were mixed with the
blood plasma of others, the cells remained separate insome cases and clumped
together in others. This was the beginning of the discovery of blood groups
which made possible the transfusion of blood from the blood vessels of one
person to those of another. Before this, the unexplained and sometimes fatal
results of transfusion made it a last resort. The clumping of the red cells of
incompatible bloods plugged up the blood capillaries and ultimately caused
death. The bloods of donor and patient, therefore, must be compatible. Tests
have shown that two kinds of antigens (agglutinogens), called A and B,
210 Till INTI.RNAL LNVIRONMnNT OF T HF. BODY Part III
occur in the red cells of dilTercnt persons and that the plasma of the blood
contains two kinds of antibodies (agglutinins), a and h. There are four main
blood groups among human beings: Group O with antibodies a and b but
no antigens is a universal donor; Group A, antibody b and antigen A; Group
B, antibody a and antigen B; and group AB, antigens A and B but no anti-
bodies in the plasma is a universal recipient (Table 12.3). Many more groups
have been described; this is a much simplified statement of complex reactions.
The characteristics of blood groups are inherited and remain constant through-
out life.
Table 12.3
Results of Mixing Red Cells and Plasma of Human Blood Groups
Blood Group
O
A
B
AB
Antigen in
Reel Cells
None
A
B
AB
d.
O
5
to
a,b
—
+
+
+
^
o
A
?1
b
—
—
+
+
.^
B
■<3
a
—
+
+
m
AB
X
None
—
—
—
—Compatible; no agglutination
-|-Not compatible agglutinates
Transfusion of Blood
When more than 40 per cent of the blood is lost within a short period, the
body cannot make up the loss. In such a case a transfusion is made, that is
an injection into a vein of whole blood, plasma, or serum from another per-
son in an effort to restore volume. Great care must be taken to choose com-
patible blood to inject into the recipient. Wrong types of blood cells block the
capillaries and later disintegrate; the pigment finally fills the tubules of the
kidney, ultimately causing death. Blood types are inherited according to
Mendelian laws (Chap. 20), and as a child of a blue-eyed and brown-eyed
parent may have either blue or brown eyes, so it is impossible to predict
the exact blood characteristics of a child from those of its parents.
Transfusions of whole blood are the only adequate treatment when loss of
blood is excessive. The need is usually immediate because cells are necessary
to take oxygen to the tissues. The question has always been how to have
Chap. 12 CIRCULATION AND TRANSPORTATION BODY FLUIDS 211
the blood ready for the emergency. In August 1944, refrigerated whole blood
was sent to the European and Pacific battlegrounds. The great impetus for
the use of whole blood that has continued in peacetime came with the dis-
covery of a special solution that would preserve whole blood at least 28 days.
The solution is known as ACD because it contains acid citrate which lengthens
the life of the cells, citrate of sodium that prevents coagulation, and dextrose
that provides nourishment. Great strides have also been made in the prepara-
tion and use of dried plasma. To prepare this, whole blood is centrifuged
as in a cream separator, thus dividing the cells from the plasma, which is
then frozen and dried. When the plasma is distributed, it is mixed with sterile
water just before using. Another important preparation, developed by Dr.
Edwin J. Cohn and his associates at Harvard University, was also used during
the war. This preparation included the isolation of the serum albumin which
constitutes about half of all the protein in plasma but occupies a very small
amount of space. Serum albumin was found to be mainly responsible for hold-
ing the balance of pressure between the capillaries and surrounding tissues,
and thus it counteracts effects of shock, such as failure of the circulation. In
severe shock in which there is a marked loss of blood-volume the effects on
the body are serious and complex. Capillaries are damaged and plasma and
blood cells escape into the tissues; circulation is slow and inefficient.
Clotting of Blood
The clotting process is a series of changes in the proteins and platelets of
the blood due to new conditions which arise when the organization of the
plasma is disturbed by a rough, jagged surface or by breaks in the blood
PLATELETS
FIBRIN
THREADS
RED BLOOD
CELLS
Fig. 12. 11. Fragment of a clot of blood highly magnified. It is a tangled mesh of
delicate filaments among which blood cells and platelets are entrapped. The fila-
ments are composed of fibrin produced during the clotting process, and appear
to radiate from groups of platelets. (From Best and Taylor: The Living Body,
ed. 3. Copyrighted by Henry Holt and Co. Reprinted with their permission.)
212 Tin; INII.RNAL INVIRONMINI Ol III! IU)I)Y Part III
vessels. The ehanges are climaxed by the formation of gel fibrin, the strands
of which hold the cell mass together in the clot (Fig. 12.11).
Some of the substances that take part in forming fibrin, such as calcium,
prothrombin, and fibrinogen are present in circulating blood; the others,
thrombokinase and thrombin are formed during the clotting process. At the
beginning of this process the exceedingly delicate platelets are injured and
in most cases the cells of the blood and the vessel walls as well. The substance
liberated by the decomposing platelets and broken cells is thrombokinase,
an cnzymelike clot-hastcner. The newly formed thrombokinase unites with
the calcium and prothrombin already in the blood and produces thrombin.
This second newly formed substance unites with fibrinogen, also already in
the blood and produces the fibrin whose strands hold together the cell mass
called the clot. The process may be summarized as follows:
Thrombokinase + calcium -\- prothrombin = thrombin
Thrombin -f fibrinogen = fibrin
Fibrin -\- cell mass =^ clot
The existence of prothrombin in healthy circulating blood depends in turn
upon the presence of the antihemorrhagic vitamin K (Table 11.1). This is
taken in with food and with the aid of bile is absorbed in the intestine, then
goes to the liver where it takes part in the formation of prothrombin. If
vitamin K is missing, the formation of prothrombin is prevented, clotting does
not take place, and bleeding results.
Abnormal Blood Clotting
Hemophilia is an inherited defect in blood clotting. Persons who suffer
from it are known as bleeders (Chap. 20).
Thrombosis is coagulation of the blood in any part of the circulatory sys-
tem. A coronary thrombosis is the stoppage of a coronary artery by a blood
clot; the coronary arteries originate near the base of the aorta and supply
the walls of the heart. Occasionally a fragment of a clot, called an embolus,
breaks ofT, is carried free in the circulation, and becomes lodged in the brain
or heart. In the brain a clot results in loss of memory, speech, and paralysis
of various parts of the body.
The Lymphatic System
Lymph originates from plasma that, except for its proteins, filters through
the walls of blood vessels. Outside them it becomes tissue fluid occupying
any spaces there may be among the tissues. As it fills these spaces and as
the pressure in them rises, it filters through the walls of the lymph vessels
and becomes lymph. Lymph capillaries end blindly. The ready entrance of
tissue fluid into lymph capillaries is due to their extreme thinness and deli-
Chap. 12 CIRCULATION AND TRANSPORTATION BODY FLUIDS 213
cacy. When the pressure becomes high in lymph capillaries and tissue spaces,
the fluid filters into the blood capillaries and joins the plasma.
Lymph flows in only one general direction, toward the heart. In its course
it runs through larger and larger vessels finally converging in the left thoracic
duct that empties into a large vein in the left shoulder — in man, at the junction
of the left jugular and subclavian veins (Fig. 12.12). Thus, lymph continually
filters out of the blood and then returns to it by a large inflow, as well as by a
refiltering through capillary walls. Blood flows away from the heart through
arteries and capillaries, but its fluid content returns not only as blood through
capillaries and veins, but as lymph through the lymph vessels.
Lymph vessels are provided with efficient bacteria traps in the many lymph
nodes that are located along the vessels (Figs. 12.13, 12.14). Each of them is
a labyrinth of lymphatic tissue in which lymphocytes are produced. In its regu-
lar course lymph flows slowly through these mazes populated with phagocytes
which attack and engulf such bacteria and foreign particles as may be passing
by. Dense lymphatic tissue is abundant about the throat (e.g., the tonsils) and
respiratory passages, and in the intestinal wall, places where bacteria abound.
In an infected thumb the lymph vessels become inflamed and hinder the
circulation of the blood so that red streaks extend up the inner side of the
arm to the elbow where there are good chances that the poisons may be
caught in the lymphatic tissue, kept out of the general circulation, and ulti-
mately destroyed.
Lymph moves slowly through the vessels pushed along by the volume
Fig. 12.12. The relationship between the blood and lymph circulations (the
latter in black). Arrows indicate the flow of blood to and from the heart, and
the flow of lymph always toward the heart and finally emptying into the blood,
in man mainly at the junction of left jugular and subclavian veins. (Reprinted from
The Machinery of the Human Body by Carlson and Johnson, by permission of
The University of Chicago Press. Copyright 1948.)
214
THE INTI RNAL ENVIRONMENT OF THE BODY
Part III
B
Fig. 12.13. A, The superficial lymph vessels of the thumb and finger. A small
part of the great network in which a balance of fluid is maintained with that in
the blood vessels and other tissues. B, Superficial lymph nodes in axil of arm
and throat region; both are incomplete but they suggest the prevalence of lymph
nodes. (Redrawn after Brash, ed.: Cunningham's Textbook of Anatomy, ed. 9.
New York, Oxford University Press, 1951.)
behind it, by breathing movements, and the contractions of muscles; valves
keep it from going backward just as valves do in many of the veins. In
mammals there are no lymph hearts as there are in frogs.
Blood Circulation in Mammals
The blood vessels form a complete series of intercommunicating tubes.
The heart is an enlarged and sharply bent part of a tube protected by the
pericardial sac. The tubular shape of the heart can be seen clearly in the de-
velopment of the human embryo and other higher vertebrates, and in adult
fishes (Chap. 19). In fishes the heart is continuous at one end with the
arteries that carry blood away from it and at the other with veins that return
blood to it. Connecting the larger vessels are the microscopic capillaries
usually between arteries and veins, but in the hepatic portal system between
veins and veins.
Chap. 12
CIRCULATION AND TRANSPORTATION BODY FLUIDS
215
cortex
afferent lymphatics
capsule
medulla
trabeculae
lymph
sinus
efferent lymphatic
Fig. 12.14. Diagram of a lymph node sectioned to show its internal structure,
the pocket endings of the vessels, and the valves within them that prevent back-
ward flow. The lymph spaces are shown with their usual contents of lymphocytes
(black). Such nodes are situated at strategic points on the lymph vessels and act
as filters removing bacteria otherwise entering the lymph. (Courtesy, Nonidez and
Windle: Textbook of Histology, ed. 2. New York, McGraw-Hill Book Co., Inc.,
1953.)
Main Circuits. Circulating blood goes through two main circuits, the pul-
monary route from the heart to the lungs and back to the heart, and the
systemic route from the heart over a long course through the body and back
to the heart (Figs. 12.15, 12.16). It is calculated that in man the complete
double circuit of the blood takes about 23 seconds.
Two factors are of great importance in the movement of blood. The first is
the pumping of the heart and the second, the resistance that the blood en-
counters along the sides of the vessels and at the forking of their manifold
branches. Blood that is pumped from the right ventricle into the pulmonary
artery to the lungs and back to the left auricle meets with little resistance in
this short circuit as compared with that in the long route through the aorta
and over most of the body. With the contraction of the left auricle, blood
flows into the left ventricle, the main pumping chamber of the heart (Fig.
12.17). The contraction of this chamber forces the blood past the semilunar
valves into the aorta. From there it begins the great systemic circuit through
several arteries: to the head through the carotids, to the arms through the
subclavians, and to the viscera, trunk, and legs through the dorsal aorta.
Two special parts of this circuit are of most vital importance. The first is
the circulation in the walls of the heart. Although the chambers of the heart
are continually receiving blood, the action of the muscle in its walls depends
upon the coronary arteries from the aorta for an income of blood, and upon
the coronary veins emptying into the right auricle for its outgo. Any reduction
of blood to these muscles cripples the heartbeat and a complete block of
210
III! INIIKNAl. LNVIUONMHNT OI rill BODY
Pari III
uppr/t
VENA -
AORTA
Fig. 12.15. Scheme of the main circulation of the human blood. The vessels
carrying well oxygenated blood are in outline; those containing blood poor in
oxygen are in heavy black. The vena cava and the aorta actually course along the
mid dorsal line of the body but have been pulled aside for labeling. Stomach and
right kidney are omitted for space. ( Reprinted from The Machinery of the Human
Body by Carlson and Johnson, by permission of The University of Chicago Press.
Copyright 1948.)
blood in them stops it. The second important part of the circuit is to the liver.
This important organ receives both arterial and venous blood; the latter repre-
sented by the hepatic-portal vein is the unique feature (Fig. 12.15). It carries
food-laden blood from the small intestine and blood from the spleen and
pancreas directly to the liver and there breaks up into the hepatic-portal sys-
tem of innumerable capillaries that eventually converge into the hepatic veins.
These carry blood into the postcaval vein and on to the right auricle.
In the systemic as in the pulmonary circuit the flow is from larger to smaller
arteries on into the capillaries; thence it goes into larger and larger veins and
on until it empties into the heart.
Whatever the region of the body, blood stays longest in the capillaries and
is there constantly engaged in exchanges with the surrounding cells and fluids
(Fig. 12.18). In the capillaries it distributes the supplies for metabolism, foods
and oxygen, and receives the products of metabolism, carbon dioxide and
nitrogenous by-products. These are but the high points in the complex capillary
cell and tissue fluid exchange.
The complexity of the internal aquatic environment of the body results
from the liquid that penetrates through capillary walls. This has never been so
Chap. 12
CIRCULATION AND TRANSPORTATION- — BODY FLUIDS
217
Exchange station
(Blood and tissue fluid)
Pump to
keep lilood
in motion
(Heart)
station
carbon dioxide
Renewal
station
hormones
(Endocrine glands)
Renewal station Removal station Removal station
Food Waste -f products Excess f heat
\
\
/
Digestive tract
Kidneys
Skin
Fig. 12.16. Diagram illustrating how a suitable environment within the body
is maintained by the circulation of the blood. (Courtesy, Woodruff and Baitsell:
Foundations of Biology, ed. 7. New York, The Macmillan Co., 1950.)
well demonstrated as by experiments with isotopes made at the Carnegie In-
stitution of Washington and reported by Dr. G. W. Corner as follows:
The use of substances such as heavy water and radio-active salts, differs little
from ordinary water and salts in their physiological activities but are easily identi-
fied as they travel through the body by their weight or radioactivity respectively.
In man, 78 per cent of the blood-plasma sodium and 105 per cent of the plasma
water is exchanged per minute with extravascular sodium and water. An amount
of water equal to a man's entire weight passes out of his blood capillaries, and is
replaced by an approximately equal amount, every 20 minutes. The capillary part
of human blood circulation, seen in the light of these facts, is a system of fine
tubules with permeable walls through which floods of water bearing salts and other
metabolic substances are pouring at every moment throughout life.*
Control of the Heartbeat. The heartbeat is under two nervous directives: a
control by the neuromuscular mechanism and a control by the central nervous
system. The neuromuscular control is the one that may act for some time after
the heart of a frog or a mammal is completely separated from the body. Thus
the neuromuscular control can act without the central nervous control, but
the latter cannot act without the neuromuscular control.
Neuromuscular Mechanism. Figure 12.19 shows the important features
of the mechanism. The sinuauricular and auriculoventricular nodes are net-
works of atypical muscle cells (Purkinje cells), just visible to the naked eye,
" From Annual Report of the Director of the Department of Embryology. Carnegie
Institution of Washington, 1948-49, p. 129.
218
nil INTLRNAL ENVIRONMENT OF THi: BODY
Part III
-aoHa
pre-
cava
righi
auricle
semilunar
valves
incuspid valve
righi ventricle
Fki. 12.17. Diagram of the human heart with the front wall removed. Heavy
stipple, poorly oxygenated blood; light stipple, richly oxygenated blood. A, Auricles
filling from veins, i.e., right, precava and postcava; left, pulmonary veins. B, Blood
entering relaxed ventricles. C, Auricles contracting, ventricles relaxed and filling.
D, Ventricles contracting, blood forced into aorta and pulmonary arteries. (Cour-
tesy, Storer: General Zoology, ed. 2. New York, McGraw-Hill Book Co., Inc.,
1951.)
that conduct the heartbeat. The sinuauricular node, or pacemaker, is named
from its origin from the edge of the sinus chamber present in mammalian
embryos and adults of lower vertebrates (see frog heart); the similar auriculo-
vcntricular node is named from its position between these chambers. Muscle
fibers from the sinus node spread through the walls of the auricles but are not
shown in the diagram because of the thinness of the walls. Muscle fibers from
the auriculoventricular node extend through the septum between the ventricles
(auriculoventricular bundle) and spread throughout the walls of the ventricles.
The auriculoventricular bundle of muscle and nerve-cell fibers is the functional
bridge between the auricles and ventricles.
The Pacemaker of the Heart. The neuromuscular mechanism is re-
sponsible for the conduction of the rhythmic contractions of the muscle of
the heart. The pacemaker is the dynamic center of the heart's action. In some
way not well understood, rhythmic stimuli develop from it and spread in
waves of contractions through the walls of the auricles. From there waves of
contractions spread through the auriculoventricular bundle of fibers in the
septum and thence throughout the ventricular walls. If this bundle is cut ex-
perimentally or damaged by disease, the ventricles either stop beating or beat
independently of the beat in the auricles.
Central Nervous Control. The heart is profoundly a part of the body
as a whole. It can beat temporarily when separated from the body, but the
way it normally beats, weak or strong, slow or fast, is affected by conditions
Chap. 12
CIRCULATION AND TRANSPORTATION BODY FLUIDS
219
Fig. 12.18. Network of blood vessels in the web of a frog's foot, darker ones
with venous and paler with arterial blood; a, arterioles; v, venules; x, a direct con-
nection between arteriole and venule. Many pigmented cells are scattered along the
capillaries. A view of the circulating blood in such a network can be easily ob-
tained by extending the moist web of a lightly anesthetized frog on the stage of
the microscope. (Courtesy, Maximow and Bloom: Textbook of Histology, ed. 6.
Philadelphia, W. B. Saunders Co., 1952.)
in parts or the whole of the body. Cold or heat, food or lack of food, work or
rest, and turns of mind, such as mirth, fear, and worry are all significant in-
fluences, and worry is the most devastating in its effects. Sensory nerve fibers
in the arteries, especially in the arch of the aorta, also contribute to the rate
of the heartbeat.
Stimulation of the paired inhibitor nerves (parasympathetic branches of the
vagus) slows the heartbeat. The impulses come from the vagus centers in the
medulla and pass over the nerve fibers into the walls of the heart, in and near
the pacemaker nodes (Fig. 12.19). Stimulation of the paired accelerator
nerves (sympathetic) quickens the heartbeat. Both nerves produce hormone-
like substances. Acetylcholine from the vagus nerves slows the action of heart
muscle and an adrenalinlike substance from the accelerator nerves quickens it.
The inhibitor and the accelerator nerves have been called the reins and the
220
TUP INTPRNAL PNVIRONMPNT OF THE BODY
Part III
whip of the heart, the inhibitor (parasympathetic) curbing its speed, the
accelerator (sympathetic) whipping it up.
Blood rrcssiire. Rvcry aspect of blood pressure depends upon the pumping
of the heart that works against the friction of the blood vessels, and gravity.
The nearer the blood is to the pump, the greater the pressure upon it. Farther
away from the pump with more and smaller vessels, the friction increases and
AV node
Pace-mahc
A-V bundle
Fig. 12.19. The neuromuscular mechanism that establishes the rhythm of the
heartbeat. Diagram of a frontal section of the heart. During each beat a wave of
contraction begins in the peculiar muscle fibers in the pace-maker (sino-auricular
node) and spreads through them in the walls of the auricle (arrows); another
wave of contraction begins at the auriculo-ventricular node (A-V), spreads through
the A-V bundle of muscle and on throughout the walls of the ventricles. If
either of the nodes is damaged, the auricles continue to beat normally but the
ventricles stop beating or beat irregularly. (Courtesy, Gerard: The Body Func-
tions. New York, J. Wiley and Sons, 1941.)
the energy of pressure is expended in overcoming it. In the capillaries the
friction is enormous and the drop in pressure correspondingly great. In the
veins no pressure is regained until just before the blood enters the heart.
Pressure in the Vessels, Arteries. In the arteries blood travels by
spurts since the pressure upon it increases each time the heart contracts
(systolic pressure) and decreases each time it relaxes (diastolic pressure).
The pulse is an expression of the uneven pressure upon blood in the arteries
(Fig. 12.20). It is a wave of the muscular contractions that begins in the left
ventricle and spreads throughout the arteries. The contraction of the left
ventricle sends a spurt of blood into the aorta that swells out the walls. The
spurt of blood is immediately squeezed forward by the rings of muscle behind
it and by the action of the elastic tissue in the arteries. By this time the
ventricle has contracted again and another lump has started along the aorta.
Thousands of these little lumps are constantly moving in processions over
the arteries. The rate at which they move past a certain spot is the pulse,
Chap. 12
CIRCULATION AND TRANSPORTATION BODY FLUIDS
221
f...^.
Endothelium
'■:'^.:^
>)^>^- Connective »'«"«^5;^^^^,^^W
''■^-■f- Muscle layer . I'^^t^Yl^^^^Wt^
Endothelium'
CAPILLARY
Endothelium
VEIN
ARTERY
Connective tissue
Muscle layer
Endothelium
Fig. 12.20. Three types of blood vessels. In the muscle layer of an artery the
contraction of the heart is actually continued (pulse); in the thinner muscle layer
of a vein it is not. In the cavity of an artery blood is under more pressure than
in the larger cavity of a vein of the same size. The walls of capillaries, but one cell
thick, make the income and outgo of substances easy. (Courtesy, Hegner and
Stiles: College Zoology, ed. 6. New York, The Macmillan Co., 1951.)
usually taken on the radial artery at the wrist; its rate is identical with the
heartbeat, in adults about 70 times per minute.
Capillaries. When blood enters the capillaries, it encounters a network
whose combined caliber is greater than that of the artery from which it came,
thus bringing it under less pressure. It drags along their walls and runs against
the forkings of their branches. Its energy of motion is continually dissipated
in the heat of friction (Chap. 2). At the arterial end where pressure is
higher, water is pushed out of the capillaries into the tissue fluid; at the
venous end where pressure is lower, water from the tissue fluid is taken back
into the capillaries. Thus the water content of the plasma is kept constant and
that in the tissue fluid is continually refreshed.
Veins. Nearer the surface of the body than arteries, veins are thin-walled,
extensible, and the larger ones are provided with valves that prevent backward
flow (Fig. 12.21 ). Blood is pushed through the veins by the pressure of more
blood coming from the capillaries, by the movements of skeletal muscles, and
by the motions of the body in breathing. Most veins are surrounded by skeletal
muscles; when they contract the veins are collapsed; when they relax, the
veins refill and the blood continues flowing toward the heart. This "milking"
motion helps the venous flow of blood just as it does the flow of lymph. It is
222 THE INTERNAL ENVIRONMENT OF THE BODY Part III
especially important in returning blood from the legs against the pull of
gravity. If a person stands still for some time, the blood in the veins is not
circulated properly and the feet and legs swell. If the same person is walking,
the contractions of muscles force the blood onward and no such swelling
results. Breathing greatly aids venous flow. When the chest muscles and
diaphragm contract, the space in the chest cavity is increased, and the pressure
within it is lowered to such an extent that air enters the lungs freely and blood
enters the right auricle.
Vasomotor Control. When one part of the body, e.g., the skeletal
Vein spread open, cups Section of vein showing
hanging from wall. valves closed preventing
backward flow.
2- n**^»\t.iffff„ffffi
Toward
heart
^i2.
2> ^
heart
^^
Valves open
Frc. 12.21. The valves of the veins which prevent a backward flow of blood.
They were used by William Harvey ( 1578-1657) in his argument that the blood
continually circulates through the body in one direction.
muscles, the brain, the stomach, is especially active, it receives an extra amount
of blood. The walls of arterioles contain smooth muscle innervated by two
sets of nerves. An increase in the number of impulses in one set of these nerves
(vasoconstrictors) causes the muscles in the walls of the arterioles to contract,
decreasing the size of the vessels, and lessening the blood supply. An increase
in the impulses in the other set (vasodilators) causes the muscles to relax and
increases the size of the arterioles and the consequent flow of blood within
them. Ordinarily, these muscles are partially contracted, due to a balance of
the impulses in both sets of nerves.
Chemical Control. Arterioles are also affected by carbon dioxide and
epinephrine. When muscles are very active, e.g., as in running or sawing
wood, their highly increased output of carbon dioxide acts on the smooth
muscle of the arterioles, causing them to relax. Arterioles are enlarged and the
blood supply to the hard-working muscles is increased. Epinephrine relaxes
the muscles in the walls of arterioles in skeletal muscles, but contracts those
in the internal organs such as the stomach and intestine.
William Harvey and the Circulation of Blood
For upwards of 2000 years human blood was believed to ebb and flow in
the vessels like the tides of the sea. Capillaries were unknown, because of the
Chap. 12 CIRCULATION AND TRANSPORTATION BODY FLUIDS 223
lack of microscopes, and arteries seemed to be always empty except for air.
In 1628 William Harvey, an English physician, showed that the blood moves
"as it were, in a circle." If wc consider the quantity of blood that is thrown
out by the heart every min-ute (approximately two ounces times 72 beats), he
said, "where can it go unless it circulates?" This argument was set forth in
Harvey's great work, De Motii Cordis {On the Motion of the Heart) published
in 1628, eight years after the Pilgrims landed in New England. This small book
opened the door to modern medical treatment. It is a record of observations
and experiments made by an adventurous and reasoning person.
13
Tlie Release oi Energy-
Respiration
The respiration of living organisms depends upon gases that originate
mainly in the atmosphere. Whether they are in the atmosphere or dissolved
in water, the conditions that govern these gases deeply affect the lives of plants
and animals.
Air. The earth is completely surrounded by the atmosphere, a covering of
mixed gases and water vapor. It is about 100 miles deep and is held to the
earth by gravity. In dry air the mixture of gases is mainly nitrogen, approxi-
mately 78 per cent, and oxygen, 21 per cent (Fig. 13.1). The other one per
cent is carbon dioxide with minute amounts of hydrogen, helium, argon, and
some other rare gases. When water vapor is abundant, the air is humid. In
dry air the proportions of gases do not change at different atmospheric levels,
but the total amount of gases does. At low levels molecules bombard one
another at close quarters. With lessened pressure the gases expand and the
molecules are not even near neighbors. At greater and greater heights there
is less and less gas in the air.
Atmosphere is piled up on the surface of the earth like hay in a stack
(Fig. 5.14). The hay at the bottom bears the pressure of all that is above it.
This pressure is evenly distributed within and without in all directions. The
distribution prevents the existence of weight in its ordinary sense although
atmospheric pressure is usually expressed in terms of weight. It is calculated
as the weight of a column of air one inch square and reaching from sea level
to the upper limit of the atmosphere. At sea level it is 14.7 pounds per square
inch. The pressure upon the air drives it into the lungs and acts as the first
step in inhaling. At 18,000 feet it is not strong enough to force the oxygen
from the lungs into the blood, the next necessary step. The effects of oxygen-
lack in the body have long been known as mountain sickness, the weakness,
224
Chap. 13 THE RLLEASE OF ENERGY RESPIRATION 225
CARBON DIOXIDE
AND OTHER GASES
Fig. 13.1. Proportions of gases in a relatively dry atmosphere at sea level.
Actually the average atmosphere contains a variable amount of water vapor,
usually one to five per cent, which slightly changes the proportions given here.
dizziness and unconsciousness that have overcome many mountain climbers.
At the top of Mount Everest 29,002 feet, the highest mountain in the world,
the air pressure is only 4.4 pounds.
The commonest way to adjust to high altitudes, especially in airplanes, is
to increase the oxygen content of the air by breathing through a mask con-
nected with an oxygen tank. Beyond 38,000 feet even breathing pure oxygen
is not enough because the atmospheric pressure is too low to drive any gas
into the blood. At this height it is necessary to have a hermetically sealed
plane, a pressure cabin, which confines the higher pressure caught in it at
lower levels. Those who have lived in low countries and later moved to high
mountains (14,000 feet or more) usually find that they are weak and short-
breathed. The red blood cells of the newcomers are too few. The usual im-
mediate reaction is in the spleen which contracts and forces its store of red
cells into the circulation, creating a sudden increase in the number of blood
cells in the peripheral blood. This is followed by further increases due to the
formation of blood cells in the red bone marrow.
Of the other atmospheric gases only carbon dioxide is directly active in
respiration. A very minute amount of it in the blood is necessary to stimulate
the mechanism of breathing; more than that is a poison and is normally elimi-
nated.
Nitrogen forms the great bulk of air, takes no part in respiration but is of
necessity inhaled and exhaled in breathing and is regularly present in the blood
as a dissolved inactive gas. When pressure on the body is suddenly lifted nitro-
gen comes out of solution and forms bubbles in the blood, in the joints and
226 Tin: INTERNAL r.NVlRONMHN T OF THI- BODY Part III
lungs, and under the skin (Fig. 13.2). The condition is well known to divers as
the bends or the caisson disease. It can be prevented by bringing them to the
surface in a series of decompression chambers, so that the adjustment of the
nitrogen content of their body fluids to ground level pressure is gradual and
harmless.
Water. Wherever water comes in contact with air it absorbs gases that be-
come dissolved in it. Thus air and water are continually being mixed at the
surfaces of all bodies of water. In lakes and seas the aerated water is rolled
under by the winds and distributed by currents to considerable depths. Green
plants, included in the microscopic plankton, contribute to the dissolved oxy-
FiG. 13.2. Bubbles of nitrogen in the veins of animals subjected to very low
atmospheric pressure. This is aeroembolism, produced by rapid decrease of pres-
sure such as occurs in aircraft flights to high altitudes and is marked by the
formation of nitrogen bubbles in the fluids and tissues of the body, especially in
fat. (Courtesy, Armstrong: Principles of Aviation Medicine, ed. 2. Baltimore,
Williams and Wilkins Co., 1943.)
gen. The respiratory gases are present in water as in air. Though there is much
less of it, dissolved oxygen takes the same important part in aquatic respira-
tions; so do small amounts of carbon dioxide. Nitrogen in water is an inactive
passenger as it is in air. Although water is a combination of hydrogen and
oxygen (H-O), this oxygen is chemically locked and living organisms cannot
utilize it for respiration.
Respiration Liberates Energy. From mankind to the simplest animals and
plants all direct or aerobic (with air) respiration depends upon free oxygen.
The more complex the animal, whether race horse or hummingbird, and the
greater its activity, the more constant is Us dependence upon respiration.
Respiration is above all the process by which plants and animals, with oxy-
gen as the key, release the energy locked up in food. The oxidation of food
is a biochemical process in which oxygen unites with carbon and hydrogen,
forms carbon dioxide and water, and sets free the energy that once came from
Chap. 13 THE RELEASE OF ENEROY^RESPIRATION 227
the sun. Everybody sees the oxidation of dead cells in a burning cigarette,
with the energy escaping in light and heat. In chemical terms it is expressed as:
C,iHi,.0« + 60, -^ 6CO, + 6H,0 + energy
carbohydrate + oxygen yields carbon -f water -f energy
dioxide
Respiration may a^so occur without air, that is, anaerobically. When at-
mospheric oxygen is absent, oxidation is incomplete; only part of the energy
is released and certain intermediate compounds are formed. Anaerobic respi-
ration is a phase of the ordinary respiratory process rather than an entirely
different kind. It occurs in certain bacteria and in yeast cells. It is well known
and important in mammalian muscle. The ability of muscles to work for a
short time without oxygen is one of their most important characteristics (Chap.
10). This doubtless always occurs in athletic contests and in horse races.
Arrangements for Respiration
The simplest respiratory arrangements are in aquatic animals, usually small
ones (Fig. 13.3). The covering of these animals is thin and outspread. The
Philodina
ROTIFER
Stentor
PROTOZOAN
Bosmina
CRUSTACEAN
Nais
ANNELID WORM
Fig. 13.3. Minute aquatic animals whose size and relatively large exposure of thin
membranes allow adequate diffusion and exchange of respiratory gases.
bodies of protozoans, planarians, rotifers, and minute worms are thread-
shaped, branched, and star-pointed, with crevices and outriggers that welcome
oxygen. In all of them respiration is direct. Gases diffuse directly from water
into the cells and vice versa. Although they are much larger animals, sponges
and jellyfishes also depend upon direct respiration. They are able to do this
because they are extremely water-saturated, and their bodies are interlaced by
passageways through which circulating water distributes gases directly to and
from the cells.
228
Tlir INTI.RNAL ENVIRONMI-NT OF THII BODY
Heart
— Carapace
Part III
Pericardia
cavity
Muscle
Branchiae
arthro
Arteries
sternal
ral sinus.
d in open
ce.
FEMALE CRAYFISH
Cross section of the body through the heart-,
arrows indicate the course of blood flow.
Fig. 13.4. Respiratory organs of invertebrates. A, Blood gills of crayfish visible
in a cross section through the thorax. After its passage from the heart and through
various arteries the blood flows free in the tissues, through the sternal sinus,
thence throughout the vessels of the gills and back to the heart. (After Storer:
General Zoology, ed. 2. New York, McGraw-Hill Book Co., Inc., 1951.)
Gills. Gills are the characteristic respiratory organs of aquatic animals. The
majority of them are outgrowths of the body wall that contain circulating
blood (Fig. 13.5). Those of immature aquatic insects contain air tubes (Fig.
13.4B). Blood gills are most commonly near the head, associated with the
pharynx as in fishes. Experiments on the larvae of Salamandra showed
the responsiveness of the gills to their environment. When these larvae were
kept in highly oxygenated water, their gills grew very slowly, while in control
animals kept in water poor in oxygen the gills grew very large, as if striving to
satisfy the body's demand for oxygen (Fig. 13.5). In spite of this attempt at
compensation the metabolism was reduced and the growth of the body was
slowed.
The gills of fishes are in the sides, apparently of the mouth cavity, actually
of the pharynx. When a fish breathes it opens its mouth and the fleshy skin
Chap. 13
THE RELEASE OF ENERGY RESPIRATION
229
Fig. 13.4. Respiratory organs of in-
vertebrates (continued). Tracheal gills of
the aquatic stage of a mayfly (Epeorus).
The tracheae contain gases but ordinarily
no fluid. The leaf-like gills extend from
the abdomen; their movements are quick-
ened whenever oxygen is sparse. The
tracheae are visible as a tracery of dark
lines upon each gill and in the body. Total
length of insect, one inch.
B
folds (oral valves) are bent backward allowing the water to pour in (Fig.
13.6). The water at once expands the cavity and presses the folds together.
The esophagus is contracted so that little or no water is swallowed and for a
moment it is also prevented from moving out through the slits at the sides
of the mouth (pharynx) by the closure of the opercula (Figs. 13.6, 13.7).
The floor of the mouth is then raised, the opercula are lifted, and the water
escapes through the gill slits. As it does so it washes the slender filaments of
the gills (attached to the gill arches) that contain the circulating blood. This is
the moment when the exchange of gases takes place. As soon as the water
passes out of the mouth (pharynx), the opercula close and another breathing
action begins. Each time that water passes over the gills, food contained in it
is caught on the strainers called gill rakers (Fig. 13.7). Fishes do not neces-
sarily close their mouths when breathing, but simply open them wider when
they inhale water.
Gills are significant only in connection with the circulation (Fig. 13.6).
The two main chambers of the heart of fishes lie below the pharynx. Venous
2.^0 Tlir INTI.RNAL r.NVIRONMINT OF Mir BODY Part III
blood passes from the general circulation into the sinus venosus, thence
into the auricle, and on into the muscular ventricle which forces it forward
via the bulbus arteriosus into the ventral aorta, and then into four pairs of
afferent branchial (or gill) arteries. Branches from each of these enter the
gill filaments where they divide still further into capillaries (Fig. 13.6).
These are the scene of the exchange of gases between the water and blood.
The blood comes to the gills with its oxygen low and its carbon dioxide
high; it leaves the gills by the efferent branchial arteries with these qualities
reversed. It flows over the body, entering the great dorsal aorta first, then
goes through many branches distributing oxygen and receiving carbon
dioxide. Finally it reaches the heart and again takes the direct route to the
gills.
It is important to note that the mouth (pharynx) of fishes is a single road
B
Fig. 13.5. Respiratory organs of vertebrates. Left, Larva of spotted salamander
(Amblystoma) with blood gills. /, gills; 2, fin; 3, balancers; 4, legs. Right, Blood
gills of salamander larvae showing responses to differences in the amounts of dis-
solved oxygen in the water: A, after living in water poor in oxygen; B, control
animal, after an equal time in water rich in oxygen. (After Drastich. Courtesy,
Krogh: The Comparative Physiology of Respiratory Mechanisms. Philadelphia,
University of Pennsylvania Press, 1941.)
for breathing and swallowing. This is true from frogs to man except that the
air route from the nose crosses the food route from the mouth (Fig. 13.8).
The crossing is awkward. Crumbs go down the windpipe when it is not quickly
covered. This happens often enough to give everybody experiences in that
variety of choking.
Lungs. More difficulties are involved in absorbing oxygen from air than
from water. Since living organisms are largely composed of water their thin
membranes are soon dried and useless if exposed to air. Except for this, air
breathing has great advantages, because air is richer in oxygen than water,
holding about 20 times more. This is a boon for greater activity and a higher
rate of metabolism, expressed especially by birds and mammals.
Lungs are the tools by which air-breathers have so successfully tapped the
oxygen supply. They have progressed toward greater efficiency by increase of
area, by greater diffusion of gases, and by efficient ventilation of the cavity of
the sac. The increased diffusion area has come with the enlargement of the
Chap. 13
THE RELEASE OF ENERGY RESPIRATION
231
Heart
A operculum removed
exposing gills
B Circulation through tieart and gills
Body Operculum Gills
wal
Upper
jow
C Detail of circulation in gill
Mouth
cavity
Capillaries
D'
Esoptiogus
Blood
from heart
to gill
from gill ' r/,^_^
to body
Gill arch
Raker
E E'
D,E Horizontal section D,E Vertical section
from right to left dorsal to ventral
D,D Intake of water E,E Outgo of water
Fig. 13.6. Diagrams to show how a fish inhales and exhales water, i.e., breathes
and where the exchange of oxygen and carbon dioxide between water and blood,
i.e., external respiration mainly occurs. A, B, C; The structures are typical ones of
a bony fish. D, £>'; As the valves on the upper and lower jaws open, water flows
in and fills the cavities of mouth and pharynx; it passes between the gills and floods
over them but momentarily cannot escape because the operculum and its membrane
stop the rear passage on each side. This is the moment of the exchange of oxygen
and carbon dioxide between the water and blood, possible because the blood is
circulating through the capillaries in the hundreds of gill filaments. E, E^; The
valves of the mouth are closed; the opercula press inward and the water lifts the
rear membranes which opens the back passages for its escape. With these move-
ments completed, the fish has taken a full breath of water and is ready for another.
232
THF- INTI.RNAL FNVIRONMENT OF TUT. BODY
Part III
esophagus
pharyngeal
gill slits
tongue
Fig. 13.7. View into the open mouth of a fish (barracuda) showing the gill
slits and arches in the walls of the pharynx. (Courtesy, Weichert: Anatomy of
Chordates. New York, McGraw-Hill Book Co., Inc., 1951.)
lung and the extension of its inner layer by partitions. In frogs the partitions
form alcoves, in toads open rooms, and in mammals the respiratory space is
completely divided up into minute cavities, the alveoli (Figs. 13.9, 13.10). In
each of these successive arrangements more area for blood capillaries is se-
cured. Increase in the numbers of blood capillaries parallels greater diffusion
of gases. Nerves, connective tissue, and lymphatic capillaries are also present
in highly developed lungs.
The ventilation mechanism differs in various classes of vertebrates. With
tightly closed mouths frogs take air through the nostrils and into the mouth,
and by contracting the throat, press it into the open glottis, actually swallow-
ing air into their lungs. Reptiles enlarge the body cavity by pulling the par-
tially folded ribs forward. Air is then drawn through the nostrils, windpipe,
and into the lungs because of the reduction of pressure in the body cavity
around them. In birds the mechanism is complicated and, for the full action
of the lungs, depends largely upon the movements produced particularly
while flying. The main body of the bird's lungs is small but the extensions
of the lungs in air sacs are relatively large (Chap. 36). The upper surface of
the lungs adheres to the ribs; indentations of the latter show clearly when the
lungs are pulled away. A special membrane ventral to the lungs is also at-
tached to the ribs. In quiet breathing intercostal and abdominal muscles
Chap. 13
THE RELEASE OF ENERGY RESPIRATION
233
Nasal
capsule
l^ood^^ <CC®
Water
A. FISH
Food
B. AMPHIBIAN
Food
REPTILE
MAN
Fig. 13.8. Diagrams of breathing and swallowing routes in aquatic and ter-
restrial vertebrates. Fishes inhale through the mouth. The routes of water and
food are parallel and entirely separate from the olfactory cavities. Beginning with
amphibians the routes of breathing and swallowing cross as they do in all other
air breathing vertebrates. The precise timing of nervous and muscular action
keeps the crossing clear for air or food. If both meet at the open trachea, choking
results.
enlarge and contract the body cavity, drawing air in and out of the air sacs,
and through the lungs. During flight the pectoral muscles (white breast meat)
provide ventilation by moving the sternum (breastbone) toward and away
from the vertebral column.
Tracheae of Insects. These airtubes extend throughout the body from open-
ings in the body wall and are the main distributors of oxygen (Chap. 30).
Human Respiration
Lungs. The human lungs begin as an outgrowth of the floor of the future
pharynx and develop a single trachea or windpipe which forks into two bron-
chial tubes (Fig. 13.10). Within each lung the bronchial tubes rebranch many
times and finally divide into minute bronchioles. Each bronchiole continues
into a small cluster of air sacs out of which minute alcoves or alveoli open
and create still further area for dift'usion of gases between air and blood
(Fig. 13.11). The bronchioles are encircled with smooth muscle innervated
234
THE INTERNAL ENVIRONMENT OF THE RODY
Part III
Esophoqus
Swim
bladder
^
u
\JI
To other
lung
FISH
SALAMANDER
FROG
To other
lung
To other
lung
TOAD
REPTILE
DETAIL OF
REPTILES LUNG
Fig. 13.9. The evolution of lungs shows a great increase in the area of lining
exposed to air, and the close association of air and blood, the latter circulating in
capillaries between the lining and the covering of the lungs. The great develop-
ment of the lining is emphasized in this figure. The lining is the membrane
through which oxygen and carbon dioxide pass to and from the blood, i.e., where
external respiration occurs.
by branches of the vagus nerves. These control the size of the passageways
through the bronchioles, many of which are closed in ordinary shallow breath-
ing. In the disease of asthma large numbers of them are closed spasmodically.
The capillary-covered alveoli are the real functional structures of the lung,
the part of it in which the major exchange of gases takes place (Fig. 13.10). It
is estimated that there are 400 million of these in human lungs and that four
to five quarts of blood pass through the lungs per minute during rest, and at
least 20 times that during violent exercise.
Passage of Air to and from the Lungs. Air is normally inhaled through the
nostrils into the nasal chambers. There it is broken into eddying currents as it
comes in touch with the warm, ciliated, mucous epithelium that covers the
turbinate bones that hang down like curtains into the nasal chambers (Fig.
13.12). This combination of structures constitutes an air conditioner, heater,
THE RELEASE OF ENERGY — RESPIRATION
235
B
Blood to
the heart
with
Bronchiole
Blood from
the heart
with C02+
onchioie
or wall)
bronchiole
t nnuscles)
Alveolus
Capillaries
around
alveolus
Fig. 13.10. A, The human lungs, each enclosed in a double walled sac and
attached to the body only by the bronchial tubes and trachea. The two lines
around the lungs represent their outer membrane and the lining of the thoracic
cavity (pleura). B, Diagram of five alveoli with their blood supply. C, Lung
tissue consists of an enormous number of bronchioles leading to microscopic air
sacs with their alveoli closely surrounded by capillaries. Two air sacs are shown
as cut in section and greatly magnified. Air is separated from the blood only by
the extremely thin walls of the blood capillaries and of the alveoli of the lungs.
It is estimated that there are 750,000,000 alveoli in the human lungs.
humidifier, and filter. With a "cold in the head," when the mucous cells are
inflamed, they greatly overdo the humidifying. Worse yet, the lining swells to
such an extent that for the time being it stops up the nasal passages entirely.
Minute particles of anything of any description that may be in the air are
caught against the moist walls of the nasal passages. That is the reason that
we smell so many things. The nose is the most democratic and hospitable of
our body structures.
It is easy to see why the nasal cavities and pharynx become infected and
how they infect adjoining cavities in the head (Fig. 13.12). Several hollow,
mucous-membrane lined cavities open out of the nasal ones, the frontal sinus
on each side above the eye, and a maxillary sinus on each side of the upper
jaw. The Eustachian tubes leading to the right and left middle ears open into
the nasopharynx just above the soft palate. The nearby tonsils and adenoids,
236
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
A. In the lung. Oxygen (Oj) diffuses
from air in the lung, and combines
with hemoglobin (Hb) in the red
blood cells to make oxyhemoglobin
(HbOJ
B. In other parts of the body
Oxygen seporotes from the loosely
bound oxyhemoglobin and diffuses
into the tissue fluid and cells.
Fig. 13.11. A, the exchange between air and blood in the lungs (external
respiration); and B, between the blood and cells of the body (internal respiration).
The rate and direction of the diffusion of a gas is determined by its pressure.
When oxygen is abundant in the lungs its pressure is high and it diffuses into the
blood. Carbon dioxide diffuses out of tissues, e.g., muscle, to blood, and then to
lungs because it passes from higher to lower pressures. Carbon dioxide is carried
by the blood in two ways; in loose combination with hemoglobin (CO^Hb); and
combined with water as carbonic acid {CO-, -f- H^O). Most of the carbonic
acid is converted into bicarbonates through neutralization by sodium or potassium
ions released when oxyhemoglobin is changed to hemoglobin. The process of con-
verting carbonic acid back into carbon dioxide to diffuse out in the lung capillaries
is speeded up by a special enzyme, carbonic anhydrase.
often large in children, are masses of lymphatic tissue in which bacteria some-
times accumulate.
Whether air enters through the nose or mouth it must pass through the
pharynx in order to reach the open trachea with its lifted cover, the epiglottis
(Fig. 13.12). In mammals, the pharynx is merely a place where the paths of
air and food cross in an awkward fashion. It is a place where indecision is
quickly punished. Either air enters it and goes straightway into an open
trachea, or food enters it, passes a closed trachea, and goes into the esophagus.
If both enter simultaneously, neither arrives and choking follows. A choke is
a forced expiration, an attempt to dislodge the crumbs that have "gone down
the wrong way." In ordinary breathing air passes quietly into the open trachea
through the larynx and enters the bronchial tubes.
Chap. 13
THE RELEASE OF ENERGY — RESPIRATION
237
Internol Nares
Uvula
Nosol Cavity
External Nores
Fig. 13.12. Human respiratory system. (Courtesy, Villee: Biology. The H union
Approach, ed. 2. Philadelphia, W. B. Saunders Co., 1954.)
Mechanism of Breathing
Inspiration. Lungs expand because the chest cavity pulls upon them from
all sides. Nerve impulses from the respiratory center in the medulla stimulate
the intercostal muscles to contract, which means lifting the ribs (Fig. 13.13).
The ribs then move outward as they are lifted, like the handle of a pail, and
thus they increase the spread of the chest. The breastbone also moves up carry-
ing the front ends of the ribs with it, and increasing the chest cavity from front
to back. At the same time impulses from the respiratory center are relayed
over the phrenic nerves to the diaphragm and make it contract, deepening the
chest cavity. Instead of a low dome pressing up into the thoracic cavity, the
238
Tlir INTHRNAL FNVIRONMnNT OF IHF BODY
Part III
RESPIRA TORY
CENTER
VAGUS NERVE
[AFFeRENT]
LUNG
J— MEDULLA
SPINAL CORD
RIB
INTERCOSTAL
NERVE-
[EFFERENT]
& MUSCLE-
Expiration
Inspiration
B
Fig. 13.13. A, Some of the nerves connected with human breathing. All of
the structures shown are symmetrical on both sides. The intercostal muscles move
the ribs out and up. Volleys of impulses are discharged rhythmically from the
paired clusters of nerve cells in the respiratory center in the medulla or brain
stem. These pass through the spinal cord to the intercostal muscles, and through
the vagus and phrenic nerves to the lungs and diaphragm. B, Intercostal muscles
lift the ribs out and up as the handle of a pail is lifted. {A, reprinted from
The Machinery of the Human Body by Carlson and Johnson, by permission of
The University of Chicago Press. Copyright 1948. B, courtesy, Gerard: The Body
Functions. New York, John Wiley and Sons. 1941.)
Chap. 13 THE RELEASE OF ENERGY — RESPIRATION 239
contraction of the diaphragm creates a flat floor. The floor now presses on the
organs beneath, the muscles of the abdominal wall relax and the abdomen
bulges.
The chest expands because the muscles contract. But why do the lungs
follow its expanding walls? In the first place lungs are free to move, since they
are attached only by the bronchial tubes and the partition between them.
Thus they can slide easily on the lining (pleura) of the lung cavity. In
addition, there are many elastic fibers in the lung, all of them stretched
and trying to shorten just as they do in the arteries. Thus their action keeps
the lungs in a state of trying to pull away from the walls about them. But
they meet the strong opposition of the low pressure in the space between
the lungs and chest wall. Within the lungs the pressure is near that of the
atmosphere, slightly below at the beginning of inspiration, the reason that
air enters them. On the other hand, in the space outside the lungs, there is
no air, only a little fluid and a suction or negative pressure. This exerts a
pull on the lungs that is stronger than they can resist. It is why they chng to
the thoracic wall as long as that is intact. When it is perforated by accident,
or in the treatment of tuberculosis to give one lung a rest that lung instantly
collapses. This is the rather well-known state of pneumothorax.
Expiration. Often called breathing out, expiration is a purely passive relaxa-
tion of muscles; the ribs are dropped; the dome of the diaphragm once more
presses upward against the lung cavity. Ordinary quiet breathing is an inspi-
ration and expiration repeated about 1 6 times per minute, the number differing
slightly in different individuals.
Chemical Control. As carbon dioxide increases in arterial blood, it acts
upon the respiratory center of the medulla and indirectly on the chemorecep-
tors of the carotid body, which are thus stimulated to discharge impulses that
quicken respiration. In contrast, a decrease in carbon dioxide affecting the
respiratory center diminishes or stops breathing. And the content of these
gases in the blood depends upon the proportions of oxygen or carbon dioxide
in the lungs.
Nervous Control. You can hold your breath but not your heartbeat, and
you cannot even hold your breath for long. But the fact that you can hold
your breath at all shows that messages come from the higher centers of the
brain and act upon the respiratory center in the medulla (Fig. 13.13). The
failure to continue holding the breath means that the chemical control by
accumulated carbon dioxide has gotten the upper hand of any nervous control.
Branches of the truly named vagus (wandering) nerves help control ordinary
breathing. During inspiration their receptor endings in the pleura are stretched
and the messages from them to the medulla are more frequent. The expansion
of the chest finally causes the respiratory center to stop sending the impulses
which stimulate inspiration. As soon as this occurs, another group of receptors
240 THE INTF.RNAL ENVIRONMENT OF Till BODY Part III
also belonging to the vagus nerve is aflected by the collapse of the lungs and
starts stimuli and stimulate a new inspiration.
Volume of Air in the Lungs
The amount of air which can be taken into or forced out of the lungs is
easily measured by a spirometer. A person at rest, breathing about 16 times
per minute, regularly inspires and expires about one pint of air. By great effort,
three pints more can be expelled in addition to the pint of the regular inspira-
tion. Even after such a forced expiration there is still about a pint left in the
lungs. There is. therefore, a reserve supply of over five pints of air with which
the fresh pint in the regular inspiration is mixed. It must not be concluded that
ail the oxygen of inspired air is extracted with each breath; expired air con-
tains about three-fourths of its previous content of oxygen. Air is breathed
over and over again. Note the possibilities in the next crowded bus!
Voice
Voice is due to the expulsion of air across the vocal cords, folds of the
lining of the larynx which contain bands of dense elastic tissue and muscle.
Although called vocal cords, they are not cords and do not resemble them.
The upper folds are called the false vocal cords and the lower ones the true
vocal cords. The larynx, characteristic of higher vertebrates, is located just
below the glottis or opening of the trachea (Fig. 13.14).
Electronic devices have proven that the deep sea is far from noiseless; fishes
have no larynx but they make grunts and kindred sounds with their swim
bladders; whales and their kin, being all good air-breathers, have an equipment
for voice. The really vociferous vertebrates are the birds and mammals. In
Hydroid
cartilage
Thyroid
cgrtiloge
('Adorns
opple")
^sr^^^^^^p
igloftis
Tracheal
ring of
cartilage
Epiglottis
Opening
nto trachea
B
Vocal
cord
Fig. 13.14. The human larynx. A, Front view from above, looking into the
throat; B, with vocal cords swung away from one another when at rest; C, cords
swung near together during speech. Loudness depends on the pressure with which
air is exhaled between them. Pitch depends partly upon the tightness of the con-
traction of the cords. Quality of voice depends upon many factors. (Redrawn
after Brash, ed.: Cunningham's Textbook of Anatomy, ed. 9. New York, Oxford
University Press, 1951.)
Chap. 13 THE RELEASF. OF ENERGY RESPIRATION 241
birds, a kind of historical larynx is present in the typical position, but without
vocal cords. Another and different kind of voice box, the syrinx, is located at
the junction of the bronchi. The whole bird chorus depends upon the syrinx,
such a range of sounds as those of parrots and thrushes, crows and robins.
Characteristic features of the mammalian larynx are its cartilaginous cover,
the epiglottis, which is quickly pulled down in swallowing, and the strongly
developed vocal cords (Fig. 13.14). The framework of the larynx is a group
of cartilages held together by muscle; the odd-shaped thyroid cartilage that
protrudes from the front of the neck is especially large in human males, and
known as Adam's apple. The contractions of muscles between the cartilages
change the shape of the larynx and vary the size of the opening between the
vocal cords. The pitch of the voice is determined by the length of the cords;
low with longer cords and high with shorter ones. Pitch can be modified
voluntarily. By persistent effort, a voice can be pulled away from the front
teeth where it sounds like an alarm clock, and placed properly in the larynx
where its tones may become clear and mellow.
Special Ways of Breathing
Coughing is a quick inspiration followed by contraction of abdominal
muscles, causing an increase of pressure in the thorax. The throat contracts
and the glottis is closed. After a certain amount of pressure gathers in the
lungs, the air escapes with a rush, pushing open the glottis and carrying with it
crumbs or other extraneous material. Coughing is generally a reflex act result-
ing from stimuli in the mouth and throat. Psychic coughing is stimulated by
hearing someone else cough.
A sneeze is a violent expiration with the air thrown into the nose and
against the hard palate.
Hiccoughs (hiccups) are due to the spasmodic contraction of the diaphragm
and a sudden inspiration cut short by the snaplike closure of the glottis. They
are often stimulated by very hot fluid taken into an empty stomach.
Sighing is a prolonged inspiration followed by a deep expiration, often with
fading voice.
Yawning is similar to sighing but is accompanied by stretching of the lower
jaw, sometimes of legs and arms.
Snoring is an accompaniment to deep breathing through the mouth. The
treble is the vibration of the soft palate.
Dyspnea is labored breathing due to choking, reduced absorptive surface of
lungs as in pneumonia, or lack of oxygen in the air.
Purring is probably caused by vibration of air drawn across the false vocal
cords by a comfortable cat, a social expression that corresponds in satisfaction
to human humming.
14
Tlie By-Prociucts
or MetaDolisni— Excretion
Excretion keeps a balanced content in the internal environment of the body.
This content is continually tipped between income, expenditure, and re-
mainder, between too little and too much. Food and water furnish the income;
activity with respiration is the expenditure; excretion removes the remainder.
Altogether this is metabolism, the continual buildup and breakdown that
liberates energy and leaves an ordinarily useless remainder.
Residues must be thrown out of the body because they are in the way and
even poisonous. The excretory organs carry on these processes; they are the
regulators of body content, keeping water, gases, salts, and other substances
from increasing beyond an essential standard. Excretion maintains a chemical
balance in the internal environment of the body; it includes separation, collec-
tion, and elimination of undesirable substances. The excretory organs of verte-
brates are the gills, lungs, liver, and the kidneys, also called renal organs
(L., ren, kidney), and in lower animals nephridia (Gr., nephros, kidney). The
responsibility for maintaining the delicate, complex adjustments of the blood
rests mainly with the liver and kidneys, the latter being the chief excretory
organs.
All living cells give off by-products of the chemical reactions that take place
within them. Since every cell surface is capable of excretion, this occurs
whether the animal has kidneys and other excretory organs, or no excretory
organs, as in hydra (Fig. 14.1). Except for the contractile vacuoles the struc-
tural arrangements of excretory organs are basically similar and the chemicals
excreted are the same. The oxidation of carbon frees energy and creates an end
product of carbon dioxide; most, but not all, of this is excreted in the gills or
lungs. Almost all excreted hydrogen is in the form of water. Nitrogen from the
242
Chap. 14 THE BY-PRODUCTS OF METABOLISM — EXCRETION 243
breakdown of proteins is usually eliminated as ammonia, urea, and uric acid.
Besides these there are other substances in very small amounts.
The Simple Excretory Organs and Their Functions
Vacuoles. In most protozoans there is no hint of a special excretory organ.
In fresh-water amebas, paramecia and others, the water constantly entering
the animal collects in contractile vacuoles along with the metabolic waste
products (Fig. 14.1). A vacuole is in no sense empty. It fills until the sur-
rounding protoplasm will stretch no more then suddenly contracts and dumps
the contents outside. Through the lower power of the microscope the vacuole,
as it were, winks at the observer. A contractile vacuole is primarily a water-
regulator that disposes of extra water diffusing into an animal because its
Fig. 14.1. The simpler excretory organs. The contractile vacuole of Amoeba
verrucosa. A, Vacuole that has reached full size and is near the surface of the body.
B, The vacuole, about to empty, is pressed against the outer covering which
stretches momentarily and forms a cone before it breaks. C, A living ameba is held
in place by a minute rod. A slightly blunt microneedle is inserted into the animal
and pushed against the contractile vacuole indenting it like a transparent rubber
ball pushed in from one side. This and other experiments have shown that a
contractile vacuole is enclosed by a transient but definite membrane. (Redrawn
after Howland: "Experiments on the contractile vacuole of Amoeba verrucosa."
J. Exp. Zool. 40:251-270, 1924.)
protoplasm is saltier than the water outside. Fresh-water fishes would have
the same trouble if they did not have means of preventing it. Marine amebas
do not have vacuoles, and when fresh-water species are kept in salt water their
vacuoles disappear or work very slowly since the salt content of protoplasm
and sea water nearly balance.
Association of Kidneys and Blood. Except for the vacuoles all excretory
organs are tubes, always intimately associated with blood or other body fluid.
In lower animals the kidney is called a nephridium and many words associated
with this term are used in connection with all kidneys, such as nephritis, a
disease of the kidney.
Fresh-water planarians have no circulating blood to transport waste and
the excretory system is a series of minute tubes whose closed ends, the flame
244 THE INTERNAL ENVIRONMENT OF THE BODY Part III
cells, are surrounded by the body fluid. These flame cells are so named because
a tuft of flagella in the funnel-shaped hollow of the cell flickers like a flame.
Actually the flagella constantly wave fluid into the tubes, ultimately to pass out
of the body through numerous fine pores. Planarians that live in fresh water
have well-developed flame cells; whereas in those living in brackish water, the
entire excretory system is reduced. As with marine amebas, the osmotic pres-
sure of salt in the surrounding water and that of the protoplasm are balanced.
The kidneys (nephridia) of earthworms repeat the essentials of kidney form
and function, tubules closely associated with blood and body fluid, each one
a guardian of the content of the blood. There are two kidneys in nearly every
segment of the earthworm (Fig. 14.2). Their inner ends are immersed in the
watery coelomic fluid; their outer ends open on the body surface; the tubules
themselves are entwined with blood capillaries. The inner end of each ne-
phridium is a funnel formed by ciliated cells arranged in beautiful symmetry
like the ribs of a palm-leaf fan, coming together at the mouth of the tubule
which receives fluid from the body cavity. The funnels draw in fluid and thus
keep down any excess of incoming water.
The kidneys of crayfishes and lobsters are hardly recognizable as such either
in shape or position, but they actually are tubular and are guardians of the
content of the blood (Fig. 14.2). In lobsters they are the paired green glands,
one on each side of the head near the eye. Each one is a two-lobed, saclike
tube whose inner end opens into a body cavity (hemocoel). The outer open-
ing is a hole easily seen on the basal segment of the antenna. Excretory sys-
tems usually include pairs of kidneys located well forward in the body like
those of lobsters and crayfishes. This does not occur in adult vertebrates but
as an embryo every vertebrate animal goes through a stage when it has "head
kidneys" (Fig. 14.3).
Kidneys of Vertebrates
Likeness of Structure and Function. The vertebrate kidney is an assemblage
of excretory tubules, always in a dorsal location and composed of many units,
each one basically similar to a kidney of an earthworm. In the kidneys of the
most primitive fishes, there are only a few of these units in one kidney; one
human kidney, however, contains at least one million of such tubules. During
their evolution the various types of kidneys of vertebrates that have appeared
are: those connected with coelomic fluid, that is, coelomic blood, free in the
main cavities, (pronephros); with coelomic fluid and circulating blood,
(mesonephros); and solely with circulating blood (metanephros), the kidneys
of adult reptiles, birds, and mammals.
Historical Succession of Kidneys — Pronephros, Mesonephros, Metanephros.
The first or pronephric kidneys are near the anterior end of the animal and
consist of a few tubules. The inner ends of these are ciliated funnels immersed
Chap. 14
THE BY-PRODUCTS OF METABOLISM EXCRETION
245
Intestine
Body cavity
ynx
rst aortic
h (heart)
Kidney
Hea
Internal openings
A. EARTHWORM
Coiled tube
entwined with
capillaries.
(latter not shown)
External opening
B. CRAYFISH
^^
Detail
of one'
Kidney
Mid gut
Kidney (Molpighian)
Hind gut
Anal
opening
C. INSECT
Anal
^ opening
f Blood
Fig. 14.2. Examples of the tubular structure of excretory organs and their
characteristic association with body fluids. A, left. Earthworm: body cavity and
first pair of kidneys seen from the dorsal side; there is one pair in each of the
one hundred or more segments behind this. Right, Body cavity of earthworm
seen from the side showing one of each pair of kidneys; the inner end opens into
the body fluid; the coiled tube is entwined with blood capillaries. B, Left kidney
of the single pair of kidneys in the crayfish seen from the side after the shell
and gills are removed. It appears as two bodies, one including the bladder and
another called the "green gland." The gland consists of a labyrinth of excretory
tubules connecting through a canal with the urinary bladder which has an ex-
ternal opening just below the eye. Blood capillaries are entwined about the tubules.
The entire crustacean kidney has been compared to one unit of the vertebrate
kidney. C, Simplified diagram of an insect's body cavity and organs. The kidneys
(Malpighian tubes) open into the gut and extend into the body cavity where they
are continually bathed by the blood. (A redrawn from Strausbaugh and Weimer:
General Biology. New York, J. Wiley and Sons, 1944.)
246
THr. INTFRNAL ENVIRON MFNT OF THF. BODY
Part III
'tloping m*$ontphr0t
•QMphric duct
'ervecord
Notochord
-MyotOTM
lotneTuluS
Sephrostome
Cardau^ v*in}
Fig. 14.3. The kidneys of vertebrates, a succession of types, pronephros, mes-
onephros, metanephros, all of them paired tubes associated with the blood. Upper
left, The pronephros of the amphibian embryo shown after the body wall and
viscera are removed. Each pronephridium opens into the body cavity by a ciliated
funnel as in the earthworm but the other end connects with the pronephric duct
leading via the cloacal chamber to the external opening. The partly developed
mesonephros is visible, a similar series of tubules that join the pronephric duct.
The pronephros degenerates and is succeeded by the mesonephros whose important
advance is the association of the blood vessels of the glomerulus and the kidney
tubule. Upper right, Diagram of a cross section of the dogfish embryo showing
that the kidney tubules and the capillaries of the glomerulus are independent.
Lower left. Diagram of a cross section of an amphibian embryo in which the
capillaries of the glomerulus are in the clasping cup of the tubule. Lower right,
Embryo of man showing the beginnings of the metanephros, the final kidney.
(Courtesy, Little: Structure of the Vertebrates. New York, Long and Smith, 1932.)
in coelomic fluid; their outer ends are joined and form the pronephric ducts,
one on each side of the vertebrae extending backward to a single opening near
the anus (Fig. 14.3). Pronephric kidneys occur in the adults of only a few of
the most primitive fishes. They develop, however, and are present a short time,
often only as rudiments, in the embryo of every vertebrate including man.
They exist for a time as the functional kidneys of young tadpoles. The meso-
nephros is the kidney of the majority of adult fishes and of amphibians and, as
the follower of the pronephros, is present and functions for a time in the
embryos of reptiles, birds, and mammals.
Most kidney tubules end in a saclike enlargement, the renal capsule, that
holds a tangle of capillaries, the glomerulus (L., a little ball). The capsule and
capillaries together constitute a working unit of the kidney, called a renal or
Malpighian body (Fig. 14.3). In every such unit, water and other products
Chap. 14 THE BY-PRODUCTS OF METABOLISM EXCRETION 247
are filtered from the blood into the renal capsule and a dilute urine is formed.
In primitive animals, a funnel of the tubule also opens into the coelomic fluid.
Altogether each unit has a two-way access to the vital fluids and water content
of the body and an equipment for the selective filtering of metabolic products
and water.
Adult reptiles, birds, and mammals all have the metanephric type of kidney
whose units are associated solely with the blood. These kidneys are provided
with large supplies of blood and consist of large numbers of kidney units held
together by connective tissue and the blood vessels. Externally they have no
resemblance to tubes, actually each kidney contains, in different species, from
a few dozen to about a million microscopic tubular units.
Human Urinary System
The human urinary system includes two kidneys, two long tubes, the ureters
which carry urine from each kidney to the bladder, a reservoir for urine, and
the urethra, a tube leading to the external opening (Fig. 14.4).
Kidneys
General Structure. The kidneys lie against the dorsal body wall beneath the
peritoneal lining. Although they appear to be in the coelom or body cavity
they are separated from it by the transparent layer of tissue which covers all
the other organs. All mammalian kidneys are bean-shaped and very similar in
structure. In the kidneys of rodents and carnivores the tubules all run toward
one point making a single pyramid. In the human and other mammalian kid-
neys the tubules come to a focus in several pyramids (Figs. 14.4, 14.5). When
split in half longitudinally the cut surface shows two parts: an outer finely
rayed band, the cortex, and a central part or medulla. The cortex contains
the renal bodies and the coiled parts of the tubule. The medulla contains
the U-shaped part of it and the collecting ducts which deliver urine through
pores in the tip of each pyramid. The urine flows through the minute open-
ings of the collecting tubules into the pelvis and drains from the pelvis into
the ureter. The ureter delivers it to the urinary bladder from whence it is
discharged through the urethra.
Circulation of Blood. The kidneys are located on the high road of circulat-
ing blood. The renal arteries bring blood directly from the heart under high
pressure, and the renal veins turn the great part of it into an easy road back to
the heart. There are no renal portal veins such as those in the frog that bring
blood from the hind legs to the kidneys (Fig. 34.18). In frogs, these help to
combat the income of water through the skin by providing the blood with
extra access to the kidneys where more water is filtered out, a process that
helps to prevent drowning from inside. Mammals and other land vertebrates
have waterproof skins and their kidneys are less important as water pumps.
248
THI INIIRNAL ENVIRONMENT OF TUP. BODY
Adrenal
Part HI
Posterior
vena cava
Cut ends
of arteries
Fig. 14.4. Human urinary system, ventral view. The kidneys are seen con-
nected with the great highways of the blood by the urinary arteries (white) and
the urinary veins (dark); the ureters open obliquely on the dorsal side of the
bladder. Half of the right kidney has been removed showing the pelvis, a cavity
through which the urine is delivered to the ureter; the cortex contains the renal
(or Malpighian) corpuscles, the functional units of the kidney; from them
bundles of collecting tubes extend toward the pelvis and deliver urine through
pores in each calyx. The adrenal glands adhere to the kidneys but have no direct
connections with them.
All through their history the behavior of the kidneys has been modified by the
necessity of keeping water in or out of the body in order to create an adequate
internal environment. They have been highly important in the evolution of
fresh- and salt-water, and land vertebrates.
Branches of the renal artery enter each kidney and there unite finally into
an arcuate artery that gives rise to the afferent arteries, one entering each
glomerulus. These divide into the capillaries of the glomerulus lying in the
renal capsule (Fig. 14.5). These capillaries join to form an efferent artery of
smaller diameter than the afferent one. After leaving the glomerulus the affer-
ent artery breaks into arterioles and capillaries that lace the walls of the
Chap. 14
THE BY-PRODUCTS OF METABOLISM EXCRETION
249
Ca'pillaries joining
artery and vein
Renal artery
Renal vein \
Ureter
Single
t'glomerulua
Artei-y
-Vein
^ Collecting-
tubules
■yi Papillary
' ' duct
Fig. 14.5. Finer structure of a mammalian kidney. Left, Cut surface showing
the veins and the arteries (black) with their many branches in the cortex and the
collecting tubes that converge and open through the calyx. Right, One functional
unit of a kidney showing a glomerulus such as those represented by dots in the
cortex in Figure 14.4. The afferent artery to the glomerulus is actually larger
than the efferent one. (Courtesy, Clendening: The Human Body. New York,
Alfred A. Knopf, Inc., 1930.)
tubules, then come together to make minute veins that empty into the arcuate
vein. This and its branches finally connect with the renal veins that open into
the vena cava which carries blood directly to the heart.
Units of the Kidney and Their Work. Each tubule with its accompanying
capillaries is one of about one million working units in each human kidney
(Fig. 14.6). In outlining the function of the kidney the parts named in the
preceding paragraphs will be mentioned again.
Each unit begins near the outer surface of the kidney where it holds the
glomerulus in a double-walled cup, the renal capsule. Both walls are very
thin and slightly separated by a space that is continued into the kidney
tubule. The plasma of the blood in the glomerulus is continually under pres-
sure by the drive from the heart, plus additional pressure due to the fact
that the afferent artery through which blood flows into the glomerulus is larger
than the efferent one through which it flows out. Except for proteins and other
large molecules the contents of the renal capsule are continuously filtered
through its thin inner wall into the cavity that leads into the tubule. Reabsorp-
tion occurs farther on where capillaries surround the many loops of the tubule.
250
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
Afferent
orferiole
To collecting
tubule
Efferent
arteriole
Region of
convoluted
tubules
Blood from
efferent
arterioles
Henle's
loop
Fig. 14.6. Diagram of the general exchange of substances in the formation of
urine. During filtration all the components of the blood plasma pass into the
renal capsule except proteins. This includes water; urea; glucose; and various salts.
During reabsorption urea is concentrated in the kidney tubule and water and
glucose are reabsorbed in the blood. Salts may or may not be reabsorbed.
It is estimated that about 45 gallons of blood are filtered per 24 hours in the
human kidneys. Of course this includes the same blood passing through the
glomeruli many times. The first filtrate is a very different fluid from urine. It
is probably like the watery urine of man's aquatic ancestors in which water
came through the skin and flooded into the body and was then filtered out of
the blood, creating the same kind of watery urine as that of present-day frogs.
Here, stimulated by a hormone released from the pars nervosa of the pitui-
tary gland, about 99 per cent of all the sugar and the water is reabsorbed into
the blood as it passes through these capillaries at comparatively low pressure
Chap. 14 THE BY-PRODUCTS OF METABOLISM EXCRETION 251
(Fig. 14.6). About one cubic centimeter per minute accounts for the three
pints or thereabout of urine usually secreted from the kidneys per day. After
reabsorption of the water, the concentration of urea and nitrogenous sub-
stances in the urine may be over a hundred times greater than in the blood
plasma. Up to a certain amount, the cells in the walls of the tubules are evi-
dently capable of taking up glucose and other useful constituents from the
excreted fluid in the tubule and passing them back to the blood. The blood
plasma and the filtrate in the tubule are always coming to a balance with one
another in their content of water, sugar, salts, and urea.
Water-saving insects, reptiles, and birds dispose of their nitrogenous waste
as semisolid uric acid. The kidneys of all land vertebrates take back by reab-
sorption much of the contents of the filtrate, the watery urine that is first made
according to the ancient ancestral pattern and then brought up to the modern
pattern. This is a roundabout way; it is also a physiological reminiscence.
Ureters and Bladder
Function. Urine is propelled through the ureters by peristaltic contractions
and enters the bladder in jets at the rate of one to five per minute. As the blad-
der becomes distended it presses against the oblique openings of the ureters,
preventing backflow into them. It also sets up afferent nerve impulses to the
spinal cord. These in turn set up impulses from the cord which stimulate
rhythmic contraction of muscles in the bladder, and eventually cause relaxa-
tion of the sphincter valve at its opening into the urethra. In very young ani-
mals this action is involuntary, but later it becomes a habit formed by volun-
tary behavior.
Conditions and Diseases Affecting the Work of the Kidneys
Nephritis. Various kinds of inflammation of the kidney tubules are called
nephritis. Although the term is used commonly it gains real meaning with the
knowledge that the nephric tubule is the essential working unit of all kidneys.
The type of nephritis commonly known as Bright's disease was described by
Richard Bright (1789-1858), a British physician, one of the great modern
pathologists. He did not theorize or experiment but did the observing upon
which theory and experiment are based. He was the first to connect with the
kidney the symptoms of a disease known since the time of Hippocrates. Bright
enjoyed life, his work, his travels, and the sketches that he made to illustrate
the accounts of them.
Floating Kidney. A floating kidney is due to a shift in the position of the
kidney either posteriorly, or tilted away from the dorsal wall. The kidneys of
fishes, reptiles, and birds fit snugly along each side of the backbone; those of
amphibians and mammals are attached loosely beneath the peritoneum.
252 THE INTERNAL ENVIRONMENT OF THE BODY Part III
Diuhetcs Mellitus. This is a condition in which sugar appears in the urine,
and is commonly called sugar diabetes. It is due to a defect in the glands
(called the isles of Langerhans) within the pancreas which secrete insulin.
Because of this the body is unable to use or to store carbohydrates and the
blood becomes loaded with sugar. So much sugar is filtered out of the glomeru-
lus that the kidney tubules are unable to reabsorb and return it to the blood,
consequently it passes out with the urine.
Diabetes Insipidus. A less common form of diabetes in which too much
water is lost but no sugar is diabetes insipidus. The kidney tubules are unable
to reabsorb the water filtered out of the blood in the glomerulus. Experiments
upon animals have shown that the water-absorptive function of the kidney
tubules is dependent upon pitressine, a hormone secreted by the pars nervosa
of the pituitary gland (Chap. 15). The disease may be controlled by in-
jections of pituitary extract just as was first done experimentally in treat-
ing the similar disease in rats and dogs.
Factors Influencing Urine Volume. The volume of blood is reduced if no
water or other fluid is taken, or blood may be lost by hemorrhage. In any such
case, the blood pressure is lowered in the kidneys; there is less filtration, and
less urine. Conversely, the more fluid that is taken, the greater the pressure in
the vessels, and the more urine produced.
Diuresis, or increased production volume of urine, is caused by a variety of
conditions and substances, such as nervous stimuli affecting the circulation,
temperature affecting the circulation, and certain stimulants. A swim in cold
water drives blood into deep vessels, increases the blood pressure in the kid-
ney, and consequently the filtration of urine. Tea and coffee act as diuretics,
especially if a person is not accustomed to them.
Other Organs that Eliminate Metabolic Wastes
Gills and Lungs. The respiratory organs remove most of the carbon dioxide
brought to them by the blood. Molecules of it diffuse into the water through
the thin membrane of the gills, and into the air within the lungs through their
equally thin membranes. Water is carried from the lungs with the expired air;
in man it usually amounts to about a cupful in 24 hours. Molecules of other
substances are carried out with the breath; those of whiskey, gin, onions, and
garlic are among the most vivid of the broadcasts.
Sweat Glands. These glands remove water, salts, traces of nitrogenous sub-
stances, and very little carbon dioxide. The amounts especially of water vary
greatly with metabolic activity. It is common expedence that the sweating
incident to high temperature and exercise stimulates drinking of quarts of
water.
Liver. The liver may be said to deal with the raw waste products of metabo-
lism since it manufactures the urea from the nitrogenous waste released by all
Chap. 14
THE BY-PRODUCTS OF METABOLISM EXCRETION
253
the cells and brought by the blood. Later the urea is released from the liver
cells into the blood and carried to the kidney where it becomes the basis of
urine in most animals.
Explorations of the Kidney
The knowledge of excretion and regulation has been and is still being built
up, especially by experiment and observations upon animals (Fig. 14.7).
The malpighian body of the vertebrate kidneys and the malpighian tubules
of insects were named for Marcello Malpighi (1628-1694), an astute ob-
server whose admiration of perfection in miniature structures was stimulated
by those in the kidney.
Although the function of the renal or Bowman's capsule was not known in
1842, Sir William Bowman (1816-1892) had a theory that the renal capsule
and glomerulus together might be a kind of filter. Proof of it came with experi-
ment. In 1920 and later years, Dr. Alfred N. Richards performed experiments
on frog's kidneys that dispelled any doubt that the capsule and specifically the
glomerulus does act as a filter. He obtained a sample of the filtrate as it was
being made in the kidneys of the living frog by inserting a fine glass pipette
into the renal capsule and drawing out some of the fluid (Fig. 14.7). What he
secured contained glucose and other constituents of plasma except the pro-
teins, a real filtrate, essentially a deproteinized plasma.
Studies made by Dr. Homer W. Smith extending over several years (1916-
Rod blocking tubule
Glomerulus*'
Pipette for
withdrawing
filtrate
Bowman's
^^ capsule
~^-K Arterioles
Fig. 14.7. Method used by A. N. Richards in obtaining a sample of glomerular
filtrate in the frog's kidney. He inserted a very fine pipette into the individual
capsules in the frog's kidney and analyzed what had passed across the mem-
branes. (Courtesy, Gardiner: General Biology. New York, The Macmillan Co.,
1952.)
254 THE INTERNAL ENVIRONMENT OF THE BODY Part III
195 1 ) have dealt with the relation of the function of the kidneys to the kind of
environment in which their owners Hve, and also with the evolution of the
vertebrate kidney in relation to the surrounding fresh or salt water, or the dry
land. His discussion of water regulation emphasizes the ecological significance
of the kidney, the part it has taken in limiting or extending the distribution of
animals.
15
Cnemical Regulation—
Endocrine Glanas
Chemical Coordination
The bodily activities of living organisms are so coordinated that every plant
and animal acts as a unit. Their chemical coordination is carried on mainly by
hormones, substances that are moved from one part of the body to another,
like messages in letters (Fig. 15.1). Contrasting with this, their nervous co-
ordination is achieved by cells with long processes over which changes (im-
pulses) move rapidly from one end to the other, like messages over a telegraph
wire. The relations of the endocrine and nervous systems are complex and
intimate.
Hormones are usually concerned with gradual changes in the body: growth,
whether to usual or to dwarf or giant size, whether to normal form and sym-
metry or misshapen; the rate of metabolism, whether oxidation is rapid and
temperature high or vice versa; the reproductive functions, those of the sex
cells and the structures connected with them, and of the animal as a whole.
Together the nervous and endocrine systems carry on a cooperative enterprise,
creating in the body an internal environment that is sensitive and adjustable
to the world outside.
Nature and Importance of Hormones
Hormones are chemical compounds that activate, maintain, or depress the
functions of particular parts or the whole of an organism; they are liberated
directly into the blood often functioning far away from their point of origin.
The name hormone (Gr., hormon, exciting) was first used in 1903 by the
British physiologists, Bayliss and Starling, who applied it to a secretion of cer-
tain cells in the intestinal wall. Since then it has appeared that the action of
some hormones is depressing, while some others under certain circumstances,
excite activity and under others depress it.
255
256
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
Pineal
body
Liver -
Gall bladder — /
Right adrenal
Duodenum
Pituitary
body
Parathyroids
Thymus
Stomach
Pancreas
Left adrenal
beneath pancreas
Ovary
Testis
Fig. 15.1. Location of the human endocrine glands. The hypophysis cerebri of
this drawing is commonly known as the pituitary gland. The functions of the
pineal body, thymus and spleen are incompletely known but may be in some way
associated with the endocrines. The total weight of the endocrine glands of the
adult human body is about one quarter of a pound.
Hormones are carried wherever the blood goes but act only when they reach
their particular targets. The relationships of endocrine glands are close, com-
plex, and often essential to life. When an animal is exposed to cold, nervous
stimuli cause one gland (the anterior lobe of the pituitary) to produce a secre-
tion (thyrotrophin) that stimulates another gland (the thyroid) to yield its
secretion (thyroxin). This in turn stimulates metabolism with accompanying
liberation of heat and energy. This complicated process is covered in the com-
mon saying, "I got used to the cold."
One group of endocrine secretions consists of comparatively simple chemi-
Chap. 15 CHEMICAL REGULATION ENDOCRINE GLANDS 257
cal substances (steroids); the other, of complex substances (proteins). They
occur in the blood in remarkably minute quantities, and as drugs are extraordi-
narily powerful. The hormone of the thyroid gland is so potent that one grain
of it in circulating human -blood will raise the rate of metabolism in an adult
by about one-third. Sparsity of only one of the hormones of the anterior pitui-
tary in a child can make it a dwarf, and an oversupply of the same hormone
can create a giant. In certain animals and human tribes these conditions have
become hereditary; among dogs, the Great Danes are giants and toy Pom-
eranians are well-formed dwarfs. Much of individuality originates in hor-
mones. They also aid and regulate embryonic development and growth.
Distribution of Hormone Production among Animals
Hormones take part in the control of essential activities in the lives of many
invertebrates and of all vertebrates.
Invertebrates. Hormones have been clearly demonstrated in arthropods,
especially crustaceans and insects. In each eyestalk of crayfishes and shrimps
there is a minute endocrine gland, the sinus gland (Fig. 15.2). Results of
experiments indicate that these glands secrete at least five hormones: three
that regulate the pigment in the chromatophores (pigment cells) of the skin,
one that stimulates the movement of pigment grains into a location in the cell
characteristic of them when the eye is adapted to full light, and one that delays
molting until a particular time.
The hormonal control of molting in insects has been definitely established.
y:^
Fig. 15.2. Left, Part of the head of a shrimp (Palaemonetes exilpes), showing
the position of the sinus glands in the eye stalks. They produce hormones which
influence the movement of pigment in the cells of the retina when the eyes change
from a dark to a light adapted state. Right, A diagram illustrating the dispersion
and concentration of pigment in cells (chromatophores) when the skin changes
from dark to pale color and the reverse; full color effect in a pigment cell occurs
when pigment granules occupy the numerous branches of the cell; the least pos-
sible display of color in the same cell when the granules are crowded into the
center. One cell may contain several kinds of pigment and the granules of each
kind may be dispersed and concentrated independently. (Left, courtesy, Turner:
General Endocrinology. Philadelphia, W, B. Saunders Co., 1948.)
258
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
It was first discovered in 1922 in the changes of the gypsy moth, Lymantria,
from caterpillar to pupa to adult. Since then it has been shown in other in-
sects, among them moths, butterflies, and beetles. When the cerebral ganglia
of the brain were removed, the caterpillars failed to pupate even though they
were sufficiently mature. However, they did pupate when the ganglia were
removed from the head but transplanted into the abdomen. This implanting
was effective only if done a few days before pupation would otherwise have
begun. The secretions of minute glands, the corpora allata, closely associated
with the brain, also take part in the regulation of rates of growth, as in the
cockroach, and the changes of form such as from pupa to butterfly (Fig.
15.3). If these are removed from the early nymphal stage of a grasshopper,
the nymphal period is shortened, molts are suppressed and adult differentiation
begins prematurely. The hormone from the corpora allata, called the juvenile
hormone, causes insects to remain youthful. There are other hormones, pro-
duced by groups of neurosecretory cells in the brain, that stimulate molting
and pupation. It appears that some insects are capable of changing their form
at any time but are kept from doing so by the hormones circulating in their
blood.
Vertebrates. Endocrine secretions are important to vertebrate animals
throughout their lives. They are effective not only in the animal in which they
develop, but in the bodies of animals into which they may be injected. They
may be taken from different species, even from different orders of animals.
Extracts of pig thyroid are commonly used for human thyroid deficiency. The
ANTENNA
BRAIN
COMPOUND EYE-
TRACHEAL TUBE
ESOPHAGUS
RECURRENT NERVE
CORPORA CARDIACA
CORPORA ALLATA
\
THORAX
Fig. 15.3. A dissection of the head of a cockroach {Periplaneta americana)
showing the paired endocrine glands, corpora allata and corpora cardiaca. The
corpora allata secrete a hormone that prevents the insect from maturing pre-
cociously, i.e., before it has grown to its typical size. The glands can be removed
surgically after which the insect becomes a dwarf adult; if extra glands are grafted
mto an insect it becomes an immature giant. (Courtesy, Turner: General Endo-
crinology. Philadelphia, W. B. Saunders Co., 1948.)
Chap. 15 CHEMICAL REGULATION ENDOCRINE GLANDS 259
endocrine glands are located in relatively similar positions in all vertebrates
(Fig. 15.1).
Endocrine Glands
Nomenclature. Endocrinology is a recent and very important study in which
many investigators have joined. New discoveries have suggested new names
until each gland and hormone has been christened and rechristened with sev-
eral names. The International Commission on Anatomical Nomenclature is
attempting to clarify this situation.
Study of the Endocrines — An Illustration of the Experimental Method.
Endocrinology stands forth among biological subjects as a peculiarly striking
example of the successful use of the experimental method of study. The only
way to find out what a gland does is to show what occurs when it is removed,
thus creating a deficiency of its hormone, or what happens if it is implanted
into the body of a healthy animal, or its secretion or an extract is injected thus
creating an excess of the hormone. Thousands of experiments have been done.
In the pioneer days of endocrinology Charles Berthold made the first experi-
mental demonstration of the chemical effects of one part of the vertebrate body
upon another. In 1849 he removed the testes from young cocks and replanting
them, found that the usual changes after castration did not occur. In 1855
Claude Bernard put forth the idea that organs liberate special substances into
the tissue fluids and coined the phrase "internal secretion." Before these
experiments were made, there was only a vague knowledge of chemical con-
trol. Although much is still to be learned about endocrines, many of their
extraordinarily complex relationships have been clearly demonstrated.
Light has been thrown upon the body at work by experimental surgery upon
living animals, especially by removing and transplanting glands. This has been
done with great care for the comfort of the animals, and the results have
proved highly important contributions to the intelligent treatment of human
diseases. That those who have "sugar diabetes" can live out their lives so suc-
cessfully is wholly due to experiments upon the pancreas of living animals.
Goiter, a serious disease of the thyroid gland, has been eliminated in many
regions thanks to the results first gained from the experimental treatment of
the goiter of fishes (Fig. 15.4). Other experiments include the culture of gland
cells outside the body under conditions which allow them to grow and to be
examined alive under the microscope.
Thyroid Gland
Form and General Activity. From the lower fishes to man, all vertebrates
have a thyroid gland. The human thyroid consists of a pair of lobes, one on
each side of the trachea joined by a band that crosses the trachea just below
the larynx (Fig. 15.1). It is supplied with many blood and lymph vessels, the
260 THE INTERNAL ENVIRONMENT OF THE BODY Part III
Fig. 15.4. Brook trout (Salveliniis fontinalis) with swollen gills and an external
goiter, a disease at one time prevalent among carnivorous fishes raised in hatcheries.
The disease was finally prevented by food and water containing iodine, largely
through the suggestions of Dr. David Marine (1910). This treatment has also
been applied with success to certain types of human goiter. (Courtesy, Marine
and Lenhart, J. Exp. Med. 12:311-335, 1910.)
former broken into capillaries which surround the follicles that compose the
bulk of the gland (Fig. 15.5). The follicles are held together by loose con-
nective tissue. The wall of each one is formed by a single layer of epithelial
cells that produce the jellylike colloidal secretion. The activity of the thyroid
depends upon the diet, temperature, and conditions of special physiological
stress, and is primarily under the control of the pars anterior of the pituitary
gland. Sea food with its high content of iodine reduces thyroid activity, and
heavy meats, fats, and proteins increase it. It also responds to conditions of
the body such as activity of the reproductive organs and to climatic changes.
The varying states of the thyroid, its diseases, and the results of experiments
all show its close relation to the general metabolism of the body. High activity
of the body, rapid oxidation, and quickened heartbeat all go with an over-
active thyroid. The secretion thyroxin (C15H11O4NI4) has been isolated and
synthesized. The adult human body contains a little less iodine than there
would be in ten drops of a medical solution of it. There is iodine in the skele-
ton, muscle, and liver, but the small thyroid gland itself contains about one-
fifth of the total iodine content of the body.
Diseases of Deficient Thyroids. Too little thyroid secretion is due to injury
or underdevelopment of the gland, to some defect in pituitary control, to
accident or disease, and commonly to lack of iodine in the food. However,
iodine should not be taken without expert advice, nor should "iodized salt"
be put into general use. The latter has nearly disappeared from the markets.
Chap. 15
CHEMICAL REGULATION ENDOCRINE GLANDS
261
Fig. 15.5. Sections of active thyroid glands of the salamander (Triturus viri-
descens). Thyroid glands are composed of vesicles or follicles lined by a single
layer of secretory cells. The cavities of the follicles contain the hormone produced
by those cells. This is absorbed into the blood through the walls of the blood
vessels between the follicles where there is connective tissue, fat and nerve cells.
Left, Section of a whole gland under low power. It is about half the size of an
apple seed. The white tips of the secretory cells are bulging with secretion. The
nuclei appear black. Right, Section of a gland under high power. The white tips
of cells full of thyroid secretion project into that which (dark) is stored ready to
be absorbed by the blood.
Goiter. It is an enlargement of the thyroid gland. There may be too little
secretion; in hypothyroid goiter the cells increase in order to bring the secre-
tion to a normal amount; in hyperthyroid goiter the gland secretes an excess
usually with a great multiplication of cells. Either of these conditions may
occur without an enlargement of the gland.
The association of the thyroid and goiter has long been known. Nearly
2,000 years ago, Juvenal, a Roman poet, remarked on the prevalence of goiter
in the Alps. In the 16th century the Swiss physician, Paracelsus, wrote of the
seriousness of goiter near the famous music center of Salzburg, and agreed
with others that the cases were caused by the mineral content of the drinking
water. Long before this, about 1180, another physician, Roger of Palermo,
had found a remedy for goiter in the ashes of sponges and seaweed. In 1910,
David Marine, a physician in a New York hospital, made a study of the goiter
occurring in hundreds of brook trout at a hatchery in the mountains of Penn-
sylvania. He placed small amounts of iodine in the runways, mixed iodine with
their food, and, like Roger of Palermo, included seafoods in their rations
(Fig. 15.4). A general recovery soon spread through the population. Follow-
ing this experiment human subjects were similarly treated in a region of Ohio
where goiter was prevalent, and again the goiter disappeared. The localities
peculiar to this commonest disease of the thyroid, all of them far from the sea,
262 THE INTERNAL ENVIRONMENT OF THE BODY Part III
gave the clue to the need of the gland for iodine, and finally led to the pre-
vention of goiter.
Cretinism and Myxedema. These diseases are both caused by thyroid
deficiency; cretinism arises before the child is born, infantile myxedema after-
ward. In cither one the children become dwarfs, misshapen, and underdevel-
oped physically and mentally, unless they are treated with thyroid hormone
(Fig. 15.6). Thyroid dwarfs are characteristically malformed (Fig. 15.7);
pituitary dwarfs are usually of normal shape but small (Fig. 15.17). Typical
myxedema occurs after adolescence.
High and Low Thyroid Types. The hypothyroid type of individual has
a low rate of metabolism and is relatively calm and slow-moving; among dogs
Fig. 15.6. Cretinism, a disease of the thyroid, and the importance of its treat-
ment. Left, A normal boy of seven years. Center, A cretin of thirteen years,
dwarfed physically and mentally subnormal. Right, The same boy after receiving
thyroid treatment for seventeen months. (Courtesy, Bronstein, Am. Jour. Med. Sc.
205:114, 1943.)
it is the Saint Bernard. The hyperthyroid type, such as the Irish terrier, has a
high metabolism, moves rapidly, and is seldom quiet.
Experimental Studies of the Thyroid. Removal of the thyroid glands from
rabbits shortly after birth produces dwarfs that are essentially like human
cretins. If, while still young, they are fed desiccated thyroid, they will grow
to normal size and maturity.
The results of many experiments have shown that the thyroid controls the
change of shape that occurs as young animals become mature. This is most
spectacular in amphibians which go through a striking metamorphosis from
larvae (tadpoles) to adults. Bullfrog tadpoles (Rana catesbiana) are literally
rushed through metamorphosis, into dwarf frogs by feeding them desiccated
thyroid or implanting crystals of iodine in their bodies (Fig. 15.8). The larval
tail is absorbed, the legs develop, the mouth widens, and the alimentary canal
changes from the long watchspring shape to the more common form of the
adult, but the young frog does not increase in size. Merely feeding tadpoles
Chap. 15
CHEMICAL REGULATION ENDOCRINE GLANDS
263
Fig. 15.7. A "thyroid dwarf"; childhood myxedema. "The Court Dwarf of Don
Balthazar Carlos" painted by Velasquez, 1631. The dwarf has the characteristic
"saddle nose" and pudgy face and body of thyroid dwarfs. (Courtesy, Boston
Museum of Fine Arts.)
with iodine or keeping them in dilute solutions of iodine also hastens metamor-
phosis.
The skin of human cretins is thickened and dry. If the thyroids are removed
from newts {Triturus viridescens) their skin likewise becomes thickened and
dry (Fig. 15.9). Newts normally shed their skins at intervals but after their
thyroids are removed they cease molting and accumulated layers of skin cover
the body or hang from it in tatters. The same effect occurs after the pituitary
is removed because the pars anterior controls the activity of the thyroid.
Parathyroid Glands
The first important discovery regarding the parathyroid glands was the
distinction between them and the thyroid glands on the dorsal side of which
they are embedded (Fig. 15.1). In the earlier treatment of goiter the para-
264
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
m -
■-~^v^
-'iv;
Fig. 15.8. The metamorphosis of tadpoles of bullfrogs (Rana catesbiana) is
hastened by implanting crystals of iodine in their bodies. A, Animal killed two
weeks after the crystals were implanted. B, The untreated control of the same age.
Compare the mouths, tails and paired appendages. In nature bullfrogs are two to
three years old before they become adults. (Courtesy, Turner: General Endo-
crinology. Philadelphia, W. B. Saunders Co., 1948.)
thyroids were sometimes removed with the thyroids, with extremely serious
results. When the thyroid was removed from cats and dogs, it almost always
resulted in tetany, an extreme cramplike contraction of the muscles, and
death, but in rabbits the same operation made hardly any disturbance. The
reason proved to be that in rabbits one pair of parathyroids was located so
far behind the thyroid gland that it was not removed with it. Parathyroids
regulate the amount of calcium and phosphorus in the blood and their
metabolism in the body (Fig. 15.10). Failure in this regulation produces ex-
treme irritability in the motor nerves and tetany. Tetany may also occur with
rickets, a vitamin-D deficiency, the softening of bones being due to lack of
calcium.
Adrenal Glands
In man one of the adrenal (suprarenal) glands is in contact with the upper
end of each kidney (Figs. 15.1, 15.1 1) and in animals generally they are near
the kidneys. The adrenal gland is actually two glands in one, a central medulla
and surrounding cortex.
Medulla. The medulla originates from cells allied to the autonomic nervous
Chap. 15
CHEMICAL REGULATION ENDOCRINE GLANDS
265
Fig. 15.9. The common spotted newt (Triturus viridescens), blackened by the
layers of skin that accumulated because a part of its pituitary gland had been
removed. Layers of skin began to slip from the head after a duplicate of the miss-
ing part of the pituitary had been engrafted into the animal and had activated the
thyroid gland to stimulate the molting process. (Photo courtesy A. E. Adams
from Adams et al., "The Endocrine Glands and Molting in Triturus viridescens,"
J. Exp. ZooL, Aug. 1932.)
system and the cortex from cells near to those that form the sex organs.
Epinephrine (or adrenalin) (CgHiaOsN), the hormone of the medulla, is
very useful but not essential to life. It has been isolated in pure crystalline
form and was the first hormone to be synthesized. Its injection causes a rise
in blood pressure and quickened heart rate; more glucose is turned into the
blood from the liver and muscles, accompanied by increased muscular
power and resistance to fatigue. After making a long series of experiments,
an American physiologist, W. B. Cannon, concluded that adrenalin acts as
an emergency stimulant in the body, especially for the muscles. It is secreted
into the blood in excitement such as fear, pain, or intense effort. Facing the
peril of fire a person breaks a window glass with the bare fist; run down by
a dog, a cat turns about with hairs up and claws ready. Analysis of the blood
of such animals in emotional crises has shown that it contains many times
the minute amount of adrenalin (1 part in 1 or 2 billion parts of blood)
ordinarily present. At such times the muscles demand more food and more
oxygen to combine with it and set energy free. These are provided by glucose,
by the increased pumping of the heart, by more rapid breathing, and the
higher arterial blood pressure.
266
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
Fig. 15.10. Effects of parathyroid deficiency. A, The normal dog before the
operation. B, The same animal the second day after the removal of the para-
thyroids in a convulsive condition with twitching muscles. C, The same animal 15
minutes after calcium was given to it. The parathyroid glands regulate the amount
of calcium and phosphorus in the body. (Courtesy, Turner: General Endocri-
noloiyy. Philadelphia, W. B. Saunders Co., 1948.)
The secretion of the medulla is controlled by the autonomic or involuntary
nervous system. The sympathetic fibers of the autonomic nervous system
produce a hormone, sympathin, which apparently is distributed to tissues
whenever impulses pass over the nerve fibers to them. The reactions to
sympathin are similar to those of adrenalin but the two substances are evi-
dently entirely separate. The colors of many fishes are due to pigment cells
or chromatophores. Their expansion and contraction are controlled partly
by nervous impulses and partly by hormones such as sympathin and adrenalin.
Cortex. The adrenal cortex produces hormones, certain of which are
essential to life; if the cortex of both adrenal glands is entirely removed, an
animal dies within a few days. Experiments suggest that there are three groups
of these hormones, all of which are steroids and some of which have been
synthesized. The first group (called the desoxycorticosteroids) controls the
Posterior
vena cova
Aorta
Blood vessel
Capsule
Fig. 15.11. Human adrenal glands. Left, A gland caps each kidney but is not
a part of it. Right, A section of an adrenal. The medulla is within the cortex like
the filling in a sandwich. The two parts are different in origin and function; the
cortex is essential to life, the medulla is not.
Chap. 15 CHEMICAL REGULATION ENDOCRINE GLANDS 267
balance of sodium and potassium in the body. The second group (the 11-
oxysteroids) includes cortisone, exercises its particular effect on carbo-
hydrate and protein metabolism, and is involved in the series of adaptations
called the alarm reaction that occurs after stresses such as shock, extreme cold,
and poisons. Cortisone remedies adrenal deficiency either in experimental
animals with both cortices removed or in persons suffering from Addison's
disease in which the cortices have atrophied. The production of these hor-
mones is under the control of the adrenocorticotrophic hormone (ACTH)
of the pars anterior of the pituitary. The third group of hormones is very
similar to the sex hormones. Excess production of such hormones, often
associated with tumors of the cortex, is responsible for the bearded ladies
of the circus.
Pancreas
Endocrine Glands of the Pancreas and Their Function. Nothing takes the
place of the versatile digestive juice of the pancreas as an all-round simplifier
of foods that otherwise would be out of reach of the body's metabolism. But
the pancreas also contains numerous endocrine glands, literally islands of
cells, the isles of Langerhans, that secrete into the blood stream the hormone
insulin and possibly lipocaic, a hormone of fat metabolism. Insulin has been
called the spark-plug of carbohydrate metabolism because, in some way, it
brings about the oxidation of sugar and the subsequent release of potential
chemical energy (Fig. 15.12).
Callblodd
CHOLECYSTOKININ
secreted by
duodenum-
results in
emptying goll-
blodder
ENTEROGASTERONE
Secreted by
lenum —
jostric
lotiljjy
GASTRIN
Secreted by
stomach-
stimulates stomach
Pancreos
SECRETIN
secreted by duodenum—
Stimulotes poncreos
ENTEROCRININ
Secreted by duodenum—
stimulotes duodenum
Fig. 15.12. Diagram of the structures from which digestive hormones originate
and the parts and processes which they stimulate. (Courtesy, Hunter and Hunter:
College Zoology. Philadelphia, W. B. Saunders Co., 1949.)
268 THE INTERNAL ENVIRONMENT OF THE BODY Part III
Sugar Diabetes and Insulin Treatment. When the cells in the isles of
Langerhans fail to produce insulin, the body cannot use its sugar, no matter
how plentiful it is or how well digested. The oxidation of glucose stops, espe-
cially in the muscles, and glycogen is no longer stored in the liver and
muscles. Sugar accumulates in the blood, is excreted in the urine, and thus
is continually thrown away. In the meantime the starving body uses first its
fats and then its proteins in the progress of the disease of diabetes mellitus
that resulted fatally up to the time when insulin became known. The insulin
treatment of diabetes was first used in January 1922. Since then thousands
of persons have been able to live successfully by means of it. The saving of
all of them has been due to knowledge gained by experiments upon ani-
mals.
In 1889 two European physicians, Oscar Minkowski and Joseph von
Mering, removed the pancreas from dogs in making studies of digestion.
Their caretaker noticed that there were unusual gatherings of flies about the
urine of these dogs and when the investigators examined it chemically, they
found that it contained quantities of sugar. They immediately tried to remedy
the diabetes by feeding the dogs extract of pancreas. But they were without
success because they were including the enzyme trypsinogen in the pan-
creatic juice, which becoming trypsin in the intestine destroyed the insulin, a
protein. In 1893, Minkowski published an account of the whole matter. Many
experiments followed, mostly unsuccessful because of the destruction of the
insulin. Later experiments brought more facts and more clues. Finally,
Banting, Best, Macleod, and Collip, investigators at the University of Toronto,
discovered a successful treatment and began it in 1922. Banting tied off the
pancreatic duct temporarily and, although this brought on bad symptoms,
those of diabetes were not among them. Collip destroyed the trypsin with
alcohol and acid and thus secured an effective extract of pancreas that in-
cluded insulin. This is essentially the same remedy which has been used ever
since. No cure for diabetes has been discovered and the extract can be taken
only by injection.
Gastrointestinal Hormones
These hormones work in series, each one preparing for the chemical action
of another secretion (Fig. 15.12).
Gastrin. The arrival of food in the stomach stimulates the secretion of
gastrin by the cells in its lining. Gastrin in turn acts as a stimulant to the
production of the gastric juice.
Secretin. When stimulated by the arrival of an acid food-mass from the
stomach, cells in the intestinal lining secrete the hormone secretin into the
blood. This in turn stimulates the pancreas to produce pancreatic juice and
the liver to secrete bile.
Chap. 15 CHEMICAL REGULATION ENDOCRINE GLANDS 269
Cholecystokinin and Delivery of Bile. Acid food from the stomach stimu-
lates other lining cells of the duodenum to secrete this hormone into the
blood. This stimulates the muscles of the gallbladder to contract and pour
bile into the intestine.
Enterogastrone, the Antiulcer Hormone. After partly digested food or
chyme leaves the stomach, the secretion of gastric juice and the contractions
of its muscles are slowed or stopped. Nervous mechanisms are probably
involved, but experiments have shown that such rest periods of the stomach
are caused by enterogastrone. Its production in the walls of the intestine is
stimulated by the arrival of the food, mainly by the neutral fat.
Enterocrinin. Extracts made from intestinal lining will stimulate the release
of secretion stored in the lining. The hormone is called enterocrinin.
Pituitary Gland
Appearance, Position, and Parts. The human pituitary gland (hypophysis)
is the size and shape of a large pea. It is located almost exactly in the center
of the head in a cradlelike space on the floor of the cranium above the soft
palate (Figs. 15.13, 15.14). It is formed by two outgrowths, an anterior part
which grows upward from the roof of the embryonic mouth and becomes
the pars anterior, pars tuberalis, and pars intermedia, and a posterior part
formed by a downgrowth of the developing brain which becomes the pars
nervosa. It remains permanently connected with the brain by the pituitary
stalk through which it is well supplied with blood vessels and nerves.
The early anatomists named the gland pituitary (L., pituitarius, phlegm)
because they thought that its nearness to the nasal cavities meant that it
poured a secretion into them. It is commonly known as the master gland
since it regulates growth, controls other endocrine glands, and affects tissues
and organs. With some variations in structure it is present in the vertebrates
from the lower fishes through the mammals.
Functions of the Pars Anterior. Although the manufacture of as many as
10 to 15 hormones has been attributed to the pars anterior, the most recent
evidence indicates that it probably produces seven. Three of them influence the
development and function of the reproductive organs, the ovaries, testes,
and the mammary glands. Three of its other hormones affect metabolism,
stimulating growth, regulating the thyroid gland and the cortices of the
adrenal glands (Table 15.1). The seventh, not completely estabhshed, may be
associated with the formation of red blood cells. All of them are proteins;
three are glycoproteins, i.e., combinations of a carbohydrate and a protein;
and three are simple proteins. Several have been extracted in fairly pure
form, but none has been synthesized. It is now believed that the influence of
the pars anterior on carbohydrate and fat metabolism which was formerly
assigned to hormones called diabetogenic, pancreatrophic, parathyrotrophic.
270
THE INTERNAL ENVIRONMENT OF THE BODY
Dura matter of ihe brain
Hypophysla ^
Part III
Frontal sinios s^
Nasal
geplum
Sphonoid-. ,
ad sinus /'^.
Tongue...
Pbna
cre-
el Ixim
Dura miter of
the spine
Isthmos of
thyroid gland'
Fig. 15.13. Median section of the head showing the location of the pituitary
gland (or hypophysis) in relation to other parts of the head. (Courtesy, Clenden-
ing: The Human Body. New York, Alfred A. Knopf, 1930.)
Pars tuberolis
Dura mater
Pars anteri
Sphenoid bone
^ \ ■44fc ;?!«■ v^SA(¥x«;iSS' m. ^ Nt- '<w.
Pia mater of brain
Floor of brain
Dura mater
Pars nervosa
Pars intermedia
Sphenoid bone
Fig. 15.14. Median section of the pituitary, its parts and their relation to the
brain and cranium.
Chap. 15
CHEMICAL REGULATION — ENDOCRINE GLANDS
271
Table 15.1
Hormones of the Pars Anterior of the Pituitary Gland
gonadotrophic hormones
Name
Symbol
Chemical
Nature
Main Functions
Follicle-stimulating hor-
mone
FSH
Glycoprotein
Stimulates development
of 1 ) egg-containing
follicles in ovaries,
which produce estro-
gen, 2) sperm cells in
testes
Luteinizing hormone
or
Interstitial-cell-
stimulating hormone
LH
or
ICSH
Glycoprotein
Stimulates development
of corpora lutea (after
ovulation), which pro-
duce progesterone
Stimulates interstitial
cells of testes to pro-
duce male sex hor-
mone, testosterone
Prolactin
Luteotrophic or
lactogenic hormone
LTH
Simple protein
Stimulates 1 ) develop-
ment of corpora lutea
which produce proges-
terone, 2) secretion of
mammary glands; re-
tards development of
follicles in ovaries
metabolic HORMONES
Growth hormone
GH
Simple protein
Promotes growth with
increase of water and
protein content in the
whole animal with a
decrease of fat; has
marked influence on
cartilage and bone of
skeleton; causes hyper-
trophy of thymus
Thyrotrophic hormone
TSH
Glycoprotein
Stimulates development
of thyroid gland and
its production of thy-
roxin
Adrenocorticotrophic
hormone
ACTH
Simple protein
Stimulates cortices of
adrenal glands to se-
crete cortical hor-
mones, e.g., cortisone
et al.: has growth-
retarding action and
so is antagonistic to
the growth-promoting
action of the growth
hormone; causes invo-
lution of thymus
272 THE INTERNAL ENVIRONMENT OF THE BODY Part III
to name only a few, is exerted by the growth and adrcnocorticotrophic
hormones.
The hormones of the endocrine glands (gonads, thyroid, and adrenal cor-
tices), whose production is largely controlled by hormones from the pars
anterior of the pituitary, also exercise some control on the production of
these controlling hormones by the pars anterior. For example, the thyrotrophic
hormone (TSH) of the pars anterior stimulates the thyroid to produce
thyroxin and release it into the blood stream; in turn a high level of thyroxin
hormone (TH) in the blood causes the pars anterior to reduce its production
of thyrotrophin (TSH), while a low level causes it to increase its production.
This see-saw relation also occurs between the gonadotrophic hormones se-
creted by the pars anterior (FSH and LH or ICSH) and the sex hormones
secreted by the gonads (estrogen and progesterone of ovaries and testosterone
of testes) and between the adrcnocorticotrophic hormone (ACTH) of the
pars anterior and the cortical hormones of the adrenal cortices.
After removal of the pituitary (hypophysectomy) of young animals, the
skeleton stops growing, the sex organs do not develop, and the thyroid and
cortices of the adrenal glands gradually shrink. However, if a fragment of
pars anterior is then transplanted daily into these animals, they will resume
growing, the sex organs will develop, and the thyroid and adrenal cortices
become normal. Extracts of pars anterior have been prepared which will
correct one or another defect caused by hypophysectomy; some of these
extracts also cause specific effects upon normal animals. Young rats and
puppies thus treated will grow to almost double the size of others in the same
litter (Fig. 15.15). Even immature mice or rats implanted with fresh pars
anterior or injected with gonad-stimulating hormones at weaning time will
become sexually mature in three to five days.
Giants, Acromegalics, and Dwarfs. Giants and acromegalics have over-
active pituitaries (pars anterior) (Fig. 15.16), Giantism begins in very early
childhood, acromegaly in adult life. In acromegaly the nose and lower jaw
become abnormally prominent and the forehead and the skin thickened. Indi-
viduals dwarfed by underactivity of the pituitary are of two types; one kind
has a body like a normal child's (Fig. 15.17), the type of dwarf usually seen
on the stage and in circuses; the other has a short, heavy body overlaid with
fat.
Function of the Pars Intermedia. In man the function of the pars inter-
media is not known. In frogs, toads, lizards, and some fishes it produces a
hormone, intermedin, which disperses the pigment in melanophores, the
ameba-shaped cells which contain black pigment (Fig. 15.18). Tadpoles from
which the pituitary has been removed are very pale but regularly darken
when the pars intermedia of normal tadpoles is implanted into them. By an
operation on embryos of the small spring-peepers {Hyla crucijer) the pars
Chap. 15
CHEMICAL REGULATION ENDOCRINE GLANDS
273
Fig. 15.15. The effect of extract of pars anterior of the pituitary upon the
growth of dogs. Normal dog and giant of the same litter that has been treated
with injections of the extract of the gland. (From The Living Body, Copyrighted
1952 by Henry Holt and Company. Reprinted with their permission.)
intermedia can be completely suppressed while the remainder of the pituitary
continues to develop. In consequence the tadpoles will metamorphose into
frogs with silvery-colored skin.
Regarding the relation of the pituitary to color, it is to be remembered that
adrenalin also concentrates the pigment in amphibian melanophores. If one
cubic centimeter of a solution of one part adrenalin to 10,000 parts of water
is injected into the dorsal lymph space of the leopard frog (Rana pipiens),
it will begin to turn pale in ten minutes and shortly afterward will become
thoroughly pallid, remaining so for a day or two.
Functions of the Pars Tuberalis. The function of the pars tuberalis in man
and other vertebrates is unknown.
Functions of the Pars Nervosa. The pars nervosa stores and releases at
least two hormones, pitressin and pitocin, often included together as pituitrin
(Table 15.1), both probably produced by neurosecretory cells of the
hypothalamus of the brain. Pitressin raises the blood pressure by directly
stimulating the contraction of smooth muscle in the arteries and arterioles.
Adrenalin achieves the same result but by the way of the autonomic (in-
voluntary) nervous system. Pitressin acts to conserve water in the body.
When animals are kept on short water rations there is so much antidiuretic
substance secreted that it appears in the urine. This holding of water in the
274
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
Fig. 15.16. Giantism is produced by overactivity of the pituitary gland (pars
anterior) beginning during the natural growth period. Photograph of Robert
Wadlow of Alton, Illinois, taken with a man of average height, in a tailor's shop
in St. Louis in 1939. His school record was excellent and his personality of high
character but he was never physically vigorous and very susceptible to infections
from which he died, July 15, 1940, at age 22. In that year he was 8 feet 11 inches
and weighed 491 pounds. The record of his growth is the best authenticated of any
giantism. (By special permission of Harold F. Wadlow from Fadner and Wadlow:
Gentleman Giant. Boston, Bruce Humphries, Inc., 1944.)
body during a time of sparse income is one more way in which the internal
environment is kept wet. Extracts of pars nervosa are given to check the
flow of urine that occurs in diabetes insipidus (Chap. 14, Excretion). The
pitocin principle of the extract of pars nervosa is administered as a stimulant
to the contraction of smooth muscle of the uterus during childbirth. Pitocin
also stimulates the smooth muscle of the intestine and bladder. It has now
been synthesized.
Nervous Control of Endocrines
The functions of the pituitary and adrenal glands are at least partially con-
trolled by the nervous system. The thyroid has a rich nerve supply, but there
is no evidence of nervous control of its secretion, although nervous tension
accompanies high thyroid activity.
Destruction of the pars nervosa of the pituitary results in increase of urine,
the consequence of the removal of the antidiuretic effect of pitressin. The
same thing occurs after cutting the nerves leading from the hypothalamus of
Chap. 15
CHEMICAL REGULATION ENDOCRINE GLANDS
275
Fig. 15.17. Dwarfism accompanies extreme
underactivity of the pituitary (pars anterior)
beginning in childhood. The type of pituitary
dwarf shown in this figure has the proportions
of a normal, not unattractive person. He is 21
years of age. The man on the right is 5 feet, 7
inches tall. (From The Living Body, Copyright
1952 by Henry Holt and Company. Reprinted
with their permission.)
the brain to the pars nervosa even though the pars nervosa itself is untouched.
And since* tumors in the hypothalamus result in abnormalities of growth, it
is probable that the pars anterior and hypothalamus are associated. The theory
concerning the role of the adrenal medulla under stress of excitement suggests
that the medulla is being controlled by the nervous system, but this is not
known.
Functions of Endocrines in the Sex Organs
In addition to producing eggs and sperm cells the ovaries and testes also
secrete hormones that aflfect allied structures and also secondary sex char-
acteristics such as voice, size, and coloration. Other endocrine glands, espe-
cially the thyroid and pituitary, also have a controlling influence on both
ovaries and testes.
The antlers of male deer and the brighter colors of male birds are familiar
secondary sex characters. If the testes are removed (castration) from a young
deer, no antlers grow; a castrated cock has a small comb and a faulty crow
or none. In such animals the secondary sex characters are lacking, all repro-
276
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
Fig. 15.18. Common grass frogs {Rana pipiens): one shows the natural light
brown and dark spots; the color of the nearly black one is the result of the power-
ful stimulation of the pigment cells by the injected intermedin, the hormone of
the pars intermedia of the pituitary. When colors are pale or hidden the pigment
granules are clumped in the center of the cell and its branches are invisible (Fig.
15.2). Injections of intermedin cause the granules to move into the branches and
the animal becomes deep brown or blackish. (Courtesy, Therapeutic Notes.
Detroit, Mich., Parke Davis & Co., April, 1935.)
ductive structures are reduced, and the body often takes on fat. These changes
are caused by the absence of the hormone testosterone (CioHaoOo) thought
to be produced by cells (interstitial) that are packed in between the tubules
of the testes in which the sperm cells develop.
Estrogen, a female sex hormone, is produced by follicular cells surrounding
the egg in the ovary, and is responsible for the estrus or heat in female
mammals. If the ovaries are removed from young females, they remain sex-
ually immature. On the other hand, if estrogen is injected into these castrated
females, the usual maturing is resumed. If it is injected into normal im-
mature females, the secondary sex structures and the estrous periods are
hastened into full development but the ovary is not affected and the develop-
ment of the eggs is not hurried. Another ovarian hormone, progesterone, is
produced by the corpus luteum formed from the cells of the Graafian follicle
which are left after an egg is shed from the ovary. Estrogen and progesterone,
working together, prepare the uterus for receiving the young embryo (Fig.
15.19). These two hormones also stimulate the enlargement of the mammary
glands in which the secretion of milk is later induced by the lactogenic hor-
mone of the pars anterior of the pituitary gland (figures and further discussion
of the sex organs, Chap. 18),
Chap. 15
CHEMICAL REGULATION ENDOCRINE GLANDS
277
Vfiginal Epithelium
Proliferative Phase Secretory Phase
Uterine Mucosa
Fig. 15.19. Diagram showing some of the hormones produced in the anterior
lobe of the pituitary gland, with especial emphasis upon those taking part in the
regulation of the cyclic activities of the female reproductive organs. (Courtesy,
Patten: Human Embryology, ed. 2. Philadelphia, The Blakiston Co., 1953.)
Uncertainties
The pineal gland located in the middorsal part of the brain is less than half
an inch in length and is shaped like a pine cone from which it takes its name
(Fig. 15.1). It has excited curiosity for more than three centuries, since the
time when it was called the seat of the soul. Its large blood supply and appear-
ance have aroused the suspicion that it may be an endocrine gland and it is
often included in figures as an uncertain member of that group. No convincing
evidence has confirmed this or established any other function for it.
Thymus Gland. The thymus lies beneath the breastbone in most mammals,
278 THE INTERNAL ENVIRONMENT OF THE BODY Part III
(Fig. 15.1). It is relatively large in infants but becomes much smaller with
adolescence. It has been suspected of being one of the endocrine glands, but
never proven to be though it is often placed with them. Like the tonsils and
other lymphatic structures, it is concerned with the production of lymphocytes.
16
Conduction and Coordination—
Nervous System
All living matter is in a unique way excitable and responsive to changes
that go on inside and outside of it. Much has been learned about its aware-
ness and response, but a great amount remains to be discovered. Facts con-
cerning it that are clearly shown in laboratory observations and experiments
are hard to admit when they are met in the courtroom and the church.
Through our nervous systems and sense organs we stand on the earth
and explore the universe, The light of the stars produces chemical changes
in the sensory cells of the eye; these start changes in the nerves and brain,
and we have ideas about the stars. We know a good deal about those chemical
processes, but of the making of the ideas we know almost nothing.
Response and Conduction
Touch an ameba at one point and a wave of motion sweeps over the animal
as it gradually draws away. But watch a smart dog pick up the sound of a
footstep, the scent of a rabbit! In the ameba the changes spread slowly
through generalized protoplasm; in the dog they are received, conducted, and
interpreted with great speed through the consummate performance of the
nervous system.
Response. It would take an extraordinary light to excite the nerve cells
whose fibers compose the optic nerve. On the other hand, an unbelievably
faint one will stimulate the rod cells in the retina of a dark-adapted eye be-
cause they are specialized receptors of light. The light changes them and the
changes are communicated to the nerve cells.
A receptor is a group of cells, one cell, or part of a cell that is particularly
sensitive to certain stimuli (Fig. 16.1). External receptors receive stimuli
from an animal's surroundings, temperature, light, sound, touch and others.
279
280
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
Proprioceptors (e.g., muscle spindles) receive stimuli arising within the
body, such as pain and variations in the tension of a muscle or tendon. Re-
ceptors are changed chemically or physically, often in both ways, by stimu-
lation. These stimuli set up impulses, actually changes going through the
nerve cells with which the receptors are in contact. Thus the receptors bring to
the nerve cells the raw materials with which they work. Many receptors,
such as those of the muscle sense of position, are associated with nerves that
end in the spinal cord and the cerebellum where their activity is below the
scnsorij
cqII
m.TjscLc
cell
cell
nerve
rtet
muscle
cell
Fig. 16.1. Diagrams of simple associations of receptor and eflfector cells, those
that receive the stimulus and those that act in response to it. Left, A receptor cell
(sensory) in direct contact with an effector; right, a more complex type in which
conducting cells (nerve net) act as middle men. Such arrangements occur in
simpler animals, e.g., sea anemones. (Courtesy, Parker: Elementary Nervous
System. Philadelphia, J. B. Lippincott and Co., 1919.)
region of consciousness. We do not decide to turn over in our sleep. Other
receptors such as sight and hearing are intimately connected with the higher
centers of the brain. Sense organs are parts of the nervous system but are
more conveniently discussed in another chapter (Chap. 17).
Conduction. A pinch at one end of a fresh frog muscle immediately starts
waves of contraction moving toward the opposite end. When a cell is injured
by a microdissection needle a "death wave" begins at the point of injury
and quickly overspreads the cell. This is the conduction that is characteristic
of all protoplasm. In paramecia and some other protozoans conductile
fibrils connect the basal bodies of the cilia with each other and with a center of
coordination near the mouth (Fig. 16.2). Conduction reaches its highest de-
velopment and speed in the nerve impulse.
Nerve Cell
Characteristics. Nerve cells or neurons are the basic units of structure in
the nervous system. Each neuron has threadhke extensions called fibers.
Chap. 16 CONDUCTION AND COOKDINAilON NERVOUS SYSTEM 281
sometimes of extraordinary length. Currents of energy, the impulses, move
along these fibers and by means of them messages are flashed to other neurons.
The nervous system contains millions of neurons like electric wires which
are protected and insulated from one another except at the tips of their
fibers (Fig. 16.3). The body of the neuron is relatively large. Their unique
Nissl granules or bodies (named for the neurologist, Nissl) disappear when
Fig. 16.2. The neuromotor system of a paramecium. The unified action of the
cilia on the surface of its body and food passage is controlled through the extraor-
dinarily fine fibrils that connect them. Changes proceed rapidly over these fibrils
and they conduct them as our own nerve cells do. A paramecium takes in food
because its cilia "agree" to wave it into the mouth. An outline of paramecium
with the mouth, gullet and posterior end of the body in the same position as in
the greatly magnified view. A cut through the body. The sharp lines of the con-
ducting fibrils are shown in the right half of the mouth and gullet and a particle of
food trapped among the fibrils at the lower end. These fibrils are visible only with
skilled preparation. To the naked eye the whole animal is only a minute white
fleck in the water. (Courtesy, Lund. Univ. of California Pubs, in Zoology, Vol. 39,
1935.)
the neurons are fatigued or injured by toxins as in poliomyelitis (infantile
paralysis), but are restored by rest or removal of the harmful agent if the
damage is not already too severe. There are two kinds of fibers, dendrites
through which nerve impulses come into the cell and the axon through which
they leave it (Fig. 16.3). In their evolution as in their embryological develop-
ment neurons originate from epithelial cells from which processes grow out-
ward.
Dendrites are commonly short with treelike branches. But there are ex-
ceptions: certain of the neurons whose cell bodies are located in the ganglia
of the spinal nerves may have a dendrite several feet long. These dendrites
carry incoming messages of sensation from skin, muscle, and other parts of
the body and compose a section of all branches of the spinal nerves. Such
dendrites are always figured in diagrams of a reflex arc (Figs. 16.9, 16.1 1 ). The
282
im INIIRNAL LNVIRONMLNT OI- THL BODY
Part III
Ccrvtrol
System.
Axon.
Cu.n.Tt\ye\vT\a.'l«d.)
CoUcttano.V
Axon.
(m.yclirwi.tad)
CoUa,-t«t»aLl
AxoTV
TTvyaUruxtacL
otrtd. coven^d VilK
■n.eupol«TT\TT\OL
NodLe of P>.a.nvl«i»
E>T»cincK«6 and Vn. tt\u%cI«
Fig. 16.3. Diagram of a multipolar nerve cell, i.e., one with more than one
dendrite (process that conducts an impulse into the cell body). Most of the nerve
cells in the human body are multipolar. The processes of nerve cells have contacts
or synapses; branches of the axon of one cell in contact with the dendrites of
another or with the cell body of another or with both. Such contacts are essential
for the coordination of the body and for memory. (Courtesy, Ham: Histology,
ed. 2. Philadelphia, J. B. Lippincott Co., 1953.)
main distinction between dendrites and axons is a purely functional one: the
dendrites conduct impulses toward the body of the nerve cell and the axons
away from it.
Axons are usually long, often several feet, since in man many of them ex-
tend from the spinal cord to the toes. The axon ends in a brush of short
branches which in the case of muscle may actually pierce the cell membranes.
The axon of the spinal ganglion cell gives off branches (collaterals) along its
course in the spinal cord, e.g., one at the level of the fourth and another
at the fifth rib, ending in a synapse with the dendrites of other nerve cells.
Many muscle cells are stimulated by impulses from one or a few nerve
cells (Fig. 16.4). One pinprick in the back starts impulses speeding over
Chap. 16 CONDUCTION AND COORDINATION NERVOUS SYSTEM 283
several dendrites via axons and collaterals to the dozens of muscles that
one uses in jerking away from the pin.
How are nerve fibers nourished, especially the slender axons that reach far
from the cell body? Whether nerve fibers are inside the brain and spinal
cord or in the nerves outside, most of them are clothed with a soft, fatty, non-
cellular substance, the myelin sheath. Outside the brain and cord, that is, in
Fig. 16.4. Diagram showing how a few nerve cells may communicate with many
others. Responses to one pin prick travel far. Sitting on a pin may cause a high
jump. (From The Living Body, Copyrighted 1952 by Henry Holt and Company.
Reprinted with their permission.)
the nerves, each fiber is further protected by a cellular sheath, the neurilemma
(Fig. 16.3). The nerve cells within the brain and cord are supported by the
processes of neuroglial cells. These resemble nerve cells but have no con-
ducting power. The fatty myelin causes the whiteness of nerves and the white
matter of the brain and cord, which is composed of great numbers of mye-
linated fibers. Regions where no myelin is present appear gray, as in the
gray matter of the brain and cord and in certain nerves of the autonomic
nervous system.
Regeneration of Nerve Fibers. A nerve fiber may be completely severed,
yet the part between the cell body and the cut may remain alive and regenerate
after the injury. On the side of the cut separated from the cell body the fiber
disintegrates since no cell body is left to nourish it, but its cellular neurilemma
tube persists and takes a remarkable part in the repair. As the regenerating
fiber grows longer, it actually enters the empty neurilemma sheath and finally
extends through its whole length. Later a new myelin sheath is formed
around each fiber, and with this healing of many fibers the function of the
nerve is finally restored. Sprouting nerve fibers sometimes cross a distance
of several millimeters to reach the neurilemma sheath without which effective
regeneration does not occur. In facial paralysis the hypoglossal nerve may be
cut and its proximal end sutured to the distal end of the facial nerve. The
284
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
hypoglossal nerve will then regenerate along the pathway of the facial nerve
and will control the facial muscles (Table 16.1).
Ganglia, Nerves, and Neuroglia
A ganglion is a group of nerve cell bodies. In invertebrates, the ventral
nerve chain is a series of ganglia connected by nerves (Figs. 16.5, 16.6). In
nerve
cords
HYDRA
PLANARIAN
EARTHWORM
GRASSHOPPER'
Fig. 16.5. Nervous systems of representative invertebrates; except in hydra,
they are on the ventral side of the body, each one a series of ganglia connected
by nerves. They show the segmentation that is characteristic of the central nervous
systems and very evident in the arrangement of the human spinal nerves. (Cour-
tesy, Storer: General Zoology, ed. 2. New York, McGraw-Hill Book Co., Inc.,
1949.)
ganglion cells
connective tissue
Fig. 16.6. A small ganglion (autonomic). A cross section showing the bodies
of eight large nerve cells; //. cut ends of fibers of nerve cells; v, blood vessel with
blood cells. The ganglion is enclosed in a sheath of connective tissue. (Courtesy,
Nonidez and Windle: Textbook of Histology, ed. 2. New York, McGraw-Hill
Book Co., Inc., 1953.)
Chap. 16 CONDUCTION AND COORDINATION NERVOUS SYSTEM
285
Fig. 16.7. A nerve from a kitten's tongue. A, Cross section showing the cut ends
of nerve cell fibers. In each one the dark center is the axon; the pale ring around
it is the sheath. The nerve is enclosed in a sheath of connective tissue. B, Longi-
tudinal section. (Courtesy, Nonidez and Windle: Textbook of Histology, ed. 2.
New York, McGraw-Hill Book Co., Inc., 1953.)
the vertebrates ganglia are prominent in the dorsal roots of the spinal nerves.
Nerves are bundles of nerve cell fibers that convey sensory and motor
impulses between the brain and spinal cord and other parts of the body.
Sensory nerves contain fibers that conduct impulses from the sense receptors
to the cord or brain, e.g., the optic nerve from the eye. Motor nerves contain
fibers that conduct impulses from the brain and cord to muscles or glands.
The trunks of the spinal nerves contain both sensory and motor fibers, as do
some of the cranial nerves (Table 16.1).
Conduction — the Nerve Impulse
The nerve impulse is not yet understood. The statements that follow may
apply to it as a whole or only to a process which accompanies it.
The nerve impulse is an electrochemical process that passes through a
neuron. It represents conduction at its highest development and speed. The
impulse enters the cell through the dendrites and passes through the cell
286 THE INTERNAL ENVIRONMENT OF THE BODY Part III
body and axon, lis rate of movement varies in different animals and within
different nerves of the same animal; in warm-blooded animals it may travel
300 feet or more per second, about the speed of a pistol shot.
Experiments and refined measurements have shown that the impulse is
not a purely electrical current as was formerly thought. It is an electro-
chemical reaction involving the consumption of oxygen, production of carbon
dioxide, the freeing of heat, and modification (depolarization) of electrical
charges on the surface membrane of the nerve fiber, followed quickly by their
restoration (Fig. 16.8). One such change starts another one just ahead
and thus the process travels along a fiber. It is something like a fuse burning
along a wire but the nerve fiber is in no way harmed by the passing of the
impulse. An impulse cannot be started unless the stimulus is of a certain
intensity, but beyond that the strength of the stimulation makes no difference
in the speed of the impulse. The stimulus is like a spark that may start
a small fire or a large one. The processes in the nerve impulse are in some
ways similar to those of muscular contraction. The nervous tissue, however,
expends an extraordinarily small amount of energy compared to muscle.
Nerve cells are not easily fatigued. Impulses pass over a nerve cell sepa-
FiG. 16.8. Diagram illustrating the membrane theory according to which the
nervous impulse is an electrochemical process that passes through a nerve cell. The
resting nerve fiber is polarized, that is, the outside is positively charged, and the
inside negatively charged. A, B, C, A stimulus passing along a fiber involves a
change in the membrane and a loss of polarization. In an interval of from one to
five one-thousandths of a second later the fiber becomes repolarized again {%) and
the fiber is ready for another impulse to pass over it. In any given nerve, stimulus
of a sense organ, perhaps a voice that is heard, results in hundreds of nerve im-
pulses each one on a nerve cell fiber that is insulated by its sheath from others
beside it. (By permission from Biology: Its Human Implications, 2nd. ed., by
Hardin. Copyright, 1952. W. H. Freeman and Company.)
Chap. 16 CONDUCTION AND COORDINATION— NERVOUS SYSTEM
287
rately and in quick succession like bullets from a machine gun. The time
between them is the interval of restoration of the electrical charges, called
the refractory period because it is the instant when progress is balked for
about one to five thousandths of a second.
Association of Nerve Cells by Synapses
Synapses are points of contact between nerve cells (Figs. 16.9, 16.10). In
passing from cell to cell every impulse must go through a synapse, but it can
do so in only one direction. (This is in contrast to the movement of an impulse
through the axons and other parts of individual nerve cells which experiment
has shown may be either toward the cell body or away from it.) A synapse
is a junction of resistance through which impulses pass more slowly than
along the nerve fibers. The passage of impulses varies in different synapses
and in different physiological conditions of the same synapse. It may be rapid
and easy or it may be almost or completely stopped. This is true in the brain
when words escape one's memory, then suddenly return. By their selective
resistance synapses determine that the proper muscles reply to certain stimuli
in an orderly fashion while others remain inactive. They are, at least in part,
the basis of the relative quickness of accustomed reaction and thinking and
also of relative slowness or nervous fatigue.
|— REFLEX ARC — ,
I synapse I
efferent i afferent
nG-uron ^_^ neutron
(iTLotor)\ ^^ (sen^orij)
cell bod-Q of aff. neuron
Fig. 16.9. Synapses, the places of communication between nerve cells. Diagram
of the synapse of two nerve cells of the earthworm. A change occurring in a sen-
sory cell in the skin is conducted over the axon to its end branches that are inter-
twined with the dendrites of the motor cell. From there it passes through the cell
body and axon of the motor cell to the muscle. At the synapse the fibers appear
continuous but observation has shown that they are only in contact. (After Parker.
Courtesy, Ham: Histology, ed. 2. Philadelphia, J. B. Lippincott and Co., 1953.)
288
THF. INTERNAL ENVIRONMENT OF THE BODY
Part III
Fig. 16.10. Photograph of a model of the body of a nerve cell from the dorsal
(or sensory) horn of the cat's spinal cord. It shows the enormous number of
fibers of nerve cells whose end bulbs are in synaptic relation with this cell. The
model was made by fitting together individual models made from serial sections
of the cell. (After Haggar and Barr. Courtesy, Ham: Histology, ed. 2. Phila-
delphia, J. B. Lippincott Co., 1953.)
Tropisms, Responses, and Reflexes
Tropisms and reflexes are responses to stimuli that have a definite standard-
ized pattern. Tropisms make up almost the whole behavior of plants and
lower animals. One sunflower plant turns toward the sun; all sunflower plants
respond to the sun in the same way. Turn a bright light on a cockroach and
it will scuttle to the nearest shadow; any other cockroach would do the same.
Almost anybody chokes when a crumb starts down his windpipe; and all
chokes have a more or less standard pattern. Tropisms are movements of
the whole body toward or away from the stimulus, as a housefly turns toward
the light. Reflexes are more often movements of a part of the body, the flick
of a cat's ear when its edge is touched, the snapback of one's hand at the
touch of a spark.
Tropisms. Insects are drawn to light or dark, that is, they are positively
or negatively phototrophic to light. But they are so physiologically attuned
that their reactions to light are changed by temperature and humidity, and
vary with particular phases of their lives. In the mating flight the queen honey-
bee rises for the first time in her life, high into the sunshine, and for only the
second time she flies with the swarm on a brilliant day. Outside of these two
occasions, both concerned with the reproduction of her species, a queen bee
stays in complete darkness within the hive.
Chap. 16 CONDUCTION AND COORDINATION NERVOUS SYSTEM 289
Responses, Unlearned and Learned. Reflexes are the prompt responses of
muscle, either voluntary or involuntary (Fig. 16.11). Naturally they are un-
learned actions, such as the wink of the eyelids at a flash of light. Learned
responses may be established through the conditioning of unlearned ones,
and are then called conditioned reflexes or responses. Human behavior is
largely made up of reflexes. They begin at birth with the first breathing, a
response to an accumulation of carbon dioxide and lack of oxygen in the
Brain
sing of 0
motor pathway.
Medulla
Lower leve
of cord -
An area of cord
not sfiown
Sensory fiber
from skin
Crossing of a
sensory pathway
Muscle t;i^L
Fig. 16.11. Diagram showing the courses of nerve fibers within sections of
spinal nerves, and in nerve cord and brain. A reflex response is represented on
the lower right side. It involves: a sensory cell carrying an impulse from the skin
to the cord; an adjustor cell carrying the impulse from the dorsal or sensory horn
of the gray matter of the cord to a cell in the ventral or motor horn; and, a motor
cell conducting the impulse to the muscle which then contracts. This response
may occur without association with the brain. The diagram also shows the course
of responses that are associated with the brain. An impulse enters the cord on
a sensory fiber. In the cord this passes to the fiber of another cell, and upward
over a succession of cells to the cortex of the brain. There, it passes to the fiber
(dendrite) of another cell and on through a succession of cells finally reaching
the one that bears it along to the muscle. The way upward is the sensory path-
way; the way downward is the motor pathway. The diagram shows the crossing
of cell fibers from the right to the left side of the nerve cord in the sensory
(ascending) pathway and from the left to the right in the motor (descending)
pathway. The right foot is pricked and moves but the order to do this comes from
the left side of the brain.
290 TMi: INTHRNAL ENVIRONMENT OF THE BODY Part III
blood. This, like other true rellexes, is an experience common to all indi-
viduals of one species or to many species within a large group. A young
mammal sucks milk whether it is cat, whale, or human. Other pure reflexes
arc the quick closure of the eyelid when something comes near the eye; the
sudden pullback of the hand that is pinched; the sharp recovery of balance
lost in a stumble.
Conditioned responses, formerly called conditioned reflexes, were demon-
strated by Ivan Pavlov (1849-1936), a Russian physiologist. Over and over
he attracted a dog's attention by the sound of a bell, then gave it food, and
observed the flow of its saliva. After being fed many times at the ringing of
the bell, then the bell was rung but no food was offered. In such cases, the
bell alone stimulated a flow of saliva from the dog's mouth. Pavlov christened
the response a conditioned reflex but the better name, conditioned response,
is taking its place. It is habit formation like eating, sleeping, and waking at
definite times.
The Functional Unit — the Reflex Arc or Reflex Response. The action of
a great number of reflexes never goes higher than various levels of the spinal
cord, never enters the brain at all. Many that seem simple actually involve
many nerves and muscles and are very complex. One may touch a hot iron
with one's hand and pull it away, skew the body suddenly, and step back.
The action is all reflex of which the brain is notified only by means of asso-
ciated nerve cells. One of the simplest of human reflexes is the knee-jerk.
This is well known, not only as an example of a simple reflex, but as the one
used in routine tests of nervous adjustments. A slight blow on the kneecap
(actually on the patellar ligament) when the legs are crossed will normally
cause the foot to jerk forward. The jerk will not occur if the sensory roots of
the spinal nerves are damaged, as in locomotor ataxia (tabes dorsalis) or
if the gray matter of the cord is damaged, as in infantile paralysis.
The basic unit of function called the reflex arc is typically carried out
by five parts: (1) a sensory receptor cell; (2) a conductor, the sensory
nerve cell; (3) a connecting or adjuster nerve cell (in the cord); (4) a con-
ductor, the motor nerve cell; and (5) an effector, muscle or gland cell
(Figs. 16.9, 16.11).
Actual Conditions of the Reflex Response. The usual diagram of a reflex
arc shows a single sensory nerve cell by which an impulse is transmitted
directly to a single motor cell or with an adjustor cell between them. Actually,
in all vertebrate animals, the simplest stimulus starts impulses through several
sensory fibers with a volley of them following one another in quick succession
along each fiber. And each sensory cell fiber is not in contact with only one
adjustor or one motor cell but with several of them. It is only when impulses
arrive at almost the same time via a number of sensory fibers that the motor
nerve cells are finally activated. A certain degree of stimulation (summation)
Chap. 16 CONDUCTION AND COORDINATION NERVOUS SYSTEM 291
must be produced at the synapse before the motor nerve cell receives and
transmits an impulse.
The Nervous System of Vertebrates
The nervous system is complex but its parts work together in complete
unity. It is divided only on the basis of location, special function, and con-
venience of description. The central nervous system is the spinal cord and
brain; the peripheral system includes the spinal and cranial nerves and their
branches, all of the surface nerves; the autonomic nerves control involuntary
functions, especially those of the internal organs (Figs. 16.12, 16.13, 16.15).
Peripheral Nerves
The trunks of the peripheral nerves issue from the brain and cord. Their
large branches extend through the arms and legs and the walls of the body
Fig. 16.12. A general rear view of
the human nervous system. It pre-
sents the brain and certain of the
cranial nerves, chiefly the facial ones,
the spinal cord and the spinal nerves
that divide and subdivide extending
to every part of the body. The fusions
of nerves at the shoulder and hip
levels are called the brachial and
lumbarsacral plexuses, respectively.
(From Vogel: Der Mensch. Leipzig,
Barth, 1930.)
292
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
Fig. 16.13. The underside of the human brain and the roots of the cranial
nerves. Lines are extended from the roots to show the structures with which each
nerve is associated. See also Table 16.1. (Courtesy, Ciba Collection of Medical
Illustrations. Drawings by Frank H. Netter, 1953.)
and divide into small branches supplying the muscles, skin, and other struc-
tures. They contain the sensory processes over which all impressions of the
environment are brought into the spinal cord and brain. They bring to us
the raw materials of mind, everything we know of the world. Axons of motor
neurons in the brain and cord carry the impulses that direct movements of
muscle which largely comprise behavior. Facial expression is muscular ex-
ercise; so is a large part of personality.
Chap. 16 CONDUCTION AND COORDINATION NERVOUS SYSTEM 293
Spinal Nerves. The spinal nerves occur in pairs, one on cither side of the
nerve cord. In the higher vertebrates, including man, each trunk branches
from the nerve cord by two roots; the dorsal one contains nerve cell fibers
that carry sensory messages into the cord, and the ventral one contains those
carrying messages to the muscles and glands (Figs. 16.12, 16.14). The cell
bodies of fibers in the dorsal root are contained in its ganglion, but those of
the ventral root are always in the gray matter of the cord. Soon after the
sensory nerve fibers have entered the spinal cord they may come in contact
with adjustor neurons and so take part in reflex responses or they may
participate in carrying messages to the brain.
White matter
Gray matter
Dorsal root
(sensory)
Synapse
Centra
cana
Communicotinq ram
Sympathetic ganglion
Intestine
(Cross section)
Skin
(receptor)
Skin
receptor)
untary)
fector)
Sensory nerve cell
Motor nerve cell
Adjustor nerve cell
O-
-f (somotic)
- (somatic)
-< (somatic)
(visceral)
(visceral )
Fig. 16.14. Diagram showing the close association of the nerve cells (autonomic
or sympathetic) that control involuntary muscles of the intestine and those that
control the voluntary or somatic muscles. Only a few are drawn out of the great
numbers of cell bodies and fibers in the cross section of the cord. Cell fibers
carrying impulses from sensory stimuli in skin or muscle enter the cord through
a sensory root; cells bearing impulses that result in contraction of muscle leave
the cord by a motor root. Cell fibers of the autonomic nerve form part of this
motor root. All of the thousands of fibers are one way passages. (Modified from
Neal and Rand: Comparative Anatomy. Philadelphia, The Blakiston Co., 1936.)
C£fiCB^OM
MIDBRAIN
LACRIMAL GlAND
SALIVARY .,0L AND
LACRIMAL GLAND
SALIVARY GLAND
STOMACH
DUODENUM
PANCREAS
URINARY BLADDER
GONADS AND SEX
ACCESSORIES
URINARY BLADDER
GONADS AND SEX
ACCESSORIES
PA^AS YMPA THE TIC
(CRANIO- SACRAL)
SrMPATHETIC
(THORA CO-L UMB A R)
Fig. 16.15. Diagram of autonomic nerves that carry impulses to various
organs. Figures of the organs are repeated on each side in order to avoid over-
lapping of the pathways. The diagram emphasizes the fact that each organ re-
ceives a double supply of autonomic nerves, the parasympathetic and the sym-
pathetic nerves carrying antagonistic impulses that increase or decrease activity.
The Arabic numerals on the cord stand for spinal nerves, the Roman numerals
for cranial nerves. (Courtesy, Turner: General Endocrinology. Philadelphia, W.
B. Saunders Co., 1948.)
294
Chap. 16 CONDUCTION AND COORDINATION NERVOUS SYSTEM 295
Branches. Each spinal nerve trunk contains sensory and motor nerve-cell
processes from the respective roots. After a spinal nerve trunk emerges from
the vertebral column it divides into several branches, one supplying the
muscles and the skin of the back, another the sides of the body, still another
branch contains fibers of spinal and autonomic nerve cells (Fig. 16.14). The
size of the spinal nerves depends upon the functional demand in the area
supplied; in man the largest ones extend into the legs.
Plexuses. Different nerves may join and form a plexus in which their
fibers are bound together (Fig. 16.12). As a result, one nerve that reaches
a muscle may contain fibers of several nerves and all of them may stimulate
the muscle.
Cranial Nerves. Most persons are more regularly aware of cranial than of
spinal nerves since the former are in control of smiles and toothaches as well
as of sight and hearing (Fig. 16.13 and Table 16.1).
Autonomic Nervous System
The autonomic (involuntary) part of the nervous system is largely in con-
trol of internal organs that are more or less continuously active, such as the
alimentary canal, blood vessels, lungs, and heart (Fig. 16.15). The activity
of most of these is essential to life. Each one, the heart for example, is in-
nervated by nerves carrying impulses that have antagonistic effects; impulses
via one nerve hasten its activity, those in the other slow it. In the autonomic
nervous system there is an almost total absence of voluntary control. The
movements of the heart cannot be slowed by willpower as the tongue can
be halted. In this system neither afferent (sensory) nor efferent (motor) fibers
are directly connected with the higher centers in the cerebral cortex. Thus
stimulation of sensory autonomic fibers does not result in any conscious
sensation such as that which results from impulses carried to the brain by
the fibers of peripheral nerves. We do not feel the dust in our lungs or food
entering the stomach.
In general the autonomic nervous system is one of multiple reflexes and
adjustments beyond the direct control of the individual, a great insurance
of safety in crises when voluntary action often fails. It is entirely absent
in the invertebrates but becomes progressively more elaborate in the verte-
brates, especially in mammals. The whole system was formerly called the
sympathetic system. It is now divided into two parts, the sympathetic and
parasympathetic systems. Of the double sets of autonomic fibers whose
impulses have antagonistic effects on various internal organs, one set is in a
sympathetic and one in a parasympathetic nerve (Table 16.2, Fig. 16.15).
In the autonomic system, two neurons always make up the efferent or motor
pathway of an impulse, a contrast to the single neuron in the motor pathway
of the ordinary reflex arc.
Tdhle 16.1
Names and Main Functions of the Human Cranial Nerves*
Number
Name
Structures Innervated by
Structures Innervated by
Motor (Efferent) Fibers
Sensor (Afferent) Fibers
I
Olfactory
None
Olfactory mucous mem-
brane of nose (smell)
II
Optic
None
Retina of the eye
(sight)
III
Oculomotor
Muscles of movement of
eyeballs, with IV and
VI
Muscles of accommoda-
tion of eye
Iris (constriction of
pupil)
Muscles lifting the eye-
lids
IV
Trochlear
Muscles of eye move-
(pulleylike)
ments, with III and
VI
V
Trigeminal
Muscles of chewing
Structures of sensation
in scalp, face, teeth.
mouth
VI
Abducens
Muscles of eye move-
(drawing aside)
ment with III and
IV
VII
Facial
Muscles of facial ex-
Taste buds of anterior
pression, salivary
two-thirds of tongue
glands
VIII
Auditory
None
Internal ear (hearing)
Vestibular
None
semicircular canals
(senses of movement,
rotation, balance)
IX
Glossopharyngeal
Muscles of pharynx
Mucous membrane of
(tongue and
(swallowing)
pharynx
pharynx)
Salivary glands
Taste buds of posterior
third of tongue
X
Vagus
Muscles of larynx
Mucous membrane of
(wandering)
(speech)
larynx
Muscles of pharynx
Lungs (reflex control of
(swallowing)
rate of breathing)
Esophagus, stomach.
Stomach (hunger sense)
small intestine
(peristalsis)
Glands of stomach
(secretion), muscles
of bronchial tubes
Heart
XI
Spinal accessory
Muscles which turn the
head
None
XII
Hypoglossal
(under tongue)
Muscles of tongue
None
* Nerves of muscle sense are omitted.
296
Chap. 16 CONDUCTION AND COORDINATION NERVOUS SYSTEM
Table 16.2
Antagonistic Action in the Autonomic System
297
Organ Innervated
Action of Sympathetic System
Action of Parasympathetic System
Digestive tract
Slows peris'talsis and decreases
activity
Quickens peristalsis and increases
activity
Urinary bladder
Relaxes bladder
Constricts bladder
Heart
Quickens heartbeat
Slows heartbeat
Arteries
Constricts arteries and raises
blood pressure
Dilates arteries and lowers blood
pressure
Muscles in bron-
chial tubes
Dilates passages
Constricts passages
Muscles of iris
Dilates pupil
Constricts pupil
Muscles of hair
root
Causes hair to stand erect
Causes hair to lie fiat
Sweat glands
Increases sweat
Decreases sweat
Sympathetic System
The sympathetic nerves originating in the thoracic and lumbar regions
of the cord have a regulating influence on a great number of structures
(Table 16.2). The cell bodies of the first of two efferent neurons are located
in lateral regions of the gray matter of the cord and their axons extend out
through the ventral roots of the spinal nerves along with the axons of ordi-
nary motor cells (Fig. 16.14). After passing through a motor root the axons
of the sympathetic neurons separate from it and become the autonomic
branch of the spinal nerve leading to a vertebral sympathetic ganglion.
These ganglia contain the cell bodies of the second of the efferent neurons
whose axons go to the internal organs. They constitute a series of pairs
with one member on each side of the spinal cord (Fig. 16.15).
Parasympathetic System. The parasympathetic group consists of nerves
with the first of their efferent neurons in the brain stem and the sacral region
of the spinal cord. Each vagus nerve which well earns its name arises from
the medulla, passes down the chest and abdomen, and mainly innervates the
heart, respiratory system, and the digestive system as far as the large in-
testine.
The peripheral ganglia, containing the second of the efferent neurons of
the parasympathetic nerves, are usually near or in the organs innervated.
These as well as the vertebral ganglia of the sympathetic nerves are the loca-
tions of synaptic connections outside the central nervous system. This is a
unique characteristic of the autonomic nervous system.
298
THF INTFRNAL ENVIRONMENT OF THE BODY
Part III
Central Nervous System
The spinal cord is the main connection between the brain and all parts of
the body except regions of the head. It varies greatly in length, extends to
the end of the body in fishes and snakes, is shortened in mammals, and
reaches only to the small of the back in the human body. It contains a
central fluid-filled canal, a remnant of the once open gutter of the embryonic
nervous system. When the cord is cut across, two substances are readily
distinguishable, a central, roughly H-shaped area of gray matter surrounding
the canal, and a border of white matter around this (Fig. 16.16). In the gray
matter there are many cell bodies, but the white matter consists of great
PRAY AISD WHITE f^ATTER OF SPmA.L CORD
Post<2PlOP
Post. Hoprv
Gfcxy
A.n.t. Koprx
Cen.-tra.l cartal
corn.rru.^'i u. r»«
WKlte
■m.ci.tt«t>
fiber
Ant. Kopn. cell
rTu.clcu.'o of
rKzarosUoL cell
r^«i?ve fiber*
r^ycUn. sK«o.tK
Fig. 16.16. Cross section of the human spinal cord showing the gray matter
containing nerve cell bodies and their fibers, and the white matter containing
their fibers only. In the brain the gray matter is outside and the white within. The
central canal is continuous with cavities (ventricles) of the brain. (Courtesy, Ham:
Histology, ed. 2. Philadelphia, J. B. Lippincott Co., 1953.)
Chap. 16 CONDUCTION AND COORDINATION NERVOUS SYSTEM 299
numbers of myelin-wrappcd fibers that extend up and down the cord. The
regions of gray and white matter are continued into the brain, but in the cere-
bellum and cerebrum their positions are reversed and thus the cortex or outer
layer of the brain, is gray matter.
The gray matter of the cord contains neurons with various functions.
Among them are connecting neurons (adjustors) that transmit impulses from
one neuron to another in the same or in different levels; motor neurons, always
in the ventral horns of the gray matter, that carry impulses to skeletal muscles
and glands; and neurons of the autonomic system that carry impulses to the
interna' organs and other structures.
In the white matter, the axons are segregated in bundles of fibers of similar
function. Great numbers of axons of cell bodies in the spinal ganglia carry
sensory impulses to the brain. These are the ascending tracts. There are also
axons from cells in the gray matter of the brain, carrying impulses to motor
cells in the ventral horns of the gray matter of the cord which then relay them
to the muscles. These are the descending tracts (Fig. 16.11 ). If we are sud-
denly pricked by a pin, we not only jerk involuntarily, which is the reflex
action, we also know about the prick and may remove the pin. The apprecia-
tion of the prick and removal of the pin depend on the sensory impulses to the
brain, the association of cells in the higher centers, and a complex of impulses
to the muscles. In part these relationships have been found out by observing
symptoms in persons whh injured nerve cords and correlating these with de-
stroyed tracts found when the spinal cord was examined after the patient's
death. With some diseases the patient cannot locate his arms and legs without
looking at them and must watch his feet in order to walk. This is due to the
destruction of the nerve cells responsible for transmitting the sense of position
of muscles and joints to the central system.
Vertebrate Brain
The brain is the master coordinator of the bodily activities of an animal and
of its awareness and adjustment to the environment. The brain and chief sense
organs are appropriately so located that wherever the animal travels, they
lead on and arrive there first. Every creeping baby has that experience.
General Description. The vertebrate brain is the bulbous front end of a
tube whose walls are composed mainly of nerve cells (Fig. 16.17). In fishes
and other lower vertebrates, its outer surface is smooth and the cavity within
it is relatively large in comparison to the thickness of the walls. In mam-
mals, and especially in man, its surface dips and bulges and the cavity within
it is relatively small compared to the thickness of the walls. The great pile
of nervous tissue that makes up the cerebral hemispheres is a comparatively
late development in animal history, and the cerebral cortex with its billions
of interrelated neurons is largely a mammalian achievement (Fig. 16.18).
300
niL inti:rnal environment of the body
Mesencephoion
Prosencephalon Rhombencephalon
Part III
A.
Telenceph.
/^ a b c ^=5aaEiiiD
>■ -^^^ ■"^-^-^ ^^^
I
Three parts
B.
C.
Myelenceph.
Optic
nerves
Cerebral
hemisphere
(Telencephalon)
Optic nerves
Five main parts
D.
Outline of
lenceph.
Mesenceph.
Cerebellum
(Metenceph.)
Medulla
(Myelenceph.)
L Her\ie cord
Fig. 16.17. Diagrams to illustrate some of the changes that transform the
smooth bulb at the anterior end of the neural tube into a highly complex brain.
A and B, Median vertical sections of brains showing the changes that occur in
given regions, e.g., the telencephalon, as they appear in certain primitive chordates
and (C) certain vertebrates. D, Outlines of the brain seen in a human embryo of
about 11 weeks' development, 50-60 mm length, about two inches. {D redrawn
and modified from Patten: Human Embryology, ed. 2. New York, The Blakiston
Co., 1953.)
During its development the nervous system is at first on the outside of the
body. At one period in the life of every vertebrate, fish or human, there is an
open groove that extends the whole length of the central nervous system. Later
when the system takes its place inside the body, only the central canal in the
cord and the communicating ventricles of the brain are left of the former
open ditch.
The working parts of the brain are composed of many types of neurons
associated in groups for general and specialized functions. Thus the human
brain is divided into districts occupied by neurons that control definite parts
Chap. 16 CONDUCTION AND COORDINATION NERVOUS SYSTEM 301
of the body (Fig. 16.17). Although the cerebral lobes are distinct and the
brain is definitely bilateral, there are great numbers of intercrossing fibers
that insure synchronous action of the parts. The left side of the brain controls
the right side of the body, and vice versa; the fibers from one side of the cord
and brain cross to the opposite side. The sensory axons (bearing impulses
to the brain) cross over in the brain, in the medulla or above; the motor
OLFACTORY LOBE
CEREBRAL
HEMISPHERES
PINEAL BODIES
OPTIC LOBES
CEREBELLUM
MEDULLA
SPINAL CORD
PERCH
FISH
FROG
AMPHIBIA
ALLIGATOR
REPTILE
PIGEON
BIRO
CAT
MAMMAL
Fig. 16.18. Five types of vertebrate brains. In the cat's brain the pineal body
and the optic lobes are present, but are hidden by the cerebral hemispheres.
(Courtesy, MacDougall and Hegner: Biology. New York, McGraw-Hill Book Co.,
Inc., 1943.)
axons (bearing impulses to the muscles) cross over in the midbrain, pons,
medulla and at various levels below it (Fig. 16.11).
Meninges. The brain and cord are enclosed in three protective coverings
of connective tissue v*'ith spaces between them filled with cerebrospinal fluid
(Fig. 16.19). The innermost one, pia mater (tender mother) is very delicate
and carries many blood vessels. It is intimately associated with the arachnoid
layer so called because of its open spaces like a spider's web; these are filled
with cerebrospinal fluid. The outermost cover, dura mater (hard mother),
is made of tough connective tissue, contains many blood vessels, and adheres
tightly to the cranium and vertebrae. Meningitis is an inflammation of the
meninges, especially the pia mater and arachnoid.
Cerebrospinal Fluid. The central canal and the ventricles are continually
moistened by the cerebrospinal fluid. Most of this is formed by vascular
glands, the choroid plexuses, located in the ventricles. Much of the fluid
makes its way through holes in the choroid plexus of the fourth ventricle and
enters the space between the two delicate coverings of the brain and cord
so that these organs are actually surrounded by a blanket of fluid (Fig. 16.20).
It is produced more or less continuously and the excess is drained off through
hollow, button-shaped projections (villi) that dip into open lakes of venous
blood in the dura mater. The cerebrospinal fluid diffuses into the blood
through the thin caps of the villi whenever its pressure is higher than that of
the venous blood.
302 THE INTERNAL FNVIRONMF.NT OF THF BODY Part III
Brain Size. In evolutionary history, the greatest increase in the size of
brains occurred as animals began to live on land. Swimming in the sea was
monotony and ease compared with clambering through the ooze and over
the hillocks on land. The adaptation to the details and variety of land living
left its mark on the brain as it did on the legs and feet. The human brain is
heavy in proportion to the weight of the body. Its weight varies with the age
and size of the individual. Except in extreme cases such as some defective
individuals there appears to be no correlation between size and weight of
the brain or number of cerebral convolutions and the degree of intelligence.
Primary Divisions of the Vertebrate Brain
The embryonic brain is the key to the structure of the adult brain. In its
earlier development, the brain is a single hollow enlargement whose cavity
is continuous with that of the nerve cord. In the third week of human life,
two constrictions indicate three regions, the fore-, mid-, and hindbrain (tech-
nically called the prosencephalon, mesencephalon, and rhombencephalon).
In the fourth week, a constriction forms two subregions of the forebrain,
the endbrain and the between brain, respectively the telencephalon and
diencephalon. In man, the hindbrain is set off into two regions, the future
cerebellum and future medulla (metencephalon, myelencephalon) about the
fifth week. The brain is then composed of five primary regions, from anterior
to posterior: (1) endbrain, (2) between brain, (3) midbrain, (4) future
Arachnoid frabecula.
Subdural space.
Arachnoid membrane
PiQ mater.
Arachnoid villus Dura moter.
Superior saqiHol smus
Endothelium
Suborochnoid space Fcix cerebri Corlex cerebri
Fig. 16.19. Relation of the meninges, the protective covers of the brain, the
pia mater (tender mother) next to the brain, the dura mater (tough mother) next
to the skull and the arachnoid (spiderweb) layer between them. The spaces in
the latter are filled with the cerebrospinal fluid, a modified tissue fluid. All of
the layers also surround the spinal cord including the spinal fluid. Diagram of the
layers as they overlie the brain. Excess cerebrospinal fluid drains through the
arachnoid villus, one of many that extend into the blood of the sinus. This occurs
in many areas of the brain. (After Weed. Courtesy, Ham: Histology, ed. 2. Phila-
delphia, J. B. Lippincott Co., 1953.)
Chap. 16 CONDUCTION AND COORDINATION NERVOUS SYSTEM
303
Arachnoid
trabecula
Arochnoid
memb'one.
Cerebrol
vein.
Pia maler.
^«o»A Feri'vQSCL/lgr
^ spoce.
L'Hinq cells of
perivascular
space.
Copil'ory wilhm
pencopillory
spoce -
Fig. 16.20. Diagram of the relations of the pia mater, the arachnoid and the
blood vessels of the brain. The pia mater dips into the channels of the larger
blood vessels. This figure shows the possibilities of broken blood vessels that create
hemorrhage of the brain. (After Weed. Courtesy, Ham: Histology, ed. 2. Phila-
delphia, J. B. Lippincott Co., 1953.)
cerebellum, and (5) medulla. Each of them is the location of sense organs
and controls for which it is nicknamed: endbrain, "nose brain"; midbrain,
"eye brain"; medulla, "ear brain." From the fishes onward through the
mammals these sense organs are located according to this plan. In higher
animals the main parts that develop from the fundamental regions are as
follows :
1. Endbrain. In fishes, this region is practically limited to the sense of
smell, whereas in mammals this sense is relatively little developed. In mam-
mals, the corpora striata have a stabiUzing effect upon the muscles in walk-
ing. In man, the cerebrum overtops the rest of the brain and its cortex is a
supreme achievement of the human species (Figs. 16.17, and 16.21).
2. Between brain. This is the main pathway of the fibers between the
spinal cord and the cortex of the cerebral hemispheres, and by means of it
all other parts of the brain and the body are connected with the higher cen-
ters of control. The main substance of the between brain is in the side walls
of the third ventricle which are collectively called the thalamus. So called by
certain early anatomists who believed the enclosure to be a room in which
vital spirits were imparted to the optic nerves. The thalamus is the anterior
end of the primitive brain stem, the oldest part of the brain, a center of the
autonomic nervous system, and such functions as temperature regulation
and the awareness and expression of emotions (Fig. 16.21).
3. Midbrain. The midbrain is a small part that connects larger ones. Its
floor is a part of the brain stem, very old in evolution. In fishes, the mid
brain is the original eye brain. In mammals, the four-fold bodies (corpora
quadrigemina) in its roof are, in a limited way, centers of visual and audi-
tory reflexes.
304
THE INTERNAL ENVIRONMF.NT OF THE BODY
Part III
gray ma'lVar
of cerebral
corVe
a qijrus
a sulcus
h.xjpo^hQlainus
optic chiasma
pi'luitarij qland-fpars
[pars p
ost.
phorioid
us
CGn'tral
carxal of
spinal cord.
Fig. 16.21. The right half of the human brain. A piece has been cut from the
front of the cerebral hemisphere in order to expose the lateral ventricle. This
ventricle and its mate in the left hemisphere, and the central third and fourth
ventricles have developed from the central canal of the primitive brain and cord.
The hypothalamus that forms part of the floor of the third ventricle is believed to
have an important part in controlling the secretion of the adrenocorticotrophic
hormone (ACTH) under conditions of stress. The cut in the cerebral hemisphere
reveals the thickness of the cortex, the shaded gray matter whose area is greatly
increased by the folds that are absent in lower animals. It is estimated that there
are 10,000 million nerve cells in the cortex of the human brain, each one having
synaptic connections with several others. The number of pathways in these highest
centers of the brain is beyond imagination. (Courtesy, Ham: Histology, ed. 2.
Philadelphia, J. B. Lippincott and Co., 1953.)
4. Hindbrain {cerebellum). On its dorsal side the hindbrain is com-
posed of the cerebellum and on its ventral side, of the floor of the ancient
brain stem. The pons, a bridge of nerve fibers including those that connect
the cerebellum and cerebral hemispheres, forms a part of the floor of the
fourth ventricle. Some of the main functions of the cerebellum are the main-
tenance of unconscious muscular coordination, and the preservation of
muscular tension or tonus. It contains numerous connections with the eyes,
ears, muscles, joints, and other parts of the body.
5. Hindbrain {medulla). The white appearance of the hindbrain is due
to the fibers of the nerve cells being on the outside as they are in the spinal
cord. It is the great passageway for nerve fibers that extend along the sides
and form the swollen cords of the pyramids. Its cavity is the fourth ventricle
and its thin roof, the choroid plexus, is one of the main sources of cerebro-
spinal fluid.
Chap. 16 CONDUCTION AND COORDINATION NERVOUS SYSTEM
305
The medulla is the entrance and exit way of nerve impulses to and from
the spinal cord and brain and to and from the last six pairs of cranial nerves,
including the widely effective vagus nerves. It is the center of the autonomic
control of important body -functions such as respiration and the rate of the
heart beat.
Table 16.3
Principal Structures of the Adult Human Brain with Their
Locations in the Basic Divisions
Forebrain
Endbrain
(Telencephalon)
Olfactory lobes, cerebral hemispheres, cor-
pora striata, corpus callosum
Contains the lateral ventricles
(Prosencephalon)
Between brain
(Diencephalon)
Thalamus, pineal eye stalk, infundibulum
(or pituitary stalk), optic nerves vi'ith their
crossing (chiasma)
Contains the third ventricle
Midbrain
(Mesencephalon)
Optic lobes
Fibers of nerve cells (white matter) form
walls and floor
Contains the cerebral aqueduct
Hindbrain
After brain
(Metencephalon)
Cerebellum, pons (a bridge of nerve cell fibers
in mammals)
Contains part of fourth ventricle
( Rhombencephalon )
Cord brain
( Myelencephaion )
Medulla
Great passageway of fibers of nerve cells
Contains part of fourth ventricle
Features of the Human Brain
The history of ihe cerebral hemispheres of vertebrates is one of the most
spectacular in comparative anatomy. It begins with them as smooth paired
outgrowths of the forebrain, the centers of olfactory sensation in the nose
brain of fishes. Later with the adoption of land life, animals had to clamber
and creep, and their cerebral lobes became large and important centers of
sensory correlation. Finally, with the mammals, the same lobes became a
great superstructure reared on the old primitive nervous system. This newer
part of the brain is the center of the nervous functions which in man have
been developed far beyond those of any other animal.
The brain is a bilaterally symmetrical organ that acts as a unit in the inver-
tebrates as well as in the vertebrates. The action of the hind legs of a grass-
hopper is as well timed for a take-off as that in the hind legs of a kangaroo.
Cerebrum. In contrast to its smoothness in other vertebrates the surface
of the mammalian cerebrum is usually increased by fissures and by folds
called convolutions (Fig. 16.21). The fact that these convolutions give more
306 THF, INTfRNAL F.NVIRONMF.NT OF THF BODY Part III
space for nerve cells is more significant than their arrangement or character
which are not unique. They are similar in higher mammals, and the brains
of normal human beings greatly resemble one another no matter how different
the mental ability of their owners. After years of study, no structure has
been found in the human brain which is actually different in kind from those
present in the brain of a chimpanzee. In man, the size of the cerebrum com-
pared to the rest of the brain is far greater than it is in the apes. An adult
human cerebrum weighs around three times that of an adult gorilla. The
layer of gray matter, called the cerebral cortex, is about one-eighth of an
inch thick. By counting the cells in small areas and using such counts as a
basis of computation, it is figured that the human cerebral cortex alone
contains some 9280 millions of nerve cells. Most of these are provided with
long nerve fibers, chiefly axons, that extend for relatively great distances and
branch in different directions, connecting each cell through the junction-like
synapses with the cells in many different centers. The total number of such
connections and nervous pathways is inconceivably great.
Fiber Tracts. The wires of a telephone exchange are grouped in cables
and distributed on a switchboard according to a system. In like manner, the
nerve fibers that have similar functions extend in bundles or tracts through
the white matter of the brain and cord (Fig. 16.11), and from there they have
synaptic connections with other nerve cells which continue into nerves. In
spite of the complexity of their arrangement, the make-up of the main nerve
tracts has been analyzed. The courses of various series of nerve cells have been
traced from receptors, such as those involved in a pinched toe, to the appro-
priate center of adjustment in the cortex. Likewise, the motor pathways
have been traced, in this case from the cortex to the muscles which move
the foot.
In general, nerve cells in the cortex of the right side of the brain com-
municate with muscles on the left side of the body, and likewise those on the
left side of the brain communicate with the right side. Throughout the white
matter of the spinal cord and brain there are intercrossing fibers. As a result
the spinal cord and the two parts of the brain are bound together structurally
and functionally by an unthinkably complex network. Untangling these facts
began centuries ago and is not finished. Much has been learned from dissec-
tions, microscopic examinations, and experimental studies. Hundreds of
observations on how the human nervous system works have been made
when injuries to it made this possible.
Functions of Cerebral Cortex. The human cerebral cortex is the location
of intelligence, of reasoning powers, of consciousness and of memory. The
brain acts as a coordinated whole. However, it is well known that different
areas of the cerebral cortex function differently.
At the extreme rear of each cerebral lobe is the visual area (Fig. 16.22).
Chap. 16 CONDUCTION AND COORDINATION NERVOUS SYSTEM 307
Destruction of this causes blindness, even though the eyes may be normal; a
blow on this part of the head makes one "see stars." The auditory area is
near the temple and injury to this causes deafness, loss of the interpretation
of sound although the sound receptors may be normal. Along and behind
the central fissure (Rolando) which extends from about the middle of the
head to the top of the ear is an area associated with various bodily sensations:
muscle sense, pressure, temperature, and pain. Patients are able to report
their sensations when this region is exposed and is stimulated by electricity. On
Medulla
Fig. 16.22. Outline of left half of the human brain with the mental functions
of certain areas indicated. For example, the visual association area contains the
cells essential to interpret and coordinate the objects seen. (After Morgan. Cour-
tesy, Boring, et al.: The Foundation of Psychology, New York, John Wiley and
Sons, 1948.)
the opposite side of the central fissure is the motor area. When different
parts of this area are stimulated, movements of the fingers, legs, or throat
can be produced. A cerebral hemorrhage or "stroke" in this region on the
left side causes paralysis of muscles in the right side of the body and vice
versa. Both of these areas, the sensory and the motor, are laid out like a
map with different places representing different parts of the body. There are
also association centers, such as the auditory and visual ones, believed to
be concerned with the remembrance of things heard and seen; these regions
have been only partially explored. Although the whole cerebral cortex is
concerned with thinking, the capacity to direct it and to lay out plans of
living appears to be located in the front regions of the cerebrum.
Brain Waves
Several waves per second of electrical activity are produced by the brain
even when a person is resting. Recordings of brain waves are obtained by
fastening electrodes to different parts of the scalp by adhesive tape and
picking up the currents by a recording apparatus. The records (electro-
308
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
encephalograms) show that brain cells are more or less continuously active.
This is clearly demonstrated by a frog's brain, which when taken from the
body and kept alive, will continue to generate electrical waves for some hours.
All brain waves are so feeble that they can be recorded only after being
received by a very sensitive apparatus and magnified millions of times. They
occur rhythmically, and sleep is the only normal condition in which they
are much altered. During sleep, the records show waves that are slower and
wider, sometimes broken by irregularities, beheved to be caused by dreams
(Fig. 16.23). Brain diseases change the brain waves, and epilepsy can be
diagnosed by characteristic wave patterns.
Sleep
The average individual spends from a quarter to a third of his lifetime in
sleep. All observations on sleep have ended in the conclusion that animals
cannot live without it. However, the actual nature of sleep is unknown and
this is especially true of the part played in it by the nervous system.
ExciTeo -
RCLAXCD
DROWSY
ASLEEP
DEEP SLEEP
I SEC
so;xv.
Fig. 16.23. Sleep and excitement in the human brain. Records of electrical
waves produced in the normal human brain. Excitement is characterized by very
frequent waves and sleep by irregular, less frequent ones. In the (upper) sleep
record there was a "sleep spindle" of frequent waves every 14 seconds. (After
Jasper. From Penfield and Erickson: Epilepsy and Cerebral Localization. Spring-
field, 111., Charles C Thomas, 1941.)
17
Responsiveness— Tlie Sense
o
r^ans
Sense organs are gateways to the mind. All that we know of our surround-
ings is brought to us through them. It is difficult to imagine our existence
without them. What would it be?
Receptors. Receptors are cells or parts of cells that are especially sensitive
to particular conditions in their surroundings. Sense organs include receptors
and associated cells. Those that are affected by external things are most
familiar for they include sight and hearing, taste and smell, touch and tem-
perature. Other receptors are sensitive to situations within the body, the
stretch and pressure of muscles, the movements of internal organs. We feel
comfortable in one chair and not in another, we feel thirsty, or we know
that we have eaten too much. We make hundreds of adjustments of the body
without being aware of any of the sensory signals concerned with them
although such signals are constantly being given by these active internal
receptors.
A receptor responds to stimuli only when they are of a certain kind and a
particular intensity called the threshold of sensation. Within limits, the inten-
sity of a given sensation increases with the stimulus to a certain point, then
there is a sensory adaptation and the sensation decreases, and a limit may be
reached when there is no sensation at all. The first piece of candy is the sweet-
est. The stronger the odor, the more quickly it fades. A solution of camphor
can be smelled for about five minutes.
External Tactile Senses — Touch, Pain
Protoplasm is in general sensitive to slight differences in pressure whether
sensory structures are present or not. Some of the nerve fibers end without
any coverings and these probably react to any stimulus from muscular cells
309
310
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
or other cells around them (Fig. 17.1). Others end in cellular capsules
containing a clear jelly. In such capsules the nerve endings can be affected
by slight or by heavy pressure without injury. In man and other mammals,
pressure receptors of this type, the Pacinian corpuscles, are located around
joints and tendons, in fingers and toes, and in deeper parts of the body,
e.g., the mesentery, wherever pressure is apt to be. The Pacinian corpuscle
is oval; when it is pressed it elongates and the nerve fiber within it is stretched
(Fig. 17.1). Nerve endings are twined around the roots of hairs that are
sensitive to the slightest pressure, even of air currents, such as, the hairs in
Meissner's corpuscle
Sebaceous gland....;
End bulbs of Krause j Hair
Smooth
muscle
t
Tactile disks Free nerve endings
Epidermis • Dermis
Nerve ending Subcutaneous Pacinian Duct of Ruffini
around hair fat corpuscle sweat gland ending
Fig. 17.1. Diagram of the nerves and end organs in a section of human skin.
Not all of these endings are to be found in any one area. Cold receptors; Meissner's
corpuscles, abundant on the palm side of the fingers. Touch receptors; nerve end-
ings entwined about the hairs often with end feet applied to them; highly developed
association with cat's whiskers. Pressure and possibly vibration receptors; Pacinian
corpuscles, abundant in skin of palms of hands and feet, internal organs and
mesentery. Heat receptors; Ruffini endings lie deep in the skin. Receptors for pain
are not shown; they consist of bare nerve cell fibers extending about and between
cells. The number of structures present in a small area of skin may be realized
in connection with the sweat glands one of which is shown in this diagram. By
counts of limited spaces at least 3000 sweat glands are calculated to be in the
skin of the palm of the hand. (Courtesy, Gardner: Fundamentals of Neurology,
ed. 2. Philadelphia, W. B. Saunders Co., 1952.)
Chap. 17 RESPONSIVENESS THE SENSE ORGANS 311
a cat's ear, and the vibrissae or whiskers of cats and rats, and less evidently
human hairs.
In the insects and other arthropods, bristles are connected with receptors.
Although ants are armored in chitin they are exquisitely in touch with their
surroundings by way of their bristles. Human skin contains several types
of sense organs. By testing a small area of skin, point by point, receptors
for touch, pain, cold, and heat can be found at different locations. Pain may
be slight or very intense. The mild pain of pinpricks can be definitely located,
but pains deeper in the body are rarely so precisely determined.
Temperature — Thermoreceptors
Little is known about temperature receptors except those in higher animals
(Fig. 17.1). Protoplasm is sensitive to temperature although no receptors
may be present. Insects are highly responsive to it; ants move their young
from one to another part of their underground nests as the temperature
changes during the day. In temperatures around 30° C. and under controlled
conditions of humidity adult mosquitoes {Culex fatigans) react to differences
as slight as five-hundredths of one degree.
Internal Senses of Muscles and Viscera
Many impulses from these receptors reach the higher centers of the brain
and consciousness, but many others end in the spinal cord and cerebellum
below the level of consciousness. Proprioceptors are sensitive to changes in
the tension of muscle and tendon. Such changes stimulate impulses to the
brain, making us aware of the position and movements of our arms and legs
and other parts of the body, the interplay through which the body is kept
in a balanced position. Interoceptors are important in regulating the activities
of the lungs, alimentary canal, and other viscera in which they are located.
They bring about reflex control of internal organs through centers in the
medulla and thalamus of the brain. Some of these impulses go through to
the higher centers of the brain and are responsible for such sensations as
having had "a good dinner."
Chemical Senses
These are the common chemical sense and the twin senses of taste and
smell. All chemical receptors are alike in their requirement that particles of
a substance must be dissolved and in actual contact with them. We can taste
sugar only when it is chemically associated with certain taste buds; we can
smell roses, skunks, cheese, or vanilla only when their essences enter the
olfactory cells.
Common Chemical Sense. The surface of the bodies of fishes and am-
phibians is sensitive to chemical substances of a very mildly stimulating
312 THE INTERNAL ENVIRONMENT OF THE BODY Part III
character. Taste and smell are closely allied to the common chemical sense
from which they sprang. The catfish, Amiurus, has taste buds along the sides
of its body and will turn and snap at bait that is suspended near its flank. Its
skin can be stimulated by very weak chemical solutions even after its taste
buds have been isolated by cutting the nerves leading to them.
Taste. The sensation of taste never acts separately as vision and hearing
do. Smell plays the largest part in what is called taste, and pressure and
temperature have their shares in it. By one or another kind of receptor we
not only perceive the sourness of lemonade, but its temperature, its weight
on the tongue, and the consistency which helps or hinders its spread over the
tongue. Substances can be tasted only when they are in solution and their
molecules are moving about freely.
Fig. 17.2. Sense of smell in honeybees. Outline
of head showing segments of antennae. Cutting an
antenna at the line aa leaves one segment that bears
sense organs of smell. Cutting at bb leaves no sense
of smell. See also figure 30.27.
Taste and smell are highly developed in insects and because of this, insects
are important to humanity both for good and bad. Bees smell and taste nectar
and pollen, and in gathering them accomplish the cross-pollination on which
the production of many fruits depends (Fig. 17.2). Their sense of smell
guides certain moths and butterflies to lay their eggs on particular host plants
on which the young caterpillars will feed. But the same moths and butterflies
will readily lay eggs on the wrong kinds of plants if they have been sprayed
with extracts of the host plants. Houseflies are quickly attracted by odors of
food, fruitflies (Drosophila) by ripening fruit, and female mosquitoes by
body odors. Ants, bees, and wasps smell through their antennae, as is readily
shown by tests made after these have been removed. Honeybees can taste
by receptors in their mouth parts and they as well as the wasps, Vespa and
Polistes, can distinguish plain from sweetened water. They also can recog-
nize sweet, bitter, and salt as separate qualities. Out of 34 sugars and related
substances, 30 are sweet to human taste, but only nine are sweet to honeybees
and all of these are in their natural foods. The sweeter the mixture of cane
sugar, the more of it the worker honeybees will drink. The sweeter the
mixture that foraging honeybees discover, the more will they excite workers
in the home hive by dancing when they return from successful foraging trips.
Chap. 17 RESPONSIVENESS THE SENSE ORGANS 313
In order to taste something, mammals must have the substance on their
tongues. Nearly all of them are adept at stretching these tongues outside their
mouths, cattle licking salt blocks and human beings licking anything.
Taste Receptors in Man. Special sense organs known as taste buds are
imbedded in the mucous membrane of the soft palate and upper surface of
the tongue. Their name comes from their bud-like shape, but they are quite
as much like bottles with small mouths, the pores that open into furrows
that surround them (Fig. 17.3).
Anything which is tasted must get into the bottle and bring about the
chemical reaction with the receptor cells with which the dendrites of the
facial or glossopharyngeal nerve are in contact. These reactions start impulses
to the brain, ending in the sour taste of pickles, or the sweet taste of sugar.
Salty substances are tasted quickly, bitter ones more slowly, due partly to the
distribution of the taste receptors. All four kinds of taste receptors are on
bitter
Fig. 17.3. A, diagrams of the human tongue and the distribution of the four
tastes, sweet, sour, bitter, salt; the central part of the tongue is insensitive to taste.
The closeness of the dots represents the number of the sense organs. The rings at
the back mark the papillae, each holding a battery of taste buds. B, section of a
papilla of the tongue (much enlarged) with taste buds in the groove that surrounds
the papilla. C, human taste bud greatly magnified. Saliva mixed with food juice, for
example onion, enters the bud through the poie at the top and chemical reactions
take place between it and the sensory cells. Impulses pass over the sensory fibers to
the brain, where the little understood process of interpretation occurs and maybe
the flavor of candy or of onion is revealed. (After Parker: Smell, Taste and the
Allied Senses in th^ Vertebrates. Philadelphia, J. B. Lippincott Co., 1922.)
314 THE INTERNAL ENVIRONMENT OF THE BODY Part III
the upper surface of the tongue, salt and sweet in bands near the tip, sour
near the sides, and bitter in the center near the root. The receptors of bitter
taste are in a few conspicuous papillae ranged in a V-shaped line far back on
the tongue. They have been called vallate papillae because each one is shaped
like a turrcted castle surrounded by a flooded moat, in this case, a tasty
flood (Fig. 17.3).
Smell. At least among mammals, smell is the most democratic of the senses.
Whatever minute particles there may be in the air, and of whatsoever kind,
they are hospitably drawn into the nostrils. The nose of mammals is not only
a heater, humidifier, and cleaner of air but through the sense of smell it is
a testing place of the chemical nature of the surroundings. In spite of all they
smell and think they smell, man and other primates are only feeble smellers
as compared with cats and dogs and other mammals. A man looks as he
walks; a dog smells as he runs.
In the human nose, the mucous membrane on each side is raised upon
three ridges of the turbinate bones that spring from the outer nasal wall.
Each cavity of the nose is thus incompletely divided into compartments. The
lower ones are passages that are open behind, and air slips through them
into the pharynx; the uppermost one is a narrow cleft directly under the
floor of the skull. The olfactory organs are pale yellow patches of cells on the
walls of this cleft (Fig. 17.4). Their location, as it were, in the attic, sets
them off the main roadway of incoming and outgoing air. Each breath of
cool air pushes the warmed air up into the olfactory attic where it is poised
over the smell receptors till more air comes in. The exposed ends of the
receptors bear slender processes that are always wet with mucus. Fibers
arising from the other ends of the receptors are grouped together in the
olfactory nerves that pass through the skull to the olfactory centers in the
brain (Fig. 16.13).
Although man's sense of smell is relatively weak, even so, it will respond
to remarkably small amounts of substance. A synthetic substitute for the
odor of violets (ionone) can be detected when it is present as one in over
30 billion parts of air. The sense of smell of a particular substance is
fatigued in a few minutes, but will then react to a new odor; all recoveries
are rapid since odors are diffused through the air. The smaller, lighter par-
ticles spread most readily, and as they are scattered farther apart the chance
of inhaling them lessens. The aroma of coffee thins quickly; the scent fades
on yesterday's rabbit tracks; the odor of last night's cigarette lingers and
changes.
Equilibrium — Statoreceptors
The great majority of animals, grasshoppers, fishes, or cows, have an up-
side and a downside, and it is very important that the owner be informed of
Chap. 17
RESPONSIVENESS THE SENSE ORGANS
315
Olfactory
hairs
sustentacular
cells
sensory
cells
basal
eel I s
Fig. 17.4. Human olfactory organ. A, side wall of nasal chamber with the pro-
jecting turbinate bones and the clefts between them. Arrows indicate the course of
air in ordinary breathing. B, sniffing the air brings it forcibly against the olfactory
organ located under the lobe marked by the circle of arrows. C, diagram of sensory
receptor cells of smell with their supporting cells (sustentacular). The sensory cell
has a single dendrite which extends to the exposed surface where it is expanded
into a bulb which bears delicate processes, the olfactory hairs. These processes ex-
tend into a film of mucus that covers the surface of the organ. Extremely minute
particles of substance inhaled, whether skunk, garlic, or lily fall into the fluid and
upon the ends of the olfactory cells. A chemical change immediately occurs, passes
through the sensory cell, and by way of an olfactory nerve to the cells in the brain
and the interpretation of the odor. {A and B after Biology: Its Human Implications,
2nd ed. by Garrett Hardin. Copyright, 1952. W. H. Freeman and Company. C
after Smith, Canadian Med. Assn. J., 1939.)
the positions of these and keep them where they belong. This is brought to
them through the statoreceptors.
The majority of active multicellular animals have these paired organs of
equilibrium, of essentially the same structure wherever they occur. A stato-
receptor is a more or less spherical sac containing fluid and freely movable
granules, the statoliths. Minute bristles that project into the fluid are attached
to sensitive cells in the wall of the sac, and these in turn touch the nerve
fibers. The statolith is attracted by gravity; it rolls about, always resting on
the downside, and its pressure upon the bristles is the stimulus of the receptors.
316
THE INTERNAL ENVIRONMENT OF THE BODY
Part m
When they molt, lobsters and crabs shed the linings of their organs of
balance together with the bristles and statoliths, and new linings and bristles
are regained and new grains of sand worked into the sacs. Recently molted
crayfishes that have been supplied with particles of iron will work them
into their sacs, and thereafter will respond to a magnet. When the magnet
is held directly above the crayfish, it pulls the particles of iron against the
bristles on the upperside of the sac. In response to the unusual position of
the particles, normally on the downside of the sac, the crayfish soon turns over
and swims on its back.
Human equilibrium is a complex affair that depends upon vision, muscle
sense (proprioceptors), sensitiveness to pressure in the soles of the feet, and
paired organs of equilibrium. Each of the latter consists of two small sacs,
the saccule and utricle, and three semicircular canals, all a part of the inner
ear but not taking any part in hearing (Fig. 17.5). Hairlike processes of
sensory cells project into the cavities of the saccule and utricle each of which
contains a minute earstone or otolith of calcium carbonate. Gravity pulls the
otolith against particular hair cells; this stimulates them and initiates im-
pulses to the brain through the nerve fibers with which they are associated.
As the head is tipped this way and that, the otoliths are rolled about,
always on the downward side. There are three semicircular canals, each
of them connected at both ends to the utricle and arranged so that each is
at right angles to the other two. Near one of the openings of each canal
Left
anterior
canal
Eusfachian tube
Left
external
canal
Left Right^
^posterior posterior'
canal canal
Right
external
canal
Fig. 17.5. Organs of balance, the human semicircular canals. There is a set of
three on each side of the head near the eardrum. Left, the three semicircular
canals shown in natural location with the bone cut away to show their nearness to
the middle ear. The coiled cochlea of the inner ear is deeply embedded in bone like
the semicircular canals but has no functional connection with them. Parts of mid-
dle ear shown here: m, malleus; /, incus; s, stapes; me, cavity of middle ear; tin,
eardrum; oe, cavity of outer ear. Right, diagram of the semicircular canals show-
ing their position with reference to the surface upon which the person stands up-
right, represented by a glass plane. The back of the head is toward the reader.
{Left, courtesy, Romer: The Vertebrate Body, ed. 2. Philadelphia, W. B. Saunders
Co., 1955. Right, courtesy, Guilford: General Psychology. New York, D. Van
Nostrand & Co., 1939.)
Chap. 17 RESPONSIVENESS THE SENSE ORGANS 317
into the utricle there is a bulbous enlargement containing sensitive "hair
cells" like those in the utricle and saccule but without otoliths. These cells
are stimulated by movements of the fluid (endolymph) contained in the
canals and their tips are lurned in whatever direction it flows. Since the
canals lie in three diff"erent planes, a movement of the head in any direction
will afi'ect one or more of them. If the outer ear is irrigated with cold water,
convection currents are set up in the fluid of the canals and the person
will turn dizzy although his head and body may be kept upright and quiet.
The receptors in the canals may be strongly afi'ected by vertical movements
like that of an elevator, and by the rolling motions of ships and planes.
Hearing — Phonoreceptors
Ears and eyes work together so closely that we scarcely realize which one
brings the news. The eye sees in straight lines in light but the ears hear in
light and dark, and around the corner. The eye sees what something is; the
ear hears what it does.
Probably relatively few invertebrates can hear. Insects that from very
ancient times have been land residents are the great exceptions; their per-
ception of vibrations of air is widespread. Fine hairs commonly borne on
the antennae of mosquitoes respond to vibrations. Crickets, cicadas, grass-
hoppers have tympanal organs or "eardrums" that vibrate in response to
the various clicks and squeaks which insects make with their legs and wings.
In the vertebrates, the ears developed and took on their special function
as these animals gradually assumed their life on land. The new organ of
hearing developed from the saccule which retains its connection with the
semicircular canals of equilibrium, but the functions of the old and new
organs remained distinct.
The Ear of Mammals — Man. The ears of mammals respond to vibrations
that are transmitted through air; this contains comparatively few particles so
that the vibrations or sound waves are relatively slow and widely diffused.
There is a great advantage in having the outer ear spread out to catch the
vibrations and a corridor to conduct them to the middle ear, from which they
are transmitted to the real sound receptors in the inner ear (Figs. 17.6, 17.7).
The outer ear includes the outgrowth of flesh called the auricle and the
passageway to the eardrum, the auditory canal. Auricles are more or less
trowel-shaped and well supplied with cartilage and muscle. Sounds are lo-
cated by the intensity of the sound waves that stimulate the receptors of the
inner ear. We turn our heads and cup our ears to catch more sound waves,
as dogs turn their heads and lift both ears or one toward the sound. The
ear catches the sound wave; the brain decides where it comes from and
what it is. Human auricles are almost immovable; we cannot prick them
forward and backward with the attention that is so becoming to dogs and
318
TUL INTERNAL ENVIRONMENT OF THE BODY
Part III
Fig. 17.6. The external ears of tree-shrew and man are strikingly similar in hav-
ing the rolled edge that is associated with their reduction in size. In some mammals,
i.e. hats, there are muscles by which the flap (tragus) can be pulled down over the
passage to the eardrum; in the human ear unfortunately this passage can only be
stopped with the fingers. The tree-shrew {Tiipaia tana) of southeast Asia is a small
generalized mammal that originated about 100 million years ago and is believed to
be an ancestor of the gorillas, man, and other primates.
horses, or drop one ear and lift the other toward the danger as rabbits do.
The sizes and patterns of auricles are correlated with the habits of their
owners, and picturesquely so, small in the burrowing chipmunks and wood-
chucks, large in horses and giraffes that gather sound waves on the open
plains, largest of all in African elephants, and most elaborate in bats that
are aware of ultrasonic sounds (Figs. 17.7, 17.8). A number of mammals,
especially seals and others living in the water, can close the entrance to the
Fig. 17.7. The enormous external ears of insectivorous bats. Left, European
long-eared bat; right, pallid cave bat of U.S.A. Bats hear ultrasonic sounds, wholly
inaudible to human ears. (After Allen: Bats. Cambridge, Mass., Harvard Univer-
sity Press, 1939.)
Chap. 17
RESPONSIVENESS THE SENSE ORGANS
319
''iiiniWJj
C1J.(W<WICRE
Fig. 17.8. A flying bat makes an ultrasonic cry completely inaudible to human
ears. The curved lines represent the sound waves of a single pulse or vibration. Bats
emit as many as 50 of these sounds per second and locate obstacles to their flight
by hearing the echoes. The sound waves are here represented in true proportion to
the size of the bat. When a bat's ears are stopped it strikes whatever is in its path.
(Courtesy, Boring et al.: Foundations of Psychology. New York, John Wiley &
Sons, 1948.)
auditory canal by a fleshy cover (tragus) that works like an eyelid. Man
and other primates have only hairs and wax to ward off insects, dust, and
water. The human ear has a cover at the entrance but has no means of pull-
ing it down. Neither human noses nor ears can close their doors.
Each middle ear is an air-filled chamber opening into the pharynx by the
Eustachian tube. The middle ear contains a chain of three little bones: the
malleus or hammer at one end of the chain is attached to the eardrum by
ligaments; the incus or anvil is the middle link; and the stapes or stirrup at
the other end of the chain is attached to the membrane of the minute oval
window in the bony capsule containing the inner ear (Fig. 17.9). Sound
vibrations are transmitted from the eardrum over the bony bridge to the
internal ear. Under the impact of faint sounds the eardrum is tightened, and
under that of loud sounds it is loosened by involuntary muscles that attach
it to the bony bridge. Thus the bridge becomes a lever transmitting light or
heavy vibrations to the inner ear. Vibrations are also transmitted by the
surrounding bone.
The inner ear contains the real mechanism of hearing, the organ of Corti,
triply protected by the membranous cochlea or cochlear duct, by surrounding
fluids, and by a casing of the hardest bone in the body (Fig. 17.9). The
structures of the cochlea are well known, but the details of the way in which
they work are still explained only theoretically. Only a bare outline of it can
be given here; books containing further details are given in the suggested
reading for this chapter.
The cochlea is divided into three fluid-filled cavities, the cochlear duct,
320
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
auricle or
pinna
semicircular canal
nerve
_ ductus
en.d.oltjiiiphaticu3
LOl sac
e diate
ot
hlecL
i lymph.
External
ear
Fig. 17.9. The human mechanism of hearing and adjacent organ of balance, the
semicircular canals. A diagram of the general structure shown by a cut through the
temporal bone. The inner ear is structurally but not functionally associated with
the semicircular canals. The pharyngo-tympanic duct (Eustachian tube) opens into
the pharynx. Pressure of air against the outer side of the eardrum is evident in
travel through a deep tunnel. It is balanced when the mouth is opened and air goes
through the Eustachian tube and presses against the inner side of the eardrum.
(Courtesy, Ham: Histology, ed. 2. Philadelphia, J. B. Lippincott & Co., 1953.)
and the vestibular cavity above and the tympanic below it. The latter two
are continuous, one into the other at the tip of the cochlea. At the base of the
cochlea the vestibular cavity comes to an end against the membrane filling
the oval window and the stapes. At the base of the cochlea, the tympanic
cavity is also ended by a membrane that closes the round window. When
the membrane of the oval window is pushed in toward the vestibular cavity
by vibrations in the middle ear, the fluid in the cavities is moved, finally
pushing against the resilient membrane in the round window (Fig. 17.9). As
the minute vibrations surge along through the fluid from the oval to the
round window, they vibrate the basilar membrane on the floor of the
cochlear duct which contains the actual organ of hearing, the organ of
Corti. Fibers of the auditory nerve extend to the receptor cells in this organ.
These cells are similar to those of taste and smell in that their hairlike
processes protrude into the fluid which floods over them. A delicate mem-
brane (tectorial) projects like a miniature porch roof over and so close to
the processes that the slightest jar of the basilar membrane brings them in
touch with it. Thus the receptor cells are stimulated, and they in turn excite
impulses that are transmitted to the brain by fibers of the auditory nerves.
Summary of Action. Sound vibrations move along the chain of bones
in the middle ear and against the membrane of the oval window, thereby
Chap. 17 RESPONSIVENESS THE SENSE ORGANS 321
pushing it inward. This starts corresponding vibrations that run through the
fluid for the length of the vestibular cavity and on into the tympanic cavity
toward the round window. As the vibrations travel along the cavities, each
one vibrates the basilar membrane and the sound receptors, more or less
strongly and in difterent regions, depending on its own character. Finally,
the vibration expends its force against the membrane of the round window
which it bends outward a little toward the middle ear. Our ability to dis-
tinguish different tones is due to the fact that the vibrations of a particular
tone pass more frequently through a certain part of the basilar membrane.
The nerve fibers ending in that part carry the impression of the tone to the
brain.
Vision — Photoreceptors
Light filters through the air in one direction; if it enters water it passes on
in a different direction. This change in direction is refraction and it occurs in
greater or lesser degree whenever light passes from one medium into another.
The amount of change in the direction depends upon the character of the new
medium and the angle at which the light enters it (Fig. 17.10).
Lenses. A lens is a transparent object with a curved surface. A drop of
water is a lens. The lens in the eye of a frog or a man contains thousands of
cells. Artificial lenses are commonly made of glass, of quartz, or of fluorite.
When the surface of a biconvex lens is properly curved all the rays that enter
it are brought to a focal point at a certain distance from it, called the focal
distance. This distance varies with the curvature of the lenses in cameras
and microscopes, as well as with the curvature of the cornea and lens in
the human eye.
There are various shapes of artificial lenses; the common foundation lens
.•,!;,
r1^t:I>^
Fig. 17.10. Formation of an image by a lens. Rays of light are reflected from
each point of a black arrow pointed end up. Rays from the right of the arrow are
intercepted by a glass lens and their courses are bent. Those from the lower end of
the arrow are turned upward; those from the upper end are turned downward.
Rays from every point of the black arrow are brought to a focus in a point beyond
the lens. These points compose a reversed image, shown by a white arrow pointed
end down. We see everything upside down. On the retina the legs of a horse point
up. The interpretation of the brain points them down. (Courtesy, Walls: The Ver-
tebrate Eye. Bloomfield Hills, Mich., The Cranbrook Press, 1942.)
322 THE INTERNAL ENVIRONMENT OF THE BODY Part III
is convex. Biconvex lenses are thick at the center and thin on the periphery.
Lenses in the eyes of fishes are spherical; in the eyes of mammals they are
usually oval and elastic. The sharper the curvature of a biconvex lens, the
shorter its focal distance, as in the nearsighted eye (Fig. 17.18). Light pass-
ing through a biconvex lens produces a reverse image (Fig. 17.10). Rays
reflected from the lower part of an object meet in the upper part of its image
and vice versa, creating a small picture that is upside-down. Likewise, the
rays reflected from the right side of the object pass to the left side of the
image and vice versa, thus the picture is not only upside-down, but its sides
are reversed. We learn the proper position of an object by experience and
after that we cannot imagine it otherwise. A cat's eyes show a mouse with
feet up, but her brain doubtless shows her a mouse with its feet down.
Image-Forming Eyes. Rays of light reflected from an object fall upon the
sensitive receptors and initiate chemical reactions within them which create
impulses in the associated nerve fibers. The impulses pass along the fibers to
the brain where they are interpreted (Figs. 16.11, 16.22). The number
and direction of the light rays and the nature of the receptors on which they
fall determine the character of the image they form. Lenses guide light rays
to form an image in the eyes of the great majority of animals. There are
three main types of image-forming eyes: (1) the exceptional pinhole eyes
of Nautilus, in which the rays are brought into diffused focus through a
minute hole in the front of each eye (Fig. 17.11); (2) the compound eyes
optic nerve Pigmented cells
of retina
Fig. 17.11. The pinhole eye of Nautilus, the paper
''"'"' sailor, a mollusk related to the octopus, is similar to
the pinhole camera which is in focus for all distances,
but only a little light is admitted and the image is dim
and foggy.
PINHOLE-CAMERA EYE
WITHOUT A LENS
of insects, spiders, and other arthropods, with a lens set into each one of the
multiple tubes so that no rays can reach the lens except from directly in front
(Fig. 17.12); and (3) the eyes of vertebrates with a single lens set in the
front of the eye where it receives light reflected at various angles from the
object (Figs. 17.13 and 17.14). The capacities of image-forming eyes are
matched by the habits and abilities of their owners to act appropriately for
what they see. A fish hawk flying a 100 feet above a lake not only sees a
fish beneath the surface but plunges unerringly after it even disappearing
into the water to clutch it. The vision of the fish hawk is significant because
Chap. 17
RESPONSIVENESS THE SENSE ORGANS
323
Twilight eye
Daylight eye
Twilight -^
eye
Daylight
eye
Ganglion
Optic nerve
Esophagus -
A. FRONT FACE
■■■■■■IS . \im
:>:■■•■• i-.
^
B. SECTION OF COMPOUND EYE
Fig. 17.12. Top, face of adult male mayfly, Callibaetis. In the compound eyes of
mayflies there are hundreds of lenses each one set so deep in a tube that no rays
can reach it except those coming from directly in front of it. The segment at the
top, "the daylight eye," provides detailed vision; the other segment, "the twilight
eye" provides images or general vision. The daylight eye of this mayfly is twice the
size of a period on this page. The majority of mayflies are twilight fliers. Bottom,
section of the eye of an adult male Callibaetis, highly magnified. {Bottom, after
Shafer: "Divided Eyes of Certain Insects," Proc. Wash. Acad, of Sciences, March,
1907.)
it fits into the bird's whole pattern of behavior (Fig. 17.13). Even if a jellyfish
had the eyes of a hawk, it would still lack the plunge of a hawk.
Chemical Reactions of the Light Receptors. Although eyes have developed
in different epochs of evolution and in widely different kinds of animals,
they almost universally contain lenses and carotenoid pigments (Figs. 17.14
and 17.16). The lens guides the light to the receptors; the carotenoid pigments
in the receptors take part in the chemical reactions that create the nerve im-
pulses passing to the centers of vision in the brain.
324
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
Fig. 17.13. Lejt, a section through the head of the White-bellied Swallow. Three
features are characteristic of birds, the most supremely eye-minded of all verte-
brates: the eyeball is relatively the largest among animals (the eye of an ostrich is
2 inches in diameter); a vascular nutrient organ, the pecten, is attached to the
retina which does not contain blood vessels; the retina, which is the essential, sen-
sory layer of the eye, is the most elaborate among animals, and the foveas or areas
of clearest vision, two in each eye are the most perfect. In the diagram the lines RT
and LT represent rays of light. They are reflected on the foveas from an object in
front of the bird and fall on the foveas in each eye creating two-eyed or binocular
vision. The lines HN and IN represent rays from objects visible on only one side
of the bird and fall only on one fovea creating monocular vision. Right, sections
of the retina of a crow and a dog, shown with the same magnification. They empha-
size the relatively great thickness of the bird's retina. The visual cells are near the
tops of the figures. Light enters from the bottom. (Courtesy, Wood: The Fundus
Oculi of Birds. Chicago, The Lakeside Press, 1917.)
Carotenoids are red and yellow pigments that are most abundant in plants,
especially carrots. They can be transformed to vitamin A (Table 11.1) and
are stored as such in the liver. Like other substances that have important
and common uses, they are plentiful and widely distributed. They are present
in light receptors of such great variety as the orange-red light spots of the
protozoan Euglena, the eyes of starfishes, squid, and crabs, and the rod and
cone cells of vertebrates (Fig. 17.17). Carotenoids are the visual pigments,
the purples, violets, and yellows often referred to in connection with the
rods and cones, especially of the human eye. They are being found in more
and more animals; three of them have recently been extracted by George
Wald from the cone cells of chickens (Table 11.1, vitamin A). Similar
studies by Wald and others indicate that the perception of light, including
color, is basically dependent upon these common pigments. Wald has stated:
Chap. 17
RESPONSIVENESS THE SENSE ORGANS
325
View Camera
sKuttec JidpKraigrn lens supporting photosensitive bldcKcned
atructunz film surface
lens scleral
coat
retina.
cKoroidL
6TS>*
Fig. 17.14. Diagrams illustrating the similarities of the human eye and the cam-
era. In both eye and camera there are two main processes: a physical one by which
rays of light are directed to a focus through one or more lenses and a chemical
reaction between light and light sensitive substances. In the eye, the latter occurs
in the rod and cone cells of the retina; in the camera it occurs within the coating
of the photosensitive film. In the camera, the glass lens can be moved nearer or
farther from the film in focusing. In the human eye, the shape of the lens as well
as its position are changed. In the camera, the amount of light is changed by the
diaphragm; in the eye, by the iris. (Courtesy, Ham: Histology, ed. 2. Philadelphia,
J. B. Lippincott & Co., 1953.)
"It seems likely now that photoreception, visual [image forming] or photo-
tropic [light turned] throughout all living organisms may be founded chem-
ically upon this single group of substances."
The Human Eye
The Eye and the Camera. The eye is a complex organ that takes pictures
again and again on the same light receptors (rod and cone cells) that re-
generate their own sensitivity and after one exposure are instantly ready for
another. The camera is a complex contrivance that takes pictures when prop-
erly operated and its light receptor, the coating of the film, can only be
exposed once because it never regenerates (Fig. 17.14). The evolution of
eyes has been going on many millions of years. The history of the camera
has been relatively short, even including the early suggestions of it in the
326 THE INTERNAL ENVIRONMENT OF THE BODY Part III
eleventh century. It has not been copied from the eye but has been built to
obtain similar results and automatically resembles the eye.
Path of Light through the Eye. In order to reach the retina light must
penetrate through: (1) the conjunctiva, the outermost covering of the front
of the eyeball; (2) the cornea, the transparent front part of the outer, tough
or sclerotic coat of the eyeball, actually a most important part of the eye
that brings rays of light to a focus (Figs. 17.14 and 17.16); (3) a transparent
fluid (aqueous humor) that fills the front chamber of the eye; (4) the
crystalline lens, important in accommodation (Fig. 17.14); and (5) the
transparent jelly (vitreous humor) that fills the back chamber and keeps
the eyeball expanded, and finally (6) the retina with its receptors, the rod and
EVE MUSCLE
WASHED
BT TEARS
EVE LIDS
OPTIC
NER\/Es
EYE MUSCLE
Fig. 17.15. The eyeball in its socket. It is set deep in a bony socket, packed
about by fat, curtained by eyelids, and washed by tears. Each eye is equipped with
six muscles by which the front of the ball is moved up or down, from side to side
and slightly rotated. (Reprinted from The Machinery of the Human Body by
Carlson and Johnson, by permission of The University of Chicago Press. Copy-
right 1948.)
cone cells. These curiously enough are seemingly turned away from the light,
a condition that can be explained by their development (Figs. 17.14 and
17.16). The back wall of the retina and the choroid coat behind it are heavily
pigmented and so absorb excess light. Rays of light pass freely through the
pupil which is surrounded by the iris, a circular curtain which is part of the
vascular or choroid coat of the eyeball.
A structure usually located in the choroid coat, called the tapetum lucidum
(L., bright carpet), acts as a fight-concentrating mirror and causes the night
eyeshine of many animals. Some tapeta, as in many hoofed animals, consist
of shimmering connective tissue fibers. In others, the cells are packed with
glimmering rodlets, as in the cat's eye, the brilliance of which encouraged the
Egyptians to reverence cats which could reflect the light of the sun even at
night.
Accommodation for Near and Far Objects. The image made by means of
the cornea and the crystalline lens is a very small picture upside-down on
the retina (Fig. 17.14). In sharpening this picture the eye accommodates,
Chap. 17
RESPONSIVENESS THE SENSE ORGANS
327
Gland
Aqueous
humor
Retina
Spot of clearest
vision
Optic
nerve
Fig. 17.16. General structure of the human eyeball cut in a vertical section, from
the top of the eye downward. The blind spot is the place where the fibers from the
cells in the retina leave the eyeball and form the optic nerve. When the eye is
directed upon an object it is placed so that the image falls upon the fovea, the area
of clearest vision.
that is, changes the focus of the rays from near or far objects by changes
in the curvature of the crystalline lens. This is made possible by stretching
or relaxing the tension upon the ring of the suspensory ligament attached at
one border to the lens and at the other inserted into the circle of ciliary
muscles. When these muscles contract they pull the choroid coat forward
and relax the tension on the ligament. The lens then becomes more convex,
taking its natural more spherical shape. Rays of light from nearby objects
are then brought to a focus on the retina in near vision (Fig. 17.14). When
at rest, the eye is adjusted for far vision. The eyeball is always distended by
the fluids within it and when the ciliary muscles are relaxed there is a chronic
pull on the suspensory ligament. This flattens the lens, and rays reflected
from distant objects are brought to a focus on the retina.
Imperfections in Convex Lenses. There are imperfections or aberrations
in biconvex lenses because the rays that penetrate their thin margins meet
in different places from those that pass through them near the center. The
spherical aberration of the lens of the eye is partially corrected by the curva-
ture of the cornea. Cameras usually have lenses with compensating curvatures
fastened to the convex lens (Fig. 17.14).
Chromatic aberration or color error is also characteristic of single lenses.
Short wave lengths are bent more strongly than longer ones. Thus rays of blue
328 THF INTERNAL ENVIRONMF.NT OF THE BODY Part III
lisht are brought to a focus sooner than those of red Hght, resulting in a
blur of white within a halo of color. All cameras are corrected for this defect
by combinations of lenses. In the human eye part of the color error is cor-
rected by the yellow tinge of the crystalline lens, actually a color filter that
passes rays of certain wave lengths, i.e., visible light, but stops the ultra
violet. Persons who have had the crystalline lenses of both eyes removed
because of cataracts can see in ultraviolet light which is not possible to
normal eyes.
The Iris — Regulation of Light. Too much light spoils the picture on a film
or retina. In the eye, excess light is stopped by the iris and reflections are
reduced by the black lining of the eyeball provided by the pigmented layer
of the retina and the choroid coat. The iris is a curtain containing a set of
circular muscle fibers that contract in bright light and decrease the pupil
and a set of radial muscle fibers that contract in dim light and enlarge the
pupil. Such responses to changes in light intensity require 10 to 30 seconds.
Flashlight photographs sometimes show the wide open pupils that did not
have time to close.
The muscles of the iris are controlled by autonomic nerves (Table 16.2).
Excitement of the sympathetic system, as in extreme pleasure, dilates the pupil.
Certain drugs affect the iris; atropine that dilates it is commonly used during
examinations of the eye.
The value of the iris as a curtain is increased by its content of dark pig-
ment. In the white races the front layers of cells of the iris are relatively free
from pigment and light passing through them appears blue, paler or deeper
depending upon the amount of black in the background. Varying amounts
of pigment distributed in the front layers of the iris are the basis for all the
varieties of hazel, brown, and black eyes. Absence of pigment lets the blood
vessels show, giving the pink eye of the albino.
The Light Receptors. The retina of man and most vertebrates contains two
kinds of light receptor cells, the rods and cones, and many associated neurons
(Fig. 17.17). The retina is connected with the brain by the bundle of thou-
sands of axons that compose the optic nerve. Its exit from the back of the
eyeball is the blind spot on the retina in which there are no receptors (Fig.
17.16).
Each receptor is composed of one part that is much like an ordinary nerve
cell and either a rod- or a cone-shaped part that is sensitive to light and con-
tains carotenoid pigments. The cone cells are responsible for vision in bright
light, for detail, and for color vision. They are distributed over the central
region of the retina and in the human eye are most abundant in a minute
spot of clearest vision, the fovea (Fig. 17.16). The rod cells are especially
equipped for vision in dim light, are insensitive to color and are numerous
in the sides and periphery of the retina. Each cone cell is usually connected
Chap. 17
Sensory cells,
reception
RESPONSIVENESS THE SENSE ORGANS
329
Nerve cells,
conduction
I Hill lllkf
\ f \ jl
— Pigmented cells
-Cone cell.
has most direct
pathway to broin ond
gives sharpest vision
Rod cell
►Brain
Optic nerve
Light comes in here
Fig. 17.17. Section of the central part of the retina (highly magnified). The
retina is composed of four layers of cells: an innermost one of nerve cells, the
ganglion cells whose long processes (axons) constitute the optic nerve; the bipolar
nerve cells that are the intermediates between the ganglion cells and the sensory
cells; the sensory rod cells and cone cells; and heavily pigmented epithelial cells.
The rod and cone cells are the receptionists of light and are chemically changed
by it. The nerve cells are the conductors of effects of those changes. The pigmented
layer is a backstop of light; pigment moves within its cells and into their processes.
Why light does not first strike the rod and cone cells is explained in the story of the
development of the eye.
with the brain by a single chain of neurons, whereas whole clusters of rod
cells are connected with the brain by a single chain (Fig. 17.17). The acute
vision of the cones seems to be related to their direct connection with the
brain, and the less vivid vision of the rods to their indirect connection with
it. Cones produce a sharp, detailed image; rods produce a soft, indefinite
one. In the starlight, we see with the rods, and the cones, which are rela-
tively insensitive to light, do not function at all. Cones begin to function
when the light is of about 1000 times greater intensity than the smallest
amount to which the eye can respond. In the gray dawn, the rods dominate
vision and there is no color; as the light increases vision is taken over by
the cones and the grass is green again. Every rod contains visual purple
(carotenoid), a light-sensitive compound related to vitamin A. When light
falls on the rods, its energy breaks the visual purple into visual yellow. If
dim light is to be perceived, several rod cells must be affected by it at once.
The impulse that is created in the associated neurons then passes over them.
In the dark, visual yellow is resynthesized into visual purple and the rod cells
are charged for another exposure to light. If one comes out of brilliant light
into a darkened room, one is completely blinded for a few minutes because
the visual purple in the rod cells has been bleached out by the bright light.
330 THE INTERNAL ENVIRONMENT OF THE BODY Part III
The blindness in the darkened room occurs when the visual yellow is being
resynthesizcd to visual purple.
Theory of Color Vision. Rod cells are better understood than cone cells,
but the latter arc known to contain visual violet. Indications seem to justify
the theory that there are at least three different kinds of cones, and that these
are sensitive to the different wave lengths of light which produce the sensa-
tions of red, blue, and green color. According to this theory, the sensation
of white results when all kinds of cones are stimulated equally, and inter-
mediate colors result when two kinds of cone cells are stimulated unequally.
Defects of Vision. The most common defects of the human eye are near-
sightedness (myopia), farsightedness (hypermetropia), and astigmatism. In
the normal eye, the retina is the proper distance behind the cornea and lens
for the light rays to come to a focus or point on the fovea. In the nearsighted
eye, the eyeball is too long and the light rays converge in front of the retina
and are diverging when they reach it; thus they produce a blurred image
(Fig. 17.18). In the farsighted eye, the eyeball is too short and the retina
too close to the lens; the rays come in contact with the retina before they
converge. With age the lens loses its elasticity, does not become more convex
in accommodation, and the eye is chronically farsighted.
Astigmatism, meaning "off the points," results from irregularities in the
curvature of the cornea or the lens. In one plane the rays are brought in focus
at different points from that of the rays in another plane. On the oculist's
chart the upright lines may look clear and black, while horizontal ones look
blurred and gray. Astigmatism is so common that this appearance on the chart
is familiar to almost anyone whose eyesight has been tested.
FARSIGHTED EYE
Corrected by
Light rays ^,^-— «>^ convex lens ji ^^-'»w
::.. Blurred I I T~ ^-^^^^ Sharp
image LJ — X^^^^^^^^ image
Corrected by
Light rays ^^*«i>^— Vv concave
Blurred
Sharp
'"^age 'v^rC^ZI^^--^ / image
NEARSIGHTED EYE
Fig. 17.18. Diagrams of some common defects of the eye. Nearsighted eye,
with elongated eyeball and rays brought to a focus in front of the retina. Far-
sighted eye, with shortened eyeball and light rays in focus behind the retina.
18
Reproauction
Living organisms have the remarkable power of producing new ones that
look and act like themselves, though never exactly so. Many do this by the
division of their substance into parts of equal size with nothing remaining
to be a parent; all amebas begin life as orphans. Many others divide into
parts of very unequal size, a large one, the parent's body and small ones, the
cells, two of which, one male and one female must unite to make a new indi-
vidual. Whatever the case, parental protoplasm is the first substance of the
new individual no matter what its kind, ameba, bird, or man.
Asexual and Sexual Reproduction
Either asexual or sexual reproduction increases the population. The main
difference is in its variety. By asexual reproduction one cell becomes two cells,
and by sexual reproduction two cells become one and this one divides asexually
into many (Fig. 18.1). Thus, generations of amebas are produced, and the
bodies of multicellular animals increase in size whether small or great, fleas
or elephants. Asexual division, the pioneer method of reproduction, has
persisted throughout the course of evolution.
Various invertebrates divide into two or more parts, each of them a new
individual. In the marine worm, Autolytus, a second head appears part way
down the body. There are soon two fully organized worms attached one be-
hind the other. For a time they swim about tandemwise, then separate and
each one swims away alone. Sometimes a chain of individuals will form and
swim about together. Fresh-water hydras put forth buds that pinch off as
independent animals, and thus stop just short of colonial life. The internal
buds (gemmules) that form within fresh-water sponges are eventually set
free to start new colonies. No higher animals produce buds; cats do not bud
off kittens.
Sexual reproduction differs from asexual in that two individuals furnish
different kinds of cells, eggs and sperms. When such cells are fully developed
331
332
THi: INTERNAL ENVIRONMENT OF THE BODY
mm//,,,
Part III
E55 cell
Tcslia
conledninj
sperm cem
Fig. 18.1. Two methods of reproduction. Top, asexual, by which one cell be-
comes two or more. A one-celled animal (Trichospherium) dividing into many
individuals; the substance of the parent is entirely divided up among the offspring.
Bottom, sexual, by which two cells, egg and sperm, unite in one cell, the first of the
multicellular body. A male and a female many-celled animal (Hydra) with the
respective sex cells. (Courtesy, Corner: The Hormones in Human Reproduction.
Princeton, N.J., Princeton University Press, 1942.)
their pattern is set; neither their form nor function can be changed, and by
itself the life span of either kind is short. When they are joined, the resulting
cell contains the potentiality for longer life, an extraordinary variety of pat-
terns of structure and actions, and unique adjustments. It may bring forth
not only the traits of its parents and grandparents but signs of its ancient
animal ancestry. Every child is a surprise.
Beginnings of Sex
Conjugation. Paramecia and many other protozoans join in a union or
conjugation that resembles the mating of multicellular animals. Ordinarily
paramecia swim through the water, passing and repassing their neighbors
Chap. 18 REPRODUCTION 333
without response. From time to time, more often in some species than in
others, this behavior changes with dramatic suddenness. Mating spreads
through the population Mke an epidemic and for hours a lone paramecium is
scarcely to be found. Couples swim about for hours, always in the same
position with parts of their oral surfaces held together by a bridge of proto-
plasm (Fig. 18.2). After preliminary divisions of the micronucleus in each
one, two micronuclei of unequal size remain in each individual. The smaller
male micronucleus, essentially similar to a sex cell, migrates over the proto-
plasmic bridge and fuses its substance with the nonmigrating female micro-
nucleus. The female micronucleus becomes a permanent part of each recipient
Paramecium. After the exchange is completed, the bridge is gradually with-
drawn and the mates (conjugants) separate, each animal carrying with it a
new strain of inheritance to be distributed to its descendants.
The frequency of conjugation varies in different species, environments, and
physiological conditions. After conjugation paramecia divide more rapidly
as if mating were the rescue from a physiological depression. However, no
such rescue is essential. In a famous experiment carried on at Yale University,
L. L. Woodruff kept a culture of paramecia (P. aurelia) for over 20 years
(12,000 generations) without conjugation simply by changing the water
daily and keeping the food and environment satisfactory.
Special mating types of paramecia were discovered by H. S. Jennings
who reared thousands of them from natural pond populations. Among them
he found certain ones that would and others that would not mate outside
their own type, such as type A and B that mated together and a type C
that would not mate with either of them. It seems that type C is not a
fixed sex but is only generally sexual; animals of this type have not become
limited and settled into the bisexual pattern. Their situation suggests that the
development of sexes might not have been restricted to two kinds. If a
general sexual type had persisted among higher animals including man,
would not social behavior have been complex beyond imagination?
Endomixis. In some species of paramecia and under certain conditions
there is a nuclear reorganization, called endomixis, and this is followed by
an invigoration similar to that after conjugation. This process takes place
entirely within one individual.
Sexual Reproduction
The Plan of the System. The bisexual reproductive systems of multicellular
animals consist of the gonads, i.e., testes in the male, ovaries in the female,
and a series of more or less elaborate tubes and glands located within the
system or in another part of the body. The gonads are the essential organs that
produce the sex or germ cells. The tubes and sacs provide for the transporta-
tion of the sex cells and the developing young that may originate from their
334
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
777/5 animal
not followeci
further;samQ
as other
1. Two paramecia come together
2. Micronucleua divides. Macronucleus begins to dis-
integrate
3. Micronuclei divide and three of foxir disappear
-c 4. Remaining micronucleua divides unequally
5. Smaller micronucleua crosses into other animal
6. Each animal with its own larger micronucleua and
smaller one from other animal
7. Two micronuclei fuse
8. Two animals separate. Each ezconjugant with
fusion micronucleua
9. Fusion micronucleua divides
10. Two micronuclei divide
11; Four micronuclei divide
12. Four micronuclei become nxacronuclei, three dis-
Micronucleua divides and animal divides
14. In each of two micro-
nucleus divides and ani-
mals divide again
Fig. 18.2. Mating of paramecia, a complicated process by which part of the
substance of heredity in the male micronuclei is exchanged between the mates and
later distributed to their descendants. The large nuclei or macronuclei, are repre-
sented by black spots. They are concerned with the bodily processes, and appear
to take no part in conjugation and gradually disappear during it. The micronuclei
that are exchanged between the mates are shown by small black dots; those that
disappear are shown by circles. (After Jennings. Courtesy, Wolcott: Animal Bi-
ology, ed. 3. New York, McGraw-Hill Book Co., 1946.)
Chap. 18 REPRODUCTION 335
fusion. The glands produce secretions that control activities of the system.
Similarities of Male and Female. The union of a male and female germ
cell is the first event in the life of the great majority of multicellular animals.
Since each of them has a male and a female parent it is not surprising that
females inherit male as well as female characteristics, and that males inherit
female ones as well as male. No animal is entirely male or female in its
chemical content, its structure, or its behavior. The pars anterior of the
pituitary gland of the male liberates the same gonad (sex organ) stimulating
hormones as that of the female. The nipples, developed in all female mammals,
are also present in the males.
The characteristics of the opposite sex appear in the sex reversal that
occurs in some animals in nature as well as in experiments. The right ovary
of most birds is ordinarily only partly developed and the left one produces
the eggs. If the left one is removed by careful operation the small and incom-
plete right one usually develops into a testis and produces sperm cells. This
is because the cortex or outer layer of the bird's fully developed ovary secretes
a male-suppressing substance that ordinarily prevents the development of
sperm cells. Without it they would form in the medulla or central part of the
ovary. In the experiment the active cortex was removed with the functional
left ovary; it was undeveloped in the incomplete right one. Thus the male part
of the right ovary was no longer repressed.
Male and Female Cells — Gametes. In many lower plants and animals, all
of them aquatic, the male and female cells are often about the same size
and shape and both may swim with tail-like flagella. Within the bodies of
multicellular animals, constituting essentially aquatic surroundings, the eggs
are moved by cilia or by muscular pressure while the sperm cells are agile,
persistent swimmers (Fig. 18.3). Eggs are the energy-conserving cells; sperms
are the energy-expending cells. Many eggs are enlarged with food stored for
the embryo (Table 19.1). We fry eggs for food, but not sperm cells.
More eggs and sperms are produced than ever fulfill their promise. A bull-
frog lays from 10,000 to 20,000 eggs at one time. Counting on one egg
matured per month, covering the period between 12 and 45 years, a woman
produces about 430 eggs. Yet, within a pair of human ovaries thousands of
eggs wait in vain to develop further. George W. Corner quotes an investigator
who counted the incompletely developed eggs in both ovaries of a 22-year-
old woman and found about 420,000. The numbers of sperm cells are
astronomical in the majority of animals. It has been estimated that during
his reproductive lifetime, a man produces about four hundred billion sperms
or about one billion to each egg released from the human ovaries.
Fertilization. The union of a sperm and egg which constitutes the beginning
of a new individual is fertilization. It may be external and occur in the open
water, as it does in starfishes and sea-urchins, most fishes, frogs and toads,
336
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
B
V
%
■PV
i ^
^.
^*
•
m^
<r ■
D
Fig. 18.3. Photomicrographs of eggs and sperm. /4, human ovum about to burst
the enclosing sac and leave the ovary. B, a similar stage of the ovum of a macaque
monkey. C, living human ovum washed out of an oviduct and photographed im-
mediately. The small whitish spots are fatty particles; the nucleus is not visible. The
human ovum is about 1/175 of an inch in diameter, barely visible to the naked eye.
D, living human sperm photographed through a phase contrast microscope. The
nucleus of the sperm cell contains the substance of 24 chromosomes, half the herit-
able material of a new individual. The human sperms are the smallest cells in the
body; estimated to take about 2500 of them to cover a period mark such as on this
page. (A, B, C, courtesy, Patten: Human Embryology, ed. 2. New York, The
Blakiston Co., 1953. D, courtesy, O. W. Richards, American Optical Company.)
Chap. 18 REPRODUCTION 337
or internal within the body of the female as in some fishes, in salamanders,
reptiles, birds, and mammals (Fig. 18.4). Fertilization can occur only in a
wet place since sperm cells are swimming cells and all cells are essentially
aquatic.
The eggs of sea-urchins and sand-dollars are beautifully translucent and
beneath the microscope much of the process of fertilization can be seen.
During the spring breeding season the common eastern sea-urchin (Arbacia
Fig. 18.4. Courtship of brook sticklebacks
(mature fish, two-and-a-half inches long); ex-
ternal fertilization of the eggs. In spring the
male leaves the school, stakes out his territory
and builds a nest, and at the same time appears
in breeding colors. The females are now ready
to lay from 50 to 100 eggs. The courtship be-
gins. The male (left) zigzags toward the female,
swims toward his nest, and repeatedly thrusts
his head into it. The female enters the nest and
lays the eggs. She leaves the nest. The male
enters and sheds the milt (sperm cells) over
them. The sperms and eggs meet in the open
water. The male fans the water over the eggs
and thus increases their supply of oxygen; lines
indicate currents in the water. (Courtesy, Tin-
bergen: Social Behavior in Animals. London,
Methuen, 1953.)
pimctulata) naturally deposits its eggs and sperm directly into the sea. If the
mature male and female animals are placed in separate dishes of sea water,
the sex cells are discharged. A few eggs, the size of coarse sand grains, can
then be slipped onto a glass slide with a little sea water. Under the micro-
scope the nucleus of the living egg appears as a rounded body about one-sixth
the diameter of the whole egg and the membrane surrounding the cell and
the grainy cytoplasm are clearly visible. If now a droplet of the sea water
containing sperm cells is added to the eggs, thousands of sperms can be seen
swimming toward one or another of the eggs. At once, the surface of almost
338 THE INTERNAL ENVIRONMENT OF THE BODY Part IIT
every egg becomes fringed by sperm cells headed toward the eggs with tail-
pieces vibrating. In less than an instant this activity passes, the sperms
cease moving all at once as if a quick shadow passed over each egg and
stopped them. Actually, one sperm has pierced the egg membrane and is on
its way to the nucleus and as this occurs a special barrier, the fertilization
membrane, instantly forms around the egg and shuts out the competing
sperms. With the union of the sperm and egg nuclei that soon follows, the
inheritance of the coming individual is decided and its sex determined. With-
out ado or hesitancy the single cell goes through the process of division
into two cells, repeating this again and again. Thus a new sea-urchin begins.
Fertilization is a kind of junction between the existence of a sperm and
an egg, each of them prepared by meiotic divisions, and a new individual in
which mitotic divisions (Chap. 3) and differentiation are preeminent. These
processes are discussed under their respective names.
Special Types of Sexual Reproduction
The bisexual method of reproduction is the usual one in higher animals, in
all of the vertebrates, and in many invertebrates, jellyfishes, nearly all insects,
starfishes, sea-urchins, and their kin. Several varieties and irregularities of
sexual reproduction occur.
Hermaphroditism. In some species, each individual normally produces
both eggs and sperm cells at the same time, and is called an hermaphrodite.
Such animals belong to a few groups of invertebrates, among them planarians
and other flatworms, earthworms, leeches, and snails. Among vertebrates,
hermaphroditism occurs only rarely. Even in hermaphroditic species, pairs of
animals mate and cross-fertilization occurs. In earthworms, the reproductive
organs are so located that the eggs of one worm can be only fertilized by the
sperm of another worm (Chap. 28).
Although rare, hermaphroditic frogs, birds and even mammals are known;
some of these animals have one testis and one ovary, or some other combina-
tion of the primary organs. More often, the animal is a partial hermaphrodite
having the primary organs of one sex and the ducts and external genitalia of
the other. Hermaphrodites with both testicular and ovarian tissue are ex-
ceedingly rare in man,
Freemartins. A freemartin is a sterile cow which was born a twin of a bull
calf. Her ovaries are usually testislike but contain no developing sperms;
the vasa deferentia and other masculine ducts are represented but the exter-
nal genitalia are mainly female. The twins are known to come from separate
eggs. The sterile condition of the freemartin is believed to occur because the
membranes (chorions) of the twins are fused in such a way that the blood
vessels are joined and there is a common circulation between them. Thus,
the hormone of the testes of the bull calf passes into the body of the heifer
Chap. 18 REPRODUCTION 339
and acts upon the ovaries. The twin heifer is never sterile unless the mem-
branes of the male and female embryos are fused. Freemartins are known
to occur only in cattle, pigs, and goats.
Intersexes. Any normal, sexually produced animal has some structures of
the opposite sex. Intersexes are individuals in which the development of such
structures is carried to a more or less marked degree, actually degrees of
hermaphroditism. Many examples of intersexes show that the plans of the
male and female bodies are fundamentally similar and delicately balanced. A
tilt in one direction lifts the maleness, in the other the femaleness.
Parthenogenesis. Eggs may develop without fertilization, i.e., partheno-
genetically. Natural parthenogenesis is known only in the invertebrates,
notably in many small crustaceans and certain orders of insects. In social
ants, bees, and wasps the queen can lay either fertilized or unfertilized eggs.
Male honeybees develop parthenogenetically. In most aphids (plant lice),
there is one generation after another of wingless females, great populations
in which every individual produces young from unfertilized eggs. In autumn,
these are succeeded by a generation of parthenogenetically produced winged
males and females from which fertilized, winter-hardy eggs are produced.
In the spring a generation of females hatches from these and the program
of the previous summer is repeated. Again every plant louse is busy on the
production line; each one means dozens more.
Artificial Parthenogenesis. Certain kinds of eggs that normally re-
quire fertilization can be stimulated artificially by chemical and physical
means to develop into embryos; some even grow into adult animals. This
can be done by jolting them in revolving egg-shakers, pricking them with a
fine needle, raising the temperature, or changing the content of the fluid
about them. Eggs of cold-blooded animals that lay their eggs in open water
were the first to be tested, those of starfishes, sea-urchins, and frogs being
easiest to manipulate. The possibilities of artificial parthenogenesis were
first clearly demonstrated in 1900 by Jacques Loeb who obtained over 200
tadpoles by stimulating frogs' eggs. About half of these lived to become
well-grown young frogs of both sexes, the famous "fatherless frogs." When
their tissues were examined, the cells of most of these proved to have the
usual double number of chromosomes characteristic of frogs of their species
and not half that number as might have been expected. More recently, eggs
taken from the oviducts of rabbits have been stimulated by changes of
fluid and temperature. These cleaving eggs were transplanted into the uteri
of unmated foster mothers where some developed normally. After birth, they
grew into adults as fatherless as the distinguished frogs of 1900. Such ex-
periments show that an egg is capable of developing without the biparental
inheritance, and that development may be started by physical or chemical
means, possibly stimulating enzymes within the egg that are ordinarily ac-
340 THE INTERNAL ENVIRONMENT OF THE BODY Part TIT
tivated by the entrance of the sperm cell. The puncture of an egg membrane
by a fine needle appears to arouse the egg as well as a puncture by a sperm
head. It seems that an egg may be as responsive to a physical starter as a
motor.
Pedogenesis is parthenogenetic reproduction by a young, incompletely
developed animal. Its normal occurrence in a species is extremely rare.
Neotcny. Under certain conditions tiger salamanders {Ambystoma ti-
grinuni ) that have not metamorphosed become sexually mature, mate, and
produce fertile eggs. This is neoteny, also a rare condition.
Human Reproductive System
Male
In man, as in other mammals, the male reproductive system consists of a
pair of testes in which the sperm cells are produced and a series of ducts and
associated glands by which they are protected, nourished, and transported
(Fig. 18.5).
Structure and Function. The testes lie in extensions of the body cavity
covered with skin, the scrotal sacs, that hang outside the body. Each testis
is the size of a walnut, about an inch long, smooth and oval. It consists of
hundreds of seminiferous or sperm-bearing tubules, each a foot or two
long, and the thickness of a coarse thread. All of them are tightly coiled in
an entanglement which requires exceedingly skillful dissection to unravel
(Fig. 18.6). Under the influence of a gonad-stimulating hormone of the
URINARY
BLAOOCR
VAS DEFERENS
PENIS
BULBOURETHRA
GLAND
TESTIS
SCROTUM
Fig. 18.5. Diagram of a section of human male reproductive organs showing
their relation to the urinary bladder and urethra. (Courtesy, Harbaugh and Good-
rich, eds.: Fundamentals of Biology. New York, The Blakiston Co., 1953.)
Chap. 18
REPRODUCTION
341
Seminal duct-
-Tube drawn out
TUbes coiled
in place
Fig. 18.6. The human testis with a piece removed and some of the seminiferous
tubules drawn out of place. The sperm cells develop within the hundreds of these
threadlike seminiferous tubules. They mature as they pass through other ducts,
especially the epididymis, the much coiled single duct that lies along the side of
each testis. (Courtesy, Corner: The Hormones in Human Reproduction. Princeton,
N.J., Princeton University Press, 1942.)
pars anterior of the pituitary gland, the sperms develop from cells in the walls
of the seminiferous tubules. They divide repeatedly, reduce their chromo-
somes to the half number, finally become very minute, and each develops
a single flagellum, a swimming tailpiece (Figs. 18.3 and 18,7). At first the
sperms cling to the supporting cells in the lining of the tubules, then they move
into the open channels, and are gradually carried toward the outer ducts. In
animals that breed the year round, such as rats, rabbits, and man, they are
produced more or less continuously. In those with limited annual breeding
seasons, such as birds, the production stops between seasons.
The testes develop in the body cavity near the lower ends of the kidneys,
locations which the ovaries occupy throughout life. Before birth, however,
they gradually slip downward into the scrotal sacs. In man, this location is
permanent. In rats, rabbits, and several other mammals, the testes slip in and
out of the abdominal cavity. They are outside in the scrotal sacs during the
breeding season, and in the abdominal cavity between those seasons. If the
testes of certain animals abnormally remain in the body cavity, its higher tem-
perature destroys the sperm cells; such testes are said to be hidden or crypt-
342
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
Sertoli cell
Undifferentiated
germ cell
Spermatogonium
Sperma
tozoon
Primary Spermatocyte
_„^„ Secondary Spermatocyte
Spermatid
Primary Spermatocyte
Fig. 18.7. Photograph of the seminiferous tubule (x 550) in which spermato-
zoa develop. The outermost dark band is the wall of the tubule, mainly connec-
tive tissue. All of the other cells are developing sperms and cells (Sertoli) which
nourish the sperms. The nearly mature spermatozoa are nearest the fluid filled cen-
ter of the tubule. Their dark, oval heads are crowded together and their exces-
sively slender tails (flagella) extend into the fluid. Between them and the wall of
the tubule are sperms in successive stages of development beginning near the wall.
They multiply; their nuclei divide by mitosis and each sperm has 48 chromosomes.
Nearer the center others that are further developed divide by meiosis and each
mature sperm has 24 chromosomes. (Courtesy, Ham: Histology, ed. 2. Philadel-
phia. J. B. Lippincott Co., 1953.)
orchid. Rarely, as in armadillos, elephants, and whales, the testes remain
permanently in the body cavity and yet are not injured by the body tempera-
ture. Under some strain of the abdominal muscles a loop of the small intestine
may be forced into the passage through which the testis slips; this is called
inguinal hernia.
The seminiferous tubules of each testis unite to form a dozen larger ducts
which in turn open into the epididymis, a single tortuously coiled duct about
21 feet long. This duct is lined by secretory cells which contribute to the semi-
nal fluid in which the sperm cells slowly mature and develop part of their
motility (Fig. 18.6). From the epididymis they move into the sperm duct
(vas deferens). These sperm ducts, one from each testis, pass upward into
Chap. 18 REPRODUCTION 343
the body cavity and, joining together, enter the urethra which extends through
the introniittent organ or penis to the external opening (Fig. 18.5).
Other glands, chiefly the seminal vesicles and the prostate gland, also con-
tribute to the seminal fluid. This fluid contains salts that act as protective
buffers against the acids in the urethra of the male and in the reproductive
passages of the female, and glucose, a nutrient. The prostate gland almost
completely surrounds the urethra near its exit from the urinary bladder. In
elderly men, this gland often enlarges. Since its outer surface is covered by an
unyielding capsule it can do nothing else but squeeze the urethra and more or
less cut off the passage of urine. The gland was named prostate (Gr., standing
before) from its position in front of the urinary bladder and is in nowise
"prostrate" as it is sometimes called.
The urethra extends through the penis to its external opening (Fig. 18.5).
It contains sperm cells only when the penis is erected, that is, when the
"spongy" tissues surrounding it are stiffened by the blood that floods into them,
and the sperm ducts contract spasmodically, forcing the sperms into it before
Fig. 18.8. The effect of a hormone of the testis on the comb of the cock, a, cas-
trated cockerel, otherwise untreated; b, a castrated cockerel after 11 days treatment
with extract of testis. Drawn from photographs by Freud and co-workers. (Cour-
tesy, Corner: The Hormones in Human Reproduction. Princeton, N.J., Princeton
University Press, 1942.)
or during copulation. At the same time urine is shut out of it. The spurts or
ejaculations of seminal fluid, a half-teaspoonful or less in bulk, are estimated
to contain about 300 million sperm cells. In the ordinary, somewhat shrunken
condition of the penis, the skin is very loose and a fold of it, the foreskin,
covers the tip. This is very often removed in babies by a simple operation
called circumcision. This is a hygienic measure and a very old religious rite.
Testicular Hormone. The testes produce the sperm cells; they also produce
fluids. Under stimulation by a hormone (gonadotrophic) of the anterior lobe
of the pituitary gland, they secrete a male hormone (androgen) that causes
and maintains the development of the secondary sex characters such as voice,
form, behavior, and sexual activity (Fig. 18.8). The male hormone is believed
to be secreted by interstitial cells lying between the seminiferous tubules. In
spite of its name and effects, androgen belongs to the same family of chemical
344 THE INTERNAL ENVIRONMENT OF THE BODY Part III
substances as the female hormones, estrogen and progesterone. This is another
aspect of the similarity of maleness and femaleness already mentioned and one
of the many cases of the likeness of substances that are active in carrying on
different functions. Since every human being inherits traits from a male and a
female parent, it is not surprising that the male hormone, androgen, figures in
the metabolism of women as well as men and that the female hormone, estro-
gen, is in men as well as women. Both hormones appear in the urine of both
sexes. Male hormones administered to animals will counteract the effects
of the removal of the testes; a castrated male treated with androgen becomes
normal except that it has no sperm cells and is of course infertile.
Castration and Sterilization. Castration of boys and men has been per-
formed for various reasons from ancient times into the present. In the past it
was done to produce the eunuchs (Fig. 18.9) who served in courts and harems
and, as late as 1870, to preserve the soprano quality of voice in boy choristers.
By true castration the testes are removed, thus sterilizing the animal; steriliza-
tion may also be produced by cutting or tying the duct (vas deferens) from
each testis, thus blocking the passage of the sperm cells which are eventually
absorbed. This type of sterilization is sometimes used to prevent the breeding
of mental defectives and, with the consent of the person involved, for other
reasons.
Female
Structure and Function. The female reproductive system of mammals is
more complicated than that of the male since it not only produces and provides
for the eggs, but gives protection and nourishment to the developing young.
The structures that take part in this double program are the ovaries, the ovi-
ducts (Fallopian tubes), the uterus, vagina and external genitalia, and the
mammary glands (Fig. 18.10). Like the testes, the ovaries also produce inter-
nal secretions.
The ovaries develop and remain in the body cavity a little below the kid-
neys. Unlike the testes they do not suffer from the high temperature within
the body. In mature women, they are the size of a shelled almond, about one
and a half inches long and an inch wide. Each one consists of a central core of
connective tissue, blood vessels and nerves, enclosed by a covering, the cortex
consisting of cords and nests of epithelial cells. This contains the developing
eggs and is covered by a single layer of cells, the germinal epithelium, from
which the eggs originate. As they develop, they become surrounded by nutrient
(follicular) cells. Each egg with its follicular sac forms an ovarian (or
Graafian) follicle (Fig. 18.11). Under the influence of an anterior pituitary
hormone (gonadotrophic), the follicle grows and a split develops between its
outer and inner layers of cells. Into this space these cells or others near them
secrete the liquor folliculi, containing the hormone estrogen that is responsible
Chap. 18
REPRODUCTION
CASTRATES
345
NORMAL
COCK
CASTRATE
CCXK
NORMAL
MEN
y
CASTRATE
HEN
?¥
NORMAL
COW
fT
CASTRATE
COW
ft
NORMAL
MAN
EUNUCHOID
GIANT
Note difference in length of A, A' and
B, B\ In normal A=B, in eunuchoid
giant B' ii greater thon A'.
Fig. 18.9. Effects of castration on the shape of the body. These do not occur
when sterilization is done by cutting or ligating sperm ducts without removing the
testes. (Courtesy, Gregory: A, B, C of the Endocrines. Baltimore, Williams & Wil-
kins Co., 1935.)
for certain changes in the reproductive tract. As the egg matures, it hangs out
into the cavity of the follicle which is swollen with fluid so that it protrudes
like a minute volcano on the surface of the ovary (Figs. 18.11 and 18.12).
At length, the wall of the follicle breaks and egg and fluid are set free; this is
ovulation. Human ovulation may occur irregularly in the right or left ovary;
ordinarily but one egg is freed per monthly cycle, but there may be two and
rarely even more. As an egg matures, the number of its chromosomes is re-
duced to half that of the parent's body. In the human egg, the number is cut
from 48 to 24. This involves two divisions (Chap. 3). The first one occurs
before ovulation.
346
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
OVIDUCT
HYMEN
Fig. 18.10. Diagram of a section of the human female reproductive organs.
(Courtesy, Harbaugh and Goodrich, eds.: Fundamentals of Biology. New York,
The Blakiston Co., 1953.)
From the surface of the ovary, the egg ordinarily enters the enlarged funnel-
shaped end of the oviduct which clasps the ovary in its soft ciliated folds
(Fig. 18.10). The ends of the oviducts contain smooth muscle and have
considerable range of position in cupping themselves about the ovaries. Even
if the egg is not discharged directly into the funnel, it is apt to be pulled
into it by the beat of the cilia always directed toward the uterus. If copulation
has recently occurred and sperm cells are present, they usually meet the egg
in the oviduct and fertilization results. Once in the oviduct whether fertilized
or not the egg is carried on to the uterus by peristaltic contractions of the walls
and the urging beat of the cilia. The second meiotic division occurs after the
sperm enters the egg. The nucleus of the egg is then ready to unite with the
nucleus of the sperm. At fertilization the addition of 24 chromosomes of the
nucleus of the egg and 24 of the nucleus of the sperm restores the regular
number of 48 in the human body cells. The sex of the individual is also deter-
mined at fertilization.
In the ovary, the broken follicle soon enlarges. Influenced by the gonad-
stimulating hormones of the anterior pituitary its cells increase in size and
number and form an endocrine gland, the corpus luteum or "yellow body"
(Figs. 18.11, 18.12). This secretes progesterone which stimulates the further
growth of the uterus. If the egg has been fertilized, the embryo developing
from it may be gradually making its way into the uterine wall. If so, substances
will be produced by its outer membranes and taken by the blood through the
Chap. 18
REPRODUCTION
347
Eqq Nests
Primary
Follicle
Double-
layered
Beqinninq
of Antrum
Atretic
Follicle
Follicle Approadiinc]
Maturity
Atretic Follicle
Mature
Follicle
Corpus
Albicans
.Ruptured
Follicle
Fully Formed/
G)rpu5 Luteum
Connective
Tissue of Ovary
Conn.Tissue
Lutein Cells
Fibrin of Clot
Coaoulated Blood
4iW^' Released
Younc]t
"Corpus
Luleum
Fig. 18.11. Diagram of a cut tfirougli a mammalian ovary showing a sequence
of stages in the growth and maturity of the ovarian follicle, the egg and its sur-
rounding sac; the rupture of the sac and release of the egg; and the transformation
of the sac into a gland, the corpus luteum. At the left, the strands of the mesova-
rium attach the ovary to the body wall. Follow the sequence clockwise around the
ovary, starting at the mesovarium. Note the atretic follicle, one which abruptly
ceases to grow before maturity and then degenerates. In the human ovary only one
ovum ordinarily matures each four weeks during the active life of the ovary. (Cour-
tesy, Patten: Human Embryology, ed. 2. New York, The Blakiston Co., 1953.)
mother's body, inevitably reaching the ovary. During the first half of preg-
nancy, the corpus luteum is affected by these substances and becomes about
the size of a grape. Under stimulation from them and the pituitary gland, the
corpus luteum conditions the uterus to hold the embryo until the time when
hormones secreted by its placenta take part in this function.
The uterus is the organ within which the mammalian embryo is sheltered
and nourished. This period (gestation) may be short or long, three weeks in
a mouse, nine months in man, two years in elephants. An embryo enters the
uterus as a minute ball of cells and leaves it via the vagina or birth canal
as a well-formed individual. The lining of the uterus superficially resembles
that of the mouth but has more glands and blood vessels and is physiologically
responsive to the embryo and to certain endocrine secretions. It takes part
in the formation of the maternal part of the placenta, the organ through
which the bodies of mother and child cooperate in the growth and develop-
348 THE INTERNAL ENVIRONMENT OF THE BODY Part III
Fig. 18.12. Photograph of a section of the ovary of a whale showing the typical
mammalian structure. In the largest follicles, the wall has split giving the appear-
ance of double sacs. In life, the minute ovum (not visible) is in the smaller sac
surrounded by fluid. There is a corpus luteum, the solid growth, at each end of the
ovary. When taken from the whale this ovary was about 14 inches long. (Courtesy,
The South Kensington Natural History Museum, London.)
*
ment of the latter. Without the embryo the reactions of the uterus are very
different; they are outlined in a later paragraph.
Ovarian Hormones. The ovaries produce at least two hormones. Both are
secreted under the influence of the gonad-stimulating hormones (the follicle
stimulating hormone FSH and the luteinizing hormone LH) of the anterior
lobe of the pituitary gland and the luteotrophic hormone (LTH) (Fig. 18.13).
Estrogen, the female counterpart of the testicular hormone, androgen, is
secreted by the follicle. Although the ovaries are the principal source of estro-
gen, it has also been extracted from the placenta, testes, cortices of the adrenal
glands, and even from certain plants. A second hormone, progesterone, is
secreted by the corpus luteum, also by the placenta and adrenal cortex. Pro-
gesterone, acting with estrogen, stimulates the uterine wall to receive and hold
the embryo; with estrogen it also stimulates the development and growth of
the mammary glands. Both hormones play important parts in the reproductive
cycles of the female, in the production of secondary sex characters, and in
sexual behavior.
Female Reproductive Cycle
In mammals generally the reproductive or estrous cycle includes the produc-
tion of one or more mature eggs and the preparations for the protection and
nourishment of one or more embryos. Fertilization of the eggs may not occur
Chap. 18
REPRODUCTION
349
Vaginal Epithelium
Proliferative Phase Secretory Phase
Uterine Mucosa
Fig. 18.13. Diagram showing hormones arising in the anterior lobe of the pitui-
tary gland that especially influence the female reproductive cycle. (Courtesy, Pat-
ten, Hitman Embryology, ed. 2. New York, The Blakiston Co., 1953.)
and no embryos be produced. Then the cycle of ovulation and preparation
will recur again and again. The human reproductive cycle is substantially the
same as that of other mammals although in some respects spectacularly differ-
ent from all except monkeys and other primates. The peculiarities of the
human cycle can be much more clearly understood against the background of
the reproductive cycle as it occurs in the majority of mammals.
Typical Estrous Cycle. This consists of a cycle of changes in the ovary,
accompanied by changes in the entire reproductive tract. As repeatedly stated,
the cycle is brought about by hormones of the pars anterior of the pituitary
gland acting upon the ovaries, and by those of the ovaries acting upon the
pituitary and on the reproductive tubes, especially the uterus, and on certain
350 THE INTERNAL ENVIRONMENT OF THE BODY Part III
glands. The ovarian cycle comes to a climax in ovulation, when one or more
eggs leave the ovaries. In rats and mice, the interval between ovulations is
four and a half or five days; in cattle, horses, and pigs, 25 days. Dogs breed in
early spring and fall, irregularly; cats in spring and early fall, sometimes more
often. In rabbits, cats, and dogs ovulation occurs only when induced by
copulation.
The events of the 21 day estrous cycle in the pig may be taken as an exam-
ple of a cycle essentially similar to others. For two and a half weeks, the extent
of the diestrous period, the pig moves about, eats, and sleeps in apparent
satisfaction. Then, in the last three days of the cycle, the estrous or "heat"
period, she becomes restless and sexually excited. At the same time, special
activity is going on in the ovary. About two days before estrus begins a certain
few ovarian follicles grow rapidly and their cavities fill with fluid containing
estrogen. On the first day of estrus, they are fully mature. By the second day
the eggs have been forced out of the follicles and are in the oviducts, due to
meet the sperm cells.
During the reproductive cycle there is a seesaw influence between the an-
terior lobe of the pituitary gland and the ovary. The follicle-stimulating hor-
mone of the pituitary excites the maturing ovarian follicles and their produc-
tion of estrogen. Estrogen stimulates the glands in the walls of the uterus and
regulates their blood supply, effects changes in the walls of the vagina and
mammary glands, and brings about the characteristic behavior of estrus. When
it reaches a certain level, it also inhibits the production of the follicle-stimu-
lating hormone and stimulates the production of the luteinizing hormone by
the pars anterior of the pituitary. Under the influence of these pituitary hor-
mones, ovulation occurs. Aided by another hormone of the pars anterior,
luteotrophin, the corpora • lutea, made from the emptied ovarian follicles,
secrete progesterone which causes further uterine secretion and growth. By
about the sixth day after ovulation, the corpora lutea produce their full quota
of progesterone. They continue for a time to make this secretion which further
stimulates the uteri (two uteri in the pig), provided embryos are developing
in them. Evidently the developing embryos contribute substances to the
mother's blood that support the corpora lutea. The placenta (Chap. 19) asso-
ciated with each embryo produces hormones that help to maintain the embryos
in the uteri and prevent more new eggs from maturing in the ovary.
If the eggs are not fertilized, they degenerate, and phagocytic cells consume
them as in all mammals. On the fifteenth day after the last ovulations the
corpora lutea also degenerate and in consequence the activity and preparations
which they stimulated in the uterus likewise subside. Their control of young
ovarian follicles is lifted and on the nineteenth day after the ovulations, an-
other group of these enlarges, and another reproductive cycle is about to begin.
Chap. 18 REPRODUCTION 351
Two features of the typical reproductive cycle of the lower mammals are
especially significant. ( 1 ) Ovulation occurs at a time of sexual excitement, and
mating will take place only during that period. (2) The degeneration of the
corpora lutea and the withdrawal of preparations for an embryo in the uterus
cause very little physiological stir.
Reproductive Cycle. As already noted, the reproductive cycle of men-
struating mammals (the human species and the closely related apes and higher
monkeys) is similar to that of other mammals except for activities associated
with ovulation and the breakdown of the uterine lining.
The changes in the ovary including ovulation proceed as in other mammals.
Usually, only one egg follicle enlarges and finally breaks, releasing its egg and
the estrogen it contains. Ordinarily, the ovary is already clasped by the ciliated
funnel of the oviduct and the egg is at once drawn into it (Figs. 18. 11, 18.14).
As already stated, even before it leaves the follicle, the number of its chro-
mosomes has been reduced from the 48 of the general human body cells to
the 24 of the human sex cells. The egg is carried slowly along the oviduct by
the currents created by cilia and by the contraction of its muscle. In other
primates, fertilization occurs in the oviducts, and this is known to be true
of the human egg. Stimulated by the pituitary (Fig. 18.13), the enlarging
follicle steadily secretes estrogen into the blood up to the time when the follicle
releases the egg. Stimulated by this estrogen, the lining of the uterus becomes
more glandular. After ovulation, the corpus luteum provides the progesterone
which further induces the enlargement of the uterine glands, their secretory
activity, and the increased blood supply (Figs. 18.11, 18.12, 18.14). All of
these changes reach their height in the second week after ovulation. If an
embryo arrives in the uterus at this time, it is surrounded by ideal conditions
for its reception and nourishment. The embryo is extremely minute. As in
the pig it produces important reactions in the uterine wall, and substances
are absorbed into the blood that prolong the existence of the corpus luteum.
As in the pig, too, the placenta provides hormones which help to stimulate the
uterus to hold the embryo.
A very different program follows if no embryo enters the uterus although
the latter is highly prepared for one. The corpus luteum degenerates for want
of stimulation via the blood from the uterus. Cut off from progesterone, the
uterus goes through the violent reactions of menstruation. Its swollen blood
vessels are disturbed and ruptured; its lining cells, glands, and inner connective
tissue break down. Blood from the broken vessels is mixed with the sloughed
off tissues, and the whole cast off debris is gradually drained away through
the vagina, a process lasting from one to seven days, but most often for five.
Even in the latter part of the period another ovarian follicle is already forming
and under the influence of its secreted estrogen the lining of the uterus and its
352
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
^1>NT •^'--^^^-S^/V.
^^V'^ morula v^^r:
aP carlv and (^
qostrula \^j^'^O^^y
JOTjRNry
cleavoqe
H
implon^aHon
beqins
"'^s,
'r?'*/;
nplontcdj/v. --IR^ ^^#1%^ ' ■'!
implontcd
9
^^n
produces chorionic
qonadolrophin, ixihich
fur>h«r m-aintoins
corpus luteum
UV
male
and
female
pronuclei
Hijoluronidase
from sperms
breaks up corona,
ond sperm enters
eqq Second
maturation
division, beqins
Fig. 18.14. Diagram showing essential steps in the beginning of a human indi-
vidual, changes in which endocrines are prominent actors in a complex scene. In
the ovory. An ovum and its follicular sac grow to maturity stimulated by F.S.H.,
the follicle stimulating hormone of the anterior pituitary. The first of the two di-
visions of maturation (meiosis) occurs here after which the chromosome number
of the ovum is reduced one-half. The luteinizing hormone, L.H., of the anterior
pituitary stimulates the follicular sac and causes it to break and release the ovum
(ovulation). With luteotrophin (L.T.H.) of the anterior pituitary follicular sac to
transform into the corpus luteum, an important gland. In the oviduct. Entrance
into and movement through the oviduct are largely due to currents produced by
cilia. The ovum is surrounded by sperm cells. An enzyme, hyaluronidase, produced
by them breaks up its covering (corona) of cells. One sperm enters the ovum. This
is a signal for the second maturation division which is completed before male and
female nuclei fuse in the fertilization process. The new individual is moved through
the tube, at first as one cell, but soon it becomes a ball of cells. In the uterus. Under
the influence of the luteinizing and luteotrophic hormones, L.H. and L.T.H. of the
pituitary, the corpus luteum is now producing progesterone. This substance is car-
ried by way of the circulating blood to the uterine wall and prepares it for the
reception of the embryo. The white bands on the uterine wall are drawn to suggest
lapses of time during the processes represented. The growing embryo is surrounded
by coverings (chorion) that produces the hormone, chorionic gonadotrophin. This
is carried about by the blood, stimulates the corpus luteum which in turn produces
the progesterone that maintains the capacity of the uterus to hold the embryo. Mis-
carriages occur without this. The time in the tube varies and probably is often
shorter than five days. (After Dickinson. Courtesy, Ham: Histology, ed. 2. Phila-
delphia, J. B. Lippincott Co., 1953.)
blood vessels are being repaired (Fig. 18.15). The physiological anticipation
of an embryo begins all over again.
The human reproductive cycle is counted from the first day of menstruation.
All of its timing is variable, especially that of ovulation which may occur at
different intervals in different individuals, and even at varying intervals within
the same individual (Table 18.1).
Chap. 18 REPRODUCTION
Table 18.1
Sequence of Events in the Human Cycles of Menstruation
AND Pregnancy
(See also Figs. 18.14 and 18.15)
353
Days after
First Dav of
Ovaries
Lining of Uterus
tion
Follicle and Egg
Corpus Liiteiim
1-4
New follicle (and egg)
Corpus luteum of pre-
Blood vessels in lining
begins to develop
vious cycle degener-
rupture in menstruation;
ating
lining sloughs off
5-11
Gradual development
Further degeneration
Resting condition followed
variable
and increase in es-
in corpus luteum of
by thickening of lining
interval
trogen
previous cycle
and increased volume of
blood and glands
12-16
Ovulation, passage of
Forms from ruptured
Uterine glands more ac-
variable.
egg into oviduct
follicle: produces
tive
average 14
(where fertilization
hormone, proges-
may occur)
terone
17-23
Egg in oviduct or
Enlarges
Fully vascular and glandu-
uterus
lar condition
If Fertilization Has Not Occurred
24-28
Egg probably begins
Gradually begins to
Marked congestion of
to disintegrate
disappear
blood vessels and
shrinkage of uterine
wall
If Fertilization Has Occurred
19-280
No ovulation during
Remains during first
Embryo grows within the
variable
pregnancy
half of pregnancy
uterine wall
Comparison of Female Reproductive Cycles
In all mammals, the most important feature of the cycle is ovulation. In most
mammals, this is accompanied by marked sexual excitement. Mating is con-
nected v.'ith the ovulation period and limited to it. In the human species and
higher primates, there are usually few or no outward symptoms of ovulation.
Mating occurs at any time and without reference to ovulation.
In most mammals, if no egg is fertilized, the preparation of the uterus for
the embryo subsides gradually without rupture of blood vessels or glands. In
the human species and certain primates, under the same conditions, the prep-
arations in the uterus are drastically destroyed with the rupture of blood ves-
sels and the sloughing off of much of the uterine lining. The physiological
advantages of menstruation are not evident. Nonmenstruating animals expe-
rience essentially the same reproductive cycles, and the uterine life of the
embryo is quite as complete as in menstruating ones.
354
THE INTERNAL ENVIRONMENT OF THE BODY
Part III
OVARIAN
CYCLE
UTERINE
CYCLE
Functional
layer of
mucosa
Basa!
layer
Day of cycle
Unfertiliied
ovum,-
Ovum
fertilized and
Regressing Corpus
^ de-geneVates i^r^J^X, ^^**"'" ^ implanted.- .
" * ^^ If '^fS^, XSJ^ corpus luteum persisfs
Corpus tuteum
of Pregnancy
V4„;
OVULATION
IMPLANTATION
Copid qrowlh of
follicle ending
in ovulation
Migration and deolh rCBTILIZATION ,
of ovum. Funclionol
corpuJ luleum OVULATION
PLACLNTATION
(Ovulolion ceasei.
Corpus luleum
conlinuej)
Menses Proliferative
Phase
Secretory Phase
Incomplete cycle
Placentation
(Menses Withheld)
ORDINARY MENSTRUAL CYCLE
MENSTRUAL CYCLE ENDING IN PREGNANCY
Fig. 18.15. Graphic summary of changes in the inner layers (endometrium) of
the uterus in an ordinary menstrual cycle and in another cycle in which pregnancy
occurs. The changes in the ovary are placed in their proper relation to the time
scale and activities in the uterus. (Courtesy, Patten: Human Embrvology, ed. 2.
New York, The Blakiston Co., 1953.)
Production of Children
Production is important to the human crop as it is to others How can pro-
duction of children be encouraged when there is plenty of space and food and,
what is more difficult, discouraged when there is not? These two questions
penetrate into every society the world around.
Behind both questions is the fact that living matter insists upon reproducing
itself. The many cells of our bodies are due to their persistent multiplication.
Hard or easy living, much or little food, heat or cold may affect reproduction,
but in general and in the long run they do not stop it. Children were conceived
and born in the worst prison camps of World War II.
The problem of inducing production of children is a complex one that for
thousands of years has been met according to the understanding of various
peoples. Social and economic influences are exceedingly important and they
as well as the physiological ones are very complex. In our own time, the gen-
eral trend of experimental evidence has shown that the sperm and egg are
capable of fertilization for a shorter time than was previously supposed.
The problem of reducing the production of children is also an old one,
dealt with in ancient times and in primitive societies. Fundamentally, it is
solved by preventing the egg and sperm cells from meeting. Almost all animals
do this for a good part of their lives because they only mate at certain sharply
Chap. 18 REPRODUCTION 355
limited and relatively very short periods. In contrast to this, as already pointed
out, in chimpanzees, the higher apes, and man mating may occur at any time.
The principal means of preventing fertilization are the mechanical and
chemical ones that keep the sperm and egg from meeting, and the "rhythm
method," a restriction of the time of sexual intercourse to the periods when
no egg is apt to be present, i.e., avoiding those near ovulation. Authorities
generally agree that ovulation occurs about 14 days after the first day of men-
struation in a 28-day cycle, when these two functions are completely regular
and standardized in time. Even so, two days before and two days after the
supposed ovulation date are usually included in the possible ovulation time.
Two facts, however, must be taken into consideration. Many exceptions and
irregularities occur in the menstrual schedule even in the same individual.
Ovulation is an unobtrusive physiological process of which few persons are
certainly aware. Completely regular and standardized cycles are rare indeed.
Part IV
Tne New Inaiviaual
19
Development
An embryo is a living organism in the early unfolding of its form and func-
tion. It has potentiality, and its possibilities for the future contrast with what
it is at the moment. This is the root of its compelling interest. The two cells
which we see through the microscope would not be so unforgettable if we did
not know that they were the first ones in the making of a rabbit. They hold the
pattern of lifted ears, of still fright and startled leaping, and of rabbits and
more rabbits for years to come.
The development of an embryo is a series of orderly changes in which cells
grow and divide and become different. Growth and differentiation are its key
processes. Embryonic development may end at hatching or at birth. Birds and
other animals that develop and hatch from eggs outside the body are called
oviparous. Those that develop from eggs retained in the body are called vivip-
arous. These include man and other mammals with such rare exceptions as
the duckbilled platypus of AustraHa.
As cells grow they become larger and heavier. They take in food and from
it make chemical substances like their own. By the time a cell is full grown
and ready to divide each of its chromosomes has assimilated food and dupli-
cated itself in quality and quantity. Multicellular animals grow by controlled
increase in cell number as well as cell size. Every human being begins as a
single cell, smaller than a pin head, scarcely visible to the naked eye. At birth,
nine months later, a baby is an organization of over 200 billion cells and
usually weighs about seven pounds. Increases in weight and cell number
are controlled and limited; men, mice, and elephants have their respective
limits.
The animal pole of the egg is the most active in physiological exchange with
its environment even while the egg is in the ovary. It usually marks the future
anterior end of the embryo. Various regions of the fertilized egg are set off by
differences in appearance and function (Fig. 19.1). The gray crescent on the
surface of the fertilized frog's egg is the scene of great activity, since its posi-
359
360
THE NEW INDIVIDUAL
Part IV
Nucleus
Follicle cells of
corona radiata
Cytoplasm
Vitelline
membra
Zona
pellucida
Pigmented
cytoplasm
Nucleus
Yolk-rich
cytoplasm
Animal pole
Vegetal pole
B
Albumen
Disc of
protoplasm
Outer shell
membrane
Air
chamber
Inner shell
membrane
White yolk
Yellow yolk
Chalaza
Shell
Vitelline membrane
D
Fig. 19.1. Eggs whose size depends upon the amount of food (yolk) that they
contain. A, human egg (x200) typical of mammals has practically no yolk and
is just visible to the naked eye. At its lower right a human sperm is drawn, very
highly magnified, even so its difference in size is striking. B, frog (after T. H. Mor-
gan); lower right, a frog's egg surrounded by jelly, natural size. C, hen's egg
(after Lillie), abundant yolk; shows disk of protoplasm from which the chick de-
velops. D, fly; yolk is in the center of the egg and the embryo forms around it.
(Courtesy, Arey: Developmental Anatomy, ed. 5. Philadelphia, W. B. Saunders
Co., 1946.)
lion at the future rear end of the body marks the first ingrowth of the digestive
tract. As the cells of the embryo multiply, those of succeeding generations be-
come different from their predecessors. The tall cells in the neural folds of the
future nerve cord are descendants of low, rounded ones. Groups of cells
acquire special shapes and abilities; potential muscle cells gradually come
to look and act like muscle. As differentiation goes on cells actually move
about, changing their positions, and by so doing affect their neighbor cells
and arc affected by them.
Differentiation transforms the potentiality of the fertilized egg into the com-
plex realities of the young animal. In 21 days of incubation the latent power
Chap. 19 DEVELOPMENT 361
of a fertilized hen's egg is changed to the liveliness of a chick that can aim a
peck at another chick's bright eye and strike it.
The Yolk Content of Eggs — Its Food Value and EflFcct on Development.
Because of their content of yolk, eggs are the largest cells in the body. Even
in the eggs of mammals a minute amount of yolk is present, a fragment of his-
tory from their egg-laying ancestors. The egg cell of a mouse (0.06 mm. in
diameter) is one of the smallest eggs of vertebrates; those of some of the huge
sharks are the largest eggs known. The egg of an ostrich (85 mm. in diameter)
is the largest of any familiar animal. It weighs three pounds and contains the
equivalent of one and a half dozen hen's eggs. The ancient birds produced the
really large eggs with enough food for a banquet in one yolk. The fossil egg
shell of the extinct bird Aepyornis holds a gallon.
Except in mammals, yolk is the complete food for embryos. Its value as
human food has greatly added to the economic importance of hens, ducks, and
ostriches. The eggs of fishes have not only food value tut, in caviar, they add
social prestige. It is the yolk that counts; "fried eggs" mean yolky hens' eggs,
never cows' eggs. The high value of yolk is due to the completeness of its food
content of proteins, fats, carbohydrates, inorganic salts, vitamins, pigments
(carotin in birds), and enzymes; water composes about half of its bulk.
Yolk changes the pattern of development because it takes no part in cell
division except as it is a hindrance. Obviously, there must be less protoplasm
in parts of the cell that are packed with yolk; there cell division is slow because
the rate of metabolism is low. Cell division must combat the inertia of yolk or
avoid it by taking a roundabout way, as it does in the early embryos of frogs,
birds and many other animals (Figs. 19.1, 19.7). Yolk accumulates in one
hemisphere of the egg of frogs, and forces the nucleus into the other. Since
yolk is heavier than protoplasm, the vegetal pole where it is most abundant is
always down and the lighter animal pole is up.
In large-yolked eggs such as those of the frog and chick, the accumulation
of yolk in one region is so great that they are known as telolecithal, "end-
yolked" eggs (Table 19.1). In small-yolked eggs, like those of amphioxus and
man, the yolk is generally distributed and they are called isolecithal, "equal-
yolked." Even in these, there is a visible difference between the poles.
Fertilization — The Prelude to Development. The main steps in fertilization
are the entrance of the sperm into the egg, and the union of the male and
female nuclei (Fig. 19.2). The sperm makes its way into the egg membrane
stimulating the rise of a cone of protoplasm that surrounds it and draws it into
the egg. At the same time, a thin layer of protoplasm, the fertilization mem-
brane, is suddenly lifted from the surface of the egg and shuts out other sperm
cells. The male and female nuclei, each with half the number of chromosomes
to be contained in the body cells of the embryo, now approach one another
and come in contact. The first cell division of the new individual follows at
362
THE NF.W INDIVIDUAL
Table 1 9.1
Part IV
Arrangements of Yolk and the Accompanying Types of Cleavage
IN the Embryo
Amount unci
ArranQcinent of
Technical Name
Type of
Technical Name
of Cleavage
Type
Familiar
Yolk in the
Egg Cell
of Egg Type
C leavage
(Division)
Examples
Little and evenly
Isolecithal
Complete,
Holoblastic
Starfish
distributed
(equal-yolked)
nearly equal
(cleavage
through whole
of the em-
bryo)
Amphioxus
Man and other
mammals, ex-
cept mar-
supials and
egg-laying spe-
cies
Medium amount.
Telolecithal
Complete, un-
Holoblastic
Frogs
less near the
(end-yolked)
equal
Toads
animal pole
Salamanders
Some fishes
Abundant, except
Telolecithal
Incomplete, un-
Meroblastic
Chick
at the ani-
(end-yolked)
equal cells in
(cleavage
Majority of fishes
mal pole
disk on large
through part
Reptiles
yolk mass
of the em-
bryo)
Birds
Egg-laying mam-
mals, e.g., duck-
bill, spiny ant-
eaters
Medium amount
Centrolecithal
Incomplete
Superficial
Insects and other
in core near
(center-yolked)
through the
arthropods, ex-
center of egg
peripheral re-
gion of egg
cept scorpions
an interval varying with the kind of animal and environment but often very
soon.
The Substance of the Embryo Arranged in the Egg. In certain kinds of eggs
there are special regions in which pigment is present or absent, or yolk is
sparse or abundant. Either by following these visibly pigmented zones or
coloring them with vital dyes they have been traced to particular destinations
in the embryo. In the fertilized egg, these future organ regions are more defi-
nite than before fertilization. The substance of the egg takes part in an active
organization for the future development of the embryo. In the unfertilized egg
of the tunicate, Styela (Cynthia), one of the lower chordates, orange pigment
is uniformly distributed through the cell. But by streaming movements of the
cytoplasm during fertilization it is later concentrated into a yellow crescent
that marks the future posterior end of the embryo (Fig. 19.3). On the oppo-
site side of the egg is the gray crescent that becomes its anterior end. During
early development, the protoplasm of the yellow crescent is distributed to
form the middle layers of cells or mesoderm; the gray crescent becomes noto-
Chap. 19
DEVELOPMENT
DEUTOPLASM
363
Fig. 19.2. Fertilization in the guinea pig. Microscopic sections of eggs taken be-
fore and soon after mating. The eggs are minute, smaller than fig-seeds. Deuto-
plasm is protoplasm that is permeated with particles of yolk. The yolk may be light
(A) or dark colored (B) depending partly upon the stain used in its preparation.
A, before fertilization. The first maturation division with the nucleus now in meta-
phase. This results in two cells, the egg with half its former number of chromo-
somes and the first polar body {/ P.B.), a minute cell that contains the other half.
The egg is enclosed in a special (vitelline) membrane. B, preparation for fertiliza-
tion. The sperm has just entered the egg. The first polar body (/ P.B.) is outside
the egg cell. The entrance of the sperm stimulates the completion of a second di-
vision (// P.B.). Changes in the position of particles show that the sperm affects
the whole egg. C, fertilization. The nuclei of egg ( 9 ) and sperm ( $ ) are almost in
contact. Each one has half the number of chromosomes that is characteristic of the
body cells of a guinea pig. The polar bodies are disintegrating outside the egg.
(Courtesy, Nelsen: Comparative Embryology of the Vertebrates. New York, The
Blakiston Co., 1953.)
chord and neural plate; the gray yolk will be the inner layers of cells or endo-
derm, and the remainder of the egg will become the outermost cells or skin
ectoderm. The identity of parts of amphibian embryos has been followed by
coloring them and tracing their future careers in the animal, and also by trans-
planting them to other regions to test the effect of changed locations. Both
methods are widely used in experimental embryology.
Development of the Lancelet
A lancelet is a transparent fishlike animal about three inches long. Its
lance-shaped tail gave it the name lancelet and its sharp-edged body the name
amphioxus (double edged) (Fig. 19.4). There are about two dozen species of
Amphioxus distributed in the warmer seas over the world, including those
along the southeastern and -western coasts of the United States. Lancelets from
the Bay of Naples have long been known to biologists and those from the
waters near Amoy, China, to fishermen who may harvest as much as a ton of
them per day. In the breeding season, the males and females leave the sand
and swarm in shallow water. Eggs and sperm are shed in the open water and
fertilization occurs there. At this time the cytoplasm of the egg is apportioned
out for particular destinations in the embryo.
The eggs are just visible to the naked eye (0.1 mm. diameter) and contain
364
THE NEW INDIVIDUAL
Part IV
so little yolk that their processes can be clearly observed through the micro-
scope. The pattern of development is, on one hand, similar to that of hydra
and the starfish, animals far older in evolutionary history, and, on the other,
similar to that of the vertebrates that are much younger in evolution.
Early Development. The early development of amphioxus proceeds on a
(5) Ground
protoplasm
(2) Grey
crescent
Fig. 19.3. Fertilized egg of the tunicate
Styela. A crescent of yellowish protoplasm
(yellow crescent) becomes the posterior
end and a crescent of grayish protoplasm
(f^ray crescent) becomes the anterior end
of the embryo. Even at fertilization the
cytoplasm of the egg becomes arranged
for particular destinations in the develop-
ing animal. (After Conklin. Courtesy,
Shumway: Vertebrate Zoology, ed. 4.
New York, J. Wiley & Sons, 1942.)
(3) Grey yolk
plan followed in essentials by all vertebrates. Development is ordinarily a con-
tinuous process. It includes stages such as cleavage, blastulation, and gastrula-
tion that blend into one another.
Cleavage and Blastulation. The first cleavage begins as a slight depres-
sion at the animal pole. This deepens and lengthens into a constriction which
divides the egg into the first two cells representing the right and left halves of
the new animal (Fig. 19.5). The second cleavage also begins at the animal
pole, at right angles to the first. The third one is at right angles to the first two
Coelom
Brown funnel
Nofochord
Brain
Spinal cord
Myomere
Dorsal fin
Cirrus
Oral hood
Caudal fin
I
Mouth
Branchial
clefts
Gonad
Atnum
?>
Anus
Fig. 19.4. An adult amphioxus with part of the body wall removed from the left
side. The essentials to be noted are the relative positions of the spinal cord, note-
chord and alimentary canal. Amphioxus is generally regarded as an ancient ances-
tor of the vertebrates. The fundamental plan of its development is followed in all
of them. Adults are two inches long. (Courtesy, Rand: The Chordates. Philadel-
phia, The Blakiston Co., 1950.)
Fig. 19.5. Early stages in the development of Amphioxus. The egg is almost
microscopic but has practically no yolk. The processes of development are direct
and easier to follow than those of the frog whose eggs contain so much yolk, a,
fertilized egg. Egg and sperm nuclei in contact. The minute second polar body is
at the top; the first one has disintegrated, b, two-celled stage with nuclei dividing
again, c, four-celled stage, two hours after fertilization. Note a temporary cavity
(segmentation c.) formed as the cells divide, d, eight-celled stage, a side view,
showing the smaller cells at the upper or animal pole, the future front end of the
animal, e, all the cells are dividing at nearly the same rate which would not occur
if any of them contained much yolk. The nuclei are in metaphase and anaphase
stages of division. The segmentation cavity, open at one end, is traced by a broken
line. /, g, h; blastula stages. Cells multiply and the embryo grows. Its cavity, the
blastocoel, is shown in the half section. /, gastrula. The embryo flattens on the side
that is finally its posterior end. This is called the gastrulation or stomach forming
stage. /, k, I; views into the right half of the embryo. /, the embryo is now shaped
like a broad raspberry; the two layers of its wall are of ectoderm that will form the
skin and nervous system, and endoderm that becomes the lining of the alimentary
canal, now an open cavity, called the archenteron. The blastocoel is squeezed out
of existence, k and /, the embryo is growing longer; at the rear, its walls draw to-
gether except for the small anal opening. Layers of cells, the mesoderm, have
spread out between the ectoderm and endoderm. Mesoderm will become skeleton,
muscle, blood and other tissues. (After Conklin. Courtesy, Hegner and Stiles:
College Zoology, ed. 6. New York, The Macmillan Co., 1951.)
365
366 THE NEW INDIVIDUAL Part IV
and results in cells of unequal size. During these divisions the cells are gradu-
ally shifted outward and a temporary cavity is created in the center of the
cluster. Cells continue to divide until 200 or more are formed. The embryo is
then a hollow sphere called a blastula and the cavity within it is a blastocoel.
The cells of the embryo can easily interchange materials with the environment
of sea water. They have a relatively high income of oxygen and outgo of
excretory products resulting from the rapid metabolism of cell division espe-
cially at the animal pole.
Gastrulation — Formation of Primitive Digestive Tract. Changes
now transform the hollow sphere of the blastula to the saclike form of the
gastrula (Gr., gaster, stomach) in which there is a new cavity, the archenteron,
or first digestive tract (Fig. 19.5).
In the early part of gastrulation, the embryo is a double layered cup such
as a soft rubber ball would be if you pressed your thumb into its side. The side
forced in would be comparable to endoderm and chordamesoderm, and the
dent to the cavity of the archenteron; the other side of the ball would be ecto-
derm and the cavity inside the ball being pushed out of existence, the blasto-
coel. The archenteron appears gradually foreshadowed by the differentiation
of cytoplasm in the fertilized egg and the shape of the blastula (Fig. 19.5).
Toward the end of the blastula stage the vegetal region begins to flatten ever
so slightly like one side of a waning moon. Hindered by their content of yolk,
the cells on the flattened side divide fewer times and thus are larger than the
others. Presently the flat region is turned inward or invaginated more and
more sharply.
The embryo is now shaped like a raspberry. The opening into the archen-
teron gradually becomes smaller due to the multiplication and inturning of
cells about its rim and finally becomes the minute blastopore. Its rim is the
transition zone between endodermal cells and chordamesoderm, and the ecto-
derm. The endoderm will line the digestive canal. The chordamesoderm will
make the notochord and the mesoderm that forms the bulk of the body, of
organs such as the liver, the lining of the body cavity, and all muscles and
bone. The ectodermal cells are the ancestors of the cells of the nervous system
and outer layers of skin. The rim of the blastopore is the germ ring, a growth
zone in which cells form rapidly especially in the important side, called the
dorsal lip of the blastopore. This is the starting place of the notochord, present
throughout life in the ancestors of vertebrates and the forerunner of the back-
bone in the early embryo and the mesoderm of every vertebrate from fish to
man. The development thus far occurs within about seven hours after the fer-
tilization of the egg. As in all eggs, it varies with the temperature and other
conditions.
Nervous System — Notochord and Mesoderm. As the archenteron con-
tinues to enlarge, the dorsal surface of the embryo flattens and a broad band
Chap. 19 DEVELOPMENT 367
of thick ectoderm extends from the lip of the blastopore to the anterior end of
the body, the former animal pole. This is the neural plate from which the
nervous system is formed.
At first, the roof of thearchenteron is flat. Gradually, three folds arise in
it and extend the length of the body. The central one of chordamesoderm be-
comes the notochord. Those on either side separate from the wall of the
archenteron and grow in between the ectoderm and endoderm (Fig. 19.5). A
cavity in each one will be part of the future coelom or body cavity. The outer
side of each fold adheres to the ectoderm and together they become the
somatopleure, the forerunner of the body wall; the inner side of each fold
unites with the endoderm to become splanchnopleure, the future wall of the
digestive canal. Mesodermal cells differentiate in these layers and form various
structures such as muscles.
Development of the Frog
Eggs. Small as they are, frogs' eggs are huge compared with those of
amphioxus and their bulk is largely yolk (Fig. 19.1, 19.6). As they float in
Polar bodies
Animal hemisphere
Groy crescent
Vegetal hemisphere
Jelly coats (3)
ViteHine membrane
Fig. 19.6. Frog's egg 35 minutes after fertilization. The protective jelly secreted
by the oviduct swells as soon as the eggs touch the water. The egg loses water dur-
ing the rearrangement of protoplasm that occurs at fertilization and the shrinkage
allows it to rotate within the fertilization membrane. The unfinished division re-
sulting in the second polar body has been stimulated to completion by the entrance
of the sperm. In one region the pigment has moved toward the entering sperm thus
creating the gray crescent. (Courtesy, Rugh: The Frog. Philadelphia, The Blakiston
Co., 1951.)
368 THE NEW INDIVIDUAL Part IV
the ponds, their white vegetal poles are turned toward the dark bottom and
their black poles toward the light. This is due to relative weight but it results
in excellent concealing coloration. Yolk supplies the tadpoles with food until
they are well beyond hatching. Embryos have the same general needs as frogs,
plenty of water and food, income of oxygen, and outgo of carbon dioxide,
water and urea. They are easily killed from the by-products of their own metab-
olism and are so sensitive to temperature that they will reach any given stage
of development almost three times faster at 20° C. than at 10° C.
Reproduction Ends — Development Begins. Reproduction ends with two
processes that are extremely important to the new individual. They are: (1)
the maturation of the sex cells whereby their chromosomes are reduced to
half the number in the body cells; and (2) fertilization with its immediate
effects upon the organization of the egg, followed by the union of the sperm
and egg nuclei and the reestablishment of the whole number of chromosomes
(Fig. 19.6).
The entrance of the sperm always occurs in the hemisphere of the animal
pole and stimulates a reorganization of the egg which makes it repellent to
other sperm cells. Even if the egg membranes have been removed, a sperm
will not enter a fertilized egg. As before mentioned, experiments have proved
that the reorganization and development of an egg can be stimulated by vari-
ous shocks, pricks, solutions, and shakings. Frogs have grown to young adult-
hood with only pricks and chemical solutions for fathers.
Among the results of the reorganization is the gray crescent, an area oppo-
site the entrance point of the sperm, from which some of the black pigment
retreats. Staining parts of the egg has shown that a plane that passes through
the axis of the egg and bisects the gray crescent usually divides the future ani-
mal into right and left halves. Since the first cleavage plane bisects the gray
crescent it follows that the bilateral symmetry of the embryo is prearranged in
the egg.
Cleavage. Successive cell divisions follow one another at intervals of about
an hour varying with the temperature. The speed with which new cell mem-
branes grow is slowed down as the membrane formation plows through the
yolk. In the animal hemisphere, the wall of the blastocoel is thin because the
cells contain so little yolk; in the vegetal pole it is thick because they contain
so much (Fig. 19.7).
Within 12 hours after fertilization (at 18° C.) the embryo, usually in the
late blastula stage, contains hundreds of cells. The speed with which they multi-
ply makes it hard to realize that with every division a nucleus with its thou-
sands of genes is accurately allotted to each daughter cell. Equal distribution
of parental genes begins with the first cell division and is repeated through
billions of divisions in the growth of animals from jellyfishes to man.
Gastrulation. Gastrulation proper in amphioxus, for example, includes only
Chap. 19 DEVELOPMENT
Animal hemisphere
Gray crescent
369
Vegetal
hemisphere'
Stage 2. 1 hr post-
fertilization. Right
side view.
Stage 3. First cleavage Stage 4. Second Stage 5 Third
at 3.5 hrs. Posterior cleavage at 4.5 hrs. cleavage at 5.4
view. Right side view. hrs. 8 cells.
Stoge 7. Fifth cleavage. Stoge 10. Earliest Stage II. Extension Stage 12. Complete
32 cells at 7 hrs. involution of dorsal of dorsal to lateral lip involution, en-
lip at 26 hrs. Pos - lips at 34 hrs. circling yolk at
terior view. Posterior view. 42 hrs.
Fig. 19.7. General survey of the early development of the leopard frog seen in
external views. The stages are selected from many intermediate ones. Sioge 2, the
fertilized egg. Polar bodies not shown. Stages 3, 4, and 5; cleavage. Continued di-
vision creates smaller cells. Where yolk is most abundant, in the vegetal hemi-
sphere, division is slower and the cells are larger. Stage 7, early blastula. Stages 10,
11, and 12; gastrula. The crescentic groove (10) becomes a ring (12) as the mi-
nute rapidly dividing cells of the ectoderm grow over and around the more slowly
dividing yolk-filled cells of the endoderm. These and the cells which will form
notochord and mesoderm are thus turned inside (involution). In 72, only the yolk
plug, a pinhead of endoderm, is visible. The opening decreases but remains for a
time as the blastopore. (Courtesy, Rugh: The Frog. Philadelphia, The Blakiston
Co., 1951.)
the processes by which the single layered blastula is converted into the animal
with a definite ectoderm and endoderm and chordamesoderm about a future
digestive cavity. In the embryos of hydra, starfish, amphioxus and others there
is little yolk in the vegetal region of the embryo to hinder the ingrowth of cells
that creates the pioneer food cavity. However, in the frog the cells in the
vegetal area are burdened by yolk and do not grow inward so readily. Actu-
ally, the embryo frog has to swallow a lump of yolky food at its rear end. This
process begins with the ingrowth of cells that results in the appearance of the
crescentic groove at the junction of the animal and vegetal hemispheres (Fig.
19.7). The crescentic groove deepens because the cells multiply so fast that
they not only turn inward, but grow farther and farther over the yolk-filled
cells which are also turning in. While this is going on, the horns of the crescent
grow toward one another and finally complete a circle. At the same time, the
rim continues to close in, and makes the circle smaller and smaller. By now
less than a pinhead of white cells, the yolk plug, is visible, and presently not
even this because the dark rim has closed the blastopore. The food-filled cells
370 THE NEW INDIVIDUAL Part IV
are now appropriately located in the floor of the enteron, the future digestive
tract (Figs. 19.7, 19.9). As this cavity enlarges, the blastocoel is practically
blotted out.
Mcsoderni and Notochord. The ingrown mid-dorsal cells are the future
notochord and mesodermal somites (Figs. 19.8, 19.9, 19.10). They form a
temporary roof of the enteron whose sides and floor are made of endoderm.
The enteron soon acquires an endodermal roof by the upgrowth and meeting
beneath the notochord of the endodermal cells that form its sides. The chorda-
mesoderm is continuous on each side with other potential mesodermal cells.
These have turned in along the lateral lips of the rim of the blastopore and lie
between the outer ectoderm and the inner endoderm.
Crevices now appear in the mesoderm along the sides of the body; these
widen and extend forward and backward, splitting it into two layers, one that
unites with ectoderm (somatopleure) and forms the future body wall, and the
other that unites with endoderm (splanchnopleure) to be the future wall of
the alimentary canal. The crevices between the layers are the beginning of the
future coelom which will contain the digestive canal, kidneys and other organs
of the body (Fig. 19.10).
Thus the three principal layers, ectoderm, mesoderm, and endoderm and
the notochord are established. Cell division, movement, and differentiation
have gone on together. All over the embryo parts are growing and changing
partly because of what their inherited genes make them and partly because of
their environment, the effects of their neighbor cells.
Nervous System and Epidermis. While the mesoderm and notochord are
being established the nervous system is also taking shape largely under their
influence. A broad band of thickened ectoderm that extends forward from the
blastopore lies directly over the notochord and its adjoining mesoderm. This
is the neural plate, the material of future brain and spinal cord (Fig. 19.8,
19.9, 19.10). Along its borders, cells accumulate in ridges, the neural folds
which gradually come together and unite to make the neural tube. The neural
tube then differentiates; the front part of it becomes brain; the remainder be-
comes nerve cord. During the closing of the neural tube some of the cells are
left along each side. These are the neural crests from which the dorsal ganglia
of the spinal nerves arise.
Cilia are now abundant on the skin ectoderm and their steady backward
beat keeps the embryo slowly turning over and over while it is still within the
egg membranes. After they hatch, tadpoles are moved smoothly forward by
their cilia.
Form and Organs of the Tadpole
The embryo grows rapidly, especially its head and tail. As it lengthens, it
loses its stumpy form and looks more and more like a corpulent fish.
Anterior
Lett
Brain region
Posterior
No 13. Early neurula. Dorsol
view. Medullary plate stage
No.14 Neural fold stage.
Dorsal view.
Body
Head
Toil
No 15. Closing neural fold.
Dorsal view.
Gill onlage
Optic
vesici*
Sucker
No. 16. Early tail bud.
Dorsal view
Nol7. Eorliest muscular response.
Loterol view.
External gills
Stage 20. 6mm I40tirs.
Gill circulation and
hatctiing.
External gills
\'
>>^>//>////^/'
Ifactory
organ
Stage 23. 9mm 2l6hrs.
Oifoctory
pit,
Spiracle
Stage25. Ilmm.284hrs. External gills
obsorbed. Left side to show opercular
fold and spiracle.
Fig. 19.8. Survey of the later development of the leopard frog, external views.
Stages 13, 14, and 15; nervous system and epidermis. In the neurula stage, the neu-
ral or medullary plate extends forward from the blastopore and lies directly above
the notochord and adjoining mesoderm. In stage 14 the neural folds are present
but are separated by an open trough, the future nerve cord and brain. In 75 the
folds gradually close together. The central canal of the mature nerve cord and the
ventricles of the brain are remainders of the once open trough. In stages 16 and
17, the body is lengthening and the developing muscles twitch spasmodically. The
optic vesicles are outgrowths of the brain that form the retinas and optic nerves.
The cells of the endodermal lining of the gut are still packed with yolk. Stages 20,
23, and 25 show a rapid increase in size due to absorption of water. At hatching,
the tadpole is about 56 per cent water; fifteen days after hatching, it reaches its
maximum of 96 per cent water. By stage 23 the tadpoles hang by their suckers
from submerged stems. By stage 25 they are eating soft plants; their bodies are
fish-shaped. The external gills of stage 23 are replaced by internal ones covered
by the opercula. The spiracle, a pore on the left side, is the only exit for water. In
leopard frogs the respiratory system changes little until the tadpole is transformed
into a frog over two months later. (Courtesy, Rugh: The Frog. Philadelphia, The
Blakiston Co., 1951.)
371
372
THE NEW INDIVIDUAL
Part IV
Future skin
e nerve
ord
Blastopore
of cells
Future yolk plug
A. LATE BLASTULA
B. GASTRULA STAGE
Future skin
Future nerve
cord
Arch
enteron
Future brain
Archenteron
Notochord
Future nerve
cord
Notochord
Yolk plug
Yolk plug
C. GASTRULA
D. LATE GASTRULA STAGE
Fig. 19.9. Internal views of frog embryos. (From Development of the Frog, as
illustrated by the Mueller-Ward Models. Courtesy, Justus F. Mueller and Ward's
Natural Science Establishment.)
Skin and Nervous System. After the neural groove is closed there is a short
passageway between the neural tube and enteron, the neurenteric canal, that
exists but a short time (Fig. 19.10). The forebrain, midbrain, and hindbrain
gradually take shape. Beneath and near the front of the forebrain a process of
superficial ectoderm extends inward. This later joins a downpushing of the
brain and together they become the pituitary body. From the ventral side of
the neural tube motor nerve cells send out processes to muscles and glands.
Processes from the cells of the dorsal ganglia extend into the cord, to the skin
and to other parts of the body. The cord and brain are gradually surrounded
by an envelope of loose mesodermal (mesenchymal) cells. In all vertebrates
such cells form the coverings or meninges of the spinal cord.
The lining of the neural tube is a center of active cell division and gradually
increasing differentiation. Two types of cells are formed, the future supporting
cells or neuroglia of the nervous system and the nerve cells or neurons. The
latter move out of the lining into the thick wall of the neural tube where they
develop into typical neurons with extended axons and dendrites (Fig. 16.3).
Chap. 19
Mid gut
Brain
Future
mouth
opening
Future heart
DEVELOPMENT
Nerve cord ^Notochord
Blastopore
osed
Rectum
373
Liver
Mesoderm
Brain
Notochord Nerve cord
Tail fin
Future
mouth
opening
Fig. 19.10. Sections of frog embryos, before and after hatching. (From Devel-
opment of the Frog, as illustrated by the Mueller-Ward Models. Courtesy, Justus
F. Mueller and Ward's Natural Science Establishment.)
Similar changes take place in the cells of the neural crest as it is transformed
into ganglia.
Sense Organs. An optic vesicle pushes out from each side of the forebrain
and makes a well-marked bulge where it is in contact with the skin ectoderm.
Each vesicle is shaped like one half of a hollow dumbbell (Fig. 19.11). Its
walls are continuous with the wail of the brain, and nerve and sensory cells
develop in them. After the vesicle has extended outward, it takes the shape of
a double-walled cup. The front or inner wall of the cup will be the sensory
layer of the retina containing the light sensitive cells and the cell bodies of the
optic nerve fibers; the outer wall will be the pigmented layer. The light-sensi-
tive cells develop from cells that were on the former outer surface of the
neural folds. Diagrams of cross sections of the same region of the brain and
vesicles at successive ages show how the cells originating on the outer surface
of the folds are finally located inside the optic vesicle (Fig. 19.11). This ex-
plains why light that comes to the retina strikes the nerve cells, and then the
sensory cells seemingly wrong end first (Fig. 17.17). As the optic vesicle
grows outward, it touches a plate of skin ectoderm which thickens and dips in
to make a sac, the lens vesicle, that fits into the cup. The lens vesicle separates
374
THE NEW INDIVIDUAL Part IV
Skin Fore brain Future lens Optic vesicle
Brain nearly closed
Future lens Future retina
Brain and cord closed
Future retina
Lens
Cornea
Skin layer over lens becomes cornea
Fig. 19.11. The development of the eye. Diagrams of cross sections of frog em-
bryos showing successive stages of the part taken by the ectoderm in the develop-
ment of the eye. Except for its part in the sense organs, the superficial ectoderm
becomes skin. A, embryo with brain open as in Figure 19.8, stages 14 and 15. B,
on each side of the head an outgrowth of the brain (optic vesicle) approaches the
lens, a thickened plate (placode) in the superficial ectoderm. C, the optic vesicle
at first shaped like a hollow dumbbell is now a shallow cup. The lens bends toward
the cup. The neural folds have closed and the future skin is separated from the
future brain. D, the lens has separated from the future skin ectoderm. The bottom
of the optic cup (vesicle) is the future retina. The lens nearly fills the top of the
cup. The superficial ectoderm outside the lens will be the cornea.
from the skin ectoderm which later becomes the cornea (Fig. 19.11). The
accessory parts of the eye, the coats, blood vessels and muscles, are developed
from mesoderm.
The sensory parts of other prominent sense organs, inner ears, nose and
taste all develop from ectoderm in fundamentally similar ways. The lateral
line system consists of a series of sense buds arranged in rows over the head
and body. Each line begins as a thickening of sensory ectoderm which later
breaks up into the sense buds that respond to vibrations in the water. Lateral
lines are conspicuous in bony fishes and in tadpoles, but they do not persist in
frogs and toads.
Digestive System. As the body grows longer, the enteron also lengthens and
Chap. 19 DEVELOPMENT 375
a ventral outpocketing of it near its front end is the first appearance of the
liver. At the posterior end the endoderm grows outward and the ectoderm in-
ward till they meet and break; the latter forms the lining of the future cloaca
and its external opening. At the anterior end, a similar ingrowth of ectoderm
which will line the greater part of the cavity of the mouth meets the endoderm
in an oral plate which also breaks through. Thus, the saclike enteron becomes
a tube.
Only the linings of the alimentary canal and its branches are endoderm. In
various regions of these, cells are gradually differentiated for their respective
functions, such as secretion and absorption. Except for the nerves, mesoderm
composes the whole outer wall of the digestive canal and its derivatives such
as pancreas and liver and their ducts. The endodermal cells lining the finer
branches of the liver ducts become the cells which secrete the bile. Like the
liver, the pancreas also arises as an outpocketing of the inner layers of cells in
the wall of the digestive canal.
The fundamental processes of ingrowth, outgrowth, and differentiation of
cells are repeated over and over again in all embryos.
Respiratory System. The respiratory organs of vertebrates are also derived
from the digestive canal. Whether their function demands exposure to water
or air, their surfaces are continually moist and are always close to the blood.
The first signs of a respiratory system in the tadpole are the outpushings
from the endoderm of the foregut, the region of the future pharynx (Fig.
19.10). There are in all six of these gill pouches on each side. The first and
last never open but about the time of hatching, the others meet the superficial
ectoderm, break, and become the gill clefts that give free passage to the water
outside. The solid bars of tissue anterior and posterior to the gill clefts are the
gill arches that support the gills. In the frog, the tissue in front of the first pair
of pouches that remain closed will form the lower jaw. In all vertebrates, these
pouches become the middle ears and the eardrum develops where the endo-
derm of the pouch meets the skin ectoderm which will line the tube of the
external ear. The Eustachian tube is derived from the part of the pouch nearest
the foregut and thus the pharynx and middle ear communicate (Fig. 17.9).
In a newly hatched tadpole, respiration is carried on by external gills that
develop as outgrowths of the skin ectoderm of the three arches (Fig. 19.10).
These external gills are later absorbed and replaced by internal gills which
also arise from the gill arches. At about this time, a fold of ectoderm, the
future operculum, arises in front of the gill clefts and grows backward, form-
ing a mantle around the internal gills and gill arches of both sides (Fig.
19.10). It has one external opening on the left side, the only exit for the water
that enters the mouth and flows over the gills as the tadpole breathes. Even
before hatching, the lungs appear as two small outpocketings from the floor
of the future esophagus and are inconspicuously present through the period
376 THE NEW INDIVIDUAL Part IV
in which the gills are functioning. Then, with the approach of metamorphosis,
the lung sacs enlarge but the endoderm at their tops is constricted in prepara-
tion for the future larynx. Although the tadpole is still a true water breather,
it is also a presumptive air breather. Before the gills are spent, the lungs are
ready to begin work. For the gills and the lungs it is a case of: "The king is
dead! Long live the king."
Mesoderm — The Bulk of the Body. The mesoderm produces the connective
tissue, the skeleton, the blood and blood vessels, the muscles and other parts
including the lining of the body cavity, the kidneys and the reproductive sys-
tem.
Metamorphosis from Tadpole to Frog
During the change to adult form in the larvae of frogs and toads, the tail
and gills are absorbed; the gill clefts are closed; legs develop; lungs become
functional; and the food cavity is changed. The horny lips with which the
tadpole scrapes algae are replaced by bony jaws and teeth; the relatively long
"watch spring" intestine is changed into a shorter one that functions with a
mixed diet of plants and animals (Fig. 34.7).
Provisions for Health and Safety of Embryos
Developing embryos are provided with water and food. They use water
continually and it forms a large part of their substance. They have prospered
in watery surroundings throughout their histories. The delicate embryos of
aquatic animals float and swim in lakes and seas. The equally delicate embryos
of most land animals develop within sacs of fluid, individual ponds that take
the place of the wider waters of their aquatic relatives.
Earthworms pass their early days within seed-like capsules. Each of these
holds a few embryos in a bath of nourishing albumen which they swallow and
also absorb through their skins. Like those of other invertebrates, these em-
bryos have no special food-sacs attached to their bodies.
Food and the Yolk Sac. The majority of vertebrates, fishes, reptiles, birds,
and mammals, have a yolk sac containing more or less food in the form of
yolk. It is a pouch-like extension of the digestive tract, an organ producing
enzymes that break the yolk into substances that pass into the blood, are car-
ried into the body of the embryo, and finally converted into its protoplasm.
In birds, the body wall closes over the yolk sac before hatching and the latter
shrinks and finally merges into the intestine. The rounded front of a one-day
chick is due to its yolk sac.
The Watery Environment and the Amniotic Sac. The amnion is a trans-
parent roomy sac that loosely surrounds the embryo (Figs. 19.13, 19.14). It
contains the amniotic fluid secreted by the membranous sac and by the embryo
itself. The fluid allows the embryo considerable free motion especially during
Chap. 19 DEVELOPMENT 377
its earlier development and acts as a protection and shock absorber. It is also
a catch basin for waste products of metabolism.
Amniotic sacs first appeared in reptiles, the first truly land animals. In them,
they are the guarantee of watery surroundings for the embryos even in the
desert where many reptiles live. The amnion is also well developed in birds
and mammals. All of these are essentially land animals and it functions in
them as it does in reptiles.
Fig. 19.12. Embryo fish and its food supply. The yolk sac is prominent for some
time after hatching in trout and many other fishes. It is a blind sac which opens
out of the alimentary canal. The body wall grows completely around it and it is as
much inside the body as the intestine. It is highly useful to the embryo in all verte-
brates except mammals; in them the yolk sac is history. (Courtesy, Bridge in Cam-
bridge Natural History, Vol. VII. London, The Macmillan Co., 1910.)
Allantois
Amnion
Embryo
Vitelline
vessels
Sinus
terminalis
Fig. 19.13. Chick of about five-and-a-half days incubation taken out of the shell
with the yolk intact. The albumen and the serosa, a membrane lying next to the
shell, have been removed. By means of the allantois the blood receives oxygen and
is relieved of carbon dioxide. The yolk sac holds the food supply of yolk easily
within reach of the digestive tract of the embryo. (Courtesy, Patten: Early Embry-
ology of the Chick, ed. 4. New York, The Blakiston Co., 1951.)
378
THE NEW INDIVIDUAL
Part IV
Fig. 19.14. Photograph of human embryo and sacs, in the eighth week of de-
velopment; the chorion has been cut away to show the embryo, about one half inch
long. The two sacs, amnion and chorion, are roomy and fluid-filled. In its natural
position, the whole chorion is covered by the tissue of the uterine wall in which it
first became embedded. The exchange of gases, food and waste between the blood
of the mother and embryo occurs through the walls of the finger-like villi of the
chorion that look so feathery in this figure. The left eye, hand, and leg of the em-
bryo are clearly recognizable. (Courtesy, Department of Embryology, Carnegie
Institute of Washington.)
The Chorion and Associated Membranes. The life processes of the embryo
depend upon the chorion with its specialized part the placenta in mammals
and in reptiles and birds with its associated sac the allantois. In birds, the
amnion and chorion arise simultaneously from a fold of the extended body
wall that first appears in front of the head and then encircles the embryo
with its edges closing together as if pulled by a drawstring. The inner part
of the fold becomes the amnion, the outer part forms the chorion. They are
united for a short time at the meeting place of the folds but the delicate join-
Chap. 19 DEVELOPMENT 379
ing usually soon gives way and the layers seem never to have been connected.
The chorion of reptiles and birds is united with the allantois which contains
many blood vessels. Together they rest closely against the porous egg shells,
and function as a respiratory organ.
The Allantois. Like the yolk sac, the allantois is an outgrowth of the
digestive tract but has a different function (Fig. 19.13). In birds, it fills most
of the space between amnion and chorion and fusing with the chorion (cho-
rioallantoic membrane) becomes an important respiratory organ. It is also
a temporary urinary bladder.
In mammals, except the guinea pig and some other rodents, and the
primates including man its walls may fuse with the chorion and become part
of the embryonic section of the placenta. It then functions in the transfer of
food, respiratory gases, and waste products between mother and embryo.
The placenta is discussed in later paragraphs that deal with the human
embryo.
Umbilical Cord. As the embryo grows, the folds of the amnion surround-
ing the stalks of the yolk sac and allantois come together in a ventral tube
(Fig. 19.15). In the higher mammals this tube is the umbilical cord that
Myometrium
Decidua
Parietalis
Decidua
Capsularis
Amnion
Chorion
Frondosum
■yj Decidua
Basalis
Yolk-sac
Anterior fornix of vagina
Fig. 19.15. Outline diagram of human uterus showing the placenta, sacs and
embryo. The placenta consists of the chorion where the villi have greatly developed
— over most of it, they have disappeared — and of the decidua basalis, a part of the
wall of the uterus. Compare with Figure 19.14. Decidua capsularis is the part which
covered the embryo when it was first implanted. Placenta and sacs are parts of the
afterbirth. (Courtesy, Patten: Human Embryology, ed. 2. New York, The Blakis-
ton Co., 1953.)
380 THE NEW INDIVIDUAL Part IV
in addition to the yolk sac and allantois also holds the large blood vessels
that connect the embryo with the placenta (Fig. 19.18).
Human Embryo
First Days of Life. DifTerent as they may be later, animals greatly resemble
one another in the earliest part of their lives (Fig. 38.7). In their youngest
stages, rabbits, monkeys, and men look very much alike, though their chromo-
somes soon tell a different story. In recent years, the microscopic living
embryos of mice, rabbits, and monkeys have been removed from the maternal
oviducts and uteri, placed in salt solution at body temperature, and photo-
graphed in still and motion pictures (Fig. 19.16). The youngest human em-
bryos yet seen, including a 2-celled one, have been removed from the oviducts
and uteri of persons undergoing operations (Fig. 19.17). The fertilization
of the human egg on a microscope slide has also been photographed.
Implantation in the Uterine Wall. With the help of cilia and contractions
of muscles in the wall of the oviduct the human embryo is rolled into the
uterus. By the time it arrives there, or soon after that, it reaches the blastocyst
stage (Fig. 18.14). This is an almost microscopic sphere, its wall a thin layer
of cells (trophoblast), mostly chorion, that contains fluid and a knot of
cells, the embryo. In this stage it is presumed to be about 4 to 5 days old,
counting from the time that the egg was probably fertilized. Within a day or
two, the blastocyst sticks to the lining of the uterus, and then sinks into it,
evidently through the effect of its own secretions upon the cells about it. In
the meantime, delicate fingerlike processes, the villi, grow out from the
surface of the little sphere into the wall of the uterus, like roots into soil
(Figs. 18.14, 19.15). There they are surrounded by blood from the uterine
capillaries whose walls have been broken during this process of implantation.
Thus the embryo's source of supplies is at once established. At first, all ex-
change of water and food and gases is by absorption through the membranes
and body of the embryo. Later, the blood vascular system develops and the
embryo's own blood transports materials always to and from the villi extend-
ing into the mother's blood. The two kinds of blood never mix.
Placenta. The placenta is a temporary organ formed from parts of two
individuals of different generations, the mother and her unborn young. Its
maternal part (or decidua) is an elaborate development of the inner layers
of the uterine wall. Its embryonic part is a specialized region of the chorion.
In the human placenta there are open spaces or sinuses between these two
parts into which maternal blood flows from uterine arteries that were first
broken during the implantation of the embryo. This blood is constantly
changed as it flows from the uterine arteries and slowly returns to the
uterine veins. Minute richly branched villi from the embryonic placenta dip
into this reservoir of blood (Fig. 19.15). Within each fingerlike villus are
c
D
Fig. 19.16. Photomicrographs of living embryos of monkeys showing early stages
of division. The fertilized ovum was washed out of the tube, cultivated in plasma
and its growth recorded in micro-moving pictures.
The numbers of hours include the time between ovulation and fertilization, and
the period of cell division which follows.
A, two-cell stage, about 29 hours after ovulation, the actual escape of the egg
from the ovary. B, three-cell stage, about 36!/2 hours. C, four-cell stage, about
31 V2 hours. D, five-cell stage, about 48 '/i hours. E, six-cell stage, about 49 hours.
F, eight-cell stage, about 50 hours. (After Lewis and Hartman. Courtesy, Patten:
Human Embryology, ed. 2. New York, The Blakiston Co., 1953.)
381
382
THE NEW INDIVIDUAL
Part IV
Fig. 19.17. A section of the two-celled stage of a human embryo taken from an
oviduct during an operation. (Courtesy, A. T. Hertig and Carnegie Institute of
Washington.)
capillaries that join vessels that reach the embryonic placenta from the em-
bryo by way of the umbilical cord. Thus, there is a double circulation in the
placenta, an embryonic part in the villi and a maternal part in the reservoir
in which the villi are immersed.
Carbon dioxide and other waste products of the embryo pass through the
membranes of the villi from the blood of the embryo into the blood of the
mother. Food and oxygen from the blood of the mother pass into that of the
embryo.
By means of radioactive chemicals, it has been shown that the smaller
molecules of matter pass through the membranes of the villi, substances such
as salts, sugars, calcium, amino acids, and certain vitamins and hormones.
The Rh factor, an antigen or substance that causes agglutination (clumping
and sticking together of red blood cells), may be present in the blood of the
embryo. This may pass through the placenta into the mother's blood. If her
blood is negative to the Rh substance, it can stimulate the production of
antibodies which return to the embryo and destroy its red blood cells.
There is no means of communication between the embryo and mother
except by substances such as those that have been named. Not a single nerve
passes from one individual to the other. In its psychology the embryo is as
independent of its environment as any other animal may be.
Hormones. The placenta is not only a filter of foods going inward to the
embryo and waste products going out, but it also produces a series of hor-
mones.
Chap. 19 DEVELOPMENT 383
As soon as the villi are well developed, they secrete a hormone (chorionic
gonadotrophin) promptly circulated by the blood and easily extracted from
the urine. Experiments have shown that human urine of pregnancy has a
stimulating effect upon the ovaries when injected into the bodies of immature
rats and mice, the basis of the Aschheim-Zondek pregnancy test. In the
Friedman test for early pregnancy, the urine is injected into the ear vein of a
rabbit. If the woman is pregnant, eggs will be shed from the ovaries into
the oviducts of the rabbit in about 24 to 48 hours. Obviously this requires an
operation on the rabbit. Physicians most commonly use the much simpler
test on frogs. Some of the urine to be tested is injected into a dorsal lymph
sac of an adult male frog (Fig. 32.20), usually the common Rana pipiens.
The frog is placed in a dry jar for two hours. Some of its urine is then col-
lected and examined with a microscope. If it contains sperm cells the preg-
nancy is regarded as certain.
I'terine muscle
Remains of volk sac
Fetal villi of
chorion
Maternal sinus
{Intervillous space
Vecidua basalis
Placental septum
Marginal sinus
Inised decidua parietalis
and capsularis
Chorion
A mnion
Fig. 19.18. Diagrammatic section through the uterus; infant just before birth in
the usual position. As in the majority of mammals the yolk sac is present only as
an inheritance from vertebrate ancestors. The placenta and other sacs are forced
from the uterus as the afterbirth. (Courtesy, Arey: Developmental Anatomy, ed.
5. Philadelphia, W. B. Saunders Co., 1946.)
384 THE NEW INDIVIDUAL Part IV
Other hormones produced by the placenta include estrogen and progesterone
which stimulate the enlargement of the uterus, the growth of the mammary
glands, and are involved in the uterine contractions that occur at birth.
Birth. The birth process begins with rhythmic contractions of the smooth
muscles in the uterine wall, joined later by the striated muscles of the ab-
dominal wall. These are timed with the stretching of the birth canal so that
the infant is forced out, normally head first, pulling the umbilical cord after it.
Similar contractions expel the afterbirth, which includes the placenta and
all the other membranes which were, for a time, of life and death importance
to the infant. Birth is in no way such a simple process as this statement sug-
gests. A complex of hormones, changed rate of blood flow, sensitivity of nerves
and muscles — a whole system of balanced forces — is concerned.
At birth, a baby meets a great crisis of its life. For nine months it has
lived in a soft-walled chamber, flooded with fluid warmed to a steady 98.6°
F., protected from jar and vibration and in total darkness (Fig. 19.18).
Food ready to use and oxygen have been filtered into its blood. Its lungs
are collapsed, without air and with only a fraction of the blood soon to
come to them. Instead of going to the lungs, the main supply of blood has
taken a short cut and bypassed them; it also has crossed the heart through
an opening between the auricles. These arrangements provide for the circu-
lation to the placenta; after birth, they would be useless and worse. If the
short routes stay open, a blue baby results because venous blood leaps through
the opening from the right to the left auricle, and through the duct from the
pulmonary artery into the aorta (Fig. 19.19).
When a baby first emerges into the air its lungs are immediately inflated
due to the negative pressure in its lungs and the positive pressure of the air.
It must breathe, at once and without practice, a complicated business in
which failure is fatal. Before birth, the baby may only swallow amniotic
fluid and whatever it contains. After its birth, it deals with food at first hand;
its digestive tract is new to this also, the reason for hiccoughs and other
digestive rebellions. A baby arrives in a changeful environment, of moving
air that may be dry or moist, of shifting temperatures, changing light, food
in variety, human neighbors, plants and animals. With unwarned suddenness
its ecology is changed and it begins adjustments that must continue through-
out its life.
Twins. Multiple births are due to the development of more than one egg
or to the division of the fertilized egg into parts each of which develops into
an infant.
Fraternal twins are the product of two different eggs which matured at the
same time and were fertilized by two different sperm cells. Fraternal twins
have different genes and are not any more alike than any children of the
same parents. They may or may not be of the same sex. There is a separate
Chap. 19
DEVELOPMENT
385
B. Postnatal
Fig. 19.19. General scheme of human circulation before (fetal) and after birth
(postnatal). Before birth. The outstandingly important organ is the placenta
through which the embryo receives oxygen, food and other substances from the
maternal blood, all of it passing through membranes. Before birth blood passes
freely from the right to the left auricles (or atria).
Supplies from the mother's blood are carried to the embryo via the placenta, the
umbilical vein and the vena cava (on left side) to the right auricle (atrium) of the
heart. Carbon dioxide and substances to be eliminated from the embryo are
brought to the placenta via the aorta (right side) and the umbilical artery. After
birth. At birth the vessels in the umbilical cord shrink and close and the placental
blood stream is abruptly cut oif. The circulation to the lungs is immediately and
completely underway. The passage (ductus arteriosus) between the two auricles is
soon closed. (Courtesy, Patten: Human Embryology, ed. 2. New York, The
Blakiston Co., 1953.)
placenta and amniotic sac for each one and there are two afterbirths (Fig.
19.20).
Identical twins come from a single fertilized egg that divides after fertiliza-
tion, begins to grow, splits in half and develops into two individuals. Each
one has the same inheritance as the other and since sex is inherited they are
always of the same sex. They share the same placenta and there is only one
afterbirth (Fig. 19.20). Siamese twins are identical twins only partly sep-
386
THE NEW INDIVIDUAL
Part IV
IDENTICAL TWINS
Are products of
A single
sperm
and
G
A single
egg
In an early stage
the embryo divides
The halves go
on to become
separate
individuals
Usually — but not always — identical
twins share the same placenta and
fetal sac
But regardless of how they develop,
they carry the same genes and are
therefore
Always of the same sex — two boys
or two girls
FRATERNAL TWINS
Are products of TWO different eggi
fertilized by TWO different sperms
They have different genes and may
develop in different ways, usually—
but not always — having separate
placentas and separate fetal sacs
Also, as they are totally different in.
dividuals, they may be
>r two girls
—Or a
mixed
pair
One
boy
One
girl
Fig. 19.20. How twins are produced. (From The New You and Heredity by
Amram Scheinfeld. Copyright, 1939, 1950 by Amram Scheinfeld, published by
J. B. Lippincott Co.)
Chap. 19 DEVELOPMENT 387
arated. Quadruplets and quintuplets may include fraternals and identicals; the
odd one in quintuplets is usually regarded as a twin whose mate did not live
long.
In man, apes and many- other mammals, only one infant is usually produced
at a time. According to estimates from statistics once in every 80 human
births, two are born at the same time, and triplets once in 512,000. Only
about 30 quintuplets have been recorded, and there are three substantiated
cases of the birth of sextuplets. At this date the famous Dionne quintuplets
of Canada and a similar series born in Argentina are the only groups of five
known to have survived.
20
Tlie Pliysical Basis or Heredity
Two influences enter into the making of every plant, animal and man —
their inheritance and their surroundings. Nature and nurture are never sepa-
rated but nature once set is steadfast and harder to change than nurture. A
hen sits on ducks' eggs and hatches ducks but no hen broods ducks' eggs into
chickens. Monkeys learn to climb trees; cows never do.
The question "Which is more important, heredity or environment?" has
started endless arguments, but it was never a sensible question for no plant
or animal can exist without both. Although inseparable, they are different.
By the time an animal has come into existence as a fertilized egg, its in-
heritance has been set, heredity is behind it. Nobody chooses his parents
and the inheritance they give him. But most of his environment is still in
front with chances of change and choice.
What is Heredity? The heredity of a plant or animal consists of the
characteristics brought to it by its ancestors. People of every kind, climate
and time, have had their own ideas and uses of inheritances. In their early
history, the Egyptians selected and artificially pollinated their date palms
and got a better crop of dates. In the middle ages, the big horses capable of
carrying the enormous weight of the armored knights were selectively bred
and became the ancestors of the English Great Horse or Shire Horse. In
later times, many new types, such as mules and Poland-China hogs, have
been produced by crossing different varieties and species. Hardy range-sheep
come from crosses of Merino and "mutton sheep." From time immemorial
human beings have looked at one another and recognized that like begets
like; so have the robins and rabbits and other animals according to their
kind.
What is Genetics? Genetics is the science of the genes, the physical units
of heredity contained in the chromosomes and believed to be protein mole-
cules. Studies of genetics are precise and analytical, usually focused on single
or small groups of inherited characters and often based on experiments.
388
Chap. 20
THE PHYSICAL BASIS OF HEREDITY
389
Beginning of Genetics
The science of genetics has had a lifetime of about fifty years, marked by
an extraordinary advance in knowledge and usefulness. From its beginning
workers in this field have used precise methods, analysis, experiments upon
large numbers of individuals, and meticulous records. The present knowledge
of heredity rests upon the discovery that the characters of an organism
are inherited independently of each other and not blended together. The
discoverer. Father Gregor Mendel, was a gardener, beekeeper, and priest
who was interested in flowers, their pollination and the part taken in it
by the bees, not only bees in general but the particular varieties that he
secured by selecting and cross breeding them (Fig. 20.1). All of his work
was illumined by enthusiasm and enjoyment. The flowers were lively and
special to him; the fuchsia was his favorite. He finally selected garden peas
for his main experiments because they were easy to raise and cross pollinate,
and he was especially interested in their inheritance of size and form. So
it came about that for his far-reaching work, his material was mainly garden
peas grown in a small plot near his monastery. Mendel's enthusiasm was
Fig. 20.1. The garden in the Koniginkloster in Brunn where Gregor Mendel
(1822-1884) carried on his experiments ( 1856-1864). Those experiments were the
foundations of genetics, the science of the gene, the unit of inheritance. (Photo-
graph by Hugo litis. Courtesy, Sinnott, Dunn, and Dobzhansky: Principles of
Genetics, ed. 4. New York, McGraw-Hill Book Co., 1950.)
390 THE NEW INDIVIDUAL Part IV
combined with a rare equipment of curiosity, precise observing and record-
ing, respect for facts and logical reasoning. His work is an inspiring example
of what observation and reason can achieve. He planned his experiments
with great care, and set them like traps to catch the facts. The basic principles
which he drew from them have been upheld by thousands of experimenters
who have followed him.
Gregor Mendel, Founder of the Science of Genetics
Gregor Mendel, 1822-1884, spent his boyhood on an Austrian (now
Czechoslovakian) farm where he grew up with orchards and gardens all
about him. At 21, he entered the monastery at nearby Brunn (now Brno),
was ordained a priest three years later, went to Vienna for a scientific train-
ing, returned to his home monastery, and for 14 years was a teacher of natural
history in Brunn Modern School. During those 14 years, he conducted the
experiments on peas that led him to believe that heritable characters are
produced by separate units, and that this separateness is a basic principle of
inheritance. Mendel was searching for laws that operate in creating species
at the same time that Charles Darwin was writing the Origin of Species. His
experiments and conclusions were published in a brief paper in The Pro-
ceedings of the Natural History Society of Brunn (1865). By this time
many people were fiercely attentive to the Origin of Species (published in
1859) and Mendel's paper went unnoticed. In addition to this, in 1868 he
met another handicap in being elected Prelate of Altbrunn, a high adminis-
trative office which consumed most of his time. With this new occupation
his work in genetics and the adventures of his mind were ended.
Resurrection of a Discovery
Mendel's conclusions remained hidden until 1900, 16 years after his death,
when three botanists experimenting in different countries made discoveries
similar to those of Mendel. In that same year, and independently of one
another, they found his paper. By that time, the first shock from the Origin
of Species had died down and the theory had begun to stimulate curiosity.
People were asking how plants and animals came to be different and how
their differences were inherited. Chromosomes had been discovered and
biologists were highly excited about their significance. It soon appeared that
these things were related to Mendel's inherited characters. Although they
were discovered before Mendel's death, he never mentioned them and perhaps
never heard of them.
Mendel's Approach to the Problem of Inheritance
Peas are naturally self-fertilizing in one flower. However, it is easy to
cross fertilize the eggs of one plant by the male cells (pollen) of another.
Chap. 20 THE PHYSICAL BASIS OF HEREDITY 391
Before the flower is quite developed the bud is opened and the stamens con-
taining the pollen are removed (Fig. 20.2). Then pollen from another plant
is placed on the pistil through which the male cells make their way to the eggs.
Mendel chose plants of two pure-line varieties, that is, one in which for
several generations the plants had been tall and another in which they had
been dwarfs, terming these the parental generation (P). He cross-pollinated
flowers from these two parent stocks. All of the resulting hybrids were tall
plants, the First Filial or Fi generation (Fig. 20.3). The dwarf character
had disappeared. However, when the plants of this (Fi) generation were
self-pollinated and another generation (F^) was produced, the dwarf ness
termed the recessive character turned up again. Not only that, but it appeared
in a regular and predictable ratio of three tall, termed the dominant characters,
to one dwarf, the recessive.
stamen
Pollen grain
Pollen tube
Sperms
Ovule
Ovary
B
Fig. 20.2. Flower of garden peas, the subjects of many of Mendel's experiments.
A, diagram of the flower with the petals, consisting of standard, wings and keel,
separated to expose the pistil and stamens. The boat-shaped keel and the wings
naturally close tightly around the pistil and stamens insuring self-pollination. B,
diagram of the pistil and Stamens of the pea showing the pollen tube that grows
downward carrying the sperm that fertilizes the egg. Other sperms unite with nuclei
in the ovules (not shown) to produce the nutritive part of the seed. (Courtesy,
Colin: Elements of Genetics, ed. 2. Philadelphia, The Blakiston Co., 1946.)
392 THE NEW INDIVIDUAL Part IV
Mendel went on rearing the plants to see if their inherited content, the
genotype, was what it appeared to be, that is, the phenotype. By analyzing the
offspring of self-pollinated plants of the F2 generation he found that one-
fourth of them were pure tails, one-fourth pure dwarfs, and one-half ap-
parently tall but actually hybrids. When crossed with one another these hybrids
produced a 3:1 ratio of tall dominants and dwarf recessives as before.
Mendel's Explanation
He explained his observations by assuming that all living things transmit
hereditary traits by means of physical particles in the sex cells of the parents.
Parents Tall (tall) X Dwarf (dwarf)
F, Tall (dwarf) X Tall (dwarf)
1
F2 Tall (tall) Tall (dwarf) ^ Tall (dwarf) Dwarf (dwarf)
Fig. 20.3. The results of Mendel's cross of garden peas of pure ancestry for
tallness with peas of pure ancestry for dwarfness. The first generation, first filial
Fj, was tall; the second generation, Fo, was tall in a proportion of three tails to one
dwarf. Mendel named these characteristics dominant and recessive, terms used
ever since. In Fj the tallness of the tall plant was visible or dominant. The dwarf-
ness of the tall plant was present in its make-up and might be inherited by its off-
spring but was invisible or recessive.
He called them "formative elements" and assumed that they were units that
acted separately. With this correct interpretation Mendel laid the foundation
of modern genetics.
Mendel's Principles
The Law of Segregation. While both members of a given pair occur in an
individual only one of these is in a single egg or sperm. Thus, characters are
segregated. The nature of the members of each pair of opposite characters,
e.g., tall and dwarf in peas, or black and white in fowls, is not affected by the
other. The black that is inherited from hybrid gray parents proves to be as
black as if from pure black ones (Fig. 20.4). Characters are units which do
not blend or mix.
The Law of Independent Assortment. Every character is inherited sep-
arately from every other character, in peas, the height of the plant from the
color of the flower.
Dominance. When organisms, each with a pure-line for opposite characters,
are crossed, one character is either completely or incompletely dominant over
the other in the offspring (Figs. 20.3, 20.4); the other is completely or in-
completely recessive. Some characters are incompletely dominant, such as
the red of the red and white plants of four o'clocks that produce the pink
Chap. 20 THE PHYSICAL BASIS OF HEREDITY 393
The 3tol Ratio Oemonstroted
PUREBRED BLACK ^ Jfj^ PUREBRED WHITE
M
BLACK GREY CREY WHITE
Fig. 20.4. The result of crossing fowls of pure lines, one with an unmixed
ancestry for black and the other for white feathering; Fj, incomplete dominance
of black resulting in dapple gray. Crossing of dapple grays produces a generation
{¥.,) in ratio of 1 black, 2 dapple gray, 1 white. The blacks are pure black, and the
whites are pure white like their grandparents. (Courtesy, Public Affairs Pamphlet
No. 165. New York, Public Affairs Committee, Inc., 1950.)
ones of the next generation (Fi). Mendel had experience with incomplete
dominance for he crossed pure early flowering peas with pure late flowering
ones and produced an Fi generation of plants with a flowering time half way
between those of their parents.
Mendel's principles have held true. Since his time, thousands of experi-
ments have been made in plant and animal breeding and the results of the
great majority have upheld his principles.
Cellular Basis of Genetics
Chromosomes. In 1902 an American biologist, W. S. Sutton, pointed out
that chromosomes are mechanisms that carry out the Mendelian principles.
It may be well to review the characteristics of chromosomes in connection
with their role in genetics (Figs. 20.5, 20.6). The behavior of chromosomes
shows a striking parallel to the dominant and recessive body characters. The
chromosomes of the body cells are paired; so are dominant and recessive
characters. A character is an inherited quality, e.g., the color black. A factor
is the gene or genes that are responsible for it. A gene is a minute part of
a chromosome. Factor and gene are used as synonyms. Experimental cross
394
THE NEW INDIVIDUAL
Part IV
Distribution of chromosomes in the developmenrt of
sperm cells. Dork chromosomes = mole inheritance.
Light chromosomes = female inheritance.
Body cell of fother _
i.e. skin, muscle, etc.
Germ cell destmed to divide
and develop into sperm cells
Spermatogonium
Primary —
spermatocyte
MITOTIC
divisions
Cell enlarges
Similar chromosomes pair
(Synapsis)
Eacn chromosome duplicates
itself. Tetrads result. Tetrads
separate into pairs. Cell divides.
MEIOSIS
Tetrads separate into pairs.
Cell divides.
A.B.MEIOTIC
divisions
Secondary
spermatocyte -
Sister chromosomes
separate.
Spermatids
Sperm
cells
Fig. 20.5. The reduction (meiosis) of the number of chromosomes from the
double (diploid) to the single (haploid) number during the formation of sperm.
For each sex cell, the process includes: increase in number of cells by MITOSIS;
reduction of chromosomes by MEIOSIS. For simplicity, six chromosomes are used
in the body cells. Cells of the human body contain 48 chromosomes.
Chap. 20 THE PHYSICAL BASIS OF HEREDITY 395
breeding has proven that the genes responsible for characters are segregated
in separate sex cells.
The number of chromosomes is normally constant for each species, but
varies in different ones. Although each species has its characteristic number,
other species may have the same number; man and tobacco plants both have
24 pairs of chromosomes. There are 100 pairs of chromosomes in crayfishes
and 24 pairs in man. This tells plainly that there is no relation between an
animal's place in evolution and the abundance of its chromosomes.
Chromosomes occur in pairs, except in mature sex cells. One member of
each pair is contributed by the egg and the other by the sperm cell of the
parents (Fig. 20.5). In the body cells the only chromosomes which may not
be paired are those which determine sex and in many species these are also
paired but of different shape and size as in man. In other species, usually in
the male parent, e.g., grasshoppers, half the sperm cells contain a sex
chromosomes and produce females, and half are without one and produce
males (Fig. 20.6).
During development of human sex cells, the double number of chromo-
somes is reduced to the single or haploid number, 24 in the human sperm
and 24 in the egg (Figs. 20.5, 20.6). Each time a developing egg divides,
one member of each pair of chromosomes is segregated in the egg or in
the polar body, and likewise for the sperms, a result that is very significant
in the ancestry of all of us, whether mouse or man.
Genes. Genes are the units of heredity, probably molecules of nucleoprotein
about five millionths of an inch long. By interaction with other genes, with
the cell content surrounding them, and the whole environment of the animal,
a gene or combination of genes controls the inheritance of such diverse
qualities as brown eyes, a soprano voice, and a way of walking. They are
contained and transmitted in chromosomes, hundreds of them being located
along the cross bands that are visible when certain chromosomes, as in the
fruit fly, are highly magnified (Figs. 20.7, 20.8). Although genes have not
been clearly seen, their places on a given chromosome have been located
exactly.
Genes of fruit flies can be "knocked out" of chromosomes by treating the
animals with radium. When the sex organs of such flies are examined micro-
scopically, empty or damaged places may be found on the chromosomes of
the sex cells. In such flies, some part of the body may be changed, a new
wrinkle in the wings, or some action may be different. Treatments and ex-
aminations are repeated over and over again until the changed structure or
action of the fly is correlated with the particular spot on the chromosome.
Thus, the gene is located. Maps of chromosomes of fruit flies on which genes
are located are the results of the combination of experimental breeding and
microscopic examination of chromosomes (Fig. 20.8).
396
THE NEW INDIVIDUAL
Part IV
XO
Grasshopper
Domestic fowl
Honeybee
Fig. 20.6. Four types of sex determination, different ways by which the chromo-
somes determine the sex of an individual. Man, cells of the body (except sex cells) ;
male and female each has 48 chromosomes. In males the members of one pair
called xy are of different sizes, y being the smaller. In females, members of the
counterpart of this pair are the same size and called xx.
All of the eggs contain an x chromosome. Half of the sperm cells contain an x
and half of them the y chromosome. Thus the sex of an individual depends upon
Chap. 20 THE PHYSICAL BASIS OF HEREDITY 397
Fig. 20.7. The fruit fly, Drosophila melanogaster. This and other species of these
common gnat-like flies have contributed more material to the study of genetics than
any other animal. They have 8 chromosomes in the body cells but the genes con-
tained in them are responsible for thousands of structures and actions. Fruit flies
were used by Thomas Hunt Morgan in his studies which constitute some of the
most important contributions to the science of genetics. (Courtesy, Morgan: The
Physical Basis of Heredity. Philadelphia, J. B. Lippincott Co., 1919.)
Genes act like enzymes in that they are able to speed up or slow down
chemical actions without themselves being used up in the process. They re-
semble viruses in being extraordinarily minute. Like them they multiply only
within living cells; they have specific effects upon cells; they may change
(mutate) in nature; and may be caused to change by exposure to x-rays.
Genes and viruses differ, genes being orderly and mainly beneficial, while
viruses, at least the well-known ones, are usually lawless and destructive.
Genes are inherited but some of the characters which they control may be
modified by environment. In man, the ability to smile is inherited, but not the
exact smile for those of fat faces differ from those of thin faces and food may
create the change.
the content of the sperm cell that happens to join the egg from which he or she
develops, xx a female, xy a male. Grasshopper, cells of the body (except sex cells);
male has 21 and female 22 chromosomes. Every egg has an x chromosome. Half
of the sperm cells have an x chromosome, and half of them have no .v chromosome.
If the latter fertilizes an egg it produces a male (20+ a:) grasshopper. Domestic
fowl, cells of the body (except sex cells) : male and female each have 18 chromo-
somes. In males the pair of sex cells are called zz; in females the members of this
pair are different and called zw.
Every sperm contains the z chromosome; half of the eggs contain z and the other
half contain the w chromosome. A z sperm fertilizes an egg with the z chromo-
some and produces a female zz. Honeybees, fertilized eggs (sperm 16 chromo-
somes and egg 16), having the diploid number of 32 chromosomes develop into
females (queen and workers); unfertilized eggs (no sperm, and egg 16 chromo-
somes), having the haploid number of 16 chromosomes develop into males
(drones). Their body cells have only half the number of chromosomes (16) that
are contained in the body cells of the females (32). (Courtesy, Winchester:
Genetics. Boston, Houghton Mifflin Co., 1951.)
398
THE NEW INDIVIDUAL
Part IV
r?-^ RIGHT ABM OF
^<,y^ \ ; \ CHAOMOSOME nL
^ X CHROMOSOME
CHROMOCENTER <<^
CHROMOSOMES FROM OVARIAN TISSUE
(Same enlarqement)
LEFT ARM OF
CHROMOSOME
Fig. 20.8. A, chromosomes in the saUvary glands of a fruit fly. Although they
belong to minute flies these chromosomes are among the largest that have been
observed. (From Altenburg: Genetics. Copyrighted by Henry Holt and Co. Re-
printed with their permission.)
Experiments Illustrating Mendelian Principles
Monohybrid Cross. A monohybrid cross may be illustrated by cross breed-
ing one parent having a long line of black ancestors (pure-line), and another
parent having a long line of white ones. The offspring produced by animals
differing in one character, such as color, are monohybrids.
A pure-line black guinea pig is bred to a pure-line white. The male may
be black and female white or vice versa (Fig. 20.9). In a pair of genes, e.g.,
the gene for white, the recessive, is expressed by w in small type; the gene
for black, the dominant, by W in capitals. The formulas for the parents (P)
Chap. 20
399
Fig. 20.8 (continued). B, A microphotograph of normal chromosomes from the
nucleus of a cell in a salivary gland of a female fruit fly, Drosophila melanogaster.
Such chromosomes, among the largest chromosomes that have been discovered in
animal cells, have greatly aided the study of the effect of x-rays on the chromo-
some. If fruit flies are irradiated, e.g., males, the chromosomes in the nuclei of cells
in the salivary glands of the first generation of offspring show various changes.
Such changes may be losses of parts, shifts in position of parts, combinations of
parts involving two or more chromosomes. Changes in the form or habit of the
animal accompany these changes, sometimes its death. (Courtesy, B. P. Kaufmann,
Carnegie Institute of Washington.)
are ww and WW since each one has the diploid number of chromosomes, the
product of two sex cells each containing gene w in one case, and gene W in
the other. Thus, in each the genes for coat color are similar or homozygous.
The sex cells of the black parent (P) each contain a gene for black {W);
those of the white parent (P) contain a gene for white (h'). In Fi only black
guinea pigs are produced because each receives one gene for black and one
for white, with black dominant. Although each animal is black, it is actually
a hybrid for color since half of its sex cells contain a gene for black and half
of them a gene for white. Because a trait is dominant in one species it does
not follow that this will occur in some other species. Black is dominant over
white in rabbits and guinea pigs but white is dominant over black in Leghorn
poultry. A different type of gene is involved in the two cases.
Backcross. A backcross (or test cross) of breeding is a method of testing
animals that appear alike in one or more characters (phenotypically) but
400
THE NEW INDIVIDUAL
Part IV
may differ genotypically. It is used commonly in analyzing F] dominants
by crossing them with their pure recessive parents, hence the name backcross.
In the preceding cross of guinea pigs, this would be a cross between a black
guinea pig of Fi and its pure white parent (Fig. 20.10). If the black is a
hybrid, the offspring are black and white guinea pigs in equal numbers. In
the hybrid black guinea pig of Fi, half of the sex cells carry a gene for black
(W) and half of them a gene for white (w). In the white parent P every sex
cell carries a gene for white (h'). The cross results in the half white and half
black of black and white animals (Fig. 20.10). This figure also shows a
similar result for another pair of contrasting characters, short-long hair, where
short is dominant to long.
Dihybrid Cross. A dihybrid cross is one between organisms that differ
Parents
[••
Black male
carrying 2
black gems
Gametes
carrying
\(me black
White female
carrying 2
white ^enis
carrying
/"^^m white ^
(O) 9^*^ (O
First
generation
Gametes
Cross these two
All black
carryin^one
black and
one white
gene
(•) ^39s (O)
Second
generation
Ratio:
3 black
to
1 white
Fig. 20.9. Monohybrids, offspring of guinea pigs which differ in one color. The
original parents, P,, are pure-line blacks and pure-line whites, black W being domi-
nant over white h'. The resulting progeny show the behavior of a dominant gene.
They also show the effects of the all-important separateness of genes, that the
character white which was lost from sight in the first or F^ generation reappeared
unaffected in the Fo generation. Each animal of F^ is in appearance (phenotypi-
cally) black but in gene content (genotypically) black and white. When the hy-
brids of Fi are crossed, their offspring F2 show the typical Mendelian ratio of
three dominants to one recessive. When animals of the F2 generation are inter-
crossed, one-fourth are pure black, one-fourth pure white, and two-fourths black
(black and white). When crossed these animals that contain genes for black and
white produce blacks and whites in the 3:1 ratio. (Courtesy, Winchester: Genetics.
Boston, Houghton Mifflin Co., 1951.)
Chap. 20 THE PHYSICAL BASIS OF HEREDITY 401
from one another in two pairs of contrasting characters. A male guinea pig
that has a pure-line ancestry for short, black hair is crossed with a female
pure-line for long, white hair, or vice versa for sex (Fig. 20.11). The genes
in the body cells are dominant black (WW) and dominant short (LL) that is
WWLL in the male, and recessive white (vv-vv) and recessive long (//) in the
female, wwll. During meiosis the genes on homologous pairs of chromosomes,
i.e., WW, or LL, or ww or //, go to different sperm or eggs as the case may
be (Fig. 20.11).
These gametes form the offspring of the Fi generation, all of them black
short haired guinea pigs {WLwl) having the dominant genes for black and
short {W and L) in their body cells as well as the recessive ones for white
and long (vv and /). The gametes of the Fi generation will contain the genes
WL, Wl, wL, wl (Fig. 20.1 1 ). If animals of the Fj generation are intercrossed
the ratio of their offspring will be: 9 black short, 3 black long, 3 white short,
1 white long. The combinations of genes in the eggs and sperm that produce
these are shown in Figure 20.11, with the combinations of genes in the body
cells. Since in each sex there are four kinds of gametes, there will be 16 pos-
sible combinations of gametes with their contained genes in the animals of
the Fo generation.
The foregoing experiment shows that whenever strains of animals differ
from one another in two or more pairs of genes the inheritance of one pair is
independent of the other (Mendel's Law of Independent Assortment).
W'Black
W" White
Black Short
Chtterozy^ous)
L'Short
WhUtLonj
tA Black hBlack %.\/hxtc ^Whik
*" Short "*Lorvg Short ''Loncf
Fig. 20.10. Back- (or test) cross, a common method of testing the gene content
of animals that look alike but may differ genetically. A backcross of a black,
short-haired guinea pig to a white, long-haired one shows that the black, short-
haired guinea pig carried genes for white color and long hair, i.e., the animal did
not breed true to type. (Courtesy, Winchester: Genetics. Boston, Houghton
Mifflin Co., 1951.)
402
Parents
THE NEW INDIVIDUAL
Black short
haind male
(.homozygous)
Part IV
White long haired
female
(homozygous)
Gametes
First
generation
Qametes
Eggs
(D(D®
^o"
Second
generation
All black
short haired
^heterozygous)
Ratio:
9 Black sliort
3 Black long
3 White slwrt
I White lonj
Fig. 20.11. Offspring of dihybrid guinea pigs, a male with short black hair and
a female with long white hair. The genes for short and black are dominant. Chart
of the combinations of genes that produce them with the combinations in the sex
cells. (Courtesy, Winchester: Genetics. Boston, Houghton Mifflin Co., 1951.)
Multiple Hybrids. Three independent pairs of contrasted characters are
governed by the same principles as two. The only difference is the greater
variety of gametes and the larger number of possible combinations of genes.
Animals in general are hybrids for hundreds of different characters; human
beings are probably the greatest mixture of all. If an animal had only ten
pairs of contrasting characters, each pair on a different pair of homologous
chromosomes, it could produce 1,024 types of gametes which could in turn
form 1,048,576 combinations. But animals actually have thousands of char-
acters. This is the reason that no two children inherit the same combination
of traits unless they are identical twins (Fig. 19.20).
Linkage of Genes
Linked genes are those that are located in the same chromosome and in-
herited together. This is an exception to MendeFs Independent Assortment of
Genes which still holds for genes that are located on different chromosomes.
Chap. 20 THE PHYSICAL BASIS OF HEREDITY 403
In his experiments, Mendel luckily dealt with no such genes. Linkage works
as a check on the independence of genes, a hold-back on too much scattering.
Arrangement of Genes on Chromosomes
Linear Arrangement. Genes are located throughout the length of each
chromosome in precise and standardized arrangement (Fig. 20.8). Maps
have been made of certain chromosomes of Drosophila showing the locations
that have been worked out for a comparatively large number of genes. How-
ever, such maps give little idea of the number of genes on a chromosome. In
one species of these little fruit flies it is well established that there are certain
chromosomes that contain some 2500 genes.
Crossing Over. This change in location of chromosomes occurs during
meiosis in the prophase stage when similar (homologous) chromosomes unite
in pairs with equivalent genes opposite to one another. A part of one of the
pair may change places with a corresponding part of the other (Fig. 20.12).
It is as if the residents of a section of one side of a street changed places with
those of a corresponding section of the other side.
Crossing over may occur in more than one section of the partner chromo-
somes in some species and not in others, and in some species only under
certain conditions. It may take place in one sex and not the other, as is the
case in the female but not ordinarily in the male of Drosophila, although it
may be induced by exposing the latter animals to high temperature or x-rays.
In most plants and animals, however, crossing over occurs in both sexes.
Thus the position of genes on different pairs of chromosomes results in their
1
<_
^
f A
B '
im.:.-:r
, ..-^.X-Sb:?-^
t
B3 c^-—^
■BZ)
B
D
(
^
iiiiil
b
1 c J
C 0 f
B
1 c )
c
)
I 2
Fig. 20.12. Crossing over of corresponding sections of the homologous partner
chromosomes during the "4-strand" phase, in the prophase stage of division of
sperm or egg cells. Upper — 1,2,3,4; example of single crossing. Lower — 1,2; ex-
ample of double crossing. Letters represent sections of chromosome strands. In
this early phase of synapsis, each member of a future pair of chromosomes has
doubled, thus forming 4 chromosomes. In these cases, crossing over occurs in
only two of them. Each of the four chromosomes will be distributed into a separate
cell in the two later meiotic divisions.
404 THE NEW INDIVIDUAL Part IV
independent assortment when sex cells are formed, but the linkage of genes
on the individual chromosomes of a pair reduces their independence of
others on the same chromosomes.
Sex Determination
Whether an animal is male or female is determined by the number and
quality of certain genes in the egg and sperm from which it originated. Some
lower animals are changed from males to females and vice versa by hormones
and variations in temperature. This does not happen in higher animals.
Sex Chromosomes. In the body cells of various animals there are either
one or two distinctive chromosomes usually smaller than the others. These
are the sex chromosomes; the others are called autosomes. Both sex chromo-
somes and autosomes carry genes influencing sex and it is the balance between
these genes that results in maleness or femaleness. In the cells of the human
body, there are 48 chromosomes and two of them are sex chromosomes (Fig.
20.13). In a woman, these are the same size, X and X; in a man the two are
II |t*«cirrr(cc
ItliKCC «•( P
A B
Fig. 20.13. Chromosomes of human cells. Those of the body cells show the
characteristic diploid number resulting from the union of male and female sex cells.
A, the normal pattern of arrangement in a body cell. B, the chromosomes ar-
ranged in pairs; the presence of x and the very small y denote a male; two x
chromosomes denote a female. (Courtesy, Baitsell: Human Biology, ed. 2. New
York, McGraw-Hill Book Co., 1950.)
different, X and the smaller Y. All human eggs have one X; half of the sperm
cells have an X chromosome; half of them have a Y. Thus, sex is determined
at fertilization; the X-egg and X-sperm result in a female XX and the
X-egg and Y-sperm in a male (XY). The X-sperms and Y-sperms result from
divisions during meiosis (Fig. 20.5). A plan similar to this occurs in many
animals.
There are other animals in which half the sperms have an X chromosome,
while the other half lacks any sex chromosome (Fig. 20.6). The resulting
body cells contain XX in the female and XO in the male. The latter animals
appear as typically male as those of the XY plan. Although fruit flies usually
have half X and half Y sperm cells, there are rare individuals in which some
of the sperms lack any sex chromosome. Male flies develop from the eg^
Chap. 20 THE PHYSICAL BASIS OF HEREDITY 405
fertilized by such O-sperms and appear typically male. However, breeding
experiments have proven that these males are sterile. In other species in
which half the sperms regularly lack a sex chromosome (e.g., grasshoppers),
the males are fertile. Y-chromosomes contain only a few genes. In fruit flies
these appear to be associated with fertility. In the XO male fruit flies that is
the main character missing.
Different as males and females are, they are also fundamentally similar.
Some invertebrates require but a slight shift in conditions, perhaps of the
genes, to tilt the organism toward maleness or femaleness. Sometimes ab-
normal chromosome numbers resulting in a different balance of the genes
may produce supermales, superfemales, or intersexes as in Drosophila (Fig.
20.14). Higher animals are seldom if ever entirely male or entirely female,
as the nipples of human males bear witness. The possible explanation may
be that every individual carries all the genes essential for both sexes and
that certain genes or conditions of the genes tip the balance toward maleness
or femaleness.
Discovery of Sex Chromosomes. Sex chromosomes were first correctly
interpreted fifty years ago (1901) by C. E. McClung during his study of the
Fig. 20.14. Sex types in fruit flies, Drosophila. Upper left, normal female; upper
right, intersex; lower left, supermale; lower right, superfemale, and chromosomes
of each type. (After Bridges. Courtesy, Snyder: Principles of Heredity, ed. 4.
Boston, D. C. Heath and Co., 1951.)
406 THE NEW INDIVIDUAL Part IV
spermatogenesis of the long-horned grasshopper. In 1905, Nettie Stevens
published an account of the sex chromosomes in a beetle (Tenebrio) and
showed that the male had 19 large chromosomes (18 autosomes plus an X)
and one small one (Y). In the same year, Edmund B. Wilson announced
similar discoveries in insects; one of them, the common squash bug {Anasa
tristis), has 22 chromosomes in the body cells of the female and 21 in those
of the male.
Sex-linked and Sex-influenced Inheritance
Sex-linked. The sex chromosomes, chiefly the X-chromosomes, carry other
genes besides those associated with sex. These are known as sex-linked genes.
Among the best known of human sex-linked characters are color blindness
and hemophilia or "bleeding."
Color blindness varies in degrees from a weakened sense of red-green to
the absolute loss of color as in late twilight. Red-green color blindness and
hemophilia have long been known to be inheritable in the same peculiar
criss-cross way (Fig. 20.15). A color blind man may transmit color blindness
through his daughters who have normal vision to half of his grandsons; a color
blind woman transmits color blindness to her sons and to her daughters who
become carriers. The gene for color blindness (c) is carried on the X-chromo-
some and is recessive to normal vision (C); females have two X-chromosomes,
males an X and Y-chromosome. A woman may be a carrier producing eggs
half of which carry the gene for color blindness though she herself has normal
vision. Color blindness shows that genes for sex and for other characters may
be associated in the same chromosome. It also emphasizes the fact that genes
on the X-chromosomes are not transmitted by a father to his sons and so
reduces the importance ascribed to a direct male line of inheritance.
Eight out of 100 persons are color blind and it is likely that accidents are
sometimes due to misinterpretation of red and green traffic signals. These
colors are an unfortunate choice for signals, red and blue would have been
distinguishable by almost everybody. Engineers and pilots and other officers
on railways, steamships, and airplanes are tested for color blindness; in some
states, automobile drivers are not.
Hemophilia, the abnormal tendency to bleed, has been widely publicized
because of its distribution in the royal families of Europe. The most famous
pedigree of hemophilia is that of Queen Victoria who jwas a carrier (Fig._ (T^^
20.16). Of her four sons, only Leopold (II. 8) who lived to be 31 was
affected. The other three sons were free from it including Edward VII (II. 2)
from whom George VI was descended. One of Queen Victoria's carrier
daughters, Alice (II. 3) was the mother of Alexandra of Russia (III.6Xwhose„
son Alexis (IV. 12) suffered severely from hemophilia. Victoria's vQther'car-
rier daughter, Beatrice (II.9), was the mother of Victoria Eugenie (III. 16)
Chap. 20
Normal
THE PHYSICAL BASIS OF HEREDITY
Color blind Color blind
407
Nocmal
Normal
<0>tC>
9
XX
o"
r
<3>m><zp-
9
KX
9
XX
XY
<o><o<o-
KK XX Xy XY
Fig. 20.15. The criss-cross inheritance of color-bUndness from a color-blind
man, via his daughter, a carrier, to his grandson.
Color-blindness is a recessive c to normal vision C. The gene for color-blindness
is carried only on X-chromosomes. A man is color blind because he inherits one
X-chromosome carrying color-blindness and no gene for normal vision in the Y-
chromosome. A woman is a carrier because she inherits two X-chromosomes, C and
c with the C of normal vision the dominant one. If the mother is a carrier and
father is color blind, their daughter may be color blind, a rare occurrence. (After
Dunn. Courtesy, Sinnoit, Dunn, and Dobzhansky: Principles of Genetics, ed. 4.
New York, McGraw-Hill Book Co., 1950.)
of Spain, two of whose sons had hemophilia, including the Crown Prince
Alfonso (IV. 16). There appears to be no record of hemophilia among the
ancestors of Queen Victoria, and the gene for the disease is believed to have
arisen as a mutation. If her consort, Prince Albert, had carried a gene he
would have had the disease.
Sex-influenced Inheritance — Baldness. There are many types of baldness;
some of them are inherited. Its most striking character is its much greater fre-
quency in men than in women (Fig. 20.17). It seems probable that hereditary
baldness is due to a gene that behaves like a dominant in men and like a
limited recessive in women. The different expression of the genes in men and
women is evidently due to a difference in hormones that makes them more
or less sensitive to their inherited genes. Eunuchs (castrated men) seldom
become bald. In women, the sparsity of male hormones is said to keep the
hair, even though the genes for baldness may be present; in men the ex-
cess of male hormone makes the hair follicles sensitive to the genes of bald-
ness.
408
THE NEW INDIVIDUAL
Part IV
THE "ROYAL" HEMOPHILIA PEDIGREE'
QUEEN VICTORIA
OF ENGLAND
PRINCE
ALBERT
(^
VICTORIA'S CHILDREN "]
SYMBOLS:
©CARRIER or
HEMOPHIUA GENE
I HEMOPHILIAC
r~I OrO ^f^^^ OF GENE
'— ' ^^ FOR. HEMOPHILIA
I
CERMAN
a-rO &tU i-rONs^H
VICTORIA
fMPRESS
OF
6ERMANY
FOWARO
OF EMGLAND
ira
OUEEN ALICE
ALEX-
ANDRA
NO DESCENPANTS OF
tpWARD (members
Of PRESEhTT BRITISH
ROYAlFAMILY) RECEIVE
HEMOPHIUA GENE
LUDWIG
12 OF
HESSE
lEOPOLD
OF
Albany
WINCE
HENRY
PRUSSIA
1
/
IRENE PRINCE
OF FRIEP-
HESSE RICH
"'6-Hii (i)-ra
HELEN BEATRICE
OF
WALOECK
VICTOKIA PRINCE ALICE
LOUIS Of (CZAR-
BATTEN- ina;
or
0
DIED
tCHjNO
CZAR Alice
NiCHOtAi
rr OF
ROSS/A
in
a
HENRY.
PRINCE OF
BAtTEMBERO
^)Ta
ALEXAN-
PER OF
TECK
VIC-
TORIA.
OUEEN
ALFON-
SO XDI
KING
OF
SPAIN
6
<ROWN
PRINCESS
ELIIABETH
CZAREVITCH
ALEXI
LORD
TREMATOM
ALFONSO 60NZAU)
PIO MANUEL
PRINCE PMILlP
MOUNTBATTEN
ALL CHILDREN FREE
OF HEMOPHILIA GENE
Fig. 20.16. Descendants of Queen Victoria, showing the distribution of hemo-
phiUa, evidently a mutation. (Data by litis. From The New You and Heredity by
Amram Scheinfeld. Copyright, 1939, 1950, by Amram Scheinfeld, published by
J. B. Lippincott Company.)
Mutations
A mutation is an inheritable change in a gene. This definition applies to
changes in the genes of sperm cells and eggs. They are the all important
mutations, the ones ordinarily meant by the term, mutation. They are the
ones discussed here. Changes in the genes of body cells do occur but are
exceptional and never inherited.
Mutation and Evolution. The evolution of living things is possible only
because a gene can change and can reproduce itself in the changed form
(Figs. 20.18, 20.19). How one gene changes into another kind is one of the
greatest problems of biology. The change in one gene on one chromosome of
an egg can establish a new kind of plant or animal which in good time may
spread over the earth.
Mutations were discovered by Hugo de Vries, one of the rediscoverers of
Mendel's pioneer paper on genetics. Since then mutations have been found
Chap. 20
THE PHYSICAL BASIS OF HEREDITY
409
Fig. 20.17. Patterns of hereditary baldness. The gene for baldness is inherited
by men and women but has different results. The most convincing theory is that
difference in hormones acting on the same kind of gene may be responsible for the
baldness in men and the usual lack of baldness in women. (Courtesy, Snyder:
Principles of Heredity, ed. 4. Boston, D. C. Heath and Co., 1951.)
Fig. 20.18. A mutation for short legs in sheep; short-legged ewe in the center,
ordinary sized sheep of the same variety at left and right.
In 1791, a Massachusetts farmer found in his flock a short-legged lamb from
which he bred a strain of sheep, valuable to him because they did not jump the
pasture walls. This variety, called Ancon sheep, still exists. (Photograph from Life
Magazine © Time, Inc. Courtesy, Storrs Agricultural Experiment Station.)
410
THE NEW INDIVIDUAL
Part IV
in many plants and animals, so frequently in some that they are known as
mutating species. In recent years, a thousand or more have been found in
fruit flies. Many times that number were examined without discovering a
structure suspected of being a mutation. And when some new feature was
found, the fly had to be bred and several generations produced in order to
show whether or not the new feature was inherited. Fortunately, fruit flies
mature and breed quickly. Their lifetime in days is about the same as the
human lifetime in years. In 1927, H. J. Muller discovered that if fruit flies
were exposed to x-rays, the mutations would occur about 150 times more
often than naturally; later treatment with radium increased them to 200 times
(Fig. 20.20). The effect of the radiation suggested that mutations might
be induced by cosmic rays. Fruit flies were taken to mountain tops where
such radiation is more intense and mutations were speeded up. In later ex-
periments, mutations were produced by certain extremes of temperature, by
chemical substances, and by other influences inside and outside the flies.
Almost every type of mutation found in nature has been induced in them
experimentally, and some once believed to be unique results of experiments
have been discovered in wild flies. Changes in the genes have gone on through
millions of years of evolution as they are continuing quietly now.
Frequency of Natural Mutations. Mutations in any one gene are rare,
estimated about one in 50,000 generations. The rate varies in different genes.
It is also estimated that a mutated gene occurs in every ten human sperms
and eggs. This seeming contradiction disappears when it is remembered that
Fig. 20.19. Mutations for lack of pigment. Albino twins, without pigment in
hair, eyes and skin, a recessive mutation in a pair of identical twins. (From Rife,
Schonfeld, and Humstead in Journal of Heredity.)
Chap. 20
THE PHYSICAL BASIS OF HEREDITY
411
Fig. 20.20. Mutations in eyes of fruit flies (Drosophila melanogaster) induced
by exposure to radium. A, normal eyes, top view; B-G, different degrees of eye-
lessness, top views; H, normal eyes, side view; l-K, different degrees of eyeless-
ness, side views. (After Hansom and Winkleman. Courtesy, Fasten: Introduction
to General Zoology. Boston, Ginn and Co., 1941.)
there are thousands of genes in one sperm or one egg and that the majority
of mutations effect such slight changes that they are not discovered. In addi-
tion, the majority of them are recessives that are carried in the animal but not
expressed for a very long time.
On the basis of observations on fruit flies, Muller has estimated that the
average time clasping without change in any particular gene may be about
100,000 years. Allowing 10 generations of fruit flies per year, any particular
fruit fly would mutate only once in something like a million generations. The
mutation rate of the disease of hemophilia in a human line of descent has
been estimated by J. B. S. Haldane as one in 100,000 generations.
Some species and some characteristics mutate more than others; fruit flies
have many mutations; certain colors of sweet peas and many other garden
flowers are mutations.
4i:
THE NEW INDIVIDUAL
Part IV
The genes in the egg and developing embryo may mutate independently
of fertilization by the male cell. This has been observed in certain partheno-
genetic animals such as waterfleas (Cladocera) by Arthur M. Banta who
reared these through 850 generations and observed many mutations.
Effects of Mutations. More mutations are harmful than helpful. Their
character shows that any desirable ones that appear are selected by the natural
conditions inside and outside the organism. Otherwise they must be preserved
by human selection, e.g., the valuable platinum or silver blue mink, the
seedless grape.
Some of the most striking effects of environment on the expression of genes
are produced by differences in temperature. At 27.5° C. the gene for "short
wing" in Drosophila has a more marked effect than at a lower temperature.
In Siamese cats the dark pigment is produced only in cooler parts of the body
(Fig. 20.21).
Fig. 20.21. Dark pigment of Siamese cats,
produced in the extremities of the body
which are below a certain level of tempera-
ture. (Courtesy, Boyd: Genetics. Boston,
Little, Brown and Co., 1950.)
Giant Plants — Giant Cells. Polyploidy is a type of mutation in which the
whole number of chromosomes, diploid or haploid, is increased two to sev-
eral times. Such increased numbers are hereditary and are accompanied by
marked changes. Polyploid plants are very large. Various garden flowers and
vegetables, crop plants and fruit trees are polyploids. Plant polyploids are
frequently found in nature, animal polyploids rarely if ever.
Many plant polyploids have been produced experimentally, largely by
means of colchicine, a solution derived from the bulbs of the autumn crocus
(Colchicum). The buds are bathed in colchicine solution which penetrates
into the developing reproductive organs and affects the cells. The chromo-
somes double their number but the rest of the cell fails to divide. In most
plants, fertilization goes on as usual except that, for example, instead of 4
Chap. 20 THE PHYSICAL BASIS OF HERKDITY 413
chromosomes in the sex cells, there are 8 and the fertilized egg has 16.
Thus the young plant starts with double the number of chromosomes and
larger cells than those of its parents. Giant tomatoes (Fig. 20.22) and
giant flowering marigolds- are polyploids. The radiant "Tetra Snaps" of cer-
tain seed catalogues are "Giant Tetraploid Snapdragons" that have giant
flowers and four times the usual haploid number of chromosomes. Animals
make a poor showing of polyploidy. It has been induced experimentally in
Drosophila, and in several species of salamanders chiefly by subjecting the
animals to low temperature (Fig. 20.23).
Inbreeding and Outbreeding
Inbreeding is the mating of near kin; cross breeding and outbreeding are
the matings of unrelated individuals. In many communities there is a great
deal of the former. Obviously, the more closely individuals are related, the
more hereditary traits they have in common; the better or worse are their
traits, and the better or worse for their descendants. Charles Darwin and
his wife Emma Wedgwood were first cousins, each with a long heritage of
desirable genes. In their case, nature and nurture joined in producing the
gifted and cultured Darwin family. Cleopatra was the descendant of six
generations of brother and sister marriages, yet the story of her life does not
imply that she was dull or helpless.
Outbreeding usually produces individuals with unlike genes in which re-
FiG. 20.22. Giant plants. Polyploidy in the tomato resulting from treatment with
colchicine, a, leaf and usual diploid number (12 pairs) of chromosomes; b, leaf
and chromosomes of triploid (3 sets of 12); c, leaf and chromosomes of tetraploid
(4 sets of 12). (After Jorgenson. Courtesy, Snyder: Principles of Heredity, ed. 4.
Boston, D. C. Heath and Co., 1951.)
414
THE NEW INDIVIDUAL
Part IV
Pentaploid
5X
Tefraploid
4X
Haploid
IX
Fig. 20.23. Giant cells. Polyploidy in salamanders (Tri turns viridescens) . The
salamander larvae are all at about the same stage of development. Since they are
about the same size, the changes in cell size due to polyploidy result in a reduced
number of body cells. (Courtesy, G. Fankhauser, Princeton University.)
cessives, often defects, are hidden by dominants. Cross breeding of plants
or animals of different varieties leads to increased vigor. This is often de-
scribed as hybrid vigor, for example, the offspring of a male ass and a mare
is a mule, a hybrid tougher than either parent.
Mistaken Ideas about Heredity
Acquired Characters. Nothing is inherited unless it changes the genes in
the sex cells. Bodily injuries do not do this; neither do acquired habits or
training — eating olives or building bridges. An overwhelming number of
experiments and arguments has been presented in fruitless attempts to prove
that effects upon muscles, nerves, and bones may be inherited. Tails of rats
bobbed for many generations have left the last generation of rats growing
tails as long as those of the first; the sex cells are untouched by the afflictions
of the tails. Only the capacity is inherited, a tail to be cut, a mind to be trained.
Does one or the other parent take a greater part in inheritance? Only if
one has the dominant members of pairs of genes and the other the recessives.
A recessive must await its chance of expression until it can pair with another
recessive.
Telegony. This is a theory that in case two or more males mate with one
Chap. 20 THE PHYSICAL BASIS OF HEREDITY 415
female the influence of an earlier mating may be carried on to the offspring
that result from the later one. This is the favorite reason that dog breeders
propose when puppies have been due to "mistakes." Similarly, among cattle
dealers there is a notion that if a "blooded" bull is mated to a "scrub" cow,
the latter may infect his offspring of later matings. Such beliefs are numerous
but have no foundation in fact.
Human Inheritance
Value of Knowledge. Knowledge of human heredity is of great practical
value, (1) in medical treatment, especially public health, (2) in forming wise
opinions and judgment of the special and economic problems that crowd the
present world, and (3) as an aid in reaching legal decisions, such as disputed
parentage. With a knowledge of heredity it is also to be remembered that chil-
dren cannot choose their parents. The parents do the choosing; the children
take the results.
The inheritance of many physical and mental defects and diseases is becom-
ing more or less clearly understood. It is important to know whether a defect
is a dominant or a recessive since no recessive even if present in one parent
will crop out in a child unless a matching recessive is transmitted by the other
parent. At present, prospective parents can secure a clearer idea than ever
before of what benefits or dangers they may pass on to their children. There
are blood tests which detect the presence of hemophilia and hereditary anemia
in carriers who otherwise give no hint of the diseases.
Genetics holds a leading role in the investigations of cancer and thousands
of experimental studies are being made in this field. Clues to the behavior of
breast cancer have been discovered in inbred mice and rats susceptible to the
disease. Globular particles (the "milk factor") visible under the great mag-
nification of the electron microscope have been isolated from these inbred mice
that regularly transmit cancer to nursing offspring.
Heredity is in the kernel of racial problems. There is at least a better chance
for clearer thinking and wiser judgment about social problems when the facts
of human inheritance are kept in sight. The facts overtop the notions of pure
human breeds and superior races. All human beings are multiple cross breeds.
All are superlative mongrels, that are like kaleidoscopes whose patterns may
be changed but only insofar as the material allows.
Knowledge of blood types has entered the courts, as in New York, where a
man claimed that he was not the father of his wife's child. The tests showed
that his blood was type O, "universal donor"; his wife's type was A, and the
child was AB. Since the parents could not pass on a combination of genes for
the AB type, the court decided with the father.
Examples of Inherited Qualities. Blood is an extremely sensitive and complex
chemical compound. Even in closely related species of animals the chemical
416 THE NEW INDIVIDUAL Part IV
compositions of blood are diflercnt. Only a little blood from an animal of one
species is harmful or fatal to an animal of another species if injected into the
vessels of the latter. The blood of different persons also differs. It is well
known that human blood is affected by the chemical composition of blood in
certain persons and not in others. This is the basis of blood groups, the in-
herited chemical compositions of blood discussed in Chapter 7. It is also the
basis of Rh, an hereditary characteristic in the chemical content of the blood.
In dealing with this the meaning of the terms antigen and antibody should be
clear.
An antigen is any substance, often one injected into the body, that stimu-
lates the formation of the chemical substances called antibodies. The toxin of
an infection is an antigen which stimulates the formation of antibodies (anti-
toxins) that turn about and work against it.
Rh Protein. Up to a comparatively few years ago, the cause of deaths of
many infants before birth or soon afterward was a mystery. However, in 1940
a new type of human blood group was discovered which proved to be the
cause. It was named the Rhesus or Rh type after the Rhesus monkeys whose
blood was used in making tests that led to the discovery. About 85% of the
human population are Rh-positive, that is their red blood cells contain the
characteristic Rh-protein, an antigen, which reacts to the tests. The Rh pro-
tein is inherited through dominant genes, Rhrh or RhRh. The remaining 15%
of the population inherit recessive genes, rhrh. Their red cells lack the Rh
substances and they are termed Rh-negative.
The connection between the Rh blood and the harm to children arises only
when the mother is Rh-negative and the unborn child is Rh-positive (Rhrh),
through inheritance from its father. The Rh-proteins (antigens) pass from the
blood of the child to its mother's blood where they stimulate the production
of "anti-Rh" substances, that is, antibodies against themselves. Eventually
some of this anti-Rh passes into the child's blood (Fig. 20.24). There it may
cause such agglutination (sticking together) of the red cells that the child can-
not survive. This does not usually happen with a first baby because not enough
anti-Rh is then produced, but more accumulates with the second or third child
usually with grave results.
The "anti-Rh" substance occurs in the blood and tissue fluid and can pene-
trate the membranes that separate the blood of mother and child. But the Rh-
protein is in the child's red blood cells which would not be expected to get
through the membrane. How this happens remains to be discovered.
Testing for Rh blood is a common procedure. When a Red Cross blood
donor is typed, the identification card includes an Rh+ or Rh— . Babies that
are born alive but with damaged blood may be saved by transfusions of Rh—
blood. The damaged blood with its dangerous anti-Rh is literally washed out
of the blood vessels by the donor's blood.
Chap. 20 THE PHYSICAL BASIS OF HEREDITY 417
Skin Color. The natural color of skin is complex and several genes take
part in its inheritance. Three pigments are involved in any human complexion,
melanin (black or brownish), carotene (carrot color), and hemoglobin (vary-
ing reds of the blood). The blue of skin, e.g., on the wattles of male turkeys, is
due to the scattering of light upon the layers of cells, not to pigment. The pre-
dominance of one or more of these pigments determines what the skin color
will be. The key genes are those which govern the melanin. The genes remain
separate and only in their effects is there any blending, as in mulattoes.
Changes of skin color may also be due to jaundice, glandular antS other dis-
turbances that may or may not be related to heredity.
Eye Color. The colors of skin, hairs, and eyes are produced by virtually
the same kinds of pigments. In eyes as in skin, the genes for the dark pigment
Rh negative
Rh positive
(heterozygous)
(-')
First child Second child Third child Fourth child
Rh positive Rh negative Rh positive Rh negative
(mother (Dies of
sensitized) erythroblastosis)
Fig. 20.24. Diagram of the possible action of the Rhesus (Rh) protein in the
bloods of mother and child. The types of children that may result from a cross of
an Rh negative woman and a man who is Rh positive; in this case only half of the
sperms carry the Rh+ genes. (Courtesy, Winchester: Genetics. Boston, Houghton,
Mifflin Co., 1951.)
418 THE NEW INDIVIDUAL Part IV
melanin play key parts. Melanin is present in all human eye colors from black
to pale blue (Chap. 17). Black and brown eyes occur in a majority of the
human race, and the genes for dark pigments were probably the pre-eminent
ones in early human history.
Table 20.1
Certain Traits Inherited in Man
Dominant
Recessive
Dark hair
Blond hair
Curly hair {incomplete dominance, wavy)
Straight hair
Black skin (incomplete dominance)
White skin
Brown eyes
Blue or gray
Hazel or green
Blue or gray
Nearsightedness
Normal vision
Blood group A, B, and AB
Blood group O
Mental Disorders. A number of mental disorders are known to be in-
herited. The inheritance of a few of these is known; for others it is suspected
and still being studied. Superior mental ability and special aptitudes run in
families but are also strongly influenced by upbringing and other surroundings.
Eugenics
The increasing knowledge of human inheritance has brought with it numer-
ous plans for racial betterment. Eugenics includes study, plans, and action for
the betterment of the human race. Eugenics may be negative with education
and regulations against the reproduction by which feeble-mindedness, insanity,
and appalling physical defects are continued. Positive eugenics encourages the
continuation of the qualities of health and good citizenship.
The greatest problem of negative eugenics is feeble-mindedness since 5 per
cent of the American population has an intelligence rating of 70 or much less.
This group includes: paupers due to laziness and inability; criminals, large
numbers of them hopelessly defective; many persons who have grown up in
institutions for defectives and must remain there; and great numbers of morons
who hang to the fringes of life but contribute nothing but inertia and children
like themselves.
The reproduction of definitely unfit persons has been to some extent pre-
vented by segregating them in institutions and by sterilization. However, con-
finement is a heavy financial load on the state and is unhappiness for the in-
dividual. Sterilization, on the other hand, is a simple operation; severance of
both sperm ducts of the male (Fig. 18.5) or both oviducts of the female (Fig.
18.10). It prevents the outlet of the sex cells, but in no way aflfects the sensa-
tions or health of the person. The operation is performed upon the advice of
Chap. 20 THF. PHYSICAL BASIS OF HERFDITY 419
committees of physicians and, where feasible, the consent of the person in-
volved. The laws of twenty-seven states provide for sterilization of the feeble-
minded and permanently insane under such well-guarded provisions. California
has carried out the law extensively and with satisfactory results. The American
performance of sterilization was settled by the Supreme Court in a decision
given on May 2, 1927, in which Judge Oliver Wendell Holmes made his
famous remark, "Three generations of imbeciles are enough."
Positive eugenics is largely education in the ideals of what good citizens
should be and the power which they have upon society. Awards for large
healthy families have been more frequent in the eagerly militaristic countries
than in America. For the most part the positive aspect of eugenics takes care
of itself.
Part V
Evolution or Animals
ANCESTRAL COELENTERATES
ANCESTRAL PROTOZOA Ancestral plants
Ancestral animcl-plants
I
Primitive protoplasm
Plate I. A suggestion of relations within the animal kingdom. See Plate II. (After
Alice et al.)
422
Birds
Amphibians
PREVERTEBRATES
Amphioxus
Mammals
Insects
Squids
Octopuses
ECHINODERMS
Starfish, etc
Arrow w
So git
ROTIFERS
Troct)elmintties
NEMERTIANS
Round worms
FLAT WORMS
Plafylielmintties
ANCESTRAL TROCHOPHORE- LI KE ANIMALS
COELENTERATES
Hydroids, Jelly fisties'
COMB JELLIES
Ctenoptiores
SPONGES
P or if era
ANCESTRAL COELENTERATES
PROTOZOA
Amoeba
ANCESTRAL PROTOZOA
Ancestrol animal~plants
I
Primitive protoplasm
Ancestral plants
Plate II. A suggestion of relations within the animal kingdom. See Plate I.
42a
21
Tlie Protozoans — Representatives
or Unicellular Animals
Living organisms are the centers of relationships that reach out and connect
with numberless other things, living and nonliving. These relationships have
muhiplied through the long past as they are still doing. This is evolution.
Learning about relationships is a universal and exciting occupation, whether
it occurs in telephone conversations, in political campaigns, in searching out
the what and wherefore of plants and animals and other things. Because of
this, protozoans have place and importance; no matter that they are little,
largely unknown, and hardly ever seen. Discoveries of their far-reaching rela-
tionships are the lively rewards of exploration into their daily lives.
Protozoans — The Pioneer Animals
The great advances in the evolution of animals occur in flights of steps on
the long stairway of living. The protozoans were the pioneers and dominating
animals on the first steps. Multicellular animals with innumerable complexities
dominated the second great flight of steps. Continuing their own evolution,
many protozoans moved into the bodies of the multicellular animals and be-
came successful parasites in these new surroundings. Others persisted in free
living, becoming adjusted for various conditions in the always-changing en-
vironment of the earth.
Gradually many kinds of animals were gathered into communities, held by
bonds of food and shelter. Organized societies appear on the highest steps,
and are still continuing to change. Social insects became prominent, and after
long ages primitive human societies developed. The human groups became
divided; some scattered widely; others intermixed. They often came together
to eat, to fight, and to acclaim their works. All of this greatly benefited the
distribution of the protozoans; gave them new places to live and easy ways to
425
426 EVOLUTION OF ANIMALS Part V
reach them. They became, and still are, successful parasites of the human
digestive tract and, with 'f't help of man's insect associates, have been widely
introduced into human blood.
Compared with multicellular animals, protozoans are only relatively simple;
many are extraordinarily complex. No near kin of the ameba has come to f!y
like a bird, but neither can birds make a living on bacteria, as many protozoans
do. The multicellular animals created opportunities for the protozoans; they
have never displaced them.
Characteristics. Protozoans are minute unicellular animals that carry on all
the fundamental processes of the life of higher animals. They live in all sorts
of places and in different ways with one limitation, that for at least part of
their life span their surroundings must be wet, actually a limitation of every
animal.
Most protozoans have animal-like structures, flagella, cilia, and special
openings for the entrance of food; some have light receptors containing the
visual pigment carotene probably present in all types of eyes; others have
neuromotor fibrils suggestive of nervous systems; many bear paralyzing trich-
ocysts that are shot out in defense or attack (Fig. 21.1). Likewise, most pro-
tozoans are animal-like in their activities, such as the digestion of food and
elimination of water, the conjugation or mating of Paramecium, and the
fiercely carnivorous behavior of Didinium. Contrasting with these are the
plant-like flagellates that are green with chlorophyll and contain cellulose,
such as the green spheres of Volvox, and the myriad euglenas that give a
pasture pond the look of a spring greensward. Euglenas carry on photosyn-
thesis as truly as maple trees yet they continually travel about, their eye-spots
in front according to the general custom of animals. It is easy to tell John Doe
from a rose bush, but it is hard to tell whether green flagellates are plants or
animals. They fit partly into each kingdom, not wholly into either.
Sizes and Numbers. All protozoans are minute. Only the larger ones are
visible to the naked eye; a colony of Volvox only large enough to be a dot of
green; Stentor coendeus to show its trumpet shape; the giant ameba {Chaos
carolinensis) of the laboratories to look like a minute splash of water, and the
white Spirostomiim ambigumn to cover a hyphen on this page (Fig. 21.2).
In general, the largest protozoans are marine radiolarians and foraminiferans,
shell-forming relatives of the ameba. There are great numbers of microscopic
protozoans; the parasitic ones are especially minute. In a human red blood cell
there may be space not only for one parasite, but for many young ones result-
ing from its division. Protozoans outnumber all other animals in individuals
and perhaps even in species. Euglenas are scarcely visible to the naked eye,
although it is common for countless millions of them to create a green layer
on an acre of pond water.
Distribution and Habitats. Protozoans live in moist and watery places. Many
Chap. 21
THE PROTOZOANS
427
RESULTS OF A LONG EVOLUTION
Glassy frame
Chambered shells
Swims, gifdes,
by cilia
Swfms by
loshing "neck"
Eye with
lens
B C
Didinium eats o Paramecium
6.
Fig. 21.1. Protozoans are the results of an evolution that was under way long
before multicellular animals appeared. Here are a few examples of their special
structures. Protecting shells. 1, Clathrulina elegans extends its delicate pseudopodia
through the openings in its glassy basket. 2, three types of the multichambered
shells of foraminiferans that are secreted and occupied, one chamber after another
until in the last one the owner reaches full size. Locomotion. 3, Kerona polyporiini
glides by means of cilia over various species of hydra. 4, Lacrymaria olor swims
by lashing movements of its swanlike neck. Sensory organs. 5, Pouchetia has a
relatively enormous light receptor, a lens and cup containing the visual pigment
carotene similar to that in eyes of multicellular animals. Weapons of attack and
defense. 6, Didinium attacks and devours a paramecium which has thrown out its
poisonous trichocysts without effect. (Courtesy, Jahn and Jahn: The Protozoa.
Dubuque, Iowa, Wm. C. Brown and Co., 1949.)
428 EVOLUTION OF ANIMALS Part V
can resist drying while in cysts or spores, but only for a time. This limitation
has not hindered their success.
In spite of their remarkably long history of life in watery environments,
they are the most widely distributed of all animals, both geographically and
ecologically; they have found the greatest number and variety of homes. They
live in the upper soil along with hordes of bacteria, worms, and rotifers. They
swarm through the surface waters of the seas, both polar and tropical. The
luminescence of Noctiluca lights the surfaces of temperate as well as tropical
seas. Protozoans live in hot springs and in the snow and ice of the Rocky
Mountains, at times covering the glaciers with pinkish films.
One of the largest protozoans
Fig. 21.2. One of the largest protozoans, Spirostomiiin ambiguum, easily visible
to the naked eye. They look like white flecks against the dark bottoms of fresh-
water pools where they are occasionally abundant. Contractile vacuole (cv) con-
nected with a canal; (fv) food vacuoles; the macronucleus (M) is shaped like a
string of beads. (Courtesy, Jahn and Jahn: The Protozoa. Dubuque, Iowa, Wm.
C. Brown and Co., 1949.)
Many live in the wet surroundings within the bodies of land or water ani-
mals, usually as parasites, sometimes only as passengers. Within flies, bees,
horses, cattle, and man protozoans can travel far and wide in the safety of a
fluid environment.
Ways of Living. Protozoans live more or less independently. They are free
Hving or in loose association with plants and animals.
Free-living ones, paramecia and others, ingest solid food — bacteria, diatoms
and other protozoans; some of them absorb food in solution through the body
covering. Those that contain chlorophyll — Euglena, Volvox, and others —
make their own food from inorganic material elaborated by photosynthesis
(Fig. 21.3).
Associations. Colonies of Vorticella and Epistylis are attached to sub-
merged objects in ponds; to the naked eye they may seem to be patches of
mold, but through a lens they are like miniature gardens of nodding flowers.
Kerona creeps louse-like over hydra (Fig. 21.1). Green paramecia {Parame-
cium biirsaria) and green stentors {Stentor polymorphum) are colored by uni-
cellular algae (Chlorella vulgaris) that live within them. There are mutual
benefits in such associations; the protozoans receive food and oxygen from the
algae, and the algae secure protection from the protozoans. Wood-eating
Chap. 21
No solid
food
THE PROTOZOANS
WAYS OF LIVING
Much solid food
429
) 5.
Colonies
Parasites
Fig. 21.3. Ways of living. Free living and solitary. 1, Euglena is brilliant green
with chlorophyll and makes its own food by photosynthesis. 2. Pelomyxa paliistris
is relatively very large and ameba-like. It consumes so many small organisms that it
may have a hundred food vacuoles at one time. /// colonies. In spheres of trans-
parent cellulose, 3, Pandorina and 4, Eudorina. 5, Giardia intestinalis and related
species live in the intestines of various vertebrates including man. A, active form
with two nuclei and eight flagella. B, side view of the active animal attached to the
lining of the intestine. C, two young animals that are about to separate. (Courtesy,
Jahn and Jahn: The Protozoa. Dubuque, Iowa, Wm. C. Brown and Co., 1949.)
cockroaches {Cryptocercus punctulatus) and termites have a similar relation
with certain flagellates.
Their minute size and preference for fluid environments open the way for
protozoans to be successful parasites. Among the parasites are species of
Plasmodium that in one stage of their lives invade human red blood cells and
cause malaria, and in another live in anopheiine mosquitoes without doing the
latter any apparent harm. Entameba histolytica, the most important intestinal
430 EVOLUTION OF ANIMALS Part V
protozoan of man, is the cause of amebic dysentery in temperate as well as
tropical climates and is estimated to inhabit 10 per cent of the world's popula-
tion. All protozoan parasites of the blood and intestines live completely im-
mersed in fluid food (Fig. 21.3).
Place of Protozoans in the Food Supply. Protozoans feed upon bacteria and
unicellular algae, mainly diatoms and desmids. They are important food, in
some places almost the sole food, of multitudes of minute animals, crustaceans,
rotifers, larval fishes, and in salt waters the ciliated swimming young of jelly
fishes and other invertebrates. This floating population (plankton) is the food
of larger animals, of medium-sized fishes that in their turn furnish food to still
larger ones. The bluefish and the cod would die in infancy if it were not for
the protozoans, and the bacteria and algae which support the protozoans.
Locomotion. Protozoans move about by means of flagella, by the flowing of
protoplasm in pseudopodia, or by cilia. All of them have one or the other of
these structures through some period of their lives, except the sporozoans
•
y
f
Fig. 21.4. Swimming motions of Euglena. The blunt end containing the reddish
eye spot is forward. The flagellum lashes sidewise and backward, pushing the body
forward in a spiral path and turning it over as it goes. Euglena swims toward the
light except when too strong. (Data from Jennings.)
which have no locomotor organs. The classes of Protozoa are arranged on the
basis of their ways of locomotion.
Flagella. The flagellum is a whip-like extension from the cell, with a con-
tractile core. Its simplest motion is like that of a swimming eel or a snake that
glides through the grass, bending its body from side to side in one plane. In
most flagellates the flagellum moves in a spiral that turns the body obliquely,
at the same time rotating it as in Euglena (Fig. 21.4).
Pseudopodia. The flowing of protoplasm is the most primitive means of
animal locomotion. It is caused by the changing states of protoplasm from
mobile watery plasmasol to the firmer plasmagel and vice versa (Fig. 2.11).
Such changes occur in response to those in the animal's surroundings and to
conditions within its body. A pseudopodium looks like a spreading spatter of
egg white. Its significance appears when the ameba moves in a definite direc-
tion, only after several small pseudopodia have been overcome by larger ones
(Fig. 21.12).
Cilia. The ciliates are the fastest, most versatile swimmers of all protozoans.
Chap. 21 THE PROTOZOANS 431
Their cilia are similar to flagella but finer and more numerous. Each one makes
a backward power stroke and a return drag, the whole movement being rapidly
repeated in unison with others (Fig. 21.5). In salt-water shallows the surface
water often teems with minute ciliated swimmers; many are protozoans; many
others are newly hatched marine invertebrates.
Structures similar to the locomotor organs of protozoans appear over and
over again in multicellular animals. In man, and in the majority of higher
animals, ameboid blood cells creep along the capillaries by outflowing proto-
plasm; sperm cells swim by means of flagella; and the cilia of the lining of the
trachea keep the way clear for breathing.
o ^
Red blood cell
Copillory
White blood cell
AMEBOID MOTION
Ameba
AMEBOID MOTION
White blood cell
Body moves forward
Cilium strokes backward
1-6 Power stroke, backward
7~i0 Return stroke
Fig. 21.5. The motion of pseudopodia and cilia is important in both unicellular
and multicellular animals. Upper, motion by pseudopodia in the ameba and in
white blood cells of higher animals. Ameboid locomotion is prevalent throughout
the animal kingdom. White blood cells are continually crawling about and in and
out the blood capillaries of the human body. Lower, diagram of the power stroke
of a cilium that pushes the animal forward, e.g., a Paramecium, and the return
stroke that is actually a hindrance. The same thing would happen in rowing if the
oars were kept in the water on the return stroke. In the lining of the human trachea
the power stroke of the cilia is toward the mouth.
432
EVOLUTION OF ANIMALS
Part V
There are five classes of protozoans:
1 . Mastigophora, or flagellates, with one or more flagella.
2. Sarcodina or rhizopods, with pseudopodia.
3. Sporozoa, with no locomotor structures.
4. Ciliata, or ciliatcs, with cilia throughout life.
5. Suctoria, with cilia in the young and tentacles in the adult stages.
Class Mastigophora or Flagellata
This class includes both the plantlike phytoflagellates that contain chromo-
plasts with chlorophyll and often other pigment, and the zooflagellates that
are clearly animals and without chlorophyll. The phytoflagellates make their
food from inorganic matter and are basically constructive organisms in what-
ever community they live. The zooflagellates take their food from plants and
other animals.
Phytoflagellates
Structure. The brilliantly colored euglenas of several species are common in
fresh waters (Fig. 21.6). Among their characteristic structures are the green
flagellum
reservorr
or gullet
poramylum
body
(starch)
pellicle
eye spot
ractile
cuole
oroplast
cleus
Fig. 21.6. Euglena, a fish-shaped green
protozoan that lives in many stagnant pools
of fresh water. It is just visible to the naked
eye but the millions of them often turn the
surface of a pool brilliant green. There are
many species, in some the mouth leads to a
gullet as in this one, others are without these
structures, and probably make all of their
food by photosynthesis. The flagellum, a
bundle of contractile fibrils bound together
in a sheath, is an efficient swimming organ.
There are many species of Euglena, in some
the body is long and slender.
chloroplasts, (disks, ovals, stars, or bands) scattered through the body and
about the central nucleus. The flagellum that arises from a minute body
(blepharoplast) in the side of the cytopharynx is associated with the control
Chap. 21 THE PROTOZOANS 433
of movements. Excess fluids and metabolic products collect in its enlarged
base from whence they are discharged from the body. The reddish, light sensi-
tive eyespot is a markedly animal characteristic. The whole body is enclosed
in a thin elastic cover or pellicle that adjusts itself easily to the organism's
squirming movements.
Nutrition. The chloroplasts are vital organs, the centers of photosynthesis by
which the carbohydrate food is formed with the help of water, carbon dioxide,
and radiant energy from the sun. Free-living flagellates also absorb dissolved
nutrient materials from the water in which they live; in fact, in nutrient solu-
tions euglenas will live and multiply even in the dark after losing their chlo-
rophyll. Their stored paramylum is a food similar to the glycogen in the tissues
of multicellular animals. Chlorophyll-bearing flagellates are the constructive
organisms of their communities. In both fresh and salt water they are the great
carbohydrate producers.
Pigments. Phytoflagellates may be yellow green, blue green, orange, and at
times some are red. The colors are due mainly to carotene and allied pigments
that cloak the chloroplasts that are then called chromoplasts. Like the related
green of chlorophyll, the pigments of carotene are generally sensitive to light.
Protozoans that contain chromoplasts usually have a reddish stigma or eyespot
similar to that of the euglenas. "Red snow" and pasture pools "colored by red
rain," common in midwestern United States, are usually due to dense popula-
tions of red euglenas {Euglena rubra) (Fig. 21.6).
Colonies. Some phytoflagellate colonies contain but a few individuals, 4, 8,
16 and thereabout, held together in jelly; others such as Volvox contain thou-
sands of them (Fig. 21.7). Many colonies show distinct polarity or difference
between the ends; in Pleodorina and Volvox the individuals at the anterior
pole are sterile while those farther back produce new colonies asexually by
repeated cell divisions. Volvox and others reproduce sexually and asexually;
some cells enlarge and become eggs, others divide and produce flagellate sperm
cells. The fertilized egg secretes a shell in which it can remain for a long period,
through drought or winter. When favoring conditions return, the egg divides
and a young colony emerges.
Dinoflagellates. Composing a large part of the microscopic surface fauna of
the sea, dinoflagellates include the luminescent noctilucas that float in coastal
waters, and the armorbearers that are typical plankton forms of both ocean
and inland waters (Fig. 21.8). Dinoflagellates usually bear two flagella, each
one originating in a groove of the body surface. Their bodies are clothed in
membranes, or in two shells or several plates. Thus they are armored and earn
their name, dino or terrible flagellates. Their nutrition is generally plant-like
but some have lost the chromoplasts, have become ameboid, and feed on small
organisms in typical ameboid fashion. Still others get their living as parasites
in the intestines of copepods and other small floaters of the sea. Larger inverte-
434
EVOLUTION OF ANIMALS
Part V
Green volvox of the ponds
Fig. 21.7. A, Volvox, a colony of thousands of cells, most of them with two fla-
gella, a red eyespot, contractile vacuole, and chlorophyll. Strands of protoplasm
unite the asexual cells and make them physiologically continuous. Certain of the
cells reproduce by division. Certain cells in some colonies enlarge and become
female sex cells; in others certain cells divide and become male sex cells. These
fuse with the large cells in the female colonies and form daughter colonies which
remain for a time within the parent colony. B, a detailed view of the surface of
Volvox highly magnified showing the protoplasmic connections between the cells.
(B, courtesy, Hyman: The Invertebrates, vol. 1. New York, McGraw-Hill Book
Co., 1940.)
brates feed upon them especially along the coasts. Epidemics of human food
poisoning have been traced to eating mussels (Mytilus californicus) which had
fed upon a species of Gonyaulax that produces an alkaloid poison (Fig. 21.8).
The "red tide" that came in along the Florida coast in 1947 brought poisonous
dinoflagellates in untold numbers and tons of dead fishes were strewn for many
miles upon the shore.
ZOOFLAGELLATES
Definitely animal-like, zoofiagellates do not contain chlorophyll, and usually
have one or two flagella. They may be solitary or colonial, and many are para-
sitic. The collar-flagellates (choano-flagellates) that live mainly in fresh water,
have transparent protoplasmic collars. The single flagellum swings forth from
within the collar and draws food against the cell along with the currents of
water that it creates (Fig. 21.9). In the sponge-like colonial Proterospongia,
the individuals are embedded in a blob of clear jelly; collared cells protrude
from the surface and collarless ameboid ones migrate into the interior of the
jelly. Collared cells are very characteristic of sponges, and Proterospongia
appears like a hesitant step in an evolution toward a structure similar to
sponges.
Chap. 21
THE PROTOZOANS
435
Ponds, lakes,
and seas
DINOFLAGELLATES
2.
Red tides,
Florida
Red waters,
Pacific Coast
Living light,
all oceans
Fig. 21.8. Dinoflagellates: armored and unarmored types. /, Ceratium, with the
typical armorlike shell and flagellum. 2, a dinoflagellate (Gymnodinium) that
often sheds its armor and becomes a naked swimmer. They occur in vast numbers
in the "red tides" of Florida. Tons of dead fishes are thrown on the beaches when-
ever these protozoans abound. 3, Gonyaulax polyhedra, a main cause of some of
the red water of the oceans. Several kinds of shellfishes feed on them after which
they are poisonous as human food. 4, Noctiluca, a relatively large translucent
sphere. They float on the sea in vast numbers, each one flashing light. Together
they create miles of bioluminescence. //, tentacle; //, flagellum; tf, flagellum; s,
groove. (Courtesy, Jahn and Jahn: The Protozoa. Dubuque, Iowa, Wm. C. Brown
and Co., 1949.)
Colony of collared cells Each cell o food frop
> 3.
Proterospongia, collored cells
unique in protozoans
and sponges
Fig. 21.9. Protozoans that suggest sponges. /. Codosiga botrytris, each individual
of the colony has a kind of food trap called a collar cell or choanocyte. 2, four
cells of the colony in different stages of catching and ingesting food; A, a particle
caught by the flagellum is whipped against the collar which contracts; B, slides
the particle against the body of the cell; C and D, finally it enters the cell body.
3, Proterospongia, so called because of the resemblance of its cells to the collared
cells (choanocytes) of sponges. (Courtesy, Jahn and Jahn: The Protozoa. Du-
buque, Iowa, Wm. C. Brown and Co., 1949.)
436
EVOLUTION OF ANIMALS
Part V
Trypanosomes
The trypanosomes are blood parasites in all classes of vertebrates, but so
far as known are pathogenic only in man and domestic animals where, in an
evolutionary time sense, they have but recently developed. They are trans-
mitted from one vertebrate to another by blood-sucking invertebrates — those
of fishes, salamanders, frogs and reptiles by leeches — those of land vertebrates
by ticks and insects. Within these carriers they go through a cycle of several
days' development without which they cannot be transmitted into their second
host (Fig. 21.10).
The trypanosome of the rat (Trypanosoma lewisi) is nonpathogenic and
common in our native wild rats, often so abundant that the blood literally
Fig. 21.10. Trypanosoma gambiense among the red cells of human blood. These
microscopic blood parasites are the cause of trypanosomiasis, the sleeping sickness
of tropical West Africa. They pass one period of their life history in the tsetse flies
that are essential for their distribution. Aside from that they are parasites in the
blood of man and certain of the wild game animals of Africa. (Courtesy, General
Biological Supply House, Chicago, 111.)
twinkles from their motions. Yet the rats thus infected show no signs of harm.
The rat becomes infected by licking its skin and thus gathers the feces of in-
fected rat fleas. After an incubadon time of two weeks the parasites appear in
the blood as typical trypanosomes and multiply enormously. Fleas suck up the
trypanosomes with every meal of an infected rat's blood. In the lining cells
of the flea's stomach, they muldply by repeated divisions and transform into
the mature trypanosomes then ejected upon the rat's skin in the feces of the
fleas.
This life history shows important characteristics of such parasites; their
great capacity to multiply, and their ability to change form and adjust them-
selves to environments in which they thrive and are carried about and dis-
tributed. This life history also displays the ability of a host animal to become
Chap. 21 THE PROTOZOANS 437
more or less immune to injury from its parasites. Natural immunities occur on
every hand commonly because of chemical content or structure, or both.
Immunities to protozoans include that of wood-eating insects which are not
only immune but are benefited by the flagellates that live in their digestive
tracts.
Trypanosomes and Sleeping Sickness. The most widely injurious of patho-
genic trypanosomes are those that cause the African sleeping-sickness of man
and domestic cattle, not to be confused with the sleeping-sickness or encepha-
litis, a paralysis, that has no relation to trypanosomes. The African disease
occurs throughout central Africa and is due either to Trypanosoma gambiense
or its near relative Trypanosoma rhodesiense. They are transmitted from man
to man or from wild mammals to man by blood-sucking tsetse flies (Glossina)
that inject the parasites into the blood when they bite just as mosquitoes inject
malarial parasites into the blood. The trypanosomes go through an essential
part of their life history in the body of the tsetse fly. This takes 14 days at
the end of which they have bored their way into the salivary glands of the fly
and are ready to enter the mammalian blood and cerebrospinal fluid (Fig.
21.10). The big game animals of Africa are the reservoirs for these parasites
and the only known transmitters are the tsetse flies. Like wild rats and fleas,
the big game animals and tsetse flies have become practically immune to tryp-
anosomes. Only man and domestic mammals are mortally harmed, an indi-
cation that for them the trypanosomes are still relatively new parasites.
Class Sarcodina or Rhizopoda
The Sarcodina — amebas, radiolarians, foraminiferans, and others — move by
means of flowing protoplasm, many of them by pseudopodia. They feed on
bacteria, microscopic plants and animals and next to the bacteria, algae, and
phytoflagellates are basic food supplies. Fresh-water species have one or more
contractile vacuoles; salt-water and parasitic species usually lack these alto-
gether. Reproduction is mainly asexual, by binary fission or by budding;
sexual reproduction is known in comparatively few species, such as fora-
miniferans.
The Ameba
Habitat. Fresh-water amebas live in ponds and streamsides, on decaying
leaves and slimy stems. All of them likely to be found there are microscopic.
The only way to see them is to collect pond water and plant debris, let it stand
several days, and then examine it bit by bit under a microscope. The rela-
tively large amebas of most laboratories have been grown in cultures, pur-
chased from specialists in rearing them. Ameba proteus and Ameba caroUnensis
(also called Chaos chaos), the giant ameba, are commonly used for study.
Appearance. At first glance, through the microscope, an ameba seems to be
438
EVOLUTION OF ANIMALS
Part V
a strangely active spatter of peppered egg white. It gives no sign that it is
carrying on the same basic essentials of living as are one's own cells. Amebas
are generally colorless, or gray to black {Pelomyxa palustris) from the bac-
teria that live in the cytoplasm. Two regions are distinguishable in the body,
a clear outer layer of ectoplasm and the central endoplasm which contains
the vital organelles and the nucleus, separated from the endoplasm by the
nuclear membrane (Fig. 21.11). There may be clusters of green particles in
the endoplasm, bacteria and diatoms in the food vacuoles which are temporary
stomachs in which digestion prepares the food for absorption and assimila-
tion. The contractile vacuole widens and vanishes only to appear again in
nearly the same place. Such vacuoles eliminate metabolic waste products and
are important guardians of the water content of the body. They are active
when the animal contains too much water, and disappear when it contains too
little. Neither marine nor parasitic amebas have contractile vacuoles. Since
their bodies and the sea water or the protoplasm of their hosts contain about
the same proportions of salt, the sea water does not flood into their bodies as
it does into the amebas of fresh water.
Locomotion. Amebas commonly move about by means of pseudopodia
although these are usually greatly reduced in parasitic species.
Emptying ond reforming
of contractile vocuole
Amebo cut through
contractile vocuole
(O^^
4 3 2
Food vacuole
Contractile vocuole
Temporory front end
, Pseudopodium forming
Ectoplasm
Endoplasm
nner
toplosm
Clear outer protoplasm
Fig. 21.11. An ameba, showing its principal structures. Inset, section of ameba
with the contractile vacuole in successive stages of emptying and refilling.
THE PROTOZOANS
439
Chap. 21
Digestion, Absorption, and Assimilation. Amebas have no permanent re-
ception place for food. When an ameba first touches an inviting particle its
protoplasm rapidly flows around it and the food is engulfed as the ameba
passes over it (Fig. 21. 12). -Amebas never ingest dry food. Each bit is filmed
with water as our own food is cloaked with air or liquid. As soon as the food
is engulfed in the endoplasm, the digestive ferments flow into this temporary
stomach from the surrounding protoplasm. These digest the food, mainly the
proteins. The digested foods, the water and ferments are gradually absorbed
into the protoplasm. The indigestible remainders, such as diatom shells, stimu-
late the wall of the vacuole which squeezes them out of the body and all signs
of the food vacuole disappear. Finally the absorbed substances are assimilated,
arranged within their kindred materials in the living ameba.
Respiration and Excretion. Oxygen held in the water diffuses through the
body of the ameba and unites with carbon and other substances within it.
This oxidation liberates energy and heat and leaves a by-product, carbon
dioxide, that either diffuses out through the body covering or collects in the
contractile vacuoles with other metabolic products, and is discharged with
them.
The natural growth of the whole animal is a constructive process of change
Fig. 21.12. "Pursuit, capture, and swallowing" of one ameba by another; escape
of the captured ameba and its recapture; final escape. The whole action took about
fifteen minutes. /, an ameba (a) from which the part b has been almost severed
by a glass rod; c is an ameba which has come in contact with part b and tries to
ingest it; 2-4. are stages in the ingestion, which are accomplished in 5 and 6 when
ameba a moves off and out of the story; 7-10 show that b is restless in the food
vacuole of c; at 7/ and 12, b escapes and moves away entirely out of contact with
c; 13, c pursues and captures b; for the second time b escapes, this time perma-
nently leaving c at 15 with temporarily vacant food vacuole. (Courtesy, Jennings:
Behavior of Lower Organisms. New York, Columbia Univ. Press, 1906.)
440 EVOLUTION OF ANIMALS Part V
and addition that goes on through its early hfc. Assimilation of food is its
essential preliminary. The extent of growth is determined by heredity and by
surrounding conditions, regulators that are as effective for an ameba as for a
horse.
Reproduction. Amebas reproduce by division into two approximately equal
parts and by mitotic division of the nucleus. At a temperature of 24° C. the
process takes about half an hour. There is no real metamorphosis; an ameba
that has just come into existence by division looks like any other one of its
kind only smaller.
Reactions to Stimuli. In their natural surroundings amebas are touching
something, are resting, or moving upon water soaked and decaying leaves. One
ameba described by H. S. Jennings touched the end of an algal filament, after
which a pseudopodium was extended along each side of the filament. Then
the protoplasm on one side stopped flowing and the filament was avoided
as part of the current was reversed and turned into another direction. If an
ameba is touched with a glass rod it behaves the same way. Reactions to con-
tact are not all negative, however. If an ameba comes in touch with a surface,
while it is still suspended in the water, it immediately spreads itself as a cat
landing from a jump will spread its toes to contact the ground (Fig. 10.1 ). In
general, amebas react positively to gravity; they creep on the bottom of a dish,
or on the mucky bottom of a pool, a contrast to the usual open water swim-
ming of paramecia. If salt solution from a very fine capillary tube diffuses
against the side of an ameba, the part affected will contract and the proto-
plasmic currents will start in another direction.
Amebas are no more responsive in one part of the body than in any other
to touch, light, or other stimuli. In general, if light interferes with their
activities, they will move away from it. If it is suddenly thrown on them when
they are feeding on a filament of alga, they will stop and even squeeze out
bits of alga that were already ingested. Over-stimulation by light makes an
ameba refuse food as interference with equilibrium and other senses makes
some persons lose their appetite.
Pursuit of Prey. In general, amebas draw away from things which would
be harmful to them and toward those that are beneficial. Most of their re-
sponses are due to direct physical or chemical stimuli from the environment.
Yet H. S. Jennings was not wholly able to analyze pursuits of one ameba by
another, although he observed them several times and devoted years of study
to the behavior of protozoans. One such pursuit and capture is described in
the figure and legend (Fig. 21.12). The captor (ameba c) pursued its
prey (ameba b) with great persistence. The climax came at numbers 11 and
12 when the captive (b) escaped completely out of contact with its captor
(c) yet the latter continued the pursuit and repeated the performance. Does
this not suggest that the ameba depends upon a primitive kind of memory?
Chap. 21
THE PROTOZOANS
441
Other Sarcodina — Shelled Amebas
A few common species of Sarcodina are presented according to their habits
of life, free living in fresh water; free living in salt water; and parasitic (Figs.
21.13,21.14).
Fresh Water. Arcellas glide over submerged pond weeds with pseudopodia
NEAR RELATIVES OF THE AMEBA
Radiolorian
Sun animalcule
Shell is secreted
Shell of microscopic
sand grains
Fig. 21.13. Ornate relatives of the amebas. /, Acanthometron, whose radiating
needles are attached by musclelike bands, the myofrisks by which needles and body
are moved. 2, the sun animalcule, Actinophrys sol, a splendid relative of the better
known amebas. This one is common on the submerged vegetation of ponds. 3,
Arcella vulgaris, an amebalike animal with a tam-o'-shanter-shaped cover. A, sec-
tion showing the two nuclei and the pseudopodia extending from under the shell.
B, view through the top of a translucent shell. 4, Difflugia makes its covering of
microscopic sand grains firmly held together by a secretion. (Courtesy, Jahn and
Jahn: The Protozoa. Dubuque, Iowa, Wm. C. Brown and Co., 1949.)
442
EVOLUTION OF ANIMALS
Part V
Fig. 21.14. Shells of marine foraminiferans. The majority of them have several
chambers. The animal secretes the shell of one chamber but as it grows it slips out
of this and secretes a larger chamber and then another until it reaches full size. The
whole collection shown here is about seven inches wide. Layers of foraminiferan
shells fallen from the surface waters are said to cover two-thirds of the floors of
the oceans. (Courtesy, South Kensington Natural History Museum, London.)
extended from beneath the mushroom-shaped shell-like tests. They are actually
shelled amebas and as easily cultured (Fig. 21.13). Difflugias, also ameba-
like, are covered by tests inset with minute sand grains. During the asexual
division one part of the animal protrudes from the test and its ectoplasm
secretes a sticky fluid to which sand grains adhere. After division, the difflugias
separate, one covered by the old test, one by the new. Heliozoans or sun
animalcules whose bodies are decked with crystal clear filaments are the
splendid relatives of amebas.
Sea Water. Foraminiferans (hole-bearing) are ameboid protozoans that
secrete many-chambered shells, most of which are chalky; others are of
chitin or silica (Fig. 21.13). The young foraminiferan makes a shell with an
opening from which it extends its body as a snail does. As it grows, its proto-
Chap. 21 THE PROTOZOANS 443
plasm flows out of the chamber, spreads over the shell it has made and secretes
another shell, a second chamber. Delicate pseudopodia extend through pores
in the shell as well as through the main opening. Adult foraminiferans are
dimorphic, that is, some individuals divide asexually into many new ones;
others divide into flagellate gametes (sex cells). The gametes fuse in pairs and
produce individuals that divide asexually when full grown. This alternation of
sexual and asexual phases suggests the "alternation of generations" of the
coelenterates (Chap. 24).
The "white cliffs of Dover," England, and the chalk-beds 1 000 feet or more
deep of Mississippi and Georgia are made of foraminiferan shells that once
dropped downward through deep seas that flooded these lands. Foraminif-
erans live in surface waters and Globigerina is one of the commonest of
them. These animals are constantly dying and their shells form the "globigerina
ooze" that covers some 40 million square miles of ocean floor.
Radiolarians are among the most beautiful objects in nature. They are a vast
array of animals with clear glassy skeletons, radiating needles and latticed
spheres of silica fashioned like delicate crystal toys. The protoplasm that is
foamy with vacuoles, holds fat drops, oil spheres, and red, yellow, and brown
pigment granules. Many of them contain "yellow cells," very minute proto-
zoans that live within them.
Radiolarians are exclusively marine, living chiefly in surface waters; some
species have been found in samples taken at depths of over three miles. Their
skeletons fall upon the sea bottom in more perfect shape than those of foram-
iniferans of the same size because the silica is so resistant to the corrosive
effects of the sea water. Probably for the same reason, radiolarians are among
the oldest and most perfect fossils known. Many of the old patterns are almost
identical with those of present-day species although their microscopic sculp-
turing must have been in the making long ages before multicellular animals
appeared.
Parasitic Sarcodina
Parasitic amebas occur in many vertebrates, in man, and in such dis-
tantly related invertebrates as hydras, leeches, and cockroaches. Practically
all of them inhabit the alimentary canal and all enter an encysted stage at
one time or another in their life history. It is then that they pass out of the
body of the host and are freely distributed. Endameba histolytica, the cause
of amebic dysentery, lives within the human intestine (colon). In its encysted
stage, it is transmitted from one person to another in drinking water and by
flies and food. Endameba gingivalis is a common parasite in the human mouth
where it lives near the base of the teeth. The colons of cockroaches often
contain numbers of Endameba blattae that ingest bacteria from the in-
testinal content.
444 EVOLUTION OF ANIMALS Part V
Class Sporozoa
Characteristics. The sporozoans are without exception the parasites with
complicated Hfe histories, often including an alternation of sexual and asexual
reproduction. Their hosts are animals of many types from protozoans to man.
As spores they commonly pass from one host to another. A spore is a young
individual or group of them (sporozoites), usually enclosed in a capsule,
capable of establishing the parasite in a new host.
In addition to malaria in man and birds, sporozoans cause the serious dis-
eases of coccidiosis in fowls and rabbits, certain fevers in cattle, and the
pebrine disease of silkworms.
Gregarines. The Gregarines are chiefly parasites in the body cavities of
invertebrates. They are common in grasshoppers, cockroaches, and in the
seminal vesicles of earthworms. The latter are easily examined and beautiful
when taken from freshly killed worms; the viscera of pickled worms are drab
and sterile. Pieces of earthworm vesicle can be teased out in a little water on
a glass slide. If they are infected, the ciliated adult parasites will swim
through the mealy debris and the spores containing the young parasites will
be scattered through it or packed in cysts.
Coccidia. The Coccidia live in the epithelial cells of many vertebrates and
a few invertebrates. Their life history is complex and, in essentials, similar to
that of the malarial parasite. That of a coccidian (Eimeria schubergi), a
parasite of centipedes, is typical of others. It is swallowed with the food and
passes on into the intestine as a cyst (oocyst) containing several young indi-
viduals (sporozoites). The sporozoites enter the cells of the intestinal lining.
They divide repeatedly producing two kinds of individuals; asexual ones that
enter cells and divide asexually; and sexual ones that enter cells and enlarge
into egg cells (macrogametes) or enlarge and undergo multiple division into
sperm cells (microgametes). A micro- and a macrogamete fuse forming a
zygote as in ordinary sexual reproduction. The zygote surrounds itself with a
secretion which hardens into a shell. Within this shell or cyst (oocyst) it
divides several times until the cyst is packed with young parasites (sporo-
zoites). While this division is going on, the cyst is either in the lower intestine
or has been thrown outside the body. There along with millions of its kind it
wins or loses a chance to be ingested by another centipede.
Hemosporidia. These sporozoans are parasites of vertebrates, blood suck-
ing insects and other arthropods. In the vertebrates, they inhabit the blood
cells and plasma. In arthropods that transmit them from one vertebrate to
another, they occur in the stomach and salivary glands. Those that most affect
human welfare are the species that cause human malaria.
Malaria. Malaria, meaning bad air, is the name of a group of infections
caused by microscopic protozoan parasites (Class Sporozoa) that live mainly
Chap. 21 THE PROTOZOANS 445
in the blood and are transmitted solely by female anophcline (Anopheles)
mosquitoes. During its complete life history, the malarial parasite passes one
part of its existence in man and another part in the mosquito. Although other
vertebrates have malaria-like parasites and symptoms, the parasites causing
human malaria have been found only in man and anopheline mosquitoes.
The paroxysms of malaria known as chills and fever may occur every day,
every other day, or every third day. These differences are due to peculiarities
in the life cycle of different species of malarial parasites which occur in human
blood. More than one of these may live in the blood at once and thus a person
may have more than one type of malaria at the same time and the distinctness
in the succession of temperature changes may be irregular.
Immunity. Human beings have some natural resistance to all malarial para-
sites and certain races show a greater degree of it than others. In the United
States, the Negro race has a greater immunity to Plasmodium vivax than the
white. Some degree of acquired immunity is evidently developed by people
living in tropical regions where they have been subjected to malaria since
babyhood. Immunity to malaria artificially acquired by a vaccine as it is in
smallpox has not been accomplished and the prospects for it are not regarded
as promising. There are several reasons for this. One of them is the existence
of so many malarial parasites each of which may create its own type of im-
munity, and the immune person is prone to carry latent infections of them.
Plasmodium Causing Human Malaria. There are four species of Plasmo-
dium that cause malaria: Plasmodium ovale is very rare; P. vivax is the cause
of "tertian" or "benign tertian" malaria, has a 48-hour cycle of development in
man, and is widely distributed in tropical and temperate zones; P. malariae
is the cause of "quartan" malaria and has a 72-hour cycle; and P. falciparum
is the cause of "malignant tertian" malaria and has a 40- to 48-hour cycle.
Life History of Plasmodium vivax. Benign tertian malaria caused by the
parasite, Plasmodium vivax, is the commonest type of the disease in the
United States. When they bite human beings, female anopheline mosquitoes
carrying these parasites introduce them into the blood in an infective stage
of development known as sporozoites (Fig. 21.15). The sporozoites travel to
the liver cells and divide for 6 to 10 days (exoerythrocytic stages). When they
are released from the liver cells they enter the red blood cells and give rise to
nonsexual and sexual forms. In a red blood cell, each sporozoite grows and
divides into from 15 to 20 new individuals, the merozoites, nonsexual forms,
within about 48 hours. During this time, the parasite splits the hemoglobin in
the blood cell, and absorbs the hematin part of it known as malarial pigment
which accumulates in the parasite. After 48 hours, the red blood cell bursts
and the contained merozoites are freed in the blood plasma along with the
debris of the broken cell. This is the period of fever and general disturbance of
temperature in the person who has the disease. The merozoites soon attack
LIFE CYCLES OF MALARIAL ORGANISM IN MOSQUITO (ANOPHELES) AND MAN
IN THE LIVER, 1st
Exoerythrocyti
1 Sporozoite enters blood
2 Sporozoite enters liver eel
3,4 Asexual cycle
5,6.7 Second asexual cycle
THE MOSQUITO
Stomach
etocytes (sex cells)
re cells
izotion
individuol (zygote)
n Woll of Stomach
Ookinete forms oocyst
, 18 Division into
sporozoites
19 Sporozoites freed
20 Sporozoites enter
salivary glands
escape mto blood
Mitti saliva
N HUMAN BODY
BLOOD
CAPILLARIES
8
9
10
II
12
THE BLOOD STREAM, 7th DAY ON
Erythrocytic stages (in red cells)
Ring stage
Ameboid stage
Schizonf stage
Merozoites escaping from blood cells
Gametocytes form (sexuol phase) 3rd week
Fig. 21.15. Life history of the parasite, Plasmodium vivax, which causes benign
tertian malaria. In the human body. An infected female Anopheles mosquito bites
and injects saliva containing the parasites into the blood of its victim. Stages 1,2;
the parasites travel in the blood and enter the liver cells. Stages 3-7; the parasites
multiply in the liver cells. Stages 8-12; the parasites leave the liver, enter the blood;
many but not all invade the red blood cells, multiply and the red cells burst;
gametocytes (sexual phase) develop in some of the red blood cells. In the mosquito.
A female mosquito of the genus Anopheles bites and sucks blood from a person
whose blood contains developing gametocytes of Plasmodium vivax. Stages 12-15;
in the stomach the male and female cells mature, fertilization occurs and young
parasites develop. Stages 16-20; phases of growth and multiplication occur in the
wall of the stomach, followed by release, migration and entrance into the salivary
glands. The parasites are now ready for distribution into any individual whom the
mosquito may bite.
446
Chap. 21 THE PROTOZOANS 447
other red blood cells and the cycle of growth and asexual multiplication just
described begins over again. Sexual development starts with a stage that dif-
fers scarcely at all from the sporozoite from which the asexual generations
develop. It also grows in. the red blood cell but instead of dividing into
merozoites it gives rise to either a male organism (microgametocyte) or
female (macrogametocyte). If left in the human body, these male and female
organisms usually die.
Description of the parasite's life in the female mosquito. If the gameto-
cytes are taken into the stomach of a female anopheline mosquito they develop
into easily recognized male and female individuals (Fig. 21.15). The nucleus
of the male gametocyte (microgametocyte) divides and within a few minutes
6 to 8 microgametes, each with a flagellum, are formed. The female gameto-
cyte (macrogametocyte) does not divide and is the macrogamete. Into it one
of the microgametes enters. The union of these two cells makes a zygote (cor-
responding to the fertilized egg in higher animals). It becomes wormlike and
is called an ookinete. The ookinete bores into the stomach wall of the mosquito
and there, surrounded by a kind of cellular capsule (oocyst), it divides into
many sporozoites. There may be more than 10,000. These grow until they
burst the capsule and are freed in the body cavity of the mosquito, usually
within 10 days to three weeks depending on the temperature. In their migra-
tion in the body cavity, many of them reach the salivary glands and bore into
them, finally lodging in the tubes which carry saliva into the mouth. As many
as 200,000 sporozoites may be packed in one mosquito's salivary glands.
When an infected female mosquito bites (only the females suck blood), she
always injects her saliva into the blood capillary which she has pierced, at the
same time injecting parasites into the blood.
Benign and Malignant Malarias. Benign malaria is characterized by periods
of fever, the malarial paroxysms, broken by periods of normal or below normal
temperatures. The period of fever consists of a seemingly cold stage of chills
during which there is actually a rise in temperature, a hot stage of high tem-
perature and a sweating stage, all of these occurring within about 10 to 12
hours.
The nonsexual cycle of the life of the parasite occurs in the period between
the paroxysms. For Plasmodium vivax of benign tertian malaria, this period
lasts 48 hours and the paroxysm occurs on the third day. Plasmodium malariae
of benign quartan malaria has a nonsexual period of 72 hours and there is a
paroxysm on the fourth day. In malignant malaria the temperature changes are
likely to be less regular than in benign types and the paroxysms last longer.
Malignant tertian malaria is caused by Plasmodium falciparum. This para-
site multiplies in very great numbers. Corpuscles containing their asexual
stages tend to clump in the capillaries. When such a clogging of capillaries
occurs in the brain ("cerebral malaria"), the patient becomes unconscious. In
448
EVOLUTION OF ANIMALS
Part V
this and certain other conditions, the symptoms of malignant tertians are quite
different from those commonly supposed to belong with malaria. "Blackwater
fever" is probably a type of malaria caused by Plasmodium falciparum.
Treatment of Malaria by Drugs. A considerable number of drugs have been
found to have antimalarial effects. The four which arrest the development of
the merozoites of all species of Plasmodium and in sufficient doses are cura-
tive in the malaria of Plasmodium falciparum are quinine, atabrine, chloro-
quine, and paludrine. More recently developed than any of these is the power-
ful antimalaria drug, darasprim, which holds the possibility of eliminating the
disease.
Class Ciliata
All ciliates bear cilia at some period of their lives; many throughout life
(Fig. 21.16). Ciliates are complex, and specialized mainly for independent
living. They live on or in many plants and animals, myriads of them in pro-
tecting capsules on grass blades. Sheep, cattle and other cud-chewers swal-
low them into the first stomach or rumen along with great numbers of bac-
teria. Ciliates and bacteria become active in the warmth and moisture of the
rumen and the bacteria provide a rich food supply for the protozoans
(Fig. 11.14). Ciliates always abound in all healthy cud-chewers after they are
old enough to eat grass. They disappear as the food is moved on into other sec-
FiG. 21.16. Two colonial protozoans that like paramecia are dependent on cilia
for the intake of food and are common residents of fresh water. Left, Vorticella,
bell animalcule. Right, Epistylis, often attached to aquatic insects. (Left, courtesy,
Conn: "Protozoa of Connecticut," Conn. State Geol. and Nat. Hist. Survey Bull.
%2, 1905. Right, courtesy, Hyman: The Invertebrates, vol. 1. New York, Mc-
Graw-Hill Book Co., 1940.)
Chap. 21 THE PROTOZOANS 449
tions of the stomach, apparently killed by the digestive fluids. It has been
estimated that two per cent of a sheep's daily protein requirement may be
met by digested ciliates. They are present in the alimentary canals of other
animals apparently sharing the food supply, but without damage to their hosts.
Most slugs and many snails, planarians and sea urchins contain them.
Paramecium and other ciliates have systems of contractile fibrils and neuro-
fibrils concerned with responses, coordination, and control of the cilia. The
trichocysts are minute poisonous rods arranged at right angles to the body
surface. They are discharged with great vigor particularly when a Paramecium
is attacked by its constant foe, Didinium. Most ciliates are peculiar in having
two kinds of nuclei, a large macronucleus important in general metabolism, and
one or more smaller nuclei that take part in conjugation. The latter is an
approach to the mating relation and the fusion of sex cells in multicellular
animals. In certain individuals, there may be a reorganization of nuclei called
endomixis that always occurs within single animals. This brings about an
upswing of physiological activity similar to that which follows conjugation.
Paramecium
Appearance. Paramecia are common animals in both ponds and labora-
tories. This "slipper animalcule" was among the "little things" which were first
seen in the seventeenth century, when the newly devised microscopes were being
tried out with great enthusiasm. A drawing of it was made by Joblot in 1718.
Paramecium came on the human stage then and has never left it. No one will
go far into the most recent studies of heredity, of variation and sex, of re-
sponses and behavior, and of populations, without finding paramecia a focus
of attention.
General Structures. Its form and structure show the definite shape, differ-
entiation of front and rear ends, a definite position of mouth and gullet, path-
way of food vacuoles, anal opening, and contractile vacuoles (Fig. 21.17). All
of these localizations suggest a trend toward permanence in the location of
organs familiar to us in multicellular animals. The endoplasm is enclosed by
ectoplasm that secretes the flexible non-living pellicle and bears the cilia that
extend through the pellicle.
Support and Movement. A Paramecium swims by the beating of its cilia.
Strong oblique backward strokes drive it forward and, in addition to the
forward movement, continually rotate the body on its long axis (Fig. 21.18).
The forward movement may stop or be reversed, yet the body will continue
to turn. The cilia in the oral groove beat more strongly than elsewhere. This
turns the anterior end away from the oral side as a boat turns toward the side
that is rowed more strongly. The boat eventually swings in circles and the
Paramecium would do the same if it were not that it rotates on its long
axis.
450
EVOLUTION OF ANIMALS
Part V
FRONT END
Coniractile
vacuole
Small nucleus
Large nucleus
Top view of
surface cilia
and network
of coordinat-
ing fibrils
Food vacuole
Contractile
vacuole
Clear outer
layer
Cilium
Stiff outer
covering
Trichocysts
Oral groove
Mouth
pore
Gullet
Food vacuole
forming
Anal pore
Cilia
REAR END
Fig. 21.17. Paramecium, a general view, with its main structures and functions
indicated. Gullet, food vacuole, and other organelles are embedded in the proto-
plasm. (Courtesy, Gerard: Unresting Cells. New York, Harper & Bros., 1940.)
Fig. 21.18. Diagram of an avoiding reaction, the basic pattern of behavior in
paramecia {Paramecium caiuiatiim) . A is the source of stimulation; 1-6 are suc-
cessive positions of the animal. The habitual rotation on the long axis of the body
is not shown. (Courtesy, Jennings: Behavior of Lower Organisms. New York,
Columbia Univ. Press, 1906.)
Chap. 21 THE PROTOZOANS 451
Nutrition. Paramecia live surrounded by swarms of bacteria. These are
swept into the oral groove and down the gullet by cilia that move so rapidly
that a microscopic stream of water seems to run through the protoplasm. For-
tunately for the Paramecium, food is not always pouring into it. Granules
containing enzymes form about the food vacuole as soon as the food creates it.
In the first stage of digestion, the content of the vacuole is acid and the micro-
organisms in it are killed. In the second stage, the granules swell and dissolve;
the content of the vacuole becomes alkaline; part of the food is dissolved and
absorbed in the protoplasm and the indigestible residue is squeezed along in a
regular circuit toward the anal pore. The vacuole disappears when its func-
tion ceases, but a successor appears in the same location as soon as more food
arrives.
Respiration, Water Content, Excretion. Oxygen is secured from the sur-
rounding water and carbon dioxide is given off into it. The fresh water that
surrounds the paramecia has a lower osmotic pressure than protoplasm and
therefore is continually diffused into them. This creates an income of oxygen,
but necessitates the outlet supplied by the contractile vacuoles or else the ani-
mal would burst. The vacuoles eliminate metabolic waste though they also
have the very important function of maintaining water balance just as the
kidneys of the frog do.
Mechanisms of Sensory-motor Functions. Complicated neuromotor systems
in paramecia and other ciliates have been demonstrated by special preparation
and high magnification. Beneath the pellicle each cilium originates in a
rounded base and these are connected with one another (Fig. 16.1). These
fibrils are associated with a latticelike network of fibrils surrounding the mouth
and gullet. It is probable that the fibrils are conductors and serve to coordinate
the cilia while ingesting food. Some of the fibrils are joined in a minute body
(motorium) located in the lattice. Destruction of this in the ciUate Euplotes
upsets the coordination of the animal.
Behavior. The behavior of a Paramecium consists of only a small number of
definite movements. By one, or another, or combinations of these few move-
ments, it responds to all the stimuli that act upon it. The basic pattern is that
of an avoiding reaction (Fig. 21.18). By means of it, the Paramecium rejects
one stimulus and accepts another. An avoiding reaction occurs immediately
after a stimulus such as contact with an object. The animal slows up, stops or
banks off, then moves in a different direction. In doing so it enters a new
place, comes upon different chemical, mechanical, and electrical stimuli, light
or temperature. The repetition of the avoiding reaction by trial and error
results in the rejection of some stimuli and acceptance of others. However
significant the process may be, it ends in a generally consistent choice of
favorable food and surroundings.
Reproduction — Conjugation and Sex. Paramecia reproduce asexually by
452 EVOLUTION OF ANIMALS Part V
transverse division with the macronucleus and one or more micronuclei, de-
pending on the species, leading the division (Fig. 21.19). The old oral groove
goes with the anterior half and a new one is formed in the posterior half. A
new contractile vacuole forms in each part. Under favorable conditions the
process takes about two hours. At temperatures of 15° to 17° C. the animals
grow rapidly to mature size and at the end of about 24 hours each one divides
again.
Conjugation is similar to fertilization, a mixture of nuclear materials from
two individuals thereby creating new hereditary combinations. Conjugation
rejuvenates the animals that take part in it, but it is not an essential process
and may not occur, Endomixis, the nuclear reorganization, that may take
place in single individuals, also rejuvenates the animals and stimulates division.
A description of conjugation is given in Chapter 18, Reproduction.
Class Suctoria
Adult suctorians have no cilia or other locomotor organs. Neither do they
have a mouth but take their food through tubular tentacles. The tips of these
tentacles are attached to other protozoans, thrust into their protoplasm which
then streams into the invader apparently by suction (Fig. 21.20).
Suctorians are common in fresh water and salt; many live as commensals,
in fresh water attached to such various objects as algae, and the shells of
turtles; and in salt water, to sea weeds and hydroids.
f. 2. 3.
Fig. 21.19. Division of Paramecium caudatum: 1, micronucleus beginning di-
vision; 2, macronucleus lengthening, micronucleus in mitosis; 3, nuclear division
continuing; cellular division beginning; 4, two animals of the next generation.
Paramecia multiply only by division. Occasionally there is a temporary union
(conjugation) with exchange of nuclear material followed by the division of each
of the partners. An individual's life span is the period between divisions; in the
natural plan a lifetime is ended by a division not by death.
THE PROTOZOANS
453
Chap. 21
Like ciliates, suctorians have two types of nuclei, larger and smaller and of
different function. The animals conjugate and all the embryos are ciliated.
Fig. 21.20. Suctorians. Podophyra fi.xa, com-
mon protozoans of fresh water. The tubular ten-
tacles are attached to a ciliate and the suctorian
sucks in the substance of its prey. (Courtesy, Jahn
and Jahn: The Protozoa. Dubuque, Iowa, Wm. C.
Brown and Co., 1949.)
Sucking tentacles
in use
Such similarities make it seem probable that suctorians were originally ciliates,
now greatly changed in structure and habits.
22
Sponges — A Side Line
or Evolution
Cellular Organization. Sponges are living waterways. Water is constantly
moving over them and into and out of them, continually flowing through the
labyrinth of canals and chambers which they contain. These countless water-
courses are keys to their liveUhood. Some sponges are radially symmetrical but
many more have fantastically irregular forms that are named after fancied
resemblances, dead men's fingers, Neptune's cup, and Venus's flower basket
(Fig. 22.4). Water is drawn through the microscopic pores that give the name
Porifera to the phylum and flows through the many passageways that are
unique among animals.
Protozoans are minute and unicellular while sponges are relatively large
and multicellular (Fig. 22.1). It is hard to find any other real difference be-
tween members of the two groups. The organization of sponges is relatively
simple, but the structure of the individual ceUs is complex and specialized.
Except for those that secrete the units of the skeleton, the cells carry on their
functions independently. Sponges have neither mouth nor digestive tract,
neither organs nor systems. There are no nerve cells or central controls as in
other multicellular animals, or as in some protozoans (Fig. 16.1). The skele-
ton is an outspread network of spicules or fibers. Except for the extensive
development of skeleton, a simple sponge resembles Proterospongia. In this
colonial protozoan the flagellated collared cells project from a blob of jelly in
which ameboid cells move about freely as they do in the jelly layer of sponges.
Although sponge cells are relatively independent, they are also deeply
cooperative in maintaining the entity of the sponge and they stay together as
the cells of young embryos do without any apparent binding. Sponges, like
early human embryos, are held together by the insistent cohesion of their cells.
Certain sponges may be pushed through a fine cloth and their cells separated,
454
Chap. 22
SPONGES A SIDE LINE OF EVOLUTION
455
Fig. 22.1. A cluster of common calcareous sponges that grew hanging from a
harbor wharf pile. The loosely branched one is Leucosolenia, each branch an in-
dividual sponge. The others are: (left) the crowned sponge, Sycon (Grantia), its
long fingers with crowned tips; and (top) a shapeless bread-crumb sponge, Hali-
chondria. (Photograph of living sponges by Douglas P. Wilson, Marine Biological
Laboratory, Plymouth, England.)
yet in favorable conditions they will come together and become perfectly reor-
ganized into their former shape (Fig. 22.9).
Sponges are undoubtedly multicellular animals. But in very ancient times
they drew away from the developments going on in other multicellular ani-
mals. In their early history they must have adopted a static existence, thor-
456
EVOLUTION OF ANIMALS
Part V
oughly adjusted and dependent upon the come and go of water, a sideline and
blind pocket in the trend of animal evolution (Fig. 33.1).
Structure. Leucosolenia, a simple sponge, illustrates the fundamental char-
acteristics of all the sponges (Fig. 22.2). Colonies of various species of
Leucosolenia grow just below the low tide mark. The body of each individual
is a sac whose open top is the excurrent opening or osculum. The current of
water that flows from this opening, carries particles outward and was the clue
by which Ellis in 1765 discovered that sponges are animals. Thousands of in-
current pores perforate the body wall, each one opening through a single pore
cell into the large central cavity or spongocoel (Figs. 22.2, 22.3). The outer
surface is covered with epithelial cells and flooded with mucous secretion that
hinders small animals from settling upon it. The central cavity is lined with
choanocytes or collared cells whose lashing flagella produce continual cur-
rents through the waterways of the sponge. Water enters through the incur-
rent pores bringing oxygen and microscopic particles of food with it. It passes
Osculum
Pore
Spicule
Water,
food
Spicule
Water, food
Outer covering
Middle layer
of jelly
B.
inner layer
collared cells
Fig. 22.2. A, a stage of a simple sponge with part of its wall cut away to reveal
the central cavity. This illustrates the fundamental characteristics of sponges. It is
a hollow vase with pores in its wall through which water and food enter a central
cavity. Water, waste and doubtless much food pass out through the main opening
(osculum). The intake and digestion of food is carried on by collared cells (choano-
cytes) that project into the cavity. B, a long section of the wall shows the lining of
the central cavity with its collared cells that catch particles of food, digest it, and
pass it on to the ameboid cells within the body wall. The spicules forming the
skeleton are each secreted by two cells that move inward from the outside layer.
The stage shown (known as Olynthus) occurs in the development of certain spicule-
bearing sponges. It is not a species. (Courtesy, Borradaile & Potts: The Inverte-
brata. Cambridge, England, The Macmillan Co., 1932.)
Chap. 22 SPONGES — a side line of evolution 457
out through the excurrent pore taking with it the various by-products of
metabolism. Each collared cell is a provider of food. Its flagellum brings in
the water that carries food; it captures and ingests the particles that the cur-
rent throws against it, and it partially digests them before they are passed on
to the ameboid cells that complete the process. Particles of solid waste are
eliminated from the various cells that perform the digestion. Each cell comes
very close to carrying on the whole process of nutrition essentially as it is in
the ameba, only a few degrees more specialized.
The outer cellular covering of the body and the lining of the spongocoel are
separated by a layer of clear jellied secretion, the mesenchyme (Fig. 22.3). It
contains the versatile ameboid cells that move about easily in the yielding jelly.
When in contact with the collared cells, certain of the ameboid ones receive
food particles from them and complete the digestion of these. Certain others
secrete the crystal clear spicules of calcium carbonate; others are often packed
with excretory inclusions and pigment granules; still other cells are filled with
food and evidently act as storage reserves.
Skeleton. Many people know sponges only as skeletons because the natural
sponges in general use are cleaned and bleached skeletons (Figs. 22.4, 22.5).
The skeleton is produced in the mesenchyme. It determines the shape of the
sponge, holds the water canals open, and is the support of the body. It is doubt-
Ascon
TYPES OF SPONGES
Leucon
Complex like bath sponge
Fig. 22.3. Body plans of three types of sponges: A, simple sponge; B, sponge
with folded wall; C, complex structure, e.g., bath sponge. Arrows denote currents
of water; short lines indicate flagella of the collared cells that line the food cham-
bers.
458
EVOLUTION OF ANIMALS
Part V
Fig. 22.4. Skeleton of the glass sponge, Euplectella, or Venus's flower basket
which has a skeleton of silicious spicules interwoven like basketwork. It is at-
tached by "glassy" fibers, in deep water. Common near the Philippine Islands.
Young shrimps often enter the basket and become permanently imprisoned there.
(Courtesy, American Museum of Natural History, New York.)
ful whether sponges could have attained their relatively large size without these
latticed frames.
The skeletons of calcareous and "glass sponges" are composed of different
material, but both are built of units called spicules. Spicules are secreted by
special ameboid cells, some of them by one cell, others by two or more to-
gether, a cooperation that is rare among sponge cells except in spicule pro-
duction. The secretion of a single shaft of spicule (monaxon) is begun within
a cell as a minute axial thread around which calcium carbonate is deposited.
Chap. 22 SPONGES — a side line of evolution 459
This cell divides into two as the process continues and when the spicule is com-
plete both cells move away. Spicules vary in shapes and sizes according to the
species. In general, they are elaborations of the single needle form. The most
beautiful spicules are the silicious ones composed of opal, a form of hydrated
silica. They are present, not only in the deep-sea glass sponges, but in the
fresh-water sponges, several of them very common (Figs. 22.6, 22.7). Bath
sponges contain interjoined fibers of spongin, a protein similar to that in hair
and feathers. The skeletons form an important basis for the classification of
sponges.
Reproduction. Sponges reproduce sexually as well as asexually. In sexual
reproduction, female cells are produced in one individual and male cells in
another. Both kinds develop in the mesenchyme from especially large ameboid
cells. The sperm cells enter other sponges, whether male or female, by way
of water currents, and in the females the eggs are fertilized in the locations
where the embryos develop. In Sycon (older name, Grantia) the egg takes
in food, enlarges and protrudes into a cavity lined with collared cells pushing
some of the food with it. During the breeding season the large numbers of
sperm cells freed from male sponges in a vicinity make it inevitable that many
of them are carried through the incurrent pores of sponges whether male or
female. When they are brought into the female they enter the collared cells
that are adjacent to the ripe eggs. In the meantime one or more of the cells
loses its collar and flagellum, becomes ameboid and applies itself to the sur-
16 cells
48 cells
hatching
stage
amphiblastula inversion
free-swimming
fixation
(seen in section)
Fig. 22.5. Development of a calcareous sponge, Sycandra: the ovum fertilized by
sperm from another sponge; the early embryo, 8, 16, and 48 cells, which is em-
bedded in the jellied middle layer (mesenchyme) of the parent's body wall; an
opening formed on the underside of the 48-celled stage functions as a mouth for
the embryo; blastula and beginning of hatching when the embryo makes its way
into the water passages of the parent; collared cells are already formed with flagella
extending into the blastocoel; amphiblastula: the embryo turns inside out by way
of an opening that first appeared in the 8-celled stage; the future upper end is up;
inversion; the future excurrent opening (osculum) is down; the larva is floating
in open water; fixation, compare Fig. 22.2. (After Schulze. Courtesy, Storer: Gen-
eral Zoology, ed. 2. New York, McGraw-Hill Book Co., 1951.)
460 EVOLUTION OF ANIMALS Part V
face of the egg. The sperm enters the modified choanocyte, its own shape
changed by the loss of its tail and capsule-like cover. It passes through the
choanocyte, enters the egg and fusion of the male and female nuclei finally
occurs.
After fertilization, the egg divides completely and at the 16-celled stage the
embryo is a disk-shaped cushion of cells (Fig. 22.5). The eight cells next to
the collared cells are the layout of the future outer cover or epidermis of the
sponge. The other eight cells are the future collared cells. The latter divide
rapidly and develop flagella. In this stage the embryo of calcareous sponges,
now a hollow sphere, makes its way into the water currents in the parent
sponge and is borne out of the excurrent opening as a free-swimming animal.
Later, the layer of collared cells bends inward and the epidermal layer grows
over it forming an outer sac around it. By this stage the young sponge has
attached itself to a rock or seaweed and settled down for its further develop-
ment and to a life of complete dependence upon the currents of the sea.
Sponges reproduce asexually by budding and branching somewhat after the
fashion of plants. This habit produces the familiar "fingers" of sponges, as in
B
Fig. 22.6. Fresh-water sponge. A, living Spongilla, spread over a stone. The out-
lines of the water canals are faintly visible. Spongilla often covers submerged twigs
and if exposed to sunlight is green with algae that grow within its cells. Bl , diagram
of a section of the wall of a fresh-water spicule-bearing sponge. B2, microscopic
spicules within the cells which formed them. Greatly enlarged. (Courtesy, Mor-
gan: Fieldbook of Ponds and Streams. New York, G. P. Putnam's Sons, 1930.)
Chap. 22 SPONGES — a side line of evolution 461
the eyed Finger Sponge {Chalina oculata) of the Atlantic coast and Leu-
cosolenia eleanor of the Pacific coast. Buds broken off and carried by currents
established themselves in protected crannies and other places, such as wharf
pilings and the backs of cjrabs.
Fresh-water sponges are mainly annual growths that die out in autumn ex-
cept for the gemmules that can resist both drought and cold. These winter
over in safety and germinate into young sponges in the spring (Figs. 22.7,
22.8). A gemmule is a ball of foodfilled ameboid cells and mesenchyme
enclosed within a capsule. The outer wall is pierced by a minute outlet through
which the growing sponge spreads forth. In autumn, the flourishing summer
colonies of Spongilla are reduced to thousands of spicules sticking to the rock
with many gemmules appearing like fig seeds packed among them.
All animals, especially invertebrates, have some power to replace lost or
injured parts. With their relatively simple organization, sponges have a great
capacity for these processes of regeneration even to the extent of a complete
rearrangement of their parts after they are separated. When certain sponges
are pushed through silk bolting cloth, their cells are nearly all separated from
one another. If the redbeard sponge, Microciona, is thus treated and its cells
allowed to fall into a large flat dish of sea water, they will spread and the
solution soon resembles tomato soup. The amebocytes immediately begin
random movements and certain of them become centers about which special
food-carrying amebocytes congregate (Fig. 22.9). Collared cells that have
been injured regrow their collars and take their proper places as living cells of
Fig. 22.7. Spongilla and other fresh-water sponges frequently overwinter as
gemmules which resist cold and drying. Held among the spicules of the summer
colony they look like fig seeds caught in the meshes of torn lace.
462 EVOLUTION OF ANIMALS Part V
the flagellate chambers. A considerable bulk of sponge should be put through
the cloth. There must be a sufficient number of cells, especially food-carrying
amebocytes, or regeneration will not occur. Collared cells will not collect
Fig. 22.8. Germinating gemmules of Spongilla.
The young colonies have surrounded the capsules of
the gemmules from which they grew. Readily reared
on glass.
except about amebocytes. If bodies of two different species of sponges are put
through the bolting cloth, their cells may at first intermingle, but soon those of
each kind congregate by themselves.
Fresh-water Sponges
All fresh-water sponges are classified in the family Spongillidae, of which
there are about 20 American species (Fig. 22.6). They grow in clean water, in
ponds, lakes, and streams, upon stones, the undersides of lily pads, and sub-
merged stems and sticks. When they are in full light they are often colored
green by Zoochlorellae, the unicellular algae, within their cells. Many are
annual growths, germinating from gemmules in the spring, reaching full size
in mid-summer and dying away toward autumn except for the new crop of
gemmules (Fig. 22.7). Fresh-water sponges are inhabited by a few minute
residents, not large or as numerous as those that live in marine sponges. Among
them are the larvae of Spongilla flies that puncture the sponge cells and suck
up the protoplasm. They are about a quarter of an inch long and match the
sponge color exactly. The best way to find them is to watch for what appear to
be bits of sponge moving about through the sponge colony. Compared with
marine sponges fresh-water ones are small, scanty growths. Nevertheless, in
reservoirs they may spread through the water pipes unless the water is chemi-
cally treated. This has occurred in the water systems of more than one large
city.
Chap. 22
SPONGES — A SIDE LINE OF EVOLUTION
SCATTERED SPONGE CELLS REORGANIZE
463
A. Red sponge
Cells reunite when
pressed apart.
3 Scattered cells
Ameboid cells:
collared cells ttiot
will regain collars.
C. Ameboid cells move about,
make contacts witti ottier cells.
D. Groups of cells ore reformed.
Fig. 22.9. Regeneration of the redbeard sponge, Microciona prolifera. A, its
natural growth; B, cells of the living sponge after it was broken up by being
pressed through a fine cloth strainer; C, random movement of an ameboid cell
(archeocyte) observed two and a half hours; D, the amebocytes have begun the
reorganization which continues until the canals and chambers are reformed and
new spicules are produced. (C, courtesy, Galtsoff, Jour. Exper. ZooL, 42:197.)
Marine Sponges
Marine sponges are notable for their characteristics and biological and
economic importance (Fig. 22.10).
Uses. The absorbent quality of sponges has long been known. Roman
soldiers carried sponges with them to use for drinking cups. The era of bath
sponges was followed by one of sponges for automobiles, and both have been
displaced by plastic sponges. Needless to say, the sponge industry has been
464 EVOLUTION OF ANIMALS Part V
oscula
mBm^
Fig. 22.10. Commercial sponge, Hippospongia, a typical sponge of commerce.
When cut open, living sponge looks like raw liver. The chief American region for
sponges is the west coast of Florida, centering at Tarpon Springs. The skeletons
of commercial sponges are composed entirely of spongin fibers that are horny and
elastic. The preparation for market consists of removing all soft matter and bleach-
ing the skeleton. (Courtesy, Brown: Invertebrate Types. New York, John Wiley
and Sons, 1950.)
greatly reduced. The sponge-fishing grounds of Florida and the Bahamas have
been overfished and sponges are subject to diseases which occasionally reduce
the growth for long periods.
Comparisons with Other Phyla
Likenesses
Simple sponges resemble the colonial protozoan, Proterospongia.
Collared cells of sponges are similar to those in Proterospongia and some
other protozoans. They occur nowhere else among animals.
The wandering amebocytes of the mesenchyme of sponges are similar in
habit and form to amebas.
The tube-shaped body, the colonial habit and attached state of sponges are
suggestive of the corals (Phylum Coelenterata).
Differences
The characteristic spicules of sponges are different from skeletal structures
in any other animals. Sponges differ from protozoans in that their cells are
more dependent upon one another than the cells of colonial protozoans. They
differ from other multicellular animals in that their cells are less dependent
upon one another.
23
Coelenterates — Simple
Multicellular Animals
Clusters of orange and yellow sea anemones, colonies of pink hydroids and
plumy sea pens well deserve their old-time names of "plant-animals" and
"gardens of the sea." No marine animals have such translucent beauty as the
coelenterates. Nor have any truly multicellular ones so long a lineage — at least
five hundred million years. They are direct descendants of the protozoans and
are the ancestors of all multicellular animals (Fig. 33.1). Sponges are also
directly descended from protozoans, but they long ago became set apart on
an offshoot of evolution.
The two basic forms of coelenterates are the polyp and the medusa. The
polyp has a cylindrical body and, in its more typical condition, has one end
that bears the tentacles and mouth and another end attached to a surface or
joined to a colony. Hydra is a polyp; so are the sea anemones and corals. The
medusa or jellyfish has an umbrella- or bell-shaped body, is usually free-
swimming, and bears the sex cells (Fig. 23.1).
Ecology. Almost all coelenterates are marine. Of about 10,000 species only
a few, mainly the hydras, live in fresh water. Coelenterates are widespread and
abundant, chiefly in surface waters and between the tide lines. Jellyfishes
thrive in sheltered coves rich in organic matter. They are carried about by
currents, great numbers of them often suddenly appearing in harbors and the
shallows along bathing beaches. There they feed heavily upon the swarms of
minute crustaceans (copepods) that become sparse soon after the jellyfishes
move in. Bathers know jellyfishes as sea nettles.
Sea anemones and corals are numerous and colorful in warm seas. A few
inconspicuous ones occur along the more northern Atlantic and Pacific coasts
of the United States. Sea anemones cling tightly to rocks and wharf pilings.
When the tide is out, they draw their tentacles in and their bodies down almost
465
466 EVOLUTION OF ANIMALS Part V
hidden against the rock surfaces and seaweeds. Corals do not flourish in water
below 66° F. or in the deep sea. The white coral Astrangia, wiiich lives on the
Atlantic coast as far north as Massachusetts, grows only in small colonies,
never in the lush growths of the corals of tropical waters (Fig. 23.1 ). Except
for corals, coelenterates of one kind or another are at home from the far north
to the equator; the giant or pink jellyfish, Cyanea capillata, of the Atlantic
is common about Greenland.
Food. All coelenterates are carnivores. Drifting jellyfishes are surrounded
by protozoans, entomostracans and numberless young larvae which they con-
stantly consume. To the sessile hydroids, sea anemones and corals, the tides
daily bring fresh supplies swept from the bottom and deeper waters off shore.
Appearance and Size. Coelenterates occur in great variety. Branching colo-
nies of little hydroids are attached like plants upon the seaweeds. Jellyfishes
are bell- or umbrella-shaped and there are sea anemones more delicate and
colorful than their namesakes. The common names of corals are descriptive
of their forms — sea pen, sea fan, organ pipe, staghorn, brain, and mushroom.
Some jellyfishes are as transparent and colorless as crystal; others are trans-
lucent brown, deep red, yellow, lavender, or milky white. Colonies of the
hydroid, Tubularia crocea, common on the Atlantic coast, are rose-pink;
polyps of the organ pipe coral, Tubipora, have bright green tentacles and the
limy pipes of their skeletons are red. The Portuguese man-of-war floats on the
sea like a great opal, one of the most beautiful of marine animals.
Hydroid polyps are usually very small, often microscopic, but colonies of
them extend over bands of seaweed for 50 yards or more. The diameter of
jellyfishes ranges from an inch, to 8 feet in the great pink Cyanea. Likewise,
sea anemones range from little ones with oral disks half an inch wide to giants
with a five-foot span. Although individual coral polyps are minute, the count-
less numbers of them in the colonies have built thousands of miles of coral
reefs and islands.
Characteristics. Coelenterates are radially symmetrical and without head or
segmentation. The body is composed of two layers of cells, the external epi-
dermis or ectoderm and inner gastrodermis or endoderm, with a middle layer
of jellied mesoglea between them. Unique stinging cells containing the nemato-
cysts occur in one or both layers. The mouth, surrounded by soft tentacles,
opens into a saclike digestive cavity, the enteron, that may be branched or
divided by partial partitions and has no other opening. The skeleton is limy,
horny, or absent. There are no blood, respiratory, or excretory organs. A net-
work of nerve cells conducts messages through the body wall. Reproduction is
commonly by alternation of generations, with asexual budding from attached
polyps (hydralike) and with sexual reproduction by sex cells in the free-
swimming medusa (jellyfish) stage.
Classes of Coelenterates. Hydrozoa. These are the little hydroids that grow
CLASS HYDROZOA
Portuguese Mon-of-War Velella
CLASS ANTHOZOA
Sea Anemone
CLASS ANTHOZOA
Sea Fan Sea Pen
CLASS ANTHOZOA
Astrongia
Fig. 23.1. Representatives of the three classes of coelenterates, all are greatly
reduced but not to the same scale. These or nearly related species live in both At-
lantic and Pacific coastal waters. Class Hydrozoa, hydroids: Eudendrium, colony
of polyps, 5 inches high; Gonionemus, medusa, % inch in diameter, cosmopolitan;
Physalia, Portuguese man-of-war, colony of polyps beneath a gas float, 10 inches
long, tentacles up to 50 feet long when fully extended, float 6 to 8 inches in diame-
ter; Velella, the "little sail" of the California coast. Class Scyphozoa, jellyfishes:
Aurelia. Class Anthozoa, sea anemones and corals: Metridium, brown anemone,
length to 4 inches; Astrangia, white coral, colonies of 5 to 30 individuals, 10 inches
diameter, Florida to Cape Cod; Gorgonia, sea fan, a colony of horny corals, in
warm waters on coral reefs; sea pen, a colony of fleshy polyps, warm coastal waters.
467
468 EVOLUTION OF ANIMALS Part V
in tufts on rocks and seaweeds (Fig. 23.1) and the hydrocorallines, among
them the "stinging corals." The class also includes the Siphonophora, the
Portuguese man-of-war, and others that live in the open sea and have no
sessile stage.
ScYPHozoA. Larger medusae or jellyfishes with notches in the margin of
the umbrella, as in the common jellyfish, Aurelia.
Anthozoa. These are either solitary or colonial coelenterates, with a great
development of the polyp and no medusoid stage. Figure 23.1 suggests the
form of the sea anemones, the brown anemone, Metridium, and the true corals.
Hydra — A Representative of Simple Multicellular Animals
Hydra is a link between older and newer ways of living. It digests its food
partly by the old method of the ameba, partly by the newer methods of the
grasshopper, frog, and man. Many of its characteristics are like those of higher
animals, but simpler.
All hydras live in fresh water. They look like bits of coarse thread frayed
out at one end, are semi-transparent and, except the green ones, are almost
colorless. Their movements are visible to the naked eye and they are easily
examined with the microscope. They are also common, widely distributed, and
easily kept in aquaria. Only when they are undisturbed in considerable space
do they display the deliberate grace of their searching tentacles and their sud-
den capture of minute water animals.
Ecology. Hydras live in sunlit pools, hanging from submerged plants and
decayed vegetation. With the help of a gas bubble at the base of the body they
are often buoyed up against the underside of the surface film (Fig. 23.2),
Enormous numbers occasionally appear in lakes as they have done at Douglas
Lake, Michigan, when the seines spread for fishes have been weighed down
by the millions of hydras clinging to them. Under certain peculiar conditions,
they may turn red, especially toward fall, and large patches of pond surface
may be colored by them. Other aquatic organisms do this; the redness of blue-
green algae gave the Red Sea its name.
Hydras reach their full activity in summer and then they frequently produce
buds asexually. They usually reproduce sexually toward the end of the season
on a lowering temperature, down to 50° F. They make a definite adjustment
to winter temperatures. Brown and green hydras collected in winter from ponds
in which the temperatures were 46° to 56° F. and placed in pond water at
35° F. contracted into balls and stayed so for two weeks, as long as the water
was kept at the same degree of cold. When it was warmed to 46° F. they
stretched out and began feeding. Active and semiactive hydras are certainly
not confined to summer conditions. Various species with flourishing growths
of buds have been found thriving beneath the ice.
Food. Hydras are carnivores that forage freely on protozoans and crusta-
Chap. 23 COELENTERATES SIMPLE MULTICELLULAR ANIMALS
469
Fig. 23.2. Hydras in natural positions on water plants and buoyed up beneath
the surface film. They swing and stretch downward like pieces of elastic thread
frayed out at their ends.
ceans. They are avid feeders commonly swallowing fingernail clams and eject-
ing the shells after the soft bodies have been digested. Attached to the side of
an aquarium they hang outward slowly swaying their bodies through the water
with their tentacles trailing. Let a water flea graze one tentacle and it instantly
shortens, carrying the water flea toward the hydra's mouth while the other
tentacles join in paralyzing the victim. The body soon bulges with the water
flea whose movements grow feebler as the digestive enzymes begin to work on
it (Figs. 23.3, 23.4).
Common Species. Of the eight species of hydra known in North America,
three are widely distributed and common. The green hydra, Chlorohydra
viridissima, owes its brilliant color to the single-celled algae called zoochlo-
rellae which live within the inner cells of its body. They are thus protected
and, during photosynthesis, they use the carbon dioxide that they and the
hydra give off in respiration (Fig. 23.4). Two other species are the gray hydra.
Hydra americana, in the eastern United States, with short tentacles and no
stalk to its body, and the brown hydra, Pelmatohydra oUgactis {Hydra fusca),
with a basal stalk and tentacles which stretch three or four times the length of
body and stalk combined. Pale-colored hydras are larger, more translucent
and better for study than the green ones.
Fresh-water jellyfishes or medusae (Craspedacusta) have bells about half
an inch in diameter. They are rare yet occasionally occur in large numbers as
they did in Gardiner's Lake, Connecticut, in the summer of 1952.
The following account of hydra applies to most of the species.
470
EVOLUTION OF ANIMALS
Part V
Movements and Locomotion of Hydra. When they are searching for food
hydras sway their tentacles and stretch them gently in all directions. They
move from place to place, imperceptibly by gliding upon their bases, some-
times by turning somersaults (Fig. 23.5). Such end-over-end steps are re-
peated again and again. Green hydras move about more than other species;
^
Fig. 23.3. A hydra which has caught
and swallowed a "full meal" of water flea.
Sketched from life.
the brown and gray ones will attach themselves and sway or hang almost
motionless in one place for long periods.
Responses and Coordination of Behavior. Hydras react to mechanical con-
tacts, light, electricity, and chemical solutions. The firmness of their attachment
to the side of an aquarium as they swing out in the water regardless of gravity
is an example of their reaction to contact. They respond to the slight current
created by a passing water flea with the simultaneous contraction of the ten-
tacles and the body, showing how quickly the reaction spreads through the
animal. In unevenly lighted jars, hydras will retreat from the dark areas as
well as from the strongly lighted ones moving about until by trial and error
they finally reach their optimum degree of light. All of these responses may be
affected by some special physiological state of the animal.
Form and Structure. General Plan. The radial symmetry of hydra is at
once conspicuous in the arrangement of the tentacles (Fig. 23.3). It has a
distinct oral end with some of the characteristics of a head. The other end
functions as the base by which it is attached and on which it glides about. One
end of hydra is permanently different from the other, a foreshadowing of the
polarity so evident in higher animals. The front end of an ameba is distinguish-
able mainly by the fact that it is forward during locomotion. Hydra's bodily
Chap. 23 COELENTERATES — SIMPLE MULTICELLULAR ANIMALS
471
Ingestion
Egesfion
■, Food
'^A It
• «
Excrefion
Respirafion
Oxygen
S^ Urea and other
P\ products of katobolism
token in
< —
Carbon dioxide
given off
Food vacuole
intracellular- -
digestion
•^Pi given off
:^\^j- — — Ingested food
ly^ mass
■^n^ — Extracellular
■^ digestion
3 '"""""" Absorption of
2 digested materials
wrnriin^
^
Fig. 23.4. Hydra, its general metabolism. Excretion is carried on by all cells;
exchange of respiratory gases likewise. (Courtesy, Mavor: General Biology, ed. 3.
New York, The Macmillan Co., 1947.)
^-
"^■3
'^-
■a;
'^■S-
Fig. 23.5. The more rapid ways by which hydras travel. Figures 1-4, by loop-
ing; 5-9 by somersaults. Drawings from work of Abraham Trembley ( 1700-1784),
a pioneer in the study of hydra and of experimental zoology. (From Trembley:
Memoires pour I'Histoire des Polypes. Leyden, Jean and Herman, 1744.)
472
EVOLUTION OF ANIMALS
Part V
tjewafocysts
Tenfvicl&
Mesogle^^
Younger bud
Older
bud
Testis
Gland cell
Ectoderm
■Flagellated cell
Ovary
£gg-cell
•Basal disc
Fig. 23.6. A long section of hydra with the bud of asexual reproduction and the
male and female organs. Such a composite is unusual; in the majority of species
the sexes are separate. (Courtesy, Wolcott: Animal Biology, ed. 3. New York,
McGraw-Hill Book Co., 1946.)
Cuticle
Entoderm
functions occur in exact places. Food enters through the mouth and nowhere
else. Stinging cells are most abundant on the tentacles which grapple the prey.
Nerve cells are most numerous near the mouth, the usual locality for a brain.
Like all coelenterates, hydra contains a single cavity called the enteron,
coelenteron, or gastrovascular cavity. Its one opening functions as an entrance
and exit for food, water, and waste (Figs. 23.4, 23.6). In all hydras, the
gastrovascular cavity is continuous into the tentacles, but is not so in the corals
and other hydroids. The body wall enclosing the cavity consists of the three
layers already mentioned, the epidermis (ectoderm), the lining of the enteron,
(endoderm), and the extremely thin gelatin-like mesoglea (Fig. 23.6). In
jellyfishes, mesoglea forms the bulk of the body and contains fibers and cells
Chap. 23 COELENTERATES SIMPLE MULTICELLULAR ANIMALS 473
which move into it from the true cell layers; in sea anemones, it is a tough
fibrous tissue.
Epidermis. The epidermis is composed of epithelial tissue containing sup-
porting cells, epithelio-muscular, glandular-muscular and glandular cells, sen-
sory nerve cells, formative and stinging cells (Fig. 23.8). The supporting cells
protect and support other cells. The outer ends of the epithelio-muscular cells
are likewise protective but their inner ends are drawn out into contractile
strands which extend along the mesoglea lengthwise of the body. When these
strands contract, the tentacles and body shorten and widen. Glandular cells
are crowded about the mouth and in the basal disk along with epithelio-mus-
cular cells. Hydras attach themselves to objects by means of a sticky secretion
and the contraction of epithelio-muscular cells. Gas is also secreted in the basal
region; a bubble of it caught in the mucus often buoys an animal up beneath
the surface film (Fig. 23.2).
The neurosensory cells reach to or near the outer surface and their processes
extend to the nerve plexus close to the mesoglea. These are the receptors of
touch and other stimuli, called neurosensory cells because they look so much
like nerve cells. Cells of the nerve plexus or "net" rest against the processes of
the epithelio-muscular cells (Fig. 23.7). The neurosensory, nerve and epi-
B. Nerve cells
Hydra
A. Nerve cells
Sea anemone
Fig. 23.7. Nerve cells. A, sea anemone. A layer of nerve cells from the oral
disk, more elaborate but similar to the layer of nerve cells in the body wall of
hydra. Note the lack of continuity of the cells. B, hydra, part of the ring of nerve
cells in the base (pedal disk) of the body. Note that these cells are not regularly
continuous. (Courtesy, Hyman: The Invertebrates, vol. 1. New York, McGraw-
Hill Book Co., 1940.)
474 EVOLUTION OF ANIMALS Part V
thelio-muscular cells equip hydra to respond to its environment. The nerve
plexus acts as a unit and the impulses appear to travel in either direction over
a given process, as in a telephone conversation the speaking goes first one way
and then the other on the same wire. In higher animals the incoming and out-
going impulses travel on different pathways. There is a concentration of the
plexus about the mouth which suggests the more prominent nerve ring around
the mouth of a starfish. Investigators have shown that in hydra the processes
of different nerve cells may touch but are not continuous. Thus, there is a
synapse, a break over which the nerve impulse jumps from one cell to another
as in higher animals.
Formative (or interstitial cells) are small cells wedged in between those of
the epidermis and gastrodermis, the lining of the enteron. They behave like
embryonic cells, still capable of developing into something different; some of
them become sex cells, many become stinging cells (Fig. 23.8). In the human
bone marrow, there are embryonic cells that differentiate throughout life into
specialized blood cells.
A stinging cell (cnidoblast) is one that forms within itself the nonliving
mechanism called a nematocyst (Fig. 23.8). In the epidermis, mature stinging
cells occur close to the outer surface, are numerous on the body, and abundant
on the tentacles. Nematocysts are microscopic harpoons expelled from the
stinging cells against the hydra's prey and enemies. They are the unique sting-
ing mechanism of coelenterates. Each one carries a charge of poison. Those
of hydra are harmless except to minute animals, but the stings of larger jelly-
fishes and the Portuguese man-of-war are very painful. The fully formed
nematocyst is a transparent capsule containing a minute coil usually termed a
thread, shown to be a tube in some species and believed to be so in all. Poison
is secreted by the stinging cell and is in some way carried by the nematocyst
when the latter is discharged. One side of the cell is ordinarily exposed and
the triggerlike cnidocil that projects from it is supersensitive to stimulation.
Stinging cells respond directly to stimuli. The threads of some nematocysts
pierce their prey (Fig. 23. 8C). There are four kinds of stinging cells each of
slightly different structure, usually not visible except by special preparation.
The expulsion of nematocysts is too sudden to be clearly observed. As the
tentacle of a living hydra is viewed through the microscope, they can be seen
each with a thread coiled within the capsule. When the tentacle is stimulated
by pressure or by weak acid, they are instantly expelled and the capsules lie
outside the tentacle with their threads uncoiled. It is believed that before ex-
pulsion the threadlike tube is inverted in the capsule like a glove-finger pulled
inward. When the nematocyst is expelled the tube is rapidly everted by the
pressure on the capsule as it is shot out of the cell.
Stinging cells are wandering cells. Many of them migrate from the epi-
dermis, across the mesoglea, go through the gastrodermis into the enteron and
Chap. 23 COELENTERATES SIMPLE MULTICELLULAR ANIMALS
475
A
Tentacle
Cnidoci
(sensitive
D
Nematocys
(stinging so
Nucleus
Fig. 23.8. The stinging capsules (nematocysts) of hydra. A, a bit of tentacle
magnified to reveal the batteries of stinging capsules. B, tail bristle of Cyclops with
the stinging capsules thrown upon it during its capture by hydra. C, Cyclops, a
favorite food of hydra, is only a white speck to the naked eye. Note its single eye,
the eggs it carries and the tail bristles. D, a stinging capsule highly magnified
within the cell that formed it. E, the stinging cell with the thread unloosed and
poison discharged. (A and B, courtesy, Hyman: The Invertebrates, vol. 1. New
York, McGraw-Hill Book Co., 1940. D and E, after Schneider. Courtesy, Dahl-
gren and Kepner: Animal Histology. New York, The Macmillan Co., 1908.)
enter the cell layers at some other point. Wherever they enter they finally
lodge in the ectoderm. Not all of them migrate; some remain in the ectoderm
where they developed. Their structures and functions are entirely different
from those of the wandering cells (macrophages) of mammals that pick up
foreign substances in the human body, yet both types move about in similar
ways. Each illustrates the flexibility of form and function that is highly charac-
teristic of living matter.
Endoderm. The gastrodermis of the enteron and its extensions in the ten-
tacles is in general similar to the epidermis. It is composed of epithelial tissue
and contains glandular, sensory, and nerve cells — the latter less frequent than
in the epidermis. There are fewer formative cells and no stinging cells except
those that migrate into it (Figs. 23.6, 23.8).
Nutritive muscular cells are the predominant cells of the gastrodermis. Their
bases are extended into muscular processes which run in a circular direction
opposite to the processes in the epidermis but like them rest against the
476 EVOLUTION OF ANIMALS Part V
mesoglea. Their contraction makes the body and tentacles more slender and
stretches the comparable processes in the epidermis. Their bases are special-
ized for movement and their inner ends contain vacuoles usually filled with
particles of food that has been partly digested in the enteron. Glandular cells
are abundant about the mouth and in the gastrodermis. In hydra, and more
evidently in sea anemones, the cells near the mouth produce mucus. A slippery
surface must ease the slide of a struggling water flea into the "stomach" (Fig.
23.3). The glandular cells also secrete digestive enzymes.
Mesoglea. In hydra, mesoglea is noncellular and so thin that in stained sec-
tions of the body it appears only as a dark line. This is far from true in jelly-
fishes whose bulk and shape are largely due to their mesoglea, but when they
are washed up on the beaches and the water evaporates only papery wisps
remain.
Digestion, Respiration, and Excretion. Food is brought to the mouth by the
tentacles and drawn into it by contractions of the body. It is partly digested
in the enteron by enzymes which reduce it to a semifluid. Any partly digested
particles of food which remain are then engulfed by the nutritive muscular
cells and digestion is completed within them. Thus hydra employs two methods
of digestion, an extracellular one like that of higher animals, and an intra-
cellular one like that of an ameba. Finally, the completely digested food is
absorbed through the cell membranes and passed on from one cell to another.
Indigestible wastes are ejected through the mouth.
There is no special "breathing mechanism" in hydra. The cells take oxygen
from the water or from one another and give off carbon dioxide likewise.
There is no transporting fluid such as the blood, and no need of it since the
body wall is thin and there is no body cavity to separate the digestive tract
from the outer cells. Individual cells eliminate nitrogenous waste but have no
contractile vacuoles or other special means of doing so.
Reproduction. Hydras reproduce asexually by budding or under unusual
conditions, by transverse division of the body, and sexually by the fusion of
male and female sex cells (Fig. 23.6).
The buds develop near the junction of the enteron and stalk, when the latter
is present. In dioecious (separate sexed) species the individuals produced
from buds have the same sex as their parent. In a well-fed hydra, a bud will
form and separate from the parent within two or three days. Before it separates
there is a free passageway between the enterons of the parent and bud, and
food swallowed by the parent may be absorbed by the buds.
The testes and ovaries develop from formative cells in the ectoderm. During
the maturation of the sex cells the number of chromosomes in each one is
reduced by half (meiosis) . When the sex cells are brought together the chromo-
some number is returned to that of the body cells.
In the ovary, formative cells are absorbed by the future egg until it becomes
Chap. 23 COELENTERATES SIMPLE MULTICELLULAR ANIMALS 477
a large food-filled cell. It is soft and irregularly shaped, with outspreading
processes which are withdrawn as the egg matures. Sperm cells swimming free
in the water go through the thin cellular sac enclosing the egg and fertilization
occurs while the egg is still- attached to the parent. The now one-celled embryo
divides many times and becomes a hollow sphere of cells (blastula), then a
double layered sac (gastrula), in the meantime slipping out of its protective
sac. The embryo secretes a capsule in which the embryo may remain dormant
for several months. There is no evidence, however, that the eggs have a definite
resting period or that they are latent over the winter except as low temperature
slows down the development of those produced in the fall.
Regeneration and Grafting. Like other coelenterates, hydras can replace
lost parts. If one is cut transversely and the parts are kept in good conditions,
a new basal piece will grow on the one bearing tentacles and a new set of
tentacles on the basal piece. Or if a central part of the body is removed it will
grow new oral and basal ends in their original relationships. If properly fed,
hydras will regain the full size of a lost part within a few days. Regeneration
follows a variety of cuts (Fig. 23.9).
Studies of Hydra. Aristotle knew that coelenterates could sting, thought
they looked like plants, and named them zoophyta along with other soft-bodied
animals. This name stayed with them for several hundred years.
Regeneration in animals was first described in hydra. In 1744 Abraham
Trembley (1700-1784) made a thorough study of hydras and published a
Regeneration of bodily portion
Regeneration of cut anterior end
Fig. 23.9. Regeneration of hydras. A, successive stages in regeneration of a
piece cut from the mid-region of body. B, regrowth of parts of heads — a five-
headed animal from original single head. (Courtesy, Fasten: Introduction to Gen-
eral Zoology. Boston, Ginn and Co., 1941.)
478 EVOLUTION OF ANIMALS Part V
monograph, "Polypes d'eau douce." He described them as animals, portrayed
their locomotion, and gave accounts of his experiments upon them. He dis-
covered that if one were cut into two, three, or four pieces, each piece would
form a new animal; and if the oral end of one were split it would form a two-
headed animal (Fig. 23.9). Hydra has continued to be a subject of experi-
mentation and R. L. Roudabush (1933) turned hydras inside out as Trembley
did. The striking result of the later experiments was the migration of cells,
discovered by studying sections of the animals killed and fixed at periods of
10 minutes, 2 hours, and 24 hours after they had been turned inside out. They
showed: the epidermis on the inside and the gastrodermis on the outside as
they had been turned in the experiment; and later also the cells of the gastro-
dermis in migration toward the inside and those of epidermis toward the out-
side; and finally, cells of the two layers in position as they were before the
experiment.
Grafting. Trembley's grafting experiments were the first of many others.
Pieces of different hydras, even those of different species, have been grafted
together. Pieces may be too small to regenerate but will fuse and grow, the
ectoderm joining with ectoderm and endoderm with endoderm.
The Invertebrates. Protozoa through Ctenophora, by L. H. Hyman, con-
tains an unequaled store of knowledge about coelenterates, including the re-
sults of the author's own extensive work on the hydras and comprehensive lists
of references.
Class Hydrozoa
The hydrozoans most frequently studied are the solitary polyp. Hydra, the
colonial hydroid, Obelia, and the hydrozoan medusa, Gonionemus. In Hydra,
only the polyp form occurs. In many hydrozoans, however, both polyp and
medusa are well developed as in Obelia (Figs. 23.10, 23.11). In Gonionemus,
the polyp is minute and rarely recognized while the medusa is well known (Fig.
23.12). Hydrozoans exhibit an extraordinary degree of division of labor, in
which different functions are performed by different kinds of individuals of
the same species, as in the Portuguese man-of-war instead of by different or-
gans in the same individual (Fig. 23.13).
Obelia — A Colonial Hydrozoan. In the fully developed colony, there are
three types of individuals: hydranths, the feeding polyps with mouths and ten-
tacles; gonangia, modified polyps that produce medusae and lack mouths and
tentacles; medusae or jellyfishes that arise as buds from the gonangia and
grow to sexual maturity as free swimming male or female individuals (Fig.
23.10). In Obelia, the sex cells are shed in the open water where the eggs are
fertilized. In some hydroids, the sperm cells are shed and the eggs are fertilized
while the medusa is still attached. In its complete cycle, the life of Obelia in-
cludes an alternation of generations. One generation, the colony, is asexual and
Chap. 23 COELENTERATES SIMPLE MULTICELLULAR ANIMALS
479
HYORANTH
TENTACLES
MOUTH
PLANULA
THE ASEXUAL GENERATION
Fig. 23.10. The structure and life cycle of Obelia, a marine colonial hydroid.
The mature colony is about one inch high with swollen joints from which the
branches, vegetative and reproductive individuals, are given off alternately. A, a
mature colony, the asexual generation. B, the minute jellyfish or medusa (greatly
enlarged), a free swimming sexual individual, of which there are males and females
developed in different colonies. C, section through a vegetative individual (hy-
dranth) showing the gastrovascular cavity that extends throughout the colony. D,
the early development of a colony, from the fertilized egg, through the free swim-
ming ciliated planula, to the young attached colony. (Courtesy, MacDougall and
Hegner: Biology. New York, McGraw-Hill Book Co., 1943.)
produces new individuals (feeding and reproductive polyps) by budding. The
next generation, medusae, is sexual and by the fusion of sex cells produces the
first polyp of a new colony. In this cycle, an individual is the image of its grand-
parents but looks like a stranger to its parents.
Habitats. Various species of Obelia live on both coasts of North America.
Colonies of them, an inch or two high, grow by millions on the long ribbons of
kelp and other seaweeds. Attached and branched as they are, even a good
observer might well take them for plants just as Aristotle did. They can be
examined satisfactorily only with a strong lens.
The Colony — Its Form and Way of Living. The colony is held fast by
the root-shaped hydrorhiza and from this springs the upright branch (hydro-
480
EVOLUTION OF ANIMALS
Part V
caulus) that forms the main stalk of the colony (Fig. 23.10). The hydrorhiza
extends along the seaweed with many colonies springing from it.
The tentacles of the feeding polyps are armed with stinging cells (Fig. 23.8).
These and the persisting motion of its tentacles are the hydranth's equipment
for catching the minute animals which swarm through the surrounding water.
The reproductive polyps (gonangia) bear the medusae which bud off from its
stalk much as buds of hydra develop from its body.
The bodies of Obelia and Hydra are essentially similar. As in hydra, the
body wall is composed of two cellular layers, epidermis and the thin layer of
Hydra
Ectoderm
•^^ Mesoglea
Endoderm
Jellyfish
a medusa
Sea anemone
Fig. 23.1 1. Ground plans of the three main forms of coelenterates: the hydroid
polyp, the medusa or jellyfish, and the polyp of the anemone, are constructed on
the same general plan. The mouths of hydra and sea anemone are held upward;
a jellyfish swims with mouth down.
mesoglea. In Obelia, the body is encased in a transparent casing (perisarc),
complete except at the tips of the polyps. The digestive processes are essentially
the same as those of hydra. The gastrovascular cavity is continuous throughout
the stalks and branches of the colony and food is shared by the community.
Lively protozoans are swept into the mouths of feeding polyps, moved along
while still in tremors through the enteron, and gradually digested and absorbed
along the way.
The Medusa — Its Form and Way of Living. Medusae are the sexual links
in the hydrozoan life cycle. Medusae are specialized individuals devoted to
reproduction in contrast to the ovaries and testes of higher animals which are
only organs of reproduction. Hydrozoan medusae are always small, and those
of Obelia are minute. They live in tide pools and shallows, swimming about
by vigorous contractions of their umbrellas. But they are powerless against
currents and are carried into harbors in enormous numbers though they are
so small that they go unnoticed. It is the larger scyphozoan jellyfishes that
everybody sees (Fig. 23.14).
Chap. 23 COELENTERATES — SIMPLE MULTICELLULAR ANIMALS 481
The medusa's umbrella-shaped body is largely the jellied mesoglea contain-
ing at least 95 per cent water and the scattered fibers and cells that have
migrated into it. Both the upper convex and under concave surfaces are cov-
ered by epidermis liberally-supplied with stinging cells and sensory nerve cells.
The mouth is at the end of a short tube (manubrium) which hangs from the
center of the under surface (Fig. 23.10). The passage from it opens into a
central cavity from which four radial canals lead to the circular canal that
extends around the margin of the umbrella. All of these canals are parts of the
enteron which in its evolution has added the distribution of food to its already
established functions of digestion and absorption. As might be expected of an
active free-living animal, the nerve cells and their associations in the medusa
are much more highly developed than in the attached polyps. A nerve ring
lies along the margin of the umbrella. This receives processes of nerve cells
acting in a simplified way like the central nervous system of higher animals.
A sensory organ of equilibrium (statocyst) is located at the base of every
other tentacle. These and similar sensory organs in other medusae are consid-
ered as the first real organs to be developed in the invertebrates.
Reproduction, The medusae of any one colony are either all male or all
female. They closely resemble one another and the ovaries and testes develop
in the same relative position beneath the radial canals. The sex cells are shed
into the open water where fertilization occurs. The embryo becomes first a
spherical blastula, then a swimming larva. Its wanderings are important to the
distribution of the species but they last only a few hours before it settles upon
rock or seaweed and the development of the colony of polyps begins (Fig.
23.10).
Gonionemus — A Hydrozoan Jellyfish. In Gonionemus, the medusa is well
developed and the polyp is diminutive. The medusa is as transparent as glass
and less than an inch in diameter (Fig. 23.12). This was formerly a common
jellyfish among the eel grasses along the eastern coast of the United States.
When feeding, Gonionemus swims toward the surface with its mouth down.
There it turns over and floats slowly downward, its mouth up and its tentacles
extended in a wide open snare for any small animals within reach of their
stinging clutch. When at rest it likewise lies mouth up, with its tentacles at-
tached to the bottom by the adhesive pads.
Other Species of Hydrozoa. The skeletons of hydrocorallines are peppered
with minute pores but the polyps are seldom seen extending from them since
they expand at night. The "stinging coral" (Millepora alicornia) well known
along the coast of Florida contributes largely to the formation of coral reefs.
The colonies of Hydractinia which live on the shells of hermit crabs have a
division of labor similar to that of Obelia but in these colonies there are
feeding polyps, reproductive polyps with medusa buds, and protective polyps
without mouths, only stubby tentacles and a great supply of stinging cells.
482 EVOLUTION OF ANIMALS Part V
Adhesive disks holding to surface
Fig. 23.12. Upper, adult Gonionemus murbachi, a beautiful jellyfish with a disk
hardly an inch in diameter and 60 to 80 tentacles that bear rings of stinging cells. It
goes through a medusa and a polyp stage, the latter so minute it is little known.
Very abundant in the quiet inlets of Cape Cod. Lower, a young jellyfish resting on
the bottom and holding fast with its suction disks. {Lower, after Perkins, Proceeds.
Academy of Natural Sciences, Philadelphia, 1902.)
The Portuguese man-of-war (Physalia pelagica) floats on the surface of
warm seas in many parts of the world and was named Portuguese only because
seamen saw it floating near Portugal (Fig. 23.13). It occurs in the Gulf Stream
from Florida northward, occasionally drifting into harbors in New England.
Its gas-filled float, about ten inches long, is translucent blue and rose-tinted,
colors that are continued in the polyps which trail backward for 1 0 to 40 feet.
Their beauty is strictly for the eye, nothing to be fondled. Colonies and pieces
of tentacles that have been picked up half dead upon the beach have caused
serious poisoning. The long defense polyps paralyze a good-sized fish and, due
to their extraordinary contractions, are able to present the fish which they have
snared at the mouths of the short feeding polyps.
The "little sail" (VeleUa) is a similar hydrozoan colony supported by a
float about two inches wide that bears an erect projection, the "little sail."
These are common drifters often whole fleets of them, in the warmer waters
of the west coast.
Class Scyphozoa
The Scyphozoans include the larger jellyfishes. Their radial symmetry is
based upon four or a multiple of four structures, such as the eight notches in
the margin of the umbrella (Fig. 23.14). The polyp stage is either lacking or
the polyps are minute, A full-grown polyp suggests a stack of diminutive
Chap. 23 COELENTERATES SIMPLE MULTICELLULAR ANIMALS
483
Fig. 23.13. Portuguese man-of-war, Physalia, eating a fish held by the feeding
polyps. The float (about 10 inches long) is tilted over on its side with the crest
toward the camera. Physalia is a colony of hydrozoan polyps fitted for different
functions — feeding, defense, reproduction. They act together in such close coop-
eration that they form an individual. Physalia frequents warm ocean currents and
is often carried to the shores of Europe and America. (Photograph courtesy,
Douglas P. Wilson, Marine Biological Laboratory, Plymouth, England.)
saucers (strobilas). Some jellyfishes are crystalline clear and colorless; others
are rose-tinted, yellow, lavender, blue, or deep red; all their swimming motions
have characteristic grace and rhythm.
Aurelia — A Scyphozoan Jellyfish. Aurelia is one of the commonest of jelly-
fishes and most often studied. Drying fragments of them litter the beaches after
a storm, great bounty for the sandpipers. The polyps are small and usually
hidden in seaweeds (Fig. 23.15).
A long folded lip trails from each corner of the square mouth (Fig. 23.14).
The edges of these are well armed with stinging cells and the fold encloses a
groove along which cilia drive minute animals toward the mouth and thence
to the four-pouched stomach. There they come in contact with gastric filaments
484
EVOLUTION OF ANIMALS
Part V
Mole
SEXUAL REPRODUCTION
Female
Fold of lip
Swimming
ephyra
Sperm
Strobilo
"^^ SCyphistoma
'0 develops
he folds
the lips
Swimming
iarvQ
Polyp attached
to rock
Fig. 23.14. The life cycle of the common jellyfish, Aurelia. During their com-
plete cycle jellyfishes have different forms and habits. The largest of these are the
male and female medusas, 6 to 10 inches across the disks in Aurelia. All the other
forms are minute. The embryo is produced by the union of sperm and egg, and
sheltered in the streamer-like lips of the parent. The larva swims by cilia and trans-
forms into a hydra-like polyp. In the following stages, scyphistoma and strobila,
the animal divides into a series of saucer-shaped young ones. Finally these separate
and as ephyras, developing males or females, they swim free.
heavily loaded with more stinging cells. Within a few hours, they are reduced
to broth by secretions strong enough even to digest chitin. Particles of food
are engulfed by nutritive cells and digestion is completed within them as it is in
the similar cells of hydra.
Jellyfishes have a very definite sense of balance. If one of them is tilted out
of horizontal position it will contract more strongly on the upper than on the
lower side and bring itself back to a horizontal position. If the organs of
balance in the notches are all removed from one side and that side is upturned
as before, the animal will not attempt to right itself. The ovaries and testes,
always borne on separate individuals, are the four horse-shoe-shaped bodies
in the floor of the central enteron, the most conspicuous structures in the
animals.
The embryo goes through its early development within the folded lips of the
female, becomes a ciliated free-swimming larva, and then a polyp that settles
upon a rock or seaweed. There it may grow for months budding off one young
Chap. 23 COELENTERATES SIMPLE MULTICELLULAR ANIMALS 485
medusa after another; the oldest one at the end the first to separate and swim
away (Fig. 23.15).
Class Anthozoa
Sea anemones and true corals. These are the fleshy sea anemones and the
limestone secreting corals. All are polyps, solitary or colonial with no medusa
stage. They are distinguished from hydrozoan polyps by the vertical partitions
or septa which partially divide the gastrovascular cavity into alcoves opening
into a central space below the gullet.
Sea-anemone — A Representative Anthozoan. Metridium marginatum is the
common sea anemone which attaches itself to wharf piles and gathers by
Fig. 23.15. An underwater photograph of living polyps of Aurelia. Polyps
(about one-half-inch long) of Aurelia aiirita, growing on a hollowed rock. Jelly-
fishes (medusae) are being formed by the transverse division of the polyps. A
young jellyfish (ephyra stage) has just separated from a polyp and is swimmmg
into open water. (Photographed from life by Douglas P. Wilson, Marme Biologi-
cal Laboratory, Plymouth, England.)
486 EVOLUTION OF ANIMALS Part V
dozens in the tide pools along our north Atlantic coast (Fig. 23.1). Metridium
has a cylindrical body topped with a crown of hollow tentacles arranged in
circlets around its slit-shaped mouth. When full grown and expanded it is
about 4 inches high and its oral disk may be three inches wide. Its skin (epi-
dermis) is soft and slimy but tough even to sharp scissors.
Partial septa extend vertically from the body wall; some are attached to the
gullet, others extend only part of the way toward it (Fig. 23.16). Their free
edges are thickened by digestive filaments containing the nutritive cells. Some
of these secrete digestive fluids into the gastrovascular cavity; others engulf
particles of food and digest them within food vacuoles in the cells. The struc-
ture of hydra, jellyfish, sea anemone, and coral is fundamentally similar
(Fig, 23.11). Stinging cells are active and abundant on the tentacles and on
the stinging threads (acontia) borne near the bases of the digestive filaments.
These threads may be shot through pores in the body wall or out of the mouth
and extended three or four inches into the water, paralyzing animals which the
tentacles cannot reach.
Ovaries and testes are in separate individuals and the young develop from
fertilized eggs. Anemones also reproduce asexually by longitudinal division
and by pedal laceration, the pinching off of fragments of the basal disk. In an
aquarium, the base of the anemone may be spread against the glass side and
Gullet
Ostta, holes in the partitions through which
water passes from one chamber to another.
Hollow tentacle
Circular muscle
Partition
Longitudinal muscle on the
partition
Reproductive organ on edges of
partitions.
Stinging filaments are shot out
through body wall.
Here the filaments overlay the
partition like threads laid on gloss
Cut into body Creeping basal disk
Gostro-vascuiar covity
Fig. 23.16. Sea anemone with a side of the body cut away to show the partial
partitions in the coelenteron.
Chap. 23 COELENTERATES SIMPLE MULTICELLULAR ANIMALS 487
firmly attached along the edges. The attachment is so strong that the central
part can be pulled away leaving a ring of torn tissue behind it. Each piece will
develop tentacles and a mouth and finally a complete minute anemone, ulti-
mately a ring of little anemones.
Anemones can glide on the pedal disk, but at the slow pace of about four
inches per hour. When conditions are good they stay in one place for long
periods. An anemone contracts its body tightly and quickly; the tentacles dis-
appear suddenly, and its mouth appears tied up like a bag. Water is squeezed
out through pores in its body wall and the acontia are also forced out through
them. It may not expand for a long time and then very slowly while water
gradually flows into the enteron through smooth ciliated furrows on one or
both sides of the gullet (Figs. 23.1 and 23.16).
The tentacles are very sensitive to stimulation and move excitedly if meat
juices are added to the surrounding water. If a water flea happens to come in
contact with the tentacles it is immediately snared in the sticky mucus, then
paralyzed by the stinging cells and brought to the mouth by the ciliated ten-
tacles (Fig. 23.17). Immediately the whole oral disk is in motion, the mouth
opens and, with the further help of tentacles and lips, it takes in the food. In
the gullet, the food comes in touch with currents of cilia, always inward when
the anemone is feeding though they may be outward at some other times.
Anemones are carnivores that will eat any animal flesh, living or dead. They
often attach themselves to crab shells and to the shells appropriated by hermit
crabs. The crab is hidden and the sea anemone rides to new feeding grounds,
foraging as it goes, probably a truly symbiotic relation.
Astrangia — A Coral Polyp. Astrangia danae form little colonies of a couple
of dozen polyps on the rocks, in sheltered places from North Carolina to
Massachusetts (Fig. 23.18). They feed upon small crustaceans and young
fishes and can be kept alive quite successfully in cold salt-water aquaria. They
are like smaller editions of the sea anemone except for the limy coral cup
secreted by the ectoderm. This is laid down at the base of the polyp, in thin
ridges and as more coral is produced the bottom of the cup is also thickened.
Astrangia is closely related to the most important builders of coral reefs.
Coral Building. In tropical waters, where they abound, coral animals have
built the foundations of large areas of land. The Bermuda Islands are at the
northern limit of coral building and are comparatively small, yet they include
more than 19 square miles of coral. The Great Barrier Reef of Australia,
crowded with coral, is 1350 miles long (Fig. 23.20). Such areas have been
built by the epidermal cells of millions of minute polyps each one slowly
secreting its cup-shaped home. Polyps die and new generations of them secrete
new cups upon the old ones. Only the surface of the coral mass is alive.
Other animals live in the crevices and chasms of the coral ledges — pro-
tozoans, sponges, boring mollusks, case-making worms, seaweeds, and bril-
488 EVOLUTION OF ANIMALS Part V
liantly colored fishes. Probably no place on earth is so replete with life as the
undersea gardens of coral reefs. All of these plants and animals leave their
remains on the coral and gradually build it up toward the surface where it then
receives the drift brought by winds and waves.
Coral Reefs. There are three main types of coral reefs and they are among
the most interesting of land masses (Fig. 23.19). A fringing reef is near the
coast, separated from it only by narrow strips of shallow water. It is a platform
of coral which projects outward from the shore and ends steeply on the sea-
ward side of the reef. Breaks occur here and there in the reef, letting currents
into the shallows, but little or no navigation is possible. Barrier reefs resemble
the fringing ones but differ in that there are wide, deep channels between the
mainland and the reef. The world famous one is the Great Barrier Reef of
Australia (Fig. 23.20). The atoll is a ring-like reef with an opening in one or
several places into a lagoon which may be less than a mile or as much as 50
Fig. 23.17. Snake-locks anemone (Anemonia sulcate). The tentacles and cilia
bring food to the central mouth. No garden is more beautiful than are colonies of
sea anemones — ivory, yellow, purple, rust-colored, and orange, with their trans-
lucent tentacles shifting and stretching in the currents. (Photograph courtesy,
Douglas P. Wilson, Marine Biological Laboratory, Plymouth, England.)
Chap. 23 COELENTERATES SIMPLE MULTICELLULAR ANIMALS 489
miles wide. None of the reefs is continuous — all of their fronts being subject to
incessantly breaking waves. The booming of surf is a characteristic voice of
the reefs.
Theories of Reef Building. Charles Darwin suggested The Subsidence
Theory: that in past ages all corals had lived in fringing reefs; that in places
7 •/ / V ^■-
- • SeptUAT?
Fig. 23.18. Upper, polyps of the coral, Astrangia, cup half an inch high. The
white Astrangia danae lives on the eastern coast, as far north as Cape Cod and the
orange and red Astrangia insignifica on the western coast of North America.
(Courtesy, American Museum of Natural History. New York.) Lower, diagram of
a coral polyp with one side of the body cut away to show the general structure.
The polyp is resting on the basal plate and partitions (or septa) of the limy cup
which it has secreted. Only the basal parts of the cup are included. The mouth,
tentacles, and walls (mesentery) of the alcove-like parts of the central cavity are
similar to those in sea anemones. (After Pfurtscheller. Courtesy, Wolcott: Animal
Biology, ed. 3. New York, McGraw-Hill Book Co., 1946.)
Fig. 23.19. Coral reefs: fringing, barrier, and atoll reefs. A, a barrier reef in
the Caroline Islands, Polynesia; the land is crosshatched. B, an atoll in the Chagos
Archipelago, Indian Ocean. C, profile of a fringing reef. Living coral cannot survive
more than brief exposures to the air and usually does not grow above the low water
line. D, profile of a barrier reef (see Fig. 23.20, corals of a barrier reef). E, profile
of an atoll reef. Taken in a different place the section might have gone through
another island like the one included. {A and B from Principles of Geology by
Gilluly, Waters, and Woodford. Copyright, 1952. W. H. Freeman and Company.
C, D, and E, courtesy, McCurdy: Manual of Coastal Delineation. Washington,
Hydrographic Office, 1947.)
490
Chap. 23 COELENTERATES — SIMPLE MULTICELLULAR ANIMALS
491
Fig. 23.20. Upper, the edge of the Great Barrier Reef and the overwash of the
sea at Heron Island, Australia. The Great Barrier Reef is 1350 miles long and in
places 30 miles wide. It is a natural factory where billions of coral animals take
lime from the sea water and build the cups that protect them and bury their an-
cestors. Lower, corals on the Great Barrier Reef at South Malle, Australia. (Cour-
tesy, Australian News and Information Bureau.)
492 EVOLUTION OF ANIMALS Part V
the land had sunk creating wide channels and barrier reefs; that in the case of
islands the land might have sunk completely out of sight and formed the
lagoon. The relatively recent Glacial-control Theory states: that during the
last glacial period the amount of water frozen in the great ice caps may have
lowered the ocean by about 200 feet. Shallows resulted covering many plat-
forms of the ocean with water too cold for corals. However, as the ice melted
and the waters were warmed coral growth began and kept pace with the rising
ocean level. This theory accounts for the uniform depths of coral lagoons
whose bottoms may represent the platforms which existed when the ocean was
at its ancient low level.
Attempts to unravel the mystery of reef formation have been made by bor-
ing deep into a reef and identifying the coral skeletons found at low levels.
This was done on Funafuti Atoll, in the South Pacific north of Fiji. One boring
about five inches in diameter was carried down 1114 feet without reaching
the base of the reef. Twenty-eight reef-building corals were identified and of
these 22 are now living on the reef in water around 100 feet deep. Borings on
other reefs have given similar results, all of them supporting the glacial-control
theory.
24
Ctenopnores — ComL Jellies
or Sea Walnuts
The ctenophores or comb bearers constitute a small phylum whose members
live in the surface waters of warm seas and ocean currents. They are commonly
taken for jellyfishes and were formerly classified with them. Their differences
from coelenterates, the absence of stinging cells and the peculiarities of sense
organs and radial bands, are now regarded as important enough to place them
in a separate phylum. They are transparent and glimmering, some pink or
bluish or orange, but many colorless except for the continually shifting coppery
bronze iridescence of their combs. All of them are luminescent and the millions
occasionally swarming through the ocean surface create fantastically beautiful
illuminations.
General Features. Ctenophores are moderately small, often about the size
of a plum. One of the smallest, Pleurobrachia is no larger than a garden pea
(Fig. 24.1). The pale violet Venus's girdle (Cestum) is a ribbon 2 to 3 feet
long. It is usually oval or globular, sometimes pear-shaped.
Its conspicuous distinguishing feature is the eight rows of combs that radiate
from the mouth at one pole of the animal and extend to the opposite one like
the ridges of a cantaloupe (Fig. 24.1 ). These rows are arranged in radial sym-
metry, but the long tentacles usually present are located one on each side of
the body and Venus's girdle is clearly bilaterally symmetrical. Ctenophores
are regarded as a higher group than coelenterates because of their tendency
toward this balance of two sides of the body. Another mark of progress is the
three-layered body wall, ectoderm, endoderm, and a middle layer closely ap-
proaching the cellular mesoderm of higher animals. In ctenophores whole cells
are muscular, not merely the processes as in the epithelio-muscular cells of
hydra. They have no stinging cells. Neither is there asexual reproduction nor
alternation of generations as in coelenterates.
493
494
EVOLUTION OF ANIMALS
Part V
Fig. 24.1. Ctenophores. A, Pleurobrachia pileus, the "sea gooseberry," named
because its size, streaks and translucence suggest a gooseberry. Common on the
northern Pacific and Atlantic coasts. B, Hormiphora plumosa, barely an inch long.
Tortugas, Florida. (Courtesy, Mayer: Ctenophores of the Atlantic Coast of North
America. Washington, Carnegie Institution, 1912.)
Ecology. Ctenophores are carried about by currents but they also swim
feebly by means of their combs. Venus's girdle swims by undulations of the
body similar to those of a leech or an eel.
Ctenophores are carnivores that feed voraciously upon any animals that
they can swallow. Swarms of billions of Uttle Pleurobrachia can sweep the
surface water clean with their tentacles that trail for several inches behind
them. In his study of the food relationships of animals in the Gulf of Maine,
H. B. Bigelow writes that "of all the members of the plankton [surface or-
ganisms], the most destructive to smaller or weaker animals are the several
coelenterates, and especially the ctenophores, genus Pleurobrachia, a pirate to
which no living creature small enough for it to capture and swallow comes
amiss." All ctenophores have a unique means of catching their prey, the glue
cells presently described.
Structure. Each comb is composed of long cilia fused together as if the teeth
of a comb were united nearly to their tips (Fig. 24.2). A ctenophore swims
Chap. 24
CTENOPHORES COMB JELLIES OR SEA WALNUTS
495
Fig. 24.2. Diagram of the digestive system of a ctenophore. 1, statocyst (sense
organ of balance); 2, anal pore; 4, aboral canal; 5, stomach; 6, transverse canal;
11, meridional canal; 12, pharynx; 22, mouth. Several labels omitted. (Courtesy,
Hyman: The Invertebrates, vol. 1. New York, McGraw-Hill Book Co., 1940.)
mouth forward. The motion in each row usually begins with the last comb at
the aboral end and goes forward like a wave. Each beat is a strong backward
flap of the comb which drives the water out from under it and helps to push
the animal forward. If a ctenophore strikes head on against an object, the beat
is at once reversed. Experiments have shown that the movement of the combs
is controlled by the nerve cells that lie beneath the rows. At the rear, or aboral
pole, of the body there is an area of nerve and sensory cells. In the center of
this is a pit containing a sense organ, the statocyst, which holds a little cluster
of limestone particles supported by tufts of cilia that are connected with sen-
sory cells. This is believed to be a balancing or steering mechanism since any
turning of the body causes the limestone to rest more heavily on one or another
of the tufts. This would stimulate the sensory cells, and the stimulus carried to
the combs would cause them to beat faster on one side than the other. From
this polar area a nerve net extends through the body and is concentrated in
eight strands, one under each row of combs.
On each side of the body is a sac into which the tentacles can be retracted.
The latter are often very long in proportion to the body and are the cteno-
phore's catch-traps for small animals. Their epidermis consists largely of glue
cells (colloblasts) each of which in action is a combination of a lasso and glue-
496
EVOLUTION OF ANIMALS
Part V
sticky
granule
Nucleus of cell
bearing granules
Spiral elastic
filament
Straight
filament
Attachment to
core of tentacle
B c
Fig. 24.3. Adhesive cells: these sticky "lasso cells" compose a large part of the
epidermis of the tentacles. Still attached to the tentacle by the lasso threads they
are thrown against unfortunate little animals that are then stuck fast to them. When
the tentacle has collected its prey, it contracts and wipes itself across the expectant
mouth of its owner. A, a section through one of the branches of a tentacle (Fig.
24.1.). The outer surface is covered with the sticky heads of the "lasso cells."
Each cell is attached by a coiled filament which acts as a spring preventing the cell
from being wrenched off by the struggling victim. B and C, sticky cells with fila-
ments uncoiled and coiled. (B and C, redrawn after Wolcott: Animal Biology,
ed. 3. New York, McGraw-Hill Book Co., 1946.)
pot (Fig. 24.3). Each cell discharges a sticky secretion. It is fastened to the
tentacle by a coiled contractile fiber encircling a straight fiber which acts as a
special holdfast while the contractile one is stretched out into the water. The
core of the tentacle contains a central strand of nervous tissue concerned with
the responses in the tentacle and a cord of muscular cells which provides for its
extreme contractility. As in coelenterates, the extensive branched enteron pro-
vides for digestion, absorption and transport of food, water, and metabolic
waste. The only opening in the enteron is the mouth which leads into a
Chap, 24 CTENOPHORES COMB JELLIES OR SEA WALNUTS 497
pharynx and stomach. From there a series of ciliated canals extends through
the body, often especially prominent in luminescent individuals (Fig. 24.4).
Reproduction and Regeneration. All ctenophores are hermaphrodites and
in most species eggs and sperm cells are shed into the open water where the
eggs are fertilized.
Ctenophores have high powers of regeneration and can repair severe in-
juries to their frail bodies. They have been subjects for many experiments on
regeneration and grafting. Any parts removed, including the statocyst, are
regrown. Whole rows of combs are replaced. Halves of an animal, cut in
either direction, will regenerate but parts containing the statocyst grow more
rapidly than others. Pieces of one animal may be grafted into another. Bands
of combs may be grafted onto another ctenophore in reverse of their natural
position. Such grafted combs will continue to beat as before in opposition to
those on the recipient animal, causing the latter to turn round and round.
Fig. 24.4. Luminescent ctenophores. Beroe photographed by daylight and in
darkness except its own light. Vividly shown here are the meridional canals and
network of intercommunicating canals. Beroe is less than two inches long and has
no tentacles. At certain seasons swarms of ctenophores illuminate wide ex-
panses of the seas. (After Panceri. Courtesy, Harvey: Bioluminescence. New
York, Academic Press, 1952.)
25
Flatworms — Vanguard oi
tne Hi^ner Animals
There is a vast difference in the relative speed of a flatworm and a race
horse, yet bilateral symmetry, always the partner of speed, had its beginning in
the flatworms. They were the first animals in the evolutionary procession to
firmly establish the likeness of two sides of the body feebly suggested in the
ctenophores (Fig. 24.1). Other features of higher animals begun in the free-
living flatworms are a definite head, a centralized nervous system, the meso-
derm or middle layer of body cells, and complex reproductive organs.
Along with their advances in build and behavior flatworms are strikingly
primitive. In the majority of the free-living species the digestive tract has but
one opening, the mouth. Instead of being located in the head, it is in the center
of the underside of the body suggesting the wheel-like symmetry of the jelly-
fishes, in which the mouth takes the place of the hub (Figs. 25.1).
Classes of Flatworms. There are three main classes in the Phylum Platy-
helminthes and each one has its particular successes: 1, Turbellaria are, for
the most part, free living, have a digestive cavity and are covered with cilia;
2, Trematoda have a digestive cavity, cuticle covering the body but no cilia,
and are parasitic; 3, Cestoda lack an enteron and have a cuticle, but no cilia.
Turbellaria — Planarians and Others. The power to regrow lost parts
permits planaria to survive and even to multiply after injury. Three planarians
may live and flourish because one was cut into three pieces. All turbellarians
are aquatic. A considerable number live in fresh water and a few on moist soil.
Most of them are marine. Some are marked with striking patterns in yellows,
reds, and black and white and all are graceful swimmers and gliders. They are
named for the turbulent movements of their abundant cilia.
Trematoda — Liver Flukes and Others. All trematodes have great
capacity to multiply. They are parasites, with a wide distribution assured them
498
Chap. 25
FLATWORMS VANGUARD OF THE HIGHER ANIMALS
499
-V051HJ
Fig. 25.1. Two flatworms: the planarian, free living, and the tapeworm, a
parasite. The planarian (mouth and pharynx extended) shows the thorough bi-
lateral symmetry of the flatworms, a feature that brought great changes into
the evolution of animals. The tapeworm has developed extraordinary reproductive
capacity by means of the many sections (proglottids) of the body almost all of
which are capable of producing hundreds of fertilized eggs. (Courtesy, Pauli:
The World of Life. Boston, Houghton Mifilin Co., 1949.)
by the animals on or in which they live. All have an outer covering of resistant
cuticle and no cilia.
Cestoda — Tapeworms. All cestodes have extreme capacity to multiply.
All of them are parasites with a world-wide distribution assured them by the
far-ranging vertebrates that are their hosts. They travel by land, sea, and air,
in goats that scale the mountains, in salmon that swim the Pacific, in bobo-
links that fly from Brazil to New England.
Class Turbellaria
Turbellaria are free-living flatworms. The most familiar of them are the
fresh-water planarians, one-half to one inch long (Fig. 25.2). They are com-
mon in streams and lakes and in many laboratories they are among the classic
subjects of experimental zoology. Some are white or translucent; others are
sober colored, gray, brown or black, a contrast to the brilliance of the marine
species.
A Representative Planarian
The commonest planarian in the United States is Dugesia tigrina {= Pla-
naria maculata) (Figs. 25.2, 25.3). It glides over alga-covered stems in ponds
500
EVOLUTION OF ANIMALS
Part V
Pharynx-
Mouth
■:m
N •
Fig. 25.2. Habit sketches of a common fresh-water planarian. Dugesia tigrina
(Planaria maculata) . 1, full grown, about three quarters of an inch long, 2, feed-
ing on the water-soaked body of a dead fly, its sucking pharynx extended through
a soft place in the insect's abdomen. The eggs of planarians are usually pro-
tected in cocoons, about the size of pinheads, and attached to submerged rocks
and leaves.
and quiet streams, its dark body so soft that it can be cut with the edge of a
leaf.
A Pioneer Head. Planarians are pioneers not only in their bilateral sym-
metry, but in having two uniquely different ends to their bodies, one of which
is a recognizable head. Dugesia cannot be credited with a neck, yet the head
is clearly set off from the rest of the body. It bears the eyes and many sensory
cells, and holds the brain. As with cats and other more astute animals, the
head of the planarian always arrives first. It is lifted slightly and bent from
side to side testing and exploring the way, a faint foreshadowing of the wise
end of a cat. On its surface are many microscopic pits containing cells that are
sensitive to chemical substances. On the pointed flaps, fancifully called auricles,
similar chemical perceptors are set in ciliated troughs. Planarians are attracted
by such foods as snail blood or crushed earthworms and in a dish of water
they will follow a capillary tube that contains them. If certain very weak acids
are used as bait, a planarian will grip the tube that holds them and push its
pharynx up into it as if the acid were a choice flavor, comparable to grape-
fruit and pickles to human taste. On the other hand, when certain other sub-
stances are offered, it will turn sharply away as if in a hurry. With one auricle
removed it travels in circles in a dish of dilute snail blood, curving its body
toward the unhurt side by which it still responds to the attractive blood. If
its brain is removed it will not react even to the most desirable snail juice.
The whole body surface will respond to a delicate touch though the head is
FLATWORMS VANGUARD OF THE HIGHER ANIMALS
Eye
gestlve
tract
Pharynx
withdrawn
in sheath
Opening of
pharynx
Mouth
501
Cut edge of
body wall
Mouth
Extended
pharynx
Opening of
pharynx
Ventrol
nerve
cord
Nerves
Fig. 25.3. Digestive and nervous systems of a planarian. A, the digestive
system, so distributed through the body that digestion and absorption take place
in every locality. B, view showing the pharynx extended to capture food. C, the
nervous system. Compared to hydra and the sea anemones, the nervous system is
distinctly centralized.
by far the most sensitive part. Planarians strive to keep their undersides in
contact with a supporting surface and in their attempts to do so, the head
takes the lead just as it does when a turtle turned wrong side up flops over to
gain a foothold. Turned onto its dorsal side, a planarian twists into a spiral
so that the ventral surface of its head comes in contact with the substratum.
The head then glides forward and the body unwinding the spiral follows after.
In the eyes of Dugesia and other planarians the pigmented cells form a cup
into which one to many neurosensory cells project. These are retinal cells com-
parable to the rod and cone cells of the human eye. Turbellarians, in general,
avoid the light; fresh-water planarians seek the darker sides of stones, the under-
sides of submerged leaves. When placed on contrasting backgrounds, such as
an experimental one of black and white circles, planarians (Dugesia lugiibris)
followed the black circles. After they were blinded, they made no distinction
between white and black.
Locomotion. Planarians glide about by means of the assembled help of
millions of cilia located on their ventral sides and by muscular contractions,
the latter more important than the cilia. The roles of the cilia and muscles have
been separated by treating planarians with lithium chloride which paralyzes
the cilia, but not the muscles, and with magnesium chloride which paralyzes
the muscles but not the cilia. The slime trail secreted by mucous cells is an
important asset for gliding. The cilia are whipped into the slime, strike against
502 EVOLUTION OF ANIMALS Part V
the underlying surface and the body is moved forward in rapidly repeated
microscopic lurches that merge into a glide.
Feeding. The majority of turbellarians are carnivorous. The smaller fresh-
water ones feed upon crustaceans and worms that are nearly microscopic, the
larger ones on snails, earthworms and insect larvae, often on their softened
remains. Even in quiet waters they can detect juicy meat two or three feet
away. As the worm recognizes the food it pauses, swings its raised head about
and starts directly toward it. First it touches, then rubs its head against the
piece and glides onto it, finally stretches and dips its pharynx into it (Fig. 25.2) .
Digestion, Assimilation and Food Storage. Flatworms are strikingly different
from other bilaterally symmetrical animals in having the mouth half way
down the body, curiously enough not in the important head region (Figs.
25.3, 25.4, 25.5). The pharynx leads into the three-forked (in triclads)
intestine whose many branches reach throughout the body. Practically any
piece that may be torn from the body takes digestive and excretory cells with
it; thus it can be nourished and can grow.
Feeding experiments and microscopic examinations of the intestine have
shown that the entire processes of digestion, absorption, assimilation and
storage of food occur within the partly ameboid cells of the intestinal lining.
A planarian grows fat in its linings, usually of the intestine; food stored there
is largely fat, rarely glycogen. Nothing is known of the actual processes by
which the stored food is transferred and used by the other cells of the body.
In one series of experiments, planarians (Dugesia) were starved for two
weeks, then fed on beef liver. At frequent intervals, some of them were killed
and examined microscopically. The partially ameboid cells began to engulf the
bits of liver as soon as they came in contact with them. Swollen with absorbed
fluid, they bulged into the intestine and embraced the food with their pseu-
dopodia. Within them the bits of food and fluid were digested in food vacuoles
like those of amebas. It took about eight hours for the content of a full in-
testine to be taken up by the ameboid cells. During digestion planarians take
in two or three times more oxygen than usual and utilize the stored fat for the
extra energy expended.
Fresh-water planarians can endure starving for six to 14 months but at the
end of that time they may be reduced to one three-hundredth of their original
size. The greatest degeneration is in the reproductive system, part of which
entirely disappears. Their condition suggests that of worker honeybees that
are chronically underfed and have undersized reproductive organs. The heads
of starved planarians are relatively large because the nervous system is not
reduced.
Respiration. Oxygen and carbon dioxide are exchanged by diflfusion through
the body as in ordinary aerobic respiration, a contrast to the anaerobic respira-
tion of parasitic flatworms (see cestodes p. 515).
Chap. 25 FLATWORMS VANGUARD OF THE HIGHER ANIMALS 503
Excretion and Water Balance. The excretory system consists of many large
flame cells each of which faces into a kidneylike (protonephridial) tubule
(Fig. 25.4). A network of these tubules opens out on the surface of the body
by minute pores. As in other animals, water is continually coming in and going
out of the body. Water that regularly diffuses into the body and collects in the
flame cells is waved into the tubules and passes through the microscopic out-
lets, thus completing the circuit. Nothing is known about the excretion of
nitrogenous waste.
Nervous System. Planarians have a bilobed brain from which two main
nerves reach backward through the body giving off frequent branches (Figs.
25.3, 25.5). By skillful operating the brain can be removed. Following this
the animals remain quiet, unless stimulated, then they move about freely show-
ing that muscular action is independent of the brain and can be coordinated
by the branches.
The sensory cells with which the head is richly supplied have already been
mentioned. Planarians are responsive to chemical substances, to changes of
Excretory bulb
or flame cell
Excretory
bulb
Excretory
pore
Excretory
canals
Nucleus
Cilia
Cavity within the cell
opens into canal
:)
Excretory
canal
Excretory
bulb
Excretory
canals
c.
Excretory
opening
(pore)
Fig. 25.4. The excretory system of a freshwater planarian. A, the entire system,
excretory bulbs (flame cells) in which excess water and metabolic waste is col-
lected and waved by cilia into the microscopic canals which finally carry it out
of the body through microscopic pores. Detail of canals: arrows mark the flow of
fluids from the bulbs. Highly magnified excretory bulb, called a flame cell from the
flicker of the cilia, which project into the cup-like cavity in the cell and create
a current of fluid into the canal. Like all kidney systems the function of this one
is the regulation of water content and the elimination of metabolic waste, espe-
cially nitrogen.
504
EVOLUTION OF ANIMALS
Part V
Digestive
cavity
Nerve net
Circular
muscle
Longitudinal
nnuscle
Mesenchyme
Excretory pore
Epidermis
sa?jflfissJs;
»^^WSS]^S3ffi!
Rhabdites
Dorso- ventral
muscles
Nerves and
nerve net
Ventral
nerve cord
Fig. 25.5. Cross section of a mature planarian. A net-like tissue of the mesen-
chyme occupies the space that in higher animals is taken by the body cavity. The
excretory organs are not shown. The glandular rhabdite cells form and discharge
minute bodies, the rhabdites, largely composed of calcium.
temperature, to water currents, to currents of electricity, and to gravity.
Reproduction. Most planarians are hermaphrodites having complete male
and female systems in the same individual. In spite of this, they mate and the
sperm cells of one are placed in the female passages of the other and vice
versa.
Both male and female systems are complex (Fig. 25.6). The male sys-
tem consists of hundreds of minute testes, each connected by a micro-
scopic tube that joins a larger tube (vas deferens) , one on each side of the body.
These connect with the median seminal vesicle which serves as a storage for
the sperm cells before they are released at mating. The eggs are fertilized as
they are discharged from the ovary. Yolk cells, from the yolk glands, adhere
to the outside of the fertilized egg and in this unique way supply it with food.
As in a hen's egg, yolk is universally inside. As the planarian's one or more
eggs with their yolk cells are moved along the oviduct, the latter secretes a
capsule about them. Such capsules are commonly fastened to the undersides
of submerged rocks; those of Dugesia resemble fig seeds on short stems.
Capsules collected from rocks usually hatch in two or three weeks if kept in
clean, cool water and subdued light at ordinary temperatures.
Planarians commonly reproduce asexually by transverse division or fission.
Fission is most common during the summer, sexual reproduction in winter and
spring. When about to divide, the animal suddenly fastens its rear end down
and pulls its front end forward, till the two separate. In a lightly greased dish,
Chap. 25 FLATWORMS VANGUARD OF THE HIGHER ANIMALS 505
a planarian is completely frustrated; it can neither fasten its body to the sur-
face nor divide.
Regeneration. The common Dugesia and certain other free-living pla-
narians have remarkable powers of regeneration. Parasitic flatworms, like
parasites in general, are unable to replace damaged parts. Experiments upon
the regeneration of sponges, hydras, and especially planarians have shown
important principles governing the organization and growth of the body. The
possibility of grafting human tissues was discovered by experimenting on
lower animals. The experiments on the regeneration of planarians carried on
by T. H. Morgan about 1 890 are among the classics of experimental zoology.
Pieces of a planarian's body maintain the natural polarity of the whole
body. Remove the head and tail leaving only the middle part of the body, and
a new head will grow from the front edge and a new tail from the hind edge
(Fig. 25.7).
WINTER
WINTER
SPRING
FALL SUMMER
ASEXUAL
FALL
SUMMER
SEXUAL
Fig. 25.6. Diagrams of the life cycles of Dugesia tigrina (Pkmaria maculata)
as they vary under different ecological conditions. Left, purely asexual repro-
duction. Transverse divisions occur throughout the warmer months. The parts of
the animals grow to a certain size; the rear end adheres to the surface and the
front part proceeds forward, pulling at the middle of the body which quickly
breaks. Right, the more common succession of sexual and asexual reproduction;
the sexual organs are highly developed in spring; many egg capsules are laid; by
midsummer sexual reproduction ceases and asexual reproduction by fission begins.
(Courtesy, Morgan: Animals in Winter. New York, G. P. Putnam's Sons, 1939.)
506
<^
EVOLUTION OF ANIMALS
Part V
<Sjt> V* r V r
11
u
If
b \/ c
1^
<fi>
\j
B.
□
0
D
Fig. 25.7. Regenerating planarians. A, their capacity to regenerate is greatest
at the anterior end; B, a regenerating piece shows its natural polarity, that is,
the head grows from the front and the tail from the rear as it does in normal
animals; C, a piece removed from the head and grafted into the body produces a
head; D, a short piece taken near the head may regenerate a head at each end.
(After Child: Patterns and Problems of Development. Chicago, University of
Chicago Press, 1941.)
The results of experiments upon planarians support C. M. Child's theory
of the axial gradient. This theory postulates that there are different rates of
metabolic activity in different regions of an animal's body, commonly the
highest at the anterior and lowest at the posterior end. Planarians confirm this
since pieces taken from the front end of a planarian grow faster and larger
than those taken from the rear. In some species, only the pieces from the front
will produce heads. Experiments show that the head dominates adjoining
regions and leads them to cooperate in their growth. If the central part of the
head of one planarian is grafted into an open wound in another planarian, it
will not only develop a whole head, but will influence adjacent tissues to pro-
Chap. 25 FLATWORMS VANGUARD OF THE HIGHER ANIMALS 507
duce a pharynx (Fig. 25.7). Tails thus engrafted are simply absorbed. A small
cross section of a planarian taken close to the head will produce a head on each
cut surface. The dominance of the head over the rest of the body is limited for
parts that are at some distance. In the natural asexual reproduction of a
planarian, the rear end gets beyond the control of the head and constricts
off as a separate; animal. Similar constriction and division can be brought on
by cutting off the head. All such behavior indicates that there is a gradation
of physiological activity from stronger to weaker and of control from front
to rear of the body, an anterior to posterior gradation of metabolism.
Other Turbellarians
Acoela. The most primitive turbellarians are the Acoela (without a cavity)
that have a mouth, but no definite digestive cavity. They swallow their food
directly into the loose mesenchyme where ameboid cells gather about it and
engulf the particles. Thus, digestion is intracellular like that of the amebas.
All the Acoela are marine, usually only one-tenth of an inch long and generally
little known.
Rhabdocoela. The Rhabdocoela, named from the rod-shaped gut, are com-
mon throughout the world in fresh waters and along sandy and muddy sea-
shores (Fig. 25.8), a few in hot springs. Most of them are less than half an
Fig. 25.8. A rhabdocoel, Stenostomum: various species of this genus are among
the commonest of invertebrates, cosmopolitan in standing waters but little known
because of their minute size. A chain of connected individuals is formed by in-
complete divisions of the body. (Courtesy, Morgan: Life of Ponds & Streams.
New York, G. P. Putnam's Sons, 1930.)
inch long, faintly colored and little noticed. The digestive cavity is straight
and unbranched. The rhabdites, rod-shaped bodies of unknown function, are
very abundant in them.
Tricladida. The Tricladida include land and marine planarians as well as
fresh-water ones. All triclads have a three parted digestive cavity. Many land
species live in the humid tropics, some of them marked with brilliant colors
and several inches long (Fig. 25.9). They are limited to localities where there
is a heavy rainfall, and much of the time lie under logs and leaves surrounded
by mucus. They travel on their own slime tracks and in tropical rain forests
they swing from the branches on slime threads as caterpillars swing on silken
ones.
Polycladida. The Polycladida have a digestive tract that is branched many
508
EVOLUTION OF ANIMALS
Part V
times. They are commonly two to six inches long and all are of leaf-like thin-
ness (Fig. 25.10). They live almost entirely on the rocky seashore, gliding over
the rocks or swimming by the undulating motions of their fluted bodies. Many
are inconspicuous; others are strikingly dappled and striped; all swim with a
peculiar grace and rhythm that has made them the "butterflies of the sea,"
competitors with a group of the snails for that name.
Class Trematoda
General Characteristics. Trematodes are called flukes (Anglo-Saxon, flok =
flat) because of their flat shape. They are built on the turbellarian plan, but
are parasites that have become extremely dependent upon other animals. The
Fig. 25.9. A cosmopolitan land planarian,
Bipalium kewense, sometimes brought to
northern greenhouses on tropical plants;
also found in Florida, Louisiana, and Cali-
fornia. It is nearly a foot long, has an ex-
panded head and is marked by long purple
to black stripes on a yellowish ground; 4,
eye; 5, creeping sole. It moves on a creep-
ing sole like the fresh-water planarians, oc-
casionally hanging off into the damp air.
(Courtesy, Hyman: The Invertebrates, vol.
2. New York, McGraw-Hill Book Co.,
1951.)
adults cling to their host by one or more suckers, and their bodies are covered
with tough cuticle. They have an enormous reproductive capacity and live
parts of their life span in alternate hosts. Like other parasites, they lack some
of the features that are present in their free-living relatives, external cilia, an
epidermis, rhabdites, and eyes.
Flukes attack large numbers of vertebrates, including domestic animals and
man. Their life cycles are complicated and their existence a gamble. Certain
trematodes have relatively direct development and one host (Order
Monogenea). Most of these are ectoparasites on the gills and skin of fresh-
water and marine fishes; some of them live mainly in the urinary bladders of
frogs. The fertilized eggs are shed into the water and there develop into ciliated
larvae that gradually become like their parents, first in their clinging habits and
then in structure including the gradual loss of ciha and of eyes.
Chap. 25 FLATWORMS VANGUARD OF THE HIGHER ANIMALS 509
Fig. 25.10. A black and white flatworm of the Pacific coast (Pseiidoceros
montereyensis) called a polyclad because of its many-branched digestive tract.
Natural size. This and other polyclads swim and glide about through the water,
the fluted borders of their bodies undulating like living ruffles. They are among
the most beautiful of marine animals, comparable to the butterflies on land.
(Courtesy, MacGinitie and MacGinitie: Natural History of Marine Animals.
New York, McGraw-Hill Book Co., 1949.)
Many other trematodes have an elaborate life history and develop in-
directly (Order Digenea). The fertilized eggs develop into young flukes that
look unlike their parents and go through several phases before they are adults.
During the life span they live in alternate hosts, the adults in a warm-blooded
vertebrate, the young ones in snails, crustaceans or other invertebrates. If there
are three hosts in one series, they are usually, first, a mammal occupied by the
adult; second, a snail; and third, a fish.
Sheep Liver Fluke
The liver fluke, Fasciola hepatica, is often chosen as a type for study because
of its large size, economic importance and its well-known life history (Fig.
25.11). The hosts of the adults are sheep, cattle and other herbivores, and
man. There are sheep flukes all over the world wherever sheep are raised,
especially in mild climates; in the United States, they are most common in the
states bordering the Gulf of Mexico. Where the cysts are thickly distributed
over pasture grass the infection of the sheep may be enormous, killing 50 to 60
per cent of a flock.
The adult liver fluke looks like a small dead leaf. At the pointed tip of its
body is the muscular mouth with which it punctures the tissues of its host and
510
EVOLUTION OF ANIMALS
Part V
Host snail
, natural
size
snail on I
grass in '
water
A- free swimming
miracidium
Fig. 25.11. Life history of the liver fluke of sheep, Fasciola hepatica. (After
Thomas. Courtesy, Storer: General Zoology, ed. 2. New York, McGraw-Hill
Book Co., 1951.)
sucks up their fluids. With minor differences, the digestive, excretory, nervous,
and reproductive systems are similar to those of planarians. They are all
hermaphrodites.
Life History. During its life cycle the liver fluke of sheep resides in two
hosts, the adults, usually in sheep, the larvae in fresh-water snails of the genus
Lymnaea. Without both of these hosts, the fluke cannot complete its life his-
tory.
The adult flukes inhabit the ducts of the sheep's liver. The fertilized eggs are
carried down the bile duct, into the intestine, and from there are cast out of
the body. One sheep may support, on an average, 200 mature flukes. Although
each of these may produce its half million eggs, only those that happen to fall
into fresh water have any chance of survival. In the water, they hatch into
minute ciliated larvae (miracidia) that are active swimmers. In order to sur-
vive, the larvae in this particular stage bore their way into the body of the
common water snail Lymnaea (Fig. 25.11). In the liver of this snail, they
transform into stationary sporocysts within which the egglike cells develop into
very minute active larvae (rediae). These work their way about in the snail,
become stationary and then produce active larvae (more rediae). Several
generations of these active larvae may develop resulting in great increases of
numbers. Instead of changing into sporocysts, the later generations transform
into active tadpole-shaped larvae (cercariae), which are discharged into the
water by the snail. In order to survive, they must reach the grass and leaves
along the shore where they enclose themselves in resistant cysts and await
their fate of being eaten by a sheep or left to perish. Billions of them are lost.
Chap. 25 FLATWORMS VANGUARD OF THE HIGHER ANIMALS 511
However, in the infected grass it now takes only the right nibble from one
sheep to insure a fluke population. In the sheep's stomach, the digestive juices
free the larvae (cercariae) which then migrate to the liver, chemically and
physically their home niche. They attach themselves by means of the ventral
suckers and in three to six weeks develop into adult flukes, the parents of an-
other generation.
The two greatest gambles in the fluke's life history are on its chances of
entering its hosts, the pond snail and sheep. Both ends are achieved by the pro-
duction of vast numbers of young, the chief tool of a parasite's existence. Prob-
ably one fluke among untold numbers secures the necessary lodging in both
hosts. Yet, the great reproductive capacity of that one hermaphroditic fluke —
half a milUon eggs from a single adult, 300 larvae from a single egg — main-
tains the exuberant success of the species.
Salmon-poisoning Fluke
The salmon-poisoning fluke, Troglotrema salmincola, is prevalent in the
extreme northwestern United States. The adult flukes live in the intestines of
dogs, foxes, bears, bobcats, and other mammals. In dogs, the parasites cause
salmon-poisoning — violent illness often resulting in death.
In order to live, the fertilized eggs must reach the water and enter their inter-
mediate host, a snail called a periwinkle {Goniobasis pUcijera). Larvae similar
to those of the sheep liver fluke develop and finally the active ones (cercariae)
make their way into the water. When these come in contact with trout or
salmon they bore into the muscles and become encysted. If a dog or other pos-
sible host eats salmon raw or semi-cooked, the young flukes are freed from
their cysts and take up their ultimate residence in the intestine and their busi-
ness of creating the next generation.
Important Human Parasites
Human flukes are frequent in tropical and Oriental countries; none is native
to North America. However, infections are occasionally discovered in per-
sons who have been residents of countries where they abound and these may
be a source of further infection. There are four main types of human parasites
in this group.
Blood Flukes. The adults live in the blood vessels of man and several do-
mestic animals. Like those of other flukes the larvae inhabit water snails. One
species. Schistosoma haematobium, is distributed in parts of southern Europe,
Asia, and Australia. It causes the disease called bilharzia in about .fifty per
cent of the population of Egypt. The fertilized eggs are expelled from the
human body in the urine. The embryos hatch in fresh water and ultimately
enter mainly one kind of snail (Bulimus) and undergo part of their develop-
ment within it. Then, the active young cercariae swim out into the water and
512 EVOLUTION OF ANIMALS Part V
the stage is set for the human infection through the skin or by swallowing
infected water. Blood flukes with life histories similar to this are encountered
in the West Indies, the Philippine Islands, China and Japan.
Lung Flukes. Known in Oriental countries, including the Philippine Islands,
and in Central America and Peru, lung flukes occasionally appear in the United
States in former residents of the Orient. The adults of one well-known species,
Paragonimus westermani, deposit their eggs in the cavities of the human lung,
and the fertilized eggs are set free in mucus coughed from the lungs. The
larvae first enter fresh-water snails, and next fresh-water crabs and crayfishes
in which they become inactive and encysted. They then have two chances to
live; meat from the crab must be eaten raw by human beings or water in
which larvae have been freed from dead crabs must be used for drinking.
Intestinal Flukes. Probably the most destructive of these is the giant intesti-
nal fluke, Fasciolopsis biiski, common in man and pigs, particularly in Central
and South China, but also encountered in India, Siam, and Malaya (Fig.
25.12). The adult flukes, about two or three inches long, inhabit the small
intestine and produce the fertilized eggs. In order to live, these eggs must reach
quiet fresh water, the haunts of several species of snails which the larvae
(miracidia) may then enter. About 50 days later, the larval flukes leave the
snails and swim about freely as cercariae. They then encyst themselves on
Fig. 25.12. Life history of the giant intestinal fluke, Fasciolopsis, abundant in
South China. In one stage the larvae are in cysts on water-chestnuts (water
caltrop) that are commonly eaten raw. (Courtesy, Mackie, Hunter and Worth:
Manual of Tropical Medicine. Philadelphia, W. B. Saunders Co., 1945.)
Chap. 25 FLATWORMS — VANGUARD OF THE HIGHER ANIMALS 513
water plants, abundantly on water-chestnuts such as those that were introduced
into the United States and have now crowded other plants and animals to
extinction in a considerable number of American waterways.
The success of this parasite's gamble for life has come with the custom of
eating water-chestnuts. The outer husk is peeled off and the succulent "nut
meat" is eaten raw, an abundant and cheap food in the Chinese summer
markets. In China, as many as 1000 larvae of giant flukes have been picked
from a single water-chestnut.
Liver Flukes. A half dozen or more species of liver flukes are frequent para-
sites of man mostly in Oriental countries. The Chinese liver fluke, Clonorchis
sinensis, is a common parasite of man, cats, and other mammals that eat raw
fish. Enclosed in minute capsules, the encysted larvae can live for many
months in the muscle of 40 different species of fresh-water fishes thus awaiting
a cat or a man to eat them. In an earlier stage, the larvae live in snails. The
great numbers of canals and the farm fish ponds in sections of South China and
Japan are ideal meeting places for the snails and fishes. The people who eat
the fishes give the parasites their final home in the liver.
Class Cestoda
The life histories of such parasites as the flukes are mystery stories com-
pared with the plain histories of their free-living relatives, the planarians. Para-
sitic living has made a still deeper mark on the tapeworm (Cestoda), especially
on their appearance. They are hardly recognizable as flatworms and are well
named after tape measures. It is believed that any vertebrate may be host to
one or another kind of adult tapeworm.
General Characteristics. The cestodes are internal parasites that are deeply
committed to the parasitic habit. Like the trematodes, they have no epidermis;
neither have they a mouth or digestive tract, either in immature or mature
stages. They have no sensory receptors except free nerve endings that are
sensitive to touch. They can move about only feebly, but are amply provided
with holdfasts such as hooks and suction cups. In a few species the body is a
unit, like those of flukes, but in the great majority it is divided into many units
or sections commonly called proglottids from some very highly imagined re-
semblance to the shape of the tongue. It is a question whether proglottids
might not be more appropriately termed segments since they are repeated as
true segments are in the earthworm. The general structure of tapeworms is too
degenerate to establish this.
Adult tapeworms inhabit the intestines of vertebrates entering as larvae,
always by way of the mouth. The total length of adults of different species
ranges from about that of an ordinary typed hyphen to 40 feet. Like the
flukes they require one or more intermediate hosts, vertebrate or inverte-
brate, to complete their life history.
514
EVOLUTION OF ANIMALS
Part V
Most tapeworms are hermaphrodites. Each proglottid contains at least
one set of reproductive organs of each sex, and in some species two sets
(Fig. 25.13). The eggs may be fertilized by sperm cells from the same
proglottid. However, mating proglottids have been observed in tapeworms
immediately after being taken from the intestine. The physiology of tape-
worms is difficult to investigate since they live only a short time outside the
intestine, even in normal salt solutions. The youngest proglottids, behind the
neck, constitute a zone of growth. Those farther back have definite organs; in
the middle parts of the worm they contain mature reproductive organs; toward
the posterior end these organs lose their form and the proglottids become sacs
filled with hordes of eggs and embryos. Although lilies and tapeworms are far
kin, the stages of development in the chain of proglottids are comparable to a
^XENIA SOLlUAf
EGGS
passed in
human feces
/
swallowed by
PIG or MAN
cystlcercus cellulosae
may lodge in brain,
eye, muscle etc.
produces
Cytlicercus cellulosae
in men
CYSTICERGUS CELLULOSAE
(larva) develops in pig Cysticercus
cellulosae
INFECTED MEAT
produces
ADULT TAENIA SOLIUM
in man
Phyllis Smith, 1944.
Fig. 25.13. Life cycle of pork tapeworm, Taenia solium. (Courtesy, Hunter
and Hunter: College Zoology. Philadelphia, W. B. Saunders Co., 1949.)
Chap. 25 FLATWORMS VANGUARD OF THE HIGHER ANIMALS 515
bud, a perfect flower, and finally a seed pod. Fertilized eggs and early embryos
are shed freely into the intestine (Figs. 25.13, 25.14). A ripe proglottid at the
end of the body occasionally separates off, carries the pregnant uterus with it,
and sets free the eggs wherever it may fall with the waste from the intestine.
Proglottids may be eaten by animals of many kinds. They will survive only
if they are swallowed by their secondary hosts. In them, they hatch out in the
intestines and bore their way into voluntary muscle where they become
encysted. Within the cysts they develop into minute bladder-shaped worms, the
cysticercus stage. Their lives now depend on having their final host feed upon
the secondary one, such as a cat or man eating raw fish or pork. The encysted
worm is then freed in the intestine and begins its growth as an adult.
Physiology and Ecology of Adult Tapeworms. Tapeworms live in the dark,
in very special chemical surroundings; shifting hosts is a gamble for life; they
endure a long waiting period (cysticercus); and they perish by thousands. This
is the price of parasitism which tapeworms pay and yet survive.
In making its home in the intestines of vertebrates, the adult tapeworm
adjusts itself within an elaborate canal that is functioning for another animal.
Such canals are in no way modified for the tapeworm. The worm must main-
tain its location against the constant shifting of the walls and the pressure
of moving food. Yet its only anchor is its minute head (scolex) hanging
attached by hooks and suction to the intestinal wall.
Tapeworms live regardless of the presence or absence of oxygen in their
environment. There is very little of it in the intestines.
The content of the host's intestine, the tapeworm's only source of food, is
absorbed through its body wall, but little is known of the process. Glycogen
constitutes about 60 per cent of the dry weight of tapeworms, however, and
is essentially similar to that stored as a reserve food in the bodies of the
majority of animals.
Pork Tapeworm
The two common tapeworms of man are the pork tapeworm and beef tape-
worm. Taenia solium and T. saginata. The latter is distributed throughout most
of the world, especially in parts of Africa and eastern Europe. The rate of in-
fection is high among the Mohammedans who merely sear the outside of large
chunks of beef. In the United States, less than one per cent of inspected beef
has been found infected. The pork tapeworm is also distributed throughout
the world, wherever raw or inadequately cooked pork is eaten. Adult pork
tapeworms rarely occur among Jews and Mohammedans since they seldom eat
pork.
The beef and pork tapeworms are similar in structure and plan of life his-
tory. Man is the only final host of the pork tapeworm and the hog the usual
intermediate host. The adult pork tapeworm lives in the human intestine with
516
EVOLUTION OF ANIMALS
Part V
vas deferen;
cirrus pouch-
cirrus.
genital
pore
genital
atrium
lateral
!rve cord
IS efferens
ieminal
iceptocle
ovary
■ odtype
vitelline
duct
vitelline
gland
transverse
excretory canal
Fig. 25.14. Mature proglottid (or segment) of Taenia pisiformis, a tape-
worm of dogs, showing the male and female reproductive systems. Male System.
The male cells are produced by many minute testes; they are carried by micro-
scopic tubes (vasa efferens) to a larger tube {vas deferens) and discharged
through the genital pore during the mating between proglottids of the same or
of different tapeworms. Female System. Great numbers of microscopic eggs are
produced in the ovaries. They are moved through the oviduct into the vagina
and are there fertilized by sperm cells received from the mating proglottid. The
eggs are then moved backward into a small structure {odtype) where they re-
ceive yolk from the vitelline gland. The fully formed eggs then pass forward
into the uterus that becomes so crowded with them that it finally fills the whole
proglottid, a bag of eggs ready to develop into young tapeworms. (After Good-
child. Courtesy, Brown: Selected Invertebrate Types. New York, John Wiley and
Sons, 1950.)
its head, about the size of a pinhead, attached to the intestinal wall. Posterior
to the short neck is the chain of proglottids which make up the body, from six
to 25 feet long in mature worms with about 1000 proglottids. Each mature
proglottid contains 150 or more testes and at least one complex ovary. Fer-
tilized eggs burst from the proglottids either before or after the latter are cast
out of the intestine. They are protected by shells and on moist soil or vegetation
the embryos may remain alive for weeks.
When swallowed by hogs or man, the embryos hatch soon after reaching
the intestine. The embryos soon pierce the intestinal wall, enter the blood
Chap. 25 FLATWORMS VANGUARD OF THE HIGHER ANIMALS 517
vessels and are distributed through the body. Parasites in general not only
have their own hosts but their particular niches in the host to which they are
chemically and physically adjusted. So it is with young tapeworms. Their par-
ticular niche is the subcutitneous tissue and muscle, usually voluntary muscle
such as that in the shoulders and back — ham and spare rib. In these tissues,
they become encysted and begin their waiting period.
Within 60 to 70 days the encysted embryos have metamorphosed into
bladder worms (about 5 mm. long and 8 mm. broad), the cysticercus stage,
often confusingly called Cysticercus cellulosae as if they were a separate species
as they were first thought to be. Bladder worms are capable of growth into
adult worms if they are freed from their enclosure in the muscle and reach
the human intestine (Fig. 25.13). This is the point at which eating infected and
inadequately cooked pork is a favor to tapeworms. In the intestine, the worm
becomes mature in five to ten weeks but it may live there for several years
continuing to produce and cast off proglottids as well as millions of fertilized
eggs free in the intestinal contents.
Larval tapeworms may make their way out of the human intestine and be-
come encysted in the muscle of the same person. They remain there a long
time and are ultimately absorbed. Cannibalism would be their only gate to
freedom.
Fish Tapeworm
The broad or fish tapeworm, Diphyllobothrium latum, is common in per-
sons living in the Baltic countries, northern Wisconsin, Minnesota, Michigan,
and regions of Canada bordering these states (Fig. 25.15). The adults live
in the human intestine. In order to progress further, the developing eggs must
reach fresh water, where the larvae, then free-swimmers, are eaten by various
IN MAN
larvae in
raw fish eaten
by man
adult worm develops
in human intesfine
copepod
eaten
by fish
egg
swimming embryo
IN MUSCLES"^OF FISH
IN COPEPOD
Fig. 25.15. Life cycle of the fish tapeworm. Diphyllobothrium latum. Adult
fish much reduced; larval stages variously enlarged. (Courtesy, Storer: General
Zoology, ed. 2. New York, McGraw-Hill Book Co., 1951.)
518 EVOLUTION OF ANIMALS Part V
species of minute crustaceans (copepods) in which they develop into the inac-
tive phase. Even so, they travel far since about 22 species of fishes feed upon
copepods. In the fishes the larvae migrate into the muscle, the "clean white
meat." The human infection occurs and the progress of the parasite goes on
when the meat is eaten without thorough cooking.
Consequences of Parasitism
Parasitism is an unbalanced relationship between organisms that has de-
veloped from a balanced one. Parasites and their hosts are close intimates. A
parasite must get on or into one host and stay, or it must have first one and
then another host. At one time or another, or all the time, it must cling to its
host. Its whole success depends upon this.
Parasites ride about on or in their hosts. Those that ride most can move
about least by themselves. In general, the more they depend on the possessions
of the host, the fewer they have of their own. Tapeworms do indeed travel
light, without locomotor organs, without mouth or digestive tract, without skin
cover, without eyes, almost without sense organs.
26
Roundworms — Tlie Tubular Pi
an
Phylum Nemathelminthes — Nematodes
Roundworms are spread over the earth in every region where animals Hve
(Fig. 26.1). Great numbers of them contribute to plant, animal, and human
welfare. Hosts of them live in the soil — minute, hidden, and little known.
Still others are parasites of plants, of invertebrate animals, and probably of all
vertebrates.
Their evolution has included structures of very great importance to higher
animals. The tube-within-a-tube plan of the body first came into existence in
them, the digestive canal as the inner tube, the body wall as the outer one.
Less obvious in a peacock or a man, the plan is as really present in them as it
is in a hookworm or a vinegar eel, both of them roundworms.
There are widely varying degrees of similarity and relationship among
roundworms. Formerly all of them were included in the Phylum Nemathel-
minthes. Now the more closely related roundworms are grouped together in a
phylum, the Nemathelminthes, by some zoologists and in a class, the Nema-
toda, by others. Still other more diverse forms are included in the small phyla
and the classes that are discussed briefly in the next chapter.
Characteristics and Structure Illustrated by Ascaris. Nematodes are slender
worms, pointed at head and tail ends, many of them microscopic, others sev-
eral inches long. The structure of the widely distributed species of Ascaris
that parasitize man and pig is typical of nematodes rn general (Fig. 26.2).
A Human Parasite. Ascaris lumhricoides is among the longest-known
human parasites and is still common in localities where the soil is polluted
with sewage. They probably became established in the human body when wild
pigs were first hunted and eaten, when agriculture was in its beginnings and
pigs were being domesticated. The human parasite {A. lumhricoides) is indis-
tinguishable except in habit from the Ascaris of the pig (A. lumhricoides,
variety suum) from which it doubtless originated. Probably infection with
519
520
EVOLUTION OF ANIMALS
Part V
Fig. 26.1. A free-living nematode worm. Vinegar eels (Tiirbatrix aceti) cul-
tured on an agar (gelatin) plate. They are minute, little longer than the width of
a pinhead. They flourish on the fungus that abounds in the "mother" of raw
cider vinegar; they also live in sour paste. (Courtesy, General Biological Supply
House, Chicago.)
Ascaris usually spreads from man to man with no other animal implicated.
Ordinarily eggs from the Ascaris of pigs do not develop in man nor those
from the Ascaris of man in pigs. However parasites may be otherwise re-
garded, they deserve respect for their sensitive discrimination of environ-
ments.
Life History of Ascaris lumbricoides. The adults live in the small in-
testine where they feed mainly upon the partly digested food of the host, also
upon blood from the intestinal walls. The mature worms mate and each female
produces over 20 millions of eggs. These are freed in the intestine and as
embryos within the thick, resistant egg shells they pass out of it with the
/at era/ //ne
dor^a/ //r?e>
c/or3a/ //p
sensory pap///a
mout/7
/atera/ //ne.
ventro-/atera/ //p
ventra/ //'ne
Fig. 26.2. Ascaris lumbricoides, a human parasite probably introduced to man-
kind when pigs were first domesticated. Upper, outline of the body of the female.
Lower, the sucking mouth guarded by three lips by which it can grasp and suck
blood from the lining of the intestine although it feeds more regularly on the
digesting food. Length of adult females, 8 to 14 inches; males, 3 to 5 inches.
(Courtesy, Curtis & Guthrie: General Zoology, ed. 4. New York, John Wiley and
Sons, 1947.)
Chap. 26 ROUNDWORMS THE TUBULAR PLAN 521
excreta. Under favoring conditions of temperature, moisture, and air the active
embryos develop in about two weeks. In another week, while they are still in
the shell, the minute worms molt and become active larvae. They are now
capable of infecting a host. When the eggs are swallowed, often on uncooked
vegetables, the larvae hatch in the small intestine. After repeated investiga-
tions upon animals which harbor the parasites for a time, it has been discov-
ered that the larvae do not continue to develop in the intestine. Instead, they
pierce the intestinal lining and enter the blood stream thus reaching succes-
sively the liver, heart, and lungs. They burrow out of the lungs, reach the
trachea and esophagus, and finally the intestine. This journey takes about ten
days during which the larvae increase from microscopic size to a length easily
visible to the naked eye. In the intestine they grow to maturity, six to 12 inches
long, the females larger than the males (Fig. 26.2). The average length of
their mature life in the intestine is about a year. The number of eggs in the
mature female may reach 27,000,000, probably more.
Knowledge of the life cycle of this species of Ascaris and the successful treat-
ment of its human host are among the thousands of benefits to human life that
have come from experimentation upon animals. These parasites have not lived
out their life cycle in any animals which have been experimentally infected
with them. Yet, the larvae will migrate through the body in mice and guinea
pigs as well as in their human host. And this was the hardest part of their life
story to discover — why and how they take their roundabout route away from
the intestine through membranes and passageways and back to the intestine
again.
General Structure. Nematodes are clothed with a tough, usually trans-
parent cuticle secreted by a layer of protoplasm in which there are nuclei but
no cell membranes (syncytium). Beneath the syncytium a layer of longitudinal
muscles is divided into four bands that extend the whole length of the body.
When the dorsal band contracts the ventral one is stretched and vice versa;
likewise, when the right side of the body is contracted the left side is stretched
and vice versa. The action of these muscles and probably some rebound
from the bent cuticle compose the entire locomotor outfit of nematodes.
It is responsible for their thrashing gait, a swinging whip in one direction,
and backward whip in the opposite. Even so, they make good progress when
they can push against particles of soil, or against food in the intestine, or as
they squirm through tissues. Water gives them little help. On a microscopic
slide a group of flexing nematodes might be taking a gymnastic exercise, much
bending and no locomotion.
Between the muscles and the digestive tube there is considerable space, a
body cavity in that it holds the organs. However, it is not lined with epithelium,
and thus not a true body cavity or coelom comparable to that of the earth-
worm and of higher animals.
522 EVOLUTION OF ANIMALS Part V
Nematodes have no special circulatory or respiratory systems. The fluid
contained in the body cavity distributes digested food and collects metabolic
waste. The microscopic nematodes of the soil evidently exchange respiratory
gases through the outer cuticle just as minute insect larvae exchange gas
through their extremely thin chitinous covering. Ascaris is mainly anaerobic,
obtaining oxygen from the body fluids of its host and energy from the break-
down of its own stored glycogen. Ascaris has a definite excretory system. Two
canals, one running along each side of the body, come together at the anterior
end and open to the outside through a ventral pore. The nervous system is a
delicate ring of nervous tissue about the esophagus. Two large nerves con-
nected with the ring extend the length of the body, one on the dorsal and one
on the ventral side with connecting branches. The higher invertebrates have
a main ventral nerve chain and the vertebrates a dorsal nerve cord. Ascaris is
not committed to either plan.
The male and female reproductive systems are in separate individuals and
in either one the organs occupy a large part of the body cavity. The eggs are
fertilized in the uterus. Each one is later surrounded by a hardy chitinous shell.
The egg shells are so resistant to chemicals that they will develop while im-
mersed in a weak formalin solution.
Free Living Soil Nematodes. Myriads of little animals find pasture on the
plants in the shallows of fresh waters. These millions feed on one another, on
the algae that cloak the living plants, and on the soft tissues of decaying ones.
Among them in untold numbers are the nematode worms recognizable under
the microscope by their glassy smoothness and translucence. Among other
wigglers of different kin, bristle worms and gnat larvae, the sweeping curves of
the nematodes are distinctive.
Numerous as parasitic nematodes may be, those that live independently in
fresh and salt water and soil probably far outnumber them. Their home niches
are astonishingly various, on lake bottoms, in hot springs, and in polar seas,
in soils, even in deserts.
Vinegar Eels. Who has seen live vinegar eels? Probably nobody who has
used only "store vinegar," pasteurized and bottled. Vinegar eels are the
nematode worms (Turbatrix aceti) of raw cider vinegar. They are about one-
sixteenth of an inch long and their characteristic nematode thrashing move-
ments are recognizable when the vinegar containing them is held up against
strong light (Fig. 26.1). They are distributed on the fruit mainly by fruit flies,
Drosophila melanogaster, the famous fly of genetics. It is also the fly of rotting
apples.
Plant Parasites
Minute nematodes bore into the roots of a great variety of plants. Some of
them, such as the sugar beet worm, Heterodera schactii, live in only a few
Chap. 26 ROUNDWORMS THE TUBULAR PLAN 523
species of plants while the closely related common garden roundworm, Melo-
idogyne marioni, inhabits plants of over 1000 varieties. The worms lay their eggs
either in the roots or in nearby soil. In either case, the young larvae bore their
way into the rootlets. The plant cells are stimulated by the foreign body and
divide rapidly, forming little galls, or root-knots, in which the parasite is walled
in by scar-tissue (Fig. 26.3). The roots soon become so deformed that they
cannot function and the plant dies. In both plants and animals, the tissues of
the hosts often develop growths or secrete substances that wall in the parasite.
Nematodes also enter leaves, usually through the breathing pores (stomata),
and move about the latticed interior, eating out the contents of the cells as they
go (Fig. 26.1). On the outside, the disturbance is marked by twists in the
leaves and by whitened trails. Nematode parasites are harbored by water as
well as land plants. Even sea weeds (Ascophyllum of the Atlantic coast) may
be burdened with nematode galls.
Fig. 26.3. Knot-root caused by a microscopic nematode. Meloidogyne marioni.
Knot-root galls cause great loss to vegetables especially cabbage and its kin,
cotton, and several of the grains: A, tomato; B and C, parsnips. Every knot-root
gall is populated by millions of nematodes. (After Jeffers and Cox. Courtesy,
Walker: Diseases of Vegetable Crops. New York, McGraw-Hill Book Co., 1952.)
524
EVOLUTION OF ANIMALS
Part V
Animal and Human Parasites
Pinworms. Many parasites are highly favored by tropical climates but one
of the commonest, the pinworm {Enterohius vermicularis) is equally abundant
in temperate climates. These are strictly human parasites, most frequent in
children of the Caucasian race. The adults live and reproduce in the intestine,
feeding only upon its content. They are most active at night and then emerge
through the anal opening and lay their developing eggs upon the skin and
clothing; eggs are also freed in the intestine. They are taken into the human
mouth via many kinds of infected objects and eventually hatch and mature
in the intestine. The effects of the infection are irritating rather than dangerous.
Hookworms. Exclusively human hookworm disease like malaria paves the
way for other diseases and often brings whole communities into distress and
poverty. Medical treatment of hookworms is relatively easy and successful.
Teaching people to avoid them is difficult. There are many parts of the world
in which hookworm disease is still an important health problem, in our own
southeastern coastal states, in the West Indies — especially Puerto Rico, in
Central America, in some parts of South America, in Egypt, and in parts of
Africa and Asia (Fig. 26.4). The disease is stopped wherever the ground is
frozen all winter.
There are two widely distributed species of hookworms — the Old World
hookworm, Ancylostoma duodenale, and the American hookworm, Necator
americanus. Their habits are essentially similar but Old World hookworms are
Fig. 26.4. Hookworms of man. A, mouth of the European hookworm. Ancy-
lostoma duodenale, armed with hooks. B, mouth of the American hookworm.
Necator americanus, armed with cutting plates and hooks.
The world distribution of hookworm. Areas that are criss-crossed and deeper
black indicate infection by two species, Necator americanus and Ancylostoma
duodenale. The -\- marks indicate Ancylostoma braziliense, in Central America.
Brazil, Africa, and Pacific Islands. (Courtesy, Craig and Faust: Clinical Para-
sitology, ed. 5. Philadelphia, Lea and Febiger, 1951.)
Chap. 26 ROUNDWORMS — THE TUBULAR PLAN 525
more dangerous to the host and more difficult to eHminate. The fertilized eggs
are extruded in the intestine and, as early embryos, pass out of it with the
feces. On moist warm soil, the larvae hatch within 24 to 48 hours. They bore
downward a little way into' the soil but never travel far in any other direction.
Their very presence on the ground or in water means that human excrement,
known as night soil, has been deposited in the immediate vicinity. This insures
an abundance of bacteria on which the larvae feed. At the end of about five
days they molt a second time although the loosened cuticle is not cast off but
stays on until it is worn away by the worm's movements against the soil.
They are now in the infective stage, with bodies that are slender, sharply
pointed, and of microscopic size. They become not only different in shape but
their appetites change. They forsake the bacteria on which they have fed, are
restless and go without food. Instead of boring downward as they did earlier,
they now squirm upward and lie as close to the surface of the soil as possible
and still keep moist. They are now prepared to bore into human skin, usually
on the feet. The country may be one in which night soil is used as a fertilizer
as is common in Asia. In that case, the larvae wander over the vegetables and
so have a good chance at the human mouth and a direct route to the intestine.
If they enter through the skin, they burrow until they reach a lymph or blood
vessel, and in the circulation they are ultimately taken to the lungs. There they
are caught in the capillaries and this particular environment stimulates them
to burrow out into the air chambers. This is nicety of discrimination at its
height. In the lungs, the upward movement of the cilia acts as an escalator
that carries them to the throat from which they are swallowed. They are then
on the way to their final stop in the intestine. There they bury themselves for
a short time between the villi, go through a third molt and develop a mouth
by which they grasp the intestinal wall (Fig. 26.4). They grow rapidly until
they are about one-quarter of an inch long and then molt for the fourth and
last time. With this molt, the mouth is changed to its final form and the worms
become mature. They are now able to clamp their mouths to the intestinal
lining, to wound the capillaries and to suck blood. Eggs begin to appear in the
feces about six weeks after a known infection, a sign that the parasites consti-
tute a growing population and are steadily drawing blood from their host. By
ingenious calculations upon the number of the female population it is figured
that each female sucks one cc. of blood from the host per day. In doing so
they are provisioning a metabolism that according to careful estimates enables
a female of Necator americaniis to produce from 5000 to 10,000 eggs per day.
Each one is fertilized internally and the embryo leaves the female body in the
four-celled stage of development.
Fortunately, this multiplicity is reduced by circumstances. The embryos will
not develop beyond four cells unless they are exposed to air. This hinders the
succession of one generation after another within the intestine. Whatever sub-
526
EVOLUTION OF ANIMALS
Part V
stance surrounds the developing embryos must be moderately warm and moist,
must contain bacteria and be well mixed with air. Temperature between 70° F.
and 85° F. is the optimum; if it is much lower or higher than that, the embryos
are injured or destroyed. Direct sunshine and drying kills them. Wriggling
through soil is rugged business and clay or salty ground injures them. Hook-
worms are not long-lived, at most about five years. Infections tend to die out
unless repeated, the chemical environment having changed, and immunity be-
ing established. Such obstacles as these are set against the daily litters of
10,000 eggs.
Trichina. Adult trichinae {Trichinella spiralis) are parasites of the intestine.
But it is young ones, not the adults, which are responsible for the serious dis-
turbance called trichinosis. Unlike most parasitic worms, they live in temperate
climates and are almost completely absent from the tropics; they occur mainly
in Europe and the United States. According to data of 1947 and more recent
estimates, the United States had three times as much trichinosis as all other
countries combined.
Trichinae most often parasitize man and pigs but can live in other animals
(Fig. 26.5). Rats and cats are easily infected, dogs are less so, and birds are
resistant to them. Human infections usually come from eating imperfectly
cooked pork, hurriedly cooked roasts with red parts left in the center, and ham
improperly cured and cooked. In the United States at this date, the prevalence
of infections in man and pigs is highest in the Atlantic States, especially in
Original Source
of
Infection
for Hogs
(Usually Garbage)
Infected Rats
Infected Hog
U^
Infected Cats, Dogs
& Other Animals
Fig. 26.5. Diagram illustrating the common methods of exposure to trichinosis
(caused by Trichinella spiralis) in the continental United States. (Courtesy, Craig
and Faust: Clinical Parasitology, ed. 5. Philadelphia, Lea and Febiger, 1951.)
Chap. 26 ROUNDWORMS THE TUBULAR PLAN 527
Massachusetts, and on the west coast. Essentially, it occurs wherever pigs are
fed on garbage that contains bits of infected pork. However, marketing of meat
products into different regions of the country does not leave any locality free
from suspicion. Uninspected pork from farms and small butchering places has
proven more dangerous than government inspected pork. Trichinae have not
been eliminated anywhere. More effective than inspection is the fact that pork
is usually refrigerated for long intervals which kills trichinae.
Life History. Trichina worms are usually swallowed as immature larvae
enclosed in cysts embedded in pork muscle (Fig. 26.6). The cysts are digested
off and the microscopic larvae bore into the intestinal wall where they grow to
maturity, mate and reproduce within five to seven days after they are swal-
lowed. The adults may or may not cause intestinal disturbances depending
upon the number of larvae that were swallowed. An ounce of heavily infected
pork sausage may contain 100,000 encysted larvae.
The embryo trichinae develop in the uterus of the mother. The microscopic
larvae are born alive, burrow into the capillaries and become numerous in the
blood between two and three weeks after their parents were swallowed. They
are distributed all over the body but finally settle into muscles that have a large
blood supply, those of the diaphragm, the thorax, the legs, but not the heart
(Figs. 26.5, 26.6).
After they enter the muscles, the larvae grow rapidly but are still practically
microscopic. They are then in the infective stage. Their only chance for life is
that the muscle which they occupy may be eaten by an animal in which they
Fig. 26.6. Drawing of microscopic cyst of trichinae about three weeks old.
The walls of cysts contained in infected pork are digested off in the human
stomach and the larvae develop into adults within five to seven days. Mating
occurs and the females produce living young, larvae that invade the body within
about three weeks, finally settling into muscles and other tissues in the encysted
state shown here. See also figure 26.1. The harm to the body is done by the
migrations of larvae, rather than by the cysts. (Courtesy, Craig and Faust:
Clinical Parasitology, ed. 5. Philadelphia, Lea and Febiger, 1951.)
528 EVOLUTION OF ANIMALS Part V
can survive. Otherwise, they die in the cysts and become calcified. The trichi-
nae in the human body constitute great populations of suicides since human
cannibaHsm is almost extinct. The survival of trichinae is kept up only by the
eating of infected scraps of meat, mainly by pigs and rats.
Trichinae differ from other intestinal parasites in that the young do not leave
their native host and take their chances for a new one. The majority of young
trichinae stay within their home hosts, although this means destruction for so
many. How well the species can afford this is shown by the prevalence of
trichinosis. The invasion of the muscle is a critical step for the larvae and in
heavy infections highly dangerous for the host. The symptoms include intense
pain in particular muscles, great difficulty in breathing, and in movements of
the eyes and jaws. The surrounding muscle fibers become inflamed and dis-
integrate. About six weeks after the original infection walls form about the
larvae then curled up among the muscle fibers. Gradually one, sometimes two
or more larvae are walled into the capsule that at first is delicate but after a
year or more becomes hard and chalky. This encystment phase is the second
dangerous one for the host. Other symptoms continue and pneumonia is often
a complication. The host does not recover until eight weeks to several months
after the infection. Even after that there is a period of a year or longer when
the jarring and stretching of the muscles is made painful by the cysts.
Filariae. With infections of trichinae the immature young are the chief cause
of disturbance; with infections of filariae the adults are the main trouble
makers. The adults, living in the human passages, produce young called micro-
filariae. The embryonic microfilariae must go through a stage of development
in a blood-sucking insect before they become infective to man (Fig. 26.7).
Filarial parasites {Wuchereria bancrojti) are widely distributed in tropical
and subtropical countries, especially in coastal regions and on islands (Fig.
26.8). In the western hemisphere they occur throughout the West Indies,
Panama, and northern South America. The adults are the cause of elephanti-
asis. They live in the lymph passages, tangled together like snarls of coarse
white threads, the females about three inches in length, the males half as long.
Life Cycle. Within the lymph passages the females give birth to the micro-
filariae. These are microscopic (about 0.2 mm.) slender squirmers that at
once bore into the blood and lymph capillaries, and are carried over the body
by the circulating blood (Fig. 26.7). Their further development depends on
their being sucked up with the blood by a biting mosquito (female) that may
belong to one of several genera. Anopheles, Culex, and others. Experiments
have shown that there must be at least 15 microfilariae per drop of blood in
order to infect the mosquito. Evidently they must be numerous enough to
condition their surroundings by their metabolic by-products. Blood containing
100 or more microfilariae per drop will kill a mosquito, even one of the trans-
mitting species. Yet, the blood of heavily infected persons commonly contains
Chap. 26
ROUNDWORMS — THE TUBULAR PLAN
529
Fig. 26.7. The microscopic filaria worms. Wiichereria hancrofti, swarming in
human blood at night. They are parasites in human lymph glands and in certain
species of mosquitoes which are essential to their complete life cycle and which
transmit them to their human hosts. They are the cause of filariasis (elephan-
tiasis). (Courtesy, Craig and Faust: Clinical Parasitology, ed. 5. Philadelphia,
Lea and Febiger, 1951.)
several hundred of them per drop. Many mosquitoes must be killed by large
meals of them. Thus, millions of microfilariae are swallowed into death traps
as surely as human muscles are death traps for trichina larvae.
Within the mosquito, the microfilariae immediately bore through the stom-
ach wall and enter the muscles of the thorax. There they develop into larvae;
their form changes from slenderness to sausage shape, and back again to
slenderness and lengthening. This takes about 10 days at the end of which
they are physiologically set for a change. They wriggle out of the thoracic
muscles of the mosquito and make their way into its mouth parts (Fig. 26.7).
The mosquito is now loaded with infective larvae. Mosquitoes that carry
microfilariae live near human dwellings, not far to go for a blood meal.
Everybody must have seen mosquitoes feel the skin for an easy place to
bite. The filaria-loaded mosquito does this like any other mosquito, and the
larvae in its mouthparts stimulated by the warmth and pressure of the flesh at
once bore their way through the mosquito's labium (lower lip) and into the
skin. The next chapter of filaria life history is almost a blank. Into what part
of the human body the larvae go and how long before they are full grown
inhabitants of the lymph passages is mostly unknown. Their arrival there is a
certainty.
530
EVOLUTION OF ANIMALS
Part V
Phylum Nematomorpha
Horsehair Worms. Adult horsehair worms writhe slowly like living wire or
he in still coils in the edge-waters of ponds. They used to be common in drink-
ing troughs and the wayfarers who saw them added their testimony to the
belief that horsehairs "turn to life" after a night in the water. Adult hairworms
are from a few millimeters to a yard in length; in shallow water they are easily
noticeable; coiled in the body cavity of a freshly killed grasshopper they are
spectacular.
The names of the genera, Gordius and Paragordius, come from the Gordian
knot that their coils suggest.
,Proboscis
Body cavity
Cement glantf
Lemniscus
ADULT MALE
Fig. 26.8. Structure of typical spiny-headed worm or Acanthocephala. These
worms are parasites of fishes, birds, and mammals in most of the world including
the Arctic and Antarctic. They range in size from less than an inch to more than
one foot. (Courtesy, Hunter and Hunter: College Zoology. Philadelphia, W. B.
Saunders Co., 1949.)
General Structure — Advance over Flatworms and Nematodes. The
body cavity is lined with epithelium and is thus a true coelom. Partitions of
loose tissue divide the cavity into compartments. It is not filled with tissue
(parenchyma) as the comparable cavity is in nematodes. A single midventral
nerve connects with the brain by way of the ring around the esophagus, an
arrangement suggesting the one in the earthworm and insects.
The adult worm is uniformly cylindrical and slender. Its covering of cuticle
is very thin but the thickness of the body wall makes the cuticle look opaque.
There is no special circulatory, respiratory, or excretory system. The digestive
canal is open throughout its length in young worms but may close or degen-
erate in adults. The sexes are separate. The eggs are shed from the ovaries
into the coelom and then pass into the oviducts which are structurally separate
from the ovaries as they are in the vertebrates.
Life Cycle and Ecology. Several stages in the life history of horsehair
worms were discovered many years ago, but the actual life cycle has been
learned only recently by controlled experiments in the laboratory, as well as
Chap. 26 ROUNDWORMS THE TUBULAR PLAN 531
observations in the natural habitats. The life cycle of hairworms is another evi-
dence of the precision with which an individual parasite must follow a fixed
schedule of life or perish. Production of great numbers is the safeguard of the
species.
After mating, the females lay their eggs in strings usually twined about twigs
submerged in the water. These are from 15 to 20 cm. long and contain an
enormous number of minute eggs. Gordius lays more than half a million eggs
and Paragordius about six million. Toward fall the adults die, the males before
the females. The microscopic larva pierces the egg shell at a point that it
softens with its own secretion. Within 24 hours after hatching it surrounds
itself with a cyst wall and becomes inactive. If it is prevented from doing this
on time, it loses its power to do so. Larvae may live for two months within
cysts submerged in water, and for a month when they are in damp air.
The cysts are swallowed by aquatic insects or by land insects, such as grass-
hoppers and crickets, that forage on the grasses at the water's edge. As soon
as the cyst walls are digested off, the larvae pierce the wall of the gut and bur-
row into fatty tissue from which they absorb abundant nourishment. There the
young Gordius grows and changes to the mature form. If the host is an aquatic
insect, the parasite escapes directly into the water. If it is a land insect, its
successful escape must await the host's visit to the waterside. Most of these
facts have been learned from experimental infections of insects.
It is noticeable that Gordius does not strictly specify its host. Well-grown
worms have been found in various species of insects; larvae are probably
swallowed and mature in several different aquatic invertebrates.
Phylum Acanthocephala
Spiny-headed Worms. Spiny-headed worms constitute a peculiar group of
about 300 species ranging in length from six to 460 mm. (IVi ft.). All are
parasites of vertebrates, from fishes to mammals. The name refers to their dis-
tinctive feature, a relatively short retractile proboscis armed with rows of stout
recurved hooks (Fig. 26.8). The worm projects this proboscis in among the
folds of the lining of the intestine of its host and holds its place with the hooks
while it absorbs nourishment through the delicate porous cuticle that covers
its body. Neither larva nor adult has a digestive tract, and no circulatory or
respiratory organs. There are two primitive kidneys, and a roomy body cavity
but, lacking a peritoneal lining, it is not a true coelom. The sexes are separate.
The eggs are fertilized internally and the embryos well developed before they
are extruded into the intestine of the host.
Life Cycle. The life cycle includes an intermediate host, usually an ar-
thropod: small crustaceans for those that are parasites of fishes and other
aquatic vertebrates; cockroaches, larvae of June beetles and other terrestrial
arthropods for those that are parasites of pigs, rats, and other land vertebrates.
532 EVOLUTION OF ANIMALS Part V
Characteristics of Ecology and Form of Nematodes
Nematodes live everywhere that animals can exist. Great numbers of minute
free-living ones stir and enrich the soil. In both soil and water they constitute
links in the food chains that reach to higher animals. As parasites, large num-
bers of them are physiologically intimate with many species of plants and
animals.
They are slender, cylindrical, and covered with a protective cuticle. They
have a functional body cavity containing organs, but not a true coelom. The
digestive tract is a canal with mouth and anal openings. The sexes are separate.
Ectoderm, mesoderm, and endoderm are present.
The movements of nematodes are distinctive; swinging and thrashing due
almost completely to the use of longitudinal muscles.
27
An Aquatic Miscellany
Ecological Intimacy. Ecologically, the animals described in this chapter are
closely related and they have shared the welfare of water for untold genera-
tions. They have gradually fitted into one or another of the numberless niches
in water, from ponds to oceans. They have many traits in common, also differ-
ences. The latter are the basis for their separation into several distinct groups.
Most of these animals are marine. As adults they creep, burrow, or are
attached to rocks and plants, but in general the young swim about freely and
are carried by the ever shifting currents of water. The free-swimming young
of several of the groups resemble one another and are also similar to those of
annelids and mollusks (Fig. 27.1). They are trochophores (Gr. trochos, wheel
-f phoriis, to bear), the minute larvae which suggest that all of them used to
resemble one another throughout their lives, though they do not now. Even
the various adults meet over the same kinds of food. They consume bacteria
and silica-coated diatoms, and themselves provide protein and minerals for
their slightly larger neighbors. Rotifers, bryozoans, brachiopods and phoronids
are food-sifters relying on the transporting power of water and their own
equipment of tentacles and cilia to bring the harvest to their mouths.
The animals of this "miscellany" do not lack conflicts and contrasts, dra-
matic in their vigor and precision. Carnivorous rotifers hunt down the water-
fleas with furious pounce. Arrow worms move up and down in the sea by the
time clock of light. In the morning and evening twilights, millions of them
swarm through the surface waters of the ocean. They arrive in them promptly,
remain while the amount of light is precisely right for them, and departing
sharply, spend other hours in the darkness of deep water. Also among the
miscellany is the lamp shell, Lingula, so like the fossils of its ancestors of
400,000.000 or more years ago, that its nickname is "living fossil."
The classification of these groups has been rearranged several times and
changes are still being made. Some groups have long been recognized as unique
533
534
EVOLUTION OF ANIMALS
Part V
enough to warrant their status as phyla. Others are named classes by certain
zoologists and phyla by others. There are facts that stimulate arguments for
both opinions. The latter one is followed here.
Trochophore Larvae. The trochophore is a pear-shaped larva, the stem
of the pear being the future posterior end of the animal (Fig. 27.1). A wheel
of cilia, encircles the body which also bears tufts of longer cilia, all of them
used in swimming. The complete U- or L-shaped digestive tube is lined with
cilia. The nervous system is relatively elaborate and there are various sense
organs such as eyes and organs of balance that might be expected on an active
animal. The trochophore larvae of several phyla of invertebrates have already
been mentioned. In annelid worms and mollusks, the trochophores are very
similar but the adult earthworm and clam into which they develop can hardly
be confused. Immature animals show likenesses; mature ones show the dif-
ferences.
Phylum Nemertinea — Ribbon Worms
Most ribbon worms live between the tide lines coiled among the rocks and
seaweeds; a few live in fresh water or moist earth. All of them are slender,
and their stretching ability is fantastic. The common Cerebratuhis lacteus of
muddy sands on Atlantic shores is three feet long when contracted and may
be 35 feet outstretched, flat and only an inch wide. Its near relative {Cerebrat-
SIMILARITY OF YOUNG MARINE INVERTEBRATES
A. Primitive
worm
B. Annelid
worm
C. Snail
Fig. 27.1. Young stages of three marine invertebrates. A, Polygordius, a relative
of annelid worms. B, Echiurus, a marine worm that as an adult (4 inches long)
burrows in sandy bottoms. C, Patella, the limpet, a snail that clings to rocks.
These animals are strikingly similar in their young stages but very different in
habit and appearance when they are mature. As transparent, ciliated larvae they
swim free in the sea making their own living; in the remote past they probably
did so throughout their lives. {A and B after Hatschek. C after Patten. Courtesy,
Hesse and Doflein: Tierbau und Tierleben. Leipzig, Teubner, 1910.)
Chap. 27 AN AQUATIC MISCELLANY 535
ulus hercLileus) of the CaHfornian coast is 12 feet long contracted and an esti-
mated 75 feet when expanded. The length of outstretched ribbon worms is
partly due to the extended proboscis that commonly reaches forward two or
more times the length of the body. Not all ribbon worms are so long; some are
minute and many measure but a few inches. Like flatworms, some are strongly
colored and patterned, many are pale and the species are difficult to identify.
The proboscis usually marks them as ribbon worms.
Unique Features. Proboscis. The ribbon worms' unique and surprising
feature is the protrusible proboscis that shoots rapidly forward, comes in con-
tact with some hapless clamworm (Nereis), twines around it, and shortening
again, pulls the prey back to its mouth (Fig. 27.2). Then the proboscis and
the clamworm both disappear. It is as if an elephant could roll its trunk out
Fig. 27.2. Ribbon worm, Linens socialis, 10 inches or more long, its body con-
tracted in a characteristic close spiral. Ribbon worms prey upon clamworms that
live among the tide washed seaweeds. (From original of figure 1, Wesley R. Coe,
/. Exp. ZooL, 54:416.)
of a short upper lip, catch a peanut on it, and telescope it inside again. When
the wandering ribbon worm (Paranemertes peregrina) of the Pacific Coast
comes upon the tunnel of an annelid it extends its slender proboscis through
it, like a "plumber's snake," finally winds it about the annelid owner and pulls
the latter out. The proboscis is withdrawn by the shortening of a retractile
muscle and pushed out when the walls of its sheath are contracted upon the
fluid in the sheath.
Regeneration. Ribbon worms have exuberant powers of regeneration.
They break easily, but they more than make up for this in their mending.
Those of different species vary greatly in the freedom with which they frag-
ment; some break into many pieces whenever they are touched. The hinder
parts of mature worms commonly break up spontaneously into pieces which
regenerate into perfect individuals, the regular method of asexual reproduction.
Many experiments in regeneration have been made upon ribbon worms by
W. R. Coe especially upon Linens socialis of the Atlantic and Linens vegetiis
of the Pacific Coast (Figs. 27.3, 27.4). If a worm 100 mm. long is cut into
100 pieces, each one mm. long, they will develop into an equal number of
minute worms in four to five weeks. Regenerated worms like whole ones can
536
EVOLUTION OF ANIMALS
Part V
Fig. 27.3. Typical stages in the regeneration of ribbon worm. Linens socialis,
from a fragment (/) taken back of the mouth. (From original of figure 4,
Wesley R. Coe, /. Exp. Zool., 54:426.)
go without food for a year pr more during which they live upon their own
constantly decreasing bodies.
Ribbon worms dwarfed by starving are commonly found in nature. Many
dwarfs have been produced experimentally. When examined with the micro-
scope they reveal a series of sacrifices. Some of the cells of the primitive
middle layer, mesenchyme, become wandering phagocytes literally devouring
the body cells, especially those of the digestive canal. Loaded with food these
cells then disintegrate and their remains furnish food for surviving cells. As
starving continues, this process is repeated over and over and the animal be-
comes smaller and smaller. In this way digestive tract, reproductive organs,
and muscles gradually disappear.
Circulatory System. Ribbon worms are the simplest animals to have a
circulatory system of the closed type with true blood vessels and spaces in the
mesenchyme continuous with the vessels. The blood is usually a colorless fluid
carrying blood cells but in various species it may be yellow, green, or red —
the red color due to hemoglobin contained in the cells as in human blood. This
system takes over the distribution of substances that in flatworms were carried
by fluid in the gastrovascular cavity. There is no special pumping organ and
the blood vessels have few branches.
Chap. 27
AN AQUATIC MISCELLANY
537
Fig. 27.4. Reproduction by natural division in Linens socialis. A, mature worm;
B, dividing; C, reconstruction of these pieces into nine complete worms; D, por-
tion of body of mature worm showing zones of division. (Courtesy, Coe,
Physiol. ZooL, 3:299, 1930.)
Structures and Functions. The important structural advances in which rib-
bon worms have progressed beyond the flatworms are the digestive canal with
mouth and anal openings present in all members of the phylum, and the circu-
latory system. The general plan of the body is otherwise similar to that of
planarians. The body is completely covered with ciliated epithelium and be-
neath it are the circular and longitudinal muscles. There is no special respira-
tory organ. The excretory system consists of a pair of lateral canals with side
branches ending in flame cells. The male and female systems are usually in
separate individuals; a few species are hermaphroditic. Eggs and sperm are
produced in many little sacs which open directly to the outside. The sex cells
are strewn into the water where fertilization occurs and the free-swimming
helmet-shaped larva (pilidium) develops (Fig. 27.1).
Ecology. Habitats. Most marine ribbon worms are bottom dwellers in
mucky sand, within holes lined with mucus; some live in parchmentlike tubes
similar to those built by annelid worms.
Feeding. Ribbon worms are carnivores — burrowers that feed chiefly on
annelid worms, especially the abundant clamworms (Nereis), and they forage
mostly at night when the latter are active. The proboscis is their chief burrow-
ing tool.
Way of Living. Many nemerteans are free-living predators. Others are
commensal, sharing "the bed and board" with a partner without affecting it.
A smaller species {Malacobdella grossa) lives in the mantle cavity of various
clams on the Atlantic coast, and in about 80 per cent of the razor-shell clams
538 EVOLUTION OF ANIMALS Part V
of the Pacific Coast. The currents of cilia that deliver the fragments of food to
the clam's mouth at the same time serve it to the worms. Certain ribbon worms
(Carcinonemertes) live on crabs, as larvae on the gills and as adults on the
eggs. In the water as on land every cranny is a home niche for a plant or
animal.
Phylum Rotifera — Trochelminthes
Their Place in Nature. Rotifers are minute animals that abound in fresh
waters throughout the world. Only a few species live in salt water. Their forms
and habits are in general similar, but in details they are varied and fit with
durable nicety into the niches that their worldwide distribution supplies. They
consume microscopic plants and animals, living or dead, and clean the water
of its least debris. In turn they are eaten by their next sized neighbors, which
are in turn eaten by larger animals and so on up to the fishes.
The body walls of rotifers are transparent, and their internal organs as easy
to see as the works of a glass clock. Both inside and out their constant activity
is visible. One of the early observers (Eichhorn 1761) wrote of Floscularia
(Fig. 27.5), "Now I come to a very wonderful animal, which has very often
rejoiced me in my observations: I call it the Catcher: extraordinarily artistic
in its structure, wonderful in its actions, rapid in capturing its prey." Anton
Leeuwenhoek, who first described rotifers in 1703, gave them their name
meaning wheel-bearers and thought that he had discovered the principle of
the rotating wheel in nature.
Rotifers are an old group with a very long evolution. They resemble flat-
worms in having ciliated excretory organs called the flame cells. As in round-
FiG. 27.5. Representative rotifers selected from the great variety of form and
habit in this typically fresh-water group. Floscularia (old name Melicerta),
Rotifers of this group create tubes of a gelatinous secretion covered with
meticulously fashioned pellets and attached to submerged plants. They are ele-
gant but precise creations — minute, yet easily visible to the naked eye. Asplanchna,
a predator on all kinds of minute animals including other rotifers. Brachionus
is an omnivorous eater, dependent upon its crown of cilia to whirl particles of
food into its grinding mastax. Polyarthra, a lake rotifer often in water that is poor
in oxygen. It moves by jerks owing to the sudden beating movements of its
long appendages. (Adapted from various sources. Courtesy, Needham and Need-
ham: Guide to the Study of Fresh Water Biology. Ithaca, N.Y., Cornell Uni-
versity Press, 1941.)
Chap. 27 AN AQUATIC MISCELLANY 539
worms, the body cavity has no special lining and hence is not a coelom.
Rotifers are composed of relatively few cells, commonly a definite number
characteristic of a species. Many of the adults are shaped like larval worms
(trochophores).
Unique Features. The corona is unique and essential in the life of rotifers
(Fig. 27.6). It is an irregular disk rimmed and banded with cilia whose beat
creates the effect of rotation, the wheels that delighted Leeuwenhoek. The
corona functions in locomotion, in gathering food, and in respiration. The
mastax or chewing pharynx is unique. In other animals food is ground after it
leaves the mouth, in the gizzard (part of the esophagus) of the grasshopper,
and in the stomach of lobsters, but only the rotifers have a chewing-throat.
Structures and Functions. Various rotifers are excessively slender, almost
spherical, turtle-shaped, and flowerlike; very many of them have figures with
profiles like those of carrots and turnips (Fig. 27.5). The anterior end is
topped by the corona. At the posterior end, the body narrows into the foot and
one or two toes. Rotifers can poise and pirouette with sure stance because in
each toe there is a cement gland that secretes a sticky temporary anchorage.
Numbers of Cells and Nuclel There is probably an approximately con-
stant number of cells in the bodies of various species of multicellular animals,
VENTRAL
Corona
Mostax
Salivary gland
Gastric gland
Ovary
Yolk gland
Excretory
bladder
Pedal gland
Toe
Corona
Mostox
Stonnach
Neohridio
Intestine
DORSAL
Fig. 27.6. General structure of a female rotifer. The majority of rotifers are
females; in many species males are unknown and reproduction is altogether
parthenogenetic. Unique features of rotifers are the corona or rotating crown of
cilia, and the mastax or chewing pharynx. (Courtesy, Robert W. Pennak: Fresh-
water Invertebrates of the United States. Copyright 1953, The Ronald Press
Company.)
540 EVOLUTION OF ANIMALS Part V
even of man. In rotifers, there is a definite number of cells according to the
species, and so few that they can be easily counted. Adults are peculiar in that
the whole body may be composed of incomplete cells (syncytia). In Hydatina
senta, there are about 1000 nuclei in every adult and in embryos of the same
species about 1000 complete cells are formed. Later, the cell membranes dis-
appear but the number and locations of the nuclei remain definite and in the
same relative position as in the embryo.
Other Functions. The corona helps in locomotion, respiration, and get-
ting food. Whether a rotifer is swimming or creeping, the strong backward
strokes of the cilia on the corona drive it forward. Rotifers are delicately
responsive to their surroundings and the activity of the cilia is quickened or
slowed accordingly. Their beat continually bathes the animal with fresh water,
provides oxygen and food, at the same time carrying away the carbon dioxide.
Particles floating in the water are brought to the mouth at the center of the
whirlpool (Fig. 27.7). The rotifer needs only to open its mouth.
Whatever the food may be, whole cells or fragments, plants or animals, it
is whirled into the grinding mastax (Fig. 27.8). It then passes through the
esophagus to the stomach where chemical digestion is carried on by the secre-
tion from two large gastric glands. Digested food is absorbed through the walls
of the stomach and intestine and on into other regions of the body. The un-
digested remains pass out through the anal opening. It is to be remembered
that rotifers swallow great numbers of diatoms, all of them encased in silicious
shells considerably harder than glass. In some species, the digestive canal does
Fig. 27.7. Currents of water produced by the cilia of the corona of a rotifer,
Proales. The fine dots represent particles drawn in a vortex toward the mouth as
the animal moves toward the right. The cilia strike backward more strongly than
forward and thus produce currents of water that pass backward from in front
of the animal to its mouth and over the surface of its body. Thus, they bring
food and a continual supply of oxygen to the surface of the body. (Courtesy,
Ward and Whipple: Fresh Water Biology. New York, John Wiley and Sons, 1918.)
AN AQUATIC MISCELLANY
541
Chap. 27
not extend beyond the stomach in either sex; in others, it is incomplete only
in the males. Undigested remains are discharged from the mouth as they are
in hydra.
Nitrogenous waste is removed by means of ciliated cells (flame cells)
located at intervals along the two excretory tubes. These primitive kidneys
that extend backward beside the digestive tube open into the contractile vesicle.
The vesicle discharges relatively large amounts of water into the cloaca. Thus,
Fig. 27.8. Left, rotifers (Brachionus) holding on to Daphnia by their
sticky toes, and collecting particles of food from the water as they ride. Right, a
fierce carnivore (Dicranophorus) eating its way into one of its neighbor caldo-
cerans. (Courtesy, Myers, "What is a Rotifer?" Nat. Hist. 25:221, 1925.)
the excretory system is a water balancer just as the contractile vacuole is in the
ameba and as the kidneys and urinary bladder are in the frog and higher
animals.
The main part of the nervous system is the brain and from it nerves pass to
various organs. There are several sense organs, usually one or two red eye-
spots, evidently strong tactile senses in the corona, and a pair of sensory tufts
on the sides of the body. The sense of touch must be elaborate in Floscularia
{Melicerta ringens) which builds its exquisite case with great precision of uni-
formly rounded microscopic pellets (Fig. 27.5).
Reproduction and Life Cycle. Female rotifers have a single ovary, a yolk
gland that supplies the eggs with food, and a short oviduct that carries them to
the cloaca in which they are fertilized (Fig. 27.6). Male rotifers are incom-
pletely developed except for the reproductive system. In some species, there
are no males. All eggs have the diploid or double number of chromosomes and
develop without fertilization into females.
An annual succession of generations typical of many summer rotifers (hav-
ing parthenogenetic generations in summer) is outlined in Figure 27.9. The
chief peculiarities of rotifers are due to the presence of the diploid number of
chromosomes in the eggs of the parthenogenetic female-producing females;
and of the haploid or single number of chromosomes in the eggs of the sexual
females. Parthenogenesis and diploid and haploid numbers are explained in
Chapters 18 and 20. The reproductive cycle has a seasonal rhythm. A stem
mother produces a generation of females, parthenogenetically. These are suc-
ceeded by several generations of females, an enormous population, all likewise
542 EVOLUTION OF ANIMALS Part V
SUMMER
Much food. Great population
Every unit a female A
reproducing asexually f
LATE SUMMER
Asexual reproduction
decreases
0-> *=VA_^ ^
^ ^^^ ^ Sexual reproduction
^ / (J) begins
SPRING \U A FALL
More food f \S\ S produces
More rotifers rx V/ some eggs (m)
Resting eggs W\ $ motes with M / I 'hat unfertilized
develop into \) and produces / i develop into
stem mothers \ fertilized / moles. |||V1
winter eggs
Al \
©
<^
sperm
WINTER
Food sparse. Population reduced
to fertilized resting eggs, ^y^
Fig. 27.9. Annual succession of generations typical of summer rotifers. Begin-
ning with the stem mothers {A I) of spring there are successive generations that
consist only of parthenogenetic females (A), all producing female young (eggs,
/) an economical arrangement for great multiplication. Under changed con-
ditions that occur in the fall a generation of sexual females (S) arises whose
unfertilized eggs (m) develop into males (M). These males mate with the sexual
females (5) of their mothers' generation and produce the fertilized resting or
"winter" eggs {W) from which stem mothers (Al) develop. In the spring the stem
mothers produce parthenogenetic females and the cycle begins again. (Based on
data for Lecane inermis by H. R. Miller, Biol. Bull, 60:345-380, 1931.
produced from unfertilized eggs containing the diploid number of chromo-
somes. Then, with a change in the environment, such as temperature, or food,
or others not fully understood, generations of sexual females appear that bear
especially small eggs. They contain the haploid number of chromosomes and
develop parthenogenetically into males. These males mate with the sexual
females, actually the generation of their mothers. The fertilized eggs that result
become the thick-shelled resting or "winter" eggs. They contain the haploid
number of chromosomes from the male plus the haploid number from the
sexual female, and thus carry a biparental inheritance. After a resting period
they develop into the stem mothers.
Seasonal Differences in Reproductive Cycles. There are striking sea-
Chap. 27 AN AQUATIC MISCELLANY 543
sonal changes in rotifers, differences among species, and variations in the form
and activities of individuals within the same species. In the perennial ones,
parthenogenetic reproduction continues throughout the year although sexual
reproduction may also occur in spring and fall. In the summer species, par-
thenogenetic reproduction occurs in summer, sexual reproduction in the fall
and the species is carried over the winter in resting eggs (Fig. 27.9). In the
winter species, there is a large parthenogenetic population in winter and the
males appear in the spring.
Cycles Changed Experimentally. The reproductive cycles are readily
changed experimentally by food and temperature. When carefully cultured
populations of rotifers (Brachionus pola) were kept on scanty food, partheno-
genetic females were produced. When the food was adequate and plentiful,
sexual females soon became superabundant. In other experiments, D. D.
Whitney fed rotifers (Hydatina) on colorless flagellate protozoans (Polytoma)
and obtained 289 successive parthenogenetic generations. By feeding them
only chlorophyll-bearing flagellates, he could obtain sexual females at any
time.
Economies. The reduction of male individuals enables rotifers to produce
large populations with a minimum consumption of food. The only function of
male rotifers is the fertilization of the resting eggs and their brief lives, with
little need of food, are entirely adequate for this function. Parthenogenetic
females eat far more than males, but every one of them produces more. Rabbit
populations are scanty compared with those of parthenogenetic rotifers.
Phylum Gastrotricha
Some gastrotrichs are marine, but most of them live in fresh water and are
often among the minute organisms swept up from the pond shallows with a
fine collecting net. Beneath the microscope they can be seen swimming, creep-
ing, even leaping rapidly about among the protozoans and rotifers with which
they consort, and in some ways resemble. Unlike the rotifers, they have no
crowns of cilia, but on their ventral sides they have bands of them which
accounts for their gliding and explains the name Gastrotricha (Gr. gaster,
belly -f trichos, hair). On the dorsal side the cuticle is scaly or hairy (Fig.
27.10). The majority of fresh-water gastrotrichs have a pair of tubes at the
end of the body, the outlets for the cement which forms their temporary hold-
fasts. In fresh-water gastrotrichs, all reproduction is parthenogenetic; no males
have ever been discovered.
Phylum Bryozoa
Their Place in Nature. Bryozoans or moss animals are minute animals,
nearly all of them living in colonies that look so much like moss that the name
544
EVOLUTION OF ANIMALS
Part V
bryozoans has replaced their other name Polyzoa. All are aquatic and upwards
of nearly 3000 species are marine; only about 35 live in fresh water. The
marine species are widely distributed in coastal waters, between the tide lines.
They grow on rocks and seaweeds, easy to see — but not to distinguish as
animals. Most of the colonies seem to be only white, yellow, or brown patches
of crust on the damp stones and seaweeds (Fig. 27.11). Other colonies might
be delicate branching seaweeds, two to four inches high, rooted to rocks and
kelp. The common fresh-water Plumatella resembles a dark vine with white-
FiG. 27.10. A typical gastrotrich, Chaetonotus. They are
many-celled fresh-water animals of microscopic size, like
their neighbor rotifers. They are so abundant, widely dis-
tributed, and striking in appearance that they demand at-
tention even among hordes of other minute animals. (Cour-
tesy, Robert W. Pennak, Fresh-Water Invertebrates of the
United States. Copyright 1953, The Ronald Press Company.)
tipped branches, actually the folded tentacles of the animals. Colonies of the
fresh-water Pectinatella magnifica live on the surface of great blobs of jelly
which they secrete about submerged stems. Algae invade the jelly and the
whole object might be a green pineapple floating in the midsummer pond. If
they are taken from the water none of these colonies gives the slightest sign
of life, but immersed in it, each animal puts forth its exquisite plumy crest on
the regular business of gathering food.
Structures and Functions. The common bryozoan Bugula grows on the east-
ern and western coasts of North America in tufts two or three inches long,
attached to seaweed. Although they are members of a colony, each individual
lives independently of its neighbors (Fig. 27.12). In this type of bryozoan,
each animal is protected within a homy tube; in others, every animal is in a
limy cup or surrounded by jelly. The characteristic and, under a lens, con-
spicuous feature of each animal is the lophophore which bears hollow flexible
tentacles astir with cilia that draw diatoms and protozoans into the mouth,
whence they are passed along the digestive canal by more cilia.
Chap. 27
AN AQUATIC MISCELLANY
545
Young Colony
Sfotoblosis
Units of Colony (x)
Moture Colony
Fig. 27.11. Bryozoan colonies. Upper, Marine. Encrusting colonies that live in
patches of their own limy deposits on rocks and seaweeds. The common Bugula
turrita that at first glance seems to be a delicate seaweed growing in tufts but
a few inches high. Lower, Fresh Water. Left, Plumatella spreads like a dark vine
over the stones in running water. Photograph of a living colony. The white
tips are the crowns of the zooecia with the tentacles withdrawn. Right, Pectinatella
magnifica, with its core of jelly, is a compound of many colonies. Floating in a
pond it appears to be a great green pineapple, each of its colonies taking the
place of the units of fruit. {Upper, courtesy, American Museum of Natural
History, New York. Lower right, courtesy. Ward and Whipple: Fresh Water
Biology. New York, John Wiley and Sons, 1918.)
Bugula and bryozoans of a similar type have a true coelom lined with a
cellular peritoneum. The coelomic fluid contains corpuscles and is the main
carrier of substances to and from the cells. There are no special organs of
respiration, excretion, or circulation. In the smaller class Endoprocta, gelat-
inous mesenchyme fills the space occupied by the coelom in the ectoprocts
such as Bugula. The ganglion or "brain" is connected by nerves with the ten-
tacles and retractile muscles.
Many bryozoans have minute pincers scattered over their outer surfaces,
believed to be very specialized individual animals rather than appendages.
546
EVOLUTION OF ANIMALS
TENTACLES
Part V
CIRCULAR
CANAL
MOUTH
NERVE
GANGLION
STOMACH
RETRACTOR MUSCLE
INFUNDIBULUM
SHELL
OR
ZOOECIUM
Fig. 27.12. Structure of one individual greatly enlarged of a bryozoan colony
such as the common marine Bugula. (Courtesy, Miner: Fieldbook of Seashore
Life. New York, G. P. Putnam's Sons, 1950.)
They are shaped like birds' heads, whence they are called avicularia. Under
a lens they can be seen snapping their bills with every vibration in the
water, and if any particle touches them they snap shut in a viselike hold.
They catch and kill the microscopic organisms that continually settle on the
bodies of the bryozoans and constitute private cleaning squads. Starfishes and
sea urchins have similar mechanisms, but they are unknown in higher animals.
No dog has pincers to trap his fleas.
Movements. A lophophore with all its tentacles can be instantly jerked out
of sight by the bands of muscle in the body cavity. But, its emergence is slow
and the tentacles spread forth seemingly with great caution, actually because
each one is expanded by fluid flowing slowly into it (Fig. 27.13).
Reproduction. Bryozoans are hermaphroditic and ovaries and testes develop
in the coelom in which the eggs are fertilized. The embryo develops in a brood
pouch that opens out of the coelom (Fig. 27.12). In the marine species, the
ciliated trochophore swims about freely for a short time, then becomes attached
to seaweed or rock (Fig. 27.1).
Chap. 27 AN AQUATIC MISCELLANY 547
Fresh-water bryozoans do not produce free-swimming larvae but bear in-
ternal buds or statoblasts that develop directly into colonies like the gemmules
of sponges (Figs. 22.7, 27.13). Most bryozoans exist only as statoblasts during
the winter. Many of these are banded with air cushions that buoy them up,
and armed with circlets of hooks that catch on the feathers and feet of ducks.
Statoblasts are carried far and wide by birds and currents of water. Occa-
sionally, they are washed out along the shores of lakes and lie in countless
numbers, long dark ribbons of them on the beaches.
Phylum Brachiopoda — Lamp Shells
Their Great Past. These animals were named brachiopods because some-
body mistook their long lips for arms, and lamp shells because their shells
suggested miniature Roman oil lamps.
Brachiopods have had a great past in numbers, diversity, wide distribution,
Air cells
Germinating
area
B. GERMINATING
STATOBLAST
A. RESTING STATOBLAST,
OR INTERNAL BUD
Esophagus
"- " I Tl III ilT
SECTION OF MATURE COLONY
Fig. 27.13. Fresh-water bryozoan, Pliimatella repens. Animals drawn greatly
enlarged with their tentacles expanded, or withdrawn; both contain developing
statoblasts. A, statoblast, about the size of a fig seed, with horny covering and
band of air cells. B, in the germinating statoblast the young animal has split the
shell revealing its body and yolky food. (After Brown, Trans. Amer. Micr. Soc.
53:427, 1934.)
548 EVOLUTION OF ANIMALS Part V
and an immensely long history all attested by their fossil remains. Un-
doubtedly, the adults were once free swimmers as their trochophore larvae are
now. However, through millions of years the adults proved the success of
their stalked food traps that contain a regulated collecting and filtering system
for gleaning particles of food from the water (Fig. 27.14). The fossils show
that their stalks extended from the posterior ends, as they do now, that the
shells opened upward, and that the long-lipped mouths expanded like the petals
of a flower. Their attached state and great abundance must have made them
food for roving predators, annelid worms, crustaceans, starfishes, and sea
snails. They constituted an important link in the food chain between the micro-
organisms they consumed and the carnivores that preyed upon them.
Fossils of over 2500 species have been discovered and a large number of
these are known from Paleozoic rocks, the oldest rocks in which fossils of ani-
mals are found. The 225 living species are only a remnant of those that are
now extinct. Of the living brachiopods, Lingula is scarcely changed from its
ancient ancestors, an animal on which evolution paused (Fig. 27.15).
Structure and Relationships. An adult brachiopod is enclosed within a pair
of shells resembling those of clams and oysters and like them, secreted by folds
of a fleshy mantle (Fig. 27.14). But the shells differ from those of mollusks in
that they cover the dorsal and ventral sides of the body, instead of the right
and left, and they swing open on a hinge at the rear end from which the body
stalk extends. In rock-dwelling brachiopods and most others, the shell is bent
upon the stalk. However, when they are burrowing, brachiopods hold their
bodies straight up, the original position with the tentacles and mouth facing
upward.
Like bryozoans, a brachiopod has no real head, its place being taken by
Digestive
Stalk gland
Heart
Adductor
yy\uscle
Lophophore
Storyiach
Mouth
MantU
Fig. 27.14. Brachiopod, or lamp shell. A marine animal, about one inch long,
that superficially resembles a giant bryozoan crowded into a clam shell. Its im-
portance is in its antiquity, its residence on the ocean bottom over 400,000,000
years ago, and its pioneer development of kidneys (nephridia) and heart. (Cour-
tesy, Pauli: The World of Life. Boston, Houghton Mifflin Co., 1949.)
Chap. 27
AN AQUATIC MISCELLANY
549
Modern (A) and Fossil (B)
Brochiopods
Fig. 27.15. A, Lingula, so like its ancestors that it is called a "living fossil,"
still so abundant on the borders of the Indian Ocean that it is used for food. It
lives in vertical burrows in the sand attached to the bottom by a stalk. B, a fossil
brachiopod shell that displays marked likeness to living brachiopods. (After Pauli:
The World of Life. Boston, Houghton Mifflin Co., 1949.)
enormous lips (lophophore), that surround the small mouth and bear rows of
ciliated tentacles. When not in action, the lips or "arms" are coiled up on each
side of the mouth. Their many tentacles have ciliated grooves through which
food and water are drawn toward the mouth. The cavity within the shell is
divided into a front chamber containing the lophophore and the lobes of the
mantle, and a posterior one containing the coelom, branches of which extend
into the mantle. It also contains the pairs of muscles by which the shell is
opened or closed and turned on its stalk, also those that work the stalk of such
burrowers as Lingula. The digestive canal usually lacks an anal opening. Any
waste which remains after digestion must be exceedingly fine, probably dis-
solved and excreted by the two relatively large nephridia. The sexes are sepa-
rate. Fertilization of the egg occurs outside the body. The free-swimming larva
is ciliated and has a general resemblance to the trochophore larvae of annelid
worms, rotifers, and moUusks (Fig. 27.1).
Phylum Chaetognatha — Arrow Worms
Their Vertical Migrations. In the morning and evening twilights, vast num-
bers of arrow worms join the plankton population of the sea. There they feed
550 EVOLUTION OF ANIMALS Part V
for a brief time on microscopic organisms — crustaceans and larval fishes, and
then return to the dark deep water. They not only furnish food to animals a
.^^^^^^^Snn
Fig. 27.16. Arrow worms (Sagitta hexaptera) swarm in open seas suddenly
visiting the surface at certain times of the year and during morning and evening
twilights. This species (length 3 inches), among the largest of the arrow worms, is
abundant off Martha's Vineyard, Massachusetts, and occurs throughout the world.
(Courtesy, Miner: Fieldbook of Seashore Life. New York, G. P. Putnam's Sons,
1950.)
little larger than themselves, but a few billion of them make a tasty catch for
the whale-bone whale whose food sifter is as efficient for gallons as that of a
rotifer for droplets.
Structures and Functions. The phylum name refers to the bristly mouths
and that of the principal genus, Sagitta, to their habits of darting like arrows.
Anus
Mouth
Cfrculotor
system
Esophog
ectum
Ovary
entory
not
Stomach
Fig. 27.17. Phoronis, a tube-dweller in
the mucky sand between the tide lines.
Diagram of its structure; the crown of
sticky tentacles is its all-important means
of getting a living. It is chiefly interesting
as a link suggesting relationships of var-
ious phyla of invertebrates and even a
remote one with the chordates because of
a notochord-like structure present in
them. (Courtesy, Hunter and Hunter:
College Zoology. Philadelphia, W. B.
Saunders Co., 1949.)
Testis
With their crystal transparency and cutting speed they are more like glass-
arrows than arrow worms. Dozens of them may swim about unseen in a glass
of water.
Chap. 27 AN AQUATIC MISCELLANY 551
The bristles and hooks that surround the head of an arrow worm are in-
stantly recognizable as the tools with which as a carnivore it seizes its prey
(Fig. 27.16). The body cavity, a true coelom, is divided into compartments,
all lined with peritoneum and filled with peritoneal fluid. Arrow worms are
pioneers in the development of a coelom, and this possibly places them among
the transitional forms from which the ancestors of the vertebrates finally
emerged. There are no special respiratory, circulatory, or excretory organs
but diffusion through the whole body carries on their work.
Arrow worms are hermaphroditic. The ovaries are in the coelomic cavities
of the trunk, and the testes in coelomic cavities in the tail. At hatching, the
young resemble the adult.
Phylum Phoronidea
The special features of phoronids are the food-catching organ, the body
fluids, the coelom and the larva (Fig. 27.17). The food collector is a lo-
phophore on a larger scale but similar to that of rotifers in its structure and
importance to the welfare of the animal. There are two body fluids, a color-
less one in the body cavity, and red blood circulating in blood vessels. Both
fluids are very different from those of higher animals, the lymph and blood
which they suggest.
28
Annelias — Pioneers in
Segmentation
Annelids are extremists. The outside of an earthworm is monotonously
austere; there are no decorations. But many among the marine worms bear
plumy gills; those of the peacock and feather duster tribe are Hke miniature
fountains shifting with iridescence (Fig. 28.1).
Annelids were pioneers in segmentation, the plan in which similar parts
of the body are repeated over and over. It is conspicuous in only two groups
of animals, the Phyla Annelida and Arthropoda, the latter known to everyone
through the lobsters, flies, and grasshoppers. Although it is less obvious, seg-
mentation is present in all higher animals, especially in the embryos but
clearly traceable in their later life. The rings of an earthworm's body and the
human vertebrae are evidences of segmentation. Both owe their origin to the
segmentation established in the ancestors of annelid worms some 550 mil-
lion and more years ago.
Annelids exist in variety — earthworms in sober colors and streamlined
form, leeches with the parasite's appetite, marine worms of flowerlike beauty,
delicacy and diversity. They are as significant in the economy of the sea and
land as they have been in the evolution of the animal body. They are respon-
sive to their environments to an extent and precision, ordinarily little credited
to "worms." Examples of it are in: the burrowing habits of earthworms; their
responses to the chemical and physical nature of the soil and their age-old
plowing of the earth; the swarming of clamworms and the famous Palolo
worms. Out of all the days of the year spent on the sea bottom, Palolo worms
come to the surface only a few hours on nights appointed by the moon and
tides, and by forces beyond our solar system. They answer to an environment
that extends very far away.
Ecology. Habitats of Annelids. AnneUds are numerous, biologically suc-
552
Chap. 28
ANNELIDS PIONEERS IN SEGMENTATION
553
Fig. 28.1. Tube-building annelids; peacock worm, Sabella pavonia (12 to 15
inches long). This and similar species live in British and North American tide
waters. The feathery plumes are glorified breathing organs and food traps that
emerge from the tubes and spread fanwise in the water like iridescent flowers.
(Photograph courtesy, Douglas P. Wilson, Marine Biological Laboratory, Plym-
outh, England.)
cessful, and widespread over the world — some 6500 species in all. They live
in soil and fresh water but are most numerous in the sea. There they live in
the shallows and between the tide lines, at the surface, and on the bottom at
great depths. Water is their natural home. Earthworms flourish in moist soil,
and punctually come to the top in warm spring rains.
Food. Annelids feed heavily on bacteria and on decayed plants; among sea-
weeds, as well as inland gardens, they clear space by eating and fertilize
it with their own bodies. There are predators among them, clamworms preying
upon smaller worms, some leeches living on smaller invertebrates, others
sucking blood. Annelids are in turn rich forage for larger predators in the
water and on land — crabs, lobsters, and fishes that hunt over the coastal bot-
toms, gulls that pick the seaweeds, robins seizing earthworms at the surface
of the soil, and ground moles catching them below it. By eating and being
eaten, they help to check the unbalance of too few or too many.
Ways of Living. Burrowing annelids are successful animals but the tube-
making ones far outdo them in variety of form and habit (Figs. 28.1, 28.2).
554
EVOLUTION OF ANIMALS
Part V
Fig. 28.2. Amphitrite johnstoni (8 to 10 inches long). The small scale worm
Gattyana cirrosa lives in the tube as a partner and takes the food that escapes the
larger worm. Various species of Amphitrite hide in sand-covered burrows but their
filamentous crimson, or crimson and yellow gills wave freely in the water. Amphi-
trite, named for the Greek goddess of the sea, has a near relative named for
Aphrodite, the goddess of beauty. (Photograph courtesy, Douglas P. Wilson,
Marine Biological Laboratory, Plymouth, England.)
The developing young ones swim about freely, but the adults are nearly all
rocking-chair travelers moving back and forth within their tubes. For various
burrowers and most tube dwellers, the sticky mucus on a thrusting proboscis
or waving gills is a means of collecting food. Many of them live alone in hard
tubes of calcium carbonate fastened to rocks, to seaweeds and oyster shells.
The majority of those in the larger, soft tubes have one or more guests, com-
mensal worms and crabs that share the house and whatever board they can
collect. On the Atlantic coast, a little crab (Pinnixa chaetopterana) lives with
Chaetopterus and moves with the worm, keeping near its mouth for the extra
"crumbs." Annelid scale worms are frequent guests. Some of these, like
parasites, will live with only one kind of host; for others, almost any tube
will do.
Characteristics. Annelids may be scarcely visible to the naked eye or several
feet in length. A seaweed feeder {Neathes brandti) of the Pacific coast of
North America is six feet long when relaxed and the giant earthworms of Aus-
tralia reach 10 and 12 feet. The characteristic structures of annelids are the
segmentation of the body before mentioned; a true coelom lined with a peri-
toneum; a central nervous system in which the brain (a pair of dorsal ganglia)
is connected with a double ventral nerve chain expanded in each segment
Chap. 28 ANNELIDS PIONEERS IN SEGMENTATION 555
into a ganglion; and chitinous bristles or setae usually present on most seg-
ments. Whenever a larval stage is characteristic of the species, it is the trocho-
phore type similar to those of many other aquatic invertebrates (Fig. 27.1).
The ancestors of annelids lived in the Cambrian Period, the early part of the
Age of Invertebrates.
Class Oligochaeta
The Earthworm
The earthworm, Lumbricus terrestris, is an immigrant from Europe that
spread through the eastern part of North America and, at least, in labora-
tories has reached the west coast, a few years ago more conspicuously than now.
Ecology. These burrowers clear their way through the soil mainly by
swallowing it. In spite of a long land residence and earthy contacts inside and
out, the bodies of earthworms are excessively water hungry (Fig. 28.3). Their
skins are too permeable for real land life. A worm that is transferred to water
absorbs 15 per cent of its initial weight in 5 hours and then levels off, water-
adapted. Conversely a water-adapted worm removed to moderately dry soil
loses water for a few hours, then levels off, semi-land-adapted. As a conse-
quence of their need for water, earthworms rarely live in dry climates and are
active only in the rainy seasons. They benefit the soil by loosening and aerating
it, swallowing and carrying top soil downward and deep soil upward. Thus,
they have plowed the land for centuries. Charles Darwin brought out the im-
portance of this in his "The Formation of Vegetable Mould, through the
Action of Worms with Observations on their Habits" — his last book, pub-
lished in 1881. It is the account of observations and experiments continued for
over 20 years in his "earth worm field" close to Downe House, his home
near London.
The Outer Tube — Protection, Locomotion, and Support. The earthworm's
mouth is overhung by a supple grasping lip, the prostomium. The flattened rear
end of the body is pressed against the inside of the burrow, a holdfast when
the worm is extended on the surface (Fig. 28.4).
Earthworms are dark colored above and pale on the underside, embar-
rassingly good examples of counter-shading although they are strictly noc-
turnal. Such examples are thorns in the theory of counter-shading which is based
on the presence of strong light from above. The conspicuous glandular swelling
is the saddle or clitellum which secretes the cocoon that contains the develop-
ing eggs. On each segment except the first and last there are four pairs of
minute chitinous setae. Each seta can be moved in several directions, also
extended or withdrawn into the flesh, and the worms catch the ground with
them as they crawl. On a quiet night the sound of moving earthworms can be
heard among dry leaves, like sandpaper catching against the edge of paper.
There are numerous microscopic openings in the skin, those of the mucous
556
Moist
EVOLUTION OF ANIMALS
SUMMER WINTER
Dry
Part V
feet
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~ '.'-;..' • ; . I'i .■••■.! '•■.'.'{•' •■■.•'.■'*■■■.'.'■;•'••;;' *•'.'•■•'»
• ••.'... ..■■.•.*>..- I ? ■ .* '.-.■• ".*••..•■•'■•.* .•> ^ .
Fig. 28.3. Seasonal locations of earthworms. Summer: worms feed, mate and
crawl about on the ground on moist nights; leave castings on the surface; burrow
in the upper soil, the depth depending on moisture; cluster in dry soil. Winter:
migrate below the frost line and hibernate; conserve moisture and heat by clus-
tering. (Courtesy, Morgan: Animals in Winter. New York, G. P. Putnam's Sons,
1939.)
cells, of the dorsal pores from the body cavity to the outside, and above the
sensory cells (Fig. 28.5). These and the outer openings of the nephridia
(kidneys) are invisible except by microscopic examination. Other passage-
ways are those of the two oviducts on the fourteenth segment; four minute
openings of the seminal receptacles in the furrows between segments 9 and
10, and 10 and 11; and the sperm ducts on segment 15 (Fig. 28.11). The
surface of the body is covered with layers of iridescent cuticle secreted by the
outer cells of the skin. Cuticle and mucus compose the trail left on the side-
walks after a night's wandering. Earthworms are sensitive to touch especially
at the ends of the body. Each contact cell has a hairlike tip that projects
Chap. 28
ANNELIDS PIONEERS IN SEGMENTATION
557
Mucous tube
and
cocoon
Fig. 28.4. Mating earthworms (anterior parts of bodies shown). Every earth-
worm has a fully developed male and female reproductive system. During mating
each animal transfers sperm cells into the seminal receptacles of the other. The
worms become bound together by mucous belts each secreted by a glandular
swollen band, the clitellum, conspicuous in all mature worms. After mating, the
mucous belts slip off over the heads of the worms, each one gathering up eggs and
the transferred sperm cells. Each belt hardens and becomes the cocoon in which
fertilization occurs and the embryos develop. The species figured Allolohophora
foetida (or Eisenia) is smaller than the familiar Lumhricus terrestris and also very
common especially in compost heaps. (Courtesy, Foot: "The Cocoons and Eggs of
Allolohophora foetida," Jour, of Morph., 14:38, 1898.)
through a pore in the cuticle and its other end is in touch with the process
of a nerve cell (Fig. 28.5). Light receptor cells are located on the front and
rear segments.
The Inner Tube — Food and Digestion. The body consists of two tubes, an
outer one, the body wall, and an inner digestive tube. The space between is the
body cavity or coelom, divided into a succession of compartments by par-
sensorc/ f/ben
6asa/ membr.
^.sensory ce//
sensort/ fiber
nuc/eus
Fig. 28.5. Gland cells and sense organs in the earthworm's skin. Left, section
of the cuticle and skin. The large glands produce the cuticle, the substance that
creates the iridescent trails left on sidewalks. Sensory cells receive stimuli and
transmit nerve impulses to adjustor nerve cells in the ventral cord. Right, light-
sensitive cell, containing a lens-like body (/) and surrounded by retina-like cyto-
plasm (r). (Left, courtesy, Curtis and Guthrie: General Zoology, ed. 4. New
York, John Wiley and Sons, 1947. Right, courtesy, Hess, Jour. Morph. & Physiol.,
41:68-93, 1925.)
558
EVOLUTION OF ANIMALS
Part V
titions of connective tissue. A layer of mesodermal cells, the peritoneum,
covers all surfaces facing the cavity, including those of the organs within it.
The cavity holds the watery coelomic fluid containing colorless amebfoid cells.
This is squeezed through pores in the septa and slowly circulated by move-
ments of the body. Some of it oozes through the dorsal pores and moistens the
body surface.
Digestive Tract and Its Functions. Earthworms consume quantities of
bacteria and minute nematodes along with the soil that contains them. At
the surface they feed largely on plant fragments. A worm can be caught by a
spotlight and camera with a sheaf of broken leaves being sucked into its mouth.
In the pharynx leaves are moistened by slippery saUva and further broken
by squeezing (Fig. 28.6).
The food then passes through the relatively long esophagus where it re-
ceives a milky fluid from the calciferous organs located in segments 10, 11,
and 12. All of these are outpocke tings of the wall of the esophagus. The first
Segment I
— Brain
Pharynx
Aortic arcti I
Esoptiogus
Sem. ves. I
Stom -int.
Dor, blood ves.
Fig. 28.6. Earthworm. The general structure of the anterior part of the body
shown with the dorsal wall removed. The nephridia (kidneys) are not shown.
(Courtesy, Calkins: Biology. New York, Henry Holt and Co., 1914.)
Chap. 28 ANNELIDS PIONEERS IN SEGMENTATION 559
pair (10) are pouches; the second and third pairs (11, 12) are glands (Fig.
28.7). The glands produce the chalky secretion, evidently from an excess of
calcium carbonate in the blood. From the glands it passes into the pouches,
trickling forward through channels created by the infolding of the lining of the
esophagus. The pouches act as storage sacs from which the secretion oozes
into the food mass as it passes by their openings. The function of the cal-
ciferous bodies has been variously interpreted. Their secretion has been said
to be a neutralizer that aids digestion. This was not borne out by the experi-
ments of J. D. Robertson who concluded that the calciferous organs eliminate
excess calcium that is absorbed from the food into the blood. The ways in
which animals have dealt with extra calcium has had far-reaching effects upon
them. Earthworms eliminate it and wriggle on with their soft, freely flexible
bodies unaffected. Snails and clams use it in protective shells with which they
are often weighed down in continual semicaptivity; vertebrates use it in their
bones.
From the esophagus, food passes into the thin-walled and elastic crop, an
expansion of the esophagus (Fig. 28.6). It opens into a muscular gizzard with
a chitinous lining. This is essentially similar to the gizzards of grasshoppers
and chickens and performs the same work of squeezing and grinding. The re-
mainder of the food tube is the intestine where the main part of digestion and
10
II
12
Cavity of esophagus
Esophageal pouch
Stored carbonaie
Esophageal glands
Wall of
esophagus
Trough of
secretion
14
Fig. 28.7. Earthworms. Calciferous organs seen from the dorsal side. The glands,
11 and 12, absorb calcium from the blood and produce a chalky secretion that
trickles forward through the special channels in the lining of the esophagus visible
in a cross section. It passes into the pouches {10) where it is temporarily stored
before it oozes into the passing food masses. (After Robertson: "Calciferous
Glands of Earthworms," Brit. Jour. Exp. Biol, 13:279-297, 1936.)
560
EVOLUTION OF ANIMALS
Part V
all of absorption occur. Its enzymes are cellulase that acts on the cellulose of
plant tissues, amylase on carbohydrates, pepsin and trypsin on proteins and
lipase on fats. Absorption of food takes place through the ciliated epithelium
that lines the intestine. The absorptive surface is increased by the bulging out
of the intestinal wall in separate segments and by an infolding of its dorsal
wall — the typhlosole. The digested food is here either taken up by the blood
into the numerous capillaries embedded in the intestinal wall, or directly into
the coelomic fluid. A greenish layer of chloragog cells, important in excretion,
covers the blood vessels and intestine.
Other Metabolic Processes. Circulation, The body fluids are the watery
coelomic fluid, the tissue fluid in direct contact with the cells, and the red
blood. The red blood consists of the red plasma with the respiratory pigment
hemoglobin in solution in it and the colorless ameboid cells. The blood circu-
5 pairs of hearts
Body wall
Dorsal
Ventral v.
Nerve cord
Sub neurol v.
Fig. 28.8. Main vessels in the forepart of the earthworm. Blood is forced
through the large dorsal vessel by waves of contractions that begin at its posterior
end and pass forward, backward flow being prevented by valves. Along the way
it is distributed to side branches, largely to the hearts which connect with the main
ventral vessel. Small branches from this carry it to the kidneys (nephridia) and
body wall. By way of various vessels it is finally returned to the posterior end of
the dorsal vessel and carried forward again.
lates through a system of tubes that branch to all parts of the body. The dorsal
vessel lying on the digestive tube is the ffiain collecting vessel (Figs. 28.8,
28.9). By its rhythmic contractions, this vessel and the five pairs of hearts
determine the direction of the flow of blood through them and backward
through the long ventral vessel that lies directly beneath the digestive tube. In
almost every segment blood flows out of the ventral vessel to the dorsal one
by way of the capillaries in the body wall, digestive tube, and kidneys. The
subneural vessel also carries blood backward, supplies the nerve cord, and has
branches that connect with the dorsal vessel. Valves in the hearts prevent back
flow as they do in the veins of higher animals.
There are special distributions of blood vessels to very vital structures: to
the skin in which respiratory gases are exchanged; to the digestive tube with
its food supply; to the kidneys concerned with water balance and excretion;
to the muscles and to the nerves that depend upon abundant oxygen.
Chap. 28 ANNELIDS PIONEERS IN SEGMENTATION 561
Respiration. Earthworms can breathe in air or water. Their wet skin func-
tions essentially like a lung or a gill in spite of its cover of cuticle. It is well
supplied with blood capillaries and under sufficient pressure oxygen passes into
the blood and combines with the hemoglobin; in the outer skin cells it prob-
ably combines directly with the protoplasm as in the ameba. Although no
exact measurements are available, it appears that earthworms can make use
of oxygen in air or water with almost equal readiness. Experiments have shown
that the oxygen-loading capacity of the earthworm's hemoglobin is low and
inefficient as compared with the hemoglobin of higher animals.
Excretion. The nephridia (kidneys) of annelid worms are tubes associated
with blood vessels and with the coelomic fluid (Fig. 28.9). Each one is a coiled
tube with a ciliated funnel opening into the coelom, and a relatively long tube
looped back and forth upon itself and ending in an enlarged bladderlike part
Dorsal vessel
Chlorogogen cells \
Endoderm
Muscle \
Peritoneum
Cuticle
I Ectoderm
I
Circular muscle
Longitudinal muscle
Peritoneum
I
I
Typhi
/
Seta
Nephridiopore
y Ventral vessel
taterol vessel
\ \
^ Ventral nerve cord
Subneural vessel
\ ''Coelom
Enteron
Fig. 28.9. Diagram of a cross section of an earthworm showing the intestine,
one pair of nephridia, the chitinous setae which are aides in locomotion and the
excretory chlorogogen (chloragog) cells. The inbent fold (typhlosole) extending
nearly the whole length of the intestine is a means of increasing the area of
absorption of digested food. None of the smaller blood vessels are shown; nets of
them cover the coils of the nephridia. (Courtesy, Mavor: General Biology, ed. 3.
New York, The Macmillan Co., 1947.)
562 EVOLUTION OF ANIMALS Part V
that opens externally. At the inner end of the bladder, muscle cells prevent the
excreted fluid from flowing back into the body. The essentials of structure in
these kidneys are the tubes and their contact with blood capillaries, arrange-
ments common to kidneys in general. Some waste substances are taken
directly from the coelomic fluid through the ciliated funnel; other by-products
are taken from the red blood. The kidney also helps to dispose of excess water.
Greenish chloragog cells surround the blood vessels and cover the intestine
where they are in contact with microscopic capillaries (Fig. 28.9). They take
up dissolved wastes from the coelomic fluid and these form the yellowish-
green granules within them. When full of such granules they are sloughed off
into the coelomic fluid. Some probably disintegrate and their substance passes
out through the nephridia; others are taken up by the highly phagocytic
ameboid cells. The latter wander into the tissues, disintegrate and their remains
are deposited as pigment in the body wall.
Nervous System — Coordination. The two ganglia that constitute the brain
are connected by nerves with another pair beneath the pharynx (Fig. 28.10).
From these ganglia the double nerve cord, with a double ganglion in each seg-
ment, extends along the ventral floor of the coelom to the last segment. The
removal of the brain has little effect upon the responses of an earthworm. How-
ever, after the subpharyngeal ganglia are removed, a worm neither burrows
nor eats. The neurons in these ventral ganglia are evidently much more im-
portant than those in the brain. The ventral cord is the coordinating center of
the body. The fibers of sensory neurons extend into it and those of motor
neurons out of it as they do in the human dorsal nerve cord. Hundreds of both
types of fibers pass through each of its branches (Fig. 28.10). Fibers from the
receptor sensory cells connect with the cord. There the impulse on the sensory
fiber passes over to an adjustor neuron and thence to a motor neuron which
carries the impulse to the effector, in this case a muscle cell. Sensory and motor
impulses pass one another on the same nerve but along separate cell fibers as
in higher animals. Sensory and motor impulses are continually relayed along
the cord over adjustor neurons.
The waves of muscular action which pass down the body as a worm crawls
must be controlled by the ganglia in various parts of the cord since any mod-
erate-sized piece of the body will crawl as well as a whole worm. If an earth-
worm is touched while outstretched from its burrow, it instantly snaps back, its
longitudinal muscles contracting throughout their length. The nervous trans-
mission is relatively rapid; it evidently passes over certain giant nerve cell
fibers visible when specially prepared sections of the cord are examined
microscopically. Experiments have shown that the speed of an impulse over
these fibers is 1.5 yards per second. The speed of an impulse over the motor
nerve cell fibers in man is about 100 yards per second.
Reproduction. The reproductive organs are located in the anterior part of
Chap. 28
ANNELIDS PIONEERS IN SEGMENTATION
563
Circumpharyngeol
Cerebral ganglion connecfive
Buccal cavity
Prosfomiom
Segmenfol nerve
iVI
Mourh
Motor fiber ending in
longitudinal muscle
Body wall
Sobphoryngea! ganglion
Ventral nerve cord
at ganglion
\ I
Septal nerve
Motor neuron cell body
Sensory fibers
_ — Body cavity
Longitudinal
muscle
Circular
muscle
Epithelium
Sensory cells
Fig. 28.10. Upper, forepart of an earthworm showing the nervous system with
the ganglia repeated in each segment. Lower, diagram of the nerve cells involved
in a simple reflex movement of the earthworm. {A, after Hess. B. after Parker.
Courtesy, Mavor: General Biology, ed. 3. New York, The Macmillan Co., 1947.)
the worm, each organ in a particular segment (Fig. 28.11). The male cells
originate in two pairs of minute testes. These are surrounded by conspicuous
seminal vesicles, sacs in which the sperm cells mature. They finally pass into
the ruffled sperm funnels and through slender sperm ducts to the two external
openings on the ventral side of segment 15. Two pairs of small sacs, the semi-
nal receptacles, open through pores, on the ventral surface of segments 9 and
10. During mating these receive sperm from the sperm ducts of the part-
ner worm. The microscopic eggs are formed in a pair of translucent ovaries in
segment 13. As the eggs mature they are shed into the funnels of the oviducts
almost in touch with the ovaries. At the side of each oviduct is a minute pouch
in which they collect. Behind the sex organs is the conspicuous clitellum
(saddle) of gland cells. These secrete the mucous belt and cocoon that later
protects the developing embryos.
The seminal vesicles very often contain large numbers of the parasitic proto-
zoan, Monocystis agilis, in various stages of development. In one stage they
564
Seminal
vesicles
Oviduct
EVOLUTION OF ANIMALS
Nerve cord
Part V
Lateral nerves
Seminal
receptacles
Vas deferens
Testis
Sperm funnel
Vas deferens
Ovary
Egg funnel
Opening,
oviduct
Opening
sperm duct
Fig. 28.11. Earthworm. The hermaphrodite reproductive system composed of
complete male and female organs. (After Vogt and Yung. Courtesy, Brown:
Selected Invertebrate Types. New York, John Wiley and Sons, 1950.)
are ciliated and constantly moving; in another they are seedlike spores packed
in a capsule.
The mating of earthworms is a complicated process, not simply the shed-
ding of the sex cells into the water as in the aquatic annelids. Although earth-
worms are hermaphrodites they usually extend the forepart of their bodies
and mate with worms of nearby burrows (Fig. 28.4). The heads of the worms
are pointed in opposite directions with the ventral sides in contact. The clitel-
lum of one worm is opposite to segments 9 to 1 1 of the other worm. Mucus is
secreted until each worm is cloaked in a mucous tube that extends from seg-
ment 9 to the hind edge of the clitellum. As the sperm cells are discharged
from the openings of the sperm ducts on segment 15, they are carried back-
ward through two grooves to the sperm receptacles of the partner. This ends
the mating process and the worms separate. The clitellum secretes a mucous
belt which is shifted forward, along with the mucous tube, and finally over the
head of one worm. As this elastic belt passes the openings of the oviducts, the
mature eggs are evidently expelled into it. Farther forward, on segments 9 and
10, it apparently receives the sperm cells deposited there by the partner worm
during mating. The sperm and eggs join and fertilization is completed within
the mucous belt that in the meantime is slipping forward and finally off the
Chap. 28 ANNELIDS PIONEERS IN SEGMENTATION 565
worm. As this occurs the edges of the belt come together and a sealed capsule
or cocoon results. Within this the young worms develop and in about three
weeks, at least one or two of them emerge and make their way into the soil
without going through a swimming stage such as the trochophore of the marine
annelids. The cocoons of young earthworms, about the size of apple seeds, are
numerous in moist compost heaps in spring and summer.
Regeneration. Adult earthworms can regenerate segments removed from
the ends of the body, accidentally or by experiment. According to recent in-
vestigations of G. B. Moment (1953), the complete number of segments is
present in the earthworm when it hatches and that number is not exceeded
either by its usual growth or by regeneration. No more than five new segments
will regenerate at the anterior end and no head will regenerate if 1 5 or more
segments have been cut off. Various combinations can be made by grafting
pieces together, fastening them by threads until they become united. It is
doubtful whether regeneration contributes to their survival as it does to
planarians and starfishes. In Tubifex, a common fresh-water oligochaete, the
posterior end of the worm regenerated 31 new segments in 32 days. This is
largely due to the totipotent cells (neoblasts) which migrate to the cut sur-
face, multiply and differentiate into one or another kind of cells during the
regeneration. Totipotent cells are those that have kept their embryonic charac-
ter and have the power to multiply with great rapidity somewhat as cancer
cells do. It is worth notice that such cells are killed by x-rays.
Other Oligochaetes
Most of the 2400 species of oligochaetes are smaller than Lumbricus
terrestris. The 10-foot giant earthworms of Australia are impressive excep-
tions. Two species of small earthworms are common all over North America;
one of them (AUoIobophoro caliginosa) lives in the soil; the other (Eisenia
[old name Allolobophora] foetida) lives in compost.
The majority of oligochaetes are aquatic. The young ones called naiads are
transparent little bristle worms familiar to anybody who examines pond-
sweepings under the microscope. Slender red worms, Tubifex tubifex, about
an inch and a half long, live in tubes with their "tails" waving above the sur-
face of mud, usually odorous from decaying organisms. Milk-white enchy-
traeids {Enchytraeus albidus) about half an inch long are sold at pet shops for
turtle and fish foods.
Class Polychaeta
The Clamworm
Ecology. Several species of clamworms live on sandy shores and clam flats
on both Atlantic and Pacific coasts of North America. The large clamworm.
Nereis virens, often a foot long, is one of the commonest annelids on the New
566 EVOLUTION OF ANIMALS Part V
England coast, mainly in the low tide range. It is a ravenous hunter, swims at
a good speed, and can grapple worms larger than itself. Clamworms construct
loose flexible tubes on a base of sticky mucus that catches the sand and
broken shells that disguise their chimneys. Like earthworms, they stretch out
of their tubes at night but day and night they are preyed upon by birds and
fishes.
The Outer Tube — Protection, Locomotion, and Support. The greenish skin
is covered by iridescent cuticle like that of the earthworm. All of the segments
are externally similar except the head (Fig. 28.12). On each side of the seg-
ments there is a flattened fleshy lobe the parapod or "side foot" bearing bundles
of bristles.
Clamworms and earthworms greatly resemble one another but are products
of unlike experience. For untold generations clamworms have lived in the
sea, swimming after their prey and away from their enemies. For an equally
long time streamlined earthworms have bored through the ground, swallowing
I.
Prostomium
a soft lip
Sucking
mouth
Head of eorthwornija herbivorous bur/ower: I. dorsal, 2. ventral view
Palp
Prostomium
Eyes
Tentacles
Head of clam worm, an active carnivore:
Dorsal views I. Jaws withdrawn, 2. Grasping jaws extended
Fig. 28.12. Upper, head of the herbivorous burrowing earthworm with only
primitive light and touch receptors and no oral armature. Lower, head of the
active predatory clamworm equipped with clutching jaws and relatively complex
eyes.
Chap. 28 ANNELIDS PIONEERS IN SEGMENTATION 567
the inert soil. Clamworms and earthworms are illustrations of the saying that
the outside of an animal tells where it has been, the inside what it is.
General Internal Structure. The internal structure of the clamworm is
essentially the same as that of the earthworm. Behind the esophagus the coelom
is divided into segments by partitions whose surfaces are covered with thin
peritoneum; there is a pair of kidneys in nearly every segment; and the nerve
chain is likewise branched.
The jaws and the protrusible pharynx which can be thrust out onto the
prey are marks of the clamworm's predacious habit; withdrawing the pharynx
is a part of swallowing the food into the esophagus into which a pair of diges-
tive glands opens. From the esophagus, the digestive tube extends to the end
of the body.
Reproduction. In Nereis and almost all polychaetes, the eggs and sperm
develop in separate individuals from certain cells in the peritoneum of most
of the segments. They are finally discharged into the water by way of the
nephridia. The breeding habits of these, like many marine invertebrates, fol-
low rhythms of the moon and the tides. In one of the smaller clamworms,
Nereis limbata, each breeding period follows a cycle of the moon. In each one
there are two peaks of abundance, also timed with phases of the moon. These
clamworms that throughout the year have lived on the sea bottom come to the
surface on certain days and hours, following a habit that probably began with
the great tides of the Cambrian Period, half a billion years ago. At Woods
Hole, Massachusetts, their swarming is a scheduled event of certain summer
nights. By a light held over the water, the throngs of swimmers can be seen
circling through the water as they shed eggs or sperm before they drop to the
bottom again. Each run begins near the time of the full moon, increases to a
maximum on successive nights, falls to a low point about the third quarter,
then increases again, and finally shortly after the new moon no worms appear.
The influences on the habits of these worms are examples of the many effects
that originate far away in space and time.
Other Polychaetes
Illustrations of a few polychaete worms may give a slight notion of their
variety and beauty. There is no hope of suggesting the translucence and play
of color of the living animals. Those that are mentioned here, or their near
relatives, live on both American sea coasts.
The sea mouse. Aphrodite aculeata, may be three to seven inches long. The
under surface of the body is a flattened creeping sole like a snail's foot but is
furrowed by segmentation. Along each side of the upper part there is a band
of iridescent, hairlike setae. Between them the back is greenish gray bordered
by green and gold setae and brown spines that hide the segmentation. At first
glance, a sea mouse looks no more like a worm than it does like a mouse.
568
EVOLUTION OF ANIMALS
Part V
It lives on muddy sea-bottoms and, climaxing its peculiarities, commonly has
one or more small guest clams, living in the furrows of its foot.
The parchment worm, Chaetopterus, is six inches long (Figs. 28.13, 28.14).
Parchment worms secrete the tough substance of their U-shaped tubes whose
chimneys project above the sand at low tide. As they lie in their tubes a steady
Fig. 28.13. Diagrams of Chaetopterus. A, animal feeding in its tube. B, dorsal
surface of the anterior end. 7, mouth; 2, wing-like structure from the edge of
which mucus is secreted; 3, mucous sac; 4, food ball, being rolled up in a ciliated
cup; 5, one of the main "fans" that with many smaller ones circulates the water
within the tube; 6, ventral suckers by which the worm holds itself to the sides of
the tube; 7, dorsal groove through which cilia carry the food ball toward the
mouth. (Courtesy, MacGinitie and MacGinitie: Natural History of Marine Ani-
mals. New York, McGraw-Hill Book Co., 1949.)
Fig. 28.14. Chaetopterus glowing in the dark. (After Panceri. Courtesy, Harvey:
Living Light. New York, Academic Press, 1952.)
Chap. 28 ANNELIDS PIONEERS IN SEGMENTATION 569
current of water goes in one end and out the other, kept in motion by the
worm's rhythmic fanning of the broad flaps near the middle of its body. Oxygen
and particles of food go in with the current. As before mentioned minute crabs
(Pinnixa) often live in the tube and share the "crumbs." The daily life of
Chaetopterus can be observed because it will live for long periods within a
glass U-tube in a salt water aquarium. In the dark it is silvery from its bio-
luminescence.
The plumed worm, Diopatra ciipraea, is 10 to 12 inches long and is one of
the most beautiful annelids of the Atlantic Coast. It constructs a tube large
enough for the worm to turn around inside, with a chimney perfectly dis-
guised by shells and seaweed. It is common in shallows below the low-tide
line, from New England to South Carolina.
Palolo worms, Eunice viridis, and their near relatives are the classic ex-
amples of spawning associated with the tides and moon. The Atlantic palolo
swarms a few hours before sunrise in June and July, shortly before the last
quarter of the moon. The Pacific palolo swarms in October and November,
near the last quarter. The Bermuda "fire worms" not only swarm but are
luminescent while they do so. In Harvey's Living Light there is an account of
their spectacular performance. A similar species {Odontosyllis phosphorea)
swarms on the western coast of North America.
Class Hirudinea
Leeches
Leeches are segmented worms that hold on by suckers. They get about by
swimming and by looping over surfaces like measuring worms (caterpillars).
Holding onto the surface with its rear sucker, the leech stretches out its body,
attaches the front sucker to the surface and pulls the body forward. The rear
sucker then releases its hold and is placed close behind the front one so that the
body forms a loop (Fig. 28.15). The common name leech means to hang on
and gain thereby. The class name, Hirudinea, comes from the hirudin that a
leech injects into the wound as it bites and thus prevents the blood from
coagulating.
Ecology. The majority of leeches live in fresh water; a few are marine;
others abound in swamps and the forests of the humid tropics. Some of them
feed on snails and worms; others are true bloodsuckers. As a group they are
wavering on the edge of parasitism but not wholly committed to it. Most of
them are predators, not more than 25 per cent are parasitic and many of these
stay on their hosts only while they are feeding.
Leeches are acutely sensitive to vibrations and to extremely small amounts
of substances dissolved in water. If you press your finger against the bottom of
a dish containing leeches, they will at once begin to creep about, exploring the
570
EVOLUTION OF ANIMALS
Part V
Fig. 28.15. Right, the common bloodsucker, American Medicinal leech {Mac-
robdella decora). Length, full grown, four inches or more. The general color is
green, mottled and lined with black and orange; the underside is rich orange. This
is one of the few leeches that regularly take human blood. It attaches itself by its
rear sucker and explores the skin with its anterior end, then attaches the oral
sucker and makes three fine painless cuts by a rotary motion of its jaws. Left,
common brook leeches, Glossiphonia complanata: one with eggs attached. They
live upon snails and are commonly called "snail leeches." A brook leech, two
inches long, may have 40 or more young leeches attached to its underside, stretch-
ing out their bodies from beneath the parent as they ride.
whole surface and if they happen to pass over the fingerprint, their excitement
shows that they detect it. In their native ponds, bloodsucking leeches are very
responsive to movement of the water. They will gather from all directions even
when one moves slowly and in high boots. They are also sharply responsive
to light. Some have one or several pairs of eyes on the head as well as light
perceptive organs on the segments.
Structure. There are 34 segments in the body of a leech but these are not
clear-cut externally for each one is furrowed by two to five circular wrinkles
or rings. Many structures of leeches are essentially similar to those of earth-
worms but their muscles are much stronger.
The bloodsucking leech (Hirudo medicinalis) has three sawlike teeth that
make a Y-shaped cut in the flesh. Glands in the wall of the pharynx secrete
Chap. 28 ANNELIDS PIONEERS IN SEGMENTATION 571
the anticoagulant, hirudin, and the muscular pumplike pharynx draws out the
blood (Fig. 28.16). The pharynx opens into an enormous crop extended by
pairs of sacs, the last of which reaches nearly to the end of the body. This can
hold enough blood for several months' food supply. Soon after a blood meal
Pharynx
Crop with pouches
Anterior sucker
Posterior sucker
Stomach
Capacity to toke in much food at one time
Fig. 28.16. Leech. A general diagram of the digestive cavity. A leech sucks in
enough blood at one time to increase its weight five times. When well inflated it
loosens its hold voluntarily but the wound continues to bleed because of the anti-
coagulant injected with its saliva.
much of the water is excreted and the concentrated blood is slowly digested
in the small stomach into which the crop opens.
Leeches are hermaphrodites and, as in earthworms, there is a mutual trans-
fer of sperm cells during mating. After mating, usually in summer, eggs
and sperm pass into cocoons produced by a clitellum. The cocoons remain in
water or earth except in one fresh-water family (Glossiphonidae) in which the
cocoon and afterward the young leeches are attached to the underside of the
parent (Fig. 28.15).
Leeches in Medical History. Leeching is an old medical treatment, so com-
mon that leech became the name for the physician as well as the treatment. The
leeches, placed on the skin, carried on the real treatment that consisted of their
sucking out a considerable amount of the "bad blood." During the early nine-
teenth century there was an enormous demand for "medical leeches." They
were reared in ponds in many parts of Europe. Broussais (1772-1838), a
French physician, was a leading advocate of "blood letting." During the year
1833 over 41 million leeches were imported into France for medical use and
a good number into the United States.
Class Archiannelida
This is a small group of inconspicuous worms (e.g., Polygordius) of the sea-
shore, that in the adult stage resemble the late larval stages of polychaetes.
Internally the adults are segmented, but externally the segments are indis-
tinct or missing. The larva is a typical trochophore. The class is merely men-
tioned here for completeness.
29
Artnropoas — Crustaceans
There are more kinds of arthropods than of all other animals together (Fig.
29.1). They are a collection of multitudes: crustaceans, hosts of little ones as
well as large lobsters and crabs; myriapods, the centipedes and millipedes;
spiders and their allies, ticks and mites; and insects by millions. Their variety
seems infinite but their basic pattern is the same, the tube within a tube plan
of body and the segmentation inherited from annelid worms. Two leading
Fig. 29.1. The relative abundance of
species of arthropods is estimated to be 80
per cent of all kinds of animals. (Courtesy,
Frost: General Entomology. New York,
McGraw-Hill Book Co., 1942.)
characteristics have developed upon this ground plan, an important and com-
plex head, and jointed appendages the unique character of which has given
the name Arthropoda {arthros, a joint -f pous, a foot) to the phylum (Fig.
29.2).
Arthropods have complex and important relationships with plants and
with other animals. Among them are the social organization of ants and bees,
the most elaborate outside of human society, the effects of insects upon agri-
culture throughout the world, constructively by cross pollinating plants, de-
structively by feeding on plants and by carrying diseases from one to another
plant, animal or man. Crustaceans provide the chief food for many fishes;
572
Chap. 29
ARTHROPODS CRUSTACEANS
573
Fig. 29.2. A crayfish (upper) and a mayfly (lower) display the leading char-
acteristic of all arthropods, segmented bodies combined with segmented append-
ages. The crayfish is an example of this plan in an arthropod that lives in the
water, and the adult mayfly of an arthropod adjusted to the land. Arthropods have
carried the basic plan into almost every corner of the earth.
small crustaceans are the main food of the great blue whale; the larger ones
are human food throughout the world.
A Connecting Link. Peripatus is the only living animal that comes near
being a common relative to annelid worms and to arthropods (Fig. 29.3). It
is a couple of inches long, has a velvety skin and resembles a caterpillar. It
belongs to a small group, the Onychophora ("claw bearing"), considered a
phylum by some and a class by others. It lives in tropical forests in such
separated regions as Australia and South and Central America, suggesting
that it once may have been widely distributed and is now disappearing. It dif-
574 EVOLUTION OF ANIMALS Part V
fers from both annelids and arthropods in being segmented only on the inside.
There is a pair of stiff peglegs for each internal segment. Among the annelid-
like structures are its thin cuticular cover, the continuous bands of muscle in
the body wall, and the excretory organs, a pair of coiled ciliated tubes in each
segment resembling the nephridia of earthworms. Arthropods lack cilia
altogether. The arthropodlike structures are chiefly the tracheal tubes of the
respiratory system carrying air directly to the tissues. A bundle of unbranched
tracheae extends into the body from each of the numerous external openings.
There is no mechanism for closing them as there is in the similar ones of
insects, and experiments have shown that body water evaporates through them
Fig. 29.3. Peripatus, a walking worm. Neither an annelid worm nor an ar-
thropod yet resembling each, it has internal segments like the worms and air tubes
like the insects and spiders. This connecting link is distributed in regions of the
Southern Hemisphere. (Courtesy, Pauli: The World of Life. Boston, Houghton
Mifflin Co., 1949.)
about 40 times more rapidly than in a caterpillar. The skin of Peripatus is
adapted to moist land life and is restricted to it. The unguarded holes are un-
safe against the evaporation of dry air. The advancement of Peripatus has
doubtless been hindered by too much ventilation.
Trilobites — The Pioneer Arthropods. Over half the fossils that date from
the first era ef invertebrates are trilobites (Fig. 29.4). They were arthropods
with 3-lobed bodies and many pairs of uniform 2-branched appendages, the
latter probably for locomotion. In the course of time new types of arthropods
developed from certain of the trilobites. The sea scorpions were among those
that became the ancestors of the spider tribe (arachnids) and the horseshoe
crabs (Limulus) which have survived into the present day. The trilobites, once
the most numerous of invertebrates, now exist only as fossils but their
descendants have more than taken their places (Fig. 29.1).
Class Crustacea
With but few exceptions, crustaceans are a great tribe of animals that
breathe by gills. Some have pioneered into fresh water and a few live cau-
tiously on land but, like their ancestors, most of them belong in the sea. There
they exist in untold numbers and in great variety of shapes and sizes. Through
all its variations the crustacean plan is evident — the segmented body bearing
jointed appendages that typically have two branches (Figs. 29.5, 29.6). Crus-
taceans range in size from water fleas that are microsopic, and barnacles an
Chap. 29
ARTHROPODS CRUSTACEANS
575
.||HH||^. ,i^
' ^^^^^^^HH^'^'**''
^^^^^^^^^HIPP^^'''^-''' .^
' ' J^^^^^^^^^^^^^^B^ k -t'^-^^-dK v^'
-IRIHI^RHHnpHHil--' v '>;^^'^V'/'^'N
m^
i
■ II' iiim^jgipiiUJi. ---h.^*^ , *■.
Fig. 29.4. Restoration of Silurian sea bottom, now the site of the city of Buflfalo,
New York. Made from a study of fossils found in the Niagara region, in that
period some 400 million years ago when it was overspread by ocean. A large
trilobite and several smaller ones creep upon the bottom showing the characteristic
furrows and triple sections of the body. A cephalopod, and two crinoids, the once
abundant stemmed echinoderms, are in the mid-ground of the scene. (Courtesy,
The Buffalo Museum of Science.)
inch wide or less, to the American lobster that holds a record weight of 35
pounds, and the giant spider crabs of Japan that measure 20 feet from tip to
tip of the first pair of legs.
Development. As in all higher animals, crustaceans pass through stages
that suggest either the adult or the immature stages of simpler animals. Like
most crustaceans, a shrimp (Penaeus) hatches into a larva called the nauplius
stage that has three pairs of appendages and a single eye (Fig. 29.7). The
nauplius transforms into a protozoea and the latter into a zoea in which the
cephalothorax appears. The zoea develops into the mysis, a stage named after
the common shrimp Mysis.
Crayfishes and Lobsters
Aristotle did well to call crayfishes "the small lobsters" of the rivers. Their
habits of living are remarkably similar considering the differences in their
576
EVOLUTION OF ANIMALS
Part V
:.;^\^
Fig. 29.5. Examples of the variety of small marine crustaceans. A, B, C; minute
copepods of the surface waters of the open sea with appendages used in swimming
and floating. D, an equally small copepod of the tide-pools which lacks any elab-
orate equipment for floating. Copepods compose an important part of the basic
food supply of surface sea waters. (Courtesy, MacGinitie and MacGinitie:
Natural History of Marine Animals. New York, McGraw-Hill Book Co., 1949.)
homes, lobsters in coastal sea waters and crayfishes in ponds and streams,
most often in limy regions. As a representative crustacean either animal is
attractive for study. Crayfishes offer the advantages of being widely dis-
tributed and in relatively small demand for food and in general structure they
are but smaller editions of lobsters. In North America crayfishes of the genus
Cambarus are common east of the Rocky Mountains and Potamobius
(Astacus) west of them.
Ecology of the Crayfishes. Crayfishes hide in dark places and forage about
on pool bottoms walking on their claws as if on tiptoes, their great pincers
held out in front for instant attack, like hands in a reception line. Some species
do not burrow, such as Cambarus bartoni, one of the common dwellers in
small clear streams. Cambarus diogenes is a well-known burrower in swamps,
Chap. 29
ARTHROPODS CRUSTACEANS
577
B
Fig. 29.6. Upper, Tidepool shrimp (Spirontocaris, length 11/2 inches). 1, an-
tennule; 2, antenna; 3, carapace or "saddle"; 4, abdomen; 5, tail fan; 6, swim-
merets; 7, walking legs; 8, pincers. Lower, Pistol shrimp, Crangon californiensis
(length, 2 inches) with pincers called pistol-hand closed. B shows the pistol-hand
"cocked." The hand is the weapon of offense and defense as these shrimps forage
in the tide pools where populations are dense and fiercely competitive. (Courtesy,
MacGinitie and MacGinitie: Natural History of Marine Animals. New York,
McGraw-Hill Book Co., 1949.)
where it digs long passages that extend away from the stream, and open above
the ground surface through chimneys (Fig. 29.8). They are inactive in winter,
eat and grow very little and molt seldom if at all. During droughts, burrowing
crayfishes take to their tunnels, stop up the openings and retire into cisterns of
ground water. In early spring they appear in the open water, usually in the
shallows, leaving their tunnels — considerable numbers about the same time —
as if they had precise appointments with the softening temperature.
A crayfish walks forward slowly with the stealth of a cat but a sudden
stroke of the tail fin sends it streaking backward. They capture aquatic insects
and fishes by lying in wait and seizing them with their claws. They notice mov-
ing objects but their other senses, touch, taste and smell, are more important
to them (Chap. 17). Frogs, turtles, and herons feed upon little crayfishes.
Pickerel and yellow perch take any size, tails in first, and stomachs of pickerel
may hold four or five packed spoonwise. The shells of one or two may be
completely dissolved off, while those of later arrivals have only thin spots in
the shells where digestion has begun.
578 EVOLUTION OF ANIMALS Part V
General External Structure. The exoskeleton of crustaceans is a secretion
of the epidermis in which lime is gradually deposited (Fig. 29.9). Exoskeleton
not only covers the outside of the body but lines the digestive tube except for
the midgut. It will not stretch except while it is soft: neither is it enlarged by
additions like the shells of clams. Periodically the crayfish sheds the old skeleton
for a new one and this introduces a crisis in its life such as all arthropods share.
Body Regions. Arthropods have fewer and far less regular segments than
their annelid relatives, the clamworm and earthworm. The differences be-
tween them are most striking at the front end, far more exciting in a crayfish
than a worm. The body of the crayfish is divided into a fused head and thorax,
the cephalothorax, and a jointed abdomen. Each part is composed of seg-
ments. In the cephalothorax, covered by the jacketlike carapace, the seg-
ments are indicated by appendages, the mouth parts and legs; in the abdomen
by the obvious segmentation of the body and the swimmerets.
Appendages. Crayfishes and lobsters can use their variously specialized
appendages in numerous ways: as feelers for exploring; as jaws for tearing
and grinding; as food handlers; as bailers for dipping water; as pincer claws
for seizing prey; as paddles for swimming; in the male for transferring sperm
cells; and in the female for carrying eggs and young ones (Table 29.1). The
Fig. 29.7. Stages in the development of the shrimp, Penaeus. Like other animals,
the higher crustaceans go through developmental stages that are in some respects
similar to the adults of simpler ones. A, nauplius has three pairs of two-branched
appendages as in certain simpler crustaceans; B, protozoea stage with six pairs of
appendages; C, the zoea stage, with a distinct head and abdomen; D, mysis stage
with more appendages on the cephalothorax; E, adult shrimp, (Courtesy, Pauli:
The World of Life. Boston, Houghton Mifflin Co., 1949.)
Chap. 29
ARTHROPODS CRUSTACEANS
579
Fig. 29.8. Section of earth showing types of crayfish burrows, c, chimney and
opening of burrow; cc, closed chimney; s, stream; wl, ground water level; x, place
from which crayfish was taken. (After Ortman. Courtesy, Robert W. Pennak,
Fresh-Water Invertebrates of the United States. Copyright 1953, The Ronald
Press Company.)
jointed appendages of arthropods are among the most versatile of nature's in-
ventions. The abdominal ones are built on the basic plan nearest the original
pattern (Fig. 29.10).
Homology and Evolution of Appendages. The appendages of crayfishes and
lobsters are homologous structures with like parts in similar relation to one
another. They are striking examples of serial homology, all of them variables
of a common pattern. In the developing young, the basic pattern is clear, espe-
cially in lobsters.
Internal Organs and Metabolism. Digestion. Food is cut, shredded and
Fig. 29.9. Female crayfish in a resting position. Eggs are carried glued to the
swimmerets. After they are hatched the young ones hold on for a time with their
pincers in the exact fashion of young lobsters.
580
EVOLUTION OF ANIMALS
Table 29.1
Part V
Paired Appendages of the Crayfish (or Lobster) — Variations of Function on
THE Theme of a 3-Piece Appendage*
Segmentf
Appendage
Specialization
1
Antenna
A sensory filament (endo) and a shield for the eye (exo)
2
Mandible
Grinding jaw and a sensory feeler
3
1st maxilla
Food handling
4
2nd maxilla
Thin plates forming scoop to bail water over gills
5
1st maxilliped
Food handling, touch, taste
6
2nd maxilliped
" (gill) +
7
3rd maxilliped
ti it (( ti a
8
1st walking leg
Pincer and great claw (chela )^grasping, touch (gill)
9
2nd walking leg
Walking and grasping (gill)
10
3rd
Walking and grasping, opening of oviduct (gill)
11
4th
Walking (gill)
12
5th
Walking, cleaning abdomen, opening of sperm duct (gill)
13
1st swimmeret
Reduced in female; in male, protopod and endopod fused
forming organ for transferring sperm
14
2nd
In female, creates currents of water, attachment of eggs
and young; in male, takes part in transferring sperm
15
3rd
Creates currents of water; attachment for eggs
16
4th
As for 3rd swimmeret
17
5th
a (6 a a
18
Uropod (Tail foot)
Swimming oarlike plates used in quick backward glide
* The fundamentals of the 3-piece appendage are a basal piece, protopodite, and two
branches, an outer one or exopodite, and an inner one or endopodite. Some authorities list
20 and others 18 pairs of these appendages, depending on interpretations. The argument
for 18 pairs is: that the antennules develop from a structure that is homologous with the
prostomium ("upper lip") of annelids not considered a segment; and that the antennules
and eyes are basically sense organs, not appendages. A gill is attached to certain of the
appendages, is moved as they move and thus washed by more water.
t Segment indicates the segment of the body represented.
ij: (Gill) means that a gill is attached to the basal piece.
ground, the maxillae and maxillipeds holding it while it is crushed by the
mandibles. It then passes through the short esophagus to the stomach. Cray-
fishes can live in aquaria very well because being scavengers they do their own
housekeeping. They seize earthworms and pieces of meat and their chewing
competes with modern meat grinders; three pairs of tools hold, cut, shred and
grind; all the motions are rapid, including the frequent spitting out of the re-
jects.
The stomach is partially divided into two chambers (Fig. 29.11). In the
larger front, or cardiac, chamber there are three hard teeth that form a grind-
ing mill moved by muscles attached to the carapace at one end and to the teeth
at the other. When the food is crushed fine it enters the pyloric chamber
through a strainer of hairlike setae which allow only liquids and fine particles
to pass. There it is digested by the pancreaticlike secretion of the large diges-
tive glands. In cooked lobsters these are always "liver," green quilted rolls
that start arguments, to eat or not to eat. The digested food is absorbed
Chap. 29
ARTHROPODS CRUSTACEANS
Generalized Biromous Appendage
581
). Antenna (loucfilng, fasting)
18. Uropod (swimming)
17. Swimmeret
(carrying of
eggs In
female)
[ Protopodite
Endopodlle
13. First abdominal
appendage of male
(copulating)
13. First abdominal
oppendoge of
female
(rudimentary)
11. Fourth walking leg (walking)
8. First walking leg (pinching)
I I Exopodite
Fig. 29.10. Homology and the evolution of appendages. The appendages of the
left side of a crayfish. All these special structures are believed to have been derived
from a generalized two-branched appendage consisting of a basal piece, prodop-
odite; an inner branch, endopodite; and an outer one. exopodite as shown in the
figures. These basic structures are adapted for the different uses noted. They are
striking demonstrations of the changes that occur in evolution. (Courtesy, Hegner
and Stiles: College Zoology, ed. 6. New York, The Macmillan Co., 1951.)
through the intestinal wall in the midgut, the part of the tract not lined with
chitin. Only a small amount of waste passes through the straight insignificant-
looking intestine. At certain times two limy bodies, the gastroliths, form in
pouches in the lining of the cardiac chamber of the stomach. These are asso-
ciated with molting to be discussed later.
Blood and Circulation. The blood plasma is a watery fluid that contains
the bluish respiratory pigment hemocyanin composed of protein, copper, and
sulfur. It is similar to the pigment that makes clam broth bluish. Suspended in
it are numerous phagocytic cells. It clots very quickly and is probably a life
saver every time a claw of a crayfish is bitten off. It distributes food through
the body, carries respiratory gases to and from the gills, and waste products
to the kidneys. As in all arthropods, the circulation is the open type in which
blood vessels open into blood cavities, the sinuses or hemocoels. Blood flows
from the heart into the arteries and from them is carried by capillaries to the
various tissues, where it passes freely through minute open spaces and gradu-
582 EVOLUTION OF ANIMALS Part V
ally accumulates in the large sternal sinus which appears like a coelom but is an
unlined blood cavity (hemocoel) (Fig. 29.12). It then flows into the gills
through thin walled incurrent vessels and out through excurrent ones. From
the gills the now fully oxygenated blood flows back through vessels (branchio-
cardiac) to the pericardium and the heart.
Respiratory System. The plumy gills of crayfishes and lobsters are pro-
tected by the sides of the carapace that covers them like a jacket (Fig. 29.12).
They are washed by water bailed back over them by the scoops on the 2nd
maxillae and are moved by the legs and mouth parts to which they are attached
(Table 29.1 ). When water in an aquarium becomes too warm, the scoop beats
more rapidly. In response to the sparsity of oxygen in the warm water — the
crayfish is "out of breath." The same response is noticeable in lobsters caged
in tanks at summer lobster pounds.
Excretion. At least for animals beyond the embryonic stage, crayfishes
and lobsters are unconventional in having kidneys, "green glands," in their
heads, each one of the pair opening on a basal segment of an antenna just
below the eye (Figs. 29.10, 29.11). Like all kidneys these are closely asso-
ciated with blood. They carry on the characteristic functions of kidneys, re-
move metabolic waste, and take part in keeping the water content of the body
normal. Each consists mainly of a sac crowded with blood vessels, minute
blood sinuses, all closely associated with the coils of microscopic kidney tu-
CAROIAC
PORTION
•STOMACH
"■"^p'oflTION DIGESTIVE GLAND
ANTENNA ^ ' ' """^
ANT^NNULE I BRAIN
DORSAL ARTERY
INTESTINE
Fig. 29.11. Internal structure of the male crayfish (very similar to that of the
lobster). The green gland is the secretory or working part of the kidney; the
"bladder" of the diagram refers to the urinary bladder that opens externally below
the eye. Note the sperm duct opening externally in the basal segment of the fifth
walking leg. Sperm cells are placed in the seminal receptacle of the female by the
slender first and second abdominal appendages here shown. These are easy recogni-
tion marks of male crayfishes and lobsters. (Courtesy, MacDougall and Hegner:
Biology. New York, McGraw-Hill Book Co., 1943.)
Chap. 29 ARTHROPODS CRUSTACEANS 583
bules, and a canal that opens into the urinary bladder which in turn opens ex-
ternally. The entire crustacean kidney is in principle comparable to one unit
of the vertebrate kidney. Although crayfishes live in fresh water, they keep
an adequately salt solution in their bodies by the water resistance of their
body cover, by water loss from the kidneys, and by the absorption of salt by
the gills. Like all fresh-water invertebrates they contain a higher percentage
of salt than the surrounding water which would flood their bodies except for
the special means of keeping it out.
Fig. 29.12. Diagram of the respiratory and circulatory systems of the crayfish
or lobster. Efferent blood vessels from gills to heart, and the arteries are unshaded;
afferent vessels to the gills, and veins are black. Left side of heart with three open-
ings; p, pericardium; h, heart; aa, abdominal aorta; ac, cephalic aorta; as, ventral
abdominal artery. (After Claus. Courtesy, Conklin: General Morphology. Prince-
ton, Princeton University Press, 1927.)
Coordination and Response. The central nervous system is similar to that
of the earthworm but obviously further developed in the head and thorax
corresponding with the more elaborate activities of the crayfish (Fig. 29.11).
In the embryo, each of the segments contains a pair of ganglia but in the
adult crayfish members of the pairs and some of the pairs are fused. Numer-
ous nerves penetrate throughout the body, all of them composed of the proc-
esses of nerve cells whose bodies are in the ganglia.
By means of sensory pits and bristles the surface of the body is more or
less responsive, the pincers particularly to touch and the antennules to taste.
The organs of equilibrium by which the crayfish keeps its upright position
are located in small chitin-lined sacs, the statocysts, one on the basal segment
of each antennule (Fig. 29.11). Within the statocyst is a sensory cushion on
which there are numerous sensory hairs innervated by a single nerve cell fiber.
Large grains of sand (statoliths) are placed in the cup by the crayfish, an
extraordinary habit. These adhere to hairs made sticky by a secretion pro-
duced below the sensory cushion. The contact of the sand with the sensory
hairs is communicated by way of a nerve fiber to the central nervous system
and thence to the muscles. The linings of the statocysts are molted with the
584
EVOLUTION OF ANIMALS
Part V
rest of the skeleton and crayfishes cannot keep their balance until they have
another supply of sand grains. An experiment made upon shrimps is easily
repeated on crayfishes. Newly molted ones are placed in an aquarium of
filtered water, clear except for a scattering of iron filings dropped into it. After
exploring the bottom a while the crayfishes pick up the filings with their pincers
and place them in the statocysts. If an electromagnet is then moved about in
the water the crayfishes will follow it. According to the position of the mag-
net, they roll from side to side or lie on their backs and stab the air with their
legs. The exercise might be the preview of a human dance.
The two compound eyes are on stalks, movable independently, one to the
right and one to the left or otherwise. Each is composed of some 2500 simple
eyes or ommatidia. Seeing through one or another of these is like seeing
through a telescope pointed at a starry sky; through one there is a star; through
another there is darkness. Many simple eyes together bring a picture put
-TESTIS
OVARY
OPENINGS OF THE OVIDUCTS
SPERM DUCT
5TH WALKING
LEG
OPENINGS OF THE SPERM DUCTS
Fig. 29.13. A, female reproductive system of the crayfish. B, male reproductive
system of the crayfish. (Courtesy, MacDougall and Hegner: Biology. New York,
McGraw-Hill Book Co., 1943.)
together like a dissected puzzle as shown by photographs which have been
taken through parts of the eyes of insects. With such movable eyes as those
of crayfishes the pictures must be different in each one. Sight is essentially the
same in crustaceans as in insects.
Reproduction and Life History. Crayfishes mate in September and through
November of their first year. At that time, sperm are passed along the spe-
cialized appendages of the male to the seminal receptacle of the female, a
cavity in a fold of cuticle on the mid-ventral line between the fourth and fifth
pairs of legs (Fig. 29.13). The eggs are laid in April while the females are
still within the burrows. Before spawning she cleans the underside of her
abdomen, picking it over meticulously with her pincers. Then she lies on her
back, with her abdomen curved so that it makes a bowl. Presently a gluey
secretion flows out from the cement glands and over the bases of the swim-
Chap. 29 ARTHROPODS CRUSTACEANS 585
merets and tail pieces (uropods) coating every surface. Following this prepa-
ration eggs pour from the oviducts and pass backward across the seminal
receptacle where they are fertilized. Further backward they spread out among
the swimmerets, and stick fast to their fringes. Crayfishes and lobsters carry-
ing eggs are said to be "in berry" (Fig. 29.9). The eggs of crayfishes hatch
in five to eight weeks but the young ones, in Cambarus — diminutives of adults,
are for some time fastened to the egg shells by delicate threads that act like
"mother's apron strings." During their first year they molt about every 12 days
and after that usually only twice a year, once in spring, and again in late sum-
mer.
Regeneration. Crayfishes can replace lost appendages but to a lesser extent
than animals more simply organized. After a leg is lost, a new one appears
partly formed at the next molt, and larger at each succeeding molt until it is
complete.
Self -amputation — Autotomy. Crayfishes and other crustaceans, especially
crabs, amputate their own thoracic legs. If a leg is injured or grasped it may
be suddenly snapped off at a definite breaking place, on the basal segment
of the great claw or at the third joint at the other legs. Across the inside of the
leg on the proximal side of the breaking place there is a partition with a small
hole in the center through which nerves and blood vessels extend to the tip of
the leg. When the leg is cast off the hole is quickly stopped by a blood clot.
Molting and Hormones. A crayfish sheds a hard exoskeleton that fits tightly
and will not stretch. It appears in a new one that is soft and elastic, and ad-
justable to increased size (Fig. 29.14). The old skeleton was brittle with cal-
cium; the new one contains relatively little of it.
Molting is a laborious process during which every smallest spine and fila-
ment of the gill is pulled from its old cover. As it proceeds, the molting animal
uses more and more oxygen until the shedding is over. Then, for a time, it is
weak and helpless. There are profound adjustments in the metabolism of cal-
cium in preparation for the discard of the old skeleton and the completion
of the new one. For some time previous to the molt, a quantity of calcium
from the old exoskeleton is absorbed and distributed by the blood especially
to the stomach where it is deposited in the gastroliths (Fig. 29.14). Experi-
ments prove that the formation of the gastrolith is under the control of an
endocrine gland. After molting, the cuticle of the new exoskeleton is hardened
by calcium brought from the gastroliths by the blood as well as from new sup-
plies absorbed from the surrounding water. Most arthropods absorb unusual
quantities of water before molting. This swells their bodies, helps to split the
old exoskeleton and partly accounts for the sudden enlargement of the "soft-
shelled" animal.
An endocrine secretion limits the number of molts. It is produced by the
minute sinus glands, one in each eyestalk of crustaceans which have eye stalks;
586
EVOLUTION OF ANIMALS
Part V
PYLORIC STOMACH-
INTESTINE.
ChlTlNOUS LINING
CASTROUTH
GASTRIC EPITHELIUM
ESOPHAGUS
•CARDIAC STOMACH
Fig. 29.14. Upper, A, the sudden increase in size of a lobster after molting. The
skeleton that was shed has a crack in the thorax through which the lobster emerged.
B, the "soft-shelled" lobster after the molt. The sudden increase in size is due to
growth before molting and expansion afterward. Lower, diagram of the stomach
of the crayfish with a part cut away to show the gastrolith in the wall of the cardiac
chamber. For some time before molting calcium from the old exoskeleton is
absorbed by the blood and is stored in the gastrolith. (Upper, after Herrick.
Courtesy, Wolcott: Animal Biology, ed. 3. New York, McGraw-Hill Book Co.,
1946. Lower, courtesy. Turner: General Endocrinology , ed. 2. Philadelphia, W. B.
Saunders Co., 1955.)
in others they lie close to the brain. It is possible to remove the glands from
the eye stalks of a crayfish without injuring other structures, and when this is
done the animals form gastroliths, absorb extra water, consume more food and
oxygen and molt repeatedly. This can be prevented, however, by grafting sinus
glands of other crayfishes into those whose own glands have been removed.
Other Effects of Hormones. The sinus glands of crayfishes in some
Chap. 29 ARTHROPODS CRUSTACEANS 587
way stimulate movement of the pigments in the retina of the eyes (Fig. 15.2).
Experiments have shown that products of the sinus glands regulate shifting
color changes in the skin once thought to be nerve controlled,
Entomostracans
Entomostracans are crustaceans, most of them small, even microscopic, and
numerous beyond imagination. They feed upon the minute plants of fresh and
salt waters and thus are the chief means of turning them into food for higher
animals. They are themselves the main food of nearly all young fishes and
the adults of several market fishes. There are three groups, the branchiopods,
copepods, and ostracods.
Branchiopods
The gill-footed crustaceans, Branchiopoda, have thoracic feet that are ex-
panded and function as gills. Most of them live in fresh water, among them
the fairy shrimps (Eubranchipus), the largest and most colorful of entomos-
tracans but not important food producers. The most common branchiopods,
of the Order Cladocera, are the almost microscopic water fleas. The body, but
not the head, is enclosed in a bivalve shell so transparent that the pulsating
heart, the circulating blood, the contracting muscles, and vibrating gill feet
can be clearly seen. Many water fleas swim by their antennae; Daphne and
others with long antennae take slow strong strokes and go through the water
in jumps; those that have short antennae make quicker strokes and progress
evenly (Fig. 29.5).
The females carry the eggs and developing young in brood sacs. In sum-
mer they reproduce parthenogenetically. Their possible productivity is sug-
gested by the calculation that, barring accident, the descendants of one female
Daphne pulex might reach 1 3 billion in 60 days. Their populations create liv-
ing soup.
Copepods
From springs to lakes, from tide pools to the open ocean hardly any body
of water is without copepods. Those of one or another species are active in
summer and winter, most abundant wherever there are diatoms, their main
food. The great populations of glassily transparent copepods, a large part of
the surface fauna of the ocean, are the main link in the food chain between
microscopic plants and large animals. The blue whale, the largest of living
animals, feeds chiefly upon Calenus. Two tons of this little copepod. believed
to be one day's swallowing, have been taken from the stomach of a blue whale.
Three simple eyes (ocelli) are often fused into one compound eye. The one-
eyed jerky copepods, Cyclops, live as well in aquaria as they do in ponds. The
developing eggs and sometimes the young ones (nauplii) are carried in brood
588 EVOLUTION OF ANIMALS Part V
sacs, one on each side of the tail. Copepods are prodigies in reproduction.
Tisbe furcata, common in salt-water aquaria, goes through its life cycle from
egg to reproducing adult in 9 to 10 days (Fig. 29.5).
Ostracods
Ostracods are minute crustaceans, about one millimeter long. An ostracod
might be mistaken for a microscopic clam were it not for the appendages that
kick out between the valves, neither in structure nor character like a clam,
Ostracods live in fresh water and salt, usually creeping over plants, occasion-
ally swimming out into surface waters. They range into new places during the
free swimming naupHus stage.
Crustaceans as Human Food
Of all the crustaceans, shrimps probably take first place as human food
with crabs and lobsters close followers and crayfishes far behind.
Texas and other Gulf states furnish most of the shrimps for the American
market. They are fished from South Carolina southward and to some extent
on the northern Pacific coast. There are several "edible" species, one or
another being more highly regarded in different regions. Those of the same
species are called prawns or shrimps depending on their size, the shrimps
being smaller. Shrimps have long been thought of as little shrunken lobsters
and their name is derived from the Old English, scrimman, meaning shrink.
Thus, somebody may be a "little shrimp." The main edible crab of the east
coast of North America is the blue swimming crab (Callinectes sapidiis, Cape
Cod to Florida). On the west coast, the edible crabs include several species;
in some of them the thorax of the adults is commonly nearly a foot wide.
The American lobster, Homarus americana, ranges from Labrador to South
Carolina, along rocky coasts, in the shallows in summer, in deeper water in
winter. The female lays her eggs in July and August, about 10,000 by a
10-inch lobster. The mating and egg laying are similar to those of crayfishes
except that the lobster carries the eggs 10 to 11 months before they hatch
and spawns only every other year. Lobster culturists claim that a modest crop
of two adults from each 10,000 eggs is sufficient to maintain the species. The
smaller spiny lobster {Panulirus interruptus) is the edible lobster of the Pa-
cific Coast. Whether it has a quality of flavor equal to the New England lobster
is difficult to discover, in New England.
30
Artnropoas — Insects, Spiders,
ana Allies
Insects are small arthropods encased in lightweight, waterproof and flexible
exoskeletons. Basic features of their success are — their ability to live on land,
their economy of space and food, and their production of many offspring.
Their exoskeleton protects them from the evaporation that would otherwise
be inevitable with small size and life in dry air. Sense organs and sensory cells
in abundance can be stimulated through the exoskeleton which thus becomes
a means of contact and adjustment to the surroundings.
Insects have always lived with human beings; fleas have shared their blood;
cockroaches, their food; and silkworms provided them with draperies. Insects
have pressed upon humanity, hundreds of thousands of species to one of man.
They have crowded over the earth for ages, far longer than man has existed.
Many of them live together socially, ants, bees and others following inborn
patterns that bear undeniable resemblances to those of human society. Not
only are insects and man associated with one another, but among all animals
they are the two paragons of social life.
Characteristics. Insects can fly. In this they are unique among invertebrates
as birds are unique among the vertebrates. There are relatively few adult
insects that cannot fly — primitive species and confirmed parasites such as lice
and fleas. Immature insects do not fly except the mayflies and these do so only
when they are in a subadult stage.
All insects are clothed in an integument, the living epidermis or "skin" and
the nonliving exoskeleton or cuticle which it secretes (Figs. 30.1, 30.3).
The exoskeleton of insects differs from that of crustaceans in the absence of
lime and importance of chitin. The terms exoskeleton and cuticle are both
used for the secreted layer but the latter suggests its chemical content and
applies especially to insects. The best-known component of cuticle is chitin, a
589
590 EVOLUTION OF ANIMALS Part V
nitrogenous polysaccharide (C:{i.H-,4N40oi )x that is insoluble in water, dilute
acids, and the digestive juices of many animals. Chitin is extremely resistant
to decay and has been analyzed from the remains of beetles that lived in the
Eocene Period of 25 million years ago. In addition to covering the body, the
cuticle lines the fore- and hindgut, the air-tubes and the ducts of surface
glands.
An insect is an air-breathing arthropod with a distinct head, thorax, and
abdomen. The in-cut sharpness with which these parts are set off suggested
Fig. 30.1. Insects live almost everywhere and in unimagined places and ways.
They represent perfection of adjustment and success. Silverfish, Thermobia
domestica, a wingless insect about half an inch long, a rapid runner and skillful
dodger. The various domestic species live in the warmest places in houses, eat glue,
starch and paper, and are pests in libraries. Out of doors other species frequent
moist fallen leaves. (Courtesy, Ross: Entomology. New York, John Wiley and
Sons, 1948.)
the name insect. The head bears most of the sense organs, the thorax includes
those of locomotion, and the abdomen those of reproduction. All adult insects
have six legs, thus the name. Class Hexapoda. As adults, they usually have
either one or two pairs of wings. The primitive wingless ones are the Thysa-
nura, silverfish and firebrats and the Collembola, springtails (Fig. 30.1).
Male and female organs are in separate individuals and fertilization is internal.
Abundance, Reproductive Capacity and Size. There are some six times as
many species of insects as of all other animals (Fig. 30.4; Table 30.1). At
least 685,900 have been described but there is no complete catalogue, and
the estimates shift with many additions and changes due to duplications. Many
new species are still being discovered, especially in the tropics. The number
in any one locality is relatively small, varying greatly with the climate. Only
15,449 are given in "A List of the Insects of New York (state)" published in
1928 (Cornell University).
Individual insects are countless. The two or three hundred tent caterpillars
in one web swell to enormous numbers when they are compounded with those
in an unsprayed apple orchard. Mayflies emerge from the water by millions,
fly for a brief period, then fall to the ground and mounds of them, accumulat-
ing under the lamps in lakeside parks, are cleared away by shovelfuls. In
some pantries and kitchens, the supply of cockroaches is like a never-failing
Chap. 30
ARTHROPODS INSECTS, SPIDERS, AND ALLIES
591
m'
it
J
Fig. 30.2. Termites. Buildings of the magnetic termites (Hamitermes meridio-
nalis) or white ants near Darwin, Australia. They are slabs of peaty soil whose long
axes lie almost exactly north and south. Within them millions of termites populate
the passageways and the chambers are filled with grass collected in the wet season
(November to April) and stored to last through the dry season (May to October).
On a smaller scale termites in the milder climates of the United States build
similar passageways in wood. (Photograph by W. Brindle. Courtesy, Australian
National Information Bureau.)
spring. Warm damp evenings murmur with mosquitoes and a meadow lighted
by fireflies tells more about their numbers than can be written. In autumn,
ladybird beetles turn gregarious and pack together in protected spots for the
winter. In northern California, ladybirds {Hippodamia convergens) go to the
mountains in winter and hide under the pine needles in sunny slopes. Two per-
sons working together can collect 50 to 100 pounds of them in a day and
since each beetle weighs about 20 milligrams, a day's catch is estimated to be
at least one to two and a half millions.
The reproductive capacity of insects depends upon the number of eggs laid
and the length of time it takes for an egg to develop into an adult. "Seventeen
year locusts" are 17 years old before they produce eggs, but most insects
mature within a year or less. A grand climax is attained by aphids with 30
generations in a single season, nearly every one wholly made up of productive
females and each generation a stepping stone to a larger generation. Aphids
must be a pleasure to mathematicians. Herrick calculated the weight of cab-
bage aphids that produce 12 parthenogenetic generations between late March
592
EVOLUTION OF ANIMALS
Part V
Fig. 30.3. Carpenter ants (Camponotus), about half an inch long, and burnished
black, in the corridors which they have cut. These ants are common in and out of
houses. Their young are here shrouded in white cocoons. (Photograph by Lynwood
Chace. Courtesy, National Audubon Society.)
and mid-August. At the end of that time, the progeny of the original female
would weigh 800 million tons providing every one were living.
Insects are like old miniatures in their perfection within small size. Some
are larger than the smallest vertebrate and others are smaller than the largest
protozoan. The smallest North American beetles can scarcely be seen with-
out a lens, yet their structure is as complex as any other insect. The Central
American rhinoceros beetle {Megasoma elephas), a relative of our common
June beetle, is five and a half inches long.
The large size of ancient animals was no more successful for insects than
for the great reptiles. Only the fossils are left to tell the old story, too much
body to feed and no place to hide. A fossil dragonfly (Meganeura) has a wing
expanse of two feet; there are no such living ones (Fig. 30.5).
Habitats and Distribution. Although their remote ancestors came from the
Chap. 30
ARTHROPODS INSECTS, SPIDERS, AND ALLIES
593
Table 30.1
Number of Described Species of Insects, Ticks, and Mites at the End of 1948*
Order
Common . Names
World
Anoplura
Coleoptera
Collembola
Corrodentia
Dermaptera
Diptera
Embioptera
Ephemeroptera
Hemiptera
Hymenoptera
Isoptera
Lepidoptera
Mallophaga
Mecoptera
Neuroptera
Odonata
Orthoptera
Plecoptera
Protura
Siphonaptera
Thysanoptera
Thysanura
Trichoptera
Zorapterat
Total
Acarina
Sucking lice (true lice)
Beetles, weevils, twisted winged insects
Springtails
Booklice, barklice
Earwigs
Flies, mosquitoes, gnats
Embiids
Mayflies
True bugs and Homoptera (cicadas, leafhoppers,
aphids. scale insects)
Ants, bees, wasps
Termites ("white ants")
Butterflies and moths
Biting lice (bird lice)
Scorpionflies
Lacewings, ant lions, dobsonflies
Dragonflies, damselflies
Grasshoppers, crickets, roaches, mantids, katydids
Stoneflies
Fleas
Thrips
Bristletails, "Silverfish"
Caddisflies
Ticks
Mites
250
277,000
2.000
1,100
1.100
85,000
149
1,500
55,000
103,000
1,717
112,000
2,675
350
4,670
4,870
22,500
1,490
90
1,100
3.170
700
4,450
19
685,900
440
8,700
North
America,
north of
Mexico
62
26,676
314
120
18
16.700
8
550
8,742
14,528
41
10,300
318
66
338
412
1.015
340
29
238
606
50
921
2
82.394
113
2,500
* Source: Insects, U.S.D.A. Yearbook of Agriculture. 1952.
t Zoraptera includes Corrodentia or booklice. Embioptera — minute tropical species, and
Protura — minute and rare species.
sea, insects have been land-adjusted for millions of years. In their immature
stages, mosquitoes can thrive in brackish water but with rare exceptions, in-
sects keep away from the sea. Those of several groups live in fresh water while
they are immature and some others remain there as adults but all breathe air
as adults and are essentially terrestrial.
Insects have spread almost all over the earth, in abundance in all tropical
and temperate countries and as parasites living on the warm bodies of birds
and mammals, in arctic and antarctic lands. One or another kind of insect
makes a living in every conceivable location in and out of buildings, in every
part of all kinds of plants, in forests and open fields. Insects are persistently
active, feeding, and flying, constantly urged to shift their places by competition
594
EVOLUTION OF ANIMALS
Part V
for food and space and changes in their microclimates. In general, they do not
make long flights. The forays of migratory grasshoppers are exceptions; so
are the seasonal migrations of butterflies (Fig. 30.6).
Insects are carried long distances by air currents. Newly hatched gypsy-
moth larvae are buoyed up in the air by small air pockets on the hairs with
Fig. 30.4. Diagram representing the
relative abundance of insects (Hexapoda)
and other animals. (Courtesy, Frost: En-
tomology. New York, McGraw-Hill Book
Co., 1942.)
which their bodies are covered. They have been captured 300 feet or more
up in the air and on strong winds they may travel many miles a day. Insects
travel far and wide on human beings and their vehicles — by water, by land,
and by air.
Molting and Metamorphosis. Molting. The young insect grows larger but
its cuticle does not. Relief comes to it only with a new and larger cuticle and
escape from the old one, that is, by molting.
As before stated, the integument of insects consists of epidermal cefls and
the cuticle that they secrete. The cuticle includes two regions of different
chemical content; the outer cuticle, mainly cuticulin, fats and waxes, is re-
sistant to injury and has an outermost waxy layer; the inner cuticle is com-
posed chiefly of chitin.
There are several steps in the preparation for molting. (1) The epidermal
cells secrete a new outer cuticle which then lies between them and the old
cuticle. (2) Specialized epidermal cells secrete molting fluid which passes out-
ward through ducts in the new outer cuticle and spreads over its surface. In
doing so it separates the new and the old outer cuticles. (3) Molting fluid
gradually digests the old inner cuticle. (4) In the meantime, the epidermal
cells are forming a new inner cuticle. The molting fluid does not digest this.
The digested substance of the old inner cuticle is absorbed back into the
body. This has been shown by the absorption through the body wall of dye
injected between the old and new cuticles. At this time the new cuticle is
permeable to water. An insect sheds its old cuticle soon after the new one is
completed. Some insects do this too quickly to be clearly observed; others
Chap. 30
ARTHROPODS INSECTS, SPIDERS, AND ALLIES
595
Fig. 30.5. A dragonfly is ancient history on wings. The form of this one was
preserved in stone over 250 million years ago, long before there were birds to fly.
Of three great steps in the evolution of insects, wings held straight out from the
body came first; wings folded to the body when at rest was second; complete change
of form in a single lifetime came third. (Courtesy, Frank M. Carpenter, Harvard
University.)
take several minutes. The insect contracts the muscles of its legs and abdomen
forcing blood into the thorax which swells accordingly. Young mayflies swal-
low air. The old cuticle cracks along the line on the head and thorax where
the inner cuticle has never formed and the other one is weak. A molting insect
bucks its thorax upward, wriggles its body free of the old cuticle, and contracts
itftpasmodically. This drives blood into the wings and legs which stiffen out
as molting is completed. Forcing the blood here and there during molting
stretches the cuticle to its utmost, leaving the softer parts in folds that are
smoothed out only after further growth. The new cuticle hardens and darkens
in a short time, but this is not due simply to exposure to air. If a part of the
new cuticle is exposed by the removal of a piece of the old one 24 hours
before molting, the new cuticle will neither harden nor darken.
Metamorphosis. The young insect that crawls out of the eggshell is usually
quite unlike the adult it will become. Between hatching and maturity insects
increase in size mainly by steps at molting time. Most of them undergo a
metamorphosis or change of form. The less the young and adult resemble one
another, the greater are the structural changes inside and outside of the body.
There are three main types of metamorphosis (Fig. 30.7). (1) With slight
change of form and no wings ever developed, e.g., the household silverfish
596
EVOLUTION OF ANIMALS
Part V
Fig. 30.6. Monarch butterflies (Donaus menippe) resting while on an autumn
migration. They rest at night and whenever the wind is strong. (Photograph by
Hugh Halliday. Courtesy, National Audubon Society.)
(Thysanura) and the springtails (Collembola). (2) With gradual or incomplete
change of form or metamorphosis — the wings developing as external pads;
in the immature stages the young are called nymphs. Grasshoppers, crickets,
cockroaches, cicadas, squashbugs, dragonflies, mayflies and others develop in
this way. Nymphs usually feed in the same manner and on the same food as
the adults. (3) With complete change of form or metamorphosis, the wings
Pupa
Fig. 30.7. Upper, types of life histories and metamorphoses: gradual meta-
morphosis of the grasshopper; incomplete metamorphosis of the dragonfly that
lives in water and breathes by internal tracheal gills during immaturity, and on
land as an adult; complete change of form in the army worm. Lower, transforma-
tion of a dragonfly. Left, the full-grown nymph has crawled onto a floating lily
pad. The adult has emerged through a crack in the nymphal skin, and is bent
backward still wet and soft, with wings tightly folded. Right, the adult rests with
stiffening wings unfurled. (Upper, courtesy, Strausbaugh and Weimer: General
Biology. New York, John Wiley and Sons, 1952. Lower, photographs by Lynwood
M. Chace. Courtesy, National Audubon Society.)
597
598 EVOLUTION OF ANIMALS Part V
developing as internal pads; in the immature stages the young are called larvae
and pupae. Bees, wasps and ants, moths and butterflies, beetles, and caddis-
flies develop in this way. The larvae eat ravenously and increase greatly in size.
At the end of several molts, the number depending upon the species, they
transform into pupae. The pupa does not eat and moves little or none. It is a
stage of transformation in which the outer form and the internal structures are
changed; the digestive tube is reshaped; the reproductive organs are developed.
Even the tissues are reorganized, and muscle is literally made over. The adult
is the final mature stage. The larvae of moths and butterflies are caterpillars,
strikingly different from the adult even in appetite. "Cabbage worms" have
insatiable appetites for cabbage leaves; cabbage butterflies follow the scent
of cabbage plants but only to lay their eggs on them, never to eat them.
Foods Habits and Mouth Parts. Insects of one sort or another eat all kinds
of food. Many are very special but altogether they fall into four general groups,
plant feeders, predators, scavengers, and parasites.
Nearly half of all insects feed upon living plants, the most reliable food there
is. Most plant feeders prefer one group of plants or they may feed upon only
one part of the plant, the leaf, stem, root, bud, flower and fruit. Plant lice
(aphids) insert their slender, piercing mouth parts into the tissues of ten-
der leaves and stems, dissolve the tissues with saliva and suck out the juices
(Fig. 30.8). In spring, the garden cut-worms (larva of noctuid moths) are
roused from hibernation in the soil and begin biting off the stems of seedling
plants — tomatoes, cucumbers and others at the surface level of the soil. Gipsy-
moth caterpillars eat oak leaves, veins and all; larvae of elm-leaf beetles take
only one layer of the leaf. Most plant feeders take their meals in daylight, but
there are some evening diners.
Predacious insects are less abundant than the plant eaters. Predators have
dash and go, or stealth. Dragonflies with their arrowy flight, clutching fore-
legs, and chewing jaws were built for predation 100 million years ago. The
larva of the ant lion (Myrmeleon) digs a trap, an inverted cone in loose dry
sand. Ants roll down the slopes of the cone and as they struggle, they are
showered with sand by a twist of the ant lion's head whose jaws await them at
the bottom. The majority of predatory insects depend upon less active vege-
tarian insects for food.
Certain insects, especially the larvae, are scavengers that eat dead and
decaying animal matter. Two familiar ones are houseflies and clothes moths.
Both are typical scavengers but the clothes moth larvae have an insatiable
craving for keratin, the hornlike substances in hair (fur, wool) and feathers.
Many insects are parasites, living on other animals, and gradually consuming
them while they are in the living state. Among them are the blood-sucking fleas,
biting lice of birds, and the parasites of other insects.
The mouth parts of insects are often specialized and elaborate. The main
Chap. 30
ARTHROPODS INSECTS, SPIDERS, AND ALLIES
599
Fig. 30.8. Biting mouth parts in action. Upper, caterpillar shearing a leaf with
its cutting jaws (mandibles). Its upper lip (labrum) and the attached piece
(clypeus) hang downward at the center. The second pair of jaws (maxillae), the
lower lip (labium) and the tongue are hidden. Lower, sucking mouth parts in
action. Plant lice feeding, a, finding a place; b, needling in the slender tube,
mainly composed of the mandibles and maxillae; c, sucking up the sap. The com-
bination of piercing and sucking is the method of feeding in such successful
insects as the plant lice, squash bugs, mosquitoes and bed bugs; and sucking is the
way of the moths and butterflies. (Courtesy, Matheson: Entomology. Ithaca, N.Y.,
Comstock Publishing Co., 1944.)
biting tools are the mandibles hinged to the head at the sides of the mouth and
operated by muscles that oppose or separate their tips, a sidewise bite. In the
lapping and sucking equipments of insects the mandibles, maxillae, labrum
and hypopharynx are stiletto-like blades combined into a beak used for suck-
ing sap or blood and other fluids (Fig. 30.8). Houseflies lap up syrup. In stable
flies (Stomoxys) the lapping organ has become needlelike and able to pierce
the flesh. The long nectar-sucking tube of moths and butterflies consists only
of maxillae that fit together and make a tube. Their mandibles and other parts
have ceased to develop. No butterfly can bite.
Representative Insects — Grasshopper and Honeybee
The Grasshopper
Grasshoppers are generalized in structure and habits, less so than cock-
roaches, but outside of agriculture more attractive in human circles. Gen-
eralized insects are comparable to the crows that can both walk and fly,
specialized ones to humming birds that can fly but scarcely walk. The ancestors
600 EVOLUTION OF ANIMALS Part V
of grasshoppers were pioneer insects in the warm dampness of the Car-
boniferous Period, when primeval forests were being slowly overspread and
were turning to coal beds. Their fossils show that since then grasshoppers and
cockroaches have changed far less than most insects.
Grasshopper. Grasshoppers belong to the family Locustidae, the locusts or
short-horned grasshoppers with antennae shorter than the body. They include
the common red-legged grasshopper (Melanoplus femur-rubrum) , the "Caro-
lina locust" (Dissosteira Carolina), the "Rocky Mountain locust" (Melano-
plus mexicanus) , and the short-winged lubber grasshopper of the south, often
studied in laboratories. The following discussion applies in general to any one
of these.
The names grasshopper and locust are confusingly applied even to the same
species. Grasshoppers are permanently resident, solitary species such as the
common red-legged one. "Locusts" are migratory grasshoppers, such as the
Rocky Mountain locusts that periodically produce enormous populations, com-
pletely exhaust the food in their own region and then move from one new
feeding ground to another. In 1933 and before and since then, "Rocky Moun-
tain locusts" have swarmed over the country from the Rocky Mountains east-
ward nearly to the Mississippi River, devastating com and wheat fields and all
ground vegetation before them.
Ecology. Grasshoppers flourish in sunlit fields of grass and grain. The
young ones hatch in early spring, by July are usually abundant, and in August
sprays of grasshoppers arise wherever long grass is disturbed.
Food and Relationships. A great element of success in life is the habit of
living on common food. The success of the tribes of grasshoppers is due to this
habit. No other invertebrates consume grass and grains in such quantities.
Toads, frogs, owls, meadowlarks, chipmunks, and ground squirrels all feed
upon grasshoppers. Parasites also beset them, young hair worms that bore into
their bodies, red mites that hang from them like brilliant beads. Enough grass-
hoppers to produce a plague would appear every year were it not for the mis-
haps that befall the eggs, the attacks of parasites, winter freezing and thawing,
spring floods, skunks and ground moles that nose them out of the ground, and
their great enemies, the larvae of blister beetles. A nicety in seizing an oppor-
tunity is exemplified by certain small wasps (Lepidoscelio) which ride about
on the females until they lay their eggs, and then deposit their own eggs be-
side them (Fig. 30.9).
External Structures and Functions. Like other agile animals, grasshoppers
are bilaterally symmetrical. The body consists of three divisions, the head, the
thorax, the abdomen (Fig. 30.10).
Head. The head is a hard capsule, composed of immovable plates or
sclerites. The eyes and antennae, mandibles, maxillae and labium are believed
to represent different segments in the wormlike ancestors. There are two kinds
Chap. 30
ARTHROPODS INSECTS, SPIDERS, AND ALLIES
601
Fig. 30.9. Successful hitchhikers, quick transport and the right landing place.
Females of wasp-like insects (Lepidoscelio) that ride about on grasshoppers until
the latter lay their eggs. Then they dismount and lay their own eggs on those of
the grasshopper in which their larvae develop. (After Brues: Insect Dietary. Cam-
bridge, Mass., Harvard University Press, 1946.)
of eyes, simple (ocelli) and compound, the latter an assemblage of simple eyes.
Insects never have but one pair of antennae; in grasshoppers they are primarily
feelers. In other insects they may have auditory, olfactory, or respiratory func-
tions.
Grasshoppers have the complete quota of mouth parts typical of insects
(Fig. 30.11). Their comparative simplicity is a contrast to the specializations
of the blood-sucking equipment of mosquitoes and the nectar-dippers of bees.
The exact shape of the jaws of grasshoppers is also well fitted to bite particular
plant tissues. Lubber grasshoppers feed upon leaves and have jagged "teeth"
that tear and shred. Another species eats seeds that it cuts and chisels (Fig.
30.12). The mouth parts include: (1 j the broad upper lip or labrum; (2)
a median tonguelike hypopharynx; (3) two heavy biting jaws, the mandibles,
so shaped that the teeth interlock; (4) two slender jaws, the maxillae whose
several parts include jointed palpi with sensory organs on their tips; and (5)
a broad median lower lip, the labium with two jointed palpi that bear
sensory organs. The opening of the salivary glands is on the edge of the
tongue or hypopharynx.
Thorax. The thorax, with the legs and wings, holds the chief muscles of
locomotion and the nerve centers that control them (Fig. 30.10). It is divided
into prothorax, mesothorax, and metathorax. On the dorsal side of the
602
EVOLUTION OF ANIMALS
Part V
Ocelli
Compound
eye
Fig. 30.10. Grasshopper. The tarsus of the hind foot is comparable to the sole
of the human foot in relation to the surface. Foothold is strengthened by claws
and non-skid pads. The hind legs are the powerful equipment for take-off in the
jump of grasshoppers as they are in kangaroos.
prothorax there is a saddle-shaped sclerite that extends forward and protects
the neck. Each of the other divisions bears one pair of spiracles and a pair of
legs and wings; in the course of evolution the sclerites in these divisions have
been greatly modified in accommodating the large muscles of locomotion.
Legs. In climbing plant stems grasshoppers pull with their front legs and
push with the hind ones. Their take-off for a jump is a relatively enormous
compound eye
maxillary palp
Fig. 30.11. Head and mouth parts of the grasshopper; outer surfaces of the
jaws (mandibles) and upper lip (labrum); inner surfaces of the maxillae and
lower lip (labium). (After Snodgrass. Reprinted from Animals Without Backbones
by Buchsbaum, by permission of The University of Chicago Press. Copyright,
1948.)
Chap. 30 ARTHROPODS INSECTS, SPIDERS, AND ALLIES 603
push which would end in a crash-landing except for the flexiblity and spread
of the middle and front legs and the jack-knife bend of the hind ones. As
animals walk and run they alternately balance and move their bodies. The bal-
ance is a momentary rest on one, two, or three feet, depending on the type,
whether human, horse, beetle, or others (Fig. 10.10). The movement, also
momentary, is a falling forward of the body or a fall coupled with a pull. As
an insect walks it balances by resting on a tripod, the first and last leg of one
side, and the middle leg of the other. The balance quickly shifts into movement
B
Fig. 30.12. Mouthparts of insects are precision tools, mandibles of two species
of grasshoppers that eat different foods. Left, the lubber grasshopper (Brachystola
magna) feeds on foliage. Right, another grasshopper {Menuaria macnlipennis)
feeds on seeds. (Redrawn from Isely. Courtesy, Brues: Insect Dietary. Cam-
bridge, Mass., Harvard University Press, 1946.)
as the other three legs are swung forward. In this latter trio, the front leg pulls
the body, the middle one lifts it, and the hind one pushes. The insect goes for-
ward in such a slight zigzag that it seems to be a straight line.
Wings. Many invertebrates can walk and crawl but only the insects can fly.
The wings of birds are highly modified front legs; those of insects have no rela-
tion to their legs. The wings of most insects are connected with the body by
flexible joints to which the flight muscles are attached. In grasshoppers and
other insects that gradually change form, wings are direct outgrowths of the
posterior dorsal edges of the meso- and metathorax (Fig. 30.7). While it
is developing, the wing pad contains tracheae, nerves, and blood. The ar-
rangement of the tracheae usually determines the future pattern of the veins.
By the time the wing is mature it is comparable to a flat envelope composed of
chitin and the dead remains of cells. Within it the walls of the tracheae are
thickened and transformed into solid rods, the veins. Although so much of the
wings is chitinous, blood continues for a time to circulate slowly through it
outward to the tip and back to the body by another route (Fig. 30.13).
The patterns of veins (wing venation) are important in showing relation-
ships between species. All of them seem to have evolved from one or a few
basic ones. The more primitive insects, mayflies, grasshoppers and others
have many veins. Specialized insects such as bees have few veins. During the
long history of insects the veins have been reduced in number but are better
placed and mechanically more efficient.
604 EVOLUTION OF ANIMALS Part V
Abdomen. Each typical segment has a dorsal and a ventral sclcrite, con-
nected at the sides by flexible membranes which allow the abdomen free
breathing movements (Fig. 30.10). The first pair of abdominal spiracles is on
the first segment, one in front of each eardrum; the others are in the same
relative positions in the next seven segments.
In the female grasshopper, the terminal segments form the ovipositor. The
ventral sclerite of the eighth segment is prolonged beyond its dorsal mate, and
extends between the prongs of the ovipositor and into the genital opening and
forms a trough, the egg guide. The most conspicuous parts of the ovipositor are
the digging tools called valves. These are closed together like scissors, pushed
into the ground and then opened, letting the eggs slip between them through the
Fig. 30.13. Circulation of blood in the hind wing of the cockroach (PeripJoneta
americana). (From Wigglesworth. Courtesy, Ross: Entomology. New York, John
Wiley and Sons, 1948.)
egg guide (Fig. 30.10). The ventral sclerites are lacking on the ninth and tenth
segments. The eleventh is represented only by a triangular piece above the
anal opening, and a pair of similar pieces, the cerci, one on either side of it.
The latter are remnants of abdominal appendages present in the ancestors of
grasshoppers when the bodies of insects were longer than now. In the male
the sternum of the ninth segment forms a hoodlike cover over the copulatory
organs.
Cuticle and Integument. Neither the outer nor inner surface of the cuticle is
smooth. On the outer one there are ridges, spines and hairs. In butterflies and
moths, there are numberless scales formed by secretions from cells in the
epidermis. Certain cells build up flexible bristles (setae), and after the bristles
are formed the cells usually die. On the inner surface of the cuticle there are
knobs, hooks and ridges to which the muscles of the body are attached and
thus it becomes a supporting framework.
Color. Insect colors are located in the epidermis, except for a few in the
cuticle. They may be chemical colors, due to pigments, or structural ones due
to the reflection and interference of fight rays on the surfaces of cells and
Chap. 30 ARTHROPODS INSECTS, SPIDERS, AND ALLIES 605
layers of cuticle as in the blue of butterflies; or pigment and structural effects
may be combined in iridescence. The blackish pigment melanin and yellow
carotin deposited in the secretion of the outer cuticle are responsible for prac-
tically all chemical colors. Following the intense muscular activity of their
flights migratory grasshoppers, ordinarily light brown, turn dark brown with
orange markings. If such grasshoppers are captured and kept quiet for a
time their original color returns; if they are restless and continually fluttering,
the dark background and orange marks remain.
Internal Structures and Functions. Body Cavity. The body cavity of in-
sects lacks the epithelial lining of a true coelom as in the frog. It contains
circulating blood and is correctly called a hemocoel.
Muscles. The muscles of insects are complicated and numerous. In man
there are 792 distinct muscles, in a grasshopper over 900. The ends of insect
muscles are attached by tendons to knobs on the inner surface of the cuticle.
Digestion and Assimilation of Food. The digestive tube runs an almost
straight course from mouth to anal opening (Fig. 30.14). In the head it is held
casthk; caicac ovarian tubulcs
BURSA
COrULATRIX
' SALIVARY
'v DUCT ,
ClRCUMlSORHAOtAl.;^; N STOMOOiAl
COMUrSSURI ; LABIUM ^^ *'"'' saLPVARY JRD "MORACIC i^^"' MiSINTIRON maL^IGHIAN "-lUM
MYPOmARYNX SUBISORHAMAI ^^j,^o GANGLION NERVE CO«D <IH ABDOMINAL TUBULES
GANGLION GANGLION
Fig. 30.14. Internal organs of the female grasshopper. The foregut extends from
the mouth to the openings of the stomach pouches (gastric caeca); the midgut
(stomach or mesenteron) from the gastric caeca to the Malpighian tubules; the
hindgut from the tubules to the anal opening. (Courtesy, Matheson: Entomology,
Ithaca, N.Y., Comstock Publishing Co., 1944.)
in place by muscles attached to the body wall, but elsewhere it is supported by
the tracheae. The foregut is lined with cuticle continuous with the outer cover-
ing of the body; the hindgut is likewise lined; the midgut has no chitinous
lining. The muscular action in the walls of each region results in the churning
movements similar to those in other digestive tubes.
Foregut. The foregut begins with the mouth cavity which receives the
saliva, continues into the curved pharynx and short esophagus that widens
into the thin-walled crop, then narrows into the thicker-walled gizzard. The
mandibles and maxillae cut and shred the food while the saliva is mixed with
it. The brown "molasses" extruded from the mouth when a grasshopper is
handled is at least partly a regurgitation from the crop mixed with fluid from
the gastric caeca.
606 EVOLUTION OF ANIMALS Part V
In herbivorous insects, the saHva contains a starch-splitting enzyme (am-
ylase) whose action begins in the mouth. Plant lice inject such saliva into the
plant tissues and digestion starts before the food is taken into the mouth. As
it is swallowed it is evidently pushed backward onto the base of the tongue
(hypopharynx). It then slips on into the crop, mainly a storage sac. The giz-
zard or proventriculus is equipped with chitinous teeth that thoroughly grind
the food by a different method but with the same result as in birds.
Midgut. At the posterior end of the gizzard a valve keeps food from passing
into the stomach before it is ground. The stomach is the main organ of
chemical digestion and absorption. In the cockroach, its lining produces the
sugar enzyme — maltase, the fat enzyme — lipase, and the protein splitter —
trypsin. All of these enzymes are catalysts that speed digestive processes, much
needed in animals with low body temperatures. Insects have no mucus to pro-
tect the lining of the stomach as the vertebrates do. In place of it certain
epithelial cells produce an extremely thin sheath (peritrophic membrane)
which in the stomach surrounds the food like a tube.
Hindgut {intestine). The excretory organs (Malpighian tubules) open into
the digestive tube at the junction of the stomach and intestines (Fig. 30.14).
The lining of the hindgut is permeable to water and, with the economy of water
usual in insects, it is there absorbed back into the body. Waste substances are
finally extruded from the body in dry pellets.
Blood and Circulation. Insect blood, like vertebrate blood, is a tissue
fluid that distributes digested food to the tissues and carries away the waste
products of their metabolism. Although it holds oxygen and carbon dioxide in
solution it contains no such efficient oxygen carrier as the hemoglobin of
vertebrates and its role in respiration is secondary. It contains proteins, glu-
cose, salts, fats and an unusual amount of amino acids. With rare exceptions
such as the larvae of chironomids (midges), it does not contain hemoglobin
but absorbs oxygen and carbon dioxide in solution. As before mentioned,
while an insect is molting, muscles in the legs and abdomen contract and fill
the thorax with blood, swelling it till the outer cover cracks open along the
midline of the back. As soon as the insect sheds the old cover it contracts the
thorax and forces blood into the wings (Fig. 30.13).
Blood Cells. There are several different kinds of blood cells, but no red ones.
They adhere to tissues and spread out often in star shapes and circulate with
the fluid (Fig. 30.15). Here, as in other animals, blood cells are deeply in-
volved in the experiences of the animal and their forms and functions change
with conditions in the body.
Functions of Insect Blood. Three functions of insect blood are well estab-
lished. The chief function of the blood cells is phagocytosis, the ingestion of
minute particles and living bacteria. Blood carries digested food to the tissues
and metabolic waste from them to the excretory organs (Malpighian tubules).
Chap. 30
ARTHROPODS INSECTS, SPIDERS, AND ALLIES
607
Alimentary
cana
rtition
Ner\/e
THORAX
ABDOMEN
Fig. 30.15. Upper, diagram of the circulatory system of an insect. B, cross
section of the thorax of the same. C, cross section of the abdomen. Arrows indi-
cate the course of the circulation. The blood flows forward through the heart, a
tube extending along the middle of the back. It pours out of the open front end
of this and turns backward flowing through open spaces (sinuses) above and
below the digestive tube. As it does this some of it turns toward the back and
enters the heart through small openings. Some turns out into the legs and wings
where it bathes the tissues directly.
It also transports hormones. Pressure upon the blood in one or another part of
the body is a part of the mechanics of molting and of moving the air in the
tracheae during breathing.
Circulation of the Blood. The only blood vessel is the heart, a tubelike suc-
cession of connecting chambers extending along the mid-dorsal line of the body,
the ?ieart proper in the abdomen, the aorta in the thorax (Fig. 30.15). Peri-
staltic contractions move in waves over the tube from rear to front. In many
species, the movement is reversed in one or another phase of life, and the
blood flows backward. As each chamber dilates, blood is sucked into the heart
through slitlike openings along the sides. These close as a wave of contraction
passes them and pushes the blood before it. At the open end of the aorta it
floods out into an open space about the brain, circulates within the head and
turns backward through the spaces (hemocoels) surrounding the internal
organs, much of it passing into the wings and legs. Minute contractile pumps
in the thorax draw it through the wings and legs. In the wings it passes out-
ward beside the veins of the front part of the wing and inward again to the
body beside other veins as it does in cockroaches (Fig. 30.13). With a micro-
scope circulating blood can be clearly seen in the flattened legs of certain
608 EVOLUTION OF ANIMALS Part V
mayfly nymphs. The blood cells dally along the muscles, are moved toward
the foot, then drift slowly back to the body and turn toward the heart.
Release of Energy — Breathing and Respiration. Skin was the original
respiratory organ of all multicellular animals but the skin of insects is covered
with cuticle. In them its place is taken by a tubular ventilating system through
which air is brought in and out by the muscular action of breathing (Fig.
30.16). The tracheal tubes open to the outside through spiracles. The structure
of their walls is similar to that of the body wall and they originate by ingrowths
of it during embryonic development. Tracheae carry oxygen directly to the
cells and bring carbon dioxide away. Their walls are permeable to gases espe-
FiG. 30.16. Diagram of the tracheal system of the grasshopper by which oxygen
is carried directly to the tissues. It finally reaches them through tracheoles, the
minute ends of the tracheae not shown here. The main tracheae and air sacs of
one side are shown with the digestive tube removed. A, main air sac; O, trachea
surrounding the compound eye; E, inner surface of ear surrounded by trachea;
S, abdominal air sacs; numbers indicate spiracles, the external openings of the
system. (Courtesy, Matheson: Entomology. Ithaca, N.Y., Comstock Publishing
Co., 1944.)
cially to carbon dioxide. The spiracles are opened and closed by valves that
control the flow of air and evaporation. The chitinous lining of the tracheae is
strengthened by spiral bands (taenidia) that with the aid of a microscope can
be unwound like the spring of a curtain roller. Tracheae divide again and
again, until they terminate in exceedingly minute tracheoles. Clusters of these,
clearly visible with the great magnification of the electron microscope, extend
from the tracheae to the cells of the body and end blindly within them or on
their surfaces (Fig. 30.17). Tracheoles are the main functional part of the
tracheal system. When oxygen is under high pressure in the tracheoles it
passes into the cells where the pressure is lower; substances in the cell com-
bine with the oxygen, energy is set free, and carbon dioxide diffuses into the
tracheoles.
Tracheae are frequently enlarged into air sacs and muscles squeeze and re-
lease these like bellows thus aiding the intake and expulsion of air through the
spiracles. Air sacs also lighten the body and must make it easier to jump and
fly. There are also air sacs in birds.
By-products of Metabolism — Excretion. The function of an excretory
Chap. 30
ARTHROPODS INSECTS, SPIDERS, AND ALLIES
609
Body wall
External
opening
or
spiracle
Fig. 30.17. A trachea, with its external opening, branches, and the tracheoles
associated with muscle cells where the exchange of oxygen and carbon dioxide
mainly occurs. (Courtesy, Ross: Entomology. New York, John Wiley and Sons,
1948.)
system is the maintenance of a good environment in the body, mainly by the
eUmination of unneeded substances from the blood.
The kidneys of the grasshopper are the thread-sized Malpighian tubes
named after Marcello Malpighi (1628-1694), an Italian anatomist, who first
described them in the silkworm. In the grasshopper, each one extends through
the blood from its opening in the intestine to its free end, a blind pocket (Fig.
30.14). Metabolic wastes, destined to form uric acid, are diffused from the
body cells into the blood. The walls of the Malpighian tubes gradually absorb
the uric acid, discharge it in a watery solution into the tubes which in turn
empty it into the intestine from whence excess water is absorbed back into the
blood through the rectal wall. This is in hne with the small animal's usual
economy of water.
Metabolism. Whether it is a grasshopper or a palm tree, the living or-
ganism is a result of chemical and physical reactions of which metabolism is
the sum total. Digestion, respiration, excretion, and other processes are parts
of metabolism. Grasshoppers become more active as surrounding temperatures
rise. With increased activity their bodily temperature and the rate of metabo-
lism also rise. Chemical reactions are increased. Heat is produced, and energy
is liberated. When grasshoppers are warm they jump, fly, and eat more.
Chemical Regulation — Hormones. The hormoneUke substances in in-
sects are briefly discussed with the endocrines (Chap. 15). One endocrine
gland, the corpus allatum, is mentioned here because its endocrine nature was
established largely by experiments on grasshoppers. It is a double body near
the brain, often taken as two glands. During the growth of young grasshoppers
its secretion, the "juvenile hormone," checks the differentiation of adult char-
acters and stimulates the retention of nymphal ones. It gives the nymphs time
610
EVOLUTION OF ANIMALS
Part V
to increase in size before they mature. In adults, its secretion partially con-
trols the growth of the eggs. This has been discovered by removing the gland
from young females in various stages of maturity. Its removal prevents the eggs
from ripening. Evidently sex does not affect the corpus allatum since a trans-
plant of one from an adult male into an adult female deprived of her own
gland will bring on the maturity of her eggs.
Coordination and Sense Organs. The nervous system is highly developed
and serves to coordinate the activities of the body with whatever is going on
inside and outside it. The central nervous system consists of a pair of dorsal
ganglia, the brain, and a series of pairs of ventral ganglia and nerves connect-
ing and branching out from all of them (Fig. 30.18). From the subeso-
phageal ganglia the ventral nerve cord extends posteriorly formed by a series
of paired ganglia and connecting nerves. Each division of the thorax contains a
pair of ganglia from which nerves extend to the legs, wings, and internal
organs. There are only five pairs of abdominal ganglia, some of the once larger
number having been fused during the evolution of grasshoppers. In addition to
the central nervous system, insects have a visceral nervous system, ganglia and
nerves concerned with the control of the purely involuntary activity of the
salivary glands and parts of the digestive canal.
The Sensitivity of Insects. In spite of their armor, grasshoppers are highly
sensitive to their surroundings. They have sense organs for the reception of
tactile stimuli, hearing, taste, smell, and sight, all of these connected with the
central nervous system.
TACTILE HAIRS. Their delicate sense of touch is due to many protruding hairs
that are in contact with sensory nerve cells. In a simple type of such an organ
three kinds of cells are concerned, the hair cell which secretes the hair, the
Subesophageal
ganglion
Thoracic
ganglia
Cut end
ol. canal
lobe
Brain
Fig. 30.18. Nervous systems of grasshopper. View after alimentary canal re-
moved. The largest ganglia are those associated with greatest activity, e.g., with
wings and legs. (After Hegner: Invertebrate Zoology. New York, The Macmillan
Co., 1933.)
Chap. 30 ARTHROPODS INSECTS, SPIDERS, AND ALLIES 611
cell forming its socket, and the sensory nerve cell. The tip of this is in contact
with the base of the hair exposed to the changes in pressure that it communi-
cates to the nerve centers. Such tactile organs are abundant on the antennae
and ovipositors of grasshoppers.
HEARING. In the red-legged, the lubber, and other common grasshoppers
there is an eardrum on each side of the first abdominal segment (Fig. 30.10).
In some species it is on the front legs. Comparatively few insects, among them
grasshoppers, crickets, and cicadas have these eardrums. In the common short-
horned grasshoppers, the eardrum is a thin cuticular drum fully exposed on
the outside and closely associated with a group of peculiar sensory cells.
CHEMICAL SENSES — SMELL AND TASTE. Smell and taste are both chemical
senses and not easy to distinguish. The chitin that covers these sensory cells is
so thin that chemical substances can easily penetrate it. Chemical sense organs
are often on minute knobs; others are in pits. Smell is located chiefly in the
antennae and the palps. Grasshoppers are sensititve to temperature all over
their bodies. They have sharp temperature preferences and as far as possible
choose their own private climates in protected sunny nooks.
COMPOUND EYES. Thcsc cyes are immovable, set well over to the side of
the head and a diflferent object is seen through each one at the same time. They
are composed of single eyes, usually thousands of them, through which pieces
of an object appear in mosaic vision as in the similar eyes of crayfishes.
Processes from the light sensitive cells of the eye continue through the optic
nerve and are associated with nerve cells in the brain. As in all animals, the
interpretation of vision occurs in the brain. That insects do interpret what
they see is evident from experiments with honeybees. On the surface of a
compound eye its units appear as many six-sided areas, each one a transparent
lenslike cornea. Directly beneath this is the crystalline cone composed of
crystal clear cells. This in turn rests upon the light receptors or retinal cells that
are sensitive to light on the sides meeting in the center of a peculiar rosette
(rhabdom). A process extends from each of the light receptor cells and
together they form the optic nerve connecting the eye with the brain. A curtain
of pigment cells keeps the light that falls on one unit from striking any other.
As more or less light falls upon the eye, granules in the pigment cells move to
different positions. This shuts out or lets in the light upon the retinal cells just
as the iris of the human eye curtains the light sensitive retina.
Reproduction. The sexes are separate in all insects. In most species they
are readily distinguishable by the external sexual structures on the abdomen.
There are two testes in which the sperm cells develop. The latter are dis-
charged into two tubes (the vasa deferentia) which unite to form the ejacu-
latory duct extending through the penis, the organ by which the sperm cells are
transferred into the female reproductive passage during mating. Each ovary
consists of a group of egg tubules within which the eggs develop (Fig. 30.14).
612 EVOLUTION OF ANIMALS Part V
Different stages of developing eggs fill each tubule of the ovary. They are
supplied with nourishment from cells in the wall of the egg tubule, ultimately
from the blood. As the oldest eggs mature they slip into the oviduct and in the
egg-laying season this becomes distended with eggs. By that time each egg has
a thin shell with a minute pore in it (micropyle) through which the sperm cell
may enter. As the eggs pass into the vagina they come to the opening of the
spermatheca which in a mated grasshopper is crowded with sperm cells.
Pressure on this sac forces out the sperm cells and fertilization of the eggs
follows.
Just before fertilization the number of chromosomes in the eggs is reduced
to half their former number (Chap. 6). A comparable reduction in chromo-
some number also occurs in the sperm cells. Thus, after the male and female
nuclei have joined, the fertilized egg begins as a new individual that will have
the same number of chromosomes present in the body cells as in those of one
or the other parent.
Egglaying and Winter Life. The grasshopper begins laying her eggs in late
summer or fall several days after mating. She digs a short tunnel in dry ground
and deposits the eggs shrouded in a sticky secretion. In common grasshoppers,
development begins immediately and continues for about three weeks (Fig.
30.19). By that time the six legs, the antennae, eyes and the segments of the
body all show clearly in the still unhatched embryo. It then enters a rest period
(diapause); consumes little oxygen; growth stops and is not resumed until
spring.
The Honeybee — A Flower-insect
Honeybees are social insects, with each bee a team worker taking a par-
ticular part in the life of its colony — an organized society. Honeybees are
wholly dependent upon flowers for nectar and pollen, their only food. Great
numbers of plants, among them the fruit trees, are in turn dependent upon
bees for cross pollination and the consequent continuance of their species.
Content of the Colony. Honeybees, Apis mellifica, were introduced into this
country in colonial times and are now widely distributed in apiaries and as
escaped wild bees that build their nests in hollow trees. The colony in a bee-
hive has continued to be essentially a copy of the nest in the hollow tree. In
summer, there may be 60,000 or more bees in a colony, but fewer in winter.
There are three easily recognized castes, the females, workers and queen, and
the males or drones (Fig. 30.20). ^
The workers constitute the great bulk of the colony — the honeybees that
are usually seen on flowers and going in and out of their hives. They are
sexually undeveloped females, highly specialized as workers for the general
welfare of the colony. They rarely produce eggs and when they do the eggs
are unfertilized and develop into males only. Workers are so called because
Chap. 30
A
ARTHROPODS INSECTS, SPIDERS, AND ALLIES
B C D
613
fertilized egg
cleavage
blasfoderm
germ band
Fig. 30.19. Development of the grasshopper. Development begins in autumn
immediately after fertilization, and continues two to three weeks till the embryo
is well formed. Then there is a rest or diapause until spring when the nymphs
hatch at the right time to feed on the young grass. A, B, C, D; nuclear division
occurs and {B) nuclei are scattered through the yolk; they migrate to the outer
surface of the embryo where each one is surrounded by a cell body. This (C) is
the blastula stage. D, cells divide rapidly on one side forming the germ band
which will be the embryo. E, F, G, H, I; continuous development proceeding
most rapidly on the ventral side where the nerve chain will be located. Stomodeum
is the layout for the mouth; proctodeum is the layout for the anal region. The
serosa is the outer covering membrane of the embryo; the amnion is the inner one.
J, development pauses for the winter (diapause). K, L, M; development begins
again; the embryo turns about so that its head is at the larger end of the egg. It
soon hatches, head first. Like other immature animals its head is relatively large.
N, O, P, Q, R; five nymphal stages. S, adult. Legs came before wings in the evo-
lutionary history of insects; they come first in young grasshoppers. (Adapted from
various sources. Courtesy, Storer: General Zoology, ed. 2. New York, McGraw-
Hill Book Co., 1951.)
614
EVOLUTION OF ANIMALS
Part V
Fig. 30.20. Types of individuals in a colony of honeybees, and the life history
of a honeybee. A, worker; B, queen; C, drone; D, portion of comb showing queen,
worker and drone cells; E, egg; F, young larva; G, old larva; H, pupa. A to C
somewhat enlarged; D, natural size; E to H much enlarged. In D several of the
honey cells are capped. (Courtesy, Phillips: Fanner's Bulletin 447. Bur. Ent. and
Plant Quar., U.S.D.A.)
they perform the labor. Young workers attend to the inside work, mold the
wax into comb, feed the larvae, keep the hive clean, and guard the entrance.
The older workers go into the field to collect nectar, pollen, and the mixture
of plant gums called propolis. They live only a month or two except those that
hatch out in the fall and live through the winter when the colony is smaller
and the housework lighter. In the colony, workers are both governors and
governed. Their treatment creates the queen; they kill unwanted queens; and
they direct the outgoing swarm yet they are bewildered and often return to
their hive if the queen is not with them.
There are few drones in a colony and they are present only in spring and
summer until after swarming time. A small group of them follows the young
queen on her mating flight and one of them mates with her. This is their only
service to the colony.
Chap. 30 ARTHROPODS INSFCTS, SPIDERS, AND ALLIES 615
The queen is the egg producer of the hive. At the height of the flower season
she lays thousands of eggs per day with clocklike regularity, placing one in
each cell. Most of the time she lays fertilized eggs, always placing them in the
smaller brood cells; these develop into females (workers); if a queen cell is
present she places the same kind of fertilized egg within it. Occasionally she
lays unfertilized eggs, placing them in the larger brood cells; these develop
into males. Thus, the eggs develop whether they are fertilized or not, but those
with the double sets of chromosomes (32) become females, and those with
the single sets (16) become males (Chap. 18). The queen is a generalized bee
with wings and legs and an ovipositor but none of the specialities of the
worker.
Special Structures and Functions of the Worker Bee. The mouth parts, legs
(Fig. 30.21) and sting are the external parts especially concerned with the
worker's activity; the digestive and respiratory systems and the wax glands are
the internal ones. Workers use their mouth parts on building materials and
food. The smooth-edged, scoop-shaped mandibles are adaptable to molding
wax as well as biting off pollen. The nectaries of plants are located deep in
the center of the flowers and reaching them is like licking syrup out of a bottle
(Fig. 30.22). The bee does this with its combination sucking and lapping
"tongue" that is folded back under the head when not in use. This remarkable
instrument is composed of the modified maxilla and labium or lower lip, the
central part of the latter forming the "tongue," actually a spoon with a tubular
handle.
Position of leg i
when cieoningi)
antenna -
Metatarsus
Torsos ^'.ii^ Pollen"'
Antenna ■ .
comb '''*"*' >^1^ Torsos
Metotarsos "^K. Torsos
PROTHORACIC LEG ^ METATHORACIC LEG
MESOTHORACIC LEG
Fig. 30.21. The legs of the worker honeybee. Some part of each one is a tool
used in collecting and manipulating pollen or wax. The wings have been removed
and no hairs are shown on the head and body. Hairs are as abundant there as they
are on the legs and the sticky pollen likewise clings to them. The pecten is a row
of bristles on the hind leg; the auricle is a lobe used as a pusher; these parts are
worked together in packing pollen into the basket. (Courtesy, Hegner and Stiles:
College Zoology, ed. 6. New York, The Macmillan Co., 1951.)
616
EVOLUTION OF ANIMALS
Part V
Fig. 30.22. Nectar is produced at the bottom of the flower and as the bees suck
it up they come in contact with the pollen. Bees in flowers of Salvia: 1, pollen-
covered anther is striking the bee's back; 2, the lower flower is being visited by a
bee which carries on its back pollen from a younger flower and is rubbing it off
on the deflected stigma. (Courtesy, Kerner and Oliver: The Natural History of
Plants. London, Gresham Publishing Co., 1904.)
Legs. There is some tool connected with pollen or wax on every leg of a
worker bee; the rights and lefts match, are mirror images. As bees gather
Jiectar from the flowers they also collect pollen that clings to the hairs on their
^yes, legs and bodies. Workers must keep combing and brushing and the tools
for this are built into their bodies. The eyebrush is a set of bristles on the first
leg and just below it is the antenna-comb, a circular comb with a movable
flap (Fig. 30.21). The bee raises its leg and draws the antenna through the
comb while the flap holds it in place. A honeybee brushes an eye with a
pollen brush as a cat curves her paw over one ear.
On each middle leg there is another pollen-brush and a wax-pick with
which the bee plucks scales of wax from the under surface of the abdomen,
and prys balls of pollen out of the pollen baskets. When a bee returns from a
pollen trip, its hind legs hang straight with the loads of pollen in the baskets
that bulge out like green and yellow saddle bags. The pollen combs on the
inner surfaces of the tarsi serve to comb out the pollen entangled on the hairs
of the body and transfer it to the pollen basket on the opposite leg. The tibia
ends in a row of spines, the pecten (comb). The pecten of one leg is scraped
across the pollen comb of the other and the pollen thus collected is packed
into the pollen basket.
Chap. 30 ARTHROPODS INSECTS, SPIDERS, AND ALLIES 617
Sting. The sting of a bee is an ovipositor modified into a weapon. Its ex-
ternal parts are two feelers that locate the point to be stung, and a needle,
composed of two barbed shafts that slide within a shaft. Connected with this
is the internal poison sac that receives the poison from adjoining glands. Bees
sting to defend the colony; thus stinging is a social act. It often kills the bee
because the shafts catch in the flesh and the whole stinging mechanism is
pulled out of the bee.
Digestive System. The special feature of the digestive system is the honey
stomach, a modified crop used as a tank to carry nectar from the flowers to
the honey cells in the comb (Fig. 30.23). A short tube (proventriculus) con-
taining a valve connects the honey stomach with the true stomach (ventric-
ulus). The valve is closed except when the bee takes some of the nectar for
itself but what signals the opening of the valve is not known. The honey
stomach is very distensible and when full of nectar, looks hke a transparent
balloon. Honeybees fly rapidly, distances of a mile or more, or make short
trips — ones with quick stops and starts from flower to flower. The supply of
oxygen in the air-sacs probably eases up on breathing during flight (Fig.
30.24).
Nervous System — Coordination. As might be expected from their be-
havior, ants, wasps, and bees have the most highly developed nervous systems
of any insects. In the bees the ventral nerve chain is characteristically shorter
and more ganglia are fused than in the grasshopper (Fig. 30.18).
The Senses and Language of Honeybees. The statements that follow give
Phorynx
Pottcerebrai
glands
Honey
•tomoch
Molpighion
tubules
Rectum
Phoryngeal
glands
Esophag us
Salivary
glands
Honey
stopper
Ventriculus
Small
intestine
Rectal gland
Fig. 30.23. The digestive system of the worker honeybee. (Courtesy, Hunter and
Hunter: College Zoology. Philadelphia, W. B Saunders Co., 1949.)
618 EVOLUTION OF ANIMALS Part V
some of the results obtained by a famous student of animal behavior, Karl
von Frisch, through years of experiment and observation. His book, Bees,
Their Vision, Chemical Senses and Language, is largely made up of lectures
given in American universities during 1949 with motion pictures of the dances
of the bees. His conclusions have been termed "of basic importance to bio-
logical science and truly revolutionary in the special field of animal behavior"
(Donald R. Griffin).
The Materials. In the course of the experiments, worker honeybees were
marked with colored symbols by which each one of a large number could be
Trocheol
Fig. 30.24. The respiratory system of the
worker honeybee with the air sacs that hold an
emergency supply of air. (Courtesy, Hunter
and Hunter: College Zoology. Philadelphia,
W. B. Saunders Co., 1949.)
Spiracle
identified. They were observed on combs among other bees in observation
hives, and at feeding stations where dishes of sugar water and control dishes
were placed on colored cards, and on flowers. The observation hives were in
diffused light and in red light (black to the bees). Experiments and observa-
tions were repeated, and often varied many times. They have also been re-
peated by others.
Are Bees Color Blind? Bees can distinguish yellow, bluegreen, blue, and
ultraviolet (Fig. 30.25). Red and black are the same to bees for they are red-
blind. They and various other insects can distinguish certain red flowers, such
as scarlet poppies because these flowers reflect ultraviolet light. Ultraviolet
appears to be a distinct color for the bees (von Frisch). Color vision of man
and the bee is different; the human eye responds to more colors but not to
ultraviolet.
Can Bees Recognize Different Shapes? They can distinguish solid ob-
jects from open ones, e.g., a solid triangle from three parallel lines (Fig.
Chap. 30
RED
BOO
ARTHROPODS INSECTS, SPIDERS, AND ALLIES
COLOR VISION OF MAN AND HONEY BEE
VIOLET
500 480 400
619
ORANGE YELLOW GREEN BLUE- BLUE
GREEN
650 600
550
J/\.
Human only/
/
/
/
/
/
/
650
/
y
Hunnan and bee
/>
COLORS AS ABOVE
400
ULTRAVIOLET
300
Bee and human
Fig. 30.25. The colors of a spectrum to the human eye and to the eye of the
honeybee. For bees the visible spectrum is shortened in the red light but is ex-
tended in the ultraviolet. Apparently bees see only four qualities of color: yellow,
blue-green, blue, and ultraviolet. The numbers indicate the wave length of light
in millimicra (one micron = 1/25000 of an inch). (Based on data from von
Frisch: Bees. Ithaca, N.Y., Cornell University Press, 1950.)
30.26). The criterion of visibility seems to be the amount of openness in the
pattern. It apparently gives a flickering impression as the bee flies past it just
as a picket fence looks to us as we ride past.
Taste, Smell, and Touch. Honeybees can distinguish salt, sour, sweet,
and bitter. There are some sense organs of taste on the mouth parts though it
is not certain that they are all there. Butterflies have them on their feet. Honey-
bees are very sensitive to degrees of sweetness. They refused low percentages
of sugar in the experimental sugar waters. Conditions modify their choices. In
the spring blooming period they may refuse to collect nectar that is less than
40 per cent sugar, but in the fall when flowers are scarce, they will accept it
with sugar content as low as 5 per cent. Honeybees are keenly responsive to
odors. The sense organs of touch and smell are very close together on the first
eight distal segments of the antenna (Fig. 30.27). As bees explore flowers
they wave their antennae about and constantly touch certain parts of them.
In bees, smell and touch may work together just as we handle something in
order to see it better.
Honeybees Broadcast News of Food. Workers perform the "round
dance" after they have collected food near the hive (Fig. 30.28). The worker
620
EVOLUTION OF ANIMALS
Part V
/
XOiM'Y
Fig. 30.26. Bees distinguish between solid and broken patterns. They do not
learn to distinguish between different shapes of solid patterns (upper row) or
between those of different broken ones (lower row). (Courtesy, von Frisch: Bees.
Ithaca, N.Y., Cornell University Press, 1950.)
sucks up the sugar water (placed there for the experiment), goes back to the
hive and walks onto the comb among hundreds of bees. First, she delivers
sugar water to some of them. After that she dances, turns a circle to the left,
turns one to the right, repeats this in one spot for a half minute or more, then
goes to another place and dances again. During the dances the nearby bees
become more and more excited. They troop behind the dancer and extend
their antennae toward her. Suddenly one of them turns away and leaves the
hive; others follow and the watcher soon sees them at the feeding place.
Workers that have been collecting food at more distant places perform the
wagging dance (Fig. 30.28). They run a little way straight forward wagging
the abdomen then turn a circle to the left, retrace the straight hne wagging
Fig. 30.27. Sense organs on one of the eight outer or distal segments of the
antennae of honeybees. Sections through the chitinous body covering (black), the
cells which produce it and the sense organs. Left, section through an organ of
touch, highly magnified. Center and right, the organs of smell. Processes from
nerve cells, in the cluster, end beneath a very thin part of the chitin and can be
stimulated by scented substances diffusing through it. (Courtesy, von Frisch: Bees.
Ithaca, N.Y., Cornell University Press, 1950.)
Chap. 30
ARTHROPODS INSECTS, SPIDERS, AND ALLIES
621
ROUND DANCE
Food near hive
WAGGING DANCE
Food distant from hive
Fig. 30.28. Honeybees broadcast the news of food by dancing on the comb
after they return to the hive with nectar. Left, the round dance performed when
the feeding place is near the hive (c. 10 meters). The bee turns around, once to
right and once to the left, repeating the circles for about half a minute in one
place. Right, the tail-wagging dance, performed when the feeding place is far from
the hive. The bee runs a short distance in a straight line wagging the abdomen,
then makes a complete 360-degree turn to the left, runs ahead once more and
turns to the right, and repeats this over and over. (Courtesy, von Frisch: Bees.
Ithaca, N.Y., Cornell University Press, 1950.)
again, turn a circle to the right, retrace the line and wag. In the wagging dance
the number of turns in a given time indicates the distance more exactly, e.g.,
for 100 meters, nine or 10 complete circles. When sugar water was set out in
nearby and in distant food stations at the same time .the bees returning from
them performed the appropriate dance for the station visited. If the farther
food station was moved closer to the hive, the same bees which had been wag-
ging, changed to the round dance.
The Diversity of Insects
Except in the Arctic and Antarctic, insects have occupied all lands. Their
numbers have so intensified their struggle for existence that no place or way
of living has been untried. Grasshoppers and honeybees meet their surround-
ings with complex and successful structures and activities that have been
merely suggested in the brief descriptions in this chapter. Thus, insects have
become of great importance to plants, to one another, to other animals and
humanity. Observation and experiments upon them have brought great con-
tributions to zoology and through it to agriculture, medicine, and sociology.
In this book it is only possible to introduce these through the books in the
Suggested Readings. Such subjects as insects and agriculture and forestry,
insects and their food, insects and disease, and social insects are included there.
Happily many of such books are readable and witty as well as informing.
622 EVOLUTION OF ANIMALS Part V
A Review of Arthropod Relations
Again and again arthropods show their ancestral connections to annelid
worms. Peripatus (Class Onychophora — "claw bearing") is the simplest living
arthropod and with its segmentally arranged excretory organs and wormlike
form most resembles the annelids. Centipedes (Class Chilopoda — lipfoot) and
millipedes (Class Diplopoda — doubled feet) have mainly uniform segments.
The voracious predatory centipedes are equipped with powerful mandibles
each with an incurved hook from the tip of which a poison gland opens. In
contrast to them the vegetarian millipedes have weak mandibles and no poison
glands. A centipede is composed of flattened segments, each with one pair of
long, jointed legs; a millipede is cylindrical and each segment is a fusion of
two embryonic ones bearing two pairs of legs. When traveling these various
legs are moved from front to rear rapidly like scales being played on a key-
board. Crayfishes (Class Crustacea) are divided into a fused head and thorax,
and abdomen and have gills, two pairs of antennae, and two-branched appen-
dages. In grasshoppers (Class Insecta), the body is divided into head, thorax,
and abdomen, and they have three pairs of legs, one pair of antennae and are
usually winged. The bodies of spiders and their allies (Class Arachnoidea) are
divided into a cephalothorax and abdomen; they are without antennae or
mandibles, have four pairs of legs, and breathe by tracheae and book lungs.
Spiders and Tlieir Relatives
Spiders are well named for the majority of the females are inveterate spin-
ners and the word spider is a descendant of the Danish word spinden, to spin.
For most spiders silk is the thread of life from the time they hatch from the
shell. Spiders are air breathers, thoroughly land animals, yet inside of silken
waterproofs a few of them live in water. Some occupy silk curtained holes in
coral rocks that are immersed at high tide. The "water spider" (Argyroneta)
of fresh waters of Europe and temperate Eurasia is a pioneer user of the diving
bell. She collects her supplies of oxygen at the water surface raising her ab-
domen and capturing bubbles of air in addition to that caught on the covering
of her body. Between repeated trips to the surface she weaves a canopy of silk
attaching it to the submerged stems of plants that grow in the shallows of ponds
and streams. After the canopy is made she continues to bring down air bubbles
and to shed them beneath the canopy replenishing the supply as it is used.
This airy chamber is the home of the female spider into which she brings her
captured prey, and where she lays her eggs. The spiderlings that hatch there
can also spin and swim and with their own silk soon repeat the performances
of their mother. The males spin only small canopies sufficient for them to
linger in the locality until they are mature. There are many spiders that fre-
quent the margins of quiet inland waters, running about on the surface film
Chap. 30 ARTHROPODS INSECTS, SPIDERS, AND ALLIES 623
foraging for water skaters and other insects. Spiders are predators that seize
and crush their prey between the chelicerae or jaws and suck the juices. They
are generally solitary with no hint of any such group organization as that of
the social insects. In the instincts that guide female spiders in the architecture
of their webs and the trapping of their prey, they are unsurpassed among
invertebrates.
General Structure. Spiders are examples of the narrow-waisted arachnids,
a contrast to the thick waisted harvestmen (Figs. 30.29, 30.30), They have
neither antennae nor true mandibles. In front of the mouth are the two special
jaws or chelicerae, each with a sharp fang through which a poison gland opens,
and behind these is a pair of pedipalps. In the female each of the latter ends
in a claw, often used in manipulating the silk. In the male the enlarged tip of
each pedipalp is the organ by which sperms are transferred to the female. The
four pairs of legs vary in size and function; some of them are important in
King Crab
Scorpion Whip Scorpion
\ ^. ./
Pseudoscorpion
XIPH08TJRA
SCOKPIONIDA
PEDIPALPI
PSEUDO-
BCOBPIONIDA
Sunspider Spider
( )
Harvestman
Tick
BOLPUGIDA
ABANEAE
PHALANGIDA
ACARINA
Fig. 30.29. Relatives in the Class Arachnoidea. King crab, a relative of the
fossil trilobites; scorpions, the oldest of land arachnids, with fossils going back
400 million years; pseudoscorpions, the largest a quarter of an inch long and
without the poisonous tail gland of the true scorpions; sunspiders of the American
southwest, an inch long or more; spiders; harvestmen, long-legged, frequently in
companies; Ucks that push their heads through the skin and gorge themselves with
blood. (Courtesy, Storer: General Zoology, ed. 2. New York, McGraw-Hill Book
Co., 1951.)
624
EVOI.UnON OF ANIMALS
Part V
etatarsus
k lungs
rrow
/ [ly JC tracheal spiracle
r- ^^spinnerets
Fig. 30.30. External anatomy of a spider. (Courtesy, Gertsch: American Spiders.
New York, D. Van Nostrand Co., 1949.)
constructing webs. Spines and other finer projections, many of them sensory,
project from the surfaces of the body. Spiders usually have eight simple eyes,
in some species fewer. The majority of spiders have poor eyesight, at its best
in the runners and jumpers. Smell and taste are also weak. They know their
environment through their extraordinary sensitiveness to touch and vibration.
Near the posterior end of the abdomen are two or three pairs of spinnerets
from the tips of which the silk glands open. Spinnerets are flexible fingers that
a spider continually extends, withdraws and manipulates as the slender streams
of silk pour from their tips.
The respiratory system also opens on the ventral side of the abdomen in
front of the spinnerets. The openings of the two leaflike book lungs are located
one on either side of the opening of the reproductive organs.
The short esophagus leads to the sucking stomach operated by powerful
muscles that attach it to the skeleton of the cephalothorax (Fig. 30.31 ). These
contract and enlarge the stomach thus creating the suction. It usually takes a
spider about an hour to suck in the juice of a fly. Digested food is absorbed
from a series of blind pouches extending from the stomach and from numerous
glandular extensions of the intestine that branch and rebranch through the
abdominal cavity. Waste substances accumulate in a pocket (stercoral) that
opens from the hindgut and are afterward discharged from the anus.
The ovaries and the silk glands make great demands for food, the ovaries
to build up a store of yolk in the eggs, and the silk glands to provide the sub-
stance, mainly protein, in the constantly expended silk. Wherever a spider goes
it plays out a silken thread, the dragline. As a house spider drops from ceiling
to floor, it descends gently on a dragline making it longer and longer as it
drops. Before a spider jumps, it fastens a dragline down to some object and
Chap. 30
ARTHROPODS INSECTS, SPIDERS, AND ALLIES
625
Pericardia]
cavity
Intestine with Mafpighian
complexly brancnea tuou/es
diverticula /tteart
Stercoral
poc/<et
Poison gland Q^^gjl^^
Pedipalpus
, Silk' , , , .
7- h/r, g'^'^ds// Anus
Chelicera'^^ / L\:^^!7 Spinnerets
Book lung \
Stvmpsoflegs Seminal oVi'dLlt
receptacle
Fig. 30.31. Internal anatomy of a female spider. The nervous system highly
developed in the head and thorax, is shown by dark stippling; the nerves in the
abdomen are too small to be shown. The alimentary canal is white; note its
branches (caeca) extending into the stumps of the legs; a network open into the
intestine from a digestive gland which is packed around the abdominal organs.
Note the prominent eggs in the ovary. The stercoral pocket, a sac in which waste
products accumulate. The malpighian tubules are kidney-like in function as in
insects. (From Comstock: The Spider Book. New York, Doubleday, Page and
Co., 1913.)
then leaps spinning the line out as it goes through the air. Spiders spin forth
yards of draglines that are carried by currents of air from tree to tree and
across streams. Young spiders and the smaller species are lifted into the air
and carried by draglines for miles over mountains and seas. The dragline is
also the trapline which a spider holds until it vibrates from the touch of an
insect caught in the web. Draglines are the outermost threads of orb webs, the
fundamental lines in their construction (Fig. 30.32). There are seven kinds of
silk glands in spiders but not all of these are present, even in any one family
of spiders. The silk that is poured out through the minute holes in the tips of
the spinnerets is of different sorts that are more or less elastic, but its final
character depends largely upon the pull to which it is subjected. The viscid
spiral lines of orb webs are two firm threads which are at first evenly covered
with a fluid silk. As she spins, the spider holds the whole thread with her hind
leg, stretching it a little but at regular intervals letting it snap back. On the
shortened line drops of the sticky silk form at regular intervals. Dew gathered
on them creates the shining beads of early morning (Fig. 30.33). An orb web
is a triumph of symmetry and it takes a spider only an hour to build it.
Spiders always develop from the fusion of male and female sex cells but in
most species the male individual is of no consequence except for the fertiliza-
tion of the eggs. The females spin the egg sacs and give the young what care
they receive. Male spiders have silk glands but spin little or none. They hunt
626
EVOLUTION OF ANIMALS
Part V
Fig. 30.32. The viscid lines of an orb web in close-up photograph. The viscid
silk collects in droplets when the tension on the basal lines is loosened. Dew
gathers on these and they are jeweled in morning sunlight. The web is a trap in
which insects are ensnared. (Photograph by Lynwood Chace. Courtesy, National
Audubon Society.)
alone and are inconspicuous because much smaller than the females. In their
courtships the males of some species are stealthy; others are acrobatic. Many
of them meet a tragic end since the female finally devours her mate.
Other Arachnids. Mites and ticks are small arachnids (Order Acarina) with
the head, thorax, and abdomen closely fused and unsegmented (Fig. 30.34,
30.35). They hatch from the eggs as active six-legged larvae that feed and molt
into eight-legged nymphs. These feed still more, molt and change into adults,
also eight-legged. Ticks and mites are similar except for certain details of struc-
ture and size, ticks being much the larger. In both types, a dartiike structure
(hypostome) below the mouth acts like an anchor when pushed into the flesh.
In ticks the outer surface of the hypostome is armed with recurved teeth; in
mites it is smooth.
Ticks are parasites of mammals, birds, reptiles, and some amphibians. A
tick lays hundreds of eggs on the ground, in birds' nests and other homes of
Chap. 30
ARTHROPODS — INSECTS, SPIDERS, AND ALLIES
627
Fig. 30.33. A complete orb web in early morning. The long trap leads to the
retreat from which the spider emerges when the trap line is moved by the struggles
of an insect caught in the web. (Photograph by Hugh Spencer. Courtesy, National
Audubon Society.)
their hosts. After hatching, the larvae immediately seek their hosts and a blood
meal. Unlucky larvae who do not find their hosts may survive eight months
without food but they cannot molt or transform without a blood meal. When
a tick bites, it cuts with its jaws, stabs with its hypostome and injects an anti-
coagulating fluid into the blood. Its whole head is buried in the flesh and
because of tearing by the reversed teeth, it should never be pulled out quickly.
628
EVOLUTION OF ANIMALS
Part V
Fig. 30.34. The "red spider," Tetranychus telariiis, of plants is a mite that covers
the leaves with silk and sucks out the sap. A, the mature female; B, the egg; C and
D, larva and nymph; E, the fully developed nymph just before its last molt and
maturity. (Courtesy, Matheson: Entomology. Ithaca, N.Y., Cornell University
Press, 1944.)
Fig. 30.35. The spotted fever tick. Dennacentor andersoni. In its immature
stage it is a parasite of rabbits, squirrels, and other rodents — as an adult, a willing
parasite of man. (Courtesy, Matheson: Entomology. Ithaca, N.Y., Cornell Uni-
versity Press, 1944.)
Chap. 30 ARTHROPODS INSECTS, SPIDERS, AND ALLIES 629
It will drop off when surfeited with blood. The danger from ticks is in the
organisms they may carry from an infected animal to an uninfected one. Some
of the resultant diseases are: relapsing fevers of certain western states, due to
a species of spirochaete; RocTcy Mountain spotted fever of rodents and man
caused by Rickettsia organisms; and tularemia, a disease of rabbits, squirrels,
rats and certain game birds, caused by a bacterium {Pastiirella tularemia)
carried by the tick {Dermacentor andersoni) . Tularemia is highly infectious
to man since the organisms pass through slight breaks in the skin when infected
game is handled.
Mites live on plants and animals and cause great damage to both. Among
those of plants are the destructive stored grain mites, the citrus bud mite of
the lemon trees of California, the mites on peas, clover, and the "red spiders"
of junipers (Fig. 30.34). The parasitic mites of animals include the "southern
chiggers" or "red bugs" whose larvae burrow just under the skin as a ground
mole burrows just under the surface of a lawn.
31
Mollusles — Specialists in Security
Most people know that clams and oysters make shells; that oysters belong
in stew, clams in chowder, and that scallops are fried. Many know the pleasant
softness of oysters on the half shell. When the novelist Thackeray ate his first
raw oyster he is said to have exclaimed that he felt as if he had "swallowed a
little baby." For the majority of mollusks, these impressions are correct. Most
of them bear shells, provide abundant food for man and other animals, and
have such soft bodies that the phylum is named Mollusca.
The group includes an enormous number of animals whose lives are deeply
affected by their shells. It contains animals of such different forms and activity
as snails and slugs, clams and oysters, swift darting squids, slow creeping
chitons, and the storied paper sailor and chambered nautilus (Fig. 31.1).
Mollusks are scattered over the lands and through the seas and fresh waters
of the world. There are over 80,000 known species. Fossils of the ancestral
mollusks are abundant in Lower Cambrian rock laid down 600 million years
ago. The free-swimming ciliated larvae of mollusks and annelid worms are so
similar that they suggest a common ancestry.
General Characteristics. The dominant structures of mollusks are the mantle,
the foot, and the spiral form. The fleshy cloaklike mantle produces the myriad
kinds of shells, takes part in forming the gills and the lung sacs of air-breathing
snails, and in many species bears cilia. The foot is the organ of locomotion
(Fig. 31.5), the traveling platform of snails, the digging tool of clams, the
head-foot from which tentacles originate in squids. Spirals or some hint of
spirality appear in many mollusks; laterally developed spirals are prominent
in the majority of snails; the symmetrical spiral is equally prominent in the
chambered nautilus; and an oblique slant in the hinges of clam and oyster
shells is noticeable. Mollusks have a true coelom but lack several prominent
features of other higher invertebrates. Although they can swim, crawl, climb,
dig and bore, they have no legs. The body is not divided into segments, and
only in the chitons is the shell segmented (Fig. 31.1).
630
Chap. 31
MOLLUSKS SPECIALISTS IN SECURITY
631
CLASS AMPHINEURA
Chiton
CLASS CEPHALOPODA
Squid Cuttlefish
Nautilus Octopus
CLASS
PELECYPODA
Teredo
Little Neck
Mytilus
Clom
Round Clom
Oyster
CLASS SCAPHOPODA
Tooth Shell
Fig. 31.1. Mollusks. the shelled animals. They are predominantly marine, except
for the snails and mussels, many of which live in fresh water, or on land. In per-
sistence, distribution and numbers Mollusks are highly successful. The majority of
them are hindered as well as helped by the safety of their shells. Except in the
cephalopods the nervous and sensory structures are only moderately developed.
632
EVOLUTION OF ANIMALS
Part V
Ecological and Economic Importance. Marine mollusks are far more nu-
merous than terrestrial ones. Their free-swimming ciliated larvae abound in
the surface plankton that forms the basic food supply of the sea. Myriads of
pteropods often crowd the surface waters. They are snails, many no longer
than cloves, with lobes of flesh that give them their name sea butterflies and
enable them to flit and glide on the surface as their namesakes do in air (Fig.
31.2). Vast schools of them swim among the icebergs around Greenland and
are strained from the water by the whalebone whales.
Hosts of small snails live on the seaweeds between the tide lines and rasp
off the tissue with their filelike tongues. Each incoming tide brings more sea-
weeds, inhabited by more snails and with each ebb tide leaves a new harvest
for the gulls and sandpipers. In ponds and lake shallows, snails forage chiefly
on the plants but from any submerged surface they scrape bacteria, protozoans,
and algae. Benefiting by this food they eventually furnish their own bodies to
the frogs and water birds.
The majority of mollusks are hampered by their shells and do not travel far
BUTTERFLIES
A. Clione limacina
Chief food of
Greenland whale
Fig. 31.2. Pteropods, the sea butterflies, are winged snails, many of them but
little longer than cloves. Each side of the foot is extended into a wing and they
skip and sail in vast schools on the surface of the sea. One of them {Clione
limacina) is the chief food of the Greenland whalebone whales. (Courtesy, Miner:
Fieldbook of Seashore Life. New York, G. P. Putnam's Sons, 1950.)
Chap. 31 MOLLUSKS — specialists in security 633
except as they cUng to boats, driftwood, and floating plants, to the bodies of
fishes, seals, and whales; and on land to the feet of birds. The striking excep-
tions are the free-swimming squids that range the seas. Sense organs are not
highly developed in mollusks, the tactile sense, and the eyes of land snails,
scallops, squids and octopuses excepted. Great aggregations of marine snails
and mussels are common. On land, slugs and snails congregate in moist places
and about decaying tissues, but there is no such variety of responses and social
relationships as in arthropods. The periwinkles and blue black mussels that
cling to rocks between the tide lines are expressive of the monotony of relative
safety and endurance that accompanies their survival. Security is expensive.
Oysters feed upon microorganisms from the muck and water of the bottom
and in turn are consumed by starfishes, oyster borers and mankind. In open
sea, enormous numbers of squids follow and feed up on schools of herring and
other fishes. Toothed whales attack the giant squids. Part of a giant squid's
arm, eighteen feet long, was once taken from a whale's stomach.
Mussels, clams, scallops, oysters, and various kinds of snails including
abalones, are all used for human food. In North America, the "American
oyster," Crassostrea virginica, that is cultured along the eastern coast, brings an
annual income of millions of dollars. The native oyster (Ostrea km da) of the
Pacific coast is commercially less important. It is small and when shucked
there may be 1600 to 2000 in a packed gallon. In late years, Japanese and
eastern American oysters have been introduced on the Pacific coast and are
thriving especially in the northwest. Scallops are harvested on both coasts but
to no such extent as the common oyster. Abalone steaks familiar in California
markets, though little known outside the state, are slices of the muscular foot
of this large marine snail whose iridescent shell figures in many collections.
Formerly great numbers of pearl buttons were cut from the shells of large
mussels of the Ohio-Mississippi River system. That industry has almost dis-
appeared since synthetic substances have captured the market.
The Classes. The five classes of mollusks have Greek names, all but one
referring to the shape or location of the foot (Fig. 31.3). These names are:
Amphineura meaning double nerve — the chitons; Scaphopoda meaning plow
foot — the tooth shells; Pelecypoda meaning hatchet foot — the clams, mussels,
and oysters; Gastropoda meaning stomach foot — the snails, conchs, slugs;
Cephalopoda meaning head foot — squids, nautiluses, and octopuses.
Class Amphineura — Chitons
Chitons are widely distributed mollusks of the many seashores. Their eight
overlapping shells are flexibly attached to one another and when a chiton is
not clinging to rock it usually rolls up in a ball like an armadillo (Fig. 31.4).
There are fossil chitons at least 400 million years old. These also have the
typical eight shells, a sign that chitons have survived long and changed little.
634
EVOLUTION OF ANIMALS
Part V
AMPHINEURA
Chiton
SCAPHOPODA
Tooth Shell
GASTROPODA
Snail
PELECYPODA
Clam
CEPHALOPODA
Squid
Fig. 31.3. Comparison of three important structures in the members of the five
classes of mollusks, the shell (heavy line), the digestive tract (solid black) and
the foot (stippled).
Mantle
Mantle
isjj;^'"
Mantle
+
Keyhole limpets
Chitons, gills withdrawn,
gills extended
^^
The eight valves of chiton
Edge of
shell
Under side on which
chiton moves about
Fig. 31.4. Chiton and keyhole limpets (snails) clamped to a rock lying between
the tide lines. Each chiton shows the ancient pattern of overlapping shells that
form a flexible roof over its body. When a chiton is turned ventral side up, the
foot is exposed, and the borders of the mantle roll up, giving it the common name,
sea cradle.
Chap. 31 MOLLUSKS — specialists in security 635
Commonly chitons are about four inches long. They are generally drab-
colored, frequent shaded rocks and seaweed, and are neither dangerous,
strikingly beautiful, nor edible. They are plant feeders that scrape fine bits
from the rocks and seaweedsand the small ones furnish picking for shore birds.
Chitons are chiefly interesting as pieces of living history.
Chitons are bilaterally symmetrical outside and inside. The shells and a
fold of the mantle project over deep grooves, one extending along each side of
the under surface of the body. These are parts of the mantle cavity with gills
located in them as they are in the larger mantle cavity of clams. The surface
of the tongue is a file, in function a replica of those on the tongues of snails.
The relatively large fleshy foot has a ciliated surface and strong muscles.
Chitons move like drifting sailboats.
Class Scaphopoda — Tooth Shells
This is a small and little known class of mollusks, with single shells usually
less than two inches, but in some species even six inches long (Fig. 31.1).
They live in sand beyond the low tide mark, some of them at great depths.
Their shells are open at both ends, larger at the head end which is pointed
forward as they burrow.
Class Pelecypoda — Bivalves
These are the clams, oysters, scallops, and other two-shelled mollusks. The
majority of fresh-water bivalves, both large and small, are widely distributed
in lakes and streams (Fig. 31.5), All bivalves are essentially similar and the
following outline of the fresh-water mussel applies in general to common
marine species such as the round clam, Venus, and the soft shell, Mya. Fresh-
water mussels practically all belong to one family. Over 500 species have been
found in the United States, but many are impossible to distinguish except by
special students of this group.
*
Fresh-water Mussel
Skin and Mantle. The mantle makes the shell; the shell protects the mantle
and together they are the main contributors to security which is the prime
achievement of mollusks (Figs. 31.6 and 31.7). The mantle is the soft cover-
ing of the body extended into folds on the ventral side, opposite the hinge of
the shell. It covers the back of the clam and folds of it hang free in front as an
open topcoat hangs free on the human body. It is different in that the mantle
of the mussel also fits close to the body even though the folds hang free. There
is a space between the open coatsides and the human body. The comparable
space in the clam is the mantle cavity in which the gills are suspended. The
borders of the mantle contain many glandular cells, are supersensitive to touch,
636
EVOLUTION OF ANIMALS
Part V
Fig. 31.5. Locomotion of the fresh-water clam is slow as in most clams except-
ing the razor shells. Blood is forced into the foot and it reaches forward. This takes
time. Finally, the muscles of the foot contract and pull the body forward. Thus, the
clam takes a step. (Reprinted from Animals Without Backbones by Buchsbaum by
permission of The University of Chicago Press. Copyright 1948.)
and freely movable for a short distance back to the pallial or fence line where
the mantle is attached to the shell. In the soft-shell clam (Mya), the borders
of the right and left folds of the mantle are grown together and form the band
of flesh prominent in steamed clams. At the rear, usually recognizable by the
more pointed end of the shell, the flaps of the mantle are joined and form a
tube with fleshy walls. This contains the siphons. In some clams, there are two
tubes but if single, the tube is divided within by a partition. Drawn in by the
cilia on the mantle and gills, water passes into the incurrent or ventral siphon
carrying microorganisms and other particles of food with it (Fig. 31.7). Part
of the water is carried toward the mouth and part of it enters the gills. After
passing through the gills it passes out the excurrent or dorsal siphon taking
away metabolic waste. Although always at the rear end, the siphon is com-
monly called the neck, long neck for the soft shelled Mya, little necks for the
round clams. The tips of siphons are heavily pigmented and black, removed
as inedible for indoor meals, eaten with relish at outdoor parties (Fig. 31.8).
Shell. The shell is composed of three layers; the outermost or periostracum
is thin, often horny; the middle one contains prisms of lime (calcium carbo-
nate), and the innermost pearly layer is composed of crystals of lime lying
irregularly parallel to the surface so that they break up the rays of light and
create iridescence (Fig. 3 1 .6) . The pearly layer is secreted by cells in the whole
Chap. 31 MOLLUSKS — specialists in security 637
surface of the mantle. The other two layers are formed only by cells in the
border which at intervals add to the edge of the shell and thus produce the
lines of growth. The main function of the shell is protection but it also neutral-
izes acid. Clams flourish where the mud abounds in organic matter, much of
it decayed. Oxygen is scarce; carbon dioxide and sulfur abound and acidity is
high. Under these conditions the calcareous shell is an important source of
neutralizer of the acid. In clams (Venus) kept out of water experimentally,
oxygen is depleted and carbon dioxide accumulates. Under these conditions
Fig. 3 1 .6. In the innermost, pearly or
nacreous layer of the shell the crystals
of lime are irregularly parallel and rays
of light are broken upon them. This is
the cause of iridescence.
If a sand grain or minute animal gets
between the shell and the mantle the lat-
ter forms a pocket around it and then
a pearly cover. Many a natural pearl is
the casket of a worm. (Courtesy, Fasten:
Introduction to General Zoology. Bos-
ton, Ginn and Co., 1941.)
t^%^mwx\av-5}« *!«s,\<s;
■Nacreous layer
Epithelium of mantle
Parasite or foreign
particle
Mantle tissue
Pearl
some of the shell is dissolved by the mantle and the calcium content of the
fluid in the mantle cavity is this increased with the necessary neutrahzer.
Respiration. If one shell and flap of the mantle are removed the gills are
conspicuously displayed hanging into the mantle cavity with their ventral edges
free. The dorsal edges of each pair are so attached that a chamber above the
gills (suprabranchial) is shut off from the large rnantle cavity below (Fig.
31.8). The incurrent siphon opens into the chamber containing the gills; the
excurrent siphon opens out of the chamber above them. The fold of each gill
is divided by partitions into narrow water tubes. Minute holes open into these
from the mantle cavity and the tubes extending from these open into the supra-
branchial chamber. Urged on by cilia, water continually enters the holes in
the gills and passes through the water tubes close to blood vessels comparable
to arteries and veins (Fig. 31.8). When breathing, a clam always extends the
siphons. It gets little or no oxygen when its shells are closed. This is the time
when it draws on the calcium carbonate of the shell to neutralize the acidity
produced by the excess carbon dioxide.
Circulation. Oxygen diffused from water in the gills, and digested food ab-
sorbed from the stomach and intestine are distributed over the body by the
638
EVOLUTION OF ANIMALS
Part V
Muscle refracts foot
Muscle
Muscfe retracts foot
closes shells
ncurrer^t
siphon
Foot
Fig. 31.7. The left mantle cavity of a clam. Movements of cilia on the gills and
surfaces of the mantle cause the currents (marked by heavy arrows) that carry
particles of food toward the mouth. Other cilia create currents (marked by
lighter arrows) that carry rejected particles outward over the folds of the mantle
as the clam lies with shells partly open when feeding. This occurs likewise in the
right mantle cavity.
slightly bluish watery blood. At the same time metabolic waste is collected
from the tissues. The heart composed of two auricles and one ventricle is in
the pericardium near the hinge of the shell (Fig. 31.9). When the ventricle
contracts it forces blood forward through the anterior aorta and backward
through the posterior one each leading to the intestine and other organs of
the body. It is finally returned to the auricles. All the blood except that reach-
Buprabranchial
chamber
inner gill
opening of
water tube'
dorsal siphon
mantle/'^ outer/
cavity gUl
ventral
siphon
Fig. 31.8. Diagram of the circulation of water through the gills of a fresh-water
clam. Movements of cilia cause continual currents of water to pass into the hun-
dreds of pores in the gills, through the water tubes, and finally out of the dorsal or
excurrent siphon. (Courtesy, Brown: Selected Invertebrate Types. New York, John
Wiley and Sons, 1950.)
Chap. 31 MOLLUSKS — specialists in security 639
ing the mantle returns through the kidneys and gills where waste substances
are eliminated and oxygen received. Many animals must hunt for their food;
clams are relaxed receivers lying quiet while cilia-driven currents of water serve
them. Most of the water coming into the mantle cavity enters the gills. The
smaller particles of food become entangled in mucus on the outer surfaces of
gills and are propelled by cilia to the lips (labial palps). These are remarkable
sorting mechanisms that separate out the usable particles which are turned
into a groove between the lips and from thence go directly to the mouth.
Food and Digestion. As before stated, clams feed upon bacteria and micro-
scopic plants and animals. Ciliated lips surround the mouth which opens into
a short passageway leading to the stomach that is surrounded by a greenish
black gland, the so-called liver, whose ducts empty a digestive secretion into it.
The intestine extends from the stomach into the foot where it is coiled about
the ovary or the testis as the case may be, then turns toward the dorsal side of
the body, extends through the heart and opens into the excurrent siphon. This
curious route is necessitated by the close quarters of the shell (Fig. 31.9).
Food is digested by secretions such as the enzyme amylase of the liver and also
within cells. Throughout the digestive canal ameboid cells are common. From
microscopic examinations it is believed that such cells make their way through
the walls of the canal, ingulf food and digest it, then leave the intestine and
return into the spaces between the tissues. Similar intracellular digestion occurs
in hydra and other invertebrates including starfishes.
Excretion. The two kidneys are close to the heart (Fig. 31.9). They are
difficult to understand without special study but two important facts can be
made out. They are tubular and they are closely associated with the blood
vessels. Thus they conform in essentials with other kidneys.
Coordination. The nervous system is mainly composed of three pairs of
ganglia and their connectives: one pair, the brain or cerebropleural ganglia, is
above the mouth, the usual location of a brain; the pedal ganglia are in the
foot; and the visceral ganglia just below the posterior adductor muscle (Fig.
31.9). The different ganglia of each side and the members of each pair are
joined by nerves. Small branches extend from the ganglia to muscles and sense
organs.
Sense organs are few and their functions uncertain, as might be expected of
an animal living in unusual security. A minute structure near the pedal ganglia
is a typical organ of balance, a cavity containing a bit of lime surrounded by
sensory ceUs. The edges of the mantle contain cells pecuUarly sensitive to
touch, those of the siphon to touch and light.
Reproduction and Development. In some bivalves, male and female organs
are in the same individual; in fresh-water clams, they are in separate ones. The
reproductive organs are in the foot packed between the coils of the intestine.
The sperm cells are shed into the excurrent siphon and carried into the open
640
EVOLUTION OF ANIMALS
Part V
ventricle
anterior
retrac
anus
adductor
muscle
ige
mantle
liver
testis
ovary
oce
kidney
adductor
mooth) muscle
nus
or
sole
Fig. 31.9. Upper, general structure of the salt water littleneck or quahog {Venus
mercenaria) . The left shell, part of the mantle and the gills are cut away. Lower,
general structure of the scallop (oyster), Pecten. The adductor muscle that pulls
the shells together is familiar as fried scallop. The brilliant blue eyes are located
along the borders of the mantle. A scallop jumps through the water by clapping its
shells together, forcing out the water between them and flying forth, hinge forward,
actually jet propelled.
MOLLUSKS — SPECIALISTS IN SECURITY
641
Chap. 31
water from whence they are usually drawn into the incurrent siphons. Sperm
and eggs ripen at the same time and the latter are shed into the mantle cavity.
Both sperm and eggs are drawn through the microscopic holes into the water
tubes of the outer gills where fertilization occurs. There millions of embryos
develop. The outer gills become swollen brood pouches, and the young clams
thrive until they are easily visible to the naked eye. They are then definitely
clam-shaped animals called glochidia. They are discharged from the excurrent
siphon and scattered on the bottom with their valves open and a sticky thread
trailing out between them. For a time they are gamblers for their existence,
and then for several months they are parasites (Fig. 31.10). The edges of the
shells are smooth in some species; armed with hooks in others. From time to
time glochidia snap their valves together, bounce upward, then drop back with
valves open. If there is any disturbing motion of the water their snaps and
bounces increase. Fish or anything savoring of fish creates the wildest excite-
ment. All of this can be seen with glochidia in a glass of water and a bit of
fish meat or blood. It is easy to remove glochidia from a ripe brood pouch — a
slight cut in it and they pour out like sand.
All of this happens in nature when fishes are near except that the glochidium
Fig. 31.10. Life history of a fresh-water clam. The embryos develop in the outer
gill. Later they are shed through the excurrent siphon (nearer the hinge), as
minute clams with one strong muscle connecting the valves and a sticky thread
dangling from them. They clap their shells at every fish that approaches and some
among the millions are able to hook themselves into the fins and gills where they
live for weeks as parasites. Finally they drop off into the mud.
642 EVOLUTION OF ANIMALS Part V
snaps its shells permanently into the skin of the fish and is gradually enclosed
in a fleshy case. Through the next weeks or months the glochidium is a parasite
receiving nourishment and protection from the fish. Finally it breaks out of the
case and falls to the bottom, now formed like its parents but still small. During
its fife in the fish, it may have traveled many miles; after that it becomes inde-
pendent and for a time at least a local resident.
Other Bivalves
The bivalves are all aquatic, mainly dwellers on the bottom, most of them
marine, and commonest between the tide Hnes. Among the rare climbing ones
are the little fingernail clams (Family Sphaeridae), many of them less than half
an inch long. A fingernail clam forages over the bottom of ponds. It also
curves its supple foot around the stems of water weeds like a pole climber with
one leg. Meanwhile, its split siphon is extended and apparently it is drawing
in some of the minute organisms which it must disturb as it climbs.
The razor-shells (Ensis), 4 to 7 inches long, are both agile and strong
burrowers that can outspeed a human shoveler. They also jump with a steel-
spring action of the foot. The common scallop {Pecten irradians) is another
lively bivalve that makes zigzag jumps by opening and forcibly closing its
valves (Fig. 31.9). One clap expels the water from the mantle cavity and drives
the scallop, hinge first, a yard or more in a straight line through the water —
sometimes out of it like a flying fish. Another clap drives it in a different direc-
tion. It is as difficult to catch as a clothes moth when it performs the familiar
zigzag trick in the air. The scallop closes its valves by its one powerful ad-
ductor or cross muscle, and the springy hinge-ligament opens them. The adduc-
tor muscles are the tasty fried scallops. Tons of scallops are harvested annually
along the Atlantic Coast and only one muscle from each animal is used. Deep
sea scallops {Pecten grandis), five inches or more wide, are most abundant
off the coast of Maine and most expensive in restaurants.
Oysters undergo rhythmical changes of sex during the individual's life-
time. There are two similar types of these changes; one type occurs in the
European oyster {Ostrea edulis), and in the Pacific oyster (O. lurida), a
species native to Japan; the second type occurs in the American oyster, Cras-
sostrea virginica (formerly Ostrea virginica), and others. In the American
oyster, the majority of the young are males and during the first spawning
season they function as males and produce sperm cells that are extruded into
the water. Before they become sexually mature however, these young oysters
may present all gradations from true males in which there are developing
sperm cells to other individuals that have complete ovaries. After the second
spawning season, the number of individuals of each sex is almost equal. The
adults usually function permanently as one or the other sex. American oysters
begin to spawn soon after the temperature of the water passes 63° F., usually
MOLLUSKS SPECIALISTS IN SECURITY
643
Chap. 31
at higher temperatures in the south. In Long Island Sound, the season is late
June to September; in Chesapeake Bay, May to October; in Puget Sound,
May to October.
Class Gastropoda — Snails and Slugs
Gastropods are distributed in almost every part of the earth — land and fresh-
water snails, great numbers of marine snails including the huge whelks and
conchs and the limpets. The soft naked land slugs are limited to moist places;
the equally naked nudibranchs are marine. There are some 30,000 living
species of gastropods and many more that exist only as fossils, among them
limpets of millions of years ago.
Structure. The common edible garden snail (Helix aspersa) is often taken
as a type (Fig. 31.11). This snail moves about on its fleshy foot leaving a trail
of mucus from the gland within it. On the prominent head there are two pairs
of tentacles, the shorter pair sensitive to smells, the longer one to light. The
single coiled shell is secreted by a mantle as in other mollusks. The organs of
the body are crowded within it, a complicated mass of twisted viscera including
a complete male and female reproductive system, and a digestive tube begin-
ning at the mouth, twisting upward into the spire and turning back toward
the head to end in the anal opening (Fig. 31.3). One section of the mantle is
an air sac whose walls are supplied with blood vessels and blood pumped by
the heart; thus it functions as a lung. Most fresh-water snails come to the sur-
face and take air into the air sac or breathe through their skin; the majority
of marine snails breathe by gills.
Activities and Functions. The snail's shell is a house into which it retreats.
Heart
Pulmonary vein
Mantle cavity
Eyes in upper tentacles;
lower tentacles sensitive to contact.
Genital pore
Pedal ganglion
Mouth
Buccal mass
with rasping
tongue
Fig. 31.11. The form and part of the general anatomy of a snail; the right side
with the shell removed; the reproductive systems, male and female, are not shown.
644 EVOLUTION OF ANIMALS Part V
In most species, there is a hard plate on the upper surface of the foot that is
last to be drawn into the shell. This is the operculum that acts as a stopper to
evaporation and keeps out intruders. The small opercula of snails were the
original eyestones passed between the eyelid and eye to bring out foreign
bodies.
Feeding. Snails scrape surfaces with the rasping tongue or radula; when a
garden snail is rasping cabbage the sound can be heard several feet away
(Fig. 31.11). The radula is a horny ribbon with ridges and teeth on its upper
surface and beneath it is a cartilage which can be pushed forward against what-
ever the snail is feeding upon. The radula is then pulled back and forth over
the cartilage to rasp a green leaf, the skin of a tadpole, seaweed, or films of
algae and bacteria depending upon the snail's habits. A great many snails are
carnivorous and in these the radula is at the end of a proboscis which can be
extended through a hole bored in a shell. The familiar and unpopular "drills"
are snails that rasp holes in the shells of edible clams and oysters and other
bivalves whose pierced shells are common on many beaches (Fig. 31.12). Sea
slugs, beautiful though too soft, feed upon sea anemones likewise soft (Fig.
31.13).
Relationships. Snails are the hosts of immature worms, including the highly
injurious flukes that as adults are parasites in birds and mammals. Both fresh-
and salt-water snails are eaten in great numbers by fishes and shore birds. Any
one who examines the stomachs of common fresh-water fishes known as suckers
will find plenty of small snails swallowed whole, with shells being slowly
dissolved by the powerful digestive juices. In the stomach of a mullet (a
name given to many small bottom-feeding fishes in fresh and salt waters), one
investigator found 35,000 little marine snails. Snails are generally unimportant
among human foods, but at European shore resorts roasted periwinkles are
sold in bags like peanuts; and steaks from the foot of abalones are sold in
California markets.
Reproduction. In about half the species of snails there is a fully function-
ing male and female reproductive system in each individual, but even so, these
snails mate and cross fertilization occurs as it does in a similar situation in
earthworms. Fresh-water snails produce relatively few eggs in blobs of crystal
clear jelly deposited on submerged stones and on the undersides of floating
leaves. Marine snails produce great numbers of eggs. The sea hares (or sea
slugs, Tethys calijornicus) of the California coast lay their eggs in gelatinous
strings. By counting and computing them, the MacGinities of the Kerckhoff
Marine Laboratory, California, found that one of these sea hares produced
478 millions of eggs in four months and one week. The animal, obviously kept
in captivity, weighed five pounds and 12 ounces.
In many mollusks, sex variations occur in the same individual. Young
marine snails of the genus Crepidula, commonly called boat shells, function at
Chap. 31
MOLLUSKS -SPECIALISTS IN SECURITY
645
Mouth
Cartilage supporting
the radula
Muscles that
rotate radula
Muscles that retract
radula and cartilage
Fig. 31.12. Upper, holes bored by snails whose rasping tongue (radula) is on
the end of a proboscis that is finally pushed into the soft body. They suggest the
number of animals consumed by snails and by boring sponges which bore holes by
dissolving the shells. Lower, proboscis of marine snail cut lengthwise to show the
rasping tongue. {Upper, courtesy, MacGinitie and MacGinitie: Natural History of
Marine Animals. New York, McGraw-Hill Book Co., 1949.)
first as males but later transform into females. This sexual transformation is
hereditary, normally occurring in all individuals.
Class Cephalopoda — Squids and Octopuses
Characteristics. These are the most highly developed and swiftest of all
mollusks (Figs. 31.14, 31.15, 31.16). The head-foot is used equally as a
head and a supporting foot. The digestive tract turns back upon itself as it
does in snails so that the mouth and anal opening are close together, but there
is no such coiling as in snails (Fig. 31.3). In cephalopods, the foot and head
with its remarkable eyes are most highly developed and the shell most
646
EVOLUTION OF ANIMALS
Part V
Fig. 31.13. The gray sea-slug feeds on sea anemones such as the one on the
left; another one at the upper right has its tentacles withdrawn. Sea-slugs are mol-
lusks that lose their shells in early life and commonly bear gill-like filaments
brightly colored, translucent and continually moved by the currents of water.
Length of slug 4 inches. (Photograph courtesy, Douglas P. Wilson, Marine Bio-
logical Laboratory, Plymouth, England.)
unevenly so. The shell of a squid is a horny quill pen embedded in flesh; that
of a nautilus is a many-chambered dwelling. All of the tribe are marine. No
other mollusks approach them in travel, the drifting of the female paper
sailor, the darting of the squids and sepias (Figs. 31.15, 31.16). Clam and
snail shells have great beauty of color, but no other mollusks can display the
Chap. 31
MOLLUSKS SPECIALISTS IN SECURITY
647
Fig. 31.14. Common octopus or devilfish, Octopus vulgaris. When they are ex-
tended the arms of this species may have a span of over six feet though they are
usually much shorter. The arms of an Octopus apollyn of the western coast of the
U. S. may have a span of 20 feet. (Photograph courtesy, Douglas P. Wilson,
Marine Biological Laboratory, Plymouth, England.)
mauve and rose, and the yellows and browns that shift over the body of an
excited squid. Some species have bioluminescence to add to their beauty of
daytime color (Fig. 31.17). In the squids and sepias, an ink gland secretes a
dark fluid that is stored in the ink sac. When the owner is disturbed it shoots
jets of ink from the siphon, creating a cloud in the water that hides its escape.
Living cephalopods are a small group, but their ancestors once swarmed the
seas and fossils of some 10,000 different species are known. The pearly nauti-
lus (Nautilus pompilius) is the only living relic of great numbers of predeces-
sors which also had spiral shells, divided into compartments by septa. As a
nautilus grows, it enlarges its shell and secretes a partition behind it so that the
whole shell comes to be a series of chambers empty except for the cord of living
tissue connecting the body to the first small chamber (Fig. 31.15). Among the
ancestors of the pearly nautilus was one whose fossil shell is 1 5 feet long. The
shell of the living nautilus measures about 10 inches.
The Squid — Loligo
The common squid, Loligo pealii, of the Atlantic, is about 10 inches long;
that of the Pacific is a little more than half that. Squids range from those that
are less than two inches long to the giant squids of the deep sea some of them
probably having an over-all length of over 50 feet — by far the largest living
invertebrates. All of them are fierce carnivores that follow and attack schools
648
EVOLUTION OF ANIMALS
Part V
TinivcMs
Dorse/ /obe
or manHe
Siphon
Sfie/t mt/scfe Marrf/e
Siphuncle
between chambers
Fig, 31.15. Cephalopods, the swiftest of all mollusks. Upper left, sepia, the
cuttlefish with one tentacle stretched forward gripping a crab with its vacuum
disks. The white "cuttlebone" fed to canaries for lime is the shell of the cuttle-
fish. The name sepia is due to the brown inklike secretion that the cuttlefish throws
off when disturbed. Upper right, paper sailor (Argonauta). Female in floating
position. Paper sailors float on the surface of the warmer waters of the Atlantic and
Pacific oceans, occasionally in coastal waters. The thin papery shell is secreted by
the flattened arms. It is not attached to the body, has no partitions and is mainly a
carrier for the eggs. The female is eight inches long, the male about one inch long.
Lower, chambered nautilus (Nautilus) cut open to show the successive chambers
that have been occupied as the animal has grown. A cord of living tissue extends
from the animal's body to the first chamber that it occupied. {Upper left after
Boulenger. Upper right after Claus and Sedgwick. Lower after Ludwig and Leunis.
of fishes. They are themselves in turn the prey of fishes, but they are swift
dodgers. They are actually jet propelled, darting with sudden speed when
water gathered in the mantle cavity is spurted out of the siphon with great
force (Fig. 31.16). They swim by undulating movements of the fins, actually
flaps of the mantle, not at all like the fins of fishes.
Structure. Squids have 10 arms, including one pair with grasping tentacles
much longer than the others. When a squid is swimming it holds the arms
close together and uses them as rudders for steering. A squid darts at its prey
arms foremost and when almost upon it spreads them like the rays of a daisy,
stretches out the tentacles, grips the prey, pulls it back against the sharp beak
in the meantime clasping it with the other arms. Next to the arms, the eyes are
the most prominent features of the head. Although entirely different in their
development, they are the camera type like those of vertebrates. The squid is
an example of the association of the active hunting habits of a carnivore
Chap. 31
MOLLUSKS SPECIALISTS IN SECURITY
649
Fig. 31.16. The common squid (Loligo pealii) photographed in an aquarium at
Marine Biological Laboratory, Woods Hole, Massachusetts. Length, about ten
inches. (Courtesy, General Biological Supply House, Chicago, 111.)
^^
*
J^ ^ y
w \^
— ~— — .^^^ ~
J^^J^
>>^^W^.--^*tf^' ■ '' . v<' .■
'^^r:.-- ^k&^ ' ,-:);;.:■■"
''^>V V'.-.: •"■■■■•;••..
^•••.. ^^~ ■■
• ■ -- - -
ilJ^S'^
Fig. 31.17. A school of bioluminescent deep-sea squids (Watasenia scintillans)
as it might appear in the darkness of the deep sea. An actual observer said of one
tropical species of squid that the eyes shone blue, the sides of the body with pearly
sheen, and the underside of the body crimson. The common squid {Loligo pealii)
is not bioluminescent. (Courtesy, American Museum of Natural History, New
York.)
650
EVOLUTION OF ANIMALS
Part V
coupled with the development of an acute sense of sight, a contrast to the lack
of vision in the lethargic clams.
The mantle is a conical envelope from which the head and siphon protrude,
the latter structure representing the front and rear of the body; the digestive
tube is bent double like a jackknife (Fig. 31.3). The shell is a quill feather-
shaped plate of chitin buried under the skin on the dorsal side of the body.
The jaws resemble a parrot's beak and with them a squid can kill a fish by
a single bite through the spinal cord or head. The ink sac is a relatively large
pear-shaped organ. It consists of a gland which secretes the ink, a sac for
storage, and a duct leading to the anal chamber from which the ink is ex-
pelled.
The sexes are separate, each with one gonad opening toward the siphon.
When mating a sperm packet is transferred by the specialized right arm of the
male to the mantle cavity of the female where fertilization eventually occurs.
The eggs are laid in long capsules of jelly from which the young ones emerge,
minute but in perfect squid form (Fig. 31.18).
Fig. 31.18. Common squid {Loligo pealii). Squids stand on their heads when
laying eggs. The gelatinous egg capsules, about three inches long, are discharged
through the opening of the siphon and attached by one end to seaweeds and rocks,
usually in clusters. They are commonly washed in upon the shore all along our
coasts.
32
Ecninoaerms — Forerunners oi tn(
Vertenrates
Their Relatives. The starfishes and their relatives are animals whose body
plan, except in the developing young ones, is utterly different from that of any
other animals. A clue to their possible kinship comes from the resemblance
of the larvae to those of certain primitive chordates clearly related to the
vertebrates. Because of this the echinoderms have been promoted, by general
but by no means unanimous opinion, to a position near the chordates.
Although mollusks and insects have reached high peaks of invertebrate spe-
cialization and would seem to belong in that place, their larvae resemble those
of annelid worms more than those of any chordate. And young animals are
tell-tales of the origins of their parents.
Characteristics. As the name of the Phylum Echinodermata, spiny-skinned,
implies, many of these animals are armed with hard, chalky and in some species
very heavy spines. Except in the larvae, they are radially symmetrical on a
plan of five or multiples of five that is unique (Figs. 32.1, 32.2). Even in
adults, however, they show signs of bilateral symmetry such as the position of
the sieve plate in the starfish through which a line may be drawn separating the
body into right and left halves. The bilateral symmetry of the free-swimming
larva (Fig. 32.8) is generally regarded as the fundamental plan upon which
the radial one has been overlaid during a long evolution.
All echinoderms live in salt water and are thoroughly and curiously adjusted
to this existence. Their bodies are continually drenched with sea water, inside
and out. Their blood is practically sea water. A starfish cannot take one step
unless its watery blood flows into its foot. Oxygen diffuses into the blood and
carbon dioxide diffuses out of it through the thin walls of hundreds of skin-gills.
There are no special excretory organs but slowly circulating fluid is con-
tinually washing the tissues and carrying away their by-products. Cilia are
651
652
EVOLUTION OF ANIMALS
Part V
CLASS CRINOIDEA
Sea Lily
CLASS OPHIUROIDEA
Brittle Star
CLASS HOLOTHUROIOEA
Sea Cucumber
CLASS ASTEROIDEA
Sea Stor
CLASS ECHINOIDEA
Sand Dollar
Fig. 32.1. Echinoderms. Animals of this group are built upon a plan of five.
Details of the plan vary greatly, but five and multiples of five appear as insistent
and clear as the theme of a symphony. Of those shown in this figure, the five is
least apparent in the sea urchin and sea cucumber. If the sea urchin were moving
however, its five rows of tube-feet would reach out to the rock; and if the sea
cucumber were active five pairs of respiratory gills would be expanded in the water.
The starfish has so stretched its stomach into the clam that the five lobes are pulled
out of their regular shape during this meal time.
numerous on the internal organs as well as on the outer surfaces of the body,
the latter a proof of complete limitation to life in the water. With the rare
exceptions of certain feather stars, only the larvae can swim. Nearly all
echinoderms live on the bottom and minute organisms and particles are always
Chap. 32 ECHINODERMS FORERUNNERS OF THE VERTEBRATES 653
falling upon them. Suffocation from these is prevented by a remarkable skin-
cleaning and trapping equipment consisting of great numbers of minute pincers
distributed over the outer surface of the body (Fig. 32.3). The smallest adult
echinoderm is half an inch in diameter. The largest starfish is 32 inches or more
across and a slender worm-shaped sea-cucumber may be six feet long.
Class Crinoidea — Sea Lilies
In ancient times crinoids {crinon meaning lily) stood like waving lilies
attached to primeval sea bottoms (Fig. 29.4). Some of those sea bottoms were
long ago lifted and now constitute inland highlands. In upper New York
starfish
sea urcnin
ECHINOIDEA
OPHIUROIDEA
CRINfqiDEA sea
cucumber^^
HOLOTHURIOIDEA
Fig. 32.2. Schematic representation of the relations of important structures in
five classes of echinoderms. T, tube feet; 5, spines; M, mouth; A, anus. (Courtesy,
Storer: General Zoology, ed. 2. New York, McGraw-Hill Book Co., 1951.)
State, slabs of stone have been found with dozens of fossil sea lilies pressed
into it as if they had been outspread by falling earth. The majority of fossil
echinoderms are crinoids but there are relatively few living species. These are
the stalked sea lilies attached on the bottom in deep waters and the feather
stars that can swim feebly on the surface. All crinoids are attached when young
(Fig. 32.1). Each arm and its branches bear central grooves through which
cilia propel particles of food toward the upturned mouth.
Class Asteroidea — Sea Stars or Starfishes
These echinoderms have flat bodies with five conspicuous arms or varia-
tions of this number. The central part of the body is relatively broad and high
and the arms short. In all starfishes, the arms are broad as compared with
those of brittle stars.
Class Ophiuroidea — Brittle or Serpent Stars, Basket Stars
The body is flattened with a very definite central disk from the under side
of which the slender, flexible, jointed arms are sharply marked off. These are
provided with strong muscles and are bent and lashed like rapidly moving
serpents. There are no tube feet, but structures comparable to them all help
to pass food toward the mouth. Brittle stars are the most agile of echinoderms,
crowding together in narrow crevices and scuttling rapidly when disturbed.
654
EVOLUTION OF ANIMALS
Part V
Their arms are easily snapped off and in turn easily regrown (Figs. 32.1,
32.9).
Class Echinoidea — Sea Urchins, Sand Dollars
Sea urchins are generally biscuit-shaped with more or less prominent spines
for which the class is named. Sand dollars are flat, cooky-shaped, with very
short usually fine spines. Instead of the separate pieces of skeleton being
Spines
Pedicellariae
Gills
Pedicellariae
A
B
Muscles
Fig. 32.3. A and B, small portion of the surface of a star fish showing the large
spines and finger-shaped skin gills through whose thin walls gases are diffused,
oxygen into the tissue fluid which they contain and carbon dioxide from tissue
fluid into the surrounding water. The minute pincers or pedicellariae cooperate in
keeping the surface clean aided by cilia which create currents of water. C, a single
pedicellaria. These pincers are very responsive to touch. Hundreds of them will
snap and clamp if a hair is drawn across the body. (After Jennings. Courtesy,
Fasten: Introduction to General Zoology. Boston, Ginn and Co., 1941.)
embedded in a muscular body wall as in starfishes, these skeletons are in-
flexible cases formed of limy plates lightly fused together (Fig. 32.1). The
spines are attached by ball-and-socket joints and sea urchins walk on them as
if on stilts. Colonies of sea urchins cling to wave-washed rocks. Sand dollars
commonly lie half burrowed in sand rich in organic matter. Relatively few tube
feet touch the surface when a sea urchin walks over flat places, but it uses feet
from every surface of the body when it climbs (Fig. 32.1). The crystal clear
eggs and developing embryos of sea urchins and sand dollars are among the
most famous subjects of embryological investigations.
Class Holothuroidea — Sea Cucumbers
Some sea cucumbers are replicas of pickled cucumbers; others are long and
slender, translucent and beautiful (Fig. 32.1). They have no skeletal frame.
What skeleton there is consists of smaU limy plates, helplessly isolated and
embedded in the thin muscular body wall. Sea cucumbers rest and travel
on their sides mouth forward; all other echinoderms except the crinoids travel
mouth down. Superficially sea cucumbers seem bilaterally symmetrical, and
J
Chap. 32 ECHINODERMS — FORERUNNERS OF THE VERTEBRATES 655
do not display the five-point plan that is evident enough when they are care-
fully examined.
The Starfish — An Example of the Echinoderms
The following general description applies to the common American star-
fishes such as Asterias jorbesi of the eastern coast south of Maine and the
Pacific starfish, Pisaster ochraceus.
Appearance. The mouth and feet of a starfish are on the down or more
correctly oral side upon which it rests and travels (Fig. 32.4). The rear or
aboral surface is up. On that side, between the bases of two of the arms is the
ciliated sieve plate through which water is continually drawn into the body
(Fig. 32.5). Its position is one of the indications of bilateral symmetry present
even in adult starfishes. The entire surface of the body is rough with the
blunt spines fastened to the units of the skeleton in the body wall. Hundreds of
these units, the ossicles, are set close together in the soft middle layer of cells
(mesoderm) that formed them (Figs. 32.6, 32.7). Covering the whole surface
of the body including the spines and pincers (pedicellariae) is a delicate skin
Fig. 32.4. Starfish. A detail of the oral surface. Rows of tube feet radiate
from the central mouth region. Most of the tube feet are extended by the
pressure of the watery body fluid; some have been retracted by the strap-shaped
muscle within each one. The tip of each foot is enlarged by a suction disk or
foot hold. (Reprinted from Animals without Backbones by Ralph Buchsbaum
by permission of The University of Chicago Press. Copyright 1948.)
656
EVOLUTION OF ANIMALS
Part V
Cut end
Radial canal
Bulb of
lube foot
(inside body wall)
Tube foot
(outside body wqII)
Fig. 32.5. Diagram of a part of the water-vascular (circulatory) system of a
starfish; three of the radial canals are cut near the base. This system takes part
in all movements. It is to the starfish what the circulation of blood is to the
human body and more — a waterway constantly receiving water through the
sieve plate and constantly expending water carrying other substances with it.
Water takes part in every movement of the starfish and in every phase of its
living. Tiedemann vesicles are not shown.
clothed with cilia, whose rapid whipping keeps currents of water moving over
the surface. Some of the pincers work like forceps, others like scissors, but all
are traps that pinch and hold until they are stimulated by some other contact
such as the touch of a neighboring pincer or a falling particle. Multitudes of
minute skin gills which freely open from the body cavity are filled with the
coelomic fluid that oozes slowly about in any open place (Fig. 32.3).
Locomotion, Circulation of the Blood, and the Water Vascular System.
Starfishes move by manipulating the fluid in the versatile water vascular sys-
tem. This contains the circulating fluid that, although largely sea water, may
still be called blood since it contains cells and is concerned with respiration.
The structures that belong especially to this system are the sieve plate, many
canals, the tube feet, and the skin gills. The ciliated stone canal leads from
the sieve plate to the circular canal around the mouth. Opening into the latter
are nine small sacs (Tiedemann vesicles) in which the ameboid blood cefls
originate. Also opening from the circular canal are five radial canals, one to
each arm (Fig. 32.5). These connect with each tube foot by a short canal.
All the tubes are passageways for the water that enters through the sieve plate
and, picking up various substances in the body, becomes the blood.
The tube feet are so coordinated through the central nervous system that
Chap. 32 ECHINODERMS FORERUNNERS OF THE VERTEBRATES 657
they are able to work together and the starfish can move in one direction.
Progress is slow and often begins only after a period of seeming disagreement
among the feet as to which direction they will go. In order to take a step, a tube
foot must receive a signal from the nervous system and be stimulated by con-
tact with a surface. First the internal bulb (or ampulla) contracts and forces
fluid into the external tubular part of the foot which is extended (Fig. 32.4).
In the meantime, a valve prevents the fluid from instantly flowing back into
the bulb. The extended foot makes a contact with the surface, muscles in the
disk or sole of the foot contract, suction is produced, and a foothold estab-
lished. Longitudinal muscles in the tube then contract and pull. This is the
pull that moves the body of the starfish when many tube feet are working. Fol-
lowing this, the longitudinal muscles of the tube relax, and circular ones con-
tract and force fluid back into the bulb. The foot is now ready for a refill and
another step. Water continually diffuses from the water vascular system into
the body cavity and this diffuses through the gills and body wall. With every
step some water is lost from each tube foot.
Nervous System. The central nervous system consists of a nerve ring sur-
rounding the mouth and connected with five radial nerves, one in each arm
(Fig. 32.6). At the tip of the arm a radial nerve gives off fine branches. This
region is highly sensitive to touch and to light through the eyespot. The cen-
anus
rectal sac
intestine
sieve plate
c stomach
ac stomach
hollow gastric gland
Sieve ca
ossicles
ring canal'^ nerve ring
radial
nerve cord
Fig. 32.6. Diagram of a vertical section, tube feet omitted, through the central
disk and base of one arm of a starfish, Asterias. A few pedicellariae are shown
to indicate their presence; actually gills and pedicellariae are abundant. The
cardiac stomach is the part that the starfish extends out through its mouth and
spreads over the soft body of a clam or oyster. (After Brown: Selected Inverte-
brate Types. New York, John Wiley and Sons, 1950.)
658 EVOLUTION OF ANIMALS Part V
tral system is associated with a network of nerves spread out below the surface
of the body.
Feeding and Digestion. Most starfishes are carnivorous, feeding principally
upon clams, oysters, scallops, and mussels. If a starfish is placed in an aquarium
with one or two clams the sensitive tube feet at the ends of the arms soon wave
excitedly in their direction. Very soon, the starfish proceeds toward them and
attacks one of them. It climbs over the clam, its body tentwise above it
with its arms so placed that the tube feet finally pull on the opposite shells.
Many tube feet pull but not all of them at the same time so that there is a relay
of continuous pulling that fatigues the muscles of the clam which eventually
opens its shells. Immediately, the arms of the starfish contract pressing fluid
against the pouched part of the stomach which is everted through the mouth
and lowered between the shells. It envelops the clam's body and digestion
.skin gi
radial
canal
nerve
Fig. 32.7. Cross section of an arm of the starfish. It shows the separateness
of the ossicles; the free passage ways between the roomy body cavity (coelom)
and the skin gills, between the radial canal and the whole extent of the tube
feet; and the openings of the gonads, the ovaries or testes whichever the sex
may be. (After Brown: Selected Invertebrate Types. New York, John Wiley and
Sons, 1950.)
Chap. 32 ECHINODERMS FORERUNNERS OF THE VERTEBRATES
659
Radial
canal
Structures of
adult become
opporent
Division of coelomic
sacs into anterior
and posterior ports
Late Gastrulo showing
Stan of coelomic pouches
Fig. 32.8. Diagrams to show the development of the starfish. This is one more
example of the similarity of the early processes of development among animals
that are later as different as worms and echinoderms. It is clearly shown here in
the blastula and gastrula stages. The diagrams of the later stages can probably
mean little without a special study of the embryology of starfishes. Even in these
stages it is clear that the starfish has a two-sided symmetry before it attains the
five-sided one. (Courtesy, Hunter and Hunter: College Zoology. Philadelphia,
W, B. Saunders Co., 1949.)
is begun. The partly digested food is sucked into the posterior or pyloric part
of the stomach into which five pairs of conspicuous digestive glands open.
They are hollow so that food passes into them freely, and their hnings are
provided with cilia that keep the contents astir. Their surfaces are greatly in-
creased by infoldings and their cells produce powerful protein-splitting
enzymes which complete the digestion of the food eventually absorbed
through their walls (Fig. 32.6). Free-moving ameboid cells are abundant in
the digestive tract and they digest food just as similar ones do in hydra and
in the clam. Practically no indigestible food is consumed by common starfishes
660
EVOLUTION OF ANIMALS
Part V
(Asterias). There are certain species that feed on small snails, taking them
into the stomach in the regular way. After the soft parts are digested these
starfishes spit out the shells, following the custom that has persisted from
ameba to man. An intestine and anal opening are practically nonfunctional.
Excretion. Many ameboid blood cells are drawn into the skin gills by the
cilia which line them. Such phagocytic cells, usually carrying waste matter,
gradually work their way through the thin membranes of the gills into the open
Fig. 32.9. Brittle-stars (Ophiothrix fragilis), the most agile of the echinoderms.
They are named for their ability to snap off their arms. This species is common
in Great Britain; others with similar habits live on rocky coasts of North America.
They are usually wedged in between rocks, tangled with seaweeds or one another.
When scattered on the bottom of a large aquarium without rocks or seaweed
brittle-stars will clump together within ten minutes and twine their arms about
one another. This and others of his experiments with brittle-stars are mentioned
by W. C. Allee in his book The Social Life of Animals. (Photograph courtesy,
Douglas P. Wilson, Marine Biological Laboratory, Plymouth, England.)
Chap. 32 ECHINODERMS FORERUNNERS OF THE VERTEBRATES 661
water. Other waste is probably carried away by escaping body fluid. There are
no kidneys.
Reproduction. With few exceptions, the sexes are separate in starfishes.
There are two ovaries or two- testes in each arm with a minute opening in each
organ near the base of the arm (Fig. 32.7). In most species, the eggs and
sperm are discharged into the open water; fertilization occurs there, and
there is no trace of parental care (Fig. 32.8). Certain of the West Coast star-
fishes brood their eggs. In one very small species, the female carries her eggs in
clusters fastened to her mouth. Others arch the center of the body and draw
the arms together making a kind of brood pouch in which they hold the eggs.
33
IntroQuction to tlie Vertetrates-
Lower Cnoraates ana Fisnes
Higher and Lower Chordates. The higher chordates are the vertebrates, the
most highly developed of all animals — fishes, amphibians, reptiles, birds, and
mammals. They are to a certain degree familiar and commonly known as ani-
mals. The lower chordates such as the worm-shaped Balanoglossus and the
tunicates formed on the same basic plan are unfamiliar, unrecognized as ani-
mals, and altogether unsuspected as relatives of the vertebrates (Fig. 33.1).
Dozens of tunicates firmly attached to a wharf pile suggest miniature hot water
bottles rather than living relatives of man. Yet they have three fundamental
characteristics that occur in every chordate including man, and in no other
animals.
Three Unique Characteristics of Chordates
1 . All have at one time or another a strong flexible notochord that extends
through a part or the whole length of the body. In lower chordates, unless lost
by retrogressive evolution, it is present throughout life. In higher chordates,
it is fully present only during embryonic stages and is replaced by the vertebral
column.
2. The central hollow nerve cord is dorsal to the digestive canal and en-
larged at the anterior end as the brain.
3. Paired gill pouches which open as gill slits, or traces of them are present
in the pharynx at some time in the life of all chordates. Up to and including
the fishes, gills on the arches between the slits serve for respiration throughout
life. In higher vertebrates, gill slits or traces of them are generally present
only in larval or embryonic stages. In mammals, the gill slits never open
and only in amphibians do they function in breathing.
The presence or absence of a notochord, and the dorsal or ventral position
662
Chap. 33
VERTEBRATES LOWER CHORDATES AND FISHES
663
PROTOZOA
Fig. 33.1. A simplified family tree of the animal kingdom suggesting the
probable relationships of vertebrates. Studies generally agree that coelenterates,
such as jellyfishes, sea-anemones, and corals are the basic stock of all animals
above the protozoans and sponges. Clues to any ancestral relationship between
invertebrates and vertebrates are still unsatisfactory. Certain similarities between
vertebrates and, strangely enough, the echinoderms have been discovered. They
are claimed to establish some affinity between the two groups though by no
means placing the echinoderms as ancestors of vertebrates. (Reprinted from
Man and the Vertebrates by A. S. Romer by permission of The University of
Chicago Press. Copyright 1933.)
of a central nerve cord and heart are invariable differences between verte-
brate and invertebrate animals (Fig. 33.4),
Lower Chordates
These constitute three subphyla of little -known animals, but they are sig-
nificant because of their relationship to echinoderms on one hand and to verte-
brates on the other (Fig. 33.2).
Hemichorda. In the Hemichorda, represented by the acorn worm Bala-
noglossus the so-called notochord is a short tubular outgrowth that extends
forward from the mouth into the proboscis (Fig. 33.3). It stiffens this muscu-
664 EVOLUTION OF ANIMALS Part V
lar burrowing organ and thus performs a skeletal function. Acorn worms are
common on muddy bottoms along both east and west coasts. The pharynx is
divided into a dorsal region, containing many pairs of gill slits, and a ventral
food passage.
Urochorda. Members of the Urochorda are called tunicates because of their
tunic-Uke covering and sea squirts because they squirt water from the pores
Fig. 33.2. Diagrammatic side views of the larvae of A, an acorn worm (a
hemichordate); B, a starfish; and C, a sea cucumber — all of them minute,
nearly microscopic. The black lines represent bands of cilia. In life the digestive
tract (stippled) is clearly seen through the translucent body. Until the life history
of the acorn worm was known the larvae of acorn worms were taken for star-
fishes. This is an example of certain similarities between chordates and echino-
derms that has led to the theory that the two groups have a common ancestry in
some minute bilaterally symmetrical animals of the ancient oceans. There is also
a striking biochemical resemblance. The amino acid, creatine occurs in all verte-
brates; among invertebrates it is known only in echinoderms. (Courtesy, Romer:
The Vertebrate Body, ed. 2. Philadelphia, W. B. Saunders Co., 1955.)
of the mantle when disturbed. The larvae, but a few millimeters long, are
tadpole-shaped with a notochord in the tail. Appropriately for their free liv-
ing, they are equipped with eyes. As they go on developing, they settle front
end down on submerged seaweeds and rocks and become permanently at-
tached. The tail and notochord waste away and the eyes disappear. These
animals are striking examples of evolution gone backward, but their abun-
dance shows that they have fitted into a niche in which they have survived
with great success.
Cephalochorda. The Cephalochorda includes the lancelet Amphioxus, the
fish-shaped burrowers, two to four inches long, that live in limited zones of sea
bottoms all over the world (Fig. 33.3). The basic pattern of these lance-
lets resembles that of the vertebrates. The development of the embryo is also
a ground plan of vertebrate development and certain studies of it are classics
in embryology (Chap. 19).
Higher Chordates
SUBPHYLUM VeRTEBRATA
The animals that attract human interest are most often the vertebrates. In-
sects are their chief competitors for attention — the only group that equals
Chap. 33
VERTEBRATES LOWER CHORDATES AND FISHES
665
PROBOSCIS COLLAR
MOUTH CILLS
Proboscis
_ Dorsal
Collar nerve cord
(Notochord)
Gill slits
Fig. 33.3. Representative lower chordates. A notochord is present at some
time in every chordate; in man only in the embryo. Upper, acorn "worm,"
Balanoglossus (adult worm), a burrower in sand, between the tides and deeper.
Middle, a sagittal section of the anterior end of the adult. If the notochord were
prolonged backward it would lie between the nerve cord and the alimentary canal
as it does in all chordates. Lower. Amphioxus — Branchiostoma lanceolatus. About
2 inches long, a burrower along the coasts of warmer seas. ( Upper, after Bateson.
Courtesy, Rand: The Chordates. Philadelphia, The Blakiston Co., 1950. Middle,
courtesy, Romer: The Vertebrate Body. ed. 2. Philadelphia, W. B. Saunders Co.,
1955. Lower, after Hesse-Doflein: Tierbau iind Tierleben. Leipzig, B. G. Teubner,
1910.)
666 EVOLUTION OF ANIMALS Part V
them in prominence. On no other group of animals has the human race de-
pended so much for food, work, transport, and companionship. Verte-
brates have a bewildering capacity for adaptability in form, size, and habit;
mouse and whale, ground mole and eagle, flying fish and antelope, flounder
on the sea bottom and squirrel on the tree trunk, penguins grand marching on
the ice and dancers in the ballroom. Differences in size do not alter the basic
pattern. Learn the anatomy of a mouse and you can understand that of an
elephant. An elephant's trunk is still a nose.
Animals in the Subphylum Vertebrata fall into 7 groups:
Class Cyclostomata. Lamprey eels.
Class Chondrichthyes. Cartilaginous fishes. Skeleton cartilaginous, dogfish,
shark.
Class Osteichthyes. Bony fishes. Skeleton more or less bony; trout, perch, true
eels.
Class Amphibia. Salamanders, frogs, and toads. Skin moist and glandular;
gills temporary or permanent, rarely lacking; five-fingered and four-toed
limbs.
Class Reptilia. Turtles, lizards, and snakes. Cold-blooded; embryo developing
in a sac (amnion); dry skin with outer horny layer of scales.
Class Aves. Birds. Feathers.
Class Mammalia. Mammals, including man. Hair and milk glands.
In addition to the three unique characteristics of all chordates the leading
ones of the vertebrates are: an internal skeleton of cartilage or bone; a verte-
bral column, replacing the notochord of lower chordates; usually two pairs of
appendages, fins or jointed limbs; a ventral heart with two or more chambers;
a closed circulatory system; a large coelom or body cavity containing essential
organs.
Lamprey Eels
The lamprey eels are usually a foot or two long with round, sucking mouths
without jaws, numerous gill clefts, no paired fins, a poorly developed skull,
and no scales in the mucous skin. Lampreys are neither eels nor true fishes,
although they have some resemblance to both. Almost every feature of a
lamprey eel is peculiar. Its most striking one is the large suction disk that
surrounds the mouth and bears circlets of horny teeth upon its surface. The
adult lamprey fastens this disk to the side of a fish, rasps the teeth against the
flesh and sucks out the blood while the fish carries its rider about as long as it
can swim (Fig. 33.5).
Most species of lampreys pass part of their fives in salt water but some are
land-locked in fresh water and all of them breed there. The lake lamprey eel,
Petromyzon marinus, is generally considered the same species as the great sea
Chap. 33
VERTEBRATES LOWER CHORDATES AND FISHES
INVERTEBRATE
667
VERTEBRATE
Fig. 33.4. Diagrams to show the difference in body plan between an invertebrate
(an annelid worm) and a vertebrate. In the latter, the inner ends of the vertebrae
(centra) are in the area occupied by the notochord in the embryo.
lamprey that became land-locked in ancient times. Lampreys are abundant in
the Finger Lakes of New York and tributary streams and it is estimated that
tons of fish are killed by them every year. Before the breeding season, when
lampreys are hungriest, the upturned body of a dead fish with its quota of
lamprey holes is a common sight in Cayuga and the other Finger Lakes.
In spring, these lampreys go up the creeks to make their nests and breed.
Like many fishes, they clear the bottom of gravel by fanning with their tails.
They pick up stones with their sucking mouths, which during this season are
turned from blood-sucking to domestic work.
Fishes
Fishes are the dominant aquatic animals, more numerous than any other
vertebrates except birds. Various kinds live in fresh, brackish and salt water
— in clean water and on mucky bottoms. Some stay near the surface, others
live at great depths where there is no light except from the light organs of
Fig. 33.5. Lake lamprey eel, Petromyzon marinus, attached to a fish. Above
the pectoral fin is a scar where another lamprey made a ragged opening with its
rasping tongue.
668 EVOLUTION OF ANIMALS Part V
animals. Their sizes are various. The whale shark grows to be 40 feet long; the
pygmy fish (Pandeka) of the Philippines is less than half an inch long.
All fishes, except the sturgeons and lung fishes, have a more or less well-
developed vertebral column. The nervous system has essentially the same
arrangement as the frog. The sense organs differ from those of the latter
mainly in degree of development. In the skin, fishes have chemical senses
similar to taste and smell; they also have organs of smell in the nostrils. The
lateral line organs are rows of pits containing cells that are very sensitive to
changes in pressure and to any commotion in the water — even a fish passing
by. To a large extent fishes can find their way by means of their skins.
Fishes are classified according to the condition of their skeletons. At one
extreme, in general the most primitive, are the sharks and rays, the elas-
mobranchs (Class Chondrichthyes), whose endoskeletons are cartilaginous
except for whatever beads of notochord still persist (Figs. 33.6, 33.7). At
the other extreme is the great group of teleosts or bony fishes (Class Oste-
ichthyes), true eels, catfishes, swordfishes, trout, perch, mackerel and scores of
others, that are familiar at least in books and the fish market. Their skeletons
are the most completely bony of any fishes. Between these two extremes are
fishes whose skeletons are partly cartilage and partly bone in various pro-
portions.
Bony Fishes. There are more than 12,000 species of bony fishes, one or
Fig. 33.6. Upper, dogfish (spiny dogfish or shark) (Squalus acanthias) . A
bottom feeder, commonly 2 to 3 feet long. Lower, dogfish shortly before birth.
The yolk sac containing the still unused yolk protrudes from the body wall for
some time after birth, but becomes gradually smaller. ( Upper, courtesy. General
Biological Supply House, Chicago, 111. Lower, courtesy. Rand: The Chordates.
Philadelphia, The Blakiston Co., 1950.)
Chap. 33 VERTEBRATES LOWER CHORDATES AND FISHES 669
another kind distributed through salt and fresh waters everywhere (Fig. 33.8).
They are the main food fishes. Seagoing fishermen catch more than 10,000,-
000,000 herrings annually to be salted, smoked, and packed. The 1947-1948
catch of sardines off the coast of Calfornia was 10,237 tons. Haddock, mack-
erel, flounders, and salmon are standards of the market among many other
Efferent
branchial artery
Gill
slit
Ventrol aorta
Afferent
branchial artery
Fig. 33.7. The arrangement of the internal organs of a dogfish shark is near to
being a living diagram of a generalized vertebrate.
fishes including those of fresh waters. Fishes are even more important as food
for other fishes than for man. From greatest to least, larger fishes eat smaller
ones. The great sport fishes are bony fishes — mackerel, tuna, and swordfishes;
in clear streams, the golden trout of the west, the rainbow trout, the eastern
brook trout and the hardier brown one (Fig. 33.9).
Skin and Scales. In bony fishes, the outermost layer of the skin is a living
layer. Except for a coating of slime, the skin is constantly in contact with a
world of water. Fishes have no eyelids and no tears, but water is always wash-
ing their eyes.
The skin secretes the first defense of the body, a slimy covering that per-
mits the fish to slide more easily through the water and protects the cells
against fungus and bacteria. With the skin, kidneys, and gills this helps to keep
an excess of water from passing in or out of the body. In the ocean, such
structures hinder the weaker salt solution of the body fluid from passing into
the stronger salt solution of sea water, thus shrinking the body. In lakes and
streams, they likewise hinder the fresh water from passing into the weak salti-
ness of the blood, thus bloating the body.
The skin produces scales, the second defense of the body, by the division of
dermal cells in its inner layer. Scales, like fingernails, are composed mainly
670
EVOLUTION OF ANIMALS
Part V
Fig. 33.8. The streamlined bodies of fishes and the variations that are usually
associated with a reduction in the efficiency of swimming and the development
of some protective mechanism. A, mackerel, a streamlined fish known to travel
more than twenty miles an hour. B, trunk fish (Ostracion) whose scales form a
rigid box; it lives in coral pools browsing on the polyps. C, sunfish (Mola), may
have a length of 5 feet or more and, whatever the advantages may be, has
managed to inhabit all temperate and warm oceans. D, globe fish (Chilomycterus)
is slow moving but has heavy armor. E, sea horse (Hippocampus) has no caudal
fin but anchors itself by its prehensile tail. F, common eel (Anguilla) that can
squirm over barriers between bodies of water. (Courtesy, Young: The Life of the
Vertebrates. Oxford, England, The Clarendon Press, 1950.)
of dead cells and like them have a growing part or quick. In most fishes, the
scales are covered by a layer of skin, so thin it is invisible, and usually worn
off at their tips. In others such as in the various species of trout, they appear
only when the surface of the body is rubbed lightly; in eels, they are deeply
hidden and it is commonly thought that there are none. In bony fishes, the
scales overlap one another like shingles. The visible part of each one is
smaller than the hidden part and always points away from the head. In black
bass and others, the scales are ctenoicl, i.e., comblike with toothed edges; in
Chap. 33 VERTEBRATES — LOWER CHORDATES AND FISHES 671
salmon, trout, and others, they are cycloid, more or less circular and smooth-
edged.
As the fish grows, scales increase in size but not in number. Within close
range, each individual has the same number as others of the same species. Each
scale enlarges by the addition of many bands or rings per year (Fig. 33.10).
The width of a ring signifies the rate of growth and is based on the metabolic
activity of the fish. In summer, when food is abundant, the bands are broad
and the lines farther apart. In winter, food is sparse and growth is slow; the
lines are close together. The age of many, though not all, bony fishes can be
told by the number of summers and winters recorded. It is believed that most
fishes grow as long as they live and usually obtain enough food to have some-
thing extra beyond routine upkeep.
The color of skin is due to saclike cells, the chromatophores, that contain
pigment. They are distributed in great numbers through the deeper layer
(dermis) of the skin. Each contains only one color, usually red, orange, yel-
low, or black and these pigments may be spread out in the cells or con-
tracted to pinpoints. White, blue, and green are due mainly to the break up of
light rays on the surfaces of crystals of guanin that are colorless metabolic
Fig. 33.9. Early stages of eastern brook trout, Salveliniis fontinalis. A, eyed
eggs showing the embryos through the egg envelopes; B, hatching; C, a group of
free swimming fry; and D, a recently hatched fry with its blood vessels outspread
through the yolk-sac (enlarged about five times). (Courtesy, Needham: Trout
Streams. Ithaca, N. Y., Comstock Publishing Co., 1940.)
672
EVOLUTION OF ANIMALS
Part V
Fig. 33.10. A scale from a seven pound female rainbow trout (Salvelinus
gairdnerii) taken at spawning time. May 20, in Paul Creek, British Columbia.
The age is indicated by winter rings (7-7) showing slowed growth in such
periods of low food supply. The ring at 4 is a typical example of a spawning mark,
when feeding stops and life is strenuous. This fish probably did not spawn in its
fifth (5) year, but did so again in its sixth (6). The dark part of the scale without
rings is embedded in the flesh. (After Mottley. Courtesy, Needham: Trout Streams.
Ithaca, N. Y., Comstock Publishing Co., 1940.)
products. The silvery sheen on the undersides of fishes also is due to guanin.
Some fishes show remarkable changes of color when against different back-
grounds. Sunfishes and others are brilliantly colored during the breeding
season.
Skeleton. Fishes are the early models of vertebrates. The main parts of
the skeleton are the skull, vertebral column, the pectoral and pelvic girdles, and
the pair of pectoral fins with the pelvic fins behind them. There are other fins
but these are the most important to the skeleton (Fig. 33.8). The pairs are far
apart in most primitive bony fishes such as trout, and closer together in the
more specialized yellow perch.
The great feature of the skeleton is the strength and flexibility of the chain
of vertebrae that form the backbone (Fig. 33.11). Its weakness is with the
paired fins and their girdles that are not attached to the backbone, but are only
embedded in the flesh. This arrangement is adequate for the fishes that do
Chap. 33 VERTEBRATES LOWER CHORDATES AND FISHES 673
not depend upon paired fins to pull or push greatly, or to carry weight. Its faults
are in its use by other animals. It started a pattern in evolution and millions
of years later, the human shoulder blades (scapulas) and collar bones
(clavicles) slip about, or break too easily and often.
Muscle. The important muscles of a fish are arranged on each side of the
body in the V-shaped blocks that are familiar on the dinner plate. The body
muscles are wholly responsible for the alternate swimming movements, con-
traction and bending to the right and to the left, repeated over and over. In
fishes, each group of muscles acts locally within a small area. The muscles that
control the fins are concerned with piloting, the tactics of locomotion.
Fishes can cut straight down or straight up through the water; they can
hang motionless, as if suspended in it; and they can maintain themselves facing
into a current with the merest flicker of their pectoral fins. Most of this is due to
the lift of water, to its density which makes it a support. A fish that in air
would weigh about 20 pounds is estimated to have a pressure or equivalent
weight of about one pound in salt water.
Digestive System. The majority of fishes have a large mouth and numerous
teeth on the jaws, on the roof of the mouth, the pharynx, and on the almost
immovable tongue (Fig. 33.11). All of these are used for gripping and strain-
ing food; fishes do not chew and they have no salivary glands. Most of them
are carnivorous and their prey is swallowed undamaged until it reaches the
stomach. There are great variations from the typical teeth. Some vegetarians
like the carp have no teeth in their mouths; in the parrot fish the front teeth
are fused into a beak with which it nibbles seaweeds; but both of these fishes
grind their food with their pharyngeal teeth.
The sac-shaped stomach is highly extensible and provided with gastric juice
that dissolves bones and shells. As in other animals, the intestines of carnivo-
LOBES
tEREBRUM
EYE
SOCKET
NOSTRILS
TONGUE
PHARYNX
Fig. 33.11. Main internal structures of a bony fish. (Courtesy, MacDougall and
Hegner: Biology. New York, McGraw-Hill Book Co., 1943.)
674 EVOLUTION OF ANIMALS Part V
rous fishes have a relatively small absorptive surface and those of herbivorous
ones a very large one. In many bony fishes, the blind pouches or caeca which
open off the intestine just behind the stomach increase the digestive and ab-
sorptive surface. Such caeca as these are found in no vertebrates above the
fishes. A catfish has none, a sunfish has seven, and the king salmon of the
Pacific has over 200. Fishes have an extensive liver, in some so stocked with
oil that it helps them float. The pancreas is in small pieces, not easy to identify;
they contain the islets of cells that secrete insulin. Fishes are peculiar in having
the anal opening anterior, instead of posterior, to the urino-genital ones as it is
in other vertebrates.
Breathing. Fishes breathe through their mouths and by means of gills. The
breathing mechanisms are shown in Figure 33.12 and are described further in
Chapter 13. Two arrangements prevent undue confusion of food and water.
The esophagus is tightly closed by circular muscles except when food is swal-
lowed. The arches supporting the gills bear inward-projecting rakers that keep
food from lodging on the gills. It is not always remembered that the oxygen
mainly available to aquatic animals is originally absorbed from the at-
mosphere. Oxygen in the composition of water, HoO, is not available.
The amount of water in the goldfish bowl is of little help if its surface ex-
posure is too small. Trout keeping close to the brook bottom on a warm day
remind one that cold water sinks and that it holds more oxygen than warm
water.
Water Content and Excretion. The gills are the main breathing organs,
but they are also excretory organs that control the salt and the water content
of the body and eliminate waste products. An important difference between
fresh- and salt-water fishes is in their water income and outgo.
Fresh-water fishes continually absorb water mainly through the gills. It
passes through the semipermeable membranes and into the salty body fluids
according to the law of osmosis (Chap. 2). Fresh-water fishes must have the
income of water controlled or their bodies swell. Much of the nitrogenous
waste diffuses out across the gills.
Salt-water fishes drink water and their stomachs are often found full of it.
Their gills excrete salt. Their kidneys eliminate ammonia and urea, but very
little water. They must conserve water because their body fluid is less salt than
the ocean water and in very small amounts water leaches through the semi-
permeable membranes wherever it can. Salt-water fishes must have their
outgo of water controlled or their bodies shrivel.
Shad, salmon, eels, and others can adjust from salt water to fresh and vice
versa, but not suddenly. Young salmon cannot be dumped into salt water any
time; only when they have silvery guanin crystals (nitrogenous excretory
products) in the skin is the change safe.
The Air Bladder and the Sounds of Fishes. The majority of fishes
Chap. 33
VERTEBRATES — LOWER CHORDATES AND FISHES
675
Heart
A operculum removed
exposing gills
B Circulation through heart and gills
Operculum Gills
Esophagus
D,E Horizontal section
from right to left
Upper
jaw
C Detail of circulation in gill
Mouth
cavity
D'
Capillaries
Blood
from heart
to gill
Blood
from gill
to body
Gill arch
Raker
E'
D,E Vertical section
dorsal to ventral
D,D Intake of water E,E Outgo of water
Fig. 33.12. Respiratory organs and breathing action of a bony fish. The circu-
lation of blood is shown in one gill; the structures in diagram (B) are shown
in the same position that they would be in the fish (A). The diagrams (D, D^
and E, £i) represent the intake and outgo of water in one complete "'breath."
676 EVOLUTION OF ANIMALS Part V
have a conspicuous air or gas bladder that lies in the dorsal part of the body
cavity parallel to the backbone. In some species, it is connected with the
pharynx by an open duct; in others, as in the perch, by a solid strand of tissue
(Fig. 33.11). Its transparent walls are plentifully supplied with blood vessels
and it is filled with oxygen, nitrogen, and carbon dioxide in varying propor-
tions, evidently originating from the blood. African lung fishes use the air
bladder as a lung. In certain deep-sea fishes, the unusually large proportion of
oxygen that has been identified in the bladder has been taken to be an extra
insurance against its sparsity in deep water. The bladder also aids the fish
in lifting and holding its body at one or another level of water as a pickerel
hangs in ambush, motionless among the pond weeds. The sharks lack air blad-
ders, but they have the lifting capacity of relatively enormous livers stocked
with oil (Fig. 33.7).
Fishes make sounds with their air bladders. Undersea noises were heard in
full strength by means of radar during World War II. Certain regions, one of
them the Chesapeake Bay, were at that time a bedlam of racket obstructing
any other sounds. The croaker ( Micropogon ) listened to by means of a hydro-
phone proved to be guilty of the noise, made by the contraction of a muscle on
the capacious bladder that acted as a resonator. Croakers are edible fishes com-
mon along our southern Atlantic coast. It has been estimated that their
population in Chesapeake Bay was at one time about 300 million. The male
weakfish (squeteague) can set its bladder in vibration and produce sounds.
The fresh-water drum or sheepshead is able to grunt by muscles working on
the gas bladder. Although fishes lack true vocal organs, they have joined the
world's chorus with what means they have.
Circulation. The heart with its auricle and one ventricle lies in the peri-
cardial sac ventral to the pharynx. It is located far forward in the body, con-
tains venous blood only, and pumps it to the gills, from whence it goes directly
to other parts of the body (Fig. 33.12). In all other vertebrates, the blood is
returned from the respiratory organs to the heart before it is distributed to
the body. Fish blood is under low pressure, is relatively thick, and does not
flow easily.
Reproduction. Almost all fishes multiply abundantly, many of them
enormously. The sexes are separate and in the majority fertilization occurs in
the open water. Mackerel gather in great assemblies of males and females and
the water swarms with sex cells. Herring do likewise and a single female
produces 30,000 to 2,500,000 eggs per year. Counts and calculations by the
U. S. Bureau of Fisheries credit the halibut with 2,000,000 eggs per year and a
codfish with 9,000,000.
In central New York state, brook trout (Salvelinus fontinalis) begin to
ascend to the spring-fed headwaters of streams in September. There the males
and females first congregate in the deeper pools below the spawning grounds.
Chap. 33 VERTEBRATES LOWER CHORDATES AND FISHES 677
By late October, the female prepares the nest. It is a basin a foot or two in
diameter, if possible placed near a spring, swept in the gravelly bottom by
vigorous brushing with the body and tail. The male takes no part in the
preparations but is always nearby. After some hours of courtship, the two
fishes vibrate their bodies above the basin and spawning occurs. The pair
then separates and the female swims a short distance upstream from the nest
and stirs up sand and gravel which the water carries over the eggs. The
average nest contains nearly 200 eggs about 70 per cent of which hatch in the
spring.
The herring and the brook trout, sunfish and other nest makers represent
extremes in deposition of the sex cells and the numbers of eggs. There are
hundreds of variations in spawning habits. The eggs of fresh-water fishes are
relatively large, fewer, and usually sink to the bottom. The newly hatched
young (fry) of a one pound brook trout is half an inch long. The newly
hatched fry of a 300-pound swordfish is one quarter of an inch long.
Nervous System and Special Senses. The discussion of the nervous sys-
tem is given in Chapter 16. Whatever is said about the special senses will
mean more if Chapter 17, Sense Organs, is read with it.
Vision. Most of us know a fish's eye as the hard white ball in the head of a
baked fish. That ball is the crystalline lens, the gatherer of light rays that has
lost its translucence but is a perfect sphere as it was when the fish was alive.
The shape of the fish's lens cannot be changed like that of the human eye. It
can only be moved backward and forward to get a Uttle better focus (Fig.
33.13). Fishes have no true eyelids and no lachrymal glands. The living cells
of the cornea are washed by the waters of lakes or oceans, not by tears. With
some exceptions, e.g., shark, there appears to be practically no control of the
amount of light that enters the pupil. An iris is present but immovable.
When the human eye is focused on objects that are close by, the lens is
nearest to spherical. The lens of the fish's eye is always spherical, always ad-
justed to close vision. Many fishes are naturally nearsighted. On the other
hand, sharks that pursue rapidly fleeing prey have lenses that are peculiarly
set for distance. The fish's eyes are on opposite sides of its face; they look in
opposite directions and see different things but only a little of what is in front
of them. If the headlights of automobiles were moved even a Uttle distance to
the sides of the hood the front view would be greatly foreshortened. Something
comparable to that has happened to the fish.
When a light ray passes from air into water its direction is changed. This
occurs when rays pass from air into the watery interior of the eye. The human
eye is adjusted to this and vision is clear, but under water human vision is
blurred. The direction of light rays is not changed as they enter the eye of a
fish because they pass from water into wet cells and a watery interior. Such
facts have been learned by experimentation and repeated observations. On
678 EVOLUTION OF ANIMALS Part V
the basis of certain trials, it appears that a fish can tell a blue fly from a red
one when either one is submerged, but not when they are on the surface. One
version of what a fish may be able to see above the surface is set forth in
The Story of the Fish, by Curtis Brian (Suggested Reading).
Hearing. Bony fishes must hear better than sharks and others with carti-
laginous skulls since bone is a better resonator than cartilage and because fishes
A. B.
ACCOMMODATION IN EYE
OF A BONY FISH
A. Position for near sight
B. Limited for sight
Fig. 33.13. Eye of a blenny. A, usual resting position for near sight. B, the
lens pulled backward, in the position for limited far sight. Fish are nearsighted
and probably their eyes and the condition in water do not allow vision to extend
more than fifty feet. As any trout fisherman knows the trout can see above the
water surface. Blennies (Blennius) live among the mussel beds on reefs of the
Pacific coast. (After Walls: The Vertebrate Eye. Bloomfield Hills, Mich., Cran-
brook Institute of Science, 1942.)
lack eardrums and middle ears. They have parts of the inner ear (utricle and
sacculus), but they do not have the cochlear duct so important in the human
ear (Fig. 17.9). Experiments have led to the conclusion that goldfishes can
hear, although within a very small range of sound. Care must be taken that
responses to vibrations received by the skin cells are not mistaken for hearing.
The sounds used to test the goldfishes were produced by a telephone inside a
submerged balloon.
Pressure. The lateral line is a tube that lies just below the skin and runs
along each side of the body from the gill openings to the tail. It often con-
tinues with several branches onto the head. The tube is filled with mucus and
at frequent intervals opens by a pore over a group of sensory cells in its floor
(Fig. 33.14). Slightly deeper in the body wall is a long branch of the tenth
cranial nerve which supplies a branch to each lateral line organ. These organs
are extremely sensitive to changes of pressure. They react to the sUght changes
of pressure that come from a passing fish and they doubtless initiate the shift-
ing of position that can be seen so often in a school of fishes idling in a slow
stream.
Touch. Sense organs of touch essentially like our own are spread over the
surface of the skin.
Chap. 33 VERTEBRATES LOWER CHORDATES AND FISHES 679
Chemical Senses — Taste and Smell. There are organs of taste on the tongues
of certain fishes and experiments have indicated that those fishes can taste salt
and bitter. Catfishes have taste organs on their whiskerUke barbels and organs
of chemical sense are distributed all through their skins. In aquatic animals
especially, taste and smell are so similar that it is hard to separate them. The
behavior of some fishes does not seem to leave the slightest doubt that they
can smell. Sharks have a keen sense of smell, or taste, but when catching its
prey a bony fish such as the trout seems to depend entirely upon its eyes.
Great Migrators. Salmons and eels are true migrators to distant places.
Fig. 33.14. Long section of the body wall of a fish showing the lateral line
sensory system. A branch of the lateral nerve runs to each sensory organ which
opens into the minute openings in the body wall and allows water to enter the canal.
By means of the lateral line organs fishes taste the water that washes their sides.
(Courtesy, Romer: The Vertebrate Body, ed. 2. Philadelphia, W. B. Saunders Co.,
1955.)
Under certain conditions in themselves and their surroundings, they journey
from their native homes to other places where they live for a time and then
in full maturity return to their native waters to spawn. The great migrations of
Atlantic salmon have become history. Salmon are now known mainly from the
Pacific Ocean and its watershed. They are hatched from the eggs, high up in
the rivers away from the sea and spend the first months of their lives there.
When they are five or six inches long, in answer to an age-old inherited habit
and state of body, they turn downstream. They feed and loiter but finally
reach the Pacific Ocean. They remain in the ocean about four years and then
as m.ature fishes ready to spawn, they collect near the mouth of a river. The
mouth of the Columbia in the state of Washington is a famous gathering place;
the mouth of the Fraser in British Columbia is another. The Chinook, blue-
back, and silver salmon enter the Columbia in early and late summer and begin
their ascent, an army of animals that cannot stop pushing against currents and
waterfalls. They swim upstream for hundreds of miles, without taking food,
often mounting 19-foot falls, until they reach their native tributary stream.
There the female lays her 10,000 or more eggs, the male sheds the milt (sperm
cells) over them, and the female covers them with sand. Within a brief time,
the exhausted fishes float downstream, dead or dying. The eggs hatch, the
young grow, and the story begins over again.
The change from fresh to salt water demands a period of adjustment. A
young salmon can be killed by being dropped into fresh water at the wrong
680 EVOLUTION OF ANIMALS Part V
age. There once were and still are Atlantic salmon. Thousands of them once
went up the Connecticut River to spawn. Now when a few swim up the river
it is an event for the newspapers. The New Englanders took too many fishes
from an easy catch.
Two federal dams now span the Columbia River, the Bonneville dam, 152
miles from the sea, and the Grand Coulee, 552 miles from it. They are in the
direct way of the salmon. Bonneville supplies fish ladders. The Federal Gov-
ernment tried education on the offspring of salmon headed for the Grand
Coulee. Eggs and sperm were collected from the migrating fishes and mixed
together for fertilization. The resulting young fishes were placed in streams that
entered the Columbia below the Coulee. In time, these fishes left the stream
and entered the ocean. In a later time, they returned to the streams below the
Coulee, known to them but not to their parents or grandparents. As an experi-
ment, at least, it was successful.
The journeyings of the eels (Anguilla), true bony fishes of the east coast of
North America and west coast of Europe, are directly opposite those of sal-
mon. They are hatched in the Sargasso Sea, northeast of the West Indies where
seaweeds (Sargassum) float in the relatively calm water. Here the spawning
grounds of American and European species are near together, yet separate,
and the young eels take their own routes to their respective continents. The
larvae are slender and thin, so different from their parents that their relation
was for a long time unknown. On the first part of their journey, the young eels
ride on the great ocean currents, the American ones chiefly in the Gulf Stream.
They are one-quarter of an inch long when they leave the Sargasso Sea. A year
later, when they reach the mouths of the North American rivers, they are 3
inches long. There they are transformed into elvers, that look and act like little
eels. In the Gulf Stream as larvae they were carried; in the rivers as elvers they
swim upstream into tributary streams and into lakes. There they live for five
years or more until they are fully mature. Then they swim downstream to the
river mouths and as silver eels probably colored from guanin crystals, they
pass out into the ocean.
The eels, true bony fishes, of the Pacific live in the coastal waters and do
not migrate to fresh water. Salmon, trout, and other fishes that go upstream to
spawn are termed anadromous, meaning up the river and eels are catadromous,
meaning down the river. Next to birds, fishes are the great travelers. These
migrations are examples of much coming and going, to and from deeper water,
in winter and summer, in daylight and dark.
34
Anipnir)ians — Tne Frog^
An Example or tlie Vertebrates
Salamanders, frogs and toads, and the little-known wormlike caecilians live
partly in water and partly on land; hence the name. Class Amphibia, from
amphibios meaning double living. All of them spend part of their life span in
the water. A very rare and specialized few stay in it all their lives. From the
fossils that picture their early history, it appears that amphibians originated
from fishes, that some of those pioneers of ancient times lost their scaliness
and became the ancestors of modern frogs while others kept their scales and
gave rise to reptiles. Amphibians are the oldest four-footed backboned animals,
once dominant in the swamps of the early Mesozoic Period 200 or more
million years ago. In times long before paddles were transformed into legs, the
air-breathing lobe-fin fishes must have been stranded in muddy water full of
gas from decaying vegetation. A few were mired in the clay and became fossils.
Others wriggled into fresh pools and shady places. After millions of years of
natural selection their descendants managed to walk on their weak legs shifting
their bodies from side to side, as salamanders still do (Fig. 34.2).
Characteristics. Amphibians are vertebrates with moist glandular skins and
no external scales. Except for the limbless caecilians, they have two pairs of
limbs used in walking or swimming (Fig. 34.1). The two nostrils connected
with the mouth cavity have valves to shut out the water. The heart has two
auricles and one ventricle. Respiration is by gills, lungs, skin, the lining of the
mouth, or combinations of these. There are gills at some phase of the life span,
e.g., in the tadpoles of frogs. The eggs are fertilized externally in frogs and
toads, internally in salamanders. No membranes are formed around the embryo
(Chap. 19).
681
I. With sticky toe pads,
climbs a tree
or window pane.
2. Loud speaker
One inch long,
heard one mile.
3. Shell headed toad
lives in burrow,
its head the stopper.
4. Defense stand of toads and frogs:
head low, eyes flat, body puffed.
5. Burrowing toads
wedge headed, barrel bodied.
6. Each egg hatches
in a pool of fluid in skin
7 An amphibian of over
two million years ago.
Fig. 34.1. Shapes and ways of frogs and toads. /, the common "tree toad,"
Hyla versicolor (length 2 inches). 2, spring peeper, Hyla crucifer (length, one
inch); its resonating vocal sac is a third the size of its body. 3, shell-headed toad,
Bufo empusus of Cuba, whose head fits perfectly as a stopper in a tubular burrow.
4, the defense, fright reaction of a toad, Bufo calamita. 5, some burrowing toads
have sharp narrow, often bony snouts, others have blunt bony heads. 6, the
Surinam toad, Pipa pipa, of South America is a purely aquatic toad as its webbed
hind feet testify. Its eggs are spread over the spongy skin of its back and the young
ones develop there in individual pouches till they are minute toads. 7, an ancient
amphibian (eryops) of North America, a partial restoration from the fossil. These
animals dragged their bodies after the fashion of present day alligators.
682
Chap. 34
AMPHIBIANS
683
Class Amphibia
There are some 2500 species of living amphibians; at least 1500 of them are
frogs and toads. This smallest of the classes of vertebrates is usually divided
into 3 groups: Caudata or Urodela (tailed); Anura (tailless); and Apoda
(limbless).
Order Urodela — Salamanders
The Urodele's body is long and slender, carried or dragged on puny legs as
if it belonged to a pigmy dachshund (Fig. 34.2). All larvae and some adults
have gills. Among the gilled adults are the common mud-puppies (Necturus)
of the eastern United States and Canada that live in rivers and creeks, crawl-
ing over the bottoms, mostly at night. It is easy to see why they are called
puppies. The "ears" are the very beautiful gills that swing rhythmically as the
puppy breathes (Fig. 34.3).
The majority of tailed amphibians are without gills in adult life. They include
the better-known and generally smaller salamanders. There is no sharp distinc-
tion between salamanders and newts or efts except that the latter are smaller
and more delicate. Newt, eft, and asker with varied spelling are old names for
salamanders, commonly taken for lizards. Like frogs and toads, they are bound
up with superstition, often with witchcraft (Fig. 34.26).
Many salamanders are abundant and some of them such as the spotted sala-
mander {Ambystoma maciilatiim) and the newts (Triturus) are subjects of
important experimental studies. The tiger salamanders {Ambystoma tigrimim)
widely distributed in the United States resemble the spotted salamanders. They
start life as typical aquatic larvae, breathing by gills. In most regions, the
Fig. 34.2. Newt (Triturus) walks on its 4 weak legs at the same time weaving
its body like a fish. Drawings from photographs of slow locomotion. (After
Evans. Courtesy, Young: The Life of the Vertebrates. Oxford, England, The
Clarendon Press, 1950.)
684
EVOLUTION OF ANIMALS
Part V
Fig. 34.3. Two of the largest salamanders in North America. Left, the "mud
puppy" (Necturus), one foot long, has gills throughout life. Common in eastern
rivers of United States and Canada. Right, the hell-bender (Cryptobranchus) ,
about a foot and a half long; the adult has no external gills but makes up for
this by loose folds of skin that function as gills. Hell-benders are usually in the
shallows of streams and are secretive, but once seen are not forgotten.
larvae transform and climb on land as air breathers. But in some localities,
such as Mexico, western Texas, the southwest, and Colorado, and under cer-
tain special conditions, they continue to grow to full size in the water, become
sexually mature but do not change their form or lose their gills (Fig. 34.4).
In such a phase they are known as axolotls.
The best-known North American newts are the "water dog" {Triturus
torosus), eight inches long, of the Pacific drainage, and the spotted newt (Tri-
turus viridescens) , half as long, of the Atlantic drainage. The "red eft" or "red
lizard" of the woods and the spotted newt of the ponds are the same animal
in different color phases (Fig. 34.5). Adult spotted newts live in ponds and
meandering streams from September or October to the next summer, perhaps
longer. The breeding season is in the spring, when an elaborate courtship pre-
cedes the egg-laying and lasts from several hours to a day or more, as readily
in aquaria as in a pond. After the pair separates, the male deposits white
jellied spermatophores containing spermatozoa, on submerged leaves and
sticks. As he moves away from a spermatophore, the female creeps over it and
takes it into the cloaca, the cavity through which the eggs must pass to be
fertiUzed and laid. Upwards of a hundred are deposited separately here and
there, usually on submerged plants. The larvae live in the water until toward
fall. Then their gills gradually shrink; they acquire lungs, their skin becomes
firmer; and their color changes from green to orange-red. They climb out of
the pond and spend at least one winter and summer on land, during which they
are the red newts of the woodland carpet. In some localities, they return to the
water in the fall, as nearly mature animals, their backs turning olive green with
Chap. 34
AMPHIBIANS
685
Fig. 34.4. Tiger salamander {Ambystoma tigrinum) . 1, adult and 2, larva,
adult 7 inches long. Under particular conditions and in certain parts of the
country the larva (axolotl) grows to full size and sexual maturity without trans-
forming.
red spots along the sides, the adult coloration. These are the animals that will
reproduce the next spring. In some parts of their range, certain ponds on Long
Island and Cape Cod, spotted newts retain their gills and do not leave the
water for a long time, if at all. If such newts are kept in cages, with little water
except that in damp moss, their gills shrink away and their skin changes in
texture to the land type. This is an example of the easy adjustment of which
amphibians are capable.
Order Anura — Frogs and Toads
Frogs and toads hop and leap with an always ready kick-off by the mas-
sive muscles of the thighs and calves that snap the bent hindlegs into ac-
tion. When salamanders walk, their body muscles work, pull and swing the
body. When frogs hop, the leg muscles work, but the weak body muscles take
little part. Frogs have the shortest backs and smallest number of presacral
vertebrae of any land-living vertebrate. Correlating with this they have very
long hip bones (ilia) extended to meet the fanlike extensions of the sacral
vertebra. The meeting place is the conspicuous hump on a frog's back, the
outward sign of a peculiar evolution.
Frogs and toads are more specialized than salamanders. Their metamor-
phosis includes not only the change from gills to lungs, but from the digestive
tube of a herbivore to that of a carnivore, and from a mouth fitted for scraping
Fig. 34.5. /i, adult spotted newts (Tritiinis viridescens) . The male {upper)
has a fin on the tail especially prominent during the breeding season and horny
pads on the hind legs. Length of male, V/i inches. It is easily recognized by the
row of scarlet spots ringed with black on each side of the body. B, diagram of
the typical life history of the spotted newt indicating its first summer in the water,
its winter and second year of residence on land when it is known as the red eft,
and the return to water in autumn in the adult green phase.
686
Chap. 34 AMPHIBIANS 687
algae to one adapted to catching insects. They also change from fishlike swim-
mers to expert jumpers. A description of the essentials of the biology of frogs
is included later in this chapter.
Order Apoda — Caecilians
At first glance caecilians might be mistaken for earthworms instead of verte-
brates. Except for a few that are aquatic, they live underground, burrowing in
moist places in Mexico, South America, and other tropical countries. Their
amphibian characteristics are unmistakable. They have gilled larvae and go
through the typical metamorphosis. The adults have well-developed lungs and
their skin is smooth and glandular.
The Frog — An Example of the Vertebrates
Frogs are nature's gift to laboratory study and experiment. They are abun-
dant, widely distributed, and live well in captivity. Hundreds of papers and
books have been written about them and important facts have been discovered
by means of them. The frogs most frequently used are: the leopard frog, Rana
pipiens (Fig. 34.6), distributed from the east coast through the western states
except California; and the bullfrog, Rana catesbiana, the largest North Amer-
ican species, with a natural range through the eastern half of the United States
and an introduced range in the west.
Ecology and Life History. Frogs are limited to lands where there is enough
moisture. They do not live in deserts, in frigid climates, or salt water. In tem-
perate climates, they commonly leave the water after their spring breeding
Fig. 34.6. Leopard frog. Rana pipiens. the frog of the laboratories. It is also
the mainstay of the edible frog business that supplies hotels and markets for
which an average expert can dress 1000 frogs per hour.
688
EVOLUTION OF ANIMALS
Part V
season, scatter off by themselves, and spend the summer in meadows and moist
woods. Their haunts vary with the species, but all are moisture seekers.
Food. Adult frogs eat any animal they can get, of any size that they can
swallow whole, mostly invertebrates — insects, spiders, earthworms, snails, fish
fry, and their own tadpoles. The latter are strict herbivores that rasp and comb
soft water plants (Fig. 34.7).
Fot body
Fat body
Fig. 34.7. Upper, mouths of tadpoles. Left, green frog, Rana clumitans: right,
spade foot toad, Scaphiopus holbrooki. Tadpoles of frogs and toads live on soft
plant food collecting it with the chitinous scrapers and combs that surround their
mouths. Different species have such different patterns of scrapers that they are
used as recognition marks. Lower, the relatively short intestine of a carnivorous
adult newt {left) and the long watchspring coil of the intestine of the herbivorous
tadpole of a bullfrog (right). The adult newt, Triturus, length 4 inches, lives on
aquatic worms, crustaceans and insects. The tadpole, length 2 to 4 inches, feeds
exclusively on algae and other soft plants of which it requires a large amount.
(Upper, courtesy, Wright: Life Histories of the Frogs of the Okefenokee Swamp,
Georgia. New York, The Macmillan Co., 1931.)
Chap. 34 AMPHIBIANS 689
Frogs have little or no defense against predators. Diving beetles suck out the
body juices of the tadpoles and catfishes swallow a hundred of them at a gulp.
Turtles, snakes, herons, raccoons, and man all prey upon them. They are used
in hotels, markets, and laboratories, a total that goes into billions per year,
chiefly leopard frogs.
Parasites and Diseases. Leeches clamp to their bodies and suck out their
blood; molds and bacteria invade their moist skin; flukes, roundworms, and
protozoans flourish within their bodies. One of the worst calamities is their
wholesale destruction due to the drying out of swamp lands by dams and
irrigation.
Seasonal Life (Fig. 34.8). Frogs and toads are like rabbits: in front they
stand; behind, they sit in continual readiness for a take-off. In the ponds, frogs
lounge with their nostrils, valves open, just above the surface. The hippo-
potamus, also semi-aquatic, does the same thing. In winter, when they are
under water they depend upon skin breathing and the lungs are nearly emptied
of air. This is adequate for long periods when the metabolism of the body is
low and the demand for oxygen decreased.
In temperate climates, frogs spend the winter in damp protected places,
mainly in the muddy bottoms of ponds and in swamps. About mid-winter,
preparation for the early breeding period begins in the reproductive organs,
supported by food stored in the fat bodies. As spring approaches, the former
increase in size and maturity. Secondary sex characters, the horny thumb pads
of the males and the vocal sacs, are prominent during the breeding season. At
its height, even the most solitary frogs become social as they gather in the
ponds in full croak. The male leopard frog and to a lesser degree the female
inflate the vocal sacs, one over each shoulder, swelling them larger and larger
by drawing air across the vocal cords as the croak is repeated. Then the air is
suddenly drawn into the lungs and the sacs collapse. The breeding season
reaches its climax in mating, and the release and fertilization of the eggs. A
leopard frog produces up to 5000 eggs per season deposited in the water in
masses of about 500 each.
After the breeding period, great changes take place in these frogs that for
months have taken no food and for weeks have been congregated in their an-
cestral home in water. Promptly they leave the ponds and scatter, each a soli-
tary land animal. This is the beginning of the summer-feeding period when fat
is accumulated in the fat bodies and glycogen is stored in the liver and muscles.
With the chill nights of autumn they stop feeding, and seek winter quarters in
the swamps, crowded together by dozens, even hundreds. For the second
period of the year they are social animals, urged by sex at one time and by
cold weather at the other.
Life History. For the development of the embryo, and the transformation
of the tadpole into the frog, see Figure 34.8 and Chapter 19.
690 EVOLUTION OF ANIMALS Part V
Structure and Function. Form, Covering, and Color. The flattened head
of a frog still suggests that of its ancient ancestors. A frog's neck is short, and
like those of other aquatic animals, the fishes and whales, is not marked off
from the rest of the body. The body is short. Its complete lack of external tail
is rare among lower vertebrates. As in all vertebrates, the skin consists of an
outer epidermis and inner dermis (Fig. 34.9). Throughout their lives frogs
molt the outermost dead part of the epidermis, casting it off in a whole piece
every few weeks except in winter when living processes are slowed and molting
is almost or entirely absent. The shed skin is pulled over the head like a sweater
and swallowed. The comparable layer of human skin is constantly shed, but
less dramatically in scalelike bits. Numerous mucous and poison glands orig-
inate in the epidermis but project down into the dermis where they are nour-
ished by the blood. Mucus becomes a lifesaver by moistening the skin and
slowing evaporation. The deeper-lying dermis includes blood vessels that func-
tion in skin respiration, many small nerves, smooth muscle cells, connective
tissue and pigment cells.
le^^
^(7/. _ Of
Fig. 34.8. Life history of the green frog, Rana clamitans, that transforms and
goes onto the land in its second year.
Chap. 34
AMPHIBIANS
691
> Epidermis
! — Mucous Gland
-"Poison" Gland
3.^' Loose Connective Tissue
of Derma
] — Pigment Cell
,,- Subcutaneous
Connective Tissue
— Muscle
Fig. 34.9. Section of tlie frog's skin. (After Haller. Courtesy, Walter and Sayles:
Biology of Vertebrates, ed. 3. New York, The Macmillan Co., 1949.)
Frogs are prevailingly green and brown, with light underparts, usually white,
often yellow. They may be paler or darker depending on the physiological
condition of the frog and its response to the environment. Although melanin
or black pigment occurs in the epidermis, the shifts of color in amphibians are
primarily due to changes in certain dermal cells called chromatophores. In
these cells, the pigment may be dispersed throughout the cell or concentrated
in its center (Fig. 34.10). There are three kinds of chromatophores arranged
from without inward in the following order: lipophores with yellow or reddish
pigment, the carotene like that in carrots; guanophores holding guanin crystals
(allied to uric acid) that reflect blue when against a dark background; and
melanophores containing brown or black pigment and always lying deepest in
the dermis. By their contraction or expansion, chromatophores hide others
from the light or expose them to it. The skin is green when the expanded black
pigment gives the guanin crystals the dark background against which they
reflect blue and the yellow pigment is expanded (Fig. 34.11). The blue and
Fig. 34.10. Black pigment cells (melanophores) of frog skin. A, with pigment
dispersed. B. with pigment concentrated in the body of the cell and the processes
appearing shrunken. (After Hewer. Courtesy, Noble: Biology of the Amphibia.
New York, McGraw-Hill Book Co., 1931.)
692
EVOLUTION OF ANIMALS
Part V
colorless
ow
ii^Mr- blue
black
SECTIONS OF SKIN TREE FROG
Fig. 34.11. Sections of skin cells of a tree frog (Hyla), showing the relations
of the pigments and blue-reflecting crystals when the skin is bright green, dark
green, and yellow. Yellow pigment is contained in the cells next to the outer
skin cells or epidermis. In the next layer inward the cells contain blue-reflecting
crystals (guanin). The cells below these contain black pigment. A, bright green.
The yellow pigment is expanded. The cells with the blue-reflecting crystals are
covered by black pigment on one side. B, dark green. The yellow pigment is only
slightly expanded. The black pigment covers much of the cells that contain
crystals. C, yellow. Black pigment is greatly contracted. Yellow pigment is ex-
panded and blue-reflecting crystals are irregularly arranged. In brown, not shown
here, the crystals are almost covered and black and yellow are expanded, with
black dominant. (After Noble: The Biology of Amphibia. New York, McGraw-
Hill Book Co., 1931.)
yellow produce the green. The skin is yellow when light is reflected only from
yellow pigment; the black pigment is then contracted and fails to give the
guanin crystals the dark background. Brown color occurs when black pig-
ment covers the guanin crystals; black and yellow are reflected and mixed.
The association of endocrine secretions and color changes is discussed in
Chapter 15.
Skeleton. The skull is roughly triangular with bones firmly joined except
the loose attachments of the lower jaw. The cranium, a narrow bony box that
holds the brain, is similar in shape to the fish's cranium and a contrast to the
Chap. 34 AMPHIBIANS 693
high-domed human one (Fig. 34.12). Between the cranium and upper jaw
are the capsules that hold the sense organs for smell, hearing, and sight. The
relatively enormous cavities for the eyes have no bony floor and, when the
eyelids are closed, the eyeballs bulge down into the mouth cavity, seeming
about to be swallowed. An opening in the posterior end of the cranium (fora-
men magnum) makes the cranial cavity continuous with the canal (neural
canal) in the vertebral column. The skull can be revolved only slightly on the
first and only vertebra of the neck.
The vertebral column consists of nine vertebrae and the urostyle which func-
tions as a balance rod swung in the crotch of the pelvic girdle. It represents a
number of tail vertebrae now fused together and unrecognizable, but believed
to be the fused remains of the vertebrae of external tails in ancient amphibians.
The human skeleton also carries the remains of a once external tail that still
shows in the fused vertebrae of the coccyx.
The pectoral girdle forms attachment places of the forelimbs and an almost
complete circlet of the body over the heart, lungs, and liver. It supports part
of the body's weight but is fastened to the vertebral column only by muscles
and ligaments which allow it to slide and act as a shock absorber in jump
landings. Dorsally, it consists of the flat shoulder blades (suprascapulas) that
Nasal opening
Ptialanges
Metacarpus — '
Carpus (wrist)
Metatarsus
(Ankle
Femur
Tibio- tarsus
Tarsus
Premaxilla
Maxilla (upper jaw)
Orbit (eye)
4^4 — Fronto parietal
(broin case)
Atlas
Scapula
7 Vertebrae
Urostyle
Iliunn
sctiium
Fig. 34.12. Skeleton of the frog.
694 EVOLUTION OF ANIMALS Part V
cover the second, third, and fourth vertebrae. Joined to these are the coracoids
and clavicles; the latter known in man as the collarbones, in chickens as the
wishbone. The upper bone (humerus) of the foreleg fits into a cavity where
the coracoid and clavicle come together. The pelvic girdle is formed by the
long innominate bones each composed of three pieces, the ilium, pubis, and
ischium, that are joined together to form the sockets for the femurs of the hind
legs. The spread of the anterior end of the girdle accounts for the hump back
of the resting frog. The solitary sacral vertebra is the only anchor for the frog's
pelvic girdle and appended hind legs. This arrangement provides for the re-
bound needed in the rear of a jumping animal. It does not support weight like
the human pelvic girdle with its attachment to the fused sacral vertebrae (Fig.
34.12).
The arrangement and in general the number of the bones of the fore and
hindlimbs of the frog are similar to those of human limbs. In the forelimb,
however, the radius and ulna are permanently crossed. A frog cannot turn its
forefoot, "palm up."
Muscles. The main kinds of muscular tissues and arrangements of muscles
are discussed in Chapter 10.
Body Cavities. Body cavities are bounded by the body wall, by mesentery
or other membranes, or by combinations of these. In the frog, the main ones
are the pericardial cavity containing the heart and the pleuroperitoneal cavity.
The latter is called pleura from the membrane that covers the lungs and lines
the spaces surrounding them, combined with peritoneum, the lining of the ab-
dominal walls (Fig. 34.13). The peritoneum is a transparent, moist, shimmer-
ing membrane that continues, as part of the mesenteries, around the stomach,
intestines, and other abdominal organs and forms a partial capsule about each
kidney (Fig. 34.14). It is so thin that tissue fluid filters through it and, becom-
ing the coelomic fluid, keeps the surfaces of the organs wet and slippery. Folds
of the intestine and lobes of the liver slide upon one another. The internal
organs of a breathing animal are slightly but continually moved.
Food and Digestion. With its tongue, a frog jerks small animals into its
mouth and throws them down its throat (Fig. 34.15). It clutches larger ones
by its maxillary teeth and by vomerine teeth on the roof of the mouth. The
movement of material through one or another part of the alimentary canal is
aided by contraction of muscles (peristalsis), lubrication by mucus, and down-
ward lashing of cilia. The pharynx begins where the mouth region narrows
backward at the level of the internal nares, and ends by gradually merging
into the esophagus. Its lining bears constantly lashing cilia. If powdered chalk
or small bits of cork are scattered on the roof of the pharynx they will instantly
begin moving into the esophagus. The latter is short, capable of great exten-
sion, with strong muscular walls that contract peristaltically and urge the food
over the slippery lining.
Chap. 34
AMPHIBIANS
695
Pericardium
Pericardium
Septum
Lung
Pleural
cavity
Liver
Peritoneal
cavity
Pleuroperifoneal
cavity
Fig. 34.13. Outlines of the body cavities of frog and man. A, in the frog, the
pericardial cavity contains the heart; the pleuroperitoneal cavity contains lungs,
alimentary canal and associated glands, and reproductive organs. There is no dia-
phragm. B, in man; the pericardium contains the heart; the thoracic cavity is
divided into two pleural cavities, each holding a lung; the abdominal cavity is
separated from the pleural cavities by the diaphragm.
Storage, digestion, and absorption of food are carried on by the stomach
and intestine (Fig. 34.16). The stomach is a pouch for temporary storage.
Muscles in its walls squeeze and mix the food, and cells in the lining secrete
the gastric juice, which begins the chemical break-up of proteins. Stomachs
are not essential to life, but wild animals must eat when food is available and
temporary storage in the stomach is important. Frogs may find a pond swarm-
ing with mosquitoes on one day and none the next. While food is being mixed
Kidney
Gonad
Peritoneol
lining
RELATION OF PERITONEAL LINING
TO ABDOMINAL ORGANS
Fig. 34.14. Cross section of the body of a male frog taken through the
abdomen showing the peritoneal cavity and the kidneys. The peritoneum covers
them as it does the other abdominal organs.
696
EVOLUTION OF ANIMALS
Part V
by the muscles of the stomach, the cardiac valve at the upper end and pyloric
valve at the lower end keep its contents from escaping in either direction. In
the meantime, the gastric juice flows into it. This contains acid that softens
shell and bone, and the enzyme pepsin which begins the digestion of proteins,
converting them into proteoses and peptones. When a sufficient stage of soft-
ness and acidity has been attained, the food mass is passed through the relaxed
pyloric ring into the intestine.
This is divided into the relatively long small intestine, in which digestion is
completed and digested food absorbed, and the shorter large intestine, in which
Fig. 34.15. Action of the tongue when a frog catches a fly. (Courtesy, Tinbergen:
Study of Instinct. London, England, Oxford University Press, 1951.)
water is absorbed from the residue of indigestible matter. Like other parts of
the alimentary canal, both intestines are attached to the dorsal wall by mesen-
tery. The acid food mass entering the small intestine immediately stimulates
glandular cells in the lining to produce the hormone, secretin. This soon enters
the circulation, reaches the pancreas and stimulates it to produce its digestive
secretion, the pancreatic juice. The pancreas and liver pour their secretions
through the common bile duct opening into the first loop of the small intestine,
the duodenum. The pancreas performs two functions; the bulk of it produces
the digestive fluid called pancreatic juice, and islets of cells within it form the
hormone, insulin. The pancreatic juice, able to act in the alkaline conditions
within the intestine, affects all classes of foods and virtually completes diges-
tion. It does this mainly by three enzymes; trypsin that breaks proteins into
peptones; amylase that changes starches into sugars; and lipase that separates
fats into fatty acids and glycerol. Cells in the intestinal lining also secrete diges-
tive enzymes, the most important of these (erepsin) breaks peptones to amino
acids, the basic constituents of proteins.
In all these processes, molecules of the food substances become smaller and
Chap. 34
AMPHIBIANS
697
Nasal covify
Brain
Testis
Adrenal gland
Kidney
Urinory
bladder
left leg
Pancreas' Small intestine
Fig. 34.16. Frog showing the relative positions of systems.
able to pass through cell membranes as they could not have done before.
Finally simple sugars, fatty acids, glycerol and amino acids are absorbed
through cells in the intestinal lining. The fats are taken up by the lymph, the
sugars and amino acids by the blood plasma, and all are distributed by these
fluids. The vertebrate liver is only indirectly a digestive gland (Fig. 34.16).
It is an excretory organ that picks waste substances from the blood and pre-
pares them for elimination, the nitrogenous waste into urea and the pigment
of worn-out red blood cells into bile pigments. It is a storage place for an
emergency food (glycogen). It produces bile that carries away waste pigments
and certain other waste products and performs important functions in the in-
testine connected with the digestion and absorption of fat. Bile aids digestion
indirectly because it stimulates the enzymes of the pancreatic juice by creating
the alkaline environment necessary for them to act. It is a lubricator and easy
slipping is essential. Excess bile is stored in the gall bladder. The liver is in
short a strainer and balancer of the blood content, having also an indirect but
essential part in digestion.
Peristaltic contractions gradually move the undigested residue of the food
into the large intestine. Its walls absorb water from this, contract upon it, and
eventually force it into the cloacal chamber and out of the body through the
698 EVOLUTION OF ANIMALS Part V
external or cloacal opening. This opening is usually called the anus, but this
term does not homologize the structure with higher vertebrates in which the
term anus always signifies the external opening of the intestine only.
Cooperating Fluids — Blood, Tissue Fluid and Lymph. Circulating
blood transports substances to cells where they are needed and away from cells
to which they are a burden. Like other vertebrates, frogs have three body
fluids; the tissue fluid that is in direct contact with the cells and through which
all substances must pass in order to reach them; and the circulating blood and
related lymph in their respective vessels. All three fluids are dependent upon
the water content of the body, especially so in frogs.
The blood consists of fluid plasma and cells. Its general functions are the
transport of oxygen and carbon dioxide, food and water, waste substances of
metabolism, and hormones. Although largely water, the plasma also includes
blood proteins, salts, and metabolic products. On account of the frog's low
temperature, its plasma carries more oxygen in solution than that of the warm-
blooded birds and mammals.
The red cells (erythrocytes) are relatively large and each is bulged out by
its prominent nucleus. There are about 400,000 per cubic millimeter, most
abundant just before the breeding season, a relatively small number compared
to the four to five millions per cubic millimeter in human blood. Their small
surface exposure and the space taken up by the nucleus combine to reduce
their efficiency in carrying oxygen. In certain salamanders (Batrachoseps)
many red cells lose their nuclei as they mature just as mammalian red cells do,
but this is very rare in amphibians. Red blood cells ordinarily develop in the
spleen. Only when metabolism of frogs is at its height in spring do red cells
arise in the red marrow of bone as in mammals. The white cells (leucocytes)
are colorless and nucleated, about 7000 per cubic millimeter of blood. Spindle
cells (thrombocytes) are nearly twice as numerous as the white cells and ex-
tremely minute, disappearing from blood which has been shed for any length
of time.
Blood Vessels and Circulation. In the frog and with few exceptions in
the vertebrates in general, blood circulates within a system of vessels, the
heart, arteries, capillaries, and veins. Lymph flowing through tubes and open
spaces provides fluid with a return route to the heart, an alternative to that of
the veins. In the frog, the characteristics and functions of the three types of
blood vessels are similar to those of other vertebrates. The reader is referred
to the discussion of these in Chapter 12 and to figures 34.17, 34.18, and
34.19.
Heart. The frog's heart is a muscular pump that pushes the blood through
blood vessels, but does not affect it in any other way. It is enclosed in a thin
but strong membranous sac, the pericardium, containing just enough fluid to
let the heart slip easily as it beats.
Chap. 34
AMPHIBIANS
699
Fig. 34.17. Networks of blood vessels in the web of a frog's foot, a, the
arterioles; v, venules and the capillaries between them; x, direct connections be-
tween arterioles and venules; pigment spots are scattered along the capillaries.
(Courtesy, Maximow and Bloom: Histology, ed. 6. Philadelphia, W. B. Saunders
Co., 1952.)
The frog is midway between fishes and higher vertebrates and its heart is
midway between the two-chambered heart of fishes and the four-chambered
hearts of reptiles, birds, and mammals. It contains two auricles and a single
ventricle (Figs. 34.18, 34.19). On its dorsal side is an important entrance
chamber, the sinus venosus, to which three great veins bring blood from all
parts of the body except the lungs. The auricles have thin, elastic walls
strengthened by narrow bands of muscle. The right one, larger than the left,
is separated from it by a partition. The ventricle has a relatively very thick
wall containing interlacing muscles. It is separated from the auricles by a par-
tition whose location is indicated on the outside by a prominent constriction.
On the ventral side a great artery, the truncus arteriosus, is the only exit for
the blood. It runs forward a short distance and divides into two trunks, the
right and left aortic arches, each of which splits into three branches that supply
the entire body.
700
EVOLUTION OF ANIMALS
Carotid arteries
Part V
Pulmonary
vein
Jugular vein
Cutaneous artery
Left
Pulmonary
artery
^Subclavian rein,
Intestine
Renal
portal
vein
Iliac artery
Fig. 34.18. Circulation of blood in the bullfrog. Veins in black. (Courtesy,
Wolcott: Animal Biology, ed. 3. New York, McGraw-Hill Book Co., 1946.)
Circulation of Blood. Blood containing various substances from the body is
poured into the sinus venosus which opens into the right auricle. At the same
time, well-oxygenated blood flows through the pulmonary veins into the left
auricle. Both auricles then contract and force their contents onward into the
ventricle. Blood is kept from going back into the sinus by blood behind it,
which pours in from the veins, and from going into the pulmonary veins by
the pressure of the distended wall of the auricles against their openings. Well-
oxygenated blood fills the left side and sparsely oxygenated blood the right side
of the ventricle with blended blood between. The ventricle then contracts.
With the valves into the auricles closed behind it, the blood takes the only free
road, into the truncus arteriosus. As it does so it passes the semi-lunar valves,
three soft cups, and the current approaches from beneath and completely
flattens them. Muscles in the truncus contract upon the blood and it fills the
cups behind it bringing their soft edges together. This creates a backstop.
Muscular contraction continues in a wave over the arteries of the body.
Blood with a low-oxygen content enters the truncus from the right side of
Chap. 34 AMPHIBIANS 701
the ventricle. It takes the path of least resistance and enters the pulmocutaneous
arches to the lungs and skin (Figs. 34.18, 34.19). The next to enter is the
partially mixed blood from the central part of the ventricle and this goes into
the systemic arches, the pair that offers next least resistance. The carotid
arches that supply the head region receive the remainder, the blood from the
left side of the ventricle, that carries the best supply of oxygen. The twisted
ribbon of tissue (longitudinal or spiral valve) in the truncus has been held
renal
portal vein
cutaneouj
artery
fiu/monart/
artery
Fig. 34.19. Circulation of the blood in a vertebrate with two auricles (atria) and
one ventricle as in the frog. (Courtesy, Curtis and Guthrie: General Zoology, ed. 4.
New York, John Wiley and Sons, 1947.)
important in keeping the blood rich in oxygen from that less well supplied
with it. This has not been supported by some recent experiments.
The circulating blood of the frog makes two partially separated circuits,
each one passing through the heart. In one of these (pulmonary), the blood
flows from the heart to the lungs and back to the heart. In the other (systemic
circulation), the blood flows to all parts of the body, except the lungs, and
returns to the heart. Since it is constantly shifting, all of the blood is able to go
through each route very often.
Lymph and Lymphatics. Lymph is a watery fluid similar to the blood
plasma. It contains colorless cells, the lymphocytes, but no red blood cells.
Several fluids contribute to its content — the plasma of the blood, tissue fluid,
and, in the frog, extra large quantities of water. It is contained in tubes, in
spaces between the tissues (lymph sinuses), and in lymph hearts. In frogs, the
lymphatic system is especially important and conspicuous. The smallest lymph
vessels have blind ends. They form networks of capillaries which join larger
and larger vessels and finally one or more main trunks that open into the veins.
Some lymph vessels are broken by lymph sacs in which the lymph is in direct
contact with the tissues. Such sacs are located directly beneath the skin, almost
surround the body and sometimes become pillowed out by abnormal accumu-
702
EVOLUTION OF ANIMALS
Part V
lations of the fluid (Fig. 34.20). In the common species of Rana, there are four
lymph hearts, each of them a two-chambered pump which forces lymph into
the blood stream through openings in the vessels.
Respiration, Breathing, and Voice, Properly speaking, breathing is ex-
ternal respiration and the chemical changes in the cells constitute an internal
respiration.
Breathing. Floating with only its nostrils above the surface, a frog breathes
air and takes oxygen from it by way of its mouth and lungs. It also takes
oxygen from the water through its skin. In winter, when there is less oxygen
demand, skin breathing alone is sufficient for life. The breathing organs of the
adult frog, lungs, skin, and lining of the mouth cavity, are abundantly supplied
with blood vessels. The lungs are thin elastic sacs with low internal folds that
greatly increase the surface between which the capillaries extend (Figs. 34.16,
13.9). The lining of the lungs is continuous with that of the larynx into the
alimentary canal. The lungs branch from a hardly perceptible trachea. Their
outer covering is continuous with the lining of the body cavity. Nerves, con-
nective tissue, and pulmonary arteries, veins, and connecting capillaries are
outspread between the covering and lining of the lungs.
As a frog breathes, the floor of its mouth rhythmically rises and falls, a
throat-breathing in which the capillaries of the lining of the mouth and throat
Caudal
Lymph
Heart
Cranial
Lymph-
Heart
Fig. 34.20. Frog's lymphatic system Sacs for the lymph which creates a fluid
coat about the frog's body. The skin has been removed from this frog. The dark
lines represent the boundaries of lymph sacs. Lower, lymph hearts in the frog
(Rana); these are pulsating lymph pumps which keep the lymph moving. (Cour-
tesy, Walter and Sayles: Biology of Vertebrates, ed. 3. New York, The Macmillan
Co., 1949.)
Chap. 34 AMPHIBIANS 703
are exposed to air. Now and then, the frog seems to swallow — a sign of lung-
breathing. Actually it pulls the floor of its mouth downward creating a partial
vacuum and air comes into this through the open nostrils. The flaps over the
nostrils are then pulled down, the floor of the mouth lifted, the glottis opens
and the air escapes the pressure by going into the lungs. At the same time, an
exchange of gases has been going on between air and blood, through the lining
of the mouth. As it exhales, the frog contracts its abdominal wall and squeezes
the lungs. The glottis is pushed open; the flaps over the nostrils are lifted; and
the air escapes. Usually, the skin is moist enough for an exchange of gases.
Experiments have indicated that more carbon dioxide is given off by the skin
than by the lungs.
Voice. Frogs and toads may have been the first animals to use vocal cords.
The sound of their spring choruses still seems to come from ancient marshes.
The vocal cords are two folds of the lining of the larynx, below and parallel
to the glottis. When a frog croaks, it keeps its mouth and nostrils tightly closed
and squeezes air back and forth between the lungs and mouth. During this
performance air escapes through slits in the floor of the mouth into the air sacs
and dilates them into balloon-like resonating organs (Fig. 34.1).
Excretion. Along with essential products, metabolism produces harmful
ones, usually accompanied with water. The waste products may be gases,
solids, or liquids. Carbon dioxide, from the oxidation of carbohydrate and
fatty foods, is eliminated through the lungs of frogs, the gills of the tadpoles,
and the skins of both. The undigested residue of food is not a metabolic
product, except as it contains bile excreted by the liver.
It is important to any animal that a standard amount of water be maintained
in its body, especially so in frogs. The skin, urinary bladder, and kidneys
maintain this. Frogs constantly absorb water from the air and soil, as well as
from the ponds. A relatively large amount passes into the lymph, blood, and
other tissues, and from the kidneys into the urinary bladder. The latter is
actually a water reservoir.
Like those of all vertebrates, the kidneys of the frog are composed of micro-
scopic tubules bound together by connective tissue, supplied with nerves and
closely associated with the blood (Fig. 34,16). The ureter of each kidney lies
along its outer edge and receives the urine from minute collecting tubes which
cross the dorsal side of it. These collecting tubes in turn receive urine from the
kidney tubules which have completed it from urea brought by the blood from
the liver.
Endocrine Glands. The frog's body is under the elaborate chemical con-
trol of the endocrine glands that produce secretions which pass directly into
the blood. Some of these influence another gland or structure; others affect the
whole organism, its behavior, rate of growth, and symmetry. The endocrine
glands of vertebrates are discussed in Chapter 15.
704 EVOLUTION OF ANIMALS Part V
Nervous System — Cellular Control, Perception. Nervous and en-
docrine systems cooperate, with the nervous system taking the lead in quick
actions. The nervous system is divided into three closely associated divisions:
the cerebrospinal, brain and spinal cord; the peripheral, all the nerves which
extend to and from the brain and cord and connect them with the sense organs,
muscles and outer parts of the body; and the autonomic (involuntary) division,
the nerves that carry messages to and from the digestive, respiratory, and
circulatory systems and the glands (Fig. 34.21). All three divisions work to-
gether to make a unified animal.
Cerebrospinal Division. The narrow cranium and the bony tube made by
the vertebrae form a first line of defense for the brain and cord. Within this
are other covers, the meninges. The colorless cerebrospinal fluid circulates
slowly about the cord, within its central canal, and through the ventricles of
the brain. Oxygen is supplied from this fluid as well as from the blood.
Spinal Cord. The spinal cord is a tube with relatively thick walls and a
minute central canal which continues into the brain where it widens into the
'Spinal cord
Sympathetic
trunk and
Sciatic nerve
Sciatic plexia
Fig. 34.21. Nervous system of the frog, ventral view; the brain and cord and
their branches; the sympathetic nerve trunks (part of the autonomic system) lie
on either side of the cord and the branches join the spinal nerves. Cranial nerves,
Roman; spinal nerves, Arabic. (Courtesy, Wolcott: Animal Biology, ed. 3. New
York, McGraw-Hill Book Co., 1946.)
Chap. 34 AMPHIBIANS 705
ventricles (Fig. 34.22). The outer part of the tube wall contains long processes
of nerve cells (white matter), whose fatty sheaths cause the whiteness. The
inner part, like a letter H surrounding the central canal, contains the bodies
of nerve cells and looks pearly gray (gray matter). The central canal is a
remnant of the open groove which was present in the brain and cord during
the early development of the nervous system (Chap. 19). The cord extends
backward from the opening in the cranium (foramen magnum) to the seventh
vertebra where it tapers into a fine thread of non-nervous tissue, the filum
terminale (Fig. 34.21). Like the nerve chain of the bee, the frog's nerve cord
is in an evolutionary process of shortening. At the levels of the front and hind
legs, it is enlarged by the large number of nerve cells and nerve cell fibers in-
volved in the movement of the legs. There are similar arrangments in other
animals — the ganglia near the bases of the wings and legs of the grasshopper
are also extra large because of the many nerve cells involved with movement.
Brain. During its development, the brain (encephalon) at first forms three
and then five enlargements with constrictions between them. These five divi-
sions are found in all vertebrates. The divisions and the structures they contain
are as follows:
TELENCEPHALON. This is composcd of the olfactory and cerebral lobes,
chiefly the latter (Fig. 34.22). To the former, nerves pass from the sensory
epithelium of the nostrils. Each cerebral lobe contains a cavity (first and second
or lateral ventricles). These are continued forward into the olfactory lobes
Epiphysis
Olfactory
lobe
Optic lobe
Cerebellum
Spinal canal
Cerebra
hemisphere
Spinal cord
Tholomencephalon
Medullo
Cerebellum
Fig. 34.22. Upper, brain of frog, side view. Lower, diagram of ventricles of the
frog's brain — V.l, V.2, V.3, Optic V., and V.4. (Upper, courtesy, Romer: The
Vertebrate Body, ed. 2. Philadelphia, W. B. Saunders Co., 1955.)
706 EVOLUTION OF ANIMALS Part V
and backward through a small hole (foramen of Munro) which opens into
the third ventricle. They are finally continuous with the central canal of the
cord. Thus, all of them are open to the circulation of the cerebrospinal fluid
and there is a serious disturbance if the passage becomes closed.
In frogs, the nerve cells of the cerebral lobes seem to function mostly in the
conduction of nerve impulses from the olfactory lobes to a more posterior
region (thalamus). If the olfactory and cerebral lobes are removed the frog
sits, jumps, and eats as usual, a contrast to the result even of a minor injury
to the cerebrum of a mammal.
BETWEEN BRAIN, DiENCEPHALON (oR thalamencephalon) . Directly be-
hind the cerebral lobes is a folded membrane, the anterior choroid plexus that
forms the roof of the median third ventricle. Its large blood supply is important
to the brain which, like the human brain, has work to do. The pineal stalk, a
delicate stemlike process, reaches up to the cranium and, in the skin above it,
is marked by the brow spot. These structures are remains of a third eye present
in the ancestral amphibians. The optic nerves from the eyes reach the dien-
cephalon just below the third ventricle. All the processes from nerve cells in
the right eye cross over to the left side of the brain, and those from the left
eye cross to the right side thus forming the optic cross or chiasma. There are
theories regarding it but the reason for this crossing is not known; in higher
vertebrates, it is only partial (Chap. 17). The sides of the diencephalon are
thickened and form the thalami over which the cell processes of the optic
nerves spread out fan-wise before entering the optic lobes. Behind the optic
chiasma, the floor of the brain projects downward toward the mouth and is
joined to a little mass of glandular cells originating from the wall of the
mouth (Fig. 34.22). This compound structure is the pituitary gland or
hypophysis (Chap. 15).
After the diencephalon is removed with the cerebral lobes, a frog seldom
moves voluntarily. It is completely blind because its optic nerves have been
cut. When placed on a tilted board it will not climb like the frog from which
only the cerebral lobes are removed. Neither can it keep its balance on the
edge of the board. Placed on a rotating disk it will try to adjust itself by turning
its head opposite to the direction of rotation.
MIDBRAIN OR MESENCEPHALON. In fishcs and amphibians, this short section
of the brain stem is expanded on its dorsal side into prominent optic lobes. On
its under surface are two ridges, the crura cerebri, literally the legs of the
cerebral hemispheres. These are composed of cell processes extending from the
medulla to the cerebral lobes. Cavities in the optic lobes communicate with
the slender central passage connecting the third and fourth ventricles (Fig.
34.22).
In lower vertebrates, the midbrain is a coordinating center and impulses
enter it through the nerves from the eyes, ears, and certain other parts of the
Chap. 34 AMPHIBIANS 707
body. Frogs with all of the brain removed except the cerebellum and medulla
can still move about more or less normally, will croak when properly stimu-
lated and can breathe regularly.
METENCEPHALON. This vcry short section is here roofed by the narrow
cerebellum; it is relatively large in higher vertebrates. Experiments show that
it is a center of muscular coordination.
MYELENCEPHALON. The sides and floor of the myelencephalon make up the
medulla oblongata which is composed of nerve cell processes extending to and
from the spinal cord and parts of the brain. Processes of its cell bodies extend
to the autonomic nervous system (parasympathetic) that controls breathing
movements and the action of the heart. It contains the fourth ventricle which
tapers posteriorly into the central canal of the cord. The former is covered by
the posterior choroid plexus, and in freshly killed frogs it is colored red by its
abundant capillaries.
After all of the brain except the medulla has been removed, a frog is in-
active apparently with comfort. It will swallow food placed well down its
throat and, properly cared for, may live for some time. Removal of the whole
medulla kills the animal since this region controls the breathing movements,
contraction of the walls of the blood vessels and the action of the heart.
Spinal and Cranial Nerves. The spinal and cranial nerves are the roadways
over which pass all the countless messages of the frog's awareness of and ad-
justment to its surroundings. Ten pairs of spinal nerves branch from the sides
of the cord and extend out through openings between the vertebrae (Fig.
34.21 ). Each nerve has two roots. The dorsal or sensory root contains nerve
cell processes (afferent) over which nerve impulses from sensory cells such as
touch pass into and up the cord. The bodies of the cells over which the im-
pulses go are grouped together in a ganglion on the dorsal root. These ganglia
are covered by white chalky deposits, the calciferous bodies, pouches of the
dura mater filled with granules of calcium carbonate. The ventral or motor
root of the same nerve contains processes over which impulses, initiated in the
brain or cord, pass from cells in the cord to the muscles and direct their move-
ment. Processes of sensory cells and motor cells lie side by side in the same
spinal nerve, but impulses from the skin always come in on the sensory ones
and impulses from the cord to the muscle always go out over the motor ones.
It is a strictly one way system, like messages passing one another on different
telephone wires. Impulses go over the complete sensory-motor circuit when
something touches a frog's foot and it moves away.
Ten pairs of cranial nerves branch from the brain of the frog. Some are
sensory, like the olfactory nerves, others are motor such as the oculomotors
through which the movements of the eyeballs are controlled (Figs. 16.13,
34.22). Most of the cranial nerves have single roots and do not occur at
such regular intervals as the spinal nerves.
708 EVOLUTION OF ANIMALS Part V
Autonomic Nerves. The autonomic nerves regulate involuntary action,
routine functions such as those of muscles in the alimentary canal, blood
vessels, and glands. Fibers of the autonomic nerve cells enter and leave the cord
and brain in the cranial and spinal nerves.
The whole autonomic system was formerly called the sympathetic system.
That term is now commonly used for the chains of nerves and ganglia which
serve the viscera. They lie on either side of the thoracic and lumbar vertebrae.
Autonomic is the word used for the entire system with reference to its in-
voluntary nature (Chap. 16).
Sense Organs. The sense organs are described in Chapter 17 and only
particular applications to frogs will be given here.
Sense organs or receptors are cells or groups of cells whose content is
changed or stimulated by particular conditions in the environment. Familiar
ones are the eye and ear; less known are the receptors of cold and heat in the
skin.
The frog's skin is sensitive to touch, to cold and heat, to pain, to acids and
other irritants in each case through different sensory cells. To some degree,
frogs taste through their skins. They can also detect odors under water as well
as in the air. The lateral line organs of balance that are well developed in tad-
poles are absent in most species of adult frogs.
The frog's eye has some markedly fishlike characters. It will not accom-
modate, that is, the shape of the lens cannot be changed nor can it be moved
nearer and farther from the retina to any such degree as the human lens. In
the air, frogs are nearsighted; in the water, they are farsighted; in either
medium they see moving objects best. Because their eyes are located so far
to the sides of the head, frogs cannot easily use both eyes on an object directly
in front of them.
Frogs have a well-developed sense of hearing. They respond to croaks
heard in the distance, also to simulated croaks. Anyone who has disturbed a
populous spring frog pond knows the sudden silence that falls upon it. Then
after a waiting time of complete quiet, one frog raises a solitary voice and, as
if that were a signal, other frogs one after another begin to call. One of the
sure proofs that a frog hears is the quickening of its throat movements when
a bell is rung in a nearby room.
Frogs have a sense of balance. This is located in the semicircular canals
associated with the inner ear (Chapter 17).
Reproduction. Female Organs. In winter, the ovaries are the most con-
spicuous objects in the body cavity. The eggs are then absorbing food from
the blood and approaching full size. Beneath the membrane of each egg a
layer of black pigment partially surrounds the yolk. After the breeding season,
the ovaries are a small fraction of their former size with the eggs of another
season hardly visible.
Chap. 34
AMPHIBIANS
709
MOUTH OF ?-
OVIDUPT
i^VIOUCT
OVIDUCTS
OPENING
OF
VRETER
.^LADDER
CLOACA OP
FEMALE
SIDE VIEW
Fig. 34.23. Excretory and reproductive organs of the frog. Male and female.
Note the vestigial oviduct in the male. (Courtesy, MacDougall and Hegner: Biol-
ogy. New York, McGraw-Hill Book Co., 1943.)
Each ovary is a lobed sac, with its interior divided by partitions into
chambers which are more or less filled with fluid (Fig. 34.23). It is covered
with epithelium continuous with the peritoneum of the mesentery (meso-
varium) that suspends the ovary from the body wall. Blood and lymph vessels
and nerves extend into it by way of the mesentery. The eggs originate from
certain cells in the lining of the ovary; certain others produce endocrine
secretions. As the eggs are enlarged with yolk, they project into the cavity of
the ovary. Cells in the lining of the ovary multiply and form a sac around each
growing egg (Fig. 34.24). Each follicular sac fits about the egg like a grape
skin around the pulp becoming a tighter fit as the egg reaches full size. Finally,
the egg is squeezed out of the sac, through the covering of the ovary and into
the body cavity. This process of ovulation occurs at about the same time for
the hundreds of eggs that mature in one season and leave the ovary within a
short interval. There are several factors which bring this about, among them
the secretion of endocrine glands chiefly of the anterior lobe of the pituitary
(Chap. 15).
After ovulation, eggs fiU the body cavity but only briefly for they begin one
by one to pass into the funnels of the oviducts in a steady procession (Fig.
34.25). The funnels are small and are located on each side of the esophagus.
710
EVOLUTION OF ANIMALS
fiUPTURE A/>£A
Part V
■BLOOD i^sssns
CAvirr OF
OVARY
INNER
MEMBRANE OF.
OVARy
crsT yyALL
FOLLICLE CELLS
VITELLINE
MEMBRANE
OUTER
OVARIAN WALL
PERI TONEUM
RUPTURED
FOLLICLE
EGC EMERGING
FROM FOLLICLE
FOLLICLE CELLS REMAINING WITHIN
POSTOVULATORf FOLLICLE
'SMOOTH MUSCLE OF CrST WALL
Fig. 34.24. Diagrammatic section through a lobe of the frog's ovary. 1,2,3, 4,
and 5 represent stages in the growth of an ovarian follicle (ovum and sac); 6,
the break of the peritoneum, the ovarian wall and the follicular sac; 7, the
emergence of the egg from the ruptured follicle; 8, the follicle after ovulation.
(Courtesy, Turner: General Endocrinology, ed. 2. Philadelphia, W. B. Saunders
Co., 1948.)
The peritoneal lining, the outer surface of the ovary, the liver, and the funnels
themselves all bear cilia, each of which waves its microscopic lash toward the
destination of the eggs in the oviducts. Motion pictures of the funnel region
in anesthetized frogs show the eggs carried inevitably as on a moving stair,
coming to the funnels of the oviducts and toppling into them. The eggs are
pushed through the oviducts by the contraction of their walls. At the same
time, each one is covered with crystalline jelly, just as hens' eggs are coated
with the "white" or albumen. They gradually collect in the expanded part of
each oviduct, the uterus. Eventually the whole mass from each uterus is ex-
pelled at one time, usually while mating. The size and numbers of eggs vary
with the species. In the family Ranidae to which leopard frogs belong, there is
a range from about 350 eggs in certain species to 20,000 in the bullfrog. The
size of the frog does not determine the size of the egg.
Male Organs. The testes are two relatively small bean-shaped bodies (Fig.
34.23). Like all organs in the body cavity their outer covering is continuous
with the peritoneal lining. Its extension out over each testis forms a mesentery
(mesorchium) by which it is attached to the dorsal wall. Each testis is a com-
pact bundle of microscopic, coiled seminiferous tubules. The spermatozoa
develop from cells in their linings, when mature, a sum total of many millions.
They pass out of the testes through threadlike tubes, the vasa efferentia that
extend into the collecting tubes of the kidney which in turn join the ureter.
They finally lodge in an expanded part of the ureter (seminal vesicle) where a
great number of them accumulate for some time before mating.
Chap. 34
AMPHIBIANS
711
Fig. 34.25. Photograph ot the body cavity and ovary of the frog, Rana pipiens,
at the height of ovulation. (Courtesy, Rugh: The Frog. Philadelphia, The Blakiston
Co., 1951.)
The finger-shaped fat bodies, present in both sexes, provide extra food for
the gonads when the sex cells are growing. The secondary sex characters of
male frogs are stouter arm and pectoral muscles and swollen, roughened
nuptial pads on their "thumbs."
Mating is preceded by springtime assemblies and congregational singing,
mostly by the males. The females come to these assemblies a little later than
the males and mating begins immediately. The male rests on the back of the
female with his forelegs around her body and mating pairs float with their
heads just above water. When the female finally expels the eggs, the male dis-
charges the seminal fluid over them. Fertilization occurs at once, and with
this process the first cell of a new individual comes into existence (Chap. 19).
Frogs in Folklore
Frogs have played a prominent part in folk tales and legends. They appear
on tribal crests and in designs wrought on dishes and clothing (Fig. 34.26).
712
EVOLUTION OF ANIMALS
Part V
The Indians of western British Columbia carved them on totem poles believing
that they would prevent the destruction of the poles. They held frogs wise and
helpful to man and beast. The great Thunderbird of the Haida Indians had
two large frogs in his celestial kingdom whose duty it was to croak loudly, to
give warning of the approach of strangers. The Thunderbird tops the totem
pole and the frog gazes upward from below. Humanity's use of totems, very
often animals, began before history and still flourishes, with the American
eagle and the British lion among them.
Fig. 34.26. Blanket border of frogs from a drawing by Chief Charlie Edensaw,
Haida Indian. Masset, Queen Charlotte Islands, B.C. (From Amphibians of
British Cohimbia by C. C. Carl. Victoria, British Columbia, Provincial Museum,
1950.)
35
Reptiles — First Land Vertebrates
The first land animals were reptiles. They were the ancestors of modern
turtles, lizards, snakes, and crocodilians, a small remnant compared with those
that once overspread the earth during the "Age of Reptiles," at least 150
million years ago. In the early part of that era, certain reptiles developed
structures and habits the like of which would eventually be those of birds and
mammals.
The name of the Class Reptilia refers to the creeping habits of many of the
group (Fig. 35.1). Reptiles originated from primitive amphibians that then
and ever since have been bound to water by their unprotected eggs that de-
velop only in watery surroundings. Unlike amphibians, the reptiles made
permanent homes on land and laid their eggs there. In the course of time, the
eggs became truly land eggs with fluid held within them by their shells. Inside
the shell the young reptile was surrounded by membranes that had various
uses. One of these was the amnion, a sac of fluid, the private pond in which
every reptile, bird, and mammal now goes through its early stage of life (Fig.
35.2).
In addition to the all important eggs, there were three other main keys to
reptilian success on land — their skins, respiratory organs, and means of loco-
motion. Necessity of being near a body of water and dependence upon warm
climate are like chains limiting the distribution of land animals. Reptiles broke
the first, but not the second chain. With their low rate of basic bodily activity
and "cold blood" they have continued heavily dependent upon a warm climate.
Only warm-blooded birds and mammals can live on the polar ice.
Three Key Adjustments to Land Life. Skin. The skins of reptiles and am-
phibians are essentially similar except for one great difference. A snake's skin
resists drying; a frog's skin does not. The difference is in the outermost horny
layer of the epidermis (stratum corneum) that in frogs is soft and permeable
to water and in lizards is tough and waterproof (Fig. 35.3).
713
714
EVOLUTION OF ANIMALS
Part V
RoHlesnoke
Fig. 35.1. Types of North American reptiles. Fence Lizard (Sceloporus).
Length, 5 to 6 inches. Gray brown to green. A dozen and a half species ranging
throughout south-central United States and in the west north to Oregon. A com-
mon pet. Horned Lizard or Toad (Phrynosoma). Length 5 inches. Several species
in western United States only. Unlike most lizards they give birth to living young.
Six-lined Lizard. Race runner (Cnemidophorus). Length to 10 inches of which
7 inches is tail. Easily identified by the prominent yellow Hnes in a brown back-
ground. Allied species common in south to south-central regions across the
continent. Gila Monster (Heloderma). Length to 24 inches. Beautifully colored
gray with rose patches and beading. The only poisonous lizard in the United
States. It lives in desert places in the southwest, especially Arizona. Common
Garter Snakes (Entema sirtalis) live in every part of America where snakes
exist, the first to come out of hibernation in spring, the last to go into it in
autumn. With several related species it ranges the north and north-central United
States. Rattlesnakes. There are 15 species of rattlesnakes in the United States
and with one or another of them their range extends over all but the northern-
most part of the country. They are all dangerously poisonous. Snapping Turtle
(Chelydra). Less protected by shell than most turtles, snappers are demons for
fighting and will snap even as they are hatching. Found in ponds. Common
snapper grows to 50 pounds or more. (Courtesy, Palmer: Field book of Natural
History. New York, McGraw-Hill Book Co., 1949.)
Respiration, Reptiles cannot breathe through their skins and they have
no gills. They do have lungs, however, with greater capacity than the most
elaborate amphibian ones. In most reptiles, the heart is incompletely four-
chambered; in crocodilians, it is completely so insuring a supply of better
oxygenated blood.
Locomotion. Reptiles long ago developed legs and speed such as never
Chap. 35
REPTILES FIRST LAND VERTEBRATES
715
Fig. 35.2. Upper, embryo of the painted turtle, Chrysemys picta, enlarged about
3 times. Lower left, embryo of the snapping turtle, Chelydra serpentina, sur-
rounded by the amnion. Lower right, snapping turtles at hatching, about natural
size. Reptiles were the first land animals. Before them all animals had been
bound to the water. Their young could not and cannot now develop without it
but now they have it in a sac. The shelled egg and the amnion, the sac of fluid
that contains the embryo, were the keys to land life for the reptiles. Shelled eggs
are all important to the birds, and the amniotic sac of fluid has continued im-
portant in birds and mammals. Every human being spends his early months in a
pond. (After Agassiz: "Embryology of the Turtle," in Contributions to Natural
History of U.S.A., vol. II, pt. III. Boston, Little, Brown, and Co., 1857.)
had been achieved by any animals before them. Many of the ancient dinosaurs
could run on their hind legs, and dig up roots, pick fruit and fight with their
forefeet (Fig. 35.15). Most snakes can travel rapidly and although they have
no appendages they can climb and swim.
716
EVOLUTION OF ANIMALS
Part V
Non poisonous snake without pit
Poisonous snake with pit.
Section through the interlocked
scales of a rattle
B
Fig. 35.3. Scales of snake. A, head of a non-poisonous pilot snake. B, side of
the body of a snake with smooth scales; the anal region and tail showing the
large ventral scales. C, head of poisonous copperhead snake. The pit between the
eye and opening of the nostril is characteristic of poisonous snakes. D, section
through the tip of the tail of a rattlesnake showing the loosely interlocked scales
which are rattled. (A, B, and C, courtesy, Surface: Serpents of Pennsylvania.
Harrisburg, Penna. State Dept. of Agric, 1906. D, courtesy, Weichert: Anatomy
of the Chordates. New York, McGraw-Hill Book Co., 1951.)
Characteristics of Reptiles. The outer layer of skin is dry and horny, usually
with small scales in lizards and snakes and very large ones (scutes) in turtles
and crocodilians. The ancient reptiles and the modern lizards, alligators, and
others have two pairs of limbs, typically with five toes that end in horny claws
(Fig. 35.5). Their bodies are low slung, adapted to running close to the
ground, to climbing in many lizards, and to crawling in alligators. Limbs are
reduced or absent in some lizards and in all snakes.
The reptilian skeleton is relatively heavy, and contains more calcium than
that of fishes or amphibians. Except in turtles and snakes, the ribs are moved
during breathing much as they are in birds and mammals. Reptiles have a
distinct neck region, and were the first vertebrates that could turn their heads
sidewise. Even the sea turtles breathe chiefly by means of lungs. Eyes and other
sense organs are adapted to life on land, always protected from exposure to
air. The temperature of the body, always the expression of its metabolism, is
low and varies with that of its surroundings. This has limited reptiles to long
hibernations or to life in subtropical regions. For example, Louisiana has
over 70 species of reptiles; northern Alberta has one, a garter snake.
Fertilization is internal, a protection of the sex cells from drying. The eggs
are large with abundant yolk, and in leathery or limy shells. The majority of
reptiles are oviparous, and their eggs are incubated and hatch outside the body.
Some lizards and snakes are ovoviviparous; the eggs, fertilized and later
supplied with shells, are incubated and hatched within the oviducts from which
the shells are later expelled. In essentials, this process is intermediate between
Chap. 35
REPTILES FIRST LAND VERTEBRATES
717
Fig. 35.4. Tuatera {Sphenodon punctatum) has features of the early ancestral
reptiles (Cotylosaurs). A relic from a remote past, existing now only on the islands
near New Zealand. Length, 30 inches. (After Blanchard. Courtesy, Rand: The
Chordates. Philadelphia, The Blakiston Co., 1950.)
the development and hatching of the eggs of birds and development and birth
in practically all mammals (Fig. 19.14).
Like other land animals, reptiles do not go through an aquatic or larval
stage. Living upon the yolk and with the help of the other membranes the
reptile embryo like the bird embryo grows to relatively large size and inde-
pendence before it hatches (Fig. 35.2). As soon as they hatch, snakes take care
of themselves, much better than do chickens.
Orders of Living Reptiles
Modern reptiles are usually classified in either four or five orders, variously
arranged and named by different workers. In contrast to this small number are
the 14 or more orders of ancient ones known only by their fossil remains.
Modern reptiles include:
Order Rhynchocephalia. Only one representative, Sphenodon, a lizard-like
connecting link between ancient and modern reptiles (Fig. 35.4).
718 EVOLUTION OF ANIMALS Part V
Order Squamata. Lizards and snakes.
Order Crocodilia. Crocodiles and alligators.
Order Chelonia. Turtles and tortoises.
Order Squamata — Lizards and Snakes
These reptiles have certain distinguishing structures not intelligible without
special study. The two suborders are easy to separate, since lizards have legs
and snakes do not. However, there are a few limbless lizards which cannot be
distinguished from snakes except by internal structures.
Lizards. In general, lizards are clean vigorous carnivores that earn their
way in the living web of their community. All are interesting. Many are beauti-
ful. The little geckos, numerous in hot countries, run about at night often on
the walls of houses, even on the ceilings to which they hold tightly by their
sticky toe pads (Fig. 35.5). In the flying dragons of the East Indies, the ribs are
Fig. 35.5. Common wall gecko (Tarentolo mauritanicus) of southern Europe.
Length, 6 inches. Geckos are a large group of lizards, four of them native to
the southern United States. Their sticky toe pads enable them to walk on ceilings.
(Courtesy, Guide to the Reptile Gallery, British Museum.)
extended beyond the sides of the body and covered by folds of skin that serve
as wings enabling their owners to take gliding flights from branch to branch
(Fig. 35.6). The wings are gorgeously colored and flying dragons are sugges-
tive of brilliant butterflies.
Chameleon is the common name of one of the most remarkable of the
families of lizards (Fig. 35.7). Because of their ability to change color the
same name is also applied to the chameleons (Anolis) of the southern United
States. The true chameleon however is found in Africa, Arabia, and southern
India. In it, the toes are joined together in two bundles; with these and its
prehensile tail it is a truly non-slip climber of extraordinary agility. It can
thrust out its tongue more than the length of its body, aim with accuracy and
bring back a fly on the sticky tip, all in motions too fast for the eye to follow
clearly. Its ability to change color receives more than its share of fame for it
is equaled or surpassed in this by other species. The lizards found in the
United States include the horned lizard or "horned toad" (Phrynosoma) of
the Great Plains and Rocky Mountains (Fig. 35.1); and one of the only two
Chap. 35 REPTILES — FIRST LAND VERTEBRATES 719
lizards known to be poisonous, the Gila monster (Heloderma) of the Texas
and Arizona deserts, marked with alternate rings of black and coral pink
Fig. 35.6. An unusual display of ribs. In the "flying dragons"
(Draco) of Malaya the ribs support wing-shaped sheets of
skin, folded when the lizard is running, spread when it para-
chutes. In the hooded cobras (Naja), the ribs support the
hood spread when the snake is excited (Fig. 35.8). (Courtesy,
Guide to the Reptile Gallery, British Museum.)
(Fig. 35.1). It has a row of poison glands along the inside of its lower
lip, holds on like a bulldog when it bites, and chews in the poison. It has
a bad reputation but is so conspicuous that human beings are rarely bitten
by it.
Caught by the tail a lizard immediately escapes leaving the captured piece
New World Chameleon
Fig. 35.7. Three well-known lizards. The new world chameleon (Anolis), about
6 inches long, common in southern United States, varies in color from gray to
green. The old world chameleon (Chameleon) may be a foot long including its
prehensile tail and is famous for its changes of color. The chuckwalla (Sauro-
malus), one foot long, is locally known in southwestern United States. (Courtesy,
Palmer: Fieldbook of Natural History. New York, McGraw-Hill Book Co., 1949.)
720 EVOLUTION OF ANIMALS Part V
behind it. Such breaks occur in definite cleavage planes through the middle
of a vertebra. The lost part of the tail is replaced on a simpler plan without
true vertebrae and often with a different kind of scales.
Snakes. Snakes travel on the ground. They also climb trees and the "flying
snake" of India is a glider. Snakes work their way into crevices and holes
made by other animals and the "earth snakes" of southern India are blind
burrowers. They swim easily and a few tropical ones spend most of their lives
in the water. Nevertheless, the real home of the great majority is the surface
of the ground, in touch with earth and plants. There they hunt their prey,
waiting for it or silently slithering after it.
Snakes are superlatively streamlined examples of an efficiency of omission.
Fig. 35.8. Skeleton of cobra. The skeletons of nearly
all snakes are without limbs, limb girdles and sternum.
Pythons are among the exceptions. (Courtesy, Rand:
The Chordates. Philadelphia, The Blakiston Co., 1950.)
Their heads are wedge-shaped without extensions of ears or feelers. They are
without limbs, limb girdles and sternum, the absence of the latter a convenience
since all snakes swallow whole animals, mice, rabbits, or sheep (Fig. 35.9).
Only primitive snakes, the pythons and boas, have vestiges of hind legs.
The complexity of the middle ear is reduced, without a membranous eardrum
and located far back on the head, an advantage since snakes open their mouths
back to their internal ears. There are no movable eyehds, but the delicate
cornea is protected by a hard transparent cover that is shed when the outer
skin is molted. The eye is never left unprotected. There are no vocal cords
and no voice. There is no urinary bladder; metabolic waste is semisolid and
water is conserved as it is in birds.
In spite of all these omissions, snakes have all the essential structures of
such wide-bodied relatives as the turtles. The pairs of lungs, kidneys, and
Chap. 35 REPTILES FIRST LAND VERTEBRATES 721
VERTEBRAL COLUMH';^
-I ANUS
■CLAW
FEMUR -"
CLAW-
Fig. 35.9. Remnants of hind legs and pelvic girdle of a python, indicating that
the ancestors of snakes once traveled on legs. A, ventral external region where
claws extend out between the scales. B, skeleton in the same region. The hip girdle
is represented only by a slender ilium, embedded in the flesh on each side. The
limbs are vigorously moved and the claws are capable of inflicting deep cuts.
Pythons and boas are constricting snakes, some of them 30 feet long, with jaws
capable of opening widely enough to take in a sheep. (Courtesy, Rand: The
Chordates. Philadelphia, The Blakiston Co., 1950.)
ovaries or testes are present, but one lung is in front of the other, one kidney
in front of its mate and so on. It is a tandem series.
Ribs play various parts in the activities of snakes. They stiffen the spreading
hoods of cobras (Fig. 35.8), and urged by the muscles that control them, they
squeeze the still-living animal that the snake has swallowed. Contractions of
muscles in the body wall, contractions of rib muscles, and of those that lift
the ventral scales all take part in locomotion. This is either a glide straight
forward or a curving slither alternately from side to side like a swimming eel.
A snake seems to slide without effort. It is not surprising that Solomon found
"the way of a serpent upon a rock" one of the things that baffled his mind.
Snakes are pure carnivores. Common garter snakes prey upon insects and
other animals up to the size of frogs. Rattlesnakes do likewise and can swallow
small rabbits. The bones of the lower jaw have elastic joints allowing the
necessary great stretch. The snake hooks its teeth into the victim, first on one
side, then the other gradually pulling its mouth over the rabbit. Teeth, espe-
cially the poison fangs, are often broken but partly developed ones behind
them immediately take their places (Fig. 35.10). A snake travels by its tongue
as a dog travels by its nose. Slipping leisurely along with its mouth tightly
closed it explores every object with this ominous, flashing, black and red,
but entirely harmless organ. It is lodged in a sheath in the floor of the mouth
and extended through the small opening formed by a notch in each jaw.
Poisonous Snakes. Of some 2500 living species of snakes about 600 are
more or less poisonous. The venom is secreted by modified salivary glands in
the upper jaw and injected into the wound by the fangs which are grooved or
tubular teeth. The venom contains poisonous proteins whose proportions vary
722
EVOLUTION OF ANIMALS
tp dig iP
Part V
Fig. 35.10. One side of the head of a poisonous snake, with the skin and cheek
muscle removed to show the duct connecting the poison gland with the tubular
fang. When the jaws are opened the fangs drop downward; when they clutch the
contraction of cheek muscles pushes poison into the fangs, ta, dig, tp, ta, muscles;
pg, poison gland; d, duct; g, sensory groove; n, nostril. (Courtesy, Gadow: "Am-
phibia and Reptiles," in Cambridge Natural History, vol. 3. London, The Mac-
millan Co., 1909.)
with the type of snake. Venoms produce two main eflfects. In one, the venom
breaks up the blood cells and injures the linings of blood vessels. In the other,
it attacks the nerve centers especially those of the respiratory system. Anti-
venins are available in certain countries but are in no wise so accessible as they
should be. First aid treatment, however, is described in almost all recent books
about snakes. Anti-venins are prepared by immunizing horses against a par-
ticular poison by gradually increasing injected doses of the venom. The clear
serum of the horse's blood with its antitoxins is then ready to be used to
inoculate patients. Snake venom is one of the most complex poisons produced
by animals and it has not been possible to prepare a general antitoxin for it.
In some cases, an antitoxin works against the venom of only one species, in
others against those of two or more. Snakes of the United States that produce
the most serious poisons are: the western diamond-backed rattlesnake, eastern
diamond-backed rattlesnake, prairie and Pacific rattlesnake, timber rattlesnake,
and water moccasin. If frequency of the bite, not strength of poison and danger,
is considered the copperheads would top this list.
Hibernation. Large numbers of snakes commonly of one species, some-
times of two or three, hibernate in one locality, in various protected holes in the
ground where the temperature stays above freezing. They congregate in autumn,
always in warm places, mate and finally retire for the winter, sometimes dozens
intertwined in clumps in which heat and moisture are conserved.
Order Crocodilia — Alligators and Crocodiles
Crocodilians are the giants among reptiles. They are ponderous, lizardlike
and clothed with exceedingly tough skin and an armor of bony plates overlaid
Chap. 35 REPTILES FIRST LAND VERTEBRATES 723
by horny scales. They are seemingly dull and slow but are capable of lightning
quick attacks (Fig. 35.1 1 ). In past ages, they were widely distributed into the
cooler regions. Now they are restricted to the tropics and semitropics. In
relatively few years excess hunting for eggs, young animals, and skins valued
for leather have dangerously decreased the alligators and crocodiles in Florida
and other southeastern states.
Crocodilians are without exception amphibious. They float partly submerged
in quiet, warm waters, but true to the habit of their group they lay their eggs
on land. They are all carnivorous, the young ones feeding upon fishes, the
older ones upon water birds and mammals. They have pointed teeth and under
jaws with a spring like a steel trap, capable of easily crunching the bones of a
dog. The feet are little used in swimming but the side-swinging of its powerful
tail sends an alligator rapidly through the water. The heart is four-chambered,
the right and left ventricles being separated in crocodilians, but in no other
reptiles. The urinary bladder is absent as it is in birds.
Alligators and crocodiles are essentially similar but the differences between
them are sufficient to place them in separate genera, the two American ones
being Alligator and Crocodilus. The most obvious difference in these two is
in the shape of the head: in alligators broad with a blunt snout; in crocodiles
narrow with a pointed snout (Fig. 35.11), Alligators are hardier, can live
farther north than crocodiles, and are able to hibernate under water as turtles
do. Crocodiles are practically helpless in water at 45° F. and soon drown.
Fig. 35.11. Left, head of alligator, blunt snout. Right, head of crocodile, pointed
snout. (Courtesy, Rand: The Chordates. Philadelphia, The Blakiston Co., 1950.)
724 EVOLUTION OF ANIMALS Part V
Order Chelonia — Turtles
Turtles can be instantly distinguished from all other animals by the shell, a
fortress so large in many of them that they can withdraw into it, head, legs,
and tail (Fig. 35.12). The order consists of over 200 species that breathe air
and lay their eggs on land with some, such as the sea turtles, that spend most of
their Uves in water. In general usage, chelonians are called turtles or tortoises
with little regard for meanings. The most common three types are:
Turtles — Semiaquatic in fresh or salt water, e.g., painted and loggerhead turtles
(Fig. 35.12).
Tortoises — Mainly or entirely land dwellers, e.g., wood turtles.
Terrapins — Edible with market value, e.g., diamondback terrapin.
In Britain, tortoise is applied to land and fresh-water species and turtle to
marine ones.
Ancestry. In the early part of the Age of Reptiles certain ones developed
horny, toothless beaks and bony casings about the body. Their descendants are
the turtles of today.
Shell. This consists of an upper carapace and lower plastron united on each
Fig. 35.12. Sea turtles probably originated from ancient marsh-inhabiting an-
cestors. They live in the warmer seas encircling the globe. Atlantic green turtle
{Chelonia mydas mydas). For the food market the most valuable reptiles in the
world, they have been exterminated from many areas by hunting them in the sea,
and collecting their eggs on land. They are still a staple food in some Caribbean
ports and a delicacy in large American and European cities. The Pacific green
turtle is very similar to the Atlantic species. The weights of green turtles now cap-
tured are from 25 to 200 pounds, formerly 500 pounds was common. (Photo-
graph by Isabelle Hunt Conant.)
Chap. 35 REPTILES FIRST LAND VERTEBRATES 725
side by a bridge of cartilage or bone (Fig. 35.13). The two are usually com-
posed of plates of bone overlaid by a mosaic of flat horny scales. In soft-
shelled turtles the carapace and plastron are partly bone and covered by a
leathery skin (Fig. 35.14). The thoracic vertebrae and ribs are fused to the
bony carapace outside the pectoral girdle. It is as if our shoulder blades
and collarbones were inside our ribs. Since only the vertebrae of the neck
and tail can be moved, the muscles of the body are greatly reduced. Only
those of the neck, legs, and tail are well developed.
The form of the shell varies with the habits of the animal. In land turtles,
it is usually high dom.ed and permits the head and appendages to be com-
pletely protected as in box turtles; in aquatic species it is low, in the snapping
turtle, so small that the head and soft parts are unprotected. The protection
afforded by the shell seems to be correlated with the disposition. Most turtles
are inoffensive, being structurally set up for retirement to their shells under
disagreeable circumstances. On the other hand, those with small or soft shells
snap and bite at the slightest excuse. Snappers are ferocious and will strike
with the speed and fury of a rattlesnake, without the poison.
Breathing. The respiratory system is typical of air-breathing vertebrates,
with nostrils, pharynx, glottis, larynx, trachea, and lungs — the latter containing
Radius
Ischium
Fig. 35.13. Skeleton of a turtle (Cestudo). The living epidermis outside the
bony plates produces the horny shell. During the embryonic development the
processes of the vertebrae and the ribs are fused with the bony plates. (Courtesy,
Wolcott: Animal Biology, ed. 3. McGraw-Hill Book Co., 1946.)
726
EVOLUTION OF ANIMALS
Part V
Fig. 35.14. Florida soft-shelled turtles (Trionyx) are highly active aquatic
turtles in which bony plates are reduced or absent, and the outer covering is a
leathery skin. (Photograph by Isabelle Hunt Conant.)
enough air chambers to furnish an abundance of exposure to air. In spite of
the unyielding shells, turtles appear to breathe somewhat like mammals.
Muscles in each leg-pocket operate like the diaphragm of a mammal, their
contractions enlarging the body cavity and allowing the lungs to expand with
air. During expiration, the viscera press against the lungs and deflate them. In
many aquatic turtles, the walls of the pharynx contain numerous blood vessels
over which water is sucked in and expelled so that the whole structure acts as a
gill.
All female turtles produce eggs either with leathery or brittle shells. These
are usually laid in holes dug by the female in soil or in decaying vegetation in
which heat aids the incubation. The number varies in different species up to
about one hundred. Incubation periods range between two and three months,
being greatly affected by humidity and temperature.
Ancient Reptiles
The story of the great Age of Reptiles is told by their fossilized remains
and by certain descendants that have changed little since then. During that
age reptiles became at home on land, in water, and in the air. Some were small,
but many were giants such as have never existed since. For this period of some
140 million years reptiles dominated the earth, but in spite of them birds,
small mammals, insects, and flowering plants were becoming established. The
reptilian promise of bird life seems to have been dramatic and convincing
Chap. 35 REPTILES FIRST LAND VERTEBRATES 727
while the promise of mammalian life was still hidden in small meat-thirsty
carnivores that ate the large yolk-filled eggs of the reptiles (Fig. 35.2).
Among the earliest reptiles were three types from which a varied host of
animals originated. One side -line of those (Cotylosauria) with sprawling legs,
heavy bodies and remarkable armor were the ancestors of turtles. In another
side line were the mammal-Uke reptiles (Synapsida) that ultimately gave rise
Fig. 35.15. Upper, a small dinosaur, the bird catcher (Ornitholestes), 5 or 6
feet in length, that lived 200,000,000 years ago, here represented in the act of
catching the first known bird (Archaeopteryx). In such agile animals the two
footed pose was finally established along with a carnivorous diet. Lower, a con-
temporary dinosaur, the four-footed Tyrannosaurus, of 50 feet total length, and a
weight 8 to 10 tons. So far as fossil remains show, this is one of the largest
animals that ever lived. Restorations from fossils, painted by C. R. Knight and in
the American Museum of Natural History. (Courtesy, Colbert: The Dinosaur
Book. New York, American Museum of Natural History, 1945.)
728 EVOLUTION OF ANIMALS Part V
to egg-laying mammals (e.g., duckbill), marsupial mammals (e.g., kangaroos),
and placental mammals (e.g., man). The central reptilian stock (Archosauria)
were the seemingly insignificant progenitors of the midgets as well as the giants
of the Reptilian Age. The amphibious dinosaurs were plant feeders that moved
heavily on four legs in their marshy homes. Certain of them were only 30 to
40 feet long, but the fossil skeleton of one measures about 80 feet. The car-
nivorous dinosaurs ran upright on their hind legs, as do some modern lizards,
pricked into speed by hunger and fighting (Figs. 35.15, 35.16). They became
increasingly large and the fossil of Tyrannosaurus shows a monster that reared
upward 19 feet, no doubt using its great teeth and front claws on the unarmed
Fig. 35.16. Drawings of three frilled lizards. (Chlamydosaurus) and another
species (Grammatophora) at right showing the bipedal habit in living reptiles.
Drawings made from photographs of exhilarated lizards running at full speed.
Millions of years ago reptiles walked on two legs. In succeeding ages nearly all
the reptiles abandoned the habit but in the birds that originated from them, walk-
ing was continued with success. An ostrich can run. (Courtesy, Young: The Life
of the Vertebrates. Oxford, England, The Clarendon Press, 1950.)
plant feeders. Some small reptiles were no larger than chickens and squirrels.
A little dinosaur whose fossilized skeleton was about one foot long was dis-
covered a few years ago near South Hadley, Massachusetts, in a region where
footprints of giant dinosaurs are found in the sandstone. Some flying reptiles
were the size of sparrows; some had wingspreads of 20 feet.
After some 140 million years, the Age of Reptiles came to an end and the
hordes of these ruling animals gradually disappeared. A cataclysm or a gradual
change of climate or great competition for food and space between the reptiles
and other animals may have brought about their disappearance. By that time
there was a host of active, warm-blooded mammals with appetites for reptilian
eggs and meat. These mammals originated from one or more strains of reptiles.
From the reptiles also had come the shelled egg which could be incubated in a
dry place, yet the developing embryo would be surrounded by fluid. The
shelled egg and the embryonic membranes were the great contributions of the
reptiles to the evolution of vertebrates.
36
Birds — Conquest or tne Air
Mastery of the Air. Birds are the only animals that have mastered the air.
Human flight is a mastery of machines. Compared with the flights of birds
those of insects are little and near the earth — cautious, fair weather travels;
even those of bats with their sure piloting by supersonic echoes are specialized
and limited. Birds swing into the air with certainty. The golden plover takes
off on an over-sea journey of 2000 miles; geese have been seen flying at a
height of 9000 feet; by slight turns of body and wings hawks ride on the air
currents; bobolinks sing as they fly skyward, then drop, tumbling almost to
the earth with the showmanship of an aviator. Birds travel by day and by night,
in soft weather and through wind above rough seas. They are the world's
greatest migrators (Fig. 36.1).
Birds are animals that have feathers. Their power and skill in flight, their
steering and balancing all depend upon feathers. They are protected from cold
and water by feathers dressed with oil; the ear openings of diving birds,
American loons and Antarctic penguins swimming under water, are roofed with
mats of oily feathers (Fig. 36.2).
Birds are the warmest of all animals. They have a usual temperature of
100° F to 110° F; that of mammals rarely exceeds 98° F to 100° F except
under special conditions. In accord with the body temperature, the rate of their
metabolism is high. The bodily activity of birds is rapid; their metabolic
build-up and use-up is swift. They eat relatively enormous amounts of food,
digest it quickly, and eliminate the waste frequently. The prompt use of
digested food is aided by oxygen from the air in the air sacs as well as in the
lungs.
The largest living birds are the ostriches (Struthis camelus) that may be
7 feet tall and weigh 300 pounds. The condors (vultures) of North and South
America have a wingspread of 10 feet. The smallest bird is Helena's humming-
bird of Cuba; it weighs one-tenth of an ounce. The bodies of birds are wedges
729
730
EVOLUTION OF ANIMALS
Part V
Fig. 36.1. Flying geese. Drawing by Peter Scott. (Courtesy of Peter Scott: Wild
Chorus. London, Country Life Ltd., 1950.)
thrust into the air in flight, streamlined, and slipping forward with no outriggers
to hinder. Walking birds, quail, pheasants, chickens, fold their wings and slip
through underbrush. The diving sea birds do likewise, driving down through
the water with arrowy velocity.
Feathers
Their covering of feathers provides birds with a light, water resistant in-
sulation from cold, a matchless equipment for flight, and a clothing whose
beauty has brought them admiration and relentless killing.
A feather is a complex, exquisitely wrought, yet durable structure composed
of the horny remains of dead cells. Its growth begins, like the scale of a bird's
leg or reptile's body, as a nipple-shaped upgrowth of the skin that soon sinks
into a depression, the future follicle or sac that holds the feather in place. The
Chap. 36
BIRDS — CONQUEST OF THE AIR
731
Fig. 36.2. King penguins. A penguin is a bird that swims with great speed
usually below the surface of the sea, and dives often and swiftly. A land animal,
it is also superbly aquatic. It is the result of an evolution of animals that swam
the sea with fins, that clambered onto the land and lived there for long ages,
climbed trees, and eventually could fly. Sometime in the succeeding millions of
years they returned to the water and now their wings work only like flippers.
Penguins cannot fly. (Photograph courtesy. New York Zoological Society.)
development of a feather is described with other outgrowths of the skin in
Chapter 8. Pinfeathers, the common name for developing ones, are a source
of vitamin requirement. They are enclosed in a horny, pointed sheath, the
"pin" which breaks as the feather grows. The sheath and other castoff bits of
feathers are eaten during the bird's frequent oiling and cleaning of plumage.
These oiled fragments have usually been exposed to sunshine. Thus, while
preening their feathers birds treat themselves to irradiated oil containing the
fat soluble vitamin D.
Feathers do not develop equally on all parts of the body. Except in a few
primitive species including the penguins and ostriches they grow in tracts
separated by bare skin (Fig. 36.3). It is likely that in the early ancestors of
birds the feathers were small and covered the whole body like those of penguins.
Types and Functions. The contour feathers are the larger ones that con-
tribute most to the form of the bird. The outer ones covering the body and
limbs are the flight feathers of wings and tail (Fig. 36.4). The bases of the
contour feathers of wings and tail are usually protected by smaller covert
feathers. The tail is primarily a rudder for steering, but it has many forms and
732
EVOLUTION OF ANIMALS
Part V
uses. A peacock's so-called tail is a gorgeous display of overgrown covert
feathers and the real tail is inconspicuous. The male turkey displays a fan of
tail feathers but, when they are folded, the tail is a rudder.
At hatching, chickens, ducklings, and many other birds are covered with
small fluffy down feathers that shut in the heat of the young animals. Each
one consists of filaments that spring from the tip of a very short quill. Down
is often abundant under the contour feathers especially in ducks and geese.
Filoplumes or thread feathers are the hairlike ones that remain after all the
others are plucked and are removed when a chicken is singed. In the course
of evolution these feathers have lost the spreading vane or web and only a
weakened shaft is left. Whippoorwills, flycatchers and others have stiff bristles
near the base of the beak. A bristle has a short quill, and a slender shaft with
a few barbs at its base.
Colors. White and the colors of feathers including iridescence are due to
structure and pigment. There is no pigment in white feathers. Reflected light
rays strike obliquely against the dried cell membranes and when there is no
pigment, no rays are absorbed — all are shattered and the surface appears
white. The microscopic bubbles in well-beaten albumen or "white of egg" are
white for the same reason; in this respect a white feather and meringue are
nearly related. A blue feather is like the white one except that the cells contain
the dark pigment melanin (Fig. 36.5), Rays of reflected light strike obliquely
Fig. 36.3. Feather tracts of a cuckoo (Geococcyx calif ornicus) . Feathers do
not develop equally on all parts of the body except in primitive birds such as
penguins. (After Shufeldt. Courtesy, Rand: The Chordates. Philadelphia, The
Blakiston Co., 1950.)
Chap. 36
CONTOUR y/^'ffc'?f?^fS
BIRDS CONQUEST OF THE AIR
shaft
733
STRUCTURE Or A
CONTOUR FEATHER
FILOPLUME
BRFSTLE
Fig. 36.4. Types of feathers. Left, the contour feathers provide the main cov-
ering of the bird, estabhsh the outUnes of its figure, and are the flight feathers of
wings and tail. Down feathers are air traps that provide insulation for nestling
birds and for older ones of certain kinds, notably the water birds. Filoplumes or
thread feathers are down feathers without the loose barbs that create the down,
the feathers that are singed off the chicken before cooking. Bristles are wiry
feathers about the mouths of the phoebe and other flycatchers. Right, detail of
a contour feather. Strength is secured by barbs interlocked by barbules. (Left
and upper right, courtesy, Storer: General Zoology, ed. 2. New York, McGraw-
Hill Book Co., 1951. Lower right, courtesy, Rand: The Chordates. Philadelphia,
The Blakiston Co., 1950.)
against the dried cell membranes; some are absorbed by the pigment; others
are shattered and the surface appears blue. The same feather appears dark
gray in direct light because the rays pass through the dark pigment and the
black is predominant. In reflected light the physical effect of structure is pre-
dominant.
There are two general kinds of pigment: melanin— blacks, browns to dull
reddish, all in minute granules soluble in acid; and lipochromes — pure yellow
and pure red, soluble in alcohol or ether. Combinations of different melanins
give the blue gray of the chickadee; those of lipochromes the orange of the
Baltimore oriole. The iridescence in the neck feathers of pigeons is due to the
pigment granules in the feather tips being perfectly spherical so that light
striking against them is broken up and rainbow tints produced.
Molting. This is a gradual, systematic process during which no part of the
734 EVOLUTION OF ANIMALS Part V
body is left bare of feathers. Its details vary in different species and within one
species with age, sex, and other physiological conditions. The molt of a
feather is the stimulus to the growth of another in its place, but the succeeding
one may be different from its predecessor. In its first winter plumage, the male
scarlet tanager is olive with brown wings and tail. In the following spring,
these feathers are replaced by scarlet and black ones. All adults of the smaller
land birds undergo at least one annual molt at the end of the breeding season
when their plumage is entirely renewed. The large and important wing feathers
Fig. 36.5. Cross sections of a barb from a blue feather of an Ant Thrush (7)
and Cotinga (2), greatly magnified. A layer of reflecting cells on the upper sur-
face of the barb is backed by cells containing black pigment. Whether shades of
blue are light or dark depends upon the amount of black pigment that is present
and how it is distributed in the cells. (Courtesy, Allen: Birds and Their At-
tributes. Boston, Marshall Jones Co., 1925.)
are molted less often than any others. Those of the wings and tail are typically
molted in symmetrical pairs making the least disturbance to flight. Molting is
mainly under the influence of the thyroid and pituitary glands.
Special Adjustments
Bill and Food. A bird's bill is its mouth, lips, teeth, and nose, and in use
takes the place of hands. With it birds get their own food, feed their young,
preen and oil their feathers, defend themselves and build their nests (Fig.
36.9).
Of all uses of bills, feeding is the most important. Birds are high-geared
engines running at a rate that in mammals would be fever heat and only
plenty of the most nutritious foods is adequate for them. These are mainly
seeds and animal tissues. Seeds are stored with oil and starch. The meat is of
many sorts, worms and insects, fish, mice and other small mammals, all of it
high in protein.
Bills tell what birds eat and where they find it (Figs. 36.6, 36.7). A crow's
bill is an all-round tool for miscellaneous food. Crows dig up corn, crack nuts,
break eggs, and pick and tear at various refuse. With the same kind of bills,
starlings are also markedly successful in getting a living.
Many of the carnivorous birds are fish eaters. The American bittern of the
watery bogs spears both frogs and fishes. The edges of the bill of the fish-
eating merganser duck are deeply saw-toothed, once in its grip the most slip-
BIRDS CONQUEST OF THE AIR
735
Chap. 36
pery fish is helpless. The pelican scoops fishes into its great pouch as into an
aquarium, lets the water strain away, then tosses the fish in the air to come
down headfirst into its gullet. Other meat eaters, the hawks, eagles, and owls
seize small animals with their feet and tear the flesh with the hooked end of
the upper mandible. Bills may be insect traps, hedged at the base with stiff
hairs as in the phoebe and other flycatchers, widely opened as well as hedged
in the night-flying whippoorwills. Delicacy of sensation is remarkable in the
bills of birds that search muddy pond bottoms with their bills or that probe for
worms in moist earth. The upper mandible of the woodcock is extremely sensi-
tive and so flexible that it can be moved without opening the angle of the jaw.
With its bill driven deep into the soil it feels about and seizes the worm. In
the meantime, its eyes set well back in the head have a clear lookout for
danger, though they are of little use in hunting wrigglers on the ground. The
seed-eating birds, sparrows, goldfinches, cardinals, grosbeaks, pigeons, and
domestic fowls, usually have simple pointed bills, that are strong at the base.
Crossbills pick the seeds from pine cones with special nutpicks, their crossed
mandibles.
The tongue is also a food-collecting tool. That of a sapsucker ends in a
brush but in the insect-eating woodpeckers it bears spines and teeth. The
tubular tongue of hummingbirds ends in two brushes suited for nectar dip-
ping. In fish-eating birds such as pelicans, the tongue is very small and well
out of the way of the fishes slipping down the throat.
Relatively few birds are pure vegetarians. As fledglings almost all are fed
on bits of animals or animal products. Both young and adults of many species
live upon a miscellany of small animals in summer and revert to buds and
-~>^v
General use
Pigeon
Seed and nut cracker
Parrot
^v*,.
General use
Blue Joy
.fl^^r^'^-<~^_
v^'"
Shucking seeds from pine cones
Crossbill
Seed and berry picker
Grouse Quail
Fig. 36.6. Beaks of birds that live on mixed or on purely plant diet. (Not drawn
to scale.)
736
EVOLUTION OF ANIMALS
Part V
Insect trop
Night Hawk
Grips slippery fishes
Merganser Duck
X..
.'"'
Chisels for insects
Woodpecker
Spears frogs, fishes
American Bittern
.<!rr^^
Scoops fishes into its pouch
Brown Pelican
..^^-^^
Tears flesh with a hook
Hawk
Probes for worms with flexible bill thot
can be moved without opening the jaw
Woodcock
^v>
Bristles help to catch small insects
Phoebe
Fig. 36.7. Beaks of birds that feed upon animals, insects to small mammals; the
structures are more specialized and striking than those of plant feeders.
seeds in winter, the only food on which they can live through the winters of
temperate and northern climates. Owls, hawks, and other predators live on
small animals the year round.
Types of Feet. No birds have more than four toes, commonly arranged
with three in front and one corresponding to our great toe pointed backward.
From this oldest pattern, feet vary with the habits of the birds and the toes
may be four, three or two. From hummingbirds to ostriches the legs and feet
Chap. 36 BIRDS — CONQUEST OF THE AIR 737
of birds are covered with strikingly reptilian scales. The heel is the first back-
ward bending joint above the part of the foot that rests upon the ground. The
forward-bending knee is covered with feathers (Fig. 36.10).
Perching birds are the crows, thrushes, warblers, swallows, larks, and others,
numbering more than half of the group. All of them have muscles so arranged
that sitting on a perch is automatic with holding to it with the feet. This
efficiency is due to the remarkable strength in the tendons which run through
each toe and enable it to clasp and to hold and balance the bird on its
branch. In all this, the hind toe is essential (Fig. 36.14). There are specialists
among the perchers. With the same arrangement of toes, American wood-
peckers clutch the surface of a tree trunk, lean back on their tails and hammer
with their bills.
A nuthatch climbs down and around tree trunks as easily as up, with
stops to pick up insects, and no help from its tail. Swallows have little feet and
telephone wires are their favorite perches. The feet of chimney swifts are still
smaller, yet weak as its feet may be, a young swift can cling fast to the vertical
face of a brick chimney. Owls are as flexible as many human "liberals," being
able to move their outer toes backward or forward and perch like a robin or
to put two toes before and two behind Hke a parrot.
A bird's feet tell where it lives (Figs. 36.8, 36.9). Herons that wade about
the shallow margins of ponds and streams have long legs that Hft their bodies
Perching
Robin
Grasping, Tearing
Howk
Scratching Earth
Pheasant
Climbing
Woodpecker
Swimming
Duck
L^-
•iP
•?"
^'%^^^^
Stockinged by Feathers:
Snow Arctic Ptarmigan
Fig. 36.8. The shapes of birds' feet are correlated with their habits and sur-
roundings. Their feet and beaks are often used like hands in finding food and
building nests.
738
EVOLUTION OF ANIMALS
Part V
I
'
'A
)
^ ^
1^
f
«vi
^^
Fig. 36.9. Process of nest-building by a weaverbird (Quelea). The arrows
show the direction in which the string is pulled. A, the points of holding by the
beak. 4, 5, and 6 show stages of cooperative work by foot and beak. Weaverbirds
are relatives of house sparrows that range through Europe, Africa, and Australia.
Birds follow a set pattern of nest building. Weaverbirds raised by hand for four
generations made perfect nests of a type they had never seen. (After Friedman.
Courtesy, Young: The Life of the Vertebrates. Oxford, England, Oxford Uni-
versity Press, 1950.)
above the water surface, and four long toes that distribute the weight on the
mud. The plover runs along the beach, swims into the waves, then scurries onto
the sand again on front toes that are partly webbed but still flexible for run-
ning. In ducks, geese, swans, and other water birds, the three front toes
are joined together by a web of skin that is outspread against the water in the
backstroke. When pulled forward the foot slips through the water, with toes
drawn together and webs folded. An ostrich runs on its third toe, the large
powerful one which supports most of its weight. The fourth or outer toe is the
small helper with a toenail only as large as the claw of a chicken.
Internal Structure
Modifications for Flight. Skeleton and Muscles. The skeleton of a bird
is modified for flight, for walking and running, for perching and for laying eggs
with hard shells (Figs. 36.10, 36.12, 36.13, 36.14). It bears several reminders
of the skeletons of reptiles.
The skeleton is light yet rigid. Except in running birds, the bones of the
skull unite early making it strong against shocks, such as those from a wood-
pecker's hammering. The importance of vision in birds is emphasized by the
large eye sockets. The neck is commonly long, 14 vertebrae in a pigeon, 25
in a swan, with peculiar joints that allow the bird to turn it freely when watch-
BIRDS CONQUEST OF THE AIR
739
Chap. 36
ing for danger, feeding, and nest building. The remainder of the vertebral
chain is rigid except for four or five caudal vertebrae which allow the tail to
act as a rudder during flight. The terminal bone, called the ploughshare or
pygostyle, is composed of "fused vertebrae supporting the tail. It is a great
contrast to the long tail (20 vertebrae) of the earliest known bird, Archae-
opteryx (Fig. 36.20).
The shoulder girdle supplies the sockets for the wings and with the keeled
Shoulder
Wing
Clovicle
wish bone
Keel
pectoral
rdle
Torso- metatarsus
Fig. 36.10. Skeleton of a bird (domestic fowl). The main skeleton of birds is
built for locomotion in the air and on land (or water). No other animals are so
perfectly adapted to travel in such different surroundings. The flexibility of the
vertebral column is almost solely limited to the neck whose turning makes it pos-
sible for a bird to see in every direction, and the tail which is a rudder. The
pectoral girdle, chiefly its keel, is concerned with air travel. The keel serves for
the attachment of the flight muscles, the "white meat" of domestic fowl, the rela-
tively huge pectoralis major muscle that lifts the wings, and the smaller pectoralis
minor that lowers them. The pelvic girdle or saddle is concerned with land travel.
Its irregular plates (pelvis in the figure) serve for the attachment of the leg
muscles; those of the "drumsticks" (dark meat) are as important to walking as
the pectoralis muscles are to flying. (Courtesy, Putnam: Animal X-Rays. New
York, G. P. Putnam's Sons, 1947.)
740 EVOLUTION OF ANIMALS Part V
breastbone furnishes the attachment for the great flight muscles. The wing
socket is formed at the junction of the shoulder blade or scapula, the coracoid
that connects with the sternum, and the spread ends of the collarbones or
wishbone. The spread of the wishbone helps to keep the shoulders sprung
apart when the wings are raised. The keel of the breastbone, familar to any-
body who has carved a chicken, is the attachment for the great flight muscles
(pectorals). In ostriches, as in other flightless birds, the breastbone is a simple
shield without a keel.
A bird's hips are mainly broad plates that form attachment places for the
great leg muscles and a saddle above the otherwise unprotected vital organs.
The presence of a pelvic saddle of bones fused together and to the vertebrae
instead of a pelvic girdle allows the passage of the large hard-shelled egg
(Fig. 36.18).
Wings and Flight (Figs. 36.11, 36.12). When a bird folds its wings the
elbows point backward like human elbows. At the same time, a bird folds its
"hands" backward in a jackknife bend with the wrists in a sharp point forward,
impossible for the human wrist. A bird's "hand" is small and rigid, reduced to
three fingers from the five of its reptilian ancestors. The inner stub next to the
ulna corresponds to the index finger, the outer stub and the bones fused to-
gether at the tip of the wing also represent fingers. In reptiles, the fingers end
Fig. 36.11. The take-off of an American egret. The bird leaps into the air,
raises its wings and stretches out its neck, thrusts the feet down. In the air as in
this picture it draws the head back; the legs balance the neck; the wings go into
the down stroke. (Photograph by Allan D. Cruickshank. Courtesy, National
Audubon Society.)
Chap. 36
BIRDS — CONQUEST OF THE AIR
C
A
741
Fig. 36.12. Pigeons (Columba) photographed during a take-off for flight with
exposures of 1/825 second. A, front and B, rear view with wings together. C,
nearly, and D, at the bottom of the downstroke; note the slight rotation and for-
ward movement of the wing. E and F, wings during the upstroke; in F the
feathers have opened and the wings move backward, their motions faster than on
the downward stroke. (After Aymar. Courtesy, Young: The Life of the Verte-
brates. Oxford, England, The Clarendon Press, 1950.)
in claws; so do the first and second fingers of the ostrich. Ancient birds had
such claws and used them in climbing. After making several downward and for-
ward strokes birds often hold their wings motionless and glide. Before a high
wind a bird can flex its wings and glide with the wind. Usually, at high eleva-
tions, it rises through the air and soars in circles without moving the wings,
']
A
Standing
Fig. 36.13. Standing and stepping. Drawings from photographs of a goose.
A, standing; B, stepping. In stepping the center of gravity is brought over the foot
on the ground by a rotation of the femur on the tibia (Fig. 36.10, knee). The
tail is shifted to the left. A similar human gait is associated with weight and cer-
tain moods. (After Heinroth.)
742 EVOLUTION OF ANIMALS Part V
taking advantage of upward rushing air currents. A bird hovers, even poises in
the air over some object, a hummingbird over a flower, a gull above the water.
Birds do other things with their wings; penguins swim with them; geese, broody
hens, and fighting cocks strike with them; and birds in general spread them
over their eggs and young.
Special Features of Digestion. Various birds obtain the same kind of food
in different ways: an osprey hovers and drops, catching the fish in its claws;
Flexor
muscles
Fig. 36.14. Mechanism of perching in birds. Leg of
crow. The flexor muscles end in tendons that pass behind
the joints, beneath a strap of ligaments at the base of the
toes, and are distributed to the toes. As a bird flexes its legs
and sits on the perch, the flexor muscles contract, pull on
the tendons and the toes automatically grip the perch.
(Courtesy, Wolcott: Animal Biology, ed. 3. New York,
McGraw-Hill Book Co., 1946.)
herring gulls swoop down and grip it with their bills; a heron stalks or stands
motionless till a fish swims by; the kingfisher makes a sudden plunge; penguins
swim rapidly under water and grip the fishes in their bills.
Cormorants, peUcans and others that eat large fishes have small tongues.
In sparrows, warblers, small seed- and insect-eaters the tongues are horny,
often with inward pointing spines along the sides that catch in the bits of food.
The hummingbird has a long cleft tongue with an inrolled membrane on each
half which is worked back and forth in the flower to take up nectar. Saliva
figures prominently in some birds; in woodpeckers, its stickiness picks up in-
sects; chimney swifts use it as glue in nest building. In all birds, digestion and
its associated processes are rapid.
The esophagus is simply a passageway, or a passageway with an expan-
sion, the saclike crop, which provides for quick filling when food happens
to be plentiful (Fig. 36.15). Chickens, pigeons, and other grain and mis-
Chap. 36 BIRDS CONQUEST OF THE AIR 743
cellaneous feeders have well-developed crops. In pigeons, the lining secretes
"pigeon's milk." This is the first food of young pigeons and they reach
down their parent's gullet to collect it. An air-filled crop is the pout of the
pouter pigeon. Some birds quickly empty their crops when they are frightened
into sudden flight. This is a bird's involuntary reaction against extra weight.
There are two divisions in the stomach, the first and smaller one (proven-
triculus) has thin glandular walls which secrete the gastric fluid. In grain-eating
birds — pigeons, chickens, and turkeys, in insect eaters, and some others, the
second section of the stomach is a well-developed gizzard. Its walls are com-
posed of two great muscles whose tendons are brilliantly iridescent. Its inner
layer of cells produces a fluid that hardens into the tough lining that is peeled
out when the gizzard is prepared for cooking. Grain-eating birds swallow small
stones and gravel that grind against the food, without which their gizzards are
useless. The great muscles contract again and again grinding the gravel against
the already softened food. Birds such as owls, hawks, gulls, and ducks that eat
flesh and plants, have poorly developed gizzards or none. In flying birds, the
large intestine is relatively short. It is kept almost clear of waste, another way
of decreasing the flight load.
Circulation of Blood. In birds, the circulation of blood differs from that of
reptiles in one very important respect. In most reptiles, the oxygen-rich blood
eiitceiiuM
CEREBELLUM^
6PT1C LOBES
OLrMTOHV
lose
URETER
OIL CLANO SPERM OUCT
CAECUM
OPENIN* OF
UKCTCR
CLOACA
OPCNINS OF
SPEKH Due
Fig. 36.15. Diagram of the general structure of a bird (except the air sacs), the
domestic fowl. The crop is a storage pouch formed by an enlargement of the
esophagus. It is highly developed in seed eaters and practically absent in fish eaters.
The stomach includes two sections, the proventiculus whose walls secrete the di-
gestive juices and the heavily muscular gizzard where grinding occurs. (Courtesy,
MacDougall and Hegner: Biology. New York, McGraw-Hill Book Co., 1943.)
744 EVOLUTION OF ANIMALS Part V
from the lungs received in the left auricle of the heart is mixed with the
oxygen-poor blood from the right auricle in the incompletely separated
ventricles. For the slow metabolism of the cold-blooded reptiles, this is enough
oxygen to supply the needs. This is not true in birds. The bird's heart is com-
pletely four-chambered and the two kinds of blood are entirely separated.
Except for those that go to the lungs, all arteries carry highly oxygenated
blood. Only a rapid and generous supply is adequate for the oxygen-hungry
body of a bird. The heartbeat of birds is incredibly rapid. The basal rates of
the heartbeat of an English sparrow, a canary, and a hummingbird have been
recorded respectively as 350, 500, and 1000 per minute. The adult human
heart beats about 70 times per minute. The red blood cells of birds are
nucleated like those of lower vertebrates; there are more of them per unit of
blood than in any other animal.
Respiration. The vocal organ or syrinx is ordinarily located where the wind-
pipe forks into the bronchial tubes, one to each lung (Fig. 36.15). The lower-
most rings of the windpipe fuse to form a tube within which are the membranes
and muscles whose vibrations produce the voice. Because of their intense ac-
tivity and high temperature, birds have the highest oxygen consumption of all
animals. This is satisfied by fast breathing, the rapid passage of air through the
small compact lungs, and the extremely swift flow of blood through them. The
lungs are expanded by the pull of the ribs to which they are closely fitted. Air
goes through them and enters the internally ciliated air sacs by way of the
bronchial tubes. The air sacs extend along the neck, beneath the wishbone, and
far back among the viscera (Fig. 36.16). Air spaces connected with them
reach into the larger bones. Air is forced out of the air sacs by the pressure
of muscles; this time, it enters the lungs directly from the sacs. It rushes
through them past the blood capillaries from which carbon dioxide is collected
and to which oxygen is contributed. The air sacs constitute a cooling system
that combats the intense heat of the bird's body produced by the muscles and
kept within it by the feathers. When a bird's air sacs are opened experimentally
it continues to live, but its temperature rises higher than the usual 100° to
110° F. In swimming birds the air sacs are helpful floats.
Excretion. Birds conserve water and excrete salts. The completed urine of
a bird is a semisolid mass of uric acid crystals cast out of the body as whitish
material adhering to the darker waste from the digestive tract. There is no
urinary bladder.
Nervous System and Sense Organs. The cerebellum and optic lobes are rela-
tively well developed. This indicates that birds have good coordination and
sight. The olfactory lobes are small and even buzzards suspect dead flesh by
sight rather than smell. As might be expected birds taste very little. They are
sensitive to touch in particular places. Woodcocks probe soft earth and feel for
worms with the tips of their bills; various ducks have sensitive sifting plates
Chap. 36
BIRDS CONQUEST OF THE AIR
745
Syrinx
(voice box)
at base of
trachea
Interclavicular
sacs
Intermediate sacs
Fig. 36.16. The respiratory organs of a pigeori. The lungs fit closely to the ribs
and do not dilate. The air sacs are extensions of the lungs. Their thin transparent
walls are freely expansible and they communicate with one another directly or by
way of the lung cavities. Air sacs constitute a ventilating system which moderates
the high body temperature of the bird. The syrinx, the unique voice box, is located
at the junction of the bronchial tubes close to the lungs. In this figure it is hidden
by the air sacs. (Redrawn and modified from Muller: The Air Sacs of the Pigeon.)
along the sides of the bills between which particles of food are strained from
the water; bristles about the mouths of the fly-catching phoebes are responsive
to contact with small insects. Next to sight, hearing is the most important sense.
Birds have no external ears, but near their peculiar bony eardrums the feathers
are especially open to currents of air. Barn owls have folds of skin near the
eardrums that they can lower or raise to make catch cups for sounds.
,.. i
Fig. 36.17. Reproductive organs of the hen. (After Duval.) The organs fully
develop only on the left side; those of the right are rudimentary. Two eggs are
shown in the oviduct at different levels; normally but one is in the oviduct at one
time. 1, ovary showing many young follicles each containing an egg; 2 and 3, suc-
cessively larger follicles containing the enlarging eggs; the dark lines are blood
vessels in the walls of the follicular sacs; the white band, 4, is the line where the
follicular sac breaks and releases the egg; 5, empty follicular sac; 6 and 7, lip and
funnel of the oviduct; 8, egg in the upper part of the oviduct; 9, region of the
oviduct in which the albumen is secreted; 10, the oviduct cut open to show the
albumen surrounding {11) the egg; 12, the germinal disk where the chicken be-
746
Chap. 36
BIRDS — CONQUEST OF THE AIR
747
Fig. 36.18. An x-ray photograph of a living hen showing an egg about to be
laid, 25 1/2 hours after the last one was laid. Note that there are no bones below
the egg. The skeleton is strikingly open at the rear, a reminder that birds are the
only animals that produce such large hard-shelled eggs. Actually birds have a
pelvic saddle, not a pelvic girdle. (Courtesy, J. A. F. Fezzard: Series (1) of
Medical and Biological Illustration. Cambridge, England, Cambridge University,
1951.)
The eyesight of birds is exceptionally keen. They can see to dart through
trees without striking a twig and to alight on one branch out of a thousand
others. This means constant shifts from far to near vision and reverse — great
power of accommodation. A sparrow hawk can drop down upon a beetle
after hovering 200 feet above it; by rapid peering this way and that chicka-
dees and warblers catch even the smallest insects on rough bark. The eyes of
gins to develop; 13 and 14, lower regions of the oviduct; the latter is the part where
the shell is secreted; 15, the alimentary canal (cut off); 16, reflected body wall;
17, external opening of the cloaca. (Fertilization of the egg occurs before it is
coated with albumen.) (From Hamilton: Lillie's Development of the Chick.
Copyrighted 1952 by Henry Holt and Co. Reprinted with their permission.)
748 EVOLUTION OF ANIMALS Part V
birds are relatively large, often enormous, and set in exposed positions. The
eyeballs are protected by bony plates embedded in the outermost coat. The
pecten, a peculiar structure shaped like a half-folded fan, is suspended in the
vitreous humor. It is crowded with blood capillaries and nerves. Although its
function is not proven it may be connected with nutrition.
Reproduction. Courtship and mating reflect the bird's generally rapid ac-
tivity. Courtships may include brief darting flights, social gatherings and cere-
monies such as those of prairie fowls, dances dignified or tempestuous (Fig.
36.19). Reproduction in birds is similar to that in reptiles. All young birds
hatch from hard-shelled eggs. Paralleling the essentially complete land life of
birds fertilization is always internal. Sperm cells developed in the testes pass
through coiled sperm ducts that open into the cloaca, and are ejected into the
cloaca of the female in the extremely brief mating contact. In the cloaca of
very young male chicks there is a small process, the rudiment of a copulatory
organ similar to one that is well developed in some reptiles. This structure is
the means by which the sex of downy chicks is determined in hatcheries.
Fig. 36.19. Incidents in the mutual courtship of the great crested grebes, marine
diving birds in which the two sexes are strikingly similar in color and form. 1,
mutual head shaking; 2, the female is displaying her plumage before the male;
3 and 4, further views of the male rising from the water after various dives; 5,
both birds have dived and brought up weeds. Then, they meet together and go
through a period of head swaying. (Courtesy, Young: The Life of the Verte-
brates. Oxford, England, The Clarendon Press, 1950.)
Chap. 36 BIRDS CONQUEST OF THE AIR 749
The female organs usually develop to maturity only on the left side, but
hawks and some others are exceptions (Fig. 36.17). During the laying season
the ovary of an ostrich may weigh three pounds or more and the egg is equal in
volume to about a dozen and a half chicken eggs. When an egg reaches full size
in the ovary it breaks out of its enclosing sac, is grasped in the soft funnel of
the oviduct and begins its travel through the tube. FertiUzation occurs in the
upper part of the tube. The albumen or white is laid over the yolk by glands in
the middle region of the tube and the shell membranes, the so-called skin,
and finally the shell are added in the latter part. Eggs are usually deposited
soon after the shell is completed. For comparison of the reproductive processes
of other animals see Chapter 18.
Distribution
Birds live on all continents, on most islands, and in all seas. They live in all
climates, and are abundant in the tropics and through the temperate zone. They
penetrate well into the Arctic and penguins thrive in the antarctic cold that
mammals cannot endure. One or another species is at home from sea level to
heights of 20,000 feet on the slopes of the Himalaya Mountains. Although
flight has given birds the vast space of the air, they still conform to the laws
of animal distribution, and each species has its own geographic range and
particular habitat. Woodpeckers range all over North America below the
Arctic, but they hunt insects on tree trunks wherever they are. Emperor
penguins endure the storms, cold, and darkness of antarctic winters because
they can secure food. Owls and woodpeckers nest in holes in trees and bank
swallows and others in the ground, but no birds are subterranean like the
woodchucks and ground squirrels. In polar regions, there are few species and
many individuals; in temperate regions, many species are resident and many
more come and go in different seasons. There are also many species in the
tropics, among them various and resplendent parrots and birds of paradise.
Migration
Birds outdo all land animals in the distance and regularity of their migra-
tions. Not all species are far travelers; chickadees, downy woodpeckers, and
blue jays are semipermanent residents in many localities. Yet, individual birds
move from one place to another, and bird banding has shown many migrants
even among so-called winter residents. Except poor-wills and certain swallows
no birds hibernate. They remain in their own locality in full activity, or they
leave it and return in a later season. The general trend of migrations is north
and south. In the Northern Hemisphere, birds move toward the north where
food and nesting places are available during the warmer months, and toward
the south to warmth and food in winter. In the Southern Hemisphere where
750 EVOLUTION OF ANIMALS Part V
the seasons are reversed, migrations are less general and occur in opposite
directions.
The times and general migration routes of North American birds are now
fairly well known. Most of the insect eating birds, flycatchers and warblers,
retire to the southern states, many of them to South America. The majority of
species either cross the Gulf of Mexico, or follow its western shore and settle
in Central and South America. Ducks and other waterfowl have definite routes,
several of them over the ocean. Certain birds migrate by day and others by
night. This was long ago discovered by pointing a telescope at the moon and
observing the silhouettes of the birds that cross it.
Many migrants follow river valleys, mountain chains and coast lines; others
launch off over the ocean, or across country where there seem to be no guide
marks. The urge to migrate is to a considerable degree affected by changes in
amount of light and other features of the environment, also by the endocrine
secretions of the reproductive organs.
Ancestors
In 1860, on a slab of limestone taken from a quarry in Bavaria, an imprint
was discovered that appeared to be the fossilized imprint of a feather. Its
identity was established a year later when in the same locality another fossil
was found, an almost complete skeleton of a vertebrate animal with feathers.
About 1 6 years later, a still better fossil of a feathered animal was found in the
same locality. The fossil record of birds is sparse. No other similar fossils
Fig. 36.20. Fossil remains of an ancient reptilian bird ( Archaeopteryx) embedded
in a slab of limestone — as they were discovered. Above the slab is a partial recon-
struction of the distal part of the wing and below the foot is represented. In life,
the bird was about the size of a crow. (After Zittel. Courtesy, Rand: The Chor-
dates. Philadelphia, The Blakiston Co., 1950.)
Chap. 36 BIRDS — CONQUEST OF THE AIR 751
have been found in Bavaria and none anywhere so perfect as the now famous
Archaeopteryx (Fig. 36.20). The skeleton is similar to that of the flying rep-
tiles of the same era. As a bird, Archaeopteryx is certified only by its feathers.
It was about the size of a large crow but more heavily built than a modern
bird. The skeleton is lizardlike; the vertebrae of the pelvic region are separate,
not fused as in birds and freely movable ones formed a long tail. Each tail
vertebra supports a pair of long feathers all of them forming an expanse that
was probably spread fan-wise in the air. The wings had free movable "fingers,"
each with a claw, and on the jaws there were true teeth set in sockets. Ages
must have elapsed between the scaly flying reptiles and a feathered Archae-
opteryx, but there is no fossil record of a development of birds in that long
period. In fossil birds of the far later Tertiary Period (Eocene), the teeth are
missing and the tail is short.
37
Mammals ana Mankind
Characteristics and Reptilian Origin. Mammals are animals that have
hair. No others, except birds, are warm-blooded, and no others, except birds,
have coverings that so well conserve the heat of their bodies. Mammals have
lungs; their breathing is always aided by the diaphragm, a muscle that works
like a bellows. Their red blood cells, without nuclei when mature, are uniquely
efficient oxygen carriers. The brain is relatively large due to the great develop-
ment of the cerebral hemispheres.
Except in the two egg-laying species the eggs are minute, are without shells,
and contain scarcely any yolk. Fertilization is always internal. The young de-
velop within the body of the mother, are born alive and are fed milk produced
by the mammary glands for which the class is named. While the embryo is
developing, it is surrounded by membranes formed on the basic patterns in-
herited from reptiles (Fig. 35.2). In the higher mammals, the placenta, a
modification of the chorion and allantois, is unique among all animals in its
provision for the developing young.
Birds and mammals arose from different branches of reptiles early in the
Reptilian Age. Mammals increased in number slowly through that long period
of 70 million years or more. Towards its end, however, one of the most conse-
quential developments in the history of life was quietly appearing, the rise of
flowering plants. After that, there were flowering trees, with edible leaves,
seeds, nuts, and fruits. Times of good feeding had come. The flowering plants
spread, especiaUy the grasses, as lands were lifted, and through seasonal
changes many climates became more livable. Swamps dried and became
grazing lands. The Great Plains of North America were coming into existence,
and grass-eating hoofed animals spread over them. The evolution of mammals
quickened and broadened following that of the plants. The great Age of Mam-
mals had begun (Figs. 37.1, 37.2, 37.3).
Mammalian Structures and Functions. For the structure and physiology of
752
Chap. 37
MAMMALS AND MANKIND
753
FLYING
^MlNG^mD^
i
Fig. 37.1. The spread of mammals in environments and habits. Mammals prob-
ably first lived in trees, climbing and leaping. From there they gradually radiated
into other habitats and activities. (Courtesy, American Museum of Natural His-
tory.)
mammals accounts such as Movement and Muscles (Chap. 10), Foods and
Nutrition (Chap. 11), and The Release of Energy — Respiration (Chap. 13)
and others should be consulted. These are units of The Internal Environment
of the Body discussed in Part 3.
The Domestic Cat — A Representative Mammal
The cat is regularly studied as a mammal and an introduction to the human
body. The discussions of organs and systems in Part 3 were prepared with
those two ends in view, especially the latter. It will be of help and interest if
they are consulted as suggested.
The study of organs should always be lifted by acquaintance with the grace
of the living animal (Figs. 10.1, 37.4). A cat is a natural carnivore and
754
EVOLUTION OF ANIMALS
Part V
hunter. It prowls in the grass, waits, and pounces. If not too hungry, it brings
the mouse home still alive, sets it free to take a crippled run, then pounces
again. Cats catch and clutch and climb — the play of their foreshoulders is
something to see and remember. Their musdes are surpassingly supple,
elaborately developed on head, neck and shoulders. Their facial expressions
Fig. 37.2. The flexibility of a mam-
mal. Gibbon, the acrobat of mammals.
At home in southeastern Asia these
long-armed apes leap and swing
through the treetops always depend-
ing greatly upon their arms. (After
Clark: History of the Primates. Cour-
tesy, British Museum Guide, 1949.)
change. Their night "eyeshine" is momentarily reflected by the headlights as
the car approaches within twenty feet of them. Then it glitters and disappears.
The angle of reflection is limited as it is in the wayside signs. Cats walk on
their toes; the hind foot is bent at the heel with a downward sag, not upright as
it is in dogs, and their step is more elastic. They are famous for turning in
the air and landing "on all fours" when dropped.
About 3000 B.C., the Egyptians tamed a certain variety of African wild cats
so that they might hunt and protect their grain. The cats did this so well that
they were for a time believed to represent one of the gods. Later, they were
exported and introduced into other countries. It is a comment on the cat's
subtlety that where a dog and cat are pets, the dog follows the owner, and the
owner follows the cat.
Chief Types of Mammals
Based on the provisions for the developing young, there are three types of
mammals: those which lay eggs; those which carry the young in a brood
pouch after a short period of internal development; and those in which the
developing young are attached by a placenta to the uterus of the mother.
Egg-laying Mammals — Subclass 1, Monotremata. Monotremes are so called
because the single opening (L, monotrema, one opening) of the cloaca re-
Chap. 37
MAMMALS AND MANKIND
755
Fig. 37.3. The speed of a mammal — portrait ot Citation. The thoroughbred
horse is developed for speed. The world's record for one mile was made by Cita-
tion of Calumet Farms, Lexington, Kentucky, who ran at Golden Gate Fields,
Albany, California in one minute and 33 and three fifths seconds, June 3, 1950.
For a human run the fastest mile to this date is three minutes and 58 seconds, by
John Landy of Australia, June, 1954. (Portrait of Citation, by Allen F. Brewer, Jr.,
equine artist, Lexington, Ky.)
ceives the urinogenital and digestive tubes, as it does in the amphibians, reptiles
and birds. Only two species have survived, the duckbill (Ornithorhynchus) — a
semiaquatic animal with soft fur, and the spiny anteater or Echidna with coarse
hair and spines that lives in dry country (Fig. 37.5). The duckbill deposits its
two leathery-shelled eggs in its burrow and crouches on them during incuba-
tion. The anteater carries her one egg in a fold of abdominal skin warmed by
her body until it hatches. The membranes of the embryo (amnion, chorion,
allantois and yolk sac) are essentially like those of reptiles. The mammary
glands produce the milk which the young ones lick from the skin; monotremes
have no nipples.
Marsupials — Subclass 2, Marsupialia. These are mammals with a brood
pouch or marsupium on the outer surface of the body, as in koalas and well
known in the kangaroos (Figs. 37.6, 37.7). Most marsupials live in Australia,
756 EVOLUTION OF ANIMALS Part V
Fig. 37.4. The joints of cats allow them great flexibility and grace of movement.
The turns of a cat's forefoot and leg during a face washing rivals those of a human
hand and arm in the same exercise. (Courtesy, Putnam: Animal X-Rays. New
York, G. P. Putnam's Sons, 1947.)
New Guinea and Tasmania, but not in New Zealand as might be expected.
Marsupial moles (Notoryctes) and others inhabit South America; and the
opossum (Didelphis virginiana) is well known in our southern states (Fig.
37.8). The majority of marsupials are plant feeders; originally, they probably
all were; now there are carnivorous ones such as the Tasmanian wolf (Thyla-
cinus) which has been nearly exterminated because of sheep killing.
Among the pouched mammals are mice, rats, squirrels, sloth-like "bears,"
koalas, bandicoots that suggest rabbits with longer tails, and kangaroos. Brood
pouches are examples of convergence in evolution, the independent origin of
similar functions in genetically unrelated plants and animals. The male sea
horse, which is a fish with a broad pouch, and the female kangaroo, a mammal,
illustrate convergence. These animals are widely different and only distantly
related, yet both carry their young in pouches.
Newborn marsupials are very small and immature. The great kangaroo,
Macropus major, is about 1 inch long when it is born and enters the pouch.
There it becomes attached to one of the nipples and milk is pumped into its
mouth by the contractions of muscles about the mammary gland. In this
kangaroo, the development before birth lasts for only 5 or 6 weeks. There is
little food in the egg and no adequate supply from the mother. After birth, the
young joey is carried in the pouch for about eight months. During the last part
of its stay, it leans out of the opening and sometimes crops grass as its mother
grazes, often jumping out and in again, reluctant to leave its carriage.
Placental Mammals — Subclass 3, Placentalia. The members of this group
include all the other mammals, the cats, elephants, polar bears, and others
throughout the earth. There are about 3500 species of placental mammals in
Chap. 37
MAMMALS AND MANKIND
757
Fig. 37.5. Upper, duckbill (Ornithorhynchus). A semi-aquatic egg-laying mam-
mal, about the size of a large cat, that lives only in Eastern Australia. Lower, five-
toed echidna or spiny anteater, also an egg layer. As adults neither duckbills nor
echidnas have true teeth; the duckbill lives on worms and small moUusks; the
echidna has a long beak with which it captures ants. During the period of rearing
young a fold of skin forms a pouch in which the one or two eggs are incubated.
After hatching, the young ones enter the pouch and from certain areas of the skin
lick the milk secreted by the milk glands which are specialized sweat glands. (Cour-
tesy, Australian News and Information Service, New York.)
758
EVOLUTION OF ANIMALS
Part V
Fig. 37.6. Koala, an Australian marsupial, at ease. It lives entirely in trees and
its feet, the spread and separation of the toes, are adapted for clinging to branches.
Koalas feed entirely on the leaves of a few species of Eucalyptus trees. Their only
water supply is from the trees and their name koala is an old Australian word
meaning "no drink." (Courtesy, Australian News and Information Service, New
York.)
contrast to the now scarcely 150 species of marsupials. The great key to their
success is the nourishment of the young before birth by means of the placenta,
the organ formed partly on the pattern of the old reptilian allantois (Fig. 35.2).
By means of the placenta, food and other needs of the growing embryo are
provided for and waste products pass through it into the blood of the mother
(Fig. 19.18). The young marsupial encounters the setbacks of a seedling that
is transplanted midway in its early growth, but for the young placental mammal
there is no transplanting.
Chap. 37
MAMMALS AND MANKIND
759
Representative Groups of Placental Mammals
One or another of these mammals is adjusted to all the major phases of
environment, air, water, and land. They can live in arctic, temperate, or tropi-
cal climates; they are fitted to manifold special niches in swamps and plains;
to life in tropical forests and rocky mountain slopes — on deserts and in the
ocean. An animal's diet, habitat, and general way of living are reflected par-
ticularly in the character of its locomotor appendages and in the number and
Fig. 37.7. Kangaroo (Macropus). The
joey is leaning out of the pouch or marsu-
pium. At this age the joey jumps out of the
pouch, crops grass as its mother does and
clambers in again. Koalas and kangaroos
are the most pictured of the marsupials of
Australia, but there are many other marsu-
pials— among them pouched rats, moles, ant-
eaters and flying opossums. They have re-
tained characteristics that were general in
mammals more than 70 million years ago.
(Courtesy, Young: The Life of The Verte-
brates. Oxford, England, The Clarendon
Press, 1950.)
type of its teeth. The placental mammals are arranged in orders, the num-
ber differing slightly with the valuations given by the classifier. Groups called
orders in one system may be suborders in another. The names, general habitat,
and diet, are given here for the orders to which the better-known placental
mammals belong.
Insectivora — Moles and Shrews (Fig. 37.9). Moles are stout-bodied bur-
rowers with pointed noses, hardly visible eyes and ears, and a hunger for
worms and insects. Their total length is five to nine inches. Shrews are the
smallest of North American mammals, high strung, constantly moving, secre-
tive, common but seldom seen, and fierce in their attacks on insects and mice.
The length of various species of shrews ranges from three to six inches.
Chiroptera — The Only Flying Mammals (Fig. 37.10). The Chiroptera in-
clude the large fruit bats of the Eastern Hemisphere and small ones, that are
chiefly insectivorous. The wings are formed of webs of skin and instead of be-
ing supported by a single long finger as in the wings of ancient reptiles, those
of bats are supported by nearly the whole hand. Bats are skilled night flyers,
avoiding all obstacles. As they fly, they constantly utter cries inaudible to the
human ear. These are reflected back from objects as ultrasonic echoes that are
detected by the bats (Fig. 17.8).
Rodentia — Gnawing Mammals. This large group includes the woodchucks
and ground squirrels, chipmunks, squirrels, mice, rats, muskrats, porcupines,
760
EVOLUTION OF ANIMALS
Part V
Fig. 37.8. Opossum {Didelphis virginiana). A prehensile tailed marsupial about
the size of a large cat, common in the southern United States. When it is born the
young opossum is strikingly undeveloped and considerably smaller than the honey-
bee. At birth, it immediately clambers into the pouch, similar to the more familiar
one of kangaroos. It climbs by hand-over-hand movement through its mother's
hair until it reaches the pouch where it remains attached to a nipple for over two
months. (Courtesy, American Museum of Natural History, New York.)
.\,/7i
Fig. 37.9. Common shrew (Sorex vulgaris). One of the smallest and commonest
of mammals. Shrews are quick and ferocious, seldom seen although they may live
in a bushy bank in the dooryard. They most nearly represent the ancestors of
placental mammals. (After Flower and Lydekker. Courtesy, Rand: The Chordates.
Philadelphia, The Blakiston Co., 1950.)
Fig. 37.10. Long-eared bat (Corynorhinus) pursues a moth. Bats are the only
mammals that have attained the power to fly and according to the evidence of
fossils they were flying 50 million years ago. In the wings of bats the thumb is
always separate from the rest of the wing. When a bat crawls its thumb helps to
hook its body along. Note the curled tail-membrane with which some bats capture
their prey. (Courtesy, Hamilton: American Mammals. New York, McGraw-Hill
Book Co., 1939.)
761
762
EVOLUTION OF ANIMALS
Part V
chinchillas, and guinea pigs. There are about 3000 species, more than in any
Other order of mammals. All of them feed chiefly on common plants and plant
products. They have one pair of chisellike incisor teeth in each jaw, and
molars but no canines. They are mainly small animals; the largest Uving rodent
is the South American capybara, about four feet long, a semiaquatic animal
that suggests an overgrown guinea pig.
Lagomorpha — Gnawing Mammals (Fig. 37.11). These rabbits, hares and
pikas were formerly classified as a suborder of Rodentia. On the basis of
certain structures they have now been placed in a separate order. The cutting
teeth include 2 pairs of upper incisors and one pair of lower ones. Pikas are not
coneys, though sometimes called so. The true coney (Procavia) of Syria and
Africa, the Hyrax and others, resemble guinea pigs with hooflike nails and
belong to the Order Hydracoidea.
Xenarthra (old name Edentata). Xenarthra meaning strange joints applies
to peculiar articulations of certain vertebrae. The order includes the tree
sloths, armadillos, and South American anteaters (Fig. 5.3).
Carnivora (Figs. 37.12, 37.13). Dogs, wolves, foxes, raccoons, the Asiatic
pandas, weasels, minks, skunks, otters, mongooses, hyenas, cats, lynxes, lions,
tigers, and panthers are all carnivores. Aside from their strong upper and
lower canine teeth, the carnivores are not peculiarly specialized. The brain is
well developed. A suborder includes the marine carnivores — sea lions, seals
Fig. 37.11. Pika (Ochotona). A little tailless rabbit, but 7 inches long and 3
inches at shoulder height. It lives in rock piles at high elevations in western North
America. Its high squeak is familiar about Lake Louise, Yosemite and other moun-
tain parks. (Courtesy, Seton: Lives of Game Animals. Garden City, Doubleday,
Doran and Co., 1929.)
Chap. 37 MAMMALS AND MANKIND 763
and walruses. When a sea lion is hungry it sinks to the sea bottom, usually in
the shallows, stands on its head and grubs up clams and sea snails with its
tusks.
Perissodactyla (Fig. 37.14). Horse, ass, zebra, tapir, and rhinoceros are
all perissodactyls (odd-toed). Animals in this and the next two orders have
hoofs. Formerly, they were included in one order Ungulata (hoofed) but are
now believed to be less closely related than their feet would suggest. All hoofed
'^ • ,-.. J- '
^M
ib^
^^^pi
PW^B^SWte'
Fig. 37.12. Mink {Putorius vison) . The mink, about the size of a slender cat,
is a crafty killer of muskrats, ducks, chickens and fishes, seldom hunting far from
a lake or stream. Its fur is soft and the glistening guard hairs have long captured
human eyes and pocketbooks. (Courtesy, Rand: The Chordates. Philadelphia, The
Blakiston Co., 1950.)
animals are herbivores. In members of this group, the main weight falls upon
the tip of the third digit. In horses, it is the only one that touches the ground;
the second and fourth are splint bones attached to it (Fig. 38.6). Tapirs have
four digits (thumb lacking) on the front feet and three on the hind ones. The
snout of the tapir is an example of similar ones in the elephant, proboscis
monkey, and others, that show convergent evolution. Tapirs are natives of Cen-
tral and South America and Malaysia.
Artiodactyla (Fig. 37.15). Pigs, peccaries, hippopotamuses, and cud-
chewers such as camels, deer, giraffes, cattle, sheep and goats, chamois, and
others are all even-toed. Their weight is carried by the third and fourth digits
which are equally developed. The teeth are usually of the plant-feeding type.
Proboscidea. Elephants are the most highly specialized of living mammals
(Fig. 37.16). With its trunk, an elephant can lift logs, dehcately examine the
texture of a leaf, pick up a peanut, suck up a drink of water and pour it into
the mouth, or give itself a shower bath (Fig. 5.3). An elephant bears its weight
on all five toes of each foot. They are bound together with connective tissue
so that the sole is a large hooflike expanse. The teeth are exceptional in that
one pair of upper incisors becomes the tusks and there are no canines.. The
development of the cheek teeth is peculiar and slow; finally, they acquire great
764
EVOLUTION OF ANIMALS
Part V
Fig. 37.13. Walrus (Odobenus). An arctic marine carnivore with a massive body
of 2000 pounds or more, small head, ill-favored face and upper canine teeth grown
into tusks 2 feet long. Above the tusks its lazy gentle disposition is apparent. It is
the original model of the "walrus mustache." (Courtesy, American Museum of
Natural History, New York.)
size, as much as three inches across the crown. Elephants eat large amounts
of herbage but do no after-meal chewing like cattle.
Cetacea. Toothed whales, porpoises and dolphins, and whale-bone whales
are all typically marine. Some are gigantic, the largest living animals. All are
streamlined, fish-shaped. The skin is extremely thick, underlaid with fatty
blubber, and almost or entirely hairless in the adults, but hairy in the young.
All of the toothed whales (Fig. 37.17), porpoises, and dolphins are car-
nivorous, having simple pointed teeth — numerous in some species, few in
others. Toothed whales are the killers; the males run in schools in the Atlantic
and Pacific oceans and far into the antarctic; the females are said to stay
in the tropics. Dolphins are small-toothed whales, five to 14 feet long; one of
them is the "killer whale," regarded as the most ferocious mammal in the sea.
The whale-bone whales feed upon the minute plants and animals that live in
surface waters. The adults have no teeth. In place of them are plates of horn,
Fig. 37.14. Living relatives of the horse. Upper, American tapir and young;
note the break of color on the young one comparable to the spots on a young
robin. Lower, African black rhinoceros, pair and young. Like horses (zebras and
asses) they are hoofed animals whose weight is borne on one hoof. (Order Peris-
sodactyla.) According to the fossils the living tapirs have not changed in essentials
since the time, at least 20 million years ago, when their ancestors resembled the
small ancient horse (Eohippus). Their only special structure is the proboscis, more
of a promise than an achievement. Rhinoceros history is more complex than that
of tapirs and many types have perished including those that could run. (Courtesy,
American Musuem of Natural History, New York.)
765
766
EVOLUTION OF ANIMALS
Part V
Fig. 37.15. Upper, northern white-tailed deer {Odocoileus virginianus) (bore-
alis). Southern New England and New York through south-eastern Canada and
westward. Lower, Virginia whitetailed deer (Odocoileus virginianus virginianus) .
From southern New Jersey to east central Florida. These are members of the
Artiodactyla, the great order of even-toed hoofed mammals that includes such
extremes as the pig and hippopotamus, and all the cud chewers whether oxen or
gazelles. (Courtesy, Mochi and Carter: Hoofed Mammals of The World. New
York, Charles Scribner's Sons, 1953.)
known as whale bone, that hang from the upper jaw like curtains, their fringed
edges sweeping down to the floor of the mouth (Fig. 37.18). When a whale is
feeding, it swims at the surface with its mouth open, collects a mouthful of the
plankton-filled water, expels the water between the close set plates, and keeps
the plankton.
Whales may dive 3,600 feet or more when wounded and doubtless do so
at other times. When harpooned, a baleen whale can carry a line straight
down for a half a mile, a depth where the pressure is half a ton — on every
Chap. 37
MAMMALS AND MANKIND
767
Fig. 37.16. African elephants, a group in the American Museum of Natural
History, New York. Mounted by Carl Akeley, one of his many examples of
taxidermy as a fine art. For a fuller appreciation of the work of Akeley and
that of others in the African Hall read Frontiers of Enchantment by W. R.
Leigh (Simon & Schuster, 1938) who was with Akeley in Africa and who painted
many of the backgrounds in the African Hall of the American Museum of Natural
History. (Courtesy, American Museum of Natural History, New York.)
inch of its body. From such depths, it can return immediately to the surface.
Yet it shows no symptoms of the accumulation of nitrogen bubbles in the
veins which afflicts human divers who rise too quickly to the surface. Whales
can stay submerged an hour or more though they usually stay down only a
fraction of this time. They have varied equipments for this; one is the quality
of the hemoglobin of their blood which has a long hold on oxygen. The spout-
FiG. 37.17. Sperm whale or cachelot {Physeter macrocephalus) . The head is a
third the length of the body which is about 65 feet. There are sharp clutching
teeth on the lower jaw but none on the upper. A great cavity in the expanded
snout holds about a ton of the highly valued oil from which vitamins are extracted
for use in margarine. Moby Dick was a sperm whale. (Courtesy, Rand: The
Chordates. Philadelphia, The Blakiston Co., 1950.)
768
EVOLUTION OF ANIMALS
Part V
Fig. 37.18. Jaws of whale-bone whale. Its food of minute animals and plants is
caught on the horny plates, called whale bone. (Courtesy, American Museum of
Natural History, New York.)
APBOREAt, INSECTIVORES
\ \
Fig. 37.19. A simplified family tree of the primates. (Reprinted from Man and
the Vertebrates by A. S. Romer by permission of The University of Chicago Press.
Copyright 1941.)
MAMMALS AND MANKIND
769
Chap. 37
ing of whales is the expiration of warm air from the lungs condensed by the
coolness of the surface water.
Primates. Lemurs, monkeys, apes and man are all primates (Figs. 37.19,
37.20, 37.21, 37.22, 37.23). Primates take hold of things with their hands.
Their coordination of eyes and hands is one of their basic characteristics. The
remote ancestors of man lived in trees, constantly climbing, gripping a branch,
aiming at another branch and leaping to it, repeatedly catching a swinging
vine and balancing upon it. They required an effectual combination of eye,
hand, and brain work. Those tree dwellers were trapeze performers with
mobile forelimbs that reached and stable hind ones that pushed. As the ages
passed some, probably the smaller ones, tree shrews, lemurs, monkeys, and
others remained in the trees; larger ones took to the ground. Among the
latter were the ancestors of the manlike apes, and after untold generations
of them there were prehistoric human beings.
Fig. 37.20. Left, lemur (Galago) and right, Tarsius — two members of the
Order Primates which includes mankind. All primates have four generalized limbs
each with five digits bearing nails. Lemurs are the most primitive of primates,
small nocturnal animals that live in trees especially in Madagascar; some are as
small as a mouse, others as large as a cat. Their right to belong in the primates is
in the shortening of the jaws and greater size of the brain. Tarsius shows signs
of relation to the higher primates, most of them associated with its arboreal life.
Like those of many nocturnal animals its eyes are very large. They are turned
completely forward as in the human face and close to the nose. Like other tree
dwellers the capacity of the eyes has increased, that of the nose decreased. The
upper lip is uncut and its shape suggests that of monkeys and man. {Left, courtesy,
American Museum of Natural History, New York. Right, after Vogt and Specht.
Courtesy, Rand: The Chordates. Philadelphia, The Biakiston Co., 1950.)
770
EVOLUTION OF ANIMALS
Part V
Fig. 37.21 Center, the gibbon is the smallest (about three feet tall) of the four
manlike apes — gibbon, orangutan, gorilla and chimpanzee. The orang {top) has
a small opposable thumb suggesting the human hand and hand-like feet. A
large male gorilla {left) weighs about 600 pounds. Right, a young chimpanzee.
(Courtesy, Museum of Comparative Zoology, Harvard University.)
Chap. 37
MAMMALS AND MANKIND
771
Prehistoric Man
Although mankind must have appeared much earlier, its history recorded
by fossils and other remains begins with the "ice age" or Pleistocene Epoch
(Table 38.1).
The Ice Age was a time of many changes; lands were lifted from the sea;
mountains were made; climates were changed; whole populations of plants
and animals were moved, many of them destroyed and new ones formed. Four
ice sheets spread over the northern lands and each time melted back. The
time since the last ice sheet is called post glacial or recent. According to some
theories of glacial origins, ice will be back again in about 50,000 years. The
dawning humanity shared in the changes of the Ice Age. Some were isolated
and subjected to special changes; others came together and interbred; many
must have emigrated toward the south. Human populations increased and be-
came diverse. They mixed and separated and mixed again as they have ever
since.
The characteristics of prehistoric man have been reconstructed from the
usually fragmentary remains which have been discovered, chiefly in Asia and
Europe. New finds are still being made from time to time. At present the pre-
historic record of human ancestry rests mainly upon the following types all
extinct.
Java Ape Man — Pithecanthropus erectus. Several bones have been found
in Java from earth of the Pleistocene Epoch about one million years ago (Table
38.1). In 1940, a skull was discovered. The Java man probably stood erect,
t PROSNaTHISM \
f S12E OF \
Canine--.
'^--CHIN
Foramen
Magnum—"''
Neck Muscle
attachments''
Fig. 37.22. Skull of gorilla showing generalized anthropoid ape characters con-
trasted with skull of man showing specialized ones. The prognathism, i.e., the
protrusion of the jaws, is strikingly greater in the ape. (Courtesy, Howells:
Mankind So Far. New York, Doubleday and Co., 1952.
772
I
EVOLUTION OF ANIMALS
Part V
Fig. 37.23. Concepts of the possible appearance of three ancient types of man
calculated from fossil remains. Hair and flesh have been added. Left, the "erect
ape man" or Pithecanthropus erectus whose remains were first found in Java, from
deposits by some estimates said to be about 500,000 years old. From other such
bones it is believed that erectus stood erect not with an apelike droop. Center,
Neanderthal man, Homo neanderthalensis was the first fossil type of man discov-
ered and is still the best known of the sub-human types. The first such fossil was
found in western Germany, in the Neander Valley. Since then more complete
remains of this race have been found at various places in Europe and the skeleton
is almost completely known. Right, Cro-Magnons, Homo sapiens. The Cro-Magnon
race may have been established by 40,000 B.C. and persisted until perhaps 13,000
years B.C. The name Cro-Magnon is from that of the French rock shelter where
a typical example of the race was found. Cro-Magnon artists wrought paintings
and carvings upon the walls of caves that are vivid and life-like after these thou-
sands of years. (Restorations by Dr. J. H. McGregor.)
but with outthrust head. Comparison of the skulls of a gorilla and the Java
man shows them both chinless, the brow-ridge of the man lower, and the
front teeth smaller though tusklike compared with those of modern man. The
brain cavity is larger than that of the gorilla, but is only two- thirds that of any
modern man. No stone tools have been found associated with the Java man's
remains.
Peking Man — Sinanthropus. Teeth and several crania from the Pleistocene
Epoch have been dug up and with them were numerous stone tools and evi-
dences of the use of fire.
Piltdown Man — Eoanthropus. Fragments of a cranium, a jaw, and a few
teeth were discovered near Piltdown, in Sussex, England, in 1908. The fore-
head is upright, and the brow-ridge slight. The upper part of the face is human,
the lower part apelike. Tools of chipped flint were discovered in the vicinity.
For several years curiosity, and respectful study were excited by the remains.
Authorities in paleontology wrote about it. Suspicions of its genuineness
finally developed. In 1953 a new examination proved the jaw to be that of a
modern chimpanzee and the worn surfaces of the teeth due to modern scrap-
Chap.«37 MAMMALS AND MANKIND 773
ing. The Piltdown Man was changed to The Piltdown Fraud. The guilty party
has not been discovered.
Heidelberg Man — Homo heidelbergensis. The remains consist of one com-
plete lower jaw with teeth. Evidently the jaw muscles were powerful.
Neanderthal Man — Homo neanderthalensis. Bones of nearly 100 individuals
come from various localities in Europe but the type is described from those
found in the Neanderthal Valley in Germany (Fig. 37.23). The impressions of
the convolutions of the brain on the interior of the cranium are simpler than in
modern man. Skeletons found on the floors of caves along with tools and
weapons of chipped stone are estimated to be about 100,000 years old.
Rhodesian Man — Homo rhodesiensis. The species is known only from a
cranium in a cave in Rhodesia, South Africa. The teeth are distinctly human.
Cro-Magnon Man — Homo sapiens jossilis. Nearly complete skeletons have
been found in southwestern Europe, along with stone implements, sculpture,
and paintings of wild animals in the famous caves of France and Spain (Fig.
37.24). Cro-Magnon paintings are startlingly realistic, especially in the effects
of motion and hunting with stone points and bows. The estimated date of Cro-
Magnbns is about 60,000 B.C.
Modern Man — Homo sapiens ( Wise Man) . All members of the human popu-
lation of the earth belong to a single species. There are no significant struc-
tural differences between them and all interbreed. Without regard for culture,
they are estimated to show 99.44 per cent of likeness and 0.56 of difference.
Homo sapiens is the only surviving species of those which laid the way for its
development, those that made the tools and weapons that are experiences of
mind preserved in stone and later in metal. These were passed on from one
generation to another and tied the past to the present. Time went on and more
tools were made; speech developed; and pictures were painted in the caves. All
of these contributed to continuity of ideas. Gradually, the species Homo sapiens
came into being, unique upon the earth, perhaps anywhere.
Fig. 37.24. Paintings made by prehistoric man in the Cavern of Font-de-Gaume
in the Dordogne region of southwestern France. On the sides and ceiling of a
smooth-walled cave the artists engraved and painted in black, red and brown, fig-
ures of more than 80 animals. In this cave paintings are made over one another
and the earliest are the crudest. The work was probably done from memory by the
light of a torch or a grease lamp. (After Breuil. Courtesy, Cleland: Our Prehistoric
Ancestors. New York, Coward McCann, 1928.)
Part VI
Evolution ana Conservation
38
Organic Evolution — Conservation
Organic Evolution
The basic resemblance of living things comes from their common origins
and countless kinships. Their extraordinary complexity and variety are due to
changes in them that have taken place during past ages and are continuing.
Living matter is known only as it appears in different species of organisms, A
species is a group of nearly related plants or animals that agree in certain
distinguishing characteristics. They interbreed freely and their characteristics
are inherited by their offspring. Species are inheritable patterns of life, re-
peated in generation after generation, though never exactly. They are patterns
and processes that require time to become established. No species of bird came
into being in a moment. Organic evolution is history.
Origin of Life
We do not know how life began. Neither do we yet know what keeps it
going.
It is certain that the novelty of Uving matter is in the way it is put together,
its organization, not the materials. Not one of these is unique (Chap. 3),
The beginning of life might have been in the organization of a complex mole-
cule containing carbon, and perhaps capable of affecting other molecules. The
changes from the organization of such a molecule to that of the simplest
protozoan would be greater than those between a protozoan and man.
Beginnings of Life
We do not know when life began. Measurements of the radioactivity of
certain minerals have placed the age of rocks containing them at two billion
years (Fig. 38.1; Table 38.1), and there is evidence that these rocks are by
no means the youngest. A billion years and more may have passed before they
777
778
EVOLUTION AND CONSERVATION
Part VI
LIFE
BEGINS
THE
FUTURE
THE
PRESENT
Fishes
Amphibians
Reptiles
Dinosaurs
Mammals
Birds
Homo sapiens- I '/a Sees.
Historic man- y^j second
LIFE CLOCK
ONE HOUR= 100,000,000 YEARS
ONE MINUTE= 1,660,000 YEARS
If life's past, present and future ore plotted on a 24-t)our
clock, modern men oppeared in ttie world about I '/j seconds ago.
Fig. 38.1. Life clock scaled to 12 hours showing the first appearance of various
vertebrates in the history of life on the earth. Only invertebrates existed in the
earlier three-fourths of the 12-hour day which represents time from the beginning
of life to the present. New estimates ( 1954) of the age of the earth place its begin-
ning at 5,000,000,000 years and the beginning of life at 3,500,000 to 4,000,000
years. (Redrawn after Ritchie, New York Times, Sept. 29, 1940.)
existed. There are no fossils of the first soft bits of living matter whose de-
velopment must have taken eons of time. The oldest fossils are those of simple
water plants that are more than one billion years old. After they appeared there
seems to have been a tremendously long period before living organisms were
numerous and varied enough to leave a continuous fossil record.
Evolution is an unimaginably long process that includes periods of pro-
found geologic change. In some of these, the currents of life seem to have
moved more rapidly; in others, they flowed slowly and evolutionary changes
were slight. Fossils in the Cambrian Period mark the beginning of an upswing
of change (Table 38.1).
The earth's past history has been divided into eras according to the evo-
Chap. 38
ORGANIC EVOLUTION CONSERVATION
779
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780 EVOLUTION AND CONSERVATION Part VI
lutionary advancement of life, such as Pre-cambrian, Paleozoic (primitive
life), Mesozoic (intermediate life), and Cenozoic (modern life); even the
Cenozoic Era extends back millions of years. The eras are divided into
periods or epochs named for the locality where the rocks formed in that period
were found or are best developed. Thus Cambrian, Ordovician, Silurian, and
Devonian take their names from ancient inhabitants of England or Wales.
Jurassic refers to the Jura Mountains in Switzerland and Cretaceous to the
chalk deposits in western Europe. The limits of all these eras, periods, and so
on are due to changes of conditions especially of climate caused by that pro-
found shifting of the earth's crust that gave rise to mountains, moved the lines
between sea and land, and caused destruction or changes in the inhabitants.
Increase of Life
From its beginning, life increased. It began spreading over the earth and
has never stopped. There were animals in the sea during the Cambrian Period
but none on land or in the air. Now, through great expanses of the earth every
handful of soil is alive with organisms, microscopic or otherwise; the tropics
and all summer airs resound with songs of birds and the hum of insects. The
history of plants and animals is the story of increase, and the invasion and
filling of habitable space (Fig. 38.2). New occupants opened as well as closed
the way to others. Wherever plants grew on land the plant-eating animals fol-
lowed, and where the plant eaters were the carnivores came and preyed upon
them. Little animals lived in the spaces between large ones. Insignificantly
small mammals hid among the giant dinosaurs of the Late Reptilian Age. Long
time residents of the water, such as the protozoans, moved into the pools where
the newly come large animals fed and lounged. Certain of the protozoans
moved into the larger animals and finally became parasites. Animals took the
places left by other animals through desertion or death. As mammals over-
spread the earth, porpoises and dolphins took possession of the seas in which
the great swimming reptiles (ichthyosaurs) had lived before them. Replace-
ments were not exact for environments changed.
All living plants and animals have behind them unbroken streams of life
that come from beginnings which we may surmise but do not know.
The Environment and the Organism
Adaptations. An adaptive structure or characteristic of an organism is one
that is useful to it under the conditions in which it lives. Two mechanisms by
which adaptations become established are inheritance and natural selection
or the selective action by the environment. This question of how living things
come to be the way they are is far from answered although many facts are
known. Adaptation is characteristic of all living organisms and is one of the
key puzzles of nature.
Chap. 38
ORGANIC EVOLUTION CONSERVATION
781
Fig. 38.2. The gradual clambering on to the land, a restoration of early amphib-
ians (labyrinthodonts) of ancient Carboniferous times, the period of coal deposits.
The history of plants and animals is the story of increase, and the invasion and
filling of habitable space. Painting by F. L. Jaques. (Courtesy, American Museum
of Natural History, New York.)
Conditions and Varieties of Adaptations. Adaptations of one kind shut
out others. Australian koalas ("teddy bears") live where eucalyptus trees
are abundant and they are adapted to a pure diet of their leaves. They cannot
live on anything else. Birds use their bills and feet to manipulate their food but
those that are highly efficient tools for one skill are worthless for some other
— the beak and talons of a hawk are poor seed pickers (Fig. 36.7).
An anteater that can poke its snout into an anthill and collect a dozen ants
on its sticky tongue could scarcely use it to catch a grasshopper (Fig. 5.3).
Such a particular tool is overspecialized, on a byroad, even a dead end. It
allows its owner only one very particular kind of food. An anteater must have
ants or starve. The zigzag course of evolution is full of byroads and pockets of
adjustments so perfectly special that they come to a standstill in their perfec-
tion. Among them are the sponges with their elaboration of water tubes and
the starfishes with their structures locked to the number five.
Adaptations of Particular Structures in Different Species. The
fore limbs of vertebrates show striking and varied adjustments to use in each
of the three basic environments — water, air and land. The relation of the bones
782 EVOLUTION AND CONSERVATION Part VI
DEVELOPMENT OF FLIPPER
k ♦ ^
Fig. 38.3. Models of the developing left front flipper of a whale. Note the five
digits in the first stage shown, more like a paw or hand than a flipper. In their
earlier development the limbs of whales are strikingly like those of their ancestors,
the land mammals. Later they become the flippers whose shape is adjusted to swim-
ming. Within the flipper, however, the bones are similar in location and relationship
to those of the ancestors of whales that lived on land. (Courtesy, British Museum,
South Kensington, London, England.)
I
to one another is essentially the same, that is, the parts are homologous in
spite of their striking differences in form and function (Fig. 38.3). The basic
fore limb of land vertebrates is five-toed and adapted for walking. It has under-
gone great changes in different environments and yet has kept a basic plan
(Figs. 38.4, 38.5). It may be close to the type, five-toed and soft padded, the
silent walking foot of cats; or farther from the type, the single tiptoe running
foot of horses (Fig. 38.6); the five-fingered grasping hand of man; the wing
of a bird with thumb and first two fingers corresponding to the human hand;
the bat's wing supported by four long fingers; and the fleshy flipper of whales
and seals (Fig. 38.3). Wings have developed three times during the evolution
of vertebrates — in the ancient flying reptiles, in birds, and in bats. They are
examples of convergent evolution in the air. The structure arose from the
same ancestral stock and retained the same ground plan but differed in ex-
pression. In other cases, a water environment offered an opportunity for
adaptation in three different classes of animals (Fig. 38.4).
Racial Long Life. Long ago certain animals reached an almost perfect
state of adaptation to environments in which there have been no essential
changes. These animals have been unstirred to further evolution. For genera-
tion after generation, through millions of years, they have scarcely changed.
Among these museum pieces of antiquity are the little reptile Sphenodon (Fig.
35.4) which closely resembles the fossils of its ancestors of the Jurassic Period
(Table 38.1), the opossum, the "living fossil" Lingula (Fig. 27.15) so like
its brachiopod ancestors that are known from their fossil remains of 400 mil-
lion years ago, and the common edible oysters very like their ancestors of
Chap. 38
ORGANIC EVOLUTION CONSERVATION
783
Fig. 38.4. Convergent evolution {upper) by a fish, the shark; (center) by a
reptile (ancient Icthyosaurus); (bottom) and a mammal, the dolphin. They all live
or did live in the sea and all are fish-shaped although only distantly related. (Cour-
tesy, Moody: Introduction to Evolution. New York, Harper & Bros., 1953.)
more than 200 million years ago. Certain of these ancient animals had offspring
that started side lines of descent. Some of these prospered and others disap-
peared. Those in the main lines lived on in uneventful safety as we see them
now.
Nonadaptive Trends. These are tendencies for certain characteristics to keep
developing until they become useless or dangerous. Great increase in size is
one of these. Growth with increase in size is universal in living organisms. It
usually reaches a slightly variable limit evidently an adaptation for the plant
or animal and this is repeated generation after generation. We think of a mouse
of one size, a horse of another. In contrast to this was the size of the dinosaurs,
with Brontosaurus, 75 feet long, hazardous especially for land animals. There
were other causes for their extinction, but giantism must have been an impor-
tant one. The heavy, multibranched antlers of deer are claimed as nonadaptive
features. In connection with adaptation, as with nonadaptation, it is reaUzed
that many structures are useless when they begin to develop and are not large
enough to be selected by the environment till long afterward. In "The Origin
of Species" Darwin pointed out that nonadaptation was an unexplained diffi-
culty in the working of natural selection in evolution.
784
EVOLUTION AND CONSERVATION
Part VI
ARBOREAL
Climbing
TERRESTRIAL
Short, pentadactyl
limbs
CURSORIAL
Running
AQUATIC
> Swimming ^«^<.«r....
^ FOSSDRIAL
Burrowing
Fig. 38.5. The structure of the Hmbs of mammals that live in different environ-
ments. At the center is a primitive 5-toed terrestrial mammal. The other figures
show other mammals related to the central one but adjusted to particular environ-
ments and ways of living in them. This is called adaptive radiation. (Courtesy,
Moody: Introduction to Evolution. New York, Harper & Bros., 1953.)
Heredity — A Force in Evolution
Heredity produces both unity and diversity. It maintains old fundamental
structures and activities and it establishes the new features known as mutations
that partly account for the entrancing variety of nature.
Inheritance of Ancestral Pattern in Embryos. Except in some special types
of reproduction, every multicellular animal begins life as one cell, a fertilized
egg which divides into two cells, and goes on according to the course of its
ancestors. The embryos of various invertebrates show striking similarities,
maintained by inheritance and expressive of kinship. Those of various groups
of animals are figured and described in Part 5, Evolution of Animals. In-
heritance of ancestral pattern in embryos includes only the oldest and most
fundamental structures. In the vertebrates, these are the notochord, the ver-
Chap. 38
ORGANIC EVOLUTION CONSERVATION
785
Fig. 38.6. Evolution of the horse. Within each section from left to right the
drawings show: a reconstruction of an ancestor of the horse in the surroundings
in which it is believed to have lived; fossil remains of the animal's fore and hind
feet displaying the progressive reduction in the lateral digits; one of the molar
teeth and the skull. Only the teeth are drawn to scale. The oldest horse (eohippus)
in the bottom section, was about the size of a fox terrier (12 inches tall). (Cour-
tesy, Rogers, Hubbell, and Byers: Man and the Biological World, ed. 2. New York,
McGraw-Hill Book Co., 1952.)
786
EVOLUTION AND CONSERVATION
Part VI
tebrae, gill slits, aortic arches, and two-chambered heart that exist for different
lengths of time.
The top Hne of drawings in figure 38.7 shows young embryos with funda-
mental structures that are common to all vertebrates — a striking presentation
of similarities. Figures in the next line show that the body form of the fish is
not shared by the cow. Finally, in each of the oldest embryos there are one or
more structures that are unique, the shell of the turtle, wing of the bird, snout
of the pig, and the domed head of man. Unity is apparent in the younger
embryos. Diversity is striking in the older ones.
Vestigial Organs. These are small, useless vestiges of structures that may be
well-developed and functional in near kin and ancestors of their owners. They
are maintained by heredity and the conservatism of living matter — treasured
clocks that have stopped ticking. The human body has a collection of several
dozen such structures. Among them are the ear muscles so feeble in man, so
active in horses; the vermiform appendix, a nuisance in man, a digestive cae-
cum in rabbits; and the nictitating membrane, a little fold of flesh in the inner
angle of the human eye, a protective membrane that may instantly slip over
the eye of a bird.
Fish
SaioAtuindcr Tortoise
Fig. 38.7. Vertebrate embryos showing the inheritance of a unified basic plan
followed by diverse structure which easily identify the animals. Embryos in three
successive and comparable stages of development. Upper row, all are in general
much alike. Middle, lower vertebrates, fish and salamander show difi'erences
sooner than mammals, pig to man. Bottom, all types are recognizable. These figures
originally after Haeckel lack detail and certain points of accuracy but they excel
in emphasizing essential agreements and ultimate differences. (Courtesy, Pauli:
The World of Life. Boston, Houghton Mifflin Co., 1949.)
Chap. 38 ORGANIC evolution — conservation 787
Mutations. Evolutionary changes consist of changes in heredity. Mutations
are probably pre-eminent among them. They are sudden inheritable charac-
teristics caused by changes in genes, the chemical units of the chromosomes.
Mutations are discussed elsewhere (Chap. 20). This note is given here in view
of their place in evolution. Probably all genes mutate at some time; some of
them much oftener than others. Their frequency may be changed experi-
mentally, and also in nature, by heat, radiation, and other influences. Muta-
tions differ in extent; they may be "large," those having the greatest effect on
the animal, such as brittle bones in man, or they may be "small" such as nar-
row nostrils in man. The effects of mutations of the genes have no evident
relationship to the adaptation of the animal. They appear to be random
changes. This is true of experimental mutations; those caused by applications
of heat have no relation to adaptation to the temperature of the environment.
For an animal that is not well adapted to its life in a certain place, some ran-
dom mutation, however, might be the very one that would improve matters.
Evolution and the Kinships of Animals and Man
By many evidences, it is clear that all protoplasm has the same basic organi-
zation, and that all living things that exist or have existed are related including
man.
Humanity is bound to other animals by many and deep kinships. Neverthe-
less man is unique among all animals, in flexibility of behavior, in control of
the environment, in social organization, in degree of reasoning power, and in
other qualities of mind and its expression.
Humanity is unique in having two inheritances that are highly different, yet
blend on their borders. One of them is concerned with organic evolution, with
features such as the build and grip of the hand, the size and activity of the
brain. The other is a newer kind of social evolution built on cultures passed
on by legends and records even through long lapses of time (Fig. 38.8). Past
experiences are preserved, available for help and warning. The records of them
have accumulated greatly and constitute a complex story of ups and downs of
human thought and deed. Human beings alone are aware of their own evolu-
tion and their possible ability to direct it. In order to do so, they must use their
double inheritance especially that of experience in which at sometime ethics
appeared.
Separate Creations
The first time a puppy sees a strange animal in the grass he shies back. Then,
curiosity overcomes him and he goes closer. From earliest times, human beings
have been afraid of things. But, as with the puppy curiosity has prevailed;
mankind has drawn closer, inquired and tried to explain.
In the sixth, fifth, and fourth centuries B.C., Aristotle and other Greek
788
EVOLUTION AND CONSERVATION
Part VI
Fig. 38.8. Cro-Magnon art. Upper, a partial restoration of what has been termed
"the earliest picture in the world because it is a composition" (After Lankester).
It is an engraving on the antler of a deer representing a group of deer advancing.
The largest stag looks backward, his mouth open and "blowing." Lower, figure of
a wild horse carved in ivory from Lourdes, France. The relatively abundant skele-
tal remains of the Cro-Magnons indicate that they may belong to our own species
Homo sapiens. They lived in Europe perhaps as early as 40,000 B.C. and their
culture seems to have persisted until about 13,000 B.C. The name Cro-Magnon
is that of a French rock shelter where several of their remains were found. (Cour-
tesy, Cleland: Our Prehistoric Ancestors. New York, Coward McCann, 1928.)
philosophers described animals and set them in a progression from imperfect
to perfect — a procession with one behind the other and few questions asked.
There was little or no meddling into the relationships between them.
Arrangements of animals according to perfection and separateness became
fixed in general thinking. For 15 centuries and more of the Christian era,
special creation, the separateness of difTerent kinds of animals, was held essen-
tial to Christian belief. It pleaded for unity on the one hand and supported
separateness on the other. Toward the end of this long era, there were now
and then signs of a change.
From Separateness to Relationship
The first general theory of evolution (1809) was that of Jean Lamarck
(1744-1829), a French zoologist. Its basic plan was the sequence of living
organisms from less to greater perfection. This had been held long before
Chap. 38 ORGANIC evolution — conservation 789
Lamarck, but not as an evolution which he proposed for the first time and for
which he deserves great credit. He observed that the progress toward perfec-
tion in no wise followed a straight line, but was uneven and branched. He held
that the results of use or disuse of a structure, an arm or an eye, would be
inherited by the offspring and succeeding generations. This easy entrance of
recent change was emphasized and the theory became known as that of ac-
quired characters. By thousands of experiments and histories of succeeding
generations it has since been shown that acquired characters are not inherited,
at least in any such way as Lamarck maintained. The tails of horses may be
docked for generations but the tails of their descendants still grow long. La-
marck's theory fell into disrepute because of its mistaken explanation. Not-
withstanding this it drew attention to adaptation, exemplified by the honeybee
that fits the flower. Such adaptation was the same material to which Darwin
later applied natural selection.
Franklin and Malthus
Roughly within the span of Lamarck's lifetime, many another person was
thinking about the multiplicity of plants and animals and the great numbers
in human populations. There is room to mention only two of them, Franklin
and Malthus. In view of the great increase in the population of the American
Colonies, Benjamin Franklin (1706-1790) concluded (1751) that there is
no bound to the prolific nature of plants and animals except that which is
caused by crowding and competition for food. A similar principle was upheld
by Thomas Malthus in his Essay on Population (1798). Unless humanity re-
stricts its own rate of increase, war and hunger will do it. Malthus had been an
Anglican priest and when the essay was written he was teaching political
economy in Great Britain. He foresaw the disapproval that his book would
excite. But time never allowed him to know the constructive interest which it
was to kindle in the mind of Charles Darwin nor the important steppingstone
that it would be for the Theory of Natural Selection.
Charles Darwin
Charles Darwin (1809-1882) proposed the most adequate and influential
theory of organic evolution which has ever been stated. His materials were
plants and animals growing in their natural surroundings in various countries
and climates. His tools were keen observation and sound reasoning. His un-
limited use of these was his genius.
Darwin's school education led him into changes in professional training,
and from his own testimony into a waste of time in taking courses, including
preparation for medicine. He was an independent observer and thinker in his
chosen field of natural history. It was through this that he became friends with
some of the great scholars of Cambridge University, especially Professor J. S.
790 EVOLUTION AND CONSERVATION Part VI
Henslow whose encouragement of Darwin was lifelong. In the British scheme
of education, students have always been expected to learn and think for them-
selves. Darwin was happy in doing this.
What he termed "the most important event of my life" began in the autumn
of 1831, a few months after he was graduated from Cambridge University, at
22 years of age. In his student days, he had called himself a naturalist (the old
name for ecologist). He now became the official naturalist on the five-year
voyage of "the Beagle" (1831-1836) (Fig. 38.9). This was to be an expedi-
tion to learn of the plants and animals of South America, its coastal waters and
the famous Galapagos Islands, and to visit Africa, Australia and New Zealand.
There Darwin saw and lived with plants and animals in their own homes. He
felt the urge and press of tropical abundance. In the rain forests, he saw
crowded plants reaching for light, heard the deafening hum and clatter of
myriads of insects, and on the coral reefs he walked over packed coral animals
in numbers beyond thinking. He had already learned to observe and think. He
kept voluminous notes of what he had seen and of what he had thought.
Fig. 38.9. Charles Darwin in his thirty-first year, 1840. From a water color by
George Richmond, R.A. On October 2, 1836 Darwin had returned to England
after his five-year voyage on "the Beagle" which was the making of the Charles Dar-
win that the world was to know. Between 1836 and 1840 ideas about the multi-
plicity of kinds of life were coming into his mind. They persisted and in the Origin
of Species (1859) brought to the world the fact that human beings are fellow
voyagers with other animals in the great kinship of evolution. (Courtesy, West:
Charles Darwin, A Portrait. New Haven. Yale University Press, 1938.)
Chap. 38 ORGANIC evolution — conservation 791
Back in England, in London for a time, and later living in nearby Down,
he pondered upon the plants and animals that grew crowded together. He also
read the essay in which Malthus told of the human populations that became
too large for the space available to them (1838). This suggested a plan. Some
organisms must be winnowed out by their natural surroundings; thus, others
would be benefited. There would be a natural selection.
In 1844, Darwin wrote a summary of his theory but continued to gather
facts from his own observations and those of others. In the meanwhile, Alfred
R. Wallace (1823-1913), another English naturalist, arrived independently
at conclusions similar to those of Darwin. He had reached his conclusion also
after an exploring trip through the tropics. By mutual agreement and especially
through the desire of Darwin, the respective views of Darwin and Wallace were
read to the Linnaean Society, in London, on June 30th, 1858. Wallace shared
with Darwin the credit of propounding the theory of natural selection and
there was sincere friendliness between the two naturalists. Now, Darwin has
become famous throughout the world for a theory supported by thousands of
observations and years of study. And now, Wallace is relatively little known
for a conclusion which he arrived at honestly, independently and quickly, but
with little critical treatment and relatively few examples for proof.
Changes Preserved by Selection. The Origin of Species by Natural Selection,
or the Preservation of Favoured Races in the Struggle of Life, by Charles Dar-
win was published in 1859. It is regarded as the most widely influential book
of the nineteenth century and the leading classic in biology. Its effect upon
sciences and society in general was due to the vital nature of the theory and
no less to the convincing presentation of facts supporting it. The following
summary contains the essence of the theory.
1. Variations occur in individuals and species.
2. The numbers of every species tend to become enormously large, yet the
population of each remains nearly constant because of the effects of climate,
competition of other organisms, and other factors that eliminate many indi-
viduals.
3. This involves a struggle for existence. During this struggle, individuals
in which variations are favorable continue to live and produce their kind
whereas those having variations that are unsuitable in nature are eliminated.
4. A process of selection by the environment or natural selection operates.
5. There is a natural preservation of those that fit into a certain niche in
nature, a survival of the fittest.
Within a year after "The Origin of Species" was published Darwin admitted
that it would have been better to use "natural preservation" as a key phrase
for the theory.
The majority of biologists accept Darwin's theory as the most adequate
statement of evolution. Disagreements with it have been based upon the better
792 EVOLUTION AND CONSERVATION Part VI
understanding of processes that have been investigated since Darwin's time,
especially inheritance. Darwin himself was aware of the gaps in knowledge
and very wisely pointed them out. One of the finest results of his theory has
been the investigation it has set in action. Among the results is the clearer
understanding of heritable changes and the ways by which they are passed on
from one generation to another. These are discussed in "The Physical Basis of
Heredity" (Chap. 20) under "Changes in the Genes — Mutations" and other
topics. Now it is known that mutations (changes) occur in genes (the physical
units of heredity), and that the chromosomes which contain them may be
rearranged. This alters the assortment of genes and hence the characteristics
that are passed on to further generations. Darwin knew nothing of all this
but he suspected that there was much to be discovered.
Conservation
Humanity is facing two very old problems, living with itself and living with
its natural surroundings. Conservation is one way of working out these prob-
lems, an appreciation and intelligent care of living things and their environ-
ments. It is applied Ecology (Fig. 38.10).
Conservation brings many rewards. The rarest of them is the interest and
enjoyment of unspoiled landscapes and the plants and animals growing in their
natural places and in relationships, made right by ages of trial. Humanity
created civilization out of the wilderness. Now that the wilderness is almost
gone, we are beginning to be lonesome for it. We shall keep a refuge for our
minds if we conserve the remnants. Psychologists suspect this; fishermen
know it.
Writers and speakers discuss food and distribution of food. They discuss
the present extraordinary rise of population and ways in which larger popula-
tions shall be fed. They calculate the space that may be necessary to raise more
wheat and cattle. They do not give enough thought to the quality and quantity
of space for human beings, spaces to whet their curiosity and adventure, to
show them natural beauty, to give them places that are far from crowds. Con-
servation must guard the open spaces. A full stomach is not a cure-all.
The results of conservation that are best known are concerned with the care
and economy of natural resources that are vital to communities, and to agri-
culture and industries. An awareness of the importance of saving the grass
roots is increasing; fortunately one of its byproducts is the saving of minds.
Natural Resources
Natural resources are everything in nature that is used to sustain life. Those
called nonrenewable resources include metals, petroleum, gas, and coal, and
the energy of the sun, abundant beyond imagination. It has taken ages to pro-
duce them and, except for atomic energy, substitutes in any practicable
amounts are not available. The program of conservation for such material is:
CERTAIN
BEETLES EAT
EGGS OF
COLLOPS
PREDACEOUS BUGS, WASPS,
LADYBIRD BEETLES,
BIRDS
BIRDS
THE ENEMIES OF ALFALFA CATERPILLAR
t
WILT
DESEASE
SPIDERS
DRAGON FLIES
ROBBER FLIES
ALFALFA CATERPILLAR
LARVA
PUPA
ADULT
B. STAGES IN THE LIFE OF THE ALFALFA CATERPILLAR
I
DAYS
1
15
.l-^A
A. ALFALFA, THE BASIC FOOD, GROWTH - HARVEST
RELATIONSHIPS IMPORTANT IN CONSERVATION. READ UP
Fig. 38.10. The relationships of alfalfa plants and the various animals associated
with them are an example of the natural control of populations. Change in one
population brings changes in others. Knowledge of such relationships is essential
for conservation. A, in favorable climates the widely cultivated clover-like alfalfa
grows to full size in 30 days. B, the lifetime, egg to adult of the orange and yellow
butterflies (C alias philodice eiiry theme) is also about 30 days. In the populations
of alfalfa plants and alfalfa butterflies however, there are always various stages of
development. The butterflies lay their eggs mainly on young plants. The cater-
pillars feed heavily on all the plants. The pupae are fastened to the stems. C, main
enemies of the caterpillars and adults. Pupae suffer least. D, the enemies of enemies
of the alfalfa pests; each group keeps other groups from the destruction of over-
population. (Based on Smith, Bryan and Allen: "The Relation of Flights of Colias
to Larval Population Density," Ecology, 30:288-297; U. S. D. A. Bull 124, and
personal communication.)
793
794 EVOLUTION AND CONSHRVATION Part VI
avoid waste of the product. Coal can be burned once; the products of the fire
do not return to coal again. What a diflfcrence in the heating bills if oil or coal
could be reburned! What a difference when atomic energy can be turned to
peaceful ends!
Renewable Resources. Soil, water, air, and living organisms of all kinds are
renewable resources. Air and water can be used over again; soil and living
organisms are in certain ways renewable. In one or another situation, all of
these need care in order to preserve their greatest usefulness; air needs the
least; soil and living organisms the most. There are excellent books that deal
with the earth's natural resources, with definite methods, e.g., of keeping
streams clean enough for fishes, and of guarding the trees in house lots as well
as forests. There are books that deal with the extraordinary increase in human
populations of the earth and its relation to space and other possessions and to
war. A few are mentioned in the Reading List for this chapter.
Only one natural resource, the soil, may be further mentioned here. It is
one of the most important and rapidly disappearing resources of them all.
Natural soil is made of particles of weathered rocks mixed with organic matter
— the scattered substance of dead plants and animals intimately associated
with living ones, myriads of bacteria, roots searching for water, and burrowing
animals, microscopic and otherwise. Such soil occurs only in the shallow upper
layers of the earth's crust. It is the fertile layer that pulsates with daily changes
of temperature and activity of life, and the deeper changes of seasonal tempera-
ture and moisture, and animal migrations. There are chemical cycles of dearth
and abundance of a given substance, e.g., perhaps calcium compounds weath-
ered from limestone and transported by water. Calcium may be picked up by
roots, locked in the plant for its lifetime, then returned to the soil from the
dead and softened tissues. Other substances come and go — carbon, nitrogen,
sulfur, and others. Soil formation is carried on by the energy of the sun and
secondarily by the energy liberated from weathering rock and broken tissues.
This fountain of energy flows upward from the fertile soil through the plants
that grow out of it, from the insects that live upon the plants, through the birds
and rodents that feed upon the plants and insects, and on into the carnivores —
shrews that devour insects, and cats that eat field mice. This upward stream of
energy flows through a chain of food. It returns to the soil in the byproducts
of living and in the dead bodies of plants and animals.
By natural methods, it takes hundreds of years to make an inch or two of
fertile topsoil. By human means, it takes work and money and years, more in
some regions than others. It is estimated that since farming started in the
United States one third of the whole area of topsoil has been lost, overworked,
carried by wind, washed into the rivers, and taken into the sea. Under the good
topsoil, there is another layer of soil, poor but present. Land may be danger-
ously hurt; but not finally destroyed. Conservation of soil is an effort to renew
its pulsating energy.
Appendix
Scheme of Classification
Example: Man — Homo sapiens
Phylum Chordata
Subphyliim Craniata — Vertebrata
Class Mammalia
Order Primates
Family Hominidae
Genus Homo
Species sapiens
A species is the smallest standard group into which plants or animals are
classified. Members of a species are alike except for relatively slight, more or
less inconstant differences and can interbreed. A genus includes a number of
species that have many features in common. Similarly, a family is a group of
genera, an order a group of families, a class a group of orders, and finally a
phylum a group of classes that have fundamental likenesses. Thus the phyla
are the largest groups into which the plant or animal kingdom is divided.
Throughout their history classifications have varied with the knowledge of
the classifier. They still vary especially in the genera and species, hence there
is no one true or best classification.
The Plant Kingdom
Several tables have been consulted and parts included in the following table;
especially those in T. I. Storer, General Zoology, 2nd. ed.. New York, McGraw-
Hill Co., 1951. C. A. Villee, Biology, 2nd. ed., Philadelphia, W. B. Saunders Co.,
1953, and Zoological Names. Prepared for Sect. F., Am. Assoc, for the Advance-
ment of Science, 1949.
Phylum Thallophyta. The simplest plants, without true roots, stems or leaves
(about 107,000 species).
Subphylum Schizophyta
Class Bacteria
Class Cyanophyceae — blue-green algae. Most primitive plants.
Subphylum Algae. Thallophytes with chlorophyll.
795
796 APPENDIX
Class Chlorophyceae — green algae, with definite nuclei and chloroplasts.
Ex. Volvox, Spirogyra
Class Phaeophyceae. The brown algae, large seaweeds.
Class Rhodophyceae. The red algae, usually marine plants.
Class Bacillariaceae. Diatoms.
Subphylum Fungi. Thallophytes without chlorophyll, either parasites or
saprophytes
Class Myxomycetes. Slime molds. The body is a blob of protoplasm con-
taining many nuclei, but not perfectly divided into cells.
Class Phycomycetes. Bread molds and leaf molds.
Class Ascomycetes. Yeasts, mildews and cheese molds.
Ex. Penicillium.
Class Basidiomycetes. Mushrooms, rusts, smuts.
Phylum Bryophyta. Multicellular plants, with a marked alternation of sexual
and asexual generations (23,000 species).
Class Hepaticae. Liverworts.
Class Musci. Mosses.
Phylum Pteridophyta. Multicellular, terrestrial plants, with true roots, stems
and leaves, and with alternation of sexual and asexual generations. The
asexual generation is more prominent.
Class Lycopodineae. Clubmosses, ground pines.
Class Equisetineae. Horsetails.
Class Filicineae. Ferns.
Phylum Spermatophyta. Multicellular plants with well-developed roots, stems
and leaves. The familiar dominant is the sporophyte or asexual plant. Trees,
shrubs, and seed plants.
Subphylum Gymnospermae. Without flowers; the seeds are borne on the
surface of the cone scales. Order Coniferales. Evergreen trees and shrubs,
pines, firs, with needle-shaped leaves.
Subphylum Angiospermae. Flowering plants with seeds enclosed in ovary.
Class Dicotyledoneae. Most flowering plants. Embryos with two seed
leaves or cotyledons.
Order Rosales, rose, apple, strawberry, cherry and others. A
dozen and more orders containing great numbers of familiar
flowering plants.
Class Monocotyledonae. Leaves with parallel veins. Embryos with one
seed leaf. Grasses, lilies, and orchids.
The Animal Kingdom
Animals rarely have stiff cell walls and do not have chlorophyll. The excep-
tions are mainly border line organisms such as Euglenas that are brilliant green.
Phylum Protozoa. The simplest animals, one-celled, microscopic, some of them
living in colonies. Many are free-living; others are parasitic. Ameba, Vorti-
cella (colonial).
APPENDIX 797
Class Flagellata. Protozoans that swim by flagella. The group probably
most nearly related to one-celled plants. Euglenas.
Class Rhizopoda. Protozoans that move by pseudopodia. Amebas,
Class Sporozoa. Parasitic protozoans. Malarial parasites.
Class Ciliata. Protozoans that move by means of cilia. Paramecia.
Phylum Porifera. The sponges, the simplest of the many-celled animals, in
many ways resembling colonies of protozoans. Fresh-water and marine
forms.
Class Calcarea. With limy skeletons. Scypha (formerly called Sycon).
Class Hexactinellida. With 6-rayed silicious spicules. Glass sponge.
Class Demispongiae. Skeletons of elastic spongin and with silicious spic-
ules. Bath sponge.
Phylum Coelenterata.
Class Hydrozoa. Hydralike animals, either single or colonial. Nearly all
marine. Hydra.
Class Scyphozoa. Jellyfishes. Aurelia. Marine.
Class Anthozoa. Corals and sea anemones. Marine.
Phylum Ctenophora. Comb jellies or sea gooseberries. Marine,
Phylum Platyhelminthes. Flatworms.
Class Turbellaria. Nonparasitic flatworms. Planarians.
Class Trematoda. Many are internal parasites. Flukes.
Class Cestoda. Tapeworms. Parasites.
Phylum Nemertinea. Ribbon worms. Free-living. Most of them marine.
Phylum Nematomorpha. Horsehair worms. Aquatic and parasitic.
Phylum Acanthocephala. Spiny-headed worms. Parasites.
Phylum Phoronida. Marine tube dwellers.
Phylum Gastrotricha. Microscopic. In fresh and salt waters.
Phylum Chaetognatha. Glassworms, arrow worms. Marine.
Phylum Brachiopoda. Lamp shells, about 225 living species from the great
numbers that once existed. Marine.
Phylum Rotifera. Rotifers, wheel animalcules. Abound in fresh water.
Phylum Bryozoa. Bryozoans, moss animals. Most of them marine.
Phylum Nematoda. Roundworms. In soil, water, roots of plants, parasitic in
animals. Trichina, hookworm.
Phylum Annelida. Segmented worms. Soil, and fresh and salt waters.
Class Polychaeta. Most of them marine. Clamworms, Nereis.
Class Oligochaeta. Fresh water and land. Earthworm.
Class Hirudinea. Fresh and salt water and land. Leeches.
Phylum Arthropoda. Joint-footed animals.
Class Onychophora. Few species known. Little known tropical animals,
intermediate between annelids and arthropods. Peripatus.
Class Crustacea. Lobsters, crabs, crayfishes, water fleas, sowbugs. Fresh
and salt water and land.
Class Chilopoda. Centipedes. On land, mainly tropical.
798 APPENDIX
Class Diplopoda. Millipedes, thousand-legged worms. Land in damp
places.
Class Arachnoidea. Spiders, scorpions, mites, ticks, horseshoe crabs.
Class Insecta. Probably the largest group of animals. Grasshoppers, ter-
mites, dragon flies, water-striders, lice, fleas, beetles, butterflies and moths
and others.
Phylum Mollusca. Mollusks. Fresh and salt water and land.
Class Amphineura. Chitons, shell composed of 8 plates. Marine.
Class Gastropoda. Snails, slugs, abalones. Fresh and salt water, and land.
Class Pelecypoda. Clams, mussels, oysters, scallops. Fresh and salt water.
Class Cephalopoda. Squids, cuttlefishes, octopuses. Marine.
Phylum Echinodermata. All marine.
Class Asteroidea. Starfishes.
Class Ophiuroidea. Brittle stars.
Class Echinoidea. Sea urchins and sand dollars.
Class Holothuroidea. Sea cucumbers.
Class Crinoidea. Sea lilies. Most of the group known only as fossils.
Phylum Chordata. Chordates, bilaterally symmetrical animals with a noto-
chord. Fresh and salt water and land.
Subphylum Hemichorda. Acorn worms. During their development they re-
semble larvae of echinoderms. Marine.
Subphylum Urochorda. Sea squirts (tunicates). Marine.
Subphylum Cephalochorda. Amphioxus. Marine.
Subphylum Vertebrata. Vertebrates. Those with a definite head, a well-de-
veloped brain and a chain of supporting bones, the vertebrae.
Class Cyclostomata. Lampreys. Vertebrates without jaws or paired fins.
Class Chondrichthyses. Sharks, rays, skates and other fishes with car-
tilaginous skeletons.
Class Osteichthys. Sturgeon, garpike, lung fish, herring, mackerel, and
other fishes with bony skeletons.
Class Amphibia. Amphibians.
Order Urodela. Tailed amphibians, newts, salamanders.
Order Anura. Tailless amphibians, frogs, toads.
Order Apoda. Caecilians. Body wormshaped; no limbs. They live
in the tropics.
Class Reptilia. Reptiles.
Order Rhynchocephalia. Primitive lizardlike reptile, only one liv-
- ing species, Sphenodon or tuatara of New Zealand.
Order Crocodilia. Crocodiles, alligators.
Order Chelonia (or Testudinata). Turtles, tortoises, terrapins.
Order Squamata. Lizards and snakes.
Class Aves. Birds. The only animals that have feathers.
Subclass Ratitae. Flightless birds. Ostrich, cassowary, emu, kiwi.
Subclass Carinatae. All can fly except the penguins and a few species
in various orders. Penguins, cormorants, swans, ducks, geese, tur-
APPENDIX 799
keys, hawks, eagles, vultures, pigeons, parrots, owls, hummingbirds,
and all the perching birds such as sparrows and thrushes.
Class Mammalia. Mammals. The only animals that have true hair.
Subclass Prototheria. Egg-laying mammals, monotremes, duckbilled
platypus, spiny anteater.
Subclass Metatheria. Pouched mammals, marsupials. Kangaroo,
opossum, and others, nearly all of them native to Australia.
Subclass Eutheria. Placental mammals. Young developed in the
body of the mother and attached to the uterus by a placenta.
Order Xenarthra. Armadillo, sloth.
Order Insectivora. Moles, shrews, hedgehogs.
Order Chiroptera. Bats.
Order Lagomorpha. Pikas, hares, rabbits.
Order Rodentia. Squirrels, rats, mice, beavers, gophers, etc.
Order Proboscidea. Elephants.
Order Hyracoidea. True coneys of Syria and Africa, e.g., Hyrax.
Superficially resemble guinea pigs but related to hoofed animals.
Order Perissodactyla. Odd-toed hoofed mammals. Horses, zebras,
rhinoceros, tapir.
Order Artiodactyla. Even-toed hoofed mammals. Pigs, hippo-
potamus, deer, giraffe, sheep, cattle, etc.
Order Cetacea. Whales, dolphins, porpoises.
Order Sirenia. Sea cows. Large plant-eating aquatic mammals.
Order Carnivora. Walruses, seals, dogs, cats, bears, weasels,
foxes, wolves, etc.
Order Primates. Lemurs, tarsiers, monkeys, apes and man.
Equivalent Measurements
Table 1
Units of Weight
Metric
Avoirdupois
1 kilogram (kg.)
or
1,000 grams (gm.)
2 pounds (lb.), VA ounces (oz.)
1 gram (gm.)
or
1,000 milligrams (
mg.)
0.035 ounces (oz.)
or
15.43 grains (gr.)
1 milligram (mg. )
or
1,000 micrograms
(Mg)
0.015 grains (gr. )
Examples
A man may weigh 75 kilograms or 165 pounds. His heart weighs about
lOVi ounces, or 300 grams. He began life as a fertilized egg about 0.1 milli-
meters in diameter and weighing about 0.5 of a microgram.
800
APPENDIX
Table 2
Units of Length
Metric
English
1 meter
or
100 centimeters (cm.)
3 feet (ft.), VA inches (in.)
1 centimeter (cm.)
or
10 millimeters (mm.)
^3 inch (in.)
1 millimeter
or
1,000 microns (ix)
1/25 inch (in.)
1 micron (/u)
or
1,000 millimicrons (m/x)
1/25, 400 inch (in.)
Examples
A 6 foot, 6% -inch man is 2 meters tall. At birth he was about 20 inches,
or 50 centimeters long. His red blood corpuscles are about 7.5 microns in
diameter.
Table 3
Units of Volume
Metric
Apothecaries' Measure
1 liter (1.)
or
1.000 cubic centimeters (cc.)
1.06 quarts (qt.)
or
2.11 pints (pt.)
1 cubic centimeter (cc.)
or
1,000 cubic millimeters (cu.mm.)
0.034 fluid ounces (fl. oz.)
or
0.27 fluid drams (fl.d.)
1 cubic millimeter (cu.mm.)
0.016 minim (min.)
(1 min. = 1 drop)
Examples
A man who weighs 165 pounds (or 75 kilograms) has about 12% pints,
or 6 liters, of blood.
Suggested Reading
The references are grouped by chapters with those in periodicals placed at the
end of each group.
The references include well-seasoned books, and new ones, selected because they are
important, well written and lively. Even in comparative anatomies there may be humor,
detectable to readers who are sensitive to it.
1. Relationships of the Living World
Menzel, D. H.: Our Sun. Philadelphia, The Blakiston Company, 1949. (Its publication
now [1955] controlled by Harvard Press, Cambridge, Mass.) A small, well-illus-
trated book based on the work of eminent astronomers.
Kalmus, Hans: "The Sun Navigation of Animals," Scientific American, 191:74-78 (Oct.
1954). Such navigations as those of bees locating the direction to food by its angle
with respect to the sun, and movements of starlings shown by experiment to be
dependent on the sun.
2. Life Is a Concern of Matter and Energy
Curie, Eve: Madame Curie. Translated by Vincent Sheean. New York. Doubleday,
Doran and Co., 1937. An account that expresses the dramatic quality of the original
discovery of radium.
Eddington, a. S.: Stars and Atoms. New Haven, Yale University Press, 1927. Astron-
omy and physics discussed with competence and appeal.
Lemon, Harvey B.: From Galileo to Cosmic Rays. A New Look at Physics. Chicago,
University of Chicago Press, 1946. Interpretation given in nontechnical language
with familiar examples. Chapters on Electrons, Positive Rays, Protons and Isotopes,
Radioactivity. Fully illustrated.
Moulton, F. R., and J.J. Schifferes, eds.: The Autobiography of Science. New York,
Doubleday, Doran and Co., 1945. Great steps in science recorded in the original
words (or translations) of those who achieved them.
Weaver, W., ed.: The Scientists Speak. New York, Boni & Gaer, 1947. Eighty-one
leading American scientists, most of them research workers in the branches they
represent, have joined in this symposium. The separate discussions, each contained
in two or three pages of this small book, are highly authoritative and clearly written.
Kamen, M. D.: "Tracers," Scientific American, 180:31-41 (1949).
3. Living Matter and Cells
De Robertis, E. D. p., W. W. Nowinski, and F. A. Saez: General Cytology, 2nd ed.
Philadelphia, W. B. Saunders Co., 1954. Advanced reference for chemical and
physiochemical organization of the cell, submicroscopic organization, functions of
organoids, plasma membrane and cell permeability, chromosomes and cell division.
Heilbrunn, L. v.: An Outline of General Physiology, 3rd ed. Philadelphia, W. B.
Saunders Co., 1952. Discussions on advanced level, with excellent examples. Chap-
801
802 APPENDIX
ters on: Chemical Aspect of Protoplasm, Osmosis, Enzymes and Metabolism,
Growth.
Snyder, L. H.: The Principles of Heredity, 4th ed. Boston, D. C. Heath & Co., 1951.
Excellent reference.
Spear, F. G.: Radiations and Living Cells. New York, J. Wiley & Sons, 1953. A small
book, clearly written and interesting to the intelligent reader. An introduction to
the action of radiation on living cells, especially those of human tissues.
WiLLMER, E. N.: Tissue Culture, 2nd ed. New York, J. Wiley & Sons, 1954. The growth
and differentiation of normal tissues in artificial media. The essentials of the
methods of culturing cells outside the body; a small book.
Wilson, E. B.: The Cell in Development and Heredity, 3rd ed. New York, The Mac-
millan Co., 1925. A classic by an important authority.
SiNNOTT, E. W., and K. Wilson: Botany: Principles and Problems, 5th ed. New York,
McGraw-Hill Book Co., 1954. An excellent and widely used book.
Bonner, James: "Chemical Warfare Among the Plants," Scientific American, 180:
48-51 (Mch. 1949). Remarkable plant relationships. Some plants have chemical
weapons with which they attack their neighbors. Penicillin is a familiar one.
SCHROCKEN, v.: "Plant Hormones," Scientific American. 180:40-43 (1949).
Wilson, M.: "Priestly," Scientific American, 191:68-73 (Oct. 1954). This article is
about Priestley as a scientist and even more as a person who struggled and suffered
in the cause of civil, political, and religious liberty. What happened to him in
1791 savors of the present times.
4. Plants Provide for Themselves and the Animals
Avery, G. S., Jr., and E. B. Johnson: Hormones and Horticulture. New York, McGraw-
Hill Book Co., 1947. Chapters on Hormones and the Rooting of Cuttings, Hormone
Treatment of Seeds.
Bonner, J. B., and A. W. Galston: Principles of Plant Physiology. San Francisco,
W. H. Freeman & Co., 1952. Excellent account of photosynthesis.
Fairchild, D.: The World Was My Garden. New York, Charles Scribner's Sons, 1938.
The world travels of a naturalist who traced plants to their original homes and
established valuable ones in this country.
Martin, A. C, H. S. Zim, and A. L. Nelson: American Wildlife and Plants. New York,
McGraw-Hill Book Co., 1951. A guide to the food habits of wildlife: the use of
trees, shrubs, and smaller plants by the birds and mammals of the United States.
It brings together the major research of the United States Fish and Wildlife Service
on American wildlife in relation to plants.
Platt, R.: This Green World. New York, Dodd, Mead and Co., 1946. Includes a highly
interesting and intelligible explanation of autumn coloration of deciduous trees.
A book to own.
Platt, R.: Our Flowering World. New York, Dodd, Mead and Co., 1947. Chapters
5 through 12 describe the adversities that plants have survived through the ages:
The Coal Age, Drifting Continents and the Ice Age. Vivid descriptions tell how
the trees and flowers of today have traveled to their present locations.
5. Animals and Their Environments
Allee, W. C, and K. P. Schmidt: Ecological Animal Geography, 2nd ed. New York,
John Wiley & Sons, 1951. Animals in their environments, giving about equal space
to sea, fresh water and land. Effect of civilization on the distribution of animals.
Carson, R. L.: The Sea Around Us. New York, Oxford University Press, 1951. Among
the chapters are: The Birth of an Island; Wind, Sun and the Spinning of the Earth.
Brief, searching accounts that create a consciousness and vision of the sea.
Clarke, G. L.: Elements of Ecology. New York, John Wiley & Sons, 1954. Excellent.
CoTT, H. B.: Adaptive Coloration in Animals. New York, Oxford University Press,
1941. An inclusive reference book with many illustrations.
MacGinitie, G. E., and N. MacGinitie: Natural History of Marine Animals. New
APPENDIX 803
York. McGraw-Hill Book Co., 1949. Firsthand observations, many but by no
means all of them made on the Pacific Coast.
Morgan, A. H.: Fieldhook of Ponds and Streams. New York. G. P. Putnam's Sons,
1930. Ponds and streams have lively populations. This book is an introduction to
them.
Morgan. A. H.: Fieldhook of Animals in Winter. New York, G. P. Putnam's Sons,
1939. Where and how animals spend the winter; "winter sleep" of hibernators,
hoarded food, migrations, winter resorts in water and on land.
Needham. J. G.: The Life of Inland Waters. Ithaca, N. Y., Comstock Publishing Co..
1937. A book whose content and grace of language make reading it a discovery
and pleasure.
Nice, M. M.: The Watcher at the Nest. New York, The Macmillan Co., 1939. The author
is a foremost authority on the behavior of birds in their home territory.
Odum, E. p.: Fundamentals of Ecology. Philadelphia, W. B. Saunders Co., 1953. Content
well chosen and arranged, concise, a small book.
Johnson, F. H.: "Heat and Life," Scientific Monthly, 191:64-68 (Sept. 1954). Life is
limited to the zone between the freezing and boiling points of water where enzymes
can exist and speed the reactions of metabolism.
Kalmus, Hans: "The Sun Navigation of Animals," Scientific American. 191:74-78
(Oct. 1954). Such navigations as those of bees locating the direction to food by its
angle with respect to the sun, and movements of starlings shown by experiment to
be dependent on the sun.
6. Mutual Relationships of Animals
Allee, W. C: "Animal Sociology," in Encyclopedia Britannica, 14th ed.. 1947.
Allee, W. C: Cooperation Among Animals. New York, Henry Schuman, Inc., 195 L
Cooperation is demonstrated in animals from protozoans to man. A brief and stimu-
lating discussion of relationships.
Allee, W. C. A. E. Emerson, O. Park. T. Park, and K. P. Schmidt: Principles of
Animal Ecology. Philadelphia, W. B. Saunders Co., 1949. Essential for everyone seri-
ously interested in the relationships of plants and animals.
Chandler. A. C: Introduction to Parasitology, 8th ed. New York, John Wiley & Sons,
1949. Excellent.
Dowdeswell. W. H.: Animal Ecology. London. Methuen & Co., 1952. Excellent; it is
brief, interesting, and inexpensive. Valuable for beginners of any age and training.
Elton, C: The Ecology of Animals. London, Methuen & Co.. 1933. By a leading
authority on populations, the Director of the Bureau of Animal Population at
Oxford University.
Tinbergen, N.: Social Behavior in Animals. London, Methuen & Co.. 1953. A small
book, clearly written, terse and interesting. Closes with hints for research in animal
sociology.
Tinbergen, N.: The Study of Instinct. Oxford, The Clarendon Press, 1951. Lectures
given in New York in 1947 under the auspices of the American Museum of Natural
History. They review the work done in animal behavior on the European continent
in recent years; not easily accessible elsewhere.
Wheeler. W. M.: Foibles of Insects and Men. New York, Alfred A. Knopf, Inc., 1928.
Observation, scholarship, and wit.
Zinsser, H.: Rats, Lice and History. Boston, Little, Brown & Co., 1935. Also paper
bound by Pocket Books, Inc. Parasites and typhus fever against a background of
human history; told with scholarship, wit, and skill.
7. Tissues
Bremer, J. L., and H. L. Weatherford: Textbook of Histology, 6th ed. Philadelphia,
The Blakiston Co., 1944. Arranged on an embryological basis.
Ham, a. W.: Histology, 2nd ed. Philadelphia, J. B. Lippincott Co., 1953. Emphasis on
function. Excellent for general and medical reference.
804 APPENDIX
Maximow, a. a., and Wm. Bloom: A Textbook of Histolof>y. 6th ed. Philadelphia,
W. B. Saunders Co., 1952. Excellent. Especially for medical reference.
Sherrington. C: Man on His Nature. London and New York. Cambridge University
Press, 1951. A small book, only for those who think. See Chapter 4, The Wisdom
of the Body.
8. An Agent of Evolution — The Body Covering
Rand, H. W.: The Chordates. Philadelphia, The Blakiston Co., 1950. See Chapter 16,
Skin of Mammals.
RoMER, A. S.: The Vertebrate Body, 2nd ed. Philadelphia, W. B. Saunders Co., 1955.
See Chapter 6, The Skin.
Hausman, L. a.: "Structural Characters of the Hair of Mammals," American Naturalist,
54:496-523 (1920). Figures show identification marks of hairs of various mammals.
Structural causes of colors of hair and gray hair.
WiSLOCKi, G. B.: "Studies on the Growth of Deer Antlers," American Journal of
Anatomy, 71:371-415 (1942). Interesting facts as well as a good example of in-
vestigation.
9. Protection, Support, and Movement — Skeletons
Rand, H. W.: The Chordates. Philadelphia, The Blakiston Co., 1950. Comparative
anatomy that portrays the evolution of the vertebrates. Excellent illustrations.
Romer, a. S.: The Vertebrate Body, 2nd ed. Philadelphia. W. B. Saunders Co.. 1955.
The structure and evolution of systems in various types of vertebrates. See Chapter
7, The Skeleton.
Simpson, G. G.: Horses. New York, Oxford University Press, 1951. An account of the
evolution of horses; for any intelligent reader.
SissoN, S.: The Anatomy of the Domestic Animals, 4th ed. Philadelphia, W. B. Saunders
Co., 1953. General and veterinary reference.
Chubb, S. H.: "How Animals Run: Some Interesting Laws Governing Animal Loco-
motion," Natural History Magazine, 29:543-551 (1929).
10. Movement — Muscles
Fulton. J. F.: Textbook of Physiology, 16th ed. Philadelphia, W. B. Saunders Co.,
1949. A standard advanced reference book.
Gray, James: How Animals Move. Cambridge, England, Cambridge University Press,
1953. Clearly expressed, highly interesting lectures with original illustrations.
Originally given to British children. Appropriate and informing to adults.
Hill, A. V.: Muscular Movement in Man. New York, McGraw-Hill Book Co., 1927.
A book to be known. The author writes with all possible simplicity about the
fundamentals of muscular movement.
Langley, L. L., and E. Cheraskin: The Physiology of Man. New York. McGraw-Hill
Book Co., 1954. A succinct presentation of the more important physiological
processes.
Prosser, C. L., et al.: Comparative Animal Physiology. Philadelphia. W. B. Saunders
Co., 1950. Discussions of muscle of invertebrates with many references. See
Chapter 16, Muscle and Electric Organs.
Rogers, C. G.: Textbook of Comparative Physiology, 2nd ed. New York. McGraw-Hill
Book Co., 1938. An old book that contains facts about invertebrates not easy to
find elsewhere. See Chapter 15, Physiology of Movement.
Schneider, E. C, and P. V. Karpovich: Physiology of Muscular Activity, 3rd ed.
Philadelphia, W. B. Saunders Co., 1948. Good reference for general structure and
function, especially for practical, commonly asked questions. Brief.
Szent-Gyorgi, a.: Nature of Life: A Study of Muscle. New York, Academic Press Inc.,
1948. Advanced.
appendix 805
1 1 . Foods and Nutrition
Babkin. B. p.: Pavlov. A Biography. Chicago. University of Chicago Press, 1949. A
Russian scientist who was devoted to fact. His most notable investigations were
made upon conditioned reflexes. In 1904 he received a Nobel prize for his work
on the digestive system.
Cannon, W. B.: The Wisdom of the Body. rev. ed. New York, W. W. Norton & Co.,
Inc., 1939. The content of the body: thirst and hunger as means of assuring
supplies, the constant balance of water, salt, sugar, protein, fat, and calcium
contents; the constancy of body temperature; the natural defenses of the body.
See Chapter 9, The Internal Environment and The Quality of Life.
Carlson. A. J., and V. Johnson: The Machinery of the Body, 4th ed. Chicago, Uni-
versity of Chicago Press, 1954. Excellent reference.
Gerard, R. W., ed.: Food for Life. Chicago, University of Chicago Press. 1952. Chapters
of note are: Preparation from Mouth to Cell; Enzymes Effective Agents; The Foods
of Animals and Men.
Sherman, H. C: Tiie Nutritional Improveiiient of Life. New York, Columbia University
Press, 1950. Traces the growth of man's awareness of nutrition; traces details of ad-
vances in major fields — energy foods, proteins and their amino acids, the mineral
elements, and the vitamins; gives basic principles of nutrition, present-day ap-
proaches to malnutrition, vitamin deficiencies; describes the human body as a
biochemical organism.
12. Circulation and Transportation — Body Fluids
Amberson, W. R., and D. C. Smith: Outline of Physiology, 2nd ed. Baltimore, Williams
& Wilkins Co., 1948. A standard reference book.
Best, H. B., and N. B. Taylor: The Living Body, A Text in Human Physiology, 3rd ed.
New York, Henry Holt & Co., 1952. A dependable, interesting, and useful book to
own.
Carlson, A. J., and V. Johnson: Machinery of the Body, 4th ed. Chicago, University
of Chicago Press, 1954.
Clark-Kennedy, A. E.: Stephen Hales (1677-1761). An Eighteenth Century Biography.
Cambridge. England. Cambridge University Press, 1929. Between his sermons Hales
made observations on the circulation of blood which rank with those of Harvey.
Classed as one of the best biographies of a scientific man written in recent years.
Ham, a. W.: Histology, 2nd ed. Philadelphia, J. B. Lippincott Co., 1953.
Harvey, William: The Motion of the Heart and Blood. (Original edition 1628).
Translated with notes by C. D. Leake. Springfield, 111., Charles C Thomas, Publisher,
1931. Harvey's own account of his experiments and conclusions.
Maximow, a. a., and W. Bloom: A Textbook of Histology, 6th ed. Philadelphia,
W. B. Saunders Co., 1952.
13. The Release of Energy — Respiration
Armstrong, H. G.: Aviation Medicine, 3rd ed. Baltimore, Williams & Wilkins Co., 1952.
Gerard, R. W.: The Body Functions. New York, John Wiley & Sons, 1941. Discussions
are stimulating, clearly written, and brief.
Krogh, a.: The Comparative Physiology of Respiratory Mechanisms. The Cooper
Foundation Lectures at Swarthmore College 1939. Philadelphia, University of
Pennsylvania Press, 1941. Excellent reference, clear, authoritative, brief.
Schneider, E. C, and P. V. Karpovich: Physiology of Muscular Activity, 3rd ed.
Philadelphia, W. B. Saunders Co., 1948. Good reference for respiration, blood
content, and circulation.
Stackpole, C. E., and L. C. Leavell: Textbook of Physiology. New York. The Mac-
millan Co., 1953. See Section 4 on respiration. Brief and meaty, an excellent book
to own.
806 appendix
14. The By-Products of Metabolism — Excretion
Ham, a. W.: Histology, 2nd ed. Philadelphia, J. B. Lippincott Co., 1953. Excellent
account of excretion.
Prosser, C. L., et al.: Comparative Aninuil Physiology. Philadelphia, W. B. Saunders
Co., 1950. See especially, excretion of crayfish, pp. 29-32.
Smith, Homer W.: From Fish to Philosopher. Boston, Little, Brown and Co., 1953. One
of the foremost authorities on the kidney traces the evolution of man by way of
the evolution of the kidney. The kidney, more than any other organ, is responsible
for maintaining the internal environment of the body. Excellent for the general
reader.
Stackpole, C. E., and L. C. Leavell: Textbook of Physiology. New York, The Mac-
millan Co., 1953. See The Role of the Kidney.
HowLAND, R. B.: "Experiments on the Contractile Vacuole of Amoeba verrucosa and
Parainoeciiim caudatiim," Journal of Experimental Zoology, 40:251-262 (1924).
Smith, Homer W.: 'The Kidney," Scientific American, 188:40-48 (1953). Excellent
account of the evolution of the kidney.
15. Chemical Regulation — Endocrine Glands
Allen, E., C. H. Danforth, and E. A. Doisy: Sex and Internal Secretions, 2nd ed.
Baltimore, The Williams & Wilkins Co., 1939. A standard advanced reference for
the foundation work on endocrines.
Avery, G. S., Jr., and E. B. Johnson: Hormones and Horticulture. New York, McGraw-
Hill Book Co., 1950. See note on Schocken and section on plant hormones.
Beach, F. A.: Hormones and Behavior. New York, Paul B. Hoeber, Inc., 1948.
Corner, G. W.: The Hormones in Hunuui Reproduction. Princeton, Princeton University
Press, 1942. An excellent account by a leading authority, well illustrated.
Ham, a. W.: Histology, 2nd ed. Philadelphia, J. B. Lippincott Co., 1953. An excellent
account of the endocrines, with a particularly clear discussion of the pituitary gland.
Hoskins, R. G.: Endocrinology, 2nd ed. New York, W. W. Norton & Co., Inc., 1950.
The more significant facts of endocrinology as known at this date. One of the best
general accounts for any intelligent reader.
Parker, G. H.: Animal Colour Changes and Their N euro-Hormones. Cambridge, Eng-
land, Cambridge University Press, 1948. Inclusive, interesting, advanced.
Stevenson, L.: Sir Frederick Banting, rev. ed. Springfield, 111., Charles C Thomas,
Publisher, 1947. Biography of the discoverer of insulin.
Turner, C. D.: General Endocrinology, 2nd ed. Philadelphia. W. B. Saunders Co., 1955.
A standard work on the endocrine glands of vertebrates and one chapter on those
of invertebrates.
WiGGLESWORTH, V. B.: Insect Physiology, 4th ed. New York, John Wiley & Sons, Inc.,
1950. Hormones chiefly in Chapter 7, Reproduction and Growth. A brief and inex-
pensive book containing chapters on the physiology of the main systems of the
insect body.
Bargmann, W., and E. Scharrer: 'The Site of Origin of the Hormones of the Posterior
Pituitary," American Scientist, 39:255-259 (1941).
Constantinides, p. C, and N. Carey: 'The Alarm Reaction," Scientific American,
180:20-23 (1949). The adrenal gland sends out its hormones in time of stress.
Gray, G. W.: "Cortisone and ACTH," Scientific American, 182:30-37 (1950).
HoAGLAND, H.: "Schizophrenia and Stress," Scientific American. 181:44-47 (1949).
Schocken, V.: "Plant Hormones," Scientific American, 180:40-43 (1949). A review of
how plant hormones have been studied and applied. Subjects such as the effect of
chemical substances (auxins) on the rooting of cuttings, stimulation of growth, and
fruiting.
Williams, C. M.: "The Metamorphosis of Insects," Scientific American. 182:24-28
(1950). Discussion of the important effects of endocrines on metamorphosis.
appendix 807
16. Conduction and Coordination — Nervous System
Adrian, E. D.: Tlie Physical Buckf^roiiiul of Perception. Oxford, England, The Claren-
don Press, 1947- A small book of lectures delivered at Magdalen College, Oxford
University, by a master of the English language as well as of his subject. They were
deemed appropriate for English University students. Among the titles are: The
Brain and the Mind, Motor and Sensory Areas of the Brain, Sight and Hearing;
The Electrical Activity of the Brain.
Best, C. H., and N. B. Taylor: The Living Body. A Text in Human Physiology, 3rd ed.
New York, Henry Holt & Co., 1952. See the following chapters: The Physiology
of Nerve and Muscle, The Central Nervous System, and The Special Senses.
Cobb, Stanley: Foundations of Neuropsychiatry, 5th ed. Baltimore, Williams & Wilkins
Co., 1952.
Dennis, W., ed.: Readings in the History of Psychology. New York, Appleton-Century-
Crofts, 1948,
Fulton, J. P.: The Physiology of the Nervous System, 3rd ed. London, Oxford Uni-
versity Press, 1949. A comprehensive advanced treatment on the functions of the
nervous system.
Garrett, H. E.: Great Experiments in Psychology, 3rd ed. New York, Appleton-
Century-Crofts, 1951.
Sherrington, C. S.: Integrative Action of the Nervous System. New Haven, Yale
University Press, 1948.
BoDiAN, D.: "The Paralytic Plague," Scientific American, 183:22-26 (1950). The virus
that causes the symptoms of poliomyelitis; its location in the brain and spinal cord;
its behavior.
17. Responsiveness — The Sense Organs
Boring, E. C, H. S. Langfeld, and H. P. Weld, eds.: Foundations of Psychology.
New York, John Wiley & Sons, 1948. Topics such as: color vision; hearing; taste
and smell; perception of space by ultrasonic cries of bats; comparison of the bats'
device with sonar instruments. Interest for general reader.
Davis, S. S., and H. Davis: Hearing, Its Psychology and Physiology. New York, John
Wiley & Sons, 1938. An advanced, comprehensive reference.
Howell, A. B.: Aquatic Mammals. Their Adaptations to Life in the Water. Springfield,
111., Charles C Thomas, Publisher, 1930. See Chapter 4, The Senses. The adjust-
ments to water of eyes, ears, and other senses of marine mammals.
Walls, G. L.: The Vertebrate Eye. Bloomfield Hills, Mich., The Cranbrook Press, 1942.
An inclusive reference, especially valuable because of its emphasis on ecology.
Wigglesworth, V. B.: The Principles of Insect Physiology. New York. E. P. Dutton
& Co., Inc., 1939. An unexcelled authority. Discussions of sense organs of insects:
Vision, Mechanical and Chemical Senses and Behavior. Many figures and extensive
reference lists.
Griffin. D. R.: "The Navigation of Bats," Scientific American, 183:52-55 (1950). A
description of the guidance of bats by the echoes of their own supersonic cries,
inaudible to human ears.
Wald, G.: "Eye and Camera," Scientific American, 183:32-41 (1950). A comparison of
the eye and camera, with a discussion of the basic physics and chemistry involved.
Excellent.
18. Reproduction
Altenburg, Edgar: Genetics. New York, Henry Holt & Co., Inc., 1948. Reference for
special topics, e.g., beginnings of sex, mating types, and reproduction of paramecia.
Bullough, W.: Vertebrate Sexual Cycles. New York, John Wiley & Sons, 1953. A
generalized account with examples from different animals. Interesting, readable,
brief.
Corner, G. W. : The Hormones in Human Reproduction. Princeton, N. J., Princeton
University Press, 1942. An excellent presentation, interesting, finely written, schol-
808 APPF.NDIX
;irly, well illustrated, and brief. This and Ourselves Unborn by the same author
are valuable to read and own. Both are for the general reader.
Stone. A., and H. Stone: Manual of Marrhif-e, rev. ed. New York, Simon & Schuster,
Inc., \^'S5. Excellent reference.
Walter, H. E., and L. P. Sayles: Biology of the Vertebrates, 3rd ed. New York, i he
Macmillan Co., 1949. A new edition of a comparative anatomy that has a long
history of usefulness.
19. Developmf.nt
Arey. L. B.: Developmental Anatomy, 6th ed. Philadelphia, W. B. Saunders Co., 1954.
A superbly illustrated up-to-date edition of a standard embryology.
Barth. L. G.: Embryology, 2nd ed. New York, Dryden Press, 1954. Brief, fully illus-
trated with original, usually simple diagrams. Valuable for its emphasis on experi-
mental embryology.
Bracket, Jean: Chemical Embryology, Trans, by L. G. Barth. New York, Interscience
Publishers, Inc., 1950. Advanced treatise. Subjects such as: the relation of metab-
olism to cell division; chemical embryology of the invertebrates; chemical embry-
ology of amphibian eggs.
Corner, G. W.: Ourselves Unborn. New Haven, Yale University Press, 1944. For the
general reader. A brief account of human development by a leading authority.
Written with clarity and grace. Illustrated with excellent photographs. A book to
own.
Patten, B. M.: Human Embryology, 2nd ed. New York, The Blakiston Co., 1953.
Patten's embryologies are highly regarded and widely used.
Patten, B. M.: Embryology of the Pig, 3rd ed. Philadelphia, The Blakiston Co., 1948.
Patten, B. M.: Early Embryology of the Chick, 4th ed. Philadelphia, The Blakiston
Co., 1951.
Shumway, W.: Introduction to Vertebrate Embryology, 5th ed. New York, John Wiley
& Sons, 1954. Amphioxus, frog, chick and mammal discussed comparatively.
Windle, W. F.: Physiology of the Fetus. Philadelphia, W. B. Saunders Co., 1940.
Functions of the body in prenatal life.
20. The Physical Basis of Heredity
CoNKLiN, E. G.: Heredity and Environment in the Development of Men, 4th ed.
Princeton, N. J., Princeton University Press, 1922. A classic not to be missed; suit-
able for those who think.
Dunn, L. C, and T. H. Dobzhansky: Heredity, Race and Society. New York, The
New American Library of World Literature, 1950. Authentic, interesting and in-
expensive.
Goldschmidt, R. B.: Understanding Heredity, An Introduction to Genetics. New York,
John Wiley & Sons, 1952. Excellent. Brief yet it includes the significant items.
Holt, R.: George Washington Carver. Garden City, N. Y., Doubleday, Doran & Co.,
1943. A fascinating story of a Luther Burbank of the south.
Iltis, Hugo: Life of Mendel, Translated by Eden and Cedar Paul. New York, W. W.
Norton & Co., 1932.
MuLLER. H. J., C. C. Little, and L. H. Snyder: Genetics, Medicine and Man. Ithaca,
N. Y., Cornell University Press, 1947. Brief, authoritative and readable, with ap-
plications to evolution and public welfare.
Pfeiffer, J.: Genetics, The Science of Heredity. Public Affairs Pamphlet No. 165.
Public Affairs Committee, 22 East 38th St., New York, 1950. Content well chosen
and written.
ScHEiNFELD, A.: The New You and Heredity. Philadelphia, J. B. Lippincott Co., 1950.
Genetics in everyday life with familiar examples.
SiNNOTT, E., L. C. Dunn, and T. Dobzhansky: Principles of Genetics, 4th ed. New
York, McGraw-Hill Book Co., 1950. A standard text.
Snyder, L. H.: The Principles of Heredity, 4th ed. Boston, D C. Heath & Co., 1951
Excellent; has good teaching quality, and liveliness.
appendix 809
21. The Protozoans — Representatives of Unicellular Animals
DoBELL, C: Antony van Leeuwenhoek and his "Little Animals." London. J. Bale Sons
and Danielsson. 1932. An account of the founder of protozoology and bacteriology
and his work (1632-1723). -
Grant. M. P.: Microbiology and Human Progress. New York. Rinehart Co., 1953. How
the world of microscopic beings surrounds and travels with human ones in modern
world affairs. The author believes that any citizen's culture and contributions to
society are enriched by an understanding of the part taken by micro-organisms in
its progress.
Hyman. L. H.: The Invertebrates, Protozoa through Ctenophora. New York, McGraw-
Hill Book Co., 1940. The standard advanced reference work in English on the
invertebrates.
Jahn. T. A., and F. F. Jahn: How to Know the Protozoa. Dubuque, Iowa, W. C. Brown
Co., 1949. A small, fully illustrated manual, interesting and easy to use, as easy
as possible to make it.
Jennings, H. S.: Behavior of the Lower Organisms. New York, The Macmillan Co.,
1915. A famous biologist's discussion of a subject on which he was a thought-
provoking scholar.
LoCY, W. A.: Biology and Its Makers, 3rd ed. New York, Henry Holt & Co., 1915.
Excellent accounts of Leeuwenhoek and other pioneers.
Mackie, T. T., G. W. Hunter, and C. B. Worth: Manual of Tropical Medicine, 2nd
ed. Philadelphia. W. B. Saunders Co.. 1954. Comprehensive and advanced discus-
sions of malaria, sleeping sickness and other diseases caused by protozoans.
Warshaw, L. J.: Malaria, The Biography of a Killer. New York, Rinehart Co.. 1949.
Interesting, inclusive account of the parasites and the disease.
Wichterman, Ralph: The Biology of Paramecium. New York, The Blakiston Co., 1952.
Allen, W. E.: "The Primary Food Supply of the Sea," Quarterly Review of Biology,
9:161-180 (1934). A general survey of a subject of increasing importance. A
valuable reference work with a list for further reading.
Allen, W. E.: "Red Water in La Jolla Bay (California) in 1945," Transactions, Ameri-
can Microscopical Society, 55:149-153 (1946). "Red water" due to dinoflagellates
(protozoans) has appeared now and again along the western coast of the United
States.
Hegner. R. W.: "The Interrelations of Protozoa and the Utricles (leaf traps) of
Utricularia," Biological Bulletin. 50:239-270 (1926). Also, "Protozoa of the
Pitcher Plant," Biological Bulletin. 50:271-276 (1926). The story of how plant
traps catch and digest protozoans and other minute animals.
Woodruff. L. L.: "Eleven thousand generations of Paramecium," Quarterly Review of
Biology, 1:436-438 (1935). Generations of paramecia in which conjugation did
not occur. Division followed endomixis.
22. Sponges — A Side Line of Evolution
BucHSBAUM. R.: Animals Without Backbones, 2nd ed. Chicago, University of Chicago
Press, 1948. This book presents the essentials of the structure and habits of sponges
clearly and vividly.
Hegner, R. W.: Invertebrate Zoology. New York, The Macmillan Co., 1933.
Hyman, L. H.: The Invertebrates, Protozoa through Ctenophora. New York, McGraw-
Hill Book Co., 1940. A leading authority, inclusive and thorough. Advanced.
Miner, R. W.: Field Book of Seashore Life. New York, G. P. Putnam's Sons, 1950.
An excellent chapter on marine sponges.
Potts, E.: The Sponges (Porifera) in Ward and Whipple's Fresh-Water Biology. New
York. John Wiley & Sons. 1918.
Ramsay, J. A.: A Physiological Approach to the Lower Animals. Cambridge, England,
Cambridge University Press, 1952. Broad generalizations in the physiological ap-
810 APPENDIX
proach to invertebrate animals. Subjects dealt with in short chapters are: Nutrition,
Circulation, Respiration, Excretion, Muscle and Nerve, Sense Organs, Coordination,
and Behavior. Brief and illuminating.
Stuart, A. H.: World Trade in 3pdnges. Washington, U.S. Government Printing Office,
1948.
23. COELENTERATES SIMPLE MULTICELLULAR AniMALS
BuCHSBAUM, R.: Animals Without Backbones, 2nd ed. Chicago, University of Chicago
Press, 1948.
Hyman, L. H.: The Invertebrates, Protozoa through Ctenophora. New York, McGraw-
Hill Book Co., 1940.
MacGinitie, G. E., and N. MacGinitie: Natural History of Marine Animals. New York,
McGraw-Hill Book Co., 1949.
Miner, R. W.: Fieldlwok of Seashore Life. New York, G. P. Putnam's Sons, 1950.
YoNGE, CM.: A Year on the Great Barrier Reef. New York, G. P. Putnam's Sons. 1930.
Roudabush, R. L.: "Phenomenon of Regeneration in Everted Hydra," Biological Bulletin,
64:253-258 (1933).
24. Ctenophores — Comb Jellies or Sea Walnuts
Hyman, L. H.: The Invertebrates, Protozoa through Ctenophora. New York, McGraw-
Hill Book Co., 1940. Ctenophora pp. 662-696, figs. 209-221.
Mayer, A. G.: Ctenophores of the Atlantic Coast of North America. Carnegie Institu-
tion of Washington, Publication 162 (1912), 58 pp. 17 pis. See also titles for
Chapter 23.
25. Flatworms — Vanguard of the Higher Animals
Child, C. M.: Patterns and Problems of Development. Chicago, University of Chicago
Press, 1941. An advanced reference.
Hyman, L. H.: The Invertebrates, Platyhelminthes and Rhyncliocoela. New York,
McGraw-Hill Book Co., 1951. Authoritative and inclusive. Extensive bibliography.
MacGinitie, G. E., and N. MacGinitie: Natural History of Marine Animals. New
York, McGraw-Hill Book Co., 1949. Original observations, well told. For general
reading.
Morgan, T. H.: Regeneration. New York, The Macmillan Co., 1901. A classic in the
subject. Out of print but in many college libraries.
26. Round Worms — The Tubular Plan
Chandler, A. C: Introduction to Parasitology with Special Reference to the Parasites
of Man, 8th ed. New York, John Wiley & Sons, 1949. It contains a general account
of animal parasites and excellent discussions of human parasites and the diseases
which they cause.
Cobb, N. A.: Free Living Nematodes. In Ward and Whipple: Fresh-Water Biology. New
York, John Wiley & Sons, 1918. The ecology, and structure and functions of the
nematodes of soil and fresh water. The keys are necessarily technical.
Craig, C. P., and E. C. Faust: Clinical Parasitology. 5th ed. Philadelphia, Lea &
Febiger, 1951. A readable and authentic account of human parasites.
Elton, C: Animal Ecology, 3rd ed. New York, The Macmillan Co., 1947. A small book
that contains ideas and principles; pithy and stimulating.
Goody, T.: Plant Parasitic Nematodes. New York, E. P. Dutton & Co., 1933.
Hyman, L. H.: The Invertebrates, Nematoda. New York. McGraw-Hill Book Co., 1951.
Stunkard. H. W.: "Parasitism as a Biological Phenomenon." Scientific Monthly,
28:349-362 (1929). An excellent survey; characteristics of parasitism illustrated
by examples.
appendix 811
27. An Aquatic Miscellany
BoRRADAiLE, L. A. et al.: The Invertehrata. New York, The Macmillan Co., 1932. An in-
clusive detailed treatment of types of animals. Discussions include functions and
relationships with ecological notes.
MacGinitie, G. E., and N. MacGinitie: Natural History of Marine Animals. New
York, McGraw-Hill Book Co., 1949. Original observations with life kept in the
records. For both American coasts, especially the Pacific.
Miner, R. W.: Field Book of Seashore Life. New York. G. P. Putnam's Sons, 1950. A
fieldbook of seashore animals from protozoans through the lower chordates.
Descriptions and illustrations of 1300 species of animals of American coastal waters
especially the Atlantic. Includes a list of references for the phyla in this chapter.
Pratt, H. S.: A Manual of the Common Invertebrate Animals Exclusive of Insects, rev.
ed. Philadelphia, The Blakiston Co., 1935. Widely used, chiefly for identifications.
SvERDRUP, H. U., M. W. Johnson, and R. H. Fleming: The Oceans, Their Physics,
Chemistry, and General Biology. Nev/ York, Prentice-Hall Inc., 1942. A detailed
treatise with extensive bibliographies for each chapter. Chapter 18, Interrelations
of Marine Organisms, contains a general discussion of food relations in the ocean.
CoE, W. R.: "Biology of the Nemerteans of the Atlantic Coast of North America,"
Transactions of Connecticut Acadenfy of Arts and Sciences, Vol. 35 (1935).
28. Annelids — Pioneers in Segmentation
Darwin, Charles: The Formation of Vegetable Mould through the Action of Worms,
with Observations on Their Habits, 1st ed. London, John Murray, 1881. Later
published as Formation of Vegetable Mould. New York, D. Appleton & Co. A
classic that reveals Darwin's methods of observation and reasoning.
Harvey, E. N.: Living Light. Princeton, Princeton University Press, 1940.
Harvey, E. N.: Biolnminescence. New York, Academic Press, 1952.
MacGinitie, G. E., and N. MacGinitie: Natural History of Marine Animals. New
York, McGraw-Hill Book Co., 1949. Many photographs. Lively, meaty accounts
by experienced observers. Authors are at the Kerckoff Marine Laboratory, Cali-
fornia Institute of Technology.
Miner, R. W.: Field Book of Seashore Life. New York, G. P. Putnam's Sons, 1950.
An inclusive, fully illustrated handbook. Selected references.
Rogers, C. G.: Textbook of Comparative Physiology, 2nd ed. New York. McGraw-Hill
Book Co., 1938. Physiology of invertebrates; e.g., earthworm — respiration, chlora-
gog cells.
LiLLiE, F. R., and E. E. Just: "Breeding Habits of the Heteronereis Form of Nereis
limbata at Woods Hole, Mass.," Biological Bulletin. 24:147-168 (1913). Observa-
tions of the spawning swarms and bioluminescence of clamworms.
Moore. J. P.: "The Control of Blood-sucking Leeches, with an Account of the Leeches
of Palisades Interstate Park," Roosevelt Wild Life Bulletin, 2:1-55 (1923).
Prosser, C. L.: "The Nervous System of the Earthworm," Quarterly Review of Biology,
9:181-200 (1934). Emphasis on experimental studies and function.
Robertson, J. D.: "The Function of the Calciferous Glands of Earthworms," Journal
of Experimental Biology (British), 13:279-297 (1936). Experimental environments
and diets and their effects on the calciferous organs,
29. Arthropods — Crustaceans
Huxley. T. H.: The Crayfish. New York, D. Appleton & Co., 1880. A classic of clear,
accurate description of structure. No attempt to present the living animal.
MacGinitie, G. E., and N. MacGinitie: Natural History of Marine Animals. New
York, McGraw-Hill Book Co., 1949. Chapter 27, Arthropoda, is a lively, well-
illustrated account that emphasizes the Pacific Coast fauna but includes much else.
Miner, R. W.: Fieldbook of Seashore Life. New York, G. P. Putnam's Sons, 1950. A
compact introduction to the invertebrate animals of the Atlantic coastal waters of
North America.
812 APPENDIX
Ward, H. B., and G. C. Whipple: Fresh-Water Biology. New York, John Wiley & Sons,
1918. Fresh-water crustaceans with abundant figures.
Andrews, E. A.: "Breeding Habits of Crayfish," American Naturalist, 38:165-206
(1904).
Herrick, F. H.: "Natural History of the American Lobster," Bulletin U. S. Bureau of
Fisheries, 29:149-408 (1911).
ScuDAMORE, H. H.: "The Influence of the Sinus Glands Upon Molting and Associated
Changes in the Crayfish," Physiological Zoology, 20:187-208 (1947). Endocrine
control of calcium metabolism, formation of gastroliths, hardening of exoskeleton.
Tack, P. I.: "The Life History and Ecology of the Crayfish, Cainbarus iminunis Hagen,"
American Midland Naturalist, 25:420-446 (1941).
30. Arthropods — Insects, Spiders, and Allies
Baker, E. W., and G. W. Wharton: An Introduction to Acarology. New York, The
Macmillan Co., 1952. An essential book for special study of mites.
Brues, C. T.: Insect Dietary, An Account of the Food Habits of Insects. Cambridge,
Mass., Harvard University Press, 1946. Facts with wit and philosophy added.
Chu, H. F.: How to Know the Immature Insects. Dubuque, Iowa, Wm. C. Brown Co.,
1949. An illustrated key for identifying the orders and families of immature insects
with suggestions for collecting, rearing and studying them.
Clausen, Lucy W.: Insect Fact and Folklore. New York, The Macmillan Co., 1954. A
highly entertaining book; various facts packed in among stories and anecdotes
of great variety.
CoMSTOCK, J. H.: An Introduction to Entomology, 9th ed. Ithaca, N. Y., Comstock Pub-
lishing Co., 1936. A highly valued standard text.
Comstock, J. H.: The Spider Book, rev. ed. by W. J. Gertsch. New York, Doubleday,
Doran & Co., 1940. Among other interesting accounts is the description of web
making.
Emerton, J. H.: The Common Spiders of the United States. Boston, Ginn & Co., 1902.
A small, approachable book by a famous authority.
Fabre, J. H.: The Life of the Spider. New York, Dodd, Mead & Co., 1917. Charles
Darwin termed Fabre an "incomparable observer."
Folsom, J. W., and R. A. Wardle" Entomology with Special Reference to Its Ecological
Aspects, 4th ed. Philadelphia, The Blakiston Co., 1934. Useful for reference.
Gertsch. W. J.: American Spiders. New York, D. Van Nostrand Co., 1949. Finely
illustrated by 32 color and 32 half-tone plates.
Matheson, Robert: Medical Entomology, 2nd ed. Ithaca, N. Y., Comstock Publishing
Co., 1950. Presents well-chosen facts with precision and clarity.
Matheson. Robert: Entomology for Introductory Courses, 2nd ed. Ithaca, N. Y.,
Comstock Publishing Co., 1951. Excellent presentation of basic facts.
Michener, C. D., and M. H. Michener: American Social Insects. New York, D. Van
Nostrand Co., 1951.
Ribbands. C. R.: The Behavior and Social Life of Honeybees. London, Bee Research
Association, 1953. Emphasis on recent research and presentation in nontechnical
language.
Rothschild, M., and T. Clay: Fleas. Flukes and Cuckoos. London, Collins, 1952. A
study of bird parasites. A revealing picture of relationships in one kind of world —
the bodies of birds. Well illustrated and written with few technical terms. Extensive
bibliography.
Steinhaus, E. a.: Insect Microbiology. Ithaca, N. Y., Comstock Publishing Co., 1946.
Sample titles of chapters: Rickettsiae; Fungi and Insects; Protozoa and Insects
except Termites; Protozoa in Termites.
Thorp, R. W., and W. D. Woodson: Black Widow. Chapel Hill, N. C, University of
North Carolina Press, 1945. A special study of a famous spider.
United States Department of Agriculture: Insects, The Yearbook of 1952. Wash-
ington, D. C, U. S. Government Printing Office, 1952. A practical book about
APPENDIX 813
useful as well as harmful insects, insecticides, and crops. Seventy-two color plates
of economically important insects. Extensive bibliography.
VON Frisch, Karl: Bees: Their Vision, Chemical Senses, and Language. Ithaca, N. Y.,
Cornell University Press. 1950. A fascinating account.
VON Frisch, Karl: The Dancing Bees. New York, Harcourt. Brace & Co.. 1955.
Wheeler. W. M.: Social Life among the Insects. New York, Harcourt Brace & Co.,
1923.
Wheeler, W. M.: The Social Insects, Their Origin and Evolution. New York, Harcourt
Brace & Co., 1928. Wheeler's books stand high in literary flavor as well as upon
his deep understanding of social insects.
Wigglesworth, V. B.: The Principles of Insect Physiology, 4th ed. London, Methuen
& Co., 1950.
Wigglesworth, V. B.: The Physiology of Insect Metamorphosis. Cambridge. England,
Cambridge University Press. 1954.
ZiNNSER. Hans: Rats, Lice and History. New York, Pocket Books, Inc., 1945. Wit,
poetry, historical and biological facts. From the preface: ". . . art and sciences
have much in common and both may profit by mutual appraisal." Among the
chapter subjects: a discussion of the relationship between science and art; on
parasites and old and new diseases; on the louse; the birth, childhood and adoles-
cence of typhus fever.
Bailey. L.: "The Action of the Proventriculus of the Worker Honeybee. Apis mellifera
L." The Journal of Experimental Biology (British), 29:310-327 (1952).
Waterman, T. H.: "Flight Instruments in Insects," American Scientist, 38:222-238
(1950).
Waterman. T. H.: "Polarized Light Navigation by Arthropods," Transactions of the
New York Academy of Sciences, 14:11-14 (1951).
31. MoLLUSKS — Specialists in Security
Black. J. D.: Biological Conservation. New York. The Blakiston Co.. 1954. Wild life is
interpreted to include invertebrates and other animals outside the game types. A
practical introduction to conservation.
MacGinitie, G. E., and N. MacGinitie: Natural History of Marine Animals. New
York, McGraw-Hill Book Co., 1949. Interesting, authentic and a pleasure to read.
Miner. R. W.: Fieldbook of Seashore Life. New York, G. P. Putnam's Sons, 1950. An
excellent fully illustrated guide to the common invertebrates of the Atlantic coast.
Morgan, A. H.: Fieldbook of Ponds and Streams. New York, G. P. Putnam's Sons,
1930. A brief chapter on the snails and mussels.
RiCKETTS, E. F., and J. Calvin: Between Pacific Tides, rev. ed. Stanford, Calif.. Stan-
ford University Press. 1948. An account of the habits and habitats of the common
invertebrates of the Pacific coast.
Alexander, A. E.: "Pearls through Artifice," Scientific American, 160:228-229 (April,
1939).
Grave, B. H.: "Natural History of the Shipworm, Teredo navulis, at Woods Hole,
Massachusetts," Biological Bulletin, 55:260-282 (1928).
GuNTER. G.: "The Generic Status of Living Oysters and the Scientific Name of the
Common American Species; Placed by Gunter as Crassostrea virginica," American
Midland Naturalist, 43:438-449 (1950).
KoRRiNGA. P.: "Recent Advances in Oyster Biology," Quarterly Review of Biology,
27:266-308; 339-365 (1952). An excellent survey of many aspects of the biology
of oysters including "The American Oyster" known in many books as Ostrea vir-
ginica recently placed by some authors in a different genus, by Korringa as
Gryphaea virginica, by Gunter as Crassostrea virginica.
32. ECHINODERMS FORERUNNERS OF THE VERTEBRATES
Agassiz, Elizabeth C: Louis Agassiz, His Life and Correspondence. Boston, Houghton,
Mifflin and Co., 1886. Agassiz kindled the spirit and built the foundation of the
teaching of zoology in the United States. He was also an investigator and teacher
814 APPENDIX
of the structure and biology of echinoderms. This biography is one of several but
none makes his time more alive.
Miner, R. W.: Fieldhook of Seashore Life. New York. G. P. Putnam's Sons, 1950.
Useful for all groups of seashore animals. Fully illustrated.
CoE, W. R.: "Echinoderms of Connecticut." Connecticut State Geological and Natural
History Survey Bull., 19:1-152 (1912). Bibliography and excellent brief accounts
and illustrations of general use on the Eastern coast of the United States.
Jennings, H. S.: "Behavior of the Starfish Asterias forreri Deloriol," University of
California Publications in Zoology, 4:53-185 (1907). Written by an authority in
animal behavior.
Mead. Albert D.: "The Natural History of the Star-fish," Washington Bull. U. S.
Bureau of Fisheries, 19:203-224 (1899). The most interesting account of the
natural history of the common starfish. Available in many libraries.
33. Introduction to the Vertebrates — Lower Chordates and Fishes
Beston. Henry: The Outermost House: A Year of Life on the Great Beach of Cape
Cod. New York, Rinehart and Co., 1949.
Breder, C. M., Jr.: Fieldhook of Marine Fishes of the Atlantic Coast. New York, G. P.
Putnam's Sons 1929. Reliable, with a content and size for ready use.
Carson, Rachel L.: The Sea Around Us. New York, Oxford University Press, 1951.
The book brings to the reader a sea as ancient and living, and as changeful as the
sea really is.
Carson, Rachel L.: Under the Sea-wind. New York, Oxford University Press, 1952.
Authentic life stories of fishes with the flavor left in — mackerel, herrmg, cod and
their neighbors. The "River and the Sea" contains a life story of the eel.
Curtis, Brian: The Life Story of the Fish: His Morals and Manners, 2nd ed. New
York, Harcourt, Brace & Co., 1949. The author was formerly in charge of biological
investigations of fresh-water fishes for the California State Division of Fish and
Game. This book is an enjoyably clear and brief account told with humor, and
based on firsthand acquaintance with living fishes.
Daniel, J. F.: The Elasmobranch Fishes, 3rd ed. Berkeley, Calif., University of Cali-
fornia Press, 1934.
Jordan, David S.: Science Sketches, 5th ed. Chicago, A. C. McClurg Co., 1916. In-
cludes a famous "Story of Salmon," a classic of American fish stories. Out of print
but in many libraries.
LaGorce. J. O., ed.: The Book of Fishes. Washington, D. C, National Geographic
Society, 1939. Chapters on fishes and fishways of the streams and coastal waters of
North America. With 443 color portraits and 162 photographs from the National
Geographic Magazine.
Smith, Homer W.: Kamongo. New York, The Viking Press, 1932. An account of the
African lung fish. The author spent a year in Africa learning about the lung fish,
then wrote Kamongo which has been read by thousands.
Walton, Isaak: The Compleat Angler, 5th ed. The classical account of the delights of a
sport that has never gone out of fashion for men and should be in greater fashion
for women.
Gage, Simon H.: "Lampreys and Their Ways," Scientific Monthly, 28:401-416 (1929).
U. S. Department of the Interior, Fish and Wildlife Service (Washington, D. C),
publishes reports and other publications that deal with fish and fisheries. Lists of
these are sent upon request. Some publications are distributed free; others are for
sale. The Transactions of the American Fisheries Society, Reports of the North
American Wild Life Conference, and the periodical Copeia contain articles on all
aspects of fishes. Various states distribute papers of interest to fishermen.
34. Amphibians — The Frog, An Example of the Vertebrates
Barbour. T.: Reptiles and Amphibians, Their Habits and Adaptations, 2nd ed. Boston,
Houghton, Mifflin & Co., 1934. Accounts of exotic amphibians by a great traveler
with unique illustrations.
APPENDIX 815
Bishop, S. C: Handbook of Salamanders of the United States, Canada and Lower
California. Ithaca, N. Y., Comstock Publishing Co., 1943. Original observations,
well illustrated. The only book on the subject.
DiCKERSON, M. C: The Frog Book. New York, Doubleday, Page and Co., 1920. An
excellent book which has' had a great career of use and is owned in personal, school
and general libraries.
Holmes, S. J.: The Biology of the Frog, 4th ed. New York, The Macmillan Co., 1927. A
standard college text.
Noble, G. K.: The Biology of the Amphibia. New York, McGraw-Hill Book Co., 1931.
An advanced reference; structure, function, life histories and classification.
RuGH, Roberts: The Frog: Its Reproduction and Development. Philadelphia, The
Blakiston Co., 1951. An embryology which contains a chapter on the reproductive
system of the adult frog. An advanced reference, finely illustrated.
Wright, A. H., and A. A. Wright: Handbook of Frogs and Toads of the United States
and Canada. Ithaca, N. Y., Comstock Publishing Co., 1949. The standard modern
work on the ecology, identification, and classification of frogs and toads. It contains
an abundance of original observations and excellent photographs.
35. Reptiles — First Land Vertebrates
Colbert, E. H.: The Dinosaur Book. New York, American Museum of Natural History,
1945. Fully illustrated by J. C. Germann and previously published drawings by
C. R. Knight. An untechnical account based largely on fossils, with photographs
of paintings in the American Museum, and including a reading list and glossary.
DiTMARS, R. L.: Reptiles of the World. New York, The Macmillan Co.
DiTMARS, R. L.: The Reptiles of North America, rev. ed. New York, Doubleday, Doran
and Co., 1936. Firsthand observations of reptiles at the New York Zoological Park
where the author was curator of reptiles; 400 photographs from life.
Pope, C. H.: Snakes Alive and How They Live. New York, The Viking Press, 1937.
Excellent photographs; an ecological viewpoint; one chapter on snake venoms.
Pope, C. H.: Turtles of the United States and Canada. New York, A. A. Knopf, Inc.,
1939. Both of the foregoing books by Pope are useful and interesting references
for general readers.
RoMER, A. S.: Man and the Vertebrates, 3rd ed. Chicago, University of Chicago Press,
1941. From the standpoint of evolution with a good allowance for reptiles. Fully
illustrated.
RoMER, A. S.: Vertebrate Paleontology, 2nd ed. Chicago, University of Chicago Press,
1945.
Schmidt, K. P., and D. D. Davis: Fieldbook of Snakes of the United States and Canada.
New York, G. P. Putnam's Sons, 1941.
Smith, H. M.: Handbook of Lizards of the United States and Canada. Ithaca, N. Y.,
Comstock Publishing Co., 1946.
Sharp, Dallas Lore: "Turtle Eggs for Agassiz," Atlantic Monthly, 150:537-545 (1932).
A classic account of a hunt for incubating turtle eggs for Agassiz's work on the
embryology of the turtle. First published, Atlantic Monthly, February, 1910.
36. Birds — Conquest of the Air
Allen, A. A.: Stalking Birds with Color Camera. Washington, D. C, National Geo-
graphic Society, 1951. A magnificent collection of color photographs and descrip-
tions by an outstanding authority.
Allen, G. M.: Birds and Their Attributes. Boston, Marshall Jones, 1925. One of the
best non-technical books on the general biology of birds. Chapters deal with charac-
teristic structures; food; ecological relations; eggs and nests; parasitic habit; senses
and behavior; flight and migration. Recommended for any student of bird life.
Aymar, Gordon: Bird Flight. New York, Dodd Mead & Co., 1935. A collection of 200
photographs.
Barton, R.: How to Watch Birds. New York, McGraw-Hill Book Co., 1954. Interesting
tips by a noted amateur ornithologist.
816 APPENDIX
Herrick, F. H.: Aiidiihon, the Naturalist. New York, D. Appleton-Century Co., Inc.,
1938. A one-volume edition of the biography published in 1917. An acquaintance
with the most eminent of pioneer American ornithologists with glimpses of the
naturalists whom he knew.
HicxEY, J. J.: A Guide to Bird Watching. New York, Oxford University Press, 1943.
An introduction to bird study; how to identify birds in the field; where and when to
look for them: how to acquire a good field glass and to keep records. Chapters on
migration and bird banding and an annotated list of bird books.
Howard, H. Eliot: Territory in Bird Life. London, John Murray, 1920. Authentic and
thought provoking. Difficult to secure except in college or special libraries. Chapters
on securing and defending the territory, its relation to song, to reproduction, to
migration.
Howard, H. Eliot: The Nature of a Bird's World. New York, The Macmillan Co., 1935.
A brief, thought provoking book by a stimulating authority. Partly takes the place
of the preceding reference.
Leopold. Aldo: Game Management. New York, Charles Scribner's Sons, 1933. It is
notable for its accumulation of facts, and clear style. It is said to be responsible
for the founding of game management as an independent science and to be one of
the most significant books in the field. The author was professor of game manage-
ment at the University of Wisconsin, a leading authority and writer on conservation.
Lincoln, F. C: Migration of Birds. Garden City, N. Y., Doubleday & Co., 1952. A
little book, up-to-date and written in direct, simple language.
Nice, Margaret M.: The Watcher at the Nest. New York, The Macmillan Co., 1939. A
thorough acquaintance with individual birds achieved by constant watching and
recording. A unique study.
Peterson, R. T.: A Field Guide to the Birds, 2nd ed. Boston, Houghton Mifflin Co.,
1947. An excellent book for general field use with short descriptions of field marks,
voice and range. It covers the area from the Dakotas and east Texas to the Atlantic
Coast.
Peterson, R. T.: A Field Guide to Western Birds. Boston, Houghton Mifflin Co., 1941.
An excellent counterpart to the author's guide for eastern birds. It covers the
western states, Washington and Oregon to New Mexico, including western Texas.
Pettingill, O. W., Jr.: A Laboratory and Field Manual of Ornithology. Minneapolis,
Minn., Burgess Publishing Co., 1945. Maps of life zones of birds. Definitions, keys,
and descriptions. Plans of study designed by an expert.
Griffin, D. R., and R. J. Hoch: "Experiments on Bird Navigation," Science, 107:347-
349 (April, 1948). Experiments on gannets et al. Results suggest: "The actual flight
paths suggest exploration rather than absolute sense of direction."
Welty, C: "Birds as Flying Machines," Scientific American, 192:88-95 (March, 1955).
An interesting article on modifications in bird structure to adapt them for flying.
Special Periodicals. The leading North American ones are: The Auk (published by
American Ornithologists Union); The Condor (Cooper Ornithological Club) for
Western North America; The Wilson Bidletin (Wilson Ornithological Club) espe-
cially for the Middle West; Bird Banding (Northeastern Bird Banding Association).
The Audubon Magazine, formerly Bird Lore is the official publication of the
National Audubon Society. The headquarters of the Society are at Audubon House,
1130 Fifth Avenue, New York City. Members of the staff' are helpful to anyone
properly interested in bird life who may wish to consult them. The library is rich
in books and periodicals.
37. Mammals and Mankind
Anthony, H. E.: Fieldbook of North American Mammals. New York, G. P. Putnam's
Sons, 1928. Excellent reference in handy size.
Cahalane, V. H.: Mammals of North America. New York, The Macmillan Co., 1947.
Deals with groups of mammals, not with species. It is stored with firsthand infor-
mation about the ways of mammals and written and illustrated to bring interest to
anyone.
APPENDIX 817
Elton, C. S.: Moles. Mice and Lenimini^.s. Oxford, England, The Clarendon Press,
1942. An important ecological study, especially of populations. Advanced and
inclusive.
Hamilton, W. J., Jr.: The Mammals of Eastern United States: An Account of Recent
Land Mammals Occurring East of the Mississippi. Ithaca, N. Y., Comstock Pub-
lishing Co., 1943. Concise and interesting. Many firsthand observations.
Hartman, Carl G.: Possums. Austin, University of Texas Press, 1952. The develop-
ment, habits, history and folklore of the opossums of the south with many illus-
trations.
Hooton, Ernest A.: Up from the Ape, 3rd ed. New York, The Macmillan Co., 1946.
Howell, A. B.: Aquatic Mammals. Springfield. 111., Charles C Thomas, 1930.
Leigh, W. R.: Frontiers of Enchcmtment. New York, Simon and Schuster, 1938. An
artist's account of the African country in which he painted scenes for Akeiey Hall.
See Figure 37.16.
Melville, H.: Moby Dick. New York, The Modern Library, 1926. First ed. in 1851.
The story of Moby Dick. A great whale is the symbol of adventure and courage.
An allegory, a tale, and now a classic.
MocHi, Ugo and T. Donald Carter: Hoofed Mammals of the World. New York, Charles
Scribner's Sons, 1953. The accurate and beautiful results of a pioneer technique in
illustration. See Figure 37.15.
Osborn, Henry Fairfield: Men of the Old Stone Age, 3rd ed. New York, Charles
Scribner's Sons, 1918.
Robertson, R. B.: Of Whales and Men. New York, A. A. Knopf, Inc., 1954. An account
of whaling as it goes on today, the sea, the ships, the whales and whalers. It tells
of the human mind and its culture unalarmed against the might of water, cold and
animals.
Romer, a. S.: Man and the Vertebrates, 3rd ed. Chicago, University of Chicago Press,
1941.
Seton, E. T.: Lives of Game Animals, 4 vols. New York, Doubleday, Doran & Co.,
1929. Abundant illustrations by the artist author. A wealth of lively description
and personal observation.
Simpson, G. G.: The Principles of Classification and a Classification of the Mammals.
New York, American Museum of Natural History, 1945. Bulletin 85 of the
museum.
Young, J. Z.: The Life of Vertebrates. Oxford, Clarendon Press, 1950. The book is what
its title says it is, the life of vertebrates. Excellent.
Kellogg, R.: "The History of Whales. Their Adaptation to Life in the Water," Quarterly
Review of Biology, 3:29-76 and 3:174-208 ( 1928). Their sight and hearing.
38. Organic Evolution — Conservation
Darwin, Charles: The Origin of Species by Means of Natural Selection, or, the
Preservation of Favoured Races in the Struggle for Life. London, John Murray
(Numerous editions, the first one, 1859).
Graham, E. H.: Natural Principles of Land Use. New York, Oxford University Press,
1944. Short, finely illustrated survey of applied ecology.
Howells, William W.: Mankind So Far. New York, Doubleday & Co., 1952.
Irvine, W.; Apes, Angels, and Victorians. New York, McGraw-Hill Book Co., 1954. The
story of Darwin, Huxley, and evolution.
Kellogg, C. E.: The Soils That Support Us. New York, The Macmillan Co., 1941. A
layman's book, by a scientist who knows the soil and how to bring its fascination
before the reader.
Leopold, Aldo: A Sand Coimty Almanac. New York, Oxford University Press, 1949.
"There are some who can live without wild things and some who cannot. These
essays are the delights and dilemmas of one who cannot." Widely known as an
authority in the fields of ecology, conservation and forestry Leopold wrote with the
integrity and flavor of the lines here quoted.
818 APPENDIX
Moody, P. A.: Introduction to Evolution. New York, Harper & Bros., 1953. "Evolution
as Seen in the Classification of Animals" is an unusual and valuable chapter in this
readable book.
OssORN, Fairfield: Our Plundered Planet. Boston, Little, Brown & Co., 1948.
Raverat, G. M. (Darwin): Period Piece; a Cambridge Childhood. London, Faber &
Faber, 1952. A thoroughly human reminiscence of the Darwin family. A fascinating
tale.
Sears. Paul B.: Charles Darwin, the Naturalist as a Cultural Force. New York, Charles
Scribner's Sons, 1950. A small and lively book that presents Darwin's way of
living in present affairs and thinking.
Simpson, G. G.: The Meaning of Evolution. New Haven, Yale University Press, 1949.
The best book on the general meaning of evolution.
Simpson, G. G.: The Life of the Past, An Introduction to Paleontology. New Haven,
Yale University Press, 1953. Excellent for biologist and general reader.
VoGT, William: Road to Survival. New York, Wm. Sloane Associates, 1948. A
dramatic analysis of human ecology and land use, a discussion of waste and the
way to a rescue.
Wald, G.: The Chemical Evolution of Vision. Lancaster, Penna., The Science Press,
1946.
West, G.: Charles Darwin, A Portrait. New Haven, Yale University Press, 1938. Ex-
cellent. It should be better known.
Eisley, L. C: "Fossil Man," Scientific American, 189:65-72 (Dec. 1953). The bones
of related animals offer no clue to the forces which caused the development of the
unique human brain.
Ind
ex
Numbers in boldface type refer to pages on which illustrations occur. Complete scien-
tific names of species are in italic type.
Abalone, 631
Abomasum, 184
Aboral surface, 655
Acanthocephala, 530, 531, 797
Acanthometron, 441
Acarina, 593
Acoela, 507
Acorn worm, 663, 664, 665, 798
Acquired characters, 414, 415, 789
Acromegaly, 272
ACTH, 267, 271, 272, 277, 304
Actinophrys sol, 441
Adaptations, 780-784, 789
Adaptive radiation, 784
Addison's disease, 267
Adenoids, 206, 235
Adipose tissue, 115
Adrenal glands, 200, 248, 256, 264, 266,
269, 271, 274, 348, 697, 709
Adrenalin, 266, 273
Adrenocorticotrophic hormone (ACTH),
267, 271, 272, 277, 304
Aeroembolism, 226
Afferent neuron, 287, 295
Age of the earth, 778
Agglutinin, 209, 210
Agglutinogen, 209, 416
Aggregation, 100, 101
Agranulocytes, 202, 205, 207
Air bladder, 674
Air sacs, 608, 729, 744, 745
Albumin, 198, 746, 749
Alimentary canal, 179
Allantois, 377-380, 752, 758
Allee, W. C, 93
Alligators, 716, 718, 722, 723, 799
Alternation of generations, 443, 478
Alveoli, 232-235, 236, 237
Ambly stoma, 230, 683
tigrimim, 685
Ameba, 3, 21-23, 34, 243, 437. 438-443
AmQha—{C ontinited)
carolinensis, A'il
motion of, 430, 438, 439
proteus, 437
reaction to stimuli, 440
reproduction of, 331, 440
Amebic dysentery, 430, 443
Amebocytes, 461, 463, 464, 562
Ameboid cells, 639, 659
Amino acids, 31, 32, 65, 79, 169, 192, 194,
198, 199, 696
Amitosis, 43
Amiurus, 31 1
Amnion, 376, 377-379, 383, 613, 666,
713
Amniotic fluid, 376
Amniotic sac, 377
Amphibia, 666. 681-713, 798
Amphineura, 631, 633, 634, 798
Amphioxus, 141, 363, 364, 365, 664, 665,
798
Amphitrite johnstoni, 554
Ampulla, 657
Amylase, 696
Amylopsin, 187, 191
Anadromous, 680
Anaerobic respiration, 227
Anaphase, 40-42, 46, 365
Anasa tristis, 406
Ancylostoma duodenale, 524
Androgen, 343, 344, 348
Anemia, 204
Anemonia sulcate, 488
Animal pole of egg, 359, 361. 367, 368
Annelida, 227, 552-571, 663, 798
Anolis, 718, 719
Anopheles, 445, 446, 447, 528
Anoplura, 593
Ant lion, 598
Anteater, spiny, 755, 756, 781, 799
Antennae, 601, 602, 611, 619, 620
comb, 615, 616
Anthozoa, 467, 468
819
820
INDEX
Antibodies, 199, 208-210, 416
Antigens, 208-210. 416
Antitoxin, 208, 416
Antivenins, 722
Antlers, 132, 133
Anura, 685
Aorta, 215, 216, 248, 701
Aortic arches, 699, 786
Aphids, 591, 598
Aphrodite aculeata, 5(il
Apis meUifica, 612
Apoda, 687
Appendages, jointed, 572, 574, 579
of vertebrates, 148
Appendicular skeleton, 146
Appendix, 194. 786
Aqueous humor, 326, 327
Arachnids, 574. 663, 798
Arachnoid layer, 301, 302, 303
Arachnoidea, 622-629
Arhcicia piinctiilata, 337
Arcella, 78, 441
Archenteron, 365-367, 372
Archeopteryx, 727, 750, 751
Archiannelida, 571
Archosauria, 728
Argonauta, 648
Argyroneta, 622
Aristotle, 477, 479, 575, 787
Army worm, 597
Arrow worm, 549-551, 797
Arterioles, 222, 699
Artery, 213, 221
Arthropods, 572-629, 663, 798
Artiodactyl, 763
Ascariasis, 100
Ascaris liimbricoides, 519, 520
life history, 520, 521
Aschheim-Zondek pregnancy test, 383
Ascon, 457
Ascorbic acid, 171
Asker, 683
Assimilation, 168
Asterias jurbesi, 655, 657, 660
Asteroidea, 652, 653, 798 ^ "
Asthma, 234
Astigmatism, 330
Astrangia, 466, 467, 489
danae, 487
Atlas, 693
Atmospheric pressure, 81, 83, 224, 225
Atoms, 1, 9, 12-16, 17
Atretic follicle, 347
Atria. 701
Auditory canal, 317
Auditory nerve, 320
Aurelia, 467, 483, 485, 797
life cycle, 484
Auricle, 317. 318, 320, 500, 700
Auriculoventricular node, 217, 218
Autolytus, 331
Autonomic nerves, 293, 294, 708
Autonomic nervous system, 295, 297
Autonomy, 585
Autosomes, 404
Aves, 666
Avicularia, 546
Axial gradient, theory of, 506
Axial skeleton, 146. 147, 150
Axolotls, 684, 685
Axon, 122, 123, 281, 282, 283, 286, 287,
292, 297, 299, 306
B
Backcross, 399, 400, 401
Balanoglossus, 662, 663, 665
Balaniis halanoides, 75
Baldness, 407, 409
Banta, A. M., 412
Banting, F. G., 268
Barbules, 733
Barnacles, rock. 75
Basilar membrane, 320, 321
Basket stars, 653
Basophils, 202, 205, 207
Bats, 759. 799
long-eared, 761
Bayliss, Sir William M., 192, 255
Beaks, bird, 734, 735, 736, 738
Becquerel, H., 12
Behavior, competition, 91. 92, 104
cooperation, 91-93, 94, 100, 104
Bends, 83, 85, 226
Beriberi. 173
Bernard, C, 259
Berthoid, C, 259
Best, C. H., 268
Between brain, 302, 303, 305, 706
Biceps muscle, 158, 159
Bile, 189-191, 212, 697, 703
Bilharzia, 100, 511
Binocular vision, 324
Biology, 4
Bioluminescence, 435, 568, 569, 647, 649
Bipalitim kewense, 508
Birds, 178, 666, 729-751, 799
Birth process, 384
Bivalves, 635
Bladder, urinary, 247, 251, 340, 343, 346,
379, 697, 709
Blastocoel, 365, 366. 368, 370, 372
Blastopore, 366, 369, 370, 373
Blastula, 365, 366, 368, 369, 372, 477,
659
Blind spot, 327
Blood, 119, 123
circulation of, 215, 216, 217, 219
human, 196
relation to lymph. 213
Blood clot, 211, 212
Blood groups, 209, 210, 415, 416
INDEX
82!
Blood pressure. 165. 197, 220. 221. 252
Blood vessels in frog's foot, 219, 699
Blubber. 764
Blue-green algae, 78, 79
Bone, 114. 115. 117, 118, 140, 143, 145
Bone marrow, 118, 119, 135, 145
Bosmina. 78, 227
Bower birds. 2
Bowman. W.. 253
Bowman's capsule. 253
Brachial plexus. 291
Brachiopoda, 533. 547-549. 797
Brain. 299-302
amphibia. 301, 373, 697
bird. 301, 743
fish, 299. 301, 669, 673
frog, 705
human. 292, 304-307, 308
mammal. 301
reptile. 301
Branchiopods. 587
Branchiostoma lanceolatiis, 665
Breathing. 237. 238, 239
Bright, R., 251
Bright's disease, 251
Brittle star. 652, 653
Bronchial tubes, 233, 235, 236, 239
Bronchioles, 233-235
Brontosaurus. 783
Brood pouches, 103, 756, 759
Broussais, F. J. V., 571
Brown, R.. 24
Brownian movement, 23, 24
Bryophyta. 796
Bryozoa, 533. 543-547. 797
Bufo calaniila, 682
empusus, 682
Bugula. 544, 545, 546
Bulimus, 51 1
Bullfrog, circulation. 700
tadpole. 688
Butterfly. 176
monarch. 596
Caecilians. 681, 687
Caecum, 194, 674, 786
Calciferol, 170
Calciferous bodies, 559
Calcium, 139. 140, 145, 174. 197-200,
212, 264, 266, 585, 636, 637, 794
Calenus, 587
Callibaetis, 323
Callinectes, 588
Cambarus, 575, 585
Cambium, 60
Camponotus, 592
Cannon, W. B., 265
Capillaries, 197, 201, 216, 221, 232, 235,
236, 246, 248, 249, 699
Carapace, 578, 724, 725
Carbohydrates. 29, 55. 65. 77. 168-170.
188, 190. 227, 252
Carbon cycle, 3, 77, 82
Carbon dioxide, 10, 57, 61, 77, 163, 224-
227. 236. 239
Carboniferous period, 600
Cardiac muscle. 121, 122
Cardiac valve, 184, 696
Carnivora. 172, 180, 762, 799
Carotene, 56, 417, 426. 427. 433, 691
Carotenoid pigments, 323, 324, 328, 329,
605
Carotid arches, 701
Carpals, 148, 150, 151
Carpenter ants, 592
Carpus, 148, 150, 151, 693
Cartilage. 114-117, 139, 143
Casinogen. 186
Castration. 275. 343-345, 407
Cat. 753-754, 756
Catadromous, 680
Cataracts, 328
Caterpillars. 598, 599
Cell differentiation, 38
Cell division, 36. 39, 43
Cell membrane, 33, 35, 53
Cell wall. 33, 37, 53
Cells, 1. 17, 23, 32, 33-35, 65
animal, 33, 36
origin of, 33
phases of, 38
plant, 33. 42. 53
polarity of, 38
reproduction of, 34, 40
shapes of, 37
sizes of, 37, 38
{See also specific cells)
Cellulose, 29, 30, 53
Cenozoic era, 779
Centipedes, 622. 663, 798
Centrioles. 36, 39. 42, 46
Centrolecithal. 362
Centromere, 39, 41
Centrosome, 33, 35, 36, 39, 40, 42
Centrosphere, 36
Centrum, 142
Cephalochorda, 664
Cephalopoda, 631, 633. 634, 645. 648
798
Cephalothorax, 575, 578, 624
Ceratium. 78, 435
Cerci, 604
Cerebellum, 270, 302-305, 705, 707
Cerebral cortex, 304, 306
Cerebral hemorrhage or "stroke," 307
Cerebral lobes, 706
Cerebratiilus herciileus, 534, 535
lacteiis, 534
Cerebrospinal fluid. 301
Cerebrum, 270, 294, 303, 306, 307, 705
822
INDEX
Cestoda, 498, 499. 513-518
Cetacea. 764, 799
Chaetognatha, 549-551, 797
Chaetonotus, 544
Chaetopterus, 554, 568, 569
Chalaza, 360
Chameleon, 718. 719
Chaos carulinensis, 426
chaos, Alil
Chelicerae, 623
Chelonia, 718. 724-726
Chelonia my das my das, llA
Chelydra. 714, 715
Chiasma. 706
Chick embryo. 377
Chigger, 97, 98. 629
Child, C. M.. 506
Chilopoda, 622. 798
Chimpanzee, 770
Chiroptera. 759
Chitin. 589, 590, 594, 603, 611
Chiton. 630, 631, 633-635, 798
Chlamydosaurus, 728
Chloragog cells, 560, 561, 562
Chlorella vulgaris, 428
Chlorine, 174
Chlorohydra viridissima, 469
Chlorophyll, 55-57, 61, 63, 203, 433, 797
Chloroplasts, 56, 61, 432, 433
Choanocytes, 435, 456, 459-461. 464
Cholecystokinin, 190, 267, 269
Cholesterol, 170, 190, 199
Chondrichthyes, 666, 668, 798
Chordamesoderm, 366, 367, 369, 370
Chordata, 141, 798
Chordates, 651, 662-666
unique characteristics of, 662
Chorion, 352, 378-380, 383, 752
Choroid coat, 326, 327
Choroid plexus. 304, 707
Chromatin, 33, 34, 38
Chromatophores, 129, 165, 257, 266, 671,
691
Chromonemata, 34, 38, 39, 42
Chromoplasts, 433
Chromosomes, 34. 38-44, 45, 65. 341,
380, 388, 390, 393, 398, 399, 402,
404, 787, 792
crossing over, 403
distribution of, 394
human cells, 404
number of, 361, 395, 412, 413
pairs of, 395
sex, 404-406
Chrysemys picta, 715
Chuckwalla, 719
Cilia, 36, 430, 431, 450, 534, 538, 651,
664
Ciliata, 432, 448, 449, 453
Circulatory system, 3, 195-223
bird, 743, 744
Circulatory system — {Continued)
crayfish, 581, 582
earthworm, 560
fish, 676
frog. 219, 698, 699, 700, 702
human, before and after birth, 385
insects, 606, 607, 608
mollusk, 637-639
ribbon worms, 536
starfish. 656
vertebrate, 701
Circumcision, 343
Cladocera, 412, 587
Clam worms, 565-567
Clams, 630, 631, 633, 638, 639, 798
fresh-water, life history, 641
Class, 795
Clathndina elegans, All
Clavicle, 148, 150, 151, 154, 158, 673
Claws, 126, 129, 131, 132, 741
Cleavage, 364, 368, 369
types of. 362
Clione limaciiia, 632
CHtellum. 555, 557, 563, 564. 571
Cloaca, 194, 375, 684, 697, 709, 743, 746,
748, 754
Clonorchis sinensis, 513
Closterium, 23
Clothes moth larvae. 598
Cnemidophorus, 714
Cnidoblast, 474
Cnidocil. 475
Cobra, hood, 721
skeleton, 720
Coccidia. 444
Coccyx. 150, 693
Cochlea, 316, 319, 320
Cockroaches, 590
Codosiga botrytris, 435
Coe, W. R., 535
Coelenterata, 465-492. 663, 797
Coelom, 370, 551, 554, 561
Cohn, E. J., 211
Colchicine, 412
"Cold blood," 713, 716
Coleoptera, 593
Coleus, 57
Colias philodice eiirytheme, 793
Collagen, 113, 139
Collateral branches, 282, 283
Collembola, 590, 593, 596
Collip, 268
Colloids, 21, 22, 26
Colon, 194
Color-blindness, 406, 407
Color vision, man and honeybee. 618-620
Commensalism. 93, 94, 452, 537, 554
Compound eyes, section of, 323
Condor, 729
Conjugation, 332, 334, 426, 449, 452, 453
Conjunctiva, 326, 327
INDEX
823
Connective tissues, 107, 111, 112-114, 123,
124, 142, 158
Conservation, 792-794
relationships in, 793
Coordination, chemical, 255 -
nervous, 255
Copepods, 576
Copperhead snake, 716, 722
Copulation, 343. 346, 350, 748
Coracoid. 148, 725
Corals, 465. 466, 487-492
Cornea, 321, 326, 327, 330, 374, 720
Corner, G. W., 217, 335
Corona, 539, 540
Coronary veins, 215, 216
Corpora allata, 258, 609, 610
Corpora cardiaca, 258
Corpora striata, 303
Corpus callosum, 270
Corpus luteum, 346, 347, 348, 350-353
Corrodentia. 593
Corti, organ of, 319, 320
Cortisone, 267, 271
Costal plate. 725
Cotylosaurs. 717, 727
Counter-shading, theory of, 555
Crab, 73, 138
Cranial nerves, 296. 707
Cranium. 692, 693
Crassostrea virginica. 633. 642
Crayfish, 141, 228, 576-586, 622
burrows. 579
female with eggs, 579
Creatine, 198, 664
Crepidula, 644
Crescentic groove. 369
Cretinism, 262, 263
Crinoidea, 652, 653. 798
Cro-Magnon man. 772, 773
Cro-Magnon art, 788
Crocodiles, 713, 716, 718, 722, 723, 799
Crop, birds, 743
grasshopper, 605
Cross breeding, 413, 414
Cross fertilizing, 338, 390
pure lines. 393
Crustacea, 78, 227, 572-588, 663, 798
Crypt of LieberkiJhn, 192
Cryptobranchus, 684
Cryptorchid, 341, 342
Ctenophores, 493-497, 797
digestive system, 495
reproduction, 493, 497
Culex fatigans, 311, 528
Curie, M., 12
Cuticle, 556, 557, 561, 589, 590, 594, 595,
604, 605
Cuttlefish, 631
Cyanea capiUata, 466
Cyclops, 78, 576, 587
Cyclostomata, 666, 798
Cysticerciis celliilosae, 517
Cytoplasm, 33-37, 41, 53
D
Dalton, J., 14
Danaus menippe, 596
Darasprim, 448
Darwin. C, 390, 413, 489, 555, 789, 790,
792
Daylight eye, 323
Decidua basalis, 379, 383
Decidua capsularis, 379
Deer, white-tailed, 766
Dendrites, 122, 123, 281, 282, 283, 285,
289
Dermacentor andersoni, 628, 629
Dermaptera, 593
Dermis, 127, 128-131, 157, 691
Desmids, 78
Deutoplasm, 363
De Vries, H., 408
Diabetes insipidus, 252, 274
Diabetes mellitiis, 191, 252, 268
Diapause, 612, 613
Diaphragm, 237, 238, 239, 726, 752
Diastolic pressure, 220
Diatoms, 51, 74, 78
Didelphis virginiana, 756. 760
Didiniiim nasutuin, 111, 174, 426, 427, 449
Diencephalon, 706
Diestrous period, 350
Differentiation, 360
Difflugia, 441, 442
Diffusion, 22, 23
Digestion, 168, 170, 177-194
Digestive cavities, 177, 178
Digestive system. 177-194
bird, 742, 743
crayfish, 579-581
ctenophore, 495
earthworm, 557-560
fish, 669, 673, 674
frog, 694-697
human, 178-194
hydra, 471, 476
insects, 605, 606, 617
mollusks, 634, 639
planaria, 501, 502
starfish, 658
Dihybrid cross, 400, 402
Dinoflagellates, 433-435
Dinosaur, 727, 783
Diphyllobothriiim latum, life cycle, 517
Diploid, 43
Diplopoda, 622, 798
Diptera, 593
Dissosteira Carolina, 600
Diuresis, 252
Dogfish, 666
Dolphins, 77, 764, 783
824
INDEX
Dominant characters, 391-393, 398-402,
414. 415, 418
Draco, 719
Dragonfly. 176
fossil, 592, 594
metamorphosis, 597
Drone bee, 614
Drosophila, 312, 403, 405. 412, 413
melunogusler, 2>91, 399. 411
sex types. 405
Duckbill, 728, 755, 757, 799
Dtigesia higiibris, 501
tigrina, 499-502, 504, 505
Duodenum, 189, 190, 193, 256, 696
Dura mater, 301, 302, 707
Dwarfs. 257, 258, 262, 263, 272, 275
Dyspnea, 241
E
Eardrum, 316, 317, 375, 602, 611, 720,
745
Earthworm, 176, 178, 555-567, 798
cross section, 561
ecology, 555
general structure, 558
mating, 557
regeneration, 565
seasonal locations, 556
Echidna, 755, 756
Echinodermata, 651-661, 663, 664, 798
Echinoderms, 575
Echinoidea, 652, 654, 798
Echiurus, 534
Ecological relationships, 3
Ecology, 2, 3, 792
Ectoderm, 363, 365-370, 372, 373, 375
Ectoparasites, 97, 98
Eels, common. 670
lamprey, 666, 667, 798
migration, 680
true, 666
Effector cells, 280, 293
Efferent neuron, 287, 295, 297
Efts, 683
red, 684, 686
Eggs, 331, 335, 360, 361
Amphioxus, 365
caddis fly, 79
crayfish, 579
Donacia, 79
frog, 367-370
hen, 747, 749
honeybee, 614
human, 336, 347
midges, 79
Psephenus, 79
snail, 79
toad, 682
tunicate, 362, 364
turtle, 726
Eggs — {Continued)
water mite, 79
whirligig beetle, 79
Egret, American, 740
Eichorn, 538
Eijkman, C, 172
Elasmobranchs, 668
Electroencephalograms, 307, 308
Electrolytes, 17, 18
Electrons, 13, 14, 16. 17
Elephant, 70, 763, 767, 799
Elephantiasis, 100, 528, 529
Ellis, 456
Elvers. 680
Embioptera, 593
Embolus, 212
Embryo, 351, 352, 359-363, 366. 368-
370. 376, 378-385
Emulsion, 19, 22
Encephalon, 705
Enchytraeus alhhiiis, 565
Endbrain, 302, 303, 305
Endocrine glands, 4, 32, 257, 259, 278, 703
human, location of. 256
secretions, 258, 267, 275, 277
Endocrinology, 259
Endoderm, 363, 365-370, 375, 376
Endolymph, 317
Endomixis, 333, 452
Endoparasites, 98
Endoprocta, 545
Endoskeletons, 136, 141, 142
Energy, 1, 9, 10, 12, 19, 29, 53, 226, 227
atomic, 10-13, 73
chemical, 10-12, 56, 57, 65, 267
conservation of, 57
kinetic, 10, 11, 24, 56
muscular, 162
nuclear, 15
potential, 10, 11, 56
Ensis, 642
Entameba blattae, 443
gingivalis, 443
histolytica, 429. 443
Entema sirtalis, 714
Enterobiasis, 100
Enterobiiis vermiciilaris, 524
Enterocrinin, 267, 269
Enterogasterone, 267, 269
Enterokinase, 187, 192. 193
Enteron, 370, 372, 374, 375, 466, 472, 474,
475, 480, 481, 496
Entomostracans. 586-588
Entrobiciihi alfredugesi, 97
Environment, 2, 3. 72
biological. 86-90
chemical conditions, 76
history of, 68
internal. 4
and size of animals, 68
types, 74
INDEX
825
Enzymes, 32. 199. 397
Eoanthropus. 772
Eohippus. 785
Eosinophils. 202, 205. 207
Ephemeroptera, 593
Epidermis. 127, 128, 129. 132. 157
frog. 691, 692
human, 310
hydra. 437
Malpighian layer. 127, 128
plant. 60, 61, 62
reptiles, 7 13
Epididymis, 341, 342
Epiglottis, 181, 182, 206, 236. 237, 240,
241
Epinephrine. 265
Epistylis. 428. 448
Epithelial tissue. 107. 108, 109, 110. Ill
Equilibrium, 314-316, 320
Erepsin, 187, 696
Eryops, 682
Erythrocytes, 200, 202, 207, 211
development of, 205
frog, 698
relative sizes, 201
Esophagus, 182-184, 206
Estrogen, 271, 276, 277, 344, 348, 350,
351, 384
Estrus. 276
Estrus cycle, 348-350
Eubranchipus, 587
Eudorina, 429
Eugenics, 418, 419
Euglena, 426, 428, 429, 430, 432, 433, 797
Eunuch, 344, 407
Eustachian tubes. 235. 319, 375
Evolution, 664, 777-794
convergent, 756. 782. 783
organic and social, 787
theories, 788-792
Excretion, 4, 242-254
Excretory organs, 242-254
bladder, 251
flame cells, 243, 244
gills, 252
kidneys, 243-247, 248, 249, 253
lungs, 252
nephridium, 243
ureters. 251
vacuoles. 243
Excretory system, 242-254
Ascaris. 522
bird, 743, 744
crayfish. 245, 582
earthworm, 245, 561
frog, 703, 709
insect, 245, 608, 609
moUusk, 639
planaria, 503
reptile, 723
starfish, 660
Exoskeletons, 136-138. 141, 578
Experimental method, illustration of. 259
Eyeball, 326, 327
Eyes, compound. 323, 584, 601, 602, 611
development of, 374
farsighted, 330
frog. 708
simple, 601, 602, 624
snail. 643
squid. 648
Eyesight of birds, 747, 748
Fairy shrimp, 587
Fallopian tubes, 344
Family, 104, 795
Fangs, poison, 721, 722
Farsighted eye, 330
Fasciola hepatica, 509
life history of. 510
Fasciolopsis buski, 512, 513
life history of, 512
Fat body. 688, 689. 697, 709, 711
Fats. 30-33, 168-170. 188. 190. 198. 199
Feathers, 126, 129, 130. 729-734, 799
types, 733
Feeding devices, 175. 176, 177
Feet of birds. 736. 737, 738
Femur. 143, 148, 150, 693, 725, 739
Fertilization, 40, 47, 54, 65, 335, 337. 339.
346. 351. 353. 361, 380, 381. 404
cross-. 338, 390, 393
guinea pig, 363
membrane, 338, 361, 367
Fibrillae, 36
Fibrin threads. 211, 212
Fibrinogen. 212
Fibula. 148, 150, 152, 725
Filariae. 528
Filariasis, 100, 529
Filoplume, 733
Filum terminale, 705
Fishes. 667-680
airbladder and sounds, 674-676
embryo, 377
eyes, 677, 678
hearing. 678
internal structures. 673
skin and scales. 669
spawning habits. 677
taste and smell. 679
(See also specific fish)
Flagella, 36, 244, 430, 432, 457, 460
Flame cells, 503, 537. 538. 541
Flatworms, 100, 498-518, 663
Flea, 97, 98
Flight. 740. 741
Floscularia, 538, 541
Flukes, 508-513, 797
blood, 511, 512
826
INDEX
Flukes — (Continued)
intestinal, 512, 513
liver, 498, 509, 513
lung, 512
salmon-poisoning, 511
Fluorine, 174
Folic acid, 171
Follicle, ovarian, 345, 347, 349, 351, 352,
709, 710, 746
Food web, 52, 87, 88, 89, 90
Foramen magnum, 693, 705
Foramen of Munro, 706
Foraminifera, 426, 427, 437, 442, 443
Forebrain, 302, 305, 372, 373
Foregut, 605
Fovea, 324, 328, 330
Franklin, B.. 789
Freemartin, 338, 339
Friedman pregnancy test, 383
Frisch, K., 618-621
Frog, 3, 80, 666, 681, 685-712
body cavities, 694, 695
development, 367, 371, 372, 374
ecology and life history. 687
egg after fertilization, 367, 369
folklore, 711, 712
food, 688
hearing, 708
parasites and diseases, 689
secondary sex characters, 689
skeleton, 692-694
skin, 691, 692
systems, position of, 697
voice, 703
Frontal sinus, 270
Fruit, 65
Fucus, 56
Gall bladder. 189, 190, 256, 697
Gallstones, 190. 191
Galvani, L., 9
Gamete, 43, 46, 335, 443
Ganglion, 284, 285, 293, 297
Gannets, 68
Gases, 19-21, 28, 224, 225
Gastric juice, 186
Gastrin, 186, 267, 268
Gastrodermis, 466, 474, 475, 478
Gastroliths, 581, 585, 586
Gastropoda, 631, 633, 634, 643, 798
Gastrotricha, 543, 544, 797
Gastrula, 365, 366, 369, 372, 477, 659
Gastrulation, 365, 366, 368
Gecko, 718
Geese, flying, 730
walking, 741
Geiger-Muller counter, 19
Gel, 21, 22, 26-28, 139
Gemmules, 461, 462, 547
Genes, 38, 39, 43, 368, 389, 393. 395, 397,
399-408, 412, 414, 417, 787, 792
arrangement on chromosomes, 403
linked, 402
sex-influenced, 406, 407
sex-linked, 406
Genetics. 388, 389, 392, 415
Genotype. 392
Genus, 795
Geococcyx californiciis, Til
Germinal epithelium, 344
Gestation, 347
Giants, 257, 258, 272, 273, 274
Giardia intestinalis, 429
Gibbon, 754, 770
Gila monster, 714, 719
Gill arches, 375, 786
Gill clefts, 375, 662, 666
Gills, 574, 674, 683
circulation in fish, 675
clam. 638
snail, 643
Gizzard, 605. 606, 743
Glands, endocrine, 4, 32, 256-259, 275-
278, 703
mammary. 129, 752
mucus, 690. 691
oil, 129, 157, 743
poison, 690, 691, 722
sebaceous, 131
sweat, 129, 134, 252
tear. 129
Globe fish, 670
Globigerina hulloides, 73, 443
Globulin, 198
Glochidia. 641, 642
Glomerulus, 246, 248. 249, 252, 253
Glossiplionio complanata, 570
Glottis, 697, 703 "
Glucose, 29, 57, 163, 169, 170, 187, 193,
198, 199, 265, 268, 343
Glue cells, 494-496
Glycerin, 170, 191
Glycerol, 170, 187, 191
Glycine, 169
Glycogen, 29, 30, 57, 159. 169, 198, 268,
689, 697
Goblet cell, 192
Goiter, 259, 260-262
Golgi bodies, 33, 36
Gonads, 333
Gonionemus, 478, 481
nuirhachi, 482
plicifera, 5 1 1
Gonyaiilax polvhedra, 434, 435
Gordius, 530, 531
Gorilla, 770, 771, 772
Graafian follicle, 344
Grammatophora, 728
Granulocytes, 202, 205. 207
Grasshopper, 3, 599, 602, 605-612
INDEX
827
Grasshopper — ( Continued)
abdomen, 604
development of, 613
ecology, 600
head, 600-602, 603
hearing, 61 1
metamorphosis, 597
thorax, 601-603
wings, 603
Gravity, 81
Grebes, courtship of, 748
Green frog, life history, 690
"Green glands," 582
Gregarines, 444
Guanin, 671, 672, 674, 680, 691, 692
Guanophores, 691
Guard cells, 60, 61
Guttation, 63
H
Hair, 126, 129, 130
human, 131
Hair follicle, 128
Hair papilla, 128
Hair shaft, 157
Haldane, J. B. S., 411
Halichondria, 455
Haploid, 43, 46
Harvestman spider, 623
Harvey, W., 222, 223
Haversian system, 117, 118
Heart, 214, 215, 786
earthworm, 560
frog, 697, 698
human, 218
insect, 607
Heartbeat, bird, 744 *
control of, 217
Heidelberg man, 773
Heliozoans, 442
Helix, 643
Hell-bender, 684
Heloderma, 714, 719
Hematin, 203
Hemichorda, 663-664
Hemiptera, 593
Hemocoel, 605, 607
Hemoglobin, 125, 199, 200, 202-204, 236,
417
Hemophilia, 212, 406-408, 411, 415
Hemosporidia, 444
Hepatic ducts, 189
Hepatic veins, 216
Herbivorous animals, 172
Herbivorous teeth, 180
Heredity, 388, 389, 414, 449, 786, 787, 792
as force in evolution, 784
human, 415-418
Hermaphroditism, 338, 339
arrow worms, 551
Hermaphroditism — (Continued)
bryozoans, 546
ctenophores, 497
earthworms, 564
leeches, 571
liver fluke, 511
planaria, 504
ribbon worms, 537
tapeworms, 514
Hernia, inguinal, 342
Herrick, 591
Hexapoda, 590, 594
Hibernation, reptiles, 722, 723
Hiccough, 241
Hindbrain, 294, 302, 304, 305, 372
Hindgut, 606
Hippocampus, 103
Hippodaniia convergens, 591
Hippospongia, 464
Hirudin, 569, 571
Hirudinea, 569-571, 798
Hirudo medicinalis, 570
Histology, 107
Holothuroidea, 652, 654, 798
Homarus americana, 588
Homo heidelbergensis, 11 "i
Homo neanderthalensis, 111, 713
Homo rhodesiensis, 113
Homo sapiens, 112, 773, 788, 795
Homo sapiens fossilis, 773
Homologous parts, 782
Honey stomach, 617
Honeybee, 3, 612-621
development, 339
salivary glands, 180
sense of smell, 312, 620
Hoofs, 129, 131, 132
Hookworm, 100, 524-526
Hormiphora plumosa, 494
Hormones, 32, 66, 199, 255-257, 267, 268,
271, 272, 275, 277, 304, 335, 349,
404, 407, 409. 609
gonadotropic, 271, 343, 344, 346, 349
luteotrophic, 348, 350, 352
placental, 382
Horned toad, 714, 718
Horns, 126, 129, 132
Horse, 763, 765, 799
evolution of, 785
speed of mammals, 755
Horsehair worms, 530, 797
Horseshoe crab, 574, 798
Human embryo, 378, 380, 382
Humerus, 143, 148, 150, 158, 693, 725
Hummingbird, 729
Hyaloplasm, 26, 35
Hyaluronidase, 352
Hybrid, 399, 400, 402, 414
Hydra, 3, 20, 38, 70, 178, 242, 465, 468-
478, 797
digestion, 476
828
INDEX
Hydra — (Continued)
ecology, 468, 469
excretion, 476
movements and locomotion, 470. 471
regeneration, 477
reproduction, 331, 332, 468, 476
respiration, 476
responses, 470
Hydra americana, 469
Hydractinia, 481
Hydranths, 478, 479
Hydrocaulus, 479
Hydrocorallines, 468
Hydrorhiza, 479, 480
Hydrozoa, 466, 467, 478-482
Hyla crucifer, 272, 682
versicolor, 682
Hymen, 346
Hymenoptera, 593
Hyoid cartilage, 240
Hypermetropia, 330
Hyperthyroid, 261, 262
Hypoglossal nerve, 283, 284
Hypopharynx, 601, 606
Hypophyses, 269, 270, 706
Hypostome, 626, 627
Hypothalamus, 200, 273-275, 304
Hypothyroid, 261, 262
Ice age, 771
Icthyosaurus, 783
Ilium, 148, 685, 693, 725
Immunity, 437, 445
Implantation, 380
Impulses, 281, 285-287, 289
Inbreeding, 413
Incus, 316, 319, 320
Independent assortment of characters, 392
Ingenhousz, J., 57
Ink sac, 650
Inner ear, 317, 319, 320
Insectivora, 759
Insects, 589-621, 663, 798
abundance and size, 590-592, 594
body cavity, 605
characteristics, 589-590
color, 604
diversity, 621
habits and distribution, 592-594
metabolism, 609
metamorphosis, 595-598
muscles, 605
number, 593
tactile hairs, 610
wing venation, 603, 604
(See also specific insects)
Insulin, 191, 198, 252, 267, 268, 696
Intercostal nerve, 238
Intermedin, 276
Interoceptors, 31 1
Interphase, 34, 36, 38-40
Intersexes, 339, 405
Intervertebral discs, 142
Intestine, large, 194
small, lining of. 192, 193
Iodine. 174. 199
Ions, 16-18
Iris, 326, 327, 328
Iron. 174. 199. 204
Ischium, 148, 693, 725
Isolecithal, 361, 362
Isoptera, 593
Isotonic, 23
Isotopes, 14-16. 19. 140
radioactive tracers, 18, 140, 217
Java ape man, 771. 772
Jellyfish, 27, 70, 465-467, 474, 480, 482,
483
Jennings, H. S., 333
Joints. 138. 142
ball-and-socket. 142. 144, 150
hinge, 139. 142, 144, 151
pivotal, 142
rotating. 142
telescopic. 139
Jugular vein, 700
Juvenal, 261
K
Kangaroo, 728, 755, 756, 759, 799
Karyosome, 33
Keel, bird, 745
Keratin, 127, 598
Kerona polypornni, 427, 428
Kidneys, 3
floating. 251
frog. 697, 709
human, 249
lobster, 582, 583
mammalian, structure of. fine, 249
general, 247
vertebrates, 246
King crab, 623
Koala, 103, 755, 756, 758, 759, 781
Kraus, end bulbs of, 310
Kymograph, 186, 188
Labium, 601, 602, 615
Labrum. 599. 601, 602
Lacryniaria olor, ^11
Lactase. 187, 192
Lactates, 199
Lactic acid. 163
Lady-bird beetles. 86, 591
Lagomorpha, 762, 799
INDEX
829
Lakes, 75
Lamarck, J., 788, 789
Laminaria, 56
Lamp shell, 533, 547, 548, 797
Lancet, 664
Landsteiner, K., 209
Langerhans, islands of, 191, 252, 267, 268
Larva, 598, 686
Larynx, birds, 241
human, 182, 206, 236, 237, 240
Lateral line, 668. 678. 679, 708
Leadership and foUowership, 102, 104
Leaf, 60, 61
Lecithin, 170
Leeches, 569, 570, 571, 798
Leeuwenhoek, A., 538. 539
Lemur, 769, 799
Lens vesicle, 373
Lenses. 321-323, 330
compound eyes, 322. 323
crystalline, 326-328, 677
development, 374
vertebrate. 322
Leopard frog. 687, 689
Lepidoptera, 593
Lepidoscelio, 601
Leucocytes, 123, 124, 201, 202, 204-208,
209. 698
Leucon, 457
Leucosolenia. 455, 456. 461
Life, beginning of. 778, 779
increase of, 780
Light. 86
Light receptors, 426, 611
Lignin, 53
Limbs, evolution of, 146
Limpet, keyhole, 634
Limulus, 574
Linens socialis, 535, 536
vegetus, 535
Lingula, 533, 548, 549, 782
Linnaea, 55
Linnaeus, C, 55, 57
Lipase, gastric, 185-187, 696
Lipocaic, 267
Lipochromes, 733
Lipophores. 691
Liquid. 19, 21
Liquor folliculi. 344
Littorina litorea, 75
Liver, 216. 252
bird, 743
development of, 375
fish. 673
frog. 697
human, 189, 256
lobster, 580
mollusk, 639, 640
Lizards, 666, 713, 716, 718. 799
fence, 714
Gila monster, 714
Lizards — (Continued)
horned toad, 714
six-lined, 714
Locomotion, 65, 135-154, 155-167
bird, 739-741
fish, 670, 673
hydra, 470, 471
planaria, 501
reptiles, 714, 715
starfish. 657
Locustidae, 600
Loeb, J., 339
Loligo. 647, 649, 650
Lophophore, 544, 546. 547, 548, 549, 551
Louse, 97, 98
Lumbarsacral plexus, 291
Lumbricns terrestris, 555-567
Luminescent ctenophores, 493, 497
Lungs, 230, 232, 236
bird. 743
evolution of, 234
frog, 697, 700, 702
human, 233. 235, 238, 239
mammals. 752
Lymantria, 257
Lymnaea. 510
Lymph, 119, 123, 195-197, 213, 215, 701
capillary, 197
origin. 212
relation to blood, 213
Lymph hearts, 701. 702
Lymph nodes, 214, 215
Lymph sinuses, 701
Lymph vessels, 194, 214
Lymphocytes, 202, 205-209, 213, 215, 701
M
McClung, C. E., 405
Mackerel, 670
Macleod, J. J. R., 268
Macrobdella decora, 570
Macrocystis, 56
Macronucleus. 428
Macrophages, 203, 475
Macropus major, 756, 759
Magnesium, 174, 199
Malaria, 100, 429, 444, 445, 447, 448
Malleus, 316, 319, 320
Mallophaga, 593
Malpighi, M., 253, 609
Malpighian body, 246
Malpighian layer, 127. 128
Malpighian tubules, 605, 606. 609. 617, 625
Maltase, 187. 192
Malthus, T., 789. 791
Mammals, 666, 752-773, 799
egg-laying, 728, 754. 799
limbs of. 784
marsupial, 728, 755, 799
placental, 728, 756, 759, 799
830
INDEX
Man, prehistoric, 111-111)
skull, 771
Mandibles, 599, 601
Mantle, 630, 635-637, 640, 643, 650
Mantle cavity, of brachiopod, 548
of clam, 638
of snail, 643
Marine, D., 260, 261
Marsupialia, 755, 756, 758
Marsupium, 755, 759
Mastax. 539, 540
Mastigophora, 432, 433, 437
Matter, 9, 10, 12, 25
living, 1
states of, 19
structure of, 13
Maxillae, 599, 601, 602, 693
Mayer, J. R., 57
Mayfly, 573, 590, 595
Mecoptera, 593
Medulla, 302-305, 311, 705, 706
Medulla oblongata, 707
Medusa, 465, 469, 478, 479, 480, 481, 485
Megasoma elephas, 592
Meiosis, 39, 43, 44, 45, 46. 394, 401, 403,
404
Meissner's corpuscle, 310
Melanin, 418, 691, 732, 733
Melanophores, 691
Melanoplus jemiir-rubriim, 600
mexicanus, 600
Mendel, G., 389, 390, 392, 393, 408
experiments, 391, 392, 403
law of independent assortment, 401, 402
Meninges, 372, 704
Meningitis, 301
Menstrual cycle, 354, 355
Menstruation, 351-353
Mering, J., 268
Merozoites, 445, 447
Mesencephalon, 706
Mesenchyme, 457, 459, 464, 536, 545
Mesentery, 190
Mesoderm, 362, 365, 366, 370, 374-376
Mesodermal somites, 370
Mesoglea, 466, 472-476, 480
Mesonephros, 244, 246
Mesovarium, 347
Mesozoic era, 779
Metabolism, 65, 66, 125, 170, 216, 242,
256, 260, 262, 264, 267, 269, 366,
368, 377
Metacarpals, 144, 148, 150, 152
Metamorphosis, 376
frog, 685
insect, 595-598
salamander, 686
Metanephros, 244, 246, 247
Metaphase, 39, 40-42, 46, 363, 365
Metatarsals, 148, 150, 152, 693
Metencephalon, 707
Microciona, 461, 463
Microsorcx hoyi winnemana, 69
Midbrain, 294, 302, 303, 305, 372
Middle ear, 317, 319, 320, 321, 375
Midgut, 606
Migration, birds, 749, 750
fish, 666, 667, 679, 680
insects, 596
Millepora alicornia, 481
Millipedes, 622, 798
Milt, 679
Mimicry, 90
Mineral cycle, 79
Mineral salts, 28
Minerals. 168, 169, 172, 174, 196
Mink, 763
Minkowski, O., 268
Minot, G. R., 204
Mites, 626, 628, 629. 798
Mitochondria, 33, 36
Mitosis, 39, 42, 43, 44, 45, 46
results of, 43, 338
Mitotic divisions, 394
Molecules, 1, 13, 14, 19, 20. 26, 35
Mollusca. 630-650. 663, 798
Molt, birds, 733, 734
crabs, 316
frogs, 690
insects. 257, 258, 594-598, 606
lobsters, 316, 585, 586
salamander, 263, 265
Monkey embryo, 381
Monocystis agilis. 563
Monocyte, 123, 202, 205-207
Monohybrid cross, 298, 400
Monotremata, 754
Morgan, T. H., 505
Mouth cavity, 178
Movement of arms and legs. 167
Mud-puppies, 683, 684
Muller, H. J., 39, 41, 410, 411
Murphy, W. P., 204
Muscle, cardiac or heart, 157
contraction. 163
fatigue. 161
heat-production, 162
involuntary or smooth, 157, 164, 165
recovery, 163
relaxation, 163
stimulation, 160
tetanus, 162
tonus, 161
voluntary, striated, skeletal, 157, 158-
161, 162. 164-166
Muscular system, 155
flexibility, 156
man, 151
Muscular tissue, 107, 119
red and white, 121
smooth. 120
striated, 120, 121
INDEX
831
Mussels, 633, 635
locomotion, 636
Mutation, 407, 408, 409-411
effects, 412
evolution, 408, 787, 792
rate, 410
Mutualism, 93-96
Myelencephalon. 707
Myelin sheath, 282, 283, 298
Myoglobin, 119, 121
Myomeres, 166
Myopia, 322, 330
Myriapoda, 663
Myrmeleon, 598
Mysis, 575, 578
Mytiliis californicus, 434
Myxedema, 262, 263
N
Naiads, 565
Nails, 126, 129, 131, 132, 769
Nais, 227
Nasal cavities, 235, 237
Nasal septum, 270
Natural selection. 791
Nauplius, 575, 578
young stage, 78
Nautilus, 322, 630, 631, 633, 646-648
Nautilus pompilius. 647
Neanderthal man, 772
Nearsighted eye, 322. 330
Neathes brandti, 554
Necator americanus, 524, 525
Nectary, 54
Necturus, 683. 684
Nekton. 74, 77
Nemathelminthes. 519
Nematocysts. 466, 474, 475
Nematode galls, 523
Nematodes, 519, 520, 797
ecology of, 532
Nematomorpha, 530. 797
Nemertinea, 534-538, 797
Neoteny, 340
Nephridia, 548, 549, 574
Nephritis, 243, 251
Nereis limbata, 567
virens, 565-567, 798
Nerocystis, 56
Nerve cells, 164, 282, 283, 285, 288, 293
course of, in spinal nerves, 289
fatigue of, 286
motor, 285, 287, 295, 301
sensory, 285, 287, 295. 301
Nervous system. 155, 156. 279-308
annelid, 554, 562
Ascaris, 522
bird, 744-748
central, 291, 298
crayfish, 583, 584
Nervous system — (Continued)
development of, 300, 370
earthworm, 284
frog, 704
grasshopper, 284
human, 291
hydra, 284
insect. 610, 617
mollusk, 639
peripheral. 291
planarian, 284, 503
starfish, 657
Nervous tissue. 107, 121, 123
Nest building, weaver bird, 738
Neural arch, 142
Neural crests, 370
Neural folds, 370
Neural groove, 372
Neural plate, 363. 367, 370
Neural tube, 370, 372
Neurenteric canal, 372
Neurilemma, 282, 283
Neuron, 122, 280, 281, 285, 292, 293, 299,
300, 372
Neuroptera, 593
Neutrons, 14, 16
Neutrophils, 202, 205-208, 209
Newts, 683-685. 688
metamorphosis of, 686
Niacin, 171
Nictitating membrane, 786
Nissl or tigroid bodies. 122, 281
Nitrogen, 168, 199, 224-226
Nitrogen cycle. 78. 82
Noctiluca, 428, 433, 435
Noses, 70
Nosopsyllus fasciatus, 97
Nostrils, 234
Notochord, 140, 141. 362, 363. 366, 367,
370. 372, 373, 662-665, 784
Nuchal ligament of horse. 114
Nucleolus, 33, 34, 39, 42, 53
Nucleus, 33, 34. 37, 53
importance of. 34
reproduction. 39
Nudibranchs, 643
Nutrition. 168-194
Nymphs, 596, 597
O
Obelia. 478. 480. 481
life cycle. 479
reproduction. 481
Ocelli, 601, 602
Octopus, 631, 633, 642, 645, 647
Octopus vulgaiis, 647
Odocoileus virginianus, 766
Odonata, 593
Olfactory lobe, 705
Olfactory organs, 314, 315
832
INDEX
Oligochaeta. 555-565, 798
Omasum, 183, 184
Ommatidia, 584
Omnivorous animals, 172, 173
Onychophora, 573, 622, 798
Oogenesis, 43-45, 46
Oogonium, 45, 46
Ookinete, 447
Operculum, 375, 644, 675
Ophiuroidea, 652, 653, 798
Opossum, 756, 760, 782
Optic lobe, 705
Optic nerve, 279, 326, 327-329. 373, 611,
706
Optic vesicle, 373, 374
Orangutan, 770
Orb web, 626, 627
Order, 795
Organ, 107
Organic compounds, 28
Organoids, 33, 35
Ornitholestes, 727
Orthoptera, 593
Osculum, 456, 459
Osmosis, 22, 23, 61, 63, 674
Osmotic pressure, 22, 26, 28
Ossicles, 655, 657, 658
Osteichthyes, 666, 668, 798
Ostracods, 587, 588
Ostrea ediilis, 6A1
liirida, 633, 642
Ostrich, 70, 728, 729
Otolith, 316. 317
Outbreeding, 413
Outer ear, 317, 318, 320
Oval window, 320
Ovary, 333, 344, 345
bird, 746
frog, 709, 710, 711
human, 256, 346, 349, 350, 352
mammalian, 347, 351
plant, 54, 64, 65
whale, 348
Oviduct, 346
bird, 746
frog, 709, 711
Oviparous animals, 359, 716
Ovipositer, 604, 605, 617
Ovoviviparous animals. 716
Ovulation, 345, 350-353, 381, 709-711
Ovules, 64, 65
Oxygen. 199. 224-227, 230, 236
Oxygen cycle, 77, 78
Oxyhemoglobin, 203, 236
Oyster, 630, 631, 633, 798
Pacemaker. 217-220
Pacinian corpuscles, 310
Palaemonetes exilpes, 257
Paleozoic era, 779
Paloio worm. 552
Palpi, 601, 602
Pancreas, 189, 191, 216, 256, 267, 268,
697
development of, 375
Pandeka. 668
Pandorina, 429
Paracelsus, 261
Piiragoniinus westermani, 512
Paragordius, 530, 531
Paramecium, 27, 36, 280, 450
appearance, 449
behavior, 451
excretion, 451
general structures, 449
nutrition. 451
reproduction. 332-334, 451, 452
respiration, 451
support and movement, 449
Paramecium aitrelia, 333
bursaria, 428
caudatiim, 450, 452
Paranemertes peregrina, 535
Parasites, 93, 95-97, 425, 426, 429, 433.
436. 437. 498. 499. 508-532, 569.
598. 600. 626. 628. 641, 644
Parasympathetic nerves, 294
Parasympathetic system, 295, 297
Parathyroid glands, 140, 200, 256, 263,
264, 266
Parchment worm, 568
Parotid, 180
Pars anterior, 260, 263, 267, 269, 270-
276, 335, 341, 349, 350
Pars intermedia, 269, 270, 272, 276
Pars nervosa. 269. 270, 273-275
Pars tuberalis. 269, 270. 273
Parthenogenesis. 99. 339. 340, 412, 539,
541, 542, 587, 591
Pasturellu tularemia, 629
Patella, 150, 152, 534
Pavlov, I., 188, 290
Peacock worm, 553
Pearl, formation of, 637
Pecten, 165, 640, 642
Pecten comb, 615, 616
Pectinatella magnifica, 544, 545
Pectoral girdle, 672
bird, 739
turtle, 725
Pedicellariae, 654, 655, 658
Pediculus human Is, 97
Pedipalps, 623
Pedogenesis, 340
Peking man, 772
Pelecypoda, 631, 633, 634-642, 798
Pelmatohydra oligactis, 469
Pelomyxa palustris, 429, 438
Pelostoina flumineum, 103
INDEX
833
Pelvic girdle. 147, 149, 150, 152, 672
bird, 739
python, 721
Pelvic saddle, 739, 747
Penguins, king. 731
Penis, 340, 343
Pepsin, 185. 187
Peptidases. 192
Perching in birds, 742
Pericardial cavity. 694
Pericardium, 695, 698
Periosteum, 143, 145
Periostracum, 636
Peripatus. 573, 574, 622, 798
Periphmeta americana, 258
Perissodactyla, 763
Peristalsis. 165. 697
Peritoneum, 558, 561, 694, 695, 709
Periwinkles, 75
Permeability, 22
Petals, 54, 64
Petromvzon marinus, 666. 667
Phagocytes, 203, 205-207, 213, 536. 581
Phagocytosis. 606
Phalanges. 144, 148, 150, 152, 693
Pharyngeal teeth. 673
Pharynx. 182, 206, 235, 236, 502, 507,
539. 605
Phenotype, 393. 399
Philodina. 227
Phloem. 59-61. 63
Phonoreceptors. 317
Phoronid. 533
Phoronida. 551. 797
Phoronis. 550
Phosphatides. 199
Phosphocreatine, 164
Phosphorus, 174, 264, 266
Photoreceptors, 321, 325
Photosynthesis, 28, 51-61, 65, 77, 78, 83,
426, 433
Phototropic. 325
Phrenic nerve. 238
Phrynosoma, 714, 718
Phylum, 795
Physalia, 467, 482, 483
Physeter macrocephalus, 767
Phytoflagellates. 432. 433. 437
Pia mater. 301. 302, 303
Pickerel. 80
Pigeons. 173, 741
Pika. 762, 799
Pilot snake, head. 716
Piltdown man. 772. 773
Pineal body. 256, 277, 304
Pineal stalk, 706
Pinfeathers, 731
Pinnixa chaetopterana, 554
Pinworms, 99. 524
Pipa pi pa, 682
Pisaster ochraceiis, 655
Pistil, 54, 64. 65
Pith. 59
Pithecanlhropus erectus, 771, 772
Pitocin. 273
Pitressin. 252, 273, 274
Pituitary gland, 250, 252, 256, 257, 260,
265, 269, 270, 271-276, 304, 335,
341, 343, 346-349, 350. 352. 705,
706, 734
Placenta. 347, 348, 350, 378-382, 383,
384, 385, 754, 758
Planaria, 178, 244, 499-507, 797
Plankton. 73, 74, 78, 84, 430. 433, 494,
549, 632
Plant lice feeding, 599
Plants, 57, 59, 60
Plasma, 196-202, 209-211. 249. 251, 253
PUismodiiim, 36, 429
falciparum, 445, 447, 448
malariae, 445, 447
ovule, 445
vivax, 99. 445, 446, 447
Plasmosome. 33
Plastid, 33, 53
Plastron, 724, 725
Platelets, 201, 207, 208, 211, 212
Platyhelminthes, 498, 663, 797
Plecoptera, 593
Pleodorina, 433
Pleucrobrachia, 493, 494
Pleura, 239
Pleural cavity, 695
Pleuroperitoneal cavity, 694, 695
Plexus. 295
Plumatella, 544, 545, 547
Pneumothorax, 239
Podophyra fixa, 453
Polar body. 45-47
Poliomyelitis, 281
Polistes. 312
Pollen. 54, 64
Pollen basket. 615, 616
Pollen brush. 615, 616
Pollination, 2, 312. 616
Polychaeta, 565-567, 798
Polyclad, 509
Polycladida, 507
Polycythemia. 204
Polygordius, 534, 571
Polyneuritis, 173
Polyp, 465
Polyploidy, 412
in salamanders. 414
in tomato plants, 413
Pond. 3. 4, 75
Population of animals. 67
Porifera. 454-464, 797
Porpoises, 764
Portuguese man-of-war, 466, 467, 468.
474. 482. 483
Potassium, 174, 197, 199, 200, 267
834
INDFX
Pouchetia. 427
Pre-Cambrian era, 779
Pregnancy. 347, 353, 354, 383
tests for, 383
Priestley, J., 57,58
Primates, 769, 770. 799
family tree, 768
Proboscidea, 763
Proboscis, 535, 537, 554, 663, 644, 765
Procoracoid, 725
Proctodeum, 613
Progesterone, 271, 276, 277, 344, 346,
348, 350-352, 384
Proglottids, 513, 515, 516
Prognathism, 771
Pronephros. 244, 246
Prophase. 39, 40, 43
Proprioceptors, 166, 280, 311, 316
Prostate gland. 340, 343
Prostomium, 555. 566
Protective resemblance, 90
Proteins. 31, 32, 65, 79, 168, 169. 187-
198, 211, 249, 257, 269, 388. 417,
696
Proterospongia, 434, 435, 454, 464
Prothrombin. 212
Protons. 14, 16
Protoplasm, 22, 25-29. 32. 35, 77, 78,
309, 311
Protozoa, 3, 71, 78, 95, 100, 227, 425-
454, 663, 797
Protozoea, 575. 578
Protura. 593
Proventriculus, 606. 617. 743
Pseudoceros montereyensis, 509
Pseudopodia. 430, 431, 437, 438, 440,
443. 502
Pteridophyta. 796
Pteropods. 632
Ptyalin, 187
Pubis. 148, 150, 725
Pulmocutaneous arches, 701
Pulse, 220, 221
Pupae, 598
Purkinje cells, 217
Purr, 241
Piitoriiis vison, 763
Pygostyle. 739
Pyloric valve. 184. 188. 696
Pylorus, 187
Python, 720, 721
Quahog, 640
Queen bee, 614, 615
Quill, 733
Rabbit, blood vessels of ears, 200, 203
Radio-ulna, 693
Radioactive elements. 12. 14-16. 19, 777
Radioactive tracers, 18
Radiolarians, 136, 426, 437, 443
Radius, 148, 150-152, 154, 158, 725
Raduia, 644. 645
Rana cateshiana, 262, 264, 687
clamitans, 690
pipiens, 273, 276, 383, 687, 711
Rattlesnake, 714, 721, 722
Reaumur. R.. 188
Receptor cells. 279, 280, 309-312, 315,
317, 320
cold, 310
hearing, 312, 317
heat, 310
light, 426. 611
pressure, 310
sight, 312, 321-325, 328
smell, 311. 312. 314
taste, 311-313, 314
temperature, 309. 3 1 1
touch, 309. 310
Recessive character. 391-393, 398, 400,
401, 406, 407. 411, 413-418
Rectum, 194
"Red snow," 433
"Red tide," 434
Reefs, coral, 487-492
Reflex arc, 287, 290
Reflexes. 288-290, 293
conditioned, 289, 290
involuntary, 289, 293
voluntary, 289. 293
Regeneration. 461-463, 477. 497. 498.
505. 506, 535, 536, 565, 585, 654,
720
Relationships, plant and animal, 52, 53
Renal arteries, 247-249
Renal capsule, 246, 248, 249
Renal vein, 249
Rennin, 185-187
Reproduction, 331-355
asexual. 99. 331. 332
plants, higher, 64
sexual, 331, 332, 333, 338
Reproductive systems, ameba, 331, 440
bird. 746, 748. 749
crayfish. 584. 585
ctenophore. 493. 497
earthworm. 562-565
fish, 676-677
frog. 709
human, 340-346, 355
hydra, 331, 332, 468, 476
insects, 61 1-612
mollusk. 639-642, 644
planaria, 504, 505
ribbon worm. 537
rotifer. 541-543
starfish, 661
INDEX
835
Reptilia, 666. 713-728. 799
ancient. 116-11^
Respiration, 4, 11A-1A\, 375, 379
bird, 232, 233, 744, 745
crayfish, 582
earthworm. 561
fish, 228-233, 675
frog, 232, 233. 702. 703
insect. 608. 609, 618
in invertebrates, air tubes. 228. 233
blood gills. 228
tracheal gills of mayfly, 229
man, 233, 237, 238
mollusk, 637
planaria. 502
plants. 59. 61
reptile. 232, 233, 714, 725, 726
salamander, 230
Responsiveness, 66
Reticulum. 183. 184
Retina, 321, 324, 326, 327, 328, 329, 373,
374
Rh factor. 204. 382, 416, 417
Rhabdocoela, 507
Rhinoceros, 765
Rhinoceros beetle. 592
Rhodesian man, 773
Rhynchocephalia, 717
Ribbon worms, 534-538
Riboflavin, 171
Ribs, 149, 721
Richards. A. N.. 253
Rickets, 118
Rickettsia, 629
Robertson, J. D.. 559
Rocky Mountain spotted fever. 629
Rod and cone cells. 326. 328, 329, 330
Rodentia, 759
Roger of Palermo, 261
Rolando, L., 307
Root hairs, 60. 62, 63
functions, 63
and root pressure, 64
system, 62
Rotifer, 78, 176, 227, 533, 538-543, 797
Roudabush. R. L., 478
Rouleaux formation, 202
Round window, 320, 321
Roundworms, 100, 519-532
Ruflini ending, 310
Rumen, 183, 184
Rutherford, 12
Sahella pavonia, 553
Saccule, 316, 317, 320
Sacculus, 678
Sagitta hexaptera. 550
Salamander, 71, 666, 681, 683-685
Saliva, 179
Salivary glands. 180
Salveliniis fontimilis. 260
Sand dollar, 652, 654
Sarcodina, 432, 437, 441, 443
Scales, 126. 129, 130, 669-672
bird, 730, 737
snake, 716
turtle, 725
Scallops, 633, 640, 642, 798
Scaphopoda, 631, 633, 634, 635
Scapula, 148, 150. 158, 673. 693, 725, 740
Scavengers, 580, 598
Sceloporus, 714
Schistosoma haeniatohiitm, 511
Sciatic nerve, 704
Sclerites, 600, 602, 604
Sclerotic coat, 327
Scorpion, 623
Scrotal sac, 340, 341
Scutes, 716
Scyphozoa, 467, 468, 482
Sea, 3, 74
Sea anemones, 70. 71, 465-467, 480, 486,
487. 797
Sea cucumber. 652-654, 664, 798
Sea hares, 644
Sea horse. 670, 756
Sea lily. 652, 653
Sea mouse. 567
Sea scorpion. 574
Sea star. 652, 653
Sea turtle, 724
Sea urchin, 652, 653, 654
Sebaceous gland, 128, 310
Secondary sex characters, 275, 276, 689,
711
Secretin, 192, 267, 268, 696
Seed, 65
Segmentation, 71, 552, 573, 574, 578, 622
Segregation of characters, 392
Self-fertihzation, 390
Semicircular canals, 316, 317, 320
Semilunar valves, 700
Seminal vesicle, 340, 343, 563
Seminiferous tubules, 341, 342, 343, 710
Sensation, threshold of, 309
Sense organs, 309-330, 708, 745, 748
Sensory nerve fibers, 161
Sepals, 54, 64
Sepia, 647, 648
Septa, 486
Serosa, 613
Serpent stars, 653
Serum, 197, 210, 211
Setae, 555, 561, 604
Sex determination, 396
Sexual coloration. 90
Shark. 666. 668, 669, 783
Shrew, common, 761
pigmy, 69
Shrimp, 577, 578
836
INDEX
Siamese cat, 412
Sieve plate. 655, 656, 657
Sigh, 241
Silicon. 174
Silk glands, 624, 625
Silurian sea bottom, 575
Silverfish. 590, 595
Sinanthropus, 772
Sinuauricular node, 217, 218
Sinus, frontal. 235, 270
maxillary, 235
Sinus gland, 257
Sinus venosus, 699, 700, 701
Siphonaptera, 593
Siphons, 636, 637, 638-640, 641, 647, 650
Skeleton, 135, 154
bird, 738, 739
fish, 672, 673
frog, 693
horse, 149
human, 148, 150
reptile, 716, 725
Skeleton content, 139
functions, 135
permanent, 137
temporary, 137, 138
Skin, 126
frog, 692
functions, 133, 134
human, 128,310
sense organs. 31 1
structure, 127
Skin muscles of horse, 164
Skull, 150, 153
Slug, 630, 631, 633, 643
sea, 646
Smell, 611. 679
Smith, H. W., 253, 254
Snail, 71, 178, 630, 631-643, 644, 798
Snakes, 666, 713, 716, 718, 799
boa constrictor, 720
cobra, 719
garter, 714, 721
rattlesnake, 714, 721
Sneeze, 241
Snore, 241
Social hierarchies, 102
Soddy, 12
Sodium. 174, 199. 200, 267
Sodium chloride, 198
Soft palate, 182, 206
Soft-shelled turtle, Florida, 726
Sol, 21, 22, 26-28
SoUd, 19-21
Solution, 19, 21
Somatopleure, 367, 370
Species, 67, 777, 795
living, number of, 69
Sperm, 331, 335, 336, 341, 405
Spermatheca, 612
Spermatogenesis, 43, 44, 46, 406
Spermatogonia, 43, 44
Spermatophores, 684
Spermatophyta, 796
Sphaeridae, 642
Sphenodon pnnctatiiiu. 111, 782, 799
Sphenoidal sinus, 270
Sphincter valve, 251
Spicules. 456, 458. 459. 464
Spiders, 622, 623-626, 663, 798
anatomy, external. 624
internal. 625
Spinal cord. 238, 251
frog, 704
human. 298
Spinal nerves, 293, 707
Spindle. 41. 42, 46
Spinnerets, 624
Spiracles, 602, 604, 608, 618
Spirometer, 240
Spirostonuiin ainbigituni, 426, 428
Splanchnopleure, 367, 370
Spleen, 216, 225, 697
Sponges, 454, 455, 456-464
reproduction, 459-462
skeleton, 457, 459
uses, 463, 464
Spongilla, 460-462
Spongin, 459
Spongocoel, 456, 457
Sporozoa, 432, 444
Sporozoites, 445, 447
Spring peeper, 682
Squamata, 718-722
Squid, 630, 631, 633, 645-650, 798
giant, 70
Stamens, 54, 64
Stapes. 316, 319, 320
Starch, 30, 32
Starfish, 70, 651, 653-661, 798
appearance, 655
arm, cross section of, 658
canals, 656, 657
development, 659
eating, 652
eyespot, 657
larva, 664
locomotion, 657
water vascular system, 656
Starling. 192, 255
Statoblast, 547
Statocyst, 481, 495, 497, 583, 584
Statolith. 315, 316, 583
Statoreceptors, 314, 315
Steapsin. 187, 191
Stentor, 111
coernlens, 426
polymorphitm, 428
Sternum, 150, 151. 720
Sterocoral pocket, 625
Steroids, 257
Stevens, N., 406
INDEX
837
Sticklebacks, courtship of, 337
Stigma, 54, 64, 65
Sting, bee, 617
Stinging cells, 480, 486, 487
Stomach, cardiac, 657
cow, 184
frog, 697
human, 185, 189, 256
Stomata, 60, 61
Stomodeum, 613
Struthis cameliis, 729
Styela (Cynthia), 362
fertilized egg, 364
Sucrase, 187, 192
Sucrose, 29
Suctoria, 432, 452, 453
Sugar, 29, 32
"Sugar diabetes," 259
Sulfur, 174
Sun. 2, 29, 72
energy of, 29, 73
Sunfish, 670
Suspension, 19, 21
Sutton, W. S., 393
Swallowing, 181, 182, 183
Swammerdam, J., 162
Sweat, 126
Sweat gland. 128
Swimmerets. 577, 578, 585
Swordfish, 77
Sycandra, 459
Sycon, 455, 457, 459
Symbiosis, 93, 95. 487
Symmetry of animals, 70-72
bilateral, 368, 493, 498, 600. 635, 651,
654. 655, 659, 664
radial, 466, 470, 651
Sympathetic nerves, 294, 295, 297, 704,
708
Sympathin, 266
Synapses, 44, 45, 46, 287, 288, 291, 293,
297, 306
Synapsida, 727
Syncytia, 521, 540
Synovial membrane, 143
Syphilis, 100
Syrinx, 241, 743, 744, 745
System, 107
Systemic arches, 701
Systolic pressure, 220
Tadpole, 178
mouths of, 688
Taenia pisiforniis, 516
saginata, 515
solium, life cycle, 514
Tapetiim liuidiini. 326
Tapeworm, 499, 513-518, 797
Tapir, 763, 765, 799
Tarentiila nmuritaniciis, 718
Tarsals, 148, 150, 152, 602, 615, 693
Tarsius, 769, 799
Tasmanian wolf, 756
Taste, 611, 679
Teeth, 153, 179, 181
carnivorous, 180
human, 181
vomerine, 694
Telegony, 414, 415
Telencephalon, 705
Teleosts, 668
Telolecithal, 361, 362
Telophase, 41, 42
Temperature, 83, 84
bird, 729
reptile, 716
Tendons, 159
Tenebrio, 406
Tent caterpillar, 93, 590
Tentacles, 467
ctenophores, 494-496
hydra, 470, 472, 474, 475
mollusk, 640
Obelia, 479
sea anemone, 488
squid. 648
Termite, 95, 103, 591
Termitoniciis mahout, 95
Territorial rights, 102
Testes, 256, 333, 340, 341, 343
frog, 697, 709, 710
hormone, effect of, 343
Testosterone, 271, 276
Tetany, 264
Tethys californicus, 644
Tetrads, 44, 45, 46
Tetranychus telarius, 628
Thalamencephalon, 705, 706
Thalamus, 303, 305, 311
Thallophyta, 795
Thalmi, 706
Thermohia domestica, 590
Thiamine, 171, 173
Thrombin, 212
Thrombocytes, 208, 698
Thrombokinase, 212
Thrombosis, 212
Thymus, 256, 271, 276
Thyroid gland, 256, 258, 259. 261, 264,
270, 274, 734
activity, 260, 262, 265, 269, 271, 272
cartilage. 206, 240, 241
Thyrotrophin, 256, 277
Thyroxin, 19, 256. 260. 271, 272
Thysanura. 590, 593, 596
Tibia, 148, 150, 152, 725
Tibio-tarsus, 693, 739
Ticks, 623, 626-628, 629. 789
Tiedemann vesicles, 656
Tissue fluid, 195-197, 221
838
INDEX
Tissues, 107
Toads, 666. 681, 685
Tongue, 313-314
bird, 735, 742
frog, 694, 697
human, 313
Tonsils, 206, 213, 235
Tooth shell, 631, 633, 635
Tortoises, 718, 724
Touch, 309-311
honeybee, 620
Trachea, 235-237, 603, 605, 608, 609, 618
human, 182, 206
Tracheoles, 608, 609, 618
Tragus, 318, 319
Transfusion, 210, 211
Transpiration, 59-61
Trematoda, 498, 508-513
Trembley, A., 477, 478
Triceps muscle, 158, 159
Trichina, 96, 98, 526, 527, 529, 797
Trichinella spirella, 98
life history, 527, 528
methods of exposure to. 526
Trichinosis. 100. 526. 528
Trichocysts, 426, 427, 449, 450
Trichoptera, 593
Trichospherium, 332
Tricladida, 507
Trilobites, 574. 575
Triturus, 166, 683, 688
torosiis, 684
viridescens, 261, 263, 265, 414, 684. 686
Trochelminthes, 538
Trochophore, 533, 539, 546-549, 555, 565,
571
Troglotrema sahnincola, 511
Tropisms, 288
Trout, 666
early stages, 671
reproduction, 676, 677
scales, 672
Truncus arteriosus, 699, 700
Trunk fish, 670
Trypanosoma gambiense, 98, 436, 437
lewisi, 436
rhodesiense, 437
Trypanosomes. 98, 436, 437
Trypanosomiasis. 436, 437
Trypsin, 187, 193, 268
Trypsinogen, 191, 268
Tsetse fly, 98, 436. 437
Tuatera, 717, 799
Tube feet, 652, 653, 655, 656, 658
Tubifex tiibifex, 565
Tubipora, 466
Tularemia. 629
Tuna, blue-fin or marlin, 77
Tunicates, 662, 664, 798
Tupaia tana, 318
Turbellaria, 498, 499, 501, 502
Turbinate bones, 206, 234, 315
Turtles, 80, 666, 713, 718, 724-726, 799
box, 725
loggerhead, 77
musk, 714
painted, embryo, 715
snapping, 714, 715, 725
soft-shelled, 726
spotted, 714
Tusks, 763, 764
Twilight eye, 323
Twins, fraternal, 384, 386
identical, 385, 386, 402, 410
Siamese, 385
Tympanum, 316, 317, 375, 602, 611, 720,
745
Typhosole. 560. 561
Tyrannosaurus, 727
U
Ulna, 148, 150-152, 725
Ultrasonic sounds, 318, 319, 759
Umbilical cord, 379, 382, 384, 385
Umbo, 640
Ungulata, 763
Urea, 78, 126, 198, 243. 251. 252
Ureters. 247, 248, 249, 251. 697, 703, 709,
710
Urethra, 247. 251. 340, 343. 346
Uric acid. 198. 243. 251
Urinary bladder, 340, 343, 346, 379
bird, 744
frog. 697
reptile, 723
Urinary system, 247-251
human. 248
Urine, 247, 251-253, 343
formation of, 250
Urochorda. 664
Urodela, 683-685
Uropods, 585
Urostyle. 693
Uterus, 344, 346-351, 384
frog, 710, 711
human. 379, 380. 383
Utricle, 316, 317, 320, 678
Uvula, 206
Vacuole. 33, 35. 53
contractile. 428, 432, 438, 541
food. 428, 438
Vagina, 344, 346, 347, 350. 612
Vagus nerve. 184, 185, 219, 238-240, 297,
305
Vallate papillae, 314
Vas deferens. 340, 342. 516. 611
Vasa efTerentia. 710
Vasoconstrictor nerves, 200
INDEX
839
Veins, 213, 221
valves of, 222
Vena cava, 248, 249
Venom. 721. 722
Ventricles, 218, 700, 701
Venus's flower basket. 458
Venus's girdle, 493
Vertebra, 141, 672, 686
Vertebral column, 140, 143, 662, 672
Vertebrata, 141. 664-680, 798
probable relationships of, 663
Vertebrate body plan, 146, 147
compared with invertebrate, 667
generalized, 669
Vespa, 312
Villard, 12
Villi, 192, 193
Vinegar eels, 520, 522
Viruses, 25, 397
Vitamins, 32, 168, 170-172
A group, 171, 172, 174, 324, 329
B group, 171, 173
Bi.., 171, 204
C group, 171
D group, 171, 731
E, 171
K, 171, 191, 212
Vitreous humor, 326, 327
Viviparous animals, 359
Vocal cords. 182, 206, 240, 241, 689
Voice. 240. 703, 720
Volvox. 426, 428, 433, 434
Vomerine teeth. 694
Vorticella, 428, 448
Weasel, 183
Welk, 631
Whale, 3, 32, 52, 129, 764, 767
blue, 69. 70
development of flipper, 782
sperm, 767
voice. 240
Whale shark, 70
Whalebone, 766, 768
Whipple. G. H., 204
White-bellied swallow, section through
head. 324
Whitney. D. D., 543
Wilson. E. B.. 406
Wings. 740, 741, 782
Wishbone, 694. 739, 740
Woodpecker, 176
Woodruff, L. L., 333
Worker bees, 612, 614, 616
"dances," 620, 621
legs of, 615
special structures of, 615, 616
Wuchereria bancrojti, 528, 529
X-rays, 12, 410
Xanthophyll. 56
Xenarthra, 762, 799
Xerophthalmia, 174
Xiphosura. 623
Xylem, 59-61, 63
W
Wald, G., 324, 325
Wallace, A. R., 791
Walrus. 764
Wutasenia scintillans, 649
Water. 4. 13-16, 27, 81, 198, 226, 227
in frog, 703
need of, 3, 63, 65, 80, 168, 195, 197, 555
Water cycle, 80
Water molecules, 20
Water striders, 20
Wax pick, 616
Yawn, 241
Yolk, 368
Yolk sac, 380, 668
Yucca lily, 94, 96
Yucca moth, 94
Zoea, 575, 578
Zooflagellates, 432, 434
Zoology, 4
Zoraptera, 593
Zygote, 47, 444, 447
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