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.B

TEXTBOOKOF

^

ZOOLOGYBY

GEORGE EDWIN POTTER,

Ph.D.

Professor of Zoology, Agricultural and Mechanical College of Texas, Formerly Professor of Zoology, Baylor University

SECOND EDITION

With 445 Text

Illustrations

-T).A.\

W!v-*ti

l-

xin^'L-v ^Iffai:i'Tfii., ^F^^'.

ST.

LOUIS1947

THE

C. V.

MOSBY COMPANY

Copyright, 1938, 1947,

By The

C. V.

Mosby Company

(All rights reserved)

Printed in U.

S.

A.

Press of

The

C. V.

Mosby Company

St. Loviis

DEDICATED TO

PROFESSOR FRANK A friend and an

A.

STROMSTEN

inspiration

to the student

/f-i

PREFACE TO SECOND EDITIONThe present edition represents a revisionof certain parts of several chapters, such as those dealing with Annelida, Genetics, Eugenics, Internal Regulation and Endocrines, Physiology, and Phylogenetic A brief section on Mammalian Development Relations of Animals. has been added. Numerous minor corrections or improvements have

been

made throughout.

Several illustrations have been added and

others improved.

In addition to the acknowledgements included in the preface to the first edition, the author wishes to acknowledge the help of Dr. Kelshaw Bonahm of the Fish and Game Department, Agricultural and Mechanical College of Texas, on the chapter dealing with Pisces. At this point recognition is made of assistance given by Mr. Gordon Gunter in revising the list of animals of the Texas Gulf Coast in the chapter on ]\Iarine Zoology. The author is also indebted to Dr. Fred L. Kohlruss, Biology Department, University of Portland (Oregon), Many valued suggestions have for numerous useful suggestions. been received from individuals in a number of other institutions wherethe first edition has been in use.

The author's indebtedness and

appreciation

expressed to Mr. Phil T. Williams who has furnished a number of the new illustrations. Finally, appreciation is expressed to Agricultural and Mechanical College of Texas for cois

also

operation in numerous ways to assist in making this revision.

George E. Potter.College Station, Texas

PKEFACE TO FIRST EDITIONThe important problems of life are common to all animals (including man) as well as to plants. It should be the purpose of a textbook in general zoology to present the animal kingdom in a logical and natural way and at the same time carry the interpretation of the factsin terms of the principles involved. strike the ideal balancefactual,It is exceedingly difficult tosufficient,

between the necessity of presenting

"type" materialknowledge of

in order that the student will have the

requisite

classification, structure, function,

development,

and organographyrules,

to appreciate the discussion of principles,

and the

opposite temptation to go into endless discussions of theories and

comprehension of which is unquestionably beyond the who has not become grounded in fundamentals of animal make-up. Of course it is usually hoped that the laboratory division of the course wiU supply this needed foundation. It seems reasonable that the ultimate aim of the teacher of introductory zoology should be to bring the student to a fundamental and well-grounded understanding of the principles involved in all of the living processes. It is extremely difficult to skim this information from the top of the entire body of zoological knoAvledge, as one can skim cream from a crock of milk, and hand-feed it to the waiting student mind. Apparently there must be a certain amount of personal acquisition of the principles of the subject through attaining a clear-cut knowledge of tlie complete biology of a series of representative animals. Each of these representatives, since it is a living organism, demonstrates certain of these principles. In order to bring this out there must be a rather close coordination between the studies of the laboratory andthe

capacity of the student

the presentation of principles

by the textbook. Based on a recognition of the above-mentioned situation and

also

on the realization that the majority of students taking elementary zoology plan to go no further in the field, the author has attempted to strike a workable combination of the two schools of teaching and still cover the fundamental knowledge of the subject. There has beena definite effort to lead the student to think of biology as related to humankind and to himself. It is hoped that the book will overlapthe laboratory studies just far enough tolift

the student out of the

laboratory into his o^vn correct interpretation of the facts discovered

PREFACEthere.It is of course

assumed that the teacher

will naturally elabo-

rate

in the course. The upon particular phases of the topics anticipation of this and limitations of space have reduced the volume

taken up

of detailed information included.

Manythis

animals from west of the Mississippi River are featured in book. There has been no attempt to limit the scope of the work

to this region,

but since many southwestern and western forms are available and serve as very good illustrative material, they have been It is hoped this wiU make the book more useful and meanutilized.ingful to students in these regions, as well as

more

teachable.

The introductiongeneration,

of chapters on

Biological

Effectsis

of Radiation,

Wildlife Conservationoutline,

a slight

Animal Anomalies, Animal ReMarine Zoologij, and departure from the usual textbook

but each of these seems to the author to have enough of special value and current interest to warrant presentation. The chapIts ters on Regulatory Glands, Animal Distribution, The Animal and Environment, Animal Parasitism, Comparative Emhrijology, AnimalBehavior, and Paleontology are also presented with the feeling that they are of exceptional general interest to all students, as well as

being thoroughly zoological.

The arrangement of the chapters on animal groups has been somewhat in the order of complexity and systematic relationships. Thechapters are written in such a way, however, that this order may be modified in any manner to suit the teacher. The chapters dealing with typical Protozoa, Hydra, Planaria, Annelida, Arthropoda, and Amphibia are somewhat amplified and include more detail because

they are so often chosen as typical groups for study. Throughout many the book the genus and species names have been italicized, and

names

of structures

and functions have

also

been italicized the

first

time they occur.

The author

is

of several teachers

indebted and extremely grateful for the cooperation and specialists who have contributed manuscript

For this service acknowledgment is made Teague Self, University of Oklahoma, Annelida; Elmer P. to: J. Cheatum, Southern Methodist University, Mollusca, and assisted with Marine Zoology; Vasco M. Tanner, Brigham Young University, Arthropoda; Mary Fielding, Public Schools, Waco, Texas, collaborafor chapters in their fields.

Newman, Baylor University, collaboraOttys Sanders, Southwestern Biological Supply Co., tion on Pisces;tion on Elasmohranchii; Rose

.

8

PREFACE

Amphibia, and assisted with Marine Zoology; Leo T. Murray, Baylor University, assisted by James E. Blaylock, Ranger Junior College, Reptilia; Helen Joe Talley, University of Oklahoma, collaboration on Regulatory Glands; T. C. Byerly, United States Bureau of Animal Industries, Ayiimal Regeneration; Titus C. Evans, University of Iowa, Medical College, Biological Effects of Radiation; Willis Hewatt, Texas Christian University, A^iimal Distrilution, and assisted with

Marine Zoology; A. 0. Weese, University of Oklahoma, The Animal and Its Environment ; Sewell H. Hopkins, Texas Agricultural and Mechanical College, Animal Parasitism ; J. G. Burr, Texas Game, Fish, and Oyster Commission, marine data; Walter P. Taylor, Texas Cooperative Wildlife Service and United States Bureau of BiologicalSurvey,Wildlife

Conservation; A. Richards, University of Okla-

homa, Comparative Embryology ; Frank G. Brooks, Cornell College, Genetics and Eugenics; Iva Cox Gardner, Baylor University, Animal Behavior; W. M. Winton, Texas Christian University, Paleontology

To Mr. Ivan Summers goes immeasurableart

credit for the excellent

work he has put

into this edition.

Dr. Titus Evans, of the

University of Iowa, Medical College, has also been of great service Mrs. Ruth M. with his excellent talent in creating illustrations.Sanders, Miss Joanne Moore, and Mr.assisted

Edward O'Malley have

each

The drawings used in XLII on Genetics and Eugenics were made by Miss Betty Chapter R. Smith of Cornell College. The author is grateful to all of theseby contributing certain illustrations.individuals for their valuable services.

The author wishes

to

acknowledge also the friendly and helpful

advice which has been offered by Professors D. B. Casteel, T. S. Painter, and E. J. Lund of the University of Texas, and Professor

Asa Chandler of Rice Institute. Finally, appreciation is expressed to Baylor University for the cooperation which has made the writing of this book possible. George E. Potter.Waco, Texas.

;

CONTENTSIntroduction The Biological Point of View, 17; Science and the Scientific Method, 18; Zoologj-, a Biological Science, 19; The Subdivisions of Zoologj-, 19; Classification of the Animal Kingdom, 25; Vital Eelations of Animals and Plants, 27; Attributes of Life, 30; Balance in Nature, 31; Zoology as Belated to Man, 33; Agriculture and Zoology, 34; Fisheries and the Application of Zoologj-, 34.

CHAPTEE ___________________I

PAGE17

History op Zoology

________________________________CHAPTERIII

CHAPTER

II

36

Protoplasm and the Cell

49

Living Matter, or Protoplasm, 49; The Cell Principle, 49; General Characteristics of Protoplasm and the Material of the Cell, 53;

Fundamental Properties or Activities of Protoplasm, 54; Physical Nature of Protoplasm, 55; Chemical Nature of Protoplasm, 56; Structure of a Typical Animal Cell, 58; Cell Division, 61.

CHAPTER IVPhylum Protozoain

General

_____________77.

65

Characteristics, 65; Classification, 65; Colonial Protozoa, 75; Tropisms

and Animal Reaction, 77; Economic Relations of Protozoa,

CHAPTER VEuglena op Class Mastigophora

____________

81

Habitat and Characteristics, 81; Structure, 81* Food and Assimilation, 81; Respiration and Excretion, 83; Reproduction and Life Cycle, 83; Behavior, 84; Locomotion and Flagellar Movement, 84.

Amoeba op

Ck, Si

m o

ttl

8

fe

o >> o CT.:"

Fig.

(From 14. Charles Darwin (1809-1882), the author of Origin of Species. Garrison, History of Medicine, published by W. B. Saunders Company.)

Gregor Mendel (1822-1884) was an Austrian monk who carried on experiments with the breeding of garden peas in the cloister garden. From his work there, he derived the original laws of heredity. His results were first published in an obscure Swiss paper in 1866 and were not really discovered and appreciated until 1900. He was the founder of genetics. He crossed different kinds of peas and found that the offspring in the first generation all resembled one parent. When these oft'spring were interbred he found that three-fourths of their progeny resembled one grandparent, and the remainder resembled the other. From these facts he referred to characteristics of the former group as dominant and those of thelatter as recessive.

The facts which he established are now knownof Heredity.

as Mendel's

Laws

46

TEXTBOOK OF ZOOLOGY

,(S,

C^^f^e^a^-^

Fig. 15. Greg-or studies of heredity.

made between 1861 and

Mendel (1822-1S84), the Austrian monk who pioneered In Plaque by Theodor Charlemont based on "Fuschia picture" 1864. The signature is his, taken from an autobiography.

(Courtesy of the Journal of Heredity.)

HISTORY OF ZOOLOGYLouis Pasteur (1822-1895) was a French scientist

47

who had been

trained in chemistry but became one of the outstanding pioneers in applied biology and medicine. In 1861 he put an end to the

controversy regarding spontaneous generation of living organisms

and established the idea that all life in present times comes from life. He showed that living organisms cause fermentation and demonstrated that these organisms and others could be killed by heating them to a certain temperature. He showed that materials thus heated and then sealed would not ferment until after they were exposed to the organisms in the air. The pasteurizationpreexisting

Fig.

16. Louis Pasteur (1822-1895), one of the benefactors of mankind. (From Garrison, History of Medicine, published by W. B. Saunders Company.)

process grew out of these experiments. He rescued the silk industry of southern Europe by discovering the organism which killed the insects, and he also discovered an immunization process and treatment for hydrophobia.

Thomas Henry Huxley (1825-1895) Avas one of the most popular English scientists of his day. He was one of the principal champions of Darwin's ideas and theories. Comparative anatomy and paleontology were greatly advanced through his influence.

August Weismann (1834-1914) was a German

zoologist

who

started

out as a physician after having been trained in that

field.

He was

48

TEXTBOOK OP ZOOLOGYfields of

an outstanding scholar in theis

for his theory, that there He plasm from generation to generation.

best

known

heredity and embryology. is continuity of germ

(1848-1935), a Dutch botanist, brought out the mutor tion theory, which is important to all modern biological conceptions. His idea was that species have not arisen through gradual selection requiring thousands of years for each but by jumps through sud-

Hugo DeVries

den, though small, transformations.

He

is

widely

known

for his

experimental studies in plant breeding and with evening primrose.omists of America.

genetics, particularly

E. D. Cope (1840-1897) was one of the greatest comparative anatHe dealt not only with living forms but with

fossil materials as well.

The work of all those mentioned and hundreds of others has given us the background for our present knowledge and grasp of zoology and medicine. History is being made so rapidly in these fields durdifficult even to catalogue the imextremely active field, particularly portant contributions. It is an The printed program in the realm of the experimental endeavors. American Zoological Society, which for the annual meeting of the is made up largely of titles and abstracts of new papers to be pre-

ing the current years that

it is

sented,

is

a small book in

itself.

The works and lives of such prominent pioneer zoologists of the Southwest as Jacob Boll, Gustaf W. Belfrage, Lincecum, Vliet, Walker, "Webb, and others have been described in the recent book by Dr. S. W. Geiser of Southern Methodist University, entitledNaturalists of the Frontier.read.

This book

is

extremely interesting to

CHAPTER

III

PROTOPLASM AND THE CELLLiving- Matter, orLittle is

Protoplasm

known concerningcalled, but

the origin of living matter, or protois

plasm, as

it is

more and more

being learned about

itsis

nature, characteristics, structure, and activities.

Living matter

ahvays active in some degree, and this activity attracted the attention of scholars at a rather early date, but serious study of thematerial was not begun until approximately one hundred years ago.

A

some of the simple microscopic animals he was studying were composed of a soft, gummy substance and called it sarcode, which means "flesh." He was able to test its solubility and its behavior with alcohol and acids sufficiently to satisfy himself that it differed from ordinary gelatin or albumin, with Avhich it might be confused. In 1840, Purkinje, a Bohemian biologist, gave living matter the name protoplasm, which comes from the Greek protos, first, and plasma, anything formed or molded. In 1846 von Mohl, a German botanist, saw in plants a granular, viscous substance similar to that already seen in animals', and called it protoplasm. He was instrumental in bringing this name into common use. During these years it had gradually dawned on biologists that this matter is found in all livtheof Dujardin, in 1835, realized that

Frenchman by

name

ing things.

TheCells

Cell Principlesuperficially described during the

had been seen and even

latter part of the seventeenth century

the eighteenth century, but their significance

and numerous times during was not realized.

Hooke, an Englishman, in 1665 in observing cork with the microscope he had made, saw the spaces in it and called them cells because they reminded him of prison cells. This name later came to be applied to the real cells. It was an unfortunate term, for cells do not have a hollow structure but are typically semisolid bodies. Leeuwejihoek saw spermatozoa and bacteria and included them with single-celled animals as "little beasties"; Malpighi had described49

50

TEXTBOOK OF ZOOLOGY

the nature and appearance microscopically of several organs of the body; Grew had made rather extensive microscopic studies of plants,

and in 1831 Kobert Brown had discovered the nucleus of the cell, but not until the work of Schleiden in 1838 and Schwann in 1839 was the cell theory formally enunciated. The former a botanist and

Fig. 17. Matthias J. Schleiden (1804-1881), noted German botanist who helped establish the cell theory. (From Locy, Growth of Biology, published by Henry

Holt and Company, Inc.)

the latter a zoologist, each

working independently, came

to the

same

conclusion and in 1839 collaborated their ideas. This theory, as they gave it, was in substance, All living things (plants and animals)are composed ofcells.

PROTOPLASM AND THE CELLIt is

51that they and

no discredit

to this

theory or these

men

manydis-

other biologists of the time had erroneous ideas concerning theessential features of the cell.

Although Brown had recently

covered the nucleus, thepart,cells since practically

cell

wall was thought to be the essential

though now we know it is not a universal structure of all no animal cells have a cell wall. The notions of the origin of cells and the functional significance were almost wholly fantastic, yet the cell theory proved to be such a unifying generalization and inspiring stimulus to investigation that it became the turning point in the development of biological study.

Fig. 18. Theodor Schwann (1810-1881^), the German zoologist who, in 1838 and 1839, collaborated with Schleiden in formulating the cell theory. (From Garrison, History of Medicine, published by \Y. B. Saunders Company.)

The bare statement that living beings are composed of cells soon became inadequate as studies of cells progressed. It was soon found that some tissues are made up not only of cellular structures but included also certain noncellular materials produced by the cells. The matrix, so abundant between the cells of cartilage, was soon found to be noncellular and to be produced by the cartilage cells which became embedded in it. This matrix is not strictly living matter since it is inactive and passive as far as life processes are concerned. Connective tissue fibers fall in the same category. Since living bodies are composed of such an abundance of this noncellular

52

TEXTBOOK OF ZOOLOGYproduced by the:

m aterial

cells,

the cell prmciple soon

came

to be

things are composed of cells and cell products. stated thus conceptions of the nature of the nucleus, the With the years, theall living

membranes, and the composition of protoplasm itself have all added their contributions to the present understanding of the meaning and application of the cell principle. The cell is now regardedcell

as a physiological unit as well as a structural one, and as almost a corollary to the original statement of the principle, namely, that

the activities of the organism equal thecells.

sum

of the activities of its

the embracing of the functional activity of the cell as a part of the principle underlying living processes comes also the inclusion Cell division, growth, tissue formaof heredity and development.

With

tion,

migration of

cells,

formation of

cell

products, chromosome rela-

and modifications have come to be recognized as being brought about in or by the cells. Through the rather rigid and constant set of developmental changes for which the cells are responsible, there is developed a new individual which usually resemblestionshipsits

parents quite closely.

cell theory on biological thinking and progress on fundamental thinking generally, can hardly as well as its effect be over-estimated. The conception of this idea was one of the great landmarks in development of biological ajid scientific thinking. It was the first great generalization in biology. It is comparable in

The influence of the

the field of biology to Newton's law of gravitation in the field of physics. Up until this time there had been no single fundamentalidea applied to living material that

was recognized

as being univer-

This conception focused the thinking of all biologists sally true. in the same direction and therefore it had a great unifying influence. Deliberation and meditation on this fundamental idea seemedto

prepare biologists for other great generalizations which followed quite rapidly. Many new problems arose with this new knowledgeof plants

and animals. Comparative morphology was extensively investigated, and physiology now has become physiology of cellsas a result of this impetus. of cellular

An understanding of the permeability membranes, the transformation of energy by chemicalcells,

reaction within

the roles of electrolytes in living substance,

and the principles of heredity are some of the results of thisconception oflife

new

embraced

in the cell theorj^

PROTOPLASM AND THE CELL

53

General Characteristics of Protoplasm and the Material of the Cell

To beginvariation

with,

it

may

be said that this substance has a variable

degree of fluidity under different conditions.

The rangeItis

of this

may

be from semisolid to semiliquid.

viscid

and

less,

It is more or less granular, nearly colorand more or less translucent; however, it is never perfectly transparent. The trauslucency causes a mass of it to have a lustrous gray appearance. As a constituent of protoplasm there is always a considerable percentage of water, which conditions the degree of

gelatinous in consistency.

viscosity.WT.:

i->.

-v

"v^^._.^.J^"^-?'^%^,^

*

-

.

Fig.

19.

structure of Theis

living protoplasm as seen in tlie starfish.

(From Wilson,

Cell,

published by The Macmillan Company.)

Protoplasmis

in a colloidal state of the

emulsoid type.

A

colloid

a substance of gelatinous nature, permeable by crystalloid solutions,

and diffusing not at all or very slowly through animal or vegetable membranes. In the emulsoid, or colloidal emulsion, the substances are distributed through the more watery or dispersion medium. A colloid is identified by the presence of particles which are groups of molecules dispersed through a more fluid or watery phase. These particles, of course, are larger than molecules, but they are too small to be seen with the ordinary microscope. It is possible for water and substances in solution to enter protoplasm from without, and this is With loss of water from the dispersion medium the disreversible. persed particles of the colloid become congested by loss of generalfluidity.

This condition

is

increased water in the dispersion

known as the gel state. When there is medium and the particles move with

greater ease in the more fluid medium, the colloidal state tends to be-

54

TEXTBOOK OF ZOOLOGYsol.

be due to chemical changes in dispersion medium of the colloid. the dispersed particles or in the The ability of protoplasm, because of its colloidal nature, to change

come

This transfer of water

may

from

sol to gel state

and back

to sol repeatedly is the basis of

many

of the vital activities, such as utilization of food, disposal of waste,

and movement.

Fundamental Properties or

Activities of Protoplasm

In addition to the general characteristics, there

may

be mentioned

andall1.

described briefly a

number

of important activities

common

to

protoplasm.

These properties are:

Irritability, which refers to the capacity present in all protoplasm for responding to changes in environmental conditions, or

external stimuli.2.

Conductivity refers to the fact that the impulses produced bycell

stimuli or irritations at one point in protoplasm are conducted to

other parts of not only a single3.

but also to adjoining

cells.

Contractility,is

which

is

the power of contraction and relaxation

that4.

common

to the substance of every cell.

Metaholism, the process of continual exchange of food and fuel

materials being built into the protoplasm, while, at the same time, materials there are being oxidized to liberate kinetic energy, such asheat and movement, and produce waste by-products.

recognized as any increase in volume. When the rate of the building side of metabolism exceeds the oxidation rate in the protoplasm, there is storage of materials in the mass of the protoplasm5.

Growth

is

and hence growth. All protoplasm has this capacity. 6. Reproduction is the capacity for producing new individuals of the same kind. All living organisms are capable of this by some means. Simple cell division is the most primitive process of reproduction

among

animals.

Consciousness, which refers to the awareness of one'sence, is frequently given as a property of protoplasm.

own

exist-

It is certain

that some protoplasm possesses consciousness, but evidence of thisqualityis

rather intangible.all

Spontaneity

is

also considered a prop-

erty of protoplasm

source of

by some. To be certain that the activity and reaction comes from within is likewise rather difficultis

of definite proof, so this

simply mentioned here as another prop-

erty which

is

often listed.

PROTOPLASM AND THE CELLPhysical Nature of Protoplasm

55

Protoplasm is a semifluid material which is heavier than water and somewhat more refractive to light. Its physical constitutionis

similar to glue or gelatin, rather than to crystalloids, such as

sugar or ordinary table salt (sodium chloride).in the

Instead of being

form of a true solution

like salt in water, it consists of sus-

pensions of relatively large molecular aggregations varying roughlyparticles keep

between 0.0001 and 0.000001 of a millimeter in diameter. These up an expression of energy in that they move against each other as though they were dancing in a limited space. This activity can be seen only with a special optical arrangement known

as the ultramicroscope and the

phenomenon

is

known

as

Brownian

movementfrom a

(characteristic of colloidal substances).

fuses slowly or not at all

Protoplasm difthrough animal membranes. It changes

fluid or sol state to a

more

solid or gel state

and may return

in the other direction.

Ordinarily the viscosity of the continuousis

phase or supporting liquidtimes that of water.that of water.

only three or four times that of water,it is

while with the dispersed particles included

only eight or ten

The

viscosity of the nuclear fluid is only twice

Since glycerin has a viscosity about a thousand times

as great as water, it will be realized thatfluid in its active state.

essential to

most protoplasm is quite Changes in viscosity accompany and are the activity and functioning of it.is

Protoplasmare

not a single compound;

it is

a colloidal system ofColloidal systems

a number of chemical compounds existing together.

known

as

disperse systems of the

emulsoid type.

The more

watery or continuous part of the system is loiown as the dispersion medium, while the particles or molecular aggregations constitute the dispersed phase. An important consequence of the colloidal systems in protoplasm is the enormous surface of particles exposedto the continuous phase.If a sphere of material

has a radius ofcentimeters.

one centimeter

its

total surface will beis

12.6 square

Now,

if

the same volume of material

in colloidal particles of the

average size given above, the total surface of these will be approximately 7,000 square meters. This increase in surface is one of the

many important

significant effects of colloidal organization of substances, because reactions occur at these surfaces. By the presence

of salt ions in the continuous phase

and these becoming adsorbed

56

TEXTBOOK OF ZOOLOGYof the colloidal particles, they acquire an electric Protoplasm exhibits these several phenomena because of

upon the surfacescharge.its colloidal

nature.

Chemical Nature of Protoplasm

Up

to

exact chemical analysis.

the present time, protoplasm has eluded complete and Nevertheless the compounds of living

matter are composed of several elements, many of them the most ordinary and abundant in the world. The list of elements necessary to make human protoplasm could be gathered in almost any locality on the face of the earth. As a rule the elements found in protoplasm are oxygen, carbon, hydrogen, nitrogen, sulphur, phosphorus, calcium, sodium, chlorine, magnesium, iron, potassium, iodine, and frequently others like silicon, aluminum, copper, manganese, bromine,

andfirst

fluorine.

part of the

The most abundant of these are found named in the list. A few of them are usually given as constitut:

ing approximately the following percentages of protoplasm

oxycarbon 18 per cent, hydrogen 10 per cent, nitrogen 3 per cent, calcium 2 per cent, phosphorus 1 per cent, and all others makijig up the remaining 1 per cent. These elements are found combined to form compounds. The organic compounds include carhohydratcs, fats, proteins, and also enzymes. The inorganic com-

gen 65 per

cent,

povmds consist of several inorganic salts and water. The carbohydrates, which include starches and sugars, are compounds of carbon, oxygen, and hydrogen. The proportion of the hydrogen to oxygen in the molecule is the same as found in water, two to one. The principal carbohydrate found in protoplasm is the monosaccharid, or simple sugar, glucose, whose formula is CeHioOeThis is actually built into some parts of the cell, but its chief function is to furnish the most available source of energy by its readyoxidation.

When

a molecule of glucose

is

burned, the potential

energy

is

released as kinetic or mechanical energy, and there are

formed

(H2O) and six molecules of carbon converted to a starchlike substance, glycogen, for storage in the various animal tissues. This substance must be reconverted to glucose before it is available for productionsix molecules of v.'ater

dioxide

(CO2).

Glucose

is

of energy. Fats, like carbohydrates, are composed of carbon, hydrogen, and oxygen but in more complex molecular arrangement. There is much more carbon and hydrogen with less oxygen, which allows the fats

PROTOPLASM AND THE CELLto

57

combine with more oxygen in oxidation and therefore release Fat is extremely well adapted as a form of material for storage, since Aveight by weight it contains more potential energy than any of the organic group. Such common substances as lard, butter, tallow, whale blubber, and cottonseed oil are good examples. Fats serve a double function in protoplasm constitution of a part

more energy.

:

of the structure of the cell and, secondly, the storage of food.

Proteins constitute the bulk of the foundation or framework of the

and are the most abundant organic constituents. and nitrogen, with the frequent addition of traces of sulphur, phosphorus, magnesium, and iron. All of the proteins have large molecules, each being comcellular structure

They

are composed of carbon, hydrogen, oxygen,

posed of thousands of atomsteinsrent,

;

as

an

illustration, take

hemoglobin of

the red blood corpuscles with its formula C7i2Hii3oN2i40245FeS2. Pro-

have a slow rate of diffusion, high resistance to electric curand usually coagulate upon heating or upon addition of acids, alcohol, or salts to form a clot. Egg albumen, gelatin, and lean meat are common examples of proteins. They are split into numerous amino acids which serve as the building stones of the stable

portions of protoplasm.

Enzymes are substances whose exact chemical nature is not yet known, but whose importance to protoplasm is probably unequaled. Chemically and physically they seem to be more like proteins than anything else. These substances are found not only in the cells, but they are also secreted by cells into the digestive tract and into the blood stream, where they act as organic catalysts. The generalfunction of the catalyzer or catalytic agent is that of facilitating and speeding up certain chemical exchanges without the agent itself

The well-known example of catalysis amount of platinum in increasing the rate at which hydrogen and oxygen combine to form water. A particular enzyme is usually specific for one kind of reaction, but not for the species of animal in which it will function. Enzymes taken from one species will usually facilitate the same kind of specific reaction in other species. The digestive enzymes may be thought of as an example. Of these, pepsin will bring about the same general reaction, whether it is in the stomach of a frog or of a man, under favorable conditions. Since many enzymes influence only one specific type of chemical reaction and since there are numerousentering into the reaction.the effect of a smallis

58

TEXTBOOK OF ZOOLOGYit

types of reactions going forward in active protoplasm,that there must be

is

seen

numerous enzymes present

in the cells of every

organism.

Water

constitutes 60 to 90 per cent of protoplasm

and maintainsefficient

many

substances in solution.

Water

is

not only a very

solvent; but it is important to protoplasm because of its comparatively high surface tension, because its presence gives the protoplasm a consistency compatible with the range of variation necesThis sary for metabolism, and because of its high specific heat. latter point is important in maintaining protection against sudden and extreme temperature changes in the living organism. Young cells contain more water than old ones, young organisms likewise contain more than old ones. The relative amounts of water in relation to other materials of the protoplasm vary in different cells andin different species.

present in considerable numbers but in They are electrolytes, and therefore split up in aqueous solution into ions, which are able to combine with all the other substances in protoplasm. The chlorides, phosphates, iodides, carbonates, and sulphates of sodium, potassium, calcium, magnesium, and iron are important salts of living cells. The relative proportion of these salts is kept at a fairly constant level, and slightsalts are

The inorganic

relatively small amount.

changes in this balance have regulatory effects on metabolism.

From

the chemical standpoint, living protoplasm

is

considered

the most complex of all systems of compounds.

Even

the proteins,

as a part of protoplasm, arestances.its

more complex than any other sub-

is quite unstable in that it changes composition in response to every change in the environment, and when active it is not the same for any two consecutive moments. The exceeding variability of protoplasm chemically, makes possible

In a sense, protoplasm

all

of the necessary adjustments of living matter to

its

environment.it is

On account

of the extreme complexity of protoplasmall

not sur-

prising that the chemistry ofpletely understood.

of its activities is not yet

com-

Structure of a Typical Animal Cell

The quantity of protoplasm comprising a single cell varies within wide limits therefore cells vary greatly in size. The majority of cells,;

but not

all

of them, require considerable magnification to be seen.

Cer-

PROTOPLASM AND THE CELLtain of the single-celled blood parasites are about as small ascells

59

known.

any They are barely seen with our highest magnifications.

At

the other extreme of size

we may

refer to another parasitic

single-celled animal, Porospora gigantea,

which

lives in the intestine

of the lobster,

and may reach from one-half to two-thirds of an

inch in length.

Egg

cells,

including the yolk,

Some

of the nerve cells,

though of less mass,long

may exceed this may be several

size.

feet

in length.

Muscle

cells are relatively

also.

Plasma /Atmirane

Eciop/asm

ChondriosomaEn