ELEMENTARY ZOOLOGY

BY

VERNON L. KELLOGG, M.S.

Professor of Entomology, Leland Stanford Junior University

SECOND EDITION, REVISED

NEW YORK

HENRY HOLT AND COMPANY

1902

Copyright, 1901,

BY

HENRY HOLT & CO.

ROBERT DRUMMOND, PRINTER, NEW YORK

[PREFACE]

It seems to the author that three kinds of work should be included in the elementary study of zoology. These three kinds are: (a) observations in the field covering the habits and behavior of animals and their relations to their physical surroundings, to plants, and to each other; (b) work in the laboratory, consisting of the study of animal structure by dissection and the observation of live specimens in cages and aquaria; and (c) work in the recitation- or lecture-room, where the significance and general application of the observed facts are considered and some of the elementary facts relating to the classification and distribution of animals are learned.

These three kinds of work are represented in the course of study outlined in this book. The sequence and extent of the study in laboratory and recitation-room are definitely set forth, but the references to field-work consist chiefly of suggestions to teacher and student regarding the character of the work and the opportunities for it. Not because the author would give to the field-work the least important place,—he would not,—but because of the utter impracticability of attempting to direct the field-work of students scattered widely over the United States. The differences in season and natural conditions in various parts of the country with the corresponding differences in the "seasons" and course of the life-history of the animals of the various regions make it impossible to include in a book intended for general use specific directions for field-work. Further, the amount of time for field-work at the disposal of teacher and class and the opportunities afforded by the topographic character of the region in which the schools are located vary much. The initiation and direction of this must therefore always depend on the teacher. On the other hand, the work of the other two phases of study can to a large extent be made pretty uniform throughout the country. For dissection, specimens properly killed and preserved are about as good as fresh material, and by modifying the suggested sequence of work a little to suit special conditions or conveniences, the examination of live specimens in the laboratory can in most cases be accomplished.

The author believes that elementary zoological study should not be limited to the examination of the structure of several types. The student should learn by observation something of the functions of animals and something of their life-history and habits, and should be given a glimpse of the significance of his particular observations and of their general relation to animal life as a whole. The drill of the laboratory is perhaps the most valuable part of the work, but as a matter of fact the high school is trying to teach elementary zoology, an elementary knowledge of animals and their life, and dissection alone cannot give the pupil this knowledge. On the other hand, without a personal acquaintance with animals, based on careful actual observations of their life-history and habits and on the study of the structural characters of the animal body by personally made dissections, the pupil can never really appreciate and understand the life of animals. Reading and recitation alone can never give the student any real knowledge of it.

The book is divided into three parts, of which Part I should be[1] first undertaken. This is an introduction to an elementary knowledge of animal structure, function, and development. It consists of practical exercises in the laboratory, each followed by a recitation in which the significance of the facts already observed is pointed out. The general principles of zoology are thus defined on a basis of observed facts.

Part II is devoted to a consideration of the principal branches of the animal kingdom; it deals with[2] systematic zoology. In each branch one or more examples are chosen to serve as types. The most important structural features of these examples are studied, by dissection, in the laboratory. The directions for these dissections consist of technical instructions for dissecting, the calling attention to and naming of principal parts, together with questions and demands intended to call for independent work on the part of the student. The directions follow the actual course of the dissection instead of being arranged according to systems of organs, and are intended for the orientation of the student and not to be in themselves expositions of the anatomy of the types. The condensation of these directions is made more feasible by the presence of anatomical plates (drawn directly from dissections). Following the account of the dissection of the type are brief notes on its life-history and habits. Then follows a general account of the branch to which the example dissected belongs and brief accounts of some of the more interesting members of the branch. In these accounts technical directions are given for brief comparative examinations and for the study of the life-history and habits of some of the more accessible of these forms.

It will not be possible, of course, to undertake with any thoroughness the consideration of all of the branches of animals in a single year. But all are treated in the book, so that the choice of those to be studied may rest with the teacher. This choice will of necessity depend largely on the opportunities afforded by the situation of the school, as, for example, whether on the seashore or in the interior near a lake or river, or on the dry plains, and on the relation of the school-terms to the seasons of the year. The branches are arranged in the book so that the simplest animals are first considered, the slightly complex ones next, and lastly the most highly organized forms. But if in order to obtain examples for study it is necessary to take up branches irregularly, that need not prove confusing. The author would suggest that whatever other branches are studied, the insects and birds, which are readily available in all parts of the country, be certainly selected, and with this selection in view has given them special attention. Indeed some teachers may find these two branches to offer quite sufficient work in classificatory and ecological lines.

Part III is devoted to a necessarily brief consideration of certain of the more conspicuous and interesting features of animal ecology. It has in it the suggestion for much interesting field-work. The work of this part should be taken up in connection with that of Part II, as, for example, the consideration of social and communal life in connection with the insects, parasitism in connection with the worms, and also with the insects, distribution in connection with the birds, perhaps, and so on.

In appendices there are added some suggestions for the outfitting of the laboratory, and a list of the equipment each student should have. Here, also, is appended a list of a few good authoritative reference books which should be accessible to students and to which specific references are made in the course of this book. Some practical directions for the collecting and preserving of specimens are also given. (Suggestions for the obtaining of material for the various laboratory exercises outlined in the book are to be found in "technical notes" included in the directions for each exercise.) The author believes that the building up of a single school-collection in which all the pupils have a common interest and to which all contribute is to be encouraged rather than the making of separate collections by the pupils. Waste of life is checked by this, and in time, with the contributions of succeeding classes, a really good and effective collection may be built up. The "collecting interest" can be taken advantage of just as well in connection with a school-collection as with individual collections.

The plates illustrating the dissections have all been drawn originally for the book from actual dissections. Most of the other figures are original, either drawn or photographed directly from nature, or from preserved specimens. Credit is given in each case for figures not original. The drawings for all of the figures of dissections and for all original figures not otherwise accredited were made by Miss Mary H. Wellman, to whom the author expresses his obligations. The thanks of the author are due to Mr. George Otis Mitchell, San Francisco, who kindly made the photo-micrographs of insect structure from the author's slides; to Professor Mark V. Slingerland, Cornell University, for electros of his photographs of insects; to Dr. L. O. Howard, U. S. Entomologist, for electros of figs. [45], [52], [56], [68], [81], [82], [83], [84], [87], [90], and [92]; to Professor L. L. Dyche, University of Kansas, for photographs of his mounted groups of mammals; to Mrs. Elizabeth Grinnell, Pasadena, Calif., for photographs of birds; to Mr. J. O. Snyder, Stanford University, for photographs of snakes; to Mr. Frank Chapman, editor of "Bird-lore," for electros of photographs of birds; to Mr. G. O. Shields, editor of "Recreation," for an electro of the photograph of a bird; to the American Society of Civil Engineers for electros of photographs of boring marine worms; to Cassell & Co., for electros of three photographs from nature; to Geo. A. Clark, secretary Fur Seal Commission for photographs of seals; and to the Whitaker and Ray Co., San Francisco, for electros of figs. [46], [59], [60], [61], [64], [65], [93], [94], [97], [98], [99], [100], [102], [119], and [166] to [172], published originally in Jenkins & Kellogg's "Lessons in Nature Study." The origin of each of these pictures is specifically indicated in connection with its use in the book.

The author's sincere thanks are also due to Mrs. David Starr Jordan and to Mr. J. C. Brown, graduate student in zoology in Stanford University, for their assistance in the correction of the MS., and in the preparation of the laboratory exercises respectively. The chapters of Part II relating to the vertebrates were read in MS. by President David Starr Jordan, whose aid and courtesy are gratefully acknowledged. Similar acknowledgments are due Professors Harold Heath and R. E. Snodgrass for reading the proofs of the directions for the laboratory exercises.

Vernon Lyman Kellogg.

Stanford University, May, 1901.

[CONTENTS]

PART I

STRUCTURE, FUNCTIONS, AND DEVELOPMENT OF ANIMALS

I.—THE STUDY OF ANIMALS AND THEIR LIFE.

Our familiar knowledge of animals and their life, [1.]—Zoology and its divisions, [2.]—A first course in Zoology, [3.]

II.—THE GARDEN TOAD (Bufo Lentiginosus).

[Laboratory exercise], [5.]—External structure, [5.]—Internal structure, [7.]

III.—THE STRUCTURE AND FUNCTIONS OF THE ANIMAL BODY.

Organs and functions, [14.]—The animal body a machine, [14.]—The essential functions or life-processes, [15.]

IV.—THE CRAYFISH (Cambarus sp.).

[Laboratory exercise], [17.]—External structure, [17.]—Internal structure, [21.]

V.—THE MODIFICATION OF ORGANS AND FUNCTIONS.

Difference between crayfish and toad, [26.]—Resemblances between crayfish and toad, [27.]—Modification of functions and structure to fit the animal to the special conditions of its life, [29.]—Vertebrate and invertebrate, [30.]

VI.—AMŒBA AND PARAMŒCIUM.

[Laboratory exercise], [31.]—Amœba, [31.]—The slipper-animalcule (Paramœcium sp.), [34.]

VII.—THE SINGLE-CELLED ANIMAL BODY; PROTOPLASM AND THE CELL.

The single-celled animal body, [36.]—The cell, [37.]—Protoplasm, [39.]

VIII.-CELLULAR STRUCTURE OF THE TOAD (OR FROG).

[Laboratory exercise], [40.]—The blood, [40.]—The skin, [40.]—The liver, [41.]—The muscles, [41.]

IX.—THE MANY-CELLED ANIMAL BODY; DIFFERENTIATION OF THE CELL.

The many-celled animal body, [43.]—Differentiation of the cell, [43.]

X.—HYDRA.

[Laboratory exercise], [46.]

XI.—THE SIMPLEST MANY-CELLED ANIMALS.

Cell-differentiation and body-organization in Hydra, [52.]—Degrees in cell-differentiation and body-organization, [54.]

XII.—DEVELOPMENT OF THE TOAD.

[Field and laboratory exercise], [55.]

XIII.—MULTIPLICATION AND DEVELOPMENT.

Multiplication, [57.]—Spontaneous generation, [58.]—Simplest multiplication and development, [59.]—Birth and hatching, [61.]—Life-history, [62.]

PART II

SYSTEMATIC ZOOLOGY

XIV.—THE CLASSIFICATION OF ANIMALS.

[Laboratory exercise and recitation], [65.]—Basis and significance of classification, [65.]—Importance of development in determining classification, [67.]—Scientific names, [68.]—An example of classification, [68.]—Species, [69.]—Genus, [70.]—Family, [72.]—Order, [72.]—Class and branch, [73.]

XV.—BRANCH PROTOZOA: THE ONE-CELLED ANIMALS.

Example: The bell animalcule (Vorticella sp.) [Laboratory exercise], [75.]

Other Protozoa.

Form of body, [78.]—Marine Protozoa, [80.]

XVI.—BRANCH PORIFERA: THE SPONGES.

Example: The Fresh-water sponge (Spongilla sp.) [Laboratory exercise], [84.]

Example: A calcareous ocean-sponge (Grantia sp.) [Laboratory exercise], [85.]

Example: A commercial sponge [Laboratory exercise], [86.]

Other Sponges.

Form and size, [87.]—Skeleton, [88.]—Structure of body, [88.]—Feeding habits, [88.]—Development and life-history, [89.]—The sponges of commerce, [90.]—Classification, [91.]

XVII.—BRANCH CŒLENTERATA: THE POLYPS, SEA-ANEMONES, CORALS, AND JELLYFISHES.

Polyps, sea-anemones, corals, and jellyfishes.

General form and organization of body, [93.]—Structure, [94.]—Skeleton, [95.]—Development and life-history, [95.]—Classification, [96.]—The polyps, colonial jellyfishes, etc. (Hydrozoa), [97.]—The large jellyfishes, etc. (Scyphozoa), [101.]—The sea-anemones and corals (Actinozoa), [102.]—The Ctenophora, [107.]

XVIII.—BRANCH ECHINODERMATA: THE STARFISHES, SEA-URCHINS, SEA CUCUMBERS, ETC.

Example: Starfish (Asterias sp.) [Laboratory exercise].—External structure, [108.]—Internal structure, [110.]—Life-history and habits, [113.]

Example: Sea-urchin (Strongylocentrus sp.) [Laboratory exercise].—External structure, [113.]

Other Starfishes, Sea-urchins, Sea cucumbers, etc.

Shape and organization of body, [116.]—Structure and organs, [117.]—Development and life-history, [119.]—Classification, [120.]—Starfishes (Asteroidea), [121.]—Brittle stars (Ophiuroidea), [122.]—Sea-urchins (Echinoidea), [123.]—Sea-cucumbers (Holothuroidea), [124.]—Feather-stars (Crinoidea), [125.]

XIX.—BRANCH VERMES: THE WORMS.

Example: The Earthworm (Lumbricus sp.) [Laboratory exercise].—External structure, [127.]—Internal structure, [129.]—Life-history and habits, [133.]

Other Worms.

Classification, [135.]—Earthworms and leeches (Oligochætæ), [136.]—Flat worms (Platyhelminthes). [137.]—Round worms (Nemathelminthes), [140.]—Wheel-animalcules (Rotifera), [142.]

XX.—BRANCH ARTHROPODA: THE CRUSTACEANS, CENTIPEDS, INSECTS, AND SPIDERS.

Class Crustacea: Crayfishes, crabs, lobsters, etc.

Example: The crayfish (Cambarus sp.). Structure, [146.]—Life-history and habits, [146.]

Other Crustaceans.

Body form and structure, [147.]—Water-fleas (Cyclops), [148.]—Wood-lice (Isopoda), [150.]—Lobsters, shrimps, and crabs (Decapoda), [151.]—Barnacles, [155.]

XXI.—BRANCH ARTHROPODA (CONTINUED).

Class Insecta: The Insects.

Example: The red-legged locust (Melanoplus femur-rubrum). [Laboratory exercise]. External structure, [157.]—Life-history and habits, [161.]

Example: The water-scavenger beetle (Hydrophilus sp.) [Laboratory exercise]. External structure, [163.]—Internal structure, [166.]—Life-history and habits, [169.]

Example: The monarch butterfly (Anosia plexippus) [Laboratory exercise]. External structure, [171.]—Life-history and habits, [175.]

Example: Larva of monarch butterfly [Laboratory exercise]. Structure, [177.]

Other Insects.

Body form and structure, [181.]—Development and life-history, [188.]—Classification, [191.]—Locusts, cockroaches, crickets, etc. (Orthoptera), [192.]—The dragon-flies and May-flies (Odonata and Ephemerida), [194.]—The sucking-bugs (Hemiptera), [197.]—The flies (Diptera), [201.]—The butterflies and moths (Lepidoptera), [205.]—The beetles (Coleoptera), [206.]—The ichneumon flies, ants, wasps, and bees (Hymenoptera), [212.]

Class Myriapoda: The centipeds and millipeds.

Class Arachnida: The scorpions, spiders, mites, and tics.

XXII.—BRANCH MOLLUSCA: THE MOLLUSCS.

Example: The fresh-water mussel. (Unio sp.) [Laboratory exercise]. Structure, [239.]—Life-history and habits, [243.]

Other Molluscs.

Body form and structure, [245.]—Development, [246.]—Classification, [246.]—Clams, scallops, and oysters (Pelecypoda), [246.]—Snails, slugs, nudibranchs, and "sea-shells" (Gastropoda), [252.]—Squids, cuttlefishes, and octopi (Cephalopoda), [255.]

XXIII.—BRANCH CHORDATA: THE ASCIDIANS, VERTEBRATES, ETC.

Structure of the vertebrates, [259.]—Classification of the Chordata, [260.]—The ascidians, [261.]

XXIV.—BRANCH CHORDATA (CONTINUED).

Class Pisces: The Fishes.

Example: The golden sunfish (Eupomotis gibbosus) [Laboratory exercise]. External structure, [263.]—Internal structure, [265.]—Life-history and habits, [270.]

Other Fishes.

Body form and structure, [271.]—Development and life-history, [276.]—Classification, [277.]—The lancelets (Leptocardii), [277.]—The lampreys and hag-fishes (Cyclostomata), [278.]—The true fishes (Pisces), [279.]—The sharks, skates, etc. (Elasmobranchii), [279.]—The bony fishes (Teleostomi), [281.]—Habits and adaptations, [285.]—Food-fishes and fish-hatcheries, [288.]

XXV.—BRANCH CHORDATA (CONTINUED).

Class Batrachia: The Batrachians.

Body form and organization, [292.]—Structure, [293.]—Life-history and habits, [295.]—Classification, [297.]—Mud-puppies, salamanders, etc. (Urodela), [297.]—Frogs and toads (Anura), [299.]—Cœcilians (Gymnophiona), [302.]

XXVI.—BRANCH CHORDATA (CONTINUED).

Class Reptilia: The snakes, lizards, turtles, crocodiles, etc.

Example: The garter snake (Thamnophis sp.) [Laboratory exercise]. Structure, [303.]—Life-history and habits, [308.]

Other Reptiles.

Body form and organization, [310.]—Structure, [311.]—Life-history and habits, [312.]—Classification, [313.]—Tortoises and turtles (Chelonia), [314.]—Snakes and lizards (Squamata), [317.]—Crocodiles and alligators (Crocodilia), [325.]

XXVII.—BRANCH CHORDATA (CONTINUED).

Class Aves: The Birds.

Example: The English Sparrow (Passer domesticus) [Laboratory exercise]. External structure, [327.]—Internal structure [Laboratory exercise], [329.]—Life history and habits, [335.]

Other Birds.

Body form and structure, [336.]—Development and life-history, [339.]—Classification, [340.]—The ostriches, cassowaries, etc. (Ratitæ), [341.]—The loons, grebes, auks, etc. (Pygopodes), [343.]—The gulls, terns, petrels, and albatrosses (Longipennes), [345.]—The cormorants, pelicans, etc. (Steganopodes), [346.]—The ducks, geese, and swans (Anseres), [347.]—The ibises, herons, and bitterns (Herodiones), [347.]—The cranes, rails, and coots (Paludicolæ), [348.]—The snipes, sand-pipers, plovers, etc. (Limicolæ), [349.]—The grouse, quail, pheasants, turkeys, etc. (Gallinæ), [358.]—The doves and pigeons (Columbæ), [351.]—The eagles, hawks, owls, and vultures (Raptores), [351.]—The parrots (Psittaci), [353.]—The cookoos and kingfishers (Coccyges), [354.]—The woodpeckers (Pici), [354.]—The whippoorwills, chimney-swifts, and humming-birds (Macrochires), [356.]—The perchers (Passeres), [357.]—Determining and studying the birds of a locality, [359.]—Bills and feet, [362.]—Flight and songs, [364.]—Nestling and care of the young, [366.]—Local distribution and migration, [367.]—Feeding habits, economics, and protection of birds, [370.]

XXVIII.—BRANCH CHORDATA (CONTINUED).

Class Mammalia: The Mammals.

Example: The Mouse (Mus musculus) [Laboratory exercise]. Structure, [373.]—Life-history and habits, [379.]

Other Mammals.

Body form and structure, [381.]—Development and life-history, [387.]—Habits, instincts, and reason, [387.]—Classification, [388.]—The opossums (Marsupialia), [389.]—The rodents or gnawers (Glires), [390.]—The shrews and moles (Insectivora), [391.]—The bats (Chiroptera), [391.]—The dolphins, porpoises, and whales (Cete), [393.]—The hoofed mammals (Ungulata), [394.]—The carnivores (Feræ), [396.]—The man-like mammals (Primates), [398.]

PART III

ANIMAL ECOLOGY

XXIX.—THE STRUGGLE FOR EXISTENCE, ADAPTATION, AND SPECIES-FORMING.

The multiplication and crowding of animals, [404.]—The struggle for existence, [406.]—Variation and natural selection, [406.]—Adaptation and adjustment to surroundings, [407.]—Species forming, [408.]—Artificial selection, [409.]

XXX.—SOCIAL AND COMMUNAL LIFE, COMMENSALISM, AND PARASITISM.

Social life and gregariousness, [410.]—Communal life, [411.]—Commensalism, [413.]—Parasitism, [415.]

XXXI.—COLOR AND PROTECTIVE RESEMBLANCES.

Use of color, [424.]—General, variable, and special protective resemblance, [426.]—Warning colors, terrifying appearances, and mimicry, [430.]—Alluring coloration, [433.]

XXXII.—THE DISTRIBUTION OF ANIMALS.

Geographical distribution, [435.]—Laws of distribution, [437.]—Modes of migration and distribution, [437.]—Barriers to distribution, [438.]—Faunæ and zoogeographic areas, [440.]—Habitat and species, [441.]—Species-extinguishing and species-forming, [442.]

APPENDICES

EQUIPMENT AND METHODS

APPENDIX I.—EQUIPMENT AND NOTES OF PUPILS.

Equipment of pupils, [447.]—Laboratory drawings and notes, [447.]—Field observations and notes, [448.]

APPENDIX II.—LABORATORY EQUIPMENT AND METHODS.

Equipment of laboratory, [450.]—Collecting and preparing material for use in the laboratory, [451.]—Obtaining marine animals, microscopic preparations, etc., [453.]—Reference-books, [454.]

APPENDIX III.—REARING ANIMALS AND MAKING COLLECTIONS.

Live cages and aquaria, [457.]—Making collections, [461.]—Collecting and preserving insects, [463.]—Collecting and preserving birds, [466.]—Collecting and preserving mammals, [470.]—Collecting and preserving other animals, [472.]

[PART I]

STRUCTURE, FUNCTIONS, AND DEVELOPMENT OF ANIMALS

CHAPTER I

THE STUDY OF ANIMALS AND THEIR LIFE

Our familiar knowledge of animals and their life.—We are familiarly acquainted with dogs and cats; less familiarly probably with toads and crayfishes, and we have little more than a bare knowledge of the existence of such animals as seals and starfishes and reindeer. But what real knowledge of dogs and toads does our familiar acquaintanceship with them give? Certain habits of the dog are known to us: it eats, and eats certain kinds of food; it runs about; it responds to our calls or even to the mere sight of us; it evidently feels pain when struck, and shows fear when threatened. Another class of attributes of the dog includes those things that we know of its bodily make-up: its possession of a head with eyes and ears, nose and mouth; its four legs with toes and claws; its covering of hair. We know, too, that it was born alive as a very small helpless puppy which lived for a while on food furnished by the mother, and that it has grown and developed from this young state to a fully grown, fully developed dog. We know also that our dog is a certain kind of dog, a spaniel, perhaps, while our neighbor's dog is of another kind, a greyhound, it may be. We know accordingly that there are different kinds of tame dogs, and we may know that wolves are so much like dogs that they might indeed be called wild dogs, or dogs called a kind of tame wolf. But how little we really know about the dog's body and its life is apparent at a moment's thought. We see only the outside of the dog, but what an intricate complex of parts really composes this animal! We see it eat and breathe and run; of what is done with the food and air inside its body, and of the series of muscle contractions and mechanical processes which cause its running, we have but the slightest conception. We see that the pup gets larger, that is, grows; that it changes gradually in appearance, that is, develops; but of the real processes and changes that take place in growth and development how little we know! We know that there are other kinds of dogs; that wolves and foxes are relatives of the dog; and we have heard that cats and tigers are relatives also, although more distant ones. We know, too, that all the backboned animals, some of them very unlike dogs, are believed to be related to each other, but of the thousands of these animals and of their relationships our knowledge is scanty. Finally, of the relations of the dog, and of other animals, to the outside world, and of the wonderful manner in which the dog's make-up and behavior fit it to live in its place in the world under the conditions that surround it, we have probably least knowledge of all.

Zoology and its divisions.—What things we do know about the dog, however, and about its relatives, and what things others know, can be classified into several groups, namely, things or facts about what the dog does, or its behavior, things about the make-up of its body, things about its growth and development, things about the kind of dog it is and the kinds of relatives it has, and things about its relations to the outer world, and its special fitness for life.

All that is known of these different kinds of facts about the dog constitutes our knowledge of the dog and its life. All that is known by scientific men and others of these different kinds of facts about all the 500,000 or more kinds of living animals, constitutes our knowledge of animals and is the science zoology.[3] Names have been given to these different groups of facts about animals. The facts about the bodily make-up or structure of animals constitute that part of zoology called animal anatomy or morphology; the facts about the things animals do, or the functions of animals, compose animal physiology; the facts about the development of animals from young to adult condition are the facts of animal development; the knowledge of the different kinds of animals and their relationships to each other is called systematic zoology or animal classification; and finally the knowledge of the relations of animals to their external surroundings, including the inorganic world, plants and other animals, is called animal ecology.

Any study of animals and their life, that is, of zoology, may include all or any of these parts of zoology. Most zoologists do, indeed, devote their principal attention to some one group of facts about animals and are accordingly spoken of as anatomists, or physiologists, systematists, and so on. But such a specialization of study should be made only after the zoologist has acquired a knowledge of the principal or fundamental facts in all the other branches of zoology.

A first course in zoology.—The first "course," then, in the study of animals should include the fundamental facts in all these branches or parts of zoology. That is what the course outlined in this book tries to cover. But no text-book of zoology can really give the student the knowledge he seeks. He must find out most of it for himself; a text-book, based on the experiences of others, is chiefly valuable for telling him how to work most effectively to get this knowledge for himself. And the best students always find out things which are not in books. Especially can the beginning student find out things not known before, "new to science," as we say, about the behavior and habits of animals, and their relations to their surroundings. The life-history of comparatively few kinds of animals is exactly known; the instincts and habits of comparatively few have been studied in any detail. The kinds of food demanded, the feeding habits, nest-building, care of the young, cunning concealment of nest and self, time of egg-laying or of producing young, duration of the immature stages and the habits and behavior of the young animals—a host, indeed, of observations on the actual life of animals, remain to be made by the "field naturalist." Any beginning student can be a "field naturalist" and can find out new things about animals, that is, can add to the science of zoology.

Fig. 1.—Dissection of the Garden Toad (Bufo lentiginosus).

[CHAPTER II]

THE GARDEN TOAD (Bufo lentiginosus)

LABORATORY EXERCISE

Technical Note.—Although this description is written for the toad it will fit for the dissection of the frog. It will be found, after casting aside a few ungrounded prejudices, that the toad is the better for class dissection. Toads are best collected about dusk, when they can be picked up in almost any garden in town or in the country. During the spring many can be found in the ponds where they are breeding. To kill the toad place it in an air-tight vessel with a piece of cotton or cloth saturated in chloroform or ether. When the toad is dead, wash off the specimen and put in a dissecting pan for study. Several specimens should be placed in a nitric acid solution for a day or so (for directions for preparing, see p. [12]) to be used later for the study of the nervous system. Also several specimens should be injected for the better study of the circulatory system. With an injecting mass made as directed on p. 451 introduce through a small canula into the ventricle of the heart. This will inject the arterial system, and with increased pressure the injecting mass may be forced through the valves of the heart, thus passing into the auricles and throughout the venous system. After injecting use the specimen fresh or after it has been preserved in 4% formalin.

External structure.—Note that the body of the toad is divided into several principal regions or parts, as is the human body, namely, a head, upper limbs, trunk, and lower limbs. As you look at the toad note the similarity of the parts on one side to those of the other, as right leg corresponding to left leg, right eye to left eye, etc. This arrangement of the body in similar halves among animals is known as bilateral symmetry. As a rule animals which show bilateral symmetry move in a definite direction. The part that moves forward is the anterior end, while the opposite extremity is the posterior end. In most animals we note two other views or aspects; that which is called the "back" and with most animals is, under ordinary conditions, uppermost is the dorsum or dorsal aspect, while that which lies below is the venter or ventral aspect. When referring to a view from one side we speak of it as a right or left lateral aspect. These terms hold good for most of the animals that we shall study.

Note at the anterior end of the toad a wide transverse slit, the mouth. What other openings are on the anterior end? Note the two large eyes, the organs of sight. Just back of each eye note an elliptical, smooth membrane. This is the tympanum of the outer ear, and through this membrane the vibrations produced by sound-waves are transferred to the inner ear, which receives sensations and transmits them to the brain. Open the mouth by drawing down the lower jaw. Note just within the angle of the lower jaw the tongue. How is it attached to the wall of the mouth? On the tongue are a great many fine papillæ in which is located the sense of taste. It has now been seen that most of the special senses of the toad have their seat in the head. Pass a straw or bristle into one of the nostrils. Where does it come out? These internal openings to the nose are the inner nares. Note in the roof of the mouth just posterior to each of the eyeballs an opening. These are the internal openings to the wide Eustachian tubes, which lead to the mouth from the chamber of the ear behind the tympanum.

Note far back in the mouth an opening through which food passes. This is the œsophagus or gullet. Note just below this gullet an elevation in which is a perpendicular slit, the glottis. This is the upper end of the laryngo-tracheal chamber, and the flaps within on either side of the slit are the vocal cords.

Note at the posterior end of the body in the median line an opening. This is the anal opening or anus. Note the general make-up of the toad. How do its arms compare with our own? How do its fore feet (hands) differ from its hind feet? Note that the body is covered by a tough enveloping membrane, the skin. In the skin are many glands which by their excretion keep it soft and moist.

Internal structure.—Technical Note.—With a fine pair of scissors make a longitudinal median cut through the skin of the venter from the anal opening to the angle of the lower jaw. Spread the cut edges apart and pin back in the dissecting-pan.

Note the complex system of muscles which govern the movements of the tongue. Observe a number of pairs of muscles overlying the bones which support the arms. These are attached to the pectoral or shoulder-girdle. Note the large sheet of muscles covering the ventral aspect of the toad. These are the abdominal muscles, which consist of two sets, an outer and an inner layer. Note that posteriorly the abdominal muscles are attached to a bone. This is the pubic bone of the pelvic girdle which supports the hind legs.

Technical Note.—With the scissors cut through the muscles of the body wall at the pubic bone and pass the points forward to the shoulder-girdle. Separate the bones of the shoulder-girdle and pin out the flaps of skin and muscle to right and left in the dissecting-pan (see fig. [1]). Cover the dissection with clear water or weak alcohol.

Note two large conspicuous soft brown lobes of tissue. These form the liver, an organ which produces a secretion that assists in the process of digestion. Note just anterior to the liver and extending between its two lobes a pear-shaped organ, the heart, which may yet be pulsating. Are these pulsations regular? How many occur in a minute? The lower end or apex of the heart, ventricle, undergoes a contraction, forcing blood out into the blood-vessels. This is followed by a relaxation of the apex and a contraction of the basal portion, the auricle. The heart is surrounded by a delicate semi-transparent sac, the pericardium. The pericardium is filled with a watery fluid, body-lymph, which bathes the heart. Note between the lobes of the liver a small bladder-shaped transparent organ of a pinkish color. This is the gall-cyst, or gall-bladder, a reservoir for the bile, the secretion from the liver. Separate the lobes of the liver and note, beneath, the long convoluted tube which fills most of the body-cavity. This is part of the alimentary canal. Is the alimentary canal of uniform character? The most anterior portion of the canal, the gullet or œsophagus, leads to a large U-shaped enlargement, the stomach. From the lower end of the stomach there extends a long, slender, very much convoluted tube, the small intestine, which is followed by a much larger one, the large intestine. This large intestine after one or two turns passes directly back into the rectum, which opens at last to the exterior through the anus. Note just ventral to the rectum a large thin-walled membranous sac. This is the urinary bladder which acts as a reservoir for the secretion from the kidneys. Notice a many-branched yellow structure with a glistening appearance, the fat-body (corpus adiposum). Now push liver and intestine to one side and note the pinkish sac-like bodies (perhaps filled with air), the lungs. The lungs are paired bodies which open into the laryngo-tracheal chamber. The toad takes air into its mouth through its nostrils, and then forces it, by a kind of swallowing action, through the laryngo-tracheal chamber into the lungs.

Now lift the stomach and note in the loop between its lower end and the small intestine a thin transparent tissue. This is a part of the mesentery, which will be found to suspend the whole alimentary canal and its attached organs to the dorsal wall of the body. Note in the loop of the stomach in the mesentery an irregular pinkish glandular structure which leads by a small duct into the intestine. This gland is the pancreas, and the duct is the pancreatic duct. From it comes a secretion which aids in the digestion of food. Near the upper end of the pancreas note a round nodular structure, generally dark red. This is the spleen, a ductless gland, the use of which is not altogether known.

Make a drawing which will show as many of the organs noted as possible.

Technical Note.—Pass two pieces of thread under the rectum near the pubic bone. Tie these threads tightly a short distance apart and then cut the rectum in two between the threads. Now carefully lift up the alimentary canal with attached organs (liver, etc.), and cut it off near the region of the heart.

How is the heart situated with regard to the lungs? The heart consists of a lower chamber with thick muscular walls, the tip, called the ventricle, and two upper thin-walled chambers, the right and left auricles. Can you make out these three chambers? The purified blood from the lungs flows into the left auricle, while the venous blood from all over the body laden with its carbon dioxide enters the right auricle. From these two chambers the blood enters the ventricle. Here the pure and impure blood are mixed. From the ventricle the blood enters a large muscular tube on the ventral side of the heart. This is the conus arteriosus, which gives off three branches on each side; the anterior ones, the carotid arteries, supply the head, the next ones, the systemic arteries, or aortæ, carry blood to the rest of the body, while the posterior vessels, the pulmonary arteries, go directly to the lungs and there break up into fine vessels (capillaries) where the carbon dioxide is given off and oxygen is taken from the air. From the lungs the blood returns through the pulmonary vein to the left auricle. Meanwhile the blood which has passed through the systemic arteries and body capillaries is collected again into other vessels going back to the heart; these are the veins, which empty into a large thin-walled reservoir, the sinus venosus, which in turn connects with the right auricle of the heart. Three large veins enter the sinus venosus, namely, two pre-caval veins at the anterior end, and a single post-caval vein at the posterior end. Trace out the larger arteries and veins from the heart to their division into or origin from the smaller vessels.

Technical Note.—Carefully remove the heart together with the lungs. The lungs may be inflated by blowing into them through the laryngo-tracheal chamber with a quill and tying them tightly, after which they should be left for several days to dry. When perfectly dry, sections may be cut through them in various places with a sharp knife, and by this means a very good idea of the simple lung structure of the lower backboned animals can be obtained. With a sharp knife cut the heart open, beginning at the tip (ventricle) and cutting up through the conus arteriosus and the two auricles. Note the valves in the heart which separate the different compartments.

Note on either side of the median line in the dorsal region a pair of reddish glandular bodies (the kidneys). From each kidney trace a tube (ureter) posteriorly toward the region of the anus. The kidneys are the principal excretory organs of the body. The blood which flows through the delicate blood-vessels in the kidney gives up there much of its waste products. These pass out through small tubules of the kidneys into the ureters, which carry the wastes toward the anus. Along one side of each kidney may be seen a yellowish glistening mass, the adrenal body.

In some of the specimens studied, the body cavity may be filled with thousands of little black spherical bodies. These are undeveloped eggs. They are deposited by the mother toad in the water in long strings of transparent jelly, which are usually wound around sticks or plant-stems at the bottom of the pond near the shore. From these eggs the young toads hatch as tadpoles and in their life-history pass through an interesting metamorphosis. (See Chapter [XII].)

Technical Note.—The teacher should be provided with several well-cleaned skeletons of the toad in order that the bones may be carefully studied. Boil in a soap solution a toad from which most of the muscles and skin have been removed (see p. [452]). Leave in this solution until the muscles are quite soft and then pick off all bits of muscles and tissue from the bones. If this is carefully done, the ligaments which bind the bones will be left intact and the skeleton will hold together.

Note that the skeleton (fig. [2]) consists of a head portion which is composed of many bones joined together to form a bony box, the skull; of a series of small segments, the vertebræ, forming the vertebral column, which with the skull forms the axial skeleton; and of the appendicular skeleton, consisting of the bones of the fore and hind limbs. Note that the skull is composed of many bones joined together, some by sutures, while others are fused. Do the limbs attach directly to the axial skeleton? The anterior limbs (arms) articulate with the pectoral or shoulder-girdle. The arms will be seen to be made up of a number of bones placed end to end. Note that the uppermost, the humerus, is attached to the pectoral girdle, while at its lower end it articulates with the radio-ulna. At the lower end of the radio-ulna is a small series of carpal bones which afford attachments for the slender finger-bones, the phalanges or digital bones. The bones of the leg are articulated with a closely fused set of bones, the pelvic girdle. The leg-bones, proceeding from the pelvic girdle, are named femur, tibio-fibula, tarsal bones, and phalanges or digits. To what bones of the arm do these correspond? Determine the other principal bones of the skeleton by reference to figure [2].

Fig. 2.—Skeleton of the garden toad.

Technical Note.—In a specimen which has been macerated for some time in 20% nitric acid dissect out the nervous system. Place the specimen in a pan ventral side uppermost and pin out. Carefully pick away the vertebræ and the roof of the mouth-cavity, thereby exposing the central nervous system, which will appear light yellow.

Examine the brain. In front of the true brain are the olfactory lobes, the nervous centre for the sense of smell. The brain itself is composed of several parts. The anterior portion consists of two elongated parts, the cerebral hemispheres; just back of these are the optic lobes or midbrain, consisting of two short lobes, which are followed by the small cerebellum, which in turn is followed by a long part, the medulla oblongata, which runs imperceptibly into the long dorsal nerve, the spinal cord. Note the large optic nerves running out to each eye. How far backward does the spinal cord extend? Note the many pairs of nerves given off from the brain and spinal cord. These nerves branch and subdivide until they end in very fine fibres. Some end in the muscle-fibres, and through them the central nervous system innervates the muscles. These are motor endings. Still others pass to the surface and receive impressions from the outside. These last are sensory endings. Note that the spinal nerves arise from the spinal cord by two roots, an anterior or ventral, and a posterior or dorsal root. Trace the principal spinal nerves to the body-parts innervated by them. These nerves are numbered as first, second, etc., according to the number of the vertebræ (counting from the head backward) from behind which they arise.

[CHAPTER III]

THE STRUCTURE AND FUNCTIONS OF THE ANIMAL BODY

Organs and functions.—The body of the toad is composed of various parts, such as the lungs, the heart, the muscles, the eyes, the stomach, and others. The life of the toad consists of the performance by it of various processes, such as breathing, digesting food, circulating blood, moving, seeing, and others. These various processes are performed by the various parts of the body. The parts of the body are called organs, and the processes (or work) they perform are called their functions. The lungs are the principal organs for the function of breathing; the heart, arteries and veins are the organs which have for their function the circulation of the blood; the principal organ concerned in the digestion of food is the alimentary canal, the function of seeing is performed by the organs of sight, the eyes, and so one might continue the catalogue of all the organs of the body and of all the functions performed by the animal.

The animal body a machine.—The whole body of the toad is a machine composed of various parts, each part with its special work or business to do, but all depending on one another and all co-operating to accomplish the total work of living. The locomotive engine is a machine similarly composed of various parts, each part with its special work or function, and all the parts depending on one another and so working together as to perform satisfactorily the work for which the locomotive engine is intended. An important difference between the locomotive engine and the toad's body is that one is a lifeless machine and the other a living machine. But there is a real similarity between the two in that both are composed of special parts, each part performing a special kind of work or function, and all the parts and functions so fitted together as to form a complex machine which successfully accomplishes the work for which it is intended. And this similarity is one which should help make plain the fundamental fact of animal structure and physiology, namely, the division of the body into numerous parts or organs, and the division of the total work of living into various processes which are the special work or functions of the various organs.

The essential functions or life-processes.—The toad has a great many different special parts in its body. Its body is very complex. It performs a great many different functions, that is, does a great many different things in its living. And the structure and life of most of the other animals with which we are familiar are similarly complex: a fish, or a rabbit, or a bird has a body composed of many different parts, and is capable of doing many different things. Are all animals similarly complex in structure, and capable of doing such a great variety of things? We shall find that the answer to this question is No. There are many animals in which the body is composed of but a few parts, and whose life includes the performance of fewer functions or processes than in the case of the toad. There are many animals which have no eyes nor ears nor other organs of special sense. There are animals without legs or other special organs of locomotion; some animals have no blood and hence no heart nor arteries and veins. But in the life of every animal there are certain processes which must be performed, and the body must be so arranged or composed as to be capable of performing these necessary life-processes. All animals take food, digest it, and assimilate it, that is, convert it into new body substance; all animals take in oxygen and give off carbonic acid gas; all animals have the power of movement or motion (not necessarily locomotion); all animals have the power of sensation, that is, can feel; all animals can reproduce themselves, that is, produce young. These are the necessary life-processes. It is evident that the toad could still live if it had no eyes. Seeing is not one of the necessary functions or processes of life. Nor is hearing, nor is leaping, nor are many of the things which the toad can do; and animals can exist, and do exist, without any of those organs which enable the toad to see and hear and leap. But the body of any animal must be capable of performing the few essential processes which are necessary to animal life. How surprisingly simple such a body can be will be later discovered. But in most animals the body is a complicated object, and is able to do many things which are accessory to the really essential life-processes, and which make its life complex and elaborate.

[CHAPTER IV]

THE CRAYFISH (Cambarus sp.)

LABORATORY EXERCISE

Technical Note.—The crayfish, or crawfish, is found in most of the fresh-water ponds and streams of the United States. (It is not found east of the Hoosatonic River, Mass. In this region the lobster may be used. On the Pacific coast the crayfishes belong to the genus Astacus.) Crayfishes may be taken by a net baited with dead fish, or they may be caught in a trap made from a box with ends which open in, and baited with dead fish or animal refuse of any sort. This box should be placed in a pond or stream frequented by crayfish. If possible the student should study the living animal and observe its habits. Crayfish which are to be kept alive should be placed in a moist chamber in a cool place. They will keep for a longer time in a moist chamber than in water. Some fresh specimens should be injected by the teacher for the study of the circulatory system. A watery solution of coloring matter or, better, of an injecting mass of gelatine (see p. [451]) is injected into the heart through the needle of a hypodermic syringe. For the purpose of injecting, a small bit of the shell may be removed from the cephalothorax above the heart. Specimens which are to be kept for some time should be placed in alcohol or 4% formalin.

External structure (fig. [3]).—Place a specimen in a pan for study. Note that the body, which of course differs much in shape from that of the toad, is also unlike that of the toad in being covered by a hard calcareous exoskeleton, which acts as a covering for the soft parts and also as a place of attachment for the muscles, just as the internal skeleton does in the case of the toad. The body is composed of an anterior part, the cephalothorax, and a posterior part, the abdomen. The cephalothorax is covered above and on the sides by the carapace, which is divided into parts corresponding to the head and thorax of the toad by the transverse cervical suture. The abdomen is composed of segments. How many? The flattened terminal segment is called the telson. Is the cephalothorax composed of segments? Where is the mouth of the crayfish? Where is the anal opening?

Fig. 3.—Ventral aspect of crayfish (Cambarus sp.), with the appendages of one side disarticulated.

At the anterior end of the cephalothorax note a sharp projection, the rostrum. Where are the eyes? Remove one of them and examine its outer surface with a microscope. A bit of the outer wall should be torn off and mounted on a glass slide. Note that it is made up of a great many little facets placed side by side. Each of these facets is the external window of an eye element or ommatidium. An eye composed in this way is called a compound eye. In front of the eyes note two pairs of slender many-segmented appendages. The shorter pair, the antennules, are two-branched. Remove one of them and note at its base a small slit along the upper surface. This slit opens into a small bag-like structure which contains fine sand-grains. The bag is protected by a series of fine bristles along the edge of the slit. This bag-like structure is believed to be an auditory organ. The longer pair of appendages are the antennæ, and in the fine hair-like projections upon the joints is believed to be located the sense of smell. Thus it will be seen that the sense-organs of the crayfish, like those of the toad, are located on the head. Beneath the basal portion of each antenna there is a flat plate-like projection, at the base of which on the upper edge will be noted a small opening, the exit of the kidney, or green gland.

Make a drawing of the surface of part of an eye; also of an antennule; and of an antenna.

Technical Note.—Stick one point of the scissors under the posterior end of the carapace on the right side, and cut forward, thus exposing a large cavity, the gill-chamber. Remove all of the mouth-parts, legs and abdominal appendages from the right side, being careful to leave the fringe-like parts, the gills, attached to their respective legs. Place all of the appendages in order on a piece of cardboard.

Examine the abdominal appendages, called pleopods, or swimming feet. How many pairs are there? Each is composed of a basal part, the protopodite, and two terminal segments, an inner one, the endopodite, and an outer, the exopodite. In the males the first and second pleopods of the abdomen are larger and less flexible than the others. In the female the pleopods serve to carry the eggs and the first two pairs are very small or absent. Note the last set of abdominal appendages. These are the uropods, which together with the telson form the tail.

Make a drawing of the pleopods of one side.

Examine the appendages of the cephalothorax. Like the appendages of the abdomen the typical composition of each includes a protopodite, an exopodite and an endopodite, but some of these appendages are much modified, and show a loss of one of these parts, or the addition of an extra part. The cephalothoracic appendages may be divided into three groups, an anterior group of three pairs of mouth-parts (belonging to the head) of which the first pair is the mandibles and the others are the maxillæ; a second group of three pairs of foot-jaws or maxillipeds, belonging to the thorax, and a third group of five pairs of walking-legs. The mandibles, lying next to the mouth-opening, are hard and jaw-like and lack the exopodite; the first maxillæ are small and also lack the exopodite; the second maxillæ have a large paddle-like structure which extends back over the gills on each side within the space, the branchial chamber, above the gills. It is by means of this paddle-like structure (the scaphognathite) that currents of water are kept up through the gill-chambers. The maxillipeds increase in size from first to third pair. Each pair of walking-legs except the last bears gills. These gills are the organs by which the blood is purified. The blood of the crayfish flows into the large vessels on the outer sides of the gill and thence into the fine vessels in the little leaf-like lamellæ. At the same time the air which is mixed with the water bathing the gills passes freely through the thin membranous walls of these lamellæ and blood-vessels, and the blood gives off its carbonic acid gas to the water and takes up oxygen from the air in the water. Thus it will be seen that the office of the gill is like that of the lung in the toad, namely, to act as an organ for the elimination of carbonic acid gas and the taking up of oxygen.

Note the pincer-like appendages of the first pair of legs. These pincers are the chelæ, with which food is torn into bits and placed in the mouth. In the basal segment of each of the last pair of legs of the male note the genital pore. In the female the genital pores are in the basal segments of the next to last pair of legs. Is the crayfish bilaterally symmetrical? Note the repetition of parts in the crayfish, that is, the recurrence of similar parts in successive segments. This serial repetition of parts among animals is called metemerism.

Internal structure (fig. [4]).—Technical Note.—With a pair of scissors cut through the dorsal wall of the cephalothorax into the body-cavity. Cut the body-wall away from both sides and remove the middle portion.

Fig. 4.—Diagrammatic median longitudinal section of crayfish (Cambarus sp.).

At the anterior end of the cephalothorax note the large membranous sac, the stomach. Attached to each end of this are sets of muscles which control its movements. To the right and left of the stomach notice attached to the shell large muscles which connect by stout ligaments at their lower ends with the mandibles. Note a yellow fringe-like structure, the digestive gland, which fills most of the region about the stomach. It connects by a pair of small tubes, the bile-ducts, with the alimentary canal. Within the posterior portion of the cephalothorax note a pentagonal sac, the heart, contained within a delicate membrane, the pericardium. Remove the pericardium and note a pair of dorsal openings into the heart, called ostia. (There are also two lateral pairs and a ventral pair of ostia.) Note passing anteriorly from the heart along the median line to the eyes a blood-vessel, the ophthalmic artery. Arising from the anterior portion of the heart are the antennary arteries, running to the antennæ. Yet another pair running anteriorly from the heart to the stomach and digestive glands are called the hepatic arteries. From the posterior end of the heart arises the dorsal abdominal artery, running back to the telson. Below this arises the sternal artery, which will be seen later.

In the region below the heart are located the reproductive organs. They are whitish glandular masses from each of which runs a tube which opens at the base of the last pair of walking-legs in the male, and at the base of the third pair of walking-legs in the female.

Technical Note.—Cut longitudinally through the dorsal wall of the abdomen on either side of the median line and remove the piece of shell.

Note the powerful muscles within which flex and extend the abdomen. By a rapid contraction of these muscles the tail is brought beneath the body, propelling the animal strongly backwards. When the crayfish crawls it generally goes forward, but in swimming it reverses this direction.

Make a drawing showing, in their natural position, the internal organs which have been studied.

Examine the alimentary canal for its whole length. Note that the large bladder-shaped stomach is attached to the mouth-opening by a short tube. What part of the canal is this? From the posterior end of the stomach is a short thick-walled part, the small intestine, followed by a long straight tube, the large intestine, which opens to the exterior through the anus.

Technical Note.—Remove the alimentary canal, detaching it from the anal end first, and working forward.

Cut the stomach open. Note an anterior portion, the cardiac chamber, and a smaller posterior portion, the pyloric chamber. Examine its inner surface. What do you find here? This structure is called the gastric mill. Food, which for the most part consists of any dead organic matter, is chewed by the "stomach-teeth" into fine bits, and is then passed into the pyloric chamber. It is here that the digestive glands empty their secretion into the food. These glands have the same office as have the liver and pancreas combined in the toad, and so they are often called the hepato-pancreas. When the stomach has been removed there will be noted in the anterior portion of the body paired, flattened bodies, already mentioned, which connect with openings at the base of each of the antennæ by means of wide thin-walled sacs, the ureters. These organs are the kidneys, or green glands. Their office is similar to that of the kidneys in the toad, namely, the elimination of waste from the body.

Technical Note.—Carefully remove all of the alimentary canal, digestive glands, and reproductive organs. This process will expose the floor of the cephalothorax. Now cut away from either side the horny floor or bridge at the bottom of the cephalothorax. If the specimen has not already been immersed, place it in clear water for further dissection.

The foregoing dissection will expose the central nervous system. It extends as a series of paired ganglia connected by a double nerve-cord along the ventral median line from the œsophagus to the last segment of the abdomen. From what points do the lateral nerves arise? Anteriorly the double nerve-cord divides, the two parts passing upward on each side of the œsophagus, where they again meet to form the supra-œsophageal ganglion or brain. Where do the nerves run which rise from the brain? What is the difference between the position of the central nervous system in the crayfish and in the toad?

Make a drawing of the nervous system.

Just beneath the nerve-cord note a blood-vessel extending the length of the body. This is the sternal artery, which arises from the posterior end of the heart and passes ventrally at one side of the alimentary canal and between the nerve-cords. Here the sternal artery divides into an anterior and a posterior branch, from which lesser branches are given off to each one of the appendages. The various arteries running to all parts of the body finally pour out the blood into the body-cavity, where it flows freely in the spaces among the various tissues and organs. After the blood has bathed the body tissues it flows to the gills on either side, passing up the outer side of the gill through delicate thin-walled vessels, where it is oxygenated as has already been described. From the gills the purified blood flows back on the inner side through a large chamber, sinus, into the pericardium, through the ostia of the heart, whence it is driven into the arteries once more. This sort of a circulatory system in which the blood in places is not enclosed in a definite vessel is known as an open system. In the toad we find the blood in a closed system, i.e., arteries leading into capillaries which in turn lead into veins, in no case allowing the blood to pass freely through the spaces of the body.

[CHAPTER V]

THE MODIFICATION OF ORGANS AND FUNCTIONS

Differences between crayfish and toad.—In the dissection of the crayfish one of the most important things in the study of zoology has been learned. It is plain that the crayfish has a body composed, like the toad's, of parts or organs, and that most of these organs, although differing much in appearance and actual structure from those of the toad, correspond to similarly named organs of the toad, and perform the same functions or processes, although with many striking differences, essentially in the same way as in the toad. But the structure of the body is very different in the two animals. The toad has an internal body skeleton to which the muscles are attached, and a soft, yielding, outer body-covering or skin; the crayfish has no internal skeleton, but has its body covered by a horny, firm body-wall to which the muscles are attached. The toad has its main nervous chain lying just beneath the dorsal wall of the body; the crayfish has its main nervous chain lying just above the ventral wall of the body. The toad has lungs and takes up oxygen from the air of the atmosphere; the crayfish has gills and takes up oxygen from the air which is mixed with the water. The toad has a single pair of jaws; the crayfish has several pairs of mouth-parts. The toad has four legs fitted for leaping; the crayfish has numerous legs fitted for crawling or swimming. The crayfish's body is composed of a series of body-rings or segments; the toad's body is a compact apparently unsegmented mass. The toad has eyes each with a single large lens and capable of moving in the head and of changing their shape and hence their focus; the crayfish's eyes are immovable and have a fixed focus, and are composed of hundreds of tiny eyes each with lens and special retina of its own. And so a long list of differences might be gone through with.

Resemblances between toad and crayfish.—But on the other hand there are many resemblances—resemblances both in structure and life-processes or physiology. Both toad and crayfish have organs for the prehension of food, its digestion and its assimilation. And these organs, the organs of the digestive system, while differing in details are alike in being composed principally of a long tube, the alimentary canal, running through the body, open anteriorly for the taking in of food, and open posteriorly for the discharge of indigestible useless matter. Both alimentary canals are divided into various special regions for the performance of the various special processes connected with the digestion and assimilation of food. Each is adapted for the special kind of food which it is the habit of the particular animal to take. The two sets of organs are essentially alike and have the same essential function or life-process to perform. But this process differs in the details of its performance, and the organs which perform this function and which constitute the digestive system of each are modified to suit the special habits or kind of life of the animal.

Both toad and crayfish have a heart with blood-vessels leading from it. In the case of the toad the heart is more complex than in the crayfish, and the system of blood-vessels is far more extensive and elaborate. But the heart and blood-vessels in both animals subserve the same purpose; their function is the circulation of the blood, this being the means by which oxygen and food are carried to all growing or working parts of the body, and by which carbonic acid gas and other poisonous waste products are brought away from these parts. But this function differs somewhat in its performance in the two animals, and the organs which perform the function are correspondingly modified in structural condition.

Both toad and crayfish have organs for respiration, that is, for breathing in oxygen and breathing out carbonic acid gas. But the toad takes its oxygen from the atmosphere about it; its respiratory organs are the lungs, the sac-like tube leading to the mouth, and the external openings for the ingress and exit of the gases. The crayfish, living mostly in the water, takes its oxygen from the air which is mechanically mixed with the water. Its respiratory organs are its gills. There is a great difference, apparently, in the structural conditions of the organs of respiration in the two animals. As a matter of fact the difference is less great than, at first sight, appears to be the case. The lungs of the toad are composed primarily of a thin membrane, in the form of a sac, richly supplied with blood-vessels. Air is brought to this thin respiratory membrane and by osmosis the oxygen passes through the membrane and through the thin walls of the fine blood-vessels, and is taken up by the blood. At the same time the carbonic acid gas brought by the blood to the lungs from all parts of the body is given up by it and passes through the membranes in order to leave the body. The air comes in contact with the respiratory membrane (which is situated inside the body) by means of a system of external openings and a conducting chamber, and by these same openings and chamber the carbonic acid gas leaves the body. In the crayfish the gills are nothing else than a large number of small flattened sacs each composed of a thin membrane richly supplied with blood-vessels. This respiratory membrane is not, in the crayfish, situated inside the body, but on the outer surface, although protected by being in a sort of pocket with a covering flap, and it comes into immediate contact with the air held in the water which freely bathes the gills. By osmosis the oxygen of this air passes in through the gill-membranes, while the carbonic acid gas brought by the blood passes out through them. Exactly the same exchange of gases is accomplished as in the toad. But because of the great difference in the conditions of life of the toad and crayfish, one living in water, the other living out of water, the character of the performance of the function of respiration, and correspondingly the structural condition of the organs performing this function, are strikingly different.

Modification of functions and structure to fit the animal to special conditions of its life.—As has been done with the organs of digestion, circulation, and respiration, so we might compare the other organs of the crayfish and the toad. There would be found not only many very marked differences between organs which have the same general function in the two animals, but we should find also numerous organs in the toad which are not present at all in the crayfish, and conversely; and this means, of course, that the toad can do numerous things, perform numerous functions, which the crayfish cannot, and, conversely, that the crayfish does some things which the toad cannot. But both of these animals agree in possessing in common the capability of performing those processes such as taking food, breathing, reproducing, etc., to which attention has been called as being indispensable to all animal life. These processes, however, are performed by the two animals in different ways and the organs for the performance of these processes, although at very bottom essentially alike, are in outer and superficial details of position, appearance and general structure markedly different. Animals are fitted to live in different places amid different surroundings by having their bodies modified and the performance of their life-processes modified to suit the special conditions of their life.

Vertebrate and invertebrate.—In selecting the toad and the crayfish as the first animals to study and to compare with each other, we have chosen representatives of the two great groups into which the complexly organized animals are divided, viz., the group of backboned or vertebrate animals, and the group of backboneless or invertebrate animals. To the vertebrates belong all those which have an internal bony skeleton (and a few without such a skeleton) and which have also an arrangement of body-organs on the general plan of the toad's body. A conspicuous feature of this arrangement is the situation of the spinal cord or main great nerve-trunk along the back or dorsal wall of the animal, and inside of a backbone. All the fishes, batrachians (frogs, toads, salamanders, etc.), reptiles (snakes, lizards, alligators, etc.), birds, and mammals (quadrupeds, whales, seals, etc.) belong to the vertebrates. The backboneless or invertebrate animals have no internal bony skeleton and have their main nerve-trunk usually along the ventral wall of the body, sometimes in a circle around the mouth, but never in a backbone. To the invertebrates belong all insects, lobsters, crabs, clams, squids, snails, worms, starfishes and sea-urchins, corals and sponges, altogether a great host of animals, mostly small.

[CHAPTER VI]

AMŒBA AND PARAMŒCIUM

LABORATORY EXERCISE

Amœba.—Technical Note.—Amœbæ are found in stagnant pools of water on the dead leaves, sticks and slime at the bottom. To obtain them, collect slime and water from various puddles in separate bottles and take them to the laboratory. Place a small drop of slime on a slide under a cover-glass. Examine under the low power first and note any small transparent or opalescent objects in the field. Examine these objects with the higher power and note that some are mere granular jelly-like specks, which slowly (but constantly) change their form. These are Amœbæ.

A teacher of zoology recommends the following method of obtaining a large supply of Amœbæ: "For rearing Amœbæ place two or three inches of sand in a common tub, which is then filled with water and placed some feet from a north window; three or four opened mussels, with merest trace of the mud from the stream in which they are taken, are partially buried in the sand and a handful of Nitella and a couple of crayfish cut in two are added; as decomposition goes on a very gentle stream is allowed to flow into the tub, and after from two to four weeks abundant Amœbæ are to be found on the surface of the sand and in the scum on the sides of the tub; small Amœbæ appear at first, and later the large ones."

Having found an Amœba (fig. [5]) note its irregular shape, and if it moves actively observe its method of moving. How is this accomplished? The viscous, jelly-like substance which composes the whole body of an Amœba is called protoplasm. The little processes which stick out in various directions are the "false feet" (pseudopodia). Note that the outer portion, the ectosarc, of the protoplasmic body is clear, while the inner, the endosarc, is more or less granular in structure. Has Amœba a definite body-wall? Do the pseudopodia protrude only from certain parts of the body? Within the endosarc note a clear globular spot which contracts and expands, or pulsates, more or less regularly. This is the contractile vacuole. Note the small granules which move about within the endosarc. These are food-particles which have been taken in through the body-wall. Note how pseudopodia flow about food-particles in the water and how these are digested by the protoplasm. If an Amœba comes into contact with a particle of sand, note how it at once retreats. Note within the endosarc an oval transparent body which shows no pulsations. This is the nucleus, a very complex little structure of great importance in the make-up of Amœba.

Fig. 5.—Amœba sp.; showing the forms assumed by a single individual in four successive changes. (From life.)

Note that Amœba has no mouth or alimentary canal; no nostrils or lungs, no heart or blood-vessels, no muscles, no glands. It is an animal body not made up of distinct organs and diverse tissues. Its whole body is a simple minute speck of protoplasm, a single animal cell. But it takes in food, it moves, it excretes waste matter from the body, is sensitive to the touch of surrounding objects, and, as we may be able to see, it can reproduce itself, i.e., produce new Amœbæ. Amœba is the simplest living animal.

It is only rarely that we can find an Amœba actually reproducing. The process, in its gross features, is very simple. First the Amœba draws in all of its pseudopodia and remains dormant for a time. Next, certain changes take place in the nucleus, which divides into equal portions, one part withdrawing to one end of the protoplasmic body, the other to the opposite end. Soon the body protoplasm itself begins to divide into two parts, each part collecting about its own half of the nucleus. Finally the two halves pull entirely away from each other and form two new Amœbæ, each like the original, but only half as large. This is the simplest kind of reproduction found among animals.

Amœbæ continue to live and multiply as long as the conditions surrounding them are favorable. But when the pond dries up the Amœbæ in it would be exterminated were it not for a careful provision of nature. When the pond begins to dry up each Amœba contracts its pseudopodia and the protoplasm secretes a horny capsule about itself. It is now protected from dry weather and can be blown by the winds from place to place until the rains begin, when it expands, throws off the capsule and commences active life again in some new pond.

The Slipper Animalcule (Paramœcium sp.)—Technical Note.—Paramœcia can be secured in most pond water where leaves or other vegetation are decaying. However, if specimens are not readily secured place some hay or finely cut dry clover in a glass dish, cover with water and leave in the sun for several days. In this mixture specimens will develop by thousands. Place a drop of water containing Paramœcia on a slide with cover-glass over it. Using a low power, note the many small animals darting hither and thither in the field. Run a thin mixture of cherry gum in water under the cover-glass. In this mixture they can be kept more quiet and be better studied.

How does Paramœcium (fig. [6]) differ from Amœba in form and movement? Has the body an anterior and a posterior end? The delicate, short, thread-like processes, on the surface of the body, which beat about very rapidly in the water are called cilia, and they are simply fine prolongations of the body protoplasm. What is their function? Note a fine cuticle covering the body. Note also many minute oval sacs lying side by side in the ectosarc. These are called trichocysts and from each a fine thread can be thrust out.

Note on one side, beginning at the anterior end, the buccal groove leading into the interior through the gullet. Observe also that by the action of the cilia in the buccal groove food-particles are swept into the gullet. Rejected or waste particles are ejected from the body occasionally. Where? Note about midway of the Paramœcium an ovoid body with a smaller oval one attached to its side, the former being the macronucleus, the latter the micronucleus. Note that there are two contractile vacuoles in the Paramœcium; also that the food-vacuoles have a definite course in their movement inside the endosarc.

Make a drawing of a Paramœcium.

In comparing Paramœcium with Amœba it is apparent that the body of the first is less simple than that of the second. The definite opening for the ingress of food, the two nuclei, the fixed cilia, and the definite cell-wall giving a fixed shape to the body, are all specializations which make Paramœcium more complex than Amœba. But the whole body is still composed of a single cell, and there is, as in Amœba, no differentiation of the body-substance into different tissues, and no arrangement of body-parts as systems of organs.

Fig. 6.—Paramœcium sp.;
buccal groove at right. (From
life.)

Paramœcium may occasionally be found reproducing. This process takes place very much as in Amœba. The animal remains dormant for a while, the micronucleus then divides, the macronucleus elongates and finally divides in two, the protoplasm of the body becomes constricted into two parts, each part massing itself about the withdrawn halves of the macro- and micro-nuclei, and lastly the whole breaks into two smaller organisms which grow to be like the original. After multiplication or reproduction has gone on in this way for numerous generations (about one hundred), a fusion of two Paramœcia seems necessary before further divisions take place. This process of fusion, called conjugation, may be noted at some seasons. Two Paramœcia unite with their buccal grooves together, part of the macronucleus and micronucleus of each passes over to the other, and the mixed elements fuse together to form a new macro- and micronucleus in each half. The conjugating Paramœcia now separate, and each divides to form two new individuals.

[CHAPTER VII]

THE SINGLE-CELLED ANIMAL BODY.—PROTOPLASM AND THE CELL

The single-celled body.—The study of Amœba and Paramœcium has made us acquainted with an animal body very different from that of the toad or the crayfish. These extraordinarily minute animals have a body so simple in its composition, compared with the toad's, that if the toad's body be taken for the type of the animal body, Amœba might readily be thought not to be an animal at all. The body of Amœba is not composed of organs, each with a particular function or work to perform. Whatever an Amœba does is done, we may say, with its whole body. But as we learn the things that this formless viscid speck of matter does, we see that it is truly an animal; that it really does those things which we have learned are the necessary life-processes of an animal. Amœba takes up and digests food composed of organic particles; it has the power of motion; it knows when its body comes in contact with some external object, that is, it can feel or has the power of sensation. Amœba takes in oxygen and gives out carbonic acid gas, and it can produce new individuals like itself, that is, it has the power of reproduction. But for the performance of these various life-processes or functions it has no special parts or organs, no mouth or alimentary canal, no lungs or gills, no legs, no special reproductive organs. We have here to do with one of the "simplest animals." With a minute, organless, soft speck of viscous matter called protoplasm for a body, the simplest structural condition to be found among living beings, Amœba nevertheless is capable of performing, in the simplest way in which they may be performed, those processes which are essential to animal life.

Paramœcium has a body a little less simple than Amœba. The food-particles are taken into the body always at a certain spot; this might be spoken of as a mouth. And the body has some special locomotory organs, if they may be so called, in the presence of the cilia. The body, too, has a definite shape or form. But, as in Amœba there is no alimentary canal, nor nervous system, nor respiratory system, nor reproductive system. The whole body feels and breathes and takes part in reproduction.

A long jump has been made from the toad and crayfish to Amœba and Paramœcium; from the complex to the simplest animals. But, as will later be seen, the great difference between the bodies of these simplest animals and those of the highly complex ones is only a difference of degree; there are animals of all grades and stages of structural condition connecting the simplest with the most complex. When animals are studied systematically, as it is called, we begin with the simplest and proceed from them to the slightly complex, from these to the more complex, and finally to the most complex. There are hundreds of thousands of different kinds of animals, and they represent all the degrees of complexity which lie between the extremes we have so far studied.

The cell.—The characteristic thing about the body of Amœba and Paramœcium and the other "simplest animals"—for there are many members of the group of "simplest animals," or Protozoa—is that it is composed, for the animal's whole lifetime, of a single cell. A cell is the structural unit of the animal body. As will be learned in the next exercise, the bodies of all other animals except the Protozoa, the simplest animals, are composed of many cells. These cells are of many kinds, but the simplest kind of animal cell is that shown by the body of an Amœba, a tiny speck of viscous, nearly colorless protoplasm without fixed form. The protoplasm composing the cell is differentiated to form two parts or regions of the cell, an inner denser part, called the nucleus, and an outer clearer part, called the cytoplasm. Sometimes, as in the Paramœcium, the cell is enclosed by a cell-wall which may be simply a denser outer layer of the cytoplasm, or may be a thin membrane secreted by the protoplasm. Thus the cell is not what its name might lead us to expect, typically cellular in character; that is, it is not (or only rarely is) a tiny sac or box of symmetrical shape. While the cell is composed essentially of protoplasm, yet it may contain certain so-called cell-products, small quantities of various substances produced by the life-processes of the protoplasm. These cell-products are held in the protoplasmic body-mass of the cell, and may consist of droplets of water or oil or resin, or tiny particles of starch or pigment, etc. The cell cannot be said to be composed of organs, because the word organ, as it is commonly used in the study of an animal, is understood to mean a part of the animal body which is composed of many cells. But the single cell can be somewhat differentiated into parts or special regions, each part or special region being especially associated with some one of the life-processes. In Paramœcium, for example, the food is always taken in through the so-called mouth-opening; the fine protoplasmic cilia enable the cell to swim freely in the water, the waste products of the body are always cast out through a certain part, and so on. But this is a very simple sort of differentiation, and the whole body is only one of those structural units, the cells, of which so many are included in the body of any one of the complex animals.

Protoplasm.—The protoplasm, which is the essential substance of the typical animal cell and hence of the whole animal body, is a substance of very complex chemical and physical make-up. No chemist has yet been able to determine its exact chemical constitution, and the microscope has so far been unable to reveal certainly its physical characters. The most important thing known about the chemical constitution of protoplasm is that there are always present in it certain complex albuminous substances which are never found in inorganic bodies. And it is certain that it is on the presence of these substances that the power possessed by protoplasm of performing the fundamental life-processes depends. Protoplasm is the primitive physical basis of life, but it is the presence of the complex albuminous substances in it that makes it so.

The physical constitution of protoplasm seems to be that of a viscous liquid containing many fine globules of a liquid of different density and numerous larger globules of a liquid of still other density. Some naturalists believe the fine globules to be solid grains, while still others believe that numerous fine threads of dense protoplasm lie coiled and tangled in the clearer, viscous protoplasm. But the little we know of the physical structure of protoplasm throws almost no light on the remarkable properties of this fundamental life-substance.

[CHAPTER VIII]

CELLULAR STRUCTURE OF THE TOAD (OR FROG)

LABORATORY EXERCISE

The blood.—Technical Note.—The blood of a frog can be studied as it flows through the small vessels in the membranes between the toes while the animal is alive. Place a frog on a small flat board which has had a hole cut near one end, and with a piece of cloth bind it to the board. Spread the web between two toes over the hole in the board and keep it in place with pins. This done, examine the distended web under the compound microscope first with low then with higher power, and observe the blood-vessels and the blood circulating in them. For a further study of the blood kill a toad or frog and place a drop of the blood on a slide with a cover-glass over it.

Put the prepared slide under the microscope and note that the blood, which as seen with the unaided eye appears to be a red fluid, is made up of a great many yellowish elliptical disks or cells, the blood-corpuscles, floating in a liquid, the blood-plasma. Here and there you may notice amœboid blood-corpuscles. These are irregular-shaped cells which move about by thrusting out pseudopodia. They look like some of the unicellular animals, as the Amœba. Can you distinguish a nucleus and cell-wall in the blood-cells?

Make drawings of these blood-cells.

The skin.—Technical Note.—Keep a live toad or frog in water for some time and note if its skin becomes loose or begins to slip away. If the outer skin, epidermis, comes off, take some of the shed skin and wash it in water, then stain for three or four minutes in a solution of methyl-green and acetic acid (see p. [451]). Cut the pieces of stained skin into small bits and examine one of these under the microscope.

With the low power of the microscope you will note that the skin is made up of a great many flat cells placed edge to edge. Each one has its cell-wall and a central darkly stained nucleus.

Make a drawing of a portion of the toad's skin.

The liver.—Technical Note.—Cut through the fresh liver of a toad, and with a knife-blade scrape from the cut surface some of the liver-cells and place them on a slide with cover-glass.

Examine under the microscope and observe many polygonal cells. Place some of the methyl-green acetic stain under the cover-glass and note, after the cells are stained, that they have definite boundaries and a central nucleus.

Draw some of these scattered liver-cells.