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.
The muscles.—Technical Note.—Take a piece of intestine from a freshly killed toad, wash it thoroughly and place it in a concentrated solution of salicylic acid in 70% alcohol for 24 hours, then gradually heat until about the boiling-point, when the muscles will fall to pieces. Transfer the preparation to a watch-crystal and tease small bits of isolated muscle with dissecting-needles. Place some of the teased muscle-fibres on a slide, cover with cover-glass, and add a drop of the methyl-green acetic acid.
Note the small spindle-shaped muscle-fibres. Each one of these fibres is a cell possessing all of the structures common to cells, namely, cell-wall, nucleus, etc.
Make a drawing of a few isolated fibres of muscle.
From this study of some of the tissues in a toad it will be noted that in the first case we had in the blood separate cells which moved about freely in the plasma. In the second case, that of the epidermis, the cells are fixed edge to edge, thus forming a thin tissue; while in the third and fourth cases, that of the liver and muscle, the cells are not only placed edge to edge, but aggregated into vast masses or bundles, in one case to form the liver and in the other case a muscle. The entire body of the toad is built up of a colony of simple units (cells) combined in various forms to make all the various tissues and organs.
[CHAPTER IX]
THE MANY-CELLED ANIMAL BODY.—DIFFERENTIATION OF THE CELL
The many-celled animal body.—In the study of certain of the tissues and organs of the toad we have learned that the body of this animal is composed of many cells, thousands and thousands of these microscopic structural units being combined to form the whole toad. This many-celled or multicellular condition of the body is true of all the animals except the simplest, the unicellular Protozoa. Corals, starfishes, worms, clams, crabs, insects, fishes, frogs, reptiles, birds, and mammals, all the various kinds of animals in which the body is composed of organs and tissues, agree in the multicellular character of the body, and may be grouped together and called the many-celled animals in contrast to the one-celled animals. This division is one which is recognized by many systematic zoologists as being more truly primary or fundamental than the division of animals into Vertebrates and Invertebrates. The one-celled animals are called Protozoa, and the many-celled animals Metazoa.
Differentiation of the cell.—It is apparent at first glance that the cells which compose the body of a many-celled animal are not like the simple primitive cell which makes up the body of the Amœba, nor are they like the more complexly arranged cell of the Paramœcium. Nor are they all like each other. The cells in the toad's blood are of two kinds, the white blood-cells, which are very like the body of Amœba, and the elliptical disk-like red blood-cells. The cells composing the muscles are, moreover, like neither kind of blood-cells, and the cells of which the liver is composed are not like the cells of the muscles. That is, there are many different kinds of cells in the body of a many-celled animal. While the single cell which composes the whole body of the Amœba is able to do all the things necessary to maintain life, the various cells in the body of a complex animal are differentiated or specialized, certain cells devoting themselves to a certain function or special work, and others to other special functions. For example, the cells which compose the organs of the nervous system, the brain, ganglia, and nerves, devote themselves almost exclusively to the function of sensation, and they are especially modified for this purpose. The highly specialized nerve-cells resemble very little the primitive generalized body-cell of Amœba. The muscle-cells of the complex animal body have developed to a high degree that power of contraction which is possessed, though in but slight degree, by Amœba. These muscle-cells have for their special function this one of contraction, and massed together in great numbers they form the strongly contractile muscular tissue and muscles of the body on which the animal's power of motion depends. The cells which line certain parts of the alimentary canal are the ones on which the function of digestion chiefly rests. And so we might continue our survey of the whole complex body. The point of it all is that the thousands of cells which compose the many-celled animal body are differentiated and specialized; that is, have become changed or modified from the generalized primitive amœboid condition, so that each kind of cell is devoted to some special work or function and has a special structural character fitting it for its special function. In the Protozoan body the single cell can perform and does perform all the functions or processes necessary to the life of the animal. In the Metazoan body each cell performs, in co-operation with many other similar cells, some one special function or process. The total work of all the cells is the living of the animal.
[CHAPTER X]
HYDRA
LABORATORY EXERCISE
Technical Note.—Hydra lives in fresh water, attached to stones, sticks, or decayed leaves. It can be found in most open fresh-water ponds not too stagnant, often attached to Chara. There are two species occurring commonly, H. viridis, the green Hydra, and H. fuscus, the brown or flesh-colored Hydra. Both are very small forms and have to be looked for carefully. Specimens should be brought to the laboratory, put into a large dish of water and left in the light. Hydra is best studied alive. Place a living specimen attached to a bit of weed in a watch-crystal filled with water or on a slide with plenty of water and examine with the low power of the microscope.
Note the cylindrical body (fig. [7], A, B) with its flat basal attachment and radial tentacles (varying in number) which crown the upper end and surround the centrally located mouth. Note the movements of Hydra, its powers of contraction, and method of taking in food.
Technical Note.—To feed Hydra, place very small "water-fleas" (Daphnia sp.) in the water with it.
Observe the method by which "water-fleas" are taken into the mouth. Food is caught on stinging cells (to be studied later) and conveyed to the mouth by the tentacles. Note that the cylindrical body encloses a cavity, the digestive cavity. How is this connected with the exterior? If Hydra captures prey too large or is no longer hungry, the prey is released.
Fig. 7.—A, Hydra fusca, with expanded body and a budding individual; B, H. fusca, contracted; C, H. fusca, part of outer surface of a tentacle, greatly magnified. (A and B drawn from live specimens, C, from a preparation) D, Grantia sp. (a sponge), three individuals; E, Grantia sp., longitudinal section; F, Grantia sp., spicules. (D, E, and F drawn from preserved specimens.)
Technical Note.—Place small slips of paper on the slide near the Hydra, put cover-glass over the whole, and examine with the low power of the microscope.
Note that the whole animal is made up of cells closely joined. Are the cells in the tentacles all alike? Note nodule-like projections above some of the cells; these are stinging cells, or cnidoblasts. In some cases a small hair-like process, the trigger hair or cnidocil, may be seen projecting above the surface of the cell. Note in some of the tentacles dark-colored particles. These are food-particles which have been taken through the mouth into the digestive cavity and have passed thence into the tentacles. The central digestive cavity communicates freely with the cavities in the tentacles, for the tentacles are merely evaginations of the body-wall.
Make drawings of the Hydra expanded and of the same individual contracted.
Technical Note.—From the preparation which you have under the microscope pull out the slips of paper, thus letting the cover-glass drop down on the specimen. With a small pipette put a drop of anilin-acetic stain (see p. [451]) on the slide at one side of the cover-glass and with a piece of filter-paper draw the water through from the other side of the cover-glass. When the stain is diffused press down the cover-glass gently and examine the tentacles first under a low power of the microscope, then under a high one.
Note the distortion that the animal has undergone through the action of the reagent. Observe the cnidoblasts of the tentacles and note that many of them have thrown out long whip-like processes (fig. [7], C). On what parts of the body do the cnidoblasts occur? Carefully examine one of the cnidoblasts which has been discharged and note a clear transparent bag-like structure within, the nematocyst, to which is attached the long whip-like process. In another cnidoblast cell which has not been discharged note that the whip-like process is coiled about inside of the bag-like structure. The whole apparatus is like the inturned finger of a glove which can be blown out by pressure from the inside. The mechanism is simple. The cnidocil or trigger-hair is touched by some animal, an impulse is conveyed to the delicate fibres interspersed among the cells (nerve-cells) which stimulate the cnidoblast cell, whereupon there is a contraction of the contents and, the cnidoblast being compressed, the inverted whip-like process turns wrong side out and impales the animal on its points or barbs.
Technical Note.—The teacher should be provided with microscopical sections, both transverse and longitudinal, of the Hydra stained in some good general stain (hæmatoxylin or borax carmine). If the teacher has no means of making such preparations, they may be procured from dispensers of microscopical supplies.
From the cross-section of the Hydra make out the general structure of the body. Note that it is a hollow cylinder consisting of two well-defined layers of cells, an outside ectoderm layer and an inner endoderm layer. Between these two is yet another thin non-cellular layer called the mesoglœa.
Thus it will be seen that Hydra is made up of two layers of cells, the outer ectoderm or skin, which is specialized to perform the office of capturing prey as well as that of protection, and the inner endoderm, which surrounds the digestive cavity and performs the function of digestion. The endoderm lines the body-cavity, particles taken in as food being digested by certain digestive cells which thrust out amœboid processes and ingest particles of food. Other cells in the endoderm have long flagellate processes which vibrate back and forth in the digestive cavity, thereby creating currents in the water containing food-particles.
Note, in a cross-section, that there are small ovoid or cuboid cells at the bases of the large ectoderm cells. These are the interstitial cells. Some of the interstitial cells become modified and pushed up between the ectoderm cells to form cnidoblast cells. Many of the endoderm as well as ectoderm cells have muscle-processes which spread out from the base of the cell and which serve to contract and expand the body.
Technical Note.—In the specimens which have been collected perhaps two methods of reproduction will be observed. Place healthy Hydræ in a wide-mouthed jar in the sunlight with plenty of water and food. In a few days active budding will take place.
Observe the method of reproduction in Hydra. Commonly the parent produces small buds, which at first are only evaginations of the body-wall, but which later develop tentacles and a mouth of their own. Subsequently the bud becomes constricted at the base, separates from the parent, and the young Hydra begins a distinct existence.
Another mode of reproduction takes place which, in distinction from the asexual method just mentioned, is called sexual reproduction. This last is the method common to most of the higher organisms. You may note that in some Hydræ there is a swelling or bulging of the ectoderm of the body-wall in the region just below the tentacles. These are the sperm-glands. Within these are produced sperm-cells which break away in great clusters to fertilize the ova, or eggs. Note a larger bulging of the body-wall nearer the lower end of the body which, under high power, has a granular appearance. This is the egg-gland, in which develops a single ovum or egg. The ovum breaks from its covering and is fertilized by sperm-cells from another individual. In forms like Hydra, where both sexes are represented in a single individual, the organism is termed monœcious or hermaphroditic. In connection with reproduction Chapter [XIII] should be studied.
An instructive experiment can be performed by cutting a Hydra into two or more parts, when (usually) each of the various parts will develop into a complete Hydræ. This process may be called reproduction by fission, but it rarely occurs naturally.
[CHAPTER XI]
THE SIMPLEST MANY-CELLED ANIMALS
Cell differentiation and body organization in Hydra.—From the examination of Hydra we have learned that there are true many-celled animals which are much less complex in structure than the toad and crayfish. The body of Hydra, like the body of the toad, is composed of many cells, but these cells are of only a few different kinds; that is, show but little differentiation. There is relatively little division of the body into distinct organs. Still, certain parts of the body devote themselves principally to certain particular functions. Thus all the food is taken in through the single "mouth-opening" at the apical free end of the cylindrical body, and there are certain organs, the tentacles, whose special business or function it is to find and seize food and to convey it to the mouth. After the food is taken into the cylindrical body-cavity it is digested by special cells which line the cavity. Some of these cells are unusually large, and each contains one or more contractile vacuoles. From the free ends of these cells, the ends which are next to the body-cavity, project pseudopods or flagella. These protoplasmic processes are constantly changing their form and number. In addition to these large sub-amœboid cells there are, in this inner layer of cells lining the body-cavity, and especially abundant near the base or bottom of the cavity, many long, narrow, granular cells. These are gland-cells which secrete a digestive fluid. The food captured by the tentacles and taken in through the mouth-opening disintegrates in the body-cavity, or digestive cavity as it may be called. The digestive fluid secreted by the gland-cells acts upon it so that it becomes broken into small parts. These particles are seized by the projecting pseudopods of the sub-amœboid cells and taken into the body-protoplasm of these cells. The cells of the outer layer of the body do not take food directly, but receive nourishment only by means of and through the cells of the inner layer. The body-cavity of Hydra is a very simple special organ of digestion.
In the outer layer of cells there are some specially large cells whose inner ends are extended as narrow pointed prolongations directed at right angles with the rest of the cell. These processes are very contractile and are called muscle-processes. Each one is simply a specially contractile continuation of the protoplasm of the cell-body. There are also in this layer some small cells very irregular in shape and provided with unusually large nuclei. These cells are more irritable or sensitive than the others and are called nerve-cells. We have thus in Hydra the beginnings of muscular organs and of nerve-organs. But how simple and unformed compared with the muscular and nervous systems of the toad and crayfish! There is no circulatory system, nor are there any special organs of respiration.
But Hydra is far in advance of Amœba or Paramœcium. Its body is composed of thousands of distinct cells. Some of these cells devote themselves especially to food-taking, some especially to the digestion of food; some are specially contractile, and on them the movements of the body depend, while others are specially irritable or sensitive, and on them the body depends for knowledge of the contact of prey or enemies. In the cnidoblast cells, those with the stinging threads, there is a very wide departure from the simple primitive type of cells. There is in Hydra a manifest differentiation of the cells into various kinds of cells. The beginnings of distinct tissues and organs are indicated.
Degrees in cell differentiation and body organization.—In the study of the cellular constitution of the tissues and organs of the toad, we found to what a high degree the differentiation of the cells may attain, and in the study of the anatomy of the toad we found how thoroughly these differentiated cells may be combined and organized into body-parts or organs. The body of the toad is made up of distinct organs, each composed of highly differentiated or specialized cells. The body of Hydra is composed of cells for the most part only slightly differentiated and hardly recognizably grouped or combined into organs. These two conditions are the extremes in the body-structure of the many-celled animals. Between them is a host of intermediate conditions of cell differentiation and body organization. When we come to the study of other members of the great branch of simple many-celled animals to which Hydra belongs (see Chapter [XVII]), it will be found that some of them show a slight advance in complexity beyond Hydra. Higher in the scale of animal life the forms will be found still more and more complex, with ever-increasing differentiation of the cells, with the combination of the differentiated cells into distinct organs, and the co-ordination of organs into systems of organs up to the extreme shown by the birds and mammals. And hand in hand with this increasing complexity of structure goes ever-increasing complexity or specialization of function. Breathing is a simple function or process with Hydra, where each body-cell takes up oxygen for itself, but it is a complex business with the toad, or with a bird or mammal, where certain complex structures, the lungs and accessory parts, and the heart, blood-vessels and blood all work together to distribute oxygen to all parts of the body.
[CHAPTER XII]
DEVELOPMENT OF THE TOAD
FIELD AND LABORATORY EXERCISE
Technical Note.—As the work of this chapter, or some similar work in getting acquainted with the postembryonic development of a many-celled animal, should be done early in the course, and as most schools open in the fall, it will perhaps be impossible to make this first study of development from live specimens in the field. In such case the examination of a series of prepared specimens, previously obtained by the teacher, must be resorted to. In the spring the development of several kinds of animals, including the toad, can be studied from live specimens in the field or in breeding-cages and aquaria in the laboratory. The eggs of the toad may be found in April and May (the toads are heard trilling at egg-laying time) in ponds. The eggs look like the heads of black pins, and are in single rows in long strings of transparent jelly, which are usually wound around sticks or plant-stems at the bottom of the pond near the shore. Bring some of these strings into the schoolroom and keep them in water in shallow dishes. Keep them in the light, but not in direct sunlight. In the dishes put some small stones and mud from the pond, arranging them in a slope, thus making different depths of water. Stones with green algæ on should be selected, for algæ are the food of the tadpoles. The eggs will hatch in two or three days, and if too many tadpoles are not kept in the dish, and the little aquarium be well cared for, the whole postembryonic development of the toad can be well observed. For the study of the development from prepared specimens the teacher should have a complete series of stages from egg to adult toad in alcohol. The specimens may be examined by the students in connection with a talk from the teacher on the life-history of the toad.
If the study is made from prepared specimens, make drawings of egg-strings, and of a single egg magnified and shaded to indicate its color. Draw each specimen of the series of tadpoles, noting in the youngest the presence of gills and tail and absence of legs and eyes; in the older the appearance of eyes, the shrivelling of the gills, shrinking of the tail and development of legs; in the still older the characteristic shape, in miniature, of the adult toad.
In observing the course of development of the living specimens there should be made, in addition to the drawings, notes showing the duration of the egg stage, and the time elapsing between all important changes (as seen externally) in the body of the young. Observations and notes on the general behavior of tadpoles should also be made; note the swimming, the feeding, the gradual leaving of the water, etc.
In addition to the easily seen external changes in the body, very important ones in the internal organs take place during development. Perhaps the most important of these concerns the lungs. The young gilled toad breathes as a fish does, but gradually its gills are lost, while at the same time lungs develop and the tadpole comes to the surface to breathe air like any lunged aquatic animal. The toad on leaving the water changes its diet from vegetable to animal food; a tadpole feeds on aquatic algæ; a toad preys on insects. Correlated with the change in habit, the intestine during development undergoes some marked changes, becoming relatively diminished in length.
For an account of the development of the toad see Gage's "Life-history of a Toad" or Hodge's "The Common Toad."
[CHAPTER XIII]
MULTIPLICATION AND DEVELOPMENT.—MULTIPLICATION OF ONE-CELLED ANIMALS
Multiplication.—We know that any living animal has parents; that is, has been produced by other animals which may still be living or be now dead or, as with Amœba, may have changed, by division, into new individuals. Individuals die, but before death, they produce other individuals like themselves. If they did not, their kind or species would die with them. This production of new animals constantly going on is called the reproduction or multiplication of animals. The process is well called multiplication, because each female animal normally produces more than one new individual. She may produce only one at a time, one a year, as many of the sea-birds do or as the elephant does, but she lives many years. Or she may produce hundreds, or thousands, or even millions of young in a very short time. A lobster lays 10,000 eggs at a time. Nearly nine millions of eggs have been taken from the body of a thirty-pound female codfish. As a matter of fact but very, very few of these eggs produce new animals which reach maturity. From the 10,000 eggs produced by the lobster each year an average of but two new mature lobsters is produced. There is always a struggle for food and for place going on among animals, for many more are produced than there are food and room for, and so of all the new or young animals which are born the great majority are killed before they reach maturity. In a later chapter more attention will be given to this great struggle for life.
In the preceding paragraph it has been stated that "we know that any living animal has parents; that is, has been produced by other animals which may still be living or be now dead." This is a statement, however, which has found complete acceptance only in modern times. It is a familiar fact that a new kitten comes into the world only through being born; that it is the offspring of parents of its kind. But we may not be personally familiar with the fact that a new starfish comes into the world only as the production of parent starfish, or that a new earthworm can be produced only by other earthworms. But naturalists have proved these statements. All life comes from life; all organisms are produced by other organisms. And new individuals are produced by other individuals of the same kind. That these statements are true all modern observations and investigations of the origin of new individuals prove. But in the days of the earlier naturalists the life of the microscopic organisms like Amœba and Paramœcium, and even that of many of the larger but unfamiliar animals, was shrouded in mystery. And various and strange beliefs were held regarding the origin of new individuals.
Spontaneous generation.—The ancients believed that many animals were spontaneously generated. The early naturalists thought that flies arose by spontaneous generation from the decaying matter of dead animals. Frogs and many insects were thought to be generated spontaneously from mud, and horse-hairs in water were thought to change into water-snakes. But such beliefs were easily shown to be based on error, and have been long discarded by zoologists. But the belief that the microscopic organisms, such as bacteria and infusoria, were spontaneously generated in stagnant water or decaying organic liquids was held by some naturalists until very recent times. And it was not so easy to disprove the assertions of such believers. If some water in which there are apparently no living organisms, however minute, be allowed to stand for a few days, it will come to swarm with microscopic plants and animals. Any organic liquid, as a broth or a vegetable infusion, exposed to the air for a short time becomes foul through the presence of innumerable microscopic organisms. But it has been certainly proved that these organisms are not spontaneously produced in the water or organic fluid. A few of them enter the water from the air, in which there are always greater or less numbers of spores of microscopic organisms. These spores germinate quickly when they fall into water or some organic liquid, and the rapid succession of generations soon gives rise to the hosts of bacteria and one-celled animals which infest all standing water. If all the active organisms and inactive spores in a glass of water are killed by boiling the water, and this sterilized water be put into a sterilized glass, and this glass be so well closed that germs or spores cannot pass from the air without into the sterilized liquid, no living animals will ever appear in it. We know of no instance of the spontaneous generation of animals, and all the animals whose life-history we know are produced by other animals of the same kind.
Simplest multiplication and development.—The simplest method of multiplication and the simplest kind of development shown among animals are exhibited by such simple animals as Amœba and Paramœcium. The production of new individuals is accomplished in Amœba by a simple division or fission of its body (a single cell) into two practically equivalent parts. An Amœba which has grown for some time contracts all of its finger-like processes, the pseudopodia, and its body becomes constricted. This constriction or fissure increases inwards so that the body is soon divided fairly in two. There are now two Amœbæ, each half the size of the original one; each, indeed, actually one-half of the original one. The original Amœba was the parent; the two halves of it are the young. Each of the young possesses all of the characteristics and powers of the parent; each can move, eat, feel, grow, and reproduce by fission. The only change necessary for the young or new Amœba to become like its parent, is that of simple growth to a size about twice its present size. The development here is reduced to a minimum. Just as the simplest animals perform the other life-processes, such as taking and digesting food, breathing and feeling, in an extremely primitive simple way, so do they perform the necessary life-process of reproduction or multiplication in the simplest way shown among animals.
In the case of Paramœcium the process of multiplication is slightly more complex than that of Amœba in the fact that sometimes before the simple fission of the body takes place the interesting phenomenon of conjugation occurs. Paramœcium may reproduce itself for many generations by simple fission, but a generation finally appears in which conjugation takes place. Two individuals come together and each exchanges with the other a part of its nucleus. Then the two individuals separate and each divides into two. The result of the conjugation, or the coming together, of two individuals with mutual interchange of nuclear substance is to give to the new Paramœcia produced by the conjugating individuals a body which contains part of the body-substance of two distinct individuals. If the two conjugating individuals differ at all—and they always do differ, because no two individual animals, although belonging to the same species, are exactly alike—the new individual, made up of parts of each of them, will differ slightly from both. Nature seems intent on making every new individual differ slightly from the individual which precedes it. And the method of multiplication which Nature has adopted to produce the result is the method which we have seen exhibited in its simplest form in the case of Paramœcium—the method of having two individuals take part in the production of a new one.
The development of the new Paramœcia is a little more complex than that of Amœba. Not only must the new Paramœcium grow to the size of the original one, but it must develop those slight, but apparent, modifications of the parts of its body which we can recognize in the full-grown, fully developed Paramœcium individual. A new mouth-opening must develop on the new individual formed of the hinder half of the original Paramœcium and new cilia must be developed. Thus there is a slight advance in complexity of development, just as there is in complexity of structure in Paramœcium as compared with Amœba. In the many-celled animals this complexity of development is carried to an extreme.
Birth and hatching.—When a young animal is born alive, it usually resembles in appearance and structure the parent, although of course it is much smaller, and requires always a certain time to complete its development and become mature. A young kangaroo or opossum is carried for some time after its birth in an external pouch on the mother's body and is a very helpless animal. A young kitten is born with eyes not yet opened and must be fed by the mother for several weeks. On the other hand young Rocky Mountain sheep are able to run about swiftly within a few hours after birth.
Most animals appear first as eggs laid by the mother. This is true of the birds, the reptiles, the fishes, the insects, and most of the hosts of invertebrate animals. This egg may be cared for by the parent as with the birds, or simply deposited in a safe place as with most insects, or perhaps dropped without care into the water as with most marine invertebrates. The young animal which issues from the egg may at the time of its hatching resemble the parent in appearance and structural character (although always much smaller) as with the birds, some of the insects, and many of the other animals. Or it may issue in a so-called larval condition, in which it resembles the parent but slightly or not at all, as is the case with the gill-bearing, legless, tailed tadpole of the frog or the crawling, wingless, wormlike caterpillar of the butterfly, or the maggot of the house-fly.
Life-history.—Any animal which hatches from an egg has undergone a longer or shorter period of development within the egg-shell before hatching. The development of an animal from first germ-cell to the time it leaves the egg, for example, the development of the embryo chick from the first cell to time of hatching, is called its embryonic development; and the development from then on, for example, that of the chick to adult hen or rooster, or that of tadpole to frog, is called the post-embryonic development. Beginning students of animals cannot study the embryonic development (embryology) of animals readily, but they can in many cases easily follow the course of the post-embryonic development, and this study will always be interesting and valuable. When the "life-history" of an animal is spoken of in this book, or other elementary text-book of zoology, it is the history of the life of the animal from the time of its birth or hatching to and through adult condition that is meant, not the complete life-history from beginning single egg-cell to the end. In all of the study of the different kinds of animals to which the rest of this book is devoted, attention will be paid to their life-history.
[PART II]
SYSTEMATIC ZOOLOGY
[CHAPTER XIV]
THE CLASSIFICATION OF ANIMALS
Basis and significance of classification.—It is the common knowledge of all of us that animals are classified: that is, that the different kinds are arranged in the mind of the zoologist and in the books of natural history, in various groups, and that these various groups are of different rank or degree of comprehensiveness. A group of high rank or great comprehensiveness includes groups of lower rank, and each of these includes groups of still lower rank, and so on, for several degrees. For example, we have already learned that the toad belongs to the great group of back-boned animals, the Vertebrates, as the group is called. So do the fishes and the birds, the reptiles and the mammals or quadrupeds. But each of these constitutes a lesser group, and each may in turn be subdivided into still lesser groups.
In the early days of the study of animals and plants their classification or division into groups was based on the resemblances and the differences which the early naturalists found among the organisms they knew. At first all of the classifying was done by paying attention to external resemblances and differences, but later when naturalists began to dissect animals and to get acquainted with the structure of the whole body, the differences and likenesses of inner parts, such as the skeleton and the organs of circulation and respiration, were taken into account. At the present time and ever since the theory of descent began to be accepted by naturalists (and there is practically no one who does not now accept it), the classification of animals, while still largely based on resemblances and differences among them, tells more than the simple fact that animals of the same group resemble each other in certain structural characters. It means that the members of a group are related to each other by descent, that is, genealogically. They are all the descendants of a common ancestor; they are all sprung from a common stock. And this added meaning of classification explains the older meaning; it explains why the animals are alike. The members of a group resemble each other in structure because they are actually blood relations. But as their common ancestor lived ages ago, we can learn the history of this descent, and find out these blood relationships among animals only by the study of forms existing now, or through the fragmentary remains of extinct animals preserved in the rocks as fossils. As a matter of fact we usually learn of the existence of this actual blood relationship, or the fact of common ancestry among animals, by studying their structure and finding out the resemblances and differences among them. If much alike we believe them closely related; if less alike we believe them less closely related, and so on. So after all, though the present-day classification means something more, means a great deal more, in fact, than the classification of the earlier naturalists means, it is largely based on and determined by resemblances and differences just as was the old classification. Sometimes the fossil remains of ancient animals tell us much about the ancestry and descent of existing forms. For example, the present-day one-toed horse has been clearly shown by series of fossils to be descended from a small five-toed horse-like animal which lived in the Tertiary age.
Importance of development in determining classification.—A very important means of determining the relationships among animals is by studying their development. If two kinds of animals undergo very similar development, that is, if in their development and growth from egg-cell to adult they pass through similar stages, they are nearly related. And by the correspondence or lack of correspondence, by the similarity or dissimilarity of the course of development of different animals much regarding their relationship to each other is revealed. Sometimes two kinds of animals which are really nearly related come to differ very much in appearance in their fully developed adult condition because of the widely different life-habits the two may have. But if they are nearly related their developmental stages will be closely similar until the animals are almost fully developed. For example, certain animals belonging to the group which includes the crabs, lobsters, and crayfishes, have adopted a parasitic habit of life, and in their adult condition live attached to the bodies of certain kinds of true crabs. As these parasites have no need of moving about, being carried by their hosts, they have lost their legs by degeneration, and the body has come to be a mere sac-like pulsating mass, attached to the host by slender root-like processes, and not resembling at all the bodies of their relatives the crabs and crayfishes. If we had to trust, in making out our classification, solely to structural resemblances and differences, we should never classify the Sacculina (the parasite) in the group Crustacea, which is the group including the crabs and lobsters and crayfishes. But the young Sacculina is an active free-swimming creature resembling the young crabs and young shrimps. By a study of the development of Sacculina we find that it is more closely related to the crabs and crayfishes and the other Crustaceans than to any other animals, although in adult condition it does not at all, at least in external appearance, resemble a crab or lobster.
Scientific names.—To classify animals then, is to determine their true relationships and to express these relationships by a scheme of groups. To these groups proper names are given for convenience in referring to them. These proper names are all Latin or Greek, simply because these classic languages are taught in the schools and colleges of almost all the countries in the world, and are thus intelligible to naturalists of all nationalities. In the older days, indeed, all the scientific books, the descriptions and accounts of animals and plants, were written in Latin, and now most of the technical words used in naming the parts of animals and plants are Latin. So that Latin may be called the language of science. For most of the groups of animals we have English names as well as Greek or Latin ones and when talking with an English-speaking person we can use these names. But when scientific men write of animals they use the names which have been agreed on by naturalists of all nationalities and which are understood by all of these naturalists. These Latin and Greek names of animals laughed at by non-scientific persons as "jaw-breakers," are really a great convenience, and save much circumlocution and misunderstanding.
AN EXAMPLE OF CLASSIFICATION.
Technical Note.—There should be provided a small set of bird-skins which will serve just as well as freshly killed birds, and which may be used for successive classes, thus doing away with the necessity of shooting birds. The birds suggested for use are among the commonest and most easily recognizable and obtainable. They may be found in any locality at any time of the year. The skins can be made by some boy interested in birds and acquainted with making skins, or by the teacher, or can be purchased from a naturalists' supply store, or dealer in bird skins. The skins will cost about 25 cents each. This example or lesson in classification can be given just as well of course with other species of birds, or with a set of some other kinds of animals, if the teacher prefers. Insects are especially available, butterflies perhaps offering the most readily appreciated resemblances and differences.
Species.—Examine specimens of two male downy woodpeckers (the males have a scarlet band on the back of the head). (In the western States use Gardiner's downy woodpecker.) Note that the two birds are of the same size, have the same colors and markings, and are in all respects alike. They are of the same kind; simply two individuals of the same kind of animal. There are hosts of other individuals of this kind of bird, all alike. This one kind of animal is called a species. The species is the smallest[4] group recognized among animals. No attempt is made to distinguish among the different individuals of one kind or species of animal as we do in our own case.
Examine a specimen of the female downy woodpecker. It is like the male except that it does not have the scarlet neck-band. But despite this difference we know that it belongs to the same species as the male downy because they mate together and produce young woodpeckers, male and female, like themselves. There are thus two sorts of individuals,[5] male and female, comprised in each species of animal. A species is a group of animals comprising similar individuals which produce new individuals of the same kind usually after the mating together of individuals of two sexes which may differ somewhat in appearance and structure.
Examine a male hairy woodpecker and a female; (in western States substitute a Harris's hairy woodpecker). Note the similarity in markings and structure to the downy. Note the marked difference in size. Make notes of measurements, colors and markings, and drawings of bill and feet, showing the resemblances and the differences between the downy woodpecker and the hairy woodpecker. These two kinds of woodpeckers are very much alike, but the hairy woodpeckers are always much larger (nearly a half) than the downy woodpeckers and the two kinds never mate together. The hairy woodpeckers constitute another species of bird.
Genus.—Examine now a flicker (the yellow-shafted or golden-winged flicker in the East, the red-shafted flicker in the West). Compare it with the downy woodpecker and the hairy woodpecker. Make notes referring to the differences, also the resemblances. The flicker is very differently marked and colored and is also much larger than the downy woodpecker, but its bill and feet and general make-up are similar and it is obviously a "woodpecker." It is, however, evidently another species of woodpecker, and a species which differs from either the downy or the hairy woodpecker much more than these two species differ from each other. There are two other species of flickers in North America which, although different from the yellow-shafted flicker, yet resemble it much more than they do the downy and hairy woodpeckers or any other woodpeckers. We can obviously make two groups of our woodpeckers so far studied, putting the downy and hairy woodpeckers (together with half a dozen other species very much like them) into one group and the three flickers together into another group. Each of these groups is called a genus, and genus is thus the name of the next group above the species. A genus usually includes several, or if there be such, many, similar species. Sometimes it includes but a single known species. That is, a species may not have any other species resembling it sufficiently to group with it, and so it constitutes a genus by itself. If later naturalists should find other species resembling it they would put these new species into the genus with the solitary species. Each genus of animals is given a Greek or Latin name, of a single word. Thus the genus including the hairy and downy woodpeckers is called Dryobates; and the genus including the flickers is called Colaptes. But it is necessary to distinguish the various species which compose the genus Colaptes, and so each species is given a name which is composed of two words, first the word which is the name of the genus to which it belongs, and, second, a word which may be called the species word. The species word of the Yellow-shafted Flicker is auratus (the Latin word for golden), so that its scientific name is Colaptes auratus. The natural question, Why not have a single word for the name of each species? may be answered thus: There are already known more than 500,000 distinct species of living animals; it is certain that there are no less than several millions of species of living animals; new species are being found, described and named constantly; with all the possible ingenuity of the word-makers it would be an extremely difficult task to find or to build up enough words to give each of these species a separate name. This is not attempted. The same species word is often used for several different species of animals, but never for more than one species belonging to a given genus. And the names of the genera are never duplicated. (There are, of course, much fewer genera than species, and the difficulty of finding words for them is not so serious.) Thus the genus word in the two-word name of a species indicates at once to just what particular genus in the whole animal kingdom the species belongs, while the second or species word distinguishes it from the few or many other species which are included in the same genus. This manner of naming species of animals and plants (for plants are given their scientific names according to the same plan) was devised by the great Swedish naturalist Linnæus in the middle of the eighteenth century and has been in use ever since.
Family.—Examine a red-headed woodpecker (Melanerpes erythrocephalus) and a sapsucker (Sphyrapicus varius) and any other kinds of woodpeckers which can be got. Find out in what ways the hairy and downy woodpeckers (genus Dryobates), the flickers (genus Colaptes) and the other woodpeckers resemble each other. Examine especially the bill, feet, wings and tail. These birds differ in size, color and markings, but they are obviously all alike in certain important structural respects. We recognize them all as woodpeckers. We can group all the woodpeckers together, including several different genera, to form a group which is called a family. A family is a group of genera which have a considerable number of common structural features. Each family is given a proper name consisting of a single word. The family of woodpeckers is named Picidæ.
We have already learned that resemblances between animals indicate (usually) relationship, and that classifying animals is simply expressing or indicating these relationships. When we group several species together to form a genus we indicate that these species are closely related. And similarly a family is a group of related genera.
Order.—There are other groups[6] higher or more comprehensive than families, but the principle on which they are constituted is exactly the same as that already explained. Thus a number of related families are grouped together to form an order. All the fowl-like birds, including the families of pheasants, turkeys, grouse and quail, all obviously related, constitute the order of gallinaceous birds called Gallinæ. The families of vultures, hawks and owls constitute the order of birds of prey, the Raptores, and the families of the thrushes, wrens, warblers, sparrows, black-birds, and many others constitute the great order of perching birds (including all the singing birds) called the Passeres.
Class and branch.—But it is evident that all of these orders, together with the other bird orders, ought to be combined into a great group, which shall include all the birds, as distinguished from all other animals, as the fishes, insects, etc. Such a group of related orders is called a class. The class of birds is named Aves. There is a class of fishes, Pisces, and one of frogs and salamanders, Batrachia, one of snakes and lizards called Reptilia, and one of the quadrupeds which give milk to their young called Mammalia. Each of these classes is composed of several orders, each of which includes several families and so on down. But these five classes of Pisces, Batrachia, Reptilia, Aves and Mammals agree in being composed of animals which have a backbone or a backbone-like structure, while there are many other animals which do not have a backbone, such as the insects, the starfishes, etc. Hence these five backboned classes may be brought together into a higher group called a branch or phylum. They compose the branch of backboned animals, the branch Vertebrata; all the animals like the starfishes, sea-urchins and sea-lilies which have the parts of their body arranged in a radiate manner compose the branch Echinodermata; all the animals like the insects and spiders and centipedes and crabs and crayfishes which have the body composed of a series of segments or rings and have legs or appendages each composed of a series of joints or segments make up the branch Arthropoda. And so might be enumerated all the great branches or principal groups into which the animal kingdom is divided.
In the remainder of this book the classification of animals is always kept in sight, and the student will see the terms species, genus, family, order, etc., practically used. In it all should be kept constantly in mind the significance of classification, that is, the existence of actual relationships among animals through descent.
[CHAPTER XV]
BRANCH PROTOZOA: THE ONE-CELLED ANIMALS
Of this group the structure and life-history of the Amœba (Amœba sp.) and the Slipper Animalcule (Paramœcium sp.) have already been treated in Chapter [VI]. Another example is the
BELL ANIMALCULE (Vorticella sp.)
Technical Note.—Specimens of Vorticella may usually be found in the same water with Amœba and Paramœcium. The individuals live together in colonies, a single colony appearing to the naked eye as a tiny whitish mould-like tuft or spot on the surface of some leaf or stem or root in the water. Touch such a spot with a needle, and if it is a Vorticellid colony it will contract instantly. Bring bits of leaves, stems, etc., bearing Vorticellid colonies into the laboratory and keep in a small stagnant-water aquarium (a battery-jar of pond-water will do).
Examine a colony of Vorticella in a watch-glass of water or in a drop of water on a glass slide under the microscope. Note the stemmed bell-shaped bodies which compose the colony. Each bell and stem together form an individual Vorticella (fig. [8].) How are the members of the colony fastened together? Tap the slide and note the sudden contraction of the animals; also the details of contraction in the case of an individual. Watch the colony expand; note the details of this movement in the case of an individual.
Make drawings showing the colony expanded and contracted.
Fig. 8.—Vorticella sp.;
one individual with
stalk coiled, and one
with stalk extended.
(From life.)
With higher power examine a single individual. Note the thickened, bent-out, upper margin of the bell. This margin is called the peristome. With what is it fringed? The free end of the bell is nearly filled by a central disk, the epistome, with arched upper surface and a circlet of cilia. Between the epistome and peristome is a groove, the mouth or vestibule, which leads into the body. Study the internal structure of the transparent, bell-shaped body. Note the differentiation of the protoplasm comprising the body into an inner transparent colorless endosarc containing various dark-colored granules, vacuoles, oil-drops, etc., and an outer uniformly granular ectosarc not containing vacuoles. Is the stalk formed of ectosarc or endosarc or of both? Note the curved nucleus lying in the endosarc. (This may be difficult to distinguish in some specimens.) Note the numerous large circular granules, the food vacuoles. Note the contractile vesicle, larger and clearer than the food vacuoles. Note the thin cuticle lining the whole body externally. A high magnification will show fine transverse ridges or rows of dots on the cuticle.
Make a drawing showing the internal structure.
Observe a living specimen carefully for some time to determine all of its movements. Note the contraction and extension of the stalk, the movements of the cilia of peristome and epistome, the flowing or streaming of the fluid endosarc (indicated by the movements of the food vacuoles), the behavior of the contractile vesicle.
Make notes and drawings explaining these motions.
Specimens of Vorticella may perhaps be found dividing, or two bell-shaped bodies may be found on a single stem, one of the bodies being sometimes smaller than the other. These two bodies have been produced by the longitudinal division or fission of a single body. In this process a cleft first appears at the distal end of the bell-shaped body, and gradually deepens until the original body is divided quite in two. The stalk divides for a very short distance. One of the new bell-shaped bodies develops a circlet of cilia near the stalked end. After a while it breaks away and swims about by means of this basal circlet of cilia. Later it settles down, becomes attached by its basal end, loses its basal cilia and develops a stalk.
"Conjugation occurs sometimes, but it is unlike the conjugation of Paramœcium in two important points: Firstly, the conjugation is between two dissimilar forms; an ordinary large-stalked form, and a much smaller free-swimming form which has originated by repeated division of a large form. Secondly, the union of the two is a complete and permanent fusion, the smaller being absorbed into the larger. This permanent fusion of a small active cell with a relatively large fixed cell, followed by division of the fused mass, presents a striking analogy to the process of sexual reproduction occurring in higher animals."
OTHER PROTOZOA
Besides the Amœba, Paramœcium, and Vorticella there are thousands of other Protozoa. Most of them live in water, but a few live in damp sand or moss, and some live inside the bodies of other animals as parasites. Of those which live in water some are marine, while others are found only in fresh-water streams and lakes.
Fig. 9.—Sun animalcule, a fresh-water protozoan with a siliceous skeleton, and long thread-like protoplasmic prolongations. (From life.)
Form of body.—The Protozoa all agree in having the body composed for its whole lifetime of a single cell,[7] but they differ much in shape and appearance. Some of them are of the general shape and character of Amœba, sending out and retracting blunt, finger-like pseudopodia, the body-mass itself having no fixed form or outline but constantly changing. Others have the body of definite form, spherical, elliptical, or flattened, enclosed by a thin cuticle, and having a definite number of fine thread-like or hair-like protoplasmic prolongations called flagella or cilia. Many of the familiar Protozoa of the fresh-water ponds always have two whiplash-like flagella projecting from one end of the body. By means of the lashing of these flagella in the water the tiny creature swims about. Others have many hundreds of fine short cilia scattered, sometimes in regular rows, over the body-surface. The Protozoan swims by the vibration of these cilia in the water.
Fig. 10.—Stentor sp.; a protozoan
which may be fixed, like Vorticella,
or free-swimming, at will, and
which has the nucleus in the shape
of a string or chain of bead-like
bodies. The figure shows a single
individual as it appeared when fixed,
with elongate, stalked body, and as
it appeared when swimming about
with contracted body. (From life.)
There is no stagnant pool, no water standing exposed in watering-trough or barrel which does not contain thousands of individuals of the one-celled animals. And in any such stagnant water there may always be found several or many different kinds or species. A drop of this water examined with the compound microscope will prove to be a tiny world (all an ocean) with most of its animals and plants one-celled in structure. A few many-celled animals will be found in it preying on the one-celled ones. There are sudden and violent deaths here, and births (by fission of the parent) and active locomotion and food-getting and growth and all of the businesses and functions of life which we are accustomed to see in the more familiar world of larger animals.
Marine Protozoa.—One usually thinks of the ocean as the home of the whales and the seals and the sea-lions, and of the countless fishes, the cod, and the herring, and the mackerel. Those who have been on the seashore will recall the sea-urchins and starfishes and the sea-anemones which live in the tide-pools. On the beach there are the innumerable shells, too, each representing an animal which has lived in the ocean. But more abundant than all of these, and in one way more important than all, are the myriads of the marine Protozoa.
Although the water at the surface of the ocean appears clear and on superficial examination seems to contain no animals, yet in certain parts of the ocean (especially in the southern seas) a microscopical examination of this water shows it to be swarming with Protozoa. And not only is the water just at the surface inhabited by one-celled animals, but they can be found in all the water from the surface to a great depth below it. In a pint of this ocean-water there may be millions of these minute animals. In the oceans of the world the number of them is inconceivable. And it is necessary that these Protozoa exist in such great numbers, for they and the marine one-celled plants (Protophyta) supply directly or indirectly the food for all the other animals of the ocean.
Among all these ocean Protozoa none are more interesting than those belonging to the two orders Foraminifera (fig. [11]) and Radiolaria. The many kinds belonging to these orders secrete a tiny shell (of lime in the Foraminifera, of silica in the Radiolaria) which encloses most of the one-celled body. These minute shells present a great variety of shape and pattern, many being of the most exquisite symmetry and beauty. The shells are perforated by many small holes through which project long, delicate, protoplasmic pseudopodia. These fine pseudopodia often interlace and fuse when they touch each other, thus forming a sort of protoplasmic network outside of the shell. In some cases there is a complete layer of protoplasm—part of the body protoplasm of the Protozoan—surrounding the cell externally.
Fig. 11.—Rosalina varians, a marine protozoan (Foraminifera) with calcareous shell. (After Schultze.)
When these tiny animals die their hard shells sink to the bottom of the ocean, and accumulate slowly, in inconceivable numbers, until they form a thick bed on the ocean floor. Large areas of the bottom of the Atlantic Ocean are covered with this slimy ooze, called Foraminifera ooze or Radiolaria ooze, depending on the kinds of animals which have formed it. Nor is it only in present times that there has been a forming of such beds by the marine Protozoa. All over the world there are thick rock strata composed almost exclusively of the fossil shells of these simplest animals. The chalk-beds and cliffs of England, and of France, Greece, Spain, and America, were made by Foraminifera. Where now is land were once oceans the bottoms of which have been gradually lifted above the water's surface. Similarly the rock called Tripoli found in Sicily and the Barbadoes earth from the island of Barbadoes are composed of the shells of ancient Radiolaria.
It is thus evident that the Protozoa is an ancient group of animals. As a matter of fact zoologists are certain that it is the most ancient of all animal groups. All of the animals of the ocean depend upon the marine Protozoa and the marine Protophyta, one-celled plants, for food. Either they feed on them directly, or prey on animals which in turn prey on these simplest organisms. A well-known zoologist has said: "The food-supply of marine animals consists of a few species of microscopic organisms which are inexhaustible and the only source of food for all the inhabitants of the ocean. The supply is primeval as well as inexhaustible, and all the life of the ocean has gradually taken shape in direct dependence on it." The marine Protozoa are the only animals which live independently; they alone can live or could have lived in earlier ages without depending on other animals. They must therefore be the oldest of marine animals. By oldest is meant that their kind appeared earliest in the history of the world, and as it is certain that ocean life is older than terrestrial life—that is, that the first animals lived in the ocean—it is obvious that the marine Protozoa are the most ancient of all animal groups.
As already learned in the examination of examples of one-celled animals, it is evident that life may be successfully maintained without a complex body composed of many organs performing their functions in a specialized way. The marine Protozoa illustrate this fact admirably. Despite their lack of special organs and their primitive way of performing the life-processes, that they live successfully is shown by their existence in such extraordinary numbers. They outnumber all other animals. The conditions of life in the surface-waters of the ocean are easy and constant, and a simple structure and simple method of performing the necessary life-processes are wholly adequate for successful life under these conditions.
[CHAPTER XVI]
BRANCH PORIFERA: THE SPONGES
THE FRESH-WATER SPONGE (Spongilla sp.)
Technical Note.—Fresh-water sponges may perhaps not be readily found in the neighborhood of the school, but they occur over most of the United States, and careful searching will usually result in the finding of specimens. They are compact, solid-looking masses, sometimes lobed, resting on and attached to rocks, logs, timbers, etc., in clear water in creeks, ponds, or bayous. They are creamy, yellowish-brown or even greenish in color and resemble some cushion-like plant far more than any of the familiar animal forms. They can be distinguished from plants, however, by the fact that there are no leaves in the mass, nor long thread-like fibres such as compose the masses of pond algæ (pond scum). When touched with the fingers a gritty feeling is noticeable, due to the presence of many small stiff spicules. Sponges should be removed entire from the substance they are attached to, and may be taken alive to the laboratory. They die soon, however, and should be put into alcohol before decay begins.
Note the form of the sponge mass. Is it lobed or branched? Examine the surface for openings. These are of two sizes; the larger are osteoles or exhalant openings, while the smaller and more numerous are pores or inhalant openings. The sponge-flesh is called sarcode. Examine a bit of sarcode under the microscope; note the spicules. Have these spicules a regular arrangement? Of what are they composed?
Draw the entire sponge, showing shape and openings; draw some of the spicules.
Embedded in the body-substance, especially near the base, note (if present) numerous small, yellowish, sub-spherical or disk-like bodies, the gemmules. These are reproductive bodies. Each gemmule is a sort of internal bud. It is composed of an interior group of protoplasmic cells, enclosed by a crust thickly covered with spicules. In winter the sponge dies down and the gemmules are set free in the water. In spring the protoplasmic contents issue through an aperture in the crust, called the micropyle or foraminal opening, and develop and grow into a new sponge.
For a good account of the fresh-water sponge, see Pott's "Fresh-water Sponges."
A CALCAREOUS OCEAN-SPONGE (Grantia sp.) (fig. [7], D, E, F.)
Technical Note.—For inland schools, specimens preserved in alcohol or formalin must be used. They may be obtained from dealers in naturalists' supplies (see p. [453]). Specimens of some species of this genus can be obtained at almost any point on the Atlantic or Pacific coasts of this country.
Examine the external structure of a specimen. Note the elongate, sub-cylindrical form, the attached base, the free end. Note the large exhalant opening, osteole or osculum, at the free end; the numerous small inhalant openings elsewhere on the surface (best seen in dried specimens). Note the spicules covering the surface of the body, and the longer ones surrounding the osculum. Cut the sponge in two longitudinally and note the simple cylindrical body-cavity, the gastric cavity or cloaca. Note the thickness of the body-wall; note the tubes running through the body-wall from cloaca to external surface. Through these tubes water laden with food enters the gastric cavity, where the food is digested, the water and undigested particles passing out through the osculum. Crush a bit of dried sponge, or boil a bit of soft sponge in caustic potash and mount on a glass slide. Examine under a microscope and note the abundance of spicules and the variety in their form. Two kinds may always be found, and sometimes three. These spicules are composed of carbonate of lime and can be dissolved by pouring on to them a drop of hydrochloric acid.
Some of the sponges may have buds growing out from them near the base. These buds are young sponges developed asexually. If allowed to develop fully the buds would have detached themselves from the parent and each would have become a new sponge.
Make drawings showing the form of a whole sponge; the appearance of the inner face of the sponge bisected longitudinally; the shape of the spicules.
A COMMERCIAL SPONGE
Technical Note.—For the study of the skeleton of an ocean-sponge with more complex body buy several common small bath-sponges without large holes running entirely through them. The teacher should have also a few specimens of small marine sponges preserved in alcohol or formalin. Such specimens should be part of the laboratory equipment (see account of laboratory equipment, p. 450), and can be readily and cheaply obtained from dealers in naturalists' supplies.
The bath-sponge or slate-sponge consists simply of the hard parts or skeleton of a sponge animal. In life all of the skeleton is enclosed or covered by a soft, tough mass composed of layers of cells. Note the many openings on the surface of the sponge. Crush a bit of the skeleton and examine it under the microscope. Note that it is composed of fine fibres of a tough, horny substance called spongin, instead of tiny distinct calcareous spicules.
OTHER SPONGES
The sponges are fixed, plant-like aquatic animals. The members of a single family live in fresh water, being found in lakes, rivers, and canals in all parts of the world. All the other sponges, and there are several thousand species known, live in the ocean. They are to be found at all depths, some in shallow water near the shore and others in deeper water, even to the deepest depths yet explored. They are found in all seas, though especially abundantly in the Atlantic Ocean and Mediterranean Sea.
Fig. 12.—The skeleton of a
"glass" sponge (skeleton
composed of siliceous spicules)
from Japan. (From specimen.)
Form and size.—The shape of the simplest sponges is that of a tiny vase or nearly cylindrical cup, hollow and attached at its base. At the free end there is a large opening. But there is a great deal of variety in the form and size of different sponges. There is, indeed, much variation in the shape and general character of different individuals of the same species. Unlike most other animals, sponges are fixed, and the character of the surface to which a sponge is attached has much influence upon its shape. If this surface is rough and uneven the sponge may follow in its growth the sinuosities of the surface and so become uneven and distorted in shape. At best, only a few kinds of sponges have any very even and symmetrical shape. Most of them are very unsymmetrical and grow more like a low compact bushy plant than like the animals we are familiar with. The smallest sponges are only 1 mm. (1/25 in.) high, while the largest may be over a meter (39 in.) in height. In color living sponges may be red, purple, orange, gray, and sometimes blue. Most sponges have the whole body of one color.
Skeleton.—A very few sponges have no skeleton at all. The others have a skeleton or hard parts composed of interwoven fibres of the tough, horny substance called spongin, or of hosts of fine needles or spicules of silica or of carbonate of lime. The siliceous skeletons of some of the so-called glass-sponges (fig. [12]) are very beautiful. The lime and siliceous sponge spicules exhibit a great variety of outline, some being anchor-shaped, some cross-shaped, and some resembling tiny spears or javelins.
Structure of body.—The skeleton of a sponge whether composed of interlacing fibres or of short spicules is always invisible from the outside when the sponge is alive. It is embedded in, or clothed by, the soft, fleshy part of the body. The soft part of the sponge is composed simply of two layers of cells, one constituting the external surface of the body, and the other lining the interior cavities and canals of the body. Between these two cell-layers there is a mass of soft gelatinous substance all through which protoplasm ramifies, and in which are embedded numerous scattered cells. There are, as seen in the case of Spongilla and Grantia, no systems of organs such as characterize the higher animals. No heart, lungs, alimentary canal, nervous system, organs of locomotion, eyes, ears, or other organs of special sense; the sponge has none of these. It is simply an aggregate of cells, arranged in two layers, and supported usually by a skeleton of horny fibres or calcareous or siliceous spicules. Its body is usually shapeless, unsymmetrical and without front or back, right or left. It is not to be wondered at that sponges were for a long time believed to be plants.
Feeding habits.—The sponges feed on minute bits of animal or plant substance and on the microscopic unicellular plants or animals which float in the water which bathes their bodies. The water entering the sponge-body through the various openings of the surface is moved along by the waving or lashing of the flagella of the cells which line the canals, and these currents of water bear with them the tiny organisms which are taken up by these same cells and digested. The incoming currents of water meet in the central cavity or cavities of the body and pass out through the large opening called the osculum at the free end of the vase-like body, or if the body is branched, through the large openings at the tips of these branches.
The same currents of water bring also oxygen for the sponge's breathing and carry away the carbonic acid gas given out by the body-cells.
As a German naturalist has said, the one necessary condition for the life of a sponge is the streaming of water through its body. All sponges have a system of canals for this water-current and all have means, in the waving flagella or cilia with which these canals are lined, for producing these currents. When a live sponge is put into a vessel of water, currents are immediately set up, and they always flow into the body through the many fine openings and out of the body through the osculum.
Development and life-history.—Although the sponge in its adult condition is permanently attached by its base to the sea-bottom or to some rock or shell, when it is first born it is an active free-swimming creature. The sponges reproduce in two ways, asexually and sexually. The asexual mode of reproduction of the fresh-water sponge by gemmules has already been described. The ocean sponges also reproduce asexually either by forming interior gemmules or external buds. In this latter method a bud forms on the outer surface of the body which increases in size and finally grows into a new sponge individual. In some species this new sponge does not become separated from the body of the mother, but remains attached to it like a branch to a tree-trunk. By the continued production of such non-separating individuals, a colony of sponges is formed which has the general appearance of a branching plant. In other species the new sponge formed by the development and growth of a bud falls off and becomes a distinct separate individual.
In the sexual mode of reproduction, male or sperm-cells and female or egg-cells are developed in the same individual. The sperm-cells are motile and swim about in the cavities and canals of the sponge-body until they find egg-cells, which they fertilize. The fertilized eggs begin to develop and pass through their first stages in the sponge-body. Finally the embryo sponge, which is usually a tiny oval or egg-shaped mass of cells, escapes from the body of the parent into the water. The young sponge has some of its outer cells provided with cilia, and by means of these it swims about. After a while it comes to rest on the ocean-floor or on some rock or shell, attaches itself, and begins to take on the form and character of the parent. It leads hereafter a fixed sedentary life.
The sponges of commerce.—The sponge-skeletons which are the "sponges" that we use all belong to a few species, not more than half a dozen. Most of the commercial sponges come from the Mediterranean Sea, though some come from the Bahama Islands, some from the Red Sea, and a few from the coasts of Greece, Asia Minor, and Africa. The commercial sponges do not live in very deep water; they are usually found not deeper than 200 feet. The living sponges are collected by divers, or are dragged up by men in boats using long-poled hooks, or dredges. "When secured they are exposed to the air for a limited time, either in the boats or on shore, and then thrown in heaps into the water again in pens or tanks built for the purpose. Decay thus takes place with great rapidity, and when fully decayed they are fished up again, and the animal matter beaten, squeezed, or washed out, leaving the cleaned skeleton ready for the market. In this condition after being dried and sorted, they are sold to the dealers, who have them trimmed, re-sorted and put up in bales or on strings ready for exportation. There are many modifications of these processes in different places, but in a general way these are the essential-steps through which the sponge passes before it is considered suitable for domestic purposes. Bleaching-powders or acids are sometimes used to lighten the color, but these unless very delicately handled injure the durability of the fibres."
Classification.—The sponges are classified according to the character of the skeleton. In one group are put all those sponges which have a skeleton of calcareous spicules, and this group is called the Calcarea. All other sponges are grouped as Non-Calcarea, the members of this group either having no skeleton at all, or having a skeleton composed of siliceous spicules or of spongin fibres. According to the absence or presence of a skeleton and the character of the skeleton when it exists the Non-Calcarea are subdivided into smaller groups.
[CHAPTER XVII]
BRANCH CŒLENTERATA: THE POLYPS, SEA-ANEMONES, CORALS, AND JELLYFISHES
The structure and life-history of an example of the polyps (the Fresh-water Hydra, Hydra sp.) has been studied in Chapters [X] and [XI].
OTHER POLYPS, SEA-ANEMONES, CORALS, AND JELLYFISHES
Technical Note.—The teacher should have, if possible, several pieces of coral and a few specimens of Cœlenterates in alcohol or formalin, which will show the external character, at least, of these animals (see account of laboratory equipment, p. [450]). If the school is on the coast, the pupils should be shown the sea-anemones of the tide-pools.
The animals which are included in the branch Cœlenterata are, at least in living condition, unfamiliar to most of us. Like the sponges, they are almost all inhabitants of the ocean; a few, like Hydra, live in fresh water. Like the sponges, too, most of the members of this branch are fixed, and in their general appearance suggest a plant rather than an animal. The name zoophytes, or plant-animals, which is often applied to these animals is based on this superficial resemblance. But many of the Cœlenterates lead an active free-swimming life. This is true of the jellyfishes which float or swim about on or near the surface of the ocean. Many of the zoophytes spend part of their life in an active free-swimming condition before settling down, becoming attached and thereafter remaining fixed. In localities near the seashore many animals belonging to this great group can be readily found and observed. The beautiful sea-anemones with their slowly-waving tentacles, the fine many-branched truly plant-like hydroids with their hosts of little buds, and the soft colorless masses of jelly, the jellyfishes, which are cast up on to the beaches by the waves are all animals belonging to the branch Cœlenterata.
General form and organization of body.—The general or typical plan of body-structure for the Cœlenterata, these animals which come next to the sponges in degree of complexity, can best be understood by imagining the typical cylindrical or vase-like body of the simple sponges to be modified in the following way: The middle one of the three layers of the body-wall not to be composed of scattered cells in a gelatinous matrix, but to be simply a thin non-cellular membrane; the body-wall not to be pierced by fine openings or pores, but connected with the outside only by the single large opening at the free end, and this opening to be surrounded by a circlet of arm-like processes or tentacles, which are continuations of the body-wall and similarly composed. Such a body-structure, which we saw well shown by Hydra, is the fundamental one for all polyps, sea-anemones, corals, and jellyfishes. The variety in shape of the body and the superficial modifications of this type-plan are many and striking, but after all the type-plan is recognizable throughout the whole of this great group of animals.
The two chief body-shapes represented in the branch are those of the polyps on the one hand, and the jellyfishes or medusæ on the other. The polyp-shape is that of a tube with a basal end blind or closed, attached to some firm object in the water and with the free end with an opening, the mouth-opening. At this mouth-end there is a circlet of movable, very contractile tentacles. The mouth may open directly into the interior of the body, which interior may be called the digestive cavity, or it may lead into a simple short tube produced by the invagination or bending in of the body-wall, which may be looked on as the simplest kind of œsophagus. This œsophageal tube opens into the body-cavity or digestive cavity. This cavity may be incompletely divided by longitudinal partitions which project from the sides into the cavity.
The jellyfish or medusoid body-form corresponds in general to an umbrella or bell. Around the edge of this umbrella are disposed numerous threads or tentacles (corresponding to the circlet of tentacles in the polyp). The mouth-opening is at the end of a longer or shorter projection which hangs down from the middle of the under side of the umbrella. The interior body-cavity or digestive cavity extends out into the umbrella-shaped part of the body, usually in the condition of canals radiating from the centre and a connecting canal running around the margin of the umbrella.
Structure.—Although the Cœlenterata show little indication of the complex composition of the body out of organs, as it exists among the higher animals, yet they do show an unmistakable advance on the simple, almost organless body of the sponges. This is chiefly shown by the differentiation among the cells which compose the body. In the polyps and jellyfishes some of the cells are specialized to be unmistakable muscle-cells, some to be nerve-cells and fibres, and so on. A very simple nervous system consisting of small groups of nerve-cells connected by nerve-fibres exists. Some very simple special sense-organs may occur. The digestive system, although in the simpler Cœlenterates consisting merely of the cylindrical body-cavity enclosed by the body-wall and opening by the single hole at the free end of the body, in some is rather complex and is composed of different parts. Those Cœlenterates which are not fixed but lead an active, free-swimming life, viz., the jellyfishes or medusæ, are the most highly organized.
The tentacles which surround the mouth-opening and serve to grasp food and carry it into the mouth, and the stinging or lasso threads with which these tentacles are provided are special organs possessed by most of these animals.
Skeleton.—Like the sponges, some of the Cœlenterata possess a hard skeleton. This skeleton is always composed of calcium carbonate and is called coral. Those polyps which form such a skeleton are called the corals. Coral will be described in connection with the account of the coral-polyps.
Development and life-history.—The polyps and jellyfishes reproduce both asexually and sexually. The asexual mode is usually that of budding. On a polyp a bud is formed by a hollow outgrowth of the body-wall. The bud grows, an opening appears at its distal end, a circlet of tentacles arises about this mouth-opening and a new polyp individual is formed. This individual may separate from the parent or it may remain attached to it. By the development of numerous buds, and the remaining attached of all of the individuals developing from these buds, a colony of polyp individuals may be formed, plant-like in appearance. The various polyp individuals of a colony may differ somewhat among themselves, and these differences are correlated with a division of labor. Thus some of the individuals may devote themselves to getting food for the colony, and these have mouth and tentacles. Others may be devoted to the production of new individuals by budding or by producing germ-cells, and may not have any mouth-opening or any food-grasping tentacles.
In case of many polyps all or some of the new individuals which arise by budding do not become polyps, but develop into medusæ or jellyfish, which separate from the fixed polyp and swim off through the water. These medusæ or jellyfish produce sperm-cells and egg-cells. The sperm-cells fertilize the egg-cells and a new individual develops from each fertilized egg. This new individual is at first an active free-swimming larva called a planula, which does not resemble either a medusa or polyp. After a while it settles down, becomes fixed and develops into a polyp. Thus a polyp may produce a medusa or jellyfish which, however, produces not a new jellyfish, but a polyp. This is called an alternation of generations, and is not an uncommon phenomenon among the lower animals. It results from such an alternation of generations that a single species of animal may have two distinct forms. This having two different forms is called dimorphism. Sometimes, indeed, a species may appear in more than two different forms; such a condition is called polymorphism.
Not all medusæ or jellyfish are produced by polyp individuals, nor do jellyfish always produce polyps and not jellyfishes. There are some jellyfishes (we might call them the true jellyfishes) which always have the jellyfish form, producing new jellyfishes either by budding or by eggs, and there are some polyps which always have the true polyp form, producing new individuals, either by budding or by eggs, always of polyp form and never of jellyfish form. That is, some species of Cœlenterata exist only in polyp form, some species exist only in jellyfish form, while some species (those having an alternation of generations) exist in both polyp and jellyfish form, these two forms appearing as alternate generations.
Classification.—The branch Cœlenterata is divided into four classes: (1) the Hydrozoa, including the fresh-water polyps, numerous marine polyps, many small jellyfishes and a few corals; (2) the Scyphozoa, including most of the large jellyfishes; (3) the Actinozoa, including the sea-anemones and most of the stony corals; (4) the Ctenophora, including certain peculiar jellyfishes.
Fig. 13.—The Portuguese Man-of-War (Physalia sp.). (From specimen from Atlantic Coast.)
The polyps, colonial jellyfishes, etc. (Hydrozoa).—To the class Hydrozoa belongs the Hydra already studied. There are a few other fresh-water polyps and they all belong to this class. The most interesting members of the class are the "colonial jellyfishes," constituting the order Siphonophora. These colonial jellyfishes are floating or swimming colonies of polypoid and medusoid individuals in which there is a marked division of labor among the individuals, accompanied by marked differences in structural character. The individuals are accordingly polymorphic, that is, appear in various forms, although all belong to the same species. Because these various individuals forming a colony have given up very largely their individuality, combining together and acting together like the organs of a complex animal, they are usually not called individuals, nor on the other hand organs, but zooids, or animal-like structures. The beautiful "Portuguese man-of-war" (fig. [13]) is one of these colonial jellyfishes. It appears as a delicate bladder-like float, brilliant blue or orange in color, usually about six inches long, and bearing on its upper surface which projects above the water a raised parti-colored crest, and on its under surface a tangle of various appendages, thread-like with grape-like clusters of little bell- or pear-shaped bodies. Each of these parts is a peculiarly modified polyp- or medusa-zooid produced by budding from an original central zooid. The Portuguese man-of-war is very common in tropical oceans, and sometimes vast numbers swimming together make the surface of the ocean look like a splendid flower-garden.
Fig. 14.—A colonial jellyfish (Siphonophora).
(After Haeckel.)
Usually the central zooid in a Siphonophore to which the other zooids are attached is not a bladder-like float, but is an upright tube of greater or less length. In the Siphonophore shown in figure [14], the compound body is composed of a long central hollow stem with hundreds or thousands of variously shaped parts, each of which is reducible to either a polyp or medusazooid, attached around it. The upper end is enlarged to form an air-filled chamber, a sac-like boat, by means of which the whole colony is kept afloat. Around the upper end of the central stem are many medusoid structures, the swimming-bells, by means of whose opening and closing the whole colony is made to swim through the water. Each swimming-bell is a modified medusa-zooid, without tentacles, without digestive or reproductive organs, but exercising the power of swimming by contracting and forcing the water out of the hollow bell just as is done by the free medusæ. Below the swimming-bells, at the lower end of the central stem, are grouped many structures presenting at first sight a confusion of variety and complexity, but on careful examination revealing themselves to be polyp- and medusa-zooids modified to form at least five kinds of particularly functioning structures. There are many flattened scale-like parts whose function is simply that of affording a passive protection, in times of danger, to the other structures. These protecting-scales are greatly modified medusa-zooids, each consisting of a simple cartilage-like gelatinous mass penetrated by a food-carrying canal. Under the broad leaves of these protecting-zooids are a number of pear-shaped bodies which have a wide octagonal mouth-opening at their free end, and possess in their interior certain digestive glands. Each one is provided with a very long flexible tentacle which bears many fine stinging-threads. The tentacle waves back and forth in the water, and on coming in contact with an enemy or with prey its poisonous stinging-threads shoot out and paralyze or wound the unfortunate animal. These pear-shaped bodies are the feeding structures, each being a modified polyp-zooid. Scattered among these dangerous structures are many somewhat similarly shaped but wholly harmless structures, the sense-structures. Each of these has a pear-shaped body but without mouth-opening, and also a long, very sensitive, tentacle-like process. The sense of feeling is highly developed in these tentacles, and they discover for the colony the presence of any strange body. These sense-structures are modified polyp-zooids. Finally there are two other kinds of structures, usually arranged in groups like bunches of grapes, which are the reproductive structures, male and female. They are modified medusa-zooids grown together and without tentacles. This whole colony, or this compound animal, floats or swims about at the surface of the ocean, and performs all of the necessary functions of life as a single animal composed of organs might. Yet the Siphonophore is more truly to be regarded as a community in which the hundreds or thousands of animals, representing five or six kinds of individuals, all of one species, are fastened together. Each individual performs the particular duties devolving upon its kind or class. Thus there are food-gathering individuals, locomotor individuals, sense individuals, and reproductive individuals. The modifications of the various kinds of individuals are more extreme than in the case of the various kinds of individuals composing a bee-community, for example, but the holding together or fusing of all into one body or corporation is a condition which makes this greater modification necessary and not unexpected. And there is no difficulty in seeing that each of these parts is really, structurally considered, a modified polyp or medusa.
Fig. 15.—A jellyfish or medusa, Gonionema vertens, eating two small fishes. (From specimen from Atlantic Coast.)
The large jellyfishes, etc. (Scyphozoa).—To the class Scyphozoa belong most of the common large jellyfishes. When one walks along the sea-beach soon after a storm one may find many shapeless masses of a clear jelly-like substance scattered here and there on the sand. These are the bodies or parts of bodies of jellyfishes which have been cast up by the waves. Exposed to the sun and wind the jelly-like mass soon dries or evaporates away to a small shrivelled mass. The body-substance of a jellyfish contains a very large proportion of water; in fact there is hardly more than 1 per cent of solid matter in it.
The jellyfishes occur in great numbers on the surface of the ocean and are familiar to sailors under the name of "sea-bulbs." Some live in the deeper waters; a few specimens have been dredged up from depths of a mile below the surface. They range in size from "umbrellas" or disks a few millimeters in diameter to disks of a diameter of two meters (2-1/6 yards). They are all carnivorous, preying on other small ocean animals which they catch by means of their tentacles provided with stinging-threads. The tentacles of some of the largest jellyfishes "reach the astonishing length of 40 meters, or about 130 feet." Many of the jellyfishes are beautifully colored, although all are nearly transparent. Almost all of them are phosphorescent, and when irritated some emit a very strong light.
The sea-anemones and corals (Actinozoa).—Almost everywhere along the seashore where there are rocks and tide-pools a host of various kinds of sea-anemones can be found. When the tide is out, exposing the dripping seaweed-covered rocks, and the little sand- or stone-floored basins are left filled with clear sea-water, the brown and green and purple "sea-flowers" may be found fixed to the rocks by the base with the mouth-opening and circlet of slowly-moving tentacles hungrily ready for food (fig. [16]). Touch the fringe of tentacles with your fingertip and feel how they cling to it and see how they close in so as to carry what they feel into the mouth-opening. A host of individuals there are, and scores of different kinds; some small, some large, some with the body covered outside with tiny bits of stone and shell so that they are hardly to be distinguished from the rock to which they cling; some of bright and showy colors. These are the most familiar members of the class Actinozoa.
Fig. 16.—Sea anemones, Bunodes californica, open and closed individuals. The closed individuals in upper right-hand corner show the external covering of small bits of rock and shell, characteristic of most individuals of this species. (From living specimens in a tide-pool on the Bay of Monterey, California.)
But in other oceans, along the coasts of other lands, especially those of the tropics and sub-tropics, there are some other members of the class which are of unusual interest. They are the corals, or coral polyps. We know these animals chiefly by their skeletons (fig. [17]). The specimens of corals which one sees in collections, or made into ornaments, are the calcareous skeletons of various kinds of the coral polyps. Some of the corals live together in enormous numbers, forming branching colonies fixed as closely together as possible, and secrete while living a stony skeleton of carbonate of lime. These skeletons persist after the death of the animals, and because of their abundance and close massing form great reefs or banks and islands. These coral reefs and islands occur only in the warmer oceans. In the Atlantic they are found along the coasts of Southern Florida, Brazil and the West Indies; in the Pacific and Indian Oceans there are great coral reefs on the coast of Australia, Madagascar and elsewhere, and certain large groups of inhabited islands like the Fiji, Society, and Friendly Islands are exclusively of coral formation. Coral islands have a great variety of form, although the elongated, circular, ring-shaped and crescent forms predominate. How such islands are first formed is described as follows by a well-known student of corals:
Fig. 17.—Skeleton of a branching coral, Madrepora cervicornis. (From specimen.)
"A growing coral plantation, with its multitudinous life, oftentimes arises from great depths of the ocean, and the sea-bed upon which it rests is probably a submarine bank or mountain, upon which have lodged and slowly aggregated the hard skeletons of pelagic forms of life. When, through various sources of increase, this submarine bank approaches the depth of from one hundred to one hundred and fifty feet from the surface of the water, there begins on its top a most wonderful vital activity. It is then within the bathymetric zone of the reef-building corals. Of the many groups of marine life which then take possession of the bank, corals are not the only animals, but they are the most important, as far as its subsequent history goes. As the bank slowly rises by their growth, it at last approaches the surface of the water, and at low tide the tips of the growing branches of coral are exposed to the air. This, however, only takes place in sheltered localities, for long before it has reached this elevation it has begun to be more or less changed and broken by the force of the waves. As the submarine bank approaches the tide level, the delicate branching forms have to meet a terrific wave-action. Fragments of the branching corals are broken off from the bank by force of the waves, and falling down into the midst of the growing coral below fill up the interstices, and thus render the whole mass more compact. At the same time larger fragments are broken and rolled about by the waves and are eventually washed up into banks upon the coral plantation, so that the island now appears slightly elevated above the tides. This may be called a first stage in the development of a coral island. It is, however, little more than a low ridge of worn fragments of coral washed by the high tides and swept by the larger waves—a low, narrow island resting on a large submarine bank."
When the coral island rises thus a little above the surface of the water, the waves break up some of the coral into fine sand, which fills in the interstices, and offers a sort of soil in which may germinate seeds brought in the dried mud on the feet of ocean birds or carried by the ocean currents. With the beginning of vegetable growth the soil is more firmly held, is fertilized and ready for the seeds of plants which need a better soil than lime sand. Flying insects find their way to the island, especially if it be near the mainland, birds begin to nest on it, and soon it may be the seat of a luxuriant plant and animal life.
For an account of coral islands see Darwin's "The Structure and Distribution of Coral Reefs."
There are over 2000 kinds of coral polyp known, and their skeletons vary much in appearance. Because of the appearance of the skeleton certain corals have received common names, as the organ-pipe coral, brain coral, etc. The red coral, of which jewelry is made, grows chiefly in the Mediterranean. It is gathered especially on the western coast of Italy, and on the coasts of Sicily and Sardinia. Most of this coral is sent to Naples, where it is cut into ornaments.
There are other interesting members of the class Actinozoa like the beautiful sea-pens, sea-feathers and sea-fans, delicate, branching, tree-like forms found all over the world.
Ctenophora.—The members of this class are mostly small, peculiar jellyfishes which do not form colonies, and are extremely delicate, being usually perfectly transparent. They swim by means of cilia. They never appear in a polyp condition, but are always medusoid in shape.
[CHAPTER XVIII]
BRANCH ECHINODERMATA: STARFISHES, SEA-URCHINS, SEA-CUCUMBERS
STARFISH (Asterias sp.)
Technical Note.—The species of Asterias are widely distributed on both coasts of the United States and may be procured on almost any rocky shore at low tide. Teachers in inland schools can obtain preserved material from the dealers mentioned on p. 453. Most of the specimens should be placed in alcohol or 4% formalin. If fresh material can be had it is well to place at least one specimen for each student in a 20% solution of nitric acid in water for two or three hours, when all of the calcareous parts will have been dissolved, and after a thorough washing the specimen will be ready for use.
External structure (figs. [18] and [19].)—In a fresh specimen or one which has been preserved in alcohol or formalin note the raying out of parts of the body from a common centre. This is characteristic of the body organization of all Echinoderms, and is known as radial symmetry. The lower surface of the body is called the oral (because the mouth is on this surface), while the upper is called the aboral surface. The central part of the body is called the disk. Note on the aboral surface of the disk a small striated calcareous plate, the madreporite or madreporic plate. In the middle (or very nearly in the middle) of this surface of the disk there is a small pore, the anal opening. The entire aboral surface as well as a greater part of the oral side is thickly studded with the calcareous ossicles of the body-wall. These ossicles support numerous short stout spines arranged in irregular rows. Note that some of the ossicles support certain very small pincer-like processes, the pedicellariæ. In the interspaces between the calcareous plates are soft fringe-like projections of the inner body-lining, the respiratory cæca. Note at the tip of each arm or ray a cluster of small calcareous ossicles and within each cluster a small speck of red pigment, the eye-spot or ocellus.
Fig. 18.—Dissection of a starfish (Asterias sp.).
Make a drawing of the aboral surface showing all these parts.
On the oral surface note the centrally-located mouth, the ambulacral grooves, one running longitudinally along each ray, and in each groove two double rows of soft tubular bodies with sucker-like tips. These are called the tube-feet and are organs of locomotion. Make a drawing of the oral surface.
Internal structure (figs. [18] and [19]).—Technical Note.—Take a specimen which has been immersed for some time in the nitric acid solution, and with a strong pair of scissors, or better, bone-cutters, cut away all the aboral wall of the disk except that immediately around the madreporite and the anus. Now begin at the tip of each ray and cut away the aboral wall of each, leaving, however, a single arm intact. When the roof of each arm has been carefully dissected away the specimen should appear as in fig. [18].
Note the large alimentary canal, which is divided into several regions. Note the short œsophagus leading from the mouth on the oral surface directly into a large membranous pouch, the cardiac portion of the stomach. By a short constriction the cardiac portion is separated from the part which lies just above, i.e., the pyloric portion of the stomach. From the pyloric portion large, pointed, paired glandular appendages extend into each ray. These are the pyloric cæca. Their function is digestive, and oftentimes they are spoken of as the digestive glands or "livers." The pyloric cæca, as well as the cardiac portion of the stomach, are held in place by paired muscles which extend into each arm. Note two sets of these muscles, one set for thrusting the cardiac portion of the stomach out through the mouth and another for pulling it back, the protractor muscles and retractor muscles, respectively. The starfish obtains its food by enclosing it in its everted stomach and then withdrawing stomach and food into the body. Note that the pyloric portion of the stomach opens above into a short intestine terminating in the anus, and observe that there is attached to the intestine a convoluted many-branched tube, the intestinal cæcum.
Carefully remove a pair of pyloric cæca from one of the rays and note the short duct which connects them with the pyloric chamber of the stomach. Note in the angle of each two adjoining rays paired glandular masses which empty by a common duct on the aboral surface. These glands are the reproductive organs. Note the small bulb-like bladders extending in two double rows on the floor of each ray. These are the water-sacs or ampullæ, and each one is connected directly with one of the locomotor organs, the tube-feet.
Make a drawing of the organs in the dissection which have so far been studied.
Technical Note.—For a careful study of the locomotor organs a fresh starfish should be injected. This can usually be accomplished by cutting one ray off squarely, and inserting the needle of a hypodermic syringe (which has been previously filled with a watery solution of carmine or Berlin blue), into the end of the radial water-tube which runs along the floor of the ray. By injecting here, the whole system of vessels, tube-feet, and ampullæ are filled.
Note a ring-shaped canal which passes around the alimentary canal near the mouth from which radial vessels run out beneath the floor of each ray and from which a hard tube extends to the madreporite. This hard tube is the stone canal, so called because its walls contain a series of calcareous rings, while the circular tube is the ring canal or circum-oral water-ring from which radiate the radial canals. In some species of starfish there are bladder-like reservoirs, Polian vesicles, which extend interradially from the ring canal.
Note that the ampullæ and tube-feet are all connected with the radial canals. By a contraction of the delicate muscles in the walls of the ampullæ the fluid in the cavity is compressed, thereby forcing the tube-feet out. By the contraction of muscles in the tube-feet they are again shortened while the small disk-like terminal sucker clings to some firm object. In this way the animal pulls itself along by successive "steps." This entire system, called the water-vascular system, is characteristic of the branch Echinodermata. In addition to the fluid in the water-vascular system there is yet another body-fluid, the perivisceral fluid, which bathes all of the tissues and fills the body-cavity.
Fig. 19.—Semi-diagrammatic figure of cross-section of the ray of a starfish, Asterias sp.
Technical Note.—Take a drop of the perivisceral fluid from a living starfish and examine under high power of microscope, noting the amœboid cells it contains.
The perivisceral fluid is aerated through outpocketings of the thin body-wall which extend outward between the calcareous plates of the body. These outpocketings have already been mentioned as the respiratory cæca (see p. [109]). Surrounding the stone canal is a thin membranous tube, and within it and by the side of the stone canal is a soft tubular sac. The function of these organs is not certainly known.
Work out the nervous system; note, as its principal parts, a nerve-ring about the mouth, and nerves running from this ring beneath the radial canals along each arm.
Life-history and habits.—The starfishes are all marine forms. They hatch from eggs, and in their early stages are very different in appearance from the adults. At first they are bilaterally symmetrical, their radial symmetry being acquired later. Thousands of eggs and sperm-cells are extruded into the sea-water, where fertilization and development take place. The young swim freely in the open sea, feeding on microscopic organisms, and then undergo very radical changes in the course of their development. The adults are for the most part carnivorous, feeding on crabs, snails, and the like. The live prey is surrounded by the extruded stomach which secretes fluids that kill it, after which the soft parts are digested. (See general account of the life-history of Echinoderms on p. [119].)
THE SEA-URCHIN (Strongylocentrotus sp.)
External structure.—Technical Note.—If fresh or alcoholic specimens or even the dry "tests" of the sea-urchin (fig. [20]) are to be had, the general characteristics of the external structure can be made out.
How does the external surface of the sea-urchin differ from that of the starfish? Can you find the very long tube-feet? Where is the mouth-opening? With what is it surrounded? Each tooth is enclosed in a calcareous framework. The whole structure is known as "Aristotle's lantern."
Technical Note.—Remove the spines from the underlying shell or test (fig. [21]) and wash the test until perfectly clean, or place in a solution of lye for a short time and then wash.
Fig. 20.—A sea-urchin, Strongylocentrotus franciscanus. (From specimen from Bay of Monterey, Calif.)
Note the characteristic radial symmetry of the shell or test. Note on the aboral aspect, diverging from the medial anal aperture, five double rows of pores. What are these for? Each of the five divisions set with pores is called an ambulacral area, while the intervening segments which support the long spines are called the interambulacral areas. Note on the aboral surface, surrounding the median-placed anal aperture, a series of small plates. Those which are located in the interambulacral areas are the genital plates. Through these plates the ducts from the reproductive organs open by small pores. Note a very much enlarged plate with a striated appearance. This is the madreporite, which, as in the starfish, is the external opening of the stone canal and water-vascular system. Note the small ocular plate at the tip of each ambulacral area. The ocular plates contain small pigment-cells and communicate with the nervous system.
Fig. 21.—"Test" of sea-urchin, Strongylocentrotus franciscanus, with spines removed. (From specimen.)
From a general inspection of the sea-urchin's shell the Echinoderm characteristics, namely, radial symmetry and the presence of the water-vascular system, are readily seen. While at first glance there is apparent little similarity between the starfish and sea-urchin, nevertheless careful examination shows that the two animals are alike in their fundamental structure. Both are radially symmetrical. The position of the anal opening makes both starfish and sea-urchin slightly asymmetrical. In both the madreporite and anus are on the aboral side, while the mouth is centrally located on the oral side. In the starfish we noted five ambulacral areas, one on the under side of each arm; similarly we find five in the sea-urchin. In both cases also we find the ocular spots at the tips of the ambulacral areas. The genital apertures are situated interradially in the starfish. In the sea-urchin they are similarly placed. The dissimilarity between the two forms is largely due to the very much developed outer spines and the dorso-ventral thickening of the disk in the sea-urchin. The starfish is carnivorous, while the sea-urchin lives on vegetable matter consisting for the most part of green algæ and the red sea-weeds. Correlated with this difference in food-habits there are certain differences in the structure of the internal organs. For example, the alimentary canal in the sea-urchin winds in about two and one-half turns within the body-cavity before it reaches the anus.
OTHER STARFISHES, SEA-URCHINS, SEA-CUCUMBERS, ETC.
Without exception all the Echinoderms, under which term are included the starfishes, sea-urchins, brittle-stars, feather-stars, and sea-cucumbers, live in the ocean. Some of them, the starfishes and sea-urchins, are among the most common and familiar animals of the seashore. Most of them are not fixed, but can move about freely, though slowly. Some of the feather-stars are fixed, as the sponges and polyps are.
Shape and organization of body.—The body-shape of the Echinoderm varies from the flat, rayed body of the starfish to the thick, flattened egg-shape of the sea-urchin, the melon-like sac of the sea-cucumber and the delicate many-branched head of the sea-lily sometimes borne on a slender stalk. But in all these shapes can be seen more or less plainly a symmetrical, radiate arrangement of the parts of the body. The Echinoderm body has a central portion from which radiate separate arm or branch-like parts, as in the starfishes and sea-lilies, or about which are arranged radiately the internal body-parts, although the external appearance may at first sight give no plain indication of the radiate arrangement. This is the case with the sea-urchins and sea-cucumbers, yet, as has been seen in the sea-urchin, the radiate arrangement can be readily perceived by closer examination of the surface of the egg- or sac-like body. The radiating parts of the body are usually five. In the body of an Echinoderm can be usually recognized an upper or dorsal surface and a lower or ventral surface. The mouth is usually situated on the ventral side and the anal opening on the dorsal. Echinoderms agree also in having a calcareous outer skeleton or body-wall usually in the condition of definitely-shaped plates or spicules fitted either movably or rigidly together. This outer body-wall or exoskeleton may bear many tubercles or spines. These spines are sometimes movable. The body-wall of the sea-urchin shows very well the exoskeleton composed of plates on which are borne movable strong spines.
Structure and organs.—As has been learned from the dissection of the starfish, the Echinoderms have well-developed systems of organs. The body-structure in its complex organization presents a marked advance beyond the structural condition of the polyps and jellyfishes. There is a well-organized digestive system with mouth, alimentary canal, and anal opening. The alimentary canal is either a simple spiral or coiled tube, or it is a tube in which can be recognized different parts, namely, œsophagus, stomach, intestine, cæca, and special glands secreting digestive fluids. This alimentary canal is not, as in the polyps, simply the body-cavity, but it is an inclosed tubular cavity lying within the general body-cavity. At the mouth-opening there is in some Echinoderms, notably the sea-urchins, a strong masticating apparatus consisting of five pointed teeth which are arranged in a circle about the opening. The nervous system consists of a central ring around the œsophagus or mouth, from which branches extend into the radiately arranged arms or regions of the body. There is no brain as in the higher animals, but the central nerve-ring is composed of both nerve-cells and nerve-fibres as in the nerve-centres of higher forms. Of organs of special sense there are special tactile or touch organs in all the Echinoderms, and the starfishes have very simply composed eyes or eye-like organs at the tips of the rays.
While some of the Echinoderms breathe simply through the outer body-wall, taking up by osmosis the air mixed with the water, some of them have special, though very simple, gill-like respiratory organs. These organs consist of small membranous sacs which are either pushed out from the body into the water, or lie in cavities in the body to which the water has access. There is also a distinct circulatory system, but the "blood" which is carried by these organs and which fills the body-cavity consists mainly of sea-water, although containing a number of amœboid corpuscles containing a brown pigment. There is no organ really corresponding to the heart of the higher animals. There are distinct organs for the production of the germ or reproductive cells. The sexes are distinct (except in a few species), each individual producing only sperm-cells or egg-cells, but the organs or glands which produce the germ-cells are very much alike in both sexes. There is no apparent difference between male and female Echinoderms except in the character or rather in the product of the germ-cell producing organs. A few species are exceptions, certain starfishes showing a difference in color between males and females.
As all of the Echinoderms except some of the feather-stars can move about, they have organs of locomotion, and well-defined muscles for the movement of the locomotory organs. The external organs of locomotion, the tube-feet (in the sea-urchins the dermal spines aid also in locomotion), are parts of a peculiar system of organs characteristic of the Echinoderms, called the ambulacral or the water-vascular system. This system is composed of a series of radial tubular vessels which rise from a central circular or ring vessel and which give off branches to each of the tube-feet. The water from the outside enters the ambulacral system through a special opening, the madreporic opening, and flowing to the tube-feet helps extend them. The tube-feet usually have a tiny sucking disk at the tip, and by means of them the Echinoderm can cling very firmly to rocks.
Development and life-history.—Differing from the sponges and the polyps and jellyfishes, the reproduction of the Echinoderms is always sexual; young or new individuals are never produced by budding, or in any other asexual way. The new individual is always developed from an egg produced by a female and fertilized by the sperm of a male. The eggs are usually red or yellow, are very small (about 1/50 in. in diameter in certain starfishes), and are fertilized by the sperm-cells of the males after leaving the body of the female. That is, both sperm-cells and unfertilized egg-cells are poured out into the water by the adults, and the motile sperm-cells in some way find and fertilize the egg-cells.
From the egg there hatches a tiny larva which does not at all resemble the parent starfish or sea-urchin. It is an active free-swimming creature, more or less ellipsoidal in shape and provided with cilia for swimming. Soon its body changes form and assumes a very curious shape with prominent projections. The larvæ of the various kinds of Echinoderms, as the starfishes, sea-urchins, sea-cucumbers, etc., are of different characteristic shapes. The naturalists who first discovered these odd little animals did not associate them in their minds with the very differently shaped starfishes and sea-urchins, but believed them new kinds of fully developed marine animals, and gave them names. Thus the larvæ of the starfishes were called Bipinnaria, the larvæ of the sea-urchins Pluteus, and so on. These names are still used to designate the larvæ, but with the knowledge that Bipinnaria are simply young starfishes, and that a Pluteus is simply a young sea-urchin. From these larval stages the adult or fully developed starfish or sea-urchin develops by very great changes or metamorphoses. The Echinoderms have in their life-history a metamorphosis as striking as the butterflies and moths, which are crawling worm-like caterpillars in their young or larval condition.
Most of the Echinoderms have the power of regenerating lost parts. That is, if a starfish loses an arm (ray) through accident, a new ray will grow out to replace the old. And this power of regeneration extends so far in the case of some starfishes that if very badly mutilated they can practically regenerate the whole body. This amounts to a kind of asexual reproduction. Some species, too, have the peculiar habit of self-mutilation. "Many brittle stars and some starfishes when removed from the water, or when molested in any way, break off portions of their arms piece by piece, until, it may be, the whole of them are thrown off to the very bases, leaving the central disc entirely bereft of arms. A central disc thus partly or completely deprived of its arms is capable in many cases of developing a new set; and a separated arm is capable in many cases of developing a new disc and a completed series of arms." In some of the sea-cucumbers "it is the internal organs, or rather portions of them, that are capable of being thrown off and replaced, the œsophagus ... or the entire alimentary canal, being ejected from the body by strong contractions of the muscular fibres of the body-wall, and in some cases, at least, afterwards becoming completely renewed."
Classification.—The Echinodermata are divided into five classes, viz., the Asteroidea or starfishes, "free Echinoderms with star-shaped or pentagonal body, in which a central disc and usually five arms are more or less readily distinguishable, the arms being hollow and each containing a prolongation of the body-cavity and contained organs"; the Ophiuroidea, or brittle-stars, "star-shaped free Echinoderms, with a central disc and five arms, which are more sharply marked off from the disc than in the Asteroidea and which contain no spacious prolongations of the body-cavity"; the Echinoidea, or sea-urchins, "free Echinoderms with globular, heart-shaped, or disc-shaped body enclosed in a shell or corona of close-fitting, firmly united calcareous plates"; the Holothuroidea, or sea-cucumbers, "free Echinoderms with elongated cylindrical or five-sided body, ... with a circlet of large oral tentacles"; and the Crinoidea, or feather-stars, "temporarily or permanently stalked Echinoderms with star-shaped body, consisting of a central disc, and a series of five bifurcate or more completely branched arms, bordered with pinnules."
Starfishes (Asteroidea).—The starfishes feed on other marine animals, especially shell-fish and crabs. They are also reputed to destroy young fish. By means of their sucking-tubes, or tube-feet with sucker tips, they can seize and hold their prey firmly. They do much injury to oyster-beds by attacking and devouring the oysters. When attacking prey too large to be taken into the mouth the starfish everts its stomach over the prey and devours it. The stomach is afterward drawn back into the body-cavity by special muscles.
Starfishes vary much in size, color and general appearance, although all are readily recognizable as starfishes (fig. [22]). The number of arms or rays varies from five to thirty or more in different species; some have the interradial spaces filled out nearly to the tips of the rays, making the animal simply a pentagonal disc. In size starfishes vary from a fraction of an inch in diameter to three feet; in color they are yellow or red or brown or purple.
Fig. 22.—A group of Echinoderms; the upper one, a starfish, Asterina mineata, the one at the right a starfish, Asterias ocracia, at the left a brittle-star, species unknown, and at bottom two sea-urchins, Strongylocentrotus franciscanus. (From living specimens in a tide-pool on the Bay of Monterey, California.)
Brittle-stars (Ophiuroidea).—The brittle-stars, or serpent-stars (fig. [22]) as they are also called, resemble the starfishes in external appearance, that is, they are flat and composed of a central disc with radiating arms (always five in number, although each arm may be several times branched). The central disc is always sharply distinguished from the arms, and the arms are usually slender and more or less cylindrical. The distinguishing difference between the brittle-stars and the starfishes is that the body-cavity and the stomach which extend out into the arms in the starfishes are in the brittle-stars limited to the central disc, or to the disc and bases of the arms. The tube-feet also have no suckers at the tips. More than 700 species of brittle-stars are known. They feed on marine shell-fish, crabs and worms.
Sea-urchins (Echinoidea).—The sea-urchins (figs. [20], [21] and [22]) of which more than 300 species are known, have no arms or rays, and they are usually not flat like the starfishes but globular, with poles more or less flattened. As has been noted in the examination of the body-wall or "shell," the radiate character of the body is shown by the five radiating zones of tube-feet. The mouth, with its five strong "teeth," is on the ventral surface, and the anal opening and madreporic opening are on the dorsal surface. The calcareous plates (seen distinctly in a specimen from which the spines have been removed) which constitute the firm part of the body-wall, are more or less pentagonal in shape and are usually firmly united at the edges. The spines which are so characteristic of the sea-urchins vary much in size and number and firmness, but are present in some form on all of them.
While most of the sea-urchins live near the shore, being very common in tide-pools, some live only on the bottom of the ocean at great depths. Their food consists of small marine animals and of bits of organic matter which they collect from the sand and débris of the ocean floor. Many of the sea-urchins are gregarious, living together in great numbers. Some have the habit of boring into the rocks of the shore between tide-lines. I have seen thousands of small beautifully colored purple sea-urchins lying each in a spherical pit or hole in hard conglomerate rock on the California coast. How they are enabled to bore these holes is not yet known. There is great variety in size and color among the sea-urchins. The colors are brown, olive, purple red, greenish blue, etc.
A few kinds of sea-urchins have a flexible shell or test. The Challenger expedition dredged up from sea-bottom some sea-urchins, and when placed on the ship's deck "the test moved and shrank from touch when handled, and felt like a starfish." The cake-urchins or sand-dollars are sea-urchins having a very flat body with short spines. They lie buried in the sand, and are often very brightly colored. Their hollow bleached tests with the spines all rubbed off are common on the sands of both the Atlantic and Pacific coasts.
Sea-cucumbers (Holothuroidea).—The sea-cucumbers (fig. [23]) show at first glance little resemblance to the other radiate animals. The body is an elongate, sub-cylindrical sac, resembling a thick worm or sausage or cucumber in shape. At one end it bears a group of branched tentacles which are set in a ring around the mouth-opening. The body-wall is muscular and leathery, but contains many small separated calcareous spicules. There are usually five longitudinal rows of tube-feet. In some species, however, tube feet are wholly wanting; in others they are scattered over the surface.
Although there are known about five hundred species of sea-cucumbers many of which live along the shores, they are much less familiar to us than the starfishes and sea-urchins. They usually rest buried in the sand by day, feeding at night. Some of them attain a large size. A great orange-red species of the genus Cucumaria, which is found in the Bay of Monterey, California, is three feet long.
The people of some nations use sea-cucumbers as food. They are called "trepang" in the orient. The trade of preparing the trepang is almost entirely in the hands of the Malays, and every year large fleets set sail from Macassar and the Philippines to the south seas to catch sea-cucumbers.
Fig. 23.—A sea-cucumber, Pentacta frondosa. (After Emerton.)
Feather-stars (Crinoidea).—The feather-stars or sea-lilies or crinoids (fig. [24]), as they are variously called, differ from the other Echinoderms in having the mouth on the upper side of the central disc, and in the fact that all of the species are fixed, either permanently or for a part of their life, being attached to rocks on the sea-bottom by a longer or shorter stalk which is composed of a series of rings or segments. The central disc is small and the radiating arms are long, slender, sometimes repeatedly branched, and all the branches bear fine lateral projections called pinnulæ. Most of the feather-stars live in deep water and are thus only seen after being dredged up. They feed on small crab-like animals, and on the marine unicellular animals and plants.
Fig. 24.—A crinoid or feather-star, Pentacrinus sp. (After Brehm.)
[CHAPTER XIX]
BRANCH VERMES:[8] THE WORMS
THE EARTHWORM (Lumbricus sp.).
Technical Note.—Obtain live earthworms of large size, killing some in 30% alcohol and hardening and preserving them in 80% alcohol, and bringing others alive to the laboratory. The worms may be found during the daytime by digging, or at night by searching with a lantern. They often come above ground in the daytime after a heavy rain. Live specimens may be kept in the laboratory in flower-pots filled with soil. "They may be fed on bits of raw meat, preferably fat, bits of onion, celery, cabbage, etc., thrown on the soil."
External structure (fig. [25]).—Examine the external structure of live and dead specimens. Which is the ventral and which the dorsal surface? Which the anterior and which the posterior end? Note the segmented condition of the body; the number of segments or somites, and their relative size and shape. Note absence of appendages such as limbs and the presence of locomotor setæ (short bristles). How many setæ are there on each segment and what is their disposition? The mouth is covered by a dorsal projection called the prostomium. The anal opening is situated in the posterior segment of the body. The broad thickened ring or girdle including several segments near the anterior end of the body is the clitellum, a glandular structure which secretes the cases in which the eggs are laid. On the ventral surface of the fourteenth and fifteenth segments (in most species) are two pairs of small pores; two other pairs of small openings (usually difficult to find), one between segments 9 and 10, and one between segments 10 and 11, are present. All these are the external openings of the reproductive organs.
Fig. 25.—Dissection of the earthworm, Lumbricus sp.
Make drawings showing the external structure of the earthworm.
Examine a live specimen placed on moist paper or wood. Note the characteristics of its locomotion, and the movements of its body-parts. How do the setæ aid in locomotion?
Internal structure (figs. [25], [26] and [28]).—Technical Note.—With a fine-pointed pair of scissors make a dorsal median incision, not too deep, behind the clitellum and cut forward as far as the first segment. Put the specimen into dissecting-dish, carefully pin back the edges of the cut and cover with clear water or, better, 50% alcohol.
Note the long body-cavity divided by the thin septa which have been torn away for the most part by the pinning process. Note the thin transparent covering of the body, the cuticle. Just beneath this note a less transparent layer, the epidermis, and underneath this a layer of muscles. The muscular layer is made up of two clearly recognizable sets, an outer circular layer and an inner longitudinal layer the fibres of which are continuous with the septa.
Note, as the most conspicuous internal organ, the long alimentary canal, of which a number of distinct parts may be recognized. Most anteriorly is a muscular pharynx, which is followed by a narrow œsophagus, leading directly into the thin-walled crop; next comes the muscular gizzard, and next the intestine which opens externally in the terminal segment through the anus. The anterior end of the alimentary canal is more or less protrusible, while the posterior portion is held more firmly in place by the septa which act as mesenteries. Surrounding the narrow œsophagus are the reproductive organs, three pairs of large white bodies and two pairs of smaller sacs.
Note the dorsal blood-vessel lying along the dorsal surface of the alimentary canal, from the anterior portion of which arise several circumœsophageal rings or "hearts." These hearts are contractile and serve to keep the blood in motion through the blood-vessels (see later). In the most anterior of the body segments note the pear-shaped brain or cerebral ganglion.
Technical Note.—Lift carefully to right and left the reproductive organs, thus exposing the œsophagus.
Note three pairs of bag-like structures projecting from the œsophagus. The front pair is the œsophageal pouches; the next two pairs are the œsophageal or calciferous glands. They communicate with the alimentary canal, and their secretion is a milky calcareous fluid.
Make a drawing that will show all the parts so far studied.
Technical Note.—Cut transversely through the alimentary canal in the region of the clitellum and carefully dissect the anterior portion of the canal away from the surrounding organs.
Note the dorsal fold of the intestine, typhlosole, extending into the lumen. This fold gives a greater surface for digestion, and in it are a great many hepatic or special digestive cells. The entire alimentary canal is lined with epithelium. Observe just beneath the alimentary canal the ventral blood-vessel, and still beneath this blood-vessel the ventral nerve-cord. There is a slight swelling on the nerve-cord in each segment of the body. These swellings are the ganglia. How many pairs of nerves are given off from each ganglion? Observe in each segment, posterior to the first three or four, the successive pairs of convoluted tubes, the nephridia, or organs of excretion. Each nephridium opens internally through a ciliated funnel, the nephrostome, within the body-cavity, while it opens externally by a small excretory pore between the setæ on the ventral surface of the segment behind that in which the nephridium chiefly lies. The function of the nephridia is to carry off waste matter from the fluid which fills the body-cavity.
Fig. 26.—Dissection to show alimentary canal in section and nephridia of earthworm.
Trace the ventral nerve-cord forward to its connection with the cerebral ganglion. Note the throat nerve-ring or circumœsophageal collar connecting the ventral cord with the brain.
Make a drawing of the nervous system showing its relation to other organs.
Fig. 28.—Cross-section of earthworm.
Life-history and habits.—The earthworm lives in soft moist soil which is rich in organic matter. Its food is taken into the mouth mixed with dirt and sand. As this mixture passes through the long alimentary canal the organic particles are taken up and digested. As we have already seen, there are in each worm two sets of reproductive glands, namely, male and female organs. Each earthworm produces both egg-cells and sperm-cells, but the sperm-cells of one worm are not used to fertilize the eggs of the individual producing them. When the eggs are ready to be discharged from the body, the clitellum becomes very much swollen and its glands begin an active secretion which hardens and forms a collar-like structure about the body of the worm. As this collar moves forward toward the anterior end of the body it collects the eggs and also the sperm-cells previously received from another worm, and finally slips off the head end of the animal. The entire structure with the contained eggs and sperm-cells as it passes off from the body becomes closed at both ends, thus forming a horny capsule which lies in the earth until the young worms emerge. Only a part of the eggs develop in each capsule, the rest being used as food for the growing young. The young earthworms, though of very small size, are fully formed before they leave the egg-capsule. Earthworms are more or less gregarious, large numbers often being found together.
For an interesting account of the habits of earthworms see Darwin's "The Formation of Vegetable Mold."
OTHER WORMS.
The branch Vermes comprises so large a number of kinds of animals presenting such great differences in structure and habit that it is impossible to give a brief statement in general or summary terms of their external body-characters, of the structural and functional condition of their various organs and systems of organs, and of the course of their development and life-history as has been done for the preceding branches. Many zoologists, indeed, do not include all the worms or worm-like animals in one branch, but consider them to form several distinct branches.
Fig. 29.—A group of marine worms: at the left a gephyrean, Dendrostomum cronjhelmi, the upper right-hand one a nereid, Nereis sp., the lower right-hand one, Polynoe brevisetosa. (From living specimens in a tide-pool on the Bay of Monterey, California.)
In certain very general characters all of the animals which compose the branch Vermes do agree. All, or nearly all, have an elongate body which is bilaterally symmetrical, that is, which could be cut by a median longitudinal cutting in two similar halves. In most of them also the body is composed of a number of successive segments or somites which are more or less alike. This kind of segmented or articulated body is also possessed by the insects and crabs. Almost all of the worms have the power of locomotion; usually that of crawling. For this crawling they do not have legs composed of separate segments or joints as do the higher articulated animals, the crabs and insects, but either have fleshy unjointed legs, or various kinds of bristles or spines, or suckers, or even no external organs of locomotion at all. As regards their internal structure they have well-organized systems of organs, which show great variety in character and degree of complexity. The special sense-organs are usually of simple character and low degree of functional development. Reproduction occurs both sexually and asexually; in some species the sexes are distinct, while in others both sperm-cells and egg-cells are produced by the same individual. Asexual reproduction is by budding or by a kind of simple division or fission. The worms live either in salt or fresh water, or in moist, muddy or slimy places or as parasites in the bodies of other animals or in plants. While most worms feed on animal substance either living or dead, some feed on living or decaying plant matter.
Classification.—There is great lack of agreement among zoologists in the matter of the classification of the worms. Not only are the various groups which by some are called classes held by others to be distinct branches, co-ordinate in rank with the Echinodermata, Cœlenterata, etc., but the limits of these groups are also constantly called in question. It will require a great deal better knowledge of the structure and life-history of these diverse animals before the matter of their classification is satisfactorily settled. We shall consider briefly four of the various groups (which we may consider as classes) which include worms either specially familiar to us or of special interest or importance. One or two examples of each group (the groups being selected primarily because of the examples) will be described in some detail. By this means we may get an idea of the extremely diverse character of the animals which are included in the heterogeneous branch Vermes.
Earthworms and leeches (Oligochætæ).—The various species of earthworms, an example of which has been studied are found in all parts of the world; they occur in Siberia and south to the Kerguelen Islands. They are absent from desert or arid regions, and some can live indifferently either in soil or in water. Some near allies of the earthworms are aquatic, living in fresh or brackish water, some in salt water near the shore. In size earthworms vary from 1 mm. (1/25 in.) to 2 metres (2-1/6 yds.) in length. All show the distinct segmentation of the body noticeable in the common earthworm already studied.
The leeches, some of which are familiar animals, are closely related to the earthworms, although at first glance the similarity in structure is not very noticeable.
Technical Note.—Some common water-leeches, alive or preserved in alcohol, should be examined by the class. The animals are not unfamiliar to boys who "go in swimming" in the small streams of the country. The body of a leech should be examined carefully, and drawings of it showing the external structural characters should be made.
The body of a leech is flattened dorso-ventrally, instead of being cylindrical as in the earthworm, and tapers at both ends. In the live animal the body can be greatly elongated and narrowed or much shortened and broadened. It is composed of many segments (not as many as there are cross-lines however; each segment is transversely annulated), and bears at each end on the ventral surface a sucker, the one at the posterior end being the larger. These suckers enable the leech to cling firmly to other animals. The mouth is at the front end of the body on the ventral surface and is provided with sharp jaws. Leeches live mostly on the blood of other animals which they suck from the body. The common leech "fastens itself upon its victim by means of its suckers, then cuts the skin, fastens its oral sucker over the wound and pumps away until it has completely gorged itself with blood, distending enormously its elastic body, when it loosens its hold and drops off." Its biting and sucking cause very little pain, and in olden days physicians used the leeches when they wanted to "bleed" a person. A common European species of leech much used for this purpose is known as the "medicinal leech." All leeches are hermaphroditic, that is, the sexes are not distinct, but each individual produces both sperm-cells and egg-cells. Most of the leeches lay their eggs in small packets or cocoons. This cocoon is dropped in soil on the banks of a pond or stream so that the young may have a moist but not too wet environment. The young issue from the eggs in four or five weeks, but they grow very slowly and it is several years before they attain their full size. Leeches are long-lived animals, some being said to live for twenty years.
Flatworms (Platyhelminthes).—Technical Note.—Collect some live fresh-water planarians (see fig. [30]), which are to be found on the muddy bottom of most fresh-water ponds, and examine them while alive in watch-glasses of water. Make drawings showing the external appearance, and as much of the internal anatomy as can be seen. The branching alimentary canal can be seen in more or less detail, and with higher power of the microscope parts of the nervous system can be seen also. Have also a tapeworm preserved in alcohol or formalin to show the very flat and many-segmented body.
The flatworms include a large number of forms which vary much in shape and habits. They are all, however, characteristically flat; in some this condition is very marked. Some are active free-living animals, as the planarians (figs. [30] and [31]), while many live as parasites in the alimentary canal of other animals, as do the sheep-fluke and the tapeworms.
Fig. 30.—A fresh water planarian, Planaria sp. (From a living specimen.)
The fresh-water planarians (fig. [30]), which live commonly in the mud of the bottom of ponds, are small, being less than half an inch long. They are very thin and rather broad, tapering from in front backwards. On the upper surface near the front they have a pair of eyes; the mouth is on the under surface a little behind the middle of the body. The alimentary canal is composed of three main branches, each with numerous small side branches. One main branch runs forward from the mouth, and the other two run backwards, one on each side of the body. There is no anal opening, and the alimentary canal thus forms a system of fine branches closed at the tips, and extending all through the body. The nervous system is composed of a ganglion or brain in the front end of the body from which two main branches extend back throughout its whole length. From these main longitudinal branches arise many fine lateral branches.
Fig. 31.—A marine planarian, Leptoplana californica. (From a living specimen.)
Of the parasitic flatworms the tapeworms are the best known. There are numerous species of them, all of which live in the bodies of vertebrate animals. In the adult or fully developed stage the tapeworms live in the alimentary canal, holding on to its inner surface by hook-like clinging organs and being nourished by the already digested food by which they are bathed. In the young or larval stage tapeworms live in other parts of the body of the host, and usually, indeed, in other hosts not of the same species as the host of the adult worm.
The common tapeworm of man, Tænia solium (there are several other species of Tænia which infest man, but solium is the common one), may serve as an example of the group. In the adult condition its body, which is found attached to the inner wall of the intestine, is like a long narrow ribbon: it may be two or three metres long. It is attached by one end, the head, which is very small and provided with a score of fine hooks. Behind the head the thin ribbon-like body grows wider. The body is composed of many (about 850) joints called proglottids. There is no mouth or alimentary canal, the liquid food being simply taken in through the skin. Each proglottid produces both sperm-cells and egg-cells; one by one these proglottids or joints with their supply of fertilized eggs break off and pass from the alimentary canal with the excreta. If now one of these escaped proglottids or the eggs from it are eaten by a pig, the embryos issue from the eggs in the alimentary canal of the pig, bore through the walls of the canal and lodge in the muscles. Here they increase greatly in size and develop into a sort of rounded sac filled with liquid. If the flesh of the pig be eaten by a man, without its being first cooked sufficiently to kill the larval sac-like tapeworms, these young tapeworms lodge in the alimentary canal of the man and develop and grow into the long ribbon-like many-jointed adult stage.
The life-history of the other tapeworms which infest the various vertebrate animals is of this general type. There is almost always an alternation of hosts, the larval tapeworm living in a so-called intermediate host, and the adult in a final host. Of the domestic animals the dog is the most frequently attacked. At least ten different species of tapeworms have been found in the dog. The intermediate hosts of these dog tapeworms include rabbits, sheep, mice, etc. Some of the domestic fowl, ducks, geese and chickens, for instance, are also infested by tapeworms, and the intermediate hosts in these cases are usually insects or small aquatic crustaceans like the familiar Cyclops.
Roundworms (Nemathelminthes).—Technical Note.—Vinegar-eels from mouldy vinegar, and hair-worms from fresh-water pools, can usually be readily obtained. They should be examined, and drawings should be made of them, showing their shape and simple external structural character. If a specimen of trichinosed pork be obtained, the encysted stage of the Trichina, described in the following account, can be shown.
The roundworms are slender, smooth, cylindrical worms pointed at both ends. They are all very long in proportion to their diameter, although their actual length may be short. Some species are of microscopic size; as the Trichina worm, which is about 1/20 in. long; while the guinea-worm, one of the worst parasites of man, may reach a length of six feet. Many of the roundworms are parasites living in the various organs of other animals. Some, however, lead an independent free life in water or in damp earth.
Fig. 32.—A
vinegar eel,
Anguillula
sp. (From
a living
specimen.)
Familiar examples of roundworms are the so-called vinegar-eels (Anguillula) (fig. [32]) to be found in weak vinegar, and other species of this same genus which live in water or moist ground or in the tissues of plants, doing much injury. The hair-worms (Gordius) or horse-hair snakes, which are believed by some people, to be horse-hairs dropped into water and turned into these animals, are also familiar examples of roundworms. They are often found abundantly in little pools after a rain, and it is sometimes said that these worms come down with the rain. They have in reality come from the bodies of insects in which they pass their young or larval stages as parasites. The hair-worms all live as parasites during their larval stage, and as free independent animals in their adult stage. Some of them require two distinct hosts for the completion of their larval life, living for a while in the body of one, and later in the body of another. The first host is usually a kind of insect which is eaten by the second host. The eggs are deposited by the free adult female in slender strings twisted around the stems of water-plants. The young hair-worm on hatching sinks to the bottom of the pond, where it moves about hunting for a host in which to take up its abode.
Fig. 33.—Trichina spiralis, encysted
in muscle of a pig.
(From specimen.)
The terrible Trichina spiralis (fig. [33]), which produces the disease called trichinosis, is another roundworm of which much is heard. This is a very small worm which in its adult condition lives in the intestine of man as well as in the pig and other mammals. The young, which are borne alive, burrow through the walls of the intestine, and are either carried by the blood, or force their way, all over the body, lodging usually in muscles. Here they form for themselves little cells or cysts in which they lie. The forming of these thousands of tiny cysts injures the muscles and causes great pain, sometimes death, to the host. Such infested muscle or flesh is said to be "trichinosed," and the flesh of a trichinosed human subject has been estimated to contain 100,000,000 encysted worms. To complete the development of the encysted and sexless Trichinæ the infested flesh of the host must be eaten by another animal in which the worm can live, e.g. the flesh of man by a pig or rat, and that of a pig by man. In such a case the cysts are dissolved by the digestive juices, the worms escape, develop reproductive organs and produce young, which then migrate into the muscles and induce trichinosis as before. But however badly trichinosed a piece of pork may be, thorough cooking of it will kill the encysted Trichinæ, so that it may then be eaten with impunity. Some people, however, are accustomed to eat ham, which is simply smoked pork, without cooking it, and in such cases there is always great danger of trichinosis.
Wheel animalcules (Rotifera).—Technical Note.—Live specimens of Rotifers can be found in almost any stagnant water. Examine a drop of such water with the compound microscope, and find in it a few small, active, transparent creatures, larger than the Paramœcium and other Protozoa in the water and which have the appearance shown in fig. [34]. They may be known by the constant whirling, or rather vibrating, circlet or wheel of cilia at the larger or head end of the body. These wheel animalcules may be studied alive by the class. Although usually darting about, the animalcules occasionally cease to move, when, because of their transparency, almost the whole of their anatomy can be made out. Their feeding habits can also be readily observed, and the food itself watched as it moves through the body. Make drawings showing as much of the anatomy as can be worked out. Note especially the "mastax" or gizzard-like masticating apparatus in the alimentary canal.
Fig. 34.—A wheel animalcule,
Rotifer sp. (From living specimen,
Stanford University.)
The wheel animalcules (fig. [34]) or Rotifers look little like the other worms we have studied. But they are nevertheless more nearly related to the worms than to any other branch of animals. They are all small, about 1/3 mm. long, and have a compact body. They are aquatic and feed on smaller animals and plants or on bits of organic matter which they capture by means of the currents produced by the vibrating cilia of the "wheel." Small as they are they have a complex body-structure, with well-organized systems of organs. For a long time, however, they were classed by naturalists with the Protozoa on account of their size. They are found all over the world, mostly in fresh water; a few are marine. More than 700 species of them are known.
An interesting thing about the Rotifers is their remarkable power to withstand drying-up. When the water in a pond or ditch evaporates some of the Rotifers do not die, but simply dry up and lie in the dust, shrivelled and apparently lifeless, yet really in a state of suspended animation. On being put into water they will gradually fill out to their full size and shape, and finally resume all their normal activities. In this dried-up condition Rotifers may persist for a long time, several years even, although otherwise their natural life is short, being probably of not over two weeks' duration. Certain other of the lower animals have this same power of withstanding desiccation.
[CHAPTER XX]
BRANCH ARTHROPODA: CRUSTACEANS, CENTIPEDS, INSECTS, AND SPIDERS
The great branch Arthropoda includes a host of familiar animals. It contains more species than any other branch of the animal kingdom. To it belong the crayfishes, shrimps, crabs, lobsters, water-fleas, and other animals which compose the class Crustacea; the centipeds and thousand-legged worms which compose the class Myriapoda; the true or six-footed insects forming the class Insecta, which includes nearly two-thirds of all the known species of animals; and the scorpions, mites, ticks, and spiders which constitute the class Arachnida. There is also a fifth class in the branch Arthropoda which includes a few species of animals unfamiliar to us but of great interest to zoologists.
All these varied kinds of animals have a body on the annulate or segmented type-plan, like that shown by most worms, but they differ from the worms in possessing jointed appendages, used for locomotion or food taking. There is typically or racially one pair of these jointed or segmented appendages on each segment of the body, but in all of the Arthropoda some of the segments have lost their appendages. The body is covered by a firm cuticle or outer body-wall called the exoskeleton. This exoskeleton serves not only to enclose and protect the soft parts of the body but also for the attachment of the body muscles. It may be flexible as in the sutures between the body-segments in most insects, or hard and rigid as in the sclerites of the segments. The firmness is due primarily, and in the insects usually solely, to a deposit in the cuticle of chitin, a substance probably secreted by the underlying cells of the true skin, or it may be due chiefly, as in the crabs, to a calcareous deposit. In such cases it becomes a veritable armor. The internal organs of the Arthropods show a more or less obvious segmentation corresponding with the segmentation of the body-wall. The alimentary canal runs longitudinally through the center of the body from mouth to anal opening. The nervous system consists of a brain lying above the œsophagus and a double nerve-chain running backward from beneath the œsophagus, along the median line of the ventral wall, to the posterior extremity of the body. This ventral nerve-chain consists of a pair of longitudinal commissures or cords and a series of pairs of ganglia, arranged segmentally. The two ganglia of each pair are fused more or less nearly completely to form a single ganglion, and the nerve-cords are partially fused, or at least lie close together. In addition there is a smaller sympathetic system composed of a few small ganglia and certain nerves running from them to the viscera, this system being connected with the main or central nervous system. In this group the organs of special sense reach for the first time a high stage of development. Compound eyes are peculiar to Arthropoda. The heart lies above the alimentary canal. Respiration is carried on by gills in the aquatic forms, and by a remarkable system of air-tubes or tracheæ in the land forms (insects). The sexes are usually distinct, and reproduction is almost universally sexual. Most of the species lay eggs.
The Arthropods are animals of a high degree of organization. The extremely diverse life-habits of the various kinds among them have led to much modification and to great specialization of structure. The course of development, too, is made very complicated by the elaborate metamorphosis undergone by many of the members of the branch.
We shall study the Arthropoda by getting acquainted with a few examples of each class and thus learning the special class characteristics.
Class Crustacea: Crayfishes, Crabs, Lobsters, Etc.
THE CRAYFISH (Cambarus sp.)
Structure.—The structure of the crayfish has been already studied (see Chapter [IV] and figs. [3] and [4]).
Life-history and habits.—Crayfish frequent fresh-water lakes, rivers, and springs in most parts of the United States. Many of them perish whenever the small prairie ponds dry up. But some burrow into the earth when the dry season comes. There may be noticed in meadows where water stands for certain seasons of the year many scattered holes with slight elevations of mud about them. These are mostly the burrows of crayfish. During the dry season the crayfish digs down until it reaches water, or at least a damp place, where it rests until wet weather brings it to the surface once more. One of these burrows, followed in digging a mining shaft, extended vertically down to a distance of twenty-six feet, where the crayfish was found tucked snugly away.
The eggs are carried by the female on her abdominal appendages. Previous to the laying of the eggs the female rubs off all foreign matter from the appendages, thus preparing them for the reception of the eggs. This cleaning is done with the fifth pair of legs. When the eggs are ready to be laid, which is during the last of March or in April in the Central States, a sticky secretion passes out of the openings at the base of the walking legs and smears the pleopods of the abdomen. The eggs as they pass out are fertilized and caught on the pleopods, where they remain attached in clusters. After some weeks the young crayfishes issue from the eggs. In general appearance they are not very unlike the adults. They grow very rapidly at this stage. As the animal is enclosed in a hard shell, growth can only take place during the period just following the molt, for the crayfish casts its skin periodically, and it is while the new shell is forming that the animal does its growing. The crayfish when it molts casts not only the exoskeleton, but also the lining of part of the alimentary canal. After the females have hatched their young many die in the shallow pools, in which places the dried-up skeletons are noticeable during the summer months.
OTHER CRUSTACEANS.
Most of the crustaceans live in water, a few being found in damp soil or in other moist places. Some are fresh-water animals and some marine. They vary in size from the tiny water-fleas, a millimeter long, to crabs two feet across the shell or sixteen feet from tip to tip of legs. They present great differences in form and general appearance of body, being adapted for various conditions of life. Some crustaceans live as parasites on other animals, in some cases on other crustaceans. Such parasitic species have the body much modified and are hardly to be recognized as members of the class.
Body form and structure.—In structural character and body organization the Crustaceans show, of course, the general characteristics already attributed to the Arthropoda, the branch to which they belong. The characteristics which distinguish them from other Arthropods are the possession of gills for respiration (some insects have gills, but of a very different kind as will be seen later), and the bi-ramose condition of the body appendages, each appendage (excepting the antennules) consisting of a single basal segment from which arise two branches made up of one or more segments. Of the form of the crustacean body few generalizations can be made.
"There is no [other] class in the animal kingdom which presents so wide a range of organization as the Crustacea, or in which the deviations in structure from the 'type form' are so striking and so interesting from their obvious adaptation to the mode of life." For this reason no attempt will be made to discuss in general terms the form of the crustacean body, but brief accounts will be given of a few of the more familiar kinds of Crustacea which will serve to illustrate this remarkable diversity of body form.
Similarly impossible is it also to give a general account of the development of the crustaceans. The sexes are distinct in most Crustacea, and there is often great difference in form between the male and female. A certain amount of metamorphosis takes place in the development of all crustaceans; that is, the young when hatched from the egg differs, often decidedly, in appearance and structure from the parent, and in the course of its post-embryonic development undergoes more or less striking change or metamorphosis. This metamorphosis is often very marked.
Water-fleas (Cyclops).—Technical Note.—The water-fleas are common in the water of ponds or of slow streams; they may often be found in the school aquarium. They are, though small (about 1 mm. long), readily seen with the unaided eye; they are white, rather elongate, and have a rapid jerky movement. Examine specimens alive in water in a watch glass. Note the "split pear" shape, broadest near the front, tapering posteriorly, flat beneath, convex above; note the forked stylets at tip of abdomen; also the two pairs of antennæ, the single median eye, the mandibles, two pairs of maxillæ, and five pairs of legs (last pair very small). There are no gills. Some of the specimens, females, may have attached to the first abdominal segment on either side an egg sac. Make drawings showing all these structural details. Watch the Cyclops capturing and feeding on Paramœcium or other small animals.
Fig. 35.—A water-flea, Cyclops sp. Female with egg-masses. (From living specimen.)
The water-fleas (Cyclops) (fig. [35]) are among the smallest of the Crustacea. They are extremely abundant, having great power of multiplication. "An old Cyclops may produce forty or fifty eggs at once, and may give birth to eight or ten broods of children living five to six months. As the young begin to reproduce at an early age, the rate of multiplication is astonishing. The descendants of one Cyclops may number in one year nearly 4,500,000,000, or more than three times the total population of the earth, provided that all the young reach maturity and produce the full number of offspring." The Cyclops feed on smaller aquatic animals such as Protozoa, Rotifera, etc. They in turn serve as food for fishes; and because of their immense numbers and occurrence in all except the swiftest fresh waters "they form the main food of most of our fresh-water fishes while young." Many aquatic insect larvæ feed almost exclusively on them.
Related to the Cyclops are a host of other kinds of minute Crustaceans. Among these the so-called fish-lice are specially interesting because of their parasitic habits and greatly modified and degenerate structure. There are many kinds of these parasitic crustaceans infesting fishes, whales, molluscs, and worms. "As on land almost every species of bird or mammal has its own parasitic insects, so in the water almost every species of fish or larger invertebrate has its parasitic crustaceans." Some of the most common of these parasites attach themselves to the gills of fishes. Here they cling, sucking the blood or animal juices from the host. In form of body they do not at all resemble other Crustaceans, but are strangely misshapen. They are often worm-like, or sac-like, without legs or other locomotory appendages. As with other parasites (see Chapter [XXX]) an inactive dependent life results in the atrophy and loss by degeneration of the body-parts concerned with locomotion and orientation.
Wood lice (Isopoda).—Technical Note.—Specimens of wood lice, pill bugs, or damp bugs, as they are variously called, may be readily found in concealed moist places, as under stones or boards on damp soil. They are often common in houses, near drains or in dark, damp places. Examine some live wood lice, and some dead specimens (killed by chloroform or in an insect-killing bottle). Note the division of the body into the head, thorax, and abdomen; find the eyes, the antennæ and the mouthparts (mandibles and maxillæ are usually pressed closely together). All the locomotory appendages are adapted for walking or running, not swimming. Note the number of pairs of legs; the structure of a leg; find gills and gill-covers. Some females may be found with eggs on the under side of the thorax near the bases of the legs, the eggs being covered by thin membranous plates. Make drawings showing the general form and character of body and details of legs, gills, etc. Compare with the crayfish and Cyclops.
Fig. 36.—A damp bug,
Isopod, species not determined.
(From specimen.)
The wood-lice (fig. [36]) are among the few Crustacea which have a wholly terrestrial life. They run about quickly and feed chiefly on decaying vegetable matter. They are night scavengers. They have the body oval and convex above, rather purplish or grayish brown, and smooth. Although they do not live in the water they breathe partly at least by means of gills (though they may breathe partly through the skin). It is therefore necessary for them to live in a damp atmosphere so that the gill membranes may be kept damp. If not kept moist they could not serve as osmotic membranes.
Lobsters, Shrimps and Crabs (Decapoda).—Technical Note.—Teachers living near the sea-shore can get specimens of live and dead lobsters, shrimps, and crabs in the markets. Schools in the interior should have a few preserved specimens for examination. These specimens should be compared with the crayfish; although differences in shape of body are evident, the character and arrangement of body parts will be found to be very similar.
The largest and most familiar Crustaceans, as the crayfishes, lobsters, shrimps, prawns and crabs, all belong to the order Decapoda, or ten-legged Crustacea. The members of this order have, including the large claws, ten walking feet; they all have eyes on movable stalks, and the front portion of the body is covered by a horny fold of the body-wall called the carapace.
The lobsters are large ocean-inhabiting crustaceans which are very like the fresh-water crayfish in all structural characters. They live on the rocky or sandy ocean-bottom at shallow depths. They feed largely on decaying animal matter. They are caught in great numbers in so-called "lobster pots," a kind of wooden trap baited with refuse. "The number thus taken upon the shores of New England and Canada amounts to between twenty and thirty million annually." Live lobsters are brownish or greenish with bluish mottling; they turn red when boiled. A single female will lay several thousand eggs. The eggs are greenish and are carried about by the mother until the young hatch. The young are free-swimming larvæ, until they reach a length of half an inch.
The shrimps and prawns are mostly marine, though some species live in fresh water. They are, like the lobsters, used for food. Some of the species are gregarious in habit, occurring in great "schools" of individuals. Like the lobsters they crawl about on the sea-bottom feeding on decaying animal matter. Shrimps are very abundant near San Francisco, where extensive "shrimp fishing" is done by the Chinese.
Fig. 37.—Some crabs and barnacles of the Pacific coast; the short sessile acorn barnacles in the upper left-hand corner belong to the genus Balanus; the stalked barnacles in the upper right-hand corner are of the species Pollicipes polymenus; the largest crab (upper left-hand) is Brachynotus nudus; the one in left-hand lower corner is a young rock-crab, Cancer productus; the crab in the sea-weed at the right is a kelp-crab, Epialtus productus, while the two in snail-shells in lower corner are hermit-crabs, Pagurus samuelis. (From living specimens in a tide-pool on the Bay of Monterey, California.)
The crabs (fig. [37]) differ from the lobsters and crayfishes and shrimps in having the body short and broad, instead of elongate. This is due to the special widening of the carapace and the marked shortening of the abdomen. The abdomen, moreover, is permanently bent underneath the body, so that but little of it is visible from the dorsal aspect. The number of abdominal legs or appendages is reduced. When the tide is out the rocks and tide-pools of the ocean shore are alive with crabs. They "scuttle" about noisily over the rocks, withdrawing into crevices or sinking to the bottom of the pools when disturbed. They move as readily backward or sidewise, "crab-fashion," as forward. They are of various colors and markings, often so patterned as to harmonize very perfectly with the general color and appearance of the rocks and sea-weeds among which they live. The spider-crabs are especially strange-looking creatures with unusually long and slender legs and a comparatively small body-trunk. They include the Macrocheira of Japan, the largest of the crustaceans. Specimens of this crab are known measuring twelve to sixteen feet from tip to tip of extended legs; the carapace is only as many inches in width or length. The soft-shelled crab is a species common along our Atlantic coast. It is "soft-shelled" only at the time of molting, and has to be caught in the few days intervening between the shedding of the old hard shell and the hardening of the new body-wall. The little oyster-crabs (Pinnotheres) which live with the live oyster in the cavity enclosed by the oyster shell are well-known and interesting crabs. They are not parasites preying on the body of the oyster, but are simply messmates feeding on particles of food brought into the shell by the currents of water created by the oysters.
Among the most interesting crabs are the hermit crabs (fig. [37]), familiar to all who know the seashore. There are numerous species of these crabs, all of which have the habit of carrying about with them, as a protective covering into which to withdraw, the spiral shell of some gastropod mollusc. The abdomen of the crab remains always in the cavity of the shell; the head and thorax and legs project from the opening of the shell, to be withdrawn into it when the animal is alarmed or at rest. The abdomen being always in the shell and thus protected loses the hard body-wall, and is soft, often curiously shaped and twisted to correspond to the cavity of the shell. It has on it no legs or appendages except a pair for the hindmost segment which are modified into hooks for holding fast to the interior of the shell. As the hermit crab grows it takes up its abode in larger and larger shells, sometimes killing and removing piece-meal the original inhabitant. Some hermit crabs always have attached to the shell certain kinds of sea-anemones. It is believed that both crab and sea-anemone derive advantage from this arrangement. The sea-anemone, which otherwise cannot move, is carried from place to place by the crab and so may get a larger supply of food, while the crab is protected from its enemies, the predaceous fishes, by the stinging threads of the sea-anemone, and also perhaps by the concealment of the shell its presence affords. This living together by two kinds of animals to their mutual advantage is called commensalism or symbiosis (see Chapter [XXX]). The hermit crabs are not true crabs, but are more nearly related to the crayfishes and shrimps than to the true broad-bodied, short-tailed crabs.
Barnacles.—Technical Note.—Specimens of barnacles may be got readily from the tide rocks or from piles in a harbor. Interior schools should have, if possible, specimens preserved in alcohol or formalin for examination. The "shells" of acorn (sessile) barnacles may often be found on oyster shells (get at restaurants).
Crustaceans which at first glance are hardly recognizable as such are the stalked or sessile barnacles (fig. [37]) which live fixed in great numbers on the rocks between the tide lines, or on the piles supporting wharves, or on the bottom of ships or even on the body-wall of whales and other ocean animals. In the stalked forms the stalk is a flexible stem or peduncle covered with a blackish finely-wrinkled skin bearing at its free end the greatly modified body of the barnacle. This body is enclosed in a sort of bivalved shell or carapace formed by a fold of the skin and stiffened by five calcareous plates. Within this curious shell is the compact, rather worm-like body-mass, showing little or no indication of segmentation. The legs, of which there are usually six pairs, are much modified, being long, feathery, and divided nearly to the base. These feathery feet project from the opened shell when the animal is undisturbed, and waving about in the water catch small animals which serve as the barnacle's food. When disturbed the barnacle withdraws its feet and closes tightly its strong protecting shell. The acorn-barnacles have no stalk, but look like a low bluntly-pointed pyramid, this appearance being due to the converging arrangement of six calcareous plates in its body-wall.
The barnacles present several unusual conditions with regard to the internal organs. They have no heart nor any blood-vessels; most of the species are hermaphroditic; and there are other indications of a degenerate condition. This degeneration of the barnacles is due to their fixed life, the results of which are like those of a parasitic life. The young barnacles when hatched from the egg are free-swimming larvæ as with the other Crustacea. They finally attach themselves and undergo the changes, some of them of degenerative nature, which produce the body-structure of the adult. It was long a belief among many people that the barnacle produced the barnacle goose. Pictures in ancient books show the young barnacle geese issuing from the opened shell of the barnacle. The early naturalists believed barnacles, on account of the shell, to be a kind of shell-fish or mollusc, but when their development was thoroughly worked out, it became evident that they belong to the Crustacea.
[CHAPTER XXI]
BRANCH ARTHROPODA (continued); CLASS INSECTA: THE INSECTS
THE LOCUST (Melanoplus sp.)
Technical Note.—Locusts or grasshoppers are common and familiar insects all over the country. The genus Melanoplus includes numerous species, one or more of which are to be found in almost any locality. The common red-legged locust (M. femur-rubrum) of the East, the Rocky Mountain migratory locust (M. spretus), of the West, the large differential (M. differentialis) and two-striped (M. bivittatus) locusts of the Southwest, are especially common species. All the members of the genus have their hind wings uncolored, and the front wings marked with a longitudinal series of small dots more or less distinct, or with a longitudinal line. There is a small blunt spine or process projecting from the ventral aspect of the prothorax. If a species of Melanoplus cannot be found, any other locust may be used, although there are some slight variations in the external structure of the various species. Fresh specimens killed in a cyanide bottle (for preparing see p. [463]) are preferable in the study of the external structure, but specimens preserved in alcohol will do.
External structure (fig. [38]).—Note that the body of the grass-hopper is composed of successive rings or segments grouped into three regions, the head (anterior), thorax (median), and abdomen (posterior). In which region of the body are the segments most readily distinguished? Of how many segments does the head appear to be composed? The thorax is composed of three segments of which the most anterior, to which is attached the front pair of legs, differs from the succeeding two, being freely movable and bearing a large hood- or saddle-shaped piece on its dorsal aspect. To the other two thoracic segments the second and third pair of legs are attached, as are also the two pairs of wings. The remaining segments of the body compose the abdomen.
Fig. 38.—The red-legged locust, Melanoplus femur-rubrum, to show external structure.
Note the smooth, rather firm and horny character of the body. This is due to the fact that the skin is everywhere covered with a cuticle in which is deposited a horny substance called chitin. The cuticle is not uniformly firm over the body. At the junction of the body segments in the abdomen, in the neck and between the segments of the legs, in fact, wherever motion is desirable, the cuticle is flexible, thus making bending of the body-wall possible. Elsewhere, however, it is hard and stiff, serving not only as a protective coat or armor over the body, but also affording firm places for the attachment of muscles.
Insects (and all other Arthropods) have no[9] internal skeleton, but, in this firm cuticle, an exoskeleton.
Although the head is apparently a single segment, it is really composed of six or seven body segments greatly modified and firmly fused together. Note that it bears a pair of large compound eyes and three much smaller simple eyes or ocelli.
Technical Note.—Strip off a bit of the outer covering of a compound eye, mount on a glass slide and examine under the microscope.
Note that, as in the crayfish, each compound eye is composed externally of many small hexagonal facets, the outer covering, the cornea, being simply the cuticular covering of the body, in this place transparent and divided into small facets. Besides the eyes, the head bears also several movable appendages, namely the antennæ, and the mouth-parts. Note the number, place of insertion, and segmented character of the antennæ. These antennæ are sense-organs and are used for feeling, smelling, and, in some insects, for hearing. Note that the mouth-parts consist of an upper, broad, flap-like piece, the[10]labrum; of a pair of brown, strongly chitinized, toothed jaws or mandibles; of a second pair of jaw-like structures, the maxillæ, each of which is composed of several parts; and of an under, freely-movable flap, the labium, also composed of several pieces. Each maxilla bears a slender feeler or palpus composed of five segments. The labium bears a pair of similar palpi, which are, however, only three-segmented. The mandibles and maxillæ, which are the insect jaws, move laterally, not vertically as with most animals.
Make drawings of the lateral aspect of the head; of a bit of the cornea; of the dissected out mouth-parts.
Of the three segments of the thoracic region of the body, the most anterior one is called the prothorax. It is freely movable and has a large hood or saddle-shaped piece, the pronotum, on its dorsal aspect, and a blunt-pointed tubercle on the ventral aspect. The foremost pair of legs is attached to the prothorax. The next segment is the mesothorax, which is immovably fused to the next thoracic segment. What appendages does it bear? The third segment is the metathorax, which besides being fused with the mesothorax in front, is similarly fused with the foremost abdominal segment behind. What appendages does the metathorax bear?
Examine one of the fore legs and note that it is composed of a series of unequal parts or segments. The segment nearest the body is sub-globular and is called the coxa; the second segment is smaller than the coxa and is called the trochanter; the third, known as the femur, is the largest of all; the fourth, tibia, is long and slender; and the next three, the last of which is the terminal one and bears a pair of claws and between them a little pad, the pulvillus, are called the tarsal segments. Most insects have five tarsal segments. Note the great size of the hindmost or leaping legs. Determine the segments of the middle and hindmost legs. Make a drawing of a fore leg.
Examine the wings. In what ways do the front wings differ from the hind wings? The front wings are known as the wing covers or tegmina. Note how the hind wings fold up like a fan, and are covered and protected by the wing covers. Draw the wings.
The abdomen is composed of a number of segments most of which resemble each other. The first segment (immediately behind the metathorax) has its dorsal and ventral parts widely separated by the cavities for the insertion of the hindmost legs. The ventral part of this segment is dovetailed into the ventral part of the metathorax and appears to be part of it. In the dorsal part of this segment there is on each side a spot where the cuticle is only a thin membrane. At these places are the auditory organs or ears of the locust. The thin membranes are the tympana. Only the various kinds of locusts and those insects closely related to them have ears of this kind. Most other insects are believed to have the sense of hearing situated in the antennæ.
The abdominal segments from second to eighth are ring-like in form and are without appendages. There is on the side of each of these segments near its front margin a tiny opening or pore called a spiracle. These spiracles are the breathing pores of the locust, which does not take in air through its mouth or any other opening in the head. There is a spiracle near each ear in the first abdominal segment, and one on each side of the mesothorax near the insertion of the middle legs.
The terminal segments of the abdomen are provided with certain processes which are different in male and female. The female has at the tip of its abdomen two pairs of strong, curved pointed pieces which compose the ovipositor, or egg-laying organ. The opening of the oviduct lies between the pieces. The male has a swollen rounded abdominal tip, with three short inconspicuous pieces on the dorsal surface.
Make a drawing of the lateral aspect of the abdomen of a female locust; also, of a male.
For a more detailed account of the external anatomy of a locust see Comstock and Kellogg's "Elements of Insect Anatomy," chap. II.
The external structure of the grasshopper should be carefully compared with that of the crayfish; pay special attention to the mouth-parts and legs.
The teacher should point out the homologies and modifications.
Life-history and habits.—The eggs of the locust are laid in the autumn in the ground in bare dry places, as roadsides, closely-grazed pastures, etc. The female thrusts her strong ovipositor into the soil, and by opening and shutting it, thus boring, pushes in the abdomen for about two thirds its length. The eggs, about one hundred, are then deposited in a capsule or pod. The young locusts hatch in the following spring. When just hatched they resemble the parent locust in general appearance and structure except that they lack wings, and are of course very small. The young locusts are gregarious, congregating in warm and sunny places. They feed on green plants and travel about by walking and hopping. At night they try to find shelter under rubbish in the fields. They feed voraciously and grow rapidly, reaching maturity in about two months. During this post-embryonic development and growth they molt (shed the chitinous exoskeleton) five times. After the first molt indications of the wings appear in the shape of small backward and downward prolongations of the posterior margins of the dorsum of the mesothorax and metathorax. With each succeeding molt these wing-pads, or developing wings, are larger and more wing-like, until after the last molting they appear fully developed. With each molting, too, there is a marked increase in size of the locust, the average length of the body just before the first moult being 4.3 mm., before the second 6.8 mm., before the third 9 mm., before the fourth 14 mm., before the fifth 17 mm., and after the fifth (the full-grown stage) about 26 mm.
The molting is an interesting process, and can be readily observed. The young locust ready for its last molt crawls up some post, weed, grass stalk, or other object, and clutches this object securely with the hind feet. The head is generally downward. The locust remains motionless in this position for several hours, when the skin suddenly splits along the back from the middle of the head to the base of the abdomen. By steady swelling and contracting and slight wriggling, lasting for half an hour to three-fourths of an hour, the old skin is completely shed, and the wings spread out. In an hour the wings are dry and the new chitinized exoskeleton firm enough for flying, or crawling about, and in another hour the locust begins to eat.
The red-legged locust does considerable damage to cultivated crops, but its injuries are insignificant compared with the tremendous losses occasioned by a near relative, the Rocky Mountain Locust (Melanoplus spretus). This locust has its breeding-grounds on the high plateaus of the Rocky Mountain region, but it sometimes migrates in countless numbers southeast over the plains and into the great grain-fields of the Mississippi valley. Such migrations occurred in 1866, 1867, 1874 (in this year eighteen hundred and forty two families in Kansas were reduced to destitution by the utter wiping out of their crops by the locusts) and 1876. With the settling-up of the regions in which the Rocky Mountain locust breeds, there seems to have come a change of conditions, so that no great migrations have occurred since 1876.
THE GREAT WATER-SCAVENGER BEETLE (Hydrophilus sp.)
Technical Note.—The great water-scavenger beetles are large, black, elliptical insects common in quiet pools where they may be found swimming through the water, or crawling among the plants growing on the bottom. They are an inch and a half long and are readily distinguishable from all other water insects except the predaceous diving beetles (Dyticus). The antennæ of Hydrophilus, however, are thickened (clavate) at the tip, while those of Dyticus are thread-like for their whole length. The beetles may be readily collected with a water-net, and kept alive in glass jars or aquaria in water containing decaying vegetation.
External structure (fig. [39]).—Is the body of the water-beetle composed of segments? Can you make out three body-regions, head, thorax and abdomen? As in the locust the metathorax is fused with the first abdominal segment and with the mesothorax, while the prothorax is freely movable, and is covered above by a strong shield. The chitin armor of the whole body is specially heavy and strong, affording a great protection to the insect.
Fig. 39.—Ventral aspect of male great water-scavenger beetle, Hydrophilus sp.
On the flattened head note the compound eyes and the peculiarly-shaped nine-segmented antennæ. Are there any ocelli? Dissect out the mouth-parts. The beetle's mouth is fitted for biting, the mouth-parts being in general character like those of the locust, with distinct flap-like labrum, dentate mandibles, jaw-like maxillæ with long, slender, four-segmented palpi and lip-like labium with three-segmented palpi. Make drawings of the antennæ and mouth-parts.
Note the character of the thoracic segments. Examine the wings and legs. The fore wings are modified into strong horny sheaths, or elytra, which completely cover and protect the folded hind wings. The hind wings are large and membranous. How are they folded? Note the adaptation of the middle and hind legs for swimming. Determine the various segments of the legs, i.e. coxa, trochanter, femur, tibia and tarsus. Note the long longitudinal median keel on the ventral aspect of the thorax.
The abdomen articulates with the metathorax by the full width of the broad first abdominal segment. It is composed of a series of segments without appendages, of about equal length but decreasing in width from in front backwards. Of how many segments does the abdomen seem to be composed when viewed from the ventral aspect? From the dorsal?
Make a drawing of the ventral aspect of the whole body.
Technical Note.—After examining the abdomen thus far, remove it from the rest of the body, and boil it in dilute potassium hydrate (KOH) in a test-tube. This will soften and partially bleach the body wall.
Examine the softened specimen, and note that at least two additional segments are to be found retracted or telescoped into the apparently last segment. The character of these terminal abdominal segments differs in male and female individuals, and specimens of both sexes should be examined. (The males can be distinguished from the females by the peculiar pad-like expansion of the last tarsal segment of the fore legs.) Pull out the retracted segments, and note that they are unevenly chitinized, parts of their surface being simply membranous. Projecting backwards are several long-pointed processes. The female has but one retracted segment. Though the females of many insects possess more or less elaborately developed egg-laying organs, this is not the case with the beetles. Look for spiracles near the lateral margins of the dorsal surface of the abdomen. How many pairs are present?
Internal structure (fig. [40]).—Technical Note.—If fresh specimens are to be had, kill by dropping into the cyanide bottle (see p. [463]). Specimens preserved in a 5% solution of chloral hydrate may be used if necessary. When putting specimens into this solution a small slit should be cut through the body wall to allow the preservative to enter the body cavity. When ready to dissect a specimen cut off the elytra and wings close to the base, and carefully remove all of the dorsal wall of the abdomen and thorax and the median portion of the dorsal wall of the head. Pin out, ventral side down, under water in a dissecting-dish.
Fig. 40.—Dissection of female great water-scavenger beetle, Hydrophilus sp., the heart and tracheæ being cut away.
Note in the median dorsal line of the abdomen a pale transparent longitudinal vessel, the heart or dorsal vessel. Note on each side of it six prominent triangles or "Vs" with apex of each directed laterally, the posterior three smaller than the anterior three of each side. These triangles are formed by respiratory tubes or tracheæ. From each spiracle or breathing-pore there extends into the body a respiratory tube or trachea. These lateral tracheæ join a main longitudinal trachea on each side, from which are given off branches, which in turn repeatedly subdivide, until all parts of the body are ramified by tracheæ, large and small, bringing air to all the tissues. The oxygen is taken up from this air, and carbonic-acid gas is given up to it, when it passes out of the body again through the spiracles. Thus in the insects oxygen and carbonic-acid gas are not carried by the blood but by special air-tubes. The respiratory system of insects is very different from that of other animals.
Mount a bit of trachea in glycerine on a glass slide and examine under the microscope. Note the fine spiral line (looking like transverse annular striations) which is a thickening of the chitinous inner wall of the tube and which by its elasticity keeps the tracheal tubes open.
The heart, already noted, is composed of a longitudinal series of very thin-walled chambers, each with a pair of lateral openings into the body-cavity and with terminal openings into the adjacent chambers. The blood, which is colorless or greenish or yellowish, is sent forward through the successive heart chambers by regular contractions until it finally pours from the most anterior chamber freely into the body-cavity. Here it bathes the body-tissues, flowing perhaps in regular paths, giving up food to the tissues and taking up food from the alimentary canal, until it finds its way through the lateral openings into the heart chamber again. There are no arteries or veins.
Note the large mass of muscles in the metathorax. Note, by attempting to remove it, that the anterior part of the muscle mass is attached to a chitinous partition-wall between the meso- and meta-thorax. Remove this partition-wall (and one between the metathorax and abdomen) and note that certain muscles run deeply down into the body. By pulling on the bits of chitin to which the muscles are attached, the muscles (if they have not been cut) can be stretched to the length of three-quarters of an inch. When released they will contract. (This stretching and contracting takes place only in fresh specimens.) What are these large and numerous muscles of the thorax for?
Remove the thin membrane stretching over the abdomen and in which the heart and tracheal "Vs" lie, and note immediately underneath it the large coiled intestine with a knot of greenish yellow threads in the centre. Carefully uncoil and pin out the intestine, cutting away the tying tracheæ, but being careful not to cut other structures. Work out the full length of the alimentary canal, noting the œsophagus, the widened crop behind it, and the long intestine. From the intestine arise several greenish yellow threads, the Malpighian tubules. These are the excretory organs of the insect. What is the total length of the alimentary canal?
The reproductive organs, consisting of a pair of glands (egg-glands or sperm-glands) with a pair of tubes which unite before reaching the body-wall and have a common external opening, may now be seen. These should be removed, thus exposing the ventral nerve-chain in the abdomen. To expose the chain in the thorax it will be necessary to pick away carefully the muscles. As in the crayfish, the central nervous system in the beetle consists of a ventral nerve-chain, a brain or supra-œsophageal ganglion and a pair of circum-œsophageal commissures connecting the brain and the foremost ganglion (infra-œsophageal) in the ventral chain. There are, in the ventral chain, four ganglia in the thorax and four in the abdomen. The large nerves running from the brain to the compound eyes and to the antennæ can be traced.
Make a drawing showing the nervous system.
Life-history and habits.—The eggs, usually about one hundred, are deposited in a silken sac or case which is spun by the female, and either floats freely or is attached to the under sides of the leaves of aquatic plants. This egg-case is not wholly filled with eggs but has a considerable air-chamber in it, causing it to float. It is oval in shape, and has a peculiar curved horn-like projection at the upper end. In sixteen or eighteen days the young water-scavenger beetles hatch as elongate, wingless, active larvæ, provided with three pairs of legs and strong jaws. They remain for a short time after hatching in the egg-case, feeding on each other! After they issue from the case they feed on flies or other insects which fall into the water, and on snails. They breathe through a pair of spiracles situated at the posterior tip of the abdomen, coming to the surface and thrusting this tip up so that the spiracles are out of water. They grow rapidly, molting three times before becoming full grown. They attain a length of nearly three inches. When full grown they leave the water, crawling out on the damp shore of the pond or stream, and burrow into the soil for a few inches. Here they molt again, or pupate as it is called, changing to a non-feeding, quiescent stage called the pupal stage. The pupa is the stage in which the great changes from wingless, crawling and swimming, short-legged, long, slender-bodied larva to winged, swimming and flying, long-legged, compact, broad-bodied adult are completed. Late in the summer or in the fall the pupal skin breaks and the adult issues. It works its way to the surface of the ground, and betakes itself to the nearest water.
The water-scavenger beetle shows in its post-embryonal development a "complete metamorphosis" as contrasted with the "incomplete metamorphosis" of the locust. Wherever among insects similar changes occur, the young issuing from eggs as larvæ only remotely resembling the parent, and these active feeding larvæ changing finally into more or less quiescent, strictly non-feeding pupæ, which finally change into the active adults, a complete metamorphosis is said to exist. All the beetles, the butterflies and moths, the two-winged flies, the ants, bees and wasps, and certain other groups of insects undergo in their post-embryonic development a complete metamorphosis. The crickets, katydids, the sucking bugs, the May-flies, the white ants and numerous other insects have, like the locust, an incomplete metamorphosis, that is, the young when hatched resemble in most respects, except in the absence of wings, their parents.
The adult water-scavenger beetle feeds chiefly on decaying vegetation in the water, but instances of the taking of other insects and of snails have been noted. Although an aquatic insect the beetle, like its larva, has no gills for breathing the air which is mixed with the water, but has to come to the surface occasionally to obtain air. This it does in an interesting way, which should be carefully observed by the pupils. The air is received and held by a covering of fine hairs on the ventral surface of the body, so that a considerable supply may be carried about by the beetle while underneath the surface. The beetles often leave the water by night, flying abroad to other ponds or streams. In winter the beetles hibernate, burying themselves in the banks of the ponds which they inhabit.
For a good account, with illustrations, of the water-scavenger beetle's life-history see Miall's "Natural History of Aquatic Insects," pp. 61-87.
THE MONARCH BUTTERFLY (Anosia plexippus)
Technical Note.—The Monarch or Milkweed butterfly is distributed all over the country. It is large, and red-brown in color, and lays its eggs on milk weeds where the greenish yellow and black-banded larvæ (caterpillars) may be found feeding. The covering of scales conceals the outlines of the various external parts, but these scales may be easily removed with dissecting needle and a small brush. In brushing the scales from the head care must be taken not to break off the mouth-parts.
External structure (fig. [41]).—Note the three body-regions, head, thorax and abdomen. Is the body segmented? Note the dark color and firm character of the chitinized cuticle.
Fig. 41.—Body of the monarch butterfly, Anosia plexippus, with scales removed to show the external parts.
Note on the head the large compound eyes. Note the tumid convex clypeus which composes most of the anterior aspect of the head. Are ocelli present? Compare the antennæ with those of the locust and water-beetle. Compare also the mouth-parts and note that they differ radically from those of the locust and beetle. They are not fitted for biting, but for sucking up liquid food (the nectar of flowers). Note the absence of a movable flap-like labrum (a minute narrow stiff piece, bearing at each latera end a small group of fine brown hairs, represents the labrum), the entire absence of mandibles, and the absence of a movable flap-like labium. The labium is a fixed chitinized triangular piece forming part of the floor of the head. Note the long slender proboscis coiled up like a watch-spring. (In fresh specimens this proboscis can be uncoiled and will be found flexible. If dried or alcoholic specimens are being studied, the head of the butterfly should be removed and softened in warm water before the mouth-parts are examined.) On either side of this proboscis is a peculiar pointed process which rises from the under side of the head. These processes are the labial palpi and serve to protect the sucking proboscis. The proboscis itself is composed of the two greatly modified maxillæ. Instead of being short, jaw-like and composed of several pieces as in the locust, in the butterfly each maxilla is a slender, flexible half tube applied against its mate on the opposite side in such a way as to form a perfect tube long enough to reach into the nectaries of flowers when in use and capable of being compactly coiled up at other times. Cut across the proboscis and note the canal in the centre. Try to separate the two maxillæ which compose it.
Make a drawing of the frontal aspect of the head with the eyes and appendages.
Compare the thorax with that of the beetle and that of the locust. The prothorax is a freely movable narrow ring or collar. The mesothorax and metathorax are fused to form a large convex mass, of which fully five-sixths is mesothorax and only one-sixth metathorax. Try to distinguish the boundaries of the two segments. Note the three pairs of legs; the differences in size among them, and the differences between them and the legs of the locust and water-beetle. In one of the legs determine the coxa, trochanter, femur, tibia and tarsal segments. Note the differences between the wings of the butterfly and those of the locust and beetle. Note that the wings are membranous, but are covered with many fine scales (fig. [42]), as is, indeed, the whole body. Rub off some of these scales on a glass slide and examine; note shape, little stem or pedicel of insertion, and longitudinal striations. Examine under microscope a bit of wing from which some of the scales have been rubbed. How are the scales attached to the wing membranes? How are the scales arranged? Note that the wing is colorless where the scales have been removed. All the colors and patterns of the wings of butterflies are produced by the scales.
Make drawings of scales; of parts of denuded wings, and of bit of wing covered with scales.
Remove all or nearly all the scales from a wing and note the arrangement of the veins (venation). Compare with venation in wings of locust.
Make drawing showing venation in the butterfly's wings.
Fig. 42.—Bit of wing of Monarch butterfly,
Anosia plexippus, magnified to show the
scales; some scales removed to show the
insertion-pits and their regular arrangement.
(From specimen.)
The venation of insects' wings is much used in insect classification, and the various veins have been given names. The names of the veins in the butterfly's wings are given in fig. [43]. When the veins in the wings of all the various groups of insects are studied, it is evident that the principal ones are the same in all insects, so that the costa, sub-costa, radius, media, cubitus and anal veins of the butterfly's wings can be compared with the corresponding veins in the wings of a beetle or wasp or fly. Noting the differences in the number and character of branching of these principal veins, and the number and disposition of the cross-veins which connect the longitudinal veins, the various kinds of insects can be to a large extent properly grouped or classified. A detailed account of the wing-veins of insects is given in Comstock and Kellogg's "Elements of Insect Anatomy," chap. VII.
Fig. 43.—Wings of monarch butterfly,
Anosia plexippus, to show venation; c,
costal vein; sc, sub-costal vein; r, radial
vein; cu, cubital vein; a, anal veins.
In addition most insects have a vein
lying between the sub-costal and radial
veins called the median vein.
Of how many segments is the abdomen composed? The first or basal segment is depressed, while the others are more or less compressed. The spiracles are, as in the locust, situated on the lateral aspects of the abdominal segments. What segments bear spiracles? The terminal segments of the abdomen differ in the two species. In the female the dorsal part of the (apparently) last segment is longer than the ventral part and is bent down over it forming a sort of hood over a space enclosed partly by this hood, partly by a bluntly-pointed projection from the ventral surface, and partly by the lateral margins of the segment. In this chamber lies the opening from which the eggs issue. In the male there are several backward-projecting, horny, thin processes.
Make a drawing of the lateral aspect of the whole body.
Life-history and habits.—The tiny, conical, yellowish-green eggs of the monarch butterfly are deposited on the under side of the leaves of milkweeds (Asclepias) and when examined under the microscope are seen to be very beautiful little objects finely ribbed with longitudinal and transverse striæ. The eggs are laid in April and May (depending on the latitude and season) by females which have hibernated in the adult condition. From the eggs the minute, cylindrical, pale-green, black-headed larvæ hatch in four or five days. As soon as hatched the larva devours the eggshell from which it has escaped and then feeds voraciously on the milkweed leaves. It grows rapidly, and in three or four days a blackish band or ring appears on each segment, and for the rest of its life it is very conspicuously colored with its black rings on a yellowish-green background. It molts three times, and in from twelve to twenty days is ready to pupate, or change to a chrysalis.
When ready to pupate the larva usually leaves the milkweed plant, and seeks some such protected place as the under side of a fence-rail or jutting rock. Here it attaches its posterior extremity by a small silken web to the rail or rock, and casting its larval skin appears as a beautiful pale-green chrysalis with ivory black and golden spots. It hangs motionless, and of course without taking food, for from a week to two weeks (according to season and temperature), when the pupal cuticle breaks and the great red-brown butterfly (fig. [165]) issues.
The butterfly feeds (as is indicated by the structure of its mouth-parts) very differently from the larva; it sucks up by means of its long tubular proboscis the nectar of flowers, nor does it confine itself at all to the flowers of milkweeds. It is a fine flyer and a great traveller. Many thousands of these butterflies often make long flights or migrations together. At other times tens of thousands of these butterflies congregate in a certain limited area, clinging sometimes to the branches of a few trees in such numbers and so closely together as to give the tree a brown color. Such a "sembling" of monarch butterflies occurs every year near the Point Pinos lighthouse on the Bay of Monterey, California. The object of this assembling together is not understood. Both the larvæ and adults of the monarch butterfly are distasteful to birds, by their possession of an acrid body-fluid. The species is thus protected against the most dangerous enemies of butterflies, a fact which chiefly accounts for the great abundance and wide distribution of the monarch (see p. [137]). For a full account of the life-history of the monarch butterfly, see "Scudder's Life of a Butterfly."
LARVA OF MONARCH BUTTERFLY (Anosia plexippus)
Technical Note.—For directions for finding and identifying the larvæ of the monarch butterfly see p. [171]. If larvæ (caterpillars) of Anosia cannot be found, those of any other butterfly or moth will do. Use naked, smooth kinds like cutworms, cabbage worms and the like, rather than hairy or spiny ones. Use large specimens. Kill the caterpillar with ether or in a cyanide bottle.
Structure (fig. [44]).—As we have learned from the study of the life-history of the locust, water-beetle and butterfly, some insects are hatched from the egg in a condition resembling that of the parents in most structural characters. This is true of the locust. Other insects, as the beetle and butterfly, are hatched in a form and condition apparently very different from that of the parents. The external appearance of a beetle or butterfly larva differs much from that of the adult or imago of the same individual. It will be of interest to examine more particularly the structural condition of one of these larvæ and to compare it with the structure of the adult.
Fig. 44.—Dissection of the silkworm, larva of the moth Bombyx mori.
Is the body segmented? Is the body composed of head, thorax and abdomen? Note the soft, flexible, weakly-chitinized condition of the body-wall. How many pairs of legs are there? Where are they situated? Is there any difference in the various legs? If so, what is the difference? Which of the legs of the larva correspond with the legs of the butterfly? Why? The prothoracic segment and the abdominal segments 1 to 8 each bear a pair of spiracles (small blackish spots on the sides). Are both compound and simple eyes present? How many eyes are there? Are there antennæ? Dissect out the mouth-parts. How do they differ from those of the butterfly? Are they more like the mouth-parts of the butterfly or more like those of the locust?
With fine sharp-pointed scissors make a shallow longitudinal incision along the whole length of the dorsal wall. In a freshly-killed specimen a drop of pale greenish blood will issue as the scissors' point is first thrust through the skin. Put a droplet of this blood on a glass slide, cover with cover glass and examine with high power of the microscope. Note that the blood is a fluid containing numerous sub-circular or elliptical bodies, the blood-corpuscles. Note at least two kinds of corpuscles: most abundant a granular, circular kind, the true blood-corpuscles; and rarer, a larger, clear, usually elliptical or oval, but sometimes irregular and amœbiform kind, generally spoken of as fat-cells.
Make a drawing of the corpuscles in the field of the microscope.
After making the dorsal longitudinal incision pin out the caterpillar in the dissecting-dish with dorsal aspect uppermost. When the edges of the skin are pinned back, the organs most conspicuous in the body-cavity will be the flocculent masses of adipose tissue, the large, simple, tubular alimentary canal usually dark or greenish because of the color of its contents, and the numerous silvery tracheal tubes. In those caterpillars which spin a silken cocoon, the silk or spinning-glands are usually long and prominent. They lie on either side of the anterior part of the alimentary canal, and open by a common duct on the labium. Rising from behind the middle of the alimentary canal may be found the long, whitish, folded and twisted Malpighian tubules. By picking away the fat masses, expose the full length of the alimentary canal. Note its great size (large diameter). Is it divided into distinct regions such as crop, proventriculus, stomach, intestine, etc.? How is it held in place? Trace the principal longitudinal tracheal trunks. Find, if you can, a pair of small compact bodies usually somewhat elongate, one lying on each side of the posterior part of the alimentary canal. These are the rudimentary reproductive organs.
Remove the alimentary canal by cutting it off at its posterior tip and also in the prothoracic segment. Work out now the ventral nerve-cord and ganglia, and the supra-œsophageal (brain) and infra-œsophageal ganglia and the commissures in the head.
In the body of the caterpillar we have found the same general disposition of organs as in the body of an adult insect, but several differences are nevertheless noticeable, viz., the presence of a large quantity of fatty tissue, the great size and simple character of the alimentary canal, and the undeveloped condition of the reproductive organs.
OTHER INSECTS
The class Insecta includes those Arthropods which have one pair of antennæ (sense appendages), three pairs of mouth-parts (oral appendages), and three pairs of legs (locomotory appendages). The insects, in further contradistinction to the crustaceans, are mostly land animals and breathe by means of tracheæ or tracheal gills. They are the most familiar of land invertebrates, and, as already mentioned, include more species than are comprised in all the other groups of animals taken together. Beetles, moths and butterflies, flies, wasps and bees, dragonflies and grasshoppers are familiar members of the class of insects, but spiders, mites, scorpions, centipeds and thousand-legged worms are not true insects and should not be so miscalled. These last belong to the branch Arthropoda but to other classes than the class Insecta. While insects are found living under most diverse conditions on land, that is, on the ground, in the leaves, fruits and stems of plants, in the trunks of trees or in dead wood, in the soil, in decaying animal or plant matter, as parasites on or in other animals, and in all fresh-water ponds and streams, they do not live in ocean water. A few species live habitually on the surface of the ocean, and a few other forms are found habitually on the water-drenched rocks and seaweeds between tide lines. The varied habits of insects, their economic relations with man, the beauty and grace of many of them, and the readiness with which they may be collected, reared and studied, renders them unusually fit animals for the special attention of beginning students of zoology.
Fig. 45.—A wingless insect; the American
spring-tail. Lepidocyrtus americanus,
common in dwelling-houses. The short
line at the right indicates the natural
size. (From Marlatt.)
Body form and structure.—The segments composing the body of an insect are grouped to form three body-regions, the head, thorax, and abdomen. The head of an adult insect appears to be a single segment or body-ring, but in reality it is composed of several segments, probably seven, completely fused. The head bears the eyes, antennæ and the mouth-parts. The thorax is made up of three segments, each segment bearing a pair of legs. From the dorsal side of the hinder two thoracic segments arise the two pairs of wings which are the most striking structural features of insects. Not all insects are winged, (fig. [45]), and of those which are a few have only one pair of wings, but the great majority of them have two pairs of well-developed wings (fig. [46]), which give them, as compared with the other animals we have studied, a new and most effective means of locomotion. The great numbers of insects and their preponderance among living animals is undoubtedly largely due to the advantage derived from their power of flight. The hindmost part of the body, the abdomen, is composed of from seven to eleven segments, only the last one or two of which are ever provided with appendages. When such posterior abdominal appendages are present they form egg-laying or stinging or clasping organs.
Fig. 46.—A four-winged insect; a stone fly, Perla sp., common about brooks. (From Jenkins and Kellogg.)
The body-wall is usually firm and rigid, with thinner flexible places between the segments and body-parts for the sake of motion. The body-wall is composed of a cellular skin or hypoderm, and an outer non-cellular cuticle in which is deposited a horny substance called chitin. This chitinous cuticle or exoskeleton serves as an armor or protective covering for the soft body within, and also as a point of attachment for the many muscles of the body.
Fig. 47.—Piece of trachea (air-tube) from the larva of the giant-cranefly. (Photo-micrograph by Geo. O. Mitchell.)
Insects vary a great deal in regard to shape and appearance of the body, and certain of the external organs are greatly modified in different insects to adapt them to the varied conditions under which they live. Especially interesting and important are the variations in the character of the mouth-parts and wings, the organs of food-getting and locomotion. In our consideration later of some of the more important groups of insects the modification of these parts will be specially referred to. Despite the great number of insects, however, and their varied habits of life, a strong uniformity of body-structure is noticeable, all of them holding pretty closely to the typical body-plan.
The most interesting feature of the internal anatomy of the insect body is the respiratory system. Insects breathe through tiny paired openings, called spiracles, in the sides of the abdominal (and sometimes the thoracic) segments (the number and disposition of the pairs of spiracles varying much in different insects). These spiracles are the external openings of an elaborate system of air-tubes or tracheæ (fig. [47]) which ramify throughout the whole body and carry air to all the organs and tissues. The blood has apparently nothing to do with respiration as it has in the vertebrate animals, where it carries oxygen to all the body tissues.
The other systems of organs are well developed and in many respects more complex and elaborate than those of any of the other invertebrates. The muscular system comprises a large number of distinct muscles, usually small and short, which are disposed so as to make very effective the various complex motions of antennæ, mouth-parts, legs, wings, and egg-laying organs. The muscles appear to be very delicate, being almost colorless when fresh, but they have a high contractile power. The alimentary canal is divided into various special regions, as pharynx, œsophagus, crop, fore stomach or gizzard, digesting stomach, and small and large intestine. From the canal just at the point of union of the digesting stomach (ventriculus) and the small intestine rise the so-called Malpighian tubules, which are excretory organs. They are long slender diverticula of the alimentary canal, and are typically six (three pairs) in number. The circulatory system is composed of a tubular vessel running longitudinally through the body in the median line just under the dorsal wall. It is composed of a series of chambers or segmental parts, which by a rhythmic contraction and expansion propel the blood anteriorly and into a short, narrow, unsegmented anterior portion of the vessel which may be called the aorta. There are no other arteries or veins, the blood simply pouring out of the anterior end of the dorsal vessel into the body-cavity. It bathes the body tissues, flowing usually in regular channels without walls. It re-enters the dorsal vessel through paired lateral openings in the chambers.
Fig. 48.—The antenna of a carrion beetle, with the terminal three segments enlarged and flattened, and bearing many "smelling-pits", the antenna thus serving as an olfactory organ. (Photo-micrograph by Geo. O. Mitchell.)
The main or central nervous system consists of a large ganglion, the "brain," situated in the head above the œsophagus, which sends nerves to the antennæ and eyes, a ganglion in the head below the œsophagus connected with the brain by a short commissure on each side of the œsophagus, and sending nerves to the mouth-parts; and a ventral nerve-chain composed of a pair of longitudinal commissures lying close together and running from the head to the next to the last abdominal segment, which bears a series of segmentally disposed ganglia, each ganglion being composed of two ganglia more or less nearly completely fused. There is, in addition, a lesser system called the sympathetic system, which comprises a few small ganglia and certain nerves which run from them to the viscera. The function of the nervous system of insects reaches a very high development among the so-called "intelligent insects" and certain extraordinarily complex and interesting instincts are possessed by many forms. The social or communal habits of the ants, bees, and wasps and the habits connected with the deposition of the eggs and the care of the young exhibited by the digger wasps and other insects are of extreme specialization. The organs of special sense are highly specialized, the sense of smell (fig. [48]) reaching in particular a high degree of perfection. One of the compound eyes (figs. [49] and [50]) may contain as many as 30,000 distinct eye-elements or ommatidia, but the sight is probably in no insect very sharp or clear. Among insects there are organs of hearing of two principal kinds. In one kind the organ for taking up the sound-waves is a group of vibratile hairs usually situated on the antennæ, as is the case with the mosquito; in the other kind, it is a stretched membrane or tympanum such as is found in the fore leg of a cricket or katydid or on the first abdominal segment of the locust (fig. [51]).
Fig. 49.—A section through the compound eye (in late pupal stage) of the blow-fly, Calliphora romitoria. In the centre is the brain, with optic lobe, and on the right-hand margin are the many ommatidia in longitudinal section. (Photo-micrograph by Geo. O. Mitchell.).
Fig. 50.—Part of cornea, showing
facets, of the compound
eye of a horse-fly (Therioplectes
sp.). (Photo-micrograph
by Geo. O. Mitchell.)
The sexes are distinct in insects, and there is often a marked sex dimorphism; in numerous species the males are winged while the females are wingless, and in a few cases this condition is reversed. Where there is a difference in size between male and female, the females are usually the larger. Fertilization of the egg takes place in the body of the female and, strangely, this fertilization is effected after the eggshell has been formed. In all insect eggs there is a minute opening in one pole of the eggshell called the micropyle through which the sperm-cells enter. In a few cases the young are born alive, but such a viviparous condition is exceptional. In a few species, too, young are produced parthenogenetically, that is, are produced from unfertilized eggs. And in the case of a few insect species male individuals are not known.
Fig. 51.—The auditory organ of a locust (Melanoplus sp.). The large clear part in centre of the figure is the thin tympanum, with the auditory vesicle (small black pear-shaped spot) and auditory ganglion (at left of vesicle and connected with it by a nerve) on its inner surface. (Photo-micrograph by Geo. O. Mitchell.)
Development and life-history.—The young insect when just hatched from the egg either resembles, except for the absence of wings, its parent in general appearance as in the case of the locust, or it may, as in the butterfly, emerge in a form very unlike the parent. In the first case the young has simply to grow, that is, to increase in size, to develop wings, and to make some other not very obvious developmental changes in order to become fully grown. But in the case of the butterfly, and similarly in the case of all other insects as the flies, beetles, bees et al., whose young hatch in a larval condition differing markedly from the adult, some radical and striking developmental changes occur before maturity is reached. Such insects are said to undergo complete metamorphosis in their development, while those insects like the locusts, the sucking-bugs, white ants, and others, the just hatched young of which resemble their parents, are said to have an incomplete metamorphosis (fig. [52]).
Fig. 52.—The young (at left) and adult (at right) of the bed-bug, Acanthia lectularia, a wingless insect with incomplete metamorphosis. (After Riley.)
In the case of insects with complete metamorphosis, the young hatches as an active grub or worm-like feeding larva which increases in size, casting its skin or molting several times in its growth. Finally after the last larval molt (fig. [53]) called pupation the insect appears in a quiescent non-feeding stage called the pupa (fig. [54]), and encased in an extra thick and firm chitinous exoskeleton. The immovable pupa is sometimes concealed underground, sometimes enclosed in a silken cocoon spun by the larva just before pupation, or is in some other way specially protected. It is in this pupal condition that the great changes from wingless, often legless, worm-like larva to winged, six-legged, graceful imago of adult stage are completed, and with the molting of the chitinous pupal cuticle the metamorphosis or development of the insect is completed. As a matter of fact many of the special organs of the adult, the legs and wings, for example, begin to develop as little buds or groups of cells in the body of the larva, and when the larva is ready to pupate these imaginal wings and legs are drawn out to the external surface of the body, and may be readily recognized as they lie on the ventral surface of the pupa folded and closely pressed to the body surface. In recent years the study of the post-embryonic development of insects with complete metamorphosis has revealed some remarkable changes of the internal organs which result in a nearly complete disintegration or breaking down of most of the internal organs of the larva (fig. [55]) and a rebuilding of the organs of the adult from primitive beginnings.
Fig. 53.—The larva of the violet tip butterfly, Polygonia interragationis, making its last molt, i.e. pupating. (Photograph from life.)
Fig. 54.—Chrysalid (pupa) of the violet tip butterfly, Polygonia interragationis. From this chrysalid issues the full fledged butterfly. (Photograph from life.)
The habits of the larvæ of insects with complete metamorphosis and of the young of some insects with incomplete metamorphosis often differ markedly from the habits of the adults, and as the habits and instincts of insects are remarkably specialized, the study of their behavior and of the structural and physiological modification which their varied habits of life have brought about is of much interest and significance. In later paragraphs this phase of insect study will be again referred to.
Fig. 55.—A cross-section of the body of the pupa of a honey-bee, showing the body cavity filled with disintegrated tissues, and (at the bottom) a budding pair of legs of the adult, the larva being wholly legless. (Photo-micrograph by Geo. O. Mitchell.)
Classification.—Much attention has been paid to the classification of insects and the 300,000 (approximately) known species have been variously grouped together into orders by different entomologists. A subdivision of the class Insecta into five orders was proposed by Linnæus about 1750 and was used until comparatively recently. Since then, however, numerous other arrangements have been proposed, all of them agreeing in increasing the number of orders by breaking up some of the old ones into two or more new ones. The classification adopted in the text-book[11] of zoology which we have made our reference in classification is an 8-order system. The latest English[12] text-book in entomology adopts a 9-order system, while the principal American[13] text-book on this subject divides the insects into nineteen orders.
The classification depends chiefly on the character of the post-embryonic development, that is, on whether the metamorphosis is complete or incomplete, and on the structural character of the mouth-parts and wings. In the following paragraphs a few of the larger insect orders, with some special representatives of each, will be briefly considered.
The best American text-book of the classification and habits of insects is Comstocks' "Manual of Insects." For an account of the structure of the wings and mouth-parts of various insects see Comstock and Kellogg's "Elements of Insect Anatomy."
Orthoptera: the locusts, cockroaches, crickets, katydids, etc.—Technical Note.—Obtain specimens of crickets or katydids, and cockroaches, and compare the external body structure with that of the grasshopper; examine especially the wings, mouth-parts, legs, and egg-laying organs. Note that the hindmost legs of the cockroach are not fitted for leaping but for running. Note the sound-making (stridulating) organs on the bases of the fore wings of the male katydids and crickets. Note the auditory organs (tympana) in the fore tibiæ of the katydids and crickets. Crickets can be easily kept alive in breeding-cages in the laboratory and their feeding habits and much of their life-history observed. The growth of the young and the development of the wings can be noted, and will be found to be essentially similar to the conditions already found in the case of the locust.
Fig. 56.—The house cricket,
male (a) and female (b). (From
Marlatt.)
Fig. 57.—A bird louse, Nirmus
præstans, from a tern,
Sterna maxima. Most birds
are infested with small,
wingless, biting insects,
called bird-lice, which are
external parasites feeding
on the feathers of the bird
host. The bird louse
figured is about 1/12 in. long.
(Photo-micrograph by Geo.
O. Mitchell.)
The locust studied as one of the examples of the class Insecta belongs to the order Orthoptera, which also includes the cockroaches, crickets (fig. [56]), katydids and green grasshoppers, the walking-stick or twig insects, the praying mantis and others. The members of this order all have an incomplete metamorphosis, and in all the mouth-parts are fitted for biting and the fore wings are more or less thickened and modified to serve as covers or protecting organs for the broad, plaited, membranous hind wings, which are the true flight organs. The hind legs of locusts, grasshoppers, crickets, and katydids are very large, and enable the insects to leap; the legs of the cockroaches are fitted for swift running; the fore legs of the praying mantis are fitted for grasping other insects which serve as their food, and the legs of the walking-stick (fig. [162]) are long and slender and fitted for slow walking. The shrill singing of the crickets and katydids and the loud "clacking" of the locusts are all made by stridulation, that is, by rubbing two roughened parts of the body together. The sounds of insects are not made by vocal cords in the throat. The male crickets and katydids (for only the males sing) have the veins of the fore wings modified so that when the bases of the wings are rubbed together (and when the cricket or katydid is at rest the base of one fore wing overlaps the base of the other) a part of one wing called the "scraper" rubs against a part of the other called the "file" and the shrilling is produced. The sounds of locusts are produced by the rubbing of the inside of the hind leg against the outside of the fore wing when the insect is at rest, or by striking the front margin of each hind wing against the hind margin of each fore wing when the locust is flying. For hearing the Orthoptera are provided with auditory organs having the character of tympana or vibrating membranes. In the locusts these ears (fig. [51]) are situated on the dorsal surface of the first abdominal segment; in the katydids and crickets they are in the tibiæ of the fore legs. The food of locusts, crickets, and katydids is vegetable, being usually green leaves; the cockroaches eat either plant or animal substances fresh or dry, while the praying mantis is predaceous, feeding on other insects which it catches in its strong grasping fore legs. The walking-stick or twig insect is an excellent example of what is called "protective resemblance" among animals. Indeed most of the Orthoptera are so colored and patterned as to be almost indistinguishable when on their usual resting- or feeding-grounds. Some of the tropical Orthoptera carry to a marvelous degree this modification for the sake of protection. (In this connection read Chapter [XXXI] referring to "Protective Resemblances".)
Odonata and Ephemerida: the dragon-flies and May-flies.—Technical Note.—Obtain specimens of adult and immature dragon-flies. The young dragon-flies (fig. [59]) may be got by raking out some of the slime and aquatic vegetation from the bottom of a small pond. Compare the external structure of the adult dragonflies with that of the grasshopper; note the large eyes, the narrow nerve-veined wings, the biting mouth-parts, and the short antennæ. Compare the young dragon-flies with the adults; note the developing wings and the peculiar modification of the lower lip into a protrusible, grasping organ which when at rest is folded like a mask over the face. Examine the interior of the posterior part of the alimentary canal to find the rectal gills. Obtain specimens of adult and young May-flies. The young may be found on the under side of stones in a "riffle" in almost any stream. They live also in ponds. They may be recognized by reference to fig. [61]. Compare adult May-flies with the dragon-flies; note the weakly chitinized, delicate body-wall, and the difference in size between fore and hind wings; note the biting mouth-parts of the young and their absence or presence in vestigial condition only in the adults.
The young of both dragon-flies and May-flies may easily be kept alive in the laboratory aquarium (fruit-jars or battery-jars with pond water in), and their feeding habits, their swimming, their respiration, and much of their development observed. The young May-flies should be got from ponds, not running streams. Put one of these semi-transparent May-fly nymphs into a watch-glass of water, and examine under the microscope. The movements of the gills, heart, and alimentary canal, and much of the anatomy can be readily made out. The emergence of the adult from the nymphal skin can be seen if close watch is kept. The young dragon-flies may be seen to capture and devour their prey. They may also transform into adults, but for this it will be necessary to obtain nymphs nearly ready for transformation.
Among the most familiar and interesting insects are the dragon-flies (fig. [58]), sometimes called "devil's darning-needles." They are commonly seen flying swiftly about over ponds or streams catching other flying insects. The dragon-flies are the insect-hawks; they are predaceous and very voracious, and are probably the most expert flyers of all insects. There are many species, and their bright iridescent colors and striking wing-patterns make them very beautiful. The young dragon-flies (fig. [59]) are aquatic, living in streams and ponds, where they feed on the other aquatic insects in their neighborhood. They catch their prey by lying in wait until an insect comes close enough to be reached by the extraordinarily developed protrusible grasping lower lip (fig. [60]). When at rest this lower lip lies folded on the face so as to conceal the great jaws. The young dragon-flies breathe by means of gills which do not project from the outside of the body, as do the gills of other aquatic insects, but line the inner wall of the posterior or rectal part of the alimentary canal. Water enters the canal through the anal opening and bathes these gills, bringing oxygen to them and taking away carbonic acid gas. The aquatic immature life of the dragon-flies lasts from a few months to two years. When ready to change to adult, the young crawls out of the water and clinging to a rock or plant makes its last molt.
Fig. 58.—A dragon-fly, Sympetrumillotum, common in California.(From life.) | Fig. 59.—The young (nymph) of thedragon-fly, Sympetrum illotum.(From Jenkins and Kellogg.) |
Fig. 60.—Young (nymph) dragon-fly, showing lower lip folded and extended.(From Jenkins and Kellogg.) | |
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Fig. 61.—Young (nymph) of
May-fly, showing (g) tracheal
gills. (From Jenkins
and Kellogg.)
Other abundant and interesting pond and brook insects are the May-flies. The young May-flies (fig. [61]) are aquatic, living in streams and ponds and feeding on minute organisms such as diatoms and other algæ. The immature life lasts a year, or even two or three in some species, and then the May-fly crawls out of the water upon a plant-stem or projecting rock and, molting, appears as the winged adult. The adult May-fly, having its mouth-parts atrophied (a few May-flies have functional mouth-parts), takes no food, and lives only a few hours or at most perhaps a few days. It has the shortest life (in adult stage) of all insects. The female drops her eggs into the water.
Hemiptera: the sucking-bugs.—Technical Note.—Obtain specimens of water-striders (narrow elongate-bodied insects with long spider-like legs which run quickly about on the surface of ponds or quiet pools in streams), water-boatmen (mottled grayish insects about half an inch long which swim and dive about in ponds and stream-pools), back-swimmers (which are usually in company with the water-boatmen, but which swim with back downwards and are marked with purplish-black and creamy white patches), cicadas (the dog-day locusts), and plant-lice (the "green fly" of rose-bushes and other cultivated plants). Compare the external structure of some of these Hemiptera with the other insects already examined; note especially the sucking beak, composed of the elongate tube-like labium in which lie the greatly modified flexible needle-like maxillæ and mandibles, the whole forming an equipment for piercing and sucking. Obtain immature specimens of some of these insects (distinguished by their smaller size and the wing-pads); note that the metamorphosis is incomplete, the young resembling the parents in general appearance. Both immature and adult specimens of water-boatmen (Corisa), back-swimmers (Notonecta), and water-striders (Hygrotrechus) can be easily kept in the laboratory aquaria- and their swimming, breathing, and feeding habits observed. Note especially the carrying of air down beneath the water.
The Hemiptera are characterized particularly by their highly specialized sucking mouth-parts, no other of the sucking insects having the proboscis composed in the same manner. The palpi of both maxillæ and labium are wholly wanting in Hemiptera and the flexible needle-like maxillæ and mandibles are enclosed in the tubular labium. This order is a large one and includes many well-known injurious species, as the chinch-bug (Blissus leucopterus), which occurs in immense numbers in the grain-fields of the Mississippi valley, sucking the juices from the leaves of corn and wheat, the grape Phylloxera (Phylloxera vastatrix), so destructive to the vines of Europe and California, the scale insects (Coccidæ) (figs. [62] and [63]), the worst insect pests of oranges, the squash-bugs and cabbage-bug and a host of others. Some of the Hemiptera, for example, the lice and bed-bugs, are predaceous, sucking the blood of other animals.
![[62]](#illus061a.jpg){kind=link}
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Fig. 64.—A water-strider, Hygrotrechus
sp. (From Jenkins and Kellogg.)
The water-striders (fig. [64]) catch other insects, both those that live in the water and those which fall on to its surface, and holding the prey with their seizing fore legs they pierce its body with their sharp beak and suck its blood. They lay their eggs in the spring glued fast to water-plants. The young water-striders are shorter and stouter in shape than the adults.
Fig. 65.—A water-boatman, Corisa
sp. (From Jenkins and Kellogg.)
The water-boatmen (fig. [65]) and back-swimmers swim and dive about in the water, coming more or less frequently to the surface to get a supply of air. This air they hold under the wings, or on the sides and under part of the body entangled in the fine hairs on the surface. The insects appear to have silvery spots on the body, due to the presence of this air. The "rowing" legs of the water-boatmen (Corisa) are the hindmost pair; in the back-swimmers (Notonecta) they are the middle legs.
Fig. 66.—The seventeen-year cicada, Cicada
septendecim; the specimen at left
showing sound-making organ, v. p., ventral
plate; t, tympanum. (From specimen.)
The cicadas (fig. [66]) are the familiar insects of summer which sing so shrilly from the trees, the seventeen-year cicada (Cicada septendecim) (oftentimes called locust) being the best known of this family. Its eggs are laid in slits cut by the female in live twigs. The young, which hatch in about six weeks, do not feed on the green foliage, but fall to the ground, burrow down to the roots of the tree and there live, sucking the juices from the roots, for sixteen years and ten or eleven months. When about to become adult, the young cicada crawls up out of the ground and clinging to the tree-trunk molts for the last time, and flies to the tree-tops.
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The plant-lice (Aphididæ) are small soft-bodied Hemiptera which have both winged and wingless individuals. In the early spring a wingless female hatches from an egg which, laid in the preceding fall, has passed the winter in slow development. This wingless female, called the stem-mother, lays unfertilized eggs or more often perhaps gives birth to live young, all of which are similarly wingless females which reproduce parthenogenetically. This reproduction goes on so rapidly that the plant-lice become overcrowded on the food-plant and then a generation of winged[14] individuals is produced from time to time. These winged plant-lice fly away to new plants. In the autumn a generation of males and females is produced; these individuals mate and each female lays a single large egg which goes over the winter, and produces in the spring the wingless agamic stem-mother. Plant-lice produce honey-dew, a sweetish substance much liked by ants, and the lice are often visited, and sometimes specially cared for, by the ants for the sake of this honey-dew. Small as they are, plant-lice occur in such numbers as to do great damage to the plants on which they feed. The apple-aphis, cherry-aphis, pear-aphis, cabbage-aphis and others are well-known pests. The most notoriously destructive plant-louse is the grape Phylloxera, which lives on the roots and leaves of the grape-vine. Immense losses have been caused by this pest, especially in the wine-producing countries of southern Europe.
Diptera: the flies.—Technical Note.—Obtain specimens of the adult and young stages of the blowfly and the mosquito. All the young stages of the blowfly may be obtained, and its life-history studied, by exposing a piece of meat to decay in an open glass jar. The larvæ of the mosquito are the familiar wrigglers of puddles and ponds, and by collecting some of them and keeping them in a glass jar of water covered with a bit of mosquito-netting, the life-history of the mosquito is easily studied. If the eggs can be obtained from the pond so much the better; they are in little black masses floating on the surface of the water, and resemble at first glance nothing so much as a floating bit of soot. The external structure of the adult flies should be compared with that of the other insects studied, noting especially the condition of mouth-parts and wings, and the substitution of balancers for the hind wings. The mouth-parts of the mosquito are in the form of a long proboscis composed of six slender needle-like stylets lying in a tube narrowly open along its dorsal surface. The tube is the labium, and the stylets are the two maxillæ, two mandibles, and two other parts known as the epipharynx and the hypopharynx. Two additional thicker elongate segmented processes lying outside of and parallel with the tube are the maxillary palpi. The male mosquito (distinguished from the female by the more hairy or bushier antennæ) lacks the pair of needle-like mandibles. The mouth-parts of the blowfly are composed almost exclusively of the thick fleshy proboscis-like labium, which is expanded at the tip to form a rasping organ.
The Diptera or true flies are readily distinguishable from other insects by their having a single pair of wings instead of two pairs, the hind wings being transformed into small knob-headed pedicels called balancers or halteres. The flies undergo complete metamorphosis, and their mouth-parts are fitted for piercing and sucking (as in the mosquito) or for rasping and lapping (as in the blowfly). Nearly 50,000 species of flies are known, more than 4,000 being known in North America alone.
The blowfly (Calliphora vomitoria) is common in houses, but can be distinguished from the house-fly by its larger size and its steel-blue abdomen. It lays its eggs on decaying meat (or other organic matter) and the white footless larvæ (maggots) hatch in about twenty-four hours. They feed voraciously and become full grown in a few days. They then change into pupæ which are brown and seed-like, being completely enclosed in a uniform chitinized case which wholly conceals the form of the developing fly. The house-fly has a life-history and immature stages like the blowfly, but its eggs are deposited on manure.
Fig. 67.—The mosquito, Culex sp.; showing eggs (on surface of water), larvæ (long and slender, in water), pupa (large headed, at surface), and adult (in air). (From living specimens.)
The mosquito (Culex sp.) (fig. [67]) lays its eggs in a sooty-black little boat-shaped mass which floats lightly on the surface of the water. In a few days the larvæ, or "wrigglers," issue and swim about vigorously by bending the body. The head end of the body is much broader than the other, the thoracic segments being markedly larger than the abdominal ones. The head bears a pair of vibrating tufts of hairs, which set up currents of air that bring microscopic organic particles in the water into the wriggler's mouth. At the posterior tip of the body are two projections, one the breathing-tube (the wriggler coming often to the surface to breathe), and the other the real tip of the abdomen. The wriggler, although heavier than water, can hang suspended from the surface film by the tip of its breathing-tube. It changes in a few days into the pupa, which, instead of being quiescent as with most flies, can swim about. It has a large bulbous head end and the posterior end of the body bears a pair of swimming-flaps. It takes no food. When ready to change to the adult mosquito the pupa (which, unlike the wriggler, is lighter than water) floats at the surface of the water, back uppermost. The chitinous cuticle splits along the back and the delicate mosquito comes out, rests on the floating pupal skin until its wings are dry, and then flies away. Only the female mosquitoes suck blood. If they cannot find animals, mosquitoes live on the juices of plants. They are world-wide in their distribution, being serious pests even in Arctic regions, where they are often intolerably numerous and greedy. Recent investigations have shown that the germs which cause malaria in man live also in the bodies of mosquitoes, and are introduced into the blood of human beings by the biting (piercing) of the mosquitoes. It is probable also that the germs of yellow fever are distributed by mosquitoes in the same way. By pouring a little kerosene on the surface of a puddle no mosquitoes will be able to escape from the water.
![[67]](#illus064.jpg){kind=link}
Fig. 68.—The house-flea, Pulex irritans; a, larva; b, pupa; c, adult. (The fleas are probably more nearly related to the Diptera than to any other order of insects.) (After Beneden.)
Lepidoptera: the moths and butterflies.—Technical Note.—Obtain specimens of a few moths, and compare with the butterfly already studied; note especially the character of antennæ. Obtain miscellaneous specimens of larvæ, pupæ, and cocoons of any moths or butterflies. Note the variety in colors, markings, and skin coverings of the larvæ; note the shape and markings of the pupæ. Rear from eggs, larvæ, or pupæ in breeding-cages any moths and butterflies obtainable (for directions for rearing moths and butterflies see Chapter [XXXIV]), keeping note of the times of molting and of the duration of the various immature stages. If the eggs of silkworms can be obtained the whole life cycle of the silkworm moth can be observed in the schoolroom. The larvæ (worms) feed on mulberry or osage orange leaves, feeding voraciously, growing rapidly and making no attempts to escape. The molting of the larvæ can be observed, the spinning of the silken cocoon, and the final emergence of the moth. The moths after emergence will not fly away, but if put on a bit of cloth will mate, and lay their eggs on it. From these eggs, which should be kept well aired and dry, larvæ will hatch in nine or ten months (if the race is an "annual").
The Lepidoptera (figs. [69]-[74]) include all those insects familiarly known to us as moths and butterflies; they are characterized by their scale-covered wings (fig. [69]) and long nectar-sucking proboscis composed of the two interlocking maxillæ. They undergo a complete metamorphosis (fig. [70]) and their larvæ are the familiar caterpillars of garden and field. These larvæ have biting mouth-parts and feed on vegetation, some of them being very injurious, for example the army-worms, cut-worms, codlin moth worms, etc. The adult moths and butterflies take only liquid food, or no food at all, and are wholly harmless to vegetation. The structure and life-history of a butterfly has already been studied, and in the more general conditions of structure and life-history there is much similarity in the many insects of this order. The eggs are usually laid on the food-plant of the larva; the larva feeds on the leaves of this plant, grows, molts several times, and pupates either in the ground or in a silken cocoon or simply attached to a branch or leaf. There are about six thousand species of moths and butterflies known in North America, and they are our most beautiful insects.
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Fig. 69.—A small, partly denuded part, much magnified, of a wing of a "blue" butterfly, Lycæna sp., showing the wing, scales and the pits in the wing-membrane, in which the tiny stems of the scales are inserted. (Photo-micrograph by Geo. O. Mitchell.)
Coleoptera: the beetles.—Technical Note.—Obtain specimens of various beetles, among them some water-beetles and June-beetles with their young stages, if possible; if not, then the young stages and adults of any beetle common in the neighborhood of the school. Of the swimming and diving water-beetles there are three families, viz., the Gyrinidæ or whirligig beetles, with four eyes (each compound eye divided in two), the Hydrophilidæ, or water-scavengers with two eyes and antennæ with the terminal segments thicker than the others, and the Dytiscidæ or predaceous water-beetles with two eyes and slender thread-like antennæ. Try to find Dytiscidæ, large, oval, shining black beetles; the larvæ are called water-tigers and are long, slim, active creatures with six legs and slender curving jaws (see fig. [76]). The June-beetles are the heavy brown buzzing "June-bugs" and their larvæ are the common "white grubs" found underground in lawns and pastures. Have live water-tigers and predaceous water-beetles in the aquarium. Note their feeding and breathing. Compare the external structure of the beetles with that of the other insects, noting especially the biting mouth-parts, and their thickened horny fore wings serving as covers for the folded membranous hind wings.
![[76]](#illus073.jpg){kind=link}
Fig. 70.—The forest tent-caterpillar moth, Clisiocampa disstria, in its various stages; m, male moth; f, female moth; p, pupa; e, eggs (in a ring) recently laid; g, eggs hatched; c, larva or caterpillar. Moths and caterpillar are natural size, eggs and pupa slightly enlarged. (Photograph by M. V. Slingerland.)
Fig. 71.—A trio of apple tent-caterpillars, Clisiocampa americana, natural size. These caterpillars make the large unsightly webs or "tents" in apple-trees, a colony of the caterpillars living in each tent. (Photograph from life by M. V. Slingerland.)
Fig. 72.—A family of forest tent-caterpillars (Clisiocampa disstria), resting during the day on the bark, about one-third natural size. (Photograph from life by M. V. Slingerland.)
The Coleoptera is the largest insect order, probably 100,000 species of beetles being known, of which 10,000 species are found in North America. They pass through a complete metamorphosis (figs. [75] and [76]), the larvæ of the various kinds showing much variety in form and habit. The pupæ are quiescent and are mummy-like in appearance, the legs and wings being folded and pressed to the ventral surface of the body. Among the familiar beetles are the lady-birds, which are beneficial insects feeding on plant-lice and other noxious forms; the beautifully colored tiger-beetles, predaceous in habit; the "tumblebugs" and carrion beetles, which feed on decaying organic matter; the luminous fire-flies with their phosphorescent organs on the ventral part of the abdomen; the striped Colorado potato-beetle and the cucumber-beetles and numerous other destructive leaf-eating kinds; the various weevils (fig. [78]) that bore into fruits, nuts and grains, and the many wood-boring beetles, destructive to fruit-trees as well as to shade- and forest-trees.
![[75]](#illus072.jpg){kind=link}
![[78]](#illus074b.jpg){kind=link}
Fig. 73.—Moths of the peach-tree borer, Sanninoidea exitiosa, natural size; the upper one and the one at the right are females. (Photograph by M. V. Slingerland.)
The predaceous water-beetles (Dyticus sp.) are common in ponds and quiet pools in streams. When at rest they hang head downward with the tip of the abdomen just projecting from the water. Air is taken under the tips of the folded wing-covers (elytra) and accumulates so that it can be breathed while the beetle swims and feeds under water. When the air becomes impure the beetle rises to the surface, forces it out, and accumulates a fresh supply. The beetles are very voracious, feeding on other insects, and even on small fish. The eggs are laid promiscuously in the water, and the elongate spindle-form larvæ (fig. [77]) called water-tigers are also predaceous. They suck the blood from other insects through their sharp-pointed sickle-shaped hollow mandibles. When a larva is fully grown it leaves the water, burrows in the ground, and makes a round cell within which it undergoes its transformations. The pupa state lasts about three weeks in summer, but the larvæ that transform in autumn remain in the pupa state all winter.
![[77]](#illus074a.jpg){kind=link}
Fig. 74.—Army-worms, larvæ of the moth, Leucania unipuncta, on corn. (Photograph by M. V. Slingerland.)
Fig. 75.—The quince-curculio (a beetle),
Conotrachelus cratægi, natural size and
enlarged. (Photograph by M. V.
Slingerland.)
The June-beetles (June-bugs) (Lachnosterna sp.) feed on the foliage of trees. Their eggs are laid among the roots of grass in little hollow balls of earth, and the fat sluggish white larvæ feed on the grass-roots. They sometimes occur in such numbers as to injure seriously lawns and meadows. The larvæ live three years (probably) before pupating. They pupate underground in an earthen cell, from which the adult beetle crawls out and flies up to the tree-tops.
Hymenoptera: the ichneumon flies, ants, wasps, and bees.—Technical Note.—Obtain specimens of wasps, both social (distinguished by having each wing folded longitudinally) and solitary (wings not folded longitudinally), and if possible of both queens (larger) and workers (smaller) of the social kinds; of ants both winged (males or females) and wingless (workers) individuals; also of honey-bees, including a queen, drones, and workers, and some brood comb containing eggs, larvæ, and pupæ. The bee specimens can be got of a bee raiser. Compare the external structure of ants, bees, and wasps with that of other insects; note the pronounced division of the body into three regions (head, thorax, abdomen); note the character of the mouth-parts having mandibles fitted for biting (ants and wasps) or moulding wax (honey-bees) and having the other parts adapted for taking both solid and liquid food; note the sting (possessed by the females and workers only). Observe the behavior of bees in and about a hive; note the coming and going of workers for food. Observe bees collecting pollen at flowers; observe them drinking nectar. Examine the honey-bee in its various stages, egg, larva, pupa, adult. Note the special structure of the adult worker fitting it to perform its various special labors; the pollen-baskets on the hind legs; the wax-plates on the ventral surface of the abdomen, the wax-shears between tibia and tarsus of hind legs; the antennæ-cleaners on the fore legs; the hooks on front margin of hind wings, etc.
Fig. 76.—Immature stages of the quince curculio, Conotrachelus cratægi; at the left, the larva natural size and enlarged; at the right, the pupa. The beetle lays its eggs in pits on quinces, and the larva lives inside the quince as a grub; the pupa lives in the ground. (Photograph by M. V. Slingerland.)
The Hymenoptera include the familiar ants, bees, and wasps, and also a host of other four-winged, mostly small, insects, many of which are parasites in their larval stage on other insects. All Hymenoptera have a complete metamorphosis, and their habits and instincts are, as a rule, very highly specialized. The parasitic Hymenoptera such as the ichneumon flies, chalcid flies, etc., are stingless but have usually a piercing ovipositor (the sting being only a modified ovipositor). The general life-history of these ichneumons is as follows: the female ichneumon fly, finding one of the caterpillars or fly or beetle larvæ which is its host, settles on it and either lays an egg or several eggs on it, or thrusting in its ovipositor, lays the eggs in the body; the young ichneumon hatching as a grub burrows into the body of its caterpillar host, feeding on the body-tissues, but not attacking the heart or nervous system, so that the host is not soon killed; the ichneumon pupates either inside the host, or crawls out and, spinning a little silken cocoon (fig. [160]), pupates on the surface of the body or elsewhere.
![[160]](#illus156.jpg){kind=link}
Fig. 77.—Water-tiger, the
larva of the predaceous
water-beetle, Dyticus sp.
(From specimen.)
Some of the stingless Hymenoptera are not parasites, but are gall-producers. The female with its piercing ovipositor lays an egg in the soft tissue of a leaf or stem, and after the larva hatches the gall rapidly forms. The larval insect lies in the plant-tissue, having for food the sap which comes to the rapidly growing gall. It pupates in the gall, and when adult eats its way out.
Fig. 78.—The plum curculio,
Conotrachelus nenuphar, a
beetle very injurious to plums.
(Photograph by M. V. Slingerland.)
The ants, bees, and wasps are called the stinging Hymenoptera, although the ants we have in North America have their sting so reduced as to be no longer usable. Among these Hymenoptera are the social or communal insects, viz., all the ants, the bumblebees and honey-bee, and the few social wasps, as the yellow-jacket and black hornet. There are many more species of non-social or solitary bees and wasps than social ones, and their habits and instincts are nearly as remarkable.
Fig. 79.—The currant-stem girdler, Janus integer, a Hymenopteron at work girdling a stem after having deposited an egg in the stem half an inch lower down. (Photograph by M. V. Slingerland.)
The solitary and digger wasps do not live in communities as the hornets do, but each female makes a nest or several nests of her own, lays eggs and provides for her own young. The nest is usually a short vertical or inclined burrow in the ground, with the bottom enlarged to form a cell or chamber. In this chamber a single egg is laid, and some insects or spiders, captured and so stung by the wasps as to be paralyzed but not killed, are put in for food. The nest is then closed up by the female, and the larva hatching from the egg feeds on the enclosed helpless insects until full grown, when it pupates in the cell and the issuing adult gnaws and pushes its way out of the ground. Each species of wasp has habits peculiar to itself, making always the same kind of nest, and providing always the same kind of food. Some of these wasps make their nests in twigs of various plants, especially those with pithy centres in the stems. For interesting accounts of the habits of several digger wasps see Peckham's "The Solitary Wasps."
The solitary bees, of which there are similarly many kinds, are like the solitary wasps in general habit, only they provision the nest with a mixture of pollen and nectar got from flowers instead of with stung insects. Sometimes many individuals of a single species of solitary bee will make their nests near together and thus form a sort of community in which, however, each member has its own nest and rears its own young. In the case of certain small mining bees of the genus Halictus, a step farther toward true communal life is taken by the common building and use by several females of a single vertical tunnel or burrow from which each female makes an individual lateral tunnel, at the end of which is a brood-chamber. Perhaps half a dozen females will thus live together, each independent except for the common use of the vertical tunnel and exit.
The bumblebees (Bombus sp.) are truly communal in habit. All the eggs are laid by a queen or fertile female, which is the only member of the colony to live through the winter. In the spring she finds a deserted mouse's nest or other hole in the ground, gathers a mass of pollen and lays some eggs on it. The larvæ, hatching, feed on the pollen, dig out irregular cells for themselves in it, pupate, and soon issue as workers, or infertile females. These workers gather more pollen, the queen lays more eggs, and several successive broods of workers are produced. Finally late in the summer a brood containing males (drones) and fertile females (queens) is produced, mating takes place, and then before winter all the workers and drones and some of the queens die, leaving a few fertilized queens to hibernate and establish new communities in the spring.
The yellow-jackets and hornets (Vespidæ), the so-called social wasps, have a life-history very like that of the bumblebees. The communities of the social wasps are larger and their nests are often made above ground, being composed of several combs one above the other and all enclosed in a many-layered covering sac open only by a small hole at the bottom. This kind of nest hangs from the branch of a tree and is built of wasp-paper, which is a pulp made from bits of old wood chewed by the workers. The brood-cells are provisioned with killed and chewed insects, the larvæ of both solitary and social wasps being given animal food, while the larvæ of both solitary and social bees are fed flower-pollen and honey. As in the bumblebees, all the members of the community except a few fertilized females die in the autumn, the surviving queens founding new colonies in the spring. The queen builds a miniature "hornet's nest" in the spring, lays an egg in each cell and stores the cells with chewed insects. The first brood is composed of workers, which enlarge the nest, get more food, and relieve the queen of all labor except that of egg-laying. More broods of workers follow until the fall brood of males and females appears, after which the original process is repeated.
The honey-bees and ants show a highly specialized communal life, with a well-marked division of labor and an individual sacrifice of independence and personal advantage which is remarkable. Their communities are large, including thousands of individuals, and the structural differences among the males, females, and workers are readily recognizable. With the ants the workers may be of two or more sorts, a distinction into large and small workers or worker majors and worker minors being not uncommon.
Fig. 80.—The honey-bee, Apis mellifica; A, queen, B, drone, C, worker. (From specimens.)
A honey-bee community, living in hollow tree or hive, includes a queen or fertile female, a few hundred drones or fertile males, and ten to forty thousand workers, infertile females (fig. [80]). The number of drones and workers varies, being smallest in winter. Each kind of individual has a certain particular part of the work of the whole community to do; the queen lays all the eggs, that is, is the mother of the entire community; the drones act simply as the royal consorts, fertilizing the eggs; while the workers build the comb, produce the wax from which the cells are constructed, bring in all the food consisting of flower-pollen and nectar, care for the young bees, fight off intruders, and in fact perform all the many labors and industries of the community except those of reproduction. There is a certain not very well understood and perhaps not very sharply defined division of these labors among the worker individuals, the younger ones acting specially as "nurses," feeding and caring for the young bees (larvæ and pupæ), the older ones making the food-gathering expeditions. The queen lays her eggs one in each of many cells (fig. [81]). These eggs hatch in three days, and the young bee appears as a white, soft, footless, helpless grub or larva that is fed at first by the nurses with a highly nutritious substance called bee-jelly which the nurses make in their stomachs and regurgitate for the larva. After two or three days of this feeding the larvæ are fed pollen and honey. After a few days a small mass of this food is put into the cell, which is then "capped" or covered with wax. The larva after using up this food-supply pupates, and lies quiescent in the pupal stage for thirteen days, when the fully developed bee issues, and breaking through the wax cap of the cell is ready for the labors which are immediately assigned it. The bee with the kind of life-history just described is a worker. It has been demonstrated that the eggs which produce workers and those which produce queens do not differ, but if the workers desire to have a queen produced they tear down two or three cells around some one cell, enlarging this latter into a large vase-shaped cell. When the larva hatches from the egg in this cell it is fed for its whole larval life with bee-jelly. From the pupa into which this larva transforms issues not a worker but a new queen. The eggs which produce drones or males differ from those which produce queens and workers in being unfertilized, the queen having the power to lay either fertilized or unfertilized eggs. When a new queen appears or when several appear at once there is great excitement in the community. If several appear they fight among themselves until only one survives. It is said that a queen never uses its sting except against another queen. The old queen now leaves the hive accompanied by many of the workers. She and her followers fly away together, finally alighting on some tree-branch and massing there in a dense swarm. This is the familiar act of "swarming." Scouts leave the swarm to find a new home, to which they finally conduct the whole swarm. Thus is founded a new colony. "This swarming of the honey-bee is essential to the continued existence of the species; for in social insects it is as necessary that the colonies be multiplied as it is that there should be a reproduction of individuals. Otherwise as the colonies were destroyed the species would become extinct. With the social wasps and with the bumblebees the old queen and the young ones remain together peacefully in the nest; but at the close of the season the nest is abandoned by all as an unfit place for passing the winter, and in the following spring each young queen founds a new colony. Thus there is a tendency towards a great multiplication of colonies. But with the honey-bee the habit of storing food for winter, and the nature of the habitations of these insects, render it possible for the colonies to exist indefinitely, and thus if the old and young queens remained together peacefully there would be no multiplication of colonies and the species would surely die out in time. We see, therefore, that what appears to be merely jealousy on the part of the queen honey-bee is an instinct necessary to the continuance of the species."
![[80]](#illus076.jpg){kind=link}
Fig. 81.—Worker brood and queen cells of honey-bee; beginning at the right end of upper row of cells and going to the left is a series of egg, young larvæ, old larvæ, pupa, and adult ready to issue; the large curving cells below are queen cells. (From Benton.)
Fig. 82.—Honey-bees building comb. (From Benton.)
For the special labors of gathering food, making wax, building cells, etc., the workers are provided with special structures, as the pollen-baskets on the outer surface of the widened tibia of the hind legs, the wax-shears between the tibia and first tarsal joint of the hind legs, the wax-plates on the ventral surface of the abdomen, etc. A great many interesting things connected with the life and industries of a honey-bee community can be learned by the student from observation, using for a guide some book such as Cowan's "Natural History of the Honey-bee."
Fig. 83.—Comb of the tiny East Indian honey-bee, Apis florea, one-third natural size. (From Benton.)
The gathering of food from long distances, the details of wax-making and comb-building, of honey-making (for the nectar of flowers is made into honey by an interesting process), the storing of food, how the community protects itself from starvation when winter sets in or food is scarce by killing the useless drones and the immature bees in egg and larval stage, and many other phenomena of the life of the bee community present good opportunities for careful observation and field study. Although the community is a persistent or continuous one, the individuals do not live long, the workers hatched in the spring usually not more than two or three months, and those hatched in the fall not more than six or eight months. But new ones are hatching while the old ones are dying and the community as a whole always persists. A queen may live several years, perhaps as many as five. She lays about one million eggs a year.
There are more than two thousand known species of ants (fig. [84]), all of which live in communities and show a truly communal life. The ant workers are specially distinguished in structure from the males and females by being wingless, and in numerous species there are two sizes or kinds of workers known as worker majors and worker minors. The life-history and communal habits of ants are not so thoroughly known as are those of the honey-bee, but they show even more remarkable specializations. The ant nest or formicary is with most species an elaborate system of underground galleries and chambers, special rooms being used exclusively for certain special purposes, as nurse-rooms, food-storage rooms, etc. The food of ants comprises many animal and vegetable substances, but the favorite food with many species is the "honey-dew" secreted by the plant-lice (Aphididæ) and scale insects (Coccidæ). To obtain this food an ant strokes one of the aphids with its antennæ, when the fluid is excreted by the insect and drunk by the ant. In order to have a certain supply of this food some species of ants care for and defend these defenseless aphids, which have been called the "cattle" of the ants. In some cases they are even taken into the ants' nests and food provided for them. "In the Mississippi Valley a certain kind of plant-louse lives on the roots of corn. Its eggs are deposited in the ground in the autumn and hatch the following spring before the corn is planted. Now the common little brown ant (Lasius flavus) lives abundantly in the cornfields, and is especially fond of the honey secreted by the corn-root louse. So when the plant-lice hatch in the spring before there are corn-roots for them to feed on, the little brown ants with great solicitude carefully place the plant-lice on the roots of a certain kind of knot-weed which grows in the field and protect them until the corn germinates. Then the ants remove the plant-lice to the roots of the corn, their favorite food-plant. In the arid lands of New Mexico and Arizona the ants rear scale insects on the roots of cactus."
Fig. 84.—The little black ant, Monomorium minutum; a, female, b, female with wings, c, male, d, workers, e, pupa, f, larva, g, egg of worker, all enlarged. (From Marlatt.)
The ants are among the most warlike of insects. Battles between communities of different species are numerous, and the victorious community takes possession of the food-stores of the conquered. Some species of ants live wholly by war and robbery. In the case of the remarkable robber-ant (Eciton), found in tropical and subtropical regions, most of the workers are soldiers, and no longer do any work but fighting. The whole community lives exclusively by pillage. Some kinds of ants go even farther than mere robbery of food-stores: they make slaves of the conquered ants. There are numerous species of these slave-making ants. They attack a nest of another species and carry into their own nest the eggs and larvæ and pupæ of the conquered community, and when these come to maturity they act as slaves of the victors, collecting food, building additions to the nest, and caring for the young of the slave-makers.
As with the honey-bee the larval ants are helpless grubs and are cared for and fed by nurses. The so-called "ants' eggs," the little white oval masses which we often see being carried in the mouths of ants in and out of an ants' nest, are not eggs, but are the pupæ which are being brought out to enjoy the warmth and light of the sun or being taken back into the nest afterward.
There are in this country numerous species of ants showing much variety of habit and offering excellent opportunities for most interesting field observations. For an account of several of the common species see Comstock's "Manual of Insects," pp. 633-643. Ants may be readily kept in the schoolroom in an artificial nest or formicary and their life-history and habits closely watched. For full directions for making and keeping a simple and inexpensive formicary see Comstock's "Insect Life," pp. 278-281. For an interesting account of some of the habits of the social insects see Lubbock's "Ants, Bees, and Wasps."
Class Myriapoda: The Myriapods, or Centipeds and Millipeds.
Fig. 85.—Peripatus
eiseni (Mexico).
(From specimen.)
Belonging to the branch Arthropoda, with the classes Crustacea and Insecta, are three other classes, of which one, the Onychophora, is represented by a single genus Peripatus (Fig. [85]), of extremely interesting animals. However, as these animals are not found in the United States we cannot study them. The other two classes are the Myriapoda, including the centipeds and millipeds or thousand-legged worms, and the Arachnida, including the scorpions, spiders, mites, and ticks. All these animals are often spoken of as insects, but though related to them they are not true insects.
![[85]](#illus081.jpg){kind=link}
Technical Note.—From under stones or logs obtain specimens of millipeds, or thousand-legged worms (large blackish, cylindrical, worm-like animals with each body-segment back of the fourth bearing two pairs of jointed legs); also specimens of centipeds or hundred-legged worms (flattened, usually brownish or pale worm-like animals with the body-segments bearing only one pair of legs each) in the same places. Examine the external structure; note number of body-rings; division into body-regions; presence of antennæ; character and number of eyes; character of mouth-parts; character and arrangement of legs. In the centipeds the first pair of legs is modified to form a pair of poison-fangs. They appear to belong to the mouth-parts. The internal anatomy will be found to be, if examined, much like that of insects and can be studied from the account of the anatomy of the water-scavenger beetle and butterfly larva. Compare the Myriapods with the Hexapods or true insects. What are the points of resemblance? what are the points of difference?
The Myriapoda are land-animals breathing by means of tracheæ like the insects. In them the body-segments are nearly uniform in character with the exception of the head, which, as in the insects, bears the mouth-parts and antennæ. There is no grouping of the body-segments into regions except as the head is opposed to the rest of the body. (In a few myriapods there are indications of a division of the hind body into thorax and abdomen.) The presence of true legs on all the segments of the hinder region of the body and the lack of the three-region division of the body are the principal external structural characteristics which distinguish myriapods from insects. The internal anatomy corresponds in general character with that of insects.
Fig. 86.—A galley-worm
(milliped),
Julus sp. (From
specimen.)
The most familiar myriapods are the millipeds, and the lithobians and centipeds. The millipeds are cylindrical in shape, have two pairs of legs on most of the body-segments and are vegetable feeders, though some may feed on dead animal matter. The galley-worms (Julus) (fig. [86]), large, blackish, cylindrical millipeds found under stones and logs and leaves and in loose soil, are familiar forms. They crawl slowly and when disturbed curl up and emit a malodorous fluid. They can easily be kept alive in shallow glass vessels with a layer of earth in the bottom, and their habits and life-history may thus be studied. They should be fed sliced apples, green leaves, grass, strawberries, fresh ears of corn, etc. They are not poisonous and may be handled with impunity. They lay their eggs in little spherical cells or nests in the ground. An English species of which the life-history has been studied lays from 60 to 100 eggs at a time. The eggs of this species hatch in about twelve days.
![[86]](#illus082.jpg){kind=link}
Fig. 87.—The skein centiped,
Scutigera forceps, natural
size, common in houses
and conservatories. (From
Marlatt.)
Fig. 88.—A centiped, Scolopendra
sp. (From specimen.)
The lithobians and centipeds are flattened and have but a single pair of legs on each body-ring. They are predaceous in habit, catching and killing insects, snails, earthworms, etc. They can run rapidly, and have the first pair of legs modified into a pair of poison-claws, which are bent forward so as to lie near the mouth. The common "skein" centiped (Scutigera forceps) (fig. [87]) is yellowish and has fifteen pairs of legs, long 40-segmented antennæ, and nine large and six smaller dorsal segmental plates. The true centipeds (Scolopendra) (fig. [88]) have twenty-one to twenty-three body-rings, each with a pair of legs, and the antennæ have seventeen to twenty joints. They live in warm regions, some growing to be very large, as long as twelve inches or more. The "bite" or wound made by the poison-claws is fatal to insects and other small animals, their prey, and painful or even dangerous to man. The popular notion that a centiped "stings" with all of its feet is fallacious. It is recorded by Humboldt that centipeds are eaten by some of the South American Indians.
![[88]](#illus083b.jpg){kind=link}
Class Arachnida: The Scorpions, Spiders, Mites, and Ticks.
Technical Note.—Obtain specimens of various spiders; the running or hunting spiders may be found on the ground, especially under stones and boards, the web-makers on their snares. Get also spiders' "cocoons" (egg-sacs). Examine the external structure of the spider; note the two body-regions; the number and character of legs; the absence of antennæ; the number and arrangement of the eyes (which are simple, not compound); the mouth-parts, especially the large mandibles; the spinnerets at the tip of the abdomen (examine a cut off spinneret under the microscope to see the spinning-tubes); note the breathing openings or spiracles on under side of abdomen. Obtain also a scorpion if possible, and some ticks and mites. Compare with the spiders and note that in the scorpion the body is plainly seen (especially in the abdomen) to be composed of segments. Note the extreme fusion of the segments and body-regions in the mites and ticks. The common red spider of hothouses and gardens is a mite; ticks may sometimes be found on dogs. Observe various kinds of spider-webs, and try to observe the process of web-making (this can be observed early in the morning or about dusk) by one of the orb-weaving garden-spiders. Live spiders can be kept in the schoolroom and their feeding habits and perhaps web-making habits observed.
Fig. 89.—A scorpion, Centrurus
sp., from California.
(From specimen.)
The class Arachnida is composed of Arthropods whose body-segments are grouped into two regions, a cephalothorax bearing the mouth-parts, eyes, and legs, and an abdomen. The segments composing these two parts are so fused that, except in the scorpions, they are usually indistinguishable. There are no antennæ, the eyes are simple, the mouth-parts fitted for biting, and there are four pairs of legs. In their internal anatomy the arachnids show in some forms a peculiar modification of the respiratory organs, the tracheæ being flat and leaf-like and massed together in a few groups rather than being tubular and ramifying through the body.
Fig. 90.—The cheese-mite, Tyroglyphus
siro, greatly enlarged.
(After Berlese.)
The dorsal vessel or heart usually has a few blood-vessels or arteries running from it. This class is divided into three orders, the Arthrogastra, or scorpions, the Acarina, or mites and ticks, and the Araneina, or spiders.
The scorpions (fig. [89]) have the posterior six segments of the abdomen much narrower than the seven anterior segments and forming a tail which bears at its tip a poison-fang or sting. This sting is used to kill prey, insects and other small animals. The tail can be darted forwards over the body to strike prey which has been previously seized by the large pincer-like maxillary palpi. Scorpions are common in warm regions, about twenty species being known in southern North America. Their sting though painful is not dangerous to man. The young are born alive and are carried about by the mother for some time after birth.
![[89]](#illus084a.jpg){kind=link}
The mites (figs. [90] and [91]) and ticks (fig. [92]) are mostly small obscure animals, which live more or less parasitically. The common red spider of house-plants as well as the sugar- and cheese-mites, the dreaded itch-mite and the chigger are familiar examples of these degraded arachnids, and the wood-ticks, dog- and chicken-ticks are common examples of the larger bloodsucking forms. The body in both mites and ticks is very compact, the two body-regions, cephalothorax and abdomen, being closely fused.
![[91]](#illus085.jpg){kind=link}
Fig. 91.—Bird mite, species undetermined, from the gnome-owl, Glaucidium gnomus. (Photo-micrograph by Geo. O. Mitchell.)
The spiders have the abdomen distinctly set off from the cephalothorax. The eyes (fig. [93]) vary in number and arrangement, the mandibles are large, each being composed of two parts, a basal hair-covered part, the falx, and a terminal smooth, shining, slender, sharp-pointed part, the fang, which is movably articulated with the falx (fig. [93]). In the falx is a poison-sac from which poison flows through the hollow fang and out at its tip. The legs vary in relative length in different spiders, and each is made up of seven joints. The spinnerets (fig. [94]), which are situated at the tip of the abdomen, are six in number (a few spiders have only four), and are like little short fingers. They have at their tips many fine little spinning-tubes from each of which a fine silken thread issues when the spider is spinning. These many fine threads fuse as they issue to form a single strong cable or sometimes a flat rather broad band. The spinnerets are movable, and by their manipulation the desired kind of line is produced. The silk comes from many silk-glands in the abdomen, from each of which a fine duct runs to a spinning-tube.
Fig. 92.—The dog or wood tick, Dermacentor americanus male, the most common tick in the Northern States. (After Osborn.)
Fig. 93.—The
eyes and jaws,
showing falx
and fang of a
spider. (From
Jenkins and
Kellogg.)
The spiders may be divided into two groups according to their habits, viz., the wandering or hunting spiders, which do not spin webs to catch their prey, and the sedentary or web-weaving spiders, which spin snares to catch their prey. The wandering spiders can spin silk, however, and often do so to line their burrows, to make nests, or to make egg-sacs.
The hairy tarantulas and the trap-door spiders of similar appearance are among the most interesting of the hunting spiders. They live in vertical burrows or tunnels in the ground which are lined with silk, and which in the case of the trap-door spider are covered with a door or lid made of silk and soil. The top of this door is always covered with soil or bits of leaves or twigs so that it is nearly indistinguishable from the surface of the ground about it. When the nest is in ground covered with moss the spider covers the door with moss. The tarantulas hunt at night and rest in the burrow in the daytime. They are very large, sometimes having an expanse of legs of 6 inches.
Fig. 96.—A running spider (Lycosidæ). (From life.)
Fig. 97.—A female running spider (Lycosidæ) carrying its egg-sac about attached to its spinnerets. (From Jenkins and Kellogg.)
The common, rather large swift black spiders found under stones and boards are hunting spiders, belonging to the family Lycosidæ and are called the running spiders (fig. [96]). They live in burrows in the ground, coming out to stalk and chase their prey. The eggs are laid in globular egg-sacs which are often carried about, attached to the spinnerets, by the female (fig. [97]). The young spiderlings after hatching, in some species, climb on to the mother's back and are carried by her for some time. Other kinds of wandering or hunting spiders are the crab-spiders (Thomisidæ) (fig. [98]), which run sidewise or backward as well as forward, and the black and red, fierce-eyed stout-bodied little jumping spiders (Attidæ) (fig. [99]), which leap on their prey.
![[96]](#illus088a.jpg){kind=link}
| Fig. 98.—A crab-spider (Thomisidæ). (From Jenkins and Kellogg.) | Fig. 99.—A jumping spider (Attidæ). (From Jenkins and Kellogg.) |