BY VERNON L. KELLOGG, M.S.
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.
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).
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.
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.
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[Pg 18]
[Pg 19] 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[Pg 22]
[Pg 23] 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 . These organs are thekidneys . Their office is similar to that of the kidneys in the toad, namely, the elimination of waste from the body.