M BOUGHT WITH THK INCOME FROM THE SAGE ENDOWMENT FUND THE GIFT OF Henrg W. Sage 1S91 A J J- f A3 Z /9/6/^f^2- Cornell University Library The original of tliis book is in tlie Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924024565396 QL 48^82°™" """*"»>' Library ii/!|n|njal forms: TWENTIETH CENTURY TEXT-BOOKS EDITED BY A. F. NIGHTINGALE, Ph.D., LL.D. FORMERLY SUPERINTENDENT OF HIGH SCHOOLS, CHICAGO cs. W '*W fl^^ ^H - ^' y;- r jit.-^r'i... ^1^ '. ;l. '^^^BHBP^^I^B^^P '^al^^S llfl .^^mwi^tBlrm/KB^(^'*^'^J^P^^^ ■^I^^^Mar '■rr»^^^"'^^^ y^BC SB^^^^^^^yr-''-''JBWB^^^Vry^j''" -"J^^HhH ^^^f^S^^^^^^SwMsi^W- '«ll^^^ jK^m^^^^P^^'^^^^HXJ^C^aSHHl 1 ^BuBs^^^n^0^''''^''^iJ^^^^BUBlfedfl^K^H^^H k ^-: 41 B^ v^.;-^ W^ PPWBjB R' JLZ^:F-- t^Hi ^S ^B^L^ jk ''*^A^' t^^'^H^lSS^Hl^^H^^I w Bv^Ih^^B^B^H^ ^fl ^^^xSjml^BE^S^S^Si< ' y^ ^^SbbHB^B ''^1 ^S^S^mM^^L jfla^^^^^^^^^^L /■jii^^^^^^^^^fflBIBE^^a^^^B a. 3 cu e s TWENTIETH CENTURY TEXT-BOOKS ANIMAL FORMS A SECOND BOOK OF ZOOLOGY DAVID S. JORDAN, M. S., M. D., Ph. D., LL. D. PRESIDEN'l^OF LELAND STANFORD JUNIOR UNIVERSITY AND HAROLD HEATH, Ph. D. PROFESSOR IN LELAND STANFORD JUNIOR UNIVERSITY NEW YORK D. APPLETON AND COMPANY 1902 /< Copyright, 1902 v Bit D. APPLETON AND COMPANY Published May, moi PEEFACE The present volume is designed to meet the needs of the beginning student of zoology. Accordingly, technical and scientific names have been avoided as far as possible, and those used are fully explained in the text or elsewhere. The opening chapters deal with the characteristics of living things, and, in contrasting animals and plants, attempt to bring into relief the distinguishing niarks of all animals. Then follows a discussion of the cell and protoplasjp, pre- paring the way for the examination of a series of animals representative of each of the great groups, from the sim- plest to the most complex. These are considered from the view-point of structure ; but considerable attention is also paid to the functions of their parts, to their habits and life- history, so that while the representatives examined are, for the sake of simplicity, relatively few in number, they are, it is believed, thoroughly typical. Hence, with a knowledge of the facts presented, the student should have a broad view of the animal kingdom, and a foundation on which to . base future study and observation. It is perhaps unneces- sary to add that from the study of books alone no one can really make such knowledge his own. A personal acquaint- ance with even a few animals in their native haunts, and an understanding of the structure and the function of their VI ANIMAL FORMS parts gained from dissection and experiment, is essential to a full comprehension of what the student learns from text- book and teacher. The greater number of illustrations are new, and have been drawn or photographed from living or preserved ma- terial. When not otherwise accredited, the drawings have been made by Miss Mary H. Wellman and J. Carter Beard, to whom the authors extend their sincere thanks. Our obligations are also due to Mr. Walter K. Fisher, who has made the drawings of the vertebrate dissections ; to Messrs. A. L. Melander and 0. T. Brues, of Chicago, 111. ; Mr. Wm. H. Fisher, of Baltimore, Md. ; Eev. H. K. Job, of Kent, Conn. ; Mr. Wm. Graham, of Pasadena, Cal. ; and Dr. E. W. Shufeldt, of New York city, for numerous photographs. David Staer Jordan, Hakold Heath. CONTENTS CHAPTER PAGE I. — Introduction 1 II. — The cell and protoplasm 7 III. — The Protozoa ... 11 IV". — The sponges . 19 V. — The Ccelenterates 29 VI. — The worms . . ..... 44 VII. — Animals of uncertain relationships ... 66 VIII.— MOLLUSKS 72 IX. — Arthropods. Class Crustacea 93 X. — Arthropods. Class Insects 114 XI. — Arthropods. Class Araohnida 133 XII. — KCHINODERMS . . 140 XIII. — The chordates ... 151 XIV.— The fishes 154 XV. — The amphibians 174 XVI. — The reptiles 184 XVII.— The birds 301 XVIII.— The mammals 265 vii ANIMAL FORMS CHAPTER I INTRODUCTION 1. Divisions of the subject. — Biology is the science which treats of living things in all their relations. It is sub- divided into Zoology, the science which deals with animals, and Botany, which is concerned with plants. The field covered by each of these branches is very extensive. Within the scope of zoology are included all subjects bear- ing on the form and structure of animals, on their devel- opment, and on their activities, including the consideration of their habits and the wider problems of their distribution and their relations to one another. These various subjects are often conveniently grouped under three heads : Morphology, which treats of the form and structure or the anatomy of organisms ; Physiology, which considers their activities; and Ecology, which in- cludes their relations one to another and to their surround- ings, jkll the phases of plant or animal existence may be considered under one or another of these three divisions. 2. The diflference between animals and plants. — Generally speaking, we have little difficulty in seeing that the objects about us are either living or lifeless ; but the boundary line between the two great divisions of living things, the animals and plants, can not always be so clearly drawn. This is es- pecially true of the simpler forms of life which frequently combine both animal and plant characteristics ; but in the 1 2 ANIMAL FORMS greater number of more highly developed species the line of separation is clearly marked. It is very easy, for example, to distinguish the oak-tree or the rose from a horse or a butterfly, and, as we shall see, the differences are not based merely on outward appearance. In the oak-tree, for example, the roots reaching down into the earth, with the branches and leaves spreading out into the air and sunlight, are admirably fitted for taking up the food, which consists of very simple materials, less com- plex than those forming the diet of an animal. This permits a continuous existence in one place, and accord- ingly we note the entire absence of locomotion and the or- gans controlling it, which form so conspicuous a part of the body of an animal. Also in the production of flowers and seeds, and in the growth of the seed into the tree, we detect many characteristics peculiar to plants. 3. Characteristics of an animal. — On the other hand, the squirrel, for example, or any other animal, is unable to sub- sist on water, air, and elements from the soil. These crea- tures demand the highly diversified materials found in the bodies of plants and of animals. Such being the case, they do not remain anchored to one spot (except in a relatively few cases), but are compelled to lead an active existence. The power of voluntary movement, or movement in response to internal impulse, is thus the first and one of the most striking peculiarities of animals. In the second place, the food of plants enters the body in a soluble condition and is readily transferred to the or- gans requiring it. While in the animals, the nutritive ma- terials pass into the body in an insoluble state and de- mand a varied preliminary treatment, usually within a special digestive tube, before they are fit to be absorbed. In the squirrel, by way of illustration, the food is first ground to a pulp by the action of the teeth, and, moistened with saliva, is swallowed and passed into the stomach, where it is subjected to the solvent action of the gastric ANIMAL FORMS juice. From the stomach it is made to enter the intestine, and is further acted upon by fluids from the liver, the pancreas, and the-glands of the intestines themselves. Thus treated it becomes changed from an insoluble state into a fluid which readily penetrates the coats of the digestive tract. Many of the organs of the body are placed at a considera- ble distance from the food as it comes through the coats of the stomach and intestine. In order to supply thern, with the necessary nourishment a distrib- uting apparatus is required. This is the office performed by the circulatory system, for as rapidly as the food penetrates the walls of the digestive tract it enters the blood, and by the beating of the heart is driven to all parts of the body, which are thus continually kept in a state of repair. The blood serves also to remove waste substances from the various structures or organs of the animal body and to transfer them to the kidneys, skin, or lungs, which effect their removal from the body. 4. Muscular and nervous sys- tems.— Owing to the fact that animals, as a rule, are compelled to move about in search of food, we find two highly devel- oped systems, the muscular and nervous, which are absent Fig. 2.— Diagram of heart and blood- vessels of the squirrel or other mammal, a.o., aorta ; A, vessels of head ; La., left auricle ; l.esr., vessels of lower extremities ; Ig., lung; l.v,, left ventricle; p.a., pulmonary artery ; p.v., pulmo- nary vein ; r.a., right auricle ; r.v., right ventricle ; v., vessels of viscera. Arteries are represented by heavy walls. INTRODUCTION 5 in the plants. The first of these, constituting what is usu- ally known as the lean meat, is a relatively complex system of organs, differing widely according to the work performed. In the higher animals — the squirrel, for example — there are not less than five hundred muscles, which are under the control of the nervous system. The nervous system consists of the brain and spinal cord, which in the squirrel are concealed and protected Pig. 3.— Skeleton of sqairrel, showing Its relation to the body. within the skull and back-bone. From them many nerves pass outward to the muscles, and as many pass inward from the eye, ear, nose, tongue, or skin. By the action of these sense-organs the animal determines the nature of its sur- roundings, detects its food, recognizes the presence of its enemies, and is thus able to direct its movements to the greatest advantage. 5. Multiplication of animals. — ^The organs thus far con- sidered serve to perpetuate the animal as an individual ; but some provision must also be made for the continuance of the race. In the economy of nature each animal before 6 ANIMAL FORMS its death should leave ofispring to take the place of the parent when it falls from the ranks. This is effected in various ways. In some of the simpler animals the body may divide into two equal parts, each of which becomes a complete individual. In other cases the animal detaches a relatively small portion of its body, much as a gardener cuts a slip from a plant, and this likewise develops into a new organism. In the greater number of animals, very clearly illustrated by the birds, eggs are produced which under favorable conditions develop into an organism resem- bling the parents. 6. Summary. — Animals are thus seen to lead active, busy lives, collecting food, avoiding enemies, and producing and and caring for their young. While the activities of all animals are directed to their own preservation and to the multiplication of their kind, these processes are carried on in the most diverse ways. The manner in which an organ or an organism is made, and the method by which it does its work, are mutually dependent one on the other. As there is an enormous number of species of animals, each differently constructed, there is, accordingly, a very great variety of habits. As we shall see, the lower forms are remarkably simple in their construction, and their mode of existence is correspondingly simple. In the higher types a much greater complexity exists, and their activities are more varied and are characterized by a high degree of elabo- ration. In every case, the animal, whether high or low, is fitted for some particular haunt, where it may perform its work in its own special way and may lead a successful life of its own characteristic type. CHAPTEE II THE CELL AND PROTOPLASM 7. CeUs. — If we examine very carefully the different parts of a squirrel under the high powers of the microscope we find that they are composed of a multitude of small structures which bear the same relations to the various organs that bricks or stones do to a wall ; and if the inves- tigation were continued it would be found that every or- ganism is composed of one or more of these lesser elements which bear the name of cells. In size they vary exceedingly, and their shapes are most diverse, but, despite these differ- ences, it will be seen that all exhibit a certain general re- semblance one to the other. 8. Shape of cells. — In many of the simpler organisms the component cells are Jelly-like masses of a more or less spherical form, but as we ascend the scale of life the condi- tion of affairs becomes much more complex. In the squir- rel, for example, we have already noted the presence of various organs for carrying on different functions, such as those of digestion, circulation, and respiration ; and, further, the cells composing these various parts have been modified in accordance with the duties they have to perform. In the muscles the cells are long and slender (Fig. 4, D) ; those forming the nerves and conveying sensations to and from all parts of the body, like an extensive telegraph sys- tem, are excessively delicate and thread-like ; in the skin, and lining many cavities of the body, where the cells are united into extensive sheets, they range in shape from high and columnar to flat and scale-like forms (Fig. 4, E, F, G). 3 7 8 ANIMAL FORMS The cells of the blood present another type (Fig. 4, B) ; and so we might pass in review other parts of the body, and con- tinue our studies with other groups of animals, always find- ing new forms dependent upon the part they play in the organism. 9. Size of cells. — Also in the matter of size the greatest variations exist. Some of the smallest cells measure less than one mioromillimeter {^jhss of an inch) in diameter. Over five hundred million such bodies could be readily stowed away into a hollow sphere the size of the letter be- ginning this sentence. In a drop of human blood of the same size, between four and five million blood-cells or cor- puscles float. And from this extreme all sizes exist up to those with a diameter of 2.5 or 5 cm. (one or two inches), as in the case of the hen's or ostrich's egg. On the average a cell will measure between .025 to .031 m.m. (y^Vo ^^^ •g^ of an inch) in diameter, a speck probably invisible to the unaided eye. While the size and external appearance of a cell are seen to be most variable, the internal structures are found to show a striking resemblance throughout. All are constructed upon essentially the same plan. Differ- ences in form and size are superficial, and in passing to a more careful study of one cell we gain a knowledge of the important features of all. 10. A typical cell. — Some cell, for example that of the liver (Fig. 4, A), may be chosen as a good representative of a typical cell. To the naked eye it is barely visible as a minute speck ; but under the microscope the appearance is that of so much white of egg, an almost transparent jelly- like mass bearing upon its outer surface a thin structure- less membrane that serves to preserve its general shape and also to protect the delicate cell material within. The com- parison of the latter substance to egg albumen can be car- ried no further than the simple physical appearance, for albumen belongs to that great class of substances which are said to be non-living or dead, while the cell material THE CELL AND PROTOPLASM 9 or protoplasm, as it is termed, is a living substance. We know of no case where life exists apart from protoplasm, and for this reason the latter is frequently termed the physical basis of life. In addition to the features already described, the proto- plasm of every perfect cell is modified upon the interior to Fie. 4.— Different types of cells composing the body of the squirrel or other highly developed animal. A, liver-cell ; /, food materials ; n, nucleus. B, blood-cell. C, nen'e-cell with small part of its fiber. D, mnscle fiber. E, cells lining the body cavity. F, lining of the windpipe. 6, section through the skin. Highly magnified. form a well-defined spherical mass known as the nucleus. Other structures are known to occur in the typical cell. Experiment shows that the nucleus and cell protoplasm are absolutely indispensable, whatever their size and shape, and 10 ANIMAL FORMS therefore we are at present justified in defining the cell as a small mass of protoplasm enclosing a -nucleus. 11. Structure of protoplasm. — When seen under a glass of moderate power protoplasm gives no indication of any definite structure, and even with the highest magnifica- tion it presents appearances which are not clearly under- stood. According to the commonly accepted view, it con- sists of two portions, one, the firmer, forming an excessively delicate meshwork (Fig. 4, A) enclosing in its cavities the second more fluid part. Therefore, when highly mag- nified, the appearance would be essentially like a sponge fully saturated with water ; but it should be remembered that in the protoplasm the sponge work, and possibly the fluid part, is living, and that both are transparent. There are reasons for thinking that the structure and the composition of protoplasm may change somewhat under certain circumstances. It certainly is not everywhere alike, for that of one animal must differ from that of another, and different parts, such as the liver and brain, of the same form must be unlike. These differences, however, are minor when compared to the resemblance^, for, as we shall see, this living substance, wherever it exists, carries on the pro- cesses of waste, repair, growth, sensation, contraction, and the reproduction of its kind. CHAPTER III THE PROTOZOA 13. Single-celled and many-celled animals, — In almost every portion of the globe there are multitudes of animals whose body consists of but a single cell ; while those forms more familiar to us, and usually of comparatirely large size and higher development, such as sponges, insects,, fishes, birds, and man himself, are composed of a multitude of cells. For this reason the animal kingdom has been divided into two great subdivisions, the Protozoa including all unicellular forms and the Metazoa embracing those of many cells. 13. Single-celled animals. — The division of the Protozoa comprises a host of animals, usually of microscopic size, inhabiting fresh or salt water or damp localities on land in nearly every portion of the globe. The greater number wage their little, though fierce, wars on one another with- out attracting much attention ; others, in the sharp struggle, have been compelled to live upon or within the bodies of other animals, and many have become notorious because of the diseases they produce under such circumstances. A few are in large measure responsible for the phosphores- cence of the sea ; and still others have long been favorite objects of study because of their marvelous beauty. Adapted for living under diverse conditions, the bodily form differs greatly, and yet all conform to three or four principal types, of which we may gain a good idea from the study of a few representative forms. 11 12 ANIMAL FORMS 14. The Amoeba. — Among the simplest one-celled ani- mals living in the ooze at the bottom of nearly every fresh- water stream or pond is the Amoeba (Fig. 5, A), whose body is barely visible to the unaided eye. Under the microscope Fig. 5.— a, the Am(sba, highly magnified, showing o.v., pulsating vacuole; /, food particle ; n, nucleus. The arrows show the direction of movement. B, shape of same individual 30 seconds later. C, an amceba-like animal (Diffiugia) partially enclosed in a shell. D, an Amceba in the process of division. E, Gromia, another shelled protozoan (after Schulze). it is seen to consist of an irregular, jelly-like mass of proto- plasm totally destitute of a cell wall. Unlike those animals with which we are familiar, the body constantly changes its shape. A rounded bud-like projection will be seen to appear on one side of the body and the protoplasm of adjacent regions flows into it, thereby increasing its extent. Similar projections iit the opposite end of the cell are withdrawn, and their substance may flow into the newly formed lobe, which gradually swells in size and pushes forward. Thus, by constantly advancing the front part of the body and THE PROTOZOA 13 retracting the hinder portion, the cell glides or flows along from place to place. Upon meeting with any of the smaller organisms upon which it lives, projections from the body are put out which gradually flow around the prey and it becomes pressed into the interior of the cell. The process is not unlike pushing a grain of sand into a bit of jelly. There is no mouth. Any point on the surface serves for the reception of food. Oxygen gas also is taken into the body all over the surface, and wastes and indigestible material are cast out at any point. Nothing exists in these simple forms comparable to the complex systems of organs that carry on these processes in the squirrel. The bodily size of animals is limited, and to this general rule the Amoeba is no exception, for upon gaining a certain size, the nucleus divides into two exactly similar portions, and very soon afterward the rest of the body separates into two independent masses of equal size (Fig. 5, D), each of which, when entirely free, contains a nucleus. In this way two daughter amcebEe are formed possessing exactly the characters of the parent save that they are of smaller size ; but it is usually not long before they reach their limit of growth, when division occurs again, and so on, generation after generation. It not infrequently happens, however, that the pond or stream, in which the Amceba and other Protozoa live, dries up for a portion of the year. In such an event the body assumes a spherical shape, develops a firm, horn-like mem- brane about itself, and thus encysted it withstands the sum- mer's heat and dryness and may he transported by the wind, or otherwise, over great distances. When the conditions again -become favorable the wall ruptures and the Amceba emerges to repeat its life processes. 15. Some relatives of the Amoeba. — All amoeba-like forms, to the number of perhaps a thousand species, possess this same method of locomotion, but many present some inter- 14 ANIMAL FORMS esting additional characters. For example, the form repre- sented in Fig. 5, 0, constructs a sac-like skeleton of tiny pebbles cemented together, into which it may withdraw for protection. Others construct similar envelopes of lime or flint, and still others, as they continue to grow, build on additional chambers, giving rise to a great variety of forms often of wonderful beauty. In the tropics, particularly, some of the shelled Protozoa are so abundant that they may impart a whitish tinge to the water, and in some places their empty shells on falling to the bottom form immense deposits. The chalk cliffs of England are in large measure made up of such shells. 16. The Infusoria.— A little over two hundred years ago it was discovered that wherever water remained stagnant it became favorable for the rapid multiplication of a large number of species of Protozoa which live in such situations. These are known as Infusoria, and, like the preceding spe- cies, are usually of microscopic size and of the most varied shapes. The first striking feature of their organization is the presence of a delicate though relatively firm external cell membrane known as the cuticle, which preserves a defi- nite shape to the body. Such a method of locomotion as exists in the preceding group is consequently an impossi- bility, but other and more highly developed structures per- form the oflBce. These latter organs are of two types, and their general characteristics may be readily understood from an examination of a few species living in the same localities as the Ammia. 17. The Euglena.— The first type exists in the common fresh-water organism known as Euglena, represented in Fig. 6, A. Here the spindle-shaped body is surrounded by a delicate cuticle perforated at one point, where a funnel- shaped depression, the gullet, leads into the soft proto- plasmic interior. From the base of this depression the protoplasm is drawn out in the form of a delicate whip-like process known as the fiagellum. This structure, always THE PROTOZOA 15 permanent in form, constantly beats backward and forward with great rapidity in a general direction represented in the diagram (Fig. 6, c). The movement from a to 5 is much more rapid than the reverse, from b to a, which results, like the action of the human arm in swimming, in driving the organism forward. Not only does the flagel- lum serve the purpose of locomotion, but it also produces currents in the water which may serve to bear minute organisms down into the gullet, whence they read- ily pass into the soft pro- FiQ. 6. — Flagellate Infusoria. A, Euglena viridis ; c, pulsating vacuole ; «, eye-spot ; g, gullet ; n, nucleus ; t, flagellum. B, Oo^ dosiga, with collar surrounding the flagellum. C, diagram illus- trating the action of the flagel- lum. All figures greatly enlarged; Fig. 7. — Parammdum aurelia, a ciliate infusorian. c, cilia; c.v,, pulsating vacuoles ; /, food particles ; g, gullet ; m, huccal groove ; n. nucleus. toplasm of the body, there to undergo the processes of di- gestion and assimilation. In some forms the protoplasm in the region of the flagellum is drawn out in the form of a collar (Fig. 6, B), whose vibratory motion also aids in con- veying and guiding food into the body. 18. The Slipper Animalcule. — The second type of loco- motor organ may be understood from a study of the 16 ANIMAL FORMS Slipper Animalcule {Paramoscium, Fig. II), abundant in stagnant water. In this form the cuticle surroundmg the somewhat cylindrical body is perforated by a great number of minute openings through which the internal protoplasm projects in the form of delicate threads. Each process, termed a cilium, works on the same principle as the flagellum, but it beats with an almost perfect rhythm and in unison with its fellows, drives the an- imal hither and thither with considera- ble rapidity. On one side of the body is a furrow which deepens as it runs backward and finally passes into the gullet {g), which leads into the interior of the body. Throughout the entire extent it is lined with cilia which create strong currents in the surrounding water and in this way conduct food down the gullet into ' the body. Embedded in the outer sur- face of the body, in among the cilia, are also a number of very minute sacks, each containing a coiled thread which may be discharged against the body of any intruder, so that this form is sup- plied with actual organs of defense. Two pulsating vacuoles {c.v.) or simple kidneys are also present, consisting of a central reservoir into which a number of radiating canals extend. 19. The Bell Animalcule and other species. — The Bell Animalcule ( Vorti- cella. Fig. 8) is often found in the same situations as the Slipper Animalcule, which in certain respects it resembles. It is generally attached by a slender stalk, and where many • Fig. 8.—Vorticella, an at- tached ciliate infusori- an, highly magnified, a, tuUy extended individ- ual ; C.V., pulsating va- cuole ; g, gullet ; n, nu- cleus, b, contracted specimen, c, small free- swimming individual, which unites with a sta- tionary individual (one partly united is shown in specimen b). THE PROTOZOA 17 are growing together they appear like a delicate growth of mold upon the water weed. The stalk is peculiar in being traversed by a muscle fiber arranged in a loose spiral, which upon any unusual disturbance contracts together with the body into the form shown in Fig. 8, b. These few examples serve to show the general plan of organization and the method of locomotion of the Infuso- ria ; but, as upward of a thousand species exist, with widely differing habits, many interesting modifications are present. Some have been driven in past time to adopt a parasitic mode of life within the bodies of other animals. At pres- ent they are devoid of locomotor organs, and as they absorb nutritive fluids through the surface of the body all traces of a mouth are also absent. The reproductive processes also are peculiar, but they do not concern us now. 20. Characteristics common to the Protozoa, — We have now studied the principal structures which serve in loco- motion among these simple one-celled forms, also'the means by which they catch their food, and we shall now glance at the internal processes, which are much the same in all. After the food has been taken into the cell, it is prob- ably acted upon by some digestive fluid, for it soon assumes a granular appearance and finally undergoes complete solu- tion. In every case the oxygen is absorbed through the general surface of the body, and uniting with the living substance, as in the squirrel, liberates the energy necessary for the performance of the animal's life work. The wastes thus produced in a large number of forms simply filter out from the body without the agency of anything comparable to a kidney, but in several species they are borne to a 'definite spot, the pulsating vacuole (Figs. 5, 7, 8, c.v.), where they gradually accumulate into a drop about the size of the nucleus. The wall between it and the exterior now gives way and the excretions are passed out. In active indi- viduals this process may be repeated two or three times a minute, but it is usually of less frequent occurrence. 18 ANIMAL FORMS The loss in bodily waste is continually made good by the manufacture of the food into protoplasm, and if the in- come be greater than the outgo growth ensues. But, as in all other forms, growth is limited, and ultimately the cell is destined to divide, resulting in two new individuals. This process may be repeated many times, but not indefinitely, for sooner or later various members of the same species unite in pairs temporarily or permanently, exchange nu- clear material, and separate again with apparently renewed energy and the ability to divide for many generations. 31. Simple and complex animals. — It is important to note that these same processes of waste, repair, growth, feeling, motion, and multiplication are the same as those of the squirrel, and, furthermore, are common to all living crea- tures, so that the difference between animals is not in their activities, but in their bodily mechanisms ; and according to the perfection of this, the animal is high or low in the scale. Comparing, for example, the Amcela and Slipper Animalcule, which are relatively low and high Protozoa, we find in the former that any part of the body serves in loco- motion and in the capture of food, while in the latter these same functions are performed by definite structures, the cilia and gullet. Now it is well known that a workman is able to make better watch-springs, when this is his sole duty, than another who must make all parts of the watch ; and likewise where a definite task is performed by a defi- nite structure, it is more efficiently done than where any and every part of the body must carry it on. So the Amoeba, in which definite tasks are performed by any part of the body indifferently, is less perfect and thus lower than the Paramcecium, where these functions are performed by special organs. As we ascend the scale of life we find this division of labor among special parts of the body more complete, the organs and therefore the animal more com- plex, and better fitted to carry on the work of its life. CHAPTEE IV THE SPONGES 22. Their relation to the Protozoa. — While the greater number of one-celled forms are not united with their fel- lows, there are several species where the reverse is true. In Fig. 9, for example, a fresh-water form known as Pandorina is represented, consisting of sixteen cells embedded in a spherical, jelly-like substance, each one of which is precisely like its companions in form and activity. The aggregation may be looked upon as a colo- ny of sixteen Protozoa united together to derive the benefit of increased locomotion and a larger amount of food in consequence. As a result of such a union they have not lost their independence, for if one be separated from the main company it' continues to exist. From such a simple colonial type we may pass through a series of several more complex forms which reach their highest development in the beau- tiful organism, Volvox (Fig. 10). In this form the indi- vidual members, to the number of many thousand, are ar- ranged in the shape of a hollow sphere. The united efforts of the greater number, which bear on their outer surfaces two flagella, drive the colony with the rolling movement 19 Fig. 9.- -Pandm-iim (from Nature). Highly magnified. 20 ANIMAL FORMS from place to place. As jast indicated, some individuals lack the flagella, and their subsequent careers show them to be of a peculiar type. Sooner or later each undergoes a series of divisions form- ing a little globe of cells, which migrates into the in- terior of the parent sphere and develops into a new colony. Within a short time the walls of the parent break, liberating the im- prisoned young, which con- tinue-the existence of the species while the parent or- ganism soon decays. Under certain circum- stances, instead of develop- ing colonies by such a meth- od, some of the cells may store up food matters and become eggs, while others, known as sperm-cells, de- velop a flagellum, and sep- arating from the colony swim actively in the sur- rounding water, where each finally unites with an egg. This union, like that of the two individuals in Vorticel- la (Fig. 8, h, c), results in Fig. lO.-A, Yolvox minor, entire colony the power of division, and (from Nature). B, C, and D, reprodnc- the egg enters Upon its de- tneceWs otVolvoxglobator. All highly i , t •,■ magnified. velopment, dividing again and again. The cells so pro- duced remain together, form a sphere, and finally develop a Volvox colony. ' THE SPONGES 21 In such associations as ' Volvox an important step has been taken beyond that of Pandor'ina, for there is a division of the labors of the colony among its various members, some acting as locomotor cells while others are germ-cells. These are now so dependent one upon the other that they are unable to exist after separation from the main com- pany, just as a part of the squirrel is incapable of leading an independent existence. A higher type of organism has thus arisen intermediate between the simple one-celled animals and those of many cells, especially the sponges — a relation which is more readily recognized after .an examina- tion of the latter. 33. Development of the sponge. — As with all many- celled animals, the sponge begins its existence as an egg, in this case barely visible to the sharp unaided eye. Fer- tilized by its union with a sperm cell, development com- mences, and the first apparent indication of the process "will be the division of the cell into two halves (Fig. 11, A, B). Each half redivides into four, these again into eight cells, and this process is repeated, giving the young sponge the general form of Pandorina. The divisions of the cells still continue and result in the formation of a hollow globe of cells (called the Uastula, Fig. 11, E, F) similar to Volvox, and at this point the young larva leaves the parent. The next transformation consists in a pushing in of one side of the sphere, just as one might press in the side of a hollow rubber ball. The depression gradually deepens, and finally results in the formation of a two-layered sac known as the gastrula (Fig. 11, G). At this stage of its existence the sponge settles down for life in some suitable spot, by applying the opening of its sac-like body to some foreign object. In assuming the final form a new mouth breaks through what was once the bottom of the sac, canals per- forate the body wall, a skeleton is developed, and the char- acteristic features of the adult are thus attained. 22 ANIMAL FOEMS 34. Distribution.— The sponges are aquatic animals, and, with the exception of one family consisting of relatively few species, all are inhabitants of the sea in every part of Fig. 11.— Diagrams illustrating the development of a sponge. A, egg-cell ; », nu- cleus. B, C, D, 2-, 4-, and 16-cell stages. E, Uastula. F, section through some- what older larviB. G, gash-ula. H, young sponge. I, section through somewhat younger larvse than H. the globe. The larger number occupy positions along the shore, becoming especially abundant in the tropics; but other species occur at greater depths, several species living THE SPONGES 23 between three and four miles from the surface. Unlike the majority of animals, all members of this group are securely fastened to some foreign object, such as rocks, the supports of wharves, or with one extremity embedded in the sand. As we have seen, the young enjoy a free-swim- ming existence and are swept far and wide by means of tidal currents, but sooner or later these migrations are terminated in some suitable locality, where the sponge passes the remainder of its existence. During this time some species may never exceed the size of a mustard-seed, while others attain a diameter of three feet, or even more. Sponges also vary exceedingly in shape, some having the form of thin encrusting sheets, others being globular, tubu- lar, cuplike, or highly branched (Fig. 13). 25. The influence of their surroundings. — In by far the larger number of cases an animal possesses the bodily form of the parent. External agencies may modify this to some extent, but usually only to a limited degree. A squirrel, for example, resembling its parent, may grow to a relatively large or stunted size according to the food supply, and it may become strong or weak according to the amount of exercise, and various other changes may result owing to outside causes ; but as a result of these influences the animal is rarely so modified that one is unable to distinguish the species. Many of the sponges, however, are exceptions to this general rule. If, for example, some of the young of a certain parent develop in quiet water or in an un- favorable locality, they will usually be low, flat, and un- branched ; while the others, growing in swiftly running waterways, develop into tall, comparatively delicate and highly branched individuals. Under such circumstances not only does the external form become modified, but the internal organization may undergo profound change. The entire organism is plastic and readily molded by the influence of its surroundings, and the consequent lack of definite characters often renders it impossible 3 24 ANIMAL FORMS to assign such forms to a definite position among the sponges. 26. Structure of a simple sponge.— In the simpler sponges the body is usually yase-shaped (Fig. 13), with the base fastened to some foreign object, while at an oppo- site end an opening leads into a comparatively large internal cavity. This lat- ter space is also put in communication with the exterior by a multitude of minute pores which pene- trate the body wall. In Fig. 13.— Various forms of sponges, natural size. (From Nature.) the living condition currents of water continually pass through these -smaller canals, and out of the large termi- nal opening, thus bringing within reach of the body minute THE SPONGES 25 floating organisms or organic remains which serve as food. The mechanism by which this process is effected, and the various other structures of the body, are in large part invis- ible from the exterior, requiring the study of thin sections of the sponge to make them clearly understood. Under the microscope such a sec- tion shows the body of a sponge to consist of an immense number of va- riously formed cells constituting three distinct layers (Fig. 14). Not only do these layers consist of different kinds of cells, but the duties per- formed by each are different. For ex- ample, a glance at Fig. 14 will show that in the inner layer certain colum- nar cells exist, provided with a fla- gellum and encircling collar, the ap- pearance being strikingly like certain of the Protozoa (Fig. 6, B). During life their whip-like processes, lashing backward and forward in perfect uni- son, produce currents of water which continually pass through the body. The food thus entering the animal is taken up by the cells of the inner layer as it passes by. The supply, however, is usually more than suflS- cient to meet the demands of this layer, and the excess is passed on to the middle and outer layers. The exact method by which this occurs is still a matter of doubt, but there seems to be little question but that each cell of the body receives its food in a practically un- modified condition, requiring that it digest as well as assimilate. The oxygen necessary to this latter process Fig. 13.— One of the sim- plest sponges ( Calcolyn- thus pHmigenius (after Haeckel). a portion of the wall has been re- moved to show the in- side. 26 ANIMAL FOKMS is absorbed by all parts of the body in contact with the water. 27. Skeleton of sponges. — When it is remembered that the protoplasm composing the cells of the sponge has about the same consistence as the white of egg, it will be readily un- derstood why the greater number of sponges possess a skel- eton. Without such Fio. 14.— Portion of wall of sponge, showing three ^ support the larger layers, e, outer layer ; i, inner layer, consisting ^ " of collared cells'; »i, middle layer, consisting of glODUiar Or Drancned irregular cells, among which are the radiate spic- f grmS COuld not ex- ules and egg-cells. . . -■ • xi, ist, and even in the smaller members there would be danger of a collapse of the body walls and consequent stoppage of the food supply, owing to the closure of the pores. So in all but a very few thin or flat forms a skeleton appears in the young sponge almost before growth has fairly begun, and this increases with the body in size and com- plexity. It is formed by the activity of the cells of the middle layer, and may be composed either of a lime com- pound resembling mar- ble, or of flint, or of a horn-like substance resembling silk, or these may exist in combination in certain species. When consisting of either of the first-named substances it is never formed in one continuous piece, but of a vast multitude of variously shaped crystal-like bodies termed spicules (Fig. 15). These occur everywhere throughout the body, firmly bound together Pig. l.?.— Different types of sponge spicules. THE SPONGES 27 by means of cells, or so interlocked that they form a rigid support to which the fleshy substance is bound and through which the numerous canals penetrate. In a relatively few species only does the skeleton con- sist of horn, though there are many in which horn and flint exist together. In the former event, if the skeleton be elastic and of sufficient size, it becomes valuable to others than the naturalist, for the familiar sponges of commerce are the horny skeletons of forms usually taken in the West Indies or in the Mediterranean Sea. In these localities the animals are pulled off by divers, or with hooks, and are then spread out in shallow water where the protoplasmic sub- stance rapidly decays. The remaining skeleton, thoroughly washed and dried, is ready for the markets of the civilized world. Examining a bit of such a " sponge " under a magnify- ing glass, it will be seen that the skeleton is not composed of various pieces, but of one continuous mass of branching fibers, which interlace and unite in apparently the greatest confusion ; yet in the living animal these were perfectly adapted to the position of the canals and the general needs of the animal. Besides being a scaffold-work to which the fleshy portions of the body are fastened, the skeleton serves also for pro- tection. In some species, needle-like spicules as fast as they are formed are partly pushed out over the entire sur- face of the body, giving the appearance of a spiny cactus ; or in other cases they are arranged in tufts about the canals, effectually preventing the entrance of any marauder. Thus perfectly protected, the sponges have but few natural enemies, and hence it is that in favorable localities they grow in great profusion. 28. Race histories and life histories. — We have now traced living things from their simplest beginnings, where they exist as single cells, and have seen that in bygone times similar forms have united into simple colonies, and these 28 ANIMAL FORMS through a division of labor among the constituent cells have resulted in Volvox-like colonies. There are the strong- est reasons for the belief that as these simple forms scat- tered into various surroundings and underwent changes to meet the shifting conditions, they assumed different de- grees of complexity that have resulted in the animal forms of the present day. It may have been noticed also that the sponge in its development passes through these stages : a single-celled egg ; later, a young form similar to Fandorina, then growing to look like Volvox, and finally assuming its permanent form. The history of the race of sponges and their development through a long line of ancestry of increasing complexity is thus told by the sponge as it develops from the egg into the adult ; and, so far as we know, all the many-celled ani- mals in their growth from the egg repeat more or less clearly the stages passed through by their forefathers. CHAPTEE V THE CCBLBNTERATES 29. General remarks. — This division of the many-celled animals includes the jelly-fishes, sea-anemones, and corals. A few species live in fresh water, but the majority are con- fined to the sea, being found everywhere from the shore- line and ocean surface to the most profound depths. Adapted to different surroundings and modes of life, they constitute a vast assemblage of the most bewildering di- versity. In some cases their resemblance to plants is re- markable, and the term zoophyte or " plant animal," occa- sionally applied to them, is the relic of former times when naturalists confounded them with plants. Even to-day certain species are sometimes collected and preserved as seaweeds by the uninformed. The general plan on which all coelenterates are con- structed is a simple sac, in some respects resembling that of the lower sponges, yet, since the modes of life of the members of the two groups are usually quite unlike, we shall find many profound differences between them. 30. The fresh-water Hydra. — The bodily plan comes out most clearly in the Hydra (Fig. 16, A, D), which occurs upon the stems and leaves of submerged fresh-water plants in this and other countries. Its body, of a green or grayish color, according to the species, scarcely ever attains a diam- eter greater than that of an ordinary pin nor a length ex- ceeding half an inch. One end of the cylindrical organism is attached to some foreign object by means of a sticky secretion, but as occasion requires it may free itself, and by 89 30 ANIMAL FORMS means of a " measuring-worm " movement travel to another place. Examined under a hand lens, the free end of the body will be found to support six to eight prolongations known as tentacles, which serve to convey food to the mouth, centrally located in their midst. This opening, un- like that of the sponges, is the only one leading directly into the large central gas- tric cavity which occupies nearly the entire animal (Fig. 16, D). As in the sponge, the cells of the body are arranged in the form of defi- nite layers, but the middle one is rep- resented only by a thin gelatinous sheet. 31. Oi^ans of defense. — These are the so-called lasso or nettle-cells (Fig. 16, C). Some of tlie cells of the outer layer possess, in addition to the elements of the typical cell, a relatively large spherical sac filled with a fluid, and also a spirally wound hollow thread Fig. 16. — The fresh-water Hydra. A, entire animal, de- veloping a new individual (enlarged 25 times). B, C, nettle-cells (after Scuneider) ; I), section through the body. THE CCELENTBRATBS 31 provided with barbs near its base. On the outer extremity of the nettle-cell projects a delicate bristle-like process, the trigger hair. These cells are especially abundant on the tentacles (Fig. 16, A, D), forming close, knob-like eleva- tions or " batteries," thus rendering it practically impossi- ble for any free-swimming organism to avoid touching them in brushing against the body. In such an event the dis- turbances conveyed through the trigger hair set up in some unknown way very rapid changes in the cell. This causes the sac to discharge the coiled thread and barbs into the body of the intruder, which is rendered helpless by the par- alyzing action of the fluid conveyed through the thread. Thus benumbed it is rapidly borne to the mouth and swal- lowed. In time new nettle-cells develop to take the place of those discharged and consequently worthless. 32. Digestion of food. — Upon the interior of the body of Hydra and all of the coelenterates the food, by reason of its large size, is incapable of being taken into the various cells. It is necessary, therefore, to break it up into smaller masses, and this is accomplished through the solvent action of the digestive fluid poured over it from some of the cells of the adjacent inner layer. When subdivided, the granules swept about the gastric cavity by the beating of the flagella (Fig. 16, D) are seized by the processes on the free surfaces of the remaining inner layer cells, where they undergo the final stages of digestion ; then in a dissolved state they become absorbed and assimilated by all the cells of the body. 33. Methods of multiplication. — Very frequently, espe- cially if the Hydra has been well fed, two or three pro- cesses arising as outpushings of the body wall may be noted upon the sides of the animal (Fig. 16, A, D). If these be watched 'from time to time they are found to in- crease in size, and finally, upon their free extremities, to develop a mouth and surrounding tentacles. Up to this point growth has taken place as a result of the assimilation of nutritive substances supplied from the parent ; but a con- 32 ANIMAL FORMS striction soon occurs ■which separates the young from the parent, and from that time on the two lead independent existences. At other times this asexual method of mul- tiplication is replaced by sex- ual reproduction, where new individuals arise from fertil- ized eggs. Both eggs and sperm arise in Hydra and in some other animals in the same individual, but in all such cases the eggs are fertil- ized by sperm vrhich escape from some other individual. The fertilized egg, surround- ed by a firm coat, separates from the parent, drops to the bottom, and after a period of rest develops into a little Hy- dra which hatches and enters upon a free existence. . 17.— Different types of Hydrozoan colonies. Prom Nature, the lower species magnified about SO diameters. THE CCELENTERATES 33 34. Hydrozoa, or Hydra-like animals. — Attention has al- ready been directed to the fact that the structure of Hydra is the simplest of the ccelenterates ; nevertheless, the thou- sand or more species belonging to this class which present a much more complicated appearance (Fig. 17) possess many fundamental nydraAik.Q characters. It is owing to this fact that this assemblage of forms has been placed in the class of the Hydrozoa, or HydraAike animals. With but very few exceptions the members of this class are marine, usually living near the shore-line, where at times their plant-like bodies occur in the greatest profusion attached to rocks, seaweeds, or the bodies of other animals, particularly snails and crabs. Fig. 17 (upper colony) gives a good idea of one of the more complex forms, whose tree- like body attains in some cases the relatively giant height of from 15 to 25 cm. (six to ten inches). In early life it bears a close resemblance to a Hydra. Buds form in much the same way, but they retain permanently their connection with the parent, and in turn bear other buds, until finally the form shown in the figure is attained. In the meantime root-like processes have been forming which afford firm attachment to the object upon which the body rests. Also during this process the cells of the outer layer form a horny external skeleton ensheathing the entire organism except the ter- minal portions (the hydranths, Fig. 18, B) bearing the ten- tacles. The gastric cavities of all communicate, and the food captured by one ministers in part to its own needs and, swept through the tubular stalks and roots, is also shared by all other members. 35. Jelly-fishes and the part they play. — During the pro- cess of growth a number of stubby branches arise which differ from the ordinary type in shape, and also in many cases as regards color. These club-like, fleshy portions de- velop close-set buds (Fig. 18, c) which early assume a bell- like shape, the point of attachment corresponding to the handle, while the clapper is represented by a short, slender 34 ANIMAL FORMS process bearing on its end an opening which becomes the mouth (Fig. 18, A). Around the margin of the bell nu- merous tentacles develop, and at the same time the gelati- nous substance situated between the outer and inner layers of the bell expands to a relatively enormous degree, giving it an increasing globular form and glassy appearance. Fig. 18.— a jelly-fish (Gonlanemus), slightly enlarged. The stalked month is shown in dotted outline. B. C, enlarged portions of a hydroid colony bearing the mouth and tentacles ; j, a capsule within which the jelly-fish develop ; D, dia- gram of jelly-fish, illustrating its method of locomotion. Finally, vigorous movements rupture the connection with the parent, and this newly developed outgrowth, usually small, becomes an independent organism popularly termed a Jelly-fish. While the external form of the jelly-fish appears to be widely different from the hydranths, a more careful study shows the difEerence to be superficial. Some zoolo- gists believe that jelly-fishes are simply buds which have become fitted to separate and swim away from the colony in order to distribute the young, as described hereafter. When the stalked colonies are very abundant the jelly- THE CCELBNTBRATES 35 fishes may be liberated in such multitudes that the upper surface of the ocean for many miles may be closely packed with them in numbers reaching far into the millions. In these positions they are carried both by oceanic currents and through the alternate expansion and contraction of the bell, a movement resembling the partial closing and open- ing of an umbrella. In the jelly-fish the contraction is the more vigorous and rapid, and as it takes place the opening in the velum or veil (Fig. 18, 5) is so narrowed that the water in the subumbrella space (a) is driven through it with con- siderable force, which results in driving the body in the opposite direction. The life of a jelly-fish is perhaps of short duration, last- ing not more than a few hours in some species, up to two or three weeks in others, but during that period they pro- duce multitudes of eggs which develop into minute free- swimming young. These settle down on some rock or sea- weed, and soon develop a Hi/dra-like body which, after the fashion described above, grows into another tree-like colony. 36. Alternation of generations. — It will be noticed that the offspring of the jelly-fishes are not jelly-fishes, but stalked colonies, and these latter forms give rise to jelly-fishes. This is known as the alternation of generations, the jelly- fish generation alternating with the colonial form. This characteristic is of the greatest service in preventing the extermination of the race. "Were the stalked forms to give rise directly to other stationary colonies, it is obvious that before long all the available space in the immediate locality would be filled. The food supply, always lim- ited, would not suffice, and starvation of some or imper- fect development of all would result ; but by means of the free-swimming jelly-fish new colonies are established over very extensive areas, and favorable situations are held by all. 37. More complex types. — As mentioned above, there are perhaps upward of a thousand species of Hydrozoa, all with 36 ANIMAL FOEMS essentially the same structure but with various modes of branching (for some of the commoner modes, see Fig. 17)- In some of the higher forms a division of labor has arisen among various members of the association ■which has led to most interesting results. For example, Fig. 19 represents a species of hydroid found investing the shells of sea-snails occupied by hermit crabs (Fig. 60). To the unaided eye its appearance is that of a delicate vegetable growth, but when placed under the microscope it is found to consist of a multitude of Hydra-like animals united by a hollow branching root system con- necting the gastric cavities of all of them (Fig. 19). Certain individuals (a) with tentacles and a mouth resemble a Hydra ; others, without a mouth and ten- tacles, are reduced to a _ ,„ . , , ,. , , , . , club-like form (b) liberally Fig. 19.— An enlarged portion of a hydroid _ ^ ^ ■' -^ colony (Hydraciinia), showing (a) the Supplied with ncttle-Cells nutritive polyp, (6) the defensive polyp, .^p^^ ^Jjgjj. f j.gg extrcmi- and (c) the reproductive polyp. . i ■ t ties; while the third type (c), likewise devoid of a mouth, possesses rudiments of ten- tacles below which are borne numerous clumps of repro- ductive cells. The first type, the only one possessing a mouth, captures the food, and after digesting it distributes the greater portion to the remaining members by means of the connecting root system ; those of the second form, de- fending the others by means of their nettle-cells against the inroads of a foreign enemy, are the soldiers of the colo- ny; while the third type produces the eggs from which new individuals develop. In some of the higher Hydrozoa, the Portuguese man- of-war (Fig. 30), this division of labor has reached a more advanced stage of development, and in addition the entire THE CCELENTERATES 37 colony is fitted for a free-swimming existence. What cor- responds ordinarily to the attached stalk in other forms terminates in a bladder-like expansion, distended with gas, that serves as a float. From it are suspended individ- uals resembling great stream- ers sometimes many feet in length, without mouths, but loaded with nettle-cells that enable them to capture the food, which is conveyed to the second type, the nutritive polyps. - Each of these is a simple tube bearing a mouth, and within them the food is digested and distributed by means of a branching gastric cavity extending throughout the entire colony. Then there are individuals likemouthless jelly-fishes which bear the eggs and care for the perpet- uation of the colony ; and be- sides these there may be some whose duty it is to defend the rest, and others whose active swimming movements, to- gether with the wind, drive the colony about. Thus uni- ted, sharing the food supply and working for the general welfare of all, the members of this colony live in greater security and with less effort than if, as separate individuals, each was fighting the battles of life alone. 38. Scyphozoa. — The greater number of the larger and more conspicuous jelly-fishes are included under this term. In general shape and locomotion they resemble those of the Fig. 20.- A colonial jelly-fish {Physalia). From Nature. 38 ANIMAL FORMS preceding group (Fig. 21), but, while the latter are generally very small, these forma are commonly from four to twelve inches in diameter, and some measure one to two meters (three to six feet) across the bell. They are also distin- guished by means of tentacles which extend from the cor- ners of the mouth sometimes to a distance of several feet, and together with the marginal tentacles are formidable weapons for capturing small crabs, fishes, and other ani- mals which serve as food. In turn these forms serve as the food of many whales, por- poises, and numerous fishes which hunt them down, though the amount of nourishment they contain is prob- ably relatively small owing to the fact that in their composition there is a large percent- age of water (99 per cent in some species). The lobed margin of the bell, the absence of a definite swimming organ or velum, and the character of several of the internal organs, distinguish the larger from the smaller jelly-fish ; but the greatest differ- ence, however, is in the method of development. 39. Development. — The eggs arise from the inner layer of the jelly-fish and drop into the gastric cavity, where each develops into a ciliated two-layered sac in some respects like that of a young sponge. Swimming away from the parent, they finally settle down, and attaching themselves (Fig. 22, a) assume the external form and habits of the sea- FiG. 21.— A jelly-fish iEhizosloma), about one- fonrth natural size. THE CCELENTERATES 39 anemones, described in the next section. In the course of time remarkable changes ensue, which first manifest them- FiG. 22.— stages in the development of a scyphozoan jelly-fish, a, the attached young, which in b has separated into a number of disks, each of which becomes a jelly-fish, c— After Kokschelt and Heider. selves in a series of grooves encircling the body. These grow deeper, and the body of the animal finally comes to resemble a pile of sau- cers with the edge of each developed into a number of lobes (Fig. 22, h). One after an- other each saucer, to preserve the simile, raises itself from the top of the pile and swims away, and - is clearly seen to be a jelly-fish, though con- siderably unlike the adult. As growth pro- ceeds, however, it un- dergoes a series of transformations which result in the adult form. 4 Fig. 23.— An attached scyphozoan jelly-fish [Haliclysius). Natural size, from Nature. 40 ANIMAL FORMS 40. Sea-anemones. — In its external appearance the sea- anemone (Fig. 24) bears some resemblance to the Hydra, but is of a much larger size (1 to 45 cm., or ^ inch to 1^ feet in diameter), and is frequently brilliantly colored. The number of tentacles is also more numerous, and the mouth leads into the body by means of a slender esophagus (Fig. 35). Numerous partitions from the body wall extend in- ward, and many unite to the esophagus, keeping the latter r;.;- Fig. 24. — Sea anemones (liie two upper figures) and solitary coral polyps. in position. Below the esophagus each partition projects into, the great cavity of the body and bears upon its inner free edge several important structures. The first of these, known as the mesenteric filaments (Fig. 25), appearing like delicate frills, plays an active part in the digestion of the food. Associated with these are long, slender threads, THE CCBLENTBRATBS 41 closely packed with innumerable lasso-cells, which may be thrown out through openings in the body wall when the animal is attacked. Lasso-cells are also very numerous on the tentacles, which are thus to some extent defensive, but are chiefly active in capturing the crabs and small fish which serve as food. The partitions also carry eggs which may undergo the first stages of their growth within the body, and when finally able to swim are sent out through /f the mouth opening by hundreds to seek out favorable situa- tions, there to set- tle down and re- main. In some spe- cies the young may sometimes arise as buds, as in Hydra (Fig. 34), and in others the animals have been described as splitting longi- tudinally into two equal-sized young. 41. Corals.— The coral polyps also belong to this group, showing a very close resemblance to the sea-anemones. In most cases they develop a firm skel- eton of lime, conamonly known as " coral," which serves to protect and support the body. In a few species the polyps throughout life are solitary, and with skeleton comparative- ly simple (Fig. 24) ; but the larger number of species be- come more complex by developing buds, which retain their connection with the parent, and in turn produce other out- growths with the ultimate result that highly branched Fig, 25.— Longitudinal section through the body of a sea-anemone, oe, esophagus; m. /., mesenterial filaments ; r., reproductive organs. 42 ANIMAL FORMS colonies are produced (Fig. 26). At the same time the outer layer of the body is continually forming a skeleton which encloses the colony as a sheath, except at the ter- mination of each branch, where the mouth and tentacles are located. In certain species — for example, the sea pens {Pennatula) and sea fans (Gorgonia)—a skeleton may be Fig. 26.— Small portions of coral colonies, with some of the polyps expanded. formed of myriads of lime spicules, somewhat like those of the sponge, which are bound together by the fleshy substance of the body; but the skeleton of most of the common forms in the ocean, and the coral found in general collections, is stony. According to their method of branching, such specimens have received various popu- lar names, such as brain, stag-horn, organ-pipe, and fun- gous corals. THE CCBLENTERATBS 43 Nearly all species, like the sea-anemones, are brilliantly colored during life, and several are highly phosphorescent. All are marine, and while they are found everywhere, from the shore-line to great depths, the more abundant and larger species inhabit the clear, warm waters of the tropics down to a depth of one hundred and sixty feet. In such regions the stag-horn corals especially grow in the wildest profusion, and become tall and greatly branched. Except in quiet water they are continually being broken by the waves, beaten into fragments, and the resulting sand is deposited about their bases. As a result of this continu- ous growth and erosion, there have been formed from coral sand mixed with the shells of moUusks and the skeletons of various Protozoa several of the islands along the Florida coast and many of those of the Pacific, some of them hundreds of miles in extent. CHAPTER VI THE WORMS 43. General Characteristics. — The bodies of the animals comprising the two preceding groups are exposed on all sides equally to the water in which they live and are radi- ally symmetrical ; but in the worms, one side of the body is fitted for creeping, and for the first time we note a well- marked dorsal (back) and ventral (under) surface. In the former, the body, like a cylinder, may be divided into simi- lar halves by any number of planes passing lengthwise through the middle ; but in the worms, the right and left halves only are exposed equally to their surroundings, and there is, accordingly, only one plane which divides the body into corresponding halves, so that these animals, like all higher forms, are bilaterally symmetrical. In creeping, also, one end of the body is directed forward and it thus be- comes correspondingly modified. It usually bears the mouth, and may be provided with eyes, feelers, or organs of touch, and various other structures which enable the worm to recognize the nature of its surroundings. The nervous and muscular systems are better developed than in the foregoing groups, and we note a greater vigor and defi- niteness in the animal's movements, and in various ways the worms appear better able to avoid or ward ofE their enemies, recognize and select their food, and in general adapt them- selves to the conditions of life. The division of the worms is a very large one, and in some respects difiQcult to define, owing to the close resem- 44 THE WORMS 45 blance which many of them show to animals in other groups. All the invertebrates, therefore, except the crabs and insects, were placed in one group until subsequent study made it possible to classify them more exactly. Ac- cording to the general shape of the body, and the arrange- ment of internal organs, worms are diyided into a number of groups, chief among which are the flatworms, the thread or roundworms, and the ringed worms or annelids. The Flatwokms 43. Form and habitat. — The flatworms, as their name indicates, are much flattened, leaf-like forms, some species living in damp places on land, in fresh - water streams or ponds, or along the seacoast, while a variety of other spe- cies are parasitic. The free forms (Fig. 27) are usually small, barely reaching a length greater than five or seven cen- timeters (2 to 3 inches), but some of the parasitic species (Fig. 31) attain the great length of six to thirteen me- ters (30 to 40 feet). The free-living forms usu- ally occur on the under side of stones, and frequently are so delicate that a touch is sufiBcient to destroy them. A few species are almost trans- parent, while many are col- ored to harmonize completely with their surroundings, so that, even though fragile and defenseless, they escape the attacks of enemies by being overlooked. The night-time or dark days are their hunting Fig. 27.— a, fresh-water flatworm (Pto- naria) ; B, marine flatworm {Lepto- plana). Enlarged, from Nature. 46 ANIMAL FORMS season, and at such periods they may be found moving about with a steady gliding motion (due to cilia covering the en- tire body), varied occasionally by a looping, caterpillar move- ment, or by swimming with a flapping of the sides of the body. When watched at such times they may sometimes be seen to snatch up small worms, snails, small crabs and insects, which serve as food. More closely examining one of these forms, for example, the species usually found on the under side of sticks and stones in our shallow fresh-water streams (Fig. 37, A), we note that the forward end is not developed into a well-defined head as in the higher worms, but is readily determined by the presence of very simple eyes and tentacles, while the lower creeping surface is dis- tinguished by a lighter color and the presence of the mouth. Through this small opening a slender proboscis (in reality the pharynx) may be extended some distance, and may be seen to hold the small organisms upon which it lives until they are suffi- ciently digested to be taken into the body. 44. Digestive system. — In the smaller flatworms, some of which are scarcely larger than many of the Protozoa, the alimentary canal is a sim- ple unbranched tube ; but in the larger forms such an ap- paratus is replaced by a greatly branched digestive tract which furnishes an extensive surface for the rapid absorp- FiG. 28.— Anatomy of fresh-water flat- worm (Planaria). exs, excretory sys- tem, with flame-cell (/). The all mentary canal Is stippled. B, nerv- ous system. THE WORMS 47 tion of food, and extending deep into the tissues of the body, carries nutriment to otherwise isolated regions. In the fresh-water forms and their allies there are three main branches of the intestine (Fig. 28), while in many of those from the sea there are several, and their arrangement affords a basis for their general classification. 45. Excretory system. — In the sponges and ccelenterates the wastes are cast out by the various cells into the gastric cavity or at once to the exterior with- out the aid of any pronounced system of vessels; but in the flatworms sev- eral of the organs are deeply buried within the tissues of the body and a drainage system becomes a necessity. This consists of a paired system of ves- sels extending the length of the ani- mal (Fig. 28) and provided with numer- ous branches, some of which open at various points on the surface of the body, while the others terminate in spaces (Fig. 29, s) among the organs in what are known as flame-cells. The substances which accumulate in these spaces are gathered up by the flame- cell, poured into the space it contains, and by means of the vibratory motion of its flagellum (/), a movement bearing a fancied resemblance to the flickering of a flame in the wind, are borne through the tubes to the exterior. 46. Nervous system and sense-organs. — In the sponges no definite nervous system is known to exist, the slight move- ments which the cells are able to undergo being regulated somewhat as they are in the Protozoa. Among the ccelen- terates certain of the cells scattered over the surface of the body are set aside as nerve-cells, and, more or less united by means of fibers extending from them, convey impulses over the body. In the flatworms the larger number of nerve-cells Pig. 29.— Flame-cell o£ flat- worm (after Lang). /, flagellum ; n, nucleus; s, spaces among the or- gans of the body ; v, waste materials. 48 ANIMAL FOKMS are collected into two definite masses (Fig. 28, B), which constitute a simple brain on which the eyes are situated and from which bundles of nerve fibers pass to all parts of the body, the two extending backward being especially noticeable. As in the squirrel, these are distributed to the muscles and other organs to regulate their actiyity, while those distributed to the skin, especially in the forward part of the body, convey stimuli produced by touch. The branches connecting with the eyes enable the animal to distinguish light from darkness, but are probably too sim- ple to allow it to clearly distinguish objects of the outside world. The sense of smell and possibly that of taste are also present, but are relatively feeble. Some other characters of this class will be noted in the consideration of the two following classes. 47. Parasitic flatworms (trematodes) — parasitism. — Men- tion has already been made of the associations of two ani- mals as " messmates " for mutual benefit, such as the Hy- dractinia growing on the surface of the shell inhabited by the hermit crab, to which it gives ^protection by means of its nettle-cells, while in turn being borne continually into regions abounding with food. More frequently, however, one animal derives benefit from another without making any compensation. For example, many species of fiatworms live within the shells of certain snails and upon the bodies of searurchins and starfishes, where they gather in their food supply safe from the attacks of enemies. Such asso- ciations are probably without much if any inconvenience to the animal thus inhabited, and it also appears probable that the tenants are transients, using the mollusk or star- fish only as a temporary home. But from this condition of affairs it is only a short step to the parasitic habit, where the association becomes permanent and the occupant is provided with various structures which prevent its sepa- ration from its host. This latter kind of union occurs throughout the group of trematodes ; all are parasitic, and THE WORMS 49 their internal organization, so closely resembling that of the free-living forms as to need no further description, in- dicates that they are descendants of the lat- ter. In the greater number the body is flat, and a few species still retain their outer coat of cilia ; but since these are no longer of service as locomotor organs they have gen- erally disappeared, and in their place numer- ous adhesive organs, such as spines, hooks, and suckers (Fig. 30), have arisen, which en- able the animals to hold on with great te- nacity. Thus attached to its host, and using it as a convenient and comparatively safe means of locomotion, the parasibe may still continue to capture small animals for food or may derive its nourishment from the tissues of the host. In addition there are numbers of internal parasites, living almost ex- clusively i,n the bodies of vertebrate animals, scarcely a sin- gle one escaping their ravages. 48. Life history. — In the external parasites the young hatch out and with comparative ease make their way to another host ; but the young of an internal parasite, inhab- iting the alimentary canal, have a very slight chance in- deed of ever reaching a similar location in another host. Fie. 30. — A parasitic flatworm (Epid mouth ; o, opening of reproductive system ; 6, sucker and spines for attachment. The di- gestive system is stippled ; nervous system black. Enlarged 8 times, from Nature. 50 ANIMAL FORMS For this reason an almost incredible number of eggs is laid, and some extraordinary measures are employed in effecting the desired result. Probably the best-known example is that of the liver fluke inhabiting the bile-ducts in the sheep. Each worm lays several hundred thousand eggs, which make their way from the host, and if they chance to fall in pools of water or damp situations may proceed to develop, other- wise not. If the surroundings be favorable, the young, like little ciliated Infusoria, escape from their shells and rest- lessly swim or move about for a short time, and if during this time they come in contact with certain species of snails living in these situations they at once bore into their bodies. Here they produce other young somewhat resem- bling a tadpole, that now make their escape from the snail. In a short time each one crawls upon a blade of grass, and surrounds itself with a tough shell, where it may remain for several weeks. If the grass on which they rest be eaten by a sheep, they finally make their way to the bile-ducts and there become adult. The life cycle is now complete ; the young form has found a new host ; and the process shows how wonderfully animals are adapted to the conditions which surround them, and how closely they must conform to these conditions in order to exist. 49. The tapeworms (cestodes). — The cestodes, or tape- worms, are also parasitic flatworms in which the effects of such a mode of life are strongly marked. They occur almost exclusively in the bodies of vertebrate hosts and exhibit a great variety of bodily forms, in some cases resem- bling rather closely the trematodes, but in others strikingly different. In the latter type the body is usually of great length (from a few centimeters to upwards of sixteen meters (50 feet)), and terminates in a "head" (Pig. 31) provided, in the different species, with a great variety of hooks and spines and numbers of suckers for its attachment to the body of the host. From the head the body extends back- ward in the gradually enlarging ribbon-like body, slender at THE WORMS 51 first and scarcely showing the segments which finally be- come so prominent a feature. When carefully examined, a two-lobed brain is found in the " head," and from it nerves extend the entire length of the body, followed throughout their course by the tubes of the excretory system ; also each segment contains a perfect reproductive system, so that even if it be separated from the others it may continue to exist for a consid- erable length of time. Furthermore, the tapeworms are surrounded by the predigested fluids of their host; a special alimentary canal is therefore superfluous, and all traces of it have disappeared. 50. Development.^As the animal clings in this passive way to the body of its host the segments, loaded with eggs ready for development, separate one after another from the free end of the body, pass to the exterior, and slowly crawling about like independent organisms, lay great numbers of eggs, which may find an intermediate host as in the life cycle of the liver fluke, and so in time find their permanent resting-place. Fortunately in all these parasitic forms, though an inconceivably great number of eggs are laid, only a comparatively few reach maturity. Even these, however, may cause at times great destruction among the higher, and especially our domestic, animals, often doing damage amounting to many millions of dollars per year. 51. The tapeworm in relation to regeneration. — It has been known for more than one hundred and fifty years that some of the lower animals possess to a surprising degree the ability to regenerate parts of the body lost through injury. The Hydra, hydroids, and some of the jelly-fishes Fig. 31.— Tapeworm (Tainia solium). In upper left- hand corner of figure is the much enlarged head. —After Leuckabt. 52 ANIMAL FORMS may be cut into a number of pieces, each of which will develop into a complete individual ; and this power of recov- ery from the injuries produced by enemies is of the great- est service in the perpetuation of the species. This ability is also present in certain flatworms, and some species are known which voluntarily separate the body into two por- tions, each of which becomes an adult. In other species a similar process results in the formation of a chain of six individuals, placed end to end, the chain finally breaking up into as many complete worms. It is possible that the tapeworm may also be looked upon as a great chain of united individuals produced by the division of a single original parent, which becomes adapted for attaching the others until they separate. These latter are capable only of a very sluggish movement, and, devoid of mouth and ali- mentary canal, are not able to digest their food, but their life work is to so lay their eggs that they may develop into other individuals, and for this they are well adapted. Nematodes (Threadworms) 52. General characters. — This class of worms is com- posed of an enormous number of diflerent species, some para- sitic, others free all or a portion of their lives, and in view of the fact that they inhabit the most diverse situations it is remarkable that they are so uniform in their structure. In all the body is slender, and the general features of its organ- ization may be readily understood from an examination of the "vinegar eel" (Fig. 32, A). This small worm (not an eel), a millimeter or two in length, lives on the various forms of mold that grow in fermenting fruit juices, especially after a little sugar or paste has been added. A tough cuti- cle surrounds the body, preserving its shape and at the same time protecting the delicate organs against the action of the acids in which it lives. Through this may be seen great bands of muscles extending the entire length of the body and producing the wriggling movements of swimming THE WORMS 53 or crawling. They also give support to a brain, which is in the form of a collar encircling the pharynx near the head, and to the great nerves which extend from it. Still fur- ther within the transparent body the alimentary canal may be distinguished as a straight tube passing directly through the ani- mal. This latter system lies freely in a great space, the body cavity, traces of which may exist in the flatworms in the form of small hol- lows among the organs into which the kidneys open. It is possible that in this form also the kidneys open into this space, and it is roomy enough besides to afford lodgment for the reproductive or- gans in addition to a large amount of fluid which is probably somewhat of the nature of blood. A space in some respects similar to this occurs in all the animals above this group, and as we shall see, it is often cu- riously modified and serves for a number of different and highly im- portant purposes. In the round- worms the fluid it contains proba- bly acts in the nature of a blood system, distributing the food and oxygen to various parts of the body and carrying the wastes to the kidneys for removal. 53. Multiplication. — In the matter of the production of new individuals the greatest differences exist. In some threadworms, for example the " vinegar eel," eggs develop within the body and the young are born with the form of the parent. In other cases the eggs are laid in the water, where they, too, may directly grow to the adult condition ; but in Fig. 32. — Thread- or round worms. A, vinegar eel (,An guillula) ; m, month ; ph. pharynx ; i, intestine ; ov. developing young. B, Tri' china. From Nature, greatly enlarged. 54 ANIMAL FORMS the greater number of species the development is round- about, and one or more hosts are inhabited before the young assume the adult condition. Such is the case with the dreaded Trichina (Fig. 33, B), which infests the bodies of several animals, particularly the rat. When these forms are introduced into the alimentary canal of the rat, for example, they soon lay a vast quantity of eggs, sometimes many millions, which develop into young that bore their way into the muscles of the body, where they may remain coiled up for years. If the body of the rat be eaten by some carnivorous animal, these excessively small young are lib- erated during the process of digestion and rapidly assume the adult condition in the alimentary canal, likewise giving rise to young which pursue again this same course of development. Another example of a complicated life history is in the Oordius or " horsehair snake " (a true worm and not a snake) frequently seen in the spring in pools where it lays its eggs. These eggs develop into young which bore their way into different insect larvae, which are in turn eaten by some spider or beetle, and the worm thus transferred to a new host. In this they grow to a considerable size, and then make their exit from the body of the host and finally become adult. 54. Spontaneous generation, — It .is only within compara- tively recent years that such life histories have been under- stood. Formerly the sudden appearance of these and other forms in various situations were accounted for on the ground that they arose spontaneously without the intervention of any living creature. Even yet we hear of the transforma- tion of horsehairs into hairworms, and of .frogs, earthworms, and several other animals from inorganic matter, but such assertions are based on superficial observations, and at the present time no exception is known to the law that living creatures arise from preexisting living parents. " All life from life " {omnium vivuni ex vivo) is a universal law. THE WORMS, 55 Annelids oe Segmented Worms 65. The earthworms and their relatives. — Leaving the groups of the parasitic animals, which have been driven from. the field of active existence and in many ways are degraded by such a mode of life, we pass on to the higher free-living worms, where brilliant colors, peculiar habits, or remarkable adaptations render them peculiarly interesting. In consid- ering first their general organization, we may use the earth- C m Fig. 33.— Earthworm (Lumbricus terrestris). m, moath ; c, girdle or clitellum. worm (Fig. 33) (sometimes called angle-worm or fish-worm) as a type because of its almost universal distribution. The body is cylindrical, shows well-marked dorsal and ventral surfaces, and, as in all of the annelids, is jointed, each joint being known as a segment. Anteriorly it tapers to a point, and the head region bearing the mouth is ill- defined, unlike many sea forms, yet serves admirably for tunneling the soil in which all earthworms live. In this process the animal is also aided by bristles or setm which project from the body wall of almost every segment and may be stuck into the earth to afford a foothold. 56. Food and digestive system. — The earthworms are nocturnal animals, seldom coming to the surface during the day except when forced to do so by the filling of their tun- nels with water or when pursued by enemies. At night they usually emerge partially, keeping the posterior end of the body within the burrow, and thus they scour the sur- rounding areas for food, which they appear, in some cases at least, to locate by a feeble sense of smell. They also frequently extend their habitations, and in so doing swallow enormous quantities of earth from which they digest out any nutritive substances, leaving the indigestible matter in 5 56 ANIMAL FORMS coiled " castings " at the entrance of the burrows. In thus mixing the soil and rendering it porous they are of great service to the agriculturist. Although earthworms are omnivorous they also manifest a preference for certain kinds of food, notably cabbage, celery, and meat, which leads us to think that they have a sense of taste. All these substances are carried within their retreats and devoured, or are used to block the entrance during the day. The food thus carried within the body is digested by a system (Fig. 34) composed of several portions. nc. " s,f •&'^iii. ■ It- N^ii Hi ^^^^^^^BISnGwRpV^w^^ tmmi WlttttKl^^s^^^;^^\ "4^lfe S"* jlj^^y'ji ^r^ ,-i5l '"■5^S$~S>^^1 ^^^ ' j i ^ i.-^^i^ff^ i£^^^^'fMi^f*"'^Jf* 7 "My rtgS ^i\l // efif^^^l^'J ►^ ^^mIh m m 1^^^ Fig. 61.— Kelp-crab {EpiaUus prodnctus) in upper part of flgnre ; to the right the edible crab (Cancer productus), and the shore-crab {Pugettia richii). connection it is interesting to note that the giant crab of Japan, the largest crustacean, being upward of twenty feet from tip to tip of the legs, is a spider-crab, constructed on Fro. 63. — The fiddler-crab (Gelasimun). Photograph by Miss Mary Rathbun. the same general pattern as our common coast forms. Between these two extremes numberless variations exist. ARTHROPODS. CLASS CRUSTACEA 105 some for known reasons, but more often not readily under- stood. And not only does the form vary, but the external surface may be sculptured or beset with spines or tubercles which frequently render the animal inconspicuous amid its natural surroundings. Such an effect is heightened by the presence of spongesj hydroids, and various seaweeds which the crab often permits to gather upon its body. 105. Pill-bugs and sandhoppers. — Finally there remain the groups of the pill- or sow-bugs (Isopods) and the sand-fleas or sandhoppers (Amphipods). In the first of these the body is usually small and compressed, the thorax more or less plainly segmented, and the seven walking (thoracic) legs are similar. In the female each leg bears at its base a thin membranous plate which extends inward and hori- FiG. 63.— laopod or pill-bug (Porcetlio laevls). zontally, thus forming on the under side of the body a brood-pouch (Fig. 63) in which the young develop. As one may readily discover in any of the common species, the abdominal segments are more or less fused, and bear appendages adapted for respiration and, in the aquatic forms, for swimming. 106 ANIMAL FORMS The marine isopods occur in the sand, under rocks, and i]i the seaweeds ; many are parasitic upon fishes ; and the ter- restrial forms (Fig. 63) are very common objects under old Pig. 64.— Amphipods or sund-fleas (Gammarus, upper species, and Caprella). logs and in cellars, where they live chiefly on vegetable mat- ter. In the sand-fleas the body is compressed from side to side, and while the thorax shows distinct segments, the legs are frequently dissimilar, and some may bear pincers. One of their most distinctive marks concerns the last three ab- dominal appendages, which are usually modified for leaping. The sand-fleas (Fig. 64) are familiar objects to any one who has collected alohg the beach and has turned over the cast-up seaweeds, while numbers of small species often oc- cur among the plants in ovir fresh-water ponds. Some most curious and highly modified forms, whose general appear- ance is shown in the lower part of Fig. 64, occur among ARTHROPODS. CLASS CRUSTACEA 107 hydroid colonies, with which their bodies harmonize in form and color. And, lastly, most bizarre creatures, known as " whale-lice," attach themselves to the skin of whales, of which each species acts as host for one or more kinds. 106. Internal organization. — Most Crustacea are carnivo- rous, preying upon almost any of the smaller animals within convenient reach ; a much smaller number live on vege- table food ; and there are many, such as the crayfishes, lob- sters, and numerous crabs, which are also notorious scaven- gers. In these latter forms the food is held in one of the large pincers, torn into shreds by the other, and transferred to the mouth-parts, where, as in all Crustacea, it is soon reduced to a pulp by their rapid movements. In many species the food is now ready for the digestive process, but not so in the higher forms. If the stomach of any of these, for example, the crabs or crayfishes, be opened, three (Fig. 65, s) large teeth operated by powerful muscles will be noted, and beyond these a strainer consisting of many closely set hairs. In operation this " gastric mill " takes the food passed on from the mouth-parts, and crushes and tears it until fine enough to pass through the strainer, whereupon it is dissolved by the juices from the liver and is absorbed as it passes down the intestine. The circulatory system is usually highly developed, and consists of a heart, in some species almost as long as the body, though usually shorter (Fig. 65), from which two or more arteries branch to all parts of the body. Here the blood, instead of emptying into definite veins, pours into a series of spaces or sinuses in among the muscles and other organs of the body, through which it makes its way back to the heart. During this return journey it is usually made to traverse definite respiratory organs, either situated upon the legs or, as feathery outgrowths, upon the sides of the body, and generally concealed under the carapace. A por- tion of the blood is also continually sent to the kidneys, which are located either at the base of the second antennae 108 ANIMAL FORMS (and known as green glands), as in the crayfishes or crabs, or on the second maxillae (shell-glands) in many of the Pig. 65.— Dissection of crayfish, b, brain ; h, heart ; i, intestine ; k, Icidney ; I, liver ; 7t, nerve-cord ; y, reproductive orp:an ; rf, stomach, showing two teeth in position. simpler crustaceans. Their method of operation is much like that of the kidneys in the earthworm. 107. Nervous system and special senses. — The nervous sys- tem also shows a decided resemblance to that of the anne- lids. The cerebral ganglia or brain is situated above the alimentary canal in the head, and connects with the ven- trally lying cord by a collar. As in the earthworm, this ventral cord is double, and bears a pair of swellings or gan- glia in each segment. In the crayfish, crabs, and other highly modified forms, where the segments tend to fuse, several of these ganglia may also unite, and except in early life their number cannot be determined. Among the less specialized Crustacea the order of intel- ligence is low, though perhaps it may prove to be higher than is usually supposed when such forms have been move thoroughly studied. The following quotation relating to the lobster applies even more to the higher forms, the crabs : " Sluggish as it often appears when out of water and when partially exhausted, it is quite a different animal when free to move at will in its natural environment on the sea- ARTHROPODS. CLASS CRUSTACEA 109 bottom. It is very cautious and cunning, capturing its prey by stealth, and with weapons which it knows how to conceal. Lying hidden in a bunch of seaweed, in a crevice among the rocks, or in its burrow in the mud, it waits until its victim is within reach of its claws, before striking the fatal blow. The senses of sight and hearing are probably far from acute, but it possesses a keen sense of touch and of smell, and probably also a sense of taste." Although enclosed in a horny and often very thick and strong armor, the sense of touch is very keen in the Crustacea and in arthropods generally. On many of the more exposed portions delicate hairs or pits connected with the nervous system occur in great abundance. Some of these, usually on the antennae, undoubtedly serve in detecting odors, but the remainder are considered to be tactile. In the higher Crustacea, such as the crayfish, lobsters, and crabs, ears are usually found, consisting of sacs lined with similar delicate hairs, and containing sev- eral minute grains of sand, which in many cases make their way through the small external opening. Vibrations com- ing through the water gently shake the grains of sand, causing them to strike against the hairs which communi- cate with the nervous system — a very simple ear, yet suffi- cient for the needs of the animals. The eyes of the Crustacea and arthropods in general are either simple or compound. The simple and frequently single eyes usually consist of a relatively few cells embedded in a quantity of pigment and connected with the nervous system. It is doubtful whether they perceive objects as anything more than highly blurred images, and perhaps they merely recognize the difference between light and darkness. The compound eyes, on the other hand, are remarkably complex structures, often borne on the tops of movable stalks, as in the common crabs and crayfishes. Each consists of an external transparent cornea, divided into numerous minute hexagonal areas corresponding to as 110 ANIMAL FORMS many internal rods of cells, provided with an abundant nerve-supply. These latter elements may perhaps repre- sent simple eyes grouped together to form the compound one ; and it appears possible that each element may form a complete image of an object, as each of our eyes is known to do. On the other hand, many hold that the complete eye forms only one image, a mosaic, each element con- tributing its share. 108. Growth and development. — As we have seen, the simpler Crustacea hatch as minute larvae (Fig. 56), and dur- ing their growth to the adult condition are especially sub- ject to the attacks of multitudes of hungry enemies. In the higher forms, such as the crabs, some of these early transformations take place while the young are still within the egg and attached to the parent. Accordingly, the little ones are fairly similar to their parents, and their later his- tory is very well exemplified by the lobster. The eggs of the lobster are most frequently hatched in the summer months, usually July, after they have been carried by the parent for upward of a year. The young, about a third of an inch in length, at once disperse, undergo four or five moults during the next month, then, ceasing their swimming habits, settle to the bottom among the rocks. At this time, twice their original size, they closely resemble their parents, and their further development is largely an increase in size. " The growth of the lobster, and of every arthropod, apparently takes place, from in- fancy to old age, by a series of stages characterized by the growth of a new shell under the old, by the shedding of the outgrown old shell, a sudden increase in size, and the gradual hardening of the shell newly formed. Not only is the external skeleton cast off in the moult and the linings of the masticatory stomach, the esophagus, and intestine, but also the internal skeleton, which consists for the most part of a complicated linkwork of hard tendons to which muscles are attached." ARTHROPODS. CLASS CRUSTACEA 111 109. Peripatus (class Onycliophora). — It is generally be- lieved that the Crustacea, insects, and spiders, together with their numerous relatives, trace their ancestry back to animals that bore a certain resemblance to the segmented worms. Most of these ancient types have long been extinct, but here and there throughout the earth we occasionally meet with them. Among the most interesting of these are a few widely distributed species belong- ing to the genus Peripatus (Fig. 66), but as they are comparatively rare we shall dis- miss them with a very brief description. They usually dwell in warm countries, un- der rocks and decaying wood, emerging at night to feed on insects, which they ensnare in the slime thrown out from the under surface of the head. Their external form, their excretory system, and various other organs are worm-like. On the other hand, the appendages are jointed, and one pair has been modified into jaws. The peculiar breathing organs characteristic of the in- sects are also present. Peripatus therefore gives us an interesting link between the worms and insects, and also affords an idea of the primitive insects from which the modern forms have descended. 110. The centipeds and millipeds (class Myriapoda).— Many of the myriapods — that is, the centipeds and thousand-legged worms — are familiar objects under logs and stones throughout the United States. The first of these (Pig. 67) are active, savage creatures, devouring numbers of small animals, which they sting by means of poison-spines on the tips, of the first pair of legs. The bite of the larger tropical Pig. 66.— Peripatus {Peripatus eiseni). Twice the natural size. 112 ANIMAL POEMS species especially causes painful but not fatal wounds in man. On the other hand, the millipeds (Fig. 68) or thousand- legs are cylindrical, slow-going animals, feeding on yegetable Fig. 67.— Centiped. One-half natural size. Fig. 68. — Thou8aiid-Je.gs or milliped (Julus). Natural size. substances without causing any particular damage, except in the case of the " cutworms," which often work great injury to crops. When disturbed they make little effort to escape, but roll into a coil and emit an offensive-smelling fluid, which renders them unpalatable to their enemies. All present a great resemblance to the segmented worms, as their popular names often testify; but, on the other hand, many points in their organization indicate a closer relationship to the insects. As in the latter, the head is distinct, and bears a pair of antennae, the eyes, and two or three pairs of mouth-parts. The trunk is more worm-like, and consists of a number of similar segments, each bearing I ARTHROPODS. CLASS CRUSTACEA 113 one or two pairs of jointed legs. In their internal organ- ization the character of the various systems closely resem- bles that of the insects, and will be more conveniently described in that connection. Among the myriapods the females are usually larger than the males. Some of the centipeds deposit a little mass of eggs in cavities in the earth and then abandon them, while others wrap their bodies about them and pro- tect them until the young are hatched. The millipeds lay in the same situations, but usually plaster each egg over with a protective layer of mud. After several weeks the young appear, often like their parents in miniature, but in other species quite unlike, and requiring several molts to complete the resemblance. CHAPTEE X ARTHROPODS {.Continued). CLASS INSECTS 111. Their numbers. — It has been estimated that upward of three hundred thousand named species of insects are known to the zoologist, and that these represent a fifth, or possibly a tenth, of those living throughout the world. Many of these species, as the may-flies and locusts, are represented by millions of individuals, which sometimes travel in such great swarms that they darken the sky. With nearly all of these the struggle for existence is fierce and unrelenting, and it is little wonder that such plastic animals have changed in past times and are now becoming modified in order to adapt themselves to new situations where food is more abundant and the conditions less severe. Owing to such modifications we find some species fitted for flying, others for running and leaping, or for a life underground, and many for a part or all of their lives are aquatic in their habits. 112. External features.— The body of an insect— the grasshopper, for example— consists of a number of rings arranged end to end, as we have seen them in the Crustacea i^nd the segmented worms. In the abdomen these are qlearly distinct, but in the thorax, and especially the head, they have become so intimately united that their number is a matter of uncertainty. These three regions— head, thorax, and abdomen — are usually clearly defined in most insects, but they are modified in innumerable ways in ac- cordance with the animal's mode of life. 114 ARTHROPODS. CLASS INSECTS 115 The head usually carries the eyes, a pair of feelers (an- tennae), and three pairs of mouth-parts which may be fash- ioned into a long, slender tube to be used in sucking, and frequently as a piercing organ ; or they may be constructed for cutting and biting. The thorax bears three pairs of legs and often one or two pairs of wings. The appendages of the abdomen are usually small and few in number, or even absent. 113. Internal anatomy. — The restless activity of insects is proverbial. Some appear to be incessantly moving about, either on the wing or afoot, and are endowed with com- paratively great strength. Ants and beetles lift many tjmes their own weight. Numerous insects are able to leap many times their own length, and others perform different kinds of work with a vigor and rapidity unsurpassed by any other class of animals. As is to be expected, the muscular sys- tem is well developed, and exhibits a surprising degree of complexity. Over five hundred muscles are required for the various movements of our own bodies, but in some of the insects more than seven times this number exist. The amount of food necessary to supply this relatively immense system with the required nourishment is correspondingly large. Many insects, especially in an immature or larval condition, devour several times their own weight each day. Their food may consist of the juices of animals or plants, which they suck out, or of the firmer tissues, which are bitten or gnawed off. Not only do the mouth-parts stand in direct relation to the habits of the animal and to its food, hut, as we have often noticed before, the internal organization is also adapted for the digestion and distribution of the nutritive substances in the most economical way. For this reason we find the alimentary canal differing widelyin the various forms of insects. In each case it extends from the mouth to the opposite end of the animal, and ordinarily consists of a number of different parts. In the insect shown in 116 ANIMAL FORMS M-C Fig. 69 the mouth soon leads into the esophagus, which in turn leads into the crop that serves to store up the food until ready for its entry into the stomach ; or in some of the ants, bees, and wasps it may contain material which may be disgorged and fed to the young. In many cases the stomach is small and ill-defined as in Fig. 69, and again it may reach enormous dimensions, near- ly filling the body. It may also bear numerous lobes or delicate hair-like processes, which afford a greater sur- face for the absorption of food. Behind the stomach are a number of slender outgrowths that are believed to act as kidneys. Beyond their insertion lies the in- testine, which, like the stomach, is the subject of many modifications in the different kinds of insects. The digested food is rap- idly absorbed through the coats of the stomach and intes- tine and enters a circulatory system which reminds us of what exists in many of the Crustacea. The heart is situ- ated above the digestive tract, and from it arteries pass out to different parts of the body. Here the blood leaves the vessels and is poured directly into the spaces among the viscera, whence it is finally conducted through irregular channels to the heart by its pulsations. In the Crustacea the blood is made to pass through a respiratory system usually in the form of definite gills, and the oxygen with which it is charged is distributed to all Fig. 69.— Cockroach, diesected to show ali- mentary canal, al. c— After Hatselek and CoRi. ARTHROPODS. CLASS INSECTS 117 parts of the body. In the insects the blood serves almost entirely to carry the food, and the oxygen is conveyed through the animal by a remarkable contrivance found only in the insects, the spiders, and a few related forms. 114. Respiratory system. — If we examine an insect, the grasshopper for example, we find a number of small brown spots on each side of the abdomen, each of which under a magnifying-glass is seen to be perforated by a narrow slit. Carefully opening the body, we find that each slit is in communication with a white, glistening tube that rapidly branches and penetrates to all parts of the animal. When the body is expanded the air rushes into the outer openings, on through the open tubes, and is distributed with great rapidity to all the tissues of the body. In many insects some of these tubes connect with air-sacs which probably serve to buoy up the insect during its flights through the air. 115. Wingless insects (Thysanura). — The simplest of all insects -are the fishmoths and springtails, relatively small organisms covered with shining scales or hairs. The first of these is occasionally seen running about in houses feed- ing upon cloth and other substances, while the latter live in damp places under stones and logs. They are without wings, but are able to run rapidly and to leap considerable distances. In addition to the ordinary appendages, the abdomen bears what are perhaps rudimentary legs, a fact which, together with their relatively simple structure, strengthens the belief that the insects have descended from centiped-like ancestors. 116. Grasshoppers, crickets, katydids, etc, (Orthoptera). — Rising higher in the scale of insect life, we arrive at the group of the cockroaches, crickets, grasshoppers, locusts, and other related insects. Four wings are present, the first pair thickened and overlapping the second thinner pair. The latter are folded lengthwise like a fan, which is said to have given the name Orthoptera (meaning straight- winged) to 118 ANIMAL FORMS this group of insects. These extend all over the world, being particularly abundant in the -warmer countries, and their strong biting mouth-parts and voracious appetites render many of them dreaded pests to the farmer. The cockroaches are nocturnal in their habits, racing about at night, devouring victuals in the pantry and gnawing the bindings of books. During the day their flat bodies enable them to secrete themselves in crevices wherever there is suificient moisture. In the grasshoppers, locusts, katydids, and crickets the body is more cylindrical, and the hind pair of legs are often greatly lengthened for leaping. The crickets and katydids are nocturnal, the former re- maining by day in burrows which they construct in the earth, the latter resting qui- etly in the trees. At night Fig. 70.— The Rocky Mountain locust.— , i » , ^ i i After R.LET, from The Insect World. they f Cast Upon vegetable matter principally, though some species are known to prey on small animals. Those insects we usually term grasshoppers (properly called lo- custs) are specially destructive to vegetation. Some spe- cies are strong fliers, and this, connecbed with their abil- ity to multiply rapidly, renders them greatly dreaded pests. They have been described as flying in great swarms, form- ing black clouds, even hiding the sun as far as the eye could reach. The noise made by their wings resembled the roar of a torrent, and when they settled upon the earth every vestige of leaf and delicate twig soon disappeared. The eggs of the majority of Orthoptera are laid in the ground, where they frequently remain through the winter. When hatched the young quite closely resemble the parents, and, after a relatively slight metamorphosis, assume the adult form. 117. Dragon-flies, may^ies, white ants, etc. (Neuroptera). — The dragon-, caddis-, may-flies, ant lions, and the white ants ARTHROPODS. CLASS INSECTS 119 possess four thin and membranous wings incapable of being folded. These possess a network of delicate nervures, giv- ing the name Neuroptera (meaning nerve-winged) to the class. Of the forms mentioned above, all but the white ants lay their eggs in the water, and the developing larvae Fig. 71.— Dragon-fly (Libellula pulc/iella). spend their lives in this medium until the time comes for their complete metamorphosis into the adult. The larvae of the caddis-flies protect themselves within a tube of stones or sticks bound together with silken threads, which they usually attach to the under side of stones in running water. On the other hand, the young of the dragon- and may-flies, pro- vided with strong jaws, are active in the search of food and very voracious. In time they emerge from their larval skin and the water in whicli they live, and after a life spent on the wing they deposit their eggs and perish. The adult ant-lion, which has somewhat the appearance of a small dragon-fly, lays its eggs in light sandy soil. In this the resulting larvae excavate funnel-shaped pits, at the bottom of which they lie concealed. Insects stumbling into their 9 120 ANIMAL FORMS pitfalls are pelted with sand, which the ant-lion throws at them with a jerky motion of the head, and are speedily tumbled down the shifting sides of the funnel to be seized and devoured. While the white ants are not in any way related to the true ants, they possess many similar habits. • Associated in great companies, they excavate winding galleries in old logs and stumps, and, further, are most interesting because of the division of labor among the various members. The wingless forms are divided into the workers, which exca- vate, care for the young, and otherwise labor for the good of the others; and into the soldiers, huge-headed forms, Fig. 72.— Ant-lion larva plowing its way through the sand (upper figure) while an- other is commencing the excavation of a funnel-shaped pit similar to one on right. Photograph by A. L. MELiNDER and 0. T. BRnES. whose strong jaws serve to protect the colony. The re- maining winged forms are the kings and queens. In the spring many of the royalty fly away from home, shed their wings, unite in pairs, and set about to organize a colony. The queen rapidly commences to develop eggs, and in some ARTHROPODS. CLASS INSECTS 121 species her body becomes so enormously distended with these that she loses the power of locomotion and requires to be fed. A single ■ queen has been known to lay eggs at the rate of sixty per minute (eighty thousand a day), and Fig. 73.— Termites or white ants, a, queen ; S, winged male ; c, worljer ; d, soldier. those destined to royal rank are so nursed that they advance farther in their development than the remaining sterile and wingless .forms. 118. The bugs (Hemiptera). — The large and varied group of the bugs {Hemiptera) includes a number of semi-aquatic species, such as the water-boatmen, often seen rowing themselves along in the ponds by means of a pair of oar- shaped legs, in search of other insects. Somewhat similar at first sight are the back-swimmers, with like rowing habits, but unique in swimming back downward. Both of these bugs frequently float at the surface, and when about to undertake a subaquatic journey they may be seen to imprison a bubble of air to take along. Closely related are the giant water-bugs (Pig. 74), which often fly from pond to pond at night. In such flights they are frequently 122 ANIMAL FORMS attracted by lights, and have come to be called " electric- light hugs." Among our most dreaded insect pests are the chinch- bugs — small black-and-white insects, but traveling in com- panies aggregating many millions. As they go they feed upon the stems and leaves of grain, which they devour with extraordinary ra- pidity. The squash-bug family is also extensive, and destructive to the young squash and pumpkin plants in the early spring. The lice are small, curiously shaped bugs, which suck the blood of other animals. The plant-lice, also small, suck the juices of plants, and are often exceedingly destructive. This is especially true of the phylloxera, a plant-louse which causes annually the loss of millions of dollars among the vine- yards of this and other countries. Even more destructive are tho scale-insects, curiously mod- ified forms, of which the wingless females may be found on almost any fruit-tree and on the plants in conservatories, their bodies covered with a downy, waxy, or other kind of covering, beneath which they remain and lay their eggs. 119. The flies (Diptera). — The group of the Diptera (meaning two-winged) includes the gnats, mosquitoes, fleas, house-flies, horse-flies (Fig. 75), and a vast company of related forms. Only a single pair of wings is present, the second pair being rudimentary or fashioned into short, thread-like appendages known as balancers, though they probably act as sensory organs and are not directly con- cerned with flight. The mouth-parts are adapted for pier- cing and sucking. The eyes, constructed on the same plan Fig. 74.— Giant water-bug (Se?-- phus dilaiatus), with eggs at- tached. ARTHROPODS. CLASS INSECTS 123 Fig. 75.— Horse-fly ( Therio- plectes). as those of the Crustacea, are comparatively large, and are frequently composed of a great number of simple eyes united together, upward of four thousand forming the eye of the common house-fly. These insects are widely distrib- uted throughout the world, where they inhabit woods, fields, or houses as best suits their needs. Their food is varied. Some suck the juices of plants, others attack ani- mals, and, while many are trouble- some pests, others, especially in the early stages of their existence, are of great benefit. 130. Familiar examples. — Owing to the widely different habits and structure of the members of this group, we shall briefly consider two examples, the mosquito and the house-fly, which will give us a fairly good idea of the characteristics of all. The eggs of the mosquito are laid in sooty-look- ing masses on the surface of stagnant pools. Within a very short time the young hatch, and, owing to their pecul- iar swimming movements, are known as " wrigglers." They are then active scavengers, devouring vast quantities of noxious substances and performing a valued service. They frequently rise to the surface, take air into the tracheal system, which opens at the posterior end of the body, and descend again. After an increase in growth and many in- ternal changes resulting in a chrysalis-like stage, they rise to the surface, split the shell, and, using the latter as a float, carefully balance themselves and soon fly away. The house-fly usually lays its eggs in decaying vegetable matter, and the young, maggot-like in form, are active scavengers. They too undergo deep-seated changes during the next few days, finally transforming into the adult. 124 ANIMAL FORMS Many of this great group of the flies spend their early life in the water or other medium acting as scavengers ; but, on the other hand, numbers attack domestic and other animals, and throughout their entire lives are an intolerable plague. 131. The beetles (Coleoptera). — Owing to the ease of pres- ervation and their bright colors, the beetles have probably been more widely collected than other insects. Fully ten Fig. re.— Long-homed borer (Ergates). Larva (left-hand figure), pupa, and adult insect. thousand distinct species are known in A'orth America alone. They are all readily recognized by the two firm, horny sheaths enclosing the two membranous wings, which alone are organs of flight. The mouth is provided with jaws, which are used in gnawing. Some prey on noxious insects or upon decaying vegetable or animal matter, and are often highly beneficial ; but others attack our trees and domestic animals, arid work incalculable damage. ARTHROPODS. CLASS INSECTS 125 In some of the stag- or wood-beetles (Fig. 76), which we may select as types, the adults are often found crawling about on or beneath the bark of trees, living on sap or small animals. The eggs laid in these situations develop into grub-like larva, which bore their way through living or dead wood, and in this condition sometimes live four or five years. They then transform into quiescent pupae (Fig. 76), which finally burst their shells and emerge in the adult form. Others, like water-beetles and the whirligig- beetles, whose mazy motions are often seen on the surface of quiet streams, pass the larval period in the water. Under somewhat different conditions we find the potato- bugs, lady-bugs, fire-flies, and their innumerable relatives, but the changes they undergo in becoming adult are essen- tially the same as those described for the other members of the order. 123. The moths and butterflies (Lepidoptera). — The moths and butterflies occur all over the world. In their mature Fig. 77. — Monarch-butterfly {Anosia plextjyptts). From photograph by A. L. Melan- DER and C. T. Bkues. state they are possessed of a grace of form and movement and a brilliancy of coloration that elicit our highest admi. ration. The mouth-parts are developed into a long pro- boscis, which may be unrolled and used to suck the nectar out of flowers, though in many of the adult moths, which never feed, it may remain unused. The wings, four in number, are covered with beautiful overlapping scales that 126 ANIMAL POEMS adhere to our fingers when handled. This feature, and the general plan of the body, which is much the same Fig. 7S.--Tlie silver-spot (Argynnis cybele). Photograph by A. L. Mklandeb and 0. T. Brues. throughout the group, enables us to recognize most of them at once. 133. Development and metamorphosis. — In some of the simplest insects, as in the bugs, the young at birth resemble their parents. In other insects the resemblance is not so close. The young grasshopper, for example, hatches, from an egg laid in the ground, with a ridiculously large head and staring eyes ; still there is no difficulty in recognizing its relationships. During the next week internal changes take place. The shell is burst, and the grasshopper emerges, looking more like its parents than before. This process is repeated four or five times during the next few weeks, and the gradual changes thus produced finally bring the young insect to the adult form. This latter state has been attained by an incomplete metamorphosis. ARTHROPODS. CLASS INSECTS 127 In the flies, beetles, butterflies, and numerous insects the differences between the newly hatched young and the adult are vastly greater. No one looking on a caterpillar or a grub for the first time would suspect its origin, and the changes they undergo have attracted attention for cen- turies. Placing any of the ordinary caterpillars with their favorite food in a glass-covered box, we may readily watch their transformations. Provided with biting mouth-parts and a voracious appetite, they devour vast quantities of vegetation for several days. Finally they cease eating, and Fig. 79.— Life-history of silk-moth {Bombyx mori). A, adult ; B, C, D, caterpillars of different ages ; E, F, G, silken cocoon and pupa ; H, eggs. suspend themselves head downward by means of a kind of cobweb. After remaining quiet a few hours, they burst their skin, and within appears a chrysalis or pupa. In the moths, for example, the silk-moth (Fig. 79), the caterpillar or silk-worm, after eating the favorite mulberry leaves, spins a silken cocoon, in which the pupa is produced. The larvae of beetles and many other insects excavate tunnels in wood or in the earth, and there undergo their transforma- tions. Invariably the pupa remains quiet for days, months, 128 ANIMAL FORMS or even years, but when the proper time arrives the fully formed insect emerges, and takes to the wing. Wonderful internal changes have been taking place during this time. The organs fitted for the proper treat- ment of the vegetable food of the caterpillar or grub are destroyed, at least in part, and new systems are produced ready for the nectar and vegetable juices which are to be the food of the adult insect. All insects that pass through a pupal quiescent stage are said to undergo a complete metamorphosis. 124. The ants, bees, wasps, etc. (Hymenoptera). — The ants, bees, and wasps are the best-known insects belonging to this order. They are characterized by four membranous wings, by biting and sucking mouth-parts, and the female is often provided with a sting. All undergo a complete metamorphosis. The eggs may be laid in the bodies of other insects, many of which are pests, and are thus de- stroyed ; or they may be deposited in the nests of other insects, the foster-parents being compelled to feed them ; or they may be placed in marvelously constructed homes, and be the objects of the greatest attention, the parents or attendants often risking or losing their lives in their defense. The members of this order have long attracted attention, largely on account of their remarkable instinc- tive powers. They live in highly organized communities and certain of their characteristics may be illustrated by a study of some of the more familiar forms. 125. The ants. — The ants live in communities consisting of anywhere from a dozen to many thousands of individuals, according to the species. Each of these colonies contains the queen, several young winged males and females, des- tined as kings and queens to found new colonies, and of a far greater number of wingless sterile females, the workers. The workers construct the greater part of the nest, which often consists of extensive galleries, nurseries, and grana- ries, excavated in wood or in the earth. They also attend ARTHROPODS. CLASS INSECTS 129 to the acquisition of food, which consists of the sweet juices of plants, of other insects, or of leaves and seeds. These may be fed at once, or placed in storehouses until times of need. Certain species of ants make carefully planned attacks upon other weaker forms. The young are carried off, at times only after a prolonged and fierce struggle, and all are soon eaten, or a few may be allowed to develop and act as slaves. Some species are unable to exist without serv- ants, which feed them, wash them, and otherwise minister to their comfort. In some of their raids numerous plant-lice (delicate, usually green, insects, such as occur on our household plants) are often captured and carried into the nest. These so-called, " ant-cows " are carefully tended, and in return yield up a tiny drop of a sugary fluid to the hungry ant that solicits it. The eggs laid by the queen develop into white worm- like creatures, which ftrdinarily spin cocoons when about to become pupse. These are incorrectly called " ant-eggs." Many, probably on account of insufficient nourishment, never develop reproductive organs. They become the neu- ters or workers. The winged royalty fly away from the colony, pair and found homes of their own, and become surrounded by a numerous progeny. 126. The bees. — Among the bees we find a considerable number which lead solitary lives, excavating tunnels in earth or wood, as in the case of many of the wasps, but, unlike them, supplying the young with honey or pollen. Others may constitute a band of worthless insects which steal their food from their more industrious relations, in whose nests they also secretly deposit their eggs, leaving the young to be nourished with food rightly belonging to others. But it is with the social bees we are most familiar — the bumble- and honey-bees. The former usually build in the ground, and form colonies consisting of the queen and from 130 ANIMAL FORMS twenty to two hundred workers. Eegular combs are not constructed, the young at first feeding on pollen masses or "bee-bread," and finally spinning cocoons. In the late summer males and females appear, but as winter comes on all perish except the queens, which seek a sheltered place, and in the spring revive to establish new colonies. In a wild state the honey-bees dwell in cavities of trees and other protected places, where they form colonies, consisting of the queen, of per- haps two hundred males or drones if the nest be examined in the spring and summer, and of a hundred times as many sterile females, the workers. These form among the most highly organized insect soci- eties known. All work for the Pig. 80.— Bumblebee (Bombus). t » , i i m i good of the colony, io each worker is assigned a definite task, which is never shirked. It must collect the honey, supply the wax for making the comb, take care of the brood, or in other ways minister to the welfare of the community. On the queen devolves the entire task of egg-laying. She may lay three thousand eggs a day and be fully occupied during the three or four years that she lives. The drones, or males, fertilize most of the eggs, and are then driven out from the hive, after a stay of a month or two. The eggs unfertilized by the drones are placed in large cells, and the young fed on pollen develop into males. The fertilized eggs may pro- duce queens or workers at the discretion of the queen. If the latter be desired, the eggs are placed in small cells with a scant amount of food, which apparently causes the repro- ductive system to remain undeveloped. The same eggs, if placed in the large queen cells and supplied with highly nutritious food, would have developed into queens. When these latter appear they are vigorously attacked and killed ARTHROPODS. CLASS INSECTS 131 by the parent if not protected by the workers. If the young queen survive, the old queen departs with many of her subjects, and collects them into a dense swarm attached to a limb of a tree, where they remain until scouts return to conduct them to their new home. 127. The wasps. — The digger-wasps are frequently to be seen gnawing tunnels in the wood or earth, at the inner end Fig. 81. — Nest of Vespa, a social wasp. Photograph hy A. L. Mblander and C. T. Brues. of which an egg is laid. In some species the developing young is nourished by food carried in to it day by day. In other cases the parent may never see her child, dying or abandoning it before its birth ; but before departing she is careful to place within reach a sufficient supply of spiders, caterpillars, beetles, or locusts that shall nourish the little one until it becomes a motionless pupa. This stage is soon over, and the adult wasp now digs its way to the surface. Passing by the familiar mud-wasps or mud-daubers, whose nests are common objects under stones or against 132 ANIMAL FORMS the rafters of bams and houses, we arrive at the social wasps. As the name indicates, these insects, such as the yellow-jackets and hornets, live together in companies which consist, as in the ants and bees, of males, femalesj and workers. They also are fond of the juices of fruits, and many of them destroy insects which may be fed to the young. Theiir nests are variously situated and constructed, but all of them agree in being composed, at least in part, of a grayish substance which is in reality a kind of paper. With their jaws they scrape off from old logs and fences small particles of wood, which they probably mix with saliva, and rolling the mass into a ball set out for home. These pellets are then flattened out into thin sheets, and worked up into hexagonal cells, in which the eggs are laid. Along with the nests of the mud-daubers one frequently notices the nests of some of the familiar wasps (PoUstes), which build cake-like nests composed of thirty or forty hexagonal cells attached by a stalk. Somewhat similar nests, though usually more extensive, are constructed by the yellow-jackets in cavities in the ground. The numer- ous combs of the hornet are surrounded by several sheets of wood-pulp, and the whole structure is attached generally to the limb of a tree. In the spring the nests of all these species of wasps are commenced by a single female, who has lived in a dormant condition through the winter. She builds a small nest and in time is surrounded by numerous workers, which live in perfect harmony, enlarging the nest and rearing the young. As autumn approaches the young males and females leave the nest ; but the males, together with the workers, all suc- cumb to the cold, and none but the females persist to found a new colony the following spring. CHAPTEE XI ARTHROPODS (Continued). CLASS ARACHNIDA 128. General characters. — In this group, comprising the spiders, mites, and a large assemblage of related species, we again meet with great differences in form and structure which fit them for lives under widely different conditions. The three regions of the body, head, thorax, and abdomen, so clearly marked in the insects, are here less plainly de- fined. The head and thorax are usually closely united, and in the mites the boundaries of the abdomen are also indis- tinct. The appendages of the head are two in number, and probably correspond to the antennae and mandibles of other Arthropods. In the scorpions and some species of mites these are furnished with pincers for holding the prey, and in other forms they act as piercing organs. Usually the thorax bears four pairs of legs, a characteristic which readily separates such animals from the insects. The internal organization differs almost as much as does the external. In many species it shows a considerable re- semblance to that of some insects, but in others, especially those of parasitic habits, it departs widely from such a type. Respiration is affected by means of trachese, or lung-books, which consist of sacs containing many blood-filled, leaf -like plates placed together like the leaves of a book. Usually, as in the insects, the young hatch from eggs which are laid, but in the scorpions and some of the mites the young develop within the body and at birth resemble the parent. Almost all of these organisms live either as 133 134 ANIMAL FORMS parasites or as active predaceous animals upon othier animals. For this purpose many are provided with keen senses for detecting their prey and poisonous spines for despatching it. 139. The scorpions.— Owing to the stout investing armor, the strong pincers, and the general form of the body, the scorpions might at iirst sight be mistaken for near relatives of the crayfish or lobster. A more careful examina- tion will show that the two pairs of pincers prob- ably correspond to the antennae and mandibles of the Crustacea that have become modified for seiz- ing the food. The swol- len part of the animal lying behind the four pairs of legs is a part of the abdomen, of which the slender " tail " consti- tutes the remainder. On the tip of the tail is a curved spine supplied with poison glands. Sev- eral pairs of eyes are borne on the dorsal surface of the head and thorax, while on the under side of the animal several slit-like openings lead into as many small cavities containing the lung-books. The scorpions are the inhabitants of warm countries, where they may be found under sticks .and stones through- out the day. At night they leave their homes in search of food, which consists chiefly of insects and spiders. These are seized by means of the pincers, and the sting is driven into them with speedily fatal results. It is doubtful if the poison causes death in man, but the sting of some of the Pig. 6 1.— Scorpion, showing pincer-liive moutli- parts and spine-tipped tail. ARTHROPODS. CLASS ARACHNIDA 135 larger species, which measure five or six inches in length, may produce certain disorders chiefly affecting the circula- tion. In this country there are upward of thirty species, most of which are comparatively small. 130. The harvestmen. — The harvestmen or daddy-long- legs are small-bodied, long-legged creatures which resemble in general appearance several of the spiders. They differ from them, however, in the possession of claws correspond- ing to the smaller ones of the scorpion, and in their method of respiration, which is similar to that of insects. During the day they conceal themselves in dark crevices or stride slowly about in shaded places ; but at night they emerge into more open districts and capture small insects, from which they suck the juices. 131. The spiders. — The spiders are world-wide in their distribution, and are a highly interesting group, owing chiefly to their peculiar habits. Examining any of our familiar species, it will be seen that the united head and thorax are separated by a narrow stalk from the usually distended abdomen. To the under side of the former are attached four pairs of long legs, a pair of feelers, and the powerful jaws supplied with poison-sacs, while eight shin- ing eyes are borne on the top of the head. On the abdo- men, behind the last pair of legs, are small openings into the lung cavities which contain a number of vascular, leaf- like projections known as lung-books. In some species a well-marked system of tracheae are also present. At the hinder end of the body are four or six little projections, the spinnerets, each of which is perforated with many holes. Through these the secretion from the glands be- neath is squeezed out in the form of excessively delicate threads, often several hundred in number, which harden on exposure to the air. According to the use for which these are intended, they may remain a tangled mass or become united into one firm thread ; and according to the habits of the animal, they may be used for enclosing their eggs, 10 136 ANIMAL FORMS for lining their burrows, or for the construction of webs of the most diverse patterns. 132. The habits of spiders.— Many species of spiders, some of which are familiar objects in fields and houses, construct sheets of cobweb with a tube at one side in which they may FiQ. S3.— A tarantala-spider (Eurypelma lentzii). Natural size. Photograph by A. L. Melahdek and C. T. Brhbs. lie in wait for their prey or through which they may escape in times of danger. In the webs of the common orb- or wheel-weavers several radial lines are first constructed, and upon these the female spider spins a spiral web. Besting in the center of this or at the margin, with her foot on some of the radial threads, she is able to detect the slight- est tremor and at once to rush upon the entangled captive. Some of the bird-spiders and their allies, living in trop- ical America, and attaining a length of two inches, con- struct web-lined burrows in the ground. Prom these they stalk their prey, which consists of various insects and even ARTHROPODS. CLASS ARACHNIDA 137 small birds. These are almost instantly killed by the poison- fangs, and are then carried to the burrow, where the juices of the body are extracted. The trap-door spiders of the southwestern section of the United States also dig tunnels, which they cover with a closely fitting lid com- posed of earth. Eaising this they come out in search of insects, but if sought in turn, they dash into the burrow, closing the door after them, and holding it with such firm- ness that it is rarely forced open. If this should hap- pen, there are sometimes blind passage-ways, also closed with trap-doors, which usually baffle the pursuer. -Finally, there are among the thousand spe- cies of spiders in the United States a considerable propor- tion which construct no definite web. Many of these may be seen darting about in the sunshine on old logs and fences, often trailing after them a thread which may sup- port them if they fall in their active leaping after in- sects. 133. Breeding habits. — The male spiders are usually much smaller than the females, and some species are only one- fifteenth as long as the female and one one-hundredth of its weight. They are usually more brilliantly colored, more active in their movements, yet rarely spinning their own webs and capturing their own food, preferring to live at the expense of the female. At the breeding season the males of several species make a most interesting display Fig, 84.— Trap-door spider and burrow ( Cteniza). 138 ANIMAL FORMS of their colors, activity, and gracefulness before the females ; and the latter, after watching these exhibitions, are said ..to select the one who has " shown ofE " in the most pleasing fashion. The life after this may be stormy, resulting in the death of the male ; but ordinarily the results are not so disastrous, and in a little while the female deposits her eggs in cases which she spins. In these the young develop, sometimes wintering here, and emerging in the spring to scamper about in search of food, or to drift through the air to more favorable spots on fluffy masses of cobweb. Few groups of animals are more interesting objects of study and more accessible. Their bites are rarely more serious than those of the mosquito — never fatal ; and a careful study of any species, however common, will undoubtedly bring to light many interesting and unknown facts. 134. The mites and ticks. — The mites and ticks are the simplest and among the smallest of the animals belonging to this group. To the at- tentive observer they are rather com- mon objects, with homes in very dif- ferent situations. Some occur on liv- FiG.8B.-The itch-mite (,sa?-- j^g ^nd decaying vegetation, in old copies scaoei). n i i -i ji flour and unrefined sugar, while oth- ers live in fresh water and a few in the sea. Almost all tend toward parasitism. Some of the insects which they pierce and destroy are a pest to man, but on the other hand some are intolerable owing to the diseases they produce. As to other parasitic organisms, degradation of structure is manifest. The respiratory system, so important to the active life of the insects, may be absent, the animal breath- ing through its skin. The circulatory system may be want- ing, the blood occupying spaces among the various organs being swept about by the animal's movements. And many ARTHROPODS. CLASS ARACHNIDA 139 other peculiarities have arisen which fit them for their difEerent modes of life. 135. The king crab (Linmlus). — The king crab may be found crawling over the bottom or plowing its way through the sand and mud in many of the quiet bays from Maine to Florida. The large head and thorax of these animals are united into a horse- shoe-shaped piece, be- hind which lies the triangular abdomen. On the curved front surface of the former are a pair of small me- dian eyes, and farther outward are two larger compound ones. On the ventral side are six pairs of append- ages, instrumental in capturing and tearing the small animals that serve as food, and functioning in con- nection with the ter- minal spine as locomo- tor organs. On the ventral surface of the abdomen are numerous plate-like flaps which serve in respiration, and in the imperfect swimming movements in which these animals occasionally indulge. These relatively large and clumsy creatures are the rem- nant of a great number of strange, uncouth animals that in- habited the earth in past ages. Many of them show a close resemblance to the scorpions. The anatomy and develop- ment also show certain points of resemblance, and by some are thought to give us an idea of the ancient type of spider- like animal from which the modern forms have descended. Fig. 86.- -The king or horseshoe crab {Limutus polyphemvs). CHAPTEE XII ECHINODBRMS 136. General characters. — The division of the echino- derms includes the starfishes, sea-urchins, serpent- or brittle- stars, sea-cucumbers, and crinoids or searlilies. All are ma- rine forms, and constitute a conspicuous portion of the animals along almost any coast the world over. From these shallow-water situations they extend to the greatest depths of the ocean, and the bodily form possesses a great number of variations, adapting them to lives under such diverse conditions; and yet there is perhaps no group of organisms so clearly defined or exhibiting so close a resem- blance throughout. At one time it was thought that their radial symmetry was an indication of a close relationship to the coelenterates, but more careful study has shown them to be much more highly developed than this latter group, and widely separated from it. A skeleton is almost always present, consisting of a number of calcareous plates embed- ded in the body-wall, and often supporting numbers of pro- tective spines, which fact has given to the group the name Echinoderm, meaning hedgehog skin. 137. External features.— The body of a starfish (Pig. 87) consists of a more or less clearly defined disk, from which the arms, usually five in number, radiate like the spokes of a wheel. At the center of the under side the mouth is located, and from it a deep groove, filled with a mass of tubular feet, extends to the tip of each arm. Innumerable calcareous plates firmly embedded in the body-wall serve 140 BCHINODBRMS 141 for the protection of the internal organs, and at the same time admit of considerable movement. In the brittle-stars (I'ig. 88) the central disk is much more sharply defined than in the preceding forms, and the long snake-like arms are capable of a very great freedom of movement, enabling the animal to glide over the sea-bottom, or through the crevices of the rocks, at a surprising rate. In several species, otherwise closely resembling those Fig. 87.— Starflsh Usterias ocracea), ventral view. One-halt natural size. in Fig. 88, the arms divide repeatedly. These are the so- called basket-stars, living in the deeper waters of the sea, where they, like other brittle-stars, act as scavengers and devour large quantities of decomposing plant or animal remains. At first sight the globular spiny sea-urchins (Fig. 90) would scarcely be recognized as close relatives of the star- fishes. A closer examination, however, shows the mouth to be located on the under-side of the body ; from it five rows of feet radiate and terminate close to the center of the dorsal side, and the arrangement of the plates forming the 142 ANIMAL FORMS skeleton indicate that the sea-urchin is comparable to a starfish, with its dorsal surface reduced to insignificant proportions. In the sea-urchins the calcareous plates possess a great regularity, and are so closely interlocked that they prevent Fig. 88. — Brittle- or serpent-stars (species undetermined). IS'atural size. any motion of the body-wall. Also, each plate is usually provided with highly developed spines, movable upon a ball- and-socket Joint. These spines serve for locomotion, and, in some instances, for conveying food to the mouth. A considerable number of sea-urchins show an irregularity in form which destroys to a corresponding degree the radial symmetry. This is due to various causes, but especially to a compression of the body, which, in the "sand-dollars," ECHINODERMS 143 has resulted in the production of a thin, cake-like form (Fig. 91). If the spherical body of a sea-urchin were to be stretched in the direction of a line joining the mouth and the center Fib. 89.— Basket-star {Astrophytmi). One-half natural size. of the dorsal surface, a form resembling a sea-cucumber (Pig. 92) would be the result. These latter organisms live among crevices of the rocks, embedded in the mud or bur- rowing in the sand at the bottom of the sea. In such situa- tions they are well protected, and a highly developed skele- ton, such as that of the sea-urchin, would not only be of little value, but a positive hindrance to locomotion. The skeleton, therefore, is much reduced, consisting of a few scattered calcareous plates embedded in the fleshy body- wall. Another peculiar feature is almost universally pres- ent, in the form of a circlet of tentacles surrounding the mouth, which serve either for the purpose of respiration, for locomotion, or to convey food to the mouth. A very good imitation of the general plan of a sea-lily or crinoid (Fig. 93) could be made by attaching a serpent- 144 ANIMAL FORMS star, especially one of the basket-stars, by its dorsal side to a stalls. In the crinoids the numerous branches of the arms are compara- tively short, and in the arrangement of the internal organs there are numer- ous differences, but for all that the re- semblance of these organisms to the other echinoderms is undoubted. 138. Haunts.— The greater num- ber of starfishes occur alongshore, slowly crawling about in search of food, or concealed in dark creyices of the rocks, where they may often be found as the tide goes out, and we know that in gradually lessening numbers other species lead similar lives at different levels far down in the dark and gloomy depths. In these same locations the sea- urchins occur, sometimes singly, but more usually associa- ted in great numbers, several species excavating hollows in the rocks, within which they obtain protection. The brit- tle-stars and sea-cucumbers may also be found occasionally in open view, but more often they make their way about in search of food buried in the sand. The crinoids are usual- ly inhabitants of deeper water, where they are found asso- ciated often in great numbers. A few species upon attain- ing the adult condition separate from the stalk, and are able to move about (Fig. 95), but the remaining species never shift their position. Fig. 90.- -Sea-urchin {Strongykicentrotus piirpuratus). Hatural size. ECHINODERMS 145 139. The organs of defense and repair of injury. — As we hare seen, the body-wall of the echinoderms is provided with a series of plates, often bearing spines which serve as organs of defense, and to protect the internal organs. The starfishes and sea-urchins also possess numerous modified spines {pedicellaria) scattered over the surface of the body, which have the form of miniature birds' beaks, fastened to slender muscular threads. During life these jaws continu- ally open and close, and it is said they clean the body of debris that settles on it ; but on the other hand there are several reasons for the belief that they also act as organs of defense. Thus protected, the natural enemies of echino- derms appear to be relatively few, and are confined chiefly to some of the fishes whose teeth are especially modified for crushing them. In this way, and owing to the action of the breakers, they suffer frequent injury, but many species exhibit to a remark- able degree the ability to re- generate lost parts. Experi- ments show that if all the arms of a starfish be separa- ted from the disk the latter will within two or three months renew the arms ; and ^'°- si-sand-doiiar, a flat sea-urchin. Natural size. a single arm with a part oi the disk is able to renew the missing portions in about the same length of time. ^ The brittle-stars, as their name indicates, are usually ex- cessively delicate, often dropping all of their arms upon the slightest provocation ; but here again the ability is present to develop the lost portions. Sea-cucumbers resent rough treatment by vigorously contracting their muscular walls and removing from the body almost the entire digestive tract, the respiratory tree. 146 ANIMAL FORMS and a portion of the locomotor system ; but some species, at least, renew them again. In some of the starfishes and brittle-stars portions of the body appear to be voluntarily de- tached and to develop into new individuals, and it is thought that such self-mutilation is a normal method of reproduction. 140. Locomotor system. — One of the most characteristic and remarkable features of the echi- noderms is the water-vascular system, a series of vessels con- taining water which serve in the process of locomotion. Their arrangement and mode of operar tion are, with slight modifica- tions, the same throughout the group, and may be readily un- derstood from their study in the starfish. On the dorsal surface of a starfish, in the angle between two of the arms, is a round, slightly elevated, calcareous plate, the madreporic iody (Fig. 95, m.p.), which under the mici'oscope appears full of holes, like the " rose " of a watering-pot. This connects with a tube that passes to the opposite side of the body, where it enters a canal completely encircling the mouth. On this ring-canal a number of sac-like reservoirs with muscular walls are at- tached, and from it a vessel extends along the under sur- face of each arm from base to tip. Each of these radial water-mains gives off numerous lateral branches that open out into small reservoirs similar to those located on the ring-canal, and a short distance beyond communicate through the wall of the body with one of the numerous Fig, 92.— Sea-cucumber {Cucu- maria sp.). Natural size. ECHINODBRMS 147 tube-feet, which, as we haye seen, are slender tubular or- gans, many in number, filling the grooves on the ventral surface of each arm. This entire system of tubes and reservoirs is full of water, taken in, it is said, through the perforated plate, and, when the starfish wishes to advance, many of the little reservoirs con- tract, forcing water into the cav- ity of the feet, with which they are in communication, thus ex- tending the extremity of the tubes a considerable distance. The terminal sucker of each foot, act- ing upon the same principle as those on the cuttlefish, attaches firmly to some foreign object, whereupon the muscles of the foot contract, drawing the body toward the point of attachment. This latter movement is similar to that of a boatman pulling him- self to land by means of a rope fastened to the shore. When the shortening of the tube-feet has ceased, the sucking disks release their attachment, project them- selves again, and this process is repeated over and over. At all times some of the feet are con^ tracting, and a steady advance of the body is the result. This method of locomotion also obtains in the sea-urchins and cucumbers, but in the serpent-stars the tube-feet have become modified into feel- ers, and the animal moves, often rapidly, by means of twist- ing movements of the arms. The feet have this character also in the crinoids, where the animal is generally without Pig. 93.— Sea-lily or crinoid. 148 ANIMAL FORMS Fig. 94. — An nnattaclied crinoid (Anifidon). half natural size. One- the power of locomotion. In some of the sea-cucumbers five equidistant rows of tube-feet extend from one end of the body to the other, and the animal crawls worm-like upon any side that happens to be down ; but certain spe- cies living in the sand, where tube - feet will not work satisfactorily, have lost all traces of them, and creep like an earthworm from place to place. In all the searcucumbers the feet, situated near the mouth, have been curiously modified to form a cir- clet of tentacles, which range in form from highly branched to short and thick structures, and in func- tion from respiratory organs and those of touch to con- trivances for scooping up sand and conveying it to the mouth. 141. Food and digestive system. — In the echinoderms the body-wall is comparatively thin (Fig. 95), and encloses a great space, the body-cavity, in which the digestive and re- productive organs are contained. As the former in various species is adapted for acting upon very different kinds of food, it shows many modifications ; but there are a few prin- cipal types which may be briefly considered. In the starfishes the mouth enters almost directly into the cardiac division of the stomach, a capacious, thin- walled sac, much folded and packed away in the disk and bases of the arms (Fig. 95, b). This in turn leads into the second pyloric portion (a), with thicker walls and dorsal, to the first, from which a short intestine leads to the exterior, near the center of the disk. Another conspicuous and im- portant feature is the so-called liver, consisting of a pair ECHINODERMS 149 of closely branched, flufEy glands (l), extending the entire length of each arm and opening into the pyloric stomach. The starfishes are carnivorous and highly voracious, de- vouring large numbers of barnacles and mollusks which hap- pen in their path. If these are small and free they are taken directly into the stomach, but when one of relatively large size is encountered the starfish settles down upon it, and, slowly pushing the cardiac stomach through the mouth, envelops it in the folds. Digestive fluids are now poured over it, and the victim is speedily despatched and in a partly digested condition is gradually absorbed into the body, leav- FiG. 95.— Dissection of stariisti to bIiow : li, pyloric stomach ; b, bile-dncts (above), cardiac stomacli (below) ; b.c, body-cavity ; /, feet ; g, spines ; i, intestine ; /, liver ; m, mouth ; m.p., madreporic plate ; /•, reservoir ; r.c, ring canal ; r.m., stomach retractor muscle ; r.-y., radial vessel ; s, stone canal ; t, respira- tory tree. ing the shell and other indigestible matters upon the exte- rior. Oysters and clams close their shells when thus attacked, but a steady, continuous pull on the part of the starfish finally opens them, and the stomach is spread over the fleshy portions with speedily fatal results. In the interior of the body the food is transferred to the pyloric stomach, sub- jected to the action of the liver, and when completely dis- solved is borne to all parts of the body; 150 ANIMAL FORMS The digestive system of the starfishes, with its various subdivisions and appendages, is in some respects more com- plicated than in the other classes. This is most strikingly the case with the serpent-stars, where the entire system for disposing of the minute animals and plants on which it feeds consists of a simple sac communicating with the exterior by a single opening — the mouth. In the sea-cucumbers large quantities of sand are taken into the body, and the minute organisms and organic mat- ter are digested from it. In the sea-urchins the mouth is provided with five teeth, and the food consists of minute bits of seaweeds, which these snip off. Such diets evidently require a comparatively simple digestive apparatus, for in both it consists throughout its whole extent of a tube of equal caliber, in which the various divisions of esophagus, stomach, and intestine are little, if at all, defined. This is usually somewhat longer than the body, and therefore thrown into several loops ; and in the sea-cucumbers its last division is expanded and furnished with more highly mus- cular walls, which aid in respiration. 142. Development. — With but a few exceptions, the eggs of the echinoderms are laid directly in the surrounding water, and for many days the exceedingly minute young are borne great distances in the tidal currents. During this period they show no resemblance to their parents, and only after undergoing remarkable transformations do they assume their permanent features. In every case they have a five-rayed form in early youth, but in several species of starfishes additional arms develop until there may be as many as twenty or thirty. CHAPTER XIII THE CHOEDATES 143. General characters. — Up to the present time we have been studying the representatives of a vast assemblage of animals whose skeletons, if they have any at all, are located on the outside of the body. In the corals, the mighty com- pany of arthropods, and the echinoderms, it is external. On the other hand, we shall find that the animals we are now about to consider, the fishes, frogs, lizards, birds, and mam- mals, are in possession of an internal skeleton. In some of the simpler fishes and in a number of more lowly forms (Fig. 96) it is exceedingly simple, and consists merely of a gristle- like rod, the notochord (Fig. 98, nc), extending the length of the body and serving to support the nervous system, which is always dorsal. This is also the type of skeleton found in the young of the remaining higher animals, but as they grow older the notochord gives way to a more highly developed cartilaginous or bony, jointed skeleton, the vertebral column. In the young of all these back-boned or chordate ani- mals, the sides of the throat are invariably perforated to form a number of gill-slits. In the lower forms these per- sist and serve as respiratory organs, but in the higher ani- mals they disappear in the adult. The chordates are thus seen to be distinguished by the possession of a dorsal nerv- ous cord supported by an internal skeleton and by the presence of gill-slits, characters which separate them widely from all invertebrates. The chordates may be divided into ten classes, seven of 11 151 152 ANIMAL FORMS which (the lancelets, lampreys, fishes, amphibians, reptiles, birds, and mammals) are true vertebrates, while the others embrace several peculiar animals of much simpler organiza- tion. 144. The ascidians. — Among the latter are a number of remarkable species belonging to the class of ascidians or sea-squirts (Fig. 96). These are abundantly represented along our coasts, and are readily distinguished by their sac -like bodies, which are often attached at one end to shells or rocks. On the opposite extremity two openings exist, through which a constant stream of water passes, bearing minute organisms serving as food. When disturbed they frequently expel the water from these pores with considerable force, whence the name " sea-squirt." While many lead solitary lives, numerous individuals of other species are often closely packed together in a jelly-like pad attached to the rocks, and others not distantly related are fitted to float on the surface of the sea. The young when hatched resemble small tadpoles both in their shape and in the arrangement of some of the more important systems of organs. For a few hours each swims about, then selecting a suitable spot settles down and ad- heres for life. From this point on degeneration ensues. Fig. 96. — Ascidian or sea-squirt. THE CHOEDATES 153 The tail disappears, and with it the notochord and the greater part of the nervous system. The sense-organs van- ish, the pharynx becomes remodeled, and numerous other changes occur, leaving the animal in its adult condition, with little in its motionless, sac-like body to remind one of a vertebrate. 145. The vertebrates. — Since the remainder of this vol- ume is concerned with the vertebrates it will be well at the outset to gain some knowledge of their more important characteristics. One of the most apparent is the presence of a jointed vertebral column, composed of cartilage or bone, which supports the nervous system. To it are also usually attached several pairs of ribs, two pairs of limbs, either fins, legs, or wings, and in front it terminates in a more or less highly developed skull. In the space par- tially enclosed by the ribs, the body-cavity, a digestive • sys- tem is located, which consists of the stomach and intestine, together with the attached liver and pancreas. The cir- culatory system is also highly organized, and consists of a muscular heart, arteries, and veins which ramify through- out the body. Breathing, in the aquatic animals, is car- ried on by means of gills, and in the air-breathing forms by means of lungs, which, like the gills, effect the removal of carbonic-acid gas and the absorption of oxygen. The nervous system, consisting of the brain situated in the head and the spinal cord extending through the body above the back-bone, even in the lower vertebrates, is far more complex than in the invertebrates. The sense-organs also attain to a high degree of acuteness, and in connec- tion with the highly organized nervous system enable these forms to lead far more varied and complex lives than in any of the animals heretofore considered. CHAPTEE XIV THE FISHES 146. General characters. — In a general way the name fish is applied to all vertebrates which spend the whole of their life in the water, which undergo no retrograde metamor- phosis, and which do not develop fingers or toes. Of other aquatic chordates or vertebrates the ascidians undergo a retrograde metamorphosis, losing the vertebral column, and with it all semblance of fish-like form. The amphibians, on the other hand, develop jointed limbs with fingers and toes, instead of paired fins with fin rays. A further com- parison of the animals called fishes reveals very great dif- ferences among them — differences of such extent that they cannot be placed in a single class. At least three great groups or classes must be recognized: the Lancelets, the Lampreys, and the True Pishes. The general characters of all these groups will be better understood after the study of some typical fish, that is one possessing as many fish-like features as possible, unmodified by peculiar habits. Such an example is found in the bass, trout, or perch. In either fish the pointed head is united, without any external sign of a neck, to the smooth, spindle-shaped body, which is thus fitted for easy and rapid cleaving of the water when propelled by the waving of the powerful tail (Fig. 97). A keel also has been provided, enabling the fish to steer true to its course. This consists of folds of skin arising along the middle line of the body, supported by numerous bony spines or cartilaginous 154 THE PISHES 155 rays. These are the unpaired fins, as distinguished from the paired ones, which correspond to the limbs of the higher vertebrates. In the bass or perch the latter are of much service in swimming, and are also most important organs in directing the course of the fish upward or downward, or for Fia. 97.— Yellow perch (Percajlavescene). df, dorsal fins ; pc, pectoral fln ; pf, pelvic fin ; V, ventral fin, aiding the tail in changing the course from side to side ; or they may be used to support the animal as it rests upon the bottom in wait for food ; and, finally, they may serve to keep the body suspended at a definite point. In addition to an internal skeleton the bass or perch, like the greater number of fishes, is more or less enclosed and protected by an external one, consisting of a beautifully arranged series of overlapping scales, which afford protec- tion to the underlying organs, and at the same time admit of great freedom of movement. These usually consist of a horny substance, to which lime is sometimes added, and are peculiar modifications of the skin, something like the feathers, nails, and hoofs of higher forms. 147. The air-bladder. — JN'aturally a fish's body is heavier than the water in which it lives, and there are reasons for thinking that the air-bladder (Fig. 106, a.bl) acts in the 156 ANIMAL FORMS bass and perch and many other fishes as a float to enable them, without much efEort, to remain suspended at a defi- nite level. By compressing this sac, partly by its own mus- cles and partly by those of the body-wall, the bulk of the fish is made less, and it sinks ; upon the relaxation of these same muscles the body expands and rises again. Deep-sea fishes, when brought to the surface, where the pressure is relatively slight, are found with their air-bladders so dis- tended that they can not sink again, and the float of surface fishes would be as useless if they were to be carried into the depths below, so that such fishes are compelled to keep within tolerably definite limits of depth. Morphologically considered, the air-bladder is a modified or degenerate lung, and in many fishes it is lost altogether. 148. Respiration. — Looking down the throat of the perch or any other fish, a series of slits (the gill-openings), usually four or five in number, may be seen on each side communi- cating with the exterior. In the sharks these outer open- ings are readily seen, but in the bony fishes they open into a chamber on each side of the head, covered by a bony plate or gill-cover that is open behind. On raising these flaps the gills may be seen composed of great numbers of bright- red filaments attached to the bars between each slit. Dur- ing life the fish may be seen to open its mouth at regular intervals, and, after gulping in a quantity of water, to close it again, contracting the sides of the throat to force it out of the gill-openings and over the gill-filaments to the exte- rior. During this process the blood traversing the excess- ively thin filaments extracts the oxygen from the water and carries it to other parts of the body. With this information, let us return to the study of the three classes of fishes. 149. The lancelet (Branchiostoma).— The lancelet, some- times called amphioxus (Fig. 98), the type of the class Lepto- cardi.i, is a little creature, half an inch to four inches long, in the different species, transparent and colorless, living in the THE PISHES 157 sand in warm seas, the nine species known being found in as many different regions. A lancelet may be regarded as a vertebrate reduced to its lowest terms. Instead of a jointed back-bone, it has a cartilaginous notochord, running from the head to the tail. A nervous cord lies above it, enclosed in a membranous sheath. No skull is present, and the nerve-cord does not swell into a brain. There are no eyes and no scales. The mouth is a vertical slit, without jaws. There is no trace of the shoulder-girdle (shoulder- blade and collar-bone) or pelvis (hip-bone) from which Fig. 98.— The California lancelet {Branchiostoma californiense). Twice the natural size, fir, gills ; I, liver ; m, mouth ; n, nerve-cord ; nc, notochord. spring the paired fins, which, in true fishes, correspond to arms and legs. The circulatory system is fish-like, but there is no heart, the blood being driven about by the contraction of the walls of the vessels. Along the edge of the back and tail is a rudimentary fin, made of fin-rays connected by mem- brane. In the character and arrangement of its organs the lancelet is certainly like a fish, but in degree of develop- ment it differs more from the lowest fish than the fish does from a mammal. 150. Lampreys (or Cyclostomes). — The class of lampreys stands next in development (Fig. 99). The notochord gives way anteriorly to a cartilaginous skull, in which is con- tained the brain, of the ordinary fish type. There are eyes, and the heart is developed, and consists of an auricle and a ventricle. As distinguished from the true fish, the lam- preys show no trace whatever of limbs or of the bones which would support them. The lower jaw is wholly want- ing, the mouth being a roundish sucking disk. The fins 158 ANIMAL FORMS are better developed, but of the same structure as in the lancelet. There is no bony matter in the skeleton, and there are no scales. The nasal opening is single on the top of the front of the head. There are about twenty-five species in this class. Some of them, called lampreys, ascend the streams from the sea Fig. 99. — Lampreys. in the spring for the purpose of spawning. The young undergo a metamorphosis, at first being blind and tooth- less. The adults feed mostly on the blood of fishes, which they suck after scraping a hole, in the flesh with their rasp- like teeth. The others, called hag-fishes, live in the sea and bore into the bodies of other fishes, whose muscles they devour. All are slender, smooth, and eel-shaped. From their structure and a few fossil remains we sup- pose that these eel-like forms existed long ago, probably be- fore the more highly developed sharks and bony fishes made their appearance, but it is difficult to determine whether their simple organization is of such long standing or is not in part the result of semiparasitic habits, or a life spent THE PISHES 159 largely in burrowing. Like the lancelet and other simple chordates, they are of the greatest interest to the zoologist who gains from them some idea of the lowly vertebrate forms that peopled the earth long ago. ISl. True fishes.— The third class, Pisces or true fishes, to which the shark as well as the bass and perch belong has a well-deTcloped skeleton, skull, and brain. The lower jaw is developed, forming a distinct mouth, and there is at least a shoulder-girdle and pelvis ; although the fins these should bear are not always developed, the general traits are those we associate with the fish. Of the true fishes, there are again several strongly marked groups, usually called sub- classes. Of these, three chiefly interest us. 153. The sharks aiid skates. — Very early in the life of the sharks (Pig. 100) and skates {Selachii or Elasmobranchii) Pis. 100.— Dogfish {Squalus acanthias). One-seventh natural size. a notochord appears, similar to that in the lancelet and the lampreys. As growth proceeds its sheath becomes broken up into a series of cartilaginous rings, which thus appear like spools strung on*a cord. As the fish grows older these " spools " or vertebrae grow solid, cutting the notochord into little disks, and great flexibility is thus secured. Cartilagi- nous appendages also grow up and cover the spinal nerve- cord lying above, and give strength to the unpaired fins ; the paired fins also have their supports. The shoulder- 160 ANIMAL FORMS girdle is placed behind the skull, leaving room for a distinct neck ; strong bars of cartilage bear the gills ; others form jaws to carry the teeth ; and a complex skull protects the brain and sense-organs, which are of a relatively high state of devel- opment. Throughout life the skeleton is of cartilage, with perhaps here and there a little bone where greater strength is required. Besides these, there are numerous minor characters which the student will readily find for himself. The sharks and skates or rays live chiefly in the sea, and some reach an enormous size, the largest of all fishes. Some are very ferocious and voracious ; others are very mild and weak, and the development of teeth is in direct pro- portion to their voracity of habit. In earlier geologic times there were many more species of them than now exist. 153. The lung-fishes.— The lung-fishes (Dipnoi) are pe- culiar forms living in some of the rivers of Australia and the tropical regions of Africa and South America. In these the air-bladder is developed as a perfect lung. During the wet season they breathe like other fishes by means of gills, but as the rivers dry up they burrow into the wet mud and breathe by means of lungs which are spongy sacs of which the air-bladder of other fishes is a degenerate representative. As we shall see, they resemble in this respect the tadpoles and some adult Amphibia (frogs and salamanders). The paired fins are also peculiar in structure, having an elongate jointed axis, with a fringe of rays along its length, a struc- ture almost as much like that of the limbs of a frog as that of a fish's fin. In fact the Dipnoi must be regarded as an ancestral type, an ally of the generalized form from which Amphibia and bony fishes have descended. Only four liv- ing species of dipnoans are known, but great numbers of fossil species are found in the rocks. 154. The bony fishes (Teleostei).— The bony fishes, or Teleosts, are distinguished by the bony skeleton, the sym- metrical tail, and by the development of the air-bladder as a more or less completely closed sac, useless in respiration. THE PISHES 161 Often this organ is altogether wanting, as in the common mackerel. About ten thousand kinds of bony fishes are known. The species swarm in every sea, lake, or river throughout the earth, and some form or another among them is familiar to every boy in the land. These fishes are divided into about two hundred families, and these may be arranged in fifteen to twenty orders. As these are mostly distinguished by features of the skeleton, we need not name them here. In Jordan and Evermann's Fishes of North and Middle America, as well as in various other books, the stu- dent of fishes can find the characters by which orders may be distinguished. 155. Sturgeons and garpikes (Ganoidea). — While the great majority of the typical fishes possess a bony skeleton, there are a few quaint types — the ganoid fishes, such as the stur- geons (Fig. 101) and garpikes — in which it is cartilaginous or partly bony. In past ages these were probably the highest type of fishes, and from their fossil remains we may con- clude that they flourished in vast numbers ; but at present they are almost extinct. In this country the ganoids are represented by several species, the best known being the sturgeons which inhabit the Great Lakes, the Mississippi, and its tributaries ; while on the East coast the common sturgeon (Acipenser sturio) often leaves the sea and ascends rivers. They are the largest fishes found in fresh water, attaining a length of ten or twelve feet, and a weight of five hundred pounds. Their food consists of small plants and animals, which they suck in through their tube-like mouth. The garpikes live in the larger lakes and rivers throughout the East and Mississippi Valley. Their bodies, from three to ten feet in length, according to the species, are covered with comparatively large regularly arranged square scales, and the upper jaw is elongated to form a kind of beak, abundantly supplied with teeth. They are carnivorous, voracious fishes, working great havoc among the more defenseless food-fishes. •I THE PISHES 163 156. The catflshes. — Lowest of all the bony fishes we may place the great group to which almost all fresh-water fishes belong. In this group the four vertebrae situated next the head are firmly united, and by means of certain small lever- like bones a connection is formed between the air-bladder and the ear of the fish, which is sunk deep in the skull. The air-bladder thus becomes a sounding organ in the function of hearing. The family of catfishes possesses this structure, and the student should look for it in the first one he catches. The catfishes are remarkable for the long feel- ers about the mouth, with which they pick their way on the bottom of a pond. There are many kinds the world over. The small ones are known as horned pout or bullhead. In these the dorsal and pectoral fins are armed each with a strong, sharp spine, which is set stiff when the fish is dis- turbed, and makes them very troublesome to handle. The catfishes have no scales. 157. The earp-like fishes. — The still greater carp family includes all the carp, dace, minnows, and chubs. They have the air-bladder joined to the ear, just like the catfish, but they lack the long feelers and the fin spines, while the soft body is covered with scales, and there are no teeth in the mouth. In the throat are a few very large teeth, which the ingenious boy should find. In the sucker family these throat teeth are like the teeth of a comb, and the mouth is fitted for sucking small objects on the river bottom. 158. The eels. — In the great order of eels the body is long and slim, scaleless, or nearly so, with no ventral fins. The shoulder-girdle has slipped back from the head, so as to leave a distinct neck, while ordinary fishes have none. Of eels there are very many kinds — some large and fierce, some small as an earthworm ; and one kind comes into fresh water. 159. Herring and salmon. — In the great order which in- cludes the herring and salmon the vertebrae are all alike, the ventral fins far from the head, and the scales smooth to 164 ANIMAL FORMS the touch. The herring and shad are examples, as also the salmon and trout. Some live in the great depths of the sea, even five miles below the surface. These are very soft in body, being under tremendous pressure. They are inky black — for the sea at that depth seems black as ink — and most of them have luminous spots which give them light in the darkness. Some species have the foi-ehead luminous, like the headlight of an engine. Most of these deep-sea fishes are very voracious, for there is nothing for them to feed on save their neighbors. 160. The pike, sticklebacks, etc. — Several small orders stand between these soft-rayed, smooth-scaled fishes and ¥1G. 103. — The blindfish and its parentage. A, Dismal Swamp fish {Chohgaster avetus), the ancestor of (B) Agassiz's cave fish. (^C/iologaster aga^sisi) and (C) cave hlindflsh (Typhlichthys subterranms). the form, like the perch and bass, which has many spines in the dorsal fin. Among these transitional forms is the pike (Pig. 103) — long, slender, circumspect, and voracious, lying in wait under a lily-pad; the blindfish, which lost its eyes through long living in the streams of the great caves ; the stickleback, small, wiry, malicious, and destructive, steal- ing the eggs and nibbling the fins of any larger fish ; the sea-horse, clinging with its tail head downward to floating 166 ANIMAL FORMS seaweed, the male carrying the eggs about in his pocket until they hatch ; the mullet, stupid, blundering, feeding on minute plants, crushing them in a gizzard like that of a hen, but withal having soft flesh, good for the table ; the flying-fishes, which sail through the air with great swiftness to escape their enemies. 161. The spiny-rayed fishes. — In the group of spiny- rayed fishes the ventral fins are brought forward and joined to the shoulder-girdle. The scales are generally rough to the touch, and the head is usually roughened also. There are many in every sea, ranging in size from the Everglade perch of Florida, an inch long, to the swordfish, which is thirty. These are the most specialized, the most fish-like of all the fishes. Leading families are the perch, in the fresh waters, the common yellow perch, familiar to all boys in the Northeastern States ; the darters, which are dwarf perches, beautifully colored and gracefully formed, living on the bottoms of swift rivers ; the sunfishes, with broad bodies and shining scales, thriving and nest-building in the quiet eddies ; the sea-bass of many kinds, all valued for the table; the»mackerel tribe, mostly swimming in great schools from shore to shore. After these come the multi- tude of snappers, grunts, weakfishes, bluefishes, rose-fishes, valued as food. Then follow the gurnards, with bony heads; the sculpins, with heads armed with thorns, the small ones in the rivers most destructive to the eggs of trout ; and at the end of the long series a few families in which the spines once developed are lost again, and the fins have only soft and jointed rays. It is a curious law of development that when a structure is once highly special- ized it may lose its usefulness, at which point degeneration at once sets in. Among fishes of this type are the cod- fishes, with spindle-shaped bodies, and the flounders, with flat bodies. The flounders lie on the sand with one side down, and the head is so twisted that the eyes come out to- gether on the side that lies uppermost. This side is col- s I 13 168 ANIMAL FORMS ored like the bottom — sand colored or brown or black — and the under side is white. When the flounder is first hatched, the eyes are on each side of the head, and the animal swims upright in the water like other fishes. But it soon rests on the bottom ; it turns to one side, and as the body is turned over the lower eye begins to move over to the other side. Finally, we -may close the series with the an- glers (Fig. 105), in which the first dorsal spine is trans- formed into a sort of fishing"-pole with a bait at the end, which may sometimes serve to lure the little fishes, which are soon swallowed when once in reach of the capacious mouth. 163. Internal anatomy. — A few fishes are vegetarians., but the greater number are carnivorous. Some swallow large quantities of sand of the searbottom and absorb from it the small organisms living there. Others are provided with beaks for nipping off corals and tube-dwelling worms. Huge plate-like teeth enable others to crush moUusks, sea-urchins, and crabs, and many are adapted for preying upon other fishes. The latter are often able to escape, owing to the presence of numerous spines, sometimes supplied with poison-glands; or their colors are protective, and a vast number of devices are present which enable them with some degree of surety to escape their enemies and capture food. Usually, without mastication, the food passes into the digestive tract (Fig. 106), which in the main resembles that of the squirrel, but varies considerably according to the nature of the food it is required to absorb. As in other animals, it is usually longer in the vegetable feeders. In most fishes the walls of the canal are pushed out at the junction of the stomach and intestine, to form numerous processes like so many glove-fingers (the pyloric cceca, Fig. 106, py.c), which probably serve to increase the absorptive surface. The same result is obtained in other ways, chiefly by numerous folds of the lining of the canal. The blood-system is much more complex in the fishes THE PISHES 169 than in any of the invertebrates. It also differs in its gen- eral plan from that of most adult vertebrates, owing to the peculiar method of respiration. In almost every case the Fie. 105. -Angler or frogflsh (Lophius 2nscatm'ius). Baskett. One-tenth natural size.— After vessels returning from all parts of the body unite into one vein leading into the heart, which consists of only one auricle and ventricle (Fig. 106). Prom the heart the blood lYO ANIMAL FORMS is forced through the gills, with all their delicate filaments, and now, laden with oxygen and nutritious substances al- ready absorbed from the coats of the digestive tract, it 01)1.1. piyblcrb,. Fig. 106. — Dissection of a bony fish, the trout {SaXmo). a.bl., air-bladder ; an., anal opening; aw., auricle; gl.st., gills; gul., esophagus; int., intestine; kd., kidney; Ir,, liver ; l.ov., ovary; opLL, brain ; py.c, pyloric "cceca ; sp-c, spinal cord ; ^l., spleen ; St., stomach ; v., ventricle. travels on to all parts of the body, continually unloading its cargo in needy districts and^ waste matters in the kid- neys before returning once more to the heart. 163. The senses of fishes. — The habits of fishes indicate that they know considerable of what is going on in the outside world, and their well-developed sense-organs show the degree of their sensitiveness. A share of this informa- tion comes through the sense of touch, which is distributed all over the surface of the body, chiefly in the more ex- posed regions sometimes especially provided with fleshy feelers, like those on the chin of the catfish. The sense of smell appears to be fairly developed, as is that of hearing ; but there is no evidence of a sense of taste. A few fishes chew their food, and may possibly taste it, but there are others that swallow it whole, and in all there are relatively a few nerves going to the tongue or floor of the mouth. THE FISHES 171 The eyes of most fishes are highly developed, and are of the greatest use at all times. Exceptions to the rule are found in certain species which live in cayes or in the dark abysses of the ocean. In some of these the eyes have dis- appeared almost completely, and the sense of touch be- comes correspondingly more acute ; in other deep-sea forms they have grown to a large size, enabling them to distin- guish objects in the gloom, like the owls and other noc- turnal animals. Embedded- in the skin of some of these deep-sea fishes, and certain nocturnal ones, are peculiar spots, composed of a glandular substance, which produces a bright glow like that of the fireflies. These may be located on the head or arranged in patterns over various parts of the body, and may serve to light the fish on its way and enable it to see its food to better advantage, or it may act as a lure to many fishes that become victims to their own curiosity. In those fishes which are active most of the time the eyes are located on the sides of the head, and in those which remain at or near the bottom they are turned toward the top ; in every case where they can be used to the best advantage. 164. Breeding habits. — Among fishes the egg-laying time usually comes with the spring, when the males of several species become more resplendent, and sometimes engage in struggles for their respective mates. In others this ceremony is performed without show of hostility. Some make nests, while others lay their eggs loosely in the water. In all the salmon family the young fishes are born in the colder fresh-water rivers, and later make their way into the sea, where they spend the greater part of their lives. When the time comes for them to lay their eggs they migrate in great companies, and make their way hundreds, perhaps thousands, of miles to the rivers in which they spent their youth. Up these streams they rush in crowds, leaping waterfalls and rapids, and, dashed and battered on the rocks, many, and in some species all, die from injuries 172 ANIMAL FORMS or exhaustion after the breeding season is passed. The eggs, like those of the chubs, suckers, sunflshes, and cat- fishes, are usually buried in shallow holes in the sand, and the males of most fishes keep a faithful watch over the young until they are able to live in safety. In some of the sticklebacks and several marine species elaborate nests are composed of grass or seaweeds ; some of the catfishes carry the eggs until they hatch in their mouths or else in folds of spongy skin on the under side of the body ; in the pipefishes and sea-horses a slender sac along the lower sur- face of the male acts as a brood-pouch, in which the female places the eggs to remain until developed ; and some fishes, such as the surf-fishes and a number of the sharks, bring forth their young alive. On the other hand, the young of many of the herrings, salmon, cod, perch, and numerous other fishes are abandoned at their birth, and fall a prey to many animals, even their parents often included. In the former cases, where the young are protected, only a relatively few eggs are produced : where they are aban- doned the female often lays many millions. In every case the number of eggs is in direct relation to the chances the young have of reaching maturity, a few out of each brood surviving to perpetuate the race. 165. Development and past history. — The eggs of the higher bony fishes are usually small (one-tenth to one-third of an inch in diameter), and the young when they hatch are accordingly little ; in the sharks the eggs are larger, the size of a hen's egg or even larger, and the young when born are relatively large and powerful. These differences, however, do not greatly affect the early development, for in every case the head and then the trunk soon become formed, gills arise, the nervous system appears, which is invariably supported by a skeleton in the form of a gristly rod — the notochord. In the lower forms of fishes this per- sists throughout life ; but in the sharks and skabes it be- comes replaced in the adult by another and higher type of THE FISHES ITS skeleton, which is much more specialized with the bony fishes. Those who study the fossils on the rocks tell us that the first fishes were very simple, and many believe that their skeleton, like that of the little growing fish, consisted only of a notochord. Many of these old forms died out long ago, while others gradually changed in one way and another to adapt themselves to their surroundings, the con- stant need of adaptation having resulted in the multitude of present-day types. Some, such as the lamprey, have probably changed relatively only to a slight extent ; others, like the sharks and skates, are much more altered ; and the bony fishes are far from their original low estate, though their development has been rather toward a greater specialization for aquatic life than an advance upward. The little fish in its growth from the egg thus repeats the history of its ancestral development; but as though in haste to reach the adult condition, it omits many impor- tant details. Moreover, the record in the rocks is not complete, and we have many things yet to learn of the ancient fishes and their development from age to age to the present day. CHAPTEE XV THE AMPHIBIANS Ik many respects the amphibians — toads, frogs, and sala- manders — resemble the fishes, especially the lung-fishes (Dipnoi). The modern amphibians are essentially fishes in their early life, but in developing legs and otherwise changing their bodily form they become adapted for a life on land under conditions differing from those of the fishes. Judging from this class of facts, we may assume that fish- like ancestors, by the development of the lungs, became fitted for a life on land, and that from these the amphib- ians of our times have been derived. 166. Development. — The eggs of the Amphibia are laid during the spring months in fresh-water streams and ponds. They are globular, about as large as shot, and are embedded in a gelatinous envelope (Fig. 107). They are either de- posited singly or in clumps, or festooned in long strings over the water-weeds. During the next few days development proceeds rapidly under favorable conditions, resulting in an elongated body with simple head and tail. In this condition they are hatched as tadpoles. As yet they are blind and mouthless, but lips and horny Jaws soon appear, along with highly developed eyes, ears, and nose. External fluffy gills arise on the sides of the head, and slits form in the walls of the throat, between which gills are attached, and over which folds of skin develop, as in the fishes. A fin-fold like that of the lancelet or lamprey appears on the tail. The brain and spinal cord, extending along the line of the back, are supported by a gristly notochord, and complete and com- 174 THE AMPHIBIANS 175 plex internal organs adapt the animal to a free-swimming existence for days to come. The tadpole is now, to all intents and purposes, a fish^— a fact most clearly recognized in its form, method of loco- FiG. 107.— Metamorphosis of the toad.— Partly after Gage, from Animal Life. motion, the arrangement of the gills, and the general plan of the circulatory system. 167. Further growth. — In the course of the next few weeks hind limbs develop beneath the skin, through which they finally protrude. In the same manner, fore limbs arise at a later date. In position these organs are like the paired fins of fishes, but they are intended for crawling or leaping on land, and are modified in accordance with this need. As in the higher vertebrates, the limbs develop as arms and legs, with long fingers and toes, between which are stretched webs of skin, which serve in swimming. 176 ANIMAL FORMS In the meantime large internal changes are also taking place. The wall of the esophagus has gradually pouched out to form the lungs. They are richly supplied with blood- vessels, closely resembling in their general features the lungs of the lung-fishes. The animal now rises to the sur- face occasionally to gulp in air, and it also continues to breathe by means of gills. At this stage of its existence, therefore, the larva is amphibious (two-living), and we have the interesting example of an animal extracting oxygen from both the water and the air. The diet of the tadpole at this time changes from vegetable to animal substances, and horny teeth give way to the small teeth of the frog, and the digestive system undergoes an entire remodeling to adapt it to its new duties. The young amphibian — whether frog, toad, or salamander^is now a four-legged creature, with well-developed head and tail, with lungs and gills, though the latter are usually fast disappearing, and is rapidly assuming those characters which will fit it for a terrestrial or semiaquatic existence. 168. The salamanders. — The changes which now ensue in such a larva in reaching the adult condition are relatively slight in the lower salamanders. The external gills often persist (Fig. 110), the lungs are also functional, and the changes are largely those of increase of size. In the larger number of species the gills disappear more or less com- pletely (Fig. 108), such species often abandoning the water for homes in damp soil or under stones and logs, returning to it only when the time comes for their eggs to be laid. The limbs are always relatively weak, never supporting the body from the ground, but serving in a clumsy way to push it from place to place. In the aquatic forms the tail con- tinues to serve as a swimming organ. In some species the hind legs become rudimentary, or even entirely lacking. A still further modification occurs in a few burrowing spe- cies, which move by wrigglings of the body, and are with- out either pairs of legs. THE AMPHIBIANS 111 In geological times many of the salamanders were of great size, several feet in length, and some were enclosed in an armor consisting of bony plates. All now living have the skin naked, and with the exception of the giant species of Japan, three feet in length, and a few similar forms in America, the modern representatives are comparatively Fis. 108.— Blunt-uosed Salamander {Amblysioma opaeum). Photograpli by W. H. Fisher. feeble and measure their length by inches. Only a few, on account of their bright colors, are particularly attractive, while the others are usually shunned and considered re- pulsive, chiefly because of their supposed poisonous char- acter, though in reality few animals are more harmless. 169. Tailless forms, — In the frogs and toads the meta- morphosis which the young undergo is almost as profound as that which takes place with the insects. The gills, to- gether with their blood-vessels, disappear completely. The tail, with its muscles, nerve-supply, and skeleton, is ab- sorbed. The cartilaginous notochord gives way to a jointed back-bone. A skull is developed ; numerous bones form in the limbs, affording an attachment for the powerful muscles which make the toad, and especially the frog, expert swim- 178 ANIMAL FORMS mers and leapers, and thus equipped they hereafter lead a wholly terrestrial or semiaquatic life. 170. Distribution and common forms. — All the Amphibia are dependent upon moisture. Almost all are hatched and developed in fresh water, and those which leave the water return to it during the breeding season. So we find repre- sentatives of the group all over the world having much the same range as the fresh-water fishes. The great majority of the salamanders are confined to the northern hemisphere, but the toads and frogs are almost universally distributed. Among the salamanders in this country only a relatively few species completely retain their external gills. This is the case with sirens and mud-puppies or water-dogs (Fig. 110), which may occasionally be seen in the clear waters of our lakes and rivers crawling slowly about in search of food, and every now and then rising to the surface to gulp in air. The remainder lose their gills more or less com- pletely, and usually leave the water for damp haunts on land. One of the blunt-nosed salamanders, known as the tiger salamander {Amblystoma tigrinwn), is found in moist localities in most parts of the United States. Besides these are numerous small species, among them the newts (Die- mydylus), ranging widely over the United States, living under logs and stones and feeding upon the small insects and worms inhabiting such situations. In several species of salamanders the lungs disappear with age, and respira- tion is performed solely through the surface of the skin. The tailless amphibians are much more abundant and familiar objects than the salamanders, and from the open- ing of spring until late in the fall they are met with on every hand. With few exceptions the frogs live in or about ponds and marshes, in which they obtain protection in troublous times and from which they derive the store of worms and insects that serve as food. On the other hand, the tree-frogs, as their name indicates, usually abandon the water and repair to moist situations in trees and other vege- THE AMPHIBIANS 1Y9 tation. Their shrill, cricket-like calls are often heard in the summer. The fingers and toes are more or less dilated into disks at their tips, enabling them to climb with con- siderable facility; and they are further adapted to their surroundings on account of their protective colors. The toads undergo their metamorphosis while very small, and approach only the water at the breeding season. During the day they remain concealed in holes and crevices, but at the approach of evening come out in search of food. 171. Means of defense. — The food of the members of this group consists chiefly of small fishes, insect larvse, snails, and little crustaceans, which are swallowed whole. On the other hand, many Amphibia prey on each other, while most of them are eagerly sought by birds and fishes. Some, as the toads, stalk their food only during the night-time or depend upon their agility to escape their enemies. Others are colored protectively, the markings of the skin resem- bling the foliage of the earth upon which they rest, and in some species, as the tree-toads, this color-pattern changes as the animal shifts its position. A few species are most brilliantly colored with red, green, yellow, or combinations of these, in striking contrast to their surroundings. They have apparently few enemies, possibly because of an un- pleasant odor or taste, and it has been suggested that their gorgeous tints are danger-signals, warning their would-be captors from attempting a second time to devour them. At the same time it is well known that the somber-hued toads emit a milky secretion from the warty protuberance of their skin which is intensely bitter, irritating to delicate skin, and poisonous to several animals. 173. Skeleton. — As in all vertebrates, the skeleton of the amphibian first arises as a cartilaginous rod, the notochord, which is afterward replaced by a jointed back-bone, to which the limbs are attached. The back-bone is anteriorly modified into a flat, usually complex, skull. In the sala- manders the number of vertebrae is sometimes very large. 180 ANIMAL FORMS and the body correspondingly long and snake-like ; but in other cases parts of the vertebras are reduced in number, and the body is rather short and thick. In the frogs and toads this reduction reaches its culmination, for only nine distinct vertebrae are present, the tail vertebrae, correspond- ing to those of the salamanders, being represented by a rod-like bone, the urostyle, made of segments grown to- gether. 173. Digestive and other systems. — In its main characters the digestive tract of the amphibian (Eig. 109) resembles p.na Fia. 109.— Dissection of toad (Bw/o), an., anal opening; o«., auricle ; 6?., bladder; duo., duodenum ; Ing., lung ; /r., liver; pn., pancreas ; ret., rectum ; spl., spleen ; St., stomacli ; v., ventricle. that of the fishes and the squirrel. The mouth is usually large, and the teeth are very small, as in the frog or sala- mander, or are lacking completely, as in the common toad. In many salamanders the tongue, like that of a fish, is fixed and incapable of movement. In most of the frogs and toads it is attached to the front of the mouth, leaving its hinder portion free, and capable of being thrown over and outward for a considerable distance. In the throat region gill-clefts may persist, but they usually close as the lungs reach their development. The succeeding portions of the canal are comparatively straight in the elongated forms, or THE AMPHIBIANS 181 more or less coiled in the shorter species. In some cases no well-marked stomach exists, but ordinarily the different portions, as they are shown in Fig. 109, are well defined. As noted aboye, the circulation in the tadpole is the same as in fishes, then lungs arise, and for a time respi- ration is effected both by gills and lungs, and the cir- culation resembles in its essential points that of the lung-fishes. This may continue throughout life, but more frequently the gills and their vessels disappear, and the circulation approaches that of the reptiles. In such forms the heart consists of two auricles and one ventricle. Into the left auricle pours the pure blood from the lungs ; into the right the impure blood from the body. To some extent these mix as they are forced into the general cir- culation by the single ventricle. The amount of oxygen carried is therefore smaller than in the higher air-breathers, the amount of energy is proportionately less, and hence it is that all are cold-blooded and of comparatively sluggish habits. In some species of salamanders the lungs may also dis- appear, and breathing is carried on by the skin, as it is to a certain extent in all amphibians. In the frogs and toads lungs are invariably present, and vocal organs are situated at the opening of the windpipe in the throat. These pro- duce the characteristic croaking or shrilling, which in many species are intensified through the agency of one or two large sacs communicating with the mouth-cavity. Although the brain is small in the amphibians, it is more complex in several respects than it is in fishes. The eyes are also usually well developed, but in some of the cave and burrowing salamanders they are concealed beneath the skin, and are rudimentary. The ear varies considerably in complexity in the different species, but in the possession of semicircular canals and labyrinth resem- bles that of the fishes. In the frogs and toads, as one may readily discover, the drum or tympanum is external, ap- 182 ANIMAL POEMS pearing as a smooth circular area behind the eye. Organs of touch, smell, and taste are likewise developed in varying degree of perfection. 174. Breedii^f-habits.' — While the great majority of am- phibians mate in the spring and deposit their eggs in the water, often to the accompaniments of croakings and pip- ings almost deafening in intensity, several species, for various reasons, have adopted different methods. Some of the salamanders bring forth young alive, and several species of toads and frogs are known in which the young are cared for by the parent until their metamorphosis is complete. In one of the European toads {Alytes) the male winds the strings of eggs about his body until the tadpoles are Fio. 1111.— SnlamaiKlcrs. The axolotl (the larva of Ajii- hbjMoma tifffinyrn) and the newt {Dieinycl ijhtt; to- - ready to hatch ; and in a few species of tree-toads the eggs are stored in a great pouch on the back of the parent until the early stages of growth are over. In the Surinam toad of South America the eggs are placed by the male on the back of the female, and each sinks into a cavity in the spongy skin. Here they pass through the tadpole stage without the usual attendant dangers, and emerge with the form of the adult. THE AMPHIBIANS 183 Sunlight and warmth are apparent necessities for speedy development. Tadpoles kept in captiTity where the con- ditions are generally unfavorable may require years to as- sume the adult form. As mentioned above, the tiger sala- mander {Ambly stoma tigrinum) occurs in most parts of the United States and Mexico, In the East this species drops its gills in early life as other salamanders do, and assumes the adult form, but in the cold water of high mountain lakes, in Colorado and neighboring States, it may never become adult, always remaining as in Pig. 110. This peculiar form is locally known as axolotl. In this condition it breeds. It is thus one of the very few examples of animals whose un- developed larvse are able to produce their kind. Owing to this trait it was at first considered a distinct species, and many years elapsed before its relationship to the true adult form was discovered. 13 CHAPTER XVI THE REPTILES 175. General characteristics. — In all the reptiles the gen- eral shape of the body, and to some extent the internal plan, is not materially different from that seen among the amphibians. In spite of external resemblance the actual relationship is not very close. It appears to be true that ages ago the ancestors of the modern reptiles were aquatic animals, possibly somewhat similar to some of the sala- manders ; but they have become greatly changed, and are now, strictly speaking, land animals. At no time in their development after leaving the egg do we find them living in the water and breathing by gills. Some species, such as the turtles, lead aquatic or semiaquatic lives, but the modifications which fit them for such an existence render them only slightly different from their land-inhabit- ing relatives. The skin bears overlapping scales or horny plates, united edge to edge, as in the turtles, enabling them to withstand the attacks of enemies and the effects of heat and dryness. Indeed, it is when heat is greatest that rep- tiles are most active. In no other class of vertebrates, and very few invertebrates, do normal activities of the body appear to be so directly dependent upon external warmth. In the presence of cold they rapidly grow sluggish, and sink into a dormant state. As in the case of all animals, habits depend upon structure, and accordingly among the reptiles we find many remarkable modifications, enabling them to lead 184 THE REPTILES 186 widely different lives. Nevertheless all are constructed upon much the same plan. 176. The lizards (Sauria), — As in the amphibians, es- pecially the salamanders, the body (Fig. Ill) consists of a relatively small head united by a neck to the trunks, FiQ. 111.— Common lizard or swift iScelopm^s undulatus). Pliotograpli by \V. U. Fisher. which, in turn, passes insensibly into a tail, usually of con- siderable length. Two pairs of limbs are almost always present, and these exhibit the same skeletal structure as in the amphibians ; but in their construction, as in the other divisions of the body, we note a grace of propor- tion and muscular development which enable the lizards to execute their movements with an almost lightning-like rapidity. The mouth is large and slit-like, well armed with teeth, and the eyes and ears are keen. Scales of various 186 ANIMAL FORMS forms and sizes, always of definite arrangement, cover the body. The scales are always colored, in some species as brilliantly as the feathers of birds, and usually harmonize with the surroundings of the animal, enabling it to escape the attacks of its many enemies. Altogether the lizards are a very attractive group of animals. As in the salaman- ders, the vertebral column is usually of considerable length, but it too presents a lighter appearance and a greater flexi- bility. Slender ribs are present, and a breast-bone and the girdles which support the limbs. Although more ossified than in the amphibians, the skull still continues to be com- posed here and there of cartilage. The roof also is yet incomplete, but with the firm plates on the surface of the head ample protection is afforded the small brain under- neath. As above mentioned, the limbs are slender and insufficient to support the body, which accordingly rests upon the ground, and by its wrigglings and the pushing of the limbs is borne from place to place. It will be recalled that some of the salamanders living in subterranean haunts and burrowing in the soil have no need of limbs, and the latter have accordingly disappeared. This condition is paralleled by certain species of lizards. The blindworms (which are neither blind nor worms, but true lizards, though snake-like in appearance) are devoid of limbs, as are also the " glass-snakes." In some species the hinder pair arise in early life, but they remain small, and ultimately disap- pear. In almost all lizards the tail is very brittle, breaking at a slight touch. In such case the lost member will grow again after a time. 177. The snakes (Serpentes).— The snakes are character- ized by a cylindrical, generally greatly elongated body, in which the divisions into head, neck, trunk, and tail are not sharply defined. As we have seen, this is also true of cer- tain lizards, but the naturalist finds no difiiculty in detecting the differences between them. Another peculiarity of the snakes is in the great freedom of movement of the bones THE REPTILES 187 not concerned with the protection of the brain. In the reptiles the lower jaw does not unite directly with the skull, as in the higher animals, but to an intermediate bone, the quadrate, which is attached to the skull. In the snakes these unions are made by means of elastic liga- ments. The two halves of the lower jaw are also held Fig. 112.— Blacksnake {Bascanion constrictor). Photograph by W. H. Fisher. together by a similar band, so that the entire palate and lower jaw are loosely hung together. This enables the snake to distend its mouth and throat to an extraordinary degree, and to swallow frogs and toads but slightly smaller than itself. Where the prey is of relatively small size, the halves of the lower jaw alternate with each other in pulling backward, thus drawing the food down the throat. The food is never masticated. The teeth are usually small and recurved, and serve only to hold the food until it may be swallowed. The latter process is facilitated by the copious secretion of the ■ salivary glands, which become very active at this time. A further character of the snakes is the absence exter- 188 ANIMAL POEMS nally of any trace of limbs. However, in some of the pythons and boas hind limbs are present in the form of small groups of bones embedded beneath the skin and ter- minating in a claw. There thus appears to be no doubt that the ancestors of the modern snakes were four-footed, lizard-like creatures, which have assumed the present form in response to the necessity of adaptation to new conditions. More than any other order of vertebrates do the snakes deserve the name of creeping things, and yet their method of locomotion enables them to crawl and swim with a ra- pidity equal to that of many of the more highly developed animals. This depends chiefly upon certain peculiarities of the skeleton, which consists merely of a skull, vertebral column, and ribs. The vertebrae, usually two or three hun- dred in number, are united together by ball-and-socket joints, and each attaches by similar joints a pair of slender ribs. These in turn are' attached to the broad outer plates upon which the body rests, and the whole system is operated by a powerful set of muscles. Upon the contraction of the muscles the ventral plates are made to strike backward upon the ground or other rough surface, which drives the body forward. Also, the ribs may be made to move back- ward and forward, and the snake thus progresses like a centiped or "thousand-legs," 178. The turtles (Chelonia). — In many respects the tur- tles are the most highly modified of all the reptiles. The body (Fig. 113) is short and wide and enclosed in a shell or heavy armor, consisting of an upper portion, the carapace, and a flat ventral plate, the plastron. The shape of the carapace varies greatly from a low, flat shield to a highly vaulted dome, remaining cartilaginous throughout life, as in the soft-shelled turtles, or becoming bony and of great strength. These two portions of the shell form a box-like armor, through whose openings the head, tail, and limbs may be extended. The latter organs are superficially unlike those of any other order of animals. The head is generally THE REPTILES 189 thick-set and muscular, and provided with horny jaws entirely destitute of teeth, like those of the birds. The limbs also are usually short and thick and variously shaped, and adapted for aquatic or terrestrial locomotion. The number of vertebrae in the body and tail are relatively few, and the thick and heavy body is devoid of the elements of grace and agility of movement characteristic of the other reptiles. On the other hand, the former enjoy a freedom from the attacks of enemies not accorded to animals in general. At first sight the appearance of a turtle does not indi- cate a close relationship to the other reptiles, but a more Fig. 113.— Box-turtle ( Terrapene Carolina). careful examination, and especially of their development, discloses a remarkable resemblance. The head, tail, and limbs are essentially similar to those of the lizards, but in the trunk region peculiar modifications have taken place. The ribs at first separate, as in other animals, flatten greatly, and unite with a number of bones embedded in the skin, thus forming one great plate overlying the back of the animal. About the circumference of the shield other dermal or skin-bones are added, which increase the area of the carapace, and at the same time still others have 190 ANIMAL FORMS arisen and nnited on the ventral surface to form the plas- tron. In this process the shoulder- and hip-girdles which attach the limbs come to be withdrawn into the body, and we have the curious example of an animal enclosed within its back-bone and ribs. This is even more the case with the box-turtles (Fig. 113), common in the eastern United States, whose ventral plate is hinged so that after the limbs, head, and tail have been withdrawn it may be made to act like a lid to completely enclose the fleshy parts of the body. Scales and horny plates are present, as in other reptiles, the former covering all parts of the body except the cara- pace and plastron, which support the plates. In nearly all species the latter are of considerable size, and in the tor- toise-shell turtles are valuable articles of commerce. They also are sculptured in a fashion characteristic of each spe- cies, and may, like the colors of other animals, render them more like their surroundings, and consequently incon- spicuous. 179. Crocodiles and alligators (Crocodilia). — The alligators, (Fig. 114) and crocodiles are much more complex in struc- ture than the lizards, though their general form is much the same. The body is covered with an armor of thick bony shields and horny scales. These, along the median line, are keeled, and extending along the length of the laterally com- pressed tail form an efficient swimming organ and rudder. The mouth is of large size, and is bounteously supplied with large conical teeth, which are set in sockets in the jaw, and not fused with it, as in many of the lizards. The nose and ears may be closed by valves to prevent the entrance of water, and a similar structure blocks its passage beyond the throat while the mouth is open. When large animals, such as hogs or calves, are captured as they come to drink, these devices enable the alligator or crocodile to sink with them to the bottom and hold them until drowned. The limbs, short and powerful, are efB'cient organs of locomo- THE REPTILES 191 tion on land, and together with the general shape of the body, are also well adapted for swimming. Pig. 114.— Alligator {Alligator mississippiensis). 180. Distribution of the lizards. — In a general way the number of reptiles is greatest where the temperature is highest. The tropics therefore abound in species, often of large size, and usually of bright coloration. As one travels northward the numbers rapidly diminish, their size is smaller, and the tints less pronounced. In all probability not less than four thousand known reptiles exist, whose haunts are of the most varied description. In K"orth America the lizards are almost exclusively confined to the southern portions, only a very few species extending up to the fortieth parallel. Among these the skinks {Eumeces) are most widely distributed. The blue- tailed skink is probably the most familiar, a small lizard eight or ten inches in length, dark green with yellowish streaks and a bright-blue tail. On sunny days it may sometimes be seen darting about on the bark of trees in search of insects, upon which it feeds. One of the most familiar lizards in this country is the "glass-snake," found burrowing in the drier soil of the southern half of the United States east of the Mississippi. 193 ANIMAL FORMS Both pairs of limbs are absent, but by wriggling movements of the body this lizard is able to force its way through light soil with considerable rapidity. It is a matter of some difficulty to secure entire specimens, for with other than the gentlest handling the tail sevprs its connection with the body, as the vertebrae in this portion are extremely brittle. This peculiarity, together with its shape, has given it the popular name of glass-snake. Many species of liz- ards will thus detach the tail, a habit which is a means of protection, enabling the animal to scamper away into a place of safety while its enemy is concerning itself with the detached member. Later on a new tail develops, though usually of a less symmetrical form. 181. Horned toads. — The horned toads {Phrynosoma) are lizards peculiar to the hot, sandy deserts and plains of Fig. U5. — Gila monster (Hdoderma suspecium). One-third natural size. Mexico and the western United States. The body is com- paratively broad and flat, almost toad-like, and is covered with scales and spines of brownish and dusky tint, so like dried sticks and cactus spines in form and color as to ren- der them difficult of detection. In captivity they readily THE REPTILES 193 adapt themselyes to their new surroundings, become tame, and feast on flies, ants, and other insects, which they cap- ture by the aid of their long tongue. The horned toads are perfectly harmless creatures, but when irritated some- times perform the remarkable feat of spurting a stream of blood from the eye toward the intruding object for a dis- tance of seyeral inches. This has been regarded by some as a zoological fable ; but there are many who have watched the horned toad in its natural state and in captivity, and they assure us that it is a fact. In the hot deserts of Arizona and Sonora is another peculiar species of lizard known as the Gila monster {Helo- derma) (Fig. 116), having the distinction of being the only poisonous lizard known. Further protection is afforded by bony tubercles on the head and by scales over the remainder of the body, all of which are colored brown or various shades, of yellow, giving the animal a peculiar streaked and blotched appearance. 183. Distribution of the snakes. — The snakes are much more common than the lizards. All over the United States one meets with them, especially the garter- or water-snakes. Of less wide distribution are the black-, grass-, and milk- snakes, and a number of less known species, all of which are perfectly harmless and often make interesting pets. Some of them when cornered show considerable temper, flatten the head and hiss violently, andximitate poisonous forms, but venomous snakes are comparatively few in num- ber in northern and eastern United States. In the south- ern portions of the country they become more abundant. Along the streams and in the swamps the copperheads, and especially the water-moccasins, often lie in wait for frogs and fish. Both these species are especially dreaded, as they strike without giving any warning sound, but the name and bad reputation of the moccasin is often, especially in the South, transferred to perfectly harmless water-snakes. On higher ground are the rattlesnakes (Grotalus), once 194 ANIMAL FORMS abundant but now in many regions well-nigh exterminated. In these species the tail terminates in a series of horny Fig. 116.— Diamond-rattlesnake (.Orotalus adamarUeus). Photograph by W. H. PiSHEK. rings that produce a buzzing sound like that of the locvist when the tail is rapidly vibrated. 183. Distribution of the turtles. — The turtles are perhaps somewhat less dependent upon warmth than other reptiles, yet they too delight to bask in the sunshine, and soon grow sluggish in its absence. In all our fresh-water streams and ponds they are familiar objects, and several species extend up into Canada. Among the turtles the soft shell, the painted and the snapping turtles have the widest distri- bution, scarcely a good-sized stream or pond from the Gulf of Mexico to Canada, and even farther north, being without one or more representatives. All are carnivorous and vora- cious, and the snapping turtles are especially ferocious, and " for their size are the strongest of reptiles." In the woods and meadows the wood-tortoise and box-turtles are occa- THE REPTILES 195 sionally met with, and at sea several turtles exist, some of them of great size. Among these is the leather-turtle, found in the warmer waters of the Atlantic, lazily floating at the surface or actively engaged in capturing food. They attain a length of from six to eight feet, and a weight of over a thousand pounds, and are sometimes captured for food when they come ashore to bury their eggs in the sand. By this same method the loggerheads, the hawkbills, and the common green turtles are also captured in consider- able numbers. These are of smaller size, and the second named is of considerable value, as the horny plates cover- FiG. 117. — Hawkbill turtle {Eretmoclielys imbricata). ing the shell furnish the tortoise-shell of commerce. These plates are removed after the animal is killed, by soaking in warm water or by the application of heat. 184. Food and digestive system. — Some reptiles, among which are a number of species of lizards and the box- and green turtles, are vegetarians, but the great majority are 196 ANIMAL FORMS carnivorous, and usually very voracious. The lizards espe- cially devour large quantities of insects and snails, together with small fishes and frogs. The latter figure largely in the turtle's bill of fare, and in that of the snakes, which also capture birds and mammals. On the other hand, many of the reptiles prey upon one another ; and they are the favorite food of hawks and owls and numerous water-birds, of skunks and weasels and many other animals, which look for them continually. Many of the turtles, owing to their protective armor, and the snakes because of their poison- ous bite or great size and strength, are more or less ex- empt, but this is not true of their eggs and young. The smaller species depend upon keenness of sense, agility, and inconspicuous tints. These latter may undergo changes according to the character of the surroundings, but usually only to a slight extent. The chameleons of the tropics and a similarly colored green lizard on the pine-trees in the Southern States are able to change with great rapidity from green, through various shades, to brown. 185. Respiration and circulation. — While still in the egg the young lizard develops rudimentary gills, and thus bears namiU '^ extna.i int.ni FiQ. 118.— Dissection of lizard (Scebyporus). an., anal opening ; au., auricle ; crb.h., brain ; coec, intestine ; kd., kidney ; Ling., left Inng ; Ir., liver ; pn., pancreas ; sp.c, spinal cord ; spL, spleen ; St., stomach ; v., ventricle of heart. evidence to the fact that its distant ancestors were aquatic ; but before hatching they disappear, and lungs arise, which THE REPTILES 197 remain functional throughout life. Corresponding to the shape of the body, these are usually much elongated and ordinarily paired (Pig. 118, Ung.). The snakes are peculiar in haying the left lung rudimentary or even lacking com- pletely, while the right one becomes greatly elongated and extends far back into the body. In nearly all the reptiles the amount of oxygen brought into the lungs is relatively large and the activity of the animal is proportionately great. The circulation of reptiles shows an advance be- yond that of the Amphibia. As in the latter, there are two distinct auricles ; but the chief difference arises from the fact that the ventricle is more or less divided by a par- tition which to a considerable degree prevents the blood returning from the lungs from mixing with the impure blood as it returns from its journey over the body. In the crocodiles and alligators the partition is complete, and the circulation thus approaches close to that of the higher animals. 186. Hibernation. — Attention has already been called to the fact that reptiles are very susceptible to cold, rapidly growing less active as the temperature lowers. When win- ter comes on they seek protected spots, and either alone or grouped together hibernate. The various activities of the body during this period are at very low ebb. The blood barely circulates, breathing is imperceptible, and stiii and insensible to the world about them they remain until the warmth again stirs them to their former activity. Some of our common turtles must also pass a somewhat similar sleep while embedded far down in the mud during the disappearance of the ponds in summer. At such times no food is taken, but owing to their loss in weight it is probable that a slow consumption of the body supplies the small amount of necessary energy. 187. Nervous system and sense-organs. — At first sight one is struck with the small size of the brain of fishes. Am- phibia, and reptiles. Their intelligence likewise is at low 198 ANIMAL FORMS ebb. Almost all the movements and operations of the body appear to be carried on by the animal with little apparent thought. Their acts, like most of the animals below them, are said to be instinctire ; yet they are sufladently well done to enable the animal to procure its food, avoid its enemies, and lead a successful life. As is true of other animals, the ability of the reptile to cope with its surroundings depends to a great extent upon the keenness of one or all of its or- gans of special sense. In the reptiles the sense of sight is perhaps sharpest, but there is considerable variation in this respect. Eyelids are present in all except the snakes, to- gether with a third, known as the nictitating membrane, a thin, transparent fold located at the inner angle of the eye, over which it is drawn with great rapidity. In the snakes eyelids are absent, giving the eye its characteristic stare. Furthermore, their sense of sight, except in a few tree-dwell- ing species, appears to be defective, the majority depending largely upon the sense of touch. In all the vertebrates a very peculiar organ known as the pineal gland or eye is situated on the roof of the brain. In several lizards its position is indicated by a trans- parent area in one of the plates of the head, and by an opening in the bones of the roof of the skull. In young reptiles, and especially in one of the New Zealand lizards {Hatteria, Fig. 119), its resemblance to an eye is decidedly striking. Lens, retina, pigment, cornea, are all present much as they are in some of the snails, but they finally degenerate more or less as the animal reaches maturity. It is a general belief that it represents the remnant of an organ of sight, a third eye, which looked out through the roof of the skull in some of the ancient vertebrates. With the possible exception of the few species of reptiles which produce sounds, probably to attract their mate, the sense of hearing is not particularly well developed. The senses of smell and taste are also comparatively feeble. The latter sense is located in the tongue, which is also popularly THE REPTILES 199 supposed to serve for the purpose of defense, and that it is in some way related to the poison-glands. This, however, is an error. The tongue is used primarily as an organ of . Fig. l\0,~Tuatt;ra (Sphenodon puTiCtaivs), touch, and in snakes especially it is almost continually darted in and out to determine the character of the animal's surroundings. 188. Egg-laying. — The eggs of the reptiles are relatively large and enclosed in a shell like a bird's egg, the shell, however, being leathery rather than made of lime. These are deposited in some warm situation, and generally left to themselves to hatch. Under stones, logs, and leaves, or buried lightly in the soil, are the positions most frequently chosen by the lizards and snakes. The turtles almost inva- riably select the warm sand at the edge of the water, and after scooping a hole lay several perfectly spherical eggs, usually at night. The alligators lay upward of a hundred eggs about the size of thoge of a goose, and guard them jealously until and even after they hatch. On the other hand, the young of many lizards and snakes are born alive, the eggs being hatched within the body. Many reptiles are surprisingly slow in attaining maturity, and live to an age attained by few other animals. It is a well-known fact that turtles live fully a hundred years, and 14 200 ANIMAL FORMS probably the same is true of the crocodiles and alligators and some of the larger snakes. Their enemies are few, and death usually results when the natural course is run. Throughout life all reptiles periodically shed their skin, as birds do their feathers and mammals their fur. In the snakes and some of the lizards the skin at the lips loosens, and the animal gradually slips out of its old slough, bright and glossy in the new one which previously developed. In the others the old skin hangs on in tatters, gradually com- ing away as they scamper through the grass. CHAPTEK XVII THE BIRDS 189. Characteristics. — Birds form one of the most sharp- ly defined classes in the animal kingdom, and the variations among the different species are relatiyely small. " The ostrich or emu and the raven, for example, which may be said to stand at opposite ends of the series, present no such anatomical differences as may he found between a common lizard and a chameleon, or between a turtle and a tortoise,'' and these we know to be relatively slight. In many respects the birds resemble the reptiles, and long ago in the world's history the relationship was much closer than now, as we know from certain fossil remains in this country and in Europe. One of the earliest of these fossil birds, that of the Archasopteryx, is a most remarkable combination of bird and lizard. Unlike any modern bird, the jaws were provided with many conical reptile-like teeth. The wings were rather small, and the fingers, tipped with claws, were distinct, not grown together, as in modern birds. The tail was as long as the body, and many-jointed, like a lizard's, each vertebra carrying two long feathers. The bird was about the size of a crow, and it probably could not fly far. Other ancient types have been discovered — principally sea-birds — many of which existed when the Pacific extended over the region now occupied by the Rocky Mountains. These were all of the same generalized type, intermediate between reptile and bird. This fact leads us to the belief that birds descended from reptilian 301 202 ANIMAL POEMS ancestors, and in becoming more perfectly adapted for an aerial life have developed into our modern forms. In the modern birds the most important peculiarities, those which separate them from all other animals, are correlated with the power of flight. The body is spindle- shaped, for readily cleaving the air. The fore limbs serve as wings. The hind limbs, supporting the weight of the body from the ground, are usually well developed. A series of air-chambers usually exists in powerful fliers. This serves a purpose analogous to that of the air-bladder of a fish, giving buoyancy. But the most characteristic mark of a bird, as above stated, is its feathers, universally present and never found outside the class. Like the scales of lizards, and probably derived from similar structures, they are of different forms, and serve a variety of purposes. The larger ones, with powerful shafts, and forming the tail, act as a rudder. Those of the wings give great expanse with but little increase in weight, and are so constructed that upon the down-stroke they offer great resistance to the air, and push the bird forward, while in the reverse direction the air slips through them readily. In flight these movements of the wing may be too rapid for us to follow, as in the humming-birds, though they are usually much slower, two to five hundred a minute in many power- ful fliers, such as the ducks, and frequently long-continued enough to carry them many hundreds of miles at a single flight. The remaining feathers are soft and downy, giving roundness to the body and enabling it to cleave the air with greater ease, and, being poor conductors of heat, they aid in keeping the body at the high temperature characteristic of birds. In most birds the body is not uniformly clothed in feathers. Naked spaces, usually hidden, intervene between the feather tracts, and on the feet tod toes scales exist. 190. Moltii^. — As we all know, the growth of feathers, unlike that of hair and nails, is limited, and after they have become faded and worn out they are shed, and new ones THE BIRDS 203 arise to take their place. This process of molting is usually accomplished gradually, without diminishing the powers of flight ; but in the ducks and some other birds all the wing- and tail-feathers drop out simultaneously, leaying the bird to escape its enemies by swimming and diving. The molting-process usually takes place in the fall, after the nesting and care for the young is over, and often when the need for a heavy winter coat commences to be felt. Many birds also don what are called courting colors, ruSs, crests, and highly colored patches, in the spring, previous to the mating season, doubtless for the purpose of attract- ing or impressing their mates. In other cases the change appears to be related to the bird's surroundings. A most beautiful example of this is the ptarmigans — grouse-like birds living far to the north. During winter they are per- fectly white and are almost invisible against the snow ; but in the spring, as the snow disappears, the white feathers gradually fall out and new ones arise. The latter so har- monize "with the lichen-colored stones among which it delights to sit, that a person may walk through a flock of them without seeing a single bird." There are also numerous birds, chiefly those that go in flocks, which possess what are known as color-calls or recog- nition-marks. These may consist of various conspicuous spots or blotches on different parts of the head or trunk, such as we see in the yellowhammer or meadow-lark ; or one or more feathers of the wings or tail may be strikingly colored, as in many sparrows and warblers. During the time the bird remains at rest these usually are concealed under neighboring feathers, but during flight they are strikingly displayed. It may possibly be true, as many have urged, that these color-signals are for the purpose of enabling various members of the flock to readily follow their leader ; but this and many other interesting questions regarding the color of birds and other animals have not yet received final answers. 20i ANIMAL POEMS In very many animals, fishes as well as birds, the tints on the under side of the body are usually relatively light colored, shading gradually into a darker tint above. This is in all probability a protective device, as was recently shown by Mr. A. H. Thayer, an American artist. His ex- periments show that the light from above renders the back less dark, and that the shadow beneath is neutralized by the light color. The bird thus appears uniformly lighted, and this efEect, together with streaks and blotches, renders them invisible at surprisingly short distances. ■ 191. Skeleton. — Turning now to the internal organization of birds, we find many points in common with other verte- brates, especially the reptiles, but many interesting modifi- cations are also present that adapt them for fiying and for collecting their food. According to the nature of the food, the beak may have a great variety of forms. The skull may be thick and heavy, or thin and fragile, but these are mat- ters of proportion of the various parts possessed by all birds. The neck also is of differing length ; but it is in the trunk region that the greatest changes have arisen, as we may see in any of our ordinary birds. For example, the vertebrae of this part of the body are more or less fused together into rigid framework, to which are attached the ribs that in ■ turn unite with the breast-bone. In the fliers the latter bears a vertical plate or keel, to which the great muscles that move the wings are attached. The tail con- sists, like that of the old-fashioned birds, of several verte- brae, but these are of small size and fused together into a little knob that supports the tail-feathers. The fore limbs are used for flight, but there are the same bones that exist in the fore limbs of other vertebrates — one for the upper arm, two for the lower, a thumb carrying a few feathers, and known as the bastard wing, and indications of several bones that form the hand. In the hind limb the resem- blance is equally apparent, though its different parts are of relatively large size to support the body. It is interest- THE BIRDS 205 ing to note that the knee has been drawn far up into the body, and that the joint above the foot is in reality the ankle. We thus see that the bird's skeleton presents the same general plan as that of the lizard, for example ; but in order to combine the elements of strength, lightness, and com- pactness essential to successful flight, it has been necessary to remodel it to a considerable degree. 193. Other internal structures. — The lungs of birds con- sist of two dark-red organs buried in the spaces between the ribs along the back. Each communicates with extensive thin-walled air-sacs extending into the space between the ' '«'1J!V Hllbffl ii i '\\m\m ii'ii'imim!' .Illl| I P I i I'illl!' ' Il 111 11 'IWIilli I ILIllli lil!!!il||l I nil, ,n iiiii 11 I' n , V' N ,' l'l"''»l'ii^W 'Il nil. 1'*'^ i'i i II 1 I ' '' " 111 1 I I 'iii;'iiV«;'ls' 1, ! 1 I'iilf It! ^I^^^^^^^^^^^^^^^i^^l^ r , nil Il 11 1 1"' ' ii il ii fi' '■- ri'ill'ill'ili' nl'i III i 1 Ml If I HI ' i 1 |ll 1 1 ' i' L. ll«« 1 UN 1 ' !ii ,11' 11 1 ill ' Il ii li'V ;iji 1 I' HH vhim 'i' -liii'ii f ■"'''' .iJjJi! 1,1,1111,1 1 11' 1 1 ! 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