liini i " I ^ , ^OF-CAIIFO S £l *Mf UDKMrt I't^r i % i § ^tUNIVER ryomwi$> .v>:llft-ANGafj:> ^t-UBRARY COMPARATIVE ZOOLOGY STRUCTURAL AND SYSTEMATIC FOR USE IN SCHOOLS AND COLLEGES BY JAMES ORTON, A.M., PH.D. LATE PROFESSOR OF NATURAL HISTORY IN VASSAR COLLEGE; CORRESPONDING OF THE ACADEMY OF NATURAL SCIENCES, PHILADELPHIA, AND OF THB LYCEUM OF NATURAL HISTORY, NEW YORK; AUTHOR OF "THE ANDES AND THE AMAZON," ETC. REVISED EDITION " The education of a naturalist now consists chiefly in learning how to compare.'1'1 — AGASSIZ NEW YORK HARPER & BROTHERS, FRANKLIN SQUARE 1886 62668 Entered according to Act of Congress, in the year 1876, by HARPER & BROTHERS, In the Office of the Librarian of Congress, at Washington. Copyright, 1883, by HARPER & BROTHKRS. QL-4 0 PREFACE. THE distinctive character of this work consists in the treatment of the whole Animal Kingdom as a unit; in the comparative study of the development and variations ^ of organs and their functions, from the simplest to the <\ most complex state ; in withholding Systematic Zoology until the student has mastered those structural affinities upon which true classification is founded ; and in being fitted for High Schools and Mixed Schools by its lan- s^ guage and illustrations, yet going far enough to constitute v a complete grammar of the science for the undergraduate course of any College. It is designed solely as a manual for instruction. It is not a work of reference, nor a treatise. So far as a book v is encyclopedic, it is unfit for a text-book. This is pre- v <^ pared on the principle of "just enough, and no more." ^ It aims to present clearly, and in a somewhat new form, k the established facts and principles of Zoology. All the- oretical and debatable points, and every fact or statement, however valuable, which is not absolutely necessary to a clear and adequate conception of the leading principles, are omitted. It is written in the light of the most recent phase of the science, but not in the interest of any par- ticular theory. To have given an exhaustive survey of animal life would have been not only undesirable, but impossible. Even Cuvier's great work must be supple- Jv PREFACE. raented by museums, monographs, and microscopes. Nat- ural History has outgrown the limits of a single book. Trial has proved the folly of giving the student so many things to learn that he has no time to understand, and the error of condemning the student to expend his strength upon the details of classification, which may change in the coming decade, instead of upon structure, which is permanent. Of course, specialists will miss many things, and find abundant room for criticism in what they regard as deficiencies ; but the work should be judged by what it does contain, rather than by what it does not. What is claimed, in the' language of inventors, is the selection and arrangement of essential principles and typical illustrations from the standpoint of the teacher. The synthetic method is employed, as being the most natural : to begin with complex Man, instead of the sim- plest forms, would give a false idea. Man is not a model, but a monstrosity, the most modified of Vertebrates. But these outlines must be filled up, on the part of the teacher, by lectures, and by the exhibition of specimens; and, on the part of the student, by observation (noting, above all, the characteristic habits of animals), and by per- sonal work with the knife and microscope. No text-book caa take the place of nature, or supersede oral instruction from a competent teacher. Suggestions and corrections from naturalists and teach- ers will be thankfully received. In a work of this character, which is but a compound of the labors of all naturalists, it would be superfluous to make acknowledgments. The works referred to on page 397 have been specially consulted. REVISER'S NOTE. IN revising the work of Professor Orton, the writer has not attempted to rewrite the book nor to introduce new ideas. His plan has been to insert such changes as the author would have been likely to make if he had lived to revise his book. On only two points has the reviser de- parted from this plan of altering only minor details. The chapter on Development has been largely rewritten, and the classification of the Invertebrates has been changed so as to separate the worms from the Arthropoda and the sponges from the Protozoa. In both these cases the change seemed imperatively demanded by the progress of Zoology in those directions. It is hoped that the altera- tions in the book will increase its accuracy and useful- ness. EDW. A. BIKGE. UNIVERSITY OF WISCONSIN. CONTENTS. INTRODUCTION. RMB Definition of Zoology, and its Place among the Sciences 11 Historical Sketch... .. 14 PART I.— STRUCTURAL ZOOLOGY. CHAPTER I. MINERALS AND ORGANIZED BODIES DISTINGUISHED 19 CHAPTER II. PLANTS AND ANIMALS DISTINGUISHED 21 CHAPTER III. RELATION BETWEEN MINERALS, PLANTS, AND ANIMALS 27 CHAPTER IV. LIFE 28 CHAPTER V. ORGANIZATION 30 1. Cells 31 2. Tissues. 32 3. Organs, and their Functions 41 CHAPTER VI. NUTRITION 45 CHAPTER VII. THB FOOD OF ANIMALS . . 47 CONTENTS. CHAPTER VIII. PAGI How ANIMALS EAT 49 1. The Prehension of Food 49 2. The Mouths of Animals 55 3. The Teeth of Animals 63 4. Deglutition, or How Animals Swallow 72 CHAPTER IX. THE ALIMENTARY CANAL 74 CHAPTER X. How ANIMALS DIGEST 91 CHAPTER XI. THE ABSORBENT SYSTEM •.. 94 CHAPTER XII. THE BLOOD OF ANIMALS 97 CHAPTER XIII. THE CIRCULATION OF THE BLOOD 103 CHAPTER XIV. How ANIMALS BREATHE Ill CHAPTER XV. SECRETION AND EXCRETION 121 CHAPTER XVI. THE SKIN AND SKELETON 127 CHAPTER XVII. How ANIMALS MOVE 154 1. Muscle. 154 2. Locomotion 157 CHAPTER XVIII. THE NERVOUS SYSTEM 166 1. The Senses 176 2. Instinct and Intelligence 184 3. The Voices of Animals 188 CONTENTS. ix CHAPTER XIX. PAG« REPRODUCTION 191 CHAPTER XX. DEVELOPMENT 197 1. Metamorphosis 207 2. Alternate Generation 211 3. Growth and Repair 214 4. Likeness and Variation 215 5. Homology, Analogy, and Correlation 217 6. Individuality : 220 7. Relations of Number, Size, Form, and Rank 221 8. The Struggle for Life 226 PART II.— SYSTEMATIC ZOOLOGY. CHAPTER XXI. THE CLASSIFICATION OF ANIMALS 231 Protozoa 238 Spongida 244 Coelenterata '. 246 Echinodermata 257 Vermes 263 Mollusca 269 Artliropoda 281 Tunicata 305 Vertebrata 306 CHAPTER XXII. SYSTEMATIC ARRANGEMENT OF REPRESENTATIVE FORMS 362 CHAPTER XXIII. THE DISTRIBUTION OF ANIMALS 371 NOTES 381 THE NATURALIST'S LIBRARY 397 INDEX.. .. 399 The first thing to be determined about a new specimen is not its name, but its most prominent character. Until you know an animal, care not for its name. — AGASSIZ. The great benefit which a scientific education bestows, whether as train- ing or as knowledge, is dependent upon the extent to which the mind of the student is brought into immediate contact with facts — upon the degree to which he learns the habit of appealing directly to Nature. — HUXLEY. INTRODUCTION. 1. Definition of Zoology, and its Place among the Sciences. — The province of Natural History is to describe, compare, and classify natural objects. These objects have been divided into the " organic " and the " inorganic," or those which are, and those which are not, the products of life. Biology is the science of the former, and Mineralogy the sci- ence of the latter. Biology again separates into Botany, or the Natural History of Plants, and Zoology, or the Natural His- tory of Animals ; while Mineralogy divides into Mineralogy proper, the science of mineral species, and Lithology, the science of mineral aggregates or rocks. Geology is that com- prehensive knowledge of the earth's structure and develop- ment which rests on the whole doctrine of Natural History. If we examine a piece of chalk, and determine its physical and chemical characters, its mode of occurrence and its uses, so as to distinguish it from all other forms of matter, we have its Mineralogy. But chalk occurs in vast natural beds : the examination of these masses — their origin, structure, po- sition, and relation to other rocks — is the work of the Li- thologist. Further, we observe that while chalk and marble are chemically alike, they widely differ in another respect. Grinding a piece of chalk so thin that we can see through it, and putting it under a microscope, we find imbedded in it innumerable bodies, about the hundredth of an inch in diame- ter, having a well-defined, symmetrical shape, and chambered like a Nautilus. We cannot say these are accidental aggre- gations, nor are they crystals : if the oyster-shell is formed by an oyster, these also must be the products of life. In- deed, the dredge brings up similar microscopic skeletons from the bottom of the Atlantic. So we conclude that chalk is but the dried mud of an ancient sea, the cemetery of count- 12 INTRODUCTION. less animals that lived and died long ago. The considera- tion of their fossil remains belongs to Paleontology, or that part of Biology which describes the relics of extinct forms of life. To study the stratigraphical position of the chalk- bed, and by the aid of its Paleontology to determine its age and part in the world's history, is the business of Geology. Of all the sciences, Zoology is the most extensive. Its field is a world of varied forms — hundreds of thousands in number. To determine their origin and development, their structure, habits, distribution, and mutual relations, is the work of the Zoologist. But so many and far-reaching are the aspects under which the animal creation may be contem- plated, that the general science is beyond the grasp of any single person. Special departments have, therefore, arisen ; and Zoology, in its comprehensive sense, is the combined re- sult of the labors of many workers, each in his own line of research. Structural Zoology treats of the organization of animals. There are two main branches : Anatomy, which considers the constitution and construction of the animal frame ; and Physiology, which is the study of the apparatus in action. The former is separated into Embryology, or an account of the successive modifications through which an animal passes in its development from the egg to the adult state ; and Morphology, which includes all inquiries concerning the form of mature animals, or the form and arrangement of their or- gans. The microscopical examination of any part, especial- ly the tissues, belongs to Histology. Comparative Zoology is the comparison of the anatomy and physiology of all ani- mals, existing and extinct, to discover the fundamental like- ness underneath the superficial differences, and to trace the adaptation of organs to the habits and spheres of life. It is this comparative science which has led to such grand gen- eralizations as the unity of structure amidst the diversity of form in the animal creation, and by revealing the degrees of affinity between species has enabled us to classify them in natural groups, and thus laid the foundation of Systematic Zoology. When the study of structure is limited to a par- ticular class or species of animals, or to a particular organ or part, monographic sciences are created, as Ornithotomy, INTRODUCTION. 13 or anatomy of birds ; Osteology, or the science of bones ; and Odontography, or the natural history of teeth. Systematic Zoology is the classification or grouping of ani- mals according to their structural and developmental rela- tions. The systematic knowledge of the several classes, as Insects, Reptiles, and Birds, has given rise to subordinate sciences, like Entomology, Herpetology, or Ornithology.1* Distributive Zoology is the knowledge of the successive ap- pearance of animals in the order of time (Paleontology in part), and of the geographical and physical distribution of animals, living or extinct, over the surface of the earth. Theoretical Zoology includes those provisional modes of grouping facts, and interpreting them, which still stand waiting at the gate of science. They may be true, but we cannot say that they are true. The evidence is incomplete. Such are the theories which attempt to explain the origin of life and the origin of species. Suppose we wish to understand all about the Horse. Our first object is to study its structure. The whole body is en- closed within a hide, a skin covered with hair ; and if this hide be taken off, we find a great mass of flesh or muscle, the substance which, by its power of contraction, enables the animal to move. On removing this, we have a series of bones, bound together with ligaments, and forming the skel- eton. Pursuing our researches, we find within this frame- work two main cavities : one, beginning in the skull and running through the spine, containing the brain and spinal marrow ; the other, commencing with the mouth, contains the gullet, stomach, intestines, and the rest of the apparatus for digestion, and also the heart and lungs. Examinations of this character would give us the Anatomy of the Horse, or, more precisely, Hippotomy. The study of the bones alone would be its Osteology ; the knowledge of the nerves would belong to Neurotomy. If we examined, under the microscope, the minute structure of the hair, skin, flesh, blood, and bone, we would learn its Histology. The consid- eration of the manifold changes undergone in developing from the egg to the full-grown animal, would be the Embry- * Numbers like this refer to the Notes at the end of the volume. 14: INTRODUCTION. ology of the Horse ; and its Morphology, the special study of the form of the adult animal and of its internal organs. Thus far we have been looking, as it were, at a steam- engine, with the fires out, and nothing in the boiler ; but the body of the living Horse is a beautifully formed, active ma- chine, and every part has its different work to do in the working of that machine, which is what we call its life. The science of such operations as the grinding of the food in the complex mill of the mouth ; its digestion in the labo- ratory of the stomach ; the pumping of the blood through a vast system of pipes over the whole body ; its purification in the lungs ; the process of growth, waste, and repair ; and that wondrous telegraph, the brain, receiving impressions, sending messages to the muscles, by which the animal is en- dowed with voluntary locomotion — this is Physiology. But Horses are not the only living creatures in the world ; and if we compare the structures of various animals, as the Horse, Zebra, Dog, Monkey, Eagle, and Codfish, we shall find more or less resemblances and differences, enough to enable us to classify them, and give to each a description which will dis- tinguish it from all others. This is the work of Systematic Zoology. Moreover, the Horses now living are not the only kinds that have ever lived ; for the examination of the earth's crust — the great burial-ground of past ages — reveals the bones of numerous horse-like animals : the study of this pre-adamite race belongs to Paleontology. The chronologi- cal and geographical distribution of species is the depart- ment of distributive Zoology. Speculations about the ori- gin of the modern Horse, whether by special creation, or by development from some allied form now extinct, are kept aloof from demonstrative science, under the head of Theo- retical Zoology. 2. History. — The Greek philosopher Aristotle (B.C. 384- 322) is called the " Father of Zoology." Certainly, he is the only great representative in ancient times, though his fre- quent allusions to famili%r works on anatomy show that something had been done before him. His " History of Animals," in nine books, displays a wonderful knowledge of external and internal structure, habits, instincts, and uses. His descriptions are incomplete, but generally exact, so far INTRODUCTION. 15 as they go. Alexander, it is said, gave him nine hundred talents to collect materials, and put at his disposal several thousand men, for hunting specimens and procuring infor- mation. The Romans accomplished little in natural science, though their military expeditions furnished unrivalled opportuni- ties. Nearly three centuries and a half after Aristotle, Pliny (A.D. 23-79) wrote his "Natural History." He was a volu- minous compiler, not an observer : he added hardly one new fact. He states that his work was extracted from over two thousand volumes, most of which are now lost. During the Middle Ages, Natural History was studied in the books of the ancients ; and at the close of the fifteenth century it was found where Pliny had left it, with the addi- tion of many vague hypotheses and silly fancies. Albertus Magnus, of the thirteenth century, and Conrad Gesner and Aldrovandus, of the sixteenth, were voluminous writers, not naturalists. In the latter half of the sixteenth century, men began to observe nature for themselves. The earliest note- worthy researches were made on Fishes, by Rondelet (1507- 1556) and Belon (1517-1564), of France, and Salviani (1514- 1572), of Italy. They, were followed by valuable observa- tions upon Insects, by Redi (1626-1698), of Italy, and Swam- merdam (1637-1680), of Holland ; and towards the end of the same century, the Dutch naturalist, Leeuwenhoeck (1632-1723), opened a new world of life by the use of the microscope. But there was no real advance of Systematic Zoology till the advent of the illustrious John Ray (1628-1705), of Eng- land. His " Synopsis," published in 1693, contained the first attempt to classify animals according to structure. Ray was the forerunner of "the immortal Swede," Linnaeus (1707— 1778), "the great framer of precise and definite ideas of natural objects, and terse teacher of the briefest and clearest expressions of their differences." His chief merit was in de- fining generic groups, and inventing specific names.* Scarce- ly less important, however, was the impulse which he gave to the pursuit of Natural History. The spirit of inquiry, which his genius infused among the great, produced voyages of research, which led to the formation of national museums. 16 INTRODUCTION. The first expedition was sent forth by George III. of Eng- land, in 1765. Reaumur (1683-1757) made the earliest zoological collection in France ; and the West Indian col- lections of Sir Hans Sloane (1660-1752) were the nucleus of the British Museum. The accumulation of specimens sug- gested comparisons, which eventually resulted in the high- est advance of the science. The brilliant style of Buff on (1707-1788) made Zoology popular not only in France, but throughout Europe. While the genius of Linnaeus led to classification, that of Buffon lay in description. He was the first to call attention to the subject of Distribution. Lamarck (1745-1829), of Paris, was the next great light. The publication of his " Animaux sans Vertebres," in 1801, was an epoch in the history of the lower animals. He was also the first prominent advocate of the transmutation of species. But the brightest luminary in Zoology was George Cuvier (1769-1832), a German, born on French soil. Before his time, " there was no great principle of classification. Facts were accumulated, and more or less systematized, but they were not yet arranged according to law ; the principle was still wanting by which to generalize them and give meaning and vitality to the whole." It was Cuvier who found the key. He was the first so to interpret structure as to be able from the inspection of one bone to reconstruct the entire animal, and assign its position. His anatomical investiga- tions revealed the natural affinities of animals, and led to the grand generalization, that the most comprehensive groups in the kingdom were based, not on special characters, but on different plans of structure. Palissy had long ago (1580) asserted that petrified shells were of animal origin ; but the publication of Cuvier's " Memoir on Fossil Elephants," in 1800, was the beginning of those profound researches on the remains of ancient life which created Paleontology. The discovery of the true relation between all animals, living and extinct, opened a boundless field of inquiry ; and from that day the advance of Zoology has been unparalleled. Special studies of particular parts or classes of animals have so rapidly developed, that the history of Zoology during the last fifty years is the history of many sciences.3 I. STRUCTURAL ZOOLOGY, COMPARATIVE ZOOLOGY. CHAPTER I. MINERALS AND ORGANIZED BODIES DISTINGUISHED. Nature may be separated into two great kingdoms — that of mere dead matter, and that of matter under the influence of life.4 These differ in the following points : ( 1 ) Composition. — While most of the chemical elements are found in different living beings, by far the greater part of their substance is composed of three or four — car- bon, oxygen, and hydrogen ; or these three with the addi- tion of nitrogen. Next to these elements, sulphur and phosphorus are most widely distributed, though always found in very small quantities. The organic compounds belong to the carbon series, and contain three, four, or five elements. The former class, comprising starch, sugar, fat, etc., are relatively stable. The latter, possessing the three elements named, with nitrogen and sulphur or phos- phorus, are very complex, containing a very large number of atoms to the molecule, and are usually unstable. Here belong albumen, myosin, chondrin, etc., the constituents of the living tissues. The formula for albumen is said to be C72H112N18SO22, or some multiple of this formula. These compounds also contain more or less water, and usually exist in a jelly-like condition, neither solid nor fluid. With these colloid substances alone is life associ- ated. Only these can undergo the rapid decomposition 20 COMPARATIVE ZOOLOGY. and recornposition necessary to the manifestation of the vital phenomena. (2) structure. — Minerals are homogeneous, while organ- ized bodies are usually heterogeneous ; i. e., composed of different parts, called tissues and organs, having peculiar uses and definite relations to one another. The tissues and organs, again, are heterogeneous, consisting mainly of microscopic cells, structures developed only by vital ac- tion. All the parts of an organism are mutually depend- ent, and reciprocally means and ends, while each part of a mineral exists for itself. The smallest fragment of mar- ble is as much marble as a mountain-mass; but the frag- ment of a plant'or animal is not an individual. (3) Size and Shape. — Living bodies gradually acquire de- terminate dimensions; so do minerals in their perfect or crystal condition. But uncrystallized, inorganic bodies have an indefinite bulk. Most minerals are amorphous; crystals have regular forms, bounded, as a rule, by plane surfaces and straight lines ; plants and animals are cir- cumscribed by curved surfaces, and rarely assume accurate geometrical forms.* (4) Phenomena. — Minerals remain internall}7 at rest, and increase by external additions, if they grow at all. Liv- ing beings are constantly changing the matter of which they are composed, and grow by taking new matter into themselves and placing it among the particles already present. Organized bodies, moreover, pass through a cy- cle of changes — growth, development, reproduction, and death. These phenomena are characteristic of living as opposed to inorganic bodies. All living bodies grow from within, constantly give up old matter and replace it by new, reproduce their kind, and die; and no inorganic body shows any of these phenomena. PLANTS AND ANIMALS DISTINGUISHED. 21 CHAPTER II. PLANTS AND ANIMALS DISTINGUISHED. IT may seem an easy matter to draw a line between plants and animals. Who cannot tell a Cow from a Cab- bage? Who would confound a Coral with a Mushroom ? Yet it is impossible to assign any absolute, distinctive character which will divide the one mode of life from the other. The difficulty of defining an animal increases with our knowledge of its nature. Linnaeus defined it in. three words ; a century later, Owen declared that a defi- nition of plants which would exclude all animals, or of animals which would not let in a single plant, was impos- sible. Each different character used in drawing the boun- dary will bisect the debatable ground in a different lati- tude of the organic world. Between the higher animals and higher plants the difference is apparent; but when we reflect how many characters the two have in common, and especially when we descend to the lower and minuter forms, we discover that the two "kingdoms" touch, and even dissolve into, each other. This border-land has been as hotly contested among naturalists as many a disputed frontier between adjacent nations. Its inhabitants have been taken and retaken several times by botanists and zoologists; for they have characters that lead on the one side to plants, and on the other to animals. To solve the difficulty, some eminent naturalists, as Hackel and Owen, propose a fourth " kingdom," to receive those living be- ings which are organic, but not distinctly vegetable or animal. But a greater difficulty arises in attempting to fix its precise limits. 22 COMPARATIVE ZOOLOGY. The drift of modern research points to this : that there are but two kingdoms of nature, the mineral and the or- ganized, and these closely linked together; that the lat- ter must be taken as one whole, from which two great branches rise and diverge. " There is at bottom but one life, which is the whole life of some creatures, arid the common basis of the life of all ; a life of simplest moving and feeling, of feeding and breathing, of producing its kind and lasting its day: a life which, so far as we at present know, has no need of such parts as we call organs. Upon this general foundation are built up the manifold special characters of animal and vegetable existence ; but the tendency, the endeavor, so to speak, of the plant is one, of the animal is another, and the unlikeness between them widens the higher the building is carried up. As we pass along the series of either [branch] from low to high, the plant becomes more vegetative, the animal more animal." 6 Defining animals and plants by their prominent char- acteristics, we may say that a living being which has cell- walls of cellulose, and by deoxidation and synthesis of its simple food-stuffs produces the complicated organic sub- stances, is a plant ; while a living being which has albu- minous tissues, and by oxidation and analysis reduces its complicated food-stuffs to a simpler form, is an animal. But both definitions are defective, including too many forms, and excluding forms that properly belong to the respective kingdoms. No definition is possible which shall include all animals and exclude all plants, or vice (i) Origin. — Both branches of the tree of life start alike : the lowest of plants and animals, as Protococcus and Gre- garina, consist of a single cell. In fact, the cycle of life in all living beings, high or low, begins in a small, round particle of matter — in plants called an ovule, in animals PLANTS AND ANIMALS DISTINGUISHED. 23 an ovum. This cell contains a semi-fluid, called proto- plasm, similar in composition and in function. In the very simplest forms the protoplasm is not enclosed by a membrane, but generally there is a cell-wall. In plants, with few exceptions, this wall is of cellulose, a substance akin to starch; in animals, with few exceptions, the wall is a pellicle of firmer protoplasm, i. e., albuminous. (2) Composition. — Modern research has broken down the partition between plants and animals, so far as chemical nature is concerned. The vegetable fabric and secretions may be ternary or binary compounds ; but the essential living parts of plants, as of animals, are quaternary, con- sisting of four elements — carbon, hydrogen, oxygen, and nitrogen. Cellulose (woody fibre), starch, and chlorophyl (green coloring matter) are eminently vegetable products, but not distinctive; for cellulose is wanting in some plants, as some Fungi, and present in some animals, as Tunicates; starch, under the name of glycogen, is found in the liver and brains of Mammals, and chlorophyl gives color to the fresh - water Polyp. Still, it holds good, generally, that plants consist mainly of cellulose, dextrine, and starch; while animals are mainly made up of albumen, fibrine, and gelatine ; that nitrogen is more abundant in animal tissues, while in plants carbon is predominant. (3) Form. — No outline can be drawn which shall be com- mon to all animals or all plants. The lowest members of both have no fixed shape. The spores of Confervae can hardly be distinguished from animalcules; the compound and fixed animals, Sea-mat and Sea-moss (Polyzoa), and Corals, often resemble vegetable forms, although in struct- ure widely removed from plants. Similar conditions of life are here accompanied by an external likeness. In free-living animals this resemblance is not found. (4) Structure. — A plant is the multiplication of the unit — a cell with a cellulose wall. Some simple animals have 24 COMPARATIVE ZOOLOGY. a similar simple cellular structure; and all animal tissues, while forming, are cellular. But this character, which is permanent in plants, is generally transitory in animals. In the more highly organized tissues the cells are so united as partly or wholly to lose their individuality, and the characteristic part of the tissue is the intercellular sub- stance, while the cells themselves are small and unimpor- tant, or else the cells are melted together and lose their dividing walls, as in striped muscles and in nerves. Ex- cepting the lowest forms, animals are more composite! than plants, i. £., their organs are more complex and numerous, and more specially devoted to particular purposes. Rep- etition of similar parts is a characteristic of plants ; and when found in animals, as the Angle-worm, is called vege- tative repetition. Differentiation and specialization are characteristic of animals. Most animals, moreover, have fore-and-aft polarity ; in contrast, plants are up-and-down structures, though in this respect they are imitated by radiated animals, like the Star-fish. Plants are continually receiving additional members; most animals soon become perfect. (5) Physiology. — In their modes of nutrition, plants and animals stand widest apart. A plant in the seed and an animal in the egg exist in similar conditions : in both cases a mass of organic matter accompanies the germ. When this supply of food is exhausted, both seek nourish- ment from without. But here analogy ends: the plant feeds on mineral matter, the animal on organic. Plants have the power to form chlorophyl, the green coloring matter of leaves, which uses the force of the sunlight to form starch out of the inorganic substances — carbon-di- oxide and water. They are able also to form albuminoid matter out of inorganic substances. A very few animals which have a substance identical with or allied to chloro- phyl have the same power/ but in general animals are de- PLANTS AND ANIMALS DISTINGUISHED. 25 pendent for their food on the compounds put together in plants. Colorless plants, possessing no chlorophyl, feed, like animals, on organic compounds. No living being is able to combine the simple elements — carbon, oxygen, hy- drogen, and nitrogen — into organic compounds. The food of plants is gaseous (carbon dioxide and am- monia) or liquid (water), that of animals usually more or less solid. The plant, then, absorbs these foods through its outer surface, while the animal takes its nourishment in larger or smaller masses, and digests it in a special cav- ity. A few exceptions, however, occur on both sides. Certain moulds seem to swallow their food, and certain animals, as the tape-worm, have no digestive tract. Plants are ordinarily fixed, their food is brought to them, and a large share of their work, the formation of organic compounds, is done by the force of the sunlight; while animals are usually locomotive, must seek their food, and are unable to utilize the general forces of nature as the plant does. The plant is thus able to grow much more than the animal, as very little of the nourishment received is used to repair waste, while in most animals the time soon comes when waste and repair are approximately equal. But in both all work done is paid for by waste of substance already formed. In combining carbon dioxide and water to form starch the plant sets oxygen free (6(COa) + 5(HaO) = C6H10O6 + 6(O2)): in oxidizing starch or other food the animal uses oxygen and sets carbon dioxide free. The green plant in the sunlight, then, gives off oxygen and uses carbon diox- ide, while plants which have no chlorophyl, at all times, and all plants in the darkness, use oxygen and give off carbon dioxide, like an animal. Every plant begins life like an animal — a consumer, not a producer : not till the young shoot rises above the soil, and unfolds itself to the light of the sun, at the touch of whose mystic rays chlo- 26 COMPARATIVE ZOOLOGY. rophyl is created, does real, constructive vegetation begin ; then its mode of life is reversed — carbon is retained and oxygen set free. Most plants, and many animals, multiply by budding and division ; on both we practise grafting ; in both the cycle of life comes round again to the ovule or ovum. Do annuals flower but to die? Insects lay their eggs in their old age. Both animals and plants have sensibility. This is one of the fundamental physiological properties of proto- plasm. But in plants the protoplasm is scattered and buried in rigid structures: feeling is, therefore, dull. In animals, the protoplasm is concentrated into special or- gans, and so feeling, like electricity rammed into Leyden jars, goes off with a flash.8 Plants never possess conscious- ness or volition, as the higher animals do. The self-motion of animals and the rooted state of plants is a very general distinction ; but it fails where we need it most. It is a characteristic of living things to move. The protoplasm of all organisms is unceasingly active.9 Be- sides this internal movement, myriads of plants, as well as animals, are locomotive. Rambling Diatoms, writhing Oscillaria, and the agile spores of Cryptogams crowd our waters, their instruments of motion (cilia) being of the very same character as in microscopic animals ; while Sponges, Corals, Oysters, and Barnacles are stationary. A contractile vesicle is not exclusively an animal prop- erty, for the fresh -water Yolvox and Gonium have it. The act of muscular contraction in the highest animal is due to the same kind of change in the form of the cells of the ultimate fibrillae as that which produces the sensible motions of plants. The ciliary movements of animals and of microscopic plants are precisely similar, and in neither case indicate consciousness or self -determining power. RELATION BETWEEN MINERALS, PLANTS, ETC. 27 Plants, as well as animals, need a season of repose. Both have their epidemics. On both, narcotic and acrid poisons produce analogous results. Are some animals warm - blooded ? In germination and flowering, plants evolve heat — the stamens of the Arum, e. g., showing a rise of 20°. In a sense, an Oak has just as much heat as an Elephant, only the miserly tree locks up the sunlight in solid carbon. At present, any boundary of the Animal Kingdom is arbitrary. " Probably life is essentially the same in the two kingdoms ; and to vegetable life, faculties are super- added in the lower animals, some of which are, here and there, not indistinctly foreshadowed in plants." " It must be said that there are organisms which at one period of their life exhibit an aggregate of phenomena such as to justify us in speaking of them as animals, while at another they appear to be as distinctly vegetable.10 CHAPTER III. RELATION BETWEEN MINERALS, PLANTS, AND ANIMALS. THERE are no independent members of creation : all things touch upon one another. The matter of the living world is identical with that of the inorganic. The plant, feeding on the minerals, carbon dioxide, water, and am- monia, builds them up into complex organic compounds, as starch, sugar, gum, cellulose, albumen, fibrine, caseine, and gluten. When the plant is eaten by the animal, these substances are used for building up tissues, repairing waste, laid up in reserve as glycogen and fat, or oxi- dized in the blood to produce heat. The albuminoids are essential for the formation of tissues, like muscle, nerve, 28 COMPARATIVE ZOOLOGY. cartilage ; but the ternary compounds help in repairing waste, while both produce heat. When oxidized, whether for work or warmth, these complex compounds break up into the simple compounds — water, carbon dioxide, and (ultimately) ammonia, and as such are returned to earth and air from the animal. Both plant and animal end their life by going back to the mineral world: and thus the circle is complete — from dust to dust. Carbonate of ammonia and water, a blade of grass and a horse, are but the same elements differently combined and arranged. Plants compress the forces of inorganic nature into chem- ical compounds; animals liberate them. Plants produce; animals consume. The work of plants is synthesis, a buiiding-up ; the work of animals is analysis, or destruc- tion. The tendency in plants is deoxidation; the tenden- cy in animals is oxidation. Without plants, animals would perish; without animals, plants had no need to be. There is no plant which may not serve as food to some animal. CHAPTER IV. LIFE. ALL forces are known by the phenomena which they cause. So long as the animal and plant were supposed to exist in opposition to ordinary physical forces or indepen- dently of them, a vital force or principle was postulated by which the work of the body was performed. It is now known that most, if not all, of the phenomena manifested by a living body are due to one or more of the ordinary physical forces — heat, chemical affinity, electricity, etc. There is no work done which demands a vital force. The common modern view is that vitality is simply a LIFE. 29 collective name for the sum of the phenomena displayed by living beings. It is neither a force nor a thing at all, but is an abstraction, like goodness or sweetness; or, to nse Huxley's expression, to speak of vitality is as if one should speak of the horologity of a clock, meaning its time-keeping properties. A third theory is still possible. The combination of elements into organic cells, the arrangement of these cells into tissues, the grouping of these tissues into organs, and the marshalling of these organs into plans of structure, call for some further shaping, controlling power to effect such wonderful co-ordination. Moreover, the manifesta- tion of feeling and consciousness is a mystery which no physical hypothesis has cleared up. The simplest vital phenomenon has in it something over and above the known forces of the laboratory.11 If the vital machine is given, it works by physical forces; but to produce it and keep it in order needs, so far as we now know, more than mere physical force. To this controlling power we may apply the name vitality. Life is exhibited only under certain conditions. One condition is the presence of a physical basis called proto- plasm. This substance is found in all living bodies, and, so far as we know, is similar in all — a viscid, transpar- ent, homogeneous, or minutely granular, albuminoid mat- ter. Life is inseparable from this protoplasm ; but it is dormant unless excited by some external stimulants, such as heat, light, electricity, food, water, and oxygen. Thus, a certain temperature is essential to growth and motion ; taste is induced by chemical action, and sight by luminous vibrations. The essential manifestations of animal life may be re- duced to three: contractility; sensibility, or the peculiar power of receiving and transmitting impressions ; and the power of assimilating food. All these powers are pos- 30 COMPARATIVE ZOOLOGY. sessed by protoplasm, and so by all animals : all move, feel, and grow. But some of the lowest forms are with- out the slightest trace of organs ; they seem to be as per- fectly homogeneous and structureless as a drop of jelly. They could not be more simple. They are devoid of muscles, nerves, and stomach ; yet they have all the fun- damental attributes of life — moving, feeling, and eating. It has been supposed that the muscular and nervous mat- ter is diffused in a molecular form; but all we can say is, that the highest power of the microscope reveals no organ- ized structure whatever — i. e., there are no parts set apart for a particular purpose, but a fragment is as good as the whole to perform all the functions of life. The animal series, therefore, begins with forms that feel without nerves, move without muscles, and digest without a stom- ach : in other words, life is the cause of organization, not the result of it. Animals do not live because they are or- ganized, but are organized because they are alive. CHAPTER Y. ORGANIZATION. WE have seen that the simplest life is a formless speck of protoplasm, without distinctions of structure, and there- fore without distinctions of function, all parts serving all purposes — mouth, stomach, limb, and lung — indiscrimi- nately. There is no separate digestive cavity, no separate respiratory, muscular, or nervous systems. Every part will successively feed, feel, move, and breathe. Just as in the earliest state of society all do everything, each does all. Every man is his own tailor, architect, and lawyer. But in the progress of social development the principle of ORGANIZATION. 31 the division of labor emerges. First comes a distinction between the governing and governed classes ; then follow and multiply the various civil, military, ecclesiastical, and industrial occupations. In like manner, as we advance in the animal series, we find the body more and more heterogeneous and complex by a process of differentiation, i. e., setting apart certain portions of the body for special duty. In the lowest forms, the work of life is carried on by very simple appara- tus.13 But in the higher organisms every function is per- formed by a special organ. For example, contractility, at first the property of the entire animal, becomes centred in muscular tissue; respiration, which in simple beings is effected by the whole surface, is specialized in lungs or gills; sensibility, from being common to the whole or- ganism, is handed over to the nerves. An animal, then, whose body, instead of being uniform throughout, is made up of different parts for the performance of particular functions, is said to be organized. And the term is as ap- plicable to the slightly differentiated cell as to complex Man. Organization is expressed by single cells, or by their combination into tissues and organs. 1. Cells. — A cell is the simplest form of organized life. In general, it is a microscopic globule, consisting of a del- icate membrane enclosing a minute por- tion of protoplasm. The very simplest kinds are without granules or signs of circulation ; but usually the protoplasm is granular, and contains a defined sep- arate mass called the nucleus, within which are sometimes seen one or two, , . , , , , FIG. 1 — Parts of a Cell : rarely more, dark, round specks, named a, », y, ceii-waii ; p, nu- nudeoli. The enveloping membrane is cleus: "• uncleolus- extremely thin and transparent, and structureless : it is only an excretion of dead matter acting as a boundary to 32 COMPARATIVE ZOOLOGY. the cell-contents." The nucleus is generally attached to the inside of the membrane, and is the centre of activity. Cells vary greatly in size, but are generally invisible to the naked eye, ranging from -^-g- to 10000 of an inch in diameter. About 4000 of the smallest would be necessary to cover the dot of this letter i. The natural form of iso- lated cells is spherical ; but when they crowd each other, as seen in bone, cartilage, and muscle, their outlines be- come angular, either hexagonal or irregular. Within the narrow boundary of a simple sphere, the cell-membrane, are exhibited all the essential phenomena of life — growth, development, and reproduction. The physiology of these minute organisms is of peculiar inter- est, since all animal structure is but the multiplication of the cell as a unit, and the whole life of an animal is that of the cells which compose it: in them and by them al' its vital processes are carried on.14 The structure of a cell can be seen in blood-corpusclt by diluting with a weak (-£ per cent.) solution of sa1' a drop of blood from a Frog, and placing it under the mi- croscope. (See Fig. 63.) 2. Tissues. — There are organisms of the lowest grade (as Gregarina] which consist of a single cell, living for and by itself. In this case, the animal and cell are identical: the Gregarina has as much individuality as the Elephant. But all animals, save these unicellular beings, are mainly aggregations of cells : for the various parts of a body are not only separable by the knife into bones, muscles, nerves, etc., but these are susceptible of a finer analysis by the microscope, which shows that they arise from the devel- opment and union of cells. These cellular fabrics, called tissues, differ from one another both chemically and struct- urally, but agree in being permeable to liquids — a prop- erty which secures the flexibility of the organs so essential to animal life. Every part of the human body, for exam- ORGANIZATION. 33 pie, is moist: even the hairs, nails, and cuticle contain water. The contents as well as the shape of the cells are usually modified according to the tissue which they form : thus, we find cells containing earthy matter, iron, fat, mu- cus, etc. In plants, the cell always retains the characters of the cell; but in animals (after the embryonic period) the cell usually undergoes such modifications that the cellular form disappears. The cells are connected together or enveloped by an intercellular substance (blastema], which may be watery, soft, and gelatinous, firmer and tenacious, still more solid and hyaline, or hard and opaque. In the fluids of the body, as the blood, the cells are separate ; i. e., the blastema is fluid. But in the solid tissues the cells coa- lesce, being simply connected, as in the epidermis, or united 4nto fibres and tubes. In the lowest forms of life, and in all the higher ani- als in their embryonic state, the cells of which they are composed are not transformed into differentiated tissues: infinite tissues make their first appearance in the Sponges, and they differ from one another more and more widely as we ascend the scale of being. In other words, the bod- ies of the lower and the immature animals are more uni- form in composition than the higher or adult forms. In the Vertebrates only are all the following tissues found represented : (1) Epithelial Tissue. — This is the simplest form of cellu- lar structure. It covers all the free surfaces of the body, internal and external, so that an animal may be said to be contained between the walls of a double bag. That which is internal, lining the mouth, windpipe, lungs, blood-ves- sels, gullet, stomach, intestines — in fact, every cavity and canal — is called epithelium. It is a very delicate skin, formed of flat or c}rlindrical cells, and in some parts (as in the wind-pipe of air-breathing animals, and along the gills 3 COMPARATIVE ZOOLOGY. of the Oyster) is covered with cilia, or minute hairs, about of an inch long, which are incessantly moving. Con- tinuous with this in- ner lining of the body (as seen on the lip), and covering the outside, is the epidermis, or cuti- cle. It is the outer layer of the " skin," which we can re- move by a blister, and in Man varies in PIG. 2.— Various kinds of Epithelium Cells: a, colum- nar, from small intestine; 3, a single cell, showi nucleus; 6, ciliated, from one of the small t tubes; rf, the same, from the windpipe, with single tllicknCSS 'f 1*0111 -3-^- ce.ll magnified about 200 times; c, sqntimous, from eyelid of a calf, showing changes of form, from the of an inch On the deep to superficial cells, 1 being the scurf. , , . . cheek to TV on the sole of the foot. It is constantly wearing off at the sur- face, and as constantly growing in the deeper portion; and in the process of growth and passage outward, the cells change from the spherical form to dead horny scales (seen in scurf and dandruff). In the lower layer of the cuticle we find the pigment cells, characteristic of colored races. Neither the epidermis nor the corresponding tissue within (epithelium) has any blood-vessels or nerves. The epithe- lial tissue, then, is simply a superficial covering, bloodless and insensible, protecting the more delicate parts under- neath. Hairs, horns, hoofs, nails, claws, corns, beaks, scales, tortoise-shell, the wings of Insects, etc., are modifications of the epidermis. The next three sorts of tissue are characterized by a great development of the intercellular substance, while the cells themselves are very slightly modified. (2) Connective Tissue. — This is the most extensive tissue in animals, as it is the great connecting medium by which the different parts are held together. Could it be taken ORGANIZATION. 35 out entire, it would be a complete mould of all the organs. It surrounds the bones, muscles, blood-vessels, nerves, and glands, and is the substance of the ligaments, tendons, "true skin," mucous mem- brane, etc. It varies in character, being soft, ten- der, and elastic, or dense, tough, and generally un- yielding. In the former state, it consists of innu- merable fine white and yel- low fibres, which interlace 111 all directions, leaving FIG. 3.— Connective Tissue, showing areolnr i j f structure, X 25. irregular spaces, and form- ing a loose, spongy, moist web. In the latter, the fibres M&y/frf/ar FIG. 4 — Connective Tissue from human peritoneum ; highly magnified ; a, blood- vessel. 36 COMPARATIVE ZOOLOGY. are condensed into sheets or parallel cords, having a wavy, glistening appearance. Such structures are the fasciae and tendons. Connective tissue is not very sensitive. It con- tains gelatine — the matter which tans when hide is made into leather. In this tissue the intercellular substances take the form of fibres. The white fibres are inelas- tic, and from -4o^u0 to 24u07 of an inch in diameter. They are best seen in the tendons. The yellow fibres are elastic, curled at the ends, very long, and from -ST-TITTO to 7oVir °f an incn ^n diameter. They are shown in the hinge-ligament of an Oyster. Connec- tive tissue appears areolar, i. e., shows r,o.5.-HyaiiI1eCa,ti,nge, interspaces, only under the microscope. (3) Cartilaginous TiSSUe.—Tllis Diagram: a, cartilage cell; 6, cell about to di- . . vide ;c, cell divided into KHOWn also 3S " gristle, IS Composed two; d, into four parts. i 11 • L jj j • i i The npnce between the 01 cells imbedded in a granular or hy- ^SZSSSS. aline substance, which is dense, elastic, stance; highly magni- bluish white, and translucent. It IS found where strength, elasticity, and insensibility are wanted, as at the joints. It also takes the place of the long bones in the embryo. When cartilage is mixed with connective tis- sue, as in the ear, it is ca]\ed Jtbro-car- tilage. (4) Osseous Tissue. — This hard, opaque tissue, called " bone," differs from the former two in having the intercellular spaces or meshes filled with phosphate of lime and other earths, instead of a 'd hyaline or fibrous substance. It may Fliage ~x°w-ya"LCin*ze be called Petrified tissue— the quantity ceil?, passing into com- of earthy matter, and therefore the brit- pact bone, c, and then ' . . spongy bone, «. tlcness of the bone, increasing with the ORGANIZATION. 37 age of the animal. If a chicken-bone be left in dilute muriatic acid several days, it may be tied into a knot, since the acid has dissolved the lime, leaving noth- ing but cartilage and connective tissue. If a bone be burned, it be- comes light, porous, and brittle, the lime alone remaining.14 Bone is a very vas- cular tissue; that is, it is traversed by minute blood- vessels and nerves, ^ ' FIG. 7. — Transverse section of a Bone (Human which paSS through a Femur), x 50, showing Haversian canals. net-\vork of tubes, called Haversian canals. The canals average ToVo- °f an inch, being finest near the surface of the bone, and larger further in, where they form a cancel- lated or spongy structure, and finally merge (in the long bones) into the central cavity, containing the marrow. Under the microscope, each canal appears to be the cen- tre of a multitude of lamince, or plates, ar- ranged around it. Ly- ing between these plates are little cavities, called lacunae, from which ra- diate exceedingly fine » 8.— Frontal Bone of Human Sknll under the microscope, showing lacuni aud canaliculi. or canaliculi. represent the original cells of the bone, and differ in shape and size in different animals. r» 0 C! 38 COMPARATIVE ZOOLOGY. True bone is found only in Vertebrates, or back-boned animals. (5) Dental Tissue. — Like bone, a tooth is a combination of earthy and animal matter. It may be called petrified skin. In the higher animals, it consists of three parts : dentine, forming the body of the tooth, and always pres- ent; enamel, capping the crown; and cement, covering the fangs (Fig. 31). The last is true bone, or osseous tissue. Pio. 9. — Highly magnified section of Dentine and Cement, from the fang of a Human Molar: a, 6, marks of the original dentinal pulp; d, dentrnal tubes, terminating in the very sensitive, modified layer, g; h, cement. Dentine resembles bone, but differs in having neither la- cunae nor (save in Shark's teeth) canaliculi. It shows, in place of the former, innumerable parallel tubes, reaching from the outside to the pulp-cavity within. The " ivory " of Elephants consists of dentine. Enamel is the hardest substance in the body, and is composed of minute six-sided fibres, set closely together. It is want- ing in the teeth of most Fishes, Snakes, Sloths, Armadillos, Sperm-whales, etc. True dental tissue is confined to Vertebrates. (6) Adipose Tissue. — Certain cells be- come greatly enlarged and filled with fat, so that the original protoplasm oc- cupies a very small part of the space within the cell-membrane. These cells . 10.— Adipnse Tissue, a; are united into masses by connective with fibres of connective . , . . , . tissue, &. tissue, in the skin (as in the "blub- ORGANIZATION. 39 ber" of whales), between the muscles (as in "streaky" meat), or in the abdominal cavity, in the omentum, mes- entery, or about the kidneys. The marrow of bones is an example. Globules of fat occur in many Molluscs and Insects ; but true adipose tissue is found only in back- boned animals, particularly the herbivorous. In the aver- age Man, it constitutes about ^V part of his weight, and a single Whale has yielded 120 tons of oil. The fat of animals has the different names of oil, lard, tallow, suet, spermaceti, etc. It is a reserve of nutriment in excess of consumption, serving also as a packing material, and as a protection against cold. (7) Muscular Tissue. — If we examine a piece of lean meat, we find it is made up of a number of fasciculi, or bundles of fibres, placed side by side, and bound together by connective tissue. The microscope informs us that each fibre is itself a bundle of smaller fibres; and when one of these is more closely examined, it is found to be enclosed in a delicate, glossy tube, called the sarcolemma. This tube is filled with ., . Fio. 11.— Striated Muscular Fibre (of the Pi"-), very minute, parallel x 200. The constituent fibres are seen at a; fibrils, averaging ^^ c isa fascicu1"8' or buudle- of an inch in diameter, and having a striated aspect. Tissue of this description constitutes all ordinary muscle, or " lean meat," and is marked by regular cross-lines, or striae. Besides this striated muscular tissue, there exist, in the coats of the stomach, intestines, blood-vessels, and some oth- er parts of Vertebrates, smooth muscular fibres, or mem- 40 COMPARATIVE ZOOLOGY. branes, which show a nucleus under the microscope, and do not break up into fibrils (Fig. 122). The gizzards of fowls exhibit this form. All muscle has the property of shorten- ing itself when excited ; but the contraction of the striated kind is under the control of the will, while the movement of the smooth fibres is involuntary.16 Muscles are well sup- plied with arteries, veins, and nerves ; but the color is due to a peculiar pigment, not to the blood. Muscular tissue is found in all animals from the Coral to Man. ( 8 ) Nervous Tissue. — Nervous matter exists under three forms : First — the cellular, con- FIQ 12 — stri d s^n£ °f nucleated cells, varying from ^\nr Muscular Fibres, to -^^ of an inch in diameter, and found in from the heart of , .„. . . Man, divided by the nerve-centres (t ig. 132), the gray por- iuto 7eprarateenu* ^on °f tne bniin, spinal cord, and other gan- cieated portions. gjja> Second — t\\efibrous, consisting of pale, flat, extremely fine filaments. They abound in the sympa- thetic nerves, and are the only nerves found in the Inverte- brates. Third — the tubular. These are much larger than the fibrous, the coarsest being -rVb-Tr of an inch in diameter. They consist of tubes enclosing a transparent fibre and a fatty substance called the nerve- marrow.17 The delicate tube itself is called iwurilem- ma, analogous to the sarcolemma of mus- F'°j cular tissue. Nerve -tubes are found only sheath, or neun- lemma; 2, med- m back -boned animals, in the white sub- uiiary substance »,•,.,. . i j j . , i of Schwaim ; 3, stance of the brain, spinal cord, and in the axis cylinder, or nerves. primitive band. A bundle of fibrous or tubular nervous matter, sur- rounded by connective tissue, constitutes a nerve. ORGANIZATION. 41 FIG. 14.— A Ganglion of the Sympathetic Nerve of a Mouse. 3. Organs, and their Functions. — Animals, like Plants, grow, feel, and move ; these three are the capital facts of every organism. Besides these there may be some pecul- iar phenomena, as motion and will. Life is manifested in certain special operations, called functions, performed by certain special parts, called or- gans. Thus, the stomach is an organ, whose function is digestion. A single organ may manifest vitality, but it does not (save in the very lowest forms) show forth the whole life of the animal. For, in being set apart for a special purpose, an organ takes upon itself, so to speak, to do something for the benefit of the whole animal, in return for which it is absolved from doing many things. The stomach is not called upon to circulate or purify the blood. There may be functions without special organs, as the Amoeba digests, respires, moves, and reproduces by its general mass. But, as we ascend the scale of animal life, we pass from the simple to the complex : groups of cells or tissues, instead of being repetitions of each other, take on a difference, and become distinguished as special parts with specific duties. The higher the rank of the animal, the more complicated the organs. The more complicated the structure, the more complicated the functions. But in 42 COMPARATIVE ZOOLOGY. all animals, the functions are performed under conditions essentially the same. Thus, respiration in the Sponge, the Fish, and in Man has one object and one means, though the methods differ. A function, therefore, is a group of similar phenomena effected by analogous structures. The life of an animal consists in the accumulation and expenditure of force. The tissues are storehouses of power, which, as they waste, is given off in various forms. Thus, the nervous tissue generates nerve-force ; the mus- cles, motion. If we contemplate the phenomena presented by a Dog, the most obvious fact is his power of moving from place to place, a power produced by the interplay of muscles and bones. We observe, also, that his motions are neither mechanical nor irregular; there is method in his movement. He has the power of willing, seeing, hear- ing, feeling, etc. ; and these functions are accomplished by a delicate apparatus of nerves. But the Dog does not exhibit perpetual motion. Sooner or later he becomes exhausted, and rest is necessary. Sleep gives only temporary relief. In every exercise of the muscles and nerves there is a consumption or waste of their substance. The blood restores the organs, but in time the blood itself needs renewal. If not renewed, the animal becomes emaciated, for the whole body is laid un- der contribution to furnish a supply. Hence the feelings of hunger and thirst, impelling the creature to seek food. Only this will maintain the balance between waste and repair. We notice, therefore, an entirely different set of functions, involving, however, the use of motion and will. The Dog seizes a piece of meat, grinds it between its teeth, sends it into the stomach, where it is digested, and then into the intestine, where it is further changed; there the nourishing part is absorbed, and carried to the heart, which propels it through tubes, called blood-vessels, all over the body. In this process of digestion, certain fluids ORGANIZATION. 43 are required, as saliva, gastric juice, and bile: these are secreted by special organs, called glands. Moreover, since not all the food eaten is fitted to make blood, and as the blood itself, in going around the body, acts like a scaven- ger, picking up worn-out particles, we have another func- tion, that of excretion, or removal of useless matter from the system. The kidneys and lungs do much of this; but the lungs do something else. They expose the blood to the air, and introduce oxygen, which, we shall find, is essential to the life of every animal. These centripetal and centrifugal movements in the body — throwing in and throwing out — are so related and involved, especially in the lower forms, that they cannot be sharply defined and classified. It has been said that every Dog has two lives — a vegetative and an animal. The former includes the processes of digestion, circulation, respiration, secretion, etc., which are common to all life; the functions of the other, as motion, sensation, and will, are peculiar to animals. The heart is the centre of the vegetative life, and the brain is the centre of the animal life. The aim of the vegetative organs is to nourish the individual, and reproduce its kind; the organs of locomo- tion and sense establish relations between the individual and the world without. The former maintain life; the others express it. The former develop, and afterwards sustain, the latter. The vegetative organs, however, are not independent of the animal ; for without muscles and nerves we could not procure, masticate, and digest food. The closer the connection and dependence between these two sets of organs, the higher the rank.18 All the apparatus and phenomena of life may be in- cluded under the heads of NUTRITION, MOTION, SENSATION. 44 COMPARATIVE ZOOLOGY. These three are possessed by all animals, but in a vari- ety of ways. No two species have exactly the same mech- anism and method of life. We must learn to distinguish between what is vital and what is only accessory. That only is essential to life which is common to all forms of life. Our brains, stomachs, livers, hands, and feet are luxuries. They are necessary to make us human, but not living, beings. Half of our body is taken up with a com- plicated system of digestion; but the Amoeba has neither mouth nor stomach. We have an elaborate apparatus of motion ; the Oyster cannot stir an inch. Nutrition, Motion, and Sensation indicate three steps up the grade of life. Thus, the first is the prominent function in the Coral, which simply " vegetates," the pow- ers of moving and feeling being very feeble. In the higher Insect, as the Bee, there is great activity with sim- ple organs of nutrition. In the still higher Mammal, as Man, there is less power of locomotion, though the most perfect nutritive system ; but both functions are subordi- nate to sensation, which is the crowning development. In studying the comparative anatomy and physiology of the animal kingdom, our plan will be to trace the vari- ous organs and functions, from their simplest expression upward to the highest complexity. Thus Nutrition will begin with absorption, which is the simplest method of taking food; going higher, we find digestion, but in no particular spot in the body ; next, we see it confined to a tube ; then to a tube with a sac, or stomach ; and, finally, we reach the complex arrangement of the higher animals. NUTRITION. 45 CHAPTER VI. NUTRITION. Nutrition is the earliest and most constant of vital op- erations. So prominent is the nutritive apparatus, that an animal has been likened to a moving sac, organized to convert foreign matter into its own likeness, to which the complex organs of animal life are but auxiliaries. Thus, the bones and muscles are levers and cords to carry the body about, while the nervous system directs its motions in quest of food. The objects of nutrition are growth, repair, and propa- gation. The first object of life is to grow, for no animal is born finished. Some animals, like plants, grow as long as they live;19 but the majority soon attain a fixed size. In all animals, however, without exception, food is \vanted for another purpose than growth, namely, to repair the waste which is constantly going on. For every exercise of the muscles and nerves involves the death and decay of those tissues, as shown by the excretions. The amount of matter expelled from the body, and the amount of nour- ishment needed to make good the loss, increase with the activity of the animal. The supply must equal the de- mand, in order to maintain the life of the individual; and as an organism can make nothing, it must seek it from without. Not only the muscles and nerves are wasted by use, but every organ in the body ; so that the whole struct- ure needs constant renewal. An animal begins to die the moment it begins to live. The function of nutrition, therefore, is constructive, while motion and sensation are destructive. 46 COMPARATIVE ZOOLOGY. Another source of demand for food is the production of germs, to propagate the race, and the nourishment of such offspring ui the egg and infantile state. This reproduc- tion and development of parts which can maintain an in- dependent existence is a vegetative phenomenon (for plants have it), and is a part of the general process of Nutrition. But it will be more convenient to consider it hereafter (chapters xix., xx.). Still another necessity for aliment ,among the higher animals is the maintenance of bodily heat. This will be treated under the head of Respiration. For the present, we will study Nutrition, as manifested in maintaining the life of an adult individual. In all animals, this process essentially consists in the in- troduction of food, its conversion into tissue, its oxidation, and the removal of worn-out material. 1. The food must be procured, and swallowed. (Inges- tion.) 2. The food must be dissolved, and the nutritious parts separated into a fluid. (Digestion.) 3. The nutritive fluid must be carefully taken up, and then distributed all over the body. (Absorption and Cir- culation.) 4. The tissues must repair their parts wasted by use, by transforming particles of blood into living matter like themselves. (Assimilation.) 5. Certain matters must be strained from the blood, some to serve a purpose, others to be cast out of the sys- tem. (Secretion and Excretion.) 6. In order to produce work and heat, the food must be oxidized, either in the blood or in the tissues, after assimi- lation. The necessary oxygen is obtained through expos- ure of the blood to the air in the lungs. (Respiration in part.) 7. The waste products of this oxidation taken up by the blood must be got rid of ; some from the lungs (car- THE FOOD OF ANIMALS. 47 bon dioxide, water), some from the kidneys (water, urea, mainly), some from the skin (water, salines). (Respira- tion in part, Excretion.) The mechanism to accomplish all this in the lowest forms of life is exceedingly simple, a single cavity and surface performing all the functions. But in the major- ity of animals the apparatus is very complicated: there is a set of organs for the prehension of food ; another, for digestion ; a third, for absorption ; a fourth, for distribu- tion ; and a fifth, for purification. CHAPTER VII. THE FOOD OF ANIMALS. THE term food includes all substances which contribute to nutrition, whether they simply assist in the process, or are actually appropriated, and become tissue. With the food is usually combined more or less indigestible matter, which is separated in digestion. Food is derived from the mineral, vegetable, and animal kingdoms. Water and salt, for example, are inorganic. The former is the most abundant, and a very essential article of food. Most of the lower forms of aquatic life seem to live by drinking: their real nourishment, how- ever, is present in the water in the state of solution. The Earthworm, some Beetles, and certain savage tribes of Men swallow earth; but this, likewise, is for the organic matter which the earth contains. As no animal is pro- duced immediately from inorganic matter, so no animal can be sustained by it. Nutritious or tissue-forming food comes from the or- ganic world, and is albuminous, as the lean meat of ani- 48 COMPARATIVE ZOOLOGY. mals and the gluten of wheat; oleaginous, as animal fat and vegetable oil ; or saccharine, as starch and sugar. The first is the essential food-stuff; no substance can serve permanently for food — that is, can permanently prevent loss of weight in the body — unless it contains albuminous matter. As stated before, all the living tissues are albu- minous, and therefore albuminous food is required to sup- ply their waste. Albumen contains nitrogen, which is necessary to the formation of tissue ; fats and sugars are rich in carbon, and therefore serve to maintain the heat of the body, and to repair part of the waste of tissues. Warm-blooded animals feed largely on farinaceous or starchy substances, which in digestion are converted into sugar. But any animal, of the higher orders certainly, whether herbivorous or carnivorous, would starve, if fed on pure albumen, oil, or sugar. Nature insists upon a mixed diet ; and so we find in all the staple articles of food, as milk, meat, and bread, at least two of these prin- ciples present. As a rule, the nutritive principles in veg- etables are less abundant than in animal food, and the indigestible residue is consequently greater. The nutri- ment in flesh increases as we ascend the animal scale; thus, Oysters are less nourishing than Fish; Fish, less than Fowl ; and Fowl, less than the flesh of Quadrupeds. Many animals, as most Insects and Mammals, live solely on vegetable food, and some species are restricted to par- ticular plants, as the Silk-worm to the white mulberry. But the majority of animals feed on one another; such are hosts of the microscopic forms, and nearly all the ra- diated species, marine Mollusks, Crustaceans, Beetles, Flies, Spiders, Fishes, Amphibians, Keptiles, Birds, and clawed Quadrupeds. A few, as Man himself, are omnivorous, i. e., are main- tained on a mixture of animal and vegetable food. The use of fire in the preparation of food is peculiar to Man, HOW ANIMALS EAT. 49 who has been called " the cooking animal." A few of the strictly herbivorous and carnivorous animals have shown a capacity for changing their diet. Thus, the Horse and Cow may be brought to eat fish and flesh ; the Sea-birds can be habituated to grain ; Cats are fond of alligator- pears ; and Dogs take naturally to the plantain. Certain animals, in passing from the young to the mature state, make a remarkable change of food. Thus, the Tadpole feeds upon vegetable matter ; but when it becomes a Frog it lives on Insects. Many tribes, especially of Reptiles and Insects, are able to go without food for months, or even years. Insects in thn larval, or caterpillar, state are very voracious ; but upon reaching the perfect, or winged, state, they eat little —some species taking no food at all, the mouth being act- ually closed. The males of some Rotifers and other tribes take no food from the time of leaving the egg until death. In general, the greater the facility with which an animal obtains its food, the more dependent is it upon a constant supply. Thus, carnivores endure abstinence better than herbivores, and wild animals than domesticated ones. CHAPTER VIII. HOW ANIMALS EAT. 1. The Prehension of Food. — (l) Liquids. — The sim- plest method of taking nourishment is by absorption through the skin. The Tape -worm, for example, has neither mouth nor stomach, but imbibes the digested food of the animal it infests. Many other animals, especially Insects, live upon liquid food, but obtain it by suction through a special orifice or tube. Thus, we find a mouth, 4 50 COMPARATIVE ZOOLOGY. ' or sucker, furnished with teeth for lancing the skin of an- imals, as in the Leech; a bristle-like tube fitted for pierc- ing, as in the Mosquito; a sharp sucker armed with barbs, to fix it securely during the act of sucking, as in the Louse ; and a long, flexible proboscis, as in the Butterfly. Bees have a hairy, channelled tongue, and Flies have one terminating in a large fleshy knob, with or without little " knives " at the base for cutting the skin : both lap, rather than suck, their food. Most animals drink by suction, as the Ox ; and a few by lapping, as the Dog; the Elephant pumps the water up with its trunk, and then pours it into its throat; and Birds (excepting Doves) fill the beak, and then, raising the head, allow the water to run down. Many aquatic animals, whose food consists of small par- ticles diffused through the water, have an apparatus for creating currents, so as to bring such particles within their reach. This is particularly true of low, fixed forms, which are unable to go in search of their food. Thus, the Sponge draws nourishment from the water, which is made to cir- culate through the system of canals traversing its body by the vibration of minute hairs, or cilia, lining parts of the canals (Fig. 189). The microscopic Infusoria have cilia surrounding the mouth, with which they draw or drive into the body little currents containing nutritious particles. Bivalve mollusks, as the Oyster and Clam, are likewise dependent upon this method of procuring food, the gills being fringed with cilia. So the singular fish, Amphioxus (the only example among Vertebrates), em- ploys ciliary action to obtain the minute organisms on which it feeds. The Greenland Whale has a mode of iri- gestion somewhat unique, gulping great volumes of water into its mouth, and then straining out, through its whale- bone sieve, the small animals which the water may con- tain (Fig. 343). HOW ANIMALS EAT. 51 (2) Solids. — When the food is in solid masses, whether floating in water or not, the animal is usually provided witli prehensile appendages for taking hold of it. The jelly- like Amceba has neither mouth nor stomach, but extemporizes them, seizing its food by means of its soft body. The food then passes through the outer wall into the softer interior, where it is digested. The waste particles with pseudopodia extended, x so. are passed out in a similar way. In the Foraminifers, thread-like projections of the body are thrown out (pseu- dopodia) which adhere to the prey. The soft jelly-like substance of the body then flows down the pseudopodinm, collects about the food, and digests it (Fig. 15). A higher type is seen in Polyps and Jelly-fishes, which have hollow tentacles around the entrance to the stomach (Fig. 193). These tentacles are contractile, and, moreover, are covered with an immense number of minute sacs, in which a highly elastic filament is coiled up spiral h7 (lasso- cells, nettle-cells). When the tentacles are touched by a passing animal, they seize it, and at the same moment throw out their myriad filaments, like so many lassos, which penetrate the skin of the victim, and probably also emit a fluid, which paralyzes it; the mouth, meanwhile, expands to an extraordinary size, and the creature is soon engulfed in the digestive bag. In the next stage, we find no tentacles, but the food is brought to the mouth by the flexible lobes of the body, commonly called " arms," which are covered with hun- dreds of minute suckers; and if the prey, as an Oyster, is too large to be swallowed, the stomach protrudes, like a proboscis, and sucks it out of its shell. This is seen in the Star-fish (Fig. 126). COMPARATIVE ZOOLOGY. A great advance is shown by the Sea-urchin, whose month is provided with five sharp teeth, set in as many jaws, and capable of being projected so as to grasp, as well as to masticate, its food (Figs. 214, 28). In Mollusks having a single shell, as the Snail, the chief organ of prehension is a strap-like tongue, covered with minute recurved teeth, or spines, with which the animal rasps its food, while the upper lip is armed with a sharp, horny plate (Fig. 29). In many marine species, as the Whelk, the tongue is situated at the end of a retrac- tile proboscis, or muscular tube. In the Cuttle-fish, we see the sud- den development of an elaborate system of prehensile organs. Be- sides a spinous tongue, it has a pair of hard mandibles, resem- bling the beak of a Parrot, and working vertically ; and around the mouth are eight or ten pow- the arm, containing nerve and ar- QUS Clip-like SUCkerS. So perfect is tery; c, cellular tissue; d, radial- . , . • . , ing fibres; h, raised margin of the adllCSlOn 01 tllCSC SUCKCl'S, that SKStf .(£SR££ it is easier to tear away a limb braiie, or "piston," i. t,|an to detach it f rOtTl itS hold. The Earth-worm swallows earthy matter and decaying leaves, which it secures with its lips, the up- per one being prolonged. Other worms (as Nereis) are so construct- ed that the gullet, which is fre- quently armed with teeth and for- ceps, can be turned inside out, to FIO.IT.— Nereis— head, with ex- . , . . . . tended proboscis : J jaws : T, form a proboSCIS for Seizing prey. tentacles; //, head -,E, eyes. HOW ANIMALS EAT. 53 The Arthropoda exhibit a great variety of means fur procuring nourishment, in addition to the suctorial con- trivances already mentioned, the innumerable modifica- tions of the mouth corresponding to the diversity of food. Millepedes, Caterpillars, and Grubs have a pair of horny jaws moving horizontally. The Centipede has a second pair of jaws, which a.re really modified feet, terminated by curved fangs containing a poison-duct. The Horse- shoe Crab uses its feet for prehension, and the thighs, or basal joints, of its legs to masticate the food and force it into the stomach. The first six pair of legs in the Lob- ster and Crab are likewise appropriated to conveying food into the mouth, the sixth being enormously developed, and furnished with powerful pincers. Scorpions have a similar pair of claws for pre- hension, and also a pair of small forceps for holding the food in contact with the mOUth. In their relatives, FIO. IS.-One of the Fan-s, or Perforated the Spiders, the claws are Mandibles, of the spider, wanting, and the forceps end in a fang, or hook, which is perforated to convey venom.40 The biting Insects, as Beetles and Locusts, have two pairs of horny jaws, which open sidewise, one above and the other below the oral orifice. The upper pair are called mandibles; the lower, maxillae. The former are armed with sharp teeth, or with cutting edges, and sometimes are fitted, like the molars of quadrupeds, to grind the food. The maxillae are usually composed of several parts, some of which serve to hold the food, or to help in divid- ing it, while others (palpi) are sensory. There is generally present a third pair of jaws — the labium — which are united in the middle line, and serve as a lower lip. They also bear palpi. The Mantis seizes its prey with its long 54 COMPARATIVE ZOOLOGY. fore-legs, crushes it between its thighs, which are armed with spines, and then delivers it up to the jaws for masti- cation. All Arthropods move their jaws horizontally. The back-boned animals generally apprehend food by means of their jaws, of which there are two, moving ver- tically. The toothless Sturgeon draws in its prey by pow- erful suction. The Hag-fish has a single tooth, which it plunges into the sides of its victim, and, thus securing a firm hold, bores its way into the flesh by means of its saw- like tongue. But Fishes are usually well provided with teeth, which, being sharp and curving inward, are strictly prehensile. The fins and tongue are not prehensile, A mouth with horny jaws, as in the Turtles, or bristling with teeth, as in the Crocodile, is the only means possessed by nearly all Amphibians and Reptiles for securing food. The Toad, Frog, and Chameleon capture insects by dart- ing out the tongue, which is tipped with glutinous saliva. The constricting serpents (Boas) crush their prey in their coils before swallowing; and the venomous Snakes have a poison-fang. No reptile has prehensile lips. All Birds use their toothless beaks in procuring food, but birds of prey also seize with their talons, and Woodpeckers, Hum- mers, and Parrots with their tongues. The beak varies greatly in shape, being a hook in the Eagle, a probe in the Woodpecker, and a shovel in the Duck. Among the Quadrupeds we find a few special contriv- ances, as the trunk of the Elephant, and the long tongues of the Giraffe and Ant-eater; but, as a rule, the teeth are the chief organs of prehension, always aided more or less by the lips. Ruminants, like the Ox, having hoofs on their feet, and no upper front teeth, employ the lips and tongue. Such as can stand erect on the hind-legs, as the Squirrel, Bear, and Kangaroo, use the front limbs for hold- ing the food and bringing it to the mouth, but never one limb alone. The clawed animals, like the Cat and Lion, HOW ANIMALS EAT. 55 make use of their feet in securing prey, all four limbs be- ing furnished with curved retractile claws; but the food is conveyed into the month by the movement of the head and jaws. Man and the Monkeys em- ploy the hand in bringing food to the mouth, and the lips and tongue in taking it into the cavi- ty. The thumb on the human hand is longer and more perfect than that of the Apes and Mon- keys ; but the foot of the latter is also prehensile. 2. The Mouths of Animals. — In the Parasites, as the Tape- worm, which absorb nourishment through the skin, and Insects, as the May-fly and Bot-fly, which do FIG- w.-Arm of the Thumbiess ni.. . ^ , , Monkey (A teles). all their eating in the larval state, the mouth is either wanting or rudimentary. The Amoeba, also, has no mouth proper, its food passing through the firmer outside part of the bit of protoplasm which consti- tutes its body. Mouth and anus are thus extemporized, the opening closing as soon as the food or excrement has passed through. In the Infusoria the mouth is a round or oval opening leading through the cuticle and outer layer of protoplasm to the interior of the single cell which makes their body. It is'usually bordered with cilia, and situated on the side or at one end of the animal. An elliptical or quadrangular orifice, surrounded witli tentacles, and leading directly to the stomach, is the ordi- nary mouth of the Polyps and Jelly-fishes. In those which are fixed, as the Actinia, Coral, and Hydra, the mouth looks upward : in those which freely move about, 56 COMPARATIVE ZOOLOGY. as the Jelly-fish, it is generally underneath, the position of the animal being reversed. In some, the margin, or lip, is protruded like a proboscis; and in all it.is exceedingly dilatable. The mouth of the Star-fish and Sea-urchin is a simple round aperture, followed by a very short throat. In the Star-fish, it is enclosed by a ring of hard tubercles and a membrane. In the Sea-urchin, it is surrounded by a mus- cular membrane and minute tentacles, and is armed with five sharp teeth, set in as many jaws, resembling little conical wedges (Fig. 28). Among the headless Mollusks, the oral apparatus is very simple, being inferior to that of some of the radiated ani- mals. In the Oyster and Bivalves generally, the mouth is an unarmed slit — a mere inlet to the oesophagus, situ- ated in a kind of hood formed by the union of the gills at their origin, and between two pairs of delicate lips. These lips make a furrow, along which pass the particles of food drawn in by the cilia. Of the higher Mollusks, the little Clio (one of the Ptero- pods) has a triangular mouth, with two jaws armed with sharp horny teeth, and a tongue covered with spiny hook- lets all directed backward. Some Univalves have a sim- ple fleshy tube. Others, as the Whelk, have an extensible proboscis, which unfolds itself, like the finger of a glove, and carries within it a rasp-like tongue, which can bore into the hardest shells. Such as feed on vegetable matter, as the Snail, have no pr'obos- cis, but on the roof of the of the common siuaii mouth a curved horny plate (Helix albolabri*). fitted t() ^ ^.^ ^ ^^ are pressed against it by the lips, and on the floor of the mouth a small tongue covered with delicate teeth. As fast as the tongue is worn off by use, it grows out from the root. HOW ANIMALS EAT. 57 The mouth of the Cuttle-fish is the most elevated type below that of the Fishes. A broad circular lip nearly conceals a pair of strong horny mandibles, not unlike the beak of a parrot, but reversed, the upper mandible being the shorter of the two, and the jaws, which are cartilagi- nous, are imbedded in a mass of muscles, and move ver- tically. Between them is a fleshy tongue covered with teeth. The parasitic Worms, living within or on the outside of other animals, generally have a sucker at one end or underneath, serving simply for attachment, and another which is perforated. The latter is a true suctorial mouth, being the sole inlet of food. It is often surrounded with booklets or teeth, which serve both to scarify the victim and secure a firm hold. In the Leech, the mouth is a triangular opening with thick lips, the upper one pro- longed, and with three jaws. In many Worms it is a fleshy tube, which can be drawn iu or extended, like the eye -stalks of the Snail, and contains a dental apparatus inside (Fig. 17). . Millepedes and Centipedes have two lateral jaws and a four-lobed lip. In Lobsters and Crabs the mouth is situated underneath the head, and consists of a soft upper lip, then a pair of upper jaws provided with a short feeler, below which is a thin bifid lower lip ; then follow two pairs of membranous under jaws, which are lobed and hairy ; and next, three pairs of foot-jaws (Fig. 250). The Horse-shoe Crab has no special jaws, the thighs answering the purpose. The Barnacle has a prominent mouth, with three pairs of rudi- mentary jaws. With few exceptions, the mouths of Insects in the lar- val state are fitted only for biting, the two jaws being horny shears. But in the winged, or perfect, state, Insects may be divided into the masticating (as the Beetle) and 58 COMPARATIVE ZOOLOGY. FIG. 21.— Month of a Locust dissected: 1, Inbrnm, or tipper lip; 2, mandibles; 3, jjiws; 4, labinm, or lower lip; 5, tongue. The appendages to the maxillae aud lower lip are palpi. the suctorial (as the Butterfly). In the former group, the oral apparatus consists of two pairs of horny jaws (mandi- bles and maxillce), which work horizontally between an upper (labrum) and an under (labium) lip. The maxilla and under lip carry sensitive jointed feelers (palpi}- The front edge of the labium is commonly known as the tongue (liguld).*1 In such a mouth, the mandibles are the most important parts; but in passing to the suctorial Insects, we find that the mandibles are secondary to the maxillfe and labium, which are the only means of taking food. In HOW ANIMALS EAT. 59 the Bee tribe, we have a transi- tion between the biting and the sucking Insects — the mandibles " supply the place of trowels, spades, pickaxes, saws, scissors, and knives," while the maxillae are developed into a sheath to enclose the long, slender, hairy tongue which laps np the sweets of flowers. In the suctorial But- terfly, the lips, mandibles, and palpi are reduced to rudiments, while the maxillae are the only useful oral organs. These aro excessively lengthened into a proboscis, their edges locking FI». 22.— Head of a wsid Bee (A . thophora return), front view: by means OI minute teeth, SO as compound eyes; 6, clypens; to form a central canal, through which the liquid food is pumped up into the mouth. Seen un- der the microscope, the proboscis is made up of innumer- able rings interlaced with spiral muscular fibres. The proboscis of the Fly is a modified lower lip ; that of the Bugs and Mosquitos, fitted both for piercing and suction, is formed by the union of four bristles, which are the mandibles and maxillae strangely al- tered, and encased in the labium when not Fie. 23.— Proboscis of u Butterfly. in US6. 60 COMPARATIVE ZOOLOGY. FIG. 94.— Mouth of the Horse-fly (7'<«6an!wttn- eola): a, antennas; m, mandibles; mx, max- illse ; mp, maxillary pnlpi; Ib, labrnm; I, labiura, or tongue. As most of the Arachnids live by suc- tion, the jaws are seldom used for masti- cation. In the Scorpion, the apparent representatives of the mandibles of an Insect are transformed into a pair of small forceps, and the palpi, so small in Insects, are developed into formidable claws : both of these organs are prehen- sile. In Spiders, the so -called mandi- bles, which move more or less vertically, end in a fang; and the club-like palpi, often resembling legs, have nothing to do with inges- tion or locomotion. Both Scorpions and Spiders have a soft upper lip, and a groove within the mouth, which serves as a canal while sucking their prey. The tongue is external, and situated between a pair of diminutive maxillae. In the Ascidians the first part of the alimentary canal is enormously enlarged and modified to serve as a gill- sac. At the bottom of this sac, and far removed from its external opening, lies the entrance to the diges- tive tract proper. Into it the particles of food enter- F.o.S5._Under Surf,,CB of Male Spider :«, ing with the water are con- c- P"i*"»-fc"s •• b- '<*'" <>,, interior mm- ' gin of mandible, f; f, Inbtnm; g, thorax; (Fig. 279). A, limbs; i, abdomen; I, spinnerets; wt, r,-,, , . iT maxillary palpus; d, dilated terminal The mouth of Yerte- joint. HOW ANIMALS EAT. 61 brates is a cavity with a fixed roof (the hard palate) and a movable floor (the tongue and lower jaw), having a trans- verse opening in front," and a narrow outlet behind, lead- ing to the gullet. Save in Birds and some others, the cavity is closed in front with lips, and the margins of the jaws are set with teeth. In Fishes the mouth is the common entry to both the digestive and respiratory organs; it is, therefore, large, and complicated by a mechanism for regulating the tran- sit of the food to the stomach and the aerated water to the gills. The slits leading to the gills are provided with rows of processes which, like a sieve, prevent the entrance of food, arid with valves to keep the water, after it has en- tered the gills, from returning to the mouth. So that the mouths of Fishes may be said to be armed at both ends with teeth-bearing jaws. A few Fishes, as the Sturgeon, are toothless ; but, as a class, they have an extraordinary dental apparatus — not only the upper and lower jaws, but even the palate, tongue, and throat being sometimes stud- ded with teeth. Every part of the mouth is evidently designed for prehension and mastication. Lips are usu- ally present ; but the tongue is often absent, or very small, and as often aids respiration as ingestion. Amphibians and Reptiles have a wide mouth ; even the insect-feeding Toads and the Serpents can stretch theirs enormously. True fleshy lips are wanting; hence the savage aspect of the grinning Crocodile. With some ex- ceptions, as Toads and Turtles, the jaws are armed with teeth. Turtles are provided with horny beaks. The tongue is rarely absent, but is generally too thick and short to be of much use. In the Toad and Frog it is sin- gularly extensile : rooted in front and free behind, it is shot from the mouth with such rapidity that the insect is seized and swallowed more quickly than the eye can fol- low. The Chameleon's tongue is also extensile. Snakes 02 COMPARATIVE ZOOLOGY. FIG. 26.— Mouth of the Crocodile: d, tongue; <•, glands; /, inferior, and g, raperiur, valve, separating the cavity of the mouth from the throat, h. have a slender forked tongue, consisting of a pair of mus- cular cylinders, which is solely an instrument of touch. Birds are without lips or teeth, the jaws being covered with horn forming a beak. This varies greatly in shape, being extremely wide in the Whippoorwill, remarkably long in the Pelican, stout in the Eagle, and slender in the Hummer. It is hardest in those that tear or bruise their food, and softest in water-birds. The tongue is also cov- ered with a horny sheath, and generally spinous, its chief function being to secure the food when in the mouth. It is proportionally largest and most fleshy in the Parrots. The main characteristics of the mammalian mouth are flesh lips and mobile cheeks." In the duck-billed Mon- otremes lips are wanting, and in the Porpoises they are barely represented. But in the herbivorous quadrupeds they, with the tongue, are the chief organs of prehension ; in the carnivorous tribes they are thin and retractile; while in the Whale the upper lip falls down like a cur- tain, overlapping the lower jaw several feet. As a rule, the mouth is terminal ; but in the Elephant. Tapir, Hog, HOW ANIMALS EAT. 63 and Shrew, the upper lip blends with the nose to form a proboscis, or snout. The mouth is comparatively small in the Elephant and in gnawing animals like the Squir- rel, wide in the Carnivores, short in the Sloth, and long in the Ant-eater. Teeth are usually present, but vary in form and number with the habits of the animal. The Ant-eater is toothless, and the Greenland Whale has a sieve made of horny plates. The tongue conforms in size and shape with the lower jaw, and is a muscu- lar, sensitive organ, which serves many purposes, assisting in the prehension, mastication, and swal- £ lowing of food, besides being an organ of taste, touch, and speech. Its surface is covered with minute prominences, called papillce, which are arranged in lines with mathe- matical precision. In the Cats, these are developed into recurved FIG 8Ti_Hnmn spines, which the animal uses in cleaning bones and combing its fur. Similar papillae occur on the roof and sides of the mouth of the Ox and other Ruminants. In some animals, as the Hamster and Gopher, the cheeks are developed into pouches in which the food may be carried. These may be lined with hair. The tongue is remarkably long in the Ant-eater and Giraffe, and almost immovable in the Gnawers, Ele- phants, and Whales. 3. The Teeth of Animals. — Nearly all animals have certain hard parts within the mouth for the prehension or trituration of solid food. If these are wanting, the legs are often armed with spines, or pincers, to serve the same Tongue and ad- jacent parts: a, lingual papillae; 6, papilla? forming V-shaped lines ; rf, fungiform papillte ; e, filiform papilte; 0, epiglottis; 7n, uvula, or conical process, hanging from the soft palate, n; o, hard palate; r, palatine glands, the mucous membrane being removed ; v, section of the lower jaw. COMPARATIVE ZOOLOGY. purpose, as in the Horse -shoe Crab; or the stomach is lined with "gastric teeth," as in some marine Snails; or the deficiency is supplied by a muscular gizzard, as in Birds, Ant-eaters, and some Insects. Even the Lobster and Crab, in addition to their complicated oral organs, have the stomach furnished with a powerful set of teeth. The Sea-urchin is the first of animate, and almost the only one below Worms and Mollusks, which exhibits anything like a dental apparatus. Five calcareous teeth, having a wedge - shaped apex, each set in a triangular pyr- amid,- or "jaw," are moved upon each other by a FIG. 28.— Echinus bisected, showing masticating apparatus, complex arranfC- ment of levers and muscles. Instead of moving up and down, as in Yertebrates, or from right to left, as in Ar- thropods, they converge towards the centre, and the food passes between ten grinding surfaces. The Rotifers (a group of minute Worms) have a curi- ous pair of horny jaws. That which answers to the lower jaw is fixed, and called the " anvil." The upper jaw con- sists of two pieces called " hammers," which are sharply notched, and beat upon the "anvil" between them (Fig. 219). The horny-toothed mandibles of Insects, already men- tioned, are prehensile, and also serve to divide the food. The three little white ridges in the mouth of the Leech are the convex edges of horny semicircles, each bordered by a row of nearly a hundred hard, sharp teeth. When the mouth, or sucker, is applied to the skin, a sawing HOW ANIMALS EAT. 65 movement is given to the horny ridges, so that the "bite" of the Leech is real- ly a saw-cut. B The dentition of _ FIG. 2W. — Teeth and Masticatory Apparatus of Gastero- the Univalve Mol- pods: A, portion of odontophore, or "tongue," of Pel- , , 4.1 o •] utina, enlarged; B, portion of odontophore of Whelk IllSKS, Or tile OliaiJS, (Biuxinum undatum), magnified — the entire tongue i a rr ATI f>ra 1 1 v 1 i r> rrn a 1 nas 10° rows of teeth : C; head and odontophore of Lim- it? &t;il dUJ miglldJ, pet (patella vulgata) \ D, portion of same, greatly mag- i. 6. it Consists of nifled, to show the transverse rows of siliceous teeth. microscopic teeth, usually siliceous and amber -colored, planted in rows on the tongue. The teeth are, in fact, the ser- rated edges of minute plates. The number of these plates va- ries greatly ; the garden Sing has 160 rows, with 180 teeth in each row. All living Birds, and some other Vertebrates, as Ant-eat- ers,24 Turtles, Tortoises, Toads, and Sturgeons, have no teeth. Their place is often supplied by a horny beak, a muscular gizzard, or both structures. In a few Vertebrates, horny plates take the place of teeth, as the Duck Mole (Ornitho- rhynchus) and Whalebone Whale. In the former, the plates consist of closely set ver- hollow tubes ; in the lat- r, the baleen, or whalebone, attaching the horny body of the ba- plates, triangular ill shape, and leen-plate, c; d, fringe of bristles ; e, v . ' fringed on the inner side, hang 5 smaller plates. QQ COMPARATIVE ZOOLOGY. in rows from the gums of the upper jaw. In some Whales there are about 300 plates on each side." True teeth, consisting mainly of a hard, calcareous sub- stance called dentine, are found only in back-boned ani- mals. They are distinct from the skeleton, and differ from bone in containing more min- ie eral matter, and in not showing, under the microscope, any minute cavities, called lacunae. A typical tooth, as found in Man, consists of a central mass of dentine, capped with enamel and surrounded on the fang with cement. The first tissue is always present, while the others may be absent. It is a mixt- ure of animal and mineral matter FIG. 31.— Section of Human Mo- •> • , i <• /• i lar, enlarged: k, crown; n, disposed in the form of extremely define* ^n^Tpu^ fine t»be8 and Cells> SO mi"»te aS tO cavity- prevent the admission of the red particles of blood. One modification of it is ivory, seen in the tusks of Elephants. Enamel is the hardest tissue of the body, and contains not more than two per cent, of animal matter. It consists of six-sided fibres set side by side, at right angles to the surfaces of the dentine. Ce- ment closely resembles bone, and is present only in the teeth of the higher animals. Teeth are usually confined to the jaws ; but the num- ber, size, form, structure, position, and mode of attachment vary with the food and habits of the animal. As a rule, animals developing large numbers of teeth in the back part of the mouth are inferior to those having fewer teeth, and those nearer the lips. The teeth of Mammals only have fangs. The teeth of Fishes present the greatest variety. In number, they range from zero to hundreds. The Hag- HOW ANIMALS EAT. 67 fish (Myxi-ne) has a single tootli on the roof of the mouth, and two serrated plates on the tongue; while the mouth of the Pike is crowded with teeth. In some we find teeth short and blunt, in the shape of cubes, or prisms, arranged like mosaic work. Such pavement-teeth (seen in some Rays) are fitted for grinding sea-weed and crush- ing shell-fish. But the cone is the most common form : sometimes so slender and close as to resemble plush, as in the Perch ; or of large size, and flattened like a spear - head with serrated edges, as in the Shark; but more often like the PIG. 32.— Jaws and Pavement-teeth of a canines of Mammals, curved Ray (Myiioia ««). inward to fit them for grappling. In the Shark, the teeth are confined to the fore-part of the mouth; in the Carp, they are all situated on the bones of the throat; in the Parrot-fish, they occupy both back and front; but in most Fishes the teeth are developed also on the roof, or palate, and, in fact, on nearly every bone in the mouth. They seldom appear (as in the Salmon) on the upper max- illary. As to mode of attachment, the teeth are generally anchylosed (fastened by bony matter) to the bones which support them, or simply bound by ligaments, as in the Shark. In a few Fishes, the teeth consist of flexible car- tilage; but almost invariably they are composed of some kind of dentine, enamel and cement being absent. Of Amphibians and Reptiles, Toads, Turtles, and Tor- toises are toothless ; Frogs have teeth in the upper jaw only; Snakes have a more complete set, but Saurians pos- sess the most perfect dentition. The number is not fixed even in the same species : in the Alligator it varies from 72 to 88. The teeth are limited to the jawbones in the higher forms (Saurians); but in others, as the Serpents, 68 COMPARATIVE ZOOLOGY. they are planted also in the roof of the mouth. With few exceptions, they are conical and curved (Fig. 33). In the Serpents they are longest and sharpest; and the ven- omous species have two or more fangs in the upper jaw. 9 m These fangs contain a canal, through which the poison is forced by muscles which compress the gland. The bones to which they are at- tached are movable, and the FIG. 33.— Poison Apparatus of the Rattle- fangs ordinarily lie flat upon the gums, but are brought the jaws ; n, nostril. the act of striking. As a rule, the teeth of Eeptiles are simply soldered to the bone which supports them, or lodged in a groove ; but those of Crocodiles are set in sockets. Eeptilian teeth are made of dentine and a thin layer of cement, to which is added in most Saurians a coat of enamel on the crown. In the majority of Mammals, the teeth are limited in number and definite in their forms. The number ranges from 1 in the Narwhal (but the longest tooth in the king- dom) to 220 in the Dolphin. The average is 32, occur- ring in Ruminants, Apes, and Man ; but 44 (as in the Hog and Mole) is called the typical or normal number, and this number is exceeded only in the lower groups. When very numerous, the teeth are of the Reptilian type, small, pointed, and of nearly equal size, as in the Porpoise. In the higher Mammals, the teeth are comparatively few, and differ so much in size, shape, and use, that they can be classed into incisors, canines, premolars, and molars. Such a dental series exhibits a double purpose, prehension and mastication. The chisel-shaped front teeth are fitted for cutting the food, and hence called incisors. These vary in number : the Lion has six in each jaw ; the Squir- HOW ANIMALS EAT. 69 rel has two in each jaw, but remarkably developed ; the Ox has none in the upper jaw, and the Elephant none in the lower ; while the Sloth has none at all.26 The canines, so called because so prominent in the Dog, are conical, and, except in Man, longer than the other teeth. They are designed for seizing and tearing; and they are the most formidable weapons of the wild carnivores. There FIG. 34. — Skull of the Babirusa, or Malayan Hog, showing growth aud curvature of the canines. are never more than four. They are wanting in all Ro- dents, and in nearly all herbivorous quadrupeds. The molars, or grinders, vary greatly in shape, but closely cor- respond with the structure and habits of the animal, so that a single tooth is sufficient to indicate the mode of life and to identify the species." In the Ruminants, Ro- dents, Horses, and Elephants, the summits of the molars are flat, like mill-stones, with transverse or curving ridges 70 COMPARATIVE ZOOLOGY. of enamel. In the Cats and Dogs, they are narrow and sharp, passing by each other like the blades of scissors, and therefore cutting, rather than grinding, the food. The more purely carnivorous the species, and the more it feeds upon living prey, the fewer the molars. In ani- mals living on mixed diet, as the Hog and Man, the crowns have blunt tubercles. Premolars, or bicuspids, are those which were preceded by milk-teeth ; the true, or back, molars had no predecessors. The dentition of Mammals is expressed by a formula, which is a combination of initial letters and figures in 7113 FIG. 35.— Teeth of the right lower jaw of adnlt male Chimpanzee (Troglodytes niger), natural size. The molar series does not form a curve, as in Mail. fractional form, to show the number and kind of teeth on each side of both jaws. Thus, the formula for Man is : *» |E| J c> T=I 5 P> lirl 5 ™, J=| = 32. The teeth of Mammals are always restricted to the margins of the jaws, and form a single row in each. But they rarely form an unbroken series.28 The teeth im- planted in the premaxillary bone, and in the correspond- ing part of the lower jaw, whatever their number, are in- cisors. The first tooth behind the premaxillary, if sharp and projecting, is a canine. Each tooth has its particular bony socket." The molars HOW ANIMALS EAT. 71 may be still further strengthened by having two or more diverging fangs, or roots, a feature peculiar to this class. The incisors and canines have but one fang; and those that are perpetually growing, as the incisors of Rodents and Elephants, have none at all. The teeth of flesh-eat- ing Mammals usually consist of hard dentine, surrounded on the root with cement and capped with enamel. In the herbivorous tribes, they are very complex, the enamel and cement being inflected into the dentine, forming folds, as in the molar of the Ox, or plates, as in the compound tooth of the Elephant. This arrangement of these tissues, which differ in hardness, secures a surface with prominent er Molar Tooth of Indian Elephant (Elephas Indicus), showing trans- verse arrangement of dentine, d, with festooned border of enamel plates, e; c, FIG. 36.— Upper Molar Tooth verse arrangement of de _, cement ; one-third natural size. ridges, well adapted for grinding. The cutting teeth of the Rodents consist of dentine, with a plate of enamel on the anterior surface, and the unequal wear preserves a chisel-like edge. Enamel is sometimes wanting, as in the molars of the Sloth and the tusks of the Elephant. In Fishes and Reptiles, there is an almost unlimited succession of teeth ; but Mammalian teeth are cast and renewed but once in life. Vertebrates use their teeth for the prehension of food, as weapons of offence or defence, as aids in locomotion, and as instruments for uprooting or cutting down trees. But in the higher class they are principally adapted for dividing or grinding the food.30 While in nearly all other 72 COMPARATIVE ZOOLOGY. Vertebrates the food is bolted entire, Mammals masticate it before swallowing. Mastication is more essential in the digestion of vegetable than of animal food ; and hence we find the dental apparatus most efficient in the herbivorous quadrupeds. The food is most perfectly reduced by the Rodents. Teeth, as we shall see, are appendages of the skin, not of the skeleton, and, like other superficial organs, are es- pecially liable to be modified in accordance with the hab- its of the creature. They are, therefore, of great zoologi- cal value ; for, such is the harmony between them and their uses, the naturalist can predict the food and general structure of an animal from a sight of the teeth alone. For the same reason, they form important guides in the classification of animals; while their durability renders them available to the paleontologist in the determination of the nature and affinities of extinct species, of which they are often the sole remains. Even the structure is so peculiar that a fragment will sometimes suffice. 4. Deglutition, or How Animals Swallow. — In the lowest forms of life, the mouth is but an aperture opening immediately into the body-cavity, and the food is drawn in by ciliary currents. But in the majority of animals, a muscular tube, called the gullet, or oesophagus, intervenes between the mouth and stomach, the circular fibres of which contract, in a wave-like manner, from above down- ward, propelling the morsel into the stomach.31 In the higher Mollusks, Arthropods, and Vertebrates, deglutition is generally assisted by the tongue, which presses the food backward, and by a glairy juice, called saliva, which facil- itates its passage through the gullet.8" Vertebrates have a cavity behind the mouth, called the throat, or pharynx, which may be considered as a funnel to the oesophagus." In air-breathers, it has openings leading to the windpipe, nose, and ears. In Man, as in Mammals generally, the HOW ANIMALS EAT. 73 process of deglutition is in this wise : the food, masticated by the teeth and lubricated by the saliva, is forced by the tongue and cheeks into the pharynx ; the soft palate keep- ing it out of the nasal aperture, and the valve-like epiglot- tis falling down to form a bridge over the opening to the windpipe. The moment the pharynx receives the food, it is firmly grasped, and, the muscular fibres contracting above it and left lax below it, it is rapidly thrust into the oesophagus. Here, a similar movement (the peristaltic) strips the food into the stomach.34 The rapidity of these contractions transmitted along the oesophagus may be ob- served in the neck of a Horse while drinking. Deglutition in the Serpents is painfully slow, and some- what peculiar. For how is an animal, without limbs or molars, to swallow its prey, which is often much larger than its own body ? The Boa-constrictor, e. g., seizes the FIG. 37.— Skull of Boa-constrictor: 1, frontal; 2, prefrontal ; 4, postfroutal; 5, basi- occipital; 6, sphenoid; 7, parietal; 12, squamosal; 13, prootic; 17, premax- illary; 18, maxillary; 20, nasal; 24, transverse; 25, internal pterygoid; 34, den- tary, lower jaw; 36, angular; 36, articular ; a, quadrate; «, prenasal; v, petrosaL head of its victim with its sharp recurving teeth, and crushes the body with its overlapping coils. Then, slow- ly uncoiling, and covering the carcass with a slimy mu- cus, it thrusts the head into its mouth by main force, the mouth stretching marvellously, the skull being loosely put 74 COMPARATIVE ZOOLOGY. together. One jaw is then unfixed, and the teeth with- drawn by being pushed forward, when they are again fastened farther back upon the animal. The other jaw is then protruded and refastened ; and thus, by successive movements, the prey is slowly and spirally drawn into the wide gullet. CHAPTER IX. THE ALIMENTARY CANAL. The Alimentary Canal is the great route by which nutritive matter reaches the interior of the body. It is the most universal organ in the animal kingdom, and the rest are secondary or subservient to it. In the higher an- imals, it consists of a mouth, pharynx, gullet, stomach, and intestine. It is a general law, that food can be introduced into the living system only in a fluid state. While plants send forth their roots to seek nourishment from without, ani- mals, which may be likened to plants turned outside in, have their roots (called absorbents) directed inward along the walls of a central tube or cavity. This cavity is for the reception and preparation of the food, so that animals may be said to carry their soil about with them. The necessity for such a cavity arises not only from the fact that the food, which is usually solid, must be dissolved, so as to make its way through the delicate walls of the cav- ity into the system, but also from the occurrence of inter- vals between the periods of eating, and th? consequent need of a reservoir. For animals, unlike plants, are thrown upon their own wits to procure food. The Protozoa, as the Amoeba and Infusoria, can hardly THE ALIMENTARY CANAL. 75 be said to have a digestive canal. The animal is here composed of a single cell, in which the food is digested. The jelly-like Amceba passes the food through the firmer outer layer (ectosarc) into the more fluid inner part (endo- sarc), where it is digested. The Infusoria, which have a cuticle, and so a more definite form, possess a mouth, or opening, into the interior of their cell-body, and at least a definite place where the excrement is passed out. But we cannot call this cell-cavity a digestive tract. In the higher animals, the alimentary canal is a contin- uation of the skin, which is reflected inward, as we turn the finger of a glove.35 We find every grade of this re- flection, from the sac of the Hydra to the long intestinal tube of the Ox. So that food in the stomach is still out- side of the true body. The simplest form of such a digestive, tract is seen in the Hydra (Fig. 191). Here the body is a simple bag, whose walls are composed of two layers of cells (ectoderm and endoderm). A mouth leads into the cavity, and serves as well for the outlet of matter not wanted. The endodermal cells furnish the juices by which the food is di- gested and absorb the nutritious portions of it. There is no rad- ical difference, how- ever, between the two 1 f 11 -f <-V> FIG. 38. — Dissected Actinia: a, the thick opaque skin :611S, IOr tlie consisting of ectoderm, lined with muscular fibres ; rnrn ci tne tubular tentacles communicating with the in- " terspaces ; *, between the membranous vertical ed inside OUt, When folds; #, g', orifices in the walls allowing passage ... of respiratory water from one compartment to an- the former ectoderm other; d, mouth leading to gastric cavity, e. 76 COMPARATIVE ZOOLOGY. has digested the food and the former endoderm has taken on the functions of the outer layer. The Polyps have also but one external opening ; but from this hangs down a short tube, open at both ends, reaching about half-way to the bottom of the body-cavity. Such an arrangement would be represented by a bottle with its neck turned inward. In this suspended sac, which is somewhat con- stricted at the extremities, digestion takes place ; but the product passes freely into all the surrounding chambers, along with the water for respiration. The Medusae, or Jelly-fishes, preserve the same type of a digestive appara- tus; but the sac is cut off from the general cavity, and numerous canals radiate from it to a circular canal near the margin of the disk. In the Star-fishes (Fig. 126), we find a great advance. The sac-like stomach sends off two glandular branches to each arm, which doubtless furnish a fluid to aid in digestion (so-called hepatic cceca). There is also an anus present in some forms, but it hardly serves to pass off the waste matter. Thus far we have seen but one opening to the digestive cavity, rejected portions returning by the same road by which they enter. But a true alimentary canal should have an anal aperture distinct from the oral. The sim- plest form of such a canal is exhibited by the Sponge, in its system of absorbent pores for the entrance of liquid, and of several main channels for its discharge. The apparatus, however, is not marked off from the general cavity of the body, and digestion is not distinct from cir- culation.3' The Sea-urchin presents us with an important advance — one cavity with two orifices; and the complicated ap- paratus of higher animals is but the development of this type. This alimentary canal begins in a mouth well pro- vided with teeth and muscles, and extends spirally to its outlet, which generally opens on the upper, or opposite, THP; ALIMENTAKY CANAL. 77 surface. Moreover, while in some of the Worms the canal is a simple tube running through the axis of the cylindri- cal body from oral ori- fice to anal aperture, the * canal of the Sea-urchin shows a distinction of parts, foreshadowing the pharynx, gullet, stom- ach,and intestines. Both mouth and vent have muscles for constriction and expansion; and, as the vent is on the sum- mit of the shell, and the latter is covered with spines, the ejected par- ticles are seized by del- icate forks (pedicella- ricB), and passed on from one to the other down the side of the body, till they are dropped off into the water." The Worms present us with a great range of structure in the digestive tract. It is sometimes almost as si-mple as that of the Hydra — a mere sac. The Earth-worm has a tube running straight through the body, divided into pharynx, oesophagus, crop, gizzard, and sacculated intes- tine. The Leech has large sacs on each side of the intes- tine. The Sea-worms have the pharynx armed with teeth, and some have glandular cceca attached to the intestine. The plan is that of a straight tube extending from mouth to anus. In Myriapods and larvae of Insects, the same general plan is continued, the canal passing in a straight line from one extremity to the other, but showing a divis- ion into gullet, stomach, and intestine." Crustacea, like Fie. 39. — Dingrammatic Section of a Sea-nrchin (Echimts) : a, month ; 6, oesophagus ; c, stom- ach'; d, intestine ; /, madreporiform tubercle ; 17, stone-canal; h, ambulacral ring; k, Polian vesicles, which are probably reservoirs of fluid; m, ambnlacral tube; o, anus; p, ambulacra, with their contractile vesicles; r, nervous ring around the gullet ; «, two nervous trunks, the right terminating, at anal pole, in a small gan- glion ; t, blood-vascular rings connected by v, the contractile heart ; w, two arterial trunks ra- diating from the anal ring; x, an ovary open- ing at the anal pole in a genital plate, y; z, spines, with their tubercles. 78 COMPARATIVE ZOOLOGY. the Lobster, have a short gullet leading to a large cav- ity, situated in the front of the animal, which is a giz- zard, rather than stomach, as it has thick muscular walls armed with teeth. A well- marked constric- tion separates this organ from the in- testine. The liver is highly devel- oped ; instead of numerous folli- cles, there is a large bilaterally symmetrical or- gan, divided into three lobes on each side, pouring its secretion into the upper part of the intestine, which is the true stomach. Among Insects, there is great vari- ation in the form and length of the mm £&§, m j£| pf Jj- Fis. 40.— Anatomy of a Caterpillar: y, h, oesophagus; h, canal. Thefollow- j, stomach ; k, hepatic vessels ; I, m, intestine ; q, r, sal- ivni-y glands ; p, salivary duct ; a, 6, c, longitndinnl • trachenl trnnk* ; rf, e, air-tubes distributed to the vis- *ng Parts Can cera; /, fat-mass; v, x, y, silk-secretors ; z, their ex- Ora11v ])P cretory ducts, terminating in t, the spinneret, or fu- G1 ™ l ««f«»- guished : gullet, crop, gizzard, stomach, and large and small intestines, with many glandular appendages. The crop, gizzard, and large intestine are sometimes absent, especially in the carnivorous THE ALIMENTARY CANAL. 79 species. In Bees, the crop is called the "honey -bag." The gizzard is found in Insects having mandibles, and is FIG. 4t.— Alimentary canal of a Beetle: a, pharynx; b, gullet, leading to crop, c, gizzard, d, and stomach, e ; f, deli- cate urinary tubes; 0, intestine; h, other secreting organs. FIG. 42. — Alimentary Canal of the Bee (Apis mellifica) : a, gullet; 6, crop; e, d, stomach ; e, small intestine ; /, large in- testine ; g, anal orifice ; h, urinary ves- sels ; i, auxiliary glands. frequently lined with rows of horny teeth, which are spe- cially developed in Grasshoppers, Crickets, and Locusts. The intestines are remarkable for their convolutions. In- sects have no true liver; but its functions are performed by little cell-masses on the inside of the stomach.39 The alimentary canal of Spiders is short and straight, the pharynx and gullet being very minute. The stomach is characterized by sending out tubular prolongations, and I' I" V" s n no' Fio. 43.— Anatomy of a Sphinx Moth : n, nervous cord ; n\ brain sending off nerves to the legs, I', I", I'", and for the wings at n" • h, dorsal vessel, or heart ; c, crop; «, stomach; «', intestines; o, reproductive organs ; o', oviduct; 8-20, segments. 80 COMPARATIVE ZOOLOGY. the intestines end in a large bladder-like expansion. Scor- pions have no stomachal cavity — a straight intestine passes directly through the body. In bivalve Mollusks, like the Clarn, the month opens into a short oesophagus which leads into the stomach, which lies imbedded in a large liver, and the intestine, describing a few turns, passes directly through the heart.40 In the univalve Mollnsks, like the Snail, the gullet is long, and frequently expands into a crop ; the stomach is often double, the anterior being a gizzard provided with teeth for mastication ; the intestine passes through the liver, and ends in the fore-part of the body, usually on the right side. The highest Mollusks, as the Cuttle-fish and Nautilus, exhibit a marked advance. A mouth with powerful man- dibles leads to a long gullet, which ends in a strong mus- cular gizzard resembling that of a fowl.41 Below this is a cavity, which is either a stomach or duodenum ; it receives the bile from a large liver. The intestine is a tube of uniform size, which, after one or two slight curves, bends up, and opena into the "funnel" near the mouth. Fishes have a simple, short, and wide alimentary canal. The stom- ach is separated Pto. 44. — Alinientiiry Caunl of the Oyster: a, Btomnch f pom the intestine laid open ; ei, liver ; 6, c, d,f, convolutions of the intes- tine , (7, anal apertnre; n, o, auricle and ventricle; /, by a nari'OVV " py- m, adductor muscle; h, k, lobes of mouth divided to . . ,, .,, show the venous canals ut the base of the gills. lOHC OrmCC, or THE ALIMENTARY CANAL. 81 valve, but is not so clearly distinguished from the gullet, so that regurgitation is easy.43 Indeed, it is common for FIG. 46. — Anatomy of a Lamellibranch (Mactra) : a, shell ; h, mantle ; e, tentacles, or lips ; d, mouth ; e, nerves ; /, muscles ; g, anterior, mid n, posterior ganglion ; h, liver; i, heart: k, stomach ; I, intestine passing through the heart; w, kidney • o, anal end of the intestine ; p, exhaleiit, aud q, iuhalent respiratory tubes, or si- phoiis ; r, gills ; 8, foot. 82 COMPARATIVE ZOOLOGY. Fishes to disgorge the indigestible parts of their food, and some, as the Carp, send the food back to the pharynx to be masticated. The stomach is usually bent, like a si- phon; but the intestine is nearly straight, and without any marked distinction into small and large. Its append- ages are a large liver and a rudimentary pancreas. In the Amphibians, as the Frogs, the digestive appara- tus is very similar to that of Fishes; but the two kinds of intestines can be more readily distinguished. The Reptiles gen- erally have a long, wide gullet, which passes insensibly into the stomach, and a short intestine (about twice the length of the body) very distinctly divided into small and large by a constric- tion.43 The vegetable -feed ing Tortoises have a comparatively long intestinal tube ; and the Serpents have a slender stomach, but little wider than the rest of IO. 47.— Aimt.miy of a Cepimiopod the alimentary canal. (diagram): a, tentacles; b, masti- m. . - . ~ ... calory apparatus; c, eye; d, sali- Ihe Stomach OI the (JrOCOdllC ?Z$^?J^J^& is more complex than any liitli- Ifc resembles Ink-bag; «, ovary; p, oviduct; p, that of the Cuttle-fish, l)llt offei'S liver ; r, gill contained in the hrnn- ... ., . chial chamber; «, branchial heart; a Still mOl'6 Striking analogy to Systemic heart 5 * mantle. the ^^ Q{ ft ^ having very thick walls, and the muscular fibres radiating pre- cisely in the same manner, so that, in this respect, the Crocodile may be considered the connecting link between Reptiles and Birds." In Crocodiles also the duodenum, with which the intestine begins, is first distinctly defined. Into this part of the intestine the liver and pancreas, or sweet-bread, pour their secretions. Furthermore, in the THE ALIMENTARY CANAL. lower animals, the intestines lie more or less loose in the abdomen ; but in the Crocodile, and likewise in Birds and Mammals, they are supported by a membrane called mes- entery. Fio. 48. — Anatomy of the Carp : br, branchiae, or gill-openings ; c, heart ; /, liver; vn, vn't swimming-bladder ; ci, intestinal canal; v, ovariuin; it, ureter; a, anus; o', genital opening; u', opening of ureter. The side-view shows the disposition of the muscles in vertical flakes. 84 COMPARATIVE ZOOLOGY. In Birds, the length of the alimentary canal varies with their diet, being greatest in those living on grain and fruit. The gullet corresponds in length with the neck, which is longest in the long-legged tribes, and in width with the food. In those that swallow large fish entire, the gullet is dilatable, as in Snakes. In nearly all Birds, the food is delayed in some cavity before digestion : thus, the Pelican has a bag under the lower jaw, and the Cormorant has a capacious gullet, where they store up fishes ; while those that gorge themselves at in- tervals, as the Vulture, or feed on seeds and grains,astheTur- key,haveapouch, called the crop, developed near the lower end of the gullet.46 The Ostrich, Goose, Swan, most of the Waders, and FIG. 49 — Stomach of the Crocodile: nt, muscular fibres ra- , ,. dialing from a ceutnil tendon, 6; d, commencement of tlle U'lllt Or in- duodeuam ; c, (esophagus ; /, intestine. gect cat jng Birds, which find their food in tolerable abundance, and take it in small quantities, have no such reservoir. Pigeons have a double crop. In all Birds, the food passes from the gullet into the proventriculus, or stomach proper, where it is mixed with a " gastric juice " secreted from glands on the surface. Thence it goes into the gizzard, an oval sac of highly inuscular texture, and lined with a tough, horny skin.48 THE ALIMENTARY CANAL. 85 The gizzard is most highly developed, and of a deep-red color, in the Scratchers and flat-billed Swimmers (as Fowls and Swans); but comparatively thin and feeble in Birds of Prey (as the Eagle). The gizzard is follow- ed by the intestines, which are longer than those of Reptiles : the small intestine begins with a loop (the duo- denum), and is folded several times upon it- self ; the large intestine is short and straight, terminating in the sole outlet of the body, the cloaca. A liver and pancreas are always attached to the upper part of the small in- testine. The alimentary ca- nal in Mammals is clearly separated into four distinct cavities: the pi larynx, or throat; the oasophagus, or gul- let ; the stomach ; and the intestines. The pharynx is more FIG. 50. —Digestive Apparatus of the Fowl: I, ,. j i • tongue: 2, pharynx: 3, 5, oesophagus; 4, crop; Complicated than in C, proventncnlus ; 7, gizzard ; 8, 9, 10, dnodenum ; "Ritvk Tt id a -frmnol 11,12, small intestine ; 13, two cseca (analogue of Ub. It IS a IL the co]on of mammals); 14, their insertion into shaDed ba0" bavin0" l'ie 'ntes*'na' tube; 15, rectum; 1C, cloaca; 17, & anus ; 18, mesentery ; 19, 20, left and right lobes Seven Openings lead- «>f liv«l-: 2L gall-bladder ; 22, insertion of pan- . creatic and biliary ducts; 23, pancreas; 24, lung; ing mtO it: tWO from 25, ovary; 26, oviduct 80 COMPARATIVE ZOOLOGY. the nostrils, and two from the ears ; one from the windpipe, guarded by the epiglottis ; one from the mouth, with a fleshy curtain called the soft pal- ate ; and one from the oesophagus. It is the nat- ural passage for food be- tween the mouth and the O3sophagus, and of air be- tween the nostrils and windpipe. Like the mouth, it is lined with a soft mucous membrane. The ossophagus is a long and narrow tube, formed of two muscular layers : in the outside one, the fibres run length- wise ; in the other, they are circular. It is lined with mucous membrane. While in all Fishes, Rep- tiles, and Birds the ven- tral chamber is one, in Mammals it is divided, by a partition called the diaphragm, into two cav- ities— the thorax, con- taining the heart, lungs, Fm. 51. — Digestive Apparatus of Man (diagram): 1, tougae; 2, pharynx; 3, oesopha- gus; 4, soft palate ; 6, larynx; C, palate; 7, epiglottis; 8, thyroid cartilage; 9, beginning of spinal marrow; 10, 11, 12, vertebne, with spinons processes; 18, cardiac orifice of stomach; 14, left end of stomach ; IS, pyloric valve; 19, 20, 21, duodenum; 22, gall-bladder ; 27, duct from pancreas ; 28, 20, jejunum of intestine; 30, ileum ; 34, cceciuu ; 36, 37, 39, colon, or large intestine ; 40.' rectum. THE ALIMENTARY CANAL. 87 etc. ; and the abdomen, containing the stomach, intes- tines, etc. The resophagus passes through a slit in the FIG. 52.— Ideal Section of a Mammalian Vertebrate: A, pectoral, or fore limb; B, pelvic, or hind limb: a, mouth ; b, cerebrum; c, cerebellum; rf, nose ; e, eye; /, ear; g, oesophagus; h, stomach ; t, intestine; j, diaphragm, or midriff; k, rectum, or termination of intestine ; I, anus ; m, liver ; r«, spleen ; o, kidney ; p, sympa- thetic system of nerves; 9, pancreas; r, urinary bladder; «, epinal cord; M, ure- ter ; v, vertebral column ; w, heart ; x, lung ; lintil ifc is exPelled tudinai muscles. from tiie ^ydy. The beginning of the small intestine is called the duodenum, into which the ducts from the liver and pancreas open. The intes- tinal canal has the same structure as the stomach, and by a peristaltic motion its contents are propelled downward. The inside surface of the small intestine is covered with a host of thread-like processes (villi), resembling the pile of velvet. HOW ANIMALS DIGEST. 91 In taking this general survey of the succession of forms which the digestive apparatus presents among the princi- pal groups of animals, \ve cannot fail to trace a gradual specialization. First, a simple sac, one orifice serving as inlet for food and outlet for indigestible matter; next, a short tube, with walls of its own suspended in the body- cavity; then a canal passing through the body, and, there- fore, having both mouth and vent; next, an apparatus for mastication, and a swelling of the central part of the canal into a stomach, having the special endowment of secreting gastric juice; then a distinction between the small and large intestine, the former thickly set with villi, and re- CL'iving the secretions of large glands. We also notice that food, the means of obtaining it, the instruments for mastication, and the size and complexity of the aliment- ary canal, are closely related. CHAPTEK X. HOW ANIMALS DIGEST. The object of the digestive process is the reduction of food into such a state that it can be absorbed into the system. For this purpose, if solid, it is dissolved; for fluidity is a primary condition, but not the only one. Many soluble substances have to undergo a chemical change before they can form parts of the living body. If albumen or sugar be injected into the veins, it will not be assimilated, but be cast out unaltered. To produce these two essential changes, solution and transmutation, two agencies are used — one mechanical, the other chemical. The former is not always needed, for many animals find their food already dissolved, as the 92 COMPARATIVE ZOOLOGY. Butterfly; but solid substances, to facilitate their solu- tion, are ground or torn into pieces by teeth, as in Man ; by jaws, as in the Lobster; or by a gizzard, as in the Turkey. The chemical preparation of food is indispensable.47 It is accomplished by one or more solvent fluids secreted in the alimentary canal. The most important, and one al- ways present, is the gastric juice, the secretion of which is restricted to the stomach, when that cavity exists. In the higher animals, numerous glands pour additional flu- ids into the digestive tube, as saliva into the upper part or mouth, and bile and pancreatic juice into the upper part of the intestine. In fact, the mucous membrane, which lines the alimentary canal throughout, abounds with secreting glands or cells. The Digestive Process is substantially the same in all animals, but it is carried further in the more highly de- veloped forms. In the Infusoria, the food is acted upon by some secretion from the walls of the body-cavity, the exact nature of which is unknown. In the Star-fish and Sea-urchin, we find two solvents — a gastric juice, and another resembling bile; but the two appear to mingle in the stomach. Mollusks and Arthropods show a clear distinction between the stomach and intestine, and the contents of the liver are poured into the latter. There are, therefore, two stages in the digestive act: first, the food is dissolved by the gastric juice in the stomach, form- ing chyme ; secondly, the chyme, upon entering the intes- tine, is changed into chyle by the action of the bile, and is then ready to be absorbed into the system. In Vertebrates, a third solvent is added, the pancreatic juice, which aids the bile in completing digestion. But Mammals and Insects have a still more perfect and elab- orate process; for in them the saliva of the mouth acts chemically upon the food; while the saliva in many other HOW ANIMALS DIGEST. 93 animals lias no other office, so far as we know, than to moisten the food for swallowing. Taking Man as an example, let us note the main facts in the process. During mastication, by which the relative surface is increased, the food is mixed with saliva, which moistens the food,48 and turns part of the starch into grape-sugar. Passed into the stomach, the food meets the gastric juice. This is acid, and, first, stops the action of the saliva; secondly, by means of the pepsin which it con- tains, and the acid, it dissolves the albumen, fibrine, and such constituents of the food. This solution of albumi- noids is called a peptone, and is especially distinguished from other such solutions by its diff usibility — i. e., the ease with which it passes through a membrane. These pep- tones, with the sugars of the food, whether original or the product of the action of the saliva, are absorbed from the stomach. The food, while in the stomach, is kept in con- tinual motion, and, after a time, is discharged in gushes into the intestine. The name chyme is given to the pulpy mass of food in the stomach. In the intestine the chyme meets three fluids — bile, pancreatic juice, and intestinal juice. All of these are alkaline, and at once give the acid chyme an alkaline reaction. This change permits the action of the saliva to recom- mence, which is aided by the pancreatic and intestinal juices. The pancreatic juice has much more important functions. It changes albuminoid food into peptones, and probably breaks up the fats into very small par- ticles, which are suspended in the fluid chyle. This forms an FIG. 59.— chyle corpuscles, x 500. emulsion, like milk, and causes the chyle to appear whit- ish. The bile has important functions, but little under- 94: COMPARATIVE ZOOLOGY. stood. It saponifies part of the fats, so that they are dig- solved, and prevents the food from decomposing during the process of digestion and absorption. The chyle is slowly driven through the small intestine by the creep- ing, peristaltic motion of its walls,49 the nutritious portion being taken up by the absorbents, as described in the next chapter, while the undigested part remaining is discharged from the large intestine.50 CHAPTER XL THE ABSORBENT SYSTEM. THE nutritive matter (chyle), prepared by the digestive process, is still outside of the organism. How shall it enter the living tissue ? In animals, like the Infusoria and Polyps, whose digest- ive department is not separated from the body -cavity, the food, as soon as dissolved, mingles freely with the tissues and organs it lias to nourish. In the higher Invertebrates having an alimentary canal, the chyle passes, by simple transudation, through the walls of the canal directly into the soft tissues, as in Insects, or is absorbed from the canal by veins in contact with it, as in Sea-urchins, Mollusks, Worms, and Crustaceans, and then distributed through the body. In Vertebrates only do we find a special absorbent sys- tem. Three sets of vessels are concerned in the general process by which fresh material is taken up and added to the blood : Capillaries, Lacteals, and Lymphatics. Only the two former draw material from the alimentary canal. It is a general law that the food is absorbed as fast as THE ADSORBENT SYSTEM. 95 it is dissolved, and, therefore, there is a constant loss in the passage down the canal. In the mouth and resoph- agus, the absorption is slight; but much of that which has yielded to the gastric juice, with most of the water, is greedily absorbed by the capillaries of the stomach, and made to join the current of blood which is rushing to the liver. Absorption by the capillaries also takes place from the skin and lungs. Medicinal or poisonous gases and liquids are readily introduced into the system by these channels. We have seen that the oily part of the food passes un- changed from the stomach into the small intestine, where, acted upon by the pancreatic juice, it is cut up into ex- tremely minute particles, and that the undigested albumi- noids and starches are digest- ed in the intestine. Two kinds of absorbents are pres- ent in the intestine, lacteals and blood -capillaries. Both the lymphatic and blood sys- tems send vessels into the velvety villi" with which the intestine is lined. The blood- .,, . ,. j .1 , Fm. 60.— Lacteal System of Mammal: a. Capillaries lie towards the OUt- descending norm, or principal artery : Ri'dp of flip villns and thp b' thoracic dllct' c- orteiu of lacteal ' vessels, g, in the walls of the intestine, lacteal in the Centre. The d' ".mesentery, or membrane attach- ins* the intestine to walls of the body ; albuminoids and SUgarS are /, lacteal, or mesenteric, glands. chiefly absorbed by the blood-vessels and go to the liver. The fats pass on into the lacteals, which receive their name from the milky appearance of the chyle. These lacteals unite into larger trunks, which lie in the mesen- tery (or membrane which suspends the intestine from the back wall of the abdomen), and these pour their contents into one large vessel, the thoracic duct, lying along the backbone, and joining the jugular vein in the neck. 96 COMPARATIVE ZOOLOGY. The laeteals are only a special part of the great lym- phatic system, which absorbs and carries to the thoracic duct matter from all parts of the body." The lymph is a transparent fluid having many white blood corpus- cles. It is, in fact, blood, minus the red corpuscles, while chyle is the same fluid rendered milky by numer- ous fat -globules. During the intervals of digestion, the laeteals carry ordinary lymph. This fluid is the overflow of the blood — the plasma and white corpus- cles which escape from the blood capillaries, and are not needed by the tissues in which they are. This sur- plus overflow is returned to the blood by the lymphatics. The current is kept up bj the movements of the body, Fio. 61.-PriMcipal Lymphatics of the Ha- an(J jn many Vertebrates, 88 man Body: a, nuion of left jugular and , «.. veins; 6, thoracic dnct ; c, FfOffS and V IsllCS, DV tmbclnvi recepracnlum chyli. The oval bodies , are glands. hearts. Like the roots of Plants, the absorbent vessels do not commence with open mouths; but the fluid which enters them must traverse the membrane which covers their mi- nute extremities. This membrane is, however, porous, and the fluids pass through it by the forces of filtration and diffusion. How the fat gets into the laeteals is not yet well understood, but the laeteals are themselves rhyth- mically contractile," and force the absorbed chyle tow- THE BLOOD OF ANIMALS. 97 ards the heart. The valves of the lymphatics prevent its return. CHAPTER XII. THE BLOOD OF ANIMALS. The Blood is that fluid which carries to the living tis- sues the materials necessary to their growth and repair, and removes their waste and worn-out material. The great bulk of the body is occupied with apparatus for the preparation and circulation of this vital fluid. The blood of the lower animals (Invertebrates) differs so widely from that of Man and other Vertebrates, that the former were long supposed to be without blood. In them the blood is commonly colorless ; but it has a bluish cast in Crustaceans ; reddish, yellowish, or greenish, in Worms ; and reddish, greenish, or brownish, in Jelly- fishes. The red liquid which appears when the head of a Fly is crushed is not blood, but comes from the eyes. In Vertebrates, the blood is red, excepting the white- blooded fish, Amphioxus™ Asa rule, the more simple the fabric of the body, the more simple the nutritive fluid. In unicellular animals (as Protozoa), in those whose cells are comparatively inde- pendent (as Sponges), and in small and lowly organized animals (like Hydra), there is no special circulating fluid. Each cell feeds itself either directly from particles of food, or from the products of digestion. In Polyps and Jelly-fishes, the blood is scarcely different from the prod- ucts of digestion, although a few blood-corpuscles are pres- ent. But in the more highly organized Invertebrates the blood is a distinct tissue, coagulating, and containing white corpuscles. The blood of the Vertebrates, appar- 7 COMPARATIVE ZOOLOGY. ently a clear, homogeneous fluid, really consists of minute grains, or globules, of organic matter floating in water. If the blood of a Frog be poured on a filter of blotting-paper, a trans- parent fluid (vdlledplas- ma) will pass through, leaving red particles, re- sembling sand, on the upper surface. Under the microscope, these particles prove to be cells, or flattened disks 6 (called corpuscles), con- Fio. 62. -Red Blood-corpnscles of Man : «, shows v . * circular contour; 6, a biconcave section; c, a taming a nucleus ; SOIIIC groupinchai118- are colorless, and others red. The red disks have a tendency to run together into piles; the colorless ones remain single. Meanwhile, the plasma separates into two parts by coagulating; that is, minute fibres form, consisting of fibrine, leaving a pale yellowish fluid, called serum.66 Had the blood not been filtered, the corpuscles and fibrine would have mingled, forming a jelly-like mass, known as clot. Further, the serum will coagulate if heated, dividing into hardened albumen and a watery fluid, called serosity, which contains the soluble salts of the blood. These several parts may be expressed thus : ( Corpuscles |CO!01f l- HP, SS I Plasma | ( albumen. c serosity=water and salts. If now we examine the nutritive fluid of the simplest animals, we find only a watery fluid containing granules. In Radiates and the Worms and Mollusks, there is a sim- ilar fluid, with the addition of a few white corpuscles. But THE BLOOD OF ANIMALS. 99 there is little fibrine, and, therefore, it coagulates feebly or not at all. In the Arthropods and higher Mollusks, the circulating fluid contains colorless nucleated cells, and coagulates.58 In Ver- tebrates, there are, in ad- dition to the plasma and white corpuscles of In- vertebrates, red corpus- cles, to which their blood owes its peculiar hue. In Fishes, Amphibians, Keptiles, and Birds, i. e., all oviparous Vertebrates, these red Corpuscles are Fm.63.-NucleatedBlood-cell8ofaFrog,x 260. nucleated ; but in those of Mammals, no nucleus has been discovered." All blood -corpuscles are microscopic. The white are more uniform in size than the red ; and generally smaller (except in Mammals), being about TrsW °f an incn in diameter. The red corpuscles are largest in Amphib- ians (those of Proteus being the ex- treme, or -j-^- of an inch), next in Fishes, then Birds and Mammals. The smallest known are those of the Musk- PIG. 64. -Elliptical corpus- deer. In Mammals, the size agrees cle of the Frog, showing , . . .. , . .. a white promiuence at the with the size of the animal only with- in a natural order; but in Birds the correspondence holds good throughout the class, the larg- est being found in the Ostrich, and the smallest in the Humming-bird. In Man, they measure -y-gV^ of an inch, so that it would take 40,000 to cover the head of a pin. As to shape, the colorless corpuscles are ordinarily glob- 100 COMPARATIVE ZOOLOGY. ular, or sac-like, in all animals; but they are constantly changing. The form of the red disks is more permanent, although they are soft and elastic, so that they squeeze FIG. 65. — Comparative Size and Shape of the red Corpuscles of various Animals. through very narrow passages. They are oval, circular, or angular, in Fishes; oval in Reptiles, Birds, and the Camel tribe ; and circular in the rest of Mammals. They are double-convex when nucleated, and double-concave when circular and not nucleated. Blood is always heavier than water; but is thinner in cold-blooded than in warm-blooded animals, in herbivores than in carnivores. The blood of Birds, which is the hot- test known, being 10° higher than Man's, is richest in red corpuscles. In Man, they constitute about one half the mass of blood. The white globules are far less numerous than the red; they are relatively more abundant in venous than arterial blood, in the sickly and ill-fed than in the healthy and vigorous, in the lower Vertebrates than in Birds and Mammals. Their number is subject to great THE BLOOD OF ANIMALS. 101 variations, increasing rapidly after a meal, and falling as rapidly. There is less blood in cold-blooded than in warm-blood- ed animals ; and the larger the animal, the greater is the &... FIG. 06.— Capillary Circulation in the Web of a Frog's Foot, X 100 : a, b, small veins ; d, capillaries in which the oval corpuscles are seen to follow one another in sin- gle series; c, pigment-cells in the skin. proportion of blood to the body. Man has about a gallon and a half, equal to one thirteenth of his weight. The heart of the Greenland Whale is a yard in diameter. The main Office of the Blood is to supply nourish- ment to, and take away waste matters from, all parts of the body. It is at once purveyor and scavenger. In its circulation, it passes, while in the arterial half of the cap- illaries, within an infinitesimal distance of the various tis- sues. The plasma, carrying the nutritive matter needed, exudes through the walls of the capillary tubes ; the tissue assimilates or makes like to itself whatever is suitable for its growth and repair ; and the lymphatics (the escape- 102 COMPARATIVE ZOOLOGY. pipes) take up any surplus, and return it to the blood. At the same time, the venous part of the capillary net- work absorbs the waste products of the tissues, expelling the gases by the lungs, and the solid matters by the skin and kidneys. The special function of the several constit- uents of the blood is not wholly known. The colorless corpuscles in Vertebrates are supposed to be the source of the red disks. The latter are the carriers of oxygen, which is taken up by their red matter (haemoglobin) in the lungs, and given up to the tissues. The same office is performed by the blue coloring - matter (baemocyanin) in the blood of certain Invertebrates, as the Squid and Lob- ster. The carbon dioxide is taken up by the plasma. Like the solid tissues, the blood, which is in reality a liquid tissue, is subject to waste and renewal, to growth and decay. The loss is repaired from the products of digestion, carried to the blood by the lacteals, or absorbed directly by the capillaries of the digestive tract. The white corpuscles are probably prepared in many parts of the body, especially the liver, spleen, and lymphatic glands. In the lower organisms, the nutritive food is prepared by contact with the tissues, without passing through special organs. Lymph differs from blood chiefly in containing less albumen and fi brine, and no red disks. Chyle is lymph loaded with fat globules, and is found in the lac- teals and vessels connected with them during the absorp- tion of food containing fat. THE CIRCULATION OF THE BLOOD. 103 CHAPTEE XIII. THE CIRCULATION OF THE BLOOD. The Blood is kept in continual motion in order to nourish and purify the body and itself. For as life means work, and work brings waste, there is constant need of fresh material to make good the loss in every part of the system, and of the removal of matter which is no longer fit for use. In the very lowest animals, where every part of the structure is equally capable of absorbing the digested food and is in contact with it, there is no occasion for any circulation, although in them even the blood is not allowed to stagnate. But in proportion as the power of absorption is con- fined to certain parts, the more is the need and the greater the complexity of an apparatus for convey- ing the nutritive fluid to the various tissues. In nearly all animals, the nutritive fluid is con- veyed to the various parts of the body by a system of tubes, called Uood-ves- , —.. •,.-, » Fl°- 67- —Venous Valves. They usually oc- SelS. The higher forms car in pairs, as represented. 104 COMPARATIVE ZOOLOGY. have two sets — arteries and veins, in which the blood moves in opposite directions, the former carrying blood from a central reservoir or heart, the latter taking it to the heart. In the Vertebrates, the walls of these tubes are made of three coats, or layers, of tissue, the arte- ries being elastic, like rubber, and many of the veins being furnished with valves.68 The great artery coming out of the heart is called aorta, and the grand venous trunk, entering the heart on the opposite side, is called vena cava. Both sets divide and subdivide until their branches are finer than hairs ; and joining these finest arteries and finest veins are intermediate microscopic tubes, called capilla- ries (in Man about -^^ of an inch in diameter)/' In these only, so of a Dog. thin and delicate are their walls, does the blood come in contact with the tissues or the air. In those Vertebrates which have lungs there are two sets of capillaries, since there are two circulations — the systemic, from the heart around the system to the heart again, and the pulmonary, from the heart through the respiratory organ back to the heart. This double course may be illustrated by the figure 8. In gill-bearing animals there are capillaries in the gills, but not a double circu- lation. There is no true system of blood-vessels below the Star-fish. The simplest provision for the distribution of the products of digestion is shown by the Jelly-fish, whose stomach sends off radiating tubes (Fig. 197). THE CIRCULATION OF THE BLOOD. 1Q5 The first Approach to a Circulatory System is made by the Star-fish and the Sea-urchin. A vein runs along the whole length of the alimentary tube, to absorb the chyle, and forms a circle around each end of the tube. These circular vessels send off branches to various parts of the body; but as they are not connected by a net-work of capillaries, there can be no circuit (Fig. 39). A higher type is exhibited by the Insects. If we ex- amine the back of any thin-skinned Caterpillar, a long pulsating tube is seen running beneath the skin from one end of the body to the other. This dorsal vessel, or heart, as it is called, is open at botli ends, and divided by valves into compartments, permitting the blood to go forward, but not backward. Each compartment communicates by a pair of slits, guard- ed by valves, with the body - cavity, so that fluids may enter, but cannot es- cape. "Circulation" is very simple. We have seen that the chyle exudes through the walls of the alimentary ca- nal directly into the cavity of the abdo- men, where it mingles with the blood already there. This mixed fluid is drawn into the dorsal tube through the FIO. 0 sal Vessel, or Heart, of valvular openings as it expands; and a cockchafer bisected : 11 ,1 «j i a, 6, muscular walls; upon its contraction, all the side-valves d, valves between the are closed, and the fluid is forced tow- SJSJ"*' "J •£ ards the head. Passing out at the front oriflces communicating 0 with the general cavity opening, it is again diffused among and of the abdomen. between the tissues of the body. The blood, therefore, does not describe a circle in definite channels so as to re- turn constantly to its point of departure. Many worms (as the Earthworm) have a pulsating tube 106 COMPARATIVE ZOOLOGY. extending from tail to head above the alimentary canal, a similar tube on the ventral side through which the blood returns, and qross-tubes in every segment. In the Lob- ster and Crab, Spider and Scorpion, the dorsal tube sends Fio. 70.— Circulation in a Lobster : a, heart : 6, artery for the eyes ; c, artery for an- tennae ; d, hepatic artery ; e, superior abdominal artery ; /, sternal artery ; g, ve- nous sinuses transmitting blood from the body to the branchiae, h, whence it returns to the heart by the branchio-cardiac vessels, i. off a system of arteries (not found in Insects) ; but the blood, as it leaves these tubes, escapes into the general cavity, as in other Arthropoda. The Lobster and Crab, however, show a great advance in the concentration of the propelling power into a short muscular sac. A third development of the circulatory system is fur- nished by the Mollusks. Comparatively sluggish, they need a powerful force-pump in the form of a compact heart. In the Oyster and Snail (Figs. 44, 45), we find such an organ having two cavities — an auricle and a ventricle, one for receiving, and the other for distributing, the blood. The auricle injects the blood into the ventricle, which propels it by the arteries to the various organs. Thence it passes, not immediately to the veins, as in higher ani- mals, but into the spaces around the alimentary canal. A part of this is carried by vessels to the gills or lung, and then returned with the un purified portion to the auricle. The whole of the blood, therefore, does not make a com- plete circuit. The Clam has a similar heart, but with two auricles. THE CIRCULATION OF THE BLOOD. 107 A still higher form is seen in the Cuttle-fish, the high- est of the Invertebrates. This animal has a central heart, with a ventricle and two auricles, and, in addition, the veins which collect the blood from the system to send it back to the heart by the way of the gills are furnished with two branchial hearts, which accelerate the circulation through those organs. Many of the arte- ries and veins are joined by cap- illaries, but not all; so that in no invertebrate animal is the blood returned to the heart by a continuous closed system of blood- vessels. As a rule, in all animals hav- ing any circulation at all, the cur- rent always takes one direction. This is generally necessitated by valves. But a curious exception is presented by the Ascidians, whose tubular heart is valveless, and the contractions occur alter- nately at one end and then the other ; so that the blood oscil- lates to and fro, and a given ves- sel is at one time a vein and at another an artery, spect it resembles the foetal heart of higher animals (Fig. 279). In Vertebrates only is the cir- culating current strictly confined to the blood-vessels; in no case does it escape into the general cavity of the bodjr. In other respects, there is Tn this rp Fla 71— Circulating Apparatus in the Fish : a, branchial artery ; b, arterial bnlb ; c, ventricle ; d, au- ricle ; e, venous sinus ; /, portal vein ; rj, intestine ; h, vena cava ; f, branchial vessels ; k, dorsal ar- tery, or aorta; I, kidneys; m, dorsal artery. 108 COMPARATIVE ZOOLOGY. no great advance in the apparatus of the lowest Verte- brates over that of the highest Mollusks. The heart of Fishes, as in the Oyster, has two cavities, but its position is reversed. Instead of driv- ing arterial blood over the body, it receives the return- ing, or venous, blood, and sends it to the gills. Re- collected from the gills, the blood is passed into a large artery, or aorta, along the' back, which distributes it by a complex system of capil- Fio. 72.— Diagram of a single Heart: rf, , . ,, . auricle; e, ventricle ; c, veins leading to lariCS among the tlSSU6S. aaricle; a, aorta, or main artery. rpj^ capillaries unjte wjth the ends of the veins which pass the blood into the auri- cle80 (Fig. 48). In Amphibians and in Reptiles generally (as Frogs, Snakes, Lizards, and Turtles), the heart has three cavities — two auricles and one ventricle. The venous blood from the body is received into the right auricle, and the puri- fied blood from the lungs into the left. Both throw their contents into the ventricle, which pumps the mixed blood in two directions — partly to the lungs, and partly around the system. Circulation is, therefore, incomplete, since the whole current does not pass through the lungs, and three kinds of blood are found in the body — arterial, ve- nous, and mixed. In many animals, however, arrange- ments exist which nearly separate the venous from the arterial blood. The ventricle of Reptiles is partially divided by a par- tition. In the Crocodile, the division is complete, so that there are really four cavities — two auricles, and two ven- tricles. But both ventricles send off aortas which cross THE CIRCULATION OF THE BLOOD. 109 one another, and at that point a small aperture brings the two into communication. The venous and arterial cur- rents are, therefore, mixed, but not within the heart, as in the other Eeptiles, nor so extensively. In the structure of the heart, as well as in that of the gizzard, Crocodiles ap- proach the Birds. The Highest Form of the Circulating System is pos- sessed by the warm-blooded Vertebrates, Birds and Mam- FIG. 73.— Heart of the Dugong, a fonr- , -vr i/ i £ i i j chambered heart, the parts being more malS. JNot a drop OI blOOd 8eparated than in higher animals: E, fan mfllcp thf> firpnif of thp right ventricle; L, left veutricle ; D, TCUlt Or tne rjght au,.icle. F, pulmonary artery; body without passing through K- left auricle ; A- aorta- the lungs, the circulation to and from those organs being as perfect as the distribution of arterial blood. The heart / 9 * 9 consists of four cavities — a right auricle and ventricle, and a left auricle and ventricle. In other words, it is a hollow mus- cle divided internally by a ver- tical partition into two distinct chambers, each of which is again divided by a valve into an auricle and a ventricle. The work of the right auricle and ventricle is to receive the blood 6 inferior vena cava; c, tricuspid from the Veins, and Send it to valve ; d, right auricle ; e, pulmona- ? ry veins; /, superior vena cava; g, the lungS ; while the Other tWO pnl monary arteries; h, aorta; k, left . . . , , . , auricle ; I, mitral valve ; m, left ven- reCCl V6 the blOOd f rOUl the lungs, and propel it over the body. The left ventricle has more to do than any other cavity. The two auricles contract at the same instant; 110 COMPARATIVE ZOOLOGY. so also do the ventricles. The course of the current in Birds and Mammals is as follows : the venous blood brought from the system is discharged by two or three large trunks" into the right auricle, which immediately forces it past a valve " into the right ventricle. The ventricle then con- tracts, and the blood rushes through the pulmonary artery past its semi- lunar valves into the lungs, where it is changed from venous to arterial, FIG. 75. — Plan of circuia- returning by the pulmonary veins to tion in Fishes: o, auri- . , ,, . , mi . ", cle: b, ventricle; e, bran- the left auricle. IhlS SCIlds it past veinir^ringin^Tiood tne m^1'^ valves into the left veutri- from the gills, d, and c]e which drives it past the semilunar uniting in the aorta, /; g, veuacava. valves into the aorta, and thence, by its ramifying arteries and capillaries, into all parts of the body except the lungs. From the capillaries, the blood, now changed from arterial to venous, is gathered by the veins, and conveyed back to the heart. The Bate of the Blood -current gener- ally increases with the activity of the animal, being most rapid in Birds." In Insects, however, it is compara- tively slow hllf tin's ifi Mammals: a, right auricle receiving venous lively slow , put tins is b]ood from the gystem . ^ ]eft anric]e receivil)2 because the air is taken arterial blood from the lungs; c, c', ventricles ; d,e, f, systemic artery, vein, and capillaries; g, tO the blOOd — the Whole pulmonary artery ; h, h, vein and capillaries. HOW ANIMALS BKEATHE. HI body being bathed in air, so that the blood has no need to hasten to a special organ. However, activity nearly doubles the rate of pulsation in a Bee. The motion in the arteries is several times faster than in the veins, but diminishes as the distance from the heart increases. In the carotid of the Horse, the blood moves. 12£ inches per second ; in that of Man, 16 ; in the capillaries of Man, 1 to 2 inches per minute ; in those of a Frog, 1. The Cause of the Blood-current may be cilia, or the contractions of the body, or pulsating tubes or hearts. In the higher animals, the impulse of the heart is not the sole means : it is aided by the contractions of the arteries themselves, the movements of the chest in respiration, and the attraction of the tissues for the arterial blood in the capillaries. In the Chick, the blood moves before the heart begins to beat ; and if the heart of an animal be suddenly taken out, the motion in the capillaries will continue as before. It has been estimated that the force which the human heart expends in twenty-four hours is about equivalent to lifting 217 tons one foot. CHAPTER XIY. HOW ANIMALS BREATHE. Arterial Blood, in passing through the system, both loses and gains certain substances. It loses constructive material and oxygen to the tissues. These losses are made good from the digestive tract and breathing organ. It gains also certain waste materials from the tissues, which must be got rid of. Of these waste products, one, carbon dioxide, is gaseous, and is passed off from the same organ as that where the oxygen is taken in. This exchange of 112 COMPARATIVE ZOOLOGY.' gases between the animal and its surroundings is called Respiration. The First Object of Respiration is to convort venous into arterial blood. It is done by bringing it to the sur- face, so that carbon dioxide may be exhaled and oxygen absorbed. The apparatus for this purpose is analogous to the one used for circulation. In the lowest animals, the two are combined. But in the highest, each is essentially a pump, distributing a fluid (in one case air, in the other blood) through a series of tubes to a system of cells or capillaries. They are also closely related to each other : the more perfect the circulation, the more careful the pro- vision made for respiration. Respiration is performed either in air or in water. So that all animals may be classed as air-breathers or water -breathers. The latter are, of course, aquatic, and seek the air which is dissolved in the water. Land-snails, Myriapods,- Spiders, Insects, Reptiles, iBirds, and Mam- mals breathe air directly ; the rest, with few exceptions, receive it through the medium of water. In the former case, the organ is internal ; in the latter, it is more or less on the outside. But however varied the organs — tubes, gills, or lungs — they are all constructed on the same prin- ciple— a thin membrane separating the blood from the atmosphere. ( 1 ) Sponges, Infusoria, and Polyps have no separate respir- atory apparatus, but absorb air, as well as food, from the currents of water passing through them. In the Star-fish, Sea-urchin, and the like, we find the first distinct respiratory organs, although none are exclu- sively devoted to respiration. There are two sets of ca- nals— one carrying the nutrient fluid, and the other, radi- ating from a ring around the mouth, distributing aerated water, used for locomotion as well as respiration. This may be called the " water-pipe system." Besides this, 'HOW ANIMALS BREATHE. 113 there are numerous gill-like fringes, which probably aid in respiration (Fig. 39). Fresh- water Worms, like the Leech and Earth-worm, breathe by the skin. The body is always covered by a viscid fluid, which has the property of absorbing air. The air is, therefore, brought into immedi- ate contact with the soft skin, underneath which lies a dense net- work of blood-ves- sels. But most water -breathing animals have gills. The simplest form is seen in Marine Worms : delicate veins projecting through the skin make a series of arborescent tufts along the side of the body; as these float in the water, the blood is purified.'4 Bi- valve Mollusks have four flat gills, consist- ing of delicate membranes filled with blood- vessels and covered with cilia. In the Oys- FlG 7T_Lob.WOrm ter, these ribbon-like folds are exposed to (Arenicoiapt*nto- rum), a dorsibran- the water when the shell opens; but in the Clam, the mantle en- !«- closes them, forming a tube, called siphon, through which the water is driven by the cilia. The aquatic Gastero- pods (Univalves) have either? tufts, like the Worms, or comb- FIS. 78.-Dia fyjng throngll the 8mall. est and most delicate organs, so that the air may follow HOW ANIMALS BKEATIIE. 115 the blood wherever it circulates. To keep the pipes ever open, and at the same time leave them flexible, they are provided with an elastic spiral thread, like the rubber tube of a drop-light. Respira- tion is performed by the movements of the abdomen, as may be seen in the Bee when at rest. This "air-pipe system," as it may be termed, is best developed in In- sects. The "nerves" of an Insect's wing con- ( sist of a tube within a tube: the inner one Tube of an insect, highly magnified, is a trachea carrying air, and the outer one, showing elastic sheathing it, is a blood-vessel. So perfect 8piral threiuL is the aeration of the whole bodjr, from brain to feet, the blood is oxygenated at the moment when, and on the spot where, it is carbonized > only one kind of fluid is, Fio. 81. — Ideal Section of a Bee: a, alimentary canal; h, dorsal vessel; f, trachea; n, nervous cord. therefore, circulating — arterial. It is difficult to drown an Insect, as the water cannot enter the pores ; but if a drop of oil be applied to the abdomen, it falls dead at once, being suffocated. The largest spiracle is usually 116 COMPARATIVE ZOOLOGY. found on the thorax, as un- der the wing of a Moth: such may be strangled by pinching the thorax. In Millipedes and Centi- pedes, the spiracles open into little sacs connected together by tubes ; in Spi- ders and Scorpions, the spiracles, usually four in P.O. 82,-sectionZIngh a bronchial tube, number, are the mouths of Lung of a Bird, magnified: a, the cavity; sacs without the tubes, aild b, its lining membrane s pportiug blood- ... vessels ; c, perforations a the orifices of the interior of the Sac IS the lobnlar passages, d e, interlobular * j • 4. j: 1 J T J spaces, containing the le miual branches gathered into folds. Land- of the pulmonary vesse supplying the Snfl4]c }lavp onp cniraolp or capillary plexus, /, to the meshes of which M Ut3' Or the air gets access by the lobnlar passages, aperture, OH the left side of the neck, leading to a large cavity, or sac, lined with fine blood-vessels. These sacs represent the primitive idea of a lung, which is but an infolding of the skin, divided up into cells, and covered with capillary veins.65 Fio. 83.— Part of a transverse section of n Pier's Bronchial Twig, x 240: a, outer fibrous layer; I, muscular layer: c, inner fibrous layer; d, epithelial layer with cilia; /, one of the neighboring alveoli. HOW ANIMALS BREATHE. Like the alimentary canal, the lungs of an animal are really an inflected portion of the outer surface; so that breathing by the skin and breathing by lungs are one in principle. Indeed, in many animals, especially Frogs, res- piration is carried on by both lungs and skin. The lungs of Yertebrates are derived from the front part of the alimentary canal. In some Fishes, air is swal- lowed, which passes the whole length of the digestive tract, and is expelled from the anus. Here the whole canal serves for respiration. In Reptiles, Birds, and Mammals the hinder part of the intestine develops an outgrowth (the allantois) during embryo -life which serves as the embryo's breathing organ (Figs. 170, 171). All Yertebrates have two kinds of respir- atory organs in the course of their life. Fishes have gills; their lung (the air-blad- der) rarely serves as a functional respiratory organ, and is sometimes wanting. Amphibi- ans have gills while in the larval state. Some keep them throughout life ; but all develop functional lungs, and also breathe by means of the skin. In the remaining Yertebrates, the allantois is the breathing organ of the embryo, and the lung is the breathing organ of the adult. The skin is of small or no importance in respiration. The lungs of Yertebrates are elastic mem- ....,» , . ,, branous sacs, divided more or less into cells to increase the surface. Upon the walls of SSSfw?! '? the cells are spread the capillary blood-ves- J^^Jj^Ji sels. The smaller the cells, the greater the nary vein; the . , lung, B, is rudi- extent of surface upon which the blood is mentary. IG. 84.— Lungi of a snake: «, 118 COMPARATIVE ZOOLOGY. exposed to the influence of the air, and, therefore, the more active the respiration and the purer the blood. The lungs are relatively largest in Reptiles, and smallest in Mammals. But in the cold-blooded Amphibians and Rep- tiles, the air-cells are few and large ; in the warm-blooded Birds and Mammals, they are exceedingly numerous and minute.86 In Birds and Mammals, the blood in the capil- laries is exposed to the air on all sides ; in the Reptiles, on one only. Respiration is most perfect in Birds ; they require, relatively to their weights, more air than Mam- mals or Reptiles, and most quickly die for lack of it. In Birds, respiration is not confined to the lungs ; but, as in Insects, extends through a great part of the body. Air- sacs connected with the lungs exist in the abdomen and under the skin of the neck, wings, and legs. Even the bones are hollow for this purpose ; so that if the wind- Fio. 95 — Lungs of a Frog: a, hyoid apparatus ; b, cartilaginous ring at root of the lungs ; e, pulmonary vessels ; rf, pulmonary sacs, having this peculiarity common to all cold- blooded air-breathers, that the tra- chea does not divide into bronchial branches, but terminates abruptly by orifices which opeu at once into the general cavity. A cartilaginous net-work divides the space into lit- tle sacs, on the wnlls" of which the capillaries are spread. FIG. 86. —Distribution of Air-tubes in Mam- malian Lungs : a, larynx ; b, trachea ; c, d, left and right bronchial tubes ; e, f, g, the ramifications. In Man the subdivision con- tinues until the ultimate tubes are one twen- ty-fifth of an inch in diameter. Each lobule represents in miniature the structure of the entire lung of a Frog. HOW ANIMALS BREATHE. 119 pipe be tied, and an opening be made in the wing-bone, the bird will continue to respire. The right lung is usu- ally the larger; in some Snakes, the left is wanting en- tirely. In most Vertebrates, lungs are freely suspended; in Birds, they are fastened to the back. The lungs communicate with the atmosphere by means of the trachea, or windpipe, formed of a series of cartilag- inous rings, which keep it constantly open. It begins in the back part of the mouth, opening into the pharynx by a slit, called the glottis, which, in Mammals, is protected by the valve -like epiglottis. The trachea passes along the neck in front of the oesophagus, and divides into two branches, or bronchi, one for each lung. In Birds and Mam- mals, the bronchial tubes, after entering the lungs, subdivide again into minute ramifications. Vertebrates are the FlG- 8T'~" only animals that breathe through the mouth or nos- trils. Frogs, having no ribs, and Turtles^, whose ribs are soldered together into a shield, are compelled to swallow the air. Snakes, Lizards, and Crocodiles draw it into the lungs by the play of the ribs.87 . Birds, unlike other ani- mals, do not inhale the air by an active effort; for that is done \)j the spririging-back of the breast-bone and ribs to their natural position. To expel the air, the breast-bone is drawn down towards the back-bone by muscles, which compresses the lungs. Mammals alone have a perfect thorax — i. £., a closed cavity for the heart and lungs, with movable walls (breast- 120 COMPARATIVE ZOOLOGY. bone and ribs) and a diaphragm, or muscular partition, separating it from the abdomen.68 Inspiration (or tilling the lungs) and expiration (or emptying the lungs) are both accomplished by muscular exertion ; the former, by raising the ribs and lowering the diaphragm, which en- large the capacity of the chest, and the air rushes in to prevent a vacuum ; the latter, by the ascent of the diaphragm and the de- scent of the ribs. As a rule, the more ac- tive and more muscular an animal, the greater the de- mand for oxygen. Thus, warm-blooded animals live fast, and their rapidly de- caying tissues call for rapid ** respiration ; while in the v cold-blooded creatures the waste is comparatively slow. Respiration is most active in Birds, and least -breathing animals. Toad respires more slowly than the busy Bee, the Mollusk more slowly than the Fish. But respi- rations, like beats of the heart, are fewer in large Mam- mals than in small ones. An average Man inhales about 300-400 cubic feet of air per day of rest, and much more when at work. Another result of respiration, besides the purification of the blood, is the production of heat. The chemical combination of the oxygen in the air with the carbon in the tissues is a true combustion ; and, therefore, the more Fio. 88.— Human Thorax: o, vertebral col- umn ; b, b', ribs, the lower ones false ; c, clavicle; e, intercostal muscles, removed on the left side to show the diaphragm, d; f, pillars of the diaphragm attached to the lumbar vertebrae ; g, muscles for elevating the ribs; ft, sternum. SECRETION AND EXCRETION. 121 active the animal and its breathing, the higher its temper- ature. Birds and Mammals have a constant temperature, which is usually higher than that of the atmosphere (108° and 100° F. respectively). They are therefore called con- stant-temper atured or warm-blooded. Other animals do not vary greatly in temperature from that of their sur- roundings, and are called changeable-temperatured or cold- Hooded. Still, their temperature does not agree exactly with that of the air or water. The Bee is from 3° to 10°, and the Earth-worm and Snail from 1£° to 2°, higher than the air. The mean temperature of the Carp and Toad is 51°; of Man, 98°; Dog, 99°; Cat, 101°; Squirrel, 105°; Swallow, 111°. CHAPTER XY. SECRETION AND EXCRETION. IN the circulation of the blood, not only are the nutrient materials deposited within the body in the form of tissue, but certain special fluids are separated and conveyed to the external or internal surfaces of the body. These flu- ids are of two kinds : some, like saliva, gastric juice, bile, milk, etc., are for useful purposes ; others, like sweat and urine, are expelled from the system as useless or injurious. The separation of the former is called secretion; the re- moval of the latter is excretion. Both processes are sub- stantially alike. In the lower forms, there are no special organs, but se- cretion and excretion take place from the general surface. The simplest form of a secreting organ closely resembles that of a respiratory organ, a thin membrane separating the blood from the cavity into which the secretion is to 122 COMPARATIVE ZOOLOGY. be poured. Usually, however, the cells of the membrane manufacture the secretion from materials furnished by the blood. Even in the higher animals, there are such secret- ing membranes. The membranes lining the nose and ali- mentary canal and enclosing the lungs, heart, and joints, secrete lubricating fluids. The infolding of such a membrane into little sacs or short tubes (follicles), each having its own outlet, is the type of all secreting and ex- creting organs. The lower tribes have nothing higher, and the apparatus for pre- paring the gastric fluid at- tains no further develop- ment even in Man. When FIG. 89.-Three plans of secreting Mem- a c]U8ter of these follicles, Or branes. The heavy hue represents the areolar-vascular layer; tlie next line is saCS, discharge their Contents the basement, or limiting membrane; , , and the dotted line the epithelial layer: by One COttlinOn dllCt, WC * SX ^ve a gland. But whether £ membrane, follicle, or gland, of compound glands. the organ is covered with a net-work of blood-vessels, and lined with epithelial cells, which are the real agents in the process. The chief Secreting Organs are the salivary glands, gastric follicles, pancreas, and liver, all situated along the digestive tract. 1. The salivary glands, which open into the mouth, se- crete saliva. They exist in nearly all Vertebrates, higher Mollusks, and Insects, and are most largely developed in such as live on vegetable food. The saliva serves to lu- bricate or dissolve the food for swallowing, and, in some Mammals, aids also in digestion of starch." SECRETION AND EXCRETION. 123 2. The gastric follicles are minute tubes in the walls of the stomach secreting gastric juice. They are found in all Vertebrates, and in the higher Mol- lusks and Arthropods. In the lower forms, a simple membrane lined with cells serves the same purpose. Under the microscope, the soft mucous mem- brane of the human stomach presents a honey-comb appearance, caused by nu- merous depressions or cells. At the bot- tom of these depressions are clusters of spots, which are the orifices of the tubu- lar follicles. The follicles are about -^^ of an inch in diameter, and number mill- ions. 3. The pancreas, or " sweetbread," so , , £ j- • FIG. 00.— Follicles fronrthe important in the process of digestion, stomach of a DO-, x when present, exists only in the Verte- SeTat i"?- brates, and perhaps in the higher Mol- inmnar epithelium, lusks. In its structure and its secretion it closely resem- bles the salivary glands. In the Cuttle-fish, it is repre- sented by a sac ; in Fish- es, by a group of follicles. It is proportionally larg- est in Birds whose sali- vary glands are deficient. The pancreatic juice en- ters the duodenum. 4. A liver in some form is found in all animals having a distinct diges- tive cavity. In Mol lusks and Vertebrates, it is the largest gland in the body. The higher the animal, the more compact the organ. Fro. 91.— Pancreas of M:in, ",- », cystic duct; e, duct from loric valve ; e, t, duodenum. •rnll-Wndfler: e liver; p, py- 124 COMPARATIVE ZOOLOGY. Thns, in Polyps it is represented by yellowish cells lining the stomach ; in Insects, by cells in the wall of the stom- ach ; in Mollusks, by a cluster of sacs, or follicles, forming a loose compound gland. In Vertebrates, the liver is well defined, and composed of a multitude of lobules (which give it a granular appearance) arranged on the capillary veins, like grapes on a stem, and containing nucleated secreting cells. It is of variable shape, but usually two, three, or five lobed, and is centrally situated — in Mam- mals, just below the diaphragm. In most Vertebrates, there is an appendage to the liver, called the gall-lladder, which is simply a reservoir for the bile when not wanted. The so-called liver of Invertebrates is probably more Fio. 92.— Liver of the Dog, F, F; D, duodenum and intestines; P, pancreas; r, spleen ; «, stomach , /, rectum ; R, right kidney ; B, gall-bladder ; ch, cystic duct; F, lobe of liver dissected to show distribution of portal vein, VP, and hepatic veiu, vh; d, diaphragm; VC, vena cava; C, heart. SECRETION AND EXCRETION. 125 like the pancreas of Vertebrates in function, as its secre- tion digests starches and albuminoids. The liver of Ver- tebrates is both a secretory and an excretory organ. The bile performs an important, although ill-understood, func- tion in digestion, and also contains some waste products. The gland also serves to form sugar from part of the digested food, and may well be called a chemical work- shop for the body. In animals of slow respiration, as Crustaceans, Mollusks, Fishes, and Reptiles, fat accumu- lates in the liver. "Cod-liver oil" is an example. The great Excreting Organs are the lungs, the kid- neys, and the skin; and the substances which they re- move from the system — carbonic acid, water, and urea — are the products of decomposition, or organic matter on its way back to the mineral kingdom.70 Different as these organs appear, they are constructed upon the same prin- ciple : each consisting of a very thin sheet of tissue sepa- rating the blood to be purified from the atmosphere, and straining out, as it were, the noxious matters. All, more- over, excrete the same substances, but in very different proportions : the lungs exhale carbon dioxide and water, with a trace of urea ; the kidneys expel water, urea, and a little carbon dioxide ; while the skin partakes of the nat- ure of both, for it is not only respiratory, especially among the lower animals, but it performs part of the work of the kidneys when they are diseased. 1. The lungs (and likewise gills) are mainly excretory organs. The oxygen they impart sweeps with the blood through every part of the body, and unites with the tis- sues and with some elements of the blood. Thus are pro- duced heat and work, whether muscular, nervous, secre- tory, etc. As a result of this oxidation, carbon dioxide, water, and urea or a similar substance, are poured into the blood. The carbon dioxide and part of the water are passed off from the respiratory organs. This process is 126 COMPARATIVE ZOOLOGY. more immediately necessary to life than any other : the arrest of respiration is fatal. 2. While the lungs (and skin also, to a slight degree) are sources of gain as well as loss to the blood, the kidneys are purely excretory organs. Their main function is to eliminate the solid products of decay which cannot pass out by the lungs. In Mammals, they are discharged in solution ; but from other animals which drink little the excretion is more or less solid. In Insects, the kidneys are groups of tubes ; in the F,«. 93. -section of Human higber Mollusks, they are represent- Kiduey, showing the tubu- ed by spongy masses of follicles : in lar portion, 3, grouped into «'* Dl/ ' cones; 7, the ureter, or out- vertebrates, they are well-developed let of the secretion. i j • u j glands, two in number, and consist- ing of closely packed tubes. 3. The skin of the soft-skinned animals, particularly of Amphibians and Mammals, is covered with minute pores, which are the ends of as many delicate tubes that lie coiled up into a knot within the true skin. These are the sweat-glands, which excrete water, and with it certain salts and gases. Besides these secretions and excretions, there are others, confined to particular animals, a'nd designed for special purposes: such are the oily matters secreted from the skin of quadrupeds for lubricating the hair and keeping the skin flexible; the tears of Reptiles, Birds, and Mam- mals; the milk of Mammals; the ink of the Cuttle-fish; the poison of Jelly-fishes, Insects, and Snakes; and the silk of Spiders and Caterpillars. THE SKIN AND SKELETON. 127 CHAPTER XYI. THE SKIN AND SKELETON. The Skin, or Integument, is that layer of tissue which covers the outer surface of the body. The term Skeleton is applied to the hard parts of the body, whether external or internal, which serve as a framework or protection to the softer organs, and afford points of attachment to mus- cles. If external, as the crust of the Lobster, it is called Exoskeleton j if internal, as the bones of Man, it is called EndoskeUtan. The former is a modification of the skin ; the latter, a hardening of the deeper tissues. 1. The Skin. — In the lowest forms of life, as Amreba, there is no skin. The jelly of which they are composed is firmer outside than inside, but no membrane is present. In Infusoria, there is a very thin cuticle covering the ani- mal. They have thus a definite form, while the Amrebae continually change. Sponges and Hydras also have no true skin. But in Polyps, the outsi'de layer of the animal is separated into two portions — ecderon and enderon71 — which may be regarded as partly equivalent to epidermis and derrnis in the higher animals. These two layers are, then, generally present. • The outer is cellular, the latter fibrous, and may contain muscular fibres, blood-vessels, nerves, touch-organs, and glands. It thus becomes very complicated in some animals. In Worms and Arthropods, the cellular layer, here called hypodermis, excretes a structureless cuticle, which may become very thick, as in the tail of the Horseshoe Crab, or may be hardened by deposition of lime-salts, as in many Crustacea. The loose skin, called the mantle, 128 COMPARATIVE ZOOLOGY. which envelopes the body of the Mollusk corresponds to the true skin of higher animals. The border of the man- tle is surrounded with a delicate fringe, and, moreover, contains minute glands, which secrete the shell and the coloring matter by which it is adorned. The Tunicates have a leathery epidermis, remarkable for containing, in- stead of lime, a substance resembling vegetable cellulose. In Mammals, whose skin is most fully developed, the dennis is a sheet of tough elastic tissue, consisting of in- terlacing fibres, and containing blood-vessels, lymphatics, sweat-glands, and nerves. It is the part converted into leather when hides are tanned, and attains the extreme thickness of three inches in the Rhinoceros. The upper surface in parts of the body is covered with a vast num- ber of minute projections, called papillae,, each containing the termination of a nerve; these are the essential agents in the sense of touch.78 They are best seen on the tongue of an Ox or Cat, and on the human fingers, where they are arranged in rows. Covering this sensitive layer, and accurately moulded to all its furrows and ridges, lies the bloodless and nerve- less epidermis. It is that part of the skin which is raised in a blister. It is thickest where there is most pressure or hard usage : on the back of the Camel it attains un- usual thickness. The lower portion of the epidermis (called rete mucosum) is comparatively soft, and consists of nucleated cells containing pigment-granules, on which the color of the animal depends. Towards the surface the cells become flattened, and finally, on the outside, are changed to horny scales (Fig. 2, c). These scales, in the higher animals, are constantly wear- ing off in the form of scurf, and as constantly being renewed from below. In Lizards and Serpents, the old epidermis is cast entire, being stripped off from the head to the tail ; in the Toad, it comes off in two pieces ; in the THE SKIN AND SKELETON. 129 br BC! E FIG. 94.— Section of Skin from Horse's Nostril: E, epidermis; D, deimi.-; 1, horny layer of epidermis; 2, rete mucosum; 3, papillary layer of dermis ; 4, excretory duct of a sudoriparous, or sweat, gland; 5, glomerule, or convoluted tube of the same ; 6, hair follicle ; 7, sebaceous gland ; 8, internal sheath of the hair follicle ; 9, bulb of the hair ; 10, mass of adipose tissue. Frog, in shreds ; in Fishes and some Mollusks, in the form of slime. However modified the epidermis, or whatever its appendages, the like process of removal goes on. Mam- mals shed their hair ; Birds, their feathers ; and Crabs, their shells. When the loss is periodical, it is termed moulting. 2. The Skeletons. — ( l ) The Exoskeleton is developed by the hardening of the skin, and, with very few excep- tions, is the only kind of skeleton possessed by inverte- brate animals. The usual forms are coral, shells, crusts, scales, plates, hairs, and feathers. It is horny or calca- reous; while the endoskeleton is generally a deposit of rthy material within the body, and is nearly confined to the Vertebrates. The exoskeleton may be of two kinds — dermal and epidermal. The microscopic particles of living jelly, called Polycis- tina and Foraminifera, possess siliceous and calcareous shells of the most beautiful patterns. The Sponge has a 9 130 COMPARATIVE ZOOLOGY. skeleton of horny fibres, which is the sponge of commerce. Coral is the solid framework of certain Polyps. There are two kinds: one represented by the common white coral, which is a calcareous secretion within the body of FIG. 05.— 1, Vertical Section, and, 2, Transverse Section, of a sclerodermic Corallite : a, mouth; 6, tentacles; c, stomach; d, intermesenteric chamber; e, mesentery; /.septum: fj, endoderm ; h, epitheca; k, theca, or outer wall; m, columella ; n, short partitions ; p, tabnla, or transverse partition ; r, eclerobase ; «, coenenchy- ma, or common substance connecting neighboring corallites; t, ectoderm; *, pali, or imperfect partitions. the Polyp, in the form of a cylinder, with partitions ra- diating towards a centre (scleroderni) ; the other, repre- sented by the solid red coral of jewelry, is a central axis deposited by a group of Polyps on the outside (sclero- lase}. The first sort is a dermal, the latter an epider- mal, exoskeleton. The skeleton of the Star-fish is a leathery skin stud- ded with calcare- ous particles and plates. The Sea- urchin is covered with an inflexible PIG. 96.— Shell of Sea-urchin (Cidaris) without its spines. , ,, f 1 1 and beautiful construction. The shell is really a calcified THE SKIN AND SKELETON. 131 skin, being a net- work of fibrous tissue and earthy matter. It varies in shape from a sphere to a disk, and consists of hundreds of angular pieces accurately fitted together, like mosaic-work. These form ten zones, like the ribs of a melon, five broad ones alternating with five narrower FIG. 97. — Structure of Sea-urchins' Spines: 1, a, spine of Cidaris cut longitudinally; t, s, ball-and-socket joint ; p, pedicellariae ; 2, 3, transverse sections of spines of Cidaris aud Echinus. ones. The former (called interambulacra) are covered with tubercles bearing movable spines. The narrow zones (called ambulacra, as they are likened to walks through a forest) are pierced with small holes, through which the animal sends out fleshy sucker-feet. The skin of the Crab and Lobster is hardened by cal- careous deposit into a "crust," or shell;73 but, instead of forming one piece, it is divided into a series of segments, which move on each other. The number of these seg- ments, or rings, is usually twenty-one — to the head, tho- rax, and abdomen, seven each. In the adult, however, the rings of the head and thorax are often soldered to- gether into one shield, called cephalo-thorax ; and in the Horseshoe Crab the abdominal rings are also united. The shell of Crustaceans is periodically cast off, for the ani- mals continue to grow even after they have reached their 132 COMPARATIVE ZOOLOGY. mature form. This moulting is a very remarkable opera- tion. How the Lobster can draw its legs from their cases without unjointing or splitting them was long a puz- zle. The flesh be- comes soft, and is drawn through the joints, the wounds thus caused quickly healing. The cast- off skeleton is a per- fect copy of the an- imal, retaining in their places the del- icate coverings of the eyes and anten- nae, and even the membrane of FIG. 98.— Diagram of nn Insect: A, head bearing the eyes aud antennae ; B, prulborax, carrying the first pair of legs; C, mesothorax, carrying the second the Stomach with its pair of legs and first pair of wings ; D, carrying the , third pair of legs and second pair of wings; E, ab- teeth. doraen, with ovipositor, P; 1, coxa, or hip; 2, tro- TKo li/~ki't r /^ine*- chanter ; 3, femur, or thigh ; 4, tibia, or shank ; 5, tar- u n 7 ( BUS, or foot-, e, claw. of Insects differs from that of Crustaceans in consisting mainly of a horny substance called chitine and in containing no lime. The head, thorax, and abdomen are distinct, and usually con- sist of fourteen visible segments — one for the head, three for the thorax (called prothorax, mesothorax, and metatho- rax), and ten for the abdomen. The antennae, or feelers, legs, and wings, as well as hairs, spines, and scales, are ap- pendages of the skeleton. As Insects grow only during the larval, or caterpillar, state, moulting is confined to that period. These skeletons are epidermal, deposited in suc- cessive layers, from the inside, and are, therefore, capable of but slight enlargement when once formed. THE SKIN AND SKELETON. 133 The shells of Mollusks are well-known examples of exo- skeletons. The mantle, or loose skin, of these animals se- cretes calcareous earth in successive layers, converting the epidermis into a "shell."74 So various and characteristic is the microscopic character of shells, that a fragment is sometimes sufficient to determine the group to which it belongs. Many shells resemble that of the Fresh-water Mussel (Vnio}, which is composed of three parts: the ex- ternal brown epidermis, of horny texture; then the pris- matic portion, consisting of minute columns set perpen- dicularly to the surface; and the internal nacreous layer, or " mother-of-pearl," made up of exceedingly thin plates. The pearly lustre of the last is due to light falling upon the outcropping edges of wavy laminae.76 In many cases, the prismatic and nacreous layers are traversed by minute tubes. Another typical shell-structure is seen in the com- mon Cone, a section of which shows three layers, besides the epidermis, consisting of minute plates set at different angles. The Nautilus is composed of two distinct layers : the outer one having the fracture of broken china ; the inner one, nacreous. Most living shells are made of one piece, as the Snail; these are called " univalves." Others, as the Clam, con- sist of two parts, and are called" bivalves." In either ease, a valve may be regarded as a hollow cone, growing in a spiral form. The ribs, ridges, or spines on the out- side of a shell mark the successive periods of growth, and, therefore, correspond to the age of the animal. The figures on the following page show tlie principal parts of the ordinary bivalves and univalves. The valves of a bivalve are generally equal, and the mnbones, or beaks, a little in front of the centre. The valves are bound to- gether by a ligature near the umbones, and often, also, by means of a "hinge" formed by the "teeth" of one valve interlocking into cavities in the other. The aperture of 134: COMPARATIVE ZOOLOGY. a univalve is frequently closed by a horny or calcareous plate, called " operculum," which the animal carries on its back, and which is a part of the exo- skeleton. The shells of Mollusks are epidermal, and are, therefore, dead and incapable of true repair. When broken, they can be mended FIG. 99.— Left Valve of a Bivalve Mollnsk (C.yf/wrea FIG. 100. — Section of a Spiral chione): ft, hinge ligament; u, umbo; Z, lunule; Univalve (Triton corrugatus) : e, cardinal, and t, <', lateral teeth; a, a', impres- a, apex; b, spire; c, suture; sions of the anterior and posterior adductor mns- d, posterior canal; e, outer cles ; p, pallia! impression ; s, sinus, occupied by lip of the aperture ; /, aute- the retractor of the siphous. rior canal. only by the animal pouring out lime to cement the parts together. They cannot grow together, like a broken bone. Imbedded in the back of the Cuttle-fish is a very light spongy " bone," which, as already observed, is a secretion from the skin, and therefore belongs to the exoskeleton. It has no resemblance to true bone, but is formed, like shells, of a number of calcareous plates. Nevertheless, the Cuttle-fish does exhibit traces of an endoskeleton: these are plates of cartilage, one of which surrounds the brain, and hence may be called a skull. To this cartilage, not to the " cuttle-bone," the muscles are attached. In Vertebrates, the exoskeleton is subordinate to the endoskeleton, and is feebly developed in comparison. It TI1E SKIN AND SKELETON. 135 is represented by a great variety of appendages to the skin, which are mainly organs for protection, not for sup- port. Some are horny developments of the ep- idermis, such as hairs, feathers, nails, claws, hoofs, horns, and the scales of Reptiles; oth- ers arise from the hard- ening of the dermis by calcareous matter, as the I f T7" 1 4-\ 1 FIG. 101.— Skeletal Architecture in the Armadil- SCalCS 01 £ IStieS, tlie bony 10] 8howing lhe relation of the carapax to the plates Of Crocodiles and vertebral column. Turtles, and the shield of the Armadillo. The scales of Fishes (and likewise the spines of their vertical fins) lie imbedded in the overlapping folds of the skin, and are covered with a thin, slimy epidermis. The scales of the bony Fishes (Perch, Salmon, etc.) consist of FIG. 102. — Diagrammatic Section of the Skin of a Fish (Carp) : a, derm, showing lam- iuated structure with vertical fibres, 6; c, gristly layer; e, laminated layer, with, calcareous granules ; d, superficial portion developing into scales ; /, scale-pit. two layers, slightly calcareous, and marked by concentric and radiating lines. Those of the Shark have the structure of teeth, while the scutes, or plates, of the Crocodiles, Turtles, and Armadillos are of true bone. The scales of Snakes and Lizards are horny epidermal plates covering the overlapping folds of the true skin. In some Turtles these plates are of great size, and are called "tortoise-shell;" they cover the scutes. The scales on the legs of Birds, and on the tail of the Beaver and Rat, have the same structure. Nails are flattened horny plates developed from the upper surface of the fingers 136 COMPARATIVE ZOOLOGY. FIG. 103.— Vertical Section of the Forefoot of the Horse (middle digit): 1,2, 4, proximal, middle, and distal, or nngnal, phalanges; 3, sesamoid, or nut-bone; 5, 6, 7, tendons ; 9, elastic tissue ; 8, 10, internal and external floor of the hoof; 11, 12, internal and exter- nal walls. and toes. Claws are sharp conical nails, being devel- oped from the sides as well as upper surface; and hoofs are blunt cylin- drical claws. Hol- low horns, as of the Ox, may be likened to claws sheathing a bony core. The horn of the Rhinoc- eros is a solid mass of epidermal fibres. " Whalebone," the rattles of the Rattlesnake, and the beaks of Turtles and Birds, are like- wise epidermal. Hairs, the characteristic clothing of Mammals, are elongated horny cones, composed of " pith " and "crust." The latter is an outer layer of minute overlapping scales, which are directed towards the point, so that rubbing a human hair or fibre of wool between the thumb and finger pushes the root- end away. The root is bulbous, and is contained in a minute de- pression, or sac, formed by an in- Fio.io4.-SectionoftheRoota.Hi folding of the skin. Hairs are usu- £r uncovered wS^ ally set obliquely into the skin. . ^^.^""SS Porcupine's quills and Hedgehog's of the shaft, being imbricated: . . the root consists of angular spines make an easy transition to ceils loaded with pigment. THE SKIN AND SKELETON. 137 feathers, which differ from hairs only in splitting up into numerous laminae. They are the most complicated of all the modifications of the epidermis. They consist of a "quill" (answer- ing to the bulb of a hair), and a "shaft," supporting the "vane," which is made up of "barbs," "bar- bules," and interlocking " process- es." The quill alone is hollow, and has an orifice at each end. Thus feather is moulded on a papilla, the shaft lying in a groove on one side of it, and the vane wrapped around it. When the feather emerges from the skin, it unfolds itself. Thus shaft and vanes together resemble the quill split down one side and spread out. The teeth of Mollusks, Worms, and Arthropods are also epidermal structures. Those of Vertebrates are mixed in their origin, the dentine be- ing derived from the dermis and the enamel from the epidermis. In all cases teeth belong to theexoskeleton. (2) The Endoskeleton, as we have seen, is represented in the Cuttle- fish. With this and some other exceptions, it is peculiar to Verte- brates. In the Cuttle-fish, and some Fishes, as the Stur- geon and Shark, it consists of cartilage ; but in all others (when adult) it is bone or osseous tissue. Yet there is a diversity in the composition of bony skeletons; that of fresh-water Fishes contains the least earthy matter, and that of Birds the most. Hence the density and ivory- vnne, or beard ; d, accessory plume, or down -. e, f, lower and upper umbilicus, or ori- fice, leading to the interior of the quill. 138 COMPARATIVE ZOOLOGY. whiteness of the bones of the latter. Unlike the shells of Mollusks and the crust of the Lobster, which grow by the addition of layers to their borders, bones are moist, living parts, penetrated by blood-vessels and nerves, and covered with a tough membrane, called periosteum, for the attach- ment of muscles. The surface of bones is compact; but the interior may be solid or spongy (as the bones of Fishes, Turtles, Sloths, and Whales), or hollow (as the long bones of Birds and the active quadrupeds). There are also cavities (called "sinuses") between the inner and outer walls of the skull, as is remarkably shown by the Elephant. The cavities in the long bones of quadrupeds are filled with marrow; those in the long bones of Birds and in skulls contain air. The number of bones not only differs in different ani- mals, but varies with the age of an individual. In very early life there are no bones at all ; and ossification, or the conversion of cartilage into bone, is not completed until maturity. This process begins at a multitude of points, and theoretically there are as many bones in a skeleton as centres of ossification. But the actual number is usually much less — a result of the tendency of these centres to coalesce. Thus, the thigh-bone in youth is composed of five distinct portions, which gradually unite. So in the lower Vertebrates many parts remain distinct which in the higher are joined into one. The occiput or back-bone of Man's skull is the union of four bones, which are seen separate in the skull of the Fish, or of a baby. A complete skeleton, made up of all the pieces which might enter into its composition, does not exist. Every Vertebrate has some deficiency. All, except Amphioxus, have a skull and back-bone ; but in the development of the various parts, and especially of the appendages, there is endless variety. Fishes possess a great number of skull- bones, but have no fingers and toes. The Snake has plenty THE SKIN AND SKELETON. 139 of ribs and tail, but no breast-bone ; the Frog has a breast- bone, but neither tail nor ribs. As the skeleton of a Fish is too complicated for the primary student, we will select for illustration the skeleton of a Lion — the type of quad- rupeds. It should be remembered, however, that all Ver- tebrates are formed on one plan. In the lowest Vertebrate, Amphioxus, the only skeleton is a cartilaginous rod running from head to tail. There is no skull, nor ribs, nor limbs. In the cartilaginous Fishes, 140 COMPARATIVE ZOOLOGY. the backbone is only partially ossified. But usually it consists of a number of separate bones, called vertebrce, ar- ranged along the axis of the body. They range in number from 10 in the Frog to 305 in the Boa-constrictor. The skull, with its appendages, and the vertebrae, with the ribs and sternum, make up the axial skeleton. The shoulder and pelvic girdles and the skeleton of the limbs constitute the appendicular skeleton. A typical vertebra consists of a number of bony pieces so arranged as to form two arches, or hoops, connected by B. Fio. 107.— Vertebrae— A, cervical; B, dorsal; 2, centrnm ; 4, transverse process, con- taining foramen, a, for artery; 5, articular process; 3, spinous process, or neural spine ; 1, neural canal ; 6, facets for head of rib, the tubercle of the rib fitting in a facet on the process, 4 ; b, laininse, or ueurapophyses. a central bone, or centrum.''8 The upper hoop is called the neural arch, because it encircles the spinal marrow ; the lower hoop is called the haemal arch, because it en- closes the heart and the great central blood-vessels. An actual vertebra, however, is subject to so many modifica- tions, that it deviates more or less from this ideal type. Selecting one from the middle of the back for an exam- ple, we see that the centrum sends off from its dorsal side two branches, or processes, called neurapophyses. These meet to form the neural arch, under which is the neural canal, and above which is a process called the neural spine. On the anterior and posterior edges of the arch are smooth surfaces, or zygapophyses, which in the natural state are covered with cartilage, and come in contact with THE SKIN AND SKELETON. 141 the corresponding surfaces of the preceding and succeed- ing vertebrae. The bases of the arch are notched in front and behind, so that when two vertebrae are put together a round opening (intervertebral foramen) appears between the pair, giving passage to the nerves issuing from the spinal cord. From the sides of the arch, blunt transverse processes project outward and backward, called diapophy- ses. Such are the main elements in a representative ver- tebra. The haemal arch is not formed by any part of the vertebra, but by the ribs and breast-bone. Theoretically, however, the ribs are considered as elongated processes from the centrum (pleurapophyses), and in a few cases a hcemal spine is developed corresponding to the neural spine. The vertebrae are united together by ligaments, but chiefly by a very tough, dense, and elastic substance be- tween the centra. The neural arches form a continuous canal which contains and protects the spinal cord ; hence the vertebral column is called the neuroskeleton. The column is always more or less curved; but the beautiful sigmoid curvature is peculiar to Man. The vertebrae gradually increase in size from the head towards the end of the trunk, and then diminish to the end of the tail. The neural arch and centrum are seldom wanting; the first vertebra in the neck has no centrum, and the last in the tail is all centrum. The vertebrae of the extremities (head and tail) depart most widely from the typical form. The vertebral column in Fishes and Snakes is divisible into three regions — head, trunk, and tail. But in the higher animals there are six kinds of vertebrae : cranial, cervical, dorsal, lumbar, sacral, and caudal. The cranial vertebrce form the skull." They are greatly modified, as the neural arches are expanded to enclose the brain. The number of distinct bones composing the skull is greatest in Fishes, and least in Birds : this arises partly COMPARATIVE ZOOLOGY FIG. 110. THE SKIN AND SKELETON. 143 BONES OF THE MAMMALIAN SKULL.* BRAIN-CASE. NASAL. NOSE. KTHHOID. FRONTAL. PARIETAL. SUPRAOCCIPITAL. LAC HRYMAL. SQUAMOSAL. ORBITOSPHENOID. EYE. ALISPHENOID. PERI- EAR. OTIC. EXOCCIPITAL. MALAR. TYMPANIC. PRESPIIEN01D. BASISPIIENOID. BASIOCCIP1TAL. rOXER. HYOID ARCH. PREMAXILLA. MAXILLA. PALATINE. PTERYGOID. LOWER JAW, OR MANDIBLE. THE SKULL OF THE DOG. FIG. 108.— Under surface. FIG. 109.— Upper surface. FIG. 110.— Longitudinal ver- tical section; one-half natural size: SO, supraoccipital : ExO, exoccipital ; BO, basioccipital ; IP, interparietal ; Pa, parietal ; Fr, frontal ; Sq, equamosal ; Ma, malar; L, lachrymal ; MX, maxilla ; PMx, premaxilla ; Art, nasal ; SIT, maxillo- tnrbinal: ET, ethmoturbinal ; ME, ossified portion of the mesethmoid ; CE, cri- briform, or sieve-like, plate of the ethmoturbinal ; VO, vomer ; PS, presphenoid ; OS, orbitospheuoid ; AS, alispheuoid ; BS, basisphenoid ; PI, palatine; Pt, pterygoid; Per, periotic ; TIJ, tympanic bulla ; on, anterior narial aperture ; ap, or apf, anterior palatine foramen ; ppf, posterior palatine foramen ; io, infra- orbital foramen ; pof, postorbital process of frontal bone ; op, optic foramen ; «/, ephenoidal fissure ; fr, foramen rotundam, and anterior opening of alisphenoid canal; as, posterior opening of alisphenoid canal ; fo, foramen ovale ; Jim, fora- men lacernm medium; af, glenoid fossa; gp, postglenoid process; paf, post- glenoid foramen; earn, external auditory meatus ; «m, stylomastoid foramen; ftp, foramen lacernm posterins ; cf, coudylar foramen ; pp, paroccipital process ; oc, occipital condyle ; fm, foramen magnum ; a, angular process^ «, symphysis of the mandible where it unites with the left ramns ; id, inferior dental canal ; cd, condyle ; cp, coronoid process ; the * indicates the part of the cranium to which the condyle is articulated when the mandible is in place ; the upper border in which the teeth are implanted is called alveolar ; sh, eh, ch, bh, th, hyoidean ap- paratus, or os linijuce, supporting the tongue. In, the skulls of old animals, there are three ridges: occipital, behind; sagittal, median, on the upper surface; and superorbital, across the frontal, in the region of the eyebrows. The last is highly developed in the Gorilla and other Apes. * In this diagram, modified from Huxley's, the italicized bones are single ; the rest are double. Those in the line of the Ethmoid form the Cranio -facial Axis: these, with the other sphenoids and occipitals, are developed in cartilage ; the rest are membrane bones. In the Human skull, the four occipitals coalesce into one. 144 COMPARATIVE ZOOLOGY. from the fact that the bones remain separate in the for- mer case, while those of the chick become united together (anchylosed) in the full-grown Bird ; but many bones are present in the Fish which have no representatives in the Bird. The skull consists of the brain-case and the face. The principal parts of the skull, as shown in the Dog's, are: 1. The occipital bones behind, enclosing a large hole, or foramen magnum, on each side of which are rounded prominences, called condyles, by which the skull articulates with the first cervical vertebra. 2. The parietal. 3. The frontal. These three form the main walls of the brain. 4. The sphenoid, on the floor of the skull in front of the occipital, and consisting of six pieces. 5. The temporal, in which is situated the ear. In Man this is one bone; but in most animals there are three or more — \\\Q periotie, tympanic, and squamosal. 6. The malar, or " cheek-bone," which sends back a process to meet one from the squamo- sal, forming the zygomatic arch. 7. The nasal, or roof of FIG. lit.— Sknll of the Horse: 1, premaxillary bone; 2, upper incisors; 3, upper canines; 4, 'superior maxillary ; 5, infrnorbital foramen; 6, superior maxillary spine; 7, nasal bones; 8, lachrymal; 9, orbital cavity; 10, lachrymal fossa; 11, malar; 12, upper molars; 13, frontal; 15, zygomatic arch; 16, parietal; IT, oc- cipital protuberance; 18, occipital crest; 19, occipital condyles; 20, styloicl proc- esses; 21, petrous bone; 22, basilar process; 23, cnndyle of inferior maxillary: 24, parietal crest ; 25, inferior maxillary ; 26, lower molars ; 27, anterior maxillury foramen; 28, lower canines; 29, lower iucisors. THE SKIN AND SKELETON. 145 the nose. 8. The maxilla; that part of the upper jaw in which the canines, premolars, and molars are lodged. 9. The premaxilla, in which the upper incisors are situated. 10. The palatine, which, with the maxillary bones, forms the roof of the mouth. There are two appendages to the skull: the mandible, or lower jaw, whose condyles, or' rounded extremities, fit into a cavity (the glenoid) in the temporal bone ; and the hyoid, situated at the root of the tongue. The simplest form of the skull is a cartilaginous box, as in Sharks, enclosing the brain and supporting the car- tilaginous jaws and gill arches. In higher Fishes this box is overlaid with bony plates and partly ossified. In Frogs the skull is mainly bony, although a good deal of the car- tilage remains inside the bones. In higher Vertebrates the cartilage never makes an entire box, and early disappears. The cervical vertebrae, or bones of the neck, are peculiar in having an orifice on each side of the centrum for the passage of an artery. The first, called atlas, because it supports the head, has no centrum, and turns on the sec- ond, called axis, around a blunt process, called the odon- toid. The centra are usually wider than deep, and the neural spines very short, except in the last one. The number of cervical vertebras ranges from 1 in the Frog to 25 in the Swan. The dorsal vertebrae are such as bear ribs, which, uniting with the breast-bone, or sternum, form a bony arch over the heart and lungs, called the thorax. The sternum may be wanting, as in Fishes and Snakes, or greatly developed, as in Birds. When present, the first vertebra whose ribs are connected with it is the first dorsal. The neural spines of the dorsal series are generally long, pointing backward. The lumbar vertebrce are the massive vertebrae lying in the loins between the dorsals and the hip-bones. The sacral vertebra lie between the hip-bones, and are 10 146 COMPARATIVE ZOOLOGY. generally consolidated into one complex bone, called sa- crum. The caudal vertebrae are placed behind the sacrum, and form the tail. They diminish in size, losing processes and neural arch, till finally nothing is left but the centrum. They number from 3 or 4 in Man to 270 in the Shark. Besides the lower jaw, hyoid, and ribs, Vertebrates have other appendages to the spinal column — two pairs of limbs.™ The fore limb is divided into the pectoral arch (or shoulder girdle), the arm, and the hand. The arch is fastened to the ribs and vertebrae by powerful muscles, and consists of three bones, the scapula, or shoul- der-blade, the coracoid, and the clavicle, or collar-bone. The scapula and coracoid are generally united in Mam- mals, the latter forming a process of the former ; and the clavicles are frequently wanting, as in the hoofed animals. The humerus, radius, and ulna are the bones of the arm, the first articulating by ball-and-socket joint with the scapula, and by a hinge-joint with the radius and ulna. The humerus and radius are always present, but the ulna may be absent. The bones of the hand are divided into those of the carpus, or wrist; the metacarpus, or palm; and the phalanges, or fingers. The fingers, or " digits," range in number from 1 to 5. The hind limb is composed of the pelvic arch (or hip- bones), the leg, and the foot. These parts correspond closely with the skeleton of the fore limb. Like the shoulder, the pelvic arch, or os innominatum, consists of three bones — ilium, ischium, and pubis. The three are distinct in Amphibians, Reptiles, and in the young of higher animals; but in adult Birds and Mammals they become united together, and are also (except in Whales) solidly attached to the sacrum. The two pelvic arches and the sacrum thus soldered into one make the pelvis. The leg-bones consist of the femur, or thigh; the tibia, or THE SKIN AND SKELETON. 147 shin-bone ; and the fibula, or splint-bone. The rounded head of the femur fits into a cavity (acetabulum} in the pelvic arch, while the lower end articulates with the tibia, and sometimes (as in Birds) with the fibula also. An ex- tra bone, the patella, or knee-pan, is hung in a tendon in front of the joint between the femur and tibia of the high- er animals. The foot is made up of the tarsus, or ankle ; the metatarsus, or lower instep ; and the phalanges, or toes. The toes number from 1 in the Horse to 5 in Man. Certain parts of the skeleton, as of the skull, are firmly joined together by zigzag edges or by overlapping; in either case the joint is called a suture. But the great majority of the bones are intended to move one upon an- other. The vertebrae are locked together by their proc- esses, and also by a tough fibrous substance between the centra, so that a slight motion only is allowed. The limbs furnish the best examples of movable articulations-, as the ball-and-socket joint at the shoulder, and the hinge-joint at the elbow. The bones are held together by ligaments,- and, to prevent friction, the extremities are covered with cartilage, which is constantly lubricated with an unctuous fluid called synovia. CHEMICAL COMPOSITION OF BONES. COD. TORTOISE. HAWK. MAN. Phosphate of Lime, with trace of Fluate of Lime., 57.29 52 66 64 39 59 63 Carbonate of Lime 4 90 12 53 7 03 7 33 Phosphate of Magnesia 2.40 0 82 0 94 1 32 Sulphate, Carbonate, and Chlorate of Soda 1.10 090 0 92 069 Glutine and Chondrine. 32 31 31 75 25 73 29 70 Oil . 2 00 1 34 0 99 1 33 100.00 100.00 100.00 100.00 148 COMPARATIVE ZOOLOGY. THE SKIN AND SKELETON. 149 150 COMPAEATIVE ZOOLOGY. FIG. 115.— Skeleton of the Tortoise (plastron removed): o, cervical vertebrae; e, dor- sal vertebrae ; d, ribs; e, marginal bones of the carapace; I, scapula; k, precora- coid; 6, coracoid;/, pelvis; i, femur; g, tibia ; A, fibula. Pio. 116. — Skeleton of a Vulture : 1, craninm — the parts of which are separable only In the chick ; 2, cervical vertebrae ; 3, dorsal ; 4, coccygeal, or caudal ; the lumbar and sacral are consolidated; 6, ribs; 6, sternum, or breast-bone, extraordinarily developed; 7, fnrculnm, clavicle, or "wish-bone;" S, coracoid; 9, scnpnla; 10, hmnerus; 11, ulna, with rudimentary radius; 12, metacarpals ; 13, phalanges of the great digit of the wing; 19, thumb ; 14, pelvis ; 15, femur ; 16, tibia-tarsus and fibula, or cms; 17, tarso-metatarsus ; 18, internal digit, or toe, formed of three phalanges ; the middle toe has four phalanges ; the outer, five ; and the back toe, or thumb, two. THE SKIN AND SKELETON. 151 FIG. 117.— Skeleton of the Horse (Equv* eaballus) : 22, premaxillary ; 12, foramen in the maxillary ; 15, nasal ; 9, orbit ; 19, coronoid process of lower jaw ; 17, surface of implantation for the masseter muscle ; there are seven cervical vertebrae, nine- teen dorsal, D-D; five lumbar, o-e; five sacral, f-l; and seventeen caudal, p-r ; 51, scapnla, or shoulder-blade ; i, spine, or crest ; ft, coracoid process (acromion wanting) ; 1, first pair of ribs (clavicle wanting, as in all Ungulates) ; e, sternum ; a, shaft of hnmerus; 6, deltoid ridge ; g, head fitting in the glenoid cavity of the scapula — near it is a great tnberosity for the attachment of a powerful muscle ; k, condyles ; 54, radius, to which is firmly anchylosed a rudimentary ulna, 55, the olecranon ; 56, the seven bones of the carpus, or wrist ; 57, large metacarpal, or " cannon-bone," with two " splint-bones ;" 68, fetlock-joint ; 59, phalanges of the developed digit, corresponding to the third finger in Man; 62, pelvis; 63, the great trochanter, or prominence on the femur, 65 ; 66, tibia ; 67, rudimentary fibula ; 68, hock, or keel, falsely called knee ; 69, metat arsals. 152 COMPARATIVE ZOOLOGY. FIG. 118.— Skeleton of the Cow (Bos taurus). FIG. 119.— Skeleton of an Elephant (Elephas Indicut). THE SKIN AND SKELETON. 153 Fio. 120.— Skeleton of the Chimpanzee (Troglodytes Niger). 154 COMPARATIVE ZOOLOGY. CHAPTEK XVII. HOW ANIMALS MOVE. 1. THE power of animal motion is vested in protoplasm, cilia, and muscles. The power of contractility is one of the ultimate physiological properties of protoplasm, like sensibility 'and the power of assimilation. Protoplasma- animals, like the Amoeba and Rhizopoda, move by the contractility of their protoplasm, as also may the germs of higher animals upon the yolk of the egg. The proto- plasm may be extended into projections called pseudopodia, by whose contraction the animal may move (Fig. 186). Infusoria, and nearly all higher animals, possess cilia (Fig. 188). These are microscopic hairs (Fig. 2, J) which have the power of bending into a sickle-shape and straight- ening out. As they bend much faster than they straight- en, and as they all work together, they can cause motion of the animal, or may serve to produce currents in the water, the animal remaining at rest. They are seen on the outside of Infusoria, and of very many embryos of higher animals, serving as paddles for locomotion ; they fringe the gills of the Oyster, creating currents for respi- ration; and they line the passage to our lungs to expel the mucus. Flagella (Fig. 189) are a sort of long cilia, which are thrown into several curves when active, resem- bling a whip-lash, whence their name. Both cilia and fla- gella seem to be wanting in Arthropods. The cause of ciliary motion is unknown. Their one- sided contraction is their property, as the straight con- traction of the muscle-fibre belongs to it. No structure can, however, be seen in them with the microscope. No HOW ANIMALS MOVE. 155 nerves go to them, yet they work in concert, waves of motion passing over a surface covered with cilia, as over a field of grain moved by the wind. But muscular tissue is the great motor agent, and exists in all animals from the Coral to Man.79 The power of contractility, which in the Amoeba is diffused throughout the body, is here confined to bundles of highly elastic fibres, called muscles. When a muscle contracts, it tends Fia. 121.— A Contractiug Muscle. to bring its two ends together, thus shortening itself, at the same time increasing in thickness. This shrinking property is excited by external stimulants, such as elec- tricity, acids, alkalies, sudden heat or cold, and even a sharp blow; but the ordinary cause of contraction is an influence from the brain conveyed by a nerve. The prop- erty, however, is independent of the nervous system, for the muscle may be directly stim- ulated. The amount of force with which a muscle contracts depends on the number of its fibres ; and the amount of shortening, on their length. As a rule, muscles are white in cold-blooded animals, and red in the warm-blooded. They are white in all the Invertebrates, Fishes, Batrachians, and Reptiles, except Salmon, Sturgeon, and Shark ; and red in Birds and Mammals, except in the breast of the com- fowl, and the like.80 It is also a rule, with some exceptions, that the voluntary muscles of Vertebrates, and all the muscles larFibre.much Scfens? ' "* 156 COMPARATIVE ZOOLOGY. of the Lobster, Spider, and Insect tribes, are striated ; while the involuntary muscles of Vertebrates, and all the muscles of Radiates, Worms, and Mollusks, are smooth. All mus- cles attached to internal bones, or to a jointed external skeleton, are striated. The voluntary muscles of Verte- brates are generally solid, and the involuntary hollow.81 This leads to another classification of muscles : into those which are attached to solid parts within the body; those which are attached to the skin or its modifications ; and those having no attachments, being complete in them- selves. The last are hollow or circular muscles, enclosing a cavity or space, which they reduce by contraction. Ex- amples of such are seen in the heart, blood-vessels, stom- ach, iris of the eye, and around the mouth. In the lower Invertebrates, the muscular system is a net-work of longi- tudinal, transverse, and oblique fibres intimately blended with the skin, and not divisible into separate muscles. As in the walls of the human stomach, the fibres are usually in three distinct layers. This arrangement is exhibited by soft-bodied animals, like the Sea-anemone, the Snail, and the Earth-worm. Four thousand muscles have been count- ed in a Caterpillar. There are also " skin-muscles " in the higher animals, as those by which the Horse produces a twitching of the skin to shake off insects, and those by which the hairs of the head and the feathers of Birds are made to stand on end. Invertebrates whose skin is hard- ened into a shell or crust have muscles attached to the inside of such a skeleton. Thus, the Oyster has a mass of parallel fibres connecting its two valves ; while in the Lobster and Bee fibres go from ring to ring, both longi- tudinally and spirally. The muscles of all Invertebrates are straight parallel fibres, not in bundles, but distinct, and usually flat, thin, and soft. The great majority of the muscles of Vertebrates are attached to the bones, and such are voluntary. The fibres, HOW ANIMALS MOVE. 157 which are coarsest in Fishes (most of all in the Rays), and finest in Birds, are bound into bundles by connective tis- sue; and the muscles thus made up are arranged in layers around the skeleton. Sometimes their extremities are at- tached to the bones (or rather to i\\% periosteum) directly ; but generally by means of white inelastic cords, called tendons. In Fishes, the chief masses of muscle are dis- posed along the sides of the body, apparently in longitu- dinal bands, reaching from head to tail, but really in a series of vertical flakes, one for each vertebra. In propor- tion as limbs are developed, we find the muscles concen- trated about the shoulders and hips, as in quadrupeds. The bones of the limbs are used as levers in locomotion, the fulcrum being the end of a bone with which the mov- ing one is articulated. Thus, in raising the arm, the hu- merus is a lever working upon the scapula as a fulcrum. The most important muscles are called extensors aud flex- ors. The latter are such as bring a bone into an angle with its fulcrum — as in bending the arm — while the for- mer straighten the limb. Abductws draw a limb away from the middle line of the body, or a finger or toe away from the axis of the limb, while adductors bring them back. 2. Locomotion. — All animals have the power of vol- untary motion, and all, at one time or another, have the means of moving themselves from place to place. Some are free in the embryo-life, and fixed when adult, as the Sponge, Coral, Crinoid, and Oyster. There may be no regular well-defined means of progression, as in the Amoe- ba, which extemporizes arms to creep over the surface; or movement may be accomplished by the contraction of the whole body, as in the Jelly-fish, which, pulsating about fifteen times in a minute, propels itself through the water. So the "Worms and Snakes swim by the undulations of the body. But, as a rule, animals are provided with special organs 158 COMPARATIVE ZOOLOGY. for locomotion. These become reduced in number, and progressively perfected, as we advance in the scale of rank. Thus, the Infusorian is covered with thousands of hair-like cilia; the Star-fish has hundreds of soft, unjoint- ed, tubular suckers ; the Centipede has from 30 to 40 jointed hollow legs ; the Lobster, 10 ; the Spider, 8 ; and the Insect, 6 ; the Quadruped has 4 solid limbs for loco- motion ; and Man, only 2. ( 1 ) Locomotion in Water. — As only the lower forms of life are aquatic, and as the weight of the body is partly sustained by the element, we must expect to find the or- gans of progression simple and feeble. The Infusoria swim with great rapidity by the incessant vibrations of the delicate filaments, or cilia, on their bodies. The com- mon Squid on our coast admits water into the interior of the body, and then suddenly forces it out through a fun- nel, and thus moves backward, or forward, or around, ac- cording as the funnel is turned — towards the head, or tail, or to one side. The Lobster has a fin at the end of its tail, and propels itself backward by a quick down-stroke of the abdomen. But Fishes, whose bodies offer the least resistance to progression through water*, are the most perfect swimmers. Thus, the Salmon can go twenty miles an hour, and even FIG. 123.— The Pins of a Fish (Pike-perch). ascend cataracts. They have fins of two kinds : those set obliquely to the body, and in pairs ; and those which are HOW ANIMALS MOVE. 159 vertical, and single. The former, called pectoral and ven- tral fins, represent the fore and hind limbs of Quadrupeds. The vertical fins, which are only expansions of the skin, vary in number; but in most Fishes there are at least three : the caudal, or tail-fin ; the dorsal, or back-fin ; and the anal, situated on the abdomen, near the tail. The chief locomotive agent is the tail, which sculls like a stern-oar; the other fins are mainly used to balance and raise the body. "When the two lobes of the tail are equal, and the vertebral column stops near its base, as in the Trout, it is said to be homocercal. If the vertebrae extend into the upper lobe, making it longer than the lower one, as in the Shark, the tail is called hetero- cereal. The latter is the more effec- tive for varying the course; the Shark, e. a., will accompany and resultant of the tw ' y ' , . . » „ ., Pulses is the straight line gambol around a ship in lull sail m front. across the Atlantic. The Whale swims by striking the water up and down, instead of laterally, with a fin-like horizontal tail. Many air-breathing animals swim with facility on the surface, as the Water-birds, having webbed toes, and most of the Reptiles and Quadrupeds. (2) Locomotion in Air. — The power of flight requires a special modification of structure and an extraordinary muscular effort, for air is 800 times lighter than water. Nevertheless, the velocity attainable by certain Birds is greater than that of any Fish or Quadruped ; the Hawk being able to go at the rate of 150 miles an hour. The bodies of Insects and Birds are made as light as possible by the distribution of air-sacs or air-cavities.82 The wings of Insects are generally four in number; 160 COMPARATIVE ZOOLOGY. sometimes only two, as in the Fly. They are moved by muscles lying inside the thorax. They are simple expan- sions of the skin, or crust, being composed of two delicate films of the epidermis stretched upon a net-work of tubes. There are three main varieties : thin and transparent, as in the Dragon-fly ; opaque, and covered with minute col- ored scales, which are in reality flattened hairs, as in the Butterfly ; and hard and opaque, as the first pair (called elytra) of the Beetle. The wings of Birds, on the other hand, are modified fore-limbs, consisting of three sets of feathers (called pri- mary, secondary, and tertiary), inserted on the hand, fore- arm, and arm. The muscles which give the downward stroke of the wing are fastened to the breast-bone ; and their power, in proportion to the weight of the Bird, is very great. Yet the Insect is even superior in vigor and velocity of flight.83 In ascending, the Bird slightly rotates the wing, striking downward and a little backward ; while the tail acts as a rudder. A short, rounded, concave wing, as in the common Fowl, is not so well fitted for high and prolonged flight as the long, broad, pointed, and flat wing . FIG. 125. — Flamingoes taking Wing. of the Eagle. The wing is folded by means of an elastic skin and muscle connecting the shoulder and wrist. Be- sides Insects and Birds, a few other animals have the power HOW ANIMALS MOVE. 161 of flight, as Bats, by means of loDg-webbed fingers ; Fly- ing Fishes, by large pectoral fins. Flying Keptiles, Flying Squirrels, and the like, have a membrane stretched on the long ribs, or connecting the fore and hind limbs, which they use as a parachute, enabling them to take very long leaps. (3) Locomotion on Solids. — This requires less muscular effort thaii swimming or flying. The more unyielding the basis of support, the greater the amount of force left to move the animal along. The simplest method is the suctorial, the animal attaching itself to some fixed object, and then, by contraction, dragging the body onward. But the higher and more common method is by the use of bones, or other hard parts, as levers. The Star -fish creeps by the working of hundreds of tubular suckers, which are extended by being filled with Fio. 126.— Diagrammatic section of Star-fish: a, mouth; 6, stomach; e, hepatic CJC- cnm: d, dorsal or aboral surface ; e, ambulacral plates; /, ovary ; g, tubular feet ; h, internal sacs for extending the feet fluid forced into them by little sacs. The Clam moves by fixing and contracting a muscular appendage, called a "foot." The Snail has innumerable short muscles on the under side of its body, which, by successive contrac- tions, resembling minute undulations, enable the animal to glide forward apparently without effort. The Leech has a sucker at each end : fixing itself by the one on its tail, and then stretching the body, by contracting the mus- cular fibres which run around it, the creature fastens its mouth by suction, and draws forward the hinder parts by 11 162 COMPARATIVE ZOOLOGY. the contraction of longitudinal muscles. The Earth-worm lengthens and shortens itself in the same way as the Leech, but instead of suckers for holding its position, it has nu- merous minute spines pointing backward ; while the Cat- erpillar has short legs for the same purpose. The legless Serpent moves by means of the scutes, or large scales, on the under side of the body, acted upon by the ribs. In a straight line, locomotion is slow ; but by curving the body, laterally or vertically, it can glide or leap with great rapidity. Most animals have movable jointed limbs, acted upon as levers by numerous muscles. The Centipede has forty- two legs, each with five joints and a claw. The Crab has five pairs of six -jointed legs; but the front pair is modified into pincers for prehen- sion. With the rest, which end in a sharp claw, the Crab moves backward, forward, or sideways. The Spider has eight legs, usually seven -joint- ed, and terminating Pi«. 127.— Feet of Insects: A, Sibio febrilia; B, jn two claWS toothed House-fly (Musca domestica); C, Water -beetle (DytiK\u). like a comb, and a third which acts like a thumb. In running, it moves the first right leg, then the fourth left ; next, the first left, and then the fourth right ; then the third right and sec- ond left together; and lastly, the third left and second right together. The front and hind pairs are, therefore, moved like those of a quadruped. The Insect has six HOW ANIMALS MOVE. 163 legs, each of five parts: the coxa; trochanter ; femur; tibia, or shank ; and tarsus. The last is subdivided usu- ally into five joints and a pair of claws. Such as can walk upside down, as the Fly, have, in addition, two or three pads between the claws.84 These pads bear hairs which secrete a sticky fluid, by means of which the Fly adheres to the surface. While the leg-bones of Verte- brates are covered by the muscles which move them, the limbs of Insects are hollow, and the muscles inside. The fore legs are directed forward, and the two hinder pairs backward. In motion, the fore and hind feet on one side, and the middle one on the other, are moved simultane- ously, and then the remaining three. The four-legged animals have essentially the same appa- ratus and method of motion. The Crocodile has an awk- ward gait, owing to the fact that the limbs are short, and placed far apart, so that the muscles act at a mechanical dis- advantage. The Tortoise is proverbially slow, for a similar reason. Both swim better than they walk. Lizards are light and agile,but progression is aided by a wriggling of the body. The locomotive organs of the mammalian quadrupeds are much more highly organized. The bones are more compact ; the vertebral column is arched, and yet elastic, between the shoulder and hip, and the limbs are placed vertically underneath the body. The bones of the fore limb are nearly in a line ; but those of the hind limb, which is mainly used to project the body forward, are more or less inclined to one another, the angle being most marked in animals of great speed, as the Horse. Some walk on hoofs, as the Ox (Ungulate) ; some on the toes, as the Cat (Digitigrade) ; others on the sole, touching the ground with the heel, as the Bear (Plantigrade). In the Pinnigrade Seal, half of the fore limb is buried under the skin, and the hind limbs are turned backward to form a fin with the tail. The normal number of toes is five ; but 164 COMPARATIVE ZOOLOGY. PIG. 128. —Feet of Carnivores: A, Plantigrade (Bear); B, Pinuigrade (Seal) ; C, Digitigrade (Lion). some may be wanting, so that we have one-toed animals (as Horse), two-toed (as Ox), three-toed (as Rhinoceros), four-toed (as Hippopotamus), and five-toed (as the Ele- phant)» The Horse steps on what corresponds to the nail of the middle finger ; and its swiftness is conditioned on the solidity of the extremities of the limbs. Horses of the greatest speed have the shoulder-joints directed at a considerable angle with the arm. Pis. 129.— Feet of Hoofed Mammals: A, Elephant; B, Hippopotamus ; C, Rhinoc- eros; 7), Ox; E, Horse, a, astragalus ; cl, calcanenm, or heel ; «, uaviculure; 6, cuboides ; <*, «', cm, cuneiform bones ; the numbers indicate the digits in use. HOW ANIMALS MOVE. 165 The order in which the legs of Quadrupeds succeed each other determines the various modes of progression, called the walk, trot, gallop, and leap. Many, as the Horse, have all these movements ; while some only leap, as the Frog and Kangaroo. In leaping animals, the hind limbs are extraordinarily developed. In many Mammals, like the Squirrel, Cat, and Dog, the fore legs are used for prehension as well as locomotion. Monkeys use all four, ?io. 130.— Muscles of the Hnman Leg: gartorius, or "tailor's muscle," the longest muscle in the body, flexes the leg upon the thigh; rectus femoris and vastus externus and intermts ex- tend the leg, maintaining an erect posture; gastrocwmius, or "calf," used chiefly in wnlking, for raising the heel. Another layer underlies these superficial muscles. Fio. 131. —Muscles of an Insect's Leg (Melolontha vulgaris) : a, flexor, and 6, extensor, of tibia ; c, flexor of foot ; d, accessory muscle; e, extensor of claw; /, extensor of tarsus. The joints are restricted to movements in one plane ; and therefore the mus- cles are simply flexors and extensors. All the muscles are within the skele- ton. 166 COMPARATIVE ZOOLOGY. and also the tail, for locomotion and prehension, keeping a horizontal attitude; while the Apes, half erect, as if they were half-quadruped, half-biped, go shambling along, touching the ground with the knuckles of one hand and then of the other. In descending the scale, from the most anthropoid Ape to the true Quadruped, we find the centre of gravity placed increasingly higher up — that is, farther forward. Birds and Men are the only true bipeds ; the former standing on their toes, the latter on the soles of the feet. Terrestrial Birds walk and run ; while Birds of flight usually hop. The Ostrich can for a time outrun the Arabian Horse ; and the speed of the Cassowary ex- ceeds that of the swiftest Greyhound. CHAPTER XYIIL THE NEKVOTJS SYSTEM. Nervous Matter exists in the form of cells, fibres, or tubes. In the cellular state it is grayish, and accumulated in masses, called ganglia, or centres, which alone origi- nate nervous force ; the fibrous and tu- bular kinds are gen- erally white, and arranged in bun- dles, called nerves, which serve only as conductors. Most nerves contain two FIG. 132. — Nerve-cells from Human Brain: A, associ- kinds of fibres, like ated with nerve-tubes aud blood-vessels; B, multi- . polar nucleated cells. in structure, bu.t B THE NERVOUS SYSTEM. 167 each having its distinct office: one carries impressions re- ceived from the external world to the gray centres, and hence is called an afferent^ sen- sory, nerve; the other conducts an influence generated in the centre to the muscles, in obedi- ence to which they contract, and hence it is called an efferent, or motor, nerve. Thus, when the finger is pricked with a pin, af- ferent nerve -fibres convey the FlG. m-Nervous system of star- impression to the centre -the Spinal COrd, which immediately each arm, ending in the eye. transmits an order by efferent fibres to the muscles of the hand to contract. If the former are cut, sensation is lost, but voluntary motion remains ; if the latter are cut, the animal loses all control over the muscles, although sensi- bility is perfect ; if both are cut, the animal is said to be paralyzed. The nerve-fibres are connected with nerve-cells in the central organs, and at the outer ends are connected with the mus- cular fibres, or with various sen- sory end -organs in the skin or other parts of the body. The nature of nerve - force is not known. As to the velocity of a nervous impulse, we know it is far less than that of electricity or light, and that it is more rapid in warm-blooded than in cold-blood- iiusk (the oasteropod Apiys- e(j animals being faster in Man : o, anterior ganglion ; c, ce- * ' phaiic; i, lateral ; g, abdominal than in the Frog. In the latter it averages about 85 feet per second, in the former over 100 feet. 168 COMPARATIVE ZOOLOGY. The very lowest animals, like the Amceba and Infuso- ria, have no nerves, although their protoplasm has a gen- eral sensibility. The Hydra has certain cells which are, perhaps, partly nervous and partly muscular in function. The Jelly-fish has a nervous system, consist- ing of a net-work of threads and ganglia scattered all over its disk. We should look for a definite system of ganglia and nerves only in those animals which pos- sess a definite muscular structure, and show definitely co-ordinat- ed muscular movements. In the Star-fish we detect the first clear specimen ot such a system. It consists PIG. 135.— Nervous Sys- Of a rjnor around the mouth, tern of Clam : c, cere- ° t ' bral ganglion ;p,ped- made of five ganglia of al ganglia ; ps, parie- , . , , . . tospianchnic ganglia; equal size, with radiating from cerebral to pedal distinguished by an irregu- ganglia;X,commis- •» sure from cerebral to larly scattered nervous sys- parietogplanchnic „,, /~1 , ., ganglia; oe, oesopha- tern. The Clam has three main pairs of connected ganglia — one near the mouth, one in the foot, and the third in the posterior region, near the siphons. In the Snail, these are united into a ring around the gullet, and there are other ganglia scattered through the body. The same is true of the Cuttle- FIG. ise.— Nervous fish, where the brain is partly enclosed in a cartilaginous box (Fig. 151). In the simpler worms there is but a sin- head- ganglion. gle ganglion or a single pair. The Earth-worm has a pair of brain-ganglia lying above the gullet, and connected by THE NERVOUS SYSTEM. 169 two cords with a ventral chain of ganglia — one pair, ap- parently a single one, for each segment. In the lower Arthropods, such as Crustacea, Centipedes, and Larval In- sects, the arrangement is substan- tially the same. In higher Insects and Crustacea, many of the gan- glia are fused together in the head and thorax, indicating a concen- tration of organs for sensation and locomotion. In Vertebrates, the nervous system is more highly developed, more complex, and more concen- trated than in the lower forms. In fact, there are some parts, as the brain, to which we find nothing homologous in the Invertebrates ; and while the actions of the lat- ter are mainly, if not wholly, au- tomatic, those of backboned ani- mals are voluntary. Its position, moreover, is peculiar, the great mass of the nervous matter being accumulated on the dorsal side, 'and enclosed by the neural arches of the skeleton. The brain and spinal cord lie in the cavity of the skull and spinal column, wrapped in three membranes. Both consist of gray and white nervous matter; but in the brain the gray is on the out- side, and the white within ; while the white of the spinal cord is external, and the gray in- ternal. Both are double, a deep fissure running from the FIG. 137. — Human Brain and Spinal Cord, one fifth natural size : a, great longitudinal fissure ; 6, an- terior lobe; c, middle lobe; d, medulla oblongata; e, cerebel- lum, /, first spinal nerve; g, brachial plexus of nerves supply- ing the arms ; h, dorsal nerves ; i, lumbar nerves ; k, sacral plexus of nerves for the limbs ; I, canda eqsiiua: the figures indicate the twelve pairs of cranial nerves, of which 1 is olfactory, 2 optic, and 8 auditory. 170 COMPARATIVE ZOOLOGY. forehead backward, dividing the brain into two hemi- spheres, and the spinal cord resembling two columns welded together; even the nerves come forth in pairs to the right and left. The brain is the organ of sensation and voluntary motion ; the spinal cord is the organ of in- voluntary life and motion. The brain, above the medulla oblongata, may be removed, and yet the animal, though it cannot feel, will live for a time, showing that it is not ab- solutely essential to life ; in fact, the brain does nothing in apoplexy and deep sleep. All of the cord, except that part containing the centres for respiration and circulation, may also be destroyed, without causing immediate death. The Brain is that part of the nervous system contained in the skull." It increases in size and complexity as we pass from the Fishes, by the Amphibians, Reptiles, and Birds, to Mammals. Thus, the body of the Cod is 5000 times heavier than its brain — in fact, the brain weighs less than the spinal cord ; while in Man, the brain, compared with the body, is as 1 to 36, and is 40 times heavier than the spinal cord. The brains of the Cat weigh only 1 oz. ; of the Dog, 6 oz. 5£ dr. ; and of the Horse, 22 oz. 15 dr. The only animals whose brains outweigh Man's are the Elephant and Whale — the maximum weight of the Ele- phant's being 10 Ibs., and of the Whale's 5 Ibs. ; while the human does not exceed 4: Ibs. Yet the human brain is heavier in proportion to the body. But quality must be considered as well as quantity, else the Donkey will outrank the Horse, and the Canary-bird, Man ; for their brains are relatively heavier. The main parts of the brain are the cerebrum, cerebel- lum, and medulla oblongata. The cerebrum is a mass of white fibrous matter covered by a layer of gray cellular matter. In the lower Verte- brates, the exterior is smooth; but in most of the Mam- mals it is convoluted, or folded, to increase the amount of THE NERVOUS SYSTEM. 171 the gray surface. The convolutions multiply and deepen as we ascend the scale of size and intelligence, being very complex in the Elephant and Whale, Monkey and Man. As a rule, they are proportioned to the intelligence of the animal ; yet the brains of the Dog and Horse are smoother than those of the Sheep and Don- key. Evidently the quality of the gray mat- ter must be taken into account. Save in the bony Fishes, the cere- brum is the largest por- tion of the brain ; in Man it is over eight times heavier than the cerebellum. The cerebellum, or "little brain," lies be- hind the cerebrum, and, like it, presents an ex- ternal gray layer, with a white interior. In Mammals, it is likewise finely convoluted, Con- sist i n o- nf trrav anrl Fl°' m ~Braln of the Horse-npper view, one ing 01 gray ano half natural size: a, medullaoblongata; &, lat- white Iamin83, and is eral and middle lobes of cerebellum ; c, inter- lobular fissure ; d, cerebral hemispheres ; «, ol- divided into two lobes, factory lobes. or hemispheres. In the rest of the Vertebrates, the cere- bellum is nearly or quite smooth ; and in the lowest Fish- es it is merely a thin plate of nervous matter. In many Yertebrates, however, it is larger, compared with the cere- brum, than in Man, since in Man the cerebrum is extraor- dinarily developed. 172 COMPARATIVE ZOOLOGY. The medulla oblongata is the connecting link between the cerebrum and cerebellum and the spinal cord. In structure, it resembles the spinal cord — the white matter being external and the gray internal. The former lies beneath or behind the brain, passing through t\\e foramen magnum of the skull, and merging imperceptibly into the cord. The latter is a continuous tract of gray matter en- closed within strands of white fibres. It usually ends in the lumbar region of the vertebral column, but in Fishes it reaches to the end of the tail. In Fishes, Amphibians, and Reptiles, the cord outweighs the brain : in Birds and Mammals, the brain is heavier than the cord. In Man, it weighs about an ounce and a half. Besides these parts, there are also the olfactory and the optic lobes, which give rise respectively to the nerves of smell and sight. The parts of the brain are always in pairs ; but in rela- tive development and po- sition they differ widely in the several classes of Ver- tebrates. In Fishes and Reptiles, they are arranged in a horizontal line ; in Birds and Mammals, the axis of the spinal cord bends to nearly a right an- Fia. m-Braiu of g]e jn passjnor through the the Perch, upper ° ° ° view: o, cerebei- brain, so that the lobes no Inm; b, optic , ... . . -. lobes; c, cere- longer lie in a straight line. r?Tbe8-°0famT I" ^an> tne fore-brain is FIG. 140. -Brain of the dull* oblongata. &Q developed that it COV. &OSEX*i era all the other lobes. In looking down upon the brain of a Perch, we see iri front a pair of olfactory lobes (which send forth the nerves of smell), behind Lop S>A olfactory lobes; He, cer- ebral hemispheres; Pn, pineal gland ; Fho and Srh, third and fourth ventricles ; Lop, optic lobes; C, cerebellum; Mo, medulla oblongata. THE NERVOUS SYSTEM. 173 them the small cerebral hemispheres, then the large optic lobes (in which originate the nerves of sight), and, last of all, the cerebellum. Not till we reach Man and the Apes do we find the cerebrum so highly developed as to overlap both the olfactory lobes in front and the cerebellum behind. Functions of the Brain. — The ^ej^bJUini_is_the_seat of in- telligence and will. It has no direct communicatiolPvvith the~outside world, receiving its consciousness of external objects and events through the- spinal cord and the nerves of special sense.8' The cerebellum seems to preside over the co-ordination of the muscular movements. When removed, the animal Pin. 141. — A, C, upper and side views of the Brain of a Lizard ; B, D, upper and side views of the Brain of a Turkey: Olf, olfactory lobes ; Hmp, cerebral hemispheres ; Pn, pineal gland ; Mb, optic lobes of the middle brain ; Cb, cerebellum ; MO, me- dulla oblongata; if, optic nerves; iv and vi, nerves for the muscles of the eye; Py, pituitary body. desires to execute the mandates of the will, but cannot ; its motions are irregular, and it acts as if intoxicated. It is usually largest in animals capable of the most compli- cated movements ; being larger in the Ape than in the Lion, in the Lion than in the Ox, in Birds than in Hep- tiles. The cerebellum of the Frog is, however, smaller than that of Fishes (Figs. 139, 140). The olfactory and op- tic lobes receive the messages from their respective nerves. 174: COMPARATIVE ZOOLOGY. The medulla oblongata is not only the medium of com- munication between the brain and the spinal cord, but it PIG. 142.— Brain of the Cat (Felis do- mestica): a, medulla oblongata; b, cerebellum; c, cerebrum. FIG. 143. —Brain of the Orang-utan, upper surface; cue third natural size. is itself a nervous centre : the brain above and the cord below may be removed without death to the animal, but the destruction of the medulla is fatal. Of the twelve pairs of nerves issuing from the contents of the skull (en~ cephalon\ ten come from the medulla oblongata. Among these are the nerves of hearing FIG. 144.— Human Brain, side view: 1, medulla oblongata ; 3, cerebellum ; 6, frontal convolutions of cerebrum. FIG. 145. — Human Brain, npper view, one third natural size : 1, anterior lobes ; 2, posterior; 3, great median fissure. and taste, and those that control the lungs and heart. Kes- piration ceases immediately when the medulla is injured. THE NERVOUS SYSTEM. 175 The spinal cord is a centre for originating involuntary actions, and is also a conductor — propagating through its central gray matter the impressions received by the nerves to the brain, and taking back through its fibrous part the impulses of the brain. In Man, thirty-one pairs of nerves arise from the cord to supply the whole body, except the head. Each nerve has an ante- rior and a posterior root. The fibres of the former go to the muscles, and hence carry the impulses which cause muscular contraction (hence call- ed motor fibres) ; those of the posterior root con- vey sensations from the exterior to the central organs (sensory). The fibres leading from the brain to the cord cross ,1 .} _, FIG. 146.— Relation of the Sympathetic and Spinal one another m the me- Nerves : c> flsgure of ^ cpord . a> anteri[;r of dlllla Oblongata, SO that if the right Cerebral . . .. . , hemisphere be diseased, the left side of the body loses the power of voluntary motion. The sympathetic nervous system is a double chain of ganglia, lying along the sides of the vertebral column in the ventral cavity. From these ganglia nerves are given off, which, instead of going to the skin and muscles, like the spinal nerves, form net-works about those internal organs over which the will has no control, as the heart, stomach, 1 nerve •> ft Posterior root, with its ganglion; a, anterior branch; p, posterior branch; *, sympathetic; «, its double junction by white and gray filaments. 176 COMPARATIVE ZOOLOGY. and intestines. Their apparent office is to stimulate these organs to constant activity, but is little understood. 1. The Senses. Sensation is the consciousness of impressions on the sensory nerves. These impressions produce some change in the brain ; but what that change is, is a darkness on which no hypothesis throws light. Obviously, we feel only the condition of our nervous system, not the objects which excite that condition." All animals possess a general sensibility diffused over the greater part of the body.88 This sensibility, like as- similation and contractility, is one of the primary physio- logical properties of protoplasm. But, besides this (save in the very lowest forms), they are endowed with special nerves for receiving the impressions of light, sound, etc. These nerves of sense, as they are called, although struct- urally alike, transmit different sensations : thus, the Ear can- not recognize light, and the Eye cannot distinguish sounds. la the Vertebrates, the organs of sight, hearing, and smell are situated in pairs on each side of the head ; that of taste, in the mucous membrane covering the tongue; while the sense of touch is diffused over the skin. Sight and hearing are stimulated, each by one agent only; while touch, taste, and smell may be excited by various substances. The agents awakening sight, hearing, and touch are physical; those causing taste and smell are chemical. Animals differ widely in the numbers and keenness of their senses. But there is no sense in any one which does not exist in some other. Touch is the simplest and the most general sense; no an- imal is without it, at least in the form of general sensibility. It is likewise the most positive and certain of the senses. In the Sea-anemone, Snail, and Insect, it is most acute in the " feelers" (tentacles, horns, and antennae),8* in the Oys- THE NERVOUS SYSTEM. 177 PIG. 147. — Antenna of Various Insects. ter, the edge of the mantle is most sensitive ; in Fishes, the lips ; in Snakes, the tongue ; in Birds, the beak and under side of the toes ; in Quadrupeds, the lips and tongue ; and in Monkeys and Man, the lips and the tips of the tongue and fin- gers. In the most sensitive parts of Birds and Mam- mals, the true skin is raised up into multitudes of mi- nute elevations, called pa- pillce-, containing loops of capillaries and nerve-filaments. ' There is a correspondence between the delicacy of touch and the development of in- telligence. The Cat and Dog are more sagacious than hoofed animals. The Elephant and Parrot are remark- ably intelligent, and are as celebrated for their tactual power. Taste is more refined than touch, since it gives a knowledge of properties which cannot be felt. It is al- ways placed at the entrance to the digestive canal, as its chief purpose is to guide animals in their choice of food. No special organ of taste can be de- tected in the Invertebrates, although all seem to exercise a faculty in se- lecting their food. Even in Fishes, Amphibians, Reptiles, and Birds this FIG. US.-Pnpillse of Human ' Palm, x 35, the cuticle be- sense is very obtuse, for they bolt their food. But the higher Verte- brates have it well developed. It is confined to the tongue, and is most delicate at the root.90 A state of solution and an actual contact of the fluid are necessary conditions. Smell is the perception of odors, i. 6., certain substances 12 178 COMPARATIVE ZOOLOGY. in the gaseous state. Many Invertebrates have this sense: Snails, e. g., seem to be guided to their food by its scent, and Flies soon find a piece of meat. In the latter the organ is probably located on the antennae. In Verte- brates, it is placed at the entrance to the respiratory tube, in the upper region of the nose. There the olfac- tory nerves, which issue from the olfac- tory lobe of the brain, and pass through the ethmoid bone, or roof of the nasal cavity, »» distributed over a moist cavity, mucous membrane. The odorous sub- stance, in a gaseous or finely divided state, is dissolved in the mucus covering this membrane. In Fishes and Kep- tiles generally, this organ is feebly developed ; Sharks, however, gather from a great distance around a carcass. In the Porpoises and Whales it is nearly or entirely wanting. Among Birds, Waders have the largest olfac- tory nerves. It is most acute in the carnivorous Quad- rupeds, and in some wild herbivores, as the Deer. In Man it is less delicate, but has a wider range than in any brute. Hearing is the perception of sound. The simplest form of the organ is a sac filled with fluid, in which float the soft and delicate ends of the auditory nerve. The vibrations of the fluid are usually strengthened by the presence of minute hard granules, call- ed otoliths. Most Invertebrates have no higher apparatus than this ; and it is probable that they can distinguish one noise from another, but neither Fro. 150.— Ear of a Mol- pitch nor intensity. The organ is gen- insk (Cycias), greatly en- erally double, but not always located larged' in the head. In the Clam, it is found at the base of the foot ; some Grasshoppers have it in the fore-legs ; and in THE NERVOUS SYSTEM. 179 many Insects it is on the wing. Lobsters and Crabs have the auditory sacs at the base of the antennae." Fia. 151.— Brain and Auditory Apparatus of the Cuttle-fish: a, b, brain ; e, auditory apparatus ; d, the cavity iu which it is lodged ; e,f, g, eyes ; 1, 2, 3, otoliths. A complex organ of hearing, located in the head, exists in all Vertebrates, save the very lowest Fishes. As com- plete in Man, it consists of the following parts: 1st. The external ear (which is peculiar to Mammals) ; the auditory canal, about an inch long, lined with hairs and a waxy se- cretion, and closed at the bottom by a membrane, called tympanum, or " drum of the ear." 2d. The middle ear, contain- ing three little bones (the smallest in the body), 'mal- leus, incus, and stapes, ar- ticulated together. The Cavity communicates with FIG. 152.— Section of Human Ear: a, external ... enr, with auditory caual ; b, tympanic cavi- tlie external ail' by means ty containing the three bones ; c, hammer, of the Eustachian tube, which opens at the back part of the mouth. 3d. visible. The internal ear, or labyrinth, an irregular cavity in the solid part of the temporal bone, and separated from the 180 COMPARATIVE ZOOLOGY. middle ear by a bony partition, which is perforated by two small holes. The labyrinth consists of the vestibule, or entrance ; the semicircular canals, or tubes ; and the cochlea, or spiral canal. "While the other parts are full of air, the labyrinth is filled with a liquid, and in this are the ends of the auditory nerve. The vibrations of the air, collected by the external ear, are concentrated upon the tympanum, and thence transmitted through the chain of little bones to the fluid in the labyrinth. Now, the essential organ of hearing is the labyrinth, which is, substantially, a bag filled with fluid and nerve- filaments. Fishes generally have but little more. In Amphibians and Reptiles there are added a tympanum, a single bone, connecting this with the internal ear, the cochlea, and the Eustachian tube; the tympanum being external. Birds have, besides, an auditory passage, open- ing on a level with the surface of the head, and surround- ed by a circle of feathers. Mammals only have an exter- nal ear." Sight is the perception of light.*3 In all animals it de- pends upon the peculiar sensitiveness of the optic organ to the luminous vibrations. In Vertebrates the optic nerve comes from the middle mass of the brain, in Invertebrates it is derived from a ganglion. Many animals are utter- ly destitute of visual organs, as the Protozoa, and the lower Radiates and Mollusks, besides intestinal Worms and the blind Fishes and other cave-animals. Around the margin of the Jelly-fish are colored spots, supposed to be rudimentary eyes ; but, as a lens is wanting, there is no image; so that the creature can merely distinguish light from darkness and color without form. Such an eye is nothing but a collection of pigment granules on the ex- pansion of a nervous thread, and the perception of light is the sensation of warmth, the pigment absorbing the rays and converting them into heat. THE NERVOUS SYSTEM. 181 Going higher, we find a lens introduced forming a dis- tinct image. The Snail, for example, has two simple eyes, called ocelli, mounted on the tip of its long tentacles, con- sisting of a globular lens,84 with a transparent skin (cornea) in front, and a colored membrane ~r J) (cboroid) and a ner- vous n e t- Fio.l53.-Eyeof work (reti- Pecteu, much en- larged: wi.mouth; Iia) behind. andnc8horoirdef ? The Scallop nerve. (Pecten)l\&S such eyes in the edge of F[(j ]54 _Head of a Snajl bisected showing its mantle (Fig. 153). Sucll structure of tentacles: a, right inferior ten- . tacle retracted within the body; b, right sn- OrganS are the Only eyes perior tentacle fully protruded ; c, left supe- i^AQQ^eeprl hv Mvviannrls rior tentacle partially inverted ; d, left inferi- Dy iviy i lapoas, or tentacle . /t optic ne,.ve . fft retractor mu8. Spiders, Scorpions, and cle ? *• °P"C ufvet in lo°8e fo'd8 ; ^ ^tractor r ' muscle of head ; k, nerve and muscle of left Caterpillars. Adult In- inferior tentacle ; I, m, nervous collar. sects usually have three ocelli on the top of the head. But the proper visual organs of Lobsters, Crabs, and In- sects are two compound eyes, perched on pedestals, or fixed on the sides of the head. They consist of an immense number of ocelli pressed together so that they take an angular form — four- sided in Crustacea, six-sided in Insects. They form two rounded protuberances variously colored — white, yellow, red, green, purple, brown, or black. Under the microscope, the surface is seen to be divided into a host of facets,95 each being an ocellus complete in itself. Each cornea is convex on one side, 55.— Head of the Bee, showiiiirconipoundeyes, the three ocelli, or stem- mata, and the antennas. 182 COMPARATIVE ZOOLOGY. and either convex or flat on the other, so that it produces a focus like a lens. Be- hind the cornea, or lens, is the pigment, having a minute aper- ture or " pupil." Next is a conical tube — one for each facet — with sides and bottom lined with pigment. These tubes converge to the optic ganglion, the fibres of which pass through the tubes to the cornea.86 Vision FIG. 166.— Eye of a Beeite (Melolontha) : A, section; |^y sucn a COmpOUlld a, optic ganglion ; b, secondary nerves ; c, retina ; * d, pigment layer ; e, proper optic nerves ; B, group eye is not a mosaic ; of ocelli; /, bulb of optic nerve; g, layer of pig- ment; h, vitreous humor ; i, cornea. but each OCellUS glVCS a complete image, although a different perspective from its neighbor. The multiplied images are reduced to one men- tal stereoscopic pict- ure, on the principle of single vision in ourselves. The eyes of the Cuttle-fish are the largest and the most Fl(,157._SeCti<)nofIIumanEye:aand6,upperand perfect amoilf Inver- '°wer lid; c, conjunctiva, or mucous membrane, lining the inner surface ; d, external membrane ; «, sheath of optic nerve; /, g, muscles for rolling the eye up or down ; h, sclerotic ; i, transparent cor- nea; j, choroid ; k, I, ciliary muscle for adjusting the eye for distance ; tn, iris and pupil ; n, canal ; o, retina ; *, vitreous humor ; t, crystalline ; », an- illg a crystalline lens terior chamber ; x, posterior chamber. with a chamber in front (open, however, to the sea- tebrates. They re- semble the eyes of higher animals in hav- THE NERVOUS SYSTEM. 183 water), and a chamber behind it filled with " vitreous humor." The eye of Vertebrates is formed by the infolding of the skin to create a lens, and an outgrowth of the brain to make a sensitive layer ; both enclosed in a white spherical case (sclerotic) made of 9 tough tissue, with a transparent front, call- 8 ed the cornea. This case is kept in shape 7 by two fluids — the thin aqueous humor filling the cavity just behind 6 the cornea, and the ielly-like vitreous hu- mor occupying the lar- ger posterior chamber. Between the two hu- mors lies the double- convex crystalline lens. On the front face of the lens is a contractile circular curtain (iris), with a hole in the cen- tre (pupil); and lin- ing the sclerotic coat is the choroid mem- ij hranp nnvprpfJ with Fm.l5S.— Section of the Human Retina, X 400: 1, brane, co\ered witn iuterna]limitiugniemb!,ine.2)0ptic.liervefibree. dark pigment. The 3. ganglion cells ; 4, internal molecular layer; 5, internal grannies ; 6, external molecular layer; 7, OptlC nerve, entering exteraalgranules;8,exten)allimitingmembrane; at the back of the eye 9' layei- of rod8 and cone8 ; 10' pigment layen through the sclerotic and choroid coats, expands into the transparent retina, which consists of several layers — 184: COMPARATIVE ZOOLOGY. fibrous, cellular, and granular. The most sensitive part is the surface lying next to the black pigment. And here is a peculiarity of the vertebrate eye : the nerve-fibres, en- tering from behind, turn back and look towards the bot- tom of the eye, so that vision is directed backward; while invertebrate vision is directly forward. In Vertebrates only, the optic nerves cross each other (decussate) in pass- ing from the brain to the eyes ; so that the right side of the brain, e. g., receives the impressions of objects on the left side of the body.87 Generally, the eyes of Vertebrates are on opposite sides of the head ; but in the Flat-fishes both are on the same side. Usually, both eyes see the same object at once ; but in most Fishes the eyes are set so far back, the fields of vision are distinct. The cornea may be flat, and the lens globular, as in Fishes ; or the cornea very convex, and the lens flattened, as in Owls. Purely aquatic animals have neither eyelids nor tears, but nearly all others (especially Birds) have three lids.98 The pupil is usually round ; but it may be rhomb-shaped, as in Frogs ; vertically oval, as in Crocodiles and Cats : or transversely oval, as in Geese, Doves, Horses, and Ruminants. Many Quadrupeds, as the Cat, have a membrane (tapetum) lining the bottom of the eyeball, with a brilliant metallic lustre, usually green or pearly : it is this which makes the eyes of such animals luminous in the dark. 2. Instinct and Intelligence. The simplest form of nervous excitement is mere sensa- tion. Above this we have sensation awakening conscious- ness, out of which come those voluntary activities grouped together under the name of Instinct; and, finally, Intelli- gence. The lowest forms of life are completely under law, for their movements seem to be due solely to their organiza- THE NERVOUS SYSTEM. 185 tion. They are automatons, or creatures of necessity. Such, also, are some actions in the higher animals, as breathing, the beating of the heart, the contractions of the iris, and all the first movements of an infant." But, generally, the actions of animals are not the result of mere bodily organization. The 'inferior orders are under the control of Instinct, i. There is little modulation in Larynx, seen in profile; «, hair brute utterance. The Opossum purrs, the of the hyoid ~, , , ~,r . TT bone; e, tra- Sloth and Kangaroo moan, the Hog grunts or squeals, the Tapir whistles, the Stag bel- lows, and the Elephant gives a hoarse trump- et sound from its trunk and a deep groan from its throat. All Sheep have a guttural voice; all the Cows low, from the Bison to the Musk-ox; all the Horses and Donkeys neigh; all the Cats miau, from the domestic animal to the Lion ; all the Bears growl ; and all the Canine family — Fox, Wolf, and Dog — bark and howl. The Howling- monkeys and Gorillas have a large cavity, or sac, in the throat for resonance, enabling them to utter a powerful voice; and one of the Gibbon -apes has the remarkable power of emitting a complete octave of musical notes. The human voice, taking the male and female together, has a range of nearly four octaves. Man's power of speech, or the utterance of articulate sounds, is due to his intel- lectual development rather than to any structural differ ence between him and the Apes. Song is produced by the vocal cords, speech by the mouth. REPRODUCTION. 191 CHAPTER XIX. REPRODUCTION. IT is a fundamental truth that every living organism has had its origin in some pre-existing organism. The doctrine of " spontaneous generation," or the supposed origination of organized structures out of inorganic parti- cles, or out of dead organic matter, has not yet been sus- tained by facts. Reproduction is of two kinds — sexual and asexual. All animals, probably, have the first method, while a very great number of the lower forms of life have the latter also. Of asexual reproduc- tion there are two kinds — Self ' - division and Budding. Self-division, the simplest mode possible, is a natural breaking-up of the body into distinct surviving parts. This process is sometimes ex- traordinarily rapid, the increase of one animal- cule (Pararnoecium) be- ing Computed at 268 Fio. 160.— Reproduction of Infusoria (Vortieel- .,,. . , T Ice and others) by fission or self-division. millions in a mouth. It may be either transverse or longitudinal. Of the first sort, Figs. 1,2, and 3 (Fig. 160) are examples; of the latter, 192 COMPARATIVE ZOOLOGY. Figs. 4, 6, 9-13. This form of reproduction is, naturally, confined to animals whose tissues and organs are simple, and so can easily bear division, or whose parts are so ar- ranged as to be easily separable without serious injury. The process is most common in Protozoa, Worms, and Polyps. Budding is separated by no sharp line from Self-divi- sion. While in the latter a part of the organs of the par- ent go to the offspring, in the former one or more cells of the original animal begin to develop and multiply so as to grow into a new animal like the parent. The proc- ess in animals is quite akin to the same operation in plants. The buds may remain permanently attached to the parent-stock, thus making a colony, as in Corals and Bryozoa (continuous budding), or they may be detached at some stage of growth (discontinuous budding). This separation may occur when the bud is grown up, as in Hydra (Fig. 191), or as in Plant-lice, Daphnias (Fig. 255), and among other animals the buds may be internal, and detached when entirely undeveloped and externally re- sembling an egg. They differ, however, entirely from a true egg in developing directly, without fertilization. Sexual Reproduction requires cells of two kinds, usu- ally from different animals. These are the gerrn-cell or egg, and the sperm-cell. The embryo is developed from the union of the two cells.107 The egg consists essentially of three parts, the germinal vesicle, the yolk, and the vitellme membrane, which sur- rounds both the first. It is ordinarily globular in shape. Of the three parts, the primary one is the germinal vesi- cle— a particle of protoplasm. The yolk serves as food for this, and the membrane protects both. When a great mass of yolk is present, it is divisible into two parts— -for- mative &ndfood yolk. The latter is of a more oily nature than the former, and is usually not segmented with the REPRODUCTION. 193 egg. The structure of the hen's egg is more complicated. The outside shell consists of earthy matter (lime) depos- ited in a net-work of animal matter. It is minutely porous, to allow the passage of vapor and air to and fro. Lining the shell is a double mem- brane (membrana putaminis} resem- bling delicate tissue-paper. At the larger end^it separates to enclose a FlG.161._Theoi.etical Egg, bubble of air for the use of the chick. or Cell: *•> viteiime mem- _ T in braue ; y, oleaginous pole ; JNext comes the albumen, or "white, o, albuminous pole; p, in spirally arranged layers, within which floats the yolk. The yolk is sermiDal. dot- prevented from moving towards either end of the egg by two twisted cords of albumen, called chalazce / yet is al- lowed to rise towards one side, the yolk being lighter than the albumen. The yolk is composed of oily granules (about ir^r of an inch in diameter), enclosed in a sac, called the vitelline membrane, and disposed in concentric layers, like a set of vases placed one within the other. That part of the yolk which extends from the centre to a white Fio. 162. — Longitudinal Section of Hen's Egg before incubation: a, yolk, showing concentric layers ; o', its semi-fluid centre, consisting of a white granular sub- stance — the whole yolk is enclosed in the vitelline membrane ; b, inner dens* part of the albumen ; &', outer, thinner part ; c, the chalazse, or albumen, twisted by the revolutions of the yolk ; d, double shell-membrane, split at the large end to form the chamber,/; e, the shell ; h, the white spot, or cicatricula. 13 194: COMPARATIVE ZOOLOGY. spot (cicatricula) on the outside cannot be hardened, even with the most prolonged boiling. The cicatricula, or em- bryo-spot— the part for which all the rest was made — is a thin disk of cellular structure, in which the new life first appears. This was originally a simple cell, but de- velopment has gone some way before the egg is laid. It is always on that side which naturally turns uppermost, for the yolk can turn upon its axis ; it is, therefore, al- ways nearest to the external air and to the Hen's body — two conditions necessary for its development. There is another reason for this polarity of the egg: the lighter and most delicate part of the yolk is collected in its upper part, while the heavy, oily portion remains be- neath. In most eggs the shell and albumen are wanting. When the albumen is present, it is commonly covered by a mem- brane only. In Sharks, the envelope is horny; and in Crocodiles it is calcareous, as in Birds. The egg of the Sponge has no true vitelline membrane, and is not unlike an ordinary amoeboid cell. An egg is, in fact, little more than a very large cell, of which the germinal vesicle is the nucleus. The size of an egg depends mainly upon the quantity of yolk it contains ; and to this is proportioned the grade of development which the embryo attains when it leaves the egg.108 In the eggs FIG. IBS.— Egg of sponge: of the Star-fishes, Worms, Insects, Mol- n, nucleus. luskg (except the Cuttle-fishes), many Amphibians, and Mammals, the yolk is very minute and formative, i. e., it is converted into the parts of the future embryo. In the eggs of Lobsters, Crabs, Spiders, Cepha- lopods, Fishes, Reptiles, and Birds, the yolk is large and colored, and consists of two parts — the formative, or REPRODUCTION. 195 germ-yolk, immediately surrounding the germinal vesicle; and the nutritive, or food-yolk, constituting the greater part of the mass, by which the young animal in the egg- life is nourished. In the latter case, the young come forth more mature than where the food-yolk is wanting. As to form, eggs are oval or elliptical, as in Birds and Crocodiles; spherical, as in Turtles and Wasps; cylindri- cal, as in Bees and Flies ; or shaped like a hand-barrow, with tendrils on the corners, as in the Shark. The eggs Fio. 1C4.— Eg£ of a Shark (the external gills of the embryo are not represented). of some very low forms are sculptured or covered with hairs or prickles. The number of eggs varies greatly in different animals, as it is in proportion to the risks during development Thus, the eggs of aquatic tribes, being unprotected by the parent, and being largely consumed by many animals, are multiplied to prevent extinction. The spawn of a single Cod contains millions of eggs ; that of the Oyster, 6,000,- 000. A Queen-bee, during the five years of her existence, lays about a million eggs. Eggs are laid one by one, as by Birds ; or in clusters, as by Frogs, Fishes, and most Invertebrates. The spawn of the Sea-snails consists of vast numbers of eggs adhering together in masses, or in sacs, forming long strings. As a rule, the higher the rank, the more care animals 196 COMPARATIVE ZOOLOGY. take of their eggs and their young, and the higher the temperature needed for egg-development. In the major- ity of cases, eggs are left to themselves. The fresh-water Mussel (Unio) carries them within its gills, and the Lob- ster under its tail. The eggs of many Spiders are envel- oped in a silken cocoon, which the mother guards with jealous care. Insects, as Flies and Moths, deposit their eggs where the larva, as soon as born, can procure its own food. Most Fishes allow their spawn, or roe, to float in the water ; but a few build a kind of flat nest in the sand or mud, hovering over the eggs until they are hatched; while the Acara of the Amazons carries them in its mouth. The Amphibians, generally, envelop their eggs in a gelatinous mass, which they leave to the elements ; but the female of the Surinam Toad carries hers on her back, where they are placed by the male. The great Am- azon Turtles lay their eggs in holes two feet deep, in the sand; while the Alligators simply cover theirs with a few leaves and sticks. Nearly all Birds build nests, those of the Perchers being most elaborate, as their chicks are de- pendent for a time on the parent.'09 The young of Mar- supials, as the Kangaroo, which are born in an extremely immature state, are nourished in a pouch outside of the body. But the embryo of all other Mammals is devel- oped within the parent to a more perfect condition, by means of a special organ, the placenta. It is a general law, that animals receiving in the embryo state the longest and most constant parental care ultimately attain the high- est grade of development. The Protozoa, which have no true eggs, have a sort of reproduction called conjugation. In this process two Amoebae unite into one mass, surround themselves with a case, in which they divide into several parts, each portion becoming a new Amoeba. The sperm-cells differ from the egg in being very small, DEVELOPMENT. 197 usually motile, and in that a large number are usually produced from a single cell of the animal, while the egg represents an entire cell. The union of the sperm-cell with the germinal vesicle (fertilization) is the first step in development, and without it the egg will not develop. But the nature of the process is unknown. CHAPTER XX. DEVELOPMENT. Development is the evolution of a germ into a com- plete organism. The study of the changes within the egg constitutes the science of Embryology ; the transforma- tions after the egg-life are called metamorphoses, and in- clude growth and repair. The process of development is a passage from the gen- eral to the special, from the simple to the complex, from the homogeneous to the heterogeneous, by a series of dif- ferentiations. It brings out first the profounder distinc- tions, and afterwards those more external. That is, the most essential parts appear first. And not only does de* velopment tend to make the several organs of an individ- ual more distinct from one another, but also the individual itself more distinguished from other individuals and from the medium in which it lives. With advancing develop- ment, the animal, as a rule, acquires a more specific, defi- nite form, and increases in weight and locomotive power. Life is a tendency to individuality. The first step in development, after fertilization, is the segmentation of the egg, by a process of self-division. In the simplest form, the whole yolk divides into two parts; these again divide, making four, eight, sixteen, etc., parts, 198 COMPARATIVE ZOOLOGY. until the whole yolk is subdivided into very small por- tions (cells) surrounding a central cavity. This stage is known as the " mulberry-mass," or blastula (Fig. 165, c). ABC FIG. 165.— First Singes in Segmentation of a Mammalian Egg: A, first division into halves, with spermatozoa around it; B and C, progressive subdivision, ultimate- ly transforming the vitellus, or yolk, iuto a " mulberry mass" of globules, or em- bryo-cells. If the yolk is larger, relatively to the germinal vesicle, the process of division may go on more slowly in one of the two parts of the egg, first formed ; or in very large eggs, like those of Birds and Cuttle-fishes, only a small part of the yolk subdivides. In some form, the process of segmentation is found in the eggs of all animals, as is also the following stage. This step is the differentiation of the single layer of cells into two parts, one for the body-wall, the other for the wall of the digestive tract. In the typical examples, this is accom- plished by one part of the wall of FIQ. lee.— Diagram of Gastm- the blastula turning in, so far as to la of a Worm (Sngitta): o, . ,1 , i , , . . c primitive mouth- b primi- convert the blastula into a sort of vZS^^eZ, double-walled cup, the gastrula (Fig. endoderm ; ec, ectoderm. 16(|). One half of the Wall of the blastula is now the outer wall of the germ, the other half that of the digestive cavity ; the original blastula-cavity is now the body-cavity, and the new cavity formed by the infolding is the stomach, and its opening is both mouth DEVELOPMENT. 199 and vent (Figs. 165, 166). Some adult animals are little more than such a sac. Hydra (Fig. 191), for instance, is little different from a gastrnla with tentacles, and one of its relatives wants even these additions. Ordinarily, however, development goes much further. From the two original layers arises, in various ways, a third between them, making the three primitive germ-layers — epiblast, mesoblast, and hypoblast. This new layer is nec- essarily in the primitive body -cavity, which it may fill up ; or usually a new body-cavity is formed, in different ways in different groups. In by far the great majority of animals the digestive tract gets a new opening, which usually becomes the mouth ; and the old mouth may close, or serve only the functions of the vent. From this point the development of each group must be traced in detail. Development of a Hen's Egg. — After the segmentation the germinal disk divides into two layers, between which a third is soon formed. The upper layer (epiblast) gives FIG. 107.— Vertical Sections of au Egg, showing progressive stages of development: a, notochord ; b, medullary furrow, becoming a closed canal in the last, rise to the cuticle, brain, spinal cord, retina, crystalline lens, and internal ear. From the lower layer (hypoblast) is formed the epithelium of the digestive canal. From the middle layer (mesoblast) come all the other organs — muscles, nerves, bones, etc. The mesoblast thickens so as to form two parallel ridges running lengthwise of the germ, and leaving a groove between them (medul- lary furrow and ridges).11" The ridges- gradually rise, carrying with them the epiblast, incline towards each oth- er, and at last unite along the back. So that we have a 200 COMPARATIVE ZOOLOGY. tube of epiblast surrounded by mesoblast, which is itself covered by epiblast. This tube becomes the brain and spinal cord, whose central canal, enlarging into the ven- tricles of the brain, tells the story of its original forma- tion. Beneath the furrow, a delicate cartilaginous thread appears (called notocliord) — the predecessor of the back- bone. Meanwhile the mesoblast has divided into two layers, except in the middle of the animal, beneath the spinal cord, and in the head. One of these layers remains attached to the epiblast, and with it forms the body- wall ; the other bends rapidly downward, carrying the hypoblast with it, and forms the wall of the intestine. The space thus left between the layers of the mesoblast is the body- cavity. At the same time, the margin of the germ ex- tends farther and farther over the yolk, till it completely encloses it. So that now we see two cavities — a small one, containing the nervous system ; and a larger one be- low, for the digestive organs. Presently, numerous rows of corpuscles are seen on the middle layer, which are subsequent- ly enclosed, forming a net-work of capillaries, Pis. 168. — Rudimentary Hearts, hnmnu : 1, venous r trunks; 2, auricle; 3, ventricle; 4, bulbus arte- called the VaSCulararea. A dark spot indicates the situation of the heart, which is the first distinctly bounded cavity of the circulatory system. It is a short tube lying lengthwise just behind the head, with a feeble pulsation, causing the blood to flow backward and for- ward. The tube is gradually bent together, until it forms a double cavity, resembling the heart of a Fish. On the fourth day of incubation, partitions begin to grow, divid- ing the cavities into the right and left auricles and ven- tricles. The septum between the auricles is the last to be finished, being closed the moment respiration begins. DEVELOPMENT. 201 The blood-vessels ramify in all directions through the yolk, making it a spongy mass, and all perform the same office ; it is not till the fourth or fifth day that arteries can be distinguished from veins, by being thicker, and by carrying blood only from the heart.111 A. A FIG. 169.— Embryo in a Hen's Egg during the first five days : A, hypoblast ; B, lower layer of mesoblast ; C, upper layer of mesoblast and epiblast united, in the last figures forming the amniotic sac; D, vitelline membrane; e, thickened blasto- derm, the first rudiment of the dorsal part (in the last figure it marks the place of the lungs); A, heart; a, b, its two chambers; c, aortic arches; m, aorta; i, liver ; p, allantois. 202 COMPARATIVE ZOOLOGY. The embryo lies with its face, or ventral surface, tow- ards the yolk, the head and tail curving towards each FIG. 170. — Hen's Egg, more highly developed. The embryo is enveloped by the am- niou, and has the umbilical vessel, or remnant of the yolk, hanging from its un- der surface; while the allantois turns upward, and spreads out over the internal surface of the shell-membrane. (From Daltou's " Physiology.") other. Around the embryo on all sides the epiblast and upper layer of the incsoblast rise like a hood over the back of the embryo till they form a closed sac, called the amnion. It is filled with a thin liquid, which serves to protect the embryo. Mean- while, another important or- gan is forming on the other side. From the hinder por- tion of the alimentary canal an outgrowth is formed FIG. 171. -Mammalian Embryo withal- wnich extends beyond the lantois fully formed: 1, umbilical vesi- * cle, containing the last of the yolk; 2, wall of the embryo proper amnion; 3, allantois, on which the fringes . . . ,. . of the placenta are developing. (From HltO the Cavity OT the amni- Dalion's "Physiology.") Qn ftnd gpreads Qut over the whole inner surface of the shell, so that it partly surrounds both embryo and inner layer of the amnion (amnion prop- DEVELOPMENT. 203 er). This is the allantois. It is full of blood-vessels, and it serves as the respiratory organ until the chick picks the shell and breathes by its lungs.115 The chorion is the out- ermost part of the allantois, and the placenta of Mammals is the shaggy, vascular edge of the chorion. The alimentary canal is at first a straight tube closed at both ends, the middle being connected with the yolk-bag. As it grows faster than the body, it is thrown into a spi- ral coil; and at several points it dilates, to form the crop, stomach, gizzard, etc. The mouth is developed from an infolding of the skin. The liver is an outgrowth from the digestive tube, at first a cluster of cells, then of folli- cles, and finally a true gland. The lungs are developed on the third day as a minute bud from the upper part of the alimentary canal, or pharynx. As they grow in size, they pass from a smooth to a cellular condition. The skeleton at the beginning consists, like the noto- chord, of a cellular material, which gradually turns to car- tilage. Then minute canals containing blood-vessels arise, and earthy matter (chiefly phosphate of lime) is deposited between the cells. The primary bone thus formed is compact: true osseous tissue, with canaliculi, laminae, arid Ilayersian canals, is the result of subsequent absorption.113 Certain bones, as those of the face and cranium, are not preceded by cartilage, but by connective tissue : these are called membrane bones. Ossification, or bone-making, be- gins at numerous distinct points, called centres; and, the- oretically, every centre stands for a bone, so that there are as many bones in a skeleton as centres of ossification. But the actual number in the adult animal is much small- er, as many of the centres coalesce.114 The development of the backbone is not from the head or from the tail, but from a central point midway between: there the first ver- tebrae appear, and from thence they multiply forward and backward. 204 COMPARATIVE ZOOLOGY. The limbs appear as buds on the sides of the body ; these lengthen and expand so as to resemble paddles — the wings and legs looking precisely alike ; and, finally, they are divided each into three segments, the last one subdividing into digits. The feathers are developed from the outside cells of the epidermis: first, a horny cone is formed, which elongates and spreads out into a vane, and this splits up into barbs and barbules. The muscle-fibres are formed either by the growth in length of a single cell, or by the coalescence of a row of cells: the cell-wall thus produces a long tube — the sarco- lemrna of a fibre — and the granular contents arrange them- selves into linear series, to make fibrillse. Nervous tissue is derived from the multiplication and union of embryo-cells. The white fibres at first resemble the gray. The brain and spinal marrow are developed from the epiblastic lining of the medullary furrow. Soon the brain, by two constrictions, divides into fore -brain, mid-brain, and hind-brain. The fore-brain throws out two lateral hemispheres (cerebrum), and from these pro- trude forward the two olfactory lobes. From the mid- dle-brain grow the optic lobes; and the hind -brain is separated into cerebellum and medulla oblongata. The essential parts of the eye, retina and crystalline lens, are developed, the former as a cup-like outgrowth from the fore-brain, the latter as an ingrowth of the epidermis. An infolding of the epidermis gives rise to the essential parts of the inner ear, and from the same layer come the olfactory rods of the nose and the taste-buds of the tongue. So that the central nervous system and the essential parts of most of the sense-organs have a common origin. Modes of Development. — The structure and embryology of a Hen's egg exhibit many facts which are common to all animals. But every grand division of the Animal Kingdom has its characteristic method of developing. DEVELOPMENT. 205 Protozoans differ from all higher forms in having no true eggs. The egg of the Hydroid, after segmentation, becomes a hollow, pear-shaped body, covered with cilia. Soon one end is indented ; then the indentation deepens until it readies the interior and forms the mouth. The animal fastens itself by the other end, and the tentacles appear as buds. In the Sea-anemone, the stomach is turned in, and the partitions appear in pairs. In the Oyster, the egg segments into two unequal parts, one of which gives rise to the digestive tract and its de- rivatives, while from the smaller part originate the skin, gills, and shell. It is soon covered with cilia, by whose help it swims about. The embryo of an Insect shows from the first a right and left side ; but the first indication that it is an Articu- late is the development of a series of indentations divid- ing the body into successive rings, or joints. Next, we observe that the back lies near the centre of the egg, the ventral side looking outward; i. e., the embryo is doubled upon itself backward. And, finally, the appearance of three pairs of legs proves that it will be an Insect, rather than a Worm, Crustacean, or Spider. The Vertebrate embryo lies with its stomach towards the yolk, reversing the position of the Articulate ; but the grand characteristic is the medullary groove, which does not exist in the egg of any Invertebrate. This feature is connected with another, the setting apart of two distinct regions — the nervous and nutritive. There are three modifications of Vertebrate development: that of Fishes and Amphibians, that of True Reptiles and Birds, and that of Mammals. The amnion and allantois are wanting in the first group ; while the placenta (which is the allan- tois vitally connected with the parent) is peculiar to Mam- mals. In Mammals, the whole yolk is segmented; in 206 COMPARATIVE ZOOLOGY. Birds, segmentation is confined to the small white speck seen in opening the shell. At the outset, all animals, from the Sponge to Man, appear essentially alike. All, moreover, undergo seg- mentation, and most have one form or other of the gastrula stage. But while Vertebrates and Invertebrates can travel together on the same road up to this point, here they diverge — never to meet again. For every grand group early shows that it has a peculiar type of construc- tion. Every egg is from the first impressed with the power of developing in one direction only, and never does it lose its fundamental characters. The germ of the Bee is divided into segments, showing that it belongs to the Articulates ; the germ of the Lion has the medullary stripe — the mark of the coming Vertebrate. The blasto- dermic layer of the Vertebrate egg rolls up into two tubes — one to hold the viscera, the other to contain the nervous cord ; while that of the Invertebrate egg forms only one such tubular division. The features which determine the subkingdom to which an animal belongs are first devel- oped, then the characters revealing its class. There are differences also in grade of development as well as type. For a time there is no essential difference between a Fish and a Mammal : they have the same ner- vous, circulatory, and digestive systems. There are many such cases, in which the embryo of an animal represents the permanent adult condition of some lower form. In other words, the higher species, in the course of their de- velopment, offer likenesses, or analogies, to finished lower species. The human germ, at first, cannot be distinguished from that of any other animal : for aught we can see, it may turn out a Frog or a Philosopher. The appearance of a medullary stripe excludes it at once from all Inverte- brates. It afterwards has, for a time, structures found in the lower classes and orders of Vertebrates as permanent DEVELOPMENT. 207 organs. For a time, indeed, the human embryo so closely resembles that of the lower forms as to be indistinguisha- ble from them ; but certain structures belonging to those forms are kept long after the embryo is clearly human.116 All the members of a group do not reach the same degree of perfection, some remaining in what corresponds to the immature stages of the higher animals. Such may be called permanently embryonic forms. Sometimes an embryo develops an organ in a rudimen- tary condition, which is lost or useless in the adult. Thus, the Greenland Whale, when grown up, has not a tooth in its head, while in the embryo life it has teeth in both jaws; unborn Calves have canines and upper incisors; and the female Dugong has tusks which never cut the gum. The "splint-bones" in the Horse's foot are unfin- ished metatarsals. Animals differ widely in the degree of development reached at ovulation and at birth. The eggs of Frogs are laid when they can hardly be said to have become fully formed as eggs. The eggs of Birds are laid when segmentation is complete, while the eggs of Mammals are retained by the parent till after the egg-stage is passed.1" Ruminants and terrestrial Birds are born with the power of sight and locomotion. Most Carnivores, Rodents, and perching Birds come into the world blind and helpless ; while the human infant is dependent for a much longer time. 1. Metamorphosis. Few animals come forth from the egg in perfect condi- tion. The vast majority pass through a great variety of forms before reaching maturity. These metamorphoses (which are merely periods of growth) are not peculiar to Insects, though more apparent in them. Man himself is developed on the same general principles as the Butterfly, but the transformations are concealed from view. The 208 COMPARATIVE ZOOLOGY. Coral, when hatched, has six pairs of partitions; after- wards, the spaces are divided by six more pairs; then twelve intermediate pairs are introduced; next, twenty- four, and so on. The embryonic Star -fish has a long body, with six arms on a side, in one end of which the young Star -fish is developed. Soon the twelve -armed body is absorbed, and the young animal is of age. Worms are continually growing by the addition of new segments. Nearly all Insects undergo complete metamor- phosis, i. e., exhibit four distinct stages of existence — egg, larva, pupa, and imago. The worm-like larva117 may be called a locomotive-egg. It has little resemblance to the parent in structure or habits, eating and growing rapidly. Then it enters the pupa state, wrapping itself in a cocoon, or case, and remaining apparently dead till new organs are developed; when it escapes a perfect winged Insect, Fio. 172.— Butterfly In the Imago, Pupa, and Larva States. or imago.118 Wings never exist externally in the larva ; and some Insects which undergo no apparent metamor- phosis, as Lice, are wingless. The Grasshopper develops from the young larva to the winged adult without chang- DEVELOPMENT. 209 ing its mode of life. In the development of the common Crab, so different is the outward form of the newly FIG. 173. — Metamorphosis of the Mosqnito (Culex pipiens) : A, boat of eggs ; B, some of the eggs highly magnified ; d, with lid open for the escape of the larva, C; D, pupa; E, larva magnified, showing respiratory tube, e, anal flue,/, autenuse, g; F, imago; a, antennae; b, beak. hatched embryo from that of the adult, that the former has been described as a distinct species. The most remarkable example of metamorphosis among Vertebrates is furnished by the Amphibians. A Tadpole — the larva of the Frog — has a tail, but no legs ; gills, in- stead of lungs; a heart precisely like that of the Fish; a horny beak for eating vegetable food, and a spiral intes- tine to digest it. As it matures, the hinder legs show themselves, then the front pair ; the beak falls off ; the tail and gills waste away; lungs are created; the diges- tive apparatus is changed to suit an animal diet ; the heart is altered to the Reptilian type by the addition of another auricle ; in fact, skin, muscles, nerves, bones, and blood- vessels vanish, being absorbed atom by atom, and a new set is substituted. Moulting, or the periodical renewal of epidermal parts, as the shell of the Lobster, the skin of U 210 COMPARATIVE ZOOLOGY. the Toad, the scales of Snakes, the feathers of Birds, and the hair of Mammals, may 'be termed a metamorphosis. FIG. 174.— Metamorphosis of the Newt. The change from milk-teeth to a permanent set is another example. An animal rises in organization as development ad- vances. Thus, a Caterpillar's life has nothing nobler about it than the ability to eat, while the Butterfly ex- pends the power garnered np by the larva in a gay and busy life. But there are seeming reversals of this law. Some mature animals appear lower in the scale than their young. The larval Cirripede has a pair of magnificent compound eyes and complex antennae; when adult, the antennge are gone, and the eyes are reduced to a single, simple, minute eye-spot. So the germs of the sedentary Sponge and Oyster are free and active. The adult ani- mal, however, is always superior in alone possessing the power of reproduction. Such a process is known as retro- grade metamorphosis. There are certain larval forms so characteristic of the DEVELOPMENT. 211 great groups of the animal kingdom as to demand notice. Most Worms leave the egg as a larva, called the trocho- sphere (Fig. 175), an oval larva, having mouth and anus, arid a circle of cilia anterior to the mouth. This larval stage is common to Worms with the most diverse adult forms and habits. It is also found in all the great groups of Mol- lusks. Clams, Snails, and Cuttle-fish all have the stage represented in their history. The Mol- lusks usually pass through a later stage called the veliger (Fig. 176), in which a circle of cilia ho- mologous to that of the trochosphere is borne by a lobed expansion on the head, called the ve- FIG. U6 — Larval Gasteropoda: A, B, Trochus; C, Ter- lum, Or Sail. TllC gipes; a, trochosphere: v, velnm ; B, veliger; d, ~ ' . , mouth ; /, foot; «, shell; C, veliger; d, foot; c, teuta- OrUStacea, WlllCn exhibit so great a range of form in the adult state, all pass through a stage iu which they are substantially alike. Forms as different in appearance as Barnacles, Entomostracans, and Prawns hatch out as Nauplii, little oval animals, with a straight intestine, three pairs of legs, and a simple eye (Fig. 177). See Figs. 253, 254, 255, 256. Fig. 256 represents the Lobster, which does not hatch as a Nauplius, but is not very unlike the Prawn. These larval forms are of great interest, because they disclose the relationships of the adult forms, as the gastrula stage hints at the common relationships of all animals above Protozoa. 2. Alternate Generation. Sometimes a metamorphosis extending over several generations is required to evolve the perfect animal; "in 212 COMPARATIVE ZOOLOGY. Fio. 177 — Nanplins of Entomoetracan (Canthocnmptua). See Fig. 255. A, first an- tenna ; An, second antenna ; a, anus ; L, labrum ; O, ocellus ; S, stomach. (From Brooks, after Hoek.) other words, the parent may find no resemblance to him- self in any of his progeny, until he comes down to the great-grandson." Thus, the Jelly-fish, or Medusa, lays eggs which are hatched into larvae resembling Infusoria — little transparent oval bodies covered with cilia, by which they swim about for a time till they find a resting-place. One of them, for example, becoming fixed, develops rap- idly ; it elongates and spreads at the upper end ; a mouth is formed, opening into a digestive cavity; and tentacles multiply till the mouth is surrounded by them. At this stage it resembles a Hydra. Then slight wrinkles appear along the body, which grow deeper and deeper, till the animal looks like "a pine-cone surmounted by a tuft of tentacles ;" and then like a pile of saucers (about a dozen DEVELOPMENT. 213 in number) with scalloped edges. Next, the pile breaks up into separate segments, which are, in fact, so many dis- tinct animals ; and each turning over as it is set free, so as to bring the mouth below, develops into an adult Me- dusa, becoming more and more convex, and furnished with tentacles, circular canals, and other organs exactly like those of the progenitor that laid the original egg. Here we see a Medusa producing eggs which develop into stationary forms resembling Hydras. The Hydras FIG. ITS — Alternate Generation: a, b, c, ova of an Acaleph (Chrysaora) ; rf, «,/, Hy- dras; ff, h, Hydras with constrictions; f, Hydra undergoing fission.; k, oue of the separated segments, a free Medusa. then produce not only Medusae by budding in the manner described, but also other Hydras like themselves by bud- ding. All these intermediate forms are transient states of the Jelly-fish, but the metamorphoses cannot be said to occur in the same individual. While a Caterpillar becomes a Butterfly, this Hydra-like individual produces a number of Medusae. Alternate generation is, then, an alternation of asexual and sexual methods of reproduction, one or more generations produced from buds being followed by a single generation produced from eggs. Often, as in the fresh-water Hydra, the two kinds of generations are alike in appearance. The process is as wide-spread as asexual reproduction, being found mostly in Sponges, Coalenterates, and Worms. It is also found in certain 214: COMPARATIVE ZOOLOGY. Crustacea and Insects. The name is sometimes limited to cases where the two kinds of generations differ in form. 3. Growth and Repair. Growth is increase of bulk, as Development is increase of structure. It occurs whenever the process of repair exceeds that of waste, or when new material is added faster than the tissues are destroyed. There is a specific limit of growth for all animals, although many of the low cold-blooded forms, as the Trout and Anaconda, seem to grow as long as they live. After the body has attained its maturity, i. e., has fully developed, the tissues cease to grow; and nutrition is concerned solely in supplying the constant waste, in order to preserve the size and shape of the organs. A child eats to grow and repair ; the adult eats only to repair.119 Birds develop rapidly, and so spend most of their life full-fledged; while Insects generally, Fishes, Amphibians, Reptiles, and Mammals mature at a comparatively greater age. The perfect Insect rarely changes its size, and takes but little food; eating and growing are almost confined to larval life. The crust of the Sea-urchin, which is never shed, grows by the addition of matter to the margins of the plates. The shell of the Oyster is enlarged by the deposition of new laminae, each extending beyond the other. At every enlargement, the interior is lined with a new nacreous layer; so that the number of such layers in the oldest part of the shell indi- cates the number of enlargements. When the shell has reached its full size, new layers are added to the inner surface only, which increases the thickness. It is the margin of the mantle which provides for the increase in length and breadth, while the thickness is derived from the whole surface. The edges of the concentric laminae are the " lines of growth." The Oyster is full-grown in about five years. The bones of Fishes and Eeptiles are DEVELOPMENT. 215 continually growing; the long bones of higher animals increase in length so long as the ends (epiphyses) are sep- arate from the shaft. The limbs of Man, after birth, grow more rapidly than the trunk. The power of regenerating lost parts is greatest where the organization is lowest, and while the animal is in the young or larval state. It is really a process of budding. The upper part of the Hydra, if separated, will reproduce the rest of the body; if the lower part is cat off, it will add the rest. Certain Worms may be cut into several pieces, and each part will regain what is needed to com- plete the mangled organism. The Star-fish can reproduce its arms; the Holothurian, its stomach ; the Snail, its ten- tacles; the Lobster, its claws; the Spider, its legs; the Fish, its fins; and the Lizard, its tail. Nature makes no mistake by putting on a leg where a tail belongs, or join- ing an immature limb to an adult animal.180 In Birds and Mammals, the power is limited to the reproduction of cer- tain tissues, as shown in the healing of wounds. Very rarely an entire human bone, removed by disease or sur- gery, has been restored. The nails and hair continue to grow in extreme old age. 4. Likeness and Variation. It is a great law of reproduction that all animals tend to resemble their parents. A member of one class never produces a member of another class. The likeness is very accurate as to general structure and form. But it does not descend to every individual feature and trait. In other words, the tendency to repetition is qualified by a tendency to variation. Like produces like, but not ex- actly. The similarity never amounts to identity. So that we have two opposing tendencies — the hereditary ten- dency to copy the original stock, and a distinct tendency to deviate from it. 216 COMPARATIVE ZOOLOGY. This is one of the most universal facts in nature. Ev- ery development ends in diversity. All know that no two individuals of a family, human or brute, are abso- lutely alike. There are always individual differences by which they can be distinguished. Evidently a parent does not project precisely the same line of influences upon each of its offspring. This variability makes possible an indefinite modifica- tion of the forms of life. For the variation extends to the whole being, even to every organ and mental char- acteristic as well as to form and color. It is very slight from generation to generation; but it can be accumulated Dy choosing from a large number of individuals those which possess any given variation in a marked degree, and breeding from these. Nature does this by the very gradual process of " natural selection ;" Man hastens it, so to speak, by selecting extreme varieties. Hence we have in our day remarkable specimens of Poultry, Cattle, and Dogs, differing widely from the wild races. Sometimes we notice that children resemble, not their parents, but their grandparents or remoter ancestors. This tendency to revert to an ancestral type is called atavism. Occasionally, stripes appear on the legs and shoulders of the Horse, in imitation of the aboriginal Horse, which was striped like the Zebra. Sheep have a tendency to revert to dark colors. The laws governing inheritance are unknown. No one can say why one peculiarity is transmitted from father to son, and not another; or why it appears in one member of the family, and not in all. Among the many causes which tend to modify animals after birth are the quality and quantity of food, amount of temperature and light, pressure of the atmosphere, nature of the soil or water, habits of fellow-animals, etc. Occasionally animals occur, widely different in struct- • DEVELOPMENT. 217 ure, having a very close external resemblance. Barnacles were long mistaken for Mollusks, Polyzoans for Polyps, and Lamprey-eels for Worms. Such forms are termed homomorphic. Members of one group often put on the outward ap- pearance of allied species in the same locality : this is called mimicry. " They appear like actors or masquerad- ers dressed up and painted for amusement, or like swin- dlers endeavoring to pass themselves off for well-known and respectable members of society." Thus, certain But- terflies on the Amazons have such a strong odor that the Birds let them alone; and Butterflies of another family in the same region have assumed for protection the same form and color of wing. So we have bee -like Moths, beetle-like Crickets, wasp-like Flies, and ant-like Spiders ; harmless and venomous Snakes copying each other, and Orioles departing from their usual gay coloring to imi- tate the plumage, flight, and voice of quite another style of Birds. The species which are imitated are much more abundant than those which mimic them. There is also a general harmony between the colors of an animal and those of its habitation. We have the white Polar Bear, the sand-colored Camel, and the dusky Twilight- moths. There are Birds and Reptiles so tinted and mottled as ex- actly to match the rock, or ground, or bark of a tree they frequent; and there are Insects rightly named "Walking- sticks" and "Walking -leaves." These coincidences are not always accidental, but often intentional on the part of nature, for the benefit of the imitating species. Gener- ally, they wear the livery of those they live on, or ape the forms more favored than themselves. 5. Homology, Analogy, and Correlation. The tendency to repetition in the development of ani- mals leads to some remarkable affinities. Parts or organs, 218 COMPARATIVE ZOOLOGY. having a like origin and development, and therefore the same essential structure, whatever their form or function, are said to be homologous ; while parts or organs corre- sponding in use are called analogous. By serial homol- ogy is meant the hoinology existing between successive parts of one animal. The following are examples of homology: the arms of Man, the fore- legs of a Horse, the paddles of a Whale, the wings of a Bird, the front flippers of a Turtle, and the pectoral fins of a Fish; the proboscis of a Moth, and the jaws of a Beetle; the shell of a Snail, and both valves of a Clam. The wings of the Bird, Flying Squirrel, and Bat are hardly homologous, since the wing of the first is de- veloped from the fore-limb only; that of the Squirrel is an extension of the skin between the fore and hind limbs ; while in the Bat the skin stretches between the fingers, and then down the side to the tail. Examples of serial homology: the arms and legs of Man; the upper and lower set of teeth ; the parts of the vertebral column, however modified; the scapular and pelvic arches; the humerus and femur; carpus and tarsus; the right and left sides of most Animals; the dorsal and anal fins of Fishes. The legs of a Lobster and Lizard, the wings of a Butter- fly and Bird, the gills of a Fish, and the lungs of other Vertebrates, are analogous. The air-bladder of a Fish is homologous with a lung, and analogous to the air-cham- bers of the Nautilus. In the midst of the great variety of form and structure in the animal world, a certain harmony reigns. Not only are different species so related as to suggest a descent from the same ancestor, but the parts of any one organ- ism are so closely connected and mutually dependent that the character of one must receive its stamp from the char- acter of all the rest. Thus, from a single tooth it may be inferred that the animal had a skeleton and spinal cord. DEVELOPMENT. 219 and that it was a carnivorous, hot-blooded Mammal. Cer- tain structures always co-exist. Animals with two occipi- tal condyles, and non - nucleated blood -corpuscles, suckle FIG. 181. FIG. 182. HOMOLOGIES OF LIMBS. FIG. 179.— Arm and Leg of Man, ns they are when he gets down on all-fours. Fio. ISO.— Fore and Hind Legs of Tapir. FIG. 181.— Fore Leg of Seal and Hind Leg of Alligator. FIG. 182.— Wing of the Bat. S, scapula ; I, ilium, or shin-bone of pelvis; H, hnmerus ; F, femur; O, olecranon, or tip of the elbow; P, patella; U, ulna ; T, tibia ; K, radius ; Fi, Fibula ; Po, pollex, or thumb ; Ha, hallex, or great toe. Compare the fore and hind limbs of the same animal, and the fore or hind limbs of different animals. Note the directions of the homologous Beg- 220 COMPARATIVE ZOOLOGY. their young, i. «?., they are Mammals. All Ruminant hoofed beasts have horns and cloven -feet. If the hoofs are even, the horns are even, as in the Ox ; if odd, as in the Rhinoceros, the horns are odd, i. e., single, or two placed one behind the other. Recent creatures with feath- ers always have beaks. Pigeons with short beaks have small feet; and those with long beaks, large feet. The long limbs of the Hound are associated with a long head. A white spot in the forehead of a Horse generally goes with white feet. Hairless Dogs are deficient in teeth. Long wings usually accompany long tail-feathers. White Cats with blue eyes are usually deaf. A Sheep with nu- merous horns is likely to have long, coarse wool. Homol- ogous parts tend to vary in the same manner ; if one is diseased, another is more likely to sympathize with it than one not homologous. This association of parts is called correlation of growth. 6. Individuality. It seems at first sight very easy to define an individual animal. A single Fish, or Cow, or Snail, or Lobster is plainly an individual; and the half of one such animal is plainly not one. But when we consider animals in colo- nies, like Corals, it is not so easy to say whether the indi- vidual is the colony or the single Polyp. Is the tree the individual, or the bud H If we say the former — the colony — what shall we say to the free buds of a Hydroid colony, living independent lives, and scattered over square miles of ocean? Are they parts of one individual? If we choose the latter as our standard, we are in equal difficul- ty; for we must then call an individual the bud of the Portuguese Man-of-war, reduced to a mere bladder or feeler, and incapable of leading an independent life. We thus find it necessary to distinguish at least two kinds of individuals — physiological individuals, applying that DEVELOPMENT. 221 name to any animal form capable of leading an indepen- dent life ; and morphological individuals, one of which is the total product of an egg. Such an individual may be a single physiological individual, as the Fish ; or many united, as the Coral stock; or many separate physiological individuals, as in the Hydroids or Plant-lice. The single members of such a compound morphological individual are called zooids, or personal, and are found wherever asexual reproduction takes place. 7. delations of Number, Size, form, and Hank. The Animal Kingdom has been likened to a pyramid, the species diminishing in number as they ascend in the scale of complexity. This is not strictly true. The num- ber of living species known is at least 300,000, of which more than nine tenths are Invertebrates. A late enumer- ation gives the following figures for the number of de- scribed species : Protozoa 2,700 Coelenterata 1,560 Vermes 5, 580 Arthropoda Echinodermata 800 Mollusca 20,210 Vertebrata 25,200 These figures are lower than those usually given. Of Vertebrates, Fishes are most abundant; then follow Birds, Mammals, Reptiles, and Amphibians. There are usually said to be about 200,000 species of Insects. The largest species usually belong to the higher classes. The aquatic members of a group are generally larger than the terrestrial, the marine than the fresh- water, and the land than the aerial. The extremes of size are an Infu- sorium, -rg-QTHj- °f an incn m diameter, the smallest animal ever measured, and the Whale, one hundred feet long, the largest animal ever created. The female is sometimes larger than the male, as of the Nautilus, Spider, and Eagle. The higher the class, the more uniform the size. Of all 222 COMPARATIVE ZOOLOGY. groups of animals, Insects and Birds are the most con- stant in their dimensions. Every organism has its own special law of growth: a Fish and an Oyster, though born in the same locality, de- velop into very different forms. Yet a symmetry of plan underlies tho structure of all animals. In the embryo, this symmetry of the two ends, as well as the two sides, is nearly perfect ; but it is subsequently interfered with to adapt the animal to its special conditions of life. It is a law that an animal grows equally in those directions in which the incident forces are equal. The Polyp, rooted to the rocks, is subjected to like conditions on all sides, and, therefore, it has no right arid left, or fore and hind parts. The lower forms, generally, are more or less geo- metrical, figures: spheroidal, as the Sea-urchin; radiate, as the Star -fish; and spiral, as many Foraminifers. The higher animals are subjected to a greater variety of con- ditions. Thus, a Fish, always going through the water head foremost, must show considerable difference between the head and the hinder end; or a Turtle, moving over the ground with the same surface always down, must have distinct dorsal and ventral sides. Nevertheless, there is a striking likeness between the two halves or any two organs situated on opposite sides of an axis. And, first, a bilateral symmetry is most com- mon. It is best exhibited by the Articulates and Verte- brates, but nearly all animals can be clearly divided into right and left sides — in other words, they appear to be double. A vertical plane would divide into two equal parts our brain, spinal cord, vertebral column, organs of sight, hearing, and smell; our teeth, jaws, limbs, lungs, etc. In fact, the two halves of every egg are identical. There are many exceptions: the heart and liver of the higher Vertebrates are eccentric; the nervous system of Mollusks is scattered; the hemispheres of the human DEVELOPMENT. 223 brain are sometimes unequal; the corresponding bones in the right and left arms are not precisely the same length and weight; the Narwhal has an immense tusk on the left side, with none to speak of on the other; Rabbits have been born with one ear, and Stags with one horn ; the Rattlesnake has but one lung; both eyes of the Flounder and Halibut are on the same side; the claws of the Lob- ster differ; and the valves of the Oyster are unequal. But all these animals and their organs are perfectly sym- metrical in the embryo state. Again, animals exhibit a certain correspondence be- tween the fore and hind parts.121 Thus, the two ends of the Centipede repeat each other. Indeed, in some Worms, the eyes are developed in the last segment as well as the first. So a Vertebrate may be considered not only as two individuals placed side by side, but also as two individu- als put end to end — the head and arms representing one, and the legs the other. In the embryo of Quadrupeds, the four limbs are closely alike. But in the adult, the fore and hind limbs differ more than the right and left limbs, because the functions are more dissimilar. An ex- treme want of symmetry is seen in Birds which combine aerial and land locomotion. There is also a tendency to a vertical symmetry, or up-and-down arrangement — the part above a horizontal plane being a reversed copy of the part below. A good example is the posterior half of a Cod. while the tail of a Shark shows the want of it. This symmetry decreases as we ascend the scale. In most animals there is consider- able difference between the dorsal and ventral surfaces ; and in all the nervous system is more symmetrically dis- posed than the digestive. Every animal is perfect in its kind and in its place. Yet we recognize a gradation of life. Some animals are manifestly superior to some others. But it is not so easy 224: COMPARATIVE ZOOLOGY. to say precisely what shall guide us in assorting living forms into high and low. Shall we make structure the criterion of rank? Plainly the simple Jelly-fish is be- neath complicated Man. An ounce of muscle is worth a pound of protoplasm, and a grain of nervous matter is of more account than a ton of flesh. The intricate and fin- ished build of the Horse elevates him immeasurably above the stupid Snail. The repetition of similar parts, as in the Worm, is a sign of low life. So also a prolonged posterior is a mark of inferiority, as the Lobsters are lower than the Crabs, Snakes than Lizards, Monkeys than Apes. The possession of a head distinct from the region behind it is a sign of power. And in proportion as the fore- limbs are used for head purposes, the animal ascends the scale : compare the Whale, Horse, Cat, Monkey, and Man. But shall the Fish, never rising above the " monotony of its daily swim," be allowed to outrank the skilful Bee? Shall the brainless, sightless, almost heartless Amphioxus, a Vertebrate, be allowed to stand nearer to Man than the Ant? What is the possession of a backbone to intelli- gence? No good reason can be given why we might not be just as intelligent beings if we carried, like the Insect, our hearts in our backs and onr spinal cords in our breasts. So far as its activity is concerned, the brain may be as ef- fective if spread out like a map as packed into its present shape. Even animals of the same type, as Vertebrates, cannot be ranked according to complexity. For while Mammals, on the whole, are superior to Birds, Birds to Reptiles, and Reptiles to Fishes, they are not so in every respect. Man himself is not altogether at the head of creation. We carry about in our bodies embryonic struct- ures. That structural affinity and vital dignity are not always parallel may be seen by comparing an Australian and an Englishman. m DEVELOPMENT. 225 Function is the test of worth. Not mere work, how- ever; for we must consider its quality and scope. An animal may be said to be more perfect in proportion as its relations to the external world are more varied, pre- cise, and fitting. Complexity of organization, variety, and amount of power are secondary to the degree in which the whole organism is adapted to the circumstances which surround it, and to the work which it has to do. Ascent in the animal scale is not a passage from animals with simple organs to animals with complex organs, but from simple individuals with organs of complex function to complex individuals with organs of simple function : the addition as we ascend being not function, but of parts to discharge those functions ; and the advantage gained, not another thing done, but the same thing done better. Advance in rank is exhibited, not by the possession of more life (for some animalcules are ten times more lively than the busiest Man), but by the setting apart of more organs for special purposes. The higher the animal, the greater the number of parts combining to perform each function. The power is increased by this division of la- bor. The most important feature in this specialization is the tendency to concentrate the nervous energy towards the head (cephalizatiori). It increases as we pass from lower to higher animals. As a rule, fixed species are inferior to the free, water species to land species, fresh-water animals to marine, arc- tic forms to tropical, and the herbivorous to the carniv- orous. Precocity is a sign of inferiority: compare the chicks of the Hen and the Robin, a Colt with a Kitten, the comparatively well - developed Caterpillar with the footless grub of the Bee. Among Invertebrates, the male is frequently inferior, not only in size, but also in grade of organization. Animals having a wide range as to cli- 15 226 COMPARATIVE ZOOLOGY. mate, altitude, or depth are commonly inferior to those more restricted: Man is a notable exception. There is some relation between the duration of life and the size, structure, and rank of animals. Vertebrates not only grow to a greater size, but also live longer than In- vertebrates. Whales and Elephants are the longest-lived; and Falcons, Ravens, Parrots and Geese, Alligators and Turtles, and Sharks and Pikes, are said to live a century. The life of Quadrupeds generally reaches its limit when the molar teeth are worn down: those of the Sheep last about 15 years; of the Ox, 20; of the Horse, 40; of the Elephant, 100. Many inferior species die as soon as they have laid their eggs, just as herbs perish as soon as they have flowered. 8. The Struggle for Life. Every species of animal is striving to increase in a geo- metrical ratio. But each lives, if at all, by a struggle at some period of its life. The meekest creatures must fight, or die. " There is no exception to the rule that every organic being naturally increases at so high a rate that, if not de- stroyed, the earth would soon be covered by the progeny of a single pair." If the increase of the human race were not checked, there would not be standing-room for the descendants of Adam and Eve. A pair of Elephants, the slowest breeder of all known animals, would become the progenitors, in seven and one half centuries, of 19,000,000 of Elephants, if death did not interfere. Evidently a vast number of young animals must perish while immature, and a far greater host of eggs fail to mature. A single Cod, laying millions of eggs, if allowed to have its own way, would soon pack the ocean. Yet, BO nicely balanced are the forces of nature, the average number of each kind remains about the same. DEVELOPMENT. 227 The total extinction of any one species is exceedingly rare. The number of any given species is not determined by the number of eggs produced, but by its conditions.1" Aquatic birds outnumber the land birds, because their food never fails, not because they are more prolific. The Fulmar-petrel lays but one egg, yet it is believed to be the most numerous bird in the world. The main checks to the high rate of increase are: cli- mate (temperature and moisture), acting directly or indi- rectly by reducing food; and other animals, either rivals requiring the same food and locality, or enemies, for the vast majority of animals are carnivorous. Offspring are continually varying from their parents, for better or worse. If feebly adapted to the conditions of existence, they will finally go to the wall. But those forms having the slight- est advantage over others inhabiting the same region, being hardier or stronger, more agile or sagacious, will survive. Should this advantageous variation become hereditary and intensified, the new variety will gradually extirpate or replace other kinds. This is what Mr. Dar- win, means by Natural Selection, and Herbert Spencer by the Survival of the Fittest. II. SYSTEMATIC ZOOLOGY. Facts are stupid things until brought into connection with some general law. — AGASSIZ. No man becomes a proficient in any science who does not transcend sys- tem, and gather up new truth for himself in the boundless field of research. —DR. A. P. PEABODY. Never ask a question if you can help it ; and never let a thing go un- known for the lack of asking a question if you can't help it. — BBECHER. He is a thoroughly good naturalist who knows his own parish thoroughly. — CHARLES KINGSLEY. THE CLASSIFICATION OF ANIMALS. 231 CHAPTER XXI. THE CLASSIFICATION OF ANIMALS. THE Kingdom of Nature is a literal Kingdom. Order and beauty, law and dependence, are seen everywhere. Amidst the great diversity of the forms of life, there is unity; and this suggests that there is one general plan, but carried out in a variety of ways. Naturalists have ceased to believe that each animal or group is a distinct, circumscribed idea. "Every animal has a something in common with all its fellows: much with many of them; more with a few; and, usually, so much with several, that it differs but little from them." The object of classification is to bring together the like, and to separate the unlike. But how shall this be done ? To arrange a library in alphabetical order, or according to size, binding, date, or language, would be unsatisfactory. We must be guided by some internal character. We must decide whether a book is poetry or prose; if poetry, whether dramatic, epic, lyric, or satiric ; if prose, whether history, philosophy, theology, philology, science, fiction, or essay. The more we subdivide these groups, the more difficult the analysis. A classification of animals, founded on external resem- blances— as size, color, or adaptation to similar habits of life — would be worthless. It would bring together Fish- es and Whales, Birds and Bats, Worms and Eels. Nor should it be based on any one character, as the quality of the blood, structure of the heart, development of the brain, embryo-life, etc. ; for no character is of equal value in every tribe. A natural classification must rest on those 232 COMPARATIVE ZOOLOGY. prevailing characters which are the most constant.™ And such a classification cannot be linear. It is impossible to arrange all animal forms from the Sponge to Man in a single line, like the steps of a ladder, according to rank. Nature passes in so many ways from one type to another, and so multiplied are the relations between animals, that one series is out of the question. There is a number of series, and series within series, sometimes proceeding in parallel lines, but more often divergent. The animals ar- range themselves in radiating groups, each group being connected, not with two groups merely, one above and the other below, but with several. Life has been likened to a great tree with countless branches spreading widely from a common trunk, and deriving their origin from a com- mon root ; branches bearing all manner of flowers, every fashion of leaves, and all kinds of fruit, and these for every use. The groups into which we are able to cast the various forms of animal development are very unequal and dis- similar. We must remember that a genus, order, or class is not of equal value throughout the kingdom. Moreover, each division is allied to others in different degrees — the distance between any two being the measure of that affin- ity. The lines between some are sharp and clear, between others indefinite. Like the islands of an archipelago, some groups merge into one another through connecting reefs, others are sharply separated by unfathomable seas, yet all have one common basis. Links have been found reveal- ing a relationship, near or distant, even between animals whose forms are very unlike. There are Fishes (Dipnoi) with some Amphibian characters, and fish-like Amphibi- ans (AxolotT). The Ichthyosaurus is a Lizard with fish- characteristics. Birds seem isolated, but they are closely connected with Reptiles by fossil forms. Even the great gap in the Animal Kingdom — that separating Vertebrates THE CLASSIFICATION OF ANIMALS. 233 and Invertebrates — is partially bridged on the one side by Amphioxus, and on the other by the Tunicates. We have, then, groups subordinate to groups, and inter- locking, but not representing so many successive degrees of organization. For, as already intimated, complication of structure does not rise in continuous gradation from one group to another. Every type starts at a lower point than that at which the preceding class closes ; so that the lines overlap. While one class, as a whole, is higher than another, some members of the higher class may be infe- rior to some members of the lower one. Thus, certain Star-fishes are nobler than certain Mollusks; the Nautilus is above the Worm, and the Bee is more worthy than the lowest Fish. The groups coalesce by their inferior or less specialized members ; e. g., the Fishes do not graduate into Amphibians through their highest forms, but the two come closest together low down in the scale. Man appears to be the goal of creation ; but even within the Vertebrate series, every step of development, say of the Fish, is away from the goal. The highest Fish is the one farthest from Man. A number of animals may, therefore, have the same grade of development, but conform to entirely different types. While a fundamental unity underlies the whole Animal Kingdom, suggesting a common starting-point, we recognize several distinct plans of structure.186 Animals like the Amoaba, with no cellular tissues nor true eggs, form the subkingdom Protozoa. Animals like the Sponge, with independent cells, one excurrent and many incurrent openings, form the subkingdom Spongida. Animals like the Coral, unlike all others, have an alimentary canal but no body -cavity, have no separate nervous and vascular regions, and the parts of the body radiate from a centre. Such form a subkingdom called Ccdenterata. Animals like the Star-fish, having also a radiating body, but a closed 234 COMPARATIVE ZOOLOGY. alimentary canal, and a distinct symmetrical nervous sys- tem, constitute the subkingdom Echinodermata.™ Ani- mals like the Angle-worm, bilaterally symmetrical, one- jointed, or composed of joints following each other from front to rear, with no jointed limbs, constitute the sub- kingdom Vermes. Animals like the Snail, with a soft, unjointed body, a mantle, a foot, a two or three cham- bered heart, and a nervous system in the form of a ring around the gullet, constitute the subkingdom Mollusca. Animals like the Bee, with a jointed body and jointed limbs, form the subkingdom Arthropoda. Animals like the Sea-squirts, sack or barrel shaped, with a mantle cav- ity penetrated by an excurrent and an incurrent opening, with heart and gills, form the subkingdom Tunicata. An- imals like the Ox, having a double nervous system, one (the sympathetic) lying on the upper side of the aliment- ary canal, the other and main part (spinal) lying along the back, and completely shut off from the other organs by a partition of bone or gristle, known as the " vertebral col- umn," and having limbs, never more than four, always on the side opposite the great nervous cord, constitute the subkingdom Vertebrata. Comparing these great divisions, we see that the Verte- brates differ from all the others chiefly in having a double body-cavity and a double nervous system, the latter lying above the alimentary canal ; while Invertebrates have one cavity and one nervous system, the latter being placed either below or around the alimentary canal. The Vermes are closely related to all the following subkingdoms of Invertebrates, most nearly to Mollusks and Tunicates, while the latter have affinities with the Vertebrates. The Echinoderms and Coelenterates are built on the common type of a star; but they differ from each other in the presence or absence of distinct alimentary, circulatory, and nervous systems. THE CLASSIFICATION OF ANIMALS. 235 But there are types within types. Thus, there are five modifications of the Yertebrate type — Fish, Amphibian, Reptile, Bird, and Mammal; and these are again divided and subdivided, for Mammals, e. g., differ among them- selves. So that in the end we have a constellation of groups within groups, founded on peculiar characters of less and less importance, as we descend from the general to the special. Individuals are the units of the Animal Creation. An animal existence, complete in all its parts, is an individual, whether separate, as Man, or living in a community, as the Coral.1" . Species is the smallest group of individuals which can be defined by distinct characteristics, and which is sepa- rated by a gap from all other like groups. A well-marked subdivision of a species is called a variety. Crosses be- tween species are called hybrids, as the Mule. G-enus is a group of species having the same essential structure. Thus, the closely allied species Cat, Tiger, and Lion belong to one genus. Family, or Tribe, is a group of genera having a simi- lar form. Thus, the Dogs and Foxes belong to different genera, but betray a family likeness. Order is a group of families, or genera, related to one another by a common structure. Cats, Dogs, Hyenas, and Bears are linked together by important anatomical features; their teeth, stomachs, and claws show carnivorous habits. Class is a still larger group, comprising all animals which agree simply in a special modification of the type to which they belong. Thus, Fishes, Amphibians, Rep- tiles, Birds, and Mammals are so many aspects of the Ver- tebrate type. Subkingdom is a primary division of the Animal King- dom, which includes all animals formed upon one of the various types of structure ; as Yertebrate. 236 COMPARATIVE ZOOLOGY. The subkingdoms are grouped into two great Series, according to their histological structure and mode of de- velopment."8 These terms were invented by Linnaeus, except Family, Subkingdom, and Series. To Linnaeus we are also in- debted for a scientific method of naming animals. Thus, a Dog, in Zoology, is called Canis familiaris, which is the union of a generic and a specific name, corresponding to the surname and the Christian name in George Washing- ton, only the specific name comes last. It will be under- stood that these are abstract terms, expressing simply the relations of resemblance : there is no such thing as genus or species. Classification is a process of comparison. He is the best naturalist who most readily and correctly recognizes likeness founded on structural characters. As it is easier to detect differences than resemblances, it is much easier to distinguish the class to which an animal belongs than the genus, and the genus than the species. In passing from species to classes, the characters of agreement be- come fewer and fewer, while the distinctions are more and more manifest; so that animals of the same class are more like than unlike, while members of distinct classes are more unlike than like. To illustrate the method of zoological analysis by search- ing for affinities and differences, we will take an example suggested by Professor Agassiz. Suppose we see together a Dog, a Cat, a Bear, a Horse, a Cow, and a Deer. The first feature which strikes us as common to any two of them is the horn in the Cow and the Deer. But how shall we associate either of the others with these? We examine the teeth, and find those of the Dog, the Cat, and the Bear sharp and cutting ; while those of the Cow, the Deer, and the Horse have flat surfaces, adapted to grind- ing and chewing, rather than to cutting and tearing. We THE CLASSIFICATION OF ANIMALS. 237 compare these features of their structure with the habits of these animals, and find that the first are carnivorous — that they seize and tear their prey ; while the others are herbivorous, or grazing, animals, living only on vegetable substances, which they chew and grind. We compare, further, the Horse and Cow, and find that the Horse has front teeth both in the upper and the lower jaw, while the Cow has them only in the lower; and going still further, and comparing the internal with the external features, we find this arrangement of the teeth in direct relation to the different structure of the stomach in the two animals — the Cow having a stomach with four pouch- es, while the Horse has a simple stomach. Comparing the Cow and Deer, we find the digestive apparatus the same in both; but though both have horns, those of the Cow are hollow, and last through life ; while those of the Deer are solid, and are shed every year. Looking at the feet, we see that the herbivorous animals are hoofed ; the carnivorous, clawed. The Cow and Deer have cloven feet, and are ruminants; the Horse has a single hoof, and does not chew the cud. The Dog and Cat walk on the tips of their fingers and toes (digitigrade) ; the Bear treads on the palms and soles (plantigrade). The claws of the Cat are retractile ; those of the Dog and Bear are fixed. In this way we determine the exact place of each ani- mal. The Dog belongs to the kingdom Animalia, sub- kingdom Vertebrata, class Mammalia, order Carnivora, family Canidce, genus Canis, species Familiaris, variety Hound (it may be), and its individual name, perhaps, is "Rover." The Cat differs in belonging to the family Felidce, genus Felis. species Catus. The Bear belongs to the family Ursidce, genus Ursus, and species Ferox, if the Grizzly is meant. The Horse, Cow, and Deer belong to the order Ungulata ; but the Horse is of the family , genus Equus, species Cdballus ; the Cow is of 238 COMPARATIVE ZOOLOGY. the family JBovidce, genus J3os, species Taurus / the Deer is of the family Cervidce, genus Cervus, species Virgini- anus, if the common Deer is meant. The diagram on the opposite page roughly represents (for the relations of animals cannot be expressed on a plane surface) the relative positions of the subkingdoms and classes according to affinity and rank.* SERIES I.— PROTOZOA. Animals without cellular tissues, and with no true eggs. The body which corresponds to the egg does not develop a blastoderm. Snbkingdom. — PROTOZOA. This division was proposed by Yon Siebold in 1845, to contain that vast cloud of microscopic beings on the verge of the Animal Kingdom which could not be received into the other subkingdoms. It is artificial and provisional. The classes composing it are not founded on a common type, but are distinguished by the absence rather than the presence of positive characters. Many stand parallel to the Protophyta of the Vegetable World, and no definite line can be drawn between them. Protozoans agree in being minute, aquatic, and exceed- ingly simple in structure, their bodies consisting mainly or wholly of the contractile, gelatinous matter called pro- toplasm, or sarcode — the first homogeneous substance which has the power of controlling chemical and phj'sical forces. They have no cellular organs or tissues, yet they take and assimilate food, grow, and multiply, which are * The student should master the distinctions between the great groups, or classes, before proceeding to a minuter classification. "The essential mat- ter, in the first place," says Huxley, "is to be quite clear about the different classes, and to have a distinct knowledge of all the sharply definable modifi- cations of animal structure which are discernible in the Animal Kingdom." THE CLASSIFICATION OF ANIMALS. r a 239 E: 2 Iff p 1 1 - & p 240 COMPARATIVE ZOOLOGY. the essential signs of life. The usual methods of repro- duction are self-division and budding. The subkingdom may be divided into four classes : Mo- nera, Gregarinida, Rhizopoda, and Infusoria. CLASS I. — Monera. These simplest living beings are organless bits of protoplasma, with no distinction of layers, round when at rest, and with pseu- dopodia when active. They are all aquatic, FIG. 183. -Pro- and some are parasitic. Such is Protamceba, tammbapri- -.-,. 1 QQ mitiva. JMg. ISd. CLASS II. — Gregarinida. The Gregarinae, discovered by Dufour in 1828, are among the simplest animal forms of which we have any knowledge. They closely resemble a cell, or microscopic egg; the only organ is a nucleus, suspended in extremely mobile granular matter; and the most conspicuous signs FIG. 184.— Oregarina gigantea, highly magnified: a, nucleus. of life are the contraction and lengthening of the worm- like body. They feed by absorption, and are all parasites, living in the alimentary canal of higher animals; as in the Cockroach, Earth-worm, and Lobster. The name is derived from the fact that they occur in large numbers crowded together. CLASS III.— Rhizopoda. The Rhizopods are characterized by the power of throw- ing out at will delicate processes of their bodies, called pseudopodia, or false feet, for prehension or locomotion. PROTOZOA. They possess no cilia. The representative forms are Amoe- bce, Foraminifera, and Polycystina. An Amoeba is a naked fresh-water Rhizopod ; an in- definite bit of protoplasm, as structureless as a speck of jelly, save that it is made of two distinct layers, and has a nucleus and a contractile cav- ity inside. It thus differs from' the Monera. It has no particular form, as it changes continually. It moves by put- ting forth short, blunt proc- FlG. ^_Aril esses, and eats by wrapping its body around the particle of food. The size ranges from -fa to -j-ginr °f an inch in diameter. Specimens can be obtained by scraping the mucous matter from the stems and leaves in stagnant ponds. A Foraminifer differs from an Amoaba in having an apparently simpler body, the protoplasm being without layers or cavity ; its pseudopodia are long and thread-like, and may unite where they touch each other. It has the property of secreting an envelope, usually of carbonate of IBO ; the 8ame animal to various Bhape8- FIG, 186.— Rhizopods: a, a monothalamou*, or single-chambered, Foraminifer (Lo- gena, striata) ; 6, a polythalamous, or many-chambered, Foraminifer (Polystomella erwpa), with pseudopodia extended ; c, a Radiolarian, one of the Polycystines (Podocyrtis Schomburgkii). * 16 242 COMPARATIVE ZOOLOGY. lime. The shell thus formed is sometimes of extraordi- nary complexity and singular beauty. It is generally per- forated by innumerable minute orifices (foramina) through which the animal protrudes its myriad of glairy, thread- like arms. The majority are compound, resembling cham- bered shells, formed by a process of budding, the new cells being added so as to make a straight series, a spiral, or a flat coil. As a rule, the many -chambered species have calcareous, perforated shells ; and the one-chambered have an imperforated membranous, porcelaneous, or are- naceous envelope. The former are marine. There are few parts of the ocean, where these microscopic shells do not occur, and in astounding numbers. A single ounce of sand from the Antilles was calculated to contain over three millions. The bottom of the ocean, up to about 50° on each side of the Equator, and at depths not greater than 2400 fathoms, is covered with the skeletons of these ani- mals, which are constantly falling upon it (ylobigerina- ooze). Their remains constitute a great proportion of the so-called sand-banks which block up many harbors. Yet they are the descendants of an ancestry still more prolific ; for the Foraminifera are among the most important rock- building animals. The chalk-cliffs of England, the build- ing-stone of Paris, and the blocks in the Pyramids of Egypt are largely composed of extinct Foraminifers. Fo- raminifera are both marine and fresh-water, chiefly marine. A Polycystine differs from a Foraminifer in secreting a siliceous, instead of a calcareous, shell, studded with spines; and the central part of the body is made up of many cells, and surrounded by a strong membrane. They are also more minute, but as widely diffused. They enter largely into the formation of some strata of the earth's crust, and abound especially in the rocks of Barbadoes and at Richmond, Va. The living forms are mostly marine, but some are fresh- water. PROTOZOA. 243 FIG. 1ST.— A Compound Monad (Uvella), X 1000. CLASS IV.— Infusoria. This unassorted group of living particles derived its name from the fact that they were first discovered in veg- etable infusions. Every drop of a, stagnant pool is crowded with them. They are all single and microscopic, yet of various sizes, the difference between the small- est and largest being greater than the difference between a Mouse and an Elephant. Some are fixed (as Vorticdla), but the majority are free, and constantly in motion, propelled by countless cilia, as a galley by its oars. The delicate body consists of two layers of sarcode (there are na cellular tissues, but the whole body represents a single cell), covered by a membrane, or skit*, having one or two contractile cavi- ties> and a nucleus. Food -granules can often be seen-. On one side i& a slight depression, or " mouth," leading to a short, funnel-shaped throat. A mouth and a rudimentary digestive cavity are among the distinctive features of these 1SS. — Infusorium (Parameciumauniia), Protozoans. Sotne have a pigment-speck X 300: m, mouth; t>, ^ . , , , . , con tractile vesicles ;n, — the simplest sense organ — and m the uucleus- stem of Vorticella the first rudiments of muscle may be found. They multiply so rapidly (chiefly by self-division), that a Pararnecium, the most common form, may become the parent of 1,364,000 in forty-two days. There are two main groups: Flagellata, or Monads, provided with one or two flagella, or long, bristle -like cilia; and Ciliata, which are famished with numerous vibratile cilia. 244 COMPARATIVE ZOOLOGY. SERIES H.— METAZOA. The Metazoa include all those animals which reproduce by true eggs and spermatozoa, whose germ develops a hlastoderm, and which have cellular tissues. There are seven subkingdoms. Subkingdom I. — SPONGIDA. The position of the Sponges has been much disputed. At first they were thought to be on the border-line be- tween animals and plants, and were assigned by some to the animals and by others to the vegetables. Later, and up to very recent years, they were assigned to the Proto- zoa. The discovery of their mode of reproduction and development has determined that they belong to the Metazoa. The Sponges are formed of an aggregate of membrane- less amoeboid or ciliated cells. They usually have a skele- ton, which may be calcareous, horny, or siliceous. They have a central cavity, with numerous incurrent orifices and one excurrent opening. They reproduce by true eggs, as well as by budding and fission. The cells of the Sponge are relatively independent, whence they have been regarded as colonies of amreboid animals, and by some naturalists are still so considered. d _ Fio. 1S9.— Hypothetical! Section of a Sponsre: a, ouperflcial layer; 6, inhalant pore* e, ciliated chamber*; d, exhalnut aperture, or osculum ; «, deeper enbstance at the Sponge. SPONGIDA. 245 They develop, however, regularly from the egg, and the cells acquire their independence only at a late date in de- velopment. Some of the cells gain cilia, or flagella, and drive the water through numerous channels into the cen- tral cavity, whence it is discharged by one opening. Each cell of the Sponge feeds itself from the particles con- tained in the water. The Sponge-individual consists of one exhalant orifice, with the channels leading into it. An ordinary bathing- Fia. 190.— Horny Skeleton of a S;>onge. sponge constitutes a colony of such individuals, which are not definitely marked off from each other. Other Sponges have only one osculum, and such are a single individual. Some few Sponges have no skeleton. Most have one of horny fibres, strengthened with siliceous spicules. These last are absent in the commercial Sponges, and in them the horny fibres are much tougher than in most Sponges. 246 COMPARATIVE ZOOLOGY. A few Sponges, as the Venus's Flower -basket (Euplec- telld), have siliceous and others have calcareous skeletons. Excepting a few small fresh -water species (as Spon- gilla), Sponges are marine. In the former, the cellular part is greenish, containing chlorophyll ; in the latter, it is brown, red, or purple. In preparing the Sponge of commerce, this is rotted by exposure, and washed out. The best fishing-grounds are the eastern end of the Medi- terranean and around the Bahama Islands. Subkingdom II. — COSLENTERATA. These radiate animals are distinguished by having a dis- tinct cavity, whose walls have, at least, two layers of cel- lular tissue, an outer (ectoderm) and inner (endoderm), and usually a middle layer (mesoderm). They have thread cells, minute sacs containing a fluid, and' connected with barbed filaments capable of being thrown out for stinging pur- poses. Most are provided with hollow tentacles around the mouth. All are aquatic, and nearly all are marine. There are two classes, represented by the Hydra and Sea- anemone. Both reproduce by budding and by eggs. CLASS I. — Hydrozoa. These Coelenterates have no separate digestive sac, so that the body is a simple tube, or cavity, into which the mouth opens. The nervous system is slightly developed. Such are the fresh-water Hydra and the oceanic Jelly-fish (Acaleph or Medusa). The body of the Hydra is tubular, soft, and sensitive, of a greenish or reddish color, and seldom over half an inch long. It is found spontaneously attached by one end to submerged plants, while the free end contains the orifice, or mouth, crowned with tentacles, by which the creature feeds and creeps. The body-wall consists of two cellular layers — ectoderm and endoderm. These surround C(ELENTERATA. 247 a central cavity with one opening. The animal may be compared to a bag with a two-layered wall,andtentacles around the opening. It buds, and also reproduces by eggs. The buds, when adult, become detached from the parent. In most of the other Hydroids the colony is permanent, and support- ed by a horny skeleton. There are two kinds of \ Polyps in each colony, FlG^_Hydra:7;ith teutacle8 fallye*^d- one for feeding and the ed; 3, creeping; 5, budding. other for reproduction. Sometimes the reproductive Polyps are separated from the stock in the form of little Jelly- fishes. The larger Jelly - fishes belong to another group — the Acalephce — and are produced as told on page 212. The Jelly-fish has a soft, gelatinous, semi-transparent,bell- shaped body, with tubes radiating from the central cavity to the circumference, FIG. 192.— Hydroid (Serfutoria) growing on a Shell, and with the margin 24:8 COMPARATIVE ZOOLOGY. FIG. 193.— Jelly-fish (Pelagia noctilnca). Mediter- ranean. FIG. 194.— Portuguese Man- of-war (Phyyalia), J natu- ral size. Tropical Atlantic. Fi». 195 — Jelly-a1«h (Aitrelia aunta), with young lu various stages. CGELENTERATA. 2i9 radiating and marginal canals. fringed with tentacles, furnished with stinging thread- cells. The radiating parts are in multiples of four. Around the rim are minute colored spots, the " eye - specks." In fine weather, these " sea - blubbers " are seen floating on the sea, mouth'down- ward, moving about by flapping their sides, like the opening and shutting of an umbrella, with great regular- ity. They are frequently phospho- rescent when disturbed. Some are quite small, resembling little glass bells ; the common Aurelia is over a foot in diameter when full-grown ; Flo.196._A Medusa, seen in while the Cyanea, the giant among Jelly-fishes, Sometimes measures eight ... feet in diameter, with tentacles one hundred feet long. When dried, nothing is left but a film of membrane weighing only a few grains. There are two representative types: the Lucernaria, the Umbrella-acaleph, having a short pedicel on the back for attachment; tentacles disposed in eight groups around the margin, the eight points alternating with the four partitions of the body -cavity and FIG. 197.— Lucernaria auricula attached to a the four COl'IierS of the piece of sea-weed; natural size. The one on ,1 , •, ,, the right is abnormal, having a uiutn tuft of niOUtil | DOt JCSS tliail eight radiating canals, and no membranous veil. The common species on the Atlantic shore, generally found attached to eel-grass, is an inch in diameter, of a green color. Discophora, the ordi- nary Jelly-fish, is free and oceanic. It differs from the Lucernaria in its usually larger size and solid disk, four 250 COMPARATIVE ZOOLOGY. radiating canals, which ramify and open into a circular vessel, and a " veil," or shelf, always running around the mouth of the disk.1" CLASS II. — Anthozoa. These marine animals, which by their gay tentacles con- vert the bed of the ocean into a flower-garden, or by their secretions build up coral-islands, have a body like a cylindrical gelatinous bag. One end, the base, is usually attached ; the other has the mouth in the cen- tre, surrounded by numerous hollow tentacles, which are cov- ered with nettling lasso -cells. This upper edge is turned in so Pio.WL-Horl.onUl Section of Ac- 8 tO fOmi * SaC Withi" * 8aC' tinia through the stomach, chow- like the neck of a bottle turned ing septa and compartments. . . . outside in. Ihe inner sac, which is the digestive cavity, does not reach the bottom, but opens into the general body-cavit}7.130 The space between these two concentric tubes is divided by a series of vertical parti- tions, some of which extend from the body- wall to the digestive sac, but others fall short of it. Instead, therefore, of the radi- ating tubes of the Aca- leph, there are radiat- ing spaces. No mem- bers of this class are . . Fid. 199. — Actinia expanded, seen from above, microscopic. All are showing mouth. CCELENTERATA. 251 long-lived compared with the Hydrozoa, living for several years. One kept in aquaria in England is now more than sixty }Tears old. 1. Soft-bodied Polyps. — The best-known representa- tive of this group is the Actinia, or Sea -anemone. It leads a single life, and is capable of a slow locomotion. Muscular fibres run around the body, and others cross these at right angles. The tentacles, which often number over two hundred, and the partitions, which are in reality double, are in multiples of six. At night, or when alarmed, the tentacles are drawn in, and the aperture firmly closed, so that the animal looks like a rounded lump of fleshy substance plastered on the rock. It feeds on Crabs and Mollusks. It abounds on every shore, especially of trop- ical seas. The size varies from one eighth of an inch to a foot in diameter. 2. Coral Polyps. — The majority of Anthozoa secrete a calcareous or horny framework called "coral." With few exceptions, they are fixed and composite, living in colonies formed by a continuous process of budding. Their structures take a variety of shapes : often dome- like, but often imitating shrub- bery and clusters of leaves. The members of a coral community are organically connected ; each feeds himself, yet is not indepen- dent of the rest. We can speak of the individual Corals, a, S, c, but we must write them down abc. The compound mass is "like FIG. 200.— organ-pipe c i . . , . . , pora musica). Indian Ocean. a living sheet of animal matter, fed and nourished by numerous mouths and as many stomachs." Life and death go on together, the old 252 COMPARATIVE ZOOLOGY. Polyps dying below as new ones are developed above. The living part of an Attract is only half an inch thick. The growth of the branching Madrepore is about three inches a year. The prevailing color of the Coral Polyps is green ; and the usual size varies from that of a pin's head to half an inch, but the Mushroom-coral (which is a single individual) may be a foot in diameter. Corals are of two kinds: those deposited within the tis- sues of the animal (sclerodermic), and those secreted by the outer surface at the foot of the Polyp (sclerobasic). The Polyps producing the former are Actinoid, resem- bling the Actinia in structure.131 The skeleton of a single Polyp (called corallite, Fig. 95) is a copy of the animal, except the stomach and tentacles, the earthy matter being secreted within the outer wall and between each pair of partitions. So that a corallite is a short tube with vertical septa radiating towards the centre.132 A sclerobasic Coral is a true exoskeleton, and is distinguished by being smooth and solid. The Polyps, having eight fringed tentacles, are situated on the outside of this as a common axis, and are con- nected together by the fleshy ccenosaro covering the Coral. ( 1 ) Solerodermic Corals. — Astrcea is a hemispherical mass covered with large cells. Meandrina, or "Brain-coral," is also globular; but the mouths of the Polyps open into each other, forming furrows. Fungia, or "Mushroom- coral," is disk-shaped, and differs from other kinds in be- ing the secretion of a single gigantic Polyp, and in not being fixed. Madrepora is neatly branched, with pointed extremities, each ending in a small cell about a line in diameter. Parties, or "Sponge-coral," is also branching, but the ends are blunt, and the surface comparatively smooth. Tiibipora, or " Organ - pipe coral," consists of smooth red tubes connected at intervals by cross-plates. The Astrcea, Meandrlna, Madrepora, and Porites are the chief reef-forming Corals. They will not live in waters CCELENTERATA. 253 . 201. — Madrepora aspera, living and expanded ; natural size. Pacific. whose mean temperature in the coldest month is below 68° Fahr., nor at greater depth than twenty fathoms. The most luxuriant reefs are in the Central and Western Pa- cific and around the West Indies. hinata, or " Mushroom-coral ;" one fourth natural size. Pacific. 254 COMPARATIVE ZOOLOGY. FIG. 203.— Astrceapallida; natural size. Fejee Islands. A coral-reef is formed by many Corals growing togeth- er. It is to the single Coral-stock as a forest is to a tree. Fio. 204.— Diplaria eerebriformw, or " Brain-coral ;" one half natural size. Bermudas. CCELENTERATA. 255 FIG. 205.— Astrcea rotulusa. West Iiidi The main kinds of reefs are fringing, where the reef is close to the shore ; barrier, where there is a channel be- Fis. 206.— Cell of Madrepore Coral, magnified. The cup-like depres- sion at the top of a coral skeleton ia called calicle. Fio. 207 — Fragment of Red Coral (Coral- Hum rubrum), showing living cortex and expanded Polyps. Mediterranean. 256 COMPARATIVE ZOOLOGY. tween reef and shore ; encircling, where there is a small island inside of a large reef; and coral islands, or atolls, where there is simply a reef with no land inside of it. All reefs begin as fringing-reefs, and are gradually changed into the other forms by the slow sinking of the bottom of the ocean. This sinking must be slower than the upward growth of the reef, else it will be drowned out. Probably the reef does not grow more than five feet in a thousand years; and, as reefs are often more than two thousand feet thick, they must be very old. (2) Sclerobasic Corals. — Corallium rubrum, the precious corai of commerce, is shrub -like, about a foot high, solid throughout, taking a high polish, finely grooved on the surface, and of a crimson or rose-red color. In the living Fio. 208. — Sea-fan (Gorgnniu) ;iud Sea-peu (PK state the branches are covered with a red coenosarc stud- ded with Polyps. Gorgonia, or "Sea-fan," differs from all the other representative forms in having a horny axis covered with calcareous spicules. The branches arise in the same vertical plane, and unite into a beautiful net- work. ECHINODERMATA. 257 CLASS III. — Ctenophora. The Ctenophora (as the Pleuro- brachia, Cesium, and Bero'e) secrete no hard deposit. They are trans- parent and gelatinous, swimming on the ocean by means of eight comb- like, ciliated bands, which work like paddles. The body is not contrac- tile, as in the Jelly-fishes. They are considered the highest of Ccelente- rates, having a complex nutritive ap-Fia.209.-Actenophore(pz«*- , » .. robrachia pileus) ; natural paratus and a dennite nervous sys- size, tern. Subkingdom III. — ECHINODEKMATA. The Echinoderms, as Star -fishes and Sea-urchins, are distinguished by the possession of a distinct nervous sys- tem (a ring around the mouth) ; an alimentary canal, com- Fio. 210. — Forms of Echinoderms, from radiate to annulose type : a, Crlnoids ; b, Ophiarans ; c, Star-fish ; d, Echini ; e, Holothurians. pletely shut off from the body-cavity, and having both oral and anal apertures ; a water-vascular system of circu- 17 258 COMPARATIVE ZOOLOGY. lar and radiating canals, connected with the outside water by means of the inadreporic tubercle, and a symmetrical arrangement of all the parts of the body around a central axis in multiples of five.133 There are four principal class- es, all exclusively marine and solitary, and all having the power of secreting more or less calcareous matter. CLASS I. — Crinoidea. The Crinoids, or " Sea-lilies," are fixed to the sea-bottom by means of a hollow, jointed, flexible stem. On the top of the stem is the body proper, resembling a bad or ex- panded flower, containing the digestive apparatus, with the surrounding arms, or tentacles. The mouth looks up- ward. There is a complete skeleton for strength and sup- port, the entire animal — body, arms, and stem — consisting of thousands of stellate pieces connected together by liv- ing matter. Crinoids were very abundant in the old geo- ologic seas, and many limestone strata were formed out of their remains. They are now nearly extinct: dredging in the deep parts of the oceans has brought to light a few living representatives. CLASS II. — Asteroidea. Ordinary Star-fishes consist of a flat central disk, with five or more arms, or lobes, radiating from it, and con- taining branches of the viscera. The skeleton is leathery, hardened by small calcareous plates (twelve thousand by calculation), but somewhat flexible. The mouth is below ; and the rays are furrowed underneath, and pierced with numerous holes, through which pass the sucker-like tenta- cles— the organs of locomotion and prehension. The red spots at the ends of the rays are eyes. The usual color of Star-fishes is yellow, orange, or red. They abound on ev- ery shore, and are often seen at low tide half buried in the sand, or slowly gliding over the rocks. Cold fresh ECHINODERMATA. 259 FIG. 211. — A liviug Crinoid (Pentacrinwt asteria) ; one fourth Indian Seas. itural size. West 260 COMPARATIVE ZOOLOGY. water is instant death to them. They have the power of reproducing lost parts to a high degree. They are very voracious, and are the worst enemies of the Oyster. Fio. 212. — Under - surface of Star-flsh (Goniaster reticu(atun), showing ambulacra! grooves aud protruded suckers. About one hundred and fifty species are known. These may be divided into three groups : (1) species having four rows of feet, represented by the common five -fingered Asterias; (2) species having two rows of feet, as the many- rayed Solaster, or " Sun-star," and the pentagonal Goni- aster; (3) species having long, slender arms, which are not prolongations of the body, and are not provided with suck- ers, as the Ophiura, or " Brittle-star," and Astrophyton, or " Basket-fish." The last are of inferior rank, and resemble ECHINODERMATA. 261 inverted, stemless Crinoids. The digestive sac is confined to the disk, and the inadreporic tubercle is concealed. FIG. 213.— Ophiocoma Russet, an Ophiuran; natural size. West Indies. CLASS III.— Echinoidea. The Sea-urchin is encased in a thin, hollow shell cov- ered with spines, and varying in shape from a sphere to a disk.1'4 The mouth is underneath, and contains a dental apparatus more complicated than that of any other creat- ure. It leads to a digestive tube, which extends spirally to the summit of the body. The spines are for burrow- ing and locomotion, and are moved by small muscles, each being articulated by ball-and-socket joint to a distinct tu- bercle. When stripped of its spines, the shell (or "test") is seen to be formed of a multitude of pentagonal plates, fitted together like a mosaic.134 Five double rows of plates, 2G2 COMPARATIVE ZOOLOGY. FIG. 214.— TTnder-surface of 11 Sea-nrchin (Echinus e lentus), showing rows of suckers amuiig the spi British seas. passing from pole to pole, like the ribs of a melon, alter- nate with five other double rows. In one set, called the ambulacra, the plates are perfo- rated for the pro- trusion of tubular feet, or suckers, as in the Star -fish. So that altogether there are twenty series of plates — ten ambniacral, andteninterambu- lacral. The shell is not cast, but grows by the en- largement of each individual plate, and the addition of new ones around the mouth and the opposite pole. Every part of an Echinus, even sections of the spines, show the principle of radiation. If the up- per surface of a Star-fish should shrink so as to bring the points of the arms to meet above the mouth, we should have a close imitation of a Sea-urchin. Echini live near the shore, in rocky holes or under sea-weed. They are less active than Star-fishes; but, like them, feed on Mollusks and Crabs. They reproduce by minute red eggs. Regular Echini, as the common Cidaris, are nearly globular, and the oral and anal openings are opposite. Irregular Echini, as the Clypeaster, are flat, and the anal orifice is near the margin. CLASS IV. — Holothuroidea. These worm-like " Sea-slugs," as they are called, have a soft, elongated body, with a tough, contractile skin contain- ECHINODERMATA.— VERMES. 263 ing calcareous granules. One end, the head, is abruptly terminated, and has a simple aperture for a mouth, en- circled with feathery tentacles. There are usually five longitudinal rows of ambulacral suckers, but only three are used for locomotion, of which one is more developed than the rest. The mouth opens into a pharynx leading to a long intestinal canal. Holothurians have the singular power of ejecting most of their internal organs, surviving FIG. 215.— Sea-slugs (Hulothuria). for some time the loss of these essential parts, and after- wards reproducing them. They occur on nearly every coast, especially in tropical waters, where they sometimes attain the length of three or four feet. As found on the beach after a storm, or when the tide is out, they are leathery lumps, of a reddish, brownish, or yellowish color. They may be likened to a Sea-urchin devoid of a shell, and long drawn out, with the axis horizontal, instead of vertical. Subkingdom IV. — YERMES. The Yermes,138 or Worms, form the lowest subkingdom of the bilaterally symmetrical animals. The group in- cludes animals very different in form and rank, and the different classes are widely separated from each other. 264 COMPARATIVE ZOOLOGY. It has also close relations with the other subkingdoms of the bilaterally symmetrical animals. Through the Poly- zoa and Brachiopoda, it approaches the Mollusca ; through the Ann elides, the Arthropoda ; and through other forms, the Tunicata, and so the Vertebrata. The subkingdom thus stands in the centre of several subkingdoms, with affinities towards all. Nor are indications of connection with Coelenterata and Echinodermata wanting. The Vermes are bilaterally symmetrical animals,with one or many segments, no jointed legs. They usually have a soft skin, and peculiar excretory organs — the segmental organs. Many of the Worms are parasitic, and most of the En- doparasites belong to this group. There are numerous classes, of which only the most im- portant are mentioned. CLASS I. — Platyhelminthes. The Flat -worms include some free forms, as the Plana- ria, common in fresh water, and the Tape- worms and Flukes among the parasites. The Tape -worm consists of the so- called head — the proper worm — and the body segments, Fio. 216 — Tape-worm (Tcenia solium) : a, head ; 6, «, d, eegments of the body. Fie. 217 — Planarian worm. VERMES. 265 which are really reproductive joints. It develops from the egg in the digestive canal of the Pig, burrows into the cellular tissue of the animal, and there becomes en- cased. It thus causes the disease " measles." If the pork be eaten by man, in an uncooked condition, this case is dissolved by the gastric juice, and the embryo develops into the Tape-worm, attaching itself to the intestine by its "head," and budding off the reproductive segments. As these become ripe and filled with fertilized eggs, they are detached, and pass off with the excrement. The disease called "rot," in Sheep, is produced by the Fluke (Distoma\ a member of this class. * CLASS II. — Nematelminthes. The Bound, or Thread, Worms include free forms, as the Yinegar-eel; parasitic forms, as the Pin-worm and Trichina ; and forms free when adult, and parasitic when young, as the Hair-worm (Gor- The Trichina is usu- ally derived by Man from the flesh of the Pig. It exists in the muscles, enclosed in mi- croscopic cases. If the meat be eaten uncooked or partially cooked, the cases are dissolved, and the Trichinae become sexually mature in the intestines. The young are produced and bur-^is. 218.— rncM»w *pt>aiw .- i, male; o, month; ,, • . . ,, e, intestine; II, capsules, with Trichinae in mns- W their Way into the cle, much enlarged. 266 COMPARATIVE ZOOLOGY. muscles, where they become encysted. In burrowing, they cause great pain and fever, and sometimes death. The adult Worm is about -^"inch long. CLASS III. — Rotifera. The Wheel-animalcules, mostly found in fresh water, are minute Worms of few segments, having on the ante- rior end a disk ciliated on the edge, whence their name. They are from z^v to uV of an inch long. They can bear drying and revivifying, like seeds. CLASS IY. — Polyzoa. These minute Worms resemble the Polyps in appearance, living in clusters, each individual inhabiting a delicate cell, or tube, and having a simple mouth surrounded with ciliated tentacles. The colony often takes a plant -like form; sometimes spreads, like fairy -chains or lace-work, over other bodies ; or covers rocks and sea -weeds in patches with a delicate film. The majority secrete car- FIO. 219. — Rotifer, or bouate of lime. A Polyzoan shows its su- " Wheel-animalcule " . . . .^. . , . , .,..., ( nydatma >, highly pcriority to the Coral, which it imitates, in possessing a distinct alimentary canal and a well-defined nervous system. The cells of a group never have connection with a common tube, as in Cceleri- terates. There are both marine and fresh-water species. This group and the next following are related to the Mollusca. CLASS Y. — Brachiopoda. These Worms have a bivalve shell, the valves being applied to the dorsal and ventral sides of the body. The valves are unequal, the ventral being usually larger, and VERMES. 267 Fio. 220. — Polyzoans: 1. FTornera licheruMes, natural size. 2. Branch of the same, magnified. 3. Discopora Skenei, greatly enlarged. more convex ; but they are symmetrical, i. e., a vertical line let fall from the hinge divides the shell into two equal parts. The ventral valve has, in the great major- ity, a prominent beak, perforated by & foramen, or hole, through which a fleshy foot protrudes to attach the ani- mal to submarine rocks. The valves are opened and shut by means of muscles, and in most cases they are hinged, having teeth and sockets FIQ. 221. — A Bracliiopod (Terebratulina septentrionali*). Atlantic coast. near the beak. The mouth faces the middle of the mar- gin opposite the beak ; and on either side of it is a long, Fio. 222.— Dorsal Valve of a Brachiopod (Tcrebratula), showing, in descending order, cardinal process, dental sockets, hinge-plate, septum, and loop support- ing the ciliated arms. 268 COMPARATIVE ZOOLOGY. fringed "arm," generally coiled up, and supported by a calcareous framework. The animal, having no gills, re- spires by the arms and the mantle. Brachiopods were once very abundant, over two thousand extinct species having been described; but less than a hundred species are now living.137 They are all marine, and fixed; but of all Worms, they enjoy the greatest range of climate and depth. CLASS VI.— Annelides. The Annelides include the highest and most specialized Worms. They have many segments, spines or suckers for locomotion, a supercesophageal brain, a ventral chain Fio. 223.— Marine Worm (Cirratulus grandis), with extended cirri. Atlantic. of ganglia, and a closed blood-system. There are three main divisions: the flattened Leeches, without definite segments or bristles, and with suckers for locomotion ; the MOLLUSCA. 269 Earth-worms and their allies, which have few bristles on each segment (Oligochcetce) ; and the Sea-worms, with nu- merous bristles, arranged in two clusters on each side of each segment (Polychcetce). These last are the largest of the Worms, and may have a distinct head, bearing tentacles and eyes. The oesopha- gus is often turned in, so as to form a proboscis, which bears horny jaws, and can be protruded at the will of the animal (Fig. 17). Subkingdom V. — MOLLUSCA. A Mollusk is a soft -bodied animal, without internal skeleton, and without joints, covered with a moist, sensi- tive, contractile skin, which, like a mantle, loosely envel- ops the creature. In some cases the skin is naked, but generally it is protected by a calcareous covering (shell). The length of the body is less in proportion to its bulk than in other animals. The lower class has no distinct head. The nervous system consists of three well-devel- oped pairs of ganglia, which are principally concentrated around the entrance to the alimentary canal, forming a ring around the throat. The other ganglia are, in most cases, scattered irregularly through the body, and in such the body is unsymmetrical. The digestive system is greatly developed, especially the liver, as in most aquatic animals. Except in the Cephalopods, the muscles are at- tached to the skin, or shell. There is a heart of two chambers (auricle and ventricle) or three (two auricles and ventricle). As in all Invertebrates, the heart is arte- rial. In Mollusks, with rare exceptions, we find no repe- tition of parts along the antero-posterior axis. They are best regarded as Worms of few segments, which are fused together and much developed. The total number of living species probably exceeds twenty thousand. The great majority are water-breathers, and marine ; some are 270 COMPARATIVE ZOOLOGY. fluviatile or lacustrine, and a few are terrestrial air-breath- ers. All bivalves, and nearly all univalves, are aquatic. Each zone of depth in the sea has its particular species. CLASS I. — Lamellibranchiata. Lamellibranchs are all ordinary bivalves, as the Oyster and Clam. The shells differ from those of Bracbiopoda in being placed on the right and left sides of the body, so that the hinge is on the back of the animal, and in being unequilateral and equivalved.138 The umbo, or beak, is the point from which the growth of the valve commences. FIG. 224.— Pearl oyster Both Brachiopods and Lamellibranchs (MeleagrinanMrgariti- in i -11 i /era); oue fourth uat- are headless ; but in the latter the month uralsize. Ceylon. towards the anterior part. The length of the shell is measured from its anterior to its posterior margin, and its breadth from the dorsal side, where the hinge is, to the opposite, or ventral, edge. The valves are united to the animal by one muscle (as in the Oyster), or two (as in the Clam), and to each other by a hinge. In some species, as some fresh- water Mussels, the hinge is simply an elastic ligament, passing on the outside from one valve to the other just behind the beak, so that it is on the stretch when the valves are closed, and another placed FI« 225. -salt- water . , ' . Mussel (Mytilus pel- between the edges of the valves, so that inMmt). Atiautic it is squeezed as they shut, like the spring in a watch-case. Such bivalves are said to be edentulous. But in the majority, as the Clam, the valves also articulate by interlocking parts called teeth. The valves are, there- fore, opened by the ligaments, and closed by the muscles. MOLLUSCA. 271 The margin of the shell on which the ligament and teeth are situated is termed the hinge-line. Lamellibranchs breathe by four plate-like gills (whence the name), two on each side underneath the mantle (Fig. 78). In the higher forms, the mantle is rolled up into two tubes, or siphons, for the inhalation and exhalation of water. They feed on infusorial particles filtered from the water. A few are fixed ; the Oyster, e. llPRfl is .FiG. 250.— Under-side of the Cray-fish, or Fresh- water Lobster (ArtunuJhwiatUi,) : a, first pair fixed tO the thorax), of antennae ; 6, second pair , c, eyes ; d, open- ing of kidney ; «, foot-jaws ; /, g, first and fifth and are Compound, pair of thoracic legs; A, abdominal feet; i, made up of about two aus ; *' caudal fln" thousand five hundred square facets. At the base of each small antenna is a minute sac, whose mouth is guarded by hairs : this is the organ of hearing. The gills, twenty on a side, are situated at the bases of the legs and enclosed in two chambers, into which water is freely admitted, in fact, drawn, by means of a curious attachment to one of the 284 COMPARATIVE ZOOLOGY. maxillae, which works like the "screw" of a propeller. The heart is a single oval cavity, and drives arterial blood — a dusky fluid full of corpuscles. The alimentary canal consists of a short gullet, a gizzard -like stomach, and a straight intestine. Crustaceans pass through a series of strange metamor phoses before reaching their adult form. They also peri- odically cast the shell, or moult, every part of the integu- ment being renewed; and another remarkable endowment is the spontaneous rejection of limbs and their complete restoration. Many species are found in fresh water, but the class is essentially marine and carnivo- rous. Of the numerous orders of this great class we will mention only four: 1. Cirripeds, distinguished by being fixed, by having a shelly covering, and by their feathery arms (cirri). Such are Barnacles PIG. 251.— Idotea robusta: a Te- tradecapod. U. S. coast so common on rocks and timbers by the sea-shore. 2. Entomostracans, which agree in having a horny shell and no abdominal limbs; repre- sented by the little Water-fleas (Cyclops) of our ponds, and Fio. 252. — Amphithoe maculata: a Sand-flea. the Brine-shrimps (Artemia), and many others. The King- crabs (Limulus) and the extinct Trilobites were formerly ARTHROPOD A. 285 Fia. 253.— Barnacles, or Pedunculate Cirripedes (Lepas anatifera). united to this class, but now are known to be widely re- moved from it. The former is by some authors removed from the Crustacea. 3. Tetradecapods, small, fourteen-footed species ; as the FIG. 254.— Acorn-shells (Balanus) on the Shell of a Whelk (Buccinum). FIG. 255.— Water-fleas : 1, Cyclops communis, 2. Cypris uni/aaciata; 3, Daphnia pulex. 286 COMPARATIVE ZOOLOGY. Wood-louse, or Sow-bug (Oniscus\ so common in damp places, the Slaters (Idotea), and the Sand-fleas (Gammarus), seen by the sea-side. 4. Decapods^ having ten legs, as the Shrimp (Cmngon\ Fie. 267— Swimming Crab (Platymychw). ARTHROPODA. 287 Cray -fish (Astacus), Lobster (Homarus), and Crab (Can- cer). Crabs differ from Lobsters chiefly in being formed for creeping at the bottom of the sea instead of swim- ming, and in the reduction of the abdomen or " tail " to a rudiment, which folds into a groove under the enormous thorax. They are the highest and largest of living Crus- tacea : they have been found at Japan measuring fifteen feet between the tips of the claws. CLASS II. — Arachnida. The Arachnids are closely related to the Crustaceans, having the body divided into a cephalo-thorax and abdo- men."8 To the former are attached eight legs of seven joints each ; the latter has no locomotive appendages. The head carries two, six, or eight eyes, smooth and ses- sile (i. e., not faceted and stalked, as in the Lobster), and approaching the eye of the Vertebrates in the complete- ness and perfection of their apparatus. The antennae, if present, are only two, and these are not "feelers," but modified to serve for the prehension of food.149 They are all air-breathers, having spiracles which open either into air-sacs or tracheae. The young of the higher forms un- dergo no metamorphosis after leaving the egg. Arachnids number nearly five thousand species. The typical forms are divided into three groups : 1. Acarina, represented by the Mites and Ticks. They have an oval or rounded body, without any marked artic- ulations, the head, thorax, and abdomen being apparently merged into one. They have no brain; only a single gan- FlG 25S._A Mlte fflion lodged in the abdomen, mm), one of the lowest Arachnids; a parasite in human hair-sacs ; X 125. They breathe by tracheae. The mouth is formed for suction, and they are generally para- sitic. The Mites (Acarus) are among the lowest of Ar- 288 COMPARATIVE ZOOLOGY. ticulates. The body is soft and minute. The Ticks (Ixodes) have a leathery skin, and are sometimes half an inch long. The mouth is furnished with a beak for pierc- ing the animal it infests. 2. Pedipalpi, or Scorpions, characterized by very large maxillary palpi ending in forceps, and a prolonged, joint- ed abdomen. The nervous and circulatory systems are more highly organized than those of Spiders; but the long, tail-like abdomen and the abnormal jaws place them PIG. 259 — Scorpion (under surface) and Centipede. in a lower rank. The abdomen consists of twelve seg- ments : the anterior half is as large as the thorax, with no well-marked division between ; the other part is compara- tively slender, and ends in a hooked sting, which is perfo- rated by a tube leading to a poison-sac. The mandibles are transformed into small, nipping claws, and the eyes generally number six. Respiration is carried on by four pairs of pulmonary sacs which open on the under surface ARTHBOPODA. 289 of the abdomen. The heart is a strong artery, extending along the middle of the back, and divided into eight separate chambers. Scorpions are confined to the warm-temperate and tropical regions, usually lurking in dark, damp places. The Harvest-men (Phalangium), frequently seen about our houses, belong to this order. They have a short, thick body and extremely long legs, and breathe by tracheae. 3. Araneina, or Spiders. They are distinguished by their soft, unjointed abdomen, separated from the thorax by a narrow constriction, and provided at the posterior end with two or three pairs of appendages, called " spin- Fia. 260.— A, female Spider ; B, male of same species ; C, arrangement of the eyes. nerets," which are homologous with legs. The office of the spinnerets is to reel out the silk from the silk-glands, the tip being perforated by a myriad of little tubes, through which the silk escapes in excessively fine threads. An ordinary thread, just visible to the naked eye, is the 19 290 COMPARATIVE ZOOLOGY. union of a thousand or more of these delicate streams of silk.160 These primary threads are drawn out and united by the hind legs. The mandibles are vertical, and end in a powerful hook, in the end of which opens a duct from a poison-gland in the head. The maxillae, or " palpi," which in Scorpions are changed to formidable claws, in Spiders resemble the thoracic feet, and are often mistaken for a fifth pair. The brain is of larger size, and the whole nervous system FIG. 26i. —spin- more concentrated than in the preceding or- der, b,°c-, a! pal- der. There are generally eight simple eyes, piform organs. rarely gjx> They breathe both by tracjiege and lung-like sacs, from two to four in number, situated under the abdomen. All the species are carnivorous. The instincts of Spiders are of a high order. They are, perhaps, the most wily of Articulates. They display re- markable skill and industry in the construction of their webs; and some species (called "Mason Spiders") even excavate a subterranean pit, line it with their silken tapes- try, and close the entrance with a lid which moves upon a hinge."1 CLASS III.— Myriapoda. Myriapods differ from Crustaceans and Spiders in hav- ing the thorax merged in the abdomen, while the head is free. In other words, the body is divided .into similar segments, so that thorax and abdomen are scarcely distin- guishable. They resemble Worms in form and in the simplicity of their nervous and circulatory systems ; but the skin is stiffened with chitine, and the legs (indefinite in number) are articulated. The legs resemble those of Insects, and the head appendages follow each other in the same order as in Insects — eyes, antennae, mandibles, max- illae, and labium. They breathe by tracheae, and have two antennae and a variable number of eyes. ARTHROPODA. 291 There are two orders : 1. Chilognatha, having a cylindrical body, each segment furnished with two pairs of legs. They are of slow loco- motion, harmless, and vegetarian. The Thousand-legged Worm (Julus) is a common representative. 2. Chilopoda, characterized by having a flattened body composed of about twenty segments, each carrying one pair of legs, of which the hindermost is converted into spines. They have longer antennae than the preceding, and the mouth is armed with two formidable fangs con- nected with poisonous glands. They are carnivorous and active. Such is the Centipede (Scolopendra, Fig. 259). CLASS IY. — Insecta. Insects are distinguished by having head, thorax, and abdomen distinct, three pairs of jointed legs, one pair of antennae, and generally two pairs of wings. The number of segments in the body never exceeds twenty. The head, apparently one, is formed by the union of four segments. The thorax consists of three — the prothorax, mesothorax, and nwtathorax — each bearing a pair of legs ; the wings, if present, are carried by the last two segments. The ab- domen is normally composed of ten segments, more or less movable upon one another. The skin is hardened with chitine, and to it, as in all Arthropods, the muscles are at- tached. The organs of sense are confined to the cephalic division of the body, the motor organs to the thoracic, and the vegetative to the abdominal. All the appendages are hollow. The antennas are inserted between or in front of the eyes. There is a great variety of forms, but all are tubu- lar and jointed. They are supposed to be organs of touch, and also seem to be sensitive to sound. The eyes are usually compound, composed of a large number of hexago- nal corneas, or facets (from fifty in the Ant to many thou- 292 COMPARATIVE ZOOLOGY. sands in the winged Insects). They are never placed on movable stalks, as the Lobster's. Besides these, there are three simple eyes, called ocelli. The mouth may be fitted for bit- —e, jng (masticatory), as in Beetles, or for suck- ing (suctorial), as in Butterflies. The mas- ticatory type, which is the more complete, and of which the other ~~~y is but a modification, consists of four horny jaws (mandibles and maxillce) and an up- per and an under lip (labrum and lalium). Sensitive palpi (max- illary and labial) are developed from the lower jaw and lower lip. The labium is also prolonged into a ligula, or tongue. The legs are invari- Fio. 262. -Under surface of a Beetle (Harpalut eali- ginotus) : a, lignla ; b, paraglossse ; c, supports of labial palpi; d, labial palpus; «, mentum; /, in- ^i • • t^_ adnlf ner lobe of maxilla; g, outer lobe; h, maxillary a°V bIX 1U tue <*ulllt, palpus ; f, mandible ; k, buccal opening ; I, gnla, fag fore - legs direct- or throat: m, bnccal sutures; n, gnlar snture; o, ° prosternum ; p, episternnm of prothorax ; p', epi- ed forward and the meron ; q, q1, q", coxae ; r, r, r, trochanters ; 8, i/, s", femora, or thighs ; t, t', t", tibae ; v, ventral abdominal segments ; tc, episternaof mesothorax ; x, mesosternnm ; y, eplsterna of metathorax ; y', epimeron ; z, metasteruum. hinder pairs back- ward. Each consists of a hip, thigh, shank, and foot.11* Some larvae have also " false legs," without ARTHROPODA. 293 joints, on the abdomen, upon which they chiefly rely in locomotion. The wings are expansions of the crust, stretched over a net- work of horny tubes. The venation, or arrangement of these tubes (called veins and veinlets), particularly in the fore-wings, is peculiar in each genus. In many Insects, the abdomen of the female ends in a tube which is the sheath of a sting, as in the Bee, or of an ovipositor, or " borer," as in the Ichneumon, by means of which the eggs are deposited in suitable places. Cephalization is carried to its maximum in this class, and we have animals of the highest instincts under the articulate type. The " brain " is formed of several gan- glia massed together, and lies across the upper side of the throat, just behind the mouth. The main cord lies along the ventral side of the body, with a swelling for each seg- ment ; besides this, there is a visceral nerve representing, in function, the sympathetic system of Yertebrates. The digestive apparatus consists of a pharynx, gullet (to which a crop is added in the Fly, Butterfly, and Bee tribes), giz- zard, stomach, and intestine. There are no absorbent ves- sels, the chyme simply transuding through the walls of the canal. The blood, usually a colorless liquid, is driven by a chain of hearts along the back, i. e., by a pulsating tube divided into valvular sacs, ordinarily eight, which allow the current to flow only towards the head. As it leaves this main pipe, it escapes into the cavities of the body, and thus bathes all the organs. Although the blood does not circulate in a closed system of blood-vessels, as in Yertebrates, yet it always takes one set of channels in go- ing from the heart, and another in returning. Respira- tion is carried on by tracheae, a system of tubes opening at the surface by a row of apertures (spiracles), generally nine on each side of the body. The sexes are distinct, and the larvae are hatched from eggs. As a rule, an Insect, after reaching the adult, or 294 COMPARATIVE ZOOLOGY. imago, state, lives from a few hours to several years, and dies after the process of reproduction. Growth takes place only during larval life, and all metamorphoses occur then. Among the social tribes, as Bees and Ants, the majority (called "workers'') do not develop either sex. Insects (the six-footed Arthropods) comprise nearly one half of the whole Animal Kingdom, or from one hundred and seventy thousand to two hundred thousand species. They are grouped into seven orders : Lower series: body usually flattened; prothorax large and } Neuroptera, squarish ; mouth-parts usually adapted for biting ; met- I Orthoptera, amorphosis often incomplete ; pupa often active ; larva Hemiptera, flattened, often resembling the adult. J Coleoptera. Higher series: body usually cylindrical; prothorax small; mouthrparts more generally formed for sucking ; meta- morphosis complete ; pupa inactive ; larva usually cylin- Lepidopterti, Hymenoptera drical, very unlike the adult. 1. Neuroptera have a comparatively long, slender body, Pro. 263.— Dragon-fly (Libellula). and four large, transparent wings, nearly equal in size, membranous and lace-like. Such are the brilliant Dragon- ARTHROPODA. 295 flies, or Devil's Darning-needles (Libelluld), well known by the enormous head and thorax, large, prominent eyes (each furnished with twenty - eight thousand polished lenses), and Scorpion - like abdomen; the delicate and short-lived May-flies (Ephemera) ; Caddis-flies (Phryga- nea), whose larvae live in a tubular case made of minute stones, shells, or bits of wood ; the Horned Corydalis (Corydalus), of which the male has formidable mandibles twice as long as the head ; and the White Ants (Termes) of the tropics. 2. Orthoptera have four wings : the front pair some- what thickened, narrow, and overlapping along the back ; the hind pair broad, net-veined, and folding up like a fan Fie. 264.— Metamorphosis of a Cricket (Gryllus). upon the abdomen. The hind legs are usually large, and fitted for leaping, all the species being terrestrial, although some fly as well as leap. The eyes are small, the mouth remarkably developed for cutting and grinding. The lar- COMPARATIVE ZOOLOGY. Pia. 265.— Metamorphosis of an Hemipter, Water-boatman (NotonecM). vse and pupae are active, and resemble the imago. They are nearly all vegetarian. Each family produces charac- teristic sounds (stridulation). The representative forms Fia. 266 — Seventeen-year Cicada (Cicada »eptendecim) : a, pupa; 6, the pame, after the imago, c, has escaped through a rent in the back ; d, holes in a twig, where the eggs, e, are inserted. ARTHROPODA. 297 are Crickets (Gryllus), Locusts (Locusta), Grasshoppers (Acrydium), Walking-sticks (Phasma), and Cockroaches (Blatta). 3. Hemiptera, or " Bugs," are chiefly characterized by a suctorial mouth, which is produced into a long, hard, beak, in which mandibles and maxillae are modified into bristles and enclosed by the labium. The four wings are irregularly and sparsely veined, sometimes wanting. The body is flat above, and the legs slender. The larva differs from the imago in wanting wings. In some species the fore -wings are opaque at the base and transparent at the apex, whence the name of the order. Some feed on the juices of animals, others on plants. Here belong the wingless Bed-bug (Cimex) and Louse (Pediculus), the Squash-bug (Coreus), Water-boatman (Notonecta), Seven- teen-year Locust (Cicada), Cochineal (Coccus), and Plant- louse (Aphis). 4. Coleoptera, or " Beetles." This is the largest of the orders, the species numbering about ninety thousand. They are easily recognized by the elytra, or thickened, Fio. 267.— «, imago, and b, larva, of the Goldsmith Beetle (Cotalpa lanigera); c, papa of June-bug (Lachnoaterna fusca). horny fore-wings, which are not used for flight, but serve to cover the hind pair. When in repose, these elytra are always united by a straight edge along the whole length. The hind wings, when not in use, are folded transversely. 298 COMPARATIVE ZOOLOGY. The mandibles are well developed, and the integument generally is hard. The legs are strong, for the Beetles are among the most powerful running Insects. The lar- vae are worm-like, and the pupa is motionless. The high- est tribes are carnivorous. The most prominent forms FIG. 268. — Sexton Beetles (Necrophoriis veapillo), with larva and nymph. They are bnryiug a mouse, preparatory to laying their eggs in it. , are the savage but beautiful Tiger Beetles (Cicindelo)\ the common Ground Beetles (Cardbus), whose hind wings are often absent; the Diving Beetles (Dytiscus), with boat-shaped body, and hind legs changed into oars ; the Carrion Beetles (Silpha), distinguished by their black, flat ARTHROPODA. 299 bodies and club-shaped antennae; the Goliath Beetles (Scardbaus), the giants of the order ; the Snapping-bugs (Elater) ; the Lightning-bugs (Pyrophorus) ; the spotted Lady-birds (Coccinella)', the showy, Long-horned Beetles FIG. 269. -Metamorphosis of the Mosquito (Culex pipiens). 300 COMPAEATIVE ZOOLOGY. (Ceranibycidce) ; and the destructive Weevils (Curculio- nidce), with pointed snouts. 5. Diptera, or " Flies," are characterized by the rudi- mentary state of the hinder pair of wings. Although having, therefore, but one available pair, they are gifted with the power of very rapid flight. While a Bee moves its wings one hundred and ninety times a second, and a Butterfly nine times, the House-fly makes three hundred and thirty strokes. A few species are wingless. The eyes are large, with numerous facets. In some forms, as the House- fly, all the mouth-parts, except the labium, are rudimen- tary ; and the labiurn has an expanded tip, by means of FIG. 270.— Metamorphosis of the Flesh-fly (Sarcophaga carnaria) : a, eggs; 6, young maggots just hatched ; c, d, full grown maggots ; e, pupa ; /, imago. which the fly licks up its food. . In other forms, as the Mosquito, the other mouth-parts are present as bristles or lancets, fitted for piercing ; the thorax is globular, and the legs slender. The larvse are footless grubs. The Diptera number' about twenty-four thousand. Among them are the Mosquitoes (Culex) ; Hessian-fly (Cecidomyid), so de- structive to wheat; Daddy-long-legs (Tipula), resembling a gigantic Mosquito ; the wingless Flea (Pulex) ; besides the immense families represented by the House-fly (Mus- 6- fypMyb*** °r terflies" and "Moths," are known chiefly by their four large wings, which are thick- ly covered on both sides by minute, overlapping: scales. Fio.271.— Scales from the Wings of varl- . ous Lepidoptera. 1 he scales are of different ARTHROPODA. 301 .colors, and are often arranged in patterns of exquisite beauty. They are in reality modified hairs, and every family has its partic- ular form of scale. The head is small, and the body cylin- drical. The legs are not used for locomo- tion. All the mouth parts are nearly obso- lete except the maxil- lae, which are fash- ioned into a " probos- cis " for pumping UP Fi«>-2T2.-Part of the Wing of a Moth (Saturnia), * re magnified to show the arrangement of scales. the nectar of flowers. The larvae, called " caterpillars," have a worm-like form, and from one to five pairs of abdominal legs, in addition to the three on the thorax. The mouth is formed for mas- tication, and (ex- cept in the larvae of Butterflies) the lip has a spinneret connected with silk- glands. There are three groups : the gay Butterflies, having knobbed or hooked Fia. ^.-Vanessa polychloros, or » Tortoise-shell But- antenn83, and flying terfly." ' * in the day only ; the dull-colored Sphinges, with antennae thickened in the middle, and flying at twilight ; and the nocturnal Moths, which generally prefer the night, and whose antennae are thread-like and often feathery. Generally, when at rest, the Butterflies keep their wings raised vertically, while 302 COMPARATIVE ZOOLOGY. FIG. 274.— Moth and Larva of Attacus pavoiiia-major. the others hold theirs horizontally. The pupa of the former is unprotected, and is usually suspended by a bit of silk :1M the pupa of the Moths is enclosed in a cocoon. FIG. 275.— Fruit-moth (Carpocapm pomonella) : b, larva ; a, chrysalis ; e, imago. ARTIIROPODA. 303 From twenty-two thousand to twenty-four thousand Lepidopterous species have been identified. Some of the most common Butterflies are the swallow-tail Papilio, the white Pieris, the sulphur- yellow Colias; the Argynnis, with silver spots on the under side of the hind wings ; the Vanessa, with notched wings. The Sphinges exhibit little variety. They have narrow, powerful wings, and are some- times mistaken for Humming- birds. The " potato -worm" is the caterpillar of a Sphinx. The most conspicuous Moths are the large and beautiful Attacus, distinguished by a ^ FIG. 2T6. — Head of a Caterpillar, from triangular, transparent Spot beneath: a, antennae; b, horny jaws; 5 , . c, thread of silk from the conical fneu- m the Centre OI the Wing; Ins, on either side of which are rudi- the white Xombyx, or " silk- mentary palpi" worm ;" the reddish-brown Clisiocampa, whose larva, " the American Tent-caterpillar," spreads its web in many an apple and cherry tree ; the pale, delicate Geometrids ; and the small but destructive Tineids, represented by the Clothes-moth. 7. Hymenoptera, comprising at least twenty-five thou- sand species, include the highest, most social, and, we may add (if we except the Silk-worm), the most useful, of In- sects. They have a large, head, with compound eyes and three ocelli, mouth fitted both for biting and lapping, legs formed for locomotion as well as support, and four wings equally transparent, and interlocking by small hooks during flight. The females are usually provided with a sting, or borer. The larvae are footless, helpless grubs, and generally nurtured in cells, or nests. Such are 304 COMPARATIVE ZOOLOGY. the Honey-bees (Apis), Humble-bees (Bombus), Wasps (Vespa), Ants (Formica)^ Ichneumon-flies, and Gall-flies. Those living in societies exhibit three castes : females, or " queens ;" males, or " drones ;" and neuters, or sexless " workers." There is but one queen in a hive, and she is treated with the greatest distinction, even when dead. She dwells in a large, pear-shaped cell, opening down- ward. She lays three kinds of eggs : from the first come forth workers, the second produces males, and the last females. The drones, of which there are about eight hundred in an ordinary hive, are marked by their great size, their large eyes meeting on the top of the head, and FIG. 277.— Honey-bee (Apis meUifica) : a, female ; 6, worker ; c, male. by being stingless. The workers, which number twenty to one drone, are small and active, and provided with stings, and hollow pits in the thighs, called " baskets," in which they carry pollen. Their honey is nectar elabo- rated in the crop by an unknown process ; while the wax is secreted from the sides of the abdomen and mixed with saliva. There is a subdivision of extra labor : thus there are wax- workers, masons, and nurses. Ants (except the Saiiba) have but two classes of workers. While Ants live in hollow trees or subterranean chambers (called formi- carium), Honey-bees and Wasps construct hexagonal cells. The comb of the Bee is hung vertically, that of the Wasp is horizontal. TUNICATA. 305 FIG. 278.— An Ascidian. Subkingdom VII. — TUNICATA. This small and singular group of animals has relations with the worms on the one hand and with the Vertebrates on the other. The most common forms (the solitary As- tidians) are enclosed in a leathery, elastic bag, one end of which is fastened to the rocks, while the other has two orifices, for the inlet and exit of a current of water for nutrition and res- piration. They are without head, feet, arms, or shell. In- deed, few animals seem more helpless and apathetic than these apparently shapeless be- ings. The tubular heart exhibits the curious phenomenon of reversing its action at brief intervals, so that the blood oscillates backward and forward in the same vessels. Another peculiarity is the presence of cellulose in the skin. The water is drawn by cilia into a branchial sac, an enlargement of the first part of the intestine, whence it escapes through openings in the sides, to the excurrent ori- fice, while the particles of food drawn in with the water are retained and passed into the intestine. The larva is active, swimming by means of a long tail. It looks like a tadpole, and has a notochord and a nervous system closely resembling Fm.279.-Diagramorsim- those of a Vertebrate. Afterwards it at- pieAscidian.- fl,s,bran- taches itself by the head, the tail is ab- chial Bac ; n, nervous « « ganglion ; », stomach ; f, sorbed, and the nervous system is re- iutestine ; o, reproduc- i •• t " « •••. -i. tive organ; A, heart, duced to a single small ganglion. 20 306 COMPARATIVE ZOOLOGY. Subkingdom VIII.— VERTEBKATA. This grand division includes the most perfect animals, or such as have the most varied functions and the most numerous and complex organs. Besides the unnumbered host of extinct forms, there are about twenty-five thousand living species, widely differing among themselves in shape and habits, yet closely allied in the grand features of their organization, the general type being endlessly modified. The fundamental distinctive character of Vertebrates is the separation of the main mass of the nervous system from the general cav- ity of the body. A transverse section of the body exhibits two cavities, or tubes — the dorsal, containing the cerebro - spinal nervous system ; the ventral, in- closing the alimentary canal, heart, lungs, and a double chain of gan- glia, or sympathetic system. This ven- tral, or haemal, cavity corresponds to the FIG. 280.— Ideal Plans of the Snbkingdoms. r, J transverse section of vertebrate type ; v, the whole body OI an In- same, inverted. Jf, transverse section of mol- , •« V. *1 *T» luscons type ; and Md. of molluscoid. A and Vertebrate ', while the Ad, transverse sections of articulate type, high r1/-,rcol rtT* npiiral IB and low. C, longitudinal section of coeleute- aorsal> °r DCUrai, ] rate type; a, alimentary canal; e, body-cavity, added. In the other figures, the alimentary canal is shaded, the heart is black, and the nervous Vertebrates are alSO distinguished by an in- ternal, jointed skeleton, endowed with vitality, and capa- ble of growth and repair. During embryo-life it is rep- resented by the notochord ; but this is afterwards replaced VERTEBRATA. 307 by a more highly developed vertebral column of cartilage or bone. The column and cranium are never absent in the craniota ; other parts may be wanting, as the ribs in Frogs, limbs in Snakes, etc.154 The limbs are never more than four, and are always articu- lated to the haemal side of the body, while the legs of Inver- tebrates are developed from the neural side. The muscles moving the limbs are attached to the endoskeleton. The circulation of the blood is complete, the arteries being joined to the veins by capil- laries, so that the blood never escapes into the visceral cav- ity as in the Invertebrates. All have a portal vein, carry- ing blood through the liver; all have lacteals and lym- phatics. The blood is red, and contains both kinds of corpuscles.1" The teeth are developed from the dermis, never from the cuticle, as in Mollusks and Articulates ; the jaws move vertically, and are never modified limbs. The Flo.281._Diagrara of Circnlation ln Hver and kidneys are always the higher Vertebrates : 1, heart; 2, m, lungs; 3, head and upper extremities; present. Ihe respiratory Or- 4, spleen; 5, intestine; 6, kidney; 7, gans are either gills or lungs, 308 COMPARATIVE ZOOLOGY. or both. Vertebrates are the only animals which breathe through the mouth. The nervous system has two marked divisions : the cerebro-spinal, presiding over the functions of animal life (sensation and locomotion) ; and the sympathetic, which partially controls the organic functions (digestion, respi- ration, and circulation). In no case does the gullet pass through the nervous system, as in Invertebrates, and the mouth opens on the side opposite to the brain. Probably none of the five senses are ever altogether absent. The form of the brain is modified by the relative development of the various lobes. In the lower Vertebrates, the cere- bral hemispheres are small — in certain Fishes they are actually smaller than the optic lobes — in the higher, they nearly or quite overlap both olfactories and cerebellum. The brain may be smooth, as in most of the cold-blooded animals, or richly convoluted, as in Man. There is no skull in Amphioxus. In the Marsipo- branchii and Elasmobranchii it is cartilaginous. In other fishes it is cartilage overlaid with bone. In Amphibians and Reptiles, it is mingled bone and cartilage. In Birds and Mammals, mainly or wholly bony. The human skull contains fewer bones than the skull of most animals, ex- cepting Birds. The skull of all Vertebrates is divisible into two regions : the cranium, or brain-case, and the face. The size of the cranial capacity, compared with the area of the face, is generally the ratio of intelligence. In the lower orders, the facial part is enormously predominant, the eye-orbits are directed outward, and the occipital con- dyles are nearly on a line with the axis of the body. In the higher orders, the face becomes subordinate to the cranium, the sensual to the mental, the eyes look forward, and the condyles approach the base of the cranium. Com- pare the " snouty " skull of the Crocodile and the almost vertical profile of civilized Man. A straight line drawn VERTEBRATA. 309 from the middle of the ear to the base of the nose, and another from the forehead to the most prominent part of the upper jaw, will include what is called the facial an- gle, which roughly gives the relation between the two re- gions, and therefore the rank of the animal.168 In the cold-blooded Vertebrates the brains do not fill the cranium ; while in Birds and Mammals a cast of the cranial cavity well exhibits the general features of the cerebral surface.167 All Vertebrates are single and free. Mammals bring forth their young alive, having directly nourished them from the mother before birth (viviparous). In almost all the others the nourishment is laid up in the egg, which is laid before hatching (oviparous), or is retained in the mother until hatched (ovoviviparous), as in some Reptiles and Fishes. There are two great divisions of the subkingdom, Acrania and Craniota, or Vertebrates without skulls and those with skulls. The Craniota are divided into five great classes : Fishes, Amphibians, Reptiles, Birds, and Mammals. The first three are "cold-blooded," the other two are "warm- blooded." Fishes and Amphibians have gills during the whole or a part of their lives, while the rest never have gills. Fishes and Amphibians in embryo have neither arnnion nor allantois, while the other three are provided with both. There are three provinces of skull-bearing Vertebrates. Fishes and Amphibians agree in having gills, in want- ing amnion and allantois, and in possessing nucleated red blood-corpuscles (Ichthyopsida). Birds and Reptiles agree in having no gills, but both amnion and allantois, in the articulation of the skull with the spine by a single condyle, in the development of the skin into feathers or scales, and in circulating oval, nucle- ated, red corpuscles (Sauropsidd). Mammals differ from Birds and Reptiles in having two 310 COMPARATIVE ZOOLOGY. occipital condyles, and their blood-corpuscles are not nu- cleated IM (Mammalia). DIVISION I. — Acrania. Vertebrates without a skull. £ CLASS. — Pharyngobranchii. 1*1' The Acrania are represented by 3_ the singular animal Amphioxus or n Lancelet. It is about two inches long, jj semi-transparent, without skull, limbs, brain, heart, or red blood-corpuscles. \ . It has for a skeleton a notochord only. \ It breathes by very numerous gill arches, without fringes, and the water \ is drawn in by cilia, which line the f • gill slits. The embryo develops into f§ a gastrula closely resembling that of | § the Invertebrates. The animal lives ! J in the sandy bottom of shallow parts .E of the ocean, and has been found in ! * the Mediterranean Sea, in the Indian i *- Ocean, and on the east coast of North ; « and South America. DIVISION II. — Craniota. Vertebrates with a distinct skull. \ CLASS I. — Pisces. .£ Fishes are the lowest of Verte- :«' brates. They fall far behind the rest ^•J in strength, intelligence, and sensi- " « bility. The eyes, though large, are | almost immovable, bathed by no tears, s and protected by no lids. Dwelling in the realm of silence, ears are little VEUTEBRATA. 311 needed, and such as they have are without external parts, the sound being obliged to pass through the cranium. Taste and smell are blunted, and touch is nearly confined to the lips. The class yields to no other in the number and variety of its forms. It includes nearly one half of all the ver- tebrated species. So great is the range of variation, it is difficult to frame a definition which will characterize all the finny tribes. It may be said, however, that Fishes are the only backboned animals having median fins (as dorsal and anal) supported by fin-rays, and whose limbs (pectoral and ventral fins) do not exhibit that threefold division (as thigh, leg, and foot) found in all other Vertebrates.169 The form of Fishes is admirably adapted to the element in which they live and move. Indeed, Nature nowhere presents in one class such elegance of proportions with such variety of form and beauty of color. The head is ABC Fia. 283.— Scales of Fishes : A, cycloid scale (Salmon) ; B, ctenoid scale (Perch) ; C, placoid scale (Ray) ; D, ganoid scales (Aniblypterus) — a, upper surface ; 6, und«r surface, showing articulating processes. ' disproportionately large, but pointed to meet the resist- ance of the water. The neck is wanting, the head be- ing a prolongation of the trunk. The viscera are closely packed near the head, and the long, tapering trunk is left free for the development of muscles which are to move the tail — the instrument of locomotion. The biconcave vertebrae, with intervening cavities filled with elastic gel- atine, are designed for rapid and versatile movements. The body is either naked, as in^the Lamprey, or covered with 312 COMPARATIVE ZOOLOGY. • polished, overlapping scales, as in the Perch. Rarely, as in the Sturgeon, it is defended by bony plates, or by minute, hard spines, as in the Shark. Scales with smooth, circular outline are called cycloid; those with notched or spiny margins are ctenoid. Enameled scales are ganoid, and those with a sharp spine, like those of the Shark, are •placoid. The vertical fins (dorsal, anal, and caudal) are peculiar to Fishes. The dorsal vary in number, from one, as in the Herring, to three, as in the Cod ; and the first dorsal may be soft, as in the Trout, or spiny, as in the Perch. Fio. 284.— Blue-fish (Temnodan saltator). All seas. If the dorsals are cut off, the Fish reels to and fro. The caudal may be homocercal, as in ordinary species ; or het- erocercal, as in Sharks. In ancient heterocercal Fishes, the tail was frequently vertebrated. The pectoral and ventral fins stand for the fore and hind limbs of other Vertebrates. As the specific gravity of the body is greater than that of the water, most Fishes are provided witli an air-bladder, which is an outgrowth from the oesopha- gus. This is absent in such as grovel at the bottom, as the Rays, and in those, like the Sharks, endowed with compensating muscular power. Fishes have no prehensile organ besides the mouth. Both jaws are movable. The teeth are numerous, and VERTEBKATA. 313 may be recurved spines, as in the Pike ; flat and triangu- lar, with serrated edges, in the Shark ; or flat and tessel- lated in the Eay. They feed principally on animal mat- ter. The digestive tract is relatively shorter than in other Vertebrates.169 The blood is red, and the heart has rarely more than two cavities, an auricle and a ventricle, both on the venous side. Ordinary Fishes have four gills, which are covered by the operculum, and the water escapes from an opening behind this. In Sharks there is no operculum, FIG. 2S5.— Salmon (Salmo salar). Both hemispheres. and each gill opens separately. The brain consists of sev- eral ganglia placed one behind the other, and occupies but a small part of the cranial cavity. Its average weight to the rest of the body may be as low as 1 to 3000. The eggs of bony Fishes are naked and multitudinous, some- times numbering millions in a single spawn ; those of the Sharks are few, and protected by a horny shell. There are about thirteen thousand species of Fishes, of which over two thirds are Teleostei. There are two sub- classes of Pisces. 314: COMPARATIVE ZOOLOGY. FIG. 286. — Lamprey (Petromyzon Americanus). turtle. SUBCLASS I.-^Marsipobranchii. The Lampreys and Hag-fish have a persistent noto- chord, a cartilaginous skull, no lower jaw, round, suctorial mouth, horny teeth, one nasal -organ, no scales, limbs, or gill- arches. The gills are pouch -like (whence the name of the class), and open separately. They are found both in salt and fresh water. SUBCLASS II. — Pisces Proper. The true Fishes have two nasal organs, and well-devel- oped jaws and gill-arches. There are four orders : 1. Elasmobranchii, having a cartilaginous skeleton, and a skin naked or with placoid scales. The gill-openings are uncovered ; and the mouth is generally under the head. The ventral fins are placed far back ; the pectorals are large, in the Kays enormously developed ; and the tail is heterocercal. Such are the Sharks, Kays, and Chimaera. Fio. 28T. -Shark (Carcharias vulgaris). Atlantic. VERTEBRATA. 315 They are all marine. The largest Shark found, and there- fore the largest Fish, measured forty feet in length. FIG. 2SS. — Thoruback (Raia clavata). European seas. 2. Ganoidei, distinguished by their enameled scales or bony plates. The endoskeleton is usually not completely ossified ; the ventral fins are placed far back ; and the tail is generally heterocercal. The gills are like those of the bony Fishes, and the air-bladder has a duct, and may aid in respiration. This was one of the largest orders in old geological history. The few modern representatives, as the Sturgeon, Gar-pike, Mud (or Dog) Fish, and Polyp- terus, are essentially fresh-water. 3. Teleostei, in- cluding all the com- mon Fishes having a bony endoskeleton FIG. 289.— Gar-pike (lepidosteusosseus). LakeOntario 316 COMPARATIVE ZOOLOGY. FIG. 290 Sturgeon (Acipenser sturio). Atlantic coast. and a scaly exoskeleton. The skull is extremely com- plicated ; the upper and lower jaws are complete, and the gills are comb -like or tufted. The tail is homocercal ; the other fins are varia- ble in number and FIG. 291.— Cat-Hsu, or Homed Pout (Pimdodw catus). position. In the American rivers. - TT i soft -tinned lishes, the ventrals are ab- sent, as in the Eels ; or attached to the abdomen, as in the Salmons, Herrings, Pikes, and Carps; or placed under the throat, as in the Cod, Haddock, and Flounder. In the spiny- h'nned Fishes, the ventrals are generally under or in front of the pectorals, and the scales ctenoid, as in the Perches, Mullets, and Mackerels. 4. Dipnoi. These Fishes connect the class with the Amphibia. They have an eel -like body, covered with cycloid scales ; an embryonic notochord for a back-bone ; FIG. 292.— Cod (Morrhua Americana). Atlantic coast. PIG. 293. — Protopteru* annectens; one fourth natural me. African rivers. VEETEBRATA. 317 long, ribbon-like pectoral and ventral fins, set far apart ; two auricles, and one ventricle ; and, besides gills, a cellu- lar air-bladder, which is used as a lung. The representatives are Ceratodus from Australia, Pro- topterus from Africa, and Lepidosiren from Brazil. CLASS II. — Amphibia. These cold - blooded Vertebrates are distinguished by having gills when young, and true lungs when adult. They have no fin-rays, and the limbs, when present, have the same divisions as those of higher animals. The skin is soft, and generally naked, and the skeleton is ossified. The skull is flat, and articulates with the spinal column by two condyles. There is no distinct neck; and the ribs are usually small or wanting. The heart consists of two auricles and one ventricle. All undergo metamorphosis upon leaving the egg, passing through the " tadpole" state. They commence as water-breathing larvae, when they re- semble Fishes in their respiration, circulation, and locomo- tion. In the lowest forms, the gills are retained through life ; but all others have, when mature, lungs only, the gills disappearing. The cuticle is frequently shed, the mode varying with the habits of the species.161 The com- mon Frog, the type of this class, stands intermediate be- tween the two extremes of the vertebrate series ; no fun- damental part is excessively developed. There are about four hundred and fifty liv- ing species, grouped in four orders : 1. Urodela have a naked skin, a tail, and two or four limbs. Some retain their gills through life, as the ^^-HW-ndffl^mo**^ Cnyt.ga 318 COMPARATIVE ZOOLOGY. Proteus of Austria, Menobranchus of tlie eastern United States, and the two-legged Mud-eel (Siren) of South Car- olina. Others drop their gills, and always have four limbs, as the aquatic Newts and land Salamanders.1" The fore limbs first make their appearance in the tadpole. 2. Labyrinthodontia, now extinct, resembled gigantic Salamanders, except in their complex teeth and exoskele- ton of bony plates. 3. Ccecilia have neither tail nor limbs, a snake-like form, FIG. 296.— Proteus anguinus. Europe. minute scales in the skin, and well-developed ribs. They are confined to the tropics. 4. Batrachia include all the well-known tailless Am- phibians, as Frogs and Toads. They have a moist, naked skin, ten vertebrae, and no ribs. As they breathe by swallow- ing the air, they can be suffocated by holding the mouth open. They have Fio. 296.— Red Salamander (Paeudotriton ruber). United States. VEKTEBRATA. 319 four limbs — the hinder the longer, and the first developed. They have four fingers and five toes. The tongue is long, and, fixed by its an- terior end, it can be rapidly thrown out as an organ of prehen- sion.183 The eggs are laid in the water en- veloped in a glairy mass ; and the tadpoles Fia' ™-Fl°s (Rana)' resemble the Urodelans, till both gills and tail are absorbed. Frogs (Rana) have teeth in the upper jaw, and webbed feet ; Toads (-Bufd) are higher in rank, and have neither teeth nor fully webbed feet. The former have been known to live sixteen years, and the latter thirty-six. CLASS III. — Reptilia. These air-breathing, cold-blooded Vertebrates are dis- tinguished from all Fishes and Amphibians by never hav- ing gills, and from Birds by being covered with horny scales or bony plates. The skeleton is never cartilaginous ; and the skull has one occipital condyle. The vertebrae are ordinarily concave in front ; and the ribs are well devel- oped. With few exceptions, all are carnivorous ; and teeth are always present, except in the Turtles, where a horny sheath covers the jaws. The teeth are never fastened in sockets, except in Crocodiles. The jaws are usually very wide. The heart has three chambers, save in Crocodiles, where the ventricle is partitioned. But in all cases a mixture of arterial and venous blood is circulated. The lungs are large, and coarsely cellular. The limbs, when present, are provided with three or more fingers as well as toes. There are about fifteen hundred species and four orders of living Reptiles : the first two have horny 320 COMPARATIVE ZOOLOGY. scales, the others have bony plates combined with 1. Ophidia, or Snakes, are characterized by the absence of visible limbs;164 by the great number of vertebrae, amounting to over four hundred in the great Serpents; by a corresponding number of ribs, but no sternum ; and 110 true eyelids, the eyes being covered with a transparent FIG. 298.— Adder, or Viper (Pelias berus). England. skin. The tongue differs from that of nearly all other Reptiles in being bifid and extensile. The mouth is very dilatable. The skin is frequently shed, and always by re- versing it. Snakes make their way on land or in water with equal facility. As a rule, the venomous Snakes, as Vipers and Rattle- snakes, are distinguished by a triangular head covered with small scales ; a constriction behind the head; two or more fangs, and few teeth; small eyes, with vertical pupil; and short, thick tail. In the harmless Snakes, the head gradu- ally blends with the neck, and is covered with plates ; the teeth are comparatively numerous in both jaws ; the pu- VERTEBRATA. 321 FIG. 299.— a, Head of a Harmless Snake (upper view) ; b, heads of various Venomous Snakes. pil is round, and the tail tapering. This rule, however, has many exceptions. 2. Lacertilia, or Lizards, may be likened to Snakes pro- vided with four limbs, each having five digits.1" The body is covered with horny scales. All have teeth, which are simple in structure ; and the halves of the lower jaw are firmly united in front, while those of Snakes are FIG. 300.— Lizard (Lacerta). 21 322 COMPARATIVE ZOOLOGY. loosely tied together by ligaments. Nearly all have a breast -bone, and the eyes (save in the Gecko) are fur- nished with movable lids. In the common Lizards and Chameleon, the tongue is extensile. The tail is usually long, and in some cases each caudal vertebra has a divis- ion in the middle, so that the tail, when grasped, breaks off at one of these divisions. The Chameleon has a pre- hensile tail. The Iguana is distinguished by a dewlap on the throat and a crest on the back. Except some of the Monitors of the Old "World, all the Lizards are terrestrial. 3. Chelonia, or Tortoises and Turtles, are of anomalous structure. The skeleton is external, so as to include not only all the viscera, but also the whole muscular system, which is attached internally; and even the limbs are Fio. 301.— Hawk's-bill Turtle (Eretmochelys imbricata). Tropical Atlantic. inside, instead of outside, the thorax. The exoskeleton unites with the endoskeleton, forming the carapace, or case, in which the body is enclosed. The exoskeleton con- sists of horny plates, known as "tortoise-shell" (in the soft Tortoises, Trionyx, this is wanting), and of dermal VEKTEBRATA. 323 bones, united to the expanded spines of the vertebrae and to the ribs, making the walls of the carapace. The ven- tral pieces form the plastron, or ster- num.188 All are toothless. There are always four stout legs; and the order furnishes the only ^j%^C~ examples of Yerte- FlG. 80S.— Box-tortoi8e~(CTWi«to Virginea). United brates lower than states. Birds that really walk, for Lizards and Crocodiles wrig- gle, and drag the body along. There are no teeth, but a horny beak. The eggs are covered with a calcareous shell. The Sea -turtles, as the edible Green Turtle and the Hawk's -bill Turtle, which furnish the " tortoise - shell " of commerce, have the limbs converted into paddles. The fresh -water forms, represented by the Snapping Turtle (Chdydra), are amphibious, and have palmated feet. Land Tortoises (Testudo) have short, clumsy limbs, fitted for slow motion on the land ; the plastron is very broad, and the carapace is arched (while it is flattened in the aquatic species), and head, legs, and tail can be drawn within it. The land and marine species are vegetable-feeders ; the others, carnivorous. 4. Orocodilia, the highest and largest of Reptiles, have also two exoskeletons — one of horny scales (epidermal), and another of bony plates (dermal). The bones of the skull are firmly united, and furnished with numerous teeth, im- planted in distinct sockets. The lower jaw extends back of the cranium. The heart has four cavities, but the pul- monary artery and aorta communicate with each other, so that there is a mixture of venous and arterial blood. They have external ear-openings, closed by a flap of the 324 COMPARATIVE ZOOLOGY. skin, and eyes with movable lids ; a muscular gizzard ; a long, compressed tail ; and four legs, with feet more or less webbed, and having five toes in front and four be- hind. The existing species are confined to tropical rivers, and are carnivorous. The eggs are covered with a hard shell. There are three representative forms : the Gavial of the Ganges, remarkable for its long snout and uniform teeth ; the Crocodiles, mainly of the Old World, whose teeth are unequal, and the lower canines fit into a notch in the edge of the upper jaw, so that it is visible when the mouth is FIG. 303.— Alligator (A. Mis8i8Sippiensis). Southern States. closed ; and the Alligators of the New "World, whose ca- nines, in shutting the mouth, are concealed in a pit in the upper jaw. The toes of the Gavials and Crocodiles are webbed to the tip ; those of the Alligators are not more than half-webbed. In the mediaeval ages of geological history, the class of Keptiles was far more abundantly represented than now. Among the many forms which geologists have unearthed are numerous gigantic Saurian s, which cannot be classi- fied with any of the four living orders. Such are the Ichthyosaurus, Plcsiosaurus, Pterodactyle, Megalosaurus, and Iguanodon. VERTEBRATA. 325 CLASS IV. — Aves. Birds form the most clearly defined class in the whole Animal Kingdom. The Eagle and Hummer, the Ostrich and Duck, widely as they seem to be separated by size, form, and habits, still exhibit one common type of struct-, ure. On the whole, Birds are more closely allied to Rep- tiles than to Mammals. In number, they approach the Fishes, ornithologists having determined eight thousand species, or more. A Bird is an air-breathing, egg-laying, warm-blooded, feathered Vertebrate, with two limbs (legs) for perching, walking, or swimming, and two limbs (wings) for Hying or swimming. Organized for flight, it is gifted with a light skeleton, very contractile muscular fibre, and a res- piratory function of the highest development. The skeleton is more compact than those of Keptiles and Mammals, at the same time that it is lighter, and the bones are harder and whiter. It contains fewer bones than usual, many parts being anchylosed together, as the skull-bones, the dorsal vertebrae, and bones of the tarsus and metatarsus. The lumbar vertebrae are united to the ilia. The neck is remarkably long (containing from nine to twenty-four vertebrae) and flexible, enabling the head to be a most perfect prehensile organ. The ribs are gen- erally jointed in the middle, as well as with the backbone and sternum. The last, where the muscles of flight orig- inate, is highly developed. The skull articulates with the spinal column by a single condyle, and with the lower jaw, not directly, as in Mammals, but through the inter- vention of a separate bone, as in Reptiles. All Birds always have four limbs, while every other vertebrate class shows exceptions. The fore-limbs are fit- ted for flight. They ordinarily consist of nine separate bones, and from the hand, fore-arm, and humerus are de- 326 COMPARATIVE ZOOLOGY. veloped the primary, secondary, and tertiary feathers of the wing. The hind-limbs are formed for progression — walking, hopping, running, paddling, and also for perch- ing and grasping. The modifications are more numerous and important than those of the bill, wing, or tail. There are twenty bones ordinarily, of which the tibia is the prin- cipal; but the most characteristic is the tarso-metatarsus, which is a fusion of the lower part of the tarsus with the meta- tarsus. The rest of the tarsus is fused with the tibia. The thigh is so short that the knee is never seen out- side of the plumage ; the first joint visible is the heel.167 Most Birds have four toes (the external or "lit- d c b tle" toe is always 9 f FIG. 304.— Principal Parts of a Bird : a, primaries ; 6, secondaries ; c, spurious wing ; d, wing-coverts ; «, tertiaries ; /, throat, or jugulum ; g, chin; h, Wj,ntn'no.\ . mnnv bill; the meeting-line between the two mandi- WanungJ , mdliy bles is the commissure; the ridge on the upper fllrpp flip hnlltiv nr mandible is called culmen; that of the lower, tUree' tl16 "c *S £ 1 2 0) ^ p o> 'O s Q ^ 1 0 7 M I J! 1 i f 3 OS •^ ^, u & 13 S 1 | i i "? £ cT 9 X 5 S 1^ 02 o 1 £ 0 £ o Hi £ er of \ fental \ ity of \, NOTES. 389 cerebrum. As a rule, also, the addition of the power to reason comes in with the addition of a cerebrum, and is proportioned to its development. Between the lowest Vertebrate and Man, therefore, we observe successive types of intelligence. Intelligence, however, is not according to the size of the brain (else Whales and Elephants would be wisest), but rather to the amount of gray matter in it. A honey-comb and an Oriole's nest are con- structed with more care and art than the hut of the savage. It is true, this is no test of the capability of the animal in any other direction ; but when they are fashioned to suit circumstances, there is proof of intelligence in one direction. 104 An exception to the general rule that the smaller animals have more acute voices. 105 It is wanting in a few, as the Storks. 06 The Nightingale and Crow have vocal organs similarly constructed, yet one sings and the other croaks. 107 These cells are detached portions of the parental organisms. Gener- ally, these two kinds of cells are produced by separate sexes ; but in some cases, as the Snail, they originate in the same individual. Such an animal, in which the two sexes are combined, is called an hermaphrodite, 108 The eggs of Mammals are of nearly uniform size ; those of Birds, Insects, and most other animals are proportioned to the size and habits of the adult. Thus, the egg of the ^pyornis, the great extinct bird of Mada- gascar, has the capacity of fifty thousand Humming-birds' eggs. 109 As a general rule, when both sexes are of gay and conspicuous colors, the nest is such as to conceal the sitting Bird ; while, whenever there is a striking contrast of colors, the male being gay and the female dull, the nest is open. Such as form no nest are many of the Waders, Swimmers, Scratch- ers, and Goatsuckers. 110 This lies at first transversely to the long axis of the egg. As the chick develops, it turns upon its side. 111 The blood appears before the true blood-vessels, in intercellular spaces. It is at first colorless, or yellowish. 112 Exactly as the blood in the capillaries of the lungs is aerated by the external air. 113 Thus, the hollow wing-bone was at first solid, then a marrow-bone, and finally a thin-walled pneumatic bone. The solid bones of Penguins are ex- amples of arrested development. 114 The thigh-bone ossifies from five centres. The bone eventually unites to one piece. 115 Muscle is mainly fibrine and myosin, while nerve is neurin. 116 For this reason, Mammals are called viviparous ; but, strictly speaking, they are as oviparous as Birds. The process of reproduction is the same, whether the egg is hatched within the parent or without. The eggs of Birds contain whatever is wanted for the development of the embryo, except heat, which must come from without. Mammals, having no food-yolk, obtain their nutrition from the blood of the parent, and after birth from milk. 117 The larvae of Butterflies and Moths are called caterpillars; those of Beetles, grubs ; those of Flies, maggots; those of Mosquitoes, wigglers. The 390 NOTES. terms larva, pupa, and imago are relative only; for, while the grub and cat- erpillar are quite different from the pupa, the bee-state is reached by a very gradual change of form, so that it is difficult to say where the pupa ends and the imago begins. In fact, a large number of Insects reach maturity through an indefinite number of slight changes. The Humble-bee moults at least ten times before arriving at the winged state. 118 Every tissue of the larva disappears before the development of the new tissues of the imago is commenced. The organs do not change from one into the other, but the new set is developed out of formless matter. The pupa of the Moth is protected by a silken cocoon, the spinning of which was the last act of the larva ; that of the Butterfly is simply enclosed in the dried skin of the larva, which is called chrysalis because of its golden spots. The pupa of the Honey-bee is called nymph ; it is kept in a wax-cell lined with silk, spun by the nursing-bee, not by the larva. The time required to pass from the egg to the imago varies greatly : the Bee consumes less than twenty days, while the Cicada requires seventeen years. 119 Compare the amount of food required in proportion to the bulk of the body, and also with the amount of work done, in youth, manhood, and old age. 180 Excepting, perhaps, that the new tail of a Lizard is cartilaginous. m The patella, or knee-pan, has no representative in the fore-limb, and. strictly, it belongs to the muscular system, rather than to the skeleton. Some anatomists contend that the great toe is homologous with the little finger, in- stead of the thumb. ua "The structure of the highest plants is more complex than is that of the lowest animals ; but, for all that, powers are possessed by Jelly-fishes of which oaks and cedars are devoid." — MIVART. m It is, however, true that the number of eggs laid is proportioned to the risk in development. 124 According to Mr. Darwin, the characters which naturalists consider as showing true affinity between two or more species are those which have been inherited from a common parent ; and, in so far, all true classification is gene- alogical ; i. e., it is not a mere grouping of like with like, but it includes like descent, the cause of similarity. In the existing state of science, a perfect classification is impossible, for it involves a perfect knowledge of all animal structure and life's history. As it is, it is only a provisional attempt to ex- press the real order of nature, and it comes as near to it as our laws do in explaining phenomena. It simply states what we now know about compar- ative anatomy and physiology. As science grows, its language will become more precise and its classification more natural. 188 The term type is also used to signify that form which presents all the characters of the group most completely. Each genus has its typical species, each order its typical genus, etc. The word is also applied to the specimen on which a new species is founded. A persistent type is one which has con- tinued with very little change through a great range of time. The family of Oysters has existed through many geological ages. 186 The Goalenterata and Echinodermata together make up the Radiata, the old subkingdom of Cuvier. JEchinoderma is probably more correct than Echinodermata, but we retain the old orthography. NOTES. 391 117 Strictly speaking, no individual is independent. Such is the division of labor in a hive, that a single Bee, removed from the community, will soon die, for its life is bound up with the whole. An individual repeats the type of its kingdom, subkingdom, class, order, family, genus, and species, through its whole line of descent. 128 These definitions of the various groups are mainly taken from Agassiz. They are not practically very useful, as they are not used by working natu- ralists. The kind and degree of difference entitling a group to a particular rank varies greatly with the naturalist, and the part of the Animal Kingdom where the group is found. Some families of Insects are separated by gaps less than those which divide genera of Mammals. 129 The Millepore coral, so abundant in the West Indian Sea, is the work of Hydroids. The surface is nearly smooth, with minute punctures. Ge- genbauer, Haeckel, and others hold that the Acalephs have no body-cavity at all, the internal system of canals being homologous with the intestiiuJ cavity of other animals. 130 This digestive cavity is really homologous to the proboscis of the Jelly- fish, turned in. It is lined with ectoderm. The "body-cavity" is not really such, but homologous to the digestive sac of the Hydra. 131 Among the exceptions are Tubipora, which have eight tentacles and no septa, and the extinct Cyathophylla, whose septa are eight or more. 132 The longer septa (called primary) are the older; the shorter, secondary ones, are developed afterwards. As a rule, sclerodermic corals are calcare- ous, and a section is star-like ; the sclerobasic are horny and solid. The latter are higher in rank. 133 Some Star-fishes (Solaster) have twelve rays. In all Echinoderms, probably, sea-water is freely admitted into the body-cavity around the viscera. 134 The shell is not strictly external, like the crust of a Lobster, but is coated with the soft substance of the animal. 135 Six hundred pieces have been counted in the shell alone, and twelve hundred spines. The feet number about eighteen hundred. They can be protruded beyond the longest spines. 136 The classification of this edition may be compared with that of the for- mer by the following table, in which the order of the groups is altered to show the relation more easily. Former Edition. Present Edition. Subkingdom. Class. Class. Subkingdom. f 4. Lamellibranchi ata. Do. M V. | 5. Gasteropoda. Do. 2- MOI.LUSCA. III. 1 G. Cephalopoda. Do. 8.J VII. MOLLUSCA. j 3. Tunicata TDNICATA. 2. Brachiopoda. Do. 4-1 (_ 1. Polyzoa. Do. 5. 1 fl. Annelida = f 1. Platyhelminthes. ! J 2. Nematelminthes. f 1 3. Rotifera. IV. VERMES. IV. ARTICULATA. , Manual of Mollusca. PACKARD, Guide to the Study of Insects. DUNCAN, Transformations of Insects. STORKB, Fishes and Reptiles of Massachu- setts. COCKS, Key to North American Birds. JOBDAN, Popular Key to the Birds, etc., of Northern United States. BAIBD, BEKWKE, and RIDQWAT, Birds of North America. BAIBD, Mammals of North America. AI.T.EN, Mammalia of Massachusetts. SOAMMON, Marine Mammals of North Pa- cific. PKSOIIEI,, The Races of Man. MAKBII, Man and Nature. Tvt.oB, Primitive Culture. NIOIIOI.SON, Palaeontology. Of serial publications, the student should have access to the American Naturalist, American Journal of Science, Popular Science Monthly, Smith- sonian Contributions and Miscellaneous Collections, Bulletins and Proceed- ings of the various societies, Annals and Magazine of Natural History, and Nature. The following German works are recommended as having no English equivalents: Ci.Ars, Grundzuge der Zoologie. PAYENBTKOUKK, Allgemeine Zoologie. BBONN, Classen und Orduungeu desThier- Also the periodicals — Zoologischer Anzeiger. reichs (unfinished and expensive, bnl indispensable to the working zoolo- gist). Biologisches Centralblatt. INDEX. In the Index the numbers in Roman type (21) refer to pages ; those in bold-faced type (40) refer to cuts. No attempt is made to analyze the statements made for each group in Part II. Reference is made for each class or prominent order to those cuts in Part I. which illustrate the group. ABSORPTION, Invertebrates, 94. Alimentary Canal, Mammals, 85. Vertebrates, 94, 60, 61. " " microscopic anatomy Acalephae, 247. of, 67, 58. see Jelly-fish. " Mollusks, 80. Acarina, 28T, 258. " " Protozoa, 74. Acarus, 287. " Spiders, 79. Acetabulnm, 147. " " stomach, 87. Acipenser, 315, 290. " " structure of, 89. Acorn-shell, 2S4, 254. " " see Intestine, Month, " see Barnacle. Stomach, Teeth, Acrania, 309, 310, 282. Allantoidea, 393. Act naria, 251, 199, 201-206. Allantois, 117, 203, 169-171. Actinoid Polyp, 251, 199. Alligator, 324, 303. " anatomy of, 38, 95, 198. " nest, 196. " blood of, 97. Alternate generation, 212. development of, 205, 208. Ambulacra, 131. liver-cells of, 124. Ammonite, 279. mouth of, 55, 199. Amnion, 202, 170, 171. nettle-cells of, 51. Amoeba, 241, 185. prehension of, 51. " conjugation of, 196. reproduction of, 192. " ectosarc of, 75. respiration of, 112. " feeding of, 55. skeleton of, 130, 95. " locomotion of, 154, 157. pkin of, 127. Amphibia, 317, 63, 65, 76, 85, 87, 294- Add r, 320, 298. 296. Adipose Tissue, 3S, 10. " blood of, 99, 63-65. ^Eolis, 274. " brain of, 172, 140. Air-bladder, 117. " circulation of, 108, 76. Albatross, 330. " lungs of, 118. Albumen, 19. " mouth of, 01. Alcyonaria, 256, 200, 207, 208. " see Frog. Alcyonium, 256, 208. Amphicoelons, 386. Alimentary Canal, 74. Amphioxus, 310, 282. " " Coelenterata, 75. " feeding of, 50. " " Crustacea, 76. " skeleton of, 139. " development of, 203. Amphithoe, 284, 252. " " duodenum, 90. Analogy, 218. " " Echiuoderms, 76. Anchylosis, 143. " Fishes, 80. Animal, defined, 22. " " lusects, 78. Animalcule, see Protozoa. 4:00 INDEX. Annelides, 268, 17, 228. Bed-bug, 297. Anodon, 78 ; see Clam. Bee, 303, 277. Anoura, 318. " alimentary canal of, 42. Ant, 304. " eggs of, 195. Ant-eater, 344, 333. " eye of, 155. Antennae, 17T, 147. " instincts of, 183. Anthozoa, 250, 38, 95, 198, 208. " mode of feeding of, 50. Ape, 357, 120, 353-357. " month of, 59, 22. Apis, 304, 277. " section of, 81. Aplysia, 274. " temperature of, 121. Apteryx, 327. " see Hymenoptera, Insecta. Arachnida, 287. Beetle, 297, 267, 268. " see Centipede, Scorpion, Spi- " alimentary canal of, 41. der. " development of, 297, 267. Araneina, 289, 18, 25, 260, 261. " eye of, 182, 156. " see Spider. " month of, 67. Ardea, 332, 313- " skeleton of, 292, 262. Arenicola, 113, 77. " see Coleoptera, Insecta. Areolar Tissue, 35, 8. Belemnite, 281. Argouauta, 280, 249. Bernicla, 810. Armadillo, 135, 344, 101, 884. Beroe, 257. • Artery, 104. Bile, 98. Arthropoda, 281. Bird-of-Paradise, 339. ' blood of, 99. Birds, 325, 304-328. ' development of, 205. " alimentary canal of, 84, 60. ' number of, 221. " anatomy of, 50, 804. 1 skin of, 12T. " beak of, 54. ' see Crab, Insect, Lobster, My- " blood corpuscles of, 99, 6k rinpoda, Spider. " brain of, 141. Ascidian, 305, 278, 279. " breathing of, 119. " circulation of, 107. " circulation of, 107, 76. " month of, 60. " distribution of, 378. " skin of, 127. " drinking of, 50. Astacus, 283, 250. " egg of, 193, 162. Asterias, 200. " embryo of, 170. " see Starfish. " eye of, 184. Asteroidea, 258, 126, 183, 212, 213. " feather of, 137, 204. 106. Astrsea, 252, 208. " flight of, 160, 125. Astrophyton, 260. " gizzard of, 84, 384, 50. Atavism, 216. " heart of, 109. Attacns, 302, 274. " locomotion of, 166. Auger-shell, 270, 288. " Inngsof.llS, 50. Auk, 329. " month of, 02. Aurelia, 24S, 195. " skeleton of, 143, 116. " fee Jelly-fish. " smell of, 118. Aves, 325, 50, 65, 76, 105, 116, 125, 162, " temperature of, 121. 170. " voice of, 189. " wings of, 160, 804. BABIRUBA, 69, 34. Bivalve, see Clam, Lamellibrauchiata, Baboon, 359. Oyster. Balsena, see Whale. Blackbird, 339. Balanus, 284, 254. Blastema, 33. Barnacle, 284, 253, 254. Blastnla, 198, 165. " metamorphosis of, 210. Blood, 97. " month of, 57. " circulation of, 103. " see Cirripedia. " corpuscles, 98, !)9, 62-66. Basket-fish, 260. " development of, 200. Batrachia, 31S, 63, 65, 76, 85, 87, 296, 297. " functions of, 97 " see Frog. " of Invertebrates, 97. Bats, 346, 339, 340. " rate of motion of, 111. Beaver. 346, 837. " temperature of, 100. INDEX. 401 Blood of Vertebrates, 9& Cat, teeth of, 70. " vessels, 104. Caterpillar, 301. Blubber, 348. " anatomy of, 78, 40. Blueflsh, 312, 284. " circulation in, 105, 69. Boa, skull of, 72, 87. " false legs of, 172. Bone, composition of, 147. " head of, 303, 276. " development of, 203. " heart of, 105, 69. " structure of, 36, 7, 8. " jaws of, 53, 276. Bos, see Ox. " locomotion of, 162. Brachiopoda, 26C, 221, 222. " muscles of, 156. Brachycephalic, 393. " nervous system of, 169, 180. Bradypus, 344. " see Butterfly, Insecta, Lepi- Brain, 170. doptera. " case of, see Skull. Catfish, 316, 291. " development of, 204 Cebus, 357, 352. " functions of, 1T3. Cell, 31,1. " parts of, 170. Cement, 66, 31. " weight of, 170. Centipede, 291, 259. Brain-coral, 232, 204. Cephalization, 225. Bronchus, 119, 86. Cephalopoda, 278, 16, 47, 151, 247-249. Bryozoa, see Polyzoa. " see Cuttlefish, Squid. Bubble-shell, 274, 231. Cephalo-thorax, 282. Bucciuum, 272, 228. Cerebellum, 171. Budding, 192. Cerebrum, 170. Bufo, 318. Ceryle, 338, 827. " see Toad. Cetacea, 348, 30, 341, 342. Bugs, mouth of, 59. " see Whale. " see Herniptera. Chalaza, 193, 162. Bulimus, 275, 233. Chameleon, 322. Bulla, 272, 231. " tongue of, 81. Butterfly, 273. Cheiroptera, 346, 339, 340. " anatomy of, 43. Chelonia, 322, 115, 301, 302. " metamorphosis of, 20S, 172. " see Turtle. " mimicry of, 217. Chelydra, 323. ". mouth of, 59, 23. Chilognatha, 291. " scales of, 271, 272. Chilopoda, 291, 259. Chimaera, 314. CAPRIS-FLY, 295. Chimpanzee, 357, 354, 856. Csecilia, 318. " skeleton of, 120. Caecum, 51. " teeth of, 35. Calcispongia, 246. Chitine,132. Camel, 352. Chiton, 276, 240. Cameo-shell, 278, 237. Chlorophyl, 23. Caualiculi, 37, 8. Choroid, 183. Canine teeth, 69, 34, 35. Chrysaora, 213, 178. Capillaries, 104, 66, 68. Chrysalis, 208, 390, 172. Caprimtilgus, 338, 323. Chyle, 93, 69. Capybara, 346. Chyme, 92. Carapace, 323, 115. Cicada, 297, 266. Cardium, 227. Cicatricula.194. Carinatae, 328. Cidarie, 262, 96, 97. Carnivora, 353, 90, 92, 106, 108-110, 128, Cilia, 154, 2. 142, 346, 350. Ciliata, 248, 188. " brain of, 142. Circulation in Arthropoda, 106. " teeth of, 70. in Ascidians, 10T. Cartilage, 36, 5. in Birds, 109. Cassis, 278, 237. development of, 200. Cassowary, 327. in Echiuodermata, 105. Castor, 346, 837. in Insects, 105. Cat, 355. in Mammalia, 109. " brain of, 142. in Mollusca, 106. 402 INDEX Circulation in Vermes, 105. " in Vertebrata, 10T, 281. " see Heart. Cirripedia, 284, 253, 254. " see Baruacle. Clam, 272. " adductors of, 46. " alimentary canal of, 80, 44, 46- " anatomy of, 46. " circulation in, 106. " ear of, 178, 150. " foot of, 161, 40. " gills of, 113, 78. " heart of, 106, 46. " hinge of, 270. " locomotion of, 161. " mouth of, 56. " nervous system of, 168, 135. " prehension of, 50. " shell of, 133, 99. " siphons of, 46. " see Lamellibranchiata, Oyster. Clamatores, 338, 322. Class, 235. Classification, 231. " Table, 239. " synopsis of; 362. Claws, 136, Cloaca, 85. Clypeaster, 262. Coagulation, 9& Cochineal, 297. Cockle, 272, 227. Cockroach, 297. Cod, 316, 292. " eggs of, 195 Ccelenterata, 246. " number of, 221. " see Actiuoid Polyp, Hydra, Jelly-fish. Coleoptera, 297, 41, 156, 267, 268. " see Beetle. Colnmbae, 323, 816. Condor, 335. Cone-shell, 276, 239. Conjugation, 196. Connective Tissue, 34, 8, 4. Coral, 130, 251, 95, 201-20S. " see Actiuoid Polyp. Corallium, 256, 207. Coral reef, 254. Cormorant, 330, 809. Cornea, 183. Corpuscles, see Blood. Correlation, 218. Cowry, 276, 284. Corydalus, 295. Crab, 287, 267. " locomotion of, 162. " vocal organs of, 188. " see Lobster. Crane, 332. Craniota, 309. Cranium, 141. Cray-fish, 282, 250. Cricket, 295, 264. Crinoidea, 258, 211. Crocodilia, 323, 303. exoskeleton of, 135. heart of, 108. locomotion of, 163. " mouth of, 61, 26. skeleton of, 149, lia stomach of, 82, 49. " see Reptilia, Crow, 339. Crustacea, 282. iiauplins of, 211, 173. " see Crab, Lobster. Cteuactis, 253, 202. Ctenophora, 257, 209. Cuckoo, 335. Cnculi, 335, 321. Curassow, 333. Cursores, 327, 305. Cuticle, 128. Cutis, 128. Cuttlefish, 280, 248. " anatomy of, 47. " alimentary canal of, SO, 47. beak of, 52, 47. " brain of, 168, 151. " circulation in, 107. ear of, 151. " eye of, 181, 151. heart of, 107. " ink-bag of, 47. " mouth of, 57. " pancreas of, 123. " prehension of, 52. " skeleton of, 134. " suckers of, 16. " see Cephalopoda, Squid. Cyclops, 284, 255. Cypraea, 276, 284. Cypris, 284, 255. Cypseli, 335, 820. DADDY-LONG-LKOS, 289, 300. Daphnia, 284, 255. Dnsyptis, 344, 884. Dasyurns, 343. Decapoda (Crustacea), 286, 70, 248, 260, 256, 257. (Dibranchiata), 280. Decussation, 184. Deglutition, 72. Delphinus, 349, 84a Demodex, 287, 268. Dental Formulae, 70. 1 Tissue, 38, 81. Dentine, 38, 66, 31. INDEX. 403 Dermis, 128. Ecderon, 127. Development, 197. Echidna, 342. by alternate generation, 211. Echiuodermata, 257. of Bird, 199. " number of species of, 221 ofblastula,198, 165. Echinoidea, 261, 28, 39, 96, 97, 214. of embryonic forms, 207. Echinus, 262, 214. ofgastrula,193, 166. " see Sea-urchin. of Invertebrates, 204. Edentata, 344, 101, 833, 334. by metamorphosis, 207. Egg, fertilization of, 197. by metamorphosis, retro- " form of, 195. grade, 210. " number of, 195. oviparous, 309. " segmentation of, 197, 165. ovoviviparous, 309. " structure of, 192, 161, 168. segmentation of egg, 197. Elasmobrauchii, 314, 287, 288. of Vertebrates, 205. " see Ray, Shark. viviparous, 309. Elephant, 350. see Metamorphosis, Repro- " brain of, 170. duction. " foot of, 129. Diaphragm, 86, 88. " skeleton of, 119. Diastema, 70, 383. " teeth of, 69, 36. Dibranchiata, 280, 16, 47, 151, 248, 249. " trunk of, 66. Differentiation, 31. " tusks of, 71, 66, 119. Digestion, chemical, 92. voice of, 189. " of Invertebrate, 92. Elytra, 297. " of Man, 93. Embryology, 197. " object of, 91. Emu, 327. " of Vertebrate, 92. Enamel, 66, 31. Digitigrade, 355, 128. Encephalon, 170. Diploria, 254, 204. Enderon, 127. Dipnoi, 316, 293. Eudoskeleton, 127. Diptera, 300, 24, 127, 173, 269, 270. Entomostraca, 284, 255. " see Fly, Mosquito. Epiblast, 199. Discophora, 220. Epidermis, 34. Distribution, 371-379. Epiglottis, 119, 159. Divers, 328. Epithelium, 33, 2. Dos?, 354. Equus, see Horse. " brain of, 171. Euplectella, 246. " skull of, 143. Excretion, 121, 125. " teeth of, 68. Exoskeleton, 127. Dolichocephalic, 393. Eye, of Invertebrates, 180. Dolphin, 349, 343. " of Vertebrates, 181. teeth of, 68. " development of, 204, Doris, 274. Dove, 333, 316. FACIAL ANGLE, 309. Dragon-fly, 294, 263. Falcon, 335. " flight of, 387. Family, 235. Duck, 331, 311. Fat, 38, 10. Duck-mole, 342, 331. Feathers, 137, 106. Dugong, 350. " development of, 204. " heart of, 78. Felis, 355. Duodenum, 90. Fertilization of Egg, 197. Fibrine, 98. EAGLE, 334, 319. Fishes, 310. Ear, 179, 387, 152. " air-bladder of, 117, 48. Ear-shell, 278, 235, 246. " alimentary canal of, 80, 48. Earth-worm, 269. " blood of, 99, 100, 65. " alimentary canal of, 77. " brain of, 172, 189. " circulation in, 105. " circulation in, 107, 51, 75. " locomotion of, 162. " eye of, 184. " nervous system of, 168. " fins of, 158, 123. " prehension of, 52. " gills of, 114, 48. 404 INDEX. Fishes, heart of, 10S, 48. " locomotion of, 159, 124. " mouth of, 61. " muscles of, 157, 48. " number of species of, 313. " ovary of, 48. " pancreas of, 123. " prehension of, 54. " scales of, 135, 102, 283. " skeleton of, 112. skull of,13S, 112. " teeth of, 61,67, 82. Fish-hawk, 335, 81& Fission, 191, 160. Flagella, 154, 187. Flagellata, 243, 187. Flamingo, 331. Flea, 300. Flight of Bats, 161. " of Birds, 160. " of Insects, 159. Fluke, 265. Flnstra, 267, 220. Fly, 300. buzzing of, 188. foot of, 127. metamorphosis of, 27ft mode of feeding of, 50. mouth of, 59, 24. see Diptera, Mosquito. Fly-catcher, 338, 322. Flying Fox, 346. Follicle, 123, 90. Food, 47^9. Foramen magnum, 172. Foraminifera, 51, 241, 15, 186. Forms of animals, 222. Fox, 355, 349. Frog, 318, 297. " alimentary canal of, 82. " blood-corpuscles of, 99, 63, 65. " breathing of, 119. " circulation in, 108, 76. " food of, 49. " heart of, 108. " lungs of, 118, 85. " lymph-heart of, 96. " metamorphosis of, 209. " respiration in, 117-119. " skeleton of, 119, 140, 145, 87. " tongue of, 61. " vertebrae of, 140, 87. Fruit-moth, 303, 275. Function, 41. Fungia, 252, 202. GAU.-BLADDEB, 124, 92. Gall-fly, 303. Ganglion, 166, 14, 140. Ganuet, 331. Ganoid ei, 315, 289, 290. Gar-pike, 315, 289. Gasteropoda, 20, 29, 45,100, 154, 176, 272. " see Snail. Gastric glands, 123, 90. " juire, 93. Gastrula, ,98, 166. Gavial, 3-.4. Gecko, 322. Gelatine, 36. Genus, 235. Germinal vesicle, 192. Gibbon, 357. Gills, 114, 125, 48. Giraffe, 353. Gizzard of Invertebrates, 7 '. " of Vertebrates. 84. Gland, 121, 89. " gastric, 123, 90. " liver, 123, 92. " pancreas, 123, 91. " salivary, 122. " sweat, 126, 94. Globigerina, 242. Glottis, 119. Glycogen, 23. Goatsucker, 338, 323. Gouiaster, 260, 212. Goose, 331, 310. Gorgouia, 256, 208. Gorilla, 357, 857. Grallatores, 332, 318. Grasshopper, 297. development of, 208. " gizzard of, 79. " month-parts of, 58, 21 " stimulation, 188. Grebe, 329. Gregariuida, 240, 184, Grouse, 333, 815. Growth, 214. Grubs, 389. Gryllus, 295, 264 Guinea-pig, 345. Gulls, 3'-«C H^MATOOBYA,393. Haematothenna, 393. Hsemocyanin, 102. Hemoglobin, 102. Hag-nsh, 64, 314. Hair, 136, 94, 104. Hair-worm, 262. Haliotis, 278, 235, 246. Hand, 359. Hare, 346, 386. Harvest-man, 289. Haversian Canals, 37, 7. Hawk, 335, 818. Hearing of Invertebrates, 178. " of Vertebrates, 179. Heart, Arthropoda, 105, 69, 70. INDEX. 405 Heart, development of, 200, 168. Individual, 220. " of Mollusks, 106. Infusoria described, 243, 160. " of Tunicates, 107, 279. " digestive cavity of, 7_5. " of Vertebrates, 107-109, 71-74. fission, 191, 160. Heat, 121. " mode of feeding of, 50. Hedgehog, 346. " motion of, 154. Helix, 2T5, 232. " mouth of, 55. Hemiptera, 297, 265, 266. " respiration of, 112. " month of, 59. skin of, 127. Heron, 332, 813. Inheritance, 217. Heterocercal, 159, 123. Insectivora, 346. Heteromya, 272. Insecta, 291. Hippopotamus, 352. " absorption of, 94. foot of, 129. " alimentary canal of, 79, 41-43. Histology, 12. " anatomy of, 43, 81. Hog, 352. " anteunse of, 147. " teeth of, 67. " chrysalis of, 172. Holothuroidea, 262, 215. " circulation in, 105, 292. Homarus, nee Lobster. " development of, 205. Homo, see Man. " ear of, 179. Homocercal, 159, 287. eye of, 181, 155, 156. Homo]ogv,217. feet and legs of, 162, 127, 131. " " serial, 218. flight of, 159. Homomorphism, 217. " gizzard of, 79. Hoofs, 136, 103. heart of, 105, 69. Hornera, 207, 220. kidney of, 126, 41, 42. Horns, 136. liver of, 124. Horse, brain of, 171, 138. locomotion of, 159, 162. " hoof of, 136, 164, 103, 129. " metamorphosis of, 207, 172, 178, " skeleton of, 151, IJi. 264-267. " skull of, 144, 111. " mouth of, 57. " splint-bones of, 207. " mouth-parts of, 53, 21-24. " stomach of, S3, 63. muscles of, 156, 131. Horse-fly, mouth of, 60, 24. " nervous system of, 169, 43 Horseshoe-crab, 284. " respiration iu, 114, 291. " jaws of, 53. " salivary glands of, 122. " skeleton of, 131. silk glands of, 40. Hummer, 335. < skeleton of, 132, 202, 98, 262. Hyalea, 274, 229. smell of, 178. Hydra, 246, 191. ' spiracle of, 114, 79. " budding of, 192- ' touch of, 177, 147. " digestive cavity of, 75. tracheae of, 114, 40, 80, 81. " nerve-cells of, 168. " wings of, 159. " repair of, 215. nsessores, 337, 322-328. Hydroid, see Hydrozoa. nspiration, modes of, 115, 119,120. Hydrozoa, 246, 178, 191-196. nstinct, 184. " development of, 205. ntelligence, 187. " metamorphosis of, 212. ntestine of Amphibian, 82. " see Jelly-fish. of Bird, 84. Hyena, 355. of Pish, 81. Hymenoptera, 303, 22, 42, 81, 277. " of Mammal, 85. " see Bee. " of Reptile, 82. Hypoblast, 199. " see Alimentary Canal. Isomya, 272. Ibis, 332. Ivory, 66. Ichneumon, 304. Ichthyopsida, 309. JAWS, 51-71. Ichthyosaurus, 324. Jay, 339. Idotia, 286, 251. Jelly-fish, 247, 193-197. Iguana, 322. " blood of, 97. Incisors, 63. development of, 212, 178, 195. 406 INDEX. Jelly-fish, eye of, 180. Lion, 355. " mode of feeding of, 51. " foot of, 128 month of, 55. " skeleton of, 106. " nerves of, 168. " stomach of, as, 55. nettle-cells of, 51. Liver, 123, 92. " reproduction of, 212. Lizard, see Lacertilia. Joints, 14T. Lobster, 106, 70, 256. Julus, 291. " alimentary canal of, 7& " anatomy of, 282. KANGAROO, 88, 343. circulation in, 106, 70. Kidney, 126, 41, 93. King-crab, see Horseshoe-crab. Kingfisher, 335, 327. Kite, 335. ear of, 179. " eggs of, 196. " gills of, 114. -' gizzard of, 64. " locomotion of, 158. moulting of, 132. LABITTM and LABUUM, 58, 21. " mouth of, 57. Labyriuthodontia, 318. " prehension of, 53, 57. Lacerta,321, 300. " respiration in, 114. Lacertilia, 321. skeleton of, 131. Lachnosterna, 29T, 267. Lob-worm, 77. Lacteals, 94, 60. Locomotion of Arthropoda, 161 Lacunae, 37, 8. of Birds, ICC Lamellibnuichiata, 270, 44, 46, 78, 99, " of Fishes, 158. 185, 150, 224-227 " of Insects, 159. eye of, 181, 168. " ot'Mollnsks, 161. " see Clam. " of Starfish, ^.Cl. Lameliirostres, 331, 311. " of Vertebrates, 163. Lamprey, 286. " of Worms, 161. Lamp-shell, 266, 221. Locust, 297. Lancelot, 310, 282. Loligo, »ee Squid. Land-snail, 275, 232. Longipennes, 329, 308. Lark, 340. Loon, 328, 307. Larynx, 189, 159. Louse, 297, 50. Lasso-cells, 61. Lucernaria, 197. Leech, 268. Lumbricus, see Earth-worm " alimentary canal of, 7T. Lungs, function of, 125. " jaws of, 64. " of Snail, 116. " locomotion of, 161. " surface of, 3S5. " mode of feeding of, 50. " of Vertebrates, 11T, Lemur, 355, 851. Lymph, 102. Lepas, 284, 253. Lymphatics, 94, 6L Lepidoptera, 300, 48, 172. Lymph-heart, 96. " see Butterfly. Lepidosiren, 317. MAOTUA, 271, 46. 226. Lepidosteus, 315, 289. Madrepore, 2r>2, 201, 206. Libellula, 294, 268. Madreporic plate, 258, 39. Life, distribution of, 372. Maggots, 389. " duration of, 226. Mammalia, uua. " nature of, 28. " alimentary canal of, 85. " struggle for, 227. " anatomy of, 87, 52. Lightning-bug, 299. blood-corpuscles of, 90, 65. Ligula, 58, 21. brain of, 171, 138, 142-145. Likeness, 215. circulation in, 109, 76, 281. Lira ax, 275, 282. " digestion of, 92, 51. Limbs, development of, 204. drinking of, 50, " skeleton of, 146. ear of, 179, 152. Limnsea, 275, 232. egg of, 19S, 165. Limpet, 278, 245. embrvo of, 202, 171. Limulus. 284. eye of, 183, 157. " see Horseshoe-cratr, hair of, 136, 104. INDEX. 407 Mammalia, heart of, 109, 73, 74. locomotion of, 163. lungs of, 118, 86. mouth of, 62. palate of, 86, 27, 51. placenta of, 190,203, 171. respiration in, 120. skeleton of, 139. smell of, 178, 149. " teeth of, 68. " touch of, 17T. voice of, 189, 159. Man, 359. blood-corpuscles of, 99, 62. brain of, 170, 171, 137, 144, 145. digestive tract of, 51. ear of, 179, 152. eye of, 198, 157. mouth of, 86, 27. muscles of leg of, 165, 130. nose of, 178, 149. Manatee, 350, 343. Mandibles, 58, 21. Mantis, 53. Mantle, 127, 46. Marsipobranchii, 314, 286. Marsupialia, 342, 332. Mastodon, 350. May-fly, 295. Meamlriua, 252. Medulla oblongata, 172. Medusa, see Jelly-fish. Megatherium, 344. Melania, 278. Menobrarichus, 317, 294. Mesentery, 83. Mesoblast, 199. Metamorphosis, 207. of Crab, 209. of Frog, 209. " of Insect, 208.. " of Grasshopper, 208. of Starfish, 208. Metazoa, 244. Millepede, see Myriapoda. Millepore, 391. Mimicry, 217. Minerals and Organisms, 19. Mite, 287, 258. Molar Teeth, 69. Mole, 346. Mollusca, 269. " absorption of, 94. " anatomy of, 45, 46, 47, 78. " circulation in, 106. " development of, 205. " digestion of, 92. " distribution of, 377. " growth of, 214. " kidney of. 126, 78. " liver of, 124. Mollnsca, locomotion of, 161. metamorphosis of, 269. mode of feeding of, 52. mouth of, 5C. " nervous system of, 16S, 134, 135, 151, 157. " number of species of. 221. respiration in, 113, 46, 47, 78. " salivary glands of, 122. shell of, 133, 385, 99, 100. " skin of, 127. " see Clam, Cuttle - fish, Snail, Squid, Monad, 243, 187. Monera, 240, 183. Monkey, 35C, 19, 352. see Primates. Monomya, 271. Monotremata, 342, 331. " mouth of. 62. Mosquito, 300, 173, 269. " metamorphosis of, 208. mode of feeding of, 50. Moth, 302, 274-276. " anatomy of, 79, 43. " metamorphosis of, 274. " see Butterfly, Lepidoptera. Motor Nerves, 167. Moulting, 128, 209. Mouse, 346. Mouth, 55. of Arthropoda, 5T. of Ascidia, 60. of Birds, 62. of Cffilenterata, 55. of Echiuodermata, 5*. of Fishes, 61. of Infusoria, 55. of Mammals, 62. of Mollusks, 56. of Parasites, 55. ofReptilia, 61. of Vermes, 57. of Vertebrata, 60. Mucous Membrane, 89. Mud-fish, 315. Murex, 278. Mus, 346. Musca, see Diptera, Fly. Muscle, 39, 12, 122. " development of, 204. " of Invertebrates, 156. " kinds of, 155. " of Vertebrates, 156. Mushroom-coral, 252, 202. Mussel, 270, 225. Myriapoda, 290, 259. " alimentary canal of, 77 " mouth of, 57. " respiration in, 116. " nee Centipede. 408 INDEX. Myrmecophnga, 344, 333. Orthoptera, 217, 295, 21, 284. Mytilus, 270, 225. " see Grasshopper. Orycteropus, 344. N\ILB, 136. Oscines, 338. Narwhal, 223, 68. Osseous Tissue, see Bone. Natatores, 328. Ossification, 37, 203. Natica, 2TS. Ostrea, 272. Natural Selection, 22T. " see Oyster. Nauplins,211,177. Ostrich, 327, 305. Nautilus, 279, 247. Otoliths, 178, 156. Nematelminthes, 265, 218. Ovipositor, 293. Nereis, 268, 17. Owls, 335, 817. Nerve-cella, 40, 132. Ox. alimentary canal of, 90. " fibres, 40, 13. " foot of, 352, 129. " kinds of, 167. f " teeth of, 352. " velocity of impulse of, 167. " see Ungulata. Nervous System, 168. Oyster, anatomy of, 80, 44. " " of Arthropoda, 169. " development of, 205. " Brain, 170. " eggs of, 195. " " development of, 199, 67. " heart of, 106, 44. " ofMollusks, 168. " mouth of, 56. " " Spinal Cord, 175. " prehension of, 50. " of Starfish, 168. " respiration in, 113. " " Sympathetic, 175, 146. " see Clam, Lamellibranchiata- " of Vertebrates, 169. " " of Worms, 168. PAT.ATE, 61. Neuroptera, 294, 263. Pallial Sinus, 271, 99. " see Dragou-fly. Palpi, 58, 21. Nenroskeleton, 141. Pahulina, 278, 244. Newt, 318, 296. Pancreas, 123, 91. Nomenclature, Zoological, 236. Pancreatic Juice, 93. Notochord, 200, 167. Pangolin, 344. Nncleolus, 31, 1. Paper Nautilus, 2SO, 249. Nucleus, 31, 1. Papilio, 303. Nummulite, 242. Papillae, 128, 148. Nutrition, 45. Paramecium, 243, 188. Nymph, 377. " see Infusoria. Parrot, 337, 320. OOKT.LI, 181. " tongue of, 62. Octopus, 280. Partridge, 333. (Esophagus, 86. Patella, 278. Olfactory Lobes, 172. Pavement-teeth, 67, 32. " Nerves, 178. Pearl-oyster, 224. Olive-shell, 278. Pectoral Arch, 146. Oniscus, 286. Pedicelhiriie, 77, 97. Opercnlum,114,134, 228. Pedipalpi, 288. Ophidia, 320. " see Scorpion. " see Suake. Pelias, 320, 298. Ophiura, 260. Pelican, 331. Opisthobranchs, 274, 352, 230, 281. Penguin, 329, 306. Opisthoccelons, 383. Pennatula, 256, 208. Opossum, 342, 882. Pentacrinns, 258, 211. Optic Lobes, 172. Pepsi n, 93. Orang-utan, 357, 853, 855. Peptone, 93. Order, 235. Perch, skeleton of, 112. Organ, 41. Perchers, 337. Organization, 30. Periosteum, 138. Organ-pipe Coral, 251, 200. Peristaltic Movement, 89. Oriole, 339. Periwinkle, 278. Ornithorhynchne, 342, 881. Petrel, 330. Orthoceras, 287. Petromyzon, 314, 286. INDEX. 409 Pharyngobranchii, 310, 282. Pseudopodia, 51, 15. Pharynx, 85. Pseudotriton, 318, 296. Pheasant, 333. Psittaci, 337, 320. Phoca, 354. Pteropoda, 274, 229. Physalia, 5246, 194. mouth of, 56. Physeter, 348, 341. Pulmonates, 274, 232, 23a see Whale. Pulse, 385. PicarifC, 335. Pupil, 183, 158. Pici, 335, 820. Pygopodes, 328, 306, 307. Pigeon, 333, 316. Piunigrade, 354, 128. QUAniUr.MANA, 356. Pisces, 310, 48, 51, 65, 75, 102, 112, 123, 124, 139, 283-293 RACCOON, 355, 846. " see Fish. Radiates, 233. Placenta, 196, 171. Radiolaria, 241. 186. Planaria, 264, 217. Rail, 332, 814. Plant, 22. Runa, see Frog. " food of, 25. Range of Animals, 378. " functions of, 24. Rank of Animals, 224. Plantigrade, 355, 128. Raptores, 334, 116, 317-319, Plant-louse, 297. Rasores, 332, 315. Plasma of blood, 98. Rat, 346. Plastron, 323. Ratitse, 327, 305. Platyhelminthes, 264, 216, 217. Rattlesnake, 68, 83. " see Tape- worm. Raven, 339. Platyonychus, 287, 257. Ray, 314, 288. Pleurobrachia, 257, 209. " teeth of, 67, 82. Plover, 332. Razor-shell, 272. Poison-fangs, 68, 38. Redstart, 338, 325. Polycistiua, 242, 186. Repair, 215. Polyp, 250. Reproduction, 191. " ace Actinia. " asexual, 191. Polyzoa, 266, 220. " by budding, 192. P.ind-snail,275, 282. " checks on, 227. Porcupine, 346. by division, 191. Porites, 252. " rapidity of, 226. Porpoise, 88, 349, 54- " sexual, 192. Portal circulation, 307, 385, 281 Reptilia, 319. Portuguese Man-of-war, 246, 194. 11 alimentary canal of, 82. Potato-worm, 303. brain of, 172,141. Ponlpe, 230. " circulation in, 108, 76. Prairie Chicken, 333, 315. " corpuscles of, 99, 66 Primates, 356, 35, 120, 143-145. 35-2-358. " distribution of, 378. brain of, 143-145. lungs of, 118, 84. " skeleton of Chimpanzee, 120. " mouth of, 61. " teeth of Chimpanzee, 85. " prehension of, 61. " see Ape, Man, Monkey. " scales of, 135. Proboscidea, 350, 36, 119, 129. " teeth of, 67. Proboscis of Butterfly, 59, 28. " voice of, 189. of Elephant, 62, 119. " see Crocodile, Lizard, Snake, Procoslons, 383. Turtle. Prognathous, 393. Respiration, 111. Prosobranchs, 278, 284-246. " in Crustacea, 114. Proteus, 318, 295. in Echinoderms, 112. " blood-corpuscle of, 99, 65. " in Fishes, 114. Protista, 21. " in Insects, 114. Protoplasm, 81. in Mollusks, 113. Protopterus, 316, 298. " rate of, 120. Protozoa, 238. in Vertebrates, 117. " number of species of, 221. in Worms, 113. " see Amoeba, Infusoria. Kete mucosum, 128. 4:10 INDEX. Retina, 183, 158. Uhea, 327. Rhinoceros, 351, 344. Rhizopoda, 240, 15, 185, 186. " skeletou of, 129. Rodentia, 345, 886, 336, 837. teeth of, 71, 385. Rotifera, 266, 219. " jaws of, 64 Rudimentary Orgaus, 207. Rnminautia, 351. " stomach of, SS, 56. " see Ox, Ungulata. SALAMANDER, 318, 296. " metamorphosis of, 174. Saliva, function of, 93. Salivary Glands, 122. Salmon, 316, 285. . Sand-flea, 284, 252. Sandpiper, 332, 312. Sarcolemina, 39, 204. Sanropsida, 309. Saurnrae, 394. Scales of Butterflies, 301, 272. " of Fishes and Reptiles,135,102,283. Scallop, eye of, 181, 153. shell of, 272. Scapular Arch, 146. Scarabseus, 299. Scarf-skin, 12& Sclerobase, 129. Scleroderm, 129. Sclerotic, 183. Scolopendra, 291, 259. Scorpion, 288, 259. " mouth of, 60. " respiration in, 116. Sea-anemone, see Polyp. Sea-blubber, 247. Sea-butterfly, 273, 229. Sea-fan, 256, 208. Sea-hare, 274. Seal, 355, 128. Sea-lemon, 274. Sea-lily, 258, 211. Sea-lion, 355, 850. Sea-slug, 274. Sea-urchin, 262, 214. " absorption by, 94. alimentary canal of, 76, 89. anatomy of, 89. circulation in, 106. digestion in, 92. growth of, 214. mode of feeding, 52. mouth of, 56. respiration in, 112. shell of, 28. skeleton of, 130, 96. spines of, 130, 97. Sea-nrchin, teeth of, 04, 28. Sea-worm, 208, 17, 223. Secretion, 121. see Gland. Segmentation of egg, 197, 16b. Self-division, 191, 160. Sensation, 176. Sense of hearing, 1 78. " of sight, ISO. " of smell, 177. " of taste, 177. " of touch, 176. Sense-organs, see Sense. " development of, 204. Sensibility, 176. Sepia, '280, 248. Serpent, see Snake. Sertularia, 247, 192. Serum, 98. Setae, -269. Setophaga, 340, 325. Seventeen-year locust, 297, 26'o. Shark, 314, 287. " eggs of, 194, 164. " gills of, 114, 287. " skeletou of, 137, 145. Shells of Crustacea, 131. " of Echiuoderms, 130. " of Mollusks, 133. Shoulder-girdle, 146. Shrew, 63, 346, 338. Shrimp, 286. Sight, of Arthropods, 181. " of Coelenterates, ISO. " ofMollusks, 181. " of Vertebrates, 182. Silk-gland, 40. Silk-worm, 303. Simia, 358, 353, 355. Sinuses, 138. Siphouophora, 24S, 194. Siphtiucle, 279, 247. Siren, 318. Sireuia, 349, 78, 348. " see Dugong. Size of Animals, 221. Skeleton, of Arthropoda, 131. of Birds, 116, 144. ofCoeleuterates, 130. " of Echinoderms, 130. of Fish, 138, 144, 112. " of limbs, 146. Lion, 139, 116. " Mammals, i;,9. 106. 114, 117- 120. Mollusks, 133. Reptiles, 118, 115. of skull, 141, 108, 111. " of Vertebrates, 134. of Whale, 114. " see Exoskeletou. INDEX. Skin of Invertebrates, 12T. Spider, web of, 2S9, 260. " of Vertebrates, 128. Spinal column, 140. Skull, 141. " cord, 1T5. Slater, 2S4, 251. Spinneret of Spider, 289, 261. Slug, '274, 232. of Caterpillar, 301, 276. Smell, ITT. Spiracle, 114, 79. S 11 nil, 272. Spongida, 244, 189, 190. " alimentary canal of, SO, 45. " alimentary canal of, 76. " anatomy of, 45. ", an atomy of, 189. " circulation in, 100, 45. egg of, 194, 103. " eye of, 181, 164 feeding of, 50, 189. " gills of, 113. respiiaiion in, 11-2. " gizzard of, 04. " skeleton of, 129, 190. " heart of, 45. Squash-bug, 297. " jaw of, 56, 20. Squid, 2SO. " larva of, 176. " locomotion of, 158. " locomotion of, 161. " see Cuttlefish. " lung of, 116, 2T4, 45. Squirrel, 346. " mode of feeding, 52. Stag, 352, 345. " mouth of, 56. Star-fish, alimentary canal of, 76, 126. " nervous system of, 1GS, 134, 154. " anatomy of, 126. " operculnm of, 114, 134, 228. ' circulation in, 105, 126. " respiraiion in, 110, 45, 228. ' classification of, 268. " shell of, 133, 100, 228, 233-246. ' development of, 208. " siphon of, 228. 1 digestion in, 92. " smell of, ITS. locomotion of, 161, 126. " teeth of, 65, 29. ' metamorphosis of, 207. " tentacles of, 1TG, 154, 228. ' mode of feeding of, 51. " see Gasteropoda. " mouth of, 56. Snake, 320, 298, 299. " nervous system of, 168, 133» " deglutition of, 73. " respiration in, 112. " locomotion of, 162. " see Echiiiodermata, " lungs of, 119, 84. Stilt, 332. 11 poison apparatus of, 68, 33. Stomach, 82-89. " scales of, 135. " digestion in, 93. " skull of, 37. Stork, 332. " stomach of, 82. Stridulation, 188. " tongue of, 61. Strom bus, 278, 243. " VertebriB of, 140. Struggle for Life, 226. " voice of, 1S9. Struthio, 327, 305. " see Boa, Ophidia, Reptilia. Sturgeon, 315, 290. Snapping-bug, 299. Snbkingdom, 233. Snipe, 332. Sun-fish, 247- Solaster, 2CO. Sun-star, 260. Somite, 392. Survival of Fittest, 226. Songsters, 338. Suture, 147. Sorex, 346, 338. Swallow, 340, 328. Sow-bug, 285. Swan, 331. Sparrow, 339. Swift, 335. Species, defined, 235. Swimmeret, 282. " number of, 221. Symmetry, 222. Sperm-whale, see Whale. Sympathetic nervous system, 175, 146 sphinx-moth, 303, 43. Synovia, 147. Spider, classification of, 289, 260. " alimentary canal of, 79. T.ENIA, see Tape-worm. " appendages of, 25. Tauager, 339. " circulation in, 106. Tapetum, 184. " fangs of, 53, 18, 25. Tape-worm, 264, 216. " mouth of, 60, 25. " feeding of, 49. " respiration in, 116. Tapir, 63, 361. " spinnerets of, 289, 25, 261. Taste, 177. 412 INDEX. Teeth, of Amphibia, 67. Turritella, 278. " of Fishes, 61, 66, 67. Turtle, 322, 301, 302. " of Invertebrates, 63. " alimentary canal of, 82. " of Mammals, 68, 70. " breathing of, 119. of Reptiles, 67. " mouth of, 61. " structure of, 3S, 66, 9, 31. " shell of, 135. Teleostei, 315, 284, 290-292. " skeleton of, 115. Telson, 282. " see Chelonia. Temperature of Animals, 121. Tusks, 383. Tendon, 36. Types, 233. Tentac.e, 51. Tent-caterpillar, 803. UNGULATA, 351, 63, 66, 103, 111, 117, 118, Tenr.es, 295. 129, 138, 344, 345. Terebra, 278, 288. feet of, 129. Terebratnla, 267, 222. Uuio, 272. Terebratulina, 267, 221. " eggs of, 196. Termite, 295. Univalve, see Snail. Tern, 329, 308. Urodela, 317, 294-296. Testudo, see Turtle. Tetrabranchs, 279, 247. VARIATION, 216. Tetradecapods, 285, 261, 262. Variety, 235. Thoracic duct, 95, 61. Veins, 104, 67. Thorax, 119, 88. Veliger, 211, 176. Thornback, 315, 288. Vena cava, 104. Thousand-legged Worm, see Julus. Venus, 272 Thrush, 340. Venns'-basket, 246. Thylaciuns, 343. Vermes, 263, 17, 77, 175. Thyroid Cartilage, 189, 159. " see Earth-worm, Worms. Ticks, 288. Vertebra?, development of, 203. Tissue, 33. " kinds of, 140, 106, 107. Toad, 318. " number of, 140. Tongue, of Batrachians, 61. Vertebrata, 306. " of Birds, 62. absorption in, 94. " of Fishes, 61. " alimentary canal of, 80. of Insects, 50, 58. blood of, 98. of Mammals, 63. " brain of, 170. of Mollusks, 52. " circulation in,10;; 281. of Spiders, 60. " development of, 205. Top-shell, 278, 242. " digestion in, 92. Tortoise, 323, 802. ear of, 179. " see Turtle. " exoskeleton of, 134. Totipalmates, 330, 809. " eye of, 183. Toucan, 335. " gastric glands of, 123. Touch, 176. " heart of, 107. Trachea, 119. " kidney of, 126. Trachese, 114, 40. 79, 80, 81. " liver of, 124. Trichina, 265, 218. " lungs of, 117. Tridncne, 272. " mode of feeding of, 54. Trilobite, 284. " mouth of, 60. Trionyx, 322. " muscles of, 156. • Triton, 318, 296. " nervous system of, 169. Tritonian, 274, 230. " number of species of, 221. Trochosphere, 211, 175. " pancreas of, 123. Trochus, 273. " salivary glands of, 128. embryo of, 211, 176. " skeleton of, 137. Trogon, 335, 821. skin of, 128. Tubipora, 252, 200. " stomach of, 80. Tnnicata, 305, 278, 279. " teeth of, 66. " see Ascidians. " tongue of, 60. Turbo, 278, 242. " see Bird, Crocodile, Fish, Turkey, 383. Frog, Mammal, Reptile. INDEX. 413 Villi, 90, 58. Viuegar-eel, 265. Vireo, 340, 826. Vitelline MembraLe, 193. Viviparous, 309. Vocal Cord?, 189. Voice, of Invertebrates, 188. " of Vertebrates, 189. Volute, 2T8, 241. Vorticella, 243, 160. Vulture, 335, 116. WALKING-STICK, 29T. Walrus, 355, 383. Warbler, 340. Wasp, 304. Water-boatman, 2!»7, 265. Water-fleas, 284, 255. Wax-wing, 340. Weasel, 355, 348. Weevil, 300. Whale, 348, 341, 342. " baleeu of, 65, 30. " brain of, 1TO. " fat of, 39. " mode of feeding of, 50. " mouth of, 62. " swimming of, 159. " teeth of, 383. Whelk, 278, 228. " see Snail. White Ant. 295. Windpipe, 119, 86. Wings, of Bats, 161, 339, 340. " of Birds, 160, 304. " of Insects, 159, 206. Wolf, 355, 347. Wombat, 343. Woodpecker, 335, 320. Worms, 263. " absorption in, 94. " alimentary canal of, 7T. " blood of, 98. eye of, 17. head of, 17. " jaws of, 17. " larva of, 175. " locomotion of, 161. mouth of, 57. number of speries of, 22L proboscis of, IV. reproduction in, 192. respiration of, 113, 77. skin of, 127. " see Earth-worm. Wren, 340. YOLK, 192. ZOOLOGICAL analysis, 236. " barriers, 375. " hibtory, 18. " provinces, 376. Zoology, 19. THE END. I Ml r; ^ _ ^HAINMtV* ^IOS«J|£ 1 1 § i lim 006 504 922 3 AA 000692740 4 S sc 5 2 £? x, g I fi s AUS-ANtfU .^l i - « i