REESE LIBRARY 
 
 UNIVERSITY OF CALIFORNIA. | 
 BIOLOGY Class 
 
 LIBRARY 
 
 G 
 
\ 
 
ANIMAL BIOLOGY 
 
t^ ^ 
 
ANIMAL BIOLOGY 
 
 AN ELEMENTARY TEXT-BOOK 
 
 BY 
 
 C. LLOYD MORGAN 
 
 PROFESSOR OF ANIMAL BIOLOGY AND GEOLOGY IN UNIVERSITY COLLEGE 
 
 BRISTOL, AND LECTURER ON COMPARATIVE ANATOMY IN THE 
 
 BRISTOL MEDICAL SCHOOL 
 
 WITH ILLUSTRATIONS 
 
 RIVINGTONS 
 WATERLOO PLACE, LONDON 
 
 MDCCCLXXXIX 
 
 {Second Edition. Revised] 
 
BIOLOGT 
 ' fBRARY 
 
 E 
 
PREFACE TO SECOND EDITION. 
 
 THE call for a second edition of this volume has enabled 
 me to revise the text, to substitute in several cases im- 
 proved woodcuts, and to add a brief Classification of the 
 Types, and a Glossary. C. LL. M. 
 
 UNIVERSITY COLLEGE, BRISTOL, 
 April 1889. 
 
 PREFACE 
 
 IN preparing this volume I have endeavoured to meet the 
 requirements of those who intend to present themselves for 
 the London Intermediate and Preliminary Scientific, for the 
 Oxford and Cambridge Local, and for other examinations 
 of like range based upon the type system. 
 
 Special attention has been paid to Embryology. 
 
 The larger portion of the work has been devoted to the 
 Vertebrate types, because I have had in view, in the first 
 place, the requirements of those who intend to follow a 
 medical career, and who will, I believe, after working 
 through such a course as is developed in this volume, be 
 able to make a better start, and thereafter more rapid 
 progress, in their study of human anatomy and physiology 
 than would be otherwise possible ; and, in the second place, 
 because there are many who are led to take a deep interest 
 
PREFACE. 
 
 in human anatomy and physiology, but who are of necessity 
 able to make their study practical only through the dissec- 
 tion of the lower Vertebrates. 
 
 The illustrations have been engraved by Mr. George E. 
 Lodge, after outline sketches of my own, chiefly from dissec- 
 tions or preparations made in the Biological Laboratory of 
 the University College, Bristol. The dissections and pre- 
 parations have in many cases been made in accordance with 
 instructions given, in text or figures, by previous authors 
 especially by Professor Howes, in his excellent Atlas of 
 Biology, by Professors Foster and Balfour in their Elemen- 
 tary Embryology, and by Professor T. J. Parker in his Zootomy. 
 In the few cases in which my indebtedness is more direct 
 I have placed the name of the author in brackets. With 
 hardly any exceptions they are to be regarded, not as 
 pictures, but as outline sketches, to serve as guides to prac- 
 tical work, and to be considered only in relation to the 
 actual objects displayed by dissection. In a few cases they 
 are to be looked upon as purely diagrammatic, to aid in the 
 comprehension of the text. In my opinion, the more widely 
 different such pure diagrams are from pictures the better. 
 
 My sincere thanks are due, and are here most gratefully 
 tendered, to Professors W. K. Parker, F.E.S. ; G. B. Howes, 
 F.Z.S. ; William Ptamsay, Ph. D. ; and Mr. G. Munro Smith, 
 for valuable aid and advice. 
 
 April 1887. C. LL. M. 
 
CONTENTS 
 
 
 CHAP. PAGE 
 
 I. INTRODUCTORY, ...... xi 
 
 PART I. 
 
 VERTEBRATE ANATOMY AND PHYSIOLOGY. 
 
 As exemplified by the Frog, the Pigeon or Fowl, and the 
 
 Rabbit, with occasional Reference to other Types. 
 
 II. Natural History and External Characters, . . 1 
 
 III. General Anatomy, . . . . . .21 
 
 IV. General Physiology, . .... 55 
 V. General Histology, ...... 63 
 
 VI. General Embryology, ... 86 
 
 VII. The Genesis of Tissues and Organs, . . .117 
 
 VIII. The Skeleton, . 139 
 
 IX. Nutrition and Metabolism, . . . . .177 
 
 X. The Heart and Circulation, . . . .194 
 
 XI. The Brain and Nerves, . . . . .215 
 
CONTENTS. 
 
 PART II. 
 
 SOME INVERTEBRATE TYPES. 
 
 CHAP. PAGE 
 
 XII. The Crayfish, ...... 231 
 
 XIII. The Cockroach, 257 
 
 XIV. The Earthworm, 278 
 
 XV. The Snail, 291 
 
 XVI. The Fresh-water Mussel, .... . 305 
 XVII. The Liver-Fluke and Tapeworm, . . .322 
 
 XVIII. The Hydra, 335 
 
 XIX. Vorticella and Paramoecium, .... 347 
 
 XX. The Amoeba, .... . 354 
 
 APPENDIX. 
 
 Classification of Types, ...... 361 
 
 Glossary, ........ 365 
 
 Index, ......... 379 
 
 
INTRODUCTORY. 
 
CHAPTER I. 
 INTRODUCTORY. 
 
 WE live in a world that is teeming with life. The air, the 
 surface of the land, the waters of ocean, river, and pond, swarm 
 with living organisms, each more or less perfectly adapted to the 
 conditions of its existence. Many problems arise with regard to 
 this world of living things, What is their form and structure ? 
 How do they move and breathe and reproduce their kind ? How, 
 and on what do they feed ; and how does the food minister to 
 their growth or their activity ] How are they distributed over 
 the earth's surface ? What is the method and manner of their 
 origin ? These, and other questions of like nature, arise in con- 
 nection with the world of life. And the science that deals with 
 these problems is the Science of Biology. 
 
 This science branches into two main departments, in accord- 
 ance with the division of living things into Animals and Plants. 
 The two branches start, indeed, from a common stem, for there 
 are certain characteristics common to plant life and animal life, 
 and the lowest plants are scarcely to be distinguished from the 
 lowest animals. But the structure and functions of the vast 
 majority of animals differ so markedly from the structure and 
 functions of the vast majority of plants, and the problems of 
 animal life differ so materially from the problems of plant life, 
 that the Science of Animal Biology, which deals with the former, 
 is justly entitled to a distinct position as a separate branch of 
 study. It is with this branch that this volume deals. 
 
 The essence of science is organisation and exactness. Most 
 
xii ANIMAL BIOLOGY. [Chap. I. 
 
 people have some knowledge of the animal world in which they 
 live and of which they form a part. But this general informa- 
 tion, useful and interesting as it is, lacks that organisation and 
 exactness which is a distinguishing mark of science. To rise 
 from general information to science, we must use the methods of 
 science, which are : (1) observation and experiment; (2) infer- 
 ence and hypothesis ; (3) verification. The beginner will, how- 
 ever, wisely rest content with repeating the observations and 
 experiments (not merely reading about them but repeating them), 
 and conscientiously verifying the inferences, of his masters in 
 science. This he must do if he is to learn science as science, and 
 not as history. To learn about .science is valuable. But to be 
 taught science itself through the direct teachings of Nature is 
 far more valuable. The student of science must learn his facts 
 at first hand, and must regard books as guides to that object. 
 Observation, experiment, and verification are to be regarded as 
 primary duties by every student of science as such. They are 
 therefore incumbent on the student of Animal Biology. 
 
 What is the nature of those living things on the scientific 
 study of which we are now to enter? We are more or less 
 acquainted with a considerable number of very different kinds of 
 living animals, such as dogs, butterflies, worms, sea-anemones, 
 star-fish, jelly-fish, and so forth. There are also simpler and 
 more minute forms of life, with which, however, the student is 
 presumably at present unfamiliar. Taking such animals as 
 these, therefore, what is there about them to distinguish 
 them on the one hand from not-living things, and on the 
 other hand from plants 1 Let us endeavour to organise and 
 make exact the general knowledge on these questions which 
 we already possess. 
 
 A marked characteristic of life is growth. But we speak also 
 of the growth of not-living things, of clouds, of the river in 
 flood, of crystals from solution, of tapioca grains on boiling. 
 What then distinguishes living growth ? We cannot, perhaps, 
 describe it better than by saying, (1) that it is an organic growth, 
 that is a growth of the various organs of the living animal in 
 due proportion ; (2) that it is a growth, not merely by the 
 
Chap. I.] INTRODUCTORY. 
 
 addition of new material, but by the incorporation of that new 
 material into the very substance of the old ; and (3) that the 
 material incorporated during growth differs from the material 
 absorbed from without, which has thus undergone a chemical 
 transformation within the animal. The growth of the organism 
 is dependent upon the continued absorption of new material 
 from without, and its transformation into the substance of the 
 body. 
 
 But, after a while, the growth of the living animal ceases. It 
 is, as we say, fully grown. Why, then, continue the process of 
 intus-susception, as it is called, that is, the incorporation of new 
 material from without ? Because, if it be not continued, the 
 animal wastes away and dies. And thus a new fact comes to 
 light, that of constant waste, which must be made good by con- 
 stant repair. So that we may say that a living animal is a centre 
 of continual waste and repair, of nicely-balanced constructive and 
 destructive processes. 
 
 Only so long as the constructive processes outbalance the 
 destructive processes does growth continue. During the greater 
 part of a healthy man's life, for example, the two processes, 
 waste and repair, are in equilibrium. In old age waste slowly 
 but surely gains the mastery ; and, at death, it sets in unchecked 
 by repair. 
 
 So far, then, a living animal is a centre of continual waste and \ 
 repair, which may or may not be accompanied by growth. Let / 
 us now look at this growth a little more closely. There is some- 
 thing more than growth in the passage of the infant into the 
 man. There is development as well. But take a more marked 
 case. In spring and early summer there is plenty of frog-spawn 
 in the ponds. A number of blackish specks of the size of 
 mustard seeds are imbedded in a jelly-like mass. They are 
 frogs' eggs. They seem unorganised. But watch them, and the 
 organisation will gradually appear. The egg will be hatched, 
 and give rise to a little fish-like creature (see Fig. 2, p. 3). This 
 will gradually grow into a tadpole, with a powerful swimming 
 tail. Legs will appear. The tail will shrink in size and be 
 drawn into the body. The tadpole will have developed into a 
 
ANIMAL BIOLOGY. [Chap. I. 
 
 frog. S@, too, in the case of the chick. We know that the egg 
 we eat for breakfast, if it had been placed for three weeks or so 
 under a hen, would have developed into a little chicken. But 
 not only the frog and the chick, but the dog, the worm, the 
 butterfly, the star-fish, are one and all developed from an egg 
 which is at first just a little speck of living matter. So that we 
 may say that living animals, during their growth, pass from a 
 comparatively simple condition to a comparatively complex con- 
 dition by a process of change which we call development. 
 
 Now it is clear that, since we have no knowledge of dead 
 matter springing into existence as living matter, life on the 
 earth would soon cease if there were not something more than 
 growth, development, decay, and death. Since death is the 
 heritage of living things, we have the necessity for reproduc- 
 tion. This process is essentially the detachment of a part of the 
 parent organism, which part itself, in turn, develops, reproduces, 
 decays, and dies. In the higher animals reproduction becomes 
 possible when growth and development are ceasing. The excess 
 of repair over waste is seen, not in growth, but in the periodic 
 detachment of a portion of the organism to continue its kind. 
 
 Another fact must now be introduced since it is one that is 
 eminently characteristic of living things. In any animal the 
 line along which the series of changes (growth, development, 
 etc.) takes place is not indeterminate, but is determined by 
 inheritance. Every mammal, e.g., begins life as a minute speck 
 of an egg. There is often nothing about the egg to tell us to 
 what particular animal it will give rise. And yet this is already 
 quite determined by inheritance. So that we may say, sum- 
 ming up so far, that a living animal is a centre of continual 
 waste and repair ; it undergoes a series of successive develop- 
 mental changes constituting its life-history, the special nature 
 of which is determined by inheritance ; it reproduces its kind 
 by the detachment of a portion of its own substance. 
 
 And what is that substance ? The essential constituent of a 
 living animal is protein, composed of carbon, hydrogen, oxygen, 
 and nitrogen, with a little phosphorus and sulphur. This, with 
 much water, forms the chief constituent of protoplasm. At pre- 
 
Chap. I.] INTRODUCTORY. 
 
 sent it is only known as the product of life. The protoplasm, 
 moreover, is not distributed throughout the body in a con- 
 tinuous uninterrupted mass, but is disposed in separate indi- 
 vidual particles called cells. If you scrape gently the inside 
 of your lower lip with a pen-knife, you will remove some of 
 the cells of that part of the body. Mounted in a little saliva, 
 and examined under the microscope, 1 they will be seen (Fig. 
 24, ii.) to be flattened plates of irregular shape, sometimes 
 curled up at the edges, and containing a rounded spot at the 
 centre, called the nucleus. Of such nucleated cells (or their 
 products), differing in form and appearance in different parts, 
 the whole body is compounded. 
 
 Is there anything further to be added ? It may be suggested 
 that animals move about and are guided by feelings. Now, 
 with regard to the feelings, it will be well to leave them on one 
 side. Each of us knows a great deal about his own feelings, 
 and very little of the feelings of his neighbours. Our know- 
 ledge of the feelings of animals is only arrived at by a complex 
 process of inference ; and with regard to the lowest animals we 
 are completely ignorant whether they have feelings or not. 
 Leaving the feelings on one side, therefore, we may notice that 
 moving about is only a particular and conspicuous manifesta- 
 tion of that general activity or vital energy which is character- 
 istic of the animal organism. A large proportion of .this activity 
 arises from the fact that the organism is eminentl^ sensitive 
 using this term not as necessarily implying feeling, but in the 
 same sense as a photographer would use the word when speak- 
 ing of a sensitive plate. The animal is sensitive in its prompt 
 and ready response and reaction to the stimulus of surrounding 
 conditions, just as a sensitive plate promptly reacts under the 
 
 1 The student must provide himself with a good working compound micro- 
 scope. The instrument I am in the habit of recommending to my students in 
 the University College, Bristol, is a stand on the Hartnach model, with a No. 2 
 or 3 Zeiss ocular, and Zeiss' objectives A and E, or a " 1-inch" and "th-inch" 
 of English make. The former is spoken of in this book as low power, the latter 
 as high power. Such an instrument may be obtained from any first-class 
 instrument maker. The student is warned against purchasing low-priced 
 second-hand instruments by inferior makers. 
 
ANIMAL BIOLOGY. [Chap. I. 
 
 stimulus of light. But while a large proportion of the activity 
 of the organism is due to its sensitiveness, a certain proportion, 
 which increases as we rise in the scale of organisation, is spon- 
 taneous, for the animal contains the springs of action within 
 itself. It will be well, therefore, for us to replace the particular 
 statement, that animals move about, by the more general, com- 
 prehensive, and exact statement, that they exhibit certain 
 activities, prompted from within or called forth by surrounding 
 conditions. So that we may now finally state the character- 
 istics of living animals as follows : A living animal consists of 
 - an aggregate of protoplasmic cells, together with certain cell- 
 products ; it is a centre of waste and repair ; it undergoes 'a 
 series of developmental changes constituting its life-history, 
 the special nature of which is determined by inheritance ; it 
 exhibits certain activities by which it maintains its relation to 
 surrounding conditions ; it reproduces its kind by the detach- 
 \ ment of a portion of its own substance. 
 
 > This definition or description applies to all the higher 
 animals; but we shall learn hereafter that there are many of 
 the lower .animals that are not cell-aggregates, but are each of 
 them constituted by a single cell. These unicellular animals 
 are called protozoa, or cytozoa, while the multicellular organisms, 
 in which, as we shall see, different cells have different modes of 
 activity, are called metazoa or histozoa. 
 
 Let us now consider the essential points in which one of these 
 higher animals the animals with which we are ordinarily 
 acquainted differs from one of the higher plants the plants 
 we see around us. But first we may note the points of resem- 
 blance. They run almost through the whole description given 
 above. Both animal and plant are cellular and protoplasmic ; 
 both grow by intus-susception ; both are centres of waste and 
 repair ; of both there is a developmental life-history ; in both is 
 reproduction similar in principle. The activities of the animal, 
 however, differ widely from those of the plant. But the main 
 difference is in the nature of the food, and the manner of its 
 intus-susception. Plants can build up protoplasmic matter out 
 of such inorganic materials as contain the requisite elements. 
 
Chap. I.] INTRODUCTORY. 
 
 Animals cannot do this. They require ready-made protoplasm 
 in the form of vegetable or animal matter. Plants raise the 
 inorganic into the organic; animals then take up the process 
 and often carry it forward to more complex products. But no 
 animal can raise the inorganic into the sphere of the organic. 
 That is the function of the plant. Plants alone can manufacture 
 protoplasm. Animals require the manufactured article. 
 
 But the manufactured article is not directly incorporated as 
 such into the substance of the consumer. The protoplasm of 
 sheep cannot become the protoplasm of man without being 
 largely taken to pieces, chemically, and then put together again. 
 Hence the necessity for digestive and assimilative organs to 
 enable the animal to do this. Even the protoplasm of the 
 mother's milk must be digested before it can be assimilated by 
 the young. In plants anything of the kind is exceptional and 
 subsidiary. They take the elements of protoplasm from the air 
 that bathes their leaves, and from the water that bathes their 
 roots. Hence the branching and spreading form of that part 
 which is exposed to the air, and the far-reaching ramifications 
 of that part which is implanted in the earth. We may sum up 
 this distinction by saying that animals differ from plants in that 
 they require protoplasmic food-stuff which must undergo a more 
 or less- complex process of digestion, within or without their 
 bodies, before it can be assimilated. 
 
 Out of this main distinction there flows a secondary distinc- 
 tion of some importance. Plants manufacture organic tissues 
 out of such inorganic raw materials as contain the requisite 
 elements. One of these raw materials is the carbonic acid gas 
 of the air, from which the green plant abstracts the carbon, 
 returning the oxygen to the air as a by-product for which it has 
 no use. But carbonic acid gas is one of the chief products of that 
 continued waste which is entailed by the ceaseless activity of the 
 animal. And to enable that waste which, as we have seen, is 
 essential to animal life to continue, the animal requires a more 
 or less liberal supply of oxygen. So that animals and the 
 higher green plants perform opposite and complementary func- 
 tions in the economy of nature. By the plant carbonic acid gas 
 
ANIMAL BIOLOGY. [Chap. I. 
 
 is decomposed, and oxygen set free into the air. In the animal 
 the carbonic acid gas is recomposed and breathed forth into the 
 air. 
 
 The higher animals thus differ from the higher plants (1) in 
 that they recompose the carbonic acid gas which the plants 
 decompose ; (2) in the nature and amount of their activities ; 
 and especially (3) in requiring protoplasmic food-stuff, which 
 must undergo a process of digestion before it can be assimilated. 
 
 Having now gained some idea of the nature of those living 
 things which it will be our task to study, let us proceed to 
 consider what is the nature of the problems that are likely to be 
 suggested by the study of animal organisation and animal life. 
 And that we may not fall into generalities beyond the reach of 
 the student,, let us consider the special case of such an animal as 
 the common frog. In the first place, the fully-grown frog has 
 a tolerably constant external form and appearance, by which 
 it may always be recognised and distinguished from other 
 organisms, such as the toad, or the edible frog of the Continent. 
 Dissection after death further shows that it has a definite and 
 tolerably constant internal structure. A number of organs such 
 as heart, liver, stomach, kidneys, are found within the body, 
 and these have a constant form and constant relations to the 
 nerves and blood-vessels which ramify throughout the body. 
 And minute examination with the microscope further shows 
 that there is a definite and constant minute structure. The 
 organs are seen to be built up of cells or cell-products aggre- 
 gated in special ways into what are called tissues. So that 
 we have to consider : (1) the general form and structure of the 
 organism ; (2) the special form and structure of the organs ; and 
 (3) the minute form and structure of the tissues. 
 . But the frog does not stand alone among organisms. Hence 
 it is necessary to compare its general structure, the structure of 
 its organs, and the structure of its tissues, with the general, 
 organic, and minute structure of other organisms, with the 
 object of ascertaining what are the points of resemblance and 
 the points of difference. By this means the range and import- 
 ance of the structural problems is enormously increased. 
 
Chap. I.] INTRODUCTORY. 
 
 Now all those problems which deal with form and structure 
 
 ^re called morphological problems ; and Morphology, which deals 
 
 with these problems, forms a well-marked department of Animal 
 
 Biology. From what has just been said, moreover, it follows 
 
 that this department comprises three divisions : 
 
 1. The morphology of organisms (zoology). 
 
 2. The morphology of organs (anatomy). 
 
 3. The morphology of tissues (histology). 
 
 But the form and structure of the frog does not remain the 
 same in all periods of its existence. Beginning life as an egg, 
 it is hatched as a tadpole, and only reaches the full stature of 
 froghood after several metamorphoses. There is, therefore, a 
 Development of the organism as a whole, of the organs within 
 the organism, -and of the tissues of which the organs are com- 
 posed t Hence there arises a second and most important set of 
 morphological problems of development, which may be divided 
 into 
 
 1. The development of organisms j f f ** "^nal (ontogeny). 
 
 ( b. of the race (phytogeny). 
 
 2. The development of organs (organology) ) embryolo gy. 
 
 3. The development of tissues (histogenesis) ) 
 
 So far we have only considered the problems that arise out of 
 a study of form or structure. We have regarded the frog 
 merely as a piece of mechanism. But every structure has its 
 special office. Every organ in the frog's body has its particular 
 work to do, its function as it is called. The muscles contract or 
 shorten, and so produce motion, the heart beats and ministers 
 to the circulation of the blood, the lungs are for respiration, the 
 eye for seeing, the glands for secreting, and so forth. In these 
 various organs, moreover, special tissues perform the special part 
 of the work, and within these tissues special cells are developed. 
 Finally, all these functions are to be made subservient to the 
 general good of an organism which is set in the midst of more 
 or less complex surrounding conditions. And then the frog 
 himself, as one organism among many, has to perform his 
 special function in the economy of Nature. Hence arises a new 
 
ANIMAL BIOLOGY. [Chap. I. 
 
 set of problems connected with functions and the chemical or 
 r^etabolic changes which accompany the exercise of function. 
 These fall within the department of Physiology. And just as 
 there is a morphology of organisms, of organs, and of tissues, so 
 too is there 
 
 1. The physiology of organisms. 
 
 2. The physiology of organs. 
 
 3. The physiology of tissues and tissue cells. 
 
 Other biological problems fall under the head of Distribution. 
 Frogs do not occur all the world over. They have a definite 
 distribution in space. Nor do they occur in all geological 
 strata. They have a definite distribution in time. Hence the 
 problems connected with the distribution of animals fall 
 naturally under two heads : 
 
 1. The distribution in space (chorology). 
 
 2. The distribution in time (chronology). 
 
 Finally, there is a fourth department of Animal Biology, which 
 deals with the causes of those facts which are studied in the 
 other three departments. This is of recent growth. To the 
 questions, What are the structures and functions of tissues, 
 organs, and organisms ? What is the series of developmental 
 changes undergone 1 What is the distribution in space and time? 
 is added a further question, How has all this been brought 
 about 1 And thus to the three departments of Biology which 
 deal with the facts of structure, the facts of function, and the 
 facts of distribution, is added a fourth, called ^twlogy, which 
 deals with the causes to which these facts are due. And this 
 fourth department is co-extensive with the other three, which 
 may be grouped together under the head of Descriptive Biology. 
 Descriptive Biology thus deals with facts, and Etiology with 
 causes. And since no fact can be said to be understood until 
 we have some knowledge of its manner of causation, it is clear 
 that ^Etiology is necessary to and supplementary to the other- 
 wise incomplete science of Descriptive Biology. Descriptive 
 Biology without ^Etiology lacks life : ^Etiology not founded on 
 
Chap. I.] INTRODUCTORY. 
 
 Descriptive Biology is baseless speculation. Both must grow 
 together and minister to each other's wants. 
 
 It only remains, in this introductory chapter, to indicate the 
 aim and scope of this volume. 
 
 The number of animals each a centre of so many biological 
 problems is well-nigh countless. So is the number of houses 
 in England. And yet, on entering the house of an acquaintance 
 whom we have never before visited, we know at once what we 
 shall find. The hall and passages, the dining-room, drawing- 
 room, library; kitchen and offices below; bedrooms above we 
 already more than half know them all. And why ? Because 
 we already know the type of house that belongs to a certain 
 grade of society. If we visit a lord or a labourer, our expecta- 
 tions are different. We know that some men have to live in 
 hovels, in which one room has to suffice for all the needs of the 
 family. We know that at the other end of the scale of social 
 life there is a separate room for every function of that more 
 complex life. So is it also with animals. There is a lowly type 
 of animal, where a single cell constitutes the whole house, and 
 all the functions of life have to be performed in and through 
 that single cell. There are others composed of many cells, in 
 which differentiation of structure and specialisation of function 
 have been carried far. But a special type of structure is 
 characteristic of each grade of animal life, just as a special type 
 of house characterises each grade of social life. And if we know 
 the type which marks that special grade, we more than half 
 know what will be the state of matters with regard to differen- 
 tiation of structure and specialisation of function in any indi- 
 vidual case. If we know, for example, the tvjpical structure of a 
 mammal, a bird, and an amphibian, we know pretty well what 
 to expect in any other mammal, bird, and amphibian, and are 
 some way on our road towards a knowledge of the structure of 
 a fish or a reptile. And so with regard to other grades of 
 animal life. 
 
 It is the object of this volume to assist the student in acquir- 
 ing such a knowledge of some of the more important facts of the 
 morphology and physiology of certain typical animals as may form 
 
ANIMAL BIOLOGY. [Chap. I. 
 
 a base-line sufficiently accurate and extensive to enable him, by 
 further study and research, to carry on his survey of the animal 
 kingdom. Space will not permit of the study of the facts of dis- 
 tribution. And with regard to Etiology, the aim will be rather 
 to pave the way for a study of causes by an accurate presentation 
 of facts than to deal at any length and more than incidentally 
 with the theory of Evolution or the Doctrine of Descent. 
 
PART I. 
 
 VER TEBRA TE A NA TOM Y A ND PH YSIOLOG Y 
 
 AS EXEMPLIFIED BY 
 
 THE FROG, THE PIGEON, AND THE RABBIT, 
 with occasional Reference to other Types. 
 
CHAPTER II. 
 
 NATURAL HISTORY AND EXTERNAL CHARACTERS. 
 
 1. The Frog. The common frog, Eana temporaries, is tolerably 
 abundant in summer in damp places and by the side of ponds. 
 In winter it is not readily to be found, for at that season of the 
 year frogs hibernate, often in groups, buried in the mud and 
 under water. The warmth of spring rouses them from their 
 torpor, and they then congregate and pair with much sound of 
 croaking. The female lays a number of eggs, about one-tenth 
 of an inch in diameter, each of which is surrounded by a thin 
 layer of albumen. And as they are laid the male pours 
 upon them a fertilising fluid. If a little of this fluid, or the 
 water into which it is shed, be examined under the micro- 
 scope (high power), a great number of minute active bodies of 
 delicate tapering form will be seen. They are the spermatozoa, 
 the essential elements in fertilisation. After the eggs have been 
 fertilised by the entrance into each egg of a spermatozoon, the 
 albuminous coating of the yolk swells to many times its original 
 thickness by the absorption of water; so that the frog-spawn 
 then has the appearance of a white gelatinous mass, made up of 
 largish jelly 4ike spheres, in the midst of each of which is the 
 dark ovum, which is seen, on closer inspection, to have a darker 
 and a lighter hemisphere. 
 
 In a few hours after fertilisation a groove forms on the darker 
 hemisphere, gradually extends round the egg, and, becoming 
 deeper and deeper until it reaches the centre, cleaves the ovum 
 into two hemispheres. Let us note clearly what has taken place 
 here. The ovum, to begin with, is a single cell, and within it is 
 a specialised portion which would seem to be of special import- 
 1 
 
ANIMAL BIOLOGY. 
 
 [Part I. 
 
 ance in cell life, the nucleus. The first thing that happens in 
 development is the division of the original nucleated cell into 
 two nucleated cells, called Uastomeres. 
 
 Once begun the cleavage rapidly continues. The two cells 
 
 become four, the four become 
 eight, the eight become sixteen 
 (Fig. 1, ii. iii. iv.), and so on, 
 until by combined vertical and 
 horizontal cleavage the primitive 
 nucleated cell of the ovum has 
 become split up into a great 
 number of much smaller blasto- 
 meres, each of which is, however, 
 a nucleated cell. The cells in 
 the darker hemisphere are smaller 
 than the cells in the lighter 
 hemisphere; but after awhile the 
 darker portion begins to encroach 
 upon the lighter, and this goes on 
 until the whole surface has become 
 dark, except a little patch, which 
 finally becomes only a small de- 
 pression, called the blastopore (Fig. 1, vi. IL). 
 
 The first indication of the future frog is a broad shallow 
 groove, the edges of which soon rise up to form ridges or folds. 
 It is known as the neural groove, and the ridges as neural folds 
 (Fig. 2, i. ng. and nf.). As development proceeds the folds rise 
 up further, and bending over meet along the middle line, so as 
 to convert the neural groove into a neural canal. The neural 
 tube thus formed will give rise to the brain and si>inal cord of 
 the future frog. 
 
 The body now begins to elongate, becoming at first oval, but 
 soon showing unmistakeable signs of head and tail. The mouth 
 is indicated by a faint depression, behind and on either side of 
 which are two well-marked suckers (5.), tending to run into each 
 other posteriorly in the middle line. In front of the mouth 
 depression (stomodceum) is a fold of skin (ii. fr.), at the upper 
 
 FIG. 1. CLEAVAGE IN FROG'S OVUM. 
 
 i.-iv. Stages with 2, 4 ; 8, and 16 blasto- 
 meres. v. A later stage when the smaller 
 blastomeres cover half the ovum. vi. A 
 still later stage when the smaller blasto- 
 meres have enveloped the whole ovum, 
 except at one spot, the blastopore, bl. 
 
Chap. II.] 
 
 EXTERNAL CHARACTERS. 
 
 angles of which are the nasal pits (na.). The position of 
 eye (opt.) and ear (au.) are marked out. At the sides of the head 
 are four bar-like elevations, indicating the position of so-called 
 visceral arches. The first (md.) is called the mandibular arch ; 
 
 Fro. 2. METAMORPHOSIS OF FROG. 
 
 i.-v. Stages in development of tadpole, au. Auditory organ, bl. Blastopore. 
 Ir. i.-ii. First and second branchial arches. 5r. a. Branchial aperture. 
 ex. Ir. External branchiee. /. I. Fore-limb, fr. Frontal process, h. I. Hind- 
 limb, liy. Hyoidean arch. m. Mouth, n. f. Neural fold. n. g. Neural groove. 
 md. Mandibular arch. na. Nasal pit. op. Operculum. opt. Optic pit. s. 
 Suckers, v. Vent. 
 
 the second (%.) the hyoidean ; the other two are branchial 
 arches, and bear the rudiments of external gills, or branchia3. 
 Posteriorly, beneath the root of the tail, is a depression for the 
 vent (proctodceum, v.). 
 
ANIMAL BIOLOGY. [Part I. 
 
 The embryo is now getting ready to leave the egg within 
 which it lies, with its tail somewhat curved to one side (ii.). At 
 last it becomes more active, breaks through the jelly-like mass 
 and is hatched. It soon attaches itself to plants, or to the out- 
 side of the remaining gelatinous material, by means of the pair 
 of suckers (5.) near the mouth, and somewhat later enters on a 
 free-swimming existence. There are now (iii. ex. 5r.) three pairs 
 of external branchiae. In front of the first, and behind the first 
 and second, there are slits or gill-clefts passing inwards to the 
 throat. Water is taken in at the mouth and passes out through 
 the clefts just as in a fish. Through the branched filaments of 
 the gills the circulation of the blood may be watched under the 
 microscope. 
 
 As the fish-like tadpole enlarges, a membranous fold (oper- 
 culum, iv., op.) is developed in front of the gills, and gradually 
 extends backwards over them. Before long it has completely 
 covered the external gills on the right side, and closely adheres 
 to and unites with the body behind the clefts. Thus the exit of 
 water on the right side is stopped. On the left side, however, 
 the fold does not altogether unite with the body, but leaves a 
 branchial aperture (v., br. a.) which remains until near the end of 
 tadpole life. Beneath the opercular membrane on each side 
 there is a branchial chamber, and the two chambers communicate 
 below. But the water which passes through both chambers, 
 makes its exit through the single branchial aperture. During 
 the formation of the branchial chambers the external gills 
 atrophy, that is, disappear by absorption ; but to take their place 
 internal gills are developed on the inner sides of the branchial 
 clefts. 
 
 Meanwhile the tadpole has become an active and vigorous 
 feeder, mostly vegetarian, but by no means despising animal 
 food. The mouth (v., m.) lies at the bottom of a somewhat pro- 
 trusible cup, with a circular lip covered over with horny 
 asperities, while within the cup the mouth is armed with two 
 great horny crescentic jaws, with which it crops the water weeds, 
 or browses on the flesh of some dead comrade. 
 
 The first sign of limbs is the appearance of two little rudi- 
 
Chap. II.] EXTERNAL CHARACTERS. 5 
 
 ments of legs near the root of the tail. They are at first 
 enclosed in the skin of the tail, and acquire a considerable size 
 before any sign of fore-legs appears, for the fore-limbs lie hidden 
 beneath the operculum, so as to be invisible without dissection. 
 In Fig. 2, v., the right foreleg (/. I.) has been pulled out through 
 a hole cut in the opercular membrane ; the left remains in- 
 visible. 
 
 Thus by acquiring legs the organism passes from a fish-like to 
 a truly amphibian condition. It is still, however, a tailed 
 amphibian, like the newt or triton of our ponds. But as the 
 legs increase in size, the tail shortens and begins to atrophy ; 
 and at last, by a final ecdysis, or throwing off of skin, the oper- 
 cular membrane is got rid of, and the fore-legs are set free ; the 
 horny jaws are lost, and the mouth loses its suctorial form ; the 
 eyes, hitherto covered by skin, become freely exposed ; the gills 
 atrophy, and the gill-slits close ; and the little frog breathes 
 entirely (he has for some time breathed partially) by means of 
 lungs. The short and stumpy rudiment of a tail gradually dis- 
 appears, and the long series of changes is complete. 
 
 The student should be careful to verify the facts for himself. 
 The frog-spawn is readily obtainable in the spring; and the 
 tadpoles are easily reared in an aquarium. 
 
 Such a series of changes as is undergone by the frog is called 
 metamorphosis, which essentially consists in the reduction and 
 atrophy of provisional embryonic organs, and the appearance of 
 adult organs in their place, the series of changes taking place 
 during the free life of the organism. It is well to restrict the 
 word metamorphosis to this use. If the changes take place 
 before birth or before hatching, the word transformation should 
 be used for the prenatal embryonic changes. 
 
 The time occupied in the hatching and metamorphosis of the 
 frog varies with the temperature. At about 15 C. the eggs are 
 hatched in ten days, and the metamorphosis is complete in 
 about seventy-three days; but at about 10 C. the eggs are not 
 hatched for twenty-one days, and metamorphosis is not com- 
 plete until about the 235th day. 
 
 Even when metamorphosis is complete the frog still continues 
 
6 ANIMAL BIOLOGY. [Part I. 
 
 to develop, that is, there is a change in relative dimensions of 
 parts. There is, for example, a well-marked hump in the back 
 of a frog. If now we take measurements from this point to the 
 tip of the snout and the end of the back in an old frog, we shall 
 find that the length of the body anterior to this point is not 
 quite twice the length behind it ; whereas in a minute frog, just 
 emerged from its tadpole state, the length in front is more than 
 three times as great as that behind. So that when metamor- 
 phosis is over there is still development. 
 
 Let us now note some of the more important points in the 
 external characters of the fully-grown frog. 
 
 The body is oval, without neck or tail. The head is large, 
 and within it can be felt a smooth bony mass, the skull. There 
 is a hump in the back, anterior to which the separate vertebra of 
 the back-bone can be felt. Behind this hump there is a smooth 
 rod of bone, the urostyle. The skull, vertebrae, and urostyle 
 constitute the axial skeleton. In the "region of the chest the 
 breast-bone (sternum) can be felt ; but there are no ribs. 
 
 There are four legs, within which the bony supports may be 
 felt. The fore-legs are small, and are jointed to the shoulder- 
 girdle : the hind-legs large and powerful, and attached to the 
 hip-girdle. The bones of the limbs and the supporting arches or 
 girdles constitute the appendicular skeleton. In the fore-limb are 
 the following parts : brachium, or arm ; antibrachium, or fore- 
 arm; and manus, or hand. The corresponding parts of the 
 hind-limb are : femur, or thigh ; crus, or shank ; and pes, or foot. 
 These corresponding parts are said to be homologous. The 
 manus has four digits, and is not webbed. The pes has five long 
 digits, and is webbed. Thickened pads (callosities) are developed 
 beneath the joints of all the digits, there being an extra one 
 just at the base of the innermost digit of the pes, where there is 
 apparently the rudiment of a sixth toe. The part of the 
 manus answering to our wrist is short and small ; but the part 
 of the pes answering to our ankle is very much elongated so 
 much so as to give the appearance of an extra division of the 
 hind limb between the shank and the foot. 
 
 The normal position of the limbs of the frog (Fig, 3, A) should 
 
Chap. II.] EXTERNAL CHARACTERS. 7 
 
 be compared with the primitive vertebrate position which is 
 well seen in the newt as it lies in a 
 sprawling attitude at the surface of 
 the water (B). In the fore-limb the 
 chief change is the bending forwards 
 of the antibrachium and inwards of the 
 manus, so that the little finger is an- 
 terior. In the hind-limb the femur 
 is swung forwards, and the whole 
 
 .... ., ,- ,., -7 FIG. 3. POSITION OF LIMBS IN 
 
 limb bent upon itself like an Z ; so FROG (A) AND NEWT (B) . 
 
 that the great toe, which in the 
 
 newt is anterior in position, is in the frog, interior. 
 
 The integument of the frog is smooth and moist, and is 
 devoid of scales, feathers, hairs, or any form of exoskeleton. On 
 the dorsal aspect (the back) the colour is yellowish, or reddish 
 brown, with dark brown or greenish spots; on the ventral 
 aspect (the belly) it is pale yellow, with fewer spots. But 
 the colour varies a good deal in different frogs ; and it also 
 varies in the same individual. In bright light the colour be- 
 comes brighter : in the dark it becomes duller. If a frog be 
 kept for some time in a dark cupboard, and then brought out 
 into a bright light, its skin will be found to be dull and pale ; 
 but soon it will become much brighter, and more diversified 
 with spots. 
 
 Examination of a small transparent piece of the skin under 
 the microscope (low power) shows that the colour is due to a 
 great number of minute specks of pigmented material, called 
 pigment cells, or chromatophores (Fig. 24, xii., p. 66). These are 
 of various colours, white, light-yellow, orange, red, brown, and 
 black; but the black will probably be the most conspicuous. 
 Some of them will be rounded or oval ; others star-shaped, or 
 arranged like a piece of network, the fibres of which are of un- 
 equal and inconstant thickness. In some places one colour, and 
 in others another colour, predominates. 
 
 It has been ascertained that the cells change their form in 
 accordance with the brightness of the light that falls upon the 
 eye of the frog. Darkness stimulates the cells to activity, and 
 
8 ANIMAL BIOLOGY. [Parti. 
 
 causes them to contract, so that the skin becomes paler, duller, 
 and more uniform in tint. Light causes their relaxation, so 
 that the skin becomes brighter and more diversified with spots. 
 But the darkness and the light do not seem to act directly on the 
 cells, but only through their effect on the eye of the living frog. 
 
 There are four apertures in the frog's body, two median and 
 two lateral or paired. In front is the large mouth with a broad 
 long gape extending back to behind the eyes. On opening 
 widely the mouth, two slit-like apertures are seen at the back, 
 one ventral, clean cut, and longitudinal, the glottis, leading to 
 the lungs : the other dorsal, irregular, and transverse, the open- 
 ing of the oesophagus, or gullet. At the hinder end of the body 
 is the vent or doacal opening, whence issue the faeces, the excre- 
 tion of the kidneys, and the products of the genital organs. 
 
 The lateral apertures are the external nares, one on either 
 side of the snout. They may be seen to open and shut as the 
 frog breathes by the alternate rise and fall of the floor of the 
 throat. A bristle passed into one of the nares passes down- 
 wards and slightly backwards, and emerges in the mouth, by the 
 posterior nares, tolerably far forwards in the roof. The nares are 
 the only lateral apertures. There are no external ear openings. 
 The membrane of the drum of the ear is close to the surface and 
 only covered over with skin. It is readily visible in the midst 
 of the characteristic triangular brown patch above and behind 
 the angle of the mouth. If a hole be pricked in this tympanic 
 membrane, and a bristle passed in, it will emerge at the side of 
 the mouth, through one of the two large Eustachian recesses. 
 
 The tongue is large and white. It is fixed by its anterior end, 
 the free posterior end being bifurcated. It is darted or slung 
 out with great rapidity. The prey (insect, worm, or slug, for 
 the adult frog feeds exclusively on animals) adheres to its sticky 
 surface, and is drawn back into the mouth. Its escape is pre- 
 vented by the teeth, which may be readily felt in the upper jaw 
 and in the roof of the mouth (vomerine teeth). There are no 
 teeth in the lower jaw. 
 
 The eyes are large and prominent, and can be withdrawn into 
 sockets, and thus beneath the upper lids. The delicate and 
 
Chap. II.] 
 
 EXTERNAL CHARACTERS. 
 
 filmy lower lids close up over the eye when thus withdrawn. 
 Round the dark oval pupil is a bright yellow iris. 
 Between the eyes is a minute Irow-spot (see p. 218). 
 
 2. The Codfish. The cod (Gadus morrhua) lives in the tem- 
 perate and Arctic regions of the ocean. It is permanently 
 aquatic, and is hatched from ova which float at the surface of 
 the sea. Its form is admirably adapted for motion through the 
 water, the head passing into the trunk directly, without any 
 neck, and the trunk gradually tapering to the tail. Motion is 
 effected by the fins, of which there are ten six median, and two 
 pairs lateral. There are three median dorsal fins (d. i., d ii.,d. iii.), 
 (in the perch two, in the herring one) along the back, and two 
 median anal fins (a. i., a. ii.), (in the perch and the herring one) 
 on the ventral surface behind the anus. The powerful caudal 
 fin (c.) forms the main part of the tail. The paired fins are the 
 pectoral (pi.) on either side, just behind the head, and the pelvic 
 (pc.) or ventral fins, some- 
 what lower down and fur- 
 ther forward. These four 
 paired fins answer to the 
 four legs of the frog. The 
 pectoral fins are supported 
 on a shoulder-girdle, and P t-m pC " P 1 - 5-' ' ^' 
 
 the pelvic On bones which FIG. 4. -Coo FISH: EXTERNAL CHARACTERS. 
 
 probably represent the hip- a . Anal aperture, a. i. a. ii. Anal fins. b. Barbule. 
 
 girdle. Note that the pel- br - m - Branchiostegal membrane, c. Caudal fin. 
 
 . . i d. i. ii. iii. Dorsal fins. e. na. External nares. 
 
 VIC fins, representing the ^ Genital aper t ure . I I Lateral line. op. Oper- 
 
 hind-legs, are carried so far cuium. pc. Pelvic fin. pi. Pectoral fin. u. IM- 
 
 f i i nary aperture. 
 
 forward as to be anterior 
 
 to the pectorals, representing the fore-legs. This is not so in all 
 fishes e.g. the salmon or the herring. The fins are supported 
 on bony fin-rays, which are, at the ends, soft and flexible. In 
 the perch the rays of the first dorsal fin are stronger, and are 
 produced upwards into sharp spikes. 
 
 There are ten apertures, four median and three pair lateral. 
 The first median aperture is the mouth. This passes back into 
 the gullet ; but at the sides of the pharynx there are five gill- 
 
ANIMAL BIOLOGY. [Part I. 
 
 clefts opening into the branchial chambers. The tongue is 
 small, and cannot be protruded. Both upper and lower jaws are 
 armed with teeth ; and, in addition, there are teeth in the roof 
 of the mouth (vomerine teeth) , and in the pharynx (superior and 
 inferior pharyngeal teeth). There are no posterior nares opening 
 into the mouth. The second, third, and fourth median openings 
 are, in order, the anal (a.), genital (g.), and urinary (u.). 
 
 Of the lateral apertures two pairs are nasal, situated close 
 together, and near the anterior end of the snout (e. na.). The 
 other lateral apertures are the openings of the gill-chambers. 
 Each lies behind a flap-like gill-cover, which is bony anteriorly 
 (operculim, op.), and softer posteriorly (branchiostegal membrane, 
 br. m.), containing bony rays the branchiostegal rays. On rais- 
 ing these gill-covers there will be seen four complete gills, each 
 supported on a bony branchial arch, and composed of a number 
 of free deep-red branchial filaments. There is a fifth more rudi- 
 mentary branchial arch which bears no gill. Attached to all 
 the branchial arches are gill-rakers, horny filaments which bound 
 the margins of the five clefts, and act as strainers. On the inner 
 side of the opercular flap is a red patch (pseudobranchia), which 
 is the rudiment of a fifth gill. 
 
 The fish breathes by gulping in water through the mouth and 
 forcing it out backwards through the clefts, over the gills, and 
 beneath the gill-cover. To prevent the water passing out again 
 through the mouth there are two flaps of skin, one on each jaw, 
 which, as the water passes inwards, fold down against the jaw, 
 but as the water attempts to pass outwards are raised and, 
 coming into contact, bar the passage. Their action may be well 
 seen in the pike in an aquarium. 
 
 The body is invested with an exoskeleton, consisting of over- 
 lapping scales, over which there is spread a thin layer of slimy 
 skin (epidermis) containing pigment cells. Each scale consists 
 of a thin oval plate, which under the microscope (low power) is 
 seen to be built up of concentric rings. The free border is 
 smooth and even (cycloid scale), and is not, as in the perch, 
 produced into a number of comb-like processes (ctenoid scale). 
 Along a definite line down each side of the fish, called the lateral 
 
Chap. II.] EXTERNAL CHARACTERS. n 
 
 line (I. .), the scales are peculiarly modified, so as probably to 
 minister in some way to special sensation. The barbule (b.) 
 beneath the chin is probably an organ of touch. The eyes are 
 large, and have no definite eyelids. There is no external 
 aperture of the ear, nor is there, as in the frog, any visible 
 tympanic membrane. 
 
 3. The Rabbit. The wild rabbit (Lepus cuniculus) frequents 
 furzy sandy heaths, taking shelter when disturbed in deep 
 burrows, which it digs in the sand. In wet soils, instead of 
 digging burrows, it forms "runs "or galleries in the matted 
 vegetation. Its food is green vegetable matter, especially the 
 young shoots of the furze. 
 
 The rabbit begins to breed at the age of six months, and has 
 several litters of from three to nine young in the year. At this 
 rate a single pair of rabbits might, at the end of about five 
 years, look round with pardonable pride on a colony of some- 
 thing like a million descendants. The mother forms a special 
 chamber in which the young are born and suckled. At birth 
 they differ considerably from the adult. The head is much 
 larger, the ears comparatively short, the tail a respectable length, 
 and the fore and hind limbs of about equal size. They have a 
 much more average mammalian appearance than the adult ; 
 specialization setting in as they grow up. They are suckled 
 for a fortnight or so, and are adult in five or six months. 
 During that time the rate of growth of the trunk is greater 
 than that of the head ; that of the hind-legs much greater than 
 that of the fore-legs. The ears grow very fast, and the tail 
 hardly at all : the former tend to droop, while the latter 
 acquires its characteristic upward curve. In all this there is 
 something more than growth there is development. But there 
 is no metamorphosis. 
 
 In the adult rabbit there is a distinct head, pretty well 
 marked off into a facial region in front, and a cranial region 
 behind. The neck is short, but distinct. The body is stout and 
 slightly elongated, and divided into a thoracic region anteriorly, 
 and an abdominal region posteriorly. The sides of the thoracic 
 
ANIMAL BIOLOGY. [Part I. 
 
 region are guarded with ribs, which meet the sternum or 
 breast-bone below. The walls of the abdominal region are 
 soft. 
 
 The whole skin is invested with an exoskeleton consisting of 
 hairs, greyish brown on the back and limbs, white on the belly 
 and under the tail. On the general surface of the body the 
 hairs are of two kinds ; larger and longer contour hairs (pili), 
 and shorter and softer fur (tana or lanugo). In the seal at the 
 Zoological Gardens we see the hair ; but in the dressed sealskin 
 the hair is removed, and we see only the prepared and dyed 
 fur. Special hairs are developed on the face : the vibrissce or 
 mystaces on the upperl ip ; the supra-orbital hairs, answering to our 
 eyebrows ; the eyelashes, and the malar vibrissce or cheek whiskers. 
 These vibrissse may be regarded as long and delicate sense- 
 levers. The hair is reflected into the mouth, so as to line the 
 inner side of the cheeks, and is continued over the under 
 surface of both fore and hind feet. 
 
 If a few hairs be mounted in glycerine, and examined under 
 the microscope (low and high power), each will be seen to have 
 three layers : (1) a delicate external layer or cuticle, composed of 
 slightly overlapping scales, best seen near the tip of the hair; 
 
 (2) beneath this a longitudinally fibrous layer, the cortex ; and 
 
 (3) a central layer of irregular structure, the medulla (Fig. 30, 
 vi. vii.). 
 
 There are nine external apertures, three median and three 
 pairs lateral. The anterior median aperture is the mouth, 
 bounded by upper and lower lips, the former having a groove 
 which passes upwards to the external nares. In front of the 
 mouth are the large gnawing or incisor teeth ; further back and 
 separated from these by a long space or diastema are the grind- 
 ing teeth (premolars and molars). In a very young rabbit there 
 are three incisors on each side of the mid-line in the upper jaw, 
 one larger and two smaller, one of the latter being just behind 
 the larger tooth. In the lower jaw there is only one incisor on 
 each side. There are three grinding teeth on each side in the 
 upper jaw, and two in the lower. In the adult rabbit there are 
 only two incisors in the upper jaw, the little ones to the side 
 
Chap. II.] EXTERNAL CHARACTERS. 13 
 
 being lost and not replaced, and one incisor on each side in the 
 lower jaw. The grinding teeth of the young have been shed 
 and replaced by permanent teeth, the premolars, and in addition 
 to these, three more grinding teeth on each side, and in each jaw, 
 are formed behind the premolars. They are the molars. The 
 arrangement of teeth in the young is called the milk dentition, 
 which consists of eighteen teeth ; that in the adult is called the 
 permanent dentition, which comprises twenty-eight teeth. 
 
 The teeth are implanted in sockets, are devoid of fangs, and 
 grow continuously during life, constant waste of substance being 
 constantly made good. Each is composed of three substances of 
 different hardness : cement (softest and bony), dentine (harder), 
 and enamel (hardest). The cement forms a thin incrusting 
 external layer. * The enamel encases the dentine except where 
 it has been infolded. In the large incisors it ^forms, for 
 example, a casing to the dentine ; but the layer of enamel is 
 much thicker along the anterior face, especially in the mid-line, 
 where it is, so to speak, tucked in along a median groove. On 
 the posterior face it is very thin or absent. In consequence of 
 this arrangement of hard and soft substance, the anterior edge 
 of the tooth wears away less rapidly than the posterior, so that, 
 by the friction against each other of the upper and lower 
 incisors, a chisel-like cutting edge is maintained. In the little 
 incisors the front edge wears away slightly more rapidly than 
 the hinder edge, so that the larger and smaller incisors together 
 form a sort of notch into which the incisors of the lower jaw fit. 
 The premolars and molars vary from each other somewhat in 
 form and arrangement of their substance ; but the four median 
 grinding teeth have, from constant use, crowns which present two 
 furrows running across the axis of the jaw. These are separated 
 by a central ridge. Enamel forms the outer layer of the tooth 
 (coated with a little cement), and is infolded along the central 
 ridge, which owes its existence to this harder infolded layer. 
 Dentine occupies the troughs of the furrows. 
 
 Two pairs of passages communicate from the mouth to the 
 nasal chambers, (1) the large posterior nares, which lie far back 
 behind the roof of the mouth, (2) the minute naso-palatine 
 
14 ANIMAL BIOLOGY. [Parti. 
 
 passages, which open from the organ of Jacobson into the mouth 
 a little behind the incisors. There are also small Eustachian tubes 
 leading to the cavity of the drum of the ear. At the back the 
 mouth communicates (1) with the lungs by the glottis, (2) with 
 the stomach by the oesophagus. The tongue is fixed behind and 
 free anteriorly. At its sides are oval wrinkled patches (papillce 
 foliatce), probably gustatory. 
 
 The other median apertures are the anus, and the urino- 
 genital opening, of which the latter is anterior, and differs accord- 
 ing to sex. Of the six lateral apertures, the most anterior pair 
 are the external nares ; the second pair the ear openings, guarded 
 by the large external ears, and passing down to the tympanic 
 membrane, which is not exposed at the surface as in the frog ; 
 and the posterior pair, ducts of the perineal glands, situated on 
 hairless spaces on either side of the anus. These glands pour 
 forth an odorous secretion. In addition to these lateral 
 apertures, there are in the female those on the teats of the 
 mammary glands. 
 
 The fore-legs are considerably shorter than the hind-legs. 
 Each is jointed to the shoulder girdle, and has a brachium, an 
 antibrachium, and a manus with five digits. The hind limb has 
 a femur, crus, and pes with four digits. Thus the great toe is 
 suppressed in the rabbit, and the thumb in the frog. The main 
 hairy cushion under the sole of each foot is the pulvmus, the 
 smaller cushion under each digit the pulvulus. The ankle is not 
 elongated as in the frog, and there is a backwardly projecting 
 heel. All the digits bear strong claws, or are unguiculate. The 
 rabbit walks on the toes of its manus, but allows the whole 
 plantar surface of the pes to touch the ground. Some animals, 
 like the cat, walk on the toes of both fore and hind feet, and are 
 called digitigmde. Others, like the bear, walk on the whole 
 surface of both manus and pes, and are called plantigrade. 
 
 In their normal position the limbs of the rabbit lie parallel 
 with the body ; but whereas the femur runs forward from the 
 hip girdle, the brachium runs backward from the shoulder- 
 girdle. So that we must regard the fore-limb as having been 
 folded backward from the typical position (Fig. 3, B.), while the 
 

 ' 
 Chap. II.] EXTERNAL CHARACTERS. \Sj, !$'/,> 
 
 _ _____ __ 
 
 hind-limb has been folded forward. Both have then been more 
 or less bent at the joints. But if you fold your own arm back- 
 wards from the typical position into 
 the position indicated for the rabbit 
 (Fig. 5), and then bend it at the 
 joints without further change, you 
 will find that the hand is palm up- 
 wards you must turn the hand 
 over to apply the palm to the sur- 
 F IG . 5.-posmoN OF L.MBS IN f ace? a mo tion that is rendered pos- 
 sible by the partial rotation of the 
 
 outer end of the fore-arm. This further change of position has 
 been effected in the rabbit so as to allow the palmar surface 
 of the maims to rest on the ground. 
 
 4. The Pigeon. The Kock Dove (Columba livia), of which 
 our tame pigeons are domesticated varieties, builds, in cliffs and 
 ruined towers, an untidy nest of sticks and leaves. Here she 
 lays two white eggs, upon which she sits for sixteen days, im- 
 parting to the embryo within them the warmth of her own 
 body. The young, when they emerge from the egg, are pro- 
 vided with patches of yellow down, like (but much scantier 
 than) that which covers the newly-hatched chick. Unlike the 
 chick, which, so soon as it is hatched, is bright-eyed, active, and 
 can feed itself, the little doves are at first quite helpless, with 
 closed eyelids, and must be tended by their parents, who feed 
 them with a creamy fluid secreted by the crop. At the end of 
 three weeks, however, they are fledged, and, after a few days' 
 education by their parents, are able to fly forth and fend for 
 themselves in the world. Those birds which, like the dove, 
 have to be nursed for a while by their parents are called 
 alt-rices ; those which, like the fowl or the duck, are able at once 
 to run or swim and feed themselves, are known as prcecoces. In 
 both development accompanies growth ; but it is more marked 
 in the al trices. 
 
 In the head of the pigeon the facial portion is produced into 
 a horny beak with upper and lower mandibles. At the base of 
 
16 ANIMAL BIOLOGY. [Parti. 
 
 the upper mandible is a swollen and featherless patch of skin, 
 the cere. The cranial division of the head is well rounded. In 
 the feathered pigeon the neck passes gracefully into the body, 
 but in the plucked bird the distinction between head, neck, and 
 trunk is obvious. The body tapers backwards, the thoracic 
 region being well developed, guarded by ribs, and provided with 
 a large ventrally-keeled sternum. The feathered tail is fan- 
 shaped ; but the plucked tail is an insignificant upturned pro- 
 tuberance. There are two pairs of limbs ; the anterior converted 
 into wings, the posterior into cursorial legs. 
 
 There is a well-developed exoskeleton consisting of feathers. 
 The beauty of form is given mainly by the contour feathers 
 (pennce), but if these be plucked there still remain the more 
 delicate plumose filoplumes. There are strong quill-feathers in 
 the wings (remiges) and in the tail (rectrices). The hues of the 
 living bird are largely due to the metallic tints of the feathers. 
 
 Although the external appearance of the pigeon would lead 
 one to suppose that the feathers are developed uniformly over 
 the whole body, yet closer inspection shows that they are 
 arranged in more or less definite feather tracts (pterylce\ sepa- 
 rated by featherless spaces (apteria). This may be better seen 
 in a young blackbird or sparrow. There is a spinal tract along 
 the mid-line of the back, broadening or bifurcating posteriorly. 
 On the ventral surface are two parallel bands constituting the 
 ventral tract, and separated by a median inferior space. The 
 spinal and ventral tracts are separated by lateral spaces. There 
 are also special feather tracts on the wings and legs. In the 
 plucked bird, which loses by this process the characteristic grace 
 and symmetry of outline, the feather tracts may be traced by the 
 scars left by the removal of the feathers. 
 
 A quill feather consists essentially of a proximal part (that is, 
 a part nearer the body), the quill, and a distal part (further from 
 the body), the feather or vane. The quill (calamm) is cylindrical 
 and hollow ; at its proximal end is a hole (inferior umbilicus) into 
 which a little fleshy feather-papilla is inserted ; at the distal end, 
 where the quill joins the shaft of the vane, there is an oblique 
 aperture (the superior umbilicus). The vane (vexillum) has a 
 
Chap. II.] EXTERNAL CHARACTERS. 17 
 
 central shaft (rachis) continuous with the quill, but differing from 
 it in being quadrate in section, grooved on its under side, and 
 filled with light white pith. On either side of the shaft are the 
 barbs, attached to it much in the same way as the teeth are set 
 on a comb, and, like them, flattened in a direction at right angles 
 to the axis. The barbs will be found to adhere together, so 
 that they cannot be separated without the application of some 
 gentle force, when they suddenly tear asunder. When the 
 continuity of the feather has thus been broken between any 
 two barbs, simple pressure of the two barbs together will not 
 readily mend it ; but if the lower or proximal part of the 
 broken vane be raised and hitched over the upper part, the 
 barbs will once more adhere together and the broken vane will 
 be mended. 
 
 The cause of this will be evident if we cut off a little piece of 
 the vane, and, after soaking it for a few minutes in alcohol, 
 examine it under the microscope (low power). Each barb will 
 be seen to give rise to smaller barbs, or barbules, arranged on 
 either side of it. The distal barbules (those nearer the feather 
 tip) carry two or three hooks apiece ; the proximal barbules 
 (those nearer the quill) are simple and without hooks. When 
 the vane is perfect, the booklets on the distal barbules hook over 
 the proximal barbules ; and when we mend the broken vane, in 
 the way above described, we hitch the invisible hooks over a 
 series of invisible bars. 
 
 In the filoplumes the vane is rudimentary, and the barbules 
 are provided with no hooks. 
 
 There are seven apertures, three median and two pairs lateral. 
 The most anterior median aperture is the mouth, guarded by 
 horny mandibles, and provided with no teeth. The posterior 
 nares open by a common longitudinal slit in the roof of the 
 mouth, bounded by palatal folds. At the posterior end of this 
 slit are the openings of the Eustachian tubes. The tongue is 
 narrow, pointed, and horny. The oval glottis and the wide 
 gullet may be seen with difficulty at the back of the buccal 
 cavity. At the posterior end of the body there is a common 
 aperture (cloaca) for the exit of the faeces and the urino-genital 
 2 
 
i8 ANIMAL BIOLOGY. [Parti. 
 
 products. The third median aperture is at the summit of a little 
 papilla above the tail. It is the orifice of the duct of the uro- 
 pygial oil-gland. The lateral apertures are the anterior nares, just 
 in front of the cere, and the ear-openings at the side of the head. 
 The fore-limb is divided into brachium, antibrachium, and manus; 
 but the manus is not divided into separate digits, such of the 
 fingers as exist being united into a continuous mass, while the 
 rudimentary pollex (thumb) forms an insignificant projection. 
 The quill feathers attached to the manus are called primaries \ 
 those attached to the antibrachium secondaries. The second and 
 third primaries are longer than the first, and form the tip of the 
 wing. The pollex bears a little group of feathers called the 
 bastard wing. The quills of both primaries and secondaries 
 are overlapped above and below by wing coverts. On the dorsal 
 surface of the brachium is the humeral feather tract, and on that 
 of the antibrachium the alar tract. 
 
 Thus the fore-limb is especially developed for flight. It is a 
 homologous organ with that of the rabbit, but not an analogous 
 organ. Homologous organs are those that are built upon the same 
 plan; analogous organs those that perform the same function. The 
 hind-limbs of the rabbit and the pigeon are both homologous and 
 analogous, but the fore-limbs are homologous but not analogous ; 
 while the wings of a bird and of a butterfly are analogous, for 
 they perform the same function, but not homologous, for they 
 have no community of plan. 
 
 The hind-limb reminds us of that of the frog, in that it has 
 four divisions. The full meaning of these divisions will only 
 become clear when we study the osteology of the hind-limb. 
 For the present we may regard them as femur, crus, and pes ; 
 the latter having an undivided portion, and a divided portion 
 with four digits. The hallux, or great toe, is directed backwards, 
 the other three toes forwards ; the fifth is absent. There is a 
 well-developed femoral tract of feathers on the femur, and a less 
 developed crural tract on the crus. The pes throughout is devoid 
 of feathers, 1 but has instead well-marked red scales. 
 
 The position of the fore-limb during flight is nearly that which 
 1 Not so iii some artificially modified pigeons. 
 
Chap. II.] EXTERNAL CHARACTERS. 19 
 
 we have seen to be primitive in the newt. In its closed position 
 it is bent upon itself Z-fashion, the 
 brachium and the manus being directed 
 backwards, and the antibrachium for- 
 wards. (Fig. 6.) In the hind limb, 
 the folding forwards which we noticed 
 in the rabbit has taken place. But the 
 undivided portion of the pes is so 
 thoroughly raised off the ground, that 
 it appears to belong rather to the leg 
 than to the foot, and, indeed, as we 
 
 shall see hereafter, belongs partly to the one and partly to the 
 other. 
 
 General Conclusions. Thus in these vertebrate types there 
 is much fundamental resemblance overlain and partially masked 
 by many well-marked differences. In the possession of a skull 
 and vertebrated back-bone ; in the mode of attachment of the 
 limbs, and their general structure in frog, pigeon, and rabbit ; 
 in the position of mouth, nares, eyes, and ears, there is resem- 
 blance. In the possession of a common cloaca, the frog and the 
 pigeon differ from the rabbit, as this in turn does from the fish, 
 in having a common urino-genital aperture. In the absence of 
 exoskeleton, the frog differs from the other types. The scales 
 . of the fish are dermal "structures formed in the deeper layer of 
 the skin. Feathers and hairs are epidermal structures formed in 
 the superficial layer of the skin. In their modes of life each 
 differs from the other. The cod-fish is aquatic and marine ; the 
 frog amphibious near fresh water ; the rabbit is terrestrial ; the 
 pigeon fitted for aerial life. Hence the special modification of 
 the limbs and the form of the body. The cod breathes oxygen 
 dissolved in sea-water, the basic salts of which absorb the car- 
 bonic acid expired : the tadpole breathes the oxygen dissolved 
 in fresh water : the frog, pigeon, and rabbit breathe the oxygen 
 of the air, but the frog requires less than one-tenth of that 
 required by the pigeon. In their food they differ widely. The 
 cod feeds on shell-fish and other marine animals ; the tadpole 
 
ANIMAL BIOLOGY. [Part I. 
 
 is herbivorous ; the frog insectivorous ; the pigeon gramini- 
 vorous ; and the rabbit herbivorous ; and their mouths are 
 to some extent fitted for their special food. The cod has 
 powerful crushing jaws and pharyngeal teeth ; the tadpole, 
 horny jaws for cropping the weeds; the frog, a protrusible 
 tongue for catching prey, and minute teeth for holding the prey 
 when caught; the rabbit, special gnawing and grinding teeth, 
 in relation to its mode of feeding; but the beak of the pigeon 
 cannot be said to be specially modified in relation to its peculiar 
 diet, though its stomach is specially modified, being converted 
 into a gizzard, with thick callous walls, in which the grain is 
 triturated by the aid of small stones swallowed for that purpose. 
 Finally, in the temperature of the body there is a marked differ- 
 ence. The temperature of the cod and of the frog varies with 
 the temperature of the water or the air by which they are bathed. 
 Hence the hibernation of the frog in winter, and its renewed 
 activity in the warmth of spring. The temperature of the 
 rabbit's body is constant at about 98 Fah., its clothing of hair 
 enabling it to maintain this temperature even in winter. The 
 temperature of the pigeon's body is 104 or 105 Fah., its thick 
 clothing of feathers, in which is entangled much air an 
 excellent non-conductor of heat aiding it in maintaining this 
 temperature. 
 
 The student is advised to lose no opportunity of comparing 
 the structures of different animals, and ascertaining for himself 
 how far they resemble in the external characters we have now 
 considered, and in the internal structure, on the study of which 
 we have next to enter the cod, the frog, the pigeon, and the 
 rabbit ; and how far they differ from these types. 
 
" 
 
 CHAPTER III. 
 
 GENERAL ANATOMY. 
 
 1. The Frog. The skin of the frog is very loosely attached 
 to the underlying parts of the body, being, indeed, separated 
 therefrom by a system of subcutaneous lymph spaces (Fig. 8, Sc. I. s.). 
 If, therefore, a frog which has been killed with chloroform be 
 pinned out under water in a dissecting dish, 1 it is an easy matter 
 to slit open the skin along the whole mid ventral line without 
 injuring the body-wall beneath. Note in doing so the parti- 
 tions between separate lymph spaces. By reflecting the flaps of 
 skin on either side, the whole of the ventral body-wall may thus 
 be displayed. It will be seen to be white, and largely com- 
 posed of fibrous bands of muscle running in various directions. 
 In the throat they run transversely; in the median line of 
 the trunk they are longitudinal; on either side of this line 
 they run downwards (i.e. ventrally) and backwards; in the 
 pectoral region they converge towards the shoulders. In the 
 mid-line of the pectoral region is the elongated sternum, on 
 either side of which may be felt the bony bars of the shoulder 
 girdle. On each reflected flap of skin is a large vein (great 
 cutaneous), the main trunk of which passes under the arm-pit. 
 It carries blood from the skin towards the heart. Through the 
 abdominal muscles there is seen a median vein (anterior abdo- 
 minal). At the posterior end of the body may be felt the hip 
 girdle. 
 
 1 A useful vessel is made of tinned iron about 8 inches square, with sides 
 slightly sloping outwards, and 2 inches high, bound with thick iron wire to give 
 firmness. A thin piece of cork nailed on to a thin sheet of lead should fit loosely 
 into the bottom. 
 
 21 
 
ANIMAL BIOLOGY. [Part I. 
 
 With the point of a scalpel a small incision may now be made 
 in the muscular body-walls just in front of the hip-girdle, and 
 sufficiently to the left (the frog's left) of the middle line to avoid 
 injuring the median vein. A small hole will thus be made into 
 the body-cavity or cozlom, which contains the chief viscera. By 
 inserting the small forceps into this hole the body-walls may be 
 raised, and an incision may be carried forward just to the left of 
 the median vein as far as the bony bar of the shoulder-girdle. 
 A similar incision may be made to the right of the median vein, 
 and with a little care it may be dissected away from the strip of 
 the body- wall to which it is attached. On raising the sternum 
 the heart will be seen lying beneath it in a membranous bag. 
 Avoiding injury to this organ, the left incision may now be 
 carried forward by dividing with strong scissors the bony bars 
 of the shoulder-girdle to the left of the sternum. Those to the 
 right may be similarly divided, and the sternum carefully removed. 
 By the removal of the ventral wall of its membranous bag the 
 heart may be displayed. After trimming away the sides of the 
 body-walls the dissection will now resemble that given in outline 
 in Fig. 7. Fig. 8 represents a transverse section of the body of 
 a frog which had lain in a solution of 1 per cent, chromic acid 
 for three or four days till the bones were softened. The student 
 should make other such sections through the eyes, through the 
 tympanic membrane, and through the shoulder-girdle. 
 
 We are now in a position to take a preliminary view of the 
 following organs, or systems of organs : 
 
 1. The Alimentary System. 
 
 2. The Eespiratory System. 
 
 3. The Heart and Circulatory System. 
 
 4. The Urino-genital System. 
 
 5. The Nervous System. 
 
 6. The Skeletal System. 
 
 7. The Muscular System. 
 
 8. The Integumentary System. 
 
 1. The Alimentary System. The alimentary canal is a tube 
 running right through the body from the mouth to the vent, and 
 
Chap. III.] 
 
 GENERAL ANATOMY. 
 
 attached to it are certain glands which pour their secretion into 
 the canal. The parts of the alimentary canal are mouth, oeso- 
 phagus, stomach (St.), small 
 intestine (Sin. In.), and large 
 intestine (L. I. ). The glands 
 attached are the liver and 
 the pancreas (Pn.). The 
 cavity in which the canal 
 and its glands lie is called 
 the pi euro-peritoneal cavity 
 or ccelom. 
 
 The stomach (St.) 
 
 EX.JU. 
 
 5.ci. 
 
 IS 
 
 elongated, and lies well 
 over to the left (the frog's 
 left) side. It is much 
 hidden by the large brown 
 liver. But if we lift the 
 lobes of the liver we shall 
 see it pass anteriorly into 
 the oesophagus. Posteriorly 
 it passes at the valvular 
 pylorus into the small intes- 
 tine (Sm. In.), which is 
 This, in turn, 
 
 FIG. 7. VISCERA OF MALE FROG. 
 
 In the female the large ovaries and the coiled 
 white oviducts occupy much space. 
 
 Ant. cib. Anterior Abdominal vein. Ao. Aortic 
 arch. At. Atrium. Ca. Carotid gland and artery. 
 SOmewhat Suddenly into the Ex. Ju. External jugular vein. G. El. Gallbladder. 
 e , 4 .- / ./T T\ j /n. Innominate vein. L. /. Large intestine. Li. 
 large intestine (L. /.), and L . ^ artery p Pulmonary artery, pe. ca. 
 
 this into the cloaca, Open- Pericardial cavity. Pn. Pancreas. S. Cl. Sub- 
 
 1-1,1 clavian vein. Sm. In. Small intestine. Sp. Spleen. 
 
 IDg OUtwardS by the Vent. Tt Am Truncus arteriosis. Ts. Testis. Ur. El. Urin- 
 
 The parts of the canal are ar y bladder, v. ventricle, 
 connected together, and with the dorsal wall of the cavity, 
 by a transparent membrane, the mesentery. The whole pleuro- 
 peritoneal cavity is, moreover, lined with a glistening pig- 
 mented membrane, the peritoneum, which forms the double 
 layer of the mesentery, and so passes on to the viscera. Thus 
 there is an outer (parietal or somatic) layer lining the walls 
 of the cavity, and an inner (visceral or splanchnic) layer re- 
 flected down the mesentery on to the alimentary organs. This 
 
24 ANIMAL BIOLOGY. [Part I. 
 
 is represented by the dark line in Fig. 8. It secretes a moisten- 
 ing serous fluid. 
 
 The stomach and succeeding (duodenal) portion of the small 
 
 TT.st. 
 
 D.tto 
 
 Sv.l.s 
 
 Pie.-. 
 
 aut.Ab. 
 
 FIG. 8. TRANSVERSE SECTION OF THE POSTERIOR PART OF THE BODY OF A 
 MALE FROG. 
 
 ant. Ab. Anterior abdominal vein. D. ao. Dorsal aorta. Du. Duodenum. 
 H. G. Hip-girdle. I. s. p. Lumbo-sacral plexus. L. Int. Large intestine. 
 I/r. Liver. Pe. Peritoneum. Pn. Pancreas. PL c. Postcaval vein. r. p. 
 Renal portal vein. Ren. Kidney. S. int. Small intestine. Sc. I s. Sub- 
 cutaneous lymph space. Sv. I. s, Subvertebral lymph space. St. Stomach. 
 Sy. Sympathetic nerve chain. Ts. Testis. U. st. Urostyle. Ur. Ureter. 
 
 intestine form a loop united by mesentery. In this loop lies a 
 triangular yellow organ, the pancreas (Pn). Through the midst 
 of it there passes a delicate tube opening into the small intestine. 
 This is the bile duct leading the product of the liver from that 
 organ, and from the gall-bladder (G. Bl.) attached to it, into the 
 alimentary canal, and receiving also the pancreatic fluid secreted 
 by the pancreas. Note how the large liver is attached to the 
 anterior end of the cavity, and how it is divided into lobes. 
 
 2. The Respiratory System. The organs of respiration of the 
 adult frog are the lungs. They lie in the pleuro-peritoneal 
 cavity on either side near its anterior end. The right lung is 
 slightly displaced in the figure (7). The layer of peritoneum 
 reflected on to the lungs is called the pleura. Air passes into 
 them from the glottis, and they may be inflated from that aper- 
 ture. The nares are the external apertures of the respiratory 
 
Chap. III.] GENERAL ANATOMY. 25 
 
 system. When the frog breathes the anterior nares are opened, 
 and the floor of the mouth is depressed. Air rushes in. Then 
 the nares are closed and the floor of the mouth is raised. The 
 air is forced into the lungs. When the nares again open the air 
 rushes out of the lungs, which tend to collapse from their own 
 elasticity. 
 
 3. The Heart and Circulatory System. The heart lies in its own 
 special pericardia! cavity (7, pe. ca.), which projects backwards 
 into the pleuro-peritoneal cavity. This cavity is lined with a 
 glistening pigmented membrane, the pericardium, which has a 
 parietal and a visceral layer, the one lining the cavity, the other 
 reflected on to the heart. The heart itself has four parts, (1) 
 sinus venosus, (2) atrium, (3) ventricle, (4) truncus arteriosus. The 
 ventricle is the posterior fleshy part of the heart (F.). The 
 truncus arteriosus (T. A.) passes forwards from the ventricle as a 
 fleshy tube. The atrium is seen in Fig. 7 (At.) on either side of 
 T. A. It is thin-walled. The sinus venosus is also thin-walled 
 and dorsal in position, lying above the atrium. It may be seen 
 by lifting up the ventricle. Into this division of the heart the 
 blood is received from the various parts of the body, and is 
 passed on to the atrium. Thence it flows into the ventricle, and 
 thence into the truncus arteriosus to be distributed throughout 
 the body. In a recently-killed frog the heart may still be beat- 
 ing. Its parts will be seen to contract in the order given. 
 
 After all the other points in the general anatomy of the frog 
 have been made out, the heart may be removed and dissected 
 under water. The atrium will be found to be divided by a 
 septum into two chambers, a right and a left auricle. The ven- 
 tricle has an undivided cavity (Fig. 9, iv. v.). The right auricle 
 receives the blood from the general mass of the body through 
 the sinus venosus (s. v.). The left auricle receives blood from the 
 lungs by the pulmonary vein (p. v.). Blood from both auricles 
 passes into the ventricle, and thence to the truncus arteriosus for 
 distribution. 
 
 The truncus arteriosus soon divides into a right ancl a left 
 branch, each of which gives origin to three vessels (Figs. 7 and 
 10). Of the three arteries into which it splits, the most anterior 
 
26 
 
 ANIMAL BIOLOGY. 
 
 [Part I. 
 
 carries blood to the head (carotid, ca.), and to the tongue 
 (lingual, //.); the mid-branch curves back and passes to the 
 dorsal side of the heart to supply the body with blood (systemic 
 aorta ao. in 7, and sy. ao. in 10); the posterior branch passes 
 
 cl.ao. 
 
 FIG. 9, iv. HEART OF FROG. 
 (i., ii., and iii. may here be disregarded.) 
 
 to the lung (pulmonary, p.], giving off also a (cutaneous) branch 
 (10, cu.) to the skin. The left branch of the two into which the 
 truncus arteriosus splits divides in the same way. The two 
 systemic aortic arches, right and left, curve round and pass to the 
 back of the heart, and after giving off a branch to the fore-limb, 
 ere long meet in the middle line beneath the back-bone, fusing 
 together to form the dorsal aorta (Figs. 8 and 10, d. ao.), 1 which 
 runs backwards in the large subvertebral lymph sinus (Sv. I. s.) shown 
 in Fig. 8. Where the two aortic arches join to form the dorsal 
 aorta (or from the left arch just before it joins its fellow), a 
 large branch (coeliaco-mesenteric, 10, c. m.) passes off to supply the 
 stomach and intestines with blood. Other branches are given 
 off further back to the kidneys (ren., Fig. 8). While posteriorly 
 the aorta bifurcates to supply the two legs (Fig. 11). 
 
 All these blood-vessels which^ supply the organs with blood 
 1 See note, p. 28. 
 
Chap. III.] 
 
 GENERAL ANATOMY. 
 
 27 
 
 direct from the heart are called arteries. When they reach the 
 organ they supply, they branch again and again, and eventually 
 break up into minute tubes called capillaries. Then these unite 
 
 ex. ju. 
 
 an. aV. 
 
 d.ao. 
 
 -c. 
 
 v - m 
 
 Fio. 10. HEART AND SOME GREAT VESSELS OF FROG. 
 
 with each other, and by repeated union give rise to larger blood- 
 vessels, which lead the blood away from the organ. These are 
 called veins. So that we have the series, (1) artery, (2) capil- 
 lary plexus, (3) vein. Where two blood-vessels unite to form a 
 common trunk they should be called factors ; and where a trunk 
 splits into two vessels they should be called branches. 
 
 The blood carried to the lungs by the pulmonary artery is 
 returned thence to the left auricle of the heart by the pulmonary 
 vein. The blood supplied to the head and fore-limbs is returned 
 to the sinus venosus by (precaval) veins (10, pr. c.), of which that 
 on the left side is shown in Fig. 7, that on the right side being 
 removed. This vein is seen to be made up of three factors 
 (external jugular, ex. ju.; innominate, in.; subdavian, s. cl.). The 
 blood from the hind-limbs may take one of two courses, either 
 by the anterior abdominal vein (ant. ab.\ in which case it passes 
 to the liver, and there breaks up into a capillary plexus, or by 
 
28 
 
 ANIMAL BIOLOGY. 
 
 [Part I. 
 
 air. 
 
 two veins (renal portal, r. p.), 1 which will be seen passing to the 
 outer faces of the kidneys, in which case it breaks up into capil- 
 lary plexuses in these organs. 
 From these organs it passes by 
 means of small vessels into a large 
 vein which lies between the kid- 
 neys (post-caval, 10 and 11, pt. 
 c.), 1 and runs up through the base 
 of the liver to the sinus venosus. 
 The blood from the stomach and 
 intestines collects into a large 
 vein (the portal, 10, por.), which 
 carries the blood to the liver. 1 
 There it breaks up into a capillary 
 . v. plexus. From the liver, the blood 
 delivered to that organ by the 
 anterior abdominal, and portal 
 veins, and by the hepatic artery (a 
 branch of the cceliaco-mesenteric), 
 is conveyed into the post-caval by the hepatic veins (10, he). 
 
 As it passes through the capillaries of the various organs of 
 the body, the watery fluid of the blood oozes out and forms the 
 lymph, which collects in irregular cavities called lymph spaces. 
 It is pumped back into the blood-stream by four lymph-hearts 
 (see p. 195). 
 
 4. The Urino-genital System. The urinary organs are the 
 kidneys (8, ren.), elongated red organs lying in the subvertebral 
 lymph space ; attached and closely applied to them are the 
 yellowish adrenal bodies. From the outer edges of the posterior 
 ends of the kidneys arise their ducts (ureters, 8, ur.), which open 
 into the cloaca by minute apertures in its dorsal wall. The 
 ureters of the male have connected with them glandular bodies, 
 the vesiculce seminales. In the ventral wall of the cloaca is a 
 
 a V. 
 
 FIG. 11. RENAL PORTAL AND PELVIC 
 VESSELS : FROG. 
 
 1 When the portal vein has been made out, the stomach, intestines, and liver 
 should be carefully removed, and the subvertebral lymph space opened out 
 without injury to the kidneys, testes, etc. The dorsal aorta, post-caval, and 
 renal portal will then be readily made out, as shown in Fig. 11. 
 
Chap. III.] GENERAL ANATOMY. 29 
 
 median aperture leading into the large bi-lobed urinary bladder 
 (7, Ur. BL). Into it the urinary fluid, therefore, does not pass 
 directly from the ureters, but indirectly through the cloaca. 
 
 The genital organs of the male are the testes (1 and 8, Ts.). 
 They are closely connected with the kidneys by a mesenteric 
 membrane (the mesorchium), in which fine white lines may be 
 seen, which are the ducts by which the seminal fluid is conveyed 
 to the kidney, to pass thence by the ureter, which should there- 
 fore be termed the urino-genital duct in the male. 
 
 The genital organs of the female are, (1) the large ovaries 
 crowded in the breeding season with large ova, and suspended 
 by the mesoarium, and (2) the coiled oviducts. The oviducts are 
 anteriorly thick and glandular and swell up in water ; posteriorly 
 they are thin-walled, and open into the cloaca by slits in its 
 dorsal wall, just in front of the apertures of the ureters. The 
 oviducts do not open into the ovaries, but open at the very 
 anterior end of the pleuro-peritoneal cavity close to the base of 
 the lungs. The ripe ova are shed into the pleuro-peritoneal 
 cavity, and escape thence by the oviducts. 
 
 The yellow finger-like processes seen in both sexes are the 
 fat bodies (corpora adiposa). 
 
 5. The Nervous System. In the subvertebral lymph space 
 there are parts of two nerve systems. 
 
 On either side of the dorsal aorta is a delicate dark thread, 
 from which minute threadlets pass off to much more conspicuous 
 white threads passing backwards beneath and on either side of 
 the urostyle. The delicate dark thread belongs to the sympathetic 
 nerve system (8, Sy.). The larger longitudinal white threads 
 belong to the cerebro-spinal nerve system. They form part of the 
 lumbo-sacral plexus (8, I. s. p.\ from which the nerves of the hind- 
 leg take their origin. Further forward, two of the spinal nerves 
 form a brachial plexus running out to the arm. Other spinal 
 nerves will be seen between these two plexuses. All these 
 spinal nerves take their origin in the myelon or spinal cord, which 
 lies in a dorsal cavity within the arches of the vertebrae of the 
 backbone. In the neighbourhood of the head there are cranial 
 nerves, taking their origin in the brain, which lies within the 
 skull. 
 
ANIMAL BIOLOGY. 
 
 [Part I. 
 
 To see this central nervous system the frog should be turned 
 over and pinned out back upwards. An incision should be made 
 in the skin, and the muscles cleared away from the vertebral 
 column. The dorsal cavity should then be opened out, begin- 
 ning at the junction of the skull and vertebral column, and 
 removing the roof of the skull and the upper portions of the 
 vertebrae. The brain and spinal cord will thus be displayed. In 
 
 FIG. 12. BRAIN OF FROG. 
 A. From above. B. From below. C. In longitudinal section. 
 
 the brain three parts will readily be seen : a, fore-brain, 1 composed 
 of two cerebral hemispheres (12, c. h.) lying side by side, and 
 passing anteriorly into the olfactory lobes (olf.), which are joined 
 in the mid-line; a mid-brain, composed largely of two large 
 rounded optic lobes (op. I. ) ; and a hind-brain, composed of a band- 
 like cerebellum (cb.), and behind this the medulla oblongata (m. 0.), 
 the upper surface of which is marked by a triangular depression, 
 covered over by a plexus of blood-vessels. Dissection will 
 readily show the cerebral hemispheres and optic lobes to be 
 hollow. 
 
 The spinal cord or myelon is continuous with the medulla 
 oblongata; it presents two enlargements where the nerves 
 (1) for the arms, and (2) for the legs, are given off. It ends in 
 a fine point, ihefilum terminale. 
 
 6. The Skeletal System. Somewhat has been said of this 
 
 1 The primitive fore-brain and its outgrowths may be regarded as forc-l>r<iin. 
 See Chapter XL 
 
Chap. III.] GENERAL ANA 7^0 MY. //^F 3I 
 
 system incidentally in the last chapter. Further study we will 
 reserve for Chapter vni, 
 
 7. The Muscular System. Of this system little can here be 
 said. Suffice it to remark that most of the muscles have their 
 origin in some bone or other part of the body which is relatively 
 fixed, and their insertion in some bone or other part of the body 
 which has to be moved. There are some muscles, however, 
 which encircle certain parts of the body, generally tubular, and 
 by their contraction serve to lessen the diameter of the tube or 
 completely close it. 
 
 8. The Integumentary System. Nothing need here be added to 
 that which was said in the last chapter. 
 
 In conclusion, attention may be drawn to the following points 
 for the sake of 'future comparison : 
 
 1. The lungs are contained in the pleuro-peritoneal cavity or 
 ccelom, together with the liver, alimentary canal, generative 
 organs, spleen (7, Sp.), etc. 
 
 2. The heart has one ventricle and two auricles; a sinus 
 venosus, and a truncus arteriosus. 
 
 3. There are two systemic aortic arches. 
 
 4. Blood is distributed to the kidney by renal portal veins 
 from the posterior end of the body, as well as by renal arteries 
 from the aorta. 
 
 5. There is a large anterior abdominal vein. 
 
 6. The kidneys lie in the large subvertebral lymph sinus. 
 
 7. There is a common duct for the testis and the renal organ 
 in the male. 
 
 8. The apertures of the urine-genital duct of the male, and of 
 the ureters and genital ducts in the female, are on the dorsal 
 side of the cloaca. 
 
 9. The urinary bladder opens into the ventral side of the 
 cloaca. 
 
 10. There is a common cloacal aperture for the exit of the 
 "faeces from the alimentary canal and for the urino-genital pro- 
 ducts. 
 
 11. Respiration is by means of gills in the tadpole and lungs 
 in the frog. 
 
ANIMAL BIOLOGY. 
 
 [Part I. 
 
 The Cod. 1 To open up the body-cavity of the cod, the fish 
 should be laid on its side, a ventral incision made from the hip- 
 girdle (under the throat) to within half-an-inch of the anus, and 
 the side wall removed from the peritoneal cavity or ccelom thus 
 exposed. The cavity is lined with peritoneum, which is reflected 
 on to the viscera. It extends backwards, somewhat behind the 
 anus, and is bounded anteriorly by fas pericardio-peritoneal septum, 
 which separates it from the chamber which contains the heart. 
 Along the dorsal wall of the cavity runs the large air-bladder, 
 terminating anteriorly in a coiled caecum, and not communicating, 
 as in some fishes (e.g. trout, herring), with the gullet. 
 
 FIG. 13. VISCERA OF COD-FISH. 
 
 a. Anal aperture, c. &. d. Common bile duct. c. d. Cystic duct. g. Genital 
 aperture. G. B. Gall bladder. 7i. d. Hepatic ducts. H. K. Head kidney, ws. 
 (Esophagus. Py. Co.'. Pyloric cteca. lien. Kidney. 8m. Int. Small intestine. 
 St. Stomach. 11. Urinary aperture. U. Z>. Urinary bladder. Ur. Ureter. 
 
 1. The Alimentary System. The gullet or oesophagus (us., 
 Fig. 13) is of about the same diameter as the stomach (St.], but 
 
 1 Instead of the cod, tlie closely-allied haddock or whiting may be dissected. 
 Where this (or any other animal) is too large for dissection under water, it 
 should be pinned out with blanket-pins (or awls) on to a dissecting board of 
 convenient size. 
 
Chap. III.] GENERAL ANATOMY. 33 
 
 has thinner walls. The stomach is bent upon itself, and where 
 it passes into the intestine there are a number of pyloric cceca 
 (Py. Cce.). A little beyond these processes is the common bile duct 
 (c. b. d.) formed by the confluence of hepatic ducts (h. d.) from the 
 liver. The cystic duct (c. d.) leading to the gall bladder (G. B.), 
 in which the bile is stored, is an offshoot from the common 
 duct. The intestine is somewhat coiled, and passes almost with- 
 out change of diameter to the anus (a.). 
 
 2. The Respiratory System. The respiratory organs are the 
 gills which we have seen in the last chapter. There are no 
 lungs. 
 
 3. The Heart and Circulatory System. To expose the heart the 
 hip-girdle must be divided in the mid-line and the upper half (in 
 the present position of the fish on its side) of both this girdle 
 and the shoulder-girdle carefully removed. The pericardial 
 cavity thus displayed is a small chamber lined with pericardium, 
 which is reflected over the heart. The heart itself has a thin- 
 walled dorsal sinus venosus (14 A, s. v.), an irregular thin-walled 
 atrium (cm.), not divided by any septum into two auricles, a 
 fleshy ventricle (v.) with a single cavity, and a whitish bulbus 
 arteriosus (b. a.). 
 
 The bulbus arteriosus passes forward into the median ventral 
 aorta (14 B, v. ao.), which forthwith sends off branches, afferent 
 branchial arteries (af. br. a.), right and left to the four gills on 
 either side. In the capillaries of the gills the blood is aerated 
 and collects into vessels (efferent branchial arteries, ef. br. a.), 
 by which it is conveyed to vessels, one on each side, which 
 run along the dorsal ends of the gill-bearing branchial arches 
 (right and left epibranchial arteries, ep. br. a.). Traced forwards, 
 these run into the (carotid, car.) arteries which supply the head 
 and brain. They are united together by a transverse vessel 
 beneath the back of the skull (tr.). Traced backwards, the 
 epibranchials fuse together to form the dorsal aorta ; but before 
 they unite each gives off an artery (subclavian, s. a.) to supply 
 the pectoral fin, and that on the right side gives off in addi- 
 tion arteries (coeliac, CM. a., and mesenteric, me. a.) to supply 
 
34 
 
 ANIMAL BIOLOGY. 
 
 [Parti. 
 
 the stomach and intestines. The dorsal aorta passes back- 
 wards beneath the back-bone, and becomes posteriorly the 
 caudal artery. The circulus cephalicus formed by the epi- 
 branchials and transverse vessel is shown very diagram atically 
 in Fig. 14, which must be taken to indicate the principle of the 
 arrangement of the vessels. 
 
 The sinus venosus receives the blood that has circulated 
 through the body. At its posterior end it receives the two 
 
 f.W ac v , 
 
 FIG. 14. DIAGRAM OF HEART AND GREAT VESSELS : COD. 
 
 hepatic veins (he. v.) from the liver. Dorsal to these, there enter 
 two large veins running downwards on either side of the gullet. 
 They are the ductus Cuvieri (d. c. ). Each results from the union at 
 its dorsal end of two factors ; an anterior factor (anterior cardinal, 
 a. c. v.) bringing blood from the head, and a posterior factor 
 (posterior cardinal) p. c. v.) bringing blood from the posterior 
 regions. The right posterior cardinal is continuous through the 
 substance of the kidney, but the left is aborted for some dis- 
 tance. Posteriorly the two cardinals unite in the caudal vein, 
 which, with the caudal artery, runs in a canal (the hcemal arch) 
 protected by bony (haemal) processes of the vertebrae. The right 
 ductus Cuvieri, slightly above the sinus venosus, receives a vein 
 
Chap. III.] GENERAL ANATOMY. 35 
 
 (inferior jugular, i. /.) from the lower parts of the head. The 
 left ductus is seen in Fig. 14 to receive a factor (spermatic) from 
 the male organs of generation. 
 
 The blood from the stomach and intestines is delivered to the 
 liver by a portal system of veins. There it breaks up into a 
 capillary plexus, and is collected again into the hepatic veins 
 which pass straight to the heart. There is a connecting branch 
 between the portal system and the caudal vein. 
 
 4. The Urino- genital System. The renal organ (13, ren.) is an 
 elongated body lying behind the dorsal wall of the air-bladder. 
 Its anterior enlargement, the head-kidney (h. k.) lies above 
 the coiled caecum of the air-bladder ; its posterior enlargement 
 lies within the anterior portion of the hasmal canal; between 
 the two there is on each side an irregular longitudinal band. 
 From the posterior mass an unpaired ureter passes down to the 
 urinary aperture (u.). 
 
 The testes of the male (soft roe) are elongated lobular bodies, 
 uniting together in the median line for the posterior quarter 
 of their length, and sending off to the genital aperture a common 
 duct. 
 
 The ovaries of the female are somewhat conical pink bodies (hard 
 roe), which also unite with each other posteriorly, and send off a 
 common oviduct to the genital aperture (g.). The ovaries are 
 hollow, and the ova are, when ripe, shed into their cavities and 
 pass out by the oviduct without entering the peritoneal cavity. 
 
 5. The Nervous System. The brain may be exposed by skin- 
 ning the top of the head, removing the muscles, and then break- 
 ing away the roof of the skull. It lies somewhat loosely in the 
 cranial cavity. In it the three divisions may readily be made 
 out. The cerebral hemispheres of the fore-brain are relatively 
 shorter than in the frog. In front of them the large optic 
 nerves form a X, and above them lie the long peduncles of the 
 olfactory lobes. The optic lobes of the mid-brain resemble those 
 of the frog. But the cerebellum of the hind-brain is much 
 larger than in the frog. The medulla oblongata passes almost 
 insensibly into the spinal chord. At the base of the brain are 
 two bean-shaped inferior lobes. 
 
36 ANIMAL BIOLOGY. [Part I. 
 
 The side of the fish that has not been dissected away should 
 be carefully skinned in order to see the distribution of two 
 of the cranial nerves. This is shown in Fig. 15. The dark 
 line is the fifth-nerve, of which one branch goes along the 
 
 back and gives off nerve 
 branches to the dorsal fins ; 
 one branch crosses the 
 body to innervate the anal 
 
 FIG. 15. -CRANIAL NERVES v AND x: COD. fins \ and a tnird P asses 
 
 downwards behind the gill 
 slits to supply the pectoral and pelvic fins. The dotted line is 
 the tenth-nerve with two branches : an upper, which follows the 
 lateral line for about two-thirds of its length and then thins 
 out; a lower, which runs along the line of division between 
 the dorsal and the ventral set of muscles, and along the pos- 
 terior third of the lateral line. 
 
 6. The Muscular System. The same Fig. (15) shows in the light 
 zigzag lines, the divisions between the myotomes or segments 
 of the body muscles. The myotomes are, however, far more 
 numerous than here figured. 
 
 Of the skeletal and integumentary systems nothing need here 
 oe said. 
 
 In conclusion, attention may be drawn to the following points 
 for the sake of comparison with other types : 
 
 1. The heart has one auricle and one ventricle ; a sinus 
 venosus and a conus arteriosus. 
 
 2. The blood after aeration is at once distributed throughout 
 the body, and is not first brought back to the heart. 
 
 3. The kidney receives blood from the caudal vein as well as 
 from the renal arteries. 
 
 4. There are separate apertures for the ureter, genital duct, and 
 alimentary canal. 
 
 5. There is a common ureter for the two kidneys, which also 
 unite in the middle line. 
 
 6. There is a common genital duct for the two testes or 
 ovaries, which also unite in the middle line. 
 
 7. There is no connection between the kidney and the testis. 
 
Chap. III.] GENERAL ANATOMY. 37 
 
 8. The urinary bladder (13, V. B.) opens on the antero-ventral 
 side of the ureter. 
 
 9. The posterior nares do not open into the mouth, nor are 
 there any Eustachian tubes. 
 
 10. Respiration is throughout life by means of gills. 
 
 3. The Rabbit. The skin of the rabbit is not separated from 
 the body walls by large lymphatic spaces as in the frog. On the 
 contrary, when we place a rabbit on its back and make a longi- 
 tudinal incision from chin to pelvis, the skin has to be separated 
 from the body walls with the aid of the flattened handle of the 
 scalpel, and even then a thin sheet of (cutaneous) muscle, con- 
 taining numerous minute blood-vessels, will probably adhere to it. 
 
 The abdominal cavity (ccelom) may now be opened out by the 
 removal of its thin muscular walls. Its anterior boundary is a 
 concave moveable partition (the diaphragm) separating it from 
 an anterior thoracic cavity containing the lungs and the heart. 
 This diaphragm behind the lungs is a very characteristic mam- 
 malian structure. The pink lungs may be seen through it. 
 And if we prick it at one side, the lung of that side will be seen 
 at once to collapse. The thoracic cavity may now be opened 
 out, as in Fig. 1 6, by cutting through the ribs with bone forceps, 
 and removing most of the sternum. A ventral bridge support- 
 ing the diaphragm should however be left. 
 
 1. The Alimentary System. Four pairs of salivary glands pour 
 their secretion into the mouth. These are 
 
 a. The Submaxillary, a reddish gland lying between the two 
 divisions (rami) of the lower jaw. The gland of either side 
 meets its fellow in the mid-line. 
 
 b. The Sublingual) small and slightly anterior to the Sub- 
 maxillary. Both these glands send ducts forward which open 
 into the floor of the mouth. 
 
 c. The Parotid, seen by dissecting away the skin in front of 
 and below the base of the external ear. Its duct passes forward 
 into the mouth. 
 
 d. The Infra-orbital, seen by removing the eye-ball. It lies 
 in the anterior and inferior division of the socket, its duct 
 
ANIZTAL BIOLOGY. 
 
 [Part I. 
 
 ex.ju. 
 s.cl. 
 
 passing downwards to the mouth. Note at the same time two 
 other glands in the eye-socket, (1) the Harderian, anterior and 
 superior; and (2) the lachrymal, posterior and superior. They 
 are not salivary. 
 
 From the mouth the oesophagus passes through the thoracic 
 cavity, pierces the diaphragm, and at once enters the large 
 
 curved thick -walled stomach. 
 (The elongated deep red body 
 near its broader end is the 
 spleen. ) Then follows the long- 
 coiled small intestine, of which 
 the first part (duodenum, du.) 
 forms a great U-shaped loop, 
 the two limbs of the U being 
 closely connected by a mesen- 
 teric fold, and the ascending 
 limb, that further from the 
 stomach, being closely con- 
 nected by a membranous fold, 
 with the terminal portion (rec- 
 tum, rm.) of the large intes- 
 tine. Within the loop of the 
 duodenum is a ramifying mass 
 of yellowish-red lobules. This 
 is the pancreas. Its duct opens 
 into the duodenum near the 
 end of the loop. Into 
 
 Ctt. 
 
 rmr 
 
 - Ts. 
 
 FIG. 16. VISCERA OF RABBIT. 
 
 ao. Aorta, ao. ar. Aortic arch. Cce. Csecum. 
 Co. Colon, di. Diaphragm, du. Duodenum. 
 ex.ju. External jugular vein. il. Ileum. I. au. 
 Left auricle. I. I. Left lung. I v. Left ven- the duodenum, shortly below 
 tricle. p. a. Pulmonary artery, p. v. Pul- ,-, T 
 
 monaiy vein. pt. c. Post caval vein, r. au. the stomach, Opens the COlll- 
 Right auricle, r. c. Right common carotid <mon bile duct, formed by the 
 artery, r. I. Right lung. r. m. c. Right pre- / i r r 
 
 caval vein. r. ..Right ventricle, rm. iectum. Union of Several factors from 
 s. cl Subclavian vein. Ts. Testes. U.S. Urinary the Various lobes of the large 
 
 liver and one from the gall 
 
 bladder. The rest of the small intestines (ileum, il.) that part 
 not included in the duodenal loop is suspended by a separate 
 fold of mesentery. At its distal end there are several raised 
 whitish oval patches (Peyer's patches). 
 
Chap. III.] GENERAL ANATOMY. 39 
 
 The small intestine passes into an oval sac (sacculus rotundus), 
 with walls resembling Peyer's patches. Just beyond this there 
 is given off a very large dark thin-walled diverticulum (the 
 ccecum, Cce.), round which there passes a spiral constriction. 
 It ends blindly in a finger-like process (appendix vermiformis), 
 the walls of which are thicker, and again resemble Peyer's 
 patches. 
 
 Just beyond the sacculus into which the small intestine passes, 
 and beyond the csecal appendage, is the commencement of the 
 large intestine, which seems at first sight to be a continuation of 
 the caecum. Soon, however, it narrows and becomes puckered 
 with lateral and median sacculations. Gradually the saccula- 
 tions of this anterior portion (colon, Co.) of the large intestine 
 become less marked, and it passes into the terminal smooth 
 portion (rectum, rm.) of the alimentary canal, which contains pill- 
 like balls of faeces. It passes from the right anterior corner of 
 the abdominal cavity backwards through the pelvic arch to open 
 externally by the anus, near which rectal glands are developed 
 on either side of the rectum. The relative lengths of the vari- 
 ous parts of the alimentary canal should be measured. The 
 total length is some fifteen or sixteen times the length of the 
 body. For the examination of its parts it must be turned over 
 in the ccelom. 
 
 2. The Respiratory System. The roof of the mouth, which 
 forms also the floor of the nasal chambers, is transversely ridged. 
 This is continued backwards as the soft velum palati above which 
 the two nasal passages become confluent. These nasal passages 
 may be regarded as the air-track, the mouth as the food-track. 
 Note that the air-track is here dorsal. The food-track continues 
 as the oesophagus, the air-track as the trachea, through the glottis, 
 which is guarded by the tongue-like epiglottis in front arid the 
 cornicula laryngi behind. Note that here the air-track is 
 ventral, so that in the pharynx the air-track and the food-track 
 cross. 
 
 Behind the glottis is the larynx or organ of voice (the brown 
 glandular mass on either side of and beneath which is the 
 thyroid gland). The air-track continues as the trachea, strength- 
 
40 ANIMAL BIOLOGY. [Parti. 
 
 ened with cartilaginous rings, strong ventrally, but incomplete 
 dorsally. Posteriorly the trachea bifurcates to form the two 
 bronchi, one for each lung, which break up within the spongy 
 substance of the lungs into a multitude of bronchial tubes. 
 
 The mechanism of respiration differs from that in the frog. 
 Each lung lies in a separate pleural cavity, lined with a delicate 
 membrane, the pleura, a visceral layer of which is reflected on to 
 the lung. This cavity the lung completely fills. The pressure 
 of the air in the lung forces it to fill this cavity, which itself 
 contains no air or other gas. Between the lungs lies the heart 
 in its pericardial cavity. When the thorax is complete, there- 
 fore, it is completely filled by the distended lungs on either side, 
 and the heart in the middle. Any alteration in the size of this 
 thoracic box will cause a corresponding alteration of size in the 
 distensible lungs. In two ways can the size of the thoracic box 
 be altered, (1) By a slight change in the position of the ribs; 
 (2) By a change in the position of the diaphragm. When the 
 size is increased air is sucked in, when it is diminished air is 
 forced out. In the frog we had a buccal force-pump. In the 
 rabbit we have a costal and diaphragmatic suction pump. 
 
 3. The Heart and Circulatory System. The pericardial cavity 
 in which the heart lies is between the pleural cavities in which 
 lie the lungs. The student should tear away a little of the 
 parietal layer of the pleura of one side, and satisfy himself that 
 the pericardium is a separate membrane. As he does so he may 
 notice between the two a white thread, the phrenic nerve which 
 passes to the diaphragm. 
 
 The heart itself is slightly conical, with its blunt apex lying 
 somewhat to the left of the median line. Posteriorly are the ven- 
 tricles (16 and 17, 1. v., r. v.), there being two distinct cavities, and 
 not only one as in the frog and the fish. They are thick-walled 
 and fleshy. Anteriorly are the two auricles (r. au., I. au.), thin- 
 walled, and distended with blood, and, especially if the rabbit is 
 young, partially hidden by a soft fatty mass (the thymus gland), 
 which should be removed. There is no truncus arteriosus or 
 distinct sinus venosus, such as we saw in the frog. 
 
 A large thick-walled artery crosses over the anterior end of 
 
Chap. III.] GENERAL ANATOMY. 41 
 
 the heart to the left ; it is the pulmonary artery (p. a.) carrying 
 blood to the lungs. Another thick-walled artery arises rather 
 to the right of this (but from the left ventricle), and after 
 giving off branches (common carotid) which may be traced 
 upwards beside the trachea to the head, and branches to the 
 arms, of which that on the right side is given off from the 
 right carotid (r. c.), passes back over (dorsal to) the heart. It 
 is the aortic arch (ao. ar.\ of which there is but one in the 
 rabbit. It curves over to the left, crossing the left bronchus, 
 and represents the left aortic arch of the frog. As it passes 
 along the roof of the peritoneal cavity, the thick-walled pink 
 aorta (ao.) gives off renal branches to the kidneys, three to the 
 stomach and intestines, one in front of and two behind the renal 
 arteries, and finally divides into two branches (18, com. il. a.) 
 for the hind-limbs. 
 
 Close by this pink thick-walled empty aorta will be seen the 
 larger thin- walled postcaval vein (pt. c.). This carries blood to 
 the right auricle of the heart from the posterior parts of the body, 
 and from the kidneys, to which there passes no renal portal 
 vein. The postcaval shortly before reaching the heart receives 
 the hepatic vein from the liver, which conveys from that organ 
 not only the blood received from the hepatic artery, but the much 
 larger quantity delivered to it from the stomach and intestines 
 by the portal vein. The right auricle, which receives the post- 
 caval vein from the posterior regions of the body, receives also 
 precaval veins (r. pr. c.) from the head and anterior regions. Of 
 these precavals the large (external jugular, ex. ju.) veins seen on 
 each side of the trachea, the smaller (internal jugular) veins lying 
 still closer to the trachea, and large (subclavian, s. cl.) veins from 
 the arms are factors. 
 
 The left auricle receives, by pulmonary veins (p. v.\ the blood 
 which has passed through the capillaries of the lungs. 
 
 The distribution of the vessels will be seen by reference 
 to Figs. 17 and 18, and will be more fully considered in 
 Chapter X. 
 
 4. The Urmo-genital System. The kidneys, which have the 
 characteristic mammalian form, are attached to the body-wall 
 
ANIMAL BIOLOGY. 
 
 [Part I. 
 
 of the dorsal part of the peritoneal cavity, the peritoneum 
 passing over their ventral surfaces. The left is a good deal 
 
 pul.tr. 
 
 JWl. V. 
 
 in. ilv. 
 
 <^ Jem. a. 
 
 ^ ia.iL a. 
 
 TM. 8. Ct. 
 
 FIG. 18. PELVIC VESSELS : RABBIT. 
 
 posterior to the right. Close to the right kidney, but some- 
 what anterior to the left, is on either side a yellowish oval 
 
Chap. III.] GENERAL ANATOMY. 43 
 
 body (adrenal). From the inner edge (hilus) of each kidney a 
 .ureter passes back and enters the bladder (16, U. B.\ which is an 
 oval sac lying at the posterior end of the coelom. It communi- 
 cates with the exterior in a manner which differs considerably 
 in male and female. 
 
 There is no connection between the testis of the male and the 
 kidney. The testes (Ts.) of the rabbit undergo, during develop- 
 ment, a remarkable change of position, by which they pass 
 out of the coelom, and come to lie in little pouches of the 
 skin below the pelvic arch, one on either side of the penis. 
 They are provided with special ducts (vasa, deferentia), which 
 will be seen to form a loop on either side over the ureters. They 
 then join the urethra, which forms the canal of the penis. 
 
 The ovaries in. the female are somewhat elongated oval bodies, 
 upon which, in the adult, are a number of minute projections, 
 the Graqfian follicles, which contain the ova. Close to each ovary 
 is the funnel-shaped opening of the Fallopian tube, into which 
 the ova are received when they are shed. This is somewhat 
 convoluted, but gradually expands to form the uterus, in which 
 the ova, fecundated by spermatozoa within the Fallopian tube, 
 undergo prenatal development. The two uteri unite in the 
 middle line into the common vagina, which opens into the vesti- 
 bule leading to the external aperture or vulva. 
 
 5. The Nervous System. At the back of the peritoneal cavity 
 may be seen nerves ; several of these in the region of the arm 
 form a brachial plexus, those in the thorax run out between the 
 ribs, and several of those in the region of the hind-limb form a 
 lumbo-sacral plexus. The sympathetic nerve chain may be seen in 
 the thoracic region, lying on the heads of the ribs on either side of 
 the vertebral column ; while in the region of the neck a flat nerve, 
 lying beneath the carotid artery, alongside a rounded cranial 
 nerve, is a continuation of the sympathetic chain. 
 
 The brain must be displayed by breaking away the roof of the 
 skull, and the spinal cord by removing the arches of the more 
 anterior vertebrae. The brain is relatively very much larger, and 
 considerably more complex in form than that of the frog, and it fits 
 much more closely into its bony case. A tough membrane (dura 
 
44 
 
 ANIMAL BIOLOGY. 
 
 [Part I. 
 
 mater) lines the skull and spinal canal, while a delicate membrane 
 (pia mater) covers the brain and myelon. Between the two is a 
 third (arachnoid) membrane. (These membranes also exist in 
 
 el,. 
 
 B. 
 
 FIG. 19. BRAIN OF RABBIT. 
 A. From below. B. In longitudinal section. 
 
 our other types.) The brain should be carefully removed, 
 together with about an inch of the spinal cord. A number of 
 nerves which proceed from the brain will have to be carefully 
 severed. Before putting it into strong spirit for future study 
 its chief parts may be noted as seen from the dorsal aspect. 
 Posteriorly is a part which looks like an enlargement of the 
 spinal cord (medulla oblong ata, Fig. 19, m. o.). Over it hangs a 
 convoluted ridged mass, with a median and two lateral portions 
 (cerebellum, cb.). The medulla oblongata and cerebellum are 
 parts of the hind-brain. In front of this is a smooth, somewhat 
 pear-shaped portion, divided in the middle line. The two divi- 
 sions constitute the cerebral hemispheres (c. h.). Anterior to these 
 are two club-like prolongations, the olfactory lobes (olf.). The 
 cerebral hemispheres and olfactory lobes are parts of the fore- 
 brain. The mid-brain is hidden from view. But on lifting the 
 posterior edges of the cerebral hemispheres, four rounded promin- 
 ences (optic lobes, na. and te.), answering to the two we saw in the 
 frog and the fish, will be seen. They are parts of the mid-brain. 
 
Chap. III.] GENERAL ANATOMY. 45 
 
 Attention may now be drawn to some of the more salient 
 points in the general anatomy of the rabbit : 
 
 1. The lungs are contained in separate pleural sacs, and these, 
 together with the pericardial sac, are contained within the 
 thoracic cavity, which is well marked off from the peritoneal 
 cavity. The coelom is thus sub-divided. 
 
 2. There is a complete diaphragm behind the lungs. 
 
 3. The heart has two ventricles and two auricles ; there is no 
 separate sinus venosus or truncus arteriosus. 
 
 4. There is one aortic arch, the left. 
 
 5. There is no renal portal vein distributing blood from the 
 posterior regions to the kidney. 
 
 6. There are separate ducts for the testis and the kidney in 
 the male. 
 
 7. There is no cloaca. The urino-genital aperture being dis- 
 tinct from the anus. 
 
 8. The urinary bladder receives the ureters and opens into a 
 nrino-genital canal. 
 
 4. The Pigeon. After plucking the ventral aspect of the 
 pigeon, tie a piece of fine string round the beak at the base of 
 the cere, insert the blow-pipe into the mouth, and tighten the 
 string. Then inflate until the crop and abdomen are moderately 
 distended, and withdraw the blow-pipe, tightening and knotting 
 the string so as to prevent the escape of air. Place the bird on 
 its back, and make a median incision in the skin from beak to 
 cloaca. Eeflect the skin and make out the structures already 
 visible in the neck. The soft thin-walled gullet or oesophagus is 
 seen to expand into a large bi-lobed crop. Over it anteriorly lies 
 the ringed trachea, which passes to the left, and so over to the 
 dorsal side of the crop (Fig. 20, Tr.). Behind the crop, in its 
 present position, lie the jugular veins gorged with blood, and 
 connected with ramifying plexuses of blood-vessels in the skin of 
 the neck. A stout nerve (the vagus) accompanies each jugular. 
 Paired oval reddish lymphatic glands will also be visible in the 
 neck. 
 
 Make an incision in the abdominal walls from the posterior 
 
46 ANIMAL BIOLOGY. [Parti. 
 
 edge of the sternum to the cloaca. The contents of the peri- 
 toneal cavity or coelom will be partially hidden by the great 
 omentum, a sheet of membrane loaded with fat. By lifting it, 
 the coils of the intestine and the membranous air-sacs will be 
 displayed. Make transverse incisions along the posterior border 
 of the sternum for a short distance on either side of the median 
 line, being careful to injure as little as possible the air-sacs. It 
 will be well at once to describe the position of the chief air-sacs. 
 They are membranous bags with transparent walls, and are con- 
 nected with the respiratory system by tubes which pass through 
 the substance of the lungs. 
 
 The air-sacs are nine in number, eight lateral in four pairs, 
 and one median, resulting from the confluence of a fifth pair. 
 There is one pair cervical, or prebronchial, lying above the lobes 
 of the crop. Then follows the median inter clavicular, resulting 
 from the union of a sub-bronchial pair, and sending out prolonga- 
 tions towards the arm-pits. It lies in the fork of the merry- 
 thought. The third and fourth pairs (anterior and posterior 
 thoracic, or intermediate) are covered over by the sternum. The 
 last pair (abdominal or posterior) are those seen on opening 
 the ccelom. The ends of the posterior thoracic are just 
 visible beneath (dorsal to) the sternum without further dis- 
 section. 
 
 The sternum must now be removed, an operation that 
 requires some little care. The central keel or carina will be 
 seen in the mid-line of the thorax. Having freed the crop from 
 its anterior edge, cut into the muscles of the breast by a longi- 
 tudinal incision on each side of the carina. This incision should 
 be just deep enough to sever the great pectoral muscle without 
 cutting the subclavian muscle that lies beneath it (on its dorsal 
 side). With the handle of the scalpel these two muscles may be 
 separated in the mid-region of the sternum, and the edges of the 
 great pectoral may then be separated from their anterior attach- 
 ment to the merrythought (furcula), and their posterior attach- 
 ment to the hinder border of the sternum. In reflecting the 
 pectorals, the large pectoral veins will be brought into view, 
 probably gorged with blood. If they be cut the blood will gush 
 
Chap. III.] GENERAL ANATOMY. 
 
 tr 
 
 Forth. 1 The subclavian muscles may next be %i^near the 
 sarina, and reflected on either side, and the reflected feps may 
 be removed. The body of the sternum will thus be exposed, 
 ind anterior to it the furcula (merrythought) and two largish 
 bones (coracoids), also belonging to the pectoral arch. With 
 strong scissors the hinder half of the body of the sternum on 
 either side of the carina may now be removed. A median sheet 
 )f membrane, the falciform ligament, will be seen to extend 
 forwards beneath (dorsal to) the carina, and the anterior and 
 posterior air-sacs may perhaps be seen. The rest of the body of 
 ;he sternum, its central keel, the furcula, and the greater part 
 )f the coracoids should now be removed with the utmost care, 
 kny clotted blood must be washed away, and the dissection 
 ileaned up by the removal of such muscle or connective tissue 
 is hides the relations of the heart, blood-vessels, and digestive 
 >rgans. The student may now make out the relations of the 
 riscera by turning them over and making such dissection of the 
 mrts as he may find necessary. 
 
 1. The Alimentary System. In Fig. 20 the alimentary canal 
 ind its glands have been removed from the body and pinned out 
 50 as to display its parts. There are no large salivary glands 
 opening into the mouth. The gullet expands into a large crop, 
 :he walls of which, in the nursing bird, secrete a white nutritive 
 luid. Behind the crop the oesophagus is wide and thin-walled, 
 ind soon expands into the pink thicker-walled proventriculus, 
 which lies dorsal to the heart and liver, and is marked dotted in 
 Fig. 20 (Prov.) as if seen through the liver. This passes into the 
 rounded hard muscular gizzard (g.\ the cavity of which is small, 
 lined with horny matter, and generally contains small stones. 
 From the gizzard the duodenum (Du.} takes its origin at a point 
 near that at which the proventricular aperture is situated. The 
 iuodenum forms a U-shaped loop, within which lies the pancreas 
 (Pit.), a compact reddish gland having three ducts, two of which 
 enter the duodenum above dd, while the third enters near the 
 g on the gizzard. The two bile ducts are marked with dark 
 
 1 They may be ligatured by tying two pieces of thread round each of them, 
 one-third of an inch apart, and then cut between the two threads. But the 
 operation is a somewhat difficult one. 
 
ANIMAL BIOLOGY. 
 
 [Part I. 
 
 lines in the figure : both start from the right lobe of the liver, 
 the shorter entering the duodenum near the gizzard, the other 
 in the distal limb of the loop. The liver itself is a large reddish 
 
 brown mass, with a right and 
 left lobe, hollowed on its dorsal 
 surface for the reception of the 
 gizzard and duodenum. There 
 is no gall bladder. Following 
 on the duodenal loop is a 
 short loop of jejunum beginning 
 at Je. ; and on this, in turn, fol- 
 lows the ileum, of which the 
 proximal part forms a coil (/'/.), 
 and the distal part a single 
 loop (il\). The junction of the 
 small and large intestines (or 
 rectum, rm.) is marked by two 
 small cceca (cce.). The rectum 
 opens into the cloaca, at the 
 dorsal side of which is, in young 
 birds, a glandular oval sac, the 
 Bursa Fabritii (B. F.) An inch 
 or so of the small intestine 
 
 Fio. 20. VISCERA or PIGEON. 
 
 . F. Bursa Fabricii. cce. Csecum. d. ap. 
 Cloacal aperture, d d Positions of two of 
 the pancreatic ducts. Du. Duodenum, g. 
 Gizzard, il. il\ Loops of ileum. Je. Jejunum. 
 Pn. Pancreas. Prov. Proventriculus. rm. 
 Rectum. Tr. Trachea. 
 
 SO 
 
 should be cut out and slit 
 open under water. It will pro- 
 bably contain a semi - fluid 
 pulpy substance, the chyme. 
 The inner walls will be velvety 
 in appearance, from numerous minute processes termed villi. 
 
 2. The Respiratory System. No definite epiglottis guards the 
 glottis which opens into the trachea, at the anterior end of which 
 is the larynx. There are, however, no vocal chords, and this 
 organ is not in birds a functional organ of voice. The trachea 
 is strengthened with bony rings, and at its lower end, just before 
 it forks to send a bronchus to each lung, some of the lower 
 tracheal rings are united together to form a kind of box, the 
 syrinx, which is the organ of voice in birds. 
 
Chap. III.] GENERAL ANATOMY. 49 
 
 The lungs are flattened and oval in shape, and are firmly fixed 
 on each side of the vertebral column. Each is made up of a 
 close plexus of air-tubes connected with the branches of the 
 bronchi. There is a direct communication through the lungs 
 with the air-sacs. The muscular fibres of the so-called dia- 
 phragm, which is totally different from that of the rabbit, arise 
 from the ribs, outside the margins of the lungs, and from the 
 vertebral column, and are attached to a strong aponeurosis 
 (flattened shining masses of tendinous connective tissue) upon 
 the ventral surface of the lungs. 
 
 The mechanism of respiration differs from that of either the 
 frog or the rabbit. In the frog the lungs are inflated by a 
 buccal force-pump ; in the rabbit they are inflated by a thoracic 
 suction-pump ; in the pigeon they cannot be inflated at all. It is 
 the air-sacs which are inflated, the air being sucked into them 
 through the lungs, as the thoracic cavity is enlarged by move- 
 ments of the ribs and sternum. Thus air passes backwards and 
 forwards through the lungs with scarcely any inflation of the lungs 
 themselves. The structure and mechanism of the lung-system are 
 as perfect as that of the rabbit. But they are on a different plan. 
 
 3. The Heart and Circulatory System. The heart, which lies in 
 its pericardial cavity, beneath the anterior end of the sternum, 
 is very large, and has four distinct chambers. The right ventricle 
 (Fig. 21, r. v.) is smaller and thinner- walled than the left (/. v.). 
 It gives off the pulmonary artery (pul. a.), which at once forks 
 to send off a right and left branch to the corresponding lung. 
 From the firmer larger left ventricle (1. v.) proceeds the aortic 
 arch (ao.), and to the left of this two large (innominate) arteries 
 (in. a.), which diverge like a V. Each sends off branches to the 
 head (carotid, c. c.), to the wing (brachial, br. a.), and to the 
 muscles of the breast (pectoral, pc. a.). The arch itself, which is 
 smaller than the innominate arteries, passes back over the right 
 bronchus, and represents the right arch of the frog, not the left 
 as in the rabbit. At the back of the abdominal cavity it 
 becomes the dorsal aorta, which gives off three branches to supply 
 the stomach and intestines, two in front of (cofliac, coe. a., and 
 anterior mesenferic, a. m. a.) and one behind the kidneys (posterior 
 4 
 
ANIMAL BIOLOGY. 
 
 [Part I. 
 
 mesenteric, p. in. a.). A large sciatic artery (sc. a.) passes through 
 the kidney on each side, and gives off branches to that organ, 
 which is also supplied with arterial blood by an anterior branch 
 
 pul.cir 
 
 FIG. 21. CIRCULATION OF PIGEON. 
 
 springing directly from the aorta. Posteriorly the aorta splits 
 up into four branches, one median (caudal, c.) ; one median and 
 ventral, the posterior mesenteric (p. m. a.) ; and one on each side 
 (hypogasfaic, hyp. ). 
 
 The blood from the stomach and intestines is delivered to the 
 liver by a portal vein, and thence passes by hepatic veins into the 
 postcaval (pt. c.). This postcaval results from the confluence of 
 two large iliac factors (il. v.), which receive blood from the hind 
 limbs by the large crural veins (cr. r.). Where the crural vein 
 
Chap. III.] GENERAL ANATOMY. 51 
 
 passes into the iliac, it is joined by a vessel which passes up 
 through the substance of the kidney, receiving en route a sciatic 
 factor (sc. v.). Traced backwards, this vessel (hypogastric. Hyp. v.) 
 emerges from the kidney, and may be traced to the roof of 
 the pelvis. The hypogastric of each side, however, is connected 
 with its fellow by a transverse vessel, and from this transverse 
 connecting vessel a median vein (the coccygeo-mesenteric vein, 
 c. m. v. ) passes forwards, and forms a direct communication with 
 the portal system. 
 
 The precavals (I. pc., r. pc.) are large trunks bringing blood 
 from the head, wings, and breast, and are formed by the union 
 of three main factors, the jugulars (ju. v.) from the head and 
 neck, the Irachials (br. v.) from the wings, and the pectorals (pc. v.) 
 from the muscles of the breast. By severing the gullet and 
 trachea a little below the head, and tracing the jugulars forward, 
 they will be seen to curve round and unite in the middle line, 
 receiving a number of small factors from the head. 
 
 4. The Urmo-genital System. The kidneys are dark-red elon- 
 gated trilobed bodies closely attached to the dorsal body-walls. 
 To their anterior ends are attached yellowish elongated adrenals. 
 The ureters pass back from the point of junction of the anterior 
 and middle lobes to the cloaca. There is no bladder. 
 
 The testes in the male have not undergone the peculiar change 
 of position we noted in the rabbit, nor do they deliver their 
 products to the kidneys as in the frog. From them run special 
 ducts (vasa deferentia\ which are convoluted and dilated pos- 
 teriorly, and which open into the cloaca by apertures at the 
 apices of conical papilla. 
 
 In the adult female there is only one ovary, that of the left 
 side. In it the mature, or nearly mature eggs, form large pro- 
 minences. There is only one oviduct, the left, a convoluted 
 thick-walled tube, with a funnel-shaped membranous opening, 
 best seen by dissection under water. It opens posteriorly into 
 the cloaca. A little process from the right side of the cloaca 
 is the rudiment of the right oviduct. 
 
 The cloaca is divided into three chambers, an anterior, a- 
 median, and a posterior. The former receives the rectum ; the 
 
ANIMAL BIOLOGY. 
 
 [Part I. 
 
 middle division receives on its dorsal side in the male the 
 openings of the ureters, and the vasa deferentia, and in the 
 female those of the ureters and the left oviduct. The posterior 
 chamber has on its dorsal wall, in young birds, the aperture of 
 the Bursa Fabricii. 
 
 5. The Nervous System. In the thoracic region, white threads, 
 the thoracic nerves, pass outwards between the ribs. The 
 anterior thoracic and posterior cervical nerves form a brachial 
 plexus, whence the wing is innervated. In the pelvic region 
 there are two plexuses : one anterior, the lumbar ; the other 
 posterior, the sacral or sciatic. The gangliated sympathetic 
 chain passes up on either side of the vertebral column, and in 
 the neck runs within a canal embraced by bony processes of the 
 vertebrae, and called the vertebrarterial canal. 
 
 The central nervous system must be displayed from the dorsal 
 aspect. Great care must be taken in exposing the brain, which 
 fits closely to the skull, the walls of which must be shaved or 
 scraped away until the brain is exposed, and then carefully 
 broken away piece by piece. The brain and an inch or two of 
 the spinal cord may be removed and placed in strong spirit for 
 future study We may note in passing the large smooth cerebral 
 hemispheres (Fig. 22, c. h.), with the small anterior olfactory lobes 
 
 C.I 
 
 FIG. 22 BRAIN OF PIGEON. 
 A. From above. B. From below. C. In longitudinal section. 
 
 (olf.) constituting the visible parts of the fore-brain ; the ridged 
 cerebellum (cb.\ and underlying medulla oblongata^ constituting the 
 hind-brainy and between the two the rounded optic lobes (op. I.} 
 of the mid-brain. 
 
Chap. III.] GENERAL ANATOMY. 53 
 
 In the spinal cord there are two enlargements; a brachial 
 enlargement between the shoulders, and a lumbar enlargement 
 between the thighs. In the latter is a rhomboidal depression 
 (the sinus rhomboidalis). 
 
 6. Attention may now be drawn to one or two salient points : 
 
 (1) The lungs are closely attached to the body-walls. They 
 are not distensible, and only their ventral surface is covered 
 by a pleural membrane. They communicate with distensible 
 air-sacs. 
 
 (2) The heart is large, and has two ventricles and two auricles. 
 There is no separate sinus venosus or truncus arteriosus. 
 
 (3) There is one aortic arch, the right. 
 
 (4) There is no definite renal-portal system, but the hypo- 
 gastric veins pass through the kidney, and perhaps give off to 
 that organ a few branches. Posteriorly the transverse branch 
 connecting the hypogastrics is in connection with the portal. 
 
 (5) There are separate ducts for the kidneys and the testes in 
 the male. 
 
 (6) There is a cloaca, in the dorsal walls of which are the aper- 
 tures of the urino-gemtal ducts. 
 
 (7) The right ovary and right oviduct atrophy in the adult. 
 
 (8) There is neither urinary bladder nor gall bladder. 
 
 5. General Anatomical Conclusions. Notwithstanding well- 
 marked points of difference, our three air-breathing types, the 
 mammal, the bird, and the amphibian, agree in possessing many 
 fundamental points of resemblance. 
 
 1. All possess a skull and vertebrated back-bone. 
 
 2. The central nervous system is dorsal in position, and pro- 
 tected by the skull and arches of the vertebrae. 
 
 3. The brain is in each case divisible into fore-, mid-, and hind- 
 brain, and the spinal cord is in each case similar in essential 
 form and structure. 
 
 4. All have a visceral nervous system, the sympathetic. 
 
 5. As we shall see hereafter, there is a fundamental similarity 
 in the distribution of the cranial nerves; and in all the heart, the 
 lungs, and the stomach are largely innervated by a cranial nerve. 
 
54 ANIMAL BIOLOGY. [Part I. 
 
 6. In all three the position of the heart is the same, just 
 above the sternum. 
 
 7. Notwithstanding pronounced differences, chiefly due to sup- 
 pression, the arrangement of the great vessels which enter and 
 leave the heart is fundamentally the same. 
 
 8. Although they differ considerably in form and structure, 
 and also to some extent in position, the relations of the lungs to 
 the trachea, and of the trachea to the oesophagus are essentially 
 the same. 
 
 9. In all. the alimentary canal is suspended in the body-cavity 
 or coelom, like a smaller tube within a greater ; but in the head 
 and neck the greater tube is wanting. 
 
 19. In all the liver and pancreas have similar positions and 
 relations. 
 
 11. In all the kidneys are similarly placed, and the ureters 
 pass backward to open near the termination of the alimentary 
 canal. 
 
 12. Although the testes of the rabbit have undergone a 
 remarkable change of position, these organs are in all three types 
 essentially similar structures ; but the connection of the testes 
 of the frog with the kidneys, and the possession by that animal 
 of a common urino-genital duct constitutes an important ana- 
 tomical difference. 
 
 13. Notwithstanding the suppression of the right ovary and 
 oviduct in the pigeon, the position and relations of these parts 
 in our three types are fundamentally similar. 
 
 14. Many of these fundamental resemblances hold good for 
 the cod also. The student should note for himself how far they 
 extend. 
 
CHAPTER IV. 
 
 GENERAL PHYSIOLOGY. 
 
 A LIVING organism is wont to be guided in its actions by im- 
 pressions received from external sources. A simple impression, 
 such as a sharp sound, a flash of light, a prick on the skin, is 
 called a stimulus. If we apply such a stimulus as a light pin- 
 prick to the leg of a frog, the animal will spring away. Thus a 
 very simple stimulus gives rise to a very complex action, one in- 
 volving a great number of exceedingly delicate muscular adjust- 
 ments. It is characteristic of the higher animals that very 
 complex activities may be the outcome of very simple stimuli. 
 
 This is sometimes explained by saying that the stimulus 
 affects the mind or consciousness of the animal, and then the 
 mind or consciousness causes the activities which follow. But 
 we know so little about the mind of frogs, that it will be well 
 rather to confess our ignorance of what goes on within the 
 organism than to accept this as an explanation. 
 
 If we kill a frog, rendered insensible by chloroform, by 
 rapidly severing the muscles between the skull and vertebral 
 column, so as to lay bare the central nervous cord, and then 
 extirpating the brain by thrusting a large pin or stout wire 
 within the skull, we shall find that it still responds to certain 
 stimuli. If, for example, its side be touched with acetic acid, 
 the hind-limb of that side will be drawn up and the foot passed 
 over the spot so as to remove the source of irritation. Although 
 the frog, as an organism, is quite dead, the tissues of its body 
 are still living. We have no reason to suppose that the brain- 
 less frog possesses anything like consciousness, and yet complex 
 activities follow a simple stimulus. Such a response is called a 
 
 55 
 
 , 
 
56 ANIMAL BIOLOGY. [Parti. 
 
 reflex action. We have reason for supposing that a wave of 
 change started by the stimulus passes along a nerve-fibre to the 
 spinal cord ; that it there stimulates into activity certain groups 
 of nervous cells within the cord ; and that as a result of such 
 stimulation, a wave of change passes down other nerve-fibres to 
 certain muscles and causes them to contract in an orderly 
 fashion. The nerve-fibre that carries the wave of change in- 
 ward to the spinal cord is called an afferent fibre ; the group of 
 cells within the cord is called a ganglionic centre ; the fibres that 
 carry the waves of change outwards to the muscles are called 
 efferent fibres. The series afferent fibre, ganglion centre, efferent 
 fibre is called a nervous arc. (ttf any part of the nervous arc be 
 incomplete, reflex action does not take place. J This much we 
 may be said to know. How the incoming stimulus wave is con- 
 verted into the outgoing wave, initiating orderly activities, we 
 do not know. We are at present ignorant of the minute phy- 
 siology of the process. But we believe that there is such a 
 minute physiology. 
 
 The experiments of competent and qualified investigators have 
 further taught us that if only the fore- and mid- brain be 
 destroyed, the hind-brain remaining intact, the frog does not die. 
 It breathes ; it may be artificially fed ; and it performs a number 
 of simple reflexes, often with great vigour. And if only the 
 fore-brain be destroyed, the hind- and mid- brain being left intact, 
 the frog will eat if you feed it ; it will jump on the stimulus of a 
 needle-prick ; it will swim if you put it in water ; if you place it 
 upon a board and then very gradually tilt the board, it will slowly 
 climb and balance itself on the edge ; if you put it on its back, it 
 will turn over again and sit up ; the male will croak and clasp your 
 finger if, in the breeding season, you stimulate the pad on the 
 manus. It is thus capable of a number of responsive actions 
 that is, actions in answer to definite stimuli. But all internal 
 spring of action is gone. It is utterly incapable of initiating any 
 action, even the simplest ; and if left to itself, it will remain 
 motionless in one position till it dies. 
 
 The inference from all which is, that hind-brain, mid-brain, 
 and fore-brain have for their physiological function the co- 
 
Chap. IV.] GENERAL PHYSIOLOGY. 57 
 
 ordination, and eventually the initiation, of successively more 
 complex activities. But in what way they are able to do this 
 what is the exact nature and order of the changes which go on 
 within them these are points which it must be left to the 
 physiology of the future fully to determine. 
 
 The activities of which we have spoken involve sense-organs, v 
 nerves, nerve-centres, and muscles. It is the function of the 
 sense-organs to be the recipients of stimuli. It is the function 
 of afferent nerves to transmit the waves of stimulation from the 
 sense-organs to the nerve-centres. It is the function of the 
 nerve-centres to receive the waves of stimulation brought by the 
 afferent nerves, to organise them, and to transmute them into 
 outgoing waves of stimulation. It is the function of the efferent 
 nerves to convey these outgoing waves to the muscles which are 
 thus stimulated to orderly contraction. The common property 
 of nerves and muscles is irritability. But the irritability is 
 differently manifested in these two different tissues. 
 
 It is not necessary here to dwell longer on this aspect of 
 general physiology. It will be sufficient to note that the whole 
 surface of the body is, so to speak, in communication with the 
 surrounding world through the sense of touch ; while the organism 
 is to some extent made acquainted with the state of its own 
 organs by means of stimuli arising within the body (e.g. hunger). 
 To these are added special senses which might perhaps be termed 
 organs of prediction. Taste predicts what materials taken into the 
 mouth are suitable for allaying hunger. Smell still further pre- 
 dicts, even before they are taken into the mouth, what materials 
 are fitted for this purpose. Hearing predicts or recognises from 
 afar pleasurable or painful sensations, and thus enables the 
 organism to seek the one or to avoid the other. Still more does 
 Sight predict the pleasurable and the painful, and enable the 
 organism to act betimes. It must be noticed that, in speaking 
 of the pleasurable and the painful, we have drifted into the 
 region of feeling. It is almost impossible, in speaking of the 
 physiology of the sense-organs and nervous system, not to do so. 
 And although we know nothing, except by inference, of the 
 feelings of animals, we shall probably not be far wrong in 
 
58 ANIMAL BIOLOGY. [Part I. 
 
 assuming that, either as the result of evolution, or in some other 
 manner at present unknown, pleasure is associated with such 
 activities as are for the good of the organism and of the race to 
 which it belongs, and that pain is associated with such activities 
 as are harmful to the organism or the race. 
 
 Leaving this aspect of general physiology, let us now note 
 that to enable sense organ, nervous arc, and muscle (which, to 
 avoid repetition, we may call the senso-motor arc) to fulfil their 
 physiological function many other processes must go on within 
 the body. For they are living structures, and their life must 
 be maintained. The body is so often likened to a cunningly- 
 wrought piece of mechanism that there is some danger of losing 
 sight of this fundamental distinction between organism and 
 mechanism. In a machine, such as the steam-engine, fuel has 
 constantly to be supplied, and it is by the combustion of the 
 material thus introduced that work can be done by the 
 mechanism. So too in an organism food has constantly to be 
 taken into the body, and it is by the slow combustion of the 
 material thus introduced that work can be done by the animal. 
 So far there is analogy. But in the case of the engine, the com- 
 bustion of the fuel is restricted to a certain part of the machine, 
 and the other parts undergo no more waste of their substance 
 than is caused by friction. In the case of the organism, on the 
 other hand, combustion is not restricted to any spot, but goes on 
 within the working parts, which parts, in and through their 
 very activity, are constantly wasting away, and therefore need 
 constant repair. So that if we liken a nerve to such a mechanical 
 contrivance as a telegraph wire, we must not forget that in 
 transmitting a wave, the nerve is partially destroyed, and there- 
 fore needs constant repair. In this it differs entirely from the 
 telegraph wire ; and in this difference we have a fundamental 
 distinction between organism and mechanism. 
 
 The parts of the senso-motor arc are, therefore, constantly 
 undergoing waste due to oxidation. Hence the necessity of a 
 blood circulation (1) to renew the wasted substance, (2) to carry 
 off the waste products, and (3) to supply the oxygen required 
 for further activity through oxidation. 
 
Chap. IV.] GENERAL PHYSIOLOGY. 59 
 
 The circulating medium is the blood. In the living state it is 
 warm and fluid. It consists of an almost colourless liquid, the 
 plasma, in which float minute solid bodies, the corpuscles. These 
 are of two kinds : (1) red corpuscles, minute, flattened, round, or 
 oval discs (see p. 64) ; (2) colourless, white, or pale corpuscles, 
 which change their form when living, and resemble a minute 
 organism called the amoeba (see Ch. xx.). Blood undergoes rapid 
 change when dying, causing clotting or coagulation. This is 
 due to the formation of fine filaments of fibrin. If the blood be 
 allowed to stand, the clot contracts, and a thin fluid, termed 
 serum, is expressed. This serum is not the same as plasma ; nor 
 is the fibrin during life merely held in solution in the plasma to 
 separate out on death. The fibrin is generated from the plasma 
 and the white corpuscles. These contain a substance, fibrinogen, 
 which, under the influence of a second substance, probably also 
 derived from the corpu&cles, gives rise to fibrin. 
 
 The blood may be venous or arterial. Our blood, for example, 
 contains about 60 per cent of gaseous constituents by volume, 
 chiefly oxygen, nitrogen, and carbonic acid gas. The carbonic acid 
 gas is almost entirely held, in a state of very loose combination, 
 by the plasma. The oxygen is bound up in the hcemoglobin of 
 the red corpuscles. The proportion of oxygen aud carbonic acid 
 gas varies according to the venous or arterial condition of the 
 blood. In us venous blood contains 12 per cent, oxygen and 46 
 per cent, carbonic acid gas ; arterial blood, 20 per cent, oxygen 
 and 39 per cent, carbonic acid gas. Thus the carbonic acid gas 
 in the blood is always in excess of the oxygen. The operations 
 of giving up the carbonic acid gas and absorbing fresh oxygen 
 seem to be quite independent. Let us note then that the red 
 corpuscles are the carriers of oxygen throughout the system ; 
 while the plasma distributes the nutritive material and carries 
 off' the waste products. 
 
 The organ for propelling the blood is the heart, which thus 
 provides for the continuity of the circulation. In the pigeon 
 and the rabbit it has two sides completely separated off from 
 each other without possibility (in the adult) of direct communi- 
 cation. Each side has two parts (1) a receiver, the auricle ; 
 
60 ANIMAL BIOLOGY. [Part I. 
 
 and (2) a pump, the ventricle. Between the two in each case 
 are valves, which permit the blood to flow from the receiver into 
 the pump, but prevent its flowing from the pump into the 
 receiver. 
 
 In the course of the circulation, beside the senso-motor arc, are 
 (1) lungs, (2) kidneys, (3) the alimentary canal. In the lungs 
 the red corpuscles are freighted with oxygen, which they convey 
 to various parts of the body, returning to the lungs empty. The 
 blood which contains the laden corpuscles, and which is bright- 
 red in colour, is called arterial ; that which contains the 
 exhausted corpuscles, and which is dull-red in colour, is called 
 venous blood. In the lungs also the plasma delivers up one 
 main product of tissue waste, the carbonic acid gas. So that the 
 lungs have a double function. In their relation to the blood 
 corpuscles they have the function of arterialisation : in their re- 
 lation to the blood plasma they have an excretory function. In 
 the kidneys a further and special product of animal metabolism 
 (see p. x), the urea, is eliminated from the plasma. In the ali- 
 mentary canal the plasma is enriched with nutritive material. 
 
 In each of these organs the blood-vessels branch and sub- 
 divide until they form a close meshwork of minute vessels 
 termed a capillary plexus. In each of the multitude of capillaries 
 which make up such a plexus the walls are so thin that tissue 
 nutriment and tissue waste is readily given up or received by 
 the blood ; and from the extreme fineness and the great number 
 of the tubes, the amount of surface thus bathed by the blood is 
 relatively enormous. The sub-division of arteries into capillaries, 
 and the union of capillaries into veins, may readily be observed 
 under the microscope in the web of a frog's foot. The frog must 
 be lightly tied to a piece of thin board or thick card-board, in one 
 edge of which a V-shaped notch has been cut. Over the- notch 
 the web is placed, threads being tied to two of the toes to keep 
 them sufficiently apart. The frog should be kept moist by 
 placing a piece of wet blotting-paper over his body. In every 
 organ of the body the blood-vessels form such capillary plexuses 
 as are thus seen spread out like a network in the web of a frog's 
 foot. 
 
Chap. IV.] 
 
 GENERAL PHYSIOLOGY. 
 
 61 
 
 We have seen that, in the pigeon or the rabbit, the heart is 
 like a double pump, or a pump with ,two completely separate 
 chambers placed side by side. One of these, the pulmonary 
 pump, forces blood into the lungs; the other, the systemic 
 
 FIG. 23. PHYSIOLOGICAL DIAGKAM. 
 
 1. Lung receiving blood from the right ventricle, 12. Here oxygen is 
 absorbed into the system, and carbonic acid gas and water are got rid 
 of by the system. 2. Left auricle receiving the arterialised blood from 
 the lungs. Thence it passes to 3, the left ventricle, to be distributed 
 by means of the arteries to the left of the figure. 4. Alimentary canal. 
 Here tissue nutriment is absorbed into the system, and is carried (1) 
 by the lacteal, 5, into the blood-stream returning to the heart ; and 
 (2) by the portal vein to the liver, 6, where bile is elaborated and 
 poured into the alimentary canal by the bile-duct, 7. The kidney, 8, 
 receives blood by a separate branch-artery. Here water and nitro- 
 genous waste are eliminated and pass out of the system by the ureter, 
 9.' Another branch artery carries blood to the senso-motor arc, 10. 
 The blood returns by veins (to the right of the figure) to the right 
 auricle, 11, whence it passes to the right ventricle, and so to the 
 ungs, 1. 
 
 pump, forces to all parts of the body the blood delivered into its 
 receiver from the lungs. From all parts of the body the blood 
 collects in the receiver of the pulmonary pump. Thus all the 
 
62 ANIMAL BIOLOGY. [Parti. 
 
 blood has to pass through the lungs. But not all has to pass 
 through either the alimentary canal to be enriched with tissue 
 nutriment or through the kidneys to be deprived of nitrogenous 
 tissue waste. This will best be seen by the diagram (Fig. 23). 
 the blood which leaves the left ventricle may either pass to 
 the alimentary canal, thence to the liver, where it undergoes 
 complex and highly important changes, and thence to the right 
 auricle ; or it may go to the kidney, and thence to the right 
 auricle ; or it may go to the senso-motor arc, and thence to the 
 right auricle. 
 
 Thus to enable the senso-motor arc to continue its functions, 
 there must be a circulation of blood, and processes of digestion 
 and absorption, of arterialisation, and of excretion, to say 
 nothing of those vital processes which go on within the liver. 
 These, together with the reproductive function, are the essen- 
 tials of the general physiology of such an animal as a rabbit or 
 a pigeon. But in addition to this essential physiology there is 
 a vast amount of accessory physiology, of which more hereafter. 
 
CHAPTER Y. 
 
 GENERAL HISTOLOGY. 
 
 STUDIED under the microscope, the organs are found to be 
 composed of tissues (muscular tissue, nervous tissue, glandular 
 tissue, etc.), and the tissues to be built up of cells, together 
 with certain cell products. 
 
 To make out the form and structure of the cells, a small piece 
 of the tissue may be teased to pieces in a dilute solution (-75 per 
 cent.) of common salt (normal saline) with two needles on a glass 
 slide. The teased tissue must then be covered with a cover-glass 
 and examined. But to make out the relations to each other of the 
 cells in a tissue, thin sections of the tissue must be cut. To bring 
 out clearly the structure of a cell or of a tissue, advantage is 
 taken of the fact that different parts of a cell, and different cells 
 in a tissue, are differently affected by certain re-agents, such as 
 dilute acid or staining fluids. The cells or tissues so prepared 
 must be mounted either temporarily for immediate study in 
 normal saline solution or glycerine, or permanently in glycerine 
 or Canada balsam. 1 
 
 1 For the guidance of the student the following hints on section-cutting may 
 be useful : 
 
 1. Cut a small fragment fresh from the organ to be examined. 
 
 2. Place in (a) osmic acid, 1 per cent., for two or three minutes; or (b) in 
 chromic acid, 5 per cent., for twenty-four hours ; or (c) in Kleinemberg's picric 
 acid for eighteen hours. The object will thus be hardened. 
 
 3. Place in 50 per cent, alcohol for twenty-four hours, then 70 per cent, alcohol 
 for twelve hours, 80 per cent, alcohol for six hours, and transfer to 90 per cent, 
 alcohol. This removes the water. 
 
 4. Place in staining fluid until sufficiently deeply stained. (1) Hsematoxalin, 
 (2) picro -carmine, (3) eosin, and (4) borax-carmine, are the most useful re-agents. 
 
 63 
 
64 ANIMAL BIOLOGY. [Part I. 
 
 1. Blood and Lymph. Examined under the microscope (high 
 power), the blood is seen to consist of a colourless fluid, the 
 plasma, in which float a great number of yellowish red corpuscles 
 and a few white corpuscles. The red corpuscles of the frog (Fig. 
 24, i. b) and the pigeon are flattened slightly bi-convex oval discs, 
 and contain a nucleus. Those of the rabbit are circular bi-concave 
 discs (Fig. 24, i. c) and contain no nucleus. The size of the 
 corpuscles is, in the frog, g-^- inch ; in the pigeon, Y J^ ; in the 
 rabbit, -g^Vo"- Treated with dilute acetic acid, the nucleus (when 
 present) becomes more obvious, the colour of the surrounding 
 parts of the corpuscle is gradually discharged, and eventually 
 that of the nucleus also. If a needle be rapidly drawn across a 
 drop of blood, some of the cells will be cut in two. There is 
 no escape of the contents from the cut halves, showing that they 
 are composed throughout of a viscid material, and not of a fluid 
 contained within a membranous wall. The manner in which 
 the corpuscles recover their shape after distortion by pressure 
 may be seen in the capillaries of the living frog (see p. 60). 
 
 The white corpuscles (leucocytes) are of much the same size (^-Q-Q 
 inch) in frog, pigeon, and rabbit, and are in all cases nucleated. 
 On a cold slide they will probably appear round ; but if the 
 slide be warmed they will show amoeboid movements, the proto- 
 plasm flowing out into irregular processes and slowly creeping 
 over the slide (Fig. 24, i. a). Sometimes they divide by fission. 
 The nucleus first splits into two, and then the two halves of the 
 
 If borax-carmine be used, transfer it to 70 per cent, alcohol, to which a few 
 drops of nitric or hydrochloric acid have been added. 
 
 5. After staining, place for an hour in 80 per cent, alcohol, and then in 
 absolute alcohol for another hour. 
 
 6. Place in spirits of turpentine for an hour, and then transfer to paraffin 
 (sold of proper hardness for this purpose of imbedding), which should be kept 
 just melted. Leave for six or eight hours, and then cast in a mould. The small 
 oblong vessels in which moist water-colours are sold answer well, Arrange the 
 specimen with warm needles. 
 
 8. Pare away the paraffin so as to bring the object into view. Cut the thinnest 
 sections you can get with a sharp razor. 
 
 9. Dissolve the paraffin in turpentine. 
 
 10. The section may now be examined or mounted permanently in Canada 
 balsam. 
 
Chap. V.] GENERAL HISTOLOGY. 65 
 
 corpuscle, each containing a nucleus, seem to creep off in 
 opposite directions, the protoplasm between them getting more 
 and more constricted, till its continuity at last is broken. In the 
 capillaries of the frog's foot the white corpuscles will be seen to 
 creep along the capillary walls. 
 
 The lymph is a colourless fluid containing, in the larger trunks, 
 leucocytes quite similar to the white corpuscles of the blood 
 In the lacteals, e.g. of the rabbit killed shortly after a full meal, 
 the lymph is milky in appearance, and is called chyle. The milki- 
 ness is seen under the microscope to be due to multitudes of very 
 minute fatty particles, which dissolve in ether, and are deposited 
 as oil-drops on the evaporation of the solvent. 
 
 In the mesentery of the rabbit a number of lymphatic glands 
 will be seen. A section of such a gland will show an outer 
 portion or cortex, divided into compartments (alveoli), and an 
 inner portion or medulla, formed of interlacing strands. In the 
 alveoli of the cortex there are roundish masses or follicles of 
 adenoid tissue formed of a network of connective fibres, the inter- 
 spaces of which are crowded with leucocytes similar to the 
 white corpuscles of the blood. The strands of the medulla are 
 formed of similar tissue, also crowded with leucocytes. The 
 Peyer's patches are composed of lymphatic follicles aggregated 
 together and full of leucocytes. From such glands, and there 
 are many such, leucocytes pass into the lymph. 
 
 2. Epithelium and Endothelium. The epithelium is the lining 
 membrane of such tubes or cavities as the mouth, stomach, 
 intestine ; the ducts and cavities of the salivary glands, testis, 
 kidney, etc. They are for the most part in communication with 
 the exterior. At the surface of the body the epithelium passes 
 into the epidermis. The endothelium is a special variety of 
 epithelium, and forms the lining membrane of such cavities 
 or tubes as the peritoneal cavity, lymph sinuses, heart cavities, 
 and blood-vessels. They are for the most part not in communi- 
 cation with the exterior. 
 
 The epithelium cells vary in form and arrangement in different 
 parts of the body. Sometimes they form a single layer (simple 
 5 
 
66 ANIMAL BIOLOGY. [Part I. 
 
 epithelium) as in the stomach and intestines, and in the tubules 
 and finer ducts of the kidneys, salivary glands, etc. ; sometimes 
 they form a tissue several layers deep (stratified epithelium) as in 
 the mouth, bladder, and epidermis. Sometimes the individual 
 cells are more or less spherical, as in the deeper layers of the 
 epithelium of the mouth, and in the peptic glands of the stomach; 
 sometimes they are flattened and squamous, as in the superficial 
 layer in the human mouth; sometimes polyhedral, as in the 
 external layer of the conjunctiva covering the eye ; sometimes 
 ciliated, as in the lining cells of the frog's mouth, and in 
 the trachea of mammalia ; sometimes columnar, as in the simple 
 epithelium of the intestine. Sometimes the cells become secreting 
 cells, as in the salivary glands or the pancreas ; sometimes horny 
 cells, as in the epidermis, in hair, and in nails; while in the 
 tubules of the testis they become seminal cells, giving rise to 
 spermatozoa. 
 
 Some of these varieties we shall become further acquainted 
 
 FIG. 24. BLOOD, EPITHELIUM, AND CONNECTIVE TISSUE. 
 
 i. a. White blood-corpuscle, i. 6. Red corpuscle (frog), i. c. Red 
 corpuscle (rabbit), ii. Squamous epithelium, human mouth, iii. Ciliated 
 epithelium, mouth of frog. iv. Corneal epithelium, rabbit, v. Columnar 
 epithelium, frog's intestine, vi. Epithelioid cells of peritoneum of frog, 
 vii. White fibrous tissue, viii. Yellow elastic tissue, ix. Branched con- 
 nective tissue corpuscles, x. Corneal corpuscles, xi. Fat cell. xii. Pig- 
 ment cells : a. passive ; &. active. 
 
 with in this chapter. At present we may content ourselves 
 with the following : 
 
 (a.) Scrape gently the inner surface of your lip, and mount in 
 saliva. Flat nucleated squamous cells will be seen (Fig. 24, ii.). 
 They form the superficial layer of the stratified epithelium of 
 
Chap. V.] GENERAL HISTOLOGY. 67 
 
 the mouth. The deeper cells are rounder. These latter con- 
 stantly multiply by division, and thus force fresh layers of the 
 squamous cells to the surface, where they are shed. In the skin 
 a similar process goes on; but the external cells are more horny, 
 and have lost their nucleus. 
 
 (b.) Scrape gently the surface of the eyeball in a dead rabbit, 
 and mount in normal salt solution. The cells are polyhedral 
 and nucleated (24, iv.). 
 
 (c.) Scrape gently the inner surface of the roof of 
 mouth, and mount in normal salt solution. The cells are c 
 along their free edges. The nuclei are readily seen .. 
 
 (d.) Scrape gently the inner surface of the intestinx*%' / $he ' J ( 
 frog, and mount in normal salt solution. The cells (24, v.) are 
 columnar, and some of them pointed at one end. In some 
 (mucous cells) a globule of mucus may be seen at the broader 
 end. The nuclei are nearer the deep end. 
 
 Endothelial or epiihelioid cells are flattened squamous cells of 
 various shapes, polygonal, elongated, or irregular. They form a 
 simple membrane one layer thick. 
 
 (e.) Kemove carefully the transparent membrane that separates 
 the pleuro-peritoneal cavity of the frog from the subvertebral 
 lymph space. Treat it for two or three minutes with a dilute 
 solution (*5 per cent.) of nitrate of silver, wash with distilled 
 water, and expose to bright light until it is stained brown. The 
 irregular endothelial calls of the peritoneum will be marked out 
 with black lines (24, vi), for the cement substance which lies be- 
 tween the cells is stained black by the silver. It may be that 
 smaller cells surrounding minute orifices (stomata) will be seen. 
 They are called germinating endothelial cells, and actively multiply 
 by division. The stomata communicate with the ccelom. 
 
 All of these cells (a.-e.) should be measured and drawn from 
 Nature. 
 
 3. Connective Tissue. This tissue is to be found in all parts 
 of the body. It forms the framework or scaffolding, within and 
 around which the various organs are built. Its fibres support 
 the specialised cells of these organs. 
 
68 ANIMAL BIOLOGY. [Part I. 
 
 A piece of tendon, by which a muscle is attached to a bone, 
 and a piece of inter-muscular tissue, the filmy glistening material 
 seen when two muscular bundles are separated, should be taken 
 from a frog's hind leg, teased apart, and mounted (a) in normal 
 saline solution, (b) in acetic acid. They may also with advan- 
 tage be stained with magenta. In those mounted in normal 
 saline, and especially in the tendon, a number of bundles of 
 ivliite fibrous connective tissue (24, vii.) will be seen. They are 
 marked with longitudinal striations, and tend to break up into 
 fibrillce. In those mounted in acetic acid the fibrous tissue has 
 been rendered almost or quite invisible. Great numbers (especi- 
 ally in the inter-muscular tissue) of fine wavy branching fibres 
 of yellow elastic tissue will be seen (24, viii.) ; and here and there, 
 with the nucleus and cell-substance more or less stained, will be 
 connective tissue corpuscles, nucleated cells, elongated in the tendon, 
 and branched in the inter-muscular tissue (24, ix.). These are 
 the living formative cells. The white fibrous and yellow elastic 
 tissue is to be regarded as inter-cellular material, analogous to the 
 cement substance in endothelium and epithelium. 
 
 The pigment cells (24, xii. ; a. passive, b. active) of the frog's skin, 
 which, as we have already learnt (p. 7), change their form in ac- 
 cordance with the intensity and quality of the light which falls 
 on the frog's eye, are modified connective tissue cells, as are also 
 the corneal corpuscles seen (24, x.) in a section of the cornea 
 treated with gold chloride. Fat-cells (which may readily be 
 obtained from the corpus adiposum of the frog) are also strangely 
 modified connective tissue cells (24, xi.). In them the flattened 
 nucleus and a small remnant of protoplasm are thrust to one 
 side of the rounded cell, which contains a relatively huge 
 globule of oil. Treat the cells with ether ; the oil will dissolve 
 and the cells collapse. 
 
 4. Cartilage. If the anterior or posterior cartilaginous expan- 
 sion of the sternum be removed from a recently killed frog, and 
 mounted in normal saline, rounded nucleated cells will be seen 
 occupying cavities in a transparent matrix. The cells which 
 occupy these cavities (lacunce) will be seen in various stages of 
 
Chap. V.] 
 
 GENERAL HI SINOLOGY. 
 
 69 
 
 division. In some cases the nucleus has divided so that there 
 are two nuclei to one cell ; in others the cell has also divided, so 
 that two cells occupy one lacuna; in others a thin layer of 
 matrix divides two cells which have but recently separated from 
 one another (Fig. 25, i.). The matrix is clearly an intercellular 
 substance, answering to the cement substance in endothelium. 
 
 FIG. 25. CARTILAGE, BONE, AND TOOTH, 
 
 i. Cartilage, ii. Dry femur of frog in transverse section, iii. Metatarsal of 
 rabbit (transverse), iv. Lacuna of same. v. Haversian system (rabbit), vi. 
 Transverse section of part of canine tooth of cat, showing, a. enamel, &. 
 dentine, vii. Part of fang of a similar tooth, showing, a. cement, ft. dentine. 
 
 The free surface of cartilage is invested by a membrane of 
 fibrous tissue, the perickondrium. Under favourable circum- 
 stances fine canals may be seen passing between the lacunse, and 
 from them to the perichondrium. It may be that by them the 
 cartilage is irrigated with lymph. 
 
 Elastic and white fibro-cartilage are varieties in which the 
 matrix tends to pass into elastic or fibrous tissue. 
 
 5. Bone. Bone sections may be prepared for microscopic 
 examination in two ways ; either (1) by grinding down (with 
 file, pumice-stone, and hone) a piece of the solid and dry bone 
 until it is sufficiently thin, and then mounting it dry ; or (2) by 
 
70 ANIMAL BIOLOGY. [Part I. 
 
 decalcifying the bone by dilute acid (chromic or hydrochloric), 
 and then cutting sections in the usual manner. 
 
 We may begin by studying a section of a frog's femur. 
 Externally the living bone is invested by a layer of fibrous 
 connective tissue, the periosteum, on the inner surface of which 
 is a layer of special bone-forming cells or osteoblasts. Within 
 the living bone is a very vascular fatty substance the marrow 
 also containing osteoblasts. A section of the dry bone (25, ii.) 
 shows a number of dark elongated lacunce, from which radiate a 
 number of fine canaliculi. They are clearly arranged in two sets, 
 an inner and an outer. The inner are smaller, and their fine 
 canaliculi pass down towards the marrow cavity. The outer are 
 larger, and their canaliculi passing towards the circumference of 
 the bone, form branching tufts intersecting and uniting to form 
 a network. The lacunae run in rings round the bone, and thus 
 mark out the lamellce. Between the inner and the outer lamellae 
 is a middle lamella, in which there are no canaliculi. In the 
 young growing bone each lacuna has a bone cell sending out 
 processes into the canaliculi. There are a few Haversian canals 
 near the nutritive foramina for the entrance of the blood-vessels- 
 
 A section of a metatarsal bone of the rabbit (25, iii.) shows us 
 an inner and an outer set of circumferential lamellce with small 
 lacunae. But between these two regions there is a wider space, 
 in which there are a number of groups of lamellae surrounding a 
 small cavity. Between the lamellae are lacunae. The central 
 cavity (from which fine radiating lines proceed outwards) is the 
 cross-section of a Haversian canal, containing blood-vessels, the 
 lamellae round which are called concentric lamellce. The Haversian 
 canals are in connection either with the medulla or with the 
 periosteum. One of the lacunae is figured under a high power 
 in 25, iv. 
 
 A section of the femur of a rabbit shows us in the main an 
 aggregate of Haversian systems, one of which is shown in Fig. 
 25, v. There may be some circumferential lamellce externally, 
 and between the Haversian systems some interstitial lamellce. 
 
 Close to the marrow cavity, especially towards the ends of 
 the growing bone, the structure is much more open than that of 
 
Chap. V.] GENERAL HISTOLOGY. 71 
 
 the compact bone above described. This is called spongy-bone. 
 Many of the Haversian canals have been hollowed out into 
 large Haversian spaces, which may show a tendency to run one 
 into another. 
 
 It is clear that the ossified matter of the bone-lamellae is an 
 intercellular substance a substance produced by the bone-cells 
 answering to that which is transparent and hyaline in cartilage, 
 and fibrillated in fibrous connective tissue. In a bone softened 
 with hydrochloric acid some of the lamellae may be torn off in 
 shreds. The matrix thus obtained shows crossing fibres ; and 
 from the lamella processes are given off which project at right 
 angles to the lamella and pass through holes in adjacent lamellae. 
 The holes for the admission of such processes will also be 
 seen. 
 
 The bone* of the pigeon is essentially similar to that of the 
 rabbit. But a cross-section of the bone of a cod-fish shows a 
 structure very different from that seen either in the frog or in the 
 rabbit. It is more like ossified or calcified cartilage, there being 
 none of the lacunas or lamellae so characteristic of true bone. 
 
 6. Teeth. Tooth sections may be prepared either from the 
 dry tooth by grinding, or from the decalcified tooth by the 
 ordinary process of section-cutting. 
 
 In the interior of the tooth, passing up from the bottom of 
 the fang, is the pulp-cavity, empty in the dry tooth, filled with 
 vascular and nervous pulp in the living state. Around this, 
 forming the mass of the tooth, is the dentine, through which 
 there run a number of slightly waving or spiral tululi, sometimes 
 giving off fine branches. In the fresh state each contains a 
 fibre connected with the cells (odontoblasts) which line the pulp- 
 cavity. At their outer ends the tubules pass into a network 
 of intercommunicating spaces. Thus the dentine is an inter- 
 cellular substance formed between the fibres of, and due to the 
 vital activity of, the odontoblasts. 
 
 The material encrusting the dentine differs according as our 
 section passes through the crown or the fang. In the crown it 
 consists of enamel (Fig. 25, vi.), composed of enamel prisms, set 
 
72 ANIMAL BIOLOGY. [Part I. 
 
 more or less at right angles to the surface, or in gently curved 
 bundles, the curves at slightly different levels intersecting each 
 other. The individual prisms may be isolated by means of 
 hydrochloric acid. In the fang the encrusting material is called 
 cement, which resembles bone in structure. In it are large 
 lacunae, the canaliculi of which communicate with the ultimate 
 endings of the tubules of the dentine (Fig. 25, vii.). 
 
 The tooth of the cod-fish differs markedly in structure from 
 the mammalian tooth above described. An inner zone has 
 large canals ; a thinner outer zone fine-branched tubules. 
 
 7. Muscle. Muscle is divided into (a) striped or voluntary 
 muscle, and (b) un striped or involuntary. 
 
 (a.) Striped. Of this kind are the most important muscles 
 of the trunk and limbs. The fibres are elongated, and are 
 surrounded and bound together by connective tissue (endo- 
 mysium\ in which run the capillaries of the blood-vessels. The 
 bundles into which the fibres are thus bound together are in 
 turn invested by stronger bands of connective tissue (peri- 
 mysium). 
 
 Fresh striped muscle of the frog (cp. also that of an inverte- 
 brate such as the water-beetle, hydrophilus, or the cockroach), 
 mounted in normal saline, is seen to be composed of glassy or 
 slightly opalescent fibres surrounded by a delicate sheath, the 
 sarcolemma. Each is transversely striated, and contains fusi- 
 form muscle corpuscles. Treatment with acetic acid renders the 
 striation indistinct, and the corpuscles more conspicuous. They 
 are embedded in the midst of the fibre in frog's muscle, but lie 
 just beneath the sarcolemma in that of the rabbit. Dead fibres 
 tend to split up into a number of separate fibrillce, and a muscle 
 that is hardened in strong spirit very readily splits up into such 
 elements. Muscle fibres hardened in picric acid tend to break 
 up into discs. 
 
 In a fresh muscle fibre teased in normal saline and examined 
 under the highest powers of the microscope, the following struc- 
 ture may, under favourable conditions, arid by careful focussing, 
 be observed : The fibre shows alternate bands of lighter and 
 
Chap. V.] 
 
 GENERAL HISTOLOGY. 
 
 73 
 
 darker substance. The lighter bands are the septal zones (Fig. 
 26, iii.), the darker the interseptal zones. Within the latter very 
 delicate longitudinal striations may be made out. In the midst 
 of the septal zone is a delicate dotted line, the septal line. The 
 space between the septal lines is termed a muscle segment. 
 
 In growing muscle spindle-shaped cells are active in the 
 formation of muscular fibres, just as spindle-shaped or branched 
 cells are active in the formation of fibrous connective tissue. 
 
 Fig. 26, i., shows a muscular fibre of the frog, the elongated 
 black spots being the muscle corpuscles. Where the fibre is 
 twisted the sarcolemma becomes obvious. 26, iii., is a 
 
 Fio. 20. MUSCLE AND NERVE. 
 
 i. Striped muscle, ii. Unstriped muscle cells, iii. Striped muscle under 
 highest power. . Septal line. &. Septal zone. c. Interseptal zone. iv. Trans- 
 verse section of sciatic nerve of rabbit, v. A portion more highly magnified, 
 vi. Nerve fibre, after chloroform, vii. Nerve fibre with primary node, after 
 osmic acid. viii. Nerve fibre with nerve corpusle, after osmic. ix. Nerve 
 cell from sympathetic ganglion of frog. x. Unipolar nerve cell. xi. Multi- 
 polar nerve cell. xii. Section of spinal cord from cervical region of kitten. 
 
 diagram to show : a. septal line, b. septal zone, <'. interseptal 
 zone. 
 
 (b.) Unstriped. Involuntary unstriped muscle occurs in the 
 walls of the alimentary canal, of the bladder, and of the blood- 
 vessels. The fibres are moderately elongated, flattened, and 
 tapering at the ends (fusiform). Each has a readily-stained 
 nucleus (26, ii.), which may also be made obvious by treatment 
 
74 ANIMAL BIOLOGY. [Part I. 
 
 with dilute acetic acid. They are embedded in a cementing 
 substance. 
 
 8. Nerve. The nervous elements are divided into (a) nerve 
 fibres, and (b) nerve cells. 
 
 (a.) Nerve fibres. Such a nerve as the sciatic consists of a 
 number of nerve bundles bound together. Each nerve bundle 
 (of which three are shown in 26, iv.) is surrounded by a fibrous 
 sheath of connective tissue (perineurium), and is bound to other 
 bundles by bands of similar tissue (epineurium), while the fibres 
 within the bundle are bound together by more delicate endo- 
 neurium. Within the bundle are a number of dots, many or 
 most of them surrounded by a transparent ring (26, v.). Nerve 
 fibres isolated at once and examined in normal saline appear 
 as delicate glassy rods, without apparent external membrane, 
 but with faint indications of a central thread. Under the 
 influence of re-agents, however, we see the nerve fibre com- 
 posed of three distinct parts a central thread, the axis fibre, 
 around this a fatty medullary sheath, and external to this 
 again a primitive sheath. Chloroform dissolves the medullary 
 sheath, and makes clearly visible the axis fibre, as in 26, vi. 
 Osmic acid stains the medullary sheath black. This re -agent 
 makes obvious certain breaks in the continuity of the medulla, 
 the primary nodes (nodes of Ranvier) of which one is shown in 
 26, vii. By the subsequent addition of a staining fluid the 
 nerve corpuscles (26, viii.) in or attached to the primitive sheath 
 are rendered clearly visible. There is generally one (sometimes 
 more) such nerve corpuscles between two primary nodes. Besides 
 the primary nodes there are also much more numerous and less 
 pronounced slanting breaks in the continuity of the medulla, 
 constituting secondary nodes, and giving to this sheath an im- 
 bricated appearance. 
 
 Near the peripheral termination of a nerve fibre the axis fibre 
 loses its medullary sheath, and becomes a non-medullated nerve- 
 fibre (Remak's fibre). The dots (in 26, v.) not surrounded by a 
 clear ring are such fibres, the others being medullated. The 
 olfactory nerve-fibres are of this non-medullated type, as are also 
 
Chap. V.] GENERAL HISTOLOGY. 75 
 
 most of the fibres of the sympathetic system. Such fibres still 
 retain the primitive sheath. But eventually even this may be 
 lost, and the axis fibre may split into fine fibrillae which may 
 terminate in specially-modified cells. In the optic and auditory 
 nerves there is a medullary sheath, but no primitive sheath, and 
 no primary nodes. 
 
 (b.) Nerve cells. Traced inwards to the spinal cord a nerve 
 is found to result from the union of two roots, a ventral or motor 
 root, and a dorsal or sensory root. On the latter is a swelling or 
 ganglion. Here there are a number of unipolar nerve cells (26, x.) 
 Each is invested with an investing sheath continuous with the 
 primitive sheath. An axis fibre may, in some cases, be seen to be in 
 connection with the cell, and to bifurcate at some little distance 
 from it. In the ganglia of the sympathetic system of the frog, 
 some bipolar cells of the type figured in 26, ix. may be found. 
 The straight axis cylinder process passes into a non-medullated 
 fibre : the spiral fibre, after leaving the axis cylinder, acquires a 
 medullary sheath and becomes a medullated fibre. 
 
 On the ventral root of the spinal nerve there is no ganglionic 
 enlargement. The fibres pass inwards into the spinal cord, and 
 there come into relation with multipolar ganglion cells such as 
 that figured in 26, xi. 
 
 9. The Spinal Cord and Brain. In Fig. 26, xii. is figured a 
 section of the spinal cord (from the cervical region of a kitten), 
 showing the origin of the dorsal and ventral nerve roots. In the 
 middle of the cord is a canal (canalis centralis) lined with ciliated 
 epithelium. Above and below this the cord is nearly cleft in 
 twain by a dorsal and a ventral fissure, into the latter of which 
 pass bands of connective tissue derived from the investing mem- 
 brane or pia mater. The substance of the cord is composed of 
 white matter externally, and grey matter internally. The white 
 matter consists of medullated nerve fibres without any primitive 
 sheath and interrupted by no nodes. They run longitudinally, 
 but form also commissures, passing from one half of the cord 
 to the other. The grey matter, besides having somewhat fewer 
 medullated fibres and more blood-vessels, has also, as a char- 
 
76 ANIMAL BIOLOGY. [Parti. 
 
 acteristic, great numbers of nerve cells of various shapes 
 and sizes, but generally multipolar and stellate, and larger 
 in the ventral of the two divisions or cormia into which 
 the grey matter of each side of the cord is divided. In 
 both white matter and grey matter the fibres and cells 
 are imbedded in a supporting connective tissue (neuroglia) 
 which forms a very large proportion of the grey matter. 
 This consists of (1) homogeneous semi-fluid matrix, (2) of 
 a network of delicate fibrils, and (3) of branched nucleated 
 cells. 
 
 The brain is composed of white matter and grey matter ; but 
 in the cerebral hemispheres and cerebellum the grey matter is 
 external to the white. 
 
 10. Nerve Endings. Nerve fibres terminate peripherally in 
 various ways. In the epithelium of the skin the axis fibre breaks 
 up into a network of fibrillae lying in the interstitial substance 
 between the cells. In striped muscle the network into which 
 the axis fibre breaks up encloses nuclei of various shapes, and 
 is known as an end-plate. And in connection with the special 
 sensations there are specialised terminations of the sensory 
 nerves. 
 
 (a.) Touch. In sections of a rabbit's or a cat's lip, the deeply 
 implanted hairs (mystaces) will be seen to terminate in enor- 
 mously enlarged bulbs, with which nerve fibrillse enter into 
 close connection. Such hairs may be regarded as special sense 
 organs. In the skin of the human finger the sensory nerves are 
 in connection with pacinian corpuscles, large oval bodies, in which 
 the central nerve thread is surrounded with a number of coats, 
 onion-fashion (Fig. 30, iii.). Somewhat similar corpuscles are 
 found in the tongue, mesentery, and elsewhere. They may be 
 tactile, but their function is not certainly known. Much smaller 
 corpuscles in the papillae beneath the epidermis of the human 
 finger are undoubtedly tactile. 
 
 (b.) Taste. On each side of the tongue of a rabbit is a plaited 
 patch, the so-called papilla foliata. Fig. 27, i. shows a portion 
 of a section across the folds of this patch. On the adjacent 
 
Chap. V.] 
 
 GENERAL HISTOLOGY. 
 
 77 
 
 sides of two folds are seen four taste-buds imbedded in the 
 epithelium and connected with nerve fibres at their inner ends. 
 At ii. one of these taste-buds is shown on a larger scale. 
 
 (c.) Smell. In the olfactory membrane of the nose certain 
 elongated spindle-shaped cells are wedged in between the more 
 
 FIG. 27. XERVE-ESDTXOS. 
 
 i. Taste-buds of papilla foliata of rabbit, ii. A bud enlarged, iii. Olfactory 
 cells (dog), iv. Retina of frog. 1. Pigmented epithelium. 2. Rods and 
 cones; between this and 3 is a structureless limiting membrane. 3. Outer 
 nuclear layer. 4. Outer molecular layer. 5. Inner nuclear layer. 6. Inner 
 molecular layer. 7. Layer of nerve cells. 8. Layer of nerve fibres. The 
 layer marked 8 is turned towards the source of light, and is bounded by 
 a structureless limiting membrane. 
 
 ordinary epithelium cells as indicated in 27, iii. These are in 
 connection with the ultimate endings of the olfactory nerve. 
 
 (d.) Hearing. Somewhat similar spindle-shaped cells produced 
 into long auditory hairs are found in special regions of the organ 
 of hearing, and are in connection with the auditory nerve. 
 
 (e.) Sight. The most complex of nerve-endings are those of the 
 optic nerve in the retina of the eye. Fig. 27, iv. gives a section 
 of the frog's retina under a high power (). There are eight 
 distinct layers : 1. pigmented epithelium : 2. the layer of rods 
 and cones ; 3. the outer nuclear layer ; 4. the outer molecular 
 layer ; 5. the inner nuclear layer ; 6. the inner molecular layer ; 
 7. the layer of nerve cells ; 8. the layer of nerve fibres. The 
 last layer, that of nerve fibres, is the most internal, nearest the 
 centre of the eyeball, and presented towards the rays of light. 
 But the light vibrations are received by the ends of the rods and 
 cones in the pigmented epithelium, and then transmitted to the 
 
ANIMAL BIOLOGY. 
 
 [Part I. 
 
 layer of nerve fibres, through the intermediate layers, and thence 
 by the optic nerve to the brain. Thus the light passes outwards 
 through the thickness of the retina mechanically as through a 
 transparent medium, and then passes backwards through the 
 retina organically as a wave of molecular change through the 
 nerve endings. The granular layers owe their dotted appearance 
 to the (stained) nuclei in the specially modified cells. 
 
 11. The Stomach and Intestine. A cross-section of a frog's 
 stomach examined under a low power (Fig. 28, i.) shows a num- 
 ber of inwardly projecting folds, and if it be stained there 
 
 FIG. 28. STOMACH AND INTESTINE. 
 
 i. Stomach of frog. ii. A portion of the same under higher power. 1. 
 Peritoneum. 2. Longitudinal muscular layer. 3. Concentric muscular layer. 
 4. Submucosa. 5. Muscularis mucosse. 6. Mucosa containing peptic glands, 
 iii. Peptic gland (dog) under still higher power, iv. Villus (dog) injected to 
 show blood-vessels, v. Villus (rabbit) stained to show cells. From sections. 
 
 will be an internal lighter and an external darker zone. Under 
 a higher power (28, ii.) six tolerably distinct layers are seen : 
 (1.) Externally the peritoneum, often lost from the section; 
 (2.) external muscular -layer with fibres running longitudinally ; 
 (3.) internal muscular layer with fibres running concentrically ; 
 
Chap. V.] GENERAL HISTOLOGY. 79 
 
 (4.) submucosa of connective tissue ; (5.) muscular layer of the 
 mucosa ; (6.) mucosa. In the mucosa of the mammalian stomach 
 great numbers of glandular tubes (the peptic glands) are seen. 
 In each gland (28, iii.) there is a single layer of secreting cells 
 continuous with the epithelial lining of the stomach. 
 
 In the intestine of the mammal or the bird there are a vast 
 number of minute processes (villi) projecting inwards, and giving 
 a velvety appearance. Fig. 28, iv. shows one of these villi, in 
 which the blood-vessels are injected, while 28, v. shows the end 
 of a villus. It is covered with columnar epithelium cells, many 
 of which are converted into goblet or mucous cells, larger and clearer. 
 Beneath this the tissue is lymphatic in structure, and is apt to 
 be crowded with leucocytes. The central blind tube in Fig. 28, 
 v. is the chyle-vessel or terminal rootlet of the lacteal system of 
 lymphatics. The layers of the intestine are essentially similar 
 to those of the stomach. 
 
 12. The larger Alimentary Glands. In favourable sections 
 of a salivary gland the alveoli (29 i. from the sublingual gland 
 
 FIG 29. SALIVARY GLAND, LIVER, AND KIDNEY. 
 
 Alveolus of sublingual gland of rabbit, ii. Duct of same. iii. Liver of rabbit 
 stained, with bile canaliculi injected, iv. Liver (cat) with blood-vessels injected. 
 v. Kidney of rabbit injected to show glomeruli. All from sections. 
 
 of a rabbit) or true secreting terminations of the ducts may be 
 made out. Their cavities are nearly or quite obliterated. In 
 the ducts, on the other hand, the cavity or lumen is generally 
 obvious, and the cells more columnar (29, ii.). 
 
 A section of the pancreas shows a somewhat similar structure. 
 
8o ANIMAL BIOLOGY. [Part I. 
 
 In the case both of the salivary glands and of the pancreas the 
 alveoli are surrounded by delicate bands of connective tissue, 
 and are grouped into lobules, surrounded by stronger bands, 
 while the lobules are further grouped into lobes separated from 
 each other by yet stronger connective tissue continuous with 
 that of the capsule which encloses the whole gland. 
 
 The structure of the liver is very different from that of the 
 salivary glands or pancreas. The substance of the organ is more 
 or less distinctly divided by connective tissue partitions into a 
 a vast number of lobules about -^ inch in diameter. In a 
 section of a liver in which the blood-vessels have been injected 
 (29, iv.), there are seen round the lobule several inter-lobular 
 venules, ultimate branches of the portal system. They send a 
 fine network of capillaries inwards towards the centre of the 
 lobule, where the blood is collected by an intra-lobular venule, one 
 of the ultimate factors of the hepatic vein. In 29, iii. we have a 
 section in which not the blood-vessels but the bile canaliculi 
 have been injected. They form a network between the liver 
 cells, which are thus clearly marked out, and seen to be poly- 
 gonal and nucleated. 
 
 13. The Lung. The trachea of the rabbit is lined with 
 ciliated epithelium. It branches into two bronchi, each of which 
 again branches, rebranches, divides, and subdivides into a vast 
 number of ciliated tubes, the finer and more delicate of which 
 are the bronchioles. Each bronchiole finally branches into wider 
 non-ciliated infundibula, which, with their small branches, form 
 the ultimate termination of this extensive system of tubes. 
 Finally, the walls of each infundibulum are closely beset with 
 minute bags, the air-cells or alveoli, which open into the infun- 
 dibulum. The air-cells are lined with flattened non-ciliated epi- 
 thelial scales, beneath which lies a dense network of pulmonary 
 capillaries. Here it is that the plasma of the blood gives up its 
 carbonic acid gas, and the exhausted red corpuscles receive 
 a fresh supply of oxygen. Each lobule is separated from its 
 neighbours by delicate bands of fibrous connective tissue, by the 
 action of which the infundibula and air-cells have a tendency 
 
Chap. V.] GENERAL HISTOLOGY. 81 
 
 to resist inflation, and to expel the contained air during 
 expiration. 
 
 In birds the minute lung structure is different. There are no 
 infundibula and no air-cells, though the finest tubuli are 
 minutely sacculated. These minute tubuli which form a close 
 plexus in the lung are placed in communication with each 
 other by minute perforations in their walls. It will be remem- 
 bered (p. 49) that the lungs are in connection with a system of 
 large air-sacs. 
 
 In the frog the lung is a hollow structure, pitted on its 
 inner surface with sacculations, formed by the infolding of the 
 walls. These sacculations are more numerous in the anterior 
 part of the lung. In their walls is the capillary plexus. Their 
 edges are often covered with ciliated cells, but the cavities of 
 the sacculations are lined with flattened epithelium. 
 
 14. The Kidney. The kidney of the rabbit is hollowed out 
 on one side, its inner edge forming a depression or hilus. Here 
 the ureter enters and enlarges into a funnel-like dilatation, the 
 pelvis, which ends in a number of smaller dilatations, the calyces. 
 Into the pelvis there projects a conical mass of kidney sub- 
 stance, the urinary pyramid. The substance of the kidney is 
 divisible into an outer portion, the cortex, which has a dotted 
 appearance, and an inner portion, the medulla, which is radially 
 striated. 
 
 In a section of a kidney in which the blood-vessels have been 
 injected, arterioles are seen running horizontally in the inter- 
 mediate region between cortex and medulla. They send up verti- 
 cal branches into the cortical layer, which in turn branch, each 
 branch ending in a convoluted tuft of blood-vessels called a 
 glomerulus (Fig. 29, v.) Each glomerulus is (as shown diagram- 
 matically to the right of the figure) enclosed within a capsule 
 (lined with flattened epithelial plates), which forms the dilated 
 end of a urinary tubule. The blood passes into the glomerulus 
 by an afferent vessel, and out by an efferent vessel. It then 
 breaks up into a delicate capillary plexus within the cortex 
 
82 ANIMAL BIOLOGY. [Part I. 
 
 around the urinary tubules, which are lined with columnar or 
 polyhedral secreting cells. The course of the tubules is peculiar. 
 After winding about in the cortex (so that in a section the cut 
 ends, of numerous coils will be seen), each tubule passes down 
 into the medullary region ; but instead of passing at once to the 
 pelvis it turns back, making a sharp loop (Henle's loop), re-enters 
 the cortex, and there again becomes convoluted, after which 
 it again dives down into the medulla, and so reaches the pelvis. 
 The size of the tubule varies, being larger in the convoluted 
 parts of its course. This course is special to the mammalia. 
 In a stained section, therefore, the cortex is characterised by 
 the coiled tubules and glomeruli, while the medullary portion 
 shows the straighter parts of the tubules, and vascular bundles. 
 
 It seems probable that from the glomeruli there passes into 
 the capsule little but water containing a certain amount of salts 
 in solution, the amount of fluid thus filtered from the blood 
 depending largely on the blood pressure. The urea is probably 
 secreted in the convoluted portions of the tubule surrounded 
 by the capillary plexus. 
 
 In the frog there are glomeruli, capsules, and convoluted 
 tubules, but the latter have not the remarkable mammalian 
 course, and there is no distinction into cortical and medullary 
 regions. There is, it will be remembered (p. 28), a double blood 
 supply. That from the renal artery passes to the glomeruli, 
 that from the renal portal vein passes to the convoluted tubules. 
 And it has been experimentally proved that here, and not in the 
 glomeruli, is urea (injected into the blood) excreted. 
 
 15. The Skin. A transverse section of the skin of the frog is 
 shown in Fig. 30, i. The external layer of the epidermis is com- 
 posed of flattened scales. A middle layer shows rounder 
 nucleated cells. A deeper layer shows larger columnar cells, 
 with their long axes vertical. Beneath this is the dermis, 
 composed largely of 'fibrous bands supporting nerves and blood- 
 vessels. In the uppermost part of the dermis are large pigment 
 cells. 
 
 Imbedded in the dermis, with their narrow funnels passing- 
 
Chap. V.] 
 
 GENERAL HISTOLOGY. 
 
 through the epidermis (out of focus in 30, i.) are large flask- 
 shaped cutaneous glands lined with oblong nucleated cells, which 
 secrete a fluid by which the skin is kept moist, 
 (especially in the toad) is extremely bitter. 
 
 FIG. 30. THE SKIX : HAIRS. 
 
 i. 8km of frog. ii. Skin of human arm. 1, 2, 3. Layers of epidermis, iii. 
 Skin of human thumb, iv. Hair follicle (negro), v. Sebaceous gland 
 (human), vi. External view, white hair of rabbit, vii. Darker hair of rabbit 
 in optical section. 
 
 Fig. 30, ii. and iii. show sections of the human skin of the 
 arm (ii.) and of the thumb (iii.). They show the three layers of 
 the epidermis : (1) An outer layer of non-nucleated horny scales ; 
 (2) a more transparent middle layer, in which nuclei can be 
 seen ; (3) an inner darker layer of distinct and nucleated cells. 
 It is by the constant multiplication of these lower cells that the 
 overlying cells become thrust upwards towards the surface, 
 becoming first flattened and then eventually mere horny scales, 
 which are continually being shed. 
 
 Beneath the epidermis is the dermis. This is raised into 
 papillce, to which the epidermis is moulded, and in the papilla) 
 of the thumb the tactile corpuscles may be readily seen under a 
 high power. Deeper in the dermis lie the large oval pacliuaii 
 
84 ANIMAL BIOLOGY. [Parti. 
 
 corpuscles with their concentric coats. The tissue to the right of 
 the oval corpuscle in iii. is fatty tissue, above which is the cut 
 end of a blood-vessel. 
 
 Passing down through the epidermis into the dermis are 
 spiral tubes, each ending in a coiled glandular knot. These are 
 the sweat glands. One of these is shown in iii., while portions 
 of three other glandular knots are cut in different planes. 
 
 Imbedded in the mammalian skin are the hairs characteristic 
 of the order. Each hair is moulded on a little papilla of the 
 dermis (iv. ) Around the root of the hair is a root-sheath com- 
 posed of an internal and an external layer continuous with the 
 deeper and superficial layers of the epidermis respectively. Out- 
 side this is a fibrous dermic layer, passing up into the papilla. 
 The whole is in iv. (from the scalp of a negro) imbedded in 
 fatty tissue. The whole tubular depression in the skin contain- 
 ing the hair is called the hair follicle. Into it there open, near 
 its external end, sebaceous glands, secreting an oily substance. 
 The position of one of these is roughly indicated in 30, v. 
 
 Fig. 30, vi. and vii. show rabbit's hairs under a fairly high 
 power. Fig. 30, vi. is from near the point of a white hair, and 
 shows the imbricating scales of the cuticle. Fig. 30, vii. shows, 
 besides these scales, the fibrous cortex and the internal irregular 
 pigmented or air-containing medulla. In each case the proximal 
 or root end of the hair is uppermost. 
 
 In the nails or claws the external epidermic scales become 
 specially soldered together, and horny or fibrous in consistency. 
 They are moulded in the rabbit on large papillae, their roots 
 being also covered over with a fold of skin so as to lie in a 
 groove. 
 
 Attention may here be drawn to the different kinds of glands 
 Avith which we have become acquainted in the skin and else- 
 where. 
 
 1. Simple tube-like depressions, e.g. peptic glands. 
 
 2. Simple flask-shaped depressions, e.g. cutaneous glands of 
 
 frog. 
 
 3. Tubes coiled at the end. Sweat glands. 
 
 4. Tubes with simple branches. Sebaceous glands. 
 
Chap. V.] GENERAL HISTOLOGY. 85 
 
 5. Long tubes (ducts) with a vast number of branches end- 
 
 ing in slight dilatations (racemose glands), e.g. salivary 
 glands, pancreas. 
 
 6. Long tubes (ducts) ending in a network of inter-cellular 
 
 canaliculi, which, in the rabbit, give off minute twigs 
 that pass into the substance of the liver cells, and 
 there terminate in vesicular enlargements. Liver. 
 
CHAPTER VI. 
 
 GENERAL EMBRYOLOGY. 
 
 THE vertebrate animal is developed from a fertilised ovum. 
 The multitude of cells of which together with cell products 
 the tissues of the adult organism are composed, is produced by 
 the repeated division and sub-division of the original oosperm. 
 Thus we may trace back the organism to the ovum with which 
 a spermatozoon has entered into fertile union ; and the develop- 
 ment of the ovum and spermatozoon in the parents we may 
 trace back to the fertilised ovum from which they also were 
 respectively developed. Omnis cellula a celluld is a law that, so 
 far as we know, holds good for all animal life. It may be that- 
 in past ages, it may be that even now' in the depths of the ocean, 
 or in stagnant pools, the origin of a living cell from not-living 
 matter has occurred or is taking place. No one has a right to 
 assert that it has not and cannot. But as a matter of science 
 such abiogenesis (or origin of living from not-living matter) has 
 never yet been proved. And therefore, as students of science, 
 we shall do well to accept the law of biogenesis omnis cellula a 
 celluld as our working hypothesis. 
 
 The ovary and testis which in the parents produce the 
 ovum and spermatozoon arise in the first instance, when the 
 parent is still an embryo, as thickenings of the sheet of cells 
 that lines the body-cavity. The thickened part of the sheet is 
 called the germinal epithelium ; and for a while it is quite similar 
 in male and female. By mutual ingrowth of co 1 ls, (1) from the 
 germinal epithelium into the substance of the underlying body- 
 walls, and (2) from the body-walls into the germinal epithelium, 
 a thickened ridge, the genital ridge, is produced. But still there 
 
Chap. VI.] GENERAL EMBRYOLOGY. 87 
 
 is no distinction between ovary and testis. Ere long, however, 
 differentiation sets in, and different lines of development are 
 followed in the two sexes. 
 
 In the male the testis becomes a mass of minute tubes, the 
 seminal tubules, twisted and convoluted in all directions, and 
 lined with large cells, probably derived from the original 
 germinal cells, which are destined to give rise to spermatozoa. 
 They are termed sperm cells. From these a number of 
 smaller cells arise by a process like budding. It is these smaller 
 cells (spermatoUasts) which are converted into spermatozoa. 
 The tubules in which they are developed open into the vas 
 deferens, by which the ripe spermatozoa are carried out of the 
 body. 
 
 In the ovary of the female there is no development of tubules. 
 Certain larg germinal cells are developed at the expense of 
 other cells. These are the ova. The cells adjoining each ovum 
 arrange themselves round it so as to form a special layer which 
 forms the lining of a sort of bag, the ovisac, in which the ovum 
 lies. When the ovum is ripe the ovisac bursts, and the egg is 
 set free into the peritoneal cavity, or passes at once into the 
 oviduct, and so out of the body. 
 
 The very young ovum, before it is mature, consists of a minute 
 spherical mass of granular protoplasm. Within it is a specialised 
 portion differing from the rest, and called the germinal vesicle, 
 and within this again may be seen one or several germinal spots. 
 The germinal vesicle is invested by a delicate membrane, and 
 stretching from this membrane to the germinal spot or spots 
 is a network of delicate fibres called the reticulum. The ovum is 
 a simple cell. The germinal vesicle may represent the nucleus 
 of the cell, and each germinal spot a nucleolus. 
 
 Such is the immature ovum. From it the mature ovum 
 differs (1) in the possession of an external membrane; and (2) 
 in the development within it of a greater or less quantity of 
 food-yolk in the form of minute masses known as yolk-spheres. 
 Both the amount and the mode of distribution of the food-yolk 
 are matters of considerable importance. The amount differs 
 markedly in our three types. The minute ovum of the rabbit 
 
ANIMAL BIOLOGY. [Part I. 
 
 (y^th in.) has but little; the larger ovum of the frog contains 
 a moderate amount ; in the much larger fowl's egg (for we will 
 here take the fowl as our typical bird) there is a relatively 
 enormous quantity. The distribution of the yolk-spheres also 
 differs in the two types in which they are freely developed. In 
 the frog they are larger and more closely aggregated at one pole, 
 than they are at the opposite pole, in which the yolk-spheres 
 are smaller. This protoplasmic pole, with the smaller yolk- 
 spheres, is termed the animal pole. In the fowl the yolk-spheres 
 are of two kinds, smaller and earlier developed white yolk- 
 spheres, larger and later developed yellow yolk-spheres. These 
 are arranged in somewhat concentric layers around a central 
 flask-shaped mass of the white yolk. 
 
 The germinal vesicle, which is relatively large in the immature 
 ovum, occupies less relative space in the mature egg. In the 
 rabbit's ovum it is near, but not at the centre. With the 
 development of food-yolk it becomes thrust over towards the 
 animal pole, near which it lies in the frog's ovum; while in 
 the fowl's egg it travels to the periphery, where, together with 
 the protoplasm round it, which remains relatively free from 
 yolk-spheres, it constitutes the germinal disc. 
 
 The phenomenon now to be described, termed the extrusion 
 of the polar cells, probably occurs in all three types. It consists 
 in the division of the germinal vesicle of the ovum (or the 
 nucleus of the cell) into two parts, of which one remains within 
 the ovum, while the other, carrying with it a portion of the 
 protoplasm, gives rise to a little protuberance, budded off from 
 the ovum, and known as the polar cell. The polar cell is then 
 constricted off, becomes separate from the ovum, and dies. A 
 second polar cell is then budded off in a similar way. Thus two 
 polar globules are produced by a process analogous to cell- 
 division. It is supposed that they remove from the ovum 
 matter detrimental to the further development of the egg. 
 Each contains a share of the nucleus of the ovum. The share 
 that remains to the ovum after the extrusion of the polar cells, 
 containing perhaps only one quarter of the original nuclear 
 matter, is called the female pronudeus. 
 
Chap. VI.] GENERAL EMBRYOLOGY. 89 
 
 After this has taken place the ovum is ready for impregnation. 
 Here again we are forced to infer, from the results of investiga- 
 tions on other forms of life, the nature of the process of fertili- 
 sation. It would seem, however, that only one spermatozoon 
 enters the ovum. This spermatozoon is essentially a cell, and 
 its head contains the sperm-nucleus. When it enters the 
 ovum the sperm-nucleus forms the structure known as the 
 male pronucleus. The male pronucleus and female pronucleus 
 now approach each other, and finally fuse, the fused product 
 being known as the first segmentation nucleus. 
 
 Upon this the process of segmentation sets in, and does not 
 cease until, by repeated cell-division, the fertilised ovum gives 
 rise to the adult organism. It is obvious that during this pro- 
 cess the cells must be nourished; (1) either by gradual absorption 
 of food-yolk j or (2) by the individual efforts of the developing 
 embryo ; or (3) by absorption of nutriment elaborated by the 
 mother. The frog, after the somewhat scanty stock of food- 
 yolk is absorbed, is hatched, and forthwith has to obtain food 
 for itself. The fowl undergoes a far larger proportion of its 
 development within the egg at the expense of the abundant 
 supply of food-yolk ; then it fends for itself, with some maternal 
 aid and advice. The rabbit is almost from the first supplied 
 with nutritive material by the mother, within whose body 
 embryonic development takes place ; and even after birth feeds 
 for some time on the mother's milk. 
 
 The process of segmentation is essentially the division of the 
 primitive cell into two cells, each of these again into two, and 
 so on until the one has become a multitude. In each case of 
 cell-division there is also nucleus-division. And it has been 
 found that the reticulum of the nucleus undergoes a remarkable 
 series of changes constituting karyokinesis. 
 
 Fig. 31, i. shows diagrammatically a cell in which the nucleus 
 is in the ordinary reticulum condition. In certain cases the 
 following changes have been shown to occur. When karyokinesis 
 sets in the capsule or limiting membrane of the nucleus tends to 
 disappear, and the protoplasm of the nucleus takes on a striated 
 appearance giving rise to a nuclear spindle, from the ends of 
 
ANIMAL BIOLOGY. 
 
 [Part I. 
 
 which star-like striations extend into the protoplasm of the cell 
 (Fig. 31, ii.). lii the middle of the spindle the fibres of the 
 
 reticulum aggregate so as 
 to form the equatorial plate. 
 This splits in the equatorial 
 line into two plates which 
 travel away from each other 
 towards the poles, the proto- 
 plasm between them having 
 a striated appearance (Fig. 
 31, iii.). Meanwhile the 
 protoplasm of the general 
 
 FIG. 31. KARYOKINESIS. 
 
 substance of 
 e.g. so far 
 
 the cell becomes 
 as nearly to divide 
 
 constricted, in Fig. 31, iv., 
 the cell into two, and even- 
 
 tually to such an extent as to cause its complete division into 
 two cells, each of which now contains a nuclear spindle, 
 which may either relapse into a reticulum or straightway re- 
 divide. Watched under the highest power of the microscope 
 the fibres of the equatorial plate are seen to group themselves 
 into the form of a rosette, which then, with sundry other 
 changes of form, separates into two rosettes, each of which forms 
 a new equatorial plate. Such are the internal changes in 
 the segmenting cells. We must now turn to the consideration 
 of the segmentation of the ovum as a whole, and the formation 
 of the so-called germinal layers to which this process gives rise, 
 remembering that each case of cell-division is accompanied by 
 karyokinesis. But before describing what takes place in frog, 
 fowl, or rabbit it will be well to pave the way by taking an 
 ideal case. 
 
 We will suppose that the ovum contains no food-yolk. It 
 first divides into two equal halves, and then each of these again 
 into two. We may call the points of intersection of these two 
 planes the poles, and the planes vertical planes. We thus have 
 four cells produced by two vertical planes, the segmentation 
 being so far similar to that shown in Fig. 33. The next piano is 
 equatorial midway between the poles. By this plane the four 
 cells are subdivided into eight. Then follow two more vertical 
 
Chap. VI.J GENERAL EMBRYOLOGY. 91 
 
 planes intermediate between the first two. By them the eight 
 cells are divided into sixteen. Then follow two more horizontal 
 planes midway between the equator -and the poles. Thus 'we 
 get thirty-two cells. So the process continues, until by fresh 
 vertical and horizontal planes of division the ovum is divided 
 into a great number of cells. 
 
 But meanwhile a cavity has formed in the midst of the ovum. 
 This makes its appearance at about the eight cell stage, the 
 eight cells not quite meeting in the centre of the ovum. The 
 central cavity so formed is thus surrounded by a single layer of 
 cells ; and it remains as a single layer throughout the process of 
 segmentation. So that the result of segmentation is the pro- 
 duction of a hollow vesicle (Fig. 32, i.) composed of a membrane 
 with a single layer of cells. Suchji vesicle is called a Uastosphere ; 
 the period during which this state of things continues is called 
 the Uastula stage ; and the cavity of the blastosphere is called 
 the segmentation cavity or Uastocode.^ 
 
 Now suppose 1 that one side of the blastosphere, which is 
 represented in Fig. 32, ii., being composed of somewhat larger 
 cells, is pushed in; just as one might take a hollow squash india- . 
 rubber ball, and push in one side so as to form a hollow two- 
 layered cup. Suppose, moreover, that the mouth of the cup 
 closes in so as to become much smaller. Then we have the 
 state of things shown in Fig. 32, iii. This is the gastrula stage. 
 Thus we have constituted, by a process termed imagination, an 
 embryo with two layers of cells, an outer layer or epiblast (ep.\ 
 and an inner layer or hypoblast (%.). They are the two primi- 
 tive germinal layers. The cavity within the embryo is the primi-- 
 tive digestive body-cavity or archenteron (a. e,). The opening to 
 the exterior is the Uastopore (U.p.}. 
 
 A transverse section through the middle of the embryo at the 
 gastrula stage shows a central cavity surrounded by a two- 
 layered body-wall. This state of things does not long continue. 
 Along a definite line on the surface of the embryo, marking off 
 a region henceforth to be known as the dorsal, the epiblast, or 
 
 1 The advanced student will ascertain for himself how far these suppositions 
 are gratuitous. 
 
ANIMAL BIOLOGY. 
 
 [Part I. 
 
 outer layer becomes somewhat thickened, forming the neural 
 plate. The edges of the thickened area rise up on either side so 
 as to form a median groove, the neural groove (32, iv. n. g.) ; 
 and then close over above the groove so as to convert it into a 
 
 - 
 
 FIG. 32. DIAGRAM OF EARLY STAGES OF DEVELOPMENT. 
 
 i. and ii. Blastula stage, iii. Gaslrula stage, iv. Establishment of mesen- 
 teron ; notochord and medullary groove, v. The closure of the medullary 
 folds and cleavage of the mesoblast. 
 
 ci^ e. Archenteron. 6. c. Body-cavity (coalom). Np. Blastopore. ch. Noto- 
 chord. ep. E pi blast, ty. Hypoblast. 1. p. Lateral plate, n. c. Neural 
 canal, n^g. Neural groove, m. p. Mesoblastic plate, tsf. Mesoblast. 
 mn. Mesehteron. s. c. Segmentation cavity, so. I. Somatic layer (somato- 
 pleure). ' sp. 1. Splanchnic layer (splanchnopleure). v. p. Vertebral plate. 
 
 neural canal (32, v. n. c.). This is the first indication of. the 
 central nervous system. It lias in the mid-line of the dorsal 
 region, and is formed from* x %pT5lastT~ Immediately beneath it 
 there is formed a linear rod of cells derived from the hypoblast 
 of the mesenteron, and called the notocJwrd (ch.). 
 
 Meanwhile a third germinal layer (Fig. 32, iv.) is being formed 
 between the epiblast and the hypoblast. It is the mesoblast. 
 The cells of which it is composed are probably derived from 
 the hypoblast; but in any case it soon forms a separate layer, 
 
Chap. VI.] GENERAL EMBRYOLOGY. 93 
 
 divided by the notochord into two separate sheets, the mesdblastic 
 plates, (m. p.) passing round and meeting below the digestive &* 
 cavity (mesenteron, mn.). 
 
 We have yet to note some further changes in the mesoblastic 
 plates. (1.) Each splits 1 into two layers, of which the inner layer 
 adjoining the mesenteron is called the visceral or splanchnic layer 
 (sp. I.), while the outer, that adjoining the epiblast, is the parietal 
 or somatic layer. The spaqe between them is the body- cavity 
 (b. c.). (2) On each side of the neural canal and the noto- 
 chord longitudinal masses of the mesoblast are separated off 
 from the mesoblastic plates. These median dorsal parts of the 
 mesoblastic plates are now known as vertebral plates (v.p.), while 
 the remainder of the mesoblastic plates are now termed lateral 
 plates. In the vertebral plates the cleavage of the mesoblast 
 and resulting, body-cavity are eventually obliterated. (3.) The 
 vertebral plates become segmented by transverse constrictions 
 into a series of more or less cubical bodies (for which in the 
 chick see Fig. 37, m. s.), the mesoblastic somites. (4.) In the lateral 
 plates the cleavage of the mesoblast is obliterated only at the 
 dorsal end, near the vertebral plates. Below this the body- 
 cavity is persistent. The splanchnic layer eventually forms the 
 muscular coat of the alimentary canal, while the somatic layer 
 gives rise to the endothelial lining of the body-walls. 
 
 The blastopore has probably by this time closed, so that 
 the mesenteron has neither anterior nor posterior openings to 
 the exterior, though posteriorly it may open into the neural 
 canal, by a passage known as the neurenteric canal (n. e. c. in 
 frog, Fig. 36, in.). External openings are formed, often com- 
 paratively late in embryonic life, by the formation of external 
 depressions which meet hollow outgrowths from the mesenteron. 
 The anterior depression, that for the mouth, is called the stomo- 
 dceum ; the posterior depression, that for the vent, is called the 
 proctodceum. The former is always in vertebrates a new invagi- 
 nation; but the latter may be in some types (perhaps in the 
 
 1 The cleavage of the mesoblast begins before the mesoblastic plates meet 
 below the mesenteron. The two processes are kept separate in the figures and 
 description for the sake of clearness. 
 
94 ANIMAL BIOLOGY. [Parti. 
 
 frog) the persistent blastopore. The lungs, and the glands of the 
 alimentary canal arise as hollow outgrowths therefrom. The 
 heart, and all the blood-vessels arise as tubes within the meso- 
 blast, from which layer are also formed all the muscles, cartilages, 
 and bones of 'the body. The hypoblast gives rise to the epithelial 
 layer of the alimentary cavity and of the glands connected there- 
 with. The epiblast forms the epidermic layer of the skin ; but 
 from it are also formed the central nervous system and certain 
 important structures connected with the organs of special sen- 
 sation. 
 
 We have thus seen how the three germinal layers, the ali- 
 mentary canal with its mouth and anus, the body-cavity, the 
 central nervous system, and the notochord may be formed in 
 the case of an ideal, but not wholly imaginary, animal. The 
 student is clearly to understand that both Fig. 32 and this descrip- 
 tion of the mode of formation of the leading structures in the body 
 are, so to speak, diagrammatic, and introduced merely to aid him 
 in comprehending how these structures are formed in the frog, 
 the fowl, and the rabbit, to which we may now turn. 
 
 Segmentation of the Ovum. (1.) In the frog the mode of seg- 
 mentation does not differ very markedly from that described. The 
 segmentation is complete (holoblastic), that is to say, the planes 
 of division run right through the whole thickness of the ovum, 
 all its substance being segmented or broken up into cells ; but 
 owing to the inequality of the distribution of the food-yolk it is 
 unequal or irregular. Where the protoplasm is most concen- 
 trated, namely, at the animal pole, segmentation is more rapid. 
 Thus the first two furrows form rapidly in the upper half of the 
 ovum, but more slowly extend to the lower half. The first 
 horizontal furrow is not equatorial, but near the animal pole 
 (Fig. 33). And throughout segmentation cell-division goes on 
 more rapidly in the animal hemisphere, so that the cells are 
 smaller there than in the opposite hemisphere, It is usually 
 found that wherever large quantities of food-yolk occur, segmen- 
 tation is more or less unequal, the velocity of segmentation being 
 directly proportional, and the size of the cells inversely pro- 
 
Chap. VI.] 
 
 GENERAL EMBRYOLOGY. 
 
 95 
 
 portional to the concentration of the protoplasm. At the close 
 of the segmentation of the frog's egg (Fig. 34, i.) the ovum 
 forms a blastosphere containing 
 an excentric segmentation cavity, 
 roofed over with several layers of 
 small cells, and resting on a con- 
 tinuous mass of larger yolk-con- 
 taining cells. 
 
 (2.) In the rabbit the segmen- 
 tation is complete and nearly 
 regular, but presents some pecu- 
 liar features. Two processes seem 
 to go on together. In addition 
 to the formation of a blastula 
 with a segmentation cavity, there 
 goes on a slipping inwards of 
 some of the cells to fill up this 
 cavity. And since the latter pro- 
 cess keeps pace with the former, 
 the cavity is filled up as rapidly 
 as it is produced. Thus at the close of segmentation there is 
 an outer layer (pu. 9 Fig. 34, ii.) of clear cells surrounding, except 
 at one spot (which we may term the pseudo-llastopore, v. l.\ an 
 inner mass of granular cells (In.}. 
 
 (3.) In the fowl, owing to the enormous quantity of food 
 yolk and the consequent concentration of the protoplasm in the 
 germinal disc, the segmentation is not only very irregular, but 
 also incomplete or partial (meroblastic). That is to say, the 
 planes of division do not extend throughout the ovum, but are 
 confined to that comparatively small area of its surface known 
 as the germinal disc. The first two furrows are at right angles, 
 but do not cross each other quite in the middle of their length. 
 Four more radial furrows follow intermediate between these, 
 thus dividing the germinal disc into eight somewhat irregular 
 cells. The next furrow is irregularly circular (or polygonal), 
 giving rise to eight smaller central and eight larger peripheral 
 cells. Henceforth the central segments divide more rapidly 
 
 v/ 
 Fia. 33. CLEAVAGE IN FROG'S OVUM. 
 
 i.-iv. Stages with 2, 4, 8, and 16 blasto- 
 meres. v. A later stage when the smaller 
 blastomeres cover half the ovum. vi. A 
 still later stage when the smaller blasts- 
 meres have enveloped the whole ovum, 
 except at one spot, the blastopore, IL 
 
9 6 
 
 ANIMAL BIOLOGY. 
 
 [Part I. 
 
 than the peripheral, while furrows beneath and parallel 
 
 the surface separate the superficial from the deeper cells. Thus 
 
 at the close of segmentation (i.e. before incubation) we have a 
 
 FIG. 34. AT THE CLOSE OF SEGMENTATION. 
 
 i. In frog. ii. In rabbit, iii. In fowl. 
 In. Inner cells. Ou. Outer cells, v. &. Pseudo-blastopore. 
 
 k. Yolk. 
 
 superficial layer of small columnar cells forming a distinct mem- 
 brane, and a deep layer of larger cells irregularly disposed, but 
 deeper around the edges of the disc than at its centre (Fig. 34, 
 iii.). 
 
 The Egg Membranes. The frog's egg is closely invested by u 
 delicate vitelline membrane. External to this is the albuminous 
 envelope secreted by the walls of the oviduct. 
 
 The fowl's egg is also closely invested by a vitelline membrane. 
 In passing down the oviduct (after impregnation) it receives a 
 thick albuminous coating of white-of-egg. At either end of the 
 egg this assumes a denser condition, and forms a twisted knotted 
 cord (chalaza). A two-layered shell-membrane, and finally 5 ' the 
 shell itself, are added in the lower parts of the oviduct. 
 
 The rabbit's egg is surrounded by a radially striated membrane 
 (zona mdiata\ which probably does not' answer to the vitelline 
 membrane of the other types, though this membrane may exist 
 
Chap. VI.] GENERAL EMBRYOLOGY. 97 
 
 in the rabbit's ovum previous to the rupture of the follicle. As 
 it passes down the Fallopian tube the egg becomes invested with 
 an albuminous envelope. 
 
 The Establishment of the Germinal Layers. (1.) We have 
 seen that in the frog there is at the close of segmentation a blasto- 
 sphere containing a comparatively small segmentation cavity 
 placed excentrically, roofed over with several layers of smaller 
 cells, and resting on a considerable mass of yolk-containing cells 
 (Fig. 34, i.). So far there is no differentiation into epiblast and 
 hypoblast. The large amount of food-yolk and the mass of 
 lower cells prevents invagination from taking place in so simple 
 a manner as in our illustrative case. Still invagination does take 
 place. The small roofing cells gradually creep over the rest of the 
 sphere, by the conversion at the edges of the roof of large yolk- 
 containing cells into small roofing cells. But having reached the 
 equatorial line, they grow in over a small arc, forming a sort of 
 lip. This marks the hinder end of the future embryo. The lip 
 extends further and further inwards and upwards so as to run 
 parallel, or rather concentric, to the dorsal surface. This exten- 
 sion of the lip is due partly to a true ingrowth of the external 
 roofing cells, but perhaps partly also to a differentiation of the 
 yolk-containing cells along its line. As it extends inwards and 
 upwards a slit-like cavity opens beneath it (Fig. 35, i.). Mean- 
 while the smaller roofing cells creep further and further round 
 the rest of the sphere, until only a small patch of the larger 
 yolk-containing cells remains uncovered. As the slit-like mesen- 
 teron enlarges, the original segmentation cavity is gradually 
 obliterated. Eventually the plug of yolk-containing cells be- 
 neath the inturned lip disappears, and the mesenteron com- 
 municates with the exterior by a small circular opening. 
 
 Here, then, we have the gradual formation of an external layer 
 of cell ^ which turns inwards so as to line the roof of a cavity 
 which eventually opens to the exterior by a posterior orifice. 
 The external layer is the epiblast ; the internal layer continuous 
 with it at the lip is the hypoblast ; the slit-like cavity is the 
 mesenteron; and the posterior orifice is the blastopore. The 
 7 
 
ANIMAL BIOLOGY. 
 
 [Part I 
 
 whole thing is comparable with what we had in the illustrativi 
 case ; but it is much modified by the presence of yolk-containing 
 cells. The hypoblast, for example, only forms the roof of th< 
 mesenteron, and only gradually grows downwards to line th< 
 sides. 
 
 FIG. 35. FORMATION OF GERMINAL LAYERS. 
 
 i. Section of frog's ovum. ii. Section of upper part of blastodermic 
 vesicle of rabbit, iii. Supervicial view of blastoderm in fowl. iv. Section 
 through, and at right angles to, the primitive groove in rabbit, v. Section 
 through, and at right angles to, the medullary groove. 
 
 Hp. Blastopore. ch. Notochord. ep. Epiblast. Tiy. Hypoblast. In. 
 Inner cells. Ip. Lip of blastopore. m. g. Medullary groove, m. p. Meso- 
 blastic plate, me, Mesoblast. mn. Mesenteron. On. Outer cells, pr. g. 
 Primitive groove, s. c. Segmentation cavity, yk. c. Yolk-containing cells. 
 
 So far we have only got epiblast and hypoblast. But th< 
 invaginated layer which gives rise to hypoblast gives rise als< 
 to mesoblast. For it is not composed of a single layer of cells 
 but of several layers, of which the lowest, that which lines th< 
 roof of the mesenteron, is hypoblast, while the others give ris< 
 
Chap. VI.] GENERAL EMBRYOLOGY. 99 
 
 to the mesoblast of this region. On the ventral side of the 
 blastopore, mesoblast either grows inward from the epiblast, or 
 arises by differentiation of the larger yolk-containing cells. 
 
 Eventually all the yolk-containing cells are converted into 
 mesoblast, or hypoblast, except a certain number, which eventu- 
 ally become enclosed in the mesenteron, by the growing of 
 the hypoblast round them above the mesoblast (me. in Fig. 36, 
 iii.), so as to bring them inside the digestive cavity, where they 
 form a small internal yolk-sac, the cells of which are gradually 
 absorbed. 
 
 The formation of the notochord, and the differentiation of 
 the mesoblastic plates into vertebral and lateral plates ; and 
 the permanent cleavage of the mesoblast in these lateral plates, 
 so as to form a splanchnic and a somatic layer, with a body- 
 cavity between them; all this takes place in a manner sub- 
 stantially similar to that described in the illustrative case. 
 
 (2.) We have seen that in the fowl's egg segmentation con- 
 verts the germinal disc into a two-layered blastoderm, the cells of 
 which are smaller in the superficial than in the deeper stratum. 
 The central part of the blastoderm (area pellucida) rests on a 
 clear fluid contained in a shallow space (Fig. 34, iii.) ; its edges 
 (area opaca) rest on unsegmented white yolk (yk.), and appear 
 as an opaquer rim. 
 
 The hypoblast is not, in the fowl, produced by imagination. 
 The two layers of the original blastoderm are directly converted 
 into epiblast and hypoblast. The former (epiblast) is constituted 
 from the first (i.e. before incubation) of smaller cells arranged 
 as a definite membrane. The lower layer is rapidly converted, 
 under the influence of incubation, into a definite hypoblastic 
 membrane of flattened cells. These form the roof of the shallow 
 space in the area pellucida, which may therefore be regarded as 
 the mesenteron. A segmentation cavity is only found at a very 
 early stage, and is soon obliterated by the rapid growth of the 
 lower layer cells. Between the hypoblast and epiblast there 
 remain some undifferentiated cells, but they do not form a 
 distinct intermediate layer. 
 
 In the fowl, then, we seem to see carried further a process 
 
ANIMAL BIOLOGY. [Part I. 
 
 that was, so to speak, begun in the frog. In our illustrative 
 case, we saw the hypoblast produced entirely by invagination. 
 In the frog we saw it partly produced by invagination, partly 
 by differentiation of cells which lie beneath the epiblast. Here 
 we find the hypoblast entirely formed by such differentiation, 
 apparently without any invagination. 
 
 At first the blastoderm covers only a small area. But it 
 gradually spreads outwards in all directions, creeping over the 
 yolk beneath the vitelline membrane, until at last (and this does 
 not take place until comparatively late in development) it 
 envelopes the whole mass of yolk. 1 This is similar to what we 
 saw in the frog ; but it is carried further. The investment of 
 the yolk is complete. There is no circular patch of yolk cells 
 left uncovered to mark the position of the blastopore. Thus the 
 blastoderm creeps over the whole surface. But the embryo is 
 entirely formed in the small area pellucida. 
 
 At the hinder end of this area a linear opacity makes its 
 appearance. This is known as the primitive streak. After a 
 while it becomes grooved (Fig. 35, iii., pr. g.). It is produced 
 by a thickening of the blastoderm and the formation of rounded 
 cells which grow downwards, and soon spread outwards in wing- 
 like masses at the sides of the primitive groove. This is shown 
 diagrammatically in Fig. 35, iv. These lateral masses are hence- 
 forth to be regarded as mesoblast. The process by which they 
 are formed is not dissimilar to that by which the mesoblast is 
 formed in the frog at the inturned lip. There we saw a hypo- 
 blast and a mesoblast layer thus formed. Here we find that 
 only the mesoblast is thus produced. This takes place in the 
 hinder third of the area pellucida. 
 
 In the anterior two-thirds the mesoblast is formed in a 
 different way. Here (as seen in Fig. 35, iii. and v.) the 
 medullary groove is beginning to appear. The hypoblast here 
 breaks up into two layers a flattened lower layer, which retains 
 the character and name of hypoblast, and a more indefinite 
 
 1 The hypoblast and mesoblast are said to be here formed mainly through 
 the instrumentality of the nucleated protoplasm of the germinal walls at the 
 edge of the area opaca. 
 
Chap. VI.] GENERAL EMBRYOLOGY. 101 
 
 upper layer of cells, which constitute the mesoblastic layer. In 
 this region, the cells in the median line (which longer retain their 
 primitive connection with the hypoblast, Fig. 35, v.) become 
 concentrated into the notochord (ch\ while the two mesoblastic 
 plates, one on each side, become more and more completely 
 differentiated. The upper parts of these plates give rise subse- 
 quently to a vertebral plate segmenting into mesoblastic somites, 
 the lower parts, or lateral plates, being cleft into two layers, 
 enclosing the body-cavity. 1 
 
 (3.) We have seen that in the rabbit, at the close of segmen- 
 tation, an outer layer of clear cells surrounds, except at one 
 spot, the pseudo-blastopore, an inner mass of granular cells. It 
 is possible, as above suggested, that the slipping inwards of the 
 inner cells is analogous to the process of invagination. But if 
 so, this process is a secondary one, and the pseudo-blastopore is 
 probably not homologous with the blast opore. 
 
 The outer cells ere long grow over the pseudo-blastopore, and 
 completely enclose the inner cells. A fissure then makes its 
 appearance between the inner cells and the outer cells, on that 
 side of the ovum which is furthest from the pseudo-blastopore. 
 The cells separate more and more, until a relatively large 
 cavity is formed. And this cavity increases more and more, 
 until the inner cells form merely a lens-shaped mass attached 
 to the inner side of the outer cells (Fig 35, ii.). Thus, by 
 an internal separation of the cells, is formed a blasto- 
 dermic vesicle The greater part of its walls consists of a 
 single layer of flattened outer cells. It rapidly enlarges, the 
 cavity being filled with an albuminous fluid ; and as it continues 
 to enlarge, the lens-like mass of inner cells spreads out and 
 becomes flattened, so that there is formed a central area where 
 the cells are two rows thick, and a peripheral area where they 
 are one row thick In the central, or embryonic area, the lower 
 layer of inner cells forms the flattened hypoblast, while the 
 upper layer of inner cells fuses with the layer of outer cells to 
 
 1 The cleavage of the mesoblast at first extends into the vertebral plates, but 
 is here, for the most part, obliterated, though even here it may subsequently 
 reappear. 
 
ANIMAL BIOLOGY. [Part I. 
 
 form the epiblast. In the peripheral area around this the outer 
 cells form flattened epiblast, and the inner cells form hypoblast. 
 The lower part of the vesicle is constituted by a single layer of 
 outer cells forming flattened epiblast. In the course of develop- 
 ment the hypoblastic layer grows outwards, so as to line this 
 part also. So that we have a state of things somewhat analo- 
 gous to that which obtains in the fowl ; but with this difference, 
 that in the fowl the blastoderm grows round and encloses a mass 
 of yolk, while in the rabbit it grows round and encloses a cavity 
 containing an albuminous fluid. And just as in the fowl it is 
 only the pellucida area which gives rise to the embryo, so in the 
 rabbit it is only the embryonic area that gives rise to the 
 developing organism. 
 
 In this area the primitive streak is formed in a manner very 
 similar to that which obtains in the fowl ; its surface becomes 
 furrowed with a primitive groove, and from it the mesoblast of 
 the posterior part of the embryonic area is produced (Fig. 35, 
 iv.). In the anterior part of this area, where the medullary 
 groove appears, the mesoblast is derived, as in the chick, from 
 the hypoblastic layer (Fig. 35, v.). From the hypoblast of the 
 axial line the notochord is developed ; and on either side are 
 formed the mesoblastic plates,, separating into (1) a vertebral 
 plate with its mesoblastic somites, in which the body-cavity, 
 arising, as in other types, by cleavage of the mesoblast, is 
 obliterated, and (2) lateral plates, in which the cleavage of the 
 mesoblast and resulting body-cavity is persistent. 
 
 The Neural Tube. In all three types the mode of forma- 
 tion of the neural tube differs in no essential from that de- 
 scribed in our illustrative case. A longitudinal neural plate 
 is formed in the mid-dorsal line* On either side neural folds 
 (lamince dorsales) grow up, and, eventually meeting above, enclose 
 a canal. In the frog this takes place along the whole dorsal 
 line (Fig. 36, i.), and the canal opens into the blastopore. In 
 the fowl and rabbit it is formed in the anterior two-thirds of the 
 embryonic area, and the two neural folds diverge posteriorly, 
 so as to enclose the front end of the primitive streak 
 
Chap. VI.] 
 
 GENERAL EMBRYOLOGY. 
 
 103 
 
 (Fig. 35, iii.). Thus the primitive streak occupies the same 
 position as the blastopore of the frog, and is very probably 
 homologous with that structure. On the dorsal side of the 
 blastopore of the frog hypoblast and mesoblast come into 
 close relation with the inflected epiblast. From the primitive 
 
 MB. 
 
 no. 
 
 bl: 
 
 FIG. 36. THE NEURAL TUBE. 
 
 i. and ii. Dorsal views of frog embryo, iii. Longitudinal section of embryo 
 in about the same stage as ii. 
 
 &?. Blastopore. ch. Notocliord. FB. Fore-brain. HB. Hind-brain. 
 liy. Hypoblast. MB. Mid-brain, nc. Neural canal, nf. Neural fold. 
 me. Mesoblast. n. g. Neural groove. n. e. c. Neurenteric canal. 
 yk. c. Yolk-containing cells. 
 
 streak of the fowl and rabbit the mesoblast grows inwards 
 (Fig. 35, iv.), and at the anterior end of the primitive 
 streak this mesoblast meets that produced in the neural 
 region from the hypoblast (Fig. 35, v.), thus effecting at this 
 point an indirect junction of epiblast and hypoblast. In 
 some of the lower vertebrate types, moreover, among those 
 which possess a primitive streak, a passage here perforates the 
 blastoderm, and leads into the primitive digestive cavity. So 
 that it is probable that the primitive streak is homologous with 
 at least the dorsal part of the blastopore. In the fowl, the point 
 where the spreading blastoderm finally coalesces, so as to enclose 
 the yolk (p. 100), possibly marks the ventral portion of the blasto- 
 pore. What relation, if any, the pseudo-blastopore of the 
 rabbit's ovum bears to the true blastopore is riot known. In 
 any case, the blastopore of the frog, and the anterior end of the 
 primitive streak of the fowl and the rabbit, mark the posterior 
 
104 
 
 ANIMAL BIOLOGY. [Part I. 
 
 end of the young embryo, its anterior end being indicated by 
 the anterior end of the neural canal. 
 
 At this anterior end the neural canal enlarges into three 
 so-called cerebral vesicles, the precursors of the fore-brain, mid- 
 brain, and hind-brain. They are seen in Figs. 36 and 37, FB., 
 MB., HB. 
 
 The folding off of the Embryo. We have seen that in the 
 rabbit and the fowl the embryo, marked out by the neural 
 tube, the underlying notochord, and the mesoblastic plates, 
 occupies but a small area of the blastoderm. In front of the 
 neural tube a crescentic groove (Fig. 35, iii.) makes its appear- 
 ance, bending round the anterior end of the embryo, with the 
 horns of the crescent pointing backwards. This marks off the 
 head-fold. Behind the neural tube a similar groove is subse- 
 quently formed, with the crescentic horns directed forward, 
 giving rise to the tail-fold, and soon the horns of the crescent 
 become connected at the sides by lateral folds. All these 
 grooves gradually become constricted beneath the embryo. 
 
 Take your knife or pencil-case and lay it upon the table. 
 Spread over it your handkerchief, and tuck the handkerchief 
 under the ends. Pass a piece of string round the ends in the 
 tucks, single knot it, and gradually draw it tight. The string 
 will form curved grooves under the ends, and straight lateral 
 grooves along the sides ; and as you pull it tight the grooves at 
 the ends become deeper and deeper, until the part of the hand- 
 kerchief containing the knife is folded off from the rest, and 
 only connected by a puckered stalk. This will illustrate in a 
 clumsy but comprehensible fashion the beautiful process by which 
 the embryo is folded off from the rest of the blastoderm, which 
 in the case of the fowl envelopes the whole of the yolk, and is 
 called the yolk-sac (Fig. 38, i. y. s.), and in the case of the rabbit 
 contains an albuminous fluid, and is called the umbilical vesicle. 
 
 It must be remembered that in both these types the hypo- 
 blast covers a cavity the primitive alimentary cavity. During 
 the process of the folding off of the embryo this cavity is not 
 obliterated, but the walls of the embryo folding in beneath it, 
 
Chap, VI.] GENERAL EMBRYOLOGY. 105 
 
 and at either end, and at the sides, convert it into a tube 
 (Fig. 38, iv.). So that we have a small tubular embryo con- 
 nected by a hollow stalk with the large yolk-sac or the umbilical 
 vesicle. 
 
 In the meanwhile the cleavage of the mesoblast is in progress. 
 The ventral and lateral plates split into two layers, of which 
 one closely adheres to the hypoblast of the digestive tube. This 
 is the splanchnic layer (sj). l>\ which, with the adjoining 
 hypoblast, forms the splanchnopleure. The other forms the inner 
 lining of the body walls. This is the somatic layer (so. L), 
 which, with the adjoining epiblast, forms the somatopleure. The 
 space between them is that of the primitive body-cavity (b. c.). 
 These are seen in Fig. 38, iii., which is a quite diagrammatic 
 transverse section of the embryo. 
 
 In the frog there is no folding off of the embryo, and no 
 yolk-sac or umbilical vesicle, though the yolk-cells beneath the 
 mesenteron are homologous with these structures. The cleavage 
 of the mesoblast takes place as in the other types. 
 
 The Heart, Cerebral Vesicles, and Mesoblastic Somites. It 
 
 will be well to describe here the appearance of embryo chicks of 
 about 24, 36, and 48 hours incubation. They may be obtained 
 either from the hen or by means of an incubator kept at a tempera- 
 ture of from 38-40 C. To prepare the embryo for observation the 
 student should place the egg in a basin of normal saline warmed 
 to 38 C., and carefully break away the shell. On exposing the 
 yolk the embryo will be clearly visible on its upper surface. It 
 lies in the midst of the area pellucida, which is surrounded 
 by a mottled vascular area containing embryonic blood-vessels. 
 With sharp scissors an incision should be rapidly carried round 
 the vascular area. The blastoderm may then be floated off from 
 the yolk, and the vitelline membrane which covers it may be 
 removed. It should then be floated on to a glass slide and 
 covered with a cover-glass, taking care to have plenty of normal 
 saline. 
 
 In Fig. 37, i., the embryo (of about 24 hours) is seen, from 
 the ventral aspect, in the midst of the area pellucida. Anteriorly, 
 
io6 
 
 ANIMAL BIOLOGY. 
 
 [Part I. 
 
 in the middle line, is seen the first cerebral vesicle or fore- 
 brain (FB.\ followed by the mid-brain (MB.), and the hind- 
 brain (HB.\ the latter passing into the myelon, still unclosed 
 behind. The line marked a is the amnion fold (see p. 108). 
 Pr. g. marks the remains of the primitive groove. The noto- 
 chord (ch.) is seen through the neural tube. On either side 
 
 Sol. U. Sju. 
 
 FIG. 37. CHICK EMBRYOS. 
 
 i., ii., Hi. Chicks of 24, 36, and 48 hours incubation, iv. Diagrammatic 
 section through anterior end of i. 
 
 a. Amnion fold au Auditory vesicle, b. c. Body-cavity (ccelom). Ch. 
 Notochord. FB, Fore-brain. H. Heart. HB. Hind-brain. MB. Mid- 
 brain, MS. Mesoblastic somites. nies. Mesenteron. op. Optic vesicle. 
 Pr. g. Primitive groove. So. I. Somatic layer. Sp. I. Splanchnic layer. 
 Sp. u. Union of splanchnic layers. V. A. Vitelline artery. V. V. Vitelline 
 vein. 
 
 of the spinal cord (myelon)) are seen the cubical masses (meso- 
 llastic somites), into which the vertebral plate is segmenting. The 
 lines So. I. and Sp. I. mark the limits of the somatic and splanchnic 
 layers of the head-fold. 
 
 As this may present a difficulty to the student the diagram 
 
Chap. VI.] GENERAL EMBRYOLOGY. 107 
 
 (37, iv.) is introduced to aid him in overcoming it. It represents 
 the head end of an embryo, the amnion-fold (see p. 108) being 
 omitted to avoid confusion. FB. MB. HB. are the three cerebral 
 vesicles; Ch. the notochord which underlies the second and 
 third. Beneath it lies the blind anterior end of the primi- 
 tive digestive cavity (mes.). The shaded part in front of and 
 beneath it is the mesoblast, which has clef there, just as it cleaves 
 at the sides of the embryo (Fig. 38, iii.), into a somatic layer 
 (So. I.), and a splanchnic layer (Sp. /.), with the body-cavity (b. c.), 
 between them. When it is remembered that the embryo is 
 being folded off at the sides, as well as in the region of the 
 head, the true meaning of the lines So. 1. and Sp. I. in 37, i., 
 will be seen. 
 
 In Fig. 37 X ii., an embryo of about 36 hours, it will be seen 
 that the number of mesoblastic somites has increased, and that 
 the heart (ZT.), has now become obvious. It lies at the point of 
 union of the two vitelline veins (V. V.). The development of 
 this organ is illustrated in Fig. 3 8 A, ii., and is described in 
 Ch. vi. 
 
 In Fig. 37, iii., an embryo of about 48 hours, there is con- 
 siderable change. The mesoblastic somites have increased in 
 number. The heart is more definite, and appears as a coiled 
 tube. Its rhythmic contraction will no doubt be seen in the 
 embryo of this age, and of the age figured in ii. Besides the 
 vitelline veins (V. V.\ the vitelline arteries (V. A.\ are clearly 
 visible, but their connection with the heart is obscured by the 
 mesoblastic somites. In the fore-brain two rounded optic 
 vesicles (op.) are seen to have budded out ; and on either side, 
 in the region of the hind-brain, are the auditory pits (au.) 9 
 from which the auditory capsules will be formed. They are 
 not, like the optic vesicles, parts of the brain, but involutions 
 of the external epiblastic layer. Sp. u. marks the point where 
 the splanchnic layers of the mesoblast of the two sides have 
 united beneath the blind end of the primitive digestive cavity. 
 Anterior to this the splanchnic investment of the mesenteron is 
 complete. Posterior to this the mesenteron has not yet been 
 converted into a tube, and the splanchnic layers diverge. 
 
io8 ANIMAL BIOLOGY. [Parti. 
 
 The Embryonic Appendages and Foetal Membranes. We have 
 now to consider certain important structures which characterise 
 the fowl and the rabbit, but which are absent in the frog. The 
 first of these is the amnion. 
 
 Eevert for a moment to our rough and ready illustration, 
 with knife and handkerchief, of the manner in which the 
 embryo is folded off. Suppose that all around our mimic 
 embryo we fold the handkerchief in a ridge, and then, thread- 
 ing a piece of cotton along the summit of this ridge, draw the 
 ends of the string together, and so pull the folds over the 
 embryo until they meet above it. This again illustrates, in 
 a rough but comprehensible fashion, the way in which, in 
 the chick embryo, the amnion folds grow up all round, and 
 eventually meet and coalesce above the embryo. This illustra- 
 tion, however, fails to mark an important distinction. It shows, 
 indeed, that in the folding off of the embryo the folds are 
 inwards, while in the formation of the amnion they are out- 
 wards. But it does not show that in the former process the 
 folds affect the whole thickness of the blastoderm, while in the 
 latter, the formation of the amnion, only the somatic layer is 
 affected. This will, however, be readily seen in Fig. 38, ii. and 
 iii., which are diagrammatic cross-sections of the embryo. In ii., 
 in which the folding off of the embryo is omitted for the sake 
 of clearness, the amnion fold (am, /.), is seen at the edge of the 
 somatic layer. In iii., in which the embryo is represented as 
 folded off, the amnion folds have nearly coalesced above the 
 embryo. The folds are hollow, and enclose a space (5. s. c.) 
 which is continuous with the body-cavity . (b. c.). It must be 
 remembered that such folds extend all round the embryo, the 
 head-fold being very well marked. The edge of its outer limb 
 is shown in Fig. 37, i., ii., iii., the inner layer being so closely 
 applied to the head as not to be distinguished. At last all 
 these folds unite and coalesce above the embryo ; the outer 
 layers unite and become continuous ; the inner layers do the 
 same ; and thus the space (5. s. c., Fig. 38, iii. and iv.) between 
 them becomes continuous. The inner layer is known as the 
 true amnion, the outer layer as the false amnion, The false 
 
Chap. VI.] 
 
 GENERAL EMBRYOLOGY. 
 
 109 
 
 amnion lies beneath and in close contact with the investing 
 membrane (zona radiata of the rabbit, and vitelline membrane 
 in the fowl), and is known as the serous membrane (s. m.). The 
 space beneath it may be called the subserous space (s. s. c.) \ it is 
 
 n/m.j'^ 
 
 FIG. 38. FCETAL MEMBRANES. 
 
 i. Embryo folded off from yolk-sac, ii. Transverse section through embryo, 
 showing cleavage of mesoblast and amnion fold. iii. Transverse section 
 through older embryo, iv. Longitudinal section at a stage a little later 
 than iii., the amnion folds having united, v. Longitudinal section to show 
 formation of allantois. vi. Embryonic rabbit with foetal membranes. 
 
 al. Allantois. am. Amnion. am. /. Amnion fold. &. c. Body-cavity (ccelom). 
 s. ?i. Serous membrane, s. s. c. Subserous cavity, so. I. Somatopleure. sp. I 
 Splanchnopleure. utrib. Umbilical vesicle, x. Point where serous membrane 
 joins yolk-sac, y. s. Yolk-sac. 
 
 continuous with the body-cavity (38, iii.). Fig. 38, iv., is a 
 diagrammatic longitudinal section (the body-cavity being 
 omitted), in which the amnion folds have coalesced, the inner 
 layer forming the true amnion (am.), the false amnion forming 
 the serous membrane (s. m.). Within the inner layer or true am- 
 nion a fluid collects, in which the embryo lies as in a water-bed. 
 In the rabbit the head of the embryo is sunk into a pit or 
 depression of the blastodermic vesicle, from the edge of which 
 the amnion fold takes its origin. In this depression, termed the 
 proamnion, the mesoblast is absent. It is shown in 38A, iii. The 
 
ANIMAL BIOLOGY. 
 
 [Part I. 
 
 illustrations'!!! this figure exhibit the relations of epiblast, hypo- 
 blast, and mesoblast in the foetal membranes. 
 
 Meanwhile beneath the embryo the yolk-sac, surrounded 
 by a thin blastodermic layer, is in contact with the investing 
 membrane. But by the continuation of the cleavage into this 
 region the blastoderm is split into two layers, of which one 
 (continuous with the splanchnic layer) remains adhering to the 
 yolk-sac, while the other (continuous with the somatic layer 
 outside its amnion fold) adheres to the investing membrane, and 
 forms the serous membrane of this region. This will, perhaps, 
 be sufficiently clear by a study of iii. and iv., Fig. 38. Note 
 
 tjro.am. 
 
 FIG. 38 A. RELATIONS OF EPIBLAST, HYPOBLAST, AND MESOBLAST. 
 I. Transverse section of Embryo before the Amnion folds have met. 
 II. Transverse section of Embryo (Amnion omitted) through Pharynx and 
 
 Heart- folds. 
 III. Longitudinal section of Babbit Embryo to show Proamnion. 
 
 Thin line, epiblast ; thick line, hypoblast ; dotted, mesoblast. 
 Allantois. am. Amnion. am. f. Amnion fold. cod. Crelom. 7t. /. 
 Placental area. so. Somatopleure. sp. 
 I mesoblast. 
 
 Heart fold. ph. Pharynx, pi. a. Pla 
 Splanchnopleure. u. m. Uncleaved me 
 
 first that in iii. the whole of the amnion fold down to x is con- 
 tinuous with the somatic layer (so. L). Then note that in iv. the 
 serous membrane, derived from the outer layer of the amnion 
 fold, forms a distinct membrane as far as x. Now, picture the 
 
Chap. VI.] GENERAL EMBRYOLOGY. 
 
 i "4> .;</ 
 
 \ o hi 
 
 yolk-sac continually shrinking as its contents are absorbed into, 
 and minister to the growth of, the embryo. But owing to the 
 shrinkage of the yolk-sac (y. s.) the sub-serous space (s. s. c.) 
 gradually extends round the yolk to the lower pole of the egg. 
 And as it does so the serous' membrane is split off from it, and 
 forms a complete layer beneath the vitelline membrane (which 
 is not represented in the ^figure, but- closely adheres to the 
 serous membrane). 
 
 Another important structure, the allantois, has still to be 
 described. From the hinder-end of the digestive tube there 
 grows out, even before the aninion folds have met, a hollow 
 outgrowth. It is lined with hypoblast, but covered externally 
 by a layer of splanchnic mesoblast. It spreads out (Fig. 38, v.) 
 as a flat sac lying over the dorsal and posterior, part of the 
 embryo between the true amnion (omitted in.tke fig.) and the 
 serous membrane. In the fowl a rich network of blood-vessels 
 is developed in it, and it performs the functions of embryonic 
 respiration. Furthermore, in the later stages of embryonic 
 growth it extends outwards and downwards until its lower folds 
 meet below, overlap, and fuse together, thus enclosing the 
 albumen in a space lying between the allantois and the yolk-sac, 
 which has been termed the placental sac. Simple villi developed 
 on the walls of this sac absorb the enclosed albumen partly 
 into the blood-vessels of the yolk-sac, partly into those of the 
 allantois. In the rabbit (Fig. 38, vi.) its mesoblastic layer 
 unites and fuses with the serous membrane. The umbilical 
 vesicle, which has become flattened out (umb.), also fuses with 
 the serous membrane. These two membranes, which become 
 practically continuous, are known as the chorion. That in the 
 dorsal region, formed from the allantois, is called true chorion, 
 that in the ventral region, formed out of the flattened umbilical 
 vesicle, is called false chorion. From the outer surface of the 
 true chorion finger-like processes (villi) grow out and fit into 
 crypts or depressions formed in the wall of the uterus of the 
 mother. Both walls become highly vascular, and nutritive fluid 
 passes from the mother to the foetus, each villus being provided 
 with an artery, a vein, and a capillary plexus connecting the 
 two. There is no transfusion of blood from the mother to the 
 offspring, but a diffusion of nutritive material from the mother, 
 
112 ANIMAL BIOLOGY. [Parti. 
 
 and of waste products from the foetus. These maternal and 
 foetal structures taken together constitute the organ known as 
 the placenta. 
 
 While the placenta is being developed the folding off of the 
 embryo becomes more complete, there being (as seen in Fig 
 38, vi.) only a narrow vitelline duct connecting the remnant of 
 the umbilical vesicle with the embryo. Near the vitelline duct 
 is seen the duct of the allantois, and around both is the somatic 
 stalk of the amnion. The common stalk, consisting of vitelline 
 duct, allantoic duct, and enveloping amnion, is now known as 
 the umbilical cord. The mesoblast of the investing amnion stalk 
 develops a gelatinous tissue which cements together the whole 
 of the contents. The allantoic arteries in the cord in some 
 mammals wind in a spiral manner round the allantoic vein. 
 
 At birth the foetal membranes, together with the uterine 
 mucous membrane with which they are so closely interlaced, are 
 delivered as the after-birth, such a placenta being called deciduate, 
 as opposed to non-deciduate where the foetal membranes separate 
 from the maternal. The mother gnaws through the umbilical cord 
 close to the embryo, and probably eats the placenta. 
 
 The vitelline duct completely atrophies ; but the allantoic 
 stalk gives rise to two structures, (1) the urinary bladder, formed 
 by a dilatation of its proximal extremity ; (2) a cord known as 
 the urachus, connecting the bladder with the wall of the body at 
 the umbilicus. 
 
 We have thus traced the rabbit to its birth. Let us now 
 return to the fowl. We have seen that the allantois grows out 
 at the posterior end of the embryo, and, applying itself closely 
 to the serous membrane, has a respiratory function. Thus does 
 the growing chick obtain the necessary oxygen. The necessary 
 food is obtained from the yolk-sac, the contents of which, how- 
 ever, do not pass directly into the -alimentary canal, for the 
 duct of the splanchnic stalk becomes early obliterated, but are 
 absorbed indirectly through the blood which circulates freely 
 in the vitelline arteries and veins. Owing to this absorption of 
 the yolk the yolk-sac gradually shrinks, and eventually, while 
 still of considerable size, is withdrawn into the body-cavity, 
 the walls of which gradually grow together at the umbilicus. 
 
 At the broad end of the egg there is, between the two shell 
 
Chap. VI.] GENERAL EMBRYOLOGY. 113 
 
 membranes, a space which contains air. As development pro- 
 ceeds it increases in size, and on the fourteenth day of incuba- 
 tion the chick moves so as to lie lengthways in the egg, with its 
 beak touching the inner wall of the air-space. On the twentieth 
 day the beak pierces this wall, and the chick begins to breathe 
 the contained air. The blood, which has hitherto been oxy- 
 genated in the embryonic respiratory organ (the allantois), is 
 now arterialised in the adult respiratory organ, the lungs. The 
 functionless allantois dries up and shrivels away, as do also the 
 serous membrane and the amnion. The chick breaks the shell 
 with its beak and is hatched. 
 
 In the frog there is neither amnion nor allantois. The 
 urinary bladder, however, arises as an outgrowth of the 
 ventral wall of the posterior end of the cloaca, and has there- 
 fore been regarded as homologous with the similar structure in 
 the rabbit. 
 
 Summary. The frog, the fowl, and the rabbit pass through 
 embryonic development under very different conditions. The 
 primitive embryo, such as that of Amphioxus (p. 114), is 
 free at the gastrula stage. It moves through the water 
 in which it is hatched by means of the cilia with which its 
 surface is freely covered. And from that time forward it has 
 to obtain its nourishment by its own individual exertions. 
 The frog embryo is provided with a supply of food-yolk suffi- 
 cient to carry it over the early stages of development. It 
 is, however, hatched comparatively soon, long before it has 
 assumed anything like its adult form. There is a certain 
 amount of transformation within the egg membrane, but a large 
 proportion of the embryonic development takes place after the 
 organism is hatched, and constitutes metamorphosis. And 
 during metamorphosis the organism is absolutely dependent on 
 its own exertions for the necessary nutriment. The fowl 
 embryo is provided with an abundant supply of food-yolk, 
 sufficient to carry it over, not only the early, but also the later 
 stages of embryonic development. The mother, however, sup- 
 plies under natural conditions the necessary warmth, without 
 which developmental changes cannot proceed. And when the 
 chick is hatched, although the hen affords it protection, and 
 8 
 
ii4 ANIMAL BIOLOGY. [Parti. 
 
 aids it in its search for food, she does not prepare for it food- 
 stuff by her own vital processes. The rabbit embryo is provided 
 with but little food-yolk. It is dependent on the mother from 
 the first, not only for warmth, but also for nutriment. It is 
 developed within the maternal organism, with which it is con- 
 nected in such a way that diffusion of nutrient fluid can take 
 place from the one to the other. And when it is born it is for 
 a while dependent on the mother, not only for protection and 
 aid, but also for food-milk, which is elaborated by her own vital 
 processes. 
 
 In all three types the essential nature of the process of 
 fertilisation is the same. In all, development results from, and 
 is dependent upon, the union of ovum and spermatozoon. And, 
 in all, this union gives rise to a process of cell-division or segmen- 
 tation. But the character of the segmentation differs. In the 
 simple alecithal ovum, in which there is no food-yolk, or in which 
 the little that there is is distributed uniformly, the segmentation 
 is complete, or holdblattic, and regular. In the frog's ovum, in 
 which the yolk is arranged in polar fashion (telolecUhal), the 
 segmentation, though still holoblastic, is unequal. In the fowl's 
 ovum, where the yolk is also arranged in polar fashion (teloled- 
 thal), but is in enormous quantity, the segmentation is partial 
 (meroblastic) and irregular. In the rabbit's ovum there is very 
 little food-yolk, and that arranged uniformly (alecithal), and the 
 segmentation is complete, or holoblastic, but slightly unequal. 
 
 At the outset we made use of a supposititious case of simple 
 embryonic development for purposes of illustration. The sup- 
 posed organism possessed an alecithal ovum, which underwent 
 holoblastic segmentation in a quite regular fashion. A hollow 
 blastosphere was formed which underwent simple invagination, 
 by which the segmentation cavity was obliterated. Thus a two- 
 layered gastrula was produced, the inner layer being hypoblast, 
 the outer layer epiblast. The gastrula-cavity was the archen- 
 teron ; the opening of the gastrula-cup the blastopore. Meso- 
 blast was formed between the two primitive layers ; and the 
 body-cavity, or ccelom, arose by cleavage of the mesoblast. 
 Along the dorsal surface of the embryo the medullary axis was 
 differentiated from the epiblast. We may now notice that, with 
 certain modifications, the little Lancelet (Amphioxus) presents us 
 
Chap. VI.] GENERAL EMBRYOLOGY. 115 
 
 with a mode of development similar to that which was there 
 described. 
 
 Let us now summarise the more important events in the 
 course of development : 
 
 1. The immature ovum consists of naked protoplasm contain- 
 ing a germinal vesicle (nucleus) and germinal spots (nucleoli). 
 
 2. The mature ovum contains a variable amount of food-yolk, 
 but little in Amphioxus, and that little uniformly distributed 
 (alecithal) ; more or less in fowl and frog, concentrated at one 
 pole (telolecithal) ; but little in the rabbit (alecithal). 
 
 3. The detached ovum is surrounded by primary and 
 secondary membranes. The primary membranes (zona radiata, 
 rabbit ; vitelline membrane, fowl and frog) are formed in the 
 ovi-sac. The secondary membranes (albuminous layer, shell 
 membranes) are added in the oviduct. 
 
 4. The nucleus, previous to fertilisation, divides twice, pro- 
 bably by karyokinesis ; one half is extruded in each case in a 
 polar cell ; the portion that remains is the female pronucleus. 
 
 5. Fertilisation is probably effected by a single spermatozoon, 
 the nucleus of which, within the ovum, forms the male pro- 
 nucleus. Fertilisation is effected after the ova are laid in the 
 frog, but within the body of the parent fowl and rabbit. 
 
 6. Female and male pronucleus fuse to form the first segmen- 
 tation nucleus. 
 
 7. Segmentation is probably in all cases accompanied by 
 karyokinesis. 
 
 In Amphioxus, holoblastic and nearly regular. 
 
 In frog, holoblastic and unequal. 
 
 In fowl, meroblastic and irregular. 
 
 In rabbit, holoblastic and slightly unequal. 
 
 8. Epiblast and hypoblast produced 
 
 In Amphioxus by simple imagination. 
 
 In frog partly by invagination, partly by differentiation. 
 
 In fowl and rabbit by differentiation. 
 
 9. Archenteron produced 
 
 In Amphioxus by invagination. 
 In frog partly by invagination. 
 
 In fowl and rabbit beneath the blastoderm of the 
 embryonic area. 
 
u6 ANIMAL BIOLOGY. [Parti. 
 
 10. Mesoblast produced 
 
 In Amphioxus from paired outgrowths of the archen- 
 
 teron. 
 In frog partly from hypoblast at point of invagi- 
 
 nation, partly by differentiation of yolk-containing 
 
 cells. 
 In fowl and rabbit partly by down-growth of cells 
 
 derived from the epiblast at primitive streak, partly 
 
 by differentiation from hypoblast. 
 
 11. Neural plate, groove, and tube formed in all four types 
 from epiblast of mid-dorsal line, the process being somewhat 
 modified in Amphioxus. 
 
 12. Notochord formed in all four types by special up-growth of 
 cells of hypoblast in the mid-line beneath the medullary axis. 
 
 13. Mesoblastic plates in all three types separate into verte- 
 bral plates and lateral plates, of which the former by transverse 
 segmentation give rise to mesoblastic somites, and the latter, by 
 cleavage of the mesoblast, to the body-cavity. 
 
 14. The embryonic body is formed in the frog by simple 
 elongation, there being no yolk-sac or umbilical vesicle. In the 
 fowl and rabbit it is folded off from the yolk-sac or from the 
 umbilical vesicle. 
 
 15. In the fowl and rabbit a protective sac, the amnion, is 
 formed over the embryo. This is absent in the frog. 
 
 16. In the fowl and rabbit an embryonic respiratory organ, 
 the allantois, is developed as an outgrowth from the hind-gut. 
 In the rabbit this comes into close relation with the uterine 
 tissues of the mother, and thus acquires also a nutritive func- 
 tion. It is absent or rudimentary in the frog. 
 
 17. Tertiary investing membranes (serous membrane in fowl, 
 serous membrane ,and chorion in rabbit) are produced in the 
 higher types. 
 
CHAPTER VII. 
 THE GENESIS OF TISSUES AND ORGANS. 
 
 IN the last chapter we considered the General Embryology 
 of the frog, the fowl, and the rabbit. We saw how the so-called 
 germinal layers were constituted, how the body-cavity was 
 formed, how the vertebral plate was segmented into mesoblastic 
 somites, how 'the notochord and neural axis were formed, and 
 how the latter expands anteriorly into three cerebral vesicles 
 foreshadowing the fore-, mid-, and hind-brain. This, however, 
 carries us but a little way towards understanding how the tissues 
 and organs of the body take their origin. Their genesis, so far 
 as the space of one short chapter will permit, will now be con- 
 sidered. 
 
 The cells of which the early embryo is composed are minute 
 nucleated masses of protoplasm. They are at first for the most 
 part undifferentiated ; but those of the epiblast and hypoblast 
 very early show signs of differentiation in the direction of 
 flattened or of columnar epithelium cells. The cells of the 
 notochord differentiate in a direction of their own, becoming 
 net-like and vacuolated ; while those of the mesoblast become, 
 in the course of development, differentiated into blood, blood- 
 vessels, muscle, connective tissue, cartilage, and bone, or are 
 instrumental in the production of these tissues. For it must be 
 remembered that, as we have seen in the chapter on General 
 Histology, there is in most tissues, besides the actual cells, a 
 greater or less amount of intercellular or of cementing substance, 
 which may, as in epithelioid tissue (endothelium), be compara- 
 tively insignificant, or may, as in cartilage and bone, be im- 
 portant. 
 
 117 
 
ii8 ANIMAL BIOLOGY. [Parti. 
 
 Blood. In the vascular area of the blastoderm of fowl and 
 rabbit some of the nucleated mesoblast cells become united one 
 with another by protoplasmic processes, so that an irregular 
 network of nucleated cells is produced. The nuclei of the cells 
 rapidly multiply by division. Some remain central, and the 
 protoplasm which surrounds each of these acquires a yellowish- 
 red colour. Vacuoles are formed in the protoplasmic network 
 in which they lie, and the vacuoles running together, there 
 results a system of branched canals enclosing a fluid in which 
 the nucleated corpuscles float in irregular clusters. Eventually 
 these clusters break up into a number of red corpuscles, each 
 with an internal nucleus and surrounding red-tinted protoplasm. 
 These primitive red corpuscles seem to be capable of amoeboid 
 movements like the white corpuscles, and of multiplication by 
 division. Thus is formed a network of canals the primitive 
 blood-vessels of the blastoderm containing red blood-corpuscles. 
 It is probable that within the embryo itself primitive blood- 
 corpuscles and blood-vessels are developed in a similar way. 
 The protoplasmic walls of these primitive blood-vessels at 
 first contain only a few nuclei irregularly imbedded in the 
 protoplasm. But subsequently the protoplasm around these 
 nuclei and their descendants becomes differentiated into the 
 flattened cells which compose the walls of the blood-capil- 
 laries, and which form the lining membrane of the larger vessels 
 in the adult. Around this epithelioid layer in the larger 
 vessels, connective tissue and muscular coats are subsequently 
 formed. 
 
 The primitive nucleated red blood-corpuscles of the mammal 
 are subsequently converted into the non-nucleated blood-discs, 
 losing their nucleus and shrinking in size. Thus the nucleated 
 red corpuscles are, during embryonic life, gradually replaced by 
 non-nucleated red discs. But there can be no doubt that red 
 discs are also produced in other ways. They are formed, for 
 example, by the separation of masses of protoplasm from the large 
 cells of the red marrow of bones. It has also been shown that 
 white corpuscles develop into nucleated red corpuscles, and 
 these into red discs. The white corpuscles would seem to be 
 
Chap. VII.] THE GENESIS OF TISSUES AND ORGANS. 119 
 
 formed during early embryonic life in the liver, but subsequently 
 in the spleen, and lymphatic glands. 
 
 Connective Tissue. The mesoblast cells between the develop- 
 ing blood-vessels become somewhat separated from one another, 
 but at the same time united by processes passing from one to the 
 other. In the interspaces there collects an albuminous fluid, 
 which will be converted into the ground substance of the tissue. 
 Whether the fibrillated bundles of white fibrous tissue are (1) 
 separated off from the outer portions of the cells, the residue of the 
 cell remaining as a connective tissue corpuscle ; or (2) developed 
 in the intercellular ground substance, the cells remaining as the 
 corpuscles, is still undetermined. It is probable that elastic fibres 
 are formed in the intercellular substance between fusiform cells. 
 
 Nerve. The mode of development of nerve-fibres and 
 nerve-cells requires further investigation. It is now generally 
 believed, however, that all the nerves arise as outgrowths of 
 epiblast cells from the central nervous system. These cells 
 elongate and arrange themselves end to end with the consequent 
 production of axis fibres. If during life an axis fibre is cut, the 
 part furthest from the central nervous system dies and decays, 
 and is replaced by the outgrowth of the cut end in living con- 
 nection with the central system. 
 
 The medullary sheath is said to be formed from mesoblast. 
 The cells are said to develop around the axis fibres. Each inter- 
 node is supposed to be formed from a single cell, the greater 
 part of which becomes converted into fatty matter, the nucleus 
 and residuary protoplasm remaining as the nerve-corpuscle 
 (see p. 74) The primitive sheath is regarded as the cell mem- 
 brane of the medullary cell. 
 
 Muscle. Unstriped muscular fibres are developed from meso- 
 blastic cells (for the most part from the splanchnic layer of the 
 lateral plates), which become elongated, fusiform, and flattened. 
 
 Striped muscular fibres, notwithstanding their extreme length 
 (up to 2 inches), would also seem to be derived from single cells. 
 At first oval, the fibre becomes an elongated spindle in which the 
 
120 ANIMAL BIOLOGY. [Parti. 
 
 nucleus subdivides, the new nucleus shifting along the length of 
 the fibre. Thus is formed an elongated fibre containing several 
 nuclei. The protoplasm of the fibre, either alongside or around 
 the nuclei, becomes differentiated into the characteristic muscular 
 fibrils, only a small remnant of indifferent protoplasm remaining 
 around the nucleus as the muscle corpuscle. 
 
 We have seen that the involuntary (unstriped) muscle of the 
 alimentary system is derived from the splanchnic layer of the 
 lateral plate. The main muscles of the trunk are derived 
 from the mesoblastic somites of the vertebral plates. 
 
 Each vertebral plate originally contains a cavity, part of the 
 primitive body-cavity, by which it is divided into an inner 
 and an outer portion. That half of the inner portion which 
 lies next to the notochord and neural canal is utilised in the 
 formation of the permanent vertebrae. The other half of this 
 portion early develops into longitudinal muscles running through- 
 out the length of each mesoblastic somite ; and this, together 
 with the whole of the outer portion (which is also converted 
 into muscles), is known as the muscle plate. 
 
 Cartilage. This tissue is developed from mesoblastic cells by 
 the formation of a capsule round each. A second capsule is 
 formed within the first, a third within that, and so on in orderly 
 succession, until a concentric system of capsules surrounds each 
 cell, the capsules first formed expanding as fresh internal capsules 
 are formed. The capsules so developed fuse together and blend 
 into the hyaline matrix of cartilage. The capsules are either 
 (1) secreted by the cells, or (2) formed by the differentiation of 
 the superficial layer of the protoplasm of the cell, or (3) devel- 
 oped in the intercellular substance around the cells. In the case 
 of elastic cartilage, elastic fibres are subsequently developed. 
 
 Bone. While many of the bones take their origin by ossifi- 
 cation in cartilage, others arise by independent ossification in 
 connective tissue. The former are termed cartilage bones, the 
 latter membrane bones. 
 
 The parietal of the rabbit, one of the roofing bones of the skull, 
 
Chap. VII.] THE GENESIS OF TISSUES AND ORGANS. 121 
 
 is a good example of a membrane bone. The first sign of ossifi- 
 cation here is the formation of a network of minute bars or 
 spicules. The bars are fibrous, and composed of osteogenetic fibres, 
 which are at first soft and pliant, but become hardened by the 
 deposition within them of calcareous salts. Between the fibrous 
 bundles are large fusiform cells, the corpuscles. These cells 
 are instrumental in forming the bone, whence they have been 
 termed osteoblasts. As ossification proceeds some of these cells 
 become completely surrounded by the osseous tissue, and thus 
 give rise to the bone corpuscles, each lying in a hollow space or 
 lacuna. 
 
 Meanwhile the bone has become invested by a definite con- 
 nective tissue membrane, the periosteum. The inner layer of 
 this (osteogenetic layer) is largely composed of osteoblasts, which 
 deposit successive lamellae of bone on the surface of the growing 
 parietal. At the same time lamellae are being deposited in the 
 meshes of the original network of minute fibrous bars. Thus 
 the interstices of the network are converted into narrow channels 
 which contain blood-vessels. And thus the whole bone is con- 
 verted into a more or less compact structure. Finally, as the 
 bone increases in thickness, its middle layer becomes hollowed 
 out by absorption of the bony matter, so as to become spongy 
 in structure. This internal spongy bone is called the diploe. 
 
 We may take the femur of the rabbit as our example of a 
 cartilage bone. In a still young rabbit this bone consists of a 
 shaft (diaphysis) containing a central space in which is lodged 
 the marrow. At each end, separated from the shaft by a carti- 
 laginous interspace, is a terminal ossification (or epiphysis). In 
 the adult rabbit the epiphyses coalesce with the shaft by ossifi- 
 cation of the intermediate cartilage. 
 
 In the embryo the minute femur consists of a rod of foetal 
 cartilage invested by a sheath of vascular perichondrium. Within 
 the substance of the cartilage the cells arrange themselves in 
 columns near the growing ends, and in groups in the more 
 central portions. In the groups, and at the deeper ends of the 
 columns, the cell cavities are seen to be enlarged by the swelling 
 
ANIMAL BIOLOGY. [Part I. 
 
 of the cells, which then tend to break down and atrophy. Mean- 
 while the tissue becomes vascularized by the introduction of 
 blood-vessels from the vascular perichondrium. The channels 
 occupied by these vessels, uniting with the enlarged and coales- 
 cent cell cavities, convert the tissue into a vascular sponge-work, 
 which becomes calcified, and thereby strengthened by the de- 
 position of granules of calcareous matter in the remaining 
 strands or trabeculse of cartilage. 
 
 So far we have merely calcified cartilage, not true bone. 
 But along the edges of the primary marrow cavities, as they 
 are termed, of the sponge-work, a number of osteoblast cells 
 may now be seen. Each secretes, and may be seen to be sur- 
 rounded by, a droplet of osteogen, from which true bone is 
 formed ; and as this true bone replaces the cartilage, the tissue 
 is converted into " spongy endochondral bone." 
 
 Meanwhile, around the blood-vessels of and beneath the peri- 
 chondrium, osteoblasts are developed, which here, too, form 
 spongy bone. The perichondrium thus becomes periosteum ; 
 and the spongy bone developed in this region is termed periosteal 
 spongy bone. 
 
 If now we transfer our attention to the centre of the shaft, 
 we shall find that the spongy endochondral bone is there being 
 absorbed or eaten away through the instrumentality of special- 
 ized cells (osteoclasts), some of which are very large (giant cells). 
 The spongy bone in the centre of the shaft is thus completely 
 absorbed, and a marrow cavity produced. 
 
 Around this cavity the two processes bone formation by 
 osteoblasts, bone absorption by osteoclasts may be seen going 
 on side by side. Large spaces or marrow cavities are excavated, 
 and are then gradually filled in with bone, or narrowed to the 
 dimensions of a small canal for the passage of a blood-vessel. 
 And thus are formed embryonic Haversian systems, which 
 differ from those of the completed bone in their greater irregu- 
 larity, and the much looser texture of the lamellae. 
 
 Finally, we have to note that beneath the periosteum a 
 delicate tube of bone forms round the middle of the shaft, and 
 
Chap. VII.] THE GENESIS OF TISSUES AND ORGANS. 123 
 
 this tube gradually spreads towards either end of the bone until 
 it becomes a delicate but compact osseous sheath. It is pro- 
 duced by an osteogenetic layer (with osteoblast cells), which is 
 developed on the inner face of the membrane. This layer of 
 compact bone, with its well-defined Haversian systems, gradually 
 increases by exogenous growth beneath the periosteum. And 
 as, with the general growth of the bone in thickness, the marrow 
 cavity widens all the spongy bone is here absorbed, and in the 
 
 \^^yi-"j^^^^-~^^ :.^^'-. J.-*^i , 
 
 FIG. 39. DIAGIIAM OF BONE FORMATION. 
 
 Longitudinal section through head of bone. 
 
 1. Cartilage. 2. Columnar cartilage. 3. Vascularized cartilage. 4. Spongy 
 calcined cartilage. 5. Region of deposition of bone lamellae. 6. Reabsorp- 
 tion of bone to form medullary cavity. 7. Compact periosteal bone. 
 
 completed femur the central shaft is entirely composed of com- 
 pact periosteal bone. At the ends of the growing bone, however, 
 endochondral bone is still in process of formation, so that the 
 
124 ANIMAL BIOLOGY. [Part I. 
 
 growth of the bone in thickness is exogenous and periosteal, but 
 in length is endogenous and endochondral. 
 . Independent ossifications taking place at the growing ends of 
 the bone give rise to the epiphyses, which remain separated 
 from the shaft by a pad of cartilage. Growth then takes place 
 mainly in this pad ; its outer (epiphysial) half increasing by 
 cartilaginous growth as fast as its inner (diaphysial) half is 
 invaded by the growth of bone from the shaft. In the adult 
 the whole pad becomes ossified and growth ceases. The whole 
 femur has become bone, except the articular surfaces which still 
 have a cartilaginous layer. 
 
 The Mesenteron. We have seen (p. 105) how, in the chick, 
 which we will here take as our type, the mesenteric tube is 
 constituted during the folding off of the embryo. By the end 
 of the third day of incubation there are three divisions of the 
 mesenteron (1) an anterior division, comprising the tubular 
 portion produced by the union of the splanchnic layers of the 
 cleaved mesoblast in the neighbourhood of, and somewhat 
 posterior to, the head-fold ; (2) a posterior division, com- 
 prising the tubular portion produced by the union of the 
 splanchnic layers in, and slightly anterior to the tail-fold ; and 
 (3) a mid division, not yet converted into a tube, and resting 
 upon the yolk-sac. The first will constitute the oesophagus, 
 crop, proventriculus, gizzard, and duodenum; the second the 
 large intestine and rectum ; the third or mid division the small 
 intestine. 
 
 In the anterior part of the first division four clefts on each 
 side open outwards to the exterior. They are the visceral clefts, 
 similar to those which are seen in the tadpole, Bounding them 
 are five pairs of visceral arches, the anterior pair nearly meeting 
 in the middle line, the succeeding pairs being successively 
 shorter. The last three clefts soon completely close. But the 
 inner ends of the first pair persist as the Eustachian tubes of 
 the adult. 
 
 Immediately behind the region of the visceral clefts a ventral 
 
Chap. VII.] THE GENESIS OF TISSUES AND ORGANS. 125 
 
 diverticulum is formed in the primitive digestive tube, from the 
 hinder end of which 1 two diverticula grow out backwards into the 
 thickened mesoblastic tissue. From each diverticulum numerous 
 branches are given off, and these, in turn, branch and subdivide 
 in the thickened mesoblast, which has now assumed the form of 
 a separate lobe, and in which a rich network of capillaries make 
 their appearance. Thus are formed the lungs. Finally, at the 
 extremities of the diverticula, and some of their main branches, 
 the characteristic air-sacs of the bird become differentiated. 
 
 In the region posterior to this the crop, proventriculus, and 
 gizzard are differentiated. 
 
 Just behind the gizzard, in the short space between it and 
 the point of union of the splanchnic layers, a pair of diverticula 
 are formed, and grow out into the thickened mesoblast. From 
 them solid cylindrical cords of hypoblast cells push their way 
 in all directions through the mesoblastic tissue, and eventually 
 fuse into a network, between the strands of which blood capil- 
 laries are formed in abundance. Thus are formed the right and 
 left lobes of the liver > median outgrowths from which, meeting 
 and fusing in the mid-line, form the wedge-like bridge be- 
 tween them. The bile canaliculi are produced either by the 
 development of a lumen in the solid cords of hypoblast, or arise 
 primitively as interspaces between the hypoblastic cells. The 
 gall-bladder is a special pouch developed from the right primary 
 diverticulum. 
 
 Slightly posterior to the hepatic diverticula a dorsal out- 
 growth gives rise to the posterior duct of the pancreas. The 
 numerous branches formed in connection with it do not form a 
 network as in the case of the liver, but give rise to the tubules of 
 the pancreas. The other ducts of the pancreas are subsequently 
 formed in a similar manner. 
 
 The mid-region of the mesenteron, that which at the end of 
 the third day is not yet closed as a tube and is still widely open 
 to the yolk-sac below, gives rise to the small intestine posterior 
 to the duodenum. Lying immediately beneath the notochord 
 
 1 The ventral diverticulum is partly formed by a pinching in of the primitive 
 tube so as to cause it to be, so to speak, double-barrelled. 
 
126 ANIMAL BIOLOGY. [Part I. 
 
 and mesoblastic somites, its dorsal attachment is at first very 
 broad. As development proceeds the body-cavity extends 
 further and further round its sides and dorsal aspect until it is 
 only attached dorsally by a thin sheet of mesoblast. Meanwhile 
 it has been, so to speak, pulled downwards away from the 
 vertebral column, and has also increased in length to such an 
 extent as to be thrown into coils. The thin suspending sheet 
 of mesoblast has, during this process, been pulled downwards and 
 folded and thus converted into the mesentery. At the same 
 time, during the progress of these changes, the splanchnic layers 
 gradually close in so as to convert this region also into a tube. 
 
 The posterior region of the mesenteron is at first in communi- 
 cation with a neurenteric canal similar to that of the frog (Fig. 
 36, n. e. c.). Ere long the communication of the neurenteric 
 canal with the medullary canal is obliterated. Meanwhile the 
 proctodaeum is being formed by special invagi nation from the 
 exterior to constitute the vent ; and the portion of the mesen- 
 teron and neurenteric canal posterior to the vent is known as 
 the post-anal gut. This part subsequently atrophies. The 
 greater part of the remainder of the posterior division of the 
 mesenteron forms the rectum. But its posterior portion that 
 which meets the proctodaeal invagination gives rise to that 
 inner part of the cloaca which receives the generative ducts, and 
 at this period of embryonic life, the allantois. 
 
 The Proctodseum. The remainder of the cloaca, that which 
 constitutes its outer moiety into which the Bursa Fabricii opens, 
 is formed by the proctodseal invagination. This part is therefore 
 lined with epiblast, while the inner division of the cloaca is 
 lined with hypoblast. 
 
 The Stomodseum and Nares. During the third day of in- 
 cubation two small depressions are formed at the anterior end of 
 the embryo. They are the nasal pits. During the next day the 
 mouth begins to be marked out as a deep stomodaeal depression, 
 somewhat behind and ventral to these pits. Fig. 40, A., shows 
 the changes which have taken place by about the sixth day. The 
 
Chap. VII.] THE GENESIS OF TISSUES AND ORGANS. 127 
 
 mouth is bounded above by the fronto-nasal process (/. n. j>.), at 
 the outer edges of which are nasal grooves passing from the 
 nasal pits (n. p.) to the mouth (m.). The lateral edges of the 
 mouth lie between the somewhat divergent superior (s. m. p.) 
 and inferior maxillary processes (i. m. p.), the latter processes 
 forming also its lower boundary. As development proceeds, 
 
 l^.v.cl. 
 
 FIG. 40. THE MOUTH. 
 
 A. Ventral view of head of chick embryo. B. Transverse section of 
 mouth (after Gegenbaur). 
 
 a. Point where fronto-nasal process overarches the nasal groove, e. Inter- 
 nasal septum, fn. p. Fronto-nasal process, i. m. p. Inferior maxillary pro- 
 cess, m. Mouth, n. Nasal chambers, n. p. Nasal pit. p. Incomplete 
 palate, s. m. p. Superior maxillary process. 1st v. d. First visceral cleft. 
 
 the fronto-nasal process overarches the nasal groove at the 
 point a, and fuses with the superior maxillary process, thus 
 converting the nasal groove at this point into a canal. From its 
 mode of formation it is clear that this canal at first opens 
 just within the primitive mouth ; but, by the forward growth 
 of the parts anterior to it to form the beak, and other changes, 
 its position becomes altered. In the fowl to some extent, but 
 more markedly in the rabbit, the stomodseal cavity becomes 
 divided into two chambers by the formation of a horizontal 
 septum developed from lateral outgrowths of the superior maxil- 
 lary processes (#>., Fig. 40, B., a diagrammatic cross section of 
 the mouth, in which the palatal processes, p., have not yet 
 met in the middle line). Anteriorly the upper or respiratory 
 
128 
 
 ANIMAL BIOLOGY. 
 
 [Part I. 
 
 chamber is completely separated by this palatal septum from the 
 lower or mouth cavity ; but posteriorly the two chambers unite 
 where the palate ceases. Finally, the upper or respiratory 
 chamber becomes more or less completely divided into two 
 chambers by a median vertical partition (e.. Fig. 40, B.) or inter- 
 nasal septum. 
 
 All the salivary and buccal glands of the rabbit are stomodseal 
 in their origin, and are therefore lined with epiblast and not 
 with hypoblast as in the pancreas. 
 
 The teeth are also stomodseal structures. The manner of 
 their formation is briefly as follows : Along the line of the jaw 
 the epithelium becomes thickened. This thickened epithelial 
 keel, projecting downwards into the tissue of the jaw, is 
 known as the primary enamel germ. At the special points 
 where teeth are to be developed, the down-growth becomes 
 more marked and special enamel organs (en. or., Fig. 41) are 
 differentiated, one for each tooth. Into each enamel germ 
 
 ' . /^rrir^s?^ \ \ 
 
 FIG. 41. DIAGRAMS OF TOOTH DEVELOPMENT. 
 
 I. Transverse section through dental ridge soon after formation of enamel 
 germ. II. Transverse section of the same at a later stage. III. Transverse 
 section through the jaw of a kitten, in which the permanent tooth is yet 
 further developed. 
 
 d. Dentine, d. c. Dental capsule, d. p. Dental papilla, e. g. p. Enamel 
 germ of corresponding permanent tooth, en. Enamel, en. c. Enamel cells. 
 en. or. Enamel organ, ep. Epithelium of gum. od. Odontoblasts. 
 
 there projects upwards a vascular dental papilla (d. p.\ and 
 round the enamel germ and its dental papilla the tissues become 
 
Chap. VII.] THE GENESIS OF TISSUES AND ORGANS. 129 
 
 somewhat differentiated, thus giving rise to the so-called dental 
 capsule (d. c.). (1) The external layer of the papilla is com- 
 posed of columnar nucleated cells, the odontoblasts. These give 
 rise to long external processes, which lie in the tubules of the 
 dentine (d.), and by which that substance is formed. (2) The 
 internal layer of the enamel germ (enamel cells) sends processes 
 inwards which are believed to give rise to the enamel prisms of 
 the enamel (en.). (3) The internal cells of the dental capsule 
 surrounding the root of the tooth form an osteogenetic layer, 
 from which the cement is formed. 
 
 Early in its development the enamel organ of the milk-tooth 
 gives rise to an outgrowth, into which a dental papilla soon 
 projects, and which is the enamel germ of the permanent tooth. 
 
 The Organs of Smell, Sight, and Hearing (1.) Smell. 
 The organ of smell arises as two nasal pits lined with thick- 
 ened epiblast. As development proceeds and the olfactory 
 chambers are formed, this layer of epiblast spreading over their 
 surface, and over the surface of the turbinal bones or cartilages 
 developed within them, becomes of considerable extent. Very 
 early in development (in the chick on the third day) the olfac- 
 tory nerve-fibres, outgrowths from the anterior end of the brain, 
 come into relation with this olfactory membrane. 
 
 (2.) Sight. Examine the fresh eye of an ox, sheep, or failing 
 these, rabbit. The anterior transparent cornea (Fig. 42, D., cw\) 
 is continuous with the opaque, fibrous sclerotic (scl.). These 
 form the capsule of the eye. . Make an incision in the cornea 
 with the point of the scalpel; a lymph-like transparent fluid 
 oozes out ; this is the aqueous humour contained in the anterior 
 chamber of the eye (aq. h.). Eemove the cornea and examine 
 the iris (z'r.), a circular curtain with a central perforation, the pupil 
 (pu.), through which the transparent crystalline lens (I.) is visible. 
 The iris contains circular and radiating muscular fibres, by the 
 contraction or relaxation of which the pupil can be enlarged or 
 diminished according as the light is dim or bright. The 
 9 
 
1 3 o 
 
 ANIMAL BIOLOGY. 
 
 [Part I. 
 
 crystalline lens is attached to the sides of the eyeball by the 
 suspensory ligament. It is contained in a delicate capsule. By 
 the action of special muscles (ciliary) the convexity of the 
 anterior face of the lens can be altered, so as to allow of the 
 focussing of the image on the retina. 
 
 Cut the remainder of the capsule in half by an incision at 
 right angles to the axis of the eye. Note the optic nerve, 
 
 FIG. 42. DIAGRAMS OF EVE DEVELOPMENT. 
 
 A. Transverse section, showing the diverticulum of the brain (o.v.) meeting 
 the invagination of the surface (o.p.). B. Transverse section at a later stage, 
 showing the lens derived from the surface invagination lying in the pushed- 
 in diverticulum of the brain. C. The optic cup derived from the diverticu- 
 lum of the brain seen from the ventral aspect. D. Section of the eye. 
 
 Isf v. First vesicle of the brain (fore-brain), aq. h. Aqueous humour. 
 ch. Choroid. ch. f. Choroid fissure, cor. Cornea, cor. ep. Corneal epithe- 
 lium, ir. Iris. I. Lens. I. r. Limits of the retina proper, o. c. Optic cup. 
 o. p. Optic pit. o. v. Optic vesicle, op. n. Optic nerve, pu. Pupil, scl. 
 Sclerotic, v. h. Vitreous humour. 
 
 which will be seen to enter the back of the capsule ; also 
 jelly-like substance (vitreous humour, v. h.) with which this 
 posterior chamber of the eye is filled. Lining the interior of 
 
Chap. VII.] THE GENESIS OF TISSUES AND ORGANS. 131 
 
 this chamber will be seen the delicate transparent retina, ending 
 anteriorly by a jagged border, the ora serrata. Between the 
 retina and the sclerotic lies the choroid (ch.), a pigmented vascular 
 membrane, black for the most part, one portion, however, the 
 tapetum, being iridescent. At the point of origin of the iris the 
 choroid is continued into a ciliary fold, which bears a number of 
 thickenings known as the ciliary processes. The optic nerve 
 which enters the back of the eye-capsule is in direct continuity 
 with the retina. In its centre runs a retinal artery and vein. 
 In the fowl this artery does not run in the centre of the nerve ; 
 and in the fowl there is a structure which is not developed in 
 the mammal. This is a peculiar fold projecting inwards from 
 the choroid through the retina, and known as the pecten. 
 
 Let us now turn to the development of the eye. From the 
 first vesicle of the brain there grows out on either side a hollow 
 outgrowth known as the optic vesicle. These optic vesicles come 
 into close relation with two depressions on the exterior of the 
 head, much resembling the olfactory pits, and known as the 
 optic pits, which soon, by the growing together of their epiblastic 
 edges, become shut sacs. Thus a hollow mass of epiblast is on 
 each side separated from the superficial epiblastic layer; and 
 this, by the growth of its cells, especially the more posterior, 
 becomes a fibrous transparent mass, the crystalline lens, the 
 cavity of which is ultimately obliterated. With this the optic 
 vesicle comes into relation in such a way as to give rise to in- 
 vagination in the manner shown diagrammatically in Fig. 42, 
 where A. shows the optic vesicle (o. v.) growing out from the 
 hollow first vesicle of the brain (1st v.) to meet the optic pit 
 (o. p.), while B. shows the embryonic lens (I.) lying in the optic 
 cup (0. c.) formed by the invagination of the optic vesicle. 
 
 The optic cup is not symmetrical, and its edge is indented from 
 below in such a way that a slit, the choroidal fissure (C., ch. /.) 
 breaks the continuity of the cup. Through this cleft meso- 
 blastic tissue grows inwards and gives rise to the vitreous 
 humour (v. h.), while in the fowl the pecten passes inwards by 
 the same fissure. After this the fissure becomes more or less 
 completely obliterated. 
 
132 ANIMAL BIOLOGY. [Parti. 
 
 Meanwhile the cavity of the original optic vesicle (seen in 42, 
 A. and B.) has become completely obliterated, while the stalk 
 has become the solid optic nerve which, in the mammal, is at first 
 somewhat flattened, and is then wrapped round the central 
 artery. Of the two layers of which the optic cup is composed, 
 the posterior (outer) becomes the pigmented layer of the retina, 
 while the rest of the retina is formed out of the anterior (inner) 
 layer. In front of the region of the retina, the limits of which 
 are marked I. r. in 42, D., the two layers become fused together, 
 and with the mesoblastic tissue of the ciliary region ; and beyond 
 this the anterior part of the optic cup forms the posterior portion 
 (or uvea) of the iris. So that the pupil marks the limit of the 
 optic cup, of which only the posterior portion is converted into 
 the retina. 
 
 The remaining parts of the eye are for the most part meso- 
 blastic in origin. The mesoblastic investment of the optic cup 
 differentiates into the choroid internally, and the sclerotic 
 externally. From the mesoblastic choroid the greater part of 
 the iris is derived. The cornea is also for the most part of 
 mesoblastic origin. But the corneal epithelium is derived from 
 the external epiblast which covers the eye after the imagination 
 of the lens, which structure is also, as we have seen, epiblastic 
 in origin. 
 
 In Fig. 42, D., the dark line represents the part derived from 
 the optic cup, which originates from the epiblast of the central 
 nervous system. The lens and corneal epithelium, which are 
 also epiblastic in origin, are cross-shaded. The mesoblastic 
 parts are left unshaded. 
 
 (3.) Hearing. In the frog there is no external ear; but 
 above, and slightly behind the angle of the mouth, the tympanic 
 membrane is stretched over a hard ring, and covered by pig- 
 mented skin. Within this membrane is the tympanum, a funnel- 
 shaped cavity, widening outwards. This communicates with 
 the mouth by the Eustachian passage, and is lined with mucous 
 membrane continuous with that of the mouth. In the tym- 
 panum lies a cartilaginous rod, bony in the middle, the columella 
 
Chap. VII.] THE GENESIS OF TISSUES AND ORGANS. 133 
 
 auris, attached externally to the tympanic membrane, and 
 inserted internally into an aperture in the periotic capsule 
 called the fenestra ovalis. This aperture leads into a cavity 
 which contains the essential part of the organ of hearing, 
 which is known as the membranous labyrinth, and lies in a cavity 
 moulded to its form in the periotic capsule. Around the mem- 
 branous labyrinth, between it and the walls of the chamber, 
 is a fluid, perilymph; within the membranous labyrinth itself 
 is a similar fluid, the endolympli. The membranous labyrinth 
 
 FIG. 43. DIAGRAMS OF AUDITORY ORGAN. 
 
 A. Labyrinth of frog. B. Mammalian cochlea in diagrammatic section. 
 C. Mid-chamber of mammalian ear. 
 
 a. Summit of spire of cochlea, am. Ampulla, a. s. c. Anterior semicircular 
 canal, c. c. Cochlear canal, di. Diverticulum. eus. Eustachian canal. 
 / o. Fenestra ovalis. /. r. Fenestra rotunda. 7i. s. c. Horizontal semicircular 
 canal, in. Incus, mal. Malleus, p. s. c. Posterior semicircular canal, s. 
 Sacculus. s. t. Scala tympani. s. v. Scala vestibuli. sta. Stapes, ty. m. 
 Tympanic membrane, itt. Utriculus. 
 
 (Fig. 43, A.) consists of: (1) a vestibule constricted into two 
 divisions, the utriculus (u.) and the sacculus (5.), on the outer 
 side of which latter is a small dilatation ; and (2) three semi- 
 circular canals, of which two are nearly vertical, and the third 
 
134 ANIMAL BIOLOGY. [Part I. 
 
 nearly horizontal in position, each of which, near the vestibule, 
 presents a bulging enlargement, the ampulla. The utriculus 
 gives off a diverticulum (di.). The sacculus contains a great 
 quantity of white crystalline calcareous otoliths. In the sacculus, 
 utriculus, and ampullae of the semicircular canals there are 
 special areas of ramification of the fibres of the auditory nerve. 
 In the sacculus and utriculus these take the form of thicken- 
 ings known as maculce acusticce, the similar thickenings in the 
 ampulla being known as cristce acusticce. The epithelium which 
 covers these is composed of columnar cells, between which are 
 wedged in spindle-shaped cells prolonged into auditory hairs. 
 
 The auditory apparatus of the rabbit is more complicated. 
 The tympanic membrane does not lie on the surface, but at the 
 end of a short canal or external auditory meatus, the opening of 
 which is partially surrounded by the large pinna. Between the 
 tympanic membrane and the fenestra ovalis is a chain of three 
 small bones (see p. 146) which take the place of the columella 
 of the frog. The membranous labyrinth is enclosed in a bony 
 labyrinth, which consists of the densely ossified walls of the 
 chamber in which the membranous labyrinth lies, The sacculus 
 and utriculus are connected by a narrow channel from which the 
 recessus is given off. Connected with the utriculus are three 
 semicircular canals resembling those of the frog. But connected 
 with the sacculus is a new structure represented in the frog (if 
 at all) by the small dilatation already mentioned. This new 
 structure is the cochlear canal (c. c.) lying in the bony cochlea. 
 
 The cochlea is like a snail-shell, with a central spindle, the 
 modiolus, around which winds a spiral canal with about two and ^ 
 a half turns. But the spiral canal is divided into three parts in 
 the manner shown in Fig. 43, B., which represents the cochlea 
 quite diagrammatically in cross section, c. c. is the cochlear 
 canal connected with the sacculus and containing endolymph : 
 s. v. is a second canal, the scala vestibuli, connected with the 
 bony vestibule in which the membranous vestibule lies, and con- 
 taining perilymph : s. t. is a third canal, the scala tympani, also 
 containing perilymph. The cochlear canal gets smaller as we 
 ascend the spire, which is supposed in the figure to be seen in 
 
Chap. VII.] THE GENESIS OF TISSUES AND ORGANS. 135 
 
 cross section, and ends blindly near the top. The scala tympani 
 and scala vestibuli also get smaller, and eventually communicate 
 one with the other at the top of the spire at a. Thus it would 
 be possible for a minute organism to swim out of the perilymph 
 surrounding the vestibule into the scala vestibuli, and so up to 
 the top of the spire, and then down again by the scala tympani. 
 At the foot of that canal his way would be blocked by a mem- 
 brane filling an aperture in the bone of the osseous labyrinth, 
 and called the fenestra rotunda. If he were to swim back into 
 the perilymph round the vestibule he would there find a similar 
 membrane filling the aperture of the fenestra ovalis. And this 
 membrane is connected by means of the chain of auditory ossicles 
 (as shown in Fig. 43, C.) with the tympanic membrane. The 
 vibrations of the tympanic membrane are therefore passed on 
 by the ossicl6s (see p. 147) to the membrane of the fenestra 
 ovalis, which sets all the perilymph in vibration, and with it the 
 membrane of the fenestra rotunda. The partition between s. t. 
 and s. v. in 43, B., contains a bony shelf, the lamina spiralis, but 
 those between these canals and the cochlear canal are mem- 
 branous, while that between s. t. and c. c., called the basilar 
 membrane, contains radial fibres over-arched by bowshaped fibres 
 of Corti, and arranged somewhat as in the key-board of a piano. 
 It has been suggested that they may have the function of vibrat- 
 ing to sounds of a certain definite pitch ; but they cannot be 
 described here. 
 
 In the fowl there is no external ear ; the tympanic membrane 
 is placed in connection with that of the fenestra ovalis by a 
 columella ; there is a fenestra rotunda ; the sacculus and utricu- 
 lus are less completely constricted off; the cochlear canal 
 is not spiral, as in the rabbit, but is a curved process with a 
 dilated extremity. The osseous cochlea is, as compared with 
 the rabbit, rudimentary. 
 
 The auditory organ, like the olfactory organ and the eye, 
 originates as a depression of the surface of the head. This 
 auditory pit becomes converted into a closed epiblastic vesicle by 
 the closure of its edges and the overgrowth of the superficial 
 epiblast. From this otic vesicle and its mesoblastic investment 
 
136 ANIMAL BIOLOGY. [Parti. 
 
 are developed the membranous and osseous labyrinths by pro- 
 cesses of differentiation. 
 
 The Urinary Organs and Generative Ducts. The first trace 
 of the excretory system in the embryo is the segmented duct, which 
 is stated to take its origin from the epiblast, but soon appears as 
 a tube in the peritoneal epithelium of the somatic layer, near the 
 dorsal border of the body-cavity. This tube opens posteriorly 
 into the cloaca, and anteriorly into the peritoneal cavity by 
 apertures far forward, just behind the branchial region. Just 
 behind the openings the duct becomes coiled upon itself, and 
 sends out a number of blind diverticula. The glandular body 
 thus constituted is called the head-kidney or pronephros. Oppo- 
 site the openings of the duct, at the root of the mesentery, a 
 highly vascular ridge is formed, which constitutes a glomerulus. 
 The portion of the body-cavity which contains this glomerulus 
 and the openings of the segmental duct, is temporarily shut off 
 from the rest of the body-cavity. Thus a provisional and 
 temporary excretory organ is constituted. In the frog it sub- 
 sequently atrophies ; but in the cod-fish it probably persists 
 throughout life. 
 
 Much further back in the body-cavity a number of coiled 
 tubes are formed. They originate in the mesoblast as solid, 
 cords, which are subsequently hollowed out into tubes, one end 
 of which opens into the segmental duct, while near the other 
 each is invaginated by a little tuft of vascular tissue, thus giving 
 rise to a glomerulus and its capsule (see p. 81). In some forms, 
 but not in the frog, there is one of these for each portion of the 
 body corresponding to a mesoblastic somite or segment ; they 
 are called segmental tubes. Secondary and tertiary segmental 
 tubes are subsequently added; they become much convoluted 
 and come into such close proximity that their primitive dis- 
 tinctness is completely lost. The large glandular body thus 
 constituted is called the Wolffian Body or Mesonephros, and 
 persists as the permanent kidney of the frog. 
 
 In the meanwhile important changes go on in the segmental 
 duct of the frog. A cord of cells separates from its ventral side, 
 
Chap. VII.] THE GENESIS OF TISSUES AND ORGANS. 137 
 
 and this cord becomes hollowed out so as to form a tube 
 which acquires an opening at its anterior end into the peritoneal 
 cavity, and in the female opens posteriorly into the cloaca, but 
 in the male ends blindly. This is known as the Mullerian duct. 
 From it the oviduct of the female is developed. In the male it 
 atrophies. The remainder of the segmental duct, from which 
 this new duct was separated off, is known as the Wolffian duct, 
 since it carries off the products of the kidney or Wolffian body. 
 
 A further change has to be noted. In the anterior part of 
 the Wolffian body outgrowths from the primary segmental tubes 
 proceed to, and eventually open into, the testis of the male, the 
 products of which are thus carried by these vasa efferentia into 
 the Wolffian duct, which, therefore, is functional as the vas 
 deferens. As it conveys the urinary product it is also termed 
 the ureter ; but it is not strictly homologous with the ureter of 
 the rabbit. It is best termed the urino-genital duct. 
 
 In the rabbit and the fowl there are notable differences in the 
 mode of development of the segmental, Wolffian, and Mullerian 
 ducts. There seems, at least in the fowl, to be a rudimentary 
 pronephros ; and in both a mesonephros is formed. The latter 
 organ, however, though it functions as a kidney during em- 
 bryonic life, does not persist as* the permanent kidney in either 
 of these types. 
 
 From the dorsal aspect of the hinder part of the Wolffian 
 duct there is said to proceed an outgrowth which extends 
 forwards and sends out collecting tubes into a specially differ- 
 entiated portion of that mass of mesoblastic tissue from which 
 the mesonephros was previously formed. The outgrowth is 
 the ureter ; the glandular mass with which it enters into relation 
 is the metanephros, or permanent kidney of the bird and the 
 mammal. The ureter does not long remain attached to the 
 Wolffian duct, but, in the fowl, acquires a separate opening 
 into the cloaca, and, in the rabbit, , opens into that portion of 
 the allantoic duct which is persistent as the urinary bladder. 
 
 In the fowl and the rabbit, as in the frog, a connection is 
 established in the male between the testis and the Wolffian 
 duct through the Wolffian body. The vasa efferentia thus repre- 
 
138 ANIMAL BIOLOGY. [Part I. 
 
 sent a remnant of the mesonephros, and the Wolffian duct 
 becomes the vas deferens of the male, while it atrophies more 
 or less completely in the female. The rest of the Wolffian 
 body is generally held to atrophy more or less completely in 
 both sexes. 
 
 Thus in the male the Miillerian ducts atrophy, but the 
 Wolffian ducts become the vasa deferentia, just as they do in 
 the frog, only in that type they have also the functions of 
 ureters. In the female it is the Wolffian ducts which atrophy, 
 while the Miillerian ducts become the oviducts. In the fowl 
 one usually the right atrophies. In the rabbit the oviducts 
 of the two sides unite in the median line to form the vagina. 
 
 The embryo rabbit has, like the adult frog and fowl, a cloaca 
 which receives the excreta from the alimentary canal, and the 
 products of the kidneys and genital organs. This, however, 
 becomes modified in such a way as to separate the opening of 
 the rectum from that part (the urino -genital sinus) which receives 
 the ureters and the genital ducts. Further changes occur which 
 differ in the two sexes : one of the most remarkable of which is 
 the passage of the testes from the peritoneal cavity into two 
 special pouches of skin, the scrotal sacs. 
 
 The Heart. This organ is formed very early by the coales- 
 cence of the layers of splanchnic mesoblast beneath the throat. 
 The cleavage of the mesoblast on each side of the ^body into a 
 splanchnic and a somatic layer is shown in Fig. 38A, i. This 
 section is through the mid region of the body. Further forward 
 the mesenteron is closed in below and converted into a tube, 
 beneath which the splanchnic layers come together. But they 
 do so in such a way as to leave a cavity or space between them. 
 This space is the cavity of the primitive heart, the muscular 
 walls of which are derived from the splanchnic mesoblast, as 
 shown in Fig. 38A, ii. 
 
 The heart is thus in the first instance a simple tube. This 
 soon becomes further differentiated, and bent on itself like an 
 S (see Fig. 37, ii. and iii.). The upper limb of the S forms the 
 atrium, which in the frog is early divided into two auricles by a 
 septum, the ventricle remaining undivided. In the fowl and 
 rabbit the ventricle is the first to be divided. 
 
CHAPTER VIII. 
 
 THE SKELETON. 
 
 THE word skeleton is used in a broader and in a narrower 
 sense. In the broader sense it comprises all those tissues which 
 are developed for the support and protection of the rest, and 
 which form the framework of the essential parts of the various 
 organs. These are, in the main, connective tissue, cartilage, 
 and bone. Were it possible by some subtle solvent to dissolve 
 out all the tissues but connective tissue, we should still have a 
 perfect model of the vertebrate body in this material. Bone 
 and cartilage are more restricted in position. They may be 
 said to constitute the skeleton in the narrower sense in which 
 it is used in this chapter ; while the dried skeleton is composed 
 of bone alone. 
 
 The skeleton may be divided into the following parts : 
 
 I. Axial Skeleton 
 
 1. The skull and branchial arches. 
 
 2. The vertebral column, ribs, and sternum. 
 
 II. Appendicular Skeleton 
 
 1. The pectoral arch and fore-limb. 
 
 2. The pelvic arch and hind-limb. 
 
 The Skull. The skull, into the composition of which there 
 enter certain elements derived from the branchial arches, is 
 developed : (1) for the protection of the brain ; (2) for the 
 lodgment and protection of the special sense-capsules ; and (3) 
 for the support and attachment of the jaw-apparatus. The 
 olfactory (o/.), optic (op.\ and auditory (au.) capsules always 
 
 189 
 
140 
 
 ANIMAL BIOLOGY. 
 
 [Part I. 
 
 bear the same relation to the brain-case, shown in Fig. 44, B. 
 The jaw-apparatus bounds the mouth in the antero-lateral region. 
 We will consider the skull under the following heads : (1) 
 brain-case, (2) olfactory chamber, (3) auditory chamber, (4) 
 optic chamber, (5) maxillary region, (6) mandible, (7) hyoid. 1 
 
 (A.) The Rabbit's Skull. (1.) Brain-case (Figs. 44 A. and 45). 
 The posterior wall of the brain-case is formed, in the adult 
 
 iG. 44. SKULL OP RABBIT. 
 
 A. Diagrammatic section. B. Diagram to show relation of sense capsules 
 to brain-case. C. Periotic mass. D. Hyoid. 
 
 In A. a. s. Ali-sphenoid. 6. o. Basi-occipital. ft.s. Basi-splienoid. cr. Cri- 
 briform plate, e. o. Ex-occipital. /. m. Foramen magnum, fl. Floccular 
 fossa, fr. Frontal, i. pa. Inter-parietal, o. s. Orbito-sphenoid. pa. Parietal. 
 p. s. Pre-sphenoid. per. Periotic. s. o. Supra-occipital, s. t. Sella turcica. 
 sq. Squamosal. 1-9. Foramina. 
 
 In B. au. Auditory capsule, ol. Olfactory capsule, op. Optic capsule. 
 
 In C. /. o. Fenestra ovalis. /. r. Fenestra rotunda, ep.o. Epiotic. op.o. 
 Opisthotic. pr.o. Prootic. 
 
 In D. a. c. Anterior cornu. p. c. Posterior cornu. bh. Basi-hyal. 
 
 rabbit, of a single bone (the occipital) pierced by a large hole, the 
 foramen magnum (9. /. m.), through which passes the spinal cord. 
 
 1 The student should prepare a young and an adult skull, with all the bones in 
 their natural connection. This is best done by dissecting away the muscle, 
 and carefully scooping out the brain, dipping the skull occasionally in boiling 
 water for &few minutes. A young skull should also be left in water to macerate 
 for a week or two. The several bones will then readily separate. They should 
 
Chap. VIII.] 
 
 THE SKELETON. 
 
 141 
 
 At the sides of this great foramen are two rounded eminences 
 (the occipital condyles, oc. c., Fig. 45) by which the skull articu- 
 lates with the first vertebra. In the young rabbit this occipital 
 segment of the brain-case is not a single bone, but is composed 
 of a single median basi-occipital (b. o.) below, a single median 
 supra-occipital ($. <>.} above, and two ex-ocdpitals (e. o.), one on 
 
 m.f. 
 
 Fia. 45. SKULL OF RABBIT (Side View). 
 
 a.s. Ali-sphenoid. o.o. Basi-occipital. o.s. Basi-sphenoid. /. ?. a. Foramen 
 lacerum anterius. /. 1. m. Foramen lacerum medium, fr. Frontal, i. p. Inter- 
 parietal, in. 1 and 2. Incisor teeth. Ic. Lacrymal. in. p. Maxillary process 
 of pre-maxilla. mx. Maxilla, na. Nasal, ne. Neck of tympanic, u. p. Nasal 
 process of pre-maxilla. o. p. f. Orbital process of frontal, o. p. p. Orbital 
 process of pre-sphenoid. oc. Occipital, oc. c. Occipital condyle. op. f. Optic 
 foramen, or. s. Orbito-sphenoid. p. p. Par-occipital process, pa. Parietal, per. 
 Periotic. p. mx. Pre-maxilla. pt. Pterygoid. s. m. f. Stylo-mastoid foramen. 
 s. p. /. Supra-orbital process of frontal, sq. Squamosal. t. o. Tympanic bulla. 
 z. p. sq. Zygomatic process of squamosaL zy. Jugal. 
 
 either side. These lateral elements are produced downwards 
 into two par-occipital processes (45, p.p.). Such is the occipital 
 segment, the hindermost of the three segments of the brain-case. 
 
 not be separated, however, until the student has some knowledge of their relative 
 positions from a study of the complete skull. An adult skull should be sawn in 
 two along the mid longitudinal line. Fig. 44, A., is a very diagrammatic view of 
 the bisected brain-case. The parts cross-shaded are where the bones have been 
 cut. The dotted lines show the boundaries of the bones in the side walls. The 
 reference numbers are to the foramina. 
 
142 ANIMAL BIOLOGY. [Part I. 
 
 The median segment is the parietal segment. Here the basal 
 bone answering to the basi-occipital is the basi-sphenoid (b. s.). 
 At its sides are two winglike bones, separable in the embryo, 
 but early ankylosed (united by bony union) the all-sphenoids 
 (a. s.). Besides its ascending process which enters into the 
 composition of the brain-case, each ali- sphenoid sends down- 
 wards two flattened laminae forming together a V with its point 
 directed forwards (see Fig. 46, pt. pa.). The median segment 
 is completed above by the parietals (pa.), two large roofing 
 bones which meet in the middle line. It is, however, incom- 
 plete at the sides, since the parietals do not reach down to the 
 ali-sphenoids. 
 
 The anterior segment of the brain-case is the frontal segment. 
 The basal bone, answering to the basi-occipital and basi-sphenoid, 
 is the pre-sphenoid (p. s.). Separable from it in the embryo, 
 but early ankylosed with it, are two winglike bones, answering 
 to the ali-sphenoids. These orbito-sphenoids (44, o. s., and 45, or. s.) 
 form the posterior and superior boundaries of the large optic 
 foramen (44, 2, and 45, op. f.) which places the two orbits (con- 
 taining in the fresh state the eye-capsules) in communication, 
 and also opens into the brain-case. The pre-sphenoid forms its 
 anterior and inferior boundary, and then sends outwards ascend- 
 ing orbital processes (45, o. p. p.) which meet descending orbital 
 processes (o. p. /.), of thefrontals (fr.) large roofing bones which 
 complete this anterior segment of the brain-case. Each sends 
 out a supra-orbital process (s. p.f.) over the orbit. 
 
 The anterior end of the brain-case is bounded by a bone 
 (cribriform plate, 44 A., c. r.) which is perforated like a sieve for 
 the olfactory nerve-threads (1). 
 
 These three segments, occipital, parietal, and frontal, form 
 the greater part of the walls of the brain-case. Between the 
 occipital and parietal segments, however, there would be, were 
 no other bones developed, a great gap at the sides, and a 
 small gap in the roof. The posterior part of the gap on each 
 side is filled in with the penotic bone, which results from the 
 ossification of the auditory capsule. The gap in the roof is 
 filled in by a small oval interparietal (45, i.p.). Furthermore, at 
 
Chap. VIII.] THE SKELETON. 143 
 
 the sides of the brain-case two other bones are affixed to the 
 exterior. These are (1) the squamosal (sq.), which fills the 
 anterior part of the lateral gap, and separates the ali-sphenoid 
 from the parietal ; and (2) the tympanic, which forms a swollen 
 rounded mass (tympanic bulla, 45, t. b.) stuck on to the outer side 
 of the periotic. 
 
 The brain- case thus constituted has the following foramina 
 (see Fig. 44, A.): 
 
 1. Olfactory ; numerous perforations in the cribriform plate for 
 the fibres of the first nerve. 
 
 2. Optic ; for the second nerve, just within the orbito-sphenoid. 
 The confluence of the optic foramina places the orbits in com- 
 munication. (This is not so in the dog or cat.) 
 
 3. Sphenoidal (or foramen lacerum anterius) ; between the 
 basi-sphenoid and ali-sphenoid. It transmits the third, fourth, 
 sixth, and first and second divisions of the fiffh nerve. (In the 
 dog or cat the second division of the fifth passes through a 
 separate orifice, the foramen rotundum.) 
 
 4. Foramen lacerum medium; for the third division of the 
 fifth nerve and the internal carotid artery. It lies between the 
 periotic and the ali-sphenoid. (In the dog or cat the third 
 division of the fifth nerve passes through a separate orifice in 
 the ali-sphenoid, the foramen ovale.) 
 
 5. Facial ; this passes through the periotic bone, and emerges 
 by a foramen (styto-mastoid\ behind and above the swollen 
 portion of the tympanic. It transmits the seventh nerve. 
 
 6. Auditory ; the eighth nerve enters the periotic close to the 
 seventh, the shallow depression common to the two being the 
 internal auditory meatus. Near it is a deeper depression, the 
 floccular fossa. 
 
 7. Foramen lacerum poster ius ; between the periotic and ex- 
 occipital, for the transmission of the ninth, tenth, and eleventh 
 nerves. 
 
 8. Condylar ; in the occipital bone ; two small apertures for 
 the twelfth nerve (one in dog or cat). 
 
 9. Magnum, within the occipital ring, for the spinal cord. 
 
 It is a fact worth remembering that the three main segments of 
 
144 ANIMAL BIOLOGY. [Part I. 
 
 the brain-case always have a definite relation to the nerves which 
 pass out. The orbito-sphenoid lies between the olfactory and 
 sphenoidal, and contains the optic foramina; the ali-sphenoid 
 between the sphenoidal and f. 1. medium, being perforated by 
 the rotundum and ovale where they exist ; the periotic between 
 the f. 1. medium and the f . 1. posterius ; the ex-occipital behind 
 the f. 1. posterius, and perforated by the condylar. 
 
 The inner surface of the brain-case is divided into three 
 cranial fossae : (1) the olfactory fossa, from the cribriform to an 
 ill-marked ridge on the frontal ; (2) the cerebral fossa, from this 
 ridge to the tentorial plane, which is marked by a ridge along 
 the periotic, and by the line of junction of the parietal and 
 supra-occipital (the tentorium, which is membranous in the 
 rabbit, is ossified in the dog or cat) ; (3) the cerebellar fossa, 
 behind the tentorial plane. The plane of the cribriform is 
 called the ethmoidal plane, that of the occipital bone the 
 occipital plane. In comparative mammalian anatomy, the 
 angles made with each other by the ethmoidal, tentorial, and 
 occipital planes are of importance. The student should, if 
 possible, compare them in the hedgehog, the rabbit, the dog, 
 and man. 
 
 In the basi-sphenoid is a depression, the sella turcica (44, A., 
 s.t.), for the lodgment of the pituitary body. It is bounded 
 before and behind by anterior and posterior clinoid processes. 
 
 On the external surface there are sutures marking the contact 
 of separable bones. (1) The frontal suture, between the two 
 frontals ; (2) the inter-parietal or sagittal suture, between the two 
 parietals ; (3) the fr onto -parietal or coronal suture, between the 
 frontals and parietals ; (4) the occipito-parietal or lambdoidal 
 suture, between the occipital and parietals. 
 
 (2.) The Olfactory Chamber. This lies in front of the brain- 
 case. It is bounded posteriorly by the cribriform plate, and is 
 divided into two compartments by the median lamina perpendicu- 
 laris, which is continued forwards by the cartilaginous septum 
 nasi. The lamina perpendicularis and the cribriform plate form 
 parts of one bone, the mesethmoid. Beneath the cartilaginous 
 septum lie the coalesced vomers, forming a trough. The roof of 
 
Chap. VIII.] 
 
 THE SKELETON. 
 
 145 
 
 the chamber is formed by the frontals posteriorly, and the 
 nasal a (45, na.) anteriorly. In front are the openings of the 
 external nares, bounded below by the pre-maxilla^ (p. mx. ), which 
 send up long ascending processes (n. p.} to form part of the 
 outer wall of the nasal chamber. This outer wall is completed 
 by the maxilla? (mx.), which are spongy in structure, and by the 
 lachrymals (k.), which are apt to drop out in a dry skull, as in 
 Fig. 45. Each is perforated for the lachrymal duct. The incom- 
 plete floor is formed by the horizontal wings of the vomers and 
 by the upper horizontal laminae of the palatines. 
 
 The olfactory chamber is filled with a spongy mass of bone 
 of threefold origin, and com- 
 posed of (1) a superior pouchlike 
 process of the 4 nasal, the naso- 
 turbhial ; (2) a median mass 
 ankylosed with the mesethmoid, 
 and in the adult with the vomer- 
 ine trough, the ethmo-turbinals or 
 parethmoids; and (3) an anterior 
 scroll-like mass in close con- 
 nection with the maxilla? the 
 maxillo-turbinals. All these tur- 
 binal bones are covered over in 
 the living rabbit with mucous 
 membrane. That covering the 
 ethmo-turbinals is supplied by 
 the olfactory nerve, while that 
 investing the maxillo-turbinals 
 is innervated by a branch of 
 the fifth. 
 
 Connected with the Olfactory Posterior palatal foramen, pa.p.mx. Palatal 
 
 chambers are the narial passages process / maxilla - * * *" Palatal 
 
 . r process of palatine, pa. p. pr. Palatal pro- 
 
 (1 Ig. 46), 01 Which the VOmers, cess of pre-maxilla. pa. Palatine, pt. 
 
 pre-sphenoid (p. s.), and part of ptei> yg id - *>* P- Pterygoid process of 
 
 1 A . .- r ali-sphenoid. p. s. Pre-sphenoid. 
 
 the basi-sphenoid (o. s.), form the 
 
 mid-line of the incomplete roof. The maxillae (mx.), palatines 
 (pa.), and the ptery golds (pt.\ which are in relation with the 
 10 
 
 FIG. 46. - PALATE OF RABBIT (viewed from 
 under side). 
 
 a. p. J. Anterior palatine foramen. 
 &. s. Basi-splienoid. mx. Maxilla, mx. p. 
 Maxillary process of pre-maxilla. p. p. f. 
 
146 ANIMAL BIOLOGY. [Part I. 
 
 inner limb of the V- sna pecl pterygoid processes of the ali- 
 sphenoids, form the side-walls and lateral portions of the 
 roof. A horizontal shelf formed by palatine processes of 
 maxillae (pa. p. mx.) and palatines (pa. p. pa.) compose the very 
 incomplete bony floor. [This bony palate is much more 
 complete in the cat or dog.] In it are the very large anterior 
 palatine foramina (a. p. /.), and the small posterior palatine 
 foramina (p. p. /.). 
 
 (3.) The Auditory Chamber. The periotic fits into the side-walls 
 of the brain-case just in front of the ex-occipital. It is very 
 irregular in shape, and differs in bone-density in its anterior 
 (petrous) and posterior (mastoid) portions. The latter is seen on 
 the exterior (45, per.) behind the tympanic bulla. On its inner 
 side are two depressions, one larger and blind, the floccular fossa 
 (44, A., /.), for the lodgment of the flocculus of the cerebellum 
 (see p. 217); the other smaller, the internal auditory meatus, 
 with nerve-foramina for the seventh and eighth nerves. Exter- 
 nally there are two openings leading into the cavity of the 
 bone, the fenestra ovalis before and the fenestra rotunda behind. 
 They are best seen on removing the bulla. Within the bone 
 are lodged, in the living rabbit, the essential parts of the organ 
 of hearing. In the embryo of mammals the periotic ossifies 
 from three centres (Fig. 44, C.), and may thus be regarded as 
 due to the coalescence of three bones, the prootic (pr. o.), epiotic 
 (ep. o.), and opisthotic (op. o.), which have the relative positions 
 and the relations to the fenestra ovalis (/. o.) and fenestra 
 rotunda (/. r.) diagrammatically shown in Fig. 44, C. (p. 140), 
 the prootic being anterior. 
 
 External to the periotic, covering all its petrous portion, is 
 the tympanic, of which there are two well-marked divisions, a 
 swollen ventral portion, the bulla, and a spout-like dorsal pro- 
 longation, the neck (Fig. 45, ne.). 
 
 Within the cavity of the bulla are several small bones, the 
 auditory ossicles. In the fresh state membranes are stretched 
 across the fenestra ovalis and rotunda, and a large tympanic 
 membrane is attached to a raised ridge which marks the junction 
 of the swollen and tubular portions of the tympanic. Attached 
 
Chap. VIII.] 
 
 THE SKELETON. 
 
 147 
 
 to the membrane of the fenestra ovalis is a little stirrup-shaped 
 bone, the stapes (Fig. 47, sta.), bearing a little knob at the 
 summit of its arch, to which there articulates, by means of a 
 minute intermediate orbicular bone, a little bone, the incus (in.), 
 shaped somewhat like a two-fanged tooth. The fangs are not 
 of the same length; that articulated with the stapes is the 
 longer (long crus), while the shorter (short crus) articulates with 
 the overhanging wall of the periotic. The crown of the tooth- 
 like incus is fashioned into the shape 
 of a saddle, with which articulates 
 the head of another small bone, the 
 malleus (mal.), the shaft of which is 
 affixed to the tympanic membrane. 
 Thus the auditory ossicles form a 
 chain of bones between the tympanic 
 membrane on the one hand, and the 
 fenestra ovalis on the other. 
 
 (4.) The Optic Chamber. The capsule 
 of the eye is not ossified, nor is it fused 
 with the bones of the skull by cartilaginous union. It is 
 enclosed in the orbit. 
 
 In the rabbit's skull there will be seen, on each side, a single 
 large cavity, the two communicating by means of the optic 
 foramen. [But in the cat's skull, or the dog's, this large cavity 
 on each side will be seen to be imperfectly divided into two, of 
 which the anterior is for the lodgment of the eye-capsule, and 
 is the true orbit, while through the posterior, the temporal fossa, 
 pass the great muscles for the lower jaw. In the human skull 
 these are quite distinct.] The orbital recess of the rabbit is 
 thus a common orbit and temporal fossa. 
 
 Its inner boundary is the wall of the brain-case, the frontal 
 sending out a supra-orbital process (see Fig. 45, s.p.f., p. 141). 
 Posteriorly the squamosal (sq.) sends out a curved and distally 
 expanded zygomatic process (z. p. sq.), running at first outwards 
 and then downwards, to be applied to and form part of a lateral 
 bar or zygoma (zy.). This is composed of a single bone forming 
 a process of the large maxilla. In the new-born rabbit, however, 
 
 FlG ' 47 - AuDITORY 0ss ' CLES - 
 
148 ANIMAL BIOLOGY. [Part I. 
 
 the hinder part of this bar is a separate bone, the malar or jugal. 
 [The incomplete partition in the dog or cat is formed by an 
 ascending process of the malar, which nearly meets a descending 
 process of the frontal.] 
 
 (5.) The Maxillary Region. The tooth-bearing bones in the 
 upper jaw are the maxillae (mx.) and pre-maxillae (p.mx.). The 
 maxillae are large and irregular. Their palatal processes have 
 already been considered. Each bears three pre-molar and three 
 molar teeth in the adult. The pre-maxillse are long, and articu- 
 late with each other closely, though they do not ankylose. 
 Each has three large processes : (1) a nasal process (n. p.} pass- 
 ing along the outer edge of the nasal bones ; (2) a maxillary 
 process (m. p.) coming into relation with the maxilla; and (3) 
 a palatine process (Fig. 46, pa. p. pr.) passing backwards along- 
 side its fellow, and partially dividing from each other the 
 anterior palatine foramina (a. p. /.). Each bears two incisor 
 teeth, one behind the other; the anterior large, the posterior 
 small. A long space, or diastema, separates the incisors from 
 the pre-molars. There are no canine teeth. 
 
 (6.) The Mandible. The lower jaw is composed of two sym- 
 metrical rami which meet, but do not ankylose, at the symphysis. 
 It articulates directly with the skull by a longitudinally elongated 
 condyle, which fits in beneath the zygomatic process of the 
 squamosal. Running up towards the condyle, from the tooth- 
 bearing or horizontal portion, is an ascending portion which 
 bears an incurved coronoid process. At the junction of the 
 two portions is an inferior dental foramen for a branch of the 
 fifth nerve. The angle is rounded, and has a slight shelf pro- 
 jecting inwards. 
 
 The lower jaw in the adult bears two incisor teeth, one in 
 each ramus ; no canines ; two pre-molars and three molars on 
 each side. The following dental formula will now be under- 
 stood. The numbers above and below the line represent the 
 teeth in the upper and lower jaw respectively those of the 
 opposite sides being separated by the : 
 
 _ ,, ., . 2-2 0-0 3-3 3-3 OQ 
 
 Rabbit, i. I 1 , c. ra , p. m. , m. 3 ^, =28. 
 
Chap. VIII.] THE SKELETON. 149 
 
 This may be compared with the dental formulae of the cat, 
 the dog, and man, all of which have canines : 
 
 , 3-3 1-1 3-3 1-1 
 
 Cat, i. ff c. r - r p. m. _, m. ^ =30. 
 
 3-3 1-1 4 4 2-2 
 
 Dog, i. ^ -, c. j, p. m. j- , m. , =42. 
 
 O O 1 1 T: 'x O O 
 
 99 11 99 3 ^ 
 
 - /r " """" " *- J- ^ ^ <-' *-* or* 
 
 Movi -i v\ -ry iyi -?/ 
 
 an > h 2^2' 1-1' P< 2-2' 3-3' 
 (7.) r/ze jEfyoid. This bone, developed from one of the visceral 
 arches, consists of a stout body or lasi-liyal (Fig. 44, D., b. h.), 
 small anterior cornua (a. c.), and larger posterior cornua (p. c.). 
 It lies in the root of the tongue. In the dog the anterior 
 cornua are longer, and consist of three elements. 
 
 (B.) The Fowl's Skull. In the skull of an adult fowl, several 
 years old, the bones of the brain-case, and some other parts, 
 become so closely ankylosed, and completely fused, that it is 
 impossible to trace the sutures. In the skull of the young 
 chick, under a month old, some of the bones have not begun to 
 ossify. In the skull of a fowl some six months old, while some 
 of the bones are already ankylosed, the ossification of the 
 pre-sphenoid has not commenced. It will be convenient to take 
 for description the skull of a recently hatched chick, and the 
 skull of an adult bird. Fig. 48 gives a side view and ventral 
 view of the adult skull, the positions of some of the obliterated 
 sutures being dotted in. 
 
 (1.) The Brain-case. This is rounded above and posteriorly, 
 but much encroached upon by the large orbits. In the chick 
 the basi-occipital, ex-occipitals, and supra-occipital are distinct ; 
 but in the adult they fuse with each other, and with the other 
 bones of the brain- case (b.o., e.o., s.o.). There is a single occipital 
 condyle, hemispherical and grooved dorsally. Anterior to the 
 basi-occipital is the basi-sphenoid. This bone is not, however, 
 visible in B., since it is hidden by a large expanded basi-temporal 
 bone (b.t.), at the anterior ends of which are the openings of the 
 Eustachian canals. The ali-sphenoids (48, A., a.s.) form part of 
 the anterior wall of the brain-case. Each sends out a strong 
 
150 
 
 ANIMAL BIOLOGY. 
 
 [Part I. 
 
 sphenoidal process which meets a process of the squamosal. 
 In the chick there is a membranous fenestra in each. The 
 parietals (pa.), on the other hand, form part of the roof of the 
 
 etli. 
 
 P4i.H 
 
 FIG. 48. SKULL OF FOWL. 
 A. From the side. B. From below. 
 
 a. n. f. Anterior nasal fossa, a. p. q. Anterior process of quadrate, a. s. 
 Ali-sphenoid. an. Angular, art. Articular. Z>. o. Basi-occipital. &. t. Basi- 
 temporal. d. Dentary. e. o. Ex-occipital. /. TO. Foramen magnum, fr. 
 Frontal, Ji. q. Head of quadrate, ju. Jugal. Ic. Lachrymal, m. eth. Meseth- 
 moid. mx. Maxilla. na. Nasal, olf. Olfactory foramen, op. Optic foramen. 
 pa. Parietal, pi. Palatine, pt. Pterygoid. p. mx. Fre-maxilla. p. s. Pre-sphenoid. 
 qu. Quadrate, q. j. Quadrato-jugal. r. Rostrum, s. a. Supra-angular, s. o. 
 Supra-occipital, sp. p. Sphenoidal process, sq. Squamosal. vo. Vomer. 
 o. c. Occipital condyle. 
 
 posterior region of the brain-case. Between these two elements 
 of the parietal segment lies the large squamosal (sq.), behind and 
 below which is the auditory capsule. 
 
Chap. VIII.] THE SKELETON. 151 
 
 Of the anterior segment of the brain-case the frontal bones 
 are the most conspicuous and the earliest formed (fr.). They 
 constitute the whole of the central arch of the skull, and give 
 rise to a prominent supra-orbital ridge, of which, however, the 
 postero-inferior portion is formed by the ali-sphenoid. From 
 the ridge each is continued sharply inward, within the orbits, 
 as the orbital plate of the frontal. These orbital plates, and 
 the ali-sphenoids which lie below them, are in the chick separated 
 from the inter-orbital septum by a membranous tract, below 
 which is the optic foramen (op.). In this tract the orbito- 
 sphenoids make their appearance between the second and third 
 months of chick life. Anterior to them is the 9artilaginous 
 inter-orbital septum which contains a large membranous inter- 
 orbital fenestra. Behind this fenestra between it and the 
 optic foramen the pre-sphenoid (ps., the reference line passes 
 through the small orbito-sphenoid) begins to ossify at about the 
 ninth month, thus completing the frontal segment. In front 
 of the fenestra is the mesethmoid ossification. Beneath the 
 mesethmoid and the inter-orbital septum, stretching back along 
 the mid-line of the base of the skull as far as the basi-temporal, 
 is a grooved splint of bone, the rostrum (r.). Anterior to this is 
 the grooved vomer (vo.). 
 
 In the adult fowl all these various skull-elements, except the 
 vomer, run together, and fuse into a continuous bony mass in 
 which the sutures are completely obliterated. The bones of the 
 three segments, the basi-temporal and rostrum below, the 
 squamosal and bones of the auditory capsules at the sides, the 
 mesethmoid in front, all unite firmly together in the skull of 
 the adult fowl, The inter-orbital septum becomes a continuous 
 plate of bone, except where the olfactory and optic foramina 
 place the two orbits in communication. And the f rentals 
 fusing with the nasals anteriorly, the cranial and facial portions 
 of the skull become continuous. 
 
 The foramina for the exit of the nerves have the same 
 essential relations as in the rabbit. The student should com- 
 pare the two. 
 
 There is no olfactory fossa. The cerebral fossa is large, and 
 
152 ANIMAL BIOLOGY. [Part I. 
 
 on each side of it there is an optic fossa for the large optic lobe. 
 The cerebellar fossa is well marked, and there is a strong 
 tentorial ridge. The floor is hollowed out for the reception of 
 the medulla oblongata. 
 
 (2.) The Olfactory Chamber. On either side of the beak are large 
 anterior nasal fossce (a. n. /.), so that the nasal chambers are less 
 completely enclosed than in the rabbit. Each fossa is bounded 
 by the nasals behind (na.) and the pre-maxillse (p. mx.) in 
 front. The mesethmoid is continued forwards in the fresh 
 skull by a cartilaginous plate, the septum nasi, which forms a 
 median vertical partition between the nasal cavities. Beneath 
 it lies the vomer. It is produced on each side into scroll-like 
 masses of cartilage, the turbinals. The anterior nares are far 
 back, just in front of the eyes ; the posterior nares open 
 almost directly beneath them in a longitudinal slit in the palate. 
 There are thus no narial passages like those in the rabbit. 
 
 (3.) The Auditory Chamber. The auditory capsule is seen on the 
 inner side of the brain-case, lying just external to the foramen 
 magnum and adjoining the optic and cerebellar fossae. It is 
 limited above by a well-marked semi-circular ridge indicating 
 the position of the anterior semi-circular canal. In the chick 
 there are three distinct ossifications : (1) the prootic, which 
 forms a distinct bone for some time and constitutes the main 
 part of the capsule ; (2) the epiotic, which fuses early with the 
 supra-occipital; and (3) the opisthotic, which fuses with the 
 ex-occipital. 
 
 No tympanic bulla covers over the tympanic chamber, which 
 is an irregular hemispherical depression. In the inner wall of 
 this cavity is a largish aperture, the fenestra ovalis, and some- 
 what behind and below this a smaller one, the fenestra rotunda. 
 In place of the chain of auditory ossicles there is a partly cartila- 
 ginous, partly osseous rod, the columella auris. This is expanded 
 at its proximal end into a plate-like stapes which fits into the 
 fenestra ovalis. The cartilaginous distal end is affixed to the 
 tympanic membrane, which is stretched across the aperture of 
 the cavity. 
 
 (4.) The Optic Chamber, The capsule of the eye within which 
 
ft 
 
 Chap. VIII.] THE SKELETON. 153 
 
 y. 
 
 a circle of small imbricating bones, the sclerotic plates, is developed 
 is enclosed in the large rounded orbit. This is bounded behind/v 
 by the anterior wall of the brain-case; on its inner side %~^. 
 the inter-orbital septum ; and anteriorly by the nasals and the 
 lachrymals (lc.), small bones with a sharp downward-directed 
 process, and not ankylosed to the skull even in old birds. (The 
 sharp process internal to that of the lachrymal is part of the 
 ethmoid bone.) Below, the orbit is bounded internally by the 
 palatines and pterygoids (pi. pt.), and externally by a slender 
 bar composed of the maxillae (mx.\ the jugal (ju.), and the 
 quadrato-jugal (q. j.), which is attached by ligament to the 
 quadrate bone (qu.). 
 
 (5.) The Maxillary and Palatal Region. There are no tooth- 
 bearing bones. The pre-maxillae (p. mx.) 9 which form the 
 greater part of the upper mandible, are, in the fresh skull, en- 
 sheathed in horn. Each has long slender nasal processes 
 running up to the frontals internal to the nasals, and stout 
 maxillary processes, each of which sends inwards a palatal shelf, 
 to which the palatine is attached in the adult. The maxillae 
 (mx.) at their anterior ends send inwards flattened maxillo- 
 palatine processes which nearly meet in the middle line ; for the 
 rest of their length they are slender. The palatines (pi.) are 
 also slender for three-fourths of their length ; but in the 
 posterior fourth they send inwards curved scroll-like processes 
 which meet in the middle line beneath the rostrum. Their 
 hinder extremities articulate with the pterygoids (pt.\ short, 
 stout, mallet-shaped bones, the handles of which pass outwards 
 and backwards to articulate with the quadrate. The quadrate 
 (qu.) is a large irregular somewhat Y-shaped bone. By one 
 limb of the Y it articulates with the dorsal part of the tympanic 
 cavity (h. q.). The other limb of the Y is seen at a. p. q. 
 (Fig. 48, B.), partially hidden by the pterygoid. At the foot 
 of the stem there is a condyle for the articulation of the lower 
 jaw, near which it articulates with the quadrato-jugal (q. j.). 
 
 (6.) The Mandible. The two rami ankylose at the symphysis, 
 and the component elements are more or less completely fused 
 together in the adult. In the chick, however, each ramus is 
 
154 ANIMAL BIOLOGY. [Part 1. 
 
 distinctly composite. There is a central core of (Meckel's) car- 
 tilage which is ensheathed anteriorly by the dentary (d.). In 
 the hinder third of the jaw are three splints of bone, the angular 
 below (an.), the supra-angular (s. a.) and the splenial on the 
 inner side. And within the cartilage, at the hinder end, is a 
 further ossification, the articular (art.). All these tend to run 
 together in the mandible of the adult skull*in which posterior 
 and internal articular processes are well-marked. 
 
 It is especially to be noted that the lower jaw does not articu- 
 late directly with the squamosal as in the rabbit, but is con- 
 nected with the brain-case by the intervention of a quadrate bone. 
 
 (7.) TheHyoid. The hyo-branchial apparatus is composed of a 
 series of slender bones, the relations of which will be seen from 
 Fig. 49. The short anterior cornua (cerato-hyals) coalesce to 
 
 FIG. 49. HYOID OF FOWL. 
 
 a. c. Anterior cornu. b. h. Basi-hyal. gl. h. Glosso-hyal. p. c. Posterior 
 cornu. u. n. Uro-hyal. 
 
 form a Y-shaped glosso-hyal. Lying behind this are two slender 
 median bones, one behind the other, the basi-hyal (b. h.) and the 
 uro-hyal (u. h.). From their point of junction spring the long 
 posterior cornua, in each of which there are two ossifications, a 
 proximal and a distal. 
 
 (0.) The Frog's Skull. In the skull of even an adult frog 
 there is much more unossified cartilage than there is in the fowl 
 or the rabbit. There is, in fact, a chondrocranium or cartila- 
 ginous skull, which, partly invaded by bone, may be obtained by 
 maceration and stripping off the splints of bone which overlie it. 
 Fig. 50, A., shows a dorsal view of half this chondrocranium; 
 B. and C. are a ventral and dorsal aspect of the right side of 
 the bony skull ; E. shows the lower jaw, and D. the hyoid. 
 
Chap. VIII.] 
 
 THE SKELETON. 
 
 '55 
 
 (1.) The Brain-Case. The three typical segments are not 
 represented in bone. In the hinder segment there is no basi- 
 occipital, and the supra-occipital is not represented. On the 
 other hand, the ex-occipitals (e. o.) are large and well developed, 
 and may comprise also the epiotic and opisthotic elements. There 
 
 p. mi, p. mi. 
 
 E. 
 
 FIG. 50. SKULL OF FROG. 
 
 A. Chondrocraniuni, with invading bone, from above. B. Skull from below. 
 
 C. Skull from above. D. Hyoid. E. Lower jaw. 
 
 a. c. Anterior cornu of hyoid. a. /. Anterior fontanelle. a. n. p. Ali-nasal 
 process, a.o.p. Antorbital process, an. sp. Angulo-splenial. aw. c. Auditory 
 capsule. &. h. Basi-hyal. d. Dentary. e. n. External nares. e. o. Ex-occi- 
 pital. /. o. Fenestra ovalis. i. n. Internal nares. w. irik. Mento-meckelian 
 (probably from lower labial cartilage), tnx. Maxilla, n a. Nasal, p. ft. 
 Pterygoid bone. p. c. Posterior cornu of hyoid. p. f. Posterior fontanelle. 
 p. fr. Parieto-frontal. p. mx. Pre-maxilla. p. n. p. Pre-nasal process. 
 pa. sp. Para-sphenoid, pi. Palatine, pr. o. Prootic. pt. Pterygoid. pt. c. 
 Pterygoid cartilage, q.j. Quadrato-jugal. r. p. Rhinal process, s. e. Sphen- 
 ethmoid. sq. Squamosal. sus. Suspensorium. vo. Vomer. 
 
 are two occipital condyles. In the parietal segment there is no 
 basi-sphenoid and no ali-sphenoid. The parietals are, however, 
 fully developed, and in comparatively young frogs completely 
 fuse with the frontals to form the parieto-frontals (p. fr.). In 
 
156 ANIMAL BIOLOGY. [Part I. 
 
 the frontal segment there are no separate sphenoid or ethmoid 
 bones ; the curious girdle-bone sphenethmoid (s. e.) represents 
 these skull-elements. The frontals are represented by the 
 anterior portions of the large parieto-frontals. The front wall 
 of the brain-case is formed by the sphenethmoid, a dice-box- 
 shaped bone pierced by the olfactory nerves. In the side walls 
 the prootics (pr. o.) are large well-ossified bones; but the squa- 
 mosal takes no share in closing in the brain-case. In the absence 
 of other basal elements, the para-sphenoid (pa. sp.) underlies and 
 strengthens the cartilaginous floor of the skull. This is a con- 
 spicuous dagger-shaped bone, the blade of which probably 
 answers to the rostrum in the fowl, while the crossguard is 
 perhaps homologous with the large basi-temporal mass. 
 
 Thus in the brain-case of the frog we have the paired parieto- 
 frontals above, the single para-sphenoid below, the sphenethmoid 
 in front, the prootics at the sides, and the ex-occipitals behind. 
 
 The positions of some of the nerve exits is seen in 50, B. 
 The foramen for the ninth and tenth passes through the ex- 
 occipital mass. That for the fourth or pathetic is very minute 
 above and slightly in front of the optic foramen. The brain lies 
 loosely in the brain-case, which has no fossae. 
 
 (2.) The Olfactory Chamber. The sphenethmoid appears in the 
 roof of the skull, between and anterior to the two parieto- 
 frontals. Separated by maceration, it is seen to be perforated 
 by two foramina for the olfactory nerves. Posteriorly these 
 foramina open into a common cavity, incomplete above, in which 
 are lodged in the living frog the olfactory lobes of the brain. 
 Anteriorly there are two cavities separated by a mesethmoidal 
 partition. This is carried forwards in the fresh skull by a 
 cartilaginous septum nasi, which forms the partition between 
 the two nasal chambers. These are roofed over by the 
 nasals (na.), somewhat kite-shaped bones, with the thinner tail 
 end directed outwards and backwards. Between these and the 
 pre-maxillse (p. mx.) are the external nares (e. n.). The internal 
 nares (i. n.) are below and somewhat behind them, immediately 
 in front of the palatines. There is no prolonged narial passage 
 as in the rabbit. On the inner side of the internal nares are 
 
Chap. VIII.] THE SKELETON. 157 
 
 two somewhat irregular tri-radiate bones, the vomers (vo.), 
 which send their longest ray backwards and inwards towards 
 the point of the para-sphenoidal dagger. Each vomer bears a 
 little patch of teeth. There are no turbinal bones or distinct 
 cartilages within the nasal chambers. 
 
 (3.) The Auditory Chamber. The anterior part of the ex- 
 occipital, that in front of the foramina for the ninth and 
 tenth nerves may represent epiotic and opisthotic elements. 
 The prootic is well developed as a separate bone, though it too 
 eventually fuses with the ex-occipitals. It sends outwards a 
 shelf (tegmen tympani) over the irregular and ill-defined tym- 
 panic cavity. In the fresh skull there is a ring of cartilage 
 over which the tympanic membrane is stretched. At the 
 bottom of the cavity is the fenestra ovalis (/. 0.), a large hole 
 in the prootic'in the dried skull, but partly filled in with fibrous 
 tissue in the fresh state. A stapes, to which the columella is 
 attached, fits into the fenestra ovalis. 
 
 (4.) The Optic Chamber. The large orbits have the following 
 boundaries : Anterior, nasals above and palatines (pi.} below, 
 the latter bones being slender transverse curved rods. Posterior, 
 prootic above and guard of the para-sphenoid below. External, 
 the pterygoids (pt.), large tri-radiate bones with the longest 
 ray abutting against, and supported by, the maxillae (mx.), and 
 the shortest bending round towards the para-sphenoid. The 
 upper and hinder part of the external boundary is formed by 
 the hammer-like head of the squamosal. Internal, parieto- 
 frontals above, para-sphenoidal blade below, and sphenethmoid 
 anteriorly. 
 
 (5.) The Maxillary fiegion. Both pre-maxillse and maxillae bear 
 teeth. The former are small bones (p.rnx.), with well-marked 
 ascending processes. The maxillae are long, and they too have 
 ascending nasal processes. They are supported by the out- 
 wardly directed palatines (pi.), and the longer limbs of the 
 tri-radiate pterygoids (pt.). A slender quadrato-jugal connects 
 the maxilla with the outer limb of the pterygoid and the handle 
 end of the squamosal. 
 
 (6.) The Mandible. The lower jaw articulates with a cartila- 
 
158 ANIMAL BIOLOGY. [Part I. 
 
 ginous suspensorium directed outwards, and so far backwards 
 that the articular surface is nearly on a level with the occipital 
 condyles. At the end of this suspensorium a quadrate ossifica- 
 tion is formed in the cartilage, and constitutes the posterior end 
 of the quadrato-jugal. Dorsally the cartilage is strengthened 
 and supported by the overlying handle of the squamosal. 
 Ventrally it is supported by the outer limb of the pterygoid, 
 the inner limb of which is moveably articulated to the para- 
 sphenoid, while the anterior limb runs alongside the maxilla. 
 
 By means of this suspensorial apparatus the articulation of 
 the lower jaw, which differs from that of both rabbit and fowl, 
 is carried away from the brain-case outwards and backwards. 
 This gives the long broad gape of the adult frog. In young 
 frogs this swinging outwards and backwards of the suspensorium 
 is not carried nearly so far. In a minute frog, the tail of which 
 has only just been absorbed, the suspensorium is directed as 
 much forwards as, in the adult frog, it is directed backwards, so 
 that the gape is very much shorter. 
 
 The lower jaw itself contains a rod of cartilage (Meckel's). 
 There are three ossifications in each ramus : (1) a small mento- 
 meckelian (m. mk.) anteriorly; (2) a dentary (d.) forming the 
 upper part of the anterior half of the remainder of the jaw; 
 and (3) an angulo-splenial, by some regarded as articulare, 
 ensheathing the rest of Meckel's cartilage. 
 
 (7.) The Hyoid. The form of the hyo-branchial plate (b. h.), 
 with its posterior and anterior cornua (p. c. 9 a. c.), will be 'seen 
 by reference to Fig. 50, D. The posterior cornua pass one on 
 each side of the glottis. The anterior cornua are confluent 
 with the periotic capsule close to the fenestra ovalis. 
 
 The Chondrocranium. The form of the chondrocranium will 
 be made out by reference to Fig. 50, A. In it we find : (1) A 
 cartilaginous brain-case interrupted by an anterior and two 
 posterior fontanelles (a. /., p. /.), which lie beneath the parieto- 
 frontal bones. The cartilage is to a large extent invaded by 
 bone forming posteriorly the ex-occipitals, and anteriorly the 
 sphenethmoid (s. e.). (2) The auditory capsules (au. c,), largely 
 invaded by the prootics and, according to some observers, the 
 
Chap. VIII.] THE SKELETON. 159 
 
 anterior part of the ex-occipitals, and fused into the general 
 mass of the brain-case. (3) The nasal chamber, with the median 
 septum nasi carrying forward the mid-plate of the spheneth- 
 moid. The septum gives off above and below horizontal wings 
 of cartilage forming the roof and floor of the cartilaginous nasal 
 chamber. The roof gives off a pre-nasal (p. n. p.) and an ali- 
 nasal process (a. n. p.), the floor a slender rhinal process (r. p.). 
 Anteriorly the septum gives off on each side a flattened vertical 
 plate meeting the horizontal wings above and below, and form- 
 ing the front wall of the chamber. (4) A bow-shaped orbital 
 bar on each side, constituted anteriorly by an antorbital or pala- 
 tine bar with a forward projecting process (a. o. p.), and laterally 
 a pterygoid bar which bifurcates posteriorly, the dorsal limb 
 passing into 4 the ventral crus of the suspensorium, and the 
 ventral articulating with the periotic capsule, and forming the 
 pedicle of the suspensorium. The pterygoid cartilage (pt. c.) is 
 partly invaded by the pterygoid bone (p. b.). (5) The suspen- 
 sorium lying between the squamosal and pterygoid bones, and 
 dividing dorsally into two crura, of which the ventral crus is 
 continuous with the pterygoid cartilage, and the dorsal crus is 
 attached to the periotic capsule (p. c.). 
 
 Development and Homologies of Parts. A generalised description 
 of the development of the skull must suffice. The main features 
 of development are the same in all craniata (vertebrates with 
 skulls), but the details differ considerably, and for them the 
 student is referred to larger treatises. 
 
 The parts of the skull are first indicated in mesoblastic tissue, 
 then developed in cartilage, and finally invaded or overlaid by 
 bone. Some parts are developed in relation to the brain-case, 
 others in relation to the sense-capsules, and others in relation 
 to the visceral arches. 
 
 The first indication of the skull is the formation of two 
 parachordal plates underlying the mid and hind brain on either 
 side of the notochord (Fig. 51, A., ch.\ with which they com- 
 bine to form a continuous basilar plate (b. p.). In front of this 
 plate two bars are carried forwards from the parachordals, with 
 which they are from the first continuous in the fowl but not in 
 
i6o 
 
 ANIMAL BIOLOGY. 
 
 [Part I. 
 
 the frog or rabbit. These trabeculce (tr.) are united behind, but 
 diverge forwards, and then curve in so as to enclose a pituitary 
 space (pit.). Anteriorly they unite again, and thus are some- 
 
 t.o. 
 
 B. 
 
 FIG. 51. DIAGRAMS OF SKULL DEVELOPMENT. 
 
 A. Plan of cartilaginous embryonic skull. B. Section of a somewhat 
 older skull. 
 
 a. s. Ali-splienoid region, aw. Auditory capsule, "b. o. Basi-occipital region. 
 &. p. Basilar plate. &. s. Basi-sphenoid region, cli. Notocliord. cr. pi. 
 Cribriform plate, e. o. Ex-occipital region, n. s. Nasal septum, ol. Olfactory 
 capsule, op. Optic capsule, o. s. Orbito-splienoid region, p. s. Pre-sphenoid 
 region, pit. Pituitary space, s. o. Supra-occipital region, tr. Trabeculee. 
 
 what lyre-shaped. They lie beneath the fore-brain, the base of 
 which, to begin with, projects downwards into the pituitary 
 space. 
 
 The basilar plate represents the future basi-occipital region of 
 the skull. Its sides grow up fusing with the periotic capsules of 
 the auditory sacs, and eventually meet above. Thus the whole 
 occipital segment is preformed in cartilage, and in this cartilage 
 are subsequently formed ossifications starting from basi-occipital, 
 ex-occipital, and supra-occipital centres (b.o., e.o., s.o.), while in the 
 periotic capsules the otic bones are subsequently formed. 
 
 The trabeculse grow together so as to almost completely 
 close the pituitary space. In the region of the fore-brain they 
 
Chap. VIII.] THE SKELETON. 161 
 
 grow up at the sides as two pairs of wings, an anterior and a 
 posterior, more or less continuous ; but the roof of the cartila- 
 ginous brain-case in this region is incomplete. In this region 
 the basi-sphenoid and pre-sphenoid (b. s., p. s.) subsequently ossify 
 in the mid-ventral line, and the ali-sphenoids and orbito-sphenoids 
 (a. s., o. s.) are formed in the side wings. Anterior to this the 
 trabecular plate narrows and forms a median vertical lamina, 
 which further forward comes into relation with the nasal sacs, 
 curving over on either side to form their roof, or sending out 
 wings beneath to form their floor, or pioviding turbinal in- 
 growths. In this region the ethmoidal and turbinal bones are 
 subsequently developed. 
 
 We have now to consider the part played by the visceral 
 arches. It will be remembered that in the tadpole there are 
 gill-slits leading outwards from the pharynx to the exterior. 
 Homologous gill-slits are formed in the fowl and the rabbit; 
 but in these organisms they never have a branchial function. 
 There are four such visceral clefts. In front of each, and 
 behind the last, is a visceral fold. Of the five visceral folds or 
 arches, the first lies just behind the mouth, and nearly meets its 
 fellow in the mid-line. This is the mandibular arch. It does 
 not remain simple, but bifurcates into two processes, a superior 
 maxillary process (Fig. 52, s.m.p.) in front of the mouth, and 
 an inferior maxillary process (i. m.p.) behind the mouth. The 
 second or hyoid arch is the strongest in the embryo. The 
 third, fourth, and fifth are progressively shorter and weaker. 
 
 The first post-oral visceral cleft persists in the adult as the 
 Eustachian tube and tympanic cavity. The other three function 
 as branchial clefts in the frog, but are early obliterated in the 
 fowl and rabbit. 
 
 The further history of the first two arches is remarkable and 
 important. In the superior maxillary process of the first a 
 cartilaginous palato-pterygoid bar is developed, in relation to 
 which the palatine and pterygoid bones are formed. In the 
 inferior maxillary process of the mandibular arch a cartilage 
 (Meckel's) is formed which constitutes the basis of the lower 
 jaw. In the frog its proximal portion forms the suspen- 
 11 
 
162 ANIMAL BIOLOGY. [Part I. 
 
 sorium. In the fowl the proximal portion forms the quadrate 
 bone, the succeeding portion in the lower jaw ossifying 
 as the articular. In the rabbit it seems probable that the 
 
 s.m.p 
 
 l^.v.cl. 
 
 Fia. 52. THE MANDIBULAR ARCH. 
 
 a. Point where fronto-nasal process overarches the nasal groove, fn. p. 
 Fronto-nasal process, i. m. p. Inferior maxillary process, m. Mouth, n. p. 
 Nasal pit. s. m. p. Superior maxillary process. 1st v. cl. First visceral cleft. 
 
 proximal portion, answering to the quadrate of the fowl, 
 separates early and ossifies as the incus, while the succeeding 
 portion, answering to the articular of the fowl, is segmented 
 off from the rest of Meckel's cartilage, and ossifies as the mal- 
 leus ; while the main mass of the lower jaw is ossified (perhaps 
 in connection with lower labial cartilages) around, but not within, 
 the remainder of Meckel's cartilage. It may be, however, that 
 ossification of the most distal portion of Meckel's cartilage takes 
 place. Concerning the homologies of the quadrate, articular, 
 malleus, and incus, more than one interpretation has been offered. 
 The uppermost end of the hyoid arch would seem to be 
 segmented off and to ossify as the stapes, while the columella of 
 the frog and fowl are also developed from this arch. The 
 remainder of the arch forms the anterior cornua of the hyoid, 
 the basi-hyal giving rise to the body. The posterior cornua of 
 the rabbit and the fowl are remnants of the first branchial arch, 
 but in the frog of the fourth branchial arch, the third and first 
 two being represented by processes of the hyoid plate, the 
 
Chap. VIII.] THE SKELETON. 163 
 
 hinder portien of which is basi-branchial in origin, and repre- 
 sents the fused basal elements of these branchial arches. 
 
 So far all the bones of the skull, with the exception of the 
 palatines and pterygoids in some forms (e.g. fowl), are cartilage 
 bones, ossification taking place within a preformed mass of 
 cartilage. All the other bones are splint bones, ossification 
 taking place in membrane without the intervention of cartilage. 
 Such are the parietals, frontals, lachrymals, nasals, pre-maxillae, 
 maxillae, vomers, squamosals, jugals, quadrato-jugals (in the fowl, 
 and their anterior portion in the frog) ; dentary, angular, supra- 
 angular, and splenial, in the lower jaw ; and para-sphenoid at 
 the base of the skull. This is a single bone in the frog with 
 rostral and basi-temporal divisions. In the fowl the rostrum 
 and basi-temporal form distinct ossifications, and in the rabbit 
 the para-spherioidal rostrum is entirely suppressed, and only 
 minute rudiments [larger in the guinea pig] of the basi-temporal 
 guard have been discovered. The question of the homology of 
 the basi-temporal, however, is one that has received more than 
 one answer. 
 
 The Vertebral Column. (1.) In the Rabbit. The vertebral 
 column of the rabbit has some 45 vertebrae. They are, reckoning 
 from before backwards, atlas, axis, 5 more cervical; 12 thoracic, 
 with free ribs; 7 lumbar; 4 sacral; about 15 or 16 caudal. 
 
 We may take the fourth thoracic vertebra for illustration 
 (Fig. 53.) Below there is a solid mass of bone, the body or 
 centrum (c.), the ends of which are flattened and vertical. From 
 this springs the neural arch (n.) through which runs the spinal 
 cord. Above the neural arch rises the neural spine (n. s.). At 
 its sides project transverse processes (tr.) which bear at their ends 
 tubercular facets (./.), with which the tubercular processes of the 
 ribs articulate. If the third and fifth be placed in position 
 before and behind it, it will be found to articulate with them by 
 means of little bony shelves in front and behind. Those in front 
 (prezygapophyses, pr. z.) face upwards and outwards ; those behind 
 (postzygapophyseSy pt. z.) face downwards and inwards. When the 
 vertebrae are thus placed together a lateral space is seen between 
 
i6 4 
 
 ANIMAL BIOLOGY. 
 
 [Part I. 
 
 the peduncles of the neural arches. It is the intervertebral fora- 
 men, by means of which the spinal nerves make their exit. And 
 when the centra are in contact a capitular facet for the articula- 
 tion of the head of the rib is formed by the juxtaposition of 
 two demi-facets (c. s.), one on each centrum. 
 
 FIG. 53. FOURTH THORACIC VERTEBRARABBIT. 
 
 A. From the side. B. From the anterior end. 
 
 c. Centrum, c. s. Capitular semi-facet, n. Neurapophysis of neural arch. 
 n. s. Neural spine, pr. z. Prezygapophysis. pt. z. Postzygapophysis. t. f. 
 Tubercular facet, tr. Transverse process. 
 
 The following points may be noted in other parts of the 
 column: The transverse processes of the cervical vertebras are 
 pierced by a vertebrarterial canal. The second cervical (axis) has 
 a peg-like odontoid process; the first (atlas) is a mere ring of 
 bone with facets for the condyles of the skull, and flattened 
 transverse processes. In the eleventh thoracic vertebra the 
 neural spine becomes vertical, sloping forward in the twelth. 
 In the vertebrae of this region there is a large process .(the meta 
 popliysis) just behind the prezygapophysis. In the first three 
 lumbar vertebrae there is a flattened process (hypapopliysis] 
 beneath the centrum, and in the first five lumbar small pointed 
 processes (anapophyses) looking backwards, beneath the postzy- 
 gapophyses. The four sacral vertebrae are fused into one mass ; 
 but only the first, or at most, the first two, come into relation 
 with the ilia by means of their expanded transverse processes or 
 ankylosed ribs. The metapophyses are here smaller, and there 
 are no ana- nor hypapophyses. The caudal vertebras gradually 
 diminish in size and in complexity until they are reduced to 
 mere centra. 
 
Chap. VIII.] THE SKELETON. 165 
 
 In the young rabbit there are cartilaginous intervals in each 
 centrum near the ends ; these separate the main mass (diaphysis) 
 of the centrum from its end plates (epiphyses) which are charac- 
 teristic of mammalian vertebrae. 
 
 It will be remembered that the first indication of the axial 
 skeleton in the body is the notochord which underlies the 
 neural axis. This becomes invested by a cartilaginous sheath, 
 which comes into relation with a number of cartilaginous 
 arches derived from the mesoblastic investment of the spinal 
 cord. As growth proceeds, the notochord is encroached upon 
 by its cartilaginous investment, which becomes differentiated 
 into vertebral and intervertebral regions. In the vertebral 
 region the notochord is finally obliterated ; but in the in- 
 tervertebral . region it persists throughout life as part of the 
 nucleus pulposus in the axis of the intervertebral ligament. 
 Before the differentiation of the vertebrae, there is split off 
 from the upper and outer part of each somite a muscle-plate 
 (see p. 120), from which is formed the voluntary muscle of that 
 part of the trunk. It is to be noted, however, that the ver- 
 tebral segmentation takes place in such a way that the centrum 
 of the newly-formed vertebra does not correspond in position to 
 the middle of the muscle-plate, but to the line separating two 
 muscle-plates; the muscles formed from which, therefore, act 
 upon two adjoining vertebras. 
 
 The odontoid process of the axis ossifies from a distinct centre, 
 and is at- first a distinct bone. It is indeed a portion of the 
 centrum of the atlas, which has lost its primitive connection 
 and become conjoined to the axis. The so-called transverse 
 processes of some (cervical, sacral) vertebrae are to be regarded 
 as ankylosed ribs. 
 
 (2.) In the Fowl there are sixteen cervical vertebrae, of which 
 the last is ankylosed to the first three thoracic. The fourth 
 thoracic is free ; but the fifth or last coalesces with a number 
 of succeeding vertebrae to form the compound sacrum. In 
 this sacral mass there are ankylosed : one (the fifth) thoracic ; 
 three lumbar; five (ribless) sacral; six (with ankylosed ribs) uro- 
 sacral; fifteen in all. Following on the sacrum are six free caudal ; 
 
166 ANIMAL BIOLOGY. [Parti. 
 
 and the vertebral column is terminated by a pygostyle or plough- 
 share bone, composed of several completely ankylosed vertebrae. 
 
 Note the extreme coalescence of parts in the column. The 
 sacrum forms a compact mass, which in the adult is firmly anky- 
 losed to the iliac bones (see Fig. 56, A) by means of the trans- 
 verse processes, coalesced ribs, and anteriorly by the expanded 
 crest of the coalesced neural spines. 
 
 The form of the centrum in birds is noteworthy. The 
 anterior face is convex in vertical section, and concave in hori- 
 zontal section ; while the posterior face is concave in vertical 
 section and convex in horizontal section. In other words, both 
 faces are saddle-shaped, but the anterior saddle is horizontal and 
 the posterior saddle vertical. 
 
 (3.) In the Frog. The vertebral column consists of nine vertebrae 
 and a posterior elongated urostyle. There is no distinction into 
 cervical, dorsal, and lumbar ; but the ninth, the expanded trans- 
 verse processes of which articulate with the ilia, may be regarded 
 as the sacral vertebra. The urostyle articulates with the last 
 vertebra by two concave facets. Along its dorsal surface runs 
 a prominent ridge, in which are two coccygeal foramina for 
 the tenth spinal nerves. The faces of the centra of the second 
 to the seventh vertebras are concave anteriorly and convex pos- 
 teriorly. Both faces of the eighth are concave. The ninth is 
 convex anteriorly .and has two convex tubercles for articulation 
 with the urostyle posteriorly. The atlas has facets for the con- 
 dyles of the skull. The centra contain vestiges of the notochord. 
 
 General Considerations. In studying the successive vertebrae in 
 one organism, or comparing the vertebrae in different organisms, 
 we are struck by the general similarity of plan in the midst of 
 differences of detail. We see specialisation, and yet adherence 
 to type. After considering all the forms presented to us by the 
 vertebrae, we may construct an ideal vertebra, which we may 
 call a typical vertebra. And then we may trace the modifica- 
 tions which have led to greater or less divergence from the 
 typical form. These modifications fall under three heads 
 (Huxley's law) : (1) Coalescence, where parts typically distinct run 
 together and become fused ; (2) Suppression, where parts typically 
 
Chap. VIII.] THE SKELETON. 167 
 
 present are undeveloped ; and (3) Metamorphosis, where there is 
 a notable departure from the typical form. The student is, 
 however, to bear in mind that the typical form has not neces- 
 sarily any existence as such in nature, but is a conception of the 
 human mind. To explain : photographs are now taken which 
 represent, e.g. the typical criminal. A number of photographs 
 of representative criminals are separately taken, and then the 
 images of all are combined on one plate. The result is a typical 
 photograph which resembles no one individual, but represents 
 all. So too the ideal vertebra resembles no one individual ver- 
 tebra, but represents all. If, therefore, we speak of a particular 
 vertebra as typical, we mean that its divergence from the ideal 
 type is comparatively slight. 
 
 The Ribs and Sternum. (1.) In the 'Rabbit. Free ribs are 
 connected with and characterise the thoracic vertebrae. There 
 are seven true ribs which, with the sternebrae below, constitute 
 inferior or haemal arches. Behind these are five false ribs, in- 
 complete below. Each true rib consists of a dorsal osseous 
 portion, the vertebral rib, and a ventral cartilaginous portion, 
 the sternal rib. The vertebral rib has a head (capitulum), which 
 articulates with the capitular facets of the vertebrae, and a 
 tubercle on the dorsal side, which articulates with th,e> tubercular 
 facet on the transverse process of the vertebra. Just external 
 to the tubercle is a dorsal process. 
 
 The sternum consists of six segments or sternebrce, of which 
 the first, manubrium, is large and keeled ventrally. The last has 
 a rounded plate of cartilage (the xiphoid process). Seven pairs of 
 ribs articulate with the sternum, the first two pairs with the manu- 
 brium, the others with the interspaces between the sternebrae. 
 
 (2.) In the Fowl there are five complete or true ribs. Each 
 has a capitulum and tuberculum, and is divided by a transverse 
 articulation into a dorsal (vertebral) and a ventral (sternal) rib, 
 both of which are osseous. At their junction they form an open 
 angle, which is widened during the process of respiration. Each 
 vertebral rib has a backward and upward process (the unci- 
 nate pr.). 
 
1 68 
 
 ANIMAL BIOLOGY. 
 
 [Part I. 
 
 Anterior to the true ribs are three free incomplete false ribs, 
 the form of which resembles the vertebral portion of the true 
 ribs. 
 
 FIG. 54. STERNUM AND SHOULDER-GIRDLE. 
 
 A. Sternum of fowl from below. B. Sternum and shoulder -girdle of frog 
 in diagrammatic section. C. The same from below, with upper part straight- 
 ened out. D. Scapula of rabbit. E. Shoulder-girdle of fowl. 
 
 a. st. Part of coracoid which articulates with sternum, ac. Acromion. 
 b. st. Body of sternum, c. b. Costal border, c. g. Coracoid groove, c. p. 
 Coracoid process in fowl. ca. Carina or keel. cl. Clavicle, co. Coracoid. 
 co. b. Coracoid border, co. p. Coracoid process in rabbit, e. x. p. External 
 xiphoid process. /. Furcula. gl. b. Glenoidal border, gl. c. Glenoidal cavity. 
 h. cl. Hypocleidium. h. co. Head of coracoid. i. x. p. Internal xiphoid pro- 
 cess, m. ac. Metacromion. m. x. p. Middle xiphoid process, o. st. Omo- 
 sternum. r. Rostrum, sc. Scapula, s. sc. Supra-scapula. s. sc. b. Supra- 
 scapular border, sp. Spine, st. Sternum, x. st. Xiphi-sternum. 
 
 The sternum of the fowl is a large and well-ossified struc- 
 ture. The central portion or body of the sternum (Fig. 54, b. st.) 
 is boat-shaped and concave dorsally, and is prolonged back- 
 wards as an elongated middle xiphoid process (m. x. p.). Ven- 
 trally there is a well-marked keel or carina (ca.). Anteriorly 
 the sternum is produced into a vertically-flattened rostrum or 
 manubrium (r. ), on either side of which, running backwards and 
 outwards along the front edge of the body, is a coracoid groove 
 (c. g.). On either side of the sternum there are three large pro- 
 cesses. The most anterior is the costal process (c. p.); behind 
 which, at the side, is the costal border for the articulation of the 
 ribs (c. b.). Posterior to this are two processes having a common 
 
Chap. VIII.] THE SKELETON. 169 
 
 stem. The more anterior is the external xiphoid, process (e. x.p.\ 
 the more posterior the internal xiphoid process (i. x. p.). 
 
 The plate of cartilage from which the sternum is formed arises 
 as two bands, which are segmented off from the ventral ends of 
 the ribs, and unite to form a common plate. 
 
 (3.) In the Frog there are no ribs. The sternum is well ossi- 
 fied, and is in close connection with the pectoral arch. Behind 
 it is an expanded plate of cartilage, the xiphi-sternum (Fig. 54, 
 x. st.). Anterior to the pectoral arch is a median bony process, 
 the omostemum (o. st.), carrying a rounded cartilaginous plate. 
 
 The Pectoral Arch. The shoulder-girdle of the frog comes 
 into close relation with the sternum and omo-sternum. Between 
 these sternal elements two bones pass outwards from each side 
 of the mid-ventral line. Of these the more posterior and stouter 
 is the coracoid (54, B. and C., o.) the more anterior and slenderer 
 is the clavicle (cl). Outside these, articulating with both, and 
 passing nearly vertically on each side, is a somewhat hourglass- 
 shaped bone, the scapula (sc.). Dorsal to this, and lying over the 
 transverse processes of the second, third, and fourth vertebrae, is 
 a flattened plate of bone and cartilage, the supra-scaputa (s. sc.). 
 At the point of junction of the coracoid and scapula is the 
 glenoidal cavity for the articulation of the fore-limb (gt. c.). In 
 Fig. 54, C., the parts are flattened out. In B. they have their 
 natural position. The inner edges of the coracoids remain 
 cartilaginous. Posterior to the clavicle (mainly a membrane 
 bone) lies a precoracoid bar of carti-lage. 
 
 In the Babbit, while the ribs form an attachment with the 
 sternum which is absent in the frog, the attachment of the 
 shoulder-girdle is much less perfect. The main element is the 
 scapula or shoulder-blade (Fig. 54, D.). The coracoid is held 
 to be represented by a process (co. p.) of the scapula. Doubts 
 have, however, recently been thrown upon the validity of this 
 view. The small clavicle is the only part which has attach- 
 ment to the sternum, being connected by ligament on the one 
 hand with the acromion (ac.), and on the other with the sternum. 
 The form and relations of the scapula will be readily made out 
 
i ;o 
 
 ANIMAL BIOLOGY. 
 
 [Part I. 
 
 with the aid of the figure, in which sp. is the crest or spine 
 which forms the acromion border ; gl. b. is the glenoidal border; 
 gl. c. the glenoidal cavity ; co. b. the coracoid border ; 5. sc. b. the 
 supra-scapular border, which is postero-dorsal in position. 
 
 In the Fowl the coraeoid is a separate bone (54, E., co.). The 
 glenoidal cavity (gl. c.) is at the junction of coracoid and 
 scapula (sc) ; h. co. is the head of the coracoid which articulates 
 with the sternum by a. st. The clavicles are represented by the 
 merry-thought or furcula (/.), of which the flattened median 
 portion (h. cl.) is called the hypodeidium. 
 
 The Fore Limb. The typical fore-limb consists of the follow- 
 ing parts : (1) a single proximal bone in the arm, the humerus 
 
 u. 
 
 
 FIG. 55. EIGHT FORE-LIMB, 
 
 A. Typical. B. Fore-limb of fowl. C. Humerus of rabbit. D. Radius 
 and ulna of rabbit. E. Carpus of rabbit. 
 
 H. Humerus. R. Radius. U. Ulna. c. Centrale. ca. Carpus, cap. Capi- 
 tellum. di. Digits, g. t. Greater tuberosity. h. Head of humerus. i. In- 
 termedium. 1. t. Lesser tuberosity. TO. ca. Metacarpals. me. i. ii. iii. Meta- 
 carpels of fowl. o. /. Olecranon fossa, ol. pr. Olecranon process, ph. i. ii. 
 iii. Phalanges of fowl. r. Radiale. sh. Shaft, sy. c. Sigmoiclal cavity, tr. 
 Trochlea. u. Ulnare. 
 
 (Fig. 55, A., H.); (2) two bones side by side in the fore-arm, 
 one pre-axial the radius (Pi.), and one post-axial, the ulna (U.); 
 (3) the carpus, composed of nine bones ; (a.) & proximal row of 
 three bones, radiate (r.), intermedium (i.), and ulnare (u.) ; (b.) 
 a single (sometimes double) bone, the centrale, between the 
 
Chap. VIII.] THE SKELETON. 171 
 
 proximal and distal row ; (c.) a distal row of five carpalia, num- 
 bered from the radial to the ulnar side ; (4) the metacarpus, com 
 posed of five metacarpalia articulated to the five carpalia ; and 
 (5), the five digits with articulating phalanges. 
 
 In the rabbit the humerus has the form shown in dorsal view 
 in Fig. 55, C., where h. is the head, 1. 1. and g. i. the lesser and greater 
 tuberosity, sh. the shaft, o. f. the olecranon fossa, tr. the trochlea, 
 and cap. the capitellum. On the ventral aspect there is a well- 
 marked deltoid ridge. The radius and ulna have the form shown 
 in D. where ol. pr. is the olecranon, and si. c. is the sigmoid 
 cavity for articulation with the humerus. The two bones are 
 closely applied, and do not admit of that change of relative 
 position which gives us the power of rotating our hand and wrist. 
 In the carpus (E.) carpalia iv. and v. coalesce to form a single 
 bone (unciform), while the centrale is wedged in between carpalia 
 ii and iii. To the left of the ulnare in E. is seen a little bone 
 (shaded) ; this is the pisiform, hitherto regarded as an extrane- 
 ous bone developed in tendon. But the homologies of these 
 parts are being re-investigated, and some change of view (e.g. in 
 the nature of the pisiform) seems probable. Special names are 
 given to the bones of the mammalian carpus. In the proximal 
 row of the carpus the bones are (in E., reading from left to 
 right) pisiform, cuneiform, lunar, and scaphoid; in the distal row 
 they are unciform, magnum, centrale (wedged in from above), 
 trapezoid, and trapezium. 
 
 In the fowl the humerus is a strong light bone. In the 
 ventral aspect the greater tuberosity is strongly marked ; in the 
 dorsal aspect, close to the lesser tuberosity, is an excavation 
 leading to the pneumatic foramen, by means of which air from 
 the interclavicular air-sac finds access to the shaft. The radius 
 and ulna (B.) do not call for especial notice. The manus is, 
 however, curiously modified in relation to flight. Of the carpus 
 only the ulnare and radiale are developed. The metacarpals i., 
 ii., iii., ankylose, and are peculiar in form. The others are 
 suppressed. Of the digits, one phalange of i., two of ii., and one 
 of iii., are developed. The rest are wanting in the adult. 
 
 In the frog the humerus is short and curved, with a deltoid 
 
172 ANIMAL BIOLOGY. [Parti. 
 
 crest more pronounced in male than female. The radius and 
 ulna coalesce to form a radi-ulna, which shows, however, at the 
 distal end signs of its composite nature. The carpus consists of 
 six bones, two articulating with the radi-ulna, three with the 
 metacarpals (of which the largest articulates with iii., iv., and v.) 
 and one having an intermediate position on the radial side. 
 There are five metacarpals, the first very small ; and four com- 
 plete digits, the pollex (thumb) being present only in rudiment. 
 For the changes of position which the fore-limbs have under- 
 gone see Chapter II., pp. 7, 14, and 19. 
 
 The Pelvic Arch. Three elements enter into the composition 
 of the haunch-bone of either side. And these three meet at the 
 acetabulum or articular cavity for the hind-limb. These three 
 are the ilium (anterior); the ischium, postero- dorsal; and the 
 pubis, postero-ventral. Internal to the pubis there is, however, 
 in some forms a fourth bone, the acetabular, which may in some 
 cases (e.g. rabbit) shut out the pubis from any share in the 
 formation of the acetabular cavity. 
 
 In the frog these bones (Fig. 56, C. D.) are tolerably easily 
 separable, though the elements of the two sides fuse somewhat 
 closely in the mid-line at the symphysis. The part marked pu. 
 is regarded as pubis. The feature to be specially noticed is the 
 great lengthening of the ilia (il.), and their articulation with a 
 single sacral vertebra, through the agency of supra-iliac car- 
 tilages. The swinging back of the point of articulation of the 
 hind-limb, brought about by the lengthening of the ilia, is more 
 marked in the adult than in the young frog. The student 
 should compare the pelvic arch of the frog with that of the 
 newt, where the acetabulum is not far removed from the sacrum. 
 
 In the rabbit (56, B.) the ilium (il.\ ischium (isc.), and pubis 
 (pu.) (together with a small acetabular, which, in the very 
 young rabbit, shuts out the pubis from the acetabulum), com- 
 pletely fuse into a single innominate bone. The two innominates 
 unite by synchondrosis, or cartilaginous union, at the symphysis 
 (sy.). They articulate with the expanded lateral plates (ankylosed 
 ribs) of the first two sacral vertebrae. In B. t. isc. is the tuber 
 
Chap. VIII.] 
 
 THE SKELETON. 
 
 173 
 
 ischii, isc. b. the ischial border, s. il. b. the supra-iliac border, 
 pu. b. the pubic border, ac. the acetabulum, ac. b. the acetabular 
 border, and ob. f. the obturator foramen. 
 
 r.ac.il. 
 
 A. 
 
 3- pu. 
 
 pa. 
 
 FIG. 56. HIP-GIRDLE. 
 
 A. Innominate bones and sacrum of fowl. B. Innominate of rabbit. C. In 
 nominate of frog, from above. D. The same from the side. 
 
 ac. Acetabulum. ac. &. Acetabular border, a. t. Anti-trochanter. ca. v. 
 Caudal vertebras, il. Ilium, i. s. f. Ilio-sacral foramen, isc. Ischium. 
 isc. 6. Ischial border. 06. /. Obturator foramen in rabbit, obturator fissure 
 in fowl. p. p. Pectineal process, pu. Pubes. pu. 6. Pubic border, pr. ac. il. 
 Pre-acetabular part of ilium, pt. ac. il. Post-acetabular part. py. Pygostyle. 
 s. il. &. Supra-iliac border. sy. Symphysis. t. isc. Tuber ischii. v. Last 
 thoracic vertebra. 
 
 In the adult fowl the ilium, ischium, and pubes, not only fuse 
 into a single innominate (56, A.), but this becomes ankylosed with 
 the compound sacrum. On the other hand, there is no symphy- 
 sis as in the frog and the rabbit. The student should endeavour 
 to obtain a just-hatched chick or a nestling pigeon, in which the 
 bones are still separate. In Fig. 56, A., pr. ac. il. is the pre- 
 acetabular part of the ilium, pt. ac. il. the post-acetabular part, 
 ca. v. the caudal vertebrae, pij. the pygostyle, isc. the ischial 
 portion, pu. the pubis, ob. f. the obturator fissure, i. s. f. the ilio- 
 sciatic foramen, a. t. the anti-trochanter, ac. the acetabulum, p. p. 
 the pectineal process, perhaps derived from an acetabular, and 
 v. the last thoracic vertebra. 
 
174 
 
 ANIMAL BIOLOGY. 
 
 [Part I. 
 
 The Hind Limb. The typical hind limb (Fig. 57, A.) consists 
 of the following parts : (1) a single proximal bone articulating at 
 the acetabulum, the femur (F.) ; (2) two bones side by side in the 
 crus, one pre-axial, the tibia (Ti.), and one post-axial, the fibula 
 
 FIG. 57. RIGHT HIND LIMB. 
 
 A. Typical. B. Tarsus of rabbit. C. Distal part of hind limb of fowl. 
 F. Femur. Fi. Fibula. Ti. Tibia. 
 
 ce. Centrale. dl. Digits, fi. Fibulare. in. Intermedium, m.t. i.-iv. Meta- 
 tarsals. t. i.-v. Tarsals. ti. Tibiale. 
 
 (Fi.) ; (3) the tarsus, composed of nine bones (a.) a proximal row 
 of three bones, tibiale (&), inter-medium (i), &nd fibulare (fi.) ; (b.) a 
 single bone, the centrale (ce.\ between the proximal and distal 
 rows ; (c.) a distal row of five tarsalia, numbered from the tibial to 
 the fibular side; (4) the metatarsus (mt.) 9 composed of five meta- 
 tarsals ; and (5) the five digits with their phalanges. The 
 student cannot fail to notice how marked is the apparent 
 homology of the parts of the fore and hind limbs, femur answer- 
 ing to humerus, tibia to radius, and so on. 
 
 In the rabbit the femur is a strong bone, with rounded 
 head, borne upon a neck, below which is the lesser trochanter, 
 and above which is the strongly-marked greater trochanter, 
 overhanging a trochanteric fossa. A third trochanter is on 
 
Chap, VIII.] THE SKELETON. 175 
 
 the opposite side of the bone to the lesser trochanter. At 
 the distal end of the bone there is an internal and external 
 condyle (the former on the same side as the head), separated 
 by the inter-condylar notch, on the pre-axial side of which 
 the small patella of the knee-cap fits. The tibia is a large 
 strong bone, with which the small fibula fuses distally on the 
 outer side. The tarsus (Fig. 57, B.) consists of six bones: a 
 large calcaneum or fibulare (fi.\ and a smaller astragalus or tibiale 
 (ti.)j in which the inter-medium is also probably merged; a cen- 
 trale, or navicular (ce.), a bone (the cuboid) on the fibular side, 
 which represents tarsale iv. and v. coalesced, tarsale iii., known as 
 the ecto-cuneiform, and tarsale ii. or meso-cuneiform. Tarsale i. 
 (ento-cuneiform) may be represented by a small process of meta- 
 tarsale ii. The homologies of the tarsus are, however, being 
 re-investigated. There are only four metatarsals, i. being sup- 
 pressed ; and four digits, the hallux (great toe) being absent. 
 
 In the fowl the femur is a stout somewhat curved bone with a 
 strong great trochanter. In front of its distal end is a small 
 patellar nodule. On the femur follows a compound bone known 
 as the tibio-tarsus, the distal end of which (Fig. 57, C.) represents 
 one or more of the proximal row of tarsal bones. The fibula 
 (Fi) forms a thin bone lying as a splint on the tibio-tarsus, but 
 with an expanded head. Upon the tibio-tarsus follows another 
 compound bone, the tarso-metatarsus. Its proximal end represents 
 the distal row of tarsal bones which becomes fused on to the ends 
 of metatarsalia ii., iii., iv., ankylosed side by side into a single 
 bone. The first metatarsal is found at the distal end of the 
 tarso-metatarsus united to it only by ligament. The fifth meta- 
 tarsal is absent. There are four digits, of which three are 
 directed forwards, and rest with their whole ventral surface on 
 the ground. The other digit, the hallux, is directed backwards, 
 and raised off the ground at the proximal end. The fifth digit is 
 suppressed. 
 
 Thus in the fowl the heel-joint is not, as in the rabbit, 
 between the tibia and the tarsus, but between the proximal and 
 distal portions of the tarsus, which are respectively ankylosed 
 with the tibia and the fused metatarsus. It is therefore a meso- 
 
176 ANIMAL BIOLOGY. [Part I. 
 
 tarsal articulation. The hallux has two phalanges ; the second, 
 third, and fourth digits have three, four, and five phalanges 
 respectively. Both the hind and fore-limb of the bird are thus 
 seen to be remarkably modified. 
 
 In the frog the femur is a thin elongated bone. The tibia 
 and fibula fuse together into a tibio-fibula, which shows signs, 
 even in the adult, of its compound origin. The astragalus and 
 calcaneum are remarkably elongated, fusing to a considerable 
 extent at their ends. They aid in giving the extreme length 
 to the pes. The distal row of the tarsals is represented by two 
 partly ossified cartilages. There are five metatarsals and five 
 well-developed digits, and, in addition to these, the cakar, which 
 is situated on the radial side ; it is composed of two or three 
 pieces, and may represent a sixth digit. 
 
 For the changes of position which the hind-limbs have under- 
 gone see Chapter II., pp. 7, 14, and 19. 
 
CHAPTER IX. 
 
 NUTRITION AND METABOLISM. 
 
 No steam-engine can continue to work without fresh supplies 
 of fuel; no organism can maintain its vital activity without 
 fresh supplies of food. It is the combustion of the fuel which 
 gives to the engine its energy or power of doing work ; and in 
 the animal it is to a kind of slow combustion that the main- 
 tenance of the vital energy is due. But the chemical processes 
 in the latter case are vastly more complex, and are not confined, 
 as in the engine, to a special part of the machine \ for the 
 animal is a cunningly wrought piece of mechanism, and the most 
 efficient machine we know. But we must not forget that the 
 processes which go on in the animal, though they are chemical, 
 physical, and mechanical, are at the same time something more. 
 They are vital or organic ; and this, not through any mysterious 
 addition of something from without, but through more com- 
 plex combination of the self-same materials and energies. 
 
 In this chapter we must consider shortly some of the metabolic 
 processes (as the chemical processes within the organism are 
 termed) upon which the continued vitality of the organism 
 depends. 
 
 The Alimentary Canal. The alimentary canal is a continuous 
 more or less coiled tube running through the body from the 
 mouth to the vent, and for a considerable part of its course 
 suspended in the body-cavity. It does not in any way com- 
 municate directly, that is, by any opening or canal, with the 
 blood-vessels. And when we remember that its lining mem- 
 brane is continuous at the lips and vent with the epidermis of 
 the skin, we shall see that the contents of the canal, although 
 12 
 
178 ANIMAL BIOLOGY. [Part I. 
 
 within the body, are still in a sense outside it. They are in the 
 same position as the fuel in a Cornish or Lancashire boiler 
 which is placed in the tube or tubes that run through the midst 
 of the boiler, but which can hardly be said to be inside it. 
 Hence the elaborated food-stuff has to be absorbed through the 
 walls of the canal before it enters the body. 
 
 But though there are no tubes which carry material from the 
 canal into the system, there are several large, and a multitude 
 of minute tubes which convey material into the canal. These 
 are the ducts of the various glands which minister to digestion. 
 They are divided into : (1) salivary glands in the region of the 
 mouth; (2) gastric glands in the stomach; and (3) intestinal glands. 
 
 The digestive tube itself has the following structure (for 
 details see Chapter V. p. 78, Fig. 28) : Externally there is a 
 peritoneal investment; then follow the muscular walls with 
 longitudinal and circular fibres; within this is the mucous 
 membrane. The epithelium that lines the tube passes up into 
 and lines the ducts of the glands, large and small. At the 
 mouth and vent it is continuous with the epidermis. 
 
 This epithelium, and its supporting mucosa and sub-mucosa, 
 does not form a smooth and even lining to the canal. In the 
 stomach of frog and rabbit it is thrown into folds or rugce. In 
 the small intestine of the rabbit it is doubled inwards so as to 
 form crescentic or nearly circular folds (valvulw conniventes). 
 And if a small portion of the small intestine of a rabbit or fowl 
 be examined under water with a lens it will be seen to have a 
 velvety appearance, due to great numbers of closely-set minute 
 processes, the villi (see Fig. 28). In the small intestine of the 
 'frog there are minute dependent processes ; but they do not 
 form so close-set a velvety pile as the villi of fowl or rabbit. 
 In the large caecum of the rabbit there is a spiral infolding 
 of the tube readily seen when this curious appendage is re- 
 moved, cleaned out by directing a stream of water through it, 
 distended with air, and dried. In the colon of the rabbit 
 the whole wall is characteristically puckered. 
 
 In the fowl, answering to the stomach of frog or rabbit, there 
 is anteriorly a proventriculus, the walls of which are richly sup- 
 
Chap. IX.] NUTRITION AND METABOLISM. 179 
 
 plied with glands, the orifices of which may be seen by careful 
 examination with a lens ; and posteriorly the gizzard, the walls 
 of which are exceedingly muscular, the fibres of either side 
 radiating from a central tendinous aponeurosis. The epithelium 
 here develops a dense horny coat. It is here in fact, and not 
 in the mouth as in the rabbit, that the food is triturated ; 
 and to aid in this grinding process the fowl swallows small 
 stones. 
 
 The Glands. We must now briefly consider the glands that 
 minister to digestion ; first their structure, and then their products. 
 It will be necessary to confine our attention to the rabbit. 
 
 The salivary glands of the rabbit are : (1) the parotid, lying 
 between the base of the ear and the angle of the lower jaw, and 
 opening forwards into the side of the mouth by the parotid 
 duct (Stenson's) ; (2) the sub-maxillary, lying between the angles 
 of the lower jaw, and opening forwards into the floor of the 
 mouth by the sub-maxillary duct (Wharton's); (3) the sub-lingual, 
 small, and lying along the inner side of the mandible, with many 
 ducts opening separately into the mouth ; (4) the infra-orbital, 
 lying in the antero-inferior region of the orbit, the duct of which 
 passes downwards to the mouth ; and (5) the small buccal and 
 other glands set in the mucous membrane of the lips, palate, 
 tongue, and pharynx. 
 
 The buccal glands are simple or branched tubes lined with 
 epithelial cells, which are flattened in the duct, but columnar 
 in the deeper secreting part or body of the gland. The other 
 salivary glands are all of the racemose or branched tubular type. 
 The duct is elongated into a long tube, which divides and sub- 
 divides a great number of times, and so gives rise to a multitude 
 of branched tubes, each of which ends in a slight dilatation 
 (alveolus). The tubes and their alveoli are imbedded in connec- 
 tive tissue, and aggregated into lobules separated by connective 
 tissue septa. 
 
 The cells which line the alveoli of the so-called serous sali- 
 vary glands, differ curiously according to the state of rest or 
 activity of the gland. After a period of rest they are large, 
 and have a markedly granular appearance, the protoplasm of the 
 
i8o ANIMAL BIOLOGY. [Parti. 
 
 cell being much reduced in size. After a period of secretion 
 the cells decrease in size, so that the lumen (or cavity), which 
 was before almost or quite obliterated, becomes obvious. The 
 cell-contents are much less granular, and the granules are re- 
 stricted to that part which lies nearest the lumen. The pro- 
 toplasm is relatively increased in amount, and the nucleus, 
 before disc-shaped and seen with difficulty, becomes more 
 clearly visible, and spherical. 
 
 The gastric glands are of two kinds, cardiac and pyloric. The 
 cardiac glands, which are developed in all parts of the stomach 
 except the pylorus, are simple tubes, of which two or three may 
 open into the same duct. They are lined with epithelial cells, 
 columnar and transparent in the body of the gland, more 
 cubical in the neck. Between these epithelial cells and the 
 supporting membrane is a discontinuous layer of large granular 
 oval cells (parietal cells). The pyloric glands have longer ducts 
 and are more curved. The epithelial cells of the body of the 
 gland are cubical, and resemble those in the neck of the cardiac 
 gland. There are no parietal cells. 
 
 Of the intestinal glands, Brunner's are practically continuous 
 with the pyloric glands, but are more decidedly branched, almost 
 racemose, in character. They are confined to the duodenum. 
 But throughout the whole of both small and large intestine there 
 are great numbers of the simple tubular glands (or crypts) of 
 Lieberkuhn which are lined with columnar epithelium. 
 
 There remain the pancreas and liver. Of these the pancreas 
 is a racemose gland resembling the salivary glands in structure, 
 the lobules containing the dilated ends of the branched tubes. 
 The liver has a different structure, each lobule containing a 
 reticulum of bile canaliculi. Each lobule has, moreover, a double 
 blood-supply. It receives arterial blood from the aorta by the 
 hepatic artery : it receives venous blood from the alimentary 
 canal by the portal vein, The blood is carried off by the 
 hepatic veins to the post-cavaL Traced backwards each hepatic 
 vein is found to result from the union of a great number of 
 factors, the small hepatic veins, and these again receive yet 
 smaller factors, the sub-lobular veins. Around these sub-lobular 
 
Chap. IX.] NUTRITION AND METABOLISM. 181 
 
 veins the lobules cluster, being aggregated round, and, to some 
 extent, supported on minute intra-lobular venules. The lobules 
 are divided from each other by connective tissue, in which there 
 run : (1) the inter-lobular venules, which are the ultimate 
 branches of the portal system; and (2) the inter-lobular arterioles, 
 the ultimate branches of the hepatic artery. Thus each lobule 
 has a double blood-supply, the blood from each source being 
 collected by the intra-lobular venule. In addition to the blood- 
 capillaries within the lobule there are also bile canaliculi. The 
 hepatic cells, of which the lobule is composed, are polygonal in 
 form, and are so arranged as to leave minute channels (the bile- 
 canaliculi) between them. At the margins of the lobules these 
 canaliculi are connected with minute tubes, the inter-lobular 
 bile ducts, which form networks in the inter-lobular connective 
 tissue, and eventually, by continued fusion one with another, 
 pass into the larger bile ducts, by which the product may either 
 be delivered directly to the alimentary canal, or may pass into 
 the storage reservoir, the gall-bladder. Thus the duct of this 
 great gland does not break up into a number of blind and 
 dilated tubes, but into a network of canaliculi. It is not a 
 racemose, but a reticulated gland. 
 
 The Gland Products. The product of the salivary glands is 
 saliva. It is a thin, watery, slightly alkaline fluid, which is seen 
 under the microscope. to contain, besides flattened epithelial cells 
 from the mucous membrane of the mouth, a number of smaller 
 rounded corpuscles which may show amoeboid movements. The 
 salivary fluid is especially secreted by the parotid and the sub- 
 maxillary, the sub-lingual secreting a mucous fluid. An important 
 constituent of the salivary fluid is a ferment called ptyalin. It 
 is one of those curious chemical bodies, a very small quantity of 
 which may induce extensive chemical changes in other sub- 
 stances. The chemical change induced by ptyalin is the con- 
 version of starch ((C 6 H 10 6 ) n ) first into its isomer dextrin (a 
 body of similar chemical composition, but with widely different 
 properties), and then into sugar (glucose, C 6 H 12 6 + H 2 0, or in 
 some cases maltose, C^H^On + HaO). When we learn or 
 remember that starch mucilage will not diffuse through a mem- 
 
182 ANIMAL BIOLOGY. [Part I. 
 
 branous partition, while with a solution of sugar this diffusion 
 (or dialysis) takes place with extreme readiness, we shall see the 
 physiological importance of this change set up by ptyalin. 
 
 A little thin starch mucilage may be prepared by rubbing a 
 few grains of starch into a paste with cold water, and then 
 boiling with more water. In such a solution the presence of 
 the starch may be shown by the deep blue coloration with a 
 solution of iodine. If now a little saliva be added to some of 
 the mucilage in a test tube, and the mixture be kept for some 
 time at a temperature of about 38C., the starch will be con- 
 verted into sugar. Iodine will no longer give a blue coloration. 
 At an earlier stage it may give a violet coloration, due to the 
 mixture of the starch-blue with a claret colour, due to the action 
 of iodine on dextrin. The presence of the sugar may be shown 
 by adding a few drops of a solution of copper sulphate, and then 
 a little potassic hydrate, when, on boiling, an orange-red pre- 
 cipitate of cuprous oxide will be formed. If, however, the solu- 
 tion containing starch and saliva be acitlified so as to contain 
 about *1 per cent. HC1, the starch will remain unchanged. Acid 
 prevents the action of the ferment if it does not destroy it. 
 
 Thus we see that saliva contains ptyalin and mucin, and the 
 question arises, how are these secretions formed in the salivary 
 and mucous glands'? The gland-cells are bathed on the one 
 hand in the plasma of the blood, and on the other hand their 
 secretion is poured forth into the lumen of the tube which leads 
 to the duct. Is the secreted ptyalin or mucin simply extracted 
 from the plasma, or is it manufactured out of the materials of 
 the plasma *? The latter seems the more probable view. They 
 seem to be formed by a process of chemical change or meta- 
 bolism in the gland-cell. Moreover, it is probable that they are 
 not formed directly, but that, during the resting condition, the 
 protoplasm of the cell, in and through its special vitality, elabor- 
 ates the plasma into an intermediate substance, from which the 
 mucin or ptyalin will be elaborated in the active stage. We 
 may call the special intermediate substance mother of mucin, 
 or mother of ptyalin ; but to such intermediate substances in 
 general the term mesostates has been given. 
 
Chap. IX.] NUTRITION AND METABOLISM. 183 
 
 There are two kinds of metabolism. In one the new mole- 
 cules formed are more complex and more unstable than those out 
 of which they are formed. This is termed anabolism, and the 
 intermediate substances which may be formed during the process 
 are called anastates. The process involves the storing up of 
 energy. In the other the new molecules are less complex and 
 less unstable than those out of which they are elaborated. It is 
 called katabolism, the intermediates being termed Jcatastates, 
 The process involves the setting free of energy in the form of 
 heat or visible motion. It is under this latter category that the 
 processes of secretion fall. They are katabolic processes, and 
 the mother of mucin and mother of ptyalin are katastates. On 
 the other hand the conversion of dextrin into glucose is an 
 anabolic process, the molecule of sugar being more complex than 
 that of dextrin, while maltose, as an intermediate substance, is 
 an anastate. 
 
 The product of the cardiac and pyloric glands of the stomach 
 is gastric juice. The fluid is strongly acid (HC1), and contains 
 the ferment pepsin, with small quantities of a rennet ferment. 
 The ferments may be extracted from the minced mucous mem- 
 brane of the stomach with four or five times its bulk of glycerine, 
 which should be allowed to act for some days. The fluid may then 
 be strained off through muslin, and added to from ten to twenty 
 times its volume of dilute (*2 per cent.) hydrochloric acid. Such 
 artificial gastric juice will be found to have no action on starch 
 
 If, however, some chopped white of egg or meat be placed in 
 the solution, and be kept in the warm (38C.), the white of egg 
 will be dissolved, and of the meat only a pulp of connective 
 tissue and fatty matter will remain. And if some of the solution 
 be carefully neutralised and added to fresh milk, the milk will 
 clot, owing to the coagulation by the rennet ferment of casein, 
 which will dissolve through the action of the pepsin if the 
 solution be again acidified and placed in the warm. . 
 
 Whereas saliva, therefore, has the property of converting 
 starch into sugar, gastric juice has the property of converting 
 proteids (e.g. albumin, fibrin, myosin, casein), and such nitro- 
 genous materials as gelatin and chondrin, into bodies called 
 
184 ANIMAL BIOLOGY. [Parti. 
 
 peptones, which are not only very soluble, but very readily 
 diffuse through such a moist membrane as lines the stomach and 
 intestinal canal. The process is one of anabolism or building up 
 of more .complex molecules out of less complex, and in the course 
 of the conversion of proteid into peptone there are probably 
 several mesostates, quite possibly of both orders, katastates and 
 anastates. That is to say, the original proteid molecules may 
 very possibly be first broken down into less and less complex 
 molecules, and then built up anew into more and more complex 
 molecules. 
 
 Of the constituents of gastric juice the hydrochloric acid is 
 probably secreted by the parietal cells of the cardiac glands, 
 which are said to select sodium phosphate and sodium chloride 
 from the blood by the interaction of which the HC1 results. 
 It is questionable, however, whether the change is so simple as 
 this, and, in any case, the reaction is brought about by the 
 inherent vitality of the cells. The pepsin is probably the pro- 
 duct of katabolism (with pepsinogen, mother of pepsin, as a 
 katastate) in the lining cells of both cardiac and pyloric glands. 
 
 The product of the small intestinal glands is termed succus 
 entericus, which is said to be a yellowish alkaline fluid. It is 
 also said that the secretion of Brunner's glands converts proteids 
 into peptones, and that of Lieberkuhn's glands is said, like 
 pancreatic juice, to act upon all the constituents of food. 
 
 The product of the pancreas is called pancreatic juice, a colour- 
 less, transparent, viscid, and alkaline fluid. A pancreatic extract 
 may be made by soaking the gland in water, mincing, and treat- 
 ing with glycerine. The strained extract should be mixed with 
 ten to twenty volumes of a dilute (1*5 per cent.) solution of sodium 
 carbonate and filtered. Such artificial pancreatic juice, best 
 prepared from the pancreas of an animal killed soon after a meal, 
 will be found (1) to act on starch in a similar way to saliva, 
 converting it into sugar. This it does by means of a ferment 
 similar to, or indistinguishable from, ptyalin. The extract 
 will also be found (2) to act on proteids in a similar way to 
 gastric juice, converting them into peptones, or even altering 
 them still further. This it does through the special ferment 
 
Chap. IX.] NUTRITION AND METABOLISM. 185 
 
 trypsin. It will also (3) curdle milk through a rennet ferment, 
 And lastly (4) it has an important action on fatty and oily sub- 
 stances. Not only does it (a.) convert them into an emulsion, 
 that is, cause them to be suspended in very minute globules, like 
 the butter globules in milk, as may be seen by shaking up some 
 oil with the extract, but it also, (b.) not improbably through the 
 action of a fourth ferment (steapsin), splits up the fats into their 
 fatty acids and glycerine. These fatty acids, with an alkaline 
 carbonate, of which there is about -75 per cent, in pancreatic 
 juice, form soluble soaps, and these soaps further aid in emulsi- 
 fying the fats. Pancreatic juice, therefore, (1) converts starch 
 into sugar; (2) converts proteids into peptones; (3) coagulates 
 casein from milk ; and (4) emulsifies and saponifies fats. 
 
 The product, or rather a product, of the liver is bile. This is 
 a viscid, slightly alkaline fluid, green in the rabbit and most 
 herbivorous animals, reddish in the carnivora. It contains (1) 
 certain organic bile salts (sodium glycocholate and taurocholate) ; 
 (2) certain organic colouring matters ; and (3) fatty matter and 
 cholesterin (an univalent alcohol), together with mucous and 
 inorganic salts. Its main functions in digestion would seem 
 to be (1) emulsifying fatty matters; and (2) precipitating the 
 gastric peptones, together with pepsin and bile acids. In 
 addition to these functions it assists digestion by moistening the 
 mucous membrane and so facilitating absorption, and by acting 
 as a natural purgative. 
 
 Like the other secretions before considered, the bile is not 
 derived as such from the blood, but is elaborated out of the 
 materials of the blood by a process of katabolism, and it is 
 possible that there may be hitherto undiscovered katastates. 
 The process of secretion goes on most rapidly during digestion : 
 but it does not cease at other times. At such times the bile, 
 instead of passing down the common bile-duct into the duodenum, 
 regurgitates through the cystic duct into the gall-bladder, where 
 it is stored up for future use. 
 
 Having thus considered the general structure of the glands, 
 the nature of their products, and the part they play in digestion, 
 we may now pass on to consider the process of digestion 
 
i86 ANIMAL BIOLOGY. [Part I. 
 
 as a whole, and the manner in which the elaborated products 
 are absorbed. 
 
 Digestion and Absorption. The organic food-stuffs, exclusive, 
 that is, of water and saline matters, are of two kinds, nitrogenous, 
 such as the proteids and gelatins ; and non-nitrogenous, such as the 
 carbo-hydrates, starch and sugar, on the one hand, in which the 
 oxygen and hydrogen are in due proportion to form water, and 
 on the other hand oil and fat, in which there is a relative 
 deficiency of oxygen. These nitrogenous and non-nitrogenous 
 food-stuffs have to be consumed in such proportion as to make 
 good the material lost to the body in nitrogenous excreta, in 
 water, and in carbonic acid gas. 
 
 In the mammalia they are prepared for further digestion by 
 mastication. In the mouth the food is mixed with saliva, which 
 has no action on the fats or proteids, but converts the starch 
 into glucose. Besides having this direct chemical action, the 
 saliva serves to moisten the food and make it more readily 
 swallowed. This process of deglutition is a somewhat com- 
 plicated one, bringing many muscles into play. The morsel of 
 food is pushed back into the pharynx, where it is grasped and 
 pushed onwards, the circular muscles of the part of the tube in 
 front of it relaxing while the muscles behind it are successively 
 contracted. Thus it passes down the oesophagus into the 
 stomach. 
 
 There it is for a while imprisoned by the firm closure of the 
 cardiac and pyloric apertures, which only momentarily open for 
 the admission of food or the emission of its contents. By the 
 action of the strong gastric muscles the food is rolled about and 
 thoroughly mixed with gastric juice, the acidity of which at once 
 stops the further action of ptyalin on starch. Gastric digestion 
 thus succeeds salivary digestion. The initiation of the con- 
 version of starch into glucose ceases, but the proteids are con- 
 verted into peptones, and the fats are, by the breaking down 
 of their proteid framework, set free, and incorporated with 
 the food, though they are not further acted upon. Thus 
 the food is converted into chyme, a fluid of the consistency of 
 pea-soup, which is allowed to pass through the pylorus into the 
 
Chap. IX.] NUTRITION AND METABOLISM. 187 
 
 duodenum. At the same time, however, the process of absorp- 
 tion begins. Probably some little soluble matter is absorbed in 
 the mouth. But in the stomach a little of the peptones there pre- 
 pared, together with the glucose prepared by the saliva, and other 
 diffusible fluids, may pass by osmosis through the membranous 
 walls of the stomach, and so entering the blood of the capillaries, 
 are carried to the portal vein, and thus to the liver. 
 
 When it passes into the duodenum the acid chyme is subject 
 to the action of the alkaline fluids, bile, pancreatic juice, and 
 succus entericus. The fats are emulsified and partly saponified. 
 Under the influence of the pancreatic fluid the conversion of 
 starch into su^ar is renewed. The bile causes the precipitation 
 of the peptones, while the gastric pepsin is also precipitated and 
 rendered inert, and thus the field is left open for the digestive 
 influence of the trypsin of the pancreatic fluid. Thus, as it passes 
 onwards through the small intestine by the wave-like peristaltic 
 contractions of its muscular walls, the three chief constituents of 
 the food, the proteid, the starch, and the fat, are so acted upon as 
 to be converted, the first two into diffusible fluids, the fat partly 
 into a soluble soap, but chiefly into an emulsion. 
 
 Absorption of these materials, which are still, it must be re- 
 membered, strictly speaking outside the body, is in the small 
 intestine effected by the villi. Each of these minute processes 
 has in its centre the termination of a lymphatic canal, which is 
 here called a lacteal ; and surrounding this is a close plexus of 
 blood-capillaries. The diffusible fluids pass through the epi- 
 thelial layer, and are caught up by the blood-stream in the 
 capillaries and hurried away to the liver. The emulsified fats, 
 and such soluble matters as have escaped absorption into" the 
 blood-stream, pass into the lacteal rootlet, and so into the lym- 
 phatic system. This passage is aided by the alternate shortening 
 and elongation of the villus. By the former process the con- 
 tained fluid is forced through the lacteals, whence its return is 
 prevented by valves : by the latter the lacteal rootlet in the 
 villus tends to suck in the surrounding material. But still the 
 question how the fats pass into the lacteals, through the cells or 
 between the cells, is not well understood. It certainly is largely 
 
i88 ANIMAL BIOLOGY. [Parti. 
 
 through the agency of the cells themselves that the process is 
 effected; and it may be through the intervention of certain 
 amoeboid cells, which lie at the bases of the columnar epithelial 
 cells, and send up long processes between the cells. In any case 
 it is eminently probable that neither the absorption of emulsified 
 fats, nor the diffusion of soluble peptones and sugar is merely 
 a mechanical process. Both are probably processes effected 
 through the vitality of the tissues ; and there are good reasons 
 for believing that the epithelial cells themselves exercise a sort 
 of digestive property upon the ingested fat. 
 
 The absorption of fatty matters is probably completed in the 
 small intestine where alone the villi are developed ; but the 
 absorption of diffusible matters may probably continue in the 
 large intestine where water is largely absorbed. And one may 
 well suppose that in the large csecum of the rabbit some further 
 process of digestion, not at present well understood, is effected. 
 As they pass onwards the contents of the large intestine become 
 more solid and acquire the character of faeces, and are finally 
 expelled from the body. Notwithstanding the very large quantity 
 of bile poured forth into the small intestine, the amount of bile 
 salts to be found in the faeces is quite insignificant. Whence it 
 has been inferred that the biliary salts are decomposed and the 
 taurine and glycine thus set free are reabsorbed. 
 
 Before leaving the subject of absorption attention may be 
 drawn to the fact that, in the lower vertebrates, the epithelial 
 cells would seem to be capable of pushing forth amoeboid absorp- 
 tive processes, and that leucocytes have been shown in some 
 cases to migrate through the epithelium into the lumen of the 
 alimentary canal, and thus come in contact with the prepared 
 food. They have therefore been called phagocytes. Other pha- 
 gocytes may take up food particles which have passed through 
 the epithelium into the connective tissue. Phagocytes are also 
 said to absorb, and thus render innocuous, noxious substances, 
 or portions of tissues which have undergone retrogressive meta- 
 morphosis within the body. 
 
 Preliminary Metabolism of Absorbed Products. We have thus 
 seen how the products of digestion are absorbed, partly into the 
 
Chap. IX.] NUTRITION AND METABOLISM. 189 
 
 blood and partly into the lymphatic system. We have now to 
 note that these products do not pass unaltered into the general 
 system of the circulation. For the blood which receives the 
 diffusible products has to pass through the liver before it reaches 
 the heart; and the chyle, as the milky fluid in the lacteals is 
 termed, has to pass through the lymphatic glands before it 
 passes by the thoracic duct into the blood-stream. 
 
 The exact changes which go on in the liver have still to be 
 fully worked out. It is certainly the seat of most important 
 chemical changes, and the high temperature of the blood which 
 leaves this organ seems to indicate that katabolic changes are in 
 excess. Of these chemical changes perhaps the most important 
 is the metabolism of nitrogenous matters. From this meta- 
 bolism, effected by the vital activity of the hepatic cells, three 
 products res'ult : (1) a non-nitrogenous starchy substance, 
 readily convertible into glucose and termed glycogen; (2) the 
 slightly nitrogenous bile : and (3) the highly nitrogenous urea-, 
 or a mesostate thereof. Of these the bile, as we have already 
 seen, is poured out into the alimentary canal ; the urea passes 
 away in the blood to be eliminated by the kidneys ; the destina- 
 tion of the glycogen is still a imtter of uncertainty. By some it 
 is held to be at once converted into glucose, which, by oxida- 
 tion in the lungs, is a source of heat to the body. By others it 
 is regarded as a store of carbo-hydrate ready to be converted 
 into sugar and drawn upon by the organism when need arises. 
 By others it is regarded as a mesostate in the formation of fat. 
 
 In any case we see that the series of metabolic changes which 
 the absorbed material has to undergo are initiated in the liver 
 before the portal blood reaches the heart. So, too, do important 
 metabolic changes take place in the chyle before it passes into 
 the blood-stream by the thoracic duct. 
 
 The lymph may be regarded as to a large extent the overflow 
 of the blood, the plasma of which has exuded into the tissues, 
 and has been drained off by the lymphatic vessels. In the 
 region of the alimentary canal, the lymph, during digestion, 
 becomes chyle, a milky, slightly alkaline fluid, the whiteness 
 and opacity of which is due to the emulsion absorbed by the 
 
190 ANIMAL BIOLOGY. [Part I. 
 
 lacteal rootlets. In the course of the lacteals there are numer- 
 ous lymphatic glands, many of which lie in the mesentery. 
 Glandular tissue of similar character is developed in the walls 
 of the intestines, and forms in parts oval white patches (Peyer's), 
 and is well seen in the appendix and sacculus rotundus. In 
 them there are rounded masses of lymphatic tissue (follicles), or 
 elongated strands (medullary cords) of similar tissue, crowded 
 with leucocytes or pale blood corpuscles. Around these 
 follicles and cords there are lymph-spaces and lymph-channels. 
 The metabolism that goes on in these lymphatic glands is 
 not well understood. But there seems no doubt that the 
 chyle from the thoracic duct (that is, after it has passed 
 through the glands), readily coagulates, owing to the formation 
 of fibrin, contains an immense number of white corpuscles 
 (leucocytes), and some red corpuscles, which may not improb- 
 ably have escaped from the vascular system in the spleen or 
 elsewhere. The chyle from the lacteals before passing through 
 the lymphatic glands does not readily coagulate, has much 
 fewer pale corpuscles, and no red corpuscles. 
 
 Further Metabolism. In the liver and in the lymphatic glands, 
 however, the metabolism of the products of digestion is scarcely 
 more than begun. By the general blood-system it is carried in 
 a more or less altered condition throughout the body, and has 
 to make good the tissue wasted by its vital activity. Of the 
 complex metabolism that takes place in the tissues we at 
 present know but little. We may, however, indicate in a few 
 words the direction in which the experiment, observation, and 
 inference of modern physiology would seem to be tending, 
 
 We have already seen that in the secreting organs the proto- 
 plasm of the secreting cells elaborates from the plasma, during 
 the resting stage, a material, the mother of ferment, from which 
 during the period of activity the ferment itself is produced by 
 further katabolic changes. In a somewhat similar way it would 
 seem that, in a muscle, the protoplasm of the muscle fibre is 
 occupied during the resting stage in elaborating from the plasma, 
 with the aid of the oxygen brought by the red corpuscles, a 
 material called inogen, which it incorporates with itself. During 
 
Chap. IX.] NUTRITION AND METABOLISM. 191 
 
 the period of activity this inogen undergoes katabolism with 
 the formation of carbonic acid and sarcolactic acid, and this pro- 
 cess is accompanied by the shortening of the muscle fibre. In 
 any case it seems clear that the katabolism of the muscle fibre 
 at the moment of contraction, by which carbonic acid and sarco- 
 lactic acid are produced, is not the result of the direct oxidation 
 of the substance of the fibre at the expense of the oxygen of 
 the blood. A muscle containing no free oxygen will contract 
 in an atmosphere perfectly free of oxygen. The change is a true 
 katabolism or breaking down of more complex into less com- 
 plex molecules. The katabolism of the inogen may be likened 
 to an explosion, where the explosive material, more or less 
 suddenly, by a rearrangement of molecular groupings, falls from 
 a state of unstable to one of stable equilibrium. 
 
 Of the metabolism of nervous tissue we know still less than 
 of that which occurs in muscular tissue. But it is probable 
 that here too, by the vital activity of the protoplasmic element 
 in each cell, there is produced a mesostate which, at the moment 
 of activity, passes by katabolism into waste products, so called, 
 of less complex composition. 
 
 A metabolism similar in principle goes on in fat cells. Here 
 the inherent vital activity of the protoplasm of certain connec- 
 tive tissue cells, causes the production of this peculiar storage 
 tissue. It is exceedingly improbable that there is anything like 
 a direct deposition in the fat cells of fat absorbed into the 
 lacteals. Each individual has, by its vital activity, to build up 
 its own tissues for itself. 
 
 So far, however, we have assumed in each cell a protoplasmic 
 element which initiates the temporary or more enduring storage 
 of a mesostate. How is this protoplasm itself elaborated ? At 
 present we do not know. We may well suppose that it results 
 from a process of metabolism, and is probably preceded by one 
 or more anabolic mesostates. We know that the protoplasmic 
 pale corpuscles or leucocytes are largely produced in the lym- 
 phatic glands. But how ? 
 
 In any case we may fairly regard the complex chemical 
 changes which go on within the body as a series of ascending 
 
192 
 
 ANIMAL BIOLOGY. 
 
 [Part I. 
 
 and descending steps, which together form a pyramid. The 
 summit of that pyramid is protoplasm, which the student must 
 not regard as a body of unchanging chemical composition, but 
 rather as a group of bodies, each the terminal product of a 
 series of anabolic processes. It is scarcely probable that the 
 protoplasm of muscle, nerve, and glandular tissue is identical. 
 But in each case it is the highest product of a special line of 
 anabolism. And in general we may say that protoplasm is the 
 summit of a step-faced metabolic pyramid. The ascending steps 
 are anastates ; the descending steps katastates. The ascent of 
 the pyramid involves the absorption of energy or work done 
 on the matter involved. The descent of the pyramid involves 
 the setting free of energy, in the form of visible motion or of 
 heat, or work done by the matter involved. But the pyramid 
 
 Living 
 Protoplasm. 
 
 Anabolic 
 Mesostates. 
 
 Katabolic 
 Mesostates. 
 
 FIG. 58. METABOLIC LIFE-PYRAMID. 
 
 is not symmetrical. Its katastatic side, with descending steps, 
 is longer than its anastatic side with ascending steps ; for the 
 food starts at a higher level of energy and complexity than that 
 
Chap. IX.] 
 
 NUTRITION AND METABOLISM. 
 
 193 
 
 of the ultimate products of waste. The katastatic surplus marks 
 the energetic capital with which the animal is credited. In a 
 complete life-pyramid, however, this will not be so ; for the 
 lower anastatic stages by which the food is raised to its high 
 level of energy and complexity falls within the life-work of the 
 plant. The accompanying figure (58) will roughly symbolise 
 this conception, the notch between plant protoplasm and animal 
 protoplasm being intended to convey the fact that in the animal 
 the food prepared by the plant is partly broken down into lower 
 products before it is finally built up into higher. 
 
 The following nitrogenous substances are among those found 
 in animal tissue. They are arranged in order of chemical 
 degradation : 
 
 Cerebrin, 
 
 Tyrosin, 
 
 Hippuric Acid, 
 
 Leucin, 
 
 Guanin, 
 
 Hypoxanthin, 
 
 Kreatin, 
 
 Kreatinin, 
 
 Xanthin, 
 
 Uric Acid, 
 
 Allantoin, 
 
 Urea, 
 
 C 9 H 9 NO.. 
 C 6 H 13 N0 2 . 
 C 6 H 6 N 6 0. 
 C 6 H 4 N 4 0. 
 
 c 4 H 9 N 3 o 2 . 
 
 C 4 H 7 N S 0. 
 
 C 6 H 4 N 4 2 . 
 C 6 H 4 N 4 3 . 
 C 4 H 6 N 4 3 . 
 CH 4 N 2 0. 
 
 13 
 
CHAPTEE X. 
 
 THE HEART AND CIRCULATION. 
 
 Preliminary Considerations. The main organ of circulation 
 is the heart ; the circulating fluids are the blood and the lymph ; 
 the channels of circulation are the blood-vessels and lymphatics. 
 The principal vessels which convey blood from the heart are the 
 arteries ; when these are much reduced in size, through oft- 
 repeated branching, they are called arterioles ; the arterioles 
 end in an anastomosing network of very fine tubes, which ramify 
 throughout the organs of the body these are the capillaries. 
 The blood returning from the capillaries collects into venules ; 
 by oft-repeated unions with each other the venules form veins, 
 which convey the blood back to the heart from which it started. 
 
 The tissues may be said to be irrigated by the blood which 
 flows within the capillaries, through the delicate. walls of which 
 some of the plasma of the blood exudes and collects in the 
 crevices and in the spaces between the connective tissue strands. 
 The fluid that thus exudes, which contains less albumen than 
 blood plasma and some products of the waste of the tissues, 
 forms the lymph, and the crevices are the so-called lymph- 
 rootlets. From the lymph-rootlets the lymph passes into fine 
 tubes with endothelial (epithelioid) lining called lymph capil- 
 laries ; the larger vessels which receive the lymph from these 
 capillaries are the lymphatics. In the course of the lymphatics 
 are the lymphatic glands. The lymphatics in the lower verte- 
 brates are not infrequently in connection with large lymph-spaces, 
 
 194 
 
Chap. X.] THE HEART AND CIRCULATION. 195 
 
 such as those beneath the skin and that at the back of the 
 pleuro-peritoneal cavity in the frog the latter communicatr j 
 by means of stomata with the coelom. 
 
 In the frog, but not in the fowl or rabbit, there are four 
 lymph-hearts, whose function it is to pump the lymph back from 
 the lymphatic system into the veins. The anterior pair lie close 
 to and beneath the transverse processes of the third vertebra. 
 They communicate with the innominate veins. The posterior 
 pair lie one on each side of the urostyle about one-third of an 
 inch from its termination. They communicate with ^ie renal- 
 portal veins. In each case valves prevent the passage )f blood 
 from the veins into the rhythmically contractile lym^li-hearts. 
 In the bird and the mammal the lymph returns to the veins 
 through the .thoracic ducts or duct without the intervention of 
 lymph-hearts. 
 
 Thus the course of the blood is, disregarding the lymph 
 overflow, heart, arteries, arterioles, capillaries, venules, veins, 
 heart. 
 
 From what has been said in the last chapter, however, it 
 will be understood that the blood is no mere inert fluid which 
 returns to the heart in much the same state in which it left 
 it, but is throughout its course undergoing continuous meta- 
 bolism. Nor are the blood-vessels simple inelastic tubes. Each 
 artery and vein has, besides its smooth lining of endothelial 
 (epithelioid) cells, a triple coat of elastic tissue, in the mid layer 
 of which unstriped muscular fibres may be developed. In the 
 arterioles these muscular fibres are numerous. In the venules 
 the elastic coat is much reduced ; and in the capillaries both it 
 and the muscular coat are absent. So that we may say that the 
 arteries are very thick-walled and highly elastic ; the arterioles 
 are moderately thick-walled, elastic, and contractile ; the capil- 
 laries have only an epithelioid (endothelial) wall; the venules 
 are thin-walled and slightly elastic ; the veins are relatively 
 thin-walled and slightly elastic. The veins (of the rabbit, 
 e.g.) are, moreover, in some parts of the body, where they 
 are liable to pressure, provided with flap-like valves which 
 offer no resistance to blood flowing towards the heart, but close 
 
I 9 6 ANIMAL BIOLOGY [Part I. 
 
 and block the way to blood attempting to flow in the opposite 
 direction. 
 
 Into this system of tubes the blood is forced at each con- 
 traction (systole) of the muscular walls of the ventricles of the 
 heart. The arteries are swollen with blood, the heart-pressure 
 being to some extent stored up in the stretched fibres of the 
 elastic coats of these vessels. After the systole of the ventricles 
 is over, the blood is passed on through the arteries and arterioles 
 to the capillaries. And now the muscular contractility of the 
 walls of the arterioles comes into play. Nerves from the sym- 
 pathetic system are distributed to their muscular walls. Sup- 
 pose, then, that an organ is quiescent and in need of but little 
 nourishment. Through the influence of the nerve-fibres the 
 muscular walls of the arterioles contract ; less blood is supplied, 
 and the blood-pressure in the capillaries falls. But now suppose 
 that the organ is active and in need of much blood and nourish- 
 ment. The walls of the arterioles are caused to relax ; a full 
 tide of blood flows to the part ; blood-pressure increases in the 
 capillaries ; there is more exusion of plasma through the epi- 
 thelioid walls, and nutrition increases. 
 
 When the blood reaches the capillaries, unless the arterioles 
 be very widely distended, the wave-like flow, noticeable when a 
 great artery is cut, has ceased and given place to a continuous 
 flow. The elasticity of the walls of the elongated tubes, and the 
 distribution of the pressure over a large area of capillaries, has 
 effected this. The pulse that we feel in an artery of the human 
 wrist is the shock of systole propagated through the vessel, and 
 must not be confounded with the onward flow of the blood. 
 
 It only remains to be noticed, in these preliminary remarks, 
 that the partial vacuum in the thorax of the rabbit, and sundry 
 other minor causes, may slightly add to or slightly impede the 
 flow of the blood. 
 
 The Cod Fish. The heart of the cod (Fig. 59) consists of a 
 thin-walled sinus venosus (s. v.\ which receives blood from the 
 hepatic veins (he. v.) and the dudus Cuvieri (d. c.), of which only 
 the right vein receives an inferior jugular factor (?'. ;. v.). Below 
 
Chap. X.] 
 
 THE HEART AND CIRCULATION. 
 
 197 
 
 and slightly in front of the sinus venosus is the auricle (au.), 
 through which all the blood passes on its way to the thick-walled 
 ventricle (v.). From the ventricle it passes into the thick-walled 
 bulbils arteriosus (b. a.), and thence to the ventral aorta (v. ao.). 
 Note that there is one undivided auricle and one undivided 
 ventricle. 
 
 .Vtt. ep.W. ef.tr.a. ac v 
 
 af.lr.7T 
 
 v. 
 
 FIG. 59. DIAGRAM OF HEART AND GREAT VESSELS OF COD-FISH. 
 a. c. v. Anterior cardinal vein. a. p. Artery supplying pelvic fins. af. br. a. 
 Afferent branchial arteries, au. Atrium or auricle. &. a. Bulbus arteriosus. 
 car. Carotid artery, cce. a. Caeliac artery, d. ao. Dorsal aorta, d. c. Ductus 
 Cuvieri. ef. "br. a. Efferent branchial arteries, ep. br. a. Epibranchial artery. 
 he. v. Hepatic vein. hy. Hyoidean artery, i. j. v. Inferior jugular vein. 
 me. a. Mesenteric artery, p. c. v. Posterior cardinal vein. ps. Pseudo- 
 branchia on inner surface of operculum. s. a. Subclavian artery, s. v. Sinus 
 venosus. sp. v. Spermatic vein. tr. Transverse vessel completing circulus 
 cephalicus. v. a. Ventral aorta, v. Ventricle. 
 
 The arrangement of the great vessels is shown diagrammatic- 
 ally in Fig. 59. The ventral aorta (v. ao.) gives off on either 
 side four afferent branchial arteries (af. br. a.) to the four complete 
 gills. From the gills there pass on each side four efferent bran- 
 chial arteries (ef. br. a.), which open into a vessel (epibranchial 
 artery ', ep. br. a.), running along the dorsal ends of the gill arches. 
 Traced forwards the epibranchials give off the carotid arteries, 
 and curving round unite to complete the circulus cephalicus. 
 Traced backwards the epibranchials unite posteriorly to 
 
198 ANIMAL BIOLOGY. [Parti. 
 
 form the dorsal aorta (d. ao.\ which runs backwards beneath 
 the spine, and becomes the caudal artery. But before thus 
 uniting, each epibranchial gives off a subclavian artery (s. a.) 
 to the pectoral fin, while the right epibranchial gives off also 
 a cceliac (cce. a.) and a mesenteric artery (me. a.) to supply the 
 stomach and intestines. 
 
 From the stomach and intestines the blood is delivered by a 
 portal vein to the liver, and thence passes by the hepatic veins 
 (he. v.) to the sinus venosus. The blood from the posterior 
 parts of the body collects in posterior cardinal veins (p. card. v.\ 
 which pass through the substance of the kidney. 1 The blood 
 from the upper parts of the head collects in anterior cardinal 
 veins (a. card. v.). The anterior and posterior cardinals of 
 each side unite to form ductus Cuvieri (d. c.), which pass down- 
 wards to the sinus venosus. The left ductus receives near its 
 dorsal aspect a spermatic or ovarian vein from the organs of 
 generation. The right ductus receives near its opening into 
 the sinus venosus an inferior jugular vein from the lower 
 parts of the head. Note that the epibranchials, by curving 
 round and uniting anteriorly, form a closed circle (circulus 
 cephalicus), which lies beneath the skull, and may be seen by 
 removing the lower jaw and the mucous membrane of the roof 
 of the mouth. In the figure the afferent and efferent branchial 
 arteries and the ductus Cuvieri are placed side by side. They 
 are connected by the capillaries of the gills, and curve upwards 
 from the ventral aspect, where the heart lies, to the dorsal 
 aspect, along which run the epibranchials and cardinals. The 
 vessels containing arterial blood are shaded. 
 
 The Frog. The heart of the frog consists of four divisions : 
 
 1. Sinus venosus: dorsal, thin- walled, receiving venous blood 
 by the precavals and postcaval. 
 
 2. Atrium : anterior, thin-walled, divided into two chambers : 
 
 (a) right auricle, receiving venous blood from the sinus venosus; 
 
 (b) left auricle, receiving arterial blood from the pulmonary 
 vein. 
 
 1 That of the left side is aborted for some distance in the kidney. 
 
Chap. X.] THE HEART AND CIRCULATION. 199 
 
 3. Ventricle : posterior, muscular, undivided, receiving venous 
 and arterial blood simultaneously from the right and left auricles. 
 
 4. Truncus arteriosus : ventral, anterior, and to the right ; 
 thick-walled and muscular, receiving successively venous and 
 arterial blood from the ventricle. 
 
 To ensure the passage of blood in the right direction there are 
 the following valves : 
 
 (a) Sinu-auricular valve : between the sinus venosus and right 
 
 auricle. 
 
 (b) Auricula-ventricular valve : between the atrium and the 
 
 ventricle prevented from flapping back into the auricles 
 by fine tendinous cords. 
 
 (c) Semi-lunar valves in the truncus arteriosus (two sets). 
 
 (d) A median longitudinal valve, attached to the dorsal wall of 
 
 the truncus arteriosus, and partially dividing it into two 
 
 The arrangement of the great vessels which enter and leave 
 the heart will be seen from the diagram Fig. 60, which shows 
 the ventral aspect. The truncus arteriosus (tr. a.) bifurcates, and 
 each branch gives rise to three arteries. The carotid (ca.) 
 anteriorly, the systemic aorta (sy. ao.) in the middle, and the 
 pulmo-cutaneous (p. cu.) posteriorly. The carotid supplies the 
 head and brain. On it is developed a carotid gland (ca. gl.\ 
 a muscular vesicle with septa in its interior. Close to the 
 carotid gland the artery gives off a lingual branch (li.) to the 
 tongue. The systemic aorta of each side curves round to meet 
 its fellow in the middle line. It gives off the vertebral (ve.) and 
 the subclavian arteries (s. cl. a.) to the vertebral column and 
 fore-limb. At or near the point where the two systemic arches 
 meet to produce the dorsal aorta which runs down the roof of 
 the pleuro-peritoneal cavity, a coeliaco-mesenteric (c. m.) is given 
 off to supply the stomach and intestines. It soon branches into 
 a codiac (c.) to stomach and liver, and a mesenteric (m.). A small 
 inferior mesenteric artery supplies the base of the large intes- 
 tine. 
 
 The blood is returned from the stomach and intestines by the 
 portal vein (por.), which breaks up into a capillary plexus in the 
 
20O 
 
 ANIMAL BIOLOGY. 
 
 [Part I. 
 
 liver. It is formed by the union of two factors ; the gastric (g.) 
 from the stomach, and the lieno-intestinal (I. i.) from the intes- 
 tines. Blood from the hinder part of the body also reaches the 
 
 s cl 
 
 FIG. 60. HEART AND GREAT VESSELS OF FROG. 
 
 an. ab. Anterior abdominal, an. Auricle, br. Bracliial vein. c. Cceliac artery, 
 c. m. Coeliaco-mesenteric artery, ca. Carotid artery, ca. gl. Carotid gland. CM-. 
 Cutaneous artery, d. ao. Dorsal aorta, ex. j u. External jugular, g. Gastric vein. 
 he. Hepatic vein. in. Innominate vein, in.ju. Internal jugular vein. U. Lingual 
 artery. I. i. Lieno-intestinal vein. lg. Lung. m. Mescnteric artery, m. c. 
 Great cutaneous vein. p. Pulmonary artery, p. cu. Pulmo-cutaneous artery. 
 por. Portal vein. pr. c. Precaval vein. pt. c. Postcaval vein. s. cl. a. Subclavian 
 artery, s. cl. v. Subclavian vein. s. sc. Subscapular vein. sy. ao. Systemic aorta. 
 tr. a. Truncus arteriosus. ve. Ventricle, ver. Vertebral artery. 
 
 liver by the anterior abdominal vein (an. ab.). A connecting- 
 branch unites the anterior abdominal and the portal, and a small 
 factor comes from the truncus arteriosus. From the liver the 
 blood passes into the postcaval vein (pt. c.) by the hepatic veins (he.). 
 The postcaval delivers to the sinus venosus blood from the 
 large kidneys ; the blood from the head and fore-limbs reaches 
 the sinus venosus by two precavals (pr. c.), one on either side. 
 
Chap.X.] 
 
 THE HEART AND CIRCULATION. 
 
 Each is formed by the union of three factors, external jugular 
 (ex. ju.) from the jaw and tongue, innominate (in.), and sub- 
 clavian (s. d. v.). The innominate has two factors, internal 
 jugular (in.ju.) from the brain, etc., and subscapular (s. sc.) from 
 the region of the shoulder. The subclavian has also two factors, 
 the brachial (br.) from the fore-arm and manus, and the great 
 cutaneous (m. c.) chiefly from the skin of the back. 
 
 Fig. 61 shows diagrammatically the arrangement of the 
 vessels in the pelvic region, seen from the ventral aspect. The 
 dorsal aorta (d. ao.) gives off vessels to the kidneys (one side 
 shown), and then bifurcates into 
 two iliac arteries (il.\ which, 
 after giving off a hypogastric 
 branch (liy.) to the bladder and 
 abdominal walls, continues on to 
 supply the leg. From the leg 
 the blood is returned by two 
 veins, the femoral running down 
 the front of the thigh, and the 
 sciatic running alongside the 
 artery in the back of the 
 thigh. These two are united by 
 an anastomosing branch (a. b.). 
 The femoral then branches into 
 two, the pelvic vein (pe.\ and the 
 renal portal (r. p.\ into which the 
 
 Sciatic also falls. The pelvic a - b - Anastomosing branch between 
 
 . , , . , , sciatic, and femoral veins, an. ab. An- 
 
 Veins, On either Side, pass towards terior abdominal vein. d. ao. Dorsal 
 
 the ventral aspect, and unite to aorta - d - l Dorso-iumbar vein. fe. v. 
 
 P . , i i i Femoral vein. hy. Hypogastric artery. 
 
 form the anterior abdominal vein, ilt Iliac artery- pe . Pelvic vein . 
 
 which, as We have Seen, passes P f - c - Postcavalvein. re. a. Renal artery. 
 ,1 -i . mi , . i re. v. Renal vein. r. p. Renal-portal vein. 
 
 to the liver. The renal-portal sc> Vm Sciatic vein> 
 
 passes to the outer edge of the 
 
 kidney, and breaks up into a capillary plexus in that organ. 
 
 It receives a dorso-lumbar vein from the spinal canal and the 
 
 body-wall. 
 
 Let us now see, with the aid of Fig. 62, I. n. m., how the 
 
 Fio. 61. RENAL-PORTAL AND PELVIC 
 VESSELS : FROG. 
 
ANIMAL BIOLOGY, 
 
 [Part I. 
 
 three great arterial trunks are developed, and how a passage is 
 made from the fish condition to the frog condition through the 
 tadpole condition. Fig. 62, i., shows the four branchial arches 
 
 a.ao. 
 
 U, 
 
 Fia. 62. DEVELOPMENT OF AORTIC ARCHES HEART (FROG). 
 
 i. Fish stage, n. Tadpole stage, m. Frog stage, iv. Heart of frog dissected 
 so as to show the internal structure, all detail being omitted. 
 
 In i., ii., in. ca. Carotid artery, d. ao. Dorsal aorta, h. Heart, liy. Hyoidean 
 artery. U. Lingual artery. 1. Lung. pu. Pulmonary artery. 1, 2, 3, 4. 
 Branchial arteries. 
 
 In iv. ca. Carotid opening, p. c. Pulmo-cutaneous opening, p. v. Opening of 
 pulmonary vein. s. ao. Opening of systemic aorta, s. v. Opening of sinus 
 venosus. v. Cavity of ventricle. 
 
 of the fish, represented here as continuous, the afferent and 
 efferent branchial vessels being, for the sake of simplicity, 
 merged into one (cp. Fig. 59). The dotted arch in front is the 
 hyoid arch, not found in the adult fish. Fig. 62, II., shows the 
 state of things in the tadpole. The hyoid arch is developed and 
 gives origin to a lingual artery, the dorsal connection with the 
 carotid part of the epibranchial being slender. The first 
 branchial arch is well developed, and gives rise to the carotid. 
 The second branchial arch is also well developed, and gives rise 
 to the systemic aorta. A slender dorsal connecting branch 
 represents this part of the epibranchial. The third branchial 
 
Chap. X.] THE HEAR7" AND CIRCULATION. 203 
 
 arch is well developed below, but gives rise to a large posterior 
 pulmonary branch running to the lung. The dorsal connection 
 with the epibranchial (systemic arch) is slender. The fourth 
 arch is incomplete above, but joins by a slender tube (in the 
 newt if not in the frog) the pulmonary branch, of which pul- 
 monary branch the fourth arch is indeed most probably the 
 true parent, the connection with the third arch being subse- 
 quent. Fig. 62, in., shows the state of things in the adult frog. 
 The hyoid arch has atrophied, and the lingual artery derived 
 from it has become connected with the carotid. The slender 
 connection between the carotid and the systemic aorta has 
 atrophied, as has also the dorsal connection of the third arch, 
 and the whole of the fourth. Thus in the adult frog 
 
 The lingual artery is derived from the hyoidean arch. The 
 carotid artery is derived from the first branchial arch. The 
 systemic aorta is derived from the second branchial arch. The 
 pulmo-cutaneous artery is believed to be derived partly from 
 the third branchial arch, and partly from the fourth branchial 
 arch. 
 
 We may now consider, with the aid of Fig. 62, iv., how the 
 heart works. The truncus arteriosus, ventricle, and auricles 
 are partially laid open. By the contraction of the atrium 
 venous blood from the sinus venosus (s. v.) and right auricle, 
 and arterial blood from the pulmonary vein (p. v.) and left 
 auricle, are received simultaneously into the cavity of the 
 ventricle (v.). By the contraction of the ventricle venous and 
 arterial blood pass successively into the truncus arteriosus. 
 The longitudinal swing-valve in the truncus arteriosus is so 
 arranged that blood passing to the pulmo-cutaneous orifice 
 (p. c.\ passes to the left, while blood passing to the carotid (ca.) 
 and systemic (s. ao.) orifices passes to the right. 
 
 Now since 
 
 1. The opening into the truncus arteriosus is well over to 
 
 the right side of the ventricle ; 
 
 2. The blood in the ventricle is arterial to the left and 
 
 venous to the right, the two not completely mixing 
 owing to the spongy texture ; 
 
204 ANIMAL BIOLOGY. [Parti. 
 
 3. (a) The resistance to the blood-flow is maximum in the 
 carotid trunk owing to the small size of the 
 vessels and the carotid gland ; 
 
 (b) The resistance to the blood-flow is minimum in the 
 
 pulmo-cutaneous trunk owing to the short course 
 and large capillaries of the lungs ; 
 
 (c) The resistance in the systemic aorta being medium ; 
 therefore it follows that 
 
 1. Venous blood passes first along the line of minimum 
 
 resistance, to the left of the swing-valve, to the 
 lungs ; then the capillaries of the lungs being 
 gorged with blood, 
 
 2. Mixed blood flows next along the line of medium 
 
 resistance to the system ; 
 finally, as these capillaries also become gorged, 
 
 3. Arterial blood flows last along the line of maximum 
 
 resistance to the brain and head. 
 
 It will be understood that, as the capillaries of the lungs 
 become gorged, the resistance in the pulmo-cutaneous artery rises 
 so as to exceed that in the systemic aorta ; wherefore the blood 
 passes along that branch. And as it flows along the right of the 
 longitudinal swing-valve, it tends to fold it down to the left 
 over the pulmo-cutaneous orifice. Then as the systemic capil- 
 laries in turn become gorged, the resistance along that line 
 becomes greater than that which was at first the maximum in 
 the carotid; which thus becomes the channel of the most 
 arterial blood. 
 
 The Rabbit. The student should lay open the thoracic cavity 
 of the rabbit and dissect away the thymous gland and other 
 tissues which hide the origin of the great vessels, so as to 
 display the heart and vessels as shown diagrammatically in 
 Fig. 63. 
 
 The heart has two auricles and two ventricles. From the 
 right ventricle (r. v.) there arises the pulmonary artery (pul. a.), 
 the part of the ventricle leading up into it being known as the 
 conus arteriosus. The pulmonary artery passes over towards 
 
Chap. X.] 
 
 THE HEART AND CIRCULATION. 
 
 205 
 
 the left side, and represents the pulmonary arch of that side 
 only. It soon bifurcates to supply the two lungs of which the 
 right is removed in the figure. From the lungs the blood is 
 returned by the pulmonary veins (pul. v.) to the left auricle 
 (L au.). Thence it passes into the left ventricle (I. v.). From 
 
 pt.r. 
 
 Fio. 63. HEART AND GREAT VESSELS RABBIT. 
 
 a. i. c. v. Anterior intercostal vein. ao. Aorta, ao. ar. Aortic arch. 
 az. v. Azygos vein. ex. ju. External jugular vein. in. a. Innominate artery. 
 in.ju. Internal jugular vein. 1. OM. Left auricle. I. c. c. Left common 
 carotid artery. I. pr. c. Left precaval vein. L v. Left ventricle, pt. c. Post- 
 caval vein. pul. a. Pulmonary artery, pul. v. Pulmonary vessels: r. au. Right 
 auricle. r. c. c. Right common carotid artery, r. v. Right ventricle. 
 s. cl. a. Subclavian artery, s. d. v. Subclavian vein. s. d. Subclavian artery 
 and vein of the right side. 
 
 this chamber it passes by the single aortic arch (ao. ar.) (which 
 answers to the left systemic aorta of the frog). At the summit 
 of the arch arises a vessel which at once bifurcates, giving rise to 
 the left common carotid (1. c. c.) and the innominate (in. a), which 
 latter soon again bifurcates to form the right common carotid 
 (r. c. c. ) and the right subclavian artery. The left subclavian artery 
 (s. cl. a.) takes its origin from the aortic arch independently. 
 
206 ANIMAL BIOLOGY. [Part I. 
 
 The aortic arch passes down behind (dorsal to) the heart, and 
 supplies the trunk, alimentary canal, and hind limbs. 
 
 The blood thus distributed throughout the body is returned 
 to the right auricle by three vessels, the postcaval (pt. c.) and the 
 right and left precavals (p. c.). The postcaval passes through the 
 substance of the diaphragm, and runs forward a little to the right 
 of the median plane. The precavals are constituted by the 
 external jugular (ex. ju.) (which nearer the head is seen to arise 
 from the union of two factors, an anterior and a posterior facial, 
 the latter being external), 1 the much smaller internal jugular 
 (in. ju.\ and the large subclavian (s. cl. v.). Nearer the heart each 
 receives an anterior intercostal vein (a. i. c. v.) (shown only on the 
 left side), collecting blood from the anterior intercostal spaces ; 
 and the right precaval receives also an azygos vein (az. v.) (probably 
 answering to the right posterior cardinal of the fish), which 
 collects blood from the posterior intercostal spaces. 
 
 Traced backwards the aorta is seen to give off a large cwliac 
 artery to the stomach, spleen, liver, gall-bladder, and anterior 
 part of the duodenum ; and a little further down a still larger 
 anterior mesenteric artery supplying the greater part of the intes- 
 tines. Next are given off the renal arteries to the kidneys, and 
 then, some way further down the body, a small posterior mesenteric 
 artery (Fig. 64, p. m. a.), supplying the rectum. After giving 
 off the spermatic or ovarian artery (sp. a.) to the testis or ovary, 
 the aorta bifurcates to form the common iliac arteries (com. il. a.). 
 To follow these arteries the lower part of the innominate bones 
 should be removed, together with the rectum and genital organs. 
 As shown in the figure, they give off ilio-lumbar branches (iL I. a.) 
 to the abdominal walls of this region, and then bifurcate to 
 form (1) the external iliacs (ex. il. a.), which, after giving off 
 branches to the bladder (and uterus), become the femoral arteries 
 supplying the leg; and (2) the internal iliacs (in. il. a.\ which 
 pass along the dorsal wall of the pelvic cavity. One more artery 
 is to be noted, the median sacral (m. s. a.), which is drawn aside 
 
 1 Occasionally the two external jugulars are united by a connecting trunk in 
 the posterior region of the neck. In the higher mammalia there is but one pre- 
 caval vein the right into which the external jugulars of both sides fall. 
 
Chap. X.] 
 
 THE HEART AND CIRCULATION. 
 
 207 
 
 to the right so as to be visible near its origin passing behind the 
 ilio-lumbar vein. It passes backwards to the pelvis and tail, 
 and is the direct continuation of the dorsal aorta, answering to 
 the caudal artery in the fish. 
 
 The veins returning blood from the pelvic region and hirid- 
 
 FIG. 64. PELVIC VESSELS RABBIT. 
 
 com. il. a., com. il. v. Common iliac artery and vein. ex. il. a., ex. il. v. Ex- 
 ternal iliac artery and vein. fern, a., fern. v. Femoral artery and vein. in. il. 
 a., in. il. v. Internal iliac artery and vein. il. 1. a., il. I. v. Ilio-lumbar artery 
 and vein. m. s. a. Median sacral artery, p. m. a. Posterior mesenteric 
 artery, pt. c. Postcaval vein. sp. a. , sp. v. Spermatic artery and vein. 
 
 limb will be understood by a study of a careful dissection, aided by 
 the figure. The internal iliacs unite into a common trunk, which 
 is successively joined by thefemorals, ilio-himbars, and spermatics. 
 
 The blood is conveyed from the stomach and intestines by a 
 portal system to the liver. The portal is composed of (1) a large 
 anterior mesenteric vein, which receives a small posterior mesenteric 
 (from the rectum) ; and (2) a lieno-gastric vein from the stomach 
 and spleen. 
 
 Turn now to the heart itself. The auricles are thin- walled, 
 especially in the atrial portion lying dorsally. Those portions 
 which project over the base of the ventricles and are visible in 
 
208 ANIMAL BIOLOGY. [Part I. 
 
 Fig. 63, are strengthened with a network of muscular bands, the 
 musculi pectinati. In the septum between the auricles there is 
 an oval area which is thinner than the rest. This is the fossa 
 ovalis, and represents a communicating aperture, the foramen 
 ovale, of the fostus. The apertures of the pulmonary veins have no 
 valves. That of the postcaval has a fold (Eustacliian valve) from its 
 posterior margin towards the septum. That of the right precaval 
 has posteriorly a semi-lunar fold. That of the left precaval has 
 no valve. Just within it is the small aperture of the coronary 
 vein, bringing blood back from the tissues of the heart itself. 
 
 Between the right auricle and ventricle there is a tricuspid 
 valve, consisting of three membranous flaps, of which the irre- 
 gular posterior edges are connected by tendinous threads, the 
 chordce tendinece, to little muscular cones on the ventricle, the 
 papillary muscles. These prevent the flaps being forced upwards 
 into the auricle. Between the left auricle and ventricle there is 
 a bicuspid or mitral valve, consisting of two such flaps, also pro- 
 vided with chordae and papillary muscles. The septum between 
 the ventricles is muscular and convex towards the right ventricle. 
 
 Both the pulmonary artery and the aorta are guarded near 
 their point of origin from the ventricle by a set of three semi- 
 lunar valves, arranged like watch-pockets. If blood attempts to 
 flow towards the ventricles the watch-pockets fill, and their flaps, 
 meeting in the middle of the channel, completely block the 
 passage. 
 
 The Pigeon (Fig. 65). The heart has two ventricles and two 
 auricles. The pulmonary artery bifurcates at once (pvl. ar.), its 
 two branches being the right and left pulmonary arches. The 
 pulmonary veins fall into the left auricle in the V-shaped space 
 between the two precavals. The aorta gives off two large 
 branches, almost immediately at its origin. These are the 
 innominate arteries (in. a.). The continuation of the aortic arch 
 is relatively smaller than these two branches. It crosses over 
 the right bronchus, and represents the right trunk in the frog. 
 Each innominate gives off a common carotid, and becomes the 
 subclavian artery (s. cl. a.), which branches into a smaller brachial 
 
Chap. X.] 
 
 THE HEART AND CIRCULATION. 
 
 209 
 
 artery (br. a.) to the wing, and a larger pectoral artery (pc. a.) to 
 the great muscles of the breast. The common carotids almost 
 
 hyp.v. 
 
 in.il. c. in - n ' 
 
 FIG. 65. CIRCULATION PIGEON. 
 
 (. m. a. Anterior mesenteric artery, a. r. a. Anterior renal artery, a. r.', 
 a. r.", a. r.'" Afferent renal veins, a. o. Aortic arch. br. a., br. v. Brachial 
 artery and vein. c. Caudal vessels, c. c. Common carotid, cos. a. Cceliac 
 artery, cr. v. Crural vein. fern. a. Femoral artery, he. v. Hepatic vein. 
 hyp. v. Hypogastrio vein. il. v. Iliac vein. in. a. Innominate artery. 
 in. il. Internal iliac vessels, isc. a. Ischiatic artery, ju. Jugular vein. 
 1. au. Left auricle. I. pc. Left precaval. I. v. Left ventricle, in. r. a. 
 Middle renal artery, pc. a., pc. v. Pectoral artery and vein. p. m. a. Pos- 
 terior mesenteric artery, p. m. v. Posterior mesenteric vein. p. r. a. 
 Posterior renal artery, pt. c. Postcaval vein. put. ar. Pulmonary artery. 
 r. au. Right auricle, r. pc. Right precaval vein. r. v. Right ventricle. 
 sc. a., sc. v. Sciatic artery and vein. s. cl. a. Subclavian artery, ver. Ver- 
 tebral vessels. 
 
 14 
 
ANIMAL BIOLOGY. [Parti. 
 
 unite in the middle line under the vertebrae. Each gives off a 
 vertebral artery (ver.). 
 
 Traced backwards the aorta gives rise to a coeliac (cce. a.) and 
 an anterior mesenteric (a. m. a.), and sends an anterior renal (a. r. a.) 
 branch on each side to the front lobes of the kidney. The 
 femoral arteries (fern, a.) are not of great size. The ischiatic 
 arteries (isc. a.) are larger, and give off renal branches to the 
 second and third lobes of the kidney, and eventually become the 
 sciatic arteries (sc. a.). At the posterior end of the body the aorta 
 divides into four small vessels, the caudal to the tail (c.), the 
 posterior mesenteric (p. m. a.) to the rectum and cloaca, and the 
 paired internal iliacs (in. il.) to the roof of the pelvis. 
 
 Turning now to the veins, the right auricle receives the two 
 precavals (r. and /. pc.) and the postcaval (pt. c.). Each precaval 
 receives as factors a large pectoral (pc. #.), a brachial (br. v.), 
 and a jugular (ju.). Traced forwards up the neck the two 
 jugulars curve round and unite in the middle line beneath the 
 head. The postcaval receives several large hepatic (he. v.) fac- 
 tors from the liver. Slightly further back it is formed by the 
 union of the two large iliac veins (il. v.), which receive afferent 
 renal veins (a.r. f , a.r.", a.r.'") from the three lobes of the kidney, 
 those from the more posterior lobes uniting into a common trunk. 
 Following the iliacs down, they are seen to be composed of a 
 large crural vein (cr. v.), and a hypogastric vein (hyp. v.) which 
 passes through the substance of the kidney and receives a small 
 sciatic vein (sc. v.). Posteriorly the hypogastrics receive internal 
 iliac veins from the roof of the pelvis. Further back they are 
 seen to arise from the division of a posterior mesenteric vein 
 (p. m. v.), which receives a small caudal vein near its point of 
 division. The posterior mesenteric vein returns blood from the 
 rectum and part of the small intestine. The blood in its 
 posterior (coccygeo-mesenteric) portion flows into the hypogas- 
 trics. That in its anterior (inferior mesenteric) portion joins 
 the portal vein, which also receives blood (1) by a gastro-duodenal 
 from the right side of the gizzard, the duodenum, and the last 
 loop of the small intestine, and (2) by the superior mesenteric 
 from the rest of the small intestine. 
 
Chap. X.] THE HEART AND CIRCULATION. 211 
 
 In the heart itself the student will make out the general 
 resemblance to that of the rabbit. The main difference is that 
 the tricuspid valve is replaced by a strong muscular fold, nearly 
 as thick as the walls of the right ventricle, which applies itself 
 over the auriculo-ventricular orifice, and converts the whole 
 ventricle, except at the orifice of the pulmonary artery, into a 
 muscular chamber. There are no chordae tendinese nor papillary 
 muscles in this ventricle. 
 
 Notes on Embryonic and Fatal Circulation in Fowl and Rabbit. 
 
 1. The aortic arch is developed from the second branchial 
 arch, the right arch being suppressed in the mammal, the left in 
 the bird. 
 
 2. The carotids are developed from the arch or arches anterior 
 to the second branchial. 
 
 3. The pulmonary arteries are developed from the arch or 
 arches posterior to the second branchial. In the bird the arches 
 of both sides persist ; in the mammal that of the left only. 
 
 4. In both bird and mammal, before lung-respiration sets in, 
 the blood flows in the main from the pulmonary into the aorta 
 by a connecting branch (dudus arteriosus), similar to that shown 
 for the tadpole in Fig. 62, II. 
 
 5. From the aorta the blood is delivered in the main by the 
 vitelline artery to the umbilical vesicle or yolk-sac, and by the 
 allantoic arteries to the allantois. 
 
 6. From the umbilical vesicle and the allantois the blood 
 returns by vitelline and allantoic veins, which unite and receive 
 a small mesenteric branch from the intestines. They then pass 
 into the liver, some of the blood being distributed to that organ, 
 but much of it passing through it by the ductus venosus to the 
 sinus venosus. (See Fig. 66.) 
 
 7. From the anterior part of the body the blood is returned 
 by anterior cardinal veins (a. c.), one on each side; and from 
 the Wolffian bodies and posterior regions by posterior cardinal 
 veins (p. c.), one on each side. The anterior and posterior car- 
 dinals on each side combine and form factors of the ductus 
 Cuvieri (d. c.) which leads the blood to the heart. 
 
ANIMAL BIOLOGY. [Parti. 
 
 8. But when the kidneys are developing, the blood from them 
 and from the hind limbs is led by a new median vein, the post- 
 caval (pL c.), into the sinus venosus. 
 
 9. As the postcaval increases in size the posterior cardinals 
 diminish and tend to atrophy. 
 
 Fia. 66.- DIAGRAM OF VENOUS CIRCULATION OF CHICK PREVIOUS TO HATCHING. 
 
 a. c. Anterior cardinal vein, the jugular of the adult, al. c. Alimentary 
 canal, al. v. Allantoic vein. als. Allantois ; this and its vein are provi- 
 sional embryonic structures. Ir. Brachial vein. cr. Crural vein. d. c. 
 Ductus Cuvieri, the precaval of the adult, d. i: Ductus venosus, leading 
 blood through the liver ; atrophies in the adult, h. Heart, lie. Hepatic 
 vein, collecting blood from the liver, fc. Kidney. Ir. Liver, m. v. Mesen- 
 teric vein. p. c. Posterior cardinal vein, bringing blood from the Wolffian 
 bodies, atrophies in the adult, p. v. Portal vein, delivering some blood to 
 the substance of the liver, but delivering more through the ductus venosus 
 to the sinus venosus. pt. c. Postcaval vein. r. Renal vein. s. v. Sinus 
 venosus. v. v. Vitelline vein. w. 6. Wolffian bodies, atrophy with the 
 growth of the kidneys, yk. s. Yolk-sac, which, with its vitelline vein, are 
 provisional embryonic structures. 
 
 All details diagrammatic : relative size of parts not indicated. 
 
 10. With the atrophy of the posterior cardinals the ductus 
 Cuvieri become the precavals, which also receive factors from the 
 wing and breast. 
 
 11. During foetal life there is a communication between the 
 two auricles by the foramen ovale, through which the arrow 
 
Chap. X.] 
 
 THE HEART AND CIRCULATION. 
 
 213 
 
 passes in Fig. 67, A. -An Eustachian valve or fold of membrane, 
 running from the opening of the postcaval (pi c.) to the foramen 
 ovale, directs most of the blood from the posterior regions of the 
 body through that orifice into the left auricle, while it pravents 
 the blood passing in by the precavals (pr. c.) from leaving the 
 right auricle. The blood from the posterior regions has been 
 arterialisecl in the allantois. 
 
 /P r.c. 
 
 l.au. 
 
 pal. a. 
 
 FlG. 67. FCETAL AND xVDULT CIRCULATION IN HEART. 
 
 A. Foetal. B. Adult. 
 
 ao. Aorta. b. a. Bulbus arteriosus. ca. Carotid artery, d. a. Ductus 
 arteriosus. 1. aw. Left auricle. I. v. Left ventricle, pr. c. Precavalvein. pt. c. 
 Postcaval vein. pul. a., pul. v. Pulmonary artery and vein. r. au. Right auricle. 
 r. v. Right ventricle. 
 
 12. The blood leaves the ventricles by a bulbus arteriosus 
 (b. a.), which is divided by a septum into two chambers, which 
 will be converted into the root of the aorta and that of the 
 pulmonary artery. 
 
 13. The blood from the left ventricle (arterialised in the 
 allantois) flows along the aortic chamber of the bulbus arteri- 
 osus, and so into the carotid (ca.) and aorta (ao.). The blood 
 from the right ventricle which has returned from the head 
 passes into the pulmonary chamber, which thus delivers blood 
 along the ductus arteriosus (d. a.) to the aortic arch, and in 
 small quantities to the as yet functionless lungs (by pul. a.). 
 
2i 4 ANIMAL BIOLOGY. [Parti. 
 
 Thus the placenta and hinder regions receive less arterialised 
 blood. 
 
 14. Note that during foetal life it is the blood that flows 
 along the aorta that will be arterialised. Hence the necessity 
 of keeping open the foramen ovale, and directing by means of 
 the Eustachian valve the blood which falls into the right auricle 
 across into the left. In adult life it is the blood that flows 
 along the pulmonary artery that will be arterialised. Hence 
 the foramen ovale closes, and the Eustachian valve becomes 
 functionless. 
 
 In Fig. 67 the foetal (A.) and adult (li) condition are com- 
 pared. For the sake of diagrammatic convenience the bulbus 
 arteriosus is placed at the lower part of the figure. 
 
CHAPTER XL 
 THE BRAIN AND NERVES. 
 
 THE central nervous system is differentiated very early in 
 development. Beginning as a neural plate, it soon, by the up- 
 growth of laminae dorsales, which bend over and eventually 
 meet in the mid-line, becomes converted into a neural tube 
 (see p. 102). We have seen that the anterior part of this tube 
 becomes differentiated into three swellings or vesicles, the fore- 
 brain, mid-brain, and hind-brain. These in the course of 
 development become further differentiated, so that we have 
 eventually five regions, viz. : 
 
 (1. Myelencephalon, or region of the medulla 
 Hind-Brain, -< oblongata. ( 
 
 ( 2. Metencephalon, or region of the cerebellum. 
 Mid-Brain, 3. Mesencephalon, or region of the optic lobes. 
 
 / 4. Thalamencephalon, or region of the thalamus^ 
 -p , -n, . J 5. Prosencephalon, or region of the cerebral 
 ' j hemispheres, with their outgrowths, the 
 
 olfactory lobes. 
 
 These regions are somewhat differently developed in Frog, 
 Pigeon, and Rabbit. We will consider them seriatim from 
 behind forwards. 
 
 (A.) Hind-Brain 1. Myelencephalon. The floor and sides of 
 the canal in this region become much thickened, while the roof 
 becomes very thin. The central canal here expands to form the 
 fourth ventricle of the brain (iv.). The pia mater covering this 
 ventricle becomes very vascular, forming the tela vasculosa. 
 Into this region of the brain the dorsal and ventral fissures of 
 
 215 
 
2l6 
 
 ANIMAL BIOLOGY. 
 
 [Part I. 
 
 C. 
 
 FIG. 68. BRAIN OF FROG. A. From above. B. From below. C. In longitudinal section. 
 olF. 
 
 FIG. 69. BRAIN OF RABBIT. A. From below. B. In longitudinal section. 
 
 C.ll 
 
 Fio. 70. BRAIN OF PIGEON. A. From above. B. From below. C. In longitudinal section. 
 
 a. c. Anterior commissure, c. ca. Corpus callosum. c. ce. Cruri cerebri. c. 7?. 
 Cerebral hemisphere, c. m. Corpus mnmmillare. c. . Corpus trapezoideum. cb. 
 Cerebellum. /. Fornix. /. m. Foramen of Monro. fl. Flocculus, inf. Infun- 
 dibulum. 1. t. Lamina terminalis. m. c. Middle commissure, m. o. Medulla 
 oblongata. net. Natis. olf. Oli'actory lobe. op. c. Optic commissure, op. ch. Optic 
 chiasma. op. I. Optic lobe. op. n. Optic nerve, op. tr. Optic tracts, p. c. 
 Posterior commissure, p. va. Pons Varollii. pn. yl. Pineal body. pit. Pituitary 
 body. te. Testis. th. Optic thalamus. v.p. Ventral pyramid, iii. Third ventricle, 
 iv. Fonrth ventricle, 
 
Chap. XL] THE BRAIN AND NERVES. 217 
 
 the spinal cord are continued. On either side of the dorsal 
 fissure are the dorsal pyramids ; on either side of the ventral 
 fissure are the ventral pyramids (v. p.). The dorsal pyramids 
 diverge at the fourth ventricle. In the rabbit there is further 
 differentiation. External to the ventral pyramids, is on either 
 side, an olivary tract, and (anteriorly) the trapezoid body (c. .), a 
 band of transverse fibres. External to the dorsal pyramids, 
 which form but narrow areas, are the restiform bodies. In the 
 medulla oblongata the white matter is external and the grey 
 internal. 
 
 2. Metencephalon. 1 Here the sides and floor of the canal are 
 thickened. In the frog and pigeon they are not well differentiated 
 from the medulla ; but in the rabbit they are differentiated into 
 strong bands of transverse fibres forming ihepons Varolii (p. va.). 
 The roof undergoes special development, giving rise to the cere- 
 bellum (cb.), in which the white matter is internal and the grey 
 external. In the frog this is a small tongue-shaped band over- 
 hanging the anterior part of the fourth ventricle. In the pigeon 
 it is large, uni-lobed, overlying the fourth ventricle behind, and 
 abutting against the cerebral hemispheres in front. It is marked 
 with ridges and grooves. At each side there is a small elevation, 
 the flocculus (fl.). It is connected with the thickened floor of 
 this region by strong peduncles. A delicate sheet of nervous 
 tissue, the mlve of Vieussens, connects it with the optic lobes 
 (op. I.). In the rabbit the cerebellum is large and tri-lobed. 
 Each lateral lobe has externally a large flocculus. A valve of 
 Vieussens connects it anteriorly with the testes (te.), and on 
 each side this band is thickened to form the anterior peduncles ; 
 middle peduncles pass downwards to the pons Varolii ; posterior 
 peduncles are continued into the restiform bodies of the medulla. 
 
 (B.) Mid-Brain 3. Mesencepkalon. In the frog the floor is 
 thickened, but not further differentiated. The roof gives rise 
 to large optic lobes (op. /.), which are apposed in the middle line. 
 
 1 Some authors apply the term metencephalon to that part of the brain which 
 has been described as myelencephalon, applying the term epencephalon to the 
 cerebellum. 
 
218 ANIMAL BIOLOGY. [Part I. 
 
 In the pigeon the thickened floor is not well differentiated from 
 that of the hind-brain. The optic lobes are pressed outwards 
 to the sides by the large cerebral hemispheres and cerebellum, 
 the band of fibres uniting them being known as the optic com- 
 missure. In the rabbit the floor is differentiated into two strong 
 diverging bands of white fibres, the crura cerebri (c. ce.), pro- 
 ceeding outwards and forwards. The optic lobes are differen- 
 tiated into two pairs nates (na.) anteriorly and testes (te.) pos- 
 teriorly forming the corpora quadrigemina. In this region the 
 central canal becomes narrowed to form the iter connecting the 
 fourth and third ventricles. The cavities contained within the 
 optic lobes of the frog and pigeon communicate with the iter. 
 In the optic lobes the grey matter is external. 
 
 (0.) Fore-Brain 4. Thalamencephalon, or primary fore-brain. 
 In the frog the hinder part of the floor in this region gives rise 
 to a funnel-shaped depression, the infundibulum (inf.), leading 
 down to a small mass of tissue (pinched off in development from 
 the stomodseum), known as the pUuita/n/ body (pit.). In front 
 of this the floor gives rise to the optic cfiiasma (op. ch.), formed 
 of optic tracts (op. tr.) posteriorly, and the optic nerves (op. n.) 
 anteriorly. The sides give rise to thickened masses of mixed 
 grey and white matter, the optic thalami (68, A., th.). The roof 
 becomes very thin anteriorlyP^orming the velum interposition. 
 Posteriorly it is thickened to form a transverse band of fibres, 
 the posterior commissure (p. c.), the anterior portion of which rises 
 on either side to fotm the peduncles of the pineal gland (pn. gl.). 
 In the pigeon the infundibulum is small, the optic chiasma well 
 marked, and the optic tracts thick. The optic thalami are 
 small, covered by the velum interpositum, and connected by 
 the posterior commissure, above which on either side are the 
 peduncles of the pineal gland. 1 In the rabbit the infundibulum 
 is small, and behind it is a rounded mass, the mammillary body. 
 (c. m.). The optic chiasma is well marked. The optic thalami 
 
 1 The function of the pineal body has long been problematical. It has recently 
 been discovered that in the reptile (Hatteria) the pineal gland is the seat of an 
 eye constructed on the invertebrate type. The saccular brow-spot between the 
 eyes of the frog is probably a vestige of the same organ. 
 
Chap. XI.] THE BRAIN AND NERVES. 219 
 
 are large, and united (1) by a posterior commissure, on either 
 side of which are the peduncles of the pineal gland ; and (2), 
 by a middle or soft commissure of grey matter (m. c.). The roof 
 forms a thin velum interpositum. 
 
 In this region the central canal expands to form the third 
 ventricle of the brain (iii.). 
 
 5. Prosencephalon, or secondary fore-brain. In the frog the 
 cerebral hemispheres (c. h.), composed externally of grey matter 
 and internally of white, are formed as outgrowing buds from the 
 anterior end of the primitive fore-brain. In their adult form 
 they are elongated anteriorly, but do not extend backwards over 
 the succeeding parts of the brain. They contain lateral ven- 
 tricles which communicate with the third ventricle by the foramen 
 of Monro "(/ m.). The outer wall of each lateral ventricle is 
 somewhat thickened. The inner faces of the two hemispheres 
 are united anteriorly in the region of the so-called olfactory 
 lobes. The anterior wall of the brain in the. mid-line is the 
 lamina terminalis (I. ), which contains a band of transverse 
 fibres, the anterior commissure (a. c.). In the pigeon the cerebral 
 hemispheres (c. h.) are rounded and smooth, and extend back- 
 wards, so as to separate the optic lobes and cover much of the 
 thalamencephalon. The cavity of the lateral ventricle is much 
 diminished, the floor and lower part of its outer wall being 
 thickened by the development of an eminence of grey matter 
 forming the corpus striatum. Above the lamina terminalis (/. t.) 
 is a band of transverse fibres connecting the corpora striata, and 
 forming the anterior commissure (a. c.). 
 
 In the rabbit the early development of the cerebral hemi- 
 spheres (c. h.) resembles that in the frog. In their later develop- 
 ment matters become much more complicated. Externally they 
 are seen to be divided by a ventral and longitudinal groove into 
 a larger superior and a smaller inierior> lobe. A slight angula- 
 tion on the ventral side of the superior lobe marks the position 
 of that which becomes in higher types the fissure of Sylvius. 
 The hemispheres grow rapidly, and extend backwards so as to 
 cover the thalamencephalon, and almost hide the optic lobes 
 (nates and testes). The floor becomes thickened, giving rise to 
 
ANIMAL BIOLOGY. 
 
 [Part I. 
 
 f.c. 
 
 the corpus striatum (Fig. 71, c. st.), which projects upwards into 
 
 the lateral ventricles (/. v.). 
 Posteriorly the corpus stria- 
 tum comes into close relation 
 L, with the optic thalamus. The 
 outer wall of the hemisphere 
 - c. sfc. thickens, while the inner wall 
 becomes relatively thinner. 
 
 FlO. TI.-DIAORAMMATIC SECTION OF FcETAL 
 
 IN (after Koiiiker). 
 
 In this inner wall two curved 
 
 ance, and project towards the 
 c. st. Corpus striatum. c/i. pi Choroid plexus, interior of the lateral veh- 
 
 /. c. Falx cerebri. fc. m. Hippocampus major. tric ] e< Qf these the Upper 
 I. v. Lateral ventricle. HI. Third ventricle. . . ri 
 
 gives rise to a thickened 
 
 mass of white nervous tissue, the hippocampus major (Fig. 71, 
 h. m.). The wall of the lower fold becomes thinned to the 
 merest film, and a vascular plexus (ch. pi.), derived from the 
 connective tissue septum between the hemispheres (falx cerebri, 
 f. c.) is formed in the fold, in the manner shown in the trans- 
 verse section of the brain of a foetal sheep (after Koiiiker) 
 Fig. 71. This constitutes the choroid plexus of the lateral 
 ventricle. It is also present in the pigeon. 
 
 In the lamina terminalis (I. t.), besides the transverse fibres of 
 the anterior commissures (a. c. ), two vertical bands of fibres are 
 developed. These constitute the anterior pillars of the fornix 
 (f. in 69, B.). They are continued doAvnwards along the floor 
 of the third ventricle, to end in the mammillary body (c. m.). 
 Traced upwards they meet in the middle line in the roof of the 
 third ventricle, thus forming the body of the fornix. Thence 
 bands of fibres (the posterior pillars of the fornix) pass back- 
 wards, arching over the foramen of Monro, to unite with the 
 hippocampus major. 
 
 The main commissure, uniting the cerebral hemispheres, is 
 the corpus callosum (c, ca.). It may be seen by carefully 
 separating the cerebral hemispheres on the dorsal side, and 
 constitutes a flat white band. It is seen in section in 
 Fig. 69, B. At its anterior end it bends slightly downwards 
 
Chap. XL] THE BRAIN AND NERVES. 221 
 
 (anterior genu), doing the same posteriorly (posterior genu\ w4|eS/: v 
 it comes into relation with the fornix. Between the two is .. 
 an area called the septum lucidum. It is formed of the inner 
 walls of the lateral ventricles of this region. In higher types 
 this area is more completely enclosed by the bending down of 
 the more marked anterior genu of the corpus callosum. In this 
 way a space between the two walls is more or less shut in, and 
 constitutes the so-called fifth ventricle, which is, however, it 
 should be noted, outside the brain. Anteriorly the cerebral 
 hemispheres give rise, as outgrowths, to olfactory lobes. In the 
 frog the so-called olfactory lobes (olf.) are relatively large, their 
 bases being closely united in the middle line. They contain 
 minute cavities, w^hich are prolongations of the lateral ventricles. 
 Anteriorly they become nerve-like cords. It is very probable, 
 however, that these so-called olfactory lobes are the anterior 
 parts of the cerebral hemispheres. In the pigeon the olfactory 
 lobes (olf.) are small and apposed, but not united in the middle 
 line. They send forward nerve-like cords. In the rabbit the 
 olfactory lobes (olf.) are small (though larger than is usual in the 
 mammalia), and are not united in the mid-line. They send out 
 many fibres through the cribriform plate. 
 
 Cranial Nerves. In the frog there are ten pairs of cranial 
 nerves : 
 
 I. Olfactory ; sensory nerves derived from the olfactory lobes 
 and passing to the organ of smell. 
 
 II. Optic ; sensory nerves arising as outgrowths of the thalam- 
 encephalon, but acquiring a secondary connection with the 
 optic lobes, and passing to the organ of vision. The fibres of 
 the optic nerves interlace at the chiasma. 
 
 III. Oculo-motor ; motor nerves arising from the floor of the 
 mesencephalon, and passing to four of the eye-muscles. 
 
 IV. Pathetic or trochlear ; motor nerves arising from the floor 
 of the mesencephalon, and passing to the superior oblique eye- 
 muscle. 
 
222 ANIMAL BIOLOGY. [Parti. 
 
 V. Trigeminal ; arising from the myelencephalon, and, just 
 before leaving- the skull, expanding into a Gasserian ganglion. 
 Outside the skull each divides into an anterior orbito-nasal 
 branch, passing to the nasal capsule and integument in its 
 neighbourhood, and a maxillo-mandibular branch which shortly 
 divides into a maxillary branch which supplies the upper jaw 
 and lower eyelid, and a mandibular branch which supplies 
 the lower lip and muscles of the floor of the mouth. 
 
 VI. Abducent; arising from the myelencephalon, entering the 
 Gasserian ganglion, and supplying certain eye-muscles. 
 
 VII. Facial; arising from the myelencephalon and entering 
 into close connection with the Gasserion ganglion. Each leaves 
 the skull just behind the trigeminal, and soon divides into 
 (1) a palatine branch, which runs forward beneath the mucous 
 membrane of the palate ; (2) a hyo-mandibular branch, which 
 passes back over the columella, and down to the angle of the 
 mouth. It then gives rise to two branches ; one, the hyoidean } 
 supplying the muscles of the anterior cornu of the hyoid, the 
 other, the chorda tympani, running along the inner edge of the 
 lower jaw. 
 
 VIII. Auditory ; arising from the myelencephalon, and passing 
 to the organ of .hearing. 
 
 IX. Glosso-pharyngeal ; arising from the myelencephalon. Each 
 leaves the skull by a foramen, just behind the auditory capsule, 
 gives off a branch which joins the hyo-mandibular branch of 
 the facial, and thus passes by a somewhat sinuous course to 
 the base of the tongue. 
 
 X. Pneumogastric or Vagus ; arising from the myelencephalon, 
 and leaving the skull in close company with the glosso-pharyn- 
 geal. Outside the skull each bears a ganglionic swelling. It 
 gives off cutaneous branches to the dorsal integument, and 
 then gives rise to (1) a laryngeal branch, forming a loop over 
 the pulmo-cutaneous artery ; (2) a splanchnic branch ; this sub- 
 
Chap. XL] THE BRAIN AND NERVES. 223 
 
 sequently gives rise to (a) a pulmonary branch, passing to the 
 lung, (b) a cardiac branch passing to the septum, between the 
 auricles of the heart, and (c) gastric branches to the walls of the 
 stomach. 
 
 The following points should be made out by practical dissec- 
 tion : 
 
 1. Place the frog upon its side and remove the integument 
 from the side of the head. On dissecting away the muscles 
 just behind the tympanum, and carefully removing the anterior 
 part of the supra-scapula, four nerve-threads will be seen, three 
 taking their origin from the skull, the fourth from the spinal 
 column. This last is the hypo-glossal (1st spinal nerve). 
 Traced downwards it is found to be distributed to the 
 muscles of .the tongue. From the ventral aspect it is readily 
 seen running beneath the thin transverse sheet of muscle (mylo- 
 hyoid), between the two rami of the lower jaw. Near it, but 
 somewhat deeper, and also distributed to the muscles of the 
 tongue, is the glosso-pharyngeal (ninth cranial). This is continu- 
 ous with one of the three cranial nerves seen just behind the 
 tympanum. The other two are the laryngeal and splanchnic 
 branches of the vagus. 
 
 2. Dissect away the integument and muscles from the rami 
 of the lower jaw. The mandibular branches of the V. lie 
 on their outer faces ; the mandibular branches of the VII. on 
 the inner. Traced backwards, one of these, the mandibular, is 
 found to run up beneath the squamosal bone to the skull. It is 
 a branch of the trigeminal. The other may be traced to the 
 angle of the lower jaw, and thence upwards over the columella 
 and forwards into the skull. It is a branch of the facial. 
 
 3. Detaching or holding widely open the lower jaw, remove 
 the mucous membrane of the palate. Two nerves (one on each 
 side) are seen running from just in front of the guard of the 
 para-sphenoid, where they emerge from the skull, to the vomers. 
 They are the palatine divisions of the facial. By carefully 
 dissecting the guard away on one side the Gasserian ganglion 
 may be exposed. 
 
 4. Just on the lower edge of the orbit, beneath the eye 
 
224 ANIMAL BIOLOGY. [Parti. 
 
 and above the mandible, is seen a nerve passing to the integu- 
 ment of the upper jaw. It is the maxillary branch of the 
 trigeminal. 
 
 5. Carefully remove the eye. To do so the optic nerve 
 supplying that organ must be severed. Passing along the 
 upper border of the orbital cavity is a nerve running forwards 
 to the nasal chamber. It is the orbito-nasal division of the tri- 
 geminal. It is not connected with the sense of smell. 
 
 In the fowl and rabbit there are twelve pairs of cranial nerves. 
 Of these the first ten are essentially similar in distribution and 
 origin to those of the frog. The eleventh is a spinal nerve, which 
 (in the rabbit for instance) takes its origin a little posterior to 
 the fifth cervical nerve. It then passes forwards in the fork 
 between the two roots of the anterior spinal nerves, receiving 
 fibres from the cord en passant, and leaves the skull, together 
 with the ninth and tenth, to be distributed to some of the 
 muscles in the neck. The twelfth, or hypo-glossal, answers 
 to the first spinal of the frog, and has a similar distribu- 
 tion, but making its exit through the skull is reckoned as 
 cranial. 
 
 Spinal Nerves. In the frog the first spinal nerve forms the 
 hypo-glossal. The second and third unite to form a brachial 
 plexus for the inner vation of the fore-limb. The fourth, fifth, and 
 sixth spinal nerves pass to the body-walls. The seventh, eighth, 
 and ninth form the lumbo-sacral or sciatic plexus, from which 
 nerves are given off to the hinder region of the body, but especi- 
 ally to the hind-limb, dividing into a small crural to the front of 
 the thigh, and a large sciatic to the back of the thigh. This 
 latter divides into the tibial, which ends on the lower surface of 
 the foot, and the peroneal which terminates on its dorsal surface. 
 The tenth spinal nerve leaves the neural canal by the coxygeal 
 foramen in the urostyle, and is distributed to the adjacent 
 parts. 
 
 In the rabbit there is a brachial plexus formed by the union of 
 the fifth to the eighth cervical and first thoracic nerves. It gives 
 rise, as is shown in Fig. 72, to a circumflex (c/.), which dives into 
 
Chap. XL] THE BRAIN AND NEltVES. 225 
 
 the muscles of the shoulder dorsal to the head of the humerus ; 
 
 a median (m.) 9 which passes 
 
 to the radial side of the fore- 
 
 arm; an ulnar (w.), which c . v . | 
 
 passes to the ulnar side of the 
 
 fore-arm : and a musculo-spiral Jy vnl i 
 
 * LIL. I. T 
 
 (m. s.), which dives to the 
 
 dorsal side of the humerUS, FIG. 72. BBACHIAL PLEXUS RABBIT. 
 
 and then passes to the radial c . v.-c. viii. Fifth to eighth cervical nerves. 
 
 side of the fore-arm. The ** * First thoracic nerve, c/. circumflex 
 
 nerve, m. Median nerve, m. s. Musculo-spiral 
 Other thoracic nerves run nerve, ul. Ulnar nerve. 
 
 out between the ribs. In 
 
 the rabbit the lumbo-sacral plexus is formed by the union 
 
 of the fifth to the seventh lumbar and first to the third sacral 
 
 nerves. It gives rise to a crural nerve to the front of the 
 
 thigh, an obturator nerve to the muscles near the acetabulum, 
 
 and a great sciatic passing out between the ischium and the 
 
 sacrum. 
 
 In the pigeon there is a brachial plexus which supplies the 
 wing ; a lumbar plexus, giving off the larger femoral nerve over 
 the front edge of the ilium to the front of the thigh and the 
 obturator nerve through the obturator fissure to the muscles 
 near the acetabulum; and a sciatic plexus giving rise to the 
 sciatic nerve which passes through the ilio-sacral foramen. 
 
 The Sympathetic System. In the frog the sympathetic system 
 has ten or more ganglia on either side, connected by longitudinal 
 connectives These lie within the subvertebral lymph-space, and 
 posteriorly are closely connected with the aorta. Each ganglion 
 receives a filament from a spinal nerve. The most anterior 
 ganglia are united by filaments with the ganglion of the ninth 
 and tenth cranial nerves, from which a delicate thread passes 
 into the cranial cavity and unites with the Gasserian ganglion. 
 Branches of the sympathetic accompany the blood-vessels, a 
 large splanchnic branch (on which is developed a cceliac ganglion) 
 passing along the coeliaco-mesenteric artery. 
 
 In the rabbit the sympathetic may be seen in the thoracic 
 15 
 
226 ANIMAL BIOLOGY. [Parti. 
 
 region on either side of the vertebral column, the ganglia lying 
 on the heads of the ribs. Traced forwards into the neck, it is 
 seen as a flat nerve, on which are developed a posterior cervical 
 ganglion a little above the subclavian artery, and an anterior 
 cervical ganglion just anterior to the larynx. Traced backwards, 
 it gives off, about the level of the ninth thoracic ganglion, a 
 large splanchnic nerve, which is much more conspicuous than 
 the slender continuation of the chain. It enters a group of 
 greyish pink transparent ganglia, constituting the solar plexus, 
 of which the coeliac ganglion and the mesenteric ganglion, one 
 on either side of the anterior mesenteric artery, are the most 
 important. 
 
 In the pigeon there is a cervical ganglion near the brachial 
 plexus. Anterior to this the sympathetic lies in the verte- 
 brarterial canal. The two sympathetic chains unite posteriorly 
 in a ganglion impar near the end of the caudal vertebrae. 
 
 The Functions of the Nervous System, The function of the 
 nervous system, as a whole, is to form a centralised means of 
 intercommunication between the various parts of the organism, 
 and thus to convert an aggregate of organs into a living unit. 
 Closely connected with the nervous system are end-organs, (a) 
 for receiving impressions from without, as in the organs of 
 special sense ; and (b) for delivering impulses, as in the muscle- ' 
 plates. Space need not here be occupied in repeating what has 
 been said in Chapter IY. on reflex action ; nor is it within the 
 scope of this volume to enter further into the nervous 
 mechanism of responsive and original action. The following 
 brief notes must here suffice : 
 
 1. Afferent end-organs (e.g. special senses) receive impressions 
 from without. 
 
 2. Efferent end-organs (e.g. muscle-plates) deliver stimuli to 
 muscles or glands and other parts. 
 
 3. Afferent and efferent nerve-fibres transmit these impres- 
 sions, the former towards, the latter awa,y from, the chief nerve- 
 centres. 
 
Chap. XL] THE BRAIN AND NERVES. 227 
 
 4. The nerve-centres correlate these transmitted impressions. 
 
 5. The spinal cord contains, besides transmitting-fibres, many 
 centres of reflex action for the performance of simple or complex 
 organic actions. 
 
 6. The medulla oblongata contains a centre for such vital 
 reflex actions as are connected with the regulation of the 
 heart-beat, with breathing, and the distribution of the blood- 
 supply. 
 
 7. The sympathetic is usually held to be a subsidiary system, 
 largely occupied with regulating the calibre of certain blood- 
 vessels. 
 
 8. The cerebellum is probably largely occupied with the co- 
 ordination of muscular movements. 
 
 9. The optic lobes are probably centres for vision and the 
 adjustment of the visual apparatus. 
 
 10. The thalami (of the rabbit) are regarded as centres by 
 which sensory impressions are transmitted, while the corpora 
 striata are centres by which motor impulses are transmitted. It 
 is possible that purely responsive non-original acts require no 
 higher centres than these. 
 
 11. The cerebral hemispheres are definitely involved in 
 original and intelligent actions. 
 
 12. Accompanying and associated with, or perhaps identical 
 with, certain nerve-actions (neuroses) going on within the body, 
 there are, in all probability, certain feelings (psychoses) about 
 which, in the frog, the pigeon, and the rabbit, we can know 
 little. 
 
PART II. 
 
 SOME INVERTEBRATE TYPES. 
 
CHAPTER XII. 
 THE CRA YFISH. 
 
 THE Crayfish (Astacus fluviatilis) is a small animal, five or 
 six inches, at most, in length, and somewhat resembling a 
 diminutive brown or greenish-brown lobster. It inhabits fresh 
 water in streams and rivulets, hiding beneath stones or in 
 burrows of its own making. It walks on four pairs of jointed 
 legs, of which the posterior pair are, during progression, directed 
 backwards, the others having a forward direction; but when 
 alarmed it darts backwards by a vigorous flap of its jointed 
 abdomen, which ends in a flattened tail-fin. In front of the 
 legs is a strong pair of jointed pincers ; on either side of the 
 mouth are jaws and " foot-jaws " ; there are two pairs of feelers ; 
 and the eyes are placed on moveable eye-stalks. 
 
 The whole body, except where a joint renders mobility 
 essential, is covered with a hard external shell ; whence the 
 crayfish and its relations are called Crustacea. This hard shell 
 is periodically thrown off or shed (by ecdysis). The new skin is 
 at first quite soft, and thus allows the animal to grow rapidly 
 for a day or two, during which time the crayfish hides from its 
 enemies. If one of the pincers or other limbs be seriously 
 injured, or if the animal become fixed by one of the pincers, 
 the limb can be cast off at the slenderest joint. A new limb 
 slowly grows in place of that which is thus lost. 
 
 Beneath the abdomen, which is broader in the female than in 
 the male, there may, in spring, be seen in the female dark berry- 
 like eggs attached to the slender appendages or swimmerets of 
 this region. These develop into young round-backed crayfishes 
 which are somewhat unlike the adult. The adult male may be 
 
 231 
 
232 
 
 ANIMAL BIOLOGY. 
 
 [Part II. 
 
 recognised by two pairs of imperfectly tubular appendages on the 
 under side of the anterior end of the abdomen, pointing for- 
 wards, beneath the unjointed thoracic region. 
 
 A. 
 
 P. 
 
 FIG. 73. CRAYFISH EXOSKELETON. 
 
 A. Dorsal view of crayfish. B. Side view, with appendages removed. 
 C. Diagram of abdominal somite. D. Cross-section of cephalo-thorax. 
 E. Endophragmal system in cross-section. F. The same from above. 
 
 ar. Articular cavity, arth. Arthrophragm. "br. c. g. Branchio-cardiac 
 groove, c. en. st. Capital of endosternite. cv. g. Cervical groove, en. pi. 
 Endopleurite. en. st. Endosternite. ep. Epimeron. h'. p'. Anterior hori- 
 zontal process, h. p. Posterior horizontal process, pi. Pleuron. st. Sternum. 
 te. Tergum. id. Telson. xv.-xx. Abdominal somites. 1-3, 10-14, 20. Appen- 
 dages visible in dorsal aspect. 
 
 External Characters. The body ef the crayfish is easily seen 
 to have the following parts (Fig. 73, A.) : 
 
 1. The Cephalo-thorax, occupying the anterior half of the body. 
 The whole of this region is encased in a continuous shell, the 
 carapace, ending anteriorly in a pointed rostrum. 
 
Chap. XII.] THE CRA YFISH. 
 
 2. The Abdomen, behind the cephalo- thorax ; jointed, com- 
 posed of six divisions or somites. 
 
 3. The Telson, forming the posterior end of the body in the 
 mid-line ; divided transversely into two parts, on the under 
 side of the first of which is the vent. 
 
 4. The Appendages, of which there are 20 pairs; 6 being 
 attached to the somites of the abdomen, and 14 to the cephalo- 
 thorax. Of these 9 pairs are visible in Fig. 73, A., viz., eye- 
 stalks, short two-whipped antennules, antennae, pincers, 4 pairs 
 of legs, and the large posterior swimmerets, which, with the 
 telson, constitutes the terminal tail-flap. 
 
 Seen from the side in the normal position of rest, the abdominal 
 region is slightly curved downwards, while the anterior end of 
 the sternal surface is bent upwards, giving rise to what is termed 
 the cephalic flexure. Hence the curve of the ventral or appendage- 
 bearing surface is as represented in Fig. 73, B. 
 
 On the dorsal (or tergal) surface of the cephalo-thorax there is 
 a curved cervical groove, from which there passes backwards on 
 either side of the median line a short longitudinal branchio- 
 cardiac groove. On the tergal surface of each abdominal segment 
 there is a smooth anterior portion, seen best when the abdomen 
 is in flexion or curved up beneath the body, which marks where 
 the segment in front overlaps it. 
 
 Fig. 73, C., shows a posterior view of the fourth abdominal 
 somite, from which the limbs and the soft parts have been 
 removed. The tergal surface (te.\ is, in a dorsal view, broadest 
 in the mid-line, narrowing away to the pleura at each side. 
 The sternum (st.) is very narrow, forming a mere transverse 
 ventral bar. On each side of the sternum is seen the articular 
 cavity (ar.) for the swimmeret. The projecting flaps at the 
 sides are the pleura (pi). Between the articulation of the limb 
 and the pleuron is a narrow area called the epimeron (ep.). 
 
 The somites of the abdomen articulate together by means of 
 pegs or pivots which fit into corresponding depressions, the peg 
 being on the anterior edge of one segment and the socket on 
 the posterior edge of the next. The pleura overlap, especially 
 in flexion. The pleura of the second abdominal somite overlap 
 
234 ANIMAL BIOLOGY. [Part II. 
 
 those of the first and third ; the third overlaps the fourth ; the 
 fourth, the fifth ; and the fifth, the sixth. Thus lateral motion 
 of the abdomen is prevented. 
 
 Fig. 73, D., shows a transverse section of the cephalo-thorax. 
 The pleura are seen to be enormously enlarged, and are here 
 called the branchiostegites. The space between the point of 
 articulation of the limb and the pleuron is also much enlarged. 
 Between this (epimeron) and the branchiostegite is enclosed the 
 branchial chamber, in which lie the gills. In a dorsal aspect 
 (Fig. 73, A.) the terga are seen to have coalesced into a con- 
 tinuous carapace. 
 
 Viewed from the ventral aspect, however, the sterna, though 
 they tend to run together in the mid-line, give evidence that 
 the thoracic region is compounded of coalesced somites homo- 
 logous with those in the abdomen. Six sterna may be readily 
 made out, of which the most posterior is a transverse plate, the 
 next two triangular, and the other three longitudinally elongated. 
 External to these sterna are the articular spaces for the four 
 legs, the pincers, and, anterior to the pincers, the posterior 
 maxillipeds. Between these articular spaces are transverse 
 calcified inter-articular bars, or arthrophragms. The sterna in 
 front of these six (i.e. between them and the mouth), are very 
 delicate. In front of the mouth the sternal region forms a 
 broad plate, with anterior spear-shaped process, the epistoma. 
 
 Projecting inwards from the ventral aspect of the thoracic 
 region is a complicated system of "pillars and bulkheads," 
 constituting the endophragmal system, which (1) gives attachment 
 to certain muscles ; (2) serves to protect the nerve-chain ; and 
 (3) assists in consolidating the cephalo-thorax. Fig. 73, E., shows 
 diagrammatically the infoldings of the cuticle, which are seen 
 in a transverse section of the region of the first pair of legs. 
 Where the arthrophragm (arth.) joins the sternum (st.) there 
 rises a little pillar, the endosternite (en. st.), with an expanded 
 capital. From the outer end of the arthrophragm and the 
 adjoining epimeron (ep.) 9 there projects inward a plate, the 
 endopleurite (en. pi.). Viewed from above the endopleurite is 
 seen to split into two horizontal processes, as shown diagram 
 
Chap. XII.] THE CRAYFISH. 235 
 
 matically in F. Each of these processes meets the expanded 
 head or capital of an endosternite (c. en. st.). So that the head 
 of each endosternite is connected with the anterior horizontal 
 process of one endopleurite and the posterior horizontal process 
 of another endopleurite, as indicated in F., which, it must be 
 remembered, only represents the upper surface of the endo- 
 phragmal system, and that quite diagrammatically. On com- 
 paring these diagrams with the real thing prepared by boiling 
 with dilute caustic potash, the essential structure will, it is 
 hoped, be understood. The minute details are beyond our 
 scope. 
 
 The Appendages. There are twenty pairs of appendages 
 which are serially-homologous or homodynamous. They are 
 
 1 pair of eyestalks or ophthalmites. 1 
 
 1 antennules. 
 
 1 antennae. ^ Cephalic. 
 
 1 mandibles. 
 
 2 ,, maxillae. 
 
 3 maxillipeds. -\ 
 
 1 chelate forceps or pincers. f 
 
 2 chelate legs. f Thoracic. 
 2 ., simple legs. / 
 
 6 ,, swimmerets, of which the pos- ) 
 
 terior are much enlarged, f AbdommaL 
 
 By comparison and careful study a typical or ideal appendage 
 may be conceived from which all the others may be derived, 
 according to Huxley's law, by suppression, by coalescence, or by 
 metamorphosis. 
 
 The student should carefully remove the appendages from 
 one side, and lay them in order on a sheet of white paper. In 
 removing the legs, forceps, and maxillipeds, care must be taken 
 not to tear away the gill from its attachment to the base of the 
 limb. The first maxilliped and the maxillae are exceedingly thin 
 
 1 The ophthalmite is, by some morphologists, not regarded as a homologue of 
 the other appendages. 
 
236 
 
 ANIMAL BIOLOGY. 
 
 [Part II. 
 
 and foliaceous. It is well to cut off the branchiostegite before 
 removing the appendages. 
 
 The typical appendage consists of a basal part, or protopodite, 
 from which arises an inner part or limb, the endopodite, and an 
 outer part or palp, the exopodite, and a gill-bearing part or epi- 
 podite. In the legs, forceps, and second and third maxillipeds, 
 the protopodite and endopodite together form a series of seven 
 joints, two of which may, however be coalescent. The student 
 
 FIG. 74. CRAYFISH APPENDAGES. 
 
 A. Second maxilliped. B. First maxilliped. C. Mandible. D. Antenna. 
 E. Antennule. F. Eye-stalk (ophthalmite). G. Swimmeret. H. Twentieth 
 appendage. I. Chela, opened to show processes for attachment of muscles. 
 K. Microscopic section of exoskeleton. 
 
 en. Endopodite. ep. Epipodite. ex. Exopodite. pr. Protopodite. 
 1. Coxopodite. 2. Basipodite. 3. Ischiopodite. 4. Meropodite. 5. Carpo- 
 podite. 6. Propodite. 7. Dactylopodite. 
 
 In I. a&. Process to which abductor muscle is attached, ad. Process to 
 which adductor muscle is attached, h. Hinge. 
 
 -ec. Ectostracum. end. Endostracum. ep. Epiostracum. 
 
 should carefully study and draw the appendages. With the 
 aid of the diagrams in Fig. 74, he will be able to trace the 
 homologies of the parts. In the diagrams the seven joints are 
 numbered. They are named 1. coxopodite; 2. basipodite ; 
 3. ischiopodite ; 4. meropodite; 5. carpopodite; 6. propodite; 
 7. dactylopodite. 
 
Chap. XII.] THE CRAYFISH. 237 
 
 Fig. 74, A. represents diagrammatically the second maxilliped. 
 The third maxilliped is similar in principle, as is also the third 
 leg. The forceps and the first two legs have the dactylopodite 
 so disposed as to form with the propodite a chelate termination 
 to the limb, as shown in I. In the fourth leg the epipodite is 
 suppressed. B. is the first maxilliped in which, as in the maxillae, 
 the protopodite is very thin and flattened. In the second 
 maxillae the divisions 1 and 2 are cleft ; the endopodite is small, 
 and the epipodite either coalesces with the exopodite to form, or 
 is itself modified into, a long curved plate, the scaphognathite. 
 In the first maxilla both exopodite and epipodite are suppressed ; 
 the endopodite is small. The mandible (C.) is stout and strong, 
 being mainly protopodite, with a small-jointed palp, supposed 
 to be the endopodite. In the antenna (D.) the endopodite is 
 enormously produced, and the exopodite forms a flattened 
 squame. In the antennule (E.) the protopodite has three 
 joints; both endopodite and exopodite are elongated. In 
 the ophthalmite (F.) we appear to have all suppressed but the 
 protopodite. 
 
 In the abdominal region the typical appendage is as in G. 
 The first two pairs of appendages in the male are specially modi- 
 fied, the endopodite of the first being so rolled upon itself as to 
 enclose an incomplete canal. The first abdominal appendage in 
 the female is very small, and sometimes wanting ; its exopodite 
 is suppressed. In the posterior pair of abdominal appendages 
 of both sexes we have much metamorphosis, the endopodite and 
 exopodite (H.) being flattened and expanded, so that the two 
 pairs, with the telson, form' the broad tail-fin. 
 
 It will be noted that many of the appendages are jointed ; 
 whence a large division of invertebrates with such jo 
 are known as the Arthropoda (arthron, a joint; 
 The joints are such as to allow the successive segments of the 
 limb free play in one plane. The third leg, for example, is 
 so hinged at its connection with the thorax as to allow the 
 coxopodite to move backwards and forwards, but not up 
 and down. Referring to the other divisions by numbers, 
 we may note that the motion between 1 and 2 is up and 
 
238 ANIMAL BIOLOGY. [Part II. 
 
 down, but not to and fro ; between 2 and 3 there is little 
 motion; between 3 and 4 the motion is to and fro; between 
 4 and 5 up and down ; between 5 and 6 again to and fro ; and 
 between 6 and 7 once more up and down. The motions though 
 nearly, are, however, not quite at right angles ; so that the 
 motion of the limb, as a whole, is tolerably free. The way in 
 which the several divisions are hinged will be readily seen on 
 examination of the limb. Fig. 74, L, shows the chela of the 
 forceps from which the soft parts have been removed, and which 
 has been opened out by the removal of its dorsal wall, except 
 where it is dotted in the diagram. The motion on the hinge- 
 line (&.), at two opposite points on the exoskeleton, is in the 
 plane of the paper. At two points of the exoskeleton, at right 
 angles to the hinge-points, there project inwards into the cavity 
 of the chela two long flat processes. To one of these (ad.) 
 is, during life, attached the adductor muscle ; to the other and 
 smaller process (db.) is attached the abductor muscle. By the 
 former the jaws are closed ; by the latter they are opened. The 
 other joints of the limbs are similar in principle. 
 
 The exoskeleton consists of a tough animal substance, chitin, 
 the product of the epithelial cells. Except where a joint renders 
 flexibility essential it is impregnated with calcareous salts (car- 
 bonate, with about one-eighth phosphate, of lime). The structure 
 of the exoskeleton (from the ophthalmite) is shown in Fig. 74, 
 K. There is an external wrinkled layer (epiostracum), below 
 which is a laminated mass in which are a vast number of close- 
 set wavy canals running outwards through its thickness. The 
 laminae are close-set externally (ectostracum, ec.), but wider 
 apart in the layers below (endostracum, en.). In the lowest 
 layers they again gradually become close-set and softer. A sec- 
 tion of the dried exoskeleton is easily made by grinding down 
 on a file or on pumice-stone, and polishing on a fine-grained hone. 
 
 The Branchial Chamber. If we cut with a pair of scissors from 
 behind forwards along the branchiocardiac groove, and then out- 
 wards along the cervical groove, a flap of exoskeleton, the 
 branchiostegite, will be removed, and the plume-like branchiae 
 
Chap. XII.] THE CRAYFISH. 239 
 
 or gills will be exposed to view. They lie in the branchial 
 cavity, which is bounded on the inner side by the delicate 
 calcified outer wall of the thorax (epimeron), and opens in front 
 by a canal in which a curved plate, the scaphognathite, in con- 
 nection with the second maxilla, is constantly working in such a 
 way as to produce a continuous current from behind forwards, 
 over the gills and through the canal. 
 
 The branchial cavity is completely filled with the gills and 
 with certain hair-like processes (setae), developed on the coxo- 
 podites of the neighbouring limbs. Of branchiae there are 
 eighteen pairs : 
 
 Six podobranchice ; these are the gill-bearing epipodites of the 
 limbs from the second maxilliped to the third leg inclusive. 
 
 Eleven arthrobranchice ; attached to the inter-articular mem- 
 branes between the basal joints of the appendages and the 
 thoracic wall ; one to that of each second maxilliped, and two 
 to those of the third maxilliped to the third leg inclusive. 
 
 One pleurobranchia ; attached to the wall of the thorax above 
 the fourth leg. There are two or three minute processes 
 representing rudimentary pleurobranchiae anterior to this. 
 
 The arthrobranchiae consist of a stem beset with branchial 
 filaments forming a plume. In the stem are two canals, an 
 afferent and an efferent. The pleurobranchiae are similar but 
 larger. The podobranchiae have a base beset with fine straight 
 hair-like setae. From this arises a stem which, above, carries a 
 plume in front, and behind a V-shaped lamina, with the point 
 of the V directed forwards. The lamina is plaited longi- 
 tudinally, and is beset with short hooked setae. 
 
 In examining the branchiae practically, lay the crayfish on its 
 side, and, after removing the branchiostegite, remove carefully, 
 one by one, the six podobranchiae. Then count the arthro- 
 branchiae, turning them carefully forwards as you do so. Note 
 that they are attached to the soft inter-articular membranes. 
 The last branchia is the pleurobranchia. Note that it is 
 attached to the calcified epimeron above the inter-articular 
 membrane. 
 
 The gills are highly vascular, the blood being separated from 
 
240 ANIMAL BIOLOGY. [Part II. 
 
 the surrounding water by a filmy chitinous membrane. Respira- 
 tion is effected by the absorption of oxygen dissolved in the 
 water, and the giving up of carbonic acid. 
 
 External Apertures. 1. On the concave upper face of the 
 basal joint of the antennule is an elongated aperture, guarded 
 by setae, which opens into the auditory sac. 
 
 2. On the basal joint of the antenna is a conical process, near 
 the apex of which is a small orifice, the opening of the green 
 gland or renal organ. 
 
 3. Between the mandibles is a large longitudinal slit, the 
 mouth. 
 
 4. On the basal joints of the second pair of legs are the 
 generative apertures of the female. 
 
 5. On the basal joints of the fourth pan\ of legs are the 
 generative apertures of the male. 
 
 6. Beneath the telson is the anal aperture. 
 
 General Internal Anatomy. When the tergal portion of the 
 cephalo-thorax and abdomen of the crayfish is removed by 
 snipping it away with scissors, and the underlying skin is care- 
 fully stripped off, the following organs are visible without 
 further dissection (Fig. 75). Anteriorly in the middle line is 
 the rounded sac-like stomach (st.). A whitish bar crosses it 
 transversely, the cardiac ossicle (c. 0s.), and to its anterior edge 
 are inserted two anterior gastric muscles, which are attached to 
 small (procephalic) processes at the base of the rostrum. Smaller 
 muscles (cardio-pyloric) pass back from its hinder edge towards 
 a second shorter transverse bar, and wing-like bars attached to 
 it. This bar is the pyloric ossicle. From this posterior bar 
 and its wings pass back two posterior gastric muscles (pg.) t 
 which are inserted in the carapace. Between them is a levator 
 muscle (I. m.). On either side of the stomach are the cut ends of 
 the powerful muscles (m. m.) by which the mandibles are closed. 
 At the sides of the anterior end of the cephalo-thorax, situated 
 low down, are two greenish oval masses, the green glands 
 (gr. gl.)^ surmounted on the inner side by a delicate sac (5.). 
 
Chap. XII.] 
 
 THE CRAYFISH. 
 
 241 
 
 Behind the levator muscle of the stomach are seen the anterior 
 lobes of the spermary or testis (a. t.\ the median posterior lobe 
 being seen further back akp. t. Between these is the heart (A.), 
 with two dorsal openings or valves (d. v.). The heart is held 
 in position by fibrous bands, the alae, fixed to its corners. 
 
 st. 
 
 r.ql. 
 
 .t. 
 
 int. 
 
 FIG. 75. CRAYFISH GENERAL ANATOMY. 
 
 a. d. g. Anterior part of digestive gland. . g. Anterior gastric muscle. 
 a. t. Anterior lobe of testis. c. os. Cardiac ossicle. c. p. Cardiopyloric 
 muscle, d. v. Dorsal valve of heart, gr. gl. Green gland, h. Heart, int. 
 Intestine. 1. m. Levator muscle, m. m. Mandibular muscle, p. g. Posterior 
 gastric muscle, p. d. g. Posterior part of digestive gland, p. os. Pyloric 
 ossicle, p. t. Posterior lobe of testis. s. Saccular part of green gland, s. a. a. 
 Superior abdominal artery, st. Stomach, v. d. Vas deferens. 
 
 Anteriorly it gives off a median ophthalmic artery and two 
 antennary arteries, the cut ends of which are figured in the 
 16 
 
242 ANIMAL BIOLOGY. [Part II. 
 
 diagram. Posteriorly it gives off a superior abdominal artery 
 (s. a. a.), which runs along the dorsal surface of the intestine 
 (int.), which takes a straight course along the median line to 
 the telson. The coiled tubes on either side of the heart are the 
 vasa deferentia (v. d.), and the masses of brown caeca belong to 
 the digestive gland or so-called liver (a. d. g., p. d. g.). 
 
 In the female there is in place of the testis an ovary of some- 
 what similar form, but shorter. In the breeding season it is 
 large and mottled with berry-like eggs. The oviducts are 
 straight thin-walled tubes running nearly vertically downwards. 
 
 The muscles of the crayfish are white, and the blood colourless. 
 
 The Alimentary System. The crayfish is a voracious feeder. 
 The food is seized by the forceps, transferred to the chelate 
 legs, which tear it to pieces and then pass it on to the maxilli- 
 peds and maxillae, the exopodites of which are in constant 
 trembling motion. The mandibles form powerful crushing 
 lateral jaws. With regard to this jaw-apparatus, as compared 
 with that of the vertebrate, it should be noted (1) that it is 
 external to the mouth ; (2) that the jaws move from side to side, 
 and not up and down. 
 
 The mouth itself is median and ventral, bounded at the sides 
 by the mandibles, in front by a shield-shaped plate or labrum, 
 and behind by the united bases of a pair of fleshy lobes which 
 form the metastoma. It leads into a short, wide oesophagus 
 lined with a thin chitinous coat continuous with the exoskeleton. 
 Salivary glands have been described as occurring in the walls of 
 the oesophagus, on the metastoma and first maxilliped, the 
 coxopodite of which is habitually tucked into the mouth. 
 
 The stomach is large and sac-like, and is divided into a larger 
 anterior cardiac chamber and a smaller posterior pyloric chamber. 
 The chitinous lining of the oesophagus is continued into the 
 stomach, and in the dorsal region gives rise to a gastric mill 
 composed of more or less calcified ossicles. Posterior to this it 
 is folded inwards into the narrowed pyloric regions as valvular 
 projections beset with hairlike setae. This fold has been termed 
 the filter or strainer. 
 
Chap. XII.] 
 
 THE CRAYFISH. 
 
 243 
 
 To facilitate his study of the ossicles of the gastric mill, the 
 student should remove the stomach with a portion of the 
 oasophagus and intestine, and should boil it in a dilute solution 
 of caustic potash to remove the softer parts. Making a ventral 
 incision along its whole length, he should then pin it out under 
 water, so as to expose the dorsal surface. Fig. 76 A. is a diagram 
 of the ossicles thus seen ; B. shows the ossicles separated in the 
 mid-line, and seen from the ventral aspect ; C. is a longitudinal 
 section of the whole stomach. 
 
 M.G. 
 
 H.G. 
 
 OPS 
 
 F.G 
 
 FIG. 76. CRAYFISH STOMACH. 
 
 A. Gastric ossicles from above. B. The same from below, separated trans- 
 versely. C. The stomach in longitudinal section. 
 
 c. Cardiac ossicle, cce. Caecum, g. Mark of gastrolith. h. d. Duct 
 of digestive gland. F. a. Fore-gut. H. G. Hind-gut. M. a. Mid-gut. 
 ces. (Esophagus, p. Pyloric ossicle, p. c. Pterocardiac ossicle, p. p. Pre- 
 pyloric ossicle, u. c. Urocardiac ossicle, z. c. Zygocardiac ossicle. 
 
 The essential parts of the gastric mill are : 
 
 1. (a.) An anterior transverse plate, the cardiac ossicle (c.), 
 
 the hinder part of which (shaded) is white and 
 
 calcareous, the part in front (dotted) transparent. 
 
 (b.) A smaller posterior transverse plate, the pyloric 
 
 ossicle (p.), the greater part of which is transparent. 
 
2*4 ANIMAL BIOLOGY. [Part II. 
 
 2. Between these two in the mid-line 
 
 (c.) An anterior median bar, the urocardiac, passing down- 
 wards and backwards, the posterior end of which 
 (see B.) has two oval brown convex surfaces. 
 
 (d.) A posterior median bar, the prepyloric, passing 
 downwards and backwards from the anterior edge 
 of the pyloric to the posterior end of the uro- 
 cardiac. Its ventral end carries a large brown 
 bifurcated median tooth. This ossicle is shown 
 in A. dotted, as seen through the transparent 
 pyloric. 
 
 3. Between the transverse plates on either side there are 
 
 (e.) On each side of the cardiac a small curved an- 
 terior lateral ossicle (pterocardiac). 
 (/.) Between the ventral or outer end of each ptero- 
 cardiac and the pyloric plate, a large posterior lateral 
 bar the zygocardiac. This has a strong projection 
 into the cardiac cavity, carrying a series of power- 
 ful brown lateral teeth. 
 
 The main muscles connected with this apparatus are (1) the 
 anterior and posterior gastric (see 75, a. g. and p. g.), which, by 
 their contraction, tend to separate the transverse plates; and 
 (2) the cardiopyloric (c. p.\ which tend to approximate these 
 plates. When the plates are drawn apart (see 76, C.) the lower 
 end of the prepyloric, with its powerful median tooth, is drawn 
 downwards and forwards. At the same time the zygocardiacs, 
 with their strong lateral teeth, are swung inwards by the curved 
 pterocardiacs ; so that the median and lateral teeth clash in the 
 middle line. This the student will readily see when he experi- 
 ments with the stomach he has removed and cleaned with 
 potash. 
 
 The filter cannot here be minutely described. The student 
 will, however, readily see how the canal is closed in by valves 
 and by the approximation of its walls, being indeed reduced to 
 a narrow triradiate slit. At the opening of the pyloric portion 
 into the intestine there is a median dorsal, and on each side two 
 lateral, pyloric valves. Here the chitinous lining ceases, and a 
 
Chap. XII.] THE CRAYFISH. 245 
 
 short section of the canal has naked epithelial walls. This is the 
 mid-gut (76, C.,M. G.). Near its commencement there is on each 
 side the wide opening of the duct (h. d.) of the digestive gland, 
 and dorsally a short caecum (cce.). 
 
 Behind the mid-gut there are six squarish elevations with a 
 chitinous investment. From them longitudinal ridges pass back- 
 wards, with a slight spiral twist, to the anus. The ridges are 
 beset with small papillae. All this part of the intestine is known 
 as the hind-gut, and is lined throughout with a chitinous mem- 
 brane continuous with the cuticle of the exterior. 
 
 The digestive gland is made up of an immense number of 
 caeca, It occupies a large space on either side of the mid-gut, 
 each lateral mass being slightly three-lobed. Each lobe ha*s a 
 main duct, the three uniting to form the common duct of that 
 side. The organ, as a whole, is to be regarded as a much-divided 
 diverticulum of the mid-gut. 
 
 Its product is a yellow fluid, which has a slightly acid reaction, 
 converts proteids into peptones, acts on starch, converting it into 
 sugar, and breaks up olive oil into an emulsion. The producing 
 cells are of two kinds; (1) hepatic cells containing oil-globules, 
 and giving rise to animal colouring matter; and (2) ferment 
 cells giving rise to the digestive secretion. It has been proposed 
 to call this digestive gland the hepato- pancreas. The student 
 must not regard it as homologous with these glands in the verte- 
 brata, but merely as partly performing the same functions. 
 
 On each side of the stomach (at g., Fig. 76, C.), there is formed 
 during the summer months a calcareous body called the gastro- 
 lith. During ecdysis these are shed into the interior of the 
 stomach, and there broken up and dissolved. If they are not 
 present, ecdysis would seem to be unsatisfactorily performed. It 
 is said that during exuviation a layer of processes, or " casting 
 hairs," is formed in such a way as to aid in loosening the old 
 shell from the new. 
 
 The Heart and Circulation. In a recently-killed crayfish the 
 systole and diastole (contraction and relaxation) of the heart 
 may be seen. This organ has the form shown in Fig. 75, k., 
 
246 
 
 ANIMAL BIOLOGY. 
 
 [Part II. 
 
 and Fig. 77, A. It lies in the pericardial sinus, being suspended 
 therein by fibrous bands, the alse. Six main valvular aper- 
 tures admit the blood from the pericardial sinus, two dorsal (75, 
 d. v.), two ventral, and two lateral (77, A., /. v.). The blood 
 
 A. 
 
 FIG. 77. CRAYFISH HEART : CIRCULATION : GREEN GLAND. 
 A. Heart from left side. B. Diagram of Circulation in the Thorax. 
 
 C. Green Gland. 
 
 a. v. Afferent vessel to gill. an. a. Antennary artery. 6. c. v. Branchio- 
 cardiac vein. c. p. Conical process at base of antenna, d. v. Dorsal valve of 
 heart, e. v. Efferent vessel from gill. gr. gl Excretory part of green gland. 
 he. a. Hepatic artery. I. v. Lateral valve of heart, op. a. Ophthalmic artery. 
 p. s. Pericardial sinus, s. a. a. Superior abdominal artery, s. Saccular part 
 of green gland, s. s. Sternal sinus, st. a. Sternal artery. 
 
 leaves the heart by seven arteries, the positions of which are 
 seen in the side view of the heart (77, A.). The median anterior 
 ophthalmic artery (op. a.) runs forward over the stomach, to 
 supply the eyes and region adjacent. On either side of this are 
 the antennary arteries (an. a.) which supply the stomach, man- 
 dibular muscles, green gland, and antennary appendages. Two 
 hepatic arteries (he. a.) are mainly distributed to the liver. 
 These five start from the anterior end of the heart. From the 
 posterior end the superior abdominal (s. a. a.) runs back along 
 the intestine, supplying it and the dorsal part of the abdomen. 
 A sternal artery (st. a.) runs downwards and slightly forwards, 
 gives off branches to the vas deferens or oviduct, pierces the 
 
Chap. XII.] THE CRAYFISH. 247 
 
 ventral nerve-chain, and enters the sternal sinus within the 
 endophragmal system. Thence one branch runs forward, sup- 
 plying the ventral region and limbs of the cephalo-thorax so far 
 as the mouth, another backward to supply the ventral region and 
 limbs of the abdomen. 
 
 The arteries break up into capillary ramifications. From 
 these, however, the blood is not collected into veins, as in the 
 vertebrate, but passes into sinuses irregularly disposed among 
 the internal organs. After passing through these sinuses it 
 eventually reaches the sternal sinus (s. s.) beneath the cephalo- 
 thorax. Thence it passes by afferent vessels (a. v. in Fig. 77, B., 
 which shows diagrammatically the circulation in the thorax) to 
 the gill-plumes ; returning by efferent vessels (e. v.) to the base 
 of the gill, whence it is carried to the pericardial sinus (p. s.) by 
 branchio-cardiac vessels (b. c. v.). Thus the blood is aerated on 
 its way to the heart. In the cod-fish it is aerated in the gills on 
 its way from the heart. 
 
 The blood itself is a clear colourless fluid, which coagulates 
 to form a clot. It contains colourless nucleated amoebiform 
 corpuscles. 
 
 The Green Glands. The excretory organs by which nitro- 
 genous waste is eliminated from the blood of the crayfish are 
 the green glands (Fig. 75, gr. gl., and Fig. 77, C.). On the basal 
 joint of the antenna there is a small conical prominence (77, C., 
 c.p.), with an aperture on the inner side of its summit. This 
 leads into the thin-walled sac (5.), which may be regarded as the 
 much-expanded end of an immensely convoluted tube, which 
 ends blindly in a triangular yellowish-brown lobule. This con- 
 voluted tube forms the body of the gland (gr. gl.) The secretion 
 contains large quantities of uric acid, with very small traces of 
 guanin a substance analogous in some respects to uric acid, but 
 less highly oxidised. 
 
 The Nervous System. The nervous system of the crayfish 
 presents us with a chain of ganglia running along the ventral 
 aspect, connected by commissures. 
 
248 ANIMAL BIOLOGY. [Part II. 
 
 After the crayfish has been opened up as shown in Fig. 75, 
 and the relations of the alimentary system, the heart, and the 
 organs of generation (p. 252) have been studied, the abdominal 
 muscles should be carefully removed, the ganglia of this region 
 being thus displayed. On removal of the viscera of the cephalo- 
 thorax it is seen that the nerve-chain passes into the canal 
 formed by the apodemata of the endophragmal system. The 
 roof of this canal should be carefully cut away so as to expose 
 the whole chain, the anterior ganglion of which lies between the 
 eyes at the base of the rostrum. The chain is so easily exposed 
 and so readily understood that a figure is here unnecessary; 
 though the student should of course, in this as in all cases, draw 
 one from his own dissection. 
 
 There are thirteen pairs of ganglia. In the abdomen there 
 are six, slightly bi-lobed, connected by commissures which, on 
 closer inspection, are seen to be double. In the thorax there 
 are six. The most anterior is obviously multiple, the result of 
 the fusion of two or more pairs. Lying behind and beneath 
 the gullet it is called sub-cesophageal. This, the second, and 
 the third thoracic ganglia, are connected by somewhat elongated 
 double commissures. Between the third and fourth the com- 
 missures are short. Between the fourth and fifth they separate 
 so as to allow of the passage of the sternal artery. The fifth 
 and sixth lie close together, almost in contact. From the sub- 
 cesophageal there pass forward two long commissures, which 
 separate to pass one on each side of the gullet, uniting anteriorly 
 in one large supra-cesophageal or cerebral ganglion. 
 
 From the ganglia and inter-ganglionic commissures nerves are 
 given off. From the cerebral ganglion two antennary, two 
 antennulary, and two optic nerves are readily traced. From the 
 sub-cesophageal ganglion nerves may be traced to the muscles of 
 the mandibles, maxillae, and maxillipeds. Thus these two main 
 ganglia innervate the first nine segments. The other eleven 
 ganglia are segmentally arranged, and each innervates its own 
 somite and appendages. 
 
 Just anterior to the gullet the commissures give origin to nerves, 
 which meet in the mid-line and pass backwards as an unpaired 
 
Chap. XII.] THE CRAYFISH. 249 
 
 nerve (anterior visceral), which breaks up into branches which 
 innervate the stomach. From the last abdominal ganglion a nerve 
 (posterior visceral) passes forwards to innervate the hind-gut. 
 
 It is probable that the original, i.e. not merely responsive, 
 activities of the crayfish depend on the integrity of the supra- 
 cesophageal ganglion, which also probably possesses an inhibitory 
 and co-ordinating power over the lower centres. The sub-ceso- 
 phageal ganglion, however, may have some co-ordinating power 
 over lower centres. If both these higher centres be removed the 
 crayfish, as an organism, is probably dead ; but the individual 
 life and activity of the legs, chelae, and swimmerets, continues 
 vigorously but quite aimlessly. 
 
 Special Senses. (1.) Touch. Scattered over the surface of the 
 body, or grouped in clusters at certain points, are a number of 
 setce (78, A. and B.) which are probably organs of touch. Each 
 is a hollow diverticulum of the body-wall and exoskeleton. 
 During life they are filled with delicate prolongations of the 
 epithelial layer of the integument. 
 
 (2.) Smell. From the seventh or eighth to the last joint but 
 one of the exopodite of the antennule, there are, on the under side 
 of each joint, two bundles of curiously-modified setae (78, A. and 
 C.). These are regarded as olfactory. 
 
 (3.) Taste. No special organs of taste have been discovered. 
 Such organs should be sought for on the metastoma, or perhaps 
 on the maxillary appendages. It is scarcely probable that this 
 sense is altogether wanting. 
 
 (4.) Hearing. The auditory sacs are lodged in the basal joint 
 of the antennule. The external aperture is in the concave upper 
 face, its outer lip being guarded by closely-set setae. Within 
 the sac are delicate feathered auditory setae (78, D.) implanted 
 on either side of a ridge. Their tips are often imbedded in a 
 gelatinous mass in which there are minute particles of sand (s.) 
 
 (5.) Sight. The eye has a convex cornea (78, E., co.) continuous 
 with the exoskeleton, and marked out into a number of facets, 
 which are square, except near the edges of the eye where they 
 become polygonal and eventually hexagonal. This is readily 
 
2 5 
 
 ANIMAL BIOLOGY. 
 
 [Part II. 
 
 seen in a dried ophthalmite under a low power. Beneath each 
 facet is the end of a crystalline cone (cr.), which narrows in the 
 deeper part of the eye, and then swells again to form a striated 
 
 D. 
 
 FIG. 78. CRAYFISH SENSE ORGANS. 
 
 A. Joints near the end of the exopodite of the antenuule, showing tactile 
 setse at a and olfactory setae at &. 
 
 B. Tactile setae. a. From the tip of the endopodite of the antennule. fc. 
 From a tuft on the antennulary protopodite. 
 
 C. Olfactory setse. a. From the side. I. From above. 
 
 D. Auditory seta, with grains of sand at s. 
 
 E. Portion of section of eye. &. TO. Basilar membrane, co. Cornea, cr. 
 Crystalline cone. g. c. Ganglion cells, n. f. Nerve-fibres. re. Pigment. 
 st. sp. Striated spindle. 
 
 F. Diagram of arthropod vision. a. 6. Transparent rods, with pigment 
 between, c. d. Object. (/. d'. Its image on the screen e. f. 
 
 spindle or rhdbdom (st. sp.) marked with delicate transverse lines. 
 At the inner end of the striated spindles there seems to be a 
 basilar membrane (b. m.\ beneath which are the nerve fibrils 
 
Chap. XII.] THE CRAYFISH. 251 
 
 and cells of the optic bulb. The connective rods between the crys- 
 talline cones and striated spindles may after treatment with 
 osmic acid and teasing be resolved into four fibres. The inner 
 ends of striated spindles are probably continuous with the nerve- 
 fibrils of the optic bulb. Pigment is developed round the outer 
 ends of the crystalline cones, the striated spindles, and the outer 
 nerve-fibrils of the optic bulb. 
 
 Two theories of arthropod vision have been advocated : (1) 
 that of compound vision ; (2) that of mosaic vision. According 
 to the former, a separate image of the object seen is formed at 
 the end of each cone and spindle, the multitude of images being 
 presumably co-ordinated by the arthropod cerebral ganglion, just 
 as two images are in us by the human brain. This view finds few 
 supporters to-day. The principle of the other view, that of 
 mosaic vision, will be understood with the aid of the diagram 
 78, F. At a. b. are a number of transparent rods separated by 
 pigmented material absorbent of light. At c. d. is an arrow placed 
 in front of them. At e. f. is a screen placed behind them. Eays 
 of light start in all directions from any point c. of the arrow ; but 
 of these only that which passes straight down one of the trans- 
 parent rods reaches the screen. Those which pass obliquely into 
 other rods are absorbed by the pigmented material. Similarly 
 with rays starting from other points of the arrow. Only those 
 which pass straight down one of the rods reach the screen. 
 Wherefore there is thrown on the screen a reduced image c'. d'. 
 of the arrow. The image is not continuous, but stippled. 
 
 So much for the general principle of so-called mosaic vision. 
 According to this theory the several points of the object 
 which excites vision stimulate corresponding points in the re- 
 cipient organ. To the question, What are the true recipient 
 end organs of the optic nerve 1 several answers have been sug- 
 gested. 
 
 1. The crystalline cones are the end organs, and are 
 continuous through the basilar membrane with the nerve- 
 fibrils. 
 
 2. The cones and spindles are simply dioptric, and beneath 
 the basilar membrane is a retina which is the true recipient. 
 
252 ANIMAL BIOLOGY. [Part II. 
 
 3. The crystalline cones are dioptric, the connective rods and 
 the striated spindles being the true end organs continuous with 
 the nerve-fibrils. 
 
 The third answer is that most generally accepted. On this 
 view the spindles, nerve-fibrils, and ganglion-cells answer to the 
 vertebrate retina. Note that, if this be so, in the arthropod the 
 recipient end-organs are turned towards the source of light, and 
 not away from it as in the vertebrate. 
 
 Organs of Reproduction. (L) In the Hale. The spermary, or 
 testis, has two anterior lateral lobes and one median posterior 
 lobe. Its position is seen in Fig. 75, a. t., p. t. From the points 
 where the anterior lobes join the median lobe there arise the 
 much-coiled vasa defer entia, which, after a convoluted course, 
 pass downwards to open on the basal joints of the last thoracic 
 appendages. 
 
 In minute structure the testis shows an immense number of 
 vesicles attached to the ducts which lead to the vasa deferentia. 
 These acini are aggregated into lobules. The large nucleated 
 cells which line the cavities of the acini multiply rapidly with 
 karyokinesis (p. 89), and are eventually converted into sperma- 
 tozoa of the peculiar and unusual form shown highly magnified 
 in Fig. 79, A. In the vasa deferentia they become invested 
 with a viscid secretion. The vermicelli-like matter thus pro- 
 duced is deposited by the male on the posterior thoracic and 
 anterior abdominal somites of the female. 
 
 (2.) In the Female. The ovary is in shape somewhat like a 
 shortened thickened testis. The short wide oviducts pass down- 
 wards from its ventral side to open on the basal joint of the 
 second leg (sixth thoracic appendage). Note, as something 
 different from what we have seen in the frog, fowl, or rabbit, 
 that the lumen of the oviduct communicates with the hollow 
 interior of the ovary. The development of the ova may be 
 thus summarised. 
 
 1. A group of cells forms a papilla projecting into the cavity, 
 and covered by a structureless membrane, the membrana propria 
 (m. pr.) Such a group forms an ovisac. 
 
Chap. XII.] 
 
 THE CRAYFISH. 
 
 253 
 
 2. The central cell in the ovisac grows at the expense of the 
 others, and becomes the ovum. Certain of the others form a 
 layer round it, and constitute the epithelium of the ovisac (ep.) 
 
 "Hfl. arch. U. 
 
 FIG. 79. CRAYFISH DEVELOPMENT. 
 
 A. Spermatozoon. B. Ovum. ep. fo. Epithelium of follicle, g. s. Ger- 
 minal spot, g. v. Germinal vesicle, m. p. Membrana propria. v. m. Vitelline 
 membrane. 
 
 C. Ovum in which segmentation has produced a blastodermic layer, 6.7. 
 v. m. Vitelline membrane. 
 
 D. Invagination of blastodermic layer. arch. Archenteron. 1)1. Blastopore. 
 ep. Epi blast, hy. Hypoblast. tn. Mesoblast. 
 
 E. Formation of fore and hind-gut. arch. Mesenteron. pr. Proctod&um. 
 st. Stomodseum. 
 
 F. View of developing embryo. ab. Abdomen, m. Stomodseal invagina- 
 tion for the mouth, op. Optic pit. pr. I. Procephaliclobe. 2,3,4. Second, 
 third, and fourth appendages. 
 
 3. The nucleus of the ovum becomes the germinal vesicle (g. v.). 
 
 4. Within it there appear numerous small corpuscles, the 
 germinal spots (g. s.). 
 
254 AINMAL BIOLOGY. [Part II. 
 
 5. The protoplasm of the ovum is largely converted into deep 
 brownish-yellow vitellus or yolk. 
 
 6. A vitelline membrane (v. m.) is formed around the ovum 
 with its vitellus (Fig. 79, B.). 
 
 7. The ovisac bursts and the ovum is set free. The vitelline 
 membrane continues to invest it. 
 
 8. In the oviduct it becomes invested with a viscid trans- 
 parent substance which sets into a tougn case. This becomes 
 suspended by a stalk of the same material to the swimmerets of 
 the female. 
 
 The exact manner in which the spermatozoa unite with the 
 ova is not known. 
 
 Development. Owing to the large amount of food-yolk segmen- 
 tation is partial or incomplete. In the fowl, where the segmen- 
 tation is incomplete, it is also irregular, owing to the fact that 
 the germinal disc of formative protoplasm is placed at one pole 
 of the egg (telolecithal). In the crayfish the segmentation is 
 regular, owing to the fact that the formative protoplasm is at 
 the circumference, the food-yolk being central in position 
 (centrolecithal). 
 
 By segmentation, a blastodermic layer (b. I.) is formed (Fig. 
 79, C.) as an investment to the food-yolk, which is arranged in 
 conical masses. It is composed of nucleated protoplasmic plates, 
 but the relation of the nuclei of these plates to the germinal 
 vesicle is not well ascertained. Out of these protoplasmic plates 
 the embryo is fashioned. 
 
 The first change is that the blastodermic layer becomes 
 thickened over an area -^ inch long in the neighbourhood of the 
 pedicle, forming the germinal disc. Invagination takes place at 
 the hinder end of this area (79, D.). Thus we have a differentia- 
 tion of the blastodermic layer into epiblast (ep.) and hypoblast 
 (hy.) with the formation of an archenteron (arch.) and a blasto- 
 pore (II). 
 
 The mesgblast cells make their appearance (m.) in the neigh- 
 bourhood of the blastopore between the hypoblast and the 
 epiblast, its cells being probably derived from the former. 
 
Chap. XII.] THE CRAYFISH. 255 
 
 Thence it gradually spreads, first over the sternal and eventually 
 over the tergal aspect of the embryo. 
 
 The blastopore completely closes. The archenteron gives 
 rise to the mid-gut, from which the digestive gland arises as a 
 diverticulum. The mouth and fore-gut arise as a stomodaeal in- 
 vagination (79, E., st.) ; the vent and hind-gut as a proctodaeal 
 invagination (pr.). These two invaginations do not at first com- 
 municate with the archenteron. 
 
 The anterior end of the embryo becomes marked out by the 
 formation of procephalic lobes (Fig. 79, F.,^>r. /.), in the midst of 
 each of which is an optic pit (op.). The posterior end is marked 
 out by a rounded elevation, the rudiment of the abdomen. 
 Between these two is a narrow groove, in the centre of which is 
 the stomodaeal invagination (m.). The sides of this groove grow 
 inwards to form two solid cords of cells of epiblastic origin. At 
 its front end a mass of cells grows inwards and connects the 
 two cords in front of the mouth. The anterior mass gives rise 
 to the cerebral ganglion ; the two cords which partially coalesce 
 in the mid-line differentiate into the post-cesophageal chain of 
 ganglia. Thus the central nervous system has an epiblastic 
 origin. 
 
 The first appendages to appear are the rudiments of the 
 antennae (2), antennules (3), and mandibles (4). In certain 
 other Crustacea these are greatly developed as embryonic oar- 
 like appendages, and the embryo at this stage becomes free. 
 Such an embryo is called a nauplius. In the crayfish these 
 appendages do not thus develop into embryonic organs, nor does 
 the embryo become free till a much later stage. But the forma- 
 tion of a delicate cuticle after the appearance of these three 
 appendages seems, in the crayfish or allied forms, to symbolise 
 the nauplius condition. The other thoracic appendages appear 
 as bud-like protuberances in regular order. All are similar at 
 first, but subsequently differentiate. 
 
 When it is hatched, the young crayfish differs considerably 
 from the adult. The cephalothorax is more convex and larger. 
 The rostrum is short and bent down. The sterna of the thorax 
 are relatively wider. The chelae are more slender, and their 
 
256 ANIMAL BIOLOGY. [Part II. 
 
 tips are incurved. The dactylopodites of the third and fourth 
 pairs of legs are hooked. The appendages of the first abdominal 
 somite are undeveloped. Those of the last are enclosed 
 within the telson. It is probably set free at the first ecdysis. 
 Thus, as we found to be the case in the vertebrate, after trans- 
 formation is over there still is development. 
 
 The student should endeavour to obtain, in the spring 
 or early summer, a female crayfish with developing embryos. 
 He will find a study of the development of their external 
 features a matter of no great difficulty and of very great 
 interest. 
 
CHAPTER XIII. 
 
 THE COCKROACH. 
 
 THE Common Cockroach (Periplaneta (Blatta) orientalis) is not 
 an indigenous insect, but an immigrant from the East that has 
 domesticated itself in European kitchens. Although commonly 
 called a black-beetle, the cockroach does not belong to the 
 true beetles (Coleoptem\ but to a less modified group of insects, 
 including the Locusts and Grasshoppers, known as Orthoptera. 
 Unlike the higher and more modified insects (Butterflies, House- 
 flies, Beetles, Bees, Ants), which undergo post-embryonic 
 metamorphoses, passing through a soft-bodied, worm-like larva 
 stage, and an inactive pupa or chrysalis stage, the cockroach, 
 though it exhibits a certain amount of development after birth, 
 undergoes no true post-embryonic metamorphosis. The minute 
 cockroaches that emerge from the egg are little white six-legged 
 creatures, with black eyes and without wings. They change 
 their skin, or moult, seven times, rudiments of wings and wing- 
 covers making their appearance in the later larval stages. After 
 the seventh ecdysis (which is said to occur when the insect is 
 about four years old) the cockroach is adult. Those insects 
 which undergo complete metamorphosis are known as Holometa- 
 bola, while those in which the metamorphosis is absent or, as in 
 the cockroach, incomplete, are grouped together as Ametabola. 
 In this respect, therefore, and in certain others, the cockroach is 
 a little-specialised or primitive insect. Its family is also a very 
 ancient one, remains of cockroaches being found in the Silurian 
 strata. 
 
 External Characters. The body is marked out into distinct 
 regions. A well-developed head is followed by a narrow neck, 
 17 
 
258 ANIMAL BIOLOGY. [Part II. 
 
 and this by a thorax divisible into a fore, mid, and hind seg- 
 ment (pro-, meso-, and metathorax). Posterior to the thoracic 
 region is the abdomen, in which ten segments may be made out, 
 while two small triangular podical plates, one on each side of the 
 vent, are held by some anatomists to represent an eleventh. 
 
 The surface of the body and limbs is invested by a chitinous 
 exoskeleton, laminar in structure and secreted by a chitinogenous 
 layer of cells (hypodermis). Although the chitinous investment 
 is continuous, it is not everywhere of the same thickness or 
 consistency. Where mobility is required, it forms a yielding 
 cuticle, elsewhere it is thickened into harder and less yielding 
 plates (sclerites) or tubes. The whole cuticle, with its thickened 
 elements, is shed at the periods of ecdysis, a median dorsal 
 longitudinal fissure being formed in the thorax, through which 
 the insect escapes from its old skin. 
 
 A front view of the head is shown in Fig. 80, A. In the 
 upper region is seen the epicranial suture (ep.), the lateral 
 branches of which end in small rounded white spaces (w. s.). Near 
 these spring the long antennce (an.) from an antennary fossa (an. /.). 
 External to these are the large compound (polymeniscous) eyes. 
 The mid-region of the face (clypeus, d.) narrows below somewhat 
 suddenly at the ginglymus,wheYQ the large mandibles (I. mn., r. mn.) 
 are articulated. The flap-like piece hanging from its lower 
 edge is the labrum (lb.), behind which lies the mouth. Below the 
 mandibles, one of which is represented as widely open, are the 
 maxillce (mx.), to which are attached long maxillary palps (mx.p.\ 
 The shorter palps (labial palps, I. p.) belong to the labiuin. 
 Looking down upon the head from above, there are seen on 
 either side of the mid-line the epicranial plates, separated by the 
 epicranial suture. On either side of these are, anteriorly the 
 eyes and posteriorly the gence, which, passing down behind the 
 eyes, form the sides of the exoskeleton of the head, reaching as 
 far as the mandibles, and ending somewhat pointedly above the 
 ginglymus. A posterior view of the head shows a large shield- 
 shaped occipital foramen, the lateral margin of which is 
 strengthened by a thickened rim continuous with the tentorium 
 or internal skeleton of the head. The top and sides of this rim 
 
Chap. XI 1 1.] 
 
 THE COCKROACH. 
 
 259 
 
 are bounded by the epicranial plates, which end below in 
 facets for the articulation of the mandibles. At the inferior 
 boundary of the foramen there is a large plate, the sub-mentum, 
 which forms part of the mouth organs. 
 
 lac. 
 
 D. 
 
 FIG. SO. COCKROACH APPENDAGES. 
 
 A. Ileiul of cockroach ; front view. B. Mouth organs removed. C. A leg. 
 D. Gtmapophyses of female. 
 
 . and &. Lateral pieces, an. Antenna, an. f. Antennary fossa, an. gon. 
 Simple gonapophysis. ca. Cardo. d. Clypeus. cox. Coxa. e. Eye. ep. 
 Epi cranium, fe. Femur, g. a. Genital aperture, ga. Galea. I. Lacinia 
 oflabrum. 1. mn. Left mandible. I. p. Labial palp. lac. Lacinia of maxilla. 
 Ib. Labrum. m. Mentum. mx. Maxilla, mx. p. Maxillary palp. no. Notum. 
 p, gl. Paraglossa. pos. gon. Bifid gonapophysis. r. mn. Right mandible, 
 s. m. Sub-mentum. sp. Opening of spermatheca. st. (in B.) Stipes. st.(m C.) 
 Sternum, ta. Tarsus, ti. Tibia, tro. Trochanter. tin. Ungues. u\ s. "White 
 spot. viii. ix. Abdominal sterna. 
 
 The mouth organs posterior to the mandibles are shown in 
 Fig. 80, B. The median part results from the fusion of a pair 
 
260 ANIMAL BIOLOGY. [Part II. 
 
 of labial appendages. It consists of a sub-mentum (s. m.) and 
 mentum (m.), beyond which the labium is bifid, there being an 
 inner part, the lacinia (/.), and an outer part, the galea or para- 
 glossa (p. gl.), on each side, while labial palps (1. p.) spring from 
 the mentum. External to the labium are the maxillce. Each 
 has a basal part, the cardo (ca.\ which articulates with a pedicel 
 arising from the occipital rim. Articulating with the cardo is 
 the stipes (st.) y beyond which the maxilla is composed of two 
 parts : the inner, with sharp point and spinose blade, is the 
 lacinia (lac.) ; the outer, softer and setose, is the galea (ga.). 
 Where the galea joins the stipes springs the five-jointed 
 maxillary palp (mx. p.) The student should find little difficulty 
 in detaching and mounting the organs as figured. To get rid of 
 the soft tissues it should be boiled in a dilute solution of potash. 
 
 It is not improbable that the mandibles, maxillse, and labium, 1 
 are homologous with the mandibles, first maxillse, and second 
 maxillse of the crayfish. It is not certain whether the antennae 
 of the cockroach and crayfish are homologous. 
 
 Within the head is a complex, somewhat cruciform, chitinous 
 framework, the tentorium, for the support of the gullet and 
 brain, and the attachment of muscles. Nerve cords pass 
 through a central aperture, 
 
 In the neck there are eight small cervical plates or sclerites : 
 two dorsal, somewhat triangular and apposed in the mid-line ; 
 four (two on each side) lateral, and somewhat oblique ; two 
 ventral, transverse and median, lying one behind the other. 
 
 In each somite of the thorax there is a distinct dorsal or 
 tergal plate. The anterior of these, the pronotum, is the largest. 
 The posterior plate (metanotum) is slightly smaller than the mid- 
 plate (mesonotum). In the adult male the meso- and metanotum 
 are thin and semi-transparent. In each somite of the thorax 
 there is also a ventral plate, the sternum. The most anterior (pro- 
 sternum) is coffin-shaped, with the long axis placed longitudinally. 
 The mid-plate (mesosternum) shows indications in the male of 
 
 1 The labrum is, in some insects (e.g. bees and their allies), regarded by 
 certain recent observers as originally double, and as an extreme example of the 
 coalescence of a pair of appendages. This is, however, doubtful. 
 
Chap. XIII.] THE COCKROACH. 261 
 
 being divided into two lateral halves, and is so divided in the 
 female. The metasternum is the largest of the three sterna, and 
 is in both sexes completely divided into two lateral divisions, 
 between which is wedged in the base of a Y-shaped chitinous 
 prop (post-furca) for the support of the nerve-chain and for 
 attachment of muscles. A similar prop (medi-furca) is connected 
 with the mesosternum. An anti-furca is, if at all, incompletely 
 represented. 
 
 Each somite of the thorax carries a large pair of legs, of 
 which the anterior are the smallest, the posterior the largest. 
 Like those of the crayfish, they are jointed (arthropod type) ; 
 the divisions being coxa (proximal), trochanter, femur, tibia, and 
 tarsus, composed of six joints (or five joints, the last being sub- 
 divided). The distal division of the tarsus bears claws or 
 ungues (80, C.). 
 
 The limbs do not articulate directly with the sterna or nota, 
 two lateral pieces intervening. These are shown in 80, C. One 
 of these (a) is, in the third limb, triangular, and is attached to 
 the coxa. The other (b) is larger, and curves round the angle 
 of a. It articulates, on the one hand, with the sternum (st.), 
 and on the other with the notum (no.). This latter piece (b) 
 seems, in the cockroach, to belong to the somite ; the former (a) 
 to the appendage. But there is some doubt about the homo- 
 logies of these parts in the insecta. 
 
 Male cockroaches have, when adult, two pairs of wings. The 
 anterior (wing-covers or tegmina) are brown and stiff, and 
 attached to the second thoracic segment; the posterior pair, 
 attached to the metathorax, are soft and membranous, and, 
 when not used in flight, carried folded up beneath the wing- 
 covers. Both tegmina and wings are strengthened by nervures, 
 united by transverse ridges. The disposition of the nervures 
 will be readily seen with the aid of a lens. It is a matter of 
 importance in the classification of these and allied insects. The 
 old-world cockroaches seem to have had transparent mem- 
 branous wing-covers or anterior wings. 
 
 In the female the wings remain in a rudimentary condition. 
 The anterior pair (wing-covers) are small, not reaching beyond 
 
262 ANIMAL BIOLOGY. [Part II. 
 
 the middle of the metanotum. The posterior pair are repre- 
 sented by a reticulated pattern on the lateral edges of the meta- 
 notum. This suggests or illustrates the view which is otherwise 
 enforced by the study of development, that the wings of insects 
 are homologous with the free edges of meso- and metathoracic 
 terga. Such a wing arises as a sac-like fold of the chitinous 
 cuticle, which passes into the condition of a double lamina, 
 which, by the coalescence of its inner surfaces, becomes a con- 
 tinuous membranous expansion, strengthened by nervures which 
 take their origin in connection with the tracheal tubes. 
 
 The abdomen of the female is broader than that of the male. 
 There are ten terga, of which the eighth and ninth are hidden 
 beneath the seventh. The tenth is large, of peculiar shape, 
 and notched posteriorly. The first sternum is small and rudi- 
 mentary ; the second large, with an irregular anterior border ; 
 the third and fourth are the broadest. The seventh is very 
 large, boat-shaped and cleft, a curious fold of integument being 
 visible when the two halves are opened out laterally. The 
 parts behind the seventh sternum are somewhat complicated. 
 This is largely due to the infolding of the cuticle to form a 
 genital pouch. The floor of this pouch is formed by the mem- 
 brane between the seventh and eighth sterna ; the back wall of 
 the pouch is formed by the large eighth sternum, which is per- 
 forated by the large genital aperture. In the dorsal wall lies 
 the ninth sternum, carrying the orifice of the spermatheca. 
 Further back is the anal opening. The parts are best studied 
 by removing all the ventral region posterior to, and including, 
 the seventh sternum, boiling in dilute caustic potash, and 
 examining under the simple microscope. When the relations of 
 the parts have been made out, the seventh sternum may be 
 carefully removed, and by cutting aloog the edges of the eighth 
 sternum and bending it back, the remaining parts may be 
 mounted, as shown in Fig. 80, D. In addition to the sterna 
 VIII. and ix., two pairs of appendages, which belong to these 
 somites, and which are termed gonapophyses, are seen. The 
 anterior gonapophyses (an. gon.) of the eighth somite are some- 
 what thumb-shaped, and are normally applied together in the 
 
Chap. XIII.] THE COCKROACH. 
 
 mid-line. Their basal moieties are broad, and serve to 
 of the roof of the genital pouch. At the base of their 
 moieties a chitinous slip passes to the eighth and ninth terga. 
 The posterior gonapophyses (pos. gon.), which normally lie above 
 the anterior pair, are divided into two portions, the inner slender 
 and hard, the outer larger and soft. In the cockroach these 
 appendages grasp the egg-capsule while it is being extruded 
 from the genital pouch. The notched dorsal edge of the capsule 
 bears the imprint of the inner portion of the posterior gon- 
 apophyses. In some insects this apparatus is curiously modified 
 to form an ovipositor, by means of which holes are bored in 
 earth, wood, or animal tissue, for the reception of the ova. In 
 the bee the sting is a specialised form of the same apparatus. 
 
 Two cigar-shaped sixteen- jointed processes, the cerci, which 
 probably have a sensory function, spring in both sexes from the 
 edges of the tenth somite. On either side of, and slightly 
 dorsal to, the vent, there are, in both sexes, triangular podical 
 plates, regarded by some anatomists as the terga of an eleventh 
 abdominal somite. 
 
 Ten terga and nine sterna can be made out in the male ; but 
 the first sternum is, as in the female, rudimentary. Attached 
 to the ninth sternum are two slender sub-anal styles. There is 
 also a complex system of plates and hooks, concealed within 
 the ninth sternum, and constituting the male genital armature. 
 
 If we take the podical plates to represent an eleventh ab- 
 dominal segment, the number of segments in the cockroach 
 would seem to be seventeen, three cephalic, three thoracic, and 
 eleven abdominal. 
 
 The apertures of the body are as follows : 
 
 1. The mouth, lying between the labrum and the labium, and 
 bounded laterally by the mandibles and maxillae. 
 
 2. The vent, lying between the triangular podical plates. 
 
 3. Twenty spiracles, ten on each side of the body, forming the 
 external orifices of the tracheal system. Eight on each side 
 are abdominal, between the sterna and terga ; two on each side 
 are thoracic, between the pro- and meso-, and the meso- and meta- 
 thorax. The thoracic stigmata are provided with external valves. 
 
264 
 
 ANIMAL BIOLOGY. 
 
 [Part II. 
 
 4. In the female, (a) the genital opening on the eighth 
 sternum ; (b) the opening of the spermatheca on the ninth 
 sternum. 
 
 5. In the male, (a) the opening of the ejaculatory duct 
 behind the ninth sternum ; (b) the opening of a subsidiary 
 gland on a double hook, which forms part of the external 
 genital armature. 
 
 At these apertures the chitinous exoskeleton is inflected 
 inwards. 
 
 FIG. 81. COCKROACH ALIMENTARY CANAL. 
 
 A. Alimentary canal dissected out. B. Part of gizzard under simple micro- 
 scope. C. One tooth of the same from the side. 
 
 6. c. Buccal cavity, cce. Pyioric cseca. co. Colon, cr. Crop. gi. Gizzard. 
 II. Ileum. Ib. Labrum. li. Lingua, mn. Mandibles, m. t. Malpighian 
 tubules, ces. (Esophagus, p. p. Podical plate, r. Rectum, s. gl. Salivary 
 glands, s. r. Salivary receptacle, ve. Chylific ventricle. 
 
 The Alimentary Canal. J The alimentary canal of the cock- 
 roach (Fig. 81) has three divisions : (1) a stomodceal section 
 anteriorly, lined with a chitinous layer continuous with the 
 
 1 The cockroach is conveniently dissected, under the simple microscope, or 
 with the aid of a watch-maker's lens, in a shallow tin or glass vessel. In the 
 bottom of such a vessel melt some paraffin, and mix in lamp-black. Allow the 
 paraffin to cool and solidify as a firm cake at the bottom of the vessel. With a 
 hot glass rod rnelt a convenient cavity and imbed therein the cockroach. Dis- 
 sect under water or alcohol. 
 
Chap. XIII.] THE COCKROACH. 265 
 
 exoskeleton at the mouth ; (2) the mesenteron, without chitinous 
 lining ; (3) a proctodceal section, lined with a chitinous infolding 
 of the posterior integument. Each section is provided with 
 appendages: (1) the salivary glands and receptacles, stomodseal; 
 (2) the ccecal tubes, mesenteric ; and (3) the Malpighian tubules, 
 proctodseal. 
 
 The mouth leads into a buccal cavity, into which there 
 projects a lingua (li.), or tongue-like process, supported by 
 chitinous plates. Beneath the lingua opens the common duct 
 of the salivary glands and their receptacles. Each salivary 
 gland consists of two principal lobes and a small (anterior) 
 accessory lobe on each side. There is also on each side an 
 elongated salivary receptacle. The ducts of both glands and 
 receptacles are lined with delicately-ribbed chitinous membrane, 
 the finer branches of the salivary ducts end in secreting bulbs 
 or acini. The student must examine a gland and its ducts under 
 the microscope. The two glandular ducts unite in the middle 
 line into a common duct ; the ducts from the receptacles do the 
 same ; then both these unite into the salivary duct which opens 
 beneath the lingua. The salivary secretion is neutral, and trans- 
 forms starch into glucose. 
 
 The gullet is narrow, pierces the nerve-ring, passes through 
 the occipital foramen, traverses the neck, and then, in the 
 thorax, gradually widens into the large crop (cr.), which is at 
 once followed by the muscular gizzard (gi.). The chitinous lining 
 of the gizzard is differentially thickened, so as to form a circlet 
 of six teeth, two of which are shown in 81, B., and one, in side 
 view, at C. Between the main teeth are finely-toothed ridges. 
 The toothed part of the gizzard is followed by six prominent 
 setose cushions. The function of the toothed part of the gizzard 
 is to act as a gastric mill, while the setose cushions act as 
 strainers. Some observers, however, contend that, both in the 
 cockroach and in the crayfish, the so-called gastric mill is to be 
 regarded as part of the straining apparatus, and not as a divider 
 of the food. In any case it would tend to squeeze the food pulp. 
 The gizzard ends in a tubular portion which protrudes into the 
 cavity of the succeeding portion of the canal. 
 
266 ANIMAL BIOLOGY. [Part II. 
 
 This succeeding portion is the chylific stomach (ve.), constituting 
 the mesenteron or mid-gut of the canal. At its anterior end there 
 are eight ccecal tubes (cce.), which have been regarded as a diges- 
 tive gland, but are more probably diverticula of the chylific 
 stomach, the lining cells of which have taken on the function of 
 secreting a feebly acid fluid, which has the digestive properties 
 of the pancreatic juice of vertebrates. The epithelial cells of 
 the chylific stomach (in which there is no chitinous lining) are 
 very probably absorbent in function. 
 
 Marking the junction between the mesenteric and proctodseal 
 segments of the canal, but properly belonging to the latter, are 
 a number (sixty or more) of long thread-like Malpigliian tubules 
 (m. t.)j inconspicuously grouped into six clusters. They are 
 lined with large epithelial cells, within which are found crystals 
 containing uric acid, and said to consist of urate of sodium. 
 Similar crystals are found in the lumen of the tube. The Mal- 
 pighian tubules are therefore undoubtedly excretory in function. 
 In another part of the body, however namely, in the fat lody 
 (corpus adiposum), which appears as a white cellular mass sur- 
 rounding the viscera when the cockroach is dissected crystals 
 containing uric acid have been found. It is conjectured that the 
 urates here elaborated escape by the blood which bathes the 
 perivisceral cavity, and are again taken up by the Malpighian 
 tubules, to be thence finally discharged into the intestine. 
 
 Posterior to the Malpighian tubules is the short, narrow, small 
 intestine orileum (il.), which is separated by an annular fold from 
 the more dilated colon (co.). This narrows and becomes somewhat 
 constricted before it passes into the rectum (r.). The rectum is 
 characterised by six longitudinal folds with largely-developed 
 epithelium cells, beneath which are tracheal tubes. Between the 
 longitudinal folds the epithelium is much thinned or absent. 
 Posteriorly the alimentary canal opens by the vent, whence the 
 inflected cuticle passes inwards to line the proctodaeal portion 
 of the canal (rectum, colon, and ileum). A transverse section 
 of the stomodseal or proctodseal segment of the canal shows 
 internally the chitinous lining, followed by an epithelial layer, 
 external to which is a muscular layer with circular and longi- 
 
Chap. XIII.] THE COCKROACH. 267 
 
 tudinal fibres. These fibres, unlike those of the alimentary 
 canal in vertebrates, are striated. In the mesenteron the 
 chitinous layer is absent, and the epithelial layer specially 
 modified. 
 
 The Heart and Circulation. The heart is a long delicate 
 tube lying beneath the terga of the thorax and abdomen. It is 
 divided into thirteen segments three thoracic, and ten abdom- 
 inal ; but some of these are inconspicuous, and can hardly be 
 made out in the common cockroach, At its posterior end each 
 of the more conspicuous segments dilates into a median dorsal 
 and two lateral lobes. The lateral lobes conceal paired lateral 
 inlets. A pear-shaped mass of cells, hanging just in front of 
 the constricted opening between any two segments, serves as a 
 valve to close at once this opening and those of the lateral 
 inlets. The heart lies in a pericardial chamber, the floor of 
 which is constituted by a membrane in which are small oval 
 openings. To this membrane fan-shaped muscular bands (alary 
 muscles) arising from the anterior margin of each tergum, are 
 attached. 
 
 The heart contracts in rhythmical waves from behind for- 
 wards ; in a young cockroach, shortly after the ecdysis they 
 have been reckoned at eighty per minute. The blood thus 
 driven forwards passes by a slender aorta along the dorsal sur- 
 face of the gullet, and terminates with a trumpet-shaped orifice 
 just in front of the nerve-ring. The blood then passes through- 
 out the body by a system of lacunar spaces. It therefore bathes 
 the various organs, and is not injected into them ; hence their 
 diffuse form, seen in the salivary glands, caecal tubes, and 
 Malpighian tubules, in the reproductive organs, and in the fat- 
 body which occupies the peri-visceral space or ccelom. The 
 blood (which is colourless, with large nucleated amoeboid cor- 
 puscles), after bathing these organs, reaches the pericardial 
 cavity through the fenestrated membrane. It is doubtful 
 whether there is any distinction into venous and arterial blood 
 a distinction perhaps rendered unnecessary by the tracheal 
 system, 
 
268 ANIMAL BIOLOGY. [Part II. 
 
 The Traclieal System. Aerial respiration is, in the cock- 
 roach, on quite a different plan from that which is found in the 
 air-breathing vertebrate. There the blood is carried to a special 
 sac, the lung, in which it is aerated ; and by the blood the 
 oxygen thus absorbed is distributed through the system. In 
 the insect a number of minute tubes (trachece) ramify throughout 
 the body and carry air directly to the various organs. 
 
 From the spiracles there pass inwards tracheal trunks. Those 
 in the abdominal region give off, (1) anterior and posterior 
 branches, which unite together to form longitudinal lateral 
 vessels ; (2) dorsal branches, which pass up into the pericardial 
 cavity and branch right and left, thus giving rise, by union of 
 the branches, to dorso-lateral arches ; (3) ventral vessels running 
 transversely, but coming into relation with ventro-lateral vessels, 
 one on each side of the nerve-chain ; (4) median branches which 
 ramify upon the alimentary and reproductive viscera. From 
 the thoracic stigmata several ventral and dorsal transverse 
 trunks take their origin, which are joined to each other by 
 longitudinal connecting vessels, there being also a well-developed 
 connection between these thoracic tracheae and the anterior 
 ventral transverse vessel in the abdominal region. From the 
 anterior thoracic stigmata four large vessels, two dorso-lateral 
 and two ventro-lateral, pass to the head. 
 
 Examined under the microscope the tracheae are seen to 
 be transversely striated, and in some cases, where a tube has 
 been torn in mounting, a spiral thread of chitinous material 
 is seen to be drawn out from the trachea. This chitinous 
 lining strengthens the tubes, keeps them open, and gives them 
 elasticity. 
 
 The thoracic stigmata are provided with external valves. 
 Those in the abdominal region have internal valvular slits, which 
 can be closed by the action of occlusor muscles on a chitinous 
 bow developed along its anterior edge. Thus the tracheal 
 system can be shut off from communication with the exterior, 
 enabling the insect to live for some time in a noxious atmo- 
 sphere. 
 
 Inspiration and expiration are effected by movements of 
 
Chap. XIII.] THE COCKROACH. 269 
 
 the abdomen, though the thoracic region in the cockroach also 
 undergoes a slight change of form. In expiration the terga 
 are flattened and depressed, the sterna being at the same time 
 slightly raised. Opposite movements effect inspiration. The 
 movements of expiration and inspiration may readily be watched 
 in a tired bee. It must not be supposed that the tracheae are 
 alternately filled and completely emptied during this process. 
 But little air enters or leaves the body ; in the ultimate ramifi- 
 cations of the vessels carbonic acid gas is exchanged for oxygen 
 by a process of diffusion. 
 
 The Nervous System. The central nervous system (Fig. 82, 
 D.) is, in the cockroach as in the crayfish, ventral, and consists 
 of a double- chain, the coalesced ganglia of which are united by 
 longitudinal connectives. 
 
 The coalescent cerebral ganglia (brain) are shown in Fig. 82. 
 There are two large hemispheres (h. s.) from which the optic 
 nerves (op.) pass off to the eyes. Small nerves proceed from this 
 part of the brain to the white spot (n. w. s.). Below the hemi- 
 spheres are the antennary lobes (an. /.), from which large anten- 
 nary nerves (an.) are given off. Below these again a nerve is given 
 off from the commencement of the commissure on each side, and 
 these unite in a frontal ganglion (/. g.) which lies on the oesopha- 
 gus ; thence the median recurrent nerve passes upwards and back- 
 wards under the hemispheres, and is seen in the dorsal view 
 shown at C. (r. n.) 
 
 Large commissures pass on either side of the gullet from the 
 cerebral ganglia to the sub-oesophageal ganglia, which also lie in 
 the head. From their ventral aspect nerves are given off to 
 the mandible, maxillae, and labium. From these ganglia com- 
 missures (con.) pass backward through the neck to the first 
 of the three coalescent pairs of the thoracic region, which inner- 
 vate respectively the pro- meso- and metathorax and their ap- 
 pendages. The nerves of the wings have a Y-shaped junction 
 with the nerve-chain, as shown in Fig. 82, D. In the abdomen 
 there are six pairs of closely-united ganglia, the last being large. 
 The abdominal commissures of the male are looped. 
 
270 
 
 ANIMAL BIOLOGY. 
 
 [Part II. 
 
 A stomato-gastric system innervates the crop and gizzard. The 
 recurrent nerve comes into relation with anterior and posterior 
 stomato-gastric ganglia (C, a. stg.^p. stg.}) the former of which is 
 connected with the ventral surface of the hemispheres. Then 
 running backwards along the dorsal surface of the crop, the re- 
 
 aii.1. 
 
 D 
 
 FIG. 82. COCKROACHBRAIN, ETC. 
 
 A. B. C. Head of cockroach opened so as to expose the brain A. from the 
 front ; B. from the side ; C. from above (after E. T. Newton). D. Nerve- 
 chain. 
 
 an. Antennary nerve, an. I. Antennary lobe. a. stg. Anterior stomato- 
 gastric ganglion, con. Commissure to thoracic ganglia. /. g. Frontal 
 ganglion. Ji. s. Hemispheres of brain, la. Labial nerve. lb. Nerve to 
 labrum. mn. Mandibular nerve, mx. Maxillary nerve, n. w. s. Nerve to 
 white spot. op. Optic nerve, p. stg. Posterior stomato-gastric ganglion. 
 r. n. Recurrent nerve, s. ce. Suboesophageal ganglion. 
 
 current nerve, about the middle of the crop, enters the apex of 
 a triangular ventricular ganglion. From the lateral angles of this 
 triangle nerves pass backwards and downwards on either side of 
 the crop, and, breaking into branches, innervate this organ and 
 the gizzard. 
 
 Each coalesced pair of ganglia has to some extent independent 
 control over its somite and appendages. The general co-ordina- 
 
Chap. XIII.] THE COCKROACH. 271 
 
 tion of muscular movements is perhaps mainly the function of 
 the so-called brain ; but it would seem to be, in part at least, 
 performed by the sub-cesophageal mass. 
 
 Special Senses in Insects. The chitinous exoskeleton of in- 
 sects rests upon a hypodermic or chitinogenous layer. Its free sur- 
 face is, in the cockroach, divided into polygonal areas, and from 
 the midst of some of these there rise large sense-hairs, developed 
 from long flask-shaped hypodermic cells. These are tactile in 
 character. The nerve filament that passes to such a seta ex- 
 pands at the base of the hair into a nucleated ganglion, a fila- 
 ment from which pierces the hypodermic cell, and is continued 
 to the tip of the hair. 
 
 The paraglossae and inner ends of the maxillae may be in the 
 cockroach, as they are in the wasp, supplied with end-organs of 
 taste. No one who has seen a cockroach spitting, as he sucks 
 from his antenna some distasteful substance, can doubt his being 
 powerfully affected by sapid substances. 
 
 The sense of smell is located in the antennae, in which there 
 are developed a great number of minute sacculi, filled with 
 serous fluid, the orifices of which are covered over with a deli- 
 cate membrane. The nerve-fibres which proceed to these sacculi 
 end in rod-like bodies. If the antennae be extirpated or coated 
 over with wax, the insect exhibits no repugnance to such sub- 
 stances as turpentine and carbolic acid, and does not eagerly 
 rush to such food substances as bread sopped in beer. 
 
 No auditory organs have been described for the cockroach. 
 But other insects possess such organs. In the grasshopper, for 
 example, a thin chitinous membrane is stretched on a chitinous 
 bow over a cavity in the tibia of the fore-leg. The cavity 
 covered over by this membrane is in communication with the 
 exterior, and within the cavity is a ganglionic mass at the end of 
 a special nerve-fibre. In the antenna of the bee, at the bottom 
 of certain open cavities, there are cone-like structures, which are 
 regarded by some anatomists as auditory in function. 
 
 The many-faceted eye of the cockroach is constructed on the 
 same principle as that of the crayfish. The organ is large, and 
 
272 ANIMAL BIOLOGY. [Part II. 
 
 occupies a considerable space on each side of the head. There 
 are about 1800 facets on each eye. These facets are derived 
 from the cuticle ; they are hexagonal (as a rule) in outline, and 
 biconvex in cross-section. Beneath each facet is a short stout 
 crystalline cone coated with dark pigment. The apex or deeper 
 part of the cone is imbedded in a so-called nerve-rod or rhabdom, 
 composed of four elements (rhabdomeres), which diverge above to 
 receive the point of the cone. Around the rhabdom is a proto- 
 plasmic sheath, inconspicuously divided into four segments or 
 retinulce, which have been found to contain " visual purple," 
 such as is found in the vertebrate retina. Beneath this re- 
 tinula layer is a basilar membrane perforated by a number of fine 
 holes. The retinulae and rhabdoms constitute the ultimate nerve- 
 endings, and are connected through the fenestrations of the 
 basilar membrane with nerve-fibrils. The basilar membrane is 
 homologous with the basement membrane of the cuticle; all 
 between this and the facets is probably hypodermic in origin. 
 
 The optic nerve given off from the cerebral hemisphere of the 
 brain gives rise, in the cockroach, before it reaches the basilar 
 membrane, to two ganglionic enlargements. In the fly and bee 
 there are three such ganglionic swellings. Each ganglion in the 
 cockroach (and the inner two in the bee) is formed of a network 
 of fine fibrils, surrounded by a cellular sheath of densely-packed 
 nerve-cells, with large nuclei surrounded by a delicate proto- 
 plasmic envelope. The fibres connecting the two ganglia decus- 
 sate or cross ; and the fibres between the outer ganglion and the 
 basilar membrane do so partially. 
 
 It has recently been urged that not only the layer of retinulse 
 and rhabdoms, but the inner and outer ganglion as well, should 
 be regarded as constituting the insect retina ; in which case the 
 retinulse and rhabdoms answer to the layer of rods and cones in 
 the vertebrate retina. The order of parts in the optical appar- 
 atus of the cockroach is therefore : Brain, optic nerve, retina, 
 dioptric apparatus. Of these the retina is constituted by inner 
 ganglion, decussating fibres, outer ganglion, partially decussating fibres, 
 retinula layer. While the dioptric apparatus may be represented 
 by corneal facets and crystalline cones. 
 
Chap. XIII.] THE COCKROACH. 273 
 
 There has been much discussion as to the nature of insect 
 vision. If we cut out the faceted cornea of a beetle's eye and 
 clean its inner surface, mount for the microscope, and having 
 focussed the facets raise the objective, we shall see, if the flat 
 side of the mirror be used, a great number of images of the 
 window or lamp, one for each facet. Some, relying on this fact, 
 maintain that the compound eye is a multiple structure consist- 
 ing, in the drone for example, of more than 13,000 eyes the 
 two compound eyes giving the brain or ganglia some 26,000 
 images to co-ordinate. It would seem, however, that in the 
 water-beetle (Hydrophilus) the focus of a corneal lens is about 
 three millimetres away and altogether behind the eye. It is on 
 the whole more probable that each dioptric element transmits 
 the image of a point, and that a number of the elements combine 
 to form a stippled image, or a picture in mosaic. 
 
 In addition to the many-faceted eyes, some insects have simple 
 eyes or ocelli ; the bee, for example, has three such, arranged 
 in a triangle on the vertex of the head (queen) or on the fore- 
 head (drone). These would seem to be used for near vision, 
 the many-faceted eyes being used for far vision. Each has a 
 lens arising by differentiation and thickening of the cuticle; 
 the crystalline cones are not developed as such ; the hypodermic 
 cells give rise to a retinal layer. 
 
 Reproductive Organs. (1.) In the Female. The paired ovaries 
 consist of groups of eight beaded ovarian tubules, the apices of 
 which are filamentous and united. In them the ova are 
 developed (Fig. 83, A.). The upper part (germogen) of each 
 tubule contains protoplasm, in which nuclei are imbedded; 
 further down separate nucleated masses are found ; and lower 
 still these nucleated masses are arranged in single file, and 
 are recognisable as ova. Around each ovum small nucleated 
 cells arrange themselves as a single-layered follicle. A vitelline 
 membrane is secreted by the inner surface of this egg-follicle, 
 and a chitinous chorion by its outer surface. The lowest egg 
 is much larger than the others The two oviducts (o. d.) unite to 
 18 
 
274 
 
 ANIMAL BIOLOGY. 
 
 [Part II. 
 
 form a short uterus, which opens by the genital aperture (e. o.) on 
 the eighth sternum into the genital pouch. 
 
 Above the eighth sternum, with its genital aperture, is the 
 ninth sternum (see Fig. 80, D.), which carries at its anterior end 
 the aperture of the spermatheca, which consists of a rounded 
 sac and coiled caecum, The spermatheca is a sperm-reservoir, 
 and always contains spermatozoa in the fertile female. 
 
 VIII. C.O. 
 
 FIG. 83. COCKROACH : REPRODUCTIVE ORGANS. 
 
 A. Diagram of female organs of generation. c. g. Colleterial glands, e. o. Genital 
 aperture, od. Oviduct, viii. Sternum of eighth abdominal segment. 
 
 B. Mushroom gland of male. d. e. Ejaculatory duct. ts. Testes. 11. 6. Utriciili 
 breviores. u. m. Utriculi majores. v. d. Vas deferens. 
 
 Into the uterus, on its under side, there open the tubular 
 accessory cement (colleterial) glands (c. g.\ asymmetrically 
 developed, the left being the larger. Within these long white 
 csecal tubes crystals of (?) oxalate of lime may be developed. 
 Their function is to form an egg-capsule, within which sixteen 
 ova, arranged in two rows, are normally laid. The capsule 
 hardens and becomes brown by exposure to the air. It presents 
 a longitudinal slit along its dorsal margin, the serrated edges of 
 which bear tooth-like impressions, due to the grasp of the 
 posterior gonapophyses. 
 
 The ova are about one-sixth of an inch in length, oval, with 
 one face convex and the other slightly concave. The chorion 
 which invests them is ornamented by being divided up into 
 polygonal areas. In its convex moiety there are numerous 
 
Chap. XIII.] THE COCKROACH. 275 
 
 micropyles, or minute holes for the admission of spermatozoa 
 from the spermatheca. 
 
 (2.) In the Male. On dissecting an adult male cockroach a 
 curious "mushroom-shaped gland" (83, B.) is seen situated at the 
 end of a short duct. This organ was for long regarded as the 
 testis, since in the adult it contains developing spermatozoa. 
 The true testes are liable to be missed, since they are only func- 
 tionally active in the very young cockroach, becoming reduced, 
 though they are not altogether obliterated, in the adult, in 
 which they are vesicular bodies (83, B., ts.) lying beneath the 
 fifth and sixth terga. Vasa deferentia (v. d.) pass from them into 
 the coalescent cavities of the mushroom-shaped gland, or vesiculce 
 seminales, which bear a number of longer and shorter finger-like 
 processes, the utriculi major es and breviores (u. m., u. b.). The 
 epithelium of the testis in the young cockroach gives rise to 
 sperm cells, which develop into hollow spermatocysts, around 
 which spermotoblasts are placed radially. These are converted 
 into spermatozoa. The later stages of the development of sper- 
 matozoa are, however, carried on in the vesiculse seminales and 
 their utricles, when the testes have become reduced. The sper- 
 matozoa pass from the vesiculse seminales by an ejaculatory duct 
 (d. e.), by which they are carried out of the body. A subsidiary 
 gland of unknown function lies on the ventral surface of the 
 ejaculatory duct. 
 
 Development. The segmentation of the ovum in insects is 
 on the centrolecithal system, as in the crayfish. The first 
 segmentation nucleus undergoes division in such a way that a 
 layer of cells forms a superficial blastodermic layer, while other 
 yolk-cells remain scattered through the central mass. 
 
 In the blastodermic layer an elongated ventral plate is formed, 
 in which the embryo begins to be developed. This broadens at 
 the anterior end ; further back it is transversely divided into 
 segments, of which the total number seems to be seventeen. 
 Indications of the appendages appear early, limb-buds being 
 formed, according to recent observers, on all the abdominal seg- 
 ments. These subsequently disappear. 
 
276 ANIMAL BIOLOGY. [Part II. 
 
 In the mid-line of the ventral plate a ventral groove is formed, 
 which, as it sinks below the general level of the epiblast, gradu- 
 ally detaches itself as a tube. The tube becomes a solid cellular 
 mass, which splits into two longitudinal bands, the mesoblastic 
 bands. 
 
 The hypollast originates, in insects, according to some ob- 
 servers, by differentiation of the internal mass of yolk-cells; 
 according to others, by differentiation of certain cells of the 
 ventral groove. 
 
 The mesoblastic bands become divided into somites, each of 
 which becomes hollow, the hollowed space separating an inner, 
 dorsal, one-layered wall from an outer, ventral, several-layered 
 wall adjoining the epiblast. By the coalescence of the cavities 
 of contiguous somites a common cavity the cwlom is formed. 
 The coalesced inner walls of the somites give rise to a splanchnic 
 layer; the coalesced outer walls to a somatic layer. Certain 
 yolk-cells seem to pass into the ccelom, and it has been sug- 
 gested that these give rise to the corpus adiposum, or fat 
 body. 
 
 The nervous system is epiblastic in origin. The cerebral 
 ganglia are developed separately as a pair of pre-oral thickenings 
 of the epiblast. The post-oral chain is developed from longi- 
 tudinal thickenings of the epiblast, which sink into the body 
 and become separated from the superficial epiblastic layer. 
 
 All the anterior part of the alimentary canal (fore-gut), lined 
 with a chitinous infolding from the edges of the mouth, is 
 formed by a stomodceal imagination. All the posterior part of 
 the alimentary canal (hind-gut), as far as, and including, the 
 Malpighian tubules, with infolded cuticle from the vent, arises 
 by a proctodceal invagination. The chylific stomach and its caecal 
 tubes (mid-gut), develops from the mesenteron. The muscular 
 layer of the alimentary canal is contributed by the splanchnic 
 layer of the mesoblast. 
 
 Perhaps the most striking peculiarity in the development of 
 insects is the formation of an embryonic membrane similar to 
 the amnion. A fold of membrane grows out from the blasto- 
 dermic layer over the germinal plate, and, coalescing in the 
 
Chap. XIII.] THE COCKROACH. 277 
 
 mid-line as does the vertebrate amnion fold, forms a double 
 investment of the plate, an inner true amnion and an outer serous 
 membrane. As the ventral plate increases in size and tends to 
 enclose the embryo, the point of origin of these membranes 
 is forced to retreat dorsally, until at last, when the plate 
 envelopes the whole embryo, the amnion and the serous mem- 
 brane become separated off, and form complete membranous 
 investments of the embryo. 
 
 The heart is mesoblastic in origin. When the cavities of the 
 lateral mesoblastic somites coalesce to form the coelom, the inner 
 wall becomes the splanchnic layer, the outer wall the somatic 
 layer. At the edges of the mesoblastic bands the layer of cells 
 which unites the somatic with the splanchnic wall forms, on 
 each side, a grooved cardiac rudiment. As the hollowed meso- 
 blastic bands grow upwards so as to approach the dorsal surface, 
 these grooved rudiments tend to meet ; and the tubular heart 
 arises by their ultimate coalescence. 
 
 The trachea! system arises from tubular infoldings of the 
 epiblast. Each stigma has at first a distinct system, but these 
 coalesce into a common system. 
 
 The reproductive organs are of mesoblastic origin. 
 
CHAPTER XIV. 
 
 THE EARTHWORM. 
 
 THE body of the earthworm (Lumbricus) is long and ringed. 
 The rings indicate a division into segments or somites, of which 
 there may be as many as 350. There is no distinct head or 
 neck, but the anterior end tapers off, while the posterior end 
 terminates bluntly. There are no definite limbs or appendages, 
 but nearly each segment is provided with chitinous bristles which 
 aid it in progression, which is effected by alternate elongation 
 and shortening of the body. The worm lives in moist earth, in 
 which it forms burrows. It is omnivorous, swallows much earth. 
 and normally consumes vegetal matter, such as leaves, fresh or 
 half-decayed. It will, however, eat animal food, and at a pinch 
 a dead comrade. Its power of regenerating excised parts is very 
 remarkable. It is stated but hard to believe, that a worm not 
 only survived the removal of the first five rings, including 
 the brain, sub-cesophageal ganglion, mouth, and pharynx, but, 
 within fifty-eight days, had completely regenerated these parts. 
 
 In the young worm, not yet sexually mature, the rings 
 throughout the body are tolerably similar, those at the anterior 
 end gradually lessening in diameter, and those at the posterior 
 end becoming somewhat flattened, In the adult worm, however, 
 certain of the rings between the twenty-fourth and thirty-sixth 
 become lighter in colour, much swollen, and coalescent dor- 
 sally, and project downwards as ridges on either side of the 
 mid-ventral line, giving rise to the cingulum or ditellum. The 
 ventral aspect of segments eight to twelve also becomes swollen 
 and tumid. 
 
 The mouth is in the anterior segment, situated beneath a 
 
 278 
 
Chap. XIV.] THE EARTHWORM. 279 
 
 slightly projecting conical lobe, the prostomium. The vent is in 
 the middle of the last body-ring. Other apertures to the body 
 are numerous. Every segment of the body except the first has 
 a dorsal pore opening into the anterior part of the ring in the 
 mid-dorsal line, and two very minute pores, one on each side of 
 the ventral line, which are the external orifices of the nephridia 
 or segmental organs, whose function is excretory. In addition 
 to these openings there are in each worm the orifices connected 
 with the generative system ; for the worm is hermaphrodite, 
 each individual having both male and female genital organs. 
 The male pores of the vasa deferentia open by tumid lips on 
 the ventral aspect of the fifteenth ring. The female pores of 
 the oviducts have a similar position in the fourteenth segment, 
 and between the ninth and tenth and tenth and eleventh seg- 
 ments respectively, are the usually paired openings of the sper- 
 mathecse or receptacula seminis. 
 
 There are eight setae or hair-like projecting spines to each 
 segment, except a few of the most anterior and most posterior. 
 They can be felt as a living worm is pulled backwards through 
 the fingers, and can be readily seen with a lens, especially in 
 a worm rigidly contracted by drowning in spirits. They are 
 arranged in two double rows on each side. The outer double 
 row is where the darker-coloured dorsal region shades off into 
 the lighter-coloured ventral region. The inner double row is 
 nearer the ventral line, Each seta is curved and pointed, and 
 is lodged in a muscular sac, so that it can be protruded or re- 
 tracted. On the tenth to the fifteenth rings, on the clitellum, 
 and on a glandular prominence of the sixth ring in front of the 
 clitellum, the spines are modified and often somewhat longer 
 and thinner than elsewhere. 
 
 General Internal Anatomy. The worm is readily killed by 
 the vapour of chloroform ; but it should not be too long exposed 
 to this vapour, since it thus becomes contracted, and satisfactory 
 dissection in this condition is impossible. It should, therefore, 
 be soon removed and immersed in water for half-an-hour. The 
 student will find it convenient to dissect the worm in a saucer 
 
2 8o ANIMAL BIOLOGY. [Part II. 
 
 at the bottom of which is a layer of paraffin, poured in liquid 
 and allowed to set. The worm should be pinned out with the 
 first twenty-eight segments well stretched out, and the segments 
 should be numbered by marking the paraffin at the fifth, tenth, 
 fifteenth, and twentieth rings. The body should be opened by 
 a median incision from the twenty-fifth to the second segment. 
 Care must be taken not to cut so deeply as to injure the blood- 
 vessels. It will now be seen that the external constrictions 
 between the rings answer to internal septa which divide the 
 body into as many compartments as there are rings. These 
 must be carefully divided near the body-wall on each side, and 
 the walls pinned out laterally so as to display the internal 
 organs, as seen in Fig. 84, A., which represents the anatomy of 
 a sexually mature worm. After the worm is thus opened up 
 under water, the water should be poured away and replaced 
 with spirits of wine, which enables some of the parts to be more 
 readily seen, and hardens some of the organs so as to render 
 their dissection more readily accomplished. 
 
 The digestive canal is seen in the middle line with buccal 
 cavity (b. c.) anteriorly, opening into a large pharynx (ph.), 
 from which fine muscular fibres stretch to the body-walls. The 
 pharynx is succeeded by a long (esophagus (as.), upon which are 
 three white protuberances in the tenth and twelfth segments. 
 These are the cakiferous glands (c. g.). In the fifteenth ring the 
 oesophagus expands into a crop (cr.), which is followed by a 
 muscular gizzard (gi.) ; after which the intestine (int.) is carried 
 to the end of the body. If a living worm, selected for its trans- 
 lucency, be held up to the light, the intestine will be seen to 
 have a slightly coiled course, except when the worm is elongated 
 almost to the point of rupture. The intestine, in Fig. 84, is cut 
 at the twenty-second ring, and turned back to show the nerve- 
 chain (n. ch.\ which is median and ventral in position. The walls 
 of the intestine are covered with soft yellowish-brown tissue. 
 
 Along the dorsal line of the alimentary canal runs a dorsal 
 blood-vessel, filled with red fluid. This gives off lateral vessels, 
 which in the sixth to the twelfth segments are much enlarged, 
 giving rise to the so-called hearts (h.). 
 
Chap. XIV.] 
 
 THE EARTHWORM. 
 
 281 
 
 c. 
 
 A. 
 
 FIG. 84. EARTHWORM : GENERAL ANATOMY. 
 
 A. Adult earthworm opened out from dorsal side. B. Sexually immature 
 worm. C. Segmental organ (partly after Gegenbaur). 
 
 In A. or B. b. e. Buccal cavity, c. g. Calciferous glands, c. v. d. Common 
 vas deferens. cr. Crop. gi. Gizzard, h. Hearts, int. Intestine, m. g. p. Male 
 genital pore, n .ch. Nerve-chain, oss. (Esophagus, ovd. Oviduct, ovy. Ovary. v 
 ph. Pharynx, pr. s. Prostomium. r. i. ii. Rosette-like openings of vasa defer- 
 cntia. se. Seta sac. s. o. Segmental organ, s. ces. g. Supra-oesophageal gan- 
 glion. S2). i. ii. Spermathecge . ts. i. ii. Testes. v. s. i. ii. iii. Vesiculse seminales. 
 1-20. Bodj'-rings. 
 
 In C. i. o. Internal opening. 1. Thin- walled tube indicated by fine line. 
 2. Thick-walled ciliated portion. 3. Muscular portion. 
 
282 ANIMAL BIOLOGY. [Part II. 
 
 On either side of each segment, behind the pharynx, is a 
 coiled tube, nephridium or segmented organ, attached to the pos- 
 terior face of the septum. They are not figured anteriorly, but 
 their position is indicated at s. o. in the last few segments dis- 
 played. The inwardly projecting processes indicated at se. are 
 the sacs of the setce. By removing one of these sacs setae may 
 readily be obtained for microscopic examination. 
 
 From the ninth to the fifteenth segment certain of the genera- 
 tive organs are very conspicuous. There are two globular white 
 sacs on each side, the spermathecce (sp.) ; and there are three 
 pairs of large lobes of the vesiculce seminales (v. s.). Both of these 
 belong to the accessory organs of generation. The essential 
 organs, ovaries, and testes, with their ducts, can only be seen by 
 careful dissection. 
 
 The Body-walls and Septa. Investing the whole body, and 
 turning inwards to line the sheaths of the setae, is a thin trans- 
 parent dense cuticle. This is easily stripped off, especially from 
 a worm that has lain for a short while in spirits. Examined 
 under the microscope it shows intersecting striae, with refringent 
 dots at the points of intersection. Beneath this is a layer 
 generally known as the hypodermis. It contains columnar cells, 
 among which are large oval glandular cells. The glandular cells 
 are very numerous in the clitellum, on the ventral aspect of the 
 eighth to the twelfth rings, at the lips of the male genital pore, 
 and in the prominence on the sixth ring in front of the clitellum. 
 The hypodermis turns inwards at the sheaths of the setae. 
 Beneath the hypodermis is a muscular layer, with circular fibres, 
 between which pigment granules are scattered. Internal to this 
 is a muscular layer with longitudinally-disposed fibres. 
 
 The septa are thin membranes of connective tissue, in which 
 there are radiating and circular muscular fibres. 
 
 The Digestive System. The mouth lies beneath the conical 
 prostomium, the rest of the first ring forming a peristomium. It 
 leads into a buccal cavity, divided by a slight constriction from 
 the muscular pharynx, into which there projects a sucker-like 
 
Chap. XIV.] THE EARTHWORM. 283 
 
 fold. Salivary glands have been described as opening into the 
 buccal cavity. 
 
 Behind the pharynx is the long narrowed oesophagus, on the 
 side of which there are the three oesophageal or cakiferous glands. 
 These, or in any case the first pair, are diverticula of the oeso- 
 phagus, the anterior pair being the largest. Their walls are 
 lamellar in structure, containing deep follicles. If the hinder 
 pairs be punctured and squeezed a number of white bodies 
 exude, which contain in their centres a little fine granular 
 matter, which, on treatment with acetic acid, disappears with 
 effervescence, and which is carbonate of lime. The anterior 
 pair of diverticula contain one or two, or a few larger calcareous 
 concretions. In winter the glands seem to diminish in size. In 
 young worms there are often but two pairs. 
 
 The oesophagus opens into an expanded crop, with muscular 
 contractile walls ; and this is followed by the very thick-walled 
 gizzard ; behind which the intestine passes, with but little change, 
 to the end of the body. The intestine is laterally sacculated, 
 and its lumen is much diminished by a thick longitudinal fold, 
 the typhlosole, which projects inwards from its dorsal wall. 
 Surrounding the intestine, especially at its anterior end, is a 
 yellowish-brown, easily ruptured tissue, which is in close con- 
 nection with the blood-vessels. Similar tissue may accompany 
 the dorsal vessel above the oesophagus. It has been described 
 as hepatic, and as producing a digestive secretion. It nowhere 
 communicates, however, with the interior of the canal, and its 
 close connection with the blood-vessels lends support to the 
 view that it is vasif active concerned in the production of some 
 constituent of the red fluid of the blood-vessels. 
 
 Little is known of the process of digestion. Worms seem to 
 drag leaves, stalk forwards, into their burrows, and then moisten 
 them with an alkaline secretion. This rapidly acts on the 
 leaves, the cells with chlorophyll losing their green colour and 
 becoming brown. It also acts on the starch granules within the 
 cells, and on the protoplasmic contents of the cell-wall. The 
 leaves are thus partially digested before they are taken into 
 the canal. The function of the calciferous glands is not known 
 
284 ANIMAL BIOLOGY. [Part II. 
 
 for certain, but it has been suggested that the function is 
 (1) excretory, to eliminate excess of carbonate of lime; (2) to 
 neutralise within the canal the humous acids produced from the 
 leaves. Absorption is mainly carried on, it may be presumed, 
 in the intestine, the sacculations and the typhlosole giving addi- 
 tional absorbent surface. 
 
 A transverse section of the intestine shows : (1) an internal 
 membrane of columnar epithelium ; (2) a thin layer of vascular 
 connective tissue ; (3) a muscular layer, with circular fibres ; (4) 
 a longitudinal muscular layer ; (5) the so-called hepatic, or by 
 some observers, vasifactive tissue. 
 
 The Nephridia. Excretion is effected by segmental organs or 
 nephridia, of which there is a pair for each segment, except the 
 anterior. Fig. 84, C. (after Gegenbaur and Howes) shows the 
 arrangement of the coiled tube. The internal opening is by a 
 ciliated funnel (i. o.) attached to the septum. Then follows a 
 thin-walled tube, indicated by the thin line at 1. This is suc- 
 ceeded by a thick- walled glandular ciliated portion 2, which is 
 followed by a muscular portion 3, which opens to the exterior 
 just external to the inner row of setae. The internal opening 
 is in the segment anterior to that which contains the tube, 
 which is connected to the posterior wall of the septum by a 
 mesentery not shown in the figure. The student should 
 examine in situ and mount several segmental organs, but must 
 be content if he makes out the differentiation into three por- 
 tions, and the ciliated funnel. ^ 
 
 The Vascular System. Two fluids in the earthworm claim 
 the title of blood : (1) a colourless corpusculated fluid which 
 occupies the cavity of the segments ; (2) a red fluid, with minute 
 corpuscles, which occupies definite vessels. 
 
 (1.) The colourless fluid resembles the blood of other inverte- 
 brates, and the lymph of vertebrates. It is found in abundance 
 in the posterior segments of the body. Its corpuscles are 
 amoebiform and nucleated. Not improbably its main function 
 is nutritive. 
 
Chap. XIV.] THE EARTHWORM. 285 
 
 (2.) The red fluid (pseudhsemal fluid) is contained in a definite 
 system of vessels. 
 
 The typical arrangement in segments, posterior to the twelfth, 
 seems to be as follows : 
 
 1. The dorsal vessel running above the alimentary canal gives 
 off, on either side 
 
 2. Commissural vessels of two kinds 
 
 (a.) Intestinal, two or sometimes (1) three in each segment, 
 which break up into capillaries on the walls of the 
 alimentary canal or in the " hepatic " tissue ; 
 
 (b.) Lateral, one in each segment, to the septa and muscles 
 of the body-wall, and thus to the subneural. 
 
 3. Sub-intestinal vessels; a pair of closely-connected longi- 
 tudinal vessels lie one above the other, between the alimentary 
 canal and the nerve-chain. The superior collects red fluid from 
 the intestinal walls, the inferior gives off vessels to the seg- 
 mental organs. 
 
 4. A sub-neural vessel, lying beneath the nerve-chain, collects 
 blood from the body-walls and septa, and from the segmental 
 organs. 
 
 5. Delicate latero-neuml vessels run one on each side of the 
 nerve-chain, and give off branches along the nerves. 
 
 From the sixth to the eleventh segments single commissural 
 vessels pass from the dorsal to the single sub-intestinal vessel. 
 These commissural vessels are enlarged to form so-called hearts. 
 
 In the twelfth segment the commissural vessels do not reach 
 the sub-intestinal, but bend forwards and become lateral vessels 
 running forward along the oesophagus. 
 
 In the first six segments, the dorsal, the sub-intestinal, and 
 these lateral vessels branch out to form a net-work over the 
 pharynx. 
 
 In the " hearts "the contraction is from above downwards. 
 But it would seem that, elsewhere, there is no definite circula- 
 tion of red fluid in one direction. Not improbably it is forced 
 hither and thither during the alternate contraction and exten- 
 sion of the body. 
 
286 ANIMAL BIOLOGY. [Part II. 
 
 The red fluid contains minute corpuscles, which would seem 
 to be free nuclei. The fluid has not improbably a respiratory 
 function. 
 
 Eespiration. There are no specially differentiated respiratory 
 organs, respiration being apparently effected by the surface 
 of the body. 
 
 The Nervous System. The "brain" or supra-oesophageal 
 mass is divided into two pear-shaped masses, the broad ends of 
 which unite in the middle line. A pair of nerves run forwards 
 from it along the buccal mass to the prostomium. Commissures 
 pass round the gullet to the sub-cesophageal ganglia, which lies in 
 the third segment. Posterior to this there runs a nerve-chain 
 of two conjoined strands, with ganglionic swellings, one for each 
 segment. The swellings are not well marked, and nerve-cells are 
 not restricted to them, but occur throughout the cord. Its 
 sheath is muscular. A pair of nerves is given out on each side 
 from each ganglion, and a single interganglionic nerve on each 
 side in relation to the septa. Around the nerve-chain the septa 
 are incomplete, allowing of communication between the segments. 
 
 The Senses. There are no eyes, but the worm is sensitive to 
 light which falls upon its anterior segments. When suddenly 
 illuminated it dashes like a rabbit into its burrow. It is also 
 somewhat sensitive to radiant heat. There is no organ of hear- 
 ing at present known, but the worm is very sensitive to vibra- 
 tions. It is exceedingly sensitive in the matter of touch, being 
 affected by a very light puff of breath. It does not seem highly 
 sensitive to unnatural odours, but finds out the presence of 
 favourite foods if buried. No organs of taste have been dis- 
 covered, but the worm can apparently distinguish red from 
 green cabbage, and exhibits a decided preference for certain 
 foods, such as carrot, cherry, celery, onion, and horse-radish. 
 
 The Organs of Generation. The earthworm is hermaphro- 
 dite, each individual having both male organs and female organs. 
 
 The Female Organs. Minute ovaries (Fig. 84, B., ovy.\ about 
 $ of an inch in length are attached to the posterior face of the 
 
Chap. XIV.] THE EARTHWORM. 287 
 
 septum, between the twelfth and thirteenth segments. The ova 
 increase in size from the attached to the free portion of the 
 organ. The oviducts (ovd.) open by ciliated funnels into the 
 thirteenth segment, pass through the septum between the 
 thirteenth and fourteenth segments, and open on to the exterior 
 on the latter segment near the exterior row of setae. Connected 
 with the funnel of each oviduct is a small sac-like receptaculum 
 ovorum. 
 
 Between the ninth and tenth, and tenth and eleventh segments 
 are the spermathecce or copulatory pouches, white spherical bodies 
 which are accessory female organs. 
 
 The Male Organs. The state of the male organs differs very 
 much in mature and immature worms. The student should not 
 fail to dissect both. 
 
 Fig. 84, B., shows the organs in a young worm in which the 
 cingulum was not swollen. There are two pairs of spermaries 
 or testes (ts. i. and ii.), minute and difficult to see, attached 
 to the posterior face of the septa between the ninth and 
 tenth, and tenth and eleventh segments. In the tenth and 
 eleventh segments are seen two pairs of rosette-like funnels 
 (r. i. and ii.), which form the internal openings of the vasa, 
 deferentia. The two vasa deferentia of each side unite into a 
 common vas deferens (c. v. d.\ which opens by the male genital 
 pore (m. g. p.) in the fifteenth segment. In addition to the 
 essential organs and their ducts there are important accessory 
 organs. These are the seminal reservoirs or vesiculce seminales. 
 In the young worm there are three pairs. The anterior (v. s. i.) 
 grow forwards into the ninth segment. The mid-pair (v. s. ii.) 
 grow into the tenth and eleventh segments from the septum 
 that divides them. The posterior (v. s. iii.) grow backward into 
 the twelfth segment. 
 
 So much for the young worm. In the worm that is sexually 
 mature the vesiculse seminales increase enormously, and form 
 the large-lobed organs shown in 84, A., v. s. The anterior pair 
 and the mid pair unite in such a way as to form a central body 
 which envelopes the anterior testes and rosettes, and which has 
 two anterior and two posterior lobes. The posterior pair also 
 
ANIMAL BIOLOGY. [Part II. 
 
 unite across the mid-line so as to envelope the posterior testes 
 and rosettes. Their lobes become much enlarged and grow out 
 backwards. Since they contain vast numbers of spermatozoa 
 in various stages of development, it is not surprising that these 
 large seminal reservoirs used to be regarded as the testes, and 
 that the true testes were overlooked. 
 
 The true testes are largely made up of spherical sperm cells. 
 Of these cells the most superficial and furthest deyeloped 
 separate from the testes and pass into the seminal vesicles or 
 reservoirs which envelop them. These reservoirs are not 
 merely hollow sacs, but are divided by septa and vascular 
 trabeculse into a vast number of incomplete compartments. 
 Here the products of the testes are received and undergo further 
 development. 
 
 The mode of development of the .spermatozoa is as follows : 
 The sperm cells, ere they leave the testis, undergo segmen- 
 tation, passing through stages in which there are 2, 4, 8, 16 
 nuclei, and so on. The nucleated masses thus formed tend to 
 stand out like buds from the cell that generates them, in the 
 midst of which there remains a passive unsegmented mass. 
 At this stage the whole is called a spermato-sphere, the bud-like 
 projections spermato-blasts, and the central bud-bearing mass the 
 blastophor. By this time the spermato-sphere has been detached 
 and is lodged in the seminal reservoir. As development here 
 proceeds each spermato-blast gives rise to a whip-like filament, 
 which continues to elongate, while the bud from which it pro- 
 ceeds becomes rod-like. Eventually each rod with its whip 
 separates off as a spermatozoon, the rod forming the head, and the 
 whip the tail. The blastophor probably atrophies. Although 
 there are differences in details, the principle involved in this 
 mode of development would seem to be typical. If, as is prob- 
 able, the sperm cell is to be regarded as equivalent to the germ 
 cell or ovarian ovum, then it must be noted that all the sperma- 
 tozoa derived from one such sperm cell are homologous with the 
 single ovum. 
 
 The fully-developed spermatozoa are conveyed outwards by 
 
Chap. XIII.] THE EARTHWORM. 289 
 
 the vasa deferentia at the period of congress. For though the 
 worm is hermaphrodite, the ova of one individual are fer- 
 tilised by the spermatozoa of a second. At such time the 
 spermatozoa are received by each worm from the other into 
 the spermathecse, and are there stored until the ova shall be 
 laid. 
 
 It has been suggested that the oviducts, vasa deferentia, and 
 spermathecse are homologous with segmental organs; which 
 have in the generative segments been specially metamorphosed 
 in subservience to the process of reproduction. The evidence 
 for this view, however, involves careful comparison with other 
 allied forms of life, and is therefore outside the limits of this 
 work. 
 
 The eggs are laid in special chitinous cases, probably secreted 
 by the cingulum. Certain so-called capsulogenous organs, modi- 
 fied from the setigerous capsules of the inner series in the 
 generative segments have, however, been assigned this func- 
 tion. Into each egg-case seminal fluid is poured from the 
 spermathecse. 
 
 Development. Segmentation is complete and nearly regular. 
 It results in a flattened blastosphere forming a disc, composed 
 of an upper layer of smaller cells and a lower layer of larger 
 clearer cells. The disc becomes folded over by a process of 
 imagination, the larger and clearer cells becoming internal to 
 the others. Thus an elongated, two layered, more or less 
 ciliated gastrula is produced. The blastopore at first forms an 
 elongated slit at the folded edges, along the ventral surface; 
 but it gradually narrows to a pore which becomes the mouth. 
 The central or enteric cavity is lined by hypoblast cells, but at 
 the region of the mouth the epiblast becomes inflected so as 
 to line the cesophageal region. Thus there is no stomodseal 
 invagination. The anus and hind-gut are, however, formed by 
 a proctodseal invagination. 
 
 The exact mode of origin of the mesoblast is a matter of 
 uncertainty. When formed it segments into quadrate masses 
 19 
 
290 ANIMAL BIOLOGY. [Part II. 
 
 somewhat resembling the mesoblastic somites of the vertebrate, 
 which become hollowed, and by apposition give rise to the 
 body segments. 
 
 The nervous system arises by two distinct thickenings of the 
 epiblast : (1) in the supra-oesophageal region, (2) in the region 
 of the ventral cord. The junction between the two takes place 
 comparatively late. 
 
CHAPTER XV. 
 THE SNAIL. 
 
 THE Common Snail (Helix aspersa) is an air-breathing terres- 
 trial mollusk. In its possession of an external shell into which 
 it can withdraw itself, in the complete absence of lateral appen- 
 dages to serve as organs of locomotion, and in the curious want 
 of bilateral symmetry, this organism differs from those which 
 we have hitherto considered. 
 
 In the fully- extended snail (Fig. 85, A.) the head is seen to be 
 fairly differentiated, though it shades off into that part, the foot, 
 on which the organism creeps, which is in turn surmounted 
 by the shell, containing the visceral sac. The head bears two 
 pairs of tentacles, a shorter anterior pair (a. t.) and a longer 
 posterior pair (p. t.}, at the expanded ends of which are the pig- 
 mented eyes. The mouth (m.) is bounded by a circular lip and 
 lateral lips, and bears a brown horny jaw above. The thick- 
 ened ridge which is seen just within the lip of the shell is the 
 collar (co.). The shell itself is spiral, and consists of several 
 ivhorls gradually diminishing in size to the apex. The line of 
 junction between adjacent whorls is a suture. . The hollow 
 spindle around which the whorls are wound is the columella. 
 The lip of the shell is called the peritreme. Externally the 
 shell is covered by an organic cuticle. Beneath this the cal- 
 careous shell is composed of two layers. The first layer has a 
 thin superficial portion showing confused striation, and a thicker 
 portion (honeycomb layer) formed of vertical prisms. The second 
 or deeper layer (nacre or mother of pearl) is colourless, and con- 
 sists of several strata of prisms arranged horizontally, the 
 axes of the prisms in successive layers being set at right angles. 
 
 291 
 
292 
 
 ANIMAL BIOLOGY. 
 
 [Part II. 
 
 Both cuticle and calcareous layers are secreted by the posterior 
 edge of the collar. 
 
 There are six external apertures. Beneath (1), the mouth, is 
 (2), the opening of the pedal gland (p. g.). Below the posterior 
 
 st. 
 
 d. d. cjl 
 
 FIG. 85. SNAIL : DIGESTIVE ORGANS, HEART, AND PULMONARY VESSELS. 
 
 A. External characters of snail. B. Snail dissected to show digestive 
 system, pulmonary chamber, heart, and renal organ. 
 
 a. t. Anterior tentacle, an. Anus. ait. Auricle, col. Collar, cr. Crop. 
 d. d. gl. Duct of the digestive'gland. d. gl. Digestive gland, d. s. gl. Duct 
 of salivary gland, g. a. Genital aperture. Ti. j. Horny jaw. int. Intestine. 
 m. Mouth, m. 6. Muscular band. o. s. d. Opening of salivary duct, oil 
 Odontophore. p. a. Pulmonary aperture, p. gl. Pedal gland, p. t. Posterior 
 tentacle, p. v. Pulmonary vessel. . r. Rectum. T. s. Radula sac. ren. 
 Renal organ, s. gl. Salivary gland, st. Stomach, v. Ventricle. 
 
 tentacle on the right side is (3) the genital aperture (g. a.). In 
 the collar there is (4) a large pulmonary aperture (p. a.) leading 
 into the lung ; and close to this, on its right side, a larger aper- 
 ture (5), the vent, and closely associated with this (6), a smaller 
 orifice, by which the duct of the renal organ terminates. 
 
Chap. XV.] THE SNAIL. 293 
 
 The snail can, on occasion, completely retract itself into its 
 shell. At such times the collar is exposed so that respiration can 
 be carried on through the pulmonary aperture, which rhythmically 
 opens and shuts. When the snail retreats into its shell for a pro- 
 tracted period, e.g. during the winter months, there is formed across 
 the opening of the shell an epiphragm or hybernaadum, consisting 
 of hardened mucous impregnated with calcareous matter. 
 
 If a snail be completely immersed its tissues absorb a large 
 quantity of water. It may then be rapidly killed by placing in 
 moderately hot water. It should thus die well extended, with 
 its tissues flaccid. After making out the external apertures 
 (except the minute orifice of the renal duct), and inserting a 
 probe into the pedal gland which extends into the foot, and 
 may be opened out now or at a later period the student should 
 remove the shell bit by bit, being careful not to injure the deli- 
 cate tissue of the digestive gland, and should then pin out the 
 snail under water. A probe inserted into the pulmonary aper- 
 ture is seen to pass into the pulmonary chamber, which is roofed 
 over by the mantle. This mantle-roof, in which a number of 
 blood-vessels are seen to ramify, unites along the collar with the 
 dorsal body -walls, except at one point which remains open as the 
 pulmonary aperture. Along the right-hand side of the pulmon- 
 ary chamber runs the rectum. 
 
 Dissection is best begun by cutting along the base of the 
 collar, so as to sever the connection between the mantle and 
 collar and the dorsal body-wall. An incision should then be 
 carried up just to the right of the rectum, so as to leave the 
 rectum attached to the mantle. The roof of the pulmonary 
 cavity may now be examined from its ventral aspect. In it is seen 
 a white tongue-shaped mass, the renal organ (Fig. 85, B., ren.), 
 to the left of which (in this position) is the pericardial cavity, 
 on opening which the auricle (au.) and ventricle (v.) of the heart 
 will be seen. The floor of the pulmonary chamber is seen to 
 be constituted by a white muscular septum, which separates it 
 from the body-cavity beneath. This should now be opened, 
 -and an incision should be carried forward with scissors to the 
 anterior end of the head. A large crop, and anteriorly a buccal 
 
294 ANIMAL BIOLOGY. [Part II. 
 
 mass, will now be seen. Just posterior to the buccal mass, and 
 encircling the oesophagus, is the nerve-collar. Besides these organs 
 there are a number of white tubes (Fig. 87), which belong to 
 the organs of generation. We will now take the systems of 
 organs in detail. 
 
 The Alimentary System. The student who has dissected sa' 
 far will, with care, not find much difficulty in displaying the di- 
 gestive organs as shown in Fig. 85, B. The genital organs are 
 removed, the mantle dissected from the foot, certain white glis- 
 tening bands (columella muscles) have been cut away, and the 
 whole of the foot posterior to the head removed. The -head 
 and buccal mass are shown in section, the nerve-collar not 
 being represented. The stomach, intestine, and digestive gland 
 readily assume this position when the integument is carefully 
 stripped off. 
 
 The mouth (m.), on the dorsal side of which is the horny jaw 
 (h.j.) y leads into a buccal cavity. Into this there projects from 
 below a rounded odontophore (od.), which carries on its upper 
 surface a tooth-bearing lingual ribbon or radula. The radula is 
 developed in a diverticulum of the buccal chamber, the radula 
 sac. In this there is a layer of large cells, from each of 
 which several hard chitinous teeth are developed. Develop- 
 ment is continuous, and the whole radula grows forward over 
 the odontophore, just as our nails grow forward over their beds 
 from a groove in the skin of the finger answering to the radula 
 sac. The radula itself should be stripped off from the odonto- 
 phore of another snail, and examined under the microscope. 
 There is a central row of sharp teeth, on either side , of which 
 are a number of lateral rows. The way in which this lingual 
 ribbon is used may be seen if a pond-snail (Limnceus) be watched 
 as it licks the conferva from the sides of an aquarium tank. 
 
 The buccal cavity leads into an oesophagus, which is soon fol- 
 lowed by an expanded crop (cr.), at the sides of which are large 
 salivary glands (s. gl.). From these the ducts (d. s. gl.) lead down- 
 wards and open into the buccal cavity above the odontophore 
 (o. s. d.). The product of the salivary gland converts starch 
 
Chap. XV.] THE SNAIL. 295 
 
 into sugar. Beyond the crop the diameter of the canal narrows, 
 but soon expands again to form the stomach (si). Here the 
 digestive tube makes a sharp turn and proceeds as the intestine 
 (int.), which makes an S-shaped loop through the tissue of the 
 digestive gland (d. gl.), and then passes into the rectum (r.). The 
 large digestive gland is divided into two lobes, one of which 
 lies in the apex of the shell. This lobe pours its secretion into 
 the stomach by a special duct. The ducts of the three divisions 
 of the left lobe (d. d. gl.) unite to form a common duct. Three 
 kinds of cells are found in the digestive gland : (1) ferment cells, 
 which blacken rapidly with one per cent, osmic acid; (2) hepatic 
 cells, which contain spherical globules soluble in ether, and 
 excrete small vesicles which have yellowish contents, and are 
 evacuated with the faeces ; and (3) calciferous cells, containing 
 granules of carbonate or phosphate of lime, which are said to be 
 utilised in the formation of the hybernaculum. 
 
 Some of the cylindrical epithelium cells lining the buccal cavity 
 are ciliated. A section of the oesophagus gives the following 
 layers: (1) an internal lining cuticle; (2) an epithelial layer; 
 (3) a lacunar mucosa layer; (4) a muscular layer, with (a) cir- 
 cular and (b) longitudinal fibres; (6) an external peritoneal 
 layer. The epithelium which lines certain depressions in the 
 stomach has been regarded as glandular, that covering certain 
 conical processes of the mucosa layer as absorptive ; and from 
 the distribution of the glandular and absorptive organs it would 
 seem that, with the exception of the buccal portion, which is 
 used for collecting food, and the terminal portion for expelling 
 faeces, no separate functions are assignable to the other divisions 
 crop, stomach, intestine of the alimentary canal. The fer- 
 ment secreted in the digestive gland would seem to be capable of 
 converting starch into sugar, and of digesting proteids. 
 
 The Heart and Circulation. The heart (Fig. 85, B.) has a 
 single auricle (au.) and single ventricle (v.), between which there 
 is a valvular orifice. It lies in a pericardial chamber, which is in 
 communication with the renal organ. From the ventricle there 
 passes forward a single aorta. This runs back, passes dorsal to 
 
296 ANIMAL BIOLOGY. [Part II. 
 
 the intestine, and almost at once splits into two trunks a pos- 
 terior aorta, which follows the course of the intestine on its 
 convex side, and an anterior aorta which, after giving off branches 
 to the genital organs, passes into the body-cavity, where it may 
 readily be traced, giving off branches to the salivary glands and 
 mid-region of the foot, and proceeding to the head, piercing the 
 nerve-collar (Fig. 86, A., ao.), and being distributed to the buccal 
 mass and neighbouring parts. The ultimate branches of the 
 arteries would seem to end in funnel-shaped dilated openings into 
 the lacunae which lie within and around the tissues, there being 
 no true capillaries. The blood finds its way into large blood- 
 sinuses, there being a large lateral sinus on each side of the foot. 
 There is also a large visceral sinus running along the concave 
 side of the body whorl. This latter is in direct communication 
 with the circular pulmonary sinus which runs round the floor of 
 the pulmonary chamber; but the communication between the 
 lateral sinus and this pulmonary circle must be indirect. From 
 the pulmonary circle afferent pulmonary vessels lead the blood to 
 the roof of the lung-chamber, whence efferent pulmonary vessels 
 form factors of the large pulmonary vein (p. v.), which conveys 
 the blood to the auricle. It will be seen that the renal organ 
 (ren.) is in the course of the blood which is finding its way to the 
 pulmonary vein, on the right side of the roof of the lung. 
 The blood is colourless, and contains nucleated corpuscles. 
 
 Respiration. The afferent and efferent pulmonary vessels in 
 the roof of the pulmonary chamber form a network, in which 
 the blood is separated from the air by the thin partition of their 
 walls. Here, therefore, carbonic acid gas is given off by the 
 blood and oxygen absorbed. Air enters and leaves the lung 
 by the pulmonary aperture, being drawn in by the contraction 
 of the muscular fibres of the septum which forms the floor of 
 the chamber, and being expelled by the relaxation of these 
 fibres and consequent rise of the septum by the pressure of the 
 viscera which lie beneath' it. 
 
 The Renal Organ. The renal organ consists of two parts, the 
 
Chap. XV.] 
 
 THE SNAIL. 
 
 297 
 
 distinction between which may readily be seen in a snail that 
 has been drowned in cold or lukewarm water. A thin-walled 
 saccular part (shaded in Fig. 85, B., ren.) will then be distended 
 with water, and its duct running alongside the rectum will be 
 visible for at least the upper part of its course. The other part of 
 the organ is yellowish white and lamellar in structure. Within 
 the lamellae are great numbers of rounded refringent granules. 
 
 The Nerve-collar. Lying over the oesophagus, as shown in 
 Fig. 86, A., are united cerebral ganglia (c. g.). Lying below the 
 oesophagus are two ganglionic masses tolerably close together. Of 
 
 FIG. 86. SNAIL : NERVOUS SYSTEM, EAR, AND EYE. 
 
 A. Dissection to show nerve-collar, its sheath being removed. B. Otocyst, 
 or auditory organ. C. Eye. 
 
 In A. ao. Aorta, aw. Auditory organ, a. t. Anterior tentacle (retracted) 
 "b. g. Buccal ganglion. 6. m. Buccal mass. c. g. Cerebral ganglion. cr. 
 Crop. e. Eye. op. ti. XJptLc nerve, p. t. Posterior tentacle (retracted). 
 pd. g. Pedal ganglion, pi. g. Parieto-splanchnic ganglion, pd. n. Pedal nerve. 
 pl. n. Pallial nerve. v. n'. Visceral nerve. 
 
 In C. co. Cornea. I. Lens. op. n. Optic nerve, r. Retina. 
 
 these the anterior is formed by the fused pedal ganglia (pd. g.), 
 while the posterior is formed by the fused parieto-splanchnic 
 
298 ANIMAL BIOLOGY. [Part II. 
 
 (pi. g.). Commissures pass on either side of the oeso- 
 phagus to both these ganglia. The three united ganglionic 
 masses are called the nerve-collar, which is invested by a loose 
 sheath. In the dissection from which the diagram was taken, 
 the anterior (a. t.) and posterior tentacles (p. t.) were both in 
 the retracted position. The optic nerve (op. n.) and the nerve to 
 the anterior tentacle have therefore a somewhat backward direc- 
 tion. The optic is distinct, and arises from a knot on the 
 cerebral ganglion. The nerve to the anterior tentacle is a 
 branch of the labial nerve running forward to the lip. A second 
 nerve has a somewhat similar course, and a small nerve passes 
 over the buccal mass to the integument in the neighbourhood 
 of the horny jaw. In addition to these an auditory nerve runs 
 between the two commissures to the auditory organ (au.\ which 
 is supposed to be seen in the diagram through the substance of 
 the parieto-splanchnic ganglion ; and a buccal nerve runs forward 
 to a buccal ganglion which lies close to the termination of the 
 salivary duct. The two buccal ganglia are united by a delicate 
 commissure. 
 
 From the pedal ganglia a number of pedal nerves (pd. n.) run 
 out to innervate the foot. From the parieto-splanchnic ganglia 
 pallial nerves (pi. n.) proceed on either side, and sooner or later 
 branch before being distributed to the neighbourhood of the 
 collar. Closely associated with the aorta (ao.) is a visceral nerve 
 (v. ??.), which innervates the organs of generation, a twig passing 
 to the heart. Another nerve, taking its origin near the aorta, 
 passes to the floor of the pulmonary chamber. 
 
 Special Senses. The eyes are borne by the posterior tentacles. 
 One of these organs is seen in diagrammatic section in Fig. 86, C. 
 Beneath the external corneal layer (co.\ is the crystalline lens 
 (/.). Beneath this is the retina (r.), the rods of which are sur- 
 rounded by pigmented material. The retina is continuous with 
 the optic nerve (op. n.) Notwithstanding the possession of so 
 well-developed an eye, the snail does not seem to be aware of 
 the presence of a small object, such as the head of a pin, until 
 it is within from Jth to ^th of an inch from the end of the 
 
Chap. XV.] THE SNAIL. 299 
 
 posterior tentacle. If in a vigorous and healthy snail the pos- 
 terior tentacle be removed, the eye is regenerated. In some 
 cases it has been found that several perfect eyes have regener- 
 ated in the same tentacle. 
 
 The auditory organ is a minute sac (Fig. 86, A. and B.), with 
 which the auditory nerve is connected. The sac contains a 
 number of otoliths aggregated into a mass. 
 
 With regard to the sense of smell there has been much 
 discussion. The anterior tentacle is often spoken of as olfactory. 
 Near its end is a large ganglion from which fibres pass to the 
 epithelium, in which there are said to be developed sensory 
 knobs. Snails, however, in -which these tentacles have been 
 removed by excision, are apparently still possessed of a sense 
 of smell. The lobate processes round the mouth have been 
 regarded as the seat of olfactory sensation. Eecent investigation, 
 however, seems to point to the glandular rather than the sensory 
 nature of these organs. Finally it has recently been contended 
 that the pedal gland is the olfactory organ. Some of the cells 
 are glandular and mucus-secreting. Others are deeper-seated 
 and oval, and give off a delicate rod which enlarges at its free 
 end into a ciliated knob. These are regarded as sensory. Thus 
 it is maintained that we have in this organ the three necessary 
 factors of an olfactory organ, the presence of a layer of sensory 
 cells, the entrance of air, and the addition of a secretion from a 
 glandular organ. 
 
 With regard to the seat of a sense of taste little or nothing 
 definite is known. 
 
 The whole body is sensitive to touch, but in the tentacles 
 this sensitiveness is much more delicate than over the general 
 surface. The nerve that proceeds to the posterior tentacle 
 bifurcates within that organ. One branch goes to the eye. The 
 other passes to a ganglion, from which fine threads pass to 
 a highly sensitive prominence at the end of the tentacle. It is 
 very probable that we have here a delicate tactile organ. 
 
 The Organs of Generation. The snail is hermaphrodite, 
 male and female organs being found in the same individual. 
 
ANIMAL BIOLOGY. 
 
 [Part II. 
 
 The essential organ is lodged in the right lobe of the diges- 
 tive gland in the apex of the shell (Fig. 87, h. gl), from which 
 it has not been extricated in the dissection from which the 
 diagram is taken. It is known as the hermaphrodite gland, 
 
 .a. 
 
 g.ap. 
 
 FIG. 87. SNAIL : ORGANS OF GENERATION. 
 
 al. gl Albumen gland, d. s. Dart Sac. fl. Flagellum. g. ap. Genital Aperture. 
 h. d. Hermaphrodite duct. h. gl. Hermaphrodite gland imbedded in terminal 
 lobe of digestive gland, m. gl. Muciparous gland, ov.d. Oviduct, pr. 
 Prostate, p. s. Penis sac. r. p. Retractor muscle of penis sac. sp. Sper- 
 matheca. vl>. Vestibule, v.d. Vas deferens. 
 
 or ovotestis. Within the minute acini, of which it is an aggre- 
 gation, both ova and spermatozoa are developed. And if an 
 ovotestis be teased with needles, and examined under the micro- 
 scope, developing or ripe ova, and sperm cells, spermatoblasts 
 attached to a blastophore, blastophores with bundles of almost 
 fully developed spermatozoa, or separated male elements may be 
 seen. 
 
Chap. XV.] THE SNAIL. 301 
 
 Much larger than the essential organ or ovotestis is the 
 white tongue-shaped albumen gland (al. gl.), between the base of 
 which and the hermaphrodite gland runs a delicate convoluted 
 tube, constituting the upper part of the hermaphrodite duct (h. d.). 
 Into this duct the albumen gland pours its secretion. Below 
 the albumen gland the duct is much folded and puckered, and 
 closely connected with its lower part is the white glandular 
 prostate. Towards its lower end it becomes partially divided 
 into two tubes by an infolded partition. Lower still it gives 
 rise to a shorter oviduct (ov. d.), and a longer vas deferens (v. d.). 
 The oviduct is joined by another tube, the spermatheca (sp.), which 
 higher up bifurcates into two tubes, one ending blindly without 
 dilatation, and in its natural position closely attached to the 
 genital duct, the other ending in a rounded dilatation, which 
 in its natural position lies in the loop of the intestine at the 
 bifurcation of the aorta. Below the point of junction of the 
 oviduct and spermatheca the common tube is joined by the 
 ducts of two muciparous glands (m. gl.), just below which there 
 is a large white oval sac, the dart sac, which, if it be carefully 
 opened, will sometimes be found to contain a sharp calcareous 
 dart, the so-called spicula amoris, on which are four slightly- 
 twisted ridge-like blades. The tube common to the oviduct, 
 spermatheca, muciparous glands, and dart sac, now passes to the 
 vestibule (vb.\ which opens on to the exterior by the genital 
 aperture (g. ap.). 
 
 The vas deferens, after diverging from the common genital 
 duct, takes a curved course. From it a long whip-like diver- 
 ticulum, the flagellum (/.), is given off. Near the vestibule the 
 tube is swollen, and if it be carefully opened it will be found 
 to contain a conical intromittent organ (p. s.). From this point 
 downwards the tube may be everted. A retractor muscle (r. p.) 
 passes back from the tube above this point, and is inserted into 
 the muscular floor of the pulmonary cavity. 
 
 The spermatozoa, after descending the vas deferens, would 
 seem to be enclosed in a sperm capsule, or spermatophore, the 
 flagellum not improbably secreting the material out of which 
 this is moulded. It is a delicate spindle, two or three inches 
 
302 ANIMAL BIOLOGY. [Part II. 
 
 in length; the clear transparent ends taper off to fine points. 
 In the middle is a white opaque portion, in which are the sper- 
 matozoa. At the time of congress the two snails exchange 
 spermatophores, which pass into the spermatheca, where they 
 seem to break up, giving exit to the spermatozoa, by which 
 the ova are fertilised, as they pass down the oviduct. The 
 muciparous glands, which give rise to a milky secretion, perhaps 
 supply them with cases. 
 
 The darts are extruded at such times. They may sometimes 
 be found sticking in the skin of the snail ; while in dissecting 
 the snail their remains may sometimes be found in or near the 
 spermatheca or the genital duct. 
 
 The ova are laid in groups, generally in June or July, and 
 in about 20 days hatch into young snails. 
 
 Development of Pond Snail. The development of the pond 
 snail (Limnceus) is more typical than that of Helix, and its ova 
 and embryos are readily obtained, and studied without difficulty. 
 If pond snails be kept in a glass aquarium tank, there will, in 
 summer, be found adhering to the glass, or to water-weed, small 
 oval patches, or elongated strips of jelly-like substance, divided 
 into a number of compartments, within each of which is an 
 ovum or an embryo. 
 
 Segmentation is holoblastic and fairly regular. First two 
 and then four segmentation spheres are formed. Then from the 
 upper surface of these four spheres there separate off smaller 
 spheres, which, by a continuation of this process and by sub- 
 division amongst themselves, become comparatively numerous. 
 In Fig. 88, A., there are four large spheres, macromeres (mac.), and 
 several smaller spheres, micromeres (mic.). Subsequently both 
 macromeres and micromeres increase in number, a segmentation 
 cavity is formed, and invagination takes place in such a way that 
 the macromeres are pushed in (Fig. 88, B.) and become hypoblast 
 (%.), the micromeres giving rise to epiblast (ep.). 
 
 The blastopore (bl.) so formed would seem to elongate before 
 it finally closes, the position of the future mouth and anus being 
 at the anterior and posterior end of the slit. Both mouth 
 
Chap. XV.] 
 
 THE SNAIL. 
 
 and anus, however, are generally held to be formed by subse- 
 quent stomodseal and proctodseal invaginations. 
 
 At an early stage of development there is formed round the 
 embryo an equatorial circlet of cilia, by means of which the 
 embryo is caused to rotate within the jelly-like walls of the com- 
 
 FIG. 88. POND SNAIL : DEVELOPMENT. 
 
 A. Ovum after segmentation into macromeres (mac.) and micromeres 
 (ro-ic.). 
 
 B. Gastrula Stage. bl. Blastopore. ep. Epiblast liy. Hypoblast. s. c. 
 Segmentation cavity. 
 
 C. Trochosphere stage./. Foot. m. Mouth, sh. gl. Embryonic shell 
 gland, v. Velum. 
 
 D. Veliger stage. coZ. Collar. /.Foot. m. Mouth, t. Tentacle, v. Velum. 
 
 partment in which it lies. Subsequently growth consists in the 
 gradual enlargement of one hemisphere, and the reduction of the 
 other. In the larger hemisphere both mouth and anus will be 
 formed. In Fig. 88, C., the embryo is in what is known as the 
 trochosphere stage. The incomplete circlet of cilia is seen at v. ; 
 the stomodaeum at m. ; and in the larger hemisphere the foot 
 (/.) is becoming marked off from the visceral hump. At the 
 posterior end of this is shown (at sh. gl.) the position of the 
 so-called shell gland. It is found in many molluscan embryos. 
 In Liiimceus it is merely an embryonic organ, the true shell 
 being formed, as in the snail, by the cells in the neighbourhood 
 of the collar. 
 
304 ANIMAL BIOLOGY. [Part II. 
 
 In Fig. 88, D., the embryo is shown at a later stage, known as 
 the veliger stage. The incomplete circlet of cilia has become the 
 velum (v.) in front of the mouth (m.). The foot has become 
 bi-lobed, and the collar is developed. The visceral hump is 
 covered with a delicate shell. At t. the tentacle is being 
 developed. 
 
 The common snail does not seem to pass through a trocho- 
 sphere arid veliger phase, these stages in molluscan existence 
 being shortened or suppressed. 
 
 Mesoblast takes its origin at the lips of the blastopore. 
 
 The nerve-collar arises as several epiblastic thickenings. 
 
CHAPTEE 
 
 THE FRESH- WA TER MUSSEL. 
 
 THE Common Fresh-water Mussel (Anodonta cygnea) is enclosed 
 in a bivalve shell which constitutes its exoskeleton. This shell 
 (Fig. 89) is greenish brown in colour and rough ; of an elongated 
 oval form blunter at one end, more tapering at the other ; and 
 
 ex. si jLjf iu. si. 
 
 ill. 
 
 Fin. SO. MUSSEL : SHELL AND EXTERNAL CHARACTERS. 
 
 ". mi. i. Anterior adductor impression, a. r. i. Anterior retractor impres- 
 sion, ex. si. Exhalent siphon. /. Foot. f. e. Inturned edge. in. si. Inhalent 
 siphon. I. r. i. Lesser retractor impressions. Ig. Ligament, p. ad. i. Pos- 
 terior adductor impression, p. p. i. Protractor pedis impression, p. r. i. 
 Posterior retractor impression, pal. 1. Pallial line. urn. Urnbo. 
 
 marked with a series of concentric lines, each of which was at 
 one time the margin of the shell. The starting-point or origin 
 of this series of curved lines, the spaces between which represent 
 20 
 
306 ANIMAL BIOLOGY. [Part II. 
 
 successive additions to the shell during growth, forms, on the 
 dorsal border of the valve nearer the more rounded anterior 
 end, a small blunt eminence, the umbo (urn.). Posterior to this, 
 the exoskeleton is uncalcified for some distance along the dorsal 
 border. This uncalcified part is called the ligament (Ig.) It 
 serves by its elasticity to keep the shell slightly agape. Fig. 89, 
 A., shows the mussel in its normal position partially buried in 
 the mud or sand. A fleshy part of the animal, the foot (/.), 
 protrudes at the anterior end. Posteriorly a short double tube 
 projects from the shell. The dorsal tubular part is smaller and 
 smoother, the ventral part is larger and provided with a number 
 of papillary processes. If the water be slightly turbid, a current 
 will be seen continually setting in at the lower part of the tube 
 (inhalent siphon, in. si.) and out through the upper part (exhalent 
 siphon j ex. si.). 
 
 The mussel may be killed by placing it in cold water and 
 gradually raising the temperature of the water until it is as hot 
 as the hand can bear without discomfort. The shell will be 
 slightly gaping and the foot protruded. On removing the 
 mussel from the water, pulling the valves somewhat further 
 apart, and looking in between them from the ventral aspect, 
 their inner surface will be seen to be lined with a delicate 
 yellowish mantle or pallium. Between the mantle and the 
 fleshy foot are seen on each side two flattened lamellar organs, 
 the gills. These lie in a large chamber (infra-branchial chamber), 
 normally closed below by the meeting across the mid-ventral 
 line of the mantle folds, but open posteriorly by the inhalent 
 siphon, the papillary walls of which are now seen to be a 
 specially differentiated part of the mantle. A probe passed into 
 the exhalent siphon enters a smaller chamber (supra-branchial 
 chamber) partially separated from the infra-branchial chamber by 
 the growing together, or concrescence, of the posterior portion 
 of the inner gill-lamellse behind the foot. The probe (Fig. 90, 
 A., pr.) will, however, emerge from the upper into the lower 
 chamber by a wide slit which separates the inner gill from the 
 foot. Anterior to the foot, between it and a strong band of 
 muscle (anterior adductor) which passes from one valve of the 
 
Chap. XVI.] THE FRESH-WATER MUSSEL. 307 
 
 shell to the other, is seen the mouth. The vent opens into the 
 supra-branchial chamber close to its exhalent aperture. 
 
 With the handle of a scalpel the mantle may now be separated 
 from its attachment to the left valve of the shell by a thickened 
 ridge, the pallial muscle. The anterior adductor muscle must be 
 cut close to the shell, and a similar muscle (posterior adductor] 
 must be cut at the hinder end. In cutting these muscles, other 
 smaller muscles will also be severed. By cutting the ligament 
 the left valve of the shell may now be removed without diffi- 
 culty. 
 
 Its inner surface is white and glistening, with a play of iri- 
 descent colours. The scars or impressions of the adductor 
 muscles (Fig. 89, B., a. ad. i., p. ad. i.) will be readily seen, and 
 smaller impressions of the anterior retractor (a. r. i, confluent 
 with a. ad. i.), posterior retractor (p. r. i.), and protractor pedis 
 (p. p. i.). From each of these impressions there runs towards 
 the umbo a fainter tapering impression which marks the gradual 
 shifting of the muscles during the growth of the shell. The 
 impression of attachment of the pallial muscle of the mantle 
 is seen as the pallial line (pal. I.). 
 
 If a thin section of a fragment of the shell be ground down 
 and examined under the microscope, the three layers of which 
 it is composed will be readily made out. Externally is the 
 horny periostracum ; beneath this is a prismatic layer of pallisade- 
 like prisms ; internal to this is a laminated nacreous layer. The 
 nacreous layer is secreted by the whole external epithelium of 
 the mantle. The outer layers are formed only at the free edge 
 of the mantle. 
 
 The mussel now lies exposed to view in its right valve. The 
 left lobe of the mantle should be cut away close to its origin, 
 and the underlying parts exposed as in Fig. 90, A. The cut ends 
 of the muscles ( a. ad., p. p., a. r. ra., p. r. m., p. ad.), the mouth 
 (m.), foot (/.), and gills (i. g., o. g.}, the right mantle lobe (pal.), 
 and the inhalent and exhalent siphonal apertures (in. si., ex. si.) 
 will be readily made out. A probe (pr.) has been passed 
 through the supra-branchial chamber into the infra-branchial 
 chamber, between the left inner gill and the foot. Two labial 
 
308 
 
 ANIMAL BIOLOGY. 
 
 [Part II. 
 
 palps, or labial tentacles (/. p.) are seen as triangular backward- 
 directed processes, on either side, just beneath the mouth. They 
 are highly vascular and richly supplied with nerves. Dorsal to 
 the gills is seen a large transparent sac filled with fluid, the 
 
 '' T>.ad. 
 
 FIG. 90. MUSSEL GENERAL ANATOMY; DIGESTIVE ORGANS. 
 
 a. ad. Anterior adductor muscle, a. r. m. Anterior retractor muscle, au. 
 Auricle, d. gl. Digestive gland, d. d. gl. Duct of digestive gland, ex. si. 
 Exhalent siphon. /. Foot. g. gl. Generative gland, i. g. Inner gill. in. si. 
 Inhalent siphon. Tc. o. Organ of Keber. I. p. Labial palp. m. Mouth, o. g. 
 Outer gill. p. Pore. p. ad. Posterior adductor muscle, p. c. Fericardial 
 chamber, p. p. Protractor pedis muscle, p. r. m. Posterior retractor muscle. 
 pal. Mantle, pr. Probe passed from exhalent siphon between the inner gill 
 and the foot. r. Rectum, re. Renal organ (organ of Bojanus). s^. Stomach. 
 ty. Typhlosole. v. Vent. ve. Ventricle. 
 
 pericardium (p. c.), within which the heart may be dimly seen. 
 Beneath the pericardium is seen through the body-wall the 
 greenish brown tissue of the organ of Bojanus or renal organ (re.}. 
 
Chap. XVI.] THE FRESH-WATER MUSSEL. 309 
 
 At the anterior end of the pericardium is seen the brownish 
 tissue of the organ of Keber (k. o.). 
 
 Alimentary System (Fig. 90, B.). For the dissection of the 
 alimentary canal it is well to work on a specimen that has lain 
 for two or three days in spirit. With the aid of the figure little 
 difficulty should be found in exposing the canal, and slitting it 
 open by passing a guarded bristle (tipped with sealing-wax), and 
 following it with scissors. 
 
 The mouth, which lies between the anterior adductor and the 
 foot, leads by a short oesophagus into the so-called stomach (st.) ; 
 an irregular sac imbedded in the brownish tissue of the 
 digestive gland (d. gl.\ the secretion of which enters by ducts 
 (d. d. gl.) near its anterior end. From the stomach the intestine 
 passes downwards and takes the coiled course indicated in the 
 figure. Into its cavity (at ty.) there projects an infolding of the 
 wall or typhlosole. The intestine passes upwards and enters the 
 pericardium at its anterior end, running backwards thence as the 
 rectum (r.), which passes through the ventricle (ve.) of the heart. 
 The rectum, in which the typhlosole is well marked, leaves the 
 pericardium near the dorsal side of its posterior end, and passes 
 over the posterior adductor muscle and along the dorsal side of 
 the supra-branchial chamber, where it ends in an anus (v.) placed 
 on a prominent papilla. 
 
 On scraping the lining membrane of the intestine, ciliated and 
 columnar cells will be found. Some of the columnar cells 
 would seem to be secreting cells. 
 
 On teasing out a small fragment of the digestive gland, it is 
 seen to be composed of branched caecal tubes. Their lining 
 epithelial cells, examined under a high power, show, in addition 
 to the protoplasm and nucleus, a vesicle which encloses spherical 
 brownish-green granules. 
 
 The food of the mussel consists of minute organisms swept 
 into the infra-branchial chamber by the inhalent current caused 
 by the lashing of the innumerable cilia on the gills. 
 
 The Gill-Plates. The plate-like gills, which give to the class 
 
310 ANIMAL BIOLOGY. [Part II. 
 
 of Mollusca to which Anodon belongs the name of Lamelli- 
 branchiata, lie, as we have seen, on either side of the foot 
 anteriorly, and hang down into the infra-branchial cavity pos- 
 teriorly. Each gill-plate consists of two lamellae, an inner and 
 an outer, or preferably a descending and an ascending lamella, and 
 each lamella may be regarded as arising from the union of a 
 great number of gill-filaments, descending and ascending, which 
 have become fused together into a perforated plate-like structure. 
 Fig. 91 will exhibit the relations of the lamellae. Fig. 91, A., is a 
 section passing through the middle of the ventricle (ve.) and the 
 posterior part of the foot (/.). The gill-filaments take their 
 origin from a gill axis (g. a.), forming in this part of the body a 
 ridge on the body-wall. From this axis there pass downwards 
 two descending filaments, an inner, forming part of the inner 
 lamella of the outer gill, and an outer, forming part of the outer 
 lamella of the inner gill. After proceeding downwards the fila- 
 ment turns sharp round and forms an ascending filament, running 
 parallel with the descending filament. The ascending filament 
 of the outer gill forms part of its outer lamella ; that of the 
 inner gills forms part of its inner lamella. The upper ends of 
 the ascending filaments of the outer gills become united by 
 concrescence along their whole length with the mantle. The 
 upper ends of the ascending filaments of the inner gills are in 
 this region free, thus giving rise to the slit-like communication 
 between the supra-branchial chamber (s. b. c.) and the infra- 
 branchial chamber (i. b. c.). 
 
 Fig. 91, B., is a diagrammatic section taken through a region 
 anterior to that shown in A., nearer the anterior end of the gill- 
 plates. It shows that here the ascending lamina of the inner 
 gill has united by concrescence with the foot. Fig. 91, C., is a 
 similar section taken through the posterior adductor muscle. It 
 shows that here, in the region posterior to the foot, the ascend- 
 ing lamellae of the inner gills have united with each other by 
 concrescence. Fig. 91, D., is taken through the siphonal cavity, 
 behind the posterior adductor. It shows that the gill-axis, 
 which in A. and B. was closely adherent to the body -wall, and in 
 C. was suspended by a suspensory ligament, has here become free. 
 
Chap. XVI.] THE FRESH-WATER MUSSEL. 311 
 
 Thus in the region through which D. passes, there is a 
 common supra-branchial chamber for the inner and outer gills 
 of each side j in the region of C. this is sub-divided into three 
 
 A 
 
 pal. m. 
 
 Fio. 91. MUSSEL TRANSVERSE SECTIONS : GILLS. 
 
 A. Through the heart and posterior end of the foot. 
 
 B. Through the middle of the foot near the anterior end of the gills. 
 
 C. Through the posterior adductor muscle. 
 
 D. Just behind the posterior adductor muscle. 
 
 au. Auricle, c. 6. s. Central blood sinus. /. Foot. g. a. Gill axis. i. &. c. 
 Infra-branchial chamber, i. g. Imier gill. Ig. Ligament, n. c. Nerve com- 
 missure, o. g. Outer gill. p. c. Pericardial chamber, pal. Mantle, pal. m. 
 Pallial muscle, re. Renal organ, s. &. c. Supra-branchial chamber, sh. 
 Shell, ty. Typhlosole of rectum, ve. Ventricle, ves. Vestibule of renal 
 organ. 
 
 In B. C. D. the cavity of the supra-branchial chamber is shaded. 
 
 parts, one median, common to the two inner gills, and two 
 lateral, one for each outer gill : in the region of A. the median 
 chamber is (a) becoming sub-divided by the foot, and (b) is in 
 communication with the infra-branchial chamber. Finally, in 
 
312 ANIMAL BIOLOGY. [Part II. 
 
 the region of B. the supra-branchial chamber is sub-divided into 
 four parts, one for each gill-plate. 
 
 Eegarding, then, the gills as composed of a great number ot 
 filaments with descending and ascending moieties and we are 
 justified by comparative morphology and development in so re- 
 garding them we have to note the large amount of fusion or 
 concrescence of parts. 
 
 1. The inner surfaces of adjoining filaments have become 
 united by a membranous expansion, the lamellar membrane, 
 perforated with oval fenestrae or windows. Thus the ascending 
 and descending filaments respectively come to form ascending 
 and descending lamellae. 
 
 2. The two lamellae of each gill become fused by inter- 
 lamellar junctions, occurring in the inner gill at intervals of 
 about twenty filaments, and in the outer gill at intervals of 
 seven or eight filaments. 
 
 3. The ascending lamella of the outer gill is concrescent with 
 the mantle along its whole length. 
 
 4. The ascending lamella of the inner gill is concrescent 
 anteriorly with the foot. 
 
 5. The ascending lamellae of the inner gills are concrescent 
 with each other posteriorly. 
 
 6. The union of the gill-axis with the body-wall may be 
 partly due to concrescence. 
 
 Two little square patches should be cut out from the descend- 
 ing (outer) lamella of the inner gill of a recently-killed mussel, 
 and be mounted in water (without cover glass), the one with its 
 outer and the other with its inner face uppermost. Examined 
 under a moderate power of the microscope, that with its outer 
 side uppermost shows a number of parallel bars the gill- 
 filaments strengthened with yellowish chitinous rods. Where 
 there is a fenestra in the underlying lamellar membrane, the 
 play of numberless cilia, working in one direction on one fila- 
 ment, and the opposite direction on the other, may readily be 
 watched. The patch with its inner face uppermost, shows 
 (especially with reflected light) the oval fenestrae in the lamellar 
 membrane. If some of the gill substance be teased and 
 
Chap. XVI.] THE FRESH-WATER MUSSEL. 313 
 
 examined under a high power, there will be seen : (1) frag- 
 ments of the chitinous rods, (2) ciliated cells, (3) irregular or 
 branched nucleated cells of the lacunar tissue, which supports 
 the ciliated cells and forms the ground substance of the gill, 
 and (4) colourless amoebiform blood corpuscles. 
 
 If the gill from which the square patches have been cut be 
 examined with a hand lens, the inter-lamellar junctions will be 
 seen. They are best seen, however, in the outer gill of a 
 mussel in which this gill has been converted into a brood-pouch 
 for embryos. 
 
 The ceaseless lashing of the cilia of the gills produces the 
 current of water which sets in through the inhalent siphon and 
 escapes through the exhalent siphon. This current passes from 
 the infra-branchial into the supra-branchial chamber, partly 
 through the fenestrse of the gill-lamellse, partly through the 
 large slit-like aperture between the foot and the ascending 
 lamella of the inner gill. It subserves two functions : (1) 
 alimentation by sweeping infusoria, diatoms, and other minute 
 organisms within the reach of the cilia of the mouth cavity; 
 (2) respiration as the water is driven past the gill-filaments 
 and through the fenestrse of the lamellar membrane. 
 
 The Heart and Circulation. The heart (Figs. 91 and 92) 
 consists of a median chamber, the ventricle (ve.\ and two lateral 
 chambers, the auricles (au.). These parts lie within the peri- 
 cardial chamber (p. c.). In a recently killed mussel the slow 
 wave-like contractions of the heart (five or six a minute) 
 may be observed. Seen from above, by opening up the peri- 
 cardial chamber from the dorsal aspect, the ventricle is seen as 
 a thick-walled oval sac, with a tendency to two lateral lobes 
 posteriorly. Through its midst passes the rectum. The 
 auricles are thin- walled conical sacs, the bases of the flattened 
 cones being applied to the sides of the pericardial chamber, 
 and their apices communicating with the median ventricle. 
 The aperture of communication between the auricle and 
 ventricle is a longitudinally elongated valvular slit. If the 
 rectum, a little anterior to the ventricle, be severed, and the 
 
3H ANIMAL BIOLOGY. [Part II. 
 
 cut end examined with a lens, the anterior aorta will be seen 
 running along its dorsal surface. And if the rectum, a little 
 posterior to the ventricle, be similarly cut and examined, the 
 posterior aorta will be seen running beneath the rectum (along 
 its ventral surface). By the anterior aorta the blood is dis- 
 tributed to the foot, labial palps, intestine, and the anterior 
 part of the mantle. By the posterior aorta the adductor 
 muscles of this region and the posterior part of the mantle are 
 supplied. These vessels break up into irregular chambers or 
 lacunar spaces, whence the blood collects into a large vessel, the 
 so-called vena cava or central blood sinus (Fig. 91, A., c. b. s.), 
 which may readily be seen beneath the floor of the pericardial 
 chamber. Thence it passes by a number of vessels to an 
 irregular plexus in the renal organ ; and thence on each side to 
 a longitudinal vessel which lies in the gill-axis. From this 
 longitudinal vessel the blood passes into vertical vessels which 
 run downwards into the descending lamellae at the points of 
 union of the inter-lamellar junctions with the lamellar mem- 
 brane. There are similar vertical vessels in the ascending 
 lamellae \ but there does not seem to be a distinct capillary 
 system with definite walls between these two, but rather a 
 loosening or incoherence of the walls of the vessels, such as to 
 admit the passage of blood from them into the substance of the 
 lacunar tissue. The vertical vessels in the ascending lamellae 
 carry the blood upwards to horizontal vessels which run along 
 their margins, by which the purified and aerated blood is con- 
 veyed to the auricles. A plexus of vessels or spaces arises near 
 the junction of these afferent vessels with the auricles, and 
 ramifies in the so-called organ of Keber (Figs. 90, A, and 92, Jc. 0.), 
 which embraces the anterior end of the pericardial cavity. The 
 function of this organ is not well understood. 
 
 Thus the general order and direction of circulation in the 
 mussel is : ventricle, aortae, smaller vessels and lacunar spaces 
 in the foot and mantle, central blood sinus, renal organ, gills, 
 auricles, ventricle. 
 
 The blood is colourless, with numerous white amo3biform 
 nucleated corpuscles. 
 
Chap. XVI. ] THE FRESH- WA TER MUSSEL. 3 1 5 
 
 The protrusion of the foot is largely effected by the forcing of 
 blood into the lacunar spaces that abound therein. According 
 to some observers there are (1) three ciliated openings in the 
 ventral surface of the foot by means of which water can enter 
 and be admitted into the blood system, and (2) pores of com- 
 munication between the blood system and the pericardial 
 chamber. This provides for an ingenious mode of flushing the 
 renal organ and sweeping out its solid contents. The writer is, 
 however, in common with other observers, unable to satisfy 
 himself of the existence of these communicating pores, internal 
 or external. 
 
 The Renal Organs. The renal organs or so-called organs of 
 Bo janus of the mussel are paired structures lying beneath the 
 pericardial cavity. Their relations are shown in the dissection 
 from the side, Fig. 91, and in the cross-section, Fig. 92, A. Each 
 consists of two parts : (1) a glandular part (re.), which com- 
 municates anteriorly with the pericardial cavity, and posteriorly 
 with (2) a vestibule, with delicate thin walls, communicating 
 anteriorly with the exterior by the external renal aperture 
 (e. re. a.). To show this aperture the gills must be cut away as 
 shown in Fig. 92. Just anterior to it is the external aperture of 
 the generative gland. 
 
 Open the pericardial cavity from the dorsal aspect, divide the 
 rectum at the posterior end of the cavity, cut through the 
 auricles near their junction with the walls of the chamber, and 
 turn the rectum and heart forwards. The floor of the peri- 
 cardium will then be displayed. Beneath it in the mid-line is 
 seen the central blood sinus, and on either side the vestibule of 
 the renal organ. At its anterior end two small apertures will 
 be seen. They are the reno-pericardial apertures. Pass a guarded 
 bristle into one as far as it will go. Remove the floor of the 
 pericardial chamber and roof of the vestibule (coalescent). The 
 glandular part of the renal organ will be seen projecting like a 
 ridge into the vestibule. At the posterior end of the vestibule, 
 which does not extend so far back as the glandular part, the 
 guarded bristle, which was passed from the pericardial chamber 
 
316 
 
 ANIMAL BIOLOGY. 
 
 [Part 1 1. 
 
 into the renal gland, may be seen to emerge by an irregular 
 opening. The vestibules communicate by an inter-renal aper- 
 ture. Anteriorly there is a small aperture in the floor of each 
 vestibule. Through it may be passed a guarded bristle, which 
 
 o.g. 
 
 ves. 
 FIG. 92. MUSSEL : HEART AND RENAL ORGAN. 
 
 a. ad. Anterior adductor muscle, a. r. m. Anterior retractor muscle. 
 au. Auricle, e. re. a. External renal aperture, g. a. Generative aperture. 
 i. g. Inner gill. i. r. a. Aperture between glandular portion and vestibule, 
 fc. o. Organ of Keber. o. g. Outer gill. p. ad. Posterior adductor muscle. 
 p. c. Pericardial chamber, p. p. Protractor pedis muscle, p. r. m. Posterior 
 retractor muscle, r. Rectum, re. Glandular part of renal organ, re. p. a. 
 Reno-pericardial aperture, ve. Ventricle, ves. Vestibule of renal organ. 
 
 will emerge by the external renal aperture close to the anterior 
 end of the inner gill. (The dotted arrow in Fig. 92 passes 
 through this aperture.) The cavity of the glandular part may 
 now be opened up, its brownish solid contents washed out, and 
 the irregular walls noted. 
 
 In a second mussel the renal organ may be dissected from 
 the side, and the parts laid open and carefully drawn with the 
 guidance of the diagram, Fig. 92. 
 
 The secretion of the renal organ has recently been shown to 
 contain uric acid and urea. These substances were also found 
 in the blood of the central blood sinus, but none was found in 
 blood taken from the gills. 
 
Chap. XVI.] THE FRESH-WATER MUSSEL. 317 
 
 The Nervous System (Fig. 93). The nervous system of the 
 mussel consists of three pairs of ganglia with their commissures. 
 On the dorsal side of the mouth there are two somewhat 
 triangular cerebral ganglia (c. g.) united by a short commissure 
 which passes over the mouth. From each of these ganglia, 
 besides local branches c.o.c. 
 
 to the labial palps and 
 
 ci.r.i 
 
 the neighbouring mus- 
 cles, there passes down- 
 wards and slightly back- , 
 
 6 * a.acl.- 
 
 wards a cereoro - pedal 
 commissure (c. p. c.\ end- c f- c -' 
 ing in the pedal ganglion 
 (p. g.). The two pedal 
 
 ganglia are Orange Fia - 93. MUSSEL : NERVOUS SYSTEM. 
 
 bodies applied to each d. Anterior adductor muscle, a. r. m. Anterior 
 
 fVi * tVia rnirl linp retractor muscle, c. g. Cerebral ganglion, c. o. c. 
 
 me ' Cerebro-olfactory commissure, c. p. c. Cerebro-pedal 
 
 Each gives Off a branch commissure. /. Foot. ol. g. Olfactory ganglion, p. ad. 
 
 tn thp Tninntp anrlitnrv Posterior adductor muscle, p. c. Pericardial chamber. 
 
 ' 01 T Pmfft Pedal ganglion, p.p. Protractor pedis muscle. 
 
 Organ, and branches to p.r.m. Posterior retractor muscle, re. Renal organ. 
 
 the neighbouring parts. 
 
 From each cerebral ganglion there also passes off, upwards and 
 backwards, a cerebro-olfactory commissure (c. o. c. ) (cerebro-parieto- 
 splanchnic of some authors), which is readily traced in the 
 region of the renal organ. It proceeds to an olfactory ganglion 
 (ol. g.) (parieto-splanchnic of some authors), which lies near the 
 surface beneath the posterior adductor muscle. 
 
 Eecent observations seem to show that there are two classes 
 of nerve-fibres connected with the great adductor muscles the 
 one class motor, giving rise to contraction, the other class 
 inhibitory, producing relaxation, when mainly by the elasticity 
 of the ligament the shell gapes. The motor nerves for each 
 muscle spring from the ganglion next it ; the inhibitory fibres, 
 on the other hand, are stated to take their origin entirely from 
 the cerebral ganglia. 
 
 Special Senses. We know little about the special senses in 
 
318 ANIMAL BIOLOGY. [Part II. 
 
 anodon. The general surface is sensitive to touch, but no special 
 end organs have been described. The labial palps have a rich 
 nerve-supply ; we may conjecture that they minister to taste 
 we cannot at present do more. The papillary processes of the 
 inhalent siphon are sensory, but to what sense they minister 
 we are uncertain. The thin layer of elongated epithelial cells in 
 connection with the olfactory ganglion is held to be an organ of 
 smell (osphradium) and the olfactory ganglion is regarded as a 
 centre for the co-ordination of sensations received in connection 
 with the inhalent current. The auditory organ is a small sac 
 lying posterior to the pedal ganglion of each side. It is difficult 
 to dissect out in the mussel. But in the small fresh-water 
 mollusc Cyclas cornece it may without difficulty be seen if the 
 transparent foot be mounted for the microscope and examined 
 under a low power. It is a small sac, containing a mobile 
 particle or otolith. The sense of sight is apparently absent in the 
 mussel. 
 
 The Reproductive Organs. The mussel is dioecious. The 
 generative opening (Fig. 92, g. o.) is just anterior to the external 
 opening of the renal organ. It opens into a duct which gives off 
 a great number of csecal branches. The reproductive organs are 
 thus similar in the two sexes, and occupy in winter and spring 
 much space in the upper part of the foot Unless a little of the 
 organ be examined microscopically it is not easy to distinguish 
 the testis from the ovary, 
 
 If a little of the testis be teased in normal saline vast numbers 
 of spermatozoa will (in winter) be seen dancing in the field of the 
 microscope, especially if the slide be warmed. They have short 
 rounded heads, and long vibratile tails. 
 
 If a little of the ovary be teased, ova in various stages of de- 
 velopment will be seen. Fig. 94, 1-3, shows such ova. Externally 
 is a vitelline membrane, produced in immature ova into a spout- 
 like tube, through the aperture of which (mwropyle) the proto- 
 plasm of the ovum was originally in connection with the ovary. 
 In developing ova the protoplasm is granular, and stains readily. 
 But an ovum that has been dehisced, or set free, is wonderfully 
 
Chap. XVI.] THE FRESH-WATER MUSSEL. 
 
 319 
 
 clear and glassy (4). In such ova which may be obtained in 
 quantity in December by making a clean incision in the upper 
 part of the foot of a recently killed mussel, and mounting the 
 
 FIG. 94. MUSSEL: OVA, SPIRAL STARS, GLOcmriA. 
 
 1, 2, 3. Young ova torn from the ovary. 4. Ripe ovum dehisced. 5. Ovum, 
 showing spiral star perhaps accompanying the formation of the polar cell. 
 6. Spiral star under high power (Gundlach's ^ inch immersion). 7. Glochi- 
 dium with byssus and tactile organs. 8, 9. Glochidia shells. 10, 11. Beak 
 of Glochidium under high power. 
 
 fluid which exudes beautiful spiral stars may be seen (5 and 6). 
 One such star rarely a second may be seen in each ovum. 
 They perhaps accompany the formation of the polar body. 
 
 Development. The ova undergo the early stages of their 
 development in the external gills of the mother. Here, in all 
 probability, they are fertilised by spermatozoa shed into the 
 surrounding water by a male anodon, and carried inwards by 
 the inhalent current. 
 
 Segmentation is unequal. The granular protoplasm of the 
 impregnated ovum buds off in succession a number of small clear 
 cells. These small cells are free from yolk, the mother cell 
 retaining all the food-yolk. Between this large yolk-segment 
 and the small clear cells a cavity the segmentation cavity is 
 formed. From the small clear segments epiblast will be formed. 
 
3 20 ANIMAL BIOLOGY. [Part II. 
 
 The single yolk-sphere will give rise to hypoblast and mesoblast. 
 The large yolk-segment next undergoes cleavage, until about ten 
 blastomeres are formed from it, two of which are larger than 
 the others. These two slip into the cavity of the blastosphere, 
 while the others flatten out and undergo involution so as to form 
 a gastrula, with an elongated blastopore. A considerable space 
 separates the archenteric wall of the gastrula from the wall of 
 the blastosphere. From the two larger cells which first slipped 
 inwards a number of small cells are budded off, which form a 
 layer applied to the archenteric wall, and subsequently develop 
 into a network which fills up the space between this wall and 
 the wall of the blastosphere. 
 
 The embryo now rapidly develops into a peculiar larval form 
 originally regarded as a parasite of the mussel known as the 
 glochidium. In the winter the external gill of the female mussel 
 will be found to be distended with immense numbers of these 
 glochidia, together with yellow granular food material. Fig. 94, 
 7-11, shows their form and structure. Each has a pair of valves 
 perforated with minute apertures, united along a straight hinge 
 line, and connected by a single adductor muscle. When 
 examined alive they may be seen to open and shut as they lie 
 within the vitelline membrane. The edge of the shell opposite 
 the hinge is produced into an incurved beak, set with sharp 
 spines. Long coiled threads- the byssus may be seen in some 
 cases hanging down from the glochidium. Within the valves 
 is seen a mass of cells forming the embryonic mantle-lobes, and 
 careful examination will disclose three pairs of peculiar tactile 
 organs, consisting of columnar cells, bearing numerous fine 
 bristles. 
 
 The glochidia remain for, a long while in the gill of the parent. 
 But if some fish (e.g. perch or sticklebacks) be placed in the 
 tank in which such brood-mothers are living, the embryos are 
 ejected in great numbers. In some cases (in all 1) they are 
 ejected through the pore marked^?, in Fig. 90, A. andB., which 
 is placed at the dorsal end of a canal, the ventral end of which 
 opens into the exhalent siphon of the supra-branchial chamber. 
 The glochidia are then set free from the egg-membrane, and 
 
Chap. XVI.] THE FRESH-WATER MUSSEL. 321 
 
 swim by flapping the valves. Ere long they become attached 
 by .the byssus-thread to a fish, and hang there snapping their 
 valves until they bury them in the epidermis of the fish. There 
 they become encysted by a morbid growth of the epiderm cells 
 of their host ; and there they undergo a post-embryonic meta- 
 morphosis. The byssus and its secreting organ atrophy, and the 
 tactile organs disappear. The single adductor also atrophies; 
 but ^before it has disappeared rudiments of the anterior and 
 posterior adductors of the adult become visible. The foot arises 
 from the point at which the byssus disappeared. The gills arise 
 as papillae covered with richly ciliated epidermis. The mantle- 
 lobes undergo great change, or are perhaps formed de novo. The 
 permanent shell is formed as two plates on the surface of the 
 still parasitic larva. Finally, the little anodon, now resembling 
 in all essential features the adult, is set free and dropped to 
 the bottom. 
 
 The glochidium embryo is peculiar to fresh-water mussels 
 (anodons and unios). The great majority of lamellibranch 
 molluscs are hatched as free-swimming embryos with a ciliated 
 velum. It is clear, however, that such free-swimming embryos 
 of fresh-water mussels, did they exist, would be liable to be 
 swept out to sea by the flow of the river water in which they 
 lived. On the other hand, if they were developed under the 
 protection of the parent until they acquired the adult condition 
 and sedentary habit, there would be little dispersal of the genus. 
 The attachment of the glochidium to the epidermis of the fish, 
 while it (1) prevents the embryos being swept away by the 
 seaward current of the river, at the same time (2) provides an 
 efficient means of dispersal. 
 
 21 
 
CHAPTER XVII. 
 
 THE LIVER-FLUKE AND TAPEWORM. 
 
 THE Liver-fluke (Fasciola (Distoma) hepatica), a parasitic 
 organism which is the cause of the liver-rot in sheep a disease 
 which, during the winter of 1879-80, caused in England the death 
 of some three million sheep is a flattened oval animal about 
 an inch in length, and provided on its ventral surface with two 
 suckers, from which it received its original name of Distoma. 
 
 General Summary of Life-history. The liver-fluke, as such, 
 is found in the liver of sheep. Here it reaches sexual maturity, 
 each individual producing many thousands of eggs, which pass 
 with the bile into the alimentary canal of the host, and are dis- 
 tributed over the fields with the excreta. Here, in damp places, 
 pools, and ditches, free and active embryos are hatched out of the 
 eggs. Each embryo (Fig. 97, C., much enlarged) is covered with 
 cilia, except at the anterior end, which is provided with a head- 
 papilla (h. p.). When the embryo comes in contact with any 
 object it, as a rule, pauses for a moment, and then darts off 
 again. But if that object be the minute water-snail, Limnceus 
 truncatulus (Fig. 97, B., nat. size), instead of darting off the em- 
 bryo bores its way into the tissues until it reaches the pulmonary 
 chamber, or more rarely the body-cavity. Here its activity 
 ceases. It passes into a quiescent state, and is now known as 
 a sporocyst (Fig. 97, E.). The active embryo has degenerated 
 into a mere brood-sac, in which the next generation is to be pro- 
 duced. For within the sporocyst special cells undergo division, 
 and become converted into embryos of a new type, which are 
 known as redice (F.), and which, so soon as they are sufficiently 
 developed, break through the wall of the sporocyst. They then 
 
Chap. XVII.] THE LIVER-FLUKE AND TAPEWORM. 323 
 
 increase rapidly in size, and browse on the digestive gland of the 
 water-snail (known as the intermediate host\ to which congenial 
 spot they have in the meantime migrated. The series of 
 developmental changes is even yet not complete. For within 
 the redise (besides at times daughter redise) embryos of yet 
 another type are produced by a process of cell-division. These are 
 known as cercarice (Fig. 97, G.). Each has a long tail, by means 
 of which it can swim freely in water. It leaves the intermediate 
 host, and after leading a short active life, becomes encysted on 
 blades of grass. The cyst is formed by a special larval organ, 
 and is glistening snowy white. Within the cyst lies the trans- 
 parent embryonic distoma, which has lost its tail in the process 
 of encystment. 
 
 The last chapter in this life-history is that in which the sheep 
 crops the blade of grass on which the parasite lies encysted ; 
 whereupon the cyst is dissolved in the stomach of the host, the 
 little liver-fluke becomes active, passes through the bile-duct into 
 the liver of the sheep, and there, growing rapidly, reaches sexual 
 maturity, and lays its thousands of eggs, from each of which a 
 fresh cycle may take its origin. The sequence of phenomena 
 which constitute such a cycle is known as alternation of genera- 
 tions, or heterogamy, of which there are many modes among the 
 lower members of both the animal and vegetable kingdoms. It 
 is characterised by discontinuity of development. Instead of the 
 embryo growing up continuously into the adult, with only the 
 atrophy of provisional organs, it produces germs from which the 
 adult is developed. Not merely provisional organs but provisional 
 organisms undergo atrophy. In the case of the liver-fluke there 
 are two such provisional organisms, the embryo-sporocyst and 
 the redia. 
 
 We may summarise the life-cycle thus : 
 
 (1) Ovum laid in liver of sheep, passes with bile into intestine, and 
 thence out with the excreta. 
 
 (2) Free Ciliated Embryo in water or on damp earth ; passes into pul- 
 monary cavity of Limnseus truncatulus, and develops into 
 
 (3) Sporocyst, in which secondary embryos are developed, known as 
 
 (4) Eedice, which pass into the digestive glands of Limnseus, and within 
 which, besides daughter rediae, there are developed tertiary embryos, or 
 
324 
 
 ANIMAL BIOLOGY. 
 
 [Part 1 1. 
 
 (5) Cercaria, which pass out of the intermediate host and become 
 
 (6) Encysted on blades of grass, which are eaten by sheep. The cyst 
 dissolves, and the young flukes pass into the liver of their host, each 
 developing there into an adult hermaphrodite 
 
 (7) Fasciola hepatica. 
 
 The Distoma Stage. The liver-fluke, which is the terminal 
 product of this long series of changes, is somewhat oval in form, 
 
 D. 
 
 FIG. 95. LIVER-FLUKE : DIGESTIVE SYSTEM. 
 (After A. P. Thomas and La Valette.) 
 
 A. Young distoma ( X 70). cli. Branched digestive canal, ex. sy. Excretory 
 system. m.cZ. Its mid duct. n. Nerve, n. g. Nerve ganglion, o. s. Oral 
 sucker, p. s. Posterior sucker, ph. Pharynx. 
 
 B. C. D. Stages in the growth of distoma ( x 7) to show relative positions 
 of suckers. 
 
 and is provided with two suckers. In Fig. 95, B., G., and D., 
 three stages of growth (after A. P. Thomas x 7 J) are shown. The 
 
Chap. XVII.] THE LIVER-FLUKE AND TAPE-WORM. 325 
 
 relative shifting of the suckers should be noted. The anterior 
 or oral sucker (o. s.) is at the anterior end of the body, and in 
 this the mouth occupies a central position. The other (p. s.) is 
 ventral, and relatively nearer the posterior end in young flukes 
 than in the adult organism. The cuticle has minute spines. 
 
 The mouth leads into a muscular pharynx (Fig. 95, A., pli.\ 
 which is followed by a short oesophagus. The alimentary canal 
 then bifurcates, one limb passing down either side of the body and 
 ending blindly. Each limb has a number of lateral branches. In 
 Fig. 95, A. (partly after A. P. Thomas x 70), these are repre- 
 sented at an early stage, viz., that shown in B. In later stages 
 the branches are more complicated, though they always end 
 somewhat bluntly, and the digestive organs are masked by the 
 organs of reproduction. There is no anus. Living, as does the 
 fluke, upon the prepared juices of its host, there is presumably 
 little indigestible matter in its stolen food-stuff. 
 
 Excretory System. At the posterior end of the body there is an 
 excretory pore which leads into a median tube (Fig. 95, m. d.). 
 From this tube are given off a number of ducts which branch 
 repeatedly, and form a net- work (ex. sy.). The fine terminal 
 vessels end in minute ciliated funnels, opening into the lacunar 
 spaces which represent the ccelom. 
 
 Nervous System. This consists of a nerve-ring round the 
 pharynx, with a single ventral and paired lateral ganglia (n. g.). 
 From the lateral ganglia arise lateral nerves (n.), running down 
 the sides of the body. 
 
 The Generative System. The liver-fluke is hermaphrodite, 
 male and female organs being developed in each individual. It 
 would seem, however, that there is cross-fertilisation, so that the 
 ova of one individual are fertilised by the spermatozoa of another. 
 
 The testes (spermaries) are large branched and multi-lobed organs 
 occupying much of the central part of the body (Fig. 96, ts.). 
 Two vasa deferentia (v. d.) pass forward and unite into a reser- 
 voir and looped duct (ductus ejaculatorius), which terminates in 
 
326 
 
 ANIMAL BIOLOGY. 
 
 [Part II. 
 
 a protrusible cirrus (ci.) lodged in a cirrus sac. This is situated 
 ventrally between the two suckers. 
 
 The ovary (ovy.) is branched and tubular. From it an ovarian 
 duct meets the ducts of the large yolk-glands or vitellcvria, which 
 
 . v.l .. t. 
 
 FIG. 96. LIVER-FLUKE : ORGANS OF GENERATION. 
 
 "br. di. Branches of digestive system, ci. Cirrus, o. s. Oral sucker, ovd. 
 Oviduct crowded with ova. ovy. Ovary, p. s. Posterior sucker, ph. 
 Pharynx, sh. gl. Shell gland, ts. Testis. vi. Vitellarium. vi. d. Duct of 
 vitellarium. 
 
 lie one on each side of the body, and are composed of a multi- 
 tude of small branches terminating in rounded masses. Beyond 
 the points of union of ovarian and vitelline ducts, an oviduct or 
 uterus conveys the ova to an aperture near the base of the cirrus. 
 A shell gland (sh. gl) is situated at the junction of the ovarian 
 
Chap. XVII.] THE LIVER-FLUKE AND TAPE-WORM. 327 
 
 and vitelliue ducts. The oviduct is usually crowded with eggs, 
 which are here undergoing the early stages of their development. 
 A canal (Laurer's) from the vitelline duct is said to open dorsally. 
 
 The Ovum. The egg is small (-13 by -08 mm.), oval, smooth, 
 and transparent, with a yellowish-brown chitinous shell. At the 
 anterior end is an operculum (Fig. 97, A., op.}. The embryo (em.) 
 occupies but a small space, the rest being occupied with yolk- 
 cells. Segmentation goes on within the uterus ; but after the 
 egg is laid, further development can only go on at a reduced 
 temperature of 23-26 C. All the eggs laid under the same 
 conditions do not appear to produce embryos at the same time, 
 some embryos emerging weeks, or even months earlier than 
 others. What conditions determine this difference is not known, 
 but such a state of things is clearly of advantage to the species. 
 The embryo within the egg increases by absorption of yolk, 
 develops a head-papilla and a double eye-spot, and becomes 
 ciliated though the cilia are at present motionless. Eventually 
 the operculum springs open and the embryo emerges. 
 
 The Free Embryo. The embryo is conical in form (Fig. 97, C.) 
 with a retractile head-papilla (h.p.) at the anterior end. The whole 
 surface (head-papilla excepted) is covered with long cilia, borne 
 by flattened ectoderm cells arranged in five or six transverse 
 rings around the body. The four cells which compose the first 
 ring are thicker than the others. Beneath the ectodermic ciliated 
 layer is an endodermic body-wall, within which are a number of 
 delicate vesicular germinal cells (g. c.). A rudimentary digestive 
 tract is visible behind the head-papilla. Over this in the figure 
 (after A. P. Thomas) is seen the eye-spot (e. s.\ shaped like two 
 crescents united back to back, and developed in the body-wall. 
 In the body-wall there are also two ciliated funnel-shaped spaces 
 of the excretory system (ex.), in each of which there is a single 
 large flame-shaped cilium. 
 
 The embryo is exceedingly active, darting backwards and 
 forwards, and rotating on its long axis. The duration of its free 
 and active life is, however, not more than some eight hours. If 
 it does not meet with a water-snail within that time it dies. 
 
328 
 
 ANIMAL BIOLOGY. 
 
 [Part II. 
 
 Fio. 97. LIVER-FLUKE : EMBRYONIC STAGES. 
 
 (After A. P. Thomas.) 
 
 A. Ovum. em. Embryo, op. Operculum. 
 
 B. Limnaeus truncatulus (natural size). 
 
 C. Free Embryo. e. s. Eye-spot, ex. Excretory vessel, g. c. Germinal 
 cells, h. p. Head-papilla. 
 
 D. Embryo preparing to become a sporocyst. g. c. Germinal cells. 
 
 E. Sporocyst. g. Gastrula. m. Morula. re. Redia. 
 
 F. Redia. 6. o. Birth opening, ce. Cercaria. col. Collar, di. Digestive 
 sac. ph. Pharynx, p. pr. Posterior processes, re. Daughter redia. 
 
 G. Cercaria. cys. Cystogenous organ, di. Digestive sac. o. s. Oral sucker. 
 p s. Posterior sucker, ph. Pharynx. 
 
Chap. XVII.] THE LIVER-FLUKE AND TAPEWORM. 329 
 
 There are thus many chances to one against further development ; 
 hence the vast number of ova. If it meets with a Limnaeus 
 truncatulus it spins round upon its axis, head-papilla protruded, 
 alternately shortening and suddenly extending its body, and 
 thus bores its way through the soft tissues into the pulmonary 
 chamber. It now becomes quiescent. The external ciliated 
 cells become swollen by absorption of water, their cilia sticking 
 out stiffly from them. The body becomes elliptical, the eye-spot 
 breaks up (Fig. 97, D.), and the organism passes into the condition 
 of a sporocyst. 
 
 The Sporocyst. This now increases in size until it becomes 
 6 mm. long, while the germinal cells within it increase in num- 
 ber, partly by division of the previously contained germinal cells, 
 partly by the multiplication and division of the cells of the 
 body-wall, some of which become detached. The cells within 
 the sporocyst thus pass into the morula stage (E., m.). One side 
 then becomes flattened, and the cells here appear to be invagin- 
 ated, so that a gastrula (g.) is constituted; but the walls of the 
 gastrula are in contact, so that there is no primitive digestive 
 cavity or archenteron. Each spore is now surrounded by a 
 delicate membrane, and becoming more quadrate in shape, passes 
 into the redia condition (E., re., and F.) At one end a number 
 of cells are arranged to form a spherical pharynx (F., ph.), 
 which leads into a blind digestive sac (di.). A little behind the 
 pharynx the surface of the body is raised into a ridge, forming a 
 ring (col.) surrounding the anterior end ; whilst near the oppo- 
 site end, two short processes grow out (p, pr'.). This secondary 
 embryo has now passed into the redia stage. 
 
 The Redia. Within such a sporocyst as is shown in Fig. 97, E., 
 there are generally one or two rediaa ready to leave, and others 
 in a more or less advanced stage of development. When it is 
 about -26 mm. long the redia bursts through the sporocyst, the 
 wall rapidly closing up again after the exit. The active little 
 redia finds its way into the digestive gland of the Limnseus, and 
 then feeds upon that organ, the annular ring or collar (Fig. 97, F., 
 
330 ANIMAL BIOLOGY. [Part II. 
 
 col.) enabling it to fix itself while it browses. There is an 
 excretory system (not figured) consisting of two groups of 
 ciliated infundibula, one just behind the collar, the other just 
 behind the posterior processes. The mouth is surrounded by 
 lips, and there is a muscular pharynx (ph.) opening into the 
 digestive sac (di.), consisting of a blind tube lined with clear 
 nucleated cells. There is also a definite birth-opening (b. o.). 
 
 At the posterior end of the redia, in the lining of the body- 
 cavity, there are large clear cells which undergo segmentation, 
 and form mulberry masses (morulce) enclosed in a delicate mem- 
 brane. The morula is detached into the body-cavity, and is 
 invaginated to form a gastrula, the walls of which are, however, 
 in contact, so that the primitive cavity (firchenteron) is obliterated. 
 Some of these would seem to develop into rediae (re.) like the 
 parent redia. Others become at first oval, and then more and 
 more elongated, whilst one end becomes narrower, and is partly 
 constricted from the remainder to become the tail of the cercaria 
 (ce.), while the rest of the tertiary embryo becomes its body. 
 
 The Cercaria. A sucker appears at the anterior end of the cer- 
 caria (Fig. 97, G-., o. s.) y in the midst of which is the mouth-opening. 
 A second sucker (p. s.) appears about the middle of the ventral 
 surface of the flattened body. The digestive tract has a less 
 marked pharynx (ph.) than the redia, and is forked; but the 
 limbs have no lateral branches. When it is about -3 mm. long 
 the cercaria makes its exit from the redia by the birth-opening. 
 In mature cercariae there are very minute spines on the anterior 
 end of the body. Within the body, at the sides, are a number 
 of opaque white granules, forming the cystogenous organs (cijs.). 
 
 The cercaria passes out of the intermediate host, and after a 
 short active life comes to rest on a blade of grass or other 
 object, assumes a spherical form, and is encased in a white cyst 
 composed of opaque white granules from the cystogenous organs. 
 The tail is pinched off during the process of encystment. 
 
 The Cyst is thus round and snowy white ; but within it lies 
 the minute transparent embryo a tailless cercaria which, if it 
 pass into the alimentary canal of a sheep, will be carried thence 
 
Chap. XVII.] THE LIVER-FLUKE AND TAPEWORM. 331 
 
 to the liver, and there gradually assume the form of an adult 
 liver-fluke. 
 
 The fluke may be developed in other forms than the sheep, 
 such as the goat, and occasionally in the larger cattle, but since 
 they are less close feeders they less frequently take in the en- 
 cysted cercaria. 1 
 
 By way of appendix to the foregoing life-history of the 
 liver-fluke, the life-history of another parasite, the tapeworm, 
 may here be given. 
 
 One form of tapeworm (Tcenia solium) infests man as its host. 
 The adult worm may be many yards in length. Anteriorly there 
 is a minute scolex or head (resembling the upper part of Fig. 
 98, C.). It .has a double circlet of spines set close together, and 
 behind this four suckers. Then follow a great number of joints 
 or metameres. These are constantly forming behind the scolex, 
 and as new ones are budded off those previously formed are 
 pushed further away from the head. Thus the metameres are pro- 
 gressively older as we proceed from the anterior to the posterior 
 end. The youngest are scarcely differentiated, and form the 
 so-called neck. Behind this they have the form shown in Fig. 
 98, D. ; further down they are as shown in E. ; near the posterior 
 end they are as represented at F. The scolex and metameres 
 is called a strobila. From time to time some of the posterior 
 metameres are detached as proglottides, and pass out with the 
 excreta. 
 
 Running down the whole strobila are canals of the excretory 
 or water-vascular system. In the scolex there is a circular 
 vessel. Four longitudinal trunks (Fig. 98, G., I. v.) run down, 
 two on each side, the whole series of metameres. At the hinder 
 end of each metamere there is a transverse vessel (tr. v.). At 
 the extremity of the strobila is a porus excretorius. From the 
 trunks fine branched vessels, with ciliated junctions, open out. 
 
 1 Mr. Bolton, of Birmingham, has supplied me with liver-flukes and 
 with cercarise of allied forms. Prepared and mounted flukes may be 
 obtained of most dealers in microscopic objects. Messrs. Watson, of High 
 Holborn, supplied me with a very well-mounted specimen. 
 
332 
 
 ANIMAL BIOLOGY. 
 
 [Part II. 
 
 Nerve ganglia have been described in connection with the water- 
 vascular ring of the scolex. 
 
 The scolex itself contains no reproductive organs. In the 
 metameres these organs are progressively developed, each meta- 
 mere being hermaphrodite, with cross-fertilisation. They are 
 
 FIG. 98. TAPEWORM. 
 
 A. Embryo with six stylets. D. Anterior metameres. 
 
 B. Invaginated blastosphere. E. Median metameres. 
 
 C. Scolex of bladder- worm. F. Posterior metameres. 
 
 G. A median metamere. g. cl. Generative chamber. I. v. Longitudinal 
 water-vascular vessel, ovy. Ovary, p. s. Penis sac. r. s. Receptaculum 
 seminis. sJi. gl. Shell gland, tr. v. Transverse water-vascular vessel, is. 
 Testis. ut. Uterus, vag. Vagina, v. d. Vas deferens. vit. Vitellarium. 
 
 H. A terminal segment in which much of the genital apparatus has atro- 
 phied, and the uterus (ut.) is distended with ova. 
 
 seen in their most perfect condition in metameres of the stage 
 shown at E. The spermaries or testes (Fig. 98, G-., ts.) consist of a 
 great number of sperm-balls. These pour their contents into 
 efferent vessels, by which they are conveyed to the vas deferens 
 (v. d.), which opens by a protrusible termination (p. s.) into 
 a genital cloaca (g. d.\ opening into the exterior by a genital 
 
Chap. XVII.] THE LIVER-FLUKE AND TAPEWORM. 333 
 
 pore, which in successive metameres is alternately on the one 
 side and the other (see E.). Into the genital cloaca there also 
 opens a vagina (vag.), which expands to form a receptaculum seminis 
 (r. s.). Closely connected with this are the apertures of four 
 structures : (1) the ovary (ovy.)-, (2) the large uterus (ut.); (3) the 
 yolk gland (vitellarium) (n.) and (4) the small shell gland (sh. gl.). 
 
 The ova passing from the ovary are impregnated by sperma- 
 tozoa in the receptaculum. They also receive yolk (or albumen ]) 
 from the yolk gland, and are encased in a shell from the shell 
 gland. Then they pass into the uterus. As the uterus becomes 
 distended with eggs it assumes the form shown in Fig. 98, H. 
 (ut.), which represents a metamere in the stage figured at F. 
 The other reproductive organs are absorbed or completely 
 masked. When these ripe proglottides break away and are 
 expelled with the excreta, they die and decay; but the eggs 
 which they contain in vast numbers do not readily lose their 
 vitality, and only await favourable conditions for development. 
 
 Before showing what these conditions are, it will be well to 
 suggest the question whether the strobila is one organism or 
 many. Is the scolex an asexual zooid which gives rise by bud- 
 ding to a great number of sexual zooids ? or have we one 
 segmented organism, of which the anterior portion is specially 
 differentiated while all the others are serially homologous, and 
 each provided with male and female reproductive organs ? The 
 former view is that generally held. In which case we have here 
 an instance of alternation of generations in which the proglot- 
 tides are to be regarded as provisional embryonic organisms. 
 On the other view we have provisional embryonic body-segments, 
 a state of things curiously intermediate between embryonic 
 organs and embryonic organisms. 
 
 Within the uterus the ova undergo segmentation with the 
 formation of a morula provided with six stylets (Fig. 98, A.), and 
 enclosed in a shell. If now a portion of a proglottis, or some of 
 the embryos, be swallowed by a pig, the egg-shell is dissolved by 
 the gastric juice of this intermediate host, and further development 
 is rendered possible. The embryo, by means of the stylets with 
 which it is provided, penetrates the tissues of the intermediate 
 
334 ANIMAL BIOLOGY. [Part II. 
 
 host, in the muscles of which it becomes encysted, having as- 
 sumed the form of a hollow bladder-like blastosphere. The 
 blastosphere seems then to become invaginated, and within the 
 caecum so formed a pre-scolex is developed (Fig. 98, B.), which 
 by evagination becomes a scolex (C.), attached to which is the 
 blastosphere. This is known as the cysticercus stage, and the 
 organism is called a bladder-worm. The pork which contains 
 such bladder-worms is called measly. Such pork, if eaten by 
 man in an uncooked or incompletely cooked state, gives rise to 
 tapeworm. The scolex attaches itself to the intestine by its 
 suckers and circlet of hooks. The bladder seems to be lost, and 
 in its place there bud out the series of metameres which convert 
 the organism into a fully-developed tapeworm strobila. 
 
CHAPTER XVIII. 
 THE HYDRA. 
 
 THE fresh-water polyp (Hydra viridis) may be found in 
 summer attached to duckweed in ponds. If some of the duck- 
 weed be placed in a glass vessel near a window, the little green 
 polyps will congregate to that side of the vessel which is turned 
 towards the light. They are minute gelatinous bodies, half an 
 inch or less in length. Each is attached to the glass or to the 
 duckweed by a flat disc, known as the foot. When the hydra 
 is fully extended the foot is more expanded than the body, but 
 when the organism is in a state of contraction the foot is 
 narrower than the rounded and swollen body. At the end of 
 the body opposite to the foot is the hypostome (Fig. 99, h. s.), 
 bearing at its summit the mouth. Encircling the hypostome 
 are the tentacles (^.), varying in number according to the age 
 of the individual, but seldom exceeding eight. By alternately 
 applying the foot and the tentacles, the hydra can creep with a 
 looping motion. The body surface generally, and the tentacles 
 especially, are irritable. If one of the tentacles be touched, it 
 is rapidly withdrawn, the irritation spreads to the others, and 
 they too are drawn towards the body, which itself contracts, so 
 that the whole organism, to the naked eye, looks like a rounded 
 speck of green jelly. In Fig. 99, C., the hydra is represented in its 
 fully elongated condition ; in A. it is moderately contracted ; in 
 B. in the condition of utmost contraction. After irritation the 
 contraction relaxes, and the hydra resumes its fully expanded 
 state. Should any minute animalcule or small water-flea come 
 in contact with one of the tentacles, it seems to stick, and 
 
 335 
 
336 
 
 ANIMAL BIOLOGY. 
 
 [Part II. 
 
 after a short struggle, its activity ceases. The tentacle is 
 meanwhile retracted (Fig. 99, C.) ; other tentacles bend round to 
 lend assistance ; and the animalcule is tucked into the mouth, 
 and thus passes into the body-cavity, which occupies the whole 
 
 \.s. 
 
 FIG. 99. HYDRA VIRIDIS. 
 
 A. Hydra half retracted, with a bud and an ovum attached to the 
 shrunken ovary. 
 
 B. A small hydra firmly retracted. C. A hydra fully extended. 
 6. Bud. /. Foot. h. s. Hypostome. ovm. Ovum. ovy. Ovary, t. Ten- 
 tacles, ts. Testis. 
 
 of the interior of the polyp. Here ere long it breaks up ; the 
 nutritive matter is absorbed by the lining cells of the cavity, 
 and the indigestible residue is got rid of through the mouth, 
 for there is no separate vent. 
 
 Histology. In Fig. 100, A., a diagrammatic longitudinal sec- 
 tion through the body-wall of hydra is given. It is divided into 
 two main layers, ectoderm (ec.) external, and endoderm (en.) 
 internal. In the ectoderm there are large conical neuro- 
 muscular cells (n. m.). If a hydra be teased (fresh, or after 
 treatment with 20 per cent, acetic acid, or 1 per cent, osmic, or 
 dilute chromic and osmic) some of these cells at C. will be 
 readily seen. At their pointed ends they give off one or two 
 
Chap. XVIII.] 
 
 THE HYDRA. 
 
 337 
 
 delicate muscle processes (m. p.], which come into close relation 
 with the thin supporting lamella (s. /.), a portion of which 
 adheres to one of the fibres in C. 2. In the interstices between 
 the neuro-muscular cells, especially in the deeper part of the 
 ectoderm, there is an interstitial tissue (int.), composed of small 
 
 D. 
 
 FIG. 100. HYDRA HISTOLOGY. 
 
 A. Diagrammatic longitudinal section through a part of the body-wall. 
 ec. Ectoderm, en. Endoderm. chl. Chlorophyll corpuscles, ci. Cilium. 
 int. Interstitial cells, m. p. Muscle processes, nc. Nucleus, n. m. Neuro- 
 muscular cell. ps. Pseudopodial process, s. I. Supporting lamella, vac. 
 Vacuole. (The regularity of the ectoderm is diagrammatic). 
 
 B. Interstitial cells. ne. Nematocysts. E. Chlorophyll corpuscle. 
 
 C. Neuro-muscular cells. s. ?. Sup- F. Thread-cells (nematocysts). 
 
 porting lamella. G. Spermatozoon. 
 
 D. Endoderm cell. H. Ovum. 
 
 cells, some of which are shown at B. In the patch of cells at 4 
 the boundaries (after osmic) could not be made out. The 
 interstitial cells are absent in the foot. 
 
 Within these interstitial cells are developed the curious 
 organs known as thread-cells or nematocysts. When the tentacle 
 of a living hydra is examined under a high power, long fine 
 22 
 
338 ANIMAL BIOLOGY. [Part II. 
 
 threads may be seen protruding here and there. On addition of 
 an irritant, such as dilute acetic acid, great numbers shoot forth. 
 They are of use in capturing (and perhaps paralysing) prey. 
 Some of these cells are shown in Fig. 100, F. The largest (1) have 
 spines at the base of the very long whip. Others have no 
 spines, and may be incompletely developed thread-cells. In the 
 resting stage (before eversion) the thread-cell is rounder, and 
 resembles a flask in which the tapering neck has been pushed 
 inwards so as to lie within the flask. Within the neck the 
 barbs appear as arrow-heads. The neck is continued into a 
 long filament regularly coiled like a ship's cable. It would 
 seem that during eversion the neck and the whole length of 
 filament is turned inside out. This process would, moreover, 
 seem to be a vital one, and within the command of the Hydra. 
 Infusorians (Euplotes) have been observed wandering over the 
 body and tentacles of Hydra fusca without causing any discharge 
 of nematocysts. The free end of a cell bearing a nematocyst is 
 generally prolonged into an elementary sensiferous process or 
 cnidocil. Connected with these cells are small stellate nerve-cells. 
 
 Internal to the muscle processes and the delicate membrane 
 (supporting lamella) with which they enter into close relation, is 
 the endoderm layer, consisting, of large irregular cells (Fig. 100, 
 A. en., and D.). Their inner free surfaces carry cilia, while their 
 deeper parts are crowded, in Hydra viridis, with the green 
 chlorophyll corpuscles (M.\ which give to this hydra its dis- 
 tinctive colour. 
 
 Concerning these chlorophyll corpuscles there has been much 
 discussion. By some they are regarded as plants (algae) para- 
 sitic (or rather symbiotic, i.e. living together for mutual advan- 
 tage) on the hydra. It is more probable, however, that they 
 are true products of the animal cells. One of these is shown at 
 E., as seen under the highest power, the parts shaded dark being 
 in the cell itself green. Very commonly the green part forms 
 (as seen in optical section) a ring surrounding a clear central 
 part of the cell. There is no nucleus. As in the case of 
 vegetable chlorophyll, the green colouring 'matter liberates 
 oxygen in sunlight, as has been proved by keeping hydras under 
 
Chap. XVIII.] THE HYDRA. 339 
 
 an inverted test-tube in sunlight and testing the bubble of gas 
 collected. 
 
 Besides chlorophyll corpuscles, the endoderm cells may contain 
 clear spaces (A., vac.), and sometimes irregular masses, which 
 resemble food particles. If this be so, we have in these cells a 
 process of intracellular digestion, solid food-particles being taken 
 into the cell just as we find to be the case in such a single-cell 
 organism as amoeba. And in this connection it is interesting to 
 note that these cells are capable of altering their form and 
 pushing forth pseudopodial processes (A., ps.). If a hydra be 
 teased up fresh, some of the endoderm cells may be seen in 
 amcebiform motion. Occasionally thread-cells may be found in 
 the endoderm. It may be that, as has been suggested, these 
 have been taken in with the food and ingested. 
 
 The body- cavity, which lies within the endoderm cells, is 
 subject to much variation of diameter. It extends from near 
 the foot to the mouth, and sends prolongations into all the 
 tentacles. It is not readily seen in green hydra (the outer clear 
 space in Fig. 99 representing ectoderm and not the whole body- 
 wall), but is more easily observed in the rarer reddish polyp, 
 Hydra fusca. 
 
 Processes of Multiplication. (1.) Budding. When well fed 
 the hydra multiplies by a process of budding. The bud appears 
 first as a little green knob, then becomes pear-shaped, and ere 
 long develops mouth and tentacles, the mouth appearing some- 
 what suddenly. It thus reaches a size which may fully equal 
 half that of the parent, after which the connection between the 
 parental body-cavity is obliterated by the ingrowth of ectoderm, 
 and the daughter hydra separates to lead a free existence. Such 
 a bud in an advanced stage is shown in Fig. 99, A. Sometimes 
 there may be several buds in various stages of development, 
 when the hydra has the appearance of a compound organism. 
 This process is wholly asexual. 
 
 (2.) fission. It is said that hydra may also multiply by 
 fission, a constriction appearing in the middle of the body, and 
 separation of the organism into two parts occurring at this 
 
340 ANIMAL BIOLOGY. [Part II. 
 
 spot. In this case the separated portion acquires a new foot, 
 and the basal portion acquires the circumoral tentacles. It is 
 somewhat doubtful, however, whether this process occurs under 
 natural conditions. But there is no doubt that if a hydra be 
 artificially divided, each separated part develops into a new and 
 perfect individual. 
 
 (3.) Sexual Reproduction. In its sexual reproductive organs the 
 hydra differs very markedly from any form we have as yet 
 studied. It is hermaphrodite and self-fertilising; but the testes 
 and ovaries are variable in number, and not definite in position 
 The testes, or spermaries, of which there may be as many as 
 four or five in the same individual, arise as thickenings of the 
 ectoderm from the active local growth of interstitial cells. The 
 neuro-muscular cells here take the form of a thin external layer. 
 Within the testes, which assume a mammiform appearance 
 (Fig. 99, ts.), swarms of spermatozoa may be seen under a high 
 power. Each has an oval head and vibratile tail (Fig. 79, G,), and 
 is developed from one out of the many clear spherical bodies 
 into which the spermatospores derived from the interstitial 
 cells subdivide. 
 
 The ovary (Fig. 100, B., ovy.} is larger than the testis. There 
 may be one or two in the same individual. It, too, originates 
 in the ectoderm, and from the interstitial cells which undergo 
 special differentiation over an area which embraces about half 
 the circumference of the body, and is often, but not always, 
 situated nearer the foot than the testes (Fig. 99, B.). As de- 
 velopment of the ovary proceeds one central cell increases 
 enormously at the expense of the others. This single ovum 
 (Fig. 100, H.) is capable of amoeboid movements, pushing forth 
 pseudopodial processes at various parts of its surface. It con- 
 tains a number of yolk-granules, and is provided with a well- 
 marked germinal vesicle containing a germinal spot. In the 
 green hydra chlorophyll corpuscles are developed within the 
 ovum. Before impregnation by the spermatozoa, the germinal 
 vesicle and its contained germinal spot are said to disappear. 
 The ovum escapes by the bursting of the thin ovarian walls, but 
 still remains attached to the shrunken remnant of the ovary 
 
Chap. XVIII.] THE HYDRA. 341 
 
 (Fig. 99, A., ovm.). Impregnation now takes place by the 
 discharge of spermatozoa from one of the testes of the same 
 individual. Both ovary and testis are ectodermal. 
 
 Development. Cleavage would seem to be holoblastic and 
 regular, the cells protruding and retracting pseudopodial pro- 
 cesses along the cleavage grooves. A morula or mulberry mass 
 is thus formed ; and at the close of segmentation the cells are of 
 two kinds, (a.) prismatic cells, constituting an outer layer, and 
 (b.) polygonally flattened cells forming an inner mass 
 
 On the outer surface of each external cell a thin chitinous 
 plate is, according to the most recent researches, formed, and 
 the plates from adjoining cells fuse together so as to give rise 
 to a continuous pellicle. When this has assumed distinctness, 
 a second is formed in a similar way, and then a third, a fourth, 
 and so on, the various pellicles fusing together to form a thick 
 laminated encasing chitinous shell, which is thus morphologically 
 one of the ectodermal tissues of the embryo. Between this cuticle 
 and the germ an elastic membrane is formed. 
 
 After this a remarkable retrograde metamorphosis is said to 
 take place. The cells undergo histolysis (cell dissolution), their 
 boundaries becoming indistinct and disappearing, and the 
 protoplasm of the cell is said to revert to a primitive unseg- 
 mented condition. Within the protoplasm there is then formed 
 a small excentric cavity, the rudiment of the body-cavity, which 
 gradually increases in size until the germ assumes the condition 
 of a hollow sac, with homogeneous walls. In this state it is set 
 free by the bursting of the external chitinous investment. The 
 walls differentiate into a clear superficial zone (ectoderm) and a 
 darker inner zone (endoderm), and within the outer of these 
 layers neuro-muscular, and then interstitial cells, are differen- 
 tiated. The embryo elongates, and the mouth is suddenly 
 formed as a star-like cleft at one end. Bud-like tentacular 
 processes make their appearance at about the same time. The 
 endoderm cells are now differentiated, the inner elastic membrane 
 is thrown off, and the young hydra is set free. 
 
 Several points seem to call for especial notice in this process 
 
342 ANIMAL BIOLOGY. [Part II. 
 
 of development, and the product to which it gives rise. In the 
 first place we may note the single ovum as the product of the 
 ovary ; (2) its amoeboid movements ; (3) the modification of part 
 of the embryo to form a chitinous case for the rest; (4) the 
 curious histolysis after a certain stage of histogenesis has been 
 reached; (5) the formation of the primitive body-layers, and 
 the contained digestive cavity, not by a process of invagination, 
 but by delamination of the walls of a hollow sac-like embryo ; 
 (6) the disruptive formation of the mouth ; (7) the absence in 
 the perfected organism of any body-cavity. This distinguishes 
 the hydra and its allies (ccelenterata) from all other metazoa 
 (ccelomata), which have at some period a body-cavity. 
 
 It will be seen, also, that the development of hydra is con- 
 tinuous, There is no alternation of generations. In this the 
 hydra is peculiar among the group of hydroid polyps to which 
 it belongs. It differs also from allied zoophytes in being solitary, 
 and not a member of a colony, group, or stock (hydrosome). Such 
 a stock consists, in general, of a stem and perhaps ramifying 
 branches, from which the polyps of the colony (hydranths) are 
 offshoots. The stem and branches are tubular, and the somatic 
 cavity within them is in free communication with the digestive 
 cavity of the hydranths. The generative organs (gonophores) 
 arise in special regions, and arise as buds either from the polyp 
 or from the tubular stem or branches. In either case it is at 
 first a sac-like diverticulum of the somatic or gastric cavity. 
 Within this diverticulum the generative products accumulate. 
 The genital cells may, however, arise in parts other than those 
 in which they are finally lodged, or even in some cases in the 
 body-wall of the polyp before the gonophores have made their 
 appearance. In such cases the generative cells subsequently 
 migrate into the gonophores. The development of the oospores 
 in some cases precedes that of the spermatospores ; and the ova 
 and spermatozoa are, as a rule, finally lodged in different gono- 
 phores. 
 
 So far we have regarded the gonophore as a simple bud or 
 diverticulum attached to the polyp, or to the axial branches of 
 the stock. But in the higher forms this develops into a medusa, 
 
Chap. XVI 1 1.] 
 
 THE HYDRA. 
 
 343 
 
 which is detached and floats off to enjoy a free and separate 
 existence as a provisional embryonic organism. 
 
 Such a medusa is figured in Fig. 101. Within the bell-shaped 
 hollow umbrella or nectocalyx there hangs a tubular manubrium 
 (m.). The cavity of the manubrium leads up into the umbrella, 
 and there branches into four or more radiating canals (r. c.), 
 which proceed outwards and 
 lead into a circular canal (c. c. ), 
 running round the rim of the 
 nectocalyx. The rim has a deli- 
 cate shelf -like inward projection 
 the velum (v.) which narrows 
 the orifice of the bell. From 
 the edge of the rim tentacles (t.) 
 take their origin; and on its 
 margin there may be sense 
 organs, called marginal bodies 
 (m. b.), which may be eyespots 
 or auditory vesicles. The sper- 
 maries and ovaries are generally 
 in different individuals, and are 
 lodged either in the walls of the 
 manubrium, or in those of the 
 radiating canals (g. o.). 
 
 The nervous system consists 
 of a double ring of nervous 
 
 matter, containing both nerve-fibres and nerve-cells, which runs 
 round the edge of the nectocalyx at the point of insertion of the 
 velum. The upper nerve-ring lies entirely in the ectoderm, and 
 is hardly differentiated from the epithelium. It sends out fibres 
 to the enveloping epithelium, and these, or the epithelium cells, 
 bear delicate sense-hairs. The lower nerve-ring, though still 
 belonging to the ectoderm, is more completely differentiated 
 from the epithelium, has larger fibres, and a greater number of 
 ganglion cells. The two nerve-rings are separated by a thin 
 membrane, through which, however, there pass strands of con- 
 necting fibres. The entire under surface of the nectocalyx 
 
 FIG. 101. MEDUSA. 
 
 c. c. Circular canal, g. o. Genital organ. 
 m. Manubrium. m. 6. Marginal body 
 (ocellus or otocyst). r. c. Radiating canal. 
 t. Tentacles, v. Velum. 
 
344 ANIMAL BIOLOGY. [Part II. 
 
 (sub-umbrella), and the surface of the manubrium, is covered 
 with an epithelial layer, beneath which is a thin sheet of 
 muscular tissue, not thicker than very thin paper. And between 
 these two there is an exceedingly delicate plexus of nervous 
 tissue, containing both nerve-fibres and ganglion cells. Beneath 
 the epithelium of the outer surface of the nectocalyx there is 
 neither muscular nor nervous layer. In the velum there are 
 concentric muscular fibres, but probably no nervous tissue. 
 
 The nectocalyx is rhythmically contractile. Hence the 
 medusa can swim lazily through the water. But if the margin 
 containing the nerve-ring be cut off, immediate, total, and per- 
 manent paralysis of the umbrella results. 
 
 It should be noted that this medusa or free gonophore is in 
 several respects more highly organised than the hydroid stock 
 from which it takes its origin. It has (1) between the endoderm 
 and ectoderm a thick mass of intermediate substance; it has (2) 
 a well-defined nervous system ; and it has (3) sense organs. 
 
 The segmentation of the ovum is complete or holoblastic. A 
 morula results, which passes into a single-layered, ciliated, 
 planula. The segmentation cavity becomes the gastric cavity of 
 the future polyp, but becomes lined with a second layer of cells 
 derived from the first by delamination. The spherical or oval 
 larva, now free, after a short locomotive existence, fixes itself 
 by its aboral pole. At the opposite end a mouth appears; 
 tentacles bud out ; fresh individuals are produced by a process 
 of budding ; these, however, do not separate as in hydra viridis, 
 but remain attached, and form part of the colonial stock. 
 
 Such, then, is the typical mode of reproduction in the hydro- 
 zoa, the sexual products ripen, and are fertilised in a separate 
 medusoid gonophore. In some cases, however, the medusa 
 never separates to lead a free existence; it is an attached medusa. 
 In others, the attached gonophore is incomplete; its essential 
 parts are present, but in an unexpanded condition; it is a 
 disguised medusa. In yet others development may go no further 
 than the production of a hollow diverticulum of the body-wall 
 of the parent ; it is a sporosac. The free medusa in the course 
 of its development passes in succession through the stages of 
 
Chap. XVIII.] THE HYDRA. 345 
 
 sporosac, disguised medusa, and attached medusa. On the 
 hypothesis of evolution these forms may be regarded either as 
 (1) stages in the gradual evolution of a perfect medusa, or (2) 
 stages in the degeneration of the perfect medusa, by arrested 
 development owing to changed conditions. The latter view is 
 the more probable. It is difficult to see how the disguised 
 medusa and attached medusa, which have all the parts of the 
 free medusa fitting it for independent existence, but never have 
 an opportunity for employing them, could have arisen by any 
 process of natural selection or otherwise ; or how the possession 
 of a swimming-bell that is never opened could be of any advan- 
 tage to the individual or the species. 
 
 And now for the bearing of these facts on the system of 
 generation in hydra. Here there is not even a sporosac. And 
 it has been recently suggested that in hydra development has 
 been arrested at a yet earlier stage, not only the medusa condi- 
 tion (free, attached, or disguised) having been suppressed, but 
 even the sporosac condition also. This might very possibly be 
 the result of a fresh- water habitat. For it has been shown that 
 fresh-water forms rarely have free and active embryos, since they 
 would tend to be swept out to sea, and this would result in the 
 perishing of the race as a fresh-water type, or in the return of 
 the race to the sea, its primitive home. That power of migration 
 at some stage of its existence, which is so important for the 
 well-being and extension of a marine species, is here abandoned 
 under new conditions of existence. 
 
 If this view of the mode of generation of hydra be correct, 
 this organism must be regarded as a degenerate type. Stellate 
 nerve-cells represent the nervous system. The muscular system 
 is not differentiated. But (on this view) Hydra is the descen- 
 dant of organisms in which nervous system, sense organs, and 
 muscular system were better developed during the medusoid 
 stage. And where a form is descended from organisms more 
 highly developed than itself it is said to be degenerate. For 
 it sometimes happens that an organism is placed in conditions 
 where the possession of organs which were of advantage to its 
 ancestors in the life-struggle are no longer of advantage. 
 
346 ANIMAL BIOLOG V. [Part 1 1. 
 
 These useless organs are therefore lost, and the form though 
 admirably suited to the meaner conditions of its life becomes 
 degenerate through the meanness of these conditions. 
 
 On the other hand, the more generally, and until recently 
 universally accepted view of the generative system of hydra is 
 that its reproductive organs exhibit a primitive condition prior 
 to the evolution of special sexual buds. In which case it has not 
 degenerated from a higher form, but has simply never become 
 so highly developed as the other hydrozoa. 
 
CHAPTER XIX. 
 
 VQRTICELLA AND PARAMCECIUM. 
 
 VoimCELLA, the bell-animalcule, has a delicate clear trans- 
 parent wine-glass or bell-shaped body, carried at the end of a 
 longer or shorter elastic thread-like stalk or pedicle. Within 
 the transparent outer sheath of this stalk is a central contractile 
 thread or fibrilla (c. /.), disposed in an extended spiral form. 
 In some of the largest forms, and under the highest powers, the 
 sheath is seen to be continuous with the thin transparent outer 
 wall of the body called the cuticle. The contractile thread is 
 seen to be continuous with a delicate contractile myophan 
 layer that passes up into the walls of the body and underlies 
 the cuticle. These constitute the ectosarc. Beneath this, and 
 perhaps hardly separable from it, is the cortical layer, which 
 passes insensibly into the more fluid central substance of the 
 body. These constitute the endosarc. 
 
 That which answers to the rim of the wine-glass is known as 
 the peristome (Fig. 102, A.,ps.). ' It is somewhat thickened and 
 turned outwards, and is beset with cilia. Within the bell-shaped 
 opening of the wine-glass is the disc, the edges of which are 
 richly ciliated. The disc is, however, not large enough to fill 
 the opening of the wine-glass, and is attached to one side. Thus 
 there is left a crescent-shaped space, the oral fossa or vestibule. 
 Into this a branch of the ciliary wreath descends. It passes 
 obliquely downwards towards the centre of the bell, thus passing 
 beneath the disc, and gradually narrows till it ends abruptly. 
 An anal aperture is described as opening into the vestibule, but 
 it is perhaps rather a weak spot, or potential anus (p. a.). 
 
 Within the body, close to the vestibule, is a single spherical 
 contractile vesicle (c. v.). This may be seen slowly to enlarge until, 
 
 347 
 
348 ANIMAL BIOLOGY. [Part II. 
 
 when it has reached a certain size, it suddenly disappears. The 
 diastole is slow, the systole rapid. When a little finely-divided 
 carmine is mixed with the water in which a vorticella is under 
 observation, the method of feeding may be watched. The fine 
 particles are driven by the cilia into the vestibule, and accumu- 
 late at its blind end. After a while they suddenly burst 
 through into the soft inner substance of the body, carrying with 
 them an equal or even greater amount of water. Thus are 
 formed food-vacuoles, which float freely for a time with a circu- 
 latory motion in the soft protoplasm of the body substance. 
 Such digestible matter as the food may contain is extracted, and 
 the indigestible refuse is passed out into the vestibule through 
 the potential anus. It is probable that the contractile vesicle has 
 the function of draining off" the excess of water thus introduced 
 within the body, in which case it must be regarded as excretory 
 At the same time it is not improbable that the water thus in- 
 troduced within the body, and circulating through its substance, 
 serves also to oxygenate the protoplasm, and thus ministers to a 
 rudimentary respiratory function. 
 
 When a vorticella is disturbed, by a sudden shock given to 
 the stage of the microscope for example, the long stem rapidly 
 coils up like a cork-screw (Fig. 102, B.), and the animal is thus 
 drawn back towards its point of attachment. At the same time 
 the lips of the peristome close in and apply themselves to the 
 disc, which is also retracted. The organism thus becomes pear- 
 shaped. Soon, however, the stalk will uncoil and lengthen out 
 again, the disc will be protruded, the peristome will open out, 
 and the cilia will recommence their orderly vibrations. Nothing 
 can be more beautiful than a colony of vorticellidae alternately 
 contracting and expanding, their stems rapidly coiling and un- 
 coiling. 
 
 The cilia are developed on the ectosarc. Each is a delicate 
 structureless process, which, when the vorticella is in the 
 expanded condition, is in constant motion, alternately bend- 
 ing and straightening. Currents are produced in the water by 
 their continuous motion, and these currents may be readily 
 observed if a little powdered carmine be added to the water 
 
Chap. XIX.] 
 
 VORTICELLA AND PARAMCECIUM. 
 
 349 
 
 beneath the cover glass. Perhaps the most striking and in- 
 teresting point with regard to this ciliary action is the co- 
 ordination of the movements, and their apparently purposive 
 
 FIG. 102. VORTICELLA, PARAMCECIUM, NYCTOTHERUS. 
 
 A. Vorticella extended. D. Free vorticella after fission. 
 
 B. Contracted. E. Embryo vorticella. 
 
 C. Vorticella undergoing fission. F. Paramoecium. 
 
 G. Nyctotherus. 
 
 an. Anal slit. c. /. Contractile fibre, c. v. Contractile vesicle, d. Disc. 
 end. Endoplast. /. v. Food-vacuole. p. a. Potential anus. ps. Peristome. 
 vs. Vestibule. 
 
 nature. This is accompanied in the metazoa by the development 
 of nerve-tissue ; but nothing of the kind can be made out in 
 vorticella. 
 
 If a vorticella be stained with iodine or magenta, the ectosarc 
 is relatively uncoloured, but the cortical layer is stained. And 
 within the cortical layer a band-like structure, before hardly 
 distinguishable, becomes evident by its deeper stain. This is 
 the endoplast (end.}. It is homologous with the nucleus of a 
 simple nucleated cell. 
 
350 ANIMAL BIOLOGY. [Part II. 
 
 One mode of multiplication adopted by vorticella is that of 
 fission (Fig. 102, C.). The animal assumes the pear-shaped con- 
 dition of contraction, and becomes somewhat swollen. A notch 
 then makes its appearance in such a way as to divide the vesti- 
 bule into two halves. The notch becomes a cleft, and the 
 cleavage is continued through the body in such a way as to 
 divide the contractile vesicle and the band-like endoplast. When 
 the cleavage is complete, there result two heads to the one stalk. 
 But the fate of the two is not the same. One remains attached 
 to the pedicle and expands into a complete vorticella. The 
 other remains pear-shaped, and develops round the posterior 
 region of the body a girdle of powerful vibratile cilia, by the 
 lashing of which the zooid tears itself away' from the parent 
 stem and swims off through the water (Fig. 102, D.). After a 
 short active existence it settles down in a convenient spot, 
 adhering by its posterior extremity. The hinder girdle of cilia 
 is lost or absorbed, a stalk is rapidly developed, the peristome 
 expands, the adoral wreath of cilia exhibit their orderly activity, 
 and the organism is a perfect vorticella. 
 
 In the case just described, the two zooids which result from 
 fission are of equal size. But in some cases the fission is very 
 unequal, so that a minute microzooid results. This swims off by 
 means of its posterior girdle of cilia, but does not merely settle 
 down and grow up into a vorticella. Its mission is different. 
 It attaches itself to the base of the bell of another vorticella, and 
 becomes completely absorbed into its larger mass. Such a pro- 
 cess is called conjugation. Perhaps we may see in this process 
 a foreshadowing of the union of spermatozoon and ovum in 
 the metazoa. The result of the process would seem to be a 
 rejuvenescence or access of vital power ; for the normal mode of 
 multiplication by binary division would seem to go on more 
 rapidly after conjugation. The endoplasts of the two conju- 
 gating organisms seem to break up and disintegrate, a new 
 endoplast being shortly afterwards reconstructed. 
 
 It is not improbable that, in some cases, conjugation is a pre- 
 paratory process to encystment. But it would seem that encyst- 
 inent may take place without any such preparatory process. 
 
Chap. XIX.] VORTICELLA AND PARAMCECI 
 
 V 
 
 The animalcule which is to be encysted separates from its 
 contracts into a spheroidal body, around which a clear structure- 
 less envelope or cyst is formed. Within the cyst the endoplast 
 and the contractile vesicle, permanently expanded, are visible. 
 But ere long the contractile vesicle disappears, the band-like 
 endoplast increases much in size, and becomes convoluted. It 
 next becomes pinched in at intervals, so as to assume a beaded 
 appearance. And at last each bead becomes separately pinched 
 off from the rest. Thus the cyst appears full of bead-like spores. 
 Then the cyst bursts, and these spores are discharged as minute 
 oval or pear-shaped free-swimming animalcules, possessing an 
 anterior crown of cilia, in the midst of which there is a potential 
 mouth. These feed and multiply by fission, but ere long attach 
 themselves by the anterior oral end (E.). The girdle of cilia is 
 now lost; disc, peristome, and vestibule are developed, a 
 gradually lengthening contractile pedicle is acquired, and the 
 organism is once more a vorticella. This mode of reproduction 
 (the occurrence of which is strongly vouched for) does not seem 
 to be common among vortieellidse. More frequently encystment 
 is merely a temporary resting stage, during seasons of drought 
 perhaps, or other uncongenial conditions, from which the vorti- 
 cella emerges simply to resume its ordinary mode of life. 
 
 Vorticella may be found adherent to the roots of duckweed. 
 
 Paramoecium is found in pond-water or in vegetable infusions. 
 From its peculiar form (Fig. 102, F.) it is often known as the 
 slipper-animalcule. There is a thin superficial cuticle richly 
 supplied with cilia. Since in paramcecium and its allies the cilia 
 are more or less completely developed over the whole surface, 
 they are known as the Holotricha, as opposed to the Peritricha, 
 to which vorticella belongs, where there is a wreath of cilia, and 
 other groups in which the cilia are differently arranged. 
 Beneath the cuticle is a cortical layer, which is fibrillated in a 
 direction perpendicular to the surface. If dilute acetic acid bo 
 added to the water in which paramcecium is under observation, 
 the organism will become quiescent, and there will shoot out 
 from the surface a number of stiff processes. These accessory 
 structures are known as trichocysts. There is a bean-shaped 
 
352 ANIMAL BIOLOGY. [Part II. 
 
 endoplast (end.), associated with which , N applied to one side, is a 
 smaller endoplastule, homologous with the nucleolus. There 
 are two contractile vesicles (c. v.), which, at the moment of 
 systole, are seen under certain conditions to become starlike, 
 as if there proceeded from them radiating canals, which thus 
 for a moment became distended. The vestibule (vs.) is a ciliated 
 funnel on the ventral surface. The process of feeding, giving 
 rise to food-vacuoles (/. v.), is similar to that in vorticella. 
 The indigestible residue is passed out by a potential anus 
 (p. a.) or weak spot in the protoplasm of the ventral surface. 
 
 The movements of paramcecium through the water show a 
 curious combination of backward or forward motion and 
 rotation. Interesting observations have been made on the effect 
 of a rise of temperature on these animalcules. It was found 
 that the pulsation of the contractile vesicle grew more rapid 
 until a maximum was reached of 30 C. Beyond this point the 
 pulsations grew less frequent, but the ciliary activity continued 
 to increase up to 35 C., when the co-ordination of the move- 
 ments of the cilia became imperfect, resulting in a peculiar aim- 
 less combination of direct and rotatory motions. At 40 C. the 
 direct movements ceased, while rotation continued to 42- 45, 
 when death ensued. 
 
 Paramcecium multiplies by fission, which is transverse. Con- 
 jugation also takes place, but diifers from that observed in vorti- 
 cella by being transient and not permanent. Two paramoecia 
 become locked together by the close application or union of 
 their oral surfaces. In this condition they may remain for five 
 or six days. During or subsequent to this process the endoplas- 
 tule becomes fusiform and striated, and the endoplast turgid 
 and enlarged. It would seem that the endoplast breaks up into 
 a number of spheroidal bodies, which are ultimately cast out of 
 the body, and not improbably constitute reproductive germs 
 from which daughter paramoecia take their origin. 
 
 Many of the ciliated infusoria are parasites or messmates. 
 In the rectum of the frog, for instance, examples of Balantidium 
 and Nydotherus (Fig. 102, G.) are almost sure to be found. 
 The former has several contractile vesicles, the latter generally 
 
Chap. XIX.] VORTICELLA AND PARAMCECIUM. 353 
 
 but one (c. v.). The vestibule (vs.) in nyctotherus is very con- 
 spicuous, and on its lower edge there is a long stiff seta; the 
 anus also forms a distinct cleft (an.). 
 
 The fact to be especially noted in the forms which we have 
 studied in this Chapter is that they are unicellular organisms, or 
 protozoa, are never possessed of sexual organs, and are thus 
 very markedly distinguished from the multicellular organisms 
 or metazoa. Metazoa are typically produced by a segmentation 
 in the ovum, by which a number of cells result, which remain 
 so connected as to be constituents of a single individual. 
 Differentiation in the metazoa proceeds by the divergent de- 
 velopment of different cells, a morphological differentiation 
 of structure accompanying a physiological differentiation of 
 function. In the higher forms the differentiated cells aggregate 
 into groups, and thus form tissues, which take their part 
 in the formation of more or less differentiated organs. In the 
 protozoa there is no segmentation of the ovum. The process of 
 fission does, indeed, in some respects resemble the first cleavage 
 of the ovum into two blastomeres, but it also serves to show the 
 fundamental distinction between the two groups of organisms. 
 For in the protozoa, after fission, each cell typically becomes a 
 distinct individual. There is thus no opportunity for that 
 physiological division of labour which is normal among the con- 
 stituent cells of the metazoan. 
 
 Differentiation, therefore, where it goes on among the protozoa, 
 must take place within the single cell of which each individual 
 is framed. The forms we have studied show the nature of this 
 differentiation. At the best they remain lowly forms of life. 
 They have not learnt the secret of merging their individuality 
 for the common good, and being content to be the constituent 
 units, each with its own narrow field of work, of a nobler 
 organism. 
 
 23 
 
CHAPTER XX. 
 THE AMCEBA. 
 
 THE Amoeba, or Proteus animalcule, consists of a minute irre- 
 gular particle of jelly-like protoplasm (Fig. 103, A.). Individuals 
 may generally be found in animal and vegetable infusions or in 
 stagnant pond water. Each looks like a little patch of some- 
 what cloudy white of egg ; but, if it be carefully watched, it will 
 be found to be constantly changing its form, and moving slowly 
 across the stage of the microscope by pushing out blunt processes 
 or pseudopodia (ps.). Careful observation discloses an external 
 cortical layer, the ectosarc or ectoplasm, enclosing the inner sub- 
 stance, endosarc or endoplasm. The ectoplasm is clear, the endo- 
 plasm granular. When a pseudopodium is pushed out the ecto- 
 plasm precedes the endoplasm, which follows with a kind of 
 rush. Within the granular endoplasm there is at least one,, 
 sometimes more than one, rounded endoplast (TI.), with a smaller 
 endoplastule within it. A rhythmically pulsating contractile 
 vesicle (c. v.) exhibits the usual slow diastole and sudden 
 systole. Oily granules, crystals, and food-vacuoles may also 
 be seen to flow hither and thither within the endoplasm in 
 vague circulation. 
 
 Food is taken in at any point of the body, and the indiges- 
 tible residue is also extruded at any point. There is not even a 
 potential mouth or anus. Sometimes a pseudopodial process 
 seems to be wrapped over the morsel of food, in which case 
 some of the ectoplasm becomes internal. It must be remem- 
 bered, however, that the differentiation into these two layers is 
 of the slightest. 
 
 The amoeba multiplies by fission. After attaining a certain 
 
 354 
 
Chap. XX.] 
 
 THE AMCEBA. 
 
 355 
 
 size the endoplast divides into two, or sometimes the endoplas- 
 tule is the first to divid-e, and then the endoplast divides also. 
 In Fig. 103, B., an amoeba is figured, in which the division 
 of the endoplast has taken place, and the two endoplasts 
 (n. n.) are widely separated. Shortly afterwards the contractile 
 
 FIG. 103. AMCEBA AND MONADS. 
 
 A. Amoeba. B. Amoeba undergoing fission. C. Amoeba encysted. C. 
 Monad. D. Heteroniita. E. Cyst resulting from fusion of two Heteromita. 
 F. Cyst bursting and giving rise to fluid full of germs. 
 
 c. v. Contractile vesicle. /. Flagellmn. yu. Gubernaculum. n. Endoplast. 
 p. s. Pseudopodium. 
 
 vesicle (c. v.) also divided, and a delicate line of division 
 seemed suddenly to make its appearance. Forty minutes 
 afterwards the two amceba were quite separate. In Fig. 103, 
 C., an amceba is shown in the encysted condition, the outer 
 layer being clear and tolerably distinct. In this case the 
 amceba subsequently resumed its usual irregular form ; but 
 it is said that in some cases there emerge from the cyst crowds 
 of minute amoebae. Conjugation is said sometimes to occur. 
 
 Let us note that we have in the amceba a simple cell that 
 has undergone very little differentiation. That cell, however, 
 performs several functions which characterise it as a living 
 organism. It responds to external stimuli, and is therefore sen- 
 sitive in the sense in which we have before (p. xv.) used that 
 term. The responses take the form of changes of shape in the 
 coll. It receives and assimilates food : so that there must be a 
 
356 ANIMAL BIOLOGY. [Part II. 
 
 rudimentary form of digestion. This process, and the energy 
 involved in the constant change of form, are the indications of 
 hidden metabolic processes which go on within the cell ; and 
 there can be no doubt that these metabolic changes involve 
 processes of respiration and the excretion of waste products. 
 Whether there is any part of the cell which merely forms a 
 framework for the support of the rest it is difficult to say. 
 Finally the simple cell is reproductive. 
 
 We have already seen that the metazoan differs from the pro- 
 tozoan in being a cell-aggregate. We may now note that as we 
 advance towards the higher metazoa we find a more and more 
 perfect division of labour among the cells special cells taking 
 upon themselves the various functions which, in the amoeba, are 
 performed in a rudimentary fashion by the single cell. Or we 
 may say to put the matter in another way that in the higher 
 metazoa there is such differentiation among the cells that special 
 cells or groups of cells are enabled to cultivate, almost exclusively, 
 some one or other of the protoplasmic faculties. Some (nerve) 
 become eminently sensitive ; others (muscle) eminently contrac- 
 tile ; yet others (liver) eminently metabolic, secretory (salivary), 
 or excretory. All or most are capable of reproducing by fission 
 other cells like themselves ; but to certain cells are intrusted 
 the all-important function of continuing the race by reproducing 
 complex individuals inheriting the peculiarities of the parent 
 organism. So important are these cells that it has been sug- 
 gested that all other tissues and organs exist that the ovary may 
 produce an ovum which shall be duly fertilised, 
 
 Be this as it may, we see that the higher metazoan individual 
 is a cell-aggregate in which there has gone on a large amount of 
 differentiation and division of labour among the constituent cells. 
 But here we must further note that accompanying the differen- 
 tiation there is also integration. It would be of no use for the 
 cells to become divergent in structure and function if, accom- 
 panying this essential process, there were not a process, equally 
 essential, by which the cells are knit together into the tissues 
 and organs of one individual body. For efficient advance in the 
 life-scale integration and differentiation must go hand in hand. 
 
Chap. XX.] THE AMCEBA. 357 
 
 And not only (1) do the parts of an organism get more different 
 and yet at the same time more intimately connected, but the 
 organism (2) tends to become more different from the environ- 
 ment, and yet, at the same time, requires to be, so to speak, more 
 closely in touch with the conditions of the environment ; and yet 
 further, (3) the organism not only becomes more different from 
 other organisms, but at the same time becomes more dependent 
 upon other organisms, so that the whole organic world becomes 
 more and more a vast system of interdependent units. 
 
 These considerations help us to see what is the meaning of 
 the terms higher and lower as applied to animals. The higher 
 animals are those in which this joint process of differentiation 
 and integration has been carried furthest. But it is well to 
 avoid instituting comparisons of this nature between animals of 
 widely different groups. Such a question as, Which is the 
 higher animal a bee or a bear ? is not very profitable. It is 
 even well to limit comparison, so far as possible, to analogous 
 cases, and to speak of higher and lower in respect to the per- 
 formance of special functions. At the same time we may per- 
 haps say generally that the more complex the conditions of life 
 to which an organism is suited, and the more perfectly that 
 organism is suited to these conditions of its life, other things 
 equal, the more advanced it is. 
 
 A final question arises out of the fact that if we prepare an 
 animal or vegetable infusion (e.g. hay or cod's head), and leave it 
 to stand for a short time, we shall find it swarming with organ- 
 isms paramoecia, amoebae, monads (Fig. 103, C. D.) and countless 
 bacteria. Whence comes all this teeming life ? Two answers 
 are or have been given to this question. The first (abiogenesis) 
 is that the living forms take their origin by a regrouping of the 
 molecules of the organic but lifeless matter in the solution, with- 
 out the aid of any parental organisms. The second (biogenesis) 
 is that the living forms are due to the presence in the solution of 
 organic living germs, the products of parental organisms. Into 
 the arguments brought forward by the advocates of these two 
 answers we cannot enter here. The balance of scientific evi- 
 
358 ANIMAL BIOLOGY. [Part II. 
 
 dence would seem to be very largely in favour of biogenesis. 
 For if a solution be sterilised by boiling, sufficiently prolonged 
 to kill all germs, and then be preserved from contact with the 
 dust- and germ-laden air by a plug of cotton wool, the molecules 
 of the organic but lifeless matter in solution are utterly in- 
 capable of regrouping themselves into living forms, and the infu- 
 sion remains barren of life. This is the negative evidence in 
 favour of biogenesis. 
 
 Day by day positive evidence in favour of the same view in- 
 creases in amount as the life-histories of these lowly forms of 
 life are patiently worked out, and it is found that they " breed 
 true." In Fig. 103, D., is figured a form of monad (Heteromita) 
 found in an infusion of cod's head. It has two whip-like pro- 
 cesses, a flagellum (/.) and a gubernaculum (gu.\ by which it 
 can anchor itself. There is an endoplast (n.\ and a contractile 
 vesicle (c. v.). Such monads multiply by fission, but also by an- 
 other process. Two individuals conjugate and fuse. A trian- 
 gular cyst (E.) results. After a while the cyst bursts, and a 
 homogeneous fluid is emitted (F.). The highest powers of the 
 microscope fail to disclose in this emitted matter any germ of 
 life, and there, at first sight, would seem to be an end of the 
 matter. But wait and watch, and there appear in the field of 
 the microscope, suddenly and as if by magic, countless minute 
 points which prolonged watching shows to be growing. And 
 when they have fully grown every distinct point is seen to be 
 a monad. Thus we see that a solution in which the highest 
 powers of the microscope disclose no trace of living matter, may 
 nevertheless be swarming with germs. Let a drop of such solu- 
 tion evaporate into the air ; there is no reason to suppose that 
 the germs perish. On the contrary, there is much to lead us to 
 believe that the germs have far greater powers of resisting high 
 temperature, desiccation, and other adverse conditions, than the 
 fully developed organism. We may thus see how the air comes 
 to be laden with germs which, should they fall into an appro- 
 priate infusion, may give rise to the teeming life which we know 
 to be so soon developed in it. 
 
 Biogenesis would seem, then, to be a law of life under the 
 
Chap. XX.] THE AMCEBA. 359 
 
 conditions attainable in our biological laboratories. Let us be 
 careful, however, not to dogmatise on this subject. Whether 
 living forms ever spring from not-living matter, in ponds or in 
 the great ocean, we do not know. Nor do we know whether 
 abiogenesis has or has not taken place in times geologically 
 remote. 
 
APPENDIX 
 
 CLASSIFICATION OF TYPES. 
 
 THE broadest division of the typical organisms considered in 
 this volume is into Protozoa, or organisms which are unicellular, 
 and Metazoa, or organisms which are multicellular, and in which 
 there is physiological division of labour and morphological differ- 
 entiation of structure among the cells. To the protozoa belong 
 the amoeba, destitute of a mouth, and possessed of the power of 
 emitting pseudopodia (Rhizopoda\ the vorticella and paramce- 
 cium (Infusoria ciliata), with ciliated bodies, and provided with 
 a vestibule or oral fossa, and the monads (Infusoria flagellata), 
 with one or more whip-like processes or flagella. The other 
 organisms here considered belong to the metazoa. 
 
 The metazoa again are divided into Codenterata, to which 
 the hydra belongs, in which there is no true body cavity and 
 in which the symmetry is radial, and Coslomata, in which a body 
 cavity or coelom is present. 
 
 The ccelomatous forms are roughly divided into the Vertebrata, 
 in which an endoskeleton is developed, the axial portion of 
 which consists of the skull and vertebral column, and the In- 
 vertebrata, in which an endoskeleton so constituted is absent. 
 The Invertebrata form a very heterogeneous group, distinguished 
 merely by negative characters. Those which we have considered 
 fall into three divisions. 
 
 (1) The Arthropods, with bilateral symmetry, segmented 
 bodies, jointed lateral appendages, and a ventral nerve-chain. 
 To this group belong the crayfish, a decapod crustacean, breath- 
 ing by means of branchiae, in which the body is divided into 
 cephalo thorax and abdomen ; and the cockroach, an orihopter- 
 ous insect, in which respiration is by tracheae, and the body 
 is divided into head, thorax, and abdomen. 
 
 (2) The Termes, with bilateral symmetry, unsegmented (liver- 
 fluke) or uniformly segmented (earthworm) body, with lateral 
 appendages which (when present) are not jointed, and with 
 
362 ANIMAL BIOLOGY. 
 
 paired excretory organs. The group is a large and diverse one, 
 including the Platylielminthes (liver-fluke and tapeworm), in 
 which the body is flattened, and is either unsegmented (liver- 
 fluke) or divided into a number of successive divisions, which 
 have a tendency to separate individualisation (tapeworm), 
 without a ventral nerve-chain, generally hermaphrodite, without 
 lateral appendages, but provided with suckers (liver-fluke) or 
 suckers and hooks (tape-worm) ; and the Cho&topoda, ringed 
 worms (Annelids), possessed of a ventral nerve-chain and setose 
 locomotive appendages, which may be numerous in each seg- 
 ment (Pofychceta), or, as in the case of the earth-worm, few 
 (Oligochoeta). 
 
 (3) The Mollusca, bilaterally symmetrical (mussel) or some- 
 what asymmetrical (snail) animals, without lateral appendages, 
 but possessed of a muscular locomotive 'foot/ unsegmented in 
 the adult form, and with paired ganglia united by commissures 
 which form an cesophageal nerve-ring,, generally possessing a 
 bivalve (mussel) or univalve (snail) shell, and a bilobed (mussel) 
 or unilobed (snail) mantle. To this group belong the mussel, 
 which has plate-like gills (Lamettibranchiata), no separate head, 
 and in which the sexes are distinct ; and the snail (Gastropoda), 
 in which the head is distinct, and the mouth is provided with 
 an odontophore (Odontophora), having a twisted univalve shell, 
 and pulmonary respiration (Pulmonata). 
 
 The Vertebrata, are bilaterally symmetrical animals, with a 
 notochord, and in nearly all cases an internal skeleton, the 
 vertebral column, and a dorsal nerve system generally protected 
 by the arches of the vertebrae. There are never more than two 
 pairs of limbs, and the jaws, which are never (as in the arthro- 
 pods) modified appendages, move up and down. They are 
 always possessed, at some period of life, of visceral arches and 
 clefts. They are divided into three great groups. 
 
 1. The Ichthyopsida, including the Pisces (cod) and the Am- 
 phibia (frog). Eespiration by gills always takes place during a 
 part (frog), and sometimes the whole of life (cod). The exoskele- 
 ton is either absent or very slightly represented. The basi- 
 occipital region of the skull is generally unossified. The 
 temperature of the blood varies with, and is not much above, 
 that of the surrounding medium. The red blood corpuscles are 
 nucleated. There are two systemic aortic arches. The urinary 
 organs are persistent Wolffian bodies. There is no thoracic 
 diaphragm. There is no true corpus callosum in the brain. The 
 hypoglossal nerve does not perforate the brain-case. The amnion 
 
CLASSIFICATION OF TYPES. 363 
 
 is absent, and the allantois rudimentary. There are no mam- 
 mary glands. 
 
 2. The Sauropsida, including the Reptilia and A ves (fowl* and 
 pigeon). There is no gill-respiration, but gill-arches are present 
 in the embryo. There is almost always an epidermic exoskeleton 
 in the form of scales or feathers. The occipital region of the 
 skull is completely ossified, and there is one occipital condyle; 
 The mandible consists of several bones, and articulates with 
 the skull through the intervention of a quadrate bone. The 
 ankle-joint is between the proximal and distal divisions of the 
 tarsus. The temperature of the blood is near (reptilia) or con- 
 siderably above (aves) the normal temperature of the surrounding 
 medium. The red blood corpuscles are oval and nucleated. 
 The left systemic aortic arch is often (e.g. in all birds) suppressed. 
 The thoracic diaphragm is never complete or functional. The 
 Wolffian bodies are replaced by true kidneys. There is no 
 corpus callosum. The right oviduct and ovary may be (birds) 
 rudimentary. There is an amnion and a well-developed allan- 
 tois, the function of which is chiefly respiratory. There are no 
 mammary glands. 
 
 3. The Mammalia , to which the rabbit belongs. There is no 
 gill-respiration, but gill-arches are present in the fcetus. The 
 epidermic exoskeleton consists of hairs. The occipital region of 
 the skull is completely ossified; and there are two occipital 
 condyles. The mandible consists of a single bone on each side, 
 and articulates directly with the squamosal of the skull. The 
 quadrate becomes an auditory ossicle. The ankle-joint is between 
 the tibia and the astragalus. The temperature of the blood is 
 more or less constant, and considerably above the normal tem- 
 perature of the surrounding medium. The red blood corpuscles 
 are generally round and not nucleated. The right systemic 
 aortic arch is suppressed. There is a complete and functional 
 diaphragm. The Wolffian bodies are replaced by permanent 
 kidneys. There is a corpus callosum in the brain. The foetus 
 has an amnion and allantois ; and in the higher forms a true 
 placenta is formed. Mammary glands supply the young with 
 nourishment. 
 
GLOSSARY 
 
 Abiogenesis. Also called spontaneous generation ; the doctrine that living 
 beings can arise de novo in the midst of not-living matter. Used in 
 contradistinction to biogenesis : the doctrine that every living cell, or 
 group of cells, is a link in the chain of life. 
 
 Acinus. The blind grape- shaped termination of a gland or hollow organ. 
 The word alveolus is used with like signification. 
 
 ^Etiology. The doctrine of causes. Evolution and Natural Selection fall 
 under this head. 
 
 Afferent Nerves. Those which convey waves of metabolic change inwards 
 to the central nerve-system ; in distinction from efferent nerves, 
 which convey these waves outwards to an organ or gland. 
 
 Alecithal. A term applied to the ovum, such as the rabbit's, in which 
 there is little or no food-yolk, or in which the little there is, is dis- 
 tributed uniformly through the ovum. Used in contradistinction to 
 telolecithal (fowl's ovum), in which the food-yolk is aggregated at one 
 pole, and centrolecithal (crayfish ovum), in which it is aggregated at 
 the centre. 
 
 Alternation of Generations. The alternation of two forms of the same 
 species. Under this general head come (1) metagenesis, when the one 
 form is sexual and the other asexual (medusa) ; (2) heterogamy, when 
 the second form (or forms) are perfectly or imperfectly sexual. The 
 liver-fluke falls under this head if we regard the masses of cells from 
 which the cercarise arise as rudimentary ovaries. When the ova 
 develop without the influence of spermatozoa, the term parthenogenesis 
 (maiden-birth) is used. 
 
 Altrices. Birds whose young are hatched in a callow, immature condition, 
 as in the pigeon, as opposed to prcecoces, in which they are active at 
 hatching. 
 
 Alveolus. See Acinus. 
 
 Ametabola. Insects which undergo no metamorphosis, or in which the 
 metamorphosis is incomplete (sometimes called hemimetabola), as 
 opposed to holometabola, when the metamorphosis is complete. 
 
 Amniota. Vertebrates in which the amnion, a f ratal membrane, is 
 developed (reptiles, birds, and mammals), in distinction from anam- 
 niota (frog, cod), in which there is no amnion. 
 
 365 
 
366 ANIMAL BIOLOGY. 
 
 Amoeboid. A term applied to cells which change their form like the 
 amoeba. 
 
 Anabolism. See Metabolism. 
 
 Analogous Organs. Organs which perform the same function, but are not 
 necessarily constructed on the same plan ; used in contradistinction to 
 homologous organs, when they are constructed on the same plan, but 
 have not necessarily the same function. The wing of the insect and 
 that of the bird are analogous, but not homologous ; the wing of the 
 bat and the paddle of the whale are homologous, but not analogous ; 
 the hind-limbs of the frog and the rabbit are both analogous and 
 homologous. 
 
 Anastates. See Metabolism. 
 
 Aponeurosis. The strong sheath of connective tissue which invests a 
 muscle. Those covering the muscles have been termed aponeuroses 
 of investment, in contradistinction to the tendinous expansions or 
 aponeuroses of insertion. 
 
 Appendicular. Applied to that part of the vertebrate skeleton which is 
 related to the appendages, as opposed to the axial skeleton. 
 
 Archenteron. The digestive cavity of the embryo at the gastrula stage. 
 After the formation of the mesoblast the term mesenteron is employed. 
 
 Arterial. A term applied to blood which has been oxygenated in the lungs 
 or gills, opposed to venous when the oxygen has been used up by the 
 tissues. The pulmonary artery contains venous blood, the pulmonary 
 vein arterial blood. 
 
 Articulation. Union at a joint. The lower jaw is articulated to the skull. 
 
 Atrophy. The wasting away of a part or organ. 
 
 Axial. See Appendicular. 
 
 Biogenesis. See Abiogenesis. 
 
 Blastoccele. Also called segmentation cavity; the cavity contained within 
 the blastosphere or hollow mass of cells (blastomeres) resulting from the 
 segmentation of the ovum. Where segmentation gives rise, as in the 
 fowl, to a cellular patch, it is termed the blastoderm. The hollow 
 mass of cells produced by segmentation in the rabbit is termed the 
 blastodermic vesicle. The blastopore is the orifice of the archenteron 
 of the gastrula, produced by the invagination of the blastosphere. 
 
 Budding. The asexual formation of new individuals, in hydra, by the 
 formation of a pear-shaped outgrowth which develops into a new 
 hydra. The process is also called gemmation. 
 
 Caecum. A blind pouch. 
 
 Canaliculi. Minute tubes arising from the lacunae of bone, or from the 
 
 repeated branching of the hepatic ducts. 
 Capillaries. The finest and most delicate blood-vessels, formed by the 
 
 ultimate subdivision of the arteries, and giving rise to the most minute 
 
 factors of the veins. 
 
GLOSSARY. 367 
 
 Capsule. A little case in which some organ or part of an organ is 
 
 enclosed. 
 Cells. The nucleated elements of which the protozoa consist, and which 
 
 in rnetazoa are built up into tissues. From the undijferentiated cells of 
 
 the embryo arise such forms as the amoeboid (white blood-corpuscles), 
 
 the columnar (epithelium), the flattened or squamous (epithelium, 
 
 endothelium), the fibrous (muscle, nerve), the branched (connective 
 
 tissue), and the ciliated (tracheal epithelium). 
 Centrolecithal. See Alecithal. 
 Ciliated. A term applied to those cells or organisms which have delicate, 
 
 motile, hair-like processes or cilia. 
 Cleavage. A term applied to the splitting, division, or segmentation of 
 
 the ovum into a number of cells or blastomeres ; also to the splitting 
 
 of the mesoblast into two layers. 
 Ccelomata. Those organisms which have a separate body cavity, as 
 
 opposed to the cozlenterata. 
 Columella. (1) A bone connected with the organ of hearing in the frog and 
 
 fowl ; (2) the axis of the shell in the snail to which the columellar 
 
 muscles are attached. 
 Commissure. A nerve band joining two ganglia or two parts of the brain. 
 
 The term is sometimes reserved for transverse bands, longitudinal 
 
 connections being termed connectives. 
 Conjugation. The union or fusion, temporary or permanent, of two protozoa. 
 
 The phenomenon occurs in connection with the reproductive process. 
 
 Development. Differential growth, involving a change in the organism as 
 it passes from the ovum to the mature state. 
 
 Dioecious. Having the sexes distinct, the individuals being male and 
 female ; in contradistinction to monoecious or hermaphrodite, when 
 both sexes are combined in the same individual. 
 
 Differentiation. The process by which cells or parts originally similar be- 
 come different in structure. It is accompanied by a specialisation of 
 function among the cells, sometimes spoken of as the physiological 
 division of labour. 
 
 Distal. Applied to the end of an appendage, bone, or limb which is 
 furthest removed from the trunk : the other end being called proximal. 
 
 Distribution. (1) The way in which organisms are distributed over the 
 earth in space and in time ; (2) the way in which nerves and blood- 
 vessels branch out to supply organs. The tenth nerve (e.g.) is distri- 
 buted to the larynx, heart, lungs, stomach, etc. 
 
 Diverticulum. A blind tube opening out of another tube. 
 
 Ecdysis. The throwing off of the outer skin or exoskeleton. 
 Embryo. The organism in the stages which intervene between the seg- 
 mentation of the ovum and birth or hatching. The term foetus is used 
 
368 ANIMAL BIOLOGY. 
 
 of mammals. Embryonic or foetal appendages are provisional 
 
 structures (such as the allantois and amnion) in connection with 
 
 embryonic or foetal life. 
 Eucystment. A phenomenon occurring among lowly organisms when they 
 
 become motionless and are surrounded by a coat, case, or cyst. 
 Endoderm. The inner layer of the hydra as opposed to ectoderm. Endo- 
 
 plasm or endosarc, the inner substance of a protozoon, as opposed to 
 
 ectoplasm or ectosarc, its outer substance. Endoplast, a term applied 
 
 to the nucleus of the protozoa. 
 Epibole. The overgrowth of epiblast cells over the other cells of the 
 
 segmented ovum, as in the case of the frog. 
 Evagination. See Invagination. 
 Eversion. A term applied to the opening out and expansion of the 
 
 peristome of the vorticella. 
 Excretion. The process by which waste products are got rid of or expelled 
 
 from the body. 
 
 Facet. (1) A smooth surface of bone on which another bone works, e.g. 
 
 tubercular facet of a vertebra ; (2) the polygonal areas of the insect's 
 
 or crustacean's eye. 
 Femur. (1) The thigh of a vertebrate ; (2) the thigh-bone ; (3) the first 
 
 joint in the leg of a cockroach. 
 Fertilisation. The union of a spermatozoon with the ovum. The process 
 
 is often called impregnation. 
 Fenestra ovalis and Rotunda. Membranous "windows" in the periotic 
 
 bone in connection with the organ of hearing. In the fowl the two 
 
 often open externally into a fenestral recess. 
 Fission. The splitting of a cell or a lowly organism into two more or 
 
 less similar parts. The cleavage of the ovum is a special form of 
 
 fission. 
 
 Foetus. Foetal membranes. See Embryo. 
 Folding off of the embryo. The process by which the embryo (e.g. fowl) 
 
 is constricted from the yolk-sac. 
 Fontanelle. A space occupied by membrane. 
 Foramen. A hole in the skull or in a bone through which nerves or 
 
 blood-vessels pass. 
 Fronto-parietal. A bone of the frog's skull, also called par ieto -frontal. 
 
 Ganglion. A group of nerve-cells, e.g. Gasserian ganglion, sympathetic 
 ganglia. 
 
 Gastrula. The two-layered condition of the embryo, resulting from a 
 pushing in or invagination of one portion of the blastosphere. 
 
 Germinal layers. The cell-layers of the embryo, hypoblast, epiblast, and 
 mesoUast. Germinal vesicle, the nuclear portion of the ovum con- 
 taining nucleolar germinal spots. The germinal disc is a thilkened 
 
GLOSSARY. 369 
 
 part of the blastoderm in which the embryo is developed. The 
 germinal epithelium is that part of the peritoneal lining from which 
 the generative organs take their origin. 
 
 Gland. An organ composed of one or more epithelial cells, the physio- 
 logical function of which is secretion or excretion. The term is also 
 applied to other organs, such as the lymphatic glands, thymus, pineal 
 gland, hermaphrodite gland, etc. 
 
 Gonad. A term applied to the reproductive organ, male (testis or 
 spermary), female (ovary) or hermaphrodite (ovotestis of snail). 
 
 Hepatic. Of or belonging to the liver. Hepato-pancreas, a term applied 
 to the invertebrate digestive gland, as in the crayfish. 
 
 Hermaphrodite. Combining male and female in the same organism (earth- 
 worm) or organ (ovotestis and hermaphrodite duct of snail). 
 
 Histology. The microscopic study of tissues. Histolysis is the breaking 
 up of a tissue during development. Histogenesis is the embryological 
 development and origin of tissues. The term Histozoa has been 
 employed as equivalent to metazoa. 
 
 Holoblastic. A term applied to segmentation, where the cleavage affects 
 the whole ovum ; opposed to meroblastic, where the cleavage is only 
 partial and restricted. 
 
 Holometabola. See Ametabola. 
 
 Homologous organs. See Analogous organs. 
 
 Impregnation. The fertilisation of an ovum by the entrance of a sper- 
 matozoon. 
 
 Ingestion. The taking in of food particles. It may be inter -cellular, 
 between the cells, or intra-cellular into the cells. 
 
 Intus-susception. The incorporation of new material with the old during 
 life and growth ; as opposed to accretion, the addition of layer upon 
 layer. 
 
 Invagination. The pushing in of a part of the surface to form a pouch; 
 the gastrula (e.g.] is produced by the invagination of the blastosphere 
 to form a two-layered cup. Evagination is the converse process, where 
 a pouch is pushed outwards, or where an invaginated pouch is turned 
 inside out. 
 
 Irritability. The property of responding to a stimulus. Specialised in 
 nerve and muscle. 
 
 Karyokinesis. A complex process undergone by the nucleus of many cells 
 
 during division. 
 Katabolism and Katastate. See Metabolism. 
 
 Lacunar. Having lacunae or spaces. 
 
 Lamella. A leaf -like plate, e.g. of bone. Lamellar structure is where 
 there are a number of leaves. 
 24 
 
370 ANIMAL BIOLOGY. 
 
 Lamina. A thin film or plate. Lamina perpendicularis, part of the eth- 
 moid bone ; Lamina spiralis in the organ of hearing ; Lamina termin- 
 alis, the front wall of the brain. Laminae, dorsales, the ridges which 
 bound the neural or medullary groove. 
 
 Larva. The freed embryo of certain organisms, such as insects. 
 
 Ligament. (1) A fibrous band of connective tissue by which the bones 
 are connected at the joints or articular surfaces ; (2) a fibrous elastic 
 band where the shell of the mussel is not calcified. 
 
 Liver. A large secreting gland of the vertebrate. The term liver is often 
 applied to the digestive gland of the invertebrate. Since this has 
 often pancreatic properties, it is sometimes called the hepato-pancreas. 
 
 Lobe. A part of an organ separately folded off. There are lobes of the 
 brain, liver, lungs, and many other organs ; a lobule is a little lobe. 
 
 Lumen. The central cavity of a fine tube, duct, or vessel. 
 
 Manubrium. (1) A process of the sternum. (2) A process of the hammer 
 bone or malleus of the ear. (3) The clapper of the bell in a medusa. 
 
 Maxilla. (1) A jaw-bone in the vertebrate skull. (2) An appendage in 
 the crayfish and the cockroach. 
 
 Medulla. (1) The pith-like substance in the axis of a feather. (2) Medulla 
 oblongata, the hinder part of the brain. (3) Medullary folds, groove, 
 and tube, the first indications of the central nervous system, now 
 often termed the neural folds, tube, etc. (4) The internal striated 
 part of the kidney, as opposed to the outer dotted cortical part. (5) 
 Medullary cords, strands of lymphatic tissue in a lymphatic gland. 
 
 Membrane. A thin sheet of tissue, forming an investment or a distinct 
 layer. 
 
 Meroblastic. See Holoblastic. 
 
 Mesenteron. See Archenteron. 
 
 Mesentery. A fold of peritoneal membrane by which the alimentary 
 canal is suspended in the body-cavity or coelom. 
 
 Mesostates. See Metabolism. 
 
 Metabolism. A term applied to the chemical changes associated with 
 living cells. Metabolism may result in the formation of more complex 
 compounds (anabolism), or of less complex compounds (katabolism}. 
 Anabolism is associated with the storage of energy in the tissues ; 
 katabolism with the activities of the organism. Intermediate com- 
 pounds are called mesostates ; as terms of a katabolic process, kata- 
 states ; of an anabolic process, anaslates. 
 
 Metameres. Serially homologous divisions of an organism. 
 
 Metamorphosis. The series of changes taking place during the free life of 
 the organism by which it passes from the embryonic to the adult state. 
 When the changes take place before birth or hatching the term trans- 
 formation is used. 
 
 Metazoa. Organisms composed of many cells, in which a physiological 
 
GLOSSARY. 371 
 
 division of labour has taken place : used in contradistinction to 
 protozoa where there is generally but one cell, or at most an aggregate 
 of similar cell units. Sometimes called histozoa. 
 
 Morphology. That branch of the science which deals with the form and 
 structure of organs or organisms : whereas physiology deals with their 
 functions ; and histology with their minute structure. Morphological 
 structure is ascertained by anatomical investigation. 
 
 Morula. A mulberry mass of cells not enclosing a cavity, and resulting 
 from segmentation. 
 
 Neural (1) Neural folds, groove, and tube (often called the medullary 
 folds, tube, etc.), the first indications of the cerebro- spinal nervous 
 system ; (2) Neural arch and spine, processes of the vertebrae. 
 
 Notochord. A rod of cells derived from the hypoblast, and underlying 
 the neural tube. It forms the primitive axial support of the body. 
 Sometimes called the chorda dorsalis. 
 
 Nucleus. The specially differentiated 'kernel* of a cell in which all vital 
 changes seem to be initiated. Within the nucleus there may be one 
 or more nucleoli. 
 
 Oral. The oral end of an organism is that part which bears the mouth. 
 The opposite end is called aboral. 
 
 Origin. (1) ^Etiological questions are spoken of as questions of origin 
 (e.g. origin of species, origin of tissues). (2) The relatively fixed 
 point to which a muscle is attached is termed its origin, in contra- 
 distinction to the relatively moveable part where it has its insertion. 
 
 Otoliths. Concretions or foreign particles in an auditory sac. 
 
 Ovo-testis. Also called hermaphrodite gland. An organ or gonad which 
 produces both ova and spermatozoa. 
 
 Ovum. The cell produced by the female, from which, generally after 
 fertilisation by a spermatozoon, a new organism arises. Such repro- 
 duction is called sexual : where the organism is reproduced without 
 ova (e.g. by budding or fission), the reproduction is called asexual. 
 When the ovum develops without fertilisation the term partheno- 
 genesis is used. 
 
 Palp. A process supposed to be an organ of touch generally in relation to 
 the mouth (e.g. labial palps of mussel, mandibular palp of crayfish). 
 
 Pancreas. A vertebrate digestive gland. The invertebrate digestive gland 
 (e.g. of crayfish) is sometimes called the hepato-pancreas. 
 
 Papilla. A small projection (e.g. of the tongue). 
 
 Parietal. (1) A bone of the skull which gives its name to one of the 
 segments of the brain-case. (2) Of or belonging to the parietes or 
 walls, e.g. parietal (or somatic) layer of the peritoneum, in contra- 
 
372 ANIMAL BIOLOGY. 
 
 distinction to the visceral (or splanchnic) layer. Parieto-frontal, also 
 called fr onto -parietal, a bone in the skull of the frog. 
 
 Parthenogenesis. See Ovum. 
 
 Periostracum. Also called, as in the crayfish, epiostracum, the outer 
 layer of the shell in the mussel. 
 
 Physiology. See Morphology. 
 
 Placenta. A vascular mammalian structure bringing the embryo into 
 relation with the uterus of the mother. Villi on the allantoic chorion 
 fit into depressions or crypts of the uterine wall, the conjoint structure 
 forming the placenta. If the foetal structures separate at birth from 
 the maternal mucous membrane, the placenta is non-deciduate ; if the 
 maternal mucous membrane is torn away with the foetal structures 
 and delivered as the after-birth, it is deciduate. According to the 
 arrangement of the villi the placenta is discoidal (rabbit), dome shaped, 
 zonary, or diffuse. 
 
 Pleura. (1) The membrane that covers the mammalian lungs. (2) The 
 lateral edges of the exoskeleton of the crayfish. 
 
 Portal. A portal vein is one which instead of carrying the blood directly to 
 the heart carries it to some organ or gland within which it breaks up 
 into a capillary plexus. All vertebrates have a hepatic-portal system, 
 with a capillary plexus in the liver. The frog has also a renal portal 
 system, with a capillary plexus in the kidney. 
 
 Process. A projecting portion proceeding from some cell, organ, or part 
 of the body; e.g. transverse process of vertebra, pseudopodial processes 
 of amoeba. 
 
 Procoelous. Of vertebrae, the centra of which are concave in front ; amphi- 
 ccelous are concave at both ends ; opisthocodous concave posteriorly. 
 
 Proctodseum. That part of the alimentary canal which arises in develop- 
 ment as a posterior epiblastic invagination, giving rise in the cray- 
 fish, e.g. to the hind-gut. 
 
 Prcecoces. See Al trices. 
 
 Protovertebrse. A term sometimes used for mesoblastic somites. 
 
 Protozoa. See Metazoa. Also called cytozoa. 
 
 Protoplasm. The elementary organic basis of living matter, sometimes 
 termed sarcode. 
 
 Proximal See Distal. 
 
 Racemose. A term applied to a gland in which the lobules contain a 
 
 number of acini opening in clusters into the extremities of a branched 
 
 duct. 
 Eamus. A branch or division, (1) of a nerve, the spinal nerves, e.g. have 
 
 dorsal and ventral rami ; (2) of the lower jaw, each lateral mandible 
 
 being termed a ramus. 
 Rejuvenescence. A term applied to the added energy of a protozoon after 
 
 conjugation. Also termed adjuvenescencc. 
 
GLOSSARY. 373 
 
 Reproduction. See Ovum. 
 
 Sac. A hollow cavity, e.g. air sac, dart sac, scrota! sac. 
 
 Segmentation. (1) The process by which an ovum is cleaved into a number 
 of cells or blastomeres. (2) The division of the body into a number 
 of divisions or segments. Seymental organs, a term applied to the 
 excretory organs or aephridia of the earthworm. Segmental tube, the 
 primitive excretory duct of the vertebrate. 
 
 Sella turcica. A depression in the basisphenoid bone of the skull, some- 
 what resembling, in the human subject, a Turkish saddle. In the 
 rabbit there is in this region a hole through the bone, the canalis 
 craniopharyngeus. 
 
 Septum. A division or partition, e.g. Septum nasi, between the nasal 
 chambers ; septum auricularum between the auricles ; septum lucidum 
 between the hemispheres of the brain. 
 
 Serous membrane. A membrane that secretes a thin serous fluid (e.g. 
 peritoneum), in distinction from a mucous membrane, which secretes a 
 thicker mucous fluid (e.g. that lining the nasal chamber). 
 
 Skeleton. The supporting framework of the body, consisting of (a) con- 
 nective tissue, (b) cartilage or gristle, (c) bone. In addition to this 
 endoskekton (or instead of it in invertebrates) there may be an 
 exoslceleton. 
 
 Spermary. A term used for the male gonad or testis. 
 
 Spinal cord. The central nervous system of the vertebrate posterior to 
 the brain. The spinal nerves are the nerves given off therefrom. 
 The spinal column is the vertebral column which forms the backbone, 
 and the arches of which protect the spinal cord. This is sometimes 
 called the spine. The neural spine is a median dorsal process of a 
 vertebra. 
 
 Splanchnic. Equivalent to visceral. The terms splanchnic and somatic 
 are the Greek-derived words answering to the Latin derivatives 
 visceral and parietal. 
 
 Spleen. A red organ attached to the mesentery. Splenial, a bone in the 
 lower jaw. 
 
 Squame. A plate-like portion of the antenna of the crayfish, also called 
 the scaphocerite. Squamosal, a bone of the vertebrate skull. 
 
 Sternum. (1) The vertebrate breast-bone. (2) The ventral part of the 
 crayfish's, or insect's exoskeleton. 
 
 Stomodseum. That part of the alimentary canal which arises in develop- 
 ment as an anterior epiblastic invagination, giving rise in.the crayfish, 
 e.g. to the fore-gut ; the mid-gut being derived from the mesenteron, 
 and the hind-gut from the proctodseum. 
 
 Stroma. The connective tissue framework of an organ such as the 
 ovary. 
 
 Sympathetic nerve system. A part of the general nerve system of the 
 
374 ANIMAL BIOLOGY. 
 
 vertebrate which differentiates from the rest, and forms a chain of 
 ganglia, connected with the cerebro- spinal nerves. 
 
 Systole. The contraction of the ventricle or other part of the heart ; the 
 relaxation is called diastole. 
 
 TelolecithaL See Alecithal. 
 
 Tendon. A fibrous band of connective tissue by which a muscle is 
 
 attached to the endoskeleton. 
 
 Tergum. The dorsal part of the exoskeleton of the crayfish and cock- 
 roach. Hence this dorsal region is often termed the tergal region, as 
 
 opposed to the ventral or sternal region. 
 Testes. (1) The male generative organs. (2) The posterior divisions of 
 
 the corpora quadrigemina of the mammalian brain. 
 Thorax. (1) The part of the coelom in front of the diaphragm in the 
 
 vertebrate. (2) The mid-region of the body in the crayfish and 
 
 cockroach. 
 Traoecula. (1) A band of tissue. (2) Special bands of tissue in the 
 
 developing skull. 
 Trochanter. (1) A process of the upper part of the femur. There are 
 
 three such processes in the rabbit. (2) The second joint in the leg 
 
 of the cockroach. 
 Typnlosole. A longitudinal fold projecting inward into the intestine. 
 
 Umbilicus. (1) The inferior and superior umbilicus are small holes in the 
 bird's feathers ; (2) The umbilical vesicle and cord are foetal structures in 
 the rabbit. 
 
 Urinogenital System. Sometimes called urogenital ; the reproductive and 
 excretory organs, the ducts of which are closely related in the verte- 
 brate . 
 
 Utriculus. (1) A division of the vertebrate organ of hearing ; (2) A finger- 
 like process of the mushroom gland of the cockroach. 
 
 Vascular. Connected with the blood. The vascular system is the circu- 
 latory system. An organ is vascular when it is richly supplied with 
 blood-vessels. 
 
 Venous. The blood is venous when the corpuscles have given up their free 
 oxygen to the tissues, and the plasma is laden with carbonic acid. 
 The pulmonary artery contains venous blood. 
 
 Ventricle. (1) The fleshy part of the heart, subdivided in the rabbit and 
 the fowl ; (2) A cavity of the brain, e.g. 4th ventricle, lateral ventricle. 
 
 Vesicle. (1) The primitive vesicles of the brain are the anterior enlargements 
 of the neural tube ; (2) The auditory vesicle is the auditory sac ; (3) 
 The germinal vesicle is the nuclear part of the ovum ; (4) The umbilical 
 vesicle is a provisional fcetal organ in the rabbit ; (5) The vesiculce 
 seminales are accessory parts of the male reproductive organs. 
 
GLOSSARY. 375 
 
 Vestibule. (1) Part of the vertebrate organ of hearing ; (2) That part of 
 the female urinogenital organs of the rabbit which lies just within the 
 vulva ; (3) That part of the urinogenital organs of the snail which lies 
 just within the genital aperture ; (4) The ciliated funnel in vorticella 
 and paramoacium. 
 
 Villus A hair-like projection, multitudes of which give the small intestine 
 of the rabbit a shaggy appearance. Villi are generally developed so as 
 to give additional surface, absorptive (intestine) or vascular (placenta). 
 
 Yolk. Yelk or vitellus. The egg of the fowl consists of (1) an external 
 shell ; (2) An outer shell-membrane ; (3) An inner shell-membrane ; (4) 
 The albumen or white of egg ; (5) The vitelline membrane surrounding 
 (6) the vitellus or yolk, in one special area of which the cicatricula or 
 blastoderm, the embryo, is developed. The great mass of the yolk is 
 composed of yellow yolk, consisting of minute yolk spheres filled with 
 numerous highly refractive granules. Besides the yellow yolk there 
 are concentric layers and an internal flask-shaped mass of white yolk. 
 The term yolk is therefore applied (1) to the vitelline mass in which 
 the embryo is formed, (2) to the yolk spheres, which are to be regarded 
 as stored food-stuff for the embryo during growth within the egg. 
 
INDEX OF TYPES 
 
 Amoeba. Structure, 354 ; method of feeding, 354 ; contractile vesicle, 354 ; 
 multiplication, 354. 
 
 Cockroach. External characters, 257 ; apertures, 263 ; appendages, of the 
 head, 258 ; of the thorax, 261 ; of the abdomen, 262 ; alimentary 
 system, 264 ; circulatory system, 267 ; respiratory system, 268 ; 
 nervous system, 269 ; special senses, 271 ; reproductive system, 273 ; 
 development, 275. 
 
 Cod. External characters, 9; apertures, 9; fins, 9; alimentary system, 
 32 ; respiratory system, 10 ; circulatory system, 33, 196 ; urinogenital 
 (excretory and reproductive) system, 35 ; nervous system, 35. 
 
 Crayfish. External characters, 232 ; apertures, 240 ; appendages, 235 ; 
 alimentary system, 242 ; respiratory system, 238 ; circulatory system, 
 245 ; excretory system, 247 ; reproductive system, 252 ; nervous sys- 
 tem, 247 ; special senses, 249 ; development, 254. 
 
 Earthworm. External characters, 278; apertures, 278-9; setse, 279; 
 body walls and septa, 282 ; alimentary system, 282 ; respiration, 286 ; 
 circulatory system, 284 ; excretory system, 284 ; reproductive system, 
 286 ; nervous system, 286 ; senses, 286 ; development, 289. 
 
 Fowl. Skull, 149; vertebral column, 165; ribs and sternum, 167; 
 pectoral arch, 170 ; fore limb, 171 ; pelvic arch, 173 ; hind limb, 175 ; 
 segmentation of the ovum, 95 ; egg membranes, 96 ; establishment of 
 the germinal layers, 99 ; formation of the neural tube, 102 ; folding 
 off of the embryo, 104 ; development of heart, cerebral vesicles, and 
 mesoblastic somites, 105 ; embryonic appendages, 108 ; amnion, 108 ; 
 allantois, 111 ; hatching, 113; gizzard and proventriculus, 178. 
 
 Frog. External characters, 1 ; apertures, 8 ; limbs, 6 ; alimentary system, 
 22 ; respiratory system, 24 ; circulatory system, 25, 198 ; urinogenital 
 (excretory and reproductive) system, 28 ; nervous system, 29 ; hind- 
 brain, 215 ; mid-brain, 217 ; fore-brain, 218 ; cranial nerves, 221, 223 ; 
 spinal nerves, 224 ; sympathetic system, 225 ; histology of blood, 64 ; 
 of epithelium, 67 ; of connective tissue, 68 ; of cartilage, 68; of bone, 
 70 ; of muscle, 72 ; of nerve, 74 ; of retina, 77 ; of stomach, 78 ; of 
 liver, 80 ; of lung, 81 ; of kidney, 82 ; of skin, 82 ; development, 1 ; 
 segmentation of the ovum, 94 ; egg membranes, 96 ; establishment of 
 the germinal layers, 97 ; formation of the neural tube, 102. 
 
INDEX OF TYPES. 377 
 
 Hydra. External characters, 335; histology, 336 ; nerve-cells, 338; pro- 
 cesses of multiplication, 339 ; development, 341. 
 
 Liver-fluke. Summary of life history, 322 ; distoma stage, 324 ; excretory 
 system, 325 ; nervous system, 325 ; generative system, 325 ; ovum, 
 327 ; free embryo, 327 ; sporocyst, 329 ; redia, 329 ; cercaria, 330. 
 
 Mussel. External characters, 305 ; shell, 307; alimentary system, 309; 
 respiratory system (gills), 309 ; circulatory system, 313 ; excretory 
 system, 315 ; nervous system, 317 ; special senses, 317 ; reproductive 
 system, 318 ; development, 319. 
 
 Pigeon. External characters, 15 ; feathers, 16; apertures, 17 ; limbs, 18 ; 
 alimentary system, 47 ; respiratory system, 48 ; circulatory system, 
 49, 208; embryonic circulation, 211; urino-genital (excretory and 
 reproductive) system, 51 ; nervous system, 52 ; hind-brain, 215 ; mid- 
 brain, 218 ; fore-brain, 218 ; cranial nerves, 224 ; spinal nerves, 225 ; 
 sympathetic system, 226 ; histology of blood, 64. 
 
 Rabbit. External characters, 11; apertures, 12; limbs, 14; hairs, 12; 
 alimentary system, 37 ; teeth, 12, 148 ; glands, 179 ; gland products, 
 181 ; digestion and absorption, 186 ; preliminary metabolism of 
 absorbed products, 188 ; further metabolism, 190 ; respiratory system, 
 39 ; circulatory system, 40, 204 ; urino-genital (excretory and repro- 
 ductive) system, 41 ; nervous system, 43 ; hind-brain, 215 ; mid-brain, 
 218 ; fore-brain, 218 ; cranial nerves, 224 ; spinal nerves, 224 ; sym- 
 pathetic system, 225 ; histology of blood, 64 ; of conjunctival epithe- 
 lium, 67; of bone, 70; of muscle, 72; of papilla foliata, 76; of 
 intestine, 79 ; of sublingual gland, 79 ; of lung, 80 ; of kidney, 81 ; 
 of hair, 83 ; of nails, 84 ; development, segmentation of the ovum, 95 ; 
 egg membranes, 96 ; establishment of the germinal layers, 101 ; forma- 
 tion of the neural or medullary tube, 102 ; foetal membranes, 108 ; 
 amnion, 108 ; allantois, 111 ; chorion, 111 ; placenta, 112. 
 
 Snail. External characters, 291 ; apertures, 292 ; shell, 291 ; epiphragm, 
 293 ; alimentary system, 294 ; circulatory system, 295 ; respiration, 
 296 ; excretory system, 296 ; nerve system, 297 ; special senses, 298 ; 
 reproductive system, 299 ; development of pond-snail, 302. 
 
 Paramcecium. Structure, 351 ; method of feeding, 352 ; movements, 352 ; 
 multiplication, 352. 
 
 Tapeworm. Scolex and proglottides, 331 ; excretory system, 331 ; repro- 
 ductive system, 332 ; life history, 333. 
 
 Vorbicella. Structure, 347 ; method of feeding, 348 ; contractile vesicle, 
 348 ; movements, 348 ; multiplication, 350. 
 
INDEX. 
 
 ABIOGENESIS, 86, 357. 
 
 Abdomen Cockroach, 262 ; Crayfish, 233. 
 
 Abdominal appendages Crayfish, 235. 
 
 Acetabular bone, 172. 
 
 Acetabulum, 172. 
 
 Acinus, 300. 
 
 Acromion, 169. 
 
 Adrenal body Frog, 28 ; Pigeon, 51 ; 
 Rabbit, 43. 
 
 ^Etiology, xx. 
 
 Afferent nerve-fibre, 56. 
 
 After-birth Rabbit, 112. 
 
 Air-bladder Cod, 32. 
 
 Air-sacs Pigeon, 46. 
 
 Alaj cordis Crayfish, 241. 
 
 Albumen gland Snail, 301. 
 
 Alecithal ovum, 114. 
 
 Alimentary system Cercaria, 330; Cock- 
 roach, 264; Cod, 32; Crayfish, 242; 
 Earthworm, 282; Frog, 22; Liver-fluke, 
 325; Mussel, 309; Pigeon, 47; Rabbit, 
 37 ; Redia, 330 ; Snail, 294. 
 
 Ali - sphenoid Fowl, 149 ; Frog, 155 ; 
 Rabbit, 142. 
 
 Allantois, 111. 
 
 Alternation of generations, 342. 
 
 Altrices, 15. 
 
 Alveolus, 79, 179. 
 
 Ametabola, 257. 
 
 Amnion Cockroach, 276 ; Vertebrate, 108. 
 
 Amoeba, 354. 
 
 Amphioxus, development of, 114. 
 
 Ampullae, 134. 
 
 Anabolism, 183. % 
 
 Analogous organs, 18. 
 
 Anapophyses, 164. 
 
 Anastates, 183. 
 
 Anatomy, xix. 
 
 Angular bone Fowl, 154. 
 
 Angulo-splenial Frog, 158. 
 
 Animal compared with plant, xvi. 
 
 Anodonta, 305. 
 
 Antenna Cockroach, 258 ; Crayfish, 235. 
 
 Antennule Crayfish, 235. 
 
 Anus, or vent Cockroach, 266 ; Cod, 10 ; 
 Crayfish, 240 ; Earthworm, 289 ; Frog, 8 ; 
 Mussel, 307; Paramoecium, 352; Pigeon, 
 17; Rabbit, 14; Snail, 292; Vorticella, 
 347. 
 
 Aorta. See Arteries. 
 
 Aortic arches, development of, 202. 
 
 Apertures, external Cockroach, 263 ; Cod, 
 
 9 ; Crayfish, 240 ; Earthworm, 279 ; Frog, 
 
 8; Mussel, 306; Pigeon, 17 ; Rabbit, 12 ; 
 
 Snail, 292. 
 Aponeurosis, 49. 
 Appendages Cockroach, 261 ; Crayfish, 
 
 233. 
 
 Appendicnlar skeleton, 6, 139. 
 Appendix vermiformis Rabbit, 39. 
 Apteria Pigeon, 16. 
 Aqueous humour, 129. 
 Arachnoid, 44. 
 
 Archenteron, 91 Crayfish, 254. 
 Area opaca, 99. 
 Area pellucida, 99. 
 Arteries Cockroach, 267; Cod, 33, 197; 
 
 Crayfish, 241, 246 ; Embryo, 211 ; Frog, 
 
 25, 199; Mussel, 314; Pigeon, 49, 208; 
 
 Rabbit, 40, 205 ; Snail, 296. 
 Arthrobranchia Crayfish, 239. 
 Arthrophragm Crayfish, 234. 
 Arthropoda, 237. 
 Articular bone Fowl, 154, 162. 
 Astacus, 231. 
 Astragalus, 175. 
 Atlas vertebra, 163. 
 Atrium. See Heart. 
 Atrophy, 4. 
 Auditory capsule, 139 ; ossicles, 146 ; pit, 
 
 135 ; region of skull Fowl, 152 ; Frog, 
 
 157 ; Rabbit, 146. 
 Auditory organ Cockroach, 271 ; Crayfish, 
 
 249 ; Mussel, 318 ; Snail, 299 ; Vertebrate, 
 
 132. 
 
 Auricle. See Heart. 
 Axial skeleton, 6, 139. 
 Axis vertebra, 163. 
 
 BARB Pigeon, 17. 
 Barbule Cod, 11. 
 
 of feather- Pigeon, 17. 
 
 Basilar plate, 159. 
 
 Basi-occipital Fowl, 149; Rabbit, 141, 
 
 160. 
 
 Basipodite Crayfish, 236. 
 Basi-sphenoid Fowl, 149 ; Rabbit, 142, 
 
 160. 
 
 Basi-temporal Fowl, 149, 163. 
 Bile, 185, 189. 
 Bile-ducts Cod, 33 ; Frog, 24 ; Pigeon, 47 ; 
 
 Rabbit, 38. 
 
INDEX. 
 
 Biogenesis, 86, 357. 
 
 Bladder Frog, 29; Rabbit, 43; Develop- 
 ment of, 112. 
 
 Bladder- worm Tapeworm, 334. 
 
 Blastocoele, 91 Earthworm, 289 ; Frog, 97. 
 
 Blastoderm Fowl, 99. 
 
 Blastodermic layer Cockroach, 275 ; Cray- 
 fish, 254. 
 
 Blastodermic vesicle Rabbit, 101. 
 
 Blastomere, 2. 
 
 Blastophor Earthworm, 288. 
 
 Blastopore, 91 Crayfish, 254 ; Earthworm, 
 289 ; Frog, 2, 97 ; Pond-snail, 302. 
 
 Blastosphere, 91. 
 
 Blatta, 257. 
 
 Blood Cockroach, 267 ; Crayfish, 247 ; 
 Earthworm, 284 ; Mussel, 314 ; Snail, 296 ; 
 Vertebrate, 59, 64, 118. 
 
 Blood circulation, 194. 
 
 Blood-vessels. See Arteries and Veins. 
 
 Body-cavity. See Ccelom. 
 
 Bojanus, organs of Mussel, 315. 
 
 Bone, structure of, 69; development of, 
 120. 
 
 Brain Cod, 35 ; Frog, 30, 215 ; Pigeon, 52, 
 215 ; Rabbit, 43, 215. 
 
 Brain-case Fowl, 149 ; Frog, 155 ; Rabbit, 
 140. 
 
 Branchiae Cod, 10; Crayfish, 239; Tad- 
 pole, 4. 
 
 Branchial aperture Tadpole, 4. 
 
 Branchial chamber Crayfish, 238. 
 
 clefts and arches Tadpole, 3. 
 
 Branchio-cardiac groove Crayfish, 233. 
 
 Branchiostegal membrane and rays Cod, 
 10. 
 
 Bran chiostegite Crayfish, 234. 
 
 Bronchi Pigeon, 48 ; Rabbit, 40. 
 
 Brunner's glands, 180. 
 
 Buccal cavity Cockroach, 265 ; Earth- 
 worm, 283. 
 
 Buccal glands Rabbit, 179. 
 
 Budding Hydra, 339. 
 
 Bulbus arteriosus, 33. 
 
 Bulla tympani Rabbit, 146. 
 
 Bursa Fabricii Pigeon, 48, 52. 
 
 Byssus Mussel, 320. 
 
 C^CAL tubes Cockroach, 265. 
 
 Caecum coli Pigeon, 48; Rabbit, 39. 
 
 Caecum of air-bladder Cod, 32. 
 
 Calcaneum, 175. 
 
 Calcar Frog, 176. 
 
 Calciferous glands Earthworm, 283. 
 
 Capillary plexus, 60. 
 
 Capitellum, 171. 
 
 Capitular facet, 164. 
 
 Capitulum of rib, 167. 
 
 Capsulogenous organs Earthworm, 289. 
 
 Carapace Crayfish, 232. 
 
 Cardiac glands Rabbit, 180. 
 
 Cardo Cockroach, 260. 
 
 Carina Fowl, 168. 
 
 Carpus, 171. 
 
 Cartilage, histology of, 68 ; development of, 
 
 120. 
 Cartilage, bones of skull, 163 ; development 
 
 of, 120. 
 
 Cement of tooth, 13, 72. 
 
 Cement substance, 67. 
 
 Centrale of manus, 170 ; of pes, 174. 
 
 Centrolecithal ovum Cockroach, 275 ; 
 
 Crayfish, 254. 
 Centrum of vertebra, 163. 
 Cephalo-thorax Crayfish, 232. 
 Cercaria Liver-fluke, 330. 
 Cerci Cockroach, 263. 
 Cere Pigeon, 16. 
 Cerebellar fossa, 144. 
 Cerebellum Cod, 35 ; Frog, 30, 217 ; 
 
 Pigeon, 52, 217 ; Rabbit, 44, 217. 
 Cerebral fossa, 144. 
 Cerebral hemispheres Cod, 35 ; Frog, 30, 
 
 219 ; Pigeon, 52, 219 ; Rabbit, 44, 219. 
 Cerebral vesicles, 104, 106. 
 Cervical ganglia Rabbit, 226. 
 Cervical groove Crayfish, 233. 
 Chalazae Fowl, 96. 
 Chelae- Crayfish, 235. 
 Chiasma, optic, 218. 
 Chitinogenous layer Cockroach, 258. 
 Chlorophyll Hydra, 338. 
 Chondrocranium Frog, 158. 
 Chorion, 111, 273. 
 Choroid coat of eye, 131. 
 Choroid plexus, 220. 
 Chromatophores Frog, 7, 68. 
 Chyle, 189. 
 
 Chylitic stomach Cockroach, 266. 
 Chyme, 186. 
 
 Cilia Paramoecium, 351 ; Vorticella, 348. 
 Ciliary processes, 131. 
 Ciliated epithelium, 67. 
 Cingulurn Earthworm, 278. 
 Circulation of blood, 60, 194 ; Embryonic. 
 
 211. 
 Circulatory system Cockroach, 267; Cod, 
 
 25, 196 ; Crayfish, 245 ; Earthworm, 284 ; 
 
 Frog, 33, 198; Mussel, 313; Pigeon, 40, 
 
 208 ; Rabbit, 40, 204 ; Snail, 295. 
 Circulus cephalicus Cod, 33, 197. 
 Cirrus sac Liver-fluke, 326. 
 Clavicle, 169. 
 
 Cleaveage of Frogs' ovum, 2, 94. 
 Cleavage of mesoblast, 93. 
 Clinoid processes Rabbit, 144. 
 Clitellum Earthworm, 278. 
 Cloaca Frog, 8, 29 ; Pigeon, 17, 51. 
 Clypeus- Cockroach, 258. 
 Cnidocil Hydra, 338. 
 Cochlea, 134. 
 Cockroach, 257. 
 Coelenterata, 342. 
 
 Coelom, or body-cavity Cod, 232; Cock- 
 roach, 276 ; Earthworm, 280 ; Frog, 23 ; 
 
 Pigeon, 46 ; Rabbit, 37 ; Snail, 293. 
 Coelomata, 342. 
 Coleoptera, 257. 
 Collar Snail, 291. 
 Colleterial glands Cockroach, 274. 
 Colon Cockroach, 266 ; Rabbit, 39. 
 Columella auris, 132, 152. 
 
 of snail's shell, 291. 
 
 Commissures of brain, 218, 219. 
 
 Condyle of skull Fowl, 149 ; Frog, 15o ; 
 
 Rabbit, 141. 
 
INDEX. 
 
 Conjugation Paramoecium, 352 ; Vorticella, 
 350. 
 
 Connective tissue, histology of, 67 ; de- 
 velopment of, 119. 
 
 Contractile vesicle Amoeba, 354; Monad, 
 358 ; Paramoecium, 352 ; Vorticella, 347. 
 
 Coracoid, 47, 169. 
 
 Cornea, 129. 
 
 Gornicula laryngi Rabbit, 39. 
 
 Coronal suture Rabbit, 144. 
 
 Coronoid process Rabbit, 148. 
 
 Corpus, or corpora. Adippsa Cockroach, 
 266 ; Frog, 29. Bigemina ; see Optic 
 lobes. Callosuin Rabbit, 218. Mamillare 
 Rabbit, 218. Quadrigemina Rabbit, 
 218. Restiformia, 217. Striata, 219. 
 Trepezoidea Rabbit, 217. 
 
 Corti, fibres of Rabbit, 135. 
 
 Coxa Cockroach, 261. 
 
 Coxopodite Crayfish, 236. 
 
 Crayfish, 231. 
 
 Cribriform plate Rabbit, 142. 
 
 Cristse acusticse Rabbit, 134. 
 
 Crop Pigeon, 45. 
 
 Crura cerebri, 218. 
 
 Crustacea, 231. 
 
 Crystalline lens, 129. 
 
 Cuticle Cockroach, 258 ; Crayfish, see Exo- 
 skeleton ; Earthworm, 282 ; Mussel, see 
 Shell ; Snail, see Shell ; Vorticella, 347. 
 
 Cysticercus Tapeworm, 334. 
 
 Cytozoa, xvi. 
 
 DACTYLOPODITE Crayfish, 236. 
 
 Dart sac Snail, 301. 
 
 Deciduate placenta Rabbit, 112. 
 
 Dental capsule, 129. 
 
 Dental papilla, 1 28. 
 
 Dentary bone Fowl, 154 ; Frog, 158. 
 
 Dentine, 13, 71. 
 
 Dennis, 82. 
 
 Development Subdivisions of, xix ; Am- 
 
 phioxus, 114 ; Cercaria, 330 ; Cockroach, 
 
 275 ; Crayfish, 254 ; Earthworm, 289 ; 
 
 Hydra, 341 ; Medusa, 344 ; Mussel, 805 ; 
 
 Pond-snail, 319 ; Vertebrate, 94. 
 Dextrin, 181. 
 Diaphragm Rabbit, 37. 
 Diaphysis, 121, 165. 
 Diastema, 148. 
 Digestion, 186. 
 Digestive gland Crayfish, 245; Mussel, 
 
 309 ; Snail, 295. 
 Digits. See Manus and Pes. 
 Diploe, 121. 
 Disc, Vorticella, 347. 
 Distribution, xx. 
 Ductus arteriosus, 211. 
 Duodenum Frog, 24 ; Pigeon, 47 ; Rabbit, 
 
 38. 
 Dura mater, 44. 
 
 EAR. See Auditory organ. 
 Earthworm, 278. 
 Ecdysis Crayfish, 231 ; Frog, 5. 
 Ectoderm Hydra, 341. 
 Ectosarc Amoeba, 354. 
 Ectostracum Crayfish, 238. 
 Egg membranes, 96. 
 
 Ejaculatory duct, 275. 
 
 Embryology, Vertebrate, 86. 
 
 Embryonic membranes, 108. 
 
 Enamel, 13, 71. 
 
 organ, 128. 
 
 Encystment AmcBba, 355 ; Vorticella, 350 
 
 Endoderm Hydra, 341. 
 
 Endolymph, 133. 
 
 Endomysium, 72. 
 
 Endoneurium, 74. 
 
 Endophragmal system Crayfish, 234. 
 
 Endopleurite Crayfish, 234. 
 
 Endopodite Crayfish, 236. 
 
 Endosarc Amoeba, 354. 
 
 Endosternite Crayfish, 234. 
 
 Endostracum Crayfish, 238. 
 
 Endothelium, 65. 
 
 Epiblast. See Germinal layers. 
 
 Epicranium Cockroach, 258. 
 
 Epidermis, 83. 
 
 Epiglottis Rabbit, 39. 
 
 Epimeron Crayfish, 233. 
 
 Epiostracum Crayfish, 238. 
 
 Epiotic Fowl, 152 ; Frog, 157 ; Rabbit, 146. 
 
 Epiphysis, 121, 165. 
 
 Epipodite Crayfish, 236. 
 
 Epithelium, 65. 
 
 Ethmoidal plane Rabbit, 144. 
 
 Ethmoturbinal Rabbit, 145. 
 
 Eustachian tube or recess Fowl, 149; 
 Frog, 8, 132, 161 ; Pigeon, 17 ; Rabbit, 14. 
 
 valve Rabbit, 208, 213. 
 
 Exhalent siphon Mussel, 306. 
 
 Exoccipital Fowl, 149 ; Frog, 155 ; Rabbit, 
 141, 160. 
 
 Exopodite Crayfish, 236. 
 
 Exoskeleton Cod, 10; Cockroach, 258; 
 Crayfish, 238 ; Frog, 7 ; Pigeon, 16 ; 
 Rabbit, 12. 
 
 Eye Cockroach, 272 ; Crayfish, 250 ; Liver- 
 fluke, 327 ; Snail, 298 ; Vertebrate, 129. 
 
 FALCIFORM LIGAMENT Pigeon, 47. 
 
 Fallopian tube Rabbit, 43. 
 
 Falx cerebri Rabbit, 220. 
 
 Fat body Cockroach, 266 ; Frog, 29. 
 
 Fat cells, 68. 
 
 FeathersPigeon, 16, 18. 
 
 Femur, 174. 
 
 Fenestra ovalis Fowl, 135, 152 ; Frog, 133, 
 157 ; Rabbit, 135, 146. 
 
 Fenestra rotunda Fowl, 135 ; Rabbit, 135, 
 146. 
 
 Fertilisation. See Impregnation. 
 
 Fibrin, 59. 
 
 Fibula, 174. 
 
 Fibulare, 174. 
 
 Filoplume. See Feathers. 
 
 Fins Cod, 9. 
 
 Fission Amoeba, 354 ; Hydra, 339 ; Para- 
 moecium, 352 ; Vorticella, 350. 
 
 Fissure of Sylvius Rabbit, 219. 
 
 Flagellum Monad, 358 ; Snail, 301. 
 
 Floccular fossa Rabbit, 143, 146. 
 
 Flocculus Rabbit, 217. 
 
 Foetal membranes, 108. 
 
 Folding-off of the embryo, 104. 
 
 Fontanelle Frog, 158. 
 
INDEX. 
 
 Food-stuffs, 186. 
 
 Foot Hydra, 335 ; Mussel, 306 ; Snail, 291. 
 
 Foramina of skull Fowl, 151 ; Frog, 156 ; 
 
 Rabbit, 143. 
 Fore-brain Cod, 35; Frog, 30, 218 ; Pigeon, 
 
 52, 106, 218; Rabbit, 44, 218; functions 
 
 of, 56, 226. 
 Fore-limb Frog, 6; Pigeon, 18; Rabbit, 
 
 14. 
 
 Fore-gut Cockroach, 276; Crayfish, 243. 
 Fornix Rabbit, 220. 
 Fossa ovalis Rabbit, 208. 
 Fossajof skull Fowl, 151 ; Rabbit, 144. 
 Frontal bones Fowl, 151 ; Rabbit, 142. 
 
 suture Rabbit, 144. 
 
 Fronto-nasal process, 127. 
 Furcula Fowl, 170 ; Pigeon, 46. 
 
 GALE A Cockroach, 260. 
 
 Gall-bladder Cod, 33 ; Frog, 24 ; Rabbit, 
 38. 
 
 Ganglia. See Nervous system. 
 
 Gastric glands, 178, 180. 
 
 juice, 183. 
 
 mill Crayfish, 242. 
 
 Gastrolith Crayfish, 245. 
 
 Gastrula, 91 Earthworm, 289 ; Liver-fluke, 
 329 ; Pond-snail, 303. 
 
 Gena Cockroach, 258. 
 
 Generations, alternation of Liver-fluke, 
 323. 
 
 Generative ducts, 136. 
 
 Generative organs. See Reproductive 
 organs. 
 
 Genital ridge, 87. 
 
 Genua of corpus callosum Rabbit, 221. 
 
 Germinal cells Liver-fluke, 329. 
 
 disc, 88. 
 
 epithelium, 86. 
 
 layers, 97 Cockroach, 275; Crayfish, 
 
 254; Earthworm, 289; Fowl, 99; Frog, 
 97 ; Hydra, 341 ; Mussel, 319 ; Pond- 
 snail, 302 ; Rabbit, 101. 
 
 spots, 87. 
 
 vesicle, 87. 
 
 walls, 100, note. 
 
 Gills Cod, 10 ; Crayfish, 238. 
 
 Gill-cleftstadpole, 4, 161. 
 
 Gill-rakers Cod, 10. 
 
 Ginglymus Cockroach, 258. 
 
 Gizzard Cockroach, 265 ; Earthworm, 283 ; 
 Pigeon, 47, 179. 
 
 Gland, albumen Snail, 301. 
 
 Brunner's Rabbit, 180. 
 
 buccal Rabbit, 179. 
 
 calciferous Earthworm, 280. 
 
 cardiac Rabbit, 180. 
 
 colleterial Cockroach, 274. 
 
 digestive - Cockroach, 266; Crayfish, 
 
 245 ; Mussel, 309 ; Snail, 295. 
 
 gastric Rabbit, 180. 
 
 green Crayfish, 247. 
 
 Harderian Rabbit, 38. 
 
 hermaprodite Snail, 300. 
 
 infra-orbital Rabbit, 179. 
 
 intestinal Rabbit, 180. 
 
 lachrymal Rabbit, 38. 
 
 Lieberkuhn's Rabbit, 180. 
 
 Gland, lymphatic Pigeon, 45 ; Rabbit, 65, 
 190. 
 
 mammary Rabbit, 14. 
 
 muciparous Snail, 301, 
 
 mushroom-shaped Cockroach, 274. 
 
 parotid Rabbit, 179. 
 
 pedal Snail, 292. 
 
 perineal Rabbit, 14. 
 
 pyloric Rabbit, 180. 
 
 rectal Rabbit, 39. 
 
 salivary Cockroach, 265 ; Rabbit 
 
 179 ; Snail, 294. 
 
 sebaceous Rabbit, 84. 
 
 shell Liver-fluke, 326; Pond-snail 
 
 303. 
 
 sublingual Rabbit, 179. 
 
 submaxillary Rabbit, 179. 
 
 sweat Rabbit, 84. 
 
 thymous Rabbit, 40. 
 
 thyroid Rabbit, 39. 
 
 yolk Liver-fluke, 326; Tapeworm, 
 
 333. 
 
 products, digestive Vertebrate, 181. 
 
 Glochidia Mussel, 320. 
 
 Glomerulus, 81, 136. 
 
 Glottis Frog, 8 ; Pigeon, 17 ; Rabbit, 14, 39. 
 
 Glucose, 181. 
 
 Glycogen, 189. 
 
 Gonophore, 342. 
 
 Gonapophyses Cockroach, 262. 
 
 Graafian follicle Rabbit, 43. 
 
 Green glands Crayfish, 240, 247. 
 
 Growth, xii. 
 
 Gubernaculum Monad, 358. 
 
 Gullet. See ^Esophagus. 
 
 HAEMOGLOBIN, 59. 
 
 Hairs Rabbit, 12, 83. 
 
 Hand. See Maims. 
 
 Hatteria, pineal body of, 218. 
 
 Head Cod, 9 ; Cockroach, 257 ; Frog, 6 ; 
 
 Pigeon, 15 ; Rabbit, 11 ; Snail, 291. 
 
 kidney, 35, 136. 
 
 Head-fold, 104. 
 
 Head-papilla Liver-fluke, 327. 
 Hearing, 57. 
 
 organ of. See Auditory organ. 
 
 Heart Cod, 33, 196; Cockroach, 267; 
 
 Crayfish, 245; Earthworm, 285; Frog, 
 
 25, 203; Mussel, 313; Pigeon, 49, 211 
 
 Rabbit, 40, 207. 
 Helix aspersa, 291. 
 Hepatic cells Earthworm, 295. 
 Hermaphrodite duct Snail, 300. 
 
 gland Snail, 300. 
 
 Heteromita, 358. 
 
 Hind-brain Cod, 35 ; Fowl, 106 ; Frog, 30 
 
 215 ; Pigeon, 52, 215 ; Rabbit, 44, 215. 
 
 functions of, 56, 226. 
 
 Hind-gut Cockroach, 276 ; Crayfish, 255. 
 
 Hind-limb Frog, 6 ; Pigeon, 18 ; Rabbit, 14. 
 
 Hip-girdle, 172. 
 
 Hippocampus, major Rabbit, 220. 
 
 Histology, ix, 63. 
 
 Histolysis, 341. 
 
 Histozoa, vi. 
 
 Holoblastic segmentation, 94, 114. 
 
 Holometabola, 257. 
 
INDEX. 
 
 383 
 
 Holotricha, 351. 
 
 Homologous organs, IS. 
 
 Humerus, 170. 
 
 Hybernaculum Snail, 293. 
 
 Hydra, 335. 
 
 Hydranth, 342. 
 
 Hydrophilus, 273. 
 
 Hydrosome, 342. 
 
 Hyoid arch, 162. 
 
 bone or cartilage Fowl, 154 ; Frog, 
 
 158 ; Rabbit, 149. 
 Hypapophyses, 164. 
 Hypoblast. See Germinal layers. 
 Hypocleidium Fowl, 170. 
 Hypodermis Cockroach, 258 ; Earthworm, 
 
 282. 
 
 Hypophragm Snail, 293. 
 Hypostome Hydra, 335. 
 
 ILEUM Cockroach, 266; Pigeon, 48; Rab- 
 bit, 38. 
 
 Ilium, 172. 
 
 Impregnation, 89, 115. 
 
 Incisor teeth, 13, 148. 
 
 Incus Rabbit, 147, 162. 
 
 Infrabranchial chamberMussel, 306, 310. 
 
 Infundibulum of Brain, 218. 
 
 Inhalent siphon Mussel, 306. 
 
 Innominate bone, 172. 
 
 Inogen, 190. 
 
 Intermedium of manus, 170, 
 
 of pes, 174. 
 
 Internasal septum, 128. 
 
 Interorbital septum Fowl, 151. 
 
 Interparietal Rabbit, 142. 
 
 Interstitial cells Hydra, 337. 
 
 Intervertebral foramen, 164. 
 
 Intestine Cod, 33 ; Cockroach, 266 ; Cray- 
 fish, 245; Frog, 23; Earthworm, 283; 
 Liver-fluke, 325 ; Mussel, 309 ; Pigeon, 48 ; 
 Rabbit, 38 ; Snail, 295. 
 
 Intestinal glands, 178 180. 
 
 Intus-susception, xiii. 
 
 Imagination, 91. 
 
 Iris of eye, 129. 
 
 Irritability, 57. 
 
 Ischiopodite Crayfish, 236. 
 
 Ischium, 172. 
 
 Iter of brain, 218. 
 
 JACOBSOX'S ORGAN Rabbit, 14. 
 
 Jaw Crayfish, 242 ; Fowl, 153 ; Frog, 157 ; 
 
 Rabbit, 148 ; Snail, 294. 
 Jejunum Pigeon, 48. 
 Jugal bone Fowl, 153; Rabbit, 148. 
 
 KATABOLISM, 183. 
 Katastates, 183. 
 Karyokinesis, 89. 
 Keber's organ Mussel, 309. 
 Kidney. See Renal organ. 
 
 histology of, 81. 
 
 development of, 136. 
 
 LABIAL CARTILAGE, 162. 
 
 palps Cockroach, 258. 
 
 Labium Cockroach, 258. 
 
 Labrum Cockroach, 258 ; Crayfish, 242. 
 
 Labyrinth of ear, 133. 
 
 Lacinia Cockroach, 260. 
 
 Lacrymal bone Fowl, 153 ; Rabbit, 145. 
 
 gland Rabbit, 38. 
 
 Lacteal, 61, 65, 187. 
 
 Lacunar tissue Mussel, 313. 
 
 Lambdoidal suture Rabbit, 144. 
 
 Lamella, supporting Hydra, 337 
 
 Lamina dorsales, 102. 
 
 perpendicularis Rabbit, 144. 
 
 spiralis Rabbit, 135. 
 
 terminalis of brain, 219. 
 
 Larynx Pigeon, 48 ; Rabbit, 39. 
 
 Lateral line Cod, 11. 
 
 plates of mesoblast, 93, 99. 
 
 ventricles of brain, 219. 
 
 Leg Cockroach, 261 ; Crayfish, 235 ; Verte- 
 brate, see Hind-limb. 
 
 Lens, 129. 
 
 Leucocytes, 64. 
 
 Lieberkuhn's crypts, 180. 
 
 Ligament Mussel, 306. 
 
 Limbs Cod, 9; Frog, 6; Pigeon, 18; 
 Rabbit, 14. 
 
 Limnseus, development of, 302. 
 
 Liver Cod, 32 ; Frog, 23 ; Pigeon, 48 , 
 Rabbit, 38. 
 
 histology of, 80, 180. 
 
 development of, 125. 
 
 Liver-fluke, 322. 
 
 Living things, nature of, ii. 
 
 Lobi inferiores Cod, 35. 
 
 Lumbricus, 278. 
 
 Lungs Frog, 24 ; Pigeon, 48 ; Rabbit, 37. 
 
 histology of, 80. 
 
 development of, 125. 
 
 Lymph, 28, 64, 189, 194. 
 
 Lymphatic gland Pigeon, 45 ; Rabbit, 65, 
 190. 
 
 MACULAE ACUSTKLE, 134. 
 
 Malar bone Rabbit, 148. 
 
 Male pronucleus, 89. 
 
 Malleus Rabbit, 147, 162. 
 
 Malpighian tubules Cockroach, 266. 
 
 Mammary glands Rabbit, 14. 
 
 Mandible Cockroach, 258 ; Crayfish, 2:55 ; 
 
 Fowl, 153 ; Frog, 157 ; Rabbit, 148. 
 Mandibiilar arch, 162. 
 Mantle Mussel, 306 ; Snail, 293. 
 Manubrium or shaft of malleus Rabbit, 
 
 147. 
 
 of sternum Rabbit, 167. 
 
 Manubrium Medusa, 343. 
 
 Manus Frog, 6 ; Pigeon, 18 ; Rabbit, 14. 
 
 osteology of, 170. 
 
 Marginal bodies Medusa, 343. 
 
 Mastication, 186. 
 
 Mastoid portion of periotic, 146. 
 
 Maturation of ovum, 87. 
 
 Maxilla Cockroach, 258 ; Crayfish, 235 ; 
 
 Fowl, 153 : Frog, 157 ; Rabbit, 145, 148. 
 Maxillary palps Cockroach, 258. 
 processes, inferior and superior. 127, 
 
 161. 
 region of skull Fowl, 153 ; Frog, 157 ; 
 
 Rabbit, 148. 
 Maxillo-palatine processes Fowl, 153. 
 
3^4 
 
 INDEX. 
 
 Maxillo-turbinals Rabbit, 145. 
 Meatus, auditory, external, 134, 146. 
 
 internal, 143, 146. 
 
 Meckel's cartilage, 161 Fowl, 154 ; Frog, 
 
 158. 
 Medulla oblongata Cod, 35 ; Frog, 30 ; 
 
 Pigeon, 52 ; Babbit, 44. 
 Medullary or neural folds, 2. 
 
 plate, groove, or canal, 2, 92, 102 
 
 Medusa, 343. 
 
 Membrane bone, growth of, 120. 
 
 bones of skull, 163. 
 
 Mento-meckelian bone Frog, 158. 
 Mentum Cockroach, 269. 
 Meroblastic segmentation, 95, 113. 
 Meropodite Crayfish, 236. 
 Mesencephalon, 217. 
 Mesenteron, 93, 98. 
 
 development of, in chick, 124. 
 
 Mesentery, 23. 
 
 development of, 126. 
 
 Mesethmoid Fowl, 151 ; Rabbit, 145. 
 
 Mesoblast. See Germinal layers. 
 
 Mesoblastic plates, 93. 
 
 Mesoblastic somites, 95, 106. 
 
 Mesonephros, development of, 136. 
 
 Mesonotum Cockroach, 260. * 
 
 Mesostates, 182. 
 
 Mesosternum Cockroach, 260. 
 
 Mesotarsal, articulation Fowl, 175: 
 
 Mesothorax Cockroach, 258. 
 
 Metabolic life-pyramid, 192. 
 
 Metabolism, 177. 
 
 Metacarpus, 171. 
 
 Metacromion Rabbit, 168. 
 
 Metameres Tapeworm, 331. 
 
 Metamorphosis, 5, 167. 
 
 Metanephros, development of, 136 
 
 Metanotum Cockroach, 260. 
 
 Metapophysis Rabbit, 164. 
 
 Metasternum Cockroach, 260. 
 
 Metastoma Crayfish, 242. 
 
 Metatarsus, 174. 
 
 Metathorax Cockroach, 258. 
 
 Metazoa, xvi. 
 
 Metencephalon, 217. 
 
 Micropyle Cockroach, 274. 
 
 Microscope, xv. 
 
 Mid-brain Cod, 35 ; Fowl, 106 ; Frog, 
 
 30, 217 ; Pigeon, 52, 217 ; Rabbit, 44, 
 
 217. 
 
 functions of, 56, 226. 
 
 Mid-gut Cockroach, 276 ; Crayfish, 243. 
 
 Modiolus Rabbit, 134. 
 
 Molar teeth Rabbit, 13, 148. 
 
 Monad, 857. 
 
 Morphology, xix. 
 
 Moru la Liver-fluke, 329. 
 
 Mounting for the microscope, 63. 
 
 Mouth Cockroach, 265 ; Cod, 9 ; Crayfish, 
 
 242 ; Earthworm, 282 ; Frog, 8 ; Hydra, 
 
 335; Mussel, 309; Pigeon, 17; Rabbit, 
 
 12, 127 ; Snail, 294. 
 Mucin, 182. 
 Mullerian duct, 137. 
 Muscle, striped, histology of, 72. 
 
 unstriped, histology of, 73. 
 
 development of, 119. 
 
 Muscle processes Hydra, 337. 
 
 Muscular impressions Mussel, 307. 
 
 Mushroom-shaped gland Cockroach, 274. 
 
 Mussel, 305. 
 
 Myelencephalon, 215. 
 
 Myelon, 29. 
 
 Myophan layer Vorticella, 347. 
 
 NACRE OF SHELL Mussel, 307 ; Snail, 291. 
 Nares, Cod, 9; Frog, 8; Pigeon, 18; 
 
 Rabbit, 13. 
 
 Narial passages Rabbit, 145. 
 Nasal bone Fowl, 152 ; Frog, 156 ; Rabbit, 
 
 145. 
 
 Nasal grooves and pits, 127. 
 Naso-palatine canal Rabbit, 13. 
 Naso-turbinal Rabbit, 145. 
 Nates of mammalian brain, 218. 
 Nauplius, 255. 
 Nectocalyx Medusa, 343. 
 Nematocyst Hydra, 337. v 
 Nephridia Earthworm, 279, 284. 
 Nerve, histology of, 74. 
 
 development of, 119. 
 
 endings, 76. 
 
 Nerves, cranial, 221. 
 
 spinal, 224. 
 
 sympathetic, 225. 
 
 Nervous arc, 56. 
 
 Nervous system Cockroach, 269 ; Cod, 35 ; 
 
 Crayfish, 247; Earthworm, 286; Frog, 
 
 29 ; Medusa, 344 ; Mussel, 317 ; Pigeon, 
 
 52 ; Rabbit, 43 ; Snail, 297. 
 
 functions of, 226. 
 
 Nervures Cockroach, 261. 
 
 Neural tube, 102. 
 
 Neural spine, 163. 
 
 Neurenteric canal, 93. 
 
 Neuroses, 227. 
 
 Nose. See Olfactory organ. 
 
 Notochord, 93, 165 Fowl, 101, 106 ; Frog, 
 
 99, 103 ; Rabbit, 102. 
 Nucleus, xv. 
 
 Nucleus pulposus Rabbit, 165. 
 Nutrition, 177. 
 Nyctotherus, 352. 
 
 OBTURATOR FISSURE OR FORAMEN, 173. 
 
 Ocullus Bee, 273. 
 
 Occipital bone Rabbit, 140. 
 
 condyle Fowl, 149 ; Frog, 155 ; 
 
 Rabbit, 141. 
 
 foramen Cockroach, 258. 
 
 plane Rabbit, 144. 
 
 Odontoblasts, 71, 129. 
 
 Odontoid process, 165. 
 
 Odontophore Snail, 294. 
 
 CE.sophagus Cockroach, 265 ; Cod, 32 ; 
 
 Crayfish, 242 ; Earthworm, 283 ; Frog, 23 ; 
 
 Mussel, 309; Pigeon, 47; Rabbit, 38; 
 
 Snail, 294. 
 Olecranon, 171. 
 Olfactory capsule, 139. 
 
 foramina Rabbit, 43. 
 
 fossa Rabbit, 144. 
 
 lobes Cod, 35 ; Frog, 30, 221 ; Pigeon, 
 
 52, 221 ; Rabbit, 44, 221. 
 
INDEX. 
 
 385 
 
 Olfactory organ Cockroach, 271 ; Crayfish, 
 249 ; Mussel, 318 ; Snail, 299. 
 
 region of skull Fowl, 152 ; Frog, 156 ; 
 
 Rabbit, 144. 
 
 Olivary tract Rabbit, 217. 
 
 Omentum Pigeon, 46. 
 
 Omosternura Frog, 169. 
 
 Operculum Cod, 10 ; Frog, 4. 
 
 Ophthalmite Crayfish, 235. 
 
 Opisthotic Fowl, 152 ; Frog, 157 ; Rabbit, 
 146. 
 
 Optic capsule, 139. 
 
 chiasma and tracts. 218. 
 
 commissure, 218. 
 
 cup, 131. 
 
 foramen Fowl, 151 ; Frog, 155 ; 
 
 Rabbit, 143. 
 
 lobes Cod, 30 ; Frog, 30, 217 ; Pigeon, 
 
 52, 217 ; Rabbit, 44, 217. 
 
 pit, 131. 
 
 region of skull Fowl, 152 ; Frog, 157 ; 
 
 Rabbit, 147. 
 
 thalami, 218. 
 
 vesicles, 131. 
 
 Ora serrata, 131. 
 
 Orbicular bone Rabbit, 147. 
 
 Orbit Fowl, 153 ; Rabbit, 147. 
 
 Orbital plate Fowl, 151. 
 
 Orbital processes of frontal Rabbit, 142. 
 
 presphenoid Rabbit, 142. 
 
 Orbito-sphenoid Fowl, 151 ; Rabbit, 142. 
 
 Organ of Jacobson Rabbit, 14. 
 
 Orthoptera, 257. 
 
 Osphradium Mussel, 318. 
 
 Ossicles of gastric mill Crayfish, 243. 
 
 Osteoblast, 70, 121. 
 
 Osteoclast, 122. 
 
 Osteogenetic layer, 121. 
 
 Otic vesicle, 135. 
 
 Otocyst Mussel, 318. 
 
 Otoliths, 134. 
 
 Ovary Cockroach, 273 ; Cod, 35 ; Crayfish, 
 242-252 ; Frog, 29 ; Earthworm, 287 ; 
 Hydra, 340 ; Liver-fluke, 326 ; Mussel, 
 318 ; Pigeon, 51 ; Rabbit, 43 ; Tapeworm. 
 333. 
 
 Oviduct Cockroach, 274 ; Cod, 35 ; Cray- 
 fish, 242, 252 ; Earthworm, 287 ; Frog, 29 ; 
 Liver-fluke, 326 ; Pigeon, 51 ; Rabbit, 
 see Fallopian tube ; Snail, 301. 
 
 Ovipositor, 263. 
 
 Ovisac, 87 Crayfish, 252. 
 
 Ovotestis Snail, 300. 
 
 Ovum, or egg Cockroach, 274 ; Cod ; 
 9 ; Crayfish, 253 ; Earthworm, 289 ; Fowl, 
 113 ; Frog, 113 ; Hydra, 340 ; Liver-fluke, 
 327; Mussel, 318; Pond-snail, 302; 
 Rabbit, 113 ; Snail, 302 ; Tapeworm, 333. 
 
 PACINIAN CORPUSCLES Rabbit, 83. 
 Palatine bones Fowl, 153 ; Frog, 157 ; 
 
 Rabbit, 145. 
 
 foramina, anterior and posterior, 146. 
 
 Pallia! line Mussel, 307. 
 
 Pallium Mussel, 305. 
 
 Palp, labial Cockroach, 258 ; Mussel, 
 
 307. 
 mandibular Crayfish, 237 
 
 25 
 
 Palp, maxillary Cockroach, 258. 
 Pancreas Frog, 24 ; Pigeon, 47 ; Rabbit, 
 
 38. 
 
 development of, 125. 
 
 Pancreatic juice, 184. 
 Papilla, foliata Rabbit, 14, 76. 
 Parachordal plates, 159. 
 Paraglossa Cockroach, 260. 
 Paramoecium, 351. 
 Paras phenoid Frog, 156. 
 
 homologies of, 163. 
 
 Parethmoid Rabbit, 145. 
 
 Parietal bone Fowl, 150 ; Rabbit, 142. 
 
 cells, 180. 
 
 segment of skull Fowl, 149 ; Frog, 
 
 155 ; Rabbit, 142. 
 Parieto-frontal bone Frog, 155. 
 Paroccipital process Rabbit, 141. 
 Parotid gland Rabbit, 37, 179. 
 Patella Rabbit, 175. 
 Pecten of eye Fowl, 131. 
 Pectineal process Fowl, 173. 
 Pectoral arch, or girdle, 169. 
 
 fin Cod, 9. 
 
 Peduncles of cerebellum, 217. 
 
 of pineal body, 218. 
 
 Pelvic arch, or girdle, 172. 
 
 fins Cod, 9. 
 
 Peptic glands, 79. See Glands, cardiac. 
 Pericardial cavity, chamber, or sinus 
 
 Cockroach, 267 ; Cod, 33 ; Crayfish, 246 ; 
 
 Frog, 25 ; Mussel, 313 ; Pigeon, 49 ; 
 
 Rabbit, 40 ; Snail, 295. 
 Perichondrium, 69, 121. 
 Perilymph, 133. 
 Perimysium, 72. 
 Perineal gland, 14. 
 Perineurium, 74. 
 Periosteum, 70, 121. 
 Periotic bone Rabbit, 142, 146. 
 Periplaneta, 257. 
 Peristome Vorticella, 347. 
 Peristomium Earthworm, 282. 
 Peritoneum Frog, 23. 
 Peritricha, 351. 
 Pes Frog, 6 ; Pigeon, 18 ; Rabbit, 14. 
 
 osteology of, 174. 
 
 Petrous portion of periotic Rabbit, 146. 
 
 Peyer's patches Rabbit, 38. 
 
 Phagocytes, 188. 
 
 Phalanges, 171, 174. 
 
 Pharyngeal teeth Cod, 10. 
 
 Pharynx, 10 Rabbit, 39. 
 
 Physiology, xx., 55. 
 
 Pia mater of brain, 44. 
 
 Pigment cells Frog, 68. 
 
 Pineal body, 218. 
 
 Pinna Rabbit, 134. 
 
 Pituitary body, 144, 218. 
 
 space, 160. 
 
 Placenta Rabbit, 112. 
 
 Plant and animal compared, xvi. 
 
 Planula Medusa, 344. 
 
 Pleura Rabbit, 40. 
 
 Pleurobranchise Crayfish, 239. 
 
 Pleuron Crayfish, 233. 
 
 Podical plate Cockroach, 258, 264. 
 
 Podobranchiae Crayfish, 239. 
 
386 
 
 INDEX. 
 
 Polar bodies, 88. 
 
 Pond-snail, development of, 302. 
 Pons Varolii Rabbit, 217. 
 Pore, dorsal Earthworm, 279. 
 
 excretory Tapeworm, 331. 
 
 Post-anal gut, 126. 
 
 Post-zygapophyses, 163. 
 
 Precoracoid bar, 169. 
 
 Premaxilla Fowl, 153 ; Frog, 157 ; Rabbit, 
 
 145, 148. 
 
 Premolar Rabbit, 13, 148. 
 Prescolex Tapeworm, 334. 
 Presphenoid Fowl, 151 ; Rabbit, 142. 
 Prezygapophysis, 163. 
 Primitive streak or groove, 100, 102. 
 Prismatic layer of shell Mussel, 307. 
 Procephalic lobes Crayfish, 255. 
 Proctodseum, 93 Cockroach, 276 ; Crayfish, 
 
 255 ; Earthworm, 289 ; Fowl, 126 ; Frog, 
 
 3 ; Pond-snail, 303. 
 Proeeoces, 15. 
 
 Proglottides Tapeworm, 331. 
 Pronephros, 136. 
 Pronotum Cockroach, 260. 
 Pronucleus, male and female, 88, 89. 
 Prootic Fowl, 152 ; Frog, 157 ; Rabbit 146. 
 Propodite- Crayfish, 236. 
 Prosencephalon, 219. 
 Prostomium Earthworm, 282. 
 ProthoraxCockroach, 258. 
 Protoplasm, xiv., 191. 
 Protopodite Crayfish, 236 
 Protozoa, xvi. 
 
 Proventriculus Pigeon, 125, 178. 
 Pseudo-blastopore Rabbit, 95, 101. 
 Pseudo-branchia Cod, 10. 
 Pseudopodia Amoeba, 354. 
 Psychoses, 227. 
 Pterygoid bones Fowl, 153; Frog, 157 
 
 Rabbit, 145. 
 Pterylse Pigeon, 16. 
 Ptyalin, 181. 
 Pubes, 172. 
 Pupil of eye, 129. 
 Pygostyle Fowl, 166. 
 Pyloric cseca Cod, 32. 
 
 glands Rabbit, 180. 
 
 Pyramid, urinary, of kidney, 81. 
 Pyramids of medlula, 217. 
 
 QUADRATE Fowl, 153 ; Frog, 158. 
 
 homologies of, 162. 
 
 Quadrato-jugal Fowl, 153 ; Frog, 158. 
 Quill-feather Pigeon, 16. 
 
 RACHIS OF FEATHER Pigeon, 17. 
 Radius, 170. 
 Radula Snail, 294. 
 
 Receptaculum ovarum Earthworm, 287. 
 Rectal gland Rabbit, 39. 
 Rectrices Pigeon, 16. 
 Rectum. See Alimentary system. 
 Redia, 329. 
 Reflex action, 56. 
 Rejuvenescence, 350. 
 Remiges Pigeon, 16. 
 
 Renal organ Cockroach, 266 ; Cod, 35 ; 
 Crayfish, 247 ; Earthworm, 284 ; Frog, 28 ; 
 
 Liver-fluke, 325 ; Mussel, 315 ; Pigeon, 51 ; 
 Rabbit, 41 ; Snail, 296 ; Tapeworm, 331. 
 
 Rennet, 183. 
 
 Reno-pericardial canal Snail, 297. 
 
 Reproduction, iv. Amoeba, 354; Hydra, 
 339 ; Paramcecium, 352 ; Vorticella, 350. 
 
 Reproductive organs Cockroach, 273 ; Cod, 
 35; Crayfish, 252; Earthworm, 286, 
 Frog, 29 ; Liver-fluke, 325 ; Mussel, 318 ; 
 Pigeon, 51 ; Rabbit, 43 ; Snail, 299 ; Tape- 
 worm, 332. 
 
 Respiratory system Cockroach, 268 ; Cod, 
 33; Crayfish, 239; Earthworm, 286; 
 Frog, 24 ; Mussel, 309 ; Pigeon, 48 ; Rab- 
 bit, 39 ; Snail, 296. 
 
 Restiform bodies Rabbit, 217. 
 
 Retina, development of, 130, 132. 
 
 histology of, 77. 
 
 Ribs, 167. 
 
 Rostrum Crayfish, 232 ; Fowl, 151. 
 
 Rugae Rabbit, 178. 
 
 SACCULUS AURLS, 133. 
 
 rotundus Rabbit, 39. 
 
 Sacrum, or sacral vertebrae Fowl, 165 
 
 Frog, 166 ; Rabbit, 164. 
 Sagittal suture Rabbit, 144. 
 Salivary glands Cockroach, 265 ; Rabbit, 
 
 37 ; Snail, 294. 
 
 development of, 128. 
 
 histology of, 79. 
 
 Scala media Rabbit, 134. 
 
 tympani Rabbit, 134. 
 
 vestibuli Rabbit, 134. 
 
 Scales Cod, 10. 
 Scaphognathite Crayfish, 237. 
 Scapula, 169. 
 
 Sclerites Cockroach, 258. 
 Sclerotic of Eye, 129. 
 Scolex Tapeworm, 331. 
 Scrotal sacs Rabbit, 138. 
 Sebaceous glands Rabbit, 84. 
 Section-cutting and mounting, 63. 
 Segmental duct and tubes, 136. 
 
 organ Earthworm, 279, 284. 
 
 Segmentation of ovum Cockroach, 275 ; 
 
 Crayfish, 254 ; Earthworm, 289 ; Mussel, 
 
 319 ; Pond-snail, 302 ; Vertebrate, 89, 94. 
 Sella turcicaRabbit, 144. 
 Semicircular canals, 133. 
 Sense capsules, 139. 
 organs Crayfish, 249 ; Earthworm, 
 
 286 ; Insects, 271 ; Mussel, 317 ; Snail, 
 
 298. 
 
 Senso-motor arc, 58. 
 Septum Earthworm, 282. 
 
 auricularum Frog, 25. 
 
 lucidum- -Rabbit, 221. 
 
 nasi, 144. 
 
 Serous membrane, 109. 
 
 Setse Cockroach, 271 ; Crayfish, 249 ; 
 
 Earthworm, 279. 
 Shell -Mussel, 307 ; Snail, 291. 
 
 membrane Fowl, 96. 
 
 gland Liverfluke, 326; Pond-snail, 
 
 303 ; Tapeworm, 333. 
 Sight, 57. 
 organ of. See Eye. 
 
INDEX. 
 
 387 
 
 Sinu-auricular valve Prog, 199. 
 Sinus rhomboidalis Pigeon, 53. 
 
 venosus Cod, 33 ; Frog, 25, 198. 
 
 Siphon, inhalent and exhalent Mussel, 
 
 306. 
 Skeleton, 139. 
 
 axial and appendicular, 6, 139. 
 
 Skin, histology of, 82. 
 Skull, 139. 
 
 development of, 159. 
 
 Snail, 29J. 
 
 Pontl, development of, 302. 
 
 Smell, 57. 
 
 organ of. See Olfactory organ. 
 
 Solar plexus Rabbit, 226. 
 Somato-pleure, 105. 
 
 Somites Crayfish, 233 ; Earthworm, 278. 
 Special senses Crayfish, 249 ; Earthworm, 
 
 286 ; Insects, 271 ; Mussel, 317 ; Snail, 
 
 298. 
 
 Sperm cells, 87 Cockroach, 275 ; Earth- 
 worm, 288. 
 Spermatheca Cockroach, 274 ; Earthworm, 
 
 279 ; Snail, 301. 
 Spermatoblasts, S7. 
 Spermatocysts Cockroach, 275. 
 Spermatophore Snail, 301. 
 Spermatosphere Earthworm, 288. 
 Spermatozoa Cockroach, 275 ; Crayfish, 
 
 252 ; Hydra, 340 ; Frog, 1, 87. 
 Sphenethmoid Frog, 156. 
 Spenoidal foramen Rabbit, 143. 
 Spicula amoris Snail, 301. 
 Spinal cord, 29. 
 
 histology of, 75. 
 
 Spiracles Cockroach, 263. 
 
 Splanchno-pleure, 105. 
 
 Spleen Frog, 23, 31 ; Rabbit, 38. 
 
 Splenial Fowl, 154. 
 
 Sporocyst Liver-fluke, 329. 
 
 Sporosac Medusa, 345. 
 
 Squame Crayfish, 237. 
 
 Squamosal Fowl, 150; Frog. 157: Rabbit, 
 
 143. 
 
 Stalk Vorticella, 347. 
 Stapes Rabbit, 147 ; Fowl, 152 ; Frog, 157. 
 Starch, conversion into sugar, 182. 
 Steapsin, 185. 
 Sternebrse Rabbit, 167. 
 Sternum Crayfish, 233 ; Vertebrate, 167. 
 Stigmata Cockroach, 263. 
 Sting, 263. 
 
 Stipes Cockroach, 260 
 Stomach. See Alimentary system. 
 
 histology of Frog's, 78. 
 
 Stomodseum Cockroach, 276 ; Crayfish. 
 
 255 ; Fowl, 126 ; Frog, 2 ; Pond-snail, 303. 
 Strobila Tapeworm, 331. 
 Subanal style Cockroach, 263. 
 Sublingual gland Rabbit, 37, 179. 
 Submaxillary gland Rabbit, 37, 179. 
 Submentum Cockroach, 259. 
 Subserous space, 109. 
 Sub-umbrella Medusa, 344. 
 Subzonal membrane. See Serous membrane. 
 Succus entericus, 184. 
 Sucker Liver-fluke, 324. 
 Supra-angular Fowl, 154. 
 
 Supra-branchial chamber Mussel, 306, 310. 
 Supra-iliac cartilage Frog, 172. 
 Supra-occipital Fowl, 149; Rabbit, 141, 
 
 160. 
 Supra-orbital process Rabbit, 147. 
 
 ridge Fowl, 151. 
 
 Supra-scapula Frog, 169. 
 Suspensorium Frog, 158. 
 Suspensory ligament of eye, 130. 
 Sutures of skull Rabbit, 144. 
 Sweat glands Rabbit, 84. 
 Swimmerets Crayfish, 235. 
 Sylvian fissure Rabbit, 219. 
 Sympathetic nerve system, 225. 
 Simphysis pubis Rabbit, 172. 
 Syrinx Pigeon, 48. 
 Systole of heart, 195. 
 
 TACTILE CORPUSCLES, 83. 
 
 organ Snail, 299. 
 
 Setae Cockroach, 271 ; Crayfish, 249. 
 
 Tail-fold, 104. 
 
 Tapetum, 131. 
 
 Tapeworm, 331. 
 
 Tarso-metatarsus Fowl, 175. 
 
 Tarsus, 174. 
 
 Taste, 57. 
 
 organ of Cockroach, 271 ; Crayfish, 
 
 249 ; Snail, 299. 
 
 Taste-buds Rabbit, 77. 
 
 Teeth Cod, 10; Frog, 8; Mammals, 12, 
 149. 
 
 development of, 128. 
 
 structure of, 71. 
 
 Tegnien tympani Frog, 157. 
 
 Tegmina Cockroach, 261. 
 
 Tela vasculosa, 215. 
 
 Telolecithal ovum, 113. 
 
 Telson Crayfish, 233- 
 
 Temporal fossa Rabbit, 147. 
 
 Tentacle Hydra, 335 ; Medusa, 343 ; Mus- 
 sel, 308 ; Snail, 291. 
 
 Tentorial plane Rabbit, 144. 
 
 Tentorium Cockroach, 258, 260. 
 
 Tergum Crayfish, 233. 
 
 Testes Cockroach, 274 ; Cod, 35 ; Crayfish, 
 252 ; Earthworm, 287 ; Frog, 29 ; Hydra, 
 34 ; Liver-fluke, 325 ; Mussel, 318 ; Pigeon, 
 51 ; Rabbit, 43 ; Tapeworm, 332. 
 
 of brain Rabbit, 218. 
 
 Thalamencephalon, 218. 
 
 Thoracic appendages Crayfish, 235. 
 
 duct Rabbit, 189. 
 
 region Rabbit, 11. 
 
 Thorax Cockroach, 258, 260. 
 
 Thread-cell Hydra, 338. 
 
 Thymous gland Rabbit, 40. 
 
 Thyroid gland Rabbit, 39. 
 
 Tibia Cockroach, 261 ; Vertebrate, 174. 
 
 Tibiale, 174. 
 
 Tibio-tarsus Fowl, 175. 
 
 Toenia, 331. 
 
 Tongue Cod, 10 ; Frog, 8 ; Pigeon, 17 : 
 Rabbit, 14. 
 
 Touch, 57. 
 
 organ of. See Special senses. 
 
 Trabeculse of skull, 160. 
 
 Trachea Frog, 24 ; Pigeon, 48 ; Rabbit, 39. 
 
388 
 
 INDEX. 
 
 Tracheal system Cockroach, 268. 
 Transformation, 5. 
 Transverse process, 163. 
 Trichocyst Faramceciiun, 351. 
 Trochanter Cockroach, 261 ; Rabbit, 174. 
 Trochlea Rabbit, 171. 
 Trochosphere Pond-snail, 303. 
 Truncus arteriosus Frog, 25, 199. 
 Trypsin, 185. 
 Tubercle of rib, 167. 
 Tubercular facet, 163. 
 Tuberosity, 171. 
 Turbinals, 145, 152. 
 Tympanic bone Rabbit, 143. 
 
 membrane, 132. 
 
 Typhlosole Earthworm, 283 ; Mussel, 309. 
 
 ULNA, 170. 
 Ulnare, 170. 
 Umbilical cord, 112. 
 
 vesicle, 104, 111- 
 
 Umbilicus of feather, 16. 
 
 Umbo Mussel, 306. 
 
 Umbrella Medusa, 343. 
 
 Uncinate process Fowl, 167. 
 
 Urachus, 112. 
 
 Urea, 189. 
 
 Ureter. See Urinogenital system. 
 
 Urethra. See Urinogenital system. 
 
 Urinogenital system Cod, 35 ; Frog, 28 ; 
 
 Pigeon, 51 ; Rabbit, 41. 
 
 development of, 136. 
 
 Uropygial oil-gland Pigeon, 18. 
 
 Urostyle Frog, 166. 
 
 Uterus Cockroach, 274 ; Liver-fluke, 326 ; 
 
 Rabbit, 43, 138 ; Tapeworm, 333. 
 Utriculi Cockroach, 275. 
 Utriculus, 133. 
 Uvea, 132. 
 
 VACUOLE Amoeba, 354 Paramcecium, 352 ; 
 
 Vorticella, 348. 
 
 Vagina Rabbit, 43 ; Tapeworm, 333. 
 Valve of Vieussens, 217. 
 Valves of heart Frog, 199; Pigeon, 211; 
 
 Rabbit, 208. 
 
 Valvulse conniventes Rabbit, 178. 
 Vane of feather Pigeon, 16. 
 Vas deferens or vasa deferentia Cockroach, 
 
 274 ; Crayfish, 242, 252 ; Earthworm, 287; 
 
 Liver-fluke, 325 ; Snail, 301 ; Tapeworm, 
 
 332 ; Vertebrate, see Urinogenital system. 
 
 development of, 137. 
 
 Vasa efferentia Rabbit, 137. 
 
 Vascular system Cockroach, 267 ; Cod, 
 
 33, 196 ; Crayfish, 245 ; Earthworm, 284 ; 
 
 Embryonic, 211 ; Frog, 25, 199 ; Mussel, 
 313 ; Pigeon, 49, 208 ; Rabbit, 40, 204 ; 
 Snail, 296. 
 
 Veins. See Vascular system. 
 
 Veliger, 304. 
 
 Velum Medusa, 343. 
 
 interpositum, 218. 
 
 Vena cava. See Vascular system. 
 
 Vent or anus Cockroach, 266 ; Cod, 10 ; 
 Crayfish, 240 ; Earthworm, 289 ; Frog, 8 ; 
 Mussel, 307 ; Paramcecium, 352 ; Pigeon, 
 17 ; Rabbit, 14 ; Snail, 292 ; Vorticella, 
 347. 
 
 Ventral plate Cockroach, 275. 
 
 Ventricles of brain, 215, 219, 221. 
 
 of heart. See Heart. 
 
 Vermiform appendix Rabbit, 39. 
 
 Vertebral column, 163. 
 
 plates, 93, 99, 120. 
 
 Vertebrarterial canal Pigeon, 226 ; Rabbit, 
 164. 
 
 Vesiculae seminales Cockroach, 275 ; Earth- 
 worm, 287 ; Frog, 28. 
 
 Vestibule Rabbit, 43 ; Vorticella, 347 
 
 of ear, 133. 
 
 Vexillum of feather Pigeon, 16. 
 
 Vibrissge Rabbit, 12. 
 
 Visceral arches and clefts, 124, 161. 
 
 Vitellarium Liver-fluke, 326 ; Tape vorm, 
 333. 
 
 Vitelline membrane, or>. 
 
 duct, 112. 
 
 Vitreous humour, 130. 
 
 Vomer Fowl, 151 ; Frog, 157 ; Rabbit, 144. 
 
 Vorticella, 347. 
 
 Vulva Rabbit, 43. 
 
 WING Cockroach, 261 ; Fowl, 171 ; Pigeon, 
 
 16, 18. 
 
 White spaces Cockroach, 258. 
 Wolffian body, 136. 
 duct, 137. 
 
 XIPHISTERNUM Frog, 169. 
 Xiphoid processes Fowl, 167, 168. 
 
 YOLK, influence of, 94. 
 
 gland Liver-fluke, 326; Tapeworm, 
 
 333. 
 sac, 104, 110. 
 
 ZONA RADIATA Rabbit, 96. 
 Zygapophyses, 163. 
 
 Zygomatic processes of squamosal- Rabbit, 
 147. 
 
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