tf j. 1*'^ \jf f /i i' vy. LV ‘ ► f I; i • «. ^ ' > •i < S •* « % • \ ^ . V Vv > • ,0 » . Si AN ELEMENTARY Text-Book of Biology, 2 4 STANDARD TEXT-BOOKS, ►XX> WORKS BY WM. STIRLING, M.D., Sc.D., BRA.CKINBDRY PROP. OP PHYSIOLOGY IN OWENS COLLEGE AND VICTORIA UNIVERSITY, MANCHESTBR ; EXAMINER IN THE UNIVERSITIES OF OXFORD, EDINBURGH, AND LONDON; AND FOR THE FELLOWSHIP OF THE ROYAL COLLEGE OF SURGEONS, ENGLAND. LANDOIS AND STIRLING’S PHYSIOLOGY. 1. HUMAN PHYSIOLOGY (A Text-book of). By Profs. Landois and Stirling. In 2 vols., Large 8vo, Handsome Cloth^ with 845 Illustrations, some in Colours. Fourth Edition. 42s. “ The MOST COMPLETE resume of all the facts in Physiology in the language. With this Text-book at his command, no Student could fail in his Examination.” — The Lancet. “One of the most practical works on Physiology ever written.’’— Medical Journal. 2. PRACTICAL PHYSIOLOGY (Outlines of). By Prof. Stirling. In Crown 8vo, extra. With 234 Illustrations. Second Edition. 9s. “One of the best manuals the student can possess.’’ — The Lancet. 3. PRACTICAL HISTOLOGY (Outlines of). By Prof. Stirling. In Crown 8vo, extra. With 368 Illustrations. Second Edition. 12s. 6d. “The volume proceeds from a master in his craft. It would be impossible to find a better practical book on the subject.” — The Lancet. By Professor A. Mx\CALISTER, F.R.S. In Large 8uo, with 816 Illustrations, Cloth, 36s. HUMAN ANATOMY, Systematic and Topographical (A Text-Book of) : including the Embryology, Histology, and Mor- phology of Man. By A. Macalister, M.D., F.R.S. , Professor of Anatomy, University of Cambridge. “ By far the most important work which has appeared in recent yenrs."— Lancet. By The MASTER OF DOWNING COLLEGE, CAMBRIDGE. THE PHYSIOLOGIST’S NO’TE-BOOK : A Summary of the Present State of Physiological Science for Students. By Alex Hill, M.A., M.D., Master of Downing College, Cambridge. In Large 8vo. With numerous Illustrations and Blank Pages for MS, Notes. 12s. 6d. The object of this work is not to supersede the larger Text-Books, still less to take the place of Lectures and Laboratory work, but to assist the student in codifying* his knowledg*c. LONDON; CHARLES GRIFFIN & CO., LIMITED, EXETER STREET, STRAND. AN ELEMENTARY TEXT-BOOK OF BIOLOGY COMPIlTSIN(^ VEGETABLE AND ANIMAL MORPHOLOGY AND PHYSIOLOGY. * BY J. K AINSWORTH DAVIS, B.A., TRINITY COLLEGE, CAMBRIDGE ; YB0EEB60R OF BIOLOGY AND GEOLOGY IN THE UNITBRSITY COLLEGE OF WALES, ABERYSTWYTH. TKHitb ‘W urn CY cue ^llustvations anb (Slossav^. ’ SECOND EDITION, REVISED AND ENLARGED. PART IL-ANIMAL MORPHOLOGY AND PHYSIOLOGY. LONDON: CHARLES GRIFFIN COMPANY, LIMITED; EXETER STREET, STRAND. 1893. [All Rights Reserved.] WELLCOMF INSTITUTE lib^^ry Coll. welMOmec Call No. (v PREFACE TO THE SECOND EDITION. The Work has been subjected to a thorough revision, and has also been much enlarged, so that most of the larger organic groups are now represented by the selected typCvS. An attempt has been made in Part II. to bridge over the gap between the courses of Zoology and Human Anatomy as successively taken in a medical curriculum. A chapter on the Distribution of Animals has also been added, and this is, of necessity, mainly based on the works of Dr. Alfred Russel Wallace. Many illustrations, new and from various sources, have been added. I have especially to acknowledge my in- debtedness to Prof. Milnes Marshall and Dr. C. Herbert Hurst, for permission to use figures of Vorticella, Earth- worm, and Amphioxus, from their practical book, while much kind assistance has been given to me by Miss Mary O’Brien, B.Sc., Mr. J. W. Marshall, M.A., Mr. J. Alan Murray, B.Sc., and Dr. C. Herbert Hurst. I also wish to thank Mr. H. W. Gilbert Williams for the great cafe with which he has executed such of the new illustrations as are original or copied. J. K. A. D. Aberystwyth, June, 189.3. TABLE OF CONTENTS. INTRODUCTION, Chapter I. — Protozoa. § 1. Amceba (Proteus Animalcule), .... § 2. Vorticella (Bell Animalcule), .... § 3. Gregarina, ...... Chapter II. — Coelenterata. § 4. Hydra (Fresh Water Polyjie), .... • « Further remarks on Hydrozoa, .... Chapter III. — Platyhelmia (Flat- worms). § 5. Distoma (Liver-fluke), ..... Further remarks on Flukes, .... § 6. Tienia (Tapeworrri), ..... Further remarks on Tapeworms, Chapter IV. — Nemathelmia (Thread-worms). § 7. Ascaris (Round-worm), . Other Thread-worms, ..... Chapter V. — Annelida (Segmented Worms). « § 8. Lumbricus (Earthworm), .... Sexual reproduction, ..... § 9. Hirudo (Leech), ...... t'i.GU 1 i 1 1 19 28 39 39 46 47 53 54 71 Chapter VI. — Arthropoha. PAGE § 10. Astacus (Crayfish), ...... 83 Other Crustacea, . . . . . . .107 Chapter VII. — Mollusca. § 11. Anodonta and Unio (Fresh Water Mussels), . . .109 ^ 12. Helix (Snail), . . . . . . .122 Chapter VIII. — Verterrata Acrania. § 13. Amphioxus (Lancelet), . . . . . .134 « Chapter IX. — Pisces (Fishes). § 14. Scyllium (Dogtish), ...... 153 Chapter X. — A.mphibia. § 15. Kami (Frog), 174 Chapter XI.— Aves (Birds). § 10. Columba livia and Callus bankiva (Pigeon and Fowl), . . 229 Chapter XII. — Mammalia. § 17. Lepus cuniculus (Kabbit), . . . . • 268 Chapter XIII.— Comparative Animal Morphology and Physio- logy, ........ 310 Chapter XIV. — Man, . . . . . . 321 Chapter XV. — Classification and DistrirutiOxV of Animals, , 339 r. 1ST O K I L IJ S T K A T I O N S. [Names in italics indicate the Sources whence derived. ) FIGURK I. *> O. 5. (>. >. S. 9. 10. n. 12. ].3. 14. 15. 16. 17. IS. 19. 20. 21. 22. 2.3. 24. 25 27. 28. 29. .30. 31. 32. 83. 1 1 Amoeba and Vorticella [Haddon, after Howes and Stein), Group of Vorticellae [Marshall and Hurst), Gregarines (after Stein and Biitschli), Hydra (Original), .... Distoma — Gut (from Claus, after Leuckart), Excretory, Nervous, and Reproductive Organs (after ^ Sommer), .... Reproductive Organs [Claus, after Sommer), „ Development [Claus, partly after Leuckart), TiTiiia [Landois and Stirling), ,, Reproductive Organs (after Sommer), ,, Ripe Egg [Landois and Stirling), . ,, Cysticerci ( ,, ,, ), . ,, ,, exerted. [Landois imd Stirling), . A scar is [von Jaksch), .... ,, (Original, and after Leuckart, Vogt and Yung, Biitschli and Van Beneden), Oxyuris [von Jaksch), Trichina ( ,, ), Lumbricus — Gut, Circulatory Organs, Nervous System (Original), ,, Reproductive Organs [Marshall and Hurst), ,, Development (after Wilson), . Asterias — Polar Cells (after Fol and Her twig), ,, Fertilization (after /’oO, Hirudo — General Dissections (after Leuckart, and Vogt and Yung), ,, Nephridium and Eye (after Vogt and Yung, and Whitman), and 26. Astacus— External Characters (Original), Astacus — Appendages (Original), . ,, Gut and Kidney (Original), ,, Reproductive Organs (Original). ,, Eye (after 6arriere), ,, Blastula and Gastrula (after Jieicheribach and Huxley), Unio — General Dissection [Claus, after Grobben), Anodonta— Nervous S3'stem (G/aus, aftey 2 h •A OK 8 12 17 21 29 .30 32 37 40 42 44 44 45 47 50 53 53 56 63 69 73 74 77 77 85 88 94 99 104 105 no 112 X LIST OF ILLUSTRATIONS. i'lGUKE 34. Anodonta -'rransverse Sections and Gill (Original), 35. Helix -Digestive Organs and Nervous System (Original), 36. ,, Buccal Mass {Claus, after Keferstem), 37. „ Reproductive Organs (Original), . 38. Amphioxus {Marshall and Hurst), 39. ,, Sections (Original), 40. ,, Blastula and Gastrula {Claus, after Hatschek), 41. ,, Sections of Embryos {Haddon, after Hatschelc), 42. ,, Larval Stages {Glaus, after Hatschek), 43. Scyllium — Skull, &e. (Original), . 44. ,, Skeleton of Pectoral Fins (Original), 45. ,, ,, Pelvic Fins (Original), 46. ,, General Dissection (Original), . 47. ,, Nerves (after Wiedersheim), 48. Diagrams of Membranous Labyrinth {Bell, after Waldeyer), 49. Bana — Skin (after Wiedersheim), . 50. ,, Endoskeleton (Original, and after ^cA:er) 51. ,, General Dissection (Original), 52. Unstriated Muscle-Fibres {Landois and Stirling), 53. Hana — Heart (after Ecker), 54. ,, Arteries ( ,, ), 55. ,, Veins ( ,, ), 56. ,, Posterior Lymph-Hearts {Ecker), . 57. ,, Muscle- Fibres froTii Heart {Landois and Stirling), 58. Small Artery showing the Coats ( ,, ), 59. Capillaries {Landois and Stirling), 60. Kana — Female Reproductive Organs (after Ecker and Wieder^ heim), ...... 61. Histology of Nerve {Landois and Stirling), 62. Rana— Sense Organs (after Ecker and Wiedersheim), 63. Diagrammatic Horizontal Section of Eye {Landois and Stirling) 64. Diagram of Layers of Retina {Landois and Stirling), 65. Rana — Cleavage of Oosperm {Haddon, after Ecker), 66. ,, Early Stages {Haddon after Gotte), 67. ,, Diagrammatic Longitudinal Section through Embryo {Haddon, after G1GURE 73. Galliis— Diagrammatic l^ongituclinal Section through Unincu]>ate(l Egg {Claus, after Balfour and AlUn Thomson), Surface Views to showCleavage in the oosperm after Coste), ...... Section through part of Unincubated Blastoderm {Haddon, after Klein), ...... Transverse Section through Front F]nd of Primitive Streak in First Day Chick {Haddon, after Balfour), Surface View of First Day (20 hrs.) Chick {Kolliker), . SurfaceView of Chick, rather later than Fig. 77 {Kolliker), Surface View of Second Day Chick {Kolliker), . Development of the Eye {Haddon, partly after Marshall), 81. Transverse Section through Embryo Duct at Third Day (from Haddon, after Balfour), ..... 82. Callus — Diagram to Illustrate the Emluyonic Appendages {Haddon, after Foster and Balfour), 83. Six Stages in the Development of VLair {Haddon, after Wledersheirn }, 84. Lepus — Endoskeleton (Original), 85. ,, General Dissection of Head and Thorax (Original), 86. ,, Histology of Liver {Landois and Stirling, after Hering), 87. Muscle-Fibres from Mammalian Heart {Landois and Stirling) 88. Structure of Mammalian Kidney {Landois and Stirling), 89. Lepus — Urinogenital Organs (Original), . 90. Histology of Striated Muscle {Landois and Stirling), 91. Diagrams of Membranous Labyrinth {Bell, after Waldeyer), 92. Lepus — Formation of Blastocyst {Haddon, after E. Van Benedtn), 93. ,, 94. „ 95. „ 96. Man 97. ,, 98. „ 99. „ 100. ,, 101. „ 102. „ 103. ,, 104. „ 105. ,, 106. 107. 108. Fllastocyst {Haddon, after Kolliker), Plead of "Ten Day Flmbryo {Haddon, after Kolliker), Embryonic Appendages {Haddon, after Bischoff), Skull {Macalister), Sacrum ( Coccyx ( Bones of Hand ( Pelvis, ( Bones of P'oot ( Caecum, &c. ( Aortic Arches ( ), ), ), ), 5 5 Development of Veins {Macalister), ,, Urinogenital Organs {Macalister) Brain-exterior {Macali‘iter), . . ,, section ( ,, ), F'cetal Membranes {Macalister, after Longet), PACK 257 258 259 259 260 261 262 263 264 26(> 270 277 283 289 292 296 297 300 304 305 307 307 309 324 325 325 327 328 328 329 330 331 332 335 336 337 AN ELEMENTAEY TEXT-BOOK OF BIOLOOY. PART IL— ANIMAL MORPHOLOGY AND PHYSIOLOGY. INTRODUCTION. 1. ZOOLOGY, the branch of Biology which deals with animals, is such an immense subject that, like Botany, it is conveniently split up into a number of subdivisions. The most important of these are — (1) Animal Morphology, dealing with the structure and form of animals; (2) Animal Physiology, which treats of their actions or functions; (3) Development, the application of morphology and physiology to the study of immature forms ; (4) Classification or arrangement; (5) Distribution, in space and time; and (6) Phylogeny, the province of which is to make out the past history of animal groups (phyla). We are mainly con- cerned in this volume with the first three branches, and, to a much smaller degree, with the questions of classification. The range of these subdivisions will be comprehended more clearly if we consider what questions they seek to answer in regard to some particular animal, say, for example, the common Frog. (1) Morphology takes note of the external characters, such as shape, colour, divisions of the body, &c., and by means of dissec- tion determines that various systems of organs (digestive, &c., &c.) are present, which have a definite arrangement, and which can 2 1 1 2 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. be more or less divided up into smaller parts visible with the eye or with the aid of a lens. The stomach, for instance, is found to have a wall consisting of certain layers or coats. So far we have had to do with External Morphology and Anatomy. But, with the assistance of the compound microscope, a further analysis is pos- sible, and the special aim of Histology is to consider minute struc- ture. Such an analysis demonstrates that the body of a Frog is composed of an exceedingly large number of miscroscopic units, cells, differing very much in size, shape, and form, like cells being aggregated into masses known as tissues, such as muscular, nervous, &c. There are, besides, other elements formed by or from cells. It is usual to regard the cell as the morphological unit, but the higher powers of the microscope prove that the cell itself is most wonderfully complex. Morphology, however, in the modern sense, is not a mere de- scriptive study, but endeavours to discover not only how things are but also why they are. In the Frog, for example, there is a slender tube, the pineal body, running from the brain to the roof of the skull. In the tadpole it stretches right up to the skin, but the formation of the skull-roof pinches off its end, which remains in the adult frog as a “brow-spot.” What, then, is the meaning of the pineal body and brow-spot ? No light is thrown upon this question by the study of the Frog only, but by employ- ing the comparative method a definite answer can be given. It is known that in certain lizards the pineal body has the structure which is characteristic of eyes, and we, therefore, conclude that the Frog’s remote ancestors possessed an unpaired eye in the top of the head, which has since disappeared, leaving an insignificant rudiment. In numberless other instances Comparative Morphology (including Comparative Anatomy, &c.) helps to clear up otherwise unintelligible matters. (2) From the physiological standpoint, a Frog is a machine capable of performing various kinds of work. Physiology is con- cerned, in fact, with the uses of the different parts of the body. It investigates, for example, the processes by which food is digested, absorbed, and built up into living tissue; the circulation of the blood ; and the functions of the brain. The problems of physiology, like those of morphology, often require comparative treatment, and they are commonly more difficult to solve. (3) Development (Embryology, Ontogeny) as applied to the INTRODUCTION. 3 Frog traces its life-history from the egg to the adult condition, and is separated from the two preceding branches merely for the sake of convenience. In the solution of many biological pro- blems it is often of the greatest possible use. An animal in the course of its development passes through a series of stages which to some extent indicate its pedigree, or, as it has been put, “ climbs up its own genealogical tree.” Thus the Frog starts active life as a gill-breathing aquatic tadpole which is practically a fish in structure, and points to descent from fish-like ancestors. (4) Classification refers the Frog to a definite place in a system of grouping, in which animals are best arranged according to those characters which denote blood-relationship. It is found possible to divide the animal kingdom into several large branches or phyla, which are again subdivided, and the process continued until the individual is reached. The relative status of the various groups may be indicated as follows : — PHYLUM SUB -PHYLUM Class ORDER Family Genus Sj^ecies Variety. The Frog is placed in the important phylum CHORDATA, which includes all animals that possess, temporarily or permanently, an elastic supporting rod, the notochord, below the central nervous system, as well as clefts which place the cavity of the throat in communication with the exterior. Mammals, Birds, Reptiles, Frogs, Fishes, and certain somewhat lower forms are here included. The Frog’s sub-phylum, that of the VERTEBRATA, embraces animals among the common characters of which are possession of a more or less complete brain-case and spinal column. It includes the above-mentioned animals except the forms lower than fishes. Vertebrates are subdivided into several classes, one of which, the Amphibia, comprises frogs, toads, newts, sala- manders, and the like, all of which pass through a tadpole-stage. 4 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. Those Amphibia which, like frogs and toads, have four well- developed limbs, but are tailless, constitute the order ANUB/1, and this includes a number of families, of which one, Ranidse, comprehends frogs with long hind limbs, webbed hind feet, teeth in the upper jaw, and tongue fixed to the front of the mouth- floor. The most important genus in this family is Eana, to which the common and edible frogs belong, and which is characterized by the rudimentary nature of the thumb, by the presence of teeth on the roof of the mouth, and the deeply forked tongue. Some forty “ kinds ” or species of frog are included in the genus Eana, and this is a convenient place to explain the principle upon which scientific names are given to animals. On the binominal system, which is universally adopted, each animal receives two names, that placed first being the generic, while the other is the specific name. The common or Grass Frog is known as Eana tempomria, and the Edible Frog as Eana esculenta. The species temper aria is marked by a number of distinctive features, among which may be placed the presence of a dark blotch on each side of the head, and the absence of croaking sacs in the male. It is scarcely possible to say what is actually meant by the term “ species,” and there is much difference of opinion in certain cases. The follow- ing definition, given by De Candolle, will serve as well as any: — “A species is a collection of all the individuals which resemble each other more than they resemble anything else, which can by mutual fecundation produce fertile individuals, and which re- produce themselves by generation, in such a manner that we may from analogy suppose them all to have sprung from one single individual.” It is generally found impossible to obtain crosses between different species, or if such crosses (hybrids) are pro- duced these are, as a rule, infertile. The Frog is not a very good illustration of varieties, to ex- plain which other animals may be taken. The Clouded Yellow Butterfly (Colias edusa), for instance, is usually orange and black, but a small proportion of yellow and black individuals are found, and these constitute a variety'" of the species edusa. Another good example is the Field Snail (Helix Jiortensis'), in which the colour and striping of the shell vary in the most remarkable way, so that the species hortensis has been split up into a large number of varieties. In such a case, however, the varieties are perfectly * Hyale. INTRODUCTION. 5 fertile among themselves, and the crosses (mongrels) between them are also fertile, though there are exceptions to this among plants. The old view as to the “ origin of species ” was that they were all separately created, but it is now almost universally held by scientists that living species have been evolved from pre-existing ones, and that varieties are to be regarded as species in process of evolution. A brief sketch of the evolution theory will be given in another place. (5) Distribution. — Eana temporaria is a very widespread species, ranging over the greater part of Europe, N. Africa, N. Asia to Japan, and N. America. It is absent, however, from Iceland and N. Scandinavia. We have less definite knowledge as to the range in time of this species, but in this country the bones of a frog closely resembling it have been found in a deposit ^ accumulated at a time Avhen the British fauna included species of Elephant, Hippopo- tamus, Ehinoceros, and Hysena. (6) Phytogeny (JEtiology) makes use of data supplied by all the other departments to work out the evolution of groups. The Amphibia must be regarded as one branch of a huge genealogical tree, its orders corresponding to subdivisions of this branch, its families and genera to still smaller ramifications, and its species to the ultimate twigs. 2. Differences between Animals and Plants. — Ho difficulty is experienced in distinguishing a higher animal from a higher plant. A Frog, for instance, differs in the compactness of its form from, say, a tree, and this difference is correlated with the nature of the food. A tree spreads out leaves for the absorption of carbon dioxide, while it takes up by its roots water charged with inorganic salts. Its food is of simple kind and gaseous or liquid in nature ; hence the diffused branching form which offers a large external absorptive surface. A Frog, on the other hand, requires much more complex food, part of which must be albu- minous in nature, and it can only get this by devouring other organisms, in this case insects; while tadpoles chiefly live on plants, upon which, indeed, all animals directly or indirectly depend. This complex organic food being solid the Frog requires an internal digestive cavity for its reception, and there is an obvious * The Cromer Forest-Bed. 6 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. necessity for powers of locomotion, which are unnecessary to higher j^lants. A compact form is obviously advantageous under the circumstances, and the possession of a well-developed nervous system and sense-organs bringing the various activities of the animal into touch with one another and the outer world becomes readily intelligible. Plants and animals, however, must be regarded as of common descent, forming as it were the two diverging limbs of a V, the higher forms being situated at the ends of those limbs, and the lower forms near the point of union. Hence, as we pass down in the scale the differences between the two kingdoms become less and less marked, until, in the simplest cases, it is often scarcely possible to say whether a given organism be plant or animal. The distinction which perhaps holds most generally is found in the nature of the food, and plants are also usually characterized by the presence of protective membranes composed of cellulose (Cg Hjq 0^)n, a substance closely allied to starch. It is, indeed, the presence of these membranes which renders it necessary that the food should be in a gaseous or liquid condition. 3. Distinctions between Living and Non-Living Matter. — An organism is bounded by curved surfaces, while masses of non- living matter are either shapeless (amorphous) or else of crystalline form, in which case they are usually limited by flat faces meeting in straight edges. An organism, too, is of excessively compli- cated physical and chemical structure, and its living part is always composed of a substance known as protoplasm, of which more will be said in the sequel. This complexity is related to a jwocess of constant chemical change (metabolism), involving continual loss of substance, which must be compensated by the intaking of food. This is built up into fresh protoplasmic mole- cules, which are intercalated between those already existing. By this process of intussusception growth may be effected, up to a certain limit in the case of each organism. A mineral mass, say, for example, a crystal of copper sulphate, is not of such exceed- ingly complex nature, nor are its molecules subject to constant down-breaking and iq>building. It may be kept in an unaltered form for an indefinite period, and since it does not eliminate waste products does not require food. If jdaced in a saturated solution of copper sulphate it exhibits a kind of growth, by PROTOZOA. 7 addition of new layers to its outside (accretion), and such increase may go on indefinitely so long as fresh solution is available. An organism, again, has a life-history, passing through a cycle of chano:es, which either terminate in death or else in loss of individuality (c/. Section on Amoeba). This is not the case with a mineral. 4. Biogenesis and Abiogenesis.— All existing organisms, so far as we know, have been derived from pre-existing individuals by processes of reproduction (biogenesis). It was, however, formerly held that some organisms could spring directly from non-living matter (abiogenesis, spontaneous or equivocal generation). This belief was gradually limited to the lowest forms of life, and even for them has now been disproved. A flask partly filled with broth or hay-infusion soon swarms with such organisms if left freely exposed to the air ; but if all germs are killed by continued boiling, and the entry of fresh ones prevented by plugging the neck of the flask with sterilized cotton-wool, none of them make their appearance. Such experiments, however, do not prove that abiogenesis has not occurred during some former period of the earth’s history. CHAPTER I.— PROTOZOA. § 1. AMfEBA (Proteus Animalcule). This is a microscopic animal, varying much in size. It is found on the surface of the mud in fresh-water pools, on damp earth, in organic infusions, and elsewhere. MORPHOLOGY. 1. External Characters. — The form of an active Amoeba is constantly changing, but it is always irregular. The semi-fluid, transparent protoplasm of which the body is wholly composed is thrust out into bluntish lobes (pseudopodia), which vary contin- ually in number and shape. In most Amoebse these can be emitted from any part of the body. . 8 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 2. Structure (Fig 1, A). — An outer perfectly transparent exo]plasm can be distinguished from a more granular internal endo- plasm. The exoplasm is apparently stiffer than the endoplasm, but there is no clear line of demarcation, and as part of the body may be outside one moment, and inside next, the apparent differ- ence is probably caused by a tendency of the granules to collect Fig. 1. — Amceba and Vorticella (from Haddon, after Howes, and Stein), enlarged. — A-0, Stages in fission of Amoeba; D and E, fission of other Protozoa; F, conjugation of Vorticella. c.v.. Contractile vacuole; n, nucleus. in the centre. Some amoebae, however, possess a firmer exoplasm in which a sort of fibrillation can be made out. Within the granular portion two important structures are present: — (1) The nucleus {n), a rounded or ovoid mass, consist- ing of a modification of protoplasm, and denser than the rest of the body. In the living animal it is inconspicuous, but becomes very obvious on treatment with weak acid or a staining solution. In some cases, at any rate, the nucleus is invested by a delicate membrane, and consists of two substances, one, chromatin, staining readily — the other, achromatin, staining with difficulty. The arrangement of the chromatin differs with the species, but part of it is frequently aggregated into a central particle, the nucleolus. (2) The contractile or pulsating vacuole {c.v.), a spherical space PROTOZOA. 9 with fluid contents, which alternately increases and diminishes in size, in a rhythmic manner. There ma)? be more than one. More or less food, surrounded by fluid, is usually present in the endoplasm, occupying spaces known as food-vacuoles. Gas- containing vacuoles have also been observed in some specimens. PHYSIOLOGY. The functions carried on by Amoeba and, indeed, by all other organisms may be conveniently considered under five headings. 1. Nutrition. — The food of Amoeba consists of small organisms and organic particles, which its pseudopodia first come into con- tact with, and then flow around. All parts of the body alike serve for the reception of food. The complex chemical composi- tion of the substances utilized, and their ingestion in the solid state are worthy of notice, as typical animal characteristics. Each mass enters the body surrounded by a small quantity of water, and a food-vacuole is thus constituted, the temporary breach in the protoplasm being at once closed up. Within the body, mechanical and chemical influences are brought to bear upon the food, it being subjected, on the one hand, to slow rota- tion in the endoplasm, while on the other, either the protoplasm itself or a digestive juice formed (secreted) by it reduces the digestible portions to a state of solution, or fine division. The food thus digested is then assimilated, that is, built up into living protoplasm, while the indigestible and undigested remnants are thrown out at any convenient point. Encystment. — If conditions of temperature or food-supply are unfavourable. Amoeba possesses the powmr of assuming a spherical form, and then secreting a firm structureless coating or cyst, which is possibly of a horny nature, and is a very considerable protec- tion. 2. Kataholism. — The complex protoplasmic molecules are con- stantly breaking down into simpler substances, and potential energy is thus transformed into kinetic. This process is, as a rule, far more rapid in animals than in plants, and greater activity is consequently displayed by the former. The products of Kataholism (Katastates) are : — a. Secretions, which are utilized in some way before passing out of the body. It is probable, for example, that Amoeba 10 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. secretes something which acts chemically upon the food, and corresponds to the digestive juices of other forms. 1). Excretions (waste-products), which pass out of the body at once. These are — water, carbon dioxide (CO2), and compounds of ammonia. Katabolism is, practically, a process of oxidation, the result being that animals, like most plants, require a supply of free oxygen, and the term Respiration, or breathing, is applied to the taking in of oxygen with concomitant excretion of carbon dioxide (and water). The contractile vacuole, which is alternately enlarged to a certain maximum size by the gradual aggregation of liquid, and obliter- ated l)y the contraction of the surrounding protoplasm, appears to communicate with the exterior. It probably serves as a recep- tacle in which can collect, at one time, oxygenated water from the exterior, at another time waste-products. The general surface of the body is also doubtless respiratory, oxygen diffusing in and carbon dioxide diffusing out. 3. Reproduction (Fig. 1, A-C). — This is asexual, and usually effected by hinary fission, the animal splitting into two equal parts. The nucleus elongates, becomes dumb-bell-shaped, and separates into two, each half being surrounded by a moiety of the proto- plasm, which has meanwhile been gradually constricted. The two new individuals grow to the adult size, divide again, and so on, there being no known limit to the process. An Amoeba, in fact, instead of dying, divides into two new and vigorous individuals. Hence it has been said to be, in a sense, immortal. 4. Contractility is one of the primary properties of protoplasm, in virtue of which a change of form is effected. The process does not involve diminution in volume, since reduced breadth in one or more directions is made up for by equivalent increase in other directions. In Amoeba there are irregular contractions of the protoplasm leading to the formation of pseudopodia. Where one of these is about to be thrust out, the exoplasm (which is probably specially contractile) is protruded as a clear knob, into which, as it increases in size, the endoplasm enters. An anterior progressing region, actively flowing into psendopodia, and a more passive following region, can be made out in some Amoebae. These regions are in ordinary cases determined by the direction of movement. PROTOZOA. 11 and simply correspond to the anterior and posterior ends for the time being. There are, however, some species which can only thrust out pseudopodia from part of the body, and which there- fore possess permanent progressing and following regions. It appears probable that a delicate film of firmer nature rapidly forms on the surface of naked protoplasm. The direction of pseudopodia may perhaps, in some Amoebae, be determined by the rupture of such a film at definite points. The movements of Amoeba are so characteristic that the special epithet, ‘‘ amoeboid,” is given to all similar movements, wherever occurring. 5. Irritability and Spontaneity. — That Amoeba is affected by external agents or stimuli may readily be noticed. Pseudopodia which come in contact with food-particles flow around them, but inedible substances are usually rejected. Increase of temperature enhances activity, but as 35° C. is neared, this is diminished, and arrested when that point is reached. Death is caused by a rise of 5° or 10° more. On the other hand, activity is diminished by cooling, and arrested at the freezing-point. Weak electric shocks cause the animal to assume the spherical form, and become quiescent. Amoeba is constantly moving, and it cannot be doubted that some of its movements are spontaneous — i.e., not directly due to external stimuli, but resulting from internal causes, such, per- haps, as chemical decomposition. § 2. VOETICELLA (Bell Animalcule). Vorticella is an animal of microscopic dimensions, which is found attached to marine and fresh- water plants, &c., and may be distinguished by the aid of a lens. MORPHOLOGY. 1. External Characters (Fig. 2). — The body is shaped some- thing like a pear or rounded cone, and, in most individuals, is attached to some object by an elongated slender stalk, into which its apex is drawn out. It is convenient to speak of the attached end of the body as proximal, the free end as distal. The animal possesses considerable powers of movement, and may be found fully expanded, fully retracted, or in an intermediate condition. 12 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 2. — Group of Vorticell^ (from Marshall and Hurst). x 220. — I., II., and III, show the animal in various positions ; IV. is a much smaller specimen drawn to the same scale ; V. shows a specimen made to contract by action of alcohol ; VI, is detached from its stalk, and swimming away freely, disc forwards ; VII,, VIII,, and IX. show three stages of fission; X., XI., and XII. show the separated individual swimming by means of the alDoral circle of cilia ; XI. is slightly con- tracted ; XII. strongly contracted after the cover-glass has been tapped. A, Food-vacuole discharging contents at anus; C, cilia of the disc; C F, contractile fibre of stalk ; D, disc ; D', disc contracted ; F V, food- vacuole ; P H, gullet ; P V, pulsating vacuole ; P, peristome ; M, myo- phan striation ; N, nucleus ; V, vestibule. PEOTOZOA. 13 When it is completely expanded the stalk is straight, or slightly curved, while the distal end, or clisc^ forms a flattish projection bordered by a thickened rim or yeristonie. Within the peristome is a groove which surrounds the disc and forms on one side a deep depression, the vestibule. In the groove is a row of delicate, hair-like, protoplasmic processes (cilia). The retracted state is initiated by the withdrawal of the disc ; then follows the folding- in of the peristome, so that the body becomes rounded, and, at the same time, the stalk is thrown into a tightly-coiled spiral. Smaller free-swimming, stalkless individuals are also met with, simpler in structure and possessing a circlet of cilia near the arboral end of the body. 2. Structure. — The body is transparent as in Amoeba, and its internal parts can therefore be readily studied. Covering the whole external surface, cilia excepted, is a thin elastic membrane, the cuticle, which may exhibit transverse striations. It is thinnest on the disc and peristome, and thickest on the stalk, of which it forms the sheath. The cuticle is secreted by the underl3dng pro- toplasm, and owing to its presence the body possesses a constant form, while the protrusion of lobe-like pseudopodia is rendered impossible. The cilia, which may be compared to permanent thread-shaped pseudopodia, pass through holes in the cuticle. The protoplasm is divided into a firm, finely granular, external ectosarc or cortical layer, and a semi-fluid endosarc. The deeper part of the ectosarc exhibits more or less distinct longitudinal striations (myophan striations), probably due to a fluting of the internal surface. This deeper part is produced into a contractile filament, which is slightly twisted in a spiral manner, and traverses the cavity of the hollow stalk, attached here and there to the firm sheath. As in Amoeba, a nucleus and contractile vacuole are present. The former is a horse-shoe-shaped band placed in the broad end of the body, just within the ectosarc ; the latter is spherical, and situated nearly the vestibule. There is a small round paranucleus near the nucleus. A rudimentary digestive apparatus can here be distinguished for the first time. The vestibule, which performs the function of a mouth, is continued into a short tube, the pharynx, which is lined by a continuation of the cuticle, and is provided with numerous short cilia. It passes down into the body with a some- 14 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. what curved course, and ends abruptly within the endosarc. A small area in the vestibule, beneath the disc, is devoid of cuticle, and serving, as it does, for the extrusion of remnants of the food, may be termed the anus. Within the endosarc numerous food- vacuoles can usually be distinguished. PHYSIOLOGY. Vorticella, like Amoeba, is an unicellular animal, but the single cell of which its body is composed is very much specialized or differentiated, since different parts of it are modified for the per- formance of different functions. 1. Nutrition. — The combined action of the cilia produces a^ current by which minute organisms and organic particles are carried down to the end of the pharynx, and, together with a small amount of water, forced into the endosarc. There, in food- vacuoles, they slowly pass down one side of the body and up the other, the water being gradually absorbed, and the nutritious parts digested as in Amoeba. The undigested remnants are ejected from the anus, which is only visible at the moment of extrusion, and are carried out of the vestibule by the ciliary current. Under unfavourable conditions Vorticella sometimes detaches itself from its stalk, and swims away by means of its cilia, becoming re-attached, and developing a new stalk, if a suitable spot is reached. As in Amoeba, unfavourable conditions may also lead to encystment, the body, either whilst attached to its stalk or after se2)aration, becoming rounded, and secreting a protective horny cyst. In one species of Vorticella, V. viridis, chlorophyll is diffused throughout its body, which no doubt enables it to live, in part, like a green jjlant, utilizing carbon dioxide as a source of carbon. o 2. Katabolism. — As in Amoeba, something akin to a digestive secretion is 2)robably formed by the protoplasm. The same waste- |)roducts, namely, carbon dioxide, water, and ammonia compounds, result from the breaking-down of the body, and, as before, the contractile vacuole is probably excretory and respiratory, waste- l^roducts passing from it into the vestibule, with which it has been observed to communicate, while, on the other hand, oxygen- ated water may be taken up into it from the exterior. The PROTOZOA. 15 ciliary current brings dissolved oxygen with it, and also carries away waste-products. 3. Eeproduction (Fig. 2). — This is asexual, and usually takes place by equal hinanj fission in a longitudinal direction. The animal broadens, its nucleus elongating, and then a furrow appears, which rapidly deepens so that the animal is cleft down to its stalk, the nucleus and contractile vacuole being halved. Two equal-sized individuals result, one of which remains attached to the stalk, while the other develops a circlet of cilia near its prox- imal end, becomes detached, and swims away, later on becoming fixed by its proximal end and developing a stalk. Thus Vorticella, although a fixed form, can readily spread from, place to place, and it is, therefore, not to be wondered at that it has a very wide dis- tribution, especially if it be remembered that the process of fission only takes an hour or two for its completion. Small free-swimming individuals (microzooids), similar in struc- ture to the large free forms, may be produced by unequal fission, in which case an ordinary zooid divides into two parts, one large, the other small, — or else continued fission may take place in which equal bipartition is immediately followed by rapid division of the half to be detached into eight microzooids. Using large letters for the larger zooids (macrozooids) and small ones for the microzooids, the different kinds of fission may be expressed as follows : — Equal Fission. Unequal Fission. . ^ f B (fixed) A T / ® (fixed) • ■( B' (free) ' ( b Continued Fission. C B (fixed) A — d _^Jcccc k * c c c c A process, known as conjugation, which has an important bearing on the origin of sexual reproduction, also frequently occurs in Vorticella. A microzooid comes into contact, by its ciliated end, with a large fixed macrozooid, at a point near the junction of the stalk. The two gradually fuse together, parts of the 16 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. paranuclei uniting. Increased vigour (rejuvenescence) appears to be imparted by this process, which generally shows itself in greater reproductive energy, as displayed in fission. More rarely, encyst- ment may follow, when the nucleus becomes larger and much longer, ultimately breaking up into a number of spores, oval bodies, each provided with a circlet of cilia at one end. These are liberated by the rupture of the cyst, and becoming attached by their ciliated ends, grow up into adult forms. Before this they may, however, increase by a process of fission. 4. Contractility.— The presence of a firm cuticle prevents the formation of pseudopodia, though protoplasmic currents are observ- able in the endosarc, by which the food-vacuoles are carried round the body, and such streamings are also seen in the firmer ectosarc, though to a much less extent. Locomotion is effected in the free forms by cilia, Avhile the currents that bring food and oxygen, and carry away waste, are also due to ciliary action. Each cilium is a delicate thread of protoplasm, protruded from the ectosarc through a pore in the cuticle. By means of the alternate con- traction of its longitudinal halves, bending and straightening are produced in turn. The retraction of the body is due to the ectosarc, and in this case contraction takes place is the direction of the myophan striation, whence it appears probable that the deeper layer is the part mostly concerned in the process. The thread which traverses the stalk contracts in such a definite way, getting shorter and broader, that it deserves to be called a specially contractile or muscle fibre. The spiral direction taken by the fibre causes the stalk to be readily thrown into spiral folds. The much slower process of expansion appears to be largely due to the elasticity of the cuticle, and this is especially true of the stalk. 5. Irritability and Spontaneity.^ — These phenomena are much more definitely exhibited than in the case of Amoeba. The animals appear sensitive to the slightest touch, which causes them to contract rapidly. The same effect may be produced by irritant solutions, such as weak acetic acid, and by other stimuli. Spontaneity is shown by the way in which the cilia work together to a common end, instead of acting irregularly. Spontaneity is further exemplified by free-swimming individuals in fixing and conjugation. PROTOZOA. 17 § 3. GREGARINA. Gregarines are Protozoa, often of worm-like form, in which a relatively large size may be attained (up to | of an inch long). They are parasitic, living at the expense of other animals, and as they are found within the bodies of these “ hosts,” may be termed endopamsites. Since they are surrounded by abundance of nutri- tious food, there is no necessity for locomotor or current-producing organs, and, in fact, there are neither pseudopodia nor cilia, and digestive organs are absent as well. Fig. 3.— Gregarines (after Stein and Butschli). Enlarged to various scales. — A-E, Stages in the life-history of Monocystis agilis ; A, adult ; CO, cortical layer (dotted) ; nucleus ; B, cyst, with developing spores ; C, cyst, with ripe spores ; D, a spore with contained falciform young; E, one of the falciform young; F-K, stages in the life-history of Gregarina blattarum ; F, young adult ; ep, epimerite ; pr, proto- merite ; deu, deutomerite containing nucleus, n ; G, cyst containino- two conjugated individuals; _H, ripe cyst; gd, gelatinous layer"; f.l, firm stratified layer, within which is a network, still enclosing spores in the centre ; sp, sporoduct ; K, a spore ; L, epithelial cells from intestine of cockroach, with attached stages (1, 2, 3) of developino- spore-contents. The vesiculse seminales of the earthworm are infested with one kind of Gregarine, to which the name of Monocystis has been applied. This has a cylindrical body (Fig. 3), which in the largest species may be \ of an inch long. It is covered with a 2 2 18 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. firm cuticle, within which is a well-marked ectosarc or cortical layer, in which an appearance of longitudinal striation may be observed, probably due to an internal fluting, as in Vorticella. AVorm-like movements can be performed by the contraction of this layer. Within the ectosarc is a more fluid endosarc, and there is a large nucleus at one end. There is no contractile vacuole, which is not surprising when the sluggish nature and consequently slow katabolism of the animal is considered. Monocystis undergoes encystment as a regular part of its life- history, and this is usually preceded b}^ the conjugation of two individuals, which come into contact by their anterior ends, some- times fuse, but in any case assume a spherical form and become surrounded by a firm cyst, said to be formed from the cuticle and an altered part of the underlying protoplasm. The contents of the cyst now break up into a large number of spindle-shaped s})ores (pseudonavicellse) invested in firm envelopes, and then, by a process of division, eight nucleated sickle-shaped cells (falciform young) are formed within each spore, a small part of the proto- plasm, however, being left unused (residual core). The falciform young are ultimately liberated by rupture of the cyst and the spore-envelopes. They are capable of movement by alternate l)ending and straightening, and make their way into those cells (spermatospores) from which the male sex-cells of the earthworm are developed, thus becoming intracellular parasites. After a time they quit these cells and gradually assume the adult form. Another common kind of Gregarine is Gregarina (Clepsidrina) blattarum, found in the intestine of the cockroach. A mature individual, when young, possesses an elongated tapering bod}^ (Fig. 3), divided into three regions, (a) an anterior epimerite (‘‘cap”), provided with hook-like cuticular processes, and passing into (b) a protomerite, separated by a transverse septum of ecto- sarc from (c) a much larger deutomerite, in which the nucleus is contained. These regions are probably not distinct cells, but merel}^ parts of one cell. Sooner or later the epimerite is thrown oft'. Two individuals unite, the anterior end of one being opposite the ])Osterior end of the other, and a complicated cyst is secreted, consisting of a number of firm layers with an external gelatinous investment. Repeated nuclear division presumably takes place, and a considerable immber of barrel-shaped spores are foi’ined CCELENTEKATA. 19 just within the wall of the cyst, migrating later on to the centre. Part of the protoplasm remains as a kind of network, and a number of tubes (sporodncts) are formed, which are at first directed inwards, but are later on turned inside out (everted), projecting from the cyst, and serving as channels through which the spores escape. This, of course, involves absorption of the firm cyst at points where sporodncts are formed. From each spore a single embiyo escapes, which makes its way into one of the epithelial cells lining the intestine of the cockroach. The regions of the body are now gradually developed, the protomerite and deutomerite projecting into the cavity of the intestine, while the epimerite remains imbedded in the cell, and is soon thrown off. The young Gregarina absorbs the digested food with which it is surrounded, and quickly groAvs to the adult size. CHAPTER IL— CCELENTEKATA. § 4. HYDRA (Fresh Water Polype). Tjiis is a small animal common in ponds, ditches, and stagnant streams. Extended specimens measure from half an inch in length downwards. There are two common kinds, the Brown Hydra {Hydra fusca), and the rather smaller Green Hydra {Hydra viridis). MORPHOLOGY. 1. External Features (Fig. 4). — The body of Hydra, when extended (A), is in the form of a hollow cylinder, usually attached to some object by its closed proximal end, which constitutes an adliesive disc (the “foot”). The distal end is terminated by a con- ical projection, the Ibypostome, in the centre of wdiich is a rounded opening, the mouth {m). Several (5 to 8) slender prolongations of the body, known as tentacles {tn), project, at regular intervals, from the base of the hypostome. Both body and tentacles may 20 AN ELEMENTARY TEXT-BOOIC OF BIOLOGY. undergo various stages of contraction or shortening. In extreme contraction, the entire animal appears (A') like a small, rounded, gelatinous knob, brown or green, according to the species. The external appearance presents certain characteristic features in connection with reproduction {see below). By looking down upon the distal end of Hydra, it is seen that a number of radiating lines, all passing througli similar parts, can be drawn from the centre of the mouth along the tentacles. This sort of symmetry, common among lower animals, is of the kind termed radial. 2. Body-wall. — Amoeba, Gregarina, and Vorticella are unicellular, each being made up of one cell or morphologiccd unit. These animals therefore possess diverse parts in virtue of the specialization, differentiation, of the protoplasm of single cells. Hydra, on the other hand, is multicellular, being made up of very numerous cells, each of which is morphologically equivalent to an Amoeba. These cells, however, are modified in various ways for the performance of diverse functions. This is the principle of physiological division of labour, and the accompanying diversity of form is termed morphological differentiation. Aggregates of cells, similar in form and mode of origin, and specially capable of carrying on a particular function or functions (instead of all functions, as in Amoeba), are known as tissues, and these again are interwoven into organs, digestive, reproductive, &c. In Hydra this special- ization is not very complete ; but, as the animal scale is ascended, physiological division of labour and morphological differentiation l)ecome more and more marked. Histology. — The body of Hydra contains a large digestive cavity, prolonged into the hollow tentacles. The wall of this cavity is the hody-ivall, and the cells composing it are divided into two distinct layers, an external ectoderm (ec), and an internal endoderm (en), about twice as thick. Between the two is a very thin structureless membrane, the mesoglcea (intermediate or supporting lamella) (id), not composed of cells. The ectoderm (ec) is mainly made up of large somewhat conical cells (C) which are broadest externally, while the narrow internal end of each of them is drawn out into one or more contractile tail-like processes, which may branch. These “tails” take a longitudinal direction, and are closely attached to the mesoglcea. Each large ectoderm cell possesses a clear external border, and CCELENTERATA. 21 contains a large nucleus with nucleolus. Adjacent cells touch one another externally, whilst, internally, spaces are left between them which are occupied by interstitial cells (i.c). These are small nucleated cells present in all parts of the body but the foot, and often crowded together so as to obscure their individual outlines. Fig. 4. — Hydra, various scales. — A, Budding specimen of II . fusca extended; A', ditto, retracted; B, tranverse section of H. fusca; B', part of same on larger scale ; C, isolated tailed cells ; D, isolated thread - cell, from H. viridis; f, foot; m, mouth; tn, tentacles; ec, ectoderm; en, endoderm; h, bud; or, typical position, &c., of ovary; t, do., of testes; i.l, intermediate lamella, the dots above this represent the cut ends of “tails;” i,c, interstitial cells ; ?i, thread-cells; /, flagella; fd, organism (in B) in digestive cavity, as food. AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 22 Within some of the interstitial cells (cnidoblasts) oval vesicles (nematocysts) are developed, each of which contains a coiled-iip filament. Nematocysts are chiefly found in the distal end of the body, especially in the tentacles, where they are aggregated into “ batteries,” giving rise to an irregular surface. If a little weak acid is added to the water which contains a Hydra, the threads will be rapidly shot out. The cnidoblasts are at first situated deeply in the ectoderm, but as their nematocysts (“ thread-cells ”) develop, force their way to the surface. Several kinds of nematocyst may be distinguished, of which the largest consist of a transparent vesicle containing fluid, and (when examined in the discharged condition) this it seen to be produced (D) into a long tapering neck, which towards its end bears a few large backwardly directed spines, and is continued into an extremely long and delicate hollow filament or thread. In an unused nematocyst, the neck is turned into the vesicle, within which the filament is coiled up. The remains of the cnidoblast persist as a layer of protoplasm surrounding the vesicle and produced externally into a slender sensitive process, the cnidocil, which slopes towards the free end of the body or tentacle, as the case may be. There are also other much smaller nematocysts with shorter, thicker threads, and without barbs. Small star-shaped nerve-cellsy connected with the protoplasmic investment of the thread-cells, are said to exist in the ectoderm. The endoderm {en) is made up of a single layer of large, irre- gular, granular cells, each containing a big flattened nucleus with nucleolus, and one or more vacuoles, often of great size. The endoderm cells abut at one end upon the supporting lamella (and appear to be here produced into transversely-i\vv2ii\ged tails), while the other end projects into the digestive cavity, and can be thrust out into pseudopodia. This end often bears one or more elon- gated protoplasmic threads (flagella*) (/), which are capable of being withdrawn, like pseudopodia. The external half of each cell contains in Hydra viridis numerous small globular bodies, coated with chlorophyll. Hydra fusca possesses small bodies of similar nature, but containing particles. Reproductive Organs. — Male and female sex-organs are differ- entiated, and, as both occur in the same animal. Hydra is said to * Flagella are longer than cilia, capable of more complex movement, and either occur singly or associated in small numbers. COELENTERATA. be hermaphrodite. They are only found, as a rule, during tlie autumn months. (1) The Male Organs consist of a varying number (1 to 20) of spermaries (testes) {f, t), usually placed near the distal end of the body. Each is a conical or rounded elevation, the tvall of which is formed by large ectoderm cells, while within is an aggregate of interstitial cells. The spermaries are, in fact, projections caused by the increase of these cells at particular points of tlie body. These contained germinal cells develop into sperms (sperm- atozoa), minute tadpole-like bodies, with oval heads and long vibratile tails. The head of the sperm is mainly formed from the nucleus of the germinal cell, covered by a thin film of its protoplasm, the rest of which is drawn out into the tail. (2) The Female Organs are known as ovaries {ov), and are typically developed at the proximal end in the same manner as the spermaries, Hydra viridis usually has but one ovary; H.fusca may possess from one to eight, Each is a rounded projection, much larger than a spermary, and, when mature, contains a single large egg-cell or ovum. The young ovary contains at first a large number of germinal cells. One of these, occupying a central position, grows more vigorously than the others, becomes amoeboid, and uses them as food, amoeba-fashion. This relatively large cell is the ovum. It assumes a spherical form on attaining its full size, and though resembling ordinary cells in structure, its parts receiA^e special names. The protoplasm is termed vitellus, and contains numerous highly refractiA e bodies, knoAvn as yolk-spherules. The large spherical nucleus, or germinal vesicle contains a nucleolus, the germincd spot. PHYSIOLOGY. As might naturally be expected, the ectoderm is more esi)e- cially concerned with the function of irritability and spontaneity, Avhile the endoderm carries on digestion. O 1. Nutrition. — The food consists of small animals, often of relatively high organization, such, for example, as the active little Crustacea known as Avater fleas. The tentacles are the agents by Avhich these are conveyed to the mouth, and their nematocysts play an important part in this connection. The folloAving de- scription applies to the large forms. In the quiescent state, the 24 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. neck and filaraent of the thread-cell are inside the vesicle, whicli contains a poisonous fluid. If now a small animal touches the cnidocil (trigger-hair), the neck is first turned inside out, everted {d), and then, by means of its spines, becomes fixed in the animal. The thread is now everted within the wound, and the poisonous fluid exerts a paralyzing influence. The mouth of the animal is very extensile, and capable of taking in morsels of considerable size (B/r/). Within the digestive cavity movement of fluids, &c., is mainly effected by the action of the endodermal flagella. The endoderm-cells vary much in size at different times, and can extend themselves so as to almost obliterate the cavity. It is stated that some of these cells are glandular, secreting a fluid which diftiises into the digestive cavity, where it dissolves and breaks down the food ; but digestion is also largely intra-ceUular, the pseudopodia ingesting solid particles in the same way as in Amoeba. AVithiii these cells the food is reduced to solution, or fine division, and readily passes to the other parts of the body by diffusion. Undigested remnants are passed out through the mouth. In Hydra riridis the })resence of chlorophyll probably enables the nutrition to be partly effected as in green plants. 2. Katabolism. — Secretions are elaborated by glandular cells in various parts of the body. All parts of the body alike give rise to waste-products, which are secreted into the surrounding water, and in the process of Respiration oxygen is taken in (<■/. p. 10). 3. Reproduction is of two kinds, asexual and sexual, the one irrespective of, the other dependent on, the reproductive organs. (1) Asexual Reproduction (A and A') is effected by gemmation^ and is not limited to any special time of year, but depends upon the food supply and temperature. A small knob grows out from the side of the body, and not only the ectoderm and endoderm but also the digestive cavity of the parent are continued into it. This bud (h) lengthens, develops a mouth and tentacles, and is finally pitched off at its proximal end, which becomes an attachment disc. Several buds in the same or different stages of develop- ment may be present on one individual at the same time, and, under favourable circumstances, these may themselves bear buds before becoming detached. Fission is not known to occur in nature, but a Hydra may be cut into several })ieces, each of which can develop into a new and perfect individual. Ca:LENTEKATA. 25 (2) Sexual Reproduction. — The wall of the ripe ovary l)ursts, and part of the surface of the now spherical ovum is exposed. By the rupture of the spermary at its apex, the sperms are liberated, and by the action of their tails move rapidly head first through the water. Impregnation or fertilization of the ovum now takes place, that is, a sperm (derived from the same or a different individual) fuses with it to form an oosperm. This union of a small active male cell with a relatively large and passive female one, is the essential part of sexual reproduction. 4. Contractility. — Amoeboid and ciliary movements are seen in the endoderm. Besides these, Hydra is capable of movements on a larger scale. The body and tentacles can be extended and contracted, their appearance varying very much in consequence, and curvings, very complex in the case of the tentacles, can also be effected. These movements may lead to locomotion, either slow by the gradual shifting of the foot, or more rapid by the alternate sucker-like attachment of the foot and the mouth. Hydra has also been seen creeping along on its tentacles, the body being held almost vertical. It can also float passively near the surface of the water, mouth downwards. The retraction of the body subserves p>rotection, whilst the ten- tacular movements are mainly concerned with the procuring of food. All these movements are the result of longitudinal and tranverse contractions, separate or variously combined. The tails of the large ectoderm cells are arranged longitudinally, and by tlieir shortening or contraction furnish the retracting element. If the endoderm-cell really possesses traversely-arranged tails, their contraction would give the necessary extending element. If this is not the case, the conical parts of the tailed-cells could effect the same purpose, by contracting traversely, the result being a corresponding increase in length. Discharge of a nematocyst is caused by the contraction of the cnidoblast. By this means the contained fluid is put under pres- sure, and everts first the neck, and then the hollow filament. There is no contrivance for effecting the reverse process, aiid nematocysts, when once used, are therefore discarded. • 5. Irritability and Spontaneity. — Hydra is extremely sensitive to external stimuli, and this is especially well seen in the move- ments of retraction and in tlie discharge of nematocysts. The cnidocils are sensory, that is to say, specially modified for the 26 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. reception of external stimnli, in this case chemical. The}’ are turned in those directions whence food is most likely to approach, i.e., the oral end of body and tips of tentacles. The external sur- faces of the large ectoderm-cells appear to receive those stimuli Avhich lead to movements. Spontaneity is best shown by the co-ordination of the cells for the performance of useful movements, such as those of locomotion, which cannot all be directly due to external influences. The co- ordinative power certainly resides in the ectoderm, either in the large cells or nerve-cells, or probably in both. The discharge of the nematocysts also seems to depend on the “ will ” of the animal. If a cnidocil is touched, an impulse of some sort appears to be transmitted to the connected nerve-cell, which, in the case of food, sends back an impulse leading to the eversion of the cell, while a particle of sand, say, causes no such disturbance. DEVELOPMENT. In each ovary, as a general rule, only one ovum is developed, which when almost mature projects freely to the exterior, as a result of the rupture of the cells which cover it. Before fertil- ization the ovum divides into two unequal parts, the smaller of which is known as a polar cell. A second polar cell is next formed by further subdivision, and the nucleus of the ovum is now known as the female pronucleus. The two polar cells perish without taking any share in the further development. Fertilization now ensues, a single sperm fusing with the ovum, and its nucleus (male pronucleus) uniting with the female pro- nucleus to constitute the nucleus of the oosperm (segmentation nucleus). The oosperm at once commences to develop, and during the earlier stages remains attached to the parent. The first process is that of segmentation or cleavage, consisting of a series of cell-divisions by which the unicellular oosperm is con- verted into a cellular mass. It is, in fact, a case of continued fission, but the products of division, instead of separating and becoming distinct organisms, as in Vorticella and the like, remain connected together and constitute the rudiment of a multicellular animal. The cleavage in Hydra consists of a series of bipartitions, so that the embryo consists of 2, 4, 8, 16, 32, &c., cells succes- sively. Cleavage is here said to be both complete (holoblastic) Ca^LENTEUATA. and nearly regular, for the whole of the oosperm divides, and the resultant cells (blastomeres) are of about the same size. At the end of cleavage the embryo is a hollow sphere or blastula (blastosphere), the cavity of which is known as the blastoccele (segmentation cavity). Cells are' now budded oh‘ into the blasto- coele from the attached side of the blastula, until the embryo becomes a solid mass, consisting of an external layer of cells, the ectoderm (epiblast) covering an internal cellnlar core, the endoderm (hypoblast). At the same time a double jwotective investment, the outer layer of which is firm and chitinous, is secreted by the ectoderm. The changes above described occupy about four days, and, after their completion, the embryo falls from the ovary into the mud, where its development is slowly completed during the winter months. The further changes consist in the formation of a digestive cavity by absorption of some of the endoderm cells, while the remaining ones constitute a layer surrounding this cavity. The ectoderm at the same time becomes differentiated, and the meso- gloea makes its appearance. Later on the outer membrane is ruptnred by increase in size and elongation of the embryo, the mouth is then formed as a perforation, tentacles grow out as hollow processes of the body-wall, and the inner membrane is cast off. The development of Hydra has been investigated by several writers, whose accounts are conflicting. The lirief outline given above is probably correct in the main. Further remarks on Hydra: — It must be remembered that although Hydra is a simple and readily obtainable type of the lower multicellular animals, it is by no means a typical example of the group, i.e. , the HydrOZOa, to which it belongs. These are for the most part colonial, the colonies being produced by bud- ding. If, in Hydra, the buds, instead of becoming detached, remained united together, something resembling one of these “ hydroid zoophytes'’ would be produced. These colonial forms give rise, in a large number of cases, to free-swimming sexual individuals, which are somewhat umbrella- shaped, and are popularly known as “jelly-fish,” technically as “medusa?.” This is a good example of “alternation of generations,” where asexual and sexual stages alternate in the life history of the same form. Thus:— A, an asexual hydroid colony gives rise to S, sexual medusa?, -which again pro- duce A, and so on, indefinitely. By taking a selected series of forms it can be shown that the sexual organs of Hydra are probably morphologically equivalent (homologous) to meduste. From (1) forms with free swimming medusae, w^e can pass to (2) forms with medusa-like buds, and thence to (.3) forms with sexual buds obscurely medusa- like, — lastly to (4) Hydra, with sexual buds as mere knobs of ectoderm. If this reasoning be correct. Hydra is a degenerate form — i.e, derived from ancestors more highly differ- entiated than itself. AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 2S CHAPTER IIL— PLATYHELMIA (Flat-worms). § 5. DISTOMA (The Liver-fluke). The best-known kind of fluke is Distoma hepaticum, the Liver- fluke. Tliis is an internal j)arasite (endoparasite) when adult about an inch and a half long, which lives in the bile-ducts of several animals, more especially the sheep, and causes the disease known as liver-rot. MORPHOLOGY AND PHYSIOLOGY. 1. External Characters. — The animal (Fig. 6) is Ulateralhj sym- metrical, right and left halves, anterior and posterior ends, and upper (dorsal) and lower (ventral) surfaces being distinguishable. The animal can be divided into corresponding halves by one plane only, the median vertical. The body is flattened and somewhat leaf-shaped, its broad dorsal and ventral surfaces passing into sharj) edges at the sides and behind. The gently curved outline is interrupted at the anterior end by the head-p)apilla, a projection which tapers to a blunt point, and ends in an adhesive cup or sucker which looks somewhat ventralwards. Behind the head-papilla the sides of the body curve outwards and backwards till the maximum breadth is attained at about the junction of the anterior and middle thirds, while the hinder two-thirds iiarrow gradually to the posterior end of the body. There is a second sucker (r.s) on the ventral surface, a little way behind the head-papilla. Four median apertures are present on the surflxce of the body. These are; — (1) The piouth (pii), in the centre of the anterior sucker; the minute excretory pore (ex.p) at the posterior end; (3) the genitoJ aperture (jj.o) on the ventral surface between the suckers; and (4) the minute opening of the vagina (Laurer’s canal) on the dorsal surface. With the exception of the smooth suckers the body surface is rough, owing to the presence of an immense number of minute backwardly directed spines, which project from the cuticle, a firm membrane which invests the body and is secreted by an under- lying epidermis, that consists of a single layer of large granular cells. The hard cuticle protects and supports the body, and its I PLATYHEOriA. 29’ spines prevent the animal from slipping back as it makes its way along the bile-ducts of its host. The digestive and other organs are imbedded in a mass of tissue, which, since it supports the various parts and connects, them together, may be termed connective tissue. Polyhedrai nucleated cells are the main constituent of this tissue in the Liver-fluke, but there is also a certain amount of non-cellular o fibrous material between these. 2. Digestive Organs (Fig. 5).^ — The mouth leads into a short tube which quickly forks, the two limbs of the fork passing back and giving off numerous branching processes, all of which end blindly. There is no anus. The oval mouth (0) is situated in the middle of the anterior sucker, and leads into a small mouth-cavity, which is followed by the oval, thick-walled ]phanj7ix that passes into a very short, straight, delicate tube, the gullet or resophagus, this again opening into a bifurcated intestine. Each half of this is a thin-walled, fairly wide tube (D), running back to the end of the body, and situated near the middle line. A number of small pouches project from its inner side, whilst a large number of pouches, mostly much branched, extend from its outer side to the edge of the body. Special muscles — i.e., bands of contractile fibres — are connected with the pharynx. A sheath of such fibres, the p^'otvactor muscle, closely surrounds this organ, and, on the other hand, is connected with the anterior sucker, while retractor muscles slant back from the pharynx to the dorsal wall of the body. Histology. — The pharynx is lined by a con- tinuation of the general cuticle. Its wall is very muscular, the fibres taking various directions. The gullet and intestine have very thin walls, consisting of an epithelial layer of cells, external to which is a structureless membrane. The term epithelium is applied to membranes, formed by one or more layers of cells, which cover external and line internal surfaces. The epithelium Fig. 5, — Distoma (from Claus after Leuchart) — Ali- mentary canal. O, Mont li, a short distance behind which is ventral sucker. D, right limb of intestine. 30 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. tn. lUi: I M w t£) c 2 c C O 50 g a>g g s ® §■ 3 0 .4^ O ^ m ^ O ^ - D a c3 .0 -t-» o tD fco'^ (— I ce ^ '-I 2 £ C-. S o> -t:; o s.s C s W S , 0^5 Cw , g c6 < 2 ■" ^ ± 00^ 0> «2 m 22 f— H i> -t ^ ^ C3 . CS c +3 c s w ^ « ■§ " m ci S ® ?3 ^ - S “ H ^ ' — ' a O O cn ;3 ^ a M M 2 ~ .-- — ' P; ci t:> o I— I H -55 P^ W o o «2 ;z; < o • «\ ^ ^ ■£ o S 'i' 'tb O 'r=3 m-h p d 50 o >> C tJO' s." 02 P5 S> O ^ +3 Oa I— I ^-1 s ..^ o «« c3 o r ■ - 'i- s- Cl. 02 pq PC 4J > -p Yc o CO to OJ o o c3 H Q P P O P5 "" fb ^ P P g +3 Q 02 5 >»U-i !pJ 50 o P 52 o’^ ■ t> to p O P '■o ^ g to - >j • s &i ;> ^ P 2 g g= o .P ^ ^ Yb!^ £ P 0) CC •'- U) p a 32 p o CO 00 P 02 5 o> p o p o3 W r-3 w w ^ CO W ^3 r— 00^ H -t?r^ -tJ P o 5~ ^- O (D P O Cm O Sp^ '?! g o P re a.s ^ -3 ^ g g ^ f-> o 9 o P -b> C3 43 ._ _p H t- O C3 P O tc • 5h -p --:; o p 0X0'+-' > p o c p P2 C p P^ -3 ) i PLATYIIELMIA. 31 in question is simple — only one cell thick — and columnar, the constituent cells being longer than broad. They are amoeboid and contain large nuclei. The food consists of the bile of the host, together with blood and disintegrated liver-substance, resulting from the rot.” This is taken in l)y the action of the pharynx, which acts somewhat like a piston. By the retractor muscles it is drawn back, when the food rushes into and fills the mouth-cavity. The mouth is now closed and the pharynx brought forwards by the protractor muscle. As, meanwhile, its cavity is enlarged by the contraction of radial fibres in its wall, food passes into it, and (the cavity being diminished by the contraction of other fibres), is then squeezed on into the gullet and intestine. These present a large surfiice which can at once absorb the fluid part of the food. The amoeboid epithelial cells effect intra-cellular digestion, and perhaps also secrete a digestive ffuid which, being poured into the gut, can bring solid food into solution or a fine state of division. Undigested matter is ejected from the mouth. The digested material diffuses through the intestinal wall to the other parts of the body. 3. Excretory Organs (Fig. 6, A). — Those organs are termed “ excretory ” (in the narrowest sense), the function of which is to get rid of nitrogenous waste. They here form a network of canals ramifying throughout the body. The smallest of these each terminate in a pear-shaped “ flame ” cell, containing a large vacuole, and named from the flickering appearance it presents in the living state, and which is probably due to the presence of a row of cilia projecting into the vacuole. The cavities of the flame-cells open into the canals Avhere they terminate, and also, perhaps, into the minute sj^aces in the tissues, which collectively represent the continuous space, or hody-cavitij, found around the organs of most animals. The small tubes branch freely, and art‘ united into a network. They open into larger tubes, which, in the anterior quarter of the body, are continuous with four still larger longitudinal trunks, two dorsal {d.h) and two ventral {v.h), and in the posterior three-quarters with a median longitudinal trunk, formed by the union of the four anterior trunks. This main dud (m.d), the largest excretory trunk, runs back, just beneath the dorsal wall of the body, to open by the minute excretory pore (ex.p). Histology. — The delicate walls of the excretory tubes are made u}) of a single elastic, structureless layer. The}^ contain a clear 32 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. T tluici, in which are numerous highly refracting granules. The presence of guanin (Cg Hv 0) has been proved. 4. Reproductive Organs (Fig, 7).^ — Distoma is hermaphrodite, and its very complex reproductive organs lie, for the most part, ventral to the intestine. The genital aperture (Fig. 6, g.o) is a small oval pore situated in the middle line between the anterior and ventral suckers, and rather nearer the latter. It leads into a shallow pit, the genital sinus (Fig. 6, B), on the right of which is the larger male aperture (cr), and on the left the female aperture (od'). (1) The Male Organs consist of two spermaries (testes) (T), one behind the other, extending over a considerable area in the middle region of the body. Each is made up of several much-branched tubes, which unite to form a tube or duct of smaller calibre, the spoi'miduct (vas deferens). The spermiducts run forwards, one on each side of the middle line, to the level of the ventral sucker (S). In the latter part of their course they converge (Fig. 6, B, B'), and at the above level open into a spindle- shaped thick - walled tube, the veskula seminalis {v. sm), which tapers into a very delicate con- voluted tube, the ejaculatory duct (ej.d), that again opens into a shorter and stouter tube, the penis- (cirrus) (cr), opening at the male aperture. Around the end of the ejaculatory duct a mass of nucle- ated cells is arranged, each of which opens by an excessively fine tube into the duct. They are the- unicellular accessory gla^ids {ac.gl). Fig. 7. — Distoma Hepaticum (from Glam after Sommer). — O, Mouth ; D, limb of intestine; S, ventral sucker, in front of which is genital opening ; T, testis, in front of which is the rounded shell - gland ; Do, yolk - gland ; Ov, oviduct ; Dr, ovary. PLATYIIELMIA. 33 A gland is an organ essentially composed of one or more epithelial- cells, in which some special secretion or excretion is formed, d he simple uni- celluLar, f.e. , one-celled, condition is seen in the accessory glands. The vesicula seminalis, ejaculatory duct, accessory glands, and penis are contained in the cirrus-sac (cr.s), a hollow oval body Avith muscular Avails, lying betAveen the ventral sucker and genital opening. (2) The Female Organs are made up of the folloAving parts: — The unpaired ovary (Dr) lies on the right side, in front of the spermaries. It is composed of several branched tubes, Avhich unite together to form the oviduct (Ov). This is at first very narroAV, then gradually enlarges and becomes considerably con- voluted, and finally narroAving, opens by the female aperture (Fig. 6, B, od'). The dilated part of the oviduct (uterus) is generally found full of eggs, in Avhich embryos are beginning to develop. '^Tavo glands are connected Avith the female organs. (1) The yolk-glands (Do), Avhich secrete a nutritive substance (yolk) for the use of the developing embryos, are paired, and each of them is made up of an immense number of minute rounded bodies (acini) of glandular nature, and extends over a lateral strip outside the other reproductive organs. Behind the spermaries the tAvo meet in the middle line. From the acini fine ducts proceed, which unite into larger ones that finally open into a longitudinal duct which runs along the inner side of the gland. Just in front of the anterior spermary a transverse duct arises from this trunk, Avhich unites Avith its felloAv in the middle line to form a small yolk-reservoir, from which the short unpaired yolk- duct runs forAvards to join the oviduct not far from its commence- ment. Just before their union a fine tube, the vagina (Laurer’s canal), through Avhich sperms are introduced into the oviduct, passes to the yolk-duct from the dorsal surface, Avhere it opens by a minute aperture. (2) The shell-gland, by Avhich the egg- shells are secreted, is similar in structure to the accessory gland. It is a rounded mass, the elements of Avhich open into the oviduct near its junction with the yolk-duct. {^See Fig. 7, just below convoluted oviduct.) Histology. — The tubules of the spermary are supported by a structureless Avail, on the outside of which filamentous contractile cells are arranged longitudinally. There appears to be a lining of germinal epithelial cells, sperm-mother-cells, some of which 2 3 34 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. become free, and give rise to tufts of sperms (spermatozoa). Only part of the substance of the mother-cells is used up in this process. Each sperm possesses a small oval head, and long vibratile tail. The walls of the vesicula seminalis, cirrus-sac, and penis contain muscle-fibres variously arranged, and the last is lined by a continuation of the spiny cuticle covering the body. The ovarian tuhules possess supporting layers, and are lined by germinal epithelium, the cells of which become ova. Each of these has a small vitellus, and large germinal vesicle, with germi- nal spot. In the acini of the yolk-glands a formation of yolk-cells occurs. These contain a large amount of nutritious matter. A yellowish fluid, which can harden into shell substance, is secreted by the shell-glands. 5. Muscular System. — This is made up of bands and sheets of contractile muscle-fibres, by means of which definite movements are effected. When these fibres contract they become shorter and broader, their ends being thus brought closer together so that the parts to which they are attached tend to approach. In the case of fibres surrounding a cavity in a circular direction (pharynx, for example), narrowing of the cavity takes place, and the reverse is effected by longitudinal fibres. The movements of locomotion are mainly due to the dermal musculature, a sheet of muscle intimately connected with the skin, and together with it making up the body-wall. The sheet is composed of three layers, external, middle, and internal, the fibres of which take respec- tively transverse (circular), longitudinal, and oblique directions. Closely connected with these are the suckers, which are muscular cups of rather complex structure. There are also a great many muscular bands running through the connective-tissue which fills up the space between the organs. These bands take a dorso- ventral direction, and are also connected together so as to form a close network. Locomotion is effected in the following way : — The ventral sucker having been fixed, the head-papilla is elongated by the contraction of its circular muscle-layer. The anterior sucker then attaches itself, the ventral sucker being at the same time loosened, and the body is dragged forward by the contraction of its longitudinal muscle-layer. 6. Nervous System (Fig. 7). — Under this term are included 1 rLATYHELMIA. 35 those organs which have more especially to do with irritability and spontaneity. | Around the pharynx is placed a nerve-ring, the plane of which Ij slopes downwards and backwards. In its dorsal part there is a li swelling, the cerebral ganglmi {c.g), on either side, and a similar !■ unpaired ventral ganglion (v.g) below. From each cerebral gang- | lion two small filaments, nerves, are given off to the head-papilla, | in which they branch, whilst a stouter lateral nerve (l.n) runs | i back almost to the end of the body, giving off branches as it I does so. It lies just within the ventral body-wall, below the I reproductive organs, and is separated from its fellow by about | one-third the breadth of the body. Excessively fine filaments j are given off from the ventral ganglion to surrounding parts. I > Histology. — Two elements make up the essential part of the j nervous system — (a) Nerve-cells (ganglion-cells), {h) Nerve-fibres. The nerve-cells are most numerous in the ganglia, but are also jt present in less abundance in parts of the nerve-ring. They are ! irregular in shape, with several projections or processes; their protoplasm is clear and they contain a very distinct refractive j nucleus. The nerve-fibres are extremely delicate threads, which 1 make up the nerves and most of the nerve-ring. They are con- 1 tinuous on the one hand with the processes of the nerve-cells, on |j the other with the various organs of the body. |jii The nerve-cells are connected with one another by means of j| their processes. • ► | DEVELOPMENT (Fig. 8). | The life-history of the Liver-fluke, like that of many other | I parasites, exemplifies alternation of generations, a phenomenon far less general in the case of animals than in the case of plants. j From the egg a young fluke does not proceed direct, but a number of asexual stages — i.e., forms capable of asexual multipli- cation, intervene between it and the sexually mature adult. The | sequence of events is as follows; — (1) A ciliated embryo escapes from the egg and becomes parasitic within the lung-chamber of | a water-snail, there degenerating into (2) a shapeless sac or sj)oro- | cyst. (3) Within the sporocyst numerous cylindrical redm are 1 ; produced by a process of internal budding, and these feed upon j ! the liver of the snail. There are usually several generations of 1 ii 3G AN ELEMENTAIIY TEXT-BOOK OF BIOLOGY. reclioo. (4) A number of tadpole-shaped cercarice are formed in the redia by budding. The cercaria makes its way out of the snail, and after losing its tail, encysts upon grass. (5) A young fluke is formed within the cyst from the remains of the cercaria. (6) If the cyst is now swallowed by a sheep the young fluke emerges, makes its way up the bile-duct into the liver, and be- comes sexually mature. Cleavage (segmentation), which occurs within the oviduct, is regular and complete. The ovum, together with a number of yolk-cells, is enclosed in a horny shell to form the egg. This is oval, with a smooth surface. The ovum lies at one end, and a small circular area of the transparent shell is here marked off as the lid or operculum. The further development occurs outside the body of the sheep, the eggs passing to the exterior with the excrement. It leads to the formation of a free embryo {a and h), which gradually comes to occupy most of the cavity of the egg, as the yolk-cells are used up. AAhen fully developed, a sudden elongation of its body causes the operculum to fly open, and it is- thus liberated. The body is somewhat conical, and its thick an- terior end possesses a short retractile head-papilla. The external layer of the body-wall is formed by flattened ectoderm cells which, except on the head-papilla, bear long locomotor cilia. The deeper layer of the body-wall is granular, with rudimentary muscle-fibres, a pair of excretory funnels, and near the anterior end a pair of eye-spots. These are two small refracting cells, placed close together, and each containing a crescentic mass of pigment. The two crescents placed back to back present a somewhat X- shaped appearance. The eye-spots are imbedded in an ectodermic thickening, the cerebral ganglion. The interior of the body is mostly filled with rounded germinal-cells, but there is also an oval mass of endoderm cells (b, D), representing a rudi- mentary and niouthless gut (digestive tube). The germinal and other cells which come between this and the ectoderm are collec- tively known as the mesoderm (mesoblast). If the free embryo finds itself in water or among damp herbage, it moves actively about by means of its cilia, and should it meet with a small water-snail, Limnaea truncatula, within about eight hours development proceeds, but not otherwise. The head-papilla is lengthened, and the embryo, revolving rapidly, bores by its means into the snail. Within this host, generally in the lung- rLATYHELMIA. 37 chamber, it becomes a sporocyst. The ciliated ectoderm-cells are lost, and the body gradually becomes an elongated and rather irregular sac. Its wall is covered externally with a cuticle, below which are feebly developed muscle-fibres, within which is a layer of epithelium lining the body-cavity. Several excretory funnels are present in the body-wall, and the eye-spots, separated from each other, can still be recognised. The sporocyst may multiply by transverse fission, and it produces asexually within it a number Fig. 8. — Development of Distoma (from Claus, partly after Leuchart). — a, Free swimming ciliated embryo: h, the same contracted, with rudimentary gut, D, germinal cells, Ov, and excretory funnels. Ex: fZ, redia; 0, mouth; Ph, pharynx; D, intestine; Ex, excretory tubule ; C, contained cercaricB : e, free cercaria ; S, ventral sucker ; D, intestine; Ex, excretory tubule; the little circles in the body represent cysto- genous cells. of the next, or redia stage, in the following manner : — The body- cavity contains a large number of germinal cells, partly those of the free embryo, partly budded off' from the lining epithelium, lying loosely within it. These become aggregated into small 38 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. solid masses, morulcB, each of which becomes a gastrula (i.e., a two-layered embryo with a mouth and rudimentary digestive cavity), by an inpushing or invagination of cells on one side. This becomes a redia, which breaks through the wall of the sporocyst, the ruptured place afterwards closing up. Redia (d). — Body elongated and cylindrical. Near the anterior end is a thickened ring or collar,” while not far from the pos- terior end a pair of blunt processes project, one on each side. The body wall has a structure similar to that of the sporocyst, but the muscle is better developed. A mouth (0) is present at the anterior end of the body, which leads into a muscular pharynx (Ph). This is continued into a simple digestive sac (D). Branched excretory trunks (Ex) commencing in ciliated funnels are pre- sent. The redia wanders over the body of the snail and feeds upon its tissues, especially the liver. The posterior processes act as foot-stumps, and the collar serves as a relatively fixed point upon which the anterior end can move when feeding. Within the body-cavity, daughter-redice, or the next, cercaria stage (c), may be developed, the former only in warm weather. In either case some of the epithelial cells lining the body-cavity enlarge and segment to produce morulse, which become gastrulse, and grad- ually assume the form of daughter-redise, or cercarise, which escape by a special hirth-opening behind the collar. The Free Cercaria [e) resembles a minute tadpole. It has a rounded flattened body armed anteriorly with minute spines, and possessing anterior and ventral (S) suckers, and a muscular tail. The hody-iccdl contains numerous large granular lime-secreting cells (cystogenous cells). A mouth, pharynx, gullet, and simple forked intestine (D) are present, as well as excretory organs (Ex). The cercaria makes its way out of the snail, loses its tail, and attaches itself to a grass-stem, the cystogenous cells pouring out a secretion which hardens into a bright Avhite cyst, within which it develops into a young fiuke. If this encysted stage is swal- lowed by a sheep, the gastric juice dissolves the cyst, and the young fluke makes its way up the bile duct into the liver, there becoming sexually mature in about six weeks. It is worthy of remark that the adult sexual stage lives in the higher vertebrate host, where it has a better chance of being preserved. PLATYHELMIA. 39 The following table summarizes the succession of stages: — 1. Sexual Fluke {in liver of sheep, 5. Cercaria (frst free, then encysted on grass). produces produces 4. Redia — {in Limnaea truncatula, ■< • may produce daughter -rediae). 2. Free Embryo. i I develops into 3. Sporocyst {in Limnaea truncatula, may produce fresh sporocysts hy fission). Further Remarks on Flukes. “Alternation of Generations” includes a number of by no means equi- valent cases. One of the simplest is that described (p. 27) for certain Hydrozoa, where an asexual hydroid (A) produces a sexual medusa (S) by budding or fission. The sequence is AS, AS, &c., and this simple alterna- tion of asexual and sexual stages may be termed metagenesis. ‘The “internal budding” by which, in the Liver-fluke, redia and cercaria are produced is not, strictly speaking, an asexual process, nor, on the other hand, is it a normal sexual process. The germinal cells appear to be pre- cociously formed ova which can develop without fertilization (partheno- genetically). Such an alternation of normal sexual reproduction with parthenogenesis may be termed hetcrogamy. Writing S for the sexual fluke, s^ for the sporocyst, and s‘-^ for the redia, this particular instance, reduced to its simplest form is s^ S“ S, s^ s- S, &c. The ciliated embryo is included in the s^, since it develops into the sporocyst, and similarly the cercaria is merged in the S. D. hepaticum is but infrequently found parasitic in man, a far more important fluke, from a medical point of view, being D. hjematobium (Bilharzia), which chiefly occurs in Egypt, Abyssinia, and S. Africa. It infests the portal vein and its branches, and is remarkable among flukes in having the sexes separate. The male is elongated and worm-like ; upon its ventral surface is a groove in which the similarly shaped female is carried. Life -history unknown. '' § 6. T.®NIA (The tapeworm). Tapeworms are endoparasites, nearly all of which, when sexually mature, inhabit the intestines of vertebrate animals. One of the best known types is Toenia solium, the Common Tapeworm, which (luring the early stages of its existence is found in the muscles of the pig (intermediate host), and, when mature, in the small intestine of man (final host). 40 AN KLEMUNTAIIY TEXT-BOOK OF BIOLOGY MOEPIIOLOGY AND PHYSIOLOGY. The taceworm is bilaterally symmetrical and made up (c/.Fig- 9) of rmiS scolcx (head ami neck), about * of an inch long, and ~ neural vessel runs below the ventral nerve-cord, and a small lateral neural vessel on each side of it. These longitudinal trunks are connected by transverse lateral vessels, of which the most important are the hearts. These are six pairs of large lateral vessels in segments 6-11, which connect the dorsal and ventral trunks, and are closely attached to the anterior faces of the septa. The typical arrangement of the transverse trunks in a segment of the intestinal region is as follows ; — {a) A pair of (commissural) vessels, receiving numerous twigs from the body- wall, connect the dorsal and sub-neural trunks; (h) The intestine is supplied by numerous afferent branches from the ventral, which break up into a network, from which the blood returns by two efferent branches to the dorsal ; (c) Each nephridium possesses a very rich plexus of vessels, supplied by a branch from the ventral and sending one to the corresponding parietal. ^0 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. A large lateral oesophageal vessel runs along each side of the gullet, opening into the dorsal vessel in segment 10, and return- ing hlood from the gullet and pharynx. Course of the Circulation. — The blood is kept circulating by means of the hearts, which act as force pumps. Their walls are muscular, and they contract rhythmically from above downwards, the hinder ones contracting first. The large vessels also have contractile walls by which the circulation is assisted. The blood flows from back to front in the dorsal vessel, and the contrary way in the ventral one. The parietal vessels return blood puri- fied in the skin and nephridia to the dorsal vessels, which also receives blood from the gut by means of the lateral oesophageal and efferent intestinal vessels. This blood j)asses forwards in the dorsal vessel and through the hearts to the ventral vessels, in which the blood flows backwards and passes by afferent trunks to the gut, nephridia, and body-wall. The delicate networks of capillary vessels Avhich complete the closed blood-system are of great physiological importance, since their extremely thin walls allow of diffusion. In this way, for example, the tissues are nourished. There are no special organs of respiration and this function is performed by the general surface of the body, the carbon dioxide diffusing out of the blood-vessels underlying and penetrating the skin, while oxygen diffuses into them. The red colour of the blood is due to licemoglohm, a complex compound of carbon, hydrogen, oxygen, nitrogen, sulphur, and iron, which enters into loose combination with the oxygen taken into the body, readily parting A^dth it again to the tissues. (2) Coelom and Coelomic Fluid. — The coelom or hody -cavity is imperfectly separated into compartments by the septa, and com- municates with the exterior by the dorsal pores, excretory tubes (nephridia), and oviducts. It contains a milky coagulable coelomic (perivisceral) fluid, in which are suspended numerous irregular nucleated cells (coelomic corpuscles), with granular protoplasm, capable of amoeboid movements. The body-cavity is lined by ccelomic epithelium composed of a single layer of flattened nucleated cells, from which these corpuscles are constantly being budded off. 5. Excretory Organs.— Each segment, with the exception of the first three and the last, contains a pair of nephridia (segmental ANNELIDA. 61 organs). Each nephridium is a very long convoluted tube, of varying calibre in its different regions, attached by a membrane to the back of the septum bounding the front of the segment, and })lacing the coelom in communication with the exterior. It is thrown into three main folds running parallel to the septum in a vertical direction, and forming an internal short loop, a middle long loop, and a wide external end loop. The nephridium com- mences with a ciliated funnel which lies in the preceding segment. The margin of the funnel is formed by a row of elongated ciliated cells, and its back by a very large crescentic cell. The slit-like internal opening (nephrostome) of the nephridium is situated within the funnel. The rest of the tube is divided into four sections : (1) An extremely long and delicate narrow section which runs back from the funnel through the septum, round the short loop, up and down one side of the long loop, and back round the short loop, after which it passes into (2) a much wider middle section, contained entirely in the long loop, and followed by (3) a wide section, which begins with a dilated part, runs along one side of the long loop, round the short loop, and across to the end loop, where it is succeeded by (4) a much larger muscular section (“bladder”) which opens to the exterior by a small ventral aperture, the nephridiopore. The greater part of the nephridium (i.e., from the funnel to (4) ) is made up of tubular “ drain-pipe cells,” placed end to end, and the cavity of this part is therefore intracellular — i.e., within and not between cells. The tubular cells differ in size and character in the different regions of the neph- ridium. Cilia are found in parts of the narrow section and throughout the middle section, while the cells of the middle and wide sections are very large and full of excretory granules. The muscular section is lined by cells (its cavity being therefore intercellular), and its thick wall contains a network of muscle-fibres. The nephridium possesses a very rich network of capillary blood-vessels, supplied from the ventral vessel and returning its blood into one of the parietals. The nephridia are specially concerned with the excretion of water and nitrogenous waste, the former entering them by the ciliated funnels, and the latter being probably excreted by the glandular drain-pipe cells of the middle and wide sections. It is not unlikely that the chloragogenous cells have to do with 62 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. excretion, for they are closely connected with the blood-vessels, resemble excretory cells in their granular character, and are known to break down into the coelom, the materials thus formed being very probably got rid of by the nephridia. The urine, as the excreted substances may be collectively termed, passes down the nephridia by the action of the cilia, and collecting in the dilated muscular section is expelled by its contraction. 6. Eeproductive Organs (Fig. 19). — The Earthworm is herma- phrodite, and its sexual organs vary much in size, according to the time of year, being largest in the summer, which is the breeding-season. The segments in which they are placed are anterior to the clitellum. Their colour is white, and they lie below and at the sides of the gut, in its oesophageal region. (1) Male Organs. — ^There are two pairs of minute flattened sperjnaries (T), produced into finger-like processes behind. These are attached, near the nerve-cord, to the back of the septa forming the anterior boundaries of segments 10 and 11. In the sexually mature state they project into the cavities of the seminal reser- voirs. The most conspicuous parts of the male apparatus are the veskula', semmales, three pairs of large white pouches which may completely overlap the oesophagus, and are situated in segments 9-12. The anterior pair (A) lie in segment 9, and are united into a median seminal reservoir (B) in segment 10. This also receives the middle (C) pair of vesiculse situated in segment 11, while the j^osterior vesiculse are situated in segment 12, and unite into a similar reservoir in segment 11. Projecting into the floor of each reservoir is a pair of large plicated seminal funnels (S F) turned towards the testes. From each funnel a delicate tube, the vas efferens, proceeds, which, after forming a coil, takes a backward course. The two vasa elferentia on each side unite together in segment 12 into a straight tube, the sper7nidud (VD), Avhich passes back through segments 13 and 14 to open by the male pore on segment 15, just external to the ventral setae. The spermary may be regarded as a thickened and specialized ])art of the epithelium lining the body-cavity. It is made up of numerous germinal-cells which, as mother-sperm-cells, pass into the vesiculae seminales, the cavities of which are traversed bv numerous strands of connective tissue. In the interstices between tliese the mother-sperm-cells undergo repeated division or seg- ANNELIDA (53 mentation to ])roduce sperm-raoruUf\ each of Avlhcli contains a central spenn-hlastophor covered hy numerous small colls, .sy/cm- atocyte^, Avhicli become sperms. Tliese possess a long cylindi-ical Fig. 19. — Earthworm (from Marshall and Hurst). Plan of tlie reproduc- tive organs as seen from above after removal of the alimentary canal. The body-wall i.s pinned dowm hat.^A, Anterior vesicula seminalis, uniting with its fellow^ to form the anterior seminal reservoir (B) which is cut open on one side to display an anterior spermary (testis) ('F) and an anterior seminal funnel. C, Middle vesicula opening into B. In segment 12 is seen one of the posterior vesicula), which unite to form the ])Osterior reservoir in 11. This is cut open to display a posterior spermary (testis) and posterior seminal funnel (S F). VD, Vas deferens or spermiduct; O, ovary; OD, oviduct; B, ootheca; S, spermotheca ; N, nerve-chain ; 9-15, ninth to fifteenth segments. G4 AN ELP]MENTARY TEXT-BOOK OF BIOLOGY. head and vibratilo tail. They remain for some time attached by their heads to the blastophor, but are finally liberated.*' (2) Female Organs. — The ovaries (O) are two pear-shaped bodies, rather larger than the spermaries, and having a similar position in segment 13. Their narrow ends are directed back- Avards. The oviducts (OD) are short, wide tubes, lying mainly in segment 14, on the ventral side of Avhich they open by the female jiores, much smaller than the male pores, but occup}dng a similar position. Each oviduct ends internally in a funnel Avhich pierces the septum between segments 13 and 14, projecting into the cavity of the former. The funnel communicates with a small pouch-like ootlieca (li) (receptaculum ovorum), in Avhich a few ripe ova temporarily accumulate. The spermothecce (receptacula seminis) are two pairs of rounded sacs, lying in segments 9 and 10 (S). They open laterally between segments 9 and 10, and 10 and 11, just on a level with the lateral setae. The ovaries consist of germinal cells Avhich develop into ova. Each ovary contains many ova in different stages of development,, the ripest being at the free end. The ovum is spherical in shape, and covered externally by a delicate vitelline membrane. The granular protoplasm contains a clear germinal vesicle., with germinal spot. The ripe ova burst out of the ovary into the body-cavity, and are taken up by the ciliated funnels of the oviducts. Although the Earthworm is hermaphrodite, self-fertilization does not occur. Two individuals copulate and fertilize each other. This takes place on the surface of the ground during the Avarm spring and summer months. The two apply them- selves by their ventral surfaces, their heads being in opposite directions. Adhesion is effected by the slender genital setae, the prominent ventral edges of the sAvollen clitella, and a firm secretion of the clitella and capsulogenous glands. The sperms pass back from the male pores of either Avorm to the spermo- thecae of the other, along grooves visible at the time. They are aggregated Avithin the spermothecae into thread-like packets (spermatophores), by means of a glutinous secretion. The ova are laid in firm capsules, probably formed by the hardening of a fluid secreted by the clitellum. In L. herculeus many ova are passed into each capsule, together Avith a feAv * The contents of the vesiculse commonly contain various stages in the life- history of the Earthworm Gregarine, Monocystis lumhrici {see p. 17). ANNELIDA. 65 spermatophores (composed of sperms derived from another indi- vidual) from the spermothecae. The egg-capsule or cocoon is passed forwards over the front end of the worm, and cylindrical to begin with immediately becomes spindle-shaped by closure of its ends. The spermatophores break down within the capsule, and the liberated sperms fertilize the ova, polar cells having pre- viously been formed. 7. Muscular System. — As in the Fluke, most of the muscle is found in the body- wall. Underlying the skin is a continuous circular layer in which the pigment that gives the dorsal surface its characteristic tint is imbedded. Delicate bands, the protractor muscles of the setae, pass from this layer to the inner ends of the setigerous sacs. Beneath the circular layer is a longitudinal layer of about the same thickness, subdivided into longitudinal bands, a broad dorsal band between the two rows of lateral setae, a much narrower ventral band between the ventral setae, a lateral band of about the same width between the lateral and ventral setae on either side, and finally four excessively thin strips (two dorso- lateral and two ventro-lateral), one between the setigerous sacs in each double row. Each band is made up of plates, perpen- dicular to the circular layer, in which the fibres are arranged so as to give a feather-like appearance in cross-section. Connected with the longitudinal layer are delicate retractor muscles of the setae, attached to the setigerous sacs where they reach the middle of the circular layer. The septa contain muscular fibres, imbedded in much connective- tissue, and taking various directions. They are covered by the peritoneal epithelium of the body-cavity, with which also the longi- tudinal muscle layer is lined. The muscle-fibres are very slender cells, united together by their tapering ends. In locomotion the circular layer contracts so as to advance the front part of the body, and the hinder part is then dragged up by contraction of the longitudinal layer. The reverse movement can also be effected, and complex curves described. The setae act as hold-fasts, their direction and amount of protrusion being regulated by their retractor and protractor muscles. 8. The Nervous System (Fig. 20, B) consists of a ring encircling the gut at the junction of the buccal pouch and pharynx, a ventral cord, and numerous nerves. The dorsal side of the ring is thick- 2 5 G6 AN ELEMENTARY TEXT-BOOK OF BIOLOGAL ened into two cerebral ganglia {c.g, A and B), while its sides, the connectives (cc), run downwards and backwards and unite together to form the ventral cord. This dilates slightly into a ganglion {g) in each segment, the ganglia being best marked in the posterior part of the body. The double nature of the cord is indicated by a longitudinal furrow running along its upper surface. A nerve passes from each cerebral ganglion to the prostomium and peristomium, numerous nerves run out from the first ventral ganglion, while behind this there are typically three pairs of nerves in each segment, two arising from its ganglion (n)^ and one (septal nerves) from the cord in front of this (n). A number of delicate nerves pass from the inner sides of the connectives to the pharynx, in the walls of which they form a delicate ganglionated network. This arrangement is called the sympathetic nervous system {sy). The nervous system is essentially made up of nerve-cells and nerve-fibres. In the nerve-ring and cord there is a very delicate connective-tissue framework (neuroglia) in which these elements are imbedded. The nerve-cells occupy the anterior part of the cerebral srandia and the under side of the ventral cord both in and between the ganglia. The ventral cord is surrounded by a firm muscular sheath in which the neural vessels run, and imbedded in the dorsal part of the sheath there are three giant-fibres, which perhaps have a supporting function, but in any case are modified nerve-fibres. The central organs certainly constitute a correlating aj^paratus, but scarcely anything is known of their mode of action. 9. Special Sense Organs appear to be represented only by “goblet-bodies,” consisting of aggregates of slender epidermal cells connected with nerve-fibres, and occurring on the prosto- mium, peristomium, and, less abundantly, on the anterior segments. The Earthworm is, however, by no means devoid of special senses. The entire surface of its body is endowed with a delicate sense of touch, while certain kinds of food {e.g., bits of onion) are specially preferred and readily discovered, which seems to prove possession of both smell and taste. The structure and position of the pro- stomial and peristomial goblet-bodies indicate that these are the special organs of taste and smell. Although earthworms possess nothing that can be described as a definite sense of sight, the}^ are sensitive to intense light, which they shun, and thus escape ANNELIDA. 67 many enemies. The anterior part of the body is specially sensi- tive to light, which perhaps, as Darwin suggested, passes through the skin and acts directly on the cerebral ganglia. It has, however, been stated that all the pigmented regions of the body are affected by blue, violet, and ultra-violet rays. The sense of hearing is entirely absent. DEVELOPMENT (Fig. 20). The egg-capsules of the Earthworm are deposited during spring and summer in damp earth, a few inches from the surface. They are olive-green in colour, spindle-shaped, and about ^ of an inch long. Each capsule contains a mass of slimy albumen and several eggs, of which, however, only one usually comes to maturity. Fertilization takes place within the capsule, after which the odsperm undergoes continued cell-division (cleavage, segmentation) to form a hollow sphere (blastula, blastosphere) with a cellular wall. The sphere is then converted into a double-walled bag or gastrula by inpushing of its wall. The inner layer of the gastrula is endoderm, the outer ectoderm. At the same time the third germinal layer, mesoderm, begins to develop by division of two large cells (mesoblasts) found near the posterior end of the body. The embryo gradually becomes more and more elongated, the various organs being at the same time differentiated from the three germinal layers. The surrounding albumen is used as food, and one embryo develops more rapidly than the others which it utilizes in the same way. In the case of capsules which are kept in the laboratory, hatching takes place in from two to three weeks. 1. Cleavage (Segmentation). — This is complete (holoblastic) and irregular, the entire oosperm dividing, but the resultant cells being of unequal size. The divisions take place in a regular way (B, C) until a 7- or 8 -celled embryo is produced, consisting of 2 large, 2 medium-sized, and 3 or 4 (probably ectodermic) small cells. Beyond this there seems to be no definite order. The com- pletely segmented oosperm is a hlastula (blastosphere) containing a large segmentation-cavity (s.c) (blastocoele), bounded on one side by small and on the other by large cells, which respectively become ectoderm and endoderm (D, E, F). Two large cells (mesoblasts) are also distinguishable (E, M) from which the whole of the 68 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. mesoderm arises later on, and even during the blastula stage they divide to produce mesodermic cells which partly block up the blastocoele (F, m.s). 2. Gastrulation. — This is effected on the third day by a modi- fication of what is known in other more typical cases as emboly or embolic invagination. This is a sort of inpushing, the nature of which may be realised by taking one of the perforated india- rubber balls used by small boys as squirts, and collapsing it so as to form a double-walled cup. The earthworm blastula first of all flattens (F), and becomes an oval plate, with an upper layer of small cells (ec) constituting the ectoderm (epiblast) and a lower layer of clear columnar cells (en) constituting the endoderm (hypoblast). Mesoderm (mesoblast) is also present (ms). The plate next becomes concave below, and its edges approach. The embryo is now a gastrula (G, H, K) with an internal diges- tive cavity, the archenteron (ar), opening by a longitudinal ventral slit, the blastopore. The embryo is elongated in the direction which correspond to the long axis of the future worm, and the two mesoblasts (M) are at its posterior end. In fact, bilateral symmetry is well established. Fig. 20. — Earthworm Development (after Wilson). Enlarged to various scales. — A, Unsegmented ovum, surrounded by the vitelline membrane, with two cell-groups formed by division of polar cells. B, First cleavage (in the plane of the polar cells). C, Six-celled stage. D, Optical section of young blastula. E, Surface view of young blastula, showing the two mesoblasts. F, Optical section of a flattening blastula, seen from the side. G, Ventral view of early gastrula (anterior end directed upwards), with wide blastopore. H, Similar view of rather older gastrula, with slit-like blastopore. K, Left lateral view (in optical section) of established gastrula, in which mesoderm bands have met above mouth. L, Bight lateral view (partly in optical section) of embryo with established germinal bands and fore-gut. M, Ventral view of same embryo. N, Transverse (slightly oblique) section of rather younger embryo, cutting through germinal bands, the left neuroblast, and one of the right nephroblasts. 0, Right lateral view (in optical section) of older living embryo, showing fore-gut, mid-gut, septa, sections of body-cavity, and cerebral ganglia. P, Left lateral view of embryo in middle of development, showing segments, prostomium, head-cavity, nerve-ring and cord, seUe, nephridia, and lateral vessel, ar, Archenteron; c.g, cerebral ganglia; coe, coelom; d, septum; ec, ectoderm; en, endoderm; l.v, lateral vessel; M, primary mesoblast ; w, mouth ; ms, mesoderm (upper ms in 0 = migratory mesoderm budded off from main baud); N, nephroblast; Nb, neuroblast; m c, neural cord (nerve-cord in P) ; np, nephridia; np.c, nephric cords; 'ps, prostomium; S,c, segmentation cavity; st, fore-gut (stomodseum). ANNELIDA. 69 70 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. The blastopore immediately closes up except in front, where it acts as a mouth, and at this stage the swallowing of albumen begins, and the embryo becomes distended and thin-walled as a result (L, 0). 3. Fate of the Germinal Layers. — The ectoderm produces the epidermis with its setigerous sacs, the nervous system, the neph- ridia (except their funnels), and the epithelium lining fore- and hind-guts. The mesoderm gives rise to nearly everything between the epidermis and the epithelium of the gut, including circulatory and reproductive organs, the funnels of the nephridia, and all the muscular and connective tissue. The endoderm^ originates the epithelium of the mid-gut. Ectoderm. — The fore-gut is formed at a comparatively early stage as an ingrowth of ectoderm (st) at the unclosed mouth-end of the blastopore (L, M). Soon after gastrulation cilia are devel- oped along a median ventral strip of the ectoderm, and by their action cause the embryo to rotate in the surrounding albumen. The fore-gut is also ciliated. Not long before hatching the hind- gut is formed as a small ectodermic involution which soon meets and fuses with the mid-gut. At the time when the fore-gut is being formed a thickened band can be made out running along each side of the body, and known as a germinal band. It is partly constituted by three longitudinal rows of ectodermal cells, each of which is derived by constant subdivision from a cell (teloblast) at its posterior end. These rows and their teloblasts are at first superficial, but later on sink in and are covered by other ectodermal cells. The middle region of a fully developed germ-band (L, M, N) consists of three layers — (a) Older layer, of ordinary ectodermal cells. {h) Middle layers, of cells derived from ectodermal teloblasts. (c) Inner layer of mesoderm. The internal teloblasts are known as neuroblasts (Nb), and the neural cords {n.c) which they produce thicken and become the right and left halves of the central nervous system, shifting down and uniting ventrally to form the cord and uniting above the mouth to form the nerve-ring (see P). The two external telo- blasts on each side are termed nephroblasts (N), because the nephridia (except funnels) grow out from the nephric cord (iipx) which they originate. The two nephric cords on each side soon ANNELIDA. 71 unite together, and the setigerous sacs as well as the nephridia grow out from the hand thus formed. Mesoderm. — The whole of this is developed from the primary mesoblasts (M). These lie behind the other teloblasts and give rise to a mesodermal band [ms in K to 0) on each side which forms the middle layer of the germinal bands. From the front ends of these bands stellate cells bud off' in front and form a mass above the mouth (see 0) by which the two bands are connected together. Similar cells are budded off' in the trunk- region, but these do not develop into any adult structures. The mesodermic bands gradually broaden and thicken, their posterior ends being smallest because they have only recently been derived from the teloblasts. In each band a series of cavities (ccb) appear from before backwards. These become the sections of the adult coelom and indicate the position of the future seg- ments. Where the bands unite an unpaired “head-cavity” (P) is developed. The mesoderm which forms the outer boundary of each cavity gives rise to part of the body-wall, that making up the inner boundary originates part of the gut-wall, while the partitions between adjacent cavities become septa. The mesodermal bands gradually get broader by the division of their cells and unite together from before backwards, first ventrally and then dorsally, making their way between the other layers, and ultimately forming a continuous sheet of mesoderm. The corresponding coelomic cavities of the two bands fuse com- pletely together dorsally, but remain separated ventrally by a thin mesentery. The blood-vessels are developed as spaces in the mesoderm which at first (except in the case of the hearts) have no proper walls of their own. The dorsal vessel has a double origin, being formed from two lateral vessels {l.v in P) which gradually shift upwards and fuse together. Each neph- ridial funnel is formed by the subdivision of a large mesodermal cell which soon becomes connected with an ingrowth from the O nephric band. The endoderm becomes the lininf;; of the mid-mit, includins; the epithelial part of the oesophageal pouches and calciferous glands. Sexual Reproduction. There is some difficulty in comparing the reproduction of unicellular animals (Protozoa) and multicellular animals (Metazoa), because in the 72 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. former case all the functions are performed by the same cell, while in the latter there are special reproductive cells. The permanent conjugation of Vorticella, however, where a small active microzoid unites with a larger more passive macrozoid, appears to be equivalent to an act of sexual repro- duction— he., to the union of a male cell (sperm) with a female one (ovum). The process in Vorticella is in reality a very complicated one, and the paranuclei (micronuclei) are alone concerned in it. The ordinary nuclei (macronuclei) appear to have as their function the regulation of nutrition. In many free-swimming ciliated Protozoa “temporary” conjugation takes place between two equal-sized individuals. They apply themselves closely by their oral surfaces and exchange parts of their paranuclei, after which separation takes place, the nuclear apparatus is reconstructed, and fission is resumed. It has been demonstrated that, in ciliated Protozoa, this process is necessary for the continuance of the species, as without it asexual reproduction cannot take place beyond a certain number of generations. In one of the simplest cases [Golpidium) the changes which take place in the paranucleus are as follows: — The division is always of the “indirect” kind (mitosis, karyokinesis), and involves more than simple elongation, constriction, and division; (1) Paranucleus divides into two, iB, id, and (2) these subdivide again id, n'-^, id, n^. (3) Of these four parts three are absorbed by the protoplasm, while the fourth again divides into two, n^, n^. (4) One of these migrates into the other individual and fuses with the cor- responding part. (5) Each individual now contains a conjugation nucleus [n^^ + 113] formed by fusion of |-th of one paranucleus with |^th of the other. This compound structure divides twice, two of the quarters passing to one end and two to the other. Meanwhile the original nuclei have been absorbed and the individuals have separated. Transverse fission now takes place, the quarters above-mentioned becoming the nuclei and paranuclei of the daughter forms. The complete process may be graphically repre- sented as follows, where N = nucleus, n — paranucleus, and the indices are written above {e.g., n^) for one individual and below {e.g., 03) for the other. Elements enclosed in parentheses are absorbed, v = subdivisions of conjugation nucleus. Individual I. Individual 2. f-H w e EH m d w Eh ( ni ^ (N) ^ (U2) (11 2) 1 (Uq) no 1 I no ns J ! ■ (N 1 i- rr N andn P" < ^ lin Nandn=|';i| e of 1 V I f r 1 1 [n'^ + nslj ^ f ^ 1 1 ~ ^ ^ N and n ion. r T new = / ^ N and n ^ ( [ns + V ) ' ! The processes of conjugation just described involve complete reconstruc- tion of the nuclear elements by (a) rejection and absorption of parts of the ANNELIDA. 73 old constituents, (6) addition of new constituents from another individual. In Vorticella there is in addition dimorphism, the conjugating elements differing in size and activity. This is a form of physiological division of labour, for a small active cell is best fitted for the mere transfer of nuclear material, and a large cell is a convenient storehouse of nutriment for developmental needs. This is the meaning of the striking differences between sperm and ovum in the sexual reproduction of the Metazoa. Again, in the maturation of both ova and sperms the elimination of nuclear material just described for ciliate Protozoa is paralleled. This is the import of the polar CGIIs or bodies which are separated off from ripening ova. An ovum is commonly invested by a cell- wall, the vitelline membrane, within which are the protoplasm and nucleus (germinal vesicle). The pro- toplasm (vitellus) consists of a delicate living network, the meshes of which enclose different substances, especially nutritive granules or food-yolk, the amount of which varies with the size of the ovum. The nucleus is covered by a delicate nuclear membrane, and it consists of a modification of pro- toplasm roughly divisible into {a) achromatin, staining with difficulty, and traversed by (6) an easily stained convoluted thread or network of chromatin, part or all of which may be condensed into a rounded nucleolus (germinal spot). Fig. 21 represents the stages in the formation of polar Fig.''21. — Formation of Polar Cells in a Starfish {Asterias glacialis). A.K, after Fol ; L, after 0. Hertwig. — A, Ripe ovum, with excentric germinal vesicle and spot. B-D, Gradual metamorphosis of germinal vesicle and spot, as seen in the living egg, into two asters. F, Forma- tion of first polar cell and withdrawal of remaining part of nuclear spindle within the ovum. G, Surfaee view of living ovum, with first polar cell. H, Completion of second polar cell. I, A later stage, showing the remaining internal half of the spindle in the form of two clear A'esicles. K, Ovum, with two polar cells and radial striie round female pronucleus, as seen in the living egg. [E, F, H, I], From picric acid prejiarations. L, Expulsion of first polar cell. cells in the case of a starfish {Asterias glacialis). In A are seen the excentric germinal vesicle and spot, which become less distinct in B, C, and D. A “nuclear spindle” is then formed, consisting of threads along which half the chromatin passes to one end and half to the other. At each end of the spindle a small “central corpuscle” or centrosoma appears, which is the centre of a radiating sun-like figure, and appears to be the seat of a special sort of protoplasm (archoplasm) that determines the division and movement of the chromatin. The spindle now places itself vertically 74 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. to the surface of the ovum, and part of it, together with a minute quantity of protoplasm, is pinched off as the first polar cell (F, L, G). A second division (H, I, K) leads to the production of a second polar cell, and the nucleus, now reduced by three-fourths, travels back to the centre of the ovum. It is now termed the female pronucleus, and does not contain enough chromatin to enable the ovum to develop further. In certain ova which can develop without fertilization {i.e., are parthenogenetic) only one polar cell is formed. Weisrnann supposes that the germinal vesicle contains two kinds of protoplasm, (1) ovogenetic substance, which presides over the growth of the ovum, and (2) germ-plasma, which enables it to develop into an embryo. He believes the first polar cell (or only one in cases of par- thenogenesis) to consist of (1), the second of half (2). A certain amount of germ-plasma is imagined to be requisite for development, and when two polar cells are formed too little is left for the purpose. Fertilization according to this would seem to mean the importation of germ-plasma in sufficient quantity to make up the deficit. In Ascaris the polar cells are not formed till after the entry of the sperm. In spermatogenesis something akin to the formation of polar cells is to be observed. The whole of a mother-sperm-cell does not become converted into sperms; there is alv^ays a residue forming a sperm-blastophor or an equivalent to it. A typical sperm is somewhat tadpole-shaped. The head contains a nucleus rich in chromatin, and covered by a thin layer of pro- to])lasm drawn out into the vibratile tail, by means of which the sperm is propelled head first. Fig. 22. — Fertilization of Ovum of a Starfish {Asterias glacialis) — (after Fol). — In A-D the sperms are represented as imbedded in the mucilaginous coat of the ovum. In A a small prominence is rising from the surface of the ovum towards the nearest sperm. In B they have nearly met, and in C they have met. D, The sperm has pene- trated the ovum, and a vitelline membrane which prevents the entry of other sperms has been formed. H, Ovum showing polar cells and approach of the $ and $ pronuclei ; the protoplasm is radially striated round the former. E, F, G, Later stages in the coalescence of the two pronuclei. Fig. 22 illustrates fertilization (impregnation) as it occurs in a starfish {Asterias glacialis). A single sperm penetrates the ovum, with the proto- plasm of which its protoplasm fuses, while its nucleus, now known as the male pronucleus, unites with the female pronucleus to form the segmentation- nucleus. The oosperm constituted by the union of the male and female cells can now develop into an embryo. The phenomena of fertilization have been studied with great care in Ascaris, and it has been shown that ANNELIDA. 75 the chromatin elements of the pronuclei become intimately united, and that when the oosperm undergoes cleavage, the chromatin is halved at each cell-division in a very exact manner. Centrosomata in the protoplasm play an important part in cleavage. § 9. HIRUDO (The Leech). The medicinal Leech {Hirudo medicinalis) is a slimy, flattened worm-like animal some 2 to 6 inches long, found abundantly in the freshwater pools and marshes of the Continent. It also occurs in this country. There is a sucker at each end of the body, and by the alternate attachment of these locomotion is usually effected. The animal can also swim by means of wave- like contractions of the body. It is a matter of ordinary knoAV- ledge that the Leech is a blood-sucking ectoparasite. MOBPHOLOGY AND PHYSIOLOGY. 1. External Characters. — The bilaterally symmetrical body, when fully extended, is somewhat strap-shaped, broadest about the middle, and Avith the dorsal surface rather more convex than the ventral. A large amount of contraction can, hoAvever, take place, by Avhich the body is much shortened and its shape altered. A number of transverse grooA^es divide the body into about 95 rings or anmdi, these, however, being superficial and not corresponding to the true segments, Avhich are 26 in number, as indicated by the structure of the nervous and excretory systems. Except near the ends each segment is made up of five annuli, of Avdiich the last is distinguished by special colour- markings, and the first by a transverse series of small Avhite papillae. There is a considerable difference in appearance betAveen the dorsal and ventral surfaces, for the former is much darker and marked on each side by three longitudinal bands, of Avhich the tAvo outer are diversified by dark dots, specially conspicuous in the last annulus of each segment. The segments making up the anterior and posterior ends are shorter than the others and contain feAver annuli. An oval ventral sucker (Fig. 23) terminates the front end, and it appears to be formed by fusion of a lip-like prostomium Avith the first tAvo annuli of the most anterior or pieristorm.ial segment. The mouth is placed Avithin this sucker, and round its dorsal 76 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. margin ten minute eyes can be recognized as black spots. A large round sucker (Fig. 23), also facing ventrally, occupies the posterior end of the body, and the small dorsal anus opens just in front of it. There are two other unpaired openings, both in the median ventral line, one, the mole aperture, in the second annulus of the 6th segment, the other being the female aperture in the second annulus of the 7th segment. There are also 17 O pairs of minute excretory p>ores by which the nephridia open in the hindmost annuli of segments 2-18. The setae of the earth- worm are quite unrepresented in the leech. 2. The Skin consists of cuticle, epidermis, and dermis. The cuticle is thin, elastic, and frequently cast and renewed. It is secreted by an underlying epidermis, which is mainly made up of a single layer of cells shaped like mallets with the handles turned inward. There are also numerous epidermal glands, each of which is unicellular and o^^ens on the surface by a very narrow tube or duct. These glands are of two kinds — (1) mucous glands, scattered all over the body and secreting the slime which makes the body of a leech so slippery, and (2) clitellar glands, occurring in segments 5-7, and secreting the materials from which the cocoons are made. There is no swollen clitellum as in the earth- worm, and a further difference is that the reproductive organs open in, not in front of, the clitellar region. The dermis is a moderately thick gelatinous layer containing a number of branched cells, and traversed by pigmented fibres and a network of capillary vessels, both of which penetrate between the internal “ handles ” of the epidermal cells. There are also muscle-fibres in the dermis. Respiration is effected by the skin {i.e., is cutaneous), and absorption of oxygen from the exterior with corresponding elimi- nation of carbon dioxide is rendered possible by the superficial position of the capillary network, as described above. 3. The Digestive Organs (Fig. 23) consist of a tube running straight from mouth to anus, and divisible into fore-gut (buccal cavity and pharynx), mid-gut (gullet, crop, and stomach), and hind-gut (rectum). The fore-gnt (stomodseum) commences with a conical buccal cavity, situated at the bottom of the anterior sucker with three flat muscular jaivs projecting into it. One of these is dorsal, the others ventro-lateral, and they are arranged in a three-rayed Fig. 23. — Leech. A (slightly altered after Leuch(xrt), Diagram of side- dissection, showing general relation of organs. B (slightly altered after Vogt and Yung), Diagram of dorsal dissection, showing relation of excretory, reproductive, and nervous systems. — p.s.. Posterior sucker; m, mouth; ph, pharynx; c^, first pouch of crop; eleventh pouch of crop; st, stomach; r, rectum; a, anus; l.v., lateral vessel; and first and seventeenth (last) nephridia; sp^ and sp^, first and ninth spermaries (testes); spd, spermiduct (vas deferens); ep, epi- didymis; p, penis; ov, ovary in sac; od, oviduct; v, vagina; c.g., cerebral ganglia ; g, g, g, g, ganglia of ventral chain. 78 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. manner, thus The cuticle covering each of them is thick- ened along its margin into a large number of sharp calcified teeth. The remainder of the fore-gut is constituted by an ovoid muscular ijharynx, with which the buccal cavity communicates by an extremely small aperture. Its wall is thick, and numerous dilator muscles radiate from it to the body-wall. The pharynx is surrounded by a large number of salivary glands, each of which is a large cell from which a slender duct runs forwards to open on one of the jaws. The mid-gut (mesenteron) commences with a short and narrow gullet (oesojihagus) leading from the pharynx to a very large crop which constitutes the greater part of the gut. The crop is a thin-walled tube which gives off 11 pairs of lateral pouches, corresponding but not limited to segments 4-14, and increasing in size from before backwards. The last pair are much the largest, and extend backwards by the sides of the intestine. A small rounded stomach terminates the mid-gut. The hind-gut (proctodseum) consists of a narrow tube, the rectum or intestine, running from the stomach to the anus. A spiral fold projects into its interior. The food consists of blood sucked from a higher animal, attach- ment being effected by the anterior sucker, and a three-rayed cut made by a sawing action of the jaws. Meanwhile a fluid is poured out from the salivary gland by which the blood of the victim is prevented from coagulating. The pharynx acts as a suction-pump, its cavity being alternately enlarged by contraction of the dilator muscles and diminished by contraction of its muscular wall. A very large amount of blood can be sucked at one time, and this is stored up in the capacious crop. Meals are often infrequent and they take a corresponding time to digest, even as much as nine months. This is accounted for by the fact that digestion and absorption take place only in the relatively small stomach. 4. The Circulatory Organs consist of a blood-system which freely communicates with a reduced coelomic system, both alike containing blood coloured red by haemoglobin. There are numer- ous colourless corpuscles. The blood-vessels are distinguished from the coelomic spaces by the possession of definite muscular walls lined by epithelium. A large lateral vessel runs along each side and is connected with ANNELIDA. 79 its fellow by transverse ventral branches. Numerous branches are given off to the gut, excretory organs, and reproductive organs. The co&lom or hodij-cavity is largely filled up with connective tissue, which unites the internal organs firmly together. The chief coelomic spaces are, a dorsal sinus, running above the gut, and a ventral sinus surrounding the nerve-cord and connected with spaces which surround the internal ends of the nephridia. These sinuses have no definite walls and are simply spaces in the con- nective tissue. The vessels and sinuses communicate together by means of capillary networks, the chief of which are found in Fig. 24. — Leech. — A (slightly altered after Vogt and Yung — enlarged), a nephridium with adjacent organs, seen from above (i) testes lobe, fol- lowed by principal lobe, which again passes into the apical lobe, 3, 3; the vesicle duct, 4, arises from junction of 1 and 2, and runs to vesicle 5, which opens to exterior ; .sp, one of right spermaries (testes) ; .speZ, right spermidiict (vas deferens) ; l.v, lateral vessel of right side; g, one of ventral ganglia. B (after Whitman), longitudinal section of eye, much enlarged ; ep, epidermis ; gl, glandular cells of epidermis ; pg, pigmented sheath; r.c, refracting cells; s.c, sense cells; n, optic nerve. the skin and surrounding the crop. The latter plexus is made up of irregular tubules, the walls of which are composed of large brown granular cells, and which constitute what is known as the hotryoidal tissue. There are no hearts as in the earthworm. The course of the circulation is not accurately known. The general movements of the body largely aid in the circulation. 80 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 5. Excretory Organs. — Seventeen pairs of complicated neph- ridia succeed one another from the second to the eighteenth segment. Their internal ends lie within, but do not open into, special sections of the body-cavity communicating with the ventral sinus, and their external ends open by ventral pores. A nephridium taken from the middle of the series is an up- wardly projecting f| 'shaped loop, the limbs of which are anterior and posterior. The walls of the loop are traversed by a compli- cated system of intracellular ducts. The anterior limb commences with a blind cauliflower-shaped end which lies in the special sinus, and then runs upwards to be succeeded by the posterior limb which ends blindly below. A narrow vesicle-duct runs from the anterior limb to a vesicle which opens by a short tube to the exterior. The vesicle is muscular and lined by ciliated e2)ithelium, which also extends into the tube leading from it. As in earth- worm, therefore, the nephridial cavity is mainly intracellular, but its last section is intercellular. The former part is closely asso- ciated with an elaborate network of capillary vessels. Nitrogenous waste has been detected in the vesicles. 6. Reproductive Organs (Fig. 23). — The unpaired ventral aper- tures of the hermaphrodite system have already been described. The male organs consist of spermaries, ducts, and a protrusible penis. There are nine pairs of spherical spermaries (testes) in the ventral region of sesiments 8 to 16. The testes on each side communicate by means of wavy vasa efferentia with a sinuous spermiduct (vas deferens) which enlarges in the sixth segment into a convoluted epididymis. From this a short duct runs to the base of the pro- trusible p>enis which lies in the same segment. The female organs consist of ovaries, oviducts, and vagina, all of which are placed in the seventh segment. The ovaries are two small thread-like structures enclosed in rounded sacs, from which two narrow oviducts proceed. These unite to form an unpaired convoluted tube which is continuous with a muscular sac-like vagina. Numerous sperms with rounded heads and vibratile tails are produced in the testes by the division of mother-sperm-cells, and passing down the spermiducts are bound into cylindrical packets (sperm-ropes or spermatophores) by a secretion of the epididymes. The penis is a copulatory organ by which these packets are intro- ANNELIDA. 81 duced into the vagina of another individual. In this process two individuals mutually fertilize each other, their ventral surfaces being applied and their anterior ends turned in opposite directions. 7. Muscular System. — The body-wall, within the skin, consists of three muscle-layers, an external, a middle, and an internal. The thin external layer is composed of fibres taking a circular direction, and within this comes the thin middle layer in which the fibres are oblique, while the very much thicker internal layer is made up of numerous bundles of longitudinal fibres. There are also numerous dorso-ventral muscle-bands, the ends of which spread out in a fan-like way within the epidermis. The suckers are made up of radial and circular muscle-fibres. The muscular tissue is composed of elongated spindle-shaped muscle-cells, each of which consists of a longitudinally striated external (cortical) part and a granular nucleated internal (medul- lary) part. The former is specially contractile, the latter proto- plasmic. Locomotion is chiefly effected by a looping movement or by swimming. In the first method the posterior sucker is attached to some object and the body stretched forwards by contraction of the circular muscle-layer. Then the anterior sucker is attached and the body is dragged forwards by contraction of the longi- tudinal layer. Swimming is a much more complex affair ; in it the body is moved in a wave-like fashion. 8. The nervous system consists of a very narrow nerve-ring round the pharynx, a ganglionated ventral cord, a sympathetic system, and nerves connected with these. The nerve-ring is thickened above into cerebral ganglia, which supply the jaws, sense-organs of the head, and other anterior structures. The sides of the ring are connectives which unite these ganglia with a pair of infra-oesophageal ganglia, from which five pairs of nerves are given off. The ventral cord is made up of two closely united longitudinal halves, which enlarge to form a pair of ganglia in the first annulus of each segment. Of the 23 pairs thus constituted the first and largest are the infra- oesophageal ganglia. Then follow 21 much smaller pairs, from each of which two dorsal and two ventral nerves are given off to the corresponding segment, and the cord is terminated by the somewhat larger 23rd pair, which supply the posterior sucker and give origin to a number of nerves. 2 6 82 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. The sympathetic or visceral system is constituted by a slender nerve running on the under surface of the crop, which it supplies with numerous branches. This nerve is probably connected in front with the ventral cord ; behind it bifurcates to supply the last pair of crop-pouches. The large pear-shaped nerve cells make up the external part of the ganglia. 9. The most important sense-organs are the ten pairs of eyes situated in the dorsal margin of the anterior sucker. Each of these (Fig. 24) is a cylindrical cup, covered by elongated trans- parent epidermal cells and perforated by a nerve near its internal end. The axis of the cup is occupied by slender cells in conti- nuity with nerve fibres, and it is lined with large cells containing refractive masses. A pigment-layer surrounds the eye except on its outer side. A large number of tactile organs are found on the upper side of the front part of the body, and there is a ring of tactile papillae on the first annulus of every segment. These all closely resemble the eyes in structure, but are devoid of pigment. It is probable that the eyes in this case are modified tactile organs. The leech possesses a very delicate sense of touch. LIFE-HISTORY. The breeding-season is spring. The fertilized ova are laid in ellipsoidal cocoons, and these are deposited in the damp earth at the margin of the water. Each cocoon is about an inch long, and has a thick wall composed of a firm inner layer and a spongy outer layer which is supposed to prevent desiccation. The cocoon contains as many as twenty eggs floating freely in an albuminous liquid. The cocoon is made from the hardened secretion of the clitellar glands, and in the first stage of its formation is a cylin- drical band surrounding the clitellar region. The leech draws its body back out of this band, the ends of which then close up. The course of development is in many respects similar to that of the earthworm, and germinal bands of the same nature are present. The embryos are soon liberated from the eggs and swim about in the surrounding albumen by which they are nour- ished. The young leeches escape from the cocoon after several weeks have elapsed. ARTHROPODA. 83 CHAPTER VL— ARTHROPODA. § 10. ASTACUS (The Crayfish). The Crayfish is a small greenish lobster-like animal, living in streams and canals, under the banks of which it burrows. An average mature specimen is about five inches long. There is only one British species, Astacus fluviatilis, a large variety of which, A . fluviatilis var. noUlis, is common on the Continent, where it is largely used as food. As the points of difference are but small, the following description will apply to either : — Crayfishes are mainly carnivorous, devouring water-snails, tad- poles, insect larvse, worms or other animals, dead or alive, but they also feed on vegetable matter. They either walk by means of their four pair of jointed legs, or else swim swiftly by alter- nately bending and straightening their powerful tails, which propel them backwards; they can also employ their large nippers for climbing. The sexes are distinct, but there is not much difference exter- nally between the male and female. The most obvious distinction is that the tail of the latter is somewhat broader, and in specimens examined during winter or spring a considerable number of rather large eggs will be found attached to its under side. These are hatched in May or June. MORPHOLOGY AND PHYSIOLOGY. 1. External Characters (Figs. 25, 2G, and 27). — The bilater- ally symmetrical body is segmented, as in the Earthworm, but the segments are definite in number, and, instead of being nearly all alike, vary much in character, and many of them are fused together. Owing to these differences the body is marked out into regions from before backwards, head, thorax, and abdomen or tail. The two first are united to form the cephalothorax, which is covered by a firm continuous shell, the carapace, dorsally and laterally. The segments of all the regions bear paired, jointed appendages of various kinds, which differ greatly in form and 84 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. function. Since the body is covered by a firm calcified cuticle, segmentation and jointed appendages are a mechanical necessity to an animal of such active habits. (1) The abdomen, as the least modified part, affords a con- venient point of departure for study. It is made up of seven segments, movably joined together, the first six of which bear appendages. The abdominal region, like all the rest of the body, is covered by a firm chitinous exoskeleton which is largely calcified, but parts of it remain soft and joints are thus formed. The 3rd, 4th, and 5th abdominal segments are most typical, and similar in the two sexes. The exoskeleton of each of these segments consists of a calcified ring connected by uncalcified parts with the segments in front and behind. The broad, strongly convex dorsal and lateral part of the segment is the tergum (t). The front part of this is overlapped by the preceding tergum, and its hinder part overlaps the one behind it. Between the two extends an uncalcified intertergal membrane, which is folded when the tail is straight. The sides of the terga of adjacent segments are united by peg and socket joints, allowing of upward and downward movements. The tergum passes down on each side into a small pointed projection \pl), the pleuron (larger in the female), which is V-shaped in transverse section, being made up of an outer and an inner limb. On the under side of the segment is a slender transverse bar, the sternum {st), and broad uncalcified intervals, the inter sternal membranes, separate adjacent sterna. The sternum passes on each side into a very short but broader epimeron (epn), the junction between the two being marked by the attach- ment of the appendage. The foregoing parts form a perfectly continuous ring with no sharp lines of demarcation. The appendages are swimmerets (Figs. 26 and 27, M), and each of them is a small elongated ^-shaped limb, consisting of a prox- imal^ stalk, the protopodite (pr), and two distaP branches, an internal endopodite and external exopodite (ex). The protopodite is made up of a very short proximal and a larger distal joint. The endopodite and exopodite both possess a relatively long proximal joint, and a rather longer distal part imperfectly divided into rings. All parts of the swimmeret are more or less beset with stiff bristles or setce. * The proximal and distal ends of an appendage, &c., are the ends respec^ tively further from and nearer to its free end. ARTHROPODA. 85 Fig. 25 Dorsal and Fig. 26 Ventral View of Male Crayfish. — r, Rostrum ; c.gr^ cervical groove ; x , placed on brancliio-cardiac groove ; hr, bran- chiostegite ; t, tergum ; st, sternum; pi, pleuron ; ep, epimeron ; pr, protopodite ; ex, exopodite ; en, endopodite ; the appendages and parts of certain segments are numbered 1 to 21, but probably the eyestalks (1) are not appendages, therefore subtract one from each number to make it correct; g.g, excretory opening; m, mouth; a, anus; g.o, genital opening. 86 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. The 6tli abdominal segment (20 in Figs. 25 and 26) differs from the preceding, mainly in the great size of the swimmerets. . The protopodite is broad, thick, and undivided, while two broad oval plates fringed with setse represent endopodite and expedite, the latter being two- jointed. The last abdominal segment or telson (21) possesses neither pleura nor appendages. It is divided into two by an imperfect transverse joint, and upon its little- calcified sternal surface the anus {a) opens as a longitudinal slit with thickened edges. The telson and large 6th pair of swim- mere ts make up together the tail-fin. The 1st and 2nd abdominal segments mainly differ in the character of their appendages, but in addition to this the former is small, possesses no pleura, and hinges on to the cephalothorax in a somewhat complicated manner. In the female, the 2nd pair of abdominal appendages are swim- merets, the 1st pair, when present, rudimentary swimmerets. The appendages of these segments are modified in the male into cojpulatmj organs (Fig. 26, 15 and 16; Fig. 27, K and L). In the cephalothorax (Figs. 25 and 26) the tergal and pleural regions are covered by a continuous carapace, produced in front into a sharp spine, the rostrum (?’). On the upper surface of the carapace is a transverse groove, which slopes downwards and forwards to its front edge. This cervical groove {c.gr) marks the boundary between the head and thorax. (2) The thorax is composed of eight fused segments, the last of which is slightly movable. In many of the lower Crustacea the thoracic segments are freely movable upon one another like the abdominal segments of the crayfish, and particular interest attaches to the genus Nehalia, which is in many ways transitional between the lower and higher forms. This is a small shrimp- like animal with a thorax composed of eight segments and covered by a large fold growing back from the head. The part of the crayfish’s carapace behind the cervical groove probably corresponds to such a fold, which in this case, however, has fused closely with the thorax. The tergal region of this fold is rather narrow, and its lateral boundaries are indicated by a hr anchio -cardiac groove ( x ) running back from the cervical groove. The pleural region of the fold on each side is a large curved plate, the hranchiostegite (hr), which bends outwards and downwards to the bases of the thoracic appendages and covers a large gill-chamber. It is united with the head along the cervical groove. The epimera are very large and ARTHROPODA. 87 thin, forming on each side a plate sloping steeply downwards, which constitutes the inner wall of the gill-chamber. Grooves converging upwards mark the boundaries of the segments. The thoracic sterna {st) are narrow elongated plates, lying between the bases of the appendages, and, except the last, closely united together. The last four of these appendages are elongated ivalking-legs (11-14), the two first of which are chelate, i.e., with pincers, and possess no exopodite. The two-jointed protopodite is continuous with a five-jointed endopodite, the last joint but one of which, in the two first, is produced distally alongside the last joint, which works against it to form a claw. The broad proximal joint of the protopodite bears in the first three walking- legs a membranous plate, the epipodite {ep., L, Fig. 27), which projects into the gill-chamber, and on which gill-filaments are arranged in a plume-like way. The male genital pore {g.o) is placed on the inner side of the proximal joint of the protopodite of the last walking-leg. The female genital pore is found in a similar position on the second walking-leg. The fourth pair of thoracic appendages are the large forceps (10). These are similar in structure to the chelate walking-legs, but their claws are very much larger. The first three pairs of thoracic appendages are relatively small and flattened, and are termed foot-jaws (maxillipedes). They are directed forwards and work against one another from side to side, their inner margins being provided with stout setae. By turning a crayfish on its back the last pair (27, H) will readily be seen, and upon removing or turning back these the smaller second pair (27, G) will come into sight. These similarly conceal the still smaller first pair (27, F). They possess all the typical regions, but there is con- siderable variation in detail as may be gathered from Fig. 27. (3) The head (Figs. 25 and 28, B) is probably made up of five segments closely united, and bears five pairs of appendages. That part of the carapace in front of the cervical groove represents the terga. It is produced in front into the rostrum, on either side of which is a deep notch, in which the stalked eye is placed. This region is much broader than the corresponding thoracic one, and bounds the front of each gill-chamber. The pleural region is rudimentary, and represented by the edge of the carapace, while the epimera are narrow and the sterna represented by several small median pieces. On the sternal surface, between the 3rd 88 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. Pig. 27. — Appendages of Astacus, Right side. — A and A', Upper and lower views of antennule ; B, antenna ; C, mandible ; D, mx. 1 ; E, inx. 2 ; F, mxpd. 1 ; G, inxpd. 2 ; H, mxpd. 3 ; I, base of third walking-leg; K and L, 1st and 2nd abdominal a])pendages of male; M, typical swimt. ; N, arthrobrauchia ; pr, protopodite; e/?, endopodite; ea:, exopodite; e^, epipodite; c.s, coxopoditic setje; 5^, scaphognathite ; au, auditory opening. ARTHROPOUA. 89 and 4th segments, is the elongated and slit-like mouth (m), with a posterior lip, the metastoma (28 A, mt), from which a pointed lobe grows out on each side, and an overhanging pointed anterior lip, the lahrum (28 A, Ih), both of which are supported by calca- reous plates. In front of the mouth the sternal region is sharply bent up, and, owing to this cephalic flexure, the anterior append- ages are directed forwards (28, B). The last three pairs of head- appendages are small jaws, covered over by the foot-jaws. The hindmost are the second maxillce (27, E), which resemble the first foot-jaws, but are more delicate. The exopodite and epipodite are fused into a relatively large elongated plate, which lies in the front of the gill-chamber, and has received the special name of scaphognatkite (sg). These appendages overlap the still weaker first maxillce (D), which are devoid of endopodite. Lying at the sides of the mouth are the mandibles (C), in which exopodite and epipodite are absent. The protopodite is a firm, transversely elongated structure, the inner end of which possesses two strongly toothed ridges. The endopodite is a small three-jointed palp. The two segments in front of the mouth bear slender forwardly directed tactile appendages. Those of the 2nd segment are the large feelers or antennce (B). Each of these consists of a proto- podite Avith tAvo stout cylindrical joints, the proximal of Avhich has a small tubercle on its under surface, upon Avhich is the excretory or renal aperture (28, B). To the protopodite a scale- like exopodite, the “scpiame,” is attached externally, and a very long slender endopodite internally. This is composed of tAvo stout proximal joints and a terminal part made up of a great many small rings, imperfectly jointed together. The small feelers or antennules (A and A') attached to the 1st segment are of much less size. They lie rather internal to the antennse, and are directed forAvards. The protopodite is three-jointed, and its proximal joint, which is much the largest, abuts against its felloAv in the middle line. It is someAvhat trihedral, and on its upper surface is a small longitudinal slit {au), the auditory aperture, over which a horizontal brush of setae attached to its outer edsre O projects. The endopodite and exopodite are two short, slender, imperfectly-jointed filaments, of which the latter is rather the longer. Between the sterna of the thorax and posterior ])art of the head a number of cuticular folds project into the body. These 90 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. apodemes are calcified and make up a kind of open framework, the endophragmal system, which imperfectly roofs over a cavity known as the sternal canal, which is traversed by the ventral nerve-cord. It will be seen from the foregoing that the body of the cray- fish may be regarded as made up of twenty segments or metameres all serially homologous — that is, reducible to the same type. The dilferences between the several regions are chiefly differences of proportion, while certain parts are suppressed in some segments, and the anterior segments have fused together. These various differences have been brought al)out by division of physiological labour. The head bears the chief sense-or2:ans and contains the ganglia which supply them. Its three posterior pairs of append- ages are jaws, the last of which help to renew the water in the gill-chambers. The anterior thoracic appendages are jaws, then follow forceps which seize food, serve as weapons, and enable the animal to climb, lastly come walking-legs. The thorax also undertakes the work of respiration, and affords firm points of origin to the powerful muscles which move the tail. This is the swimming organ, and it also has secondary functions in relation to reproduction. 2. Skin. — The true skin is formed by an epidermis with a thin underlying dermis. The former secretes the exoskeleton, a many- layered pigmented cuticle, differing in degree and not in kind from that of a worm, and largely impregnated with salts of lime (seven-eighths carbonate and one-eighth phosphate). The numer- ous setae are cuticular structures. The epidermis is a single layer of columnar cells, the outlines of which are indistinct. Slender prolongations of the epidermis traverse the cuticle and end at the bases of the setae. Numerous tubular cement-glands, lined by glandular epidermal cells, open on the ventral surface of the abdomen in the female. The thin dermis is composed of connective tissue, in which delicate blood- vessels and nerves run. The exoskeleton is made up of numerous closely-united layers, which are traversed by an immense number of delicate vertical “ pore-canals.” Most of the setae are two- jointed pinnate bristles slightly sunk in small pits of the outer surface. Each of these setae is hollow and contains a granular core, which is only separated by a thin transverse cuticular layer from the epidermic process which occupies the underlying pore- canal. ARTHROPODA. 91 3. The muscular system is conveniently considered here, since its arrangement is dependent upon the structure of the exoskeleton, to the inside of which the muscles are attached. Locomotion consists in swimming, walking, or climbing. The first results from alternate flexion (bending ventralwards), and extension (straightening), of the abdomen. In accordance with this, two great flexor muscles arise from the roof of the sternal canal, and are inserted into the abdominal sterna, and two much smaller extensor museles have their origin and insertion in the side- walls of the thorax and abdominal terga respectively. During fiexion the intertergal membranes are stretched and the inter- sternal ones folded, rice rersct during extension. The lateral peg- and-socket joints only permit upward and downward movements, while excessive extension is prevented by the overlapping terga. Movements of the walking-legs and forceps effect the other two kinds of locomotion, and the successive segments of these append- ages are connected by hinge-joints, the axes of which all take different directions, so that the limb can be bent in various planes and execute complex movements. Some of the limb-muscles are extrinsic, taking origin from the exoskeleton of the body ; others are intrinsic, having both origin and insertion within the limb. The jaws are moved to and from the middle line by adductor and abductor muscles. The mandibular adductors are easily seen in dissection. The claws of the first three pairs of thoracic limbs are opened and shut by abductor and adductor muscles which work the terminal joint against the produced part of the last joint but one. In many cases it is advantageous for a large muscle to be attached to a small and definite area. This is managed by means of a firm tendon, formed as a cuticular infolding to which the muscle-fibres are connected. The muscles are made up of numerous slender inuscle-flbres resulting from the modification of cells. Each of these fibres is invested by a delicate membranous sheath (sarcolemma), and, owing to the regular alternation of dark and clear bands, is transversely striated. Hence the term “ striped ” or “ striated muscle. Longitudinal striations also occur, and along these the fibre can be split, after death, into primitive flhrillce. Beneath the sarcolemma a number of longitudinal rows of muscle-corpuscles are arranged, each of which consists of a nucleus surrounded by a small quantity of protoplasm. 92 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. The motor nerve-fibres which supply muscle come into very close relation with its fibres. The primitive-sheath becomes continuous with the sarcolemma, and the axis-cylinder passes into a granular, nucleated end-plate, resting upon the muscle substance. 4. Digestive Organs (Fig. 28, A). — The forceps and clawed walking-legs are accessory to digestion, and the six pairs of jaws overlapping one another outside the mouth are still more closely connected with the same function. But all these are more or less modified appendages, and outside the alimentary canal. This is a tube which runs straight from mouth to anus and consists of a capacious fore-gut (gullet and stomach), a very small mid-gut into which a large digestive gland opens, and an elongated tubular hind-gut (intestine). Tlie ventral mouth (m), which possesses anterior and posterior lips (/ and mt), leads into the fore-gut (stomodseum). This consists of a short, wide, upwardly-directed gullet (oesophagus) (ce), and a large sac, the stomach, into which this opens. The stomach fills most of the cavity in the head, and is divided into a larger cardiac (ca) part in front and a smaller pyloric {py) part behind, the two l^eing separated by a deep constriction. The fore-gut is an ingrowth from the exterior, and it is not, therefore, surprising to find it lined by a firm cuticle, continuous at the mouth with the exoskeleton. The cuticular lining of the stomach is locally thickened and calcified into hard plates and bars (sclerites) which form a chewing-apparatus or gastric mill, situated for the most part in the posterior part of the cardiac divdsion. Numerous projecting setae constitute a strainer in the pyloric part of the c stomach. The sclerites of the gastric mill are united together into an elastic hexagonal framework with anterior and posterior sides connected by a jointed rod. These can be diagrammatically represented as in annexed plan, the arrow pointing to the front. A broad cardiac sclerite (c) lies transversely in the dorsal wall of the cardiac division. Attached to the middle of this is a small uro-cardiac sclerite (u), directed downwards and backwards in the front wall of the constriction separating the two regions of the stomach. These two sclerites form a T-shaped arrangement. A second and somewhat similar _L-shaped combination is formed by a pyloric sclerite (p) placed across the dorsal wall of the pyloric Pty/ I U \^t + pr P ARTHROPODA. 93 part of the stomach. To this is jointed a pre-pyloric sclerite (^r), forming the stem of the ±, which takes a downward and back- ward course in the back wall of the constriction above-mentioned, and joins the end of the uro-cardiac sclerite. At the junction of the two there is a red-coloured median tooth {^), which projects into the cardiac chamber. The diagram here shown indicates the relation of these five sclerites as seen from the side. From the extremity of the cardiac sclerite on either side a small ptero-cardiac sclerite {pt) runs back, and unites with a zygo- cardiac sclerite (z) that runs forwards from the end of the pyloric sclerite. The inner side of each zygo-cardiac sclerite is thickened into a red elongated lateral tooth (f), which projects into the cardiac cavity, and is marked by numerous transverse ridges. Two anterior gastric muscles (28, a.g.m) take their origin on the inner side of the carapace, and run backwards and downwards to be inserted into the cardiac sclerite, while two posterior gastric muscles (p.g.m) run downwards and forwards from a similar origin to be inserted into the pyloric sclerite. By the contraction of the anterior and posterior gastric muscles the cardiac and pyloric sclerites are pulled away from each other, which involves — (1) the conversion of the hexagon into a rect- angle, causing the lateral teeth to approach; (2) the pulling out of the sharp fold made by the urocardiac and prepyloric sclerites, so that the median tooth passes downwards and forwards. Hence the three teeth meet together in the centre, and effectually chew anything that comes between them. The elasticity of the frame- work then comes into play and separates the teeth. In addition to the sclerites forming the gastric mill, others of less importance are found in the cardiac wall ; and in summer a round button-like calcareous gastrolith is often seen projecting into the cardiac cavity on either side. The communication between the cardiac and pyloric cavities is very narrow, partly owing to the external constriction and partly to hair-fringed projections. This is the commencement of the strainer, the rest of which is formed by hairy cushions project- ing into the pyloric cavity and reducing it to a fissure, anchor- shaped in cross-section. A valve composed of five flaps separates the stomach from the mid-eut. 94 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. The extremely short mid-gut {m.g) is not lined by cuticle. Its dorsal wall is produced upwards and forwards into a short blind tube, the ccecuin {m). A large brownish three-lobed digestive gland {li.p) (liver, hepato-pancreas) lies on each side within the cephalo-thorax, and opens by a short, wide duct (bile duct) (h.d) into the mid-gut, of which it was originally an outgrowth. A Fig. 28. — Alimentary Canal and Excretory Organs of Crayfish. — A, Alimentary canal ; m, mouth ; Ih, labrum ; mt, metastoma ; ce, gullet ; ca and py, cardiac and pyloric ends of stomach; c.o, cardiac sclerite; the horizontal shading just below reference-line shows the commence ment of the ptero-cardiac sclerite, which tapers to a point below; py.o, pyloric sclerite, running into the zy go-cardiac sclerite, which joins the ptero-cardiac ; a.g.m and p.g.m, anterior and posterior gastric muscles; m.g, mid-gut; crobably continuous with the cells of the retinula, which appear to be the end-organs for sight, while the corneal lens, crystal cone, and rhabdom are refracting elements. The crystal cones form collectively a “ vitreous body.” The action of the eye is best explained by the theory of “mosaic vision,” according to which each ommatidium is only affected by rays which correspond with it in direction, so that if the animal ARTHROPODA. 105 perceives the form of external objects, it must be by the combined action of the ommatidia, which almost certainly do not act like 80 many simple eyes. DEVELOPMENT, 1. Early Stages (Fig. 31). — The ovum is centrolecithal, con- taining a central mass of food-yolk. Cleavage (segmentation) is regular, but incomplete and peripheral, chiefly concerning the external protoplasmic part of the oosperm. This is owing to the food-yolk; but this, though it impedes division, is for the •most part affected by the cleavage, being divided into radiating Fig. 31. — Blastuca and Gastrula of Crayfish (after Reichenhach and Huxley). — A, Blastula (segmented ovum); hi, blastoderm; v, yolk. B, Gastrula ; ep.h, ectoderm (epiblast) ; m.g, archenteron (which becomes the mid-gut), bounded by endoderm (hypoblast), shaded darkly ; hp, blastopore. yolk-pyramids, continuous at first (A) with the segments of the peripheral protoplasm, but soon (B) becoming separated. The blastula, at the end of cleavage, consists of a central mass of yolk (v), enveloped by the blastoderm, a single layer of small cells (bl), which become thickened over a small oval area, the germinal disc, that marks the ventral surface of the embryo. The posterior part of this is invaginated (B) to form a small pouch {m.g) sunk in the food-yolk. The embryo has now reached the gastrula stage, and is practically a double-walled pouch, the inner wall of which is the endoderm (hypoblast) and the outer wall the ectoderm (epiblast), while its small cavity is the archenteron, and 106 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. its wide mouth the blastopore {h]o), which soon narrows, and even- tually closes. Owing to the presence of food-yolk, which the ectoderm envelops, this layer is much more extensive than the endoderm. The mesoderm (mesohlast) is recognizable at an early stage as a solid mass of cells arising in front of the archenteron, and derived from its wall. It gradually extends, by division of its elements, until it forms a complete layer surrounding the body within the ectoderm. 2. General Growth. — The germinal disc is the first trace of the ventral surface of the embryo. From its hind end the abdomen grows out as a small knob, and on its front end and sides little elevations appear, the rudiments of the eyes, labrum, and append- ages of the head and thorax. The swollen part containing food- yolk corresponds to the dorsal region of the cephalothorax, and it gradually becomes smaller as its contents are used up in the processes of growth. The young crayfish are hatched at or near the commencement of summer. They resemble the adult in most respects, but the first and last abdominal appendages are wanting, and the ends of the forceps are sharply hooked, thus serving as a means of attachment to the cement with which the swimmerets are more or less covered. Owing to the firm exoskeleton, growth cannot take place in the same way as in soft-bodied animals, and the adult size is only reached after a series of moults or ecdyses, by which the cuticle (including the endophragmal system and the linings of the fore- and hind-guts) is thrown off, a fresh one being subsequently secreted. Moulting occurs as often as eight times the first year, five the second, and after that less frecpiently. 3. Fate of the Germinal Layers. — (1) The ectoderm (epiblast) develops into the epidermis, with the exoskeleton and its various types of setse. The epithelium and cuticle of the fore- and hind- guts are also derived from this layer. They both arise as deep ])its which become connected with the archenteron to form a continuous alimentary canal. The nervous system is formed from ectodermal thickenings, and the auditory sacs from ectodermal pits. Each eye (except the pigment, which is mesodermal) is developed from (1) an ectodermal thickening, and (2) an optic pit or fold of ectoderm internal to this. All external to the retinul^e arises from (1), while (2) becomes isolated from the ARTHROPODA. 107 surface as a solid mass consisting of an outer layer, from Avhicli the retinulse originate, and an inner layer which is converted into nerve-fibres connecting the eye with the optic ganglion. (2) The endoderm (hypoblast) is converted into the epithelium of the mid-gut, and of the digestive gland Avhich grows out from each side of this. (3) The mesoderm (mesoblast) does not divide into regular mesoblastic somites (except in the tail at a comparatively late period). Irregular spaces are formed in it which become the venous sinuses. The circulatory and respiratory organs (except the epithelium and cuticle), the excretory ('?), and reproductive organs, together Avith the muscular system and the connective tissue uniting the various parts, all arise from this layer. Further Remarks on the Development of the Crayfish : — It is a noteworthy fact that most Invertebrates which inhabit rivers have a direct development, that is the young, when hatched, closely resemble the adults in appearance. Marine forms, on the contrary, are commonly hatched as free-swimming larvce, leading an independent exist- ence for some time, and differing more or less from their parents, which they ultimately come to resemble by passing through a series of changes known as a metamorphosis. This is the case, for example, with crabs and lobsters, and a larval form is of obvious advantage to theni since it affords a means of widening their area of distribution. But to most fluviatile animals free-swimming larv?e would be a positive disadvantage, since they would be very liable to be swept down to sea by the current and so perish. There is some danger of this even in the case of the young crayfish, but the risk is reduced to a minimum by their attachment immediately after hatching to the swimmerets of the mother, and it is stated that even after this she shelters them for a time under her tail, when danger threatens. Other Crustacea. The Lobster {Homarm) closely resembles the crayfish in structure, but differs in the following respects: — Ext. Chars. — Last thoracic segment completely fused with cephalothorax. Telson undivided. 1st abd. app. of $ two-jointed, with spoon-shaped end ; 2nd ditto with plate-like endo- podite. Well-developed swimmerets on 1st abd. segment of $ . Antenna with small squame. Digestive Organs. — Mid-gut with small bilobed cjecum. Intestine of a smooth anterior and ridged posterior part, with a dorsal caecum at the junction of the two. Digestive gland extends far forwards. Respy. Organs. — Podobranchs arranged as in crayfish, but the gill-plume is not fused with the epipodite. Arthrobranchs one less in number, the one found on the second thoracic segment of the crayfish being absent here. Pleurobranchs four in number. Reproductive Organs. — Tw o tubular sper- Abdomen. Thorax. Head. 108 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. TABLE OF CRAYFISH SEGMENTS. No. of Segment. Skeleton. Pro-sto- miuin I. II. III. IV. V. VI. VII. VIII. IX. X. XI. XII. XIII. XIV. XV. XVI. XVII. XVIII. XIX. XX. Cara- ' pace. r Appendages. Apertures. Antennules Antennae Mandibles Mx. I Mx. 2 Mxpd. 1 Mxpd. 2 Mxpd. 3 Forceps I Chel- Leg2 \ Leg 3 Leg 4 i $ $ l[Sw. 1] Cop>' Auditory Excretory Mouth genital genital Gills. Pod. Sca- phog Epi- pod 1 1 1 1 1 1 Arth. Sw.2 ! Swmt. App. Free. Swmt. Swmt. Swmt. (very large) 0 Anus 1 9 2 9 W 9 W 2 Pleu, Ganglia. [r] r r 1 Cerebral (Gang. 1) 1- Post- oesopha- geal (Gang. 2) Gang. 3 Gang. 4 Gang. 5 Gang. 6 Gang. 7 Gang. S Gang. 9 Gang. 10 Gang. 11 Gang. 12 ! Gang. 13 Sense- Organs. Eyes Tactile 1 Olfactory V Auditory) Tactile o o a o ^ I o Ti C3 !=! ce ® . o l-H CD CO d °° 0) tc'd a> o 02 ^ ‘1^ o 02 fco o ■TJ 0) d4 c! TO 02 D Q - . •*— * " d a> r = rudiment; [ ] signify present or absent. MOLLUSCA. 109 maries united by a transverse commissure ; spermiducts comparatively short. Nervous System. — Post-oesophageal ganglion comparatively small. Development, — Hatched as a somewhat shrimp-like Afysis larva, the last five pairs of thoracic limbs of which are provided with slender exopodites, afterwards lost, and which has no abdominal appendages. The Sea Crayfish or Norway Lobster {Nephrops) closely resembles the common lobster, but the antenna possesses a large squame, and the gill- plume of the first podobranch (that of mxpd. 2) is rudimentary or absent. The number of gills on each side in the three forms is, therefore, as follows, writing p, a, and pi for podo-, arthro-, and pleurobranchs, and omitting rudiments: — Astacus, 6p 4 11a -f \pl = 18. Homarus, 6/> + 10a 4 ‘^pl = 20. Nephrops, ^p 4- 10 a + 4 pi = 10. CHAPTER VII.— MOLLUSCA. § 11. ANODONTA and UNIO (Fresh Water Mussels). These are bivalve molluscs found in considerable numbers in the mud at the bottom of many ponds, streams, and canals, in this and other countries. Mature specimens may be as much as five or six inches long. MORPHOLOGY AND PHYSIOLOGY. 1. External Characters. — The bilaterally symmetrical body is unsegmented, flattened from side to side, and entirely enclosed in a calcareous bivalve shell, cuticular in nature, and consisting of a right and a left piece (valve). These two valves are hinged together dorsally by the ligament, an uncalcified band of horny elastic fibres, which in the closed shell are on the stretch. In dead specimens there is nothing to keep the valves closed, and they consequently “ gape,” owing to the elasticity of the ligament, but in the living animal their edges can be closely approximated by means of two strong adductor muscles, running across from valve to valve. Each valve is oval with a rounded anterior and a more pointed posterior end, a strongly curved ventral margin, and a straight dorsal margin near the front end of which is the umbo or oldest part of the shell. Around the umbo lines of growth 110 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. are concentrically disposed, which mark successive additions made during the life of the animal. The outside is olive green in colour, owing to the periostracAim, a thin horny membrane, which also forms a flexible margin to the shell. Within the periostracum there is a prismatic layer, and the shell is lined by a pearly layer, marked by a number of muscular impressions. Near the anterior end there is an oval anterior adductm' impression, and near the })osterior end a similar but larger posterior adductor impression. ^The two are connected by a pallial line running parallel to the Fig. 32. — General Dissection of Unio (from Claus, after Grohhen). — V.S and H.S, anterior and posterior adductors; M.S, labial palp; F, foot; Mt, mantle; K, gills; C.g, cerebro-pleural ganglion; P.g, pedal ganglion; M.g, visceral ganglion; O, mouth; M, stomach; L, digestive gland; Kr.S, crystalline style; D, intestine; Af, anus; Gi[, genital gland; A, exhalent aperture; E, inhalent aperture; N, nephridium; Vh, auricle; Hk, ventricle ; V. A, anterior aorta ; H. A, posterior aorta ; P, organ of Keber. ventral edge of the shell. Immediately behind the anterior adductor impression is a much smaller anterior retractor impression^ liehind which again is a small protractor imp'ession. J ust in front and above the posterior adductor impression, there is a small posterior retractor impression. From the various places above mentioned lines of shifting converge towards the umbo, indicating the successive positions occupied by the impressions during earlier stages of growth. MOLLUSCA. Ill In Unio the valves are connected at the hinge-line by small projeetions, teeth ^ which tit into corresponding sockets. Ill removing; the animal from its shell the various muscles attached to the impressions are cut through. They have corre- sponding names. The adductor muscles (Fig. 32, V.S and H.S) are broad bands of fibres which run transversely across from one valve to the other, and by their contraction keep the shell closed. Each shell is lined by a flap, the right or left lobe of the mantle. These lobes have thickened edges which are attached to the shell along the pallial line by means of muscle-fibres (Fig. 32, Mt). They are continuous dorsally with the wall of the body, from which they grow out, are fused together above and behind the posterior adductor (H.S), and immediately beyond this are closely apposed to bound two oval openings. These are — a smaller exhalent or cloacal aperktre (A) above, and a larger inhalent aperture (E) below, the edges of which are fringed with short tentacles. When at rest the mussel is completely imbedded in the mud, with its posterior end projecting, so that water can j)ass in at the inhalent and out at the exhalent aperture. In this way the animal is supplied with food and oxygen on the one hand, and gets rid of waste on the other. The mantle-lobes enclose between them a large space, the mantle-cavity, which is divided into two parts, a large branchial chamber below, and a small supra- branchial chamber above. It must not be supposed that the mantle lobes are fused posteriorly to bound the inhalent and exhalent apertures, or ventrally to bound the branchial chamber. If the animal is placed on its back the lobes can be separated without cutting anything, as in Fig. 33, where by this means the contents of the branchial chamber are exposed. In this figure the smooth sides of the exhalent aperture have been separated, and are seen just above A, on either side of which letter are the fringed right and left sides of the inhalent aperture. A large oval visceral mass (mesosoma), compressed from side to side, hangs down into the branchial cavity, but there is no distinct head, nor anything comparable to the lateral appendages of a crayfish. The lower edge of the visceral mass is produced into a yellow ploughshare-shaped muscular expansion, the foot, which projects forwards (Fig. 32, F, and Fig. 33, P), and can be protruded from the shell, serving as a locomotor organ. On either side of the I 11 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. visceral mass is a gill (ctenidium), made up of two elongated plates (Figs. 32 and 33, K). These extend behind the visceral mass l)elow the posterior adductor, and in this region the gills of opposite sides are united to- gether. In Fig. 33 this union has been cut through in order to expose part of the supra- branchial chamber. At the front end of the visceral mass, and just beneath the anterior adductor, is a transversely elongated mouth (0), on each side of which are two tri- angular folds, the labial palps (Fig. 33, Se), which are united together at their bases to form ridge -like anterior and pos- terior lips. The exhalent aperture opens out of a cloacal chamber, which is the posterior part of the supra - branchial chamber, and is floored by the united gills. The last part of the intestine or rectum, which ends in the anus (Fig. 32, Af, and Fig. 33, A), runs dorsal to the posterior adductor, and projects into this chamber. Upon either side of the vis- ceral mass, near the attach- ment of the gills, a small male or female genital opening is Fig. 33. — Nervous System of Ano- DONTA (from Claus after Keber). — O, Mouth; A, anus; K, gills; P, foot; Se, labial palp; Gg, cerebro- pleural ganglion; Pg, pedal ganglion; Vg, visceral ganglion ; G, part of genital gland ; Oe', opening of ditto ; Oe", opening of excretory organ. placed (Fig. 33, Oe'), and close to this is a rather larger excretory or renal opening (Oe"). 2. Skin. — The most important part of this is the mantle, which forms a sort of flap drawn out on either side, and secretes most of the shell. The whole external surface of the mantle and dorsal region of the body can add to the pearly-layer, but the periostracum and prismatic layer can only be increased by the thickened edge of the mantle, and hence if worn away from 1 MOLLUSCA. 113 the older part of the shell, as commonly occurs in the region of the umbo, cannot be renewed. Pearls are formed by the deposition of matter similar to that making lip the pearly-layer around grains of sand, &c., that get into the shell. The pearl-fisheries of Britain were once famous, the pearls being obtained from a species of Unio. In China, small metal images of Buddha are placed within the shells of fresh-water mussels, and thus receive a pearl}' coating. The mantle is covered externally and internally by a layer of simple columnar epithelium, which on the outer side contains many glandular cells, and on the inside is ciliated. The suli- stance of the mantle between these layers of epithelium is made of connective tissue, traversed by muscle-fibres, blood-channels, and a complex network of nerves. The skin covering the visceral mass and foot is very glandular and closely connected with underlying layers of muscle. The labial palps are, like the inside of the mantle, covered by ciliated epithelium. The shell consists of an organic basis impregnated (except in the periostracum) with carbonate of lime. The prismatic layer is made up of polygonal prisms packed closely together, and arranged obliquely to the surface. The pearly-layer is composed of numerous thin laminae, the edges of which constitute a series of delicate ridges which by diffraction produce an iridescent appearance. 3. The digestive organs consist of a convoluted tube running from mouth to anus and of a large digestive gland (Fig. 32). The mouth (0), on each side of which are the transversely striated labial palps (MS), leads into a very short gullet (oesophagus), which runs almost directly upwards, just behind the anterior adductor, and passes into a dilated stomach (M). This is succeeded by a narrow thin-walled intestine (D), which coils about (mostly in the median vertical plane) within the visceral mass, where it is closely surrounded by the genital gland (G), finally curving upwards and running back dorsally as the rectum through the ventricle of the heart, and over the posterior adductor to terminate in the anus. The cavity of the stomach is partially subdivided by irregular folds, and the ventral wall of the rectum is longi- tudinally infolded to form a typhlosole. A lobed digestive gland (L), dark brown in colour and made up of numerous branched 2 8 114 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. tubules, closely surrounds the stomach, into which it opens by several ducts. The alimentary canal is lined throughout by simple columnar epithelium, which is partly ciliated, partly glandular. External to this are muscular layers. The tubules of which the digestive gland is made up are lined by glandular epithelium, the cells of which are cuboidal with brown granular contents. By the succes- sive unions of these tubules the ducts are ultimately formed. The ciliated epithelium, lining the mantle-lobes and covering the gills and labial palps, sets up currents in the mantle-cavity, and causes water to enter the branchial chamber by the inhalent orifice. Some of this is conducted along the groove between each pair of labial palps to the mouth, into which the small organisms it contains are carried. These constitute the food, and by the contraction of the muscular walls of the alimentary canal gradu- ally pass backwards within it. The fluid secreted by the digestive gland contains ferments by the action of which fats are emul- sified, while starch and proteid are respectively converted into grape-sugar and peptone. The length of the intestine increases the surface for absorption, and this is augmented by the typhlosole. The digested parts of the food diffuse into the blood-vessels of the intestinal walls, and the refuse is ejected at the anus, being afterwards carried out of the cloacal chamber by the currents which flow from the exhalent aperture as a result of ciliary action. In specimens examined during autumn a transparent elastic rod, the crystalline style, will be found in the stomach. It is of albu- minous nature and probably serves as a store of nutriment, for it is gradually used uj) during the winter. Some material of similar kind, only less compacted, is found in the intestine at the same period. 4. Circulatory Organs (Figs. 32 and 34). — A blood system alone is present, containing colourless blood in which are sus- pended numerous nucleated amoeboid corpuscles devoid of colour. This system is constituted by heart, arteries, veins, and irregular * blood-spaces. The heart, which is contained within a thin-walled pericardial cavity situated on the dorsal side of the body, consists of a central oval muscular ventricle, into which a thin-walled auricle opens on each side. The auricles are funnel-shaped, with the narrow end attached to the ventricle, the broad end to the MOLLUSCA. 115 ventro-lateral wall of the pericardial cavity. An aiiriculo-ventricular valve is placed at the point of union of each auricle with the ventricle. It is made up of two small flaps, which j)ermit blood to pass into but not out of the ventricle. The ventricle (through which the rectum runs) gives ofl* two arteries, an anterior aorta in front running forwards above the rectum, and a posterior aorta behind running backwards below the rectum. The anterior aorta divides into branches, which supply the labial palps, the greater part of the alimentary canal, the foot, the anterior adductor, and adjacent parts; while the posterior aorta supplies the hinder part of the rectum, the posterior adductors, and the greater part of the mantle. The ultimate branches of these arteries end in minute irregular spaces (lacunae) from which veins arise. These are vessels in which the blood is flowing towards the heart. The foot and organs of the visceral mass return their blood to the vena cava, a longitudinal vein lying in the middle line immediately below the floor of the pericardial chamber. The blood passes from the vena cava into the neph- ridium on either side, where it enters a close network of channels. From this network vessels run to the gills, opening on each side into an afferent branchial vein, which runs along the junction of the outer and inner gill-plate. Numerous branches pass into the gill-substance from this vein. The blood is returned to each auricle by an efferent branchial vein, which takes a course along the attachment of the outer lamella of the outer gill-plate on that side. Efferent veins from the mantle-lobes also enter the efferent branchial veins at each end. Course of the Circulation. — The heart is systemic, receiving pure blood from the gills and mantle-lobes, which passes into the auricles, and thence into the ventricle. The former contract together, filling the ventricle, which then also contracts in a wave-like manner, and forces the blood through the arteries to the body at large. From the smallest arterial branches the blood passes into the lacunae, and thence into venous channels, which are arranged as above described. The result is that the blood from the visceral mass and foot is purified in the nephridia, and then in the gills, before reaching the auricles, while the blood from the mantle-lobes is oxygenated there, and returned to the auricles without passing through either nephridia or gills. 4. Respiratory Organs (Figs. 32, 33, and 34). — These consist 116 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. Fig. 34. — Mussel, $ — A, Section passing through pedal ganglia, showing stomach, st, surrounded by digestive gland, d.g; labial palps, l.jy, and foot, f, projecting into branchial chamber, laterally bounded by mantle, m. B, Section passing through heart. Above is seen the ventricle, vn, traversed by rectum, r, with auricle, aw, on each side. Below pericardium are seen ureters, w, with vena cava, v.c, between them ; and kidneys, k, with cerebro-visceral connectives (black) be- tween them. Projecting into branchial cavity is visceral mass showing intestine, i, cut through in six places, and foot, f ; also inner and outer gill-plates, i.g and o.g, on each side ; ax, gill axis on each side traversed by afferent branchial vessel (black) ; the outer gill-plates are full of larvse, and x placed in each of the outer supra-branchial passages, the smaller inner passages are seen above the inner gill-plates. The branches of the ovary, which fill up the greater part of the visceral mass, are omitted. C, Section passing through visceral ganglia, v.g. The rectum, r, is seen above the posterior adductor, p. add. The inner gill-plates have united together behind the visceral mass ; a is placed in each outer supra-branchial passage, and in the median passage formed by union of two inner ones. In a section still further back these three channels would be found to have coalesced to form the cloacal chamber. Gill axes as in B. D, Small part of longitudinal section through outer gill (enlarged), showing the two gill-plates formed by union of flattened gill-filaments, /, supported by chitinous rods (dark dots). Between the plates are seen two water-tubes, iv.t, separated by an interlamellar junction, i.l.j, in which is a blood-vessel, h.v. The plates are per- forated by numerous small apertures, leading into the water- tubes. E, Outer parts of six gill-filaments, further enlarged, showing supporting rods. MOLLUSCA. 117 of the mantle and of a gill {ctenidiiLm) on each side, composed of an inner and an outer gill-plate, each of these being again made up of an outer and inner lamella united together ventrally. The outer lamella of the outer gill-plate is attached by its dorsal edge to the inner side of the mantle, close to where it joins the body- wall. The inner lamella of the outer gill is united above with the outer lamella of the inner gill, and the common union of the two (gill-axis) is attached above to the wall of the body. The inner lamella of the inner gill-plate is attached anteriorly to the body-wall, is then free for a short distance, and behind the visceral mass unites with its fellow in the middle line to form the floor of the cloacal chamber. The gill-lamellae are vertically striated, and between the striations numerous minute apertures are present, leading into an interlamellar space present in the interior of each plate, which is divided into a number of vertical water-tubes by the union of the two lamellaB along a corresponding* number of narrow vertical strips hj interlamellar junctions. The visceral mass divides the anterior part of the supra-branchial chamber into right and left halves. Each of these, owing to the dorsal attachment of the gill-axis, is again subdivided into two. There are, therefore, four channels above the gill- plates in this region, into which the four sets of water-tubes open. Behind the visceral mass the two inner channels coalesce with one another, and. Anally, in the cloacal region, the supra- branchial chamber is undivided. These various relations will be understood by examining Fig. 34. In Fig. 33 the cohering inner plates have been separated so as to expose part of the supra-branchial chamber. The gills of Anodon are unusually complicated. They appear to have arisen in the following way : — From the gill-axis on either side two longi- tudinal rows of filaments grew out. The ends of the outer row turned sharply outwards and upwards ; those of the inner row inwards and upwards. Thus in cross-section a W-shaped outline would be seen. These filaments then fused into two folded lamellae, numerous apertures being left, however. The descending and ascending halves of each lamella then became connected to form a gill-plate. Union of the plates with one another and with surrounding parts increased the complexity, and brought about the distinction between branchial and supra-branchial chambers. Each gill is covered by a single layer of ciliated epithelium. The external ridges (representing gill-filaments), which appear to the naked eye as vertical striae, are supported by slender horny 118 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. rods, two of which run vertically within each ridge. Beneath the epithelium the gills are composed of loose connective tissue, everywhere permeated by blood-containing lacunae. The inter- lamellar junctions are traversed by more definite blood-channels. Respiration is said to be effected more by the mantle than the gills, which are elaborate current-producing organs, and, as usual, essentially consists of gaseous interchange between the blood and the surrounding medium, oxygen being taken up and carbon dioxide eliminated. Water entering the branchial chamber by the inhalent aperture partly passes to the mouth and partly into the water-tubes of the gills through the numerous small apertures in the lamellae. It then runs upwards into the supra-branchial passages, and backwards through the exhalent aperture to the exterior. All the currents are the result of ciliary action. 6. Excretory Organs (Figs. 32 and 34). — A pair of JMneys (organs of Bojanus) are present, each of which is essentially a tube folded upon itself, and communicating on the one hand with the pericardial cavity, which appears to represent a much-reduced coelom, and on the other with the exterior by the renal opening noted above. The tube is divided into a thick-walled glandular part, the lining of which is raised into numerous ridges, and a thin-walled ureter (non-glandular part). The vena cava lies between the two ureters, which open in front by the renal openings, and communicate posteriorly with the glandular parts which underlie them. The glandular part is broadest behind, where it abuts upon the posterior adductor, and narrows in front wdiere it opens into the floor of the pericardium. The kidneys should perhaps be regarded as nephridia, like the excretory organs of earthworm, since they are excretory tubes opening out of a coelomic body-cavity. The glandular parts of the kidneys are lined by glandular epithelium, the cells of which are granular, and often contain concretions, in which guanin (C^H^N^O), uric acid (C5H^N^03), and urea (CH^N20), have been stated to occur. The organs of Keber, consisting of glandular tubules, and situated one on each side of the pericardium, into which they have been said to open, may also have an excretory function. 7. Reproductive Organs. — The sexes are distinct, and the female is somewhat thicker from side to side. The genital glands {spermaries or ovaries, as the case may be) are much- MOLLUSCA. 119 branched structures, surrounding the coils of the intestine in the visceral mass (Fig. 32, G). They are yellowish in the male, reddish in the female, and consist of an immense number of blind tubules, from which ducts pass on either side, uniting together into a short spermiduct or oviduct which opens to the exterior by the genital aperture (Fig. 33, Oe^). The tubules of the gonads are lined by germinal epithelium. These in the male produce large numbers of sperms, each of which possesses a cylindrical head and motile filiform tail. In the female ova are developed, each of which is covered by a vitelline membrane, which at one point, the micropyle, is incom- plete. The vitellus, which contains a moderate amount of food- yolk, is separated when ripe by albuminous fluid from the vitelline membrane. It contains a large germinal vesicle with a germinal spot Each ovum while still in the ovary is contained in a follicle, to .the wall of which it is attached by a protoplasmic stalk passing through the micropyle. The ova pass back from the genital openings to the cloacal chamber, and then forwards into the water-tubes of the outer gill-plates, where they are fertilized by sperms brought in by the inhalent current. The sperm enters the ovum by the micropyle. The breeding season is in the summer. 8. Miuscular System. — The skin is intimately connected with underlying muscle, and there are also definite muscles, of which the adductors have already been described. The foot is almost entirely made up of muscular tissue, and slender protractor, anterior retractor, and posterior retractor muscles, which take origin from the corresponding impressions on the shell are inserted into it. The muscles are composed of fusiform muscle-cells, exhibiting somewhat indefinite transverse striations. The foot serves as an organ of locomotion by which the animal can move forwards in the mud, leaving a furrow-like trail behind it. The retractor muscles by their contraction draw the foot back, and the pro- tractors pull it forwards. 9. The nervous system (Figs. 32, 33, and 34) consists of a wide oesophageal nerve-ring, a long visceral loop connected with this on the dorsal side, and numerous nerves. Two pairs of ganglia (cerebro-pleural and pedal) are developed upon the ring, and one pair (visceral) upon the loop. It is convenient to use the term commissure for a nerve-cord connecting sranelia of the same name. 120 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. and the term connective for a nerve-cord connecting ganglia of a different name. Both kinds are here exemplified, but most of the nerve-cords are connectives, not commissures. On each side of the mouth, just beneath the skin, a cerebro-jpleural ganglion is situated, and the dorsal part of the nerve-ring constitutes a short com- missure above the mouth connecting these two ganglia. A pair pedal ganglia lie close together at the junction of the foot and the visceral mass, resting upon the muscle of the former nearer its anterior than its posterior end. Each side of the nerve-ring may be regarded as a cerebro-pedal connective, since it unites a cerebro-pleural ganglion with the pedal ganglion of the same side. The visceral loop runs back along the dorsal side from the cerebro- pleural ganglia to the under side of the posterior adductor, where it thickens into a closely apposed pair of visceral {parieto-splanclinic) ganglia. Each side of the loop constitutes a very long cerebro- visceral connective running back from one cerebro-pleural ganglion to the visceral ganglion of the same side, to reach which it first traverses the digestive glands and then takes a course along the inner side of the kidney. The cerebro-pleural ganglia supply the lips, labial palps, anterior adductor, and probably the otocysts. They also give off anterior pallial nerves which break up into a network in the margin of the front part of the mantle. The pedal ganglia supply the muscular tissue of the foot, and from the visceral ganglia three chief pairs of nerves run out to the gills and mantle. These are (1) in front, branchial nerves, then (2) lateral pallial nerves, and (3) posterior pallial nerves, which supply the tentacles of the inhalent aperture, and break up to form a complex nervous network in the margin of the hinder part of the mantle, continuous with the similar network arising from the anterior pallial nerves. The visceral ganglia also send nerves to the posterior adductor and the rectum. A visceral or gastric nerve runs to the digestive gland and stomach from each cerebro-visceral connective, not far from its anterior end. As usual, ganglion-cells and nerve-fihres are the essential parts of the nervous system, the former being entirely confined to the outer part of the ganglia and nerve-origins. 10. Sense Organs. — (1) Tactile Organs. — The labial palps act as feelers, and their epithelium comes into close relation with fibres from the cerebral ganglia. The tentacles surrounding the MOLLUSCA. 121 inhalent aperture are well supplied with fibres from the visceral ganglia, and appear to be specially sensitive. The palps, tentacles, and mantle margin are provided with tactile cells, each of which is elongated and provided at its external end with a bunch of delicate setse. (2) Gustatory Organs. — The labial palps (Q possibly subserve the function of taste. (3) Olfactory Organs. — Each of the branchial nerves is covered at its beginning by a patch of sensory epithelium known as an osphradium, and usually considered as a sort of olfactory organ which perhaps tests the quality of the water entering the pallial chamber. (4) Auditory Organs. — Two minute vesicles, the otocysts, are found just behind the pedal ganglia, each of them being placed at the end of an auditory nerve which comes off from the lower end of the corresponding cerebro-pleural connective, and the fibres of which probably run up the connective to the cerebro- pleural ganglion of the same side. The otocysts are lined by ciliated columnar epithelium of sensory nature, and each of them contains a rounded otolith made up of concentric layers of carbonate of lime. The otocysts are difficult to study in Anodon, but are readily seen under the microscope in a living specimen of Cyclas, a small freshwater bivalve abundant in many streams and canals. The mussel never at any time possesses eyes. LIFE-HISTORY. The ova are fertilized and the oosperms developed, up to a certain point, in the water-tubes of the outer gills of the female, the interlamellar junctions of which secrete a nutritive substance. It is an interesting fact that the shell commences as a single saddle-shaped plate on the dorsal surface. As the mantle-lobes are formed, this is divided into the two valves, which remain connected by the ligament, so that this must be regarded as an uncalcified part of the shell. Each valve is triangular, and its lower pointed end is bent inwards into a sharp spine. In the hinder part of the body a pit is formed, the hyssus gland, which secretes a long sticky filament or hyssus. 122 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. Many shell-fish, e.gr., the salt-water Mussel, are attached during life to surrounding objects by threads forming the byssus. The embryo now passes out of the gill of the parent, being ejected from a small opening bounded by the coalesced mantle- lobes, and situated some distance above the exhalent aperture. The young mussel is very unlike the adult. It was formerly thought to be a distinct mollusc, and named Glochidium. This, however, was afterwards found to be only a larva — i.e., a free- living embryo, very dissimilar to the mature form, and undergoing a series of changes known collectively as a metamorphosis before becoming adult. The Glochidium has special larval sense organs, by which it is enabled to detect the presence of fish, and it swims freely by flapping the valves of the shell. The long byssus trails behind, and if it comes into contact with the skin of a fish adheres to it, and the sharp spines on the valves then effect a firmer hold. In this position the larva is covered over by outgrowths from the skin of the fish, and undergoes metamorphosis. The byssus and larval sense organs are lost, while the foot, gills, and internal organs are gradually developed. The host is then left. This peculiar parasitic habit of the larva prevents it from being washed down to sea by the current (see p. 107). § 12. HELIX (The Snail). The two species of Helix most available in Britain for dissec- tion are Helix aspersa, the common snail, and Helix pomatia, the Boman snail. H. pomatia is characterized by its much larger size, and lighter colour. In the essential features of its organ- ization, however, it differs but little from H. aspersa^ which, as the commoner kind, will be described here. MORPHOLOGY AND PHYSIOLOGY. 1, External Characters. — The most striking feature is the pos- session of a shell, into which the animal can entirely withdraw MOLLUSC A. 123 itself. It consists of one })iece only; in other words, it is uni- valve. If a hollow cone with a very small angle is thought of as wound round and round into a right-handed spiral with closely adherent coils, a good notion will be formed of its structure. By the juxtaposition of the inner edges of the coils or whorls^ a hollow pillar, the columella, is produced, which forms the axis of the spiral, and opens to the exterior below. The opening of the shell i^mouth, or j)endome) has a prominent smooth rim, and the blind end is termed the aj)ex. During winter the snail passes into a torpid condition (hyber- nates) when the aperture of the shell is closed by means of a thick calcareous plate, the epiphragm or hyhernaculum. When the animal is fully expanded, the part of the body protruding from the shell is bilaterally symmetrical, terminated in front by a distinct head, and produced ventrally into the broad sole-like foot, an elongated muscular expansion, tapering behind to a point, and ending in front just below the mouth. The animal crawls by wave-like contractions of this organ. The dorsal part of the body projects as a spirally coiled visceral hump, always contained within the shell, and attached to the columella by a special muscle (spindle muscle) by the contraction of which the animal can be completely withdrawn into its shell. The anterior boun- dary of the visceral hump is marked by a thickened edge, the collar. The head bears two pairs of tentacles or feelers, which are hollow^, and can be retracted into the body much as the finger of a glove can be turned into the glove’s interior. Each of the posterior longer pair, the optic tentacles, bears a small black dot, the eye, on the outer side of its tip. The anterior shorter pair are the olfactory tentacles. In the front of the head is the mouth, guarded by an inferior and two lateral lips. Below the mouth there is a small pore, the opening of the supra-pedal gland. Not far below the right optic tentacle the rounded genital opening is situated. In the thickened anterior edge of the mantle, on the right side, there is a deep depression, into the left side of which the large valvular resjnratory aperture opens, and on the right side of this a smaller anus. The respiratory opening leads into a spacious mantle-cavity, which acts as a lung and is roofed in by the mantle, a vascular flap-like outgrowth of the body- wall. 2. Skin. — The skin is smooth on the under side of the foot, 124 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. and on the visceral hump, where it is so thin and transparent that many of the internal organs can he seen through it. Else- where the surface of the body is corrugated. The skin is very glandular and secretes the characteristic slime. The skin consists of an epidermis made up of a single layer of cells, and a connective tissue dermis, closely united with under- lying muscular tissue. The epidermal cells are flat on the visceral Fig. 35. — Digestive Organs anu Nervous System of Snail. — A, General view ; B, left side of stomach, &c. ; C and D, side and hind views of buccal mass; f, foot; t, tentacle; m, mouth; h.m, buccal mass; r.s, radular sac ; ce, gullet ; cr, crop ; st, stomach ; i, intestine ; r, rectum ; a, anus; s.gl, salivary glands; s.d, salivary ducts; l.l, l.r, left and right lobes of digestive gland; l.h.d, r.b.d, ducts of ditto; c.g, cerebral ganglion ; v.g, ventral ganglionic mass ; b.g, buccal ganglion ; t.n, tentacular nerve; b.n, buccal nerve; o.t, hermaphrodite gland; h.d, hermaphrodite duct ; g, place from which genitals have been cut away. hump, cylindrical elsewhere. Many of them constitute uni- cellular mucous and pigment glands, while few-celled pigment and calcareous glands are also present. These various glands project into the dermis, which is also traversed by muscle-fibres, blood-spaces, and nerves. MOLLUSCA. 125 The shell consists of three layers. There is a thin external chitinons periostracum, which is pigmented, and gives colour to the shell. Below this is a dense p)rismatic layer, and internally a thinner pearly layer, composed of numerous laminae, and with a smooth and polished internal surface. The last layer can he secreted by all parts of the epidermis covering the visceral hump, hut the two others can only be formed by the collar, which, as the animal "rows, adds successive increments to the mouth of the shell, the boundaries between which are indicated by lines of growth, 3. The digestive organs (Figs. 35, 36) consist of a convoluted gut running from the anterior ventral mouth to the unsym metri- cally placed anus, and receiving the secretions of salivary and digestive glands. The gut is divisible into buccal mass (pharynx), gullet, crop, stomach, intestine, and rectum. The mouth leads into a large mouth-cavity, contained in an oval muscular buccal mass. Immediately within the lips, dorsally, is a crescentic, horny, toothed structure, the jaw. But the most important organ connected with the mouth is the odontophore. This consists of an elevation rising up like a tongue from the floor of the mouth, on which a horny ribbon (the radula), bearing innumerable minute pointed teeth, is spread out from back to front, passing behind into a pouch, the radular sac, which lies at the back of the buccal mass, and forms a small rounded pro- tuberance of whitish colour. Into the outside of the buccal mass special muscles are inserted, the buccal protractors and retractors. From the upper side of the buccal mass a narrow, thin-walled gullet (oesophagus) passes back, and merges into a spindle- shaped crop, the thin walls of which are marked by longi- tudinal striations. The crop narrows behind, and is succeeded by a rounded stomach with moderately thick walls. This turns sharply upon itself, and is then followed by the narrow, thin- walled intestine, which first bends ventrally, and then, after coiling a little, passes into the rectum, which takes a straight course along the right side of the lung-chamber to open by the anus. Projecting into the intestinal cavity is a longitudinal fold. The salivary glands are paired branching structures, placed one on each side of the crop, to which they are attached by connective tissue. A slender salivary duct runs from each of them down the 1l>G AN ELEMENTARY TEXT-BOOK OF BIOLOGY. Fig. 36. — Section through Buccal Mass of Snail (from Claus, after Kefersttin).-0, Mouth; Mh, mouth- cavity; M, muscles; Rd, radula; Kn, supporting cartilage ; Z, radular sac; Kf, jaw; Oe, oesophagus. side of the gullet, and, dilating slightly, opens into the side of the mouth-cavity. The digestive gland (“liver”) is a large brown organ making up a considerable part of the visceral hump. It is divided into right and left lobes, of which the latter (Id) is deeply three-cleft, and much larger than the other, which occupies the final coils of the visceral hump. Two ducts (bile-ducts) run from the corresponding lobes to the right and left sides of the stomach. The left duct is extremely short, and formed b}'' the union of three branches from the subdivisions ■of its lobe. The gut is lined by a single layer of epithelial cells, which are, for the most part, of the simple columnar type. The radula is a cuticular ribbon, developed by the epithelium lining the radular sac. Upon its upper surface are numerous longitudinal rows of minute pointed, backwardly-projecting “teeth.” Teeth of the same age are at the same level, so that a clear arrangement into transverse rows is also seen. The central row contains sym- metrical median teeth (uncini), while the lateral (rachidian) teeth of the other rows are asymmetrical. The arrangement may be expressed by the formula qo . 1 . go , in which the 1 — the single longitudinal row of median teeth and co = the numerous longi- tudinal rows of lateral teeth on each side of this. As the radula is worn away in front, it grows forwards (like a finger-nail on its bed) upon, and connected with, the suh-radular membrane, which covers the projection from the floor of the mouth. This membrane is formed by epithelium, together with underlying connective tissue, and is to some extent movable upon the central part of the odontophore, which is supported by two masses of gristle, the odontoplioral cartilages. These serve as the origins of minute muscles, which are inserted into the sub-radular membrane in front and behind. The cartilages consist of a clear matrix, in which are imbedded numerous branched cartilage cells. The MOLLUSCA. 127 wall of the alimentary canal, outside the epithelium, contains muscular layers, an internal longitudinal and an external circular. The salivary glands are aggregates of unicellular glands, each of which is sharply marked off from its neighbours, and has its own minute duct. The digestive gland is composed of branched tubules, ending blindly, and lined by glandular epithelium, in which three chief types of cell can be distinguished, ( 1 ) granular cells (liver-cells), containing yellowish granules, (2) pear-shaped ferment cells, (3) large calcareous cells. The snail chiefly feeds upon the fresh leaves, stems, &c., of plants, from which it rasps off small fragments by means of its radula, in the following way; — The buccal mass is pulled forwards by means of its protractor muscles, when the front of the radula on its cushion projects a little from the mouth. Appropriate contractions of the small muscles within the cushion move the sub-radular membrane, and with it the radula, backwards and forwards, the jaw meanwhile holding the food firmly, and serving as a relatively fixed part against which the radula works. The particles of food scraped off pass back into the mouth-cavity, partly by the agency of the flexible lips, and partly as a result of the backward movement of the odontophore, which acts to some extent like a suction-pump. The salivary secretion has been stated to contain a ferment which converts starch into sugar, and the acid secretion of the digestive gland is known to bring about fermentative changes of this kind and others involving the conversion of proteids into peptone. The food gets mixed up and gradually passed backwards by the contractions of the muscular walls of the gut, the length of which gives a consider- able absorbing surface, augmented by the longitudinal fold in the intestine. The digestive gland has other functions besides that of aiding digestion. The granules in its granular cells are most likely of excretory nature, and the material secreted by its calcareous cells is used in the construction of the epiphragm. 4. Circulatory Organs. — As in the mussel, a blood system alone is present, and the reduced coelom is represented by the pericardial cavity. Heart, arteries, and venous system can be distinguished. The hlood is of a bluish tinge owing to the presence of hoemo- cyanin, and consists of plasma and amoeboid colourless corpuscles. The muscular heart is situated in the posterior part of the 128 AN ELEMENTARY TEXT- BOOK OF BIOLOGY. limg-chamber, in close proximity to the kidney, and contained in a pericardial cavity bounded by a firm translucent ^pericardium. It is oval in form, and made up of an anterior thin-walled auricle, which communicates by a valve with a posterior ventricle. Arteries. — The ventricle is continued into a large artery, the aorta, which almost immediately gives off an important visceral artery to the visceral hump, then runs forwards, supplying the body-wall, muscles, viscera, &c., finally perforating the ventral nerve-mass, and breaking up into branches for the head. Venous System. — The smallest arteries form networks (? capil- laries), from which the blood passes into minute spaces (lacunae), which communicate on the other hand with large venous sinuses. Of these the most important are — the spacious hody-cavity sur- rounding those viscera which are not contained in the visceral hump, two lateral sinuses in the foot, a visceral sinus along the inner edge of the coiled visceral hump, and a pulmonary sinus with which this communicates, running round the floor of the lung-chamber. From the pulmonary sinus numerous afferent pulmonary vessels are given off, which branch in the roof of the lung, and from these branches efferent pulmonary vessels arise, which unite together to form the pulmoriary vein opening into the auricle. A number of the afferent trunks enter the kidney, and form a network within it, from which one large and several smaller renal veins run to the pulmonary vein. Course of the Circulation. — Blood, oxygenated in the lung, and (part of it) purified in the kidney, enters the auricle during its diastole by the pulmonary vein. It then passes into the ventricle during the auricular systole. The ventricle, owing to its muscular walls, contracts more vigorously, and the blood, prevented by the auriculo-ventricular valve from returning to the auricle, is forced into the arteries. From the fine ramifications of these it passes into the lacima3, and thence into the venous sinuses, ultimately reaching the pulmonary sinus. From this the blood, now carbon- ated and loaded with nitrogenous waste, passes by the afferent pulmonary vessels into the roof of the lung. Here it is oxygen- ated, and passes into the efferent pulmonary vessels from which the pulmonary vein arises. Part of the blood traverses the kidney before entering the pulmonary vein, and thus gets rid of nitrogenous waste, some of which has been previously eliminated INIOLLUSCA. 129 in the digestive gland. The ha3mocyanin of the blood is a copper- containing proteid which readily takes np oxygen from the air in the lung into loose chemical combination, and parts with it as readily to the tissues. It acts, therefore, as an oxygen-carrier. 5. The Respiratory Organs are represented by a true air- breathing lung (pulmonary sac) placed on the upper side of the visceral hump, behind the collar, and communicating with the exterior by a rounded valvular opening on the right side of the body. The delicate roof and side- walls of the lung are formed by the mantle, which presents internally a network of ridges, in which the vessels already described ramify. The floor is thin, but muscular, and immediately overlies the crop and the bulk of the reproductive organs. When at rest it is strongly convex upwards, but it becomes flattened by contraction so that the lung-cavity is increased in size, and air consequently passes in. This is inspiration^ the converse of which, expiration, is effected by the floor simply ceasing to contract. The pulmonary opening is valvular, and thus the supply of air is regulated and desiccation prevented. The essential part of respiration consists in carbon dioxide diffusing out of and oxygen diffusing into the vessels ramifying in the lung-roof The part played by haemocyanin is explained above. 6. Excretory Organs. — A large, cream-coloured kichiPAj, some- what triangular in form, with the apex forwardly directed, is closely united to the posterior part of the lung-roof. It contains a cavity (the surface of which is increased by the projection into it of numerous lamellae) which communicates with the pericardium by a minute opening, and with the exterior by an ureter. This arises from the anterior end of the kidney, passes along its right side, and then runs along the side of the lung above the rectum to open close to the anus. The kidney is equivalent to one of the renal organs in the mussel, and is perhaps to be regarded as homologous with one of the nephridia in such a form as the earth-worm. It is plentifully supplied with blood, from which its glandular epithelium separates nitrogenous waste in the form of ammonium and calcium urates. The digestive gland has also been shown to take part in the work of nitrogenous excretion. 7. Reproductive Organs (Fig. 37). — The snail possesses a very complicated set of hermaphrodite reproductive organs, mostly of a whitish colour. (I) The hermaphrodite gland is a small kidney- 2 9 130 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. shaped organ imbedded in the inner side of the coiled right liver- lobe. From it a much convoluted hermaphrodite duct proceeds. (2) Special Male Organs. — The herma})hrodite duct splits as it were into male and female ducts, or spermiduct and oviduct, which convey the s})erms and ova respectively. The spermiduct Fig. 37. — Reproductive Organs of Snail.— o-G Hermaphrodite gland ; li.d, hermaphrodite duct; a.gl, albumen gland; v.d, spermiduct (vas deferens); ov, oviduct; va and I'S, upper and loM’^er parts of vagina; d.s, dart-sac; m.gl, mucous glands ; g.o, genital opening; sp, spermo- theca; sp' and sp'\ left and right branches of ditto; p, penis; r.p, retractor penis; Jl, flagellum. (vas deferens) is a narrow tube running at first along the side of the much larger oviduct, the cavities of the two being incom- pletely separated.* In this part of its course it is beset with numerous small eminences collectively constituting a prostate gland. Further forwards the spermiduct separates from the * The two together often receive in this region the name of COmmon duct. MOLLUSCA. 1 3 1 oviduct, and, after pursuing an independent course for some time, opens into a hollow muscular tube, the penis, which receives at the same point the fiagellum, a long and hollow filament. The penis 0})ens into the genital atrium, a shallow depression common to both male and female ducts, and opening to the exterior below the optic tentacle by the genital opening. A narrow banddike muscle, the retractor penis, takes origin in the lung-floor, and is inserted into the penis. (3) Special Female Organs. — The oviduct is a wide, somewhat twisted tube, with folded and pouched walls, along one side of which the spermiduct runs. The tongue-shaped alhumen-gland, which varies very much in size, according to the time of year, opens into its commencement. Where the spermiduct assumes an inde[)endent course, the oviduct merges into a muscular, smooth- walled tube, t\\Q vagina, which opens into the genital atrium. A tubular organ, the spennotheca (receptaculum seminis), consisting of right and left branches, opens into the hinder end of the vagina. The right and shorter branch ends in a berry-like dilatation. In Helix pomatia the left branch is only represented by a small projection. Two tuft-like mucous glands open into the side of the vagina somewhat further forwards, and in front of this an extremely muscular pouch, the dart-sac, which can be everted from the genital opening, communicates with it. This sac contains an elongated calcareous body, the spiculiim amoris, which is sharply pointed and possesses four slightly twisted lateral ridges. It is doubtful whether the dart-sac really belongs to the female organs, but its position would seem to indicate this. The hermaphrodite gland is made up of numerous branched tubules lined by germinal epithelium. Some of these germinal cells develop into rounded ova, which possess a well-marked germinal vesicle with germinal spot, but are devoid of a vitelline membrane. Other germinal cells pass into the cavities of the tubules, and as mother-sperm-cells divide repeatedly to produce bunches of sperms (spermatozoa), each of which has an irregularly oval head, and long vibratile tail. The breeding season is early summer. As in most hermaphro- dite animals cross-fertilization takes place, and self-fertilization is prevented, in this case by the sperms maturing before the ova. 132 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. The ripe sperms pass down the spermiduct and into the llagellum, by a viscid secretion of which they are bound into a thread-like packet (spermatophore). Two individuals mutually fertilize each other, the eversible penis being used as a copulatory organ, by which the spermatophore is conveyed into the sperm otheca. A preliminary stimulus is given by the ejected darts, which may be found sticking in the skin. After some days the spermatophore disintegrates and the liberated sperms fertilize the ova as they pass down the oviduct. Each oosperm is surrounded by an albu- minous investment secreted by the albumen gland, and, external to this, by a tough calcareous shell, with the formation of which the mucous glands appear to be concerned. The eggs, thus con- stituted, are about a quarter of an inch long in H. 'poniatia. They are laid in damp earth during June or July. 8. Muscular System. — The foot is almost entirely made up of muscle-bands, arranged in a complicated way, and bringing about creeping movements by their contraction. The spindle-musch serving to pull the body into the shell is a firm band taking origin in the columella, and, dividing into right and left halves, again subdivided into numerous slips which are inserted into the foot. The retractor of the buccal mass, by which the snail’s head is drawn in, is a branch of this muscle. The contrary movement is effected by protractors of the buccal mass which take origin in the foot, and there are also depressors, having a similar origin, which pull the buccal mass down. A branch of the spindle-muscle on each side constitutes a tentacular retractor which bifurcates into two slips traversing the corresponding ten- tacles and inserted into their tips. When these muscles contract they draw the tentacles back into the body-cavity, invaginating them. The retractor penis has been mentioned above (p. 131). The muscle-fibres are composed of slender spindle-shaped cells^ which, except in the odontophore, are unstriated. 9. Nervous System (Fig. 35). — This is remarkable for its great concentration, and is chiefly localized in the head, where a nerve- ring enclosed in a firm sheath is found surrounding the gullet, immediately behind the buccal mass in the extended state. When the animal retracts itself, this is drawn backwards through the nerve-ring, wdiich is, therefore, then found further forwards than usual. The ring is thickened dorsally into two cerebral ganglia, together by a broad commissure, and ventrally into a ganglionic :\[OLLUSCA. 133 liiass, formed by the coalescence of several pairs of ganglia. This ventral mass is divisible into j)ostero-dorsal pleuro-visceml ganglia, and antero-ventral pedal ganglia, respectively connected with the cerebral ganglia by a posterior cerebro-pleural connective and an anterior cerebro-pedal connective, on each side. The head is innervated by branches from the cerebral ganglia, which give off five pairs of nerves. (1) The tentacidar nerves, which supply the optic tentacles. Each runs within the corresponding tentacle, gives off an optic nerve to the eye, and ends in a tentacular ganglion, from which branches run to the skin covering the end of the tentacle. (2) Two pairs of labial nerves, one of which gives off to the short tentacle a tentacular nerve ending in a tentacular ganglion. These nerves supply the lips and neigh- bouring parts. (3) An auditory nerve passes down on each side between the two connectives to the otocyst. (d) A buccal nerve comes off from the front of each cerebral ganglion, and runs forwards to a buccal ganglion placed in the angle where the gullet joins the buccal mass. The buccal ganglia innervate the pharynx, gullet, and salivary glands. They are connected together by a commissure ventral to the gullet. An unpaired nerve runs to the penis from the right cerebral ganglion. The body-walls and viscera are supplied by the pleuro-visceral ganglia, while the nerves of the foot come off from the pedal ganglia. The nervous elements are as usual ganglion-cells and nerve-fibres ' — the former are confined to the outer parts of the ganglia, and usually possess only one process. 10. Sense Organs — (1) Tactile Organs. — The sense of touch is possessed by the surface of the body generally, but is specially localized in the head, tentacles, and sides of the foot. The tactile cells are narrow cylinders with tapering external ends formed by an aggregation of hair-like processes. (2) Olfactory Organs. — The snail possesses a keen sense of smell which enables it to detect the presence of various kinds of food and to avoid certain strongly odorous liquids, such as turpentine. This discriminative power is lost if the tentacles are removed, and there is a patch of modified epithelium at the tip of each of them, apparently of olfactory nature and containing numerous flask-shaped cells closely related to the corresponding tentacular ganglion. An olfactory function has also been ascribed to the supra-pedal gland, a tubular organ, lined with columnar 134 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. epithelium, lodged in the foot, and opening to the exterior by a small pore beneath the mouth. (3) Auditory Organs. — A minute spherical otocyst or auditory sac is placed on each side of the ventral nerve-mass, and is con- nected by an auditory nerve with the corresponding cerebral ganglion. The sac possesses an outer firm investment, and is lined by ciliated columnar epithelium, composed of auditory cells. It is filled with numerous calcareous particles or otoliths, suspended in fluid. (4) Visual Organs. — Each optic tentacle bears on the outer side of its tip an eye, which appears as a black dot, and is innervated by the optic nerve. It is placed immediately below a small transparent area of the epidermis, and essentially consists of a spherical vesicle,, enclosed in a firm sheath, and containing a large globular lens, devoid of structure. The vesicle is made up of a single layer of cells, which in front are short and transparent, constituting a cornea, while behind they are much more elongated and form a sensitive retina. The retinal cells are of two kinds, unpigmented and pigmented. Each of the former is produced into a flask- shaped visual rod next the cavity of the vesicle, and is surrounded by several of the latter, which are pigmented in their outer parts only, while internally they taper to transparent processes ensheath- ing the visual rod. It has been shown that the eyes of the snail can only clearly distinguish the form of external objects when at a distance of from 1^5 lo of an inch. The pigmented skin appears to be sensitive- to light, helping the animal to distinguish between light and darkness. CHAPTER YIIL— A^ERTEBRATA ACRANIA. § 13. AMPHIOXUS (The Laucelet). Amphioxus is a semi-transparent somewhat fish-like animal, not exceeding twm inches in length, and laterally flattened. There is a free-swimming larva, but the adult animal is a shallow Avater marine form generally found vertically buried in the sand, from which only its anterior end jirojects, but also capable of swimming by eel-like movements of its body. It is abundant round many YERTEBllATA ACJllA N T A. 135 coasts, as, for example, the Mediterranean, and has been found oft* our own shores. All the types hitherto described belong to the luvertebrata, or animals devoid of backbone, while Amphioxus, Dog-fish, Frog, Pigeon, and Kabbit, the remaining animals to be dealt with, are examples of the Vertehrata, or backboned group, using the word in a very broad sense, and by “ backbone ” understanding a firm rod, not necessarily of bone, underlying and supporting the central nervous system. To avoid confusion it is perhaps advisable to drop the name Vertebrata as used in the broader sense, replacing it by the term Chordata, and styling the Invertebrates Non- Chordata for the sake of uniformity. All Chordate animals possess, temporarily or permanently — (1) A t\x\>w\2iV . central nervous system running along the dorsal side of the bodj^, and not perforated by the gut as in many higher Non-Chordates {e.g., Earthworm, Leech, Crayfish). (2) A firm elastic rod, the notochord, underlying the central nervous system for part or all of its length. (3) [Except in one instance] A laterally perforated pharynx, serving in the lower forms as a respiratory organ. The Chordata are classified as follows : — CHOEDATA ( = VEKTEBEATA in wider sense). I. Hemichorda. Small notochord in anterior part of body. — A small group of worm-like forms. II> Urochorda, Small notochord in tail of larva, generally absent in adult. — A group of degenerate forms, the Ascidians or Sea- Squirts, which are usually fixed when adult. III. Cephalochorda. Notochord extending from one end of the body to the other. ^ — Amphioxus. IV. Vertehrata (in narrower sense). Noto- chord ends anteriorly below the middle of the brain. A more or less complete ver- tebral column (“backbone”) in which segmentation is always indicated, and which usually encroaches more or less upon the notochord. -Dogfish, Frog, Lizard, Fowl, Rabbit. = VERTEBRATA ACRANIA. Nobrain-case, limbs, or jaws. = VERTEBRATA CRANIOTA. A brain-case. Limbs, when present not more than two pairs. 136 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. MORPHOLOGY AND PHYSIOLOGY, 1. External Characters (Fig. 38). — The scientific and popular names of Amphioxus are alike derived from the fact that the flattened body is pointed at both ends. There is no distinct division into head, trunk, and tail, as is the case in a fish, and, at the first glance, it does not seem easy to say which is the anterior and which is the posterior end. The former, however, is recognizable on a cursory examination by the presence of the ventral mouth, an oval opening bordered by numerous stiff ciliated processes, the buccal cirri. Further back along the ventral surface is seen another median aperture, the atriopore, situated on a pro- minent papilla. It is the outlet of a large atrial cavity, by which the perforated pharynx is surrounded. A thin, laterally flattened fin runs from the mouth round the front end of the body along the dorsal surface, round the posterior end, and forwards along the ventral surface as far as the atriopore. About the middle of this ventral section of the fin a small opening, the anus, is to be found on the left of the median line. This is one of several particulars in which Amphioxus deviates from strict bilateral symmetry. The fin is rather larger round the posterior end, constituting a caudal fin, while that part of it which runs along the upper side of the body in front of this is known as the dorsal fin, and the part between anus and atriopore as the anal fin. All these are perfectly continuous. The ventral surface between mouth and atriopore is broad, gently convex, and marked by a series of longitudinal ridges. It is bounded on each side by a longitudinal fold (metapleural fold), the lateral fin, which unites with its fellow just behind the atriopore, at the point Avhere the anal fin begins. Amphioxus is a segmented animal, and this is indicated exter- nally by a number of > shaped lines on the sides of the bod}', corresponding to a division of the lateral muscles of the body into muscle-segments (myomeres, myotomes) Avhich in Amphioxus lanceo- latus, the species commoidy used in laboratories, are 61 or 62 in number. The atriopore corresponds to the 36th myomere, Avhile the anus is situated between the 51st and 52nd. There is a small ciliated jfit (? olfactory) on the left side of the liead above the anterior end of the mouth. VERTEBKATA A CRANIA. 137 ■ a: -ai ' d QJ CU . Cu 02 03 QJ P xn iS § 03 P 2. . << '*Ti cfi ~ Cw ' d> 03 W ^ <31'' O io fl a c3 I w ^ o o a> fl • r M I Cd ;4 ^ : i-t ^ 7^ 'G ?-l . O G 03 o 9 ® o i; O 'Gi ^ H3 £- • O ^ ^ C3 r^ •r: rt • ^ O G3 p G pH O '' r/r ^ 03 03 f= G ? ^ • 2—' 03 G bG 'G C3 C ? O (j. 03 ^ C ^ "-H 9 ^ .G 03 P O O G -+^ ra 03 f-i ro ^ 4-3 . . Ph G G G 02 G g; Xl 03 _ t-P G o rP 03 C3 .P _ O G o ;>3Q3 O .-I -P 4-0 03 r G 'Gi 02 a J> C G . .G G Ph^Q <; 13 03 * G G G O . P G 03 Jz J=5 ^ .G G rGbi G ^ S •-! Cr ::s -M p:3 T3 ^ G 5 ,03 •P 02 ® G3 S 03 13 2 rG ^ Ph O G G«1 G g:: 03 o 2 i7^ ►-0 b0 2 ^ S 2 -p ^ ^ a 'S o ►eS. jG ^ G _ 4-3 Gij -P <5 . ^-H " ^ P ^ ha d^ o-^ 03^“^ 2 ^ '-G P G4 ^ :2 d ^ . o O p g3 . G I .S-I G5 C} G ^ O G 03 G rG o C ^ rf) k-H ••— ' ••n oiW • CC « shaped muscle-segments (myomeres, myotomes), separated by connective- tissue septa. Those on opposite sides do not correspond. The constituent muscle-fibres run longitudinally. Swimming move- ments, consisting in bending the body first to one side and then VERTEBRATA ACRANIA. 145 to tlie other, can be effected by alternate contraction of the lateral muscles on each side. Transverse muscles run across the floor of the atrial cavity, and these no doubt assist in the expulsion of water through the atriopore. The muscle-fibres are transversely striated rhomboidal plates. 9. The nervous system consists of a dorsal nerve-cord and of nerves connected with this. The nerve-cord (spinal cord) constituting the central part of the nervous system is a thick-walled tube exactly fitting the neural sheath above the notochord, and extending the greater part of the length of the body. In front, however, it does not reach so far forwards as the notochord, but terminates bluntly just above the front end of the mouth. The cavity of the nerve- cord forms an exceedingly narrow central canal, except anteriorly, where it dilates into a much larger ventricle which opens to the exterior on the left-hand side within the olfactory pit. Behind the ventricle, a narrow slit, the dorsal fissure, divides the upper part of the cord, above the central canal, into right and left halves. The nerve-cells are grouped round the central canal and dorsal fissure ; in a stained section they appear as a deeply-coloured dorso-ventral streak, readily seen under the low power. As in other cases they are produced into processes by which union with one another and with nerve-fibres is effected. The smallest nerve-cells have but one process {i.e., are unipolar), the larger ones generally have several processes (ie., are multipolar). A double longitudinal series of small irregular masses of black pigment- c^lls is imbedded in the floor of the central canal. The greater part of the nerve-cord is made up of slender nerve-fihres, most of which take a longitudinal direction, while others pass out into the nerves. Numerous segmentally-arranged nerves take origin from the nerve-cord and constitute a pteripheral nervous system. Most of them correspond to myomeres, being consequently arranged asymmetrically and not in regular pairs. The nerves are of two kinds — (1) single-rooted and (2) multiple-rooted. The single- rooted nerves (except the first two) arise from the dorsal surface of the cord as single bundles of nerve-fibres, and the first six of them constitute three regular pairs. Behind this, however, those 2 10 I ' 146 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. V \ • * \ I' *• i on one side alternate, like the myomeres, with those on the other f side. The first pair have a ventral origin from the extreme front | I of the nerve-cord, and, together with the second pair, which arise I dorsally behind the ventricle supply the parts in front of the mouth, while the third pair supply the mouth-region. These ! single-rooted nerves are partly sensory, innervating the skin, and j ; partly motor, giving off twigs to the muscles. Their sensory ; branches divide repeatedly, and their finest ramifications ulti- mately unite together to form a delicate nerve-plexus below the ( epidermis. Such a nerve-plexus, investing the whole body, is : j characteristic of certain non-chordates, and must be regarded here ; ; as a primitive feature inherited from non-chordate ancestors. i The multiple-rooted nerves all arise from the ventral side of the S . _ nerve-cord for the greater part of its extent, alternating with the j I dorsal nerves. Each of them consists of a longitudinal series of I slender bundles (roots) which remain independent of one another, and branch out in the adjacent muscles. These nerves, therefore, are of motor nature. ' 10. The sense-organs of Amphioxus are ill-developed and for the most part of problematical nature. A considerable number of the epidermal cells, especially in the anterior part of the body, probably serve as taetile organs. These cells are comparatively slender, continuous internally with sensory , nerve-fibres and produced externally into a stiff process. A gustatory function has been ascribed to the following struc- tures : — ( 1 ) Small aggregates of sense-cells, similar to those described in the last paragraph and situated upon the buccal cirri. The external processes of each group form a conical pro- jection. (2) Circlets of sense-cells occurring in the velar tentacles. (3) A sac lined by sensory epithelium and opening on the left side of the roof of the buccal cavity. It is exceeding doubtful whether the so-called olfactory pit, \ situated on the left side of the head, has anything to do with smell. It is a depression lined with ciliated epithelium and communicating with the ventricle. A pigmented mass imbedded in the front wall of the ventricle lias received the name of eye. VERTEBRATA ACRANIA. 147 DEVELOPMENT. The breeding-season commences a1)out the end of March and lasts throughout the summer. The eggs are laid about an hour after sunset, and are fertilized at once by sperms shed over them by the male. Development begins an hour later and at first goes on very rapidly, so that by sunrise next morning free-swimming flagellated embryos escape from the eggs. Twenty-four hours later the flagellated embryo has become an elongated unsym- metrical larva, with mouth, anus, and one gill-slit. This completes what may be termed the einhryonic devdo])mmt. The larval development which now succeeds extends over a much longer period. The free-swimming larva is for some time extremely unsymmetrical, but gradually l)ecomes less so. At the same time the adult structure is slowly assumed ; ultimately the free-swimming life is given up, and this closes the larval period. I. Embryonic Development. Cleavage (Segmentation). — The ovum, in spite of its small size of an inch in diameter), contains a considerable number of volk-granules. It is covered by a delicate vitelline membrane, ■which becomes separated from it as soon as fertilization has been effected. Only one polar body has been observed, and this rests on the upper pole of the oosperm. Cleavage is complete (holoblastic) and nearly regular. It occupies from three to four hours. The first division takes place in a vertical plane and results in two cells of equal size. These are then equally halved by a second vertical division at right angles to the first. The third division is horizontal (equa- torial) and separates four rather smaller upper cells from four rather larger lower cells in which more numerous yolk-granules are present. These eight cells are now bisected by two vertical divisions making angles of 45° with the earlier vertical ones. The I6-celled stage so constituted becomes a 32-celled one by two new equatorial divisions, the planes of which are respectively above and below the first equatorial division. From this point cleavage proceeds more irregularly. It ultimately results in a spherical blastula (blastosphere), which may be compared to a hollow ball with a large central cavity (blastocoele, segmentation cavity) and a wall composed of a single layer of cells, which in the upper half are smaller than in the lower half (Fig. 40). The 148 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. former constitute the ectoderm (epiblast), the latter the endoderm (hypoblast). G-astrulation (Fig. 40), by which the single-walled blastula is converted into the double-walled gastrula, follows cleavage and occupies about six hours. The endoderm cells undergo a gradual inpushing or invagination (emboly, embolic invagination) until the blastocoele is obliterated, and the embryo has become cup- shaped, with a central digestive cavity (archenteron) opening by a wide blastopore. The endoderm cells now line the archenteron and are covered by the ectoderm, each cell of which develops a ilagellum. The cup-shaped gastrula soon assumes an ovoid shape with a flattened dorsal and a convex ventral surface. Meanwhile Hatschek). — A, In optical section. A, Blastula with flattened lower pole of larger cells. B, Commencing invagination. C, Gastruiation completed ; the blastopore is still widely open, and one of the meso- dermic teloblasts is seen at its ventral lip. The flagella of the ecto- derm cells are not represented. the blastopore has narrowed to a small rounded aperture, and is now situated at the posterior end towards the dorsal surface. Two endoderm cells on the ventral side of the blastopore are distinguished ,by their relatively large size. These are the mesodermic telo- blasts, which subsequently originate a part of the mesoderm. The completed gastrula escapes from the vitelline membrane and swims freely, front end first, by means of the ectodermal flagella. Origin of the Mesoderm, Coelom, Muscles, Notochord, and Nervous System (Figs. 41 and 42). — The foundations of all these are simultaneously laid during the twenty-four hours which succeed hatching. The mesoderm in the anterior part of the body is constituted by the walls of myocodomic pouches which successively grow out VERTEBRATA ACRANIA. 149 Pi > ^ o ^ / o /o j ojo \3y- \ ^ • CQ • <1 s.^ s g I c3 -(-i ^ Ph’ ^ 7n . O P s o ^ ■t) n g; _:T >« ^S&|l fl o P cS-2 - QJ Cl, *+H aj :S }-i 14-I C« O Ip ^ p o t^_| 'So ~ ci from the archenteron. The first of these pouches is unpaired and median, the remainder are paired and lateral. They develop in order from before backwards. All these outgrowths become separated from the gut (which may now be called the mesenteron), and are known as meso- clermic (mesoblastic) somites, since they in- dicate the segmenta- tion of the body. Other such somites are added later as en- dodermic outgrowths, but their cavities never communicate with that of the gut. The mesodermic telo- blasts give rise to part of the mesoderm of the tail-region. The somites soon become divided into dorsal sections, the cavities of which collectively constitute t\\Qmyocoel.e^ and ventral sections, the cavities of which collectively constitute the splanchnocmle. The outer wall of each sec- tion consists of somatic mesoderm, and its inner wall of splanch- nic mesoderm. The splanchnic walls of the dorsal sections are converted into the lateral muscles, and their somatic walls into the dermis. The sheath of the notochord, neural sheath, and septa are formed from oTitgrowths of these dorsal sections. The myocoele is mostly obliterated in the adult, except in the ^ O £ rP ^ P 2 P o ® '-'g 3^ o 'X rt (p O (—• CQ rj p. ce 0) S 'o b -g g S 2 ^ '"jp M .3 ^ -b r^; P o ^ .11 .Jp (P 3 per and lower eyelids, are placed on the sides of the head above the posterior corners of the mouth. Just behind each of them is a small round hole, the siyiracle, opening out of the pharynx. It is of the same nature as five oblique gill-slits which are seen further back immediate!}' in front of the pectoral fin. Two minute apertures on the top of the head communicate with the auditory organs. A large number of regularly-arranged pores can be seen upon the head, especially in its anterior part. They are the openings of sensory tubes (jelly-tubes, mucous canals), which lie under the skin. Another sensory structure underlies the groove-like lateral line which runs along each side of the body. The dogfish is of a whitish colour ventrally, grey with dark brown spots dorsally and laterally. The fins are spotted as well as the body. This colouration must make the animal extremely inconspicuous when seen from above in its natural surroundings. The body is entirely covered by small sharp, placoid scales, imbedded in the skin, but with projecting, backwardly directed points. The scales near the mouth closely resemble the teeth. 2. The skin consists of an epidermis composed of stratified epi- thelium, and an underlying dermis made up of connective-tissue traversed by blood-vessels, lymphatics, and nerves. The only important glands connected with the skin, if the jelly- tubes are excluded, are the clasper-glands of the male, each of which is a pouch underlying the skin between the pelvic fins and opening backwards into the groove of the corresponding clasper. The small placoid scales are developed in the skin. Examined with a powerful lens, or under a low power of the microscope, each of them is seen to consist of a four-rayed basal plate, and of a much larger spine attached to it. The end of the spine is leaf- shaped and directed obliquely backwards with its flat side upper- most. The basal plate is bone-like, the spine composed of hard dentme, capped by exceedingly hard enamel secreted by the epi- dermis. The rest of the scale is developed by the dermis. 156 AN ELKM ENTARY TEXT-BOOK OF BIOLOOY. The very young scales are covered by the epidermis, through which the spines later on force their way. 3. Endoskeleton. — In an embryo dogfish a firm cellular rod, the notochord, underlies the central nervous system, much as in Amphioxus, but in this case only extending as far forwards as the middle of the brain. A cartilaginous sheath is soon formed round the notochord ; in the head-region the floor of a firm cartilaginous brain-case is developed in connection with this ; in the trunk and tail the sheath is transversely segmented into a series of joints, flexibility being thus much increased. Further modifications and additions result in a complicated endoskeleton, composed almost exclusively of cartilage. hr. hr. Fig. 43. — Uo(4FiSH. Skull and part of vertebral column (reduced). — An, Auditory capsule; Na, nasal capsule; //. , III., IV., d'c., nerve-exits; J.J, upper jaw; L.J, lower jaw; H.m, hyo-mandibular ; Ger.Ji, cerato- hyal; B.hy, basi-hyal; Ph.hr, pharyngo - branchials ; Ep.hr, epi- branchials; (7er.&?% cerato-branchials ; i>*. basi-branchials ; c, centra, continued up into neural plates; Sp, neural spines; Int, intercalary pieces; F.d, F.r, foramina for dorsal and ventral roots of spinal nerves, indicated for two nerves. It is convenient to consider the skeleton under two headings : 1. Skeleton of the body — axial skeleton ; II. Skeleton of the fins — appendicular skeleton. I. Axial Skeleton (Fig. 43). — This is divisible into (1) Skull, (2) Vertebral Column and Ribs. (1) The skull is characteristic of Yertebrata proper (Vertebrata Craniota). It includes a cranium or brain-case — olfactory and auditory capsules ; and the visceral skeleton, consisting of jaws and respiratory skeleton. PISCES. 157 The cranium is a somewhat rectangular box of cartilage enclos- ing the brain, and incomplete clorsally, where there is a large gap or fontanelle closed by membrane. Posteriorly the cranium articulates immovably with the vertebral column by two rounded projections (condyles), between which is a large aperture (foramen magnum), where the spinal cord and brain are united. There are nerve-exits in front and at the sides, while the cranial floor is continued forwards as a median nasal septum bearing in front a slender pointed rod. The olfactory capsules are large thin-walled structures sepa- rated from one another by a median septum, open below, and fused with the front of the cranium. The auditory capsules are much firmer. They enclose the organs of hearing, and are fused with the sides of the cranium in its hinder region, each of them appearing as a squarish projection. Visceral Skeleton. — In the embryo dogfish seven thickenings,. visceral arches, appear on each side of the neck, and between them six openings, visceral clefts, placing the cavity of the pharynx in communication with the exterior. The arrangement may be indicated as follows for the left side, using strokes for the arches, and numbers for the clefts ; the arrow points to the front. Beginning in front, the arches are termed mandibular, hyoid^ Is^, 2nd, 3rd, ith, and 3th branchials. The first cleft is the hyomandibular, and becomes the spiracle ; the rest are named like the arches which bound them behind, and become the gill- clefts. Curved supporting rods of cartilage are developed in the vis- ceral arches, and become converted into the visceral skeleton. The mandibular bars are converted into two cartilages which support the lower jaw. From the upper end of each a forward outgrowth is developed, which becomes separated off and supports the corresponding half of the upper jaw. The hyoid bars are segmented into upper pieces, the hyo- mandibular cartilages, which suspend the jaw-cartilages from the auditory region of the skull, and lower pieces (cerato-hyals) which unite with a median ventral basi-hyal cartilage. A skull like this, in which the jaws are suspended by means of hyo- 158 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. mandibulars, is said to be kyostylic. The upper jaw is also connected with the skull by two strong fibrous bands (ligaments), one in front (ethmopalatine ligament) and one behind (pre- spiracular ligament) running in front of the spiracle and contain- ing a small pre-spiracular cartilage. Each of the five branchial rods on each side becomes jointed into a series of segments, named, from above downwards, epi-^ cerato-, and hypo-hranchiaU. A median ventral basi-branchial cartilage lies between and connects together the arches of opposite sides. Slender cartilaginous gUl-rays for the support of the gill-folds radiate backwards from the posterior margins of the cerato-hyals and first four pairs of cerato-branchials. The visceral skeleton includes a few other cartilages besides the above. Tliey are (1) a pair of rod-like labials at each corner of the mouth, (2) three flattened rods, the extra-hranchials, on each side, external to the three middle branchial arches. (2) Vertebral Column and Ribs. — The vertebral column con- sists of a series of joints or vertebrae, which are united together to form a fiexible rod. The vertebrje are of two kinds, trunk vertebrse and tail vertebrae ; the latter are the more typical, and each of them consists of a deeply biconcave (amphiccelous) body or centrum lined by bone, a neural arch under which the spinal cord runs, and a haemal arch protecting the blood-vessels of the tail, and j)roduced downwards into a hcernal spine. The trunk vertebrae chiefly differ in the absence of complete haemal arches ; those in front possess horizontal projecting transverse j^rocesses which bear short ribs ; those behind have downwardly projecting hcernal jwocesses ( = transverse processes of those in front, and sides of haemal arches of tail vertebrae). The neurcd arches of both kinds of vertebra are similar. Each consists of four pieces, a lateral neural plate projecting from the centrum on each side, and two small rounded dorsal cartilages [neural spines) placed one behind the other. The spaces between the neural plates are filled up by hexagonal intercalary cartilages. The notochord persists throughout the entire extent of the verte- bral column, but by the development of the vertebral centra is deeply constricted at regular intervals, though not quite divided into a series of separate sections. II. Appendicidar Skeleton, — Each unpaired fin is in typical cases PISCES. 159 supported ])V a series of cartilaginous rod-like fin-rays, bearing small plates of cartilage at their distal ends. The fin-skeleton is completed by a large number of horny fibres developed in the skin and running in the same direction as the fin-rays. The skeleton of either anterior or posterior paired fins is divisible into a proximal ])art, the limi-girdle, connected with the body, and a distal j)art which supports the free limb. (1) Pectoral Jins (Fig. 44). — Each shoulder girdle is a curved fiat cartilage Fig. 44. — l)0(;iiSH. Skeleton of pectoral fins seen from below (rednced). — Co, Coracoid region; Sc, scapular region; p.pt, propterygium; me.pt, mesopterygium ; mt.pt, metapterygium; p.r, me.r, 7nt.r, corresponding fin-rays. Fig. 45. — Dogfish. Skeleton of pelvic fins seen from below (reduced). — Ph, Pubic region; il, iliac process; basijDterygium, continued back into Cl, clasper skeleton. Fin-rays seen externally. consisting of a dorsal half, the scapular region, and a ventral half, the coracoid region, the junction of the two being marked by the attachment of the free limb. The two girdles are fused together ventrally. The base of the free fin is supported by three cartilages, named from before backwards propterygium, mesopterygium, and metapterygium, the last being much the largest. To these elements a number of fin-rays succeed, one propterygial, one mesopterygial split into two or three distally, and about a dozen metapterygial. The fin- skeleton is completed by rows of polygonal plates and by IGO AN ELEMENTARY TEXT-BOOK OF BIOLOGY. horny fibres like those of the unpaired fins. (2) Pelvic fins (Fig. 45). — The two 'pelvic girdles are fused together into a transverse bar of cartilage lying a little way in front of the cloaca. Most of the bar {pubic region) lies between the attach- ment of the free fins, but there is a small projection {iliac process) external to this on each side. The inner side of the free limb is supported by an elongated basipterygium ( = metapterygium) which in the male is continued into the clasper. A series of fin-rays are attached to the outer side of this cartilage, and one ray directly to the girdle. The fin is completed by small plates of cartilage and by horny fibres. 4. The digestive organs (Fig. 46) consist of the gut or alimentary canal running from mouth to cloacal aperture, and of appended glands. The sections of the gut are mouth-cavity, pharynx, stomach, intestine, and cloaca. The glands are the liver and pancreas. The margins of the mouth are beset with several rows of small, sharply-pointed teeth, which must be regarded as modified placoid scales. The mouth-cavity is spacious, and upon its floor there is an ill-developed tongue, supported by the basi-hyal cartilage, and with a forwardly-directed rounded end. The pharynx or respiratory section of the gut, which next succeeds, communicates with the exterior by means of the spiracles and gill-slits, and merges into a short, wide gullet (oesophagus) which enters the abdominal cavity, and is there continuous with a large U-shaped stomach. This is followed by the intestine, which is divided into {a) a short, moderately-large bursa Entiana; (b) a much larger and longer section, into which a shelf-like spiral valve projects ; and (c) a short narrow rectum opening into a good-sized cloaca which also receives the excretory and genital ducts. The liver is a large brown organ attached to the front end of the abdominal cavity and divided into two long backwardly- directed lobes. The secretion of the liver (bile) is carried away by a bile-duct which opens into the middle section of the in- testine, on the right side, not far from the beginning of the spiral valve. A large gall-bladder connected with the duct is imbedded in the left lobe of the liver near its origin. The pancreas is a small, pale, flattened gland situated in the angle between the stomach and the bursa Entiana. The pan- PISCES. 161 creatic dud is a short tube carrying off the pancreatic secretion and opening into the left side of the intestine about the same level as the bile-duct. A short tul)e with thickened walls, the rectal gland, opens into the dorsal, side of the rectum. The abdominal cavity, in which most of the digestive organs are contained, is lined by a thin membrane, the 'periiomim, which leaves the body- wall in the median dorsal line to form a double sheet, the inesentery, the halves of which diverge and wrap round the gut, liver, &c., constituting suspensory folds, which, however, are for the most part very incomplete. The dogfish is a very voracious animal, feeding upon other fishes, Crustacea, and molluscs. In some districts, at any rate, it is especially abundant during the herring season. The rows of sharp backwardly-pointed teeth assist in securing the prey. By means of contractions of the muscular walls of the gut the food is gradually passed backwards, and the force expended during this process, combined with the softening and chemical action of the digestive juices, serves to disintegrate it. The chief digestive juices are the gastric juice, pancreatic juice, and bile, of which the first is secreted by small glands in the wall of the stomach and contains a ferment which converts proteids into soluble diffusible peptones. The pancreatic juice, also by ferment action, completes the digestion of proteids, converts starch into sugar, and emulsifies fats. Bile assists in the last kind of digestion. The digested food diffuses into the blood-vessels and lym- phatics which ramify in the wall of the gut. An increased absorptive surface is given by the spiral valve, which also prevents the contents of the intestine, from passing backwards too rapidly. The comparative shortness of the gut is correlated Avith the easily digestible animal diet. 5. The circulatory organs of the dogfish comprise (I.) a blood system, and (II.) a lymphatic system. (I.) The blood system (Fig. 46) is a closed set of tubes con- taining red blood, consisting of coagulable plasma, in which colourless corpuscles and red corpuscles are suspended. The former are amoeboid and nucleated, the latter are oval discs, containing a well-marked nucleus, and coloured red by haemoglobin. A heart, arteries, veins, and capillaries can be distinguished. 2 11 162 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. The heart lies in a pericardial cavity which is separated from the abdominal or peritoneal cavity by a transverse septum, but the two cavities communicate with each other by a pair of pericardio-peritoneal canals. The pericardium is immediately above the ventral portion of the pectoral girdle, and its dorsal wall is supported by the basi-branchial cartilaginous plate. The heart, which contains only impure blood blood poor in oxygen, and loaded with CO9), consists of sinus venosus, a single auricle, a single ventricle, and a conus arteriosus. The sinus venosus is a transverse tube which receives blood at its two ends from the chief venous trunks. From this the blood passes through a valved aperture into the auricle^ which is by far the largest division of the heart, and occupies the dorsal half of the pericardial cavity. The thin walls of the auricle are 1)1*0 vided with a plentiful mesh work of muscles, which by their contraction drive the blood throush a valved auriculo-ventricular aperture into the ventricle which lies on the^ ventral wall of the pericardium. The ventricle is a nearly globular sac with a very thick muscular wall, which by its contraction drives the blood forwards into the conus through a valved aperture. The conus arteriosus is a muscular tube running horizontally forward from the ventricle to the anterior wall of the pericardium, from which point it is continued forwards by the cardiac aorta. Within the conus are two series of valves, viz. : — the series already referred to guarding the aperture from the ventricle, and a Fig. 46. — Dogfish. General dissection of $ , semi-diagrammatic (re- duced).— na, Nostrils; s.t, openings of sensory tubes; gl, gill-slits, widened in both directions by means of scissors ; j).f, right pectoral fin; pl.J, right pelvic fin — the left one is cut away ; cl, cloacal aper- ture ; ah.p, abdominal pore ; the tail has been cut off. The floor of the mouth-cavity and pharynx has been cut through and its left half removed, and the walls of these cavities have been dissected to show blood-vessels; g, cut end of gullet; st, stomach; int, intestine cut open to display spiral valve; ret, rectum; Ir, liver, the bile-duct, h.d, is seen crossing the bursa Entiana; pa, pancreas, the pancreatic duct is seen at x ; rct.gl, rectal gland. Heart, ht, with ventral aorta, v.ao, and afferent branchial arteries (shaded with transverse lines). The dorsal aorta is seen to be formed by the union of efferent branchial arteries (darkly shaded), arising from loops surrounding gill-clefts, X X X X X ; sp, internal opening of spiracle; ca, carotid; a.ca, anterior carotid ; .sc/, subclavian ; ccc, coeliac ; a. w, anterior mesenteric. The ovary has been removed ; ovd, right oviduct with oviducal gland, od ; ovd', common abdominal opening of oviducts. ft i PISCES. 163 Fig. 46. [From a Dissection ’oy S. T. Parkinson.] tk 164 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. second set about the middle of its length. Each series consists of three pouch-like flaps (pocket-valves) attached to the wall by their posterior edges, their anterior edges being free. All the valves of the heart are so arranged as to allow blood to flow freely in the direction described, but not in the opposite direction. The arteries are well-defined tubes with largely muscular walls. The blood leaving the heart passes through a mid- ventral cardiac aorta to be distributed to the gills, where the blood receives a supply of oxygen from the water by which the gills are bathed, and gets rid of its CO^. It is then collected by a series of efferent branchial vessels into a median dorsal aorta, from which arise arteries distributing blood to all parts of the body wdth the exception of the head. The head is supplied with blood by arteries arising direct from the foremost of the efferent branchial vessels. The blood-vessels of the gills are: — (1) the afferent branchial arteries bringing the blood from the cardiac aorta, of which one supplies the gills of each gill-arch, the three hinder pairs arising separately from the cardiac aorta, while those supplying the two foremost pair of arches arise by one pair of vessels, each of which then divides into two. (2) The efferent branchial arteries, carrying pure, blood into the dorsal aorta, and arising from loops which surround each of the gill-clefts except the last. The blood from the gill of the last (or fifth) gill-cleft is received by an efferent vessel which opens into the efferent loop of the fourth cleft. Each efferent loop communicates also by means of a short vessel with the loops in front of and behind it. The dorsal aorta, formed by the union of the efferent branchial arteries, runs back below the vertebral column to the posterior end of the body, becoming the caudal artery in the tail. It also has a^rward continuation which divides into two branches that are connected with the carotid arteries. Branches of the Dorsal Aorta. — (1) A pair of subclavian arteries to pectoral fins. (2) A coeliac artery, supplying liver, anterior end of stomach, begin- ning of intestine, liver, and pancreas. (3) An anterior mesenteric artery to intestine and reproductive organs, (4) A lieno-gastric artery to stomach, spleen, and pancreas. (5) A posterior mesenteric artery to rectal gland. (6) Numerous pairs of small parietal arteries to body walls. (7) Numerous pairs of small renal arteries to kidneys. Each half of the head is supplied with pure blood by: — (1) a carotid artery, running forwards from the top of the first efferent loop and dividing into (a) external carotid to upper jaw and snout; (6) internal carotid to brain. PISCES. 165 (2) A hyoidean artery, running from the middle of the same loop to supply pseudobranch and brain. (3) Small vessels to the floor of the mouth from the ventral ends of the efferent loops. The veins are partly regular tubes, partly irregular sinuses, all of which have thin walls. They may be divided into : — (1) Sys- temic veins. (2) Portal systems. (1) Systemic Veins. — The impure blood from the anterior part of the body is brought back on each side by an anterior cardinal sinus running above the gill-clefts to the level of the sinus venosus, where it unites with a much larger posterior cardinal sinus bringing forwards the impure blood from the body behind the pectoral fins. By the union of the anterior and posterior cardinal sinuses on each side a very short Cuvierian sinus is formed, which, after receiving a jugular sinus from the floor of the mouth and pharynx, is merged in the corresponding side of the sinus venosus. Factors of Cardinals. — An orbital sinus surrounding the eye opens behind into a short post-orbital sinus, which again communicates with the anterior cardinal. A hyoidean sinus opens into the front end of the anterior cardinal. The posterior cardinals begin between the kidneys, and each of them, before it unites with the corresponding anterior cardinal, receives a lateral vein from the body-wall, a genital sinus from the gonad, and a subclavian vein from the pectoral fin. (2) Portal Systems. — Hepatic portal and renal portal systems are present, which respectively supply the liver and kidneys with impure blood. Hepatic Portal System. — The blood from the abdominal digestive organs and spleen, containing most of the products of digestion, enters the hepatic portal vein., which breaks up into branches in the substance of the liver. This organ returns its impure blood direct to the sinus venosus by means of two hepatic sinuses. Eenal Portal System. — The impure blood of the tail passes into a caudal vein which runs forwards to the posterior ends of the kidneys and divides into right and left renal portal veins, which divide up in the kidneys, from which the blood is returned to the posterior cardinals by numerous small renal veins. The capillaries are minute tubes arranged in plexuses, in which the smallest arteries end and the smallest veins bemn. O (II.) The lymphatic system consists of minute spaces, small lymphatic vessels, and large cavities, all containing lymph, IGG AN ELEMENTARY TEXT-BOOK OF BIOLOGY. wliich resembles blood in many respects but possesses no red corpuscles. There are also certain “ ductless glands ” connected with the lymphatic system, of which the largest is the spleen, a reddish body attached to the bend and distal limb of the stomach. The largest lymph-space is the body-cavity or coelom, which includes the abdominal and pericardial cavities. 6. The respiratory organs of the dogfish are gills, adapted for l)reathing the oxygen dissolved in the surrounding sea- water. They consist of vascular folds arranged upon the posterior side of the hyoid arch, and both sides of the first four branchial arches. Water is taken in at the mouth and expelled through the gill-slits to the exterior, so that a continual stream passes over the gills, in the small vessels of which the blood is purified. The luemoglobin of the red blood-corpuscles acts as an oxygen- carrier, taking ip) a certain amount of free oxygen from the exterior into a state of loose chemical combination, and parting with it again to the tissues. A rudimentary gill (pseudobranch) is found on the anterior wall of the spiracle. 7. The excretory and reproductive organs are so closely con- nected that it is best to consider them under the common heading of urino-genital organs. The male dogfish possesses a pair of elongated narrow kidneys extending nearly the whole length of the abdominal cavity and situated close together below the vertebral column and above the peritoneum. Each kidney is divided into a number of segments, and its anterior half is distinguished as mesonephros (Wolffian body) from its posterior half or metanephros, the excretory pro- ducts of these being carried off by distinct mesonephric (Wolffian) ducts, and metanephrlc ducts (ureters). Each mesonephric duct, which, since it also acts as a spermi- duct, may be termed urino-genital duct, is a com^oluted tube running along the ventral side of the corresponding mesonephros, dilating into a vesicula seminal is, and finally opening into an urino- genital sinus that communicates with the cloaca by a small aperture placed on the end of a dorsally situated urino-genital papilla. The metanephrlc duct on each side is formed by the union of several smaller ducts and opens into the dorsal side of the urino- genital sinus. PISCES. 167 The spermaries (testes) are two soft flattened bodies, each connected by a number of small ducts (vasa efferentia) with the front end of the corresponding mesonephros. The kidney is made up of numerous glandular tubules, and the sperms have to traverse some of these before they can reach the urino-genital duct. The urino-genital sinus is forwardly produced into two blindly- ending S2)erm sacs situated on the ventral side of the kidneys. The claspers serve as copulatory organs by which the sperms are introduced into the oviducts of the female. Two short tubes, with a common opening; into the abdominal cavity, can be seen on the ventral side of the gullet. These are rudimentary Mullerian ducts, equivalent to the oviducts of female specimens. The kidneys in the female exhibit the same regions as in the male, but the mesonephros is not so well developed. The two mesonephric ducts are straight and unite to form a urinary sinus, which receives a number of distinct metanephric ducts and opens into the cloaca on a dorsal urinary papilla. The reproducthm organs of the female are not so intimately connected with the urinary organs as in the male. There is a large unpaired ovary from which large ova in various stages of development can be seen projecting. When ripe these may exceed half an inch in diameter, their large size being due to the presence of abundant food-yolk. The oviducts (Mullerian ducts) have a common anterior open- ing into the abdominal cavity, situated on the ventral side of the gullet in front of the liver. Each of them curves back and soon dilates into an ovoid ovklucal gland, after which it runs back as a good-sized tube towards the cloaca, just before reaching which it unites with its fellow to open by a median dorsal aperture. The ova are fertilized in the oviduct, after which each of them is surrounded by an albuminous fluid and enclosed in a horny case secreted by the oviducal gland, and with four corners pro- duced into tendril-like threads. In this condition the efir^s are OO laid, the threads serving to attach them to seaweeds, e^c. De- velopment takes place at the expense of the food-yolk, which after a time is found stored in a vascular sac, the yolk-sac, attached to the ventral side of the embryo. In most dogfishes the entire embryonic development takes place in the oviduct, and in one species (Mustelus loevis) the vascular yolk-sac is thrown 168 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. into folds which interlock in corresponding folds of the oviduct, so that a kind of placenta is formed — i.e., an embryonic structure by which the blood- systems of parent and embryo are brought into close relation. Scyllium, therefore, is oviparous, a term applied to cases where most or all of the development takes place external to the body — other dogfish are viviparous. 8. The most obvious part of the muscular system is a great lateral mass extending along each side of the body from the neighbourhood of the spiracle, and segmented into a succession of myomeres, with zigzag boundaries. The lateral muscles are also divisible into dorsal and ventral sections along the boundary between which runs the lateral line. These muscles effect swimming movements, during which the body is not merel}^ bent from side to side but thrown into sinuous curves. There are also special muscles for moving the fins, lower jaw, &c, 9. The nervous system (Tig. 48) consists of (1) cerebro-spinal axis, (2) cranio-spinal nerves, and (3) sympathetic system. (1) The cerebro-spinal axis is a thick- walled tube invested in a delicate vascular membrane (pia mater) and contained in the neural canal of the skull and vertebral column. The canal is lined by a firm membrane (dura mater) between which and the pia mater there is a large lymph-space. The front end of the cerebro-spinal axis is dilated into a brain, lying within the cranium and a spinal cord, running along the spinal canal above the vertebral centra. The cavity of the neural tube constitutes an extremely small central canal in the spinal canal and larger spaces known as ventricles in the brain. The brain at an early stage exhibits three successive swellings, the anterior, middle, and posterior cerebral vesicles. These, together with outgrowths from them, become the fore-, mid-, and hind-brains of the adult. The fore-hrain consists of a central thalamencephalon, with a large antero-dorsal outgrowth, the prosencephalon, with which are connected two olfactory lobes. The thalamence'phalon is somewhat cylindrical and contains a large cavity (3rd ventricle) with thin roof but thickened floor and side walls. The floor is produced downwards into a pro- jection (infundibulum) in which are two oval swellings (lobi inferiores). Two structures of non-nervous nature are connected with the thalamencephalon. They are known as the pineal and pituitary bodies. The pineal body is a stalk-like structure with riscES. 169 a swollen end, running forwards and upwards from the roof of the 3rd ventricle. It corresponds to what appears to have been an unpaired dorsal eye (pineal eye) in ancestral forms (see p. 2). The pituitary body is a problematic organ attached to the infun- dibulum, and consisting of two thin-walled sacs (sacci vasculosi) and a central tube. The prosencejdialon ( = cerebral hemispheres of higher verte- brates) is the largest part of the brain. It is a rounded mass l)resenting indications of divisions into right and left halves, and containing a ventricle connected with the 3rd ventricle. An expanded olfactory lobe abuts on the posterior wall of each olfactory capsule, and is connected behind with the prosen- cephalon by a short stalk. It contains an olfactory ventricle which communicates Avith the brain-cavity last mentioned. The short, thick mid-brain which next succeeds contains a ventricle (iter or Sylvian aqueduct), the roof of which is raised up into two ovoid optic lobes containing optic ventricles. The axis of the hind-brain is constituted by a long cylindrical bulb (medulla oblongata) containing a cavity (4th ventricle) with a thin roof. A large hollow body, the cerebellum, of elongated oval shape is attached by the middle of its length to the dorsal side of the bulb, Avhich is also produced on each side into a conspicuous lobe (restiform body). The spinal cord is cylindroidal in shape and somewhat flattened from above dowmvards. It is deeply furrowed by two median fissures — one dorsal, the other A^entral. (2) Cranio-spinal Nerves. — These may be divided into cranial nerves and spinal nerves, which respectively take origin from the brain and spinal cord. There are ten pairs of cranial nerves, as follows: — I. Olfactory, each of which consists of tAvo bundles of fibres arising from the corresponding olfactory lobe and supplying the lining of the olfactory sac on the same side. II. Oytic, running to the eyeballs. They unite to form an X-shaped chiasma on the A^entral side of the thalamencephalon. The posterior limbs of the X (optic tracts) end in the oj)tic lobes. The III., IV., and VI. nerves are knoAvn as the eye-muscle nerves. III. Oculomotor. — These arise from the A^entral side of the mid-brain and supply most of the eye-muscles. 170 AN elemp:ntary text-book of biology. IV. The 2)athetic nerves are the only cranial ones which have a dorsal origin. They arise nearly in the middle line just behind the optic lobes and supply the superior olilique muscles. All the remaining nerves take origin from the bulb. Fig. 47. — Dogfish. — A, Diagram of cranio-spinal nerves (slightly altered after Wkdernheim). — M, Mouth; Or, margin of orbit (indicated by dotted line); All, auditory capsule ; Sp, spiracle; +tt + +, gill-slits; II., optic ; III., oculomotor ; IV., ])athetic ; V®, ophthalmic branch of trigeminal ; *, its course in front of orbit to join VIP ; V^ and Vq maxillary and mandibular branches of trigeminal ; VI., abducent ; Vll'h ophthalmic branch of facial ; VII., main trunk of facial, where palatine branch runs forward and hyomandibular backward behind spiracle, giving off prespiracular branches ; IX. , glossopharyngeal, forking over first gill-slit; X., vagus, giving off branchial branches which fork over gill-slits, a lateral line branch {Lat), and then supply- ing viscera ; 1-14, first fourteen spinal nerves, forming brachial plexus {Br.pl), from which pectoral fin is supplied. B, Dorsal view of brain. C, Ventral view of brain. — ol. Olfactory lobes; pr, prosencephalon; th, thalamencephalon ; pin, pineal body; pij, pituitary body; Li, lobi inferiores ; o./, optic lobes; r6, cerebellum ; r.b, restiform body; h, bulb ; sp, spinal cord ; II. -X., cranial nerves. PISCES. 171 \l. The abducent nerves, which innervate the external rectus muscles, arise almost mid-ventrally from the anterior part of the bulb. [iY.i>. — These nerves are mentioned out of their order for the sake of convenience.] Y. The trigeminal, which, like all its successors, arises from the side of the bulb, is a large nerve which has 3 chief branches: — a. Ophthalmic, to sensory tubes on upper side of snout ; [3. Maxillary, to sensory tubes on under side of snout ; y. Mandihdar, to muscles of the lower jaw. VII. The facial nerves arise close behind the trigeminals, and like them have three chief branches : — a. Ophthalmic, which first runs parallel to the similarly named division of the fifth, then fuses with it and has the same distribution ; f3. Palatine, to roof of mouth ; y. Hyoidean (post-spiracular), which runs down behind the spiracle, supplying muscles, and gives off small pre-spiracular branches. YIII. The auditory nerves run into the auditory capsules to supply the membranous labyrinths. IX. The glossopharyngeal nerve on each side arises just behind the eighth, traverses the floor of the auditory capsule, and forks over the first branchial cleft into an anterior (hyoidean) branch and a posterior (branchial) branch. X. The vagus (pneumogastric) is a large nerve arising by several roots and taking a backward course. It gives off a lateral line nerve, which supplies the similarly named sense-organs, four branchial nerves, which fork over the last four branchial clefts, and finally divides into branches for the heart and viscera. The spinal nerves arise segmentally from the spinal cord, each by two roots, a dorsal (upon which is a small ganglion) and a ventral, which pierce the wall of the spinal canal and then unite together. Each segment is supplied by the corresponding pair of nerves, the fibres of the dorsal root going to the skin, those of the ventral root to the muscles. The regularity is somewhat dis- turbed by the presence of the paired fins, which are supplied from plexuses formed by certain of the anterior and posterior nerves. (3) The sympathetic system consists of two longitudinal cords situated ventral to the vertebral column and dilated into segmentally - arranged ganglia which are connected with the adjoining spinal nerves. Each cord is connected in front with 172 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. the corresponding vagus. The sympathetic ganglia supply the viscera and vascular system. Histologically, the nervous system is essentially made up of the two usual elements, nerve-cells (ganglion cells) and nerve fibres. , In the brain and spinal cord these elements are respectively ’ aggregated into what are known as grey and white matter. The former exists in the spinal cord and bulb as an axial core, in the prosencephalon and cerebellum as an external crust. Masses of it are also found in the optic lobes, side-walls of the thalam- encephalon, basal part of the prosencephalon, and in the olfactory ’ lobes. 10. The chief sense organs of the dogfish are those connected with the skin, the olfactory sacs, the ears, and the eyes. All i of these essentially consist of end-organs connected with nerve- j fibres. .1 The skin is abundantly provided with groups (end-buds) of projecting sense-cells, probably of tactile nature, and it also possesses lateral line organs and sensory tubes. The indistinct ' groove termed lateral line in describing the external characters corresponds pretty much in position with a tube which underlies the skin and opens to the exterior at intervals. It is lined by ] epithelium, many of the cells of which are mucus-secreting goblet \ cells, while others are sense-cells (hair-cells), each provided with a slender hair-like process. The sensory tubes (jelly tubes) which open on the snout under- lie the skin of the head and each of them ends in a rounded sac (ampulla) divided into compartments, and partly lined by hair-cells. These tubes contain a gelatinous substance, secreted by the lining epithelium. The ampullae are supplied by the ophthalmic branches of the fifth and seventh cranial nerves, the fibres of which run to the hair-cells. The large olfactory sacs, contained in the olfactory capsules « opening ventrally by the nostrils, have their lining raised into numerous transverse folds, the epithelium of which contains elongated olfactory cells, supplied by the olfactory nerve. The ear (Fig. 48) on each side is a membranous sac (labyrinth) of complicated shape contained within the auditory capsule, and ‘ surrounded by a large lymph-space. It is lined by epithelium, part of which is sensory, and is filled with fluid in which are suspended numerous calcareous particles. The membranous ■ I PISCES. 17S- labyrinth originates as an ectodermic pit, and in the adult still retains a connection with the exterior by means of a narrow tube (aqueductus vestibuli) opening on the top of the head. The labyrinth is divided into a central region (vestibule) and three curved tubes, the seiiiiciTculciv cancils. The vestibule is again divided into an anterior utriculus, and a posterior sacculus with which the aqueductus vestibuli communicates and which is produced into a process, the cochlea. The semicircular canals Fig. 48.— Diagrams of the Membranous Labyrinth (from Bell, after Waldeyer). — A, Fish; B, Bird; C, Mammal. Internal side of left labyrinth. — us, Utriculus and sacculus ; v, utriculus ; s, sacculus ; c, cochlea. are known from their position as anterior vertical, posterior vertical, and external horizontal. They open at both ends into the utriculus, and each has a small swelling (ampulla) at one end. This is anterior in the first and last named, posterior in the horizontal canal. The ampullae and parts of the vestibule possess patches of auditory hair-cells, with which fibres of the auditory nerve are connected. Each of the eyes is flattened externally and rounded internally. Its wall consists of three coats, the most external of which is the firm sclerotic^ supported by cartilage and exhibiting externally a transparent oval area, the cornea. The middle coat (choroid) is a pigmented vascular membrane lining the rounded part of 174 AN ELEMENTARY TEXT-BOOK OE BIOLOGY. the eye, and continued into a partition, the iris, by which the cavity of the eye is divided into a small outer and a large inner chamber. The iris is perforated by an oval slit (the pupil) for the transmission of light. The innermost coat of the eye is a delicate membrane, the retina, which contains the end- organs for sight (rods and cones). The optic nerve perforates the sclerotic and choroid on the inner side of the eye and branches out in the retina, its fibres becoming indirectly con- nected with the rods and cones. The interior of the eye is occupied by refracting structures, by which the light is focussed on the sensitive retina. These are : — a watery fluid (aqueous humour) external to the iris, a jelly-like substance (vitreous humour) occupying the internal chamber, and a spherical trans- parent lens suspended on the inner side of the iris. The eyelids have already been mentioned (p. 155). Other important accessory parts are the six small band-like muscles by which the eyeball is moved. Four of these take origin from the hinder part of the orbit, and are inserted into the upper, lower, anterior, and posterior sides of the eyeball, being respec- tively known as superior, inferior, internal, and external rectus muscles. The other two are known as superior and inferior oblique muscles, taking origin from the front end of the orbit, and inserted respectively into the upper and lower sides of the eyeball. CHAPTER X.— AMPHIBIA. § 15. RANA (The Prog). The two commonest kinds of Frog are Rana temporaria, the Common Frog, and Rana escuUnta, the Edible Frog. Both are found on the Continent, but only the former in this country. The following description applies to both, any important dif- ferences being noted : — AMPHIBIA. 175 MORPHOLOGY AND PHYSIOLOGY. 1. External Characters. — The bilaterally symmetrical body exhibits no external trace of segmentation, and is divided into head and trunk, between which no neck intervenes. There is also no tail. Fore and hind limbs are present, and these, unlike the paired fins of dogfish, to which they are homologous, are transversely jointed, and split at their distal ends into digits. The surface of the body is soft and moist, and there is no general investing exoskeleton. Owing to the presence of pigment in the skin the body is of a yellowish-brown, and is mottled dorsally. The ventral surface is much smoother and paler than the dorsal. The colour varies with the surroundings. The flattened head is bluntly triangular, with a forwardly- directed apex. The mouth is extremely wide, and extends back- wards to the posterior angles of the head. On the dorsal surface, near the front and widely separated, are two small valvular aper- tures, the nostrils or external nares. Behind these are the large projecting eyes, with small, immobile upper eyelids, and delicate, semi-transparent lower eyelids, capable of considerable movement. The space between the eyes is broader in JR. temporaria than in R. e.scu~ lenta, and while Hat or convex in the forrrier, is concave in the latter. Behind each eye is a circular space, the tympanic area (larger in Pi. esculenta), which in R. temporaria is inside a dark patch of pig- ment, that tapers to a point behind. In the male Pi, esculenta a pair of vocal sacs are found, which, when inflated, appear as rounded projections near the angles of the mouth. The trunk is somewhat oval, tapering to a blunt point behind, where a small rounded cloacal aperture is found. Hard parts can be felt through the skin along the entire dorsal surface, but this is only the case with the anterior part of the ventral surface. In this way a thoracic region in front can be distinguished from an abdominal region behind. The fore-limb commences immediately behind the head, and is divided into (1) Brachium (arm), (2) Antebrachium (fore-arm), and (3) Manus (hand). If the fore- or hind-limb of an animal is spread out in the primltivt position — t.e., at right angles to the body, with the “palm” or “sole” side I 176 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. downwards, dorsal and ventral surfaces can be distinguished. There are also anterior and posterior edges, which, being in front and behind an axial line running down the centre of the limb, are termed preaxial and postaxiaX edges. {Gf. dogfish, p. 154.) The nianus possesses four well-developed digits, and, in addition, a rudimentary one concealed under the skin, and corresponding to thQ ])ollex (thumb) or digit of other forms. The remaining fingers are 2nd, 3rd, 4:th, and 5th respectively, reckoning from the preaxial side (thumb side). None of the digits possess nails or claws. In the male Frog a thickened pad, especially prominent during the breeding-season, is found on the preaxial side of the palm of the hand. It is black in IL temporaria. The hind-limb is also divisible into three parts — (1) Femur (thigh), (2) Crus (leg), (3) Pes (foot), consisting of a short cylin- drical tarsus (ankle), and five slender digits, united together by a delicate web. The hallux (great toe), or 1st digit, is preaxial, and on its outer side there is a small, horny elevation, the calcar which is a rudimentary sixth toe ” (praihallux). All the preceding regions, especially the pes, are much longer than the corresponding parts of the fore-limb, but there is obvious serial homology between them. The typical number of digits for manus and pes in terrestrial Vertelrates is five, so that the pentadactyle pes of the frog must be regarded as more typical than the tetradactyle manus. Using ^ for rudiment, the state of things in the frog may be represented by 4 */5 Position of Body. — When at rest the frog assumes a squatting position, with the ventral surface near the ground posteriorly. The back is humped, marking the union of the sacrum and ilia. The elbow is directed backwards and outwards, and the hand rests with its preaxial side turned sharply inwards. In the hind- limb the knee is directed outwards and slightly forwards, while the crus is bent back parallel to the femur, by which the ankle- joint is thrown behind the body. The pes runs outwards and forwards with its preaxial side internal. 2. Skin (Fig. 49). — The skin is thin, and raised on the dorsal surface into an immense number of minute wart-like protuber- ances. It is only loosely united to the body, by bands of con- nective tissue along certain definite lines. The intervening subcutaneous spaces, over which the skin is baggy, are filled with lymph. 1 AMPHIBIA. 177 A superficial epidermis can be distinguished from an underlying dermis, which is considerably thicker. The epidermis is made up of several layers of cells, and hence is classified as stratified'^ epithelium. The most external cells are very flat, without evident nuclei, and make up a horny layer, below which are rounded cells with well-marked nuclei and granular protoplasm. Those abutting upon the dermis, forming the Malpighian layer, are columnar. A small number of irregular pigment cells, containing dark colouring -matter, are scattered through the epidermis. Numerous glands open upon the surface Fig. 49. — Vertical Section of Skin of Frog (after Wiedersheim), much enlarged. — ep, Epidermis; h.l, horny layer; m.l, Malpighian layer; w, warts; gl, cutaneous glands; p.c, pigment cells; De, 1, 2, 3, layers of dermis. of the epidermis. They are of two kinds, serous and mucous. The former chiefly occur in the skin of the back, and appear to secrete *The chief kinds of Epithelium may be thus classified; — {1. Simple. One cell thick. Endo- thelium is a variety of this, composed of flattened cells, and lining heart, vessels, body-cavity, &c. 2. Stratified. — More than one cell thick. Epidermis, &c. n. Columnar. — Composed of ( cells more or less elongated at 1 1. Simple. — Stomach and intestine, right angles to surface. May be \ 2. Stratified. — Mouth-cavity of frog, ciliated. ( III. Glandular. — Composed I . of spherical or cubical cells, I For the most part Simple. — Peptic which elaborate a secretion or j glands, kidney tubules, &c., &c. excretion. 2 12 178 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. a substance of irritant nature. The latter, which are more numerous and widely distributed, elaborate a slimy secretion which makes the skin moist and slippery. These glands are rounded vesicles, lying in the outer part of the dermis, but lined by glandular epidermal cells. They are large and numerous. The dermis is mainly composed of fibrous"^ connective tissue, the fibres of which are mostly parallel to the surface, but also form vertical bands. The glands are imbedded in the external part (1) of the dermis, which also contains a large number of pigment-cells (chromatophores) that give to the skin its charac- teristic colours. These vary owing to the fact that the chroma- tophores contract under the influence of the nervous system. In the fully contracted condition the pigment is limited to a relatively small area, and the skin appears light. The reverse is true when the chromatophores are non-contracted. Since these colour-changes protect a frog by causing it to harmonize with its surroundings, thus making it inconspicuous to its enemies, we have here a case of protective general resemblance. But since also the arrangement conceals the frog from its prey to a greater or less extent, the general resemblance is aggressive as well as protective. The glands are surrounded by unstriated muscle-fibres, which also form a layer in the deeper part of the dermis, where also occur networks of blood-vessels, lymphatics, and nerves. Many of the nerve-fibres end in touch-corpuscles; small oval flattened bodies mostly forming groups underneath the epidermal warts. 3. The endoskeleton in the frog is mainly made up of gristle, or cartilage, and hone, both of which are modifications of connective tissue. The bones are largely connected at the joints by fibrous bands {ligaments), which resemble tendons in structure (see p. 208). Those parts of the endoskeleton belonging to the head and trunk * Connective Tissue — This permeates the whole body, binding together the other tissues. In higher animals generally it consists of three elements, associated together in different proportions. These are — (1) Connective-tissue corpuscles, nucleated cells, often branched (of which pigment-cells are modi- fications), most abundant in young tissues. (2) White fibres — delicate, and wavy, yielding gelatin on boiling. (3) Yellow elastic fibres, much branched, or forming networks. Unaffected by boiling. The fibres are developed from the cells. All are imbedded in a structure- less ground substance or matrix, which is semifluid and albuminous. All these elements are found in the skin and in the bands uniting it with the body-wall. AMPHIBIA. 179 may be conveniently termed axial, while those supporting the limbs are appendicular. (1) The axial endoskeleton consists of the skull, backbone or vertebral column, and breastbone or sternum. (a) The groundwork of the skull is made up of cartilage, constituting the chondrocranium or primordial cranium. With this are connected bones of two kinds, named, from their mode of development, cartilage-hones and memhrane-hones. The former replace pre-existing cartilage, the latter pre-existing connective tissue. It is convenient to consider the skull under the separate headings of cranium, sense-capsules, jaws, and hyoid apparatus. The membrane bones are flattened structures investing the other parts, and will be taken last in each section. The cranium or hrain-case is a narrow cartilaginous tube, the upper side of which is broader than the lower, and possesses three spaces, fontanelles, a larger anterior and two smaller posterior, where cartilage is wanting. These are filled in by connective tissue. The cavity of the brain-case opens behind by the large foramen magnum. The region around this is known as occipital, either side of which is occupied by an ex-occipital bone, upon which is a projection, or occipital condyle, with an oval smooth surface. The two ex-occipitals do not completely bound the foramen magnum, a small cartilaginous strip being left above and below. Each of these bones is perforated by a vagus foramen through which the vagus and glossopharyngeal nerves leave the skull. The cartilaginous side- wall of the brain-case is perforated by an optic foramen, and smaller foramina are also present. The cartilage forming the front end of the brain-case is replaced by a bony ring, the sphenetlimoid or girdle bone, which also supports the hinder part of the olfactory capsules that lie in front of the skull. The sphenetlimoid somewhat resembles a dice-box in shape, and its cavity is divided into two halves by a transverse partition marking the anterior boundary of the brain-case, and is perforated by an olfactory foramen on each side for the passage of the corresponding nerve. The front half of the bone is divided by a vertical longitudinal partition, into right and left halves. Membrane Bones. — The roof of the brain-case is invested by two long, flat parieto-f rentals, which are bent over behind so as to protect the upper part of its side walls. The floor of the brain- 180 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. case is covered by the parasphenoid, a dagger-shaped bone, with its blade ” running forwards. Sense Capsules. — Two pairs of these are fused with the skull, (a) auditory capsules behind and (/5) olfactory capsules in front. (a ) The auditory capsules are closely fused with the hind end of the brain-case, one on each side. They are largely cartilaginous, but each is invaded behind by the ex-occipital of its side, while in front the cartilage is replaced by a large pro-otic bone, which also assists to form the side-wall of the brain-case, and is notched below by the trigeminal foramen, through which the trigeminal and facial nerves pass. The auditory capsule contains a complex cavity, in which the essential organs of hearing are contained. On its outer side is a depression, in which is a small opening, the fe7iestra ovalis, filled up with membrane in the recent state. Another and smaller opening on the hinder border, the fenestra rotwada, is filled up in the same way. Fig. 50. — Endoskelkton of Frog (A, A', and C, after Ecker). Cartilage dotted. — A and A', Skull from above and below, membrane bones being stripped off on one side; fontanelles; f.m, foramen magnum; ex-oc, ex-occipital; c, condyles; v.f, vagus foramen; pr-o, pro-otic; i.f, trigeminal foramen; op.f, optic foramen; g-h, girdle bone; orb, orbit ; ol, nasal capsules ; e-p.h, ethmopalatine bar ; pt. b, pterygoid bar ; sp, suspensorium; q-j^ quadrato-jugal; pt, pterygoid; pa-f, parieto- frontals; p-s, parasphenoid, half cut away ; sg, squamosal ; na, nasals; V, vomer; pr-mx, pre-maxilla; mx, maxilla; pi, palatine; e.n, external naris ; i.n, position of internal naris. B, Columella, enlarged. — p and d. Ends fitting into fen. ovalis and fixed to tymp. membrane, respectively. C, Left half of Mandible, from outside. — m-mk, Mento-meckelian ; c, condyle; a-sjo, angulo-splenial ; deutary. D, Hyoid apparatus. — b-hy. Body of hyoid; a.c, anterior cornua; br, remains of branchial arches. E, Spinal Column and Pelvis (top view). Numbers refer to vertebriB, and are placed near transverse processes. — ust, Urostyle; il, ilium; is, ischium; ac, acetabulum. F, Various Vertebrce. — 1, Atlas, front view; 2, side view of two adjacent vertebrae; 3, back view of vertebra; c, centrum; n.s, neural spine; t.p, transverse process; a-z, pre-zygapophj^sis ; p-z, post-zygapophysis ; n. c, neural canal ; i.f, inter- vertebral foramen. G, Side View of Pelvis. — il. Ilium; pb, pubis; is, ischium ; ac, acetabulum. II, Sternum, Shoulder Girdle, and Fore-limb [Ventral View). — o-s, Omo-sternum; st, sternum proper; x-st, xiphi- sternum; gl, glenoid cavity; s-sc, supra-scapula ; ep-co, epi-coracoid ; pr-co, pre-coracoid sheathed by clavicle; h, humerus; hd, head of ditto ; tLr, deltoid ridge ; r-?i, radio-ulna; ra, radiale; uZ, ulnare; cw, centrale; c 1-5, carpalia; I-V, metacarpals; 1-5, phalanges. I, Hind Limb, s- f. Fig. 50. 182 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. (/3) The olfactory capsules, situated in front of the cranium, are separated from one another by a vertical plate of cartilage con- tinuous with the longitudinal partition of the sphenethmoid.^^The walls of the capsules are mostly composed of cartilage. Membrane Bones. — On the upper side of the olfactory capsules are two triangular nasal bones, which form the posterior boundary of the external nares. The cartilaginous floor of each olfactoiy capsule is partly covered by a somewhat triangular vomer, which is widely separated from its fellow. Each vomer bears a patch of sharp vomerine teeth, and partly bounds the internal naris of its side. Jaws. — Both upper and lower jaws consist of a cartilaginous basis and of several bones, which in the case of the former are all membrane bones. (a) Upper Jaw. — Owing to the projection of the sense-capsules a sort of bay, the orbit, in which the eye lies, is left on each side of the brain-case. It is bounded in front by a flat piece of cartilage, the ethmo-palatine bar, which runs out transversely from the floor of the olfactory capsule, sends a process forwards, and becomes continuous with a slender pterygoid bar, which passes back on the outer side of the orbit, and fuses behind "with the quadrate cartilage (suspensorium). This is a cartilaginous rod directed outwards, downwards, and backwards. Its proximal end is forked, the short limbs of the fork being attached, above and below, to the outside of the auditory capsule. The lower jaw is “ suspended ” to its distal end, which presents an articular hollow. These parts constitute the cartilaginous basis of the upper jaw and its supports. Its actual margin is formed by a series of membrane bones, of which the most anterior are the two pre- maxillce, small tooth-bearing elements meeting in the middle line in front of the olfactory capsules. On each side an elongated curved maxilla, which bears most of the teeth, runs back from the pre-maxilla and unites behind with a narrow quadrato-jugal extending posteriorly to the quadrate cartilage. Three bones on each side connect the upper jaw with the cranium and keep it firmly in position. These are the j)alatine, pterygoid, and squa- mosal. The palatine is a slender transverse bone moulded on the hinder edge of the ethmo-palatine bar, while the pterygoid bar is largely ensheatlied by the anterior ray of the three-rayed pterygoid bone, AMPHIBIA. 183 which touches the external end of the palatine in front. The other two rays are shorter. One of them runs inwards, the other along the under side of the quadrate cartilage. A T-shaped bone, the squamosal, the stem of which covers the quadrate cartilage, partly covers the auditory region. This hone supports the annular tympanic cartilage, over which the tympanic membrane is stretched. {(3) The lower jaiu (mandible) consists of two strongly-curved halves, each of which is traversed by an axial cartilage, Meckel's cartilage, ossified in front into a small mento-meckelian bone, uniting with its fellow in a median symphysis. Behind it presents an oval projection, the condyle, which articulates with the quadrate cartilage. Meckel’s cartilage is strengthened below and on its inner side by a long angulo-splenicd bone, from the posterior part of which a small elevation, the cor onoid process, projects upwards. Each half of the mandible possesses a membrane-bone, the dentary, which is a thin splint covering the outside of Meckel’s cartilage for rather more than its anterior third. A series of structures forming the hyoid apparatus are con- nected with the skull, and usually described with it. They are partly related to the auditory apparatus, and partly to the floor of the mouth. The columella is a small rod, bony in the centre, which is fixed into the fenestra ovalis by one end, while the other is attached to the inside of the tympanic membrane. The remainder of the hyoid consists of a quadrangular plate of cartilage, the body of the hyoid, supporting the floor of the mouth. Its angles are produced into anterior and posterior processes. In front of the former two slender, curved rods of cartilage, the anterior cornua, arise, each of which runs backwards round the angle of the mouth to be attached to the auditory capsule just beneath the fenestra ovalis. From the posterior end of the hyoid body two short bony posterior cornua or thyrohyals run back, which diverge and enclose between them the laryngo-tracheal chaml^er. (^) The vertebral column is a hollow rod running back from the skull along the dorsal side of the body. The spinal cord lies in its cavity. The anterior part of the column is segmented, being made up of nine rings, the vertehrw, while the posterior part, termed the urostyle, is unsegniented. All are cartilage bones. The vertebrae are very similar, except the first and last. Each is a ring, the thickened ventral part of which is the body or centrum. 184 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. while the rest forms the neural arch. The successive centra are united firml}^ together, and are jprocodous — i.e., concave in front and convex behind. Both surfaces are covered by a thin layer of cartilage. (The ^th vertebra is a'm,phicodous, concave on both faces.) Dorsal and lateral spaces are left between adjacent arches. The former are filled up by connective tissue, while through the latter, or intervertebral foramina, spinal nerves take exit. From the upper side of each arch a small neural spine projects upwards and backwards in the middle line, while on each side of the arch a stout, cartilage-tipped bar, the transverse p}'t'0cess, runs outwards. Four small projections with smooth articular surfaces project from the front and back of the arch. By these articular processes or zygapophyses the adjoining arches are linked together. Two, the proe-zygapophyses, are anterior, and their articular surfaces face upwards and inwards, while the other two, post-zyg apophyses, are posterior, and their articular surfaces face downwards and out- wards, overlapping the prse-zygapophyses of the following vertebra. The Is^ vertebra, or atlas, is devoid of tran verse processes and prse-zygapophyses, and the neural spine is rudimentary. The centrum is somewhat thin, and projects forwards between the occipital condyles, which articulate with two large concave facets on the front of the vertebra. The ^th vertebra, the sacrum, has very large and strong trans- verse processes directed outwards and backwards. The centrum is convex in front, and presents a pair of articular tubercles behind, which articulate with corresponding concavities on the front of the urostyle. This is a bony rod, somewhat trihedral, and taper- ing behind to a point tipped with cartilage. It may be regarded as representing a number of vertebrsc fused together, of which a ridge running along its dorsal surface corresponds to the united neural spines. Two small foramina open, one on either side, into the canal of the urostyle, a short distance behind its anterior end. Through these the coccygeal nerves take exit, and they, therefore, correspond to intervertebral foramina. The part of the urostyle in front of them is the first of the fused vertebrie, and may possess more or less distinct transverse processes. (c) The sternum consists of several cartilage bones and cartilages placed in the middle line on the ventral side of the thoracic region. In the extreme front a flat, narrow bone, the omo-sternum, is found, to the anterior end of which a rounded flap of cartilage (the epi- AMrillBIA. 185 sternum) is attached, while posteriorly it broadens out and abuts upon a pair of narrow cartilaginous epi-covctcoids. These are succeeded by the sternum proper^ a flattened rod of bone, with a core of cartilage. To its hinder end a large, thin, deeply-notched piece of cartilage, the xiphi-sternum, is attached. (2) Appendicular Skeleton. — The endoskeleton of either the fore- or hind-limb is divisible into — (1) Limh-girdle, which is firmly attached to the trunk, and (2) Free limb, articulated to the girdle. (a) Fore-Limb. — The two shoulder-girdles form an incomplete ring almost encircling the body just behind the atlas vertebra. They are firmly fused to the epi-coracoids, ventral! y, while dorsally they are attached by muscles to the skull and vertebral column. Each girdle is made up of a dorsal and a ventral moiety. At their point of union is a shallow, articular, glenoid cavity, affording attachment to the free limb. The dorsal part is made up of — (a) The supra-scapula, a quadrangular plate of cartilage, more or less calcified and ossified, broad above, and narrowing downwards to join (b) the scapula, an hour-glass-shaped bone, the lower end of which partly forms the glenoid cavity. The ventral part is composed of — (a) The coracoid bone, similar in shape to the scapula, and completing the glenoid cavity. In front of the coracoid there is an oval space filled with connective-tissue, the coracoid fontanelle. This is bounded in front by (b) the j^'^'f^-coracoid (pr-co), a trans- verse bar of cartilage largely ensheathed by (c) the clavicle, which is the only membrane bone in the appendicular skeleton. The free limb is m.ade up of bones supporting the upper arm, fore-arm, and hand. The humerus is a long bone belonging to the first of these. Like the long bones of the limbs generally, it consists of a hollow marrow-containing shaft, and an enlarged epiphysis at each end, covered b}^ a thin layer of cartilage. The epiphyses remain for a long time distinct from the shaft, with which, however, they ultimately fuse. The proximal end of the humerus forms a rounded head, articulating with the glenoid cavity. A well-marked deltoid ridge, more prominent in the male, runs from this half way along the anterior (ventral) surface of the bone. The distal end presents a spheroidal surface with which the next bone articulates, and above and below this (pre- and post-axially) are condylar ridges. The ante-brachium is sup- ported by a short, stout radio-ulna. This is a compound bone. 18G AN ELEMENTARY TEXT-BOOK OF BIOLOGY. made up of a pre-axial radius, fused with a post-axial ulna. The boundary between these is marked by a groove at the distal end. The proximal end is excavated to receive the corresponding })rojection on the humerus, with which it forms the elbow- joint, and produced back behind the last into the olecranon process, belonging to the ulnar half of the bone. There are two articular projections, one radial, the other ulnar, on the distal end, which help to make up the wrist-joint. The endoskeleton of the manus is made up of the wrist or carpus, and the bones of the digits. The typical or theoretical carpus, deduced from comparison of numerous cases, consists of 9 elements, — 3 proximal, 1 central, and h distal, the relative position of which is as follows: — [E = radial or pre-axial side ; U = ulnar or post-axial side.] E U radiale intermedium ulnare centrale carpale 1 carpale 2 carpale 3 carpale 4 carpale 5 In the frog there are six small bones in the carpus, three prox- imal and three distal. Two of the former, corresponding to radiale and ulnare, articulate with the radial and ulnar facets respectively. The third proximal bone, the displaced centrale, is on the inner (pre-axial) side of the radiale. The three distal bones correspond to the carpalia. The first supports the rudimentary 1st digit and = carpale 1, the second supports the 2nd digit and = carpale 2, while the much larger third bone represents carpalia 3, 4, 5, fused together, and supports the remaining digits. Following the wrist- bones are five slender metacarpals, one to each digit. The 1st metacarpal is very small, but is all that represents the 1st digit, while digits 2, 3, 4, 5 are terminated by 2, 2, 3, 3 slender pkala?iges respectively^ (d) Hind-Limb. — 'The hip-girdles are closely united to form the pelvis, which resembles in shape a two-pronged fork with an extremely short handle. Its posterior part is a rounded plate, laterally compressed, and presenting on either side a deep oval cup, the acetabulum, with a prominent margin. This cup is for the articulation of the free limb. Nearly half of the plate and acetabulum are formed antero-dorsally by the broad hinder ends AMrniBiA. 187 of the two prongs ” or ilia fused in the middle line. Each ilium is continued forwards to the sacrum, to the corresponding transverse process of which its cartilage-tipped end is united. This part of the ilium is laterally flattened, with somewhat concave ventral and convex dorsal edges. Two other elements on either side, j)ubis and ischium,, of which the first is cartilaginous, unite with their fellows to form the middle and posterior parts of the plate and acetabulum. The pubes are triangular, and their apices extend to the upper margin of the acetabulum, above which the ilia and ischia unite. The free limb is composed of bones belonging to the thigh, leg, and foot. The first is supported by a long bone, the femur, the slender shaft of which possesses a slight double or sigmoid curve. Its proximal end is enlarged into a rounded head, which articulates with the acetabulum to form the hip-joint, and its distal end also presents an articular expansion. The bone of the leg, tibio-fihda, like that of the fore-arm, is compound. It is made up by the fusion of a pre-axial tibia with a post-axial fibida, the boundaries of which are indicated by grooves and a double marrow-cavity. The shaft is curved, and the ends are expanded into transversely elongated pulleys, which assist in the formation of the knee- and ankle-joints respectively. The bones of the foot are partly those of the ankle, tarsus, and partly those of the digits. The typical or theoretical tarsus contains the same number of elements as the theoretical carpus, and these are similarly arranged, as folloAvs : — [T = tibial or pre-axial side ; F = fibular or post-axial side.] tarsale 1 tarsale 2 tarsale 3 tarsale I tarsale 5 The tarsus of the frog is very much elongated, and composed of four bones, two of which are proximal and two distal. The proximal ones, astragalus and calcaneum ( = tibiale + intermedium, and fihulare), articulate with the tibial and fibular sides of the articular surface on the distal end of the tibio-fibula. They are united to each other at either end by their epiphyses. The distal bones, equivalent to the tarsalia, are extremely small. One, T tibiale intermedium centrale F fibulare 188 AN ELEMENTARY TEXT-ROOK OE BIOLOGY. representing tarsale 1, supports the calcar and 1st digit, the other represents tarsale 2 + tarsale 3, and supports the 2nd and 3rd digits. Tarsalia 4 and 5 are not distinctly represented, and there is no centrale. The tarsus is succeeded by the metatarsus. The base of the calcar is formed by a minute extra metatarsal, while the slender metatarsals 1-5 belong to the corresponding digits. A small flattened phalanx completes the calcar, and 2, 2, 3, 4, 3 phalanges terminate digits 1, 2, 3, 4, 5, respectively. Cartilage and bone may both be considered as modiflcations of connective tissue in which the matrix is very plentiful. The most typical kind of cartilage is made up of a clear, homo- geneous matrix^ in which are numerous small cavities, lacunae, connected with one another by fine channels, and containing nucleated cartilage-cells. By the division of these, and the depo- sition of fresh matrix, cartilage grows, especially near its surface, which is covered by a connective-tissue membrane, the perichon- drium, in which nerves, lymphatics, and blood-vessels run. The matrix of bone is densely fibrous connective tissue, impreg- nated with lime salts, principally carbonate and phosphate. In this are imbedded numerous much -branched hone-cells, which lie in lacunce connected by fine tubules, canaliculi, into which the cell-processes are continued. The matrix is traversed by blood- vessels, contained in Haversian canals. Bone may be either spongy, as in the epiphyses of long bones and the interior of flat and short bones, or compact, as seen in the shafts of long bones and the exterior of flat and short bones. A membrane, the periosteum, similar to perichondrium, covers the outer surface of bones. Its inner part is made up of rounded cells, osteoblasts, which during growth add fresh layers of bone to the outside. Ossification always starts from definite centres. Membrane bones commence as calcified networks of connective-tissue fibres covered by osteo- blasts, while cartilage bones are pre-formed in solid cartilage, covered by periosteum. Into this solid cartilage processes of the osteoblastic part of the periosteum penetrate to form the marrow, which first absorbs the cartilage, and then replaces it by spongy bone. In the shafts of long bones this is absorbed in its turn to form a continuous marrow-cavity. Cartilage bones can continue to lengthen by growth at their ends. The new bone is formed in the Avay just described, from the cartilage found there. 5.x AIMPHIBIA. 189 Bones thicken by the addition of layers to the outside, formed by the periosteum. The vertebral centra contain peculiar cellular cores, the last remnant of an important embryonic structure, the notochord. 4. The digestive organs (Fig. 51) consist of a convoluted tube, the gut or alimentary canal, and of glands connected with this. The gut presents the following regions : — mouth-cavity, gullet, stomach, small intestine, and large intestine opening into a cloaca which also receives the excretory and reproductive ducts. The most important annexed glands are the liver and pancreas. The wide mouth, which reaches back as far as the tympanic area, and possesses a narrow iipper lip, leads into a spacious mouth-cavity, the back part of which is termed the pharynx. More than 100 minute double-pointed teeth are affixed to the inner side of the upper jaw. They are attached to the pre- maxill^e and maxillce, and are placed in a furrow bounded by the upper lip externally, and a fold of mucous membrane internally. The mucous membrane is a pale, soft, extremely glandular layer which lines the alimentary canal. On the roof of the mouth are two small patches of vomerine teeth, forwardly placed, and borne by the vomers. They are similar to the others. No teeth are present in the lower jaw. On the roof of the mouth, in the extreme front, a number of minute pores are present, the openings of the intermaxillary glands. The vomerine teeth are a little way behind this, and external to- each patch is a small, transversely oval opening, the internal naris. Still further back two large rounded prominences, caused by the eyes, project into the mouth. Near the angle of the jaw, on each side, is a good-sized opening, that of the Eustachian tube, which leads to the tympanic cavity. On the floor of the mouth is a narrow, elongated muscular tongue attached in front. In the quiescent state its forked end is directed backwards. Numerous • small elevations cover its surface, some of which, filiform papillae, are narrow-ended ; others, fungiform papillce, broad-ended. Near the angle of the mouth in the male E. esculenta an oval opening is present, which leads into a rounded, dilatable vocal sac. » Posterior to the end of the tongue there is a longitudinal ; chink, the glottis, with firm swollen edges. It leads into the respiratory organs. 190 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. The mouth-cavity passes behind into a short, wide gullet or oesophagus, and this merges without sensible demarcation into a narrower tube, the stomach. This passes backwards, gradually narrowing as it does so. The lining of both oesophagus and stomach is raised into longitudinal ridges. At the posterior or pi/Ioric end of the latter there is a well-marked constriction Fig. ol. — !Side-Dissection of Male Feog. — 2, 3, 4, 5, Digits; to, tongue; st, stomach, upon which are blood-vessels; py, pylorus; d, duodenum; i, rest of small intestine ; Li, large intestine ; cl, cloaca, opened ; r.l, and LI, right and left liver-lobes; g, gall-bladder; x, opening of bile-duct, which is represented by the black line ; p, pancreas, s. v. Sinus venosus, near which is cut end of right precaval ; r.a, right auricle, above which are the cut ends of carotid arch, systemic arch, and cutaneous artery, while the pulmonary artery is seen running along lung; i\ ventricle; t.a, truncus arteriosus, gl. Glottis; Ig, right lung, k. Right kidney, the dark space above which represents the subvertebral lymph-sinus ; u-g, right urinogenital duct, which opens on a papilla in the cloaca (just above the L in cl)’, hi, bladder (its opening is seen just below I in cl), t. Spermary (testis) above wdiich the vasa efferentia are seen running in the mesorchium; v..?, vesicula seminalis. ol. Right ol- factory lobe, from which right olfactory nerve is seen running forwards ; ch, right cerebral hemisphere; o.l, right optic lobe, passing below and in front into optic tract and nerve; cb, cerebellum; m.o, medulla oblongata; sp, spinal cord, na. Plight nasal sac. marking the position of the pylorus or point at which the cavity of the stomach communicates with that of the small intestine. This is a narrow thin-walled tube, the first part of which, the duodenum, forms, together tvith the stomach, a U-shaped loop, AMPHIBIA. 191 while the rest is thrown into several coils. Its lining is raised into transverse folds connected by fainter longitudinal ridges. The end of the small intestine suddenly dilates into a short thin-walled large intestine, which narrows behind and becomes continuous with a chamber, the cloaca, opening externally by the small circular cloacal aperture. The cavity of the large intestine is separated by an annular valve from that of the small. Its lining is at first raised into delicate intersecting ridges, and then into longitudinal folds which pass back to the end of the cloaca, the posterior part of which is lined by ordinary skin. Two important glands are connected with the alimentary canal — the liver, and pancreas. The liver is a very large reddish-brown organ, occupying a considerable space near the front of the body-cavity. It arises as an outgrowth from the alimentary canal, which quickly be- comes bilobed. In accordance with this the adult liver is divided into right and left halves, connected by a narrow strip of liver- substance. The left half is again subdivided into two, so that altogether three lohes are present. These are convex ventrally, somewhat concave dorsally, blunt-ended in front, and elsewhere thinning off into edges. The liver secretes the bile, a bright yellowish-green fluid. Closely attached to the dorsal surface of the right lobe is a rounded thin-walled sac, the gall-bladder, full of bile, in the fresh state. From it a short tube, the cystic duct, proceeds, which is connected with a fine network of hepatic ducts, arising from the liver. From this a bile-duct is given off, which is reinforced by other hepatic ducts, and runs in the U formed by stomach and duodenum. Anally opening into the latter about half an inch beyond the pylorus on the inside of the U. The pancreas is an elongated yellowish mass lying in the U-shaped loop, and produced irito several tapering processes. With it the bile duct is closely connected, receiving from it a number of small pancreatic ducts. This gland secretes the pan- rreatic juice. The digestive and other organs of the frog are contained in the large plenro-peritoneal- or body-cavity (coelom). This narrows in front, where it is bounded by the heart in its pericardium, and a kind of muscular partition, the diaphragm. It also narrows behind. Its dorsal wmll is formed by the back- 192 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. bone and associated muscles, its ventral and lateral walls, in the thoracic region, by hard parts and muscles, by muscles alone in the abdominal region. The body-cavity is lined ventrally and laterally by a thin pigmented membrane, the pUuro-peritoneumy which dorso- laterally becomes free, leaving a space, the suh- vertehral lyni])h-sinus, beneath the vertebral column. The two lateral halves of the pleuro-peritoneum run to the middle line, forming a floor to this sinus, and then unite to make a double sheet, the mesentenj, the halves of which diverge, surround the alimentary canal, and then become continuous. The alimentary canal is thus suspended from the dorsal wall of the body-cavity, and, strictly speaking, is outside it. The same may be said of the other contained organs. The walls of the oesophagus, stomach, and intestines are com- posed of four coats, which are, commencing from the outside : — • (1) serous coat; (2) muscular coat; (3) sub-mucous coat ; (4) mucous mem- brane. (1) The serous coat is in reality the pleuro* peritoneal investment of the gut. It consists of a layer of simple squamous epithelium, with a thin underlying stratum of connective tissue in which run the blood-vessels, lymphatics, and nerves that supply the other coats. (2) The musailar coat^ best developed in the stomach, is divided into an outer longitudinal and an inner circular layer. It is made up of unstriated muscle- fibres. These are spindle-shaped cells (Fig. 52), each containing an elongated nucleus, which dovetail together by their Muscle - Fibres (from tapering ends. Landois and Stirling), (3) The suh-mucous coat is also best SoUtedT'nffn 7rols’- developed in the stomach, and consists section. of loose connective tissue traversed by numerous blood-vessels and lymphatics. (4) The mucous membrane is the part which is raised into folds internally. It is exceedingly glandular, and is made up of {a) a thin external layer of unstriated muscle, the muscularis Fig. 52. — Unstriated AMPHIBIA. 193 mucosm ; (h) a layer of connective tissue with glands, blood- vessels, and lymphatics; (c) a layer of simple columnar e}oi- thelium, adjoining the cavity of the gut.’ The epithelium is of most interest. It is ciliated in the oesophagus and beginning of the stomach, and many of its cells, known from their sliape as goUet-cells, are unicellular mucus- secretin" "lands. Besides this there are tubular multicellular O O glands lined by the epithelium and lying in the connective- tissue layer of the mucous membrane. They are of three kinds : — a, cesopliageal glands in the gullet ; j5, peptic or gastric glands in the stomach; y, glands of Lieherkilhn in the small intestine. The deeper parts of the peptic glands are lined by cuboidal cells, which secrete the gastric juice. There are several points of interest in the histology of the mucous membrane lining the mouth. Its epithelium is strati- fied near the margins, elsewhere simple, columnar, and ciliated. Numerous goblet-cells are present. The tubules of the inter- maxillary glands are lined by mucus-secreting epithelium. The teeth are developed in the mucous membrane of the mouth. The projecting part or crown^ which is forked, is separated by a constriction from the rest of the tooth, or socket, which contains a pulp-cavity, in which is the pulp, a small mass of vascular connective tissue. The tooth is mostly made up of dentine, the constituents of which are wavy tubules filled by prolongations of the pulp. The crown is covered by a cap of extremely hard enamel, exhibiting a layered structure, and containing prolon- gations of the dentine tubules. It is secreted by the epithelium of the mouth. The socket is covered by a layer of cement, resembling bone in structure. The teeth are all very similar, and during life are constantly replaced by new ones, which grow up from their bases. The liver (cf. Fig. 51) is mostly made up of polyhedral glan- dular hepatic cells, granular, nucleated, and containing fat drops. Between these cells minute tubes, the hile-capillaries, run, forming a complex network from which the ultimate branches of the hepatic ducts arise. The vessels supplying the liver with blood break up into capillaries within it, and in these the branches of the hepatic veins, which carry blood from the liver, take their origin. 2 13 194 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. The ducts of the pancreas subdivide considerably, and finally terminate in blind tubules lined by glandular epithelium. Nutrition. — The food of the adult frog consists mainly of insects. These are secured by the tongue, which can be rajiidly protruded and retracted. Its peculiar mode of attachment enables it to be put out to some distance, and being covered with a viscid secretion, insects are easily secured. The secretion of the inter- maxillary gland is especially sticky, and the tip of the tongue brushes past its openings when protruded. The teeth are not used for chewing, but to prevent the escape of prey. The cilia of the mouth-cavity work so as to carry the food to the gullet, and from this point the contractions of the muscular wall of the alimentary canal propel it backwards. The chemical agents acting upon the foods are principally — (1) Gastric juice; (2) Pancreatic juice; and (3) Bile. The gastric juice is secreted by the i)eptic glands, and in virtue of a ferment, which it contains, converts some of the proteids into peptones. The ferment can only act in an acid solution, and the secretion contains a small amount of free hydrochloric acid. The culioidal cells of the peptic glands secrete pepsin, while the acid is formed by the ovoidal cells. The pancreatic juice contains ferments which (ct) convert starch into grape-sugar; {h) juoteids into peptones; and (c) split up fats. Alkalinity is necessary for this action, and the bile neutralizes any acid passing over from the stomach. It also emulsifies fats, and facilitates their absorption. The ridges and folds into which the lining of the alimentary canal is raised largely increase its absorptive surface. Some of the products of digestion pass at once into the blood-system, others reach it indirectly by way of the lacteals, subvertebral sinus, and lymph-hearts. The refuse is ejected at the cloacal aperture. Most of the products of digestion, by 'svay of the i)ortal vein, pass through the liver, and this organ absorbs the carbo-hydrates, stores them up (as nlycogen), and returns them to the system as required. 5. The circulatory organs consist of a closed hlood-system with which a lymph-system communicates. (1) Blood System. — The Uood is a bright-red, coagulable fluid, composed of a clear plasma in which float colourless corpuscles of AMPHIBIA. 195 the usual type, and larger and more numerous red corpuscles, which owe their colour to haimoglobin. They are oval and flat, with a large central nucleus of similar shape, projecting some- what on both surfaces of the corpuscle. The blood circulates in a closed system of tubes, consisting of heart, arteries, veins, and capillaries. The heart (Figs. 51 and 53) is situated in the anterior part of the thoracic region, on its ventral side. It is enclosed in a membranous bag, the pericardium, which is composed of an inner layer closely adherent to the heart, and a loose outer layer. A space, the cavity, which is a separated part of the €) 4 to 0 small m'inogenitat arteries wliicli at once bifurcate, and run to urinogenital organs and fat-bodies; (c) small paired lumhar arteries to adjoining muscles; (d) a small liamiorrhoidal artery to the large intestine. (3) Each iliac artery gives off (a) a hypogastric artery to the bladder, and {h) epigastric arteries to the ventral body- wall. It then becomes the sciatic artery of the thigh, which divides into peroneal and tihial arteries for the leg and foot. (3) The pulmo- cutaneous arches carry impure blood to the lungs and skin. Each of them quickly divides into a pulmonary artery running down the outer side of the lung and a cutaneous artery which ramifies in the skin. The veins (Fig. 55) bring back blood from the various parts of the body to the heart. They may best be considered under the following headings: — (1) Caval system; (2) Portal systems; (3) Pulmonary veins. The caval system consists of three caval veins with their branches — i.e., two precavals (anterior or superior vense cavse) in front, and a postcaval (posterior or inferior vena cava) behind. These pour their blood into the sinus A^enosus, Avhich may be regarded as formed by the fusion of their ends (see Fig. 55). The 'preearal on each side is formed by the union of three veins — (1) The external jugular bringing back blood from the lower jaAV, floor of the mouth, and tongue ; (2) The innominate formed by the union of the internal jugular and suhscapular veins which respectively return blood from the brain, and shoulder with back of arm ; and (3) The subclavian. This is a large vein made up of the brachial vein from the fore-limb, and the musculo-cutaneous vein, Avhich brings back blood from the muscles and skin of the sides and upper surface of the body and head. The gmtcaval lies A'^entral to the dorsal aorta. It commences betAveen the kidneys, and is made up by the union of several pairs of renal veins from those organs, and veins from the genital glands and fat bodies. Just before entering the sinus A^enosus it receives tAvo hepatic veins from the liver. Portal Systems. — A portal vein is one Avhich, instead of pouring its blood into a larger trunk, breaks up into capillaries Avdthin the substance of some organ, supplying it Avith impure blood. Both kidneys and li\"er in the frog possess such a supply, and in accordance Avith this, (1) renal portal and (2) hepatic portal systems can be distinguished. O AMPHIBIA. 190 (1) llenal Portal — The principal veins of the hind- limb are the femoral and sciatic, running along the front and back of the thigh. The femoral vein is connected by a cross-branch ('''') with the sciatic just before reaching the trunk, and then divides into a 'pelvic vein which runs to the middle ventral line to unite with its fellow, and a renal portal vein which runs to the outer side of the kidney, and divides into small veins breaking up in the kidney- substance. The renal portal vein receives the sciatic vein, dorso - lumbar veins from the dorsal abdominal walls, and, in the female, small veins from the oviduct. (2) Hepatic Portal. — The liver is supplied with impure blood by two veins, (a) the anterior abdominal and (h) the portal. {a) The anterior abdominal is formed by the union of the two pelvic veins, receives veins from the bladder, and runs forwards in the middle line of the ab- dominal wall as far as the liver. Frog (after rleie it bifurcates, the two dlVl- Sinus venosus; 7A A, right auricle; sions going to the right and left L.A, left auricle ; V, ventricle ; halves" of the liver. (h) The external poital vein is formed by the lar vein; s-sc, sub-scapular vein ; in, innominate vein ; s-cl, sub- clavian vein; hr, brachial vein; m-ct, musculo-cutaneous vein; p-c, post- caval ; sc, sciatic vein ; /, femoral vein ; *, cross-branch connecting sciatic and femoral ; p.v, pelvic veins ; r.p, renal portal vein ; d-l, dorso-lumbar veins; o, veins from oviduct; r.v, renal veins ; a, ah, anterior abdominal vein ; hi, veins from bladder ; + , small vein from truncus arteriosus ; p, portal vein; h., hepatic veins ; h, kidneys ; i, represents alimentary canal with its capillaries ; I, represents capillaries of liver ; pi, pulmonary veins. 200 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. union of the gastric vein, bringing blood from the stomach, with the lieno-mesenteric vein returning it from the spleen and in- testines. It runs to the left half of the liver, and is connected by a cross-branch with the point of bifurcation of the anterior abdominal. A pulmonary vein runs up the inner side of each lung, and unites with its fellow to form a short common trunk, which opens into the left auricle. The capillaries are excessively fine tubes forming networks in nearly all parts of the body, by means of which the ultimate branches of veins and arteries are united. A vein commences, an artery ends, and a portal vein both commences and ends, in capillaries. (2) Lymph System. — The lymph is a colourless fluid which resembles blood, differing, however, in the absence of red cor- Fig. 56. — Posterior Lymph-Hearts of Frog (after EcTcer). L, Lymph-heart. puscles. The lymph-system consists of lymphatic vessels of larger and smaller size, which ramify in the skin, intestinal wall, and other parts of the body, and lymph-spaces with which these vessels are connected. The lymph-spaces include the irregular lacunae between the tissues, and also larger cavities, such as the pleuro- .peritoneal, pericardial, subvertebral, and subcutaneous spaces. The lymphatics of the intestine, which receive the special name of lacteals, traverse the mesentery and open into the subvertebral sinus. The lymph is propelled by two pairs of lymph-hearts, small oval sacs, with rhythrnically-contractile walls. The anterior lymph-hearts are situated one on each side between the trans- AMPHIBIA. 201 verse processes of vertebrse 3 and 4, and are connected with the subscapular veins. The posterior lymphdiearts (Fig. 56) are placed one on each side of the urostyle, not far from its posterior end, in a triangular space between the muscles. Each com- municates with a small vein which opens into the cross-branch between the sciatic and femoral veins of its side. Fig. 57. — Muscle-Fibres from Heart of Frog Stirling), much enlarged. (from Landois and Fig. 53,- Small Abtery showing the Coats (from Landois and Stirling), a, Fndothelium ; h, elastic membrane; c, muscular coat; d, connective-tissue coat. Closely connected with the circulator}^ system are several small organs known as ductless glands. These are the spleen and the thymus and thyroid glandsr The spleen is a small rounded body, of dark red colour, lying in the mesentery, near the commence- ment of the large intestine. The thymus glands are two small 202 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. bodies situated one on either side, near the end of the quadrate cartilage, and the thyroid glands are similar structures, two or more in number, placed near the posterior hyoid cornua. The cavities of the heart and blood-vessels are lined by endu~ tJielium, which alone forms the capillary walls (Fig. 59). The large lymph-cavities are similarly lined, and patches of granular germinal epithelial cells occur in them, from which colourless corpuscles are budded off. The heart is mainly composed of unicellular, spindle-shaped muscular fibres (sometimes branched), which are transversely striated, but possess no sarcolemma Fig. 59. — Capillaries (from Landois and Stirling), much enlarged. — The outlines and nuclei of the endothelial cells making up their walls are clearly shown. (Fig. 57). The veins and arteries possess three coats outside their endothelial lining (Fig. 58). These are, beginning from the inside — (1) a membrane formed by elastic connective tissue; (2) a muscular coat made up for the most part of unstriated muscle- fibres arranged circularly ; and (3) a connective-tissue sheath. These coats are thicker in arteries than in veins, and in large vessels than small. They gradually thin out as capillaries are approached, till at last only endothelium is left (Fig. 59). Circulation. — The heart receives oxygenated blood from the AMPHIBIA. 203 lungs by the pulmonary veins, which open into the left auricle. The caval veins pour impure blood from the general body into the right auricle. The blood in the precavals is partly oxygenated, as they receive the cutaneous veins from the skin, while that in the postcaval is largely free from nitrogenous waste. The blood is pumped out of the heart by the contraction of its walls. The sinus venosus first undergoes systole, then the auricles together, then the ventricje, and lastly the truncus arteriosus. Systole is followed by dias^tole, and the various valves prevent resfurgitation. Although the ventricle is single, and receives both kinds of blood, yet these do not completely mix, and the carotid, systemic, and pulmo-cutaneous arches are supplied by oxygenated, mixed, and impure blood respectively. This is explained as follows : — The cavity of the ventricle is transversely elongated. Into its left side the auricles open (their common aperture being rendered iwactically double by the free edge of the auricular septum), and out of its right side the truncus arteriosus. Moreover, its spongy wall absorbs much of the blood received by the auricular systole, and renders mixing more difficult. Just previous to the ventri- cular systole, the three arches offer different degrees of resistance to the passage of blood — the pulmo-cutaneous least, owing to the small extent of their branches, the carotid most, on account of the spongy carotid glands. AVhen the ventricle contracts, the blood nearest the truncus is impure, as the right auricle fills the right side of the ventricle. This blood takes the direction of least resistance — i.e., runs on the left of the longitudinal septum of the truncus into the pulmo-cutaneous arches. As these are filled, their resistance increases, and blood (now mixed, as sufficient time has elapsed for mixture to commence in the ven- tricle) flows on the right side of the septum, flapping it over the pulmo-cutaneous aperture, and enters the systemic trunks. Their resistance also increases, and the last portion of blood runs into the carotid arches. This is oxygenated blood from the left auricle, which has been stored up in the spongy wall of the left side of the ventricle. The large amount of elastic and muscular tissue in the arterial Avails serves two useful purposes. (1) Part of the force of the systole of the ventricle and truncus is expended in dilating the 204 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. arteries, which subsequently contract, and continue the pumping work during diastole, so that jerking is avoided. (2) The blood- supply to any part can be regulated. Owing to the influence of the sympathetic system the muscle is usually kept in a semi- contracted (tonic) state. The same system can cause them to contract more or to contract less, thus diminishing or increasing their calibre. The thin walls of tlie capillaries readily allow the blood-plasma to soak out of them, and waste products to enter them in the tissues, and leave them in the excretory organs. The lymph-hearts pump lymph into the blood-system. Blood- plasma exuding in excess through the capillaries is thus carried back again, and some of the products of digestion also enter the blood in this way. 6. Respiratory Organs (Fig. 51). — The opening of the glottis leads into a small larynx, which is firmly fixed between the posterior hyoid cornua. Its walls are strengthened by two triangular arytenoid cartilages bounding the glottis, and below these by a ring-like cricoid cartilage with several processes. Projecting from the sides of the cavity into its interior are two elastic folds, the vocal cords, one on each side. The larynx opens behind into the two lungs, which lie freely in the dorsal part of the thoracic region. Each is an elongated thin-walled bag, which first dilates and then tapers to a smooth tip. Its inner surface is raised into a j^rominent series of ridges, which form a close honeycombing. The skin must also be considered as a respiratory organ. The larynx is lined by a continuation of the ciliated epithelium of the mouth-cavity, and the lungs, for the most part, by simple squamous epithelium. The basis of these organs is fibrous connective tissue, with many elastic fibres and a good deal of unstriated muscle. A network of capillaries immediately under- lies the epithelium. Respiration is said to be buccal,” because inspiration (i.e., intaking of air) is effected by the agency of the mouth-cavity, as follows : — The mouth being shut, and the walls of the gullet contracted, appropriate muscles lower the floor of the mouth, and thus cause air to rush in by the nostrils. These are then closed, the floor of the mouth is elevated, and air is thus forced into the lungs. Expiration is mainly brought about by the elasticity of AMPHIBIA. 205 these organs, and the contraction of the muscle-fibres they contain,. The contraction of the abdominal walls may also assist. The thin-walled lung-capillaries are only separated from the- air contained in the lung-cavities by a layer of epithelium, and, therefore, carbon dioxide can readily diffuse out of, and oxygen into, the blood which they contain. A similar interchange takes place between the blood in the capillaries of the skin and the- oxygen in the surrounding air or water. This cannot take place in air unless the skin is damp. The licemoglohin of the red corpuscles plays the usual important part in respiration. Voice is produced by the vocal chords, the edges of which can be brought parallel to one another by certain muscles, and then thrown into vibration by the expired air. The vocal sacs in the male, K esculenta, Berve as resonators. 7. TJrinogenital Organs, including excretory and reproductive organs. The excretory organs (Figs. 51 and 60) are two elongated,, flattened, reddish-brown kidneys, symmetrically disposed in the posterior part of the subvertebral lymph-sinus, and covered with pleuro-peritoneum on their ventral surfaces only. A slender tube, urinary duct in the female, urinogenital duct in the male, runs from the outer side of each kidney to open into the dorsal side of the cloaca, close to its fellow, by a minute slit.. Directly opposite this there is a rounded aperture in the ventral cloacal wall which leads into the urinary bladder, a large bilobed sac with delicate membranous walls. Closely connected with the excretory organs are two anomalous struc- tures, the adrenals and fat-hodies. The former are two narrow bodies, of yellowish colour, one of which is closely imbedded in the ventral surface of each kidney. The fat-body (corpus adiposum) is a blight, orange- coloured tuft of finger-like processes attached to the front end of the kidney of either side. It consists of a network of connective tissue, in> the meshes of which numerous fat-cells are imbedded. These are spherical bodies full of fat, and covered by a thin layer of protojdasm with a nucleus- on one side. They result from the metamorphosis of ordinary connective- tissue cells. % The kidney is mostly made up of a large number of uriniferous tubules. Each of these commences in a thin-walled Bowmans capsule, lined by delicate squamous epithelium, into which has been pushed from the outside, so to speak, a tuft of capillaries,. 206 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. the glomerulus, formed by the breaking up of a branch of one of the renal arteries. The capsule and glomeridus together are known as a Maljihigian body. A convoluted tubule in which four iregions can be distinguished succeeds Bowman’s capsule. These Tegions are (1) a short necl:, lined with ciliated epithelium; (2) a much convoluted glandidar part, lined by columnar epithelium which is pigmented and granular ; (3) a second ciliated section ; and (4) a collecting portion. The urinary dud is formed by the union of the collecting portions, and is at first imbedded in the kidney on its outer side. 'The renal-portal vein breaks up into capillaries which closely surround the glandidar })arts of the uriniferous tubules, and are joined by the small veins coming off from the glomeruli. The nitrogenous Vvaxste of the body (as urea, CH^A^O), together Avith much Avater and A^arious salts, is excreted by the kidneys. The AA^ater and salts are filtered from the glomeruli into BoAvman’s capsules, Avhile the urea is excreted by the glandular parts of the kidney-tubules. The uriiiary bladder serves as a temporary receptacle for the urine. Male Reproductive Organs (Fig. 51). — An oA'al, yelloAvish, spermary (testis), enveloped in pleuro-peritoneum, is closely con- nected l)y a double fold of the same, the mesorchmm, Avith the inner side of each kidney. In this fold a number of delicate tubules, the vasa efferentia, run from one organ to the other. The tAVO spermaries are commonly unequal in size. In A. esculenta the urinogenital dud is considerably dilated just as it leaves the kidney, and in li. temporaria an oval glandular mass, the vesicida seminalis, is placed on its outer side immediately before it enters the cloaca. The male Frog is distinguished externally by the characters mentioned on pp. 175 and 17G. The spermary is essentially composed of seminiferous tubules, lined b}^ e])ithelium, and opening into a central caAuty. From this the vasa efferentia arise, and, after traversing the mesorchium, enter a longitudinal canal imbedded in the inner edge of the kidney, and giving off numerous transverse tubules, Avhich join the collecting parts of the uriniferous tubules just before they enter the urinary duct. The sperms (spermatozoa) are thread- like bodies, Avith narrow cylindrical heads and vibratile tails. They are developed from spindle-shaped sperm-mother-cells, AMPHIBIA. 207 which largel}^ make up the epithelial lining of the seminiferous tubules. Each sperm-mother-cell originates a tuft of sperms, an unused stump (sperm-blastophor) being left over. Female Eeproductive Organs (Fig. GO). — The ovaries are two flattened sacs, varying much in size according to the season, and situated similarly to the spermaries. The pleuro-peritoneum invests each of them and forms a double suspensory fold, the mesovarium. Ova, of various sizes, enclosed in follicles, project on the outer surface of the ovary. The oviducts are two much- coiled tubes invested in pleuro-peritoneum, and each with a narrow anterior end opening near the root of the lungs by a ciliated funnel. The two funnels are close together {the organs in the Fig. have been taken out of the hodg), and open into a common ciliated depression. The oviducts widen behind, and each of them finally dilates into a thin-walled uterus, opening on a prominent papilla on the dorsal wall of the cloaca. The two papillae are situated close together, just in front of the urinary apertures. The ovary contains numerous ova enclosed in follicles, by the rupture of which they escape into the body-cavity. The mature ovum is about jV of an inch in diameter. It is covered by a delicate Fdelline membrane, and possesses gran- ular pigmented vitellus, and a large germmal vesicle with numerous germinal spots. Two polar cells are formed before fertilization. The wall of the oviduct contains numerous branched tubular glands O Fig. ()0. — Female Repro- ductive Organs of Frog (after Ecker and Wieder- .sheim). Right ovary re- niov’ed. — ov, Left ovary; od, oviduct; od', funnel of ditto; ut, uterus; uf', opening of ditto in cloaca ; kidneys ; u,u, openings of ureters; cl, cloaca. which secrete a glairy substance, cap- able of swelling up immensely l)y imbibition of water. The wall of the uterus contains numerous unstriated muscle-fibres. At the commencement of the breeding-season (early spring for E. temporaria), the males affix themselves to the backs of the 208 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. females, in which process the roughened pads on the hands afford firm attachment. This occurs in the water. The ova burst out from the ovaries, are taken up by the ciliated funnels of the oviducts, and pass down those structures to the uteri, receiving on their way gelatinous investments. Here they are aggregated into “ clumps,’’ and expelled by the contraction of the muscle in the uterine and cloacal walls. As they are extruded from the body, sperms are shed over them by the male. Fertilization occurs in the usual way by fusion of a sperm with each ovum. 8. Muscular System. — In a tadpole there are lateral muscles divided into myomeres, as in a fish, but only traces of this seg- mentation are to be found in the adult. The muscles are very numerous, and have a very complicated arrangement. The following table will give a rough notion of their classification (after Huxley) : — A. Muscles of Head \ Epishdetal, superficial to the endoskeleton. and Trunk. \ Hyposheletal^ within the endoskeleton. ! Intrinsic, taking origin in the limbs themselves (including girdles). _ Extrinsic, taking origin outside the limbs. The firm endoskeleton affords points of attachment, and this is often effected by the intermediation of cord-like tendons, almost entirely made up of white connective-tissue fibres. The firm sheaths, aponeuroses, by which muscles are covered, may also' serve for the attachment of other muscles. This is best seen in flat muscles, such as some of those forming the abdominal wall. The muscles are made up of bundles of transversely striated fibres, each of which is invested by a delicate sar colemma, and contains numerous muscle-corpuscles — i.e., nuclei surrounded by small quantities of protoplasm. These fibres also exhibit longi- tudinal striations. From a physiological point of view the muscular tissue of Vertebrates is divided into — (1) Involuntary, not under the control of the will, including the unstriated fibres of the viscera and blood-vessels, as well as the striated fibres of the heart ; and (2) Voluntary, under the control of the will, including the striated fibres of the ordinary muscle. The minute structure of muscle and its physiological import have long been much vexed questions. The views here adopted AMPHIBIA. 209 are those of Melland and C. F. Marshall, as set forth in the following summary given by the latter : — “ 1 . In all muscles which have to perform rapid and frequent movements, a certain portion of the muscle is differentiated to perform the function of contraction, and this portion takes on the form of a very regular and highly modified intracellular network. “ 2. This network, by its regular arrangement, gives rise to certain optical effects, which cause the peculiar appearances of striped muscle. “ 3. The contraction of the striped muscle-fibre is probably caused by the active contraction of the longitudinal fibrils of the intracellular network ; the transverse networks appear to be pas- sively elastic, and by their elastic rebound cause the muscle to rapidly resume its relaxed condition when the longitudinal fibrils have ceased to contract ; they are possibly also paths for the nervous impulse. “ 4. In some cases where muscle has hitherto been described as striped, but gives no appearance of the network on treatment with the gold and other methods, the apparent striation is due to optical effects, caused by a corrugated outline in the fibre. “5. In muscles which do not perform rapid movements, but whose contraction is comparatively slow and peristaltic in nature, this peculiar network is not developed. In most if not all of the invertebrate unstriped muscles there does not appear to be an intracellular network present in any form, but in the vertebrate unstriped muscle a network is present in the form of longitudinal fibrils only ; this possibly represents a form of network inter- mediate between the typical irregular intracellular network of other cells and the highly modified network of striped muscle. 6. The cardiac muscle-cells contain a network similar to that of ordinary striped muscle.” The motor nerve-fibres may terminate in end-j)lates (compare Fig. 90), within the sarcolemma of striated muscle-fibres, but more frequently the axis-cylinder breaks into a brush of fibrils which run longitudinally in the muscle- substance. 10. The nervous system (Fig. 51) may conveniently be sub- divided into — (I) cerebro-spinal axis; (2) cranio-spinal nerves ; and (3) sympathetic system. (I) The cerebro-spinal axis is made up of the brain and spinal 2 14 210 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. cord, contained in the neural canal. They are invested by a delicate pigmented vascular membrane, the pia mater, and the neural canal is lined by a firm fibrous membrane, the dura mater, Between the two is a lymphatic arachnoid space, the walls of which are formed by a delicate arachnoid membrane. The brain is the anterior enlargement of the cerebro-spinal axis, contained in the cavity of the brain-case. At an early stage of development it consists of three vesicles, anterior, middle, and posterior. The adult parts derived from these constitute the fore-, mid-, and hind-brains. That part of the fore-brain which corresponds to the original anterior vesicle is known as the thalamencephalon, and contains a vertical slit-like cavity, the third ventricle, the anterior boundry of which is a thin lamella of brain-substance, the lamina terminalis. The thin roof of the third ventricle, is covered by a vascular membrane, the choroid plexus, and in the young tadpole is connected with a small rounded body, the pinecd gland by a hollow pinecd stalk, directed upwards and backwards. The formation of the roof of the skull pinches off the pineal gland, which in the adult underlies the skin covering the top of the head. The floor of the third ventricle is produced downwards in its hinder part into a funnel-like projection, the infundibulum, with which a flattened sac, the pituitary body, is connected. The much-thickened side- walls [optic thalami) are connected together behind by a small band, the posterior commissure. A similar anterior commissure runs transversely across through the substance of the lamina terminalis, and connects the corpora striata [see below). The rest of the fore-brain is made up of the structures deve- loped from a pair of lateral outgrowths of the anterior cerebral vesicle. These are — (1) the cerebral hemispheres continuous with the antero-lateral parts of the thalamencephalon, which structure they partly overlap, and in front of which they extend ; and (2) the olfactory lobes into which the hemispheres pass anteriorly. The cerebral hemispheres are smooth, ovoid bodies, broadest behind, and closely approximated in the middle line. Each contains a lateral ventricle, the inner wall of which presents an elevation, the corpus striatum, and which communicates with the third ventricle behind by a small aperture, the foramen of Monro. The olfactory lobes are two small bodies in front of and con- tinuous with the cerebral hemispheres, from which they are not AMPHIBIA. 211 very clearly marked off. They are closely united in tlie middle line, and each contains an olfactorif ventricle continuous with the lateral ventricle of its side. The mid-hrain is a small axial part lying immediately behind the thalamencephalon, with which it is directly continuous. It contains a narrow canal, the Sfilvian aqueduct^ opening in front into the third ventricle, and with roof and side-walls swollen into two large ovoid optic lobes, each of which contains an optic ventricle opening into the aqueduct, while its thickened floor is formed by the crura cerebri, two masses of longitudinal fibres. The hind-brain is mainly constituted by the bulb (medulla oblongata) which is continuous in front with the mid-brain, and behind with the spinal cord, which it much resembles in structure. The bulb contains a relatively large fourth ventricle, connected by the aqueduct with the third ventricle. Its roof is shaped like a triangle with forwardly-directed base, and is constituted by a vascular membrane. On the under side of the thickened floor is a median groove, the ventral fissure. The cerebellum is a small, solid, transverse ridge, just behind the optic lobes, which arises as a dorsal outgrowth from the posterior vesicle, and together with the bulb makes up the hind-brain. The spinal cord is a thick-walled tube, somewhat flattened from above downwards, which is contained in the spinal canal. It merges into the bulb in front, and from this point gradually tapers backwards (dilating, however, where the limb-nerves come off), and ends in a filament, the filum terminate, which occupies the canal of the urostyle. The small central canal of the spinal cord opens into the fourth ventricle. Dorsally, the cord exhibits a deep cleft, the dorsal fissure, and there is a similar ventral fissure which passes on to the base of the hind-brain. (2) There are twenty pairs of cranio-spinal nerves — ten cranial belonging to the brain, and ten spinal belonging to the spinal cord. Cranial Nerves. — The two first pairs belong to the fore-brain. I. The olfactory arise from the outer and front sides of the olfactory lobes, pass through the foramina in the transverse septum of the sphenethmoid, and supply the olfactory epithelium of the nasal sacs. II. The optic nerves. On the ventral side of the thalam encephalon there is an X -shaped structure, the optic chiasma. The posterior thickened limbs of this, the optic tracts, 212 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. curve backwards and upwards to fuse with the optic lobes. The anterior thinner limbs are the optic nerves. They pass through the optic foramina to reach the eyeballs. The connection with the optic lobes is a secondary one — that is, the optic structures do not originally grow out of them, but fusion occurs later. The very small third and fourth pairs belong to the mid-lmin. III. The oculo-motor nerves arise, near the middle line, from the front of its floor, and pierce the side-walls of the skull by special foramina to supply most of the eye-muscles. The pathetic (trochlear) nerves are given off from the dorsal surface of the brain just behind the optic lobes. They pass through foramina in the cranial wall, and supply the superior oblique muscles of the eye. The remaining six pairs belong to the iLind-hrain, and, with the exception of the sixth, arise from the sides of the bulb. V. The trigeminal are the largest cranial nerves. Each dilates into a Uasserian ganglion, leaves the skull by the trigeminal foramen, and immediately divides into two — (1) The ophthalmic branch, which runs along the inner side of the orbit, and then divides to supply the olfactory capsule and skin of the snout ; and (2) The maxdlo-mandilmlar branch. The latter at once bifurcates into — («) The maxUlary nerve, supplying the margin of the upper jaw ; and {h) The mandihdar nerve, running round the angle of the mouth and along the outside of the mandible, supplying the skin of that region. Branches from this nerve are also distributed to the muscles of the mouth-floor and to certain muscles which elevate the lower jaw. A^I. The very delicate abducent nerves arise close together from the ventral surface of the bulb near its front end. Each comes into close connection with the Gasserian ganglion, separates from it, and leaves the skull by a small aper- ture in front of the trigeminal foramen to supply the external rectus and retractor bulbi muscles of the eye, and also the iris. A^ll. The facial nerve on each side arises just behind the tri- geminal, and passes out of the skull with it, having previously united closely with the Gasserian ganglion. It divides at once into — (1) The palatine nerve, which runs along the floor of the orbit, on its inner side, and supplies the mucous membrane of the mouth-roof ; and (2) The hyomandihular nerve. This runs back round the auditory capsule, crosses the columella, and then descends in the posterior wall of the tympanic cavity to the AMPHIBIA. 213 angle of the mouth, supplying the regions of the tympanic mem- brane and jaw-articulation. It then divides into — (a) The man- (libular nerve, running along the inner side of the mandible, giving branches to its superficial muscles ; and (h) The hyoid nerve, which passes along the anterior hyoid cornu and innervates its muscles. VIII. The auditory nerves arise immediately posterior to the facials, and enter the auditory capsules to supply the essential organs of hearing. IX. The glossopharyngeal nerve on each side takes origin a little way behind the root of VIIL, and in close connection with that of X. After leaving the skull by the vagus-foramen, it is connected by a commissure with the facial, and then runs down to the floor of the mouth, along which it runs with a tortuous course, supplying the mucous membrane of the pharynx and tongue. X. The vagus (pneumo- gastric) nerve of each side arises in close connection with IX., passes through the vagus-foramen and dilates outside the skull into the vagus-ganglion. It takes a downward and backward course, and gives off l>ranches to the larynx, heart, lungs, and stomach. The ten pairs of spinal nerves have this in common, that each arises from the spinal cord by two roots — a dorsal, upon which is a ganglion, and a ventral. These unite together to form the nerve-trunk in the corresponding intervertebral foramen, which serves as a point of exit. The roots of the anterior nerves pass directly outwards, but the posterior ones slope more or less backwards within the spinal canal before reaching their foramina. The last few roots thus form with the filum terminale, a brush- like cauda eguina. The ganglia on the roots are covered ventral ly by calcareous sacs. Each nerve sends oft’, near its commencement, a small dorsal ramus to part of the muscles and skin of the back, while the remaining part, the ventral ramus, is connected by a delicate commissure, ramus communicans, with a sympathetic ganglion. The first or hypoglossal nerves run forwards in the throat- muscles and innervate the muscles of the tongue. The second and third unite to form a brachial nerve, which supplies the fore- limb. Nerves 4, 5, and G supply the body-wall in their region, while nerves 7, 8, and 9 unite to form a network, the sciatic plexus, which gives oft’ branches to the posterior viscera, and a very large sciatic nerve to the hind-limb. The small tenth or 2U AN ELEMENTARY TEXT-BOOK OF BIOLOGY. Fig. 61. — Histology or Nerve (from Landois and Stirling). — 1, Primi- tive fibrilljB. 2, Axis-cylinder. 3, Non-medullated fibres. In their course are seen numerous nerve-corpuscles. 5, 6, Medullated fibres ; c, primitive sheath, internal to which the medulla, h, is seen ; t.f, nodes of Ilanvier— between the two a single nerve-corpuscle ; a, axis- cylinder. 7, Eleven medullated fibres in cross-section, surrounded by connective tissue ; axis-cylinders dark, surrounded by white areas re- presenting medulla. 8, Medullated fibre at node of Ranvier ; medulla not shown. I., Multipolar ganglion-cell from spinal cord; s, process passing into axis-cylinder ; y, branched process — to right of this ganglion-cell is a bipolar one. HI., Bipolar ganglion-cell from sym- pathetic of Frog, surrounded by sheath, m : n, o, the two processes. AMPHIBIA. 215 coccygeal nerve proceeds from the foramen in the urostyle, and supplies the posterior viscera. It is also connected with the sciatic plexus. (3) The sympathetic system is made up of a slender cord on each side, lying just beneath the backbone. It is dilated at intervals into ten sympathetic ganglia, each connected by one or more rami communicantes with the corresponding spinal nerve. In front it passes through the vagus-foramen, and is united with the Gasserian ganglion, while in the region of the dorsal aorta the two cords are closely bound to the dorsal side of that vessel. The internal organs, including the vascular system, are supplied by nerves coming off from the sympathetic ganglia, and breaking up into plexuses. Histology (Fig. 61). — The ganglion cells of the cerebro-spinal axis are mostly multipolar. They make up the greater part of the grey matter^ which forms a central core to the spinal cord, medulla oblongata, optic lobes, and optic thalami, and an external crust to the cerebral hemispheres. The corpora striata are also composed of grey matter. Bipolar ganglion-cells of peculiar type, in which one process is twisted spirally round the other, have been found in the sympathetic ganglia, and unipolar cells in the spinal ganglia. The processes of the ganglion cells either become continuous with the axis cylinders of nerve-fibres or branch, and form networks. The nerve-fibres are either non-medidlated or medidlated. The non-medullated (3) are made up of axis-cylinder and primitive sheath with nerve-corpuscles. They are the only kind found in the sympathetic system and olfactory nerves, besides which they are common in the other parts of the nervous system. The medullated fibres possess a layer of fatty matter, the medullary sheath, between the axis-cylinder and primitive sheath. It is broken up into short segments at the nodes of Schmidt, and at less frequent intervals nodes of Banvier occur where the primitive sheath is constricted and touches the axis-cylinder. Each hder- nodal segment, bounded at each end by one of these last, generally possesses a single nerve-corpuscle. Medullated fibres make u{) most of the white matter of the cerebro-spinal axis, which forms the external part of the spinal cord, medulla oblongata, optic lobes, and . optic thalami, and the internal part of the cerebral hemispheres. The crura cerebri are also of white matter. 216 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. Both sorts of nerve-fibre are connected centrally, directly or indirectly, with ganglion-cells. Peripherally their axis-cylinders may break up into fine plexuses, or those of medullated fibres may terminate in various end-hodies, such as sense-cells, and, in the case of striated muscle, end-hrushes and end-jjlates (Fig. 90). Non-nervous structures are intimately connected with the nervous system, especially connective tissue, which forms the investing membranes, and binds the nerve-fibres into smaller and larger bundles, while the fibres and cells of the central organs are imbedded in an excessively delicate connective-tissue network, the neuroglia. The pineal gland and pituitary body are both non-nervous. The former is a rudimentary eye. The nerve-fibres are physiologically divisible into afferent and efferent, along which impulses respectively pass to and from the central organs. The majority of the former are sensory, since they are connected with end-organs, the stimulation of which lead, in many cases, to a sensation, judging from analogy. A large number of efferent fibres, since they supply muscles, are known as motor. The brain and sjoinal cord are, in the first place, centres for reflex actions — i.e., those which are independent of volition, and dependent on external stimuli. The apparatus involved in such an action normally consists of (a) end-organs, (h) an afferent nerve, (c) a nerve-centre, {d) an efferent nerve terminating commonly in {e) muscular or glandular tissue. The spinal cord alone, after the removal of the brain, enables fairly complicated and purposeful movements of the body to be effected, in answer to external stimuli. Thus, for example, pinching a foot causes the corresponding leg to be drawn up, and the placing on the skin of the back a bit of blotting-paper dipped in acid leads to leg-movements directed to its removal. The cord also serves as a channel by which afferent impulses can travel to the brain, and efferent impulses from it. If the cerebral hemispheres only are removed, still more complicated reflex actions can be evoked by external stimuli, such, for example, as swimming, croaking, and leaping. The medulla oblongata, next to the hemispheres, is the most important part of the brain. It regulates respiration, and has much to do (through the sympathetic) with the alimentary canal and circu- latory organs. It is also concerned with the co-ordination of many muscular movements. Spontaneity entirely resides in the cerebral hemispheres. When these are removed, movements only occur after the appli- AMPHIBIA. 217 cation of stimuli. They are also the seat of consciousness and intelligence. Aflereiit. Efferent. 1 I. Olfactory. Sensory fibres for • « • • • • smell. II. Optic. Sensory fibres for sight. III. IV. VI. Oculomotor. I Pathetic. > Abducent. ) • • • * • • Motor fibres for eye- ball muscles. V. Trigeminal. Sensory fibres to skin Motor fibres to floor of head and nasal of mouth, and cer- sacs. tain muscles rais- ing mandible. VII. Faxial. Sensory fibres to roof Motorfibres to super- of mouth. ficial muscles of mandible, and some hyoid muscles. VIII. Auditory. Sensory fibres for • • • • • • hearing. IX. Glossopharyngeal. Sensory fibres for « • • • • • taste. X. Vagus. Afferent fibres (some Efferent fibres (some sensory). motor). J To parts supplied. 1 The cranio-spinal nerves are made up of afferent and efferent fibres, of which either or both may occur in the same nerve. The preceding table exhibits the leading features of the cranial nerves. All the spinal nerves are of mixed character, the fibres derived from the dorsal roots being sensory, those from the ventral roots motor. The sensory-fibres supply the skin, the motor the voluntary muscles of the body ; in the case of the hypoglossal some of those belonging to the tongue. The preceding facts have been ascertained by cutting the roots and stimulating the ends, it being remembered that fibres only carry impulses one way, and hence no demonstrable results follow on stimulating the peripheral ends of sensory, or the central ends of motor, fibres. Therefore, the fibres of the dorsal root are sensory, those of the ventral, motor. (See next page.) The sympathetic system supplies and largely regulates the internal organs. By its means involuntary muscular contrac- 218 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. Severed. Result. Central end stimulated. Peripheral end stimulated. 1 Dorsal Root. Loss of sensation in area supplied, but muscles can be moved at will, or reflexly. Pain (comes to same thing as over-stimula- tion of parts supplied). No effect. Ventral Root. Sensation remains in parts supplied, but muscles cannot be moved at will, or reflexly. No effect. Muscles sup- plied con- tract. tions, such as those of the alimentary canal, circulatory organs, &c., are mainly effected. It is, however, quite subordinate to the brain and spinal cord. Sense Organs. — (1) Tactile Organs. — The sense of touch is pos- sessed by the skin generally, and the mucous membrane of the olfactory sacs and mouth-cavities. The nerve-fibres connected with this sense are derived from the trigeminal in the head and spinal nerves in the rest of the body. They break up into plexuses, and in the dermis numerous groups of oval flattened touch-corpuscles are present. These are most numerous beneath the wart-like elevations, and nerve-fibres are continuous with them. (2) Gustatory Organs. — Scattered throughout the mouth-cavity, and especially numerous around the vomerine teeth, and on the fungiform papillae of the tongue, are small groups of elongated forked gustatory cells. They belong to the epithelium, and fibres of the glossopharyngeal nerves terminate in them. (3) Olfactory Organs. — Two olfactory sacs (Fig. 51), separated by a median partition, the nasal septum, are lodged in the front of the head. Each contains a complicated internal cavity, com- municating with the exterior and mouth-cavity by the external and internal naris respectively, and with glandular walls largely lined by olfactory epithelium, containing numerous spindle-shaped olfac- tory cells (Fig. 62), in which fibres of the olfactory nerve end. From these cells a stiff process or bundle of olfactory hairs projects into the nasal cavity. (4) Auditory Organs. — The ear on each side is made up of two parts — (1) Middle ear, and (2) Internal ear. AMPHIBIA. 219 The middle ear consists of a small tympanic cavity, communicating by the short Eustachian tube with the back of the mouth-cavity, and lined by a continuation of its epithelium. It is closed externally by the rounded tympanic membrane, which is stretched over a cartila- ginous ring connected with the squamosal and covered externally by skin. The rod-like columella (Fig. 50, B) is attached at one end to the tympanic membrane, and stretches across the cavity to the fenestra ovalis, into which its other end is inserted. , The internal ear is made up of the membranous labyrinth (Fig. 62), contained in the cavity of the auditory capsule which, however, it does not fill. Numerous strands of connective tissue unite it with the walls of this cavity, which is filled by a clear fluid, the perilymph. The labyrinth is of complicated shape, and consists of vestibule and semicircular canals: — (1) The vestibule is divided into (a) the utri~ cuius, an irregular bag, the cavity of which is imper- fectly bisected by a fold and communicates by a small aperture with that of ((3) the sacculus, an under- lying oval bladder. Three small swellings on the pos- terior side of the sacculus receive collectively the name of cochlea. (2) The semicircular canals are three curved tubes — anterior vertical, posterior vertical, and external horizontal, connected with the utri- culus. They lie in planes mutually at right angles. The first and last dilate into swellings {ampullce) in front, where they join the utriculus. The posterior vertical unites in front with the anterior vertical to form a common tube opening into the utriculus, and dilates behind into an ampulla. 1 I Fig. C2.— Sense Organs of Frog (after Mcher and Wiedersheim), various scales. A, Cells from olfactory epithelium ; ol.c olfactory-cell. B, Right Membranous Labyrinth, viewed from outside. The arrow points to the front. The branches of the auditory nerve are shown (partly dotted), ut^ Utriculus; -sc, sacculus; a.i', p.i\ and h, anterior vertical, pos- terior vertical, and horizontal semi- circular canals; ap, ampullce; co, coch- lea. B', 1, auditory cells, w’ith auditory hairs, a,h\ 2, supporting-cells. 220 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. The labyrinth is filled with clear endolympli, in which are sus- pended, especially near the sensory patches, a large number of <1, external chamber, with aqueous humour; e, iris; f,f, pupil; A, lens; /*, junction of cornea and sclerotic; m, choroid; n, retina; o, vitreous humour; No, optic nerve; q, nerve sheath; p, nerve fibres, piercing outer coats and becomins; continuous with retina. calcareous otoliths. It is lined throughout by simple epithelium, mostly squamous, but in eight sensory patches, largely made up of AMPHIBIA. 221 elongated auditory-cells, closely con- nected with fibres of the auditory nerve. From each a tapering auditory '‘hair” projects into the labyrinth- cavity. The sensory patches are disposed as follows : — One in each ampulla on a projecting fold, crista^ acustica, the remaining five, macw/ff’, in utriculus, saccuhis (2), and three cochlear projections. (5) Visual Organs (Fig. 63). — The eye is relatively large, and projects not only externally but also internally into the mouth-cavity. It is lodged in the orbit, on the side of the skull. The eyeball is rounded internally but flattened externally, and its wall is made up of three concentric coatings. The outermost of these, the firm, white sclerotic, strengthened by cartilage, exhibits externally a circular trans- parent area, the cornea. The second much thinner coating, the choroid, is pigmented externally and very vas- cular internally. This layer does not line the cornea, but, at its margin, becomes continuous Avith a vertical partition, the iris, which presents a central perforation, the pupil. By the iris the eye is divided into a small external and a large internal chamber. The latter is lined by the retina, Avhich is the third and most delicate coating. In it eight distinct layers (Fig. 64) can be distinguished. The optic nerve pierces the scler- otic and choroid coats on the inner side of the eyeball to reach the retina, in which it breaks up into a feltwork of fibres. These form a Fig. 64. — D I A (t! ram o k Layers of Retina (from Landois and Stirling). — Pi, Pigment-cells ; St, rods and cones continuous by various intermediate struc- tures with Ggl, ganglion- cells connected with o, fibres of optic nerve; Li, internal limiting mem- brane which comes just outside vitreous humour (not shown). AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 00 0 .taJ W ^ layer on the internal side of the retina {i.e., the side next the eye- chamber), and are connected through various intermediate layers with elongated sense-cells, the rods and cones^ which occupy the outside of the retina. Refracting structures are contained within the chambers of the eye. A clear, watery aqueous humour fills the external chamber, while a gelatinous vitreous humour is contained in the internal chamber. Immediately behind the iris and connected with its outer margin is a firm spheroidal body, the lens. A number of accessory parts are connnected with the eye, of which the two eyelids have already been mentioned. Continuous Avith the lining of these is a delicate transparent membrane, the conjunctiva, which closely covers the cornea. A small Harderian gland, which secretes a fatty substance, is situated in front of and below the eye. The eye is moved by a number of small muscles, the most im- j)ortant of which take origin in the walls of the orbit, and are inserted into the eyeball. They are — (1) The retractor hulhi, a muscular sheath surrounding the optic nerve ; (2) the four recti muscles, superior, inferior, internal, and external rectus, inserted into the upper, lower, anterior, and posterior sides of the eyeball. All the preceding take origin from the inner wall of the orbit, near its posterior end ; (3) the two oblique muscles, superior and in- ferior, are inserted into the upper and lower sides of the eyeball, taking origin close together in the anterior part of the inner wall of the orbit. DEVELOPMENT. The frog is a good example of an animal in Avhich there is a free-living embryo or larva, very unlike the adult stage, into which it is converted by a gradual metamorphosis. Frog larvae are familiar to every one as fish-like tadpoles, which hatch out during March and April from the frog ‘‘ spawn,” Avhich consists of fertilized ova surrounded by gelatinous matter. 1. Early Stages. — Cleavage (segmentation) (Fig. G5) is com- plete but unequal. The ovum is pigmented on one side which is termed the upper pole, the unpigmented end being the lower pole. Division may therefore be spoken of as “ meridional ” or “ equa- torial.” The two first divisions are meridional, dividing the oosperm into two and four. Then division into eight is efiected AMrillBIA. 223 Fig. 65. — Cleavage of Oosperm of Frog (from Jf addon, aitev Ecker).— The numbers indicate the number of segments. ])otted lines show position of succeeding planes of segmentation. Fig. 66. — Blastula and Gastrula of Frog (after GOtU). All are sections. — A, Partly segmented oosperm. B, Completely segmented oosperm. C, D, E, Stages in formation of gastrula ; al, archenteron (mesenteron) ; hi, segmentation cavity ; hip, blastoywre ; ep and ep', nervous and superficial layers of ectoderm ; hy, endoderm ; m and m!, mesoderm ; y.hy, lower layer cells. 224 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. by ail equatorial furrow nearer the upper than the lower pole. Divisions of both kinds now proceed, the final result being a hlasfula (blastosphere) (Fig. 66, B), with a small Uastoccde (seg- mentation cavity) near the upper pole. This is roofed by small cells, while its floor and most of the blastula are made up of large yolk-cells, containing food material. Invagination (C, D, E) is partly effected by an inpushing of tlie small cells at one point, and partly by their growing over the yolk-cells elsewhere, leaving, however, for a long time an un- covered circular area, the blastopore. Meanwhile the blastocoele is gradually pushed aside by the archenferon, which occupies the upper pole of the ovum, and is formed partly by the inpushing of the small cells, as above stated, and partly by an absorption of the yolk-cells to form a cavity, and their differentiation to give it definite walls. The small cells, now almost covering the embryo, may at this juncture be termed ectoderm (epiblast), the contained yolk-cells endoderm (hypoblast). The mesoderm (mesoblast) is formed as a layer several cells thick, commencing at the edges of the blastopore and gradually extending over the rest of the ovum below the ectoderm. It is apparently derived from the endoderm. The notochord is formed at the same time, as a median dorsal thickening of the roof of the archenteron. 2. General Growth. — The ectoderm on the upper side of the embryo thickens into a neural (medullary) plate, broad in front, and narrowing behind to the blastopore. The axis of this plate corresponds to the longitudinal axis of the future tadpole, the blastopore being posterior. A delicate neural (medullary) groove appears on the medullary plate. Its side-walls, the neural (med- ullary) folds, grow up and unite together to form a neural tube, the rudiment of the brain and spinal cord. The dorsal side of the embryo is thus indicated, and its head and tail soon become evident, the former being marked by the dilatation of the neural tube into the three brain vesicles in front. The ventral surface bulges out, owing to the food material contained in the endoderm, but it gradually becomes less and less conspicuous. The blastopore closes up, and the cavities of neural tube and gut are then con- nected by a short neurenteric canal. As growth proceeds, the embryo acquires a strong curve, the dorsal side being concave. The sense organs become established, as also do an oval suctorial mouth with horny beaks, and a cloacal aperture. On each side AMPHIBIA. 9 9f) of the throat six thickened bars, the visceral arches, passing from above downward, are developed. These are termed mandibular, hyoid, 1st, 2nd, 3rd, and 4th branchial arches respectively. From branchial arches 1 and 2 a branched tuft-like external gill grows out on either side, and after hatching, which occurs about this period, another proceeds from the third branchial arch. The tail now assumes great size and importance as a locomotor organ. In front of each branchial arch a visceral cleft, leading into the throat, makes its appearance, and a fold of skin, the operculum, grows back from the hyoid arches over the external gills. By Fig. 67. — Diagrammatic Longitudinal Section through a Frog Em- bryo (after Gotte). — Ectoderm unshaded, endoderm shaded, mesoderm (m) darkly shaded. nc. Neural canal contained -within neural tube, which on the left is dilated into the rudimentary brain already exhibiting flexure; al, alimentary canal, with mass of yolk {yk) be- neath; X, placed at blastopore, the reference line runs to posterior end of neural canal, which is here continuous with alimentary. the union of this fold with the body-wall a branchial chamber is formed on each side. The right one soon closes, but remains connected by a cross-passage under the throat with the left, which opens to the exterior by a small rounded opening. The external gills now shrink and disappear, being succeeded by inter- nal gills, which are vascular folds on the walls of the clefts. As the lungs develop, these too disappear, and the visceral clefts close up. The limbs have been arising meanwhile as bud-like out- growths, the anterior pair being hidden at first by the operculum. They appear at the surface after a shedding of skin which follows 2 15 226 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. the atrophy of the gills. At the same time the eyes become evident, and the horny jaws are cast off. The adult form is now rapidly assumed, the tail gradually shrinking, while vegetable food is given up, and exclusively aquatic life abandoned. 3. Fate of the Germinal Layers. — (1) The ectoderm is early differentiated into superficial and nervous layers (Fig. 66). The former gives rise to the horny layer of the epidermis, the deeper part of which is formed from the nervous layer. The epithelium of the mouth-cavity (stomodseum) and posterior part of the cloaca (proctodseum) are also ectodermic, these structures arising as pits of the external surface which ultimately communicate with the mesenteron to form a continuous alimentary canal. The neural plate from which the cerebro-spinal axis originates arises as a thickening of the nervous part of the ectoderm. The essential parts of the sense organs are ectodermic. The nasal sacs commence as pits in this layer, while the membranous laby- rinths are originally depressions of the nervous part of it, which are separated off as vesicles and grow into their complicated adult form. The lens of the eye (c/. Fig. 80, 1) is formed as a similar vesicle, which becomes solid. An outgrowth from the anterior cerebral vesicle, the optic vesicle, grows towards the embryonic lens, which meets and pushes in its end, forming a double-walled optic cup. The part connecting this with the brain narrows to constitute the optic nerve, while the inner wall of the cup becomes the essential part of the retina, the outer part its pigment layer. The rest of the eye is mesodermic. (2) The endoderm gives rise to the epithelium of the mesen- teron and its outgrowths (Fig. 67, at) — ie., lungs, liver, and pancreas. The visceral clefts are also formed as pouches of the mesenteron. Four of them open as above described to the exterior. In front of these is a pouch which never opens on the surface and becomes the tympanic cavity. From the endoderm the first rudiment of the endoskeleton is formed. This is a cellular rod, the notochord, which underlies the nervous system as far forwards as the pituitary body. Remains of it are found in the centra of the adult vertebrse. (3) The mesoderm at first forms a sheet on each side of the notochord below tlie ectoderm, and also extends into the front of the head. Each sheet splits into an outer somatic layer, and an inner splanchnic layer, the two remaining united dorsally. The AMPHIBIA. 227 layers of the two sides become continuous ventrally, the split between them forming the body-cavity. The mesoderm on each side becomes divided, posterior to the head, into two longitudinal parts. The upper of these, the vertebral plate, lies alongside the neural tube and notochord, and becomes divided transversely into mesodermic somites (protovertebrae). The lower part is known as the lateral plate, the somatic mesoderm of which unites with the ectoderm to form the somatopleure (wall of body), while its splanchnic layer unites with the endoderm to form the splanch- nopleure (wall of alimentary canal). Both mesodermic somites and lateral plate are made up of somatic mesoderm externally, and splanchnic mesoderm internally. The dermis is developed from somatic mesoderm, mostly from that of the lateral plates. The axial endoskeleton is formed from splanchnic mesoderm, that of the mesodermic somites giving rise to the vertebral column, which, with the hinder part of the base of the brain-case, is moulded round the notochord. The visceral arches become supported by rods of cartilage, similarly named. Their fate is as follows : — I. Mandibular — Suspensoria and cartilaginous basis of the jaws. II. Hyoid, \ /Columellse and anterior I I cornua. III. Branchial 1, (Fuse ventrally to| (Anterior angles of IV. Branchial 2, form hyoid ap-j ( hyoid plate. Y. Branchial 3, paratus. I Posterior angles. VI. Branchial 4, j \ Posterior cornua. The rest of the skeleton is also mesodermic. The alimentary canal and its outgrowths are formed (except the epithelium) from the splanchnic layer of the lateral plate. Important changes take place in the circulatory oi'gans in con- nection with the successive appearance of gills and lungs. The heart at first possesses an undivided auricle, and pumps impure blood to the gills by afferent branchial arteries corresponding to the last four visceral arches (branchial arches), and breaking up into capillaries. After the blood has been oxygenated in these gill-capillaries it passes into efferent branchial arteries, which open above into a loop from which the dorsal aorta arises. These vessels may be numbered 3, 4, 5, 6, like the visceral arches with 228 AN ELEMENTAIIY TEXT-BOOK OF BIOLOGY. which they are connected. As the lungs develop they are supplied by branches of the 6 th afferent arteries, and, gradually becoming functional, pour their blood into the auricle, which becomes divided. At the same time the afferent and efferent vessels become directly united to form continuous arches. The internal gills meanwhile atrophy, and the 4th arches alone remain connected to form the dorsal aorta. These changes are best exhibited tabularly. 1 1st stage. 2nd Stage. 3rd (Adult) Stage. arches, •ith arches, 7Uh arches, Qth arches. > / Supply ex- t ternal gills. { Supply in- ( ternal gills. Become carotid arches. Become systemic arches. Disappear. Become pulmo-cut. arches. The adult kidney is preceded by a rudimentary excretory organ, the pronephros (head-kidney), consisting of a glandular con- voluted tube, with three openings into the body-cavity. A longitudinal pronepliric duct (= archinephric or segmental duct) runs back from this and opens into the cloaca. The adult kidney, more properly termed the Wolffian body or mesonephros, is develo]^ed as a number of tubules, at first comparatively simple,, which become connected with the body-cavity on the one hand, and the pronephric duct on the other. This duct becomes the urinogenital duct of the male and the ureter of the female. The ovary and spermary are developed as thickenings of the peritoneum lining the body-cavity. The oviduct (Mullerian duct) commences on the dorsal wall of the body-cavity as a groove which is subsequently converted into a tube. The voluntary muscles are formed from the somatic layers of the mesodermic somites. AVKS. 229 CHAPTER XL— AVES. § 16. COLTJMBA LIVIA (The Rock-Pigeon). The common Rock-Pigeon is here selected for description, but it must be remembered that the very numerous fancy breeds have all been derived from this form. In some few places the structure of the Fowl (Callus bankiva) is alluded to, where it differs markedly from the Pigeon. MORPHOLOGY AND PHYSIOLOGY. 1. External Characters. — The bilaterally symmetrical body, in addition to head and trunk, possesses a long flexible neck, and a short stumpy posterior process, the tail (uropygium). Fore- and hind-limbs are present, the former modified into wings, the latter set on to the body very far forwards, thus securing equilibrium, and facilitating bipedal progression. The body is largely, but not entirely, covered with feathers, arranged in definite tracts (pterylce), there being intervening bare spaces (apteria). There are several kinds of feathers, the largest, quill-feathers, present on the wings and tail, are those principally used in flight. The wing-quills (remiges) are 19 in number on each side. They are overlapped, above and below, by smaller wing-coverts. The tail- quills {rectrices), 12 in number, are attached to the uropygium, and are overlapped on both sides by tail-coverts. The small soft feathers, which invest all parts of the body, and confer the characteristic outlines, are contour-feathers. In addition to these a very great number of minute rudimentary feathers are present, the filoplumes. The head is well rounded behind, while in front it tapers into the beak, the upper and lower parts of which are encased in horn, and bound the large mouth. A bare elevated area, the cere, is present at the base of the upper division of the beak, and in front of this two slit-like nostrils {external nares). The large eyes possess upper and lower featherless eyelids, and each is provided as well with a thin translucent third eyelid [nictitating membrane) which can be quickly drawn over the eye, or folded 230 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. up in its anterior angle. A little way behind and below the eye is a rounded auditory opening concealed by feathers. It opens into a short tube, the external auditory meatus. The trunk is somewhat boat-shaped, presenting a prominent ridge along the middle of its ventral side, except for a small posterior area where soft abdominal walls can be left. A large transverse cloacal aperture with swollen edges, opens ventrally at the base of the tail. Upon the dorsal surface of the latter structure there is a small papilla, upon which the oil-gland opens (Fig. 70, u.gl). The fore-limb (c/. Fig. 69) is divided into upper arm, fore- arm, and hand (hrachium, antebrachium, and manus), which are of about equal length, and folded closely one upon another when not extended for flight, but, when so extended, approximating to the primitive position. The manus is tridactyle, but the small thumb [pollex) is the only digit well-marked externally. It bears a small tuft of feathers, the bastard iving. Eleven of the remiges are attached to the hand. These are the primary quills. The remaining eight remiges, secondary quills, are attached to the fore- arm. The hind-limb [cf. Fig. 69) is divided into thigh, leg, and foot [femur, crus, and|;es). The last is made up of a cylindrical tarsomdatarsus, about as long as the thigh (the leg is much longer), and four digits, of which the first, or great toe (hallux), is directed backwards, the others forwards. The foot is covered by overlapping scales, and the digits are terminated by claws. Position of Body. — The directions assumed by the divisions of the fore-limb have already been mentioned. In the standing posture the thigh, leg, and tibio-tarsus are arranged in a longi- tudinal vertical plane with the knee directed forwards and the mesotarsal ankle-joint backwards. The great toe is also turned backwards, and owing to its free metatarsal is more mobile than the other forwardly directed digits. 2. The extremely thin skin is divided into epidermis and dermis, the former possessing horny and deeper layers. The scales and claws are made up of coalesced epidermic cells, and the feathers are also modifications of this layer. A feather consists of a central stem, the proximal part of which is a hollow quill, and the distal part a solid shaft, forming the central part of the expanded vane, the rest of which is made up of a row of AVES. 231 narrow harhs on each side, flattened at right angles to the axis of the feather. Each barb possesses a proximal and a distal row of minute harhules. These overlap and hold the barbs together, and, for this purpose, the distal barbules are provided with diminutive booklets, A minute aperture, the inferior umbilicus ^ leads into the proximal -end of the quill, and a similar superioi' umbilicus into its distal end on the ventral side of the feather. Ventral side = Z>eZoM? in quill-feathers, next body in the others. The shaft is longitudinally grooved on this side. In the fowl, but not in the pigeon, a small after-shaft which resembles the vane in structure is attached near the superior umbilicus. The filoplumes possess a minute thread-like stem, but the barbs and barbules are only represented by a tuft of disconnected processes. The oil-gland belongs to the skin. It is made up of numerous branched tubules, epidermic in origin, and lined by glandular-cells. The most important histological point is the way of develop- ment of the feathers. A small papilla is formed on the surface, the base of which sinks down into the dermis and thus becomes enclosed in a pit, the feather-follicle^ from which its pointed end grows out. The papilla is known as the feather-germ. It con- tains a vascular core of dermis, and the epidermis covering its surface is gradually moulded into the feather. The quill is formed around the base of the germ, the vane around the free part, from which it splits off and expands. The inferior umbilicus is the point where the vascular core entered the feather, the superior umbilicus shows where it left the unsplit quill-part to form the centre round which the vane was once folded. This description refers to the first-formed feathers. The papillae of new feathers are formed in connection with the old follicles, and never project on the free surface. The feathers are renewed periodically during life, the old ones being cast off at moulting. The dermis contains a network of muscle-fil)res attached to the featlier-follicles. By the contraction of this the feathers can be erected. Blood-vessels, lymphatics, and nerves are present, and a large number of ovoid touch-corpuscles. 3. The endoskeleton (Figs. G8 and G9). — This, as in the frog, AN elemj:ntary text-book of biology. ^ U is mainly made up of cartilage, cartilage-bones,*^ and membrane- bones, f There are also a few sesamoid bones, e.g.^ patella, developed in the course of tendons. The bones are distinguished by their spongy character, and the shafts of the long bones contain, in the adult, air instead of marrow. The skeletons of the trunk and limbs may be distin- guished as axial and appendicular. (1) The axial endoskeleton consists of skull, vertebral column, ribs, and sternum. — (a) The mature skull contains compara- tively little cartilage, and a chondro-cranium is only to be made out in the embryo. The bones in the adult are fused together so as to make the determination of boundaries a difficult matter. A rounded cranial iwrtion behind may be distinguished from a tapering facial portion in front, the two being connected together by an imperfect joint which permits a small amount of up and down movement. A large rounded orbit is present on each side near the junction of the two. The cranial part is mostly made up of the rounded brain-case or cranium. This exhibits a large foramen magnum behind, bounded by the occipital region, to form which four bones are fused together, — two ex-occijntals at the sides, a supra-occipital above, and a hasi-occqntal below. The last bears a median rounded occipital condyle, for articulation with the vertebral column. The roof of the brain-case is completed by two pairs of bones, the parietals behind and the frontals anteriorly. The side-wall of the brain-case is partly formed by the squamosal, below which is a depression, the tympanic cavity, bounded internally by the auditory capsule. This is formed by the union of three elements, epi-otic, and opisth-otic, of which the lirst is most important. They are placed in front, above, and behind respectively. About the centre of the capsular wall are two small openings, one above the other, separated by a very narrow interspace. * Cartilage-hones : — Basi-, ex-, and supra-occipitals, pro-, epi-, and opisth-otics, hasi-, pre-, all-, and orhito-sphenoids, mesethmoid, quadrate, orticxdar, columella, hasi-hyal, basi -branchial, posterior cornua of hyoid. Ve.rtebrce. Sternum. Bibs. Appendicular skeleton (except furcula). f Membrane-bones : — Parietal, frontal, squamosal, parasphenoid, lachry- mal, basi-temporal, vomer, nasal, premaxilla, maxilla, jugal, quadrato-jugal, pterygoid, palatine, angular, supra-angular, dentary, splenial. Furcula. AVES. 233 The upper is the fenestra ovalis, the lower the fenestra rotunda. Into the front of the tympanic cavity the Eustachian tube opens. Further forwards the brain-case is bounded laterally by an orhital plate, which also forms the hinder and upper parts of the wall of the orbit, and is made up below by the ali-sphenoid, and above by the orhital process of the frontal. The floor of the brain- case in front of the basi-occipital is formed by the hasi-sphenoid. From this a pointed rod, the para-sphenoid {basi-sphenoidal rostrum), projects forwards. It is fused with the lower edge of a thin bony plate, the interorhital septum, continuous behind with the ali-sphenoids. The posterior part of the septum represents three distinct bones, a median pre-sphenoid, and two lateral orhito- sphenoids. The upper and anterior parts of the septum are, in the young bird a distinct bone, the mesethmoid. The anterior margin of the orbit is formed at this point by the lachrymal. The basi-sphenoid is overlapped by a thin plate of bone, the hasi-temporal, which forms the lower boundary of the tympanic cavity on either side. It tapers somewhat in front, and between it and the basi-sphenoidal rostrum is a median Eustachian open- ing, from which an Eustachian tube leads back on either side to the corresponding tympanic cavity. The basi-temporal is probably equivalent to the posterior part of the frog’s para-sphenoid. The cavity of the brain-case closely corresponds to the shape of the brain. Its floor is very steep, rapidly ascending towards the front. The foramen magnum looks downwards in accordance with the upright position of the neck. Nerve Exits. — The olfactory foramen (I.) is in the front of the brain-case. A vacuity in the dry skull at the upper margin of the inter-orbital septum marks the course of the olfactory nerves to the nasal capsules. Below the olfactory foramen is an 0/ die foramen (II.), and at this point, in the dried skull, the two orbits are placed in communication by a hole. The oculomotor and pathetic nerves have exit by special small foramina near the oj)tic, the fifth and sixth by a larger aperture behind the ali-sphenoid. The eighth nerve pierces the inner side of the auditory capsule, and, just in front of it, there is a small foramen for the seventh. Behind the tympanic cavity there is a small opening on each side at the posterior angle of the basi-temporal for the ninth, 234 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. tenth, and eleventh (IX. and X.), while the twelfth passes out by a small condylar foramen situated in the ex-occipital near tlie condyle (XII.) The facial part of the skull consists of the nasal capsules and jaws. The nasal capsules contain a good deal of cartilage, and are separated by a cartilaginous nasal septum, which is a con- tinuation of the mesethmoid. A scroll-like fold, the turbmal, projects from it on either side. Below this is a slender vomer, just in front of the basi-sphenoidal rostrum. The capsules are partly roofed in by the nasals, which unite with the frontals behind, and are deeply notched in front. The notches partly bound the external nares. The margin of the upper jaw is formed in front by the jjrg- maxillce, each of which unites behind with a delicate rod-like maxilla, from the anterior end of which a flat maxillo-palatine process projects towards the middle line. The maxilla fuses behind with a styliform jugal. These two last bones form the greater part of a sub-orbital bar, which is completed posteriorly by a quadrato-jugal , the rounded posterior end of which is united by a fibrous band with the quadrate, a short thickened bone, Fig. 68. — Endoskeletox of Pigeon (Cartilage dotted).— A and A', Skull, side and under views ; s-oc, supra-occipital ; ex-oc, ex-occipital ; b-oc, basi-occipital ; c, condyle; pa, parietal ; sq, squamosal ; as, ali-sphenoid ; fr, frontal; fr', orbital process of ditto; i-o.s, inter-orbital septum; la, lachrymal; b-t, basi-temporal ; ps, para-sphenoid; v, vomer; na, nasal; p-mx, pre-maxilla; mx, maxilla; mx-p, maxillo-palatine process; j, jugal; q-j, quadrato-jugal; pi, palatine; pt, pterygoid; (/d, quadrate ; d, dentary ; ar, articular ; an, angular ; s-an, supra-angular ; ty, tympanic cavity (reference line runs to fenestra ovalis); Eu, Eustachian opening; f-m, foramen magnum. I., II., &c. , Nerve exits. (V., just above for- amen for fifth.) B, Left columella, enlarged; pi, plate fitting into fenestra ovalis. C, Hyoid apparatus, from above; g-hy, glosso-hyal; b-ky, basi hyal; b-br, basi-branchial ; a.c, anterior cornua; q).c, posterior cornua. D and D', ^'•Sacrum''' wad pelvis, under and side views, num- bers refer to vertebrae ; t, thoracic ; I, lumbar ; S, sacral ; c, caudal ; c', free caudal; psh, plough-share bone; r, rib; il, ilium; pb, pubis; is, ischium ; ac, acetabulum ; ant, anti-trochanter ; ob, obturator fissure; isf, ilio-sciatic foramen. E, 1-6, Vertebrje. 1, Atlas: 2, axis: 3, two free cervical, from above : 4, ditto, from side : o, ditto, from front: 6, first thoracic ; c, centrum; n.sp, neural spine; Lp, transverse process ; hy, hypapophysis ; a-z, pre-zygapophyses ; p-z, post-zyga- pophyses ; o.p, odontoid peg; i.l, cervical rib; n.c, neural canal; v.c, vertebrarterial canal ; r, vertebral rib ; c.j), head ; th, tubercle ; un, uncinate process : 7, sclerotic ring. A tiS(j AN ELEMENTARY TEXT-BOOK OF BIOLOGY. raovably articulated at one end by two convex surfaces with the squamosal and pro-otic, while the other presents a transverse articular condyle for the mandible. Upon the ventral surface of the skull an X-shaped structure is formed by four bones, pterygoids behind and palatines in front. The 'pterygoid arti- culates behind with a facet on the quadrate, in front with a facet on the side of the parasphenoid, at its base. AVhen the facial part of the skull is moved up and down, the pterygoids readily slide forwards and backwards on these smooth facets. The palatine is a flattened rod, broadest behind, where it meets its fellow and the pterygoid, and tapering to a point in front, where it fuses with the maxillo-palatine process. The lower jaw or mandible consists of two almost straight halves (rami), each of which articulates behind with the corre- sponding quadrate, and unites with its fellow in front to form a median symphysis. The axis of the ramus is formed by Meckel’s cartilage, which persists, more or less, throughout life, and is ossified proximally into the articular^ which is jointed on to the quadrate. Angular and supra-angular elements ensheath the ramus below and above in its proximal half, the latter being produced above into a small cm'onoid process, while the distal half is covered ventrally and externally by the dentary, internally by the splenial. Hyoid and First Branchial Arches. — The proximal end of the hyoid arch is formed by the columella, which helps to transmit sound-waves to the internal organs of hearing. Its inner end is bony and expanded into an oval plate fitting into the fenestra ovalis, Avhile its outer cartilaginous end is firmly fixed to the inner side of the tympanic membrane. The “ hyoid apparatus ” consists of the body of the hyoid, supporting the base of the tongue, and anterior and posterior cornua. The body is composed of three median pieces : — {a) A cartilaginous glosso-hyal, shaped like an arrowhead, from which the short anterior cornua also of cartilage, diverge backwards; (&) a short bony hasi-hyal ; and (c) a styliform bony hasi-hranchial. Between (b) and (c) the slender three-jointed posterior cornua diverge backwards and upwards. 'the basi-branchial and posterior cornua represent the Jirst branchial arches. AVES. 4) O - (h) Vertebral Column and Ribs. — The vertebral column is divided into regions, arranged as follows : — Xo. of Vertehrce. 1. Cervical, 1. Thoracic— 13 or 14 Free. (a) Anterior, 4 Fused. (b) Posterior, 1 3. Lumbar, 2 or 3 f Fused to form 4. Sacral, . . 5. Caudal — 4 or 3 f ‘‘ Saa'um.’’’ (a) Anterior, 6 \b) Middle, . 6 Free. (c) Posterior, Several / Fused to form plough- t share bone. The cervical vertebrae are those supporting the neck. Each is made up of two parts : — {a) An elongated centrum,, articulating with neighbouring centra by saddle-shaped surfaces conferring great flexibility to this region, and with a median ventral process ijnjpapophysis). {h) A neural arch, produced dorsally into a plate- like neural spine, and connected with adjacent arches hj pre- and post-zygapop)hyses in the same way as in the frog. Intervertebral foramina are left between the arches for the transmission of the spinal nerves. The last cervical vertebra possesses a strong transverse process running out from each side of the neural arch, and bears a pair of cervical ribs, each of which is a small curved bone, with a forked proximal end attached to the vertebra, and a free distal end. From its hinder edge a small flat uncinate gorocess projects. The two limbs of the forked end terminate in articular projections termed head (capitulum) and tubercle. The former articulates with a small capitular facet on each side of the centrum near its front end, the latter with a similar tubercidar facet on the under side of the transverse process. The last vertebra but one also bears free ribs, these, however, are devoid of tubercles and uncinate processes. In the remaining vertebrae which possess them, the cervical ribs are very short, and completely fused with the transverse process and centrum, an aperture being left, however, which, with those 238 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. of the other vertebrae, forms the vertehr arterial canal in which certain soft structures run. The first and second vertebrae, atlas and axis, are very small and possess neither ribs nor transverse processes. The atlas (1) is a ring, without anterior zygapophyses. Over its centrum’s upper surface a small peg, the odontoid process, projects from the front end of the centrum of the axis (2). The occipital condyle fits into an articular pit formed above by the odontoid process, and below by the atlas-centrum. The odontoid process is at first a distinct bone. It is part of the atlas- centrum which fuses with that of the axis. The thoracic vertebrce bear free thoracic ribs, and the commence- ment of the series is marked by the first vertebra bearing ribs which unite ventrally with the sternum. The four anterior vertebrae are closly united with one another by their centra and the processes of their arches. The thoracic ribs are made up of two segments, one, the vertebral rib, similar in character and mode of attachment to the last cervical rib, the other, sternal rib, arti- culated distally, with a facet on the sternum. The “ sacrum ” of the adult bird is made up of vertebrae belonging to four regions closely fused together. The single thoracic vertebra is easily re- cognised by its rib. It possesses a large bilaminar hypapophysis, and its stout transverse processes abut against the ilia. The lumbar vertebrae have short and strong, the sacral, lamellar trans- verse processes, while the first caudal possesses a ventral caudal rib as well as a dorsal transverse process. T\\q free caudal vertebrae are small, and their transverse pro- cesses flattened. The ploughshare bone is a laterally flattened plate, bent up sharply on the rest of the vertebral column. (c) The sternum (Fig. 69) is a very broad, plate-like bone, concave above, supporting the ventral wall of the thorax and part of the abdomen. It is produced in front into an obtuse process, the manubrium, and behind into a narrow jdate, the middle xiphoid process. At the sides are paired internal and external xiphoid processes. On each side of the front of the ster- num there is an excavation for the end of the coracoid, and behind this comes the costal process, bearing four facets for the articulations of the sternal ribs. The ventral side of the sternum AYES. 239 is produced downwards into a prominent vertical plate, the keel, wliich is deepest in front. (2) Appendicular Endoskeleton. — The long bones possess epi- ptliijses, as in the frog. Girdle and free limb can be distinguished in both fore- and hind-limbs. {a) Fore-Limb — Shoidder-girdle (Fig. 69). — The scapula is a backwardly-directed, blade-like bone, united closely in front with a rod-like coracoid, the other end of which articulates with the sternum. These two bones make with one another an acute angle, open behind, and the glenoid cavity is situated at their point of union. In front of this the slender clavicle is attached, which curves backwards and downwards, and unites with its fellow in the middle line to form the furcula (“ merry-thought ”). The union is effected (?) through the intermediation of a small inter- clavicle, which forms in the Fowl a small, laterally flattened disc. Free Limh (Fig. 69). — The humerus possesses a proximal head for articulation with the glenoid, and a distal, pulley-like trochlea, which presents preaxial and postaxial articular surfaces for the radius and ulna. Two distinct bones, radius and ulna, support the antebrachium. The preaxial radius is straight and slender, the postaxial ulna relatively stout. The proximal end is pro- duced into an olecranon process. The carpus is made up of two small bones, the radiate and idnare, which articulate with the radius and ulna respectively. First, second, and third metacarpals are present, closely united proximallj^ The first is small, and projects but little beyond the fused part. The second is long and stout, and the third long and slender. The three digits are completed by phalanges, the first and third possessing a single small one, while two large phalanges terminate the second digit. (b) Hind-Limb — Hip Girdle (Fig. 68). — Each half, or os innominatum, is separate from the other, and made up of three elements, ilium, ischium, and pubis, all three of which contribute to the formation of the acetabulum, a rounded cup, not floored by bone, on the outer side of the girdle. The large ilium extends forwards and backwards above the acetabulum. It meets and fuses with the anterior ends of the ischium and pubis to bound the acetabular cavity, above which it presents a smooth surface, the anti-trochanter. The sacrum ” is closely connected with the 240 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. inner concave surface of the ilium. The ischium is a flat rod running back from the acetabulum parallel to the lower edge of Fig. 69. — Endoskeleton of Pioeon. — A, Sternum and shoulder-girdles; c.st, corpus sterni; h, keel; x.a, x.m, x.p, external, internal, and middle xiphoid processes ; r, sternal ribs ; co, coracoids ; sc, scapulae ; J, furcula ; gl, glenoid cavity. B, Left Wing — h, humerus ; hd, head of ditto ; r. t and u. t, radial and ulnar tuberosities ; r, radius ; ul, ulna, showing scars left by quills; ra, radiale; u, ulnare ; I. -III., meta- carpals; 1-3, phalanges. C, Left Leg (patella gone, fibula not seen) — f, femur; hd, head; g.t, great trochanter; t-t, tibio-tarsus ; t.p, proxi- mal part of tarsus; t.d, distal part of tarsus; t-mt, tarso-metatarsus ; l.-IV., metatarsals; 1-4, phalanges. AYES. 241 the ilium, beyond which it extends. The pubis is a slender rod which, in the acetabulum, fuses with tlie ilium and ischium. It runs back below the latter, the obturator fissure separating the two, and extends for some distance l)ehind it. A wide space separates the pubes and ischia of opposite sides. Free Limb (Fig. 69).- — The femur articulates with the acetabu- lum by a preaxial head at right angles to the shaft, while its proximal end is terminated by a facet which works against the anti-trochanter, and on the postaxial side of which is a consider- able elevation, the great trochanter. The distal end of the femur is pulley-shaped. The main bone of the leg is the preaxial tibio- tarsus with which the small rod-like postaxial fiFula is partly fused. The tibio-tarsus is a stout bone, much longer than the femur, and with a pulley-shaped distal end. In front of the knee-joint and fibula there is a small irregular bone, the patella. In the adult there is no separate tarsus, but in the embryo it is represented by two cartilages, the proximal of which unites with the preaxial tibia and forms the distal end of the tibio- tarsus, Avhile the distal fuses with the metatarsus. The ankle- joint is thus between the two tarsal elements, and is said to be mesotarsal — i.e., in the middle of the tarsus. Four digits are present — first, second, third, and fourth. The metatarscds of the three last fuse with the distal part of the tarsus to form the tarso-metatarsus. This is a rod of bone, the distal end of which possesses three very distinct pulley-like sur- faces for the three corresponding metatarsals. The small first metatarsal is attached to the preaxial side of this end. The four digits, of which the first is directed backwards, are completed by 2, 3, 4, and 5 phalanges respectively. The terminal phalanges support claws. 4. The digestive organs (Fig. 70) consist of gut and appended glands. The former is divided into mouth-cavity, gullet with crop, proventriculus, gizzard, small intestine, and large intestine opening into a cloaca. The glands are liver and pancreas. The mouth is bounded by the horny margins of the beak, and, owing to the double joint by which the mandible is connected with the skull, can be opened very widely. It leads into a large month-cavity, upon the roof of which are two narrow slits, the interned nares, largely overlapped by folds of mucous membrane. Behind them is a small median Eustachian aperture, and posterior 2 16 24:^ AN ELEMENTARY TEXT-BOOK OF BIOLOGY. to this the end of the roof is marked hy a backwardly directed fringe. The large tongue attached to the floor of the mouth, has a sharply-j)ointed tip directed forwards, and its hinder part is produced backwards into fringed processes, behind which is the glottis, a narrow slit, with edges also somewhat fringed. The back part of the month-cavity, or pharynx, is continued behind into a very long gullet (cesophagus), for the most part wdth thin walls, which runs back on the ventral side of the neck to reach the thorax. In the posterior part of the neck it dilates into the crop, a very large bilobed sac, with extensible Myalls. Behind this the gullet narrows, and its walls are much thicker. It passes in the thorax into a small oval chamber, the proveutri- culus, of somewhat larger calibre, and with thick soft walls, upon the inner surface of vdiich are a number of small rounded ai)crtures, those of the peptic glands. The proventriculus opens, on the left side, into the dorsal border of the gizzard, a large rounded, somewhat flattened structure with extremely thick hard walls, the lining of which is horny. It contains numerous small j)ebbles and other foreign bodies which have been swalloM'ed. Proventriculus and gizzard together are equivalent to a stomach. A narrow tube, the small intestine, comes off from the right side of its dorsal border, forms a U-shaped loop, the duodenum, and, after making several loops and coils, merges into the short, straight, large intestine, the junction between the two being marked by a pair of small projections, the intestinal cceca. These are very large in the fowl. The large intestine is continued posteriorly into a small cloaca which opens externally l)y the cloaca! aperture. The cloaca is divided by inwardly 2)ro- jecting folds into three compartments, internal, middle, and external (Fig. 71). The first receives the large intestine, the second the urinary and genital ducts, while into the dorsal wall of the third a small thick-walled pouch, the hursa Fabricii, opens in young birds. The lining of the small intestine is raised into innumerable delicate thread-like processes, the vdli, which gradu- ally die away posteriorly and are replaced by longitudinal ridges. The lining of the large intestine and cloaca is smooth. The liver is a large, brown organ, lying on the ventral side of the body, and presenting depressions above and in front into which the heart, duodenum, and gizzard fit. It is divided into rigid and left lobes, of which the first is the larger. There are AVES. 243 Eig. 70. — Genekai. Dlsskotion of Male Pigeon.— c, Cere, close to which is external naris, ex.n. ; u.gl, opening of uropygial gland; il, cut edge of ilium ; st, keel of sternum. A large part of this bone has been removed together with ribs, h, Beak ; t, tongue ; p.w, internal nares ; fjl, glottis ; (E, gullet, which sw^ells into the crop in the middle of its course ; pr, proventriculus ; gz, gizzard ; d, duodenum. Most of ileum has been cut away, at i its junction with large intestine is seen. This point is marked externally by the intestinal coeca, one of w'hich is shown; cl, cloaca; LI, placed on left liver-lobe at point where right lobe was removed ; x , x , placed on points where bile-ducts enter duodenum ; p, pancreas ; ***, placed on points where the pancreatic ducts enter duodenum; r.a, right auricle of heart, into which the cavals run ; pr-c, right precaval, formed by the union of {jn) jugular and subclavian, the short vessel just over pr-c, which the brachial (1) and pectoral (2,2') veins combine to form; p-c, postcaval, the other cut end of which is close to t; r.v, right ventricle ; i.a, right innominate artery bifurcating into common carotid above, and sub- clavian, the latter at once dividing into brachial (above) and pectoral (below). The dorsal aorta (d.ao) is seen running back from heart. The cut end of the coeliac artery is just above pr, and that of the anterior mesenteric to the right of t; tr, trachea; I', k' , k” , anterior, middle, and posterior lobes of right kidney cut through ; v.d, right spermiduct (vas deferens), along inside of which right ureter runs ; t, left spermary (testis) the right is removed ; ol, placed over olfactory lobe, and olfactory nerve which runs forwards from it; cJi, cerebral hemisphere; o.l, optic lobe; ch, cerebellum; m.o, medulla oblongata. II., Optic nerve, just above which is optic tract; .sy, right sympa- thetic ganglia of thorax, between which are cut ends of ribs. The ganglia are connected by double commissures, which, with connected nerve branches, are represented by black lines. 244 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. two hile-ilucts, the left short and ivide, running into the duodenum near its commencement, while the right, long and narrow, opens into its distal limb near the beginning of its distal third. There is no (jall-Uadder. A gall-bladder is present in the Fowl. The bile and pancreatic ducts are also arranged differently. The pancreas is a compact, elongated body, lying in the loop of the duodenum, and sending three imncreatic ducts into its distal limb. As in the frog, the alimentary canal is suspended from the dorsal wall of the body-cavity by mesenteric folds. The mem- branous lining of the abdominal region (which, however, is quite continuous with that of the thorax) is the peritoneum. This closely lines the dorsal side of the cavity before leaving its walls to form mesenteric folds. The "wall of the alimentary canal is made up of the usual four coats — mucous, submucous, muscular, and serous. The mueous membrane possesses an epithelial lining varying in the different parts of the canal. That found in the mouth-cavity and parts of the oesophagus is stratified squamous epithelium, and that lining the crop glandular. The lower parts of the gullet, the proventriculus^ and the intestines are lined by simple columnar epithelium, con- tinued into the simple tubular peptic glands of the second, and there becoming glandular. Glands of Lieherkilhn are present in the small intestine. The epithelium of the gizzard secretes a thick horny cuticle. The muscidar coat varies very much in thickness, and typically consists of two layers, an internal circular and an external longi- tudinal, the relative position of these being reversed, however, in the gullet. This coat is immensely thickened in the gizzard, and has there a complicated arrangement. The serous coat is an epithelial and connective-tissue layer present from the proventriculus backwards, and formed by the investing mesentery. The liver and pancreas present no important deviations from the structure of the same organs in the frog (pp. 193, 194). The food, chiefly consisting of grain, accumulates in the crop, from which it passes on through the proventriculus to the gizzard, which grinds it up, the process being aided by the foreign bodies AYES. 245 present. The three chief digestive juices are, gastric juice, bile, and pancreatic juice, secreted respectively by the peptic glands, liver, and pancreas, and acting as in the frog. An important addition to the absorptive surface is afforded by the villi. 5. Circulatory Organs. — Blood and lymph systems are present, and well developed. I. Blood System. — The bright red Uoocl, which coagulates very quickly, is made up of plasma, with amoeboid colourless corpuscles and much more numerous red coipuscles. Both kinds are nucleated, and the latter are small, flattened, and oval, with ends somewhat pointed. The blood circulates in a closed system of tubes made up of heart, arteries, veins, and capillaries. The large conical heart (Fig. 70), contained in its pericardium, lies in the anterior part of the thorax, and is made up of four chambers, two auricles and two ventricles. Its forwardly directed base is chiefly made up of the dark thin- walled right and left auricles, its apex by the thick-walled right and left ventricles, the latter being the firmer, and alone reaching the extreme apex. The cavities of the auricles are separated by a thin auricular sepdum, upon which is a depression, the fossa ovalis, where the septum is thinnest. Within the right auricle are the three openings of the caval veins, that of the postcaval being guarded by a muscular flap, the Eustachian valve, and, posteriorly, the crescentic right auriculo- ventricular opening leading into the right ventricle. From the right side of this opening a muscular flap, the right auricido-ventricular valve, projects into the ventricular cavity, to the walls of which it is united by fibrous cords {chordae tendinece). The right ventricle is separated from the left by the ventricular septum, which bulges into its cavity, the walls of which are raised into muscular ridges {columnce carnece). The pulmonary artery, which arises from this division of the heart, has its origin guarded by tXwQo, pndmonary semilunar valves (pocket-valves). Within the left auricle, dorsally, is the small opening of the ])ulmonary veins, and, j)osteriorly, the rounded left auricido-ventricular opening. This is guarded by a mitral or bicuspid valve, made up of two membranous flaps, situated in the cavity of the left ventricle, and connected by chordae tendineae to papillary muscles, conical elevations, whicli may be considered as modified columnae carneae. The aorta arises from the anterior end of the left ventricle, and its orij^in • *1 ' O IS guarded by three aortic semilunar valves (pocket-valves). 246 AN ELEMENTARY TEXT-ROOK OF BIOLOGY. Arteries. — -A single aortic arch is present, the aorta, the first part of which (arcli of aorta) curves round to the right, giving oft‘ Ijranches which supply the head, neck, wings, and pectoral muscles, and then, reaching the middle dorsal line, becomes the dorsal aorta. This runs straight to the tail, where it becomes the caudal artenj, giving off branches on its way to the viscera, hind- limbs, and body-wall. From the arch of the aorta, large right and left innominate arteries are given off close to the heart, after which the calibre of the aorta diminishes considerably. Each innominate divides very quickly into common carotid and subclavian arteries. The common carotid runs along the neck, soon giving off the vertebral artenj (which supplies the brain and spinal cord, and occupies the vertebrarterial canal), and, at the angle of the jaw, divides into internal and external carotids. The former supplies the brain, the latter the head generally. The subclavian divides almost at once into the small brachial and large pectoral arteries, which respectiv^ely supply the wing and muscles of the chest. The dorsal aorta gives off an unpaired coeliac artery, the branches of which run to the proventriculus, gizzard, spleen, duodenum, pancreas, and last loop of the small intestine; while, slightly behind this, an unpaired anterior mesenteric artery comes off, which supplies the rest of the small intestine. Still further back the small anterior renal arteries, for the anterior kidney-lobes; the femorcd arteries, for the extensor muscles of the thigh ; and the sciatic arteries, for the flexor muscles of the thigh and for the rest of the hind-limb, are given off in suc- cession. Branches of the sciatic, the widdle and posterior renal arteries, supply the corresponding kidney-lobes. Just behind the kidneys the dorsal aorta gives off an unpaired posterior mesenteric artery to the large intestuie and cloaca, and paired internal iliac arteries to the hinder part of the pelvis, and then runs, as the caudal artery, into the tail. The pulmonary artery arises from the right ventricle, and soon divides into right and left branches for the right and left lungs. Veins. — These may be dealt with under the headings of caval system, hepatic portal system, and pulmonary veins. (1) Caval System. — There are two precavals (superior vena3 cava3) and a postcaval (posterior vena cava), all opening into the right auricle. Each precaval is a large, short vein, formed by the union of three trunks— jugular, brachial, and pectoral, return- ing the blood from one side of head and neck, one wing, and chest-muscles of one side, respectively. The jugular runs back from the base of the head (where it is united by a cross-trunk with its fellow), along one side of the neck, and not far from its union with the other two trunks, receives the vertebral vein. The postcaval is a large vein returning the blood from the posterior part of the body, formed just in front of the kidneys. AVES. 24 7 by the union of two iliac veins, and running through the liver, from which it receives two hepatic veins, to the heart. The iliac on each side commences between the anterior and middle kidney-lobes where the femoral vein, bringing back blood from the greater part of the hind-limb, unites with the hypogastric vein to form it. The latter traverses the substance of the kidney, behind which it is connected with its fellow by a cross-branch, that receives the caudal vein from the tail, and internal iliac veins from the pelvis, and is also united with the hinder end of the posterior mesenteric vein. The sciatic vein carries blood from the hinder part of the leg into the hypogastric, at the junction of the middle and posterior kidney-lobes. These lobes return their blood by a large superficial renal vein to the iliac vein, which also receives a small vein from the anterior lobe. (2) Hepatic Portal System. — This, the only portal system present in the pigeon, is entirely made up of vessels running to the liver from the alimentary canal and spleen. Into the left liver-lobe two small left gastric veins take blood from the left side of the gizzard, while the large portal vein divides into two branches, one for each liver-lobe. This vein is formed by the union of three others — gastro-duodenal, returning blood from the right side of the gizzard, duodenum, pancreas, and last loop of the small intestine; anterior mesenteric, from the rest of the small intestine; and posterior mesenteric from the large intestine and cloaca. (The hypogastric veins receive part of their blood from this vessel. ) The spleen pours its blood directly into the portal trunk. (3) One or two short pulmonary veins from each lung unite together and open by a single aperture into the left auricle on its dorsal side. Circulation. — The two sides of the heart do not directly com- municate, and therefore no mixing of blood occurs, as in the frog. The impure blood from the body is poured into the right auricle by the caval veins, and passes into the right ventricle, by which it is forced into the lungs. Thence the oxygenated blood is returned to the left auricle, and, passing into the left ventricle, is pumped througli the aorta to the body at large. The movements of the heart are very vigorous. The auricles contract together, and their systole is followed by a ventricular systole, both ventricles contracting together. The auriculo- ventricular valves prevent the blood from passing back into the auricles, and the chordae tendineae prevent the valvular flaps from going too far. The chordae would be slackened during the ven- tricular systole by the approach of the ventricular walls, were this not compensated for by the papillary muscles, which con- 248 AN ELEMENTARY TEXT-BOOK OF BIOLOOYL tract at the same time and pull the chordae taut. The semilunar valves only allow blood to pass out of the ventricles. II. The lymph system mainly differs from that of the frog (p. 200) in the absence of lymph-hearts, and the greater definite- ness of the lymphatic vessels {lymphatics of general body, lacteals of gut). These resemble small veins in structure and ultimately open into two delicate tubes, the thoracic ducts, lying just beneath the vertebral column in the thoracic region, and communicating with the veins at the junction of the jugular and subclavian on each side. Minute lymph-spaces are found in all parts of the body, and there are also large lymph-spaces of which the most imj)ortant is the coelom, which includes the general body-cavity and the pericardial cavity. Two cervkcd lymjdiatic glands are found at the base of the neck, and a small red ovoid spleen is attached to the right side of the proventriculus, 6. The respiratory organs (Fig. 70) consist of lungs, to which the air gains access through a trachea, and of accessory structures. An organ of voice results from modification of part of the air- passages. The glottis leads into a small larynx, the walls of wdiich are supported by several cartilages, and which is the commencement of the windpipe or trachea, a long tube which runs along the neck to the thorax. Numerous firm rings surround and support the trachea, which bifurcates in the thorax into a right and a left hronchus, one going to each lung. At the point of bifurcation is situated the syrinx, which is here the organ of voice. The end of the trachea and commencements of the bronchi enclose the tynijianum or syringeal cavity. Into this an elastic fold, the memhrana semilunaris, projects from the point of bifurcation, and is supported by a slender bar of cartilage, the pessulus, running dorso-ventrally. The inner side of each bronchus, at its commencement, is membranous, and forms the memhrana tym.panifoi'mis interna. Special muscles are connected with the syrinx. The lungs are spongy bodies, of dark-red colour, which lie in the anterior part of the thorax. They are closely attached to the ventral side of the vertebral column and ribs, and a thin membrane, the pleura, continuous with the peritoneum, covers their ventral surface. The bronchus which runs to each lung enters it, becomes membranous, and, after dilating into a vestibule, runs towards the external side of its posterior end, bifurcating AVES. 249 into two secondary bronchi. Other secondary bronchi come oft from the vestibule. All these tubes are placed near the A^entral surface of the lung, the dorsal part of which organ is mainly made up of their branches, delicate tubes with blind endings. Closely connected with the lungs are a number of thin-walled air-sacs into which the secondary bronchi open. On each side of the body, in the abdominal region, between the kidneys and the intestines, there is a large posterior air-sac, communicating with the hinder end of the lung. In front of this are two pairs of intermediate air-sacs, situated on the ventral side of the body-cavity. The posterior intermediate air-sacs overlap the hinder part of the lungs, and communicate with them by openings close to those of the ])osterior sacs, while the anterior intermediate air-sacs are situated ventral to the anterior two-thirds of the lungs, and open into them near the middle of their length. In the region of the clavicle a large, unpaired inter clavicular air-sac is present (formed by the fusion of two sub -bronchial air-sacs), communicating with either lung by an aperture near the entrance of the bronchus. This air-sac is bilobed, and each lobe passes out of the body-cavity near the origin of the wing to communicate with the hollow shaft of the humerus, while dorsal to it there is a small j^f'cbronchial air-sac, connected with the anterior end of the lung. The most important histological point to be noticed is the finer structure of the delicate tubules making up the greater part of the lungs. The walls of these are raised up internally into intersecting ridges, which form a complicated honeycombing, traversed by networks of capillaries, and covered by simple squamous epithelium. The epithelium lining the bronchi and trachea is largely ciliated. The lungs are very immobile and take but little part in the respiratory movements. Expiration is effected by a contraction of the body-walls, by which the highly elastic air-sacs are com- pressed. Inspiration is passively effected by the elasticity of the body-walls, which expand and cause air to rush into the air-sacs. Owing to the presence of these, a large amount of air passes through the main passages of the lung. The essential part of respiration takes place in the small tubes making up the bulk of the lung, the walls of which are raised into folds, and the air is kept pure in them by diffusion. .Respiration is very vigorous, in accordance with the rapid oxidation (metabolism) of the tissues. A great deal of heat results from this latter process, and the body is maintained at a very high temperature (I03°-I04° F.) 250 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. The organ of voice is here the syrinx. 7. The nrinogenital organs (Fig. 71) include the excretory and reproductive organs. Excretory Organs. — The reddish-brown kidneys, covered ven- trally with peritoneum, are situated just behind the lungs, and closely applied to the under side of the ilia and “ sacrum.” Each Fig. 71. — UmxoGENiTAL Organs of Pigeon. — A, female; B, male; k", anterior, middle, and posterior lobes of kidney ; u, ureters ; u', openings of ditto; cl, cl', cl", internal, middle, and external divi- sions of the cloaca, which has been cut through on its ventral side and opened out; t, spermaries (testes); v.d, spermiducts (vasa deferentia); v.s, vesiculie seminales; g.j), genital papilhe, on the end of which are the male genital openings ; ov, ovary ; od, oviducts (right a mere rudi- ment); o5l', funnel of left oviduct; od", openings of oviducts into cloaca; ad, adrenals. possesses three lobes, anterior, middle, and posterior. Between the two first, on the ventral side, a small thin-walled ureter arises, which runs to the middle division of the cloaca, into the dorsal side of which it opens. The adrenals (Fig. 71, B, ad) are small elongated bodies, of yellowish colour, closely counected with the origin of the iliac vein on either side. The uriniferous tubules are not ciliated. Each of them begins AVES. 251 in a glomerulus-containing Bowman’s capsule, which passes into a dilated glandular portion, that, after a somewhat convoluted course, is succeeded by a narrow looped part. The terminal portions of the tubules form the ureter by successive unions. The water and salts are strained off in Bowman’s capsules, while the nitrogenous waste is excreted, mainly as uric acid (CgH^N^Og) by the glandular parts of the uriniferous tubules. The urine is semi-solid. Male Eeprodnctive Organs (Figs. 70 and 71). — The sper- maries (testes) are two oval, whitish bodies, situated ventral to the kidneys, and near their anterior ends. They are invested and held in place by folds of peritoneum {mcsorchia). From the inner side of each an opaque white tube, the spermiduct (vas deferens), repeatedly and sharply bent from side to side, runs on the outer side of the corresponding ureter to the cloaca, dilating at its end into a vesicida seminalis, and opening on a small genital papilla placed just external to the opening of the ureter. The spermaries are made up of a large number of much- convoluted seminiferous tubules formed by the continued branch- ing of the spermiduct. The tubules are lined by epithelium, many of the cells of which are sperm-mother-cells, producing sperms (spermatozoa), with cylindrical heads and motile tails. Female Eeprodnctive Organs (Fig. 71). — These are only developed on .the left side. The ovary is a very irregular body (similarly situated to the spermaries), from the outer surface of which globular ova of various sizes project, enclosed in follicles. It is covered by peritoneum, which forms a suspensory fold (mesovarium). The oviduct is a thick- walled convoluted tube, communicating with the body-cavity at one end by a delicate membranous funnel situated near the ovary, and at the other entering the cloaca by a large aperture external to the opening of the left ureter. The right oviduct may be represented by a short blind tube having a similar situation. The cloaca is larger in the female. The ovary consists of a connective-tissue framework, the stroma, richly supplied with blood-vessels. The ova are contained in ovarian follicles, lined by follicular epithelium, which is again in- vested by the stroma. These ovarian ova are, when ripe, yellow spheres rather less than an inch in diameter. Such an ovum {cf. Fig. 73) is covered by 252 AN ELKMEN'I'AHY TEXT-ROOK OF BIOLOGY. a delicate vitelline' membrane, within which is the vitellus, contain- ing a germinal vesicle placed close to the surface. That part of the vitellus in which the germinal vesicle is imbedded, the germinal disc, is a small lens-shaped mass of clear protoplasm, while the rest is mainly made up of food-yolk, through which protoplasm is sparingly diffused. The yolk is of two kinds — yellow, composed of granular spheres, and ivhite, formed of smaller spheres enclosing highly refracting spherules. The yellow yolk is more abundant, while the white yolk is arranged in a flask- shaped mass running from the germinal disc to the centre of the ovum, and several thin lamellse concentric to this. The oviducts possess muscular walls, and are lined by glandular and ciliated epithelium. The cloacal chambers of the two copulating individuals are partially everted and the sperm passed into the oviduct. The ova burst out of their capsules into the body-cavity, and are taken up by the funnel of the oviduct. T3efore passing doAvn very far, they are fertilized by the fusion of a single sperm with each. 8. Muscular System. — The muscles may be classified as in the frog (p. 208), but their arrangement is more complicated. The presence of air-sacs largely increases the surface for attach- ment of the muscles of flight, the most important of which take origin in the sternum and its keel, and are inserted into the humerus. The specific gravity of the body is diminished by the presence of air-sacs, as also by the nature of the bones, and flight is thus indirectly aided. Both unstriated and striated muscles are present, as in the frog, and their structure and distribution are substantially the same. 9. The nervous system (Fig. 70) consists of cerebro-spinal axis, cranio-spinal nerves, and sympathetic system. (1) The cerebro-spinal axis consists of brain and spinal cord, enclosed in the neural canal and covered by membranes, but closely fitting the cavities in which they are placed. The brain is large and rounded. As l)efore (p. 210), it may be divided into fore-, mid-, and hind-brains ; — [a) Fore-brain. — The that amenceyl talon contains a large slit-like third ventricle, bounded by the lamina termiiialis in front, and the otitic thalarni at the sides (united behind by a posterior commissure), while a AVKS. 253 stalked pineal gland is connected with its thin roof, and a pituitary body with the infunddmlum formed by its floor. This part of the fore-brain is overlapped by the large smooth cerebral hemispheres. These are ovoid bodies closely applied to each other in the middle line, broadest behind and bluntly pointed in front. Each contains a large Icderal rentricle, communicating Avith the third ventricle by a foramen of Monro, and having its floor raised into a considerable elevation, the corpus striatum. The two corpora striata are connected by the anterior commissure running in the lamina terminalis. A small pointed olfactory lobe is connected with the anterior end of each hemisphere, close to the middle line on the ventral surface, and it contains a small olfactory ventricle, continuous Avith the lateral A^entricle. (b) Mid-brain. — The optic lobes are very large. They are AAudely separated from one another, and situated laterally, but remain united by a dorsal opjtic commissure. Each contains a good-sized optic ventricle, opening into the Sylvian aqueduct, Avhich connects the third and fourth ventricles, and possesses a thick floor formed by the crura cerebri. {c) Hind-brain. — The thickened cylindrical bulb (medulla ob- longata) possesses shalloAv dorscd and ventral fissures, and the roof of the fourth ventricle is extremely thin. The cerebellum is a large rounded projection, flattened laterally, which overlaps the mid- brain and medulla. It is marked by deep transverse furroAvs,. and on each side presents a small rounded elevation, flocculus. The cerebellum is united Avith the bulb by a large cylindrical peduncle on each side. The spinal cord is continuous in front Avith the bulb, a sharp' A^entral flexure marking their union, and tapers gradually back to the caudal region, exhibiting, hoAvever, considerable brachial and lumbar enlargements Avhere the limb-nerves come off. Dorsal and ventral fissures are present, and a central canal, Avhich in the lumbar enlargement expands into a sinus rhomboidalis, the lozenge-shaped roof of Avdiich is covered by membrane only. (2) Cranio-Spinal Nerves. — There are tAvelve pairs of cranial nerves, the first ten of which correspond in origin and distribution to those of the frog (p. 211). The optic chiasma is very large, and the optic tracts very Avide. The (V.) trigeminal nerve arises by tAvo roots, upon the larger of which is the Gasserian ganglion.. OAAung to the elongation of the neck, a very long course is taken. 254 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. ])y the (X.) vagus to reach the heart, lungs, &c. The (XL) spinal accessory, a nerve not present in the frog, is made up of fibres arising from the side of the spinal cord and bulb, it leaves the skull with the vagus, and supplies some of the neck-muscles. The (XII.) hypoglossal nerve corresponds to the first spinal of the frog, and has a similar distribution, but it arises from the ventral surface of the bulb. For the nerve exits see p. 233. The spinal nerves arise by dorsal and ventral roots (the former ganglionated) from the spinal cord, and }>ass transversely outwards by the intervertebral foramina. They are named cervical, thoracic, &c., in the corresponding vertebral regions. The brachial plexus, from which the wing is supplied, is produced by the union of the last three cervical with the first thoracic. A luinhar plexus is formed by the last lumbar and first sacral nerves, and a sciatic plexus by the five succeeding nerves. These two plexuses supply the hind-limbs and their girdles. (3) Sympathetic System (Fig. 70, Sy). — The main part of this is a delicate ganglionated cord running close to the vertebral column on each side. The ganglia are connected by rami com- muiiicantes with the commencements of the spinal nerves. The two cords unite behind in an unpaired ganglion {ganglion impar), while the anterior part of each of them is double, half of it run- nins: in the vertebrarterial canal. O The same histological elements are present as in the frog, and their structure and arrangement are similiar {cf. Fig. 61). There are, however, no bipolar ganglia in the sympathetic of the peculiar kind described on p. 215. The external part of the cerebellum is composed of grey matter, its internal part and peduncles of white matter. The most important advance upon the frog is found in the large size of the cerebral hemispheres, and this is associated with increased intelligence. The large cerebellum has apparently the correlation of muscular movements for its main function. 10. Sense Organs. — (1) Tactile Organs. — Many of the nerves ramifying in the skin terminate in ovoid touch-corpuscles. (2) G-nstatory Organs. — Gustatory cells, supplied by the fibres of the glossopharyngeal nerve, are present in the hinder part of the tongue and roof of the mouth. AYES. 255 (3) Olfactory Organs.^ — The olfactory sac on each side, which communicates by an external naris with the exterior, and by an interrial naris with the mouth-cavity, is partly lined by olfactory epithelium. This covers the projecting turbinal, the rolled shape of which increases the surface. The olfactory nerve breaks uj) below the olfactory epithelium to supply the olfactory cells, of which this is largely made up. (4) Auditory Organs. — External, middle, and internal ears are present. The external ear consists of a short tube, the external auditory meatus, opening below and behind the eye. It is separated by the tympanic membrane from the middle ear or tym- Fig. 72. — Diagrams of the Membranous Labyrinth (from Pladdon). Internal side of left labyrinth. — A, Fish. B, Bird. C, Mammal; us, utricnlus and sacculns ; u, iitriculus ; s, sacculus ; c, cochlea. panic cavity. This communicates with the mouth-cavity by an Eustachian tube, while the columella stretches across it, having one end attached to the tympanic membrane and the other inserted into the fenestra ovalis, below which is the fenestra rotunda. The internal ear is essentially composed of the membranous labyrinth (Fig. 72), enclosed in a somewhat larger cavity (filled with perilymph) of closely corresponding shape. This cavity is con- tained in the auditory capsule, and is surrounded by a thin, dense layer of bone, forming the bony labyrinth. The utriculus and sacculus are ill-marked ofi‘ from one another. With the former three semicircular canals are connected, arranged as in the frog, 250 AN ELEMENTARY TEXT-BOOK OF BI(M.O(iY. with the exception that the posterior vertical takes an outward course, and crosses the horizontal before it dilates into an ampulla. A slightly-curved tube, the cochlea, is connected with the anterior part of the sacculus. The membranous labyrinth contains endo- lymph with otoliths, and is lined by epithelium, patches of which are largely made up of auditory cells, connected with auditory nerve-fibres. It has been determined by experiment that the semicircular canals of the ear have to do with the perception of position in space. In this connection the fact that their planes are mutually at right angles deserves notice. (5) Visual Organs. — The eye agrees in all essential particulars with the description on p. 221, but differs in certain points. The inner half of the eyeball is hemispherical, while its outer part is somewhat conical and terminated by the very convex cornea around which the sclerotic is strengthened by a circlet of small, flat sclerotic j^lates (Fig. 68). The lens is flattened, and a vascular, pigmented, longitudinally plaited fold, the yecten, pro- jects into the vitreous humour below the entry of the optic nerve. The accessory parts connected with the eye are the three eyelids, conjunctiva, two glands, and the eye-muscles. The glands are (1) the Harder ian glayid in front, and (2) the lachrymal gland behind and above. The eye-muscles are arranged much as in the frog. DEVELOPMENT. The common fowl has been most studied, and, as birds differ but little in their development, will be here described. 1. Early Stages. — The fertilized ovum, in passing down the oviduct, is covered by several structures secreted by its walls (Fig. 73). These are: — (1) The ivhite of the egg, mainly com- posed of semifluid proteid material, and containing a somewhat convoluted cord, the chalaza, at each end, (2) the double-layered shell-membrane, covered by (3) the shell, formed of an organic matrix hardened by salts of lime. The entire egg is elongated, and broader at one end than the other. Cleavage (segmentation) (Fig. 74) is unequal and also partial (meroblastic), being confined to the germinal disc. It commences AVES. 257 ill the lower part of the oviduct, and by a succession of furrows, some at right angles and others parallel to the surface, the germinal disc is converted into a many-celled hlastodenn. This is placed on one side (Fig. 73, Bl), and always remains upper- most, the most favourable position for development, which requires a good deal of Avarmth, supplied in nature by the body of the hen. Before incubation (Fig. 75) the blastoderm consists of a superficial layer, one-cell thick, of columnar ecto- Fig. 78. -Diagrammatic Lonottudinal Section Through Unincubateh Hen’s Egg (from Claua, after Balfour aud Allen Thomson). — Bl, Blastoderm ; GD, yellow yolk ; WD, white yolk ; DM, vitelline mend wane ; EW, “white;” Ch, chalazai ; S, shell membrane; KS, shell ; LR, air-chamber. derm (epi blast), and a more irregular thickened mass of lower layer celh, which are rounded, granular, and of larger size. BeloAv them is a space, the earliest rudiment of the alimentary canal. Previous to laying, a blastOCCele (segmentation cavity) can be seen between ectoderm and lower layer cells. The blastoderm is at first circular, and the more transparent central part, known as the area j)elludda, is surrounded by a darker rim, the area opaca {ef. Fig. 77). If the egg is placed with its broad end to the right, the diameter of the blastoderm, which is to become the long axis of the embryo, will run across the egg, the posterior end being towards the observer. It now remains to describe the formation of the endoderin and mesoderm. These are first clearly differentiated in the 2 17 AN ELEMENTARY TEXT-B(30K OF BIOLOGY. i o o posterior part of the l)lastoderm. Many of the lower layer cells here become much flattened and unite to form a thin membrane, Fig. 74. — Surface Views to Show Cleavage in the Fowl’s Oosperm (from Kolliker, after Coste). — Cleavage commencing in 1, completed in 6. the endoderm (hypoblast) (Fig. 76, liy). In the hinder part of the area pellucida an opaque longitudinal strip, the primitive AVKS. L>59 streak (Fig. 77, ZV), appears. This is a local thickening (Fig. 76), where ectoderm and endoderm are continuous, and from which a sheet of mesoderm (mesoblast) grows out on each side, Fig. 75. — Section Thkoccii Part of Untncubated Fowl’s Blastoderm (after Klein).— a. Ectoderm; h, lower layer cells; /, archenteron ; below this, yolk. Fig. 70. — Transverse Section Tiirouoii Front End of Primitive Streak in First Day Chick (see Fig. 77) — (after Balfour). — Columnar ectoderm above, two cells thick. Pv, Region of primitive streak, where rounded mesoderm cells are originating from the ectoderm ; /i.v, endoderm, above it, at the side, are stellate mesoderm cells of endo- dermal origin. N.B. — Pv is jdaced over the middle line. being reinforced by those lower layer cells which have not united to form endoderm. In the anterior part of the blastoderm, i.c., in front of the primitive streak, the lower layer cell form — (1) a thin layer of endoderm (hypoblast), (2) two lateral sheets of mesoderm (mesoblast), (3) an axial rod of cells, the notochord, at first closely united with the hypoblast. 260 AN ELExMKNTAKV TEXT-I500K OE JilOLOGY. The blastoderm thus comes to be three-layered, both in front and behind. The closure of the blastopore in a frog-embryo produces a small primitive streak, in 'svhich the germinal layers are continuous. There can be no doubt that the primitive streak in the chick-embryo is equivalent to a closed blastopore of slit-like form. If the slit were to re-open it would lead into the space figured at f in Fig. 75, and which = an archenteron. The gastrula of the fowl, therefore, is very much modified, owing to (1) closure of blastf>i)ore, (*2) enormous accumulation of yolk on ventral side. ^ Fig. 77.— Surface View of First Day (20 hrs.) Chick (from Kollilcer). — A.o, Area opaca, bounding A.j), area pellucida ; Pr, primitive streak ; Af, head -fold. 2. General Growth. — The blastoderm is at first a watch-glass shaped plate resting upon the yolk. It generally grows in all directions, and ultimately completely encloses the yolk. This enclosure is mainly effected by the area opaca. The body of the embryo is formed in the area pellucida. This elongates (Fig. 77), AVES, 261 becoming somewhat })ear- shaped, the broad (head) end being anterior. In front of the primitive streak a neural (medullary) plate is formed (cf. p. 224), upon which is a longitudinal neural (medullary ) groove. This is bounded at the sides by neural (medullary) folds (Fig. 78, Mn), which unite above to form the neural tube. The folds first unite in the region of the mid-brain. The head now projects from the surface of the yolk, and makes with it, as seen from the left side, an S -shaped headfold. A similar tail-fold is formed later, and then lateral folds. The embryo, at first flattened out on the curved yolk - surface, thus gradually comes to assume the form of the chick, and the ever diminishing yolk re- mains attached to its ventral surface (Fig. 82). In a chick at the end of the second day of incubation (Fig. 79), the cerebral vesicles (Vh, Mh, and Ilh) are present, and the eyes and ears are commencing. The front part of the brain begins to bend round the end of the notochord. At the end of the third day, the front part of the embryo is twisted round so as to lie with its left streak ; vAf, liead-fold. either side of Rf. Fig. 78. — Surface ,Vienv of Cjiick, ItATHER Later than Fig. 77— Magnified 39 diameters (from Kol- liker). — M71, Medullary folds uniting in the head region ; P?-, primitive Tiiree Jmesodermic somites are visible on WmiL- A viT 'k'.-r. «^#8gsli IW IfciK ,' '/^ r. 2G2 AN ELEMENTARY TEXT-ROOK OF RIOLOGV. side on the yolk, and the cranial flexure round the notochord is ’well-marked. Five visceral arches and four visceral clefts have made their appearance (Fig. 80), one less than in the frog. The most })osterior of these are very small. The pro- minent heart is situated just under the throat, close to the Fig. 79. — 8u Rivv(’K ViKW OF Second Day CurcK -^higiiitied rather more than 50 diameters (from KoUiher). — Ap, Area pelliicida ; Vh, Alh, Hh, fore-, mid-, and hind-brains. The neui'al groove has closed as far back as omr. Pr, Primitive streak. Eight mesodermic somites are shovn. ventral ends of the visceral arches. The eye and ear are much more advanced, and the olfactory sacs are commencing as pits. On the fourth day the limbs appear as buds. The chick is hatched on about the twentieth day of incubation, breaking the shell with its beak, which possesses a small knob for the purpose. AA'ES. 263 3. Fate of the Germinal Layers. — These give rise to the same organs as in the frog (p. 226), but there are differences which will be here briefly noted. (1) The ectoderm (ei)iblast) does not become divided into superficial and nervous layers till comparatively late, and the membranous labyrinth commences as a pit open externally. (2) The endoderm (hypoblast) presents no important differences, but the notochord disappears more com- Fig. 80. — Development of the Eye (from Iladdon, A after MarsliaU).— 1 A and B sections through head of third day chick. In A the optic j vesicle (o.c) from the fore-brain [f.h) is becoming cup-like, I is the i developing lens. In B the cup and lens are more distinct. Stalk of the former not cut through. C. Later stage — Inner wall of optic cup j (o.c) forming most of retina, r — outer wall, p, forming pigment layer j of ditto; I, leus (quite separated off). These sections incidentally * illustrate other points. In all, owing to the cranial flexure, both fore- and hind-brains have been cut through {f.h, h.h). In C, c.h, l.t, and I inf, indicate cerebral hemispheres, lamina terminalis, and infundi- ij bulum. Near the last is the pituitary body, pit. In A and B the ;i pharynx {ph) is cut through, and the first two visceral clefts {v.d and 'i p.c") are shown, a.a, aortic arches; a.c.v, anterior cardinal vein; 1 nch, notochord ; olf, olfactory pit. ! jiletely in the adult. (3) The mesoderm (mesoblast) becomes ■ divided (cf. p. 227) into vertebral and lateral plates in the region of the J trunk, the former being again divided into mesodermic (mesoblastic) i somites (Figs. 78 and 79). The cadom (Fig. 81) is as before the i; split between the somatic and splanchnic layers of the lateral plates, the former uniting with the ectoderm to form the body-wall ! (somatopleure), the latter with the endoderm to form the wall of | J AN ELE]MENTAKY TEXT-BOOK OF BIOUJOY. the gut (s])lanchnoi)leure). When tlie embryo is spread out flat oji the yolk, the soinatopleure and splanchnopleure belonging to opposite sides diverge widely, but, as the embryo is folded off, they gradually approach, unite, and form a continuous boundary to the body and gut respectively. The latter meets in front and ])ehind with inpushings of ectoderm that form the mouth-cavity and posterior part of the cloaca (stomodseum and ju’octodseum). Fig. 81. — TpvxVnsvekse Section through Embryo Buck at Third Day {dSter Balfour). — The external layer is ectoderm; hy, endoderm; meso- derm, a thick layer between the two; am, lateral arnniotic fold; .so, soinatopleure ; sj), splanchnopleure (the reference letters lod and sh are placed in coelom); .sy>.o, spinal cord, at the side of which is section through mesodermic somite; ch, notochord (the reference letters roduced backwards into the anterior cUnoid processes which bound the sella turcica in front. After limiting the optic Joramen below, the pre-sphenoid ends in two diverging lamellie which articulate above with the orbital processes of , the frontal, and helj) to form the inner wall of the orbit. A Mung-like orhito-sphenoid is firmly fused with the pre-sphenoid on either side, form- ing the hinder and upper boundaries of the optic foramen, and uniting with the orbital process of the frontal above, and the ali-sphenoid behind. The narrow front end of the brain-case is filled in by the cribriform plate, a lamella perforated by numerous holes and forming the party-wall between the cranial and nasal cavities. It is the hinder part of the mesethmoid. The side- wall of the skull is largely formed in its posterior region by the squamosal, which fills u[) the gap between parietal and ali-sphenoid, unites in front with the frontal and orbito-sphenoid, and behind overlaps the descending process of the parietal. A delicate backward process from the squamosal helps to keep the large, irregular periotic in place, and a large zygomatic process runs outwards and forwards from it just behind the orbit. The auditory capsule is formed by the bone which results from the early fusion oi pro-, epi-, and opisthotic elements. The outer and hinder mastoid portion of this bone is porous, and a tapering mastoid, process runs down from it in front of the par-occipital process. The inner side of the mastoid part presents a deep rounded pit, the Jloccular fossa, for the flocculus of the cerebellum. The petrous portion which forms the rest of the periotic is very dense, and contains the mem- branous labyrinth. Upon its outer surface there is a smooth projection, the promontory, above which is the elliptical fenestra ovalis, and behind it tlie more irregular fenestra rotunda. Closely applied to the outer surface of the periotic is the tympanic, a l)one with a dilated lower part (hulla) and a short part directed upwards and backwards. The inner side of both is incomplete, for which the periotic makes up, forming the inner wall of the tympanic cavity, which is contained in the bulla. The tubular part supports the external auditory meatus, and, at its junction with the bulla, the tympanic membrane is stretched over a flattened rim which projects into the latter. Extending across the tympanic cavity are four minute auditory ossicles. (1) The malleus, which possesses two slender processes (one, the man- ubrium, attached to the tympanic membrane), which are given off from a rounded head. This articulates by a saddle-shaped surface with the head of (2) the incus which gives off a small process backwards and a 2 18 274 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. slender one downwards. The latter is bent up at its tip, and connected by a minute disc (3), the os orbiculare, with (4) the stirrup-shaped stapes, the oval base of which fits into the fenestra ovalis. The Eustachian tube enters the tympanic cavity below and in front by a very short canal, formed by the apposition of a groove in the tympanic to the surface of the periotic. The par-occipital and mastoid processes overlap the tympanic behind, and hold it in place. The cavity of the brain-case closely corresponds to the shape of the brain, and is divided into a small olfactory fossa in front, ill-marked off from the large cerebral fossa, which extends back to the anterior limit of the periotics and supra-occipital, where a sharp ridge separates it from the cerebellar fossa. Nerve-Exits. I. Olfactory. II. Optic. III. Oculomotor. IV. Pathetic. VI. Abducent. V. Trigeminal — a. Ophthalmic. b. Maxillary. c. Mandibular. VII. Facial. Enters periotic by Leaves skull by VIII. Auditory — Enters periotic by IX. Glossopharyngeal X. Vagus. XI. Spinal Accessory. XII. Hypoglossal. 1 Numerous olfactory foramina, j Optic foramen. iSjdienoidal fissure (foramen lacerum anterius) Foramen lacerum medium ' Aqueductus Fallopii. Meatus auditorius internus. i Foramen lacerum posterius. Condylar foramina (2). Cribriform plate. A large unpaired aper- ture, bounded by jire- and orbito-sphenoids, which places hinder end of orbits in com- munication with each other and the cranial cavity. Between basi- and ali- sphenoid on either side. Between ali-sphenoid and periotic. Just below the floccular fossa. Between mastoid pro- cess and tympanic. Just behind aqueductus Fallopii, and sunk in a depression with it. Between periotic and ex-occipital. In ex-occipital, near the condyle. MAMISIALTA. The facial part of the skull consists of the olfactory capsules and jaws with associated parts. The olfactory capsules are separated from the cranial cavity by the cribriform plate, from which a vertical partition, bony behind {laminci peTpendiculaTis), cartilaginous in front (septum 7iasi), extends forwards and separates the two capsules. This partition, together with the cribriform plate, constitutes the mesethmoid, and its lower edge rests in a deep furrow on the upper surface of the elongated wmer. The posterior wall of each capsule is partly formed by the small lachrymal^ which also makes up part of the anterior orbital wall. Its outer side is deeply notched to transmit the lachrymal duct. The capsule is partly roofed in behind by the frontal, but mainly by an elongated flat vasal, which, with the premaxilla, largely bounds the external naris. The cap- sular side-wall and floor are princi})ally formed by the premaxilla and maxilla. The nasal cavity contains the ethmo-turhinal behind and above, a bone made up of thin lamella? disposed in a complex manner, which is fused with the cribriform plate, and a similarly constructed but more delicate maxillo-turhinal in front. From the nasal bone a delicate curved fold, the naso-turhinal projects into the cavity. The margin of the upper jaw is formed by premaxilloi and maxdlce. With the latter a zygomatic arch, running along the lower side of the orbit, is connected externally, while internally they come into relation with palatine and pterygoid bones. The premaxillcp. contain sockets for the upper incisor teeth. Each sends an elongated nasal process upwards and backwards along the outer edge of the nasal, a delicate ^aZa^me process backwards along the roof of the mouth, and a stout maxillary process backwards and outwards to unite with the maxilla. This is a large, extremely irregular bone that forms the jaw- margin with the premaxilla, a little way behind which bone it dilates to contain the sockets of the grinding teeth, and projects into the front part of the orbit. The maxilla also forms a large part of the side-wall of the nasal capsule, and in this region is very loose in texture. From its external side, above the grinding teeth, a stout zygomatic process projects, which is connected by the laterally flattened jugal with the zygomatic process of the squamosal to make up the zygomatic arch, a bony bar forming the lower boundary of the orbit. A small, horizontal palatine process runs from the inner side of the maxilla, and unites with its fellow in the middle line to form the front part of the hony palate, a transverse bridge of bone between the first four grinding teeth of opposite sides. The inner surface of the maxilla behind this is covered by the palatine, a thin vertical plate uniting above with the orbital process of the pre-sphenoid. The bony palate is completed behind by the horizontal palatine process of this bone which unites with its fellow. The posterior nasal chamber is bounded laterally by the palatines, which are united behind with the downward processes of the ali-sphenoids, and with the pterygoids. Each pterygoid is a small plate, behind, and in the same plane as the palatine. It is produced below into a curved hamular process, and connected above with the junction between basi- and ali-sphenoids. 27G AN ELEMENTARY TEXT-BOOK OF BIOLOGY. ! i j i The lower jaw or mandihle consists of two halves or rami, j each of which unites in front with its fellow in the mandibular I symphysis, and articulates behind by a longitudinally elongated • condyle with the glenoid fossa, a smooth hollow on the under side ■ of the z}^omatic jwocess of the squamosal. It is laterally flat- ly tened and composed of an anterior horizontal part bearing sockets for the lower teeth, and a posterior ascending part ending in the condyle above — in front of which is the coronoid process — and uniting with the horizontal j^ortion in the rounded angle. Hyoid and First Branchial Arches. — The proximal end of the hyoid arch is formed by the stapes. The “ hyoid apparatus ” consists of a small hasi-hyal imbedded in the root of the tongue, and with which are connected short anterior and long posterior cornua. The anterior cornua represent the hyoid arches of the embryo, and are connected by ligaments with the skull, close to the mastoid processes. In this region a small part of the hyoid arch (tpmpanohyal) is fused to the skull. The posterior cornua represent the first hranchial arches. Fig. 84. — Exdoskeleton of Rabbit. — A, Skull {jugal bone removed) ; .s-oc, supra-occipital ; ex-oc, ex-occipital ; c, condyle ; par, par- occipital process ; s-m.f, stylo-mastoid foramen ; ty, tympanic ; per, periotic ; i-p, inter-parietal ; pa, parietal ; as, ali-sphenoid ; sg, squa- mosal ; z.s, zygomatic process of squamosal ; pt, pterygoid ; fr, frontal ; or.s, orbito-sphenoid ; prs, pre-sphenoid; pi, palatine; mx, maxilla; zygomatic process of ditto (with cut end); pre-maxilla ; la, lachrymal ; na, nasal ; op-f, optic foramen ; x placed at junction of basi- and pre-sphenoid. A', Outside of periotic, after removal of tympanic; ina, mastoid portion; m.pr, mastoid process; pr, promon- tory ; f.ov, fenestra ovalis ; f.ro, fenestra rotunda. A", Auditory ossicles from inside ; m, malleus ; i, incus ; st, stapes. B, Mandible ; c, condyle ; a, angle ; c.pr, coronoid process. C, Hyoid apparatus ; h, body ; a.c, anterior cornu ; p.c, posterior cornu. D, Various vertebrae ; l, atlas (front view); 2, axis ; 3, typical cervical (front view) ; 4, typi- cal lumbar ; c, centrum ; n. s, neural spine ; t, transverse process ; a.f, (in atlas), articular facet for condyle; a.z, prezygapophysis ; p.z, post- zygapophysis ; n.c, neural canal ; v.c, vertebrarterial canal ; o.p, odon- toid peg; i.l, cervical rib ; ep, epiphysis ; mt, metapophysis ; an, ana- pophysis. E, Hip-girdles and Sacrum. 1, 2, 3, Sacral vertebrje ; s.r, sacral rib ; il, ilium ; ph, pubis; is, ischium (reference letter placed in obturator foramen) ; ac, acetabulum. F, Right tarsus ; as, astragalus ; ca, calcaneum ; cn, centrale ; t, 1-5, tarsalia ; 2-5, metatarsals; *, placed by process of met. 2, representing hallux. G, Left Scapula — s-sc.b, co.b, gl.b, supra-scapular, coracoid, and glenoid borders; .9p, spine; ac, acromion ; mtc, metacromion ; co, coracoid process ; gl, glenoid cavity. H, Left Carpus — r, Radiale ; i, intermedium ; u, ulnare ; cn, centrale; c, 1-5, carpalia ; 1-5, metacarpals. 3 Z 8 i i 1 I 278 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. Skull of the Dog . — This agrees in all essential matters with the rabbit’s skull, but there are differences in general shape and also in certain details. Most of these variations are correlated with the larger development of the jaw-muscles, as may be seen, for instance, in the presence of strong ridges on the brain-case and the width of the zygomatic arches. The skull is much stronger, and the face is in line with the cranium, whereas in the rabbit it is bent slightly downwards. The spongy areas ])resent in the skull of the rabbit (especially on the outer side of the maxilla) are absent in the dog. Other differences are the following: — (1) Cranium. — Plane of occipital region vertical, instead of slanting somewhat downwards ; interparietal represented by a process of supra-occipital. (2) Olfactory capsules. — Naso- turbinal represented by uppermost lamella of ethmo-turbinal. (3) Bony palate very large and long. (4) Mandible, wdth short ascending part, well- marked angle, large coronoid, and transverse condyle, articulating with glenoid fossa to form a Avell-niarked hinge-joint. All these last features are connected with the great size of the muscles used in biting. The hinge- joint only allows of snapping movements, giving no lateral play. (5) Nerve- exits. — The 2nd and 3rd branches of the trigeminal exit by special openings, foramen rotwidum and foramen ovale, situated one behind the other in the ali sphenoid, betw’een the sphenoidal fissure and the foramen lacerum medium. (6) Teeth, see p. 285. {h) Vertebral Column and Ribs (Fig, 84). — The vertebral column is divisilde into cervical, thoracic, lumbar, sacral, and caudal regions, corresponding to neck, thorax, loins, pelvis, and tail. Except in the sacral region the vertebrae remain distinct from one another, and the more or less flat ends of the centra are connected together by cartilaginous discs, intervertebrae, which come between them. The front and back of each centrum ossify independently as thin bony plates, epiphyses, which fuse later on with the middle part. The spinal nerves exit as in the other types by intervertebral foramina between adjoining arches, and neural spines and zygapopkyses of the usual kind are present. The cervical vertebrae are seven in number. Their centra are short, and their neural spines small. The cervical ribs are not free but fused, as in most of the pigeon’s cervical vertebrae (c/. p. 237). The apparent transverse processes are thus perforated at their bases to form the vertehrarterial canal, and are divided into two parts, the upper of which is true transverse process, the lower (^inferior lamina) the projecting part of the fused rib. The atlas or first cervical vertebra is ring-like, possesses large transverse processes (but no inferior laminae), and a thin narrow centrum. It presents in front two large oval articular surfaces for the occipital condyles, and behind two smaller concavities for the axis, but there are no true zygapophyses. MAMMALIA. 279 The axis or second cervical vertebra has an odontoid peg (which ossifies separately), projecting forwards above the centrum of the atlas. A convex articular surface is present on the under side of the peg, which, plays upon a corresponding surface on the upper side of the atlas-centrum, and there are also two large convexities on its base which fit into the two shallow cups on the hind end of the atlas. The neural spine of the axis is a prominent ridge which bifurcates behind. The last (seventh) cervical vertebra has a half-facet on the side of the centrum, at its hinder end. The thoracic vertebrae are 12 (or 13) in number, and are characterized by the possession of free thoracic ribs. They increase in size from before backwards, their centra, which are thick from above downwards, elongating. The neural spines of the first 9 are slender and backwardly directed, that of the tenth is vertical, and those of the remainder slope forwards like the lumbar spines. From the ninth backwards metapophyses appear, stout processes directed upwards and for- wards in the region of the pre-zygapophyses. The transverse processes of the first 9 possess tubercular facets on the under side of their tips, and capitular half -facets on the sides of their centra, one in front, the other behind. The last 3 (or 4) have entire capitular facets on the sides of their centra, situated near the front. The ribs are curved flattened rods, increasing in length up to the sixth, and then shortening. Each consists of a bony vertebral portion, possessing two articular processes (tubercle and capitulum), and a much shorter sternal part of more or less ossifled cartilage. The tubereles of the ribs articulate with the tubercular facets, and the capitulum of each of the first 9 articulates with a capitular facet, formed in part by the anterior half-facet of its own vertebra, in part by the posterior half-facet of the preceding vertebra (the seventh cervical in the case of the first rib). The last 3 (or 4) possess no tubercles, and their capitula arti- culate with the corresponding facets on their own vertebrie. The sternal parts of the first 7 ribs unite distally with the sternum ; those of the remain- ing ones do not. This is the distinction between “ true ” and “ false ” ribs. The lumbar vertebrae are 7 (or 6) in number. They are large, with elongated centra, laterally flattened neural spines, directed forwards, and strong elongated transverse processes running down- wards and forwards. They also possess large metapophyses, stout processes overhanging the pre-zygapophyses, and small anapophyses, backwardly directed processes given ofi below the post-zygapophyses. The first two also have hypapo- j)hyses, unpaired processes running downwards from the under side of the centra. 280 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. The sacral vertebrae are 4 in number, closely fused together. The two first possess sacral ribs, which form lateral wing-like expansions, articulating with the inner sides of the ilia. The caudal vertebrae, about 15 in number, gradually get smaller towards the end of the tail, losing first their processes, and then their neural arches. (c) The sternum is a narrow rod, made up of six laterally com- pressed segments or sternehrce, and placed in the mid-ventral line of the thorax. The first segment (manubrium) is much larger than the others, and possesses a prominent ventral ridge. It is composed of two segments completely fused together, and this is indicated in the adult by the attachment of the first sternal ribs to the middle of its length. The remaining six pairs of sternal ribs are connected with the junctions of the sternebrse, one pair each to the first four, and two pairs to the last. The hindmost sternebra is elongated, slender, and terminated by a rounded plate of cartilage, with which it forms the xipliisternum. (2) Appendicular Endoskeleton. — The long bones are terminated as usual by epiphyses. As in other cases the skeleton of fore- or liind-limb is divisible into girdle and free part. (a) Foredimb (Fig. 84). — The sAoziWer is mainly composed of the triangular scapula, placed external to the anterior ribs, where it is held in place by muscles and ligaments. Its enlarged apex presents a shallow glenoid cavity for the head of the humerus. The base, anterior, and posterior sides of the triangle are termed su])ra-scapidar, coracoid, and glenoid borders respectively. With the first a strip of cartilage, the suyra-scapida, is connected. A con- spicuous ridge, the spine, runs along the outer surface of the scapula, and is produced below into a freely projecting process, the acromion (ac), from which a more slender metacromion is given off behind. The coracoid border is continued below into a hook-like coracoid process. This is originally distinct, and repre- sents the coracoid bone of lower Vertebrates. A ligament, in the centre of which is a slender curved clavicle, runs from the tip of the manubrium to the acromion. To the sternal end of this ligament a minute nodule of cartilage is attached which represents a precoracoid. Free Limb. — The humerus presents a proximal head, on its upper (dorsal) surface, for articulation with the glenoid cavity, and a distal pulley-like trochlea, which assists to form the elbow- MAMMALIA. 281 joint. The radius and ulna, which support the antebrachium, are immovably articulated, but not fused together in the position of jironation. Where, as in Man, the radius and ulna are movable, they may assume two main positions: — [a) Supination, when they are parallel, the palm of the hand is upwards, and the thumb outside (like the radius), {h) Prona- tion, when the radius crosses over the ulna towards the inside, the palm of the hand is downwards, and the thumb inside (like the distal end of the radius). The preaxial radius possesses a proximal head for articulation with the trochlea, and two distal concavities for the carpal bones. The postaxial ulna has a proximal sigmoid cavity for articulation with the trochlea, and a convex distal end. Proximally the ulna is produced into the olecranon, a process which, in the extended limb, fits into the olecranon fossa, a pit situated above the trochlea. The carpus consists of a proximal and a distal row of small bones, and a diminutive centrcde. proximal row is made up of four bones, which are (beginning on the inner (preaxial) side) — (a) radiale (scaphoid), and (/3) intermedium (lunar), articulating with the concavities on the radius, (y) ulnare (cuneiform), and (S) pisiform (not shown), articulating with the convexity on the ulna. The distal row is made up of four carpalia, which are, beginning as before, carpale 1 (trapezium), carpale 2 (trapezoid), carpale 3 (magnum), carpale 4 + carpale 5 (unciform). Five digits are present supported by five metacarpals, articulating with the corresponding carpalia, and completed by phalanges, of which the short pollex (1st digit) possesses 2, the others 3 each. The terminal phalanges are conical, and grooved to support the claws. ih) Hind-limb.— -The hip-girdle is formed by an innominate hone on each side, made up of four fused bones, ilium, ischium, puhis, and cotyloid. Upon the outer side of each innominate is a deep cup, the acetabulum, the floor of which is complete, and in the young rabbit, marked by the three-rayed junction of the ilium, ischium, and cotyloid. The ilium is placed above and in front. It is laterally flat- tened, and the sacrum articulates with its inner surface. The ischium is placed above and behind. It is separated from the pubis by the oval obturator foramen, and presents posteriorly a rough and thickened tuberosity. A plate-like expansion runs 282 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. from its hinder part towards the mid-ventral line, and unites there with its fellow to form the posterior i)ortion of the ischio- ]) uhic symphysis. The cotyloid is a very small hone helping to form the ventral side of the acetabulum, from which it shuts out the pubis. The small pubis runs from the cotyloid bone towards the mid-ventral line, and unites with its fellow to form the anterior part of the ischio-pubic symphysis. Free Limb. — The elongated femur presents a proximal head upon its inner (preaxial) side, which articulates with the aceta- bulum, and two distal condyles which articulate with the tibia to form the knee-joint. The actual proximal end of the femur is a large projection known as the great trochanter. There is a lesser trochanter below the head, and opposite this on the outer (post- axial) side a third trochanter. A large patella is present on the front of the knee-joint, at the back of which are two other sesamoid bones, the fabellae. The crus is su})ported by the large preaxial tibia, which pos- sesses two })roximal concavities for the femur, and two distal surfaces for the tarsus. With the tibia a small, postaxial, rod- like fibula, is completely fused distally. The ankle-joint in the rabbit is between the distal ends of the above bones and the tarsus. The tarsus is made up of proximal and distal rows partly separated by a large centrale (navicular). The proximal row contains two bones, the preaxial (a) tihialo- intermedkim (astragalus), and postaxial (/3) fihidare (calcaneum) produced into a large projecting heel. The distal row is com- posed of three bones, which are, beginning preaxially, (a) tarsale 2 (mesocuneiform), ()3) tarsale 3 (ectocuneiform), and (y) tarsale 4 -j- tarsale 5 (cuboid). There are four digits, the first or hallux being absent. Each possesses a metatarsal, the first of which is produced proximally into a process, originally separate, which may represent the miss- ing hallux. Each digit is completed by three phalanges, similar to those of the manus. Small nodular sesamoid hones are developed in the tendons, opposite the joints on the under (ventral) side of manus and pes. 4. The digestive organs (Fig. 85) consist of an alimentary MAMMALIA. 283 canal (mouth-cavity, pharynx, gullet, stomach, intestine) Avith appended glands (salivary glands, liver, pancreas, and rectal glands). They begin in the head and thorax, but are contained, Fig. 85. — General Dissection of Head and Thorax of Rabbit (reduced). — a, Atlas; ax, axis; 1 to 11, placed on centra of thoracic vertebrie ; h-o, basi-occipital ; pt, is placed on basi-sphenoid, and 2, on pre-sphenoid ; l.p, lamina perpendicularis ; e-t, ethmo-turbinal, and m-t, maxillo-turbinal, of right nasal cavity, the turbinal bones of left cavity having been removed and the septum nasi cut through ; p-mx, pre-niaxilla ; h.p, bony palate ; x-st, xiphisternuin ; d,d,d, diaphragm ; incisors ; t, tongue ; p.f, papilla foliata, above which is seen a circum- vallate papilla ; * placed just below tonsil ; ph, pharyux ; p.n, internal nares, the opening of p.n.c, the posterior nasal chamber, which is mostly floored by soft palate — the letter n is placed just under Eustachian opening ; a>, oesophagus ; l.a, left auricle ; l.r and r.v, left and right ventricles ; pr-c, left pre-caval ; e./u, right external jugular, formed in front by union of facial veins, and running back to help to form right pre-caval ; p-c, ])Ost caval ; ao, arch of aorta — left common carotid is seen running forwards along trachea {tr is placed just above it) ; 8-d, left subclavian artery ; ao', dorsal aorta ; pa and pv, pulmonary artery and veins — left lung has been removed ; ep, epiglottis ; x , placed on vocal chord ; tr, trachea ; a little way behind ao, opening of right bronchus, end of trachea, and left bronchus cut open, are seen ; Uj, lresents a convex margin, the (jreater curva- ture, and anteriorly a concave margin, the lesser curvature, to the left of which is the cardia, or opening of the gullet. The pylorus where the duodenum commences, is at the extreme right of the pyloric end. The mucous membrane lining the stomach is raised into irregular longitudinal ridges, or rugae. Between stomach and duodenum there is an inwardly projecting rim, the pyloric valve. The stomach is succeeded by a much convoluted intestine, some 15 or IG times the length of the body, and divided into small and large. The first part of the thin-walled small intestine,* the duodenum, forms a longitudinally directed U-shaped loop nearly the length of the abdominal cavity. It passes insensibly into the much longer ileum, in the wall of which there are oval thickenings, Feyers jMtches, here and there. Tliis part of the intestine finally opens into the csecum {see below), and at this point its wall is much thickened to form the sacculus rotundus. The mucous membrane of the small intestine is raised into numerous irregular transverse ridges, tlie vcdvulm conniventes, from which project an immense number of villi. The large intestine is divided into csecum, colon, and rectum. The ecec'um is a very large thin-walled sac, nearly 2 feet in length, interposed between the ileum and colon. Its wall is marked externally by a spiral groove. One end, near which the ileum opens, is continuous with the colon, while the other terminates- blindly in a thick-walled finger-like vermiform appendix. A spiral fold, corresponding to the external groove, projects into the cavity of the cascum, with which cavity the sacculus rotundus com- municates by a small rounded aperture (ileo-colic valve), and which opens- by a larger aperture into the colon. The lining mucous membrane is raised into minute elevations ; in other words, is papillose. The colon is a moderate-sized thin-w^alled tube with baggy walls, along which three smooth longitudinal bands run. It passes insensibly into the narrow rectum, which, after some con- volutions, runs back within the pelvic cavity, below the backbone, and above the urinogenital organs, to the amts. The mucous membrane lining the colon is papillose, that lining the rectum * In human anatomy this is divided, somewhat arbitrarily, into three parts — duodenum, jejunum, aud ileum. The two last are here included in. the term “ ileum.” MAMMALIA. 287 is smooth at first, but raised into prominent longitudinal ridges near the end. The following large glands are connected with the alimentary canal — salivary glands, liver, pancreas, and rectal glands. Four pairs of salivary glands open into the mouth-cavity, — parotid, submaxillary, sublingual, and infra-orbital. (1) The parotid glands are irregular pinkish masses, extending, on each side, from the origin of the pinna to the higher end of the mandible. From each a delicate (Stenson’s) duct runs forwards just below the skin, turns inwards, and opens into the side of the mouth-cavity opposite the last upper molar. (’J) The suh -maxillary glnnds are oval and comj)act bodies, of reddish colour, lying near together between the rami of the mandible, in front of the larynx. A delicate (Wharton’s) duct runs from the outer side of each to the floor of the mouth, in front of the tongue, (o) The small red suh-lingual glands are elongated and flat. They lie in front of and above the sub-maxillary glands among the muscles of the tongue. (4) The infra-orhital glands are irregular and lobulated. They lie, one on each side in front of and below the eye, partly within the orbit, A duct runs down- wards from each and opens into the side of the mouth-cavity, opposite the last upper pre molar. The liver is u very large reddish-brown organ, closely applied to the concave abdominal side of the diaphragm in front, and largely overlapping the stomach behind. Right and left halves may be distinguished in the liver. The former is divided into two lobes, right central, — and caudate, hollowed out for, and abutting against, the right kidney. The left half is divided into three lobes, left central, left lateral, and the very small Spigelian, fitting into the an- terior concavity of the stomach. A pear-shaped, thin-walled gall-bladder lies in a slit on the posterior surface of the right central lobe. From it a short cystic duct opens into the bile-duct, a small tube formed by the union of hepatic ducts from the various liver-lobes, and opening into the duodenum a little way beyond the pylorus. The pancreas is not compact as in the frog and pigeon, but diffuse — i.e., made up of numerous scattered lobules. These resemble small masses of fat, and are found in the mesentery between the limbs of the U formed by the duodenum. Delicate ducts proceed from the various lobules, and by their successive unions the pancreatic duct is formed, which opens into the distal limb of the duodenum, about 3 inches from the bend. The rectal glands are two compact elongated bodies lying alongside and above the rectum near its termination, and opening into it. The alimentary canal is lined as usual by mucous memhraney 288 AN ELEMENTARY TEXT-BOOK OF BIOLOGY, external to which are submucous and muscular coats, and, in the abdominal organs, there is a thin serous layer outside all. The mucous membrane consists of epithelium internally, which, as far as the stomach, is stratified squamous, but in that organ and the intestines, simple columnar. Gohlet cells are everywhere common, peptic glands are found in the stomach, and glands of Lieberkuhn throughout the intestines. Each peptic gland opens on the surface of the stomach by a narrow neck, lined with columnar epithelium. It branches at its deep end, and the branches are lined by granular cuboid chief cells, among which are scattered (in the glands of the cardiac end) a much smaller number of ovoid cells. Beneath the epithelium is a layer of connective tissue surrounding the glands, and the external limit of the mucous membrane is marked (in the ■stomach and intestines) by a very thin sheet of unstriated muscle, the muscularis inucoste. The submucous coat is made up of loose connective tissue, traversed by vessels and nerves. The muscular coat varies very much in thickness, attaining its maxi- mum in the stomach. It consists of an inner circular and an outer longi- tudinal layer in the gullet and succeeding parts. The fibres are striated in the gullet and pharynx, and the tongue is mainly composed of such fibres ; but the muscle in the walls of the stomach and intestines is made up of imstriated fibres. The salivary glands and pancreas consist of a number of lobules united together by connective tissue. Each lobule is composed of numerous acini, blindly ending tubes lined by glandular epi- thelium, from each of which a duct, lined by simple columnar epithelium, proceeds. These ducts unite again and again to form the main duct of the gland. The preceding type of structure is termed racemose. The liver, as in the frog and pigeon, is mainly made up of polyhedral, granular, hepatic cells (Fig. 86), placed in the meshes of the network formed by the hile-capillaries, from which the hepatic ducts arise. The hepatic cells are aggregated into small .lobules, around each of which is an interlobular capillary network formed by the ultimate branches of the ^lortal vein. From this network veinlets pass into the centre of the lobule, and there unite into a small intralobular vein which carries away its blood. By the union of these small vessels sublobular veins are formed, which are the factors of the hepatic veins. The food consists of vegetable substances, and the great length “of the alimentary canal is correlated with this, an immense absorptive surface, largely augmented by the caecum with its .spiral valve, being given. The food is divided by the incisor teeth, and ground up by MAMMALIA. 289 the molars and premolars. The hard tongue, working against the ridged palate, assists in this process of mastication. Saliva is poured by the salivary glands into the mouth-cavity, and not only lubricates the food, but also, in virtue of a ferment, ptyalin, which it contains, converts more or less of the starch into soluble grape-sugar. The (jastvic juicc^ bile, and pancvcatic juice act as in preceding cases (p. 194). 5. Circulatory Organs. — Well developed blood and lym])h systems are present. (1) Blood System. — The blood consists of liquid plasma, in which are suspended colourless corpuscles of the usual type, and Bile-capillary. Nucleus of a liver-cell. Bile- capillary divided. Bile-capillary. Fig. 86. — Histology of Rabbit’s Liver (after Hering). — Relation of liver-cells, bile-capillaries, and blood-capillaries. a much larger number of smaller red corpuscles, which are bicon- cave, non-nucleated discs. The blood-containing tubes comprise heart, arteries, veins, and capillaries. The conical heart (Fig. 85) enclosed in its jiericarclium, lies between the lungs on the ventral side of the thoracic cavity, with its apex directed backwards. It contains four chambers, two thin-walled auricles, and two thick-walled ventricles. The broad base of the heart is chiefly made up of the dark, thin-walled right and left auricles, to each of which is attached a plaited fold, the auricular appendix, while the rest of the heart is made up by the firm 2 19 290 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. right and left ventricles. The internal structure of the heart is in most ])articulars like that of the pigeon (p. 245), but the right auriculo- ventricular valve is not muscular but made up of three membranous flaps (giving the name tricuspid valve), connected by chordae tendinese with papillary muscles in the right ventricle. These muscles are also found in the left ventricle, and are more numerous than is the case in the pigeon. The opening of the postcaval is guarded by a fold, the Eustachian valve, and that of the left precaval by a similar Thebesian valve. Arteries. — The aorta takes origin from the left ventricle, and its arch curves round to the left to form the dorsal aorta, giving off vessels which supply head, neck, and fore-limbs. The dorsal aorta runs back ventral to the backbone, supplying the trunk with its contained viscera, and finally forks into two iliac arteries, which supply adjacent parts and continuations of which run into the liind-liml )S. The aorta dilates at its origin into the 3 sinuses of Valsalva, situated behind the pouches of the semilunar valves. Two coronary arteries, for the supply of the heart-walls, are given off here. Near the beginning of the arch a short innominate artery runs off, which at once gives off the left carotid, and soon after divides into right subclavian and right carotid arteries. The left subclavian arises from the left side of the arch. Each carotid runs forwards on one side of the trachea, giving off branches to the neck as it does so, and divides in front into internal carotid for the brain, and external carotid for the outside of the head. The subclavian arteries supply the fore-limbs, on entering which they become the brachial arteries. A vertebral artery runs dorsal wards from each subclavian near its com- mencement, and enters the vertebrarterial canal to supply the brain and spinal cord. The dorsal aorta gives off in the thorax small paired intercostal arteries to the thoracic walls. In the abdomeu a large unpaired cediac artery takes origin a little way behind the diaphragm, and quickly divides into the hepatic artery for the liver, and lieno-gastric artery for the spleen and stomach. Another unpaired artery, the anterior mesenteric, for the small intestine, pancreas, caecum, and colon, arises a short distance posterior to the coeliac. Paired renal and spermatic (or ovarian) arteries are next given off to the kidneys and spermaries (or ovaries), aud still further back a small unpaired posterior mesenteric artery to the rectum. The dorsal aorta bifurcates at the posterior end of the abdominal cavity into the iliac arteries, each of which, after giving off an ilio-lumbar artery to the body- wall, divides into an internal iliac artery for the pelvic region and an external iliac artery, which, after giving off a branch that supplies the bladder (and uterus in the female), enters the thigh as the femoral artery for the hind-limb. Shortly before the aorta bifurcates, it gives off from its upper surface a small median sacral artery, which runs back into the tail. The pulmonary artery arises from a forward prolongation of the right ventricle on the ventral side, and, curving round the MAMMALIA. 291 front of the heart, divides into right and left branches for the corresponding lungs. Veins. — Caval, hepatic portal, and pulmonary systems can be distinguished. (a) Caval System. — Kight and left precavals (anterior venae cava3) and a postcaval (posterior vena cava) carry the impure blood from the general system into the right auricle. The precavals drain the head, neck, and fore-limbs, the postcaval the rest of the body. Each precaval is formed by the union of an external jugular, with a subclavian vein. The former runs back alono; the side of the neck from the angle of the jaw, where it is formed by the union of facial veins returning blood from tlie outside of the head. Just before uniting with the sub- clavian it receives the small internal jugular vein from the brain. The subclavian vein brings back blood from the foredimb. The right precaval is joined by an unpaired azygos vein, which lies just beneath the vertebral column in the thorax, and carries off blood from a large part of its walls. The left precaval receives a coronary vein from the heart-walls, just before entering the right auricle. The postcaval is a very large vein formed at the posterior end of the abdomen by the union of the internal iliac veins from the backs of the thighs. It is joined almost at once by the external iliac veins, which return blood from the hind-limbs and bladder (also uterus in the female). They are direct continuations of the femoral veins of the thighs. The postcaval runs forwards in the abdomen close to and on the right of the aorta, receiving successively llio-lumbar, spermatic (or ovarian), and renal veins, from the body- walls, spermaries (or ovaries), and kidneys respectively. Before reaching the diaphragm it turns ventral wards, runs through the dorsal part of the liver (from which it receives four chief hepatic veins), and pierces the diaphragm ventral to the gullet. In the thorax it runs forwards between the lungs and finally enters the right auricle. (b) Hepatic Portal System. — The portal vein is formed by the union of Ueno-gastrlc, duodenal, anterior mesenteric, and posterior mesenteric veins, which return blood from the stomach and spleen — duodenum and pancreas — ileum and most of large intestines — and last part of rectum, respectively. The portal vein soon divides into branches, which break up in the various liver-lobes. (c) Pulmonary System. — There are two pulmonary -veins from each lung. All four open into the dorsal side of the left auricle. The muscle of the heart (Fig. 87) is composed of transversely- striated fibres, devoid of sarcolemma, which are united into a close network. This is marked into short lengths corresponding to the constituent cells, and in each of these a muscle-corpuscle, not placed superficially, is imbedded. The arteries and veins are 292 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. constructed as described on p. 202. The veins contain numerous valves, which are flap-like j)rojections of their inner coat. The capillaries are networks of fine tubes (c/. p. 202), con- necting the ultimate branches of arteries and veins. Course of the Circulation (c/. p. 247). — The impure blood of the body is returned by the caval veins to the right auricle, tlience passing into the right ventricle and into the lungs through the pulmonary artery. The oxygenated blood is returned to the left auricle by the pulmonary veins, then enters the left ventricle to reach the aorta, the branches which take it to all parts of the 1)0 dy. (2) Lymph System. — The lymph, as in other cases, resembles blood, were it not for the absence of red blood-corpuscles. It is contained in the minute lymph-spaces found in most of the tissues, larger lymph-cavities, and lymph-vessels opening into a thoracic duct. The lymphatic vessels resemble small veins in structure, and are of two kinds — (a) lymphatics, belonging to the body generally ; (/3) lacteals, belonging to the gut, and com- mencing in blind branched tubules, one of which is found in [Fig. 87. — Muscle-Fibres from Mammalian Heart (from Landois and Stirling), much enlarged. — 8, Side view; 9, cross-section. each intestinal villus. The thoracic duct, to which the lacteals and most of the larger lymphatic trunks run, is a slender tube lying- above the dorsal aorta in the thorax, and opening anteriorly into the junction between the left subclavian and external jugular veins. Some of the lymphatics unite to form a much smaller trunk, opening similarly on the right side. The most important large lymph-space is the ccelom, here divided up into abdominal,, pleural, and pericardial cavities. MAMMALIA. 293 Numerous small lymphatic glands, which resemble the cervical glands of the pigeon, are present in various situations. Those belonging to the lacteals are known, from their position, as mesenteric glands. An elongated flattened spleen, of a dark-red colour, is attached by a flap of mesentery to the cardiac end of the stomach. The thymus gland is a fat-like mass, largest in young animals, closely connected with the base of the heart. The thyroid gland consists of two small elongated bodies of reddish colour, closely applied to the sides of the front end of the trachea, and con- nected together by a ventral bridge of tissue. New colourless corpuscles are developed in the lymphatic glands, thymus, and spleen, new red corpuscles in the red marrow of bones, and from pre-existing colourless corpuscles. The worn- out red corpuscles are probably broken down in the spleen. 6. The respiratory organs (Fig. 85) are lungs contained in the thorax, and communicating with the exterior by a trachea, the front end of which is modified into an organ of voice (larynx). The slit-like glottis, situated at the base of the tongue, and guarded in front by an elastic flap (epiglottis), leads into the larynx. This is supported by a broad thyroid cartilage (which is a bent plate, incomplete dorsally), and, posterior to tliis, by a ring-like cricoid cartilage, the dorsal side of which is thickened. Two elastic folds, the vocal chorals, project from the sides of the larynx. They are attached below to the thyroid cartilage, above to two small arytc7ioid cartilages, which articulate dorsally with the front edge of the cricoid. The larynx may be regarded as the modified front end of the windpipe or trachea, which runs along the neck ventral to the oesophagus, and is supported by cartilaginous hoops. The trachea bifurcates within the thorax, just anterior to the base of the heart, into two bronchi. The lungs are two spongy bodies, pink in colour, which mainl}' fill the thoracic cavity, as long as its walls remain intact. The left lung is subdivided into two, the right lung into four lobes of very unequal size. Each lung is enveloped by a delicate mem- branous pleura, which consists (like the peritoneum) of a visceral layer, closely applied to the surface of the lung, and a parietal layer, lining one half of the thorax. The two layers are continuous at the root of the lung. 294 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. The parietal layers of the two pleurae meet together in the middle line to form, in the posterior part of the thorax, a longitudinal vertical partition, the mediastinum. The two sheets of which this is composed diverge further forwards, and leave between them the mediastinal space, occupied by the heart in its pericardium. The bronchi enter the anterior end of the lungs, and, like the trachea, are supported by cartilage. Each of them forms a bronchial tree in its lung by giving off’ alternating right and left branches. The diaphragm is a thin partition which forms the posterior boundary of the thorax, and separates it from the abdomen, towards which it is strongly concave when at rest. The centre of the diaphragm is transparent and tendinous [i.e., composed of white connective-tissue fibres), while its margins are muscular, especially on the dorsal side, where they are continued into two muscular pillars, which take origin on the under sides of the lumbar vertebrae. Histology. — The branches of the bronchial tree divide repeatedly (the supporting cartilages being at the same time gradually lost), to form delicate bronchial tubes. These end in clusters of dilations, the infundibula, with sacculated walls. They are lined by simple squamous epithelium, beneath which is a very rich network of capillaries. The trachea and larger air-passages are lined by stratified columnar epithelium, ciliated in ])art. The lungs contain a large < quantity of elastic connective tissue and unstriated muscle. Respiration. — Inspiration and expiration are }>artly effected by movements of the ribs, and partly by movements of the diaphragm, and respiration is hence said to be both costal and diaphragmatic. It must be remembered that each lung is enclosed in an air- tight bag, the goleura, and is obliged to follow all the movements of the thoracic walls, or else a A^acuum Avould be formed between the two laA^ers of this. The elastic tissue in the lungs causes them to be always trying to contract, but this is prevented by the pressure of the air in the lung-passages. If, however, the pleura is perforated, this })ressure is counterbalanced and the lung at once contracts. Inspiration. — The ribs and sternum are moved dowinvards and forwards, and thus increase the dorso-ventral capacity of the tliorax. At the same time the muscular parts (especially the pillars) of the diaphragm contract, thus fiattening it and increas- ing the antero-})osterior capacity of the thorax. MAMMALIA. 295 Air consequently rushes into the larger air-passages from the exterior. Expiratim is the exact converse of this, but is largely passive, the various muscles ceasing to contract. It is aided by the elasticity of the lungs and walls of the thorax. As a result, the air rushes out again. The air passes directly in and out of the larger air-passages only. The rest is effected by diffusion, and the essential part of respiration (exchange of gases) is carried on in the infundibula. Kespiration is less active than in the pigeon, but here also the body is maintained at a temperature (about 100°F.) much above that of the surrounding medium. 7. The urino-genital organs consist of excretory and repro- ductive organs. Excretory Organs (Fig. 89). — In both sexes two oval, compact, reddish-brown kidneys are closely applied to the dorsal wall of the abdominal cavity, the right rather further forwards than the left. The}'’ are covered ventrally by peritoneum. Each kidney presents a notch, the hilus, on its inner margin, from which the ureter, a narrow tube with muscular walls, proceeds. The ureters open obliquely into the dorsal side of the pear-shaped urinary bladder, towards its narrow portion or necl\ The walls of the bladder are translucent and muscular. It projects into the body-cavity just in front of the symphysis pubis. A median horizontal section of either kidney shows that it is differentiated into a marginal cortex, dark red in colour, with numerous small dots (Malpighian bodies), and a central paler medidla, which presents a striated appearance. The striae con- verge internally towards a conical eminence, the urmary j^yTamid, which projects into the^e/m, or dilated end of the ureter. The kidney is essentially made up of uriniferous tubules (Fig. 88), each commencing with a BoicmaEs capstde, which with its glomerulus constitutes a Malpighian body, and is situated in the cortex. The nech of the capsule passes into a thickened glandular part, also in the cortex, from which a narrow part runs into the medulla, and then loops back into the cortex to dilate into a second glandular part, which passes into a collecting part. The collecting parts run towards the urinary pyramid, often uniting in their course, and finally open upon it. * Fig. 88. — Diagram of Blood-vessels and Ukinifekous Tubules of I Kidney (from Landois and Stirling). — A and B, Capillaries of cortex i and medulla. On the left numerous glomeruli are shown with (1) I afferent artery ; (2), efferent vein. The glomeruli are enclosed in Bowman’s capsules, x, x, x, and n, n, 7i, glandular parts, and o, o, o, collecting parts of tubules ; 0, opening on urinary pyramid. MAMMALIA. 297 The renal artery divides within the kidney, and from its branches small afferent arteries run to Bowman’s capsules and break up into glomeruli. A small efferent rein arises from each glomerulus. The efferent veins now break up into a network of capillaries surrounding the glandular parts of the tubules, and the factors of the renal rein take origin in this network. Fig. 89. — Ukinogenital Organs of Eabbit. — A, male; B, female; C, horizontal section of right kidney ; co and md, cortex and medulla of kidney (in C) ; x , placed on urinary pyramid; «r, ureter; u.h, urinary bladder; r, rectum; r.gl, rectal gland; a, anus; p.s.^ perineal space; p.gl, perineal gland (in A, the aperture is shown by a dot); t, sper- maries (testes); ep and ep', caput and cauda epididymis; gu^ guber- naculum; v.d, spermiduct (vas deferens); ut.m., uterus masculinus; pr, pr\ and pr'\ lobes of prostate; u.-g.c^ urinogenital canal; c.c, cor- pus cavernosum; gl.p, glans penis; ov, left ovary; F.t, left Fallopian tube; F.t\ funnel of ditto; left and right uteri, the latter cut short; o.xl and o.u\ the mouths of the left and right uteri, to show which the vagina, va, has been cut open; vh, vestibule; gl.cl, glans clitoridis. Excretion (c/. p. 206). — The nitrogenous waste-products ai-e urea (CH4N2O) and hippuric acid (CgHf^NOg). The male reproductive organs (Fig. 89) consist of spermaries (testes), from which spermiducts (vasa deferentia) proceed. These communicate with a urinogenital canal traversing a cylindrical penis. Other accessory parts are present. I 298 A\ ELEMP’.NTARY TEXT-BOOK OF BIOLOGY. The oval spermaries (testes) are contained in the scrotal sacs^ which, in the l)reeding season, project at the sides of the urino- genital aperture. Each spermary is attached to its sac by a short fibrous cord {guhernaculum), and is also suspended by a mesenteric fold (mesorchium). Attached to the spermary is a compact mass of coiled tubules {epididymis), divided into the caput epididymis in front, and the cauda epididymis behind. These cap the anterior and posterior ends of the testis, and are con- nected by a tubule running along its surfixee. The spermatic artery and vein are connected with the anterior end of the testis. They form, with nerves, connective tissue, &c., the spermatic cord. The spermiduct (vas deferens) proceeds from the cauda epidi- dymis, runs forwards, and then curves over the ureter to enter the uterus masculinus, a large, slightly bilobed sac, which opens into the dorsal side of a urinogenital canal (urethra), a backward continuation of the neck of the bladder. After receiving the uterus masculinus, the ducts of a five-lobed prostate gland, and, further back, the 0})enings of two small Coivper^s glands, the urinogenital canal traverses the penis, an elongated backwardly- directed structure, the ventral side of which is strengthened by two filjrous rods, the corpora cavernosa, attached in front to the ischia. The dorsal side of the penis is made uj) of a mass of vascular tissue, tlie corpus spongiosum, through which the urino- genital canal runs, and which projects beyond the corpora caver- nosa as the glans p)cnis, a soft conical body, upon which is the slit-like urinogenital aperture. The spermary is essentially composed of seminiferous tubules lined l)y germinal ei)ithelium, the cells of which divide frequently to form sperms, each producing several, but not being entirely ' used up in the process. The sperms (spermatozoa) have oval tiattened heads and vibratile tails. The tubules of the spermary unite to form rasa efjerentia, which make up part of the epididymis. The female reproductive organs (Fig. 89) consist of ovaries,' oviducts, vagina, and urinogenital canal, together with accessory parts. The ovaries are whitish, oval bodies, placed one on either side of the dorsal abdominal wall, posterior to the kidneys. In a mature female, Graafictn (ovarian) follicles appear as clear, rounded projections on their surfaces. Near each ovary is the fringed MAMMALIA. 299 funnel-shaped opening of the corresponding oviduct. This con- sists of a narrow convoluted Fallopian tuhe, which merges into a cylindrical uterus, with thick muscular walls, which may be of very large size in the pregnant female, and then exhibits a series of oval swellings, in each of which an embryo is contained. The two uteri open by separate apertures {ora uterorum) with rounded projecting margins, into the vagina, a large tul)e which rims back dorsal to the bladder, and becomes continuous with the urino- genital canal (vestibule), a somewhat smaller but still wide tube, which runs back from the bladder in the same relative position as the narrow urinogenital canal of the male. The walls of this tube are very vascular, and it ends in an elongated urinogenital aperture (vulva). Imbedded in the ventral wall of the vestibule is a small elongated body, the clitoris, supported by corpora cavernosa, similar to, but very much smaller than, those of the penis, and similarly situated. The clitoris ends in a soft, flattened glans clitoridis. Small Coivper^s glands open into the dorsal wall of the vestibule. The ovary consists of Graafian follicles in various states of development imbedded in a connective-tissue basis or stroma. The mature follicles project from the surface of the ovary, and each, when mature, is a fluid-containing vesicle, the wall of which consists of a fibrous coat, internal to which are two or more layers of columnar epithelial cells (membrana granulosa). The ovum is placed on the outer side, and is surrounded by several layers of these cells, with which it forms a mass, the discus p)roligerus, that projects into the cavity of the follicle. The ripe ovum is invested by a radiately striated membrane, the zona radiata, l)ut no distinct traces remain of the vitelline membrane present in the young ovum. The vitelhis contains hardly any food-yolk, and within it is a large germinal vesicle with germinal spot. The ova burst from their follicles into the body-cavity, and are taken up by the funnels of the Fallopian tubes. Fertilization is effected in the upper part of these tubes, by fusion with sperms previously ejected l)y the male into the female passages, the penis being used as a copulatory organ. 8. Muscular System (c/.p. 208). — The muscles are very numerous and arranged in a very comjdicated manner. The voluntary muscles are from their appearance divided into red and white, the darker colour of the former being due to their richer blood-supply. 300 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. The striated muscle-fibres are smaller than those of the frog {cf. p. 208), and the muscle-corpuscles are situated superficially, just beneath the sarcolemma. The fibres of the red muscles are larger than the rest. 9. The Nervous System (Fig. 85) consists of cerebro-spinal axis, cranio-spinal nerves, and sympathetic system. Fig. 90. — Histology of Striated Muscle (from Landois and Stirling), all much enlarged. — 1. Part of a fibre — Q, transverse striations; F, primitive fibrillje ; K, muscle-corpuscles; S, sarcolemma; N, an entering nerve-fibre, with a, axis cylinder, and e, end-plate. 2. Part of a fibre in cross-section — K, muscle-corpuscle. 3. Primitive fibrillce. (1) The cerebro-spinal axis comprises brain and spinal cord contained in the neural canal and invested by three membranes, — pia mater carrying blood-vessels for supply of the nersmus tissue, arachnoid, a delicate mesh-work traversed by lymph-spaces, and firm dura mater lining the neural canal. MAMMALIA. 301 The brain is elongated, but not particularly well developed. It presents the usual division into fore-, mid-, and hind-brain. Fore-Brain. — The thalamencephalon possesses two j)arts not present in preceding types. One is the middle commissure, a broad band of grey matter connecting the side- walls of the third ventricle between the anterior and posterior commissures. The other is a rounded eminence, the corjms mammillare, seen on the under surface immediately behind the j^^luitary hody, which is attached as previously to the infundihdum. With the hinder part of the thin roof of the third ventricle the pineal gland is connected by a hollow bifurcated stalk. The cerebral hemispheres are broad behind and taper in front to blunt points. Externally they are almost smooth, but each is divided by grooves into three lobes, frontal, parietal, and temporal, placed respectively in front, postero-dorsally and postero-ventrally. As, in the frog and pigeon the hemispheres are closely applied in the middle line, but here, in addition, they are firmly united by an elongated band of transverse fibres, the corpus callosum. Below this and continuous with it behind is another fibrous band, the body of the fornix, which divides in front into four smaller bands, t\\Q pillars of the fornix. Two of these, the anterior -pillars, pass downwards in the lamina terminalis and substance of the optic thalami to reach the corpus mammillare, while the other two, posterior pillars, curve outwards into the hemispheres. Each lateral ventricle is divisible into three parts — a narrow anterior cornu in front, and towards the inner side — a posterior cornu, similarly placed behind, and a descending cornu, which passes down into the temporal lobe. A prominent oval mass, the hippocampus major, projects into the last-named cornu, and along the anterior edge of this the posterior pillar of the fornix runs. In front of this pillar there is a vascular fold, the choroid plexus, which extends into the ven- tricle from the pia mater. The corpus striatum forms the outer side and floor of the anterior cornu. The cerebral hemispheres fuse where they meet in front of the lamina terminalis, and form a party-wall, the septum lucidum, to the hinder parts of the anterior cornua, immediately in front of the foramen of Monro on each side. This partition is not solid, but contains a small slit-like ventricle. This is not, therefore, like the other ventricles, a part of the cavity of the original neural tube of the embryo. These were originally named 1st and 2nd {i.e., the two lateral), 3rd and 4th ventricles, and when another brain-cavity was discovered it naturally received the name 5th ventricle. The olfactory lobes are elongated and dilated at their ends. They are attached to the under sides of the frontal lobes, and run forwards in front of them to rest on the cribriform plate. 302 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. Each contains a small olfactory ventricle continuous with the corresponding anterior cornu. The mid-brain is traversed hy the Sylvian aqueduct. Above it presents the optic lobes (corpora ((uadrigemina) which are over- lapped by the cerebral hemispheres. Each of them is subdivided into two, so that there are four elevations, tv^o anterior and larger nates, and two posterior testes. There are no optic ventricles. Two very distinct longitudinal masses of fibres, the crura cerebri, form the floor of the mid-brain. Hind-brain. — The bulb (medulla oblongata) presents well- marked dorsal and ventral fissures bounded by narrow bands, the dorsal and ventral qyyramids. The dorsal pyramids diverge in front and sweep round the sides of the thin roof of the fourth ventricle, on each side of which is a projection, the corpus resti- forme. On the under surface the ventral fissure and pyramids are interrupted by a broad l)and of transverse fibres, the ])ons Varolii, just behind the crura cerebri. Immediately behind this there is a rectangular area, the corpus trapezoideum, on each side of the ventral pyramid, occupied by transverse fibres. The cerebellum is very large and made up of a median vermis marked by deep transverse furrows, and a pair of much convoluted lateral lobes, to each of which a small rounded fiocculus similarly furrowed, is attached. The cerebellum is united to the rest of the brain by three ])airs of commissures or peduncles. The anterior peduncles connect it with the testes, the middle pedun- cles wdth the pons Varolii, and the posterior peduncles with the corpora restiformia. The spinal cord is cylindrical, and possesses dorsal and ventral fissures. There is no sinus rhomboidalis. (2) Cranio-Spinal Nerves. — There are twelve pairs of cranial nerves, as in the pigeon, which take origin and are distributed in a similar manner. The olfactory lobes send numerous fibres through the pores in the cribriform plate to the olfactory mucous membrane. The optic nerves form a chiasma, but the optic tracts are relatively narrow. The trigeminal nerves arise from the sides of the pons, and the facial and auditory from the sides of the corpora trapezoidea. The abducent nerves come off from the extreme front of the ventral pyramids. The exits of the cranial nerves have already been described (p. 274). Outside the skull it may be noted that the mandibidar ramus of the fifth MAMMALIA. 3U3 sends a lingual branch to the tongue. The vagi run back along the neck by the sides of the common carotids. Each gives oft* to the larynx a su2)erior laryngeal nerve, from which a depressor branch runs, closely united with the sympathetic, to the heart. At the posterior end of the neck a recurrent laryngeal nerve is siven oft* which curves round the aorta on the left, or the sub- clavian on the right, and runs forwards to the larynx. Behind this the vagi run through the thorax and pierce the diaphragm, finally breaking up into branches for the abdominal viscera. The spinal nerves arise by two roots as usual, and are named cervical, thoracic, according to the region of the vertebral column to which they correspond. The last four cervical and the first thoracic nerves form the brachial pdexus, from which the fore-limb is supplied, and from which also arises the gdirenic nerve, which runs back to supply the diaphragm. The lumbosacral plexus for the hind-limb is constituted by the last three lumbar and the first three sacral nerves. (3) The sympathetic system is formed by a ganglionated cord on each side {cf. p. 215) of the body, beginning in the head, traversing the neck, and then running ventral to the vertebral column through the thorax and abdomen into the tail. These are connected by rami communi cantes witli some of the cranial nerves and with the spinal nerves. They are also connected by commissures with each other. In the neck there are only three ganglia, the posterior and middle cervical at the root of the neck, and the anterior cervical placed near and connected with the vagus ganglion. In the thorax two large splanchnic nerves run oft* from the cords, and, piercing the diaphragm, fuse witli a large caeliac ganglion, })laced just in front of the origin of the anterior mesenteric artery, and united with an anterior mesenteric ganglion just behind it.'" This latter ganglion is again connected with a posterior mesenteric ganglion situated near the origin of the posterior mesenteric artery. From these ganglia numerous branches run oft' to the abdominal viscera. The sympathetic also supplies the vascular system. 10. Sense Organs — (I) Tactile Organs. — Numerous touch-coi'- are present in the skin (p. 271), especially at the bases of the vibrissae. * Ccdiac ganglions anterior mesenteric ganglion = semilunar ganglion of human anatomy. 304 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. (2) Gustatory Organs. — Imbedded in the sides of the circum- vallate papillae and papillae foliatae are numerous taste-hudsy con- taining elongated taste-cells, connected with glossopharyngeal fibres. (3) Olfactory Organs. — The nasal cavities (Fig. 85) are largely lined by olfactory mucous membrane, which extends over the ethmo- turbinals, and contains elongated olfactory cells (cf. Fig. 62). (4) Auditory Organs. — (a) The external ear consists of must not be confounded with the cleft upper lip of rabbit and hare. The upper boundary of the mouth is formed to begin with by a median fronto- nasal process and a maxillary pro- cess growing forward on each side (c/. Fig. 94) from the mandibular arch. This is the permanent condition in a dogfish (Fig. 46), but in chick, rabbit, and man, the fronto-nasal and maxillary processes fuse together and con- stitute a continuous ridge within which premaxillae and maxilla? develo]). Failure of this union on one side is the cause of sincrle o hare-lip, Avhile failure on both sides results in double hare-lip. The cleft in a rabbit’s lip is median, and does not affect the hard jmrts. There arc no papilla? foliata? on the tongue, the circumvallate papilla? are from 7 to 12 in number, and arranged on the pos- terior part of the back of the tongue in the form of a V with backwardly directed apex. The upper and lower teeth are arranged in two horseshoe- shaped curves. There are no gaps, and the canines are not dis- proportionately large. The dental formula is — Fig. 102. — Max. Junction of Small and Large Intestines (from Mac- aZ/s^er.—l, Colon ; 2, c£ecum ; 3, vermiform appendix ; 4, ileum. O 9 9 9 ’ p.m. 9 _ 9 mJ ^ 9 — 3-3 ^‘3 1 9 C l-I 1-1 ’ m 330 AN Er.EMKNTARY TEXT-BOOK OF BIOLOGY. The premolars are termed bicuspids. The caecum and appendix vermiformis are extremely small as compared with the same structures in rabbit. Fig. 103. — Man. Diagram to explain fate of Aortic Arches (from Macalister). — The parts in l)lack are obliterated. The large glands which open into the alimentary canal are sali- vary glands, liver, and i)ancreas. Infra-orbital salivary glands are not represented, the liver is not so deeply cleft into lobes, MAN. 331 and the duct of the compact pancreas joins the bile duct in the wall of the duodenum. 4. Circulatory System. — Three arteries come off from the aortic arch, (a) innominate, from which right subclavian and right common carotid arise, (b) left common carotid, (c) left subclavian. The fate of the embryonic aortic arches is shown in Fig. 103. 1st (= mandibular) arches become ophthalmic branches of external carotids. middle ) 2nd ( = hyoid) 3rd ( = 1st branchial) 59 meningeal ) ’■ internal carotids. 4th (=2nd branchial) aortic arch represented by a very variable arteria aberrans. 5th ( =3rd branchial) left \ f pulmonary artery, right J ” ” (^obliterated. left right 55 1 ) Veins. — The development of the caval system is illustrated by Fig. 104. There are to begin with anterior and posterior car Fig. 104. — Man. Diagram to explain Development of Systemic Veins (from Macalister). — Three successive stages are represented, R.C. and L.C, Cuvierian veins ; R.V.I., L.V.I., vena innominata ; S.V.C., superior vena cava; sup. i, superior intercostal; h.az, hemi-azygos. dinals, which, as in the adult Dogfish (cf. p. 165),^unite together on each side to constitute a Cuvierian vein which opens into the 332 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. heart. The anterior cardinals persist as the external jugulars; internal jugulars and subclavians are developed later on. So far there is agreement with the Rabbit, but in Man an oblique cross- connection is established between the junction of the left jugular and subclavian and the middle of the right Cuvierian vein. This cross-piece becomes the left innominate vein, the part of the right Cuvierian vein distal to it becomes the right innominate, and the part proximal to it becomes the superior vena cava. The left Cuvierian vein partly aborts, but its proximal end persists as a coronary sinus, which receives the coronary veins and opens into the right auricle, while its distal end becomes the superior inter- costal vein. Fig. 105. — Man. Development of Urinogenital Organs (from jLanrfoj.s and Stirling). — I, Undifferentiated condition — D, Gonad, resting on tubules of mesonephros, W ; M, Mullerian duct ; S, urinogenital sinus. II, Transformations in the female — F, Funnel, with hydatid (h'), of Fallopian tube, T; U, utero- vaginal region formed by fusion ; O, ovary, with parovarium, P, &c. Ill, Tranformations in male — h, Hydatid of Morgagni ; u, uterus masculinus ; H, spermary (testis), wdth vasa efferentia running to epididymis, E ; a, vas aberrans ; V, spermiduct (vas deferens). 4, Shows bladder (a) and rectum {d) opening into cloaca. 5, Later stage, where bladder {b) and urinogenital sinus (S) are separated from end of rectum (M) by a perineum (m). The posterior cardinals atrophy in the middle of their course ; their hinder ends become the internal iliac veins. The anterior part of the right posterior cardinal is converted into the a::ygos MAN. vein, Mdtli which is connected a hemi-o'jygos vein developed from part of the left posterior cardinal. The inferior vena cava (posterior v.c.) only becomes important when the hind-limbs have been developed to soine extent. It is formed by the union of iliac veins. 5. Urinogenital System. — The excretory organs of the embryo consist, as in the rabbit, of a mesonephros (Wolffian body) on each side, from which a mesonephric duct (Wolffian duct) runs back to open into an urinogenital sinus. The adult kidney is a metanephros, and the ureter (metanephric duct) is an outgrowth from the posterior end of the mesonephric duct. As before, the bladder is the proximal end of the allantois. The reproductive organs at first consist of a pair of gonads and two Mullerian ducts, and later on the excretory organs of the embryo in part become genital ducts, and in part are reduced to rudiments, which are of considerable importance from a medical point of view, since they may enlarge and become the seat of disease. The following table exhibits the most important details regarding the fate of the embryonic excretory organs and Mullerian ducts. Kudiments are italicized. As compared with the Kabbit it should be noticed that (1) in the male there arevesiculse seminales, while the uterus masculinu& is relatively small. (2) In the female the most important part is the single uterus, though in abnormal cases the Miillerian ducts may fuse less completely. 6. Nervous System. — The brain (Figs. lOG and 107) is exceed- ingly large, one result of which is to profoundly influence the shape and proportions of the skull, as already explained. This increased size is mainly due to the enormously developed cerebral hemispheres, which overlap and largely conceal the other regions, extending in all directions to such an extent that nothing else is visible in a brain viewed from above. In the rabbit the hemi- spheres leave the cerebellum quite uncovered, while they are nearly smooth and a division into lobes is only indicated. In each hemisphere of the human brain, on the contrary, six chief lobes can be distinguished, and its surface is marked by elabor- ately arranged convolutions (gyri) separated from one another by furrows (sulci). In this way the surface is 5^ times as great as if it were smooth, and the extent of the cortex is correspondingly increased. It is an established fact that the intelligence of an 334 AN ELEMENTARY TEXT-BOOK OF Adult Male. V^asa efferentia (coni vasculosi), Vasa aberrant ia and parepididymis (or- gan of Giraldes). Blindly ending tub- ules connected with the epididymis, Epididymis, . . . 8permiduct(vasdefe- ( rens) and branched < vesicula seminalis, { * Hydatids of Morgagni (one more constant and of larger size than rest). Situated between spermary (testis) and caput epididymis, Aborts, ? U nites with its fellow to form a small uterus masculinus (sinus prostaticus), Embryo. I. Mesonephros. 1. Anterior tubules, 2. Posterior tubules, . II. Mesonephric Duct. 1. Anterior part, . . 2. Middle part, . | 3. Posterior ,, . \ • • III. Mullerian Duct. 1. Anterior part, . . 2. Middle part, . . . 3. Posterior part, . . BIOLOGY. Adult Female. Parovarium (organ of | Rosenmilller, epob- phoron). A series of tubules (some of which may be con- verted into hyda- tids) connected with i front end of ovary. Paroophoron; amuch i smaller rudiment i lying near the hind j end of the ovary. Parovarian duct, ter- minating anteriorly in a * hydatid, and sometimes contin- ued back as Gairtner's duct. Fallopian tube, to the j fimbriated funnel of | which a * hydatid is attached. Unites with its fellow to form a pyriform uterus. ^ I Unites with its fellow j to form vasrina. O * Hydatids are small stalked pear-shaped bodies. MAN. 335 animal is proportional to (1) the size of its hemispheres, and to (2) the complexity of their convolutions. It must be remembered, however, that the absolute size of the animal has also an influence on these factors, as might be expected, since the hemispheres are V of left hemisphere (from Macalister). — S, Sylvian fissure, running nearly to Po ; c, Rolandian fissure ; A, B, S' S", convolutions of opercular lobe ; F, frontal lobe, including convolutions Fj, F«, F.> ; P, parietal lobe, including convolutions Pj, P^, Po> ; T, temporal lobe, including con- volutions T], T.j, To ; O, occipital lobe, including convolutions Oj, 0„, O;}. N.B. — These lobes also include convolutions on the inner flattened side of the hemisphere (see Fig. 107). The bulb and cerebellum are also shown. not concerned with sensation and intelligence alone, but also contain the highest motor centres. In fact, the cortex has been partly mapped out into motor centres, regulating definite groups •of muscles, and sensory centres, concerned with specific sensations. 336 AN ELEMENTARY TEXT- BOOK OF BIOLOGY. The most important sulcus is the Sylvian fissure (just indicated in the rabbit), Avhich marks off the temporal lobe below. Above this another important sulcus, the cevdral or llolamdian fissure, the 0) •42 C ® 'r' ai .2 > G o - s ^ ed o .2 C 5 ^ .m-5 c3 l, • —1 c3 O > G o» ^ - « G -2 ^ CO O CO ^ O %> '■K> g4 ?-4 0) o « S ■ y cd _r O c I— < cd ^ ^ O ^ G^ O ^ r-^ ;_i •- c3 •22 • c 02 2 O "C ® &0> .,-r G S 2 •''h:!- G c5 -u ;: 4S td0 2 -i •Gq a G G 3 s G^G «.2^ 3 O g o © g ^cd © ed «} _, o Cd^ G © G a > c >> c 02 •: ^ 2 G. . - ® O < ^ ^ ^ . #s S ^ G I ® O ‘ O B > 2 ^ G K*” © tJO pt, MAN. 337 convolutions bounding which constitute the opercular lobe.^ The frontal lobe is anterior to this, the parietal lobe posterior to it, and there is an occipital lobe at the extreme back of tlie hemisphere. Besides this, there is a small central lobe (island of Iteil) which can only be seen by separating the lips of the Sylvian fissure. The opercular lobe has been mapped out into lower, middle, and upper motor areas for the muscles of the face, leg, and arm respectively. The following sensory centres have also been Fig. 108. — Max. Embryo in Uterus, showing P'cetal Membranes (from Macalister, after Longet). — af Stalk of allantois ; arn, amnion ; c, neck of uterus ; ch, chorion ; dr, decidua reflexa ; ds, decidua serotina ; du, decidua vera ; nb, yolk-sac ; z, placental villi ; z'^, transient villi. recognized (with less certainty) in other lobes, (a) Sight, occipital lobe ; (b) Hearing, uppermost convolution of temporal lobe (T) ; (c) Smell, inner side of temporal lobe, just below Sylvian fissure. The corpus callosum is relatively very large, and it is much more strongly curved than in the rabbit. The anterior commissure is smaller, and so are the olfactory lobes. There are two corpora mammillaria (c. albicantia). The cerebellum is well-developed, but, in comparison with the * Many anatomists do not recognize the opercular lobe as a distinct sub- division, and take the Rolandian fissure as the line of demarcation between frontal and parietal lobes. 9 9 9 338 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. liemisplieres, smaller than in the rabbit ; its lateral lobes are far larger than the median one, and the floccnli are insignificant. There are no corpora trapezoidea. 7. Placenta. — This is of the tj^pe known as metadiscoidal, for, although it is circular like that of the Rabbit, yet, to begin with, villi are developed over the entire surface of the chorion, though they only persist over a limited area. These villi, too, are much branched, and their relation to the maternal tissue is complex. It will also be observed that the placenta is opposite* the ventral surface of the embryo, the reverse being the case in the Rabbit (Fig. 95). The yolk-sac (umbilical vesicle), again, is of less importance. The thickened mucous membrane lining the uterus is known as the decidua, because it is shed at birth. That part of it which forms the maternal portion of the placenta is the decidua serotina, that lining the rest of the uterus is the decidua vera, and a decidua reflexa surrounding the embryo is formed by the outgrowth and fusion of folds. Comparison of Man with the Highest Apes. The apes which come nearest to Man in structure are the Gibbons (Ihjlohates), Orang-outan {Simia satyrus), Chimpanzee {Troglodytes niger), and Gorilla (T. gorilla). In none of these is the erect attitude maintained without the assistance of the fore- limbs, the easiest and most habitual position. Consequently there are not the same perfect adaptations to this attitude which are formed in Man, and of which the most striking are the following: — (1) Double S-shaped curve of the vertebral column, (2) hind-limbs longer than fore-limbs, (3) feet completely special- ized for terrestrial locomotion, (4) pelvis broad and short. Both pollex and hallux are proportionately much larger in Man, but the latter is not opposable as in. apes. The dental formula of Man and the higher apes is the same, but in Man the teeth form a continuous curve above and below (there being no diastema), and the canines are not so well developed. The canines of apes are disproportionately large ; when the jaws are closed their points fit into gaps between the incisors and canines above, between the canines and premolars below. , The most distinctive anatomical features of Man, besides those CLASSIFICATION AND DISTRIBUTION OF ANIMALS. 339 mentioned, are found in the relatively large cranium and brain. In the Gorilla, for example, which is better developed in these respects than other apes, the brain volume and brain weight are 30-35 cubic inches and about 24 oz., as compared with 55-115 cubic inches and over 40 oz., in Man. The cerebellum of apes is larger in comparison with the cerebral hemispheres. The cranial capacity of Man is, of course, proportionately large, while the facial part of the skull is relatively small (c/. p. 322), and the cranio-facial angle is less than in apes. The nasal bones of Man, however, project more, and there is a mental prominence (chin) at the mandibular symphysis. CHAPTER XV.— CLASSIFICATION AND DISTRIBUTION OF ANIMALS. The following is a brief outline of the classification of animals (<•/. p. 3) DIVISION A— PROTOZOA. Unicellular animals (or if multicellular, as CoUozoum, the cells are all similar and independent), not reproducing by means of sperms and ova. GROUP 1. RHIZOPODA [Amceha). — Protozoa devoid of cuticle, and possessing pseudopodia. GPiOUP 2. INFUSORIA (Vorticella). — Protozoa covered by a cuticle and possessing flagella or cilia. GROUP 3. SP OROZ OA (Gregarinida) (Gregarina). — Endoparasitic spore-producing Protozoa, covered by a cuticle but devoid of cilia or flagella. DIVISION B.— METAZOA. Multicellular animals, made up of tissues, and reproducing by sperms and ova. 340 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. A. Diploblastica. — Embryo two-layered. PHYLUM I. CCELENTERATA Radially symmetrical Metazoa with body-wall composed of ectoderm, endoderm, and mesogloea. There is a digestive cavity opening by a mouth, but no body-cavity. B. Triploblastica (Coelomata). — Bilaterally symmetrical Metazoa, usually with a body-cavity, and possessing mesoderm in addition to ectoderm and endoderm. PHYLUM II. PLATYHELMIA {Distoma, Unseg- mented usually flattened worms, without lateral appendages or distinct body-cavity. Nervous system of cerebral ganglia, nerve- cord {not as double ventral cord), and nerve-plexus. Life-history often complex. PHYLUM III. NEMATHELMIA Elongated cylin- drical unsegmented worms, usually with complete alimentary canal, but no blood-system. Cuticle well developed ; ciliated epithelium absent. Excretory organs as two iinbranched lateral canals opening by a common anterior ventral pore. Sexes distinct. Gullet surrounded by a nerve-ring from which two principal nerves (lateral) run forwards and two (a dorsal and a ventral) backwards. PHYLUM IV. ANNELIDA. — Elongated segmented worms, with paired nephridia, a nerve-ring, and a ventral ganglionated nerve-cord of double nature. Possess both blood-system and a coelom. Class 1. Chaetopoda {Lumbrkus). — Annelids possess- ing setfe, and with a well-developed coelom. Class 2. Hirudinea {Hirudo). — Annelids devoid of setae, with segmentation obscured by secondary annu- lation, and with a reduced coelom communicating with the blood-system. PHYLUM V. ARTHROPODA {Astacus). — Segmented animals possessing well-marked body-regions as a result of the differen- tiation of segments. Lateral jointed appendages, and a nervous system of the same type as in Phylum IT., but with better- developed ganglia, especially anteriorly. Blood-system largely lacunar, replacing the coelom. CLASSIFICATION AND DISTRIBUTION OF ANIMALS. 341 PHYLUM VI. MOLLUSCA. — Unsegmented animals possessing a mantle and (usually) a shell. The body is ventrally produced into a muscular locomotor appendage, the foot. The blood- system is largely lacunar, and the coelom is reduced to a peri- cardial cavity communicating with the exterior by two tubular kidneys (1 nephridia), one of which is often aborted. Nervous system more or less concentrated, and mainly formed of ganglion- ated oesophageal ring. GROUP 1. LAMELLIBRylNCIIIATA (Anodo7ita, Unio). — Molluscs without a distinct head or an odontophore. Shell bivalve and lateral. Nerve-ring long. GROUP ODONTOPHORA Molluscs with odontophore, and a distinct head bearing sense-organs and containing a short nerve-ring. Mantle never bilobed. A univalve shell commonly present. PHYLUM VII. CHOKDATA. — Segmented animals possessing a notochord and visceral clefts, transitorily or permanently. Central nervous system dorsal and (usually) tubular. (See p. 135). SUB-PHYLUM L HE MIC HO PDA (Bcdanoglossus, Cepha- lodiscus,Rhabdopleura). — A small group including three genera of worm-like Chordates possessing a proboscis supported by a small notochord. [Rhabdopleura has no visceral clefts.] SUB-PHYLUM II. UROCHORDA (Ascidums).—A group of degenerate Chordates in which the larva possesses a tail .supported by a notochord, both, however, disappearing as a rule before the adult stage is reached. SUB-PHYLUM III. CEPHALOCHORDA {Amphioxus). — A Chordate group containing a single genus, and charac- terized by asymmetry, very complete segmentation, a noto- chord extending from end to end, and the presence of an atrial cavity. SUB-PHYLUM IV. VERTEBRATA.—Qhovd.MQ^ with never more than two pairs of limbs, and usually possessing jaws that move up and down and are not constituted by modified appendages. Skin consists of a several-layered epidermis and a connective tissue dermis traversed by vessels, 342 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. = nerves, &c. The notochord does not extend further forward than the middle of the ventral side of the brain. It is su}>- plemented, or more or less replaced by a well-developed endoskeleton consisting of at least a vertebral column, brain- case, and sense-capsules; also supports for limbs when these are present. The blood contains red as well as colourless corpuscles, and it is enclosed in a system of tubes comprising a ventral heart, arteries, veins, and capillaries. There is a lymph-system, communicating with the blood-vessels, and consisting of the large coelomic body-cavity, smaller lymphatic spaces, lymphatic vessels, and “ ductless ” glands, of which the most conspicuous is the spleen. The body may be regarded as made up of two tubes, (a) a smaller dorsal tube enclosing brain and spinal cord, (/3) a larger ventral tube (coelom) containing most of the other organs. Two compact kidneys are present, each provided with a special longitudinal duct. Ova in most cases shed into coelom and are carried off b}" two oviducts. When the mesonephros persists it not only performs excretory functions, but may also serve as a channel for carrying olf the sperms. When entirely replaced by a metanephros it is converted into a conducting-apparatus for sperms. The outlets of the intestine, excretory and repro- ductive organs, are situated close together and ventrally. There is a distinct brain, outgrowths from which develop into the retinae of two eyes, each of which also possesses a lens formed as an ectodermic involution, and a muscular iris of mesodermic origin. Two olfactory sacs and two mem- branous labyrinths are present, all of which are developed from ectodermic involutions. A. Anamniot a. — Embryo without amnion. Allantois, when present, is not an embryonic appendage. GROUP L ICHTIIYOFSIDA.—YevtGhT^tes with epi- dermic exoskeleton slight or absent. Lateral line sense- organs and a median fin present at some period of life. The notochord often persists, and the vertebrie, when present, are without epiphyses. The cranium always pos- sesses extensive tracts of cartilage, there is never a well- developed basisphenoid, and the cranial membrane bones, when present, include a parasphenoid. The gut either ends CLASSIFICATION AND DISTRIBUTION OF ANIMALS. 3 4, in a cloaca or ojiens by an anus in front of the urinogenital apertures. Blood at about same temperature as surrounding medium, and containing large oval nucleated red corpuscles. Heart two- or three-chambered, and aortic arches never less than two. Gills, and at least four pairs of branchial arches, always present at some j>eriod of life. The coelom is not divided into thoracic and abdominal sections by a respiratory diaphragm. The mesonephros persists. The cerebral hemi- spheres are never united by a conspicuous corpus callosum, the hypoglossal nerve does not perforate the brain-case, and the sclerotic coat of eye, when present, is cartilaginous or bony. Class 1. Pisces [Scyllmm). — Ichthyopsida with fin-like limbs, the margins of which are supported by fin-rays, as also are the unpaired fins. A well-developed exo- skeleton mainly of dermal origin. No postcaval vein and no allantois (see p. 153). Class 2. Amphibia {liana). — Ichthyopsida in which the unpaired fins always present in the larva and some- times in the adult are not supported by fin-rays. The limbs are transversely jointed and their extremities divided into digits. There is no epidermic exoskeleton, and the skin is very glandular. A cloaca and an allan- toid bladder. B. A m n i o t a. — Embryo with amnion and respiratory allantois. Gills never present. GEOUP 2. SAUEOPSIDA. — Air-breathing Amniota, with epidermic exoskeleton of scales or feathers. Tlie ver- tebrae are without epiphyses, the cranium is well ossified and articulates with the vertebral column by a single occi- pital condyle, partly belonging to the basi- and partly to the ex-occipital region. There is an interorbital septum, the otic bones do not fuse into a periotic before uniting Avith other elements, there is no separate parasphenoid, and the complex mandible articulates with a quadrate bone. The ankle-joint is between the proximal and distal tarsal bones. The oval nucleated red corpuscles are smaller than those of the Ichthyopsida, the heart possesses two auricles and one 34 4 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. (physiologically double) or two ventricles, and the aortic arches are seldom more than two. There is never a complete respiratory diaphragm. The kidneys are metanephric, and their ducts open into a cloaca, together with the intestine and sexual ducts. The cerebral hemispheres are not united by a well-marked corpus callosum, and the hypoglossal nerve perforates the brain-case. The sclerotic coat of the eye is supported by cartilage or bone, the cochlea is straight or slightly curved, and there is a columella. The large telole- cithal ova are invested in firm calcareous shells, the cleavage is meroblastic, and most of the development takes place, with few exceptions, outside the body of the parent. Class 1. Eeptilia {Reptiles). Class 2. Aves {Columba). — Warm-blooded Sauropsida with fore-limbs modified into (usually) functional wings, and a reduced tail-region (uropygium). There is an epidermic exoskeleton, in the form of feathers. The cervical vertebrae possess saddle-shaped articular surfaces, the true sacral vertebrae are without expanded ribs, and fuse Avith numerous other vertebrae to form a pseudo- sacrum, and the posterior caudal vertebrae (usually) unite together into a ploughshare-bone. The manus never possesses more than two free carpals nor more than three digits ; there is a carpo-metacarpus. The ilia extend far behind and in front of the perforated acetabulum, and the pubes and ischia are directed backwards, the former rarely, the latter never meeting in a ventral symphysis. The tarsal bones are never separate, but fuse with tibia proximally and metatarsus distally to form a tibio-tarsus and tarso-metatarsus ; digits of pes, never more than four. The jaws are covered by a hornj^ sheath, functional teeth never being present in recent forms ; a bursa Fabricii opens into the cloaca. The heart is four-chambered, and the right auriculo-ventricular ATilve muscular. There is a single aortic arch, curving to the right. The lungs are immo- bile, and the bronchia communicate Avith air sacs. The right oA^ary and oviduct are rudimentary. The optic lobes are disjfiaced to the sides of brain. CLASSIFICATION AND DISTRIBUTION OF ANIMALS. 345 GROUP 3, MAMMALIA. Class 1. Mammalia. — Warm-blooded Amniota in which an epidermic exoskeleton, in the form of hair, is always present at some period of life. The vertebrae possess epiphyses, and the cervical vertebrae are usually seven. The occipital region is well ossified, and there are two occipital condyles. The otic bones early fuse together into a periotic. The mandible is of two pieces only, and articulates directly with the squamosal. The lips are usually definite and muscular, and the teeth, when present, are only replaceable to a limited extent by new ones. The large intestine is very long, and usually opens independently, behind a urinogenital aperture. The red corpuscles are non- nucleated, the heart four-chambered, the right auriculo- ventricular valve membranous, and there is a single aortic arch, curving to the left. There is an epiglottis, the lungs are mobile, there are no air-sacs, the bronchial tubes end in infundibula, and there is a complete respi- ratory diaphragm. The cerebral hemispheres are united by a corpus callosum, a third commissure and corpus mammillare are present, and the optic lobes are double. The sclerotic coat of eye is fibrous, there are three auditory ossicles, and the cochlea is (almost always) spirally coiled. Usually viviparous, and the young always nourished by milk for some time after birth. Sub-Class 1. Prototheria (Ornithodelphia). — Mam- mals devoid of functional teeth, and possessing distinct coracoids, epicoracoids, interclavicle, and epipubic (marsupial) bones. There is a cloaca, the young are hatched from large eggs with a calcareous shell and much food-yolk, the cleavage being conse- quently meroblastic. The mammary glands have no teats. Order 1. Monotremata.—Ornithorhynchus, Echidna, Proechidna. The remaining sub-classes of Mammals include vivi- parous animals which usually have functional teeth, 346 AN ELEMENTAPwY TEXT-BOOK OF BIOLOGY. but do not possess distinct interclavicle, epicoracoids and coracoids, the last being represented by a process of the scapula. There is no cloaca. The mammary glands possess teats. Sub -Class Metatheria (Didelphia). — Mammals with inflected angle to the mandible and epipubic (marsupial) bones. The oviducts are distinct in typical cases, and there is not an allantoic placenta. The female usually possesses a pouch. Order 1 . Marsupialia. — Pouched animals : — Kan- garoo, opossum, wombat, &c. Sub-Class S. Eutlieria (Monodelphia). — Mammals in which the angle of the jaw is not inflected, and there are no epipubic bones. The oviducts are more or less fused, and there is an allantoic placenta. Orders of Eutlieria. — (\) Edentata — Armadillo, sloth, &c. (2) Ungulata — {a) Artiodactyla, none of the digits bilaterally sym- metrical— Swine, hippopotamus, ruminants ; if) Perissodactyla, with a bilaterally symmetrical 3rd digit in each manus and pes — Tapir, rhinoceros, horse, &c. (3) Sirenia — Dugong, manatee. (4) Bodentla — Pabbit, rat, porcupine, squirrel, guinea pig, Ac. (5) Proboscidea — Elephant. (6) Hyracoidea — Hyrax. (7) Insectivora — Hedgehog, mole, shrew, &c. (8) Cheiroptera — Bats. (9) Carnivora — Hog, cat, bear, seal, Ac. (10) Frosimice — Lemurs. (11) Primates — (a) Arctopitheci, Marmozets ; (b) Platyrrhini, lower monkeys ; (c) Catarrhini, higher monkeys (Cynomorpha) and man-like apes (Anthropomorpha), see p. 338; (d) Anthropidie — Man. PBINCIPLES OF DISTRIBUTION. Explanatory Theories. I. Theory of Special Creations^ now abandoned. According to it : — 1. Organisms were created where now found. 2. The fauna and flora of any particular region must be better adapted to it CLASSIFICATION AND DISTRIBUTION OF ANIMALS. 347 than any other fauna and flora. 3. Climate, soil, and position explain all the phenomena of distribution. II. Theory of Evolution, generally accepted. Kegards modern distribution as the result of innumerable changes that have affected — (1) organisms; (2) the surface of the globe. 1. Geological history shows that there has been a succession of faunas and floras passing gradually into one another, old species becoming extinct, and new ones being evolved by the combined influence of variation and heredity, which respectively originat’e and accumulate new characters. Upon the whole a gradual advance in complexity has taken place, but the geological record is extremely imperfect, especially as regards land organisms. The process of change is still going on, and examples of modern extinction {e.g., the gigantic wingless New Zealand bird Dinornis and Sirenian form Ehytina) are well known, but the detection of newly evolved species involves greater difficulties. Species once established have extended themselves over smaller or larger areas, according to their powers of migration and sur- rounding conditions — i.e., their environment. Physical barriers, such as oceans, mountains, climate, and soil, have played an important part in limiting such extension, but the competition of other forms has had a still greater influence. Introduced forms often increase prodigiously, and even supersede the indi- genous ones, whence it follows that these last are not necessarily the best adapted. Exs. Kabbits in Australia ; the brown rat in England, which has almost ousted the indigenous black rat. 2. Owing to the wearing away or erosion of the land by various agencies (chiefly the different forms of water), and the action of subterranean forces by which upward and downward movements of the earth’s crust are produced, the distribution of land and sea has constantly varied. Europe and North America, for example, have most likely been connected at various times by land occupying part of what is now the North Atlantic, and Australia appears to have been once united with Asia. On the other hand, evidence is found on every continental land surface of the former presence of the sea. In spite of what has been said, the theory of “ permanence of oceanic and continental areas ” finds much support. According to it the great oceans are of extreme antiquity, and, on the lohole, more or less land ha's 348 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. existed from very remote times within the present continental areas. It is perhaps best to accept this theory only for the deeper parts of the great oceans. An accurate knowledge of the contours of the ocean floor is important in this connection, and serves as an important check upon speculations regarding former land-unions. On this basis islands have been divided into oceanic and continental^ which are believed respectively to have been always isolated, and to have been connected with an adjoining continent. Oceanic Islands are : — (a) generally remote from continents ; (ll) separated from them by very deep (usually over 1,000 fathoms) water; (c) of volcanic or coral nature; {d) inhabited by forms which possess powders of migration capable of carrying them, actively or passively, over more or less broad ocean tracks ; (e) characterised by numerous peculiar species. Exs. The Azores, St. Helena, Ascension, coral islands of Pacific. Continental Islands are: — (a) comparatively near a continent; {b) separated from it by comparatively shallow (under 1,000 fathoms) water ; (c) of similar geological structure, and not entirely volcanic or coral ; {d) inhabited by similar organisms, irrespective of powers of migration. Such islands are : — (1) Ancient continental, separated from the nearest continent by fairly deep (over 100 fathoms) water, and presenting only a general resemblance in the fauna and flora ; many peculiar species. Exs. Madagascar, Celebes. (2) Recent continental, separated from the adjacent continent by shallow (not more than 100 fathoms) water, and with closely similar fauna and flora ; very few peculiar species. Exs. British Islands, Japan. Both (1) and (2) have presumably been united with the adjoining continents, the latter at a recent date, geologically speaking. The surface of the globe has also undergone numerous muta- tions as regards climate. The temperate parts of N. America and Europe, for example, were at a geologically recent period passing through a glacial epoch (the “ great ice age ”), as proved by ice-worn and scratched rock-surfaces and rocks, boulder-clay, &c. On the other hand, fossil plants evidencing subtropical conditions have been found in the Arctic regions. Many theories have been advanced to account for secular changes of climate. The most satisfactory is one by Wallace which attributes them to geographical revolutions (previously suggested by Lyell), influ- CLASSIFICATION AND DISTRIBUTION OF ANIMALS. 349 enced hy astronomical changes (variations in excentricity of earth’s orbit and movements of precession, as advanced hy Croll). All the preceding changes must have exerted a profound influence upon organisms, and throw liglit upon many problems of distribution. Areas of Distribution. — May bo mapped out for species, genera, families, and orders. In all cases : (1) Size and nature of bound- aries very variable. (2) Need not be continuous. (a) Exs. of Limited Areas. — The marmot only found in the Alps. A species of humming-bird confined to the crater of the extinct volcano Chiriqui in Veragua. Six genera of Lemurs are peculiar to Madagascar. The family Galeopithecidce (including the single genus Galeopithecus) is limited to Malacca, Sumatra, Borneo, and the Philippines. The order Monotremata only occurs in Australia., Tasmania, and New Guinea. (h) Exs. of Extensive Areas.’ — The Leopard is distributed through the whole of Africa and S. Asia to Borneo and E. China. The genus Felis (cat, lion, leopard, &c.) ranges over most of the globe except Australia, the Pacific Islands, W. Indies, Mada- gascar, and the more northerly parts of North America and Asia. The family Vespertilionidce, including 200 species of small insect- eating bats, occurs everywhere within the tropical and temperate zones; while the family Miiridce (rats, mice, Ac.) is only absent from Polynesia and New Zealand. 2. Discontinuity is generally a sign of antiquity, the two or more parts being remains of a once continuous distributional area, in part of which extinction has occurred. Changes in the distribution of land and sea have broken up many once continuous areas. Examples. — The variable hare (Lepus variabilis), Europe and Asia N, of 55° ; Alps, Pyrenees, and Caucasus. The genus Tapirns, S. America, S. E. Asia. Centetidce (a family of the Insectivora), Madagascar, Cuba. Hayti. Ganoid fishes are now represented by genera with the fol- lowing distribution : — Acipenser, N, temperate and Arctic regions. Most species marine, others are found in the Casjhan Sea, Black Sea, and N. American lakes, with their rivers, also in the Danube, Mississippi, and Columbia. Scaphirliynchus, Mississippi and tributaries. Polyodon, Mississippi and Yang-tse-Kiang. Polypterus, Nile and W. African rivers. Calamoichtbys, rivers of Old Calabar. Amia, fresh- water. United States. Lepidosteus, AN ELEMENTARY TEXT-BOOK OF BIOLOGY. fresli-water, ]ST. America to Mexico and Cuba. Ganoid fishes are of great geological antiquity, and were formerly a widely spread marine group. Most of the forms now surviving have gradually accommodated themselves to a life in rivers, lakes, &c., where the struggle for existence is less severe. The peculiar distribution of Dipnoi (p. 153) can be explained similarly. The Marsupialia and Edentata (p. 354) are also good examples of interrupted areas of distribution. Zoological Regions, characterized by the presence of peculiar families and genera, and by the absence of other families and genera, have been formed for sea and land. The most useful division of the latter is chiefly based on the Mammalia, but applies very well to birds and reptiles and fairly to other groups. The regions thus established are six in number. I. Palcearctic Region. — Temperate Europe and Asia and N. temperate Africa. Extends Mb to Iceland, the Azores, and Cai)e Verde Islands, and E. to Behring Straits and Japan. Southern boundary somewhat indefinite, — tropic of Cancer in Africa and Arabia, river Indus, Himalayas, Nanling mountains. II. Ethiopian Region. — Africa and Arabia south of the trojiic of Cancer, and including Madagascar. III. Oriental Region. — Asia, S. of Region I., and the western part of the E. Indies. The eastern boundary of this region {IFallaces line) passes between Bali, Borneo, and the Philippines on the one hand, Lombok and Celebes on the other. The former islands are therefore in the Oriental Region, the latter in the Australian Region. IV. Australian Region. — Australia, New Guinea, New Zealand, with the smaller islands from AVallace’s line to the Marquesas and Low Archipelago, and the tropic of Cancer to the Macquarie Islands. V. Neotropical Region. — S. America, the W. Indies, and tropical N. America, with the exception of the central part of the Mexican table-land. VI. Near die Region. — Arctic and temperate N. America, with the central part of the Mexican table-land. I CJLASSIFICATION AND DISTRIBUTION OF ANIMALS. 351 DISTKIBUTION OF MAMMALS. I. Palcearctic Hegion. — Thirty-five families represented. Peculiar Genera. The camel, six deer, the yak, six antelopes (including the chamois), and all wild sheep and goats, except two species. Six p.g. of Muridoi (rats and mice), two of mole-rats, and one other ; dormice and pikas (calling hares) are almost confined to this region. Six p.g. of moles ; the remaining two genera of which (Talpa, Urotriclms) extend, respectively, into N. India and N. W. i America. Five p.g. of Carnivora, including the racoon-dog, a seal, and the badger; the last just enters the Oriental region in China. - II. Ethiopian Region. — Fifty families represented, of which nine a^re peculiar — i.e., Orycteropoclidce. Ilippopotamidce ; Camelopardidce. PotamogaUdcB (including Potamogale, an otter-like Insectivore) ; Chrysochloridce (golden moles). Cryptoproctidce (Cryptoprocta is a small civet -cat -like form peculiar to Madagascar) ; Protelidce, (Proteles, the aard-wolf, is allied to the hyaenas and weasels) ; Cheiromyidce (contains Cheiromys, the aye-aye, peculiar to Mada- gascar). Peculiar Genera (besides those in the above families), — Potamo- chaerus (river hog), and Phacochaerus (wart hog) ; Hyomoschus (a small deer-like form), twelve p.g. of antelopes. Thirteen p.g. of Muridce; Pedetes (a jerboa or jumping mouse); Anomalurus (a flying squirrel) ; three other p.g. of Rodents. Three p.g. of elephant-shrews, and the Insectivorous family Centetidce, except one genus from Cuba and Hayti. Three p.g. of bats. Seventeen p.g. of Viverridoe (civets and ichneumons), two p.g. of dogs, and two p.g. of Mustelidce (weasels, otters, Ac.) Nine p.g. of Lemurs. Eight p.g. of apes and monkeys, the most important being Trog- lodytes (gorilla and chimpanzee). Peculiar Species. — Among these are several species of Manis, the two-horned African rhinoceroses, the zebras, African elephant, I and lion. Absent Palcearctic Forms. — The genera Bos (wild ox), and Sus I (wild boar), camels, deer, goats and sheep, moles, bears. I III. Oriental Region. — Forty-two families represented, of which I two are peculiar, and one other almost so — i.e., Galeopithecidce I (including G-aleopithecus, the flying lemur), and Tupaiidce. (tree- shrews), among Insectivora. Tarsiidce, a family of lemurs (includ- 352 AN FJLEMENTARY TEXT-BOOK OF BIOLOGY. ing only Tarsius spectrum, found in Sumatra, Banca, and Borneo; also outside the Oriental region in Celebes). Peculiar Genera (besides those in the above families). — Tragulua (chevrotain) ; Cervulus (a deer) ; Bibos (wild cattle) ; three p.g. of antelopes. Platanista (a dolphin found in Ganges and Indus). Three p.g. of Muridce; Pteromys (a flying squirrel) ; Acanthion (a porcupine) ; Gymuura (a hedgehog). Eleven ]).g. of bats. Twelve p.g. of Viverriclce ; Cuou (a dog); five p.g. of Mustelidce; two p.g. of bears. Loris and Nycticebus (lemurs). Four p.g. of apes, including Simia (orang-utan), and Hylobates (gibbon). Peculiar Species. — Among these are the Indian tapir, several species of rhinoceros, and the Indian elephant. IV. Australian Piegion. — Twenty-eight families represented, of which eight are peculiar — f.e., seven out of the eight families of Marsupials, and the two families of Monotremes. All of these, however, are absent from Polynesia and New Zealand. Peculiar Genera (besides those in the above families). — Babirusa (a hog), and Anoa (a small kind of cow) in Celebes. Five p.g. of Muridce in Australia, and one of these in Tasmania also. Three p.g. of bats. Absent Forms. — Australia and New Guinea possess no non- aquatic Mammals higher than Marsupials, except some bats, mice, and rats. This points to extremely long-continued isolation, which has afforded time for the Marsupials to become modified in the most diverse directions, thus enabling them to fill places elsewhere occupied by other orders. New Zealand is remarkable for the absence of all indigenous Mammals, so far as certainly known, with the exception of two bats. V. Neotropical Region. — Thirty-seven families represented, of which seven are peculiar — ?*.c., Bmch/podidce (sloths), Dasypodidce (armadilloes), and Myrmecopliagidce (true ant-eaters). Chinchillidce (chinchillas) and Caviidce (cavies) among Rodents. Cebidce (New World monkeys) ; Ilapalidce (marmozets). The Phyllostomidce (leaf-nosed bats) are peculiar, with the exception of a Californian species. Peculiar Genera (besides those in the above families). — Chiro- nectes and Hyracodou (opossums). Dicoctyles (peccary, also in Texas) ; Auchenia (llama) ; Elasmoguathus (a tapir). Inia (a dolphin, upper part of Amazon basin). Six p.g. of Bluridce; six p.g. of Octodontidce (rat-like forms), two of them peculiar to W. CLASSIFICATION AND DISTRIBUTION OF ANIMALS. 353 Indies ; eight p.g. of Echimyidce (spiny rats) ; two p.g. of Cercola- bidce (tree porcupines). Solenodon (a hedgehog-like form from Cuba and Hayti). Twenty-six p.g. of bats, including the Vampires. Five p.g. of Canidce.; three p.g. of Mustelidce ; Nasua (coati) and Cercoleptes (Kinkajou) ; Tremarctos (spectacled bear) ; Otaria (an eared seal). Peculiar Species. — Among these are twenty species of Didelphys (to which genus most opossums belong). The American tapir. A species of racoon (genus Procyon). Absent Forms. — Ungulata are scarce, deer and llamas being the only ruminants, tapirs and peccaries the only non-ruminants. The only insectivores are Solenodon and a species of shrew (Sorex). The Vwerridoi are absent. VI. Nearctic Fiegion. — Thirty-two families are represented, of which one is peculiar, while one other is almost so — i.e., Haplo'6- dontidce (rat-like forms allied to beavers and marmots), and SaccomyidcE (the pouched rats, of which one genus ranges into the N. of the Neotropical region). Peculiar Genera (besides those in the above families). — Antilo- capra (prong-horned antelope), Aplocerus (a goat-like antelope), and Ovihos (the musk-sheep). Three p.g. of Muridce; Jaculus (a jerboa), Cynomys (the so-called prairie dog), and Erethizon (the tree porcupine). Three p.g. of moles ; two p.g. of bats ; two p.g. of Mustelidce; Eumetopias (an eared seal) ; Halicyon (a seal). Peculiar Species. — Among these are two of Didelphys, a peccary, several deer, the American bison, racoons, and the grizzly bear (Ursus ferox). Absent Forms. — Ungulates are ill represented, deer, the American bison, two antelopes, a sheep, and the musk-sheep being the only ruminants, while a peccary (Texas to Red River) is the only non-ruminant. Hedgehogs, Viverrida’, and monkeys are all unre- presented. Distribution of Orders. 1. Monotremata. — Consists of only three genera, limited to part of the Australian region. Ornithorhynchus, Australia and Tasmania; Echidna, Australia, Tasmania, and N. New Guinea; Proechidna, S. New Guinea. 2 23 354 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. No fossil forms are found elsewhere, so that the place of origin of this order is unknown. 2. Marsiq^ialla. — Consists of eight families (comprising thirty- seven genera), of which only one, the Opossum family (including three genera), occurs outside the Australian region. The Opossums are Neotropical and Nearctic. Fossil opossums occur in the Pleistocene of America, and in much older European deposits (Eocene to Miocene). The secondary rocks of Europe contain a number of small forms, which jDrobably resemble the ancestors of the Australian Marsupials. We may therefore suppose that this order originated in the Palsearctic 'region, and then extended into what is now Australia (at that time united by land with Asia), isolation occurring soon after, followed by specialization in various directions. The Opossums seem first to have existed in Europe, from whence they spread into America by former northerly land-connections. 3. Edentata. — This order is now chiefly limited to S. America, but Orycteropus is peculiar to the Ethiopian region, while Manis is found both in that and the Oriental region. The geological evidence is in favour of considerable development in Africa, whence the order would spread north to the Oriental and Pal^earctic regions, and thence on to America. The compe- tition with higher forms has caused its extinction in most areas, and Edentates appear to be most abundant in S. America, because the competition with other animals is there comparatively small. The peculiar burrowing or climbing habits of most of the genera also tend to j:)reserve them, and these habits no doubt represent attempts to escape from the severe competition with higher forms. The size of existing Edentates is insignificant compared with that of Pleistocene S. American and European genera. 4. Ungulata. — (a) Artiodactyla. Non-ruminantia. Swine are only represented in America by peccaries (Dicotyles); true swine are found in all the other regions, but only extend into the Australian as far as New Guinea. These animals are first known in the European Eocene, and during Miocene and Pliocene times were as common in N. America as Europe, but since then have almost entirely disappeared from the former area. The hippopotamus is now limited to the Ethiopian region, but fossil forms occur in Europe (Pliocene and Pleistocene) and India (Miocene). CLASSIFICATION AND DISTRIBUTION OF ANIMALS. 355 Riiminantia. — Recent Camelidce are only found in the Neotropical and Palaearctic regions, but numerous forms occur in the Miocene and later deposits of N. America, where the group originated. Tragulidce or mouse-deer have also a discontinuous area of distribution — W. Africa (Hyomoschus) and Oriental region (Tragulus). This is accounted for by the presence of Miocene forms in Europe, whence the family extended south. Deer occur in all the regions except the Ethiopian, but do not extend far into the Australian region. They appear to have taken origin in the Old World, from whence they reached N. America in Miocene times, and afterwards passed to S. America. Giraffes are at the present time confined to the Ethiopian region, but fossil forms are known from S. Europe and India, and a northern temperate origin is probable. Bovidce (oxen, sheep, antelopes, &c.) are present in all the regions except the Neotropical, though they only just pass into the Australian, and are scarce in the Nearctic. The family appears to have originated in the Palaearctic and Oriental regions during Miocene times. (h) Perissodactyla. Tapirs present a striking example of dis- continuous distribution, being found, on the one hand, in the Malay peninsula, Sumatra, and Borneo, and on the other, in S. and Central America. True tapirs occur in the W. of Europe as far back as Miocene times, but in America are not found further back than the Pleistocene. Migration from the Palsearctic region is thus indicated. Rhinoceroses are now only Ethiopian and Oriental, but they appear to have originated in the Palsearctic region, where they extend back to the Miocene period. In Pliocene times they also ranged into N. America. The genus Equus (horse, ass, zebra) is now limited to the Ethiopian and Palaearctic regions. It appears to have originated in the latter area during Miocene times, and then migrated not only into the Ethiopian, but also into the Oriental, Nearctic, and Neotropical regions, as proved by fossil forms. 5. Sirenia. — Manatus (manatee), E. coast of S. America, and W. coast of Africa. Halicore (dugong), shores of Indian Ocean and Red Sea. 6. Rodentia. — Very widely distributed ; occurring in all the 356 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. regions, but in Madagascar and Australia only represented by Muridce. They attain their largest development in S. America. The order is of great geological antiquity, for some living genera extend back to the Eocene. Rodents probably originated in the Palaearctic region, whence migrations took place at an early date to S. America and S. Africa, allowing time for great special- ization. 7. Prohosciclea. — Elephants are now limited to the Ethiopian and Oriental regions, but formerly had a much wider extension. Palsearctic forms occur from Miocene to Pleistocene times, and elephants have lived in India since the Miocene period. Numerous fossil examples occur in the Pliocene and Pleistocene deposits of N. and S. America. 8. Ilyracoidea. — Almost entirely limited to the Ethiopian region, but range northwards as far as Syria. 9. Insectivora. — Very widely distributed, and represented by numerous specialized forms. Absent from S. America and Australia. This order is a very ancient one, as shown by the fact that Miocene forms mostly belong to existing families. Extinction appears to be slowly taking place, and has led to many cases of discontinuous distribution — e.g., Ce7itetidce, represented by Solenodon in the W. Indies, and Centetes, with four other genera, in Mada- 2:ascar. O 10. Cheiroptera. — Bats, as might be expected, are found in all the great areas, but the Frugivora are absent from the Nearctic and Neotropical regions, as are the Horse-shoe bats (Phinolophidce) among insectivorous forms. On the other hand, the Leaf-nosed bats {Pkyllostomidce) are almost exclusively Neotropical. Fossil bats, very like recent species, date from Eocene times, and the order is undoubtedly one of extreme antiquity. 1 1. Carnivora. — (a) Fissipedia. Occur in all the regions, except, perhaps, the Australian (the native dog ” of Australia is only doubtfully indigenous), but are especially characteristic of the Ethiopian and Oriental, which possess almost all the Viverridoe and HycenidcO; with a great many of the Felidce and Mustelida\ Two genera, Cryptoprocta and Proteles, constituting distinct families, are limited respectively to Madagascar and S. Africa. Bears, however, are absent from the Ethiopian region, and are only represented by one species in the Neotropical region, which CLASSIFICATION AND DISTRIBUTION OF ANIMALS. 357 is also very poor in other Carnivora. The Procyonklce are small bear-like forms, found in the Nearctic and Neotropical regions, and include the racoons (Procyon), coatis (Nasua), and kinkajous (Cercoleptes). Fossil Carnivores date back to the Lower Eocene, but the recent families were not then differentiated. The order appears to have originated in the northern half of the Old World. {h) Pinnipedia. — Seals are limited to cold and temperate seas, and are also found in the Caspian, Sea of Aral, and Lake Baikal, all of which, at no very distant epoch, were connected with the Arctic Ocean. Walruses characterize the North Polar regions. 12. ProsimioB. — (a) Cheiromyini. Only one form, the aye-aye (Cheiromys), which is restricted to Madagascar. (b) Lemurini. — Practically limited to the Ethiopian and Oriental regions. Indris, Lemur, and four other genera are only found in Madagascar. Tarsius constitutes a distinct family, and is limited to Sumatra, Banca, Borneo, and Celebes. Lemurs date back to the Eocene in Europe. 13. Primates. — (a) Arctopitheci and (h) Platyrrhini are confined to the Neotropical region. (c) Catarrhini are found only in the Old World. C y n o m o r p h a are especially Ethiopian and Oriental, but also extend into the Palsearctic region, and into the Australian region as far as Timor. Anthropomorp h a present a marked example of discontin- uous distribution. Gibbon — S. E. Asia and Mala}^ Archipelago. Orang — Borneo and Sumatra. Chimpanzee and Gorilla — W. Africa. The order dates back to the Eocene. Origin and Migrations of the Mammalia. —The Class, and most likely all the Orders, originated in the Northern HemisjDhere. Australia was isolated at a very early date, and therefore has preserved a very ancient Mammalian fauna. S. America and S. Africa were severed somewhat later, to be afterwards reunited, and they also have preserved some ver}^ ancient forms. The northerly connection between the Eastern and Western Hemi- spheres was then broken, not only by submergence of land, but also by a lowering of temperature. The Oriental and Ethiopian regions were also marked off by the formation of the Himalayas and the desert zone stretching from the Sahara to Central Asia. INDEX-GLUSSAKY TO PART II. A Abdomen, Astaciis, 83, 84 ; Lepiis, 269. Abduction {ah, from ; duco, ductum, draw, lead) of appendages — Movement away from the middle line, 91. Abiogenesis («-, not ; d/os, life, yeVeo-is, birth) — Derivation of living from non-living matter, 7. Aboral {ah, away from ; os, mouth) — Situated at the end further away from the mouth, 19. Absorption, 5, 194. Accretion {ad, to ; cresco, cretum, grow) — Growth by addition of layers to the outside, 7. Acetabulum {acetahulum, properly a vessel for vinegar ; the socket of the hip bone) — The socket into which the head of the femur fits, 186, 239, 281, 327. Achromatin (a-, not ; colouring matter) — The part of the nucleus which does not readily take up stain, 8, 73. Acinus (aKiros, acinus, a berry, a grape) of a gland — One of the ultimate (spherical or tubular) subdivisions, lined by secreting cells, 33, 288. Acipenser, 153, 349. Acrania, 135. Acromion {aKpwuiov, used with the modern meaning) in Mammalia — A spine-like process of the scapula, 280. Action, 102. Adduction {ad, to; duco, ductum, draw) of appendages— Movement towards the middle line, 91. Adrenal, 205, 250. Aetiology {alTiov, a cause; Xoyos) = Phylogeny, q.v. Afferent {ad, to ; fero, I carry)— Of nerve-fibres in conducting impulses to a ganglion cell or to the central nervous system, 102, 146, 217 ; of vessels carrying impure blood to gills, 164. Africa, 349. After-shaft, in the Fowl— A minute vane attached to the superior umbilicus of an ordinary feather, 231. INDEX-GLOSSARY. 359 Air -sac, 249, 252. Alimentary Canal. See Digestive Organs. Allantois (aWa?, aWavT-, a sausage), in the Bird and Mammal — An embry- onic appendage which grows out from the posterior part of the intestine, 309. Alps, 349. Alternation of Generations, 39 ; Hydra, 27 ; Distoma, 35, 39. Alveolus {alveolus, a little hollow) — A socket for a tooth, 284. America — North A., 349, 350, 353; South A., 349, 350, 352. Amia, 349. Amnion {a/xviov, used with the modern meaning), in the Bird and Mammal — A sac-like appendage by which the embryo is surrounded, 265, 309. Amniota [aixvwv), 342. Amoeba {aixoi^o^, changing), 7, H, 311. Amoeboid — Resembling an amoeba, especially as regards movement by slow protrusion of pseudopodia, 11, 60, 318. Amphibia (a/jupw, both; life), 343 ; 3, 4, Chap, x., 174. Amphicoelous {afxuXXoi/^ a leaf) — The characteristic green colouring matter of plants, 14, 22, 44. Chondrocranium (xoT^pos, a eartilage; cranium) — The cartilaginous ground- w’ork of the skull ; the primordial cranium, 179, 232. Chordinas tendincae — Firm bands of connective tissue uniting the flaps of an auriculo-ventricular valve with the wall of the ventricle, 245, 288. Chordata (xopof;, a string), 341 ; 3 ; Chap, vii., 1.34, et scq. Chorion, .338. Choroid (xopior, skin, membrane ; £l(5os, to-, appearance), 174, 221 ; c. plexus, 210. Chromatin {xpoofxdnvo^^ coloured) — The more deeply staining part of a nucleus, 8, 73. INDEX-GLOSSARY. 367 Chromatopliore colouring matter ; fl)opuepw, I bear)— A pigment-bearing cell, 178. Cilium {dlium, an eyelash)— A short vibratile thread of protoplasm on the free surface of a cell, 13, 61, 312. Circulatory system — Lumbricus, 59 ; Hirudo, 78 ; Astacus, 97 ; Anodonta, Unio, 114 ; Helix, 127 ; Amphioxus, 142; Scyllium, 161; Rana, 194; 202; Tadpole, 227 ; Columba, 245 ; Chick, 267 ; Lepus, 289 ; Homo, 331 ; Comparison of, 314. Cirrus, in Distoma and Tsenia — The penis, 32, 42 ; buccal c., 136. Cirrus-sac, 33. Clasper, 154, 167. Classification of Animals, 1,3; Chap. xv. , 339. Claw, 176, 230, 241. Cleavage (Segmentation) — The early stages of cell division occurring in a developing oosperm, 26, 67, 105, 147, 222, 256, 306. Clefts, visceral, in Chordata — Lateral apertures by which the pharynx com- municates with the exterior, 3, 157, 225, 226, 262, 308. Clepsidrina, 18. Climate, 34S. Clitellum {clitellce, a pack-saddle), 55, 64, 76. Clitoris, 299. Cloaca {cloaca, a sewer) — The chamber into which the intestine, genital, and, usually, urinary ducts open, 48, 49, 160, 191, 242. Cnemial crest, 290. Cnidoblast, 22. Cnidocil, 22. Coagulation, 165. Coats, of Alimentary canal, 192, 244, 288 ; of Artery, 202. Coccyx, 326. Cockroach, 18. Cocoon, 82. Cochlea {cochlea, a snail-shell) — A posterior outgrowth from the mem- branous labyrinth, coiled in higher forms, 173, 219, 256, 304. Cod, 153. Coelenterata, 340; Chap, ii., 19. Cmlom (/coiXos, hollow) — A body cavity which typically (1) does not contain blood, (2) communicates at some period with the exterior by excretory tubes, (3) has parts of its lining thickened into gonads, (4) arises early in development, 56, 60, 79, 95, 143, 150, 191, 248, 263. Ccelomata, 340. Ccenurus, 46. Collozoum, 339. 368 INDEX-GLOSSARY. Colon, 286. Colpidium, 72. Columba, 344 ; 229-268. Columbia, 349. Columella, 123, 219. Columnse carnece, in the heart — Internal projections of the ventricular wall, 245. Commissure {commissura, a joining together) — (1) In Invertebrates: a nerve-band uniting transversely two ganglia of the same name, 119; (2) in the Vertebrate brain : bands of connecting fibres ; Anterior c., 253, 301, 337 ; Posterior c., 210, 252, 301; Middle c., 301. Comparative Animal Morphology and Physiology, Chap, xiii., 310. Condyle — A rounded projection, 276; Mandibular, 183; Occipital c., 179, 232, 238, 272, 322. Cones — Crystal c., 104; of Retina, 174, 221. Conjugation, in Protozoa — The temporary or permanent fusion of two individuals, 15, 18, 72, 317. Conjunctiva {conjtingo, conjunct-, join together), in Vertebrates — A trans- parent membrane, covering the front of the eyeball, and connecting the eyelids, 222, 256, 305. Connectiv^e — A nerve cord joining ganglia of different name to each other or to the main cord, 66, 81, 119. Continental Islands, 348. Contractility, 318; Amoeba, 10; Vorticella, 16; Hydra, 25. Conus Arteriosus, in many Fishes — A contractile tubular part of the heart succeeding the ventricle, 162. Copulatory Organ, 86, 154. Coral Is., 348. Cornea {corne.us, horny), in the eye — The transparent area forming the front part of the sclerotic, 103, 134, 174, 221. Cornua — (1) in Brain, of lateral ventricle, 301 ; (2) of hyoid apparatus, see Hyoid apparatus. Corpora quadrigemina, in Mammalia — The optic lobes, each being divided into natis and testis, 302. Corpus albicans = c. mammillare, ([.v. Corpora cavernosa, in Mammalia — The two firm cylindrical structures supporting the clitoris and penis, 298, 299. Corpus adiposum, 205. Corpus callosum, in Mammalia — A band of nerve-fibres uniting the cerebral hemispheres, 301, 337. Corpus mammillare, in the Rabbit — A small rounded projection situated on the base of the brain, just behind the infundibulum, 301. INDEX-GLOSSARY. 369 Corpus restiforme, in the Rabbit — A projection situated on either side of the roof of the fourth ventricle, 302. Corpus spongiosum, in Mammalia — A mass of vascular tissue making up the dorsal part of the penis, and traversed by the urinogenital canal, 298. Corpus striatum, in the fore-brain — A mass of grey matter projecting into the front part of either lateral ventricle, 210, 253, 301. Corpus trapezoideum, in the Rabbit — A rectangular area situated ventrally and anteriorly on either side of the medulla oblongata, 302, 338. Corpuscles — Blood c., 59, 78, 161, 195, 245, 289, 293; Central c. (centro- soma), 73 ; Colourless c., 161 ; Muscle c. , 208, 291, 300 ; Red c., 161 ; Touch c., 178, 218, 254, 271, 303. Cortex, of kidney, 295 ; of brain, 333. Corti, Organ of, 304. Coverts — Wing c., 229 ; Tail c., 229. Cranio -facial angle, 272, 339. Craniota, 135. Cranium {Kpavlov, the skull) — The brain-case, 179, 232, 271, 323. Cranium — Primordial. See Chondrocranium. Crayfish = Astacus, q.v. Crista acustica — A projection in the ampulla of a semicircular canal, upon which is a patch of auditory epithelium, 221. Croll, 349. Crop— Part of the mid-gut serving for the storage of food, 57, 58, 242. Cross, 4. Crura cerebri— Two longitudinal masses of nerve-fibres on the floor of the mid-brain, 211, 253, 302. Crus [crus, a leg), in Vertebrates— The part of the leg between thigh and ankle. See Hind limb. Crustacea, 86. Crystal cone, 104. Crystalline style, 114. Ctenidium (dim. of KTivosj a comb) — The gill of Molluscs, typically plume-like, 112. Cuba, 349. Cuticle ( cuticula^ dim. from cutis, skin ) — A structureless membrane covering the epidermis, 13, 28, 29, 48, 55, 76, 312. Cyclas, 121. Cyclostomata, 153. Cyst — A protective case in which some animals are enclosed during a dormant stage, 9, 14. Cysticercus, 44, 46. 2 24 370 INDEX-GLOSSARY. D Danger signal, 268. Danube, 3-19. Dart-sac, 131. Darwin, 58. Death, 10, 11. De Candolle, 4. Decidua, 338. Deer, 355. Delamination — Formation of epiblast and hypoblast, by division of the segmented ovum parallel to the surface. Dentine — The part of a tooth or scale developed from the dermis, 155. Dermal — Belonging to the dermis, or, in general, to the skin, 34. Dermis — The deeper part of the skin derived from mesoderm, 76, 90, 138, 178, 227, 231. Deutomerite, 17, 18. Development — Direct d., 107 ; Indirect d. = Metamorphosis, q.v. ; of nervous system, 150; of feather, 231; of hair, 270; Hydra, 26; Distoma, 35 ; Tsenia, 43 ; Ascaris, 53 ; Lumbricus, 67 ; Hirudo, 82 ; Astacus, 107 ; Anodonta, Unio, 121 ; Amphioxus, 147 ; Scyllium, 167 ; Bana, 222 ; Columba, 256 ; Lepus, 305 ; Homo, 338 ; Comparison of, 317. Diaphragm, (1) in the Frog — The muscular anterior boundary of the body- cavity, 191 ; (2) in Mammalia — A muscular and tendinous partition separating the thoracic and abdominal cavities, 294. Diastema an interval) — A gap between the incisor and pre- molar teeth, 284, 329, 338. Diastole, in the heart — The relaxation which follows each contraction, 97. Didelphia (^/-, two ; tiXipl;, the womb), 346. Differentiation — Morphological d., 20. Digestion. See Nutrition. Digestive System — Vorticella, 13; Hydra, 27; Distoma, 29, 36 (embryo), 38 (cercaria) ; Taenia, 41 ; Ascaris, 48 ; Lumbricus, 55 ; Hirudo, 76 ; Astacus, 192; Anodonta, Unio, 113; Helix, 125; Amphioxus, 139; Scyllium, 160 ; Bana, 189 ; Columba, 241 ; Bana, 282 ; Homo, 329 ; Comparison of, 313. Digit, 176, 186, 239, 241, 281, 282,327, 329. Dimorphism (^ts, doubly ; iJ^opcpn, a shape) — The occurrence of bi-sexual and uni-sexual forms in the life cycle of one animal, 73. Dinornis, 347. Diploblastica {^tTrXoO?, double ; /3\a^ov^ a living creature), 15, 72. Macula {macula^ a spot), in the membranous labyrinth — One of several sensor^'- patches situated in the utriculus, sacculus, and (wlien rudi- mentary) the cochlea, 221, Madagascar, 348, 349. Malacca, 349. Malpighian Body, in the Kidney — A Bowman’s capsule with its glomerulus, 206, 295. Malpighian layer, 177. Mammalia, 345; Chap, xii., 268, et seq. : Distribution of, 357 ; Origin of, 357. The lower jaw, 183, 236, 276, 325. Man = Homo, q.v. Mandible — In Crustacea : the third pair of head appendages, which act as jaws, 89. Mantle, in Mollusca — A fiap-like outgrowth of the body- wall which usually shelters the gills. 111. Manubrium — (1) A process of the sternum, 238, 280, 326; (2) a process of the mallens, 273. Manus — The hand, 176, 186, 230, 239, 281. Marmot, 349. Marrow — The central soft tissue contained within bone, 188, 293. Marsupialia, .346, 349, 352, 354. Mastoid — A term applied to the spongy part of the periotic, 32, 273. Matrix — Intercellular substance in which tissue elements are imbedded, 126, 188. Maxilla {maa:iZZa, a jaw) — (1) In Crustacea: one of the second or third jaw-like appendages belonging to the head, 89; (2) In Vertebrata: one of the membrane bones of the upper jaw, 182, 234, 275, 325. Maxillipede — A flattened jaw-like appendage belonging to the head, 89. Meatus {meatus, a passage) — External auditory m., 230, 255. Mediastinum — A median partition in the thorax, formed by the apposition of the parietal layers of the two pleur£e, 294. Medulla — In nerve fibre : the fatty layer surrounding the axis cylinder, 81 ; m. of hair, 270 ; m. of kidney, 295. Medulla oblongata — The axial part of the hind-brain ; the bulb, 169, 211, 216, 253, 302. Medusa, 27. Membrane — Arachnoid, 210; m. granulosa, 299; Mucous, 189, 192, 241, 384 INDEX-GLOSSARY. 289; Nictitating, 229 ; m. semilunaris, 248; Shell m., 256; Subzonal, 309; Tympanic, 219, 255, 273, 304; m. tympaniformis interna, 248; Vitelline, 73, 252, 299. Mesenteron (^ueo-os, middle ; evTspov^ a bowel) — That part of the alimentary tube which is lined by epithelium of hypoblastic origin ; the mid-gut, 48. Mesentery (/xea-os ; evrtpopos, a kidney) — The third-formed excretory organ — typically developed in the Pigeon and Rabbit, 265, 316, 333, 334. Metapleural, 136. Metapophysis (/uera, in addition; dirocpva-i^^ a projecting part), in Vertebra?— A process on the anterior end of the arch, above the praezygapophysis, 273. Metapterygium after; TTreovyioi/^ a fin), 159, 160. INDEX-GLOSSARY. 385 Metastoma (^sra, behind ; /rrofia, a mouth), in the Crayfish — The lower lip, 89. Metatarsus, 188. Metatheria, 346. Metazoa (^sra, after, later ; living creatures) — Animals consisting of more than one cell, 339, Chap. ii. to end, 71. Micronucleus, 72. Micropyle little; -ry'Xif, door), 119. Microzooid, 15, 72. Mid-brain — The part of the brain developed from the second cerebral vesicle, 169, 211, 253, 302. Mid-gut (Mesenteron) — That part of the alimentary canal which is lined by epithelium of endodermic origin, 48, 57, 78, 94. Mitosis {/x'tTo;, a thread) = Karyokinesis, r/.r. Mississippi, 349. Mollusca {mollis, soft), 340; Chap. vii. , 109, 161. Monocystis, 17, 18, 64. Monodelphia, 346. Monotremata, 345, 349, 352, 353, Morphology a shape; Xoyoi, speech, discourse), 1, 2; Amoeba, 7 ; Vorticella, 11; Hydra, 19; Distomum, 28; Tsenia, 40; Ascaris, 47; Lumbricus, 54 ; Hirudo, 75 ; Astacus, 83 ; Anodonta, Unio, 109 ; Helix, 122 ; Amphioxus, 136 ; Scyllium, 153 ; Rana, 175 ; Columba, 229 ; Lepus, 268 ; Homo, 322. Morula (dim. of morum, a mulberry) — A solid blastula, 38. Mosaic vision, 104. Motor, of Nerves — Supplying muscular tissue and conveying impulse leading to its contraction, 102, 146, 209, 216. Moults, 106. Mouth, 19, 29, 38, 47, 54, 57, 70, 76, 92, 113, 125, 157, 241, 268, 284, 313, 329. Mouth-cavity = Cavity, buccal, q.v. Mud-fish, 153. Multipolar, 145, 214. Muridse, 349. Muscle, 209; Involuntary m.,208; Striped or striated m., 91, 145, 202, 208, 209 ; Unstriped m., 132, 192 : Voluntary m., 208 ; Abductor, 91 ; Adductor, 91, 109 ; Circular, 65, 81 ; of Eye, 174, 222, 256, 305 ; Ex- tensor, 91 ; Flexor, 91 ; Gastric, 93 ; of Heart, 291 ; Longitudinal, 65, 81; Oblique, 81, 174, 222; Papillary, 245; Protractor, 29, 65, 119; Rectus, 174, 222; Retractor, 29, 81, 119; Spindle m., 123, 132. Muscle-cell, 81, 2 25 386 INDEX-GLOSSARY. Muscle- corpuscle, 91. Muscle-fibre, 91, 192, 201, 209. Muscularis mucosae, in Vertebrates — A thin layer of unstriated muscle traversing the mucous membrane of the stomach and intestines, 192, 288. Muscular system — Distoma, 34; Taenia, 43; Ascaris, 48; Lumbricus, 58, 64; Hirudo, 81; Astacus, 91; Anodonta, Unio, 119; Helix, 122; Amphioxus, 144; Scyllium, 168; Rana, 208; Columba, 252; Lepus, 299. Musculature — The muscle system, .34. Mussel (salt-water), 122. Mussel (fresh-water), 100-122. Mustelus, 167. 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Constituting, we believe, the only English work that satisfactorily treats of the manufacture of oils, colours, and pigments.’ — Chemical Trades' fournal. “ Throughout the work are scattered hints which are invaluable to the intelligent reader. ” — Invention. GARMENT DYEING AND CLEANING. A Practical Book for Practical Men. With Numerous Illustrations. General Contents. — Technology of the Textile Fibres — Garment Cleaning — Dyeing of Textile Fabrics— Bleaching— Finishing of Dyed and Cleaned Fabrics — Scouring and Dyeing of Skin Rugs and Mats— Cleaning and Dyeing of Feathers— Glove Cleaning and Dyeing— Straw Bleaching and Dyeing— Glossary of Drugs and Chemicals — Useful Tables. LONDON: EXETER STREET, STRAND. SCIENTIFIC AND TECHNOLOGICAL WORKS. ^5 COAL-MINING (A Text-Book of): FOR THE USE OF COLLIERY MANAGERS AND OTHERS ENGAGED IN COAL-MINING. BY HERBERT WILLIAM HUGHES, F.G.S., Assoc. Royal School of Mines, Certificated Colliery Manager. Second Edition. In Demy 82/0, Handsotne Cloth. With very Numerous Illustrations^ mostly reduced from Working Drawings. 1 8y. "The details of colliery work have been fully described, on the ground that collieries are more often made remunerative by perfection in small matters than by bold strokes of engineering. ... It frequently happens, in particular localities, that the adoption of a combination of small improvements, any of which viewed separately may be of apparently little value, turns an unprofitable concern into a paying one.” — Extract from Authors Preface. GENERAL CONTENTS. Geology; Rocks — Faults — Order of Succession — Carboniferous System in Britain. Coal: Definition and Formation of Coal — Classification and Commercial Value of Coals, Search, for Coal : Boring — various appliances used — Devices employed to meet Difficulties of deep Boring — Special methods c\f Boring— Mather & Platt’s, American, and Diamond systems — Accidents in Boring — Cost of Boring — Use of Boreholes. Breaking Ground; Tools — Transmission of Power: Compressed Air, Electricity — Power Machine Drills — Coal Cutting by Machinery Cost of Coal Cutting — Explosives — Blasting in Dry and Dusty Mines— Blasting by Electricity — Various methods to supersede Blasting. Sinking: Position, Form, and Size of shaft — Operation of getting down to " Stone-head” — Method of proceeding afterwards— Lining shafts — Keeping out Water by Tubbing— Cost of Tubbing — Sinking by Boring — Kind - Chaudron, and Lipmann methods — Sinking through Quicksands — Cost of Sinking. Preliminary Operations : Driving underground Roads — Supporting Roof : Timbering, Chocks or Cogs, Iron and Steel Supports and Masonry — Arrangement of Inset. Methods of Working: Shaft, Pillar, and Subsidence — Bord and Pillar System — Lancashire Method — Longwall Method — Double Stall Method — Working Steep Seams — Working Thick Seams — Working Seams lying near together — Spontaneous Combustion. Haulage: Rails — Tubs — Haulage by Horses — Self-acting Inclines — Direct-acting Haulage — Main and Tail Rope — Endless Chain — Endless Rope — Comparison. Winding; Pit Frames — Pulleys— Cages — Ropes — Guides — Engines — Drums — Brakes — Counterbalancing — Expansion — Condensation — Compound Engines — Prevention of Overwinding — Catches at pit top — Changing Tubs — Tub Controllers — Signalling Pumping: Bucket and Plunger Pumps — Supporting Pipes in Shaft — -Valves — Suspended lifts for Sinking — Cornish and Bull Engines— Davey Differential Engine — Worthington Pump — Calculations as to size of Pumps — Draining Deep Workings — Dams. Ventilation: Quantity of air required — Gases met with in Mines — Coal-dust — Laws of Friction — Production of Air-currents — Natural Ventilation — Furnace Ventilation — Mechanical Ventilators — Efficiency of Fans — Comparison of Furnaces and Fans — Distribution of the Air-current — Measurement of Air- currents. Lighting: Naked Lights — Safety Lamps — Modern Lamps — Conclusions — Locking and Cleaning Lamps— Electric Light Underground — Delicate Indicators. Works at Surface; Boilers — Mechanical Stoking — Coal Conveyors — Workshops. Preparation of Coal for Market: General Considerations- — Tipplers — Screens — Varying the Sizes made by Screens— Belts— Revolving Tables — Loading Shoots — Typical Illustrations of the arrange- ment of Various Screening Establishments— Coal Washing — Dry Coal Cleaning —Briquettes. " Quite THH BEST BOOK of its kind ... as practical in aim as a book can be . . « touches upon every point connected with the actual working of collieries. The illustrations are EXCELLENT.”- “A Text-book on Coal-Mining is a great desideratum, and Mr. Hughes possesses ADMIRABLE QUALIFICATIONS for Supplying it. . . . We cordially recommend the work.” — Colliery Guardiatt. " Mr. Hughes has had opportunities for study and research which fall to the lot of but few men. If we mistake not, his text-book will soon come to be regarded as the STANDARD WORK of its kind.” — Biryningham Daily Gazette. Note. — The first large edition of this work was exhausted within a few months of publication. LONDON: EXETER STREET, STRAND. i6 CHARLES GRIFFIN vo. Fully Illustrated. 1. STEAM AND STEAM-ENGINES (A Text-Book on). For the Use of Students: preparing for Competitive Examinations. With over 200 Illustrations, Folding Plates, and Examination Papers. Tenth Edition. Revised and Enlarged, 8/6. “ Professor Jamieson fascinates the reader by his clearness of conception and SIMPLICITY OF expression. His treatment recalls the lecturing of Faraday.” — Athenceum. “The Best Book yet published for the use of Students.” — Engineer. “ Undoubtedly the most valuable and most complete Hand-book on the subject that now exists.” — Marine Engineer, 2. MAGNETISM AND ELECTRICITY (An Advanced Text- Book on). Specially arranged for Advanced and “ Plonours ” Students. 3. APPLIED MECHANICS (An Advanced Text-Book on). Specially arranged for Advanced and “ Honours” Students. PROFESSOR JAMIESON’S INTRODUCTORY MANUALS. In Crown 8z/o, Cloth. With very niwierous Illlustrations and Examination Papers. 1. STEAM AND THE STEAM-ENGINE (Elementary Text- Book on). Specially arranged for First-Year Students. Fourth Edition. 3/6. “ Quite the right sort of book.” — Engineer. “ Should be in the hands of every engineering apprentice.” — Practical Engineer. 2. MAGNETISM AND ELECTRICITY (Elementary Text- Book on). Specially arranged for First-Year Students. Third Edition. 3/6. “ A capital text-book . . . The diagrams are an important feature.” — Schoolmaster. “A THOROUGHLY TRUSTWORTHY Text-book. . . . Arrangement as good as well can be. . . . Diagrams are also excellent. . . . The subject throughout treated as an essentially practical one, and very clear instructions given.” — Nature. 3. APPLIED MECHANICS (Elementary Text-Book on). Specially arranged for First- Year Students. Second Edition. 3/6. “Nothing is taken for granted. . . . The work has very high qualities, which may be condensed into the one word ‘clear.’” — .Science and Art. A POCKET-BOOK of ELECTRICAL RULES and TABLES. FOR THE USE OF ELECTRICIANS AND ENGINEERS. Pocket Size. Leather, 8s. 6d. Tenth Edition, revised and enlarged. (See under Munro and Jamieson.) LONDON : EXETER STREET, STRAND. SCIENTIFIC AND TECHNOLOGICAL WORKS. 17 “ The MOST VALUABLE and useful work on Dyeing that has yet appeared in the English language . . . likely to bo the Standard Work of Reference for years to come.” — Textile Mercury. In Two Large 8vo Volumes, 920 - ’ pp., with a SUPPLEMENTARY Volume, containing Specimens of Dyed Fabrics. Handsome Cloth, 45s. A MANUAL OF DYEING: FOR THE USE OF PRACTICAL DYERS, MANUFACTURERS, STUDENTS, AND ALL INTERESTED IN THE ART OF DYEING. E. KNECHT, Ph.D., F.I.C., Head of the Chemistry and Dyeing Department ®f the Technical School, Manchester; Editor of “The Journal of the Society of Dyers and Colourists ; ” BY CHR. RAWSON, F.I.C., F.C.S., Late Head of the Chemistry and Dyeing Depatment for the Technical College, Bradford ; Member of Council ot the Society of Dyers and Colourists ; And RICHARD LOEWENTHAL, Ph.D. General Contents.— Chemical Technology of the Textile Fabrics— Water— Washing and Bleaching — Acids, Alkalies, Mordants — Natural Colouring Matters— Artificial Organic Colouring Matters— Mineral Colours Machinery used in Dyeing — Tinctorial Properties of Colouring Matters — Analysis and Valuation of Materials used in Dyeing, &c., &c. This MOST VALUABLE WORK . . Will be Widely appreciated.” — Chemical J^ews. '^^^s ^^thoritative and exhaustive work . . . the most complete we have yet seen on the subject."— Textile Manufacturer. The MOST exhaustive and complete work on the subject extant.” — Textile Recorder. , distinguished authors have placed in the hands of those daily engaged in the dye- nouse or laboratory a work of extreme value and undoubted utility . . . appeals ^^‘^^“Ologist, colour chemist, dyer, and more particularly to the rising dyer generation. A book which it is refreshing to meet \v\tb." —American Textilt LONDON: EXETER STREET, STRAND. i8 CHARLES GRIFFIN d; CO.S PUBLICATIONS, ELECTRO-METALLURGY (A Treatise on): Embracing the Application of Electrolysis to the Plating, Depositing, « Smelting, and Refining of various Metals, and to the Repro- duction of Printing Surfaces and Art- Work, &c. By WALTER G. M‘MILLAN, E.LC., F.C.S., Chemist and Metallurgist to the Cossipore Foundry and Shell-Factory ; Late Demonstrator of Metallurgy in K ing's College,, London. With numerous Illustrations. Large Crown 8vo. Cloth 10s. 6d. General Contents. — Introductory — Sources of Current — General Condition to be observed in Electro-Plating — Plating Adjuncts and Disposition of Plant — Cleansing and Preparation of Work for the Depositing- Vat, and Subsequent Polishing of Plated Goods — Electro -Deposition of Copper — Electrotyping — Electro-Deposition of Silver — of Gold — of Nickel and Cobalt — of Iron — of Platinum, Zinc, Cadmium, Tin, Lead, Antimony, and Bismuth ; Electro-chromy — Electro- Deposition of Alloys — Electro-Metallurgical Extraction and Eefining Processes — Recovery of certain Metals from their Solutions or Waste Substances — Determination of the Proportion of Metal in certain Depositing Solutions — Appendix. “ This excellent treatise, . . . one of the best and most complete manuals hitherto published on Electro-Metallurgy.” — Electrical Review. “ This work will be a standard.” — Jeweller. “Any metallurgical process which reduces the cost of production must of necessity prove of great commercial importance. . . . We recommend this manual to all who are interested in the practical APPLICATION of electrolytic processes.” — Nature. Second Edition. Enlarged, and very fully Illustrated. Cloth, 4s. fid. STEAM - BOILERS; THEIR DEFECTS, MANAGEMENT, AND CONSTRUCTION. By R. D. MXJNRO, Chief Engineer of the Scottish Boiler Insurance and Engine Inspection Company. This work, written chiefly to meet the wants of Mechanics, Engine- keepers, and Boiler-attendants, also contains information of the first import- ance to every user of Steam-power. It is a practical work written for prac- tical men, the language and rules being throughout of the simplest nature. “ A valuable companion for workmen and engineers engaged about Steam Boilers, ought to be carefully studied, and always at hand.” — Coll. Guardian. “ The subjects referred to are handled in a trustworthy, clear, and practical manner. . . . The book is very useful, especially to steam users, artisans, and young engineers.” — Engineer. By the same Author. KITCHEN BOILER EXPLOSIONS: Why they Occur, and How to Prevent their Occurrence? A Practical Hand- book based on Actual Experiment. With Diagrams and Coloured Plate, Price 3s. LONDON : EXETER STREET, STRAND. SCIENTIFIC AND TECHNOLOGICAL WORKS. 19 MUNRO & JAMIESON^S ELECTRICAL POCKET-BOOK, Tenth Edition, Revised and Enlarged. A POCKET-BOOK OF ELECTRICAL RULES & TABLES FOR THE USE OF ELECTRICIANS AND ENGINEERS. BY JOHN MUNRO, C.E., & Prof. JAMIESON, M.Inst.C.E., F.R.S.E. With Numerous Diagrams. Pocket Size. Leather, 8s. 6d. GENERAL Units of Measurement. Measures. TaSTING. Conductors. Dielectrics. Submarine Cables. Telegraphy. Electro-Chemistry. “Wonderfully Perfect. . . give it.” — Electrician. “The Sterling Value of Messrs. Electrical Review. CONTENTS. Electro-Metallurgy. Batteries. Dynamos and Motors. Transformers. Electric Lighting Miscellaneous. Logarithms. Appendices. Worthy of the highest commendation we can Munro and Jamieson’s Pocket-Book.” — MUNRO (J. M. H., D.Sc., Professor of Chemistry, Downton College of Agriculture) : AGRICULTURAL CHEMISTRY AND ANALYSIS: A Prac- tical Hand-Book for the Use of Agricultural Students. NYSTROM'S POCKET-BOOK OF MECHANICS AND ENGINEERING. Revised and Corrected by W. Dennis Marks, Ph.B., C.E. (YALE S.S.S.), Whitney Professor of Dynamical Engineering, University of Pennsylvania. Pocket Size. Leather, 15s. Twentieth Edition, Revised and greatly enlarged. LONDON : EXETER STREET, STRAND. 20 CHARLES GRIFFIN CO.^S PUBLICATIONS, Demy 8vo, Handsome cloth, 18s. Physical Geology and Palaeontology, OM THE BASIS OF PHILLIPS. BY HARRY GOVIER SEELEY, F. R. S., PROFESSOR OF GEOGRAPHY IN KING’S COLLEGE, LONDON. TWUtb ^frontispiece in Cbromos^Xitbograpb^, anb illustrations. “ It is impossible to praise too highly the research which Professor Seeley's ‘ Physical Geology ’ evidences. It is far more than a Text-book — it is a Directory to the Student in prosecuting his researches.” — Presidential Ad- dress to the Geological Society, Rev. Prof. Bonney. D.Sc., LL.D., F.R.S. “ Professor Seeley maintains in his ‘ Physical Geology ’ the high reputation he already deservedly bears as a Teacher. ” — Dr. Henry Wood- ward, F.R.S., in the “ Geological Magazine.'''' “ Professor Seeley’s work includes one of the most satisfactory Treatises on Lithology in the English language. ... So much that is not accessible in other works is presented in this volume, that no Student of Geology can afford to be without it.” — American Journal of Engineering. Demy 8vo, Handsome cloth, 3Jfs. Stratlgraphlcal Geology k Palsontology, OJf THE BASIS OF PHILLIPS. BY ROBERT ETHERIDGE, F. R. S., OF THE NATURAL HIST. DEPARTMENT, BRITISH MUSEUM, LATE PALAEONTOLOGIST TO THE , GEOLOGICAL SURVEY OF GREAT BRITAIN, PAST PRESIDENT OF THE GEOLOGICAL SOCIETY, ETC. TRUltb /Sbap, IFlumerous ^Tables, anb plates. Prospectus of the above important work — perhaps the most elaborate of its kind ever written, and one calculated to give a new strength to the study of Geology in Bj itain — may be had on application to the Publishers. “No such compendium of geological knowledge has ever been brought together before." — Westminster Review. “ If Prof. Seeley’s volume was remarkable for its originality and the breadth of its views, Mr. Etheridge fully justifies the assertion made in his preface that his book differs in con- struction and detail from any known manual. . . . Must take high rank among works OF RKFKRENCK.” — Atheneeum. LONDON: EXETER STREET, STRAND. SCIENTIFIC AND TECHNOLOGICAL WORKS. 21 Thikd Edition. With Folding Plates and Many Illustrations. Large 8vo. Handsome Cloth. 36s. ELEMENTS OF METALLURGY: A PRACTICAL TREATISE ON THE ART OF EXTRACTING METALS FROM THEIR ORES. By J. ARTHUR PHILLIPS, M.Inst.O.E., F.C.S., F.G.S., portioned to the various branches of the subject, according to their relative importance. 3. The MacArthur-Forrest Cyanide Process is fully described for the first time. By this process over £2,000,000 of gold per annum (at the rate of) is now being extracted, or nearly one-tenth of the total world’s production. The process, introduced in 1887, has only had short newspaper accounts given of it previously. The chapters have been submitted to, and revised by, Mr. MacArthur, and so freed from all possible inaccuracies. 4. Among other new processes not previously described in a text-book are — (1) The modern barrel chlorination process, practised with great success in Dakota, where the Black Hills district is undergoing rapid development owing to its introduction. (2) New processes for separating gold from silver — viz., the new Gutzkow process, and the Electrolytic process ; the cost of separation is reduced by them by one-half. .5. A new feature is the description of exact methods employed in particular extraction works — Stamp-batteries of South Africa, Australia, New Zealand, California, Colorado, and Dakota; Chlorination works also, in many parts of the world ; Cyanide works of S. Africa and New Zealand. These accounts are of special value to practical men. 6. The bibliography is the first made since 1882. “Mr. Rose gained his experience in the Western States of America, but he has secured details of gold-working from all parts of the world, and these should be of great service to practical men. . . . The four chapters on C'Wortn«G'o«, written from the point of view alike of the practical man and the chemist, teem with considerations hitherto unrecog- nised, and constitute an addition to the literature of Metallurgy, which will proye to be of classical value.” — Nature. “The most complete description of the chlorination process which has yet been published. —Mining Journal. LONDON; EXETER STREET, STRAND. 28 CHARLES GRIFFIN A CO:S PUBLICATIONS. SCHWACKHOFER and BROWNE: FUEL AND WATER: A Manual for Users of Steam and Water, By Prof. FRANZ SCHWACKHOFER of Vienna, and WALTER R. BROWNE, M.A., C.E., late Fellow of Trinity College, Cambridge, Demy 8vo, with Numerous Illustrations, 9/. General Contents. — Heat and Combustion — Fuel, Varieties of — Firing Arrange- ments : Furnace, Flues, Chimney — The Boiler, Choice of — Varieties — Feed-water Heaters— Steam Pipes — Water: Composition, Purification — Prevention of Scale, &c., &c- “The Section on Heat is one of the best and most lucid ever written.” — Engiruer. “ Cannot fail to be valuable to thousands using steam power." — Railway Engineer. SEXTON (Humboldt, F.I.C., F.C.S., F.R.S.E., Prof, of Metallurgy, Glasgow and West of Scotland Technical College) : • METALLURGY (An Elementary Manual of). With numerous Illustrations. Crown 8vo, extra. 6s. OUTLINES OF QUANTITATIVE ANALYSIS. For the Use of Students. With Illustrations. Fourth Edition. Crown 8vo, Cloth, 3s. “A COMPACT LABORATORY GUIDE for beginners was wanted, and the want has been well SUPPLIED. ... A good and useful book.” — Lancet. OUTLINES OF QUALITATIVE ANALYSIS. For the Use of Students. With Illustrations. Third Edition. Crown 8vo, Cloth, 3s. 6d. “The work of a thoroughly practical chemist.” — British Medical Journal. “Compiled with great care, and will supply a Journal of Education. SHELTON-BEY (W. Vincent, Foreman to the Imperial Ottoman Gun Factories, Constantinople) : THE MECHANIC’S GUIDE : A Hand-Book for Engineers and Artizans. With Copious Tables and Valuable Recipes for Practical Use. Illustrated. Second Edition. Crown 8vo. Cloth, 7/6. SMITH (Robert H., M.Inst.Mech.E., Prof, of Engineering, Mason Science College, Birmingham) : GRAPHIC TABLES for the Conversion of Measurements (English and French). 43 Diagrams for the Mutual Conversion of Measurements in Different Units of Lengths, Areas, Volumes, Weights, Stresses, Densities, Quantities of Work, Horse- Powers, Temperatures, &c. For the Use of Practical Engineers, Surveyors, Architects, and Contractors. 4to, Price 7s. 6d. LONDON: EXETER STREET, STRAND. SCIENTIFIC AND TECHNOLOGICAL WORKS. 29 Eleventh Edition. Price 18s. Demy 8vo, Cloth. With Numerous Illustrations^ reduced from Working Draurings. A MANUAL OF MARINE ENGINEERING: COMPRISING THE DESIGNING, CONSTRUCTION, AND WORKING OF MARINE MACHINERY. By A. E. SEATON, M. Inst. C. E., M. Inst. Meeh. E., M.Inst.N.A. GENERAL Part I.— Principles of Marine Propulsion. Part II.— Principles of Steam Engineering. Part III.— Details of Marine Engines: Design and Cal- GONTENTS. culations for Cylinders, Pistons, Valves, Expansion Valves, &c. Part IV.— Propellers. Part V.— Boilers. Part VI.— Miscellaneous, “In the three-fold capacity of enabling a Student to learn how to design, construct, and w'ork a modern Marine Steam-Engine, Mr. Seaton’s Manual has NO rival as regards comprehensiveness of purpose and lucidity of treatment.” — Times. “The important subject of Marine Engineering is here treated with the thorough- ness that it requires. No department has escaped attention. . . . Gives the results of rnucH close study and practical work.” — Engineering. “ By far the best Manual in existence. . . . Gives a complete account of the methods of solving, with the utmost possible economy, the problems before the Marine Engineer.” — Athenceum. “The Student, Draughtsman, and Engineer will find this wwk the most valuable Handbook of Reference on the Marine Engine now in existence.” — Marine Engineer. Second Edition. With Diagrams. Pocket-Size, Leather. 8s. 6d. A POCKET-BOOK OF MARINE ENGINEERING RULES AND TABLES, FOR THE USE OF Marine Engineers, Naval Architects, Designers, Draughtsmen, Superintendents and Others. BY A. E. SEATON, M.I.O.E., M.I.Mech.E., M.I.N.A., AND H. M. ROUNTHWAITE, M.I.Mech.E., M.I.N.A. “Admirably fulfils its purpose.” — Marine Engineer. LONDON: EXETER STREET, STRAND. 30 CHARLES GRIFFIN A CO.’S PUBLICATIONS. By PROFESSORS J. J. THOMSON & POYNTING. In Large 8vo. Fully Illustrated. A TEXT-BOOK OF PHYSIOS: COMPRISING PROPERTIES OF MATTER; HEAT; SOUND AND LIGHT; MAGNETISM AND ELECTRICITY. BY J. H. POYNTING, J. J. THOMSON, SC.D., F.R.S., Late Fellow of Trinity College, Cambridge ; Professor of Physics, Mason College, Birmingham. and r.R.s., Fellow of Trinity College, Cambridge; Prof, of Experimental Physics in the University of Cambridge. Second Edition, Revised and Enlarged. Focket-Sise, Leather, also for Oflce Use, Cloth, 12s. BOILERS, MARINE AND LAND; THEIR CONSTRUCTION AND STRENGTH. A Handbook of Rules, Formula, Tables, &c., relative to Material, Scantlings, and Pressures, Safety Valves, Springs, Fittings and Mountings, &c. jfor tbe 'dse of all SteamsUlBerB, By T. W. TRAILL, M. Inst. 0. E., E. E. R. NT., Engineer Surveyor-in-Chief to the Board of Trade. In the New Issue the subject-matter has been considerably extended, and Tables have been added for Pressures up to 200 lbs. per square inch. “Very unlike any of the numerous treatises on Boilers which have preceded it. . . . Really useful. . . . Contains an Enormous Quantity of Information arranged in a very convenient form. . . . Those who have to design boilers will find that they can settle the dimensions for any given pressure with almost no calculation with its aid. ... A most useful volume . . . supplying information to be had nowhere else.”— P/ie Engineer. “As a handbook of rules, formulae, tables, &c., relating to materials, scantlings, and pressures, this work will prove most useful. The name of the Author is a sufficient guarantee for its accuracy. It will save engineers, inspectors, and draughtsmen a vast amount of calculation.”— iYafure. “By such an authority cannot but prove a welcome addition to the literature of the subject. . . . We can strongly recommend it as being the most complete, eminently practical work on the subject.” — Marine Engineer. “To the engineer and practical boiler-maker it will prove invaluable. The tables in all pro- bability are the most exhaustive yet published. . . . Certainly deserves a place on the shelf in the drawing office of every boiler shog. — Practical Engineer. LONDON; EXETER STREET; STRAND. SCIENTIFIC AND TECHNOLOGICAL WORKS. 31 WORKS BY DR. ALDER WRIGHT, F.R.S. FIXED OILS, FATS, BUTTERS, AND WAXES: THEIR PREPARATION AND PROPERTIES, And the Manufacture therefrom of Candles, Soaps, and Other Products. BY C. R. ALDER WRIGHT, D.Sc., F.R.S., Late Lecturer on Chemistry, St. Mary’s Hospital School ; Examiner in “Soap” to the City and Guilds of London Institute. In Large 8vo. Handsome Cloth. With 144 Illustrations. 28s. “Ur. Wright’s work will be found absolutely indispensable by every Chem^ist. Teems with information valuable alike to the Analyst and the Technical Chemist.” — The A7ialyst. “Will rank as the Standard English Authority on Oils and Fats for many years to come.” — Industries and Iron. Second Edition. With very Numerous Illustrations. Handsome Cloth, 6«l Also Presentation Edition, Gilt and Gilt Edges, 7s. 6d. THE THRESHOLD OF SCIENCE: Simple and Amusing* Experiments (over 400) in Chemistry and Physics. *** To the New Edition has been added an excellent chapter on the Systematic Order in which Class Experiments should be carried out for Educational purposes. “Anyone who may still have doubts regarding the value of Elementary Science as an organ of education will speedily have his doubts dispelled, if he takes the trouble to understand the methods recommended by Dr. Alder Wright. The Additions to the New Edition wiU be of great service to all who wish to use the volume, not merely as a ‘ play-book,’ but as an instrument for the TKAiNiNG of the mental faculties.”— “ Step by step the learner is here gently guided through the paths of science, made easy by the perfect knowledge of the teacher, and made flowery by the most striking and curious experiments. Well adapted to become the treasured FRIEND of many a bright and promising lad.” — Mamhester E^miner. LONDON : EXETER STREET, STRAND. CHARLES GRIFFIN Jc CO.’S PUBLICATIONS. 32 WELLS (Sidney H.,Wh.Sc., Assoc. Mem. Inst. C.E., Assoc. Mem. Inst. Mech. E.; Principal of, and Head of the Engineering Department in, Battersea Polytechnic Institute, late of Dulwich College) : ENGINEERING DRAWING AND DESIGN. A Practical Manual for Engineering Students. With very numerous Illustrations and Folding- Plate. In Large Crown 8vo. VoL. I. — Practical Geometry, Plane, ani> Solid. 3s. VoL. II. — Machine and Engine Drawing and Design. 4s. 6d. “A THOROUGHLY USEFUL WORK, exceedingly well written. For the many Examples and Questions we have nothing but praise.” “The Examples are WELL chosen and treated in an able manner. The Illustrations do GREAT CREDIT to the publishers.” — Science a}id Art. “A CAPITAL TEXT-BOOK, arranged on an excellent system, calculated to give an in- telligent grasp of the subject, and not the mere faculty of mechanical copying. . . . Mr. Wells shows how to make complete working-drawings, discussing fully each step in the design.” — Electrical Review. Twelfth Annual Issue. Handsome cloth, 7s. 6d. THE OFFICIAL YEAR-BOOK OF THE Scientific anC) Xearnet) Societies of Great :fi3ritain auD 5reIanD* COMPILED FROM OFFICIAL SOURCES. Comprising {together with other Official Information) LISTS of the PAPERS read during 1894 before all the LEADING SOCIETIES throughout the Kingdom engaged in the following Departments of Research ; — § I. § 2. § 3- §4- §5- Science Generally : i.e., Societies occupy- ing themselves with several Branches of Science, or with Science and Literature jointly. Mathematics and Physics. Chemistry and Photography. Geology, Gepgraphy, and Mineralogy. Biology, including Microscopy and An- thropology. 7- 8. Economic Science and Statistics. Mechanical Science and Architecture. Naval and Military Science. Agriculture and Horticulture. Law. §11. Literature. §12. Psychology. §13. Archccology. § 14. Medicine. § 5 § § 9. § 10. “ The Year-Book of Societies is a Record which ought to be of the greatest use for the progress of Science.”— Lyon Playfair, F.R.S., M.P., Past-President of the British Association. “ It goes almost without saying that a Handbook of this subject will be in time one of the most generally useful works for the library or the desk.”— Times. “British Societies are now well representea in the ‘Year-Book of the Scientific Learned Societies of Great Britain and Ireland.’”— (Art. “Societies” in New Edition of “ Encyclopaedia Britannica,” vol. xxii.) Copies of the First Issue, giving an Account of the History, Organization, and Conditions of Membership of the various Societies, and forming the groundwork of the Series, may still be had, price 7/6. Also Copies of the following Issues. The YEAR-BOOK OF SOCIETIES forms a complete index to THE SCIENTIFIC w’^ORK of the year in the various Departments. It is used as a ready Handbook in all our great Scientific Centres, Museums, and Libraries throughout the Kingdom, and has become an indispensable book of reference to every one engaged in Scientific Work. ^ ^ ^ LONDON : EXETER STREET, STRAND.