PRINCIPLES OF COMPARATIVE PHYSIOLOGY. PRINCIPLES OF COMPARATIVE PHYSIOLOGY. BY WILLIAM B. £ARPENTER, M.D., P.R.S., F.G.S., EXAMINER IN PHYSIOLOGY AND COMPARATIVE ANATOMY IN THE UNIVERSITY OF LONDON; PROFESSOR OF MEDICAL JURISPRUDENCE IN UNIVERSITY COLLEGE J PREsT&l$NT OF THE MICROSCOPICAL SOCIETY OF LONDON, ETC. ETC. WITH THREE HUNDRED AND NINE WOOD ENGRAVINCtS. A NEW AMERICAN, FROM THE FOURTH AND REVISED LONDON EDITION PHILADELPHIA BLANCHARD AND LEA 1854. PHILADELPHIA; T. K. AND P. G. COLLINS, PRINTERS. TO SIR JOHN F. W. HERSCHEL, BART., K.H., F.R.S., L., AND E., ETC., OTf)tsi Unlttme IS MOST RESPECTFULLY DEDICATED, AS A TRIBUTE DUE ALIKE TO HIS HIGH SCIENTIFIC ATTAINMENTS AND MORAL WORTH, AND AS AN EXPRESSION OF GRATITUDE FOR THE BENEFIT DERIVED FROM HIS "DISCOURSE ON THE STUDY OF NATURAL PHILOSOPHY," BY THE AUTHOR. EDITOR'S NOTICE. The connection of the subscriber with the other works of Dr. Carpenter has, at the request of the American publishers, induced him to undertake the supervision of the present volume, during its passage through the press. In this, his attention has been principally directed to obtaining an accurate reprint of the corrected sheets as they were furnished by the author. A few additional cuts have been introduced where they seemed to aid in illustration; any additions to the text would have marred the symmetry and completeness of the volume. FRANCIS GURNEY SMITH, M.D. 291 Spruce Street, Philadelphia, August, 1854. PEEP ACE. " SCIENCE IS THE KNOWLEDGE OF MANY, ORDERLY AND METHODICALLY DIGESTED AND ARRANGED, SO AS TO BECOME ATTAINABLE BY ONE." Sir John F. W. Herschci. Although three Editions of the Author's " Principles of General and Comparative Physiology" have appeared in England—in the years 1839, 1841, and 1851 respectively—no reprint of these took place in the United States. By an arrangement effected, however, between the American and the English publishers, the Third Edition was printed on joint account; and its reception in America was such as to confirm the former in their intention of reprinting the next English Edition, in order that it might range with their reprint of the Author's Human Physiology, of which five Editions have appeared in the United States. The present "Volume, however, is not altogether a new edition of the original "Principles of General and Comparative Physiology;" for the success of the previous edition of that work —as evinced, not merely by its rapid sale, but by the numerous expressions of high appreciation which it drew forth from those most competent to judge of its merits, both in England and America —has encouraged the Author to carry into effect a change of plan which had suggested itself to him during its preparation. For having been led on by the desire of rendering his work as complete as possible, to enlarge it to the utmost admissible dimensions of a single volume, he felt that it would be impossible to do justice to any subsequent extensions which its subject might receive, without making some alteration in its form. And this conclusion acquired a greatlyincreased force, when the demand for a new Edition led him to survey the deficiencies, which, notwithstanding all his care, had been left in the former one; and to estimate the amount of new matter, not only deserving but requiring notice, which the diligence of observers in various departments of this comprehensive Science PREFACE. X had accumulated in the short interval. Instead of dividing the entire Treatise into two Volumes, however, as suggested to him by many of his friends, the Author has preferred to divide its subjects, so as to treat of them separately though connectedly; and he has chosen, for various reasons, to proceed first with the "Comparative Physiology." The portion of the former edition which treated of this subject has been largely augmented, and carefully revised throughout; and the Author ventures to think that this Treatise more completely represents the state of the Science at the period of its publication, than it has done on any preceding occasion. He can honestly say that he has spared no time or labor in its preparation, which it has been in his power to bestow. And he looks with hope, therefore, to a continuance of that friendly indulgence with regard to errors and shortcomings, which has been so liberally afforded on previous occasions. As to certain points on which his opinions have undergone modification, he can again refer with satisfaction to the following passage in the Preface to his former editions: " Truth is his only object; and, even if his own doctrines should be overthrown by more extended researches, he will rejoice in their demolition, as he would in that of any other error. The character of the true philosopher as described by Schiller — one who has ahvays loved truth letter than Ms system —will ever, he trusts, be the goal of his intellectual ambition." In attempting to embody in a Systematic Treatise the general aspect of Physiology or any other Science of like comprehensiveness, it will be obvious that an Author, however extensive his own range of acquirement, must largely avail himself of the labors of others; and that the scientific character of such a treatise must depend, not so much on the amount of original matter it may contain, as on the degree in which " the knowledge of many" has been "orderly and methodically digested and arranged, so as to become attainable by one." It is by this standard that the Author desires his work to be tried; and he cheerfully leaves the verdict to the judgment of those, who are qualified by their own knowledge of the subject to pronounce it. He feels it due to himself, however, to state that he has devoted considerable time and attention to the verification of the statements of other observers, especially on points under dispute—a kind of labor which is but little appreciated by those, who contemptuously designate works like the present as "mere compilations;" and that a large amount of materials, drawn from his own original inquiries, is scattered through the work. It would have been easy for him to bring these last into greater prominence, PREFACE. XI had he been so disposed ; but as his constant aim has been, to work out his general plan harmoniously and methodically, rather than to force any one portion of it into undue prominence, he has generally preferred to allow his own contributions to pass undistinguished, rather than to be continually obtruding his personal claims upon the attention of his readers. He would remark, moreover, that originality may be as much shown in the development of new relations between facts and phenomena observed by others, as in the first discovery of such facts; and he believes that by the mode in which he has combined and arranged his materials, he has frequently been enabled to impart a new and unexpected value to statements, which, in their previously isolated condition, were of comparatively insignificant import. Although, in the selection of these materials, the Author has endeavored to avail himself of the best and most recent information he could procure upon each department of the subject, it is scarcely to be expected that he should be equally well-informed upon every point; and those who have followed particular departments into detail, will doubtless find scope for criticism in what, they may regard as deficiencies, or even as errors. Here, again, the Author must beg that his work may be estimated by its general merits; and rather by what it does, than by what it does not contain. It would have been far easier to expand it by mere compilation to twice its present dimensions, than it has been found to compress the accumulated mass within the space which it even now occupies. It has been the Author's endeavor, wherever practicable, to draw the materials, both for his text and for its illustrations, direct from original Treatises and Monographs; and thus to avoid the errors which too frequently arise from second-hand transmission. To have attempted, however, to assign each individual fact to its original discoverer, each doctrine to its first enunciator, would have augmented the bulk of the volume far beyond the dimensions appropriate to a Text-Book; and while most desirous to avoid taking credit for what is not his own, the Author has felt himself compelled to limit his references, for the most part, to those new facts and doctrines, which cannot be yet said to have become part of the common stock of Physiological Science. The number of such references has been largely increased in the present edition; and the "Index of Authors" which has been added, will, it is hoped, be found useful in enabling the reader at once to turn to the notice of any original observation that he may desire to retrace. The Illustrations not his own, which likewise have received numerous important additions, XII PREFACE. are referred to their originals in the list at the commencement of the Volume; and this list will also afford useful assistance to those who may desire to carry out their inquiries in any particular direction. The Author cannot bring his task to a conclusion, without expressing the great obligations under which he lies to his friend, Mr. T. H. Huxley, not only for many valuable suggestions, but also for the readiness which he has on all occasions evinced, to impart to him whatever he might seek from his own extensive stores of original and acquired information; nor without paying his tribute of regard to the memory of his lamented friend, Mr. G. Newport, whose premature death has deprived British Science of one of its most ardent and disinterested votaries, at a time when he was beginning to reap, in the appreciation of his discoveries on the Impregnation of the Amphibia, 1 the credit so justly due to his laborious, accurate, and sagacious researches, in the new field to the cultivation of which he had latterly applied himself. It is the Author's intention to reproduce the " General Physiology" of his former Edition, as a companion volume to the present, so soon as the numerous demands upon his time may permit him to bestow upon that part of his revision the careful attention which it requires. University Hall, London, June 1, 1854. 1 In a Postscript to the work referred to in the Note to p. 532, written almost contemporaneously with Mr. Newport's decease, Prof. Bischoff states that he has himself confirmed Mr. N.'s observation of the penetration of the Spermatozoon into the ovum of the Frog, and full credit for the determination of this important fact. TABLE OE CONTENTS. 5. Progress from General to Special in Function . . . . . 155 4. Animal Functions separately considered . . . . .154 3. Organic Functions separately considered . . . • 151 2. Mutual Relations of Organic and Animal Functions . • . 148 1. Analysis and Classification of Phenomena presented by Vital Action . .144 GENERAL VIEW OF THE VITAL OPERATIONS OF LIVING BEINGS, AND OF THEIR MUTUAL RELATIONS. CHAPTER II. 7. Geological Succession of Organic Life . . . . . .134 Monstrosities ....... 132 Rudimentary Organs ...... 129 6. Progress from General to Special in Development . . . .123 Vertebrata (Fishes, Reptiles, Birds, Mammals) . . .98 Articulata (Entozoa, Annelida, Myriapoda, Insects, Crustacea, Arachnida) . . . . ... 87 Mollusca (Bryozoa, Tunicata, Brachiopoda, Lamellibranchiata, Gasteropoda, Pteropoda, Cephalopoda) . . . .79 Radiata (Polypifera, Acalephae, Echinodermata) . . .70 Protozoa (Porifera, Rhizopoda, Infusoria) . ... .69 5. General Survey of Animal Kingdom. Phanerogamia . . . . . . .62 Acrogens (HepaticEe, Mosses, Ferns) . . . .58 Thallogens (Algae, Lichens, Fungi) . . . . .53 Prolophyta . . . ... . . .51 4. General Survey of Vegetable Kingdom. 3. Diversities in Grade of Development . . . . . . 48 2. Conformity of Structure of each group to General Design or Archetype:— Progress from General to Special in its various modifications . . . 41 1. Analysis and Comparison of Phenomena afforded by Organic Structure:— Homology and Analogy .......... 33 ON THE GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. CHAPTER I. CONTENTS. XIV CHAPTER III. OP ALIMENT, ITS INGESTION, AND PREPARATION. 1. Sources of Demand for Aliment . • • • • .157 2. Nature of the Alimentary Materials . . • • • .163 3. Ingestion and Preparation of Aliment in Plants .... 174 4. Ingestion and Preparation of Aliment in Animals . . . -176 Agastric Animals . . • • • • .176 Unicellular animals . • ¦ • • • 1'« Polystome Animals ....... 180 Oral Apparatus . . • • • .181 Prehensile Appendages . . • . . .184 Reducing Apparatus . . . . . .187 Digestive Apparatus . . . . . .191 Nature of Digestive Process ..... 203 CHAPTER IY. OF ABSORPTION AND IMBIBITION. 1. General Considerations . . . . . . . 206 2. Absorption in Vegetables ....... 212 3. Absorption in Animals . . . . . . .218 CHAPTER Y. OF THE CIRCULATION OF NUTRITIVE FLUID. 1. General considerations ....... 230 2. Circulation in Vegetables ....... 231 3. Circulation in Animals . . . . . . . . 238 Absence of Special. Circulation in Certain Classes . . . 242 Circulation in Radiata: Echinodermata ..... 244 Circulation in Articulata: Annelida . . . . . 246 Myriapoda ..... 251 Insects ...... 254 Arachnida ..... 256 Crustacea ..... 258 Circulation in Mollusca: Bryozoa ..... 261 Tunicata . . . . .261 Brachiopoda ..... 264 Lamellibranchiata . . . . 265 Gasteropoda ..... 267 Cephalopoda ..... 269 Circulation in Vertebrata: Fishes ..... 270 Reptiles ... • . 273 Birds ...... 277 Mammals ..... 277 Forces which move the Blood . .. . . . . 279 Development of Circulating Apparatus ..... 283 Malformations of Circulating Apparatus . . . / . 291 CONTENTS. XV CHAPTER VI. OF RESPIRATION. 1. General Considerations . . . . . , . . 293 2. Respiration in Plants ... ..... 29G 3. Respiration in Animals ....... 803 Aquatic Respiration : Protozoa, Zoophytes, and Acalepbae . . 806 Echinodermata ..... 307 Water-Vascular System: Rotifera . . . . . .309 Entozoa . . . . . .310 Branchial Respiration: Bryozoa ..... 312-13 Tunicata . . . . . .313 Brachiopoda . . , . .316 Lamellibranchiata . . . .316 Gasteropoda . . . . . 318 Cephalopoda . . . 319 Annelida . , . . . 319 Crustacea . . . . .321 Fishes ...... 323 Batrachia ..... 326 Atmospheric Respiration: Myriapoda ..... 327 Insects . '. . . . 328 Arachnida . . . . .332 Fishes (air-bladder) .... 333 Perennibranchiata .... 334 Reptiles . . . . . 335 Birds . . . . . 337 Mammals ..... 339 Development of Respiratory Apparatus ..... 341 Alterations effected by Respiratory Process .... 343 CHAPTER VII. OF THE EXHALATION OF AQUEOUS VAPOR. 1. General Considerations ....... 348 2. Exhalation in Plants . . . . . . . .348 3. Exhalation in Animals ........ 354 CHAPTER VIII. OF NUTRITION. 1. General Considerations . . . . . . 359 Term of Duration of Individual Parts ..... 360 Assimilation and Formation ...... 364 2. Nutrition in Vegetables . . . . . . .868 Growth and Multiplication of Cells ..... 368 Assimilating Process in Vascular Plants ..... 377 Production of Vegetable Organic Compounds .... 379 CONTENTS. XVI . . . . , . 385 8. Nutrition in Animals . Assimilation of Nutritive Materials . • • • 'or Chyle and Lymph . • • • • .387 Vascular Glands . . . 390 Composition and Properties of Blood of Vertebrata . 391 Nutritive Fluid of Zoophytes . . . .397 Echinodermata . . . 397 Articulata .... 398 Mollusca . . • .400 Growth and Multiplication of Cells - • • • .401 Production of Animal Organic Compounds . . . .405 Conditions of Nutritive Activity in Animals . . . .409 CHAPTER IX. OF SECRETION AND EXCRETION. 1. General Considerations . . • ¦ • • 411. 2. Secretion in Vegetables . • - * • . . 412 3. Secretion in Animals . . - - • • • .414 Structure of Glands in general . . . . . . 415 The Liver, and .he Secretion of Bile . . . . . 420 Biliary Apparatus of Invertebrata ..... 420 Vertebrata . . . . .424 Development of Liver . . . . . . 427 Properties and uses of Bile ...... 428 Of the Kidneys and the Urinary Excretion . . . .431 Urinary Apparatus of Invertebrata . . . .431 Vertebrata ..... 432 Development of Kidney ...... 435 Composition and Properties of Urine . . . • 436 Cutaneous and Intestinal Secretions ..... 439 Special Secretions 440 Metastasis of Secretion . . . . . . .441 CHAPTER X. EVOLUTION OF LIGHT, HEAT, AND ELECTRICITY. 1. General Considerations ... . . . . 442 2. Evolution of Light . . . . . . .443 Evolution of Light in Vegetables ...... 443 Evolution of Light in Animals ...... 444 Luminosity of the Sea ...... 445 Luminous Insects ....... 447 CONTENTS XVII 3. Evolution of Heat . ,,¦ , . ... . . 450 Evolution of Heat in Vegetables ...... 450 Evolution of Heat in Animals ...... 458 Cold-blooded Animals ...... 454 Insects . . . . . . . 455 Warm-blooded Animals ...... 458 Conditions of Evolution of Heat ...... 460 4. Evolution of Electricity . . . . . . .461 Evolution of Electricity in Vegetables . . . . . 462 Evolution of Electricity in Animals ..... 462 Electric Fishes ....... 467 CHAPTER XI. OF GENERATION AND DEVELOPMENT. 1. General Considerations . . . . . . . . 472 Developmental and Regenerating Power . . . . .476 Multiplication by Gemmation . . . . . . 479 True Generative Process . . . . . . . 480 Alternation (so called) of Generations . . . . . 481 2. Generation and Development in Plants ...... 482 Multiplication of Phytoicls . . . . . . .483 Generation and Development of Protophyta . . . . 484 Alga) . . . . . 489 Characeoe ..... 493 Lichens . 495 Fungi . . . . .497 Hepaticoe and Mosses . . . 500 Ferns . . . . .503 Equisetaceae .... 509 Lycopodiaceae . . , . 509 Marsileaceae .... 510 Gymnospermeae . . . .511 Angiospermous Phanerogamia . . 513 3. Generation and Development in Animals ..... 524 Multiplication of Zooids ....... 525 Development and Actions of Spermatozoa ..... 527 Development and Structure of Ova ..... 530 Fecundation of Ova, and subsequent Changes . . . .531 Generation and Development of Protozoa . . . . . 535 Infusoria ..... 535 Porifera . . . . 538 Generation and Development of Radiata ..... 541 Polypifera . . . .539 Compound Hydroida . . . 513 Acalephse . . . . . 554 Echinodermata .... 556 2 XVIII CONTENTS. Generation and Development of Mollusca . . • • • 562 Bryozoa . . . • 882 Tunicata . . . . . 5G3 Brachiopoda .... 508 Lamellibranchiata . . . 568 Gasteropoda .... 570 Cephalopoda .... 573 Generation and Development of Articulata .... 578 Entozoa ..... 578 Eotifera . . . . .582 Annelida . . < . 583 Myriapoda .... 587 Insects . . . . 588 Crustacea . . . 593 Cirrhipcda .... 596 Arachnida . . . . 598 Generative Apparatus of Vertebrata ..... 600 Fishes . . . . . 601 Reptiles . . . . .602 Birds . . . .603 Mammals ..... 605 Embryonic Development of Vertebrata .... 609 Area Germinativa . . . . . .610 Formation of Amnion . . . . .612 Development of Allantois . . . . .614; Formation of Placenta . . . . .615 Conditions determining Sex . . . . . .618 Lactation: Composition of Milk . . . . . . 620 4. On the Laws of the Exercise of the Reproductive function . . . 621 Species and Varieties ....... 621 Hybridity . . . . . . 623 Modifying influence of External Conditions .... 623 Origination of New Varieties ...... 625 Transmission of Acquired Peculiarities . . . . . 627 CHAPTER XII. OP THE SENSIBLE MOTIONS OF LIVING BEINGS. 1. General Considerations ........ 629 2. Motions of Plants . . . . . . . .630 3. Motions of Animals . . . . . . .633 CHAPTER XIII. OF THE FUNCTIONS OF THE NERVOUS SYSTEM. 1. General Considerations . . . . . . . . 636 2. Comparative View of the Nervous System in the Animal Series . . 639 No Evidence of Nervous System in Zoophytes .... 640 CONTENTS. XIX Nervous System of Radiata ...... 641 Acalephco ...... 041 Echinodcrmata ..... 641 Nervous System of Mollusca ...... 642 Bryozoa ...... 643 Tunicata . . . . " . 643 Brachiopoda . . . •*»' • • Lamellibranchiata 644 Gasteropoda . . . . . . 645 Cephalopoda ...... 648 Nervous System of Articulata ...... 650 Entozoa . . . . . . 656 Annelida . . . . . . 657 Myriapoda . . . . . 657 Insects . . . . , . 657 Crustacea ...... 659 Arachnida ...... 660 Nervous System of Vertebrata . . . . . .601 Fishes . . . . . . 663 Reptiles . . . . .667 Birds ....... 667 Mammalia . . . . . . 668 History of Development of Brain .... 665—670 Functions of the Cranio-Spinal Axis .... 671 Spinal Cord . . . . .672 Medulla Oblongata ..... 675 Sensory Ganglia . . . . . .676 Functions of the Cerebellum ..... 681 Functions of the Cerebrum . ... . . 682 Functions of the Sympathetic System . . . . 688 General Summary . . . . . . . . 689 CHAPTER XIV. OP SENSATION, AND THE ORGANS OF THE SENSES. 1. Of Sensation in General . . . . . 693 2. Of the Sense of Touch, and its Instruments ..... 696 3. Of the Sense of Taste, and its Instruments ..... 698 4. Of the Sense of Smell, and its Instruments ..... 699 5. Of the Sense of Hearing, and its Instruments . . . . 702 6. Of the Sense of Sight, and its Instruments ..... 708 CHAPTER XV. OF THE PRODUCTION OF SOUNDS BY ANIMALS. 720 LIST OF ILLUSTRATIONS. FIG. PAGE 1. Pterodactylus crassirostris (xxvi.) . . . . . .85 2. Different forms of Anterior Member ...... 37 3. Diagram, illustrating the Nature of Limbs (lxiv.) . . . .40 4. Group of Analifa laevis (xxiii.) . . . , . .43 5. Anatomy of Anatifa laevis (xxiii.) . . . . . .43 6. Development of Bolanus balanoides (vn.) . . . . .45 7. Comparison of Leucifer with Lcpas (xxv.) . . . . .45 8. Ophiura texturata , , . . . \ .50 9. Pentacrinites briareus (xvi.) . . . . . , .50 10. Ifermospora transversalis (lxix.) . . . . . .52 11. Bangia velutina (lxix.) ...... 52 12. Mesogloia vermicularis (lxix.) . . . . . . .54 13. Zonariaplantaginea (lxix.) . . . . , 54 14. Dasya kuelzingiana (lxix.) . . . . . , .55 15. Marginaria gigas (lxix.) ...... 55 10. Parmelia acetabulum (lxix.) . . . . . , .56 17. Sphserophora coralloides (lxix.) ..... 56 18. Slysanas caput-medusae (lxix.) . . . . . .56 19. Clavaria crispula (lxix.) . . . . , . .57 20. JEcidium ticssilagi?tis (lxix). . . . . . . . 57 21. Marchantiapolymorphia (lxix.) . . . . . .58 22. Fissidens bryoides (lxix.) . . . . . . \. 58 23. Marchantiapolymorpha with antheridia (lxix.) . . . .59 24. Marchantia polymorpha with pistillidia (lxix. ) . . 59 25. Polytrichum commune (lxix.) . . . . . . .59 26. Trichomanes speciosum (lxix.) . . . . . 60 27. Frond of Scolopendrium vulgare (lxix.) . . . . .60 28. Frond of Osmunda regalis (lxix.) . . . . . .60 29. Equisetum arvense (lxix.) . ... . . . .60 30. Lycopodium cernuum (lxix.) . . . . , .61 31. Marsilea quadrifolia (lxix.) . . . ... . .62 32. Ideal Plant (lxxviii.) . . ..... 63 33. Amoeba princcps (xxxv.) . . . . . . .69 84. Ilydria fusca (xc.) . . . . . . . .72 35. Diagrammatic section of Actinia (lxxx.) . . . . .73 36. Structure of Cyancea aurita (xxxvi.) . . . . , .75 37. Anatomy of Asterias aurantiaca (lxxxviii.) . . . . .76 38. Comatula rosacea (xxxix.) . . . . . . .77 LIST OF ILLUSTRATIONS. XXII FIG. PAGE 39. Echinus mammillatus (xxin.) . . • • • .77 40. Anatomy of Holothuria tubulosa (xxin.) . • • • .78 41. Chitoncllus and Chiton (xvin.) . • • • .79 42. Salpa maxima (xxiii.) . • • • . . .79 43. Anatomy of Maclra (xxxiv.) ....... 80 44. Anatomy of Paludina vivipara (xxn.) . . . . .81 45. Shells of Gasteropod 3Iollush (xviii.) . . . • .82 46. Hyalcea, Criseis, and Clio (xviii.) . . . . . .82 47. Sepia officinalis (xviii.) ....... 82 48. Embryos of Nudibranchiate Gasteropoda (n.) . . . .82 49. Laguncula repens (xcv.) ....... 83 50. Anatomy of Aplysia (xxn.) ....... 84 51. Botryllus violaceus (xxiii.) ....... 86 52. Anatomy of Strongylus gigas (xi.) . . . . . .87 53. Nephthys Ilombergii (xxiii.) . . . . . .89 54. lulus (xxxiv.) ... . . . . . .91 55. Scolopendra (xxxiv.) . . . . . . . .91 56. Section of the trunk of Melolontha vulgaris (lxxxiv.) . . .93 57. Ideal section of Sphinx ligustri (lxiii.) . . . . .95 58. Anatomy of Cancer pagurus (xxxiv.) . . . . .96 59. Inferior surface of Limulus moluccanus (xxxiv.) . . . .97 60. Cyclops quadricornis (vi.) ....... 97 61. Diagram of Archetype Vertebral Skeleton (lxv.) . . . .100 61*. [Skeleton of Cobra] . .• . . .101 62. Ideal Section of a Mammal (lxiv.) ...... 102 63. Oblique view of Vertebra of Cod (lxxiv.) . . . . .104 63*. [Skeleton of Sea Perch] . . . . . .105 64. Bimanus and Seps (xxxiv.) . . . . . . .107 64*. [Segment of Carapace and Plastron] . . . . . 108 65. Emysaura Serpentina (xxvu.) . . . . . . .108 66. Skeleton of Ichthyosaurus (xxvi.) . . . . . .109 67. Skeleton of Plesiosaurus (xxvi.) . . . . . .109 68. Skull of Mososaurus (xxi.) . . . . . . .111 69. Portion of jaw of Megalosaurus (xxi.) ..... Ill 70. Portion of lower jaw and teeth of Iguanodon (lvii.) . . . 112 70*. [Skeleton of the Swan] . . . . . . .114 70f. [Skeleton of the Sloth] . . . . . .118 70j. [Skeleton of Whale] ....... 119 70§. [Skeleton of Giraffe] . . . . . . . 120 70||. [Skeleton of Mole] ........ 121 71. Lower jaw of Phascolotherium Buchlandii (lxvii.) .... 122 72. Molar tooth of Asiatic Elephant (xxi.) . . . . .122 73. Duplicative subdivision of cells of Chlamydomonas (xxxv.) . . .123 74. Early stage of Mammalian Ovum (xix.) and Young of Volvox . . 123 75. Nicothoe astaci (xcvi.) .... 127 76. Ogygia Buchii (xxn.) and Limulus moluccanus (xxxiv.) . . .135 77. Metamorphosis of Carcinus mssnas (xx.) . . , ' 135 78. Homocercal and Heterocercal Tails of Fish . . . . .136 79. Skeleton of Palseotherium magnum (xxi.) ..... 137 80. Molar tooth of Mastodon (xxi.) . . . , . 138 81. Caryocrinites ornalus (xv.) ..... . 189 LIST OF ILLUSTRATIONS. XXIII TIG. . PAGE 82. Lingula anatina (xviii.) . . . . . . . 139 83. Section of Shell of Nautiluspompilius (xviii.) . . . 140 84. Exterior view and section of Orthoceratite (xviii.) .... 140 85. Cephalaspis Lyellii . . . . . . . . 141 86. Skull of Labyrinthodon . . . . . . . 141 87. Skull of Rhyncosaurus (lxviii. ) ...... 142 88. Skeleton of Mylodon (lxvi.) . . . . . .142 89. Pitchers of Dischidia Rafflesiana (c.) ..... 175 90. Polygastric Animalcules, according to Ehrenberg (xxxv.) . . . 179 91. Section of a young branch of Alcyonium stellatum (xxxiii.) . .180 92. Digestive apparatus of Rhizostoma (xxin.) . . . . .181 93. Digestive apparatus of Thaumantias (xxxvni.) . . . .183 94. Holothuria phantapus (xxin.) . . . , m .184 95. Anatomy of Echinus lividus (xxin.) . . . , , .187 96. Rotifer vulgaris (xxxv.) . . . . , ¦ 189 97. Compound stomach of Sheep . . , . . .190 98. Section of part of the Stomach of Sheep (xxxvn.) . . .191 99. Portions of Campanularia gelatinosa, natural size, and magnified (xciv.) . 192 100. Sections of Alcyonian Polype (xxxin.) ..... 194 101. Structure of Polycelis lasvigatus.(Lxxi.) ..... 195 102. Cydippepileus (xl.) and JBeroe Forskalii (xxiii.) . . . .196 103. Digestive apparatus of Annelida (xxiii.) . . . . .196 104. Eolis Inca, a Nudibranchiate Casteropod (n.) .... 198 105. Digestive apparatus of Ammotheapycnogonoides (lxx.) . . . 198 106. Digestive apparatus of 3fygale (xxin.) . . . . .198 107. Digestive apparatus of Common Fowl (xxxiv.) .... 200 108. Villi of Human Intestine .... '" 220 109. Longitudinal Section of Stem of Italian Reed (lxxviii.) . . .232 110. Laticiferous Vessels (lxxxi.) .... 236 111. Bloodvessels of Frog's foot (xcix.) ...... 240 112. Formation of Capillaries in Tail of Tadpole (l.) .' . . .242 113. Circulating apparatus of Terebella conchilega (xxx.) . . . 249 114. Circulating apparatus of Eunice sanguinea (xxx.) .... 249 115. Circulating apparatus of Arenicolapiscatorum (xxx.) . . . 251 116. Dorsal vessel of Scolopendra (lxiii.) . . . . . 252 117. Circulating system of Scolopendra (lxiii.) . . . . 253 118. Circulating system of Buthus (lxiii.) . . . . .256 119. Heart of Mygale (xxiii.) ....... 258 120. Circulating system of Lobster (xxviii.) ..... 259 121. Anatomy of Amarouciumproliferum (xxix.) .... 262 122. Circulating system of Salpa maxima (xxin.) .... 264 123. Circulating system of Pinna marina (xxxn.) .... 266 124. Circulating system of Snail (xxxiv.) ..... 267 125. Circulating system of Octopus (xxin.) . . . . .269 126. Circulating system of Fish (xxiii.) . . . , , 271 127. Anatomy of Amphioxus (lxiv. ) . . . . ' . 272 128. Circulating system of Lizard (xxxiv.) ..... 274 129. Respiratory Circulation in Tadpole (xxxiv.) .... 275 130. The same in transition state (xxxiv.) ..... 275 131. The same in the perfect Frog (xxxiv.) ..... 276 132. Diagram of the Circulation in Birds and Mammals (xxxiv.) . . 278 LIST OF ILLUSTRATIONS. XXIV PIG. PAGE 133. Vascular area of Fowl's egg (xcix.) ..... 284 134. Diagram of formation of great Arterial trunks in Fowl . . . 286 135. Diagram of Circulation in Human Embryo (en.) .... 289 136. Water-vascular system of Taenia solium (xi.) .... 310 137. Anatomy of Fasciola hepaticum (xi.) ..... 311 138. Anatomy of Perophora (lvi.) . . . . . .314 139. Portion of Branchial sac of Perophora (lvi.) . . . . 315 140. Respiratory apparatus of Pholas crispata (m.) .... 316 141. Branchial lamina? of Pholas crispata (in.) . . . . .317 142. Portion of gill of Doris Johnstoni (n.) ..... 318 143. Doris Johnstoni, showing tuft of external gills (u.) .... 318 144. Cephalic tuft of Sabella unispira (xxxiv.) ..... 320 145. Branchial arch and leaflets of Fish ...... 824 146. Lamprey, showing branchial orifices . . . . . 325 147. Proteus anguineus, showing external branchiae (xxvn.) . . . 827 148. Axoloti, showing external branchiae (xxvn.) . , . . 327 149. Tracheal system of Nepa cinerea (xxxiv.) . . . . . 329 150. Lepidosiren paradoxa (xxxiv.) ...... 335 151. Respiratory organs of Frog (xxxvn.) ..... 335 152. Lungs of Bimanus, Pipes, and Coluber (xxxvn.) .... 336 153. Section of Lung of Turtle (xn.) . . . . . .337 153*. Pulmonary apparatus of Pigeon (xxxvn.) ..... 838 154. Capillaries of air-cells of Human Lung ..... 340 155. Vertical Section of leaf of LAlium album (xiv.) . . . . 349 156. Under surface of leaf of Lilium album (xiv.) . . . . 349 157. Surface and Section of frond of Marchantia polymorpha (lviii.) . . 350 158. Sudoriferous Gland from Human Hand (xcix.) .... 355 159. Section of leaf of Agave, showing primordial utricles of cells (xlii.) . 368 160. Portions of Nitella flexilis, natural size and enlarged (lxxxi.) . . 369 161. Circulation of fluid in hairs of Tradescantia Virginica (lxxxi.) . . 370 162. Various stages of development of Haematococcus binalis (xliii.) . . 371 163. Frocess of cell-multiplication in Conferva glomerata (lix.) . . . 372 164. Development of zoospores of Achlyaprolifera (xcin.) . . 374 105. Multiplication of Cartilage-cells by subdivision (liii.) . . .402 166. Section of branchial Cartilage of Tadpole (lxxix.) . . . 403 167. Endogenous cell-growth in cells of Meliceritous Tumor (xli.) . - . 403 168. Cells with radiating fibres (i.) ...... 405 169. Capillary network around follicles of Parotid Gland (vin.) . . . 415 170. Glandular follicles of Stomach (lxi.) . . . . 41G 171. Mammary Gland of Ornithorhyncus (lxi.) ..... 417 172. Rudimentary Pancreas, from Cod (lxi.) ..... 417 173. Lobule of Parotid Gland of Human Infant (xcix.) .... 417 174. Biliary tubuli of Musca carnaria (lii.) ..... 422 175. Hepatic caecum of Astacus affinis (lxi.) ..... 423 176. Lobules of Liver of Squilla (lxi.) ...... 423 177. Glandular cells of Human Liver .... 424 178. Arrangement of Bloodvessels in Human Liver (xlviii.) . . . 426 179. Connection of Lobules of Liver with Hepatic Vein (xlviii.) . . 426 180. Early stage of Development of Liver of Fowl (lxi.) . . 427 181. Kidney of Foetal Boa (lxi.) . . . . , .432 182. Section of Kidney of Coluber (lxi.) ..... 432 LIST OP ILLUSTRATIONS. XXV FIG. - PAGE 183. Pyramidal fasciculus of Tubuli uriniferi of Bird (lxii.) . . . 433 184. Section of Human Kidney (on.) . . • • • • 433 185. Portion of Tubulus Uriniferus with tessellated epithelium (xcix.) . . 434 186. Distribution of Vessels of Kidney (xni.) ..... 434 187. Corpora Wolffiana, from Chick (lxii.) . . . . . 435 188. Nocliluca miliaria (lxxii.) ... . . . . 444 189. Pelagia nocliluca (xxxiv.)* . . . . . . 446 190. Electric Apparatus of Torpedo (lxxvi.) ..... 469 191. Diagram of Generative Process in Plants . . . . . 483 192. Multiplication of Coccochloris by subdivision (xliii.) . . . 484 193. Conjugation of Euastrum oblongum (lxxiii.) .... 485 194. Conjugation of Eunotia lurgida (lxxxvii.) ..... 486 195. Development of Spores in Aulacoseira (lxxxvii.) .... 487 196. Conjugation of Zygnema (li. ) ...... 488 197. Generative apparatus of Fucus platycarpus (lxxxvi.) . . . 491 198. Tetraspores of Carpocaulon mediterraneum (li.) .... 493 199. Generative apparatus of Char a fcetida (lxxxi.) .... 494 200. Generative apparatus of Collema pulposum (xci.) .... 496 201. Generative apparatus of Tremella mesenterica (xcn.) . . . 497 202. Generative apparatus of Agaricus campestris (lxxvii.) . . . 498 203. Development of Torula cerevisias ...... 499 204. Development of Archcgonia of Marchantia (lxix.) .... 500 205. Archegonia of Jungermannia (xliv.) ..... 500 206. Sporangia on lobed receptacles of Marchantia (lxix.) . . . 501 207. Gemmiparous conceptacles of Marchantia (lviii.) . . . . 502 208. Development of prothallium of Ptcris (liv.) .... 504 209. More advanced prothallium of Pteris (liv.) ..... 504 210. Development of Antheridia and Antherozoids of Pteris (liv.) . . 505 211. Development of Archegonium of Pteris (liv.) .... 506 212. Development of Embryo of Polypodium (liv.) . • . . 507 213. Fructification of Equisetum arvense (lxix.) ..... 509 214. Generation and development of Lycopodium (xliv.) . . . 510 215. Sporocarp of Marsilea quadrifolia (lxix.) . . . . . 510 216. Germination of Marsilea Fabri (lxix.) . . . . .511 217. Generative apparatus of Coniferae (xliv.) . . . . . 512 218. Development of Embryo of (Enotheracese. (xlv.) .... 515 219. Embryos of Potamogeton and Amygdalus (xlvn.) .... 517 220. Germination of Zanichellia and Acer (xlvii.) .... 519 221. Constituent parts of Mammalian Ovum (xix.) . . . - 580 222. Segmentation of vitellus of Ascaris acuminata (v.) . . . . 533 223. First segmentation of vitellus of Mammalian Ovum (xix.) . . . 534 224. Early stages of development of Coregonus (xcvn.) .... 535 225. Fissiparous multiplication of Chilodon cucullulus (xxxv.) . . . 536 226. Group of Vorticellse in various states (xxxv.) .... 537 227. Development and Metamorphosis of Vorlicella microstoma (lxxxiii.) . 538 228. Gemmation of Hydra fusca (xc.) . . . . . 540 229. Generative apparatus of Actinia (lxxxv.) ..... 541 230. Development of polype-bud of Campanularia (xciv.) . . . 543 231. Generation and Development of Cordylophora lacustris (iv.) . . 545 232. Development of Medusa-buds from Perigonimus (lxxv.) . . . 546 233. Medusiform gemma) of Campanularia (xciv.) .... 547 XXVI LIST OF ILLUSTRATIONS. TIG- PAGE 23-1. Strobila (or polypoid state of Medusa) propagating by gemmation (xxxv.) . 549 235. Group of Strobilae in process of Medusan gemmation (xxiv.) . . 551 236. Development of Medusa-buds from Strobila (xxiv.) . . . .551 237. Development of 3Iedusee from Strobila-gemma3 (xxiv.) . . . 552 238. Gemmiparous multiplication of Cytseis (lxxv.) .... 553 239. Structure of Velella limbosa (xi/vx.) ...... 555 240. Crinoid state of Comatula rosacea (xxiii.) ..... 556 241. Development of embryo of Echinaster rubens (lxxv.) . 557 242. Bipinnaria asterigera, or larva of Star-fish (lxii.) .... 559 B43. Embryonic development of Echinus (lxii.) ..... 560 244. Origin of Ophiura from Plutcus (lxii.) . . . . . 561 245. Gemmiparous extension of Laguncula (xcv.) .... 5G3 246. Development of embryo of Amaroucium (xxix.) . . . . 565 247. Anatomy of more-advanced embryo of Amaroucium (xxix.) . . 566 248. Development of embryo of Acteon viridis (xcvu.) . . . .572 249. Male Argonaut, showing Hectocotylus-arm (lx.) .... 575 250. Successive stages of development of Sepia (xlix.) .... 577 251. Generative apparatus of Taenia solium (xi.) ..... 578 252. Generative apparatus of Distoma hepaticum (xi.) .... 580 253. Generative organs of Nais filiformis (xxin.) . . . . 535 254. Early stages of development of Terebella nebulosa (xxxi.) . . . 586 255. Ideal Section of Larva of Sphinx ligustri (lxiii.) .... 592 250. Ideal Section of Pupa of Sphinx ligustri (lxiii.) . . . . 592 257. Generative apparatus of Fowl (xxxvn.) . . . . . 604 258. Later stage of segmentation of Mammalian Ovum (xix.) . . . 609 259. Germ and surrounding parts from Uterine Ovum (xix.) . . . 610 260. Ovum of Coregonus, in early stage of development (xevn.) . . .611 261. Incipient formation of Amnion (xcix.) . . . . .612 262. Embryo of Coregonus (xevn.) . . . . . 613 263. The same more advanced (xevn,) . . . . . . 614 264. Further development of Amnion in Human Ovum (xcix.) . . . 615 265. Incipient formation of Allantois in Human Ovum (xcix.) . . .615 266. Further development of Allantois in Human Ovum (xix.) . . . 616 267. Formation of Vascular Tufts in Human Ovum (xix.) . . .616 268. Extremity of Villus of Human Placenta (xn.) , . . . .617 269. Side view of Embryo of Dog (ix.) . . . . 617 270. Front view of Embryo of Dog (ix.) ..... 618 271. Nervous System of Solen (x.) . . . . 644 272. Nervous System of Argonauta argo (xxin.) . . . . .648 273. Nervous Centres of Octopus (xxin.) ..... 649 274. Nervous Centres of Sepia, (xxiii.) ...... 649 275. Gangliated Column of Larva of Sphinx ligustri (lxiii.) . . .650 276. Portion of ganglionic tract of Folydesmus (lxiii.) . . . .652 277. Parts of Nervous System of Centipede and Sphinx (lxiii.) . . .654 278. Nervous System of Iidus terreslris (lxiii.) ..... 655 279. Nervous Centres of Spider (xxin.) . . . , 660 280. Nervous System of Androclonus (lxiii.) . . . . .661 281. Nervous Centres of Frog (lv.) . . . . 662 282. Brains of Fishes (lv.) ... 664 283. Brain of Turtle (lxxxii.) . . 667 284. Brain of Buzzard (lv.) . . . 668 LIST OF ILLUSTRATIONS. XXVII FIQ. PAGE 285. Early stages of Development of Brain of Human Embryo (lxxxix.) . GGS 286. Brain of Squirrel (lxxxii.) . . . . . . . 869 287. Brain of Rabbit (lv.) 669 288. Later stages of Development of Brain of Human Embryo (lxxxix.) . 670 289. Capillary loops of Papilla? of Human Skin (vin.) .... 696 290. Capillaries of Fungiform Papilla? of Human Tongue (vin.) . . 699 291. Structure of the Organ of Hearing in Man (xxxiv.) . . . 706 292. Front view of Head of Bee, showing Compound and Simple Eyes . .710 293. Section of Compound Eye of Melolontha (lxxxiv.) .... 710 294. Faceted Eye of Asaphus (xvi.) . . . . . 711 295. Section of Globe of Human Eye (en.) ..... 713 296. Skull of Ichthyosaurus, showing sclerotic osseous plates (xvi.) . . 714 297. Diagram of the course of the rays in the Eye . . . .716 298. Stereoscopic figures . . . . . _ 719 299. View of Human Larynx from above (ci.) ..... 723 300. Artificial Larynx (01.) . . . . . . t 724 TREATISES AND MEMOIRS REFERRED TO IN THE LIST OF ILLUSTRATIONS. L Addison, The Actual Process of Nutrition demonstrated: Part ill. li. Alder and Hancock, Monograph of the British Nudibranchiate Mollusca. in. On the Branchial Currents in Pholas and Mya. (Ann. of Nat. Hist., 2dSer., Vol. vm.) iv. Allman, On Cordylophora lacustris. (Phil. Trans., 1853.) v. Bagge, De Evolutione Strongyli et Ascaridis. vi. Baird, Natural History of Entomostraca. vn. Bate (C. Spence), On the Development of the Cirripedia. (Ann. of Nat. Hist., 2d Ser., Vol. vm.) Viil. Berres, Anatomie der Mikroskopischen Gebilde des Menschlichen Korpers. ix. Bischoff, Entwickelungs-geschichte des Hunde-Eies. x. Blanchard, Sur le Systeme Nerveux des Mollusques Acephales Testaces. (Ann. des Sci. Nat., 3 e Se>., Zool., Tom. in.) xi. —: Sur 1'Organisation des Vers. (Ann. des Sci. Nat. 3° Ser., Zool, Tom. vn.—xn.) (See also xxin.) xn. Bojanus, Anatomie Testudinis Europeoe. xin. Bowman, On the Structure and Use of the Malpighian Bodies of the Kidney. (Phil. Trans., 1842.) Xiv. Brongniart, Sur la Structure, &c, des Feuilles. (Ann. des Sci. Nat., l e Se"r. Tom. xxi.) xv. Buck (L. von), Ueber die Cystideen. xvi. Buchland, Bridgewater Treatise on Geology, xvn. Burmeister, On the Organization of Trilobites. xviii. Chenn, Traite" de Conchyologie. xix. Coste, Histoire Gene"rale et Particuliere du Developpement des Corps Organises. xx. Couch (R.), On the Metamorphosis of the Decapod Crustaceans. (Report of Cornwall Polytechnic Society for 1843.) xxi. Cuvier, Ossemens Fossiles. xxn. . - sur les Mollusques. xxin. Regne Animal. (Edition de Planches gravees par une reunion de Disciples de Cuvier.) XXX TREATISES AND MEMOIRS REFERRED TO IN ILLUSTRATIONS. xxiv. Dalyell, Rare and Remarkable Animals of Scotland. xxv. Darwin, Monograph of the Cirripedia. xxvi. D'Orbigny, Cours E16mentaire de Paldontologie. xxvn. Dumeril et Bibron, Histoire Naturelle des Reptiles. xxviii. Edwards (Milne), Histoire Naturelle des Crustaces. xxix. Sur les Ascidies Composees. xxx. Sur la Circulationchez les Annelides. (Ann. des Sci. Nat., 2 e Ser., Zool., Tom. x.) xxxi. Sur le Ddveloppement des Annelides. (Ann. des Sci. Nat., 3 e Ser., Zool., Tom. in.) xxxn. Sur la Circulation chez les Mollusques. (Ann. des Sci. Nat., 3 e Ser., Zool., Tom. vin.) xxxiii. Recherches sur les Polypes. xxxiv. Cours Elementaire de Zoologie. (See also xxiii.) xxxv. Ehrenberg, Die Infusionsthierchen. XXXVI. Des Leucthen des Meeres. (Abhaldlungen der Kon. Akad. der Wissenschaften zu Berlin, 1835.) xxxvn. Flourens, Memoires d'Anatomie et de Physiologie Comparees. XXXVIII. Forbes, Monograph of the British Naked-eyed Medusas. xxxix. • History of British Starfishes, &c. xl. On Beroe pileus. (Ann. of Nat. Hist., Vol. n.) xli. Goodsir, Anatomical and Pathological Observations. XLfi. Ilarting, in Mulder's Chemistry of Animal and Vegetable Physiology. xliii. Ilassall, History of British Fresh-water Algae. xliv. Iloffmeister, Vergleichende Untersuchungen der Keimung,. Entfaltung und Fruchtbildung Hd'herer Kryptogamen. xlv. Sur la Fecondation chez les OEnothe're'es. (Ann. des Sci. Nat., 3 e Ser., Botan., Tom. ix.) xlvi. Bollard, Sur l'Organisation des Velelles. (Ann. des Sci. Nat., 3 e Ser , Zool., Tom. in.) xlvii. Jussieu, Cours Elementaire de Botanique. xlviii. Kiernan, On the Structure of the Liver. (Phil. Trans., 1835.) xlix. Kblliker, Entwickelungs-geschichte der Cephalopoden. L - ~ Sur le Developpement des Tissus ches les Batraciens. (Ann. des Sci. Nat. 3 e Ser., Zool., Tom. vi.) li. Kutzing, Phycologia generalis. lii. Leidy, On the Comparative Structure of the Liver. (Amer. Journ. of Med. Sci., Jan. 1848.) HH - 0n Articular Cartilages. (Op. cit., April, 1849.) liv. Leszczyc-Suminski, Entwickelungs-geschichte der Farrnkriiuter. lv. Leuret, Anatomie Comparee du Systeme Nerveux. lvi. Lister, On Tubular and Cellular Polypi, and on Ascidiai. (Phil. Trans., 1834.) V lvii. Mantell, On Iguanodon. (Phil. Trans , 1848.) lviii. Mirbel, Sur la Structure et la Developpement de la Marchantia polymorpha. (Nouv. Ann. du Musee, Tom. in.) MX. Mohl, Vermischte Schriften botanischen Inhalts. lx. Muller (Henrich), Zur Hectocotylus Argonautse. (Siebold and Kolliker's Zeitschrift, June, 1852.) lxi. (Johann), De Glandularum secernentium structura penitiori. TREATISES AND MEMOIRS REFERRED TO IN ILLUSTRATIONS. XXXI lxii. Mutter (Johann), Ueber die Larven und die Metamorphose der Echinodermen. (Abhald. der Konig. Akad. der Wissenschaften zu Berlin, 1816— 1852.) lxiii. Newport, On the structure of Insects, Myriapoda, and Macrourous Arachnida. (Phil. Trans., 1839—1843.) lxiv. Owen, Lectures on Comparative Anatomy. lxv. On the Homologies of the Vertebrate Skeleton. lxvi. Memoir on the Mylodon. lxvii. British Fossil Mammalia. lxviii. On the Rhyncosaurus. (Trans, of Cambr. Phil. Soc, Vol. vn.) lxix. Payer, Botanique Cryptogamique. lxx. Qualrefages, Sur l'Organisation des Pycnogonides. (Ann. des Sci. Nat., 3 e Se"r., Zool., Tom. iv.) lxxi. Sur quelques Planarie'es marina. (Op. cit, Tom. iv.) Lxxn. Sur les Noctiluques. (Op. cit., Tom. xiv.) lxxiii. Ralfs, Monograph of the British Desmidese. lxxiv. Roget, Bridgewater Treatise on Physiology. lxxv. Sars, Fauna littoralis Norvegia;. lxxvi. Savi, Etudes Anatomiques sur la Torpille. ' lxxvii. Schleiden, Principles of Scientific Botany. lxxviii. The Plant, a Biography. lxxix. Schwann, Mikroscopische Untersuchungen uber die Ueberreinstimmung in der Structur und dem Wachsthum der Thiere und Pflanzen. lxxx. Skarpey, Art. Cilia. (Cj'clop. of Anat. and Physiol.) lxxxi. Slack, On the Elementary Tissues of Plants, and on Vegetable Circulation. (Trans, of Soc. of Arts, Vol. xxxix.) lxxxii. Solly, The Human Brain: its Structure, Physiology, and Diseases. lxxxiii. Stein, On Metamorphoses of Vorticella. (Siebold and Kolliker's Zeitschrift, Band in.) lxxxiv. Strauss-Durckheim, Conside"ratione G6ne"rales sur 1'Anatomie comparee des Animaux Article's. lxxxv. Teale, Anatomy of Actinia Coriacea. (Trans, of Phil, and Lit. Soc. of Leeds, Vol. i.) lxxxvi. Thuret, Sur les Antheridics des Cryptogames. (Ann. des Sci. Nat., 3 e Ser., Bo tan., Tom. xvi.) lxxxvii. Thwaites, On the Conjugation of the Diatomacese. (Ann. of Nat. Hist., 1st Ser., Vol. xx., and 2d Ser., Vol. i.) Lxxxvni. Tiedemann, Anatomie der &c. lxxxix. Sur le Developpement du Cerveau. xc. Trembley, Memoires pour servir a 1'Histoire d'un genre de Polype d'eau douce. xci. Tulasne, Sur les Lichens. (Ann. des Sci. Nat., 3 e Ser., Botan., Tom. xvn.) xcn. Sur les Tremellinees. (Op. cit., Tom. xix.) xcin. linger, Recherches sur l'Achlya prolifera. (Ann. des Sci. Nat., 3 e Ser., Botan., Tom. n.) xciv. Van Beneden, Memoire sur les Campanulaires de la Cote d'Ostcnde. (Mem. " de l'Acad. Roy. de Bruxelles, Tom. xvn.) xcv. -t Recherches sur les Bryozoaires de la Cote d'Ostende. (Mem. de l'Acad. Roy. de Bruxelles, Tom. xvni.) xcvi. Van Beneden, Sur le Developpement et l'Organisation des Nicothoes. (Ann. des'Sci. Nat., 3 e Se>., Zool., Tom. xm.) XXXII TREATISES AND MEMOIRS REFERRED TO IN ILLUSTRATIONS. xevn. Vogt, Embryologie des Salmones. XCVttl. Eecherches sur l'Embryogenie des Gasteropodes. (Ann. des Sci. Nat., 3 e Se"r., Zool., Tom. VI.) xcix. Wagner, Icones Physiologies?, c. Wallich, Planta? Asiatics? Rariores. Ci. Willis, On the Organs of Voice. (Trans, of Cambridge Phil. Soc, Vol. iv.) ext. Wilson, Anatomist's Vade-Mecum. CHAPTER I. ON THE GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. 1. There are few things more interesting to those who feel pleasure in watching the extraordinary advancement of knowledge at the present time, than the rapid progress of philosophical views in every department of Biological Science; the pursuit of which has until recently been made to consist, almost exclusively, in the mere collection and accumulation of facts, with scarcely any attempt at the discovery of the ideas of which they are but the expressions. The laws of Life were long considered beyond the reach of human investigation; and the mind shrank from attempting to analyze its complex and varied phenomena, which, though constantly under observation, must be reduced to their simplest form, before any inductive reasoning can be founded upon them. It is recorded, however, of Newton, that, whilst contemplating the simplicity and harmony of the plan according to which the Universe is governed, as manifested in the relations which his gigantic mind discovered between the distant and apparently unconnected masses of the solar system, his thoughts glanced towards the organized creation; and reflecting that the wonderful structure and arrangement which it exhibits, present in no less a degree the indications of the order and perfection which can result from Omnipotence alone, he remarked, "I cannot doubt that the structure of animals is governed by principles of similar uniformity." (" Idemque dici possit de uniformitate ilia, quae est fn corporibus animalium.") "Why," asks Cuvier, in his eloquent discourse on the revolutions of the globe, "should not Natural History some day have its Newton ?" 2. Although the labors of the Naturalist and Comparative Anatomist have not yet unveiled more than a small part of that general plan, the complete discovery of which may perhaps be reserved for another Newton, many subordinate principles have been based on a solid foundation, and many more, which were at first doubtful, are daily receiving fresh confirmation. Several of these laws are alike important from their extensive range, and interesting from the unexpected nature of the results to which they frequently lead; and though their application may sometimes appear forced, and inconsistent with the usual simplicity of Nature, further investigation will generally show that the difficulty is more apparent than real—frequently arising solely from our own prejudices, and diminishing in proportion as we fix our attention upon that combination of unity o f plan with variety of purpose, by which is produced the endless diversity united with harmony of forms, so remarkable in the animated world. 3. In comparing phenomena of any kind, for the purpose of arriving at a principle common to them all, it is necessary to feel certain that they are of a similar character. Indeed, the sagacity of the philosopher is often more displayed in his discovery of that relation amongst his facts, which allows of their being compared together, than in the inferences to which such com- 3 34 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. parison leads him. The brilliancy of Newton's genius was shown in the perception, that the fall of a stone to the earth, and the motion of the moon around it, were comprehensible under the same law; not in the mere deduction of the numerical law from the ratios supplied by those facts.—In the sciences which have Life for their subject, the apparent dissimilarity of the facts which are made the objects of comparison, often prevents the true relation between them from being readily detected. Here it is that the mental training which the previous cultivation of Physical science affords, becomes peculiarly valuable to the Physiologist. " The most important part of the process of Induction," says Professor Powell, 1 " consists in seizing upon the probable connecting relation, by which we can extend what we observe in a few cases to all. In proportion to the justness of this assumption, and the correctness of our judgment in tracing and adopting it, will the induction be successful. The analogies to be pursued must be those suggested from already-ascertained laws and relations. Thus, in proportion to' the extent of the inquirer's previous knowledge of such relations subsisting in other parts of Nature, will be his means of guidance to a correct train of inference in that before him. And he who has, even to a limited extent, been led to observe the connection between one class of physical truths and another, will almost unconsciously acquire a tendency to perceive such relations among the facts continually presented to him. And the more extensive his acquaintance with Nature, the more firmly is he impressed with the belief that some such relation must subsist in all cases, however limited a portion of it he may be able actually to trace. It is by the exercise of unusual skill in this way that the greatest philosophers have been able to achieve their triumphs in the reduction of facts under the dominion of general laws." 4. The first group of phenomena encountered by the Biological student, is that presented in the many hundred-thousand diverse forms of organic structure, of which the Animal and Yegetable kingdoms are made up; and it is necessary, at the very outset of the inquiry, to settle the principles upon which these are to be compared. In many instances, it is true, there can be no room for hesitation; the general type of conformation of two or more organisms being obviously the same, and the differences in detail never obscuring the resemblances between their component parts. But this holds good to only a very limited extent; and we are soon forced to recognize such essential differences, alike in the general types of conformation, and in the form and structure of the component parts, that we feel the need of some guiding principle according to which we may arrange these phenomena for comparison.—Now from the time of Aristotle, downwards to the commencement of the present century, Anatomists were in the habit of regarding similarity in external form and in evident purpose, as indicating the analogies between different parts. But although this mode of estimating the character of organs is perfectly correct, when they are considered as instrumental structures —that is, when we are inquiring into the conditions of the function performed by them—it totally fails, when we are in search of the plan of organization, according to which their evolution has taken place; since it is frequently found that two organs which are not unlike in external form, and which have corresponding functions in the system, originate from elements entirely different, and are therefore fundamentally dissimilar; whilst, on the other hand, organs which at first sight present little or no resemblance to each other, and are applied 1 "Connection of Natural and Divine Truth," p. 33. BASIS OF COMPARISON OF PHENOMENA. 35 to very different purposes in the economy, may be really modifications of the same fundamental components. 5. If, for example, we take a cursory glance at the organs of support or motion in the air, with which different Animals are furnished, we shall observe a community of function, and a general similarity of external form, concealing a total diversity of internal structure and of essential character. Amongst all the classes which are adapted for atmospheric respiration, we encounter groups of greater or less extent, in which the resistance of this element becomes the principal means of progression; and even among aquatic animals, there are instances in which the function of locomotion is partly dependent upon the same agency. Wherever true wings exist among the Vertebrata, some modification of the anterior member serves as their basis; but there is considerable variety in the mode in which the apparatus is constructed. Thus, in the Bat (Fig. 2, e), the required area for the surface of the wing is formed by an extension of the skin over a system of bones, of which those of the hand form by far the largest part; and this membrane is extended also from the posterior extremity, and is attached to the whole length of the trunk, as well as to the tail where one exists. In the Bird (Fig. 2, b), on the contrary, the wing is formed by the skin and its appendages attached to the anterior member alone; and here the bones of the hand are developed in a comparatively slight degree, those of the arm and fore-arm being the principal support of the expansion. From what is preserved of the Pterodactylus, it seems that the wing of this extraordinary animal was extended, not over the whole member as in the Bird, nor over the hand as in the Bat, but over one of the fingers only, which was immensely elongated in proportion to the rest (Fig. 1). In the Flying-fish, again, the pectoral fins may be regarded as, in some sort, its wings (though it does not appear that the animal has the power of raising itself by means of their action on the air, the impulse being given at the moment of quitting the water) ; these fins evidently represent the anterior members of higher Vertebrata; but the bones of the arm and fore-arm are Fig. 1. Pterodactylus crassirostris. scarcely developed, while the hand is expanded, and joined immediately, as it were, to the trunk.—A very different structure prevails among those imperfect wings, which serve rather to support the animals which possess them, in their movements through the air, than to propel them in that me- 36 GENERAL PLAN OP ORGANIC STRUCTURE AND DEVELOPMENT. dium. Thus, in the Hying Squirrels, Flying Lemurs, and Phalangers or "flying opossums," there is an extension of the skin between the fore and hind legs, which, by acting as a parachute, enables these animals to descend with safety from considerable heights; in the Draco volans, on the other hand, the wings are affixed to the sides of the back, being supported by prolongations of the ribs, and are quite independent of the extremities. Here, we have still the same function and general form; but it would evidently be absurd to say that the organs are of the same structural character.—A still greater departure from the type with which we are familiar among the higher animals, is presented by the wings of Insects; for these are formed by an extension of the superficial tegumentary membrane over a framework that is not derived from an internal osseous skeleton, but is an extension of the denser subjacent layer of the external integument; and this framework is penetrated, throughout its ramifications, by "trachea?" or air-tubes, communicating with those of the interior of the body, and also (at least in the early state of these organs) by vessels or passages for the circulation of blood. As regards their essential structure, in fact, these wings correspond closely with the external respiratory organs with which certain aquatic Larva? (as that of the Ephemera) are provided; and hence they have been not inappropriately designated by Oken as "aerial gills." 1 They may, in fact, be regarded as an excessively developed form of those external appendages of the lower Articulata, which are subservient to locomotion and to respiration jointly; and it is a very interesting example of the similarity of modification which very different plans of structure may undergo, when a common purpose is to be fulfilled, that, in the wing of the Bird, as well as in that of the Insect, there should be a special prolongation of the respiratory passages into the framework which supports it. 6. Many similar examples might readily be adduced from the Animal kingdom; but the Yegetable world affords them in even greater abundance. To take a very simple case; —the expanded foliaceous surface through which the Plant obtains from the atmosphere the principal part of the solid materials of its growth, though usually afforded by the leaves, which are appendages to the axis developed for this express purpose, is sometimes provided, as in the Cactacece, by the extension of the surface of the stem itself, which remains soft and succulent; whilst in many of the Acacias of New Holland, as in the sub-aquatic leaves of the Sagittaria of this country, it is given by the laminal compression of the petiole or leaf-stalk. So, again, the tendril, which is an organ developed for the purpose of supporting the plant by twining round some neighboring prop, is in the Vine a transformation of the peduncle or flower-stalk, in the Pea a prolongation of the petiole or leaf-stalk, in the Cucumber a transformation of the stipule, and in the Gloriosa the point of the leaf itself; whilst in the singular genus Strophanthus, it is actually the point of the petal which becomes a tendril and twines round other parts. 1. We can scarcely select any example of diversity of external conformation and of function, superinduced upon an essential unity of organization, so appropriate as that which is afforded by the comparison of those different modifications of the limbs or members, and especially of the anterior pair, 1 The attempts of a generation of Entomologists now passing away, under the influence of the erroneous idea already referred to (§ 4), at bringing into comparison, as similar organs, the wings of Insects, and the anterior members of the flying Vertebrata, can now only excite a smile on the part of the Philosophical Anatomist. Such attempts, however, have exercised a most injurious influence on the progress of science, by drawing off the attention of Naturalists from the true method of philosophical research. 37 BASIS OP COMPARISON OF PHENOMENA. by which the several species of Vertebrated animals are adapted to the most diversified modes of life. No Comparative Anatomist has the slightest hesitation in admitting that the pectoral fin of a Fish (Fig. 2, a), the wing of a Bird (b), the paddle of a Dolphin (c), the fore-leg of a Deer (d), the wing of a Bat (e), and the arm of a man (f), are the same organs, notwithstanding that their forms are so varied, and the uses to which they are Fig. 2. Different forms of Anterior Member: —A, Fish; b, Bird; c, Dolphin; n, Deer; E, Bat; f, Man applied so unlike each other. For all these organs not only occupy the same position in the fabric, but are developed after the same manner; and when their osseous framework is examined, it is found to be composed of parts which are strictly comparable one with another, although varying in number and in relative proportion. Thus, commencing from the shoulderjoint, we can almost everywhere recognize without difficulty the humerus, it being only in fishes that this is so little developed as not to intervene between the scapula and the bones of the fore-arm; next we have the radius and ulna, whose presence is always distinguishable, although one of them may be in only a rudimentary condition; then, beneath the wrist-joint (through which a dotted line is drawn in the figure), we find the bones of the carpus, which are normally ten in number, forming two rows, but which may be reduced by non-development to any smaller number—three, two, or even one; next, we find the metacarpal bones, which are normally five, but are sometimes reduced among the higher vertebrata to four, three, two, or one, whilst in Fishes they may be multiplied to the number of twenty or more; and lastly we have the digital bones, of which there are normally five'sets, each consisting of three or more phalanges, but which are subject to the same reduction or multiplication as the metacarpal.—It is entirely from the differences of conformation which these osseous elements gradually come to present in the course of their development, that those special adaptations arise, which fit their combination in each case for the wants of the particular species that possesses it; enabling them to be used as an instrument for terrestrial, aquatic, or aerial progression, for swimming and diving, for walking and running, for climbing and flying, for burrowing and tearing, or for that combination of refined and varied 38 GENERAL PLAN OP ORGANIC STRUCTURE AND DEVELOPMENT. manipulations which renders the hand of Man capable of serving as the instrument wherewith to execute the conceptions of his fertile intellect. 8. We may have recourse to the Respiratory system, for another instance, which will bring the contrast between functional similarity, or analogy, and organic correspondence, or homology,' 1 into clear view.—An uninstructed observer would scarcely perceive any resemblance between the gills of a Fish (Fig. 145), and the lungs of a Quadruped (Fig. 154), or between the elegant tufts on the head of a Sabella (Fig. 144), and the airtubes ramifying through the body of an Insect (Fig. 149) ; and those who are in the habit of forming exclusive notions upon a hasty survey, might be led to deny that any real analogy could exist. When the character of the function is investigated, however, with the structure which it requires for its performance, it becomes evident that, in order to bring the circulating fluid into the due relation with the atmosphere, all that is needed is a* permeable membrane, which shall be in contact with the air on one side, and with the fluid on the other. And this key, applied to the examination of the several forms of respiratory apparatus which exist in the Animal kingdom, shows that they all possess the same essential nature as instrumental structures, and that their modifications in particular instances (which will hereafter be specially described) are only to adapt them to the plan and conditions of the organism at large. There is therefore, functionally considered, a relationship of analogy amongst all these organs; although they are not really the homologues of one another. Thus, the gills of the Fish, and the branchial tufts of the Sabella, are external prolongations of the tegumentary surface, whilst the trachea? of the Insect, and the lungs of air-breathing Yertebrata, are internal reflexions of that surface; and further, the two former set of organs, as the two latter, differ from each other in regard to the part of the surface from which the prolongation or inversion takes place. In the Perennibranchiate Batrachia, moreover, both lungs and gills are present; and their essential difference of character is most apparent, whilst their correspondence as instruments of the same functional operations is equally evident. Further, in the air-bladder of the Fish, we have an apparently anomalous organ, the only known use of which is to assist in locomotion; yet when a comparison is made between its most developed forms and the simplest pulmonary sacs of Amphibia, no doubt can remain that it is to be regarded as a rudimentary lung; and the study of its development leads to the same conclusion. (See Chap. VI.) 9. It would be easy to adduce numerous parallel examples from the Yegetable kingdom; wherein organs which correspond in structure, connections, and development, and which are therefore homologous, are observed to assume the most varied forms, and to perform the most different functions. It will be sufficient, however, to advert to the well-known fact, that the underground "creeping roots" (as they are commonly accounted) 1 In earlier editions of this work, the terms functional and structural analogy were used to express the mutual relation of parts, on the one hand as instrumental structures, on the other as fundamentally or organically correspondent. It will be found convenient to limit (as Prof. Owen has done) the use of the term Analogy to functional resemblance, and to employ Homology as indicative of structural correspondence. Thus by Analogue we now understand "a part or organ in one animal, which has the same function as another part or organ in a different animal;" whilst by Homologue is implied "the same organ in different animals under every variety of form and function." (Prof. Owen's "Hunterian Lectures," Vol. I. Glossary.) Thus, for example, the wing of an Insect is the analogue of that of a Bat or Bird, but not the homologue; whilst the latter is homologous with the arm of Man, the fore-leg of a Quadruped, and the pectoral fin of a Fish. 39 HOMOLOGY AND ANALOGY. of the Couch-grass, the subterranean'" tuber" of the Potato, the "rhizoma" or "root-stalk" of the Iris, the solid "cormus" of the Colchicum, the "bulb" of the Hyacinth, and the "runner" of the Strawberry, are not less truly stems, than are the lofty trunks of the Palm or Elm, notwithstanding the variety in their form, texture, and mode of growth; for they all constitute the ascending axes of the Plants of which tfiey respectively form part, and have the power of developing the foliaceous appendages which become leaves or flowers, whilst the radical fibres, which constitute the essential part of the roots, grow downwards from their base. 10. In all these cases, we might with perfect propriety found any inquiries regarding the functional power of the organs respectively compared, upon their capacity as instruments adapted for a particular purpose. For example, we might estimate the respective force with which Birds, Bats, and Insects, could be propelled through the air, by ascertaining the superficial area of their wings, and the energy and rapidity with which these are moved; or we might judge of the respiratory power of an Animal or Plant, by the extent of surface through which the nutritive fluid comes into relation with the atmosphere, by whatever portion of the fabric that surface might be afforded. But the Philosophical Anatomist, who seeks to determine the organic relation of these parts, must first consider their internal conformation, and examine into the structural elements of which they are composed. In the cases just alluded to, he would find not merely the osseous elements, but the muscles, nerves, bloodvessels, &c, presenting essentially the same disposition in the arm of Man, the fore-leg of the Quadruped, or the wing of the Bat or Bird. But on passing to the Insect, he would encounter, as we have seen, an entire change in the plan of structure; the same purpose being fulfilled by instrumental means of a very different order, corresponding, in fact, to those which in the Articulata generally are made subservient to the respiratory process.—The next step in the determination of homologies, is to trace the connections of the organs compared, which frequently enables the real nature of parts to be recognized, which would be otherwise obscure. For it is a principle of very extensive application, that similar parts are connected with similar parts, in different animals of the same type. Thus, we never find a hand or foot springing directly from the spinal column of a Vertebrate animal; the connection being always established by other bones, which, whatever may be the variety in their size and shape, are never wholly wanting. Hence, where we find, as in the Fish, the hand excessively developed, and no external trace of an arm or fore-arm, we expect to find it supported internally by a radius and ulna, and these again to be connected with the scapular arch by the intermediation of a humerus. Now the bones of the fore-arm are generally distinctly developed, whilst the humerus very commonly coalesces with the coracoid, so that its presence might be easily overlooked; yet even this is found in certain species to be present as a separate bone.',—Great assistance, again, in the determination of the homologies of organs, is afforded by the examination of transitional or intermediate forms. Thus, it has been by the regular progression exhibited in the structure of the pulmonic apparatus, from the simple, closed, undivided air-sac of most Fishes, through the higher forms which this organ presents even in that class, and through the various 1 See Prof. Owen's "Lectures on Comparative Anatomy," Vol. II. p. 120. The two bones supporting the Fin in Fig. 2, a, are considered by Prof. Owen to be elongated carpals, not (as usually supposed) radius and ulna. If this be the case, the member should have been so placed in the figure, that tho dotted line which marks the place of the wrist-joint should have passed above instead of below them. 40 GENERAL PLAN OP ORGANIC STRUCTURE AND DEVELOPMENT. phases which it exhibits in the Perennibranchiate Batrachia, that the homology of the swimming-bladder of the Fish with the lung of the air-breathing Yertebrata has been established. In like manner, it has been by tracing-out the intermediate forms of the bones of the extremities (Fig. 3, b, d), that Prof. Owen has succeeded in proving the complex limbs of the Fig. 3. Diagram illustrating the Nature of Limbs: —A, posterior view of the occipital vertebra of Lepidosiren annectens; —b, posterior view of the occipital vertebra of Amphiuma didactylum ; —c, posterior view of tho pelvic vertebra of Lepidosiren ; —d, posterior view of the pelvic vertebra of Proteus anguinus. In the several diagrams, the following references indicate corresponding parts; c, centrum; n, neurapophyses; «, neural spine; pi, 51, pleurapophysis of the occipital vertebra, or scapula; h, 52, haainapophysis of the occipital vertebra, or coracoid bone; o, 53-57, diverging appendages of the occipital vertebra, or anterior limbs; pi, 62, pleurapophysis of the pelvic vertebra, or iliac bone; h, 63, ha3mapophysis of the pelvic vertebra, or ischiac bone: a, 65-69, diverging appendages of the pelvic vertebra, or posterior limbs. higher Yertebrata, to be homologous with the simple rod-like members (a, c) of the Lepidosiren (Fig. 150) ; whilst these last serve as the connecting-link, whereby the homology of the scapular and pelvic arches with the haemal or visceral arches of other vertebral segments is indicated; the bones of the limbs, being at the same time shown to be homologous with their "diverging appendages" (of which we have examples in the backward projections that spring from the ribs of Birds), and the scapular arch with its anterior members being thus found to be the completion of the occipital segment, whose centrum and neural arch enter into the composition of the cranium. So, again, the identity of composition between the jaws and the true legs of the Crustacea, is shown by the transitional gradations presented by the feet-jaws. And turning to the Yegetable kingdom, we find the mutual relations of the parts of the flower, and their homology' with the leaves, to be indicated in those cases in which there is a gradational passage from the leaf to the bract, from the bract to the sepal, from the sepal to the petal, from the petal to the stamen, and from the foliaceous type to the carpel (§ 30).—But it is most certain that, of all the means of discovering the structural relations of organs, the study of their development is most important; since this, if carefully pursued, will probably never fail to clear up whatever doubts may be left by other modes of investigation. It is in this manner that the true solution has been at last attained, of many 41 MEANS OP DETERMINING HOMOLOGIES. of the most difficult and most controverted questions in the science;—questions which have reference, not merely to the nature of particular organs, but to the relations subsisting between different groups of living beings. And it is in this path, therefore, that the Philosophic Naturalist can press forward with the most assured prospect of success, in the search for that general plan of Organization, which it is his highest object to discover. 11. Thus we are led by the study of Morphology (that is, by the recognition of "homologous" organs, under whatever forms they may present), to the perception of that great general truth, which is, perhaps, the highest yet attained in the science of Organization, and which is even yet far from being fully developed; that in the several tribes of organized beings, we have not a mere aggregation of individuals, each formed upon an independent model, and presenting a type of structure peculiar to itself; but that we may trace throughout each assemblage a conformity to a general plan, which may be expressed in an "archetype" or ideal model, 1 and of which every modification has reference either to the peculiar conditions under which the race is destined to exist, or to its relations to other beings. Of these special modifications, again, the most important themselves present a conformity to a plan of less generality; those next in order to a plan of still more limited extent; and so on, until we reach those which are peculiar to the individual itself. This, in fact, is the philosophic expression of the whole science of Classification. For, to take the Vertebrate series as our illustration, we find that Fishes, Reptiles, Birds, and Mammals agree in certain leading features of their structure, which constitute them vertebrated animals; but this structure is displayed under diversified aspects in these classes respectively, which constitute their distinctive attributes. Thus, of the general vertebrated type, the Fish presents one set of special modifications, adapted to its peculiar mode of life; the Reptile, another; the Bird, a third; the Mammal, a fourth. So again, in each of these classes, we find its general type presenting subordinate modifications in the respective orders; thus, for example, the Reptilian type exhibits itself under the diverse aspects of the Frog, the Snake, the Lizard, and the Turtle; the Mammalian under those of the Whale and the Bat, the Sloth and the Deer, the Elephant and the Tiger, the Kangaroo and the Monkey, the Ornithorhyncus and Man. Each order, again, is subdivided into families, in accordance with the subordinate or more special modifications which the type of the order presents; every one of these families displaying the type of the class and order, with distinctive variations of its own. Each family consists of genera, in every one of which the family type is presented under a somewhat diversified aspect. Each genus is made up of an aggregation of species, which exhibit the generic character under a variety of modifications; these, however slight, being uniformly repeated through successive generations. Lastly, each species is composed of an assemblage of individuals, every one of which repeats the type of its kingdom, subkingdom, class, order, family, genus, and species, through its whole line of descent. 12. Thus, in assigning to any particular being its place in the Organized Creation, we have to proceed from the general to the special. —We will suppose an unknown body to be brought for our determination; the first business is to' ascertain whether it be an Organized fabric, or a mass of Inor- 1 For an admirable exposition of this doctrine, as it respects the osseous system of Vertebrated Animals, see Prof. Owen's treatise on " The Archetype and Homologies of the Vertebrate Skeleton." 42 GENERAL PLAN OP ORGANIC STRUCTURE AND DEVELOPMENT. ganic matter. This is soon discriminated, in the majority of cases, by an appeal to those most general characters which distinguish all organized structures from inorganic masses; and the next question is to determine its Animal or Yegetable nature, by the aid of those .characters which are not common to both, but are distinctive of each respectively. We will suppose the Animal nature of our unknown body to have been ascertained; the question next arises, to which of the four sub-kingdoms it shall be referred; and this, again, has to be ascertained by an appeal to characters which are less general than the preceding, not being common to all organized structures, nor yet to all animals, but being restricted to each of the four subkingdoms respectively. Then, having ascertained that it is a Yertebrated, Molluscous, Articulated, or Radiated animal, as the case might be, the Naturalist would determine its order by characters of still less generality, which are peculiar to that order; its family, by features which are still more limited; its genus, by those modifications of family character which are presented by the several genera it includes; and lastly, its species, by characters which are the most special of all, that is, which are limited to that race alone. 13. Now if our classification were perfect, it would be comparatively easy to determine the "archetype" or ideal model of each group; because we should have all the forms before us, by the comparison of which the Philosophical Zoologist seeks to educe what is common to the whole. But in practice it has often been found extremely difficult to determine what shall be considered as characters of classes, what of orders, and so on; since their respective values are very commonly mistaken by those who are imperfectly acquainted with the true principles of classification, and sometimes even by the instructed Naturalist. Thus in popular ideas, a Bat ranks as a Bird, because it flics by wings through the air; whilst the Whale ranks as a fish, on account of its fish-like form, habitation, and mode of progression. But the scientific Zoologist has no hesitation in placing the Bat amongst the Mammalia, because it presents all the characters which are essential to that class, and which distinguish it from that of Birds; namely, its viviparous and placental generation, its subsequent nurture of its young by lactation, its covering of hair, the dental armature of its mouth, its diaphragmatic respiration, its highly-developed cerebrum, and many other peculiarities of conformation; whilst its apparent resemblance to the class of Birds merely results from the adaptation of the Mammalian type to an aerial life. So, again, notwithstanding its fish-like habits, and the peculiarities of structure which adapt it to these, the Whale is a Mammal in all which is essentially characteristic of the class, and which distinguishes it from that of fishes; namely, its atmospheric respiration, its complete double circulation, its warm blood, its viviparous generation and subsequent lactation, its well-developed Cerebrum, its osteological and many other peculiarities.— Here, then, the determination is easy to those who possess but a smattering of Zoological knowledge. But we will take another case, in which a fundamental error was committed even by a great Master in modern science, owing to his misapprehension of the value of characters. Following too closely the indications afforded by the teeth (which are valuable in so far only as they serve as a key to the general plan of conformation), Cuvier placed the Marsupial Mammalia in the first instance as a subdivision of his order Garnaria; and even when he subsequently raised them to the rank of a distinct order, he gave them a position intermediate between the Carnaria and the Rodentia. Likewise, on account of the absence of teeth, he associated the Monotremata with the Sloths and Ant-eaters, in his order 43 DIFFICULTIES IN DETERMINATION OF ARCHETYPES. Edentata; satisfying himself with indicating that they presented a certain degree of affinity to the Marsupiata. Now the mutual resemblance of these two orders is extremely close; and their unlikeness to all other Mammalia, in the structure of their cerebrum, and in the mode in which the genital function is performed (two characters of fundamental importance), is such as unquestionably to require their detachment as a distinct sub-class.—So, again, it is now coming to be perceived, that the adaptation of the Mammalian structure to a fish-like habit of life, is not of itself sufficient to assemble all the animals which present it into a distinct order; for whilst the greater part of those which agree in possessing the Cetacean form, agree also in structure and carnivorous habit, there are certain genera (the Dugong, Manatee, and Stellerine), which have been until recently ranked with them, but which are found rather to correspond in the more essential peculiarities of their organization with the great series of herbivorous Mammals, and to be connected with that series by forms now extinct. 14. Many other examples might be cited, illustrative of this difficulty, which is one that especially presents itself among the lower classes of animals, with whose structure and physiology the acquaintance of the Naturalist is as yet very imperfect. It is one, however, which is continually lessening with the progress of research; and whilst, therefore, we should avoid placing too much confidence in existing systems of classification, and in existing ideas of what really constitute natural groups, we may look forward with hope, if not with absolute confidence, to the gradual accumulation of those materials, which shall enable the Philosophic Naturalist to do that for each group, which has been already effected, in great measure, for the Yertebrated series. In the determination of the relative importance of charac- Fig. 4. Fig. 5. Anatifa Icevw; —A, group of animals of different ages, as attached in tho living state; —b, interior structure, enlarged, as shown by the removal of the valves of one side; a, peduncle; b, mantle; c, cephalic portion of the body; d, mouth; e, articulated members; f, flabelliform appendages (or branchiae); g, abdominal appendage. 44 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. ters, it is certain that great assistance may be expected from the study of development; although we may not perhaps go the full length with those who maintain, that it should constitute the sole basis of all classification (§ 76). —It would be scarcely possible to adduce a more apposite illustration of the essential importance of this knowledge, to the determination of the true position and relations of a group of Animals distinguished by characters that seem to isolate it from all others, than is afforded by the case of the Cirrhipeds, or Barnacle tribe (Fig. 4). By the earlier Naturalists, this group was unhesitatingly referred to the Molluscous sub-kingdom; being allied to the classes of which that division is composed, in the softness of its body and appendages—in the inclusion of these within a hard casing, that is not fitted upon them, like the "test" of a Crab or Lobster, but loosely envelops the whole, like the "shell" of a Mussel or Limpet— and in the fixity of the animal to one spot during (apparently) the whole of its existence, the Barnacle being anchored by a long flexible tubular peduncle, as a Pinna is anchored by its byssus, whilst the Balanus is attached, like the oyster, by the adhesion of the shell itself to some solid basis of support. Even Cuvier left them in this position; although he had been led by the study of the anatomy of the animal inhabitants of the shells, to recognize their strong affinities to the Articulated series. For he perceived that their bodies are quite symmetrical, and present indications of division into a longitudinal succession of segments, each of them furnished (Fig. 5) with a pair of articulated appendages; their mouth he observed to be furnished with lateral jaws; he found their heart to lie in the dorsal region; whilst along their ventral region he detected the double ganglionic nervous chain, so characteristic of Articulated animals. Those Naturalists who considered this last assemblage of characters to possess a higher value than the preceding—as being more indicative of the essential nature of these animals, whilst their relations to the Molluscous series are rather such as adapt them to a particular mode of life—transferred the Cirrhipids to the Articulated series; and the propriety of this transference was made manifest by the discovery (first announced by Mr. J. V. Thompson in 1830 1 ), that the Cirrhipeds in their early state are free-moving animals, conformable in all essential particulars to the Crustacean type; and that they only attain their adult form and character after a series of metamorphoses, which progressively remove them to a greater and greater distance from it, and Avhich, while they constantly tend to evolve the peculiar conformation that distinguishes the Cirrhiped group, adapt the animal, in each of its stages, to maintain its own existence. The researches of Mr. Thompson, with the extensions which they have subsequently received from others, show that there is no essential difference between the early forms of the sessile and of the •pedunculated Cirrhipeds; but that both are active little animals (Fig. 5, a), possessing three pairs of legs and a pair of compound eyes, and having the body covered with an expanded shield, like that of many Entomostracous Crustaceans, so as in no essential particular to differ from the larva of Cyclops (Pig. 60). After going through a series of metamorphoses, one stage of which is represented in Fig. 6, b, these larva? come to present a form d, which reminds us of that of Daphnia, another Entomostracous Crustacean; the body being inclosed in a shell, composed of two valves, which are united along the back, whilst they are free along their lower margin, where they separate for the protrusion of a large and strong anterior pair of prehensile limbs provided with an adhesive sucker and hooks, 1 "Zoological Researches," No. III. 45 IMPOETANCE OP THE STUDY OP DEVELOPMENT. and of six pairs of posterior legs adapted for swimming. This bivalve shell, with the prehensile and natatory legs, is subsequently thrown off; the Fig. 0. Development of Balanua balanoidea; —A, earliest form; —B, larva after second moult; — c, side view of the same; —d, stage immediately preceding the loss of activity; a, stomach (?); b, nucleus of future attachment (?). animal then attaches itself to its head, a portion of which becomes excessively elongated into the peduncle of the Barnacle (Fig. 7), whilst in the Fig. 7. Comparison of Leucifer, a Stomapod Crustacean, with Lepas; —in the former, A, the abdomen, which becomes rudimentary in Cirrhipeds, is represented in outline; —in the latter, b, the antenna) and eyes, which really exist in the larva, are represented as if they had been retained, and'had continued to grow; to marks the position of the mouth in both. Balanus it expands into a broad base or disk of adhesion; the first thoracic segment sends backwards a prolongation which arches over the rest of the body so as completely to inclose it (no uncommon occurrence among the Crustacea), and the exterior layer of this is consolidated into the "multi- 46 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. valve" shell; whilst from the other thoracic segments are evolved the six pairs of cirrhi which are characteristic of these animals in their adult state.— Hence, whether we consider the peculiarities of the group, in the fullydeveloped condition, as sufficient to entitle it to take rank as a distinct class, or whether we regard it as constituting merely a section of the great Crustacean class, there can be no longer any question that the Cirrhipeds bear any extremely close affinity to the latter, and that they must be placed near its borders (whether within or beyond them), as an aberrant form of the higher Articulate type, adapted to a life essentially Molluscous. 1 15. Much of the controversy which has taken place among Physiologists, in regard to the general doctrines which may be deduced from the comparison of different plans of organization, has been due to a neglect of this difference between functional and structural correspondence, that is, between analogy and homology; whereby phenomena which are essentially dissimilar, have been brought under the same category. But by some who have clearly recognized organic identity as the basis of their reasoning, it has been attempted to show that the law of Unity of Composition has an unlimited application; it having been maintained that the same elementary parts exist alike throughout the Vegetable and Animal Kingdoms, and that the difference between the several classes of each lies solely in the respective development of these parts. Such a doctrine, however, can only be supported by assertion, since Nature affords no sanction to it; as the most cursory survey of these two of her kingdoms will at once make obvious. 16. If, for example, we commence by comparing the various tribes of Flowering-Plants with each other, we find that they may all be referred to a certain " archetype" or ideal form, consisting of an ascending axis or stem with its foliaceous and floral appendages, and of a descending axis or root with its absorbent fibres. Their most obvious diversities are generally attributable to the deficiency or excess of some or other of these component parts; thus many of the trees most remarkable for the massive perfection of their stems, have the less essential parts of their flowers undeveloped ; whilst many of the plants most remarkable for the beauty and luxuriance of their blossoms, never form a true woody stem. But amid this general conformity, the Botanist recognizes two very distinct though subordinate types, each of them including a long series of gradational forms, from the lofty tree to the humble plant; the difference between which consists rather in the diversity of the plan on which the very same elementary parts are combined and arranged, than in any superior elevation possessed by one over the other. Thus if we compare the Palm and the Oak, which may be considered as presenting typical examples of the Endogenous and Exogenous stems, we find that the same materials—cellular tissue, woody fibre, and ducts of various kinds—are worked-up, as it were, on two different patterns ; and as a like difference of plan extends itself also to the arrangement of the elementary parts of the leaf and to the number of the components of the flower, and even shows itself in almost the earliest stage of the life of the embryo, it becomes apparent that the diversity is one which belongs to the fundamental nature of the two groups. There are instances, it is true, in which there is such a general conformity in external appearance between certain of their members (between Cycads and 1 For the most complete account of these metamorphoses, as well as of the Anatomy, Physiology, and Classification of the pedunculated division of the group, see Mr. C. Darwin's "Monograph on the sub-class Cirripedia," published by the Ray Society, 1851. —It is hoped that this admirable work will be soon completed, by a like description of the sessile division. DIVERSITY OP TYPES OP ORGANIZATION. 47 Palms for example), as might deceive a mere superficial observer; yet there is no assumption of the essential characters of the latter of these groups by the former, the stem being exogenous and the embryo polycotyledonous. So, again, although there are certain Flowering-plants (such as Lemna, duckweed, and Zostera, sea-wrack) which, alike in habit and in general simplicity of structure, correspond with aquatic Cryptogamia, these are at once recognized as degraded forms of the Phanerogamic type, when their generative apparatus is examined; reduced, though this is, to a condition of extreme simplicity.—The Cryptogamic series cannot be referred with equal propriety to a single "archetype," so diversified are the types of structure, as well as of grades of development, which its principal groups present. Still, the plan on which their generative apparatus is constructed, though not so dissimilar to that of Phanerogamia as was formerly supposed (since no reasonable doubt can now remain of their true sexuality) present a certain fundamental uniformity; whilst its several modifications serve to distinguish the subordinate groups of Ferns, Mosses, Liverworts, &c. Although the Cryptogamia as a whole rank below Flowering plants, yet no one can help recognizing in a Tree-Fern a far more elaborate structure than that of Lemna or Zostera; so that the essential distinctions between the two series lies, not in grade of development, but in type of conformation. So among the Cryptogamia themselves, we find parallel series, such .as those of Algce, Lichens, and Fungi, through each of which a pertain distinctive type is preserved, notwithstanding that between their several varieties of grade there is a close correspondence.-—Hence we see that although, from the comparatively small number of distinct organs which the Plant possesses, and from the less complete separation even of these, there is not by any means the same scope for varieties in plan of organization as we shall find in the Animal Kingdom, it is not the less certain that a considerable number of distinct types of structure exists, which cannot be reconciled to any other theory of fundamental unity, than that which refers them all to their common starting-point—the single cell. 17. Turning, now, to the Animal Kingdom, we find that even a slight general survey affords ground for the recognition of those four very distinct plans of structure, which Cuvier was the first to mark out clearly—namely, the Radiated, the Molluscous, the Articulated, and the Vertebrated; and these are found to be more and more clearly distinguishable from each other, the more profoundly we examine into the fundamental peculiarities of each, and the more fully we become acquainted with the history of its development. For by accurately studying and comparing the various modifications under which these respectively present themselves, we see that, beneath the apparent mixture of characters which occasionally presents itself (as, for example, in the case of the Cirrhipeds, 14), there is an essential conformity to one type, and that the departure from the ordinary aspect is merely superficial, being such as adapts the animal or group of animals to a particular mode of existence. Now since modifications of a similar kind may take place in groups of animals belonging to different types, they may come to present very striking resemblances to each other in their adaptive characters (as is the case between Birds and Insects), although there is no conformity whatever in their general plan of structure. Taken as a whole, no animal belonging to any one of these types can be likened to any animal belonging to another; although comparisons may be legitimately made between their individual organs. Thus, as Von Baer justly observes, "metamorphose a Cephalopod as you will, there is no making a Fish out of it, save by building up all the parts afresh;" yet in many portions of 48 GENERAL-PLAN OP ORGANIC STRUCTURE AND DEVELOPMENT. their organization, Cephalopoda are unquestionably intermediate between the lower Mollusks and the typical Fishes. Again, although the higher Cephalopoda indubitably take a more elevated rank as Animals than the lowest Fishes, and in this respect might be considered as approximating more closely to Man, yet in the conformity of its organization to the Yertebrated type, the lowest Fish bears far more resemblance to Man, than does the highest Cephalopod.—Moreover, it is to be observed, that the general type of construction manifests itself not merely in the mode in which the organs are grouped together; but also in the conformation of the organs themselves. Thus we shall hereafter see, that whilst there is a remarkable correspondence between the condition of the circulating apparatus in the two series of Articulated and Molluscous animals—especially as regards the imperfection of its vascular system, and its communication with the visceral cavity—there is a type which is peculiar to each, and which shows itself in the structure of the heart, as well as in the general distribution of the bloodvessels. For the type of the heart, in the Articulated animal, is the elongated dorsal vessel, which, if divided at all, has a repetition of similar chambers for the several segments of the body; whilst in the Mollusk it is a concentrated organ, with much thicker walls, usually having the auricle or receiving cavity separated from the ventricle or impelling cavity, and presenting no other repetition of similar parts than the occasional doubling of the auricle, where the two sets of gills (whence the blood returns to the heart) are placed wide asunder. So, again, in the various Glandular organs of the Articulata, the required extent of surface is usually afforded by the elongation of a small number of narrow tubes; whilst in the Mollusca, the same extension is provided for by the multiplication of short and wide follicles. Yet we find that in certain Crustacea, which are adapted in many respects to the conditions of the Mollusk, both the heart and the glandular apparatus present a very striking approximation to the Molluscous type; whilst no such approximation is seen in the general plan of the fabric, which is as obviously articulated in the Crustacea, as it is in the Insect. 18. But although it is in type, or plan of organization, that the most essential differences lie, among the several forms of Plants and Animals, it is not the less true that they are distinguished by very marked diversities in yrade of development; by which is to be understood, the degree in which the several parts that make up the entire fabric are characterized by specialities of conformation, so that each becomes a distinct organ, adapted to perform a function, more or less different from that which other parts can discharge. The lower we descend in the scale of being, whether in the Animal or in the Yegetable series, the nearer approach do we make to that homogeneousness which is the typical attribute of inorganic bodies, wherein every particle has all the characters of individuality, so that there is no distinction either of tissues or of organs. Thus in Sponges and Sea-weeds, even when of considerable size, every part resembles other parts in intimate structure, and differs but slightly from them even in external configuration ; so that the whole mass is little else than a repetition of the same organic components. On the other hand, as we ascend the scale of being, we find the fabric—whether of the Plant or the Animal—becoming more and more heterogeneous; that is, to use Yon Baer's language, "a differentiation of the body into organic systems, and of these again into separate more individualized sections," presents itself. Thus, as we ascend from the lowest towards the highest forms of Yegetable life, we find that out of the homogeneous aggregation of cells which forms the simple frond of the 49 REPETITION OF SIMILAR PARTS IN LOWER ORGANISMS. humble Alga? (§ 22), a differentiation gradually arises between the " axis" and the "appendages to the axis;" that in the axis, there is a gradual separation established between the ascending portion, or stem, and the descending portion, or root; and that among its appendages, the foliaceous organs become more and more completely separated from the generative apparatus. Even in the highest Plants, however, we find an extensive repetition of similar parts ; and there is always, too, a close correspondence in the intimate structure of even the most antagonistic organs, such as the roots and leaves.—The differentiation, both as regards external conformation and intimate structure, proceeds to a far wider extent in the Animal kingdom, in virtue of the much greater variety of purposes to be attained in its existence; and we see this carried to its highest degree in Man, in whose organism the principle of specialization everywhere manifests itself, no part being a precise repetition of any other, except of the correspondingpart on the opposite side of the body. 1 19. It is only, however, by a very gradual succession of steps, that this elevation is attained. The simplest Animals are precisely upon a level with the simplest Plants, as regards their homogeneity of character; and no sooner does a differentiation of organs show itself, than these are in the first instance almost indefinitely repeated, so that, however numerous may be the parts of which the entire organism is composed, they are (so to speak) the facsimiles of one another. Thus not only in Zoophytes, but also in the lower Mollusca and Articulata, we find this repetition extending to those entire groups of organs, which, when detached from the rest, can maintain an independent existence, and are therefore commonly accounted distinct individuals. But we find the same to hold good, as regards individual organs, in the highest members of each of the Invertebrated subkingdoms, and even (though to a less extent) among Yertebrated animals. Thus among the Echinodermata, there is a precise repetition of similar parts around a common centre; and although this repetition is limited to Jive in the highest forms of the class, yet it extends to a much greater number in those of inferior organization—as we see in comparing the Ophiura with its five simple arms (Fig. 8), and the Pentacrinus (Fig. 9), whose ten arms all subdivide into such numerous branches, that the aggregate number of pieces in the whole is estimated at above a hundred thousand. So, again, in the Cephalopoda, which constitute the highest division of the Molluscous series, we find the tentacula surrounding the mouth to be almost indefinitely multiplied in the lower or tetrabranchiate division (Nautilus and its allies) ; whilst they are reduced to eight or ten in the dibranchiate order (Cuttlefish, Fig. 47), at the same time acquiring a much higher individual development, and often having one pair differentiated from the rest, for some special purpose. So in the Annelida and other inferior groups of the Articulate series, we find the locomotive, respiratory, and other important organs almost indefinitely multiplied in the longitudinally-repeated segments; but as we ascend towards the higher Articulata, the number of segments becomes strictly limited and greatly reduced, even where these divisions are still* little else than repetitions of one another, —being only twenty-two in the Centipede, and thirteen in the Insect-Larva; whilst in 1 This fact is most curiously exemplified in the speciality of the seats of election of those disorders of nutrition, which obviously depend upon the presence of a materies morbi in the blood, rather than upon any primary local disturbance.—See Dr. William Budd's Memoir on "Symmetrical Diseases," in the " Medico-Chirurgical Transactions," Vol. XXV.; Mr. Paget's "Lectures on Surgical Pathology," Am. Ed., p. 27 et seq., and the Author's "Principles of Human Physiology," 5th Am. Ed., p. 209. 4 50 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. the perfect Insect, the differentiation is carried to its highest extent, the locomotive apparatus being restricted to the three thoracic segments, and all the other organs, even when repeated throughout, being unequally developed in the several parts. The same principle of gradual differentiation Fig. 8. Fig. 9. Ophiura. Pentacrinitea briareus. shows itself most remarkably in the conformation of the members of Yertebrata: for, taking the many-jointed but single rod-like appendage of the Lepidosiren (Fig. 3, a, and Fig. 150) as their lowest type, we find this simply repeated even to the extent of a hundred-fold or more, in the digital rays supporting each of the pectoral and ventral fins of Fishes; as we ascend thence, through the extinct Enaliosauria (Ichthyosaurus, Plesiosaurus, &c.) to the typical Reptiles, we find the number of these multiplied digits diminishing, until it settles down at five, and the number of joints in each also reduced, until it becomes restricted to the six rows (two carpal, one metacarpal, and three phalangeal) which characterize the hand (or foot) of Man; in Birds and Flying Mammals, there is a most marked differentiation between the anterior and posterior extremities, as there is also (though in a less degree) in Man; and in the Quadrumana, we begin to see that specialization of the first digit (this being usually common to all their members) which is carried to its highest point in the hand of Man, whose other digits, also, have their distinctive capabilities, whereby this member as a whole becomes the most highly-organized of all instruments, in virtue of the unequalled variety of actions which it is adapted to perform. 20.. Thus we see that, whether we trace the '•Archetype" of each great subdivision of the Animal kingdom into those modifications which it presents in the more restricted groups—or whether we follow any organ or system, from the form under which it first presents itself, to that which it assumes in its state of most complete development:—we recognize one and the same plan of progression, namely, from the general to the special; and, as Yon Baer justly remarks, the relations of any organized fabric to any other, must be expressed by the product of its type with its grade of development Neither alone suffices to characterize it ; for under the same type, GENERAL VIEW OE VEGETABLE KINGDOM. —PROTOPHYTA. 51 different grades of development may present themselves; whilst conversely, a like grade of development may be attained under different types. And this general fact needs to be constantly borne in mind, not merely when a Plant or Animal is being considered as a whole, but also when we are studying the evolution of any individual organ or system in the ascending series; since it is no more possible to follow this system through one unbroken progression, than it is to arrange the entire assemblage of beings composing either kingdom in a single linear series.—It may in some degree assist the reader in his perusal of the subsequent pages, if we here pause to take a general survey of the principal types of Vegetable and Animal conformation, and of the chief diversities in grade of development which present themselves under each. 21. Vegetable Kingdom. —If we commence by examining any Plant of high organization, we observe, in the first place, that there is a complete differentiation between its organs of Nutrition and its organs of Reproduction; and further, that its principal organs of Nutrition, the root and the leaf, are separated from each other by the interposition of the stem or axis, around which the various appendages are arranged with a considerable degree of regularity. Further, we notice that a corresponding differentiation presents itself, as to the intimate structure of these several organs; for whilst the parts most directly concerned in the vital operations of the organism are chiefly made up of aggregations of cells, which resemble in all essential particulars those of which the simpler forms of vegetation entirely consist, these are supported upon a framework of woody fibre, an extension of that which gives strength and solidity to the stem and roots; and further, in order that air and liquids may the more readily find their way from one part of the structure to another, than they could do by transmission from cell to cell, a set of ducts is interposed, which establish a ready communica-r tion through the stem between the roots and the leaves. These organs are all mutually dependent and connected; and contribute, each in its own special manner, to the life of the Plant as a whole. But since all the most essential organs are many times repeated, the loss of some of these does not involve the destruction of the entire organism; and even the separated parts may develop the organs in which they are deficient, and may thus evolve themselves into entire plants, and maintain an independent existence. 1 In this way a multiplication of the products of the original germ may be effected; but these, as will be shown hereafter (Chap. XL), are not distinct individuals in the highest sense of that term; and the process by which they are evolved is simply a modification of the ordinary Nutritive operation, and is so far from being a form of true Generation, as to be essentially antagonistic to it. This distinction is one of much importance; since on it depends the recognition of the organs in Cryptogamia, which are homologous with those of Flowering-Plants. 1 This is usually the case under favorable conditions in regard to leaf-buds, which can put forth rootlets, and then evolve a stem, from which other leaf-buds and their flower-buds are developed. But there are some plants, as Bryophyllum, which have the same power in every leaf, or even in every fragment of a leaf; a small portion, laid upon damp earth, or suspended in a humid atmosphere, gradually evolving itself into the entire organ, and at the same time developing the other parts most essential to the performance of its nutritive operations, from which the reproductive apparatus is subsequently put forth. 52 GENERAL PLAN OP ORGANIC STRUCTURE AND DEVELOPMENT. 22. Having thus determined, by the analysis of one of the highest Plants, what it is that constitutes the most complete type of Vegetable organization, we shall commence with the lowest division of the series, and endeavor to trace out the principal lines of ascent by which that type is attained. This can only be accomplished, at present, in a very imperfect manner; since it is only within a very recent period, that the homologies of the reproductive apparatus of Phanerogamia have been discovered among Cryptogamia; and little more than a guess can be as yet made, as to the conditions which these present in some of the humbler forms of Cryptogamic life. The lowest type of vegetable existence is afforded by those organisms, which either consist of single cells, or of aggregations of similar cells, each of which can maintain an independent existence, living for and by itself, and not needing the co-operation of other cells, save for the purpose of generation, of which the reunion of the contents of two cells, by an act of "conjugation," is an essential condition. Any one of these cells may multiply itself indefinitely by subdivision, the results of which process are seen in the accompanying example (Fig. 10); but those products are all mere Fig. 10. Hormospora transveraalis. repetitions one of another, and often detach themselves spontaneously, so that the descendants of a single cell may cover a very extended area, as is the case, for example, with the Protococcus nivalis, or "red-snow." There is here, therefore, not the least show of differentiation ; no special cells being set apart even for the performance of the generative act. Where the multiplied cells remain in continuous connection with each other, being imbedded in a common substratum of gelatinous substance, so as to form but a single mass (Fig. 10), this may be perfectly homogeneous throughout; no definite form being presented by it as a whole, and no trace of "organs" being distinguishable in any part of it. The first indication of progress towards a higher grade, is given by the limitation of the direction in which the increase takes place: so that, instead of an amorphous aggregation of cells, we find a linear series (Fig. Fig. 11. Bangia velutina. 11, a) which is formed by successive transverse subdivision; and this filament may increase in breadth by longitudinal subdivision (b), so as at last to produce a laminar expansion, such as that of the common Ulvce, which is termed a thallus. In the simplest forms of this thallus, we do not meet with the slightest trace of differentiation ; and every one of its component cells appears to live as much for and by itself, as if it were completely detached from the rest. Every one of them, moreover, seems able to multiply itself, not merely by subdivision, but also by the GENERAL VIEW OP VEGETABLE KINGDOM. ALG2E. 53 emission of a portion of its contents inclosed in a cell-wall, in the condition of a "spore" or detached gemma; and this in the tribe now under consideration, being usually furnished with cilia, and endowed with the power of spontaneously moving for a time, is termed a "zoospore." When the zoospore has been thus carried to a distance from the organization from which it proceeded, it begins to develop itself into a similar organism by the process of duplicative subdivision; and in arriving at the highest of these stages of development, it passes through the simpler forms which remain permanent in yet humbler grades of vegetation. The true Generation of the plants of this group, to which the term Protophytes may perhaps be advantageously restricted, seems to be always accomplished by the process of "conjugation," in which any or all of the component cells may alike participate; but we see, in its higher forms, a tendency to the distinction between the "sperm-cell" and the "germ-cell," that is, to the differentiation of sexes into male and female—the only mark of heterogeneousness which yet presents itself. The product of this act is a new cell, from which a new plant originates by duplicative subdivision, as in the case of the zoospore. Here, then, we find that each individual (understanding by this term the aggregate result of a generative act) is made up of an indefinite number of cells, which, being precisely similar to each other, have no relation of mutual dependence; so that the Life of the whole is merely the sum of the lives of the component parts, and not, as in higher organisms, the product of it. 23. In the next stage of development, the differentiation of parts begins to manifest itself more decidedly; but this not so much in a distinction of organs adapted to separate offices in the act of Nutrition, as in the limitation of the Reproductive act to particular portions of the organism, and in the setting apart of special organs for its performance. For we have as yet no real distinction between stem, roots, and leaves; although some semblance of such a distinction may present itself. The primordial cell, by repeated subdivision, extends itself into a "thallus," whose form has but little definiteness, and whose tissue is nearly homogeneous throughout, being entirely composed of cells of various forms, without either woody fibres or vessels of any kind; and it is chiefly by its apparatus of fructification, which presents itself under many different aspects, that this group, which may be designated by the term Thallogens, is distinguished from the preceding. Nearly the same degree of general development is presented by three tribes of these humble Cryptogamia^—namely, Algce, 1 Lichens, and Fungi — which, nevertheless, are fitted to exist under very diverse conditions, and which present corresponding diversities of structural type ; and all of them seem to agree (according to the most recent investigations, of which an account will be given hereafter, Chap. XI.) in the possession of a special generative apparatus, in which the distinction of sexes is clearly marked. This consists of a set of "sperm-cells" developed in certain parts of the organism, and of a set of " germ-cells" evolved elsewhere, usually (but not always) in the same individual; the product of the former is a " sperm - atoid" body, which comes into contact with the latter and fertilizes its contents; and the result is the formation of a germ, which must be considered as the commencement of a new generation. This germ, however, 1 The group of Algse, as here limited, does not include the Protophytes described in the preceding paragraph; for although these, being mostly aquatic plants, are usually ranked in it, yet their type of reproductive apparatus is so distinct from that of the higher Algse, as to require that they should be separately considered. 54 GENERAL PLAN OP ORGANIC STRUCTURE AND DEVELOPMENT. frequently remains for some time in connection with the parent, and multiplies itself by duplicative subdivision at the expense of the nutriment which it draws from it, so as at last to evolve itself into a collection of " spores" contained in a special envelop, every one of which, when liberated from the parent, may develop itself into a new plant in which the same processes are repeated. It is by the general relation of this apparatus of fructification to that of nutrition, that the three groups already named are most distinctively characterized. 24. Thus the Algce vegetate exclusively in water or in damp situations; they require no nutriment but such as is supplied by water and by the air and inorganic substances dissolved in it; they absorb this nutriment equally by every part of their surface; and they show a great tendency to the extension of the ' thallus' by the multiplication of cells in continuity with the existing fabric, so that it frequently attains most extraordinary dimensions. In some of the simpler forms of the group, we find but a slight advance upon those aggregations of similarly-shaped cells, of which the fabrics of the Protophyta are made up. Thus in Mesogloia (Fig. 12), although we have a distinct axis with radiating appendages, the former is composed of elongated cells very loosely adherent, while the latter consist of single rows, Fig. 12. Fig. 13. Mesogloia vermicularis. Zonaria plantaginea. bearing the generative cells at their extremities; and in Zonaria (Pig. 13), it is only the character of the fructification that raises it above the type of an Ulva. In the highest Algae, however, we find some differentiation in the texture of their interior and exterior substance; and there is also a certain foreshadowing of the separation between the stem, the roots, and the leafy expansion or frond; but there is nowhere a departure from the simple cellular type, nor is there any real specialization of function, save that the fructification is evolved from the frondose portion, and not from the stemlike or root-like axis. Most Alga? are provided with a special apparatus (such as the stichidum of Dasya, Fig. 14, a) for the evolution of free gemma?, which are sometimes ciliated like the zoospores of Protophyta, and which multiply the original fabric independently of any true generative act. The proper generative organs are frequently very obscure, and are otten buried m the general substance of the frond; occasionally, however they form conceptacles, which are prominent externally (Fig. 15), or are developed on particular branches only. The embryo-cells, which are the products of the fertilization of the germ-cells by the contents of the sperm- GENERAL VIEW OP VEGETABLE KINGDOM. —LICHENS. 55 cells, do not usually undergo any great amount of subdivision into " spores," 1 before each spore that has originated from it begins to develop itself into a new plant. Hence it is obvious that the whole nisus of vital activity in Fig. 14. Fig. 15. Dasya leuetzingiana. Marginaria gigas. the Alga?, is towards Nutrition rather than Generation—the multiplication of independent organisms of the existing generation, rather than the origination of new series by the proper generative act. 25. On the other hand, Lichens grow upon living Plants, upon rocks and stones, upon hard earth, or other situations in which they are sparingly supplied with moisture, but are freely exposed to light and air. They derive their food from the atmosphere, and from the water which this conveys to them; but this they do not seem to absorb equally over the whole surface, the least exposed side being the softer, and being probably the one through which most liquid is imbibed, whilst it is rather through the other that carbon is drawn in from the air. The "nisus" or tendency of development is here to form a hard crust-like thallus, of slow growth, and of rather limited dimensions, but of great durability (Figs. 16, 11) ; and in the several layers of this thallus, there is considerable diversity of texture, although (as in the Alga?) there is no departure from the simple cellular type. As in the Alga?, moreover, we usually find a special arrangement for the production of free gemma? (soredia), by which the number of independent organisms of the same generation may be multiplied; and the evolution of these has been frequently considered as the true reproductive process. It is now almost certain, however, that in this ill-understood group, both "spermcells" and "germ-cells" exist, although usually buried in the substance of the 1 The term'"spore" has been used to designate many things homologically different. The Author believes that it will be most accordant with existing usage, to continue to apply it to the bodies contained in the capsules of Mosses, Ferns, &c, which are immediate or remote products of the subdivision of the embryo-cell, and to those bodies in Alga;, Lichens, &c, which are homologous with them. On the other hand, the germcells which themselves take part in the generative act, and from which the embryo-cells originate, should never be designated by the term spore. 56 GENERAL PLAN OP ORGANIC STRUCTURE AND DEVELOPMENT. thallus; and that of the clusters of "spores" which make their appearance within special conceptacles, each, as in the Alga?, is the result of the subdivision (to a limited extent) of a single embryo-cell produced by the generative act. These conceptacles are sometimes buried in the substance of the thallus, although their presence usually makes itself known by the prominence which it causes (Fig. 16, 11). Some tribes of Lichens very closely Fig. 17. Fig. 16. Parmelia acetabulum. Spliceroplioron coralloides. approximate to Alga?, both in their conditions of growth, and in their general character; whilst others present an equally close approximation to Fungi; so that, as some botanists have ranked this group with the former, and others with the latter, it seems reasonable to regard it as an intermediate section, the types of which are equally far removed from both. 26. The group of Fungi differs from both the preceding, in requiring as the most favorable, if not as the absolute condition, for the development of the Plants belonging to it, the presence of dead or decaying organic matter, Fig. 18. Stysanus caput-medunai. which shall afford by its decomposition a larger supply of carbonic acid and ammonia than the atmosphere and its moisture would alone furnish; their growth is favored by darkness rather than by light; and, like higher plants when not acted on by light, they absorb oxygen and set free carbonic acid. Their simpler forms (Fig. 18) strongly remind us of the lower Alga? (compare Fig. 12) in their grade of development, the nutritive and reproductive portions not being differentiated ; but in the higher we find a very marked separation between these, the reproductive apparatus being here as predominant, as is the nutritive apparatus in the Alga?. The vegetative thallus of these plants, which extends itself indefinitely in situations favorable to its development, has a very loose flocculent texture, and is composed of elongated branching cells interlacing amongst each other, but having no intimate connection (Fig. 19, a); and this mycelium, as it is termed, has such a want of definiteness of form, and varies so little in the different tribes of Fungi, that no determination of 57 GENERAL VIEW OF THE VEGETABLE KINGDOM. — FUNGI. species, genus, or even family, could be certainly made from it alone. Although any portion of this mycelium will continue to vegetate when separated from the rest, it does not appear that there is any provision for the spontaneous detachment of free gemma for the multiplication of the individual. The whole nisus of vital activity in the Fungi seems to be concentrated upon the Generative apparatus, which when fully developed, separates itself completely from the nutritive, and constitutes all that commonly attracts notice as the Plant (Fig. 20). Late observations render it proba- Fig. 19. Clavaria cris2mla; —a, portion of the mycelium magnified, ble that Fungi possess a true sexual apparatus, certain cells of the mycelium being developed into sperm-cells, and others into germ-cells; and that what is known as the "fructification" is the product of an act of conjugation, the Fig. 20, JEcidium tussilaginis: A, portion of the plant magnified:— b, section of one of the concep tacles with its sporecles. immediate result of which is the formation of an embryo-cell, which afterwards subdivides almost indefinitely, so as to produce an immense mass of "spores." These become detached from each other; and, being usually of extreme minuteness, are carried about in the atmosphere, so as to become deposited fn remote soils, and to give rise to vast numbers of separate beings constituting a new generation. 1 1 It is interesting to observe that the mode of evolution of many of these Thallogens is greatly influenced by the conditions under which it takes place. Thus, if Lichens be removed from the influence of light, and be over-supplied with moisture, they show a tendency to the extension of the vegetative or foliaceous portion of the thallus, with a 58 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. 27. The next important mode of elevation consists in the differentiation of the parts of the Nutritive apparatus, and in their gtiil more complete separation from the Generative. In ascending through the series formed by Hepaticce, Mosses, and Ferns, we observe a progressive approximation to that distinction between the •'axis" and its "appendages," which is characteristic of the highest forms of Vegetable life; but the growth of the axis is limited to one or both of its extremities, the part already formed being subject to very little, if any, increase; and from this character it has Fig. 21. Frond of Marchantia polymorpha. been proposed (by Dr. Lihdley) to distinguish this higher division of the Cryptogamic series by the title Acrogens, significant of growth at their points alone. •—The lower forms of the Hepaticce (such as the Ricciacea?) closely abut upon the Lichens, and differ from them but little as regards the organization of their nutritive apparatus, although their fructification evolves itself after a different type. In the common March- antia (Fig. 21), however, the soft green thallus now assumes more of the structure and aspect of a leaf, having an upper and under cuticle (the former perforated with stomata), and an intervening soft, loose parenchyma; and distinct radical fibres are thrown out from the lower surface, for the imbibition of moisture. In the Jungermannia there is a distinct axis of growth, on which the foliaceous appendages are symmetrically arranged; these are not completely differentiated from it in some species, but in others they are quite separated, and have an indication of a central mid-rib ; the stem, however, still trails on the ground, and radical fibres are developed from every part of it.—A slight elevation in this type brings us to that of the Mosses, which always have a distinct axis of growth, commonly more or less erect, with the foliaceous appendages symmetrically arranged upon it (Fig. 22). A transverse section of this axis shows an indication of a separation between its cortical and its medullary portions, by the intervention of a layer of elongated cells, that seems to prefigure the wood of higher plants; and from this layer, prolongations pass into the leaves, in which they form a kind of mid-rib. Fig. 22. The leaves, however, do not themselves present any considerable advance towards the more perfect type, being merely solid homogeneous aggrega- non-development of the fructification; and the thallus often assumes the byssoid form, of the mycelium of Fungi, so that it might he readily mistaken for this. So, again, if the simpler forms of Fungi develop themselves in liquids, they show an unusual tendency to the extension of the mycelium; and may even take on so much of the characteristic appearance and mode of growth of Algae, that their true nature becomes apparent only when the fructification is evolved.—See the description of "a Confervoid state of Mucor clavatus," by the Rev. M. J. Berkeley, in the "Magazine of Zoology and Botany," Vol. 59 GENERAL VIEW OF THE VEGETABLE KINGDOM. — MOSSES. tions of cells. And no proper root is yet evolved as a descending continuation of the axis, radical-fibres being put forth from every part of the lower Fig. 24. Fig. 23. Marchantia polymorpha) with peltate receptacles bearing antheridia. Marchantia polymorpha, with lobed receptacles bearing pistillidia. portion of the axis (Fig. 25), and even from the under-surfaces of the leaves. Both in Hepaticae and Mosses, we find a special arrangement for the multiplication of the plant by the formation of detached gemma; and some species owe their dispersion and perpetuation much more to this mode of propagation, than to the regular generative operation. There is no longer any doubt that both these tribes of plants possess true sexual organs; namely, antheridia containing "sperm-cells," and pistillidia or arehegonia containing "germ-cells." In Marchantia, these are borne upon distinct plants, and both are sufficiently conspicuous (Figs. 23, 24); in Mosses, on the other hand, they are usually very obscure, and are generally combined in the same individual. The product of the fertilization of one of the germ-cells by the spermatoid bodies set free from the sperm-cells, is an embryo-cell which develops itself into a capsule containing a mass of " spores;" and this, in the Mosses, is raised by the elongation of its foot-stalk, far above the original situation of the pistillidium, and becomes the only ostensible fructification of the plant (Fig. 25). In any one of the spores thus formed by the duplicative subdivision of the embryo-cell, a new plant may originate.—It is chiefly by specialities in the structure of their generative apparatus, that the preceding groups are distinguished from each other; each having its own peculiar type of fructification, whilst presenting (as we have just seen) a tolerably regular gradation in the development of the organs of nutrition. 28. Passing from these to the Ferns, we find such a Fig. 25. Polytrichum commune. rapid elevation in the character of the apparatus of nutrition, as causes the group to approximate closely in this respect to the Phanerogamic division; indeed, its members may be said to be more highly organized in most respects than the inferior Phanerogamia, although the type of their generative apparatus being essentially Cryptogamic, they must be considered as belonging to the lower rank in the Vegetable scale. It is in the Tree-Ferns that we have the most perfect evolution of the characters of the 60 GENERAL PLAN OP ORGANIC STRUCTURE AND DEVELOPMENT. group • and here we find, not only an ascending axis or stem, around which the foliaceous appendages are symmetrically arranged in a spiral, but a Fig. 26. Fig. 27. Trichornanea. Frond of Scolopendrum. proper descending axis or true root, from which alone the radical fibres are given off. In the stem, the cortical portion is separated from the medullary Fig. 28. Frond of Oamunda rega.lia;—a, sterile or foliaceous portion; b, fertile portion:—-a, part of the latter enlarged, to show the thecje. Fig. 29. JSquiaetum arvenae. GENERAL VIEW OP THE VEGETABLE KINGDOM. — PERNS. 61 by the interposition of bundles composed of woody fibre and vascular tissue; and the principal difference which exists between these and the woody layers of Exogenous stems, lies in the absence of any tendency to regular increase, except in length. From the fibro-vascular bundles in the stem, prolongations are given off, which pass into the leaf-stalks, and thence into the midrib and lateral branches of the foliaceous appendages, to which they form a kind of skeleton, as in the leaves of Phanerogamia. These organs, which are distinguished as "fronds," on account of their combining the character of a leaf with that of an apparatus of fructification, are constructed upon the same type with the leaves of Flowering-Plants; being composed of a cellular parenchyma, inclosed between two layers of epidermis, and having air-chambers to which access is given by stomata; and they can scarcely be less complete as organs of nutrition, although still made to bear a share in the function of reproduction. Even in this respect, however, a differentiation exhibits itself in certain Ferns, as the Osmunda regalis (Fig. 28) ; whose fructification is restricted to particular fronds, or parts of fronds, hence designated "fertile," which lose their foliaceous character; whilst the remainder bear no fructification, and are hence designated as " sterile," performing the functions of leaves alone. The ostensible organs of fructification are far from constituting (as they were until lately supposed to do) the real generative apparatus; for this is evolved at a period in the life of the plant, at which its appearance was totally unexpected. Each of the " spore- Fig. 30. Lycopodium cemuum. cells" which are set free from conceptacles on the under surface of the fronds (Fig. 27), when received upon a damp soil, extends itself, by duplicative subdivision, into a frondose body closely resembling the thallus of the Marchantia ; it is in this that the " sperm-cells" and " germ-cells" are evolved, and that the fertilization of the latter, by self-moving spermatoid filaments set free from the former, takes place; and from the embryo-cell, which is the product of this operation, there arises—not, as in the Mosses and Liverworts, a coneeptacle filled with spores, each of which may give origin to a 62 GENERAL PLAN OP ORGANIC STRUCTURE AND DEVELOPMENT. separate plant—but a single young Fern, which, having attained its full development by duplicative subdivision, detaches certain. of its cells, as " spores," to continue the race by the same process. In this departure from the plan which prevails among the inferior Cryptogamia, we have an obvious tendency towards that of the Flowering-Plants: the entire product of each generative act being worked up (so to speak) in the Fern, as in the Flowering-Plant, into the diversified parts of a single organism; instead of being subdivided, as in the inferior Cryptogamia, amongst an indefinite number of independent fabrics, which are mere repetitions one of another. Still, the type of the generative apparatus in the Ferns is essentially Cryptogamia—That of the Equisetacece (Fig. 29) appears to be essentially the same; but in Lycopodiaceae (Fig. 30), Isoetacece, and Rhizocarpece (Fig. 31), there is a still closer approximation to the Phanerogamic type, the "sperm-cells" ("small spores") being directly produced by the parent-struc- Fig. 31. Marsilea quadrifolia. ture, and the " germ-cells" alone being evolved after the detachment of the "large spores," upon the " prothallium" into which each of these develops itself. 29. The distinctive character of the Phanerogamia or "Flowering-Plants" is not the possession of what are commonly designated as "flowers," since these may be reduced to a condition in which they are scarcely distinguishable from the fructification of the Cryptogamia. In fact, the group of Rhizocarpeae, in which the concurrent action of the small and large spores had been ascertained to be necessary for the production of an embryo, was referred by many Botanists to this division, at a period when the existence of distinct sexes had not been recognized among the Cryptogamia generally, and when it was, in fact, not merely doubted, but usually denied. Still, it is in the peculiar type of their Generative apparatus, that the essential distinction lies; for the fertilizing process is performed among them in a manner not elsewhere seen, namely, by the emission of a long tube from the "germ-cell" (pollen-grain), which finds its way (often through a distance of some inches) to the "sperm-cell" buried in the ovule; and it is among them alone that a true seed is produced, in which, with the embryo, a store of ready-prepared nutriment is laid up for its early development. This subdivision of the Vegetable kingdom includes a vast range of species that differ very greatly in the degree of development, both of their nutritive and their generative apparatus; but for our present purpose, it will be sufficient to sketch the typical plan, which is more or less obviously manifested in the conformation of the entire group.—If we analyze the fabric of any common Phanerogamous Plant, we find that it consists essentially of an axis and appendages; the former being made up of an ascending portion or stem, and of a descending portion or root, with their respective ramifications; and the latter being distinguishable into foliaceous and floral organs, which will be presently shown to be modifications of the same fundamental parts. The axis (Fig. 32, a, a a) is composed of cellular parenchyma, with a larger or smaller proportion of fibro-vascular tissue; and it is upon the mode in which these components are arranged relatively to each other, and in which progressive additions are made to the diameter 63 GENERAL VIEW OF VEGETABLE KINGDOM. —PHANEROGAMIA. of the axis, that the distinction is founded between the Endogenous and Exogenous types, which together with corresponding distinctions in the structure of the leaves, flowers, and seeds, affords a basis for the subdivision of the Phanerogamia into two primary classes. Prom the central axis, bundles of fibro-vascular tissue pass down into the root-fibres which form the ultimate ramifications of its descending portion; these are enveloped in firm tissue, that limits their absorbent power to their extremities, which, being still soft and succulent, are known as "spongioles." On the other hand, the fibro-vascular bundles of the ascending portion of the axis pass into the footstalks of the leaves; and their ultimate ramifications form the skeletons of these organs, the interstices being filled up with cellular parenchyma, and the whole being clothed with an epidermis, quite distinct in texture from the parenchyma it covers, and perforated by the peculiar apertures termed "stomata" (Fig. 155). Various modifications present themselves in the form of the leaves, and in the arrangement of their component parts; but none of these effect the essential character of the organs. The modes, too, in which they are arranged on the stem, present a great apparent variety; but they seem all reducible to one fundamental type, namely, a spiral, which is the result of the radiation of the appendages, not from a single point, but from a longitudinal axis. When this plan is characteristically exhibited, the leaves come off at regular intervals along the axis, but not in a vertical line one with another—the second not being above the first, but a little to one side of it— the third holding the same relation to the second—and so on; in such a manner that a line carried through the points of origin of the successive leaves, which are termed "nodes," will not only ascend the stem, but Fig. 32. A, Ideal Plant, after Schleiden; a to a* 1 , the axis, a being the root, a'< a"> a m > a iT > and a v the successive internodes of the stem, and a Ti the terminal development of the axis into an ovule; b. rootlets; c to c Tii the successive foliaceous appendages to the axis, c being the cotyledons, c"> e">and e iu the ordinary leaves, c iv the outer floral leaves or sepals, c T the inner floral leaves or petals, c yi the stamens, and c Tii the carpellary leaves; d, leaf-buds: —b, carpel inclosing an ovule, seen externally and in section, showing a, the stigma, b the style, c the ovary:—c, leaf-buds, as seen externally at d'h and in section at d l K will gradually turn round it, and will at last pass through a point directly above the origin of the first leaf. The leaves whose origin has been intersected by this line, whilst it makes one turn round the stem, are said to form a cycle; and the number of leaves which this cycle contains, is subject to great variations. Thus in Dicotyledonous plants generally it may be said 64 GENERAL PLAN OP ORGANIC STRUCTURE AND DEVELOPMENT. to he Jive; that is, the sixth leaf will be directly above the first, the eleventh directly above the sixth, and so on. In Monocotyledons, however, the typical number is three; the fourth leaf being above the first, the seventh above the fourth, and so on. There are cases in which the cycle seems to consist of only two leaves; each leaf springing from the side of the stem precisely opposite to that from which the leaf below it, as well as the one above it, arises. The most common departures from the spiral type, shown in the disposition of the leaves, are those which are known as the opposite and the verticillate (or radiate) arrangements. These may be reconciled with it in three modes, each of which has some evidence to recommend it; and perhaps the deviation does not always take place in the same way. 1 30. The complete Jloral apparatus of Phanerogamia consists externally of a " perianth," composed of a series of verticils of foliaceous organs, which do not depart widely, except in color, from the ordinary type of the leaf, and are arranged according to the law of spiral development round the axis. For the first or outermost layer of the " perianth," in a perfectly regular flower, is formed of a whorl of bracts ; the calyx is composed of a whorl of sepals (Fig. 32, c iv ) alternating with the preceding; and the corolla, in like manner, consists of a whorl of petals (c v ), which alternates with that of the sepals, but corresponds with that of the bracts. These whorls, in many flowers, are considerably multiplied, and the spiral arrangement of their component parts is often very obvious; and, when such is the case (as in the Garden Pseony), we may observe such a gradual passage from the type of the ordinary leaf, through the succession of bracts and sepals, to the most characteristic petal, that the essential conformity of this last to the same general type with the preceding cannot be for a moment doubted. In the flowers of Dicotyledons, the typical number of components of each whorl, as of that of the cycle of ordinary leaves, is Jive, whilst in the Monocotyledons it is three. The regularity of a flower may be interfered with by the suppression or by the multiplication of whorls ; but the greatest departures from archetypal simplicity are those which result from the unequal development of different parts of the same whorl, some being very imperfectly evolved or even entirely suppressed, whilst others are extraordinarily augmented in size, and strangely altered in figure and character. The scientific Botanist, however, can seldom be at a loss in the investigation of their real nature, if he proceed on the morphological principles 1 Thus, "opposite" leaves would he produced in a plant whose "cycle" consisted only of two, by the non-development of every alternate segment, or "internode" of the stem, so that each leaf and its successor on the opposite side come to be developed from the same part of the stem, whilst separated by an interval from the next pair. But this explanation does not suit those cases, in which the successive pairs of leaves are arranged on the stem at right angles to each other; and this arrangement may either be attributed to the development of two opposite leaves from each node, the successive pairs being then arranged in a cycle of four; or to the existence of two spirals proceeding up the stem simultaneously.—In like manner, a "verticil" of five leaves originating from the same point of the stem, may be conceived to result from the non-development of the internodes between five successive nodes; and it sometimes happens that leaves which have a verticillated arrangement at one part of the stem, are spiral at another, being separated by the development of the intermediate internodes. But this does not account for the fact, that the successive whorls themselves usually alternate with each other; each leaf of the verticil being over the spaces between the leaves of the verticil beneath it.—And here again it would seem necessary, either to imagine that all the leaves of one verticil may originate from a single internode, or to suppose several spirals to be passing round the stem. In either way, however, this very common arrangement is reconcilable with the general theory of spiral development, which is thus readily carried into application as regards the disposition of the parts of the Flower. GENERAL VIEW OF VEGETABLE KINGDOM. PHANEROGAMIA. 65 already explained ; and he continually finds his determinations justified by the occurrence of "monstrosities," which exhibit a more or less complete reversion to the archetypal form (§ 82). The non-essential character of the perianth is indicated by the deficiency of one or more of its whorls in many tribes of Plants, which are nevertheless truly Phanerogamic. It is interesting to remark, however, that the group of Gymnospermce, in which the deficiency is most complete, really form a transition-step to the higher Cryptogamia, in virtue of certain peculiarities in their proper generative apparatus, which will be explained hereafter (Chap. XI). —It is within the protection of the perianth, that the true generative organs are developed; and these consist of the anthers (Fig. 32, c Ti ) from which the " sperm-cells" (here termed pollen-grains) are evolved, and the carpels (c yii ), whose aggregation forms the pistil, containing the ovules (« vi ), each of which includes a " germ-cell" imbedded in a mass of nutritious matter, the whole invested by two or more seed-coats. Now the anthers, which with their supporting " filaments" constitute the stamens, depart more widely than do the sepals and petals from the ordinary condition of the leaf; but it is quite certain, alike from the history of their development, from the series of intermediate forms which some flowers (as the Nymphoea alba, or white water-lily) present, and from their occasional reversion in monstrous flowers to the form of petal or sepal, or even to that of the ordinary leaf, that they too belong to the same type of structure. The carpels (b), again, may be regarded as leaves folded together at the edges ; as is indicated by their frequent retention of much of the leafy character, even in the normally-developed flower, and by their occasional more or less complete reversion to the type of the leaf in monstrous blossoms, sometimes when (as in the common " double cherry") the stamens have undergone a less complete transformation. If the Gymnosperms, indeed, the carpellary leaves are not folded together so as to inclose the ovules, which are developed upon their internal surfaces ; and merely protect them during their immaturity, by their own mutual adhesion.—It is the general rule for the two kinds of sexual organs to be developed in the same organism; and where, as is most commonly the case, every flower contains both stamens and carpels, it is said to be hermaphrodite. There are certain cases, however, in which, by the suppression of one or other of these whorls, the flowers become unisexual; when the staminiferous or male flowers are borne on the same plant or tree with the pistilline, it is said to be monoecious; whilst if the two sets of flowers are developed by different individuals, the species is said to be dioecious. This last arrangement, in which the generative apparatus attains its highest degree of differentiation, is comparatively infrequent; but we find examples of it in several groups of Cryptogamia, as well as among Phanerogamia. 31. The "embryo-cell," which is formed within the germ-cell, after the admixture of the contents of the sperm-cell with its own by the means already adverted to, develops itself by duplicative subdivision, just as among the lowest Cryptogamia; but the nourishment which it requires for the continuance of this operation is furnished by the store previously laid /tup m the ovule; and the entire mass of cells thus formed, instead of subdividing to constitute a multitude of independent organisms, remains connected so as to form but a single fabric ; and this exhibits at a very early period a tendency to become heterogeneous, by the development of distinct organs, every kind of organ, however, being very numerously repeated. For, at the time that the seed is detached, as a self-sustaining structure, from the parent, the embryonic rudiments of the stem and root are already formed, and a temporary leaf-like expansion, the single or double cotyledon 5 66 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. (Fig. 32, a, c) is prepared to evolve itself; whilst a supply of nutriment for its further development is stored up within it, either forming a separate albumen external to the embryo, or being contained within its cotyledons, which are in that case thick and fleshy. The subsequent evolution of the plant, of which " germination" is the first stage, consists in the progressive development of the ascending and descending axes and of their respective ramifications, these remaining permanent; and in the evolution, from the ascending axis, of a succession of mutually similar appendages, foliaceous and floral, which have only a temporary existence, each set being in its turn replaced by another. Thus the individuality of the whole fabric is maintained, whilst a continual change is taking place in certain of its component parts. 32. It is with the performance of the true generative act, and the consequent production of a new embryo-cell, that each "new generation" originates. But it is not in this mode alone, that Phanerogamic Plants (for the most part at least) are multiplied. For each leaf-bud usually possesses within itself the capacity of putting forth roots, when separated from the parent stock and placed in circumstances favorable to its growth, so that it thus becomes capable of maintaining an independent existence, and of developing itself into a perfect Plant; and there are some Phanerogamia which spontaneously detach leaf-buds or "bulbels," and which thus multiply themselves after a manner analogous to that which prevails so remarkably among the lower Cryptogamia. This is pre-eminently the case, for example, with the common Lemna (duck-weed), each plant of which consists of but a single foliaceous body, with a root-fibre hanging from its under surface; this puts forth buds from its margin; and these buds, early detaching themselves from their stocks, henceforth maintain an independent existence, so that the plant thus becomes rapidly multiplied by gemmation, large surfaces of water being covered by the growth proceeding from a single individual, without the intervention of any process of generation. It is interesting to remark that this little plant seems to hold almost the same relation to Phanerogamia, that the lowest Protophyta do to Cryptogamia. For it scarcely presents any distinction of parts, the leaf and stem being fused together into a single flattened lobe, whilst the organs of reproduction are reduced to their very simplest form, being developed in a slit in its edge. Its texture, too, is of the simplest kind, being composed of scarcely anything but ordinary cellular tissue. And the developmental process here, as in the Protococci, consists in the multiplication of organs which repeat each other in every particular, and which, having no relation of mutual dependence, can exist as well detached as coherent; instead of tending, as in the higher forms of Yegetable life, to the evolution of a single fabric, whose several parts present a marked differentiation of external form and of internal structure, and have such a functional dependence on one another, that they can only exist as living bodies so long as they remain mutually connected. 33. Animal Kingdom. —Turning, now, to the other great division of the Organized Creation, we shall in the first place examine, as in the previous ease, what is the highest form under which its life expresses itself. The whole nisus of Vegetative existence consists in the activity of the organs of Nutrition and Reproduction; but on the other hand, the nisus of Animal life tends towards the evolution of the faculties of Sensation and of Self-determined motion, and, in its highest manifestation, to that of the Intelligence and Will. The instruments of these faculties, however, are in the first place developed, and are afterwards sustained, by the Organic 67 GENERAL VIEW OP ANIMAL KINGDOM. apparatus with which they are connected; whilst, in their turn, they become subservient to its operations ; so that, in those forms of Animal existence, in which there, is the greatest differentiation of organs, there is at the same time the closest relation of mutual dependence in their actions ; and everything tends to render the entire product of each generative act a single individual, in the most restricted sense of that term, no multiplication by the subdivision of that product ever taking place (save as a monstrosity), but the whole of it evolving itself into a congeries of different but mutually related organs. It is only in the higher forms of Animal existence, however, that we meet with this complete individualization, and this marked predominance of the animal over the vegetative. In a large proportion of the beings composing this kingdom, the apparatus which is subservient to the strictly animal functions is scarcely differentiated from that which ministers to organic life ; in many of the cases in which the former is separately distinguished, it seems but a mere appendage to the latter ; and it is only in the highest or Vertebrate type, that we find the general plan of the fabric distinctly arranged with special reference to the manifestations of Animal power, which involve the exercise of its highest attribute—Intelligence. The nearest approach to this is made in the higher forms of the Articulated series; in which a very remarkable degree of development is given to the instruments of the lower animal powers, especially the locomotive apparatus; and in which the general plan of structure, and the arrangement of the nutritive apparatus, have evident reference to this. But in the Mollusca, we find a marked predominance of the Vegetative apparatus ; it being in only a small proportion of the group, that there is any considerable power of movement. And in the Radiata, it becomes obvious that the general plan has reference rather to the " vegetative repetition" of the organs of Nutrition and Reproduction than to any manifestation of the higher Animal powers; the apparatus for which, in so far as it is developed, exhibits a like repetition of similar parts.—Notwithstanding the diversity of these types of structure, however, and the marked differences which they present in regard to the relative development of their several organs, we observe in the higher forms (at least) of each of them, a differentiation of all the most important parts by which the Animal is especially characterized. For we find in each type a digestive cavity for the reception and preparation of aliment; chyliferous channels or vessels, into which the liquid prepared by the digestive process transudes from this cavity, to be conveyed to the remoter parts of the organism ; a circulating system, by which the distribution of the nutritive fluid is effected, the surplus materials brought back, and the waste or refuse matter removed from the tissues and conveyed for elimination to appropriate organs ; a respiratory surface, through which the circulating fluid is exposed to the influence of atmospheric air; secreting glands for the separation of certain products from the blood, either for its purification, or for special uses in the economy, or for both purposes combined; generative organs, in which "sperm-cells," or "germ-cells," or both, are developed, the latter being inclosed (as in Phanerogamous Plants) in a store of nutriment prepared for the nutrition of the germ, so as to constitute an ovum ; organs of support and protection, forming a "skeleton" of some kind, either external or internal; organs of sensation; organs of consciousness and self-direction; and organs of locomotion. 34. It is true that in the least developed forms of each type, we may find some or other of these organs but little distinguished from the general structure, or even entirely absent; but the proportion of such forms is 68 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. smaller, the higher we ascend in the scale. Thus, in a large part of the Radiated series, there is but little differentiation of the several parts of the nutritive apparatus; and although the reproductive is nearly always very distinct from it, yet even this is scarcely segregated in the lowest examples of the type : whilst even the very slight development which the organs of animal life attain in the higher Radiata, is altogether wanting in the lower, among which they are not distinguishable by any structural mark.—But in the Molluscous series, it is only among the very lowest that we have a difficulty in distinguishing all the essential parts of the apparatus of nutrition and reproduction, the absorbent and circulating apparatus being usually that which is most imperfectly developed; and although the organs of sense and locomotion are not evolved in the same proportion, we never fail to find a nervous ganglion, which must be considered as marking the existence of some degree of consciousness.—On the other hand, in the lowest forms of the Articulated series, it is the imperfection of the nutritive apparatus which most strikes us ; and although distinct sensori-motor organs are there also very deficient, yet they present themselves very prominently in higher parts of this series, in which th» type of nutritive system is still comparatively low. In both these sub-kingdoms, however, it is only in a small proportion of each series respectively, that we fail to discern all the essential parts of the assemblage of organs just now enumerated; those higher forms of each, in which the differentiation is complete, constituting the great bulk of its entire series, instead of being, as among the Radiata, exceptional as to number, and probably to be so considered in regard to type likewise. 1 — Now, in the Vertebrated series, the complete differentiation of all these structures is nearly the invariable rule; it being only in one of the very lowest fishes (the Amphioxus) that we meet with such an imperfect development of any of the systems above enumerated, as reminds us of those simpler organisms in which they are absolutely deficient. There is another point of interest nearly related to the preceding, in regard to which these primary types of Animal conformation present a marked contrast; and this is the degree in which they are severally capable of being multiplied by gemmation. This power exists among Zoophytes in exactly the same degree as among the higher Plants; for whilst the gemmae, in the former, as iu the latter, usually remain connected with the parent-stock, they are capable of maintaining their existence if detached, and are regularly thrown off in some species, so as to become independent organisms, possessing all the capabilities of that from which they have separated themselves; and in the very simplest Zoophytes (as the Hydra), we even find a capacity for reproducing the entire fabric to lie in every fragment of the body, just as a'fragment of the leaf of Bryophyllum will give origin to an entire plant (§ 21, note). A like capacity exists in the lowest group of the Mollusca, which, in this and in many other particulars, closely borders upon Zoophytes. It is. only among a very small number of the lowest Articulated animals, however, that this method of multiplication presents itself. And among Yertebrata it seems entirely wanting as a regular habit, although there is reason to think that it may occasionally occur as an abnormality, at that early period of the evolution of the germ when its grade of development has not advanced beyond the Zoophytic stage (Chap. XL). 35. Underlying these well-marked types of Animal organization, how- 1 In the Author's opinion, the Zoophytes, not the Echinodermata, are the types of the Radiated series;— Gasteropods of the Molluscous ;—Insects of the Articulated; and 3Iammals of the Vertebrated. 69 GENERAL VIEW OP ANIMAL KINGDOM. —PROTOZOA. ever, there is a group of beings which cannot be regarded as presenting even a rudiment of the plan of conformation that is characteristic of anyone of them, and in which scarcely any differentiation of organs is to be discerned—a group, in fact, which holds a rank in the Animal kingdom, that is precisely parallel to that of the Protophyta in the Yegetable (§ 22), and which may therefore be appropriately designated Protozoa. Between these two groups, indeed, no definite line of demarcation can be drawn; and the same beings have been reckoned as Plants or as Animals, according to the particular views of the classifier in regard to the mode in which they should be distinguished. A large proportion of the Protozoa consists of single cells, or of aggregations of cells in which there is no differentiation of character ; and in the lowest forms of them, there is not even that distinctness of the cell-wall from the cell-contents which exists in every completelydeveloped cell, but the whole forms one mass of living jelly (Fig. 33). The animal character of this, however, is marked in its mode of nutrition; for it does not draw its aliment, like the Protophytes, from the surrounding air and moisture, but is dependent for its support upon organic substances Fig. 33. Amceba princeps, in different forms, A, B, C, previously elaborated by other beings, which it envelopes with its own.jellylike substance, and of which it gradually dissolves and appropriates that which is fitted for its own increase. The animal character of this body is also indicated by its movements; for although the " zoospores" of the Protophyta and lower Alga? are rapidly propelled through the water by ciliary action, yet they do not exhibit that motion of one part upon another, which is often seen in the simplest Protozoa. But there are as yet no special instruments either for sensation or for motion. As every part of the body is equally adapted for digestion, for absorption, for circulation, for respiration, and for secretion, so does every part appear equally capable of receiving impressions made upon it, and of responding : to them by a contractile movement. From this starting-point we may proceed in either of two directions;. for we find in the Infusory Animalcules a tendency to the individualization of the single cell, which seems to attain in them its highest development as a separate entity; whilst in the Rhizopoda (Foraminifera) and Porifera (Sponges) we find aggregations of gelatinous bodies (which present more or less distinctly the characters of true cells) assuming cer- 70 GENERAL PLAN OP ORGANIC STRUCTURE AND DEVELOPMENT. tain definite types of form, and approaching the individuality of higher organisms.—In the true Animalcules (excluding the Rhizopods and the Protophyta which have been confounded with them) we find an obvious distinction between cell-wall and cell-cavity; there is a definite opening into the latter, through which food is introduced, instead of its being received into any part of the mass; and there is frequently, also, a second orifice, through which indigestible particles are expelled. Moreover, the locomotion of these beings is performed, as in the Protophyta, by the agency of cilia; these being prolongations of the cell itself, to which the contractile power is especially delegated. Their multiplication is ordinarily accomplished, like that of the Protophyta, by duplicative subdivision; and in this way a vast number of similar beings may be produced, each of which is a repetition of the rest, and lives altogether independently of them. But it seems probable that, like the Protophyta, they have a proper generative process, consisting in the "conjugation" of two similar cells; no sexual distinctions as yet manifesting itself between these, and both of them apparently contributing in the same manner and degree to the production of the germ.—In the Rhizopoda, we find the simple jelly-like mass extending itself by gemmation, and at the same time very commonly forming a calcareous envelop upon its exterior; whilst through apertures in this are put forth extensions (pseudopodia) of the soft substance in its interior, through which the introduction of nutriment into the body seems to be chiefly effected. Notwithstanding the small amount of differentiation which appears to exist among the several products of gemmation, yet a strong tendency to individualization in the entire aggregate is shown in the very definite plan of growth which each species exhibits, as is most obviously seen in Nummulites and other higher forms of Poraminifera. Of the mode of multiplication of these animals, nothing is yet known. In the Porifera, or Sponges, there is, with less definiteness of configuration in the aggregate mass produced by gemmation from the single primordial cell, a much higher degree of mutual interdependence; for we now find the component particles so arranged as to form the rudiments of differentiated organs, whilst the general plan of structure approaches that which we meet with among the lower Zoophytes, in whose fabrics the individuality of the components is still more completely merged in that of the organism as a whole. Por, in the first place, we have a marked distinction between the internal fibrous skeleton and the soft flesh which clothes it; and these components have a very definfte and characteristic arrangement, which varies in different parts of the mass; being dissimilar, near the external surface, and around the internal canals, to that which prevails in the intervening substance. Again, in the system of absorbent pores for the entrance of liquid, and of ramifying canals for its discharge, we have the first rudiment of a digestive and circulatory apparatus, not yet marked off, however, from the general cavity of the body. And although the organs of nutrition do not present any further specialization, yet those of reproduction are differentiated from them, and are limited to particular parts of the mass. Even in this lowest form of an aggregate Animal, there is reason to believe that a true ovum is produced ; so that we here already advance to the same essential type of generation, as that which prevails in the highest plants. 36. Among the four definite types of structure under which all the higher forms of Animal organization may be ranked, the Radiated, as already remarked, unquestionably holds the lowest rank: in virtue alike of the close conformity of its general plan to that which prevails in the higher Plants; of that predominance of its Vegetative or Nutritive apparatus over that of 71 GENERAL VIEW OF ANIMAL KINGDOM. PROTOZOA. Animal life, which is conspicuous even in its higher types; and of that very imperfect differentiation of the organs of the former, which prevails through the larger part of the group. Each of these points will now be noticed in some detail.—The radial symmetry must be regarded as in itself a vegetative character, for it corresponds with that which is seen in the disposition of the appendages around the axis in the leaf-buds and flower-buds of plants; and it is intimately connected with another vegetative character, the repetition of similar parts. Thus, in the animals in which it prevails, we find the central mouth to be surrounded externally by a circular series of prehensile appendages; which may be mere oral tentacles, as in the Polypes (Figs. 34, 35), the Medusa (Fig. 93), and the Holothuria (Fig. 40), true arms, as in the Ophiura and Comatula (Figs. 8, 38), or divisions of the body itself, as in the Star-fish (Fig. 37). In the arrangement of the internal organs, a similar character is exhibited; that is, a circular disposition of parts which precisely repeat each other. There are, it is true, modifications of the radial type in certain aberrant forms of the group, which tend towards a bi-lateral symmetry; but these are comparatively rare exceptions, which it is only necessary here to mention. It is not only in their radial symmetry, however, that the animals of this division are conformable to the type of the higher portion of the Vegetable kingdom; for this conformity is equally shown by a large proportion of the group, in the development of composite structures by gemination. From a single polype, as from a single leaf-bud, an arborescent structure may be evolved, bearing hundreds or even thousands of polype-bodies, all originating from the first, and maintaining an intimate organic connection with each other; thus bearing a close physiological resemblance to a tree, and requiring to be considered (like it) as a single individual, although its several members have no relation of mutual interdependence, and can maintain a separate existence if detached. It is not to be wondered at, then, that the older Naturalists, who were only acquainted with the skeletons of Zoophytes, should have considered them as vegetable structures, and that many of them should even now be popularly regarded in that light; whilst even the movements exhibited by the living polypes, not being apparently very different in nature from those performed by the Sensitive-Plant, or the Venus's Fly-trap, did not seem sufficient to establish their animal nature. This extension of the original fabric by gemmation may take place among Zoophytes to an indefinite extent; and the mode in which it occurs is the chief determining cause of the particular type or plan of growth which is traceable in each species, but which is liable to great variation from the influence of external conditions. In nearly all the members of the class of Acalephce, it seems to take place at some period of life or other; for although we find few traces of it in the fully developed Medusa, yet (as will be shown hereafter, Chap. XI.) multiplication by gemmation takes place to an extraordinary extent during the early stages of their existence; and in some of the lower forms of the group, especially those which closely approximate to the Zoophytic type, it continues during the whole of life, and gives rise to those composite fragments of the Oirrhiyrade and Physograde orders, which, until the recent discovery of their true character, have been a source of so much 1 perplexity to Naturalists. In the class Echinodermata, multiplication by gemination very seldom takes place; but its members retain throughout their lives an extraordinary measure of that power of reproducing lost parts, of which the production of an entire organism by gemmation is only a higher manifestation. 37. The low development of the proper Animal powers in Radiated 72 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. animals, as compared with their Vegetative activity, is one of the most remarkable features of the group taken as a whole; nor are there any exceptions to this general character. In none of the true Zoophytes is the nervous system differentiated from that general fibro-gelatinous tissue of which the entire bodies are composed; every part seems more or less impressionable and contractile, although these attributes are most strongly displayed in the oral tentacula; and there is no evidence that the respondence to external impressions which is probably the source of all their movements, proceed from any distinct consciousness of these impressions. It is in the Acalephce, that the first traces present themselves of nervous system, and of organs peculiarly fitted to receive sensory impressions; but it is probable that a large part of the movements executed by even these animals, are not dependent upon any influence transmitted through this apparatus. In the Echinodermata, whose -organs and tissues attain a far higher grade of development, the nervous system is more clearly marked out; and the distinction between nerve-cords and ganglionic centres, which has not yet been clearly established in the Acalephae, may be unmistakably affirmed Fig. 34. A, Hydra fusca, or Brown Fresh-water Polype, attached to a piece of stick, with its arras extended, ns in search of prey; a, the mouth surrounded by tentacula; b, foot or base, with its suctorial disk: at b is seen a portion of one of the arms near its . origin, and at c another portion near its termination, more highly magnified. to exist. There are also rudiments of eyes in certain members of this class; and there is some evidence that their movements are directed by visual impressions received through these organs. 38. Between the lowest and the highest members of the Radiated series, there is a very marked contrast in regard to the differentiation of the principal organs of Vegetative life; but a number of intermediate gradations present themselves, which establish a tolerably complete transition from the one condition to the other.—Commencing with the Hydra (Fig. 34), we find the digestive apparatus reduced to a state of the greatest simplicity, the whole body seeming to be nothing else than a stomach, with a circle of prehensile tentacula around its orifice, which, being single, and serving alike for the reception of food and for the ejection of its indigestible portions, must be considered as representing in itself the cardiac and pyloric orifices of the stomachs of higher animals. The wall of this cavity and the general integument of the body are so closely connected together, as to seem like two layers of one and the same membrane; there are, however, some lacunar spaces between them, constituting the first indication of that " general 73 GENERAL VIEW OP ANIMAL KINGDOM. —ZOOPHYTES. cavity of the body" which exists in almost every other animal, and which performs, as we shall see, very important functions; and these lacunar spaces communicate with similar cavities in the interior of the tentacula. There does not yet appear to be any decided structural or functional differentiation between the layer which lines the stomach and that which clothes the body; since each can perform all the offices of the other, as is shown by the result of Trembley's well-known experiment. No circulating apparatus is yet distinguishable, the nutritive liquid, which is the product of the digestive operation, being at once absorbed from the parietes of the stomach into the general substance of the body and arms; nor is there any special respiratory or secretory apparatus. Even the generative organs, which are usually the first to be differentiated from the rest of the fabric, cannot here be distinguished; for ovules and sperm-cells are evolved in the substance of the ordinary tissue; and the only indication of their specialization is afforded by the restriction of their production to particular situations, the sperm-cells usually making their appearance just beneath the arms, whilst the ovules protrude nearer the foot. The homogeneousness of the entire body, however, is most remarkably evinced in the facts, that gemnice which develop themselves into new Hydra? sprout almost indifferently from any part of it, and that a minute fragment from any region will (under favor- able circumstances) regenerate the whole. In the composite fabrics which are formed after the Hydraform type (Fig. 99), the consolidation of the external integument necessitates several other changes; amongst the rest, the evolution of a special reproductive apparatus, and the separation (within the polype-cells) of the wall of the stomach from the external integument, so as to commence the formation of the "general cavity of the body." This, however, is carried much further in the Actinia (Fig. 35), and in all the Polypes formed upon its type; for in these we find the stomach suspended (as it were) in a large space, which is subdivided by radiating partitions ; and it is in the chambers thus formed (which are prolonged into the interior of the tentacula) that the generative apparatus is Fig. 35. Diagrammatic section of Actinia, showing its internal structure; — a, a, base or foot; b, b, oral disk j c, c, tentacula; d, mouth; e, stomach; g, g, k, k, vertical partitions cut across in different directions; g',g', apertures in these; h, passages opening into the tentacula; I, I, testes or ovaria; m, m, filiferous filaments. situated. Very distinct organs for the production of sperm-cells or of ova are here evolved; these organs (according to late researches, Chap. XI.) not being combined in the same individuals. There is still a direct connection between the interior of the digestive sac and the general cavity of the body, by an aperture at the bottom of the former; and through this, the nutritive products of digestion find their way into the surrounding cavity, mingled with the water which is introduced through the mouth. This is the only mode in which the tissues are nourished, as there is not yet any special circulating apparatus; and, in like manner, it is only by the 74 GENERAL PLAN OP ORGANIC STRUCTURE AND DEVELOPMENT. expulsion of the fluid that has remained for some time in the general cavity, that the excretory products which have found their way into it from the tissues, can be carried out of the body, in those species which have no orifices at the extremities of the tentacula. Thus the very same liquid answers all the purposes, in these simply-formed animals, which are served in Vertebrata by chyme, chyle, arterial blood, and venous blood; and it also serves as a medium for respiration, the external integument being usually so thickened and hardened, that the amount of aeration of the interstitial fluids which takes place through it must be extremely limited, in comparison with that which will be carried on through the delicate membranes clothing the internal surfaces. Thus, with some very important points of differentiation, the general type of these animals remains extremely low; and their power of multiplying by gemmation, and of reproducing lost parts, in which they are only inferior to the Hydra, is what we might anticipate from their general homogeneousness. In the composite Actiniform Zoophytes, a certain degree of connection remains between the general cavities of the Polypes which have budded off one from another; but this connection is more intimate in the Alcyonian Zoophytes (Fig. 100), among which the "general cavity" extends throughout the polypidom, forming a branching system of canals which strongly resembles that of Sponges. In fact, when we compare the two organisms, we can scarcely fail to perceive that the Alcyonium is essentially a Sponge of which certain parts have been differentiated from the rest, and evolved into special organs. And this view is confirmed by the circumstance, that when a bud is put forth from one of those polypidoms, it has all the ordinary characters of a Sponge, except that its canals do not open upon the external surface (Fig. 91) ; the formation of a polype-mouth and stomach not taking place until a later period. 39. The lower forms of the class of Acalephce carry us back to the grade of development proper to the composite Hydraform Zoophytes. But in the higher, such as the ordinary Medusa (Fig. 36), there is a far less amount of repetition of similar parts, the gemma? detaching themselves from each other at an early stage of development, and subsequently maintaining an entirely independent existence. There cannot be here said, any more than in the Hydra, to be any "general cavity;" for the space between the walls of the digestive sac and of the ovarial chambers which surround it, and the external integument, is occupied by homogeneous solid tissues. But a series of gastro-vascular canals, commencing from the stomach, radiates towards the margin of the disk; and these serve the double purpose of conveying the nutritive product of the digestive operation to the remoter parts of the body for the supply of their wants, and of subjecting it to the aerating influence of the surrounding medium. In its return to the centre, the fluid will of course carry back with it whatever excretory products it may have received from the tissues through which it has passed; and thus, like fluid of the stomach and general cavity of the Actinia, it answers to the chyme, chyle, arterial blood, and venous blood, of Vertebrated animals. In the Beroe (Fig. 102), and certain allied forms, the digestive cavity has an anal as well as an oral orifice; and there also appears reason to think, that in its system of gastro-vascular canals a difference already exists between the afferent and efferent tubes, the fluid passing forth from the stomach by one set, and returning to it by the other. The generative apparatus in this class always exhibits a very well-marked differentiation; its type being in many respects higher than that of the true Zoophytes. For in the Medusa, the four ovaries or testes (b, b) are lodged in cavities round the mouth, each 75 GENERAL VIEW OF ANIMAL KINGDOM. —ECHINODERMATA. of which has its own proper outlet (c, c), so that the mouth is no longer (as it is in those species of Actinia the extremities of whose tentacula are closed) the only channel for the escape of the fertilized ova-or of the rudi- Fig. 36. Structure of Cyanwa aurita. —Disk seen from above, showing the quadrilateral mouth a, tbe four ovaries bbbh, the four orifices of tho ovarian chambers c c c c, tho stomach dddd, and its radiating prolongations, tho eight anal [?] orifices e e, &<:., and the eight ocelli [?]//, &c. mentary young. The sexes are here distinct, the ova and testes not being combined in the same bodies: and this is true also of many of the composite forms, which develop medusa-like buds containing sexual organs, each individual producing buds of only one sex, as in dioecious plants; in others, however, male and female meclusa-buds are developed on the same stock, as in monoecious plants, although in no case are the two sets of generative organs combined in the same medusoid body. 40. In the class Echinodermata, the Asterias (Fig. 31) holds by no means an elevated rank; yet we find it in a very marked advance upon either of the types previously described. The stomach with its single orifice, suspended in the midst of the " general cavity of the body," reminds us of that of Actinia; but it is entirely cut off from that cavity, which consequently remains closed. The nutritive products of digestion probably find their way into it, however, by transudation through the walls of the stomach; and it is thence taken up by a regular system of vessels, the distribution of which, however, is very limited, so that the fluid of the general cavity seems still to take the largest share in the nutritive operation. 1$, is interesting to remark, that in this class we already meet with a differentiation, however imperfect, not only between the fluid of the gastric cavity, or chyme, and that of the surrounding visceral cavity, or chylaqueous fluid, 1 but also 1 The term chylaqueous fluid, introduced by Dr. T. Williams, appears to the Author to be well adapted to designate the fluid of the "general cavity," when (as in Echinodermata and Annelida) this is distinct alike from that of the digestive sac, and from 76 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. between the latter and the blood contained within the proper circulating system. A special provision appears to be made for respiration in these Fig. 37. Asterias awantiaca, with the upper side of the hard envelop removed: — a, central stomach ; b, ca;ca upon its upper surface (salivary glands?); c c, ca>cal prolongations of the stomach into rays; et, the same empty; d, the same laid open; e, tho under surface, seen from within after the removal of the caaca showing tho vesicles of the tubular cirrhi;/, the same in a contracted state, showing the skeleton between them. animals, by the transmission of the "fluid of the general cavity" into a multitude of short delicate caecal tubes, which pass between the pieces of the calcareous framework and project externally in little tufts, and which are lined with cilia that keep up a constant movement in their contents. And there are various secretory organs possessing a distinct glandular character, whose special uses are not yet certainly known. The generative apparatus here attains a high development, the ovaries and testes (as in the higher Acalephae) being no longer combined in the same individuals, and liaving separate orifices for the discharge of their products; it is interesting to remark, that in Comatula (Fig. 38), whose digestive apparatus is framed upon a higher type than that of Asterias, the ovaries are dispersed that of the proper circulating system. It is far more extensively employed, however, by Dr. Williams in his ingenious Memoir, "On the Blood proper and Chylaqueous Fluid of Invertebrated Animals," in the "Philos. Transactions," 1852; being there applied to the immediate product of gastric digestion which passes directly into the "general cavity" of the Actiniform and Alcyonian Zoophytes, and even to that which is confined within the stomach and gastro-vascular canals of Medusas. 77 GENERAL VIEW OP ANIMAL KINGDOM. —ECHINODERMATA. in isolated spots through the integument of the arms. The Star-fish exhibits a series of elaborate provisions for locomotion, in the beautiful arti- Fig. 38. Comatula rosacea. culation of the plates of the calcareous skeleton, in the contractility of the general integument of the body, by which its lobes (misnamed "arms") are moved in various directions, and in the multiplication of tubular cirrhi furnished with suckers, by the contraction of which, when the suckers (forced out by the injection of fluid into the cirrhi from the " general cavity") have taken an attachment, the body is drawn towards the points to which they have adhered.—The chief feature of advance in the Echinus (Fig. 39) is the conversion of the digestive sac with a single orifice, into an aliment- Fig. 39. Echinus mammillatus. ary canal with a separate mouth and anus; and around the mouth we find a very elaborate dental apparatus, furnished with distinct muscles, such as do not make their appearance in any lower forms of organization. The locomotive apparatus, too, is still more highly developed; for the body being now inclosed in an immovable case, so that its parts are not themselves capable of flexure, a new set of instruments is evolved, namely, the calcareous spines, which project from the surface, and are put in motion by the contractile integument, upon the ball-and-socket joints at their base.—The Holothuria presents us with certain interesting features of more complete differentiation, without, however, any very decided advance upon the type of the Echinus. The absence of a solid "test" enables its movements to 78 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT Fig. 40. Anatomy of Holothuria tubulosa; — a, anus; b, mouth, surrounded by 20 tentacula; c, cloaca, surrounded by muscular dilators c J ; %, intestinal tube; m, mesentery; ml, ml, longitudinal muscles; mt, transverse muscles lining the entire inner surface of the integument; o, ovary; ap, ca?cnl appendages, probably seminiferous; p, contractile vesicle, probably a heart; r,r, respiratory apparatus, originating in the cloaca; t, oral tentacula; t', caecal reservoirs; va, annular vessels surrounding the mouth and supplying the tentacula; ve, external intestinal vessel, giving off a large anastomotic branch va' which enters another part of the same trunk; vi, internal intestinal vessel, with contractile dilatations; vl, longitudinal tegumentary vessels, giving off transverse branches vl', seen by removing the longitudinal muscles; vm, mesenteric vessels, connecting the branches of the external intestinal vessels with those of the respiratory system of vessels, vr. be performed by the flexure of the body generally; and for this a regular series of longitudinal and transverse muscular bands (Fig. 40, m I, m t,) is provided, reminding us of those of the Worm-tribe. The alimentary canal (i) does not yet present any distinction of parts into oesophagus, stomach, or intestine, but remains of nearly the same diameter throughout its length; it i* held in its place in the midst of the general cavity of the body, however, by a regular mesentery, upon which the bloodvessels are minutely distributed. The circulating system is more complete than among other members of this class, especially in its peripheral portion; and it is furnished with a v pulsatile vesicle (p), whose contractions assist the onward movements of their fluid. For respiration there are two special provisions; the fluid of the circulatory vessels being aerated by transmission to the branching oral tentacula (t) ; whilst that of the "general cavity" receives the same influence from the water introduced through the respiratory tree (r, r). The restriction of the outlet of the genital apparatus (p) to a single aperture (the five equal and separate portions of 79 GENERAL VIEW OP ANIMAL KINGDOM. — MOLLUSCA. this apparatus in the Echinus and Asterias having each its own outlet) is a very decided character of elevation; which seems to have been presented also by the extinct group of Cystidea (Fig. 81), notwithstanding that in the attachment of these animals by a stalk to a fixed basis, they (in common with the Crinoidea) showed a decidedly zoophytic tendency. 41. The Molluscous sub-kingdom, like the radiated, is remarkable for the high development of its apparatus of vegetative life in comparison with that of animal life; but its type of conformation is altogether different. It is true that, in the lowest group of this series, there is such a close apparent conformity to the Zoophytic type, that the animals belonging to it were, until recently, unhesitatingly ranked under that designation. But it is now perceived that the resemblance is only superficial; being dependent, in part upon the mode in which these animals extend themselves by gemmation, so as to form arborescent structures very analogous to those of true Zoophytes; and being partly caused by the state of degradation to which various organs are reduced, whereby their true type is obscured.—Taking it as a whole, the Molluscous series is characterized rather by the absence, than by the presence, of any definite or symmetrical form. In the Zoophytoid Mollusks, it is true, we are reminded of the radiated type by the circular arrangement of organs around the mouth (Fig. 49, a); whilst in Fig. 41. A, Chitonellua —b, Chiton. the family of Chitonidce, we meet with a division of the external skeleton into segments (Fig. 41), which reminds us of the articidated type. But these are peculiar exceptions; and a Molluscous animal is essentially a bag of viscera, enveloped in a skin which is thickened in parts by muscular Fig. 42. Salpa maxima; a, oral orifice,: I, vent; c, nucleus, composed of the stomach, liver, &c. • d branchial lamina; e, the heart, from which proceeds the longitudinal trunk/, sendin'o- transverse branches across the body; g,g, projecting parts of the external tunic, servino- to unite the different individuals into a chain. fibres that are not arranged after any constant plan. In the "archetype" Mollusk, the mouth and anus are situated at the two extremities of the sac • and the various organs are disposed symmetrically on the two sides of a, longitudinal median plane, just as in a Vertebrate or Articulate embryo; the centres or principal trunks of the circulating apparatus being on the dorsal aspect, (which may hence be termed the "hasmal,") whilst°the prin- 80 GENERAL PLAN OP ORGANIC STRUCTURE AND DEVELOPMENT. cipal centres and trunks of the nervous system are on the ventral aspect, (which may hence be termed "neural.") But this simple and symmetrical arrangement is very commonly obscured by subsequent inequalities in the development of particular regions, so that an entire change takes place in the relative position of the different organs, and the types of conformation thus evolved seem to have little or no affinity to one another. 1 —-The nearest approach to the archetype is presented on the whole by those of the Tunicata, in which the two orifices retain their original positions at the poles of the body (Fig. 42); and their chief peculiarities consist, in the first place, in the enormous development of their pharynx to form the branchial sac, and secondly, in the inversion of their integument around the anal orifice, so as to form an immense cloacal cavity, the wall of which extends so far into the interior, and so completely envelops the general mass of the body, as to constitute what is known as their "inner tunic. m —In the Bivalve Mollusks, on the other hand, the principal extension of the integument takes place externally; a duplicature of the thickened glandular skin of the "dorsal" or "hasmal" region (here termed the mantle) being pro- Fig. 43. Dorsal Margin. Ventral Margin. Anatomy of Mactra; — a, anus; b, posterior muscle; e, branchial ganglion; d, ovary; et, intestine;/, shell; g, nervous cord, connecting oesophageal and branchial ganglia; h, stomach; i, heart; h, liver; I, anterior or oesophageal ganglia; m, anterior adductor muscle; n, nervous filaments; o, mouth; p, one of the oral tentacula; q x, mantle; r, marginof the shell; «, foot; w, branchial lamella;; y, oral siphon; z, anal siphon. longed on either side into two lobes, which enwrap the body like a cloak, and form the valves of the shell upon their outer surface (Fig. 43). Again, a special development of muscular tissue in the integument of the "ventral" 1 See Mr. Huxley's admirable Memoir "On the Morphology of the Cephalous Mollusca," in the "Philos. Transact. 1852." 2 Such is Mr. Huxley's very ingenious account of the production of this tunic, as given in his "Report on the Tunicata" to the British Association, 1852. See also his Memoirs "On the Anatomy of Salpa and Pyrosoma," and his "Remarks upon Appendicularia and Doliolum," in the "Philos. Transact." for 1851. 81 GENERAL VIEW OF ANIMAL KINGDOM. — MOLLUSCA. or "neural" region constitutes the "foot?' of those Lamellibranchiata which possess such an organ.—In the Gasteropoda this foot assumes the form of an expanded disk (Fig. 44, a), upon which the animal can crawl; the two extensions of the upper part of the integument are wanting; but the form of the body itself is entirely altered by the extraordinary and commonly unsymmetrical development of the hindmost portion of the hsemal region into a "post-abdomen," which contains the heart and a considerable part of the alimentary canal, and from the mantle of which a shell (Fig. 45) is very frequently produced. In the pulmonated Gasteropods (Fig. 124), however, the development takes place before instead of behind the anus; so that an "abdomen" is formed instead of a post-abdomen.—This is also the case in the Pteropoda, in which the "foot" proper is but little developed, whilst two lateral expansions (epipodia) sent off from it constitute the wing-like appendages (Fig. 46) from which the group receives its designation.— Finally, in the Cephalopoda, the abdomen is so peculiarly developed that the alimentary canal is quite doubled upon itself, so as to bring the anus into immediate proximity with the mouth (as happens also in the Bryozoa at the opposite extremity of the series); the margins of the foot are prolonged into those prehensile processes (Fig. 47) which are termed "arms;" and the posterior epipodial lobes, by their cohesion, form the "funnel" that Fig. 44. Paludina vivipara, -withdrawn from its shell and laid open;— a, foot; b, operculum; c, one of the tentacula with its ocellus; d, siphon ; /, border of the mantle; g, pectinated branchiae; h, V, the oviduct dilated for the retention of the ova; h", portion of it situated within the spire of the shell; i, termination of the intestine; I, canal for the urinary [?] secretion; n, heart; o, liver; p, proboscis; q„ q', oesophagus; r, stomach; s, s f , s", intestine, lying at t within the branchial cavity; u, u, cephalic ganglia; v, v, salivary glands; x, the principal muscular nerve. serves for the discharge of the respiratory current and of the matters ejected from the intestine.—Thus each of the subordinate types that we recognize in the Molluscous series, presents us with its own special character 6 82 GENERAL PLAN OP ORGANIC STRUCTURE AND DEVELOPMENT Fig. 45. Shells of Gasteropod Mollusks: —A, Achatina; b, Sigaretus;—c, Vcrmetus; —D, Scalaria. of differentiation from the general "Archetype;" and there is no real transition from the one to the other. 1 Fig. 46. Fig. 47. Existing forms of Pteropoda.—A, Hyalaja; b, Criseis; c, Clio. Sepia officinalis, or Cuttle-fish- 1 When it was first discovered that the embryo-forms of Gasteropods (Fig. 48) pos- sess a pair of ciliated lobes corresponding in general position with those of Pteropods, the notion was entertained by many, that the animals of the latter group must be considered in the light of permanent embryoes of the former: this, however, is inconsistent with the fact pointed out by Mr. Huxley, that the ciliated lobes of the embryo Gasteropods are homologous with the anterior portion of the epipodium, whilst it is the middle portion alone which is developed into the "alas" of Pteropods; and that a more fundamental distinction lies in the development of an Embryoes of Nudibranchiate Gasteropods, "abdomen in Pteropods, whilst it is a " post-abdomen'' which is developed in G asteropods. 83 GENERAL VIEW OP ANIMAL KINGDOM. — MOLLUSCA. 42. Turning now to the internal organization of the animals of the Molluscous sub-kingdom, we find that the alimentary canal almost invariably presents a distinct separation between the oesophagus, the stomach, and the intestinal tube; this separation being as obvious in the zoophytoid Laguncula (Fig. 49), as in the Gasteropod Aplysia (Fig. 50). The mouth, or entrance to the oesophagus, is not situated, in the lower Mollusca, on a prominent part of the body, nor is it surrounded by organs of special sense; and hence these are distinguished as acephalous. In the higher classes, however, it is situated on a head, which projects from the trunk, and which is usually furnished with well-developed eyes, and with rudimentary organs of smell and hearing. In the lower Mollusca, again, there is a want of any prehensile or reducing apparatus, the food-particles being drawn in by ciliary currents, which are also subservient to the respiratory function ; but in the higher, the mouth is furnished with a complex apparatus for the reduction of solid food (Fig. 50, a), and prehensile instruments are added in the Pteropods and Cephalopods. In addition to this, a portion of the stomach is frequently developed into a gizzard-like structure, with very firm walls, adapted still further to crush and comminute the food; and this is found in many Bryozoa, as well as in several Gasteropods (Fig. 50, i), and' in Cephalopods generally. The liver is always recognizably present; and although in the Bryozoa it consists of nothing else than an assemblage of iso- Fig. 49. Laguncula re-pens, as seen in its expanded state at A, and in its contracted state, in two different aspects, at b and c. The same references answer for each figure:— a a, tentacula clothed with vibratile cilia; b, pharyngeal cavity; c, valve separating this cavity from d the oesophagus; e, the stomach, with /its pyloric valve, and g the circle of cilia surrounding that orifice ; h, wall of the stomach with biliary follicles; i, the intestine, containing k excrementitious matter, and terminating at I the anus ; m, the testicle ; n, the ovary; o, an ovum set free from the ovary; p, openings for the escape of the ova; q, spermatozoa freely moving in the cavity that surrounds the viscera; r, retractor muscle of the angle of the aperture of the sheath; s, retractor of the sheath; t, retractor of the tentacular circle; w, retractor of the oesophagus ; v, retractor of the stomach; w, principal extonsor muscle; x, transverse wrinkles of the sheath; y, fibres of the sheath, themselves probably muscular; z, muscles of the tentacula; ct (at the base of the tentacular circle in a) nervous or oesophageal ganglion; &, stem.—n, a portion of the tentacular circle shown separately on a larger scale: a a, the tentacula clothed with cilia; b b, their internal canals ; c, muscles of the tentacula; d, transverse muscles forming a ring at the base of the tentacula; e, muscles of the tentacular circle. 84 GENERAL PLAN OP ORGANIC STRUCTURE AND DEVELOPMENT. lated follicles, lodged in the walls of the stomach (Fig. 49, b, h), yet as we ascend the series, we find it gradually becoming more and more detached from that organ; and in the higher Mollusks it is developed into a compact viscus (Fig. 50, /, I), which frequently bears a very large proportion to the general mass of the body. As we ascend from the lower to the Fig. 50. Aplysia laid open to show the arrangement of the viscera: a, upper part of fheWjophagus; b, penis; c, c, salivary glands ; d, superior or cephalic ganglion ; e, e, inferior or subcesophageal ganglia;/, entrance of the oesophagus into g, g, the first stomach or crop ; h, the third or true digestive stomach; i, the second stomach or gizzard; k, intestine; I, I, I, liver; m, posterior or branchial ganglion; n, aorta; o, hepatic artery; p. ventricle of heart; q, auricle; r, s, branchiae ; t, testis; u, lower part of intestine; v, ovary ; w, anus. higher parts of the series, moreover, we find other secreting structures connected with the alimentary canal, such as the salivary glands and pancreas, presenting themselves in a more and more specialized condition; and the 85 GENERAL VIEW OP ANIMAL KINGDOM. —MOLLUSCA. general type which these attain in Cephalopods, closely approximates to that under which we find them in Fishes. In no instance does the alimentary canal possess any direct communication with the " general cavity of the body," in the midst of which it is suspended; but it may be aflirmed with certainty that a transudation of nutritive material takes place through the walls of the former into the latter, and that this is the channel through which this material finds its way into the circulating system. For in the Bryozoa, there is absolutely no other means by which the body at large can be nourished, no true circulating apparatus existing in this group; so that the extension of the visceral cavity throughout the body, and even into the tentacula and stalk, constitutes the sole means by which the products of the digestive operation can be applied to the nutrition of the parts remote from the alimentary canal. Where a distinct vascular system exists, it communicates freely with this "general cavity of the body;" so that the blood in one part of its circulation is freely discharged into it. In the higher Mollusks, however, the viscera themselves occupy so large a proportion of this cavity, that the remaining space is greatly reduced in size, and presents so much of the character of an ordinary venous sinus, that its true nature has not been until recently discovered. Notwithstanding this very important feature of degradation, we find the heart or central impelling organ of the circulation rapidly becoming more and more specialized as we ascend the series. No trace of it, of course, is to be found in the Bryozoa ; in the Tunicata it is generally but little more than a pulsatile dilatation of one of the principal trunks (Fig. 42, e), the direction of whose action is not definitely settled ; in the Gonchifera, we first meet with a differentiation of auricle and ventricle; and this distinction becomes still more strongly marked, both structurally and physiologically, in the higher classes. With the exception of some of the lowest forms of the inferior types, we everywhere find a special provision for the aeration of the circulating fluid by means of a distinct respiratory apparatus; but the position of this varies more than that of any other organ in the body; and it is seldom that any other means are provided for renewing the water in contact with the respiratory surface than the movement of the cilia with which it is clothed. No distinct urinary apparatus can be detected in the lower Mollusca; but its presence becomes distinctly recognizable in the higher. 43. Not only the Bryozoa, but by far the larger proportion of the proper Tunicata, possess a capability of multiplying by gemmation ; the degree of connection, however, that continues to exist between the gemma? and the stock from which they have been put forth, varies in different groups. Thus in the Bryozoa a continuity is frequently preserved, as in Laguncula (Fig. 49), between the "general cavity of the body" of one Zooid 1 and another, through the whole of life ; in Perophora (Fig. 138), the continuity is maintained by the vascular system, which is here in such free communication with the general cavity of the body that it may be almost regarded as a prolongation of it; in Botryllus (Fig. 51), the buds are .formed in the first instance by an extension of the " general cavity of the body" of the stock, but when they have attained an advanced