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PRINCIPLES
OF
COMPARATIVE PHYSIOLOGY.
BY THE SAME AUTHOR.
PRINCIPLES OF HUMAN PHYSIOLOGY.
Fourta Epitton. 8vo. 28s.
A MANUAL OF PHYSIOLOGY.
Szeconp Epirion. Fcap. 8vo. 12s. 6d.
PRINCIPLES
COMPARATIVE PHYSIOLOGY.
WILLIAM B. CARPENTER, M.D., E.R.S., F.GS.
EXAMINER IN PHYSIOLOGY AND COMPARATIVE ANATOMY IN THE UNIVERSITY OF LONDON 5 PROFESSOR OF MEDICAL JURISPRUDENCE IN UNIVERSITY COLLEGE} PRESIDENT OF THE MICROSCOPICAL SOCIETY OF LONDON,
ETC. ETC.
With Three Hundred Wood Gngrabings.
Fourth Edition.
` LONDON: JOHN CHURCHILL, NEW BURLINGTON STREET. (Established in Princes Street, Soho, 1784.) M DCCC LIV.
N ) è A f
€
43 | 3 ; m a C > & = i Us $e $ liz. ee = S o -e e E oi 3
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TO
SIR JOHN F. W. HERSCHEL, BART
K.H. F.R.8, L. AND E. ETC, THIS VOLUME 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.
PREFACE.
‘í SOIENCH IS THE KNOWLEDGE OF MANY, ORDERLY AND METHODICALLY DIGESTED AND ARRANGED, SO AS TO BECOME ATTAINABLE BY ONE.” —Sir John F. W. Herschel.
THE success of the Third Edition of the “Principles of Physiology, General and Comparative,”—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,—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 greatly-increased 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 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, and to treat of them Separately though connectedly.
The present Volume, therefore, consists of the “Comparative Phy- Siology” of the last Edition, extended from 530 pages to 744, and with 300 illustrations instead of 130. The First Chapter (the Eighth of the © former Edition) has been entirely re-written, with the view of bringing into greater prominence the general doctrine of Progress from the General to the Special; the enunciation of which by Von Baer (nearly thirty years ago) appears to the Author to mark a most important era in the Philosophy of Physiology, although it is far from having received the
notice which it deserved from various subsequent writers who have
Ma me RCN EMRE eM RARE a et 7 ae ye te
Vill PREFACE.
followed in the same track. The principal additions and alterations in
the succeeding Chapters (as to the general arrangement of which no change has been found necessary) occur in the part of Chapter VI. which relates to the Water-Vascular System, and in the part of Chapter XI. which treats of the Sexuality of the Cryptogamia; in regard to which latter point the Author would observe, that the course of recent discovery has fully borne-out the anticipations he expressed in the former edition. The whole work, however, has been most care- fully revised; and the Author ventures to think that the present Edition more completely represents the state of the Science at the period of its publication, than any of its predecessors have done. He can honestly say that he has spared no time or labour 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 short- comings, 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 always loved truth better than his 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 acquire- ment, must largely avail himself of the labours 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 labour 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
PREFACE, ix
own original enquiries, is scattered through the work. It would have been easy for him to bring these last into greater prominence, had he been so disposed; but as his constant aim has been, to work-out his Seneral 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 3 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 endea- voured to avail himself of the best and most recent information he could procure upon each department of the subject, it is scarcely to be “xpected 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,
Tt has been the Author’s endeavour, 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 3 and while most desirous to avoid taking credit for what is not his Own, the
self 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
x PREFACE.
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, 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 enquiries 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,* 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.
* In a Postscript to the work referred-to in the note to p. 536, written almost contem- poraneously with Mr. Newport’s decease, Prof. Bischoff states that he has himself con- firmed Mr. N.’s observation of the penetration of the Spermatozoon into the ovum of the Frog, and gives him full credit for the determination of this important fact.
TABLE OF CONTENTS.
CHAPTER I.
ON THE GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT.
PAGE . Analysis and Comparison of Phenomena afforded by Organic Structure :—
Homology and Analogy : : : ; ; ; -SM 2. Conformity of Structure of each group to General Design or Archetype :— : Progress from General to Special in its various modifications . eis . Diversities in Grade of Development. : i ; : 5 i ke - General Survey of Vegetable Kingdom. Protophyta : ; : ; ; ; ‘ ; RE Thallogens (Algæ, Lichens, Fungi) : : : ; -S28 Acrogens (Hepaticæ, Mosses, Ferns) . ; ; RPS Phanerogamia . ; ; : See ; . . 38 . General Survey of Animal Kingdom. Protozoa (Porifera, Rhizopoda, Infusoria) Radiata (Polypifera, Acalephz, Echinodermata) 2 : Mollusca (Bryozoa, Tunicata, Brachiopoda, Lamellibranchiata, Gasteropoda, Pteropoda, Cephalopoda) ; í À Articulata (Entozoa, Annelida, Myriapoda, Insects, Crustacea, Arachnida) ; ; : ; Vertebrata (Fishes, Reptiles, Birds, Mammals) . Progress from General to Special in Development Rudimentary Organs Monstrosities i - Geological Succession of Organic Life
CHAPTER II.
GENERAL VIEW OF THE VITAL OPERATIONS OF LIVING BEINGS, AND OF THEIR MUTUAL RELATIONS.
l. Analysis and Classification of Phenomena presented by Vital Action ss Nagi OE 2. Mutual Relations of Organic and Animal Functions 123 3. Organic Functions Separately considered 125 . Animal Functions Separately considered - Progress from General to Special in Function
CONTENTS.
CHAPTER III. OF ALIMENT, ITS INGESTION, AND PREPARATION.
. Sources of Demand for Aliment . . Nature of the Alimentary Materials . Ingestion and Preparation of Aliment in Plants . Ingestion and Preparation of Aliment in Animals Agastric Animals Unicellular Animals . Polystome Animals Oral Apparatus Prehensile Appendages Reducing Apparatus Digestive Apparatus . Nature of Digestive Process
CHAPTER IV.
OF ABSORPTION AND IMBIBITION.
. General Considerations . Absorption in Vegetables . Absorption in Animals
CHAPTER V. OF THE CIRCULATION OF NUTRITIVE FLUID.
. General Considerations . Circulation in Vegetables . Circulation in Animals J 3 f à x Absence of Special Circulation in Certain Classes Circulation in Radiata :—Echinodermata Circulation in Articulata :—Annelida Myriapoda Insects Arachnida Crustacea . Circulation in Mollusca :—Bryozoa Tunicata Brachiopoda Lamellibranchiata Gasteropoda Cephalopoda Circulation in Vertebrata :—Fishes Reptiles Birds Mammals . Forces which move the Blood Development of Circulating Apparatus . Malformations of Circulating Apparatus
CONTENTS,
CHAPTER VI.
OF RESPIRATION,
l. General Considerations 2. Respiration in Plants 3. Respiration in Animals i i ; k ; ; s Aquatic Respiration :— Protozoa, Zoophytes, and Acalephæ Echinodermata ; Water-Vascular System :— Rotifera Entozoa Branchial Respiration :—Bryozoa Tunicata Brachiopoda . Lamellibranchiata . Gasteropoda . Cephalopoda . Annelida, Crustacea Fishes . Batrachia, Atmospheric Respiration :—Myriapoda Insects Arachnida Fishes (air-bladder) Perennibranchiata Reptiles Birds Mammals Development of Respiratory Apparatus . Alterations effected by Respiratory Process
CHAPTER VII.
OF THE EXHALATION OF AQUEOUS VAPOUR. - Genera] Considerations - Exhalation in Plants 3. Exhalation in Animals
CHAPTER. VIII.
OF NUTRITION,
- General Considerations i : Term of Duration of Individual Parts Assimilation and Formation ,
- Nutrition in Vegetables À ; Growth and Multiplication of Cells Assimilating Process in Vascular Plants Production of Vegetable Organic Compounds .
CONTENTS.
3, Nutrition in Animals ; : Assimilation of Nutritive Materials Chyle and Lymph. Š Vascular Glands . š : > ; : Composition and Properties of Blood of Vertebrata . Nutritive Fluid of Zoophytes Echinodermata . Articulata Mollusca Growth and Multiplication of Cells Production of Animal Organic Compounds Jonditions of Nutritive Activity in Animals .
CHAPTER IX.
OF SECRETION AND EXCRETION.
1. General Considerations 9. Secretion in Vegetables 3. Secretion in Animals ; Structure of Glands in general The Liver, and the Secretion of Bile Biliary Apparatus of Invertebrata
Vertebrata . Development of Liver Properties and uses of Bile Of the Kidneys and the Urinary Excretion Urinary Apparatus of Invertebrata Vertebrata Development of Kidney š ‘ Composition and Properties of Urine . Cutaneous and Intestinal Secretions Special Secretions Metastasis of Secretion
CHAPTER X.
EVOLUTION OF LIGHT, HEAT, AND ELECTRICITY.
J. General Considerations 2. Evolution of Light . : À : Evolution of Light in Vegetables . Evolution of Light in Animals Luminosity of the Sea Luminous Insects
CONTENTS.
3. Evolution of Heat ; Evolution of Heat in Vegetables Evolution of Heat in Animals
Cold-blooded Animals
Insects i ;
Warm-blooded Animals Conditions of Evolution of Heat
4. Evolution of Electricity : : Evolution of Electricity in Vegetables Evolution of Electricity in Animals
Electric Fishes .
CHAPTER XI.
OF GENERATION AND DEVELOPMENT.
1. General Considerations $ : 3 Developmental and Regenerating Power Multiplication by Gemmation True Generative Process :
Alternation (so-called) of Generations 2. Generation and Development in Plants Multiplication of Phytoids : Generation and Development of Protophyta Algee Characesze Lichens Fungi i; Hepaticæ and Mosses Ferns Equisetaceæ . Lycopodiaceæ Marsileaceæ. Gymnospermeæ ; ? $ Angiospermous Phanerogamia 3. Generation and Development in Animals Multiplication of Zooids ; ; Development and Actions of Spermatozoa Development and Structure of Ova : Fecundation of Ova, and subsequent Changes Generation and Development of Protozoa Infusoria Porifera
Generation and Development of Radiata Polypifera x Compound Hydroida Acalephæ Echinodermata
CONTENTS,
Generation and Development of Mollusca Bryozoa Tunicata Brachiopoda . Lamellibranchiata . Gasteropoda . Cephalopoda Generation and Development of Articulata Entozoa Rotifera Annelida Myriapoda Insects . Crustacea Cirrhipeda Arachnida Generative Apparatus of Vertebrata Fishes . Reptiles Birds Mammals Embryonic Development of Vertebrata Area Germinativa Formation of Amnion Development of Allantois Formation of Placenta Conditions determining Sex . Lactation :—Composition of Milk ; 4, On the Laws of the Exercise of the Reproductive Pinetion Species and Varieties . Hybridity Modifying influence of P RIR AE T Origination of New Varieties i Transmission of Acquired Peculiarities .
CHAPTER XII.
OF THE SENSIBLE MOTIONS OF LIVING BEINGS.
. General Considerations . Motions of Plants . Motions of Animals
CHAPTER XIII.
OF THE FUNCTIONS OF THE NERVOUS SYSTEM.
. General Considerations . Comparative View of the ee System i in ‘ais Activa ies No Evidence of Nervous System in Zoophytes
CONTENTS, XVii
PAGE Nervous System of Radiata x : 3 7 : 2 i - 653
Acalephze $ ; 2 x ; : ODS
Echinodermata 7 3 : 3 ; - 653
Nervous System of Mollusca i : ; ; 5 à = p94 Bryozoa . ; ; Sits : ; = 655
Tunicata : ; : ; ; A - 655
Brachiopoda . : ; ; i : 2656
Lamellibranchiata . ; ; ; ; = 606
Gasteropoda . ; ; ; : 2 . 658
Cephalopoda . y ; : - : -600
Nervous System of Articulata À A ; : ; - 663 Entozoa . : ; ; : : i 1 O70
Annelida : : : ; s j 225 O70
Myriapoda ; : : : ; ey ORO
Insects . : ; : : ; ; 5 0
Crustacea : ; x z : ——<—OF2
Arachnida ; > ; x F OS
Nervous System of Vertebrata : i : 5 i R 675 Fishes. ty : : ; : = O19
Reptiles . i ; ; ; Z ; OSH
Birds . : ; i 7 ‘ ; - > 681
Mammalia ; 5 : s y az 682
History of Development of Brain : : : . 679—684 Functions of the Cranio-Spinal Axis . : ; : . 685
Spinal Cord ; : ; ; ; ; ar O84
Medulla Oblongata . ; : ; ; - 689
Sensory Ganglia . : : ; ; i . 690
Functions of the Cerebellum : F : : : . 696 Functions of the Cerebrum : 3 ; 2 ; . 698 Functions of the Sympathetic System . 5 : ; = 103 General Summary ; : E ; : ; z <n FOL
CHAPTER XIV.
OF SENSATION AND THE ORGANS OF THE SENSES.
- Of Sensation in General è ; E - Of the Sense of Touch, and its Instruments
- Of the Sense of Taste, and its Instruments . - Of the Sense of Smell, and its Instruments
- Of the Sense of Hearing, and its Instruments - Of the Sense of Sight, and its Instruments .
CHAPTER XV,
OF THE PRODUCTION OF SOUNDS BY ANIMALS
TORDE ~~ SAT SIE me ee
Dor ww eo
Se SOR Dy Sh
LIST OF ILLUSTRATIONS.
. Pterodactylus crassirostris (XXV1.)
. Different forms of Anterior Member > : . Diagram illustrating the Nature of Limbs (LxtIv.) . Group of Anatifa levis (XXIT.) .
. Anatomy of Anatifa levis (xxi11.)
Development of Balanus balanoides (v11.) - Comparison of Leucifer with Lepas (xxv-) -
. Ophiura texturata . Pentacrinites briareus (xv1.) . Hermospora transversalis (LXIX. )
. Bangia velutina (LXIX.) :
. Mesogloia vermicularis (LXIX.) -
. Zonaria plantaginea (LXIX.)
. Dasya kuetzingiana (LXIX.)
. Marginaria gigas (LXIX.) .
. Parmelia acetabulum (LXIX.)
. Sphærophora coralloides (LXIX. )
. Stysanus caput-medusæ (LXIX. )
. Clavaria crispula (LXIX.) .
. Acidium tussilaginis (LXIX.)
. Marchantia polymorpha (LXIX.)
. Fissidens bryoides (LXIX.) . : : i . Marchantia polymorpha with E AS N (LXIX.) . . Marchantia polymorpha with pistillidia (LXIX.) . . Polytrichum commune (LX1X.)
. Trichomanes speciosum (UXIX.) .
. Frond of Scolopendriwm vulgare (LXIX.)
. Frond of Osmunda regalis (UXIX.)
. Equisetum arvense (LXIX.) .
. Lycopodium cernuum (LXIX.)
. Marsilea quadrifolia (LXIX. )
. Ideal Plant (LXXVIII.)
. Ameba princeps (XXXV.)
. Hydra fusca (x0.)
. Diagrammatic section of shoe gee )
_ Structure of Cyanæa aurita (XXXVI. )
. Anatomy of Asterias aurantiaca (LXXXVIII. )
. Comatula rosacea (XXXIX.)
LIST OF ILLUSTRATIONS,
- Echinus mammillatus (XXIII. ) 3 Anatomy of Holothuria tubulosa (XXIIT. ) - Chitonellus and Chiton (XVIIt. ) - Salpa maxima (XXIII. ) - Anatomy of Mactra (XXXIV. ) - Anatomy of Paludina vivipara (xx11.) - Shells of Gasteropod Mollusks (XVIII. ) - Hyalea, Oriseis, and Clio (XVIII. ) - Sepia officinalis (XVIII. ) : i $ . Embryoes of Nudibranchiate Gasteropoda (11.) - Laguncula repens (xov.) - Anatomy of Aplysia (XXII. ) - Botryllus violaceus (XXIII. ) . Anatomy of Strongylus gigas (x1.) - Nephthys Hombergii (XXIIT. ) - Tulus (xxxtv.) - Scolopendra (xxxtv.) ‘ $ ; $ ; . Section of the trunk of Melolontha vulgaris (LXXXIV.) . T. Ideal Section of Sphina ligustri CR) - Anatomy of Cancer pagurus (XXXIv.) ; . Inferior surface of Limulus moluccanus (XXXTY.) . . Cyclops quadricornis (v) Í i : - Diagram of Archetype Vertebral Skeleton (LXv.) . - Ideal Section of a Mammal (LXIv.) 9. Oblique view of Vertebra of Cod (LXXIv. ) - Bimanus and Seps (XXXIV.) - Emysawra Serpentina (XXVII.) . - Skeleton of Ichthyosaurus (XXVI.) - Skeleton of Plesiosawrus (XXvI.) - Skull of Mososaurus EATA - Portion of jaw of M; egalosaurus (Xxi.) : - Portion of lower jaw and teeth of Iguanodon (LvII.) - Lower jaw of Phascolotherium Bucklandii - Molar tooth of Asiatic Elephant (xx1.) - Duplicative subdivision of cells - Early stage of M ammalian Ov - Nicothoe astaci (xov1.) ; š j $ . - Ogygia Buchti (xvir.) and Limulus moluccanus (XXXIV) - Metamorphosis of Carcinus monas (XX.) - Homocercal and Heterocercal Tails of Fish - Skeleton of Palæotherium magnum (XXI. ) - Molar tooth of Mastodon (XXI.) - Caryocrinites ornatus (xy.) - Lingula anatina (xvim.) : ; - Section of shell of Nautilus Pompilius (xy - Exterior view and section of Orthoceratite . Cephalaspis Lyellié . Skull of Labyrinthodon - Skull of Rhyncosaurus (LXVUI. )
(LXVII. )
of Chlamydomonas (XXXv.) . um (xrx.) and Young of Volvox
ui) (XVIII. )
LIST OF ILLUSTRATIONS.
. Skeleton of Mylodon (uxv1.) .
. Pitchers of Dischidia Rafflesiana (0.) . . Polygastric Animalcules, according to xii genev. Ve . Section of young branch of Alcyonium stellatum (XXXIII.) . . Digestive apparatus of Rhizostoma (XXIII. )
. Digestive apparatus of Thawmantias (XXXVIII. )
. Holothuria phantapus (XxXI1I.)
. Anatomy of Echinus lividus (xx111.)
. Rotifer vulgaris (xxxv.)
. Compound stomach of Sheep
. Section of part of the Stomach of Sheep inves )
. Portions of Campanularia gelatinosa, natural size, and vaiki a ) . Sections of Aleyonian Polype (XXx11I.)
. Structure of Polycelis levigatus (LXXI.) . z
. Cydippe pileus (xu.) and Beroe Forskalii (XXIIT. )
. Digestive apparatus of Annelida (XXIII. )
. Eolis Inca, a Nudibranchiate Gasteropod (11.) .
. Digestive apparatus of Ammothea pycnogonoides (LXX.)
. Digestive apparatus of Mygale (xx111.) y
. Digestive apparatus of Common Fowl (xxxtv.)
. Vili of Human Intestine
. Longitudinal Section of Stem of Talm Reed ERER )
. Laticiferous Vessels (LXXXI.) .
. Blood-vessels of Frog’s foot (xorx.)
. Formation of Capillaries in Tail of Tadpole (1. )
. Circulating apparatus of Terebella conchilega (xxx.)
. Circulating apparatus of Kunice sanguinea (xxx.)
. Circulating apparatus of Arenicola piscatorum (XXX.)
. Dorsal vessel of Scolopendra (LX111.) : -
. Circulating system of Scolopendra (LXIII. )
. Circulating system of Buthus (LXII. )
. Heart of Mygale (xxtt.)
. Circulating system of Lobster (XXVIII. )
. Anatomy of Amarouciwm proliferum (XXIX.) .
. Circulating system of Salpa maxima (XXIII) .
. Circulating system of Pinna marina (XXXII.) .
. Circulating system of Snail (xxxtv.)
. Circulating system of Octopus (XXIII.)
. Circulating system of Fish (XXIII.) .
. Anatomy of Amphioxus (LXIV.)
. Circulating system of Lizard (XXXIV.)
. Respiratory Circulation in Tadpole (xxxIv.)
. The same in transition state (xxxIv.)
. The same in the perfect Frog (xxxiv.)
. Diagram of the Circulation in Birds and M boii ( XXXIV. in) . Vascular area of Fowls egg (xcrx.) x
. Diagram of formation of great Arterial trunks in Fowl
. Diagram of Circulation in Human Embryo (c11.)
. Water-vascular system of Tenia solium (x1.) .
FIG.
137 138 139 140 141 142 143 144 145 146 147 148 149 150 i51 152 153
LIST OF ILLUSTRATIONS.
- Anatomy of Fasciola hepaticum (x1. )
. Anatomy of Perophora (uv1.) .
- Portion of Branchial sac of Perophora (uv1.)
- Respiratory apparatus of Pholas crispata (111.) - Branchial lamine of Pholas crispata (111.)
- Portion of gill of Doris Johnstoni (11.) í - Doris J ohnstont, showing tuft of extegnal gills (11.) .
- Cephalic tuft of Sabella unispira (XXXIV. ) .
- Branchial arch and leaflets of Fish .
- Lamprey, showing branchial orifices - .
- Proteus anguineus, showing external branchize (XXVII. ) - Axolotl, showing external branchiæ (XXVII.)
- Tracheal system of Nepa cinerea (XXXIV. )
» Lepidosiren paradoxa (XXXIV.)
. Respiratory organs of Frog (XXxvit.) :
. Lungs of Bimanus, Bipes, and Coluber (XXXVII. )
. Section of Lung of Turtle (x11.)
153*, Pulmonary apparatus of Pigeon (XXXVII)
154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 Enz 178 179 180 181
. Capillaries of air-cells of Human Lung
- Vertical Section of leaf of Lilium album (xrv.)
- Under surface of leaf of Liliwm album (XIv.) ; : - Surface and Section of frond of Marchantia polymorpha (LVIII.) - Sudoriparous Gland from Human Hand (xcrx.) : ; - Section of Leaf of Agave, showing primordial utricles of cells (XLII. ) - Portions of Nitella Jexilis, natural size and enlarged (LXXXI.)
- Circulation of fluid in hairs of Tradescantia Virginica (LXXXI. ) - Various stages of development of Hæmatococcus binalis (XLII. ) - Process of cell-multiplication in Conferva glomerata (urx.)
- Development of zoospores of Achlya prolifera (XcrII. )
- Multiplication of Cartilage-cells by subdivision (III. )
- Section of branchial Cartilage of Tadpole (LXXIX.)
- Endogenous cell-growth, in cells of Meliceritous Tumour GERS - Cells with radiating fibres (1.) 5 : . .
- Capillary network around follicles of Parotid Gland (virr.)
- Glandular follicles of Stomach (LXI.)
- Mammary Gland of Ornithorhyncus (uxt. )
- Rudimentary Pancreas, from Cod (a 3
- Lobule of Parotid Gland of Human Infant (xorx.)
- Biliary tubuli of Musca carnaria (LII.) . :
- Hepatic cæcum of Astacus afinis GX)
- Lobules of Liver of Squilla (uxt.)
- Glandular cells of Human Liver : s 3
. Arrangement of Blood-vessels in Human Liver (xvumt.)
. Connection of Lobules of Liver with Hepatic Vein (XLVIII. )
. Early stage of Development of Liver of Fowl (uxz.) .
- Kidney of Foetal Boa (Lx1.) $
182. Section of Kidney of Coluber (LXI, )
183 184
. Pyramidal fasciculus of Tubuli uriniferi of Bird (Grate . Section of Human Kidney (ctz.)
LIST OF ILLUSTRATIONS.
Portion of Tubulus Uriniferus with tesselated epithelium (xcrx.) Distribution of Vessels of Kidney (xt11.)
. Corpora Wolffiana, from Chick (ux11.)
. Noctiluca miliaris (LXXII.)
. Pelagia noctiluca (XXXIV. )
. Electric Apparatus of Torpedo (LXXVI.) .
. Diagram of Generative Process in Plants .
. Multiplication of Coccochloris by subdivision (XLIII. `
. Conjugation of Zuastrum oblongum (LXXIIL.) .
. Conjugation of Zwnotia turgida (LXXXVII. )
. Development of Spores in Aulacosetra (LXXXVII.)
. Conjugation of Zygnema (L1.) : P > :
. Generative apparatus of Fucus platycarpus (LXXXVI.) Tetraspores of Carpocaulon mediterraneum (I. )
. Generative apparatus of Chara fatida (LXXXI. )
. Generative apparatus of Collema pulposum (xcI.)
. Generative apparatus of Tremella mesenterica (XCII. )
. Generative apparatus of Agaricus campestris (LXXVIL )
. Development of Torula cerevisie . e è
. Development of Archegonia of M archantia (LXIX.)
. Archegonia of Jungermannia (XLIV.) ; í
. Sporangia on lobed receptacles of Marchantia (LX1X.)
. Gemmiparous conceptacles of Marchantia (uv11.)
. Development of prothallium of Pteris (uiv.)
. More advanced prothallium of Pteris (uIv.) is : 3
. Development of Antheridia and Antherozoids of Pteris (LIV.)
. Development of Archegonium of Pteris (utv.) .
. Development of Embryo of Polypodium (11v.) .
. Fructification of Equisetum arvense (LXIX.) :
. Generation and Development of Lycopodium (Xu1v.)
. Sporocarp of Marsilea quadrifolia (LXIX.)
. Germination of Marsilea Fabri (LXIX.) -
. Generative apparatus of Conifere (xuIV.)
. Development of Embryo of Ænotheraceæ (XLV.)
. Embryoes of Potamogeton and Amygdalus (XLVU.) .
_ Germination of Zanichellia and Acer (XLVII) -
. Constituent parts of Mammalian Ovum (xIx.)
. Segmentation of vitellus of Ascaris acuminata (v.)
. First segmentation of vitellus of Mammalian Ovum (x1x.)
. Early stages of development of Coregonus (XCVIL.) :
. Fissiparous multiplication of Chilodon cucullulus (XXxv.) .
. Group of Vorticelle in various states (xxXXV.) . š 5 :
. Development and Metamorphosis of Vorticella microstoma (LXXXII. ) .
_ Gemmation of Hydra fusca (xc.)
_ Generative apparatus of Actinia (LXXXV.) 5 230. Development of polype-bud of Campanularia (Xcly.)
. Generation and Development of Cordylophora lacustris (Iv.)
. Development of Medusa-buds from Perigonimus (LXXv.)
33. Medusiform gemmee of Campanularia (xcIv.) .
LIST OF ILLUSTRATIONS, XXI
PAGE - Strobila (or polypoid state of Medusa) propagating by gemmation (xx1v.) 555 - Group of Strobile in process of Medusan gemmation (xx1v.) . . 556 - Development of Medusa-buds from Strobila (xxrv.) . ; : . 556 - Development of Meduse from Strobila-gemmee eae, oes : = -OD% . Gemmiparous multiplication of Cytcis (LXXv.) ; s 3 = 558 - Structure of Velella limbosa (XLVI.) 9 ; 3 : ; = 560 - Crinoid state of Comatula rosacea E 26/3 3 ; . 562 - Development of embryo of Echinaster rubens (LXXv. ) 3 : ~ 663 - Bipinnaria asterigera, or larva of Star-figh Uae) ass ; ; = 065 - Embryonic development of Echinus CERI nae . $ : — 566 - Origin of Ophiura from Pluteus (LxII.) . : : ‘ j - 568 - Gemmiparous extension of Laguneula (xov.) . > i - . 569 - Development of embryo of Amaroucium (XXIX. ) 5 i d Bl: - Anatomy of more-advanced embryo of Amaroucium (XXIX. ) i = S72 - Development of embryo of Acteon viridis Cavin.) 2% ; x = 078 - Male Argonaut, showing Hectocotylus-arm TIE : ‘ Oe . Successive stages of development of Sepia (XLIX. ) 584 . Generative apparatus of Tenia solium (x1.) 585 . Generative apparatus of Distoma hepaticum (x1. ) 588 - Generative organs of Nais filiformis XIL.) ; ‘ 7 2093 . Early stages of development of Terebella nebulosa (XXX1.) . : . 594 - Ideal Section of Larva of Sphine ligustri (LXII) 600 - Ideal Section of Pupa of Sphine ligustri (LXIII. ) 600 - Generative Apparatus of Fowl (XXXVII.) . 613
. Later stage of Segmentation of Mammalian Ovum (aie); ; E.
- Germ and surrounding parts from Uterine Ovum (XIx. ) ; - 620 - Ovum of Coregonus, in early stage of development (xovir.) : PE: - Incipient formation of Amnion (xcrx.) 622 - Embryo of Coregonus (xov) 624 - The same more advanced (XOVII.) 5 ; ; 624 - Further development of Amnion in Human Ovum (XcIx.) . 625 - Incipient formation of Allantois in Human Ovum (xorx.) . 625 . Further development of Allantois in Human Ovum Ix.) 626 - Formation of Vascular Tufts in Human Ovum (XIX. ) 626 - Extremity of Villus of Human Placenta (XLI.) 627 - Side view of Embryo of Dog (zx.) 628 - Front view of Embryo of Dog (1x.) . 629 - Nervous System of Solen Eon 657 - Nervous System of Argonauta argo (xxit1.) 661 - Nervous Centres of Octopus (XXIII. ) 662 - Nervous Centres of Sepia (XXIII. ) : à ; 663 - Gangliated Column of Larva of Sphing ligustri (LX. ) : . 664 - Portion of ganglionic tract of Polydesmus (EXN. ) 3 : 5 . 665 . Parts of Nervous System of Centipede and Sphinaw (uxit.) : - 667 - Nervous System of Zulus terrestris (LXIII) é j 668 - Nervous Centres of Spider (
XXIII.) , 673 . Nervous System of Androctonus (ux. ) j 4 : : = 674 - Nervous Centres of Frog (uv.) ; ; : . . . - 675 . Brains of Fishes (HR Ss; i : : : . . . 678
LIST OF ILLUSTRATIONS,
. Brain of Turtle (LXXXII)
. Brain of Buzzard (uv.) .
. Early stages of Development of Tuas of H uman iy Gr ) . Brain of Squirrel (LXXXII)
. Brain of Rabbit (uv.)
. Later stages of Development of D of p m — ae ) . Capillary loops of Papille of Human Skin (v111.)
. Capillaries of Fungiform Papille of Human Tongue (vIn. ).
. Structure of the Organ of Hearing in Man (XXXIY.)
. Front view of Head of Bee, showing Compound and Simple es
. Section of Compound Eye of Melolontha (uxxxiv.)
. Facetted Eye of Asaphus (xv1.) :
. Section of Globe of Human Eye (c1t.) :
. Skull of Jchthyosawrus, showing sclerotic osseous nae mn jan
. Diagram of the course of the rays in the Eye
. Stereoscopic figures ‘
. View of Human Larynx from N (01.).
. Artificial Larynx (o1.)
TREATISES AND MEMOIRS REFERRED-TO IN THE LIST OF ILLUSTRATIONS,
- Addison, The Actua] Process of Nut Atu
(Ann. of Nat. Hist., 2nd Ser., Vol. VIII.)
- Allman, On Cordylophora lacustris, (Phil. Trans., 1853.)
- Bagge, De Evolutione Strongyli et Ascaridis,
» Baird, Natural History of Entomostraca,
- Bate (C. Spence), On the Development of the Cirripedia. (Ann. of Nat. Hist., 2nd Ser., Vol. VIII. )
- Berres, Anatomie der Mikroskopischen Gebilde des Mens
- Bischoff, Entwickelungs-geschichte des Hunde-Eies.
: Blanchard, Sur le Systéme N erveux des Mollusques Acéphales Testacés, (Ann. des Sci, Nat., 3° Ser. » Zool., Tom. III.)
Do Oon des Vers. Zool., Tom. VII.—x1r, )
oe (See algo XXIII.)
- Bojanus, Anatome Testudinig Europes.
- Bowman, On the Structure and Kidney. (Phil. Trans., 1842.)
: Brongniart, Sur la Structure, &c., des Feuilles, 1° Sér., Tom. XXI.)
- Buch (L. von), Ueber die Cystideen.
i Buckland, Bridgewater Treatise on Geology.
Burmeister, On the Organisation of Trilobites.
- Chenu, Traité de Conchyologie. -
- Coste, Histoire Générale et Particuliére dy Développement des Organisés.
- Couch (R.), On the Meta: of Cornwall Pol
- Cuwier, Ossemeng Fossiles.
Mémoires ur | Régne Animal.
chlichen Körpers.
(Ann. des Sci. Nat., 3° Sér.,
Use of the Malpighian bodies of the
(Ann. des Sci. Nat.,
Corps
es Mollusques. (Edition accom
pagnĉe de Planches Sravées, par
XXVi TREATISES AND MEMOIRS REFERRED-TO IN ILLUSTRATIONS.
xxv. Darwin, Monograph of the Cirripedia. xxvi. D Orbigny, Cours Elémentaire de Paléontologie. XXVII. Dumeril et Bibron, Histoire Naturelle des Reptiles. xxvii. Edwards (Milne), Histoire Naturelle des Crustacés. XXIX, Sur les Ascidies Composées. XXX. —— Sur la Circulation chez les Annélides. (Ann. des Sci. Nat., 2 Sér., Zool., Tom. x.) KIKI. Sur le Développement des Annélides. (Ann. des Sci. Nat., 3° Sér., Zool., Tom. m1.) xxxi. —————— Sur la Circulation chez les Mollusques. (Ann. des Sci. Nat., 3° Sér., Zool., Tom. vit.) XXXIII. — Recherches sur les Polypes. XXXIV. —— Cours Elémentaire de Zoologie. —— (See also XXIII.) xxxv. Ehrenberg, Die Infusionsthierchen. XXXVI. — Des Leucthen des Meeres. (Abhaldlungen der Kon. Akad. der Wissenschaften zu Berlin, 1835.) xxxvit. Flourens, Mémoires d Anatomie et de Physiologie Comparées. xxxvi. Forbes, Monograph of the British Naked-eyed Meduse. LOO History of British Starfishes, &c. XL. On Beroe pileus. (Ann. of Nat. Hist., Vol. 11.) xt. Goodsir, Anatomical and Pathological Observations. xiu. Harting, in Milder’s Chemistry of Animal and Vegetable Physiology. xu. Hassall, History of British Freshwater Algæ. xutv. Hofmeister, Vergleichende Untersuchungen der Keimung, Entfaltung und Fruchtbildung Héherer Kryptogamen. XLV. . Sur la Fécondation chez les Hnothérées. (Ann. des Sci. Nat., 3° Sér., Botan., Tom. 1x.) xivt. Hollard, Sur Organisation des Vélelles. (Ann. des Sci. Nat., 3° Sér., Zool., Tom. 111.) XLVII, Jussieu, Cours Elémentaire de Botanique. ; xuv. Kiernan, On the Structure of the Liver. (Phil. Trans., 1835.) xux. Kölliker, Entwickelungsgeschichte der Cephalopoden. L. Sur le Développement des Tissus chez les Batraciens. (Ann. des Sci. Nat., 3° Sér., Zool., Tom. vI.) LI. Kützing, Phycologia generalis. uu. Leidy, On the Comparative Structure of the Liver. (Amer. Journ. of Med. Sci., Jan., 1848.) On Articular Cartilages. (Op. cit., April, 1849.) . Leszczyc-Suminski, Entwickelungs-geschichte der Farrnkräuter. . Lewret, Anatomie Comparée du Système Nerveux. . Lister, On Tubular and Cellular Polypi, and on Ascidiæ. (Phil. Trans., 1834.) . Mantell, On Iguanodon. (Phil. Trans., 1848.) . Mirbel, Sur la Structure et la Développement de la Marchantia poly- morpha. (Nouv. Ann. du Musée, Tom. 111.) . Mohl, Vermischte Schriften botanischen Inhalts. . Müller, (Heinrich), Zur Hectocotylus Argonautæ. (Siebold and Köl- liker’s Zeitschrift, June, 1852.)
TREATISES AND MEMOIRS REFERRED-TO IN ILLUSTRATIONS. xxvii
LXI. LXII.
LXIII.
LXIV., LXV. LXVI. LXVII. LXVIII. LXIX. LXX.
LXXI. LXXII.
LXXIII.
LXXIV.
LXXV.
LXXVI. LXXVII. LXXVIII. LXXIX.
LXXX. LXXXI.
LXXXII.
LXXXIII.
LXXXIV.
LXXXV.
LXXXVI.
LXXXVII.
LXXXVIII. LXXXIX.
XO.
XCI.
= o Reade
Müller, (J ohann), De Glandularum secernentium structura penitiori. Ueber die Larven und die Metamorphose der Echinodermen. (Abhald. der Konig. Akad. der Wissenschaften zu Berlin, 1846-1852.) Newport, On the structure of Insects, Myriapoda, and Macrourous Arach- nida. (Phil. Trans., 1839-1843.) Owen, Lectures on Comparative Anatomy. —— On the Homologies of the Vertebrate Skeleton. —— Memoir on the Mylodon. —— British Fossil Mammalia. On the Rhyncosaurus, (Trans. of Cambr. Phil, Soc., Vol. vit.) Payer, Botanique Cryptogamique. Quatrefages, Sur P Organisation des Pycnogonides. (Ann. des Sci. N at., 3° Sér., Zool., Tom. Iv.) Sur quelques Planariées marins. (Op. cit., Tom. ry.) ————— Bur les Noctiluques. (Op. cit., Tom. XIv.) Ralfs, Monograph of the British Desmideze. Roget, Bridgewater Treatise on Physiology. Sars, Fauna littoralis N orvegize. . Savi, Etudes Anatomiques sur la Torpille. Schleiden, Principles of Scientific Botany. eee es Peat a Biography. Schwann, Mikroscopische Untersuchungen über die Ueberreinstimmung in der Structur und dem Wachsthum der Thiere und Pflanzen. Sharpey, Art. Cilia (Cyclop. of Anal. and Physiol.) Slack, On the Elementary Tissues of Plants, and on Vegetable Circula- tion. (Trans. of Soc. of Arts, Vol. XXXIX.) Solly, The Human Brain, its Structure, Stein, On Metamorphoses of Vorticella. schrift, Band 11r.) Strauss-Durckheim, Considérations des Animaux Articulés, Teale, Anatomy of Actinia Coriacea. Leeds, Vol. 1.)
Thuret, Sar les Anthéridies des 3° Sér., Botan., Tom. XVI.) Thwaites, On the Conjugation of the Diatomaceæ. (Ann. of Nat. Hist.,
Ist Ser., Vol. XX., and 2nd Ser., Vol. 1.) Tiedemann, Anatomie der Rohrenholothurie, &c. ——— Sw le Développement du Cerveau.
Trembley, Mémoires pour servir à P Histoire d'un genre de Polype d'eau douce.
Physiology, and Diseases. (Siebold and Kölliker’s Zeit-
Générales sur l Anatomie comparée (Trans. of Phil. and Lit. Soc. of
Cryptogames. (Ann. des Sci. Nat.,
Tulasne, Surles Lichens. (Ann. des Sci. N at., 3° Sur les Trémellinées,
Unger, Recherches sur V Ach] 3° Sér., Botan., Tom. II.)
Van Beneden, Mémoire sur leg
(Mém. de P Acad. Roy. de Brux
Sér., Botan., Tom. XVII.) (Op. cit., Tom. KG)
ya prolifera, (Ann. des Sci. Nat.,
Campanulaires de la Côte @ Ostende. elles, Tom. xvir.) S sur les Bryozoaires de la Côte @ Ostende,
(Mém de l Acad. Roy. de Bruxelles, Tom. XVIII.)
xxviji TREATISES AND MEMOIRS REFERRED-TO IN ILLUSTRATIONS.
xovi. Van Beneden, Sur le Développement et l'Organisation des Nicothoés. (Ann. des Sci. Nat., 3° Sér., Zool., Tom. x1r.) xovit. Vogt, Embryologie des Salmones. XOVIII. Embryogénie des Gastéropodes. (Ann. des. Sci. Nat., 3° Sér., Zool., Tom. vI.) xorx. Wagner, Icones Physiologice. o Wallich, Plantæ Asiaticæ Rariores. cı. Willis, On the Organs of Voice. (Trans. of Canigo Phil. Soc., Vol. rv.) 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 im 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 Jacts, with scarcely any attempt at the discovery of the ideas of which they are but the expressions. The laws of Life were long considered
eyond the reach of human investigation; and the mind shrank from attempting to analyse its complex and varied phenomena, which, though Constantly under observation, must be reduced to their simplest form,
etore any inductive reasoning can be founded upon them. It is recorded, owever, of Newton, that, whilst contemplating the simplicity and har- mony of the plan according to which the Universe is governed, as mani- fested in the relations which his gigantic mind discovered between the distant and apparently-unconnected masses of the solar system, his thoughts glanced towards the organised creation; and reflecting that the wonderful structure and arrangement which it exhibits, present in no C88 a degree the indications of the order and perfection which can result from Omnipotence alone, he remarked, “I cannot doubt that the struc- ture of animals is governed by principles of similar uniformity.” ¢“Tdem- que dici possit de uniformitate illâ, quæ est in corporibus animalium.” asks Cuvier in his eloquent discourse on the revolutions of the should not Natural History some day have its Newton?
2. Although the labours 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 con- firmation. 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 in-
vestigation will generally show that the difficulty is more apparent than real, —frequently arising solely from our own prejudices, and diminishing m proportion as we fix our attention upon that combination of unity of
SS E E A A PE E
2 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT.
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 comparison 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 com- parison, often prevents the true relation between them from being readily
detected. Here it is that the mental training which the previous culti- | |
vation of Physical science affords, becomes peculiarly valuable to the Physiologist. “The most important part of the process of Induction,” says Professor Powell,* “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-ascer- tained 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 Vegetable kingdoms are made up; and it is necessary, at the very outset of the inquiry, to settle the prin- ciples upon which these are to be compared. In many instances, it is true, there can be no room for hesitation ; the general type of conforma- tion of two or more organisms being obviously the same, and the differ- ences 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 recognise 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;
* ¢¢ Connection of Natural and Divine Truth,” p. 33.
t t t € t W. my
B'S eS ma
Is So St. ct
BASIS OF COMPARISON OF PHENOMENA. 3
Anatomists were in th
and in evident
parts. But although this mode of est perfectly correct, when th
that is, when we are
rom elements
Similar little or
in the system, originate fore fundamentally dis-
ke a cursory glance at the organs of support which different Animals are furnished, we shall y of function, and a general similarity of external
form, concealing a total diversity of internal str Amongst all the classes which are respiration, we encounter groups of greater or l
character.
resistance of this element becom and even among aquatic animals, t lon is
ucture and of essential adapted for atmospheric ess extent, in which the
es the principal means of progression ; there are instances in which the func- dent upon the same agency. Wherever
brata, some modification of the anterior
member serves as their basis; but there is consid
erable variety in the
a in which the apparatus is constructed. Thus, in the Bat (Fig. 2, £),
e required area for the surface of the win
g is formed by an extension of
© Skin over a system of bones, of which those of the hand form by far
the largest part; and this membrane is extended also fr extremity, and is attached to the whole len
om the posterior gth of the trunk, as well as to
In the Bird (Fig. 2, B), on the contrary, the
n and its ap
pendages attached to the anterior
the bones of the hand are developed in a com-
Support of the eXpansion. 1t seems that the
he fingers only, w st (Fig. 1).
may be regarded as,
that the
- but the bones of hand ig expanded, very different str
the Flying Squirrels, sums,” there is an ext which, by acting as a
parachute, enables these animals t
arm and fore-arm b From what is preserved of
‘flying opos- between the fore and hind legs,
o descend with Be
4 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT.
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 pro-
Pterodactylus crassirostris.
longations 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 mem- brane 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 ramifica- tions, by ‘ trachee’ 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 struc- ture, in fact, these wings correspond closely with the external respiratory organs with which certain aquatic Larvee (as that of the Ephemera) are provided; and hence they have been not inappropriately designated by Oken as ‘aerial gills? * 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
* 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.
BASIS OF COMPARISON OF PHENOMENA. 5
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 Vegetable world affords them in even greater abun- dance. 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 Cactacew, 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 pur- pose of supporting the plant by twining round some neighbouring prop, is in the Vine a transformation of the peduncle or flower-stalk, in the Pea a prolongation of the petiole or leat-stalk, in the Cucumber a trans- formation of the stipule, and in 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.
7. We can scarcely select any example of diversity of external con- formation and of function, superinduced upon an essential unity of organization, $o appropriate as that which is afforded by the comparison of those different modifications of the limbs or members, and especially of the anterior pair, by which the several species of Vertebrated animals are
y
Different forms of Anterior Member :—a, Fish; B, Bird; c,
Dolphin; D, Deer; x, Bat; r, Man,
adapted to the most diversified modes of life.
has the slightest hesitation in admitting that the pectoral fin of a Fish
Tees, A), the wing of a Bird (B), the paddle of a Dolphin (6), the fore-
No Comparative Anatomist
6 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT.
leg of a Deer (p), 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 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 frame-work 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 shoulder-joint, 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 rudi- mentary 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.—lt is entirely from the differences of conformation which these osseous elements gradu- ally 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 it to be used as an in- strument 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 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,* 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 air- tubes 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
* In earlier editions of this work, the terms functional and structwral 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 con- venient 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 “í 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 thatof a Bat or Bird, but not the homologue; whilst the latter
pie a with the arm of Man, the fore-leg of a Quadruped, and the pectoral fin of a Fish.
HOMOLOGY AND ANALOGY. 7
the function is investigated, however, with the structure which it requires for its performance, it becomes evident that, in order to bring the circu- lating fluid into the due relation with the atmosphere, all that is needed 18 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 m the Animal kingdom, shows that they all possess the same essential hature as instrumental structures, and that their modifications in par- ticular instances (which will hereafter be specially described) are only to adapt them to the plan and conditions of the organism at large. There 1s 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 trachez of the Insect, and the lungs of air-breathing Vertebrata, are internal reflexions of that surface ; and farther, the two former sets 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 opera- tions 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 Vegetable kingdom; wherein organs which correspond in structure, con- nections, 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 com- monly accounted) 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 Straw-
Orry, are not less truly stems, than are the lofty trunks of the Palm or
Im, notwithstanding the variety in their form, texture, and mode of growth; for they all constitute the ascending axes of the Plants of which they 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 mi inquiries regarding the functional pared, upon their capacity as instr For example, we might estimate
1 s
Qu
os mh dO
ght with perfect propriety found any power of the organs respectively com- uments adapted for a particular purpose.
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
8 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT.
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, blood-vessels, &e., 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 principleof 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 deve- loped, 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 phases which it exhibits in the Perennibran- chiate Batrachia, that the homology of the swimming-bladder of the Fish with the lung of the air-breathing Vertebrata 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 higher Vertebrata, to be homologous with the simple rod-like members (A, €) of the Lepidosiren (Fig. 150); whilst these last serve as the connecting-link, whereby the homology of the sca- pular and pelvic arches with the hemal or visceral arches of other verte- bral 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
* 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 the dotted line which marks the place of the wrist-joint should have passed above instead of below them.
MEANS OF DETERMINING HOMOLOGIES. 9
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
Fig. 3.
Diagram illustrating the Nature Lepidosiren annectens :—B
; pl, 62, pleu- ic vertebra, or _ r limbs.
composition between the by the transitional
which there is a gradational passage fro
bract to the sepal, from the sepal to
stamen, and from the foliaceous type
most certain that, of all the means of di
of organs, the study of their developme
if carefully pursued, will
may be left by other mo
the true solution has been at last attained, of
and most controverted questions in the scie
reference, not merely to the nature of pa between differ
» by the
rms they may pre- at general truth, which is, perhaps, the
of Organisation, and which is even yet
10 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT.
far from being fully developed; that in the several tribes of organised 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,* 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, adapting it 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 pre- senting 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. Hach family consists of genera, in every one of which the family type is presented under a some- what diversified aspect. Each genus is made up of an aggregation of species, which exhibit the generic character under a variety of modifica- tions; these, however slight, being uniformly repeated through successive generations. Lastly, each species is composed of an assemblage of indi- viduals, every one of which repeats the type of its kingdom, sub- kingdom, class, order, family, genus, and species, through its whole line of descent.
12. Thus, in assigning to any particular being its place in the Organised 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 Organised fabric, or a mass of Inorganic matter. This is soon discriminated, in the majority of cases, by an appeal to those most general characters which distinguish a% organ- ised structures from inorganic masses.; and the next question is to deter- mine its Animal or Vegetable 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
* 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.”
DIFFICULTIES IN DETERMINATION OF ARCHETYPES, 1]
it shall be referred ; and this, again, to characters which are less general than the common to all organised. structures, nor restricted to each of the four sub ascertained that it is a Vertebrat animal, as the case might be, th y characters of still less general. that order ; its _ family, by features which are its genus, by those Modifications of family charact nted by the several senera it includes; and ] Most special of all
13. N
has to be ascertained by an appeal
developed c
then, the determi Zoological knowle mental error was
to the Marsupiata, extremely close ;
12 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT.
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 corre- spond in the more essential peculiarities of their organisation with the great series of herbworous 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 continu- ally lessening with the progress of research; and whilst, therefore, we should avoid placing too much confidence in existing systems of classifi- cation, and in existing ideas of what really constitute natural groups, we may look forwards with hope, if not with absolute confidence, to the gradual accumulation of those materials, which shall enable the Philo- sophic Naturalist to do that for each group, which has been already
Fra. 4. Big. De A
Anatifa levis ;—a, group of animals of different ages, as attached in the living state :—p, 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 ; J, flabelliform appendages (or branchiæ); g, abdominal appendage.
IMPORTANCE OF THE STUDY OF DEVELOPMENT. 13
effected, in great measure, for the Vertebrated series. In the determina- tion of the relative importance of characters, 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 t of a group of Animals distin from. all others, than
tribe (Fig. 4),
tatingly
classes of
ase of the Cirrhipeds, or Barnacle ¢ turalists, this group was unhesi-
recognize For he perceived that
t t
Cirrhipeds ; 5 possessing
three pairs ng the body tomostracous m no essential
the larva of
fter going a series of metamorphoses, one
” “í Zoological Researches,” No. III.
14 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT.
stage of which is represented in Fig. 6, B, these larvee come to present a form D, which reminds us of that of Daphnia, another Entomostracous
Development of Balanus balanoides;—a, earliest form ;—z, larva after second moult ;—c, side view of the same ;—Dp, stage immediately preceding the loss of activity; a, stomach (?) ; b, nucleus of future attachment (?). Crustacean ; the body being enclosed 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, and of six pairs of posterior legs adapted for swimming. This bivalve shell, with the prehensile and natatory legs, is subsequently thrown off; the animal then attaches itself to its head, a portion of which
Comparison of Leucifer, a Stomapod Crustacean, with Lepas ;—in the former, A, the abdo- men, which becomes rudimentary in Cirrhipeds, is represented in outline ;—in the latter, B, the antennæ and eyes, which really exist in the larva, are represented as if they had been retained and had continued to grow; m marks the position of the mouth in both.
DIVERSITY OF PRIMARY TYPES OF ORGANISATION: 15 becomes excessively elongated into the peduncle of the Barnacle (Fig. 7), whilst in the 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 enclose it (no uncommon occurrence among the Crustacea), and the exterior layer of this is conso- lidated into the ‘multivalve’ shell; whilst from the other thoracic Segments are evolved the six pairs of these animals in their adult state.— peculiarities of the gro entitle it to take rank Stituting merely a sectio
Cirrhipeds b hey must be ple or beyond them), as an aberrant adapted to a life es 15. Much of the
3
y phenomena which are essen- een brought under the same category. But by recognized organic identity as the basis of their
been attempted to show that the law of Unity of Com-
nlimited application ; it having been maintained that the same elementary parts exist alike th:
roughout the Vegetable and Animal
ingdoms, and between the several classes of each
lies solely in the respective development of these parts. Such a doctrine,
owever, 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,
6. If, for example, Flowering-Plants with to a certain ‘ar ending axis or stem with its fo appendages, and of a descending axis
Their most obvious diversities are
any
if we compare presenting typical * For the most complete ace phoses, as well as of the Anatomy Physiology, and Classificati division of the group, see Mr. © Darwin’s E i ia,” published by the R
?
Cyn
ee
16 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT.
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 funda- mental 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 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 reco- gnized as degraded forms of the Phanerogamic type, when their generative apparatus 3 is anias; reduced, though “Abie 3 is, to a condition a extreme simplicity.—The Cryptogamie series cannot be referred with equal pro- priety 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, &e. 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 distinction between the two series lies, not in Frade of development, but in type of conforma- tion. So among the Cryptogamia themselves, we find parallel series, such as those of Algw, Lichens, and Fungi, through each of which a cer- tain 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 organisation as we shall find in the Animal Kingdom, it is not the less contest that a considerable number of distinct types ¢ of structure exists,
which eannot 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 dia- tinet plans of structure, “which Cuvier was the first to mark-out clearly,— namely, the Radiated, the Molluscous, the Articulated, and the Verte- brated; and these are found to be more and more clearly distinguishable from each other, the more profoundly we examine into the fcadeanenial 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 them-
DIVERSITY OF TYPES OF ORGANISATION. 17
Selves, 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 tom 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 their organisa- tion, Cephalopods are unquestionably intermediate between the lower Mollusks and the typical Fishes, Again, although the higher Cepha- lopods indubitably take a more elevated rank as Animals than the lowest Fishes, and in this respect might be considered ag approximating more closely to Man, yet in the conformity of its organisation to the Verte- brated 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 correspon
dence between the condition of the Circulating
apparatus in the two series of Articulated and Mollusco especially as regards the imp munication with the visceral each, and which shows itself in the general distribution of the blood-ve Articulated animal, is the elongated dors has a repetition of similar ch
us animals,— erfection of its vascular system, and its com- cavity,—there is a type which is peculiar to
structure of the heart, as well asin the ssels. For the type of the heart, in the al vessel, which, if divided at all, ambers for the several segments of the body ; a concentrated organ, with much thicker walls, e or receiving cavity separated from the ven-
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 approxi- mation is seen in the
i general plan of the fabric, which is as obviously arti- culated in the Crustacea, as it is in the Insect.
18. But although it is in type, or plan of organisation, 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 diver- sities in grade of development ; by which is to be understood, the degree in
Cc
whilst in the Mollusk it is usually having the auricl tricle or impelling cavity,
Pe er rr pr a R
18 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT.
which the several parts that make up the entire fabric are characterised 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 Vegetable 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 indivi- duality, 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 litile 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 Von 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 Vege- table life, we find that out of the homogeneous aggregation of cells which forms the simple frond of the humble Algæ (§ 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 ascend- ing 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 differentia- tion, both as regards external conformation and intimate structure, pro- ceeds 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 specialisation everywhere manifests itself, no part being a precise repe- tition of any other, except of the corresponding part on the opposite side of the body.*
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 fac-similes 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 com- monly accounted distinct individuals. But we find the same to hold good, as
* 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,” Vol. I. p- 17 et seg., and the Author’s ‘Principles of Human Physiology,” 4th Ed., p. 192.
19 REPETITION OF SIMILAR PARTS IN LOWER ORGANISMS.
regards individual organs, in the highest members of each of the set brated sub-kingdoms, and even (though to a less extent) among Vert brated. animals, Thus among the LHchinodermata, there is a Aoa repetition of similar parts around a common centre; and although this repetition is limited to
fwe in the highest forms E ons of the class, yet it extends
to a much greater num-
ber in those of inferior
organisation,—as we see
in comparing the Ophiura
with its five simple arms
— BES oD) zee RON go = LO, amie? (Fig. 8) and the Penta- AD crinus (Fig. 9), whose ten / arms all subdivide into
such numerous branches, (EES f; A
that the aggregate num- ber of pieces in the whole is estimated at above a hundred thousand. So, again, in the Cephalopo- da, which constitute the highest division of the Molluscous series, we TA find the tentacula sur- cae
rounding 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 (Cuttle-fish, Fig. 47), at the same time acquiring a much higher individual development, and often hav- ing one pair differentiated from the rest, or Some special purpose. So in the Annelida and other inferior groups of the Articulate Series, we find the loco- motive, respiratory, and other important organs almost indefinitely multiplied in the longitudinally -repeated segments ; ut 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 the perfect Insect, the differentiation is carried to its highest extent, the locomotive apparatus being restricted to the three thoracic Segments, tay, and all the other organs, even when re-
peated throughout, being unequally de- Pentacrinites briareus, veloped in the several parts. The same principle of gradual differ
c 2
entiation
20 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT.
shows itself most remarkably in the conformation of the members of Verte- brata: 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 Lnaliosawria (Ichthyosaurus, Plesiosaurus, &c.) to the typical Reptiles, we find the number of these multiplied digits dimi- nishing, 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 characterise the hand (or foot) of Man; in Birds and Flying Mammals there is a most marked differen- tiation 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 specialisation 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-organised 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 at first presents itself, to that which it assumes in its state of most complete development,—we recognise one and the same plan of progression, namely, from the general to the special ; and, as Von Baer justly remarks, the relations of any organised fabric to any other, must be expressed by the product of its type with its ų grade of development. Neither alone suffices to characterise it; for under the same type, 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 through one unbroken progression, than it is to arrange the entire assem- blage of beings composing either kingdom in a single linear geries.—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 Repro- duction; 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 con- siderable 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 opera-
2 GENERAL VIEW OF VEGETABLE KINGDOM.—PROTOPHYTA. 21
tions 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 inter- posed, which establish a ready communication 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 develope the organs in which they are deficient, and may thus evolve themselves into entire plants, and maintain an independent existence.* In this way a multi- plication of the products of the original germ may be effected ; but these, as will be shown hereafter (CHAP. xI.), 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.
22. Having thus determined, by the analysis of one of the highest Plants, what it is that constitutes the most complete type of Vegetable organisation, we shall commence with the lowest division of the series, and endeavour 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 re-union of the contents of two cells, by an act of ‘conjugation, is an essential condi- tion. Any one of these cells may multiply itself indefinitely by sub- division, the results of which process are seen in the accompanying ex- ample (Fig. 10); but these products are all mere 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,
* This is usually the case under favourable conditions with 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.
22 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT.
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
Fra. 10. the multiplied cells remain in continuous connexion
with each other, being imbedded in a common sub-
stratum of gelatinous substance, so as to form but a
single mass (Fig. 10), this may be perfectly homoge-
neous throughout; no definite form being presented
by it as a whole, and no trace of ‘ organs’ being dis-
tinguishable in any part of it. The first indication
satin of progress towards a higher grade, is given by the transversalis. limitation of the direction in which the increase takes place : so that, instead of an amorphous aggre-
gation of cells, we find a linear series (Fig. 11, a) which is formed by successive trans-
verse subdivision ;
and this filament
may increase in
breadth by longitu-
dinal subdivision(s),
so as at last to pro-
duce a laminar ex-
pansion, such as that
of the common Ulva,
which is termed a
thallus. In the
simplest forms of
this thallus, we do
: not meet with the
Bangia velutina. 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 sub- division, but also by the emission of a portion of its contents enclosed 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 organism from which it proceeded, it begins to develope 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
GENERAL VIEW OF VEGETABLE KINGDOM.—ALGA. 23
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 swm 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 appa- ratus 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, Alge,* Lichens, and Fungi,—which, nevertheless, are fitted to
which present corresponding hem seem to agree (according an account will be given here- 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 indi- vidual; the product of the former is a‘ spermatoid’ 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, 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 within a Special envelope, every one of which, when liberated from the parent, may develope itself into a new plant in which the same processes are repeated. I+ is by the general relation of this apparatus of fructification to that of nutrition, that the three groups already named are most dis- tinctively characterized. : i i
24. Thus the Algæ vegetate exclusively in water or in damp situa- tions; 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
exist under very diverse conditions, and
diversities of structural type; and all of t to the most recent investigations, of which after, CHAP. x1.) in the possession of a
* The group of Algæ, as here limited
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 Alge, as to require that they should be Separately considered.
, does not include the Protophytes described in the
24 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT.
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 adhe- rent, while the latter consist of single rows, bearing the generative cells at their extremities; and in Zonaria (Fig. 13), it is only the character of the fructification that raises it above the type of an Ulva. In the highest
SUDOC TOL EA
AQ BOSEN
> A J Sse li i : Mesogloia vermicularis. Zonaria plantaginea,
Algæ, however, we find some differentiation in the texture of their inte- rior and exterior substance; and there is also a certain foreshadowing of-
X
eti)
Dasya kuetzingiana. Marginaria gigas.
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 1s there any real specialization of function, save that the fructification
GENERAL VIEW OF VEGETABLE KINGDOM.—LICHENS. 25
is evolved from the frondose portion, and not from the stem-like or root- like axis. Most Algz are provided with a special apparatus (such as the stichidium of Dasya, Fig. 14, a) for the evolution of free gemme, which are sometimes ciliated like the zoospores of Protophyta, and which mul- tiply the original fabric independently of any true generative act. The proper generative organs are frequently very obscure, and are often buried in 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-cells, do not usually undergo any great amount of subdivision into ‘spores,* before each spore that has originated from it begins to develope itself into a new plant. Hence it is obvious that the whole nisus of vital activity in the Algæ, 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 pro- bably 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, 17); and in the several layers of this thallus, there is consi-
Fig. 16. Fie. 17.
Parmelia acetabulum. Sphærophoron coralloides.
* 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, &¢., which are imme- diate or remote products of the subdivision of the embryo-cell, and to those bodies in Algæ, Lichens, &c., which are homologous with them. On the other hand, the germ- cells which themselves take part in the generative act, and from which the embryo-cells originate, should never be designated by the term spore.
26 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT.
derable diversity of texture, although (as in the Algee) there is no departure from the simple cellular type. As in the Algæ, moreover, we usually find a special arrangement for the production of free gemmæ (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 ‘ sperm-cells’ and ‘germ-cells’ exist, although usually buried in the substance of the thallus; and that of the clusters of ‘spores’ which make their appearance within special conceptacles, each, as in the Algæ, 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 (Figs. 16, 17). Some tribes of Lichens very closely approximate to Algæ, both in their conditions of growth, and in their general character; whilst others present an equally close approxi- mation 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 favourable, if not as the absolute condition, for the develop- ment of the Plants belonging to it, the presence of dead or decaying organic matter, 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 favoured
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 Alge (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 predo-
minant, as is the nutritive apparatus in the
Algæ. The vegetative thallus of these plants,
which extends itself indefinitely in situations
favourable to its development, has a very
i loose flocculent texture, and is composed of
Styjectnus eapanimseduern. elongated branching cells interlacing amongst
each other, but having no intimate connec-
tion (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 species, genus, or even family, could be certainly made from it alone. Although any portion of this mycelium will con- tinue to vegetate when separated from the rest, it does not appear that there is any provision for the spontaneous detachment of free gemme for the multiplication of the individual. The whole nisus of vital activity in the Fungi seems to be concentrated upon the Generative apparatus,
GENERAL VIEW OF THE VEGETABLE KINGDOM.—FUNGI. 27
which, when fully developed, separates itself completely from the nutri- tive, and constitutes all that commonly attracts notice as the Plant
Clavaria crispula ;—a, portion of the mycelium magnified.
(Fig. 20). Late observations render it probable 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
lA i Ii m, N í
\ “AN
Æcidium tussilaginis :—a, portion of the plant magnified :—s, section of one of the concep- tacles with its sporecles.
‘fructification’ is the product of an act of conjugation, the immediate result of which is the formation of an embryo-cell, which afterwards sub- divides 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 in remote soils, and to give rise to vast numbers of separate beings consti- tuting a new generation.*
* Tt 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 ten- dency to the extension of the vegetative or foliaceous portion of the thallus, with a non- development of the fructification; and the thallus often assumes the byssoid form of the mycelium of Fungi, so that it might be readily mistaken for this. So, again, if the Simpler forms of Fungi develope themselves in liquids, they show an unusual tendency to the extension of the mycelium ; and may even take-on so much of the characteristic appear- ance and mode of growth of Algæ, 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. II. p. 340.
SOS eee ee
28 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT.
27. The next important mode of elevation consists in the differen- tiation of the parts of the Nutritive apparatus, and in their still more complete separation from the Generative. In ascending through the series formed by Hepatice, 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 been proposed (by Dr. Lindley) to distinguish this higher division of the Cryptogamic series by the title ACROGENS, signi- ficant of growth at their points alone—The lower forms of the H epaticæ (such as the Ricciaceæ) closely abut upon the Lichens, and differ from them
but little as regards the or-
ganisation of their nutritive
apparatus, although their
fructification evolves itself
after a different type. In
the common Marchantia
(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 interven-
Frond of Marchantia polymorpha. ing 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 folia- ceous appendages are symmetrically arranged; these are not completely differentiated from it in some species, but in others
they are quite separated, and have an indica-
tion 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 eleva-
tion in this type brings us to that of the If osses,
which always have a distinct axis of growth, com-
monly more or less erect, with the foliaceous ap-
pendages symmetrically arranged upon it (Fig. 22).
A transverse section of this axis shows an indi-
cation 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, pro-
longations pass into the leaves, in which they
form a kind of mid-rib. The leaves, however,
do not themselves present any considerable ad-
vance towards the more perfect type, being
Drsni iia, merely solid homogeneous aggregations 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
GENERAL VIEW OF THE VEGETABLE KINGDOM.—MOSSES. 29
the lower portion of the axis (Fig. 25), and even from the under-surfaces of the leaves. Both in Hepaticee and Mosses, we find a special arrangement for the multiplication of the plant by the formation of detached gemmæ; and
Big, 24.
Marchantia polymorpha, with peltate receptacles Marchantia polymorpha, with lobed recep- bearing antheridia, tacles bearing pistillidia.
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 Fre. 25. containing ‘ sperm-cells, and pistillidia or archegonia ix containing ‘ germ-cells.’ In Marchantia, these are _. hy) 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 gene-
rally combined in the same individual. The pro-
duct 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 pistilli-
dium, 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—TIt is
chiefly by specialities in the structure of their gene-
rative 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 Polytrichum commune.
in the development of the organs of nutrition.
28. Passing from these to the Ferns, we find such a 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
press SA pena anA a ea -
30 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT,
sea
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
Sri ean E
cnt
. i i | | hi
1 z Sin eo Trichomanes. Frond of Scolopendrum. the group; and here we find, not only an ascending axis or stem, around
which the foliaceous appendages are symmetrically arranged in a spiral,
Fie. 28.
Frond of Osmunda regalis ;—a, sterile or folia- ceous portion ; b, fertile portion :—a, part of the latter enlarged, to show the thecæ. Equisetum arvense.
GENERAL VIEW OF THE VEGETABLE KINGDOM.—FERNS. 31
but a 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 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 mid-rib 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, enclosed between two layers of epidermis, and having air-chambers to which access is given by stomata; and they can scarcely be less com- plete 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 fructi- fication 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-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-
Lycopodium cernuum.
92 ĠENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT.
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 conceptacle filled with spores, each of which
may give origin to a separate plant,—but a single
young Fern, which, having attained its full deve-
lopment by duplicative subdivision, detaches cer-
tain of its cells, as ‘spores, to continue the race
by the same process. In this departure from the
plan which prevails among the inferior Crypto-
gamia, 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 Cryptogamic.—That of the
Marsitea quadrifolke, Liquisetacee (Fig. 29) appears to be essentially
the same; but in Lycopodiacece (Fig. 30), Isoé-
tacee, and Rhizocarpee (Fig. 31), there is a still closer approximation
to the Phanerogamic type, the ‘ sperm-cells’ (‘ small spores’) being directly
produced by the parent-structure, and the ‘ germ-cells’ alone being evolved,
after the detachment of the ‘large spores, upon the ‘prothallium’ into which each of these developes 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 Rhizocarpes, 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 pro- duced, in which, with the embryo, a store of ready-prepared nutriment is laid-up for its early development. This sub-division 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 appa- ratus; but for our present purpose, it will be sufficient to sketch the typical plan, which is more or less obviously manifested in the conforma- tion of the entire group.—If we analyse the fabric of any common Phanero- gamous Plant, we find that it consists essentially of an axis and appendages;
GENERAL VIEW OF VEGETABLE KINGDOM.—PHANEROGAMIA. 33
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 pre- sently shown to be modifi- cations of the same funda- mental parts. The axis (Fig. 32, a, a æ) is composed of cellular parenchyma, with a larger or smaller proportion of fibro-vascular tissue; and it is upon the mode in which these com- ponents are arranged rela- tively to each other, and in which progressive additions are made to the diameter of the axis, that the dis- tinction is founded between the Endogenous and Exo- genous types, which, toge- ther with corresponding distinctions in the struc- ture of the leaves, flowers, and seeds, affords a basis for the sub-division of the Phanerogamia into two pri- mary classes. From the central axis, bundles of
fibro-vascular tissue pass wn i ff A, Ideal. Plant, after Schleiden ; a to avi, the axis, a being do y into the root-fibres the ‘root, ai, aii, aiii, viv, and av the successive internodes of which form the ultimate the stem, and avi the terminal development of the axis into a le; b tlet to evii th ome foli sean oe . : n ovule; 2, rootlets; c to evii the successive foliaceous ap- ramifications of its descend- pendages to the axis, ¢ being the cotyledons, ci, cii, and ciii
ing portion; these are en- fhe ordinary or hel ev the samen, and tho veloped 1n firm tissue, that carpellary leaves; d, leaf-buds :—B, carpel enclosing an limitstheirabsorbentpower gvm, seen externally and in seeen, Sho oth externally at to their extremities, which, d, and in section at &”.
being stillsoftand 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 affect 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
D
34 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT,
point, but from a longitudinal awis. 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 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 Dicoty- ledonous plants generally it may be said to be five ; 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.*
30. The complete floral apparatus of Phanerogamia consists externally of a ‘perianth,’ composed of a series of verticils of foliaceous organs, which do not depart widely, except in colour, 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 com- posed of a whorl of sepals (Fig. 32, ci) alternating with the preceding ; and the corolla, in like manner, consists of a whorl of petals (c"), 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
* Thus, ‘ opposite’ leaves would be 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 ‘vertici’ 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 ver- ticillate 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. 35
such is the case (as in the Garden Peony), 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 con- formity 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 five, whilst in the Monocotyledons it is three. The regularity of a flower may be interfered-with by the suppression or by the multiplica- tion 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, how- ever, can seldom be at a loss in the investigation of their real nature, if he proceed on the morphological principles already explained; and he continually finds his determinations justified by the occurrence of ‘mon- strosities, which exhibit a more or less complete reversion to the arche- typal form (§ 82). The non-essential character of the perianth is indi- cated 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 Gymnosperme, 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. x1).—It is within the protection of the perianth, that the true generative organs are developed; and these consist of the anthers, (Fig. 32, c^) from which the ‘sperm-cells’ (here termed pollen-grains) are evolved, and the carpels (c%), whose aggrega- tion forms the pistil, containing the ovules (a), 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 support- ` ing ‘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 Vymphea 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 m 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. In the Gymnosperms, indeed, the carpellary leaves are not folded together so as to enclose the ovules, which are deve- loped upon their internal surfaces; and merely protect them during their immaturity, by their own mutual adhesion.—lt 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 wniseawal; when the staminiferous or male : D 2
SR EE Rs
36 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT.
flowers are borne on the same plant or tree with the pistilline, it is said to be monecious ; whilst if the two sets of flowers are developed by dif- ferent individuals, the species is said to be diwcious. This last arrange- ment, 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, developes 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-up in 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 struc- ture, 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 (Fig. 32, a, c), is prepared to evolve itself ; whilst a supply of nutriment for its further development is stored-up within it, either form- ing 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 favourable 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
GENERAL VIEW OF ANIMAL KINGDOM. 37
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 im its edge. Its texture, too, is of the simplest kind, being composed of scarcely any- thing 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 Vegetable 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 Organised Creation, we shall in the first place examine, as in the previous case, 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, how- ever, are in the first place developed, and are afterwards sustained, by the Organic apparatus with which they are connected; whilst, in their turn, they become subservient to és 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 depen- — dence in their actions; and every thing tends to render the entire pro- duct of each generative act æ 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 individualisation, 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 mani- festations of Animal power, which involve the exercise of its highest attri- bute,—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
38 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT.
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, how- ever, 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 characterised. 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 diges- tive 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 brou ght back, and the waste or refuse matter removed from the tissues and conveyed for elimi- nation 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 ats 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 enclosed (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 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 develop- ment which the organs of animal life attain in the higher Radiata, is altogether wanting in the lower, among which they are not distinguish- able 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 Articu- lated 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 the type of nutritive system is still com- paratively low. In both these sub-kingdoms, however, it is only ina 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, constitut- ing the great bulk of its entire series, instead of being, as among the
GENERAL VIEW OF ANIMAL KINGDOM.—-PROTOZOA. 39
Radiata, exceptional as to number, and probably to be so considered in regard to type likewise.*—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 confor- mation 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 gemme, in the former as in 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 asa 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 Vertebrata 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 develop: ment has not advanced beyond the Zoophytic stage (CHAP. xa)
35. Underlying these well-marked types of Animal organisation, how- 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 any one 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 Vegetable (§ 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, accord- ing to the particular views of the classifier in regard to the mode in which they should be distinguished. A large proportion of the Protozoa consist 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 completely-developed 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 Proto- phytes, from the surrounding air and moisture, but is dependent for its support upon organic substances previously elaborated by other beings, which it envelopes with its own jelly-like substance, and of which it gradually dissolves and appropriates that which is fitted for its own
* Tn the Author’s opinion, the Zoophytes, not the Echinodermata, are the types of the Radiated series ;—-Gasteropods of the M olluscous ;—Insects of the Articulated ;—and Mammals of the Vertebrated.
40 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT.
increase. The animal character of this body is also indicated by its movements; for although the ‘ zoospores’ of the Protophyta and lower
as | Soa)
A \ 3) Y
Ameba princeps, indifferent forms, aj ya:
Algæ 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 respira-
tion, and for secretion, so does every part appear equally capable of receiving impressions made upon it, and of responding to them by a con- tractile movement. From this starting-point we may proceed in either of two directions; for we find in the Z nfusory Animalcules a tendency to the individualisation of the single cell, which seems to attain in them its highest development as a separate entity; whilst in the Rhizopoda (Fora- minifera) and Porifera (Sponges) we find aggregations of gelatinous bodies (which present more or less distinctly the characters of true cells) assum- ing certain 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. More- over, 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 sub- division; and in this way a vast number of similar beings may be pro- duced, each of which is a repetition of the rest, and lives altogether inde- pendently 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 distinction as yet manifesting itself between these, and both of them apparently contributing in the same manner and
GENERAL VIEW OF ANIMAL KINGDOM.—PROTOZOA. 41
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 envelope upon its exterior ; whilst through apertures in this are put forth extensions (psewdopodia) 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 individualisa- tion 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 Foraminifera. 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 pro- duced 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. For, 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 definite 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 specialisation, 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 ovwm 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 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. Hach of these points will now be noticed in some detail—The radial sym- metry must be regarded as in itself a vegetative character, for it cor- responds 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 inti- mately 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 pre- hensile appendages; which may be mere oral tentacles, as in the Polypes (Figs. 34, 35), the Meduse (Fig. 93), and the Holothuria (Fig. 40), true
42 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT.
arms, as in the Ophiura and Comatula (Figs. 8, 38), or divisions of the body itself, as in the Starfish (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, modifi- cations 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 gemmation. 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 origi- | nating 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. Tt is not to be won- dered at, then, that the older Naturalists, who were only acquainted with the skeletons of Zoophytes, should have considered them as vege- table 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 per- formed 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 Acelephe, it seems to take place at some period of life or other; for although we find few traces of it in the fully developed Meduse, yet (as will be shown hereafter, cmAP. x1.) 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 fabrics of the Cirrhigrade and Physograde orders, which, until the recent discovery of their true cha- racter, have been a source of so much perplexity to Naturalists. In the class Echinodermata, multiplication by gemmation 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. animals, as compared with their Vegetative activity, is one of the most remarkable features of the group taken as a whole; nor are there are 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
GENERAL VIEW OF ANIMAL KINGDOM.—ZOOPHYTES. 43
the respondence to external impressions which is probably the source of all their movements, proceeds from any distinct consciousness of these impressions. It is in the Acalephe, that the first traces present them- selves of a nervous system, and of organs peculiarly fitted to receive sen- sory 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 Lchinodermata, 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 Acalephe, may be unmistakeably affirmed to exist. There are also rudi- ments 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 Fra, 34.
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 grada-
tions present themselves, which
establish a tolerably complete
transition from the one condi-
tion to the other.—Commencing
with the Hydra (Fig. 34), we
find the digestive apparatus re-
duced to a state of the greatest
simplicity, the whole body seem-
ing to be nothing else than a
stomach, with a circle of pre-
hensile tentacula around its ori-
fice, 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 in-
tegument of the body are so
closely connected together, as to
seem like two layers of one and A, Hydra fusca, or Brown Fresh-water Polype,
the same membrane; there are, attached to a piece of stick, with its arms extended, 1 as in search of prey; a, the mouth surrounded by however, some lacunar spaces tentacula; b, foot or base, with its suctorial disk: at B
ituting is seen a portion of one of the arms near its origin, and between them 4 constit me the at c another portion near its termination, more highly first indication of that ‘general magnified, 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
44 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT.
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 pro- duct 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 gene- rative 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 gemme which develope themselves into new Hydree sprout almost indifferently from any part of it, and that a minute frag- ment from any region will
(under favourable circum-
stances) regenerate the whole.
In the composite fabrics which
are formed after the Hydra-
form type (Fig. 99), the conso-
lidation of the external in-
tegument 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 com-
mence 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
Diagrammatic section of Actinia, showing its interna formed upon its type; for in cin; aa eect Lo aata these we find the stomach ent ee enegan ; g I» aaa eni suspended. (as it were) In a oraria; m, m, diiferous filaments,” tt °F Tange space, which is subdi- vided 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 situated. Very distinct organs for the production of sperm-cells or of ova are here evolved ; these organs (according to late researches, CHAP. XL,) 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
NS a
GENERAL VIEW OF ANIMAL KINGDOM.—ACALEPH. 45
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 expulsion of the fluid that has remained for some time in the general cavity, that the excre- tory 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 aération of the interstitial fluids which takes place through it must be extremely limited, in com- parison with that which will be carried on through the delicate mem- branes 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 through- out 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 ordi- nary 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 Acalephe 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 gemme 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, 1s occupied by homo- geneous solid tissue. But a series of gastro-vascular canals, commencing from the stomach, radiates towards the margin of the disc; and these serve the double purpose of conveying the nutritive product of the diges- tive 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
46 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT.
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
Fie. 36.
ih y X) : ‘ atl nd
Wp 2
Y
Structure of Cyanea awrita.—Disk seen from above, showing the quadrilateral mouth a, the four ovaries 60d b, the four orifices of the ovarian chambers cc ec, the stomach ddd d, and its radiating prolongations, the eight anal [P] orifices e e, &c., and the eight ocelli EIA &e.
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 (6,6) are lodged in cavities round the mouth, each 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 rudimentary 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 develope medusa-like buds containing sexual organs, each indi- vidual producing buds of only one sex, as in dicecious plants; in others, however, male and female medusa-buds are developed on the same stock, as in moneecious plants, although in no case are the two sets of genera- tive organs combined in the same medusoid body.
40. In the class Hehinodermata, the Asterias (Fig. 37 ) holds by ne means an elevated rank; yet we find in it 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
Ni PE i Ki t 1 |
ee
GENERAL VIEW OF ANIMAL KINGDOM.—ECHINODERMATA. 4T
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
Fie. 37.
Asterias aurantiaca, with the upper side of the hard envelope removed :—a, central stomach ; b, ceca upon its upper surface (salivary glands P) ; ec, cecal prolongations of the stomach into rays ;c’, the same empty ; d, the same laid open; e, the under surface, seen from within after the removal of the ceeca, showing the vesicles of the tubular cirrhi; f, thesame in a contracted state, showing the skeleton between them,
fluid of the general cavity seems still to take the largest share in the nutritive operation. It 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,* but also between the latter and the
* Theterm 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 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 Actini- form and Alcyonian Zoophytes, and even to that which is confined within the stomach and gastro-vascular canals of Meduse.
48 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT.
blood contained within the proper circulating system. A special pro- vision appears to be made for respiration in these animals, by the trans- mission of the ‘fluid of the general cavity’ into a multitude of short delicate cæcal tubes, which pass between the pieces of the calcareous
Vike SE, Ke
AAU
Comatula rosacea.
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 or-
gans possessing a dis-
tinct glandular charac-
ter, whose special uses
are not yet certainly
known. The genera-
tive apparatus here
attains a high develop-
ment, the ovaries and
testes (as in the higher
Acalephe) being no
longer combined in the
same individuals, and
having separate orifices
Echinus mammillatus. for the discharge of
their products ; it is in-
teresting to remark, however, thatin Comatula (Fig. 38), whose digestive apparatus is framed upon a higher type than that of Asterias, the ovaries are dispersed in isolated spots through the integument of the arms. The Star-fish exhibits a series of elaborate provisions for locomotion, in the beautiful articulation of the plates of the calcareous skeleton, in the con- tractility of the general integument of the body, by which its lobes (misnamed ‘arms’) are moved in various directions, and in the multipli-
GENERAL VIEW OF ANIMAL KINGDOM,—ECHNINODERMATA. 49
cation of tubular cirrhi furnished with suckers, by the contraction of which, when the suckers (forced out by the injec- tion of fluid into the cirrhi from the ‘general ca- vity’) have taken an attachment, the body is drawn towardsthe 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 alimentary 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 ap- pearance in any lower forms of or- ganisation. The locomotive appa- ratus, too, is still more highly de- veloped ; for the body being now enclosed in an im- movable case, so that its parts are not themselves capable of flex- ure, a new set of instruments is evolved, namely, —the calcareous
Anatomy of Holothuria tubulosa ;—a, anus; b, mouth, surrounded by 20 tentacula; c, cloaca, surrounded by muscular dilators c’; 2, intestinal tube ; m, mesentery; ml, ml, longitudinal muscles ; mt, transverse muscles lining the entire inner surface of the integument; o, ovary; ap, cecal appen- dages, probably seminiferous ; p, contractile vesicle, probably a heart; r, r, respiratory apparatus, originating in the cloaca; é, oral tentacula; t’, cecal reservoirs; va, annularvessel surrounding the mouth and supplying the ten- tacula; 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 vessel, giving off transverse branches vl’, seen by removing the longitudinal muscles; vm, mesenteric vessels, connecting the branches of the external intestinal vessel with those of the respiratory system of vessels, vr.
E
50 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT.
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 be performed by the flexure of the body generally ; and for this a regular series of longitudinal and transverse muscular bands (Fig. 40, m J, m t) is provided, reminding us of those of the Worm-tribe. The alimentary canal (7) does not yet present any distinc- tion of parts into cesophagus, stomach, or intestine, but remains of nearly the same diameter throughout its length; it is held in its place in the midst of the general cavity of the body, however, by a regular mesentery, upon which the blood-vessels 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 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 (¢) ; whilst that of the ‘general cavity’ receives the same in- fluence from the water introduced through the respiratory tree (r,r). The restriction of the outlet of the genital apparatus (0) to a single aperture (the five equal and separate portions of 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 Morruscovus 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 dif- ferent. 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 belong- ing to it were, until recently, unhesitatingly ranked under that designa- tion. 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 ana- logous 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 charac- terized 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 the family of Chitonide, we meet with a division of the external skeleton into segments (Fig. 41), which reminds us of the articulated type. But these are peculiar excep- tions; and a Molluscous animal is essentially a bag of viscera, enveloped in a skin which is thickened in parts by muscular 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
GENERAL VIEW OF ANIMAL KINGDOM.—-MOLLUSCA. 51
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 ‘hemal’), whilst the prin-
cipal centres and trunks of the ner-
vous system are on the ventral aspect
(which may hence be termed ‘ neural.)
But this simple and symmetrical
arrangement is very commonly obs-
cured by subsequent inequalities in
the development of particular regions,
so that an entire change takes place
in the relative position of the dif-
ferent organs, and the types of con-
formation thus evolved seem to
have little or no affinity to one
another.* — The nearest approach
to the archetype is presented on the A, Chitonellus.—B, Chiton.
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
Salpa maxima; a, oral orifice; b, vent; c, nucleus, com osed of the stomach, liver, &c.; d, branchial lamina; e, the heart, from which proceeds the longitudinal trunk f, sending trans-
verse branches across the body; g, g, projecting parts of the external tunic, serving to unite the different individuals into a chain.
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 envelopes the general mass of the body, as to constitute what is known as their ‘inner tunic. t—In the Bivalve Mollusks, on the other hand, the principal extension of the integument takes place externally ; a duplica- ture of the thickened glandular skin of the ‘ dorsal’ or ‘ hemal’ region
* See Mr. Huxley’s admirable Memoir ‘On the Morphology of the Cephalous Mollusca,’ in the ‘‘ Philos. Transact. 1852.” +} 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. ý E
52 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT.
(here termed the mantle) being prolonged 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 Fie. 43.
Dorsal Margin.
Anterior extremity. *AQTUI0I4X9 IOLIOISO J
Ventral Margin.
Anatomy of Mactra ;—a, anus; b, posterior muscle; e, branchial ganglion; d, ovary; e, t, intestine ; f, shell; g, nervous cord, connecting esophageal and branchial ganglia ; h, stomach ; i, heart ; k, liver; /, anterior or cesophageal ganglia; m, anterior adductor muscle; n, nervous filaments; o, mouth ; p, one of the oral tentacula; q æ, mantle ; r, margin of the shell ; s, foot; w, branchial lamelle; y, oral siphon; z, anal siphon.
tissue in the integument of the ‘ ventral’ 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 unsymme- trical development of the hindmost portion of the hemal 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
GENERAL VIEW OF ANIMAL KINGDOM.—MOLLUSCA. 53
are prolonged into those prehensile processes (Fig. 47) which are termed ‘arms; and the posterior epipodial lobes, by their cohesion, form the
Paludina vivipara, withdrawn from its shell and laid open ;—a, foot; b, operculum ; c, one of the tentacula with its ocellus; d, siphon ; f, border of the mantle ; 9, pectinated branchiz ; h, ies the oviduct dilated for the retention of the ova; k”, portion of it situated within the spire of the shell; i, termination of the intestine; l, canal for the urinary (?) secretion ; n, heart ; 0, liver; p, proboscis; q, q', esophagus; ”, stomach; s, s’, 8”, intestine, lying at ¢ within the branchial cavity; u, u, cephalic ganglia ; v, v, salivary glands ; œ, the principal muscular nerve.
funnel’ that 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
Fie. 45.
ene E te we ee
Shells of Gasteropod Mollusks :—a, Achatina ;—B, Sigaretus ;—C. Vermetus ;—D, Scalaria.
54 GENERAL PLAN OF ORGANIC STRUCTURE. AND DEVELOPMENT.
special character of differentiation from the general ‘Archetype; and there is no real transition from the one to the other.*
Existing forms of Pferopods.—a, Hyalea; B, Criseis; c, Clio. Sepia officinalis, or Cuttle-fish.
42. Turning now to the internal organization of the animals of the Molluscous sub-kingdom, we find that the alimentary canal almost in- variably presents a distinct separation between the esophagus, 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 cesophagus, is not situated, in the lower Mollusca, ona prominent part of the body, nor is it surrounded by organs of special
* When it was first discovered that the embryo-forms of Gasteropods (Fig. 48) possess a pair of ciliated lobes corre- sponding 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 incon- sistent 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 ‘ale’ of Pteropods ; and that a more fundamental distinction lies in the development of an ‘ abdo-
Embryoes of Nudibranchiate Gasteropods men’ in Pteropods, whilst itis a ‘post-abdomen’ which is deve- loped in Gasteropods,
GENERAL VIEW OF ANIMAL KINGDOM,—MOLLUSCA. 55
sense; and hence these are distinguished as acephalous. In the higher
classes, however, it is situ- ated on a head, which pro- jects from the trunk, and which is usually furnished with well-developed eyes, and with rudimentary or- gans of smell and hearing. In the lower Mollusca, again, there is a want of any prehensile or reducing apparatus, thefi ood-particles being drawn-in by ciliary currents, which are also subservient to the respira- tory function; but in the higher, the mouth is fur- nished with a complex ap- paratus for the reduction of solid food (Fig. 50, a), and prehensile instruments are added in the Pteropods and Cephalopods. In addi- tion 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 com- minute the food; and this is found in many Bryozoa, as well as in several Gas- teropods (Fig. 50, 2) and in Cephalopods generally. The liver is always reco- gnizably present; and al- though in the Bryozoa it consists of nothing else than an assemblage of iso- 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 de- veloped into a compact viscus (Fig. 50, J, J), which
EF NORI o=JUUUUSU
A fe
Laguncula repens, 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 :—4 @, tentacula clothed with vibratile cilia; b, pharyngeal cavity; ©, valve separating this cavity from d the esophagus; e, the stomach, with „f its pyloric valve, and g the circle of cilia surrounding that orifice; h, wall of the stomach with biliary follicles; 7, the intestine, containing & excrementitious matter, and terminating at l, the anus; m, the testicle; n, the ovary; 0, an ovum. set free from the ovary ; p, openings for the esape 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 tenta- cular circle; u, retractor of the cwsophagus; v, retractor of the stomach; w, principal extensor muscle; æ, transverse wrinkles of thesheath; y, fibres of the sheath, themselves pro- bably muscular ; z, muscles of the tentacula; a (at the base of the tentacular circle in A) nervous or cesophageal ganglion ; b, stem.—D, a portion of the tentacular circle shown separately on a larger scale; aa, the tentacula clothed with cilia: b b, their internal canals; c, muscles of the tentacula; d, trans- yerse muscles forming a ring at the base of the tentacula; e, muscles of the tentacular circle.
frequently bears a very large proportion to the general mass of the body. As we ascend from the lower to the higher parts of the series, moreover,
r aS
56 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT.
we find other secreting structures connected with the alimentary canal, such as the salivary glands and pancreas, presenting themselves in a more
i | ANN
Aplysia laid open, to show the arrangement of the viscera :—a, upper part of the œæsophagus ;
b, penis; ¢, c. salivary glands; d, superior or cephalic ganglion; e, e, inferior or subwsophageal
Poncia ; J, entrance of the oes into g,g, the first stomach or crop; k, the third or true
igestive stomach ; 7, the second stomach or gizzard; k, intestine ; J, Z, l, liver ; m, posterior or branchial ganglion; n, aorta; o, hepatic arter
rt i Y; P, ventricle of heart; q, auricle; r, s, branchie ; t, testis; u, lower part of intestine; v, ovary ; w, anus,
and more specialized condition; and the 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
GENERAL VIEW OF ANIMAL KINGDOM.—-MOLLUSCA. 5T
communication with the ‘general cavity of the body; in the midst of which it is suspended; but it may be affirmed 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 re- duced in size, and presents so much of the character of an ordinary venous sinus, that its true nature has not been until recently discovered. Not- withstanding 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 Conchifera, we first meet with a differentiation of auricle and ventricle ; and this dis- tinction becomes still more strongly marked, both structurally and physio- logically, 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 be- comes 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 connexion, however, that continues to exist between the gemme 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* 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 ad- vanced stage of development they become entirely separated from it and from each other, although still enclosed within a common envelope ;
_. * The term Zooid has been suggested by Mr. Huxley, as an appropriate designation for each of the independent and self-maintaining organisms, which collectively result from a single generative act.
58 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT.
and in Salpa (Fig. 42), the buds developed from the ‘stolon’ or creeping stem in the interior
Fie. 51. of the stock, become
; detached at a very
early period, and swim
forth freely, although
connected into chains
by the mutual adhe-
sion of their bodies.
No multiplication by
gemmation is known
to exist in any of the
higher Mollusca; but
a peculiar generative
zooid is detached from
the male of certain
Cephalopods, to con-
Botryllus violaceus :—4, cluster on the surface of a Fucus:—B, por- F th the female his OE eT E a spermatic fluid.—The true generative appa-
_ ratus is very distinctly evolved throughout the series. In its lowest classes, _ the two sets of organs are united in the same individual, in such a manner that its ‘sperm-cells’ may impregnate its ‘ germ-cells,’ and thus produce fer- | tile ova, without any special operation. In certain of the Bivalves, however, the sexes are distinct; but the fertilization of the ova is provided-for without any special congress of two individuals. In the Pulmonated Gasteropods | both sets of organs are present in each individual, but they are not usually self-impregnating ; for the generative act is ordinarily effected by the con- gress of two individuals, each fertilizing the ova of the other by means of a highly-developed intromittent apparatus. In all the other Cephalous Mol- lusca, the sexes are distinct; andaregular sexual congress usually takes place. 44. No part of the organization of Molluscous animals exhibits the principle of differentiation more remarkably, than does the Nervous system. For whilst, in the Bryozoa and Tunicata, we find it to possess but a single ganglionic centre, which answers all the purposes required by the low de- velopment of their animal functions, a progressive multiplication of gan- glionic centres manifests itself as we ascend the series; this multiplication not being dependent, as in Radiata and Articulata, upon the repetition of similar ganglia (save in certain special cases), but having reference to the greater variety of purposes which this system is called-on to effect, chiefly in virtue of the development of more special organs of sensation and motion. In the Acephalous Mollusks, there is an almost entire absence of visual organs, and no trace of auditory or olfactive; and the movements of such of them as are not fixed to one spot, are of the simplest and least varied nature, being effected either by the agency of the ciliary currents, or by general contractions of the muscular sac, or (in the Bivalves) by the con- traction of those special collections of muscular fibres, which constitute the adductor muscles and foot. In the Cephalous Mollusks, the rudimentary eyes found in some Acephala are progressively developed into organs fitted for distinct vision; rudimentary organs of hearing begin to show themselves, which are evolved among Cephalopods into a proper auditory
GENERAL VIEW OF ANIMAL KINGDOM.—ARTICULATA. 59
apparatus; indications of a specialization of a part of the surface for -olfactive purposes are also perceptible; and conjointly with this advance in the sensorial apparatus, we find the capacity for locomotion,—which is so feeble in most of the Gasteropods, that the term ‘sluggish’ derived from one of the best known members of that class is applicable to the whole of it,—greatly augmented in the Pteropoda and Cephalopoda, many of the latter being nearly as active as Fish. 45. The plan of construction presented to us in the assemblage of animals constituting the sub-kingdom ARTICULATA, is much more definite than that which we have traced through the Molluscous series; and its leading features are in general more easily recognized, since the depar- tures from the ‘archetype’ form are seldom such as to interfere with the manifestation of its fundamental idea. Thus even in the Cirrhipeds (Figs. 4, 5), which constitute its most aberrant group, the Molluscoid charac- ters are superficial only, whilst the prevalence of the Articulated type through every part of the internal organization is at once revealed by anatomical research.—The body of every Articulated animal is composed. of a succession of segments arranged longitudinally; the division being usually indicated externally by a differentiation in the consistence of the tegumentary skeleton, and by the repetition of the appendages (where such exist) which each segment bears. There is a manifest predominance, in the greater part of the series, of the organs of animal life over those of organic or vegetative life; for the apparatus which is subservient to the locomotive powers, occupies, in all the higher Articulata, a very prominent position; and it is kept in a state of high activity under the guidance of senses of remarkable acuteness. As it is by the external skeleton alone, that fixed points can be afforded for the attachment of the muscles and for the fulcra of the levers by which motion is given to the body, the degree of its consolidation generally corresponds with the development of the locomotive apparatus; the chief exceptions being presented by those cases in which (as in the common Crab) there is an extraordinary development of a solid zest for the purpose of protection only.—In some of the lowest grades of this type (belonging to the group of Entozoa), the successive segments which are indicated externally by constrictions of the body, so exactly repeat each other, that each can maintain an independent exist- ence, and can reproduce the entire body by gemmation ; so that, being indefinite in number, and physiologically distinct, they are nearly on the same footing with the independent zooids of a Botryllus. Tt is interest- ing to remark, moreover, that among these the Molluscous nature so far predominates, that there is scarcely a trace of locomotive organs, and the integument is soft throughout, so that the segmental division is chiefly indicated by the repetition of the organs of nutrition and reproduction. Ascending to the Nematoid Worms (Fig. 52), we find the segments united into one continuous body, not only externally but internally ; and in these we find some of the leading features of the Articulated type dis- played in their simplest condition. The body does not externally pre- sent any true annulations, but a transverse wrinkling of the integument is generally perceptible; and the muscular fibres which line this integu- ment are disposed in regular transverse bands, which are often sufficiently developed to endow these animals with a power of active movement. The oral orifice (a), situated at one extremity of the body, leads to an
60 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT.
alimentary canal (c,c,c), which runs through the axis of the cylinder to the anal aperture (d) at its op- posite extremity; presenting in its course scarcely any differen- tiation of parts into cesophagus, stomach, or intestine, and no distinct glandular appendages. There are two pairs of longitu- dinal vessels, one of which is supposed to be arterial and the other venous; but there are not yet any special respiratory or- gans, the low degree of aeration which their circulating fluid requires, being still attained through the intermediation of the soft integument. The ner- vous system chiefly consists of a pair of longitudinal trunks which run along the ventral region of the body, and are connected with a pair of very minute gan- glia on each side of the cesopha- gus; but no distinct ganglionic enlargements present themselves in the course of these trunks; nor is there the least indication of the presence of organs of, special sense. The two sexual | divisions of the generative ap- | paratus, which are not only combined in the lower Entozoa Strongylus gigas (female) laid open to show its inter- in the same individuals, but nic nal structure ;—a, mouth; b, cesophagus ; c, 6,0, intes- even repeated through their suc- tinal canal; d, anus; e, e, e, ovary ; f, uterine dilata- ‘ i tion; g, narrow oviduct ; h, its orifice. cessive segments, are here as- signed to distinct individuals; and the form alike of the ovary (e, e, e) and testes usually participates in the general elongation, which may be considered as resulting (like that of the digestive and vascular apparatus) from the ‘fusion’ of the parts proper to each segment.—Lence the differentiation of parts is here almost at its minimum ; for there is nothing that can be properly termed a head; and with the exception of the two terminal segments and that which contains the genital orifice, there is scarcely one that differs from its fellows in any essential particular of external configuration or internal structure. 46. In the class of Annelida we meet with a decided advance in the degree of specialization of the several organs, both of Animal and of Vege- tative life, without, as yet, any marked differentiation of the regions of the body, which is usually composed of a large number of segments pre- senting a close external resemblance (Fig. 53, a). In the lower forms, which nearly approximate to the higher Entozoa, the segmental division is obscured by the general softness of the integument, and by the absence
GENERAL VIEW OF ANIMAL KINGDOM.—ANNELIDA. 61
of locomotive or branchial appendages; but in proportion as these are developed, and as the integument becomes consolidated, the anmulose
character is made obvious, by the alternation of firm rings with soft intervening membrane. So, again, in the lower forms of this group, the head is scarcely more differentiated from the body than it is in the Nematoid Entozoa; whilst in the higher, it is furnished with proper eyes and antenne, and the mouth is provided with an elaborate apparatus, consisting either of one, two, or three pairs of jaws, or of an evertible proboscis, for the prehension and re- duction of food (Fig. 53, B, c). Where locomotive appendages are developed, as in the tribe of Wereids, they are almost precisely repeated from one end of the body to the other; and this is the case, also, with the respi- ratory organs, save where the condi- tions of existence require that these should be especially developed from some particular region, as is the case with the cephalic branchial tufts of the Sabella (Fig. 144); in fact, the re- spiratory and locomotive organs in this group are by no means com- pletely differentiated from one an- other, each being subservient in great- er or less degree to both purposes. These appendages vary in different genera; but we usually find them based on two fleshy tubercles on either side of each segment, which are termed the ‘dorsal oars’ (Fig. 53, D, A’), or the ‘ ventral oars’ (B’), ac- cording as they project from the upper or under half of the segment. Each oar commonly possesses, attach- ed to the base of its tubercle, a long soft cylindrical appendage, or cirrhus (D, æ, e), homologous with the anten- niform appendages of the cephalic segment; whilst its summit bears a tuft of setæ or bristles, which serve as the instruments of locomotion when
A A
A, Nephthys Hombergii; B, its proboscis; ©, the same laid open, to show the horny teeth it contains; D, one of the feet, showing 4’, dorsal oar; B’, ventral oar ; æ, dorsal cirrhus ; b, mem- branous lobe of dorsal oar; c, tentaculiform ap- pendage; d, branchial appendage; e, ventral cirrhus ; f, membranous lobe of ventral oar.
the animal is crawling over solid surfaces. In the ordinary Nereids, more- over, each oar has also a membranous lobe (D, b, 738 which is its instrument of propulsion in water. All these appendages, together with the pre-
62 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT.
hensile and tactile organs which are developed around the mouth of many species, are probably subservient in greater or less degree to the aeration of the nutritive fluid transmitted to them; but a more special respiratory organ (d), consisting either of a flattened vesicle or of a branching tuft, is developed from the under side of the dorsal oar; this is not usually repeated, however, through the entire length of the body. 47. Notwithstanding the elaborateness of the buccal apparatus, the alimentary canal still retains much of its primitive simplicity, being an almost straight tube without any obvious division into cesophagus, stomach, and intestine; it is, however, furnished with cecal appendages of various kinds, apparently ofa glandular character. The condition of the sanguiferous system in this group is very peculiar; for whilst in the higher Articulata, as in the Mollusca, it communicates freely with the general cavity of the body, so that one and the same fluid circulates through both, the blood-vessels here form a completely closed circuit, as in the Echinodermata; and the general cavity is occupied by a true ‘chylaqueous’ fluid, which is kept in pretty constant motion by the movements of the body. As in the Echinodermata, too, there appear to be distinct provisions for the aeration of the blood-proper and for that of the chylaqueous fluid ; for whilst the latter penetrates into the locomotive, prehensile, and tactile appendages, and is freely exposed through their parietes to the surrounding medium, it is the blood alone which is trans- mitted to the special respiratory organs. The generative apparatus in the Nereids, as in the Cestoid Entozoa, is completely repeated in each © successive segment; but in the Zerricole (Karth-worms, &c.) it is more localized, having only a single external orifice, as in the Nematoid worms ; and although both male and female organs are developed in the same individual, yet the congress of two is necessary, as in the terrestrial Gasteropods, each impregnating the other. The multiplication of parts by gemmation takes place to a great extent among the Annelida; for it is in this way that the extraordinary elongation of the body is effected, which is characteristic of many species; the number of segments being thus augmented, from the single one which presents itself in the earliest stage of development, to four or five hundred. And there are certain species in which the body spontaneously divides itself into parts, each of which becomes a complete organism; whilst in others, portions of the body endowed with locomotive and sensory organs, but unprovided with a nutritive apparatus, are budded-off, for the purpose of dispersing the products of the generative act, with which they are loaded. The nervous system here presents a far higher development than in the Nematoidea, as might be expected from the presence of distinct organs of sense, and of locomotive appendages; for the double ventral cord is now studded with ganglia, disposed at regular intervals, and equal in size, thus conforming to the general similarity of the segments themselves ; whilst the cephalic ganglia exceed the rest in size, and acquire a directing power over them, in a degree proportionate to the development of the organs of special sense.—It is worthy of note, that even in this group, which, as a whole, is characterized by its locomotive activity, there is an entire order adapted to lead the sedentary life of Molluscous animals; some of them even forming shelly tubes, which can scarcely be distin- guished from those of certain Gasteropods. Various modifications of
GENERAL VIEW OF ANIMAL KINGDOM.—MYRIAPODA. 63
structure are required for this purpose, especially the concentration of the respiratory apparatus about the head; but it is to be remarked, that these special modifications begin to make their appearance at an advanced stage of development,—these Tubicolæ, up to a certain point, not only leading the errant lives of the Nereids, but exhibiting an almost exact conformity to their type of conformation.
48. Disregarding the more aberrant forms of the Articulated sub- kingdom, and restricting ourselves to that tolerably regular series which will best illustrate the principle of progressive differentiation, we now come to the class Myriapoda, in which