GENERAL OUTLINE THE ANIMAL KINGDOM. LONDON : PRINTED BY SAMUEL BENTLEV, Bangor House, Shoe Lane. GENERAL OUTLINE THE ANIMAL KINGDOM, MANUAL OP COMPARATIVE ANATOMY. BY THOMAS RYMEB JONES, F.Z.S. PROFESSOR OP COMPARATIVE ANATOMY IN KING'S COLLEGE, LONDON} FULLERIAN PROFESSOR OF PHYSIOLOGY TO THE ROYAL INSTITUTION OF GREAT BRITAIN, &C. &C. ILLUSTRATED BY THREE HUNDRED AND THIRTY-SIX ENGRAVINGS. LONDON: JOHN VAN VOORST, ], PATERNOSTER ROW; M.DCCC.XLT. ., K- To RICHARD OWEN, ESQ., F.R.S. &c. &c. &c. THE FOLLOWING PAGES ABE INSCRIBED BY HIS SINCERE FRIEND, THE AUTHOR. M367753 PREFACE. THE object of the writer of the present work has been twofold ; first, to lay before the Naturalist a complete view of the organization and physiological relations of every class of living beings ; and secondly, to offer to the Ana- tomical Student a succinct account of the structure and developement of the vital organs through all the modifi- cations that they present in the long series of the animal creation. Extensive indeed is the field of study that offers itself to the zealous cultivator of Natural History, if he would step beyond the limits that not unfrequently too narrowly circumscribe his views of animated nature. Needlessly to multiply specific distinctions, or to arrange trivial groups of external forms in imaginary circles, is an easy occupa- tion to the superficial Zoologist, — easier perhaps than it would be to one more deeply conversant with the anato- my and intimate composition of the creatures thus sum- marily classified; and, accordingly, it is by no means un- common in the present day to see the most strenuous supporters of this or that theory resolutely shutting their eyes against all evidence deducible from the laws of phy- siology, and stoutly maintaining that outward form is in itself enough for the purpose they have in view, namely, the establishment of some favourite principle or fancied Ylll PREFACE. analogy. Discussions of this kind have been carefully avoided in the following pages : to collect from every available source the ascertained facts connected with anatomical structure, and to arrange the grand divisions of the animal world in conformity with progressive de- velopement as we advance from humbler to more com- plex types of organization, has been the chief aim of the Author; and, if he has at all succeeded in divesting so important a subject of those technicalities which not un- frequently impede the progress of the general reader, his labour has not been thrown away. To the Physiologist little apology is necessary for the production of a work intended to exhibit at one view the leading facts of Comparative Anatomy. In this country, unfortunately, so extended a view of Nature is considered as being by no means essential to a correct intelligence of the laws of animal life, and as a branch of professional education has been hitherto completely neglected. Our illustrious countryman John Hunter entertained a dif- ferent opinion. May the fire which he first kindled amongst us, and which has since his time been kept alive by the fostering care of that College, the depository of his invaluable works, soon burst forth, and irradiate the realms of science as brightly as the great founder of Comparative Physiology foresaw that it might ! PHYSIOLOGICAL INDEX. NERVOUS SYSTEM. General classification of Animals, in accordance with the condition of ACRITA ........ NEMATONEURA ...... . Anatomy of the nervous system in Linguatula tamioides ..... Ascaris lumbricoides . . . . . . Notommata clavulata ..... Actheres percarum ...... Asterias ....... Echinus ....... Holothuria ...... Siponculus ....... HOMOGANGLIATA ....... Anatomy of the nervous system in Hirudo medicinalis . . . . . . Myriapoda ...... Insecta ....... Changes that take place in the condition of the nervous system during the metamorphosis of Insects Crustacea ....... Motor and Sensitive tracts in the nervous centres of Homogangliata HETEROGANGLIATA . . . . . . — . Anatomy of the nervous system of Cirrhopoda . . . . . . . Brachiopoda ...... Tunicata . . Conchifera ...... .Gasteropoda ....... Pteropoda ...... Cephalopoda ....... Nautilus Pompilius ..... VERTEBRATA . ...... Anatomy of the nervous system of Fishes ....... Reptiles ....... Birds Mammalia SENSE OF TOUCH in Polyps in Holothuria in Siponculus in Leech ORGANS OF THE SENSES. Sense of touch in Insects in Crustacea Tentacula of Gasteropoda Pteropoda Page Sec. 6— 8 6— 8 99—132 101—135 103—140 124—163 132—173 158—199 171—211 178—219 183—225 184—227 198—240 229—273 270—312 303—349 336—372 340—375 351—387 355—391 367_400 372—408 391—425 415—452 427—468 457—499 457—500 484—520 521—557 576—633 607—675 691—796 22— 28 178—219 180—181 199_240 225—267 274—317 343—378 402—439 425—466 X PHYSIOLOGICAL INDEX, Tactile organs of Cephalopoda Sense of touch in Reptiles in Birds in Mammalia SENSE OF TASTE in Insects ..... in Crustacea ..... in Cephalopods ..... in Fishes ..... in Reptiles ..... in Birds ..... in Mammals ..... SENSE OF SMELL in Insects . . • in Crustacea ..... in Nautilus Pompilius .... in Fishes ...... in Reptiles . . in Birds ...... in Mammals ..... SENSE OF VISION. Red specks observable in Acrita . in Rotifera in Lamproglena pulchella Eyes of Leech . Simple ocelli of Insects Compound eyes of Insects . Eyes of Crustacea .... the Scallop (Pecten) . Snail .... other Gasteropoda Nautilus Pompilius Cuttle-fish . Fishes .... Reptiles . - . Birds .... . — Mammalia . . . . . SENSE OF HEARING in Insects in Crustacea . in Cuttle-fishes ..... in Fishes ...«•• in Reptiles ..... in Birds . . . • • in Mammalia ..... Page Sec. 460—501 584—645 608—676 703—825 275—318 343_378 462—503 510—544 555-611 608—677 673—752 275—319 343_378 463-504 521—558 578—633 608—678 692—800 64— 92 124—162 134—177 199_241 276—321 277—321 343—379 391—426 402—439 416—452 464—506 466—507 522—560 578—634 609—679 695—805 275—319 344—380 470—512 526—568 580—638 614-684 699—819 LOCOMOTIVE ORGANS. Condition of muscular system in Acrita Coelelmintha Bryozoa Rotifera Epizoa Suckers of Star-fishes Suckers of Echini Spines of Echini Suckers of Holothuria Muscular system of Siponculus Locomotive organs of the Leech . . Earthworm Setae of Dorsibranchiate Annelidans 6— 8 102—139 111—147 121—159 132—172 149—190 163—204 163—205 173—214 180—221 191—234 201—224 213—258 PHYSIOLOGICAL INDEX. Feet of Julus ..... Scolopendra . Legs of Insects ..... Mechanical structure of the feet of Insects Wings of Insects ..... Muscular system of Insects . . Spinning organs of Arachnidans Locomotive organs of Crustacea Muscles of Cirrhopoda .... Arms of Brachiopoda . Mantle of Ascidia ..... Foot of Conchifera . Byssus of Mussel and Pinna Apparatus for opening and closing the shells of Conchifera Muscular system of Snail .... Locomotive organs of Pteropoda Tentacula and suckers of Cuttle-fishes Sails (so called) of Argonaut . . Arms and float of Nautilus Pompilius . . Locomotive apparatus of Argonaut Fins and muscular system of Fishes Limbs of Reptiles . Muscular system of Reptiles Locomotion of Birds . . . • • Muscular system of Mammalia XI Page Sec. 225—268 228—272 240—282 241—283 246—287 250—292 316—363 319—364 356—392 369—402 381—415 383—417 384—418 395_430 424—465 431—474 435—477 437—479 443—485 500—537 542—590 555—610 591—657 660—740 SKELETONS OF INVERTEBRATA. Horny and calcareous framework of Sponges Shells of Infusoria .... Polyparies of Polyps ...... Fungidae .... . Cortical Polyps ..... Tubiporidae ..... Sertularidae . ..... Internal plates of Velella and Porpita Cells of Bryozoa ...... Shells of Rotifera ..... Skeletons of Echinodermata. Crinoidae ...... Asteridae ...... Echinidae ...... Skeletons of Homogangliata ..... Myriapoda Insecta . Arachnida ..... Crustacea ...... Structure and growth of the shells of Mollusca : Cirrhopoda . Brachiopoda ..... Tunicata ...... Conchifera ..... Gasteropoda ...... Cephalopoda ..... First appearance of an internal skeleton (Endo-skeleton} 13— 16 53— 74 30— 42 18— 21 29— 40 34— 48 45— 63 69— 97 110—146 118—155 136—180 147—189 160—202 184—228 224—266 238—279 306—351 319—364' 352—389 363—398 369—402 384—419 418—458 440—482 439—481 SKELETONS OF VERTEBRATA. a. Cuticular skeleton, or Exo-skeleton. Dermo-skeleton of Fishes Growth of hair and other epidermic appendages Horns of the Deer 506—541 687—788 688—790 Xll PHYSIOLOGICAL INDEX. b. Osseous skeleton, or Endo- skeleton : General view of the skeleton of Vertebrata Osteology of Fishes Reptiles Birds Mammalia . Page Sec. 477—518 489—522 544—593 592—658 633—710 NUTRITIVE SYSTEM. DIGESTIVE ORGANS OF ACRITA. Sponges ..... Fungiae ...... Alcyonium ..... Hydra viridis ..... Cortical Polyps .... Tubipora musica .... Sertularidae ..... Actinia ..... Polygastrica .... Acalephae ..... Hydatid Trichina ..... Taenia Distoma ..... Planaria ..... Diplozoon • . . Echinorynchus .... DIGESTIVE ORGANS OF NEMATONEURA. Linguatula taenioides .... Ascaris lumbricoides . . Bryozoa ..... Rotifera ..... EPIZOA. Adheres percarum .... Lamproglena pulchella ECHINODERMATA . Asterias . Echinus ..... Holothuria ..... Siponculus ..... DIGESTIVE ORGANS OF HOMOGANGLIATA. ANNELIDA. Leech Earthworm .... Dorsibranchiata ..... Tubicola ... . MVRIAPODA. Julus ...... Scolopendra .... INSECTA. Mouths of Insects .... Alimentary canal of Insects of Arachnid a CRUSTACEA 15— 17 18— 21 27— 37 23— 29 34— 46 35— 48 45— 64 40— 55 56— 79 71— 99 81—110 83—114 84—115 87—118 89—119 92—122 95—126 100—134 103—141 108—145 122—160 131—171 134_177 151—192 166—207 174_215 180—222 192—235 203—246 215—260 221—265 226—269 229—274 254—295 260—301 310—356 328—367 PHYSIOLOGICAL INDEX, Xlll DIGESTIVE SYSTEM OF HETEROGANGLIATA. Cirrhopoda Brachiopoda Tunicata Conchifera . Gasteropoda. Snail Mouths of Gasteropoda . Alimentary canal, &c. Pteropoda Cephalopoda DIGESTIVE SYSTEM OF VERTEBRATA. FISHES. Teeth of . Digestive apparatus of REPTILES. Teeth of ... Alimentary system of BIRDS . MAMMALIA. Teeth of ... Alimentary apparatus . , Page Sec. 356—393 365—399 370—405 378—413 395—430 410—445 413—448 424—466 445—487 510—544 515—545 556—611 561—619 597—664 663—744 677—759 RESPIRATORY AND CIRCULATORY SYSTEMS. Rotifera ........ 125—164 Asterias ....... 155 — 195 Echinus ........ 170—209 Holothuria ....... 175 — 216 Siponculus . . . . . . . ___ 181 — 223 Leech ........ 195—237 Earthworm ........ 204—247 Dorsibranchiate Annelidans ..... 217 — 262 Insects . . . . . . . . 264—306 Arachnidans ....... 311—358 Crustacea ........ 329 — 368 Cirrhopoda ....... 357—394 Brachiopoda . . . . . . . 366—400 Tunicata ....... 370 403 Oyster ........ 378—412 Snail 397—433 Gasteropoda ....... 403 — 440 Pteropoda ....... 427 — 467 Cephalopoda ........ 451 494 Fishes ........ 517—553 Reptiles ........ 564—626 Birds ........ 602—671 Mammalia ........ 682 — 778 GENERATIVE SYSTEM. FIISSPAROUS GENERATION in Polygastrica in Annelida . GEMMIPAROUS GENERATION in Sponges in Fungia in Hydra viridis . 59— 85 211—254 16— 19 19— 23 25— 33 XIV PHYSIOLOGICAL INDEX. GENERATIVE SYSTEM IN Page Sec. Tubipora ....... 36— 49 Acalephae .... 78—107 Sterelmintha ...... . 82—111 Ccelelmintha. Ascaris lumbricoides ..... . 105—142 Hermaphrodism of Syngamus trachealis Rotifera ..... 106—144 . 127—166 Epizoa ....... 133_174 Asterias ....... . 158—198 Echinus ..... 172—212 Holothuria ..... . 178—218 Siponculus Leech . . 183—226 . 200—242 Earthworm 207—249 Nais . 209—252 Dorsibranchiate Annelidans 219—263 Myriapoda ... . Insects ....... . 230—275 279—323 Arachnidans . . ... . 315—361 Crustacea . , . . 344_381 Cirrhopoda . . . 357—395 Brachiopoda Tunicata ... . 367—401 . 403—407 Conchifera ...... 393—427 Snail . 399—435 Gasteropoda 417—453 Pteropoda ... Cephalopoda ...... . 428—470 472—513 T1- 1 Fishes ....... . 530—576 Reptiles ....... 585—648 . 616—687 Mammalia • • • . • • 706—833 LYMPHATIC SYSTEM. In Fishes ....... . 517—552 In Reptiles ...... In Birds ...... 563—624 . 602—670 In Mammalia ... • 682—776 URINARY SYSTEM. 520—555 . 584—646 In Birds . • 615—685 In Mammals . 704—831 DEVELOPEMENT OF THE EMBRYO. In Adheres percarum Ova of Earthworm Metamorphoses of Myriapoda Tnfifcts 133—175 . 209—251 227—271 . 288—336 Crustacea 348—386 . 361—397 475—516 . 539—589 572—630 . 620—697 707—834 . 714—839 728—858 Embryo of Cuttle-fish Metamorphosis of the Tadpole Changes in vascular system of Tadpole Developement of the chick in ovo • Anatomy of the ovum of Ornithorynchus paradoxus Anatomy of marsupial ovum Developement of the placental fetus GENERAL INDEX. CLASSIFICATION OF THE ANIMAL KINGDOM. ACRITA. Page Sponges ........ 12 Polyps . ..... 17 Polygastrica . . * . . . .50 Acalephae ........ 64 Sterelmintha ...... 79 NEMATONEURA . .... 99 Coelelmintha ........ 99 Bryozoa . . . . . . . . 107 Rotifera . . . . . . .117 Epizoa .... ... 128 Echinoderraata . . . . . . . 135 HOMOGANGLIATA .... . . 184 Annelida . . . . . — . .188 Myriapoda ........ 224 Insecta . . . . . .- . 231 Arachnida ........ 306 Crustacea .... ... . 319 HETEROGANGLIATA . . 351 Cirrhopoda . . . . . . 352 Brachiopoda ........ 362 Tunicata ........ 368 Conchifera ........ 375 Gasteropoda ....... 394 Pteropoda . . . . . . . . 423 Cephalopoda ....... 430 VERTEBRATA . .... .476 Fishes ........ 488 Reptiles ........ 537 Birds .... .... 591 Mammalia . . . . . . .632 A GENERAL OUTLINE OF THE ANIMAL KINGDOM. CHAPTER I. ON CLASSIFICATION. (1). FROM the earliest periods to the present time, the great desideratum in Zoology has been the establishment of some fun- damental system of arrangement, which, being universal in its application, should distribute the countless beings surrounding us into natural groups or divisions, such as might be subdivided into classes, orders, and genera, by obvious differences of structure in the tribes composing them, and thus enable the Zoologist at once to indicate the position which any unknown animal ought to occupy in the scale of existence, and its relations with other creatures. (2.) Aristotle, the father of our science, was the first who at- tempted a scientific division of the animal world ; * the outlines of his system were rude in proportion to the necessarily limited knowledge at his disposal, although his efforts were gigantic, and still excite our warmest admiration. This acute observer ad- mitted but two great sections, in one or other of which all known beings were included, the highest comprehending creatures pos- sessed of blood, (i. e. red blood,) corresponding to the vertebrata of modern authors ; the lowest embracing animals which in his view were exsangueous, or provided with a colourless fluid instead of blood, and corresponding to the invertebrata of more recent Zoologists. (3.) Linnaeus, like Aristotle, selected the circulatory system as * Historia Animalium. 2 ON CLASSIFICATION. the foundation of his arrangement,* dividing the animal creation into three great sections, characterized as follows : I. Animals possessed of warm red blood, and provided with a heart containing four compartments, viz. two auricles and two ventricles. Such are the mammalia and birds. II. Animals with red cold blood, their heart consisting of but one auricle and one ventricle, as he believed to be the case in reptiles and fishes. III. Animals possessed of cold white sanies instead of blood, having a heart consisting of a single cavity which he designates an auricle : under this head he includes insects and all other inverte- brate animals, to which latter he gives the general name of vermes, worms. We shall not in this place comment upon the want of anatomi- cal knowledge conspicuous in the above definitions, or the insuffi- cient data afforded by them for the purposes of Zoology. The appa- ratus of circulation, being a system of secondary importance in the animal economy, was soon found to be too variable in its arrange- ment to warrant its being made the basis of zoological classification, and a more permanent criterion was eagerly sought after to supply its place. (4.) Among the most earnest in this search was our distinguished countryman John Hunter, who, not satisfied with the results ob- tained from the adoption of any one system, seems to have tried all the more vital organs, tabulating the different groups of animals in accordance with the structure of their apparatus of digestion, of their hearts, of their organs of respiration, of their generative organs, and of their nervous system, balancing the relative im- portance of each, and sketching out with a master hand the outlines of that arrangement since adopted as the most natural and satisfactory. •(• The result of the labours of this illustrious man cannot but be of dee*p interest to the zoological student, and accordingly an epitome of his ideas upon the present subject is here concisely given. The apparatus of digestion appears to be among the least efficient for the purpose of a natural division; as the separation * Systema Naturae Vindobonae, 1767. Thirteenth Edition. t Descriptive and illustrated Catalogue of the Physiological series of Comparative Anatomy, contained in the Museum of the Royal College of Surgeons in London, — Vol. III. Fart I.— 1835. ON CLASSIFICATION. 3 of animals into such as have a simple digestive cavity, receiving and expelling its contents by the same orifice, and such as have an aperture for the expulsion of the contents of the alimentary canal distinct from that by which food is taken into the stomach, is by no means of practical utility, although this circumstance, as we shall afterwards see, has been much insisted upon. Hunter's arrangement of the animal kingdom in conformity with the structure of the heart, was a great improvement upon that of Linnseus, founded upon the same basis. He divides in this manner all animals into five groups. I. Creatures whose hearts are divided into four cavities — Mam- malia and Birds. II. Those having a heart consisting of three cavities — Rep- tiles and Amphibia.* III. Animals possessing a heart with two cavities — Fishes and most Mollusca. IV. Animals whose heart consists of a single cavity — - Articu- lated Animals. V. Creatures in which the functions both of stomach and heart are performed by the same organ, as in Medusce. We shall pass over Hunter's sketches of arrangements founded on the respiratory and reproductive organs, as offering little satis- factory ; but the researches of this profound physiologist upon the employment of the nervous system for the purpose of zoological distribution, did much to approximate a more natural method of classification, afterwards carried out with important results. (5.) The appearance of the " Animal Kingdom distributed in accordance with its organization" of Cuvier, formed a new and im- portant era in Zoology. In this we find all creatures arranged in four great divisions, VERTEBRATA, MOLLUSCA, ARTICULATA, and R ADI AT A. These divisions, with the exception of the first, are named from the external appearance of the creatures composing them, nevertheless the three first are defined by characters exclu- sively drawn from their internal organization, the arrangement of the nervous system being essentially the primary character of dis- tinction, and have been found to be strictly natural ; whilst the last division, characterized by the appellation of R ADI AT A, in the formation of which the structure of the nervous system has * For the important discovery that the heart of the Amphibia is divided into three cavities, instead of being composed of a single auricle and ventricle, we are indebted to Professor Owen. Vide Zool. Trans. Vol. I. 4 ON CLASSIFICATION. been allowed to give place in importance to other characters of secondary weight, obviously embraces creatures of very dissimilar and incongruous formation. The VERTEBRATA are distinguished by the possession of an internal nervous centre or axis, composed of the brain and spinal cord, which is enclosed in an osseous or cartilaginous case, and placed in the median plane of the body, giving off symmetrical nerves, which are distributed to all parts of the system. This general definition indicates a large division of the animal world, which, by secondary characters drawn from the structure of their organs of respiration and circulation, is separable into mammals, birds, reptiles, amphibia, and fishes. The MOLLUSCA have a nervous system constructed upon a very different type, and do not possess any vertebral column or articu- lated skeleton. The nervous centres consist of several detached masses placed in different parts of the body, without regularity of distribution or symmetrical arrangement ; and the entire group is obviously natural, although Cuvier has ranged in it some creatures which, in the structure of their nervous system, differ essentially from those comprised in his own definition. The class of ARTICULATED ANIMALS is likewise well cha- racterized by the nervous system, which, in all the members of it, is composed of a double series of ganglia or masses of neurine, arranged in two parallel lines along the abdominal surface of the body, united by communicating cords, and from which nerves are given off to the different segments of which the body consists. But the fourth division of Cuvier, namely, that of ZOOPHYTES or RADIATED ANIMALS, is confessedly made up of the most hete- rogeneous materials, comprising animals differing in too many important points to admit of their being associated in the same group ; and the efforts of subsequent Zoologists have been mainly directed to the establishment of something like order in this chaotic assemblage. (6.) The evident relation which the perfection of the nervous system bears to that of animal structure, and the success of Cuvier in selecting this as the great point of distinction in the establish- ment of the higher divisions of the animal kingdom, necessarily led succeeding naturalists still to have recourse to this important part of the economy in making a further subdivision of the Radiata of Cuvier. In some of the radiated forms, indeed, nervous filaments are distinctly visible, and such are among the ON CLASSIFICATION. 5 more perfectly organized of the group ; these, therefore, have been classed by themselves, and designated by Mr. Owen the NEMATONEUROSE* division of the animal world ; while those which are apparently without the least trace of distinct nervous matter, have been formed by Mr. M'Leay into a group by them- selves, to which he has given the denomination of ACRITA.J- (7.) There can be no doubt that the nervous matter must be regarded as the very essence or being of all creatures, with which their sensations, volition, and capability of action are inseparably connected ; and such being the case it is a legitimate inference, that the capacities and powers of the several tribes are in im- mediate relation with the developement and perfection of this supreme part of their organization, and their entire structure must be in accordance with that of the nervous apparatus which they possess. The nature of the limbs and external members, the existence or nonexistence of certain senses, the capability of loco- motion, and the means of procuring food, must be in strict cor- respondence with the powers centred in the nervous masses of the body, or in that arrangement of nervous particles which represents or replaces them. Granting the accuracy of the above view, it is obvious, that if exactly acquainted with the structure and elaboration of the nervous apparatus in any animal, we might to a great extent pre- dicate the most important points in its economy, and form a tolerably correct estimate of its powers and general conformation. But, unfortunately, such knowledge is not always at our disposal : in the lower forms of the animal world especially, we are far from being able to avail ourselves of such a guide, and it will probably be long ere our improved means of research permit us to apply to practice the views which Physiology would lead us to adopt. The grand divisions of the animal kingdom, grounded upon the principal varieties in the arrangement of the nervous system, we shall, however, proceed to consider, leaving to future occasions those comments which a consideration of the structure of par- ticular groups will force upon our notice. , a thread ; N-upov, a nerve. t «, priv.; notva, to discern. ON CLASSIFICATION. 1st Division. — ACRITA* (M'Leay); Cryptoneura, (Rudolphi)-f- Protozoa,]. Oozoa.§ (8.) In animals belonging to this division, no nervous filaments or masses have been discovered, and the neurine or nervous matter is supposed to be diffused in a molecular condition through the body, mixed up with the gelatinous parenchyma of which they consist. Possessing no brain or central mass, to which external impressions can be transmitted, or nervous filaments calculated to conduct sensations to distant points of the system, or associate muscular movements, they are necessarily incapable of possessing those organs which are dependent upon such circumstances ; instruments of the external senses are therefore totally wanting, or their ex- istence at least is extremely doubtful ; the contractile molecules of their bodies are not as yet aggregated into muscular fibre. The alimentary apparatus consists of canals or cavities, permeating the parenchyma of the body, but without distinct walls, as in the higher divisions, where it floats in an abdominal cavity. The vascular system, where at all perceptible, consists of reticulate channels, in which the nutrient fluids move by a kind of cyclosis. Their mode of reproduction is likewise conformable to the diffused state of the nervous and muscular systems ; not only are most of them susceptible of being multiplied by mechanical division, but they generate by spontaneous fissure, as well as by gemmae, ciliated gemmules, and true ova. Many appear to be made up of a repetition of similar parts, forming compound animals of various forms, and different degrees of complexity. In this division are included 1. Sponges. 2. Polyps. 3. Polygastric animalcules. 4. Acalephse. 5. Parenchymatous Entozoa or Sterelmintha. * Horae Entomologicae, Vol. I. Part II. page 202. We adopt the term, however, according to its improved application by Mr. Owen, viz. to the exclusion of the higher organized Polyps and Entozoa, and the admission of part of the Radiata of Macleay. t Beytriige sur Anthropologie. 1812. J U^uros, first ; 2*>ov, animal. § '(lov, an egg ; 2&J«v, animal, so called by Carus, because they resemble the eggs or rudiments of more perfect forms. ON CLASSIFICATION. Second Division. — NEMATONEURA (Owen).* (9.) In the second division of the Radiata of Cuvier, the nervous matter is distinctly aggregated into filaments, and in some cases nuclei of neurine, which may be regarded as rudimentary nervous centres, have been noticed. It is to be lamented, however, that in this most interesting group of animals, in which we have the first developement of most of the organs subservient to the vital functions, the extreme minuteness of some genera, and the diffi- culty of distinctly observing the nervous system in the larger species, has prevented our knowledge regarding their organization, in this particular, from being of that satisfactory character which it is to be hoped it will hereafter attain to. Owing to the want or imperfect condition of the nervous centres, the nematoneura are necessarily incapable of possessing external organs of the higher senses, the general sense of touch being as yet the only one of which they are indubitably possessed ; yet in their muscular system they are much more efficiently provided than the acrite orders, as the developement of nervous threads of communi- cation renders an association of muscular actions possible ; and therefore, co-apparent with nervous filaments, we distinguish in the structure of the nematoneura distinct fasciculi of muscular fibre, and powers of locomotion of a much more perfect description. The digestive apparatus is no longer composed of canals merely excavated in the parenchyma of the body, but is provided with distinct muscular and membranous walls, and loosely attached in an abdominal cavity. The circulation of the nutritious fluid is likewise carried on in a separate system of vessels, distinct from the alimentary apparatus, yet still unprovided with a heart, or exhibiting pulsations for the forcible impulsion of the contained blood. The fissiparous mode of reproduction is no longer witnessed, an obvious consequence of the increased complexity of struc- ture, and these animals are for the most part androgynous, or capable of producing fertile ova, without the co-operation of two individuals. Among the nematoneura, therefore, we include * Cyclopaedia of Anatomy and Physiology. Article, ACRITA. 8 ON CLASSIFICATION. 1. Bryozoa, or Polyps, with ciliated arms. 2. Rotifera. 3. Epizoa. 4. Cavitary Entozoa or Ccelelmintha. 5. Echinodermata. The reader will perceive, that this division, however well sepa- rated from the preceding by physiological characters, is, in a zoological point of view, principally composed of groups detached from the members of other orders. The Bryozoa are evidently dismemberments of the family of Polyps, from which they differ in their more elaborate internal organization. The Coelelmintha are more perfect forms of the Parenchymatous Entozoa. The Roti- fera, formerly confounded with the Infusoria, exhibit manifest analogies with the articulated Crustaceans, as in fact do the Epizoa. The Echinodermata alone appear to form an isolated group, properly belonging to the division under consideration. Third Division. — HOMOGANGLIATA (Owen) ; Articulata (Cu- vier)*; Annulosa (Macleay) ; Diploneura (Grant). ~f* (10.) The articulated division of the animal kingdom is charac- terized by a nervous system, much superior in developement to that possessed by the two preceding, indicated by the superior propor- tionate size which the ganglionic centres bear to the nerves which emanate from them. The presence of these central masses of neurine, admits of the possession of external senses of a higher class than could be expected among the Acrita or Nematoneura, and gives rise to a concentration of nervous power, which allows of the existence of external limbs of various kinds, and of a complex muscular system capable of great energy and power of action. The nervous centres are arranged in two parallel lines along the whole length of the body, forming a series of double ganglia or brains, belonging apparently to the individual segments of which the animal is composed. The anterior pair placed invariably in the head above the oesophagus, and consequently upon the dorsal aspect of the body, seems more immediately appropriated to the higher senses, supplying nerves to the antennae, or more special in- struments of touch, to the eyes, which now manifest much com- plexity of structure, to the auditory apparatus where such exists, * The Cirripecla are excluded from the Articulata of Cuvier. -f- The Entozoa and Rotifera are included in the Diploneura of Dr. Grant. ON CLASSIFICATION. 9 and probably to the senses of taste and smell. This dorsal or anterior pair of ganglia, which evidently is in relation with the higher functions of the economy of the creature, is brought into communication with the series of nervous centres placed along the ventral aspect, by means of filaments which embrace the oesophagus, and join the anterior pair placed beneath it ; the whole system may therefore be regarded as a series of independent brains destined to animate the segments of the body in which they are individually placed. Such a multiplication of the central organs of the nervous system, is obviously adapted to the elongated forms of the vermi- form orders, but from the want of concentration which such an arrangement implies, this type of structure is still very inferior in its character. As the articulata become more perfect in their out- ward form, the number of the brains becomes diminished, while their proportionate size increases ; and thus in the carnivorous Insects, Arachnida and Crustacea, they are all united into a few great masses, which, becoming the general centres of the entire system, admit of a perfection in their external senses, a precision in their movements, and an energy of action, of which the detached character of the ganglia in the lower tribes was incapable. (11.) This dependence of the perfection of the animal upon the concentration of the central masses of the nervous system, is strik- ingly proved by the changes perceptible in the number and arrange- ment of the ganglia, during the progress of an insect through the different stages of its existence. In the elongated body of the worm- like caterpillar, each segment possesses its appropriate pair of ganglia, and the consequence of such diffusion of its nervous apparatus, is apparent in its imperfect limbs, its rude organs of sense, its sluggish movements, and general apathy, but as it successively attains to more mature forms of existence, passing through the different me- tamorphoses which it undergoes, the nervous ganglia gradually coalesce, increase in power, as they diminish in number, until in the imago or perfect state, having arrived at the greatest concen- tration compatible with the habits of the insect, we find it endued with new and far more exalted attributes, the organs of its senses are more elaborately formed, it possesses limbs which previously it would have been utterly incapable of wielding, its movements are characterized by their activity and precision, and its instincts and capabilities proportionately enlarged and exalted. The Homogangliate division of the animal world is extremely natural, and includes the following classes : — 10 ON CLASSIFICATION. 1. Cirripeda. 4. Insecta. 2. Annelida. 5. Araclmida. 3. Myriapoda. 6. Crustacea. Fourth Division. — HETEROGANGLIATA (Owen) ; Mollusca (Cuvier)*; Cyclogangliata (Grant). (12.) The characters of this division are well defined, and the irre- gular and unsymmetrical forms of the bodies of most of the genera which compose it, in exact relation with the arrangement of the nervous apparatus. As in the articulata there is a large nervous mass placed above the oesophagus, which supplies the principal organs of sense, but the other ganglia are variously dispersed through the body, although always brought into communication with the supracesophageal portion by connecting filaments. Throughout all the forms, we find a distinct relation between the size and developement of the nervous centres, and the perfection of the animal, indicated by the senses and organs of motion with which it is provided. This division includes 1. Tunicata. 4. Gasteropoda. 2. Conchifera. 5. Pteropoda. 3. Brachiopoda. 6. Cephalopoda. Fifth Division. — VERTEBRATA (Cuvier); Myelencephala (Owen); Spinicerebrata (Grant). (13.) The arrangement of the nervous centres in the highest or vertebrate division, indicates the greatest possible concentration and developement. The ganglionic masses assume a very great pro- portionate size when compared with the nerves which emanate from them, and are principally united into a long chain, denominated the cerebro-spinal axis or cord, which is enclosed in a cartilaginous or bony canal, occupying the dorsal region of the animal. The anterior extremity of the cerebro-spinal axis is made up of those ganglia which are more especially in relation with the principal senses and the higher powers of intelligence, forming a mass denominated, from its position in the skull which encloses it, the encephalon. It is with the increased proportionate developement of this portion, that the intelligence of the animal becomes augmented ; in the lower tribes, the cerebral masses scarcely exceed in size those * The Cirripeda are included in the Mollusca of Cuvier. ON CLASSIFICATION. 11 which form the rest of the central chain of ganglia, but as we advance from fishes towards the higher forms of the vertebrata, we observe them to preponderate more and more in bulk, until at last in man they assume that extraordinary developement adapted to the exalted position which he is destined to occupy. It is in the cerebral ganglia, therefore, that we have the representative of the supracesophageal masses of the articulated and molluscous classes, which, as we have already seen, preside especially over the senses, and correspond in their proportions with the capabilities of the tribes of animals included in those divisions. The spinal cord, as the rest of the central axis of the nervous system of vertebrata is denominated, is made up of a succession of ganglia, in communi- cation with symmetrical pairs of nerves connected with them, and which preside over the generally diffused sense of touch, and the voluntary motions of the body. But besides the cerebro-spinal sys- tem, we find in the vertebrated classes another set of nervous centres, to which nothing corresponding has been satisfactorily identified in the lower divisions ; namely, the sympathetic system, which mainly controls the involuntary movements of the body connected with the vital functions. The vertebrata are further distinguished by the possession of an internal organized skeleton, either composed of cartilage or bone, which is made up of several pieces, and serves as the general support of the frame, forming a series of levers upon which the muscles act. This last division of the animal world embraces the following classes : — 1. Fishes. 4. Birds. 2. Amphibia. 5. Mammalia. 3. Reptiles. Such will be the classification which we shall adopt in the following pages ; and although, perhaps, the definitions of the five great groups may be considered by the scientific reader as some- what scanty, enough, we trust, has been said to render intelli- gible the terms which we shall hereafter have frequent occasion to employ. (14.) A question naturally presents itself in this place which re- quires consideration : — May we expect, as we advance from the lower types of organization to such as are more perfect, to be led on through an unbroken and continuous series of creatures, gradually rising in importance and complexity of structure, each succeeding 12 ON CLASSIFICATION. tribe of beings presenting an advance upon the preceding, and merging insensibly into that which follows it ? A very slight investigation of this matter will convince us of the contrary. Each group, in fact, will be found to present points of relationship with several others, into all of which it passes by connecting species ; as a circle would, at different points of its circumference, touch others placed around it. This, however, will be best illustrated as we proceed. CHAPTER IT. ON SPONGES. Porifera,) Grant — Amorphozoa (Blainville). (1 5.) The great circles to which we may compare the animal and vegetable kingdom, like the smaller circles to which allusion was made at the close of the last chapter, touch each other ; or, in other words, there are certain forms of organization so closely allied to both, that it is difficult to say precisely in which they ought to be included. Such are the sponges, which, although by common consent admitted into the animal series, will be found to be excluded, by almost every point of their structure, from all the definitions of an animal hitherto devised. What is an animal ? How are we to distinguish it as contrasted with a mineral or a vegetable ? The concise axiom of Linnaeus upon this subject is well known, — " Stones grow ; vegetables grow and live ; animals grow, live, and feel." The capability of feeling, therefore, formed, in the opinion of Linnaeus, the great characteristic sepa- rating the animal from the vegetable kingdom ; yet, in the class before us, no indication of sensation has been witnessed ; contact, however rude, excites no movement or contraction which might indicate its being perceived ; no torture has ever elicited from them an intimation of suffering ; they have been pinched with forceps, lacerated in all directions, bored with hot irons, and attacked with the most energetic chemical stimuli, without shrinking or exhibit- ing the remotest appearance of sensibility. On the other hand, in the vegetable world we have plants which apparently feel in 1'ORIFERA. 13 this sense cf the word. The sensitive plant, for example, which droops its leaves upon the slightest touch, would have far greater claims to be considered as being an animal than the sponges of which we are speaking. The power of voluntary motion has been appealed to as exclusively belonging to the animal economy : yet, setting aside the spontane- ous movements of some vegetables, the sponge, rooted to the rock, seems absolutely incapable of this function, and the most micro- scopic scrutiny has failed to detect its existence. The best definition of an animal, as distinguished from a vege- table, which has as yet been given, is, that whereas the latter fixed in the soil by roots, or immersed perpetually in the fluid from which it derives its nourishment, absorbs by its whole surface the nutriment which it requires ; the animal, being generally in a greater or less degree capable of changing its position, is provided with an internal receptacle for food, or stomachal cavity, from which, after undergoing the process of digestion, the nutritious matter is taken up. But in the case of the sponge no such reservoir is found; and in its place we find only anastomosing canals which permeate the whole body, and convey the circumam- bient medium to all parts of the porous mass. The last circumstance which we shall allude to as specially appertaining to the animal kingdom, is derived from the chemical composition of organized bodies. Vegetables contain but a small proportion of azote in their substance, whilst in animals this element exists in considerable abundance, causing their tissues when burned to give out a peculiar odour resembling that of burned horn, and in this particular sponges differ from vegetable matter. (16.) The common sponge of commerce is, as every one knows, made up of horny, elastic fibres of great delicacy, united with each other in every possible direction, so as to form innumerable canals, which traverse its substance in all directions. To this structure the sponge owes its useful properties, the resiliency of the fibres com- posing it making them, after compression, return to their former state, and leaving the canals which they form open, to suck up surrounding fluids by capillary attraction. The dried sponge is, however, only the skeleton of the living animal : in its original state, before it was withdrawn from its native element, every filament of its substance was coated over with a thin film of glairy semifluid matter, composed of aggregated transparent 14 PORIFERA. globules, which was the living part of the sponge, secreting, as it extended itself, the horny fibres which are imbedded in it. The anastomosing filaments which compose the skeleton of such sponges, when examined under a microscope, and highly magnified, appear to be tubular, as represented in fig. 1. c. Many species, although exhibit- ing the same porous structure, have none of the elasticity of the officinal sponge, a circumstance which is due to the difference ob- servable in the composition of their skeletons or ramified frame-work. In such the living crust forms within its substance not only tenacious bands of animal matter, but great quantities of crystallized spicula, sometimes of a calca- reous, at others of a silicious nature, which are united together by the tenacity of the fibres with which they are surrounded. On destroying the softer portions of these skeletons either by the aid of a blow-pipe or by the caustic acids or alkalies, the spicula re- main, and may readily be examined under a microscope : they are then seen to have determinate forms, which are generally in rela- tion with the natural crystals of the earths of which they consist ; and as the shape of the spicula is found to be similar in all sponges of the same species, and not unfrequently peculiar to each, these minute particles become of use in the identification of these bodies.* Crystallized spicula of this description form a feature in the structure of the sponge which is common to that of many vege- tables, resembling the formations called Raphides by botanical writers. Some of the principal forms which they exhibit are de- picted in fig. 1 a b, which likewise will give the reader a general idea of the appearance of the silicious and calcareous sponges, after the destruction of their soft parts has been effected by the means above indicated. The figures d, e, f9 and g, exhibit detached spicula of different forms highly magnified. The most * Savigny (Jules Caesar) Zoologie d'Egypte — gr. fol. Paris, 1809. PORIFERA. 15 convenient method of seeing them is simply to scrape off a few particles from the incinerated sponge upon a piece of glass, which, when placed under the microscope, may be examined with ordinary powers. (17.) On placing a living sponge of small size in a watch-glass or small glass trough filled with sea-water, and watching it attentively, something like a vital action becomes apparent.* The entire surface is seen to be perforated by innumerable pores and aper- tures, some exceedingly minute, opening on every part of its peri- phery ; others of larger dimensions, placed at intervals, and gene- rally elevated upon prominent portions of the sponge. Through the smaller orifices the surrounding water is continually sucked as it were into the interior of the spongy mass, and it as constantly flows out in continuous streams through the larger openings. This con- tinual influx and efflux of the surrounding fluid is produced by an agency not yet discovered, as no contraction of the walls of the canals, or other cause to which the movement may be referred, has ever been detected ; we are as- sured, however, that it is from the currents, thus continually permeating every portion of its substance, that the general mass is nourished. The annex- ed diagram, fig. 2 a, will give the reader an idea of the most usual direction of the streams : the entering fluid rushes in at the countless pores which occu- py the body of the sponge ; but, in its progress through the canals in the interior, becomes directed into more capacious channels, communicating with the prominent larger orifices, through which it is ultimately ejected in equable and ceaseless currents. Organized particles, which ne- cessarily abound in the water of the ocean, are thus introduced into the sponge on all sides, and are probably employed as nutri- ment, whilst the superfluous or effete matter is continually cast out with the issuing streams as they rush through the fecal ori- fices. The growth of the sponge is thus provided for, the living * Dr. Grant, in the New Edinburgh Philosophical Journal, 1827. 16 POHIFERA. gelatinous portion continually accumulates, and, as it spreads in every direction, secretes and deposits, in the form peculiar to its species, the fibrous material and earthy spicula which characterise the skeleton. (18.) From this description of the structure of a sponge, it will be apparent that all parts of the mass are similarly organized : a necessary consequence will be, that each part is able to carry on, independently of the rest, those functions needful for existence. If therefore a sponge be mechanically divided into several pieces, every portion becomes a distinct animal. (19.) The multiplication of sponges, however, is effected in another manner, which is the ordinary mode of their reproduction, and forms a very interesting portion of their history.* At certain seasons of the year, if a living sponge be cut to pieces, the chan- nels in its interior are found to have their walls studded with yel- lowish gelatinous granules, developed in the living parenchyma which lines them ; these granules are the germs or gemmules from which a future race will spring ; they seem to be formed in- differently in all parts of the mass, sprouting as it were from the albuminous crust which coats the skeleton, without the appearance of any organs appropriated to their developement. As they in- crease in size, they are found to project more and more into the canals which ramify through the sponge, and to be provided with an apparatus of locomotion of a description which we shall fre- quently have occasion to mention. The gemmule assumes an ovoid form, fig. 2 B, and a large portion of its surface becomes covered with innumerable vibrating hairs or cilia, as they are de- nominated, which are of inconceivable minuteness, yet individually capable of exercising rapid movements, which produce currents in the surrounding fluid. As soon therefore as a gemmule is suffi- ciently mature, it becomes detached from the nidus where it was formed, and whirled along by the issuing streams which are ex- pelled through the fecal orifices of the parent, it escapes into the water around. Instead, however, of falling to the bottom, as so appa- rently helpless a particle of jelly might be expected to do, the cease- less vibration of the cilia upon its surface propels it rapidly along, until, being removed to a considerable distance from its original, it attaches itself to a proper object, and, losing the locomotive cilia which it at first possessed, it becomes fixed and motionless, and * Professor Grant — loc. cit. ON POLYPS. developes within its substance the skeleton peculiar to its species, exhibiting by degrees the form of the individual from which it sprung. It is curious to observe the remarkable exception which sponges exhibit to the usual phenomena witnessed in the reproduc- tion of animals, the object of which is evident, as the result is admirable. The parent sponge, deprived of all power of movement, would obviously be incapable of dispersing to a distance the numerous progeny which it furnishes ; they must inevitably have accumulated in the immediate vicinity of their place of birth, without the possi- bility of their distribution to other localities. The seeds of vegetables, sometimes winged and plumed for the purpose, are blown about by the winds, or transported by various agencies to distant places ; but, in the present instance, the still waters in which sponges grow would not have served to transport their progeny elsewhere, and germs so soft and delicate could hardly be removed by other creatures. Instead therefore of being helpless at their birth, the young sponges can, by means of their cilia, row themselves about at pleasure, and enjoy for a period powers of locomotion denied to their adult state. CHAPTER III. ON POLYPS. Zoophytes of old Authors — Phytozoa (Ehrenberg). (20.) IT is not surprising that many members of the extensive family upon a consideration of which we are now entering, should have been regarded by the earlier naturalists as belonging to the vegetable kingdom, with which, in outward appearance at least, numerous species have many characters in common.* Fixed in large arborescent masses to the rocks of tropical seas, or in our own climate attached to shells or other submarine sub- stances, they throw out their ramifications in a thousand beautiful and plant-like forms ; or, incrusting the rocks at the bottom of the ocean with calcareous earth separated from the water which bathes them, they silently build up reefs and shoals, justly dreaded by the navigator, and sometimes giving origin, as they rise to the surface of the sea, to islands which the lapse of ages clothes with * Tournefoit, Institutiones Rei Herbaria*, 4to. 1719. 18 ON POLYPS. luxuriant verdure, and peoples with appropriate inhabitants. Va- rious indeed are the forms which these creatures offer to the zoolo- gist ; and the classification of them, even at the present day, is a subject of much doubt and uncertainty. Without entering fur- ther into the subject of their division into groups and families than is connected with our purpose of examining the main features of their economy, we shall select some of the most marked varieties for description, commencing with the simplest and least elabo- rately formed. (21.) We have already seen that in the Sponges the living portion of the animal was composed of a gelatinous film, which, without any apparent organization, was possessed of the power of extracting nutri- ment from the water around it, of deriving from the same source ani- malized materials and earthy particles, which were deposited within its texture, and used in constructing a porous frame-work or skele- ton ; and, moreover, that the same semifluid parenchyma could de- velope from its substance germs, which became ultimately expanded into other beings resembling that from which they sprung ; we shall therefore be prepared to find, in the class upon which we are enter- ing, like results produced by equally simple means. Among the calcareous structures, derived from the tropical seas, which are usually known by the general terms of Madre- pores, Corals, &c. and which, from the beauty of their structure, form the ornaments of our cabinets, few are more common than those denominated Fun- gise and Meandrinse, — animals belonging to the group Madrephyllicea of systematic zoologists. These masses consist of thin plates or laminse of various dimensions (fig. 3.) disposed in different directions in different species, but in the Fungia Agariciformis, which we have selected as an example, radiating from a common centre, and forming a circular mass resem- bling a mushroom. When living in its native element, every part of the surface of this stony skeleton was encrusted with a film of animal Fig. 3. ON POLYPS. 19 matter, dipping down into the interstices of the plates, and cover- ing the whole frame-work. In the figure, the darker portion indi- cates the living crust ; whilst from the lighter parts it has been re- moved, to show the stony skeleton itself. There are no arms or moving parts adapted to the prehension of food, and no separation of organs adapted to the performance of the vital functions has hitherto been described ; the thin membranous film apparently absorbs the materials of its support from the water of the ocean, and deposits within its substance the calcareous par- ticles which it secretes, moulding them into the form peculiar to its skeleton, which it gradually enlarges as its own extent increases. (22.) The gelatinous investment, however, gives certain dubious indications of vitality, and possesses the power of contracting itself so as to retire between the laminae of its skeleton when roughly handled, and thus conceal itself from injury. Upon the surface of the soft crust are seen a number of vesicles indicated in the figure, which were regarded formerly as rudimentary tentacula, from the circumstance of their being able to contract and vary their dimen- sions ; recent observations however lead to the belief that they are cavities filled with air, and serving an important purpose in the eco- nomy of the creature, — namely, that of preventing it from being turned upside down by the occasional agitation of the ocean, — as in such case the animal has been found by experiment to have no power of restoring itself to its former position, and consequently perishes : these air-vessels may therefore be looked upon as floats, which, ren- dering the upper surface more buoyant than the inferior, materially assist in preventing such an accident ; for, as it lies quite loose and unattached upon the surface of the sand, it is subject to be lifted up from its bed by any sudden roll of the sea, and deposited at a considerable distance from its former place. (23.) The reproduction of fungise is effected by the developement of sprouts or gemmae, which pullulate from the animal substance as buds issue from a plant, and remain for some time fixed to the parent by a species of foot-stalk, which sustains them until they have attained to a considerable size ; the young fungiae being up- wards of an inch in diameter before they become detached. When mature, they separate from the top of the stony peduncle which hi- therto supported them ; and at this time, the skeleton of the young fungia, when divested of its fleshy part, shows a circular opening beneath, through which the radiating plates of the upper surface 20 ON POLYPS. are visible. In a short time a deposit of calcareous matter takes place, which cicatrizes the opening, the marks of which however can be traced for a considerable period, until at length the increase of this secretion continuing with the growth of the animal, entirely obliterates all appearance of its having existed. In the earliest period of its developement, the foot-stalk by which the young is united to the parent, as well as its radiating disc, is entirely enveloped with the soft parts of the animal ; but as the upper portion spreads, and assumes its characteristic form, the pedicle is left naked, and the gelatinous coating extends only to the line where the separation afterwards takes place. (24.) It is generally supposed that the calcareous matter which forms the skeleton of these madrepores is perfectly external to the liv- ing crust which secretes it, and accordingly is absolutely inorganic, and removed from the future influence of the animal which produced it. Such a supposition appears, however, at variance with the facts above stated, and incompatible with many circumstances connected with the history of the lithophytous polyps. On trying to detach the soft envelope from the surface of the skeleton, the firmness of their adherence would render such a want of connexion improbable, — they appear to be, as it were, incorporated with each other ; and besides, the separation of the fungia from the peduncle which joined it to its parent during its earlier growth, necessarily supposes a power of removing the calcareous particles after their deposition. It is therefore almost demonstrable that the earthy matter secreted by the polyp is deposited in the tissue of its substance, and still remains, in a greater or less degree, subject to absorption and removal : of this, however, we shall have fuller evidence hereafter. (25.) It is astonishing how nearly the animal and vegetable king- doms approximate each other in the lower orders of these calcareous zoophytes. Admitting the animal nature of fungia, we find calcareous skeletons, essentially similar in their chemical composition, produced by a large tribe of organic forms, long classed with the creatures we are now considering, which modern observations have clearly shown to be of vegetable nature.* These are the Corallines, (Linn.) which, although so nearly re- sembling the skeletons of polyps, that Cuvier, Lamarck, and others, scrupled not to admit them into the animal circle, have been proved * Schweigger, Anatomische Physiologische Untersuchungen liber Corallen. Berlin, ON POLYPS. by microscopical researches to possess the cellular structure apper- taining to vegetable organization, and are thus placed beyond the limits of our present investigations. (26.) We have hitherto spoken of animals which do not appa- rently possess any stomach or oral aperture, — any apparatus for the purpose of the digestion or prehension of food. Before describ- ing the more complex forms of polyps, we will now select a group of that class of animals, in which the organs provided for these pur- poses are easily recognisable ; and, as the simplicity of their orga- nization will well exhibit the principal points in the physiology of the acrita,we shall detail at some length the facts known concerning them. The HYDR.E, or fresh- water polyps, are common in the ponds and clear waters of our own country ; they are generally found creeping upon confervse which float upon the surface, and may readily be pro- cured in summer for the purpose of investigating the remarkable cir- cumstances connected with their history. p^ 4 The body of one of these simple animals consists of a delicate gelatinous tube, con- tracted at one extremity, which is terminated by a minute sucker, and furnished at the op- posite end with a variable num- ber of delicate contractile fila- ments, placed around the open- ing which represents the mouth. In the Hydra viridis, (jig- 4, 1,) the species most common amongst us, the tentacular fila- ments are short, and, when elon- gated to the utmost, are not equal to the length of the body; but in the long-armed species Hydra fusca, (Jig. 4, 2,) they are much prolonged, and of extreme tenuity. If placed in a small glass tube, one side of which is flattened, these animals may readily be sub- mitted to microscopical examination, and, from their transparency, their entire structure is easily made out. When highly magnified, the whole body is seen to consist of a granular substance, generally of a greenish hue, the granules being loosely connected by a semifluid albuminous matter ; but the most minute research reveals no fur- 22 ON POLYPS. ther appearances of organization : there is no trace of muscular fibre or of nervous substance, not the slightest indication of vessels of any kind, nor any apparatus destined to the function of repro- duction; such is the hydra, offering in every particular a good example of the acrite type of structure. The young naturalist would scarcely be prepared to see an animal of this description waging continual war with creatures much more perfectly organized than itself; endowed with consi- derable capability of locomotion ; possessed not only of a refined sense of touch, but able to appreciate the presence, and seek the influence of light ; and exhibiting moreover a tenacity of life and power of reproduction almost beyond belief : a little observation, however, will convince him that it possesses all these attributes, and enable him to share in some degree the astonishment with which Trembley, their enthusiastic discoverer, first witnessed and described them.* (27.) The hydra is not like most other polyps, fixed and station- ary; but can roam about and change its situation according to circum- stances. Its usual mode of progression is by creeping along the stems of aquatic plants, or upon the sides of the glass in which it is confined : attaching first the little tubercle at its posterior ex- tremity to the surface upon which it moves, it slowly inflects its body (fig. 4, 3), and fixing its oral tentacles, moves along in the manner of a leech, by a succession of similar actions. This method of advancing is, from the small size of the animal, necessarily slow ; and a march of a couple of inches will require several hours for its performance : but, when arrived at the surface of the water, it adopts a more speedy course ; suspending itself by the tail as by a minute float, and hanging with its mouth downwards, it rows itself about with its tentacles, or, wafted by the wind, can travel to a consider- able distance without effort. (28.) When left free, the hydrse are found to select positions most exposed to the influence of light, assembling at the surface of the ponds which they inhabit, or seeking that side of the glass in which they are confined, that is most strongly illuminated. That they are able to appreciate the presence of light is therefore indubitable ; yet with what organs do they perceive it ? We are driven to the supposition that, in this case, the sense of touch supplies to a certain extent the want of other senses, and that the hydrse are able, as * Trembley, Memoires pour servir a 1'Histoire des Polypes d'eau douce. Leyde, 1744. ON POLYPS. 23 an Italian author elegantly expresses it, 6C palpare la luce," to feel the light. (29.) The tentacles placed around the mouth are eminently sensi- tive, and the smallest particles which impinge upon those organs in their expanded state appear to excite a perception of their presence ; yet their movements, as well as those of the whole body, are extremely slow and languid : it would be difficult therefore to imagine that creatures apparently so helpless should be able to obtain other prey than such as had no power of resistance ; and we could scarcely believe, were it not a matter of continual observation, that the most active little animals, entomostraca, the larvse of insects, and even minute fishes, form their usual food. When the hydra is watching for prey, it remains expanded, (fig. 4, 1, 2,5,) its tentacula widely spread and perfectly motionless, waiting patiently till some of the countless beings which populate the stagnant waters it frequents, are brought by accident in contact with them : no sooner does an animal touch one of the filaments than its course is arrested as if by magic ; it appears instantly fixed to the almost invisible threa,d, and in spite of its utmost efforts is unable to escape ; the tentacle then slowly contracts, and others are brought in contact with the struggling prey, which thus seized is gradually dragged towards the orifice of the mouth, that opens to receive it, and slowly forced into the interior of the stomach. (30.) We are naturally led to ask, what is the nature of the action by which a passing animal is thus seized ? Trembley supposed that the filamentary arms were besmeared with an adhesive secretion like bird-lime, by which the victim became glued to the tentacle ; this however can hardly be the case, as the exercise of the power of retaining prey seems quite under the control of the hydra : when hungry, seven or eight monoculi* will be captured and swallowed in succession ; but when thus gorged with prey, or when indisposed to take food, although these animals may touch the tentacula again and again, they escape with impunity. (31.) Arrived in the stomach of the polyp, the animal which has been swallowed is still distinctly visible through the transparent body of the hydra, which seems like a delicate film spread over it : (Jig* 4, 4,) gradually the outline of the included victim becomes indistinct, and the film which covers it turbid ; the process of diges- tion has begun ; the soft parts are soon dissolved and reduced to a * Minute crustaceous animals, possessing considerable strength and agility. 24 OX POLYPS. fluid mass, and the shell or hard integument is expelled through the same aperture by which it entered the stomach. We will not even hazard a conjecture concerning the process by which digestion is effected in this case, our knowledge of animal physiology is by no means sufficiently advanced to render any attempt at explanation useful ; we will rather pass on, and enquire in what manner the nutritious parts of the food are conveyed into the system of the polyp. We have already observed that no traces of vessels of any kind have as yet been detected in the granular parenchyma of which the creature seems to be composed ; coloured globules are seen floating in a transparent fluid, which, in the Hydra viridis, are green, although in other species they assume different tints. When the food has been composed of coloured substance, as, for example, red larvae, or black planarite, the granules of the body are seen to acquire a simi- lar hue, but the fluid in which they float remains quite trans- parent ; each granule seems like a little vesicle into which the coloured matter is conveyed, and the dispersion of these globules through the body gives to the whole polyp the hue of the prey which it has devoured; sometimes the granules thus tinted are seen to be forced into the tentacula, from whence they are driven again by a sort of reflux into the body, pro- ducing a kind of circulation or rather mixing up of the granular matter which distributes it to all parts. If, after having thus digested coloured prey, the polyp is made to fast for some time, the vesicles gradually lose their deepened hue and become com- paratively transparent. The granules, therefore, would seem to be specially connected with the absorption and distribution of nutriment. (32.) Rapid as is the action of the stomach upon food introduced into it, it has no effect upon other parts of the animal when immersed in its cavity : the arms, for example, of the long-armed hydra are frequently coiled around its prey during the process of its solution, without receiving the slightest injury. This circumstance may not appear very remarkable, but it has been found that other polyps of the same species are equally able to resist the solvent action. Trembley once saw a struggle between two of these creatures which had seized upon the same animal ; both had partially succeeded in swallowing it, when the largest put an end to the dispute by swallowing its opponent as well as the subject of con- tention. Trcmbley naturally regarded so tragical a termination ON POLYPS. 25 of the affray as the end of the swallowed polyp's existence, but he was mistaken ; after the devourer and his captive had digested the prey between them, the latter was regurgitated safe and sound, and apparently no worse for the imprisonment. (33.) We will now proceed to consider the mode of reproduction of these simple animals. When mature and well supplied with food, minute gemmules or buds are seen to become developed from the common substance of the body ; they spring from no particular part, but seem to be formed upon any portion of the general sur- face. These gemmsB appear at first like delicate gelatinous tu- bercles upon the exterior of the parent polyp ; but, as they increase in size, they gradually assume a similar form, become perforated at their unattached extremity, and develope around the oral aperture the tentacula characteristic of their species. During the first period of the formation of these sprouts, they are evidently continuous with the general substance from which they arise ; and even when considerably perfected, and possessed of an internal cavity and tentacula, their stomach freely communicates with that of their parent by a distinct opening, so that food digested by the latter passes into the stomach of the young one, and serves to nourish it. As soon as the newly-formed hydra is capable of catching prey, it begins to contribute to the support of its parent ; the food which it captures passing through the aperture at its base into the body of the original polyp. At length, when the young is fully formed and ripe for independent existence, the point of union between the two becomes more and more slender, until a slight effort on the part of either is sufficient to detach them, and the process is completed. This mode of increase, when the animals are well supplied with nourishment, and the temperature is favourable, is extremely rapid ; sometimes six or seven gemmse have been observed to sprout at once from the same hydra, and, although the whole process is concluded in twenty-four hours, not unfrequently a third generation may be observed springing from the newly-formed polyps even before their separation from their parent : eighteen have in this manner been seen united into one group, so that, provided each individual when complete exhibited equal fecundity, more than a million might be produced in the course of a month from a single polyp. (34.) But perhaps the most remarkable feature in the history of the hydra is its power of being multiplied by mechanical division. If a snip be made with a fine pair of scissors in ON POLYPS. the side of one of these creatures, not only does the wound soon heal, but a young polyp sprouts from the wounded part ; if it be cut into two portions by a transverse incision, each soon deve- lopes the wanting parts of its structure ; if longitudinally di- vided, both portions soon become complete animals ; if even it be cut into several parts, every one of them will rapidly assume the form and functions of the original ; the inversion of its body, by turning it inside out, does not destroy it ; on the contrary, the ex- terior surface assumes the office of a stomachal cavity, and that which was originally internal will give birth to buds, and take upon itself all the properties of the skin. (35.) Cortical compound Polyps. — From what we have said con- cerning the two preceding families of polyps, — one composed of animals consisting entirely of a gelatinous crust which invests a fixed and immoveable skeleton ; the other exhibiting active and hungry creatures, provided with an internal digestive cavity, and endowed with the capability of seizing and devouring living prey, — we are prepared to examine the more complex structure of compound polyps, which combine in themselves the characteristics of both families. The compound polyps consist of a mass of gelatinous matter, which indicates, by its power of contraction upon the appli- cation of stimuli, a degree of sensation ; and of a great number of hydrseform polyps, which spring from the surface of the common body, and are individually capable of seizing and digesting prey, the nutriment thus gained being appropriated to the nourishment of the general mass. The animals of this division are provided with numerous mouths and stomachs, each endowed with a power of independent action. Although essentially similar in their habits, the compound polyps present various modifications of structure, which natu- rally leads them to be grouped in distinct families. Some- times the central common mass is entirely soft and gelatinous, its surface being covered with minute cells in which the polyps are lodged; such are the Alcyonidas. Sometimes the common body secretes large quantities of calcareous matter in the same man- ner as the Fungia, which, being deposited in its interior, forms arborescent masses, presenting upon their surface multitudes of cells, generally distinguishable after the removal of the outer crust, in each of which when alive a polyp existed : these form the family of Madrepores. The central axis is not unfrequently quite solid and smooth upon the surface, offering no cells for the ON POLYPS. lodgment of the hydrseform mouths ; being sometimes composed of hard and dense calcareous substance, or else flexible and horny in its texture : such are the Corallida or family of corals, properly so called. The internal central axis is, moreover, in another family, composed of several pieces united together by the living crust which secretes them ; and such individuals, being free and unat- tached, are probably able to change their position at pleasure : these form the family of Pennatula. These groups are, however, merely modifications of the same general type of structure, although differing in certain minor points of their organization, so as to render an examination of each form needful for our purpose. (36.) Alcyonidce. — This family includes several genera, known by the names of Alcyonium, Lobularia, Cydonium, &c., being charac- terized by having no solid axis developed in the interior of the com- mon body. The Cydonium Fig. 5. Mulleri (Jig. 5, 1,) will give t the reader a good idea of the general appearance of one of these compound animals. The central mass, or polypary, is entirely soft, being of a gelati- nous or rather subcartilagi- nous texture. Its density varies with the state of the animal, being more firm when the crea- ture is contracted or hardened by immersion in spirits of wine, than when alive and ex- panded. Upon cutting into it, it is found to be intersected by tough fibrous bands, and not unfrequently contains calcareous spicula dispersed through its substance ; no muscular fibre or nervous matter has ever been de- tected in its composition, and its interior is permeated by nume- rous wide canals variously disposed. The alcyonidse, therefore, may justly be looked upon as intimately related to the sponges in the structure of their common body, differing from them principally in the polyps which occupy the cells upon their surface. (37.) The polyps which fill these cells resemble so many hydra in their external configuration, from which, however, they differ in the number of tentacula surrounding the mouth. In the hydra we 28 ON POLYPS. find sometimes five, sometimes six, or more of these appendages ; but in all the cortical polyps there are eight. The tentacles, also, are not unfrequently pinnated or slightly fringed on each side, but never provided with moveable cilia. The body of the polyp, when withdrawn from its cell, is somewhat globular, and more complex in its structure than that of the hydra. In Jig. 5,2, a diagram is given, representing the Alcyonium exos, in which the following parts may be distinguished. The stomach* is considerably dilated, and terminates inferiorly in a tubular prolongation, b, which ex- tends into the substance of the common mass, into which it most probably conveys nourishment. But the main difference observ- able between the alcyonidse and the hydra consists in the possession of a reproductive organ or ovary, in which the germs of its progeny are developed. This consists of a tubular filament, c, lodged in the cell which the polyp inhabits, which opens by one extremity into the bottom of the stomach, into which the ova when mature are conveyed, and they are ultimately ejected through the mouth, a, as represented in the figure. (38.) Few objects exhibit to the naturalist a more beautiful spec- tacle than the compound animals of which we are speaking. When found upon the shore contracted and deformed, it would be diffi- cult to imagine that they were really organized beings, much less possessed of the elaborate conformation we have described ; yet, on placing one of them in a tumbler of sea-water, and watching it attentively with a magnifying glass, its true nature is gradually re- vealed : the central mass expands in all directions, exhibiting the cells upon its surface, from which in time the countless flower-like polyps are protruded, and, stretching out their arms in all directions, wait for the approach of prey. A scene like this naturally leads us to make a few observations upon some points of physiology con- nected with their economy : several questions obtrude themselves upon us, which, although applicable to the whole group of com- pound polyps, may be well discussed in this place. (39.) That there is a community of nutrition, — or, in other words, that food taken and digested by the individual polyps is appropriated to the support of the general body, — appears to be indisputable, and is generally admitted ; but is there a community of sensation so as to render the entire mass one animal, capable of consentaneous move- ments, or is each polyp independent of the rest in its sensations and actions ? Upon this there are different opinions : some regard- * Spix (Jean), Memoire pour servir a I'histoire de 1'Alcyonium exos. ON POLYPS. ing the whole as a single animal, each part being in communication with the rest, and thus participating in the feelings and movements of the others ; whilst some consider each polyp as a distinct crea- ture, independent of the rest. The solution of this problem is a matter of some difficulty ; but there are several facts recorded by observers, which may in some measure enlighten us upon the sub- ject. From the absolute want of nervous filaments, which might bring into communication distant points of the body, we might theoretically deny the possibility of any combina- tion of actions ; and experiment teaches us that the assumption is correct. If when one of these animals is fully expanded, transparent and soft, any point of its surface be rudely touched, the whole body does not immediately shrink, but the point only where the irrita- tion was applied appears to feel the impression ; this part shortly becomes more dense, opaque, and a depression is seen gradually to appear. If the shock be severe, and extensively diffused over the body, the contraction slowly extends to the whole mass ; the most violent local injury, indeed, seems to be totally unperceived at re- mote parts of the body : whilst a general shock, such as striking the vessel which contains the expanded polyp, produces a simultaneous contraction of the whole.* The polyps, however, exhibit much greater irritability, and their movements, from their rapidity, form a striking contrast to the languid contractions of the connecting central mass ; but that they have a community of life appears im- probable : they seem to act quite independently of each other ; when one is touched and suddenly retracts itself within its cell, it is true that those in the neighbourhood will likewise not unfre- quently retire, but this circumstance may be accounted for by the sudden movement of their neighbour ; for, as the polyps often touch each other with their tentacles, there is no cause for urging a com- munity of substance to explain it."f" (40.) Madreporidce. — Were we to imagine one of the alcyonidse capable of secreting not merely the calcareous spicula which are mixed up with the softer portions of its body, but abundant quan- tities of carbonate of lime, which, being stored up in the centre of its substance, should form a dense calcareous axis encrusted with the uncalcified part of the living animal, and perforated at its sur- * Professor Grant, Lectures on Comparative Anatomy, — Lancet for 1833-4, vol. ii. p. 261. t Quoy et Gaimard, Zoologie du Voyage de I'Uranie. Paris, 1834. 30 ON POLYPS. face so as to form innumerable cells or lodges containing the polyps which provide nourishment for the general mass, we should have a good general idea of the structure of the tribe of polyps which now comes beneath our notice. The shallower parts of the tropical seas contain countless forms of madrepores, known to us, unfortunately but too often, only by the earthy skeletons which the beauty of their appearance induces the mariner to bring to our shores. These calcareous masses assume more or less an arborescent appearance, spreading to a considerable extent, so as to cover the bottom of large tracts of the ocean, and not unfrequently they play an important part in pro- ducing geological changes which are continually witnessed in the regions where they are abundant. (41.) The extent of our knowledge of the animals themselves is, unfortunately, but very limited. That the entire skeleton, whatever its form, is encrusted with living substance ; that the cells contain polyps, resembling more or less those of the aleyonidse, and which provide for the nutrition of the whole, — is pretty much the extent of our information concerning them : and should the scientific naturalist ever be placed in circumstances where he can more closely examine them in their living state, there is scarcely a department of science in which his labours could be more beneficially employed than in the investigation of their structure and history. (42.) That the madrepores, from the immense masses of chalky material which they accumulate in the regions inhabited by them, not unfrequently become the cause of excessive danger to the mariner, by raising the bottoms of the shallow seas which they frequent, so as to render regions once covered with deep water no longer navi- gable, or filling up by their accumulation the bays and harbours of the South Seas, — is undeniable; and a knowledge of this fact justly makes the navigator cautious in passing through the localities where they most abound. Yet the imagination of authors has not seldom far exceeded the truth in detailing the circumstances connected with them. That the harbour of Tinian, so extolled in the Voyages of Lord Anson and others, is now choked up with the skeletons of madreporegynous polyps, is readily credited ; that islands are gra- dually formed, where none existed, by the agency of these creatures, is equally authenticated ; and that madrepores are found in strata much elevated above the level of the seas in the neighbourhood, is a fact attested by many voyagers. Yet when we are told of coral reefs, some hundred miles in length, entirely formed by the agency ON POLYPS, 31 of these apparently insignificant creatures, — of perpendicular cliffs rising from immense depths, which are altogether the produce of their secretions, — we have only to turn to the details in our posses- sion, concerning their habits and mode of increase, to assure us of the inaccuracy of such statements.* In the hot climates in which the saxigenous corals abound, they are found to frequent shallow bays and sheltered spots, where they can enjoy the full influences of light and air, un exposed to the agitation of the ocean, which, were it to beat continually upon them, would infallibly destroy their delicate substance : in such situations, the sub-marine rocks become gradually encrusted with the calcareous skeletons which they produce ; and if undisturbed, in the lapse of years, successive generations will of course deposit such large quantities of calcareous matter as to form beds of considerable thickness. That there are at the bottom of the ocean bold and precipitous cliffs, rising from a depth of 1000 or 1200 feet, their broad tops approximating the surface of the ocean, every one will admit, without having recourse to the labours of madrepores to account for their formation, although the sheltered portions of the summits of such mountain ridges afford an eligible position for their increase. In such situations, therefore, they accumulate, and slowly deposit continually increas- ing masses of earth upon the brow of these sub-marine mountains, until at last the pile approaches the surface of the sea, and even at low water remains uncovered by the waves. The further elevation of the rock, as far as the polyps are concerned in its construction, here ceases ; but a variety of causes tends gradually to heap materials upon the newly appearing island : storms, which tear up the bottom of the sea, perpetually throw to the surface sand and mud ; which becoming entangled among the madrepore, and matted together with sea-weed, forms a solid bed over which the waves have no longer any power. The circumference of the islet is perpetually augmented by the same agency: sea-weeds and vegetable sub- stances cast upon it, by their decay cover its top with vegetable mould ; and if its proximity to other land permit the united action of winds and currents to bring the germs of vegetation from neigh- bouring coasts, they take root in the fresh soil, and soon clothe with verdure a domain thus rescued from the ocean. (43.) The coasts described by Cook and Bougainville, whereon strata of coral are found much elevated above the level of the sea, are undoubtedly of volcanic origin. The bottom of the ocean, * Quoy etGaimard, Op. cit. ON POLYPS. crusted over by thick masses of madrepore, has been suddenly heaved up by one of those stupendous convulsions of nature, pro- bably produced by the sea finding its way into some sub-marine volcano ; and rocks and corals, raised from their beds by the tre- mendous explosion so produced, give birth to islands and elevated tracts of country, such as are met with in the South Seas. CORALLID.E. — The Corallidse are compound polyps of appa- rently more perfect organization than those forming the last family. The polypary or central axis, which supports the external or living crust, is solid, without cells, and variously branched ; the larger species resembling shrubs of great beauty, frequently coloured with lovely hues, and studded over their whole surface with living flowers, for such the polyps which nourish them were long consi- dered even by scientific observers. The central stem of these zoophytes differs much in its composition in different families ; sometimes being of stony hardness, in other cases it is soft and flexible, resembling horn ; and not unfrequently it is formed of both kinds of material : it is however always produced by the living cortex, which secretes it in concentric layers, the external being the last deposited. The example which we shall select for special description is the Coral of commerce, Corallium rubrum, (Jig. 6.) from which we derive the material so much prized in FiS- 6- the manufacture of ornaments. (44.) The red coral is principally obtained in the Mediterranean. When growing at the bottom of the sea, it consists of small branched stems, en- crusted with a soft living investment, by which the central axis is secreted, and studded at intervals with polyps possessing eight fringed arms, and capable of being contracted into cells contained in the fleshy covering, but not penetrating the stem itself. The skeleton or polypary of the coral is of extreme hardness, and susceptible of a high polish ; a circumstance to which the estima- tion in which it is held is principally owing. But in other genera of this family, the central axis, instead of being con- structed of calcareous matter, is formed of concrete albumen, and resembles horn both in appearance and flexibility ; such are the ON POLYPS. Gorgonise of the Indian Ocean. In the Isis Hippuris (jig. 7, B) the central axis is alternately composed of both these substances, exhibiting calcareous masses united at intervals by a flexible mate- rial, allowing the stem to bend freely in every direction. The object of such diversity in the texture of the polypary of the Coral- hdce will be at once apparent when we consider the habits of the different species : the short and stunted trunks of Corallium, composed of hard and brittle Fig. 7. substance, are strong enough to resist injuries to which they are exposed ; but in the tall and slender stems of Gor- gonia and Isis, such brittle- ness would render them quite inadequate to occupy the si- tuations in which they are found, and the weight of the waves falling upon their branches would continually break in pieces and destroy them ; this simple modifica- tion, therefore, of the nature of the secretions with which they build up the skeleton which supports them allows, them to bend under the passing waves, and secures them from otherwise inevitable destruction. (45.) Upon making a transverse section of one of these poly- paries, (Jig. 7, A,) the solid axis is distinctly seen to be made up of layers arranged in a somewhat undulating manner around the centre, and successively deposited by the living cortex : the growth of the stem, in the harder species at least, is very slow, and several years are necessary to its maturity ; a circumstance whicFlias ren- dered it needful to impose strict laws, forbidding the Mediterranean coral-fishers to disturb too frequently the same localities, which are only visited at stated periods. (46.) The deposition of solid matter in the soft bodies of these polyps is not confined to the production of the central stem, but in many even of the Keratophyta * cretaceous particles are extensively * An old name for polyps with a horny axis, x'^xf, horn; Qvrov, a stem ; as distin- guishing them from the stony polyps, Lithophyta, *.i0a{, a stone ; vrav. D 34 ON POLYPS. diffused through the cortex, which not unfrequently is likewise gorgeously coloured by secretions of different hues. In the Gor- goniae, a section of one of which (Gorgonia verrucosa) is repre- sented in Jig. 7, A, the earthy matter in the crust is so abundant, that, even when dried, it will retain in some measure its natural form, and exhibit the tints peculiar to the species. The structure of the individual polyps of the Corallidse, as far as we are acquainted with their history, resembles that of one of the polyps of the Alcyonidse already described (§ 36) ; and the prey obtained by each, goes to the support of the general mass. Their reproduction is undoubtedly from germs developed in in- ternal filamentary ovaria, which escape either through the mouth, as in Alcyonium, or else, as Cavolini* supposed, through apertures placed between the origins of the tentacles. (47.) Pennatulidce. — This family belongs likewise to the divi- sion of cortical polyps, and agrees with the two last in most points, the principal distinction consisting in the character of the internal axis which supports the body. In some species this part is reduced in fact to a ligamentous mass, interspersed with calcareous granules ; but, in the most typical forms, the skeleton consists of several pieces, capable of moving upon each other. The whole animal, in such cases, resembles a feather, the stem supporting lateral branches, upon which the polyps are arranged. From the circum- stance of these compound animals being unattached to any foreign support, they have been supposed to be capable of swimming at large in the sea, by the voluntary movements of their articulated branches, a fact strongly contested by many modern zoologists ; but, as we can say nothing from our own observation upon this subject, we must leave the question open to future investigation. Many species are eminently phosphoric. Tubiporidcc. — We now have to speak of a class of polyps very different in their construction from those which have been described. Instead of encrusting an internal solid skeleton, the Tubiporidse are enclosed in a calcareous or coriaceous sheath or tube, from the ori- fice of which the polyp is protruded, when in search of prey : these are named by authors Vaginated Polyps. (48.) The Tubipora musica (jig. 8, a) is the species which has been most carefully studied, and the details connected with its or- ganization will be found of the highest importance, as affording a * Cavolini (Philippe), Memorie per servire alia storia diPolipi marini. 4to. Naples, 1785. ON POLYPS. clue to the investigation of other forms, to be mentioned hereafter.* The Tubiporse live in society, but do not appear to be organically united as the compound polyps; a group of these animals presents Fig. 9. several stages of tubes, placed one above another ; the tubes are ge- nerally straight, and nearly parallel to each other, but appear slightly to diverge, as ra- diating from a common centre ; they are separated by considerable intervals, and reciprocally support each other by horizontal laminae of the same substance as the tubes themselves, which unite them. From each tube issues a little membranous animal of a brilliant grass- green colour, the mouth being surrounded by eight tentacles, which are furnished along their edges with two or three rows of minute fleshy papillae. Within the mouth of the specimen examined by M. Lamouroux, was found an * Anatomic de Tubipore Musical, par M. Lamouroux,— in the Zoology of Quoy et Gaimard, Voyage de 1'Uranie. 36 ON POLYPS. oval membranous sac, but not in sufficient preservation to be properly described. This was most probably the stomach. (49.) Around this sac, alternating with the tentacles, are eight triangular filaments, (Jig. 9 ; 1 e,) which are at first free and floating, but they soon become attached to a membrane which lines the calcareous tube ; and, gradually diminishing in size, they extend through its whole length. These filaments are analogous to the ovaries of the Corallidse and Pennatulidse ; their inner sur- face, in mature individuals, is studded with ova of different sizes attached to them by short pedicles (Jig. 9 ; 8). (50.) At the point where the ovigerous filaments reach the ten- tacles, a membrane is observable which assumes the shape of a funnel when the animal retires into its shell, and at the open end of the funnel the membrane is seen to fold outwards, and become continuous with the calcareous tube; (Jig. 9 ; 1, &;) its inner sur- face indeed is prolonged under the form of a thin pellicle over all that part of the interior of the tube which is inhabited by the polyp, terminating at a kind of diaphragm composed of the same hard substance as the tube itself. The remains of these diaphragms are found in the interior of old tubes at various distances from each other. The funnel-shaped membrane does not terminate suddenly at its point of junction with the calcareous tube ; the latter, indeed, is a continuation and product of the first, the calcareous substance being evidently deposited in this gelatinous membrane, in the same man- ner as phosphate of lime is deposited in the bones of very young subjects, changing its soft texture into hard, solid substance. The manner, therefore, in which this tube is formed, cannot be compared to the mode of formation of the shells of Serpulce or the shells of mollusca; in the latter case it is a secretion from the skin, almost an epidermic product, but in these polyparies there is a real change of soft into solid substance, which is effected gradually, but not deposited in layers. (51.) When the tube has acquired a certain height, the animal forms the calcareous horizontal plate which unites it to those around ; the still membranous upper part of the tube extends itself horizontally outwards around the aperture, (Jig. 9 ; 2, 6,) doubling itself so as to form a circular fold ; this part of the membrane is no longer irritable ; its internal surfaces unite so as not to interrupt the continuity of the tube ; carbonate of lime is gradually deposited within it, and soon a prominent partition, com- posed of two lamellae, soldered together through almost their entire ON POLYPS. 37 extent, surrounds the tubular cell. Generally many polyps of the same polypary form these partitions at the same time and upon the same plane. In this case the gelatinous margins of the folded mem- brane unite, no space is left ; and they ultimately become most inti- mately soldered together, and the solid plane or stage (fig. 8) is formed. If the animal constructs its partition against a tube already perfect and solidified, it fixes its collar to its sides, so that the point of junction is imperceptible ; but when it is quite insulated, as at &, Jig. 8, the horizontal collar is still formed, and it then assumes somewhat of an octagonal shape. The tube-forming membrane exhibits no appearance of vessels or other traces of organization. When the polyp is withdrawn within its cell, its tentacles form a cylindrical fasciculus {Jig- 9, c) ; the papillae which partially cover them being laid upon each other like the leaflets of some mimosa when asleep. The protrusion of the creature from its tube is accomplished by the contraction of the membrane, 6, inserted into its neck. (52.) How the eggs formed upon the oviferous filaments issue from the polyp, has not been ascertained : it is most probable, from their size, that they are not expelled during the life of the parent ; but that, when it dies, the eggs all come out of the tube, except one, which developes itself in the old cell ; the rest fixing themselves upon the neighbouring stage, there to form a new story of tubes. The germs, during the first period of their developement, have no organs distinguishable, not even the rudiment of a tube ; each ap- pears to consist of a simple gelatinous membrane folded upon itself, (Jig. 9 ; 4, c,) and forming upon the stage upon which it is fixed a little tubercle resembling a small Zoanthus or other naked zoophyte. This tubercle gradually elongates, and assumes the form of a polyp, provided with all its organs ; but the sac which encloses it is still gelatinous at its upper part, and membranous near the base, (Jig. 9 ; 4, 6,) where it gradually diminishes in thickness, and, becoming calcareous, gives to the animal the general appearance of its original. (53.) In Tubularia indivisa the structure of the tentacula around the mouth is different from what has been described in Tubipora mu- sica, although in the principal points of its structure the resemblance between the two is very great ; when the Tubularia is expanded, its protruded portion is seen to be furnished with two circles of arms, one placed around the opening of the mouth, the other at a consi- derable distance beneath it, (Jig. 10 ; 1,) and nearly on a level with the inferior circle a second aperture (Jig> 10 ; 1, a) is observable, ON POLYPS. Fig. 10. communicating with that portion of the body which is lodged within the tube, and resembling a second mouth. A remarkable action has been observed to take place in these parts of the polyp, producing a continual variation in their form ;* a fluid appears at intervals to be forced from the lower compartment into the space intervening be- tween the two rows of tentacula, which becomes gradually dilated into a globular form (Jig. 10 ; 2 and 3.) This distension continues for about a minute, when the upper part, contracting in turn, squeezes back the fluid which fills it into the lower compartment through the opening a, which then closes preparatory to a repeti- tion of the operation. The intervals between these actions were, in the specimen observed by Mr. Lister, very evenly eighty seconds. In Tubularia indivisa the sheath or cell, &, which en- closes the polyp, is perfectly diaphanous, allowing its contents to be readily investigated under the microscope. When thus examined, a continual circu- lation of particles was visi- ble, moving in even, steady currents in the direction of the arrows (fig. 10; 1) along slightly spiral lines represent- ed in the drawing. The par- ticles are of various sizes, some very minute, others apparently aggregations of smaller ones ; some were globular, but they had gene- rally no regular form. In fig. 3, d, a series of longitu- dinal lines are perceptible, which most probably are ovi- gerous filaments, resembling those of Tubipora musica. Actiniadce. — The next family of polyps, from the fibrous character which the substance of their bodies assumes, have been named by zoologists " Fleshy Polyps.'1'' They differ indeed re- markably from the soft gelatiniform structures which have hitherto come under our notice, exhibiting traces of muscular fibre which are not to be mistaken. * Lister, on the structure and functions of Tubular and Cellular Polypi. — Philoso- phical Transactions, 1834. ON POLYPS. Fig. 11. Fig. 12. Although the genera composing this division are exceedingly numerous, and vary much in their external characters, they will be found more or less to conform in the essential points of their organization with the subject which we have chosen as the type of this extensive tribe, and of which, being common upon our own coasts, the reader will have little difficulty in procuring specimens for examination. (54.) The body of an Actinia when moderately expanded, (fig. 11?) is a fleshy cylinder, attached by one extremity to a rock, or some other sub- marine support ; whilst the opposite end is sur- mounted by numerous tentacula, arranged in se- veral rows around the oral aperture (fig. 1£) . When these tentacula are expand- ed, they give the animal the appearance of a flower, a resemblance which is rendered more striking by the beautiful colours which they not unfrequently as- sume ; and hence in all countries they have been looked upon by the vulgar as sea-flowers, and distin- guished by names indica- tive of the fancied resem- blance. Their animal na- ture is however soon rendered evident by a little attention to their habits ; when expanded at the bottom of the shallow pools of salt-water left by the retreating tide, they are seen to manifest a degree of sensibility, and power of spontaneous movement, which we should little an- ticipate from their general aspect. A cloud veiling the sun will cause their tentacles to fold, as though apprehensive of danger from the passing shadows : contact, however slight, will make 40 ON POLYPS. them shrink from the touch ; and if rudely assailed, they com- pletely contract their bodies so as to take the appearance of a hard coriaceous mass, scarcely distinguishable from the substance to which they are attached. (55.) It is in seizing and devouring their prey however that the habits of the Actiniae are best exemplified ; they will remain for hours with their arms fully expanded and motionless, waiting for some passing animal which chance may place at their disposal, and when the opportunity arrives, are little inferior to the Hydrse in their voracity or powers of destroying their victims. Their food generally consists of crabs or shell-fish, animals apparently far superior to themselves in strength and activity, but even these are easily over- powered by the sluggish yet persevering grasp of their assailant. No sooner are the tentacles touched by a passing animal than it is seized, and held with unfailing pertinacity ; the arms gradually close around it ; the mouth, placed in the centre of the disc, ex- pands to an extraordinary size ; and the creature is soon engulph- ed in the digestive bag of the Actinia, where the solution of all its soft parts is rapidly effected, and the hard undigestible remnants speedily cast out at the same orifice. The Actiniae, although exceedingly voracious, will bear long fasting :* they may be preserved alive for a whole year, or per- haps longer, in a vessel of sea- water, without any visible food ; but when food is offered, one of them will devour a crab as large as a hen's egg, or two muscles in their shells : in a day or two the shells are voided through the mouth, perfectly cleared of the soft parts which they contained. (56.) The Actiniae, like the Hydras, possess the power of chang- ing their position : they often elongate their bodies, and, remaining fixed by the base, stretch from side to side as if seeking food at a distance ; they can even change their place by gliding upon the disc which supports them, or detaching themselves entirely, and swelling themselves with water, they become nearly of the same specific gravity as the element which they inhabit, and the least agitation is sufficient to drive them elsewhere ; Reaumur even asserts that they can turn themselves so as to use their tentacles as feet, crawling upon the bottom of the sea ; but this mode of pro- gression has not been observed by subsequent naturalists : — when they wish to fix themselves, they expel the water from their dis- * Encyclopaedia Londinensis, art. Actinia. ON POLYPS. tended body, and sinking to the bottom attach themselves again by the disc at their base, which forms a powerful sucker. (57.) From this sketch of the outward form and general habits of these polyps, the reader will be prepared to examine their internal economy, and the more minute details of their structure. On ex- amining attentively the external surface of the body, it is seen to be covered with a thick mucous layer resembling a soft epidermis, which extending over the tentacula, and the fold around the aper- ture of the mouth, is found to coat the surface of the stomach itself; this epidermic secretion forms in fact a deciduous tunic which the creature can throw off at intervals. On removing this, the walls of the body are seen to be made up of fasciculi of mus- cular fibres, some running perpendicularly upwards towards the tentacula ; and others, which cross the former at right angles, pass- ing transversely round the body ; the meshes formed by this in- terlacement are occupied by a multitude of granules apparently of a glandular nature, which give the integument a tuberculated aspect : these granules are not seen upon the sucking disc at the base. The tentacula are hollow tubes, composed of fibres of the same description. The stomach is a delicate folded membrane, forming a simple bag within the body ; it seems to be merely an extension of the ex- ternal tegument, some- what modified in tex- ture ; it is closed infe- riorly, the same orifice serving both for the in- troduction of food, and the expulsion of effete or indigestible matter. (58.) On making a section of the animal, as represented in jig. 13, the arrangement of these parts is distinctly seen : a being the muscular integument ; b the ten- tacula formed by the same fibrous membrane ; and c the stomach, which is apparently de- ON POLYPS. rived from it. Between the digestive sac c, and the fibrous ex- terior of the body a, is a considerable space d, divided by a great number of perpendicular fibrous partitions, /, into numerous com- partments, which however communicate freely with each other, and likewise with the interior of the tentacula, as seen at e. Every tentacle is perforated at its extremity by a minute aperture £, through which the sea-water is freely admitted into these compart- ments, so as to bathe the interior of the body ; and when from alarm the animal contracts itself, the water so admitted is forcibly expelled in fine jets through the holes by which it entered. There can be no doubt that the surrounding fluid, thus copiously taken into the body, is the medium by which respiration is effected ; and every one who has been in the habit of keeping Actiniae in glass vessels for the purpose of watching their proceedings, must have noticed that as the fluid in which they are confined becomes less respirable, from the deficiency of air, the quantity taken into the body is enormous, stretching the animal until it rather resembles an inflated bladder than its original shape. (59.) It is in the compartments which are thus at the will of the creature distended with water, that we find the organs of reproduc- tion, which here assume a developement far exceeding what we have noticed in other zoophytes. On raising a portion of the mem- brane which forms the stomach, as aty, we see lodged in each par- tition an immense number of ova attached to a delicate transparent membrane, and arranged in large clusters, g. The ovigerous mem- brane which secretes these eggs is represented unravelled at h ; it is through its whole extent bathed with water admitted into the compart- ment in which it is lodged, a circumstance which provides for the re- spiration of the ova during their developement. The convoluted ovary is seen to terminate by a minute aperture near the bottom of the sto- mach k, into which when mature the young escape. The eggs found in the ovaria are round and of a yellow colour, resembling minute grains of sand : it is probable that sometimes they are hatched after their ex- pulsion, but it is likewise asserted by numerous authorities that the young are not unfrequently born alive. The manner in which the ova are extruded has been long a matter of controversy, and perhaps cannot yet be regarded as definitively ascertained. Our own dissec- tions would lead us to concur with those anatomists who describe them as escaping from the ovaria into the bottom of the stomach, whence they have been seen to escape by the mouth fully formed : it is possible, however, that they may likewise be expelled with the ON POLYPS. 43 streams of water forced by the contractions of the animal through the orifices at the extremities of the tentacula. The Abbe Dicquemare* relates several curious experiments on the multiplication of these animals by mechanical division. When transversely divided, the upper portion still stretched out its ten- tacles in search of food, which, when seized, sometimes passed through its mutilated body, but was occasionally retained and di- gested. In about two months tentacles grew from the cut ex- tremity of the other portion, which soon afterwards began to seize prey. By similar sections he even succeeded in making an animal with a mouth at each end. (60.) The entire organization of the Actinia is evidently very supe- rior to that of any animals which have been described in the preceding pages ; the muscular fasciculi, now for the first time distinctly recog- nisable, give an energy to their contractions very different from the languid movements of the gelatinous polyps. The Actinia can in- deed hardly be classed in the acrite division of the animal kingdom ; the developement of muscular fibre which it presents, presupposes the existence of nervous filaments, and we might a priori infer their existence. Spix, many years ago, described a nervous sys- tem, which he believed he had discovered, in the neighbourhood of the base, or sucking disc by which the animal attaches itself to foreign bodies ; in which situation he was led to look for it, by ob- serving that when galvanic shocks were sent through the body, convulsive movements were excited most distinctly in this part, — and also from the supposition that the organ of attachment, here placed, must necessarily be the most abundantly endued with sen- sibility, j* Having raised the longitudinal muscles by a slight incision near the middle of the base or disc of attachment, he thought he perceived an interlacement formed by some pairs of nodules, disposed around the centre, which communicated by several cylindrical threads ; from each nodule two filaments ran forwards, one accompanying the lon- gitudinal fleshy fasciculi, the other penetrating to the internal longitudinal septa, which have likewise a muscular character. Suc- ceeding anatomists have, however, totally failed in their endeavours to detect the arrangement here described ; and which indeed, did it exist, would be contrary to every analogy with which we are ac- quainted. It is more probable that the nervous system consists in * Philosophical Transactions, 1773. t Spix (Jean) Annales du Museum, tome 13. 44 ON POLYPS. a delicate thread, which we are pretty well convinced we have de- tected running round the roots of the tentacles, embedded in a strong circular band of muscle which surrounds the orifice of the stomach, and acts the part of a powerful sphincter in closing the aperture. (61.) After the account which has been given of the general structure of the Actinia, the mechanism by which the tentacula are expanded and withdrawn will be easily understood : these do not, like the horns of a snail, become inverted and rolled up within the body, but owe their different states of extension entirely to the forcible injection of water into the cavities which they contain. We have seen already that the interior of each tubular arm communi- cates freely with the space which intervenes between the stomach and the external integument, a space which, at the will of the animal, is filled with sea-water drawn through the orifices seen at the extremity of each arm : when these minute orifices are closed, and the body of the creature contracted, the water, being violently forced into the tentacula, distends and erects them, as when watching for prey ; and, on the other hand, when emptied of the fluid thus injected, they shrink and collapse. This circumstance, so easily seen in the Actiniae, will probably enable us to account for similar phenomena observable in other polyps, the internal economy of which is by no means so conspicuous. (62.) The next tribe of polyps which presents itself to our notice, differs widely from the preceding families in outward form, as well as in many important features of internal structure. It would seem, indeed, to comprise animals distinguished from each other by so many important circumstances, and yet so intimately related by ex- ternal configuration, that it is difficult to separate them, or to leave them in the same group. It was imagined a few years ago, before accurate researches had been made concerning the internal structure of these zoophytes, that in all the compound species the polyps or mouths of the general mass were in their essential structure analogous to the Hydra, being simple digestive sacs, without more complication of structure than we have found those of the cortical polyps to possess. Recent investigations, however, have shown that amongst the species ranged by Cuvier under the head of Tubular Polyps, " Polypes a Tuyaux" many are exceedingly complex in their organization, possessing the outward form of the simpler kinds, but ON FOLYPS. 45 furnished with a complete digestive canal, and approximating in their general economy very superior orders of animals. These latter would appear to be distinguishable by the nature of the tenta- cles around the mouth, which, in all the families as yet examined, we have found to be smooth or merely fringed, as they are indeed in some of the tubular polyps hereafter to be noticed ; but, in the more perfect species, the arms are covered with vibratile hairs or cilia, forming important agents in securing prey : such have been separated by Ehrenberg into a distinct class, under the title of BRYOZOA, and have been recently designated by Dr. Arthur Farre, ClLIOBRACHIATE POLYPS. Further observation is necessary before the boundaries of these important divisions can be accurately laid down ; we shall neverthe- less, without entering upon a question foreign to our present sub- ject, arrange them in conformity with the analogies of their internal structure, rather than of their outward general form, and defer the consideration of the ciliobrachiate division to another place. (63.) In the unciliated tubular polyps, the common body of the animal, instead of encrusting a solid skeleton, is enclosed in a horny sheath, which it traverses like the pith of a tree, follow- ing all the ramifications of the branched stem of the polypary : to the central part are attached, at intervals, cells opening exter- nally, in which the polyps which provide nourishment for the whole are lodged. Zoophytes of this description are readily found on our own coasts, and the microscopic observer can scarcely enjoy a richer treat than the examination of them affords. In order to study them satisfactorily, it is necessary to be provided with several glass troughs, of different depths, in which the living animals immersed in their native element may be placed : in this situa- tion, if the water be carefully renewed at short intervals, they will live for some time. (64.) On examining a piece of one of these polyparies with a good glass, the tubular horny envelope is seen to be filled with granular matter ; and, on attentively watching it, globules will be seen moving in different directions, producing a sort of circulation or cyclosis very much resembling what is observable in some plants. The glo- bules thus moving do not appear to be contained in vessels, but steal in slow currents, ascending along the sides, and returning down the middle in an opposite direction, as represented by the arrows in fig- 14. 46 ON POLYPS. (65.) It has been generally stated that the living pith exuded from its surface the horny matter which, by its concretion, forms the tube or external skeleton investing the whole ; the accuracy of such a supposition, however, may well be questioned. We have already seen, in the Tubipora musica, that the calcareous tube investing that polyp was produced by the interstitial deposit of earthy matter in the membrane which formed originally its outer case. In the tribe of zoophytes which we are now speaking of, we shall find the exterior tube to be formed in a way precisely similar. On referring to the diagram, (fig. 14,) the mode of its growth will be rendered in- telligible: the soft part or living axis of the polypary is seen to be contained in two distinct layers ; the inner one composing the digestive sac of the polyp, and embracing the granular matter, which seems to be the special seat of the nutritive process ; the outer or tegumentary layer, i, after leaving the tentacula, may be traced down the sides of each polyp to the bottom of the cell, where its course is arrested by a slight partition, at which point it turns outwards, lining the interior of the cell as far as its margin, where, as in the Tubipora, it is seen to be continuous with the horny matter itself. It is this tegumentary membrane, then, which forms by its develope- ji». 14. ment the entire skele- ton : as it expands, it gives origin to the cells and branches character- istic of the species; and, from being at first quite soft and flexible, it gra- dually acquires hardness and solidity by the de- position of corneous matter in its sub- stance. The cells thus formed are inhabited by polyps analogous to those which provide nourishment for the cortical families ; but differing in the number and appearance of the tentacula, which are ON POLYPS. 47 here studded with minute tubercles, but never provided with cilia. Few objects are more admirable than these polyps, when watched with a good microscope : protruding themselves beyond the mouths of their cells, they inflect their bodies in all directions in quest of prey, waiting till some passing object impinges upon their tenta- cula, which is at once seized and conveyed into the stomach with a rapidity and dexterity almost beyond belief. The multiplication of these singular animals appears to take place in three different modes : — 1st, by cuttings, as in plants ; Sndly, by off-shoots, or the formation of new branches bearing polyps ; 3dly, by gemmules capable of locomotion. (66.) The first mode strikingly resembles what is observed in the vegetable kingdom ; for as every branch of the plant-like body contains all the parts necessary to independent existence, it can hardly be a matter of surprise that any portion, separated from the rest, will continue to grow and perform the functions of the entire animal. (67.) The second mode of increase, namely, by the formation of new branches and polyps, seems more like the growth of a plant than the developement of an animal. We will consider it under two points of view : first, as regards the elongation of the stem ; secondly, as relates to the formation of fresh cells containing the nutritive polyps. On examining any growing branch, it will be found to be soft and open at the extremity, and through the terminal orifice, the soft tegumentary membrane above described as forming the tube by its conversion into hard substance is seen to protrude ; the skeleton is not therefore merely secreted by the enclosed living granular matter, but it is the investing membrane, which continually shoots upwards, and deposits hard material in its substance, as it assumes the form and spreads into the ramifications peculiar to its species, (68.) Having thus lengthened the stem to a certain distance, the next step is the formation of a cell and a new polyp, which is accom- plished in the following manner :* the newly formed branch has at first precisely the appearance and structure of the rest of the stalk of the zoophyte, (Jig. 15, 1,) being filled with granular matter, and exhibiting in its interior the circulation of globules already described, moving towards the extremity along the sides of the tube, and in an opposite course in the middle ; the end of the branch, however, before soft and rounded, soon becomes perceptibly * Lister, Philosophical Transactions, Loc. cit. ON POLYPS. dilated. After a few hours the branch is visibly longer, its extremity more swollen, and the living pith is seen partially to Fig. 15. f have separated itself from the sides of the tube, the boundaries of which become more defined and undulating (2). The growth still proceeding, the extremity is distinctly dilated into a cell, in which the soft substance seems to be swollen out, so as to give a rude outline of the bell-shaped polyp (3), but no tentacula are yet distinguishable ; a rudimentary septum is now visible stretching across the bottom of the cell, through the centre of which the granular matter, now collected into a mass occupying but a portion of the stem, is seen to pass. The polyp and cell gradually grow more defined, (4, 5, 6,) and the tentacula become distinguish- able ; the cell, moreover, is seen to be continued inwards by a mem- branous, infundibular prolongation of its margin (7), which imme- diately reminds us of the funnel-shaped membrane of Tubipora (§ 50), and its office is no doubt similar. As the developement proceeds, the tentacles become more perfect (8), and the polyp at length rises from its cell to exercise the functions to which it is destined. (69.) The third mode of multiplication, or that by reproductive gemmules, seems to be specially adapted to the diffusion of the ON POLYPS. 49 species ; and as it is peculiar to zoophytes of this description, we shall dwell upon it at some length. At certain periods of the year, besides the ordinary cells which contain nutritive polyps, others are developed from different parts of the stem, which may be called female or fertile polyps, although usually simply termed the vesicles. The cells of this kind are much larger than the nutritive cells, and of very different forms ; they are moreover deciduous, falling off after the fulfilment of the office for which they are provided. They are produced in the same manner as the rest of the stem, by an extension of the tegumentary mem- brane, (fg. 14,6,) which, as it expands into the form of the cell, becomes of a horny texture ; it may be traced, however, over the opening of the cavity, where it sometimes forms a moveable oper- culum. The cell being thus formed from the expansion and subse- quent hardening of the tegumentary membrane, it remains to explain the origin of the reproductive germs which soon become developed in its interior.* These are seen to spring from the inner or nutritive layer of the polyp (a), to which they are attached by pedicles, re- garded by authors as fulfilling the office of umbilical cords during their early growth. As the germs expand, they gradually advance towards the opening of the cell, where, as they are protruded, each becomes covered with a layer derived from the tegumentary membrane ( f) which closed the orifice, and protruding externally, has very much the form and appearance of a young polyp, for which indeed it has often been mistaken. We are assured, how- ever, that this supposition is erroneous, and that the polypiform bodies are only external capsules inclosing the real germs (e), from which young polyps are to be formed.^ On tearing open one of these capsules when the included germs are ripe, the latter are seen to be rounded grains of a gelatinous appearance, covered externally with minute cilia, which, like those of the gemmules of the sponge, enable them to swim about at pleasure in search of a proper locality whereon to fix their permanent habitation. These ciliated gemmules are highly irritable, and frequently contract their bodies into different shapes during their progress through the water ; but at length, when about to fix itself, each gemmule becomes flat and circular, and assumes a radiated appearance, resembling a minute grey star, having the interstices between the rays filled with * Lcefling. Miiller's Archives, 1826. — Lister, Loc. cit. t Professor Grant, Edinb. New Philosoph. Journal, 1827. Observations on the spontaneous motions of the Campanularia Dichotoma, &c. 50 POLYGASTKICA. a colourless transparent matter, which seems to harden into horn. The grey matter swells in the centre, where the rays meet, and rises perpendicularly upwards, surrounded by the transparent horny substance, so as to form the trunk of the new zoophyte. The rays first formed are obviously the fleshy central substance of the roots ; and the portion of that substance which grows perpendi- cularly upwards forms the fleshy part of the stem, from which in due time polyps become developed. CHAPTER IV. POLYGASTRICA. ANIMALCULA INFUSORIA. — Auct. (70.) Previous to the discovery of the microscope, it was little sus- pected that animals existed of such minute size as totally to elude the search of unassisted vision ; much less that every drop of water in which animal or vegetable substances have been allowed to decay, swarms with numberless forms of living beings ; that countless millions inhabit every stagnant pool or running stream ; nay, that every drop of the surface of the ocean is in itself a little world, peopled by innumerable active creatures, as various in their out- ward forms as they are elaborately adapted by their internal organi- zation to the circumstances in which they live. The terms Infusoria and Animal cula, as first used by the earliest discoverers of these beings, were applied to an immense number of creatures widely differing from each other in every particular except in the minuteness of their size, which had previously concealed them from observation. The germs of embryo polyps, the larvae of insects, and all microscopic forms of being, including the won- derful tribes of living atoms which inhabit various secretions in the interior of other animals, were thus thrown together in one heteroge- neous and chaotic group, without reference to the structure, rela- tions or habits of the creatures so denominated. This motley assemblage has, however, by subsequent laborious investigations, been separated and arranged so as in some measure to enable us to acquire accurate notions concerning the animals formerly confounded under one common designation. POLYGASTRICA. 51 (71.) The character which distinguishes the class of microscopic creatures which first offers itself for consideration, is derived from the nature of the digestive apparatus with which the creatures com- posing it are provided ;* this consists of a number of internal sacs generally regarded as stomachs, which are easily distinguishable with the microscope, and form a feature in their economy so peculiar, that they are from this circumstance alone at once recognised as an exceedingly natural and well-defined group, allied with each other in the general details of their history, and exhibiting most astonishing powers, not met with in other forms of being. In order to investigate the facts which will be hereafter stated, connected with the history of these animals, the young naturalist must be provided with a good microscope, furnished with glasses capable of magnifying objects from 200 to 1000 diameters, — the last will be seldom needed ; but a power of one-fourth of an inch focus will be indispensable. As some practice and dexterity is requisite in prosecuting researches of this description, a few hints relative to the best methods of procur- ing and observing animalcules will not be improper in this place. It would be needless to advert to the situations in which they are to be found ; every stream and stagnant pool contains some forms in countless numbers ; but, in order to obtain many uncommon species, a little care is necessary. The lemna or duck-weed should be skimmed from the surface of ponds which are exposed to the rays of the sun, or the green film, which not unfrequently covers stagnant waters ; and from these sources examples of most tribes may readily be collected : or else recourse may be had to infusions of various vegetable substances, — of hay, chopped straw, or the leaves of plants, which, if left in open glass vessels, and fully exposed in the open air to the influence of the sun, will in a few days swarm with polygastric animals, sometimes not to be pro- cured by other means. A drop of water derived from any of these sources, if placed upon a thin plate of glass, and covered with a film of talc, will rea- dily enable the observer to examine the beings which inhabit it ; or if it be deemed advisable to insulate the larger species, they may be separated from the rest with a feather, and placed in small tubes or flat troughs in filtered water, and their developement and mode of increase watched from day to day. (72.) We shall now proceed to describe some of the most common forms which the Polygastrica thus procured exhibit. In all water * Ehrenberg. E 2 POLYGASTRICA. containing putrefying vegetable matter, innumerable moving points are visible, scarcely distinguishable except under the highest powers of the microscope, but, when magnified to the utmost, assum- ing the appearance represented at Jig. 16, 1 : these have been termed Monads; and, as they F- lg may well be supposed to be the smallest creatures in ex- istence, have been regarded as the limit of the animal world ; their minuteness, in- deed, is incalculable. Dr. Ehrenberg * has described monads which are not larger than from ToW to T^O o of a line, and which appeared to be separated from each other by intervals not greater than their diameter. Each cubic inch of the water in which they are found must contain, therefore, 800,000 millions of these animalcules, estimating them to occupy but one-fourth of its space. A single drop, brought under the field of the micro- scope, and not exceeding one cubic line in diameter, will there- fore contain 500 millions, equal to the whole number of human beings upon the surface of the globe. Well may the mind, overwhelmed with wonder at such an astounding fact, launch into visionary speculations when contemplating it ; and we are little surprised to see the fertile imagination of Buffon figuring all animal and vegetable bodies as composed of aggregations of these living particles, believing them to be the primitive materials of which organized substances are made up. (73.) The Proteus, (Am ACALEPH^E. 71 Fix. 27. ties of sea-water, our knowledge of their internal structure is at present extremely imperfect. The annexed figure of Diphyes cam- panulifera (^g". 26) will give the reader a general idea of their form. The two bell-shaped portions of which the creature may there be seen to consist, are constantly found united together, and seem to compose but one animal, although they might readily be conceived to be distinct creatures ; the apex of the posterior part is received into a cavity in the other portion, but the connection between the two is so slight, that, when preserved in spirits at least, the slightest touch is sufficient to tear them asunder ; their principal bond of union appears to be a delicate filament, which, arising from the anterior compartment, passes through the whole length of the posterior portion. This strange compound body, concern- ing the structure of which our knowledge is very imperfect, swims through the water with consider- able rapidity, urged forward by the alternate contractions of the two campanulate halves, which con- tinually take in and eject the cir- cumambient fluid, with sufficient force to propel the creature in an equable and uniform course. (99.) Interesting as the acalephse may justly be considered when we contemplate the singular beauty of their external configuration, and the wonderful design conspicuous in their locomotive organs, a more in- timate acquaintance with their habits and economy will be found to disclose many facts not less curious in themselves than important in a physiological point of view. In the higher animals we are accustomed to find the nutritive apparatus composed of several distinct systems ; one set of organs being destined to the prehension of food, another to digestion, a third to the absorption of the nutritious parts of the aliment, a fourth provided for its distribution to every part of the body, and a fifth destined to ensure a constant exposure of the cir- culating fluid to atmospherical influence. These vital operations 72 ACALEPH.E. are carried on in vessels specially appropriated to each ; but, in the class of animals of which we are now speaking, we find but a single ramified cavity appropriated to the performance of all these functions, and exhibiting in the greatest possible simplicity a rough outline, as it were, of systems afterwards to be more fully developed. In the Pulmonigrade acalepha we have the best illustration of this arrangement : in these the stomach or digestive cavity is ex- cavated in the centre of the disc, and is supplied with food by a mechanism which differs in different species. In Rhizostoma, (fig. 21), which receives its name from the nature of the communication between the stomach and the exterior of the body,* the organ destined to take in nourishment consists of a thick pedicle, composed of eight foliated divisions, which hang from the centre of the disc. Each of these appendages is found to contain ramifying canals, opening at one extremity by nu- merous minute apertures upon the external surface, whilst at the opposite they are collected into four large trunks communicating with the stomach ; as the Rhizostoma therefore floats upon the waves, its pendent and root-like pedicle absorbs, by the numerous oscules upon its exterior, such food as may be adapted to its nutrition, finding most probably an ample provision in the mi- croscopic creatures which so abundantly people the waters of the ocean. The materials so absorbed are conveyed through the canals in the interior of the arms into the stomachal cavity, where their solution is effected. But it is not upon this humble prey that some of the medusae feed ; many are enabled, in spite of their apparent helplessness, to seize and devour animals which might seem to be far too strong and active to fall victims to such assailants : Crustacea, worms, mollusca, and even small fishes are not unfrequently destroyed by them. In- credible as this may seem when we reflect upon the structure of these feeble beings, observation proves that they are fully competent to such enterprises. The long tentacula or filaments, with which some are provided, form fishing-lines scarcely less formidable in arresting and entangling prey than those of the Hydra ; and, in all probability, the stinging secretion which exudes from the bodies of these medusae speedily paralyzes and kills the animals which fall in their way. The mouth of these acalephse is a simple aperture leading into the gastric cavity, and sometimes surrounded with * 'Pi&, a root j frozen, a mouth. AC ALE PILE. tentacula, which probably assist in introducing the food into the stomach. In Cassiopea Borbonica, the principal agents in procuring nourishment are numerous retractile suckers, (Jig. 28, a,) terminat- ing in small violet- F/g.28. coloured discs, which are dispersed over the fleshy appendages to the under surface of the body ; the stem of each of these suck- ers is tubular, and conveys into the sto- mach nutritive mate- rials absorbed from animal substances to which they are at- tached during the process of imbibing food. (100.) The above examples will suffice to give the reader an idea of the most ordinary provisions for obtaining nourishment met with in the Pulmonigrada : we will therefore return to consider the structure of the stomach itself, and of the canals which issue from it, and convey the digested nutriment through the system. In Cassiopea Borbonica, which will serve to exemplify the general arrangement of these parts in the whole order, the stomach (Jig- 28) is a large cavity placed in the centre of the inferior surface of the disc, and is apparently divided into four compartments by a delicate cruciform membrane arising from its inner walls. Into this receptacle all the materials collected by the absorbing suck- ers are conveyed through eight large canals, and by the process of digestion become reduced to a yellowish pulpy matter, which is almost fluid, and which is the pabulum destined to nourish the whole body. From the central stomach sixteen large vessels arise, (Jig. 29, c,) which ra- diate towards the circumference 74 ACALEPH.E. of the disc, dividing and subdividing into numerous small branches, which anastomose freely with each other, and ultimately form a perfect plexus of vessels as they reach the margin of the mush- room-shaped body of the creature. The radiating vessels are moreover made to communicate together by means of a circular canal (Jig. 29, e) which runs around the entire animal, so that every provision is made for an equable diffusion of the nutritive fluid derived from the stomach through the entire system. Now, if we come physiologically to investigate the nature of this simple apparatus of converging and diverging canals, we cannot but perceive that it unites in itself the functions of the digestive, the circulatory, and the respiratory systems of higher animals : the radiating canals, which convey the nutritive juices from the stomach through the body, correspond in office with the arteries of more perfectly organized classes ; and the minute vascular ramifications in which these terminate, situated near the thin margins of the locomotive disc, as obviously perform the part of respiratory organs, in as much as the fluids which permeate them are continually exposed to the influence of the air contained in the surrounding water, the constant renewal of which is accomplished by the perpetual contractions of the disc itself. (1 01.) Before closing our description of the alimentary system of the Pulmonigrade acalephse, we must mention some accessory organs of recent discovery which are in connection with it. Eschscholtz* describes a series of elongated granular bodies, placed in little de- pressions around the margin of the disc, which seem to be of a glandular nature, and apparently communicate by means of minute tubes with the nutritious canals : these he regards as the rudiments of a biliary system. Other observers assign a similar office to a cluster of blind sacculi or caeca, which are connected in some species with the commencement of the radiating tubes ; it is, however, scarcely necessary to observe that such surmises relative to the function of minute parts are but little satisfactory. (102.) The Ciliograde acalephse, although their digestive system varies considerably in its general arrangement from what has been described in the Pulmonigrade division, will be found to exemplify in an equally perfect and perhaps more striking manner the for- mation of the vascular and respiratory systems from an extension of the nutritive canals. In the Beroeform species (Jig. 22) the * System der Acalephen. Berlin, 1829. — Annales des Sciences Nat. vol. xxviii. p. 251. ACALEPH.E. 75 alimentary canal passes straight through the globular or barrel- shaped body, commencing at one extremity by two prominent and sensitive lips. No apparatus of prehension is here needful ; for, as these animals swim along by the action of their cilia, the water passes freely through this capacious channel, and brings into the stomach materials proper for food. From both extremities of the digestive cavity arise vascular canals which empty themselves into two circular vessels, one surrounding the oral, and the other the anal portions of the body : from these two rings eight double vessels arise, which run longitudinally from one pole to the other of the creature beneath each of the cartilaginous ribs upon which the cilia are placed ; and from these, others more minute arise, which are distributed in a delicate network through the sub- stance of the animal. In the Beroe, therefore, we must regard the vessels which convey the nutritive juices beneath the ciliated arches, not merely as arteries, but as organs of respiration ; for, thus placed close beneath the outer surface of the body, the water, which is perpetually made to rush over them by the ciliary move- ments, will serve to aerate the fluid contained within. The Cestum Veneris (fig. 23) is nearly allied to the Beroe in the arrangement of its nutritive apparatus, notwithstanding the difference of form observable in these Ciliograde medusae. In Ces- tum the digestive cavity, which is exceedingly short in comparison with the length of the animal, passes transversely across the body in a straight line from one side to the other, as represented in the engraving (fig. 23) ; but the details of its structure, and the nature of the vessels aris- ing from it, will be best understood by a reference to the enlarged diagram of these parts given in the annexed figure (fig. 30). The mouth (i) is a rhomboidal depression seen near the centre of the body, between the two lateral rows of locomotive cilia which extend from one end of the animal to the other. From the mouth arise two tubes, (/,y,) which terminate in a globular cavity common to both ; these would seem to constitute the digestive apparatus : and a straight and narrow tube (o), prolonged to the margin of the body opposite to that which the mouth occupies, may be regarded Fig. 30. 76 ACALEPH.E. as an intestine through which the residue of digestion is discharged. From around the oral extremity of the stomach, and from the glo- bular cavity in which the two principal canals terminate, arise ves- sels, (£, t, £,) which diverge so as to form a cone at the base of which they all empty themselves into two circular canals, one surrounding the mouth, and the other encircling the anal aperture ; which pre- cisely correspond with the vascular rings already described in the Beroe : and, from these, four long vessels, or branchial arteries as they might be termed, (/?, p ; 84 STERELMIXTHA. matter, which escapes when the worm is torn asunder (d) ; but whether it possesses a true alimentary tube, is not as yet satis- factorily determined. (115.) The Tania., or tape-worms, are among the most inter- esting of the Sterelmintha, whether we consider the great size to which they sometimes attain, or the singular construction of their compound bodies. Several species of these worms infest the human body, and many other forms of them are met with in a variety of animals. They are usually found in the intestinal passages, where, being amply provided with nutritious aliment, they fre- quently grow to enormous dimensions, being not unusually twenty or thirty feet in length, and some have been met with much longer ; it is therefore manifest how prejudicial their pre- sence must prove to the health of the animals in which they reside, and we are little surprised at the emaciation and weak- ness to which they generally give rise. The T