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the same field, and his researches, which were thus entirely independent, led him to the conclusion, “that these apparent plants were ramified animals in their proper skins or cases.” In 1755, he published his famous ‘Essay on the Natural History of Corallines,” a work which in profuseness and fidelity of observation, in lucidity of description, and in pictorial illustration, seemed to leave little else to be accomplished. Linnaeus, who, as we have just seen, met the animal theory only half way, was never entirely convinced; he continued, too, for some time, to have his followers, but Ellis had sapped the very foundations of the vegetable theory, and in a few years, notwithstanding bitter opposition from some isolated quarters, the question was finally set at rest in the general admission of the animality, not only of the true corals and madrepores, but of all those flexible and horny productions whose plant-like form was at such variance with every previously conceived notion of animal existence. It was not yet suspected that among these curious “zoophytes,” so like one another in external form, there were still two totally distinct types of animal organization; and the attention of naturalists was now chiefly directed to the comparison of external characters for the determination of species, and as the grounds of classification. Numerous systems were accordingly from time to time proposed, which, however, were all more or less artificial, and involved the fundamental error of assuming the external calcareous or horny covering as a character of primary importance, and, as a necessary consequence, the association of forms of a widely different plan of structure. In the mean time the number of known species had greatly increased, and collections, both on our own and foreign shores, had enriched this department of natural history to an extent of which few others could boast. Important improvements too had taken place in our means of observation, and the value of anatomy in the determination of the true rank of organic beings had been very generally recognised. Zoologists were thus prepared to appre

ciate the importance of the new light which was about to be thrown upon the structure of

these plant-like productions. In March, 1827, Professor Grant read before the Wernerian Society a Memoir on the structure of Flustrae.f. In this Memoir the author has described the locomotive embryos of Flustrae ; he also gives an account of the animals of Flustra carbased and F. foliacea, and shows them to be quite different from the hydroid polype of the Sertulariae ; but he seems as yet to have had but an imperfect knowledge of them, and I cannot find anything in the Memoir to justify the belief that this excellent zoologist was acquainted with the complete intestinal canal of the animal. In September of the following year MM. Auduin and Milne-Edwards presented to the Royal Academy of Sciences, in Paris, a summary of their researches on the invertebrate animals of the Chausey Isles, a group of small rocky islands off the coast of France. Among these researches the investigations of M. Edwards into the Flustrae hold a prominent place. No one could have come better prepared for the task than the celebrated French zoologist. Already long devoted to the study of the invertebrate animals, and just fresh from a series of

+ Essay towards a Natural History of the Corallines and other Marine productions of the like kind, commonly found on the Coasts of Great Britain and Ireland,’ London, 1755.

+ Observations on the Structure and Nature of Flustrae, “Edinb. New Philosophical Journal,” vol. iii, 1827.

most valuable investigations into the Compound Ascidiae, M. Edwards, without any knowledge of the labours of Grant in the same field, not only demonstrated in a very complete manner the anatomy of the Flustrae, but was the first to call attention to the close connection of their structure with that of the Ascidiae, and the important bearing of this connection on their systematic rank." Cuvier had already determined a striking difference of structure between the Sertularian Polypes and those of Alcyonium, and a third type of structure, widely different from both, was now pointed out by M. Edwards as characterising the Flustrae and their allies, which he proposed to distinguish under the name of “Polypes tuniciens.” At the same time that the British and French zoologists were engaged in these interesting discoveries, an accomplished and philosophic naturalist, Mr. J. W. Thompson, then officially stationed at Cork as deputy-inspector-general of hospitals, was making a series of observations on the marine productions of the coast. These observations had been commenced several years before, and had already resulted in the celebrated discovery of the metamorphosis of the Crustacea; it was not, however, until December, 1830, that they were given to the public, In this year the author published the first number of a projected periodicalt which was to contain a series of original memoirs on zoological subjects. Among the many important papers contained in this publication was one “On Polyzoa, a new animal discovered as an . inhabitant of some Zoophytes.” Thompson had examined the animals of Bowerbankia imbricata, Walkeria cuscuta, W. pustulosa, Vesicularia spinosa, and other allied forms, and the difference of their structure from that of the Sertulariae, with which they had been hitherto classed, struck him in its full force. He traced the entire course of the alimentary canal, and made himself master of almost every important point in their structure. He also perceived their close relation to the compound Ascidiae, and was the first to designate them by a distinct name which no longer assumed their connection with the polypes. The name thus proposed by Thompson for the group whose structure he had so well investigated was “Polyzoa.” Residing in a remote part of Ireland, and in a great measure cut off from intercourse

* Dumortier and Van Beneden (“Hist. Nat. des Pol. comp. d'eau douce') assign to Lamouroux the merit of having been the first to detect the affinity between the Polyzoa and the Ascidiae. Lamouroux's views, however, upon this point are exceedingly vague and incorrect; the difference between the Polyzoa and the true radiate zoophytes never occurred to him, and he imagines the affinity with the Mollusca to be possessed by the entire group of his polypes a polypiers. “J’ai dit,” says he (‘Exposition Méthodique des Genres de l’Ordre des Polypiers', Preface, p. vii), “que les polypes a polypiers ne pouvaient en aucune manière se comparer aux Hydres d'eaux douce, sous le rapport de l'organisation; qu'ils étaient plus voisins qu'on ne le pensait de la nombreuse famille des Mollusques, et qu'avec le temps on en ferait peutétre une division de cette grande classe. Les nouvelles observations que les circonstances m'ont permis de faire me confirment dans cette idée, et je me doute plus que les animaux des polypiers ne soient des étres aussi compliqués dans leur organisation que les Mollusques Ascidiens.” He then goes on to compare the walls of the body cavity in Gorgonia, Isis, Alcyonium, &c., to the tunics of an Ascidian or the mantle of an ordinary Mollusc, a comparison which plainly shows that he had no idea whatever of the relation in question.

+ ‘Zoological Researches and Illustrations.” By John V. Thompson. Cork, 1830. The date is not printed on the title-page; but is to be found on the paper wrapper in which the publication was originally stitched.

with scientific men, Thompson was quite unaware, when he published the result of his researches, that Grant and Edwards had been before him in the field; his observations are, therefore, original and independent, and, as he tells us they were made in the summer of 1820, it would seem that it was only the delay of publication that has deprived him of the honour of being the first to record a discovery so important in the history of zoology. In 1834 Ehrenberg published his “Memoir on the Corals of the Red Sea.” In this work he proposed a new classification of the so-called polypes, dividing the entire group into two great sections—the Anthozoa and the Bryozoa the former embraced the true radiate forms, the latter corresponded to the Polyzoa of Thompson. Though the term Bryozoa had already been used by him in a number of the ‘Symbolae Physicae,’ published in June, 1831, the priority of publication is still left with Thompson's name, and though Ehrenberg's term is in general use upon the Continent, and is largely adopted even by English writers, simple justice and the laws of natural-history nomenclature demand the adoption of the term Polyzoa, and it is it, therefore, which I have employed in the present Memoir.t Notwithstanding, however, the completeness with which the ascidian type of structure had now been recognised in the Polyzoa, naturalists had not yet emancipated themselves from the old notion that the closest affinities of these animals were still with the Polypes, and the Polyzoa, therefore, long continued to be classed with the Polypes, of which they were still considered as a group, though with distinct peculiarities, through which the Polypes manifested an affinity with the Tunicata. It is not easy to say to whom we are indebted for the first absolute withdrawal of the Polyzoa from the Radiate sub-kingdom, and their location among the Mollusca. The obvious justice of the step must have simultaneously presented itself to every naturalist who had made the matter a special subject of study, while the important division of the Molluscan subkingdom by Milne Edwards into the two primary sections of the Mollusca and the Molluscoida, the latter including the Tunicata and the Polyzoa, leaves nothing now to be desired in the systematic location of the Polyzoa. In the history of progressive discovery which has thus been sketched, eight distinct epochs must be noted, each characterised by some one step which has more or less directly led to the views at present entertained of the true affinities and systematic position of the Polyzoa. 1. The assertion by Imperato of the animality of coral. 2. The discovery by Marsigli of the polypes of coral, which he mistook for its flowers. 3. The determination of the true nature of these polypes by Peysonelle. 4. The discovery of the Hydra by Leuwenhoeck. 5. The discovery of the “Polype à Panache,” and the determination of its structure by Trembley and Baker. 6. The determination of the structure of certain marine Polyzoa by the independent and nearly simultaneous labours of Grant, Edwards, and Thompson; and the recognition of the affinity of these productions with the compound Ascidians by Edwards and Thompson. 7. The designation, by a common independent name, of these animals by

Thompson. 8. The entire withdrawal of the Polyzoa from the Radiata and their association with the Mollusca.

* “Beiträge zur physiologischen kenntniss der Corallen-thiere im algemeinen, und besonders des Rothen Meeres.”

t See an admirable criticism by Busk on the Priority of the term Polyzoa, in the ‘Annals of Nat. Hist.,’ vol. x, 1852.

ANATOMY AND PHYSIOLOGY.

1. PRELIMINARY MATTER. General conception of a Polyzoon.

PLAN of A Polyzoon. Fig. 1. Exserted. Fig. 2. Retracted. * Region of mouth, surrounded by tentacles. b. Alimentary canal. c. Anus. d. Nervous ganglion. e. Membranous investing sac. f. Testis. f*. Ovary. g. Retractor muscle.

Let us imagine an alimentary canal, consisting of oesophagus, stomach, and intestine, to be furnished at its origin with long ciliated tentacula, and to have a single nervous ganglion situated on one side of the oesophagus. Let us now suppose this canal to be bent back upon itself towards the side of the ganglion so as to approximate the termination to the origin. Further, let us imagine the digestive tube thus constituted to be suspended in a fluid contained in a membranous sac with two openings, one for the mouth and the other for the vent; the tentacula alone being external to the sac. Let us still further suppose the alimentary tube, by means of a system of muscles, to admit of being retracted or protruded according to the will of the animal, the retraction being accompanied by an invagination of the sac so as partially or entirely to include the oral tentacula within it; and if to these characters we add the presence of true sexual organs in the form of ovary and testis occupying some portion of the interior of the sac, and the negative character of the absence of all vestige of a heart, We shall have perhaps as correct an idea — apart from all theoretical considerations of homology or derivation from an archetype—as can be conveyed of the essential structure of a Polyzoon in its simplest and most generalised condition.

To give, however, more actuality to our ideal Polyzoon, we may bear in mind that the immediately investing sac has the power, in almost every case, of secreting from its external

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* 8 DEFINITION OF TERMS.

surface a secondary investment of very various constitution in the different groups; and we may, moreover, conceive of the entire animal with its digestive tube, tentacula, ganglion, muscles, generative organs, circumambient fluid, and investing sacs, repeating itself by gemmation, and thus producing one or more precisely similar systems holding a definite position relatively to one another, while all continue organically united, and we shall then have the actual condition presented by the Polyzoa in their fully developed State.

Definition of Terms.

The old notion, which, by mistaking the zoological rank of the Polyzoa, erroneously referred them to the class of the Polypes, caused the same terms to be applied to them which were also used to designate the various parts of the true Polypes. The recognition, however, of a type of structure in the Polyzoa totally distinct from that of the Polypes proper, necessitates a change in the terminology employed in their description. On these grounds I have ventured to substitute some new terms for those previously used, while our increased knowledge of polyzoal structure necessitates the use of certain additional terms of which we have no representatives in the descriptive terminology of previous authors. For the term Polype, therefore, originally applied not only to the polypoid Radiata, to which its use ought to be confined, but also to the retractile portion of the Polyzoa, I have substituted in the following memoir that of Polypide.” To the common dermal system of a colony, which, as well as the solid basis of the true polypes, was formerly known under the names of Polypary and Polypidome, I have applied the term Canacium.t The coenoecium is composed almost universally of two perfectly distinct tunics; to the external I have given the name of Ectocyst, { and to the internal that of Endocyst." The sort of disc or stage which surrounds the mouth and bears the tentacula, I have called Lophophore. The Epistome" is a peculiar valve-like organ which arches over the mouth in most of the fresh-water genera. The Periyastric" space is the space included between the walls of the endocyst and the alimentary canal.

The terms now enumerated are such as I believe the nature of the subject strictly requires. I employed most of them in my “Report on Fresh-water Polyzoa' published in 1850, and though I am fully aware that the changing of an established terminology is highly objectionable where it can possibly be avoided, yet in the present case, where new facts have been accumulated requiring new words for their expression, and where the very same terms have been in two different classes of animals loosely applied to organs in no respect homologous, the purposes of a rigidly scientific description can, I believe, only be served by some such change as that suggested.

Besides these terms, and some which will be explained as they occur, two others in common use ought to be here defined. The cells are the little chambers of which the coenoecium is made up, and in each of which a polypide is lodged. The part of the cell through which the polypide admits of protrusion and retraction is the orifice of the cell.

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