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Historical view of the facts which finally led to the establishment of the Polyzoa as a
It was in the last year of the sixteenth century that the Neapolitan, Ferante Imperato, asserted that corals possessed the nature of animals.* The announcement had passed away nearly unheeded; and if at the end of more than a hundred years afterwards any lingering douhts as to the vegetability of these productions still remained, it was deemed that the discovery of the Count de Marsigli, who, in the year 1706,+ declared that he had seen coral in flower, must have totally dissipated them.
Not so, however, thought Jean Andre Peysonelle, physician at Marseilles; he soon saw that the flowers described by Marsigli were nothing more than the beautiful starry polypes of the coral, and he maintained that these were true animals of the same nature as the Actiniae of the rocks, whose animality was sufficiently obvious to leave no room for doubt, and that the hard stony coral was a peculiar habitation built by these creatures for their protection.
But Peysonelle had few followers; even Reaumur, to whom he had intrusted a communication on this subject, with the intention of having it presented to the Academy of Scie nee at Paris, not only strongly opposed the views of Peysonelle, while laying them before the Academy, but, through consideration for the reputation of the author, deemed it right to suppress his name in connection with so absurd and visionary a doctrine. |
Among those, however, who saw something more than the mere dreams of a visionary in the doctrines of Peysonelle, was the celebrated botanist, Bernard de Jussieu. He felt their importance; and, in 1741, he visited the coast of Normandy, in order to subject them to the test of actual observation. De Jussieu had here an opportunity of examining the Alcyoniums, Sertulariae, Flustrae, and other flexible and plant-like productions to which the observations of Peysonelle had not extended, and the result was a complete conviction of their animality, and a firm adherence to the doctrines of Peysonelle.
In communicating to the Royal Academy of Sciences $ the result of his observations, B. de Jussieu employed the word polype to designate the various productions whose animal nature had thus been so satisfactorily determined, a term which up to the present day has been in general adoption.
It was just at this time that Abraham Trembley was engaged in his famous experiments on the Hydra, which Leeuwenhock had originally discovered, attached to the leaves of Lemna3 along with various fresh-water Infusoria; and of which he had in 1703 communicated a notice to the Royal Society of London. ||
The close relation of the Hydra with the marine polypes was now abundantly apparent, and the important light which its study shed upon the nature of these polypes, has invested its discovery with a peculiar interest as marking out a distinct epoch in the progress of zoology.
* Ferante Imperato, ' Historia Naturale/ Napoli, 1599.
t In a letter to the Abbe Bignon. See Marsigli, 'Brieve ristretto del Sagio fisico intorno alia Storia del Mare,' Venezia, 1711.
% 'Mem. de l'Acad./ 1727. § 'Mem. de l'Acad./ 1742. || 'Phil. Trans./ 1703.
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 appreciate 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 carbasea 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 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.
* '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.
t Observations on the Structure and Nature of Flustrae, 'Edinb. New Philosophical Journal,' vol. iii, 1827.
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 ' Symbola? 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.f
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 a 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.
* 'BeitriLge zur physiologischen kcnntniss der Corallen-thiere im algemeinen, und besonders des Rothen Meeres.'
f See an admirable criticism by Busk on the Priority of the term Polyzoa, in the 'Annals of Nat. Hist.,' vol. x, 1852.