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THE FRESH-WATER POLYZOA.
AMONG the most beautiful and interesting forms of invertebrate animals are those strange phytoidal productions which, long confounded with the polypes, were at last, by the nearly simultaneous investigations of several naturalists, separated as a distinct group, and described by Thompson under the name of Polyzoa, and shortly after indicated by Ehrenberg under that of Bryozoa. They are chiefly inhabitants of the sea, where they may be witnessed under numerous plant-like guises; now spreading like a lichen over submerged stones, or old shells, or the broad fronds of Laminaria and other sea-weeds; now forming soft, irregular, fungus-like masses, or hard, calcareous, branchy growths, like diminutive trees; and now again presenting the appearance of the most delicate and exquisitely formed sea-weed or moss, offering, even to the unassisted eye, in the endless repetition of the same element of form, objects of surpassing symmetry and beauty.
The Polyzoa, however, are not by any means exclusively confined to the ocean; and though by far the greater number are marine, yet in the still and running waters of the land—in the broad river and the rushing stream—in the pure, cold mountain lake and the stagnant waters of the moory fen, species are to be found, which in interest yield not one jot to their brethren of the sea, and offer to the naturalist an inexhaustible source of gratification, in the beauty of their forms and the wonders of their organization.
It is to these fresh-water species, which, independently of their peculiar habitat, possess certain characters entitling them to be viewed as a group apart from the marine representatives of the class, that the present work is to be devoted. It will be well, however, before entering into the detailed treatment of our subject, to take a general historical view of the facts which led to the establishment of the Polyzoa as a distinct class of the animal kingdom. These facts are so intimately mixed up with the gradual development of correct views as to the nature of the true Polypes, with which the Polyzoa had, until recent times, been confounded, without any suspicion of the wide interval by which they were really separated, that their historical statement will necessarily involve a rapid glance at the progressive steps made by the earlier naturalists in the determination of the animality of corals.
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 doubts 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 André 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 Actiniæ 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 Science 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, Sertulariæ, Flustræ, 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 Lemna, 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.
The celebrity conferred on Trembley by his researches into the structure and economy of Hydra, is known to every one in the least acquainted with the literature of zoology; but the assiduity of the famous historian of the fresh-water polype was destined to be rewarded by a discovery perhaps just as important as that of the economy of Hydra. It was in the month of April, 1741, that Trembley, while engaged in his researches on Hydra, discovered, in the fresh waters near the Hague, an animal form then quite new to science. It consisted of a lobed jelly-like mass, from which protruded numerous polypoid bodies, each characterised by the possession of an elegant crown of tentacula borne on the margin of a crescent-shaped disc. This beautiful tentacular plume is one of its most striking features; and as Trembley naturally supposed his animal to be intimately related with the polypes, it suggested the name of "Polype à Panache," by which he subsequently designated it.*
Almost immediately after this the same species was detected in England by Baker, who subsequently described it under the name of "Bellflower animal." Both Trembley and Baker bestowed upon their new animal a careful and accurate examination, and have thus made us acquainted with a very remarkable type of structure-a type, however, whose significance was destined to remain for nearly a century unrecognised, and it was not until a similar one in certain marine polypoid animals arrested the attention of naturalists, that its importance, and its true bearing on systematic zoology, began to be appreciated. The investigations of Trembley and Baker, however, having clearly demonstrated, in the "Polype à Panache," all the essential characters of polyzoal structure, must be viewed as marking out another most important epoch in the progress of zoological research.
Among the converts which the discoveries of Trembley and Jussieu had made to the animal theory of corals, was Reaumur, who, convinced by their reasoning, withdrew the opposition with which he had met the announcement of Peysonelle, and now ranged himself among the most strenuous supporters of the new doctrine. Still, however, assent was far from universal, and the greater number of naturalists continued to believe in the vegetability of corals, and denounced the new opinions as false and absurd. Even the celebrated Linnæus, though he admitted the animality of the stony corals, or lithophytes as he termed them, could never bring himself to express unqualified belief in the animality of those horny and flexible forms which embraced the Tubularidæ, Sertularidæ, Gorgoniæ, &c., and most of the Polyzoa of modern zoologists; and accordingly he took a sort of middle ground, maintaining that these productions possessed a double nature, that their stems and branches grew by a true vegetation and possessed the essential characters of plants, while their polypes were certain inflorescences or developments of the vegetable axis in which the vitality had become exalted from the vegetable into the animal.
Even this partial admission by Linnæus of the flexible corals and Polyzoa into the animal kingdom was due to the discoveries of John Ellis, a London merchant, who, amid the engrossing cares of his counting-house, could yet throw open his heart to the love of Nature, and find time for the cultivation of science. In the year 1752 Ellis presented to the Royal Society the result of his first observations on the nature of these creatures. He seems to have then known but little of the labours of his predecessors in
* Mémoires pour servir à l'Histoire d'un Genre de Polpes d'Eau douce,' Leide, 1744, Mém. III.
+ Employment for the Microscope,' London, 1753.
Mémoires pour servir à l'Histoire des Insectes,' Paris, 1742, tome xvi, Preface.
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.
Linnæus, 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 Flustræ. In this Memoir the author has described the locomotive embryos of Flustræ; 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 Sertulariæ; 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 Flustræ 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 Flustræ, Edinb. New Philosophical Journal,' vol. iii, 1827.
most valuable investigations into the Compound Ascidia, 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 Flustræ, but was the first to call attention to the close connection of their structure with that of the Ascidia, 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 Flustræ 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. V. 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, Valkeria cuscuta, V. pustulosa, Vesicularia spinosa, and other allied forms, and the difference of their structure from that of the Sertulariæ, 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 Ascidia, 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 Ascidiæ. 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 à polypiers. "J'ai dit," says he (Exposition Méthodique des Genres de l'Ordre des Polypiers', Preface, p. vii), "que les polypes à 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 peutêtre une division de cette grande classe. Les nouvelles observations que les circonstances m'ont permis de faire me confirment dans cette idée, et je ne doute plus que les animaux des polypiers ne soient des êtres 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.
The date is
+ 'Zoological Researches and Illustrations.' By John V. Thompson. Cork, 1830. not printed on the title-page; but is to be found on the paper wrapper in which the publication was originally stitched.