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The following work contains the result of many years' careful study of the fresh-water re. presentatives of a group of animals which, in all their relations, are full of interest for the philosophic naturalist. The highly curious modification of the Molluscan type which the Polyzoa present, their singular repetition in this type of the physiognomical features and habits of a totally different one, the great beauty of their forms, and the facility with which they can in general be observed in a living state, cannot but render them special favorites for every lover of Nature; and for the more profound student must confer on them a peculiar significance, and invest their study with a scientific interest which is scarcely surpassed by that of any other group of animals; while the fresh-water species, by certain remarkable peculiarities of structure, throw an unexpected light on the general plan and affinities of the class. In the preparation of the monograph no trouble has been spared to render it as complete as possible, and the subjects of which it treats have been considered under every point of view of which they seemed susceptible—zoographically, zootomically, homologically, and historically. Nearly every species has been carefully examined in a recent state, while the anatomical observations have been over and over again repeated for the purpose of verification, and many hundred specimens have thus passed beneath the dissecting needle. Of all the known fresh-water genera there are but two, namely, Pectinatella and Urnatella, which I have not yet had an opportunity of examining in a living condition. Both these genera, each consisting of a single species, are confined to the United States of America, where they were recently discovered by Dr. Leidy, who has given us a description of them, which, however, is purely zoographical. To Dr. Leidy's promised anatomical account of Urnatella, we cannot but look forward with impatience, while, in the mean time, I have had a woodcut prepared from a pencil sketch kindly furnished me by Dr. Leidy, so that I am enabled to introduce into the present work, a figure representing some of the more important features in the structure of this remarkable Polyzoon. All the figures upon the eleven lithographic plates which accompany this volume have been drawn from Nature, and contain careful representations of every species which I have seen. They have been engraved by Mr. Tuffen West, who has spared no pains in rendering the original drawings as faithfully as possible. In every case I have given a figure of the species both in its natural size, and magnified, and I have never omitted to draw the polypide as well as the coenoecium, believing that the latter will by itself convey but a very imperfect idea of the real character of these beautiful little animals. With regard to the few species which I have not had an opportunity of personally examining, I have availed myself of the existence of published drawings whenever they were to be found, and under the description of these species have given a woodcut copy of the drawing. The zoographical portion of the volume has thus been rendered as complete as possible for the practical identification of the species. In fixing the exact limits of the species, some difficulty has been experienced. I have however, deemed it best to describe as a distinct species every well-marked variation of form which I could not find connected by intermediate gradations with other forms, and which could not, with probability, be referred to the accidental influence of external agencies. It is possible that some of the forms here described as distinct species may afterwards prove to be only varieties of one and the same specific group; but as they all possess a real existence, and are truly distinct forms, it was thought necessary, especially in so small a group as the present, to bring them definitely before the student, even though the future discovery of intermediate forms may disentitle them to a proper specific rank. While the present work, in its purely zoographical relations, is entirely confined to those species which inhabit the fresh water, its anatomical details have a much more extended application. The Polyzoa constitute an exceedingly natural group, and possess great uniformity of structure, and as the fresh-water species afford fine typical examples of the class, a work devoted to the anatomy of these will apply in all essential points to that of the entire chass, while such points of structure as are peculiar to the fresh-water forms will only tend to illustrate and explain the structure of the marine ones; so that the present monograph, in its anatomical relations, may be fairly regarded as a general treatise on polyzoal organization. While the Fresh-water Polyzoa have been carefully studied on the Continent, especially by Van Beneden and Dumortier, they have hitherto (if we except an excellent paper by Mr. Albany Hancock) received in this country but little attention; and yet, even apart from its scientific interest, few departments of microscopic observation will be found to possess more attractiveness. There is scarcely a pond or canal of clear water where some of the species may not be found; most of them occur often in great abundance in the waters round London, and a slight acquaintance with their habits, as described in the present work, will render them very easy of detection; while few objects are capable of affording greater pleasure than these beautiful little molluscoids when examined in a living state under a moderate power of the microscope. Notwithstanding the large proportion of species which will be found described and figured for the first time in this monograph, I have little doubt that the discovery of many more will reward a patient exploration of their habitats. I have endeavoured to render the work as practically useful as possible, and one of the advantages which I hope to see result from its publication will be the placing in the hands of the naturalist a manual which may facilitate further study of a group so full of interest, and among which I doubt not that many new facts still remain to be discovered. To the many friends who, by the communication of specimens, and by much valuable information, have kindly assisted me in the preparation of this monograph, I beg once for all to return my best thanks. My acknowledgments are especially due to Mr. Bowerbank, Mr. Quekett, Mr. Busk, Mr. Albany Hancock, Mr. Huxley, Dr. Dickie of Belfast, Dr. Hassall of London, Mr. Wigham and Mr. Brightwell of Norwich, Professor Bailey of West Point, United States, and Dr. Leidy of Philadelphia. To the Council of the Ray Society I must also express my grateful acknowledgments for the ready and obliging manner with which they have invariably complied with my requests, and for the patience with which they have submitted to the delay which has taken place in the publication. The work, indeed, ought to have been long since in the hands of the Society, but has been unavoidably delayed by the accession of new and largely increased professorial duties, which rendered it impossible for me to have it ready for the press so soon as I had hoped, and which must now plead my excuse for the lateness of its appearance.

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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
distinct class.

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,t 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 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 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.f 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 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.

* Ferante Imperato, ‘Historia Naturale,” Napoli, 1599.

+ In a letter to the Abbé Bignon. See Marsigli, ‘Brieve ristretto del Sagio fisico intorno alla Storia del Mare,” Venezia, 1711.

f : Mém. de l'Acad.,’ 1727. § ‘Mém. de l'Acad.,’ 1742. | Phil. Trans.,’ 1703.

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.”f 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 Linnaeus, 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 Tubularidae, Sertularidae, Gorgoniae, &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 Linnaeus 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. f : Mémoires pour servir à l’Histoire des Insectes,’ Paris, 1742, tome xvi, Preface.

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