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name of Fredericella, called in honour of Frederic Cuvier, and he gives a tolerably extensive synonymy of the species of fresh-water Polyzoa, which he distributes under the five genera, Cristatella, Alcyonella, Plumatella, Paludicella, and Fredericella; he, however, still maintains the generic identity of Lophopus and Plumatella. He divides the whole, as he had done in his previous paper, into two sub-classes, Polypiaria hippocrepia and P. infundibulata. In the first volume of the Supplement to the “Dictionnaire des Sciences Naturelles,' published in 1840, M. Gervais, in the article Alcyonella, again gives us the result of his researches into this subject; he still rejects the generic distinctness of Lophopus, and even views Alcyonella and Plumatella as mere species of the same generic form. In 1841, M. Coste read to the Académie des Sciences a paper on the organization of the fresh-water Polyzoa; * though unaccompanied by figures, M. Coste's memoir gives a very good account of most of the important points in the anatomy of the hippocrepian Polyzoa; it describes the muscular, digestive, and nervous systems with much exactness and with considerable detail; and maintains the expediency of removing the Polyzoa out of the Radiata, and dlacing them among the Mollusca. A few months later, M. Coste addressed to the Académie a short note on the “Tubulaire sultane” (Fredericella sultana),t in which he announces that the organization of this Poylzoon is in general quite conformable to that of the hippocrepian species described in his former communication. In the year 1842, MM. Dumortier and Van Beneden read, before the Royal Academy of Brussels, the first of a series of papers, which these naturalists proposed presenting conjointly, on ‘the Compound Polypes of fresh water.'t This memoir consists entirely of a historical introduction to the subject; it is most elaborate and learned, and I have derived much assistance from it in drawing up the present historical outline. In 1843, I read to the Royal Irish Academy a memoir “On the Muscular System of Paludicella and other Ascidian Zoophytes of fresh water.'s The paper is chiefly occupied with the description of the muscles of Paludicella. They are divided into three groups—one belonging to the alimentary canal, one to the tentacular sheath, and one to the walls of the cell. Being then an inexperienced observer, and possessing but an imperfect microscope, I was unable to pursue the anatomy of the animal as far as could be desired, and several interesting points of structure escaped me at the time. The retractor muscles of the alimentary canal in Plumatella repens, and the radiating muscles of its sheath, are also described in this memoir, though I then overlooked the fact that the latter are divisible into an anterior and a posterior set; while Dr. A. Farre's account of the action of certain muscles in the Polyzoa is examined, and an attempt made to explain such action in a manner somewhat different from the views entertained by this anatomist.

* CostE, Propositions sur l'organisation des polypes fluviatiles. ‘Comptes Rendus,’ 1841.

+ Costs, Observation relative à la Tubulaire sultane. “Comptes Rendus,’ 1841.

# DuMontier et VAN Beneden, Histoire Naturelle des Polypes composés d'eau douce. “Nouveaux Mémoires de l'Acad. Roy. de Bruxelles,’t. xvi.

§ AllMAN, On the Muscular System of Paludicella and other Ascidian Zoophytes of fresh water. • Proc. Roy. Irish Acad.’ 1843.

At the meeting of the British Association for the Advancement of Science, held at Cork, in the same year, I read a paper on Plumatella repens.” In this paper it was attempted to reduce to some sort of order the chaotic accumulation of synonyms by which the species is encumbered. This was facilitated by distinguishing two variations which P. repensis found to assume, according as it grows upon surfaces of small or large extent, and from not attending to which much confusion had arisen; one of these variations is presented in the “Federbuschpolyp” of Rösel, the other in the “Kammpolyp” of Schäffer, which Müller afterwards assumed as the form on which he founded his species Tubularia repens. The digestive apparatus was also described, as well as the muscles which act on the alimentary canal and invaginated tunic; it was shown that the latter muscles consisted of two distinct sets, an anterior and a posterior, whose peculiar arrangement was pointed out.

On the same occasion I presented to the meeting a short paper, containing a synopsis of the genera and species of fresh-water Polyzoa occurring in Ireland.t In this paper two new species of Plumatella and one of Fredericella are described, and the peculiarities of Paludicella were deemed of sufficient importance to raise this genus to the type of a distinct family among the fresh-water Polyzoa.

In 1846, I described, in a paper read before the Royal Irish Academy, the locomotive ciliated embryos of Plumatella fruticosa;+ and at the meeting of the British Association for the same year, presented to that body the result of some observations I had made on the structure of Cristatella.S The nervous ganglion of this Polyzoon was described, and the muscular fibre was shown to be striated, and to have a tendency to break into discs. The statoblasts (or the ova, as I then, in common with other observers, believed them to be), were described as enclosed, during their young state, in a ciliated membranous sac, within which the hooked spines, of which they are at first destitute, are afterwalds developed.

In the second edition of the ‘History of British Zoophytes,’ published in 1847, Johnston subjects the portion of his work which treats of fresh-water Polyzoa to a complete revision, and now distributes the British species under the genera Cristatella, Alcyonella, Plumatella, Fredericella, and Paludicella; he, however, still maintains the specific identity of the “Polype à Panache” with the Alcyonella stagnarum.

In 1848, we obtain from M. Van Beneden a very valuable memoir on the fresh-water Polyzoa of Belgium. The author here enters into important details of the anatomy and classification of the fresh-water Polyzoa, and gives excellent descriptions and copious synonyms of the several species. After the lapse of nearly a century, he restores to the Alcyonella staynarum of Lamarck the specific name fungosa originally bestowed on it by Pallas. This is but an act of justice to its discoverer, and ought to be followed by subsequent systematists. Van Beneden also makes the addition of a second species of Alcyonella, which he describes under the name of A. flabellum.

* ALLMAN, On Plumatella repens. “Reports of British Association,’ 1843. " f ALLMAN, Synopsis of the genera and species of Zoophytes inhabiting the fresh waters of Ireland. “Reports of British Association,’ 1843; and ‘Ann. and Mag. of Nat. Hist,” May, 1844. st ALLMAN, On the Larva state of Plumatella. ‘ Proc. R. I. Ac.,’ 1846. . § ALLMAN, On the Structure of Cristatella mucedo. “Reports of British Association,’ 1846.

| WAN BENEDEN, Recherches sur les Bryozoaires Fluviatiles de Belgique. ‘Mém. de l'Acad. Roy. de Belgique,' tome xxi, 1848.

In the same year, MM. Dumortier and Van Beneden presented, conjointly, to the Royal Academy of Sciences at Brussels, a second and very important memoir on the “Natural History of the fresh-water Polyzoa.' * This memoir is occupied with the anatomy of the genera Paludicella, Fredericella, Alcyonella, and Lophopus, and accompanied by numerous wellexecuted figures. It contains extensive and careful anatomical details of all the genera treated of in the memoir. It describes and figures with much minuteness the development of the bud in Paludicella, and mentions the occurrence of a peculiar winter bud in this Polyzoon, occupying the position of the ordinary buds, but destined to remain during the winter months in an undeveloped state. The structure of the testicle in Alcyonella is examined, and the spermatozoa, with their vesicles of evolution, demonstrated; but these cells are not sufficiently distinguished from the contained spermatozoon. The ciliated embryos of Alcyonella are also described and figured, but the authors do not pursue their development into much detail, while they consider them as identical with statoblasts in a particular stage of evolution, and deprived of their external shell. The statoblasts themselves in Alcyonella and Fredericella are described, but the essential structure of an ovum is attributed to them, while some confusion has arisen with regard to the statoblast of Lophopus crystallinus, the body described as such being manifestly the statoblast of Cristatella. A statement formerly made by M. Dumortier, that the tentacula of Lophopus crystallinus are deprived of cilia, is repeated here; it is asserted that, instead of ciliary vibrations, the tentacula of this Polyzoon present a moniliform current, which ascends one side and descends the other of each tentacle; the appearance of these currents is compared to that of an endless chain in uninterrupted motion, and attention is drawn to the analogy of this phenomenon with that of the decomposition of water by the galvanic battery. I have no doubt, however, that the phenomenon thus described is truly a case of ciliary vibration, and that the cilia have merely escaped the observer in consequence of some defect in the microscope employed in their investigation. I have repeatedly had under my own observation a species of Lophopus, which I do not hesitate to refer to M. Dumortier's species, and yet I found the cilia in all cases perfectly distinct. An opinion previously expressed by M. Van Beneden, when he thought he had seen apertures (“bouches aquiferes”) for the admission of water into the perigastric space, is here given up, and the source of the error pointed out. Further, M. Van Beneden, now finding a testicle in the same cell with the statoblasts, modifies his previous views as to the unisexualism of Alcyonella, and, comparing this Polyzoon to the plants belonging to the twenty-third class of Linnaeus, he suggests that male, female, and hermaphrodite individuals may all coexist in the same coenoecium. On the whole, this memoir of the learned Belgian naturalists, though in some respects incorrect, must be regarded as the most important, in an anatomical point of view, of any which had as yet appeared.

In the same year (1848), Sir J. G. Dalyell published the second volume of his “Rare and Remarkable Animals of Scotland,'t and described in this work several species of fresh-water Polyzoa, as inhabitants of that part of the British Islands. Dalyell is a truthful observer and a graphical describer of the habits of the lower invertebrate animals, but he is not

* DuMoRTIER et VAN BENEDEN, Hist. Nat, des Polypes composés d'eau douce, 2° partie. Complément au tome xvi des ‘Mém. de l'Acad. Roy, des Sciences et Belles-lettres de Bruxelles,’ 1848.

t DALYELL, “Rare and Remarkable Animals of Scotland, represented from living subjects.” Lon. don, 1847-8.

always acquainted with the labours of others in the same field; and in the case of the fresh-water Polyzoa, it is often extremely difficult to identify his species—a difficulty much enhanced by the want of exactness in his numerous figures. Under Lamarck's name of Cristatella vagans, he describes the C. mucedo (Cuvier), and is the first after Gervais to figure fully-developed specimens of this beautiful Polyzoon. Under the name of Alcyonella gelatinosa he describes the true Alcyonella fungosa, while his Alcyonella stagnarum appears to be the young condition of a Plumatella. His Plumatella repens would seem to include more than one species, but neither from his descriptions nor figures is it possible to determine the exact animal intended. At the meeting of the British Association for 1849, I noticed the addition to the Irish Fauna, of Lophopus crystallinus, which I had found abundantly in the pond of the Zoological Gardens near Dublin,” and described a new species of Plumatella (P. coralloides of the present monograph); and on the same occasion I described the distribution of the nerves in Plumatella repens.t In January, 1850, I presented to the Royal Irish Academy, a memoir on the “Natural History of the genus Alcyonella.' This memoir contained a historical introduction to the subject, recorded the addition to the British Fauna of the A. flabellum of Van Beneden, and gave a detailed account of the anatomy of A. fungosa. The muscles of this Polyzoon were divided into eight distinct sets, the distribution of the nervous system was demonstrated, and the structure of the locomotive embryos was described, and certain errors in the description given by Meyen of these bodies were pointed out. In the same year, Mr. Albany Hancock, already well known by numerous important papers on the anatomy of the Mollusca, but especially by his association with Mr. Alder in their beautiful ‘Monograph on the Nudibranchiata,' published an admirable paper on certain species of fresh-water Polyzoa obtained in a small lake in Northumberland. In this paper the author gives a very full account of the anatomy of Plumatella, Fredericella, and Paludicella, characterised by great accuracy, and illustrated by excellent figures, though in some points, as already noted in the anatomical portion of the present work, I have found reason to differ from his conclusions. He also draws attention to the resemblance between the arms of the lophophore in the hippocrepian Polyzoa and the oral arms of the Brachiopoda, and compares the arrangement and action of certain muscles in the two groups—important points tending to throw light on the affinities of the Polyzoa. The same paper contains descriptions of new species, which the author records under the names of Plumatella punctata, P. Allmani, and Paludicella procumbens. After carefully considering Mr. Hancock's description of his Paludicella procumbens, I cannot satisfy myself that the characters on which the species is founded are sufficient to entitle it to be considered distinct from P. Ehrenbergi;

* ALLMAN, On Lophopus crystallinus. “Reports of British Association,’ 1849.

+ AllMAN, On the Nervous System and certain other points in the Anatomy of the Bryozoa. • Reports of British Association,’ 1849.

# AllMAN, The Natural History of the genus Alcyonella. Proceedings of Royal Irish Academy,’ 1850.

§ HAN.cock, On the Anatomy of the fresh-water Bryozoa, with descriptions of three new species. • Ann. and Mag. Nat. Hist.,’ March, 1850.

the most important difference, consisting in the greater number of tentacula, is founded on a figure of Paludicella Ehrenbergi, given by myself some years before, and which having been incorrectly engraved with too many tentacles, has thus unfortunately become a source of error in Mr. Hancock's determination of the Northumberland species. In the same year, I presented to the British Association a report on the state of our knowledge of the fresh-water Polyzoa,” in which it was my object to give a detailed account of the anatomy of these animals, and a synopsis, with diagnoses, of all the known species; and in the year 1852, I read, before the Royal Irish Academy, a memoir on the homologies of the organs in the Tunicata and Polyzoa,t in which the fresh-water hippocrepian forms were adduced as affording a means of clearing up some difficult points in the homological relations of the two groups. In 1851, we find several communications on the subject of the fresh-water Polyzoa of Pennsylvania, presented by Dr. Joseph Leidy to the Academy of Natural Sciences of Philadelphia. In these the author describes some new species of Plumatella, and two new genera (Pectinatella, Leidy, and Urnatella, Leidy) of fresh-water Polyzoa. In 1854, Leidy presented to the Academy an additional notice,$ in which, with an amended diagnosis of Urnatella, he confirms the claim of this animal to rank as a distinct genus of fresh-water Polyzoa; while, at the same time, he describes another new species of Plumatella. Leidy's account of Urnatella is confined to a simple diagnosis, but the author proposes to give, hereafter, a full description of the genus. Leidy's communications on the subject of the fresh-water Polyzoa must be regarded as among the most important contributions in a zoographical point of view which have of late years been made to this department of natural history. For some years past, I have continued to make the fresh-water Polyzoa the subject of careful study, and the result has been the acquisition of many new facts, and the correction of some errors into which I had previously fallen. The later additions which I have thus succeeded in making to our knowledge of these animals have been hitherto unpublished, and are now, in the present monograph, for the first time made known.

2.
HABITs of the FRESH-water Polyzoa.

Besides presenting well-marked differences in form, the fresh-water Polyzoa differ also considerably from one another in their habits. Some delight in the pure clear water of

* ALLMAN, Report on the present state of our knowledge of the fresh-water Polyzoa. “Report of British Association,’ 1850. + ALLMAN, On the Homology of the Organs of the Tunicata and the Polyzoa. “Transactions of

the Royal Irish Academy,’ vol. xxii, 1852. + Leidy, in “Proceedings of the Academy of Natural Sciences of Philadelphia,’ vol. v., pp. 261,

265, 321.

§ Id., vol. vii, p. 191.

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