Imágenes de páginas
[ocr errors][merged small][merged small][merged small][merged small][merged small][ocr errors]



D.D., M.R.I.A., F.R.A.S.,


[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small]


Edinburgh, May 2, 1857.


The following work contains the result of many years' careful study of the fresh-water representatives 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 physiognonical 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 cænoecium, 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 class, 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.

« AnteriorContinuar »