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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.

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