viii PREFACE. 



which I could not find connected by intermediate gradations with other forms, and which could 

 not, with probabiUty, 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 appli- 

 cation. The Polyzoa constitute an exceedingly natural group, and possess great uniformity of 

 structure, and as the fresh-water species afibrd 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 ckss, 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 aflbrding greater pleasure than these beautifid 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. 



T'o the many friends who, by the communication of specimens, and by nmch 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. Allxany Hancock, Mr. Huxley, Dr. Dickie of Belfast, 

 Dr. Hassall of London, Mr. Wigham and Mr. Brightwell of Norwich, Professor Bailey of 

 AVest 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. 



