PREFACE. 



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 tlie monograph no trouble has been spared to render it as complete 

 as possible, and the subjects of which it treats have been considered mider 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 observa- 

 tions 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 Vrnatella, 

 which I have not yet had an opportunity of examining in a livhig 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 TJrnatella, we caimot 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. Tuften 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 



