PREFACE. 



THE progress of our knowledge of the Class of Acalephs is at 

 present so closely linked with every new observation Avhich may 

 be brought up in the history of the development of these animals, 

 that it has been thought advisable to extend this Catalogue some- 

 what, and not make it simply an enumeration of the Acalejihs in the 

 collection of the Museum of Compai'ative Zoology at Cambridge. It 

 has, however, been limited to the North American species ; and even 

 many of the Sertularians, Campanularians, and Tubularians in the col- 

 lection are not described or mentioned here, because our information 

 with regard to them is too scanty to be available. The mere enu- 

 meration, with short descriptions, of Hydroids, the development of 

 which has not been fully traced, would probably only add, in the 

 course of a few years, synonymes to some of the Medusa?, the adult 

 stages of which may be well known, and would not advance in the 

 least degree our acquaintance with the North American Acalephs. 

 To make this Catalogue useful to American students, a few species 

 described by other authors, of which there are no specimens in the 

 Museum collection, are added, to facilitate further investigations. This 

 is done with the less hesitation, as it is hoped that in a short time 

 most of the species thus enumerated will have been figured in the 

 diagrams of the Museum. 



In the descriptions of the species, constant reference has been made 

 to the bearing of the facts discussed, on the classification of Acalephs, 

 and consequently much has been introduced which would be out of 

 place in a descriptive catalogue. The wood-cuts, with the exception 

 of a few borrowed from the Contributions to the Natural History of 

 the United States by Professor Agassiz, have all been drawn on wood 

 from nature by myself, and, though not highly finished, will yet 

 generally give a better idea of the Acalephs, in this simple outline, 

 than could have been done by a more finished wood-cut. Such an 

 elaborate catalogue of Acalephs may seem somewhat out of place 



