Limitation of Genera among the Hydroida. 349 



we may fairly assume the number presented by the marginal 

 tentacles in the young Medusa as representing their permanent 

 condition, as, for example, in the single long tentacle of the 

 Medusa in Corymorpha, where we find, by going back to the 

 early stages of the development of this Medusa, that the pecu- 

 liar asymmetrical form which, in a later stage, finds its expres- 

 sion so decidedly in the great development of a single tentacle 

 is quite apparent before any trace of a tentacle can be detected. 



With regard to nomenclature, I am convinced that, except in 

 certain special cases, we must give to our Hydroid the name 

 under which it was first described, whether this name may have 

 been originally given to the trophosome or to the gonosome. 

 The fact of our giving as a generic name to the complete Hy- 

 droid that by which the Medusa had been previously known 

 needs not prevent our employing the same name for all those 

 similar Medusae whose trophosome has not yet been discovered ; 

 but we must keep in mind that the name, when used in this 

 sense, is purely provisional, and liable to be changed when the 

 discovery of the trophosome shall determine the true genus of 

 our then no longer incomplete Hydroid. 



It is upon the principles here urged that I have drawn up the 

 following synopsis of the genera and species of the Tubularian 

 and Campanularian Hydroids. I have confined myself, how- 

 ever, entirely to those forms in which the trophosome is known, 

 the numerous free Medusae which have not been traced to a 

 trophosome, or been proved to originate by direct development 

 from the egg, holding places in our system which must for the 

 present be regarded as altogether provisional. 



In the generic descriptions I have adopted as far as possible a 

 uniformity in the selection of characters and in the order in 

 which these characters are noted; and I have further, by availing 

 myself of terms already in use, and by introducing one or two 

 new ones, been able to avoid tedious circumlocution, and to 

 condense the descriptions without sacrificing their precision. 



In order that the synopsis may be more easily followed, it 

 will be well to give here definitions of the principal terms used, 

 while for a fuller exposition of the terminology of the Hydroida 

 I must refer to Prof. Huxley's ' Oceanic Hydrozoa,' published 

 by the Ray Society, and to some papers of my own, more espe- 

 cially a paper "On the Structure and Terminology of the 

 Reproductive System in the Corynidse and Sertulariadse,'' pub- 

 lished in the ' Annals and Magazine of Natural History ' for 

 July 1860, and a "Report on the Reproductive System of the 

 Hydroida," in the Report of the Newcastle Meeting of the 

 British Association, 1863. 



The terms " trophosome" and "gonosome" have been already 



