Limitation of Genera among the Hydroida. 347 



Histoiy Review^ (1861-63), where they appear under the form 

 of a review of the work of Agassiz just referred to — papers in 

 which there is no difficulty in recognizing the pen of an accom- 

 plished zoologist who holds a chair in an Irish University, and 

 is already well known by his valuable contributions to the lite-^ 

 rature of the Coelenteraia. 



I believe that henceforth no classification of the Hydroida 

 will be admitted by the zoologist which does not include in the 

 conception of every Hydroid both those parts which are destined 

 for the nutrition of the colony and those which are destined for 

 the sexual perpetuation of the species, whether these last are in 

 the form of fixed sacs or of free locomotive Medusae. 



It must be borne in mind that every Hydroid whose life™ 

 history has come fully before us consists (with only a single 

 positively proved exception*) of two sets of zooids. One of 

 these is destined for the nutrition of the colony, and has nothing 

 to do with true generation; while the other is, on the con-; 

 trary, destined for true generation, and has nothing to do witl^ 

 the nutrition of the colony. For the whole assemblage of the 

 former I have elsewhere f proposed the term " trophosome," and 

 for that of the latter the terra " gonosome ;" and whether the 

 gonosome remains permanently attached to the trophosome or 

 becomes in whole or in part free, attaining thereby an indepen- 

 dent existence, it is equally necessary that it should take its 

 place in our diagnosis of genera and species. An adequate con- 

 ception of the Hydroid can thus only be obtained by regarding 

 it as the product of two factors, one of them finding its expres- 

 sion in the trophosome, and the other in the gonosome. 



Now the characters to which we shall be justified in assigning 

 a generic value will be found in both of these factors. The 

 trophosome will present them chiefly in the form of the poly- 

 pite, including the arrangement and structure of the tentacles 

 (whether these be scattered or in one or more verticils, or whether 

 they be filiform or capitate), in the solitary or associated condir 

 tion of the polypites, and in the nature and extent of the chiti- 

 nous periderm. In the gonosome, characters of generic value 

 will be found in the mode of origin of the gonophores and in 

 their general form — whether they be in the condition of a fibced 

 sac {adelocodonic) or of a developed Medusa {phanerocodonic) ; 

 while each of these forms of gonophore may itself present dif- 

 ferences which will afford characters of value in the limitation 

 of our genera. It is true that among the adelocodonic forms it 

 it is rare to meet with any differences so well marked as to be- 



* See my " Report on the Reproductive System of the Hydroida," in 

 the Report of the Newcastle Meeting of the British Association, 1863. 

 t hoc. cit. 



23* 



