THE HYDROSOMA. 



23 



apparently exceptional — case here referred to, and which will ])C afterwards more particularly 

 described, those which have been adduced as affording exceptions to this phenomenon are in 

 every instance based upon incomplete observations ; that they may or may not be confirmed when 

 opportunities of more extended observation shall have been afforded ; and that accordingly, in the 

 present state of our knowledge, we are not justified in accepting them. 



The associated zooids are always of two kinds. In one {a, b), the zooid is destitute of all 

 power of true or sexual generation, and has as its proper function the general nutrition of the colony. 

 The other group of zooids {/,(/) has nothing to do with the general mitrition of the colony ; it has as 

 its proper function true generation, and the zooids which compose it give origin to the generative 

 elements— ova or spermatozoa — either directly or after having first developed a special sexual bud. 

 For the whole assemblage of the former, or nutritive zooids, with their common connecting basis, 

 I propose the name of trophosome, while I shall designate the entire association of generative zooids 

 by the name of gonosome. Every hydroid, therefore, with whose life-history we are acquainted 

 consists essentially — with the solitary exception already alluded to — of a trophosome destined for 

 the preservation of the individual, and of a gonosome for the perpetuation of the species. 



Fig. 2. 



Hydrosoma of Campanularia Johnstoni. 



a, b, c. d, e. Various portions of the trophosome ; /, /, /, g, of the gonosome, a, Ilydranth expanded ; li, hydranth 

 contracted; c, empty liydrotheca ; d, free portion of hydrophyton (hydrocaulus) ; e, adherent portion of hydrophyton 

 (liydrorhiza) ; /,/,/, gonangia, containing generative buds,' which in this genus are blastochemes ; g, a blastocbeme just after 

 its escape from tlie gonangiuoi. 



