102 MORPHOLOGY. 



It is very seldom, liowcver, that the trophosome consists of only a single zooid. Such rare 

 instances are presented by Corijmorplia, and by certain allied forms whose trophosomes never become 

 developed into a colony of mutually dependent hydranths, and I believe it better to regard the 

 hydrorhizal fibres here as elsewhere in the light of mere extensions of the hydrorhizal base rather 

 than in that of proper zooids — a view supported by their mode of development in the primordial 

 hydranth. In almost every other case, on the contrary, the hydranths composing the trophosomes 

 become greatly multiplied by budding ; and in this respect Hydra affords no exception, though 

 here the troj)hosome, by the subsequent detachment of the buds, may become restored to its 

 original condition of a simple hydranth. 



Still less tendency is there in the gonosome to present an absolutely simple condition. 

 Indeed, the gonosome is perhaps never limited in its normal state to a single zooid, and we fre- 

 quently find hundreds and even thousands of zooids entering into the composition of this portion 

 of the hydroid colony. 



But the zooids of which the colony is thus composed, whether in its trophosome or its gono- 

 some, may not only be numerous, but may also vary inform. Those, indeed, which constitute the 

 trophosome are always of a different form from those of the gonosome. In the trophosome it is 

 rare to find any other form of zooid than that of the proper hydranth. In Hydradinia, however, 

 there is associated with the ordinary hydranths the peculiarly modified ones, whose spiral form 

 confers upon the trophosome of this genus so striking a featm'e ; while the nematophores of the 

 Phimularidte can scarcely be regarded otherwise than as special zooids, whose morphological dif- 

 ferentiation from the other zooids of the colony is carried to an extreme. 



While the type of heteromorpldsm or variety of form among the zooids is fixed for every 

 species, i\\e polymerism or simple multiplication of the component zooids is indefinite, and varies 

 with the age, perfection of nutrition, &c., of the individual. 



If we specialise the general expression already given (I), so as to make it directly applicable 

 to particular cases of heteromorphic succession in the life of the hydroid, we shall obtain the 

 following formulas, where // is used for hydranth, his for blastostyle, hlch for blastocheme, ijph 

 for gonophore : 



II. < )■ h-{-gph X h-\- gph X 

 J 



jy_ I I A -f i/s + bich + ffph X h -{- Ms -{- blch -f gph X 



CJJ 



