98 PROFESSOR ALLMAN ON THE GENETIC SUCCESSION 



Let t be the trophosome, and g the gonosome, then 



I. t +g x t + y x t + g x &c, 



will be the general expression for the genetic succession in the life of the 

 hyclroid, the sign + indicating succession by zooidal development, and x by 

 embryonal. 



It is very seldom, however, that the trophosome consists of only a single 

 zooid. Such rare instances are presented by corymorpha (fig. 1), and by cer- 

 tain 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 or 

 fixed end of the colony, rather than in that of proper zooids — a view supported 

 by their mode of development in the primordial hydranth. In almost every 

 other case, the hydranths composing the trophosome become greatly multiplied 

 by budding. 



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 frequently find hundreds and even thousands of zooids 

 entering into the composition of this portion of the hyclroid colony. 



But the zooids of which the colony is thus composed may not only be 

 numerous, but may also vary in form. 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 Hydractinia, however, there is associated with the ordinary 

 hydranths the peculiarly modified ones, whose spiral form confers upon the 

 trophosome of this genus one of its most striking features, while the nemato- 

 phores of the Plumalaridce can scarcely be regarded otherwise than as special 

 zooids whose morphological differentiation from the other zooids of the colony 

 is carried to a maximum. 



In the gonosome, on the other hand, the usual condition is that of variety 

 of form among its component zooids ; and it is quite common to find in one 

 and the same gonosome, three different kinds of zooids, each with its special 

 form among the associated zooids, and its special duty in the generative 

 functions of the hydroid. 



While the type of heteromoiphism, or variety of form, among the zooids is 

 fixed for every species, the pdlymerism, or simple multiplication of the com- 

 ponent 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 



