CCELENTERATA. 183 



traced to a division of labour in the colonial systems of zooids 

 so characteristic of these types. 



In the Hydromedusae an interesting series of relations 

 between alternation of generations and the division of the zooids 

 into gonophores and trophosomes can be made out. In Hydra 

 the generative and nutritive functions are united in the same 

 individual. The generative swellings in these forms cannot, as 

 has been ably argued by Kleinenberg, be regarded as rudimen- 

 tary gonophores, but are to be compared to the generative bands 

 developed in the Medusae around parts of the gastro-vascular 

 system. A condition like that of Hydra, in which the ovum 

 directly gives rise to a form like its parent, is no doubt the 

 primitive one, though it is not so certain that Hydra itself is a 

 primitive form. The relation of Hydra to the Tubularidae and 

 Campanularidae may best be conceived by supposing that in 

 Hydra most ordinary buds did not become detached, so that a 

 compound Hydra became formed ; but that at certain periods 

 particular buds retained their primitive capacity of becoming 

 detached and subsequently developed generative organs, while 

 the ordinary buds lost their generative function. 



It would obviously be advantageous for the species that the 

 detached buds with generative organs should be locomotive, so 

 as to distribute the species as widely as possible, and such buds 

 in connection with their free existence would naturally acquire a 

 higher organization than the attached trophosomes. It is easy 

 to see how, by a series of steps such as I have sketched out, a 

 division of labour might take place, and it is obvious that the 

 embryos produced by the highly organized gonophores would 

 give rise to a fixed form from which the fixed colony would be 

 budded. Thus an alternation of generations would be estab- 

 lished as a necessary sequel to such a division of labour. To 

 test the above explanation it is necessary to review the main 

 facts with reference to alternations of generations amongst the 

 Hydromedusae. 



Hydromedusae 1 . In many instances amongst the Tubula- 

 ridae, Sertularidae and Campanularidae medusiform buds are 

 produced which become detached and develop sexual organs. 



1 For a full account of this subject the reader is referred to the beautiful memoir 

 of Allman (No. 149). 



