HYDROZOA. 



67 



tive organs ; thus eventually becoming similar to 

 the huge reproductive body, from whose fertilised 

 ovum the primitive Hydra- tuba was produced. 

 This, and the stock which it developed, does not, 

 however, perish, but ma}^ again, by growth and 

 fission, give rise to fresh successions of generative 

 bodies. 



Fig. 15. 



Development of Chrysaoea : — a, ova, with gelatinous invest- 

 ment, from Chrysaora hysoscella ; h and c, free ova ; d, young 

 Hydra-tuba, -with four marginal tentacles, developed therefrom ; 

 e, the same, with eight tentacles ; /, Hydra tuba, in its ordinary 

 condition ; g, another Hydra-tuba, marked with constrictions ; 

 h, a more advanced form, with deeper constrictions ; i, a speci- 

 men undergoing fission, in which the tentacles are seen to arise 

 from below the constricted portion, while its upper segments 

 separate, and become free-swimming zooids (k). 



Similar to the above appears the life -history of 

 Cephea and Cassiopeia, notwithstanding the very 

 different structure of the detached reproductive 

 zooids which these genera present. On the de- 

 velopment of Rhizostoma itself accurate observa- 

 tions are wanting. 



In the Lucernariadce proper, no free zooids are 

 produced, but the generative elements are formed 



