HTDROZOA. 



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 may again, by growth and 

 fission, give rise to fresh successions of generative 

 bodies. 



Fig. 15. 



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

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

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

 e, the same, with eight tentacles ; f. 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 



F 2 



