50 



HYDROZOA. 



reproductive bodies. Exceptions, however, occur 

 in Hydra itself, in Cordylophora, in Plumularia 

 pinnata, in many Physophoridoe and Calyco* 

 phoridce, Diphyes being an exception. The re- 

 productive zooids of the Lucernaridce, except 

 in the case of Chrysaora, appear to be unisexual, 

 but it is not yet ascertained whether generative 

 bodies of dissimilar sexes can be produced by the 

 fission of one primitive hydrosoma. 



As in other animals, fecundation is effected by 

 the contact of ova and spermatozoa : the sper- 

 maria and ovaria, when fully developed, becoming 

 wholly resolved into these essential elements. 

 The spermatozoa present the form of ovate cor- 

 puscles, from the broad end of which a filament 

 projects. The ova are, in most cases, spherical, 

 destitute of vitelline membrane, with distinct 

 germ-vesicle and germ-spot. Diffusion of the 

 spermatozoa in the surrounding water seems, in 

 the present class, the usual prelude to the act of 

 fertilisation. But, in Cordylophora, it has been 

 supposed that the male elements can alone obtain 

 access to the ova by reaching them from within 

 along the general cavity of the body. 





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