HYDEOZOA. 45 



2. That of a well-defined process, or sporosac, as 

 in Ilydractinia. 



3. That of a manubrium with closed gonocaly- 

 cine investment, in which case the medusoid 

 structure is said to be " disguised," as in the 

 gonophores of Cordylophora and numerous other 

 forms. 



4. That of a manubrium, with open gonocalyx 

 and well-developed canal system. Such " medusi- 

 form gonophores " may either remain attached, 

 as in Hippopodius and Vogtia, or become free, 

 as in Velella, Porpita, and many of the fixed 

 Hydrozoa. 



The same gonophore does not contain more 

 than one kind of generative elements, and these 

 are situated either between the ectodermal and 

 endodermal layers of the manubrium, or in the 

 walls of the gonocalycine canals. When male and 

 female gonophores differ externally in form, the 

 special terms ' androphore ' and ' gynophore ' are 

 employed to distinguish them. But, apart from 

 such sexual distinctions, two kinds of gonophores 

 appear occasionally to be produced by the same 

 Hydrozoon, while, on the other hand, similar 

 gonophores may arise from the bodies of appa- 

 rently different species. 



So much, then, for the structure of the gono- 

 phores ; next, as to their position. They may be 

 seated either — 



1. on the polypites ; or, 



2. on special processes termed *gonoblas- 



tidia;' or, 



3. directly on the coenosarc. 



The first of these methods is characteristic of 



