GENITAL DUCTS. 753 



a pore to the exterior. The sack is lined by an epithelium, 

 continuous with the epidermis, the cells of which give rise to the 

 ova or spermatozoa. The duct of these organs is obviously 

 hardly differentiated from the gland ; and the whole structure 

 might easily be derived from the type of generative organ 

 characteristic of the Hydromcdusae, where the generative cells 

 are developed from special areas of the ectoderm, and, when ripe, 

 pass directly into the surrounding medium. 



If this suggestion is correct we may suppose that the genera- 

 tive ducts of the Echinodermata have a different origin to those 

 of the majority of 1 the remaining triploblastica. 



Their ducts have been evolved in forms in which the 

 generative products continued to be liberated directly to the 

 exterior, as in the Hydromedusae ; while those of other types 

 have been evolved in forms in which the generative products 

 were first transported, as in the Actinozoa, into the gastrovascular 

 canals 2 . 



1 It would be interesting to have further information about Balanoglossus. 



2 These views fit in very well with those already put forward in Chapter xin. on 

 the affinities of the Echinodermata. 



B. III. 48 



