INVERTEBRATA. 365 



vertebrates for example, or the earthworm, we find that 

 the coelom is a cavity on the walls of which are situated 

 the generative cells, and which is connected with the 

 exterior by the generative ducts. In the sporocyst and 

 redia the generative organ is one large cavity occupying 

 most of the interior of the body with reproductive cells on 

 its walls, but here there are no definite ducts, unless the 

 birth-opening of the redia be regarded as such. The 

 cavities of the generative organs in the liver-fluke, then, 

 correspond to the coelom, and the coelom generally is to be 

 regarded as primarily the generative cavity. 



It was formerly supposed to be characteristic of the 

 nephridia that they had an internal opening into the 

 coelom, as in the earthworm, but the excretory tubules of 

 the liver-fluke evidently resemble nephridia, and are now 

 held to be homologous with them. The connection of the 

 ultimate tubules with flame-cells is now regarded as the 

 primitive condition of true nephridia, and similar structures 

 have been discovered in many Polychaeta, and even in the 

 excretory tubules of Amphioxus, which do not open into 

 the coelom as formerly supposed. Q-enerative ducts or 

 gonaducts, therefore, are not modified nephridia as formerly 

 held, but are organs of primarily distinct origin and 

 nature. 



