750 DERIVATION FROM EXCRETORY ORGANS. 



function ; while in other instances, e.g. Bonellia, such modifica- 

 tion is very considerable. 



The generative ducts of the Oligochaeta are probably derived from 

 excretory organs. In the Terricola ordinary excretory organs are present in 

 the generative segments in addition to the generative ducts, while in the 

 Limicola generative ducts alone are present in the adult, but before their 

 development excretory organs of the usual type are found, which undergo 

 atrophy on the appearance of the generative ducts (Vedjovsky). 



From the analogy of the splitting of the segmental duct of the Vertebrata 

 into the Miillerian and Wolffian ducts, as a result of a combined generative 

 and excretory function (vide p. 728), it seems probable that in the genera- 

 tive segments of the Oligochaeta the excretory organs had at first both an 

 excretory and a generative function, and that, as a secondary result of this 

 double function, each of them has become split into two parts, a generative 

 and an excretory. The generative part has undergone in all forms great 

 modifications. The excretory parts remain unmodified in the Earthworms 

 (Terricola), but completely abort on the development of the generative ducts 

 in the Limicola. An explanation may probably be given of the peculiar 

 arrangements of the generative ducts in Saccocirrus amongst the Poly- 

 chaeta (vide Marion and Bobretzky), analogous to that just offered for the 

 Oligochaeta. 



The very interesting modifications produced in the excretory 

 organs of the Vertebrata by their serving as generative ducts 

 were fully described in the last chapter ; and with reference to 

 this part of our subject it is only necessary to call attention to 

 the case of Lepidosteus and the Teleostei. 



In Lepidosteus the Miillerian duct appears to have become 

 attached to the generative organs, so that the generative 

 products, instead of falling directly into the body cavity and 

 thence entering the open end of a peritoneal funnel of the 

 excretory organs, pass directly into the Miillerian duct without 

 entering the body cavity. In most Teleostei the modification is 

 more complete, in that the generative ducts in the adult have no 

 obvious connection with the excretory organs. 



The transportation of the male products to the exterior in all 

 the higher Vertebrata, without passing into the body cavity, is 

 in principle similar to the arrangement in Lepidosteus. 



The above instances of the peritoneal funnels of an excretory 

 organ becoming continuous with the generative glands, render it 

 highly probable that there may be similar instances amongst the 

 Invertebrata. 



