GENITAL DUCTS. 617 



A better established and more frequent mode of exit of the gene- 

 rative products when dehisced into the body cavity is by means 

 of the excretory organs. The generative products pass from the body 

 cavity into the open peritoneal funnels of such organs, and thence 

 through their ducts to the exterior. This mode of exit of the genera- 

 tive products is characteristic of the Cha;topoda, the Gephyrea, the 

 Bi'achiopoda and the Vertebrata, and probably also of the Mol- 

 lusca. It is moreover quite possible that it occurs in the Polyzoa, 

 some of the Arthropoda, the Platyelminthes and some other types. 



The simple segmental excretory organs of the Polychaita, the 

 Gephyrea and the Brachiopoda serve as generative canals, and in 

 many instances they exhibit no modification, or but a very slight one, 

 in connection with their secondary generative function; while in other 

 instances, e.g. Bonellia, such modification is very considerable. 



The generative ducts of the Oligoclifeta are probably derived from 

 excretory organs. In the Terricola oi'dinary excretory organs are present 

 ill 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 ap])earance of the generative ducts ( Vedjovsky). 



From the analogy of the splitting of the segmental (hict of the Vertebrata 

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

 and excretory function {vide p. 600), it seems probable that in the genera- 

 tive segments of the Oligoclueta 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 xTnmodified in the 

 Earthworms (Terricola), but completely abort on the development of the 

 generative ducts in the Fiimicola. An explanation may probably be given 

 of the peculiar arrangements of the generative ducts in Saccocirrus amongst 

 the Polychaeta {vide Marion and Bobretzky), analogous to that just offered 

 for the Oligocbreta. 



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 j and with reference to this part of our 

 subject it is only necessary to call attention to the case of I^epidosteus 

 and the Teleostei. 



In Lepidosteus the Miillerian duct appears to have become at- 

 tached 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 



