GENITAL DUCTS. 



749 



the methods of transportation of the genital products to the 

 exterior in many of the higher types. 



It has been already pointed out that the body cavity in a 

 very large number of forms is probably derived from parts of a 

 gastrovascular system like that of the Actinozoa. 



When the part of the gastrovascular system into which the 

 generative products were dehisced became, on giving rise to the 

 body cavity, shut off from the exterior, it would be essential that 

 some mode of transportation outwards of the generative products 

 should be constituted. 



In some instances simple pores (probably already existing at 

 the time of the establishment of a closed body cavity) become 

 the generative ducts. Such seems probably to have been the 

 case in the Chaetognatha (Sagitta) and in the primitive 

 Chordata. 



In the latter forms the generative products are sometimes dehisced into 

 the peritoneal cavity, and thence transported by the abdominal pores to the 

 exterior (Cyclostomata and some Teleostei, vide p. 626). In Amphioxus 

 they pass by dehiscence into the atrial cavity, and thence through the gill 

 slits and by the mouth, or by the abdominal pore (?) to the exterior. The 

 arrangement in Amphioxus and the Teleostei is probably secondary, as 

 possibly also is that in the Cyclostomata ; so that the primitive mode of 

 exit of the generative products in the Chordata is still uncertain. It is 

 highly improbable that the generative ducts of the Tunicata are primitive 

 structures. 



A better established and more frequent mode of exit of the 

 generative 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 generative products is characteristic of the 

 Chaetopoda, the Gephyrea, the Brachiopoda and the Vertebrata, 

 and probably also of the Mollusca. 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 Polychaeta, 

 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 



