GENITAL DUCTS. 751 



As has been already pointed out by Gegenbaur there are 

 many features in the structure of the genital ducts in the more 

 primitive Mollusca, which point to their having been derived 

 from the excretory organs. In several Lamellibranchiata 1 

 (Spondylus, Lima, Pecten) the generative ducts open into the 

 excretory organs (organ of Bojanus), so that the generative 

 products have to pass through the excretory organ on their way 

 to the exterior. In other Lamellibranchiata the genital and 

 excretory organs open on a common papilla, and in the remain- 

 ing types they are placed close together. 



In the Cephalopoda again the peculiar relations of the 

 generative organs to their ducts point to the latter having 

 primitively had a different, probably an excretory, function. 

 The glands are not continuous with the ducts, but are placed in 

 special capsules from which the ducts proceed. The genital 

 products are dehisced into these capsules and thence pass into 

 the ducts. 



In the Gasteropoda the genital gland is directly continuous 

 with its duct, and the latter, especially in the Pulmonata and 

 Opisthobranchiata, assumes such a complicated form that its 

 origin from the excretory organ would hardly have been 

 suspected. The fact however that its opening is placed near 

 that of the excretory organ points to its being homologous with 

 the generative ducts of the more primitive types. 



In the Discophora, where the generative ducts are continuous 

 with the glands, the structure both of the generative glands and 

 ducts points to the latter having originated from excretory 



organs. 



It seems, as already mentioned, very possible that there are 

 other types in which the generative ducts are derived from the 

 excretory organs. In the Arthropoda for instance the generative 

 ducts, where provided with anteriorly placed openings, as in the 

 Crustacea, Arachnida and the Chilognathous Myriapoda, the 

 Poecilopoda, etc., may possibly be of this nature, but the data 

 for deciding this point are so scanty that it is not at present 

 possible to do more than frame conjectures. 



The ontogeny of the generative ducts of the Nematoda and 



1 For a summary of the facts on this subject vide Bronn, Klassen u. Ordmtngen d. 

 Thierrcichs, Vol. in. p. 404. 



