r.lx DERIVATION FKOM EXCRETORY ORGANS. 



organ becoming continuous with the generative glands, render it 

 highly probable that there may be similar instances amongst the 

 Invertebrata. 



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 

 remaining 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 dif- 

 ferent, 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 Opistho- 

 branchiata, assumes such a complicated form that its origin from the 

 excretory organ would hardly have been suspected. The fact how- 

 ever 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, Arach- 

 nida and the Chilognathous Myriapoda. the Po3cilopoda, 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 the 

 Insecta appears to point to their having originated independently of 

 the excretory organs. 



In the Nematoda the generative organs of both sexes originate 

 from a single cell (Schneider, Vol. I. No. 390). 



This cell elongates and its nuclei multiply. After assuming a 

 somewhat columnar form, it divides into (1) a superficial investing 

 layer, and (2) an axial portion. 



In the female the superficial layer is only developed distinctly in 



1 For a summary of the facts on this subject ride Broan, Klaxsen u. Ordmmgen <1. 

 Tliierreichs, Vol. in. p. 404. 



