THE NEPHRIDIA OF THE POLTOH^TA. 731 



be urged as a weighty objection to the homology, since any 

 day such an opening may be discovered by some observer 

 fortunate enough to find these Avorms in the act of shedding 

 their genital products. If such a pore does not exist at all 

 we have a ready explanation at hand in the already quoted 

 example of Clistomastus (p. 726), where its absence is un- 

 doubtedly due to degeneration of the genital funnel, accom- 

 panying the adoption by this worm of another method of 

 shedding its germ-cells. 



The position, then, is this : we find two separate organs in 

 the Oligochgeta, Hirudinea, and some Polycheeta, the one 

 excretory, the other genital. I contend that they are respec- 

 tively homologous in the three groups. Surely the burden 

 of the proof lies entirely on the side of those who would deny 

 this homology. If the genital funnel of the Polycheeta 

 be merely a secondary derivative of the nephridium, not 

 homologous with the genital duct of the others, then it 

 should be explained why the latter duct is absent in these 

 worms, and we might even reasonably expect to be shown a 

 trace of it in some family. 



To answer that in the Oligochgetes and Hirudinea them- 

 selves these genital ducts are merely modified nephridia or 

 their derivatives, is only to throw the question back a step 

 farther. For then it may be asked, where are the represen- 

 tatives in the Annelids of the genital ducts of the Platy- 

 helminths and Nemertines ? What has become of these 

 ducts, have they disappeared, leaving no trace behind them, 

 to be replaced by organs of exactly similar relations, but not 

 homologous ? 



Perhaps a stronger argument still against such a view, is 

 that it is directly opposed to the evidence of comparative 

 anatomy and embryology, which goes to show that, as a 

 matter of fact, the genital ducts or funnels are not modified 

 nephridia at all, and have originally nothing to do with 

 nephridia ; that their function is, and always has been pri- 

 marily that of carrying the genital products to the exterior, 

 and that consequently they must always have been in con- 



