ON THE CCELOM, GENITAL DUCTS, AND NEPHRIDIA. 495 



anterior segments) develop in the same way as the posterior, 

 but never pass beyond the pronephridial stage (head-kidneys) ; 

 they end blindly internally with a typical flame-cell (fig. 26). 

 Meyer figures as many as five pairs of such pronephridia in 

 Nereis cultrifera (80)^. Although the head-kidneys of the 

 Mollusca undoubtedly occasionally open internally, v. Drasche 

 and Hatschek seem to have been mistaken in describing an 

 internal opening in these organs in the Chsetopods (see Frai- 

 pont, 40; Meyer, 80). 



Fraipont appears to attribute a very similar history to the 

 wide-mouthed trunk "nephridia" of Polygordius as Meyer has 

 described for the trunk " nephridia " of the Tubicolous Poly- 

 chaetes, though his statements are less precise : " Le meso- 

 blaste est represente de plus au niveau des muscles obliques 

 par une masse de cellules assez confuse. C'est dans ce groupe 

 de cellules situees au dessus des muscles obliques contre les 

 champs musculaires longitudinaux que ce difFerencient les 

 entonnoirs des organes segmentaires et plus tard encore les 

 organes sexuels. C'est un simple epaississement du peritoine " 

 (40). 



We see, then, that in the Polychgeta nephridia are developed 

 from large cells, which may be compared to the '' funnel cells,'* 

 giving rise to the nephridia in the Oligochseta. Whilst, 

 however, in the latter the pronephridium acquires an opening 

 into the coelomic follicle independently of the peritoneal funnel, 

 which acts as a genital duct ; in the former, the Polycheeta, the 

 pronephridium may acquire an opening into the coelomic 

 follicle in the region where the peritoneal funnel is formed, 

 fuse with it, and become an organ of double function — excretory 

 and genital. In many cases division of labour leads to the 

 restriction of the genital function to one set of coelomic follicles 

 and their funnels, and of the excretory function to another set 



' There can now, I think, be no doubt that the head-kidneys are simply the 

 precociously developed nephridia of the first segment ; they do not open into 

 the ccelom for the very good reason tliat at this stage there is, as a rule, no 

 ccelom for them to open into. They preserve the same relations as the Platy- 

 helminth nephridia (Hatschek, 48, 51, 54 ; Meyer, 80). 



