538 CRESSWELL SHEAKER. 



VI. The Fifth Nephrididm. 



The fifth nephridium I have had no opportuuity of exam- 

 ining personally tbroug-h lack of material as already stated. 

 According to Harmer (8) this nephridium is situated in the 

 fifth segment of the female " on the ventral side of the 

 intestine (behind the caecal end of the stomach)." In general 

 structure it resembles the other nepbridia just described. 

 In the male Harmer suggests it is represented by the vesicula 

 seminalis, and this view I v^^ish to discuss under the present 

 section. This suggestion of Harmer's concerning the vesiculge 

 has found considerable favour among subsequent investi- 

 gators, so that it is important to consider the grounds on 

 which it is based. Schimkewitsch (23) thinks it highly 

 probable that the seminal vesicles of the male represent the 

 modified fifth pair of nephridia of the female, while in the 

 female the nephridia of the sixth segment are represented by 

 the oviducts. Hasvvell (9) in the allied form of Histriob- 

 della states that "in the fourth segment the nephridia are 

 probably represented in the female by the oviducts, in the 

 male by the vasa deferentia." 



The grounds which Harmer (8) has advanced in support of 

 this contention are partly anatomical and partly embryo- 

 logical. The anatomical reasons are — that the seminal 

 vesicles in the male occupy the position of the fifth pair of 

 nephridia of the female; in the second place these vesicles 

 are furnished with ciliated funnels, opening into the cavity 

 of the testis, which resemble the ciliated appendage of the 

 nephridium (which Harmer considered probably opened in 

 the primary body-cavity by funnel-like apertures) ; thirdly, 

 cases occur among Annelids in which we know the nephridia 

 are transformed into genital ducts and functions as such in 

 the adult animal. 



The embryological reasons are derived from the study of 

 the immature vesicles of young male worms. One of these 

 is shown in Harmer's (8) figure 5, and concerning which 



