302 CRESSWELL SHEARER. 



only one segment, in every case terminatii)g' in the next 

 segment to that in which they arise in the manner similar to 

 the nephridia in Annelids. 



In both sexes the first three pairs of nephridia hold the 

 same position, but the fourth varies according to sex. In 

 the male it is situated much farther back — at the junction of 

 the generative with that of the following segment, while in 

 the female it is much more forward — in the anterior portion 

 of this segment. In the female the third and fourth pairs 

 overlap and cross one another in different planes. 



The most posterior nephridium in Stratiodril us is in 

 that segment that would correspond to the fifth in Histrio- 

 b del la. In neither of the sexes are organs found in this 

 region in Histriobdella. 



Foettinger, in figuring a pair of nephridia in the region of 

 the penis, evidently mistook the motion of the cilia in the vas 

 deferens, or the slit in the vesiculae, for the ciliary motion 

 of excretory organs. The slit in the vesiculas was first 

 described by Haswell in Stratiodrilus, and as such was 

 evidently overlooked by Foettinger. They are even better 

 developed in Histriobdella than in Stratiodrilus. They 

 are edged with very stout cilia, that could i-eadily be mistaken 

 for nephridial flagella. With methyl-blue it is easy to 

 determine, however, that no excretory organs exist at this 

 point in Histriobdella. 



In Histriobdella, unlike Stratiodrilus, the nephridia 

 partake to some extent of the metamerism of the body. In 

 the third and fourth segments this is masked in the male by 

 th^ great, development of the reproductive organs. In the 

 case of the nephridia there has resulted a pushing forward in 

 the female of the fourth organ, while in the male this has 

 been reduced in size and moved backwards. [ 



Unlike Dinophilus, we do not find the sharp specialisation 

 of the different parts of the nephridial canal into a thick- 

 walled anterior excretory portion and a thin-walled posterior 

 part. It is more uniform throughout in structure. 



