ON THE ANATOMY OF HISTlllOBDELLA HOMARf. 299 



posterior portion of this segment, and terminate in a similar 

 manner in the third segment. The third pair arise in the 

 third segment to terminate in the fourth. In tlie female the 

 third and fourth pairs overlap, while in the male the fourth 

 pair arises much farther back between the posterior portion 

 of the fourth and the anterior border of the fifth segment. 



In Stratiodril us, on the contrary, according to Haswell, 

 the nephridial system would seem to extend into the head 

 region. Each nephridium at its anterior end divides into an 

 external and an internal branch. The external branch runs 

 forward into the head, while the internal crosses over to join 

 the internal branch of the opposite side. From the fact that 

 the motion of the cilia of this pair of organs is always from 

 behind forward, their openings are probably in the head. 

 The other nephridia are not branched. " In the female an 

 apparently continuous line of cilia is traceable backwards on 

 each side from the head canals to a point some little distance 

 behind the second cirrus, where a canal is clearly traceable, 

 which, after bending round in a loop, opens on the exterior 

 on the ventral side. But as the direction of the movement of 

 the cilia is from before backwards in the posterior part of this 

 line, it would appear probable that there are two pairs of 

 canals in this anterior region in the female. In the male, on 

 the other hand, there is no such evidence of division, the pair 

 of nephridia which branch in the head being traceable back- 

 wards, without change in the dii-ection of the cilia, nearly as 

 far as the bases of the second cirri, at which point they bend 

 in and terminate in the coelom in the middle line." In the 

 fourth segment, according to Haswell, it is probable that the 

 oviducts represent the nephridia, while in the male they are 

 represented by the vasa deferentia. In both sexes, in the 

 fifth segment there is a pair of organs (beginning in a loop in 

 the male) which run back in the caudal region to terminate 

 near the anus. The direction of the movement of the cilia in 

 these oi'gans is from behind forwards. Thus, in the male 

 there are three pairs of organs, while in the female there are 

 four ; so that the nephridia do not partake of the metamerism 



