STRUCTURE OF THE NEPHRIDIA OF DJNOPHILUS. 525 



species^ and I cannot find more than four pairs in tlie female.^ 

 It is certain their number is not five, as in Dinophilus. 

 Haswell (9), in Stratiodrilus, which is almost identical 

 with Histriobdella, finds four nephridia in the female. It 

 is plain in Dinophilus, the number of nephridia are remark- 

 ably constant, while the same cannot be said of the number 

 of metameres. 



As already stated the nephridia in the male consist of four 

 pairs placed very much in the positions Harmer has indicated 

 — that isj the head of the first nephridium is very slightly 

 behind the second pre-oi'al ring of cilia on a level with the 

 anterior margin of the muscular pharynx organ, the second 

 at the anterior end of the stomach, the third about the middle 

 of the stomach, while the fourth lies at the posterior end of 

 the stomach close at its junction with the intestine. Their 

 ducts run outwards and round the segments to open on the 

 ventral surface not very far from the median line, not in the 

 segment to which they belong as stated by Harmer (8), but 

 in the subsequent following segment as in Annelids. This 

 point is difficult to exactly determine as the field of vision is 

 so limited under an oil immersion objective (with which U is 

 necessary to examine this terminal part of the canal), that it is 

 thus hard to tell where the segment septa lie in relation to the 

 end of the nephridial canals. A number of successful pre- 

 parations, however, seemed to clearly show the canals running 

 some distiince backwards into the next segment before ending 

 under the epidermis, and I think I am quite right in saying 

 that the canals end in the segment following that to which 

 they belong, as in Annelids. The closed internal ends of the 

 canals bearing the solenocytes project slightly into the irregu- 

 lar space surrounding the gut — the so-called body-cavity 

 (fig. 17). This space is of variable dimensions, being lined 

 by no distinguishable membrane, and is traversed in many 



' As in Dinophilus tliey project into tlie so-called body-cavity where 

 they appear to end as a single flame-cell. In every respect they resemble 

 the structure of the head-kiduey of the Trochophore larva, the whole organ 

 of which is quite comparable to a single very enlarged solenocyte. 



