ON A NEW HISTRIOBDELLID. 315 



dorso-ventrally compressed. In the male the cord just 

 behind this ganglion and immediately in fi'ont of the penis 

 gives off a pair of strong branches, and then becomes greatly 

 reduced, passiug towards the dorsal side as a double cord, to 

 run between the intestine and the penis, in which position it 

 enlarges into a small ganglion, the fifth, completely divided 

 into two lateral bodies ; this gives oif a pair of nerves to the 

 ganglia at the bases of the third pair of cirri. In the 

 female the nerve-cord becomes extremely attenuated behind 

 the fourth ganglion (in the neighbourhood of the dilated 

 median part of the ovary), but retains its ventral position ; 

 the fifth ganglion is dorso-ventrally compressed in its anterior 

 portion, but loses this character further back where it gives 

 origin to a pair of nerves passing to the ganglia at the bases 

 of the cirri of the third pair. 



In the caudal region the ventral chain may be described 

 either as represented by a single elongated ganglion imper- 

 fectly divided into five or six portions, or as consisting of five 

 or six imperfectly separated ganglia. Nerve-cells clothe the 

 nerve-cord ventrally and latei'ally through all this portion of 

 its extent, except that there are about five narrow intervals in 

 which they are almost absent, the breadth of the intervals 

 differing in different specimens according to the degree in 

 which the body is contracted. Throughout this posterior 

 region the cord is deeply cleft. From the posterior end of 

 the cord nerves pass to the posterior legs, and to a pair of 

 ganglia situated at the bases of the cirri which they bear. 



The account of the nervous system of Histriobdella 

 homari given by Foettinger corresponds fairly closely with 

 what I have found in Stratiodrilus as descinbed above, except 

 that in the former in the caudal region there are only three 

 ganglia, somewhat better defined than those in the corre- 

 sponding region of Stratiodrilus, and that the lateral ganglia 

 are absent, or at least have not been recognised. A more 

 important point of difference is that the nerve-cord of His- 

 triodrilus would appear to be in complete continuity with the 

 epidermal layer, while in Stratiodrilus it is much more dis- 



