ON THE ANATOMY OF HISTIUOBDELLA HOArAKI. 325 



the contrary, the cord and ganglia are much the same as in 

 the male (fig. 15). 



In Stratiodrilus the cord and ganglia are much the same 

 as in Histriobdella. In the male the fourth ganglion is 

 opposite the claspei'S. After this the cord is very much 

 reduced where it passes dorsalwards over the penis. In 

 Histriobdella this reduction is not so marked. In the 

 caudal region also the ganglia are smaller. Hasvvell remarks, 

 regarding the nervous system of the caudal region of 

 Stratiodrilus, that "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" (p. 315). The nerve- 

 cells are arranged on the ventral surface of the cord, and 

 the lateral ganglia are much better developed in Histrio- 

 bdella, especially those of the generative region. They send 

 fibres into the cirri. The second ganglion would seem to be 

 double. 



Haswell has drawn attention to the fact that the nervous 

 system in Stratiodrilus does not show the complete union 

 with the body-wall tissues as does that of Histriobdella. 

 I think, however, no great importance can be attached to this 

 point. The separation shown by the nervous system in 

 Stratiodrilus is due in great part to the thinness of the 

 body-wall as compared with Histriobdella, and not to a 

 more highly difi'erentiated condition of the system itself. 



It is of considerable interest to compare the nervous system 

 of Histriobdella with that of the Archiannelid it resembles 

 most, that is, Dinophilns. From the study of a species 

 closely allied to D. gyrociliatus, Nelson (25) has deter- 

 mined the main structure of the central nervous system in 

 considerable detail. In the first place there is a marked 

 separation of the two parts of the ventral nerve-cord in the 

 intersegmental regions, much more so than in Histrio- 

 bdella. Unlike Histriobdella they do not unite to form 

 the ganglia, but are joined by commissures, the two portions 

 of cord remaining separated throughout their course. There 



