38 BRISTOL. [Vol. XV. 



A Typical Neuromere. 



In order to analyze the " brain " and the " anal ganglia " 

 it is necessary to know the component parts of a typical neu- 

 romere and to grasp their relations to each other under normal 

 conditions. 



The general shape of a ganglion is that of a flattened ellip- 

 soid, the long axis of which is parallel to that of the body ; the 

 ventral surface being slightly more convex than the dorsal (PL 

 VI, Fig. 9). Each ganglion gives rise to two nerves on each 

 side which leave the ganglion and proceed for a short distance 

 in a horizontal plane, and then branching, go to the dorsal or 

 ventral side. 



The anterior nerve, however, is not a single nerve. It 

 results from the fusion of a ventral and a dorsal root, the 

 fusion taking place almost immediately after their departure 

 from the body of the ganglion (PI. VI, Fig. 9). This fact 

 enables us to homologize the two lateral nerves of Nephelis 

 with the three of Clepsine as follows : I and II in Clepsine are 

 represented by I in Nephelis. Ill in Clepsine is II in Nephe- 

 lis. This homology is also shown by the correspondence of 

 the areas innervated by I in Nephelis and I and II in Clepsine 

 (Pll. IV and V). I have not been able to find evidence of the 

 similar origin of the anterior nerve in Hirudo or Macrobdella, 

 and this fact suggests that Nephelis is an intermediate form 

 between the Clepsinidae and the five-ring leeches. 



Between each pair of the lateral nerves, and near the ganglion, 

 lies a bipolar cell, the principal prolongations of which pass 

 outward along the trunks of the lateral nerves for a short 

 distance and then fuse with them so as to be indistinguish- 

 able from them. This cell is found in other Hirudinea and 

 has been called from its discoverer " Leydig's cell." Its pres- 

 ence throughout the entire ganglion chain, its variation under 

 different conditions, and its possible relations to other extra- 

 ganglionic cells are of sufficient interest to demand for it 

 separate consideration. 



The nerve cells of the ganglion, with the exception of 

 " Leydig's cell," are gathered into six groups or clusters lying 



