No. I.] THE METAMERISM OF NEPHELIS. 39 



outside of the central fibrous portion in capsules as in the other 

 Hirudinea. PI. VI, Fig 9, shows the general arrangement. 

 Two clusters lie on the ventral surface in the median line (one 

 anterior and the other posterior) and two clusters on each 

 lateral face. The anterior lateral clusters are anterior to the 

 anterior nerves, and the posterior lateral clusters lie between 

 the two nerves. These lateral clusters rise slightly above the 

 dorsal surface of the body of the ganglion, and their posterior 

 edges are notched by the lateral nerves as they pass out from 

 the ganglion (PI. VI, Fig. 12). The number six is constant in 

 the whole chain and the position is also constant except in the 

 supra-oesophageal ganglia or "brain." In the "anal ganglia" 

 the lateral clusters tend to become dorsal towards the posterior 

 portion owing to compression, but they are perfectly recogniz- 

 able and referable to their proper neuromeres (PL VI, Figs. 

 14 and 15). 



Leydig s Cell. 



Lying between the two nerve trunks of either side of the 

 ganglion, and nearly in contact with the posterior lateral 

 capsule, lies a large bipolar cell whose prolongations follow 

 along the lateral nerves as they pass outward, and finally fuse 

 with them (PI. VI, Fig. 9). This cell was described first by 

 Leydig in Hirudo mcdicinalis ; it was called " Leydig's cell" 

 by Hermann (11), and is found in all the Gnathobdellidae in 

 the same relation to the ganglion. I have not been able to 

 find any trace of fibers going to the ganglion ; so far as I 

 have traced them they all pass outward along the nerves. This 

 cell, the significance of which is as yet entirely unknown, is 

 constant throughout the central nervous system. I have found 

 it in every neuromere of the body. In the first four neuromeres 

 it lies upon the fused nerves at some distance anterior to the 

 " brain " (PI. V, Fig. 2). In the fifth neuromere (PI. VI, Fig. 1 1) 

 the cell is found at the angle formed by the separation of the 

 hitherto fused portions of the nerve trunks. In the sixth 

 neuromere, the last nerve of the sub-oesophageal mass, the cell 



" Ernst Hermann : Das Central Nervensystem von Hirudo medicinalis. 

 Miinchen. 1875. 



