324 CIMilSSWELL SHEARKii. 



seen from the reconstructions shown in figs. 15, 21, and 28. 

 In these figures the nervous system is seen from the ventral 

 side. The outlines of the cord and ganglia have been 

 measured from the nerve-cel!s, as far as these could be 

 roughly differentiated from the surrounding tissues.' From 

 fig. 15 it Avill be seen that the main mass of the fourth gan- 

 glion lies just in front of the penis, but many of its cells extend 

 backwards in the region dorsal to the penis. Here they 

 would almost seem to form a second division of the ganglion. 

 I have not attempted to determine its structure, which differs 

 considerably from that of the other ganglia, on account of the 

 great size of ils lateral parts. A few of its cells are distri- 

 buted on the penis sheath. Past the fourth ganglion the cord 

 diminishes rapidly, but enlarges again rather suddenly in the 

 interior part of the caudal region. It is the second in this 

 segment that is the largest. In the posterior region the cord 

 divides to run into the posterior feet. At this point a number 

 of nerve-cells are arranged, forming quite a mass. It is diffi- 

 cult to decide whether each of these ganglia is to be considered 

 as representing a segment. If so, then there are three main 

 o-ans'lia in the segment itself, and counting- the mass of cells 

 at the termination of the cord, it would be composed of four 

 see-ments. Foettinger came to the conclusion that it was one 

 segment, formed by the partial fusion of three metameres. 



In the female there are a number of differences in the con- 

 figuration of the nervous system, due to the somewhat different 

 size of the segments as compared with the male. This is 

 most pronounced in the generative region. The absence in 

 the female of the penis and accessory glands results in the 

 almost complete disappearance of the cord and ganglia in the 

 posterior part of the generative region, and throughout this 

 portion of the body the cord and its ganglia are much less 

 prominent than in the male. In the absence of the penis the 

 cord retains its ventral position. In the caudal region, on 



' In the reconstiiiction of the male nervous system shown in fig. 15 

 uo allowance has heen made for the dorsal ciu-vatm-e of the cord in 

 the generative region. 



