118 THE SNAIL 



e. The spermatheca, or receptaculum seminis, is a long 

 caecal diverticulum from the proximal end of the 

 vagina. It lies alongside the common duct, and 

 its hinder end is expanded into a globular dilata- 

 tion just behind the pericardium and kidney. It 

 is of about the same length as the flagellum, and 

 serves to lodge the spermatophore received from 

 another snail. 



A small diverticulum is very commonly given 

 off about a third of its length from the vagina. 

 In H. aspersa this is largely developed and forms 

 the longer portion of the spermatheca. 



Care must be taken not to mistake the sperma- 

 theca for the male duct, or to cut away the glo- 

 bular dilatation during the dissection of other 

 parts. 



V. DISSECTION OF THE NEEVOUS SYSTEM. 



The nervous system consists of paired ganglionic masses 

 placed above and below the alimentary canal, and joined 

 together by nerve-commissures and connectives so as to form 

 a collar encircling its anterior end. When the head is 

 protruded this collar surrounds the oesophagus immediately 

 behind the buccal mass ; but when the head is retracted the 

 buccal mass is pulled back through the collar, which then 

 liefe in front of it. 



From these principal ganglia, nerves radiate outwards to 

 the various parts of the body. The ganglia are enveloped in 

 a rather dense layer of connective tissue, which must be 

 removed before their shape can be determined. 



1. The supra-cesophageal or cerebral ganglia are a pair of 

 pyriform masses closely applied to each other in the 

 median plane to form a broad transverse band across 

 the dorsal surface of the alimentary canal. 



From the outer ends of the ganglia large nerves 

 run to both pairs of tentacles and to the lips. 



