NERVOUS SYSTEM 119 



has very thick walls, and contains a quadrangular 

 calcareous spicule. 

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

 caecal diverticulum from the hinder 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 vas deferens, nor to cut away its glo- 

 bular dilatation during the dissection of other 

 parts. 



V. DISSECTION OF THE NEEYOUS 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 coUar encirchng its anterior end. When the head is 

 protruded this coUar surrounds the oesophagus immediately 

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

 buccal mass is pulled back through the coUar, which then 

 lies 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-oesophageal or cerebral ganglia are a parr of 

 closely apposed pyriform masses, forming a broad 

 transverse band across the dorsal surface of the ali- 

 mentary canal. 



From the outer ends of the ganglia large nerves 

 run to both pairs of tentacles and to the lips. 



