134 THE SNAIL. 



In H. aspersa this is largely developed and forms 

 the longer portion of the spermotheca. 



Care must be taken not to mistake the spermo- 

 theca for the vas deferens, nor to cut away its 

 globular dilatation during the dissection of other 

 parts. 



V. DISSECTION OF THE NERVOUS SYSTEM. 



The nervous system consists of paired ganglia, of commis- 

 sures and connectives uniting them, and of nerves running 

 from these central organs to all parts of the body. The 

 chief ganglia with their commissures and connectives form 

 a nerve-collar encircling the anterior end of the alimentary 

 canal. 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 lies in front of it. 



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 pair 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. 



From their under or ventral surface a pair of buc- 

 cal nerves run forwards alongside the oesophagus, 

 and end in a pair of small buccal ganglia, which lie 

 at the outer sides of the salivary ducts close to their 

 openings into the buccal mass. The buccal ganglia 

 are connected by a transverse commissure, and from 

 them nerves arise supplying the buccal mass. 



From the posterior and outer border of each 

 supra-cesophageal ganglion two stout connectives 

 pass downwards and slightly backwards to the sub- 



