126 THE SNAIL. 



globular dilatation during the dissection of other 

 parts. 



V. DISSECTION OF THE NEBVOUS 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 thfiir 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- oesophageal ganglion two stout connectives 

 pass downwards and slightly backwards to the sub- 

 oesophageal ganglia. Between these two on each 

 side is the slender auditory nerve. 



2. The sub-oesophageal ganglia are a number of closely 



approximated ganghonic masses, the boundaries of 



