136 THE SNAIL. 



the vesicle is partially filled by a large cuticular lens, 

 which projects into it from its anterior wall. 



2. The * auditory ' organs are a pair of small spherical sacs 

 imbedded into the substance of the pedal ganglia, 

 and containing numerous minute calcareous otoliths. 

 They are plabed at the ends of the auditory nerves, 

 which connect them with the supra-cesophageal 

 ganglia. 



VII. DISSECTION OF THE BUCCAL MASS. 



Remove the nerve-collar. Cut across the oesophagus and 

 salivary glands about a quarter of an inch behind the buccal 

 mass, and fix down the buccal mass firmly by pins through its 

 retractor muscles. Note the horny jaw, seen as a dark trans- 

 verse hoop across the front of the buccal mass : and the sac 

 of the radula, which forms a rounded projection from its hinder 

 end. 



Slit up the oesophagus with scissors along the mid-dorsal 

 line, and continue the cut forwards along the buccal mass as 

 far as the jaw. Separate the two sides so as to expose the cavity 

 of the buccal mass. 



1. The radula is a brownish-yellow chitinous ribbon lying 

 on the floor of the buccal cavity, and having its free 

 surface produced into an immense number of minute 

 backwardly directed teeth, arranged in very regular 

 transverse rows. 



The hinder end of the radula is folded longitudi- 

 nally on itself, and lodged in a tubular sac, which 

 projects freely behind the buccal mass, and within 

 which the radula is formed as a cuticular secretion of 

 its epithelial lining. The anterior end is opened out 

 and rests on a cartilaginous cushion. By means of 

 protractor and retractor muscles the organ can be 

 protruded from the mouth, and worked backwards 

 and forwards with a licking or rasping action. As 

 it wears away in front, it is replaced by growth 

 forwards from its hinder end. 



