120 THE SNAIL 



VI. THE SENSE OEGANS. 



1. The eyes are placed one at the end of each of the larger 



tentacles, and are only exposed when these are fully 

 protruded. Each receives a branch from the large 

 nerve supplying the tentacle. 



Each eye is a small spherical vesicle, the wall of 

 which is strongly pigmented in its posterior portion, 

 and lined by a layer of retinal cells. The cavity of 

 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 in the substance of the pedal ganglia, and 

 containing numerous minute calcareous otoliths. 

 They are placed at the ends of the auditory nerves, 

 which connect them with the supra-cesophageal 

 ganglia. 



8. The pedal gland is a tubular sac extending along the 

 greater part of the length of the foot, and opening 

 in front by an aperture just below the head. 



It has glandular walls, and is lined by a ciliated 

 epithelium. The cells of the ventral wall resemble 

 sensory cells in character, and the gland has hence 

 been regarded by some writers as, at any rate in 

 part, a sensory organ, possibly olfactory in function. 



Pass a seeker into the external aperture of the pedal gland, 

 and lay it open along its whole length. 



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 odontophore, which forms a rounded projection from its 

 hinder end. 



