ALIMENTARY SYSTEM. 351 



layer of pigmented and non-pigmented retinal cells, filled 

 with a clear vitreous body perhaps equivalent to a lens, 

 and closed in front by a transparent " cornea," and 

 strengthened all round by a firm " sclerotic." How much 

 a snail sees we do not know, but it detects swift move- 

 ments. Though the eye is by no means very simple, 

 the snail soon makes another if the original be lost, and 

 this process of regeneration has been known to occur twenty 

 times in succession. 



The otocysts appear as two small white spots on the pedal 

 ganglia. Each is a sac of connective tissue, lined by 

 epithelium which is said to be ciliated in one region, contain- 

 ing a fluid and a variable number of oval otoliths of lime, 

 innervated by a delicate nerve from the cerebral ganglia. 



Though no osphradium or smelling patch, comparable to 

 that which occurs at the base of the gills in most Molluscs, 

 has been discovered in Helix, the snail is repelled or 

 attracted by odours ; it shrinks from turpentine, it smells 

 strawberries from afar. This sense of smell seems to be 

 located in the horns, for a dishorned snail has none. The 

 tips of both pairs of horns bear sensory cells connected 

 with ganglionic tissue and nerve fibres within. 



Other sensory cells, probably of use in tasting, lie on the 

 lips ; and there are many others, which may be called tactile, 

 on the sides of the foot, and on various parts of the body. 

 In short, the snail is diffusely sensitive. 



Alimentary System. 



The snail files the leaves of plants by means of the radula 

 or toothed ribbon which lies in the mouth, and it grasps the 

 debris with its lips. 



The radula is a long strip of membrane, bearing several 

 longitudinal rows of minute chitinoid teeth. It rests on a 

 cartilaginous pad on the floor of the mouth cavity, and is 

 moved (backwards and forwards, and up and down) in a 

 curve, by protractor and retractor muscles. The whole 

 apparatus, including radula teeth, membrane, and pad, is 

 called the odontophore. The radula wears away anteriorly, 

 but is added to posteriorly within a radula sac which 

 projects from the floor of the buccal cavity. Its action on 



