342 MOLLUSC A. 



equivalent to a lens, closed in front by a transparent "cornea,'' and 

 strengthened all round by a firm "scleiotic." How much a snail 

 sees we do not know, but it detects quick movements. Though the eye 

 is by no means very simple, the snail soon makes another if the origina. 

 be lost, and this process of regeneration has been known to occur 

 twenty times in succession. 



The octocysts 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, containing a fluid and a variable number of 

 oval otoliths of lime, and 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 tur- 

 pentine, 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. — In cutting a piece of leaf, the snail 

 uses two instruments — the crescentic jaw-plate on the roof 

 of the mouth, and the toothed ribbon or radula on the floor. 

 This radula is like a flexible file, — a short and broad 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 teeth, mem- 

 brane, 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 leaves may be compared very roughly to that 

 of a file, but its movements within the mouth also produce 

 a kind of suction which draws food particles inwards. In 

 this suction the muscular lips and the cilia in the mouth 

 cavity assist. 



The ducts of two large salivary glands open on the 

 dorsal surface of the buccal cavity, and there are numerous 

 distinct glandular cells close to the entrance of the two 

 ducts. The salivary glands are large lobed structures, and 

 extend far backward on the crop. They consist of hundreds 

 of glandular cells or unicellular glands, which secrete a clear 



