386 PHYLUM MOLLUSCA. 
non-pigmented retinal cells, filled with u clear vitreous body perhaps 
equivalent to a lens, 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 quick movements. 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, containing a fluid and a variable number of 
oval otoliths of lime, and innervated by a delicate nerve from the cere- 
bral 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.—lIn 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 
