NERVOUS SYSTEM OF SNAIL 



439 



(a) those of the foot ; (h) those which retract the animal 

 into its shell, and are in part attached to the columella ; 

 (c) those which work the radula in the mouth ; (d) the 

 retractors of the horns : and (e) the retractor of the penis. 

 The muscle fibres usually appear unstriated. There is 

 much connective tissue, some of the cells of which contain 

 glycogen, pigment, and lime. 



Nervous system. — This is concentrated in a ring 

 around the gullet. Careful examination shows that this 

 ring consists dorsally of a pair of cerebral ganglia, con- 

 nected ventrally with a pair of pedals and a pair of pleuro- 

 viscerals, which, according to some authorities, have a 

 median abdominal ganglion lying between them. 



The cerebrals give off nerves to the head, e.g. to the 

 mouth, tentacles, and otocysts, and also two nerves which 

 run to small buccal ganglia, lying beneath the junction of 

 gullet and buccal mass. The pedals give off nerves to 

 the foot ; the pleuro-viscerals to the mantle and posterior 

 organs. 



Sense organs. — An eye, innervated from the brain, is situated on 

 one side of the tip of each of the two long horns. It is a cup invaginated 

 from the epidermis, lined posteriorly by a single layer of pigmented and 

 non-pigmented retinal cells, filled with a 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. 



