322 MOLLUSCS. 



those in the foot, {b) those which retract the animal into its 

 shell, and are 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. 



The Nervous System is concentrated in a hardly analysable 

 ring round the gullet! Careful examination shows that this 

 ring consists dorsally of a pair of cerebral ganglia, connected 

 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 a pair of small buccal ganglia, lying beneath the 

 junction of gullet and buccal mass. The pedals give off 

 nerves to the foot, the 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, 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 is easy to prove that it 

 detects swift movements. Though the eye is by no means 

 very simple, the snail can readily make 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, con- 

 taining a fluid and a variable number of oval otoliths of 

 lime, innervated by a very delicate nerve from the cerebral 

 ganglia. 



No osphradium or smelling patch, comparable to that 

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

 been discovered in Helix. But every one can see that the 

 snail is repelled or attracted by odours; it shrinks from 



