MUSCULAR AND NERVOUS SYSTEMS. 369 



fused to the body-wall. The result is to form a respiratory 

 cavity, which is as much outside the body as is the gill- 

 chamber of the crayfish. It is important to realise that the 

 snail has an "enlargement of the liver " and a great rupture- 

 like hump of viscera on the dorsal surface, that this has been 

 coiled spirally, and that there is the yet deeper torsion 

 forward to the right. 



A great part of the hump consists of the greenish-brown 

 digestive gland, in which the bluish intestine coils ; behind 

 the mantle chamber, on the right, lies the triangular and 

 greyish kidney ; the whitish reproductive organ lies in the 

 second last and third last coil of the spiral. 



Skin. --This varies greatly in thickness. It consists of a 

 single-layered epidermis and a more complex dermis, 

 including connective tissue and muscle fibres. There are 

 numerous cells from which mucus, pigment, and lime are 

 secreted ; those forming pigment and lime are especially 

 abundant on the collar, where they contribute to the growth 

 of the shell. 



Muscular system. Among the important muscles are 

 (a) those of the foot; (l>} 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 con- 

 sists 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 (Fig. 188). 



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 



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