MUSCULAR AND NERVOUS SYSTEMS. 385 



(the collar) is 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 great rupture-like hump of viscera on the 

 dorsal surface 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; (b) 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; (ct) 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. 



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 sicje 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 



