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; (4) 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. 
The cerebrals give off nerves to the head, eg. 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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