350 MOLLUSC A. 



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 left lies the triangular and 

 greyish kidney ; the whitish reproductive organ lies in the 

 second last and third last coil of the spiral. 



The Skin. 



The skin 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 shell. 



Muscular System. 



The most important muscles are (a) those of the foot; 

 (&) 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, 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. 118). 



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 offnerves 

 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 



