438 PHYLUM MOLLUSCA 



As the shell is made, the inner walls of the coils form a central 

 pillar (columella), as on a staircase, to which the animal is bound by 

 a strong (columellar) muscle. Many Gasteropods bear on the foot a 

 lid or operculum, of conchin or of lime, which closes the mouth of the 

 shell. In Helix there is none ; the " epiphragm " with which the 

 shell is sealed in winter consists of hardened mucus, plus phosphate 

 and a smaller quantit}^ of carbonate of lime. It is formed very quickly 

 from the collar region when cold weather sets in, has no organic 

 connection with the animal, such as binds the operculum to the foot of 

 the whelk, and is loosened off in the mildness of spring. 



Sinistral shells, with left-handed spiral, occasionally occur as 

 variations. The shell, held summit upwards and mouth towards the 

 observer, has the mouth to the left. The internal organs are inverted, 

 and at the start there is a reversal of the cleavage planes of the egg. 



Appearance after the shell is removed. — If the shell 

 is removed carefully, so that nothing is broken except the 

 columellar muscle, many structures can be seen without 

 any dissection. The skin of the head and foot should 

 be contrasted — {a) with the thick collar of the mantle ; 

 (b) with the mantle itself, which forms the loose roof of 

 the pulmonary chamber ; {c) with the exceedingly delicate, 

 much-stretched, and always protected skin of the visceral 

 hump. The mantle is a downgrowth of the skin of this 

 dorsal region. It is peculiar in the snail, in that its margin 

 (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. -7- Among the important muscles are — 



