STRUCTURE OF THE SNAIL. 321 



the uppermost and oldest part. As the shell is graduallymade, 

 the inner walls of the coils form a central pillar (columella), 

 such as you may see on a staircase, and to this the animal is 

 bound by a strong muscle. Many Gasteropods bear a horn- 

 like shell lid (operculum) on their foot, but Helix has 

 none ; the " epiphragm," with which the shell is sealed in 

 winter, consists of hardened mucus, plus phosphate and a 

 smaller quantity 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 an 

 operculum to the foot of the whelk, and is loosened off in 

 the mildness of spring. 



External 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 con- 

 trasted, (a) with the thick collar of the mantle, {b) with 

 the loose roof of the mantle or pulmonary chamber, {c) 

 with the exceedingly delicate, because much stretched and 

 always protected, skin of the visceral hump. It is im- 

 portant 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 

 a 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 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. — We have just noticed that the skin varies 

 greatly in thickness. It consists of a single layered epi- 

 dermis and a more complex dermis, including connective 

 tissue and muscle fibres. There are numerous cells from 

 which mucus, pigment, and lime are secreted, the two latter 

 are especially abundant on the collar, where they contribute 

 to the shell which is gradually increased in that region. 

 The mucous glands of the skin must not be confused with 

 those which are very numerous in the foot and form a 

 secretion which oozes out by a duct opening just beneath the 

 mouth. 



Muscular System. — The most important muscles are — (a) 



X 



