THE SNAIL. 



349 



opening of the ureter. The food canal ends sHghtly below 

 and to the right of the pulmonary aperture. All the three 

 openings are close together. The attterior termination of 

 ureter and food canal is one of the results of the twisting of 

 the visceral mass forwards to the right. But still further 

 forward, at the end of a slight groove which runs along the 

 right side of the neck, indeed quite close to the mouth, is 

 the genital aperture. Lastly, an opening just beneath the 

 mouth leads into the large mucus gland of the foot. 



The Shell. 



The shell, a right-handed spiral, is a cuticular product made 

 and periodically enlarged by the mantle margin. Chemically 

 it consists of carbonate of lime and an organic basis 

 (conchiolin). The outermost layer is coloured, without 

 lime, and easily rubbed off : the median layer is thickest, 

 and looks like porcelain ; the innermost layer is pearly. 

 The twisted cavity of the shell is continuous, and the 

 viscera extend to the uppermost and oldest part. 



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

 central pillar (columella), as on a staircase, and to this the animal is 

 bound by a strong (columellar) 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. 



Exie7-nal Appearance after the Shell is Re7noved. 



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 

 mande; {b) with the loose roof of the mantle or pulmonary 

 chamber; (c) with the exceedingly delicate, much stretched, 

 and always protected skin of the visceral hump. It is 

 important to realise that the snail has an "enlargeinent 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 

 the yet deeper torsion forward to the right. 



