340 MOLLUSC A. 



side of the expanded animal, close to the anterior edge of 

 the shell, there is a large aperture through which air passes 

 into and out of the mantle cavity. Within the same 

 aperture is the terminal opening of the ureter. The food 

 canal ends slightly below and to the right of the pulmonary 

 aperture. All the three openings are close together. The 

 anterior 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. 



Shell. — The right-handed spiral shell is a cuticular 

 product made and periodically enlarged by the collar. 

 Chemically it consists of carbonate of lime and an organic 

 basis (conchiolin). The outermost layer is coloured, with- 

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

 mildness of spring. 



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 

 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 



