384 PHYL UM 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 farther 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 (conchin). 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 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 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 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 with its summit towards the observer, has 

 its aperture 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 



