SHELL OF SNAIL 



437 



On slight provocation the animal retracts itself within its 

 shell, a process which drives air from the mantle cavity, and 

 thus helps indirectly in respiration. Around the mouth of 

 the shell is a very thick mantle margin or collar, by which 

 the continued growth of the shell is secured. On the right 

 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 



Fig. 250. — Vertical section of the shell of a species 



of Helix. 



M., Mouth of shell ; A., apex ; C, columella. 



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 genitaf 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 hme 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 andj.'oldest part. 



