STRUCTURE OF THE SNAIL. 319 



immersed in water. Their food consists of leaves and 

 other parts of plants, but they sometimes indulge strange 

 vagaries of appetite. They are hermaphrodite, but their 

 sexual relations are by no means simple. The breeding 

 time is spring, and the eggs are laid in the ground. In 

 winter, snails bury themselves, cementing the mouth of 

 their shell with hardened mucus and a little lime, and fall 

 into a state of " latent life " in which the heart beats very 

 feebly. In such a state they have been known to survive 

 for years. 



General Appearance. — A snail actively creeping shows a 

 well-developed head, with two pairs of retractile horns or 

 tentacles of which the longer bear eyes. W^ may see and 

 even hear the snail browsing. The foot, by the muscular 

 contraction of which the animal creeps, is very large ; it 

 leaves a slimy trail of mucus. The viscera are extruded as 

 usual in a dorsal hump, and this hump is spirally coiled, and 

 is protected by the spiral shell within which the entire 

 animal retracts itself on slight provocation. 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 virtually together. 

 The anterior termination of ureter and food-canal is one of 

 the results of the torsion which we have already described. 

 But still further forward, at the end of a slight groove which 

 runs along the right side of the neck, indeed qtiite close to 

 the mouth, is the genital aperture. Lastly, an opening just 

 beneath the mouth leads into the large mucous-gland of the 

 foot. 



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 confinuous, and the viscera extend to 



