SNAILS. 



207 



fore-part of the back, where it covers the pulmonary cavity. It often encloses 

 in its thickness a small flat shelly plate. These animals are herbivorous ; they 

 feed principally upon young plants, fruits, mushrooms, c., and are most 

 voracious towards evening. During the heat of the day they remain concealed 

 under stones, or beneath some heap of half-decayed leaves, or even in the 



earth, and they seldom 

 come out, except in the 

 morning and evening, 

 when the air is numid; 

 they are especially abun- 

 dant after rain. During 

 the cold season they bury 

 themselves in the ground 

 and remain torpid. 



The Snails (Helix) 

 have a complete and ap- 



II I parent shell. Their struc- 



ture ditiers very little from 

 that ot the slugs, and their 

 habits are nearly the same. 

 In summer they are very 

 FIG. 212. GARDEN SNAIL. voracious and destructive, 



but in autumn they eat 



little. On the approach of winter they retire into some hole and draw them- 

 selves into their shell, shutting up the aperture with a kind of door secreted 

 by the edge of the mantle. The species of snails are very numerous : they 

 are found in all parts of the world. 



The Aquatic Air-breathing G-asteropods have but tii-o 

 tentacles: their -mode of respiration obliges them to come fre- 

 quently to the surface of the water to breathe. They conse- 

 quently cannot keep at great depths, and ordinarily reside in 

 fresh waters or near the mouths of rivers. 



The Pond-Snails (Liumccus^ common in ever)' pond, live upon vegetables 

 and the seeds of water-plants, and for this purpose are provided with a strong 

 muscular gizzard. 



FlG. 213. LlMN.-EUS 



2:4. Pl.ANO^SIS CORNEL'S. 



The Flat Coils (Planorbis) are recognized by their shells being rolled up 

 spirally in the same plane, like a French horn. Their habits are similar to 

 those of the pond-snails, of which they are the constant companions. Their 



