A LAND SNAIL AND OTHER MOLLUSKS 



159 



tral axis, the columella. The oldest part, as in the bivalves, is 

 the tip where the growth rings are the smallest. It is built 

 large enough to accommodate the entire animal. When prop- 

 erly stimulated, the snail is retracted into its shell by a muscle 

 attached to the columella. The composition of the shell is like 

 that of the mussel, and on account of the necessity of obtaining 

 calcium carbonate with which to build it, snails are only able 

 to live in regions where chalky or limestone soil exists. 



Locomotion. — The food of the snail consists of bits of leaves. 

 It must therefore be able to crawl about and must possess the 

 proper sense organs 

 for becoming aware 

 of its surroundings. 

 Snails are notori- 

 ously slow-moving 

 creatures, but while 

 they move only at 

 a " snail's pace," 

 still this is rapid 

 enough to enable 

 them to reach their 

 food which is, of 

 course, abundant 

 everywhere. 



The locomotor organ of the snail is a simple mass of muscle, 

 the foot (Fig. 89, F), similar to that of the clam. It is used very 

 differently, however. The foot glides along by means of a series 

 of wavelike contractions which start at the posterior end and 

 move forward. No matter how smooth or rough the surface 

 over which the animal is moving, the speed is always the same. 

 This is explained by the fact that as the snail moves along it 

 secretes near the anterior end of its foot a band of slime or mucus 

 upon which the rest of the foot glides along. Thus the amount 

 of friction is always the same regardless of the roughness of the 

 surface. 



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Fig. 



Diagram showing the structure of a snail. 



A., anus; At., respiratory aperture, the entrance to 

 mantle cavity indicated by arrow; D., intestine ; 

 F., foot; Fii., tentacles; Ko., head; M., mouth; Mh., 

 mantle cavity ; Mt., mantle ; R.Mt, free edge of 

 mantle; Sch., shell. (From Schmeil.) 



