MOLLUSKS 



81 



striking peculiarity is in the univalve shell, with numerous 

 whorls, into which the animal may at any time withdraw 

 completely. Ordinarily this is carried on the back of the 

 spindle-shaped body, which is fashioned beneath into a great 



FIG. 48. The slug (Ariolimax) and common snail (Helix). From life. 



flat sole or creeping surface that bears on its forward bor- 

 der a wide opening through which mucus is continually 

 issuing to enable the snail to slip along more readily. Slime 

 also exudes on other points on the surface of the body and 

 affords a valuable protection against excessive heat and 

 drought. 



Unlike the clams, the forward end of the body is devel- 

 oped into a well-marked head bearing the mouth and a 

 complicated mechanism for gathering and masticating food, 

 together with two pairs of tentacles, one of which carries the 

 eyes. On the right side of the animal, some distance behind 

 the head, is the opening of the little sac-like mantle cavity 

 (Fig. 48) which contains the respiratory organs, and into 

 which the alimentary canal and the kidneys pour their 

 wastes. The relation of these organs to the mantle cavity 

 is the same as in the clams, though the cavities differ much 

 in size and position. 



82. Other snails. The shell. Extending our acquaint- 

 ance to other species of snails, we find the same general 

 plan of body, although somewhat obscured at times by 



