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 hfe. 
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 
