THE MOLLUSCA 103 



characterized by an asymmetrical and usually coiled body. 

 Most of the species live in a coiled shell, but in some forms, 

 such as the limpets, the shell is a sort of cap; in the slugs 

 it may be reduced to a small rudiment imbedded in the 

 mantle while in some forms no trace of the shell remains in 

 the adult although a shell gland is present in the embryo. 

 The Gastropoda are common in the sea, in fresh water, 

 and on land. One can in most places easily obtain a 

 fairly typical gastropod in the familiar garden snail, 

 Helix. The body is furnished below with a broad, flat, 

 muscular base, the foot, on which the animal creeps. The 



f 



FIG. 86. Body of snail in creeping position after removal of shell. 

 /, foot; go. opening of sex organs; h, head; al, opening into lung cavity; 

 mw, wall of mantle; es, visceral sac; ti, tn, and tin, tentacles, the upper pair 

 with eyes at the tip. (From Meisenheimer.) 



head bears, in addition to the short feelers over the 

 mouth, a pair of long, retractile tentacles at the end of 

 which are the eyes. When the latter are irritated they 

 may be drawn into the tentacles much as a person could 

 pull in the end of a ringer of a glove by a string attached 

 to the inner side of the tip. In the mouth of the snail 

 there is a ribbon-like structure armed with rows of minute 

 chitinous teeth which are used in rasping off bits of food. 

 On the right side of the body near the head are the open- 

 ings of the sex organs, and further back a larger aperture 

 which leads to the breathing cavity or lung. While in 

 the marine gastropods this cavity contains gills which are 



