THE CLAM AND OTHER B1VALVED SHELL-FISH. 73 



disturbed it will push a great fleshy mass out of the aper- 

 ture of the shell, and soon the head and foot will unfold; 

 the feelers or tentacles will be thrust out, and the snail 

 will walk off on its " foot," which is a broad, creeping disk, 

 very different from the clam's foot. The foot can be best 

 seen by allowing the snail to creep up the glass; the outline 

 of the foot can be observed, as well as the peculiar gliding 

 motion of the snail as it moves about. Also the mouth, 

 which is situated at the front end of the foot; as well as 

 the opening (Fig. 76), which leads into the lung-cavity or 

 gill-like organ. In the land-snail the spire of the shell is 

 very short, while the lip is 

 usually very thick and white. 

 Nothing is more interesting 

 than to collect a lot of pond- 

 snails, which may be found any- FIG. vs. pity. , common pond- 

 where in roadside ditches or 



in pools in swampy places, and to watch their movements. 

 Our commonest kind is the Phi/sa, Fig. 73 representing on 

 the right an empty shell, and on the left the animal in the act 



FIG. 74. Slug (natural 

 size), hanging by a 

 slimy thread from a 

 branch. 



FIG. 75. a, jaw; b and c, side 

 and top view of teeth of lingual 

 ribbon of the snail, Helix ulbo- 

 lubris. 



of creeping on its long pointed foot, its two tentacles ex- 

 tended out in front. 



It will be seen that the land-snails have f~-ur tentacles 



