TBE OLAM AND OTHER BIVAL7ED SHELL-FISH. 73 



disturbed it will push a great flesliy 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 

 wiU walk off on its " foot," which is a broad, creepiag 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 snaU as it moves about. Also the mouth, 

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

 the opening (Fig. "iQ), 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 iateresting 

 than to collect a lot of pond- 

 snails, which may be found any- 

 where in roadside ditches or 

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

 Our commonest kind is the Physa, Fig. 73 representing on 

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



FiQ. 73. — Physa, common pond- 

 snail. 



Fio. 74.— Slug (natural 

 size), hanging by a 

 slimy thread from a 

 branch. 



Fio. 75. — a, jaw; b and c, side 

 and top view of teeth of lingua 

 ribbon of the snail, Helix aXbc 

 labris. 



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 



