FRESH-WATER MOLLUSKS. 



565 



Among the snails collected, tliere will probably be found some 

 which have a peculiar scale on the hinder part of the body. When 

 the snail crawls, this scale will be seen just behind the shell, as in 



A view from the back, 

 or dorsal view. 



side-mew. 



ventral view. 



Fig. 3. A Feesh-Wateb Snail seen feom above, peom the Side, and from below. 

 1 1, Tentacles ; e e, Eyes ; b. Breathing-orifice ; m. Mouth. 



Fig 



5, 0. 



This scale is called the operculum, and when the snail has 

 contracted, or dr&wn within the shell, the operculum is seen to fit the 

 aperture of the shell, closing the shell as a stopper closes the mouth 

 of a bottle. 



Nearly all sea-snails, that is, snails which live in salt-water, and 

 many species of fresh-water snails, and also many snails which live in 

 damp places on the land, and which are called land-snails, have an 



