FRESH-WATER SNAILS. 9 



The broad, creeping disk upon which the snail rests, and 

 by which it retains its hold to the glass, is called the foot. 

 The snail moves about, and crawls or glides slowly along, by 

 means of the foot. 



The two little horns or feelers, in front, are called Unta^ 

 cles, and, as the snail moves, the tentacles are seen stretched 

 out in front, and occasionally bending, as if the creature were 

 feeling its way along. The eyes are seen at the base of the 

 tentacles, as two minute black dots. The mouth is between 

 the tentacles, and below. The part from which the tenta- 

 cles spring is called the head, and the opposite end of the 

 body is called the tail. The surface upon which the snail 

 rests is called the ventral or lower surface, and consequently 

 that portion of the body which is above is called the dorsal 

 surface, or back. 



9. The pupil, in watching the habits of the snails 

 he has collected, will notice some of them crawling to 

 the surface of the water to breathe air. The snail ac- 

 complishes this by raising the outer edge of the aperture 

 to the water's edge, and then opening a little orifice in 

 the side, through which the air enters to the simple lung 

 within. 



This orifice is on the right side in those snails having 

 dextral shells, and on the left side in those snails having sin- 

 istral shells. 



Many kinds of snails which live in fresh water are called 

 air-breathers, because they are forced to come to the surface 

 of the water to breathe air. In doing so they first expel a 



