IX MOLLUSCA 111 



the case of other genera of snails which are heavier than 

 water, and which therefore cannot rise in this way, the 

 mucus is attached first at the top to some object, or even 

 merely to the surface film of the water, and then drawn out 

 as the snail sinks to the bottom, and fastened there, forming 

 a convenient, short, and rapid means of rising to the surface 

 when air is needed. We see, then, that although the snail 

 has only one, very slightly specialised organ of locomotion, it 

 is able to move in several different ways, and — by its rapid 

 vertical rising or sinking in the water — to escape some of 

 the dangers resulting from its aquatic life. 

 Mouth and These snails will eat almost any water-weed. 

 Method of and, failing anything else, they will scour the 

 Feeding, glass of the tank in which they are living in order 

 to get any microscopic plants that may be adhering to it. 

 When doing this, the action of the mouth and tongue can 

 be well observed. "^ 



The mouth is bounded by the soft upper and lower lips, 

 the lower lip being deeply cleft, so that the boundary of the 

 lips forms a T-shaped mark when closed. Set in these lips 

 are horny jaws, one in the upper lip, and also a small lateral 

 jaw in each lobe of the lower lip. Within the mouth there 

 is a remarkable tongue or rasp (radula) which is the main 

 organ used in obtaining food. This rasp is a strap-shaped 

 horny structure, attached through- 

 out its whole length to the floor 

 of the mouth. The floor is very 

 uneven, for there is a projecting 

 cushion near the mouth supported 

 by a little mass of cartilage, and 

 behind this cushion a deep cavity, 

 into which the rasp is continued 

 (see Fig. 62). Down the whole 



length of the rasp are transverse Fig. 62. — Longitncliual median 



rows of sharply -pointed curved section through the head of a 



, _, ^ •' ^. , , .7 . Pond Snail. (Diagrammatic.) 



teeth. Ihere are sixteen teeth in 



1 1 - /^ rt wt'^t) mouth ; r, radula : c, cartilage : 



each row, and over oOO rows. ^ tongue sac. 



The number and shape of the teeth 



in each row vary in each genus of snail ; the variety of the 



' They are said to like animal food also, and even to attack live animals 

 PCCiVsiopallyj but I have never observed this in those I have kept. 



