244 



ZOOLOGY 



After Bulletin American Museum of Natural History 



FIG. 61. Pyramidula ralslonensis, a fossil snail from the Eocene of Wyoming. 



Enlarged about five diameters. 



structure of garden or greenhouse snail, we observe that the body, 

 when extruded from the shell, is elongated, with the 

 head at one end. There is always, of course, a portion 

 of the animal within the shell. The flat surface on 

 which the animal moves is called the foot, and the move- 

 ment is by wavelike undulations, as can be seen if the 

 snail is caused to walk on a piece of glass. It is difficult 

 at first to believe that the substance of the foot is not 

 flowing from one end to the other, just as waves on the 

 ocean give the appearance of masses of water moving 

 rapidly forward. As the snail moves, slime is secreted 

 by the slime glands, and thus the creature travels on a 

 track of its own laying. The head is marked by four 

 tentacles, the upper long ones bearing eyes at the end. 

 These eye-bearing tentacles can be retracted by the 

 contraction of internal muscles ; they turn outside in, 

 as do the fingers of a hastily removed glove. Below the 

 tentacles is the mouth, which is furnished with a trans- 

 versely placed chitinous plate called the/<m>. The jaw 

 moves up and down, and cuts the tissue of plants. In 

 certain carnivorous slugs, which devour their prey 

 alive, there is no jaw. In addition to the jaw is a 

 delicate rasping structure, the lingual membrane. This 



Jaw and 



lingual 



membrane 



