SNAILS, OR GASTEROPODS. 197 



SNAILS, OR GASTEROPODS. 



The term Gasteropod means stomach-footed, and is 

 given to these animals because the lower side serves 

 them as a sort of foot, by means of which they creep 

 along. But this "foot" is in no way related to the 

 feet of the backboned animals. Most of the Gastero- 

 pods have a shell; and as this is made of only one 

 piece, or valve, they are often called Univalves. Some, 

 however, have no shell in the adult state, though all 

 have a shell when first hatched. Most Gasteropods 

 have a lid or door, called the operculum, with which 

 they close the opening to the shell when they withdraw 

 within. It is a horny plate, sometimes strengthened 

 by shelly matter. Their eyes are two, and generally 

 on long stalks, as seen.in Figure 356 ; they perceive light, 



Spire. W, whorls. 



Umbilicus 



Canal. 



Fig. 364. Names of the parts of a Gasteropod Shell. 



but probably cannot distinguish objects. Many of the 

 Gasteropods have horny jaws ; but one of the most 

 curious parts of these animals is the tongue, or lingual 

 ribbon, which is a band armed with a great number of 



