2IO 



MOLLUSKS: GASTROPODS. 



SNAILS, OR GASTROPODS. 



The term Gastropod means stomach-footed, and is 

 eiven 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 Gastro- 

 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 Gastropods 

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

 they close the opening to the shell when they with- 

 draw within. It is a horny plate, sometimes strength- 

 ened by shelly matter. Their eyes are two, and often 

 on long stalks, as seen in Figure 354. Many of the 



I Spire. W, whorls. 



Umbilicus 



Canal. 



Fig. 362. — Names of the parts of a Gastropod Shell. 



Gastropods 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 

 glossy silicious teeth, arranged in rows in the most regu- 



