MOLLUSKS. 



325 



in the shell after the head and the foot are drawn in. In 

 Fig. 255 you see one of these animals with the head and 

 the foot out of the shell. 



558. Many of the Gasteropods are remarkable for an 

 abundant supply of flinty teeth. Sometimes these are on 

 the palate, and in some species even the stomach has 

 teeth scattered over its inner surface. The tongue, in 

 some, is remarkable for its length, and for the teeth which 

 are all along on its upper surface. The tongue of the 

 common Limpet, Fig. 256, is an example. It is from two 



to three inches long, and this 

 is longer than the whole an- 

 imal. When not in use, it is 

 turned backward down into 

 the stomach. It is spoon- 

 shaped at the end. In its 

 whole extent it is armed with 

 rows of teeth, four in each 

 row, and between each two 

 rows there are two three- 

 pointed teeth. These two 

 sets of teeth are represented 

 in a magnified portion of the 

 tongue in the figure. The 

 part of the tongue toward its 

 root generally has its edges 

 turned over so as to meet, thus making a tube. The 

 whole instrument is therefore an efficient rasper, and also 

 a proboscis. 



559. Of the Gasteropoda, some are terrestrial and some 

 live in fresh water, but most of them are found in the 

 sea. The terrestrial Gasteropods are Snails and Slugs. 

 In the common Slug there is a prominent head with four 

 tentacula, which can be drawn inward by a process like 

 the inversion of the finger of a glove. At the ends of 

 the longer pair of the tentacula are the eyes. On the 

 back there is a kind of shield formed by the mantle, 



Fig. 256. Limpet's Tongue. 



