MOLLUSKS. S23 
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. 
