PALEONTOLOGY. 



CHAPTEE XXIV. 



GASTEROPODA. 



The Gasteropods are Molluscs in whicli tlie, hody is furnished 

 with a distinct head, and the mouth is provided with a masti- 

 catory apparatus or " lingual ribbon" Locomotion is effected 

 by means of a broad, horizontally-flattened, ventral disc — the 

 "foot " — or by a vertically-flattened, fin-like modification of the 

 same. The body is never included in a bivalve shell; and nuty 

 be naked. Usually, hoivever, there is a " univalve " shell, or in 

 some cases a " multivalve" shell. 



This class includes all those Molluscous animals which are 

 commonly known as " Univalves," such as Land-snails, Sea- 

 snails, Whelks, Limpets, &c. In the Chitons, however, the 

 shell is composed of eight pieces (" multivalve ") ; and in the 

 Slugs the shell is minute and completely concealed in the 

 mantle ; whilst in the Sea-slugs and Sea-lemons the animal 

 is " naked," and is destitute of a shell. 



In their habits the Gasteropods show great differences, 

 most of them being free and locomotive, though some are 

 sedentary. The typical forms move about more or less 

 actively by the successive contractions and expansions of a 

 muscular organ developed upon the ventral surface of the 

 body and known as the " foot." In many cases the posterior 

 VOL. II. A 



