GASTEROPODA. 241 



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 is 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 por- 

 tion of the foot secretes a calcareous, horny, or fibrous plate, 



Fig. 203. Amfiullaria cajuiliczdata, one of the Apple-shells, o Operculum ; 

 s Respiratory siphon. 



which is called the " operculum " (fig. 203, o), and which serves 

 to close the aperture of the shell when the animal is retracted 

 within it. Lastly, in one aberrant group of the Gasteropods 

 (Heteropodd) the animal is fitted for swimming in the open 

 ocean, by the conversion of the " foot " into a vertically- 

 flattened fin. 



The respiratory process in the Gasteropods differs consider- 

 ably in different cases ; and the class may be divided into two 

 principal sections, according as the animal is fitted for breath- 

 ing air directly or through the medium of water. The air- 

 breathing Gasteropods are known as the Pulmonata or Pul- 

 monifera, and comprise forms which either live on land (Snails, 

 Slugs, &c.), or which inhabit fresh water (Pond-snails, &c.) 

 The water-breathing Gasteropods are mostly provided with 

 distinct gills or " branchiae," and they form the section of the 

 Branchifera. They are mostly inhabitants of the sea; but 

 some of them inhabit fresh water. 



