2 GASTEROPODA. 



portion of the foot secretes a calcareous, horny, or fibrous 

 plate, which is called the "operculum" (fig. 382, 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 {Hcteropoda) the animal is fitted for swim- 

 ming in the open ocean, by the conversion of the " foot " 

 into a vertically-flattened fin. 



The respiratory process in the Gasteropods differs con- 

 siderably in different cases ; and the class may be divided 



Fig. 3S2. — Ampullaria canalimlaki, one of the Apijle-sliells. o, Operculum ; 

 s, Respiratory siphon. 



mto two principal sections, according as the animal is fitted 

 for breathing air directly or through the medium of water. 

 The air-breathing Gasteropods are known as the Pulmonata 

 or Puhnonifera, 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. 



Shell of the Gastero'poda. — The shell of the Gasteropoda is 

 composed either of a single piece (univalve), or of a number 

 of plates succeeding one another from before backwards (mul- 

 tivalve). The univalve shell is to be regarded as essentially 

 a cone, the apex of which is more or less oblique. In the 

 simplest form of the shell the conical shape is retained with- 

 out any alteration, as is seen in the common Limpet {Patella). 

 In the great majority of cases, however, the cone is consider- 

 ably elongated, so as to form a tube, which may retain this 



