190 STRUCTURE OF THE MOLLUSCA. 



CLASS IIL-GASTEEOPODA. 



The Gasteropodous Mollusca, figs. 3, 4, are distinguished 

 by having a fleshy foot or disk attached to the lower part of 

 the body, sometimes, as in slugs, to its whole length, but 

 more frequently only to its anterior part. One may form a 

 correct enough idea of the general form of all the Gaster- 

 opoda by examining a slug or a snail. Their body is elon- 

 gated, and terminated anteriorly by a head, fig. 3, e, which 

 projects beyond the mantle, and usually bears two or four 

 retractile tentacula, fig. 3, c c, inserted above the mouth, 

 and very small eyes, fig. 3, d, of a very simple structure. 

 Their body is covered beneath by a fleshy mass, fig. 3, f; 

 fig. 4, gy, which generally presents the form of a broad disk, 

 and is the instrument by which the animal crawls along the 

 ground, but which sometimes constitutes a vertical fin. 

 Their back is furnished with a mantle, which is sometimes 

 bare, but generally covered by a shell. In most cases, the 

 shell is large enough to contain the animal entirely, when it 

 contracts itself, and it generally has the form of a tube 

 twisted upon itself. In many species a horny or calcareous 

 plate, called the operculum, is attached to the foot behind, 

 fig. 4, h ; fig. 8, b, and serves to close the aperture of the 

 shell, when the animal has withdrawn. 



The organs of respiration are sometimes adapted for air, 

 more frequently for water ; but in those species which have 

 a twisted shell, they are always lodged in the last or body 

 convolution, and receive the ambient element either by a 

 hole in the mantle, or by a wide opening between the body 

 and the mantle, which is also sometimes prolonged into a 

 canal or tube, by means of which the animal can respire 

 without protruding either its head or its foot. In this latter 

 case, the shell has a notch or canal, for the respiratory tube, 

 situated at the fore part of the aperture of the shell, near the 

 end of the columella. 



