128 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



right to left), 29 around a central axis, or " columella " 

 (Fig. 297). When the columella is hollow (perforated), 

 the opening in the end is called the " umbilicus." When 

 the whorls are coiled around the axis in the same plane, 

 we have a discoidal shell, as the Planorbis. The mouth, 

 or " aperture," of the shell is " entire " in most vegetable- 

 feeding snails, and notched or produced into a canal for 



FIG. 88. Whelk {Buccinutri) , showing operculum, o, and siphon, s. 



the siphons in the carnivorous species. The former are 

 generally land and fresh-water forms, and the latter all 

 marine. In some gastropods, as the river snails and 

 most sea snails, a horny or calcareous plate (operculum} 

 is secreted on the foot, which closes the aperture when 

 the animal withdraws into its shell. In locomotion, the 

 shell is carried with the apex directed backward. 



The body of most gastropods is unsymmetrical, the 

 organs not being in pairs, but single, and on one side, 

 instead of central. The mantle is continuous around 

 the body, not bilobed, as in lamellibranchs. A few, as 

 the common garden snail, have a lung; but the vast 

 majority breathe by gills. The head is more or less 

 distinct, and provided with two tentacles, with auditory 



