Gasteropoda 103 



flattened. The last and largest whorl is called the body whorl, in 

 which is the aperture or mouth of the shell through which the animal 

 extrudes the foot and the parts concerned with feeding. All the 

 whorls together, exclusive of the body whorl, form the spire. To 

 count the whorls place the shell with the aperture downward and 

 count every whorl up to but not including the apical whorl. 



The whorls show great variety in shape and sculpture. Some 

 are flat and smooth, hardly showing microscopic markings, and 

 from this simplicity the range broadens to include forms of the 

 greatest complexity. The purpose of this sculptural decoration is 

 considered to be protective, though nothing is certainly known of the 

 reasons for such wealth of form, pattern, and color as is exhibited 

 by gasteropod shells. 



The aperture may be of any shape between round or oval and 

 the long, narrow slit with parallel margins as seen in Conus and Oliva. 

 The outer lip may be thin or thickened, plam, dentate, or alate. 

 There may be a notch at either or both the top and the lowest part 

 of the aperture, and one or both of these notches may be prolonged 

 into canals. Sometimes the upper (posterior) notch is absent, and 

 often the lower (anterior) notch is produced into a canal of variable 

 length which may be open along one side or closed throughout its 

 entire extent. The columellar or inner lip may be perfectly simple and 

 smooth or folded into plications which are often characteristic of 

 the group to which the shell belongs. An entire, simple aperture de- 

 notes a vegetable feeder, but the rule is not invariable. Most pro- 

 boscidiferous mollusks whose shells have canaliculated apertures are 

 carnivorous and prey upon their own relatives, especially bivalves, 

 whose shells they attack at the most vulnerable point and nearest 

 to the essential organs. 



The columella is the central pillar or column which passes 

 through the shell from base to apex. It is the axis about which the 

 whorls revolve and near its apex is affixed the columellar muscle 

 which is the sole attachment of the mollusk to its shell. Contraction 

 of this muscle retracts the animal into its shell and brings the op- 

 erculum into position to close the aperture. 



Most gasteropods have opercula. The operculum is a special 



