492 MOLLUSCA 



SIPHONODENTALITJM Sars. Smaller orifice of shell with 2 slight 

 notches on each side. 



S. lobatum Sowerby. Shell smooth, shining, thin; 10 mm. long: 

 Arctic seas, south to New England; Europe. 



CLASS 3. GASTROPODA.* 



Snails. Asymmetrical mollusks with usually a spirally coiled shell, 

 a distinct head, and a broad, flat foot (Fig. 740, B). 



External Structure. The shell is a spiral cone (Fig. 772), the apex 

 of which represents the dorsal side of the animal. In most snails the 

 spiral twists to the right, and when the apex is uppermost the opening or 

 aperture of the shell will be on the right. In some, however, the twist 

 is normally to the left, and in the right-handed species occasional indi- 

 viduals are found which are left handed. The axis of the shell, around 

 which whorls are coiled is called the columella. When this axis contains 

 a cavity the space is called the umbilicus : sometimes it is partially filled 

 by a thickening called the callus. In certain species, as the slugs, the 

 shell is rudimentary or wanting, and in a few forms, as the limpet, is 

 conical without forming 'a spiral. In most prosobranchs and some 

 opisthobranchs the aperture of the shell is closed when the animal is 

 withdrawn within it by a disc called the operculum. 



The head (Fig. 771) bears one or two pairs of tentacles and a pair 

 of eyes; at its forward end is the mouth, which, in many species, is con- 

 tained in a long proboscis. The foot represents the ventral side of the 

 body, and is large and has usually a broad sole fitted for creeping on flat 

 surfaces. In the pelagic heteropods and pteropods it is modified to form 

 the fins, and in certain species it is wanting. The foot may secrete a 

 great amount of slime, either from its entire surfaces or from localized 

 glands, which facilitates locomotion. 



The visceral mass makes up the greater part of the spiral body, and 

 is covered by the mantle, which extends from the apex to the foot in a 

 single conical fold. The lower edge of the mantle is called the collar 

 (Fig. 771, 7) ; it can, in many prosobranchs be prolonged to form a 

 siphon. The shell is secreted by the mantle, and follows its spiral wind- 

 ings. The mantle unites with the visceral mass on all sides but one, 

 where a space called the mantle cavity is left between them. In certain 

 primitive snails this is posterior in position, but in the majority of them 

 it has shifted to the right or to the anterior side (Fig. 740,3). In the 

 mantle cavity are the gills or lungs, and in most snails the external open- 



* See "The Mollusca of the Chicago Area : The Gastropoda," by Frank Baker, 

 Natural History Survey, Bull. Ill, Part II, 1902. 



