41 



CLASS III. GASTEROPODA. 



Body partially covered by the mantle, and generally 

 furnished with a shell; head distinct, tentaculate; a 

 muscular disk, or foot, for creeping, attached to the 

 body beneath. 



TracheUpoda and Heteropoda, Lamarck. Paracephalo- 

 phora and Polyplaxlphora, Blainville. Gasteropoda, 

 Cuvier. rao-r^p, the belly ; TTOV?, a foot. 



Body elongated ; the back furnished with a mantle, 

 which is sometimes bare, more frequently covered by 

 a shell. Head distinct, protruded beyond the mantle. 

 Mouth surrounded by contractile lips, sometimes armed 

 with teeth inserted on the palate. Stomach, intestine 

 and liver, with the other viscera lodged beneath the 

 mantle ; the anus generally on the right side of the 

 body anteriorly. Circulatory apparatus a systemic heart, 

 and an arterial system, together with pulmonic arteries 

 and veins. Respiratory organs sometimes pulmonic, 

 more frequently branchial. Generative system various. 

 Nervous system composed of ganglia and filaments ; 

 eyes very small, sometimes adhering to the head, some- 

 times to the base, or side, or tip of the tentacula, some- 

 times wanting ; two or four retractile tentacula, which 

 are organs of touch, attached to the head, always above 

 the mouth. Organ of locomotion a ventral fleshy mass, 

 generally in the form of an oblong disk, sometimes a 

 vertical fin. The shell generally large enough to con- 

 tain the entire animal, and usually tapering and spirally 

 convoluted. The mass of the viscera always enclosed 



