110 MOLLIISCA. 



CLASS GASTEROPODA. 



These " belly-footed " inolhisks are the types of the subking- 

 doni. The characteristic mode of locomotion is exemplified by 

 the common Snail, which crawls by the alternate expansion and 

 contraction of its foot. The head is usually furnished with eyes, 

 tentacles and odontophore. Respiration is either by gills, exposed 

 or covered, or by a skin-lung (in the Pulmonata). Gasteropods 

 are terrestrial or aquatic ; bisexual or hermaphrodite ; oviparous 

 or ovo-viviparous. 



Bilateral symmetry is marked in the naked gasteropods, but 

 with few exceptions, the visceral part is asymmetrically coiled up 

 in a shell, usually spiral and univalve. The apex of the cone 

 (save in the Nucleobranchs), is always oblique and eccentric ; 

 this departure from a perfect spiral is owing to the unequal de- 

 velopment of the two sides of the animal. The spirals are 

 mostly right-handed. No shell is multilocular like the Ammon- 

 ites, but it maybe simply a cone {Patella), or multivalve {Chiton). 

 The muscular impression is crescent-shaped. The operculum is 

 equivalent to the dextral valve of Lamellibranchs. 



The class is divided into orders distinguished chiefly by the 

 form and position of the respiratory organs. The living species 

 have world-wide range, inhabiting the bottom and surface of the 

 sea, sea-shore, fresh water and dry land, and in number rank next 

 to insects. The fossil species are found in every fossiliferous 

 rock from the Cambrian upwai-d. The marine forms with entire 

 (unnotched) apertures (Holostomata), mostly vegetarian, predomi- 

 nate in the PaltBozoic strata ; the siphonated species (Siphonosto- 

 mata), indicated by the notched or produced apertures, mainly 

 carnivorous, are more abundant in the Mesozoic and later time, 

 and are at their maximum to-day. The remains of gasteropods 

 are highly important to the geologist, as they afford him un- 

 equivocal evidence of the terrestrial, fluviatile, lacustrine or 

 marine condition under which strata were formed. 



