24 



Class 2. GASTEROPODA. 



Animal ; hodij elongated, anterior extremity furnulied ivith a more 

 or less prominent head, posterior extremity mostly developed 

 into an attenuating spiral, invested with a lubricous mantle, 

 producing a shell of varied structure and solidity ; head mostly 

 furnished with tentacles, varying from two to six in number, 

 with a pair of eyes situated at different parts of the tentacles ; 

 mouth furnished with hard parts, or with a flexible retractile 

 trunk of various length, armed at the extremity with small teeth. 

 Branchice respiring air or water, either concealed or exposed. 



Shell ; calcareous, in some instances horny, either simply conical, 

 or convoluted into an enlarging spire, mostly enclosing the ani- 

 mal, but sometimes internal, or covering only a limited portion 

 of it. 



The Gastropods comprise that extensive series of moUusks which acquire 

 motion by the alternate dilatation and contraction of a gastric or ventral 

 disc. Their tj^^ncal structure is that of a long di-awn out conical mass, of 

 wliich the attenuated portion, contained in an enlarging spiral shell, is oc- 

 cupied by the soft and visceral parts, wliilst the broad extremity, protruding 

 from the aperture of the shell, comprises the head, mantle, nerves, muscles, 

 gills, &c., and also a broad fleshy muscular expansile disc, mostly attached 

 to the neck, called the foot, by which they acquire motion. 



By far the greater portion of the class are of tliis structure ; those inha- 

 biting a spiral shell are, indeed, necessarily so. In the spiral Gastropods, 

 the abdominal portion of the moUusk is contained within the shell, whilst 

 the fleshy disk, attached to the neck, is exserted for the purpose of locomo- 

 tion • but in the non-spiral Gastropods, such as the Cliitons, Limpets, and 

 Slugs, the abdominal parts are aU of a mass, and not separated from the 

 disk. Lamarck confined the term Gasteropoda to the latter division ; the 

 former section he distinguished by the title of Trachelipoda^ or neck- 

 moving MoUusks, but the affinity between the Snail [Helix) and the Slug 

 [Limax) is of too intimate a character to allow of so important a separation. 



The Gastropods are extremely numerous in species, and of considerable 

 interest, both on account of their extraordinary varieties of form, colour, 

 and sculpture, and of their curious diversity of habit. Most of them are 

 marine, some are fluviatile, some dwell in lakes and stagnant pools, wliilst 

 a large proportion are terrestial, or arboreal ; and as their breatliing appa- 



* From Tpax>]\os (trachclos) ticck. aud jrovs (pous) foot. 



