24 



Class 2. GASTEROPODA. 



Animal ; body elongated, anterior extremity furnished with 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. 

 Branchia 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 mollusks which acquire 

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

 disc. Their typical structure is that of a long drawn out conical mass, of 

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

 cupied by the soft and visceral parts, whilst 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 winch they acquire motion. 



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

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

 the abdominal portion of the mollusk 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 Chitons, Limpets, and 

 Slugs, the abdominal parts are all 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 Trachrtipoda* or neck- 

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

 [Li?nax) 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, whilst 

 <i large proportion are terrestial, or arboreal ; and as their breathing appa- 



liuiu rpux'/Xov (trachelos) neck. wad novs (iious) foot. 



