144 INVERTEBRATA OF MASSACHUSETTS. 



Class GASTEROPODA, Cuvier. 



The Gasteropoda form much the most numerous class of 

 Molluscs. They are distributed over the whole surface of the 

 globe, and live on the land, and in all waters. They receive their 

 name from the pecuhar conformation of their organ of progres- 

 sion. The under part of the belly is flattened out into a smooth, 

 elongated disk, and on this they glide along. 

 Some of them have also the power of swim- 

 ming at the surface of the water in an in- 

 verted posture. They have a distinct head, 

 furnished with one or two pairs of tentacula, /^^^P^^^P^ 

 and almost always they have rudimentary [Paiud\nadecisa,8^^.^ 

 eyes situated on or near one pair of them. 



The shell, almost without exception, consists of only one 

 piece (univalve) , and is more or less revolving in a spiral coil. 

 Into it the animal is capable of withdrawing entirely, and is 

 usually provided with a horny or bony cover (operculum) at- 

 tached to the hinder part of the foot, which accurately closes 

 the aperture of the shell after the body has wholly receded. 



The arrangement of the respiratory organs is various, and af- 

 fords good characters for distribution into families. In some of 

 them the respiratory orifice is at one side, under the edge of the 

 mantle. In others the mantle is folded so as to form a long flexi- 

 ble proboscis or siphon ; and the shells of these have a notch or 

 groove in the front of the aperture, in which the siphon lies. 



All those which live on the land or in fresh water feed on de- 

 caying vegetables ; while a great portion of those living in the sea 

 feed on animal food, and devour not only dead animals, but many 

 of them are real cannibals, and have the power, either by their 

 jaws, or by the application of an acid, to perforate the shells of 

 their fellow testacea, without respecting even their own species, 

 and thus suck out the juices of the living occupant. 



Not a few of them are destitute of shells, and, in fact, the 

 greater part of the naked moUusca belong to this class. Of the 

 few that have been observed in this region, some notice has 



