114 SEA-SHORE LIFE 



Acanthopleura gmnulatu, Fig. 7S, is a large chiton aljDut three 

 inches long, which is abundant on rocky shores, between tide limits, 

 in the Bahama Islands and West Indies. 



Another great group of mollusks are the snails, sings and 

 limpets, wliich are all grouped under the scientific name of Gastero- 

 ixdn. In all of these the shell, when present, consists of a single 



f^i.^- 7^: ^i:awMo//(fi() a .g'rrt««/a/a the commonest Chiton in the West Indies. From 

 Andros Island, Uahamas. 



piece, not of a series of shingles, as in the chitons. In some GVis- 

 teropoda, however, the shell has become enclosed by an overgrowth 

 of the edges of the mantle, and may even disappear in the 

 adult state. 



In snails we find that the viscera are protected by being cov- 

 ered with a spirall)' coiled shell, within which the entire foot and 

 head may be withdrawn. Indeed, in many of the sea snails we 

 find a horny or calcareous plate called the "operculum," wliieh is 

 developed on the upper side of the posterior part of the foot, and 

 when the creature draws in its head and foot this completely closes 

 the aperture of the shell. The o]3erculum is very rarely seen in fresh 

 water snails. The sea snails have at least one feathered gill, but 

 the land and fresh water species have lost their gills and the mantle 

 edge has fused with the side of the body, making a large, air-filled 

 cavity which opens at one point, usually on the right side, and 

 serves as a lung. One can easily see this breathing pore in slugs 

 and fresh water snails, and the latter must come to the surface at 

 regular intervals to let out a bubble of air, and take in a new sup- 

 ply. Slugs, land and fresh water snails are hermaphrodites, and 

 may bring forth their young in a well developed stage, although 



