114 SEA-SHORE LIFE 
Acanthopleura granulata, Fig. 78, is a large chiton about 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, slugs and 
limpets, which are all grouped under the scientific name of Gastero- 
poda. In all of these the shell, when present, consists of a single 

Fig. 78; Acanthopleura granulata the commonest Chiton in the West Indies. From 
Andros Island, Bahamas. 
piece, not of a series of shingles, as in the chitons. In some Gas- 
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 spirally 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head may be withdrawn.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,” which 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 operculun 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 ina well developed stage, although 
