82 BRANCH MOLLUSCA 



into the mantle cavity. Tlie foot is broad and flat and is used 

 for locomotion. Respiration is accomplished through the wall 

 of the mantle cavity, or by one or two plume-like gills or 

 ctenidia in the mantle cavity. In the air-breathing forms there 

 may be simply a pulmonary sac. 



The shell is a spiral, either flat or elongated (Fig. 62), and is 

 usually closed by a flap or operculum (a horny plate growing 

 on the posterior portion of the foot) for jirotection. 



apex 



whorls forming the spire 



body whorl 



aperture 



Fig. 62.— A snail shell. (Morse.) 



Some Gasteropods are marine, some are fresh-water forms, 

 and still others are terrestrial. 



The limpets (PateVlidce) are uncoiled forms with open conical shells. 

 They are found adhering to rocks between tide-marks. The foot acts as 

 a sucker, enabling the animal to resist a force of a thousand limes its weight 

 when one attempts to detach it. The common limpet {Patella vulgata) is 

 used as food. It feeds upon seaweeds. 



The ear-shells {Haliot'ido'), found on our western coast, have a row of 

 perforations near the margin of the shell through which the tentacles 

 pass to the exterior. The shells are much used in inlaid work on account 

 of their beautiful iridescent color. They are also used as food, and the 

 shells are used for making buttons. 



1 The cowries {Cyprce'idee) have richly enameled shells with small open- 

 ings. They are beautiful and are sold for ornaments, some species being 

 much prized. A beautiful yellow shell, an inch or less long, which abounds 

 in the East Indies, is used as money in Siam and in parts of Africa: 6400 

 cowries are equal to about 36 cents. The cowries are tropical, but a few 

 species are found in temperate seas. 



