236 



ZOOLOGY 



same nature as the substance lining the shell. The latter 

 is the mother-of-pearl of commerce, and is used for knife- 

 handles, buttons, and the like. The iridescence is not due 

 to the chemical composition of the secreted substance, but 

 to the fact that it is deposited so as to leave fine lines on the 

 surface which diffract the light. 



The scallop-shells (Pectinidse ') occur along our whole coast. 

 They live in shallow water, and are capal^le of rapid movement 

 in the water by clapping the valves. The adductor muscle 



Fig. 228. — The Eastern oyster (Ostrea virginiana). One-third nat. size. 

 Photo, by W. H. C. P. 



of the valves alone is eaten by man. The common species is 

 Peden irradians, which has a ribbed shell of varjdng color 

 (Fig. 226). 



The lidas (Ledidte) include certain elongated shells with 

 a shiny epidermis, and a large number of teeth on the hinge, 

 which are found chiefly in mud in rather deep water. The 

 animal has a large foot (Fig. 227) with which it burrows rapidly. 

 It can also move rapidly through the water as the scallops do. 



The oysters (Ostreidis^) include the most important inver- 

 tebrate food fishery, being valued at twenty to thirty million 

 dollars per year for our Atlantic seaboard. Oysters are found 



' pecten, comb. 



• From Greek name for oyster. 



