186 



ZOOLOGY 



to have been fished for two thousand years. The finest 

 pearls come from the Persian Gulf. They are gathered 

 there by native divers, who, after taking several deep 

 inspirations, either dive down unaided or descend upon 

 weighted ropes. When they can no longer remain under 

 water, they either ascend again unaided or are rapidly 

 drawn up to the boats above. The pearl oysters brought 

 up to the surface are then examined for pearls. The pearl 

 is the secretion of the oyster about a small foreign body, 



such as a grain of sand. 

 This secretion prevents the 

 irritant from injuring the 

 tissues. It is of the same 

 nature as the substance lin- 

 ing 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 Pectinidae 1 include the scallop-shells. These occur 

 along our whole coast. They live in shallow water, and 

 are capable of rapid movement in the water by clapping 

 the valves. The adductor muscle of the valves alone is 

 eaten by man. The common species is Pecten irradians. 

 It varies greatly in color, from bluish to reddish and 

 orange (Fig. 173). 



1 pecten, comb. 



FIG. 173. Pecten irradians, scallop, 

 left or upper valve. Nat. size. 

 Photo, by W. H. C. P. 



