16 Our Food Mollusks 



their value. Their hue and iridescence also are im- 

 portant. Most of the pearls of commerce come from the 

 so-called pearl-oyster of the Indian Ocean. This mollusk, 

 however, is only distantly related to edible oysters. 

 Pearls have been found in the bodies of most bivalves, 

 and those from several species are valuable. Fresh water 

 clams, especially in the streams and lakes of the central 

 states, produce pearls of great beauty. Every one has 

 found them in our oysters. These are usually small, 

 though sometimes very symmetrical in outline, but are 

 not valuable, as they are not iridescent. 



Pearls, really abnormal shell growths, are formed by 

 the introduction of some foreign object between the 

 mantle and the shell. This body becomes a nucleus about 

 which the sticky secretion of the mantle accumulates. 

 Just as in the case of the shell layers, lime is deposited 

 in this sticky coating. Successive layers are added and 

 the pearl gradually increases in size. The foreign bodies 

 acting as centers about which the pearly layers are ac- 

 cumulated, have been shown, in some cases, to be small 

 parasitic worms. It is easy to determine experimentally, 

 however, that an inert body like a grain of sand, will also 

 become coated with pearly layers. Professor Brooks, in 

 his book on the oyster, writes of the miraculous origin of 

 the sacred clam shells of the Chinese Buddhists. He 

 says : " The inside of the shell has a beautiful pearl 

 luster, and along it is a row of little fat images of Bud- 

 dha, squatting with his legs crossed under him, and his 

 elbows on his knees : they are formed of pearl precisely 

 like that which lines the rest of the shell, a little raised 

 above its surface, and outlined in faint relief, but they 

 are part of the shell, with no break or joint. In the 

 process of manufacturing them, the shell of the living 



