250 



ELEMENTARY ZOOLOGY 



but it is a member of the same class, that is, it is a bivalve 

 mollusc. Pearls are obtained from a number of different 

 " pearl-oysters, ' ' but the finest pearls and mother-of-pearl 

 come from the tropical species Meleagrina margaritifera. 

 This pearl-oyster "has an extensive distribution, being 

 found in Madagascar, the Persian Gulf, Ceylon, Australia, 

 Philippine Islands, South Sea Islands, Panama, West 

 Indies, etc." Mother-of-pearl is simply the inner lining 

 of the shell, which is composed of numerous thin layers of 

 carbonate of lime so arranged that the edges of the suc- 

 cessive layers produce many fine striae very close together. 

 The beautiful iridescence of this inner shell-lining is 

 caused by the complicated diffraction and reflection (inter- 

 ference effects) of the light by the fine striae and the 

 translucent superposed thin plates of shell material. 

 Pearls are simply isolated deposits of shell material usually 

 around some particle of foreign substance which has found 



FIG. 106. Pholas sp., a mollusc, burrowing in sandstone. (Photograph 

 by C. H. Snow; permission of Amer. Soc. Civil Engineers.) 



lodging in the mantle-cavity. Sometimes small objects 

 are purposely introduced into the shell in order to stimu- 

 late the formation of pearls. The pearl-fishers go out in 

 boats and dive to the bottom, filling baskets with pearl- 

 oysters. These are piled up in a bin and left to die and 



