ACEPIIALA. 261 



layer of the shell has a pearly appearaoce, and these forms 

 also produce, like some other molluscs, the precious pearls. 

 These pearls are really the shell-forming secretions of the 



FIG. 119. Scallop (Pecten irradians). From Binney's Gould. 



mollusc around some foreign body, and they receive their 

 beauty from the way in which the shell is deposited around 



FIG. 130. Salt- water mussel (Mytilus edulis). 



the centre. Fresh-water mussels, to be referred to a few 

 lines below, also form pearls of value. The shell of the 

 pearl-oyster also has its value, for it furnishes the mother- 



