THE PHYLUM MOLLUSCA 



257 



secreted all the way around it in concentric layers. The edge of the 

 mantle, which makes the growing edge of the shell, secretes a chalky 

 kind of shell, but the inner part of the mantle that thickens the shell 

 secretes a harder, pearly material. The quality of the pearl depends upon 

 the quality of the shell lining normally produced. The common mussel 

 produces a lustrous, irridescent shell lining, and is sometimes infested 

 with parasites around which pearls are secreted. Although there are 

 often dozens in every mussel, none of them becomes larger than a tiny 

 sand grain. 



The Japanese have mastered the technique of culturing pearls. 

 Pearl oysters are collected, small particles are introduced into the mantle, 

 and then they are put out to sea in cages for several years. When the 

 pearls have had time to reach a suitable size, the oysters are taken in 

 and opened. 



The large fresh-water bivalves are a group of mussels that no 

 longer attach, but live buried in the sand like clams. They are adapted 



Pecl^en. 



O^trca. 



Mele-stdrixia, 





Figure 14.11. Some common pelecypods. Pecten, the scallop (after Johnson and 

 Snook). Mya, the steaming clam (after Verrill, 1873). Ostrea, the oyster (drawing by Hair- 

 ston). Meleagrina, the pearl oyster (after Fischer, 1887), showing pearly "warts" in a shell. 



