ECONOMIC IMPORTANCE 



101 



have such natural enemies as the starfish removed, and, in 

 the case of oysters, brush and shells are added that they 





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Figure 102. — Stages in Life History of Oyster. 



may fasten to these rather than sink to the bottom, where 

 they become covered with mud. . 



The culture of oysters and clams near the mouths of 

 rivers contaminated with sewage is unsanitary, and dis- 

 ease may be caused by eating such mollusks raw. This 



Figure 103. — Barnacles and Clams Growing on Oysters. 



