296 



THE SEAS 



for the first few weeks of their existence and rise near the 

 surface of the sea where they are carried in all directions 

 by tides and currents (Fig. 59). At this stage they have 

 a pair of tiny shell valves between which project a crown 



Fig. 59. — Larval stage of oyster, greatly enlarged. 



of fine hairs by the rapid movement of which they swim 

 and collect their food. At the end of their brief period 

 of freedom the little oysters, or such of them as have 



