ORDER OSTRACK^E ; OYSTER. 397 



manifested ; the bivalve shell never covers the whole of the 

 body ; but frequently the respiratory tubes themselves secrete 

 shelly matter from their external surface, forming a tube with 

 which the valves are often blended ( 963). 



ORDER I. OSTRACE^. 



937. The shell of these Mollusca is formed of two unequal 

 valves, connected together by a hinge on which there are no 

 teeth or ridges, and which is therefore of the simplest character. 

 The shell is attached by the most convex of its valves to rocks, 

 to pieces of wood, and even to others of its own kind. This 

 attachment is formed by the exact adaptation of each layer of 

 shell, prolonged beyond the margin of the former one, to the 

 inequalities of the surface upon which it lies ; and sometimes 

 the margin is actually made to re-curve backwards, to enter some 



FIG. 587. ANATOMY OF THE OYSTER : v, one of the valves of the shell ; v', its hinge ; 

 i, one of the lobes of the mantle ; m', a portion of the other Jobe folded back ; c, adduc- 

 tor muscle ; br, gills ; b, mouth ; t, tentacula, or prolonged lips ; /, liver ; i, intestine ; 

 a, anus ; co, heart. 



furrow by which it may hold more firmly. The animal itself is 

 of very simple structure. No vestige of a foot can be seen ; and 



