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FIRST BOOK OF ZOOLOGY. 



Fig. 4S. — Eight Valve of an Otster, the Dark Semicircular Mark near the Middle of 

 the Shell being the Adductor Muscular Impression, the Pallial Line showing faintly. 



valves, as in the clams, mussels, and oysters. All of tliem 

 increase the size of their shells, or grow, by the addition of 

 layers of shell-material to the edge of the apertm^e, or the 

 margins of their valves, and these layers are indicated by 

 delicate lines seen on the outside of the shell, and called lines 

 of growth. They all, excepting the oyster and a few other 

 forms, have the creeping disk or foot. In the snails, this is 

 broad and flat ; in the mnssel and clam the foot is flattened 

 sideways, and variously shaped. In the snails, the creature 

 projects, with the foot, a head furnished with feelers, or ten- 

 tacles, and the mouth is possessed of certain hard parts by 

 v/hich food can be eaten. In the mnssels and clams there is no 

 definite head, the mouth being hidden away within the mantle, 

 and the creature projecting, from the forward end, only the 

 foot. In all of these animals thus far studied there is a cavity 

 within, containing the gills to which water has accessj or else 



