48 



FIRST BOOK OF ZOOLOGY. 



FIG. 4S. RIGHT VALVE OF AN OYSTER, 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 them 

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

 layers of shell-material to the edge of the aperture, 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 have the creeping disk, or foot (except- 

 ing the oyster). In the snails, this is broad and flat ; in the 

 mussel 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 tentacles, and the mouth is 

 possessed of certain hard parts by which food can be eaten. 

 In the mussels and clams there is no definite head, the mouth 

 being hidden away within the mantle, and the creature pro- 

 jecting, 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 access, or else there is a simple 



