THE OYSTER. 



59 



along a short area, the area of the hinge, upon the 

 dorsal surface, where the two valves are in contact. 



The two shells continue to grow at their edges, and 

 soon become large enough to cover up and project a 

 little beyond the surface of the body, and at the same 

 time muscular fibres, Fig. n, make their appearance. 

 They are so arranged that they can draw the edge of 

 the body and the velum in between the edges of the 

 shell. In this way that surface of the body which 

 lines the shell becomes converted into the two lobes of 

 the mantle, and between them a mantle cavity is 

 formed, into which the velum can be drawn when the 

 animal is at rest. While these changes have been 

 going on over the outer surface of the body, other im- 

 portant internal modifications have taken place. 



Soon the outer wall of the body becomes pushed 

 inward, to form the mouth. The digestive cavity now 

 becomes greatly enlarged, and cilia make their appear- 

 ance upon its walls ; the mouth becomes connected 

 with the chamber which is thus formed, and which 

 becomes the stomach, and minute particles of food are 

 drawn in by the cilia, and can now be seen inside the 

 stomach, where the vibration of the cilia keeps them in 

 constant motion. Up to this time the animal has 

 developed without growing, and is scarcely larger than 

 the unfertilized egg, but it now begins to increase in 

 size. 



Soon after the mouth has become connected with 

 the stomach this becomes united to the body wall at 

 another point a little behind the mouth, and a second 



