THE OYSTER. ' 53 



rated from each other from the first, and make their 

 appearance independently. 



Soon after they make their appearance the embryos 

 cease to crowd to the surface of the water, and sink to 

 various depths, although they continue to swim actively 

 in all directions, and may still be found occasionally 

 close to the surface. The region of the body which 

 carries the cilia now becomes sharply defined, as a 

 circular, projecting pad, the veliini, Figs. 7, 8 and 9, 

 and this is present and is the organ of locomotion, at 

 a much later stage of development. 



The two shells grow rapidly and soon become quite 

 regular in outline, but for some time they are much 

 smaller than the body, which projects from between 

 their edges, around their whole circumference, except 

 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. 9, m?ike 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 important 

 internal modifications have taken place. 



Soon the outer wall of the body becomes pushed 

 inward, to form the mouth. The digestive cavity now 



