THE OYSTER. 57 



which is continuous around the opening with a second 

 layer, which forms the outer wall of the body. This 

 fact, together with the fact that animals of the most 

 widely separated groups pass through a gastrula stage 

 of development, has led certain naturalists to a gener- 

 alization, which is known as the ' gastrula theory." 

 This theory or hypothesis is that all animals, except the 

 Protozoa, are more or less direct descendants of one 

 common but very remote ancestral form, whose body 

 consisted of a simple two-walled vase, with a central 

 digestive cavity opening externally at one end of the 

 body. 



Soon a small, irregular plate makes its appearance 

 on each side of the body. These little plates are the 

 two valves of the shell, and in the oyster they are sepa- 

 rated from each other from the first, and make their 

 appearance independently. 



Soon after the shells make their appearance the em- 

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

 sink to various depths, although they continue to swim 

 actively in all directions, and mav still be found, occa- 



* *> 



sionally, close to the surface. The region of the body 

 which carries the cilia now becomes sharply defined, as 

 a circular, projecting pad, the velum, Figs. 8, 9, 10, n ; 

 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 

 6 



