THE OYSTER. 5 I 



cavity which opens externally by a mouth-like open- 

 ing, and an outer layer which is continuous with the 

 inner around the margins of the opening, is of very 

 frequent occurrence, and it has been found, with modifi- 

 cations, in the most widely separated groups of animals, 

 such as the starfish, the oyster, and the frog, and some 

 representatives of all the larger groups of animals, ex- 

 cept the /V^/(?^^^, appear to pass during their develop- 

 ment through a form which may be regarded as a more 

 or less considerable modification of that presented by 

 our oyster-embryo. This stage of development is 

 known as the gastriila stage. 



Certain full-grown animals, such as the fresh-water 

 hydra and some sponges, are little more than modi- 

 fied gastrulas. The body is a simple vase, with an 

 opening at one end communicating with a digestive 

 cavity, the wall of which is formed by a layer of cells, 

 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- 



