56 THE OYSTER. 



may fail to develop. On one surface of the body at 

 this stage there is a well-marked groove, and when a 

 specimen is found in a proper position for examination, 

 the opening into the digestive tract is found at the 

 bottom of this groove. The embryo now consists of a 

 central cavity, the digestive cavity, which opens ex- 

 ternally by a small orifice, the primitive mouth, and 

 which is surrounded at all points, except at the mouth, 

 by a wall which is distinct from the outer wall of the 

 body. Around the primitive mouth these two layers 

 are continuous with each other. 



This stage of development, in which the embryo 

 consists of two layers, an inner layer surrounding a 

 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 Protozoa, 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 gastrula 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, 



