50 THE OYSTER. 



of the disk-shaped embryo is not flat, but very slightly 

 concave. This concavity is destined to grow deeper 

 until its edges almost meet, and it is the rudimentary 

 digestive cavity. A very short time after this stage 

 has been reached, and usually within from two to four 

 hours after the eggs were fertilized, the embryo under- 

 goes a great change of shape. Plate IV, Fig. 6. 



A circular tuft of long hairs, or cilia, now makes 

 its appearance at what is thus marked as the anterior 

 end of the body, and as soon as these hairs are formed 

 they begin to swing backward and forward in such a 

 way as to constitute a swimming organ, w^hich rows 

 the little animal up from the bottom to the surface of 

 the water, where it swims around very actively by the 

 aid of its cilia. This stage of development, which is 

 of short duration, is of great importance in rearing the 

 young oysters, for it is the time when they can best be 

 siphoned off into a separate vessel and freed from the 

 danger of being killed by the decay of any eggs which 

 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 

 externally 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 



