Development 47 



Soon after the embryo begins to swim, the valves of 

 the shell appear as minute secreted plates one on each 

 side of the body. In other bivalves, the shell rudiment is 

 at first unpaired. These shells grow rapidly at their 

 edges, and soon cover the body. Sheets of cells grow 

 out to line the shell valves on their inner surfaces, and 

 become the mantle folds. The part of the body bearing 

 the cilia projects as a rounded disk which is called the 

 velum. Muscle fibers form by the elongation of cells in 

 mantle and velum, and by their contraction, these parts 

 may be entirely withdrawn within the shell. 



In the meantime, internal changes have occurred. Ac- 

 cording to Professor Brooks, the primitive mouth closes. 

 In about the same locality, the surface of the body then 

 pushes in to form a pit. The bottom of this unites with 

 the anterior end of the stomach. These walls are broken 

 through, and the stomach and the new mouth become 

 connected. Posterior to the mouth a second pit from the 

 surface of the body touches the stomach. Its cavity in 

 the same way becomes connected with it, and there are 

 formed the anal opening and the rudiment of the intes- 

 tine (X and XI, oysters about three and six days old 

 respectively). 



Up to this time, the embryo has subsisted only on the 

 food yolk deposited in the ovum during its growth in 

 the parent's body. Consequently it has not increased in 

 volume. With the formation of mouth and anus, it be- 

 gins to capture organisms in the water and to digest 

 them, and rapidly increases in size. 



An interesting feature of the development is the for- 

 mation of a foot which is never used, because the embryo 

 becomes attached before it is large enough to make use of 

 a creeping organ. An anterior as well as a posterior ad- 



