[25] FISHERIES OF THE UNITED STATES. 209 



stage of development at all, but forms a digestive cavity in another 

 way. 



" The edges of the primitive mouth of the oyster continue to ap- 

 proach eacli other, and finally meet and unite, thus closing up the 

 opening, as shown in Fig. 36, and leaving the digestive tract without 

 auy communication with the outside of the body, and entirely sur- 

 rounded by the outer layer. The embryo shown in Figs. 32 and 36 

 are represented with the dorsal surface below, iu order to facilitate 

 comparison with the adult, but in Fig. 37 and most of the following 

 figures the dorsal surface is uppermost, for more ready comparison with 

 the adult. The furrow in which the primitive mouth was placed still 

 persists, and soon a small irregular plate makes its appearance at each 

 end of it. These little plates are the two valves of the shell, and in the 

 oyster they are separated from each other from the first, and make 

 their appearance independently. 



"Soon after they make their appearance the embryos cease to 

 crowd the surface of the water and sink to various depths, although 

 they continue to swim actively in all directions and may still be found 

 occasionally close to the surface. The region of the body which carries 

 the cilia now becomes sharply defined, as a circular projecting pad, the 

 velum, and this is present and is the organ of locomotion at a much 

 later stage of development. It is shown at the right side of the figure 

 in Fig. 37, and in Fig. 45 it is seen in surface view, drawn in between 

 the shells, and with its cilia folded down and at rest, as they are seen 

 when the little oyster lies upon the bottom. 



"The two shells grow rapidly, and soon become quite regular in out- 

 line, as shown in Figs. 37 and 44, but for some time they are much 

 smaller than the body, which projects from between their edges around 

 their whole circumference, except along a short area, the area of the 

 hinge, upon the dorsal surface, where the two valves are in contact. 



"The two shells continue to grow at their edges, and soon become 

 large enough to cover up and project a little beyond the surface of the 

 body, as shown in Fig. 44, and at the same time muscular fibers make 

 their appearance and are so arranged that they can draw the edge of 

 the body and the velum in between the edges of the shell, in the man- 

 ner shown in Fig. 45. In this way that surface of the body which 

 lines the shell becomes converted into the two lobes of the mantle, and 

 between them a mantle cavity is formed, into which the velum can be 

 drawn when the animal is at rest. While these changes have been 

 going on over the outer surface of the body other important internal 

 modifications have taken place. We left the d>estive tract at the 

 stage shown in Fig. 36, without any communication with the exterior. 



"Soon the outer wall of the body becomes pushed inwards, to form 



the true mouth, at a point (Fig. 37) which is upon the ventral surface, 



and almost directly opposite the point where the primitive mouth was 



situated at an earlier stage. The digestive cavity now becomes greatly 



2444— Bull. 27 14 



