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HANDBOOK OF INVERTEBRATE ZOOLOGY. 



unite, thus closing up the opening, as shown in Fig. 168, 

 and leaving the digestive tract without any communication 

 with the outside of the body, and entirely surrounded by 

 the outer layer. The embryos shown in Figs. 166 and 168, 

 are represented with the dorsal surface below, but in Fig. 

 169, and most of the following figures, the dorsal surface 

 is uppermost, for more ready comparison with the adult. 

 The furrow in which the orifice of invagination was placed 

 still persists, and soon a small, irregular plate (Fig. 

 168, s), 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 trom the first, 

 and make their appearance independently. 



FIG. 168. FIG. 169. 



FIG. 168. A somewhat older embryo, in the same position as Fig. 

 166. 



s. Shell. r>. Velum, a. Posterior dorsal angle of body. 



FIG. 169. A somewhat older embryo with the dorsal surface above. 

 m. Mouth, ec. Ectoderm, en. Endoderm. 



Soon after they make their appearance, the embryos 

 cease to crowd to 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 (Fig. 168, f), and this is present, and is 



