THE DEVELOPMENT OF LAMELLIBRANCHS. 



323 



digestive cavity, with a small opening (g). This is the 

 gastrula stage, and a comparison with the sea-urchin will 

 show that it is essentially like the sea-urchin gastrula, 

 although it is not formed in precisely the same way. In 

 the sea-urchin segmentation is total and perfectly regular, 

 the segmentation cavity appears very early, and the endo- 

 derm cells are similar at first to the ectoderm cells, while 

 in lamellibranchs, segmentation, although total, is irregular, 

 the segmentation cavity does not appear until much later, 

 and the micromeres, which are to form the ectoderm, are, 

 from the first, quite different from the macromere, which is 

 to form the endoderm. 



* 



FIG. 16o. Fl<t. 166. FIG. 167. 



FIG. 165. Embryo about thirty hours after fertilization, seen from 

 the side in optical section. 



ec. Ectoderm, en. Endoderm. g. Orifice of invagination. sg. Seg- 

 mentation cavity. 



FIG. 166. Side view; and Fig. 167, Optical section of an embryo a 

 few hours older, in the gastrula stage. 



ec. Ectoderm, en. Endoderm. . Velum, g. Orifice of invagina- 

 tiou. a. Posterior dorsal angle of body. 



In the sea-urchin the orifice of invagination becomes the 

 anus of the pluteus, but in the oyster it soon closes up, 

 and the anus is afterwards developed on the opposite side 

 of the body. 



The edges of the orifice of invagination of the oyster 

 continue to approach each other, and finally meet and 



