PHYLUM ANNULATA 



473 



micromeres is very marked. In some Serpulids, however, the differ- 

 ence is very slight, and the two sets of cells are at first scarcely 

 distinguishable. In such cases the cells arrange themselves in 

 such a way as to form the wall of a hollow sphere, the blastula, 



OV 



FIG. 386. Spirorbis laevis, a hermaphrodite tubicolous Polychaet. Lateral view of entire 

 animal, ant. neph. anterior nephridium ; br. branchiae ; ces. oesophagus ; op, operculum with 

 developing embryos in its interior ; ov. ova ; sp. sperms ; st. stomach. (After Claparede.) 



with an internal closed cavity, the segmentation-cavity. The 

 megameres, which may or may not have been distinct from the 

 first, lie on one side of the blastula ; and soon this side becomes 

 invaginated (Fig. 387, A), the result being the formation of an 



