PHYLUM MOLLUSCOIDA 



317 



become free from one another and move about in the body-cavity 

 or in its prolongations into the tentacles. There is no spermiduct, 

 and it is doubtful if the sperms pass to the exterior. The ovary 

 (ov) is a small rounded body formed from the parietal layer of the 

 parenchyma about the middle of the zocecium ; it consists of only 

 a small number of cells of which only one at a time becomes a 

 mature ovum, certain smaller cells forming an enclosing follicle. 

 The mature ovum is perhaps fertilised in the ccelome ; it passes 



C 



end. 



end. 



FIG. 256. Early stages in the development of Bugula. cmt, central mass of cells ; cor. corona; 

 n-t, ectoderm ; nut, eudoderm ; scg, segmentation cavity. (After Vigelius.) 



into the interior of a rounded outgrowth of the zooecium the 

 ocecium (ocec) lined with parenchyma, and forming a sort of brood- 

 pouch in which it undergoes its development. 



Development. Segmentation (Fig. 256) is complete and nearly 

 regular. A blastula is formed having the shape of a bi-convex lens. 

 In the interior of the blastocoele or cavity of the blastula, four cells 

 the primitive endoderm cells become distinguishable: these 

 increase in number by division and form a mass of free cells which 

 almost completely fill the blastocoele ; this mass apparently re- 

 presents both endoderm and mesoderm. Small cavities which 

 appear in it subsequently unite together to form the primitive 



