DEVELOPMENT IN PATIRIA MINIATA. 



by his figures. I must admit, however, that in normal material I 

 never could see even the faintest suggestion of such a structure, 

 while in subnormal material I occasionally observed what appeared 

 to be a vesicle of some sort, but even in preserved material it never 

 exhibited anything like complete continuity of outline. Heath de- 

 scribes this so-called " anterior vesicle " in great detail, dwelling on 

 its supposed mesenchymal origin and its lack of communication 

 with the archenteron. If, however, such a vesicle is absent in 

 normal larvae, discussion as to its exact structure loses significance. 

 A third peculiar structure is described by Heath : " a relatively 



22 



25 26 



FIG. 22. Right lateral view of larva with single posterior enterocoele vesicle. 

 FIGS. 23, 24, 26 show larvae with paired posterior enterocoele vesicles. FIG. 25 

 shows in detail the alimentary tract of a larva with but one posterior entero- 

 coele, on the left. 



