42 DISCOPHORA. 



cavity, which constitute a germinal epithelium (fig. 15). Very 

 frequently (Aphrodite, Arenicola), as is so common in other 

 types, these tracts of germinal cells surround the blood-vessels. 



FIG. 15. A PARAPODIUM OF TOMOPTERIS. (From Gegenbaur.) 

 o. Collection of germinal epithelial cells lining the body cavity. 



In some cases the germinal epithelium thickens to form a 

 compact organ, for which the outermost cells may form a 

 more or less definite membranous covering (Oligochseta, etc.). 

 The ova are formed by the enlargement, accompanied by other 

 changes, of these germinal cells. During their early development 

 the ova are frequently surrounded by a special capsule, which is 

 often stalked, and provided at its attachment with a large micro- 

 pylar aperture. In Aphrodite and Polynoe this arrangement, 

 which is clearly connected with the nutrition of the ovum, is very 

 easily seen. The ovum is dehisced into the body cavity by the 

 bursting of its capsule or the rupture of the stalk. The capsule 

 is always eventually thrown off; but a vitelline membrane is 

 frequently developed after the detachment of the ovum into the 

 body cavity. The vitelline membrane of Spio and other Poly- 

 chasta is provided with an equatorial ring of ampulliform 

 vesicles. 



DISCOPHORA. 



(36) H. Dorner. " Ueber d. Gattung Branchiobdella." Zcit. f. wiss. Zool., 

 Vol. xv. 1865. 



(37) R. Leuckart. Die menschlichen Parasitcn. 



(38) Fr. Leydig. "Zur Anatomic v. Piscicola geometrica, etc." Zcit.f. miss. 

 Zoo!., Vol. I. 1849. 



(39) C. O. Whitman. " Embryology of Clepsine." Quart. J. of Micr. Set'., 

 Vol. xvin. 1878. 



The ovary of the Discophora is formed of a mass of cells en- 

 veloped in a membranous sack. In Branchiobdella there is 



