494 



COMPARATIVE ANATOMY 



CHAP. 



D. Ophiupoidea. 



In structure and development the genital organs in this class 

 strongly resemble those of the Asteroidea. The gonad is connected 

 with the axial organ by means of an aboral ring-like strand, and both 

 the gonads and this strand are surrounded by coslomic sinuses, which 

 communicate with the axial sinus. 



The onlj' important difference in the genital organs of the two 

 classes is caused by the fact that, in the Ophiuroidea, the gonads do not 

 open outward directly, but by means of five pairs of large sac-like 



Fifi. 38S.— Stomaoli and bursas of a young Ophioglypha albida, in its natural position in 

 tlie disc, the dorsal wall of which is removed. 1, Bursai ; 2, cavity of the disc ; 3, interradial ; 4, 

 radial bulgings of the digestive sac (after Ludwig). 



invaginations of the body wall into the coelom of the disc, these 

 sacs themselves communicating with the exterior through five pairs 

 of slit-like apertures lying at the sides of the bases of the arms on the 

 lower (oral) side of the disc. These sac-like invaginations of the 

 body wall are the bursse or bursal pockets, their outer slit-like 

 apertures being known as the bursal (genital) apertures, which have 

 already been mentioned (Figs. 245, 246, and 314, pp. 300, 301, 

 and 359). 



1. The Bursse (Figs. 388 and 389). 



These are large sacs within walls, which fill up the body cavity of 

 the disc round the digestive sac. Their walls are attached to that of 



