OPHIUROIDEA. 203 



not seem to be any representative of the inner oral perihaemal 

 ring of Asteroids. 



The vascular tissue seems to be arranged very much as in 

 Asteroids. In some forms the radial vessels are said not to be 

 present. 



The generative organs. Most Ophiurids are dioecious, but 

 Amphiura squama-ia is hermaphrodite. The generative glands 

 are simple sacs which open, in all except Ophiopus and Ophiactis 

 virens, into the genital bursae. 



The genital bursae are five pairs of sac-like invaginations of 

 the interradial portions of the lower wall of the disc, and project 

 into the body-cavity between the bulgings of the stomach. 

 They open by slit-like apertures (double in Ophiura) placed one 

 on either side of the insertions of the arms into the disc (Fig. 

 142). They have thin walls, lined by a ciliated epithelium and 

 often containing calcareous matter. The gonads are small sacs 

 placed on the walls of the bursae and opening into them. Each 

 generative sac is connected with a branch from the generative 

 rachis, the course of which has been described. The aboral 

 sinus is continued with the rachis to each gonadial sac and 

 invests it as in Asteroids. The generative cells pass into the 

 bursae and outwards by their slit-like apertures. In many 

 Ophiurids (Amphiura squamata and magellanica, Ophiacantha 

 vivipara and marsupialis ; Ophiomyxa vivipara, etc.) the bursae 

 act as brood pouches and the eggs develop in them ; but 

 the principal function of the bursae seems to be respiratory, 

 water being continually drawn in and ejected by the ciliary 

 currents, and in some cases by the muscular elevations and 

 depressions of the dorsal surface. In Ophiactis virens the bursae 

 are absent * and the gonads open directly on the lower surface 

 of the disc. They are replaced by the canals of Simroth, which 

 have been already described (p. 201). 



The power of regenerating lost parts, e.g. arms, even a portion 

 of the disc, is considerable, but apparently the disc cannot be 

 regenerated from a single arm as in Asteroids. The brittle stars 

 readily lose their arms, so that the power of regeneration is very 

 important. 



Reproduction by fission through the centre of the disc has 

 been observed in a few genera (e.g. Ophiactis). 



* Cuenot, Arch. Biologic, 11, 1891, p. 303. 



