XVII OPHIOTHRIX — WATER-VASCULAR SYSTEM 487 



surface in a groove on tlie outer side of the dorsal lateral knob 

 on the distal side of the ossicle. As in Asteroidea there are 

 valves, which regulate the entrance of fluid into the tube-foot. 

 The stone-canal is a curved tube of simple circular section and 

 excessively narrow bore which extends from the water-vascular 

 ring downwards to the madreporite (Fig. 213, mp) situated on 

 one of the scuta buccalia. The madreporite, in Ophiothrix as in 

 most Brittle Stars, is an exceedingly rudimentary structure, con- 

 sisting of one or two pores leading into as many pore-canals. 

 From each interradius, except that in which the stone-canal lies, 

 a large Polian vesicle hangs down from the water-vascular ring 

 into the coelom. 



We saw that in the Asteroid the ampulla was used like the 

 bulb of a pipette to force the fluid in the tube-foot down into 

 the tip, so as to press the sucker against the substratum. But 

 when the tube-foot is used as a sense-organ, a few circular fibres 

 around its upper end suffice to bring about all the extension that 

 is needed. Since the extension is no longer a very vigorous act, 

 the loss of fluid by transudation has probably been rendered 

 insignificant, and hence the stone-canal and madreporite, whose 

 function it is to repair the loss, have been reduced in size. The 

 curious ventral curvature of the stone-canal is, however, due to 

 another cause. In the very young Ophiuroid the madreporite 

 is on the edge of the disc, and the stone-canal extends horizon- 

 tally outwards ; and in some Asteroidea there is a similar outward 

 direction in its course. As development proceeds the dorsal 

 interradial areas of the disc of the young Ophiuroid grow out into 

 lobes, building up the conspicuous adult disc and forcing the 

 madreporite, and with it the stone -canal, downwards towards 

 the ventral surface. 



The pores of the madreporite in Ophiothrix, like some of those 

 in the Asteroid, open not directly into the stone-canal but into 

 the axial sinus (Fig. 213, ax). This is a large ovoid sac, lined 

 with thin epithelium, lying between the stone-canal and the 

 mouth -frame, since of course it has shared in the ventral rotation 

 of the stone-canal. Its open connexion with the stone-canal 

 was easily recognised by Ludwig, who termed it, on this account, 

 the " ampulla." ^ The name " axial sinus " was bestowed mis- 



1 "Neue Beitr. zur Anat. d. Oi)Iiiurideii," Zcitschr. wiss. Zool. xxxiv. 1880, 

 p. 340. 



