THE STELLEROIDEA 



265 



either Asteroids or Echinoids. The mouth is situated in the centre 

 of the lower surface of the disc, and from it the buccal fissures 

 radiate. The mouth opens to a short oesophagus, above which 

 is a large digestive sac, occupying nearly the whole cavity of the 

 disc. Neither anus nor diverticula along the arms are present. 



The Water-vascular System consists essentially of a circum- 

 oesophageal ring which gives rise to five radial vessels bearing tube- 

 feet, and five vesicles, one of which communicates with the exterior. 

 In Ophiura ciliaris the circumoesophageal ring lies over the syn- 

 gnaths, through which pass branches (Fig. XVII. o.v) leading to 



o.t. L 



J;P- 



r. 11. 



Fio. XVII. 



Diagram of circumoral region of Ophiura ciliaris (after Miiller). c.ce.n, circumoesophageal 

 nerve ring; c.ce.v, circumoesophageal water- vascular ring; e.n, nerve branch to integu- 

 ment; i.v.m, intervertebral muscles; i.v.n, intervertebral nerve; j, jaw ; j.p, jaw plate; 

 o.n, oral nerve; o.p, oral papillae; o.t, oral tentacle; o.v, branch of water vessel to oral 

 tentacle; p.n, podia! nerve; p.v, Polian vesicle; r.n, radial nerve; t, teeth; t.g, tentacular 

 groove; t.n, tentacular nerve ; r.o, vertebral ossicles. 



the mouth-tentacles (o.t) ; radial vessels run along the under sides 

 of the arms. In each of four of the interradial (or interbrachial) 

 spaces there is a "polian vesicle" (p.v); in the fifth interradius there 

 is a short expanded stone-canal (Fig. XVIII. s.c) which opens to the 

 exterior by a single madreporite on a buccal shield. The radial 

 vessels lie in the lower canal furrows of the vertebral ossicles (Fig. 

 XV. If) ; branches from the radial vessel pass through the vertebral 

 ossicles; they emerge at the lower angle of the ossicle, and the 

 podia pass to the exterior through a space between the shields. 

 There are no ampullae, but the flow of water is regulated by 



