290 



THE ECHIN01DEA 



tally from the upper end of the pharynx to the lobed stomach (s), 

 which is twisted in a spiral twice round the body cavity ; it con- 

 tracts above to the rectum (r), which opens to the exterior by an 

 anus (a), situated in the centre of the apical system. 



The Water -vascular System. Above the top of the dental 

 apparatus a vessel may be seen running round the oesophagus. 

 This is the circumoesophageal ring of the water-vascular system 

 (c.ce.v). From it five radial vessels pass downwards outside the 

 masticatory apparatus to the peristomial membrane; there they 

 bend upwards, pass beneath the arches of the perignathic girdle 



sp. 



tb. 



pdc. 



FIG. VII. 



Diagrammatic vertical section through Echinus, a, anus; amp, ambulacral plate; ap, 

 ampulla ; fc, branchiae ; c.oc.v, circumoesophageal vessel of water-vascular system ; d.o, dorsal 

 organ ; g, gonad ; m, madreporite ; int, mouth ; n.r, nerve ring ; ce, oesophagus ; p.a, arch of 

 perignathic girdle ; pd, podia ; ph, pharynx ; pdc, pedicellaria ; pp, periproct ; p.r, ridge of 

 perignathic girdle ; pst, peristome; py, pyramids of masticatory apparatus ; r, rectum; r.n.c, 

 radial nerve cord; s, stomach; s.e, stone canal; si>/i, sphaeridia ; sp, si>. e; st, Stewart's 

 organ ; tb, tubercle ; r, Polian vesicle on circumoesophageal vessel. 



(p.a), and one runs up the inside of each ambulacrum. Branches 

 from the radial vessels are given off alternately to right and left ; 

 one of these branches passes to each pore-pair, below which it opens 

 to a pair of pocket-shaped vesicles or ampullae (ap). From each 

 ampulla a small tube passes through both pores ; the two tubes 

 unite on the exterior to form the shaft of the tube-foot or 

 podion (pd). 



The water-vascular system round the peristome of an irregular 

 Echinoid is shown in Fig. XLIV. The water-vascular ring (w.v.r) 

 lies above the peristomial membrane, and just above the circumoeso- 



