THE HOLOTHURIOIDEA 221 



The nervous system consists of a " superficial " ring surrounding the 

 mouth and giving off five radial nerves which run backwards to the 

 posterior end. The tentacles and viscera are supplied by nerves 

 from the ring ; the podia by branches from the radial nerves. The 

 ring and the radial nerves are sunk below the surface, and lie on 

 the inner side of the cutis. An epineural canal lies outside the 

 radial nerves. Following each radial nerve on its inner surface is 

 a small " deep " nerve, from which are supplied the muscles of the 

 body- wall (Fig. II. 4). 



The water -vascular system consists of a circular canal sur- 

 rounding the oesophagus behind the calcareous ring, and giving 

 off five radial canals (Fig. II. 5, c.c and r.c). Each radial canal 

 passes forward between the calcareous ring and oesophagus, and 

 then outwards, through the notch in the radial piece, to the body- 

 wall ; it then runs backwards below the radial nerve. The radial 

 canals send off a branch on either side to supply a pair of tentacles, 

 each tentacle being provided with a long ampulla freely projecting 

 into the body -cavity (Fig. I. 5, t.a, and Fig. II. 5). A Polian 

 vesicle is attached to the circular canal in the left ventral, in- 

 terradius. Branches are given off from the radial canals in the 

 body - wall to supply the podia and papillae ; small ampullae are 

 here present. A twisted stone - canal lying in the median dorsal 

 line leads from the circular canal to a madreporite, pierced with 

 many small apertures, and lying in the body-cavity at the anterior 

 edge of the dorsal mesentery (Fig. I. 1 and 5, s.c and md). 



The alimentary canal is looped (Fig. I. 1, i, and Fig. II. 1), 

 coiled in the direction of the watch-hand as viewed from the anterior 

 end, and supported almost throughout by a mesentery attached to the 

 body- wall. The mouth leads into a wide oesophagus, which narrows 

 on passing backwards out of the calcareous ring. A scarcely dis- 

 tinguishable stomach succeeds the oesophagus and passes into the 

 long intestine, which finally ends in a rectal " cloaca " opening at 

 the anus. The oesophagus, stomach, and part of the intestine 

 form the first region of the alimentary canal, running backwards 

 and supported by the median dorsal mesentery. Near the hinder 

 end of the body the intestine and mesentery cross over from the 

 dorsal interradius to the left lateral dorsal interradius, up which 

 they run. Near the anterior end this second section of the 

 intestine, with its mesentery, crosses to the right ventral inter- 

 radius, down which the third and last portion of the intestine runs 

 straight to the anus. Round the oesophagus, bounded externally 

 by the calcareous ring and the radial canals, a portion of the coelom 

 is incompletely shut off from the general body -cavity (Fig. II. 

 5, pr); connective tissue strands run across it from one wall 

 to the other. The enlarged rectum, or cloaca, is fastened on all 

 sides to the body -wall by muscular strands (Fig. I. 1, d). 



