viii ECHINODERMATA WATER VASCULAR SYSTEM 419 



FIG. 358. Diagram of the organisation of a regular Echinoid. Section in the direction of 

 the principal axis. The surface of the section lies interradially on the left and radially on the 

 right. The left half is incomplete. 1, External gill (this would not exactly come into this section, 

 since there are five pairs of interradially placed gills) ; 2, seizing pedicellaria ; 3, oral integument ; 

 4, tooth ; 5, mouth ; 6, cushion of connective tissue ; 7, nerve ring of the superficial system ; 8, 

 deeper oral nervous system ; 9, radial epineural canal ; 10, radial blood vessel ; 11, arch of the 

 ambulacral apophysis (auricula) ; 12, sphseridium in its niche ; 13, radial canal of the water vascular 

 system ; 14, radial nerve trunk (of the' superficial oral system); 15, 'radial pseud ohsemal canal ; 16, 

 circular ganglion at the base of the spine ; 17, spine ; 18, glandular pedicellaria ; 19, ambulacral 

 tube-feet with terminal disc ; 20, 21, ambulacral tentacles (without terminal disc) ; 22, terminal 

 feeler or tentacle emerging through the pore in the radial (ocular) plate ; 23, apical (genital) ring 

 sinus ; 24, perianal sinus of the coelom ; 25, anus ; 26, sinus, into which a process (27) of the axial 

 organ projects ; 27, aboral process of the axial organ ; 28, 'madreporite ; 29, genital aperture on the 

 genital papilla ; 30, genital duct ; 31, madreporitic ampulla, into which the stone canal and axial 

 sinus enter from below ; 32, axial organ ; 33, stone canal ; 34, part taken by the blood lacuna in 

 the formation of the Polian, vesicle ; 35, root of the tooth ; 36, muscle of the forked radii (Fig. 348, 7) 

 cut through ; 37, arch (arcus) of a jaw pyramid of the masticatory apparatus ; 38, lantern mem- 

 brane ; 39, ligament of a forked radius; 40, adductor muscle of the teeth; 41, interambulacral 

 apophysis ; 42, general body cavity (coelom) ; 43, pyramid ; 44, peripharyngeal sinus, lantern sinus 

 of the coelom ; 45, part taken by the water vascular system in the formation of the Polian vesicle ; 

 46, circular vessel of -the blood lacunar system ; 47, water vascular ring ; 48, oesophagus ; 49, axial 

 sinus of the coelom ; 50, hind-gut ; 51, perirectal sinus of the coelom ; at 52 and 54 the section is not 

 quite radial, so that it does not, as at 22 and 56, take in the radial canal of the water vascular 

 system, but passes transversely through its lateral canals which lead to the ampullae ; in 53 the 

 plane of the section lies still more to the side, so that the ampulla is taken in (cf. Fig. 353) ; 57, 

 abductor muscle of the teeth ; 58, Stewart's organ ; 59, muscles between the pyramids ; 60, inter- 

 mediate plate ; 61, forked radius ; 62 and 65, intestinal vessels ; 63, accessory intestine ; 64, prin- 

 cipal intestine. The accessory intestine in reality runs on the axial side of the principal intestine. 



