200 



PHYLUM ECH1NODERMATA. 



ring as well as with the radial cords. The latter are swollen 

 into ganglia at the points where the nerves come off. 



The deeper oral system consists of two trunks in each arm 

 placed close together and just above the radial trunks of the 

 superficial system (Fig. 140, 9). They innervate the inter- 

 vertebral muscles (Fig. 140) and are continued into a circum- 

 oral band in the disc. The apical nervous system appears not 

 to be represented, unless the genital nerve ring which runs in 

 the wall of the aboral sinus belongs to it. The tube-feet are 

 all sensory structures, and are supplied by a branch from the 

 ambulacral nerve (Fig. 140, 4), which dilates into a ganglion at 



aBPX KW 



FIG. 144. Diagrammatic vertical sectioa through the disc and one radius of an Ophiurid 

 (0 phioglypha), from Perrier, after .Ludwig. A 1 peristomial plate (1st ambulacral) . ; A 2 - 

 A6 2nd to 6th ambulacral plates ; aB generative rachis ; B l -B^ the 1st to the 6th ventral 

 plates (under arm plates) ; Bi mesenterial filaments attaching the stomach to the body- 

 wall ; Br radial blood-vessel ; D stomach ; ePH circumoral perihaemal sinus ; iPH 

 perioesophageal part of peri visceral cavity ; KW body wall ; L body-cavity of disc ; L 1 of arm ; 

 Li lip ; M, MV, Mi^ muscles of the oral skeleton ; ME oral angle plate ; M F and MF 1 first 

 and second oral feet ; MS buccal shield ; N nerve ring ; Nr radial nerve ; mouth ; oB 

 circular blood-vessel ; P polian vesicle ; PH aboral sinus ; rPH radial perihaemal canal ; 

 S septum which separates the perioesophageal sinus from the rest of the perivisceral cavity ; 

 T torus angularis ; W water -vascular ring ; Wr radial water vessel ; Z teeth. 



their base. There is no eye, but the ambulacral nerve trunk 

 becomes epithelial in position on the terminal tentacle. 



The general arrangement and relations of the coelom are the 

 same as in Asteroids, the principal difference consisting in the 

 ventral position of the madreporite. 



The general body-cavity or perivisceral cavity is in relation 

 with the stomach in the disc and is prolonged into the arms on 

 the dorsal side of the ambulacral ossicles (Fig. 140). The portion 

 in the disc is traversed by connective tissue strands (Fig. 144), 

 and is divided into two parts by a septum connecting the 

 oesophagus with the oral skeleton (S). A small perioesophageal 



