VIII 



EOHINOBERMATA— MORPHOLOGY OF SKELETON 



359 



or channels, one of which receives the nerve ring and the other the 

 water vascular ring. 



In AsifojiJii/ton part of the water vascular ring is entirely enclosed 

 within the oral-angle plates. 



Closer examination reveals the fact that each oral-angle plate 

 consists of two fused plates, a proximal and a distal. The former, 



pfei> 



am^-^ad^ 



Fig. S14.— Oral skeleton of the OpUopya longispinus, Lym., from within; above, an inter- 

 radial. region of tlie cover ot the di.sc. rs, Eadial shields ; nm, vertebral ossicle ; ami, peristomal 

 plates ; pteS, depressions for the oral tentacles ; ceMij+ocZi, oral-angle plates ; fh, bursal apertures ; 

 ta, torus angularis ; H, teeth ; ihr; interbrachial region ; sj/c, bursal scale ; fflj, genital plate 

 (after Lyman). 



directed towards the centre of the mouth, fuses with the corre- 

 sponding piece of its associated oral-angle plate, the two forming the 

 oral angle. The distal plate at its distal end is in contact with a 

 corresponding plate on the opposite side of the buccal fissure. The 

 former of these constituents of each oral-angle plate is regarded as an 

 adambulaeral plate of the first brachial segment, taking part in the 

 formation of the oral skeleton, while the distal plate is regarded as an 



