vin ECHINODERMATA MORPHOLOGY OF SKELETON 359 



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

 water vascular ring. 



In Astrophyton 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, 



ta 



O.7TI. 



D 



FIG. 314. Oral skeleton of the OpMopya longispinus, Lym., from within; above, an inter- 

 radial , region of the cover of the disc, rs, Radial shields ; am, vertebral ossicle ; ami, peristomal 

 plates ; ptcb, depressions for the oral tentacles ; am^+ach, oral-angle plates ; fb, bursal apertures ; 

 ta, torus angularis ; D, teeth ; ibr, interbrachial region ; sge, bursal scale ; gp, 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 



