THE OPHIURIDEA. 



483 



tween the lateral plates are the apertures by which the pedi- 

 cels make their exit. The oral aperture is surrounded by five 

 oral angles^ each of which consists of five pieces. The two 



Fig. 139.— a, ventral, B. lateral, views of a ray of O^hhtratextitraia. (After Muller.) 

 C. transverse section ; a, axial or " vertebral" ossicle of ray ; 6, antambulacral 

 plate ; c, lateral plate; d, ventral or superambulacral plate. D, section of a ray of 

 an Asterid, AstropecUn aurantiacus (after Gaudry) : a, ambulacral or " verte- 

 bral " ossicles ; b. adambulacral ossicles ; c, c', marginal ossicles ; d, paxillae of 

 antambulacral surface. 



constituents of the axial ossicle which lies at the oral end of a 

 ray become movably articulated with one another, while 

 each anchyloses with an interambulacral piece. Transverse 

 muscles connect the two interambulacral pieces, the oral 

 edg-es of which are articulated with a long, narrow plate, the 

 torus angidaris (Fig. 140,/*). The free surface of the torus 

 angularis lies in the walls of a sort of vestibule in front of 

 the mouth. A number of short, flat processes, the paloe angu- 

 lares, are articulated with it, and moved by special muscles. 

 They doubtless perform the function of teeth. Rudimentarj'' 

 representatives of the calcareous ring of the Solothuridea 

 and of the parts of the lantern of the Echinidea exist as deli- 

 cate calcareous plates, which lie on the circular ambulacral ves- 

 sel. The latter is usually provided with csecal appendages, 

 or Polian vesicles. The madreporic canal ends on the sur- 

 face of one of the scuta huccalia y the radial ambulacral ves- 

 sels run in the arch between the axial ossicles and the super- 

 ambulacral plates. The nerve lies superficial to the super- 



