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 



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Fig. 139.— A, ventral, B, lateral, views of a ray of O^hiuratextvrata. (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. Astropecten avrantiacus (after Gaurtry) : a, ambulacral or " verte- 

 bral " ossicles ; b, adauibulacral 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 

 edges of which are articulated with a long, narrow plate, the 

 torus angularis (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 palai angu- 

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

 They doubtless perform the function of teeth. Rudimentary 

 representatives of the calcareous ring of the Holothuridea 

 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 cascal appendages, 

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

 face of one of the scuta buccalia ; 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- 



