TYPE EGHINODERMA. 



563 



zontal plane, except in Astrophyton and its allies, in wliich 

 the arms may be coiled up over the oral surface, in a manner 

 similar to what is found in the Crinoids. These ambulacral 

 ossicles seem to correspond with the similarlj^-named plates 

 of the Asteroidea. 



In the neighborhood of the mouth certain modifications 

 in the arrangement of some of these plates occur. The two 

 halves of each first ambulacral ossicle (Fig. 258, A^ are widely 

 separated, and come into close relation with the similarly- 

 separated ossicles of adjacent radii, forming a buccal shield. 

 The plate so formed rests upon the aboral surface of the first 

 adambulacrals {Ad J, which unite in pairs in a similar manner, 



Ad, 



A, 



A. 



Ad, 



Fig. 258. — Diagram to show the arrangement of the circtjmobal 



PLATES OF AN OPHIURAN (after LddWIG). 



A = ambulacral plates. p = pala angularis. 



Ad = adambulacrals. T = torus. 



/ = intei'iadial. t = oral tentacles. 



icr = radial hydrocoel-vessel. 



forming a triangular plate, termed an oral angle-piece, lying in 

 an interradius, and partly covered on its oral surface by a 

 buccal shield. At the sides of the buccal shield are the so- 

 called lateral buccal shields (Ad^), which are in reality the 

 second adambulacrals of adjacent arms, and cover in the 

 second ambulacrals (A^), which serve as supports for the 

 oral angle-piece. Along the margins of the oral surface of 

 this are a series of spines, the buccal papillce, while, at the apex 

 of the triangle, are the dental papillce. The vertical edge of 

 the piece is furnished with a number of stout projections, the 



