THE STELLEROIDEA 



263 



muscle fields. Above the lower furrow there is a depression to 

 receive a prominence on the distal face of the adjoining ossicle, arid 

 on each side of the depression there is a small knob, which similarly 

 fits into depressions on the adjacent ossicle. On the distal or 

 aboral surface there is a deep " umbonal socket " (w.s), and also 

 the prominence and two hollows already referred to; the two 



it. 



1. 2. 



FIG. XV. 



Vertebral ossicle of Ophiura cilittris (after Miiller). 1, aboral surface; 2, adoral surface, 

 w, umbo; l.m, lower muscle field; u.f, upi>er, and /./, lower canal furrows; u.s, umbonal 

 socket ; c, canal for the podion (shown by removal of part of muscle Held on right side). 



lower angles are occupied by broad expanded surfaces, under 

 which pass the tubes of the podia (the surface on one side in figure 

 is broken away to show the canal for the podion, c). 



Passing from the arms to the Disc, the skeleton is seen to 

 consist of two sets of plates, one belonging to the external integu- 

 ment, and the other to the oral system. 



The Oral Skeleton is complex. It may be most readily con- 

 ceived as resulting from the fusion of the elements of two segments 

 in each of the arms. The entire oral 

 skeleton surrounds the mouth, and con- 

 sists of as many segments as there are 

 arms. Each segment is roughly triangu- 

 lar in shape, the apex pointing inwards, 

 and being separated by the deep " buccal 

 fissures." The principal element in each 

 segment is a pair of " syngnaths " (Fig. 

 XVL), also known as oral angle-plates, FIO. xvi. 



iaws, mouth-plates, scutella or alia, etc. ; syngnatiw of opu 



7T*. . , , V , ' (after Miiller). j, jaw ; m.f, mouth 



the name Syngnath IS Suggested for them, frame ; n.g, groove for circum- 



aa thmr nrmeiat f\f twn narfc o-Pnprallv oesophageal nerve ring; p.d, pore 



as tney COnSlSt OI tWO parts generally and depression for oral tentacle. 



completely fused together. The larger 



piece of the syngnath is the mouth frame (m.f), which unites with 



the corresponding plate of the next segment across the buccal 



