OPHIOTHRIX MOUTH-SKELETON 



483 



are a pair of plates termed " peristomial plates." These discs 



of which there are two in each radius, one on each jaw which 

 flanks the radius possibly represent the separated halves of the 

 first vertebra, the apparent first vertebra being really the second. 

 On the flank of the jaw there is dorsally a groove for the water- 



Fig. 2 11. Diagrams to 

 show the modification 

 of the ambulacral and 

 adambulacral ossicles 

 to form the armature 

 of the mouth. A, Aste- 

 roid ; B, Ophiuroid. 

 AyA 4 , the first four 

 ambulacra ossicles ; 

 Ad x -Ad it the first four 

 adambulacral ossicles ; 

 J v the first plate of 

 the interradius (in the 

 Ophiuroid the scutum 

 buccale) ; P, the spines 

 borne by the jaw (in 

 the Ophiuroid the 

 teeth) ; T, the torus 

 angularis ; W, the 

 water - vascidar ring ; 

 Wr, the radial water- 

 vessel ; I, II, the first 

 two pairs of tube-feet. 

 (After Ludwig.) 



vascular ring and nerve-ring (Fig. 212, n.r), and beneath this a 

 groove for the first tentacle and a pore for the second, both of 

 which spring directly from the ring-canal ; below these, in most 

 Ophiuroidea, but not in Ophiothrix, there is a row of blunt 

 triangular spines called " mouth -papillae " (Fig. 212, jfy 



The words " jaw and " tooth " are misleading. There is no 

 evidence that the jaws of a Brittle Star are ever used for crushing 

 food, but by means of the muscles attaching them to the first 



