OPHIOTHRIX MOUTH-SKELETON 



483 



are a pair of plates termed " peristomial plates." Tliese 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- 



A^ 



A^ 



^d4y, Asteroid 



A3 



f^c:\ 



Ophiuroid 



Fig. 211. — Diagrams to 

 show the moditication 

 of the ambulacral and 

 adambxilaeral ossicles 

 to form the armature 

 of the mouth. A, Aste- 

 roid ; B, Ophiuroid. 

 A^-A^, the first four 

 aiul)ulacra ossicles ; 

 A(\-Ad^, the first four 

 adamljulacral ossicles ; 

 Jj, the first plate of 

 the interradius (in the 

 Ophiuroid the scutum 

 huccale) ; P, the spines 

 borne by the jaw (in 

 the Ophiuroid the 

 teeth) ; T, the torus 

 angularis ; W, the 

 water - vascular ring ; 

 Wr, the radial water- 

 vessel ; 7, //, 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, j9^). 



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 



