492 



ECHINODERMATA OPHIUROIDEA 



allies they are short and smooth, and are borne by the hinder 

 edge of the arm and directed backwards ; but in the larger 

 number of genera they are borne nearer the centre of the plate, 

 and are directed outwards at right angles to the arm. They 

 may be covered by small asperities, as in Ophiothrix (Fig. 215, 

 C), when they are said to be rough ; or these asperities may 

 become secondary spines, as in Ophiacantha (Fig. 215, B), when 

 they are said to be thorny. In Ophiopteron all the spines borne 

 by a single plate are united by a web of skin so as to constitute 

 a swimming organ. The small plates guarding the ends of the 

 tentacles (tentacle-scales) may be absent, or more rarely double. 

 In Cladophiurae there is a regular transition from tentacle-scale 

 to arm-spine ; the tentacle-scale being merely the smallest of the 

 series of lateral spines. 



True pedicellariae are unknown amongst Ophiuroidea, since 

 there is no longer a soft ectoderm to protect, but in some cases, 

 as for instance in Ophiohelus, small hooks movable on a basal 

 piece attached to the arms are found which may represent the 

 vestiges of such organs (Fig. 216). Similar hooks are found in 

 the young Ophiothrix fragilis just after metamorphosis and in all 

 Cladophiurae, replacing in the latter case the arm-spines in the 

 distal portion of the arm. 



Mouth-Frame. — In its broad outlines there is practically no 



-tooth-papillae 

 ,..- mouth-papillae 



mouth-papillae -.. -j^W 



~^^— a^ 

 ^ 



pj^r^ 



Fig. 215. — Three types of mouth-frame found in Zygophiurae. A, Ophioscolex, x 10 ; 

 B, Ophiacantha, x 6 ; C, Ophiothrix, x 6. (After Lyman.) 



variation in this organ throughout the group, but in respect of 



