XVII 



CLASSIFICATION SKELETON 



495 



the spines, which are borne on the flanks of the jaws (mouth- 

 papillae) and on their apices (teeth and tooth-papillae) there is 

 very great variation. Teeth are always present. Mouth-papillae 

 are very frequently present, tooth-papillae are rarer, and it is 

 only in a restricted number of genera (Ophiocoma and its allies) 

 that both mouth -papillae and tooth-papillae are present at the 

 same time. 



Skeleton of the Disc. This is typically composed of a 

 mosaic of plates of different sizes, but in some cases (Ophiomyxa, 

 most Streptophiurae, and Cladophiurae) these, with the exception 

 of the radials and genitals, are entirely absent, and the disc is 



Fig. 216. A portion of an 

 arm of Ophiohelus umbella, 

 near the distal extremity, 

 treated with potash to 

 show the skeleton, x 55. 

 The vertebrae are seen to 

 consist of two curved rods 

 united at their ends. The 

 triangular side-plates bear 

 a row of movable hooks 

 which articulate with 

 basal outgrowths of the 

 plate. (After Lyman. ) 



then quite soft and covered with a columnar epithelium, the 

 persistent ectoderm. Even the scuta buccalia may disappear. 

 Eadial shields are absent in Ophiohelus. In many cases 

 (Ophiothrix and Ophiocoma) all the dorsal plates except the 

 radials are concealed from view by a covering of small spines. 

 In some genera (Ophiopyrgus) there are five large plates in the 

 centre of the upper part of the disc, which have been termed 

 " calycinals " from a mistaken comparison with the plates forming 

 the cup or calyx of the Pelmatozoa, but there is no connexion 

 between the two sets of structures. 



The madreporite is usually quite rudimentary, but in Clado- 

 phiurae there may be five madreporites, each with about 200 

 pores, and, of course, five stone-canals. 



