1892.] CLASSIFICATION OF OPHIUROIDS. 177 



Ophiuroids, however, as are, like Astroschema, capable of twisting 

 or twining their arms round a straight Gorgonian, the saddle-shaped 

 faces are well developed, hut the limiting pits and processes are 

 absent " ^. The former plan of structure may be spoken of as 

 zygospondyline and the latter as streptospondyline ; there can be no 

 doubt that the latter is the simpler, and there is much evidence 

 to support the view that this simplicity is archaic and not second- 

 arily acquired. For example, no Astrophytid, all of which exhibit 

 the streptospondyline type, has the investiture of the central arm- 

 ossicles differentiated into upper, lower, and side arm-plates ; the 

 madreporites are inconstant in number and position, and pedicellarise, 

 never known among Ophiurida, may be present. 



If the possession of streptospondyline ossicles is an archaic 

 character in the Astrophytidse, it is so also in the Ophiuridse. Have 

 any of them other archaic characters 1 Ophioscolex has no upper 

 arm-plates ; Neoplaiv has a single, incomplete, upper arm-plate ; 

 species of Ophiomyxa have or have not arm-plates, which, when 

 present, may be in two pieces ; the tentacle-scales, which are so 

 characteristic of most Ophiurids, are wanting from Ophiomyxa 

 and Ophiobyrsa, are small and single in Neoplax, small and narrow 

 in Opliiochondriis ; the teeth and teeth-papillse of Ophiobyrsa are 

 spiniform ; and the teeth-papillae are wanting in Ophiomyxa, Ophio- 

 chondnis, Sigsbeia, and Semieuryale. 



Such a combination of characters points to the forms just men- 

 tioned as the simpler of the class ; they might have led to the 

 vegetatively multiplying Gorgon's-head or to the more highly 

 differentiated Ophiothrix. 



Before coming to any definite opinion, let us consider the value 

 of the evidence of the calycinal plates. But little is known of the 

 development of any streptospondyline Ophiurid ; indeed, all that 

 we do know is, I think, contained in one passage in Mr. Lyman's 

 ' Challenger ' Report. There we read of the young Gorgonocephalus 

 (p. 252) : " Above there is in the centre a group of six or seven 

 primary plates, each encircled by a superimposed line of grains." 

 Later on, the " disk-plates " become obliterated. Mr. Lyman's 

 observations shovr that there is no regularity of the plates, which, 

 as he calls them primary, we may suppose to be the representatives 

 of the calycinal plates of recent Echinoderm Morphology. 



But, after all, this is what may well be expected ; now that we 

 are, as I hope, delivered from the theory of the pelmatozoic " origin 

 of the Echinoderms, we may go a step further and recognize, as the 

 Cystidea teach us to do, that the calyx did not appear at once with 

 all the diagrammatic regularity that it has retained during the 

 manifold changes in name that its parts have suffered. 



It is, then, among those Cystid-like forms in which a definite 

 pentamerous arrangement was not permanently established ^ that we 

 must seek for the ancestor of the Ophiurid. At present, palseonto- 



1 Bell, Comp. Anat. & Physiology, p. 316. 



^ See Auu. & Mag. N. H. viii. (1891) pp. 206 et seq. 



3 Cf. Bather, Quart. Journ. Geol. Soc. 1889, p. 166. 



