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Show 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, but the limiting pits and processes are absent" 1. 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 secondarily 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 pedicellariae, never known among Ophiurida, may be present. If the possession of streptospondyline ossicles is an archaic character in the Astrophytidae, it is so also in the Ophiuridae. Have any of them other archaic characters ? Ophioscolex has no upper arm-plates ; Neoplax has a single, incomplete, upper arm-plate; species of Ophiomyxa have or have not arm-plates, which, when present, m a y 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 Ophiochondrus ; the teeth and teeth-papillae of Ophiobyrsa are spiniform ; and the teeth-papillae are wanting in Ophiomyxa, Ophiochondrus, Sigsbeia, and Hemieuryale. Such a combination of characters points to the forms just mentioned as the simpler of the class ; they might have led to the vegetatively multiplying Gorgon's-head or to the more highly differentiated Ophiotlirix. 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 show that there is no regularity of the plates, which, as he calls them primary, we m a y 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 pelmatozoic2 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 3 that we must seek for the ancestor of the Ophiurid. At present, palaeonto- 1 Bell, Comp. Anat. & Physiology, p. 316. 2 See Ann. & Mag. INT. H. viii. (1891) pp. 206 ct seq. 3 Cf. Bather, Quart. Journ. Geol. Soc. 1889, p. 166. |