Classification of Ophiuroidea.
Before proceeding to study the classification of Brittle Stars, it is necessary to give some account of the range of structure met with in the group.
Number of Radii.—The number of arms is rarely increased, and hardly ever exceeds six; a few species (each an isolated one in its genus) have six arms, and in one case (Ophiactis virens), at any rate, this is associated with the power of transverse fission. In many Cladophiurae the arms fork repeatedly, so that although there are only five radii, there is quite a crowd of terminal branches.
Vertebrae.—The vertebrae differ in the manner in which they articulate with one another. In Ophiothrix fragilis taken as the type, which in this respect resembles the vast majority of species (Zygophiurae), the knobs and pits on the faces of the vertebrae prevent the arms from being coiled in the vertical plane. In Ophioteresis (Fig. 210, A) and some allied genera (Streptophiurae) the knobs are almost obsolete, and the arms are free to coil in the vertical plane; whilst in Gorgonocephalus and Astrophyton (Cladophiurae) the arms are repeatedly branched and the vertebrae have saddle-shaped articulating surfaces, so that they have quite a snake-like capacity for coiling themselves round external objects. In Ophiohelus (Fig. 216) each vertebra consists of two rod-like plates placed parallel with the long axis of the arm and fused at both ends, but divergent in the middle, leaving a hole between them.
Covering Plates of the Arms.—The upper arm-plates are the most variable. They may be surrounded by small supplementary plates (Ophiopholis) or double (Ophioteresis). In all (?) Cladophiurae and most Streptophiurae they are absent, being replaced by minute calcareous granules. Under arm-plates are absent in Ophioteresis and in the distal portion of the arms in many Cladophiurae. Side arm-plates are constantly present, and in most Cladophiurae meet in the middle line below.
Arm-Spines.—The spines borne by the lateral covering plates of the arms vary greatly in character. In Ophiura and its 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.
Fig. 215.—Three types of mouth-frame found in Zygophiurae. A, Ophioscolex, × 10; B, Ophiacantha, × 6; C, Ophiothrix, × 6. (After Lyman.)
Mouth-Frame.—In its broad outlines there is practically no variation in this organ throughout the group, but in respect of 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.