OCR Text |
Show 1892.] CLASSIFICATION OF OPHIUROIDS. 179 the saddle-shaped face of the Astrophytid ossicle is not seen here ; we have merely a generalized Ophiurid ossicle, without knobs or pits. The most remarkable character of this Ophiurid (see Plate XI. figs. 3 and 4) is the complete absence of a ventral plate ; no other existing brittle-star is known to want this plate. The upper plates are definitely double, and the side-plates, instead of lying flat against the side of the central ossicles, are wider than long and stand out from the sides of the arm. The radial shields are very large and extend almost to the centre of the disk ; they have the form of right-angled triangles, the hypothenuses of which face, but do not touch, one another ; there are no other plates on the surface of the disk. It is necessary to form a new genus for this form, which may be called Ophioteresis \ Definition of the Genus and Species. Ophioteresis is a streptospondyline Ophiurid in which the covering- plates of the arms are double above, wanting below, and wedge-shaped at the sides; the radial shields are well developed, and there are ordinary teeth and teeth-papillse. Ophioteresis elegans has the disk more or less distinctly pentagonal, of moderate size; arm-spines five. Elegantly coloured, the upper surface of the arms and the margins of the disk green, the central portion of the disk dark, with an irregular pattern of meandering white lines; interradial portions of lower surface of disk dark, with white lines ; the rest of the lower surface yellow. Hab. Seychelles, 4-12 fins. In coll. B. M . From this simple form differentiation would seem to have preceded along two lines ; there has been an increase in complexity of articulation, associated with the fixation of certain ossicles and spines, or there has been vegetative repetition and branching with a more primitive inconstancy and irregularity of anatomical characters. Around the primitive stock some forms-those which Mr. Lyman calls the " Astrophyton-like Ophiurans"-have remained, such as Ophioscolex, Ophiobyrsa, Neoplax, and Ophioteresis. 4. THE SUBDIVISIONS OF OPHIUROIDS. It will perhaps be found convenient to give distinctive names to the three groups; for brevity's sake I add here the definition of Ophiuroids which I ventured to publish last September2. The Ophiuroidea are caliculate, actinogonidial, eleutherozoic, azygopodous Echinoderm3, in which there is no distinct ambulacral groove. The " arms" are sharply marked off from the disk, are very rarely more than five in number, and are sometimes elaborately branched. The digestive system, which is aproctous, and the generative are confined to the area of the disk, as is also the specialized respiratory apparatus, which takes the form of deep clefts. The Streptophiurae are Ophiurids in which the ambulacral 1 Trjpqais, alertness. 2 T. c.p. 214. |