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Show 176 PROF. F. JEFFKEY BELL ON THE [Mar. 1, on the north-west coast of Australia, are a number of young Echinoderms; in many cases it is not possible to assign them a definite specific place, but to the morphologist they will offer charms less patent to the systematise Among them there is an Ophiurid which is remarkable for the large size of what are now generally regarded as the plates of the calycinal area, and which m y lamented friend P. Herbert Carpenter in his valuable essay 1 called respectively centro-dorsal, under-basals, and radials. These plates are so well marked that it is quite impossible for the most sceptical to regard them as anything else than the components of a vestigial calyx, and I think their relations to the rest of the organism are perhaps better shown in the drawing given herewith than in any previously published figure of an Ophiurid (Plate XI. figs. 6, 7). It is certain that the specimen is the young of a species of Pectinura or of some form closely allied to that genus. 2. CLASSIFICATION OF OPHIUROIDS. Since the year 1867, when Dr. Ljungman2 published his still valuable classification, no serious attempt has been made to classify the Ophiuroidea, and it is possible that some doubts remain as to the relations of the genera that compose that class ; the question whether the simple-armed Ophioderma or the much-branched Astrophyton has the more archaic characters is one which systematists have neither asked nor answered. The majority of naturalists would probably confess that their impression was that the many-branched forms had succeeded those with simple arms. At any rate all are agreed that there are two equivalent orders or groups-the Ophiurae and the Euryalee of Johannes Midler, the Ophiuridae and Astrophytidae of Theodore Lyman ; if these two groups are really sharply separated from one another, it will follow that we must look upon one as derived from the other and now separated from it by the disappearance of the connecting-links, or we must suppose that they had long ago a common ancestor and have since been evolved along distinct lines; the latter is the view adopted by Prof. Haeckel in his 'Generelle Morphologic' Mr. Lyman, though retaining the bifid division of the class, recognizes the resemblance of some of the Ophiuridae to the Astrophytidae, for his "group m . " is called " Astrophyton-like Ophi-urans." One striking point in which Sigsbeia and Hemieuryale, for example, two members of the group, resemble Astrophyton is the power of rolling their arms. And the function has a corresponding similarity of structure. In most brittle-stars the " several ossicles of the arm have a certain power of movement on one another, but this is limited by the development of processes and pits analogous to the zygosphenes and zygantra of the Ophidian vertebrae. In such 1 Quart. Journ. Micr. Sci. xxiv. (1884) pp. 1-23. 2 Ofv. Vet.-Atad. Forhandl. xxiii. (1867) p. 303. |
