Abstract
DEVELOPMENT OF OPHIOPHOLIS AND ECHINARACHNIUS. In the last series of studies from the Newport Marine Zoological Laboratory we find a memoir by Mr. Walter Fewkes, on the development of an Ophiuroid (Ophiopholis aculeata, Gray) and of an Elypeastroid (Echinarachnitts parma, Gray). But few observations have been published on the metamorphosis of Ophiopholis, and these often misleading. The eggs would appear to be extruded separately into the water, and the young pass through a metamorphosis in which a pluteus-larva is formed; the development of this pluteus is different from that of any described Ophiuran, though allied to that in Ophiothrix. The ova were voluntarily shed by the female on August 17; they were fertilised outside the body, and appeared to be very hardy. The yolk has a central and a peripheral region, which are dis tinguishable in the eight-cell and previous stages of segmentation. The cleavage is like that of other Echinoderrns. A gastrula is formed by the invagination of the blastoderm, and consequently the stomach of the pluteus is an infolded wall of the blastoderm, and not formed by delamination from the cells in the cavity. The mesoderm-cells originate in two lateral clusters. The oldest pluteus observed was a little more than three days old; they, however, appeared to be easily raised, and it is to be hoped that they will be yet traced to an adult form. In Echinarachnius the sexes are distinct, and in some cases there were colour-distinctions. In the experiments on the ovum of E. parma, artificial fertilisation was resorted to from the middle of July to the end of August; it was easily effected. In its mode of seg mentation it resembles that of other Echinoderms. It has no polar globules, but possibly these may be formed while the egg is in the ovary. As in some other Echinoderms, a gastrula is formed by invagination. The pluteus figured by A. Agassiz in the revision of the Echini as probably that of Echinarachnius proves to belong to this species at about a week old. The de velopment of the young Echinarachnius on the water-tube of the pluteus resembles that of other sea-urchins. The rosette-form of the water-tubes described in other Echinoderms also occurs. The first-formed calcareous deposits of the test are trifid in form, and vary in number in different specimens. The extremity of each trifid division bifurcates later in its growth, and the cal careous body thus formed appears to be inclosed in a transparent wall, which has a spherical outline. Spines are very early formed, and are proportionately very large as compared with those of the adult. The various stages are illustrated in nume rous figures on eight, in several cases folding, plates.—Bull. Mus, Comp. Anat. Harvard College, vol. xii. No. 4, March 1886,
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Biological Notes . Nature 34, 132–133 (1886). https://doi.org/10.1038/034132a0
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DOI: https://doi.org/10.1038/034132a0