OCR Text |
Show 26 MR. H. N. MOSELEY ON SOME CORALS. [Jan. 20, layer of the body-wall of the animal, but that it is a secondary structure formed within the body-cavity by the gradual coalescence of the outer extremities of the calcareous septa. In this conclusion he is supported by the fact that in transverse sections of the coral-wall ot many species of corals suture-like lines are to be made out, separating the calcareous tissue composing it into a series of masses which are apparently nothing else than the swollen peripheral portions of the septa themselves. " Dr. Koch, in a paper on the skeleton of corals, lately published in ' the Morphologisches Jahrbuch' *, exhibits his results in three diagrams. The diagrams represent sections of the same coral, a Caryophyllia, at various heights, and are believed by the author to exhibit also the process by which the actual development of the hard skeleton or corallum takes place. In the first the septa are seen quite separate from one another and occupying the centres of the inter-mesenterial spaces. In the second, the septa have coalesced by means of lateral outgrowths, and a complete calcareous wall is formed with the continuations of the mesenteries and intermesenterial spaces beyond it, these being shown much larger than in nature for the sake of clearness. In the third section, taken towards the bottom of the cups, the tissues external to the calcareous wall have perished and disappeared- this perishing of the lower parts of the soft tissues on the outside of the coral's cup at its base as growth proceeds at the summit being a normal process in the case of many corals, but not by any means in all. "The calcareous parts are covered everywhere, both according to Dr. von Koch's observations and m y own and those of other investigators, with a layer of mesodermal tissue, within the substance of which doubtless they are deposited. " It will be seen that the outer chambers and mesenteries are found by Dr. von Koch to exist only in the upper part of the coral. In his first diagram there is nothing to be seen but what would have been expected: the exsert calcareous septa rise above the wall of the coral-cup ; and thence they only are cut across in a superficial section. In order to explain what is seen in the second section, it may possibly not be necessary to assume that the soft tissues in which the calcareous wall is embedded do not belong to the wall of the animal. What seems to be the case is that the intermesenterial cavities lap over a short distance beyond the edge of the coral-cup, and are thus exposed in section beyond it when the entire coral is cut across. The soft tissues of the disk are to be seen in a living expanded Caryophyllia rising far above the summit of the corallum. No doubt, in specimens preserved in alcohol, the tissues are drawn down to a certain abnormal extent on the outside of the corallum as well as into its interior by contraction. The soft tissues in which the calcareous wall is developed may perhaps still be regarded as derived from the body-wall, although they do not quite coincide with tbe outer portion of it towards the summit of full-grown corals. In many simple corals the 1 "Bemerkungen iiber das Skelet der Korallen," Morph. Jahrbuch, Bd. T. p. 316. |