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Show 1899.] G E N U S P L E U R O C O R A L L I U M. 979 growing any direction other than a vertical one, whatever might be the position of the coral on which the embryos had settled ? M a y we not hold it as certain that they had grown upright, that is towards the surface of the sea ? If so, the Pleurocorallium must have taken the same direction. Another piece of evidence having the same bearing is afforded by some specimeus of a simple Madreporariau coral (Desmophyllum). All four examples were attached by their bases to the front of the coral, but two of them had twisted themselves round and had pushed their calyces between its branches to the other, that is the posterior side. It is not probable that the Desmophylla would have acted so if the supporting coral had possessed a horizontal position, because their calyces would then have been directed downward to the bed of the sea. Again, it is observable that in a specimen of another species of Pleurocorallium in m y possession where a small branch by some accident had been broken off the living coral, it had fallen upon the spreading base, to which it had in course of time been made to adhere by an extension of the growing ccenenchyma of the base. It is not easy to understand how the fractured branch could have lodged upon the base in the manner it has done unless the coral had been upright. In view of these facts, I do not see how we can adopt Dr. Gray's hypothesis, however plausible it may appear at first sight, or come to any other conclusion than that these corals assume in their growth a vertical, not a horizontal position. But if this is so. what is the meaning of the fact that the polype-cells are confined to one face of the branches ? If w e suppose that the habitat of the corals is in that part of the sea's bed where a constant current is flowing, it is clear that it would be more beneficial to the colonies if all their polypes were turned towards the direction from which their food comes, than if half of the polypes were turned in the opposite direction. A colony will obviously obtain the largest possible supply of nutriment when all its members face the current that carries it, we may say, into their mouths. To summarize what has been said: in order to account for the fan-like mode of growth of the Pleurocorallia and the unilaterality of the polype-cells, Dr. Gray maintained that the corals grow in a horizontal position. As such a position is not easily reconciled with the facts above stated, the suggestion is now put forward that the corals grow upright in the path of a submarine current with all their polypes opposed to the on-coming stream. |