1899.] GEXUS PLEtTBOCORALLIXiM. 979 



growing any direction other than a vertical one, whatever might 

 be the position of the coral on which the embryos had settled ? 

 May we not hold it as certain that they had grown upright, that 

 is towards the surface of the sea ? If so, the Pleurocoralliiim must 

 have taken the same direction. 



Another piece of evidence having the same bearing is afforded 

 by some specimens of a simple Madreporarian coral (Desmojihylluni). 

 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 BesmopTiylla would have 

 acted so if the supporting coral had possessed a horizontal position, 

 because their calyces would then have been directed downwai'd to 

 the bed of the sea. 



Again, it is observable that in a specimen of another species of 

 Pleurocorallium in my 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 coenenchyma of the base. 

 It is not easy to understand how the fractui*ed 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 hoz'izontal 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 we 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. 



