REPORT ON CORALS — HELIOPORID^E. 109 



In no part of the growing points of the corallum of Heliopora is there any trace of 

 the calcareous tissue being built up of the fusion together of a network of spicules, as 

 occurs in the case of the corallum of Corallium mbrum and also in that of Tubipora, as 

 was shown to be the case by Perceval Wright, 1 and as may be seen at once by examining 

 the growing end of the tube of a spirit specimen of Tubipora. In this respect 

 Heliopora differs most markedly from both Corallium and Tubipora. The structure of 

 the hard tissue of Heliopora is, however, in many respects very like that of the sclerites 

 of Primnoa. 



Blue Coloration of the Corallum of Heliopora ccerulea. 



The corallum of Heliopora is coloured of a deep blue, and has always been 

 regarded as remarkable amongst corals for this fact. Now that it is known to be an 

 Alcyonarian structure the fact is less exceptional, since both Corallium and Tubipora 

 have a deeply coloured corallum, and many other Alcyonarians have coloured 

 spicules. Amongst Madreporaria such a condition is exceptional, but the coralla 

 of some Fungias and Dcsmophyllums is coloured madder red by a peculiar colouring 

 matter which I have termed " polyperythrin," 2 and which occurs also abundantly in 

 many Medusae and other Ccelenterata. In the case of certain Eupsammidse also the 

 corallum is red. 



The blue tint is seen in sections of the corallum of Heliopora ccerulea to be diffused 

 within the hard tissue. The colour is faint or almost absent in the freshly-growing tips 

 of the corallum, and pale in the most recently-formed superficial structures generally ; it 

 is darkest in the layer lying immediately beneath these, that is to say, in the most 

 recently matured tissue. In transverse sections it is seen to be darkest at the surfaces of 

 the walls of the tubes and calicles. In vertical sections of the corallum the continuation 

 of the dark blue line marking the margin of the wall of each tube enables the line of the 

 tube to be traced past the superadded tabula, and marks the boundary between the two 

 structures. ■ Very exceptionally, intensely blue streaks are developed more internally on 

 cither side of the central canal, as in Plate II. fig. 6, where B marks such a blue band. 

 The usual distribution of the colouring is that shown in Plate I. fig. 4, where the dark 

 zone at the margin of each tube seen in section represents intense colouring. The tabulae 

 are almost colourless. 



When the corallum is boiled for a long period in caustic potash the blue colour 

 remains unaltered. When the calcareous matter is removed from the corallum by means 



1 J. Perceval Wright, On the Animal of the Organ-pipe dual, Ann. and Mag. Nat. Hist., vol. iii. p. 377, 4 ser., 

 18G9. 



2 H. N. Moseley, On the Colouring Matters of various Animals, and especially of Deep-Sea Forms dredged by 

 II. M.S. Challenger (Quart. Journ. of Micro. Sci., new ser., Jan. 1877, vol. xvii. p. 2). 



