194 THE GREAT BARRIER REEF. 



As so far investigated, the coral is shown to represent in its normal condition a most interesting 

 example of what is known as " commensalism," or the sharing of a common domicile by two 

 entirely distinct organisms. Illustrations of the corallum of Hdiopora can-idea with its zooids and 

 commensal worms will be found on Plate X., Figs, i to 5, of the chromo-lithographic series. 

 The entire coral, growing in situ, forms a conspicuous object in the foreground of the photo- 

 graphic view of a portion of one of the Madge reefs, Thursday Island, reproduced in the 

 photo-mezzotype Plate No. XIX. In this reef-scape, the single conspicuous corallum of 

 Heliopora occupies an isolated position among an otherwise unbroken expanse of the more 

 normal leathery Alcyonarian polyparies. On other reefs, in the vicinity of Thursday Island, this 

 interesting type has been observed by the author growing more abundantly, though not to the 

 extent asserted by Jukes in his narrative of the voyage of H.M.S. Fly, who says that it repre- 

 sents the most plentiful coral genus on the reefs of Bramble Cay, a small rock and coral islet 

 situated in the extreme north-east of the Torres Strait limit of the Great Barrier system. 



Attention may now be directed to some few of the many flexible or leathery-coralled Alcyo- 

 naria (typical Alcyonidee) that enter conspicuously into the composition of the tidally-exposed 

 reefs of the Barrier system. As previously remarked, these flexible Alcyonaria rapidly disin- 

 tegrate on death, and can contribute little, bej'ond the drifted accumulation of their component 

 calcareous spicules, to the building up of the permanent reef rocks. Moreover, as these spicules, 

 are of microscopic minuteness, it is an open question whether they are not dissolved by certain 

 products of decomposition of the organic tissues, in relation to which they were originally 

 secreted. While thus disappearing with the suspension of vitality, they play a very important 

 part, in their living condition, in the function of reef-construction. A reference to the photo- 

 graphic views reproduced in Plates XX. a and XX. b, in addition to that of Heliopora, No. XIX., 

 last named, will suffice to substantiate this assertion. In each of these illustrations, it will be 

 seen that members of the group monopolise the greater part, if not the entire surface, of 

 the reef-scape. As thus viewed, in their high and dry condition, with all their polyps completely 

 retracted, the coralla of these leathery corals present a general appearance which resembles that 

 of many fungi. After the manner of those cryptogamic plants, they have a tendency to spread 

 themselves insidiously over all neighbouring objects, represented in their case by the reef rocks 

 and more solid Madrepores. The contours and colours of these tidally-exposed Alcyonaria vary 

 among the different species, and also among individual growths of the same species. The 

 commonest form, Sarcophyton glancum, represented in the left-hand foreground of Plate XIX., has 

 a corallum which, in its earliest growth phase, is sub-orbicular in shape and attached by a thick 

 fleshy stalk, after the manner of an ordinary mushroom. As it increases in age, the stem remains 

 relatively short, while the circumference of the disk becomes irregularly lobed and convoluted, 

 and so increases in diameter that the adjacent coralla overlap one another, as illustrated in 

 the plates referred to. The colour of the coralla of this species varies, most commonly from a 



