[62 THE GREAT BARRIER REEF. 



A genus of corals that possesses structural features very closely resembling those of 

 Mussa, and whose colony-stocks play an important role in the constitution of the great Barrier 

 reefs, is that of Symphyllia. In this genus, the full-grown coralla are represented by large solid, 

 dome-shaped, masses, that may measure many feet in diameter, which, from their sinuous 

 surface patterns, considerably resemble the typical Brain-stone corals or Meandrinae. On near 

 examination, the meandering ridges and valleys of Symphyllia are found to exhibit a structure 

 which may be aptly compared with colony-stocks of Mussa, having their corallites so closely 

 compressed, that their boundary walls, or " thecas," have become completely soldered together, 

 while the corallites, instead of being sufficiently elongated to enclose only two or three oral 

 openings, are continuous with one another in simple or branched sinuous series of indefinite 

 extent. 



The expanded living polyps of Symphyllia, excepting for their extended serial union, resemble 

 those of Mussa in all essential respects ; the tentacles being subulate, transparent, and pro- 

 jecting in the form of a fringe along each peristomial border. It has been further observed by 

 the author that the polyps of this genus, in common with those of Mussa, rarely expand in the 

 daylight, and must be examined by night to be witnessed in a condition of full extension. This 

 fact was established, by preserving examples alive for a considerable time in an improvised aqua- 

 rium on board H.M.S. Rambler. During the daytime, the polyps remained persistently closed, 

 but they expanded in full filmy splendour as soon as night set in. The same phenomenon was 

 found to obtain among a large number of the ordinary Star-corals, Astrasaceae, hereafter noticed, 

 and would appear to account for the fact that, while the living membranes of the oral disks 

 and intervening areas, which are exposed to the light when the tentacles are retracted, are 

 always distinctly, and also brilliantly, coloured, the tentacles themselves, which only expand in the 

 dark, are transparent or colourless. Examples of reef photographs in which the coralla of the 

 genus Symphyllia constitute a conspicuous feature are furnished by Plates II., III., VI., VII. of 

 the photo-mezzotype series. As is the case with the genus Mussa, the dominating hues of the 

 investing membranes of the living coralla are of various shades of green and brown. The 

 commoner species, Symphyllia hemispherica, which forms the large domes previously referred 

 to, is usually a dark liver-brown throughout, while in the rarer 5. siiuiosa, or its near ally, 

 represented by Chromo plate No. V., Fig. 17, the va'leys, or polyp centres, are a rich velvety 

 green, and the intervening septal ridges golden-brown. As with Mussa, the members of the 

 genus Symphyllia are distributed throughout the length and breadth of the Barrier district. 



The extensive group of the Star-corals, or Astraeaceas, demand attention next. The members 

 of this tribe are, however, so numerous, that a few only of the more conspicuous Barrier species 

 can be here enumerated. The feature common to all the typical representatives of this coral 

 group is the solid, imperforate structure of the calcareous corallum, which consists of 

 innumerable corallites, most frequently of polygonal outline, closely united to one another 



