28 THE GREAT BARRIER REEF. 



discernible in tliis nearer view of the same reef that has not been previously noticed. This is 

 Madrcpora rosoria, var. diiinosa, of which a highly characteristic illustration is given among the 

 bleached coral specimens illustrated by Fig. i8 of Plate I. of the photographic series. The life 

 colours of this variety are usually olive-green, with the exception of all the terminal "cells" and 

 growing points, which, together with the associated polyps, are, by way of contrast, pale prnii- 

 ro.se. In the more robust normal growth of this species, represented by Figs. 2 and 5 of the 

 plate last quoted, the ground tint is commonly flesh-pink, and all the terminal corallites and grow- 

 ing apices are white or pale yellow. 



PLATE XVIII. 



ORGSN-PIPE CORAL REEF, THURSDAY ISLAND. 



The interest attached to this illustration is associated with the fact that all the coral- 

 growths visible upon it are those of the so-called Organ-pipe or music-coral, Tubipom mxisica. 

 This well-known species belongs to the group or order of the Coelenterata, which is technically 

 distinguished by the title of the Alcyonaria. All its members, as explained in a special 

 chapter, differ from the ordinary stony-corals, or Madreporaria, in respect that their polyps 

 invariably possess eight tentacles only, in place of the twelve, or of the very much greater num- 

 ber found on all Madreporarian polyps. These eight tentacles of the Alcyonarian polyp are, 

 moreover, in most instances, distinctly fringed or pinnate. The majority of the Alcyonarise 

 are characterised by their association with a more or less flexible polypary, and, as far 

 as they occur on exposed reefs, they usually take the form of encrusting or lobulate masses 

 of various patterns, certain of which have been previously referred to. The Organ-pipe coral 

 is unique of its kind, it representing, with but one exception — the Blue coral, Heliopora carnlea, 

 figured and described in connection with the next succeeding plate — the only type of its 

 class that develops a rigid corallum. This corallum consists of an irregular nodular mass of 

 closely associated calcareous tubes of a deep crimson hue, which are bound together vertically 

 by transverse plates, of the same substance, in such manner as to present a highly suggestive 

 resemblance to the pipes of an organ with the supporting platforms. The polyps of the 

 living corallum of Tubipora mitsica are a pale emerald-green, and have their eight tentacles 

 very distinctly fringed. The photograph of a corallum of the natural size, with its polyps 

 expanded, is reproduced in Plate XXVI., while a small fragment of the same coral, with the 

 associated life colours, is included in Plate X. of the chromo-lithographic series. 



It may be observed of the Organ-pipe coral that, in common with other more ordinary 

 Alcyonaria, it nourishes in areas where there is less tidal scour, and consequently more 

 sedimentary matter, than is favourable to the growth of the typical reef corals. As a matter 

 of fact, both here and in most other places where this type was observed, the coralla were 



