CORALS AND CORAL-ANIMALS. 201 



of /5(5 Jiippiiris, consisting throughout of densely aggregated calcareous spicules. The colour 

 of this corallum is more usually a bright brick-red on the surface, shading off to ochreous 

 yellow, or a lighter tint of red, internally. The cortical substance is very sparsely repre- 

 sented in this type, and consists only of loosely aggregated spicules, which fall apart almost 

 immediately after the death of the associated polyps. 



Melitodes ochracea is of more than ordinary interest, since it represents an intermediate 

 type between the ordinary flexible Gorgoniaceas and the precious coral of commerce, Coralliuin 

 rnbrum. In the last-named species, the central solid corallum is originally constructed of 

 loosely aggregated spicules, which become so closely amalgamated that its texture assumes 

 the density of marble, and is capable, like that substance, of taking the highest polish. The 

 corallum of Melitodes, while apparently solid to superficial vision, is found, on near examina- 

 tion, to be minutely porous, and is, in consequence, incapable of taking a polish. Although 

 no variety of the red coral of commerce has as yet been obtained from Australian waters, 

 the conditions would appear to be favourable to its growth throughout a large area of the 

 Queensland coast-line ; and, if not indigenous, it might, under suitable auspices, be artificially 

 introduced and made the subject of a new and highly profitable industry. 



CLASS HYDROZOA. 



The polyps belonging to the Hydrozooid subdivision of the Coelenterata are distinguished by 

 the possession of a much simpler structural organisation than that of either the Actiniae, Alcyonaria, 

 or coral-producing Madreporaria. The most essential character that distinguishes the polyps 

 of the Hydrozoa, from those belonging to any of the orders above enumerated, is associated with 

 the fact that the body of the Hydroid polyp encloses a single central cavity, while in all 

 Actinozoa the cavity in question gives rise to a series of outgrowths, which more or less 

 completely surround it, and whose walls give rise to its contained mesenteries, or sarcosepta. 

 The Hydroozoon body may be compared to a sac, with its mouth so inverted, or doubled back 

 within itself, that in diagrammatic transverse section it would be represented by a double tube, 

 the outer one corresponding with the body-wall of the polyp, and the inner one with its ali- 

 mentary tract. 



The greater number of the animals referable to the class Hydrozoa are included in the ranks 

 of the ordinary jelly-fishes, or Medusae, and in that extensive group of attached, seaweed-like, 

 organisms popularly entitled Sea-firs or Hydroid Zoophytes. There are a few, more exceptional, 

 types belonging to this class, that secrete solid coral-like structures, which, being represented in 

 the Great Barrier fauna, have to be taken into account in this volume. The only generic type, in 

 point of fact, that may be said to contribute to any material extent towards reef-construction in 

 the Barrier system, is that of Millepora. As many as three species of this genus have been 



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