I90 THE GREAT BARRIER REEE. 



As an order, it embraces organisms presenting as many modifications in its siceletal products 

 as all the remaining sections of the Actinozoa. Thus, some few Alcyonaria {e.g., Monoxenia, 

 Hartea, and Haimea) are simple and skeletonless, like the simple Anemones or Actinaria. Some 

 others, such as Claveliaria and Cornularia, correspond essentially with the sea-anemone-like polyps, 

 Zoantharia, which, while secreting no skeleton, are united together in social colonies. The Anti- 

 patharia, distinguished by their possession of a horny or chitinous axial skeleton, are typified 

 with countless modifications by the extensive group of the Gorgoniaceae ; while, finally, the 

 calcareous-skeletoned Madreporaria find their equivalent in Heliopora and, in a modified sense, 

 in Tubipora. Beyond this, the order of the Alcyonaria includes supplementary groups, such as the 

 Alcyonida; and Pennatulidae, that possess no approximate analogues in the remaining Actinozoarian 

 orders. In accordance with this very comprehensive range of modification of the Alcyonaria, 

 it is sometimes elected to divide the entire class of the Actinozoa into two sub-sections only, 

 of equivalent value, distinguished by the titles of the Hexacoralla and Octocoralla, or Hex- 

 actiniae and Octactiniae, with reference to the number of their tentacular organs and correspond- 

 ing internal, mesenterial, structures. One of the moot questions among biologists at the present 

 day, is the probable derivation of all existing Actinozoa from a primitive hexaradiate or octoradiate 

 ancestral type. The teachings of embryology appear to suggest a decision in favour of the 

 octoradiate or, more probably, of a tetraradiate ancestry. It is significant in this association that 

 certain highly-specialised sea-anemones, such as Cerianthus, begin their existence with four 

 tentacles and four mesenteries (Huxley: Anatomy of Invertebrata, p. 162), and that a tetrameral 

 radiate plan is distinctive of the entire class of the simpler organised Hydrozoa, including 

 the Hydroid polyps and jelly-fishes. Little or no consideration appears to have been given, 

 hitherto, to these earlier tetrameral Hydrozoan affinities. The extensive, but somewhat ill- 

 defined. Paleozoic group of the Rugosa, furthermore, is established on Madreporaria-like corals 

 which have their septal elements disposed in tetrameral order ; and one of the last scions of this 

 ancient race, Moseleya, has, as recorded on a previous page, turned up within recent years in 

 Australian waters. 



So far as the order of the Alcyonaria enters into the composition of the Great Barrier 

 coral-reef, its contributions are for the most part represented by that group of the typical 

 Alcyonidas, which is characterised by the possession of a flexible corallum of almost leather- 

 like consistence. This corallum, on examination under the microscope, is found to consist 

 chiefly of minute calcareous spicules of various shapes, embedded in the animal substance. 

 Coralla of this description, on the death of the associated polyps, necessarily fall to pieces, 

 and do not contribute substantially to the construction of the reef rocks, except, it may be, 

 by the local accumulation of their disintegrated spicules. In the living state, however, as 

 shown in a number of the accompanying reef-views, they may enter very extensively into 

 the compo.sition of the coral landscape. Before proceeding to an enumeration of the more 



