140 THE GREAT BARRIER REEF. 



niollusca, as abundantly represented on the reefs by the ponderous-shelled bivalves, Tridacna and 

 Hippopus, the mother-of-pearl shells, oysters, and a host of other forms, contributes probably 

 the most considerable portion of the supplementary lime supplies. The Echinodermata, includ- 

 ing the several groups of the star-fishes, sea-urchins, and Holothuridae or Beche-de-mer, all of 

 which arc the possessors of more or less substantial calcareous skeletons, abound on every 

 reef, and rank second, probably, to the mollusca as accessory lime contributors. The crustacasa, so 

 called with relation to their indurated calcareous armour, are, as far as the larger species are con- 

 cerned, conspicuously scarce among the reefs, not being represented even by the smaller species 

 of crabs usually present in crowds upon ordinary rocky shores. The interpretation of this 

 phenomenon, it is suggested, may be found in the fact that the crabs have no chance of 

 establishing colonies on the coral-reefs, since their helpless larvae or " Zoaese " would fall 

 immediate victims to the extended tentacles and hungry mouths of the countless millions of 

 polyps that clothe the entire superficies of the living areas. As an interesting commentary on 

 his suggestion, a crustacean is hereafter described, and figured in Chromo plate II., whose 

 chief abiding-place and citadel of refuge is within the mouth-portals of a huge anemone. 



Of organic groups, other than those mentioned, which contribute their modicum of calcareous 

 debris to the reef-mass, mention must be made of the Protozoic group of the Foraminifera, whose 

 calcareous shells or tests, mostly of microscopic dimensions, are present in myriads on every 

 reef and at every depth throughout the Barrier district. One special generic type, Orbitolites, 

 represented by a flat discoidal test that is commonly from one-quarter to three-eighths of an inch 

 in diameter, is so abundant among the reefs that it constitutes the chief mass of the material of the 

 white so-called "sandy-patches" that intervene between the coral banks, and which is brought up ' 

 adhering to the anchor-flukes by vessels halting for the night, as is customary, at stages in the 

 more intricate northern moiety of the charted course. The shells or tests of this and other species 

 of Foraminifera enter very extensively into the composition of the finer-grained coral-rock such 

 as is represented by Plate XXXII., Fig. 5, wherein, in the original specimen, the embedded 

 shells of Orbitolites and many smaller species can be readily detected with the aid of a magnifying 

 glass. 



There is also a small vegetable group, that furnishes a considerable quota towards the 

 composition of the characteristic coral-rock. It is that of the peculiar seaweeds or lower algae 

 known as Corallines or Nullipores. They are distinguished by the encrustment of their tissues 

 with carbonate of lime, to such density that their vegetable nature is completely disguised ; and, 

 excepting for the absence of the characteristic pores, they might in many instances be mistaken 

 for the coralla of the Hydroid coral Millepora. These Nullipores, referable to the genus Melo- 

 besia, form either encrusting lichen-like expansions, irregularly nodulated, or short obtusely- 

 branched growths, that often more or less completely cover the surfaces of the indurated coral- 

 rock to a height considerably above that whereon Madreporaria are found growing. The colour 



