42 THE GREAT BARRIER REEF. 



PLATE XXVIII. 



SUBMERGED SEA-URCHINS, FILM ISLANDS REEF. 



This plate, by way of comparison with the preceding ones, subserves the purpose of illus- 

 trating the very varied nature of the fauna that inhabit distinct reefs, or even separate areas 

 of the same reef. The prominent form in this instance is the long, slender-spined Sea-urchin, 

 Diadniia sctosa, which abounds, in social clusters, just beneath the surface at low ebb-tide, over 

 large areas of the Palm Islands fringing reef In addition to the conspicuous groups occupying 

 the central foreground, many scattered individuals may be detected, with the assistance of a hand- 

 glass, ensconced among the more remote dead and living coral boulders. A wade through a reef, 

 thickly tenanted with these organisms, necessitates treading circumspectly ; and it is almost impos- 

 sible in the course of an enthusiastic search for novelties among the coral-stocks to escape scathe- 

 less. Their spines are slender, eight or ten inches long, and sharper than needles. Their 

 owners, apparently, possess an instinctive faculty for concentrating their serried ranks, and 

 so present an impregnable clicvau.x-dc-frisc towards the approaching foe. Nevertheless, and 

 notwithstanding accumulated experiences, these magnificent Echini always seemed to exert a 

 lascinating influence, compelling the writer to make the attempt to pick one up with unprotected 

 hands. Invariably, however, and though the creature was approached with the greatest pre- 

 caution, the spines pierced one's fingers at, seemingl}', some distance before they were visibly 

 reached. It was a common notion among the Beche-de-mer fishers that the animals could 

 elastically, or telescopically, extend their spines to meet the intruding hand. The true inter- 

 pretation of the phenomenon is, probably, to be found in the fact that the distal ends of the 

 spines are of such extreme tenuity, that they are imperceptible through the surface of the 

 water. In some instances, doubtless, a lack of knowledge of the common law of water refraction 

 is accountable, in the case of an inexperienced experimenter, for the uncomtemplated precursory 

 impalement and accompanying expostulations. The points of the spines of this Sea-urchin, though 

 so easily embedded in the flesh, are very difficult to extract. Left alone, they in a week or 

 two apparently disappear, and the author was of the opinion that, being almost pure carbonate 

 of lime, they probably dissolved in the blood. Professor A. C. Haddon has, however, informed 

 him that the spine points, like incepted needles, have, in his own experience, after a year's 

 interval, worked their way out at remote distances from where they entered. The capacity for 

 lime absorption probably varies in the blood of different individuals. 



A coloured representation of one of these long-spined Diademae is included in Plate XI. of 

 the chromo-lithographic series. It serves to illustrate a feature conspicuous in the submerged 

 living organism that does not appear to have been commonly observed. Reference is here made 

 to the spheroidal structure located among the spines on the \ipper surface of the test. This 



