. Discovery. Science. DISCOVERY 177 devastated. Such exceptional instances explain the occurrence in the Pacific of reefs whose broad surface is stre\\Ti only with dead corals, with a few colonics beginning to work in again from the edge. There is, apart from this, a marked difference to be traced between the oceanic shores of an island and those within a harbour or estuary where muddy invasions may be usually expected in the wet season. In the latter position the beautiful Acroporas and the encrusting Lithothamnion are entirely absent. The massive brain corals and the flabby Alc\-onarian cora
. Discovery. Science. DISCOVERY 177 devastated. Such exceptional instances explain the occurrence in the Pacific of reefs whose broad surface is stre\\Ti only with dead corals, with a few colonics beginning to work in again from the edge. There is, apart from this, a marked difference to be traced between the oceanic shores of an island and those within a harbour or estuary where muddy invasions may be usually expected in the wet season. In the latter position the beautiful Acroporas and the encrusting Lithothamnion are entirely absent. The massive brain corals and the flabby Alc\-onarian corals (relatedto our "Dead Men's Fingers ") are the dominant forms of such impoverished reefs, and these corals are provided with an efficient mechanism to get rid of the suspended mud which threatens to choke them. Many interesting cases of distribution according to environment are to be found among the general invertebrate fauna of a coral reef. The wave-pounded edge would seem to offer a singularly ineligible shelter for any lodger, and yet characteristic forms are found there. The sea-urchin, Heterocentrotus, is widespread in the tropics, and a familiar but puzzling form in collections. Seen in its natural habitat, the meaning of its huge mace-like spines becomes apparent. They are pushed into tiny hollows and fissures of the coral blocks, and with the aid of the sucking " tube feet " are quite adequate to maintain the sea-urchin in posi- tion against a heavy wave and its backwash. Of. photograph by W. H. LongUy. Fig. 3.—schooi, of pork fishes, massive coral and sea- urchins icextrechlxus), portugas. entirely different appearance is the equallv characteristic long-spined urchin, Centrechinns, which is found only at moderate depths or in the quiet pools of the reef (Fig. 3). The variations in the form of the spines of urchins can be thus roughly compared with those of the corals themselves. Of the active population of a coral reef, the fishes are without d
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