ZOANTHARIA. 187 



semblance to the convolutions of the hraia .which its popular name of 

 Brain-stone Coral has been devised to indicate " (Greene). 



As to their habitat and distribution in space, all the living 

 Zoantliaria sclerodermata are inhabitants of the sea, and there 

 is no reason to suppose that any of the fossil forms were 

 other than marine. At the present day, also, they attain 

 their maximum development in warm seas ; and this was 

 probably the case in past times too. In existing seas, further, 

 as has been specially insisted on by Professor Martin Duncan, 

 we find two great groups of the Sclerodermic Zoantliaria 

 viz., those which inhabit tolerably deep water, and those which 

 build the great masses of coral which are known as " coral- 

 reefs." The deep-sea corals, though often attaining, as in- 

 dividuals, a considerable size, and though often compound, 

 never form massive aggregations or " reefs." This is due to 

 the fact that, when composite, the separate corallites are not 

 united together by a lax cellular coenenchyma, so that the 

 colony cannot increase to an indefinitely large size. The 

 deep-sea corals seem to have existed in all the great geological 

 periods, from the Silurian upwards. The chief genera of this 

 group at the present day are Caryopliyllia, Balanopliyllia, 

 Flaldlum, Desmophyllum, and Sphenotrochus, amongst the 

 simple forms ; and Lopholielia, Amphihelia, Dendropliyllia, 

 and Astrangia, amongst the compound forms. 



The reef-building corals, when simple, are provided with 

 special structures which enable the polypes to grow rapidly. 

 The great majority of the reef-builders, however, are com- 

 pound, and owe the large size to which they attain to the 

 fact that the corallites are mostly united by a loose cellular 

 ccenenchyma. The chief genera of reef-building Zoantharia 

 in Mesozoic, Kainozoic, and Eecent times, belong to the 

 families of the Astrceidce, Poritidce, and Madreporidce, though 

 the Oculinidce and Fungidce also contribute to the formation 

 of reefs. 



In the Palteozoic period, if true "reefs" can be said to 

 have existed at all, they were built up essentially by Eugose 

 corals. In Mesozoic times, however, true coral-reefs existed 

 towards the close of the Trias in Western Europe, and largely 



