172 Recent Views regarding Coral Reefs. [Sess. 



this latter source that marine organisms, including coral 

 polyps, secrete the carbonate of lime of which their hard 

 structures mostly consist. I used to teach this in my geology 

 classes in London in 1887-88, but it is only right to mention 

 that the point was not definitely proved until Messrs Irvine 

 and Woodhead conducted their now well-known experiments 

 bearing upon this matter, which were published in the ' Proc. 

 Eoy. Soc. Edin.,' vol. xvi., to which the reader may well be 

 referred. The authors just named show that marine animals 

 which secrete carbonate of lime structures get that carbonate 

 chiefly from the sulphate, as mentioned above. In the case 

 of the coral polyp the animal pumps in sea-water into its 

 body-cavity, sifts out of that water what is to be had in 

 the way of food, extracts oxygen for respiratory purposes, 

 and takes in some of the sulphate of lime. Within the 

 animal this latter-named salt is converted into an emulsion, 

 which forms a kind of lime-soap, and out of this there 

 eventually is formed carbonate of lime. This collects within 

 what may be termed the " skin " in the lower part of the 

 body, close to the base of attachment to the rock. It 

 accumulates at first as a mere film, whose shape is regulated 

 by that of the animal's body at that part. But as time goes 

 on this film thickens into a cushion, — still moulded on the 

 plan of the basal parts of the animal, — and this, eventually, 

 into a gradually lengthening column. This structure from 

 first to last bears the impress of the structural characters of 

 the polyp, of whose basal portion, indeed, this stony matter 

 may well be regarded as a cast in carbonate of lime. Thus it 

 is that the corallum of a coral polyp is formed. There are, 

 of course, many modifications in matters of detail ; but the 

 general idea of the animal building up a steadily lengthening 

 column of carbonate of lime, which separates the animal 

 farther and farther from its original base of attachment, will 

 quite suffice for the purpose here in view. It may be re- 

 marked here that a most admirable and simply worded 

 description of the growth of a coral, by one who knew his 

 subject well, will be found in Martin Duncan's edition of 

 Cassell's Natural History, 



So far as the larger divisions of corals are concerned, it may 

 suffice here to group them into four chief categories. In the 



