194 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1917. 



Fig. 3. — Oorallium rubra m Lamarck. 

 Lacaze-Ditiiieus. 



Afteii 



The skeletons of Alcyonaria of this kind further differ from those 

 of the Madreporaria in possessing, according to F. W T . Clarke and 

 W. C. "Wheeler, from 6.18 to 15.73 per cent of carbonate of magnesia. 

 In the red organ-pipe coral, genus Tubipora (pi. 4, figs. 1, la), the 

 spicules are sufficiently cemented together to form tubes. The skele- 

 ton of the blue coral, Helio- 

 pora coerulea (pi. 4, figs. 2, 

 2a ) , looks very much like one 

 of the Madreporaria, and it 

 is composed of almost pure 

 carbonate of lime, but the 

 polyps have the anatomical 

 characteristics of the Alcyo- 

 naria. 



One of the Irydroids, Mille- 

 pora (pi. 2, figs. 3, 3a), is 

 usually considered with the 

 corals, although zoologically 

 it is not one of them. The 

 figures of the skeleton show 

 that it has no distinct septa, and that there are two kinds of pores 

 corresponding to two kinds of polyps, also called zooids. The larger 

 pores, the gastroporcs, lodge the larger nutritive polyps; while the 

 smaller pores, dactylopores, lodge the smaller, the food-capturing, 

 zooids. The skeleton of MUhpora, according to Clarke and Wheeler, 

 is composed of almost pure carbonate of lime. 



DIFFERENCES IN THE CORALS ON THE LAGOON (THE QUIET 

 WATER) AND ON THE EXPOSED (THE ROUGH WATER) SIDES OF 

 A CORAL REEF. 



Darwin in his Structure and Distribution of Coral Reefs, * gave 

 an excellent description of the difference between the corals in the 

 lagoon .of Keeling atoll and those on the exposed reef. A few years 

 ago Dr. F. Wood Jones spent 15 months in the Cocos-Keeling 

 Islands, and in his book. Coral and Atolls, produced a far more de- 

 tailed account of the relations of the corals in those islands to their 

 environmental conditions than that of Darwin, but what Darwin 

 said is correct. Dr. Wood Jones sent me his collection, which is now 

 the property of the United States National Museum, and I have been 

 able to publish a detailed account of it. 2 The Cocos-Keeling Islands 

 are classic ground for the students of reef corals and coral reefs, and 

 it seems appropriate to begin the consideration of the relations of. 



1 See 3d ed., pp. 1-19, 1889. 



2 Vaughan, T. W., Some shoal-water corals from Murray Island (Australia), Cocos- 

 Keeling Islands, and Fanning Island : Carnegie Institution, Washington, Publ. 213, 

 pp. 49-234, pis. 20-93, 1918. 



