I20 SECTION A. GENERAL ZOOLOGY. 



were obtained from depths exceeding 30 fathoms, and in these the 

 abundance of zooxanthellae and the absence of foreign organisms, 

 serving for food, were equally marked. 



In Prionastraea, from which I obtained the greatest amount of 

 oxygen, there is, so far as the preservation of the material will 

 allow me to see, no gastro-vascular cavity between the mesenteries, 

 and the stomodoeum is but small. The tentacles are not well 

 marked, and the nutriment of the polyps appears to have been 

 obtained entirely from the commensal algae. 



If the zooxanthellae are the main source, as I contend, of food 

 for the polyps of most species of Madreporaria, the vertical distri- 

 bution then of these corals probably depends entirely on the depth 

 to which light of sufficient intensity can penetrate sea water. 



The chief organisms, so far as I have seen, engaged in building 

 up coral reefs are massive and incrusting nullipores, and the depth 

 at which such algae can grow depends likewise on the light. 



In my paper, previously referred to, I pointed out that photo- 

 graphic plates are affected in fresh water, which is not nearly so 

 translucent as sea water, at a depth of 120 fathoms, and that 

 nullipores were dredged by the Penguin below 75 fathoms off 

 Funafuti. The greater translucency the nearer the observer is to 

 the equator is, 1 think, an obvious fact. The depth from which 

 a reef could grow up to the surface would be at the latitude 

 and longitude of Funafuti, I should suppose, about 150 fathoms. 

 Generalh' speaking, the lower the latitude the greater would be the 

 depth, and the higher the less, but in the latter case another factor 

 comes in — probably temperature — preventing the formation of 

 reefs at higher latitudes than 30". The rate of growth of reefs 

 would depend, then, on the intensity of the light and the tempera- 

 ture, supposing other conditions equal. 



If this view is a correct one', the depth from which a reef could 

 be directly built up to the surface by the reef organisms must be 

 greatly increased, bringing necessarily a large number of submarine 

 mountains and elevations into its limits. 



With regard to the foundations of atolls, I have nothing new 

 to add to what Murray, Wharton, Agassiz and other observers have 

 pointed out. Mounds can be and are being, I believe, slowly built 

 up on submarine mountains to the bathymetrical limits of the reef 

 organisms. The numerous dredging expeditions show that animals 

 naturally congregate on any elevations, and by the accretion of their 

 remains and those of pelagic organisms, mounds would be built up 

 much in the same way as the raised limestone of Viti Levu, Fiji, 

 appears to have been formed. A few corals are found in this lime- 



^ The marked decrease in the steepness of the slope at a depth of 120 — 150 fathoms 

 in sections off atolls in the Pacific Ocean gives a general support to this view, but it is not 

 clear how far it may not rather be due to the talus spreading outwards on the mound on 

 which the atoll is built. The sudden change however is equally well marked off the 

 Tizard, Macclesfield and other banks, which have not yet reached the sea level and where 

 the talus is not probably of much importance. 



