422 J. STANLEY GARDINER, 



carried out. These served fully to confirm ray previous work on the subject, but most 

 reef-corals evidently ingested a certain araount of organic food, although the vast majority 

 undoubtedly obtained their nutriment principally through the agency of their commensal 

 algae. The polyps of such genera as Euphyllia and Prionastraea, in which no definite 

 gastrovascular cavity seems to be present, seized fragments of weed and animal matter when 

 placed by me within theii- reach. The presence of dead organic matter in about 1 or 2 

 per cent, of the preserved polyj^s of Pocillupora and Astraea points to the same method 

 of feeding sometimes occurring in nature, although there seems no doubt but that these 

 same corals feed mainly by means of their commensal algae. 



The occasional death of all the species of corals over considerable areas of the reefs 

 by silting up after heavy gales and cyclones is well-kno\vn, and was observed by us in 

 several places. I would emphasize the fact that in such cases practically all the coral 

 colonies of all the genera found in the area are killed. In addition separate coral colonies 

 may be here and there found killed by such a cause, probably by some purely local change. 

 Usually the absence of corals, etc. on reef-flats, in other respects suitable for their growth, 

 seems to be due to the waves and currents in such places moving fine material from the 

 reefs and shores. A series of experiments on transplanting corals to barren areas was entirely 

 unsuccessful so far as the growth of the animals was concerned, the colonies, when not 

 washed away, being in every single place killed by silt. 



A number of experiments were carried out on the exposure of colonies of several genera 

 of reef-corals to the air and sun. The living masses of various genera differed enormously, 

 but all, so long as they had some part of their polyp layers kept constantly moist by 

 {i.e. immersed in) the water, appeared to be able to live for at least 5 hours, even when 

 exposed to the full force of the sun. The colonies of most genera when placed on the 

 beach for the whole night or for 1 — 2 hours in the full force of a tropical sun were quite 

 capable of expanding again when I'eturned to the water. These experiments point to the 

 fact that the ordinary exposure — or even the extraordinary exposure after heavy winds — of 

 corals on the reef at low tide to the air and sun does not kill or otherwise destroy such 

 coral colonies as may stand up out of the water. 



Yet the death of coral colonies on reefs is of common occurrence, and has been mentioned 

 by most authors. In the Pacific, as well as subsequently in the Maldives, I frequently observed 

 separate, dead coral colonies, as well as the death of all the colonies of individual species of 

 corals over relatively wide areas. In the majority of these cases the separate corallites did 

 not appear to have been in any way silted up, nor were many of the colonies in such positions 

 that they could by any possibility have been killed by exposure either to air or sun. Separate 

 dead colonies also were often found where neighbouring colonies of the same and other species 

 of all sizes were flourishing. They were generally of considerable size, and in the absence of any 

 visible environmental cause, I am inclined to assign their death to senile decay, a phenomenon 

 I have found well-marked in Flabellum and other corals. 



An entirely different case is where the dead skeletons only of a single species of coral 

 were found on one reef, while on the next the colonies of the same species were perchance 

 growing luxuriantly. At the time I did not aj^preciate the importance of this phenomenon, 

 but I recorded .in my notes a large number of instances of the absence from particular areas 

 of living colonies of individual common species of corals, while the dead coralla of the same 

 species were common. I have already given a few instances and, to some degree, discussed 



