338 J. STANLEY GARDINER. 



assistance as maintaining open holes in the living corals, which frequently become later centres 

 of decay. 



Of other animals some Echinids and Patellids wear out holes in the rock, where they 

 normally take up their positions in daylight. Ophiurids creep into any holes, however small, 

 preventing them from being filled in with sand. BuneUia lives in cavities of the reef, often 

 1 to 2 feet below the surface, but is a rare form, and many Nemertines retire during the 

 day to holes. None of these animals, however, are so far as I am aware actual borers. 



3. Sand-feeding Organisms. 



There are on reefs a very large number of animals that swallow sand, living presumably 

 on the extremely small percentage of organic matter that it contains. It seems quite certain 

 that the majority of these forms play some considerable part in the grinding up of the sand, 

 but in most cases the process, by which they do so, is not quite clear. It will be seen later 

 that all such sand-consuming forms have peculiar modifications of the gut, which adapt them 

 in a singularly efficient manner for the consumption of such food as the sand contains. 



It should here be pointed out that none of these sand-swallowing animals feed directly 

 on living corals, breaking off branches, etc. The polyps are thoroughly well able to protect 

 themselves against most free-living organisms. Indeed, so far as I have seen, a few Gastropods 

 alone make a practice of consuming the polyj^s, leaving dead tracks over the otherwise living 

 colonies. I do not know what author is responsible for the now common text-book statement 

 that Holothurians browse on corals'. Even considering the anatomy of these animals, it is 

 obvious that the statement cannot be accepted. Holothurians might, possibly, be able to 

 suck off parts of the pol}"]) layer, but that they could regularly break off coral branches, etc., 

 is clearly impossible. Bits of recently dead corals may occasionally be found in their intestines, 

 but I found by experiment that small living solitary corals and fragments of living coral were 

 always rejected by the ordinary Holothurians of the reef at Minikoi. The actual animals that 

 conceivably could feed on the reef corals of the Indian and Pacific oceans are extremely few, 

 and certainly Holothurians are not among their nvimber. 



Holothurians are, however, from their great abundance on reefs the most important animals 

 engaged in .sand-consumption. The sand, wherever it is present under the stones of the boulder 

 zone at Minikoi, commonly has living on its surface two or three species of brown and white 

 forms, while in the sand itself a few small, transparent Synaptids are sure to be caught. A form, 

 resembling Synapta ooplax, is common on lagoon beaches, often in enormous numbers. Holothurids, 

 related to Stichopus chloronotus and Holothuria atra, are abundant on every reef The former 

 generally affects the seaward reef-flats and the encircling reef of the atoll, while the latter is 

 found on all sand-flats and reefs within the lagoons. The two species, though, may sometimes 

 be observed on the same area. Other forms of every conceivable colour up to 2 feet or more 

 in length are also to be seen on the surface of the sand, many covered by its grains so as to 

 be scarcely distinguishable. Large Synaptids, further, come out from overhanging coral masses 

 every night, retiring during the heat of the day. 



Most of the above are surface animals, but in addition to these there are numbers of species 

 that live in and bore into the sand. A relatively thin, white species with dark brown spots, 



' Vide "Coral Beefs," by Cbas. Darwin, 3rd ed., p. 20. The statement, surely, does not rest on this one observation. 



