﻿DUERDEN] morphology of coral polyps 95 



understood, but the possibility of the ultimate destruction of an 

 infested skeleton by this means is beyond doubt. 



While the corroding algal filaments arc thus general within the 

 living superficial areas of the corallum, they arc even mure prevalent 

 in the dead older parts, being particularly plentiful in so-called 

 "rotten coral." Here the) arc associated with other superficial or 

 boring organisms, such as boring mollusks, echiurids, sponges, and 

 other agents of coral disintegration. The algae continue their cor- 

 roding activity even on the separate particles of dead coral after 

 fragmentation, for fresh coral sand when dissolved in acids yields 

 filaments in a living condition. 



The universal occurrence of coral-boring algse, possibly resulting 

 in the ultimate disintegration and corrosion of the infested blocks, 

 has manifestly some bearing upon the much discussed subject of the 

 origin of the various forms of coral reefs, with which are associated 

 the nanxs of Darwin, Dana, Murray, Agassiz, and others. To 

 account for the formation of many of the barrier reefs and atolls of 

 coral seas it is necessary to assume a constant disintegration or 

 removal of the older coral growth as the outer living edge of the 

 reef continues its progress outward. In this destructive work the 

 ever-present corroding activity of boring algae is undoubtedly to be 

 accorded an importance along with the more obvious coral-boring 

 organisms, supplemented by actual solution in sea-water. 



COLORATION 



Almost all writers on coral reefs have emphasized the brightness 

 and variety of hues exhibited by the living polyps. Different tints of 

 brown, green, yellow, and blue are intermingled in never-ending 

 variety, and even the same species may present much diversity in 

 passing from one locality to another. Yet the color of living corals 

 is in part incidental, that is, independent of the true nature of the 

 polyps. Over West Indian coral areas the different shades of yellow 

 or brown predominate, and these are found to be due mainly to the 

 presence of zooxanthelke or yellow algal cells within the endoderm 

 of the polypal tissues. These are symbiotic unicellular plants, bearing 

 yellow chromoplasts, and as a rule are distributed throughout the 

 inner layer of the polypal wall, giving a light-yellow tinge to the 

 colony where sparse, and a dark-brown effect where crowded. The 

 two closely allied genera, Astrangia and Phyllangia, are the only 

 forms in which zooxanthelkne have not been found, and the living 

 tissues of these are characterized by an appearance of extreme 



