7IO COELENTERATA—II. CNIDARIA. 



a margin. At high tide, except when a retreating wave reveals a small 

 portion of rock, the only sign of the presence of the reef is the line of 

 breakers along its outer edge, where the shallow water suddenly passes into 

 deep water, the outer side of the reef sloping steeply down to a great depth. 

 Between this outer edge and the land a shallow basin or channel arises, and 

 here the water is usually very clear, and brilliantly-coloured corals are seen 

 covering the rocky bottom. 



Prom such a "fringing reef" there arises, in process of time, a "barrier 

 reef." The channel between the edge of the reef and the shore tends to 

 widen, the corals along its bottom not flourishing so well 

 The Barrier Reef. as those along the outer edge. The water, too, in which 

 they are condemned to live receives its chief additions from 

 rivers or streams of fresh water washing down mud and sand from the land. 

 The corals here, indeed, cannot grow fast enough to repair the continual 

 disintegration which is going on. The coral colonies at the edge of the reef, 

 on the contrary, grow lu%xuriantly, continually raising the level of the reef 

 and carrying it out to sea. It seems at first sight strange that corals exposed 

 to the full fury of the breakers should thus flourish ; this is no doubt due to 

 the constant supply of food in the shape of minute organisms which swarm 

 in the open sea. Even the storms which often break up the coral colonies 

 into fragments help on the work they are doing, as the broken pieces are 

 thrown up on the reef and fill up all interstices left in it, rendering it thus 

 only the more solid and complete. Even the fragments broken oflf can give 

 rise to new colonies if only the living polyps retracted within their stony 

 cells have escaped undamaged, and provided also that their new position is 

 favourable in the matter of food. 



Barrier reefs are found rising up like ramparts against the waves even as 

 much as one hundred miles from the shores where they must have originated. 

 They thus cover vast areas of the sea-bottom. The Great Barrier Keef off 

 the N.E. coast of Australia is 1100 miles long, the distance from land of its 

 outer edge being usually from 20 to 30 miles, and in places over 100 miles. 

 Saville Kent computes the total area of this reef, built up by coral polyps, as 

 at least 80,000 square geographical miles ! Over the whole of this enormous 

 area of coral rock, the water remains comparatively shallow, while the outer 

 edge dips down almost suddenly into as much as 1000 fathoms. The whole 

 surface of such a reef is covered, wherever the circumstances are favourable, 

 with living masses of coral, while, in the large areas where the corals cannot 

 flourish, the reef is a vast conglomerate, the sand and fine mud, swept about 

 by the tides, filling up the interstices between the skeletons of original coral 

 colonies. 



When reefs form round small islands, the edge of the reef rising a little 



above the water, a coral island is produced, enclosing a lake or lagoon out of 



which rises the original island. Such circular reefs are 



Coral Islands and also found surrounding a clear lake of still water without 



Atolls. any island in the middle ; these latter are known as 



"atolls." These wonderful products of the coral polyps 



are a very marked feature of tropical seas, to the beauty of which they 



greatly add. The reef encircling an island or smooth lagoon becomes raised 



above the level of the surrounding sea by the debris washed up upon it ; as 



a rule, however, it does not rise more than a few feet above the surface, the 



sea often dashing, at the narrower parts, over into the enclosed lagoon. In 



other cases, it may rise 10 or 12 ft. above high tide and may be covered 



