SPONGES AND CORALS 



extraordinary rate of three inches a year. But most 

 authorities give the average rate of coral growth as about 

 half this : one and a half inches a year, upward, over 

 comparatively small areas, and a very much slower rate 

 when the polyps spread out. Many centuries have elapsed in 

 the building of the great reefs from the sea-bed. 



Corals are free-swimming creatures only as embryos, 

 and then for brief periods. They quickly imprison them- 

 selves in their limestone structures, secreted usually on 

 their ancestors' skeletons. As time passes the individual 

 structures are firmly cemented into masses, and these are 

 even more firmly bound together by certain coralline 

 algae. In the daytime, those living corals which are on 

 the surface shrink into their stone fortresses. When 

 darkness falls they extend tentacles to catch and 

 poison planktonic creatures, which are carried to their 

 mouths. 



In every pool over the thousands of square miles of the 

 Great Barrier Reef a kaleidoscopic variety of sea life 

 swarms — gaudy coral fishes, tube worms gently waving 

 their plumed gills, sea-stars, spiny urchins and countless 

 other animals. 



Clams of the genus Tridacna up to three feet long occur 

 over the entire Reef Although clams are eyeless they 

 have organs which are sensitive to light, so that they can 

 detect a man's moving shadow. When they sense that 

 humans are passing near them in this way they close their 

 shells quickly — the action in some cases being so violent 

 that a stream of water may be shot several feet into the 

 air. When they feel that danger is past they open again, 

 and their gorgeous mantles emerge over the edges of 

 their valves. Clams along the Great Barrier Reef are of 

 many colours and sizes. Some of them have mantles of 

 brilliant green, which, as they emerge from the valves, 

 look like squirming snakes. Starfishes abound on the Reef 

 — moving with hundreds of suckered feet, which are 

 fixed in grooves running along each arm. Together with 



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