COELENTERA TA 



47 



and hence they are chiefly found in warm latitudes, and 

 arise fairly close to land where the water is shallow. They 

 are, however, known at greater depths 

 and in colder water. 



The fact that coral reefs are found 

 at a great depth was explained by 

 Charles Darwin by the theory that, 

 in such cases, the coral polyps which 

 originated the reefs started to grow at 

 the sea-bottom, in shallow water near 

 some coast, but that the land and sea- 

 bottom were steadily sinking, and con- 

 tinued to do so for long ages, but at 

 such a slow rate, that though the 

 bottom of the reef became uninhabit- 

 able to the polyps, those above still 

 flourished, multiplied, and spread up- 

 wards, and so the reef grew continu- 

 ously, based on the foundations built 

 by preceding generations ; such sub- 

 sidence of the land would also cause 

 the reef to become more and more 

 widely separated from the coast until 

 many miles of sea might intervene.^ 

 The most striking example of such a 

 coral reef is the Great Barrier Reef, which runs for 1200 

 miles parallel to the N.E. coast of Australia, and distant 

 about 80 miles from it, whilst the reef itself is 50 miles in 

 width, a tremendous structure to have been built up by 

 organisms so minute and insignificant individually as these 

 little coral polyps. 



Coral reefs often surround or fringe islands in tropical 

 seas. In some cases they have formed a fringing reef right 

 round an island which has subsequently disappeared owing 

 to subsidence, while the coral reef, which continued to grow 

 upwards, was apparently raised again, so that it now projects 

 above the water, and surrounds a shallow salt lake. Such 

 a ring-like coral island is called an Atoll. 



' This subsidence theory is now much questioned in some cases ; see 

 J. S. Gardiner, The, Fauna and Geography of the Maldive and Laccadive 

 Archipelagoes, 1902, vo), i. pt. ii. p. 172. 



Fig. 23. — A Branch of a 

 Colony of Madrepora. 



u, Central axis, the broken 

 end showing the canal up 

 the centre ; b, cup of one 

 polyp. 



