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58 liYDR^, JELLYFISHES, SEA ANEMONES, CORAL POLYPS 



It must be borne in mind tliat coral reefs are not always 

 built by coral polyps alone. Other animals and often- 

 times certain plants aid greatly in building up reefs. Never- 

 theless, the principal reef builders belong to tlie group of 

 animals known as coral ])olyps. Some of the species of 

 corals grow and branch like a tree (Fig. 26), finally attain- 

 ing a great size and 

 ^^^ producing an im- 

 mense coral forest, 

 as it were, which, in 

 time, may reach the 

 surface of the sea. 

 In such an event the 

 ends of the living 

 branches of coral 

 may extend out of 



^ tlie water at low tide. 



B The waA^es then bi-eak 



_^_^_ _ __ off ]neces of coral, 



^--^ ^.a«^. s~«, _ ., which, together with 



- j.i'^'* - '^-•^ ^ ^ 



^ _ "y^- other debris floating 



, — -».-^ w^^^ .==^^4^= =^--'^ -^ on the sea, fill uji the 



- ^, s]oa.ces between the 



' ends of tlie branches 



Fig. 27. — Diagram of coral reefs. j •] ii i r ii 



until the top of the 

 reef becomes a solid, smooth surface. More material 

 lodges on top, seeds fall there, and finall\- tlie reef liecomes 

 clothed wit) I verdure. 



There are three kinds of coral riM-fs; \vi. frincrine 

 barrier, and circular reefs, or atolls. A fringing reef 

 (Fig. 27, /I) lies close to the shore of an island or conti- 

 nent. A barrier reef (Fig. 27, B) is separated from the 



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