58 HYDK^E, JELLYFI8HES, SEA ANEMONES, C(.)RAL POLYPS 



It must be borne in mind that 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 the group of 

 animals knowm as coral polyps. 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 

 the water at low tide. 

 The wa-\'es then break 

 off pieces of coral, 

 which, together with 

 other debris floating 

 on the sea, fill up the 

 spaces between the 

 ends of the branches 

 until the top of the 

 More material 



Fig. 27. 



-Diagram of coral reefs. 



reef becomes a solid, smooth surface. 



lodges on top, seeds fall there, and finally the reef becomes 



clothed with verdure. 



There are three kinds of coral reefs; viz. fringing, 

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

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

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



