154 



METAZOAN PHYLA 



polyps build masses of coral which after long periods of time become 

 very extensive and are known as reefs. These when they margin the 

 shore are called fringing reefs but when they lie at a distance from shore, 

 inclosing a lagoon, are known as barrier reefs. The Great Barrier Reef 

 of Australia (Fig. 74) is between 1100 and 1200 miles in length, and the 



Fig. 72. — Several types of anthozoan corals. A, brain coral, Meandrina sinuosa 

 Lesueur. X }{. B, rose coral, Meandrina meandrites (Linnaeus). X %. C, portion of 



an elk-horn coral (Acropora sp.). X ^3. D, portion of another branching coral (Oculina 



sp-). X H- E< mushroom coral {Fungia sp.). X%. F, part of an organ-pipe coral 

 {Tuhipora sp.). X %. The last is made by an animal related to the sea fans and sea 

 pens; the rest fall into another group, the corals of which are known as stony corals. 



lagoon it incloses is in places 30 miles wide and reaches a maximum 

 depth of 25 fathoms. When a barrier reef surrounds a submerged island, 

 producing a circular reef with a lagoon in the center, it is known as an 

 atoll (Fig. 73). 



183. Distribution and Economic Importance. — Coelenterates are 

 distributed generally throughout all seas. A few representatives, none 

 of which builds a skeleton, occur in fresh water. Coral polyps are 

 much more abundant in the tropical regions and disappear entirely 



