Chap. 23 COELENTERATES SIMPLE MULTICELLULAR ANIMALS 489 



miles wide. None of the reefs is continuous — all of their fronts being subject to 

 incessantly breaking waves. The booming of surf is a characteristic voice of 

 the reefs. 



Theories of Reef Building. Charles Darwin suggested The Subsidence 

 Theory: that in past ages all corals had lived in fringing reefs; that in places 



7 •/ / V ^■- 



- • SeptUAT? 



Fig. 23.18. Upper, polyps of the coral, Astrangia, cup half an inch high. The 

 white Astrangia danae lives on the eastern coast, as far north as Cape Cod and the 

 orange and red Astrangia insignifica on the western coast of North America. 

 (Courtesy, American Museum of Natural History. New York.) Lower, diagram of 

 a coral polyp with one side of the body cut away to show the general structure. 

 The polyp is resting on the basal plate and partitions (or septa) of the limy cup 

 which it has secreted. Only the basal parts of the cup are included. The mouth, 

 tentacles, and walls (mesentery) of the alcove-like parts of the central cavity are 

 similar to those in sea anemones. (After Pfurtscheller. Courtesy, Wolcott: Animal 

 Biology, ed. 3. New York, McGraw-Hill Book Co., 1946.) 



