COELENTERATA 



65 



however. In one group of stony corals, the zooids are differen- 

 tiated, certain smaller individuals, the siphonozooids, lacking 

 longitudinal muscles, tentacles and gonads. Red coral is fashioned 

 into jewelry for children, while the white and rose pink Japanese 

 corals bring high prices. (See page 70.) 



The living polyps are significant since they may become the 

 foundation of islands, or protective barrier reefs (Figure 28), but 

 frequently menace shipping when they grow to a point just below 



Fig. 28. Great Barrier Reef of Australia. (Courtesy of Amer. Mus. of Nat. Hist.) 



the surface. In general, the corals that build reefs are found in a 

 zone extending about 30° on each side of the equator. For the 

 most part, since they cannot live in water below a temperature of 

 about 60°, corals are found in tropical waters, near the coast, ranging 

 no lower than 20 fathoms, and never found in brackish or fresh water. 

 Formation of Coral Islands. — Charles Darwin suggested that an 

 island surrounded by a coral reef might subside,^ thus accounting 

 for an atoll and its enclosed lagoon. (See Figure 29.) Sir John 

 Murray and A. Agassiz have staunchly supported the erosion theory. 

 According to this theory coral reefs form around an oceanic island. 

 The reef grows but the soil and rock of the island are washed away 

 until an atoll with its lagoon are left. The few lagoons that serve 



^ Davis, W. M., 1928, in his book The Coral Reef Problem, Am. Geog. Soc, agrees 

 with Darwin. The glacial control theory of Daly is the only serious rival to the sub- 

 sidence theory. 



