36 



FIBBT LBBSOm IN ZOOLOGY. 



Of the larger corals the 

 are the true reef-builders. 



Irl 





"^is 



^ 



\\ 



f 



that i(^e polyp has a more 

 hollow cylinder; the mouth 



Madreporaria in the mam 

 They are confined to waters 

 in which through the coldest 

 winter month the tempera- 

 ture of the water does not 

 fall below 68° F., though 

 usually the waters are much 

 warmer than this, the mean 

 annual temperature being 

 about 73i° F. in the North 

 Pacific and 70° F. in the 

 ' South. Coral-reefs are abun- 

 ' dant in the West Indies, but 

 still more so in the Central 

 Pacific, where there are a 

 much greater number of spe- 

 cies of corals. Along the 

 Brazilian coast, as far south 

 as Cape Frio, are coral-reefs. 

 In depth liying coral-reef- 

 builders do not extend more 

 than fifteen or twenty fa- 

 thoms below the surface. 



Coral-reefs are divided 

 into outer or barrier reefs 

 and inner reefs (Fig. 34). 

 The barrier-reefs are formed 

 from the growth of corals 

 exposed to the open seas, 

 while the inner or fringing 

 reefs are formed in quiet 

 water, between a barrier-reef 

 and the island. 



Upon comparing a polyp 



with a sponge it will be seen 



definite shape; it is a rude 



at the upper end, and sur- 



