36 



FIRST LESSONS IN ZOOLOGY. 



Of the larger corals the Madreporaria in the main 

 are the true reef-builders. They are confined to waters 



in which through the coldest 

 winter month the tempera- 

 ture of the water does not 

 fall below 8 F., though 

 usually the waters are much 

 warmer than this, the mean 

 annual temperature being 

 about 73 F. in the North 

 Pacific and 70 F. in the 

 South. Coral-reefs are abun- 

 clant 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 living 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 

 that the polyp has a more definite shape; if is a rude 

 hollow cylinder; the month at the upper end, and sur- 



