30 



BRANCH CCELENTERATA 



the solitary mushroom coral being sometimes of the exceptional 

 size of 1 foot in diameter. 



These myriads of coral polyps (Fig. 20) secrete great quanti- 

 ties of lime, the waves break off the branches, grind them up, 

 mix them with sand and shells, and thus build up coral reefs 

 and islands of vast extent. These are confined to warm regions 



Fig. 21. A sea-fan. 



about 30 degrees on each side of the equator, since coral colonies 

 cannot live in temperature below 60 F., and for a full luxuriance 

 a higher temperature is necessary. 1 They are also shallow water 

 animals, living from the high-water-mark to a depth of not 

 more than 20 fathoms. They must also have salt water, hence 

 they cannot live at the mouth of a river. 



1 Scott's "Geology." 



