CORALS, SEA-ANEMONES AND JELLY-FISHES 67 



anemones. All these belong to the class Actinozoa (Gr. aktis, 

 ray; zoon animal). 



To this class belong also the interesting coral polyps that are 

 found in all tropical and sub-tropical seas. The individual 

 coral polyps are not unlike the sea-anemones in general ap- 

 pearance and structure, but they usually live in great colonies, 

 forming large irregular or branching tree-like masses. As 

 the animals grow they secrete a strong skeleton of carbonate 

 of lime. The corals with which most of us are familiar are 



FIG. 17. Branching coral, Acropora muricata, from Samoa. (Much 



reduced.) 



really only the calcareous skeletons of innumerable little 

 polyps. The living coral is quite different in appearance from 

 this hard stony skeleton. The surface is often soft and velvety, 

 pink, green, yellow, brown or purple, and covered with the 

 small waving tentacles of the numerous little polyp individuals 

 that make up the colony. As each individual polyp dies and 

 leaves its skeleton it is adding its small mite toward the for- 

 mation of the great coral rocks or reefs or islands charac- 

 teristic of the tropical seas. Coral polyps usually do not live 



