80 STRUCTURAL AND SYSTEiMATIC ZOOLOGY 



or atolls, where there is simply a reef with no land in- 

 side of it. The Great Barrier Reef off the east coast of 



FIG. 32. Astrcza rotulosa. West Indies. 





-' 



FIG. 33. Cell of Madrepore Coral, 

 magnified. The cuplike depres- 

 sion at the top of a coral skele- 

 ton is called calicle. 



FIG. 34. Fragment of Red Coral (Coral- 

 Hum rubrutii) , showing living cortex 

 and expanded polyps. Mediterranean. 



Australia is 1250 miles in length. All reefs begin as 

 fringing reefs, and are gradually changed into the other 



