Ccelenterates 55 



hold their own. In certain parts of the 

 tropics navigation is in fact rendered 

 hazardous owing to the continually 

 changing contours of the countless reefs. 

 In a few parts of the world coral rocks 

 form a staple building material, and in 

 the Bahamas and Bermudas certain har- 

 bour structures are composed entirely of 

 finely ground coral skeletons. 



