144 THE AQUARIAN NATURALIST. 



fathom to fifty. The summit of the exterior margin 

 of the reef or wall is usually composed of various 

 species of living Porites and Millepora. The Porites 

 form irregular rounded masses of from four to eight 

 feet broad, and of nearly equal thickness : other parts 

 of the reef consist of thick, vertical plates of the 

 Millepora complanata, intersecting each other at 

 various angles, and forming an exceedingly strong 

 honeycombed mass. The dead parts of these cal- 

 careous skeletons are often incrusted over with a 

 layer of the marine vegetable called Nulliporaj which 

 can better bear exposure to the air. 



This strong barrier is well fitted to receive the first 

 shock of the heavy waves of the fathomless ocean 

 without, and, what at first appears surprising, in- 

 stead of wearing away at its outer edge, it is only 

 here that the solid wall increases. The coral animals 

 thrive best in the surf occasioned by the breakers. 

 Through their agitation, an ever-changing and aerated 

 body of water washes over their surface, and their 

 respiration is maintained in the highest state of 

 activity. Abundant animalcules, and the like objects 

 of food, are thus constantly brought within the 

 sphere of the tentacula of the hungry polyps. Their 

 reproductive gemmules are rapidly and extensively 

 dispersed amongst the crevices of the calcareous 

 mass, and thus rising colonies are planted wherever 

 fit situations occur for their support and protection. 



By the force of unusual storms, this outer reef is 

 occasionally breached, and huge masses are torn off 

 and driven towards the lagoon, where they form an 

 inner barrier or reef. The broken surface becomes 



