SECT. iv. REEF-BUILDING CORALS. 139 



abstracting the small quantity of carbonate of lime that 

 the sea-water contains, that little if any animal matter 

 remains ; and as this process is continually repeated, one 

 generation of polypes perishes after another, the inert 

 matter increases indefinitely, and the surface at which 

 the consolidation is actually going on is the only part 

 that is alive. 



The surfaces of the dense convex masses of many of 

 these Astrsean corals are entirely covered with deep hex- 

 agonal stars, whose rays extend upwards all round, and 

 end in narrow, sharp, and elevated lines formed by the 

 junction of the rays of the adjacent stars ; in other 

 species the rays are often crowded together, and the 

 columella only shows a few points in the deep hollows. 

 Through these deep cups the polypes protrude their 

 circular disks and tentacles in quest of food, the nutri- 

 tious products of which maintain the polypes as well as 

 the general living fabric which unites them, and the 

 refuse is ejected from their mouths ; for each polype has 

 an independent life of its own besides the incidental life 

 that it possesses as part of a compound being. In many 

 of the corals the polypes show great sensibility, shrink- 

 ing into their cells on the slightest touch, yet no ner- 

 vous system has been discovered. 



The variety of compact and branching corals far ex- 

 ceeds description : 120 species are inhabitants of the Eed 

 Sea alone, and an enormous area of the tropical Pacific 

 is everywhere crowded with the stupendous works of 

 these minute agents, destined to change the present 

 geological features of the globe, as their predecessors 

 have done in the remote ages of its existence. 



Four distinctly different formations are due to the 

 coral-building polypes in the Pacific and Indian Oceans, 

 namely, lagoon islands or atolls, encircling reefs, barrier 

 reefs, and coral fringes, all nearly confined to the torrid 

 zone. 



