256 



COMPARATIVE ZOOLOGY. 



tween reef and shore ; encircling, where there is a small 

 island inside of a large reef ; and coral islands, or atolls, 

 where there is simply a reef with no knd inside of it. All 

 reefs begin as fringing-reefs, and are gradually changed 

 into the other forms by the slow sinking of the bottom of 

 the ocean. This sinking must be slower than the upward 

 growth of the reef, else it will be drowned out. Probably 

 the reef does not grow more than five feet in a thousand 

 years; and, as reefs are often more than two thousand 

 feet thick, they must be very old. 



(2) Sclerobasic Corals. — Cor allium ritbrum, the precious 

 coral of commerce, is shrub -like, about a foot high, solid 

 throughout, taking a high polish, finely grooved on the 

 surface, and of a crimson or rose-red color. In the living 



Fig. 208. — Sea-fan (Gorgonia) and Sea-pen {^ennututa). 



state the branches are covered w T ith a red coenosarc stud- 

 ded with Polyps. Gorgonia, or " Sea-fan," differs from 

 all the other representative forms in having a horny axis 

 covered with calcareous spicules. The branches arise in 

 the same vertical plane, and unite into a beautiful net- 

 work. 



