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 land 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 rubrum , 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 
J O 
Fig. 208.— Sea-fan (Gorgonia) and Sea-pen ( Pennatula ). 
state the branches are covered with 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. 
