96 INVERTEBRATE ANIMALS, 
moved from the equator, being found chiefly on the east coast of Africa 
and the shores of Madagascar, in the Red Sea and Persian Gulf, 
throughout the Indian Ocean and the whole of the Pacific Archipelago, 
around the West Indian Islands, and on the coast of Florida. The 
headquarters, however, of the reef-building corals may be said to be 
around the islands and continents of the Pacific Ocean, where they often 
form masses of coral many hundreds of miles in length. According to 
Darwin, coral-reefs may be distinguished into three principal forms— 
viz., Fringing-reefs, Barrier-reefs, and Atolls, distinguished by the fol- 
lowing characters :— 
1. Lringing-reefs (fig. 36, 1).—These are reefs, usually of a moderate 
size, which may either surround islands or skirt the shores of continents. 
These shore-reefs are not separated from the land by any very deep 
channel, and the sea on their outward margins is not of any great 
depth. 
Fig. 36.—Structure of Coral-reefs. 1, Fringing-reef; 2. Barrier-reef; 3. Atoll: a 
Sea-level ; 4 Coral-reef; c Primitive land; d@ Portion of sea within the reef, 
forming a channel or lagoon. 
2. Barrier-reefs (fig. 36, 2).—These, like the preceding, may either 
encircle islands or skirt continents. They are distinguished from fring- 
ing-reefs by the fact that they usually occur at much greater distances 
from the land, that there intervenes a channel of deep water between 
them and the shore, and soundings taken close to their seaward margin 
indicate great depths. 
As an example of this class of reefs may be taken the great barrier 
