CCELENTERATA 



79 



paratively smooth. Tubipora, or " organ-pipe coral," 

 consists of smooth red tubes connected at intervals by 

 cross plates. The Astrcea, Meandrina, Madrepom, and 

 Porites are the chief reef-forming corals. They will 

 not live in waters whose mean temperature in the 

 coldest month is below 68 Fahr., nor at greater depth 

 than about twenty fathoms. The most luxuriant reefs 



FIG. 31. Diploria cerebriformis, or " Brain Coral " ; one half natural size. Bermudas. 



are in the central and western Pacific and around the 

 West Indies. 



A coral reef is formed by many corals growing to- 

 gether. It is to the single coral stock as a forest is to 

 a tree. The main kinds of reefs are fringing, where 

 the reef is close to the shore ; barrier, where there is a 

 channel between reef and shore; encircling, where there 

 is a small island inside of a large reef ; and coral islands, 



