HYDRit;, JELLYFISHES, SEA ANEMONES, CuKAL POLYPS 69 



mainland by an intei'vening lagoon of water of varying 

 depth anfl width. An atoll is, generally speaking, an oval 

 reef inclosing a lagoon of water but no land (Fig. 27, C). 



Coral polyps, with few exceptions, cannot live more than 

 fifteen or twenty fathoms below the surface of the water, 

 or in water that goes below 68° F. in temperature, or in 

 fresh or in nuuldy water. Therefore, m order that the 

 polyps may ];egin a reef, they must have rock for a founda- 

 tion that is n(jt over one hmidred 

 and twenty or thirty feet below 

 the surface of the clear, briny sea. 



Not all corals, by any means, 

 build reefs. Some polyps bud in 

 such a manner as to form hemi- 

 spherical masses of coral, varying 

 from a few inches to several feet 

 in diameter. Such an aggregation 

 of polyps is called a head coral 

 (Fig. 26). Other globular masses 

 have their outer surfaces covered 

 with serpentine furrows which 

 cause them to resemble brains, 

 hence they are called brain corals. 



The organ-pipe coral (Fig. 28) builds up a series of 

 deep red tubes arranged side by side and held together 

 by horizontal floors. 



Finally, there is the red coral of commerce, which 

 was formerly so largely used for ornament. Of course 

 the red part worn is but the hard lime secretions of the 

 little polyps. When living, these red branches are wrapped 

 in a layer of flesh much as the wood of a tree is wrapped in 

 bark. The polyps, which are creamy white and have eight 



ri 



2S — (>i(5"iii I II c Coral 

 {Tubiporaniusica), Indian 

 Ocean. 



