CORAL REEFS 159 



green tentacles, which project from the free ends of the 

 numerous little upright tubes. As a matter of fact, only 

 the surface region of the coral contains living tissues, the 

 skeleton below being abandoned as the colony grows, and 

 becoming merely a supporting structure and, incidentally, 

 the home of innumerable worms, crabs, sponges, seaweeds 

 and many other forms of life. Unlike the stony corals and 

 Millepora, the skeleton of this coral is not solid, but is 

 composed of many tiny calcareous spicules all fused together. 

 Exactly similar spicules are found in our own dead-men's 

 fingers, but there they are scattered about in a matrix of 

 spongy tissue. Another interesting coral of this type is the 

 Blue coral, Heliopora, which forms massive colonies often 

 several feet across ; if the branches are cut through, 

 however, the skeleton is seen to be composed of many parallel 

 tubes like the Organ-pipe coral. The skeleton of the Blue 

 coral is remarkable in that it is composed, not of fused 

 spicules, but of a solid mass of crystalline carbonate of lime. 

 Besides the floating Foraminifera, so common in the 

 surface waters of the oceans, there are others which live on 

 the bottom and form irregularly branching skeletons, and 

 these, together with the empty shells of the floating kinds, 

 form an important constituent of coral reefs in many parts. 

 But the chief factor in the formation of reefs, apart from 

 the various types of corals — and in some areas of equal 

 or even greater importance — are the calcareous coralline sea 

 weeds or Nullipores. Most of these belong to the red 

 algae, the most important of them being Lithothamnion 

 (Plate 61), types of which, as we saw, frequently cover the 

 sea bottom off our own coasts, Melobesia andLithophyllum. 

 They are all reddish in colour when alive, although the last- 

 named turns white when dried. They are all very wide- 

 spread, Lithothamnion, for example, occurring in great 

 abundance in the Arctic Seas, off the British Isles, in the 



