CORAL REEFS 157 



Caryophyllia, of our own seas, i.e., like anemones but with a 

 surrounding skeleton, the majority of the reef building 

 varieties are of the " colonial " type. They form great 

 colonies consisting of large numbers of individuals, the 

 skeletons of which are all fused together to produce a great 

 stony mass. The bleached corals we see in our museums 

 or such as are shown in Plates 58 and 59, are really only 

 these empty calcareous skeletons, which in life are covered 

 with the delicately coloured tissue of the coral itself, while 

 under water the beautiful feeding polyps, each surrounded 

 by a ring of tentacles, open and expand. The openings 

 from which these polyps project can easily be distinguished 

 in the coral skeleton, and will be found to lead not into a 

 smooth rounded opening, such as one can imagine an 

 anemone occupying, but into a tube the sides of which are 

 raised into a series of sharp ridges known as septa, and 

 which penetrate almost to the middle of the opening. The 

 presence of these septa is an important point of difference 

 between the Madreporarian and the other types of corals, 

 some of which we must now describe. 



Although many scientists prefer to reserve the name 

 reef -building coral to the " stony " Madreporarian corals, 

 yet these, though the most important, are by no means the 

 only organisms concerned with the formation of coral reefs. 

 There are a number of corals allied to the little hydroids, 

 from which they differ essentially only in the possession of a 

 thick calcareous skeleton forming at least four-fifths of the 

 entire mass, through fine apertures in "which project the 

 tiny polyps which are all united to a common canal within. 

 The commonest of these corals is one called Millepora, 

 which is found in coral reefs all over the world. It has two 

 kinds of polyps which have a perfectly definite arrange- 

 ment. If a piece of the whitened stony skeleton of this 

 coral be examined it will be found to be covered with a series 



