120 CORALS 



calcareous tubes which thev secrete may perforate the 

 corallum in all directions (see Figs. 51 and 52), and are so 

 numerous that they might be mistaken for a character of 

 the coral. Specimens of Heliopora from the Maldive Archi- 

 pelago are said to be free from this worm associate. 



There is no record at present of the colour and appear- 

 ance of the expanded polyps of Heliopora, and observations 

 that have been made at low tide in the day time suggest 

 that they are never expanded in such conditions. It is 

 probable that like many other polyps they only expand at 

 night. 



Heliopora is a curiously isolated genus in the system of 

 the Alcyonaria. It is the only recent genus of the Order 

 Coenothecalia to which it belongs. It has no near relations 

 among the Alcyonaria of the present day, but if we judge 

 from the character of its skeletal structures, it may be closely 

 related to a number of corals {e.g. Heliolites, Polytremacis, 

 etc.) which, in the early history of the world, flourished on 

 the reefs, but have long since become extinct. 



Heliopora itself can be traced back through the Eocene 

 to the Cretaceous period, but Heliolites and many allied 

 genera died out before the close of the Palaeozoic period, 

 and Polytremacis and others survived only to early Tertiary 

 times. Heliopora is therefore the only survivor of a long 

 line of ancestors with a pedigree extending back to the 

 earliest times of which we have any record of corals, and so 

 far as we can judge from its abundance on some reefs and 

 the massive size it attains shows no signs of following its 

 ancestors to extinction. 



The survival of Heliopora is a matter of special interest, 

 because most of the common corals of modern reefs, such as 

 Tubipora, Millepora, Madrepora, and Porites, are of com- 

 paratively recent origin. 



Isis. — The coral that was called by the older writers the 

 King Coral is the first of the few examples we shall consider 

 in this chapter of the Polypiers coralligenes flexihles of 

 Lamouroux. In general structure it presents similar features 

 to those of Corallium. There is a hard axis covered by a 

 thick coenenchym bearing the polyps, but in Isis the polyps 



