6o 



CORALS 



three categories, (i) the fohaceous Astraeids, (2) the dendritic 

 Astraeids, and (3) the sohtary Astraeids. 



Merulina. — This coral (Fig. 22), found in the Indo- 

 Pacific Oceans, is one of the commonest of the foliaceous 

 Astraeids. Its general form is that of a huge cabbage-like 

 vegetable, attached by a thick stem, and sending off, more 

 or less horizontally, a few large leaves or fronds. 



When the upper surface of one of these fronds is examined 



it has the appearance of 

 a raised map of a moun- 

 tainous country, a com- 

 plex of hills and valleys 

 with a general inclina- 

 tion from the base to 

 the periphery of the 

 frond (Fig. 22). Here 

 and there on the surface 

 of the fronds there are 

 irregular raised patches, 

 which would correspond 

 with high mountain 

 peaks. When the slopes 

 of the valleys are ex- 

 amined more carefully 

 with a magnifying glass, 

 they are found to be 

 traversed by a series of 

 parallel laminae, which 

 can be recognised as 

 the septa of an Astraeid 

 coral. In some places 

 there may be found little round pits or oval depressions, 

 where the septa have a tendency to radiate as from a 

 common centre, but there are no other indications of any- 

 thing corresponding with discrete calices. 



We have, in fact, in Merulina as in the Brain corals, a 

 complete continuity of the calyx units that are so well 

 defined in the more primitive compound Astraeids. 



The general form and surface markings of Merulina might 



Fig. 22. — Merulina. The upper surface of 

 a part of a frond. Nat. size. 



