ZOANTHARIA MADREPORARIA 395 



represent the most recent types of Madreporarian structure, but 

 the anatomy of the zooids points to a contrary conclusion. The 

 zooids are of very simple structure ; the mesenteries are found 

 only iu bilateral pairs, and all the new mesenteries formed after 

 the protocnemes originate in one of the directive chambers. 

 These are characters indicating a very ancient history, suggesting 

 affinities with the Edwardsiidea on the one hand, and some 

 ancient type of Cerianthidea on the other. There can be little 

 doubt that it was owing to the evolution of a porous skeleton of 

 rapid growth that these corals have caught up and passed the 

 Astraeidae and other more specialised forms in the struggle for 

 predominance on the coral reefs. 



Fam. 1. Madreporidae. — The calices of the corallum are 

 small and contain a few perfectly distinct septa. The coenosteum 

 is porous and contains a plexus of the coenosarcal canals, which 

 connects the cavities of neighbouring zooids. This family is 

 divided into a number of sub-families, but it is only necessary 

 here to mention the peculiarities of a few of the well-known 

 genera. 



Madrepora. — This genus is represented by an immense number 

 of forms on the coral reefs of both the old and new world. 

 Attempts have been made at various times to divide these forms 

 into specific groups, and a large number of species have been 

 defined and named. The differences between these species, how- 

 ever, are such as may be due to varying conditions of life upon 

 the reefs and not to characters transmitted from generation to 

 generation by heredity. There can be no doubt that when our 

 knowledge of the soft tissues of these corals is extended the 

 number of species will be greatly reduced. There are, however, 

 three principal forms of growth or fades in the genus. 



1. The flabellate or palmate colonies with large flat or concave 

 fronds, radiating from an encrusting base : Forma palmata. 



2. Much branched colonies, several branches radiating obliquely 

 from a common centre : Forma prolifera. 



3. Large and more erect colonies, less branched except towards 

 the periphery : Forma cervicornis. 



On some reefs one of these forms of growth predominates, and 

 for miles the reef seems to be built up mainly of corals of this 

 shape. On other reefs two or sometimes all three of these forms 

 may be found within a stone's throw of one another. ISTotwith- 



