The Relationship of the Tetracoralla to the Hexacoralla. 159 



THE TWO THEORIES OF THE ORIGIN OF 

 HEXACORALLA. 



As has been shown, there are two theories accounting for the 

 origin of the Hexacoralla, one of which depends for its proof 

 upon comparative anatomy of living corals, the other upon the 

 study of the geological sequence and relationships of fossil 

 forms. 



THEORY OF COMMON ANCESTRY. 



The first theory states that the Hexacoralla arose early in the 

 Paleozoic from a stem which also gave rise to the Tetracoralla 

 and Tabulata, but that the Hexacoralla did not commonly secrete 

 a skeleton until the Mesozoic. A large amount of the history 

 of the Hexacoralla would accordingly be lost, and as the phylum 

 was already a very old one when the lime-secreting habit became 

 firmly established in the Triassic, the early ontogenetic stages 

 reflected in the skeleton of Mesozoic forms would be of doubt- 

 ful phylogenetic value. The only test of the theory is an 

 exhaustive study of the analogy of morphological characters — 

 a method difficult to apply to an extinct group in which there is 

 no direct contact of the vital organs with the skeleton, though 

 much can be learned from the characters of living corals. 



Brown (1915 : 542) has stated this theory as follows: 



"i. All Anthozoa, Paleozoic, Mesozoic, and Modern, are 

 derived from one common stem in which the zooids were 

 bilaterally symmetrical and probably had eight mesenteries. 



"2. One branch from this common stem, arising early in 

 the Ordovicic, leads up to the modern Alcyonaria (Octo- 

 coralla). * * * 



"3. Another branch from this common stem, likewise arising 

 early in the Ordovicic, embraces the typical tetrameral corals 

 of the Paleozoic — the Rugosa. * * * 



"4. Another branch from the common stem gave rise to the 

 Mesozoic and later zoantharians — Actinians, Scleractinians, 

 Zoanthids and Cerianthids." 



This explanation carries an assumption which is very difficult 

 to accept. The advocates of this theory assume that throughout 

 the Paleozoic there developed side by side a group of skeleton- 



