i6o 



McMURRICH. 



[Vol. V. 



considered from the opposite side. The forms with only the 

 six primary mesenteries may be regarded as the more primitive, 

 additional mesenteries having been added in some forms. The 

 fact that in all the genera only six tentacles are present, tends 

 to support this view, and it furthermore does not necessitate 

 any degradation theories. On this hypothesis the Antipatharia 

 would be the descendants of an ancestor which intervened be- 

 tween the primitive Scyphistoma-like form and the primitive 

 octamerous form, and would therefore be of very ancient stock. 

 The absence of fossil Antipatharia seems to oppose this view ; 

 but on the other hand, it is probable that, as in the Alcyonaria, 

 the colonial habit and the formation of an axial skeleton has 

 been acquired within the limits of the group and perhaps at a 

 geologically recent date. 



However, the present evidence is too imperfect to permit of even 

 an approximation to a solution of the question, but I am inclined to 

 consider the six mesenteried condition to be the more primitive. 



The following scheme may represent diagrammatically the 

 line of descent which has been proposed in the preceding pages 

 for the Actinozoa : — 



Tetramerous form 



Octamerous form 



Rusrosa? Edvvardsiae 



Dodecamerous form 



Protactinias 



Antipatharia ? 



Alcyonaria 



Ceriantheae 



Zoantheae 



Hexactiniae (including Madreporaria) , 



