THE ANTHOZOA 



the Alcyonarians, called the Octactiniae. It is now known that 

 the number six is not nearly so constant as was formerly supposed, 

 and that where it does occur, the mesenteries of the first cycle are 

 not developed simultaneously nor in the couples which are eventually 

 established. In fact, the six-rayed symmetry which was supposed 

 to be so characteristic of the Zoantharia is not a primary but a 

 secondary feature. The development of the mesenteries in a six- 

 rayed Actinian may be said to proceed in two stages. Firstly, the 

 six couples of primary mesenteries are formed, not simultaneously, as 

 are the eight mesenteries of Alcyonarians, but irregularly, one after 



FIG. XX 



1. Diagram showing the developmental sequence of the mesenteries in Actinia equina, 

 Sagartia bellis, and Bunodes gemmcweus. 



1. Shows the sequence of mesenterial development in Rhodactis, Halcampa, and Manicina. 



3. Shows the sequence of mesenterial development in Aiptasia diaphana. 



In all the figures the numerals i, n, in, etc., denote the order in which the mesenteries make 

 their appearance. The eight mesenteries first formed, the so-called " Bdwardsian " mesenteries, 

 are drawn in thick lines, those formed subsequently in thin lines, s, sulcus ; si, sulculus. 



the other. This first cycle being once established, the mesenterial 

 couples of each succeeding cycle are formed synchronously, in a 

 regular manner, in the exocoeles of the cycles previously existing. 



The first cycle of six couples is formed differently in different 

 genera. In Actinia equina, Sagartia bellis, and Bunodes gemmaceus, 

 the order of succession is as follows : 



At the period when the stomodaeum is established, and the 

 mouth has taken on an elongate shape two mesenteries are formed, 

 marked I, I, in the diagram (Fig. XX. 1). They divide the 

 coelenteron into a larger sulcular and a smaller sulcar chamber. 

 It will be seen that these mesenteries originate in the neighbour- 



