42 



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.— Diagrain .showing the developmental se(iuence of the mesenteries in Actx-nia eqvina, 

 Saynrtiu iHilin, and Itunixle.s fiemmacru.i. 



2. — Shows the sequence of mesenterial development in lihndaiti.'s, Holrampa, and Manicina. 



3.— Shows the sequence of mesenterial development in Aijitasin dinjihana. 



In all the tigiues the numerals i, ii, iii, etc., denote the order in which the nie.senteries make 

 their appearance. The ei^'ht mesenteries lirst formed, the so-called " Eilwardsian " 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 C3'cle 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 eqniiia, Sofjartia hellis, and Itunodcs (jcmmaceus, 

 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- 



