AFFINITIES OF THE GENCS MADEEPORA. 357 



polyp-cavity into endocoeles and exocceles is only applicable to 

 forms in wliicli the mesenteries are arranged on the Hexactinian 

 plan, i. e. in so-called pairs consisting of adjacent mesenteries. 

 The development of mesenteries in such pairs only commences 

 after the first twelve are formed. Up to that point the arrange- 

 ment is bilateral and members of a pair are situated on opposite 

 sides of the stomodseum ; the only ones which come to form 

 pairs in the Hexactinian sense are the directives. It therefore 

 now appears necessary to make use of another term to distinguish 

 the inter-mesenterial chambers in bilateral types with 6, 8, 10, 

 12, or more mesenteries, and the word "amphiccele" appears 

 suitable. I would use the term in all cases of Anthozoa where 

 the lateral divisions of the eoelenteron are brought about by 

 the formation of mesenteries arising from opposite sides of 

 the stomodseum and where in consequence the chambers are 

 simply intermesenterial ; in other words, in homocoelic as dis- 

 tinguished from heterocoelic types. In such cases the position 

 of the retractor muscles of the mesenteries is variable and may 

 or may not correspond with the Hexactinian arrangement. In 

 the case of Madrepora the arrangement is Hexactinian, that is 

 to say the mesenteries are arranged in real or apparent pairs, 

 having the retractor muscles on their outer surfaces in the case 

 of the directives and on the inner surfaces in all other cases. 

 Using Fowler's terminology the primary septa (6) are, in Madre- 

 pora, endocoelic, the secondary cycle (6) exocoelic. Although 

 the development of Madrepora is not known, I think one is 

 justified from our knowledge of the development of Hexactinians, 

 and Haddon's observations on the embryo of EupJiyllia, in 

 concluding that in the final arrangement only the directive 

 mesenteries arose as pairs and that the other pairs consist of 

 mesenteries situated on opposite sides of the stomodseum. In 

 this case the arrangement of the retractor muscles does not indi- 

 cate the true relationship of the lateral mesenteries and all the 

 septa should be described as amphiccelic. 



I would also suggest that the septa situated between the 

 " directive " mesenteries be known as the " directive" septa, as they 

 indicate the long axis of the stomodseum and are the first to 

 indicate a bilateral arrangement of parts in the skeleton. It is 

 often stated as characteristic of the genus Madrepora that the 

 directive septa are more prominent than the other primaries ; 

 but this is by no means always the case, nor is the feature con- 



