34 GILBERT 0. BOURNE. 



that the number of mesenteries, and with them of the septa, is 

 inconstant in several genera, and that a hexameral arrange- 

 ment of the parts is by no means an invariable rule, so that 

 the name Hexactinia as applied to the group is extremely mis- 

 leading. In most cases the mesenteries are marked out into 

 pairs by the situation of their longitudinal muscles, which are 

 so arranged that in all but two pairs these muscles are 

 attached to that side only of a mesentery which is turned 

 towards its fellow. At the two ends of the long axis of the 

 stomodseum, however, this arrangement is reversed, and these 

 two pairs of " directive" mesenteries have the muscles on their 

 opposite faces. This arrangement, which is typical of the 

 Actiniae, is found in nearly all the Madreporaria hitherto 

 examined ; but the latest researches of Fowler and myself show 

 that directive mesenteries are absent in Lophohelia among the 

 Oculinidse, and in Mussa and Euphyllia among the Astrseidee. 

 Excepting for this peculiarity Lophohelia and Mussa do not 

 differ from the Actinian type; but Euphyllia is modified in an 

 extraordinary manner, as has been described in the first part of 

 this paper, and need not be recapitulated here. The other 

 forms in which there is any important deviation from the 

 normal type are Pocillopora, Seriatopora, and Madrepora 

 Durvillei. 



The anatomy of Seriatopora and Pocillopora was first inves- 

 tigated by Professor Moseley (this Journal, new series, xxii, 

 p. 391), and his results have since been confirmed by Fowler. 

 In these forms the main deviation from the normal Actinian 

 type consists in the fact that of the twelve mesenteries only 

 two bear mesenterial filaments, and these two do not belong to 

 the same pair, but are the dorsal mesenteries of the right and 

 left infero-lateral pairs respectively. In correlation with the 

 development of the filaments on these two mesenteries the 

 intermesenterial chambers or entocceles of the pairs to which 

 they belong project far deeper into the calyx than the remain- 

 ing chambers in Seriatopora; but this feature is not so well 

 marked in Pocillopora. If the mesenteries are numbered 

 from left to right, beginning with the left ventral or abaxial 



