GENEEAL EEMARKS. 16 



supported by the directive mesenteries, and the interval between these by four mesenteries on 

 each side. The directive msenteries and one mesentery on each side (nos. 4 and 9 of 

 Fowler's figure 8) — which, as in Antipatharia, may be termed the transverse mesenteries and 

 are the first to be developed — are more important than the others and extend to a lower level. 

 The transverse mesenteries are the longest, and are the only ones which bear reproductive 

 organs. Similar elongate mesenteries occur in Alcyonaria, in Antipatharia, and in Seriatopora 

 and Pocillifera amongst the Madreporaria. In Antipatharia, as in Madrepora, they are 

 the only ones which bear reproductive organs. The free extremities of the longer mesenteries 

 bear a trilobate mesenterial filament. The retractor muscles of the mesenteries are arranged 

 on the hexactinian plan — that is to say, they occur on the outer surfaces of the directive 

 mesenteries and on the inner surfaces of all the other so-called pairs. If we use Fowler's 

 terminology, the primary septa are entocoelic in position, and the secondary cycle exocoelic. 

 I have elsewhere * suggested the use of the term amphicoelic for bilateral types, and that 

 term is probably preferable in this case. If the primary septa are prominent, each septum 

 causes a fold to be formed in the wall of the stomodaeum in which its distal extremity is 

 lodged. Presumably in cases where the directive septa or all the primary septa meet in 

 the middle line and become fused together, the stomodaeum may be divided into com- 

 partments. With the development of a septum the members of a so-called pair of mesenteries 

 become pushed further apart, so that in transverse sections of decalcified specimens which 

 possess no well-developed septa of the second cycle the entocoeles are very broad and the 

 exocoeles narrow. 



The case of dimorphism is recorded by Fowler to occur in M. durvillei. I have stated 

 elsewhere my reason for believing that the species is a variety of M. tubigera. In any case it 

 is identical with the fragments recorded by Quelch in the ' Challenger ' Reef Corals under the 

 name M. capillaris, Klunz. In the species in question there are two types of polyps, both of 

 which appear to be irregularly distributed. The one type is quite normal in structure, but 

 the other (type A of Fowler) has six of the mesenteries (viz. the axial directives and alternate 

 septa afterwards) much thickened. These septa (nos. 2, 4, 6, 7, 9, and 11 of Fowler's fig. 8) 

 bear a tube lined by epiblast, which forms a U-shaped canal opening at both ends into the 

 stomodaeum. Such polyps are charged with symbiotic algae. This type of polyp appears 

 more digestive than the normal one ; but both are also reproductive, although the specialized 

 type bears very few ova. 



CLASSIFICATION. 



The classification of the species of Madrepora proposed by Dana is based primarily on 

 habit, whilst the form and condition of the radial corallites afford the necessary characters 

 for further subdivision. Seven principal divisions are enumerated, with the following 

 characters : — 



* " Notes on the Affinities of the Genus Madrepora," Proo. Linn. Soc. Lend., Zool. 1893. 



