114 



On the Morj^hoJogy of the Madreporaria. 



Carlgren (1897) upon the arrangement of the musculature 

 would seem to indicate the same, as only one pair of directives 

 are represented. The most important characteristic of the 

 Ceriantliepe in the present connexion is that the later mesen- 

 teries are always developed within the axial entocoele on the 

 asulcar aspect of the polyp, and that the organs in the adult 

 remain as bilateral pairs. Carlgren has shown that the feeble 

 retractor muscles in each mesentery are always on the face 

 turned away from the directives — that is, the mesenteries do 

 not constitute unilateral pairs in which the faces bearing the 

 retractor muscles arc vis-a-vis, as in all other Actiuians and 

 corals. 



The numerous researclies of the late H. de Lacaze-Duthiers 

 and those of G. von Koch have demonstrated that the proto- 

 septal stage of many modern corals is hexameral ; I have also 

 established the same for several West-Indian species, and a 

 like plan would seem also to have been characteristic of the 

 extinct Rugosa. It is with the appearance of the metasepta 

 that fundamental divergences are introduced. These seem to 

 afford hope that they may serve as a means for arranging the 

 Madreporaria on a more strictly morphological basis than is 

 at present possible. At least three fundamental types are 

 now known : — «, the cyclical, characteristic of most modern 

 corals ; b, the bilateral exocoelic, in Lophophijllum and 

 probably other Rugosa ', c, the bilateral eutocoelic, now 

 demonstrated for Porites and Madrepora. 



The essential results may be now summarized : — 



1. In most of the polyps of the genus Madrepora only the 

 six bilateral pairs of primary mesenteries aje developed, of 

 which four pairs are complete and two incomplete. On any 

 colony a few^ enlarged polyps may possess a greater number 

 of mesenteries. The new mesenteries beyond the primary six 

 pairs are added in complete or incomplete bilateral pairs at 

 only the two axial extremities^ the entocoele of the dorsal and 

 ventral directives. The cyclic disposition is never assumed ; 

 the directives form isocnemic pairs, but the other pairs are all 

 anisocneinic. 



2. The pinnate method of mesenterial increase is distinct 

 from that characteristic of most recent corals ; in these the 

 metacnemes are added in isocnemic pairs all round the peri- 

 phery of the polyp, within the six primary exocoeles, and in 

 the end constitute one or more distinct alternating cycles. 



3. The method of mesenterial increase beyond the proto- 

 cnemic stage occurring in Madrepora is also characteristic of 

 the genus Porites, except that in any one polyp of Porites the 



