THE ANATOMY OF THE* MADREPORARlA. 1 



The mesenteries 4, 9, run very much deeper into the corallum than 

 the others. 



Type B, of about the same diameter as A, is of the normal Actinian 

 structure. The twelve mesenteries are simple, and exactly like those 

 unmodified in Type A. Most of them die out after a very short 

 course, but those numbered 2, 4, 6, 7, 9, 11, on the same notation 

 as in Fig. 8, present a more developed filament than the other six, 

 and extend further down into the corallum, and of these 4 and 9 

 have by far the longest course, and are the only ones that bear ova. 



We have thus two distinct types of polyp, the one distinguished 

 only for entire normality ; the other with a hitherto undescribed 

 form of mesentery. In both is observable a differentiation affecting 

 the same six mesenteries, exhibited in the one case as a tendency 

 to a longer course, and to the more complete development of the 

 filament ; in the other as the peculiar modification described above ; 

 and in both types two of these six have of all the longest course, and 

 are, so far as I have observed, the only ones that bear reproductive 

 organs. 



Neither type is confined to certain areas of the branch, but both 

 appear to be irregularly distributed. 



Tentacles are not recognisable in my specimens, but it is probable 

 that in the living animal they occur as slight evaginations of the 

 chambers, and have shrunk under the action of the alcohol in which 

 the polyps were killed. 



Muscles are obviously present on the mesoderm lamella of the 

 mesenteries, but owing to their minute size it is impossible to detect 

 how they are arranged. I see no reason to doubt that they agree 

 with Actinia. So far as it is possible to judge without this clue, the 

 septa are entoccelic. 



o. Histology. — There is but little to be said under this head, 

 except as regards the modified mesentery, an almost transverse 

 section of which is represented in Fig. 9. The state of the specimens 

 did not allow of an exhaustive study of cell structure, but those 

 cells, the elongation of which causes the peculiar swelling on both 

 surfaces of the mesentery, are apparently simply lengthened, much 

 vacuolated and amoeboid at their free ends. No food particles wore 

 detected in them, or indeed in any other part, but many zooxanthelluj 

 are embedded among.st thorn. These cells puss gradually into the 



