28 SIDERASTREA RADIANS. 



the synapticular region which Bourne describes for Fungia and Fowler (1888, 

 p. 9) for Stephanophyllia jormosissima ; muscular degeneration proceeds along 

 with that of the mesentery in the West Indian form. 



The actual process of resorption of the mesenteries, from the periphery 

 inwards, can be readily traced. It commences in the peripheral region of the 

 polyp even as high as the level of the retracted stomodseum. The part of 

 the mesentery undergoing degeneration narrows and breaks up into separate 

 fragments ; the mesoglcea is often very thin for some distance, and the mus- 

 cular fibrils lining it form only an irregular, discontinuous layer. Generally 

 the portion of the mesentery some little distance from the peripherally fixed 

 part is the first to pass away, thus leaving an interval in the actual continuity 

 of the organ. The endoderm may disappear first, and the naked mesoglcea 

 then extends a little beyond and thins out (plate 7, fig. 43), but usually the 

 epithelium persists the longest. Free fragments of the degenerating tissue 

 are often found in the interseptal chambers, and other fragments are occa- 

 sionally seen absorbed by the endoderm lining the chamber. 



Mesenterial resorption must of course take place proximally in all corals 

 which continue to grow upwards as the lower part of the polyp is cut off 

 below by dissepiments. But in Siderastrea the peripheral parts begin to 

 disappear first, and the disappearance as a whole extends upwards propor- 

 tionally much further than in most corals. Perhaps this is in some way 

 connected with the presence of the synapticula in the peripheral region. In 

 other coral polyps the lower edge of the mesentery appears to be resorbed 

 more uniformly. 



The mesenteries are too small and too completely inclosed within the 

 interseptal chambers to permit of their being dissected out and examined as 

 a whole, but by means of serial sections it can easily be seen that the synap- 

 ticula actually perforate the mesenteries. In the first place, the complete 

 loculus shown on plate 6, fig. 33, reveals that the synapticula really per- 

 forate the walls, and necessarily anything inclosed within the two lining 

 walls in the same areas. Then certain of the synapticular perforations 

 included in plate 6, fig. 34, show that a mesentery is continued from each 

 wall bounding the perforation, passing centrally on the one hand and 

 peripherally on the other. On following such a mesentery both upward 

 and downward in serial sections it is found that when the synapticular 

 perforation disappears, having been followed through its vertical extent, the 

 two moities of the mesentery again become continuous. Clearly such condi- 

 tions can result only from the presence of actual fenestrse in the mesentery. 

 Similar relationships can be also followed in the series of sections represented 

 on plate 9. 



