474 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



to the left side most of the cells have become enlarged and glandular. In lower sections the 

 whole of the filament takes on this latter character, becoming at the same time much larger 

 (fig. 145). 



The glandular cells in Mseandrina differ from those in OrbiceUa only in the fact that the 

 contents of the cells are a brighter yellow in color; being unaffected by stains, the_y stand out as 

 very conspicuous areas in microscopic preparations. In Famia fragum also the contents are 

 bright yellow. 



The glandular modification appears to be somewhat generally distributed, having been 

 found in OrbiceUa acrqpora, Mseandrina labyrmthica, Favia Jragum, and ColpophyUia gyrosa. 

 Its occurrence appears to be somewhat sporadic. Only a few of the filaments in any one polyp 

 undergo the alteration, and its vertical extent is always very limited. In Orbicdla and Favia 

 two or three mesenteries, out of the usual twelve pairs, are thus distinguished, and in Mseandrina 

 the proportion is much the same. In one instance, in a portion of the brain coral, the two 

 mesenteries of a pair were thus altered. 



A still further development in the same direction is presented by the filaments of Mseandrina 

 (fig. 145). In transverse sections of ordinary filaments, the endodermal lobes immediately 

 behind are not sharply marked off from the rest of the mesenterial epithelium; the cells arc 

 exceptional in the amount of vacuolization, and the comparative paucity of the zooxanthellse, but 

 are not essentially different from the ordinary mesenterial endoderm (fig. 143). Where the 

 glandular alteration has taken place, the cells, not only of the filament, but also of the mesenterial 

 epithelium for some distance behind, are nearly all of a uniform character, and the elongated 

 nuclei of the supporting cells are arranged in a zone. In fig. 145, three, coarsely granular, 

 gland cells are represented, the granules staining very deeply, but the remainder of the cells are 

 tilled with an extremely fine granular matter, which stains but slightly. The nuclei of the cells 

 are oval, and distributed through the tissue with an approximate uniformity, and perfectly 

 clear gland cells are altogether absent. Instead of the filament being separated from the 

 mesentery by a distinct groove on either side, as is the case elsewhere, its cells are directly 

 continuous with those of the mesenterial epithelium, and these latter have undergone a like 

 glandular modification for some distance, passing gradually into the ordinary mesenterial 

 endoderm. 



Thus the elements of two different tissues — filamental and mesenterial endoderm — may 

 assume a like specialized character. 



MESENTERIAL FILAMENT* OF MADREPORA AND PORITES. 



The histological characters of the mesenterial filaments of Madrepora are such as to call for 

 special note. A transverse section of one of these is represented on PI. II, tig. 14. The meso- 

 glieal lamella from the mesentery enters a short distance into the base of the filament, and there 

 bifurcates; the two halves are directed backward into the lateral regions, where they thin out 

 and are lost. The filament thus becomes divided into three distinct areas, a larger antero-lateral 

 area, and two smaller posterior crescentic regions, each characterized by special histological 

 elements. In front the cells consist of long, narrow, supporting cells, amongst which are 

 numerous clear and granular cells, and a few small thick-walled nemtocysts, though the latter 

 are plentiful only in the proximal region. The cells of the posterior crescentic areas are all of 

 one kind — narrow, ciliated, supporting cells, with the deeply-staining nuclei wholly restricted to 

 the inner two-thirds of the cells (tig. 135); the ciliation is also stronger than anteriorly. Though 

 some such differentiation between the middle and posterior areas of the filaments is found in 

 other coral polyps, the distinction is rarely so marked as in Madrepora. In sections stained in 

 borax carmine the posterior regions are an intense red, and present a sharp contrast with the 

 rest of the filament. 



The middle region corresponds in histological detail with the glandular streak of the middle 

 lobe of the Actinian filament; while histologically the crescentic areas most distinctly recall 

 the ciliated bands of the lateral lobes of the Actinia 1 . In these latter the ciliated bands are 

 constituted wholly of ciliated supporting cells, and the condition in Madrepora serves to 



