MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 551 



color-bearing cells are much shorter than the height of the ectoderm layer, and occur at all levels 

 within it. from the free surface to the mesogloea. They readily separate in macerations, and are 

 very irregular in outline; the various -tains have little effect upon them, except that the nucleus 

 comes out distinctly (fig. 31). The superficial ciliation of the column is preserved in most 

 places. The endoderm is narrower than the ectoderm, and also contains numerous yellow 

 granular cells, while zooxanthellse are very abundant. There is no evidence in sections of any 

 ectodermal or endodermal musculature, though in all probability weak endodermal fibers are 

 present, and bring about the overfolding of the column wall on retraction. 



The stoniodanim exhibits the usual histological structure as regards its ectoderm. Toward 

 the lower termination the latter becomes slightly thinner intermesenterially. and passes 

 backward for a short distance up the endodermal surface. Where the mesenteries are attached 

 it extends outwardly along both faces, and then appears to be continued downward as the 

 mesenterial filament. In retracted polyps the ectodermal layer is often folded vertically in a 

 very regular manner (tig. •!*•). 



In practically all the polyps examined only six pairs of mesenteries occur, and of these 

 four pairs are usually complete in the upper region, while the two remaining pairs never reach 

 the stomodseum, and extend for only a short distance below the stomodseal region. The sulcular 

 or dorsal directives are sometimes free throughout, and in other cases generally cease their 

 connection with the stomodsaum in advance of the other three pairs (tig. 30), which always 

 remain attached as far as the aboral termination. The three pairs of complete mesenteries 

 alone bear mesenterial filaments, and continue their course some distance below the others. 

 Toward their lower extremity the first pair of mesenteries may become slightly convoluted, 

 but the mesenteries as a whole have a comparatively short vertical extent, being practically 

 limited to the upper half or third of the polyp. 



The retractor muscles of the mesenteries are moderately well developed on small mesoglceal 

 plaitings, and readily permit of the paired arrangement being established. The form of the 

 plaitings varies in different mesenteries, and even in different parts of the same transverse 

 section, as shown in tig. 38. The mesenterial epithelium is distinguished by the predominence of 

 large, clear, gland cells; these constitute in places nearly the whole thickness of the endoderm. 

 while toward the free margin accumulations of protoplasm, nuclei, and zooxanthelhe are to be 

 found. In a portion of one colony the contents of the glands had evidently been discharged into 

 the gastro-ccelomic cavity just before the death of the polyp, for, on being stained with hema- 

 toxylin -and sectionized. the whole of the upper region of the polyps appeared as if embedded in 

 a gelatinous mass, the secretion itself staining feebly. On preservation the polyps exude a 

 large amount of mucus. Zooxanthella? and yellow pigment cells, and an occasional large oval 

 nematocyst, also occur in the mesenterial endoderm. The mesogloea i- very narrow and homo- 

 geneous, but forms slender outgrowths for the support of the musculature. 



The mesenteries in transverse sections are sometimes observed to be free al their peripheral 

 end for a short distance vertically, and rounded off (tig. : J >*). This is found opposite a radial 

 canal, so that where these outgrowths are formed mesenterially. the mesentery is without of 

 any peripheral support. 



Immediately below the stomodseal region, the mesenterial filaments on the first three 

 developmental pairs are merely incipient (tie-. 29), and never get beyond this stage except in the 

 first and second pairs of mesenteries. In these the filament becomes more or less definitely 

 rounded off from the mesentery for a very short distance, and the mesogloea bifurcates within 

 it (fig 39). Comparatively few nematocysts occur in the filaments, and are all of a medium size. 



The skeletotrophic tissues comprise by far the greater proportion of the soft parts of a 

 colony, lining the walls of the corallite and the whole of the canal system, while the mesenterial 

 tissues scarcely extend beyond the upper half of the polyps (PI. V. fig. 40). The endoderm is a 

 comparatively broad layer, and below undergoes certain modifications; no cellular distinctions 

 can be made out. but the whole layer, with the exception of the free margin, exhibits a vacuolated 

 condition. Fine granules along the walls of the nieshwork stain with aniline blue, but not 

 readilv with carmine; occasionally a yellow pigment cell or a zooxanthella may also occur. 

 Alone- a narrow zone, at the margin of the layer, nuclei, zooxanthella?, pigment granules, and 



