MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 547 



arising from the former. No intervening discal tissue occurs between the origin of one tentacle 

 and that of tlic next, as is the case in the axial polyp- In the particular section represented the 

 larger tentacles are practically all of the same size, but in other sections the anterior tentacle 

 predominates over the others, or at some levels may even be the only one represented; the 

 smaller tentacles, in their varying dimensions, correspond with their proportions in the living 

 polyps* 



The tentacular ectoderm (fig. 10) is a uniformly broad layer, and in sections is roughly 

 divisible into three zones: (1) An outer, nearly colorless zone, with numerous nematocysts, ."(.id 

 very tew gland cells; (•_') a middle, deeply-staining nuclear zone; and ('■'•) an inner, less defined 

 fibrillar zone, terminating in a layer of delicate muscle fibrils, longitudinally arranged. Conical 

 cnidocils occur, seen especially in the living tentacle. The nematocysts are of the same form as 

 in tic coenosarc, hut the long variety with a thin wall and strongly marked spiral thread is most 

 abundant. The nematocysts are distributed nearly uniformly throughout the length of the 

 tentacular wall, not restricted to special batteries as in most corals; proximally they are some- 

 what less numerous than above. In some sections a distinct nerve layer is manifest, situated a 

 little distance from the mesoglcea. 



The mesoghea is a thin supporting lamella, slightly thickened proximally. It may be partly 

 folded in retracted specimens, but nowhere becomes plaited for the purpose of affording additional 

 support to the musculature. 



The endoderm exhibits marked variations, as in the case of the column wall. Where 

 zooxanthellse are not present in the cells the layer is very regular, and much thinner than the 

 ectoderm; the cells are tilled with protoplasm which stains slightly, and the nuclei are compara- 

 tively large and arranged in a very regular row. Where symbiotic algae occur the layer as a 

 whole becomes much broader, the cells are more vacuolated, and the internal limitations are very 

 irregular. The endodermal circular musculature is comparatively well developed. 



Sometimes the endoderm of one tentacle will be entirely without zooxanthelhe. while 

 another of the .same polyp will be crowded with them alone- the whole of its length; in other 

 eases they may be absent from the distal region of a tentacle and occur proximally. Corre- 

 sponding variations have been noted in the coloration of the tentacles in the living polyp. 



Below the tentacular region of retracted polyps the disk extends vertically for a short distance, 

 and then nearly horizontally, passing into the vertical stomodseum (tig. 11). The peripheral 

 discal area presents histological details similar to those of the tentacles, but the more central 

 region becomes narrower and nematocysts are rare in the ectoderm, while granular gland cells 

 occur here and there. Zooxanthellse are usually sparsely distributed, or altogether absent from 

 the discal endoderm. 



The six tentacles in the axial polyps are entoccelic in position, and therefore correspond with 

 the larger members of the radial polyps. They are larger than in radial polyps, and in transverse 

 sections appear as triangular ridges of the disk; the longitudinal and circular musculatures are 

 somewhat better developed than in the radial polyps, and the mesoglcea is slightly plaited to 

 afford additional support for the endodermal fibers. The disk also shows the ectodermal 

 musculature very clearly: zooxanthelhe are entirely absent from the endoderm. otherwise the 

 histological details of both the ectoderm and endoderm are the same as those of the radial polyps. 



The stomodseum is narrow and approximately circular in transverse sections, and the walls 

 hang vertically for some distance within the ccelenteron (fig. -2). They are of equal thickness 

 throughout, and present no indications of a gonidial groove. In the figure the wall narrows a 

 little toward its free extremity on the left side and terminates abruptly; on the right side of the 

 section the wall is seen to be in continuity with the filament of a mesentery, and the tissues of 

 the two pass insensibly into one another. 



The surface of the stomodseum is strongly and uniformly ciliated; nematocysts and granular 

 gland cells are numerous, some of the latter staining very deeply. An extremely weak ectodermal 

 musculature and nerve layer can be detected in transverse sections, and circular endodermal 

 muscle fiber- in radial sections. Isolated typical cells are represented in fig. 13. 



Transverse sections through the stomodseal region of both radial and axial polyps reveal 



