584 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



protoplasm of the endodermal cells are more obvious peripherally. Only the faintest indication 

 of an endodermal musculature occurs, even in the most distal region of the column. 



The tentacles are seen in transverse and vertical sections as simple outgrowths of the margin 

 of the disk, and a wide lumen remains in the partly expanded condition (tig. 139). The cells 

 containing- pigment granules are here much less numerous than in the column wall, and are mainly 

 restricted to the proximal region. The ectoderm is broad; and long, narrow nematocysts occur 

 in patches along the walls and at the apex, rendered very obvious by the internal spiral thread. 

 The tentacular ectodermal and endodermal musculatures are moderately developed, and toward 

 the apex a very distinct ectodermal nerve layer occurs. 



The disk presents no histological features distinguishing it from the column wall, except 

 that the endodermal musculature is somewhat better devekmed, and pigment granules are more 

 numerous in the middle regions than toward the periphery. 



The jtomodseum is remarkable for the prominence of the vertical ridges. In transverse 

 sections they stand out as very definite rounded projections of the wall, opposite the insertion 

 of the mesenteries, and histologically they differ somewhat from the intervening intermesenterial 

 depressions (tig. 147). The mesogloea at the base of the ridges is a little swollen, and sends 

 processes among the ectodermal cells. The latter are mainly long, ciliated, supporting cells, the 

 nuclei of which form a deeply-staining zone. In the deeper parts of the ridges are found numbers 

 of pigmented jranular cells; large oval nematocysts with a spiral thread, along with a second 

 much smaller form, occur peripherally, along with granular gland cells. The ectoderm and 

 mesogloea of the grooves are narrow, the former containing but few granular cells and nema- 

 tocysts; the ciliation is uniform all round the stomochcum, or may be somewhat stronger in the 

 grooves than in the ridges. A muscular layer of the weakest character can be distinguished on 

 the endodermal surface of the mesogloea. In partly retracted specimens the lower portion of 

 the stomodauun extends horizontally for some distance between the mesenteries, while as 

 these latter cease their connection they become tipped with a tissue resembling the ectoderm 

 of the stomodseal ridges, and directly continuous with the mesenterial filaments. 



The mesenteries are arranged in unilateral pairs throughout, but vary much in size. By far 

 the majority of the pairs reach the stomocheum, but incomplete pairs occur here and there, some 

 large and some small; these will evidently in time also become complete. In the several 

 stomodseal systems represented in fig. 141 all the pairs were complete, the number of mesenterial 

 pairs inserted on each mesentery being variable. The separation between one polvpal system 

 and another is always in the entoccelic plane, on each side, as already described in the section on 

 fission (p. 513). 



In transverse sections through the soft tissues covering the most distal part of the calicular 

 ridges the mesenteries on opposite sides of adjacent polypal systems may or may not correspond 

 with each other (tig. 138). They are arranged at practically equal distances apart, so that the 

 entoccelic and exocadic chambers are about equal. In tin 1 upper region the septal invaginations 

 are both entoccelic and exoceelic, but occasionally the latter invaginations are wanting; in the lower 

 part of the polyp only entoccelic ingrowths occur (tig. 142). In the corallum it is seen that the 

 small exosepta have a corresponding short vertical range. As shown in tig. 138, the edge of the 

 mesenteries after leaving the column wall has a free course before becoming adherent to the 

 skeleton; some of the mesenteries, as toward the right end of the section, are becoming 

 attached to the skeletotrophic tissues while others are yet free. The boundary groove of two 

 column walls is therefore not attached directly to the skeletotrophic tissues, but through the 

 intermediation of the mesenteries. In tig. 139 all the mesenteries are connected with the skeleton. 



The mesenterial mesogloea is comparatively well developed, and on the entoccelic face is finely 

 plaited to afford additional support to the musculature, while the exoceelic surface is smooth; 

 here, as elsewhere in the mesogloea, included connective tissue cells are common. The muscular 

 fibrils are very delicate, and in the upper region extend nearly in the same direction on each 

 face. Among the many mesenterial pairs passed in review no directives have been observed. 



The mesenterial epithelium is crowded with clear gland cells, and zooxanthelhe are plentiful. 

 In the lower region of the polyps certain of the mesenteries become greatly convoluted; the. 



