584 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



protopla.sm of the endo(l(>riiuil colls are move obvious jieriphei'ally. Only the faintest indication 

 of iin 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 maroin 

 of the disk, and a wide Uimen remains in the partly exi)anded condition (tig. !;>!>). 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 bi-oad; and long, narrow nematocvsts occur 

 in patches along the walls and at tlie apex, rendered very obvious by the internal s])iral 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 cohunn wall, except 

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

 numerous in the middle regions than toward the periphery. 



Th(> itomodivum is remarkable for the prominence of the vertical I'idges. In transverse 

 sections the}' stand out as very dehnite rounded projections of the wall, opposite the insertion 

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

 depressions (tig. 147). The mesoghea 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 j'-aiui'ar cells; large oval nematocvsts with a spiral thread, along with a second 

 nuich 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- 

 tocvsts; the ciliation is uniform all round the stomoda'um, 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 stomodiVHim 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 stomoda?al ridges, and directly continuous with the mesenterial filaments. 



The mesenteries are arranged in iniilat(>ral pairs throughout, but vary iiuich in size. By far 

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

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

 stomoiheal systems represented in fig. l-tl all the pairs were complete. th(^ number of mesenterial 

 pairs inserted on each mesentery being xariable. The separation between one polypal system 

 and another is always in the entocoelic plane on each side, as ali'eady described in the section on 

 fission (]). 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 (fig. ISS). They are arranged at practically etjual distances aj)art, so that the 

 entoccelic and exocojlic '■hauibers are about equal. In the upper region the septal invaginations 

 are both entocoelic and exoccelic. but occasionally the latter invaginations are wanting; in the lower 

 part of the polyp only entoccelic iiigi'owths occur (fig. 142). In the corallum it is seen that the 

 small exosepta have a corresponding short vertical range. As shown in fig. 13S, the edge of the 

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

 skeleton; some of the mesiMiteries, 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 skeletoti'ophic tissues, but through the 

 intermediation of the mesenteries. In fig. 1?>'.} all the mesenteries are connected with the skeleton. 



The mesenterial mesoglcea is comparatively well developed, and on the I'litoccelic face is finely 

 plaited to afi'ord additional supj)ort to the nuisculature. while the exoccelic surface is smooth; 

 here, as elsewhere in the mesoghea, included connective tissue cells are conunon. The muscidar 

 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 dii'ectives have been observed. 



The mesenteritil epithelium is crowded with clear gland cells, and zooxanthelliv aiv plentiful. 

 In the lower region of the [jolyps certain of the mesenteries become greatly convoluted; the 



