MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 565 



The ectoderm of the tentacles is somewhat broader than thai of the column wall, and the 

 apical region is crowded with narrow nematocysts, showing distinctly the internal spiral thread. 

 Among these are a few larger cysts, which stand out very prominently on account of the highly 



refractive character of the wall. axis, and spiral thread; usually one or i e lateral batteries 



are also seen in sections. The longitudinal ectodermal and circular endodermal musculatures are 

 both clearly determinable, and outside the former a nerve layer is sometimes displayed. 



Pigment granules occur very sparingly in the ectoderm of the disk, and in the living polyps 

 this area is usually more transparent than the column wall; clear mucous eland cells are 

 numerous, and nematocysts are developed here and there. A very delicate ectodermal and 

 endodermal musculature is present, and the external filiation is sometimes persistent in preserved 

 material. The mesogloea is a rather broad layer, and numerous zooxanthellse are present in the 

 endoderm. In strongly retracted specimens the discal ectoderm is intumed for a short distance 

 within the stomocheal cavity (tie-. t'>7). 



The stomodseum extends hut a short distance vertically, and at its lower termination is 

 folded outwardly and upwardly, so that in transverse sections it may appeal- twice, the two 

 being a wide distance apart. Its ectoderm comprises at least two kinds of narrow uematocysts, as 

 well as a few examples of a large, oval, thin-walled variety. Different stages in the development 

 of the latter are easily recognized by the deeply-staining character of both the cyst wall and the 

 contents. Large, oval or spheroidal, unicellular gland cells occur, charged with large, colorless 

 or slightly yellow, spheroidal granules, which do not stain in hematoxylin or borax carmine: 

 they are prominent features in ;>ll the internal tissues, both mesenterial and skeletotrophic. In 

 addition to these are numerous gland cells with finer granular contents which stain intensely. 



Owing to the shortness of the stomodffium, the backwardly folded condition of its walls in 

 retracted polyps, and the great number of mesenterial prolongations and septal invaginations, 

 the arrangement of the mesenteries can be determined only with difficulty. Two polyps section- 

 ized transversely contained twelve pairs of mesenteries, of which six pairs are complete and six 

 incomplete; among the former, two pairs, situated at opposite extremities, are directives. In 

 retracted polyps, where the column wall becomes overdrawn within the caliee, transverse sections 

 pass through the latter twice, and all the mesenteries extend from one wall to the other. The 

 imperfect mesenteries stretch for some distance centrally, but are never convoluted like the 

 complete members. Fully developed filaments are borne only by the mesenteries of the first 

 order, but at the free edge of the others a small number of cells with deeply-staining nuclei occur, 

 very readily distinguished from the undifferentiated mesenterial epithelium, though passing 

 gradually into it. From its close resemblance to the mesenterial filaments in the early larval 

 stages the tissue manifestly represents as an incipient filament. 



The mesenterial epithelium is a broad layer, characterized by an abundance of gland cells 

 with large granules (tig. 72); many clear gland cells also occur, and nematocysts rather sparsely. 

 The mesoghea is thickly developed in retracted polyps, and the face on which the retractor muscle 

 occurs exhibits rounded folds or narrow plaits for giving additional surface to the musculature. 

 The foldings are somewhat irregularly disposed, and scarcely alike on any two mesenteries, and 

 are more strongly developed in some regions than in others (tig. 68). The oblique musculature 

 on the opposite face of the mesentery is distinct in the upper regions of the polyp, and where 

 the mesenteries shorten below, the libers are nearly the same, in direction all the way round. 



The perfect mesenteries are larger and become much convoluted in the middle region of the 

 poly]), nearly tilling the gastro-ecelomic cavity, but the proximal region of the polyp is altogether 

 devoid of mesenteries. In vertical sections the organs are seen to be restricted to the upper 

 half or two-thirds of the polyp (tig. 67). 



In the upper region the perithecal portion of the mesenteries extends wholly across the 

 space from the column wall to the skeletotrophic tissue; but toward the lower termination some 

 become free from the column wall, remaining attached only to the skeletal lining. The muscu- 

 lature is about equally developed on each face in the lower regions, and each set of fibrils 

 extends in the same direction, the mesogloea remaining nearly smooth; above, the face bearing 

 the retractor muscle is slightly plaited. The endoderm as a rule is much swollen toward the 

 insertion of the mesentery on the column wall. 



