ADULT COLONY. 21 



two lateral walls wliicli line the adjacent faces of two contiguous septa, and 

 inclose between them a narrow chamber, broken up by the synapticular 

 perforations in such a manner as to present a complex canal-like condition 

 (plate 6, fig. 34, and plate 7, figs. 38, 39). The chambers represent the inter, 

 septal locnli less the thickness of the lining wall, but will, however, be spoken 

 of as interseptal spaces. The upper half of each contains a single mesentery, 

 but below they are empty. Moreover, only at the uppermost extremity of 

 the polyp is the mesentery found to extend outwardly as far as the peripheral 

 boundary. In the upper region of the polyp the inner border of each lamella 

 is open so that the chamber, along with all the others, communicates with the 

 central cavity of the polyp, while below all the chambers are closed centrally, 

 or are only in communication toward their centripetal edges (plate 7, figs. 

 38, 39). Where the chambers are wholly cut oflF from one another the 

 calcareous septa extend throughout the radial length of the polyp, and 

 are centrally united with others or with the columella (plate 7, fig. 39) ; 

 when the chambers are only partly separated, communicating towards the 

 center, then the septa between them do not extend as far as the middle of 

 the calice. 



ANATOMY AND HISTOLOGY. 

 COLUMN WALL AND DISC. 



In radial sections of retracted polyps the column wall and disc appear 

 as a continuous layer, and, histologically, the two are nearly alike through- 

 out. The wall as a whole is very thin, except where the knob of a tentacle 

 is included, when the ectoderm becomes thickened (plate 7, fig. 40, and plate 

 9) fig- 53); to a less degree the layer is also thickened along the line of 

 attachment of the mesenteries. In sections the three ccelenterate layers, 

 ectoderm, mesogloea, and endoderm, together, measure only about 0.07 

 mm. in thickness. 



The chief constituents of the ectoderm of the column wall are support- 

 ing cells and clear mucous gland cells, while small nematocysts occur some- 

 what sparsely. The unicellular gland cells are most conspicuous toward the 

 periphery of the layer, where they appear oval, with clear or vesicular con- 

 tents. The nuclei of the various cells are oval, and arranged as a whole 

 within the inner half of the ectoderm, a few, more circular in form, being 

 found near the mesogloea. There is no trace of any ectodermal musculature 

 on the column, but a system of weak fibrils is developed over the disc, their 

 direction being radial. No evidence of external ciliation can be detected 

 even in the best preserved examples. 



The mesogloea in the column, as elsewhere throughout the polyp, is 



