MEMOIRS OF THE NATIONAL A.CADEMY OF SCIENCES. 415 



guishable as a separate layer. The walls arc then much more nearly transparent than in the 

 retracted state. In sections the column wall varies from 0.1 millimeter across in Tsophyllia 

 dipsacea to 0.023 millimeter in Agaricia fragilis. 



The three polypal layers will now be described in more detail. 



ECTODERM. 



The ectoderm of the column of Madreporarian polyps is a regular, often ciliated, columnar 

 epithelium, constituted mainly of unicellular eland cells, supporting cells, and scattered 

 nematocyst-bearing cell--: muscle and nerve fibrils are rarely it' ever recognizable in sections. 

 The nuclei of most of the cells are arranged at nearly the same height in the layer, ami in sections 

 of moderate thickness give rise to a very definite nuclear band or zone. The nuclei thus regularly 

 distributed are mainly those of the lone- narrow supporting cells; the nuclei of the eland cells 

 and nematolilasts are less restricted and occur nearer the mesoglcea. 



The ciliation of the column wall is by no means so pronounced as in the case of the stomodseal 

 ectoderm and mesenterial filaments, and few observations have been made to determine its 

 general distribution in the living polyp, or the conditions of its activity. Traces of cilia somo- 

 times remain in preserved material, and the effects of its activity are often noticeable on tin 1 living 

 polyp. When light particles of foreign matter are dropped on the large discal area of a coral 

 like Mimicina, they are seen to be slowly transferred to the margin of the disk, but. instead of 

 merely dropping over, they are dragged in a definite manner along the column, and only 

 discarded, as it were, when they reach its lower termination. When similar particles are dropped 

 on other living polyps they are likewise set in movement in a more or less definite manner, but 

 no such action could lie distinguished on the living tissues of Fa/oia fragum. 



The glandular cells of tin 1 columnar ectoderm are mainly oval shaped toward the periphery 

 of the layer, and narrow internally: the base is generally fibrillar and rests upon the mesoglcea 

 (tie-. 8). The contents are nearly homogeneous and rarely stain, usually appearing quite clear; 

 at other times they are finely granular and stain more readily. The cells are mucus secreting. 

 and. their different behavior toward reagents probably indicates different stages in the develop- 

 ment of the cell and its secretions. In addition to ths (dear mucus cells, lone-, narrow gland cell-. 

 occur of which the contents are coarsely granular, and these take up most stains with great 

 avidity. They seem to be different in character from the other eland cells, and. as a rule, are but 

 sparsely represented. 



In most cases the eland cells occupy the greater proportion of the layer, so much so that 

 in tangential sections through the outer portions of the ectoderm the cells form a close 

 polygonal network, the interstices being occupied by a few supporting cells (PI. X. figs. 76-78). 

 Quantities of clear, colorless mucus are given out by most corals upon disturbance, as. for instance, 

 when a fragment from a large colony is broken off; also upon preservation in a limited quantity 

 of sea water sufficient mucus may be extruded to give a jelly-like consistency to the liquid. The 

 presence of the mucus upon tin' surface of a colony often interferes with the proper preservation 

 of the polyps. This i^ especially the case with Pontes, where both the ectoderm and endoderm 

 are highly glandular (PI. IV). 



A- a rule the column wall of coral polyps contains a few scattered nematocysts, which, 

 however, are never aggregated into distinct batteries such as occur on the tentacles. They 

 are always small, of two or three kinds, and are easily distinguished from the lone-, narrow, 

 tentacular form, or the large oval variety more characteristic of the endoderm. 



In the genera Isophyllia and Maxindrina, and to a less degree in certain others, the 

 superficial tis.-ues in the living condition appear dense and almost opaque. Histological examina- 

 tion reveals that the mesoglcea of the column wall in these is a little thicker than usual, but the 

 chief cause of the opacity evidently lies in tin;' contents of the ectodermal cells. This is illustrated 

 by the genus Orbicella (PI. VIH, fig. 65). ('lear mucus-secreting cells occur with comparative 

 rarity, and the chief cellular constituents of the layer are long supporting cells, the nuclei of 

 which are elongated and arranged in a very regular zone, so closely that in places they appear to 



