MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 415 



gui.shable as a separate layer. The walls are then much more iieai-ly transparent than in the 

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

 dijjsacea to 0.023 millimeter in A(jarlcia fragllix. 



The three poh'pal layers will now be described in more detail. 



ECTODERM. 



The ectoderm of the column of Madreporarian polyp.s is a regular, often ciliated, columnar 

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

 nematocyst-bearing cells; muscle and nerve tibrils are rarely' if ever recognizalile in sections. 

 The nuclei of most of the cells are arranged at nearl)' the same height in the layer, and in .sectionij 

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

 distril)uted are mainly those of the long narrow supporting cells; the nuclei of the gland cells 

 and nematoblasts are less restricted and occur nearer the mesoglcEa. 



The ciliation of the column wall is bj' no means so pronounced as in the case of the stomodaeal 

 ectoderm and mesenterial iilaments, and few observations hive been made to determine its 

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

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

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

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

 merely dropping over, \hey 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 dro})ped 

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

 no such action could be distinguished on the living tissues of Favlafragmn. 



The glandular cells of the columnar ectoderm are mainlj' oval shaped toward the periphery 

 of the layer, and nai;row internally; the base is generall}' fibrillar and rests upon the mesogloea 

 (fig. 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 clear mucus cells, long, narrow gland cells 

 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 gland cells, and, as a rule, are but 

 sparsely represented. 



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

 in tangential sections through the outer portions of the ectoderm the cells foi-m a close 

 polygonal network, the interstices t)eing occupied by a few supporting cells (PI. X. tigs. 7(i-7S). 

 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 sutficient mucus may be extruded to give a jelly-like consistency to the liquid. The 

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

 of the polyps. This is especially the case with Porites^ where both the ectoderm and endoderm 

 are highlj- glandular (PI. IV). 



As a rule the column wall of coral polyps contains a few scattered nematocysts, ^vhich, 

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

 are alwaj-s small, of two or three kinds, and are easily distinguished from the long, narrow, 

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



In the genera IxopliylJhi and MiriUKh'iiia, and to a less degree in certain others, the 

 superficial tissues 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 the contents of the ectodermal cells. This is illustiatcd 

 bj' the genus OrhiceUa (PI. VIII, fig. 65). Clear nuicus-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 apjieiir to 



