438 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



are densely colored, arc found to be quite pale when living- in the shady places often selected by 

 these forms. The presence of zooxantbellse does not seem to be at all essential to the life of 

 coral polyps, seeing that colorless individuals in the shade flourish apparently as well as those in 

 fully exposed places. 



The degree of aggregation of the yellow cells likewise determines the intensity of the 

 coloration. The tissues of fully expanded polyps are generally lighter in color and more 

 transparent, and under these conditions zooxanthelhe are proportionately less numerous in a 

 given area than during the retracted condition. When the polyps retract strongly, the alga? 

 become closely aggregated within the thicker endoderm, and the tissues are darker and less 

 transparent. 



During the early stages of maceration of corals such as Madrepora, within a white porcelain 

 basin, the zooxanthellae will separate freely from the tissues, and accumulate on the bottom of the 

 dish, giving to it a distinctly yellow appearance. 



When coral polyps are preserved in alcohol a brownish yellow or golden yellow pigment 

 is first extracted, but after a few hours, if transferred to colorless alcohol, tin 1 liquid is colored 

 in the same manner as by the green coloring matter of plants. 



The column wall and disk of the species of FsophyUia, Mseandrina, and less so of Manicina, 

 ( blpophyllia, and Orbicella, are often characterized by the presence of small, superficial, opaque, 

 granular spots and patches. These are recognizable by means of a lens on the living polyps, and 

 persist for some time after preservation. When the polyps are retracted the exposed tissues are 

 practically opaque, and seem dense in comparison with the nearly transparent walls of such forms 

 as Wadrepora, Oculina, or Cladocora. The opacity is. however, limited to the regions of the 

 column wall and disk which are fully exposed during retraction: the infolding margin of the 

 wall, the tentacles, and the peripheral region of the disk, which are non-exposed regions, seem 

 thinner and are more transparent. Any other marked superficial colors which may he present 

 in these genera are likewise practically restricted to the exposed areas. 



Examination of the outer tissues of these genera, by means of sections and macerations. 

 reveals the presence of much finely granular, colored, non-transparent matter. The granular 

 matter is mostly concentrated in the deeper regions of the ectodermal layer, hut at certain points 

 extends throughout the thickness, strongly distinguishing the areas from the remainder of the 

 layer. Occasionally, as represented in the section of the column wall of Isophyllia (PI. XVII, 

 tig. 122), the accumulation occurs at fairly regular intervals. On the other hand, some regions, 

 such as the upper margin of the column wall, the tentacles, and the periphery of the disk, are 

 without the granular cells, these being the more transparent areas in the living polyp. 



The small dense spots, referred to above as seen on the living tissues, evidently represent 

 the points at which the granular matter is most concentrated, so as to extend throughout 

 the thickness of the ectoderm, while the general opacity of the tissues is due to its more 

 diffuse, distribution within the deeper parts of the layer. 



On maceration the contents of the ((disappear as a finely granular substance, usually yellowish 

 in color, and unacted upon, or only very slightly, by stains and acids. The cells thus differ from 

 the more usual granular gland cells of the Zoantharia, which are alwavs best developed toward the 

 tree surface of the layers, and take up stains, such as hematoxylin and carmine, with great avidity. 



It may be conceived that the opacity has for its function the regulation of the amount 

 of light passing to the endodermal tissues in corals living in more shallow waters. Certainly the 

 forms in which the granular cells are best developed are among the least active of the coral 

 polyps, and the coloration is most dense, over the exposed areas. The endoderm shows no 

 peculiarities which can be connected with the ectodermal opacity; zooxanthelhe occur as 

 numerously as in other species. 



Manicina iircohdtt affords some interesting variations in the presence or absence of its dense 

 superficial coloration, which seem to indicate some connection with the depth at which the corals 

 occur. In colonies inhabiting very shallow waters, such as those of Kingston Harbor and 

 Bluefields Bay, the superficial pigmentation — yellow, brown, or dense opaque white — is strongly 

 pronounced, and in small or large irregular patches; but in young polyps and large colonies from 



