MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 437 



never so pronounced as in the larger Actinian polyps. The histological ditferentiation where 

 ridges and grooves are strongly developed has l)een alluded to above. 



The ectoderm of the stoinodivuni terminates nicsentei'ially in direct continuity with the 

 mesenterial lilaments of the complete mesenteries, and for some di.stance the histological details 

 of the two arc alike. The layer may be also partly reflected on the endodermal surface, and 

 continued a short distance along the edge and both faces of the mesenteries (PI. VI, fig. 51). 

 This reflection of the stomoda?al ectoderm plays an important part in discussions of the origin 

 of the mesenterial filaments, and is again referred to on page 477. 



The stomodieal mesoglo?a is usually thin, and uniform in chai'acter all round. As a rule the 

 endoderm presents no features which distinguish it from the same layer covering the whole of 

 the upper part of the internal cavity. 



COLORATION. 



All descriptions of coral reefs allude to the great variety, richness, and beauty of color of 

 the living coral. On any coral patch around Jamaica, the predominating colors are difterent 

 shades of brown — light, dark, yellow, or green. This is largely due to the great abundance of 

 colonies of Mudrepora, MUlepora^ and Meenndrlna, all of which exhibit one or other of these 

 brown tints. Adding variety to these ai-e the rich yellows, greens, and blues of the different 

 species of Poritts. Of the less massive corals — Sole7iastrxa, Cladocom, Oculina, and Favia — 

 yellowish-brown is likewise the prevailing color. Even where the general coloration of the 

 colonies is nearly black or steel gray, as in some species of OrhieeUa and Tsoj>hyIUu, a closer 

 examination, especially when the polvps are fulh^ expanded, indicates that yellowish-brown is 

 the fundamental color. 



The prevalence of the yellow-brown color is easily understood when an examination is made 

 of the polypal tissues. For in all instances in which it occurs, the endoderm is found to be more 

 or less crowded with the so-called "yellow-cells"' or zooxanthellie, which are unicellular 

 symbiotic alga?, the chromophores of which are yellow or yellowish-green. That these are the 

 main cause of the external coloration may be readily proved from colonies of Madrepora. 

 In this genus the polyps toward the apex of growing branches are nearly colorless, the white 

 skeleton showing through the perfectly transparent tissues, and on a microscopic examination 

 of the endodermal layer zooxunthelhe are found to be absent, while they are present in abun- 

 dance in the endoderm of the older, strongly jiiguiented regions. When a contracted living 

 tentacle is viewed under the microscope, the margin is quite colorless as far as the thickness of 

 ectoderm, M'hile on focussing within, the endoderm is found to be almost black and opaque. The 

 interior of fully expanded tentacles is lighter, and the individual j'ellow cells can be seen. 



In the few instances in which zooxanthellaj are nearly or wholly absent from a species, 

 as in AxtraiH/ia solitnrht and PlujlUtngia (imericana, the polypal tissues appear peculiar 

 delicate, and are wholly colorless and transparent, except for the occurrence of delicate 

 superficial colors, such as rose and green. The transparency of the tissue appears to l)e very 

 general in the members of this grouj) of corals. In th(> "■ Introductorj' Notice" of the anatomy 

 of Astra iigid duna', Fewkes (1SS9) also descril)cs the color of the expanded poh'ps as ■" white, 

 almost transparent, resembling an Edwardsia or small white Actinia; when contracted th(> color 

 shows a gieen or bluish tinge." An examination which I have made of the tissues of this species 

 reveals an al)s(Mice of zooxanthellie. 



Again, the polyps on the under. un('xpt)sed surface of colonics living in shady jilaces 

 are nearly always devoid of color, although the individuals on the exposed area of the 

 same colon}' are deeply pigmented. A remarkable instance of this occurs on the piles supi)orting 

 the broad wharves at Port Royal. Numerous clumps of the corals Ood/'/ni and Cladocom grow 

 attached to the ])iles; the outer exposed colonies are of th(^ usual brown color, while those living 

 on the inner i)illai-s, which are cut off from the strong sunlight, are perfectly white, the corallum 

 alone showing through the transparent tissues. It is manifest that a chloi-ophyll-bearing alga 

 could not flourish under conditions where it is more or less deprived of light; but except for 

 this absence of coloration the coral polyps appear normal. Colonies of Agur'uiia, which usually 



