MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 437 



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

 ridges ami grooves arc strongly developed lias been alluded to above. 



The ectoderm of the stomodreum terminates mesenterially in direct continuity with the 

 mesenterial filaments of the complete mesenteries, and for some distance the histological details 

 of the two are alike. The layer may l>e also partly reflected on the endodermal surface, and 

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

 This reflection of the stomodseal ectoderm plays an important part in discussions of the origin 

 of the mesenterial filaments, and is again referred to on page 4-77. 



The stomodseal mesoglcea is usually thin, and uniform in character 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 different 

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

 colonies of Madrepora, Millepora, and Maeandrma, all of which exhibit one or other of these 

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

 species of Porites. Of the less massive corals — Solenastrsea, ( 'ladocora, Oculma, 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 Orbicella and Tsophyllia, a closer 

 examination, especially when the polyps are fully 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 zooxanthellse, which are unicellular 

 symbiotic alga 3 , 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 zooxanthellse are found to be absent, while they are present in abun- 

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

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

 ectoderm, while on focussing- within, the endoderm is found to be almost black and opaque. The 

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



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

 as in Astra/ngia solitaria and PhyUangia americana, 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 be very 

 general in the members of this group of corals. In the "Introductory Notice" of the anatomy 

 of Astrangia dame, Fewkes (1889) also describes the color of the expanded polyps as " white, 

 almost transparent, resembling an Edwardsia or small white Actinia: when contracted the color 

 shows a green or bluish tinge.' 1 An examination which I have made of the tissues of this species 

 reveals an absence of zooxanthellse. 



Again, the polyps on the under, unexposed surface of colonies living in shady places 

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

 same colony are deeply pigmented. A remarkable instance of this occurs on the piles supporting 

 the broad wharves at Port Royal. Numerous clumps of the corals Oculina and Gladocora grow 

 attached to the piles: the outer exposed colonies are of the usual brown color, while those living 

 on the inner pillars, which are cut oil' from the strong sunlight, are perfectly white, the corallum 

 alone showing through the transparent tissues. It is manifest that a chlorophyll-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 Agaricia, which usually 



