MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 439 



the deeper water on the reefs this pigmentation is wholly wanting, and the tissues are of the 

 more prevalent yellowish brown eolor. produced by the endodermal zooxanthelias. 



The supertieiiil or ectodermal coloration of corals varies greatly within the same species, 

 especiall}' in different areas, and is of small importance for purposes of specific determination. 



The polyps of Dichocania stol'esi exhibit some exceptional conditions of coloration as regards 

 the internal tissues. Within the ectoderm cells of the column wall are found highly refractive 

 granules distributed throughout the layer with approximate uniformity. There is no evidence 

 of the granules being aggregated within limited groups, nor of concentration toward the deeper 

 regions of the layer. They are colorless in preserved material, but are green in the living polyp, 

 and are no doubt the chief cause of the green color of the polj'ps, and the general opacity 

 presented by the external tissues. On decalcitication of preserved material the lower two-thirds 

 of the embedded polypal tissues is also of a dark green color, contrasting strongly with the 

 upper colorless walls. Microscopic examination reveals that, as usual, the lower .skeletotrophic 

 endoderm is greatlj' thickened, and densely crowded with granules of various sizes. Most of the 

 particles, however, are a bright green, exactly recalling the chlorophyll granules in plants; in 

 decalcitied mounted sections the green color is still intense after two years. The granules in the 

 gland cells of the lower region of the mesenterial filaments are also a strong green, and even the 

 large nematocysts in the filaments have a green tinge. 



Another form of pigment cell is found in the tissues of Porites. Some colonies of Pm'ites 

 clavaria are an intense bright yellow in their living condition, and a lemon color is often met 

 with in living colonies of P. astrxotdes, while the polyps of P. dtvaricata as a rule exhibit only 

 the pale brown due to the internal zooxanthella?. Macerations and sections of polyps of the two 

 first-mentioned species reveal that both the ectoderm and the endoderm are loaded with bright 

 yellow, pigment-bearing cells, very variable in form and dimensions. So abundant are they in 

 P. clavaria that in i^laces it is almost impossible to make out any of the other histological 

 elements; but in P. astrxokhn they are more sparsely distributed, and their relationships 

 can be better determined. In general the chromophore cells are shorter than the supporting cells 

 and gland cells of the ectoderm, and occur at all heights within the layer. 



AVhen the ectoderm is macerated the pigment cells separate somewhat readily from the 

 other histological elements, and are very protean in shape, scarcely any two being of the same 

 form. A few are represented on PI. IV, fig. 37. The contents are very finely granular, and are 

 scarcely affected by stains, but with borax carmine a very distinct nucleus becomes evident. 



The chromophore cells occur most abundantly in the outer tissues, and in both ectoderm and 

 endoderm, but are also met with sparingly throughout the polyp, including the epithelium of the 

 mesenteries and communicating canals. ZooxanthelUe are found within the endodermal cells in 

 their usual numbers. The polyps of the genus Pontes vary in color more than any other form 

 examined. 



Another factor which probably inlluences the living appearance of many corals is the color 

 given to the skeleton by the presence of perforating filamentous algfe, belonging to both the 

 green and red groups. The coralla of all the species examined are found to l)e infested with 

 boring algie. After decalcification the filaments appear fresh and green near the surface, and 

 contain protoplasm and chlorophyll granules, but are colorless and apparently dead in the more 

 iivternal ivoions. So dense is the foreign growth that in some instances the superficial portion of 

 the corallum is rendered bright green or pink by its presence, either Mholly or in part. The 

 skeletal color is best seen on freshly macerated specimens, as after a few months' exposure the 

 coralla become more or less completely bleached. The upper superficial areas of the fi'esh coral- 

 lites of Orhicdiu and extracalicular regions of Colj^ioplnjllia presented green patches of various 

 sizes, while the color was usually more uniform in Agaricia, but almost any species of coral may 

 exhibit lai'ge or small affected areas. Decalcification also proves that the perforating algre may 

 be present where a superficial examination of the corallum gives no indication. 



By way of contrast with the more prevalent green color, the coi-alla of hophyllia dipsacea 

 after maceration of the soft tissues were a delicate pink, evidently from the presence of some red 

 alga. This also gradually disappeared in the course of two or three months. Siderastrsea siderea 



