MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 575 



Under ordiiKirv conditions the diseal areas are deeply depressed, and flat or si iohtly concave; 

 on full expansion they are raised a ji'reat height above the corallum. The surface appears 

 very coarse, owing to the presence of much granular matter and the verrucse over the septal 

 spines; near the periphery, in the area more or less hidden by the overhanging column wall, 

 the disk seems thinner, and the dense pigmentation is almost wanting. 



Usually the oral apertures appear as narrow slits, about 3 mm. in length, with the long axis 

 along the length of the disk; at other times they are situated at the apex of a conical peristome. 

 Wtien open the mouth is oval or nearly circular in shape, displaying the intense white stomodival 

 walls. The latter are strongly ridged and furrowed, the number of ridges varying in different 

 polyps from twelve to twenty-four: when the mouth is partly opened a sharp line of 

 demarcation exists between the disk and stomodanun. The mouths are about 7 nuii. apart in 

 the living condition, and about 5 mm. in preserved colonies. 



The i)revailing colors are dark green, brown, and yellow, with minute, opacjue white, 

 superficial granules, distriluited practically i«ll over. These latter interfere somewhat with the 

 distinctive characters of the other colors. The j'ellow color predominates along the thecal ridges, 

 and the green along the valleys. In-egular, opaque white, cream, or green patches are sometimes 

 present on the disk, ending in streaks toward the periphery — that is, in the region covered bv 

 the overfolding coluum wall." 



On irritation numerous prolongations of the mesenteries and filaments are extruded through 

 various reu'ions of the bodv wall; sometimes the greater part of the colony will be thus covered, 

 presenting a very Iteraggled appearance. On withdrawal, the apertures through which the 

 mesenteries protruded may be so large as to be visible with the aid of a lens, and remain open for 

 .some time; afterwards they close and leave no external evidence of their former presence. The 

 thin transparent mesentery can be easily distinguished from the dense white filament in any 

 protruded portion, and the former is often greenish in color. 



Anatomy (ind hiatology. — The ectoderm of the superficial bod}- wall is remarkable for the 

 abundance of a linelv granular pigment substance within the cells, and for the comparative fewness 

 of the clear gland cells. This condition is no doubt the principal cause of the dense opacity of 

 the outer tissues already described. The pigment matter is unaffected by carmine stains and 

 hrematoxjdin, and appears yellowish brown or greenish in sections, and in macerated tissues. It 

 is mainly restricted to the deeper regions of the layer, where it is either continuous or distributed 

 in more or less isolated irregular patches (tig. 122). Toward the tentacular region of the column 

 wall clear gland cells are more numerous than elsewhere. 



The mesoglcsa is of moderate thickness, and contains numerous connective tissue cells 

 distributed throughout. Sometimes their processes are seen in connection with the endoderm, 

 sometimes with the ectoderm, or may even stretch across from one layer to the other. A slight 

 difference in consistency in the mesogloea i.s also apparent in preparations stained with aniline 

 blue; lighter, tube-like portions extend across the whole laj'er, or in other sections appear as 

 so many circular disks staining less deeply than the surrounding mesogloea. 



Zooxanthelhi? occur in large numbers in the endoderm of the column, while the musculature 

 is strongh' developed in the upper region, more so than in any other species here described. 

 The inner surface of the mesogloea, for some distance, forms pointed, rounded, or dendriform 

 plaits for its support, and the muscular tibrils themselves are somewhat large in transverse 

 section, constituting what must be regarded as a definite endodermal sphincter muscle (fig. 121). 

 The whole form resembles -vhat has been described in certain Actiniaria as a "restricted" 

 sphincter muscle. 



The tentacles in retracted polyps are crowded under the overhanging thickened edge of 

 the column wall, and may or may not be introverted. Histologically they differ much from 

 the column wall. The ectoderm is a deep layer, containing numerous gland cells and a mai'ginal 

 zone of very narrow nematoc3^sts; the graiuilar pigment matter is absent from the more 



"Prof. A. E. Yerrill (1901) alludes to the very varied colors of the IsophylUa at Bermuda. He notes that some 

 specimens were pliosphorescent at nijfht, and that this property seemed to lie related to the white pigment. 



