556 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



thin, broad, basal extension of the coralluin. while others are wholly' isolated. In a series of eight 

 polj'ps, none was distant more than 1.5 mm. from what might be regarded as the eentral pol\p; 

 three were still connected by a delicately ridged, band-like skeletal deposit, and of these one polj-p 

 was much larger than the othoi' two. The column wall of one appeared to have just rounded 

 itself otl', liut thf other was still united basally with the larger polyp. 



The column is usually circular, })ut sometimes is slightly oval, and often oblique to the 

 surface of attachment, so that one side of the polyp is longer than the other. The column wall is 

 smooth, subcylindrical, and short, extending but a short distance over the outer edge of the 

 corallum, the remainder of the skeleton being hidden bv various kinds of Nullipores and other 

 foreign growths. The column can l)e overdrawn within the calice, so as to leave no central 

 aperture, and completely hide the disk. The lower terminal margin of the edge-zone in isolated 

 potyps is circular. Structurally the wall is very thin and transparent, and duiing full expansion 

 is situated some distance from the corallum; the internal mesenteries and the skeleton are seen 

 very distinctly through it. 



The tentacles in the partly expanded condition are short and stout, with a broad base; they 

 become long and narrow when fully extended, terminating in a small opacjue swelling or knob. 

 The surface is tubcrculated, owing to the presence of minute nematocyst liatteries; otherwise 

 the walls are very delicate and perfectly transparent. The tentacles are about 8 nun. long on 

 full expansion, and difl'erent cycles exhibit but slight variations in length. Thirty were 

 countetl on one polyp, thirty-six on another, and tifty-four on a third. Owing to their closeness, 

 and small difference in size, it is practically impossible to determine from the tentacles themselves 

 their cyclic arrangement, but it is readil}' seen that one occurs over each septum, and these follow 

 the hexameral plan. 



The disk is smooth, generally oval, verj- thin walled, and .so transparent as to allow of the 

 internal mesenteries being clearly seen. The peristome is usuall}' much elevated; the mouth is 

 slit-like, and extends nearly across the naked part of the disk. The stomoda-al walls are sharply 

 marked off' from the disk, and present very deep ridges and furrows; in four polyps the 

 I'idges were twelve in number, and in two other polyps eighteen ridges occurred. At each angle 

 of the mouth the disk is deeply rose-colored, thus affording a strong contrast with the whiteness 

 of the stomodfeal wall. 



The tissues of the polyps as a whole seem veiy delicate, and on expansion are perfectlj' 

 transparent, mainly owing to the absence of zooxanthelhe. The proximal region may be slightly 

 brown, while the upper part of the column on partial extension is a rich dark brown. The discal 

 area is a delicate rose color. The angle at each end of the mouth is a much deeper ro.se, while 

 the tips and the lateral nematocyst batteries on the tentacles may be a delicate green, and the 

 general surface perfectly colorless and transparent. The ridges on the stomoda?al walls are a 

 dense white, and the intervening grooves darker. In fully retracted specimens the coloration 

 is much deeper, mostly a rich dark olive, except over the septal edges, which always stand out as 

 very distinct white ridges. 



At night, in the laboratory, the polyps are seen distended to their utmost, raised a few 

 millimeters beyond the corallum, and the tentacles extended vertically upwards, horizontally, 

 or overhanging. In the ordinary day condition the polyps are partly expanded, raised ))ut little 

 above the corallum; the tentacles may then be erect or mingle with one another centrally, and 

 the mouth is open or closed. Sometimes the mouth will close quite suddenly, and then slowly 

 open; or it maj' open to such an extent as to permit of the walls of the stomodicum being visible 

 for the whole of their length, and even allow the actual interior of the polyp to be seen. On 

 irritation the mouth opens suddenly, and afterwards closes slowly. 



Aiiutimty and hixtology. — The column wall as a whole is very thin, becoming a little thicker 

 toward the tentacular region. The ectoderm comprises mainly clear glandular cells and 

 supporting cells, with a few small nematocysts in the upper region. The mesoglcea appears as a 

 clear homogeneous layer. The endoderm is very narrow, and its free surface is even; zooxanthellaj 

 were wholly absent from the specimens examined. 



Such a histological structure at once explains the clear transparent delicacy of the living 



