MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 589 



tentacles on the other hand are simple, and the apical swelling lies over the apparent inner 

 termination of the septum below (tigs. 151, 154, 155). The organs remain exposed, the column 

 wall being incapable of closing over them, but so minute are they that in completely retracted 

 preserved material it is often impossil)le to distinguish them, even with the aid of a lens. When 

 fullj^ expanded the tentacles are short; the common stem of the bifurcated form extends but a 

 shoi-t distance, and the apex of the bifurcations is rather pointed, and beai"s a white nematoblast 

 area. The simple tentacles have a short, thick stem, and the apex is rounded, tipped with a 

 battery of nematocysts. 



The hexameral cyclic arrangement of the tentacles can be determined with a little care. No 

 difference in size can be determined among the bifurcated examples, and these are disposed so as 

 to form two or three alternating cycles; but the twelve members which should constitute the 

 third cycle are not always present. The simple, outermost tentacles represent a fourth cycle, 

 more or less polygonal in form, and equaling in number the sum of the three inner C3'cles, a 

 multiple of six being rarely present. 



In rare cases one or more entotentacles of a fourth cycle may be developed, as in the polyp 

 from which tig. 150 was taken, even though the third cycle is not completed in all the other 

 systems. In such cases the exotentacles would be considered as the fifth cj'cle. 



The naked portion of the disk is smooth, and very limited in extent in comparison with the 

 broad tentacular zone. During partial retraction the peristome is elevated, the mouth is long 

 and oval, and the white lips contrast strongly with the dark- brown disk. The stomodffium is 

 smooth, without permanent ridges and furrows. 



On the sea floor the colonies as a whole appear a characteristic reddish-brown color. On 

 closer examination the disk is found to be somewhat darker than the rest of the polyp; the areas 

 along the lines of union of adjacent polyps and also over the septa are lighter, the corallum partly 

 showing thi-ough. The tentacles are a little paler: but, on the whole, the polyps are remarkably 

 uniform in color. The young polyps on a colony are for some time much lighter colored than 

 the rest. When a living colony is broken across, the superficial part of the skeleton for about a 

 centimeter in depth is frequently of a pink color, contrasting strongly with the corallum ])elow, 

 which is a dense white. 



New polyps arise along the line of union of adjacent polyps, and for some time they usually 

 project slightly above the general surface of the colony. The extrusion of mesenterial filaments 

 through the mouth or polypal wall has not been observed. 



Anatomy and lihtoJogy. — The ectoderm of the column wall contains numerous clear gland cells, 

 and here and there a small oval neniatocyst in which the axis is clearly distinguishable. The 

 mesoglcea is everywhere extremely thin except along the line of attachment of the mesenteries. 

 The endoderm contains numerous zooxanthella?, and only the merest trace of any circular 

 nmsculature can be detected. 



The tentacles have a very characteristic relation in conformity with what has been noted 

 amongst the external characters. In transverse sections through the uppermost region of 

 retracted polyps, passing through the sloping disk, the outermost series of tentacles are first 

 come upon, appearing as simple, nematocyst-bearing swellings of the ectodermal laj'er, directly 

 overlying the .septal ridge (fig. 15-lr). A little below these the bifurcated tentacles appear in 

 section, but in this case each knol) of the tentacle is situated laterally, one along each side of the 

 septal invagination, and the intermediate connecting tissue, which passes over the septal edge, 

 resembles that of the disk (fig. 155). Each half of the ajaical portion of the tentacles stands out as 

 a wing-like thickening of the superficial wall, and outwardly is crowded with long, narrow, 

 stinging cysts; but the peduncle, as such, wholly disappears, becoming involved in the discal 

 tissues. No ectodermal or endodei-mal muscle fibers have been recognized on the walls of the 

 tentacles. 



The disk presents no histological characters distinguishing it from the column wall, except 

 that a slight musculature is developed in connection with both the ectoderm and endoderm. 



The stomoda?um is folded both vertically and horizontally in retracted polyps, and the aboral 

 termination is directed outwardly and backwardly. Twelve complete mesenteries are attached 

 Vol. S— No. 7 13 



