MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 589 



tentacles on the other band are simple, and the apical swelling lies over the apparent Inner 



termination of the septum below (figs. l.M. 15-1. l.".;,). 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 impossible to distinguish them, even with the aid of a lens. When 

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



short distance, and the apex of the bifurcations is rather pointed, and hears a white nematoUast 

 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 he determined witli a little care. No 

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

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

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

 more or less polygonal in form, and equaling in Dumber the sum of the three inner cycles, a 

 multiple of six being rarely present. 



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

 from which tie-. 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 cycle. 



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 stomodanim 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 through. The tentacles are a little paler; hut, 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 ahout a 

 centimeter in depth is frequently of a pink color, contrasting strongly with the corallum helow, 

 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 histology. — The ectoderm of the column wall contains numerous clear gland cells, 

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

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

 The endoderm contain- numerous zooxanthellse, and only the merest trace of any circular 

 musculature can he 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 layer, directly 

 overlvine- the septal ridge (fig. 154). A little helow these the bifurcated tentacles appear in 

 section, hut in this case each knob 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 apical portion of the tentacles stands out as 

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

 stinging cyst-: hut the peduncle, as such, wholly disappears, becoming involved in the discal 

 tissues. No ectodermal or endodermal 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 stomodseum is folded both vertically and horizontally in retracted polyps, and the aboral 

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

 Vol. 8 No. 7 13 



