MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 583 



verruca' form alternating larger and smaller rows, corresponding with the larger and smaller 

 septa below, and are round, lighter in color, and closely arranged; they may appear contiguous 

 in the larger rows, hut arc more distant on the shorter. A narrow, smooth area at the base of 

 the walls indicates the line of union between one column wall and another, while distally it 

 passes uninterruptedly into the tentacles. When the body wall is lying upon the skeleton the 

 discal valleys are about 4 mm. across. 



The tentacles form a narrow fringe along the two margins of each discal area. They are 

 short and dieyclic. the members of the inner row slightly larger than those of the outer; the 

 former correspond with the 1 larger entosepta, and the latter with the small exosepta (tig-. L39). 

 During ordinary extension the inner tentacles are 2 mm. long: they are broad at their origin, 

 and either terminate bluntly or are slightly knobbed. On full expansion the knob is displayed 

 as a thickened, lighter, opaque area, the tentacles as a whole being shortened hut more .swollen. 

 The surface of the stem is almost covered with small, oval or irregularly shaped, white, urticating 

 spoi^. none, however, so large as that at the apex. 



The naked portion of the disk is very narrow, about 2 mm. across in the retracted condition, 

 and is usually depressed. Its surface is verrucose in parallel or slightly radiating rows, the 

 rows corresponding with the septa below, while the grooves between correspond with the 

 mesenteries. 



The oral apertures are very small, oval, or slit-like, or ma}' be circular when opened to their 

 full extent. The larger axis is along the length of the discal areas, and is about a millimeter 

 long; during retraction the apertures are separated from one another by a distance of 2 or 3 

 mm. The stomoda?al wall is thrown into deep vertical ridges and furrows, varying in number 

 from three or four to eight on each side (fig. 147). 



The color of the colonies in general is dark or grayish brown when the polyps are retracted, 

 or green may predominate. The discal areas often show a superficial, opaque green, or may be 

 dark brown: the column walls for some distance on each side of the line of union may also be 

 bright green. The tentacles are a transparent dark brown. All the external tissues become 

 more translucent and lighter in color when the polyps are fully expanded, appearing then as a 

 pale brown. Small grayish spots usually occur over the verrucse along the middle of the ridges. 



On irritation, or under unfavorable conditions, white mesenterial filaments can be emitted 

 through the mouth, disk, and column wall, sometimes in such profusion as to almost cover the 

 whole of the colony. 



The usual method of reproduction consists in the formation of additional oral apertures on 

 the discal areas, each aperture having a distinct stomodaeum and mesenterial system associated 

 with it. No further polypal separation as a rule takes place, the tentacles and column wall being 

 part of the general system. Occasionally the column wall may grow transversely, and thus cut 

 on a portion of the disk bearing one or more apertures. 



Anatomy and histology. — The ectoderm of the column wall is characterized in places by an 

 abundance of cells containing finely granular pigment, which gives a relative opacity to the 

 sections wherever it occurs. The granules are situated mostly in the deeper portions of the 

 layer, but are sometimes continued as far as the surface. They no doubt influence the external 

 coloration, and give rise to the comparative opacity of the tissues already noticed in the living 

 retracted polyps. In addition to the granuliferous cells, numbers of clear, unicellular, mucous 

 glands occur, extending across the whole thickness of the layer, and rendered very conspicuous 

 by reason of the perfect transparency of their contents. The long supporting cells constitute, 

 as it were, a matrix in which these broader, granular, and mucous cells are embedded, and the 

 nuclei form an interrupted zone just within the margin; small, narrow nematocysts also occur. 



The mesoglcea is a comparatively well-developed layer in some polyps, but thin in other-, 

 varying with the state of expansion or retraction of the polyps. Included connective tissue cells 

 are somewhat numerous, and a delicate fibrous and vacuolated appearance is presented by sections 

 stained in picro-carmine. 



The endoderm is of about the same thickness as the ectoderm, and its cells contain zooxanthellse. 

 These are mostly restricted to the inner (niesoglu-al) two-thirds of the layer, while the nuclei and 



