MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 583 



verrucas form alternating larger and smaller rows, corresponding with the larger and smaller 

 septa helow, and are round, lighter in color, and closely arranged: they may appear contiguous 

 in the larger rows, hut are 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 distallj- 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 dicyclic, the members of the inner row slightly larger than those of the outer; the 

 former correspond with the larger entosepta, and the latter with the small exosepta (tig. 131»). 

 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, opa(iue area, the tentacles as a whole being shortened but more swollen. 

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

 spots, 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 may 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 hy 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 (tig. 14-7). 



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

 or green may predominate. The discal areas often show a superticial, 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 verruciv along the middle of the ridges. 



On irritation, or under unfavorable conditions, white me.senterial hlaments 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 stomodseum 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 

 off a portion of the disk l>earing one or more apertures. 



AiKitoniy aud Iuiit'iIo<jy. — 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 poi'tions of the 

 layer, but are sometimes contiiuied 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, nuicous 

 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 broadei-. granular, and mucous cells are embedded, and the 

 nuclei form an interrupted zone just within tiie margin; small, narrow nematocysts iilso occur. 



The mesogla?a is a comparatively WvMl- developed layer in some polyps, but thin in others, 

 varying with the state of exjjansion or I'etraction of the polyps. Included connective tissue cells 

 are somewhat numerous, ami a delicate tibrous and vacuolated appearance is presented b\- sections 

 .stained in picro-carmine. 



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

 These are mostly restricted to the inner (mesoglceal) two-thirds of the layei'. while the nuclei and 



