MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 573 



around the niargiu of the colonies. The polyps are here more closel}' arranged than toward 

 the middle of the colony, and different stages in the separation of the daughter polyps can be 

 followed. 



The height of decalcified polyps is about 5 mm., and in fresh material the lower one-third of 

 the embedded tfssues is of a dark-green color, contrasting strongly with the upper, pale-colored 

 tissues. The number of septal indentations is very irregular; all luunbers from six or eight to 

 twenty may occur, and exhibit different degrees of radial intrusion from above downward. An 

 indentation may be double in the upper region and single below. 



Andfomy and histology. — The ectoderm of the column wall is constituted of long columnar 

 cells, among which are few mucous cells and no nematocysts. Small, highly refractive pigment 

 granules occur, distributed throughout the layer with an approximate uniformity. There is no 

 evidence of the granules being restricted within limited groups, or of concentration toward the 

 the inner portions of the layer. The granules are colorless in preserved material, but green in 

 living polyps, and are no doubt the chief cause of the green coloration of the polyp, and the 

 general opacity presented by the external tissues. The mesoglea is very thin, but thickens a 

 little along the line of attachment of the mesenteries. The endoderm is a broad layer with an 

 abundance of small zooxanthelhe, as well as a few cells with colored granules. 



The ectoderm of the tentacles is a greatly thickened layer, and, except proximall}', is 

 crowded peripherally with nematocysts of two kinds — the long narrow form, and a short, stout, 

 highlj^ i-efractive variety; pigment granules are sparsely distributed throughout. The ecto- 

 dei'mal musculature is well developed, and the nerve layer is sometimes seen at some little distance 

 from the mesogheal surface. The endoderm tills the lumen in I'etracted tentacles, and is crowded 

 with zooxanthellffi; a weak endodermal muscle is also distinguishable. The ectodermal and 

 endodermal musculatures of the disk are clearl}^ i-ecognizable, and nematocysts are found in the 

 peripheral region. The stomodanmi is short in vertical sections, and deeply folded transverse! \'; 

 large nematocysts occur in the outer zone of the ectoderm, and gland cells with granular 

 contents are scattered about. The stomodteal ectoderm terminates in continuitj^ with the 

 mesenterial filaments. 



A small polyp sectionized transverselj' contained only four pairs of complete mesenteries, 

 and four pairs of incomplete mesenteries alternating with these (tig. 119). In the upper region 

 eight corresponding entocoslic septal invaginations occur, practically equal in size; below, how- 

 ever, a slight in\'agination is found within the exocceles, thus constituting a third order of septa. 

 In some instances a menaber of a third cj^cle of mesentei'ies is developed. Another polyp 

 contained eighteen pairs of mesenteries, but no pairs of directives were found. The mesen- 

 teries are long in transverse sections, narrow above and broad below, whei'e they branch 

 considerably. The mesenterial epithelium is greatly swollen on the side bearing the retractor 

 muscle; the mesogloea is perfectly homogeneous, and is much and deeply folded to give an 

 increased surface for the retractor nuisde (fig. 120). The oblique musculature is also strongly 

 developed, and toward the insertion of the mesenteries the mesogloea becomes folded for its 

 support. Sections of the muscle fibers indicate that thej' are practically vertical on each side 

 of the mesentery. 



The mesenterial tilaments are simple and rounded above, in places sharplj^ marked off from 

 the endodermal enlargements behind; in many, however, there is no sharp line of distinction 

 between the filament and the mesenterial endoderm. Proximally, where the mesenteries branch 

 greatly, thefflaments are likewise much developed, and here they bear large nematocysts and gland 

 cells with coarsely granular green contents. The lower skeletotrophic tissues are characterized 

 l\y an abundance of chlorophyll-like granules within the swollen endodermal layer. Even to 

 the naked eye these give a strong green appearance to the lower third of the polypal tissues upon 

 decalcification, and in sections the granules are seen to be thickly distrilnited throughout the 

 layer. The granules vary a little in size and are more refractive than the surrounding cellular 

 constituents. They appear to be of a similar nature to the granules occurring within some- of 

 the glandular cells of the mesenterial filaments, and are found only sparingly in the upper 

 poh'pal endoderm; zooxanthelhe here appear to take their place. 

 Vol. 8— No. T 12 



