MEMOIRS OF THE NATIONAL ACADEMY OF SCI FACES. 573 



around the margin of the colonies. The polyps arc here more closely arranged than toward 

 tlic 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 tissues is of a dark-green color, contrasting strongly with the upper, pale-colored 

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

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

 indentation may l>e double in the upper region and single below. 



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

 cells, among which are few mucous cells and do 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 alone- the line of attachment of the mesenteries. The endoderm is a broad layer with an 

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



The ectoderm of the tentacles is a greatly thickened layer, and. except proximally, is 

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

 highly refractive variety: pigment granules are sparsely distributed throughout. The ecto- 

 dermal musculature is well developed, and the nerve layer is sometimes seen at some little distance 

 from the mesoglceal surface. The endoderm fills the lumen in retracted tentacles, and is crowded 

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

 endodermal musculatures of the disk are clearly recognizable, and nematocysts are found in the 

 peripheral region. The stomodseum is short in vertical sections, and deeply folded transversely; 

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

 contents are scattered about. The stomoda'al ectoderm terminates in continuity with the 

 mesenterial filaments. 



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

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

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

 ever, a slight invagination is found within the exocceles, thus constituting a third order of septa. 

 In some instances a member of a third cycle of mesenteries 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, where 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 muscle (tie-. 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 they are practically vertical on each side 

 of the mesentery. 



The mesenterial filaments are simple and rounded above, in places sharply 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, thetilaments are likewise much developed, and here they bear large nematocysts and gland 

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

 by 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 distributed 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 

 polypal endoderm; zooxanthellse here appear to take their place. 

 Vol. 8— No. 7 li' 



