THE ACTINIANS OE PORTO RICO. 
387 
form of the column, the vertical bilateral arrangement of the tubercles, and the peculiarities of the 
capitulum are repeated is a little remarkable. The polyps are rarely seen in an expanded condition. 
One of the specimens sectionized longitudinally displayed the same histological and anatomical 
characters, including the form of the sphincter muscle, as have been already given by McMurrich and 
myself. Tangential sections through the ectoderm revealed very clearly the manner in which the 
layer is broken upon into distinct cubical blocks by mesogloeal partitions, extending from the sub- 
cuticula to the middle layer (fig. 26). The ectoderm cells in each block have lost their usual columnar 
form and become rounded, and are charged with zooxanthellse and nematocysts; the endodermal 
bays are strongly developed, but not the ectodermal. 
A polyp sectionized transversely revealed the normal microtypic arrangement of the mesenteries, 
w r it-h ten complete mesenteries on one side and nine on the other (fig. 23). The sulcar groove was not 
strongly defined, but the ectoderm was smooth, while along each side the stomodaeal ridges were well 
pronounced. The mesenteries on the sulcar aspect were the first to become free. The section repre- 
sented in figs. 23 passes through two of the tubercles of the wall; the one on the upper right hand is 
long and narrow, and the ccelenteron is prolonged into it. The ventral tubercle, though smaller, has 
a distinct bay-like character. The polyps represented in transverse section in figs. 24, 25 each possessed 
ten complete mesenteries on each side. 
External characters . — The polyps are firmly adherent to rocks and stones by means of an expanded, 
irregular base. Although associated in groups they are only rarely found actually united with one 
another, the slight basal expansion representing all there is of a coenenchyme. The column is cylin- 
drical or clavate, greatly swollen toward the base, and overhangs, so as to present a concave and a 
convex border. The proximal part is smooth, with narrow, incomplete annuli, but ftie distal region 
bears rows of large, rounded or conical tubercles on the convex aspect, while the concave aspect is 
smooth, thus producing a marked external bilaterality. Four principal rows of from five to eight 
tubercles alternate with other rows which possess only two or three smaller protuberances. The 
column of young specimens may be wholly smooth. The disk is entirely hidden in retracted 
specimens, but the capitulum is flattened and simulates a disk. It forms the terminal truncated part 
of the retracted polyp, and is placed obliquely, a small central aperture remaining in the middle, from 
which radiate numerous ridges and furrows. Around the margin of the apparent disk occurs an 
incomplete ring of eight or nine tubercles, separated by deep depressions, and diminishing in size 
toward each extremity of the partial circle. In the living condition the column-wall is firm, tough, 
and partly transparent. 
The basal coenenchyme is colorless or irregularly greenish brown, apparently due to adhering 
algse. The column may be dark granular brown, mottled with green and black. 
The diameter of the column varies from about 2 cm. at the base to 0.6 cm. in the middle, where it 
is narrowest. Different polyps vary in height from 2.2 cm. to 4.2 cm. The retracted tentacles are 
0.3 cm. long in section. 
Anatomy and. Histology . — The column-wall is provided nearly throughout with a thick subcuticle 
connected with the mesoglcea by strands across the ectoderm. The outside cuticle is practically devoid 
of adhering foreign matter. The ectodermal layer is broad, and its cells have lost their columnar 
character, the mesogloeal strands dividing it into cubical or spheroidal blocks, in which large thick- 
walled zooxanthelhe and nematocysts occur (fig. 26). The subcuticle disappears toward the upper 
extremity of the column, and the ectoderm then becomes continuous and columnar in character. The 
boundary between the ectoderm and mesoglcea is not clearly defined, portions of the former being, as 
it were, cut off and isolated within the mesoglcea as cell-inclosures or islets and becoming farther and 
farther removed inwardly. 
The mesoglcea is very broad, and contains numerous cell-inclosures and uniformly distributed 
isolated cells with granular protoplasm. In the lower part of the column the cell-inclosures take the 
form of small communicating canals. 
The endoderm is narrow and contains numerous zooxanthellse and small nematocysts. A 
circular endodermal muscle occurs, and endodermal bays are met with at different levels, extending 
nearly as far as the ectoderm and evidently corresponding with the thin annuli noticed among the 
external characters. The tubercular projections on the upper part of the column are seen in sections to 
