THE ACTINIA NS OF PORTO RICO. 
833 
be a little swollen above or remain of the same diameter throughout, and terminate distally in a 
rounded or slightly conical manner; a central aperture remains, from which radiate delicate capitular 
ridges. On partial retraction a deep groove is often seen dividing the capitulum into an inner and an 
outer part, but this becomes interned when the column-wall is completely infolded. 
The tentacles are short, digitiform, overhang during extension, and are arranged in two alternating 
rows of about thirty in each. The naked portion of the disk is depressed below the tentacular margin, 
smooth, thin-walled, and exhibits radiating lines. During expansion the oral cone is much elevated, 
and the mouth is slit-like. The stomodieal groove is not clearly seen in living polyps. The disk and 
capitulum become greatly enlarged during full expansion, so that when all the polyps in a colony are 
in this state their margins are in contact, and the mutual pressure produces a polygonal outline. 
The ccenenchyme is smooth, continuous, lamellar, and adheres closely to the rocks and stones, 
following the various irregularities of their surface. Occasionally it becomes ribbon-shaped, when the 
polyps are more widely separated. 
The color of the lower part of the elongated polyps is pah; buff, the white mesenterial lines showing 
through the partly transparent walls; the upper part is olive blue, the capitulum being a little lighter 
and usually exhibiting green radiating lines. The tentacles are nearly always dark brown, but may 
be green or olive. The disk is generally a bright green, with lighter radiating lines; sometimes it is a 
pale green or yellow. A darker triangular area often extends from the two axial extremities of the 
mouth, and sometimes one is more pronounced than the other. The peristome in many is a bright 
pink, in others a bright green, more rarely yellow. The stomodffeum sometimes appears green, with 
white radiating lines showing through. An olive-brown color is first extracted on immersing the 
colonies in alcohol, and the polyps become a uniform green, due to the presence of numerous zooxan- 
thellEe within the endodermal cells. 
The average diameter of the column in living specimens is 6 mm.; the diameter of the capitulum 
on full expansion varies from 8 to 10 mm. The length of the column is variable, depending mostly 
upon the position of the polyp in the colony. An average length is 5 mm.; some may attain a length 
of nearly three times this, while the mammiform polyps may extend only 3 to 4 mm. above the level of 
the coenenchyme. The tentacles are from 2 to 3 mm. in length. Colonies are often met with from 
20 to 30 cm. across. 
Anatomy and Histology . — The column-wall is protected on the outside by a subcuticle, which is 
partly coated with a layer of foreign matter, mostly diatom frustules and fine mud. This is best seen 
in the lower part of the polyp, but may extend nearly the whole length. The ectoderm is very narrow, 
and its cells have lost their columnar character except in the distal region. Below, the layer is largely 
vacuolated, numerous nuclei occur, and the protoplasm is arranged in strands, and is granular and 
stains deeply. Connecting mesogloeal strands pass from the subcuticle to the middle layer, the ecto- 
derm appearing as if embedded in irregular mesogloeal chambers. Toward the base the subcuticle is 
better developed and the ectoderm becomes thinner. 
The mesogkea is thickest in the region of the capitulum and also toward the ccenenchyme. It 
contains isolated connective-tissue cells, and fine processes are seen extending from the ectoderm 
to the endoderm. Cell-islets also occur, some of which contain dark, granular pigment matter. They 
are largest and most numerous toward the endodermal border, where they sometimes appear as 
forming an interrupted encircling sinus. Often a canal occurs opposite the insertion of each mesentery. 
Vacuoles, or circular spaces with only a single nucleus, are not infrequent in the mesoglcea toward 
its ectodermal border, and the margin is not always well defined. 
The endoderm is a very narrow layer and is crowded with zooxanthelke and small oval 
nematocysts. On its mesogloeal border it gives rise to a weak musculature. 
The coenenchyme contains numerous irregularly distributed canals connecting the coelenteron 
of one polyp with that of others. These are lined with a strongly cilated endoderm, and possess a 
weak musculature. 
The mesogloeal sphincter muscle is strongly developed and consists of a small upper or distal portion 
and a larger lower or proximal portion, the two separated by a deep groove in retracted polyps (fig. 14) . 
The upper part consists of a number of small, closely arranged, rather elongated, irregular cavities, 
lined by muscle fibers. The mesogloeal spaces in the lower part of the sphincter are also very closely 
arranged, narrow, and at the broadest part extend nearly across the middle layer. Below, the muscle 
passes toward the ectodermal border and then turns inward, terminating in a number of small 
chambers toward the endodermal border of the mesogkea. 
