THE ACTEN1ANE OF PORTO RICO. 
341 
Among the abundance of Jamaica material which has passed under my examination I have only 
determined what seem to be two distinct species, the differences being based mainly upon the number 
of capitular ridges, which corresponds with the number of tentacles and mesenteries. In one series of 
colonies practically all the polyps possessed from 14 to 16 capitular ridges, and in another series 18 to 20. 
Scarcely any other constant differences, however, were forthcoming. 
Palythoa caribsea Duchassaing & Michelotti. Pis. II, VIII; Figs. 7, 28. 
Palythoa caribseorum, Duchassaing et Michelotti, 1860, p. 329. 
Palythoa caribxa, Duchassaing et Michelotti, 1866, p. 141, pi. vi, fig. 11; Duerden, 1898, p. 365, pi. xvn a, fig. 9, pi. xix, 
figs. 5-7. 
A single Palythoa colony of about thirty strongly retracted polyps was found in the Porto Rican 
collections from the light-house reef at Playa' de Ponce. The external characters avail but little 
toward its determination, but a careful comparison of sections with the Jamaican specimens which I 
identify as Palythoa caribxa reveals no important distinctions, and as the mesenteries are seventeen I 
identify it with this species rather than with P. mammillosa. 
External characters. — The polyps are smooth, rigid, cylindrical, closely associated, and arranged 
within the coenenchyme in an irregular manner. Distallv they are free from the coenenchyme for a 
short distance, the free portion being rounded or conical in retraction. On very strong retraction the 
upper surface of a colony may be nearly flat. At the periphery of the colonies the outlines of the 
marginal polyps are clearly indicated, and new individuals arise both at the margin and between 
the previously existing polyps. The capitular ridges and furrows vary from 14 to 17. 
The tentacles are dicyelic, smooth, pellucid, very short, acuminate, and slightly entacmseous, 
the inner row opposite the capitular ridges. They vary in number from 28 to 34. 
The disk is circular, depressed during partial expansion and overhanging on full expansion, being 
cup-shaped or saucer-shaped according to degree of expansion. Peripherally it is thin, transparent and 
non-incrusted, with rounded ridges and furrows; the central part is dome-shaped, contains a few small 
incrusting particles, and bears the slit-like mouth at apex. The gonidial groove is not clearly seen. 
In the living condition the polyps are a nearly uniform cream color, or sometimes brown; they 
appear white when the ectoderm is rubbed off. The stomodseal walls are white. 
The length of the polyp, as also the thickness of the coenenchyme, varies much, and may be 
anywhere from 0.3 to 1.8 cm., but is usually about 0.7 cm.; the diameter of the polyp in section is 
0.35 cm. The diameter of the disk on full expansion is about 1 cm. and on retraction 0.4 cm.; the 
distance of the centers of contiguous polyps is 0.5 cm.; the tentacles are about 0.2 cm. long. Colonies 
vary in size from a few centimeters across to several hundred. 
Anatomy and Histology. — The lower part of the column-wall is indistinguishable from the general 
coenenchyme in which the polyps are embedded, but except in the most strongly retracted polyps a 
free columnar region occurs above. The ectoderm spreads as a uniform layer over the whole external 
surface and is readily rubbed off in living colonies. The layer is continuous, but is not distinctly 
limited oil its mesogloeal aspect, appearing to pass insensibly into the cell-inclosures of the latter, 
ft contains zooxanthelke and narrow, elongated nematoeysts, as well as a large oval form. The 
foreign incrusting material is limited to the deeper part of the ectoderm. 
The mesogloea is thick and densely crowded with calcareous sand grains; a few siliceous sponge 
spicules, radiolarians, and an occasional test of a foraminifera also occur. Isolated cells and large 
and small cell-islets are scattered throughout, the latter containing nematoeysts and pigment granules. 
The endoderm is uniformly narrow, except in the upper region where the mesenteries are closer, 
when it becomes broader. It contains much granular pigment matter and many zooxanthelke, and a 
weak endodermal muscle is present. 
The single sphincter muscle is contained within an elongated series of small mesogloeal cavities; 
proximally, they are situated close to the endodermal border, but become more central above, where 
the cavities are a little larger (fig. 28). 
The tentacles possess a broad ectoderm crowded with long narrow nematoeysts, zooxanthelke, 
and pigment granules; the mesogloea and endoderm are both thin. The ectodermal musculature is 
well developed on mesogloeal plaitings, but the endodermal is weak. 
The discal wall is much like that of the tentacles, but incrustations occur toward the middle 
where the mesogloea thickens. 
