366 J. E. Duerpen—Jamaican Actiniaria: Part I.—Zoanthee. 
some distance down the column, are quite colourless and hence readily counted. 
The tentacles, inturned disc, and edges of the mesenteries are likewise altered in 
colour. 
Dimensions.—The length of the polyps and the thickness of the eeenenchyme 
differ very much, may vary from 0°3 or 0:4 cm. to 1:8 cm., usually about 
07 cm.; diameter of disc in partial extension 0-5 cm., in full extension 0°9 em., 
in retraction 0:4 cm.; distance of centres of contiguous polyps 0°5 em.; height 
of free portion above the level of the ccenenchyme in partial retraction about 
05 em.; tentacles about 0:2 cm. long; diameter of polyps in section 0°35 em. 
Activities—Quantities of bubbles of gas are given off when the colonies are 
exposed in the water to the direct rays of the sun. The polyps do not appear to 
open so readily as in P. mammillosa. 
Locality.— Jamaica: Numerous colonies form flat expansions covering con- 
siderable areas of coral rocks, at all the Cays outside Kingston Harbour. 
Range.—St. Thomas (Duchassaing and Michelotti). 
Column-wall (Pl. x1x., fig. 5).—The lower boundary of the column-wall of the 
individual polyps in a colony can not be distinguished from the ccenenchyme in 
which the polyps present the appearance of being embedded, but above it is quite 
distinct. The ectoderm is continuous, and spreads as a uniform layer over the 
surface of the whole colony; a thin, well defined cuticle occurs on the outside. 
It is not readily separable from the mesoglea, appearing to pass insensibly into 
the cell-enclosures of the latter; narrow elongated nematocysts occur, as well as 
very large oval nematocysts, which do not stain; zooxanthelle are present; 
foreign incrusting material is met with only in the deeper part of the ectoderm. 
The mesoglcea is densely crowded throughout its whole thickness with cal- 
careous sand-grains ; a few siliceous sponge spicules, Radiolarians, and rarely a 
Foraminifera occur ; most of the material can be dissolved out by acids. Isolated 
cells and large and small cell-islets are scattered throughout; the large nema- 
tocysts, pigment granules, and densely staining tissue fill up the islets. 
The endoderm is very thin and uniform in height, except in the upper region 
where the mesenteries are closer, when the endoderm elongates in the middle and 
appears triangular in section. It contains abundant granular pigment matter and 
zooxanthelle ; a weak endodermal muscle is present on slight plaitings of the 
mesogloea, especially in the upper region. 
Sphineter muscle (Pl, x1x., fig. 5).—The single sphincter muscle is contained in 
a very regular series of small mesoglceal cavities; proximally they are situated 
close to the endodermal border, but are more central above, where also the cavities 
are not in such a single series and become a little larger. The muscular lining 
is thick, but does not quite fill the cavities. 
