366 J. E. Duerden — Jamaican Actiniaria : Part I. — Zoanthece. 



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 coenenchyrae 

 differ very much, may vary from 0*3 or 0*4 cm. to 1*8 cm., usually about 

 0*7 cm. ; diameter of disc in partial extension 0*5 cm., in full extension 0-9 cm., 

 in retraction 0'4 cm. ; distance of centres of contiguous polyps 0*5 cm. ; height 

 of free portion above the level of the coenenchyme in partial retraction about 

 0*5 cm. ; tentacles about 0'2 cm. long ; diameter of polyps in section 0'35 cm. 



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 (PI. xix., fig. 5). — The lower boundary of the column-wall of the 

 individual polyps in a colony can not be distinguished from the coenenchyme 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 mesoglcea, 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 ; zooxanthelhe 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 

 zooxanthellse ; a weak endodermal muscle is present on slight plaitings of the 

 mesoglcea, especially in the upper region. 



Sphincter muscle (PL xix., 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. 



