198 J. E. DuERDEN — Jamaican Acfdniaria : 



circular, light-coloured discs, or low mammiform jDrominences, distributed with 

 considerable regularity over the surface of massive, dark-coloured sponges. What 

 may be regarded as a very narrow border of coenenchyme surrounds each indivi- 

 dual polyp. Only occasionally are two or more polyps still united as a result of 

 reproduction by budding, and all stages in the separation of one from the other 

 can be observed. A thin coenenchyme connects two polyps before their isolation, 

 but chain-like colonies are never produced, as in the next species. The column- 

 wall is smooth, but with a lens, minute, white, incrusting granulations are seen. 

 These give a certain rigidity to the polyps, so much so that, in preserved material, 

 the walls readily split in two. 



Retraction is complete in most examples ; only a very small circular depression 

 remains above, not sufficiently large to allow the mouth or disc to be seen. The 

 capitular ridges are wedge-shaped, and number from twelve to sixteen, twelve 

 being the most usual. The tentacles are extremely short, and, as seen in 

 sections, are almost invariably twenty-four, arranged in two cycles. The disc is 

 circular and semi-transparent, and exhibits radiating grooves corresponding with 

 the internal attachment of the mesenteries. The peristome is raised, the mouth 

 slit-like. 



The coenenchj'me and column are dull white, due to the numerous included 

 calcareous sand-grains; the tentacles and disc are dark brown. 



The diameter of retracted polyps is about 2'5 mm., the height 1*5 mm. 



Anatomy and Histology. 



All that portion of the wall of the polyp which is embedded in the sponge may 

 be regarded as the base, and discloses a different histological character from that 

 of the free column-wall. It is convex in outline, but somewhat flattened and 

 expanding peripherally, and is sharply marked off from the surrounding sponge 

 tissue, showing that there is no intimate cellular relationship between the two. A 

 very thin cuticle can also be traced (PI. xiv., fig. 4). 



The ectoderm in places is not readily distinguishable from the mesogloea, the 

 latter layer being so crowded with cells as to render the ground substance almost 

 unrecognizable (PI. xiii., fig. 8). The ectodermal cells are large and more or 

 less spherical in outline, not forming a columnar epithelium ; their protoplasm 

 stains very strongly. 



The mesogloea is broad, and, as a whole, stains very deeply, the result of the 

 presence of the cellular constituents, mostly in the form of cell-islets. Cells 

 are included to an extent greater than I have met with in any other Actinian ; 

 they are all large, and the cytoplasm, in addition to the nucleus, readily takes up 



