352 J. E. Duerden — Jamaican Actiniaria : Part I. — Zoanthew. 



length, and one to two feet broad. Incrusting sponges grow freely on the coenen- 

 chyrne and amongst the polyps, and Ophiuroids meander around. 



The specific name has reference to the amount of variation met with in the 

 various external features of the polyps. 



Column-ivall (PI. xvin. A, fig. 7). — The cuticle of the column-wall is thickly 

 coated below with a layer of foreign matter, principally diatoms. The ectoderm is 

 continuous, and presents irregular internal limitations, especially towards the upper 

 part of the column. This is due partly to the presence of incrustations, but also to 

 the ectoderm passing insensibly into the cell-enclosures of the mesoglcea. Numerous 

 zooxanthellse occur, and occasionally large colourless oval nematocysts, showing the 

 coiled internal thread. The incrustations are sand grains, sponge spicules, and 

 tests of Radiolarians, and extend from the inner border of the ectoderm to beyond 

 the middle of the mesoglcea. 



The mesoglcea is thicker above and below than in the middle ; numerous cells 

 occur bearing elongated processes, and cell-enclosures of various dimensions 

 uniformly distributed. Fine radiating processes extend from the endodermal 

 boundary, apparently throughout the layer ; the large cell-islets contain zooxan- 

 thellse and large oval nematocyts, as in the ectoderm. 



The endoderm is of medium height, and contains zooxanthellse and pigment 

 granules ; the circular endodermal muscle is easily distinguished. 



Sphincter muscle (PL xviii. a, fig. 7). — The sphincter muscle is single and enclosed 

 in an extended series of small mesoglceal cavities, varying but slightly in size, 

 shape, and distance apart. It is situated nearer the endoderm. The cavities in 

 the upper part are a little larger, and the lining muscle-fibres are arranged in 

 various directions, many being cut obliquely. A few small spherical cells are also 

 present in the middle of the cavities. 



Tentacles. — The ectoderm is thick and has an outer layer of small narrow 

 stinging cells, and below this abundant deeply staining oval nuclei and nume- 

 rous glandular cells. The mesoglcea is broad, and contains isolated cells and 

 foreign incrusting matter. An ectodermal and an endodermal musculature occur. 



Disc. — The ectoderm is very broad and contains zooxanthellse and glandular 

 cells. The mesoglcea is nearly as thick as that of the column-wall, but contains 

 no foreign inclosures ; minute cellular strands and a few cell-islets with large oval 

 nematocysts occur in it. In the peripheral part of the disc, the mesoglcea is very 

 thin, while the ectoderm is a little thicker than in the more central region. The 

 endoderm is low and contains zooxanthellse ; a weak endodermal muscle on plait- 

 ings of the mesoglcea occurs. 



(Esophagus. — The oesophagus is oval-shaped in transverse sections, with a well 

 marked truncated oesophageal groove, the sulcar directives extending from the 

 corners. The ectoderm is thrown into longitudinal folds, not followed by the 



