164 J. E. Duerden— Jamaican Actiniaria : 



the disc with the usual colours, demonstrating that we are dealing with a mere 

 colour variety, between which and the normal every gradation may occur. 



The diameter of the base is about 5 cm., and the height of the column 4 cm. 

 The diameter of the disc is 10 to 12 cm., or maybe even more. The tentacles are 

 about 06 cm. in length, and vary but little in different regions of the disc. They 

 are often largest in diameter at the tips, where they may measure 0*2 cm. 

 across. The diameter of the naked part of the disc across the mouth is 2 - 5 cm. 



Prof. M c Murrich's figure (1889, PL i., fig. 8, Discosoma anemone) represents the 

 usual appearance of the Jamaican specimens. 



Anatomy and Histology. 



The column-wall is of only moderate thickness, the mesogloea being often 

 narrower in section than the ectoderm. The latter is deeply folded, the mesogloea 

 partly following. In the ectoderm the nuclei of the supporting cells are distri- 

 buted with considerable uniformity in sections, not limited to a zone as is generally 

 the case. Very numerous, long, granular gland cells are included among the 

 supporting cells. There is no trace of any ectodermal musculature. The 

 mesogloea shows a delicate, fibrous structure, and numerous included cells. On 

 its endodermal border it presents narrow, slightly branching plaits for the support 

 of the circular musculature, and very fine fibrils pass into the denser peripheral 

 part of the endoderm, in some places giving rise to a distinct nerve layer. 

 Ganglionic cells are recognizable between the muscular and nervous layers. The 

 endoderm is much thinner than the two other layers, and contains many zooxan- 

 thellse and granular gland cells. 



Where sections pass through verrucae, the ectoderm undergoes certain modifica- 

 tions : gland cells are absent, and the region stains more densely than the ordinary 

 ectoderm. Very delicate processes, like cnidocils, also appear on the surface, and 

 the remains of the layer of cilia are more obvious than elsewhere. 



The sphincter muscle is a strong, circumscribed, endodermal represen- 

 tative. It is recognized as a large outgrowth from the column-wall, a little 

 below the outermost cycle of tentacles, and is made up of several lobes. The 

 pedicle is broad and short, and a narrow mesoglceal axis extends nearly the whole 

 length. From this axis delicate processes are given off — sometimes on one side, 

 sometimes on the other, or on both together — for the support of the musculature. 

 The lobes are so deeply separated that often a portion of the coelenteron is enclosed 

 in sections. The surrounding endodermal layer resembles that of the column-wall, 

 and contains numbers of gland-cells and zooxanthellse. Owing to the lobed 

 character, the ajjpearance presented by the muscle varies in different sections. 

 As indicating the possible amount of this variation, the figure given by M c Murrich 

 (1889, pi. in., fig. 15) should be compared with that on (Plate xiv., fig. 1). 



