170 J. E. DuERDEN — Jamaican Actiniaria^: 



many included cells. On its inner border it gives rise to short, branching 

 processes for the support of tlie weak endodermal muscle. The endoderm is 

 much the narrowest of tlie three layers, and, in places, shows a distinct nervous 

 layer. 



The column-wall is much and deeply folded, and is of only medium thickness, 

 the mesogloea throughout being of about the same breadth as the ectoderm. The 

 fine ridges noticed among the external characters are seen in sections to be 

 produced either by coarse bulgings, or by long, narrow jirocesses of the middle 

 layer. Long gland cells with granular, non-staining contents occur in the 

 ectoderm in much greater abundance than at the base. No ectodermal 

 musculature can be distinguished. The inner border of the mesogloea is thrown 

 into very delicate, branching processes for the support of the endodermal circular 

 muscle ; this extends the whole length of the column, and is often most strongly 

 developed in the lower part of the column. At the actual apex, however, it is 

 better developed, and gives rise to a feeble sphincter muscle of the restricted type. 

 The endoderm is fibrillar or reticular on its mesogloeal aspect ; towards the free 

 border the cells bear zooxanthellBe, the nucleus of which stains deeply and is 

 highly refractive. From the lower stomodseal region downwards, the algae are 

 practically absent from the columnar endoderm of the column, and none occur at 

 the base. 



Considering the magnitude attained by the polj^ps, the sphincter muscle 

 (PI. xir., fig. 6) is remarkably feeble. The fibres are arranged on a few, narrow, 

 branching mesogloeal processes, developed for some little distance along the apex 

 of the column-wall, the whole being intermediate in form between a circumscribed 

 muscle, such as that of Stoichactis, and a diffuse sphincter, as in Corynactis. In 

 trul}^ radial sections, the middle mesogloeal processes are a little longer and more 

 branching than are represented in the partly tangential figure given, very closely 

 resembling those of Radianthus macrodactijlus (H. & S.), figured by Haddon (1898, 

 pi. xxxi., figs. 2, 3). In complete retraction of the polyps, the disc can be entirely 

 hidden. 



The tentacles are all alike in structure. The apex is crowded with long 

 narrow nematocysts showing the internal spiral thread ; an occasional granular 

 gland cell also occurs. A marked histological difference is apparent between the 

 capitulum and the stem, nematocysts occurring only in the former. In the stem 

 the ectoderm is narrower, and gland cells are more numerous, while the mesogloea 

 thins towards the apex. The endoderm is a thick layer, with irregular internal 

 boundaries ; small zooxanthellse are abundant, and less so glandular cells with 

 highly refractive contents. Both the endodermal and ectodermal muscles are 

 very feeble, and connected with the latter, a fibrillar and a nervous layer show 

 very distinctly at the capitulum. Nematocysts are absent from the ectoderm of 



