170 J. E. Duserpen—Jamaican Actiniaria’: 
many included cells. On its inner border it gives rise to short, branching 
processes for the support of the weak endodermal muscle. The endoderm is 
much the narrowest of the 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 processes 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 mesoglceal aspect; towards the free 
border the cells bear zooxanthellze, the nucleus of which stains deeply and is 
highly refractive. From the lower stomodzal region downwards, the alge are 
practically absent from the columnar endoderm of the column, and none occur at 
the base. 
Considering the magnitude attained by the polyps, the sphincter muscle 
(Pl. xu., fig. 6) is remarkably feeble. The fibres are arranged on a few, narrow, 
branching mesoglceal 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 
truly radial sections, the middle mesoglceal processes are a little longer and more 
branching than are represented in the partly tangential figure given, very closely 
resembling those of Radianthus macrodactylus (H. & S.), figured by Haddon (1898, 
pl. 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 mesogleea 
thins towards the apex. The endoderm is a thick layer, with irregular internal 
boundaries; small zooxanthelle 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 
