142 J. E. Duekden — Jamaican Actiniaria : 



the chief constituents, but a few gland cells are also intermingled, though 

 not by any means so thickly as in the ectoderm of the column-wall. Towards 

 the mesogloea a fibrillar layer is very clearly displayed. No ectodermal muscle 

 is present, or is only of the weakest character. The mesogloea is typical 

 of its condition throughout the polyp, being very fibrous, and containing numerous 

 cellular constituents. To its shrinkage is perhaps due, in some degree, the fine 

 display of both the ectodermal and endodermal fibrillar la}^ers which occur 

 throughout all the polyps sectionized. 



As shown in the ectoderm of Pl.xi., fig. 1, very fine fibrils, arranged in an almost 

 parallel manner, extend from the delicate muscular layer, and afterwards unite 

 to form an extremely thin layer. This latter usually appears as if made up of 

 very delicate interlacing fibrillse; and on its outer side another series of fibrils, 

 irregularly arranged, are given off, and are connected with the cells of the 

 ectoderm. For the sake of distinction I restrict the term nerve layer to the 

 delicate, interlacing layer, and speak of that between it and the muscle layer as 

 the fibrillar layer. Sometimes, as is represented in the endoderm of the same 

 figure, the fibrils extending from the muscle fibres do not unite to form a definite 

 nerve laj^er, but interlace and are reticular in section. What appear to be 

 Ganglionic cells are distributed among the fibrillse. Appearances similar to the 

 above are also represented in the section through a portion of the tentacle 

 and also of the gonidial groove of Homostichanthns anemone (PL xv., fig. 1 ; 

 PL xiv., fig. 2), and can be made out in sections of most species. 



The endoderm of the base contains many zooxanthellae, which, along with 

 the cell protoplasm and nuclei, are mostly concentrated towards the free border of 

 the layer. A well-marked fibrillar stratum extends for some distance from the 

 endodermal muscle, and the latter is arranged on fine mesoglceal plaitings. 



The column-wall throughout is of only medium thickness, and becomes 

 thinner towards the apex ; the mesogloea is nowhere much broader than the 

 ectoderm or endoderm. The ectoderm is very deeply ridged in places, the 

 elevations being partly followed by long processes of the mesogloea, often 

 branching, and much longer than the whole thickness of the wall. The supporting 

 cells are scarcely so long as at the base ; but unicellular glands, some with 

 granular contents and others quite clear, are abundant. Maceration preparations 

 reveal the presence of numbers of small nematocysts. A very weak longitudinal 

 ectodermal muscle is present, and a nervous layer is readily distinguishable in 

 places. The endoderm is a deep layer containing zooxanthellae ; its constituent 

 cells are considerably elevated between the mesenteries where these are closely 

 arranged. Throughout the column the endodermal circular musculature is 

 developed with exceptional uniformity, the mesogloea being finely plaited for its 

 support, and a nervous layer is clearly seen in some parts (PL xi., fig. 1). 



