204 J. E. Duerden — Jamaican Actiniaria : 



relations of one cavity to another. More internal than these cell-spaces the 

 mesoglcea is practically homogeneous, and affords a sharp boundary line with the 

 endoderm. 



The endoderm is broad and contains numerous zooxanthellae ; its cells are 

 much elongated in the narrow mesenterial spaces. Only faint indications of an 

 endodermal circular musculature can be made out. It is unfortunate that in the 

 capitular region, where the sphincter muscle should occur, the walls are so thin 

 and the large sponge spicules so closely aggregated, as to render suitable sections 

 a matter of practical impossibility. The delicate walls in every case readily 

 break away with the inclusions. 



From a knowledge of related forms it can, I think, be safely inferred that if 

 a sphincter is developed it will be of an endodermal, diffuse, and extremely weak 

 type. The power of retraction is not possessed to the same degree as in the 

 previous species. 



The ectoderm of the tentacles is a broad, columnar layer ; small, narrow 

 nematoc3^sts occur peripherally, and occasionally one of the larger oval forms 

 similar to those in the column-wall. The merest traces of an ectodermal and 

 also of an endodermal musculature can be detected. The mesoglcea is extremely 

 narrow ; the endoderm is loaded with zooxanthellse and with deeply-staining 

 granules of various sizes, and, in retracted examples, completely fills the lumen. 



The disc in all its three layers is a very thin structure. In retracted 

 specimens it is deeply concave outwardly, affording space above for the tentacles, 

 while below it comes almost in contact with the floor of the ccelenteron, nearly 

 obliterating the ccelenteric cavity. In consequence of the extreme shortness of 

 the polyps as a whole, and of this approximation of the disc and base, the study 

 of the paired arrangement of the mesenteries is almost fruitless. 



The peristome remains elevated, and the stomodseuru is comparatively large 

 in sections. As shown in PL xin., fig. 9, transverse sections pass at the same 

 time through the column-wall, tentacles, elevated peristome, and stomodseum ; only 

 exceptionally can a mesentery be traced from the column-wall to the stoniodasuin. 



The stomodseum is usually oval-shajDed in tranverse sections, and the single 

 gonidial groove is clearly indicated ; the lateral walls may be thrown into a few 

 vertical folds. The ectodermal epithelium consists mainly of ciliated supporting 

 cells with nematocysts and gland cells ; the two latter are practically absent at 

 the sulcar end where the groove occurs. Large granular gland cells, the contents 

 of which do not stain in borax carmine, are also met with. The mesoglcea is a 

 very thin layer, thickening somewhat at the groove ; the endoderm is broad and 

 crowded with zooxanthellae. 



In vertical sections the stomodseal ectoderm is seen to be in continuity with the 

 filamental tissue of the mesenteries. The lower termination of the stoinodaeum is 



