J. E. Duerden — Jamaican Actiniaria : Part I. — Zoanthece. 373 



is well defined ; the ectoderm continuous and variable in height, with irregular 

 internal limitations. Excepting a narrow zone immediately below the cuticle, the 

 ectoderm cells are crowded with abundant yellow pigment granules of various 

 sizes and numerous medium-sized, oval, colourless nematocysts. It is practically 

 free from inclosures, these being limited to the mesoglcea. 



The mesoglcea shows a very marked division into two parts. The outer is a 

 thick layer of variable dimensions, and crowded with foreign inclosures and 

 abundant yellow pigment granules, limited internally by the encircling sinus. 

 This latter is broken here and there by strands of mesoglcea, and has very 

 irregular limitations ; the cavities are filled with deeply staining tissue and pig- 

 ment. The inner layer of the mesoglcea is clear and nearly homogeneous, devoid 

 of incrustations and pigment granules, and plaited internally to support the 

 endodermal muscle. The incrusted part in retracted specimens is enlarged a 

 little below the middle of the column, and contains cell-enclosures. The incrus- 

 tations consist of siliceous and a few calcareous sand grains, and sponge spicules. 

 The mesoglcea is too crowded with incrustations and pigment matter to allow of 

 any connecting canals which may be present between the ectoderm and the 

 encircling sinus being distinguished. 



The endoderm cells are high, especially between the mesenteries, and contain 

 abundant yellow pigment spheres and granules; a little below the middle of the 

 column, they give rise to a well developed circular endodermal muscle supported on 

 folds of the mesoglcea. 



Sphincter muscle (PL xx., fig. 5). — The sphincter muscle is diffuse and endo- 

 dermal, and formed as a greater concentration of the ordinary endodermal muscle 

 of the column-wall. Distally it is so deeply sunk in the folds of the mesoglcea that 

 in some sections it appears to be entirely cut off from the endoderm, and to 

 become a mesoglceal muscle enclosed in separate cavities. 



Tentacles. — In transverse sections of retracted polyps, the tentacles are so 

 closely arranged as to become polygonal in outline. The ectoderm is thick, and 

 has an outer zone of narrow nematocysts capable of staining ; below is an irregular 

 zone of pigment granules. An ectodermal muscle on slight mesoglceal plaitings is 

 seen in transverse sections. The mesoglcea is only a thin layer, and internally is 

 thrown into folds for the support of the circular endodermal muscle. 



The endoderm has abundant pigment spheres and fills the lumen in contrac- 

 tion. The distinction between the granular pigment matter in the ectoderm and 

 the spherical form in the endoderm, although of the same colour, is very marked. 

 A similar difference is found in the ectoderm and endoderm of the column, but not 

 to such a degree as in the tentacles. All the three layers of the disc are but little 

 developed, presenting a marked contrast to the tentacles. 



(Esophagus. — The ectoderm of the oesophagus is richly ciliated and folded; the 



