J. E. Dusrpen—Jamaiean Actiniaria: Part I.—Zoanthee. 373 
is well defined; the ectoderm continuous and variable in height, with regular 
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 mesoglea. 
The mesogleea 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 mesoglea, 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 mesogloea. 
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 mesogloeal plaitings is 
seen in transverse sections. The mesogloea 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 ot the disc are but little 
developed, presenting a marked contrast to the tentacles. 
Csophagus.—The ectoderm of the esophagus is richly ciliated and folded ; the 
