J. E. Durrpen—Jamaican Actiniaria: Part I.—Zoanthee. 837 
Column-wall (Pl. xviit.a, fig. 1).—A cuticle and sub-cuticla are present, the 
former having much adhering matter, such as diatoms; the sub-cuticla is in 
places minutely convoluted, evidently as a result of the excessive shrinkage of 
the mesoglosa. This is more especially seen in longitudinal sections. 
The ectoderm of the column-wall is broad above, but very narrow below. 
It is much vacuolated, with only a small amount of cellular tissue remaining in 
the form of strands passing from the outer to the inner boundary; rarely a 
connecting strand of mesogloea is seen. Some examples are not so highly 
vacuolated, particularly in the lower part. Abundant medium-sized, oval, non- 
staining nematocysts are present, the inner thread showing distinctly ; pigment 
granules are met with at its internal boundary. 
An irregular layer of spherical lacunze appears in the mesogloea, immediately 
below the ectoderm. Proximally the empty spaces extend further into the meso- 
gleea. The mesoglcea is broad in the region of the sphincter muscle, but narrows 
much below. Cells with long, fine processes are distributed sparingly throughout ; 
delicate fibrils can also be easily seen passing from the ectoderm to an irregular, 
much broken, encircling sinus. The latter, situated either about the middle or 
very near the endodermal border of the mesogloea, is formed of spaces varying 
in dimensions and form. They contain a small quantity of cellular tissue, and 
are connected with one another by larger or smaller canals, and also by canals 
with the ectoderm and the endoderm. The cells are multipolar in character. 
The endoderm is low and contains abundant zooxanthellae; nematocysts, 
similar to those in the ectoderm are present, and a weak circular muscle. 
Sphincter musele (Pl. xvut.a, fig. 1),—The sphincter muscle is mesogloeal 
and double, the two halves being distinctly separated. The upper or distal is 
smaller, and located in large irregular cavities extending almost across the 
mesogloea, diminishing both proximally and distally ; the liming of muscle cells 
is thin. The lower or proximal part of the muscle is contained in a large 
number of small, scattered, mostly circular, mesoglceal cavities; the distal ones, 
however, are elongated, more like those in the upper. The muscle fibres are 
very small in section, and only a little loose tissue is present in addition. 
Tentacles—The ectoderm of the tentacles is without cuticle or sub-cuticla, 
and shows two kinds of nematocysts—an outer thick zone of the usual narrow 
form, and an occasional medium-sized, oval-shaped form similar to those in the 
ectoderm of the column-wall. The mesoglcea is very thin. 
The endoderm is well developed, and crowded with zooxanthellae. Both 
the ectodermal and endodermal musculatures are weak. 
Dise.—The ectoderm of the disc is almost devoid of nematocysts. The 
mesoglcea is a little thicker, and the endodorm much thinner than in the tentacles. 
The endodermal muscle is seen in longitudinal sections. 
TRANS. ROY. DUB. SOC., N.S. VOE. VI., PART XIV, 3F 
