370 J. HK. Durrpen—Jamaican Actiniaria : Part I.—Zoanthee. 
Harbour in water of not more than half a fathom in depth, and only a few yards 
from the shore. The polyps are very sensitive and active, retracting immediately 
on being touched. 
Column-wall (Pl. xx., figs. 1, 2).—The outline of the column-wall, owing to the 
presence of incrusting material, is very irregular in sections, especially in the 
lower part; in the region of the marginal denticulations it is sinuous, and thicker, 
and the incrusting matter is aggregated opposite the inner circle of tentacles. 
Where perfect, the ectoderm is continuous; in most places, it is broken up or 
absent. It is covered on the outside by a cuticle with an adhering layer of 
foreign material, mostly diatom frustrules and fine mud. 
The mesogleea varies in thickness, being much better developed proximally. 
It contains isolated cells with long processes, cell-islets, and irregular communi- 
cating canals. The incrustations are sparsely distributed, and are mostly siliceous 
sand grains and a few sponge spicules. 
The endoderm is very thin and regular, and the transverse muscle is well 
developed. 
Sphincter muscle (Pl. xx., fig. 1)—The single mesogloeal sphincter muscle is 
small, and formed in a few, irregular, closely set cavities, extending about half 
way across the mesogloea, and situated just at the base of the outer row of 
tentacles. The lining muscle-fibres are weak, and other rounded cells partially 
fill up the cavities. : 
Tentacles (PI. xx., figs. 2, 3).—The ectoderm of the tentacles is thick compared 
with the two other layers, and the ectodermal muscle is well developed on small 
mesoglceal plaitings ; numerous small oval nematocysts occur, and pigment 
granules in places. The nervous layer is clearly distinguished between the 
nucleated zone and the muscle fibres, and sends connecting strands to each. The 
mesoglcea and endoderm are both very thin. An endodermal muscle layer is 
present, seen in longitudinal sections. 
Disc (Pl. xx., fig. 2).—The structure of the disc is much like that of the ten- 
tacles, but the ectoderm is not so well developed, and its outer grooved portion is 
in places loaded with pigment granules. 
Csophagus.—In extended specimens the cesophagus is almost circular in out- 
line; the cesophageal groove is quite shallow. In longitudinal sections the wall is 
thrown into transverse folds. ‘The ectoderm is avery regular, ciliated layer, with 
abundant gland-cells and a few small nematocysts; pigment granules occur in 
the deeper parts; it is reflected above the lower termination of the cesophagus, 
and below forms the mesenterial filaments in the usual manner. The mesoglea 
and endoderm are very thin, especially the latter. 
Mesenteries (Pl. xx., fig. 4).—Sixteen pairs of mesenteries, presenting the usual 
