Part I1.—Stichodactyline and Zoanthee. 195 
that the calcareous incrustations are limited towards the periphery of the wall, 
while the siliceous sponge spicules and a few Radiolarian tests are more internal 
and more distal. 
The endoderm cells are loaded with zooxanthellz. Proximally there is only 
a faint indication of an endodermal circular musculature, but towards the apical 
region the fibres become stronger and more concentrated, and constitute a weak, 
diffuse, endodermal sphincter muscle, the mesogloea forming deep, closely-arranged 
bays for its reception. No part of the muscle, however, becomes actually enclosed 
in mesogloea, except at the places where the mesogloea of the mesenteries is united 
with that of the column-wall (Pl. xm., fig. 7). 
The ccenosare surrounding the Hydroid stem, and connecting one polyp with 
another, contains inclusions similar to those of the column-wall. Irregular channels 
with a thick lining of endoderm serve as a means of communication between the 
ccelenteron of one polyp and another. In sections the Hydroid stem is completely 
embedded in mesogleea ; this latter also contains abundant cells and cell-islets. 
The ectoderm of the tentacles discloses a peripheral zone of small, narrow 
nematocysts throughout its length. The mesoglcea is thin and very slightly 
plaited for the support of a weak ectodermal and endodermal musculature, and a 
nervous layer connected with the former is distinguishable. The endoderm 
is loaded with zooxanthelle, and completely fills the lumen in contracted 
tentacles. 
The dise is extremely thin-walled, but becomes a little thicker near the 
tentacular region, where nematocysts and gland cells occur. 
The stomodzum is of small vertical extent. A single gonidial groove is 
indicated, very shallow in some examples and deeper in others, while the 
walls rarely display any vertical folding. The ectoderm is constituted of the 
usual ciliated supporting cells, granular gland cells, and but few nematocysts; in 
the region of the groove, glandular cells are very scarce. An ectodermal and 
endodermal musculature can be made out, though but feebly developed; an ecto- 
dermal nervous layer is also displayed. ‘The mesogloea is much thinner than 
the ectoderm, and undergoes no additional thickening at the groove. The 
endoderm is broad, and its cells contain many zooxanthelle. 
At its lower termination the wall of the stomodzum is backwardly and out- 
wardly directed for a short distance; and the ectoderm is in continuity with the 
tissue of almost exactly similar nature which runs radially along the edge of the 
perfect mesenteries, and, as the ‘‘ Reflected ectoderm,” passes for a very short 
distance up each face of the perfect mesenteries. 
The reflected ectoderm is not developed to the same extent ow all the 
mesenteries, and very rarely presents a similar appearance on the two faces of the 
same mesentery. It is constituted of extremely narrow ciliated supporting cells, 
