Part I1.—Stichodactyline and Zoanthece. 199 
any stain. The matrix is scarcely discernible except at the inner border, where 
it may sometimes be observed in connexion with a mesentery. Sponge spicules 
are present in both the ectoderm and mesogleea, though not to the same extent 
as might have been expected from the nature of the commensalism. Lacune 
also occur in decalcified specimens, indicating where calcareous sand-grains had 
been included. The endoderm is a broad layer, crowded with zooxanthellee. 
No basal musculature has been detected. 
At the boundary of the base and column occurs an expansion of the wall, 
there being, as noticed amongst the external characters, a slight formation of 
coenenchyme (PI. x1v., fig. 4). 
The ectoderm of the column-wall is a broad continuous layer, the columnar 
character of the cells not being clearly indicated in sections. _ Its internal limits 
are ill-defined, partly owing to the foreign inclusions tending to break up the 
layer, and also to the fact that abundant cells pass from it into the mesoglea. A 
small oval nematocyst, which does not stain, is scattered sparingly and irregularly 
throughout. 
The mesoglcea contains small, rounded or elongated cells with granular proto- 
plasm, and also cell-islets, not, however, to the same extent as in the mesoglcea 
of the base. ‘Towards its internal border a very irregular, narrow, encircling 
sinus occurs, and beyond this it is much more homogeneous. Owing to the 
strongly cellular nature of the outer region of the mesogloea the encircling sinus 
is not so distinct as in most of the species investigated by Haddon and Shackleton, 
nor as in P. tunicans, where the mesogleeal matrix is much more uniform. ‘The 
cells included in the sinus possess very granular protoplasm, and abundant nema- 
tocysts similar to those in the ectoderm; these latter are particularly numerous 
in the distal region of the polyps. 
Numerous cellular connexions can be traced between the irregular internal 
limits of the ectoderm and those of the encircling sinus. 
The whole mesoglceal layer contains foreign inclusions, more abundant, how- 
ever, peripherally; they are mainly calcareous sand-grains which are dissolved 
out by acids. Silicious sponge spicules are particularly abundant in the upper 
region of the column, and always remain in microscopic preparations. The 
sand-grains are more restricted in their distribution to the region of contact of 
the ectoderm and mesoglea. 
The endoderm of the column-wall resembles that of the base, but above is 
much thickened between the mesenteries, while it is narrow below. An extremely 
weak, endodermal musculature extends the whole length of the column. At the 
capitulum, the mesogloea becomes sinuous in sections, and the muscle fibres are 
here a little stronger and represent the sphincter muscle (Pl. x1v., fig. 4). 
The sphincter is of a diffuse endodermal character; the mesogleal folds 
22 
