REPORT ON THE TETRACTINELLIDA. 
125 
maximum height, was brought up on a fishing line used from the shore ; it is now divided 
into two parts by a transverse section, but no part is missing. The surface is more or 
less incrusted with foreign bodies ; on the upper surface these are chiefly small fragments 
of shell, on the sides and lower surface small pebbles, which are attached by strong 
fibrous bands. 
There are as many as six large oscules on the upper surface, the largest measuring 12 
by 8 mm. Each opens through a thick membranous margin into a large cloacal cavity, 
Avithin which a species of Ophiurid is sometimes found. The walls of the cloaca are pro- 
duced into ridges and thick membranous partitions, between which the excurrent canals 
open by round or oval apertures, which are spanned over by a fibrous network with large 
oval meshes. 
The ectosome, as in other species of Pilochrota, extends further inwards than in most 
corticate sponges, in so far as it surrounds the canals, which in other sponges would be 
knoAvn as the subcortical crypts ; its total thickness varies from IT mm. to 1*6 mm. ; the 
distance from the surface to’ the homologues of the subcortical crypts is 0’5 mm. to 
0‘65 mm. on an average. Beneath the outer epithelium is a layer of chiasters, followed 
by a darkly stained fibrous felt, usually about 0T6 mm., but sometimes reduced to 
0’03 mm. in thickness; this passes into a layer of collenchyme 0’24 mm. thick, contain- 
ing numerous fusiform cells; oval vesicular cells 0'012 mm. in diameter, and apparently 
empty except for a very evident nucleus, 0'004 mm. in diameter; and faintly or not at 
all stained granule-cells, which occur singly or in groups of from two or three to a great 
number, forming oval or round clusters 0’06 mm. to OT mm. in diameter. These last 
cells, when occurring singly, are oval in outline, and about 0’012 mm. to 0'016 mm. in 
diameter. The component granules are spherical and of very uniform size, about 
0’002 mm. in diameter ; they appear to be stained at the edges, but not in the middle, 
an appearance which may be due to the presence of an intracellular protoplasmic network. 
An oval granular space, about 0'004 mm. in diameter, is sometimes present amidst the 
granules, and is often more deeply stained than they are ; it may represent a nucleus. 
When aggregated in clusters the cells become polygonal by appression, and are sometimes 
separated from each other by thin deeply-stained partitions, which form a kind of inter- 
cellular framework. Though most abundant in the outer half of the cortex, these cells 
and cellular aggregates are not confined to it, but occur generally throughout the 
sponge, and are especially noticeable in the walls of the larger water canals. Occasionally 
they spread out immediately below the lining epithelium of a canal in a single layer, 
which has somewhat the appearance of a layer of granular epithelial cells, like those 
figured by Polejaeff in his Eeport on the Calcareous Sponges? 
The middle collenchymatous layer of the cortex passes into an inner fibrous felt, 
0’24 mm. thick, and beneath this lie the intercortical canals, which are homologous with 
1 Polejaeff, Zool. Chall. Exp., part xxiv. pi. viii. fig. 8. 
