132 ARTHUR DENDY. 
towards the base of the sponge. This is the only situation in 
which I have detected quadriradiate spicules, and even here 
they are few in number, and the apical rays, which project 
towards the gastral cavity, are very feebly developed. The 
widely extended oral rays of the subgastral sagittal triradiates 
may also be regarded as taking part in the formation of the 
gastral cortex, but these spicules are best considered in con- 
nection with the next portion of the skeleton. 
Skeleton of the Chamber-bearing Layer.—The part 
of the sponge lying between the gastral aud dermal cortex and 
containing the flagellated chambers is, in the piece examined 
by me, a little over 4 mm. in thickness, the total thickness of 
the sponge-wall being about one sixth of an inch. Its skeleton 
is very strongly developed, and (excluding the triradiates which 
line the large exhalant canals, and which resemble more or less 
those of the gastral cortex) it consists of the following parts: 
(a) The subgastral sagittal triradiates. Very well 
developed and abundant; occupying the normal position, with 
widely extended oral rays lying beneath the gastral cortex, and 
long straight basal rays penetrating the chamber layer more 
or less vertically or obliquely (fig. 4). 
(b) The spicular fibres (figs. 3—5, fi.). These consist 
of long bundles of the characteristic slender, elongated, 
“tuning-fork” spicules. The component spicules are closely 
packed together side by side, parallel with one another (fig. 4). 
So closely are they packed together that in a stout fibre it is 
very difficult to make out the outlines of the individual 
spicules. There does not seem to be any special connecting 
substance analogous to the spongin of siliceous sponges, but 
the spicules appear to be held together simply by the gela- 
tinous ground-substance of the mesoderm. The arrangement 
of the fibres is very similar to that of the spicular fibres in 
many siliceous sponges. They do not simply run through the 
wall of the sponge from gastral to dermal surface, but they run 
in every direction, and, by frequently coming in contact 
and crossing one another at all sorts of angles, give rise to a 
loose, irregular network (fig. 3). The thickness of the fibres 
