176 HYALONEMA (HYALONEMA) AGASSIZI. 
Just below the level in which the lateral rays of the dermal pinules of the 
outer surface extend, the paratangentially situated lateral rays of hypodermal 
pentactines are met (Plate 42, fig. 37a; Plate 45, fig. 23). In form C these 
lateral pentactine rays extend in the beams of the superficial network above 
described. The apical rays of the hypodermal pentactines extend radially in- 
ward. Inform A asuperficial zone about 0.6 mm. thick, underlying the dermal 
membrane, is occupied by dense masses of more or less radially arranged unci- 
nates and irregularly scattered microhexactines and microhexactine-derivates 
(Plate 45, fig. 23). This zone contains no spicules besides these and the proxi- 
mal rays of the hypodermal pentactines, which traverse it. Below this zone 
hexactine megascleres begin to make their appearance. Those lying nearest 
the surface are quite small, towards the interior they increase in size. Though 
often irregularly disposed in the sections, these spicules are, in the living sponge, 
in all probability regularly arranged in such a manner that two opposite rays 
extend longitudinally upward and downward, two radially outward and inward, 
and two paratangentially and laterally to the right and left. In most of the large 
and in a good many of the smaller hexactines the two opposite longitudinally 
extending rays are longer than the other four. Masses of large macramphidises 
are met with a little below the level where the hexactines begin to make their 
appearance. In some places these form but a thin layer, in others they extend 
a considerable distance, 2 mm. or more, into the interior of the choanosome. 
The inner parts of the sponge are occupied by the large hexactine mega- 
scleres referred to above, and also by rhabd-megascleres, uncinates, microhexac- 
tines, microhexactine-derivates, amphidises, and spheres. 
The large inner hexactines usually have two opposite, longitudinally extend- 
ing, greatly elongated rays and four shorter transverse rays. The rhabds of the 
axial part of the sponge are situated longitudinally and form a kind of axial 
column, which extends upwards to the summit of the gastral cone. Loose 
strands of rhabds diverge from this axial column and extend upwards and out- 
ward. Below, in the interior of the choanosome, these diverging rhabd-strands 
dissolve into scattered, obliquely situated, isolated rhabds; above they join to 
form distinct layers lying below the dermal and the gastral surfaces of the thin 
frill-like marginal part of the gastral wall. In the forms B, C, and D masses of 
longitudinal rhabds also occupy the vertical radial plates connecting the gastral 
cone with the gastral wall. Most of these rhabds are very blunt amphioxes or 
amphistrongyles; but sharp-pointed amphioxes, amphityles, styles, and tylo- 
styles also occur among them. In the axial column of form A both large and 
