File:The Quarterly journal of the Geological Society of London (12736007363).jpg

W. J. SOLLAS ON PHARETROSPONGIA STRAHANI. 247
sen ted in PL XI. fig. 6, where a fibre is longitudinally striated
throughout except at its rounded termination, where the striae are
replaced by circles.
This is due to the section crossing a fibre which at one part of its
course lies in the plane of the transparent slice, but afterwards
bends somewhat abruptly at right angles to it, and so presents us
with a transverse section of the fibre, in obvious continuity with a
longitudinal one (diagram, Fig. 6) ; and as the longitudinal section
Fig. 6.
Lateral view of a fibre bending at c, at right angles to the plane of the section,
a, b, thus producing the appearance shown in the section Plate XI. fig. 6.
of the fibre passes into the transverse one, the longitudinal acicular
section of the spicules passes correspondingly into the transverse
circular one.
It is noticeable that the spicules are not confined to the axis of the
fibre, as in the recent Chalinidse, but are uniformly crowded through-
out it ; the spicules, indeed, are the fibre, the granular material which
fills the interstices being no greater in amount than that required to
fill up the spaces between a number of cylindrical rods closely pressed
together. Nor do the spicules differ in size to any important extent ;
they are all, so far as one can see, of the same size and shape ; nor
do they follow two chief directions, one transverse and the other
longitudinal to the fibre, as in the Echinodermata, but are bound
together in parallel bundles like the Roman fasces.
It is difficult to find an example of a complete spicule sufficiently
isolated from the rest to determine its dimensions and the nature of
both its ends, though there are a few very exceptional cases in which
this can be accomplished. At the place, for instance, where two or
more fibres anastomose, the contained spicules cross one another at
various angles, and so become separately visible to a much greater
extent than when they all lie in a sheath side by side (PI. XI.
fig. 12, example from a recent sponge) ; occasionally also a single
spicule deviates from the general trend of the rest, sometimes even
to the extent of lying athwart them at right angles ; and then also
one has an opportunity of examining a spicule separately. In speci-
mens where the matrix of the network consists of the phosphatic