REPORT ON THE TETRACTINELLIDA. 
xlix 
supposing these to be oxeas ; a funnel-shaped cavity soon appears, its apex being directed 
towards the centre of the spicule, within this the axial rod is freely exposed (PL XLIII. 
fig. 18), and the increasing length of the liberated portion of the axial rod enables 
one readily to judge of the progress of the solution. Although at first sight the acid 
appears to remove all the substance of the spicule except the axial rod, careful 
observation will show that this is not the case, for a delicate film of organic matter 
also remains behind ; it has the form of a hollow sheath, corresponding in form and 
position with the outermost boundary of the original spicule ; between it and the axial 
rod the whole of the spicule is completely removed. The spicule thus consists of a 
central organic axis, surrounded by concentric layers of opal, the outermost of which is 
invested in a spicule sheath of organic matter or rather of organic matter in intimate 
association (chemical union ?) with silica. I long ago discovered this sheath by finding 
it as an insoluble residue after boiling some of the spicules of Plocamia 'plena, Sollas, 
in caustic potash, but did not then recognise it by this name.^ 
In making observations on the behaviour of spicules under the action of hydrofluoric 
acid, one has to provide first for the safety of the object-glass of the microscope and next 
to ensure the transparency of the preparation, for unless precautions are taken it will 
become obscured by the products of the action of the acid on the glass of the slide and 
the cover slip ; the glass slide is therefore protected by coating it with a film of Canada 
balsam, which is hardened by drying in the water-oven ; one side of the cover-slip is 
protected in the same way. The spicules isolated by treatment with nitric acid in the usual 
way are then placed along with a drop of water on the slide, and a drop of acid added, a 
ring of moderately fluid balsam is then run round the acid, the cover slip laid on — balsam 
face downwards and plenty of balsam run round the edge ; in this way the spicules may 
be examined with safety under a Zeiss objective “ D.” 
In all but the minutest microscleres, which are structureless and homogeneous, the 
spicule presents the structure just described, i.e., a central organic axis, which is con- 
centrically surrounded by successive layers of silica of very uniform thickness ; the 
latter are excessively numerous and consequently of extreme thinness, they are readily 
distinguished by transmitted light and when viewed by oblique light at certain angles 
give rise to interference colours, which render them iridescent. In all true spicules 
the axial rod of organic matter extends close to the termination of the spicule, and in 
some cases can be traced extending a little beyond it. In the Lithistid desmas, however, 
the axis is continued comparatively only a short distance into the desma, and the mass of 
the structure consists of concentric layers of silica only ; a difference can be discerned 
however between the axial and the peripheral portion of the more or less rod-like portion 
of the desma, a diflference not altogether dissimilar to that which exists between the central 
and peripheral parts of the horny fibres of the Ceratosa ; thus the central part is faintly 
1 Sollas, Ann. and Mag. Nat. Hist., ser. 5, vol. iv. p. 51, fig. 3, 1879. 
(zooL. CHALL. EXP. — PART Lxiii. — 1888.) Rrr 
