EEPORT THE TETRACTINELLIDA. 
clxv 
is poured on, and if after examination under tlie microscope the spicules are found to be 
clear and dry, they may be at once mounted in balsam ; if they have a dirty appearance 
they must be again washed with alcohol, but if the process has been carefully followed 
out this will seldom be found necessary. There are many advantages in this method, one 
of the chief being the fact that a complete sampling is obtained of the spicules of the 
Sponge, but few even of the most minute being lost. It is also speedy, but has the defect 
of not furnishing duplicates ; if duplicates be desired it is best to bod the piece of Sponge 
in a watch-glass and to wash the residual spicules in water and absolute alcohol before 
transferring to glass slides ; or the contents of the watch-glass may be emptied into a 
conical wine-glass filled with water, which may be left to siphon off through a wide 
capdlary glass tube, the last traces being removed by a triangular piece of blotting paper 
supported vertically, absolute alcohol is added as before and the spicules transferred to 
glass slides by a dipping tube. 
In the case of Lithistid Sponges, the spicules of which are grown together into a 
dense network, additional steps are necessary. A fragment of the Sponge is first boiled 
in nitric acid in a watch-glass, this liberates all the loose spicules, including the young 
forms of the desma, and an occasional almost adult example not yet completely 
incorporated with the skeleton. The skeleton itself remains as a coherent network, 
which may be removed with the forceps and washed in a beaker of distilled water. 
The nitric acid is removed from the loose spicules in the way already described in the 
case of the Choristida. The skeletal network is cut with a razor into fairly thin slices, 
which are thrown into water to separate useless chips, the slices are then removed, some 
are dried and mounted at once in balsam, others are subjected to further treatment iu 
order to isolate the component desmas, with a view to studying their general form. 
This may be accomplished by boiling in caustic potash in a silver vessel, or by treatment 
with hydrofluoric acid. The later is the simplest plan, but both yield equally good 
results. In treating with hydrofluoric acid the thin slice of skeletal network is placed 
on a clean silver coin (a three-penny piece answers the purpose), it is covered with water 
and a drop of the acid added ; after a few minutes, the exact time of course depends 
upon the size of the slice, the desmas fall apart with the slightest teasing, and imme- 
diately this happens the further action of the acid must be arrested ; this is most simply 
accomplished by plunging the coin into a watch-glass filled with water, the water is 
then siphoned off with a wide capillary tube, and the last traces by blotting paper ; a 
second washing with water follows, and after this is removed .the spicules are washed 
out of the watch-glass by absolute alcohol ; to accomplish this with thoroughness 
the glass should be held vertically and the alcohol delivered into it by means of a 
pipette ; the edge of the watch-glass should touch the glass slide so that the alcohol as 
it flows out may form a continuous bridge between the slide and the watch-glass. This 
ensures complete transference of all the desmas to the slide. 
