so, each opening on a little eminence, as
it were. These apertures, bear in mind, we call _oscula_. They are the
exits of the sponge-domain. But a close inspection of a sponge shows
that it is riddled with finer and smaller apertures. These latter are
the _pores_, and they form the entrances to the sponge-domain.

On the banks of the canal you may see growing plentifully in summer
time a green sponge, which is the common fresh-water species. Now, if
you drop a living specimen of this species into a bowl of water, and
put some powdered indigo into the water, you may note how the currents
are perpetually being swept in by the pores and out by the oscula. In
every living sponge this perpetual and unceasing circulation of water
proceeds. This is the sole evidence the unassisted sight receives of
the vitality of the sponge-colony, and the importance of this
circulation in aiding life in these depths, to be fairly carried out
cannot readily be over-estimated.

[Illustration: WHERE SPONGES GROW.]

Let us now see how this circulation is maintained. Microscopically
regarded, we see here and there, in the sides of the sponge-passages,
little chambers and recesses which remind one of the passing-places in
a narrow canal. Lining these chambers, we see living sponge-units of a
type different from the shapeless specks we noted to occur in the
meshes of the sponge substance itself. The units of the recesses each
consist of a living particle, whose free extremity is raised into a
kind of collar, from which projects a lash-like filament known as a
flagellum.

This lash is in constant movement. It waves to and fro in the water,
and the collection of lashes we see in any one chamber acts as a
veritable brush, which by its movement not only sweeps water in by the
pores, but sends it onwards through the sponge, and in due time sends
it out by the bigger holes, or oscula. This constant circulation in
the sponge discharges more than one important function. For, as
already noted, it serves the purpose of nutrition, in that the
particles on which sponge-life is supported are swept into the colony.

Again, the fresh currents of water carry with them the oxygen gas
which is a necessity of sponge existence, as of human life; while,
thirdly, waste matters, inevitably alike in sponge and in man as the
result of living, are swept out of the colony, and discharged into the
sea beyond. Our bit of sponge has thus grown from a mere dry fragment
int