iving parts. These living parts consist of minute masses of that
living jelly to which the name of _protoplasm_ has been applied. This,
in truth, is the universal matter of life. It is the one substance
with which life everywhere is associated, and as we see it simply in
the sponge, so also we behold it (only in more complex guise) in the
man. Now, the living parts of this dried cast-away sponge were found
both in its interior and on its surface. They lined the canals that
everywhere permeate the sponge-substance, and microscopic examination
has told us a great deal about their nature.

[Illustration: FIG. 1. DEVELOPMENT OF A SPONGE (_Olynthus_). 1. The
egg. 2, 3, and 4. The process of egg-division. 5 and 6. The
gastrula-stage. 7. The perfect sponge.]

For, whether found in the canals of the sponge themselves, or embedded
in the sponge-substance, the living sponge-particles are represented
each by a semi-independent mass of protoplasm. So that the first view
I would have you take of the sponge as a living mass, is, that it is a
colony and not a single unit. It is composed, in other words, of
aggregated masses of living particles, which bud out one from the
other, and manufacture the supporting skeleton we know as "the sponge
of commerce" itself. Under the microscope, these living sponge-units
appear in various guises and shapes. Some of them are formless, and,
as to shape, ever-altering masses, resembling that familiar animalcule
of our pools we know as the _Amoeba_. These members of the
sponge-colony form the bulk of the population. They are embedded in
the sponge substance; they wander about through the meshes of the
sponge; they seize food and flourish and grow; and they probably also
give origin to the "eggs" from which new sponges are in due course
produced.

More characteristic however, are certain units of this living
sponge-colony which live in the lining membrane of the canals. In
point of fact, a sponge is a kind of Venice, a certain proportion of
whose inhabitants, like those of the famous Queen of the Adriatic
herself, live on the banks of the waterways. Just as in Venice we find
the provisions for the denizens of the city brought to the inhabitants
by the canals, so from the water, which, as we shall see, is
perpetually circulating through a sponge, the members of the
sponge-colony receive their food.

Look, again, at the sponge-fragment which lies before us. You perceive
half a dozen large holes or