[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 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.]