Sponges are simple on the surface.
They don't look like animals at all, just blobs on the sea floor with holes in them. Looking closer at their anatomy, we see a set of chambers and tubes. Sponges continually pump water through their bodies. It is pumped through a network of tubes branching through the body of the sponge, and flows into large chambers, and out through large holes to the outside. Sponges have a skeleton.