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. Other sponges have little hard needles - spicules to stiffen their skeleton. Washing with one of these sponges would be like using sandpaper in the bath. Looking still closer, a sponge is made up of three basic sorts of cells.