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. There are sheets of cells that cover the surfaces of the sponge. There are cells that line that pump water and cover the walls of pumping chambers in the tubes that run through the sponge's body.