Tiles on a sponge
Sponges are animals that live in the ocean (and some in lakes and rivers). They stay stuck in one spot and do not move around. They have no brain, no heart, and no stomach.
The outside of a sponge is covered by thin, flat cells called pinacocytes (say "pin-AK-oh-sites"). They fit together edge to edge, like tiles on a floor. Other pinacocytes line the water tunnels inside the sponge.
They are a lot like your own flat lining cells, the ones that line your mouth and blood vessels.
Wow!
Scientists have watched some sponges slowly "sneeze". The whole sponge squeezes, very slowly, to push dirt out of its tunnels.