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. There are also cells that move around inside the body of the sponge. Looking closer still - sponge cells are really bizarre.
Sponges have many mobile cell types.
Each of these cells is a specialist. Some cells wander around the body of the sponge secreting spongin to make the skeleton. Other cells secrete spicules, others pick up food from where it is captured and transport it to the rest of the body, others collect wastes from other cells. The cells that pump the water through the sponge also capture food from the water. One particular kind of mobile cell can come by, pick up some of this food, move out to the sheets of cells that cover the surface of the sponge, and feed them.