Figure 1.  Diagram showing the general structure of a common sponge, and a detail drawing of a simple sponge showing how water flows through the animal.   Redrawn from Barnes 1980. 

Water and microscopic food particles are drawn into a sponge by the action of choanocyte cells located in the interior walls of a sponge.  Each choanocyte has a tail-like flagellum that, together with thousands of others, beats to create a water current that draws water in through the porocytes into the atrium, and out the osculum.  Food is filtered from the water by cells in the atrium.