Digestion in porifera, or sponges, occurs in individual cells, which envelop and break down food particles. Sponges lack tissues, organs and organ systems, so they don't have the same digestive systems as more complex animals. The food particles absorbed are tiny and include single-celled plankton, bacteria, fragments of larger organisms and particles of waste matter.
The main type of cell that captures food in a sponge is called a collar cell. These cells use flagella to pull water through pores in the surface of the sponge, filtering out any food particles carried on the resulting current. The collar cells are embedded in a collagen skeleton the cells excrete, which is reinforced by small, hard structures made of calcium or silica compounds.
Different groups of sponges vary in the complexity of their structures. The simplest types are tubes open at one end. The collar cells line the interior cavity, and the water is pulled in and forced out of the opening of the tube. A more complex design is similarly shaped, but the body walls are thicker, and the pores are lengthened into channels lined with collar cells. The most complex sponges have more variable shapes, and their canals lead to chambers lined with flagellated cells.