The body plan of a sponge is simple: a single outer layer of cells, the pinacoderm,  separates the inner cellular region known as the mesohyl from the outside. The pinacoderm lines the internal canals and is eventually replaced by the choanoderm, a layer of flagellated collar cells (choanocytes) grouped in chambers.  Choanocytes make up the principle ‘pump’ and’ filter’ of the system, driving water, suspended bacteria and other particulate food through the sponge, which is then digested. The flow of water inside a sponge is one-way: the water is drawn in through tiny pores (ostia) in the pinacoderm and exits through one or more larger openings (osculae).  The pumping  system of a sponge is usually supported by a combination of one or both of these skeletal elements: mineral spicules (either calcareous or siliceous) and special protein fibers (spongin).                                                                       Adapted from the Tree of Life - Porifera