Covering the sponge is a layer of cells known as the pinacoderm, which is composed of pinacocytes. In a sponge, pinacocytes are a thin, elastic layer which keeps water out. Between the pinacocytes, there are the porocytes that allow water into the sponge. Myocytes are small muscular cells that open and close the porocytes. Once through the pores, water travels down canals. The opening to a porocyte is a pore known as an ostium.