Covering the sponge is a layer of cells, very similar to skin, but it's slightly different. To scientists, these cells are known as pinacocytes. In a sponge, pinacocytes are a thin, elastic layer which keeps water out. Between the pinacocytes, there are the porocytes. These let water into the sponge. Myocytes, little muscular cells, open up the porocytes and close them. Once through the pores, water travels down canals. Using the food and oxygen in the water the sponge cells stay alive and carry out other processes such as making new sponges and repairing cells. The opening porocytes surround is called an ostium.