epidermis - the layer of cells that covers the outer surface of the sponge. The thin, flattened cells of the epidermis are called pinacocytes. holdfast - root-like tendrils that attach the sponge to rocks. osculum - the large openings in a sponge through which water flows out of the sponge. Sponges may have more than one oscula. water flows in - water flows into a sponge through pores (holes) located all over its body. The sponge obtains its nutrients and oxygen by processing this flowing water. water flows out - water flows out of a sponge through large openings called oscula.