Osculum The osculum is an excretory structure in the living sponge, a large opening to the outside through which the current of water exits after passing through the spongocoel. Wastes diffuse into the water and the water exits through the osculum at a velocity of nearly 8.4 cm/second, carrying away with it the sponge's wastes. The size of the osculum is regulated by the myocyte. Its size, in turn, determines the amount of water flowing through the sponge. References