The osculum (pl.: oscula) 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 is pumped through the osculum carrying away with it the sponge's wastes. Sponges pump large volumes of water: typically a volume of water equal to the sponge's body size is pumped every five seconds.The size of the osculum is regulated by contractile myocytes. Its size, in turn, is one of the factors which determines the amount of water flowing through the sponge. It can be closed completely in response to excess silt in the water.
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Osculum in the context of Gastrointestinal tract
The gastrointestinal tract (also called the GI tract, digestive tract, and the alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The tract is one of the largest of the body's systems. The GI tract contains all the major organs of the digestive system, in humans and other animals, including the esophagus, stomach, and intestines. Food taken in through the mouth is digested to extract nutrients and absorb energy, and the waste expelled at the anus as feces. Gastrointestinal is an adjective meaning of or pertaining to the stomach and intestines.
Most animals have a "through-gut" or complete digestive tract. Exceptions are more primitive ones: sponges have small pores (ostia) throughout their body for digestion and a larger dorsal pore (osculum) for excretion, comb jellies have both a ventral mouth and dorsal anal pores, while cnidarians and acoels have a single pore for both digestion and excretion.