Final answer:
The large central opening in the body of a sponge through which water exits is called the osculum.
Step-by-step explanation:
The large central opening in the parazoan body, specifically in the body of a sponge, is called the osculum. It serves as the exit through which water leaves the sponge's body. The body of a sponge is organized around a central cavity called the spongocoel, and water flows into this cavity through numerous small openings called ostia. After passing through the spongocoel, water is expelled out of the sponge through the osculum. This simple flow system is part of what characterizes sponges as members of the phylum Porifera.