Final answer:
The osculum is the opening in a sponge's body through which water containing wastes is expelled after the water has been filtered for nutrients and oxygen. It is an essential part of the sponge's excretory system.
Step-by-step explanation:
Sponges are simple multicellular organisms that filter water to obtain nutrients. The structure of a sponge includes a large central cavity known as the spongocoel, which occupies the inside of what can often be an irregular cylindrical shape. Water enters the spongocoel through many small pores called ostia. The water carries microscopic food particles and oxygen into the sponge.
After the water is filtered, it is expelled out of the sponge through a large opening known as the osculum. This opening is crucial for the sponge's excretory and circulatory functions, as sponges do not have a circulatory system, and gases, nutrients, and wastes are exchanged by diffusion. In addition to expelling filtered water, the osculum also serves as the exit point for waste products generated by the sponge.