The correct answer is the flow of water through the body of a sponge.
The sponges are multi-cellular species in which the organ systems like the nervous system, digestive system, circulatory system, and others are absent. They possess a specific feeding system in which they maintain the continuous flow of water through their bodies. This water movement helps them to attain oxygen and food and remove waste.
The movement of water all through the body of the sponge is unidirectional in nature, it gets inside the body of sponge via ostia, and exit through the osculum. With the external pumping of water, the cells found in the walls filter out the food present in the water.
