Be prepared to make comparisons of the three different phyla. Compare important characteristics of the three phyla. The pattern of water circulation through a sponge is inward through the pores and outward through the osculum. The pores have a greater total cross-sectional area then the osculum, so more water can move through them. Pumping of water by flagela is mechanically more efficient in small tubes than in large tubes.
Small tubes have a greater total surface area then large tubes.
The flow pattern is a secondary result of the evolutionary history of the sponges.