Porifera Phylum
Animals within the Porifera Phylum usually carry out the process of circulation in a different way that most phylums. The animals within the Porifera Phylum carry out the process of circulation through diffusion. In the circulatory system, when the body aquires oxygen diffusion takes place. In the capillaries, oxygen diffuses into the bloodstream, while carbon dioxide diffuses out. Without diffusion the animals could not aquire oxygen to got to the heart and circulate through out the body.
The only animal within the Porifera Phylum that has does not have a circulatory system but carries out a form of circulation is a Sponge. Sponges do not have a circulatory system. They depend on maintaining a constant water flow through their bodies to obtain food and oxygen and to remove wastes, and the shapes of their bodies are adapted to maximize the efficiency of the water flow. Throughout the body of the sponge, there are canals through which water flows. There is various variation in the complexity of these canals. The canals have openings to the outside which are called pores, where the water enters the sponge system these pores are usually small and are called "ostia" and where the water leaves the sponge system the pores are larger, often singular and are called "oscula" (singular osculum). Many if not most of these canals are lined with special flagellated cells called "choanocytes". These choanocytes keep the water flowing through the canals in the correct direction by beating their flagellum, they are also important in trapping food items.