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.