Porifera
Animals belonging to the Porifera phylum generally have a different operation of circulation that most phylums. Animals in the Porifera phylum use diffusion as the method for carrying out circulation. When a Porifera animal's body acquires oxygen, diffusion occurs in their circulatory system. Their capillaries act as the location for oxygen's diffusion into the bloodstream, while carbon dioxide diffuses out of the bloodstream. Animals belonging to the Porifera phylum could not acquire the required oxygen to go to the heart and circulate through out the body without the process of diffusion. Animals of the Porifera phylum are simple, as they have no distinct circulatory system.