The Annelida phylum consists of a very diverse group of worms, which range from the simple Earthworm, to the majestic Feather Duster Worm. Unlike the previous two Phyla, the Annelida group contains more detailed circulatory system, which includes a series of blood vessels, and sometimes a heart. This means that unlike, Flatworms and Roundworms, The Annelida do not gain nutrients by diffusion of nutrients which go against a concentration gradient.