Annelids are the first animals to show this type of organization. Circulatory systems can be open or closed. Open systems lack capillaries and the blood perfuses through the tissues of the animal's body. Closed systems have capillary beds which connect arteries and veins. Annelids have closed circulatory systems.
Arthropods have open systems.
Echinoderms lack circulatory systems. Chordates have closed systems. This term describe the hollow body cavity of animals that forms from within the mesoderm itself. Because of this, the entire body cavity is lined with a layer of mesoderm surrounding both the inside of the body wall and the internal organs. The most simple digestive systems are closed or incomplete with no digestive glands.