This is seen to a small degree in nematode, but to a much larger degree in annelids, arthropods and chordates. This terms describes the way that animals bodies are organized into repeating units called segments. 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.