Practice question

Closed circulatory system confines blood exclusively within continuous circuit of arteries, capillaries and veins lined by endothelium maintaining pressure gradient produced by muscular hearts, permitting efficient oxygen delivery to distant tissues, characteristic of annelids and chordates. In earthworm dorsal vessel collects blood caudal to cephalic direction, ventral vessel distributes, commissural aortic arches in anterior segments contract rhythmically acting as auxiliary hearts, capillary networks between. Insects, most molluscs and arthropods show open hemocoel, thus earthworm among listed taxa exemplifies closed circulation pattern.