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Simplified structures of different types of hearts, showing schematic diagrams of hearts and directions of blood flow. (A) Simple chordates have tubular hearts, some of which pump bidirectionally. The hearts of ancestral bilaterians probably had a similar structure. Fish hearts (B) have a single atrium and ventricle, whereas the hearts of reptiles, birds, and mammals (C) have two atrial and two ventricular chambers. Oxygenated blood is shown in red and deoxygenated blood in blue.
