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...some [[invertebrate]] groups have an open cardiovascular system. The most primitive animal [[phylum|phyla]] lack circulatory systems. The lymphatic system, on ...ns return the now oxygen-rich blood to the heart, where it enters the left atrium before flowing through the [[mitral valve]] into the left ventricle. Then,

Precursor = [[primitive ventricle]], [[bulbus cordis]] | ...receives [[Oxygenation_(medical)|deoxygen]]ated [[blood]] from the [[right atrium]] via the [[tricuspid valve]], and pumps it into the [[pulmonary artery]] v

* [[Primitive atrium]]