capable of blood circulation primary circulatory organ Gene notes: Bmp, Nkx, Gata note we reserve the subclass 'heart' from the vertebrate multi-chambered heart. 'The first heart-like organ is believed to have appeared 500my ago in an ancestral bilaterian'. Amniotes: four-chambered heart. Amphibians: two atria, one ventricle, pulmonary; fish: single atrium and ventricle; amphioxus: tubular, non-striated, closed, unidirectional; ascidians: tubular, striated, open, bidirectional; arthropods: tubular, open; C elegans: contractile pharynx; Cnideria: striated muscle cells associated with gastrodermis circulatory organ