part of part of has part has part in taxon develops from contributes to morphology of Caenorhabditis elegans Drosophila melanogaster Vertebrata <vertebrates> heart Taxon notes:" the ascidian tube-like heart lacks chambers....The ascidian heart is formed after metamorphosis as a simple tube-like structure with a single-layered myoepithelium that is continuous with a single-layered pericar- dial wall. It lacks chambers and endocardium.... The innovation of the chambered heart was a key event in vertebrate evolution, because the chambered heart generates one-way blood flow with high pressure, a critical requirement for the efficient blood supply of large-body vertebrates... all extant vertebrates have hearts with two or more chambers (Moorman and Christoffels 2003)" DOI:10.1101/gad.1485706 mesoderm-derived structure heart tube cardiac chamber cardiovascular system thoracic segment organ primary circulatory organ structure with developmental contribution from neural crest heart plus pericardium