542-488 million years ago

urochordate with blood vessels

The tunicate heart is homologous to that of vertebrates and shares features of vertebrate hearts such as pacemaker cells, gap junctions, the requirement of calcium by cardiac muscle, and a pericardium.

The tunicate heart is surrounded by both a pericardium and pericardial fluid that contribute to the pressures which move blood (Webster, 1974, p. 47).
Urochordates possess a primitive heart in which the blood flow periodically reverses due to peristaltic waves which can travel in either direction over the heart muscle, even when the heart is removed from the body (Harris; Hoar, 1983). There is no endothelial layer lining the interior of the tunicate heart and striated muscle contacts the blood directly. The muscle cells in tunicates require calcium for their contraction (as in vertebrates) and are joined by tight junctions and gap junctions. (Webster, 1974, p. 48-9; Burighel, from Harrison, 1997). There are pacemaker cells at either end of the heart and peristaltic waves move blood in the hearts of tunicates (Webster, 1974, p. 46; Prosser, 1973). There seems to be no nervous control of heart rate (Prosser, 1973).