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,
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).