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The longitudinal veins of the trunk of the Port Jackson shark exhibit low venous pressures and blood flow is facilitated by four subsidiary mechanisms. The sucking action of the heart is augmented by the presence of single flap valves at the central ends of certain longitudinal veins. The flexion of the trunk in swimming transfers blood from the dorsal aorta to the caudal vein; both the segmental arteries and the segmental veins are valved at their origins from the main vessels. Movement of the median dorsal fins and of the tail pumps blood from cutaneous veins to the caudal vein by the compression and dilation of valved venous reservoirs located close to radial muscles. Movement of the rectum generates negative pressures in certain cutaneous veins. A division of the trunk venous system, into abdominal and postpelvic regions is suggested on functional and anatomical grounds.