As lymph is a filtrate of blood, it closely resembles the plasma in its water content.
The posterior pair is on either side of a long, rod-like bone called a urostyle, formed by the fusion of the last few vertebrae.
The other pair at the end of the vertebral column pump lymph into the iliac vein in the legs.
[2] These hearts vary in size from microscopic in lungfish to an estimated 20-liter capacity in some of the largest dinosaurs.
Conversely, reptiles have a single pair of lymph hearts in the pelvic area.
[citation needed] In flightless (ratite) birds, the lymph heart function is less clear and the two almond-sized hearts located near the spinal column close to the hip joint are thought[by whom?]
to be involved in inflating and deflating the phallus with lymph, which is of a significant size in both sexes of emus and ostriches.