Lymph heart

A lymph heart is an organ which pumps lymph in lungfishes, amphibians, reptiles, and flightless birds back into the circulatory system. In some amphibian species, lymph hearts are in pairs, and may number as many as 200 in one animal the size of a worm, while salamanders have as many as 23 pairs of lymph hearts.
In frogs.
The lymphatic system of a frog consists of lymph, lymph vessels, lymph heart, lymph spaces and spleen. Some mast cells can also be found in the lymphatics of the tongue of some of the frog species.
Lymphatics and lymph.
As lymph is a filtrate of blood, it closely resembles the plasma in its water content. Lymph also contains a small amount of metabolic waste and a much smaller amount of protein than that of blood. Lymph vessels carry the lymph and, in the frog, open into the four lymph hearts. These lymph hearts are located on the dorsal side of frog's body. The front pair is situated below the shoulder blades. 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 anterior pair opens into the subclavian vein and the posterior pair into the femoral vein. The pair near the third vertebra pumps lymph into the jugular vein. The other pair at the end of the vertebral column pump lymph into the iliac vein in the legs.
Mast cells in the lymphatics of the tongue.
"Rana catesbiana" and "Rana nigromaculata" have mast cells in the lymphatics of the tongue. These are round and in the monocellular layer of the lymphatic walls either in close adhesion or in contact with the cytoplasmic process.
Mechanism of the lymph hearts.
The lymph hearts rhythmically and slowly pump to drive the lymph into the veins. It is possible to see the lymph hearts beat by looking on the dorsal surface on either side of the urostyle. In the toad, the normal lymph heart rate is about 50 beats per minute. Thus the lymph emerging out of blood ultimately merges into the blood. It returns the proteins back to blood.
Lymph heart size and pump rate.
These hearts vary in size from microscopic in lungfish to an estimated 20-liter capacity in some of the largest dinosaurs. In frogs and turtles they pump at rates higher than the blood heart and the volumes pumped are quite remarkable. In toads and frogs, this volume can amount to about 1/50 the output of blood from the heart. In amphibians, lymph hearts lie at vein junctions. Frogs and salamanders have 10 to 20 lymph hearts, while caecilians have more than 100. Conversely, reptiles have single pair of lymph hearts in the pelvic area. 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 to be involved in inflating and deflating the phallus with lymph, which is of a significant size in both sexes of emus and ostriches.