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unicameral or bicameral muscular enlargements of the lymphatic vessels, at the points of entry of the vessels into the veins, found in cyclostomes, the majority of fishes, all amphibians and reptiles, and the embryos of all birds (in some birds, they are also preserved in the adults). The openings leading into the veins are valved. The walls consist of three layers: endothelium, striated muscle, and connective tissue. The pulsation of the lymph hearts helps to force lymph into the circulatory system. The pelvic lymph hearts of frogs contract 30–40 times a minute. The activity of the lymph hearts in fish and amphibians is regulated by the central nervous system.