The caudal heart of fish: not a lymph heart.

The caudal heart is traditionally regarded as part of the lymphatic vessel system in fish. Recently, however, a secondary blood vessel system has been demonstrated in this class of vertebrates, while the existence of lymphatics was questioned. In corrosion cats of the secondary system it is shown that the caudal heart functions as a venous pump in the secondary circulation. Contrary to current views fish do not have true lymphatic vessels. Theories on the phylogeny of lymphatics therefore should be revised accordingly.