Original work on the renal structures in three classes of lower vertebrates is presented. By the application of light and electronmicroscopy, histochemistry and the compilation of available information on renal physiology and morphology, further evidence for the homology of nephron segments is forwarded. The results have been correlated with the habitat and evolutionary kinship of the lower vertebrates. A general evolutionary line can be drawn from early vertebrates (e.g. Petromyzontia) via archaic fresh-water fish (e.g. Polyteridae) and lungfish to the amphibian nephron. Renal structures of...