Gills are the primordial respiratory organs. Pharyngeal gills are a longstanding prototypical morphological feature of the phylum Chordata. External gills are rare in adult fish but play an important respiratory role in amphibian tadpoles and in water-breathing neotenic forms, e.g. Necturus maculatus. Development of external gills of larval amphibians is determined by the oxygen tension in water. In hypoxic water, the gills of Rana temporaria increase in size while in well-oxygenated water they regress. In amphibians, the contribution of gills to the overall respiratory process differs within and among species. The most elaborate gills are the internal ones of the bony fish. The mucous covering of the gills has been associated with various functions that include protection from mechanical damage, invasion by pathogens, and absorption and expropriation of toxic heavy metal ions. Fish can regulate the respiratory and osmoregulatory functions of gills. The various functions of gills occur at specific sites of a well-differentiated epithelium.