External gills

External gills are the gills of an animal, most typically an amphibian, that are exposed to the environment, rather than set inside the pharynx and covered by gill slits, as they are in most fishes. Instead, the respiratory organs are set on a frill of stalks protruding from the sides of an animals head. External gills are the gills of an animal, most typically an amphibian, that are exposed to the environment, rather than set inside the pharynx and covered by gill slits, as they are in most fishes. Instead, the respiratory organs are set on a frill of stalks protruding from the sides of an animals head. This type of gill is most commonly observed on the aquatic larva of most species of salamanders, lungfish, and bichirs (which have only one large pair), and are retained by neotenic adult salamanders and some species of adult lungfish. They are present on non-transforming salamander species, such as most members of the Proteidae family (the olm and mudpuppies) and the Sirenidae family, which naturally never metamorphose into an air-breathing form. The embryos of frogs and caecilians also develop external gills at some point in their development, though these are either resorbed before or disappear shortly after hatching. Fossils of the distant relatives of modern amphibians, such as Branchiosaurus and Apateon, also show evidence of external gills.