Anseriformes is an order of birds that comprises about 180 living species in three families: Anhimidae (the screamers), Anseranatidae (the magpie goose), and Anatidae, the largest family, which includes over 170 species of waterfowl, among them the ducks, geese, and swans. In fact, these living species are all included in Anatidae except for the 3 screamers and a magpie goose. All species in the order are highly adapted for an aquatic existence at the water surface. The males, except for the screamers, also have a penis, a trait that has disappeared in Neoaves. All are web-footed for efficient swimming (although some have subsequently become mainly terrestrial).