Anseriformes is an order of birds also known as waterfowl that comprises 178 living species of birds in three families: Anhimidae, Anseranatidae, and Anatidae, the largest family, which includes the other 174 species of waterfowl, among them the ducks, geese, and swans. Most modern species in the order are highly adapted for an aquatic existence at the water surface. With the exception of screamers, males have penises, a trait that has been lost in the Neoaves, the clade consisting of all other modern birds except the galliformes and paleognaths. Due to their aquatic nature, most species are web-footed.