Sirenia is an order of fully aquatic, herbivorous mammals that inhabit rivers, estuaries, coastal marine waters, swamps and marine wetlands.
Sirenians, including manatees and dugongs, possess major aquatic adaptations: arms used for steering, a paddle used for propulsion, and remnants of hind limbs (legs) in the form of two small bones floating deep in the muscle. They look ponderous and clumsy but are actually fusiform, hydrodynamic and highly muscular, and mariners before the mid-nineteenth century referred to them as mermaids.