Biped

Biped (Lat., 'two-footed'), a descriptive term, sometimes applied to man, but more frequently to birds. It may be used in two ways—(a) in reference to habit only (physiological), when animals use only their two hind-limbs for moving along the ground—e.g. man, kangaroo, bird; (b) in reference to anatomy (morphology), when the typical number of four limbs is reduced to two. Thus among mammals the order of whales (Cetacea) is marked by the absence of hind-limbs; among birds the fore-limbs are considerably reduced in the running birds of the ostrich sub-class, and especially in the New Zealand Kiwi (Apteryx); among reptiles, some serpents (e.g. Pythons) retain traces of hind-legs, while all the others have lost both pairs, and a few lizards have either only hind-feet (Pseudopus, Ophiodes), or only fore-feet (Chirotes); among amphibians, a few (e.g. Siren) have only fore-feet; and the same is true of numerous fishes (e.g. among Siluridae), and especially of those which live to a large extent in mud.