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Quadrupedalism or pronograde posture is a form of terrestrial locomotion in animals using four limbs or legs. An animal or machine that usually moves in a quadrupedal manner is known as a quadruped, meaning "four feet" (from the Latin ''quad'' for "four" and ''ped'' for "foot"). The majority of quadrupeds are vertebrate animals, including mammals such as cattle, dogs and cats, and reptiles, like lizards. Worms, jellies, crustaceans, gastropods, cephalopods, echinoderms (sea stars, brittle stars, sea urchins, sea cucumbers etc.), millipedes, centipedes, arachnids, insects, fish, caecillians, snakes, birds, cetaceans (whales, dolphins etc.), and humans are usually not quadrupeds, with some exceptions; for example, among the insects, the praying mantis is a quadruped. A few birds may use quadrupedal movement in some circumstances; for example, the shoebill will sometimes use its wings to right itself after lunging at prey. ==Quadrupeds vs. tetrapods== Although the words ''quadruped'' and ''tetrapod'' are both derived from terms meaning "four-footed", they have distinct meanings. A tetrapod is any member of the taxonomic unit ''Tetrapoda'' (which is defined by descent from a specific four-limbed ancestor) whereas a quadruped actually uses four limbs for locomotion. Not all tetrapods are quadrupeds and not all quadrupeds are tetrapods. The distinction between quadrupeds and tetrapods is important in evolutionary biology, particularly in the context of tetrapods whose limbs have adapted to other roles (e.g. hands in the case of humans, wings in the case of birds, and fins in the case of whales). All of these animals are tetrapods, but none is a quadruped. Even snakes, whose limbs have become vestigial or lost entirely, are nevertheless tetrapods. Most quadrupedal animals are tetrapods but there are a few exceptions. For example, among the insects, the praying mantis is a quadruped. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Quadrupedalism」の詳細全文を読む スポンサード リンク
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