Five classes of related arthropods are believed to have a common origin distinct from the chelicerates and crustaceans. These five groups are collectively called Uniramians due to their apparently unbranched appendages. The first four are also known collectively as myriapods. The most distinct uniramian characteristic is a single pair of antennae. Their mouth parts include a pair of unjointed mandibles, which are unlike the jointed mandibles of crustaceans.
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