| These taxa together form a paraphyletic group at the base of the clade of biramous arthropods. |
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| Crustacean biramous appendages have a basal or first portion referred to as the protopod. |
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| This problem is reflected in the ongoing uncertainty about the number of biramous appendages in the trilobite cephalon. |
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| The anteriormost five abdominal appendages are, almost without exception, biramous. |
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| An answer lies, he says, in the origin and modification of the ancestral biramous limb. |
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| A biramous limb typically has a basal part, or protopodite, bearing two branches, an inner endopodite and an outer exopodite. |
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| I assume that, as in Phacops and older trilobites, the cephalon bore one pair of antennae and three pairs of biramous limbs, and each thoracic segment a pair of limbs. |
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| The appendages are primitively branched, and although this condition is modified in many species, adults always have at least some biramous appendages. |
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| About ten limbs appear to have been present, which may have been biramous. |
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| Nevertheless, in spite of these differences the basic biramous division of the limb into exopodite and endopodite was consistently maintained throughout the group. |
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| At the end of the abdomen is a pair of slender biramous appendages, the uropods. |
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| The abdomen bears on each but the last segment a pair of ventral, or ventrolateral, biramous limbs called pereopods, or pleopods, which are primarily used in swimming. |
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| Trilobites, for instance, also possessed biramous appendages. |
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| It is unclear whether the biramous condition is a derived state which evolved in crustaceans, or whether the second branch of the limb has been lost in all other groups. |
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