biramous

Biramous describes a type of arthropod appendage that divides into two distinct distal branches. The two branches, typically called the endopod and exopod, arise from a common proximal base known as the protopod. This branching pattern contrasts with uniramous limbs, which consist of a single series of segments without a bifurcation.

In crustaceans, the biramous limb is a standard feature. The endopod and exopod are usually accompanied by additional segmentation and may bear setae or specialized structures. The two branches can be of different sizes and forms, reflecting functional specialization such as walking, swimming, or feeding. Although most crustacean limbs are biramous, some lineages show reduction or modification of one branch.

Biramous limbs are a defining characteristic of many crustaceans and are often used in arthropod systematics to distinguish crustaceans from other subphyla, such as insects and myriapods, which typically have uniramous appendages. However, evolutionary transitions do occur, and some groups exhibit partial loss or simplification of one branch in particular appendages.

In summary, biramous appendages are two-branched limbs sharing a common base, a key feature in crustacean morphology and a reference point in studies of arthropod evolution and functional anatomy.