exopodite

An exopodite is the external or outer branch of a biramous arthropod limb, one of the two main branches that make up the limb in many crustaceans and related groups. The other branch is the endopodite, and both typically arise from a common proximal base known as the protopodite. The term combines exo- meaning outside and -podite from the Latin root for foot or limb.

In crustaceans, exopodites are often segmented and can bear setae (bristles) along their margins. They may be relatively slender or broaden into lamellate or paddle-like structures, depending on the taxon and its mode of life. The exopodite frequently participates in locomotion, propulsion, and in some species in respiration or filter feeding, either as a swimming tagma or as a limb component adapted for specific ecological roles.

Developmentally, the exopodite forms as a distinct branch from the proximal limb basis or protopodite during the ontogeny of the limb, maintaining its separate articulation with the endopodite. Variation among groups ranges from well-developed, multi-segmented exopodites to reduced or modified forms in lineages where limb morphology has changed significantly.

Etymology reflects its position as the outer branch of the limb. The presence, absence, or modification of exopodites is a useful feature in comparative anatomy and phylogenetic studies of arthropods, helping to distinguish relationships among crustaceans and other fossil and living groups.