An appendage (or outgrowth) is an external body part, or natural prolongation, that protrudes from an organism's body. In arthropods, an appendage refers to any of the homology (biology), homologous body parts that may extend from a body segment, including antenna (biology), antennae, arthropod mouthparts, mouthparts (including Mandible (arthropod mouthpart), mandibles, maxilla (arthropod mouthpart), maxillae and maxillipeds), gills, animal locomotion, locomotor arthropod leg, legs (pereiopods for walking, and pleopods for aquatic locomotion, swimming), sexual organs (gonopods), and parts of the tail (uropods). Typically, each body segment carries one pair of appendages. An appendage which is modified to assist in feeding is known as a wikt:maxilliped, maxilliped or gnathopod. In vertebrates, an appendage can refer to a locomotor part such as a tail, fish fin, fins on a fish, limb (anatomy), limbs (leg#Tetrapod legs, legs, flipper (anatomy), flippers or wings) on a tetrapod; exposed sex organ; defensive parts such as horn (anatomy), horns and antlers; or sensory organs such as auricle (anatomy), auricles, proboscis#Vertebrates, proboscis (Elephant#Trunk, trunk and snout) and barbel (anatomy), barbels. Appendages may become ''uniramous'', as in insects and centipedes, where each appendage comprises a single series of segments, or it may be ''biramous'', as in many crustaceans, where each appendage branches into two sections. ''Triramous'' (branching into three) appendages are also possible. All arthropod appendages are variations of the same basic structure (''homologous''), and which structure is produced is controlled by "homeobox" genes. Changes to these genes have allowed scientists to produce animals (chiefly ''Drosophila melanogaster'') with modified appendages, such as legs instead of antennae.

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