An appendage is, in general, an external body part that projects from the body, or a natural prolongation or projection from a part of any organism, such as a vertebrate's limb.
In invertebratebiology, "appendage" is a general term that covers any of the homologousbody parts that may extend from a body segment. These include antennae, mouthparts (including mandibles, maxillaeand maxillipeds), wings, elytra, gills, walking legs (pereiopods), swimming legs (pleopods), sexual organs (gonopods) and parts of the tail (uropods). Typically, each body segment carries one pair of appendages.
Appendages may be uniramous, as in insectsand 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 and controlled by "homeobox" genes. Changes to these genes have allowed scientists to produce animals (chiefly Drosophila melanogaster) with unnatural appendages, such as legs instead of antennae.
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It uses material from the http://en.wikipedia.org/wiki/Appendage Wikipedia article Appendage.