An appendage (or outgrowth) is an external body part or natural prolongation that protrudes from an organism's body such as an arm or a leg. Protrusions from single-celled bacteria and archaea are known as cell-surface appendages or surface appendages. In many kinds of eukaryotic cells, the protrusions are known as membrane protrusions or cell appendages (examples include microvilli and cilia).
In , an appendage can refer to a locomotor part such as a tail, fish fin on a fish, limbs (, legs, flippers or ) on a tetrapod; exposed sex organ; defensive parts such as horns and ; or such as auricles, proboscis (trunk and snout) and barbels.
All , a mollusc class, have flexible appendages known as . They may have further extensions as suckers.
In , an appendage refers to any of the homologous body parts that may extend from a body segment, including antennae, mouthparts (including mandibles, maxillae and ), , locomotory arthropod leg ( for walking, and for swimming), (), and parts of the tail (). All arthropod appendages are variations of the same basic structure (being homologous with one another), and which structure is produced is controlled by "homeobox" genes; alterations to these genes have allowed scientists to produce genetically modified animals, such as fruit flies ( Drosophila melanogaster) with legs growing where the antennae would have on the head.
Appendages may become uniramous, as in and , where each appendage comprises a single series of segments, or it may be biramous, as in many , where each appendage branches into two sections. Triramous (branching into three) appendages are also possible. In the past this difference in leg development became the basis of arthropod classification, with Uniramia being a former group within Arthropoda.
Archaella are the similar structures to bacterial flagella, serving the same function in motility, particularly swimming, but with a different composition and action. Pili are used for attachment to surfaces, possible communication between cells enabling cell-to-cell contact allowing genetic transfer, and the formation of . A type IV pili model is used in the assembly of several cell surface structures. The bindisome is made up of sugar binding proteins to facilitate sugar uptake. So far studies are limited to S. solfataricus. Appendage fibres described as Iho670 fibres are unique to Ignicoccus hospitalis.
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