Appendage

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).

Types in animals

In echinoderms an appendage called a pedicellaria is found. The end of the pedicellaria consists of valves that give a jaw-like appearance and is thought to be used to clear the external body surface. Echinoderms also possess podia known as tube feet. Tube feet form part of the water vascular system and are used for locomotion, food and waste transportation, and respiration.

In vertebrates, an appendage can refer to a locomotor part such as a tail, fins on a fish, limbs (arms, legs, flippers or wings) on a tetrapod; exposed sex organ; defensive parts such as horns and antlers; or sensory organs such as auricles, proboscis (trunk and snout) and barbels.

In protostomes

In annelid worms, lateral protrusions from the body are called parapodia. In the class of polychaetes, chitinized lateral protrusions are called chaeta.

All cephalopods, a mollusc class, have flexible appendages known as cephalopod limbs. They may have further extensions as suckers.

In arthropods, an appendage refers to any of the homologous body parts that may extend from a body segment, including antennae, mouthparts (including mandibles, maxillae and maxillipeds), gills, locomotory legs (pereiopods for walking, and pleopods for swimming), sexual organs (gonopods), and parts of the tail (uropods). 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 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. In the past this difference in leg development became the basis of arthropod classification, with Uniramia being a former group within Arthropoda.