Appendage

An appendage (or outgrowth) is an external body part, or natural prolongation, that protrudes from an organism's or microorganism's body.

Types in animals
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, locomotor legs (pereiopods for walking, and pleopods for 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 maxilliped or gnathopod.

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.

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.

Types in prokaryotes
A number of cell surface appendages are found in prokaryotes – bacteria and archaea, and include flagella, pili, and prosthecae also called stalks.

Archaea
A number of surface appendages may be present on different archaea. Two types of appendage are species-specific; cannulae are specific to Pyrodictium species, and hami are specific to Altiarchaeum. Other various types of surface structure include pili, archaella (archaeal flagella), structures called bindisomes that bind sugars, and posttranslationally modified archaellins and pilins.

Archaella are the similar structures to bacterial flagella with the same function in motility particularly swimming, but with a different composition and action. Pili are used in attachment to surfaces, possible communication between cells enabling cell to cell contact allowing genetic transfer, and the formation of biofilms. 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.

Bacteria
Bacterial surface appendages include flagella, pili, short attachment pili known as fimbriae, and on some species curli fibres. Some bacteria also have stalks known as prosthecae.

Types in plants
A leaf is the main appendage of a plant stem. Prosthechea is a genus of orchids named for the prostheca appendage on the back of the column. Hair like structures known as trichomes are found on many types of plants.