The phylum Annelida, segmented worms, is characterized by the possession of a coelom (body cavity), setae (moving bristles), and a body divided into segments by transverse rings. The class Polychaeta, meaning "many hairs", is a large and extremely diverse group with more than 6,000 living species. It consists of earthworms, leeches and marine worms. The marine worms are then divided into two orders: the Errantia, free-moving form, and the Sedentaria, sedentary (tube dwelling) form. This convenient and widely used arrangement based on morphology is not always accepted, for some believe that it is an unnatural classification. However, there has been no other proposed system of classification that has widely gained acceptance.

A tube worm then, is any sedentary polychaete worm that inhabits a self-made tube (Iverson, 2000). Sedentary tube worms are usually marine and are isosmotic with the surrounding seawater. They secrete and live in their own encasing tube that is made of substances from specialized cells of their epidermis. This tube "base" may contain calcium carbonate, complex mixtures of proteins and polysaccharides, and/or sticky mucus, to which sediment such as sand and broken shells then adheres (Campbell, 1996). Often, tubes are doubled, with the outer part consisting of the newly added material, and the inner part being of mucus (Mill, 1978).

Tubeworms do not have the locomotory ability to "wriggle" or "walk", as exhibited by errant species (Barnes, 1993). Instead, during times of stress, the giant nerve cord of the tube worm transmits impulses from the anterior to posterior end of the body, causing the longitudinal muscles to contract. The worm then rapidly contracts into the tube as an escape reaction, or in reaction to changes in light intensity. The tube provides the worm with protection and support, and often times also provides a home for other commensal organisms, such as the scale worm (Iverson, 2000). When groups of these tube worms grow together a tube "reef" may form.

The body of a tube worm can be described as a tube within a tube, with the inner digestive tract separated from the outer body wall by the coelom (body cavity). The prostomium (head region) may be distinct or indistinct, and those with distinct heads often lack appendages. The next segment that surrounds the mouth is the peristome. Tube worms also may have eyes, which may be present on gills, on tentacles, on the peristome, or along the sides of the body. The peristome is followed by a series of segments on the thoracic and abdominal regions. These segments are normally covered in pairs of parapodia, which are paddle-like appendages that run down the sides of the body, ending in setae (sets of stiff bristles). However, in sedentary polychaetes parapodia are greatly reduced, highly modified or absent (Pechenik, 2000). Finally, tube worms are able to excrete waste through a fecal tube near the posterior tip. The movement of the water through the tubes for respiration, feeding and waste removal is accomplished by ciliary action, the motion of the modified parapodia, or waves of muscle contraction (Pechenik, 2000).

Tube worms are predominantly filter feeders and have branchiae (gills), which are simple filaments at the anterior end. They also serve in respiration and food gathering. Some species of tube worms filter feed by extending colorful, spiraling, feather-like tentacles into the water to fan for fine particulate matter (i.e. Sabella, the feather duster worm). Other feeding techniques include that of the deep-sea beardworms, which have no gut and instead use their tentacles to directly uptake dissolved organic material. Giant tube worms are unique in that they grow only near hydrothermal ventcracks deep in the ocean floor. There they enjoy pressures of a few thousand pounds per square inch (Svitil, 1998), and are able to synthesize hot minerals and chemicals of volcanic origin as their primary food source (Ocean Planet, 2000).

Tube worms display very simple systems of reproduction. Most have separate sexes, but indistinct gonads. In the coelom, the undifferentiated cells develop and mature, and the gametes are usually then shed through excretory openings into the water (Campbell, 1996). In some cases however, the entire worm may leave the tube before the gametes are released. Many tubeworms undergo epitoky, a morphological transformation that prepares them for reproductive activity. The epitoke is a male or female that is highly specialized for swimming and reproduction (Pechenik, 2000). Asexual reproduction is also known to occur in a few species such as Sabella. Hermaphroditism does not usually occur, as it does in oligochaetes (earthworms).

Finally, tube worms are capable of regeneration, however this regenerative ability is usually restricted to the anterior end. Few longevity studies have been carried out, and therefore the approximate lifespan of the tube worm is not known. Some polychaetes are able to reproduce once and then have the tendency to die (their sluggish nature after spawning provides predators with an easy target). Most tube dwellers though are able to survive the post-spawning period, but it is not known how many times.

Barnes, R.S.K., Calow, P. and Olive, P.J.W. 1993. The Invertebrates: A New Synthesis.

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Campbell, Neil A., 1996. Biology 4^th Edition. California: The Benjamin/Cummings

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http://www.eb.com:180/bol/topic?…

The Giant Tube Worms are Coming! Fast!!. Ocean Planet: Popular Science for Kids,

Smithsonian Institution. Accessed: 12 October 2000.

Available at: htttp://seawifs.gsfc.nasa.gov/OCEAN_PLANET/HTML/

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Iverson Invertebrate Glossary. Iverson Software. Accessed: 15 October 2000.

Available: http://iversonsoftware.com/business/invertebrate/Tube%20worm

Mill, P.J. 1978. Physiology of Annelids. Academic Press Inc., London.

Pechenik, J.A. 2000. Biology of the Invertebrates. McGraw Hill, Boston.

Svitil, K.A.. 1998. Bringing the tube worms back alive. Discover. Vol. 19, No. 5: 22-23.