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Polychaeta, composed of about 10,000 species, is the larger (and apparently not monophyletic) of the two generally recognized major groups of segmented worms (phylum Annelida) – the other being the Clitellata (earthworms and leeches).
Polychaete worms are characterized by an elongated, metameric body usually bearing a pair of appendages called parapodia on each metamere (segment), as well as tufts of chaetae (spines served by muscles which typically can be extended and retracted; often the polychaetes are called bristle worms, and their name derives from the Latin for many bristles). Parapodia show vast diversity of form and function, serving purposes such as locomotion, gas exchange, protection, attachment, controlling water flow within a tube, or can be reduced or lost altogether. The polychaete head can be adorned with a multitude of sensory structures such as tentacular palps, antennae, and cirri. Predatory carnivores often have large pharyngeal jaws. At the end of the segmented body is the tail, called the pygidium, which houses the anus (Brusca and Brusca 2003)
Some polychaetes are free-living, with well developed muscles that allow them to move by swimming, crawling, or burrowing, often aided by parapodia adapted as paddles or legs. Burrowers often have a muscular proboscis to aid in digging. In contrast, sedentary polychaetes feed from permanent tubes or burrows, often by suspension feeding, selective deposit feeding, or feeding on detritus. Their parapodia are often adapted for circulating water in the tube. Permanent tube-dwellers have softer and less muscular bodies and frequently lose the septa between segments. This allows for adjustment of hydrostatic pressure within the worm, which is important for functions such as anchoring the end of the body housed in its tube. As well as providing protection, tubes also function as external support for these worms. Tubes can be soft, parchment-like forms constructed from sand and mucus, or hard calcareous tubes, which when many worms are together, form reef structures (Brusca and Brusca 2003)
Reflecting the large diversity of lifestyles and degree of independence of body segments, polychaete circulatory and respiratory systems also show many variations among taxa. Gas exchange in many polychaetes is facilitated by distinct gills, but in others it occurs across the entire body surface (especially in small or sedentary taxa with no parapodia, or in worms with no or partial internal body septa to separate coelomic spaces). Some taxa increase surface respiratory areas with feathery protrusions of the body surface through which the coelom extends. In these taxa, the circulatory system is reduced, and most oxygen and nutrients are distributed in the coelomic fluid. Some taxa have pumping structures to increase blood flow, especially sedentary worms that do not use body movements to circulate the blood. Most (but not all) polychaetes have oxygen-carrying pigments in their circulatory fluid and coelomic fluid, usually a form of haeomoglobin. Polychaetes display a large array of different sensory structures, including touch receptors; photoreceptors which may be developed into one or more pairs of anteriorly positioned eyes or distributed around the body; chemoreceptors, and statocysts.
(Brusca and Brusca 2003; Kozloff 1990)