Sedentary Polychaetes
Sedentary polychaetes are usually adapted to living permanently in tubes or burrows; some attach themselves to rocks or piers. Many sedentary polychaetes, like the lugworm, Arenicola, live in burrows in sand or mud. The majority, however, are tube builders. Tubes of different species vary greatly in their composition and structure. They may be composed of sand, shell, or other particles held together with mucus, or made entirely of organic substances secreted by the worm that harden on contact with water. The tubes may be straight, branched, spiraled, or U-shaped. Most are permanently attached to a substrate, and the worm seldom or never ventures outside; however, the tube worm Cistenides moves about the seafloor, dragging along its delicate tube of sand grains. Sedentary polychaetes have greatly modified head regions for specialized feeding habits. Many are adapted for feeding on organic matter deposited on the ocean floor. For example, the lugworms have a simple, thin-walled, jawless proboscis, which is used to draw sand into the gut, where organic matter is removed. Other worms have feeding tentacles that extend from the tube opening and creep along the mud or sand, picking up organic deposits. Still others of the Sedentaria are filter feeders: the beautiful feather-duster worms have a crown of feathery, ciliated tentacles that extend from the tube opening to sweep small planktonic organisms from the water. The tentacles are quickly withdrawn if the animal is startled. The parapodia are reduced in the sedentary polychaetes, and the setae of many tube-dwelling forms are hooked to help the worm hold itself to the wall of its tube.
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