The Polychaeta or Polychaetes are a class of annelid worms, generally marine, with a pair of fleshy protrusions on each body segment called parapodia that bear many bristles, called setae, which are made of Chitin. Polychaeta means "many-bristled" (as opposed to the Oligochaeta which are "few-bristled"), and indeed the polychaetes are sometimes referred to as bristle worms. More than 10,000 species are described in this class, but common representatives are the lugworm (Arenicola marina) and the sandworm or clam wormNereis.
The polychaetes' paddle-like and highly vascularized parapodia are used for movement and act as the animal's primary respiratory surface (parapodia can be thought of as a kind of external gills that are also used for locomotion). Polychaeta also have well_developed heads compared to other annelids.
Taxonomically, the polychaetes are thought to be paraphyletic, meaning an assemblage of not very closely related forms, but all of which come from the same source.
Polychaetes are extremely variable in both form and lifestyle and include freeliving (with many swimming among plankton), burrowing, and tube-dwelling species as well as commensals. The free living forms or Errantia tend to have well-developed sense organs and jaws, while the Sedentaria (or stationary forms) lack them but may have specialized gills or tentacles used for respiration and filter-feeding, similar to the fanworms.
One notable polychaete, the Pompeii worm (Alvinella pompejana) is endemic to the hydrothermal vents of the Pacific Ocean. Pompeii worms are thought to be the most heat-tolerant complex animals known.
Bristleworms can be, and usually, are a real problem for reef hobbyists.
Whereas small bristleworms may look really harmless, larger specimens that have grown to sizes of 24 inches or more in length are quite impressive and can cause serious damage.
It may trap some real small worms but it will, in the majority of cases, not trap the real large ones, the ones you really need to get rid of as they are the ones that cause most of the damage.
The fishes may learn not to attack the worms (although this will vary from fish to fish; my mature Premnas female either is a masochist or has intelligence measured in negative numbers as she continues to persist in attacking the worms) and such learning is likely good for years.
The eunicid worms found in aquaria all have five antennae, with a large pronounced antennae arising from the center of the "forehead." Large eunicid worms from two distinct groups are found in aquaria.
After the prey is immobilized the worm extends a proboscis from the bottom of the head into the mucus and the jaws grasp the body of the prey.
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