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Polychaetes are a class of ubiquitous segmented worms, mostly marine, though a few species have adapted to terrestrial life in humid areas. They are annelids, the marine counterparts of terrestrial annelids like earthworms. "Polychaete" means "many hairs," a reference to the chitinous hairs that protrude from either side of their bodies, with an identical set of hairs per segment. Like terrestrial annelids, polychaetes have an advantage over simpler worms due to their segmentation, which helps give them a more rigid structure more conducive to secondary adaptations.
Like a few other common animal phyla, polychaetes have been around since the Lower Cambrian, approximately 540 million years ago. Definitive polychaete fossils have been found in the Sirius Passet Lagerstatte, alongside primitive arthropods. They may date back to even earlier, as segmented hollow skeleton tubes (Cloudina) have been found from the Ediacaran, reminiscent of the tubes used by some modern polychaetes, though consensus is absent on the matter. Cloudina tubes are among the earliest mineralized skeletons, and the earliest fossils to show evidence of predatory boreholes.
There are about 10,000 species of polychaetes, with great diversity in lifestyles and adaptations. Some, like the lugworm, spend their entire lives in U-shaped burrows in the intertidal zone. Coiled castings discarded above the surface are clearly visible on beaches when the tide goes out, and fisherman sometimes dig the worms out to use as bait.
Another typical polychaete is the common clam worm, which scavenges for algae and other worms on the sea bottom, and provides an important food source for crustaceans and bottom-dwelling fish. When it is time for the clam worm to reproduce, it engages in a reproductive mode unique to polychaetes, epitoky. During epitoky, the worm's body suddenly changes drastically, developing stronger swimming appendages, producing ova and sperm, and enhancing sensory and motor centers to the detriment of the digestive system. It swims from the bottom up to the pelagic zone, where the plankton live, and ejects its packet of gametes, where it mixes with gametes from other worms to produce larvae. These larva feed within the plankton, eventually sinking to the bottom and changing into worms.
Some remarkable polychaetes have learned to adapt to the most unusual of environments. The bone-eating snot flower, for instance, is a recently discovered genus which subsists on whale falls, the corpses of whales which sink to the bottom of the sea. It burrows into bones with the help of bacteria, which it is so symbiotic with that it requires no stomach or mouth, instead absorbing nutrients with an odd root-like structure. The Pompeii worm, another bottom-dwelling polychaete that lives on bacteria around hydrothermal vents, is among the most heat-tolerant of organisms, capable of withstanding temperatures up to 80 degrees C (176 degrees F). Another polychaete, Lamellibrachia, is one of the longest-living animals in the world, with ages of up to 250 years.
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