Polychaeta

Article By:

Eeckhaut, Igor Laboratory of Marine Biology, University of Mons-Hainaut, Mons, Belgium.

Hartman, Olga Allan Hancock Foundation, University of Southern California, Los Angeles, California.

Last reviewed:January 2021

DOI:https://doi.org/10.1036/1097-8542.533900

The largest class of the phylum Annelida (multisegmented worms) with about 13,000 described species. Polychaetes (Fig. 1) range from less than 1 mm (0.04 in.) to up to 3 m (10 ft) in length. Represented worldwide, they are predominantly marine organisms and occur from the intertidal zone to the greatest ocean depths at all latitudes. Some notable exceptions occupy nonmarine habitats such as supralittoral zones (beyond the high-tide mark) and freshwater lakes. The oldest known entire fossilized species dates back to the mid-Cambrian, but records on trace fossils suggest that the origin of the group predates this period. The only synapomorphy (a derived trait shared by two or more taxa that is believed to reflect their shared ancestry) that distinguishes them from other annelids is the pair of nuchal organs, chemosensory structures located on the prostomium (the portion of the head anterior to the mouth). See also: Annelida; Deep-sea fauna

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