Polychaeta is relatively primitive annelids, apparently very close to the stem species of the Articulata. A general definition of the Polychaeta does not exist, except that they are worm-like, soft-bodied invertebrates with a metamerically organized body. The more than 10.000 species of polychaetes can be arranged into about 80 mostly well-defined families, which, however, often show little similarity. The meiofaunal polichaetes do not represent a single taxonomic group. They evolved in several independent lines from larger species which may have in or on top the sediment.
Length of polychaete species range between 3 m to less than 1 mm; the smallest wpecies are those of the genus Diurodrilus (about 300 um). There are several families which are exclusively meiofaunal: Dinophilidae, Diurodrilidae, Nerillidae, Protodrilidae, Protodriloidae, Saccocirridae, Polygordiidae, Parergodrilidae, Psammodrilidae, nearly all Psionidae and many Syllidae are meiobenthic.
Length of polychaete species range between 3 m to less than 1 mm; the smallest wpecies are those of the genus Diurodrilus (about 300 um). There are several families which are exclusively meiofaunal: Dinophilidae, Diurodrilidae, Nerillidae, Protodrilidae, Protodriloidae, Saccocirridae, Polygordiidae, Parergodrilidae, Psammodrilidae, nearly all Psionidae and many Syllidae are meiobenthic.
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