Order Poecilostomatoida Thorell, 1859
   01 Family Catiniidae Bocquet & Stock, 1957
   02 Family Echiurophilidae Delamare-Deboutteville & Nunes-Ruivo, 1955
   03 Family Ergasilidae von Nordmann, 1832
   04 Family Gastrodelphyidae List, 1889
   05 Family Mytilicolidae Bocquet & Stock, 1957
   06 Family Serpulicolidae J.-s. Ho et al., 2006, nom. null. see Family Serpulidicolidae Stock, 1979
   07.1 Family Philichthyidae Vogt, 1877
   07.2 Family Vahiniidae Humes, 1967
   08.1 Family Clausidiidae Embleton, 1901
   08.2.1 Family Erebonasteridae Humes, 1987
   08.2.2.1 Family Lubbockiidae Huys & Böttger-Schnack, 1997
   08.2.2.2 Family Oncaeidae Giesbrecht, 1892[1893]
   09.1.1 Family Shiinoidae Cressey, 1975
   09.1.2 Family Eunicicolidae G.O. Sars, 1918
   09.2.1 Family Corallovexiidae Stock, 1975
   09.2.2 Family Lamippidae Joliet, 1882
   09.2.3.1 Family Antheacheridae M. Sars, 1870
   09.2.3.2 Family Ventriculinidae Leigh-Sharpe, 1934
   09.2.4.1 Family Mesoglicolidae de Zulueta, 1911
   09.2.4.2 Family Micrallectidae Huys, 2001
   09.2.4.3 Family Phyllodicolidae Delamare-Deboutteville & Laubier, 1961
   09.2.4.4 Family Pionodesmotidae Bonnier, 1898
   09.2.4.4.0 Family "Pionodesmatidae" J.-s. Ho et al., 2006, nom. null. see Family Pionodesmotidae Bonnier, 1898
   09.2.4.5 Family Saccopsidae Lützen, 1964
   09.2.4.6.1 Family Lernaeosoleidae Yamaguti, 1963
   09.2.4.6.2.1 Family Chitonophilidae Avdeev & Sirenko, 1991
   09.2.4.6.2.1.0 Family Nucellicolidae Lamb et al., 1996 see Family Chitonophilidae Avdeev & Sirenko, 1991
   09.2.4.6.2.2 Family Herpyllobiidae Hansen, 1892
   09.2.4.6.2.3 Family Xenocoelomatidae Bresciani & Lützen, 1966
   10.1 Family Anomoclausiidae Gotto, 1964
   10.2.1.1.1 Family Taeniacanthidae C.B. Wilson, 1911
   10.2.1.1.2 Family Intramolgidae Marchenkov & Boxshall, 1995
   10.2.1.2 Family Synaptiphilidae Bocquet, 1953
   10.2.2.1 Family Bomolochidae Sumpf, 1871
   10.2.2.2.1 Family Telsidae Ho, 1967
   10.2.2.2.2.1 Family Clausiidae Giesbrecht, 1895
   10.2.2.2.2.2.1 Family Tegobomolochidae Avdeev, 1978
   10.2.2.2.2.2.2.1 Family Corycaeidae Dana, 1852
   10.2.2.2.2.2.2.2 Family Tuccidae Vervoort, 1962
   11.1.1.1.1.1.1 Family Synapticolidae Humes & Boxshall, 1996
   11.1.1.1.1.1.2 Family Sabelliphilidae Gurney, 1927
   11.1.1.1.1.2 Family Kelleriidae Humes & Boxshall, 1996
   11.1.1.1.2 Family Pseudanthessiidae Humes & Stock, 1972
   11.1.1.2 Family Rhynchomolgidae Humes & Stock, 1972
   11.1.1.3 Family Lichomolgidae Kossmann, 1877
   11.1.2.1 Family Urocopiidae Humes & Stock, 1972
   11.1.2.2 Family Sapphirinidae Thorell, 1860
   11.1.3 Family Myicolidae Yamaguti, 1936
   11.1.4 Family Macrochironidae Humes & Boxshall, 1996
   11.1.5 Family Anthessiidae Humes, 1986
   11.1.6 Family Anchimolgidae Humes & Boxshall, 1996
   11.2.1.1.1 Family Thamnomolgidae Humes & Boxshall, 1996
   11.2.1.1.2 Family Octopicolidae Humes & Boxshall, 1996
   11.2.1.2.1 Family Spiophanicolidae J.-S. Ho, 1984
   11.2.1.2.2 Family Splanchnotrophidae Norman & T. Scott, 1906
   11.2.2.1.1.1 Family Polyanchylidae see Family Polyankylidae Ho & I.H. Kim, 1997
   11.2.2.1.1.2 Family Chondracanthidae H. Milne Edwards, 1840
   11.2.2.1.1.2.0 Family Pharodidae Illg, 1948 see Family Chondracanthidae H. Milne Edwards, 1840
   11.2.2.1.2 Family Philoblennidae Izawa, 1976
   11.2.2.2.1 Family Xarifiidae Humes, 1960
   11.2.2.2.2.1 Family Paralubbockiidae Boxshall & Huys, 1989
   11.2.2.2.2.2.1 Family Nereicolidae Claus, 1875
   11.2.2.2.2.2.2.1 Family Entobiidae Ho, 1984
   11.2.2.2.2.2.2.2 Family Umazuracolidae J.-s. Ho et al., 2006