• Aberrantospermata Jondelius, Wallberg, Hooge & Raikova, 2011
• Achoerus Beklemischev, 1914
• Achoerus caspius Beklemischev, 1914
• Achoerus ferox Beklemischev, 1937
• Achoerus pachycaudatus Dörjes, 1968
• Acoela
• Acoelomorpha accepted as Xenacoela
• Acoelopelophila Ceccolini & Cianferoni, 2022
• Acoelopelophila lutheri (Westblad, 1946)
• Actinoposthia An der Lan, 1936
• Actinoposthia beklemischevi Mamkaev, 1965
• Actinoposthia beklemishevi Mamkaev, 1965 accepted as Actinoposthia beklemischevi Mamkaev, 1965
• Actinoposthia biaculeata Faubel, 1974
• Actinoposthia caudata An der Lan, 1936
• Actinoposthia haplovata Dörjes, 1968 accepted as Childia haplovata Dörjes, 1968
• Actinoposthia longa Faubel, 1976
• Actinoposthia pigmentea Faubel, 1976
• Actinoposthiidae Hooge, 2001
• Adeloposthia An der Lan, 1936 accepted as Paraproporus Westblad, 1945
• Adeloposthia elegans An Der Lan, 1936 accepted as Paraproporus elegans (An der Lan, 1936)
• Adenocauda Dörjes, 1968
• Adenocauda helgolandica Dörjes, 1968
• Adenopea Antonius, 1968
• Adenopea cenata (Du Bois-Reymond Marcus, 1955)
• Adenopea chuni (Brauner, 1920)
• Adenopea illardata Löhner & Micoletzky, 1911
• Adenopea illardatus (Lohner & Micoletzky, 1911)
• Aechmalotus Beklemischev, 1915
• Aechmalotus pyrola Dörjes, 1968 accepted as Aechmalotus pyrula Beklemischev, 1915
• Aechmalotus pyrula Beklemischev, 1915
• Afronta Hyman, 1944
• Afronta aurantiaca Hyman, 1944
• Afronta rubra Faubel, 1976
• Alluna Faubel & Regier, 1983
• Alluna sublitoralis Faubel & Regier, 1983
• Alluna vulgaris Faubel & Regier, 1983 accepted as Alluna sublitoralis Faubel & Regier, 1983
• Amphichoerus Graff, 1891 accepted as Amphiscolops Graff, 1904
• Amphichoerus cinereus (Graff, 1874) accepted as Amphiscolops cinereus (Graff, 1874)
• Amphichoerus langerhansi (Graff, 1882) accepted as Heterochaerus langerhansi (Graff, 1882)
• Amphichoerus virescens Graff, 1902 accepted as Aphanostoma virescens Ørsted, 1845
• Amphiscolops Graff, 1904
• Amphiscolops australis (Haswell, 1905) accepted as Heterochaerus australis Haswell, 1905
• Amphiscolops bermudensis Hyman, 1939
• Amphiscolops blumi Achatz, Hooge & Tyler, 2007 accepted as Heterochaerus blumi (Achatz, Hooge & Tyler, 2007)
• Amphiscolops carvalhoi Marcus, 1952 accepted as Heterochaerus carvalhoi (Marcus, 1952)
• Amphiscolops cinereus (Graff, 1874)
• Amphiscolops evelinae Marcus, 1947
• Amphiscolops fulgineus Peebles, 1915 accepted as Amphiscolops fuligineus Peebles, 1913
• Amphiscolops fuligineus Peebles, 1913
• Amphiscolops gardineri Graff, 1911 accepted as Anaperus gardineri (Graff, 1911) accepted as Thalassoanaperus gardineri (Graff, 1911)
• Amphiscolops gemelliporus Marcus, 1954
• Amphiscolops japonicus Kato, 1947
• Amphiscolops langerhansi Graff, 1882 accepted as Heterochaerus langerhansi (Graff, 1882)
• Amphiscolops marinelliensis Beltagi & Khafagi, 1984
• Amphiscolops mosaicus Kozloff, 1998
• Amphiscolops oni Asai, Miyazawa, Yanase, Inaba & Nakano, 2022
• Amphiscolops potocani Achatz, 2008
• Amphiscolops sargassi Hyman, 1939 accepted as Heterochaerus sargassi (Hyman, 1939)
• Amphiscolops trifurcatus (Beltagi, 1983)
• Amphiscolops virescens (Örsted AS, 1845) accepted as Aphanostoma virescens Ørsted, 1845
• Amphiscolops zeii Riedl, 1956
• Amphiscolopus Kato, 1957 accepted as Amphiscolops Graff, 1904
• Amphiscolopus japonicus Kato, 1957 accepted as Amphiscolops japonicus Kato, 1947
• Anaperidae Dörjes, 1968 accepted as Convolutidae Graff, 1905
• Anaperus Graff, 1911 accepted as Thalassoanaperus Hernandez, 2018
• Anaperus australis Westblad, 1952 accepted as Thalassoanaperus australis (Westblad, 1952)
• Anaperus balticus Meixner, 1938 accepted as Anaperus tvaerminnensis (Luther, 1912) accepted as Thalassoanaperus tvaerminnensis (Luther, 1912)
• Anaperus biaculeatus Boguta, 1970 accepted as Thalassoanaperus biaculeatus (Boguta, 1970)
• Anaperus diploposthia (Steinböck, 1931) accepted as Anaperus rubellus Westblad, 1945 accepted as Thalassoanaperus rubellus (Westblad, 1945)
• Anaperus gardineri (Graff, 1911) accepted as Thalassoanaperus gardineri (Graff, 1911)
• Anaperus ornatus Beltagi, 2001 accepted as Thalassoanaperus ornatus (Beltagi, 2001)
• Anaperus rubellus Westblad, 1945 accepted as Thalassoanaperus rubellus (Westblad, 1945)
• Anaperus singularis Hooge & Smith, 2004 accepted as Thalassoanaperus singularis (Hooge & Smith, 2004)
• Anaperus sulcatus Beklemischev, 1914 accepted as Thalassoanaperus sulcatus (Beklemischev, 1914)
• Anaperus trifurcatus Beltagi, 1983 accepted as Amphiscolops trifurcatus (Beltagi, 1983)
• Anaperus tvaerminnensis (Luther, 1912) accepted as Thalassoanaperus tvaerminnensis (Luther, 1912)
• Ancyclocirrus Kozloff, 2000 accepted as Ancylocirrus Kozloff, 2000 accepted as Praeaphanostoma Dörjes, 1968
• Ancylocirrus Kozloff, 2000 accepted as Praeaphanostoma Dörjes, 1968
• Ancylocirrus ornatus Kozloff, 2000 accepted as Praeaphanostoma ornatus (Kozloff, 2000)
• Anthroposthiidae accepted as Antroposthiidae Faubel, 1976
• Antigonaria Dörjes, 1968
• Antigonaria arenaria Dörjes, 1968
• Antigonariidae Dörjes, 1968
• Antroposthia Faubel, 1974
• Antroposthia axi Faubel, 1974
• Antroposthia unipora Faubel, 1974
• Antroposthiidae Faubel, 1976
• Antrosagittifera Hooge & Tyler, 2001
• Antrosagittifera corallina Hooge & Tyler, 2001
• Aphanostoma Ørsted, 1845
• Aphanostoma actuosus (Kozloff, 1965)
• Aphanostoma adherens Meixner, 1938 (uncertain > nomen dubium)
• Aphanostoma album Dörjes, 1968
• Aphanostoma armatum Steinböck & Lanser, 1932 (unaccepted > nomen nudum)
• Aphanostoma aurantiacum Verrill, 1893 (uncertain > nomen dubium)
• Aphanostoma auratiacum Dörjes, 1968 accepted as Aphanostoma aurantiacum Verrill, 1893
• Aphanostoma auritum Westblad, 1946 accepted as Eumecynostomum westbladi (Dörjes, 1968)
• Aphanostoma auritum flavescens Westblad, 1946 accepted as Eumecynostomum flavescens (Dörjes, 1968)
• Aphanostoma auritum glandulosum Westblad, 1946 accepted as Mecynostomum auritum glandulosum (Westblad, 1946) accepted as Pseudmecynostomum glandulosum (Westblad, 1946) accepted as Postmecynostomum glandulosum (Westblad, 1946)
• Aphanostoma auritum typica Westblad, 1946 accepted as Eumecynostomum westbladi (Dörjes, 1968)