Aartsenia candida (Møller, 1842) (additional source)
Abietinaria abietina (Linnaeus, 1758) (additional source)
Abietinaria filicula (Ellis & Solander, 1786) (additional source)
Abothrium gadi Van Beneden, 1871 (additional source)
Acanthochondria cornuta (Müller O.F., 1776) (additional source)
Acanthocythereis dunelmensis (Norman, 1865) accepted as Actinocythereis dunelmensis (Norman, 1865) (additional source)
Acanthonotozoma inflatum (Krøyer, 1842) (additional source)
Acanthonotozoma rusanovae Bryazgin, 1974 (additional source)
Acanthonotozoma serratum (O. Fabricius, 1780) (additional source)
Acanthonotozoma sinuatum Just, 1978 (additional source)
Acanthostepheia malmgreni (Goës, 1866) (additional source)
Acartia bifilosa (Giesbrecht, 1881) represented as Acartia (Acanthacartia) bifilosa (Giesbrecht, 1881) (additional source)
Acartia clausi Giesbrecht, 1889 represented as Acartia clausii Giesbrecht, 1889 represented as Acartia (Acartiura) clausi Giesbrecht, 1889 (additional source)
Acartia longiremis (Lilljeborg, 1853) represented as Acartia (Acartiura) longiremis (Lilljeborg, 1853) (additional source)
Acartia tonsa Dana, 1849 represented as Acartia (Acanthacartia) tonsa Dana, 1849 (additional source)
Acaulis primarius Stimpson, 1853 (additional source)
Aceroides (Aceroides) latipes (G.O. Sars, 1883) (additional source)
Achelia spinosa (Stimpson, 1853) (basis of record)
Acirsa borealis (Lyell, 1841) (basis of record)
Aega psora (Linnaeus, 1758) (additional source)
Aeginopsis laurentii Brandt, 1838 (additional source)
Aequorea Péron & Lesueur, 1810 (additional source)
Aetea anguina (Linnaeus, 1758) (additional source)
Aetideus Brady, 1883 (additional source)
Aetideus armatus (Boeck, 1872) (additional source)
Aglaophamus malmgreni (Théel, 1879) (additional source)
Alcyonidium cellarioides Calvet, 1900 (additional source)
Alcyonidium diaphanum (Hudson, 1778) (additional source)
Alcyonidium gelatinosum (Linnaeus, 1761) (additional source)
Alcyonidium hirsutum (Fleming, 1828) (additional source)
Alcyonidium parasiticum (Fleming, 1828) (additional source)
Alteutha depressa (Baird, 1837) (additional source)
Alteutha oblonga (Goodsir, 1845) (additional source)
Alvania moerchi (Collin, 1886) (additional source)
Alvania pseudoareolata Warén, 1974 (additional source)
Alvania verrilli (Friele, 1886) (additional source)
Amage auricula Malmgren, 1866 (additional source)
Amauropsis islandica (Gmelin, 1791) (additional source)
Amblyops abbreviatus (G.O. Sars, 1869) (additional source)
Amblyops kempi (Holt & Tattersall, 1905) (additional source)
Ameira divagans divagans Nicholls, 1939 (additional source)
Ameira parvula (Claus, 1866) accepted as Amphiascus parvulus (Claus, 1866) (additional source)
Amonardia arctica (Scott T., 1899) (additional source)
Ampelisca declivitatis Mills, 1967 (additional source)
Ampelisca eschrichtii Krøyer, 1842 (additional source)
Ampelisca macrocephala Liljeborg, 1852 (additional source)
Ampharete acutifrons (Grube, 1860) (additional source)
Ampharete baltica Eliason, 1955 (additional source)
Ampharete finmarchica (M. Sars, 1865) (additional source)
Ampharete goesi Malmgren, 1866 (additional source)
Ampharete lindstroemi Malmgren, 1867 sensu Hessle, 1917 (additional source)
Amphiascoides debilis (Giesbrecht, 1881) (additional source)
Amphiascus tenuiremis (Brady, 1880) accepted as Sarsamphiascus tenuiremis (Brady & Robertson, 1876) (additional source)
Amphiblestrum flemingii (Busk, 1854) (additional source)
Amphiblestrum minax accepted as Ramphonotus minax (Busk, 1860) (additional source)
Amphiblestrum septentrionalis (Kluge, 1906) accepted as Ramphonotus septentrionalis (Kluge, 1906) (additional source)
Amphiblestrum solidum (Packard, 1863) (additional source)
Amphicteis gunneri (M. Sars, 1835) (additional source)
Amphilochopsis Stephensen, 1925 accepted as Amphilochus Spence Bate, 1862 (additional source)
Amphilochopsis hamatus Stephensen, 1925 accepted as Amphilochus hamatus (Stephensen, 1925) (additional source)
Amphilochus manudens Spence Bate, 1862 (additional source)
Amphilochus tenuimanus Boeck, 1871 (additional source)
Amphimonhystrella Timm, 1961 (basis of record)
Amphipholis torelli Ljungman, 1872 (additional source)
Amphiporus lactifloreus (Johnston, 1828) (additional source)
Amphithopsis longicaudata Boeck, 1861 (additional source)
Amphitrite Müller, 1771 (additional source)
Amphitrite cirrata Müller, 1776 (additional source)
Amphiura fragilis Verrill, 1885 (additional source)
Amphiura otteri Ljungman, 1872 (additional source)
Amphiura sundevalli (Müller & Troschel, 1842) (additional source)
Anaitides groenlandica (Örsted, 1842) accepted as Phyllodoce groenlandica Örsted, 1842 (additional source)
Anaitides maculata (Linnaeus, 1767) accepted as Phyllodoce maculata (Linnaeus, 1767) (additional source)
Anaitides mucosa (Örsted, 1843) accepted as Phyllodoce mucosa Örsted, 1843 (additional source)
Ancistrosyllis groenlandica McIntosh, 1878 (additional source)
Andaniella pectinata G.O. Sars, 1883 (additional source)
Andaniexis abyssi (Boeck, 1871) (additional source)
Andaniopsis nordlandica (Boeck, 1871) (additional source)
Anisakis simplex (Rudolphi, 1809) (additional source)
Anobothrus gracilis (Malmgren, 1866) (additional source)
Anomalotrema koiae Gibson & Bray, 1984 (additional source)
Anonyx debruynii Hoek, 1882 (additional source)
Anonyx lilljeborgi Boeck, 1871 (additional source)
Anonyx ochoticus Gurjanova, 1962 (additional source)
Anonyx sarsi Steele & Brunel, 1968 (additional source)
Anoplostoma Bütschli, 1874 (additional source)
Anthobothrium variabile (Linton, 1889) (additional source)
Antomicron Cobb, 1920 (additional source)
Aphanodomus terebellae (Levinsen, 1878) (additional source)
Apherusa bispinosa (Spence Bate, 1857) (additional source)
Apherusa glacialis (Hansen, 1888) (additional source)
Apherusa megalops (Buchholz, 1874) (additional source)
Aporocotyle simplex Odhner, 1900 (additional source)
Aquilonaria turneri Dall, 1886 accepted as Lacuna turneri (Dall, 1886) (basis of record)
Arabella Grube, 1850 (additional source)
Arabella iricolor (Montagu, 1804) (additional source)
Araeolaimus de Man, 1888 (additional source)
Arenicola marina (Linnaeus, 1758) (additional source)
Argis dentata (Rathbun, 1902) (additional source)
Argissa hamatipes (Norman, 1869) (additional source)