Aartsenia candida (Møller, 1842) (additional source)
Abietinaria abietina (Linnaeus, 1758) (additional source)
Abietinaria filicula (Ellis & Solander, 1786) (additional source)
Abietinaria turgida (Clark, 1877) (basis of record)
Abothrium gadi Van Beneden, 1870 (additional source)
Acanthobothrium coronatum (Rudolphi, 1819) Blanchard, 1848 (basis of record)
Acanthochondria cornuta (Müller O.F., 1776) (additional source)
Acanthocyclops vernalis (Fischer, 1853) represented as Acanthocyclops vernalis vernalis (Fischer, 1853) (basis of record)
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 forcipata Thompson I.C. & Scott A. in Thompson, Scott & Herdman, 1897 represented as Acartia (Acartia) forcipata Thompson I.C. & Scott A., 1897 in Thompson, Scott & Herdman, 1897 (basis of record)
Acartia hudsonica Pinhey, 1926 represented as Acartia (Acartiura) hudsonica Pinhey, 1926 (basis of record)
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 scabra Wilson, 1880 (basis of record)
Achelia spinosa (Stimpson, 1853) (basis of record)
Acmira catherinae (Laubier, 1967) represented as Aricidea (Acmira) catherinae Laubier, 1967 (basis of record)
Actinauge cristata Riemann-Zürneck, 1986 (basis of record)
Actinocythereis dawsoni (Brady, 1870) (basis of record)
Aega psora (Linnaeus, 1758) (additional source)
Aeginina aenigmatica Laubitz, 1972 (basis of record)
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 circinata (Verrill in Smith & Harger, 1874) (basis of record)
Aglaophamus malmgreni (Théel, 1879) (additional source)
Albionella fabricii Rubec & Hogans, 1988 accepted as Neoalbionella fabricii (Rubec & Hogans, 1988) (basis of record)
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)
Aligena elevata (W. Stimpson, 1851) accepted as Kelliopsis elevata (W. Stimpson, 1851) (basis of record)
Alteutha depressa (Baird, 1837) (additional source)
Alteutha oblonga (Goodsir, 1845) (additional source)
Alvania moerchii (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) (additional source)
Ameira spinipes Nicholls, 1939 (basis of record)
Americorchestia longicornis (Say, 1818) (basis of record)
Amicula vestita (Broderip & G. B. Sowerby I, 1829) (basis of record)
Amonardia arctica (Scott T., 1899) (additional source)
Ampelisca abdita Mills, 1964 (basis of record)
Ampelisca declivitatis Mills, 1967 (additional source)
Ampelisca eschrichtii Krøyer, 1842 (additional source)
Ampelisca macrocephala Liljeborg, 1853 (additional source)
Ampelisca vadorum Mills, 1963 (basis of record)
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 Hessle, 1917 (additional source)
Amphiascoides debilis (Giesbrecht, 1881) (additional source)
Amphiblestrum flemingii (Busk, 1854) (additional source)
Amphiblestrum minax accepted as Ramphonotus minax (Busk, 1860) (additional source)
Amphiblestrum osburni Powell, 1968 (basis of record)
Amphiblestrum solidum (Packard, 1863) (additional source)
Amphicteis gunneri (M. Sars, 1835) (additional source)
Amphilochus manudens Spence Bate & Westwood, 1862 (additional source)
Amphilochus tenuimanus Boeck, 1871 (additional source)
Amphimonhystrella Timm, 1961 (basis of record)
Amphipholis torelli Ljungman, 1872 (additional source)
Amphiporeia lawrenciana Shoemaker, 1929 (basis of record)
Amphiporus lactifloreus (Johnston, 1828) (additional source)
Amphithopsis longicaudata Boeck, 1861 (additional source)
Amphitrite Müller, 1771 (additional source)
Amphitrite cirrata Müller, 1776 (additional source)
Amphiura canadensis Verrill, 1899 (basis of record)
Amphiura exigua Verrill, 1899 (basis of record)
Amphiura fragilis Verrill, 1885 (additional source)
Amphiura otteri Ljungman, 1872 (additional source)
Amphiura sundevalli (Müller & Troschel, 1842) (additional source)
Ampithoe longimana Smith, 1873 (basis of record)
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) accepted as Andaniopsis pectinata (G.O. Sars, 1883) (additional source)
Andaniexis abyssi (Boeck, 1871) (additional source)
Andaniopsis nordlandica (Boeck, 1871) (additional source)
Anisakis simplex (Rudolphi, 1809) Dujardin, 1845 (additional source)
Anobothrus gracilis (Malmgren, 1866) (additional source)
Anomalocera opalus Pennell, 1976 (basis of record)
Anomalotrema koiae Gibson & Bray, 1984 (additional source)