• Abietinaria abietina (Linnaeus, 1758)
• Abludomelita gladiosa (Spence Bate & Westwood, 1862)
• Abludomelita obtusata (Montagu, 1813)
• Abra alba (W. Wood, 1802)
• Abra longicallus (Scacchi, 1835)
• Abra nitida (O. F. Müller, 1776)
• Abra prismatica (Montagu, 1808)
• Abra tenuis (Montagu, 1803)
• Abyssoninoe hibernica (McIntosh, 1903)
• Abyssoninoe scopa (Fauchald, 1974)
• Abyssoninoe scopa aequilobata (Winsnes, 1981) accepted as Abyssoninoe hibernica (McIntosh, 1903)
• Acanthocardia (Rudicardium) tuberculata (Linnaeus, 1758) accepted as Acanthocardia tuberculata (Linnaeus, 1758)
• Acanthocardia echinata (Linnaeus, 1758)
• Acanthocardia tuberculata (Linnaeus, 1758)
• Acanthochitona crinita (Pennant, 1777)
• Achelia echinata Hodge, 1864
• Acidostoma nodiferum Stephensen, 1923 accepted as Acidostoma obesum (Spence Bate & Westwood, 1861)
• Acidostoma obesum (Spence Bate & Westwood, 1861)
• Acidostoma sarsi Lincoln, 1979 accepted as Acidostoma nodiferum Stephensen, 1923 accepted as Acidostoma obesum (Spence Bate & Westwood, 1861)
• Acrocnida brachiata (Montagu, 1804)
• Acromegalomma vesiculosum (Montagu, 1813)
• Acteon tornatilis (Linnaeus, 1758)
• Adamsia carciniopados (Otto, 1823) accepted as Adamsia palliata (Fabricius, 1779) accepted as Calliactis palliata (Müller, 1776)
• Adamsia palliata (Fabricius, 1779) accepted as Calliactis palliata (Müller, 1776)
• Adyte pellucida (Ehlers, 1864) accepted as Subadyte pellucida (Ehlers, 1864)
• Aeolidia papillosa (Linnaeus, 1761)
• Aequipecten opercularis (Linnaeus, 1758)
• Aetea anguina (Linnaeus, 1758)
• Aglaophamus agilis (Langerhans, 1880)
• Aglaophamus malmgreni (Théel, 1879)
• Aglaophamus pulcher (Rainer, 1991)
• Aglaophamus rubella (Michaelsen, 1897) accepted as Aglaophamus agilis (Langerhans, 1880)
• Akanthophoreus gracilis (Krøyer, 1842)
• Akanthophoreus longiremis (Lilljeborg, 1864) accepted as Parakanthophoreus longiremis (Lilljeborg, 1864)
• Alcyonidioides mytili (Dalyell, 1848)
• Alcyonidium diaphanum (Hudson, 1778)
• Alcyonidium mytili Dalyell, 1848 accepted as Alcyonidioides mytili (Dalyell, 1848)
• Alcyonium digitatum Linnaeus, 1758
• Alentia gelatinosa (M. Sars, 1835)
• Alitta virens (M. Sars, 1835)
• Alpheus glaber (Olivi, 1792)
• Alpheus macrocheles (Hailstone, 1835)
• Altenaeum dawsoni (Jeffreys, 1864)
• Alvania abyssicola (Forbes, 1850) accepted as Alvania testae (Aradas & Maggiore, 1844)
• Alvania jeffreysi (E. Waller, 1864)
• Alvania lactea (Michaud, 1830)
• Alvania subsoluta (Aradas, 1847)
• Alvania testae (Aradas & Maggiore, 1844)
• Amaeana trilobata (Sars, 1863)
• Amage auricula Malmgren, 1866
• Amblyosyllis formosa (Claparède, 1863) accepted as Amblyosyllis spectabilis (Johnston in Baird, 1861)
• Amblyosyllis spectabilis (Johnston in Baird, 1861)
• Ammodytes tobianus Linnaeus, 1758
• Ampelisca aequicornis Bruzelius, 1859
• Ampelisca brevicornis (A. Costa, 1853)
• Ampelisca diadema (A. Costa, 1853)
• Ampelisca gibba G.O. Sars, 1883
• Ampelisca macrocephala Liljeborg, 1853
• Ampelisca spinipes Boeck, 1861
• Ampelisca tenuicornis Liljeborg, 1856
• Ampelisca typica (Spence Bate, 1857)
• Ampharete acutifrons (Grube, 1860)
• Ampharete baltica Eliason, 1955
• Ampharete falcata Eliason, 1955
• Ampharete finmarchica (M. Sars, 1865)
• Ampharete grubei Malmgren, 1865
• Ampharete lindstroemi Hessle, 1917
• Ampharete octocirrata (Sars, 1835)
• Amphibalanus improvisus (Darwin, 1854)
• Amphiblestrum auritum (Hincks, 1877)
• Amphiblestrum flemingii (Busk, 1854)
• Amphicteis gunneri (M. Sars, 1835)
• Amphicteis midas (Gosse, 1855)
• Amphictene auricoma (O.F. Müller, 1776)
• Amphilepis norvegica (Ljungman, 1865)
• Amphilochus manudens Spence Bate & Westwood, 1862
• Amphilochus neapolitanus Della Valle, 1893 accepted as Apolochus neapolitanus (Della Valle, 1893)
• Amphilochus spencebatei (Stebbing, 1876) accepted as Apolochus spencebatei (Stebbing, 1876)
• Amphioxus lanceolatus (Pallas, 1774) accepted as Branchiostoma lanceolatum (Pallas, 1774)
• Amphipholis squamata (Delle Chiaje, 1828)
• Amphiporus allucens Bürger, 1895
• Amphitrite birulai Ssolowiew, 1899 accepted as Paramphitrite birulai (Ssolowiew, 1899)
• Amphitrite cirrata Müller, 1776
• Amphitrite edwardsi [subsequent misspelling of 'edwardsii'] accepted as Neoamphitrite edwardsii (Quatrefages, 1866)
• Amphiura (Acrocnida) brachiata (Montagu, 1804) accepted as Acrocnida brachiata (Montagu, 1804)
• Amphiura (Ophiopeltis) securigera (Düben & Koren, 1846)
• Amphiura chiajei Forbes, 1843
• Amphiura filiformis (O.F. Müller, 1776)
• Ampithoe rubricata (Montagu, 1808)
• Amythasides macroglossus Eliason, 1955
• Anaitides citrina (Malmgren, 1865) accepted as Phyllodoce citrina Malmgren, 1865
• Anaitides groenlandica (Örsted, 1842) accepted as Phyllodoce groenlandica Örsted, 1842
• Anaitides lineata (Claparède, 1870) accepted as Phyllodoce lineata (Claparède, 1870)
• Anaitides longipes (Kinberg, 1866) accepted as Phyllodoce longipes Kinberg, 1866
• Anaitides maculata (Linnaeus, 1767) accepted as Phyllodoce maculata (Linnaeus, 1767)
• Anaitides mucosa (Örsted, 1843) accepted as Phyllodoce mucosa Örsted, 1843
• Anaitides rosea (McIntosh, 1877) accepted as Phyllodoce rosea (McIntosh, 1877)
• Anaitides subulifera Eliason, 1962 accepted as Phyllodoce rosea (McIntosh, 1877)
• Anapagurus hyndmanni (Bell, 1845)
• Anapagurus laevis (Bell, 1845)