• "Lower Heterobranchia"
• [unassigned] Buccinidae (temporary name)
• [unassigned] Caenogastropoda (temporary name)
• [unassigned] Cardiidae (temporary name)
• [unassigned] Cladobranchia (temporary name)
• [unassigned] Enteropneusta (temporary name)
• [unassigned] Euheterodonta (temporary name)
• [unassigned] Heterobranchia (temporary name)
• [unassigned] Lucinidae (temporary name)
• [unassigned] Neogastropoda (temporary name)
• [unassigned] Oegopsida (temporary name)
• [unassigned] Seguenzioidea (temporary name)
• [unassigned] Pectinidae (temporary name)
• Aartsenia Warén, 1991
• Aartsenia candida (Møller, 1842)
• Aberranta Hartman, 1965
• Aberranta enigmatica Hartman, 1965
• Aberrantidae Wolf, 1987
• Aberrantospermata Jondelius, Wallberg, Hooge & Raikova, 2011
• Abietinaria Kirchenpauer, 1884
• Abietinaria abietina (Linnaeus, 1758)
• Abietinaria filicula (Ellis & Solander, 1786)
• Abietinaria kincaidi (Nutting, 1901)
• Abietinaria pulchra (Nutting, 1904)
• Abietinaria turgida (Clark, 1877)
• Ablennes Jordan & Fordice, 1887
• Ablennes hians (Valenciennes, 1846)
• Abludomelita Karaman, 1981
• Abludomelita amoena (Hansen, 1888)
• Abludomelita obtusata (Montagu, 1813)
• Aborjinia Özdikmen, 2010
• Aborjinia corallicola Westerman, de Moura Neves, Ahmed & Holovachov, 2021
• Abothrium Van Beneden, 1871
• Abothrium gadi Van Beneden, 1871
• Abra Lamarck, 1818
• Abra lioica (Dall, 1881)
• Abra longicallus (Scacchi, 1835)
• Abra longicallus americana Verrill & K. J. Bush, 1898 accepted as Abra longicallus (Scacchi, 1835)
• Abudefduf Forsskål, 1775
• Abudefduf saxatilis (Linnaeus, 1758)
• Abudefduf sexfasciatus (Lacepède, 1801)
• Acalyptrata
• Acanella Gray, 1870
• Acanella arbuscula (Johnson, 1862)
• Acantharia
• Acanthephyra A. Milne-Edwards, 1881
• Acanthephyra acutifrons Spence Bate, 1888
• Acanthephyra eximea Smith, 1884 accepted as Acanthephyra eximia Smith, 1884
• Acanthephyra eximia Smith, 1884
• Acanthephyra pelagica (Risso, 1816)
• Acanthephyra purpurea A. Milne-Edwards, 1881
• Acanthephyridae Spence Bate, 1888
• Acanthobothrium Blanchard, 1848
• Acanthobothrium coronatum (Rudolphi, 1819) Blanchard, 1848
• Acanthocardia Gray, 1851
• Acanthocardia echinata (Linnaeus, 1758)
• Acanthocardiini Kafanov & Starobogatov, 1977
• Acanthocarpus Stimpson, 1871
• Acanthocarpus alexandri Stimpson, 1871
• Acanthocephala
• Acanthochaenus Gill, 1884
• Acanthochaenus luetkenii Gill, 1884
• Acanthocheilidae Wülker, 1929
• Acanthocheilinae Wülker, 1929 accepted as Acanthocheilidae Wülker, 1929
• Acanthocheilus Molin, 1858
• Acanthocheilus rotundatus (Rudolphi, 1819)
• Acanthochitonina
• Acanthochondria Oakley, 1930
• Acanthochondria cornuta (Müller O.F., 1776)
• Acanthocolpidae Lühe, 1906
• Acanthocorbis S.Hara & E.Takahashi, 1984
• Acanthocorbis apoda (Leadbeater) Hara & Takahashi, 1984
• Acanthocotyle Monticelli, 1888
• Acanthocotyle verrilli Goto, 1899
• Acanthocotyle williamsi Price, 1938
• Acanthocotylidae Monticelli, 1903
• Acanthocybium Gill, 1862
• Acanthocybium solandri (Cuvier, 1832)
• Acanthocyclops Kiefer, 1927
• Acanthocyclops vernalis (Fischer, 1853) represented as Acanthocyclops vernalis vernalis (Fischer, 1853)
• Acanthocyclops vernalis vernalis (Fischer, 1853)
• Acanthocythereis Howe, 1963
• Acanthocythereis dunelmensis (Norman, 1865) accepted as Actinocythereis dunelmensis (Norman, 1865)
• Acanthodoris Gray, 1850
• Acanthodoris pilosa (Abildgaard, 1789)
• Acanthodoris subquadrata (Alder & Hancock, 1845) accepted as Acanthodoris pilosa (Abildgaard, 1789)
• Acanthoecida
• Acanthoecidae Norris, 1965
• Acanthoecidae [Tectiform] (temporary name)
• Acanthogorgia Gray, 1857
• Acanthogorgia armata Verrill, 1878
• Acanthogorgiidae Gray, 1859 accepted as Paramuriceidae Bayer, 1956
• Acanthohaustorius Bousfield, 1965
• Acanthohaustorius intermedius Bousfield, 1965
• Acanthohaustorius millsi Bousfield, 1965
• Acanthoica Lohmann, 1903
• Acanthoica monospina Schiller, 1914
• Acantholiparis Gilbert & Burke, 1912
• Acantholiparis opercularis Gilbert & Burke, 1912
• Acanthomysis Czerniavsky, 1882