• [unassigned] Caenogastropoda (temporary name)
• [unassigned] Cladobranchia (temporary name)
• [unassigned] Euheterodonta (temporary name)
• [unassigned] Heterobranchia (temporary name)
• [unassigned] Pectinidae (temporary name)
• Abietinaria Kirchenpauer, 1884
• Abietinaria abietina (Linnaeus, 1758)
• Abludomelita Karaman, 1981
• Abludomelita obtusata (Montagu, 1813)
• Abra Lamarck, 1818
• Abra alba (W. Wood, 1802)
• Abra tenuis (Montagu, 1803)
• Acalyptrata
• Acanthocardia Gray, 1851
• Acanthocardia echinata (Linnaeus, 1758)
• Acanthocardiini Kafanov & Starobogatov, 1977
• Acanthoceras Kützing, 1842 accepted as Ceramium Roth, 1797
• Acanthochondria Oakley, 1930
• Acanthochondria cornuta (Müller O.F., 1776)
• Acanthochondria soleae (Krøyer, 1838)
• Acanthocodium W.F.R.Suringar, 1867 (uncertain)
• Acanthodasys Remane, 1927
• Acanthodasys aculeatus Remane, 1927
• Acanthodoris Gray, 1850
• Acanthodoris pilosa (Abildgaard, 1789)
• Acantholaimus Allgén, 1933
• Acantholaimus ewensis Platt & Zhang, 1982 accepted as Acantholaimus polydentatus Gerlach, 1951
• Acantholaimus polydentatus Gerlach, 1951
• Acanthomysis Czerniavsky, 1882
• Acanthomysis longicornis (Milne Edwards, 1837)
• Acanthostega
• Acanthostegomorpha
• Acartia Dana, 1846
• Acartia (Acanthacartia) Steuer, 1915
• Acartia (Acanthacartia) bifilosa (Giesbrecht, 1881)
• Acartia (Acanthacartia) tonsa Dana, 1849
• Acartia (Acartiura) Steuer, 1915
• Acartia (Acartiura) clausi Giesbrecht, 1889
• Acartia (Acartiura) discaudata (Giesbrecht, 1881)
• Acartiidae Sars G.O., 1903
• Acasta Leach, 1817
• Acasta spongites (Poli, 1791)
• Acastinae Kolbasov, 1993
• Achaeta Vejdovský, 1878
• Achaeta littoralis Lasserre, 1968
• Achelia Hodge, 1864
• Achelia echinata Hodge, 1864
• Achelia laevis Hodge, 1864
• Aciculata accepted as Errantia
• Acinetosporaceae Hamel ex J. Feldmann, 1937
• Acipenser Linnaeus, 1758
• Acipenser sturio Linnaeus, 1758
• Acipenseridae Bonaparte, 1831
• Acipenseriformes
• Acipenserinae
• Acoelomorpha
• Acontiaria Stephenson, 1935
• Acotylea
• Acrochaetiaceae Melchior, 1954
• Acrochaetiales
• Acrochaetium Nägeli, 1858
• Acrochaetium savianum (Meneghini) Nägeli, 1862 accepted as Colaconema savianum (Meneghini) R.Nielsen, 1994
• Acrochaetium secundatum (Lyngbye) Nägeli, 1858
• Acrocirridae Banse, 1969
• Acrocnida Gislén, 1926
• Acrocnida brachiata (Montagu, 1804)
• Acrothoracica
• Actinia Linnaeus, 1767
• Actinia candida Müller, 1776 accepted as Metridium senile (Linnaeus, 1761)
• Actinia equina (Linnaeus, 1758)
• Actiniaria
• Actiniaria incertae sedis (temporary name)
• Actiniidae Rafinesque, 1815
• Actinioidea Rafinesque, 1815
• Actinioidea Rodríguez & Daly in Rodríguez et al., 2014 accepted as Actinioidea Rafinesque, 1815
• Actinonema Cobb, 1920
• Actinonema celtica Boucher, 1976
• Actinonema pachydermatum Cobb, 1920
• Actinopoda
• Actinopteri
• Actinopterygii
• Actinostolidae Carlgren, 1932
• Actinostoloidea Carlgren, 1932
• Actinothoe Fischer, 1889
• Actinothoe sphyrodeta (Gosse, 1858)
• Actinotrocha Müller, 1846 accepted as Phoronis Wright, 1856
• Actinotrochidae Poche, 1903 accepted as Phoronidae Hatschek, 1880
• Actitis lliiger, 1811
• Actitis hypoleucos (Linnaeus, 1758)
• Acuariidae Railliet, Henry & Sisoff, 1912
• Acuariinae Railliet, Henry & Sisoff, 1912
• Acuarioidea Railliet, Henry & Sisoff, 1912
• Adapedonta
• Adenorhynchinae Ax & Heller, 1970
• Adenorhynchus Meixner, 1938
• Adenorhynchus balticus Meixner, 1938
• Adeonidae Busk, 1884
• Adeonoidea Busk, 1884
• Adesmata
• Adoncholaiminae Gerlach & Riemann, 1974