mantle cavity of early molluscs. It is likely that more complex life cycles evolved through a process of terminal addition, whereby digeneans survived predation of their mollusc host, probably by a fish. Other hosts were added by the same process until the modern bewildering diversity of life cycle patterns developed. Important families Digenea includes about 80 families. They are listed below, organised by order. Digenea Diplostomida Suborder Diplostomata Superfamily Brachylaimoidea Joyeux & Foley, 1930 Brachylaimidae Joyeux & Foley, 1930 Leucochloridiidae Poche, 1907 Superfamily Diplostomoidea Poirier, 1886 Brauninidae Wolf, 1903 Cyathocotylidae Mühling, 1898 Diplostomidae Poirier, 1886 Proterodiplostomidae Dubois, 1936 Strigeidae Railliet, 1919 Superfamily Schistosomatoidea Stiles & Hassall, 1898 Aporocotylidae Odhner, 1912 Schistosomatidae Stiles & Hassall, 1898 Spirorchiidae Stunkard, 1921 Plagiorchiida Apocreadiata Apocreadioidea Skrjabin, 1942 Apocreadiidae Skrjabin, 1942 Bivesiculata Bivesiculoidea Bivesiculidae Yamaguti, 1934 Bucephalata Bucephaloidea Poche, 1907 Bucephalidae Poche, 1907 Nuitrematidae Kurochkin, 1975 Gymnophalloidea Odhner, 1905 Botulisaccidae Yamaguti, 1971 Fellodistomidae Nicoll, 1909 Gymnophallidae Odhner, 1905 Tandanicolidae Johnston, 1927 Echinostomata Echinostomatoidea Looss, 1902 Calycodidae Dollfus, 1929 Cyclocoelidae Stossich, 1902 Echinochasmidae Odhner, 1910 Echinostomatidae Looss, 1899 Eucotylidae Cohn, 1904 Fasciolidae Railliet, 1895 Himasthlidae Odhner, 1910 Philophthalmidae Looss, 1899 Psilostomidae Looss, 1900 Rhytidodidae Odhner, 1926 Typhlocoelidae Harrah, 1922 Haplosplanchnata Haplosplanchnoidea Poche, 1925 Haplosplanchnidae Poche, 1926 Hemiurata Azygioidea Lühe, 1909 Azygiidae Lühe, 1909 Hemiuroidea Looss, 1899 Accacoeliidae Odhner, 1911 Bathycotylidae Dollfus, 1932 Derogenidae Nicoll, 1910 Dictysarcidae Skrjabin & Guschanskaja, 1955 Didymozoidae Monticelli, 1888 Gonocercidae Skrjabin & Guschanskaja, 1955 Hemiuridae Looss, 1899 Hirudinellidae Dollfus, 1932 Isoparorchiidae Travassos, 1922 Lecithasteridae Odhner, 1905 Ptychogonimidae Dollfus, 1937 Sclerodistomidae Odhner, 1927 Sclerodistomoididae Gibson & Bray, 1979 Syncoeliidae Looss, 1899 Heronimata Heronimoidea Ward, 1918 Heronimidae Ward, 1918 Lepocreadiata Lepocreadioidea Odhner, 1905 Aephnidiogenidae Yamaguti, 1934 Deropristidae Cable & Hunninen, 1942 Enenteridae Yamaguti, 1958 Gorgocephalidae Manter, 1966 Gyliauchenidae Fukui, 1929 Lepidapedidae Yamaguti, 1958 Lepocreadiidae Odhner, 1905 Liliatrematidae Gubanov, 1953 Monorchiata Monorchioidea Odhner, 1911 Lissorchiidae Magath, 1917 Monorchiidae Odhner, 1911 Opisthorchiata Opisthorchioidea Braun, 1901 Cryptogonimidae Ward, 1917 Heterophyidae Leiper, 1909 Opisthorchiidae Looss, 1899 Pronocephalata Paramphistomoidea Fischoeder, 1901 Cladorchiidae Fischoeder, 1901 Mesometridae Poche, 1926 Microscaphidiidae Looss, 1900 Paramphistomidae Fischoeder, 1901 Pronocephaloidea Looss, 1899 Labicolidae Blair, 1979 Notocotylidae Lühe, 1909 Nudacotylidae Barker, 1916 Opisthotrematidae Poche, 1926 Pronocephalidae Looss, 1899 Rhabdiopoeidae Poche, 1926 Transversotremata Transversotrematoidea Witenberg, 1944 Transversotrematidae Witenberg, 1944 Xiphidiata Allocreadioidea Looss, 1902 Acanthocolpidae Lühe, 1906 Allocreadiidae Looss, 1902 Batrachotrematidae Dollfus & Williams, 1966 Brachycladiidae Odhner, 1905 Opecoelidae Ozaki, 1925 Gorgoderoidea Looss, 1901 Callodistomidae Odhner, 1910 Dicrocoeliidae Looss, 1899 Gorgoderidae Looss, 1899 Haploporoidea Nicoll, 1914 Atractotrematidae Yamaguti,