| Summary: | 30,530 specimens of freshwater molluscs belonging to eleven genera and eighteen species including gastropods and bivalves, have been examined for infection with larval trematodes. Thirty species of cercariae have been found. Of these two are described as 'Monostome', three 'Gymnocephalous', one 'Pleurolophocercous', eleven 'Furcocercous', one 'Microcercous', one 'Cystocercous', five 'Echinostome', and six Xiphidiocercaria'. These specimens were collected on different dates from 21 different places in West Yorkshire. Eighteen of these cercariae are considered to be new to science, one is described for the first time in Great Britain and the remaining eleven have already been described in this country by previous authors. A short review of each group of cercariae found during this study is presented. The life cycles of eight species have been completed under laboratory conditions. The species involved are: Echinoparyphium recurvatum (Linstow, 1873) Hypoderaeum conoideum (Bloch, 1782) Notocotylus imbricatus (Looss, 1893) Szidat, 1935 Notocotylus attenuatus (Rudolphi, 1809) Plagiorchis farnleyensis n. sp. Plagiorchis kirkstallensis n, sp. Sphaeridiotrema wintersettensis n. sp. Dolichosaccus rastellus (Olsson, 1876) The life cycle of the latter species is the first record from Great Britain.
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