Out of 550 Lymnaea peregra (Muller) collected from a pond near Glasgow, Scotland, 29% were infected with sporocysts and cercariae of Cotylurus cornutus (Rudolphi, 1809), 0·9% with sporocysts and cercariae of Apatemon gracilis minor (Yamaguti, 1953), 0·18% with sporocysts and cercariae of Diplostomum spathaceum (Rudolphi, 1819), 0·8% with sprocysts and cercariae of Diplostomum gasterostei Williams, 1966, 9·0% with rediae and cercariae of Notocotylus attenuatus (Rudolphi, 1809), 3·6% with rediae and cercariae of Hypoderaeum conoideum (Bloch, 1782), 7·27% with sporocysts and cercariae of Haplometra cylindracea, (Looss, 1899), and 13·6% with metacercariae of Echinoparyphium recurvatum (Linstow, 1873).
The pattern of development of Cotylurus cornutus in the duck is described.