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On the occurrence, ecology and behaviour of Onchidella celtica (Gastropoda, Onchidiacea) in the littoral of CornwallOnchidella celtica is the only onchidiacean to occur in Britain; it is a littoral pulmonate gastropod of controversial taxonomic placement and an historica1 review of the group is therefore presented. The habitat, spatial dispersion, local distribution and geographical range of O.celtica are each described, with particular reference to Cornish shores. The nature of the substratum, exposure and the adverse affects of low temperatures are suggested as the principal parameters controlling the observed distributions, which are considered in detail. O.celtica lays capsular spawn and examination of these has enabled development times and hatching rates to be assessed. Juvenile growth rates have also been investigated to aid the interpretation of population age-structure in shore-line samples, and the dynamics and longevity of the population are discussed. Reproduction is shown to be inhibited by low temperatures. The various reproductive strategies of the onchidiaceans are discussed. Likely causes of mortality are considered, the likelihood of predation by intertidal carnivores is assessed and defensive strategies, including glandular secretions, rhythms of activity and cryptic habits, are shown to be important in evading predation. Various aspects of the behaviour of O.celtica are investigated quantitatively, including aggregation, crevice selection and foraging, and their importance as strategies in successfully exploiting the exposed littoral environment is discussed. A detailed investigation of homing behaviour shows this to be accomplished without obvious trail-following. and remote olfaction is suggested as the principal mechanism of homing. Ambient temperature has a marked effect on the frequency of individuals foraging, and the role of other environmental variables is considered. The feeding behaviour and diet of O.celtica are described, and the possibility of food selection is considered. A preliminary histochemical examination has elucidated the distribution of certain classes of enzymes in the gut of O.celtica.