Abstract
The emergence pattern of cercariae of Trichobilharzia ocellata from Lymnaea stagnalis was examined at 2-h intervals from infected snails acclimatized to 12 h light – 12 h dark periods. A marked diel pattern of emergence was found during the periods of illumination; this was reversed when the light–dark regime was reversed.A method for the continuous recording of the locomotory activity of L. stagnalis is described. Snails acclimatized to either normal or reversed dark–light regimes showed a consistent diel pattern of activity during illumination. Peak activity occurred in the first few hours of illumination.It is proposed that the diel activity pattern results from an internal rhythm in which the snail becomes entrained to the lighting regime. The close association between the diel activity pattern of the snail and the emergence of T. ocellata indicates that host movement stimulates cercarial emergence. Light is thought to influence the cercariae directly and indirectly through the snail.
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