Observations on excystment in vitro ofCotylurus variegatus metacercariae (Trematoda: Strigeidae)

Excystment in vitro ofCotylurus variegatus metacercariae requires a trypsin-bile salt medium for digesting the cyst wall, and events are triggered by a rise in environmental temperature of up to 41°C. Pepsin pretreatment, while effective in removing extraneous host tissues, is not a prerequisite for excystment in vitro. The metacercaria plays an active role in its release, contractile movements being stimulated by bile salts over a temperature range of 30–41° C. Synthetic sodium taurocholate was only one third effective in excystment compared with sodium taurocholate from ox bile. Excystment was maximal at pH 7.5, but conditions of osmolarity, redox potential or gas phase were not of apparent importance.