In vivo and in vitro excystation of Zygocotyle lunata (Trematoda) metacercariae and histochemical observations on the cyst.

Encysted metacercariae of Zygocotyle lunata (Trematoda) excyst within 2 hr postexposure in the lower ileum of the domestic chick. Optimal in vitro excystation of this species occurs following pretreatment of the cyst for 15 min in 1% acidified pepsin, treatment in 0.02 M sodium dithionite (a reductant) for 1 to 2 min and then 2 hr treatment in an excystation medium containing 1% sodium glycocholate plus 1% trypsin in Earle's BSS adjusted to pH 8.8 with tris and maintained at 41 C. The cyst of this species is a dome-shaped hemisphere containing an inner and outer wall. The outer wall contains mainly acid mucopolysaccharides, whereas the inner wall is mainly proteinaceous. The cyst contains a ventral lid which only was visualized during excystation.