Morphology and ultrastructure of the redia and pre-emergent cercaria of Opisthorchis viverrini (Trematoda: Digenea) in the intermediate host Bithynia siamensis goniomphalus (Prosobranchia: Bithyniidae).

The intramolluscan stages of the human liver fluke Opisthorchis viverrini were investigated by light, scanning, and transmission electron microscopy. The morphology of the tegument, and the excretory system of the redia and pre-emergent cercaria as well as glandular cells, tail structure, and the eye spots of the cercarial stage are described. Possible environmental adaptations are discussed. The redia is characterized by a tegument which is folded concentrically and covered with densely arranged microvilli. This tegument is composed of a typical syncytium with submerged tegumental cells. The pre-emergent cercaria is characterized by a long tail with prominent fin folds and obliquely striated muscles. The oral sucker is well developed and protrusible, whereas the ventral sucker is faintly developed. Proteinaceous tegumental spines which are arranged transversally and different kinds of presumed sensory structures cover the cercarial body surface. Two well pigmented eye spots with rhabdomeric photoreceptors are located in the anterior part of the body. An extended protonephridial system is connected to a voluminous excretory bladder. The excretory tube opens at the end of the body and does not lead into the tail. Three types of glandular cells could be differentiated according to their vesicles, which are shed into the syncytial layer.