Excystment of the Metacercariae of Echinoparyphium serratum (Trematoda: Echinostomatidae)

Abstract

1. Metacercarial cysts of Echinoparyphium serratum occur in the pericardium of the snail Isidorella brazieri. Each cyst is surrounded by a nucleated, syncytial host capsule containing granules which are presumed to be melanin. The cyst wall consists of two layers and both contain acid and neutral mucopolysaccharide but little or no protein. No modification of the cyst wall coincident with the point of emergence of the metacercaria has been located.2. Excystment of the metacercaria takes place in two main stages when pepsin is omitted from the initial pretreatment solution: stage I (emergence from the cyst wall through the escape aperture) is an active process (exogenous enzymes are not necessary), and a high percentage of specimens reaching this stage is dependent on the combined effects of a þH of 2, reducing conditions, elevated temperature (37–39°C) and sodium cholate. Temperature and sodium cholate are absolute requirements for this stage as is pretreatment of between þH 2 and 4 when sodium cholate is included with the reductant (sodium dithionite) rather than in the excystment medium with trypsin. The possibility that CO2 is involved has not been established.