Morphology, development and probable systematic position of Cercariaeum crassum Wesenberg-Lund, 1934 (Digenea), a parasite of Pisidium amnicum in eastern Finland.

Cercariaeum crassum Wesenberg-Lund, 1934 is redescribed at the cercariaeum stage and the daughter-rediae and cercaria are also described on the basis of new material from Pisidium amnicum collected in the Liikasepuro River (eastern Finland). The species is allocated to the family Allocreadiidae, although its generic affiliation remains unknown. The probable life-cycle (based on the developmental stages observed in daughter-redia) appears to eliminate the cercarial stage and, instead, a cercariaeum (a type of cercaria without a tail) may develop directly from germ balls or, rarely, through the stages of an ophthalmoxiphidiocercaria that transforms into a young caudate cercariaeum. Their morphology and development are shown to be consistent with the family Allocreadiidae. The probable lack of a second intermediate host in the life-cycle is discussed.