ABSTRACT
The Digenea is probably the largest group of internal metazoan parasites. According to the databases maintained at the Parasitic Worms Division of the Natural History Museum, London, there are, at present, about 150 recognized families containing nearly 2700 nominal genera and about 18 000 nominal species. The general description of a digenean, although exceptions abound, is of a platyhelminth with a complex life-cycle, with three hosts in the cycle, the first being a mollusc, the second a wide variety of invertebrates and vertebrates, and the final 'definitive' host being a vertebrate. The unique intra-molluscan stages are closely associated with the mollusc, often highly specific to a host-species or lower taxon, and are generally viewed as consisting of three 'generations', the latter two having been produced asexually by parthenogenesis or budding (the precise process remaining controversial). The final stage emerges from the mollusc as the motile form known as the 'cercaria', a stage unique to the Digenea. The cercaria develops into a metacercaria in its second intermediate host, which is ingested by the definitive host, in which the sexually reproductive stage occurs. Included in the group are major human (e.g., schistosomes) and domestic animal (e.g., paramphistomes) pathogens as well as numerous minor pathogens and worms whose pathogenic effects have not been established. The ubiquity of these parasites and the large numbers of host-species which may be affected by the life-cycle of a single digenean species, indicate that these worms are likely to be major players in many ecosystems.
