Digeneans are endoparasites. The life cycle involves two to four hosts and a number of developmental stages, including two types of free-living larvae. The definitive host of the adult is always a vertebrate. Snails are common intermediate hosts.
Clockwise from top left:
Dicrocoelium dendriticum and Opisthorchis sp. liver flukes of mammals;
blood flukes, Schistosoma spp., are among the most widespread and serious parasites of humans.