The selection of habitats among the monogenea.

The pressures responsible for the selection of "non-ectoparasitic" habitats in the Monogenea are discussed. It is suggested that certain monogeneans have become internal to reduce the pressures exerted by predators of ectoparasites rather than to avoid competition with other ectoparasites or to have access to better resources. The fact that these movements have concerned the Monopisthocotylea rather than the Polyopisthocotylea could be related to the different mode of invasion by oncomiracidia in these two groups. When Monogenea are internal in "non-fish" hosts, it is suggested that this has allowed them to survive the colonisation of terrestrial environments by vertebrates, a process of coevolution which was disturbed by lateral transfers.