The most ancient trematode life cycle is dixenic. In this
case final hosts became infected with hermaphroditic stage
(marita) after swallowing of free-swimming larvae produced
by parthenogenetic stages (parthenitae) in intermediate host
- mollusk. In modern trematodes such archaic cycles disappeared.
The further evolutionary transformations of dixenic
life cycle were connected with changes in biology of dispersive
stages. The larvae gained ability to encystment in the environment,
and the adolescaria stage was added in the cycle (Notocotylidae).
The next step is appearance of trixenic cycles;
in most cases it was connected with replacing of free-living
adolescaria by parasitic metacercariae (Echinostomatidae,
Heterophyidae, Renicolidae, Microphallidae).
The further transformations of the life cycles were connected
with decreasing in host number. Passage back to dixenic cycles
took place in the case a mollusk may act as both the first
and the second intermediate hosts (Microphallidae). Monoxenic
life cycles in hemiurids resulted from the progressive decrease
in number of the hosts in the typical trixenic cycles or dixenic
ones where molluscs combined functions of two hosts. (See
details in Galaktionov
& Dobrovolsky, 2004)