Editing Digenea (section)

==Life cycles==
There is a bewildering array of variation on the complex digenean life cycle, and plasticity in this trait is probably a key to the group's success. In general, the life cycles may have two, three, or four obligate (necessary) hosts, sometimes with transport or [[paratenic]] hosts in between. The three-host life cycle is probably the most common.  In almost all species, the first host in the life cycle is a [[mollusc]].<ref name=":0">{{Cite web|url=http://www.biology.ualberta.ca/parasites/ParPub/text/text/general/digen01t.htm|title=Principles of Parasitism: Digenea|website=www.biology.ualberta.ca|access-date=2020-01-09}}</ref> This has led to the inference that the ancestral digenean was a mollusc parasite and that vertebrate hosts were added subsequently.

The alternation of sexual and asexual generations is an important feature of digeneans. This phenomenon involves the presence of several discrete generations in one life-cycle.

A typical digenean trematode life cycle is as follows.  Eggs leave the [[vertebrate]] host in [[faeces]] and use various strategies to infect the first [[parasitic life cycles|intermediate host]], in which sexual reproduction does not occur. Digeneans may infect the first intermediate host (usually a [[snail]]) by either passive or active means. The eggs of some digeneans, for example, are (passively) eaten by snails (or, rarely, by an [[annelid]] worm),<ref name=":0" /> in which they proceed to hatch. Alternatively, eggs may hatch in water to release an actively swimming, ciliated larva, the [[miracidium]], which must locate and penetrate the body wall of the snail host.

After post-ingestion hatching or penetration of the snail, the miracidium metamorphoses into a simple, sac-like ''mother sporocyst''. The mother sporocyst undergoes a round of internal [[asexual reproduction]], giving rise to either ''rediae'' (sing. redia) or ''daughter sporocysts''. The second generation is thus the daughter parthenita sequence. These in turn undergo further asexual reproduction, ultimately yielding large numbers of the second free-living stage, the ''cercaria'' (pl. cercariae).

Free-swimming cercariae leave the snail host and move through the aquatic or [[sea water|marine]] environment, often using a whip-like tail, though a tremendous diversity of tail morphology is seen. Cercariae are infective to the second host in the life cycle, and infection may occur passively (e.g., a [[fish]] consumes a cercaria) or actively (the cercaria penetrates the fish).

The life cycles of some digeneans include only two hosts, the second being a vertebrate. In these groups, sexual maturity occurs after the cercaria penetrates the second host, which is in this case also the [[parasitic life cycles|definitive host]]. Two-host life cycles can be primary (there never was a third host) as in the [[Bivesiculidae]], or secondary (there was at one time in evolutionary history a third host but it has been lost).

In three-host life cycles, cercariae develop in the second intermediate host into a resting stage, the ''metacercaria'', which is usually encysted in a [[cyst]] of host and parasite origin, or encapsulated in a layer of tissue derived from the host only. This stage is infective to the [[parasitic life cycles|definitive host]]. Transmission occurs when the definitive host preys upon an infected second intermediate host. Metacercariae excyst in the definitive host's gut in response to a variety of physical and chemical signals, such as gut [[pH]] levels, digestive [[enzyme]]s, [[temperature]], etc. Once excysted, adult digeneans migrate to more or less specific sites in the definitive host and the life cycle repeats.