A miracidium is a small free-living larval stage
of parasitic
flatworms
in the class
Trematoda. It is
released from eggs which are usually shed in the
faeces of its vertebrate
host. When
an egg is immersed in water, its operculum liberates the
miracidium, which swims using
cilia to find the first host in
its life cycle, a
mollusc. Miracidia are
transmission stages that do not feed and, if they do not find a
molluscan host, do not survive much beyond 24 hours. It goes
through various stages in its mollusc host; the details vary with
species:-
- Miracidium: settles in the mollusc and becomes a sporocyst.
- Sporocyst: produces either more sporocysts, or rediae.
- Redia: produces either more rediae, or cercariae.
- In some species the redia stage is missed out, and sporocysts
produce cercariae.
- Cercaria: This is somewhat like a small adult, but has a large
swimming tail somewhat like a tadpole's (but without a
notochord or backbone, as it is not a
chordate). It finds and
settles in a host, and becomes an adult, or a mesocercaria, or a
metacercaria, according to species.
- Mesocercaria: A cercaria little modified but resting.
- Metacercaria: A cercaria encysted and resting.
- Adult.
With some species of Trematoda the cercaria
develops into an adult in that host.
With other species of Trematoda (for example
Ribeiroia) the
cercaria
encysts, and
waits until the host is eaten by a third host, in whose gut it
emerges and develops into an adult.
In its final host, it eventually lays eggs which
are discharged in its host's faeces. From there the eggs hatch in
the presence of free water and the miracidium stage of life is
reached again.
miracidium in Czech: Miracidium
miracidium in German: Miracidium
miracidium in Japanese: ミラシジウム
miracidium in Polish: Miracidium
miracidium in Portuguese:
Miracídio