Miracidium

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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.

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