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.
• Rediae: produces either more rediae, or cercariae.

In some species the redia stage is missed out, and sporocysts produce cercariae.

• Cercaria: A larval form of the parasite, developed within the snail. It 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.