Miracidium
From Biocrawler, the free encyclopedia.
The miracidium is the small free-swimming larva of some Trematoda including the Schistosoma. It normally hatches from eggs present in its host's faeces. When hatched in free-standing water the miracidium typically swims and finds a particular species of snail to continue its life cycle in.
In its snail host it becomes a sporocyst, which produces cerceriae, which emerge and search for a host according to species.
The cercaria 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).
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.
Iin 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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