A cercaria (plural cercariae) is the larval form of the parasite Trematode. It develops within the germinal cells of the sporocyst or redia. A cercaria has a tapering head with large penetration glands. It may or may not have a long swimming "tail", depending on the species. The motile cercaria finds and settles in a host where it will become either an adult, or a mesocercaria, or a metacercaria, according to species.
Rotifers (Rotaria rotatoria) produce a chemical, Schistosome Paralysis Factor, suppressing cercaria swimming and reducing infections.
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