A cercaria (plural cercariae) is the larval form of the parasite Trematode. Within snails, the miricidium develops into a cercaria. Humans become infected after free-swimming cercaria liberated from infected snails penetrate the skin. These embryos move to the brood sac and mature into cercaria. On penetration, the head of the cercaria transforms into an endoparasitic larva, the schistosomule. The rediae burst out from the sporocyst to become the next-stage larvae called cercaria. A cercaria has a tapering head with large penetration glands. Each cercaria has a large head and a long tail. Rotifers ("Rotaria rotatoria") produce a chemical, Schistosome Paralysis Factor, suppressing cercaria swimming and reducing infections. The cercaria has two suckers&colon; a ventral sucker and rounded subterminal sucker. The cercaria of "B. mytili" were described in 1935 occurring in "Mytilus edulis" in Wales. Each cercaria has a biforked tail with which it swims to find a human host. In other species, the cercaria develops into an adult within the same host. Bacteria "Neorickettsia risticii" was detected in cercaria from "Semisulcospira libertina" in Korea. After digestion of the broodsac, sporocysts will become cercaria and further develop into adults.