Fig. 1. Life cycle of Ichthyophthirius multifiliis parasitizing many species of freshwater fish. 1 Swarmer penetrates the skin of a fish and becomes encysted (EN) by host tissues. 2 The encysted swarmer grows to be a trophozoite, reaching a diameter of up to 1 mm; the skin then presents with a large grayish pustule at these places. 3 Upon rupture of the pustules the trophozoites, which have numerous contractile vacuoles, are liberated and swim about feebly. Upon coming to rest on the bottom of the pond the trophozoite secretes a thick-walled gelatinous cyst wall about itself. 4–8 Within an hour of encystation the mother trophozoite starts reproduction by simple transverse division, this being repeated until up to 1024, 30–50 μm long, pear-shaped swarmers (with a single contractile vacuole) are formed. 9 After rupture of the cyst, the liberated swarmer attaches to the skin of fish within 1 day (unattached ones die during the second day). CI, cilia; CV, contractile vacuole; CW, cyst wall of cysts on the bottom of the pond; CWS, cyst wall in skin; CY, cytostome; EN, encysting swarmer; LC, long terminal cilium; MA, macronucleus; MI, micronucleus; OP, opening rupture of CW; SW, swarmer