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a protozoan of the family Bodonidae of the class Flagellata. The body is in the form of a droplet, measuring 6–20 μm long and 3–10 μm wide. The nucleus is vesiculate, and there are four flagellae of various lengths. This protozoan is found everywhere. It is a dangerous parasite of various types of fish, especially of the fry, and causes the serious disease costiasis. Under unfavorable living conditions, Costia necatrix forms cysts.