They occur worldwide and range in size from about 5 millimetres (0.2 inch) to several centimetres; most do not exceed 100 millimetres (4 inches) in length.Flukes parasitize members of all vertebrate classes but most commonly parasitize fish, frogs, and turtles; they also parasitize humans, domestic animals, and invertebrates such as mollusks and crustaceans.