Light micrograph of Collotheca sp. rotifer on an "Aufwuchs"-slide, exposed 6 days in a garden pond in the botanic garden of Cologne. These animals are related to roundworms. They are usually freshwater organisms though a few marine species are known. Collotheca secretes transparent mucous tubes into which they can retreat when disturbed. Microscopic contrast method : Differential interference contrast. Magnification 180x when printed at 10 centimetres wide.