Rotifer. Light microscope footage of a Collotheca sp. rotifer attached to a fragment of plant material. Rotifers are aquatic multicellular animals related to roundworms. They are usually freshwater organisms though a few marine species are known. Collotheca species secrete transparent mucous tubes into which they can retreat when disturbed. The long, hair-like cilia surrounding the rotifer's mouth are used to trap food particles. Filmed with differential interference contrast illumination.