Stentor with vorticella. Light microscope footage of ciliate protozoa Stentor sp. (top, partly visible) and Vorticella sp. (centre). Both are filter-feeding freshwater protozoa. Stentor species can reach lengths up to 2 millimetres, and are among the largest known unicellular organisms. Some species live symbiotically with certain species of green algae (numerous green spheres). After being ingested, the algae live on and continue to photosynthesize, while their host absorbs nutrients produced, and the algae receive protection and feed on the Stentor?'?s metabolic wastes. Vorticella consist of a bell-shaped head and a contractile stalk that is anchored to the substrate. Cilia (tiny hair-like structures) attached to the head are used to create currents to capture food and also for movement.