Stentor protozoan, light microscopy footage. Stentor are filter-feeding freshwater protozoa which can reach lengths of up to 2 millimetres. They are among the largest known unicellular organisms. A ring of prominent cilia surrounds the anterior part of the cell to sweep in food and aid in swimming. Some species live symbiotically with certain species of green algae. After being ingested, the algae live on and continue to photosynthesize, while their host absorbs nutrients they produce. The algae receive protection and feed on the protozoan's metabolic wastes.