Stentor. Light microscope footage of ciliate protozoan Stentor sp. swimming. Stentor species are filter-feeding freshwater protozoa which can reach lengths up to 2 millimetres, and 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, and the algae receive protection and feed on the Stentor's metabolic wastes.