Light microscopy of a group of Stentor sp. ciliates swimming in fresh water. Each possesses a macronucleus that appears as a string of beads. Other structures include food vacuoles and contractile vacuoles. Like many protozoa, Stentor has the ability to change shape, and can range in form from pear shapes to trumpet shapes to spheres. The bluish-green colour comes from the pigment stentorin. Stentor can be found in lakes, ponds and streams throughout much of the world. Stentor species may reach almost two millimetres in length, making them one of the largest single-celled organisms. Recorded under enhanced-colour Nomarski differential interference contrast illumination.