Vorticella protozoa. Differential interference contrast (DIC) light micrograph of Vorticella sp. ciliate protozoa. These single-celled organisms consist of a bell-shaped head, and a stalk that is anchored to the substrate (not shown). The stalk contains a contractile fibril called a myoneme. When stimulated this shortens, causing the stalk to coil like a spring (seen here). Magnification: x56 when printed at 10 centimetres wide.