Paramecium bursaria, ciliated protozoans measuring about 150 microns in length, swimming about in pond water. This Paramecium species is unique in that it is the only one that forms a symbiotic relationship with zoochlorella, a green alga. The alga provides the Paramecium food and the Paramecium provides the alga a home and transportation. The Paramecium move by beating their short, hairlike cilia which cover their bodies, but are only visible along their outer edges in light microscopic views. Recorded under Nomarski DIC illumination.