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. A number of zoochlorella cells can be seen floating freely having recently departed a deceased Paramecium. Recorded under Nomarski DIC illumination.