When Paramecium comes in contact with bacteria, rows of cilia lining the oral groove move the food down to the end of the groove where it becomes engulfed in a food vacuole by the process of phagocytosis. Food vacuoles created in this way merge with the cell's lysosomes which dump in enzymes to digest the bacteria. Food vacuoles circulate within Paramecium moving first to the anterior end and then to the posterior end. In the photo to the left, several paramecia feed on a large cluster of filamentous bacteria.

In this photo, the oral groove is visible just above the center of the photo, where it looks like a flap of cell membrane folded over. At the end of the groove, a concentrated group of cilia force food into the food vacuole as it forms.

Here, the food vacuoles have been stained red. Elimination of undigested bacterial cell wall is through a temporary anal pore at the posterior end.

Lying just within the outer cell membrane covering (pellicle) are the encapsulated trichocysts, thread-like structures that fire upon contact with predators. The trichocysts also function to stabalize the cell during feeding. In the photo to the left, a Paramecium was stained and then stimulated to eject its trichocysts

The refractile crystals that can be seen in this photo are excretory products. They show up as bright colored objects in ‘polarized light’.

Paramecium bursaria is capable of farming certain algae within its own cytoplasm! It receives some of the products of the alga's photosynthesis in return for providing a home for the algal symbiont, Chlorella. In this photo, the green symbionts are seen through Paramecium's clear cell membrane. In a dish lit from one side, P. bursaria gathers toward light, while its colorless relatives tend to be found in the opposite side. This is a good relationship for the algae; safely contained within the larger organism they are not as likely to be eaten. However, if there are no other food sources available, P. bursaria will digest its algal symbionts for a source of nutrition.

A detailed learning program about Paramecium is available
on our new video-DVD program, The Branches on the Tree of Life: Protists