Paramecia are single-celled microorganisms that can be found in freshwater ponds and other standing water. Aside from the amoeba, the paramecia are probably the most widely known protozoan because of their interesting slipper shape and the fact that they are generally taught in school biology classes as a typical representative of the ciliates.

Like most other ciliates, Paramecia are fairly large for one-celled creatures. At between about 100 to 200 micrometers long, they can almost be seen without a microscope. They are fun to watch under low-power magnification because of their graceful swimming habit and the way they react to obstacles in their path. Long and streamlined, the paramecium glides through the water while twirling around its long axis. When a barrier is encountered, the paramecium reverses motion, swings around to a different angle and then moves forward again. This trial and error strategy is repeated until the paramecium has maneuvered around the obstacle. Just how the paramecium can coordinate the movements of their thousands of individual cilia to accomplish this locomotion, obstacle avoidance and feeding behaviour has yet to be explained.

These ciliates are also interesting in how they reproduce. They have two means of reproduction. One is simple cell division (binary fission or mitosis), where one cell splits itself up into two identical daughter cells. The other reproductive mechanism is conjugation, in which two paramecia come into contact and exchange genetic material in a fairly complex process.

There are several species of paramecia. Most of them don't have many noticeable differences from the others, but one, P. Bursaria, is both strikingly different and very interesting as well. Bursaria are green. That's not because they contain chloroplasts like Euglena or the plants, but because their cells are filled with chlorella, which live inside the paramecia as endosymbiotes.