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 distinctive movement when observed under a microscope and the fact that they are generally taught in school biology classes as a typical representative of the ciliates.
Like many other ciliates, most Paramecia are large for one-celled creatures. The largest are the P. multimicronucleatum, which can be as much as 350 micrometers long and are almost visible without a microscope. The smallest are the P. putrinum (a.k.a., P. trichium), which can be as little as 70 micrometers long (about the width of a blond human hair).
Paramecia 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.
Ciliates reproduce in two ways. 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 many 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 contain livinng chlorella, a kind of green algae that are not digested, but inside the paramecia as endosymbiotes. The algae takes nutrients from the host's protoplasm and returns oxygen. Another interesting symbiosis involves P. caudatum and a kind of bacteria called Holospora obtusa. This bacterium lives only inside the macronucleus of this one species of paramecium, so it's almost like a part of the host, but not all paramecium are infected. The bacterium somehow gives its host an ability to withstand higher temperatures.
Phylum: Ciliata (Ciliaphora)