Rotifers are of the animal kingdom but they are not single celled. In fact, they are among the smallest of the multi celled animals. Moreover, all Rotifers of the same species have the identical number of cells, and therefore, are valued by the scientific community for their role in cancer research.
Paramecia can divide as often as two or three times a day. Assuming that they divide only once a day - if you began with just two Paramecia, at the end of 113 days you would have a pile of Paramecia (where in heaven's name to keep them would be a challenge) whose mass would equal the mass of the earth. I don't know the weight or mass of a single paramecium, or of the earth at the moment, but I'm sure someone does. If you're interested in doing the calculations we can get that information.
The Didinium is a protozoa that feeds exclusively on Paramecia. If a culture of Paramecia is innoculated with just two Didinium, the Didinium will wipe out the colony of Paramecia in just a few days, regardless of how large the culture is. It takes about one minute for the Didinium to consume a Paramecium, after which it moves on to consume another - sort of non-stop gluttony. The more they eat, the bigger they get, and the bigger they get the sooner they split into more of themselves. The Paramecia don't have a chance. Interesting though - if a Didinium cannot find a Paramecium within a 4 hour period, it will encyst (form a cyst around itself for protection and as a form of suspended inactivity) and wait for a more bountiful day. Scientists believe that this encystment, in addition to preserving the Didinium, also prevents the Didinium from completely eliminating all the Paramecia.
Paramecia have a weapon defense system that they use to ward off their enemies. When attacked, they release dart-like objects from capsules scattered underneath their bodies. It might be noted that this defense system is totally ineffective against the Paramecia's deadly and vicious enemy - the Didinium.
Paramecia have a small circular cavity on their upper side called a "water expelling vesicle". The vesicle draws water from the Paramecium's cytoplasm and expels it, by contraction, as waste water through a hole in the vesicle's top, thereby acting as an excretory vehicle. The water drawn into the vesicle and the amount expelled is always in critical balance. Should an imbalance occur, because of changes in the surrounding environmental conditions, the Paramecium is in danger. If the water drawn from the cytoplasm into and out of the vesicle is too rapid, the Paramecium could become dehydrated and actually die of thirst (humanly speaking). If the vesicle cannot contract and expel the water fast enough, the vesicle and the channels that feed it will enlarge and eventually explode, killing the Paramecium. An animation in the section "Let's Talk About Size" shows the contracting and expanding vesicle.
The movement of a Paramecium can be controlled by effecting an electric current in the fluid (water) where they are being cultured. A positive and negative electrode are attached to two 1.5 volt batteries with a polarity reversing switch. The paramecia will always swim to the negative pole. If polarity is switched, they will swim to the other (-) pole. They do not turn around, but rather the cilia will bend and face toward the positive pole, so the animal actually swims backward toward the negative pole (if it was facing forward beforehand). This peculiar mechanism is called galvanotropism. Many of the ciliates are galvanotropic, some positively and some negatively.
There are several ways in which these pond critters are equipped to protect themselves and ensure the survival of their species. One method is encystment - an example is referred to above. The cyst is formed of a hard, protective outer shell that houses and protects the creature from the hazards of the environment, when the food supply diminishes or even if the environment disappears completely. If the water (pond, puddle) is in danger of drying up, the creature will form a cyst. These cysts will dry and can then be carried by the wind, where some will land in other bodies of water. If there is a food supply present, the creature will excyst (somehow it knows food is present) and begin its new life. Some critters, like the Paramecium, don't form cysts. They get transported to other bodies of water most commonly by water fowl or simply by common song birds that stop for a drink or a bath in a body of water where Paramecium and other microbes are present. As the birds fly to another pond, puddle or bird bath the protozoa fall off and begin again - in a new environment.
The Blepharisma (images in the image section) is a common ciliate found in most any pond. It is similar in size and shape to the Paramecium, but displays an unusual characteristic, albeit a potentially fatal one. It is easily spotted because, unlike most of its community pond inhabitants, it is not transparent, but rather a striking pink color under normal conditions. If it lives in bright sunlit ponds it is usually colorless. When exposed to an intense artificial light, such as that from a micro-projector, the pink pigment emits a poisonous toxin that completely disintegrates the creature.
Most of us picture Amoebas as protoplasmic, amorphous blobs of jelly-like substances that slide about capturing prey by engulfing them. Most are of that type. Some, however, have developed an unusual (and not so simple) mechanism to protect themselves. They construct transparent domes around their bodies, usually from sand particles found on the pond bottoms. The dome is open on the bottom so that the creature can feed and also extend its locomotion blobs - its feet, actually. In the image section, is a picture of four Arcella, or shelled Amoebas, feeding on what appears to be an egg sac of another microbe species. An Arcella on its back is in serious trouble, unable to either feed or move. In order to turn over, it generates a gas bubble inside the dome which it forces to one side. This side then rises, standing the dome on its opposite end. The Arcella then extends a foot downward, attaches the foot to the bottom surface, and pulls itself over and upright.
Glassworms, by any standard, are not among the most attractive of pond creatures. Perhaps that's why they try to remain hidden - keep a low profile, so to speak. Glassworms are the larvae of several kinds of midges (flies), and during their larval stage they live encased in a tube - a tube they construct from their own excrement. They carry the tube with them, and only extend their heads when munching on algae and decaying vegetable matter.
Protozoa die every day by the zillions. They are very sensitive to environmental changes, both physical and chemical. They also eat one another and are eaten by other aquatic animals in enormous numbers. Changes in ph, O2 levels, water temperature and especially drying up of the water kill them quickly. Despite this astronomical death rate, the number of protozoa in the world (not in a local environment) remains somewhat constant. Fortunately for the protozoa, each species is equipped with reproductive mechanisms that on average, over time, balance the death rate.
Most of the one celled protozoa reproduce from some form of cell duplication called "binary fission". It is the most common and least complex of the reproductive processes of Protozoa - the others being "multiple fission", and "external and internal budding". In the image section (and in the pic above on this page) there is an image of a Paramecium dividing into two. The "splitee" will become another complete Paramecium, which might even duplicate itself before the day ends. The Vorticella (image section) also divides by fission, developing another body from its trailing stalk. Since they are not communal creatures, the new Vorticella will break off from the parent stalk and develop its own stalk as well as its own independent life - for as long as it lasts!
The ciliates, in addition to fission, also multiply by another form of reproduction known as "conjugation". The image page contains a link in the Paramecium section to an image of two Paramecia reproducing by conjugation. The image also contains a narrative explaining the process.
It's true, the one celled Protozoa don't have organs, but they have what are called organelles - specialized parts of the cell that perform the same basic functions as organs do in the higher life forms. The Paramecium is probably the most complex of the single celled creatures, and the best of the Protozoa to see how several of these organelles function together.
The oral groove, visible in part in the image above of the Paramecium dividing, is the cilia lined channel that directs food (bacteria) into the gullet, and from there to the food-digesting vesicle, where the food is digested and the undigested food is discharged. So, the oral groove acts as the mouth and throat, the gullet as the esophagus, and the food-digesting vesicle as the stomach.
There is one other complex organelle, the water expelling vesicle, that performs both the functions of respiration and excretion. Refer to the section above on the "Paramecium" for an explanation of how this organelle keeps the Paramecium alive and functioning. All of these organelles can be seen in the different images of the Paramecium, Stentor, Amoeba, and Vorticella in the image section.