The kingdom Protista is used to group most single-celled organisms, except bacteria and blue-green algae. Protista is a large and variable group containing both plant and animal characteristics. This group includes about 50,000 species of protozoans (first animals) and between 8,000 and 12,000 species of algae (simple plants).
Protists are mostly microscopic and have no organs or tissues. They are single-celled but may occur in colonies. They may be free-living on land or in water, or live in association with other plants and animals. Locomotion is achieved by waving tiny hair-like threads. Different organisms obtain nutrients by one of three methods: direct absorption from the surrounding environment, eating other plants and animals, or by making their own through photosynthesis.
In the kingdom Protista, bacteria are sizable chunks of food and predators are also microscopic in size. Protists occur from deep oceans to shallow puddles and even in the water film between soil particles. All aquatic ecosystems depend on protozoans and algae as the basic links in food chains and nutrient cycles.
An amoeba is an organism that is readily found in pond water and gets it's name from the Greek word for change, amoibe. The outline of the single-celled body changes constantly as it moves and has no permanent shape. The freshwater amoeba is typical of this group. Oxygen and carbon dioxide for respiration pass freely through the delicate cell membrane. Amoebas multiply by simply dividing in two. If food or water becomes scarce, they can survive as inactive cysts until conditions improve.
Amoebas feed themselves by trapping food particles in the surrounding water. Their cell parts can become more fluid to stretch out and form false feet around food bits. They are predators of other protozoans and minute animals. They can detect vibrations, and when stalking active prey, they will avoid contact until their false foot has surrounded the prey completely.