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The protozoa are all unicellular heterotrophs. They obtain their nutrition by ingesting other organisms or dead organic material. The word protozoa comes from the Latin word for first animals. The protozoans are grouped into various phyla based on their modes of locomotion. They may use cilia, flagella, or pseudopodia. Some protozoans are sessile, meaning they do not move. These organisms are parasitic, since they cannot actively capture food. They must live in an area of the host organism that has a constant food supply, such as the intestines or bloodstream of an animal. The protozoans that use pseudopodia to move are known as amoebas, those that use flagella are called flagellates, those that use cilia are known as the ciliates, and those that do not move are called the sporozoans.

The amoebas belong to the phylum Rhizopoda. These protists have no wall outside of their cell membrane. This gives the cell flexibility and allows it to change shape. The word amoeba, in fact, comes from the Greek word for change. Amoebas use extensions of their cell membrane (called pseudopodia) to move, as well as, to engulf food. When the pseudopodium traps a bit of food, the cell membrane closes around the meal. This encasement forms a food vacuole. Digestive enzymes are secreted into the food vacuole, which break down the food. The cell then absorbs the nutrients. Because amoebas live in water, dissolved nutrients from the environment can diffuse directly through their cell membranes. Most amoebas live in marine environments, although some freshwater species exist. Freshwater amoebas live in a hypotonic environment, so water is constantly moving into the cell by osmosis. To remedy this problem, these amoebas use contractile vacuoles to pump excess water out of the cell. Most amoebas reproduce asexually by pinching off a part of the cell membrane to form a new organism. Amoebas may form cysts when environmental conditions become unfavorable. These cysts can survive conditions such as lack of water or nutrients. Two forms of amoebas have shells, the foraminiferans and the radiolarians.

The foraminiferans have a hard shell made of calcium carbonate. These shells are called tests. Foraminiferans live in marine environments and are very abundant. When they die, their shells fall to the ground where they become a part of the muddy ocean floor. Geologists use the fossilized shells to determine the ages of rocks and sediments. The shells at the ocean floor are gradually converted into chalky deposits, which can be uplifted to become a land formation, such as the white cliffs of Dover in England. Radiolarians have shells made of silica instead of calcium carbonate. Both organisms have many tiny holes in their shells, through which they extend their pseudopodia. The pseudopodia act as a sticky net, trapping bits of food.

The flagellates have one or more flagella and belong to the phylum Zoomastigina. These organisms whip their flagella from side to side in order to move through their aquatic surroundings. These organisms are also known as the zooflagellates. The flagellates are mostly unicellular with a spherical or oblong shape. A few are also amoeboid. Many ingest their food through a primitive mouth, called the oral groove. While most are motile, one class of flagellates, called the Choanoflagellates, is sessile. These organisms attach to a rock or other substrate by a stalk.

The ciliates are members of the phylum Ciliophora. There are approximately 8,000 species of ciliates. These organisms move by the synchronized beating of the cilia covering their bodies. They can be found almost anywhere, in freshwater or marine environments. Probably the best-known ciliate is the organism Paramecium. Paramecia have many well-developed organelles. Food enters the cell through the oral groove (lined with cilia, to "sweep" the food into the cell), where it moves to the gullet, which packages the meal into a food vacuole. Enzymes released into the food vacuole break down the food, and the nutrients are absorbed into the cell. Wastes are removed from the cell through an anal pore. Contractile vacuoles pump out excess water, since paramecia live in freshwater (hypotonic) surroundings. Paramecia have two nuclei, a macronucleus and a micronucleus. The larger macronucleus controls most of the metabolic functions of the cell. The smaller micronucleus controls much of the pathways involved in sexual reproduction. Thousands of cilia appear through the pellicle, a tough, protective covering surrounding the cell membrane. These cilia beat in a synchronized fashion to move the Paramecium in any direction. Underneath the pellicle are trichocysts, which discharge tiny spikes that help trap prey. Paramecia usually reproduce asexually, when the cell divides into two new organisms after all of the organelles have been duplicated. When conditions are unfavorable, however, the organism can reproduce sexually. This form of sexual reproduction is called conjugation. During conjugation, two paramecia join at the oral groove, where they exchange genetic material. They then separate and divide asexually, although this division does not necessarily occur immediately following the exchange of genetic material.

The sporozoans belong to the phylum Sporozoa. These organisms are sessile, so they cannot capture prey. Therefore, the sporozoans are all parasites. As their name suggests, many of these organisms produce spores, reproductive cells that can give rise to a new organism. Sporozoans typically have complex life cycles, as they usually live in more than one host in their lifetimes.

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