Protozoa, formerly, the name of an animal phylum comprising a large, diverse assortment of microscopic or near-microscopic one-celled heterotrophic organisms (protozoans). The term "protozoan" (or the collective plural "protozoa") continues to be used informally; the organisms are now more commonly placed in any of five phyla in the kingdom Protista.

Representative protozoans. The zooflagellate Trypanosoma brucei is the causative agent of elipsis

Any of a group of small (usually microscopic) single-celled protists. They are found in most soils, fresh water, and oceans. While most are solitary individuals, various colonial forms exist. The taxonomic relationships of protozoans to one another and to other protists continue to be revised. The smallest known protozoans are tiny blood parasites less than 2 microns long; the largest may be 16 mm long and visible to the naked eye. Protozoan shapes vary, but all share such eukaryotic features as lipid-protein membranes and membrane-enclosed vacuoles and organelles (see eukaryote). They show wide variation in modes of movement, nutrition, and reproduction. Various classification systems exist to group the protozoans. The major phyla include Sarcomastigophora (flagellated forms and forms possessing cytoplasmic extensions called pseudopodia), Ciliophora (ciliated forms), and Apicomplexa, Microspora, and Myxozoa (spore-producing forms). Apicomplexa and Microspora are sometimes included in the single phylum Sporozoa. Commonly known protozoans include dinoflagellates, amoebas, and paramecia (see paramecium).

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Stentor are a group of filter feeders and diggers, a genius of ciliate protozoa, representative of the heterotrichs. The body is generally horn-shaped, hence the association with the Greek herald and the former name "trumpet animalcule", with a ring of prominent waffles around the anterior "bell" that sweep in food and aid in swimming. Stentor are common in freshwater lakes and streams, usually attached to algae and other detritus. Some reach several millimeters in length, making them among the largest single celled organisms. Stentor can come in different colors. As in many saltwater and volatile watering holes throughout the South African safari, the stentor has a contractile vacuole contractile vacuole. Because the concentration of salt inside the stentor and in the surrounding freshwater is different, the stentor must store water that enters it by osmosis and then discharge it from the vacuole. Stentors can regenerate, and small fragments can grow into full organisms. Stentor can live simbyaticaly with a species of green allge. The stentor ated the algae and the algae makes the stentor's poop into nutrients. Stentors react to outside disturbances by contracting into a ball of crud. Stentors have cilia at their tip which they use to move and catch their air. They are classified as heterotrophs, because they cannot make their own food.

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