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Bacteria
Under the three-domain system, bacteria now have their own domain while single-celled organisms with nuclei make up the Protista kingdom, a smaller taxonomic division. Five-kingdom systems separate protists and bacteria, while other systems still classify them together. Bacterial shapes are simpler than those of eukaryotic (nucleus-containing) cells. Three shapes typify almost all bacteria: the sphere, the rod and the spiral. Sphere-shaped bacteria, or cocci, include Streptococcus and Staphylococcus strains. Scientists refer to rod-shaped bacteria as bacilli; Escherichia coli have this shape. Spiral bacteria may be rigid or flexible depending on their protein sheaths. The Treponema pallidus organism that causes syphilis is a spirochete or spiral bacterium.
Diatoms
These unicellular algae share one unusual characteristic: their silica-based cell wall. Their shapes fall into two broad categories, radial symmetry or bilateral symmetry. These organisms' rigid cell walls form simple and complex geometric shapes. The rods, spheres, ellipses and triangles that diatoms' silica sheaths form may have further refinements to help the cells maneuver through their environment; spines, perforations and frills help diatoms colonize both aqueous and terrestrial habitats. When the cells within them die, the silica cell walls remain.
Radiolarians
Amoebic protists that form mineralized shells around themselves, radiolarians create such complex shapes that they may look more like crystalline snowflakes than like living organisms. Their overall shapes may resemble goblets, geodesic domes, spiky anemones, bells, cacti, flowers or spiral shells. One of the earliest organisms to evolve on Earth, radiolarian remains date back to the Cambrian Period of over 500 million years ago. The ornate remains these creatures leave behind provide plentiful fossil evidence for their development through the ages.
Amoebas
Amoebas have no set shape, instead freely changing shape to meet their needs of the moment. These single-celled organisms reach out pseudopods, or "false feet," to engulf food or investigate their environment. Their name comes from a Greek word meaning "change" because they live in a constant state of shape-changing flux. The largest members of this genus of protists can reach nearly a millimeter in length, a massive size for a single-celled protozoan.
Other Protists
Paramecia, aquatic protists that move through their fluid environment with cilia, often have a distinctive shoe-sole shape. Students look at paramecia as examples of protists because the relatively large organisms have highly visible organelles. Another group of organisms that find their way into classroom slides is the genus Euglena; these protists have whip-like cilia to propel them through water, yet they also photosynthesize. These elliptical green creatures have characteristics common to both animal and plant cells, making them a biological curiosity.