Bacteria (Bacterium) and Archaea (ArchaelCell) are classified by direct examination with the light microscope according to their morphology and arrangement. The basic morphologies are Spheres (coccus) and round-ended or rod-shaped Cylinders (bacillus). But there may be others such as helically twisted cylinders (spirochetes), cylinders curved in one plane (selenomonads) and unusual morphologies (such as the square, flat box-shaped cells of the archaean genus Haloquadratum). Arrangements include pairs, tetrads, clusters, chains and palisades. [from Wikipedia]
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