Prokaryotic cells have various shapes; the four basic shapes of bacteria are:
* Cocci – A bacterium that is spherical or ovoid is called a coccus (Plural, cocci). e.g. Streptococcus, Staphylococcus.
* Bacilli – A bacterium with cylindrical shape called rod or a bacillus (Plural, bacilli).
* Spiral bacteria – Some rods twist into spiral shapes and are called spirilla (singular, spirillum).
* Vibrio – comma-shaped
The archaeon Haloquadratum has flat square-shaped cells.