text: 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.