Almost all cells – whether prokaryote or eukaryote – have tails. Eukaryotes evolved from a tail-bearing prokaryote. Only seed-producing plants have cells without tails, though their genomes retain the knowledge.
Tails may be a single whip-like flagellum or solitary sensory cilium, or a multitude of short hair-like cilia on the cell lining. Regardless of type, appearance and motion, complex cell tails are structurally identical, and their uses the same.
Cell tails serve two possible purposes: motility and perception. Tails tell tales of goings on while beating a wave-like motion. Cilia move in coordinated fashion.
Regardless of type, each tail has fibers, or microtubules, which rapidly shift positions among each other, generating movement. This set of fibers are enclosed in the cell membrane.
The proteins that make up cell tails differs between bacteria, archaea, and eukaryotes. This tells that tails are an instance of convergent evolution: coming to the same solution independently.
Bacterial flagella work differently than eukaryotic tails, rotating like a propeller rather than beating back and forth. These flagella are helical, and revolve 200 – 1,000 times per minute, propelling bacteria as fast as 60 cell-lengths per second. A cheetah, the fastest land animal, has a top per-second speed of 25 body lengths.
Tails are crucial cellular sensors. Depending upon type, cell cilia can sense fluid movement, chemicals, osmotic pressure, temperature and gravity.
Cilia are critical to inter-cellular communication. In animals, they help maintain organ function via continuous feedback loops.
The asymmetry of human bodies is established within a few hours of embryotic development. This is done by the tails of cells sweeping clockwise, generating a net leftward flow, which tells left from right, and determines situs solitus: the position of organs.
Damaged tails often spells cell death. If the cilia of cells cannot function, disorders arise. Badly behaved or non-functioning cell tails are instrumental in many diseases.
Philip Beales, “Cell tails: a surprising number,” New Scientist (30 October 2013).