Research Highlights: Severed pieces of planarian behave like a whole animal

By Eduard Solà – Own work, CC BY-SA 3.0,

Severed pieces of planarian behave like a whole animal

  • Planarian is a free-living flatworm known for its soft, broad, leaf-shaped ciliated body and a three-branched digestive tract.[3]
  • Planarian is well-known for its regeneration capabilities.
  • The behaviors of freshwater planarians have been extensively studied for many years.
  • Planarian behavior has been used to study the animal’s development, regeneration, molecular evolution, memory, and many more.
  • The nervous system of the planarian is one of the simplest of bilaterally symmetric animals.
  • Bilateral symmetry is an animal with body shapes that are mirror images along a midline, such as a butterfly.[1]
  • Planarian has an anterior brain connected to ventral nerve cords interconnected by multiple commissures.
  • Commissures are nerve tissues that connect the hemispheres of the brain and the two sides of the spinal cord.[2]
  • Researchers found that when an intact planarian is stimulated by mechanical means and electromagnetic radiation, the head turns, the tail contracts, and the trunk elongates.
  • When the planarian was cut into three pieces, the front of the headless middle part behaved like the head and started to turn instead of elongating.
  • Additionally, the end of the head and midbody sometimes contracts instead of elongating.
  • Each of the body pieces seems to behave like a whole animal.
  • The results suggest that the planarian nervous system can quickly reorganize to fix functional problems complementing the slower regeneration processes.
  • The reorganization allows the planarian to survive while regenerating the other tissues.


Le, D., Sabry, Z., Chandra, A., Kristan, W. B., 3rd, Collins, E. S., & Kristan, W. B., Jr (2021). Planarian fragments behave as whole animals. Current biology : CB31(22), 5111–5117.e4.




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