Invertebrates
Planarian
Tricladida · also called planarians, flatworms
Cut a planarian into a dozen pieces and, within about two weeks, you may end up with a dozen complete, fully functional worms — including fragments that had no brain tissue at all before the cut, yet regrow one from scratch. It manages this through a body packed with stem cells capable of becoming almost any tissue the worm needs to rebuild.
Overview
Planarians, order Tricladida, are small flatworms found in freshwater streams, ponds, and damp habitats worldwide. They hunt small invertebrates and scavenge dead matter through a single opening on the underside of the body that serves as both mouth and anus, with two closely spaced eyespots giving many species a cross-eyed look.
Regeneration without real limits
Powered by stem cells called neoblasts spread throughout the body, a planarian can regrow a complete individual, brain included, from a small fragment — a feat of regeneration far beyond almost any other animal. This has made the worm a major model organism for stem cell and regeneration research, and even studies probing whether memory can survive a regrown brain.
Status
Least Concern — common and widespread across clean fresh water and damp habitats globally, though sensitive to pollution, which makes some species useful indicators of water quality.
Planarian: common questions
How is it possible for a planarian to regrow a whole new worm from a small piece?
Planarians carry a population of specialised adult stem cells called neoblasts distributed throughout their bodies, and these cells are able to divide and transform into essentially any type of cell the worm needs, whether muscle, skin, nerve, or reproductive tissue, in order to rebuild whatever structures are missing after the body is damaged or cut. When a planarian is cut into a fragment, nearby neoblasts proliferate rapidly at the wound site and organise themselves into a new, complete, properly proportioned individual within roughly one to two weeks in most cases.
Can a planarian really regrow a working brain?
Yes, even a fragment of the body that originally contained no brain tissue at all is capable of regenerating a fully functional brain and nervous system from its neoblast stem cells, correctly reforming the eyespots, nerve cords, and brain structures in the right positions and proportions relative to the rest of the regrown body. This ability to regenerate an entire functioning nervous system essentially from scratch is one of the reasons planarians are studied so intensively by researchers interested in regeneration, since almost no other animal group can achieve anything close to this level of neural regrowth.
Why do planarians look cross-eyed?
Most planarian species have two simple eyespots, light-sensitive organs rather than true image-forming eyes, positioned close together near the front of the head and often angled slightly inward, which produces the distinctive cross-eyed appearance many people notice when looking at one under magnification. These eyespots cannot form detailed images but are effective at detecting the direction and intensity of light, which helps the worm move away from brighter areas toward the dark, sheltered, damp conditions it generally prefers.
Why are planarians used so much in scientific research?
Their combination of being simple, small, easy and inexpensive to keep and culture in large numbers, and capable of extraordinary tissue and even nervous system regeneration makes planarians an unusually convenient and informative model for studying the biology of stem cells and regeneration in a laboratory setting. Some researchers have gone further and investigated whether certain simple learned behaviours might persist in a planarian even after it regenerates a substantial portion of its body, including brain tissue, research that touches on some genuinely open questions about how memory might be stored in a nervous system.