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Go to CuriosityStream.com/SciShow to learn more. [INTRO ♪]. The planarian is a saltwater and freshwater flatworm known primarily for its regenerative abilities.

Cut off their tail, and they'll grow it back; cut them in half and they'll become two whole flatworms—even though they have a distinctive head region with a brain in it. What's really amazing is that even after losing its brain, a planarian can remember things from before it was beheaded. Planarians are pretty simple animals, but they do have brains that control things like sensory reception.

And this brain is considered a true brain, not just a bundle of neurons— it has two lobes, it controls nervous function throughout the body, and it has specialized regions. This means their brains probably look a lot how ours once did. And this makes them a great analog for studying brain evolution, as well as asking fundamental questions about things like memory.

It's hard to know what memories a planarian has, of course. But you can train them to perform new, unnatural behaviors, and then see if they retain that memory under different conditions. Planarians have simple needs: they like raw meat, and they dislike light and change.

Which ... I get it. They're willing to get over their light aversion for a tasty meal, but if other things change at the same time, they just kind of freak out.

Like, if you take them out of petri dishes with smooth glass and put them into textured petri dishes, they are too weirded out at first to go for a piece of liver in the middle with a bright light shining on it. But, if you put the flatworms in textured dishes for a while beforehand, you can passively train them to be familiar with that environment. Then, when you add the spotlight and liver, they go for it quickly—providing researchers with enough evidence of successful training.

And when scientists have done this training regimen, they've found the planarians can retain such memories for at least 14 days. That raises the question, of course: what happens when you cut off their heads? We know that within a week, the flatworms grow new, usable heads they can eat with and everything—and they can make it that long without eating, so they're fine.

But, presumably, if you have a brain, that's where your memories are—so since the 1950s, scientists have been trying to figure out what happens when the flatworms lose and then regrow their brains. And most strangely, they seem to remember previous training. In a 2013 study, for example, the trained flatworms approached the food faster, even though their training happened with their former heads.

The difference between the trained and untrained planarians was not as big as it was pre-decapitation, so the researchers concluded that this memory retention isn't quite one hundred percent. But still, it's bizarre that they could remember the training at all. As for how they retain those memories, well… we don't really know.

One intriguing idea is that memories of certain habitual behaviors are partially transferred to neural tissue outside of the brain. So, somehow there are memories stored in other neurons in the body. Which is just so completely against everything we thought we understood about memories.

I mean, we talk about “muscle memory”, but even that is thought to occur in the parts of the brain that control how you move your extremities, not in those extremities themselves. A lot more research is needed to fully understand this phenomenon, and to figure out if it's unique to planarians or true of all brainy creatures. Because if human memories also exist outside of our brains in some way, understanding how could lead to better treatments for certain brain injuries, memory loss, or dementias.

And that would be some pretty awesome stuff to learn from a headless flatworm. Speaking of learning awesome stuff, if you liked hearing about these amazing flatworms, I have a feeling you'll like the videos you can watch with CuriosityStream. CuriosityStream is a subscription streaming service that offers over 2,400 documentaries and non­fiction titles from some of the world's best filmmakers, including exclusive originals.

And they have all sorts of videos that give you a deeper dive into this planet's quirkiest inhabitants. You might check out The Year of the Hedgehog, for example—it follows these adorable weirdos over the course of a year while explaining what makes them and the other animals they encounter so wonderfully unique. You can watch it and all of the other videos CuriosityStream offers for as little as $2.99 a month.

And for our audience, the first 31 days are completely free if you sign up at curiositystream.com/scishow and use the promo code ‘scishow' during the sign-up process. [OUTRO ♪].