In the 1960s, a behavioral psychologist named James McConnell made a startling claim: planarian worms could be classically conditioned, just like Pavlov’s dogs, and the memories of that training could be transferred from one worm to another through cannibalism. The scientific community was captivated, and the experiments were replicated in labs and high school science fairs across the nation. But then, as suddenly as they began, the worms stopped learning. Now, six decades later, researchers are struggling to understand why.
The story begins with a philosophical concept known as qualia — the subjective qualities of our experiences, like what it’s like to see blue or feel delight. These are the “ways things seem to us,” as philosopher Daniel Dennett described. But McConnell’s work suggested something more tangible: that memories might have a physical, transferable form. The idea was so compelling that it sparked a wave of research into memory transfer, including attempts to extract and inject memory molecules from trained animals into naive ones.
Today, neuroscientist John Gershman and his team at the University of Texas at Austin are trying to recapture that magic. But they’ve hit a wall. Despite exhaustive efforts — collecting planarians from streams in Oregon, lakes in Michigan, and even following maps from a vintage guidebook called The Fresh-Water Triclads of Michigan — the worms simply won’t learn. They refuse to be conditioned, as if the very phenomenon has vanished from the natural world.
Gershman’s lab is better known for its work on ancient bacterial and viral defenses and the behavior of the unicellular ciliate Stentor coeruleus — an organism that, like planarians, has shown surprising signs of learning and memory. But the worm experiments have become a personal obsession. “We’ve tried everything,” Gershman says. “Different water sources, different temperatures, different training protocols. Nothing works.”
The mystery is compounded by the fact that McConnell’s original experiments were replicated by other scientists in the 1960s. The phenomenon seemed robust. But as the decades passed, interest waned, and the work was largely forgotten — until now. Gershman’s team is one of a handful attempting to reproduce the results, with little success.
Why have the worms stopped learning? Some speculate that environmental changes — pollution, climate shifts, or even the loss of specific microbial partners — could have altered the worms’ biology. Others wonder if the original experiments were flawed, perhaps due to unconscious bias or poor controls. But the consistency of the early replications argues against simple error.
Another possibility is that the worms’ ability to learn is tied to a specific genetic or epigenetic state that has since been lost. Planarians are known for their remarkable regenerative abilities, and they can regrow their entire bodies from tiny fragments. This process involves complex molecular signaling that could also influence learning and memory. If that signaling has changed over time, the worms might have lost their capacity for conditioning.
The implications of this mystery extend beyond planarians. If memories can be transferred through cannibalism, it would challenge our understanding of how memory is stored and retrieved. It would suggest that memory is not solely a function of neural networks but might involve molecules that can be passed between organisms — a concept that, if true, would have profound implications for the limits of scientific knowledge and our understanding of the mind.
For now, the worms remain silent. But Gershman isn’t giving up. He plans to continue the hunt, searching for planarians that still possess the mysterious ability to learn. “If we can find them,” he says, “we might finally understand what McConnell saw — and why it disappeared.”
As philosophers continue to debate the nature of qualia, scientists like Gershman are reminded that the most profound questions often hide in the most unexpected places — even under a rock in a Michigan stream. The search for transferable memories may seem like a relic of a bygone era, but it speaks to a deeper curiosity about what it means to remember, and whether those memories can ever truly be shared.