Are Memories Transferable — or Edible?

In the 1960s, worm-training experiments and their strange implications captivated the nation. Columnist Claire L. Evans follows the neuroscientists who attempted to recapture the magic.

Lisett Ledón for Quanta Magaizine

Introduction

I

t was the dead of winter in Boston. The surface of the Charles River was frozen solid. But Zachary Kelso braved the biting cold to finally put to rest a mystery that has haunted neuroscience labs for over half a century.

To do that, Kelso, a research assistant in the Harvard lab of the neuroscientist Sam Gershman , needed some worms. Specifically, planarians: arrow-headed flatworms, which are among the simplest creatures to possess a brain and a nervous system with bilateral symmetry like ours. Normally, labs order these widely used model organisms from biological supply companies. But the mail-order worms weren’t up to snuff. So Gershman had dispatched Kelso to the Charles’ icy banks to catch some wild ones. “I thought, ‘I’m going to look crazy because I’m using a hammer to beat through the ice,’” Kelso recalled. “So I wore the more business end of business casual.”

It wouldn’t be the last time Kelso found himself in this situation. The Charles River planarians, it turned out, didn’t cut it either. Neither did the worms he sourced while stream-hopping around Eugene, Oregon, in March 2025. Nor did the ones he fished from Michigan lakes that June — this time in thigh-high waders — while picnicking families gawked from shore. Kelso diligently turned over rocks, angled with bits of meat tied to a string, and even followed maps from a vintage guidebook called The Fresh-Water Triclads of Michigan . But his adventure was fruitless. Sure, he caught plenty of planarians. But back in Gershman’s lab, none of them would do what they were supposed to do.

In the 1960s, an eccentric behavioral psychologist named James McConnell convinced the scientific establishment that planarian worms, like Pavlov’s dogs, could be classically conditioned — and that memories of this training could be transferred from worm to worm through cannibalism. These bizarre findings were replicated by other scientists, and worm training became a staple of high school science fairs. Now, 60 years later, the worms have stopped learning, and nobody knows why.

I first learned about this scientific mystery while reporting another piece for this magazine about what a cell can remember . As I dug into the historical literature on memory research, I kept coming across McConnell’s strange worm experiments, which captivated a generation of scientists before disappearing entirely. Planarian memory had itself been forgotten. I was content to dismiss it as a fluke of history until Gershman mentioned, in passing during an interview, that in addition to their work with the unicellular ciliate Stentor coeruleus , his lab was attempting to reproduce some wacky worm experiments from the 1960s. Had I heard of them?

Planarian worms have remarkable regenerative capacity. A fragment that’s 1/279 of the original worm can regrow into a normal adult in weeks.

Ernest Cooper

Gershman, I learned, was keen to pick up where McConnell had left off. As part of a growing cohort of cognitive scientists looking beyond the brain for clues to the origins and basis of memory, he’s fascinated by any creature that seems to remember without the benefit of neural, synaptic networks. Little Stentor coeruleus , for example, can modify its behavior based on previous experience — quite a feat for a single-celled creature that can’t possibly have a neuron. Planarian worms, if McConnell’s findings were to be believed, might be the next great model organism for memory research.

The trouble was, it wasn’t going well. In fact, no matter how hard Gershman tried to train them, none of his planarians would learn a thing.

Can a worm learn? When McConnell posed the question in the early 1950s, the notion that memory had something to do with synaptic associations between neurons in the brain was just beginning to gain currency. McConnell, then a graduate student in psychology at the University of Texas, reasoned that planarians — among the simplest creatures with true neurons — should therefore be able to learn.

His early worm experiments were not particularly novel. He simply substituted worms for rats in what were, at the time, standard classical conditioning studies: repeatedly shocking the worms while exposing them to a bright light. After a period of this training, the worms came to associate the light with the shock and scrunched their bodies in anticipation whenever the light flashed. Voilà: worm learning!

Planarians have stranger features to offer for experiments. If a planarian is chopped in half, both halves will regrow into a new worm — the tail will grow a new head, and the head will grow a new tail. A fragment as small as 1/279 of the original worm can regrow into a completely normal adult worm in a matter of weeks, a regenerative capacity so p…