Minds of small things · Article

Memory in planaria

Planarians are flatworms that regrow lost heads and brains, which makes them a test case for where information is kept in a body.⁸ The held texts say little about trained memory itself. They describe regeneration to a stored form¹⁷, behaviour that varies from moment to moment²³, and adaptation to a toxin⁹.

Earliest held: 1901, Morgan, T. H.
Most discussed in: Regeneration, 1901
In the library: 640 passages in 59 works
Rewritten: 2026-10-05

Early behaviour studies

Morgan (1901) reported that if the anterior end of a freshwater planarian is cut off at any level, a new head is produced.¹ Jennings (1906), summarizing Pearl's work, described a flatworm that moves its head from side to side as it glides, so that it seems to feel its way.² He added that a worm resuming movement after rest begins again with the same testing movements.³ Washburn (1908) wrote that Planaria maculata gives negative and positive reactions to mechanical or chemical stimuli, strong stimuli giving the first and weak stimuli the second.⁴ She linked these to unpleasantness and pleasantness rather than to different sensations.⁴

Competing readings

Loeb (1918) described where planarians gather in a circular vessel. Daphnia collect at the lit or the dark side, according to the sign of their heliotropism, while the planarians collect where the intensity of light is a minimum.⁵ Loeb treated this as a different kind of conduct from orientation to the light. He also named Jennings's view of avoiding reactions and judged the term "fright reaction" improper.⁵ Washburn used the language of pleasantness, Loeb the language of intensity and position.⁴⁵ The held passages do not settle between them.

Memory and stored form

Blackiston and colleagues (2010) listed memory and learning among the uses of planarians as a research system. They called planaria a model for the regeneration of a functional nervous system and the restoration of cognitive structures after injury.⁶ The held passages report no training results. Emmons-Bell and colleagues (2015) argued that perfect regrowth after amputation requires the storage and sharing of morphological and physiological information across many cells.⁷ Neuhof, Levin and Rechavi (2016) noted that planarians recreate any removed portion, including the brain, and called the animal a challenge to the Weismann barrier.⁸

Adaptation to barium

Fields and Levin (2022) described a case of problem solving. Barium, a non-specific potassium channel blocker, makes planarian heads degenerate. Tails kept in barium regrow heads that are barium-insensitive.⁹ They reported that the wild-type and adapted heads differ in only a handful of transcripts. They argued that barium is not met in the wild, so selection could not have prepared a response. They also argued that planarian cells do not turn over fast enough for a bacteria-like trial of all responses.⁹

SourcesEach quotation was checked word for word against the passage it opens.

Amongst the flatworms the fresh-water planarians show remarkable powers of regeneration.Morgan, T. H., 1901 · Regeneration · open at passage 27
As the flatworm glides along by means of its cilia, the head is held upward (Fig. 135) and moved frequently from side to sideJennings, H. S., 1906 · Behavior of the Lower Organisms · open at passage 689
When later the animal resumes its active progression, this begins again with the testing movements of the head.Jennings, H. S., 1906 · Behavior of the Lower Organisms · open at passage 690
The negative and positive responses are given either to mechanical or to chemical stimuli, the former being produced by strong, the latter by weak stimulation.Washburn, M. F., 1908 · The Animal Mind: A Textbook of Comparative Psychology · open at passage 137
the planarians collect at c and d where the intensity of light is a minimumLoeb, J., 1918 · Forced Movements, Tropisms, and Animal Conduct · open at passage 162
Thus, planaria are a unique model organism for studying the mechanisms that underlie the regeneration of a functional nervous system and the restoration of cognitive structures after injury by adult stem cells.Blackiston D, Shomrat T, Nicolas CL, Granata C, Levin M, 2010 · A second-generation device for automated training and quantitative behavior… · open at passage 9
This remarkable example of spatial and temporal cellular organization requires the storage and sharing of morphological and physiological information across a diverse and widely dispersed population of cells.Emmons-Bell M, Durant F, Hammelman J, Bessonov N, Volpert…, 2015 · Gap Junctional Blockade Stochastically Induces Different Species-Specific Head… · open at passage 7
Planaria have advanced mechanisms of regeneration, and are able to coordinate their resident population of stem cells to recreate any portion of the animal that is surgically removed, including their brain, throughout adulthoodNeuhof M, Levin M, Rechavi O, 2016 · Vertically- and horizontally-transmitted memories - the fading boundaries… · open at passage 5
Remarkably, when kept in a barium solution, the remaining tails regenerate new heads which are barium-insensitive [71].Fields C, Levin M, 2022 · Competency in Navigating Arbitrary Spaces as an Invariant for Analyzing… · open at passage 13

Linked ideas

Regenerationrelated to

Planarians regenerate the brain after removal (Neuhof et al.), which raises the question of what is retained across regeneration.

Polarityrelated to

Child holds that cells' environment, not heredity alone, sets which body region they form; Durant et al. link neurotransmitters to polarity.

Target morphologyrelated to

Emmons-Bell et al. describe planarians reproducing their target morphology and say this needs stored, shared morphological information across cells.

Ion channels and pumpsrelated to

Fields and Levin describe barium, a potassium channel blocker, destroying planarian heads, and regrown heads that tolerate it.

Gap junctionsrelated to

Emmons-Bell et al. use gap junction blockade to alter planarian morphology and neoblast distribution.

Behaviour of single cellsparallels

Fábregas-Tejeda and Sims cite habituation in Stentor as learning that needs no neurons, a memory case outside nervous systems.

Basal cognitionparallels

Both treat memory as something found in simple or non-brain systems, and the Abramson and Levin primer ties memory and learning to basal cognition.

Determinism in physiologyrelated to

Planaria serve as the test organism for physiological determination: Child seeks conditions fixing cell fate, and Durant times wound responses, while Jennings's state-dependent reactions and Emmons-Bell's stochastic octanol outcomes show variability.

Bioelectric prepatternpart of / contains

Passages present planarian head-number/polarity as a bioelectric prepattern stored as a rewritable, persistent memory; the frog 'electric face' is another prepattern, so planarian memory instantiates the prepattern concept.

Neurotransmitters before neuronsrelated to

Planaria have a true brain with human-like neurotransmitters and are memory models. Their regeneration pattern is also regulated by neurotransmitter and gap-junction signaling, with serotonin proposed as a head-shape signal.

Homeostasis and the internal environmentrelated to

Passages treat planarian anatomical homeostasis as a rewritable pattern setpoint, with goal states encoded bioelectrically and two-headed forms persisting through regeneration, a memory-like trait. They never explicitly link it to behavioural memory.

Left-right asymmetryrelated to

Both depend on bioelectric/gap-junctional signaling for body-wide patterning; planaria also show cryptic left/right asymmetry and lateralization. The passages don't link asymmetry to memory directly.

Trial and errorprecursor of / follows

Jennings read the planarian's head-waving and testing movements as varied behaviour that samples conditions, an early account of behaviour that can later be modified.

Criteria of mindin tension with

Washburn read planarian reactions as pleasant or unpleasant, but not as different sensations. Whether such reactions count as mind is disputed.

Habit and plasticityrelated to

Habit and memory in the objective sense, a modified response to past stimuli, is the question the flatworm work takes up.

Axial and metabolic gradientsrelated to

Child's 1915 passage describes susceptibility/inhibition gradients along axes, including in Planaria; the planarian passages concern head-shape regeneration and bioelectric patterning, not memory or metabolic gradients directly.

Synthetic living machinesparallels

Both are cited as examples of genome-independent problem solving: the same planarian genome builds other species' heads, the same DNA builds tadpoles or Xenobots, and barium-adapted planarian heads regenerate.

Conditioned reflexrelated to

Another test of memory in a simple organism, which Abramson and Levin's behavioural primer would apply to.

Electrical activity of the brainrelated to

Durant and colleagues cite an electroencephalogram of the planarian brain in describing an animal with a centralized brain and complex behaviour.

Turing patterns, reaction-diffusion and autocatalysisrelated to

Passages apply Turing-inspired reaction-diffusion (Meinhardt-Gierer self-activation, lateral inhibition) to planarian regeneration, polarity and scaling; Pietak notes size-dependence limits. They don't address planarian learning or memory.

Tropismin tension with

Loeb and Jennings read planarian responses to light differently, one by intensity and orientation, the other by avoiding reactions.

Exploratory adaptation and random-network searchrelated to

Barium-adapted heads show a response to a toxin the animal has no evolutionary history with.

Germ-plasm theory and chromosome individualityrelated to

Neuhof and colleagues raise the barrier question for planaria, where new organisms form from neoblasts without a segregated germline.

Neoblasts and planarian stem cell systemsrelated to

Neuhof and colleagues discuss planarian regeneration and neoblast communities in the context of transmitted memory.

Cephalization and physiological dominance of the anterior regionrelated to

Passages tie planarian head and brain regeneration to the cephalic apical region: short planarian pieces form heads alone, and depressed conditions yield minute heads. Neither passage addresses memory directly.

Differential susceptibility as a measure of metabolic raterelated to

Both involve Planaria: passages say Child's first susceptibility-gradient observations used Planaria species. They say nothing about memory, so the link is shared organism, not shared mechanism.

Memory reinstatement after erasureparallels

Gershman and colleagues cite McConnell's planarian studies as an earlier claim that memory is stored in molecules and can be transferred.

Gradient modification by depressing agents and developmental acclimationrelated to

Both concern planarians. Child's passage describes susceptibility and inhibition gradients in Planaria along body axes; the passages say nothing about memory, so the link is the shared organism and axial form.

Memory as adaptationrelated to

A modern case of memory in a body without a conventional brain, which fits the view that memory is not specific to neural tissue.

Bacterial memory and history-dependent behaviorparallels

Neuhof, Levin and Rechavi discuss persistence of memory outside the brain; bacterial work asks the same question at cell scale.

Multi-stimulus integration and behavioral priorityrelated to

Both concern the planarian brain and its behavior. The passage says planarians have a simple brain and ask whether it integrates multiple signals. It says nothing about memory or regeneration.

Bacterial chemotaxis and the CheY signalling systemrelated to

Both concern taxis/chemotaxis-style directed behaviour; Driesch questions how much 'taxis' is real in protozoa and flatworms, while Inoue shows planarian chemotaxis requires brain neural activity. No CheY mechanism is discussed.

Memory retention through metamorphosisparallels

Another case where memory is asked to persist through large bodily change, there regeneration rather than metamorphosis.

Ion channels in stem cell differentiationrelated to

Passages show planarian regeneration, the basis of A's stored form, is regulated by ion channels and gap junctions (Durant, Emmons-Bell), and Cav channel modulation alters stem cell behaviour (Chan), linking to B.

Where it is discussedPassages matching planaria, planarian, planarians, flatworm

1901	Regeneration · Morgan, T. H.	64
1915	Individuality in Organisms · Child, C. M.	63
1924	Physiological Foundations of Behavior · Child, C. M.	58
1906	Behavior of the Lower Organisms · Jennings, H. S.	47
2012	Modeling planarian regeneration: a primer for reverse-engineering the worm · Lobo D, Beane WS, Levin M	40
2016	Physiological controls of large-scale patterning in planarian regeneration: a… · Durant F, Lobo D, Hammelman J, Levin M	39
2019	Neural control of body-plan axis in regenerating planaria · Pietak A, Bischof J, LaPalme J…	38
2015	Gap Junctional Blockade Stochastically Induces Different Species-Specific Head… · Emmons-Bell M, Durant F, Hammelman J…	34
2016	Vertically- and horizontally-transmitted memories - the fading boundaries… · Neuhof M, Levin M, Rechavi O	33
2017	Planarian regeneration in space: Persistent anatomical, behavioral, and… · Morokuma J, Durant F, Williams KB…	26