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Minds of small things  ·  Article

Memory in planaria

Planarians are free-living flatworms with a brain, a nervous system and a large capacity to regenerate. Texts from 1901 to 2022 describe them testing their surroundings, rebuilding lost heads and carrying information about body form through injury.127 The held passages say little about training experiments themselves.

Earliest held
1901, Morgan, T. H.
Most discussed in
Regeneration, 1901
In the library
617 passages in 50 works
Rewritten
2026-10-03
18501875190019251950197520002025Morgan, T. H., 19011Jennings, H. S., 19062Jennings, H. S., 19063Washburn, M. F., 19084Blackiston D, Shomrat T, Nicolas CL…, 20105Lobo D, Beane WS, Levin M, 20126Emmons-Bell M, Durant F, Hammelman J…, 20157Neuhof M, Levin M, Rechavi O, 20168Fields C, Levin M, 20229
01

Early behavioural observations

Morgan (1901) reported that when the anterior end of a freshwater planarian is cut off at any level, a new head is produced.1 Jennings (1906), following Pearl (1903), described worms that move the head from side to side as they glide, so that the sensitive anterior end meets varied conditions.2 Jennings added that testing movements precede coming to rest and begin again when the animal resumes activity.3 Washburn (1908) reported that Planaria maculata gives negative, positive and feeding reactions. She argued that strong and weak stimuli suggest unpleasantness and pleasantness, not different kinds of sensation.4 None of these authors reports a learning experiment in the held passages.

02

Brain, learning and regeneration

Blackiston and colleagues (2010) list memory and learning among the uses of planarians, alongside regeneration, cancer and the study of drug addiction.5 They present the worm as a model for how a functional nervous system and its cognitive structures are restored after injury by adult stem cells.5 Lobo, Beane and Levin (2012) stress the same capacity in structural terms. A trunk fragment regrows a whole worm, with a new head and tail in the original orientation.6 The held passages do not report training results from either work.

03

Stored form and adaptation

Emmons-Bell and colleagues (2015) argued that the shrinking and regrowth of planarians, like regeneration after injury, requires the storage and sharing of morphological and physiological information across many cells.7 Neuhof, Levin and Rechavi (2016) chose planaria as an asexually reproducing animal that presents a challenge to the Weismann barrier, in a paper on memories passed between generations and between animals.8 Fields and Levin (2022) describe worms kept in barium. Their tails regenerate heads that tolerate it, and they argue that neither natural selection for this toxin nor selection among cell clones explains the change.9

SourcesEach quotation was checked word for word against the passage it opens.
  1. If the anterior end is cut off at any level, a new head is produced (Fig. 4, C}.Morgan, T. H., 1901 · Regeneration · open at passage 27
  2. These movements of course serve to test the environment on each side ; in other words, they subject the sensitive anterior end to varied conditions.Jennings, H. S., 1906 · Behavior of the Lower Organisms · open at passage 689
  3. 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
  4. Hence they do not suggest correlation with qualitatively different sensation contents, but rather with unpleasantness and pleasantness.Washburn, M. F., 1908 · The Animal Mind: A Textbook of Comparative Psychology · open at passage 137
  5. 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
  6. A trunk fragment cut from the middle of an adult planarian will regenerate into a whole worm, always growing a new head and new tail in the same orientation as the original worm.Lobo D, Beane WS, Levin M, 2012 · Modeling planarian regeneration: a primer for reverse-engineering the worm · open at passage 4
  7. 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
  8. One asexually reproducing animal, on which we will focus in this paper, which presents an interesting challenge to the Weismann barrier, is planaria.Neuhof M, Levin M, Rechavi O, 2016 · Vertically- and horizontally-transmitted memories - the fading boundaries… · open at passage 5
  9. 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.
Emmons-Bell et al. describe planarians reproducing their target morphology and say this needs stored, shared morphological information across cells.
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.
Fábregas-Tejeda and Sims cite habituation in Stentor as learning that needs no neurons, a memory case outside nervous systems.
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.
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.
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.
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.
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 memory in the objective sense, a modified response to past stimuli, is the question the flatworm work takes up.
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.
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.
Another test of memory in a simple organism, which Abramson and Levin's behavioural primer would apply to.
Durant and colleagues cite an electroencephalogram of the planarian brain in describing an animal with a centralized brain and complex behaviour.
Where it is discussedPassages matching planaria, planarian, planarians, flatworm
1901Regeneration · Morgan, T. H.64
1915Individuality in Organisms · Child, C. M.63
1924Physiological Foundations of Behavior · Child, C. M.58
1906Behavior of the Lower Organisms · Jennings, H. S.47
2012Modeling planarian regeneration: a primer for reverse-engineering the worm · Lobo D, Beane WS, Levin M40
2016Physiological controls of large-scale patterning in planarian regeneration: a… · Durant F, Lobo D, Hammelman J, Levin M39
2019Neural control of body-plan axis in regenerating planaria · Pietak A, Bischof J, LaPalme J…38
2015Gap Junctional Blockade Stochastically Induces Different Species-Specific Head… · Emmons-Bell M, Durant F, Hammelman J…34
2016Vertically- and horizontally-transmitted memories - the fading boundaries… · Neuhof M, Levin M, Rechavi O33
2017Planarian regeneration in space: Persistent anatomical, behavioral, and… · Morokuma J, Durant F, Williams KB…26