A mammalian brain holds billions of neurons, most of them small and none reliably identifiable as the same cell in a second individual. Aplysia has roughly twenty thousand neurons in its central ganglia, and the largest are big enough to see without a microscope. Crucially, many are the same recognisable cell in every specimen, so a researcher can record from the identical neuron across animals.
Eric Kandel used this to study the gill-withdrawal reflex, in which the animal retracts its gill when its siphon is touched. Repeated harmless touches weaken the response, habituation; a noxious stimulus strengthens it, sensitisation. Kandel's group traced these changes to measurable alterations at specific synapses, showing that memory has a physical substrate at the level of single connections. He shared the 2000 Nobel Prize in Physiology or Medicine for this work.
The finding is often compressed into the claim that Aplysia explained human memory. It did not. It established mechanisms of short- and long-term synaptic change in a simple circuit, which later work extended, with modification, to more complex nervous systems.