Irving Kupfermann and I first wanted to study the simplest possible behavior of Aplysia. We examined the animal’s behavioral capabilities and delineated a very simple defensive reflex: the withdrawal of the gill upon stimulation of the siphon, an action that is like the quick withdrawal of a hand from a hot object. In aplysia, the gill is a respiratory organ that lies exposed in the mantle cavity. When the animal is in a normal, relaxed state, the gill is partially covered by a sheet of skin., which ends in a fleshy spout, the siphon. When a weak tactile stimulus is applied to the siphon, both the siphon and gill are withdrawn into the mantle cavity and for protection under the mantle shelf. Kupfermann, Harold Pinsker, and later Tom Carew, Robert Hawkins and I found that this simple reflex could be modified by three different forms of learning: habituation, sensitization, and classical conditioning.