Surprising as it sounds, octopuses can be hypnotized. The Dutch zoologist, J. ten Cate, proved this with the common octopus. It was not an easy subject, but when it succumbed it was found that the most effective way was to hold the octopus in one hand with the mouth upwards, and with the arms and body hanging downwards. The great difficulty was to prevent the arms from touching his hand or arm, as this completely broke the spell. But when he was able to prevent such contact, and to hold the octopus in the required position long enough, it was hypnotized perfectly. It breathed easily, and the arms hung limply, with no trace of movement.
When the octopus was thus hypnotized an arm could be lifted, and when released it fell as lifeless as a piece of rope – as great a contrast with the normal behavior of an arm as could possibly be imagined. The octopus could be thrown from hand to hand and showed no more reaction than if it had been a football. A heavy pinch with surgical nippers, or even more drastic treatment, was required to awaken it.
Kingdom of the Octopus p. 84, Frank W. Lane (1960, Sheridan House, NY)