1. Octopuses will make detours down a corridor with opaque walls and will make a correct left-right choice at the far end in order to attack a crab seen to one side of the passage through the transparent windows of a home compartment.
2. In all, 1071 trials were carried out. In 883 of these a detour was completed, rightly or wrongly; in the remaining trials the octopuses failed to complete detours within 5 min. of the start of the trial.
3. The percentage of errors rose with the time spent in the maze. Animals that completed their runs within 20 or 30 sec. of entering the corridor rarely made a mistake; animals that took 2 min. or more, whether due to imposed delays (animal shut in the corridor) or to slow exploration of the maze, made as many errors as correct responses.
4. After removal of the vertical lobe from the brain the octopuses made more errors, particularly in the slower runs. There was also a higher proportion of trials at which they failed to complete a detour at all. These failures are not due to a failure of interocular transfer or to locomotor defects.
6. The results are discussed in relation to the function of the vertical lobe, interocular transfer, the nature of representations of recent events within the optic lobes and the establishment of more permanent memory traces in discrimination experiments.