1. 1.

    Six octopuses were trained on a series of visual discrimination tasks where the discriminanda were presented simultaneously, and where different tasks alternated in a single training session.

  2. 2.

    All the octopuses could make discriminations based on brightness and orientation over the same period of time.

  3. 3.

    Over a period in which all octopuses discriminated a black from a white vertical rectangle none discriminated a yellow vertical rectangle from a grey (of matching brightness).

  4. 4.

    Over a period in which all octopuses discriminated a vertical from a horizontal grey rectangle none discriminated a violet vertical rectangle from a matching grey.

  5. 5.

    The behaviour of the octopuses was qualitatively different in the hue versus grey situations, their attacks being considerably slower and less certain.

  6. 6.

    These results, which agree with previous behavioural findings and a variety of morphological, physiological and biochemical evidence, make it increasingly probable that Octopus vulgaris is colour blind.

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