THE DERMAL LIGHT SENSE

<jats:title>Summary</jats:title><jats:p>1. Dermal light reactions are responses to illumination of the body surface of animals due to stimulation of diffusely distributed photoreceptors.</jats:p><jats:p>2. They are found in all major phyla, being most common in aquatic animals with non‐waterproof skins. They are rare in terrestrial arthropods, cephalopods and amniotes.</jats:p><jats:p>3. An animal may possess a dermal light sense in addition to eyes or other localized photoreceptors.</jats:p><jats:p>4. Responses to dermal stimulation may be local or general, graded or all‐or‐none. They include direct responses to light of some chromatophores, slow orientations by bending movements and many fast withdrawal reflexes.</jats:p><jats:p>5. Locomotion in many animals is controlled photokinetically by dermal receptors. The relation between intensity and the speed of locomotion is usually an orthokinesis but in some planarians and the ammocoete larva it is a klinokinesis.</jats:p><jats:p>6. There is evidence for orientated control of locomotion by dermal receptors in planarians and some lower vertebrates.</jats:p><jats:p>7. Reaction times of dermal responses are relatively long. Except in <jats:italic>Metridium</jats:italic>, the reaction time consists of a sensitization and a latent period, the relationship between which is complex.</jats:p><jats:p>8. Dermal and optic responses exhibit similar physiological characteristics. Dermal light reactions adapt to repeated and continuous stimuli, show considerable powers of dark adaptation and obey Weber's Law over a limited range of intensity.</jats:p><jats:p>9. The absolute sensitivity of dermal photoreceptor systems is several thousand times less than that of well‐developed eyes.</jats:p><jats:p>10. Action spectra are generally similar to those of eyes, with maximum sensitivity usually between 470 and 530 mμ. The photochemical systems responsible for all light reactions are believed therefore to be of a similar nature.</jats:p><jats:p>11. No single receptor system at the cellular level of organization can account for all types of dermal light reactions.</jats:p><jats:p>12. There is evidence that the protoplasm of a variety of cells in Metazoa is sensitive to light. The parietal muscles of <jats:italic>Metridium</jats:italic>, the ectodermal nerve net of <jats:italic>Diadema</jats:italic> and some types of chromatophores have been found to be light‐sensitive.</jats:p><jats:p>13. The light reactions of blind cave‐dwelling teleosts show some unusual features which may be related to a comparatively recent loss of functional eyes.</jats:p><jats:p>14. There is no convincing evidence of fundamental differences in the processes responsible for dermal and optic light responses, nor for the existence of more than one type of dermal photoreceptors in any species.</jats:p><jats:p>15. A common structural basis for all light reactions, if it exists, must be sought in the fine structure of cells.</jats:p><jats:p>I am most grateful to my colleagues who have read the manuscript. I wish to acknowledge my gratitude to Professor P. B. Medawar, F.R.S., for hospitality in the Zoology Department of University College, London, where much of the work on this article was done.</jats:p>