Sensitivity to light and the reactions to changes in light intensity of the Echinoid Diadema Antillarum Philippi

Abstract

The reactions of Diadema antillarum to directional illumination and changes in light intensity are described, and the responses of the spines, tube feet and pedicellariae are reviewed, and shown to depend on a process of adjustment, so that light- and dark-adapted phases can be recognized. The responses of the spines to decreases in light intensity are examined in detail and shown to be reflexes, the receptive surface for which extends over the entire surface of the test but not the spines. The degree of sensitivity is correlated with the degree of dispersion of the pigment in superficial melanophores. The areas with most melanin (tube feet and ambulacral margins) are most sensitive, the so-called ‘eyes’ appearing to be least sensitive. The radial nerve cords and their branches appear to be affected by changes in light intensity. No morphologically or histologically differentiated receptors have yet been found, and it is suggested that the nervous system may be influenced directly by light. The findings are reviewed in relation to existing knowledge of the similar phenomena in other animals, and a comparison made between the entire surface of Diadema and certain features of the photoreceptive surface in highly differentiated eyes.