The Covering Reaction of Sea-Urchins

Abstract

Lytechinus variegatus (Lamarck) covers the parts of its skin that are exposed to light with fragments taken from its surroundings. The covering is taken up by the tube feet, assisted by the spines, and held in place by the tube feet acting in relays. It may be orientated with respect to the light source. There are indications of adaptability of behaviour where the covering pieces offer resistance to being lifted. Covering is related to light and to diurnal light changes, being assumed in strong light and rejected, after a varying interval of time, in darkness. Both continuous bright light and decreases in light intensity evoke covering. The tube feet react to the same stimuli and the speed of their extension is roughly proportional to the change of intensity. The tendency to cover is increased after a sojourn in darkness and is greater in pale individuals than in dark ones. Urchins can be photosensitized by injection of dyes so that they cover in dim light. The prehension and holding of covering does not involve the oral and aboral nerve rings. The relation of covering to light and environment favours the idea that it acts as a screen against strong light.