The shadow reaction ofDiadema antillarumPhilippi. IV. Spine movements and their implications

Abstract

The aboral primary spines of the echinoidDiadema antillarumreact to localized shadows by a movement that is frequently constant in direction, followed by continued oscillation. Both direction and the degree of constancy are determined by the positions, relative to the spine, of the stimulating light (surface factor), oral pole (oral directing factor) and radial nerve (ambulacral directing factor). The oral and ambulacral factors appear to be an expression of the route by which nervous excitation reaches the spine from the radial nerve. The surface factor does not involve the radial nerve and appears to interact with the other factors at the spine base. Transmission of excitation over the skin can occur at differing speeds, suggesting that there are at least two divisions of the superficial nervous system, which has characters inappropriate to an anastomosing network. There is evidence that transmission does not always depend on proprio- ceptive relays at the spine base. The area of influence of a radial nerve extends at least half-way around the test. The frequency with which the spines oscillate is governed, at least in part, by excitation from these nerves. This effect, which is decremental, together with the propensity of the radial nerves to generate oscillations of the spines by spontaneous discharge and their capacity to serve as centres of interaction between excitation and inhibition, suggest that the nerves are more than simple local reflex centres. Their properties resemble those of a more elaborate central nervous system.