The Retraction and Protraction of the Tube Feet of the Starfish, (Asterias Rubens L.)

Abstract

1. An intact starfish can show spontaneous retractions and protractions of the tube feet. Under certain conditions these responses can be coordinated so that the feet contract together. 2. Using the tube foot as a muscle and the radial nerve cord as a nerve, a nerve-muscle preparation can be set up. 3. There is a definite threshold to electrical stimulation and there is a response to the first stimulus. 4. Increase in the frequency of stimulation gives no increase in the height of retraction. Increase in the intensity of the stimulation above that of threshold gives, within certain limits, a response roughly proportional to the intensity. 5. If an intense shock is interpolated amongst a series of low intensity shocks, the increased retractions caused by the former can often be maintained by the latter. This would effect the nervous economy in the maintainance of tensions as in the opening of a bivalve shell. 6. Most of the apico-basal conduction along the arms is carried out through the tracts in the radial nerve cord. Other methods of conduction along the radial nerve cord are summarised in fig. 7. 7. The dorsal surface cannot conduct excitation along the length of the arm to any considerable extent, nor can it conduct excitation from one side to the other. 8. Methods are mentioned by which the tube feet can be made to protract. 9. The manner of conduction of excitation from the site of electrical stimulation to the tube feet and the relationship of the intensity of stimulation and the response, indicate that the asteroid nervous system behaves more like a series of nerve tracts than a simple nerve net.