The Mechanisms of Coordination of the Starfish Tube Feet

1) There are two concepts of the way in which the tube feet of a starfish can be coordinated to point in a given direction. The one suggests that the coordinated pointing is effected through the central nervous system, the other suggests that it is achieved by mechanical tensions acting upon the tube feet. This paper is an attempt to evaluate the two concepts. 2) Starfishes were examined whilst their tube' feet were pointing but not actually stepping in a given direction. The animal was placed so that directive stimuli were reduced to the minimum. Two types of pointing are described; positive pointing where the tube feet point towards a given direction though no directional stimulus is applied, and negative pointing where the tube feet point directly away from a given direction as a result of the application of directional stimuli. 3) The tube feet can point in any direction but are most stable when in the direction of the main radii. 4) A quantitative method of analysing the coordination between different arms is described and it shows that the excitatory states responsable for cooperation can pass around the nerve ring in both directions, and also along the radial nerve cords. Conduction is decremental, due most likely to the many synapses in the nervous pathway. 5) Cutting the deep tracts of the radial nerve cord prevents the tube feet apical to the cut from taking part in the orientation changes that occur on the rest of the animal. This is true for both positive and negative pointing. 6) Additional evidence is presented in favour of the view that there is a centre that controls pointing in the direction of a given arm, the centre being located at the junction of the radial nerve cord and the circum-oral nerve ring. These five circum-oral or circum-oesophageal centres play a role similar to that of the brain in the other invertebrates. 7) A single cut made in the circum-oral ring does not prevent the arms on either side of the cut from showing coordinated pointing. This means that the diagram presented by SMITH (1945) requires to be modified so that paths from each centre go round the nerve ring in both directions. 8) The radial nerve cord of Astropecten and Asterias can be partially cut so that the part apical to the cut will respond to some orientation changes but not to others. This holds for both positive and negative pointing and suggests that there are certain tracts in the nerve cord specifically concerned with specific directions of orientation. 9) Cases of aberrant negative pointing are described in which the operation on the nerve cord affects the feet distal to the cut so that they make regular systematic misinterpretations of the transmitted messages. 10) Experiments involving the application of tension to inverted and normal animals indicate that the central nervous system is not the only factor in determining the orientation of the tube feet. It is highly probable that two systems, one via the central nervous system, the other via the body tensions, influence the orientation of the tube feet during normal locomotion.