The Forces Exerted by the Tube Feet of the Starfish During Locomotion

1. Doubt has existed as to whether the tube feet of a starfish in locomotion act as a system of levers or whether they form a traction system, attaching to the substratum and contracting so as to pull the animal along. 2. This problem has been investigated by recording the forces exerted by a single tube foot. The recording apparatus is described. 3. It is shown that both lever and traction methods can be used according to circumstances. ( a ) When the starfish is walking on a horizontal surface it is shown that the tube feet certainly act as struts and that they probably also act as levers. ( b ) When the starfish is climbing upon a vertical surface (or meeting with resistance upon a horizontal surface) the tube feet attach themselves to the substratum and form a traction system.