The Geotropic Response in Asterina

Upon a surface inclined at angle alpha Asterina gibbosa orients upward during negatively geotropic creeping until the average angle (theta) of the path is such that Delta sin theta/Delta sin alpha = const. This is true also in positively geotropic movement. The direction of orientation may be temporarily reversed by mechanical disturbance. The variation of theta is greater at low slopes. Tests with directed impressed pulls, due to an attached cork float, show that the pull upon the tube feet is of primary consequence for the determination of theta. When the component of gravitational pull in the direction of movement reaches a fraction of the total pull which is proportional to the gravitational vector parallel to the surface, the laterally acting component is ineffective. On this basis, it follows that Delta sin theta/Delta sin alpha = const.