Is there an orientation-dependent excursion of the Muller body in the "statocystoid" of Loxodes?

The ciliate Loxodes possesses a number of vesicles at its anterior dorsal margin. These so-called Muller vesicles contain a spherical inclusion (Muller body) in which lie crystals of barium salts. The Muller body is connected via a stalk to the wall of its vesicle. It is presumed to function as a stato-organelle and to respond by visible motions to changes of the direction of gravity. We have attempted to document the motion of the Muller body with respect to the direction of the gravity vector. Living cells moving in a horizontal or a vertical plane have been viewed under the light microscope with differential interference contrast, documented on video film, and sequences of single frames have been evaluated. Apparent excursions of the Muller body by about 1.5 μm, corresponding to a deviation of 10° at the base of the stalk, are observed in cells moving in a horizontal plane. No larger excursions have been seen in the vertical plane. Implications of this result for a model of the stato-organelle are discussed.