598 SENSORY SYSTEM 



slow deformation due to continued or static pressure of starch-grains 

 against the ectoplast constitutes an effective gravitational stimulus ; for 

 in geotropic experiments the characteristic curvature results even when 

 precautions are taken to eliminate all vibration. 



Geotropic stimulation begins as soon as an organ is deflected from its 

 stable position, so that a few starch-grains press upon the ectoplasts 

 occupying the walls which are underneath in the new position ; an 

 actual rearrangement of the starch-grains is therefore not an essential 

 condition of stimulation. As a matter of fact, the starch-grains do very 

 soon migrate onto the physically lower walls, when a positively or 

 negatively geotropic organ is placed horizontally, with the result that 

 the intensity of stimulation gradually increases, attaining its maximum 

 value when all the falling starch-grains have moved on to the lower 

 region of the ectoplast. The time required for the complete rearrange- 

 ment of the statoliths may be termed the period of migration ; its average 

 length varies from five to twenty minutes for different organs. The 

 geotropic presentation-time, that is, the shortest period of time during 

 which a geotropic stimulus has to be applied in order subsequently to 

 induce an appreciable response, varies within similar limits. It is true, 

 that if the first stages of geotropic curvature are determined with the aid 

 of a microscope instead of by naked- eye observations, the presentation- 

 time assumes a smaller value ; this result, however, merely shows that 

 the displacement of a portion of the statoliths suffices to produce effec- 

 tive stimulation. 



In all specialised statocysts the starch -grains are highly movable; 

 hence they are readily acted upon by gravity, and rapidly accumulate 

 upon the lower walls of the sensory cells. As already explained, actual 

 migration of the statoliths is not necessary. Even where the position 

 of the starch-grains in the cell is practically unaffected by gravity, 

 owing to the high viscosity of the cytoplasm, so that they are always 

 uniformly distributed over the various cell-walls, those grains which lie 

 against the physically lower walls must necessarily exert a definite 

 pressure upon the corresponding portion of the ectoplast. Geotropic 

 stimulation is therefore quite loossible, even when the starch-grains are 

 immovable. But mobility of the starch-grains, resulting as it does in 

 a rapid accumulation of all the statoliths upon the lower wall, is for 

 various reasons highly advantageous, and hence indicates a relatively 

 specialised condition of the entire sense-organ. 



Mobility of the statoliths first of all ensures that all the starch-grains 

 in each statocyst are utilised for stimulation ; in this way the total 

 intensity of the stimulus is increased and the curvature correspondingly 

 accelerated. In this sense, therefore, the evolution of movable statoliths 

 may be regarded as an illustration of the principle of economy of material. 



