120 



GRAVITY AND PROTOPLASM 



[CH.V 



8 is the angular deviation of the slug from 45 towards 90, 

 expressed in degrees ; 6 is the angle of inclination of the plate 

 to the horizontal, and a is a constant. 



In inquiring into the cause of geotaxis in animals it seems 

 best to consider chiefly the phenomenon as exhibited in Protista, 



for in the higher animals this capacity 

 seems bound up with the possession of 

 special organs of orientation. In this 

 group the first and apparently most 

 important part played by gravity is 

 the determination of the axis of the 

 individual, which comes to lie vertical 

 and with the head end up or down 

 according to the conditions of the 

 protoplasm. After the positions of 

 the axis and poles are determined, or- 

 dinary locomotion produces the geo- 

 tactic phenomena. That gravity may 

 determine a vertical position without 

 locomotion occurring is shown in the 

 ciliate infusorian Spirostomum (Fig. 

 26), which at times occurs in large 

 numbers in ordinary aquaria, sus- 

 pended almost motionless in mid- 

 water, having a distinctly vertical 

 position and with the head end 

 directed upward. They cannot be 

 said to be strictly motionless, since 

 by carefully attending to them one 

 can see them slowly rising or falling 

 or alternately, perhaps, rising and falling in their almost im- 

 perceptible movements. Miss JULIA B. PLATT, who has 

 studied carefully the movements of Spirostomum, found that 

 of 78 individuals observed all but 7 had the anterior extrem- 

 ity directed upwards and the 7 exceptional individuals were 

 all moving downwards. It therefore seems quite certain that 

 Spirostomum tends in water to orient itself with reference to 

 gravity, although without aggregating at the upper surface. 



FIG. 26. Spirostomum ambi- 

 guum, side view, az, ado- 

 ral zone of cilia ; o, mouth ; 

 os, gullet; n, nucleus; ck, 

 contractile canal; cv, con- 

 tractile vacuole; , anus. 

 Magnified about 120 diam- 

 eters. ( From BUTSCHLI 

 [BRONN'sThier-reich: Pro- 

 tozoa], after STEIN.) 



