170 



THE BEHAVIOR OF LOWER ORGANISMS. 



surface becomes applied to the substratum. It is evident that the sur- 

 face of the Amoeba is increased in extent by the pushing out of these 

 waves. 



On the upper surface of the Amoeba there is a forward current of the 

 outer layer, as indicated by the arrows. This current extends back- 

 ward to the posterior end, where it is continued as a movement upward 

 from the lower surface. This upward movement stops, at any given 

 movement, at about the point jy, though, of course, the point where 

 it ceases cannot be precisely fixed. That part of the lower surface 

 which is in contact with the substratum, from the anterior end to x, is 

 quiet. That part of the lower surface which is not in contact with the 

 substratum (from x to y) is moving slightly forward owing to the con- 

 traction in this region, as described on pages 166-168. This movement 

 is comparatively slight, as indicated by the small arrows. Within the 



xx' <* b 



FIG. 58.* 



Amoeba are currents moving forward in the same direction as the 

 current on the upper surface. 



The posterior end as a whole moves forward, so that it comes to 

 occupy later the position shown by the dotted line. The point x, 

 where the lower surface of the Amoeba becomes free from the sub- 

 stratum, moves forward an equivalent amount to x' '. The entire 

 Amoeba thus moves forward in the direction indicated by the large 

 arrow above. 



Thus far our account has been purely descriptive, containing only 

 what has been demonstrated by observation and experiment, and intro- 

 ducing nothing hypothetical. We must now endeavor to form a con- 

 ception of the location and direction of action of the forces at work in 

 producing the movements. Discussion of the ultimate character of 

 the forces will be reserved for the section on the theories of amoeboid 

 movement. 



One of the primary phenomena is evidently the pushing forward of 



* FIG. 58. Diagram of the movements of Amoeba, as seen in side view. A, 

 anterior end ; -P, posterior end ; a, , c, successive positions occupied by the an- 

 terior edge. The large arrow above shows the direction of locomotion ; the 

 other arrows show the direction of the protoplasmic currents, the longer arrows 

 indicating the more rapid currents. For further explanation see text. 



