1 66 THE BEHAVIOR OF LOWER ORGANISMS. 



In an advancing Amoeba the posterior end moves forward at about 

 the same rate as does the anterior, since the distance between the two 

 remains about the same. Leaving out of account for the moment 

 specimens with the wrinkled appendage, there is a continual current 

 forward from the posterior end. Nevertheless, the latter remains on 

 the average of about the same size. The material which flows out of 



it above is supplied from 

 beneath. As we have 

 seen, a layer of material 

 at the under surface of 

 the Amoeba is at rest. 

 The main portion of the 

 body passes over this 

 layer, dragging the pos- 



FlG ' 55> * terior end. The latter 



takes up as it passes the resting layer which was against the 

 substratum. This gradually becomes fluid, and passes for- 

 ward again in the advancing current. All this may be 

 clearly seen by observing the course of individual particles 

 in the protoplasm and on the surface, and is fully set 

 forth in the preceding pages of this paper. 



The unattached posterior portion steadily contracts as it 

 moves forward. Particles on its upper surface are moving forward, as 

 we have seen in detail. But this is not all. Particles on its sides and 

 under surface likewise move forward ; there is an actual contraction 

 independent of the streaming already described. The movement of 

 substance due to this contraction is more striking and rapid as we 

 approach the posterior end. As this contraction is an important fact, 

 it will be well to give some details of the observations which show it 

 to exist. 



Particles attached to the lower surface, or to the lateral margins of 

 the Amoeba, next to the substratum, in the anterior part of the body, 

 remain quiet for a long time. But this lasts only till they have reached 

 that portion of the body which is free from the substratum ; then they 

 begin to move slowly forward as a result of the contraction just 

 described. Of two such objects, one nearer the posterior end, the 

 hinder one moves the more rapidly, so that the distance between the 

 two slowly but distinctly decreases. Though such objects on the bot- 

 tom or sides move forward, they do so less rapidly than does the 

 posterior end. The latter, therefore, in time overtakes them, and they 

 are finally pulled around the posterior end to the upper surface, where 

 they pass forward, as we have already seen in detail. 



* FIG. 55. An Amoeba dra*wn backward by a thread of its viscid secretion. 



