UVING PROTOPLASM ; THE AMCEBA 35 



body is identical with the particle scraped from the cheek, 

 except that it has no boundary waU. It is, in fact, a single 

 ceU without a cell wall, and is one of the lowest forms 

 of animal life known. 



But ,we have not yet noted the most important feature 

 of this tiny fluid body. If we watch it a moment we shall 

 be able to see that it moves. The cytoplasm will be seen 

 to flow out first in little projections (pseudopodia) which 

 then fill as the rest of the mass flows into them, and in 

 this way the body progresses across the slide. That this 

 is not mere physical flowing like the running down hUl of 

 water is shown by the fact that it can change its direction. 

 This power of automatic movement is possessed by no 

 other fluid known, and in itself is sufficient to make this 

 fluid remarkable. 



If now we press down the cover glass with a needle or 

 pin just above the animal, we shall note that the movement 

 stops and that the animal draws back to form a round 

 baU. If not disturbed further, it wiU expand and flow 

 as before. In this simple action two more properties of 

 protoplasm are shown; the power of expansion and con- 

 traction (contractility) and sensitiveness to a stimulus 

 such as touch (irritability). Now watch the amceba until 

 it comes in contact with a particle in the water. It flows 

 over it and engulfs it, but curiously enough, if the particle 

 is of vegetable matter it is carried along with the amceba, 

 while if sand, it is soon cast out. The reason for this 

 action is that the amoeba, in selecting what to retain and 

 what to reject, actually makes a distinction between 

 foods and substances which are not foods. Further, if 

 the particle is retained it is soon broken up in one of the 

 round tubbles (food vacuoles) and this is slowly revolved 



