PROTOZOA. 155 



4. Many of these single-celled animals are so like the 

 single-celled plants that no sharp line can be drawn be- 

 tween them. 



5. Many of them have the remarkable power of dry- 

 ing up, if the w r ater fails, and of resuming activity when 

 it returns. This enables them to be blown about as dust 

 and thus to be scattered over the earth. 



172. The Atneba. This animal is common enough, but it is not' 

 always easy to find when wanted. It is most likely to be found at 

 the surface of ooze in ponds or in vessels containing mud and decay- 

 ing organic matter in the laboratory. It appears and disappears 

 in a very puzzling way. 



It is a clear, jelly-like object and is liable to be overlooked even 

 when on the slide. 



The ameba is one of the simplest of these simple animals. It is 

 a single mass of protoplasm without a cell wall (Figs. 35 and 36). It 

 is therefore very free to move. It will be seen to send out rounded 

 or pointed (according to the species) projections or temporary 

 "feet," in the direction in which it is moving. The inner and 

 more granular protoplasm (endoplasm) streams out into this foot 

 which at first consists largely of the clearer outer protoplasm (ecto- 

 plasm). In the meantime the protoplasm at the opposite side of 

 the body is being withdrawn and the result is a slow motion in the 

 direction of this growing "foot" (pseudo podium, Fig. 35). 



When the ameba comes in contact with a small organic particle, 

 such as it is accustomed to use as food, the protoplasm flows in such 

 a way as to engulf it in the body. It does not enclose about a grain 

 of sand. Thus it is seen to have some power of recognizing and 

 choosing its food. 



Within the endoplasm may be seen three different classes of objects : 

 (i) granules and food masses of different sizes, sometimes inclosed 

 in clearer circular spaces (food vacuoles) ; (2) a rounded nucleus; 

 and (3) a clear, rounded object that contracts and expands (pul- 

 sating vacuole) . It is not always easy to see the last two objects. 



After growth to certain limits, the ameba divides into two prac- 

 tically equal parts (Fig. 14). The nucleus and vacuole divide 

 and one half goes to each of the new cells so that there are two 

 perfect small-sized individuals as the result. This is the usual 

 mode of reproduction, and is as simple as can well be conceived. 



Under certain conditions the ameba cannot continue this active 



