THE LIFE OF THE SIMPLEST ANIMALS 5 



3. What the primitive cell can do. The body of one of 

 the minute animals in the water-drop is a single cell. The 

 body is not composed of organs of different parts, as in the 

 body of the horse. There is no heart, no stomach ; there 

 are no muscles, no nerves. And yet the protozoan is a liv- 

 ing animal as truly as is the horse, and it breathes and eats 

 and moves and feels and produces young as truly as does 

 the horse. It performs all the processes necessary for the 

 life of an animal. The single cell, the single minute speck 

 of protoplasm, has the power of doing, in a very simple and 

 primitive way, all those things which are necessary for 

 life, and which are done in the case of other animals by 

 the various organs of the body. 



4. Amoeba. The simple and primitive life of these 

 Protozoa can be best understood by the observation of 

 living individuals. In the slime and sediment at the 

 bottom of stagnant pools lives a certain specially interest- 

 ing kind of protozoan, the Amoeba (Fig. 2). Of all the 

 simplest animals this is as simple or primitive as any. The 

 minute viscous particle of protoplasm which forms its 

 body is irregular in outline, and its outline or shape slowly 

 but constantly changes. It may contract into a tiny ball ; 

 it may become almost star-shaped ; it may become elongate 

 or flattened ; short, blunt, finger-like projections called 

 pseudopods extend from the central body mass, and these 

 projections are constantly changing, slowly pushing out or 



others believe that protoplasm exists as a foam work ; that it is a vis- 

 cous liquid containing many fine globules (the granule-appearing spots) 

 of a liquid of different density and numerous larger globules of a liquid 

 of still other density. It is a foam in which the bubbles are not filled 

 with air, but with liquids of different density. This last theory of the 

 structure of protoplasm is the one accepted by a majority of modern 

 naturalists, although the other theories have numerous believers. But 

 just as with what little we know of the chemical constitution of proto- 

 plasm, the little we know of its physical structure throws almost no 

 light on the remarkable properties of this fundamental life substance. 



