24 STUDIES IN PHYSIOLOGY 



living forms that hurry across the field of our vision with 

 bustling activity. Here and there are little creatures that 

 move more slowly, often by performing a "series of somer- 

 saults. If we examine the drop very closely, rod-shaped 

 organisms of exceeding minuteness (bacteria, see p. 32) be- 

 come visible, which are bumping against one another in their 

 tremulous activity. 



The special object of our search in this whirlpool of life 

 is a microscopic animal about one one-hundreth of an inch 



in diameter, called the 

 a-mce'ba. This minute 

 creature is a droplet of 

 colorless, semi-fluid sub- 

 stance, at first perhaps 

 more or less spherical in 

 form. But as we look at 

 our specimen, we notice 

 that its shape is chang- 

 ing. On one side some 

 of the material of which 



FIG. 5. - An' amoeba, highly magnified. the amoeba is composed 



c.vac = contractile vacuole (probably for IS slowly streaming out 



excretion). to form a bulging pro- 



= SS2ft (pooped.) Action or process. This 



extension is called a, false 



foot, which may increase in size until all of the substance 

 of the animal has passed into it. By pushing out these 

 processes in front and pulling up its body matter from 

 behind, the amoeba moves slowly from one part of the slide 

 to another. This characteristic method of locomotion is 

 known as amoeboid. If we suddenly jar the slide, all the 

 semi-fluid substance, in the extended false feet is drawn 

 back toward the center of the animal, and the amoeba again 

 assumes its spherical form. 



Structure of a Cell. The amoeba is one of the simplest 

 of living creatures. It is known as a one-celled animal. 



