Chap. 21 THE PROTOZOANS 431 



Their cilia are similar to flagella but finer and more numerous. Each one makes 

 a backward power stroke and a return drag, the whole movement being rapidly 

 repeated in unison with others (Fig. 21.5). In salt-water shallows the surface 

 water often teems with minute ciliated swimmers; many are protozoans; many 

 others are newly hatched marine invertebrates. 



Structures similar to the locomotor organs of protozoans appear over and 

 over again in multicellular animals. In man, and in the majority of higher 

 animals, ameboid blood cells creep along the capillaries by outflowing proto- 

 plasm; sperm cells swim by means of flagella; and the cilia of the lining of the 

 trachea keep the way clear for breathing. 



o ^ 



Red blood cell 

 Copillory 



White blood cell 



AMEBOID MOTION 

 Ameba 



AMEBOID MOTION 

 White blood cell 



Body moves forward 



Cilium strokes backward 



1-6 Power stroke, backward 

 7~i0 Return stroke 



Fig. 21.5. The motion of pseudopodia and cilia is important in both unicellular 

 and multicellular animals. Upper, motion by pseudopodia in the ameba and in 

 white blood cells of higher animals. Ameboid locomotion is prevalent throughout 

 the animal kingdom. White blood cells are continually crawling about and in and 

 out the blood capillaries of the human body. Lower, diagram of the power stroke 

 of a cilium that pushes the animal forward, e.g., a Paramecium, and the return 

 stroke that is actually a hindrance. The same thing would happen in rowing if the 

 oars were kept in the water on the return stroke. In the lining of the human trachea 

 the power stroke of the cilia is toward the mouth. 



