CONTRACTION 



121 



has at some point on its surface one or two protoplasmic 

 projections which are larger and stronger than cilia and 

 which are called flagella — singular, flagellum — (Fig. 

 28). These flagella sweep back and forth propelling the 

 organism forward very much as a fish propels itself by 

 strokes of its tail. 



Contraction of Protoplasmic Strands. — The fourth 

 type of movement is found in Protozoa that are attached, 

 and serves to save them 

 from threatened harm. In 

 this type of motion there 

 can be seen in the proto- 

 plasm one or more thick- 

 ened strands which reach 

 from the spot where the or- 

 ganism is attached out -to- 

 ward the free end (Fig. 29). 

 It is by the shortening of 

 the protoplasmic strand or 

 strands that the body is 

 pulled down out of harm's 

 way. This form of motion 

 is interesting because the 

 strand of protoplasm which 

 has the power of shortening 

 represents the beginnings 

 of the method by which motion is carried on in all the 

 higher animals, namely, through muscular contraction. 



Motion in Many-Celled Animals. — In the higher 

 animals both amoeboid and ciliary motions occur to some 

 extent, and in many instances accomplish very important 

 effects. For example, in our own bodies the windpipe and 

 bronchial tubes are lined with cells which have cilia on 

 the surface fronting on the tubes. These cilia whip con- 

 stantly in such a way as to sweep up toward the throat 

 any mucous or dust particles that may lodge on the moist 

 surface of the respiratory passages. One can readily ap- 



FiG. 28. — Chlamydomonas, 

 one-celled plant showing flagella. 



