150 



THE ANIMAL KINGDOM 



muscle cells in many ways, it is not known for certain whether the 

 flagellar fibril is made ot actomyosin, the contractile protein of verte- 

 brate muscles. In a typical swimming movement, the flagellum lashes 

 stiffly to one side from an extended position and returns relaxed and 

 bent. The flagellum may also undulate, with waves passing from tip to 

 base, thus pulling the animal forward. Cilia are structurally similar to, 

 but much smaller than, flagella. 



Some protozoa can creep on the bottom with wormlike movements. 

 These animals have just beneath their surfaces a layer of contractile 

 fibrils which form an organelle comparable to the muscular body wall 

 of worms. Other protozoa "slide," moving along slowly with no apparent 

 means of propulsion. These also have a surface layer of contractile fibrils, 

 and it has been presumed (with no direct evidence) that they move by 

 passing minute waves of contraction along the fibrils, after the fashion 

 of a snail's locomotion. Finally there is locomotion by ameboid move- 

 ment, described in Chapter 5. This kind of movement apparently lacks 

 a specific functional organelle. 



Conductlle Organelles. Other organelles are conductile, their func- 

 tions being comparable to those of the nervous systems of higher 

 animals. The flagellates have, at the base of each flagellum, a basal body 

 (Fig. 8.2). If flagellum and basal body are removed intact from the animal, 

 flagellar activity continues, but as soon as the two are separated move- 

 ment usually stops. The basal body apparently stimulates and controls 



Piemen 

 spot 



Figure 8.2. The flagellum. A, Details of flagellar structure in the flagellate, 

 B, Successive positions of flagellum in a typical stroke. 



4 



Euglena, 



