196 - The Cell 



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Fig. 11-5. A living Vorticella, expanded condition. Note the myoneme that 

 extends along the stalk. (Bausch and Lomb Optical Co., Inc.) 



foreign particles are carried from the kings 

 up through the trachea into the throat. In 

 the Protozoa, however, the direction of the 

 ciliary beat may be reversed temporarily, as 

 when a paramecium backs away from an 

 obstacle. 



An active flagellum displays a whirling 



.MYONEME 



Fig. 11-6. Voriicella, a stalked ciliated protozoan; 

 A, expanded; B, contracted. The contraction is effected 

 by the shortening of the myoneme. (Other intracellular 

 structures— nucleus, vacuole, etc.— are omitted from 

 the figure.) 



movement, especially at the free end, like the 

 twilling of a whip. In some cases this action 

 pulls the cell forward, as a propeller pulls 

 an airplane, but in others it pushes the cell 

 as a propeller pushes a steamship. 



Recent electron microscope studies, ini- 

 tiated by Keith Porter of the Rockefeller 

 Institute for Medical Research, show that the 

 cilia and flagella from a wide variety of 

 plants and animals display a uniform struc- 

 ture of considerable complexity. Always there 

 seem to be ten pairs of fibrillae that extend 

 lengthwise out into the cilium (or flagellum). 

 The arrangement of these ten double fila- 

 ments always conforms to a similar pattern, 

 which is best shown in cross-sectional view 

 (Fig. 11-8). Probably, these ciliary fibrillae 

 are comparable to myofibrillae (p. 435), the 

 contractile elements of muscle fibers, but ex- 

 perimental evidence is not available. 



Amoeboid Movement. The amoeba moves 

 by means of pseudopodia, which are strictly 

 temporary protoplasmic extensions, projected 

 from the surface of the cell. During locomo- 

 tion, the protoplasm of the amoeba continues 

 to flow forth into the leading pseudopodium, 

 so that the cell moves as the pseudopodium 

 extends. The pseudopodia also play a part in 

 ingestion, when the amoeba engulfs small 

 food particles from the surrounding medium. 



