MORPHOLOGY OF ANIMALS 



B 



c yi 



Fig. 9. Types of Motor Cells. A, amoeboid cell, arrows show direction of flow ; B, ciliated 

 cells, showing successive stages in the stroke (1-7) and in the recovery (7-12) ; C, smooth 

 muscle cell, the peripheral portion of cell converted into contractile fibrillae ; D, striated 

 muscle cell, with four nuclei, one side of cell filled with fibrillae, each consisting of a series 

 of nodes and internodes, the latter with a granule at its middle. 



multicellular animals they are limited to the free borders 

 of certain epithelial cells. The beating of a cilium includes 

 two movements, — the stroke, which is rapid and by which 

 the cilium is sharply bent in one direction, and the re- 

 covery of the original position which is relatively slow 

 and weak. It is probable that the cause of this beating is 

 the unequal contraction of the protoplasm on different 

 sides of the cilium, by which it is bent first in one direction 

 and then in another. All the cilia covering a free surface 

 beat in unison, the stroke being in one direction, and the 

 movement is so timed that beginning at one end of a 

 ciliated tract it seems to pass in a wave-like movement to 

 the other end (Fig. 9, B). 



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