CONTRACTILE TISSUES 57 



is increasing and the other diminishing as the transfer 

 goes on. 



Ciliated Cells. Many one-celled animals have upon 

 their surface a sort of nap or pile which is made up of 

 very fine contractile extensions of the cell substance. 

 The suggestion is of bristles set in a brush. Every 

 individual " bristle" the term inevitably suggests 

 something a great deal coarser than the reality is 

 swinging back and forth. A balanced, pendular move- 

 ment of this kind would slightly stir the water nearest 

 to the infusorium but could not materially affect the 

 situation. But the movement of the cilia, as these tiny 

 processes are called, is not balanced and pendular. It 

 has a most curious unsymmetric character. The stroke 

 made in one direction is sharp and decisive, the recovery 

 is slower and more gentle. One is reminded of the 

 handling of a whip or of an oar; in either of these cases a 

 forcible stroke in one direction is followed by a less 

 energetic return. 



A ciliated infusorium when not attached to any 

 anchorage is propelled here and there by its waving 

 cilia. The action is like that of the many oars of a 

 Roman galley. If, however, the animalcule is fixed in its 

 position the movement brought about by the cilia is not 

 in the cell but in the adjacent water. Currents are 

 maintained which are in many cases so directed as to 

 bring food particles within reach of the infusorium. The 

 same currents must assist in ministering to respiration 

 by sweeping away water which has received carbon dioxid 

 and replacing it with a fresh portion containing available 

 oxygen. 



The ciliated cells which are found in the higher animals 

 are usually arranged in mosaic fashion to form epithelial 

 surfaces. The cilia are upon the exposed aspect and 

 their beating is effective in a direction that is the same 

 for all the assembled cells. Cilia in such localities are 

 undoubtedly overlaid by a film of moisture or mucus and 

 this is kept travelling by their rapidly repeated strokes. 



