4 



LIFE. 



and crush it between two plates of glass bsneath the 

 microscope, the substance is presently resolved into a 

 multitude of oval pellucid granules, each of which for a 

 short time maintains a spontaneous motion, sometimes 

 rotating upon itself, but more commonly jerking or 

 quivering irregularly. These are the primary cells, and 

 their motion is, doubtless, to be attributed to the presence 

 of certain hairs, called cilia ; for we cannot believe that it 

 is at all connected with currents in the fluid that surrounds 

 them, to which it has sometimes been referred. 



Cilia play an important part in the economy of all 

 animals. Even in the highest forms, many of the inter- 

 nal surfaces are furnished with them, and nearly all the 

 motions which do not depend upon muscular contraction - 

 are produced by them. In the lower tribes, especially £ 

 those which are aquatic, the office of these organs becomes^ 

 more important and more apparent, until in the very 

 lowest we find all movement originating with them. 



The form of these essential organs is that of slender, 

 tapering hairs, commonly arranged in rows, resembling the 

 eyelashes, whence their name. The base of each hair is 

 attached to the surface of the body to which it belongs, its 

 whole length besides being free.* During life each cilium 

 maintains an uniform motion of a waving or lashing kind, 

 bending down in one direction and then straightening itself 

 again. This movement is not performed by all the cilia 

 together or in unison, but in rapid succession : for example, 

 the instant after one has begun to bend, the next begins, 

 then the next, and so on ; so that before the first has re- 



* Perhaps it would be most correct to consider a cilium as formed by the 

 wall of a cell drawn out to a fine point. 



