4 
LIFE. 
and crush it between two plates of glass beneath 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 line point. 
