1292 THE STORY OF THE UNIVERSE 



erally toward the interior of the organism, it is 

 sometimes reversed, and, instead of beating the par- 

 ticles inward, the cilia energetically beat them back 

 if they attempt to enter. Fatal results would ensue 

 if this were not so. Our air-passages would no 

 longer protect the lungs from particles of sand, coal- 

 dust, and filings flying about the atmosphere; on 

 the contrary, the lashing hairs which cover the sur- 

 face of these passages would catch up every particle 

 and drive it onward into the lungs. Fortunately 

 for us, the direction of the cilia is reversed, and they 

 act as vigilant janitors, driving back all vagrant par- 

 ticles with a stern "No admittance, even on business!" 

 In vain does the whirlwind dash a column of dust in 

 our faces in vain does the air, darkened with coal- 

 dust, impetuously rush up the nostrils; the air is 

 allowed to pass on, but the dust is inexorably driven 

 back. 



The swimming apparatus of the Opalina has led 

 us far away from the little animal who has been 

 feeding while we have been lecturing. At the men- 

 tion of feeding you naturally look for the food that 

 is eaten, the mouth and stomach that eat. But I 

 hinted just now that this ethereal creature dispenses 

 with a stomach, as too gross for its nature, and, of 

 course, by a similar refinement, dispenses with a 

 mouth. Indeed, it has no organs whatever except 

 the cilia just spoken of. 



And this leads us to consider what biologists 

 mean by an organ: it is a particular portion of the 

 body set apart for the performance of some particu- 

 lar function. The whole process of development is 



