CILIARY WAVES. 211 



extreme flexure to uprightness have their tips more 

 crowded. Now, though under the microscope we can- 

 not detect the individual cilia,, we can detect the 

 effect of this alternate separation and union of many ; 

 the former produces a more transparent, the latter a 

 more opaque spot, as its optical expression ; and these 

 opaque spots, the crowded part of each wave, are the 

 dark points which seem to perform their incessant 

 and amusing gymnastics. 



Every cilium, thus, is perpetually occupied in strik- 

 ing the water ; and, like a trireme of a thousand oars, 

 the skilful rowers, as we have seen, keep the most 

 perfect time. If the galley were free, these vigorous 

 strokes, making up in cumulative energy what they 

 lack in individual force, would row it bodily through 

 the water. In many aquatic animals, the cilia are 

 effective implements of locomotion ; but here they 

 subserve no such purpose, but another, still more 

 indispensable to life. They produce, by their lashing 

 action, powerful currents in the water, which is thus 

 driven uniformly to and fro across the gill-leaves, 

 yielding up its precious burden of vitalising oxygen 

 to the blood, which permeates the thread. 



And now let us return to our little Tubuliporce. If 

 we had the good fortune to see one of these in its con- 

 dition of life and activity, we should discern protruding 

 from the mouth of the shelly tube a microscopic -coro- 

 net of diverging filaments. Some dozen or so of exqui- 

 site threads, of what you might suppose to be spun 

 glass, from the transparence and brilliance of their 



