352 NATURE'S TEACHINGS. 



wings along the shaft, so as to give it a rotatory motion 

 as it passed through the air. The hollow cones of his darts 

 were made of india-rubber, but the shape of the two was 

 identical. 



IF the reader will refer to the left-hand figure of the illus- 

 tration, he will see a beautiful example of the Conical Stopper 

 as existing in Nature. 



This is the " Stopper," as it is popularly called, and, scien- 

 tifically, the " infundibuliform operculum." I prefer the former 

 term myself, as being less liable to misapprehension. 



The Serpula lives in a shelly tube of its own construction, and 

 has the power of protruding itself when it desires to obtain 

 food, and of withdrawing itself within the tube when alarmed. 



ANTENNA Of SBBPULA. CONICAL STOPPER. 



This movement is performed so rapidly, that the eye can 

 scarcely follow it, and the mechanism by which it is done has 

 already been described when treating of War and Hunting. 



When it withdraws itself, the Stopper closes the mouth of 

 the tube with perfect exactness, so as to leave the inhabitant 

 in safety. The reader will see, on referring to the illustration, 

 how exactly similar is the Conical Stopper of Art to that of 

 Nature, and how the inventor of that article, as well as of the 

 self-fitting , candle, the candle-fixer, the blow-gun arrow, and 

 the vent-peg, might have found prototypes of their inventions 

 in Nature, if they had only known where to look for them. 



THE FILTER. 



EVEN in a state of uncivilisation man has been driven to 

 invent a Filter of some kind. 



The simplest kind of Filter is that which is used by the 



