316 NATURE'S 'I-EAOHTNGS. 



its surface is thickly covered with spines. In some species 

 these spines are as thick as ordinary drawing pencils ; but in 

 most of those which are found on our shores they are very slight 

 and scarcely longer than darning-needles. They are in almost 

 porpetual motion, and generally have a sort of revolving move- 

 ment, the base being the pivot. 



Now, if we take a dried shell of the Sea-urchin, we shall 

 find that the spines will come off with a touch, and, indeed, to 

 preserve one with all the spines complete is a most difficult 

 business. Let us, therefore, pull one from its attachment, and 

 examine its base. This will be found to be swollen into a cup- 

 like form, as seen in the illustration ; and, if we look at the 

 spot whence it came, we shall see that there is a little, rounded, 

 polished prominence, exactly fitting into the cup, just as the 

 ball of the human thigh-bone fits into the acetabulum. It 

 has also its ligament to keep it in its place, and its same set of 

 muscles that move it, and is altogether a most wonderful piece 

 of mechanism. There are in some species of Echinus about 

 four thousand of these spines. 



THE legs of an insect afford excellent examples of the ball- 

 and-socket principle, the socket being on the body, and the 

 ball on the base of the leg. Some of our largest insects such, 

 for example, as the common Stag-beetle exhibit this principle 

 very well. I have now before me a Stag-beetle which has been 

 dead for many years, and is quite dry and hard. Yet I can 

 rotate the legs almost as freely as if the beetle had been 

 just killed, so easily do the joints work. Even the antennao, 

 which are aftixed to the head by a similar joint, move about 

 by their own weight on merely changing the position of the 

 insect. 



These are only a few of the many natural examples of the 

 Ball-and-socket joint, but they are sufficient for our purpose. 



THE TOGGLE OR KNEE JOINT. 



ANOTHER most useful invention now comes before us, called 

 the Toggle-joint, or Knee-joint, the latter name being given 

 to it on account of its manifest resemblance to the action of 

 the human knee. 



