72 HOMES UNDER THE SEA. 



teeth are very much harder than the sheath in which 

 they play. 



In the rodent, the teeth are constantly worn away at 

 the tips, fresh tooth matter being formed at the bases. 

 Precisely the same provision of nature is found in tho 

 teeth of the Urchins. 



Now, however, we find ourselves face to face with a 

 curious mechanical problem. 



In the rodent, the upper jaw is fixed and the lower jaw 

 moves up and down, carrying the teeth with it ; but in 

 the radiated animals there can be no jaw like that of a 

 vertebrate, and so there must be some other mode of 

 comminuting the food. 



On examining the dental apparatus more closely, we 

 shall find that instead of being fixed in the sheath, as the 

 rodent tooth is fixed in the jaw, the sheath is fixed, and 

 the tooth slides up and down in it. Suppose that we cut 

 oft' three or four inches from the tip of a scimitar or 

 curved sabre sheath, and draw the blade up and down 

 so that the point is alternately protruded and withdrawn, 

 we shall see how the tooth of the Urchin is worked. 



To each tooth is fixed a double set of very powerful 

 muscles, the stronger serving to pull the tooth down- 

 wards, and the weaker to draw it upwards. 



Another point now comes before us. 



There are five of these chisel-like teeth, all passing 

 through a circular aperture, and meeting together at 

 their tips. Now, if we were to set five ordinary chisels 

 in like manner, they could not meet at their tips, and 



