SEA URCHINS. 71 



points are distinct from the portion immediately above 

 them. In the actual object, the distinction is very con- 

 spicuous, the points being shining white, Avhile the upper 

 portions arc dead greyish white. These latter are seen 

 within the shell in Fig. 5, two being shown in full, and a 

 portion of a third behind them. Their round points arc 

 the tips of the real teeth, and the dull white structures 

 are the sheaths in which the teeth work. 



Now we come to a curious and unexpected analogy 



Fig. 5. SECTION OF SEA URCHIN. 



between the teeth of the Echinus and those of the rabbit, 

 rat, or any other rodent animal. 



In the rodent, the gnawing teeth are very long and 

 curved, and are almost entirely buried in the jaw, only 

 their tips projecting from the bone. So are those of the 

 Echinus, the sheath taking the place of the jaw. 



In the rodent, the teeth are very much harder than 

 the jaw in which they are set, and in the Echinus the 



