SPIN3S. 



139 



seen protruding from the mouth, and their 

 extreme hardness may be tested by the finger 

 without any danger. The entire arrangement 

 of teeth and muscles, and bony scaffolding, is so 

 exceedingly complicated, that even with the help 

 of diagrams it would be difficult to explain the 

 structure^ and without their aid quite impossible. 

 I may, however, mention, that there is some re- 

 semblance between the teeth of the Echinus and 

 those of rodent quadrupeds, there being a pro- 

 vision for adding fresh substance to the tooth as 

 fast as it is worn awa}'' by use. 



If the reader will now examine the interior 

 of an Echinus shell, he will see that it is marked 

 out into five equal parts, by five double rows of 

 perforated plates, containing many hundreds of 

 very minute apertures. Through these apertures 

 protrude sucker-feet, just like those of the star- 

 fish, which have already been described, and 

 worked in the same manner. 



Eeverting now from the interior to the ex- 

 terior, we shall find its surface thickly studded 

 with spines, which, as well as the suckers, are 

 employed as a means of locomotion, and there- 

 fore must be freely movable. If a single spine 

 be removed, and note taken of the part which 

 it previously occupied, it will be seen that on the 



