ITS SPINES. 181 



their points, as the mast-heads of a ship do among the 

 clouds in a gentle swell, when seen from the deck. 

 Professor Agassiz fell into the egregious blunder of 

 supposing that the spines were the organs of locomo- 

 tion in the Urchins, denying, with much contempt, the 

 theory which attributed this office to the suckers. One 

 can only wonder whether he ever saw a living Urchin in 

 motion, as one moment's glance at the phenomenon is 

 sufficient to prove how utterly his theory was false, as 

 Forbes has well shown. 



However, let us once more observe the empty denuded 

 box from the cabinet ; and now on the exterior. Be- 

 tween the principal bands of pores (called ambulacra, 

 from their fancied resemblance to walks in a garden), 

 as also between the two series which constitute each 

 band, the space is studded with hemispherical warts, 

 of very diverse size, which look as if turned in ivory, 

 unpolished ; and each wart is crowned with a smaller 

 wart of like material, but bearing the most perfect 

 polish. 



At the bottom of each spine there is a cavity exactly 

 fitting this second wart, and equally highly-polished 

 in its interior. During life the spine was seated on the 

 wart, not united to it, but moving upon it in all direc- 

 tions, with perfect smoothness and freedom ; a ball and 

 socket joint, in fact. It was held in its place by an 



