SEA URCHINS. 309 



Manner of moving Tenacious of life. 



derable length, are disposed in a somewhat cir- 

 cular form, and the whole body is supported 

 entirely by a set of upright bones or columns. 

 On the outside of the shell is a prodigious num- 

 ber of sharp movable spines, of a dull violet and 

 greenish color, curiously articulated, like ball or 

 socket, with tubercles on the surface, and con- 

 nected by strong ligaments to the skin or epi- 

 dermis with which the shell is covered. The 

 spines are the instruments by which the animal 

 conveys itself at pleasure from one place to an- 

 other ; and by means of these it is enabled to 

 move at the bottom of the water with great 

 swiftness. It generally employs those about the 

 mouth for this purpose, keeping that opening 

 downward ; but it is also asserted to have the 

 power of moving forward by turning on itself 

 like a wheel. When any thing alarms these 

 animals, they immediately move all their spines 

 toward it, and wait an attack, as an army of 

 pikemen would with their weapons. The num- 

 ber of muscles, fibres, and other apparatus neces- 

 sary to the proper management of these must be 

 very gxeat, and are exceedingly wonderful. So 

 tenacious are the sea urchins of the vital prin- 

 ciple, that, on opening one of them, it is no 

 uncommon circumstance to observe the several 

 parts of the broken shell move off in different 

 directions. The ancients, according to Oppian, 

 believed that if they were even cut up with 

 knives, and the mangled parts thrown into the 



