SEA URCHINS. 



are apt to pierce the hand and then snap off, so that they 

 cannot be extracted without the use of the knife. 



In the Cake Urchins they are short, and as fine as human 



hair. Yet each of them has its own ball-and-socket joint 

 with the corresponding muscle, and, when viewed under 

 the microscope, each is seen to be formed of a number 



