Echinoderms 61 



whilst still attached to the inside of the 

 shell are regarded in Southern Europe as 

 a great delicacy. The eggs liberated in 

 the spring give rise to free-swimming 

 larvae, which presently sink to the 

 bottom and give rise to perfectly formed 

 urchins the size of a pin's head. 



Innumerable changes are rung upon 

 the comparatively simple shape of the 

 urchin. Many species have immensely 

 long spines capable of inflicting painful 

 stings and breaking off in the flesh of 

 the aggressor. In some of the tropical 

 forms the spines are as long and thick 

 as large cigars, and these carry the 

 urchin over the sea floor at a considerable 

 speed. Curious are the shield urchins 

 known as " sand dollars " in America, 

 for they are almost as flat as a coin, and 

 by reason of their extreme compression 

 cannot right themselves when placed on 

 their backs as can the more globular 

 and spinose forms. " Sand dollars " are 



