SEA-URCHINS 



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kinds, and proceed from the knobs on the exoskeleton, over 

 which they fit, forming ball-and-socket joints, which enable them 

 to move in any direction. The long spines are ribbed, and seem 

 to have no other function than that of protection. The second 

 set, the pedicellarice, are very peculiar small organs scattered over 

 the surface in great numbers, and consisting of a head bearing 

 three bill-like blades mounted on a long, flexible stalk. The office 



Test of sea-urchin, with the spines removed to show the arrangement of the 

 plates, lateral view : Amb, ambulacral zone with its perforated plates ; Ap, apical 

 (aboral) pole ; Int. ami)., interambulacral zones. (Prom Bronn's " Thierreich.") 



of some of the pedicellarise is to remove waste from the excretory 

 opening; this is passed down regular lines and dropped into 

 the water, thus keeping the body clean. Others are constantly 

 opening and shutting their forks, reaching in all directions and 

 grasping and removing anything which may have become entan- 

 gled in the spines. They also capture floating bits of seaweed, 

 which they drag over the body to conceal it. A third set of 

 projections are the sphmridia, small globular bodies said to be 

 connected with perception. 



The spines vary greatly in size, number, and form in different 

 species, and are such conspicuous features as to make the identi- 

 fication of species easy. In some the spines are solid and fluted ; 



