THE SEA URCHINS 



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this modification is shown in the illustration of the shield urchin {Echinarachnius} , 

 and a fully developed one in the heart urchin (Brissopsis} , with its long tube feet 

 extended in the act of walking toward the left. These heart urchins, as they move 

 along through the sand and mud, scoop it up into their mouths, and pass it through 

 the intestine, extracting on its passage such nutriment as the minute organisms it con- 

 tains can afford. To enable them to scoop it up in this way, the hinder margin of the 

 mouth is produced forward in a kind of shovel shape, as is shown in the illustration 

 of a Pourtalesia test from which the spines have been removed. These animals live 

 at very great depths in the sea, and are the urchins most modified in this particular 



FIDDI.E HEART URCHIN, MOVING OVER THE SAND TOWARD THE I.EFT 



(Natural size.) 



direction. Urchins of the heartshaped type have short delicate spines, and move 

 almost entirely by their long tube feet, in the manner described; but the greater 

 number of the regular urchins progress chiefly by the aid of their spines, which 

 are much stouter, while the tube feet often have the suckers very imperfectly devel- 

 oped. The spines of sea urchins also serve as organs of protection; but their efficacy 

 varies much in different forms. For instance, Diadema setosum has fine sharp 

 spines, eight or ten inches long, which prick one almost before one can see them, 

 and can pierce the stoutest boot; their danger being increased by the gregarious 

 habit of the animals. Some sea urchins have poison glands attached to their spines. 

 It is the smaller spines that are protective, and they are placed for this purpose near 



