SEA URCHINS 317 



Aristotle," because the philosopher so named first likened it to a lantern. 

 The test or shell of the sea urchin is composed of some 600 five-sided plates 

 of lime. 



The Common Sea Urchin (Echinus esculentus) is nowhere frequent in the 

 shore pools, but must be sought for just below low-tide mark. Its Latin 

 name shows us that it can be used for food. 



The Purple-tipped Sea Urchin (E. miliaris) is much smaller and commoner. 

 It has its spines tipped with purple. This urchin has a habit of covering itself 

 with fragments of weed, shell, etc., for the purpose of concealing itself, so that 

 very often it escapes observation. 



Although the most familiar sea urchins are those which are covered like 

 a hedgehog with spines, there are others which are almost bare, or covered 

 with fine downy hairs. Among these are the very light and fragile Heart- 

 or Shield-shaped Urchins often found on the seashore. They are mud-loving 

 species, to which a thick array of spines would be most inconvenient ; so it 

 comes about that the spines have vanished or been modified in the scanty 

 flattened golden down. They are sluggish movers, feeding on seaweeds and 

 organisms in the mud. The Heart-shaped Smooth Urchins belong to the genus 

 Spatangits ; the Shield-shaped, to Clypeaster. 



