200 



ZOOLOGY 



nearly spherical form. The sea-urchins eat various small 

 animals, and get food also from small bits of organic matter 

 in the mud which they swallow. They live more concealed 

 than the starfish, for some burrow in the mud and others 

 -such as our green sea-urchin of the Maine coast (Fig. 

 185) grind out pockets in the rocks by means of their 

 spines. Others cover themselves with seaweed, and thus 

 become inconspicuous. Besides the green sea-urchin, 



FIG. 186. Arbacia, the Easteru black sea-urchin. Tube feet retracted. Nat. 



size. Photo, by W. H. C. P. 



which is found north of Cape Cod, we have a black sea- 

 urchin (Arbacia, Fig. 186), which extends south as far as 

 North Carolina. We have on the east coast also two kinds 

 of flat sea-urchins (sand-dollars), which live in the sand 

 from low water to one hundred fathoms. Their spines 

 are small and silky (Fig. 187). Some sea-urchins lose 

 their strictly radial form and become bilateral, having a 

 ] jointed anterior end (Fig. 188). 



