232 



GENERAL ZOOLOGY 



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stalk which grows from the center of the aboral surface and 

 sends root-like branches in among the rocks at the bottom. 



The mouth and the arms lie on 

 the oral surface, which is upper- 

 most. A water- vascular system 

 is present, but it is of no service 

 in locomotion. Movement in 

 Pentacrinus is limited to the 

 arms. Food is brought to the 

 mouth by the wave-like action 

 of cilia on the inner edge of all 

 the arms. 



Sea Urchins and Sand 

 Dollars 



Sea urchins and sand dollars 

 have no arms. The entire ani- 

 mal (Fig. 126) is contained in a 

 hard shell which in the urchin is 

 roughly hemispherical, and in 

 the sand dollar is flat and disk- 

 shaped as the name indicates. 

 There is a large opening in the 

 shell on the oral surface for the 

 mouth, and fine double rows of 

 small openings for the tube feet. 

 The body is covered with mov- 

 able spines. The sea urchins live 

 chiefly on rocky ocean shores, 

 where they feed upon seaweed 

 and upon both living and dead 

 animals. The sand dollars bury 

 themselves in the sand and feed chiefly on the microscopic 

 plants and animals living in the sand and mud. 





FA 



Fig. 125. Sea lily 

 From Report of H. M. S. Challenger 



