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ANIMALS WITH SPINY SKINS— THE ECHINODERMS 



The starfish has one of the most interesting- methods of digestion 

 found in the animal kingdom. Its food consists almost completely of 

 mollusks which have hard protective shells around their bodies. If you 

 have ever tried to pry open the shell of a clam or oyster you may wonder 

 how such a small and clumsy animal as a starfish could possibly feed on 

 these mollusks. The starfish, however, can open these shells with com- 

 parative ease through a principle of applied physiology. As an illustra- 

 tion, we will sav that it finds an ovster that it wishes to eat. It will settle 



Photo by W. K. Fisher 



Fig. 20.3. A starfish opening a clam. The starfish has attached its tube feet to the 

 shell of the clam and, through continued pulling, will eventually open it so that the soft 



parts inside may be devoured. 



itself down over the portion of the shell that opens and spread its arms 

 down on either side of the shell and attach the tube feet by suction and 

 begin to pull. The oyster has very powerful muscles which close the 

 shell and it would appear rather secure against the slight pull which the 

 starfish is able to exert. However, the starfish is a patient animal so it 

 continues pulling and when one arm gets tired it will rest it while pulling 

 with another. It is a well-known principle of muscle physiology that 

 continuous contraction cannot be maintained for a very long period of 

 time, and so it is only a few minutes before the muscles of the oyster 



