CHAPTER V 



EXPERIMENTS ON ECHINODERMS 



i. The nervous system of the starfish consists, first, 

 of a central nerve-ring around the mouth (Fig. 16), 

 and, second, of the peripheral nerves radiating from 

 this ring into each of the arms. 



It is a well-known 

 fact that if such an ani- 

 mal be laid on its back 

 it soon rights itself. In 

 species like that repre- 

 sented in Fig. 1 6 the 

 ambulacral feet found 

 on the ventral surface 

 in great numbers ex- 

 ecute the righting. 

 These little feet are 

 muscular tubes, which 

 end in a plate. By 

 means of this plate the 

 foot, like the sucker of 

 the leech, can cling to solid bodies. If a starfish 

 be laid on its back, the tube-feet of all the arms 



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FIG. 16. NERVOUS SYSTEM OF A 

 STARFISH. 



a, central nerve-ring that surrounds the mouth, 

 />, peripheral nerves of the arms. 



