62 COMPARATIVE PHYSIOLOGY OF THE BRAIN 



are stretched out at once and are moved hither and 

 thither as if feeling for something-, and soon the 

 tips of one or more arms turn over and touch the un- 

 derlying surface with their ventral side (Fig. 17). 

 The tube-feet of these arms attach themselves to this 



FIG. 17. MECHANISM OF THE TURNING OF A STARFISH THAT HAS 

 BEEN LAID ON ITS BACK. 



The tube-feet of the three arms at the left are pulling while the other two arms are quiet. 

 This causes the animal to turn a somersault toward the left which brings it again into 

 the natural position. 



surface, and the animal is then able to turn a somer- 

 sault and regain its normal position. For this 

 result, it is essential that all five arms do not attempt 

 simultaneously to bring the animal into the ventral 

 position. Should the tips of all five, or even four, 

 arms tug simultaneously, it would be impossible for 

 the animal to turn over. In normal starfish having 

 five arms, not more than three begin the act of turn- 

 ing ; the other two remain quiet. If we, however, 



