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Vniversity of California PuhUcations. [Zoology 



size of the animal. Throughout the experimental work a Zeiss- 

 Greenough stereoscopic microscope was used and proved to be 

 an invaluable instrument for this kind of work. 



As soon as the animal meets an obstruction all the cilia re- 

 verse, and at the same time there is a contraction of the myoneme 

 fibers on the left side and a bending of the posterior end sharply 

 to the left. As the backward movement proceeds the posterior 

 end in its curved position acts like the rudder of a ship. Its 

 resistance to the water and its curvature cause the animal to take 

 a curved path, which if continued far enough would describe a 

 circle. The bending of the posterior end of the body is always 

 to the left and never in any other direction, except while swim- 

 ming when it is directed to the left and slightly ventrad. 



II. 12.- 



Fig. 11. — Au animal executing the backward movement of the motor reac- 

 tion. The small arrows show the direction of stroke of the body cilia, 

 the large arrows the resultant movement of the body. 



Fig. 12. — The bending of the anterior end to the right at the end of the 

 motor reaction. 



Fig. 11 illustrates the part the body plays in this reaction. 

 This is a typical case where the whole body is affected by the con- 

 traction of the myoneme, the posterior end being bent more than 

 the rest. That this bending does play a most important part in 

 the direction which the animal takes is shown by the fact that 

 the path of movement follows the curvature of the body. This 



