73 



But this locomotor mechanism (of Euglena) is imperfect, it forces 

 the organism to move in a spiral, and always to turn toward a structurally 

 determined side. There are many organisms which swim in spirals and 

 become oriented by turning toward a structurally defined side. Jen- 

 nings and Mast include all such orientations under " trial and error " 

 and contrast them with the direct orientation of such animals as the 

 amphipods in which the turning may be either toward the left or the 

 right. Let us now consider whether the orientation of Euglena is more 

 like the selection of random movements (which we would all agree may 

 justifiably be called " trial and error "), or whether it is more like the 

 orientation of the terrestrial amphipods studied by Holmes. 



I think that all students of behavior including Jennings and Mast 

 believe that in the case of these amphipods we have direct heliotropic 

 orientation. If the right eye of such a positively heliotropic amphipod 

 be covered with asphalt varnish it will execute circus movements towards 

 the left. The usual explanation is that the main nervous connection is 

 between the eye on one side and the legs on the opposite side of the body. 

 The light shining on the uncovered eye brings about a condition of in- 

 creased muscular tonus in the legs of the opposite side, which is not 

 present in the legs connected with the covered eye. Consequently the 

 right legs push more strongly and the amphipod turns towards the left. 



Suppose now we remove some or all of the left legs from an amphipod 

 of this kind so that it will always turn toward the left, and transfer it 

 to water in which it must be supposed to swim in a spiral path. We 

 will then have an organism which would become oriented in essentially 

 the same way that Euglena does. The animal would always swerve 

 toward the left. But, when the spiral course brings it into such a posi- 

 tion that the light shines directly on the left eye, the muscular tonus of 

 the right legs would be increased and the swerving toward the light would 

 increase. Thus orientation would be effected in just the same way that 

 it is in Euglena. 



While these hypothetical changes that must be made in the amphipod, 

 to make it react like Euglena, are considerable, they concern only the 

 details. The fundamental nature of the photochemical substances, the 

 nature of their stimulation and the character of their connection with 

 the locomotor organs have none of them been modified. All that has 

 been done is to make an asymmetrical organism swimming in a spiral 

 out of a bilateral one. a These changes are much less fundamental than 



a Swimming in a straight line. 



