416 FRANK W. BANCROFT 



Euglena does orient as directly as its locomotor mechanism 

 allows it. 



But this locomotor mechanism is imperfect, it forces the organ- 

 ism to move in a spiral, and always to tm*n towards a structur- 

 ally determined side. There are many organisms which swim in 

 spirals and become oriented by turning towards a structurally 

 defined side. Jennings and Mast include all such orientations 

 under 'trail and error' and contrast them with the direct orienta- 

 tion of such animals as the amphipods in which the turning may be 

 either towards 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 ('01). 



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 exe- 

 cute 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 increased muscu- 

 lar 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 towards 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 ani- 

 mal would always swerve towards the left. But, when the spiral 

 course brings it into such a position that the light shines directly 

 on the left eye, the muscular tonus of the right legs would be 

 increased and the swerving towards the light would increase. 

 Thus orientation would be effected in just the same way that it 

 is in Euglena. 



