CHAPTER VII 



ASYMMETRICAL ANIMALS 



It was necessary for us to begin our analysis with 

 symmetrical animals since as the result of tins analysis 

 tiie conduet of asymmetrical organisms oilers no difficulty. 

 The result of tiie asymmetry consists merely in a change 

 in the geometrical character of the path in which an animal 

 is compelled to move to or from the source of energy. 

 While this path is a straight line in a symmetrical and 

 positively heliotropic organism it is a spiral around this 

 straight line as an axis in an asymmetrical organism, 

 like Euglena. Suppose a positively heliotropic animal to 

 have slightly asymmetrical appendages which give it a 

 tendency to deviate to the left. Let us suppose that the 

 plane of symmetry of the animal goes at the beginning of 

 the experiment through the source of light and that the 

 animal is swimming toward the light. After a few strokes 

 the head of the organism will have deviated slightly to the 

 left on account of the asymmetry in the activity of the 

 appendages. As soon as the median plane of the animal 

 deviates to the left, the left eye is less illuminated than 

 the right one. As a consequence, a difference in the ten- 

 sion of the muscles on the two sides of the animal will be 

 produced which will compensate the natural lack of sym- 

 metry in the muscles and the animal will cease to deviate 

 further to the left; and this compensating effect of the 

 unequal illumination of the two eyes will continue until 

 the animal is actually oriented in the right way again, i.e., 

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