62 L. H. BITTNER, G. R. JOHNSON, AND H. B. TORREY 



"trial" movements, a source of confusion that has already been 

 discussed in the paper on Porcellio to which we have just 

 referred. 



The earthworm comes midway between the sow bug (Porcel- 

 lio) and the leech in the freedom with which it bends its body 

 when reacting to light. It has been shown 3 that the first move- 

 ments of Porcellio after stimulation are away from the source of 

 light. The body moves stiffly as a whole. The photoreceptors 

 are anteriorly placed paired eyes. Holmes cites observations on 

 the leech Glossosiphonia that show a wide range of mobility in its 

 response to light, dependent upon its characteristic locomotion. 

 The earthworm does not react stiffly, like Porcellio, nor are 

 more than a very few anterior segments concerned in what- 

 ever random movements may be observable under photic stim- 

 ulation. Holmes was led to believe that the method or orienta- 

 tion of the leech is, in principle, the same as that of the earth- 

 worm. He calls especial attention to the characteristic waving 

 of the body, preliminary to fixation of the anterior end. Our 

 observations, however, encourage us to place emphasis on the 

 resemblance of the reactions of the earthworm to the behavior 

 rather of Porcellio than of Glossosiphonia. The random move- 

 ments of the earthworm have thus appeared to us to be less 

 significant elements in its orientation to light than the obser- 

 vations of Holmes indicated. 



It is characteristic of the earthworm when advancing in dif- 

 fused light, to protrude its anterior end first on one side and 

 then on the other, with successive extensions, in fairly regular 

 alternation. A distinct tendency thus exists for this end, when 

 bent to one side, to bend to the opposite side at the next exten- 

 sion. Mechanical causes, such as tensions in muscles and skin, 

 are probably responsible for it. It is natural to expect evidence 

 of this tendency in experiments on earthworms where relatively 

 low intensities of light are employed unilaterally. Mast, indeed, 

 asserts that in active worms, " the anterior end is simply turned 

 sharply in the direction opposite to that in which it is when it 

 receives the stimulus. . . . Thus it is turned toward the 

 light about as often as from it, regardless of the light inten- 

 sity." 4 Sluggish individuals, however, reacted quite differently. 



3 Torrey and Hays, 1914. 



4 Light and the Behavior of Organisms. 1910, p. 200. 



