136 DYNAMICS OF LIVING MATTER 



pulsory and irresistible, and is brought about automatically or mechani- 

 cally by the light itself. On the other hand, there are compulsory and 

 mechanical reactions to light which are not cases of heliotropism ; 

 namely, the reaction to sudden changes in the intensity of light. When 

 the Serpula stretch out their gills, they instantly withdraw them if an 

 opaque body passes between the animal and the source of light. Spiro- 

 gr aphis behaves similarly. Instead of casting a shadow upon the 

 animal, the same reaction may be produced by suddenly closing the 

 shutters of the windows. It is thus evident that we are dealing here 

 with the effect of a sudden decrease in the intensity of light compara- 

 ble to the twitching of a muscle upon the breaking or sudden decrease 

 in the intensity of a current. It should be noticed, however, that I 

 never succeeded in bringing about the sudden contraction of Serpula 

 or Spirographis by a sudden increase in the intensity of light. 



There are other forms which react as well upon a sudden increase as 

 upon a sudden decrease in the intensity of light, e.g. fresh-water Plana- 

 rians and earthworms. In these forms a sudden increase in the in- 

 tensity of light causes restlessness, while the reverse change causes the 

 animals to come to rest. This may lead to the gathering of the animals 

 in such parts of the vessel as represent relative minima in the intensity 

 of illumination. When such an animal comes from a bright spot to 

 a darker spot, it comes to rest ("falls asleep")- In consequence of 

 this fact such a relative minimum must act like a trap in which the 

 animals are caught. The consequence is that the number of animals 

 collecting in such a place must always increase, inasmuch as any ani- 

 mal which gets to such a spot by chance must remain there because its 

 motions cease. 



The fact that we are dealing here with the gathering of animals 

 caused by light might easily mislead an investigator to mistake these 

 reactions for negative heliotropism. It was a long time before I real- 

 ized myself that I was dealing here with an effect of light which was 

 specifically different from heliotropism. In the latter case the results 

 are a function of the constant intensity, in the former a function of the 

 quotient of the change of intensity over time. It is, however, easy to 

 demonstrate the difference between the two kinds of gathering, experi- 

 mentally. If negatively heliotropic animals be put into a cylindrical 

 glass jar, and it be placed near a source of light, the animals move in the 

 direction of the rays gathering at the negative end of the jar (b, Fig. 27). 

 If fresh- water Planarians are put into such a circular glass dish, they show 

 very little or no tendency to move in the direction of the rays of light, 

 creeping along in an irregular manner and gathering not at the nega- 

 tive or positive side of the jar, but on both sides, c and d (Fig. 27), 



