68 EARTHWORMS AND THEIR ALLIES [ch. 



proved that earthworms can distinguish between 

 degrees of intensity of light. This obviously indicates 

 a more complete 'visual' sense. He illuminated 

 the light-box of the former experiment with light 

 admitted through a ground glass screen, thus di- 

 minishing its intensity. The other chamber was left 

 as before but the screen was removed, thus admitting 

 full daylight. In this experiment the number of 

 worms in each compartment proved to be different. 

 The results were not so striking as before, since only 

 rather more than one-half were found in the more 

 dimly illuminated chamber. It is a well-known fact 

 that if earthworms are abroad at all from their 

 burrows, it is during the night that this movement 

 takes place, the numbers decreasing towards morning 

 though worms are often seen crawling about well 

 after sunrise. Some experiments have been made in 

 attempt of explanation of this apparent anomaly. It 

 would appear from these experiments that while 

 worms are negatively phototropic to strong and 

 moderate light as has already been pointed out, they 

 are positively phototropic to very dim light ; hence 

 the advent of evening calls them forth from their 

 burrows. It will be noted that this perception is of 

 very great advantage to the worm since its more 

 active enemies above ground are diurnal. It was 

 held originally that the head end of the worm only was 

 thus sensitive to light ; but more recent experiments 



