BEHAVIOUR 183 



demonstrated to parallel the development of the nervous system. 

 Earthv^orms show responses to light, particularly blue light, to 

 heat, to bodily contact (thigmotaxis) and to touch. They show an 

 ability to discern food materials and changes in them by their 

 grazing on different leaves in litter on the surface of the ground. 

 Acidity of soils affects colonization of sites and stimulation of acid- 

 sensitive sense organs may account for migration from burrows of 

 earthworms after rain. Activity can be modified by training in a 

 maze. In normal conditions diurnal rhythms of locomotory activity 

 are known. 



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