When Does An Earthworm Turn?

It is innately entrenched within the psychology of earthworms that whenever they spot some unfamiliar object within their vicinity, their muscles will contract instinctively and automatically the body will turn in order to eschew the object and the prospective threats that it poses.  Other factors that contribute in making a worm turn are heat, light and sometimes sound.  Earthworms are creatures that are used to living in habitats where it's cold and dark and quiet.  Any type of heat and sound would be an alarming change for them in the presence of which they will feel uncomfortable.   In comparison to heat, light and sound affect the earthworm on a lesser degree.  While sound and heat cause an earthworm to retreat, light is one factor that attracts the earthworm.  The turn in the terms of detecting light is to determine where the source of light is and almost impulsively the worm crawls in that direction.