I'm trying to identify the basic biological mechanisms for sight.    
  
Could you help me with the least evolved organism that can see? Insects have fairly complex visual mechanisms (compound eyes of flies), so is there any other lesser evolved organism that has an even more basic visual perception and has that visual perception and the way its eye+brain perceives images been studied?  
  
EDIT: The organism should have the capability to see and identify objects or living beings or food and take decisions accordingly.