Re: planarian worms

  This is a question that is very difficult to answer exactly.  Planarians 
do have nervous systems.  They can respond to environmental stimuli such as 
touch and light and they can also "smell" but whether they actually have 
the sensation we call pain is unclear.  Pain is really a 
"hyper-stimulation" of one of our sensory systems.  Too much heat, too much 
cold, too much sound or too much "touch" can all cause what we call pain.  
  If we define pain this way, then perhaps all organisms with nervous 
systems (including ants & mosquitos) can have the experience of pain when 
their nervous systems are "hyper-stimulated".