this is a natural defense posture for chitons but it probably shouldn't last for weeks on end... pick them up gently and give em a sniff.  if they stink they're dead, if not put them back.  Do snails live in your tank okay?  if they do then I would imagine most chitons should fare just as well as any snail.  In which case, just give them more time i guess, and give the sniff test every few days. 
		
	
		
		
		
		
		
	
		
		
	
	
	 |