Chameleons and seahorses are able to move each of their eyes independently - they can swivel their eyes so that they point directly backwards, and thus see behind them without turning their heads.
Owls can see behind them without turning their bodies, since their heads can swivel almost 180 degrees. This is an adaptation to the fact that they cannot move their eyes - they are so huge that there is no room inside the skull for muscles to move them. They must therefore turn their heads to change their line of sight.
src: Yahoo! Answers
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