Iris aperture shape

[Mike Jones]

Does anyone have an explanation to a non-biologist why the aperture in 
my iris is circular whether open up or shut down, whereas cats, sheep, 
and donkeys have a slitted iris opening, which in some cases become 
circular under low light levels. Also what is the reason for varying the 
axis of slit orientation, ie. vertical in cats - horizontal in other 
creatures?

Mike Jones