in mammals, including humans.

The simplest eyes are pit eyes. They are eye-spots which may be set into a pit to reduce the angle of light that enters and affects the eye-spot, to allow the organism to deduce the angle of incoming light.

Eyes enable several photo response functions that are independent of vision. In an organism that has more complex eyes, retinal photosensitive ganglion cells send signals along the retinohypothalamic tract

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