Suppose proto-Flatlanders developed light-sensitive bits on their tasting organs, as another way of obtaining information about whether something was edible or not. (Not entirely unreasonable; light-sensitivity is fairly common, for example, some butterfly genitalia!) Colour vision would be most useful at close range, so let's make this colour vision.
Opening the mouth wwould give the equivalent of the cup eye. Almost closing it would give a pinhole camera and an actual image, but only with enough sensitivity.
Adding a lens would improve things. Maybe a clear tongue?
Eating is still a vulnerable moment, but better to be blind while eating than blind all the time. Having mates to watch while you chewed would be a spur to social organization.
Rationale
Suppose proto-Flatlanders developed light-sensitive bits on their tasting organs, as another way of obtaining information about whether something was edible or not. (Not entirely unreasonable; light-sensitivity is fairly common, for example, some butterfly genitalia!) Colour vision would be most useful at close range, so let's make this colour vision.
Opening the mouth wwould give the equivalent of the cup eye. Almost closing it would give a pinhole camera and an actual image, but only with enough sensitivity.
Adding a lens would improve things. Maybe a clear tongue?
Eating is still a vulnerable moment, but better to be blind while eating than blind all the time. Having mates to watch while you chewed would be a spur to social organization.