Are there animals with one eye?

Why do we need two eyes?

The main reason - due the peculiarities of stereoscopic vision. To be more precise, with one eye we can hardly determine how far away the object is.

The European plaice, a fish which survival depended on how low it can lie on the ground and stay still, during its evolution got it's second eye moved on the top side of its body. And now in the process of development from the eggs to the young, it swims like all ordinary fish. However, when a certain size is reached, a metamorphosis takes place and its eye shifts to the other side.

There is only one kind of creatures that have only one eye. The Caridea - to be more specific - the Cyclopidae.

Instead of eyes, a nature gave Cyclops 4 pairs of developed legs. The fifth pair in males is transformed into an organ for retaining the female during the sexual process. They also do not have a heart. There are no blood vessels. The organs are washed with colorless hemolymph, which is promoted by pulsation of the intestine. Breathes the entire surface of the body.

So this creatures can't be called animals in the way we understand animals. Which means there are no animals with one eye in the nature (unless there was a bad accident, winking or an masquerade of course).

Thank you @bakabou159704 for the question that led to writing this article