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Do all animals (of a certain size and not thinking about worms) have the possibility to perceive depth?

Do all mammals have at least two eyes? Are there mammals with more than two eyes?

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    $\begingroup$ Not all two-eyed mammals have depth perception. Many herbivores have eyes on the opposite sides of their head to increase their range of vision for predators, and are so widely separated that they barely overlap at the front, if at all. $\endgroup$ – MattDMo Jan 8 '16 at 15:08
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    $\begingroup$ Also, please clarify "all animals (of a certain size and not thinking about worms)". $\endgroup$ – MattDMo Jan 8 '16 at 15:09
  • $\begingroup$ @MattDMo Thank you for your first comment; I didn't think of that. For the second: I am aware of the fact that some worms (all?) don't have eyes. Also, I'm not interested of animals which consist of a very small number of cells (by a quick search I found Myxozoa as an example). So for the sake of clarity, lets say the animal has to have at least 5cm of height when standing on the ground. $\endgroup$ – Martin Thoma Jan 8 '16 at 15:30
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All mammals have exactly two eyes, though the eyes are vestigial in some species, some of which are even blind.

Small fossorial (burrowing) mammals generally have very tiny externally visible eyes or eyes that are covered with skin (e.g. golden moles). Some river dolphins (e.g. "blind dolphins") also have very small, vestigial eyes.

Some reptiles (and amphibians?) have a "third eye" called a parietal eye. However, it isn't a true eye. (See How Humans Lost Our Chance at a Third Eye).

I'm not aware of any vertebrates that have just one eye. As far as I know, all vertebrates have two eyes. The anableps is a fish that is nicknamed "the four-eyed fish," or something like that, though it actually has just two eyes.

However, there are one-eyed invertebrates, including copepods. (One copepod genus bears the scientific name Cyclops.) Answering your first question, copepods reportedly lack depth perception. There may be other one-eyed invertebrates, but I'm not an expert on inverts (there are too many species to count let alone study!).

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  • $\begingroup$ I'd imagine that large animals would have more use of depth perception, so a simple mutation for two eyes instead of one would be very advantageous. But perhaps on small, insect-level scales depth-perception gives little advantage. $\endgroup$ – Jam Jun 14 '18 at 11:25
  • $\begingroup$ Fish are usually not considered tetrapods. Did you mean birds? $\endgroup$ – Anixx Jun 14 '18 at 11:53
  • $\begingroup$ @Annix - My mistake; I meant to write "vertebrates." $\endgroup$ – David Blomstrom Jun 14 '18 at 20:48

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