system
Given the following post, respond as if you're commenting on Reddit:
human
Is there any organism that can't perceive three dimensions?

I figured that there might be some places on this earth where depth is irrelevant, maybe on some liquid surface some small creatures are floating around in a world similar to the Plato "cave experiment" ( en.m.wikipedia.org/wiki/Allegory_of_the_Cave). Are there any such creatures?

EDIT: Question depends on definition of perceiving, I'll try to clarify what I was intrigued by. 

For example, imagine a species living on a liquid surface. The individuals will be very flat and unable to dive, jump or even wobble back and forth. Their food and reproduction will be handled on surface-level, just as the rest of their lives. To those creatures, what happens in the third dimension would be totally irrelevant, and thus evolution might "remove their perception of the third dimension". I guess what I'm asking is; are there any creatures that operate only in two dimensions, and that care so little about dimension n.3 that Darwin/Plato just as well could steal their understanding of three dimensions? I imagine that it would be hard for us humans to prove that "this and this species are just as 2D as Mario".

Of course, unlike Mario, their thickness will vary (not be a constant number of atoms), but I wouldn't say that is relevant.
gpt
This really depends on your definition of "perception". In theory, if a flat organism is utilizing chemotaxis to perceive the world, only two dimensions can be recognized at a time. However, there is no reason that a third axis would be completely unrecognized -- if a drop of that same chemical trigger is placed above the organism, a similar response would occur as if it were to the side, although movement may or may not be entirely successful. Hence, perception is present, but the ability to respond is not.

If you take it down to the simplest of things, then the inability to perceive would be the only way an axis of dimension can be ignored. For example, a free-floating microbe that relies on currents to attain food and other resources would not be able to, or even need to, recognize the three dimensions. 

I guess at this point, you may as well say that this is a philosophical question, as you implied: does perception matter if you cannot recognize and react to it?