[mbq]

Amoebae have a very complex 3D structure; check out some SEM images, like http://www.sciencephoto.com/search?subtype=keywords&searchst... or http://www.photomacrography.net/forum/viewtopic.php?t=21719 . Moreover, most of their molecular machines require third dimension to work.

[StavrosK]

Sure, but if we're assuming there's life in two dimensions, the amoeba's digestive system is much simpler than zipper organs.

[mbq]

It seems so on a first glance, but if you think about how this 2D phagocyte can be formed (especially on a macroscopic scale) and how would it interact with other structures it becomes much less obvious.