@KoiwiGal - I was always surprised when they classified the skin as an organ, to be honest. When I was a kid I thought organs were basically just the individual bits that did stuff in the body, and that skin and bones and muscles were more like the packaging that just held it all together.
But skin is actually pretty complex stuff. I did a biology paper a few years ago and I can remember being kind of blown away by how many different functions the skin has.
So, anyway, I guess it depends on what your definition of an organ is. The cells in our guts do actually have quite a complex role in our bodies. I'd say they were more complex than even the eye, or possibly anything except the brain.