Na'Vi navels

OK, yes, I've been watching "Avatar" a few times on HBO, even though it's 2D and not 3D. I hadn't really paid attention before, but in addition to the female breasts (which I did in fact notice), the Na'Vi also have navels. Now, director James Cameron is on record (Playboy interview, Playboy Web site) as saying that they aren't placental mammals, so one really has to wonder what other function the navel would indicate. And all we can do is wonder right now.

If you look closely you can see a couple of Na'vi navels in the picture below.