I was messing around the creek at Craig Park the other day, and I found a bunch of planaria under a rotting, submerged leaf. I've always loved these little guys, ever since high school biology class when we kept them as "pets", and fed them bits of raw liver. I think in nature, these common flatworms eat other invertebrates, and decaying matter.
And there really isn't a phenomenon. I just like that both words are Latin and end with "n".