What is this strange thing that lives on my rocks?

[fraggalrock]

It looks a bit like a prehistoric trilobyte and when I first saw it thought it was part of the rock but the next day it was gone.Saw it again yesterday and took a picture.You can barely see it move,it has a hard boney shell and does no harm that I can see.Anyone ever see one before? You have to look really close.

Sherry
[IMG][/IMG]