This marine invertebrate is more like a scorpion or a spider than a crustacean. A true 'living fossil', it started proliferating in the Paleozoic era (about 200 million years ago) and is still found today on the east coast of the United States and in Asia. It feeds mainly on sea worms that it unearths by burrowing into the sand or mud. Today, scientists are very interested in their blue blood, which is being used in the development of screening tests against meningitis.