[Don Joe]

Trilobites were primative arthropods that existed on Earth in the Cambrian through the Devonian, with some rare species lasting until the end of the Paleozoic in the late Permian (roughly 500 million years ago to 180 million years ago).

Or, even now. Don't forget the Horseshoe Crab.

[capitan_refugio]

I thought the horseshoe crabs were more related to spiders, scorpions and the now-extinct eurypterids (thankfully extinct, because some species were like 15-foot upside-down swimming scorpions). Horseshoe crabs are in the order Xiphosurida, sort of all by themselves taxonomically. I seem to recall they had "blue blood" ... is that true?