Eurypterids, or sea scorpions are an extinct group on predatory arthropods which ranged in size from a few centimeters to 3 meters in length. The prosoma generally 5 pairs of "walking limbs" the most posterior of which was usually highly modified to form a paddle-like swimming limb. Like all chelicerates, the eurypterids had one pair of pre-oral appendages (chelicerae). The opisthosoma had 12 segments, apparently with no appendages. Like horseshoe crabs, eurypterids had a long "tail spike" (telson) attached to the most posterior segment of the opisthosoma.