Notes: This aquatic animal is not a member of the crustacean order. Its closest relatives are the scorpions and extinct sea scorpians. Though the Horseshoe Crab has survived for millions of years, today only four species survive. It's front shell is used as a plow for burrowing., This aquatic animal is not a member of the crustacean order. Its closest relatives are the scorpions and extinct sea scorpians. Though the Horseshoe Crab has survived for millions of years, today only four species survive. It's front shell is used as a plow for burrowing.