The horseshoe crab, Limulus polyphemus, at one time
were considered true crustaceans, thus the name "crab".
However, these dark brown Arthropods are only distantly related
to such crustaceans as the true crabs, shrimps and lobsters.
By examining their underside, we can clearly see many similarities
between the horseshoe crabs, members of the class Merostomata,
and spiders, scorpions and ticks, which belong to the class
horseshoe crab it has no known natural enemies. There are
reports that during the sixteenth century, the coastal
Indians of South Carolina attached the sharp tail (more
properly called a telson) to the end of their fish spears.
Learn more here and here.