The horseshoe crab is a marine arthropod.

The horseshoe crab, a marine arthropod belonging to the order Swordtail of the class Limnostomatidae, the horseshoe crab is shaped like a crab, with a greenish-brown or dark-brown body, encased in a hard armor, and has four eyes, two of which are compound eyes , two small eyes 0.5 millimeters at the front end of the cephalothoracic armour, which is the most sensitive to ultraviolet light, and is only used to perceive the brightness of the head, and a pair of large compound eyes on both sides of the cephalothoracic armour. Although horseshoe crabs can swim by slapping their gill plates dorsally to propel their bodies, they usually bend their bodies into an arch, burrow into the mud, and then propel their bodies forward with their tail swords and last pair of stepping feet.