Limulidae is the only recent family of the order Xiphosurida and contains all four living species of horseshoe crabs: Carcinoscorpius rotundicauda (the mangrove horseshoe crab), Limulus polyphemus (the Atlantic horseshoe crab), Tachypleus gigas and Tachypleus tridentatus.
Horseshoe crabs have hard shell that protects their whole body. They also have two large compound eyes and multiple smaller simple ones atop the carapace. Beneath the carapace they look quite similar to a large spider. They have six pairs of legs for walking, swimming and moving food into the mouth. The long, straight, rigid tails can be used to flip themselves over if they are turned upside down.
Mesolimulus was a Jurassic horseshoe crab.