The Class Merostomata is a small group of arthropods that includes horseshoe crabs and their extinct relatives. These animals have a hard exoskeleton, a long, pointed tail spine (telson), and jointed appendages. They are primarily marine, bottom-dwelling scavengers and predators, feeding on worms and small invertebrates. Horseshoe crabs are notable for their blue blood, which contains limulus amebocyte lysate used in medical testing. They are considered living fossils, having changed little over hundreds of millions of years.