The Order Xiphosurida comprises the horseshoe crabs, marine arthropods often called “living fossils” because their body plan has changed little over hundreds of millions of years. They have a hard, horseshoe-shaped exoskeleton, a long, pointed tail spine (telson), and a body divided into a prosoma (head) and opisthosoma (abdomen). Horseshoe crabs are benthic scavengers and predators, feeding on worms and small invertebrates, and they play a vital ecological role as a food source for shorebirds and fish. Xiphosurids are also important in biomedical research because their blue blood contains limulus amebocyte lysate (LAL), used to detect bacterial endotoxins in vaccines and medical devices.