Merostomata is a class of marine arthropods that includes horseshoe crabs. These ancient creatures have existed for over 450 million years, making them one of the oldest living species. They are characterized by their hard exoskeleton, long tail spine, and a unique body structure that includes a fused head and thorax. Horseshoe crabs are primarily found in shallow coastal waters and are known for their blue blood, which contains a substance used in medical testing. They play a crucial role in the ecosystem as both predators and prey, feeding on small invertebrates and serving as food for birds and other animals.