Horseshoe crab is also called horseshoe crab. Four kinds of marine arthropods, found on the east coast of Asia and North America. Although it is also called horseshoe crab, it is not a crab, and it is related to scorpions, spiders and extinct trilobites. Horseshoe crab is an animal as old as trilobites (only fossils are available now). The ancestors of horseshoe crabs appeared in the Paleozoic Devonian in geological history, when dinosaurs had not yet risen and primitive fish had just come out. With the passage of time, its contemporaries have either evolved or become extinct, but only horseshoe crabs have retained their primitive and ancient appearance since they came out more than 400 million years ago, so horseshoe crabs are called "living fossils". But also has high medicinal value.