The Horseshoe Crab is considered a living fossil because it is one of very few animals that is still around that you can find fossils of them dating back 500 million years.
They were around about 100 million years before the dinosaur roamed the earth and survived the ice age with little problem.
They are descendants of the trilobite, but have remained relatively unchanged in 300 million years. There are only 4 species of horseshoe crabs known to still exist.
Three of the species live in the coastal waters of India, Japan and Indonesia. The fourth is found on the east coast of North America, between Maine and the Yucatan Peninsula.
This species is referred to as Limulus Polyphemus. These are the ones that I have pictures of on this site. I am lucky to live on the east coast of Florida, near Mosquito Lagoon, the home of many horseshoe crabs.