Horseshoe crabs are sometimes known as "living fossils" because they've been around so long, 450 million years or so, but really that's not that impressive. Crustaceans have been around just as long, but no one calls them "living fossils." We call them "tempura." Horseshoe crabs are celicerates, which means they're more closely related to spiders and scorpions than crabs. What makes them living fossils is they're just about the last of celicerata to live in the water, which is where all celicerata originated. The only other marine celicerata are sea spiders, which live in the Mediterranean and may not be celicerata at all. They're not spiders either, as a Greek fisherman is bound to inform you in a superior tone if you mention one around him.
Horseshoe crabs have ten legs, which is something you'd know if you ever turned one over and counted, which I bet you haven't. People don't turn horseshoe crabs over if they can help it; the pointy tail is harmless, but looking at a horseshoe crab's underbelly is enough to give you a case of the cold-robbies. Another disturbing thought is that horseshoe crabs like to swim upside down.
Think of that next time you're paddling in the surf. In addition to jellyfish and sharks, there are horseshoe crabs, their ten little legs pumping for all they're worth, turned belly-up all the better for your toe to brush across one. This is perfectly harmless, I know, but still. Oog.
So to recap. Horseshoe crabs live in the sea. Humans live on land. The ocean reaches depths of over six miles. A lot of it filled with weird shit you'd rather not think about. For example, horseshoe crabs. We've been around in our current for about 200,000 years. Horseshoe crabs for about two thousand times as long. We have two legs. They have ten. We swim rightide-up. They swim upside-down. And they won't give a darn if they brush against you.
