In fact it's the marine chelicerate arthropod. All the other chelicerate arthropods are terrestrial and have been since about 445 million years ago. There are tens of thousands of them, including such familiar animals as spiders, scorpions, and ticks. There are only four extant marine species, and they're all horseshoe crabs. So what compelled all the other chelicerate arthropods to crawl out of the water while the horseshoes stayed behind? No one knows. But they seem to be doing all right for themselves. Four hundred and forty-five million years is also about as long as it's been since they felt the need to evolve.
I've always liked these guys. When I was a kid I used to find their molted shells on the beach near where my family spent the summers. I spent forty-eight weeks of the year with no wild animals to observe except for squirrels and pigeons, and here were these strange objects from a mystery world full of life. I didn't know at the time just how strange they were. When you don't evolve for 445 million years you retain some weird kludges, like the aforementioned ten eyes: the relatively large lateral eyes, one on each side, with a smaller backup eye behind each one; three tiny eyes on the front to detect UV light; two eyes on the bottom next to the crab's mouth for some reason; and a lot of photoreceptors on that spiny tail (called the telson) that turn it into one big tenth eye.
A lot of these eyes are mainly there to help the crab find other crabs to mate with. Horseshoes mate during spring and early summer, when a single female may be swarmed by several males. That's less pornographic than it may sound because horseshoe crabs don't actually copulate. The process begins in the shallow waters just offshore, when the lucky male affixes himself to his mate by means of a pair of specially modified claws called pedipalps that resemble boxing gloves. Sometimes another male will grab on to him and so on, creating a kind of horseshoe conga line. Once a male has grabbed on, the female will drag him (and whoever else is back there) to the waterline and lay up to twenty thousand eggs in shallow holes in the sand. Once she has laid the eggs, the male clinging to her (and any other males who happen to be hanging around) fertilize them. The female will repeat this process several times during a mating season. Of the ninety thousand eggs (or more) that she produces in a season, perhaps ten will become adult crabs.
There are two more things I especially like about the horseshoe crab. First, it has gills called book gills because they are arranged in thin flaps that really do look like the pages of a book. Not only can the crab breathe with them, and spend short periods on land with the moisture they retain, but in an emergency it can even flip itself upside down and flap those pages in order to swim. (Of course when it stops it's still upside down, and it lands that way, but it can use its telson to right itself.) The last pair of legs even has a special leaflike appendage that the crab uses to sweep debris out of its gills. The second thing is that it chews with its shoulders. Its mouth is positioned oddly in the middle of its underside, surrounded by its six pairs of legs. When a crab happens upon a piece of food it will stand on the food and run in place, grinding the food up with spines on its legs before popping it into that toothless mouth.
But perhaps the most amazing thing about this creature is its blood. Horseshoe crabs have copper-based blood that when exposed to oxygen turns a vivid semi-opaque blue, like laundry detergent. This strange blood protects the crab from infection by forming a clot when exposed to bacteria -- that's the entirety of the crab's immune system. Fortunately for humans, the list of stuff crab blood neutralizes, like E. coli, is remarkably similar to the list of stuff that makes us sick. So bleeding crabs and extracting the clotting factor is now a multimillion-dollar industry. A single quart of horseshoe blood is valued at fifteen thousand dollars, and for good reason -- the FDA requires that all intravenous drugs be given the Limulus amebocyte lysate (LAL) test for dangerous endotoxins. If the blood clots, the drug is contaminated. (Crab blood has even been to space!) Fortunately the FDA forbids killing the crabs to get their blood (also for good reason -- no one has been able to synthesize LAL, so we need to keep the crab population strong). Crabs are trapped, washed, strapped into a rack, bled, and then returned to the water. They don't get those "Be nice to me, I gave blood today" stickers, but most of them survive and recover completely though I'm sure they find the whole experience alarming. I'm sure I would, but I can't give blood on account of I had jaundice when I was fourteen. Thanks for picking up the slack, horseshoe crabs!