Because its basic body design has changed so little over the ages, the horseshoe crab is often referred to as a "living fossil.” They have as much genetic variation as many other species, but are so well adapted to their ecological niche that large-scale species radiation simply hasn't occurred. As biologist Mark Botton of Fordham University puts it, "They seem to have hit upon a body plan that worked a long time ago, and hasn't really changed."

A Curious Body
The body of this extraordinarily well-adapted animal has highly unusual features. Though named after a one-eyed character, Limulus (meaning "sidelong") actually has ten eyes: one oval lateral eye on each side of its shell, two small ones in the center, five light-receptive organs beneath its shell, and one in its tail. The larger eyes are compound, like those of insects, allowing the animal to see in all directions.