If everything about a horseshoe crab seems strange and ancient, it should, because everything about the animal is strange and ancient. For starters, its history stretches back into deep geological time to the Ordovician, an age predating the dinosaurs. So when I had a chance to observe these living fossils last summer in the Intracoastal Waterway near West Palm Beach, Florida, I jumped at the chance.

The species encountered was the Atlantic Horseshoe crab, which is not a crab at all but is more closely related to spiders and scorpions. Watching their tanklike trajectory plow along the surface feeding on mollusks, crustaceans, worms, carrion and probably anything else in their path, I felt like a time traveler diving into the distant past.

Curiosity getting the best of me, I turned a specimen over. Packed beneath the protective headshield are five sets of jointed walking legs and a pair of small pincer arms used for conveying food to a centrally located, jawless mouth. Behind, on the abdomen, sit five paired flaps, known as book gills, common features of their arachnid kin. Teses ancient leaflike respiratory organs allow gas exchange underwater, as well as on beaches where horseshoe crabs by the thousand clamber each spring to lay eggs. One function of the tail became readily apparent when the upturned animal wedged it into the sand and righted itself in a snap.