Horseshoe crabs are among the world's oldest and most fascinating creatures. They are estimated to be at least 300 million years old.
The earliest horseshoe crab species were crawling around the Earth's shallow coastal seas for at least 100 million years before the dinosaurs even arrived (which was about 200 million years ago). Since that time, the Earth's land masses have shifted dramatically, thousands of other species have come and gone, but horseshoe crabs have survived and today remain much as they were those millions of years ago.

Horseshoe crabs have been used by people for centuries. More recently, they have been instrumental to scientific research that has contributed significantly to human health. Still today, when people see for the first time this strange creature with an armored shell and spiked tail, it is often with trepidation, along with the question, "What is that thing?" Meet the remarkable horseshoe crab!

http://www.ocean.udel.edu/horseshoecrab

Or visit: http://www.beach-net.com/horseshoe/Bayhorsecrab.html