Horseshoe crabs were
misidentified as crabs hundreds of years ago. They spend most of
their time crawling on the sea floor and have a crab-like shell
roughly resembling a horseshoe. However, they are grouped with
arachnids.
Horseshoe crabs date back 500 million years as a species. These
prehistoric survivors, who perhaps never evolved to flourish on land
like the rest of their cousins, can grow up to 0.6 meters (2 ft)
wide and use their long tails as a tool to dig for food or to turn
over. The horseshoe crab has 10 eyes on its back and sides, can
replace lost body parts, and has blue blood. The blood is medically
valuable and is used to detect bacteria, for cancer research,
diagnosing leukemia, and vitamin B12 deficiency.