Horsehoe Crabs and Spiders

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.