Blue-blooded and armoured with 10 spindly legs, horseshoe crabs were first thought to be closely related to crabs, lobsters and other crustaceans. In 1881, evolutionary biologist E. Ray Lankester placed them solidly in a group more similar to spiders and scorpions. Horseshoe crabs have since been thought to be ancestors of the arachnids, but molecular sequence data have always been sparse enough to cast doubt.
University of Wisconsin-Madison evolutionary biologists, in their recent study(by analyzing troves of genetic data) published in the journal Systematic Biology, helps firmly plant ancient horseshoe crabs within the arachnid family tree. Horseshoe crabs possess a mix of physical characteristics observed among a variety of arthropods. The animals' genome has repeatedly shown them to be related to arachnids such as spiders and scorpions. They are hard-shelled like crabs but are the only marine animals known to breathe with book gills, which resemble the book lungs spiders and scorpions use to survive on land.