Blue-blooded and armored with 10 spindly legs, horseshoe crabs have perhaps always seemed a bit out of place.
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.
Image: https://scienceblog.com
