Sometimes when a new shell turns up, hermit crabs will form a line, biggest to smallest, to see which animal fits the new shell. The next smallest will take that crab’s hand-me-down home ...

They’re in the class Merostomata, which means “legs attached to the mouth” because their mouths are literally in the center of all 10 legs. Horseshoe crabs also have 10 eyes—a pair of ...