There are four species of horseshoe crabs in the world, with the most familiar to Americans being Limulus polyphemus, the Atlantic horseshoe crab. Here’s a group of them mating:
They are arthopods, but not “true” crabs, which are in the subphylum Crustacea. Horseshoe crabs are in a different subphylum, the Chelicerata, more closely related to spiders and scorpions than to the crabs we know.
A good website on the group describes its morphology, behavior, and medical uses (yes, they’re useful in making drugs; see here). Plus they’re really cute. Here’s a description of the legs of modern animals:
The horseshoe crab has 6 pairs of appendages on the posterior side of the prosoma. Five pairs of walking legs or pedipalps enable the horseshoe crab to easily move along benthic sediments. Each has a small claw at the tip except the last pair. The last pair of legs has…
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