The term crab is sometimes applied to several different
groups of short (nose to tail) decapods with thick exoskeletons,
but only members of the Brachyura are true crabs; other
taxa, such as hermit crabs, porcelain crabs, king crabs,
and horseshoe crabs are, despite superficial similarities,
not crabs at all. Hermit crabs, king crabs and porcelain
crabs belong to the Anomala and can be distinguished from
true crabs by counting the legs - in Anomala, the last
pair of pereiopods (walking legs) is hidden inside the
carapace and so only four pairs are visible (counting
the claws), whereas uninjured true crabs always have five
visible pairs.
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