Crab

Crabs are decapod crustaceans, either the Brachyura or various groups within the closely related Anomura, characterised by having a heavily armoured shell, their tail segments concealed under the body, the ability to run sideways, and the habit of hiding in rocky crevices. They do not form a single natural group or clade, but have convergently evolved multiple times from the ancestral decapod body plan through the process of carcinisation. As a group they are thus polyphyletic.