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Brachyura


Diagnosis

Brachyurans are the true crabs. Crabs have large pincers (enlarged, chelate first pereiopods, to sound professional), but the third pereiopods are never chelate. The cephalothorax is flattened and wide and the antennae and antennules are heavily reduced. In living (extant) forms, the pleon has been reduced to a simple flap which holds the eggs and is held firmly under the body. This has not always been the case in all fossil forms.

Origin

The most derived group of decapods, perhaps unsurprisingly, appears as one of the latest, first turning up in the Cretaceous (which began 145 million years ago)

Images from www.nhm.org/guana/bvi-invt/bvi-surv/deca-inf.htm