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
