True crabs are crustaceans
in the Order Decapoda, which also contains lobsters, shrimp, prawns,
hermit crabs and a number of less well-known groups. They have
five pairs of walking legs (the first of which is modified into
a pair of claws or chelae) and typically a flattened shell. In
all but a few crabs (for example, Raninoida), the abdomen is folded
under the cephalothorax. The form of the abdomen usually reveals
the sex of the crab - males have a narrow abdomen, while females
have a much wider abdomen, under which they carry their eggs.
Crabs are a very diverse group, mostly found in salt water, but
with some groups living in freshwater or on land. Although famed
for their tendency to walk sideways, crabs are in fact able to
walk in any direction.
within the crabs is traditionally based on the position of the
gonopores, whether they are found on the legs or on the thorax.
In the two "primitive" sections (sometimes called collectively
the "Podotremata"), the gonopores are found on the legs
(as in all other decapods); in the Heterotremata, the male gonopores
are on the legs, and the female gonopores are on the sternum;
in the Thoracotremata, all gonopores are on the sternum.
Above Images Came From The NOAA