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.

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Classification 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