Normal crabs have eight legs that can walk, and two chelicerae that do not have the function of walking. The crab's chest has five pairs of appendages called pectoralis. The front pair of appendages are equipped with strong pincers, which are used for foraging. The other four pairs of appendages are the crab's legs. The walking appearance of these four pairs of appendages is unique and interesting. Most of them walk sideways instead of straight.

Most types of crabs live in the sea or offshore areas, and some live in fresh water or land. They rely on female crabs to give birth to baby crabs, and each time the female crab lays a large number of eggs, the number can reach millions or more. Crabs rely on the geomagnetic field to determine direction. The genetic material of such organisms is complex and highly diverse, often as many as 100 pairs.