Crab's body is divided into head, chest and abdomen. The back of the head and chest is covered with head and chest armor, and the shape varies from species to species. There are 1 and 2 pairs of antennae in the center of the forehead, and compound eyes with stalks on the outside. The mouthparts include 1 pair of jaws, 2 pairs of jaws and 3 pairs of jaws. There are five pairs of chest feet on both sides of the head breastplate. The abdomen is degenerated, flat and tortuous on the ventral surface of the head and chest.
Male abdomen is narrow and long, mostly triangular, and only the first two pairs of appendages are deformed into connectors; The female has a broad abdomen, with 1 pair of double-branched appendages in the 2nd to 5th segments, which are densely covered with bristles for holding eggs. Most crabs are marine, with the most species in tropical shallow waters.
A small number of crabs grow and breed completely in fresh water, and most of them live under rocks in mountain streams. There are also a few crabs, such as ground crabs, which can adapt to land life, burrow in wet mud holes and migrate to the sea during breeding. Most crabs are omnivorous. Some crabs are carnivorous, such as Portunus can prey on fish, shrimp and mollusks. A few crabs, such as window crab, scrape or filter algae and organic debris.