It's easy for people to move their legs forward or backward. That's because of the way they're attached to the body. People's knees bend towards the front, and that also makes it easy to walk forward. But a crab's legs are attached to the sides of its body. And its joints, unlike our knees, bend so that the crab can walk sideways. That's why crabs walk the way they do. an we do it? Crabs have five legs on each side of their shell. The front pair are for grasping, and remaining four pairs are walking or swimming. Each leg has seven joints, arranged so that the crab walks sideways. Frog crabs and spider crabs however, have legs designed to enable them to walk forward.