KnowHOW team explains: A crab’s legs are attached to the side of its body, like human arms. While the human hand has multiple articulations allowing freedom of movement (shoulder, elbow, wrist and finger joints), crabs hardly have any.
Their leg joints flex only slightly in the front-rear direction, but they flex exceedingly well at right angles to it. In this respect, they are similar to fingers, which can move well up and down, but only slightly from side to side (try this with your palm facing down to mimic a crab).
The ball-and-socket joint of your hips and shoulders is a mechanical improvement that somehow never took place in crabs; they have chosen a different development path, favouring joint strength over flexibility.
They have developed an exoskeleton that protects them from predators at the expense of limiting the movement of their upper limbs.
Evolution shows that crabs with more protective exoskeletons and less leg movement are better able to survive. Thus crabs walk awkwardly, but they are apparently safer than they would have been with more flexible joints.
The question was sent by A.P. Bakshi from Durgapur