- Self - Host: [using his pen to investigate a crab's lost arm and leg] Many invertebrates can do something that animals like us can't. If they lose their arm or leg, they can grow a whole new one. Take a look at this. This is a crab shell from a crab that got in a pretty bad fight. He got pinched by another crab. Look, he lost a lot of his shell over here and his whole left claw is gone. His whole left arm is pinched off. Now crabs grow by shedding their shell so they molt. So this is the first shell that the crab shed after he got in that bad fight. The next time he shed his shell, look, this part of the shell is repaired and has a whole new left claw, a brand new arm. Anytime his shell shed after that, his left claw's almost full-size, and his body is just about back to normal. See, vertebrates can do this because their bodies aren't too complicated.