Naturally occurring abnormalities (Bruchdreifachbildungen) in decapod crustacean appendages are described. They are similar to the range of structures experimentally produced by cutting notches in the sides of insect legs (Bohn, 1965). It is argued that they result from failure of wounds to heal. Regeneration from a free surface along the proximodistal axis is always in a distal direction. Surfaces regenerating circumferentially can regenerate in either direction around the circumference. Regeneration will proceed until the two surfaces of the wound meet. Then, where the two surfaces on either side are non-congruent, intervening tissues will be intercalated. This explanation accounts for the range of structures observed after notching experiments (Bohn, 1965) and seen in crustacean Bruchdreifachbildungen. The explanation says that regeneration will occur wherever wounds fail to heal. This avoids the difficulties of the complete circle rule (French, Bryant & Bryant, 1976) and explains why appendages with mirror-image symmetry are often capable of regeneration.