Filipino child with an additional set of legs. Polymelia is a birth defect involving limbs (a type of dysmelia), in which the affected individual has more than the usual number of limbs. In humans and most land-dwelling animals, this means having five or more limbs. The extra limb is most commonly shrunken and/or deformed. Sometimes an embryo started as conjoined twins, but one twin degenerated completely except for one or more limbs, which end up attached to the other twin. No photographer credited, undated.