Vestigial twin

A vestigial twin is a form of parasitic twinning, where the parasitic "twin" is so malformed and incomplete that it typically consists entirely of extra limbs or organs. It also can be a complete living being trapped inside the host person, however the parasitic twin is anencephalic and lacks consciousness.

Most vestigial limbs are non-functional, and although they may have bones, muscles, and nerve endings, they are not under the control of the host. The limbs can pose problems in redirecting necessary resources such as blood to the nonfunctional limbs while the body lacks these resources. Additionally, the presence of six or more digits on the hands and feet (polydactyly) is typically caused by a genetic or chromosomal anomaly and is not considered a case of vestigial twinning.