vestigial
adjective uk/vesˈtɪdʒ.i.əl/ us/vesˈtɪdʒ.i.əl/
[ before noun ]残留的，残余的
being a small remaining part or amount
medical specialized（尤指身体部位）发育不全的，退化的
used to describe something, especially a part of the body, that has not developed completely, or has stopped being used and has almost disappeared
a vestigial organ/limb/tail 退化的器官／上肢／尾巴