Monopteralism is an extinct, primitive form of life characterized by an extended body plan including many pairs of jointed appendages, most resembling those of insects. Moreover, monopteral life exhibits a number of other primitive traits., including the lack of a head and thorax, although some may have possessed antennae. These features are often used to classify monopteral life as a subgroup of the eukaryotes (living things with cells containing a nucleus), although this classification remains controversial.