Homology between the lacinia mobilis of peracarid crustaceans (Malacostraca) and movable appendages of the mandibular edge in other crustaceans, hexapods, and myriapods has been advocated in classical as well as recent phylogenetic studies, and in some cases this feature has been attributed major significance in arthropod systematics. A comparative SEM survey of the lacinia mobilis in Peracarida and its alleged homologues (prostheca of Hexapoda, internal tooth of Diplopoda, ‘lacinia mobilis’ of Symphyla and Remipedia) rejects the primary homology of these varied structures. The lacinia mobilis of Peracarida can be characterized precisely by asymmetry on the left and right mandibles, as a strong tooth-like structure on the left mandible which is oriented at a right angle to the remaining edge and as a stalked, spine-like structure on the right mandible. A fundamental difference to other Malacostraca is that the peracarid lacinia mobilis characterizes the adult mandibles. Supposed homologues of the peracarid lacinia mobilis are instead a modified pectinate lamella (Diplopoda, Chilognatha) or a movable appendage that is associated with the pars molaris rather than the pars incisivus (Symphyla), or an inner separation of the incisor process (Remipedia). The detailed structure of the prostheca in Campodeoida and Ephemeroptera weakens interpretations of its origin as a separation of the incisor process.