Within the animal kingdom, Bilateria were first recognized by Ernst Haeckel at the end of the 19th century as a major crown group on the basis of their bilateral symmetry (either as embryos or adults) [1,2]. In all Bilateria, the central nervous system (CNS) contains subsets of axons which interconnect neurons on both sides of the nervous system [3,4]. These important fascicles of axons projecting across the CNS midline are named commissures. In vertebrates, the molecular mechanisms controlling the decision of axons to cross or not to cross the CNS midline have been studied for 3 decades and reviewed extensively elsewhere [5].
