Symmetry of axon fascicles. Bilaterally symmetrical fascicles are coded in red. The only strongly asymmetrical fascicle (in yellow) is the ventral nerve cord (VNC), which is lateralized by the hypodermal ridge (see also panel d for cross-section). Circumferential motor axon commissures also show sidedness; that is, more can be observed on the right side than on the left side12. b,c | Schematic diagram of left–right (L–R) symmetry of axon placement in the nerve ring and ventral ganglion in an adult animal. Each circle presents a cross-section through an axon. Note that as well as similar cell positions (Fig. 1b), L–R bilaterally homologous neurons also share a similar axonal neighbourhood. d | Schematic diagram of laterality in the VNC. A ridge composed of hypodermis separates the left and right VNC, and is conceptually analogous to the fly and vertebrate midline glial cells. Three patterns of laterality can be found in the VNC. First, a subpopulation of axons shows strict bilateral symmetry, such that axons that derive from bilaterally symmetrical neurons are separated in a L–R symmetrical manner by the midline (blue and green). Second, a number of bilaterally symmetrical neurons send their axons exclusively along one side of the VNC (yellow); note that these axons pairs are in immediate proximity to one another, hinting at some homophilic affinity between them. Third, axons from unilateral neurons (the most prominent being VNC motor neurons) are on the right side of the VNC (red circle). For full details of L–R symmetries in the ventral ganglion and the VNC, see online Figs 1 and 2.