Fig. 6 | Alignment of extant euarthropod and Cambrian 'great appendage' stem euarthropod brains. In living euarthropods, combinatorial expression of homo-logous genes (a) defines three asegmental domains of the cerebrum: b these are: ce1 (prosocerebrum); ce2 (protocerebrum); and ce3 (deutocerebrum). The first true segment of the trunk is T1, specified by collier39. All cerebral traits are inde-pendent of the presence or absence of tagmatized trunk segmentation, which in extant mandibulates is typified by segments T2-T4 providing gnathal appendages. c, d Each domain in extant Mandibulata is characterized by its sensory systems and computational centers.In both Scutigeridae and Drosophilidae ce1 is dominated by its central complex. In Drosophilidae and other pancrustaceans10,11 ce1 is supplied by the rostral visual system (ocelli/naupliar eyes) and provides the paired labra. Domain ce2 supports the protocerebral compound eyes and its nested visual centers and paired mushroom bodies amongst other centers18,23. The ce3 domain is served by uniramous antennae and contains chemosensory and mechanosensory neuropils24. e, f Homologous sensory systems supply corresponding domains in two divergent 'great appendage' stem euarthropods. Jianfengiidae (e) is equipped with ocelli whose central projections define ce1. As in mandibulates, the Jianfengiid compound eyes and centers define ce2 and the uniramous 'great appendages' define ce3. Leanchoiliidae (f) differ in that two pairs of single lens eyes, one forward viewing the other viewing laterally, supply contiguous ce1 and ce2 domains. Paired 'great appendages' define a substantial ce3. g Summary of the main sensory and computational attributes of each cerebral domain, as known from studies of extant euarthropods. These are applicable to all investigated mandibulate and nonmandibulate lineages24. Alignments of extant and extinct taxa along the shared nonneural endomesodermal interface identifies an invariable order of non-segmented ce1-ce3 domains, their lineage-specific neuropils, and their associated sensory traits, suggesting an ancient organization of domain-specific functionality.