Except in species that have undergone evolved loss, paired lobed centers referred to as "mushroom bodies" occur across invertebrate phyla. Unresolved is the question of whether these centers, which support learning and memory in insects, correspond genealogically or whether their neuronal organization suggests convergent evolution. Here, anatomical and immunohistological observations demonstrate that across phyla, mushroom body-like centers share a neuroanatomical ground pattern and proteins required for memory formation. Paired lobed or dome-like neuropils characterize the first brain segment (protocerebrum) of mandibulate and chelicerate arthropods and the nonganglionic brains of polychaete annelids, polyclad planarians, and nemerteans. Structural and cladistic analyses resolve an ancestral ground pattern common to all investigated taxa: chemosensory afferents supplying thousands of intrinsic neurons, the parallel processes of which establish orthogonal networks with feedback loops, modulatory inputs, and efferents. Shared ground patterns and their selective labeling with antisera against proteins required for normal mushroom body function in Drosophila are indicative of genealogical correspondence and thus an ancestral presence predating arthropod and lophotrochozoan origins. Implications of this are considered in the context of mushroom body function and early ecologies of ancestral bilaterians.