The first brain: anatomical evidence for the emergence of the central nervous system in early metazoan evolution

Cnidarians are thought the first animals in Metazoan evolution holding a simple nervous system, with an overlap of diffuse neuronal networks, ring-like concentrations of sensory cells encircling mouth or bell periphery of adult stages. We report here on planula larvae showing bilateral locomotory behaviour and extraretinal photoreception. By immunohistochemistry and confocal microscopy, in these larvae we observed the occurrence of an anterior concentration of different types of sensory cells, together with distinct commissural and longitudinal pathways of interneurons and motoneurons towards the posterior end of the larval body. These findings indicate the occurrence in cnidarians of a structure - consisting of different types of intertwined neurons - having the morphological properties of a brain, including an anterior plexus, transversal commissures, and longitudinal fibers. Both anatomical and behavioural evidences do not contradict the hypothesis of the occurrence of an early central nervous system before the emergence of Bilateria.