Fig. 1.

Schematic phylogeny of major metazoan lineages based on genome sequences. Purple lines indicate lineages containing nervous systems. The nervous system evolved early in the evolutionary history of metazoans, but the divergence order of sponges and ctenophores is contentious. It is therefore unclear if the nervous system predates radiation of all extant metazoans as depicted here (and was lost in sponges), evolved after the divergence of sponges or evolved independently in the ctenophore lineage (Ryan, 2014). Functionally polarized neurons with distinct axons (magenta) and dendrites (blue) are characteristic of bilaterians and have been well characterized at the cellular level in both deuterostome and protostome invertebrate model organisms. Cnidarians form a sister group to the bilaterians, but the functional polarity of their neurites has not been examined on a molecular level. It is therefore unknown if true axons and dendrites predate the cnidarian/bilaterian divergence.