Since otp + cells also exist in the apical organ of the snail Patella [84] and are
overlapping with six3 expression in the apical plate of the
late Nematostella planula [28], we hypothesize that otp +
sensory-neurosecretory cells projecting to the apical plexus formed part of the apical plate in neuralian ancestors and that subsets of these cells became part of the apical organ in the bilaterian lineage. Another gene demarcating the neuropeptidergic sensory-neurosecretory cells, among other cells, is nkx3.