Besides the neuropeptidergic cells, apical serotoninergic cells represent a third type of neurosecretory cell projecting into the apical plexus. They exist in mollusks [11], echinoderms [78], enteropneusts [98] and other bilaterian phyla [115]. Serotonergic cells are also enriched in apical
body regions of the hydrozoan cnidarian Phialidium gregarium, where the release of serotonin has been reported to trigger larval settlement [116].