Confocal micrographs of the serotonergic neurogenesis in Aeolidiella stephanieae. Anterior faces upwards and scale bars represent 20 μm in all aspects. All aspects are in lateral view. (A) Veliger larva showing few cells in the pedal (pg) and the cerebral ganglion (cg), which is above the apical organ (ao); cil marks the cilia, e the eye, mp the metapodium, and v the velum. (B) Elaborated nervous system of a slightly later stage as in A with cerebral ganglion joined by several connectives with the pedal ganglion, which innervates the larval foot. Note the visceral loop and the fibres in the larval foot; pp marks propodium. (C) Late veliger larva with additional cells in the pedal ganglia innervating the foot. (D) Larva in metamorphosis showing the cerebral and pedal ganglion. Note that there is no serotonergic positive signal in the apical organ. (E) Early juvenile stage with autofluorescent jaws (jw), the fused cerebropleural ganglion, the pedal ganglion and the pleurovisceral loop. Note the three pedal nerves (asterisk) innervating the foot. (F) Magnification of the anterior head-foot of an early juvenile Aeolidiella stephanieae showing cell clusters in the cerebropleural ganglia and a nerve that is running anteriorly (arrow).