Confocal micrographs of the FMRFamidergic neurogenesis from the trochophore to the metamorphosis stage of Aeolidiella stephanieae. Anterior faces upwards and scale bars represent 50 μm in all aspects expect the inset in C, in which it equals 10 μm. All aspects are in lateral view except for A and B which are in dorsal view. (A) Early veliger larva showing two round cells (arrowheads) of the apical organ in the anlage of the velum. (B) Slightly older larva as in A showing cells in the future cerebral (cg) and pedal ganglia (pg), two round apical cells (arrowheads) that lie above the cerebral commissure (cc) and project into the respective lobe of the velum (v), and a FMRFamidergic cell (arrow) in the visceral part of the body. (C) Veliger larva with three sensory cells in the apical organ (arrowheads, inset), the median cell is penetrating the epidermis (ep, inset). Note the FMRFamide positive cells in the larval foot (double-headed arrows); mp marks metapodium. (D) Late veliger larva showing elaborated cerebral, pedal, pleural ganglia (pl), cell clusters in the larval foot (double-headed arrow), and one cell cluster in the visceral body part (arrow). Note the remaining two cells of the degenerating apical organ. (E) Larva at the beginning of the metamorphosis showing the resorption of the velum (v), cells in the cerebral, pedal and pleural ganglia as well as cell clusters in the pro- (pp) and metapodium (double-headed arrow). (F) Larva at the metamorphosis showing the FMRFamidergic nervous system consisting of paired cerebral, pedal, pleural ganglia, and cell clusters in the larval foot (double-headed arrow).