Confocal micrographs of serotonergic cells in the apical organ of Aeolidiella stephanieae. Anterior faces upwards and scale bars represent 10 μm in all aspects expect A, in which it equals 50 μm. All aspects are in dorsal view. (A) Early veliger stage (5% of development) showing two flask-shaped, sensory apical cells (1, 2) above the apical neuropil (an); v marks the velum. (B) An additional median flask-shaped, sensory cell (3) in the apical organ of the early veliger. (C) Veliger larva showing a fourth, round cell in the apical organ (4). (D) Same specimen as in C, showing a fifth, round cell (5) laterally to the fourth cell in the apical organ.