We found that the apical tuft was formed by two basket-shaped cells with intracellular tubulin support structures (Figure 6C); cells with a very similar morphology, referred to as ampullary cells, have previously been described in mollusk larvae [11]. These cells persisted deep in the medial brain at later stages in the center of a massive commissural and neurosecretory neuropil (Figure 6J), and may thus represent a structural organizing center for the juvenile nervous system, as suggested for other polychaete larvae [50]. Dorsal to the ampullary tuft cells, we found another set of large cells with multiple motile cilia in a crescent-moon shape, known as crescent cells (Figure 6D). Two serotonergic cells have also been found in the apical organ region by 30 hpf [31]. Closest to the tuft was a serotonergic inter- neuron (white arrow, Figure 6D) lacking sensory den- drites. This cell was located deep in the epithelium, adjacent to an assembly of previously described sensory- neurosecretory flask-shaped cells (Figure 6E) that, mor- phologically, resemble chemosensory cells [12] (called parampullary cells in mollusks [11]). More ventral to the parampullary cells, we detected a median pair of cells bearing short, stiff and curly sensory cilia resembling mechanoreceptors (Figure 6F).