Figure 6B; [31]). 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 interneuron
(white arrow, Figure 6D) lacking sensory dendrites.
This cell was located deep in the epithelium,
adjacent to an assembly of previously described sensory neurosecretory
flask-shaped cells (Figure 6E) that, morphologically,
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).