a, Gene expression during early gastrulation and elongation, and in late larvae of T. transversa. The gene nkx2.2 is expressed ventroposteriorly (black arrowhead) and in the pedicle lobe of the larva (arrow). The gene nkx6 is detected in two bilateral symmetrical ectodermal posterior clusters (arrowheads) and in the archenteron wall. In the larva, nkx6 is expressed in the pedicle lobe (arrow) and midgut. pax3/7 is first detected in two ventrolateral domains at the prospective apical-trunk boundary (arrowheads), and in the ventral anterior region of the larva. The gene msx is first expressed dorsally, in the future mantle ectoderm (arrowheads), and in the mantle of the larva. b, In 2-day-old post-metamorphic juveniles, the CNS comprises a main serotonergic anterior commissure (white arrowhead; dorsoventral view) that innervates the developing lophophore. The schematic drawing is not to scale, and the blue line represent the commissure. c, The gene nkx2.1 is expressed in the anterior region (arrowhead), between the lophophores in 2-day-old juveniles. The genes nkx2.2 and nkx6 are expressed in the pedicle (arrowheads), and nkx6 is also detected in the gut (arrow). The gene pax6 shows no expression, pax3/7 is detected in the neural commissure (arrowhead), and msx is expressed in the cells at the edge of the mantle (dotted line). d, Neuronal markers in late larvae of T. transversa. The serotonergic marker tph is expressed in the anteroventral condensation of the mantle lobe (arrowhead) and in dorsal ectodermal cells of the apical lobe (arrow). No expression is detected for Hb9, and the genes dbx, VAchT and ChAT are all detected in the anterior apical neuroectoderm (arrowheads). e, Gene expression during early gastrulation and elongation, and in late larvae of N. anomala. The gene nkx2.2 is expressed in the anterior blastoporal lip at the onset of axial elongation, and it is not detected in the late larva. The gene nkx6 is asymmetrically expressed around the blastopore, in the putative anteroventral ectoderm (arrowhead). As the blastopore closes, the expression extends posteriorly and concentrates along the midline of the larva (arrowhead). The gene pax3/7 is detected in the posterior mesoderm at the onset of axial elongation (arrow). The gene msx is expressed in the prospective mantle lobe ectoderm (arrowheads) and in the dorsal shell-forming epithelium of the late larva. The asterisks indicate the animal/anterior pole and white dashed lines in a and d mark the region of background noise caused by probe trapping in the shell-forming ectoderm. Panel orientations are indicated in the first row/column and apply to the rest of the panels in the same column/row. Scale bar, 100 μm in b.