like expression, see above), nkx3 (

like expression, see above), nkx3 (Figure 2Q’), foxj (Figure
2K’) and hox1 (Figure 2L’). In addition, the irx gene is
expressed in the dorsal patch of cells devoid of six3
expression (Figure 2M’). Notably, nkx3 [28], foxj [30], the
hox gene paralog anthox1 [28] and irx [28] are also
expressed in the apical region devoid of six3 in Nematostella;
foxj matches the region devoid of six3 expression in
sea urchin [38] and apical irx expression has been documented
in hemichordates, although broader than in annelids
[25]. An apical region devoid of six3 expression, ‘filled’
by the restricted expression of foxj, nkx3, hox and irx, thus
represents a larval-specific transcription factor signature
that molecularly defines the apical organ in several distinct,
phylogenetically remote neuralian groups such as
anemone and annelid (Figure 3) and lends strong support
to its evolutionary conservation (see Discussion).