Retinal determination orthologs are expressed within sea urchin, ( Strongylocentrotus purpuratus, Sp ), and sea star mesoderm. (A-N) WMISH. (A and B) SpEya, (C and D) SpPax6 and (E and F) SpSix1.2 are expressed at the tip of the archenteron in gastrulae (Figures 5A, 5C and 5E) and in a mesodermally derived coelom in early pluteus larvae (Figures 5B, 5D and 5F). PmEya (Figures 5G-5I) and PmPax6 (Figures 5J-5L) expression is first detected in the mesoderm of the archenteron bulb in midgastrulae (Figures 5H and 5K; arrows) and then more prominently in a mesodermally derived coelom in late gastrulae (Figures 5I and 5L; arrow). PmPax6 transcripts are also found broadly throughout the ectoderm (Figure 5K), with more pronounced expression in the apical ectoderm (Figure 5L). SpOpsin1 (Figure 5M) and SpOpsin4 (Figure 5N) are expressed in 1-week-old larvae. (O-R) FISH in 1-week-old larvae. SpEya (Figure 5O) is shown relative to the skeletal marker SpSm50, which was used to orient the embryo. Figures 5P-5R show transcripts of SpOpsin1 (green) colocalizing with those of SpEya (red). Colocalization is shown in yellow. Arrows in Figures 5P, 5Q and 5R point to expression. Additional areas of expression within the 1-week-old larvae may be a result of nonspecific staining.