Exposure to retinoic acid (RA) or the RA receptor (RAR) antagonist BMS009 disrupts patterning and formation of the amphioxus pharynx. The anterior portions of amphioxus larvae at 36 hours post fertilization (hpf) are shown in lateral views. The right side of the larva is shown on top (with anterior to the right), with the two gill slits (gs), club-shaped gland (csg), endostyle (en) and ciliated pit (cp) in focus, and the left side is shown below (with anterior to the left), with the open mouth (m) in focus. Arrows highlight the first photoreceptor and associated pigment cell in the central nervous system. The 50-μm scale bar in A is applicable to the entire figure. Control larvae at 36 hpf are shown (A), (B), as are 36-hpf larvae treated with 10-6 M RA from 16 hpf (A’), 20 hpf (A”) and 24 hpf (A”’) or 2 × 10-6 M BMS009 from 16 hpf (B’), 20 hpf (B”) and 24 hpf (B”’).