Fig. 1. Right (A) and left (A') side views of 24-hour larvae of amphioxus (Branchiostoma floridae) showing the mouth (M) on the left and the primordium of the first gill slit (1) on the ventral right side. The first pigment spot (arrowhead) in the nerve cord is at the level of the primordium of the future third gill slit. Right (B) and left (B') views of a 3-day larva showing gill slits 1 and 2 opening on the right (gill slit 3 has not yet opened) and the mouth (M) opening on the left. Just anterior to the first gill slit is the club-shaped gland (CSG) and the endostyle (E). Scale bars: A, A'=50 µm; B, B'=100 µm.