FIGURE 1 ∣.

The time sequence of head regeneration of an individual Ptychodera flava. Note that P. flava worms are very hydraulic and the same animal may be extended and thin one moment and contracted and thick the next. (A) The uncut, intact animal. Arrowheads mark the site at posterior end of the gill basket where the animal was severed. Scale bar = 2 mm in all frames. (B) 0 days post-amputation (dpa); gaping anterior wound where the body wall has been severed. (C) 1 dpa; edges of the severed body wall are slightly swollen and smoothened and the wound has begun to close. (D) 3 dpa; wound has closed and a tiny blastema can be detected dorsal to the closure site (arrow). (E) 5 dpa; the blastema is growing rapidly (arrow). (F) 7 dpa; the blastema is shaped into a nascent proboscis (arrow) with a collar (arrowheads). (G) 11 dpa, the new head, approaching final size to match the original body, is attached to the original anterior cut stump. All views of the left side, except (G), which is more ventral. Arrowheads show the newly regenerating collar. p, proboscis; c, collar; b, branchial region; h, hepatic region.