Fig. 2

Morphology and organ arrangement of D. lineata. a External view of a D. lineata polyp. Transparency of the ectodermal cell layer allows us to see black mesenteries and orange-pigmented stripes at PDs, P1s, and P2s, while orange pigmentation sporadically appears at P3s (star). Scale bar indicates 1 mm. b Longitudinal actinopharynx and siphonoglyphs located on the oral disk surrounded by tentacles. Fan-shaped white pigmentation (outlined with gray dashed lines) appears at the base of the tentacles centered around the siphonoglyph. Scale bar indicates 1 mm. te: tentacles. c Horizontal histological section demonstrating orange pigmentation at the endodermal cell layer of the PD, P1, and P2 walls. Scale bar, 200 μm. d Schematic view of a horizontal section of a biradially symmetrical 12-striped (orange) individual. Green regions indicate P3s and pouches adjacent to P3s. Orange regions indicate P1s, P2s, and PDs. e, f External view of red fluorescence protein colocalized with orange stripes at (e) the root of tentacles on the oral disk (f). Scale bar indicates 1 mm. g Horizontal histological section demonstrating mesentery, muscle, and siphonoglyph arrangement. Nuclei are labeled with 4′,6-diamidino-2-phenylindole (white). Scale bar indicates 200 μm. h–k Enlarged views of histological sections of G showing a (h) MD, (i) M1, (j) M2, and (k) M3 (white dotted lines indicate the outline of muscle, blue lines indicate the mesenteries). Scale bars, 50 μm. ap: actinopharynx, mu: muscles, M1: first mesentery. PD, P1, P2, and P3 denote directive, first, second, and third gastric pouches, respectively. Asterisks indicate siphonoglyphs